Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session G1: Invited Session: Anderson-Higgs Boson in Condensed Matter Physics
Sponsoring Units: DCMPChair: Subir Sachdev, Harvard University
Room: Ballroom I
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G1.00001: The ``Higgs'' amplitude mode at the two-dimensional superfluid-Mott insulator transition Invited Speaker: Immanuel Bloch Spontaneous symmetry breaking plays a key role in our understanding of nature. In relativistic quantum field theory, a broken continuous symmetry leads to the emergence of two types of fundamental excitation: massless Nambu--Goldstone modes and a massive `Higgs' amplitude mode. An excitation of Higgs type is of crucial importance in the standard model of elementary particle physics, and also appears as a fundamental collective mode in quantum many-body systems. Whether such a mode exists in low-dimensional systems as a resonance-like feature, or whether it becomes overdamped through coupling to Nambu--Goldstone modes, has been a subject of debate. Here we experimentally find and study a Higgs mode in a two-dimensional neutral superfluid close to a quantum phase transition to a Mott insulating phase. We unambiguously identify the mode by observing the expected reduction in frequency of the onset of spectral response when approaching the transition point. In this regime, our system is described by an effective relativistic field theory with a two-component quantum field, which constitutes a minimal model for spontaneous breaking of a continuous symmetry. Additionally, all microscopic parameters of our system are known from first principles and the resolution of our measurement allows us to detect excited states of the many-body system at the level of individual quasiparticles. This allows for an in-depth study of Higgs excitations that also addresses the consequences of the reduced dimensionality and confinement of the system. Our work constitutes a step towards exploring emergent relativistic models with ultracold atomic gases. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G1.00002: Higgs Excitations in Dimer Antiferromagnets Invited Speaker: Christian R\"uegg In three-dimensional dimer antiferromagnets a generic quantum critical point (QCP) separates a quantum-disordered ground state with a spin gap from a phase with long-range antiferromagnetic order and finite ordering temperature. While this QCP and related phases have been studied intensely in theoretical and numerical work using among other methods bond-operators and quantum Monte-Carlo, real materials in which they can be explored experimentally are rare. Structurally dimerised antiferromagnets are located on the disordered or ordered side of the QCP and application of pressure offers a way to control the ration of exchange interactions in the material across a critical value, if the compressibility and pressure dependence of the exchange are favourable. In TlCuCl$_{3}$ this QCP was realised for the first time and was studied in great detail by neutron scattering. These experiments provide unprecedented insights into the effects of thermal and quantum fluctuations, and of the elementary excitations near QCPs. A unique phenomena is the emergence of longitudinal modes near the QCP, which are the Higgs exceptions in dimer antiferromagnets proposed by S. Sachdev and coworkers. These Higgs exceptions follow precisely scaling predications and are involved in both the quantum and thermal melting of order in such systems. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 1:03PM |
G1.00003: Higgs Bosons in Superconductors Invited Speaker: Chandra Varma Spurred by some strange experimental observations in some superconductors, the theory of a new collective mode\footnote{P.B. Littlewood and C.M. Varma, Phys. Rev. Lett. {\bf 47}, 811 (1981); Phys. Rev. B {\bf 26}, 4883 (1982). } in superconductors and how it can be experimentally found very easily under certain circumstances was provided in 1981. It was called the ``Amplitude Mode'' to distinguish it from the ``Phase Modes'' which provide Josephson effects and which in homogeneous superconductors are coupled to charge density fluctuations and are at the energies of the plasmons. More generally,\footnote{C.M. Varma, J. Low Temp. Phys., {126}, 901 (2002). } this mode is the amplitude mode of a particle-hole symmetric $U(1)$ field, i.e the model treated by Higgs and others in the1960's whose generalization have played an important role in the standard model of particle physics. Recently the amplitude or Higgs mode for d-wave superconductors have also been discussed,\footnote{Y. Barlas and C.M. Varma, arXiv:1206.0400.} where its various cousins may also be found. I will tell the story of the above and why such modes were missed in the theory of superconductivity for so long and the applications of the ideas to modes for cold bosons and fermions in optical lattices. I will also comment, as a very interested outsider and an enthusiast, on the Higgs in particle physics being discovered at LHC from the point of view of the theory of superconductivity. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:39PM |
G1.00004: Fate of the Higgs mode near quantum criticality Invited Speaker: Daniel Podolsky The Higgs mode is a ubiquitous collective excitation in condensed matter systems with broken continuous symmetry. It is expected in antiferromagnets, short coherence length superconductors, charge density waves, and lattice Bose condensates. Its detection is a valuable test of the corresponding field theory, and its mass gap measures the proximity to a quantum critical point. However, since the Higgs mode can decay into low energy Goldstone modes, its experimental visibility has been questioned. Here we show that the visibility of the Higgs mode depends on the symmetry of the measured susceptibility. Furthermore, we investigate the evolution of the Higgs mode upon approach to the Wilson-Fisher fixed point in 2+1 dimensions and demonstrate that the Higgs mode survives as a universal resonance in the scalar susceptibility arbitrarily close to the quantum critical point. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 2:15PM |
G1.00005: The Higgs Mode in Two Dimensional Superfluid Invited Speaker: Nikolay Prokof'ev |
Session G2: Invited Session: Superconductivity in Topological Insulators
Sponsoring Units: DCMPChair: Nitin Samarth, Pennsylvania State University
Room: Ballroom II
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G2.00001: High-temperature superconductivity in topological insulators Invited Speaker: Alex Hayat Interest in the superconducting proximity effect has been reinvigorated recently by novel optoelectronic applications as well as by the possible emergence of the elusive Majorana fermion. However, all previously studied structures were based on low-T$_{\mathrm{c}}$ materials. We have produced high-temperature superconductivity in topological insulators Bi$_2$Se$_3$ and Bi$_2$Te$_3$ via proximity to Bi$_2$Sr$_2$CaCu$_2$O$_{\mathrm{8+\delta}}$, using our new mechanical bonding technique. We have shown proximity-induced superconductivity up to a temperature of at least 80K -- an order of magnitude higher than any previous observations. We have also demonstrated hybrid high-Tc-superconductor-semiconductor tunnel junctions combining Bi$_2$Sr$_2$CaCu$_2$O$_{\mathrm{8+\delta}}$ with graphite, with bulk semiconductors and with semiconductor quantum wells. Our approach provides a simple method of constructing high-T$_{\mathrm{c}}$ tunnel junctions which can conceptually facilitate tunneling spectroscopy studies of novel materials. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G2.00002: Topological superconductivity in IV-VI semiconductors Invited Speaker: Liang Fu |
Tuesday, March 19, 2013 12:27PM - 1:03PM |
G2.00003: The Coexistence of Superconductivity and Topological Order in the Bi2Se3 Thin Films Invited Speaker: Jin-Feng Jia |
Tuesday, March 19, 2013 1:03PM - 1:39PM |
G2.00004: Gate-tuned superconducting transport at the surface of a topological insulator Invited Speaker: Alberto Morpurgo |
Tuesday, March 19, 2013 1:39PM - 2:15PM |
G2.00005: Majorana Bound States and Disclinations in Topological Crystalline Superconductors Invited Speaker: Jeffrey Teo We prove a topological criterion for the existence of zero-energy Majorana bound-state on a disclination, a rotation symmetry breaking point defect, in topological crystalline superconductors (TCS). We first establish a complete topological classification of TCS using the Chern invariant and a few integral rotation invariants. By analytically and numerically studying disclinations, we algebraically deduce a Z$_2$-index that identifies the parity of the number of Majorana zero-modes at a disclination. Surprisingly, we also find weakly-protected Majorana fermions bound at the corners of superconductors with trivial Chern and weak invariants. [Preview Abstract] |
Session G3: Invited Session: Progress in the New Energy Frontier
Sponsoring Units: GERAChair: George Crabtree, Argonne National Laboratory
Room: Ballroom III
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G3.00001: Nanoscience by the megaton: Scalable technologies for a sustainable future Invited Speaker: Peter Littlewood The US uses on average 3 TW of power, which is the average solar insolation on 10,000 km2 of desert. To harvest the solar spectrum, or its energy converted into wind, wave, and rain, we will have to develop a range of linked energy technologies for efficient generation, storage, transmission, and use. These provide many research targets for new materials and processes, where physics dictates we must control electrons on the nanoscale so as to reach acceptable performance levels, and cost requires manufacturing by the square mile. Perhaps with the exception of metal wires, we have no experience in developing functional materials technologies on the scale needed. With an eye on the pairing of photovoltaics and electrical storage, I will outline some of the challenges and the long-term efforts that will be needed to resolve them. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G3.00002: Thermoelectric Phenomena, Materials, Devices, and Applications Invited Speaker: Eric Toberer Thermoelectric materials, which can generate electricity from waste heat or be used as solid-state Peltier coolers, could play an important role in a global sustainable energy solution. However, advanced materials with improved conversion efficiency are required for widespread implementation. Improving thermoelectric efficiency requires reconciling competing electronic and thermal transport properties - a material must have both a large carrier effective mass and mobility and low lattice thermal conductivity. Historically, this has been achieved through engineering carrier scattering rates. This talk will focus on new approaches that achieve these conflicting properties through modifications of the electron and phonon band structures. Example materials such as Yb$_{14}$MnSb$_{11}$ and Ba$_{8}$Ga$_{16}$Ge$_{30}$ will be discussed and pathways towards further material improvements will be highlighted. Such tailored control of transport properties will be vital to realize the next generation of energy materials. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 1:03PM |
G3.00003: When is a polymer conjugated? Invited Speaker: John Lupton When considering the nanoscale structure of an electronic material, one typically focuses on the arrangement in space and the interactions between different molecules. The molecule itself is thought of more in terms of a black box. Yet extrapolating from the chemical structure of a macromolecule, such as a conjugated polymer, to its physical function is by no means trivial. How can one be sure that all pi-bonds really are of the type insinuated by quantum chemistry? Time and frequency-domain spectroscopy - most notably pump-probe, upconversion and photon-echo techniques; and the single-molecule approach - have uncovered surprising heterogeneity in intramolecular couplings within nominally homogeneous pi-conjugated systems. The problem with any spectroscopic approach, however, lies in the fact that one and the same experiment is employed to extract both electronic and conformational information, which are intrinsically interrelated. We reverse this conventional approach of adopting the spectroscopy to a particular material and instead focus on a unique set of model systems with predefined physical shape in order to reveal the intricacies of electronic structure. Shape-persistent conjugated macrocycles can be synthesized with molecular weights comparable to those of short polymers, yet with unparalleled physical control over the actual pi-electron system. Such rings reveal, using single-molecule techniques, the effect of dynamic conjugation: spontaneous symmetry breaking of the pi-system due to interactions with the environment. Chromophores, the electronically-active subunits of pi-conjugated macromolecules, are found to form dynamically, leading to rapid jumps in the polarization of light emitted from such symmetric molecules. This insight reveals that nanoscale structure fundamentally begins at the level of individual carbon bonds, which can exhibit pronounced fluctuations. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:39PM |
G3.00004: Materials for Electrochemical Energy Storage Invited Speaker: Michelle Johannes Electrochemical energy storage is a primary concern of both the consumer and public energy sectors. Energy, once generated, must be stored, transported and retrieved efficiently. This is commonly done through the use of various kinds of batteries and recently through the use of capacitors. Optimal energy storage involves the complete electrochemical system, but many of the performance properties can be understood in terms of the constituent materials that make up the anode, cathode and electrolyte. In this talk will give a brief overview of electrochemical energy storage systems and the role of materials in improving them. Using computational methods as a framework, I will discuss how discuss how macroscopic properties, such as capacity, conductivity, voltage, and stability are determined by fundamental materials properties at the quantum mechanical level. Using the knowledge gained from understanding the underlying processes, I will discuss some common battery materials, such as LiFePO$_4$, layered transition metal oxides, and oxide electrolyte materials. I will show how predictions for better materials can be made using computational tools to save time and money by circumventing expensive screening in the laboratory. I will also discuss how tailoring the morphology of materials, for example by synthesizing at the nanoscale, can have extreme benefits for battery materials performance. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 2:15PM |
G3.00005: Interfacial Effects in Polymer Membranes for Clean Energy Invited Speaker: Christopher Soles Polymeric membranes are critical components in several emerging clean energy technologies. Examples include proton exchange membranes for hydrogen fuel cells, anion exchange membranes for alkaline fuel cells, flow batteries, and even block copolymer membranes for solid electrolytes/separators in lithium ion and other battery technologies. In all of these examples the function of the membrane is to physically separate two reactive electrodes or reactants, but allow the transport or exchange of specific ions through the membrane between the active electrodes. The flow of the charged ionic species between the electrodes can be used to balance the flow of electrons through an external electrical circuit that connects the electrodes, thereby storing or delivering charge electrochemically. In this presentation I will review the use of polymeric membranes in electrochemical energy storage technologies and discuss the critical issues related to the membranes that hinder these technologies. In particular I will also focus on the role the polymer membrane interface on device performance. At some point the polymer membrane must be interfaced with an active electrode or catalyst and the nature of this interface can significantly impact performance. Simulations of device performance based on bulk membrane transport properties often fail to predict the actual performance and empirical interfacial impedance terms usually added to capture the device performance. In this presentation I will explore the origins of this interfacial impedance in the different types of fuel cell membranes (proton and alkaline) by creating model thin film membranes where all of the membrane can be considered interfacial. We then use these thin films as a surrogate for the interfacial regions of a bulk membrane and then quantify the structure, dynamics, and transport properties of water and ions in the confined interfacial films. Using neutron reflectivity, grazing incidence X-ray diffraction, and positron annihilation lifetime spectrocopy, we demonstrate that there can be substantial differences in the structure of the ion transport domains in these interfacial region. However, in-situ measurements including dynamic swelling with X-ray and neutron reflectivity, dynamic quartz crystal microbalance of mass uptake/loss, and dynamic phase modulated infrared absorption measurements and generally support both a reduced solubility and diffusivity of the ionic species in the interfacial region, consistent with enhanced interfacial impedance. [Preview Abstract] |
Session G4: Invited Session: Frontiers in Nanomanufacturing: Atomic Scale Metrology, Large Scale Industry Technology Challegnes and Inherent Device Limitations
Sponsoring Units: FIAPChair: Ernesto Marinero, Hitachi Research Center, San Jose
Room: Ballroom IV
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G4.00001: Time-Resolved, Atomic-Resolution Imaging of Metastable Atom Configurations Invited Speaker: Christian Kisielowski In the recent past significant initiatives are dedicated to the exploration of sustainable energy solutions. Certainly, related research must address a rich diversity of challenges because it is not only the static arrangement of matter that must be understood at a single atom level but also the collective behavior of molecular assemblies that leads to functionality. Moreover, hybrid materials are commonly employed that contain hard and soft matter components to artificially stimulate complex behavior. Electron microscopy is often considered a method of choice that may address these challenges if further improved. This paper reports on the development of in-line holography for atomic-resolution electron microscopy, which makes use of dose rates as low as a few atto Amperes per square {\AA}ngstrom and of variable acceleration voltages between 20 kV and 300 kV [1]. The approach allows for enhancing resolution in radiation sensitive materials and is especially well suited to study the time evolution of nanoscale objects with single atom sensitivity. For the first time temporary displacements of single atoms from their equilibrium lattice sites into metastable sites across a projected distance of only 0.07 nm and 0.10 nm are directly captured in images with a time resolution around one second. These temporary excitations seem relevant to the irreversible transformation of graphene into carbene and to self-diffusion in catalysts. In suitable experimental conditions, however, atom displacements of 0.05 - 0.1 nm are entirely reversible. Exploiting the reversible nature of such excitations, it may become feasible to probe for conformational object changes in beam sensitive materials at improved spatial resolution. \\[4pt] [1] B. Barton, B. Jiang, C.Y. Song, P. Specht, H. Calderon, C, Kisielowski, Atomic-resolution phase-contrast imaging and in-line electron holography using variable voltage and dose rate, \textbf{Microsc. Microanal. 18} (2012) 982--994 [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G4.00002: TBD Invited Speaker: Thomas Albrecht |
Tuesday, March 19, 2013 12:27PM - 1:03PM |
G4.00003: Scalable fabrication of nanostructured devices on flexible substrates using additive driven self-assembly and nanoimprint lithography Invited Speaker: James Watkins Roll-to-roll (R2R) technologies provide routes for continuous production of flexible, nanostructured materials and devices with high throughput and low cost. We employ additive-driven self-assembly to produce well-ordered polymer/nanoparticle hybrid materials that can serve as active device layers, we use highly filled nanoparticle/polymer hybrids for applications that require tailored dielectric constant or refractive index, and we employ R2R nanoimprint lithography for device scale patterning. Specific examples include the fabrication of flexible floating gate memory and large area films for optical/EM management. Our newly constructed R2R processing facility includes a custom designed, precision R2R UV-assisted nanoimprint lithography (NIL) system and hybrid nanostructured materials coaters. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:39PM |
G4.00004: to be determined Invited Speaker: Asen Asenov |
Tuesday, March 19, 2013 1:39PM - 2:15PM |
G4.00005: Imagining and Imaging Future Devices: A Physicist's Dream Invited Speaker: Scott List In the past device scaling followed conventional Dennard scaling with recent introductions of stress to enhance mobility and high k dielectrics to reduce leakage. Future devices will initially need improved electrostatic confinement with associated geometrical complexity, mobility improvements through new materials, steeper sub-threshold slopes through bandgap engineering and 3D system integration. Eventually new state variables beyond electron charge will be necessary to provide both extremely low power and non-volatility. To enable these changes, improved atomic resolution metrology techniques for both complex 3D geometries and new state variables will be required. While there is still plenty of room at the bottom for the physics of these devices, we are more rapidly running out of room for measuring and controlling these devices. Physicists will have an increasingly important role for both imagining and imaging these devices. [Preview Abstract] |
Session G5: Focus Session: Computational Discovery and Design of New Materials: Thermodynamics and Mechanical Properties
Sponsoring Units: DMP DCOMPChair: Hyongki Park, Ohio State University
Room: 301
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G5.00001: Materials Design based on Predictive Ab Initio Thermodynamics Invited Speaker: Joerg Neugebauer A key requirement in developing predictive multi-scale modeling is the availability of accurate computational tools determining energies not only at T = 0 K but also under realistic conditions, i.e., at finite temperature. Combining accurate first principles calculations with mesoscopic/macroscopic thermodynamic and/or kinetic concepts allows now to address this issue and to determine free energies and derived thermodynamic quantities such as heat capacity, thermal expansion coefficients, and elastic constants with an accuracy that matches and often even rivals available experimental data. In the talk a brief overview of the fundamentals and recent developments of combining modern fully parameter-free ab initio methods with thermodynamic concepts will be given with special emphasize on structural materials. The flexibility and the predictive power of these approaches and the impact they can have in developing new strategies in materials design will be discussed e.g. for modern high strength TWIP/TRIP steels, for understanding failure mechanisms such as hydrogen embrittlement, or for identifying chemical trends in the performance of light weight metallic alloys. Work has been done in collaboration with Fritz Kormann, Blazej Grabowski, and Tilmann Hickel. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G5.00002: Metastable Al-rich phases in the Al-Sm system: A genetic-algorithm study Feng Zhang, Zhuo Ye, Ian McBreaty, Mikhail Mendelev, Ryan Ott, Eun Soo Park, Matt Kramer, Cai-Zhuang Wang, Kai-Ming Ho Metallic glasses formed by Al and about 10{\%} rare earths such as Sm are important high-strength-low-density materials. Various metastable crystalline phases are formed in the early stages of the devitrification of Al90Sm10 glasses. Identification of these phases is crucial to understand the phase selection during amorphization and devitrification processes, and thus provides critical information for the control of microstructures in order to obtain desired mechanical properties. In this study, we use a genetic algorithm to systematically study the low energy Al-rich phases of the Al-Sm system. We discovered a new Al5Sm phase that matches excellently with the experimentally detected M1 phase in lattice parameters as well as diffraction patterns. In addition, we established the energy landscape as a function of Al composition on the Al-rich side of the phase diagram, and found key geometries of Sm-centered local clusters which could serve as building blocks for other metastable phases. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G5.00003: Prediction of previously unreported 18-electron ABC materials via first-principles thermodynamics Xiuwen Zhang, Andriy Zakutayev, Arpun Nagaraja, Thomas Mason, David Ginley, Alex Zunger The eighteen electron s2p6d10 ABC compounds derived from an s2p6 binary plus a column X element such as Ni,Pd,Pt have recently been proposed as new topological insulators and thermoelectric materials. Yet, many potentially stable 18 electron compounds are not reported in standard compilations, raising the question if they are stable or unstable. Here we use ``first-principles thermodynamics'' [1] to evaluate the thermodynamic stability of the 401 currently undocumented s2p6d10 ABC materials in the groups I-X-VII, II-X-VI, III-X-V, IV-X-IV, II-IX-VII, III-IX VI, IV-IX-V, and V-IX-IV (but excluding the elements Cs, B, Tl, and Po). The calculation follows three steps: (1) Establishing the lowest energy structure of the ternary. (2) Calculating the energies of documented and undocumented potentially competing phases in the A-B-C system. (3) Examining the dynamic stability of the new compound. We report the stable structures, formation enthalpies (corrected for DFT errors) of the new stable compounds, and document the competing phases of the predicted unstable compounds. Recently one of the predicted stable compounds--TaCoSn--has been successfully synthesized. [1] X. Zhang, L. Yu, A. Zakutayev, and A. Zunger, Adv. Funct. Mater. 22, 1425, (2012). [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G5.00004: Computational Study of the Phase Diagram of Tungsten-Nitride Michael Mehl, Daniel Finkenstadt, Christian Dane, Stefano Curtarolo, Kesong Yang, Gus Hart There has been considerable interest in the Tungsten Nitride (WN) system, as various calculations predict that structures of WN$_2$, WN$_3$, and WN$_4$ may be ultra-compressible and perhaps as hard as diamond. The predicted stability of these structures is based on vibrational stability and a negative formation enthalpy ($\Delta$H) relative to $\alpha$N$_2$ and bcc W. A negative value of $\Delta$H does not guarantee a compound exists, as it may be above the tie-line constructed from other compounds with lower enthalpies. The determination of the tie-line is complicated by the fact that the ground state of WN is not well determined by experiment or theory. We have used AFLOW, extended to include a large variety of Nitrogen containing structures, to compute the formation energy of the WN system over a large range of compositions. In this talk we discuss the ground state of the WN system, the relationship of our predicted structure to experiment, and the relative stability of the possible ``superhard'' WN$_x$ compounds. We find that this behavior depends on whether a GGA or LDA functional is used, probably because of the inability of these functionals to handle van der Waals forces in N$_2$ crystals. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G5.00005: Elastic Constants and Phonons of Tungsten-Nitride from First Principles Christian Dane, Daniel Finkenstadt, Michael Mehl, Stefano Curtarolo Certain Tungsten Nitride (WN) crystal structures have been found to exhibit tendencies for exceptional hardness. Some researchers [S. Aydin et al., J. Mater. Res. 27, 1705 (2012)] have made the claim that these structures have hardness qualities that rival diamond. There are three specific structures with unique compositions that are of interest. By calculating the bulk and shear moduli as well as analyzing phonon dispersion plots, the properties of these structures can be compared to known structures like diamond. We used VASP density-functional methods implemented within the MedeA software package to strain each structure in a series of directions in increasing amounts. A simple linear fit of stress vs. strain found that the leading structure in terms of thermodynamic stability has elastic constants of C$_{11}$ = 753 GPa, C$_{12}$ = 126 GPa, and C$_{44}$ = 172 GPa. These constants, while high, are significantly lower than diamond's. This indicates that previous calculations may have been mistaken in predicting the qualities of the WN system. Some of the difference between our results is due to the exchange-correlation functional chosen, namely, LDA vs. GGA. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G5.00006: Building Symmetry During Crystal Structure Prediction Kyle Michel, Christopher Wolverton Unconstrained crystal structure prediction is difficult in large cells since the number of free variables increases rapidly with the number of atoms that are included. We describe a method that builds symmetry on the fly during crystal structure prediction and uses this symmetry to reduce the dimensionality of the search space. We apply this method to Monte Carlo-based crystal structure prediction and show that simulations that build symmetry greatly outperform those that do not, both in average and fastest times to find the ground state structure. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G5.00007: Simulations of the Structure and Properties of Large Icosahedral Boron Clusters Based on a Novel Semi-Empirical Hamiltonian Paul Tandy, Ming Yu, C.S. Jayanthi, Shi-Yu Wu A successful development of a parameterized semi-empirical Hamiltonian (SCED-LCAO) for boron based on a LCAO framework using a sp$^{3}$ basis set will be discussed. The semi-empirical Hamiltonian contains environment-dependency and electron screening effects of a many-body Hamiltonian and allows for charge self-consistency. We have optimized the parameters of the SCED-LCAO Hamiltonian for boron by fitting the properties (e.g., the binding energy, bond length, etc.) of boron sheets, small clusters and boron alpha to first-principles calculations based on DFT calculations. Although extended phases of boron alpha and beta have been studied, large clusters of boron with icosahedral structures such as those cut from boron alpha are difficult if not impossible to simulate with ab initio methods. We will demonstrate the effectiveness of the SCED-LCAO Hamiltonian in studying icosahedral boron clusters containing up to 800 atoms and will report on some novel boron clusters and computational speed. [1] C. Leahy, et al, Phys. Rev. B 74,155408 (2006). [2] P. Tandy, et al, Bulletin of the APS, 2009 APS March Meeting Vol. 54, Num.1, Sess. D26, [3] Ming Yu, et al, J. Chem. Phys. 130,184708 (2009). [4] Ming Yu, S.Y. Wu, and C.S. Jayanthi, Physica E 42, 1 (2009). [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G5.00008: The Structure, Stability, and Properties of a One-Dimensional $\alpha $-Boron Structure Cherno Baba Kah, Paul Tandy, Ming Yu, C.S. Jayanthi, S.Y. Wu Boron is an electron deficient element exhibiting a complex and versatile chemistry. In this work we have performed a preliminary study on the structural stability and electronic properties of one-dimensional $\alpha $-boron structures based on the SCED-LCAO molecular dynamics scheme (MD) [PRB 74, 15540 (2006)]. The one-dimensional $\alpha $-boron structures were generated by constructing icosahedra B$_{12}$ clusters, referred as $\alpha $-boron balls, and arranging them in one-dimension. Such structures were stabilized through the simulated annealing based on the SCED-LCAO MD. We found that: (1) the $\alpha $-boron ball is compressed in comparison to its bulk counterpart ($\alpha $-phase); (2) the distance between ``$\alpha $-boron balls'' is shorter in the center of the chain than that at the two ends and decreases as the length of the chain increases; (3) the HOMO-LUMO gap is very small ($\sim$1 meV) in the finite chains, but it opens up when the chain length becomes infinite. The optimized lattice constant of the infinite $\alpha $-boron chain was found to be 2.998 {\AA} and its energy gap is found to be 0.74 e. The stability and properties of ring-shaped one-dimensional $\alpha $-boron structures will also be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G5.00009: A framework for the design of nanomaterials with targeted applications Vladan Mlinar Engineered nanomaterials are the key building blocks of modern optoelectronic devices and understanding their structure-property relationship can lead to breakthroughs in device design. Even if we suspect that there may be many structures that are consistent with the targeted property, finding a single one may be a daunting, but often also unnecessary, task because of the enormous space of material parameters. Here, I will show how to parametrize the structure via a set of structural features using the targeted physical property as a constraint. Structural features are extracted from our procedure such that they are relevant to the targeted property/ies and linked to the underlying full atomistic structure. I will discuss the conditions under which the representation of the structure via structural features can be very similar, ideally equivalent, to the full structure \emph{only} relative to our targeted properties. Finally, I will demonstrate the importance and validity of the approach using the example application of engineered nanomaterials for third-generation photovoltaics solar cells. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G5.00010: Optimizing the design of artificial lattices Liang Z. Tan, Steven G. Louie Artificial crystal lattices are a powerful tool for studying other condensed matter systems because they are easily tunable and may be controllably fabricated in the laboratory. For example, artificial graphene can be created out of arrays of CO molecules arranged on a Cu(111) surface. We generalize the idea of artificial graphene, and propose new, unusual band structures that can result from different types of artificial lattices. Because of the high degree of freedom in creating artificial lattices, the task of systematically designing artificial lattices that exhibit these unusual band structures is non-trivial. We address this optimization problem, and show that new physics can be observed in presently feasible artificial crystal lattices. This work was supported by NSF grant No. DMR10-1006184 and U.S. DOE under Contract No. DE-AC02-05CH11231. Computational resources have been provided by NERSC and XSEDE. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G5.00011: Modeling fracture of random media via stochastic molecular mechanics Leon Dimas, Tristan Giesa, Markus Buehler Inspired by recent experimental results suggesting that the heterogeneous distribution of the elastic modulus in bone tissue leads to increased toughness, we determine the toughness modulus of a flawed discrete particle system with stochastic elastic properties. We consider an elastic solid in plane strain conditions in uniaxial tension with a Young's modulus distribution modeled as a 2-d Gaussian process with covariance modeled as an exponential kernel. We solve the problem from a continuum perspective, both employing spectral methods with stochastic finite elements and Monte Carlo methods with conventional finite elements. We also analyze an equivalent discrete particle system, modeled as a spring bead network of FCC-lattices. Our results validate the persistence of the Cauchy Born rule in a stochastic system. We then analyze a flawed discrete particle system to assess the effect of heterogeneity on fracture properties. By studying the fracture mechanics of this system with a range of variance and correlation length parameters in the exponential kernel we gain fundamental insights in to the essential length scales of heterogeneity critical to enhanced fracture properties. This validated stochastic molecular mechanics framework further supports the inverse computation of local elastic properties, not accessible with continuum mechanics, to tailor global mechanical properties such as the fracture toughness. Specifically, Markov Chain Monte Carlo can be used to infer the elastic and geometric parameters. Our work sets the foundation for stochastic modeling in a micromechanical environment and unveils mechanisms by which mechanical behavior can be tailored due to increasingly heterogeneous mechanical properties. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G5.00012: The unsuspected origin of gold's nobleness Marisol Alcantara Ortigoza, Sergey Stolbov Understanding the ``inertness'' of Au toward oxidizing agents - appreciated since long before the beginning of recorded history -- has remained a challenge. Its nobleness has long been attached to its weak interaction with adsorbates, which contrasts with the fact that Au forms stable alloys and can be made reactive. Density-functional-theory (DFT) calculations of the binding energy (BE) of O on (111) surfaces, in fact, have shown that Au stands out for rendering the weakest BE. Here, we reveal the origin of gold's unique inertness by revising the adsorption of this prototype oxidizing agent on several (111) metal surfaces. We show via DFT that, judging by BE of O on Au(111) and Ag(111), e.g., \textit{both} the d-band-center argument \textit{and} analysis of the electronic density of states fail to describe the relatively low reactivity of Au. Nevertheless, we establish that, rather than failure of the above paradigms, a key element to understand BE of adsorbates has been left behind so far. Namely, we demonstrate that, although BE of O is higher on Ag(111) than on Au(111), (1) The local Au-O bonds are \textit{indeed stronger} than the Ag-O ones; (2) the low BE of O on Au is, paradoxically, caused by an unusually large perturbation on Au-Au bonds upon O adsorption. [Preview Abstract] |
Session G6: Focus Session: CVD Graphene - Doping and Defects
Sponsoring Units: DMPChair: Abhay Pasupathy, Columbia University
Room: 302
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G6.00001: Dopants in Chemically Doped Monolayer Graphene Invited Speaker: Liuyan Zhao In monolayer graphene, substitutional doping during chemical vapor deposition (CVD) growth can be used to alter the electronic properties of graphene. To gain full understanding of such chemically doped monolayer graphene, it is essential to learn how the dopants distribute from at atomic scale up to at micron-meter scale, how the dopants modify the electronic structures of the graphene, and how the quasiparticles in graphene behave in the vicinity of the dopants. We use Scanning Tunneling Microscopy/Spectroscopy (STM/S), Micro-Raman spectroscopy, and X-ray absorption spectroscopy to address these questions. In this presentation, we will first show both Nitrogen and Boron atoms dope graphene in the graphitic form, and contribute electron and hole carriers into graphene respectively. Secondly, we will discuss the nature of inter-valley and intra-valley scattering in Nitrogen doped graphene due to the presence of graphitic Nitrogen dopants. Finally, we will show that Nitrogen dopants show sub-lattice clustering and avoid structural features such as domain boundaries of a graphene polycrystal. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G6.00002: Nitrogen incorporation into epitaxial graphene formed on SiC Edward Conrad, Wang Wang, Gang Liu, Sara Rothwell, Leonard C. Feldman, Phil Cohen Substitutional doping is an important way to modify the electronic, chemical, optical and magnetic property of graphene. A significant body of work has shown that nitrogen can be introduced into the graphene structure during CVD growth or by plasma treatments [1,2]. These methods produce a variety of nitrogen defect sites. We present new results on the direct incorporation of nitrogen into graphene as it grows from SiC. The starting material is a sub-monolayer of N at the SiC/SiO2 interface introduced by NO annealing at 1175C [3]. The oxygen is chemically removed to leave $\sim$0.5 ML nitrogen layer that is stable on the SiC(000-1) surface up to 1550C. When heated to 1450C, nitrogen is introduced into the graphene as it grows from the SiC. Post growth studies with Raman Spectroscopy, ARPES, XPS, and LEED show that the N-doped graphene is entirely pyridinic and has a small finite bandgap. This method has an advantage in that the SiC/nitrogen surface can be pre-patterned to high resolution prior to graphene fabrication.\\[4pt] [1] Zhao, L. Y. et al. Science 333, 999-1003 (2011); [2] Lin, Y. C. et al. Appl. Phys. Lett. 96, 133110 (2010); [3] J. Rozen et al, IEEE Transactions on Electron Devices, 0018-9383, 2011 [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G6.00003: Nitrogen-doped graphene: beyond single substitution and enhanced molecular sensing Simin Feng, Junjie Wang, Ruitao Lu, Qing Li, Andr\'es R. Botello-M\'endez, Xavier Declerck, Aur\'elien Lherbier, Ayse Berkdemir, Ana Laura El\'Ias, Rodolfo Cruz-Silva, Morinobu Endo, Humberto Terrones, Jean Christophe Charlier, Minghu Pan, Jun Zhu, Mauricio Terrones Large-area ($\sim$ 4 cm$^{2})$ and highly-crystalline~monolayer nitrogen-doped graphene (NG) sheets have been synthesized on copper foils by ambient-pressure chemical vapor deposition (AP-CVD) method. Scanning tunneling microscopy (STM) and spectroscopy (STS) reveal that the nitrogen dopants in as-synthesized NG samples are separated by one carbon atom and sit consequently on the same sub-lattice of graphene. Based on our first principles and tight binding calculations, this unbalanced distribution of dopants on one of the graphene sub-lattices will promote the opening of an electronic band gap. We control the synthesis parameters and use Raman spectroscopy and electrical transport measurements to monitor the nitrogen doping levels. Finally, we will demonstrate that NG behaves as an efficient molecular sensor, especially when performing graphene-enhanced Raman scattering (GERS) of various organic and bio-molecules. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G6.00004: Temperature-dependent gap opening in doped graphene Choongyu Hwang, Chris Jozwiak, Swanee J. Shin, Eugene H. Haller, Alessandra Lanzara Fundamental physical properties of a material are strongly affected by electronic correlations, which typically reveal their origins through a temperature-dependence study. By using angle-resolved photoemission spectroscopy, we study unusual gap opening in doped graphene as a function of temperature, which poses a strong constraint on the charge carrier scattering mechanism in this system. Our finding provides a potentiality to realize new correlated states with unusual properties. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G6.00005: Mapping the Electron Transport of Graphene Boundaries Using Scanning Tunneling Potentiometry Kendal Clark, Xiaoguang Zhang, Ivan Vlassiouk, Guowei He, Gong Gu, Randall Feenstra, An-Ping Li The symmetry of the graphene honeycomb lattice is a key element for determining many of graphene's unique electronic properties. Topological lattice defects, such as grain boundaries and step edges, break the sublattice symmetry and can affect the electronic properties, especially the transport of graphene. A complete understanding of the physical and electronic properties of defects and boundaries of graphene is needed for future applications. Using a scanning tunneling potentiometry method with a low temperature four-probe scanning tunneling microscope, two-dimensional maps of electrochemical potentials have been measured across individual grain boundaries of graphene films on SiO$_{2}$, as well as across 1ML to 1ML substrate steps and 1ML to 2ML transitions of graphene on SiC. An Atomic Force Microscope (AFM) is implemented to image the grain boundary that forms between coalesced individual graphene flakes on insulating surfaces where as a Scanning Tunneling Microscopy (STM) is implemented for characterizing the SiC grown graphene samples. Results of the influence that various boundaries have on the electronic transport of graphene will be presented. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G6.00006: Chemically decorated line defect as a transport barrier in graphene Carter White, Smitha Vasudevan, Daniel Gunlycke Graphene exhibits itinerant electrons propagating ballistically across its surface. To control electrons injected at a source contact, one needs transport barriers. With reliable transport barriers, electron flow could be directed and modified, a key requirement in nanoelectronics applications. In this presentation, we show that chemically decorated line defects in graphene could act as effective atomically-thin transport barriers. The considered 5-5-8 line defect has both been observed and controllably fabricated. Our density functional theory calculations indicate that diatomic hydrogen, oxygen, and fluorine react exothermically with the 5-5-8 defect inducing effective potentials along this line defect. Transport calculations show that these potentials reduce the electron transmission probability across the line defect converting it from semi-transparent to highly reflective to incoming electrons. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G6.00007: Characterization of line defects in CVD graphene films with scanning plasmon interferometry Zhe Fei, Aleksandr Rodin, Will Gannett, Siyuan Dai, William Regan, Alexander McLeod, Martin Wagner, Benji Aleman, Mark Thiemens, Gerardo Dominguez, Antonio Castro-Neto, Alex Zettle, Fritz Keilmann, Michael Fogler, Dimitri Basov Line defects that are omnipresent in graphene films fabricated with chemical vapor deposition method (CVD) were studied with scanning plasmon interferometry (SPI)---a technique capable of convenient nano-characterization of graphene devices in ambient conditions. The characteristic SPI patterns of line defects are plasmonic twin fringes, which are generated due to interference between surface plasmons (SPs) of graphene launched by a scanning probe and reflected by the line defects. The twin fringes allow us to visualize and distinguish various types of line defects including cracks, wrinkles, and even grain boundaries. Further modeling of the twin fringes provides detailed information on the electronic properties associated with these line defects. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G6.00008: Direct Determination of the Chemical Bonding of Individual Impurities in Graphene Myron Kapetanakis, Wu Zhou, Micah Prange, Sokrates Pantelides, Stephen Pennycook, Juan-Carlos Idrobo Using a combination of Z-contrast imaging and atomically resolved electron energy-loss spectroscopy on a scanning transmission electron microscope, we show that the chemical bonding of individual impurity atoms can be deduced experimentally. We find that when a Si atom is bonded with four atoms at a double-vacancy site in graphene, Si 3d orbitals contribute significantly to the bonding, resulting in a planar sp$^{2}$d-like hybridization, whereas threefold coordinated Si in graphene adopts the preferred sp$^3$ hybridization. The conclusions are confirmed by first-principles calculations and demonstrate that [U+2028] chemical bonding of two-dimensional materials can now be explored an experimental probe at the single impurity level. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G6.00009: Effect of Defects on the Intrinsic Strength and Stiffness of Graphene Ardavan Zandiatashbar, Gwan Hyoung Lee, Hamed Parvaneh, Sung Joo An, Sunwoo Lee, Nithin Mathew, Catalin R. Picu, James Hone, Nikhil Koratkar Mechanical properties of defective mono-layer graphene sheets have been studied using experimental and computational tools. In experiments, elastic properties and breaking strength of free standing monolayer defective graphene membranes are measured by nanoindentation using an atomic force microscope. Defects have been introduced by exposure of membranes to oxygen plasma. Density of defects has been quantified using Raman and Auger electron spectroscopy, and also Transmission electron microscopy. Molecular dynamics simulations have been used to investigate the mechanical properties of free standing monolayer graphene membranes using reactive force fields. The effect of boundary conditions, as well as presence of defects in form of vacancies and bonded epoxide groups has been investigated and compared to experiments. Both experiments and simulations show decrease in Young's modulus and strength of graphene membranes by increasing defect density. However, the change in the elastic modulus is small below a certain defect density, which shows defective graphene membrane can still carry load and stay functional in different applications like decorated carbon based MOSFETs and graphene based nanocomposites. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G6.00010: Stability and Electronic Structures of Al-, Si- and Au-incorporated Divacancy Graphenes: A First-principles Study Na-Young Kim, Eui-Sup Lee, Yong-Hyun Kim C, N, and O decorated divacancy pores in graphene have been reported as well. Especially, the N4 divacancy pore can strongly bind with the divalent 3d transition metals (TMs) because of the large enough pore size and the strong p-d hybridization. Recently, the Si- and Au-incorporated divacancy pore have been also proposed, but understanding of the stability or electronic properties is largerly lacking. In this work, we invesgated the stability and electronic structure of Al-, Si- and Au-incoporated divacancy graphenes decorated with reactangular CmNn, NnOl, and OlCm, based on first-principles density-functional theory (DFT) calculations. We found that the Al-CN3, Si-C2N2, and Au-CN3 are most stable configurations for each cations because the unpaired electrons of edge atoms of divacancy pore could be completely passivated. The binding energies are also higher than cohessive energies due to the strong p-p or p-d hybridization. Because of the strong hybridizaition, the restoration of $\pi $-network of graphene or small band-gap opening near the fermi-level are also observed. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G6.00011: Impact of point defects in graphene systems Miguel Moreno Ugeda, Antonio Mart\'Inez-Galera, Iv\'an Brihuega, Jos\'e Mar\'Ia G\'omez-Rodr\'Iguez Topological defects strongly influence the mechanical, electronic and even magnetic properties of low dimensional carbon-based systems. Taking advantage of the key role of defects in these systems, a unique route based on defect engineering is being developed to broaden the functionalities of graphene. In particular, vacancy-type defects are of an extraordinary importance as they are the key ingredient to understand the new properties shown by functionalized graphene after irradiation. While the role played by these vacancies as single entities has been extensively addressed by theory, experimental data available only refer to statistical properties of the whole heterogeneous collection of vacancies generated in the irradiation process. Scanning tunneling microscopy has great potential in this arena since it enables characterization of point defects at the atomic level. In our work, we first created well characterized individual vacancies on graphene layers by Ar$+$ ion irradiation and then, using low temperature scanning tunneling microscopy/spectroscopy, we individually investigated the impact of each type of such vacancies on the electronic, structural and magnetic properties of several graphene systems [1-3]. [1] M. M. Ugeda, et al, Phys. Rev. Lett 104, 096804 (2010). [2] M. M. Ugeda, et al. Phys. Rev. Lett 107, 116803 (2011). [3] M. M. Ugeda, et al Phys. Rev B,85, 121402 (R) (2012). [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G6.00012: Defect-induced amorphization of single-layer graphene: structure and mechanical properties Corinne Carpenter, Ashwin Ramasubramaniam, Dimitrios Maroudas Defect engineering of graphene provides a potential route for tuning its mechanical, electronic, and chemical properties. While individual defects in single-layer graphene have been investigated in much detail, collective interactions of multiple defects are less well understood. In this work, we address the effects of introducing populations of vacancies in single-layer graphene using classical molecular-dynamics simulations based on reliable bond-order potentials. We study random distributions of vacancies in a single graphene layer with vacancy concentration and temperature being the key parameters in the analysis. We demonstrate that a crystalline-to-amorphous structural transition occurs at vacancy concentrations of 5-10{\%} leading to complete loss of long-range order in the graphene layer. We conduct a systematic parametric study of this phenomenon accompanied by a detailed structural analysis of the defective sheets. We also present systematic studies of tensile tests on these defective graphene sheets and identify trends for the ultimate tensile strength, failure strength, and toughness as a function of vacancy concentration. The implications of our findings for tuning the mechanical and electronic properties of single-layer graphene are discussed. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G6.00013: Heterogeneous Catalysis on Defect-Engineered Graphene M. Samy El-Shall Graphene has attracted great interest for a fundamental understanding of its unique structural and electronic properties and also for important potential applications in nanoelectronics and devices. The combination of thermal, chemical and mechanical stability with the high surface area offers many interesting applications in a wide range of fields including heterogeneous catalysis where metallic and bimetallic nanoparticle catalysts can be efficiently dispersed on the graphene sheets. We have developed facile and scalable chemical and laser reduction methods for the synthesis of defect-engineered graphene, as well as metal and semiconductor nanoparticles dispersed on graphene. We recently discovered a remarkable catalytic activity of metal nanoparticles supported on defect-engineered graphene in a variety of chemical transformation including carbon-carbon cross coupling reactions and Fischer-Tropsch Synthesis of long chain liquid hydrocarbons. The results demonstrate the role of the defect sites on the graphene surface in providing favorable nucleation sites for the selective deposition of the metal nanoparticles and as a result, play a major role in imparting exceptional catalytic properties. [Preview Abstract] |
Session G7: Focus Session: Graphene Devices V
Sponsoring Units: DMPChair: Masa Ishigami, University of Central Florida
Room: 303
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G7.00001: Coarse-grained quantum transport simulation for analyzing leakage-mobility antagonism in GNRFET Masakatsu Ito, Shintaro Sato, Naoki Yokoyama, Christian Joachim Since it became clear that graphene transistors based on the classical MOSFET principle suffer from serious performance problems, researchers have explored new graphene device design using quantum transport simulations. A first-principle quantum transport simulation, however, still takes unaffordable computational cost to deal with a realistic size of graphene transistor ($> 10^4$ atoms). This motivated us to import ESQC (elastic scattering quantum chemistry) technique from the research field of molecular electronics and to develop its coarse-grained version. To eliminate the atomic scale details, we reformulated ESQC technique using the continuum limit description of graphene charge carriers, which is given by the massless Dirac equation. Since the potential function in this Dirac equation is electrostatic potential distribution, it can be obtained from Poisson equation with the boundary conditions of gate voltages in a self-consistent manner. We are now applying this coarse-grained quantum transport simulation to GNRFETs (graphene nanoribbon field effect transistors) for resolving the mobility-leakage antagonism, where opening a bandgap in a graphene channel improves its switching ability but at the same time deteriorates the electron channel mobility. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G7.00002: Density of States and Its Local Fluctuations Determined by Capacitance of Strongly Disordered Graphene Xiaolong Chen, Wei Li, Lin Wang, Yuheng He, Zefei Wu, Yuan Cai, Mingwei Zhang, Yang Wang, Yu Han, Rolf W. Lortz, Zhao-Qing Zhang, Ping Sheng, Ning Wang We demonstrate that local fluctuations of the density of states (DOS) in strongly disordered graphene play an important role in determining the quantum capacitance of the top-gate device geometry. Depending on the strength of the disorder induced by metal-cluster decoration, the measured quantum capacitance of disordered graphene could dramatically decrease in comparison with pristine graphene (previous work on transport of metal-cluster decoration has been published on Phys. Rev. B 84, 045431, 2011). A quantitative model for correlating fluctuations of local density of states with the disorder strength and quantum capacitance is presented and discussed. The DOS of disordered graphene obeys a non-universal power law. By measuring the quantum capacitance of disordered graphene, we simultaneously determined both the DOS and its local fluctuations, which is in agreement with the lognormal distributions reported previously for localized samples. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G7.00003: Understanding Quantum Transport and the Kondo Effect in 2D Carbon Systems Ross McIntosh, Dmitry Churochkin, Somnath Bhattacharyya The rich physics surrounding correlations between conduction electrons and local spins in quantum dot systems is of significant interest towards the development of spintronic quantum information devices. In this study we establish the Kondo effect in reduced graphene oxide (RGO) films through a metal-insulator transition in resistance versus temperature interpreted within the Fermi liquid description of the Kondo effect and negative magnetoresistance which scales with a Kondo characteristic temperature. With a microstructure consisting of intact graphene nano-islands embedded within residual functionalized regions where local magnetic moments may form, RGO is effectively a disordered quantum dot system. This work is augmented with a theoretical study of transport through nano-scale multiple quantum dot devices. Solving within a Keldysh formalism we scrutinize quasi-bound state formation in a range of geometrical quantum dot configurations in order to interpret coherent quantum interference effects. We demonstrate negative differential conductance and control over device parameters such as the characteristic time. This tandem approach illustrates the promise of innovative low dimensional carbon spintronic devices. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G7.00004: An Essential Mechanism of Heat Dissipation in Nanocarbon Electronics Slava V. Rotkin, Alexey G. Petrov Nanocarbon materials were proposed and have been already used for fabricating electronic devices. These nanocarbon devices are not unlike other semiconductor devices and are subject to Joule losses. However the physics of heat dissipation in those materials is unlike the classical thermal physics. This talk focuses on near-field radiative heat dissipation mechanism in nanocarbon materials, the associated component of the heat transport across the interface with the substrate and remote Joule losses. Analytical theory was derived which allows one to trace the origin of anomalously large strength of the effect and predict dependence on the materials parameters. It was predicted to be very substantial and for certain surface morphology dominate over other mechanisms, as it was recently shown experimentally. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G7.00005: Determination of dominant scatterer in Graphene on SiO$_2$ using atomic hydrogen Jyoti Katoch, Duy Le, Talat Rahman, Masa Ishigami We have measured the impact of low energy atomic hydrogen (\textless\ 250meV) on the transport property of graphene sheets as a function of hydrogen coverage and initial, pre-hydrogenation field-effect mobility. In order to understand the correlation between the field effect mobility and the apparent affinity of atomic hydrogen to graphene, we have performed a detailed temperature programmed desorption study on hydrogen-dosed graphene sheets. Atomic hydrogen is found to be desorbing with three different desorption energies. The physisorbed atomic hydrogen on graphene with desorption energy of 60 $\pm$ 30meV (consistent with our theoretical calculations), is found to be correlated to the native scatterers in graphene. The associated charge transfer expected for such small desorption energy indicates that atomic-scale defect sites are not responsible for determining the mobility of graphene on SiO$_{2}$ and that charged impurities, presumably in substrates, define the transport property of graphene on SiO$_{2}$. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G7.00006: The zero-voltage conductance of nano-graphenes: Simple rules and quantitative estimates Matthias Ernzerhof, Yongxi Zhou, Didier Mayou Zero-dimensional graphenes, also called nano-graphenes ($NGs$), consist of fragments of graphene with a finite number of hexagons. $NGs $ can be viewed as a subset of the polycyclic aromatic hydrocarbons (PAHs) that continue to attract chemists attention. We develop a simple theory for the ballistic electron transport through $NGs$ which combines elements of the electronic structure theory of graphene, intuitive methods for the calculation of the molecular conductance, and chemical concepts such as Kekul\'{e} structures. This theory enables one to analyze the relation between the structure of $NGs$ and their conductance. General formulas and rules for the zero-voltage conductance as a function of the contact positions are derived. These formulas and rules require at most simple paper and pencil calculations in applications to systems containing several tens of carbon atoms. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 1:03PM |
G7.00007: Graphene devices and its performance limitations and opportunities Invited Speaker: Tony Low Being a two-dimensional membrane, its mechanical properties such as morphology, strains, phonons can significantly modifies the electronic properties of graphene. In fact, the coupling between the mechanical and electronic properties of graphene plays a key limiting role in the performance of electronics and optoelectronics devices. Modeling studies of several in-house experiments will be discussed to exemplify this point. Next, I will discuss how the modification of electronic properties of graphene via mechanical strains might lead to interesting electronics and electromechanical devices. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G7.00008: Epitaxial Graphene on SiC for Ultra-high Frequency Transistors Zelei Guo, Rui Dong, Partha Sarathi Chakraborty, Nelson Lourenco, James Palmer, Yike Hu, Ming Ruan, John Hankinson, Jan Kunc, John Cressler, Claire Berger, Walt deHeer Electronic devices and systems operating at ultra-high frequencies have recently generated significant interest. Graphene is considered a promising candidate material for high-frequency electronics, due to its intrinsic low dimensionality, high carrier mobility and large carrier velocity. Field effect transistors made of exfoliated graphene flakes as the channel material have shown cut-off frequency (f$_{\mathrm{T}})$ above 400 GHz. However, the maximum oscillation frequency (f$_{\mathrm{max}})$ of graphene transistors, which sets the practical limit on useful circuit operation, to date have not exceeded 45 GHz. We report here record intrinsic f$_{\mathrm{max}}$ of 70 GHz, with f$_{\mathrm{T}}$ exceeding 100 GHz, for transistors based on epitaxial graphene on SiC. In addition to setting a new performance record for graphene technology, these epitaxial graphene transistors were fabricated using well-developed, robust, top-down processes compatible with a mass-production-compatible platform. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G7.00009: Effect of back-gate bias on Graphene RF device performance Wenjuan Zhu, Damon Farmer, Yanqing Wu, Bruce Ek, Keith Jenkins, Phaedon Avouris Graphene is very promising for RF devices due to its high carrier mobility. High cut-off frequency graphene RF devices using CVD grown graphene and epitaxially grown graphene have been reported. Here we report the effect of the back-gate bias on the FET cut-off frequency and current saturation. We found that there are two peak cut-off frequencies corresponding to electron peak trans-conductance and hole peak trans-conductance maxima respectively, as we sweep the top-gate bias. The electron peak cut-off frequency can be significantly increased by applying a positive back-gate bias. The higher the voltage, the larger the maximum cut-off frequency. This can be explained by the additional electron doping introduced by the back-gate bias in the under-lap region, which forms an n-n$+$-n configuration. Similarly, the hole peak cut-off frequency can be significantly enhanced by applying negative back-gate bias to form the p-p$+$-p configuration. The shorter the channel, the more pronounced this effect. We also found that the current saturation is also improved by introducing the same type of carrier as the channel in the under-lap region. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G7.00010: Can we reduce the OFF currents of graphene without hurting their ON currents? Frank Tseng, Gianluca Fiori, Avik Ghosh The current-voltage characteristics of graphene can be understood from Landauer-Keldysh theory. The low bias conductivity is dominated by tunneling through closely spaced modes. The mid-voltage regime is dominated by Coulomb scattering from charge puddles and remote optical phonons that compete with each other towards quasi-saturation. Finally, the high bias physics is given by band-to-band (Zener) tunneling. The overall I-V, consistent with experiments is limited by the lack of a band-gap that compromises the overall gain and inverter characteristics, also seen experimentally. Conversely, structural band-gaps increase the effective mass of the electrons as well as their phase space for scattering, reducing their overall mobility. We show that a way around this dilemma is to engineer sequences of gates that stagger the Dirac point regions in the separately gated graphene segments (equivalently, bandgapped regions for nanoribbons and nanotubes) so as to effectively increase the transmission gap and suppressing subthreshold conduction by two orders of magnitude and extending current saturation without overall ON-current. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G7.00011: Electrical Transport Properties of Graphene Oxide Transistor Using Step-by-Step Reduction Seung Jae Baek, Min Park, Byung Hoon Kim, Yongseok Jun, Yung Woo Park Step by step reduced graphene oxide(GO) thin film transistors were electrically characterized as a temperature and gate voltage. The GO transistors were prepared by thermally reduced using step by step method in same samples. The reduction temperature were subtracted from the inflection point of thermogravimetric analysis(TGA) plot and their points are 88, 158, 185, 215, 250, 300 degree Celsius. All GO condition at various reduction temperatures were defined using Raman spectroscopy and atomic force microscopy(AFM). Temperature dependence electrical measurements were carried out using two terminal technique and various temperatures up to unmeasurable condition. Our charge transport behaviors well fitted to 2 dimensional variable-range hopping(2D VRH) mechanism and fluctuation induced tunneling(FIT) model. Also the conductivity level of each step was increased more than 10$^{4\, }$times. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G7.00012: Mechanism of the doping dependence of 2D Raman band: Dirac-cone migration Ken-ichi Sasaki, Yasuhiro Tokura, Satoru Suzuki, Tetsuomi Sogawa The Raman G and 2D bands are informative characterization tools. The G band can be used to determine whether or not the position of the Fermi energy $\mu$ is close to the Dirac point, since the width broadens when $\mu\simeq 0$, which is known as the Kohn anomaly effect. By contrast, the width of the 2D band sharpens when $\mu\simeq 0$ [1]. We have explored the origin of the difference between the $\mu$ dependencies of the G and 2D bands, first intuitively by employing a concept of a shifted Dirac cone, and then more rigorously in terms of self-energy taking electron-phonon coupling into account[2]. By considering a direct transition in shifted Dirac cones, we clarified that the spectral features of a phonon show varieties of behavior that depend strongly on the value of the phonon momentum $q$. In particular, the resonance decay of a phonon satisfying $v_{\rm F} q > \omega$ ($\omega$ is phonon's frequency) into an electron-hole pair is suppressed when $2\mu < \hbar v_{\rm F}q-\hbar\omega$. The idea of shifted Dirac cone can be applied to a general phonon with a nonzero $q$, including the defect induced D and D$'$ bands, which are of prime importance in recent studies on graphene edges.\\[4pt] [1] Das et al., Nature Nano. (2008).\\[0pt] [2] Sasaki et al., arXiv:1204.4543, PRB (RC) in press. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G7.00013: Effect of gate-induced doping on the Raman spectra of disordered graphene Isaac Childres, Luis A. Jauregui, Yong P. Chen We report a Raman spectroscopy study of graphene field-effect transistors (GFET) after exposure to electron-beam irradiation, used to introduce a controlled amount of defects in graphene. Raman spectra are taken over a range of temperatures (4-300 K), back gate voltages and electron-beam exposures. We observe that the intensity ratio between Raman ``D'' and ``G'' peaks,$ I_{D}/I_{G}$, commonly used to determine the amount of disorder in graphene, not only changes with the irradiation dosage, but also with gate-induced doping. At low temperature (4 K), we observe a peak in the plot of $I_{D}/I_{G}$ versus back gate voltage at the Dirac point of the GFET. As the temperature increases, the back gate voltage at which this peak occurs decreases relative to the Dirac point. Our findings may be valuable for understanding the Raman spectra and electron-phonon physics in doped and disordered graphene. [Preview Abstract] |
Session G8: Graphene: Nanoribbons, Dots, and Strain
Sponsoring Units: DCMPChair: Vincent Meunier, Rensselaer Polytechnic Institute
Room: 307
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G8.00001: Towards a Rigorous Proof of Magnetism on the Edges of Graphene Nanoribbons Hamed Karimi, Ian Affleck A zigzag edge of a graphene nanoribbon supports localized zero modes, ignoring interactions. Based mainly on mean field arguments and numerical approaches, it has been suggested that interactions can produce a large magnetic moment on the edges. By considering the Hubbard model in the weak coupling limit, $U\ll t$, for bearded as well as zigzag edges, we argue for such a magnetic state, based on Lieb's theorem. Projecting the Hubbard interactions onto the flat edge band, we then prove that resulting 1 dimensional model has a fully polarized ferromagnetic ground state. We also study excitons and the effects of second neighbor hopping as well as a potential energy term acting on the edge only, proposing a simple and possibly exact phase diagram with the magnetic moment varying smoothly to zero. Finally, we consider corrections of second order in $U$, arising from integrating out the gapless bulk Dirac excitations. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G8.00002: Quantum Monte Carlo study of edge-state magnetism on chiral graphene nanoribbons Michael Golor, Thomas C. Lang, Stefan Wessel We investigate the edge-state magnetism of chiral graphene nanoribbons using projective quantum Monte Carlo (QMC) simulations and a self-consistent mean-field approximation of the Hubbard model. Previous QMC simulations support edge-state ferromagnetism in sufficiently wide zigzag terminated ribbons. We extended these calculations to include the class of chiral graphene nanoribbons and investigate the influence of chirality and ribbon width on spin-spin correlations. The static magnetic correlations are found to rapidly increase with the width of the ribbons for all chiralities, such that already for ribbons of moderate widths we observe a strong trend towards mean-field-type ferromagnetic correlations along the edges. We extract dynamical edge state signatures which can be used to detect edge-state magnetism by scanning tunneling microscopy. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G8.00003: Strain induced magnetism in graphene Lucian Covaci, Francois Peeters Electron-electron interactions are believed not to be very important in graphene since the strength of the on-site Hubbard repulsion is moderate and the electron density of states is small near the Dirac points. Even so, graphene is believed to be in the proximity of a phase transition between a conducting state and an insulating one (antiferromagnetic or spin liquid). Finding a way to bring graphene across the transition is thus an important issue. We consider the effect of inhomogeneous strain from deformations induced by imperfections or steps in the substrate or from specific strain configurations that give almost constant pseudo-magnetic fields. We perform self-consistent mean field calculations for a tight-binding Hamiltonian where we consider only a repulsive on-site Hubbard term. We show that due to strain induced modifications of the kinetic energy, the staggered magnetization will become finite near regions where the strain is large. We also uncover that near deformations, spin-polarized states will appear, in a similar way the spin-polarized states appear at zig-zag edges of graphene nanoribbons. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G8.00004: Friction, Adhesion, and Elasticity of Grahene Edges D. Patrick Hunley, Tyler Flynn, Tom Dodson, Abhishek Sundararajan, Mathias Boland, Douglas Strachan Frictional, adhesive, and elastic characteristics of graphene edges are determined through lateral force microscopy. Measurements reveal a significant local frictional increase at exposed graphene edges, whereas a single overlapping layer of graphene removes this local frictional increase. Comparison of lateral force and atomic force microscopy measurements shows that local forces on the probe are successfully modeled with a vertical adhesion in the vicinity of the atomic-scale graphene steps. Lateral force microscopy performed with carefully maintained probes shows evidence of elastic straining of graphene edges which are consistent with out-of-plane bending of the edges. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G8.00005: Single point defect states in an armchair-graphene nanoribbon Chi-Hsaun Chiu, C.S. Chu We investigate in detail the electronic states induced by a single or a few defects in an armchair-graphene nanoribbon (AGNR). A semi-analytical approach is developed for the Lippmann-Schwinger formulation within the tight-binding model. The dependences of the local density of states (LDOS) in the vicinity of the defects on both the defect locations and the nanoribbon widths are explored. In particular, the LDOS characteristics in the gapped or gapless AGNR will be discussed. Our results are compared with exact diagonalization approach. The effects of these point defect states on the transport property of the AGNR will also be presented. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G8.00006: Ab initio electronic structure and transport studies of N$_2^{AA}$-doped armchair and zigzag graphene nanoribbons Jonathan Owens, Eduardo Cruz-Silva, Vincent Meunier Recent work by Lu, et al. (Nature Scientific Reports, DOI: 10.1038) on large sheets of nitrogen-doped graphene, determined that a highly predominant amount of nitrogen dopants (80 \%) form in pairs on the same sub-lattice. Graphene nanoribbons, which are essentially narrow strips of graphene, have a natural band gap and tunable electronic properties, making them a promising candidate for scalable nanoelectronics. In this work we explore various electronic structural (density of states, local density of states, and STM images) and transport properties of armchair (aGNR) and zigzag (zGNR) graphene nanoribbons under different orientations of the N$_2^{AA}$ dopants with respect to the ribbon growth direction. For all configurations of zGNRs and aGNRs, we see a substantial decrease in conductance due the dopants, as well as spatially localized states opening around the dopant sites. Most notably, however, we observe the emergence of a new stable spin configuration, wherein the spin-up spin-down polarizations of the edges in zGNRs (denoted the antiferromagnetic state) flip near the doping sites, while being in the normal zGNR AFM ground state away from the dopants. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G8.00007: Atypical structural, electronic, and thermoelectric properties of assembled graphene nanoribbons Liangbo Liang, Vincent Meunier, Eduardo Cruz-Silva, Eduardo Gir\~ao Highly ordered assembly of individual graphene nanoribbons (GNRs) into graphene nanowiggles (GNWs) has been recently demonstrated using a surface-assisted bottom-up chemical approach. GNWs are characterized by a periodic repetition of wiggle-like junctions where armchair- or zigzag-edged GNRs sectors alternate. We employed both density functional theory (DFT) and Tight-Binding+U to demonstrate their versatile electro-magnetic properties [Gir\~ao et al, Phys. Rev. Lett. 107 (2011) ]. The coexistence of parallel and oblique sectors leads GNWs to offer a broader set of geometrical parameters to fine tune the electronic band gap from 0.0 eV to 1.7 eV than GNRs [Gir\~ao et al, Phys. Rev. B 85 (2012) ]. Also, the presence of wiggle-like edges dramatically degrades thermal conductance but retains excellent electronic conduction, resulting in significant enhancement of the thermoelectric performance [Liang et al, Phys. Rev. B 86 (2012) ]. Finally, many-electron GW calculations show quasiparticle band gaps of GNWs generally more than twice of their DFT band gaps, reaching 3.7 eV. Furthermore, the gold substrate where GNWs are synthesized is found to lead to band gap reduction owing to substrate polarization effect, consistent with experiments [Liang et al, Phys. Rev. B 86 (2012)]. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G8.00008: Quasiparticle Band Gap modulation in Graphene Nanoribbons Supported on Weakly interacting Surfaces Xueping Jiang, Neerav Kharche, Paul Kohl, Timothy Boykin, Gerhard Klimeck, Mathieu Luisier, Pulickel Ajayan, Saroj Nayak Low dimensional nanostructures such as graphene nanoribbons(GNRs) and hexagonal boron nitride (hBN) have been successfully synthesized in experiments and attract a lot of attention recently. The strong electron-electron interactions due to quantum confinement could alter band gaps of nanostructures, which has been studied thoroughly for GNRs. Band gaps could also be changed by the effect of dielectric screening arising from the surrounding materials such as the substrate. However, this effect has not been thoroughly investigated for GNRs. In contrast, in almost all the experiments GNRs are deposited on different dielectric substrates leaving a gap between theoretical estimates and experimental measurements. The effect of dielectric screening cannot be captured in an effective single particle theory such as the density functional theory (DFT) and the many-body approaches such as \textit{GW} are required. We show the band gaps of the free standing GNRs are reduced as much as 1 eV in spite of weak van der Waals interactions between the GNR and the underlying substrate. This non-local effect can be explained by a semi-classical image charge model and such understanding is critical to the band gap engineering of graphene based devices. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G8.00009: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G8.00010: Snake states and Majorana's in graphene quantum dots in the presence of a p-n junction Francois Peeters, M. Zarenia, J.M. Pereira, Jr., G.A. Farias We investigate the magnetic interface states of graphene quantum dots that contain p-n junctions. Within a tight-binding approach, we consider rectangular quantum dots in the presence of a perpendicular magnetic field containing p-n, as well as p-n-p and n-p-n junctions. The results show the interplay between the edge states associated with the zigzag terminations of the sample and the snake states that arise at the p-n junction, due to the overlap between electron and hole states at the potential interface. Remarkable localized states are found at the crossing of the p-n junction with the zigzag edge having a dumb-bell shaped electron distribution. These states are localized Majorana states. The results are presented as function of the junction parameters and the applied magnetic flux. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G8.00011: Evidence for edge state photoluminescence in graphene quantum dots Kiran Lingam, Ramakrishna Podila, Haijun Qian, Steve Serkiz, Apparao M. Rao For a practical realization of graphene-based logic devices, opening of a band gap in graphene is crucial and has proved challenging. To this end, several synthesis techniques including unzipping of carbon nanotubes, chemical vapor deposition and other bottom-up fabrication techniques have been pursued for the bulk production of graphene nanoribbons (GNRs) and graphene quantum dots (GQDs). However, only a limited progress has been made towards a fundamental understanding of the electronic and optical properties of GQDs. In particular, the origin of strong photoluminescence (PL) in GQDs, which has been attributed to the presence of emissive surface traps and/or the edge states in GQD, remains inconclusive to date. Here, we experimentally show that the PL is independent of the functional groups attached to the GQDs. Following a series of annealing experiments, we further show that the PL in GQDs originates from the edge states, and an edge-passivation subsequent to synthesis quenches PL. These results are consistent with comparative studies on other carbon nanostructures such as GNRs and carbon nano-onions. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G8.00012: Spontaneous Gap Formation in an Uniaxially Strained Graphene Anand Sharma, Valeri N. Kotov, Antonio H. Castro Neto We study the condition of spontaneous gap generation due to Coulomb interaction between anisotropic Dirac fermions in an uniaxially strained graphene. The gap equation is realized as a self-consistent solution for the self- energy i.e., Dyson- Schwinger equation, within static Random Phase Approximation. The mass gap not only depends on the momentum due to long- range nature of the interaction but also on the anisotropy due to uniaxial strain. Using standard numerical analysis we solve the integral equation on a finite grid. We evaluate the mass gap as a function of dimensionless coupling constant for different values of anisotropy parameter and obtain the critical coupling at which the gap becomes non-zero. Our study indicates that with an increase in uniaxial strain in graphene, the critical coupling decreases which is in agreement with our perturbative renormalization group analysis. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G8.00013: Band Gap Opening in Periodically Modified Graphene Marc Dvorak, Zhigang Wu The gapless electronic structure of graphene must be modified to allow a meaningful on-and-off ratio for use in field-effect transistors. Many attempts to create semiconducting graphene have been made; among them, application of periodic structural modifications, such as patterned defects or nanoscale perforation creating a graphene nanomesh, is particularly promising. Extensive theoretical efforts have been spent to investigate such graphene structures, but the precise role of periodic perturbation on band gap opening remains unclear. Here, we show analytically that band gap opening in graphene under a periodic perturbation can be accurately predicted by mapping the perturbative reciprocal lattice vectors onto Dirac points. The modified graphene alternates between a semi-metal and a semiconductor with 8/9 gapless and 1/9 semiconducting. Furthermore, semiconducting modified graphene can be mapped to exactly two corresponding semimetallic carbon nanotubes or graphene nanoribbons. These predictions reveal the fundamental physics of band gap opening in periodically defected graphene and are in excellent agreement with previous and present first-principles results for graphene nanomeshes. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G8.00014: Gauge fields for rippled graphene membranes under central load Salvador Barraza-Lopez, James V. Sloan, Alejandro A. Pacheco, Cedric M. Horvath, Zheng Fei Wang Gauge fields on graphene are invariably expressed in the language of continuum elasticity. Following an approach where the atomic positions play the preponderant role, a model of strain on graphene was developed where all relevant quantities -including gauge fields- are directly expressed in terms of atomic displacements only. Suspended, rippled graphene membranes under cetral load by a sharp object were studied using this approach. The effects from both the pseudo-magnetic field and the deformation potential were included in calculations of the electron density at different spatial locations (the deformation potential acts as an on-site potential energy). The deformation potential -neglected without proper justification in many published works- appears to modify the electronic spectrum dramatically in a qualitative way. Discussion of experiments relevant to the model will also be given. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G8.00015: Theory of electromechanical coupling in dynamical graphene Mircea Trif, Pramey Upadhyaya, Yaroslav Tserkovnyak We study the coupling between mechanical motion and Dirac electrons in a dynamical sheet of graphene. We show that this coupling can be understood in terms of an effective gauge field acting on the electrons, which has two contributions: {\it quasistatic} and purely {\it dynamic} of the Berry-phase origin. As is well known, the static gauge potential is odd in the $K$ and $K'$ valley index, while we find the dynamic coupling to be even. In particular, the mechanical fluctuations can thus mediate an indirect coupling between charge and valley degrees of freedom. [Preview Abstract] |
Session G9: Invited Session: Broadening Participation in Physics and Other STEM Fields
Sponsoring Units: FEdChair: Paul Cottle, Florida State University
Room: 308
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G9.00001: Broadening participation in Natural Sciences and Mathematics at the University of Maryland Baltimore County Invited Speaker: Philip Rous Over the past two decades, UMBC has undertaken a series of efforts to broaden participation in the natural sciences and mathematics, beginning with the establishment of the Meyerhoff program. Using as examples the multiple initiatives that followed, and with a focus on the challenge of increasing access and success of all students who enter as both freshmen and transfer students, I will describe a model of culture change that we have employed repeatedly to understand and guide our efforts in broadening participation. Particular attention will be paid to the concept of cultural capital, the role of innovators and the challenge of scaling small-scale innovations towards institutional change. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G9.00002: APS Initiatives for Broadening Participation Invited Speaker: Theodore Hodapp Women currently earn only about 20{\%} of physics degrees, while African Americans and Hispanic Americans combined -- representing 34{\%} of the US population in their 20's -- earn only 9{\%} and 5-6{\%} of the Bachelor and Doctoral degrees respectively. To address these disparities, and improve conditions for everyone who studies physics, the APS devotes significant resources to addressing these concerns and to enabling individuals and groups to work with the APS to advance these goals. In this presentation, I will outline several of our most significant programs, give data that informs decisions to adopt programs, and describe current plans. Included in this is the new APS Bridge Program (www.APSBridgeProgram.org) for increasing underrepresented minority participation at the PhD level, the APS Conferences for Undergraduate Women in Physics (go.aps.org/cuwip), and the APS Minority Scholars Program (www.MinoritiesInPhysics.org). Please bring your ideas and concerns for how we might improve participation for all. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 1:03PM |
G9.00003: Drawing minority students into the physics community Invited Speaker: Paul Gueye In the past few years, the number of African-American undergraduate physics students in the US had a steady decrease with dramatic consequences at many physics departments within Historically Black Colleges and Universities (HBCUs). A similar trend seems to also appear at the graduate level. HBCUs have been known to graduate more than 50{\%} of undergraduate physics majors within this community for many years, a role that is now evaporating. The US African-American community cannot lose the historical and sometimes unnoticed impact of HBCUs in the physics community. The ability for these institutions to recruit, maintain and graduate students with the highest degree has turned a corner and is endangered with the recent closings of many programs. We not only must reverse this trend but also implement a sustainable growth for the future. This is an enormous task for the education community. While there are many outstanding and successful programs that have been developed over the years to target particular areas ranging from early K-12 exposure to producing MS and PhD students, each community/culture is different: one cannot transport someone else's experience and/or program and infuse it into another community. Moreover, the focus must now be comprehensive and not anymore single-centered. This talk will outline some ongoing efforts within the National Society of Black Physicists aimed at infusing a global approach to this problem that targets school districts (K-12) and after school programs, undergraduate and graduate programs within HBCUs, and the larger physic and scientific community. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:39PM |
G9.00004: Drawing Women In: Engaging in Science and Engineering Disciplines Invited Speaker: Senta Greene Recent data on the participation of women in the scientific, technological, engineering, and mathematical (STEM) disciplines shows a landscape that is somewhat different from our expectations in the past. For example, women who earn bachelors' degrees in physics go on to earn PhDs, be hired to faculty positions, and achieve promotions at the same rate as their male counterparts. However, such gains do not foretell equal participation of women in physics since, although girls make up about half of high school physics classes, the fraction of bachelor's degrees earned by women has been flat at around 20{\%} for about a decade. This remains true even with significantly increased awareness of the need to attract more women to STEM fields and despite various interventions to attract and retain talented women. This talk will present an overview of data on women's participation in STEM disciplines, provide possible explanations for the continued failure to attract women to some STEM fields, and give a brief description of some current interventions. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 2:15PM |
G9.00005: How Undergraduate Women Choose STEM Careers Invited Speaker: Roxanne Hughes In 2010 women represented half of the US population and over half of current graduates from college (57{\%}) but less than a third of undergraduate degrees in science and engineering (STEM). This underrepresentation is worse in certain fields such as physics (21{\%}), and engineering (22{\%}) compared to 52{\%} in chemistry. This underrepresentation is not only a social and cultural issue, but it is also cause for alarm in regard to the United States' ability to maintain its technological and economic dominance in the global economy. STEM fields provide valuable contributions to the nation's economic and environmental security (Augustine, 2005; Chang, 2009; Riegle-Crumb and King, 2010; Robelen, 2010; Tessler, 2008), paying practitioners well and bringing in revenue for successful businesses and governments (National Science Board [NSB], 2008; Riegle-Crumb and King). Consequently, addressing the underrepresentation of women and increasing their persistence in STEM fields will increase the number of scientists and engineers contributing to these fields, which could, in turn, improve the nation's economy, safety, and technological revenues. Research indicates that there are internal and external factors that affect the ability of women to see future success in STEM and to identify with the STEM and consequently persist. This presentation will summarize the current literature on issues affecting undergraduate women's retention in STEM as well as present strategies to improve this retention. Part of this presentation will draw from my own research studies in this area. The findings from my study and others reveal that only women who participate in redefinition strategies related to their marginalized status are able to persist; those who cannot redefine their marginality in relation to the dominant discourse of STEM begin to lose interest or doubt their competence in the field, resulting in their departure from STEM. [Preview Abstract] |
Session G10: Invited Session: Physics for the Public: Advice From the Pros
Sponsoring Units: FOEPChair: James Kakalios, University of Minnesota
Room: 309
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G10.00001: 6 Things Scientists Can Learn from Science Journalists Invited Speaker: Maggie Koerth-Baker When you talk about your research, do you feel like you're talking to yourself? Have ever accidentally left a lay person more confused than they were before they met you? Does your left eye go twitchy every time a journalist calls? Communicating science is scary. Fortunately, the same lessons that turn cringe-worthy journalism into smart science reporting can help you do a better job of communicating your own work--whether directly to the public, or to journalists, themselves. Don't freak out. Don't give up. Instead, come to this presentation. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G10.00002: Explaining Physics in a Minute (Or Two) Invited Speaker: Henry Reich It's usually assumed that youtube is just for kittens, babies, and music videos. However, youtube is also one of the highest-traffic sites on the internet and it turns out it's actually a darn good place to teach people about physics! We'll start with the story and analysis of how the video series Minutephysics grew from a fun weekend project to one of the top channels on youtube, then discuss how media and technology (especially videos) can facilitate good (and bad) communication, and finally talk about how you can harness the power of the internet in your own physics outreach. Of course we'll watch a few cool videos along the way. As a primer, feel free to check out www.youtube.com/minutephysics (this abstract based on http://pirsa.org/11110110) [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 1:03PM |
G10.00003: Capturing Science in Action: From Exploring the Origin of the Universe to a Journey to the Ends of the Earth Invited Speaker: Paul Steinhardt The public, including aspiring young scientists, seldom gets a sense of what science really feels like as it is happening -- the doubts, the fears, the twists and turns, the joy of victory and the agony of defeat. Even if the science is still uncertain, insights of this type have both inspirational and historic value. This talk will explore this issue using two very different examples from the speaker's own experience: an attempt to develop a theory of the origin of the universe that challenges the conventional big bang picture and a geological expedition to one of the most remote places on the planet in search of the first natural quasicrystal. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:39PM |
G10.00004: All the Data That's Fit to Print: How Does the New York Times Cover Physics? Invited Speaker: Kenneth Chang |
Tuesday, March 19, 2013 1:39PM - 2:15PM |
G10.00005: Science for the Public Through Collaboration and Humor Invited Speaker: Richard Wargo The transformation of all things media and information into a dynamic environment of user access has created what seems infinite possibilities to inform the public in many different ways - as well as seemingly infinite possibilities to confuse. This talk will describe a rather non-conventional collaboration between two different creative cultures and its significance to maintaining scientific accuracy and devising strategies important to audience engagement - among them humor. While focusing on the award-winning effort ``When Things Get Small'' created by University of California Television producer R. Wargo in collaboration with condensed matter physicist I.K. Schuller and actor Adam J. Smith, with both NSF and private support, the case study provides insight into a model and modes which can be used successfully by other scientists to engage the public in what they do. [Preview Abstract] |
Session G11: Invited Session: Concurrent Multiple Length-Scale Modelling
Sponsoring Units: DCOMPChair: Qing Peng, Rensselear Polytechnical Institute
Room: 310
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G11.00001: Quantum Mechanics Based Multiscale Modeling of Materials Invited Speaker: Gang Lu We present two quantum mechanics based multiscale approaches that can simulate extended defects in metals accurately and efficiently. The first approach (QCDFT) can treat multimillion atoms effectively via density functional theory (DFT). The method is an extension of the original quasicontinuum approach with DFT as its sole energetic formulation. The second method (QM/MM) has to do with quantum mechanics/molecular mechanics coupling based on the constrained density functional theory, which provides an exact framework for a self-consistent quantum mechanical embedding. Several important materials problems will be addressed using the multiscale modeling approaches, including hydrogen-assisted cracking in Al, magnetism-controlled dislocation properties in Fe and Si pipe diffusion along Al dislocation core. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G11.00002: \textit{Ab initio} prediction of environmental embrittlement at a crack tip in aluminum Invited Speaker: Derek Warner This talk reports on our \textit{ab initio} predictions of environmental embrittlement in aluminum. We have used an atomistic-continuum multiscale framework to simulate the behavior of a loaded crack tip in the presence of oxygen and hydrogen. The multiscale simulations and subsequent analysis suggest that electronegative surface impurities can inhibit dislocation nucleation from a loaded crack tip, thus raising the likelihood for incremental brittle crack growth to occur during near-threshold fatigue. The metal-impurity bonding characteristics have been analyzed using a Bader charge transfer approximation, and the effect of this bond on the theoretical slip distribution has been investigated using a continuum Peierls model. The Peierls model, which is a function of the position dependent stacking fault energy along the slip plane, was used to estimate the effects of several common environmental impurities. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 1:03PM |
G11.00003: Concurrent multiscale modeling of amorphous materials Invited Speaker: Vincent Tan An approach to multiscale modeling of amorphous materials is presented whereby atomistic scale domains coexist with continuum-like domains. The atomistic domains faithfully predict severe deformation while the continuum domains allow the computation to scale up the size of the model without incurring excessive computational costs associated with fully atomistic models and without the introduction of spurious forces across the boundary of atomistic and continuum-like domains. The material domain is firstly constructed as a tessellation of Amorphous Cells (AC). For regions of small deformation, the number of degrees of freedom is then reduced by computing the displacements of only the vertices of the ACs instead of the atoms within. This is achieved by determining, a priori, the atomistic displacements within such Pseudo Amorphous Cells associated with orthogonal deformation modes of the cell. Simulations of nanoscale polymer tribology using full molecular mechanics computation and our multiscale approach give almost identical prediction of indentation force and the strain contours of the polymer. We further demonstrate the capability of performing adaptive simulations during which domains that were discretized into cells revert to full atomistic domains when their strain attain a predetermined threshold. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:39PM |
G11.00004: Coarse-graining molecular dynamics models using an extended Galerkin method Invited Speaker: Xiantao Li I will present a systematic approach to coarse-grain molecular dynamics models for solids. The coarse-grained models are derived by Galerkin projection to a sequence of Krylov subspaces. On the coarsest space, the model corresponds to a finite element discretization of the continuum elasto-dynamics model. On the other hand, the projection to the finest space yields the full molecular dynamics description. The models in between serve as a smooth transition between the two scales. We start with a molecular dynamics (MD) model, $m_i\ddot{\mathbf x}_i= -\frac{\partial V}{\partial \mathbf x_i}$. First, let $Y_0$ be the approximation space for the continuum model. By projecting the MD model onto the subspace, we obtain a coarse-grained model, $ M \ddot{\mathbf q} = F(\mathbf q)$. Using the Cauchy-Born approximation, this model can be shown to coincide with the finite element representation of the continuum elastodynamics model. This model has limited accuracy near lattice defects. One natural idea is to switch to the MD model in regions surround local defect. As a result, one creates an interface between the continuum and atomistic description, where coupling conditions are needed. Direct coupling methods may involve enforcing constraints or mixing the energy or forces. Such an approach may suffer from large phonon reflections at the interface, and introduce large modeling error. In order to seamlessly couple this model to MD, we successively expand the approximation space to the Krylov spaces, $ K_\ell = Y_0 + A Y_0 + \cdots + A^\ell Y_0$. Here $A$ is the force constant matrix, computed from the atomistic model. Due to the translational invariance, only a smaller number of such matrices need to be computed. By projecting the MD model onto this new subspace, we obtain an extended system, $M \ddot{\mathbf q} = F_0(\mathbf q, \xi_1, \cdots, \xi_{\ell}), \ddot{\xi}_1= F_1(\mathbf q, \xi_1, \cdots, \xi_{\ell}), \cdots \cdots, \ddot{\xi}_{\ell}= F_\ell(\mathbf q, \xi_1, \cdots, \xi_{\ell}).$ The additional variables $\xi_j$ represent the coefficients in the extended approximation space. Using this systematic approach, one can build a hierarchy of models with increasing accuracy, each of which is a well-posed model. At the top of the hierarchy is the continuum model, represented on a finite element mesh. Then, on the same mesh, we obtain higher order approximations of MD. In the limit $\ell \to N$, the full MD description is recovered. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 2:15PM |
G11.00005: Seamless bridging of quantum-mechanics with mechanics and electronic structure calculations at macroscopic scales Invited Speaker: Vikram Gavini Defects play a crucial role in influencing the macroscopic properties of solids---examples include the role of dislocations in plastic deformation, dopants in semiconductor properties, and domain walls in ferroelectric properties. These defects are present in very small concentrations (few parts per million), yet, produce a significant macroscopic effect on the materials behavior through the long-ranged elastic and electrostatic fields they generate. The strength and nature of these fields, as well as other critical aspects of the defect-core are all determined by the electronic structure of the material at the quantum-mechanical length-scale. Hence, there is a wide range of {\it interacting} length-scales, from {\it electronic structure to continuum}, that need to be resolved to accurately describe defects in materials and their influence on the macroscopic properties of materials. This has remained a significant challenge in multi-scale modeling, and a solution to this problem holds the key for predictive modeling of complex materials systems. In an attempt to address the aforementioned challenge, this talk presents the development of a {\it seamless} multi-scale scheme to perform electronic structure calculations at macroscopic scales. The key ideas involved in its development are (i) a real-space variational formulation of electronic structure theories, (ii) a nested finite-element discretization of the formulation, and (iii) a systematic means of adaptive coarse-graining retaining full resolution where necessary, and coarsening elsewhere with no patches, assumptions or structure. This multi-scale scheme has enabled, for the first time, calculations of the electronic structure of multi-million atom systems using orbital-free density-functional theory, thus, paving the way for an accurate electronic structure study of defects in materials. The accuracy of the method and the physical insights it offers into the behavior of defects in materials is highlighted through studies on vacancies and dislocations. Current efforts towards extending this multi-scale method to Kohn-Sham density functional theory will also be presented. [Preview Abstract] |
Session G12: Focus Session: Complex Oxide Interfaces - Polar interfaces III
Sponsoring Units: DMPChair: Guneeta Singh Balla, University of California, Berkeley
Room: 314
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G12.00001: Gated LaAlO$_3$/SrTiO$_3$ based superconducting nanowires Michelle Tomczyk, Guanglei Cheng, Joshua Veasey, Shicheng Lu, Chang-Beom Eom, Patrick Irvin, Jeremy Levy Oxide heterostructures have been shown to support a metal-insulator transition; additionally, below T$_c$, interface transport becomes superconducting. Control of this transition has been demonstrated at the nanoscale level in LaAlO$_3$/SrTiO$_3$ by AFM lithography\footnote{Cen, C. \textit{et al}. \textit{Nature Mater}. \textbf{7}, 298--302 (2008).}. Electrical transport at the 2D interface can be controlled through backgating. Tunability of one dimensional nanostructures created by AFM lithography is demonstrated through backgating of the heterostructure and through local capacitive effects from side gates. Side gates running parallel to the main channel can tune the Fermi level within the channel, facilitating modulation of a normal-superconducting transition in the wire. Local tuning of the carrier density may enable novel superconducting-normal junctions that could be useful for topological quantum computation. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G12.00002: Tuning the carrier density at SrTiO3/LaAlO3 interface by La1-xSrxMnO3 capping layer Yujun Shi, Di Wu The observation of a high-mobility quasi-two-dimensional electron gas (q2-DEG) at the interface between the insulators of SrTiO3 (STO) and LaAlO3 (LAO) has gained significant attention in recent years. The carrier density at these interfaces is usually tuned by controlling the growth conditions or applying an electric field in a three-terminal device. According to the polar catastrophe model, which is used to interpret the origin of the q2-DEG at the LAO-STO interfaces, the carrier density and the critical thickness of LAO for the metallic interfaces are related with the net charge of LaO and AlO2 layer in LAO. Here, we systematically study the growth of La1-xSrxMnO3 (LSMO-x), whose net charge is 1-x in each layer, on LAO (\textless\ 4 u.c.)/STO to tune the interfacial carrier density and critical thickness. For LAO (3 u.c.)/STO, we found that the threshold thickness of LSMO (x$=$0.33) for the observation of q2-DEG is 2 u.c. The LAO (3 u.c.)/STO interfaces show a metal-insulator transition for x between 2/3 and 7/8. Importantly, the carrier density monotonically decreases as increasing Sr doping. Our results strongly support the polar catastrophe model and provide a new approach to tune the interfacial carrier density.. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G12.00003: Simultaneous electromechanical and capacitance characterization of top-gated LaAlO$_3$/SrTiO$_3$ heterostructures Feng Bi, Mengchen Hung, Chung Wung Bark, Sangwoo Ryu, Chang-Beom Eom, Jeremy Levy LaAlO$_3$/SrTiO$_3$ (LAO/STO) heterostructures exhibit a sharp, hysteretic metal-insulator transition (MIT) with enhanced capacitance beyond the geometric limit when the interface is tuned by a biased top gate. To understand the physical origin of these behaviors, we investigate the electromechanical response and capacitance spectroscopy of top-gated LAO/STO heterostructures. Piezoelectric Force Microscopy (PFM) measurements demonstrate local variations in the hysteretic response, and capacitance measurements show carrier density changes at the LAO/STO interface as the top gate bias is varied. A strong correlation between PFM signals and capacitance signals is established by doing simultaneous measurements. The enhanced capacitance at the MIT is correlated with charging/discharging dynamics of nanoscale conducting islands at the interface, which can be imaged by spatially-resolved PFM. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G12.00004: A Gate-tunable Polarized Phase of Two-Dimensional Electrons at the LaAlO$_{3}$/SrTiO$_{3}$ Interface Arjun Joshua, Jonathan Ruhman, Sharon Pecker, Ehud Altman, Shahal Ilani We show using anisotropic magnetoresistance and anomalous Hall effect measurements that the LaAlO$_{3}$/SrTiO$_{3}$ interface has an unconventional phase diagram in the space of electron density and magnetic field. At high densities and fields we observe a polarized phase with crystalline anisotropy. Surprisingly, below a density-dependent critical field the polarization and anisotropy vanish and the resistivity sharply rises. This behavior, unobserved in other magnetic systems, indicates strong coupling between itinerant electrons and localized magnetic moments, enabling gate-tunable magnetism at this interface. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:39PM |
G12.00005: Two-dimensional electron gas at the atomically smooth LaAlO$_{3}$/SrTiO$_{3}$ (111) interface Invited Speaker: Chang-Beom Eom The two-dimensional electron gas (2DEG) at the LaAlO$_{3}$/SrTiO$_{3}$ (001) heterointerface has been widely investigated due to its diverse functionalities such as conductivity, ferromagnetism, and superconductivity. In this orientation, the SrTiO$_{3}$ is nonpolar, with charge-neutral AO and BO$_{2}$ planes, while $+$e of charge is transferred between AO and BO$_{2}$ planes in the LaAlO$_{3}$ layer. The (111) orientation is, however, qualitatively different in that the AO$_{3}$ and B lattice planes in both materials exhibit charge transfer between layers, and both have in principle a polar character. We have found that LaAlO$_{3}$ deposited on the (111) SrTiO$_{3}$ polar surface also supports an interfacial 2DEG. An atomically smooth step and terrace structure of (111) SrTiO$_{3}$ surface was prepared by buffered-HF and heat treatment. The step height of the treated (111) SrTiO$_{3}$ is $\sim$2.25{\AA}, which is 1/3 of the diagonal of the cubic SrTiO$_{3}$ lattice along the [111] direction, consistent with the thickness of one AO$_{3}$/B (111) bilayer. LaAlO$_{3}$ was grown epitaxially in a layer-by-layer growth mode, with one oscillation of the reflection-high energy electron diffraction (RHEED) specular spot corresponding to this single step height. The (111) interfacial 2DEG shows a higher carrier concentration than LAO/STO (001) at room temperature. We find a LaAlO$_{3}$ critical thickness between 11.3 and 16 {\AA}, with the transition between insulating and conducting regimes broader than that of LAO/STO (001). Surface X-ray diffraction with COherent Bragg Rod Analysis (COBRA) has been carried out to explore the possible structural reconstruction of (111) SrTiO$_{3}$. We will discuss the origin of 2DEG at this polar-polar interface. This work has been done in collaboration with S. Ryu, C. W. Bark, T. Hernandez, M. S. Rzchowski, H. Zhou and D. D. Fong, T. R. Paudel and E.Y. Tsymbal. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G12.00006: Two-dimensional electron gasat the interface between two polar oxide materials Tula Paudel, Evgeny Tsymbal Following the discovery of a two-dimensional electron gas (2DEG) at the interface between polar LaAlO$_{3}$ (LAO) and non-polar SrTiO$_{3}$ (STO) grown in the [001] direction many related heterostructures with interesting physical phenomena have been proposed and explored. Here using the first-principles theory, we investigate the electronic band structure of the interface between two polar oxide materials -- a wide materials group that can broaden the field for designing conducing interfaces with novel properties. As a model system, we consider a LAO/STO heterostructure stacking in the [111] direction. In this direction both free standing LAO and STO are polar with alternatively charged planes -- (LaO$_{3})^{3-}$ and Al$^{3+}$ in LAO and (SrO$_{3})^{4-}$ and Ti$^{4+}$ in STO leading to inevitable interface reconstruction. Simple electrostatic arguments suggest that at the Ti/LaO$_{3}$ terminated interface of the LAO/STO(111) heterostructure this reconstruction may be achieved through depositing electron surface charge of 0.5e/$\surd $3a$^{2}$ at the interface. This is by a factor of $\surd $3 smaller than that for the LaO/STO(001) interface which is expected to lead to a larger critical thickness of LAO(111) compared to LAO(001). These arguments are consistent with our first-principles calculations which predict a critical thickness of LAO(111) to be eight (LaO$_{3}$-Al) bilayers. Our findings are consistent with the experimental studies performed by S. Ryu, C. W. Bark, T. Hernandez, M. S. Rzchowski, H. Zhou, D. D. Fong, and C.-B. Eom. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G12.00007: Effect of Surface Engineering on LaAlO$_3$/SrTiO$_3$ Interfaces Sanjay Adhikari, Chang-Beom Eom, Michael klopf, Cheng Cen Carrier properties at the LaAlO$_{3}$/SrTiO$_{3}$ interfaces are highly sensitive to potential profile generated by LaAlO$_{3}$ top surface termination. In ambient environment, the uncontrolled surface exposure may introduce randomly distributed charge or polarization and therefore significantly impact interfacial transport by disorder related effect. As evidence, local fluctuation in carrier density and mobility has been observed in nanostructures defined by atomic force microscope (AFM) lithography. Here we report controlled modification of LaAlO$_{3}$ surface by solvent deposition. Surface desorption is first carried out by sample annealing in O$_{2}$ environment. The annealed LaAlO$_{3}$ surfaces are later coated with various solvents of controlled thicknesses by pulsed laser deposition using frozen targets. Coated surfaces are analyzed by pulsed force and frictional force microscopy. AFM lithography is also carried out to locally alter the surface charge state and modulate the potential disorder level. Effect of different controlled surface coatings on interface are studied by magneto-transport measurement [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G12.00008: A spectroscopic study of the superconductor at the LaAlO$_3$/SrTiO$_3$ interface Hans Boschker, Christoph Richter, Jochen Mannhart The electron liquid at the LaAlO$_3$/SrTiO$_3$ interface is a model system for the study of superconductivity as it provides a two-dimensional superconductor whose properties can be tuned with an electrical gate field. We developed planar tunnel junctions to study the superconductivity spectroscopically. Our tunnel junctions give access to two important physical parameters: the size of the superconducting gap and the electron-phonon spectral function. We will present measurements of both as a function of the electric gate field. The likelihood of the convential electron-phonon coupling mechanism for superconducting pairing will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G12.00009: Nature of Charge Compensation Mechanism in Devices with Polar Catastrophe Raphael Tsu, Wattaka Sitaputra Polar catastrophe (PC) is well-known in surface science driving a charge compensation mechanism (CCM) at surface/interface, with properties unfound in natural solids [1]. Combining PC with superlattice and resonant tunneling, new device opportunity is wide opened. The strange results of oscillations in conductance, between two limits of $G=gG_{o} $ with $g=2,...$in units of $G_{o} =e^{2}/h=39\mu S$, and hysteresis, were observed in nano-sized (tens of nm) crystalized silicon in amorphous silicon matrix, having native oxides, can be explained [2]. Recent observation of substantial enhanced mobility for very large transfer of carrier from Gd$_{2}$O$_{3}$ (100) / Si(100), in the order of $n\sim 10^{20}$ cm$^{-3}$ near the interface may apply to high current MOSFET [3]. The field of PC is dominated by heterostructures. 3D structures are lacking defined interfacial orientation; it is compatible with the CCM incorporating resonant tunneling.\\[4pt] [1] Hwang et al., Nat Mater 11 (2), 103(2012).\\[0pt] [2] Thesis, X. Li, 1993, UNC Charlotte; Thesis, A. Bowhill, 1994, UNC Charlotte; Tsu, Superlattice to nanoelectronics, 2$^{\mathrm{nd}}$ (2011) Elsevier\\[0pt] [3] W. Sitaputra, R. Tsu, (2012). Submitted. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G12.00010: Band alignments at the interface of complex oxides Lars Bjaalie, Anderson Janotti, Chris G. Van de Walle The realization of a two-dimensional electron gas at the SrTiO$_3$/LaAlO$_3$ interface has spurred interest in the development of electronic devices based on complex oxides. In the design of such devices it is crucial to know the band alignment at the interface of the different oxides, a key quantity that governs carrier barrier heights and carrier confinement. Reported values for the valence-band alignment at the SrTiO$_3$/LaAlO$_3$ interface vary by more than 1 eV. Using first-principles calculations based on a hybrid functional we calculate the band alignment at the interface between various complex oxides, including the band insulators SrTiO$_3$, SrZrO$_3$, LaAlO$_3$, CaTiO$_3$, and GaAlO$_3$ and the Mott insulators GdTiO$_3$ and YTiO$_3$. This choice of materials allows us to analyze the effects of cation size, lattice parameters, band gaps, and lattice orientation on the band alignment. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G12.00011: Understanding polarity compensation across polar LaAlO$_3$ films G. Singh-Bhalla, P. Rossen, S. Jaganath, G. Palsson, D. Yi, A. Dasgupta, J. Ravichandran, V. Ruiz, J. Heron, C. Fadley, A. Yadav, R. Pentcheva, R. Ramesh Dipole screening mechanisms for polar crystals can manifest in a variety of ways depending on bandgaps, surface energies and environmental conditions. Here we study the polarity compensation process in LaAlO$_{3}$ thin film grown on the two different surface terminations of [001] SrTiO$_{3}$ crystals (SrO and TiO$_{2})$. An electron gas that appears at the interface between LaAlO$_{3}$ and TiO$_{2}$-SrTiO$_{3}$ surface (n-type) potentially screens the LaAlO$_{3}$ polarity, while the interface between LaAlO$_{3}$ and SrO-SrTiO$_{3}$ (p-type) remains insulating. To understand this asymmetry, we probed the LaAlO$_{3}$ surface using a variety of element-specific probes and observe a change in the LaAlO$_{3}$ stacking structure in the p-type geometry. Tunneling measurements reveal remnants of a built-in field reflective of LaAlO$_{3}$'s intrinsic polarity across the n-type structure, but no such signatures are detected for the p-type structure. When combined with density functional theory simulations, the results suggest that while free charge screens the LaAlO$_{3}$ dipole in the n-type geometry, a change in LaAlO$_{3}$'s structure during growth nullifies the dipole in the p-type geometry. In essence, SrTiO$_{3}$ surface layers drastically affect LaAlO$_{3}$ polarity compensation and in turn the electronic properties. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G12.00012: Stoichiometry, defects, and the polar catastrophe in LaAlO$_3$ thin films on SrTiO$_3$ C. Stephen Hellberg Careful growth of LaAlO$_3$ thin films on SrTiO$_3$ by molecular beam epitaxy has shown that the La/Al ratio of the nominal LaAlO$_3$ layer is key to the formation of a two-dimensional electron liquid at the interface---metallic conductivity is only observed in Al-rich films. The interfacial electron liquid forms due to the polar catastrophe, the diverging potential caused by the atomic layer arrangement at the interface when polar LaAlO$_3$ is grown on TiO$_2$-terminated non-polar SrTiO$_3$. The system eventually reconstructs, moving negative charges to the interface to screen the diverging potential. I will present density functional calculations of the defects that form in LaAlO$_3$ on SrTiO$_3$ to accomodate variations in stoichiometry. In La-rich films, the lowest energy defects are extended and allow cation vacancies to move to the interface to screen the diverging potential. Thus the interface between La-rich LaAlO$_3$ and SrTiO$_3$ remains insulating. In Al-rich films, the defects are localized and block cation motion. In this case a conducting electron liquid forms to screen the diverging potential. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G12.00013: What can we learn from AC impedance study about the bipolar resistive switching effect in LaAlO$_3$/Nb:SrTiO$_3$ heterostructures Xingli Jiang, Yonggang Zhao, Xin Zhang, Meihong Zhu, Huiyun Zhang, Dashan Shang, Jirong Sun Recently, resistive switching (RS) effect has attracted much attention due to its importance in potential applications in resistance random access memory. It has been shown that traps play an important role in RS effect. However, a direct and in-depth study on the characteristics of traps is still lacking so far, including the spatial and energy distribution of traps, relaxation of trapped carriers and transport of carriers via traps, especially the effect of historical process on the transport of carriers, which are important for understanding the mechanism of RS effect and also essential for optimizing devices. We studied the RS effect in heterostructures composed of LaAlO$_3$ (LAO) and Nb:SrTiO$_3$ (NSTO) from 80 to 300 K by using AC impedance technique. It was demonstrated that the bipolar RS effect originates from the LAO/NSTO interface and the resistance states are controlled by the filling status of traps via the trapping/detrapping of electrons. Moreover, the spatial and energy distributions of traps and the effect of history on the transport of carriers were obtained. A model was proposed to explain the experimental results. This work demonstrates that AC impedance technique is powerful for uncovering the mechanism of RS effect. [Preview Abstract] |
Session G13: Topological Insulators: Theory II
Sponsoring Units: DCMPChair: Roman Lutchyn, Microsoft Station Q
Room: 315
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G13.00001: The strong index classification of reflection symmetric topological insulators and superconductors Ching-kai Chiu, Hong Yao, Shinsei Ryu We discuss the topological invariants of topological insulators and superconductors protected by spatial reflection symmetry in any spatial dimensions. In the presence of both (non-spatial) discrete symmetries in the Altland-Zirnbauer classification and reflection symmetry, we introduce two new topological invariants: a mirror integral number and a binary integral number, which is determined by the larger one of the $Z$ number and mirror integral number. We claim that the topological states are characterized by one of `$0$', $Z_2$, $Z$, and the two new topological invariants. Furthermore, those topological invariants are also determined by commutation or anticommutation relations between the discrete non-spatial symmetry operators and the reflection symmetry operator. By using the construction of bulk Dirac Hamiltonians, we provide the complete classification, which still has the same dimensional periodicities with the original Altland-Zirnbauer classification. When a boundary is introduced, which is reflected into itself, these non-trivial topological insulators and superconductors support gapless modes localized at the boundary. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G13.00002: Symmetry protected topological phases from decorated domain walls Xie Chen, Yuan-Ming Lu, Ashvin Vishwanath Symmetry protected topological phases are gapped quantum phases with gapless edge excitations protected by certain symmetries of the system. While SPT phases in lower dimensions (especially 1D) are relatively well understood, less is known about higher dimensional (2D and 3D) SPT phases including what their edge excitations are like and how to detect them. In this work, we present a construciton of $d$ dimensional SPT phases with $Z_2\times G$ symmetry by decorating the $Z_2$ domain walls in the $d$ dimensional bulk with $d-1$ dimensional SPT phases with $G$ symmetry. Such a construction not only provides a simple understanding of higher dimensional SPT phases starting from lower dimensional ones, but also reveals a special topological feature of such SPT phases. That is, on the boundary of the system, the domain wall end points/loops carry gapless edge states of the $d-1$ dimensional SPT phase with $G$ symmetry. We discuss in detail a 2D SPT phase with $Z_2 \times Z_2^T$ symmetry and a 3D SPT phase with $Z_2\times Z_2$ symmetry, which illustrate a more general hierarchical structure of SPT phases related to the cup product of group cohomology. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G13.00003: Lattice model for the surface states of a topological insulator Marcel Franz, Dominic Marchand A surface of a strong topological insulator (STI) is characterized by an odd number of linearly dispersing gapless electronic surface states. It is well known that such a surface cannot be described by an effective two-dimensional lattice model (without breaking the time-reversal symmetry), which often hampers theoretical efforts to quantitatively understand some of the properties of such surfaces, including the effect of strong disorder, interactions and various symmetry-breaking instabilities. Here we describe a lattice model that can be used to describe a pair of STI surfaces and has an odd number of Dirac fermion states with wavefunctions localized on each surface. The Hamiltonian consists of two planar tight-binding models with spin-orbit coupling, representing the two surfaces, weakly coupled to each other by terms that remove the redundant Dirac points from the low-energy spectrum. The utility of this model is illustrated by studying the magnetic and exciton instabilities of the STI surface state driven by short-range repulsive interactions. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G13.00004: 3D Dirac Electrons on a Cubic Lattice with Noncoplanar Multiple-$Q$ Order Satoru Hayami, Takahiro Misawa, Youhei Yamaji, Yukitoshi Motome Noncoplanar multiple-$Q$ orders often lead to new low-energy excitations and/or topologically nontrivial states. In particular, triple-$Q$ orders have attracted much interest due to the emergence of topological (Chern) insulators and associated anomalous quantum Hall effects. In the present study, we explore the possibility of such multiple-$Q$ orderings on a simple cubic lattice and their influence on the electronic structure. We find that a four-sublattice triple-$Q$ magnetic order significantly affects the low-energy single-particle spectrum which is described by the three-dimensional massless Dirac electrons. In order to clarify the stability of such noncoplanar magnetic order in microscopic models, we investigate the ground-state phase diagram of an extended periodic Anderson model on a cubic lattice by mean-field approximation. As a result, we find that the triple-$Q$ phase appears in a wide range of parameters at 3/2 filling. The 3D Dirac nature gives rise to a characteristic gapless surface state. We discuss the bulk and surface electronic states in details. We also discuss a possible realization of a topological insulating phase by opening an energy gap in the triple-$Q$ phase. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G13.00005: Fermi loop in interface states and surface flat bands in diamond lattice models Ryuji Takahashi, Shuichi Murakami Previously we have shown the gapless interface states between two topological insulators with different chiralities by means of the mirror Chern number [1]. In this presentation we use the Fu-Kane-Mele tight-binding model on diamond lattice {\it with} the spin-orbit interaction, and calculate their gapless interface states. We find that when the particle-hole symmetry is imposed in the whole system the Fermi surface of the gapless states becomes a loop in the interface Brillouin zone. We show how to characterize the existence of such Fermi loop in terms of topology. Next we report flat band states in the surface of the diamond lattice model with anisotropic hopping integrals {\it without} the spin-orbit interaction. When anisotropy is not so strong, the surface flat band exits in some part of the Brillouin zone. Moreover when the anisotropy becomes sufficiently strong, the surface flat bands cover the whole surface Brillouin zone. [1] R. Takahashi, S. Murakami, Phys. Rev. Lett. 107,166805 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G13.00006: Effect of electron-phonon interaction on the velocity renormalization of the surface state of 3D topological insulator Qiuzi Li, Sankar Das Sarma Explicitly taking into account of electron-phonon interaction, we consider the velocity renormalization of the surface state of 3D topological insulator. The velocity renormalization is shown to be strongly dependent on the carrier density of the system. We compare our theoretical calculation to recent experimental data. We further consider the correction to the compressibility arising from electron-phonon coupling, and discuss its implication in experiments. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G13.00007: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G13.00008: 2D compressibility of surface states on 3D topological insulators David Abergel, Sankar Das Sarma We develop a theory for the compressibility of the surface states of 3D topological insulators and propose that surface probes of the compressibility via scanning single electron transistor microscopy will be a straightforward way to access the topological states without interference from the bulk states. We describe the single-particle nature of the surface states taking into account an accurate Hamiltonian for the bands and then include the contribution from electron-electron interactions and discuss the implications of the ultra-violet cutoff, including the universality of the exchange contribution when expressed in dimensionless units. We also compare the theory with experimentally obtained $\frac{d\mu}{dn}$ as extracted from angle-resolved photoemission spectroscopy measurements. Finally, we point out that interaction-driven renormalization of the Fermi velocity may be discernible via this technique. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G13.00009: Dislocations in topological phases of matter and their topological terms Akihiro Tanaka, Toru Kikuchi When dislocations are present in topological insulators/superconductors and their variants, they are known to endow subgap boundstates. We revisit their physics from the viewpoint of topological field theories, discussing sevral issues among which are 1) the interplay of the Nieh-Yan torsional invariant with other topological terms, 2) possibile appearance of Nieh-Yan-like terms in nonlinear sigma models of competing orders, 3) the subtle controversy on the absence/existence of Callan-Harvey-like anomaly-inflow in the dual formulation. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G13.00010: Massless Axions: the Callan-Harvey effect revisited Toru Kikuchi, Akihiro Tanaka Axion-like degrees of freedom appear in the low energy physics of various condensed matter systems, which range from quantum spin systems and superconductors to topological insulators and their variants. When topological defects such as domain walls and vortices are formed by the axion fields, their responses to external fields are dominated by the current inflow from the surrounding bulk (Callan-Harvey effect). However, a dual reformulation due to Izquierdo-Townsend is known to present a controversy regarding the existence of this inflow in the case when axions are massless, and can have important consequences. We revisit this problem and discuss its possible relevance to condensed matters. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G13.00011: Semi-metal-insulator transition at the surface of a topological insulator with in-plane magnetization Flavio Nogueira, Ilya Eremin We discuss the role of quantum fluctuations when the surface of a topological insulator (TI) is used as a substrate for a layered ferromagnetic (FM) material. As is well known, an out-of-plane magnetization gaps the surface states and modifies the Landau-Lifshitz-Gilbert equation in an essential way, due to the topological magnetoelectric effect. On the other hand, for the case of in-plane magnetization the surface states are gapless. We show that quantum fluctuations may modify this picture if the exchange interaction between the TI and the FM is sufficiently large. Indeed, we will show that a gap is dynamically generated in this case, turning in this way the semi-metallic state into an insulating one. Another situation of interest where a similar mechanism applies involves the Coulomb interaction between the fermions at the interface between the TI and the FM. The interplay between the magnetization dynamics and the Coulomb interaction is discussed in detail. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G13.00012: Theory of a quantum critical phenomenon in a topological insulator: (3+1)-dimensional quantum electrodynamics in solids Hiroki Isobe, Naoto Nagaosa We study theoretically the quantum critical phenomenon of the phase transition between the trivial insulator and the topological insulator in (3+1) dimensions, which is described by a Dirac fermion coupled to the electromagnetic field. The intriguing result is the recovery of the Lorentz invariance in the infrared limit, and the electrons in solids obey the conventional QED. In detail, the renormalization group (RG) equations for the running coupling constant $\alpha$, the speed of light $c$, and electron $v$ are derived by using perturbative RG method to one-loop level. The almost exact analytic solutions to these RG equations are obtained to reveal that (i) $c$ and $v$ approach to the common value with combination $c^2v$ being almost unrenormalized, (ii) the RG flow of $\alpha$ is the same as that of usual QED with $c^3$ being replaced by $c^2v$, and (iii) there are two crossover momentum/energy scales separating three regions of different scaling behaviors. The dielectric and magnetic susceptibilities, angle-resolved photoemission spectroscopy (ARPES), and the behavior of the gap are discussed from this viewpoint. Reference: H. Isobe and N. Nagaosa, Phys. Rev. B {\bf 86}, 165127 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G13.00013: Nonequilibrium Transport Through a Gate-Controlled Barrier on the Quantum Spin Hall Edge Roni Ilan, Jerome Cayssol, Jens Bardarson, Joel Moore The quantum spin Hall insulator is characterized by the presence of gapless helical edge states where the spin of the charge carriers is locked to their direction of motion. In order to probe the properties of the edge modes, we propose a design of a tunable quantum impurity realized by a local gate under an external magnetic field. Using the integrability of the impurity model, the conductance is computed for arbitrary interactions, temperatures and voltages, including the effect of Fermi liquid leads. The result can be used to infer the strength of interactions from transport experiments. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G13.00014: Magnetic translation algebra with or without magnetic field Christopher Mudry, Claudio Chamon The magnetic translation algebra plays an important role in the quantum Hall effect. Murthy and Shankar have shown how to realize this algebra using fermionic bilinears defined on a two-dimensional square lattice. We show that, in any dimension d, it is always possible to close the magnetic translation algebra using fermionic bilinears, be it in the continuum or on the lattice. We also show that these generators are complete in even, but not odd, dimensions, in the sense that any fermionic Hamiltonian in even dimensions that conserves particle number can be represented in terms of the generators of this algebra, whether or not time-reversal symmetry is broken. As an example, we reproduce the f-sum rule of interacting electrons at vanishing magnetic field using this representation. We also show that interactions can significantly change the bare band width of lattice Hamiltonians when represented in terms of the generators of the magnetic translation algebra. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G13.00015: Spin-orbital Texture in Topological Insulators Chaoxing Liu, Haijun Zhang, Shou-Cheng Zhang Relativistic spin-orbit coupling plays an essential role in the field of topological insulators and quantum spintronics. It gives rise to the topological non-trivial band structure and enables electric manipulation of the spin degree of freedom. Because of the spin-orbit coupling, rich spin-orbital coupled textures can exist both in momentum and in real space. For three dimensional topological insulators in the Bi2Se3 family, topological surface states with pz orbitals have a left-handed spin texture for the upper Dirac cone and a right-handed spin texture for the lower Dirac cone. In this work, we predict a new form of the spin-orbital texture associated with the px and the py orbitals. For the upper Dirac cone, a left-handed (right-handed) spin texture is coupled to the ``radial'' (``tangential'') orbital textures, whereas for the lower Dirac cone, the coupling of spin and orbital textures is the exact opposite. A spin-resolved and photon polarized angle-resolved photoemission spectroscopy experiment is proposed to observe this novel spin-orbital texture. [Preview Abstract] |
Session G14: Focus Session: Magnetic Nanoparticles II
Sponsoring Units: GMAG DMPChair: Brian Kirby, National Institute of Standards and Technology
Room: 316
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G14.00001: Strong exchange coupling in conventional and inverse ferrimagnetic hard/soft and soft/hard core/shell heterostructured nanoparticles Invited Speaker: Josep Nogues Bi-magnetic core/shell nanoparticles are becoming increasingly appealing for diverse fields such as for permanent magnets, microawave absortion, biomedical applications, sensing applications, or future magnetic recording media. Ferrromagnetic (FM)/ antiferromagnetic (AFM) core/shell nanoparticles (or inverted AFM/FM) have been extensively studied. However, exchange coupled hard/soft, or inverse soft/hard, core/shell nanoparticles have been far less investigated. Interestingly, most bi-magnetic core/shell systems are derived by simple partial oxidation of the core, e.g., Co/CoO (FM/AFM) or FePt/Fe$_{3}$O$_{4}$ (hard/soft) and only few studies of heterostructured (where core and shell are formed by different magnetic ions) can be found in the literature. We have investigated conventional hard/soft and inverted soft/hard core/shell hetroestructured nanoparticles based on magnetically soft iron oxide (Fe$_{3}$O$_{4})$ and magnetically hard manganese oxide (Mn$_{3}$O$_{4})$. The core/shell samples were synthesized by seeded growth using either Fe$_{3}$O$_{4}$ or Mn$_{3}$O$_{\mathrm{4}}$ nanoparticles as seeds. Subsequently, thin layers of the complementary material were grown by thermal decomposition of the corresponding metallorganic precursors. The structure characterization (X-ray diffraction and electron diffraction) confirms the presence of cubic (Fe$_{3}$O$_{4})$ and tetragonal (Mn$_{3}$O$_{4})$ phases both at the bulk and local levels. In addition, high resolution transmission electron microscopy (HR-TEM) with electron energy loss spectroscopy (EELS) mapping confirms the core/shell structure of the nanopartciles. Magnetic characterization and element-selective hysteresis loops obtained by x-ray magnetic circular dichroism (XMCD) reveal a strong exchange coupling between the core and the shell which results in homogeneous loops with moderate coercivity. Moreover, the magnetic properties can be tuned by controlling the core diameter or shell thickness. However, the results depend only weakly on the hard/soft or inverse soft/hard morphology. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G14.00002: Magnetic Reversal of Onion-Like Fe$_{3}$O$_{4}$\textbar MnO\textbar $\gamma $-Mn$_{2}$O$_{3}$ Core\textbar Shell\textbar Shell Nanoparticles Kathryn Krycka, Julie Borchers, Mark Laver, German Salazar-Alverez, Alberto Lopez-Ortega, Marta Estrader, Santiago Surinach, Maria Baro, Jordi Sort, Josep Nogues Magnetic nanoparticles offer potential for biomedical and data storage applications, especially with exchange bias to overcome the superparamagnetic limit. Here we study the role of an antiferromagnetic layer sandwiched between a soft ferrimagnetic core and hard ferrimagnetic shell. The nanoparticles studied consist of 3 nm (diameter) Fe$_{3}$O$_{\mathrm{4}}$ \textbar 50-60 nm thick MnO shell \textbar 5 nm thick $\gamma $-Mn$_{2}$O$_{3}$ shell [1]. Small-angle neutron scattering (SANS) probes both structural and magnetic morphology. SANS reveals that during reversal from 5 T to -5 T at 5 K, there is an increase in spins oriented perpendicular to the applied field. As the temperature is increased to 150 K (above the 123 K N\'{e}el temperature of MnO) evidence of an enhanced magnetism from within the MnO shell is observed. Finally, the scattering pattern shifts (indicating a change in the relative magnetism as a function of radius) between 5 K and 50 K. \\[4pt] [1] A. L\'{o}pez-Ortega \textit{et al}., Nanoscale 4, 5138 (2012); Salazar-Alvarez \textit{et al}., J. Am. Chem. Soc., 133, 16738 (2011) [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G14.00003: The Heisenberg Pentamer: Understanding the inelastic neutron scattering selection rules for magnetic clusters Jason Haraldsen Assuming Heisenberg interactions and the symmetric case of a spin S-S' pentamer, the energy eigenstates can be determined exactly. With the energies known, the inelastic neutron scattering intensities are then calculated for the special case of a 1-1/2 pentamer. Through an analysis of these results, two main insights are gained. (1) Because of symmetry constraints, not all $\Delta S_{tot}$ = $\pm$1 transitions are accessible by inelastic neutron scattering (INS). This constrains the standard selections rules for magnetic excitations. (2) The INS signatures of magnetic clusters are directly dependent on the state and component that is excited. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G14.00004: Chemical attachment of magnetic nanoparticles through ``click chemistry'' Yue Liu, Andrew Y. Teplyakov, George C. Hadjipanayis Iron nanoparticles were used as a test system to explore the functionalization and attachment of magnetic nanoparticles with two different functionalities through ``click chemistry.'' Two different samples of iron nanoparticles were modified with 5-azidopentanoic acid and with 5-hexynoic acid, respectively. This modification was followed by click chemistry to change the morphology of agglomeration. A combination of density functional theory calculations, Fourier-transform infrared spectroscopy, and X-ray photoelectron spectroscopy was used to monitor each step of the process. Spectroscopies confirmed the success and completion of click reaction. Scanning electron microscopy images showed the change in size and morphology of the iron nanoparticles before and after click chemistry. Vibrating sample magnetometer study showed the majority of the magnetic properties were retained following functionalization and click reaction. Exploring similar approach for two types of materials with functionalization and attachment of hard magnetic materials and soft magnetic materials will be presented based on our initial studies of SmCo nanoparticles in a combination with iron nanoparticles. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G14.00005: Magnetic Quenching of Plasmon-photonic Activities in Fe$_3$O$_4$-Elastomer Composite Danhao Ma, Dustin Huss, Pralav Shetty, Richard Bell, Mauricio Terrones, Kofi Adu We report for the first time, a systematic study of polarization dependence and the effect of particle size on the optical response of Fe$_{3}$O$_{4}$-silicone elastomer composites in the presence of external magnetic field. The optical response of composites containing 2wt{\%}, 5wt{\%} and 15wt{\%} of 20nm$\le $d $\le $30nm, 40 nm$\le $d$\le $ 60nm and d$\le $ 500nm Fe$_{\mathrm{3}}$O$_{\mathrm{4}}$ particles were aligned in- and out-of-plane in the elastomer host. We observed a systematic redshift in the optical response of the out-of-plane composite samples (containing nanoparticles 20nm$\le $d$\le $30nm) with increasing static magnetic field strength, which saturated near 600 Gauss. There were no observable shifts in the in-plane samples, suggesting that the orientation (polarization) of the magnetic dipole and the induced electric dipole play a crucial role in the optical response. However, we observed a dramatic suppression to near quenching of the plasmonic activities in the micron size particles (d \textless\ 500nm) elastomer composite, suggesting particle size limitations in modulation of plasmon-photonics by external magnetic field. Dipole approximation model is used to explain the quenching phenomenon. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G14.00006: Magnetic properties of Fe and Fe-Pt nanoparticles: application of nano-DFT$+$DMFT Alamgir Kabir, Volodymyr Turkowski, Talat S. Rahman We apply a combined density-functional theory and dynamical mean-field theory (DFT $+$ DMFT) approach [1] to handle reliably nanosized systems which display strong electron correlations. The code that we have recently developed allows one to examine systems containing several hundred atoms with feasible computational time. In particular, we calculate the magnetization of iron and iron-platinum nanoparticles by changing the system size (from 27 to 147 atoms), shape and composition. We demonstrate that the experimentally observed non-monotonous dependence of the magnetization as function of nanoparticle size can be rather accurately reproduced within DFT$+$DMFT, contrary to DFT and DFT$+$U approaches.\\[4pt] [1] V. Turkowski, A. Kabir, N. Nayyar and T.S. Rahman J. Phys.: Condens. Matter 22, 462202 (2010); J. Chem. Phys. 136, 114108 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G14.00007: A comprehensive study of the structure and magnetic properties of Gd13 Cluster Kun Tao, Puru Jena Several experimental and theoretical studies of Gd13 cluster have led to confusing results. While experimental studies using Stern-Gerlach technique yield different magnetic moments, theoretical studies provide different spin orientations and structures. We have carried out a comprehensive study of the structure-magnetic property relationship of Gd13 cluster by examining different isomers. Our calculations are based on density functional theory with GGA$+$U and takes into account spin-orbit interactions and spin canting. The cluster with icosahedra structure and collinear spins has the lowest energy irrespective of the level of theory used. However, the magnetic coupling between the central and surface atoms does depend upon the value of U. For U$=$0 the magnetic coupling in the ground state structure is antiferromagnetic between the central and surface atoms. The coupling changes to ferromagnetic when U \textgreater 4. The effect of temperature on the observed magnetic moment is also studies using Monte Carlo simulation. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G14.00008: Microwave absorption properties of BaGd$_{\mathrm{x}}$Fe$_{\mathrm{12-x}}$O$_{19}$ nanoparticles synthesized by wet milling process Mehmet Burak Kaynar, Sadan Ozcan, S. Ismat Shah It is a big demand to have a wide band, easy to synthesize microwave absorption materials with a high absorption ratio according to their weight. As a solution, nanoparticles are used for the couple of years because of their tunable frequencies by just changing their particle size. Most interesting nano structures for this objective are ferrites. In this work as a microwave absorber, BaFe12O19 and BaGd2Fe10O19 nanoparticles with different particles size are synthesized by the wet milling process. Their crystal structure analyzed by XRD, mean particle sizes were calculated from XRD patterns using rietveld analysis and from TEM images. Magnetic properties are analyzed by using Quantum design VSM. Microwave absorption properties are measured by using coaxial transmission method with an Agilent E5071 VNA. With the change of the last milling time from 0 to 20-hour crystalline sizes are changed from 48 nm to 13 nm. Decrease of particle size give rise to a decrease at coercivity and saturation magnetization of the samples. Change at the hysteresis loops gives a clue to the change of the microwave absorption frequency which is directly observed from the microwave measurements. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G14.00009: Investigation of Local Structures and Magnetism in (Y, Co) codoped CeO2 Nanoparticles T.S. Wu, H.D. Li, Y.W. Chen, S.F. Chen, S.L. Chang, Y.L. Soo Nanocrystals of (Y, Co) codoped CeO2 with different Y concentration prepared by a Polyol method were studied by X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), Raman spectroscopy, x-ray absorption fine structures (XAFS), and superconducting quantum interference device (SQUID) techniques to monitor the structural and magnetic variations of the samples. As revealed by the XRD data, all nanocrystal samples under investigation have similar average particle size. The concentration of O vacancies in the samples was found to increase with Y doping level as indicated by the Raman spectroscopy and XAFS data. Such increase of O vacancies is also accompanied by enhanced ferromagnetism as observed by SQUID measurements. Our experimental results demonstrate clear correlation between magnetism and O vacancies induced by Y doping and therefore are consistent with the bound magnetic polaron model. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G14.00010: ABSTRACT HAS BEEN MOVED TO U43.00003 |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G14.00011: Ferrofluid based micro-electrical energy harvesting Viswas Purohit, Baishakhi Mazumder, Grishma Jena, Madhusha Mishra Innovations in energy harvesting have seen a quantum leap in the last decade. With the introduction of low energy devices in the market, micro energy harvesting units are being explored with much vigor. One of the recent areas of micro energy scavenging is the exploitation of existing vibrational energy and the use of various mechanical motions for the same, useful for low power consumption devices. Ferrofluids are liquids containing magnetic materials having nano-scale permanent magnetic dipoles. The present work explores the possibility of the use of this property for generation of electricity. Since the power generation is through a liquid material, it can take any shape as well as response to small acceleration levels. In this work, an electromagnet-based micropower generator is proposed to utilize the sloshing of the ferrofluid within a controlled chamber which moves to different low frequencies. As compared to permanent magnet units researched previously, ferrofluids can be placed in the smallest of containers of different shapes, thereby giving an output in response to the slightest change in motion. Mechanical motion from 1- 20 Hz was able to give an output voltage in mV's. In this paper, the efficiency and feasibility of such a system is demonstrated. [Preview Abstract] |
Session G15: Focus Session: Quasi-Triangular Frustrated Magnets
Sponsoring Units: GMAG DMPChair: John Schlueter, Argonne National Laboratory
Room: 317
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G15.00001: Phase transition in Ba$_{2}$Ti$_{13}$O$_{22}$ with Ti$^{3+}$ quasi-triangular lattice T. Katsufuji, K. Takayama, T. Koyama, S. Mori, J. Fujioka, Y. Tokura In Ba$_{2}$Ti$_{13}$O$_{22}$, Ti$^{3+}$ ($3d^{1}$) ions form quasi-triangular lattices, and three layers of them (``trilayer'') compose a building block for the crystal structure. We found that this compound exhibits a phase transition at $T_{c} \sim 200$ K, below which the electrical resistivity increases and magnetic susceptibility decreases. We found by electron diffraction measurement that the space group changes at $T_{c}$ from $Cmce$ to $C2/m$, which means that one trilayer and the next trilayer become inequivalent. We also found that a pseudogap appears in the optical conductivity spectra below 0.3 eV at low temperatures. These experimental results suggest that the phase transition is caused by the formation of charge density wave (CDW). However, the almost $T$-linear dependence in the decrease of the magnetic susceptibility below $T_{c}$ is not what is observed in the conventional CDW state, and suggest an exotic nature of the state below $T_{c}$ in the present compound. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G15.00002: Magnetic-filed and angular dependence of magnetism in the triangular Mott insulator k-(BEDT-TTF)2Cu2(CN)3 investigated by 13C NMR Kazuya Miyagawa, Kentaro Umeda, K. Inui, Kazuahi Kanoda The organic conductors, $\kappa$-(BEDT-TTF)$_{2}$X, are prototype for investigating Mott physics and spin frustrations. The X=Cu[N(CN)$_{2}$]Cl is a Mott insulator which undergoes an antiferromagnetic phase transition. On the other hand, title compound which has a triangular lattice does not show a long range magnetic ordering. This suppression is believed to deeply relate to strong spin frustrations. While X=Cu$_{2}$(CN)$_{3}$ does not show magnetic ordering, we previously reported anomalous behaviors in $^{13}$C NMR around 6 K, where heat capacity, thermal conductivity and lattice constant show anomalies as well. So, the 6 K anomaly is a key phenomenon for understanding the origin of absence of magnetic order. We have measured external-filed angular dependence of $^{13}$C NMR under magnetic fields up to 15 T for clarifying the origins of the line broadening and the $1/T_{1}$ anomaly around 6K. At room temperature, angular dependence of spectra is well explained by a crystal structure. We will show the detailed experimental results and discuss the low temperature states. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G15.00003: Frustration dependence of elementary excitation in a quantum spin liquid M. Yamashita, K. Ueda, H. Cui, R. Kato, H.M. Yamamoto, T. Fukunaga, T. Terashima, S. Uji A quantum spin liquid state (QSL) with a magnetic gapless excitation has been found in the organic Mott insulator EtMe$_3$Sb[Pd(dmit)$_2$]$_2$ with nearly identical 2D triangular lattices of $S = 1/2$ [1]. To examine the nature of the QSL, it is essential to determine the phase diagram, especially how the gapless QSL evolves when the degree of frustration is changed. Although the gapless QSL is shown to be robust against deuteration of the cation EtMe$_3$Sb [1], the difference of frustration caused by the deuteration is not clear. We study the frustration dependence of the elementary excitation in the mixed-cation materials (Me$_4$Sb)$_{1-x}$(EtMe$_3$Sb)$_x$[Pd(dmit)$_2$]$_2$ in which the degree of frustration is directly reduced by mixing the smaller cation. Magnetic torque measurements showed that spin susceptibilities of the mixed cation ($x$ = 0.32 and 0.35) were temperature independent down to 30~mK and were almost the same with that of $x = 1$, indicating that the QSL exists as a quantum critical phase, rather than a point, when the frustration is varied. We will also present magnetic torque and thermal transport measurements of mixed-cation materials with different $x$.\\[4pt] [1] D. Watanabe \textit{et al.}, Nat. commun. \textbf{3}, 1090 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G15.00004: Spin freezing in the quasi-triangular layered magnet, Cu$_2$(OH)$_3$NO$_3$ S.A. Solin, F.M. Werner, Jason Gardner, Georg Ehlers We have investigated the structural and magnetic properties of the spin S $=$ 1/2 antiferromagnetic quasi-triangular lattice materials: Cu$_{\mathrm{2(1-x)}}$Zn$_{\mathrm{2x}}$(OH)$_{3}$NO$_{3}$ (0 \textless\ x \textless\ 0.65) using a.c. susceptibility, heat capacity [1,2] and neutron scattering. The spin 1/2 Cu planes in these layered compounds form a very slightly ($\sim$ 1{\%}) distorted triangular lattice. We will briefly describe the techniques for synthesizing the hydrogenated, deuterated and intercalated forms of these compounds and also present a brief introduction to the bulk properties of this family of materials. We will discuss recent neutron scattering results from the pure compound. The temperature dependence of the quasielastic scattering reveals an abundance of slow spin dynamics at elevated temperatures. This scattering collapses as the system is cooled through its ordering temperature (11 K) and several magnetic Bragg reflections and a Q-independent mode are observed at finite energy. We will contrast these results with those seen in triangular systems with a Kagome motif. \\[4pt] [1] J. Wu, et. al., Europhys Lett, 93, 67001 (2011).\\[0pt] [2] J. Wu, et. al., J. Phys.: Condens. Matter 22, 334211 -- 334222 (2010). [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G15.00005: Microscopic models of Pd(dmit)$_2$-based organic charge transfer salts 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$_{n}X$[Pd(dmit)$_2$]$_2$ ($X$=N, As, Sb) salts and investigate how well these systems are described by an anisotropic triangular lattice. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G15.00006: Spin-liquid versus spiral-order phases in the anisotropic triangular lattice Luca F. Tocchio, Helene Feldner, Federico Becca, Roser Valenti, Claudius Gros We study the competition between magnetic and spin-liquid phases in the Hubbard model on the anisotropic triangular lattice, which is described by two hopping parameters $t$ and $t'$ in different spatial directions and is relevant for layered organic charge-transfer salts. By using a variational approach that includes spiral magnetic order, we provide solid evidence that a spin-liquid phase is stabilized in the strongly-correlated regime and close to the isotropic limit $t'/t=1$. Otherwise, a magnetically ordered spiral state is found, connecting the (collinear) N\'eel and the (coplanar) $120^\circ$ phases. The pitch vector of the spiral phase obtained from the unrestricted Hartree-Fock approximation is substantially renormalized in presence of electronic correlations, and the N\'eel phase is stabilized in a wide regime of the phase diagram, i.e., for $t'/t < 0.75$. We discuss these results in the context of organic charge-transfer salts [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 1:03PM |
G15.00007: Magnetic Soft Modes in the Distorted Triangular Antiferromagnet $\alpha$-CaCr$_{2}$O$_{4}$ Invited Speaker: Bella Lake We have explored the phase diagram and excitations of a distorted triangular lattice antiferromagnet. The unique two-dimensional distortion considered here is very different from the ``isosceles''-type distortion that has been extensively investigated. We show that suprisingly it is able to stabilize the 120$^{\circ}$ spin structure (typical of the undistorted triangular antiferromagnet) for a large range of exchange interaction values, with new structures found only for extreme distortions. A physical realization of this model is $\alpha$-CaCr$_{2}$O$_{4}$. Despite its highly symmetric 120$^{\circ}$ spin structure, the magnetic excitation spectrum of $\alpha$-CaCr$_{2}$O$_{4}$ is very complex. The unique pattern of nearest-neighbor exchange interactions as well as the substantial next-nearest-neighbor interactions place it close to the phase boundary of the 120$^{\circ}$ structure as is clearly revealed by the presence of low energy modes acting as soft modes of the neighboring structure. Indeed, fitting to linear spin-wave theory favors a set of exchange parameters within the nearby multi-$k$ phase in contradiction to the observed 120$^{\circ}$ order, and quantum fluctuations may be necessary to stabilize $\alpha$-CaCr$_{2}$O$_{4}$ within the 120$^{\circ}$ phase. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G15.00008: Phase diagram and unusual magnetic excitations in distorted triangular lattice antiferromagnet $\alpha$-$CaCr_20_4$ Samuel Ducatman, Natalia Perkins While it is well known that the ground state of the isotropic Heisenberg model on a triangular lattice is the so called 120$^\circ$ structure, its appearance on the distorted triangular lattice is rather unusual. This case has been recently observed in the distorted triangular lattice antiferromagnet $\alpha$-CaCr$_2$O$_4$ [S. Toth et al, PRB 84, 054452 (2011)] which shows the onset of the 120$^\circ$ long-range magnetic order below $T_N = 42.6 K$. Recent neutron scattering experiments also revealed that this compound has unusual magnetic excitations with a dispersion with roton-like minima at momenta different from those corresponding to its 120$^\circ$-magnetic order [S. Toth et al, PRL 109, 127203 (2012)]. Motivated by these experimental findings, we calculate a magnetic phase diagram and excitation spectrum of anisotropic Heisenberg Hamiltonian on triangular lattice. We showed that at the parameters characterizing $\alpha$-CaCr$_2$O$_4$ compound, the ground state is indeed the 120$^\circ$-structure, however, other possible magnetic orderings are very close in energy. We compute the dispersion of magnetic excitations to order 1/S and compare it with the neutron scattering data. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G15.00009: Spin dynamics of the triangular lattice antiferromagnet ${\alpha}$-SrCr$_2$O$_4$ M. Mourigal, J.-J. Wen, Y. Wan, S. Koohpayeh, R. Vald\'es Aguilar, N.P. Armitage, O. Tchernyshov, C.L. Broholm, S. Dutton, R.J. Cava, T. Birol, H. Das, C.J. Fennie, L. Lin, J.-M. Liu, M.B. Stone, W. Tian We study the spin dynamics of the layered $S=3/2$ triangular lattice antiferromagnet ${\alpha}$-SrCr$_2$O$_4$ by means of inelastic neutron scattering on powder and single-crystal specimen. While the incommensurate long-range order observed below $T_{\rm N}$=43K resembles the usual 120$^\circ$-structure predicted for the perfect triangular lattice antiferromagnet, a spin-wave theory fit to the entire single-crystal dataset reveals strongly distorted exchange interactions. The extreme sensitivity of direct-exchange interactions to the small static Cr$^{3+}$-Cr$^{3+}$ distance variations reported by neutron diffraction, is quantitatively confirmed by {\it ab-initio} calculations that corroborate the spin-wave theory results. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G15.00010: Low-lying magnetic excitations in the distorted triangular lattice antiferromagnet $\alpha$-CaCr$_2$O$_4$ Michael Schmidt, Zhe Wang, S. Toth, B. Lake, A.T.M.N. Islam, A. Loidl, J. Deisenhofer We will discuss our results on $\alpha$-CaCr$_2$O$_4$ obtained by FIR and Terahertz spectroscopy. This compound orders below $T_{\mathrm{N}}$ = 42.6 K in a proper screw 120$^\circ$ magnetic order, but shows additional low-lying magnetic modes indicative for the vicinity of a more complex magnetic order [1-2]. Our spectra obtained by FTIR and THz-TD spectroscopy show several optical magnons appearing below the magnetic ordering with anomalous temperature dependence. We will discuss their polarization dependence and a possible magnetoelastic coupling of these modes.\\[4pt] [1] S. Toth et al., Phys. Rev. B 84, 054452 (2011)\\[0pt] [2] S. Toth et al., PRL 109, 127203 (2012) [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G15.00011: Raman Evidence for Symmetry Breaking in SrCr$_{2}$O$_{4}$ Michael Valentine, Si\^an Dutton, Seyed Koohpayeh, Robert Cava, Collin Broholm, Turan Birol, Hena Das, Craig Fennie, Natalia Drichko Raman spectra from 400 cm$^{-1}$ to 650 cm$^{-1}$ were acquired from single crystalline SrCr$_{2}$O$_{4}$ to probe magneto-elastic effects on its frustrated magnetism. The compound contains two-dimensional sheets of CrO$_{2}$, where spin-3/2 Cr atoms with direct exchange interactions form a distorted triangular lattice with helical magnetic order below 43K [1]. Even in the paramagnetic phase, the spectra show mode splitting beyond predictions for space-group P$_{mmn}$ that describes powder x-ray diffraction data. This splitting occurs at the 480 cm$^{-1}$ A$_{g}$ mode and is enhanced below T$_{N}$, which suggests it may be associated with magneto-elastic effects. \\[4pt] [1] S E Dutton, E Climent-Pascual, P W Stephens, J P Hodges, A Huq, C L Broholm, and R J Cava, J. Phys.: Condens. Matter 23 (2011) 246005 [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G15.00012: Geometric frustration on a 1/9$^{th}$ site depleted triangular lattice John Hopkinson, Jarrett Beck In the searches both for new spin liquid and spin ice (artificial and macroscopic) candidates, geometrically frustrated two-dimensional spin systems have played a prominent role. Here we present a study of the classical antiferromagnetic Ising (AFI) model on the sorrel net, a 1/9$^{th}$ site depleted and 1/7$^{th}$ bond depleted triangular lattice. The AFI model on this corner-shared triangle net is found to have a large residual entropy per spin $\frac{S}{N} = 0.48185 \pm 0.00008$, indicating the sorrel net is highly geometrically frustrated. Anticipating that it may be difficult to achieve perfect bond depletion, we investigate the physics resulting from turning back on the depleted bonds ($J_2$). We present the phase diagram, analytic expressions for the long range partially ordered ground state spin structure for antiferromagnetic $J_2$ and the short range ordered ground state spin structure for ferromagnetic $J_2$, the magnetic susceptibility and the static structure factor. We briefly comment on the possibility that artificial spin ice on the sorrel lattice could by made, and on a recent report [T. D. Keene $\it{et}$ $\it{al.}$, Dalton Trans. ${\bf{40}}$ 2983 (2011)] of the creation of a 1/9$^{th}$ depleted cobalt hydroxide oxalate. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G15.00013: Hints of possible spin-liquid state in the spin-1/2 triangular-lattice Heisenberg antiferromagnet Nikolay Prokofiev, Sergey Kulagin, Oleg Starykh, Boris Svistunov, Christopher Varney We calculate magnetic susceptibility of the triangular-lattice quantum antiferromagnet in the correlated paramagnet regime and reveal surprising microscopic correspondence between quantum and classical models at all accessible temperatures $T>0.375J$. Namely, we observe a perfect match between the quantum static (zero Matsubara frequency) response $\chi (r)$, where $r$ is the spatial coordinate, and its classical counterpart calculated at temperature $T_{cl}(T)$. The correspondence curve is rather featureless and smoothly extrapolates to a finite value of $T_{cl} = 0.28J$ when $T/J \to 0$. If this extrapolation indeed holds true, then finite value of $T_{cl}(0)$ implies that spins are not ordered in the ground state and form a spin liquid. Existing numerical evidence would $not$ be in contradiction with the spin liquid state because the spin correlation length for the classical Heisenberg model at $T_{cl} \approx 0.28J$ is $>1000$ lattice periods and simulations dealing with small system sizes $L< 10$ would misidentify the ground state as ordered. Our results are based on the high-order skeleton Feynman diagrams within the fermionization framework. [Preview Abstract] |
Session G16: Focus Session: Quantum Spins
Sponsoring Units: GMAG DMPChair: Mark Meisel, University of Florida
Room: 318
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G16.00001: Excitations in a perfect magnetized quantum spin ladder Andrey Zheludev, David Schmidiger, Sebastian Muehlbauer, Gvasaliya Severian, Pierre Bouillot, Corinna Kollath, Thierry Giamarchi, Tatiana Guidi, Robert Bewley, Georg Ehlers The strong-leg $S=1/2$ Heisenberg spin ladder system C$_7$(D$_{10}$N)$_2$CuBr$_4$ is investigated in applied magnetic fields using inelastic neutron scattering anf DMRG calculations. The spectrum in the high-field Tomonaga-Luttinger spin liquid phase is found to be qualitatively different from that in the low-field spin gap phase. In the former, numerous spectral featrures, including incommensurate excitations and multi-spinon continua are identified. In contrast, the latter is dominated by long-lived magnon excitations and two-magnon bound states [1]. An unprecedented quantitive agreement between experiment and numerical claculations is achieved.\\[4pt] [1] D. Schmidiger, P. Bouillot, S. Muhlbauer, S. Gvasaliya, C. Kollath, T. Giamarchi, A. Zheludev, Phys. Rev. Lett. {\bf 108}, 167201 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G16.00002: The Phase Diagram of the Quantum Magnet SrCu2(BO3)2 Sara Haravifard, Arnab Banerjee, Jonathan Lang, George Srajer, Daniel Silevitch, Stefan Klotz, Bruce Gaulin, Thomas Hansen, Hanna Dabkowska, Thomas Rosenbaum SrCu2(BO3)2(SCBO) is one of the few real-world materials that corresponds to the Shastry-Sutherland model, with corner-sharing Cu S=1/2 dimers lying on a square lattice. The application of pressure can be used to tune the ground state of the system. High-resolution x-ray synchrotron experiments on SCBO at pressures up to 6 GPa reveal new structural peaks as a result of lattice distortions at low temperatures that we associate with long-range antiferromagnetic order. Additionally we have conducted high-pressure neutron diffraction measurements at pressures up to 7 GPa investigating the magnetic structure of SCBO and its link to structural distortions as a function of temperature. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G16.00003: Asymmetric thermal linehape broadening in a dimerised antiferromagnet - evidence for strong correlations at finite temperature B. Lake, D.L. Quintero-Castro, A.T.M.N. Islam, E.M. Wheeler, C. Balz, M. Mansson, K.C. Rule, S. Gvasaliya, A. Zheludev In the conventional picture of thermal effects in magnetism, the excitations are long-lived at low temperatures and their lifetime decreases with temperature. The explanation is that thermally activated excitations collide with each other limiting their lifetimes - observed experimentally as a symmetric Lorentzian energy broadening of the lineshape. This is confirmed for gapless magnets with long-range magnetic order. Here the excitations interact only weakly and fluctuate among the large range of available states in an uncorrelated manner. The damping is due simply to loss of coherence associated with the reduced lifetime. The concept of thermal decoherence and symmetric Lorentzian linewidth broadening is assumed to apply to all magnetic systems. This presentation will discuss Sr$_{3}$Cr$_{2}$O$_{3}$ which is 3-dimensional network of antiferromagnetic dimers with gapped magnon excitations. High resolution inelastic neutron scattering reveals that its lineshape broadens $asymmetrically$ with increasing temperature. This indicates that far from becoming increasingly incoherent with temperature, the excitations behave collectively like a strongly correlated gas of hard-core Bosons. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G16.00004: Magnetism of Ba$_4$Ru$_3$O$_{10}$ revealed by density functional calculations: Structural trimers behaving as coupled magnetic dimers Andres Saul, Guillaume Radtke, Yannick Klein, Gwenaelle Rousse From a simple ionic picture, the only magnetically active ions in this compound are the three Ru$\sp{4+}$ atoms which form trimers of faced shared RuO$_6$ octahedral. The Ru atom in the middle of the trimer (named Ru(1)) is cristallographically inequivalent to the ones at the corners (named Ru(2)). A na\"ive analysis of the magnetic properties of this compound compatible with the expected low spin magnetic configuration of the Ru ions would predict a complicate magnetic order at low temperature involving the Ru(1) and Ru(2) ions and a high temperature susceptibility corresponding to three S=1 ions per unit cell. In spite of that, we demonstrate in this work, from density functional calculations, that under the influence of Ru-Ru covalent bonding, the structural trimers behave in an extended range of temperature from 0 to 600K, as strong ($S=1$) antiferromagnetic dimers. Our calculations of the effective exchange interactions show a strong intra-dimer interaction and a weaker inter-dimer one which explains the antiferromagnetic order observed below $T_N = 105K$ and the magnetic susceptibility in the intermediate and high temperature range (from $T_N$=105K up to 612~K). [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G16.00005: Wilson ratio of a Tomonaga-Luttinger liquid in a spin-1/2 Heisenberg ladder Tao Hong, K. Ninios, Y.H. Kim, T. Manabe, C. Hotta, G. Tremelling, S.N. Herringer, M.M. Turnbull, C. Landee, H.-J. Kang, K.P. Schmidt, G.S. Uhrig, H.B. Chan, C. Broholm, Y. Takano We report a comprehensive study of a strong-leg spin-1/2 ladder compound (C7H10N)2CuBr4 (DIMPY) by specific heat, magnetocaloric effect, magnetization and inelastic neutron scattering measurements. DIMPY is shown to be a perfect one-dimensional Heisenberg antiferromagnet with a spin gap$=$0.32 meV. Above a critical field Hc and at temperature below 1 K, the specific heat exhibits asymptotic linear-T behavior, characteristic of a Tomonaga-Luttinger liquid (TLL). In this field and temperature region, the specific heat in conjunction with the susceptibility yields the Wilson ratio $R_{W}$. The result supports the relation $R_{W}=$4$K$, where $K$ is the TLL parameter. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G16.00006: High magnetic field studies of a spin-half dimmer Kim Modick, Ross McDonald, John Singleton, Paul Goddard, Jamie Manson We present high magnetic field studies of an organic molecular magnet system comprising of spin half copper dimmers. DC and pulsed field magnetometry combined with EPR indicate a small ($\sim$ 2 K) singlet triplet gap, and can be used to infer the sign of the triplon dispersion. Furthermore the low magnetic-exchange energy scales combined with the relatively soft organic framework of exchange pathways, indicate that the magnetic order can be readily tuned by temperature, magnetic field and pressure. The anisotropy between the effective mass of the top and bottom of the triplon band are analyzed in terms of the relative upper and lower critical fields for the onset of triplon condensation and magnetic saturation respectively. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 1:03PM |
G16.00007: Magnetic engineering with molecular bricks Invited Speaker: Stephen Blundell Magnetic materials can be constructed using molecular components to build up novel and unusual architectures. This approach provides an exciting opportunity for exploring the physics of magnetism. Gaining control of the building blocks of magnetic materials and thereby achieving particular characteristics will make possible the design and growth of bespoke magnetic devices. While progress in the synthesis of molecular materials, and especially coordination polymers, represents a significant step towards this goal, the ability to tune the magnetic interactions within a particular framework remains in its infancy but promising advances are being made, including the production of single molecule magnets and a variety of extended structures. We have recently found a chemical method which achieves dimensionality selection via preferential inhibition of the magnetic exchange in an S = 1/2 antiferromagnet along one crystal direction, switching the system from being quasi-two- to quasi-one-dimensional while effectively maintaining the nearest-neighbour coupling strength [1]. We have also demonstrated that single molecule magnets can be used to store quantum information and have devised a strategy for extending the spin coherence time by chemical adjustment [2]. Very recently we have found that introduction of a molecular spacer layer can produce a greater than fourfold enhancement in the superconducting transition temperature of iron selenide [3]. The experimental techniques used in this work include ESR, muSR and high magnetic fields. \\[4pt] [1] P. A. Goddard, J. L. Manson, J. Singleton, I. Franke, T. Lancaster, A. J. Steele, S. J. Blundell, C. Baines, F. L. Pratt, R. D. McDonald, O. E. Ayala-Valenzuela, J. F. Corbey, H. I. Southerland, P. Sengupta, and J. A. Schlueter, Phys. Rev. Lett. 108, 077208 (2012);\\[0pt] [2] C. J. Wedge, G. A. Timco, E. T. Spielberg, R. E. George, F. Tuna, S. Rigby, E. J. L. McInnes, R. E. P. Winpenny, S. J. Blundell, and A. Ardavan, Phys. Rev. Lett. 108, 107204 (2012); \\[0pt] [3] M. Burrard-Lucas, D. G. Free, S. J. Sedlmaier, J. D. Wright, S. J. Cassidy, Y. Hara, A. J. Corkett, T. Lancaster, P. J. Baker, S. J. Blundell, S. J. Clarke, Nature Materials, 11, December 2012. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G16.00008: Low-temperature studies of a 2D Quantum Heisenberg Antiferromagnet Christopher Landee, Fan Xiao, Mark Turnbull, Juan Bartolom\'e A recent inelastic neutron scattering experiment of a 2D Quantum Heisenberg Antiferromagnet (2DQHAF) in an applied field [1] revealed novel features in the energy spectrum but the field was limited to \textless 0.3 H$_{\mathrm{SAT}}$ due to the exchange strength (J $=$ 17.5 K) of the material under study. (Quinolinium)$_{2}$CuBr$_{4}$$\cdot$2H$_{2}$O is known [2] to be a molecular-based version of a strongly 2D QHAF with a significantly smaller exchange strength of 6.2 K and a saturation field of 15 T. We report the low-temperature properties (T \textless\ 1.8 K) of (Quinolinium)$_{2}$CuBr$_{4}$$\cdot$2H$_{2}$O and discuss its applicability for further investigations.\\[4pt] [1] N. Tsyrulin, T. Pardina, R. R. P. Singh et al, \textit{Phys. Rev. Lett}. \textbf{102}, 197201: 1-4 (2009).\\[0pt] [2] R. T. Butcher, M. M. Turnbull, C. P. Landee et al, \textit{Inorg. Chem.} \textbf{49}, 427-434 (2010). [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G16.00009: Control of Crystal Structure and Magnetism in Copper(II) Fluoride Based Coordination Polymers John Schlueter, Saul Lapidus, Jamie Manson Whereas magnetic copper (II) halide (halide $=$ chloride or bromide) coordination polymers have been frequently studied, the copper(II) fluoride analogs have been much less investigated. This is due in part to synthetic challenges associated with solubility and reactivity. In analogy to cuprates, Cu-F-Cu linkages are expected to provide strong magnetic superexchange. The magnetic structure of such systems can be tuned by choice of ancillary ligand. Herein, we describe the use of various pyridines, diazines, and triazoles that have the ability to define the structural and magnetic dimensionality. Hydrogen bonding to the fluoride ligand provides an additional opportunity for designing molecule-based materials through a `crystal-engineering' approach. Competition for intermolecular interactions frequently enables stimuli responsive behavior, including pressure-induced phase transitions. This will be illustrated for the CuF$_{\mathrm{2}}$(H$_{\mathrm{2}}$O)$_{\mathrm{2}}$(pyrazine) coordination polymer and the five-coordinate CuF$_{\mathrm{2}}$(H$_{\mathrm{2}}$O)$_{\mathrm{2}}$(3-chloropyridine) molecular solid. The use of new synthetic methods, including the use of high pressure, will be described. Subtle changes in reaction conditions leads to significant changes in structural and magnetic properties. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G16.00010: Finite-temperature valence-bond-solid transition of quantum spins in two dimensions Songbo Jin, Anders Sandvik The $S=1/2$ Heisenberg model on the 2D square lattice with four- or six-neighbor spin interactions (JQ model) hosts a quantum phase transition between N\'eel and valence-bond-solid (VBS) ground states. The deconfined quantum critical (DQC) point, predicted by the theory of Senthil \emph{et al.}[1], may be realized in this model [2]. Here we study the finite-temperature phase transition between the VBS ($Z_4$ symmetry breaking) to the paramagnetic state. We find continuously changing exponents with the correlation-length exponent $\nu$ close to the Ising value far from the $T=0$ critical point, and diverging when the critical temperature $T_c \rightarrow 0^+$. This is in accord with the DQC theory, according to which the transition for $T_c \rightarrow 0^+ $ should approach a Kosterlitz-Thouless fixed point.\\[4pt] [1] T. Senthil, L. Balents, S. Sachdev, A. Vishwanath, and M. P. A. Fisher, Phys. Rev. B {\bf 70}, 144407 (2004).\\[0pt] [2] R. K. Kaul, R. G. Melko, A. W. Sandvik, arXiv:1204.5405. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G16.00011: Ordering in weakly coupled random singlet spin chains Matthias Thede, F. Xiao, Ch. Baines, C. Landee, E. Morenzoni, A. Zheludev We study the effect of bond randomness on long range magnetic ordering in quasi-one-dimensional antiferromagnets, where the introduction of arbitrary weak bond randomness gives rise to the so-called random singlet phase. We investigated weakly coupled spin chain systems by local (muon spin rotation/relaxation) and bulk measurements (susceptibility and specific heat). The material Cu(py)$_2$(Cl$_{1-x}$Br$_x$)$_x$ is an organic tunable spin chain which has an average intrachain coupling constant between J = 2.3 meV (x=0) and J = 4.5 meV (x = 1). The disorder free end materials order magnetically at T$_N$ = 1.15 K (x=0) and T$_N$ = 0.72 K (y=0), respectively. Bond disorder strongly affects the magnetically ordered phase. In apparent contradiction with chain mean field theory [1] bond randomness strongly suppresses both the ordered moment and the ordering temperature T$_N$ [2]. We will also report about similiar results in BaCu$_2$(Si$_1-x$Ge$_x$)$_2$O$_7$.\\[4pt] [1] A. Joshi et. al, Phys. Rev. B 67, 174403 (2003).\\[0pt] [2] M. Thede, et. al, arXiv:1208.6479 [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G16.00012: Pressure-Induced Ferromagnetic Interactions in the Molecule-based Magnet Mn(dca)$_2$ P.A. Quintero, M.K. Peprah, M.W. Meisel, D. Rajan, D.R. Talham Using SQUID magnetometry, we have studied the pressure dependence of the magnetization of the three-dimensional antiferromagnetic coordination polymer Mn(N(CN)$_2$)$_2$, referred to as Mn(dca)$_2$, up to 1.2 GPa and down to 5~K. The isostructural compounds M(dca)$_2$, where M = Fe, Co, and Ni, have been previously studied by others and are known to show variations in their transition temperatures of up to 26\% for pressures as large as \mbox{1.7 GPa.}\footnote{C. J. Nuttall \textit{et al.}, Mol. Cryst. Liq. Cryst. \textbf{343} (2000) 227.} Our results on Mn(dca)$_2$ indicate a linear dependence of the transition temperature on the applied pressure, where a change of $48\%$ is measured at 1.2 GPa. In addition, a marked difference in the behavior of the magnetization is observed above and below 0.8 GPa. Specifically, for P $<$ 0.8 GPa, the magnetization decreases with increasing pressure, and for P $>$ 0.8 GPa, the behavior is inverted. These results indicate that external pressure changes the angle along the \mbox{Mn-[N(1)-C-N(2)]-Mn} superexchange path, thereby favoring ferromagnetic interactions.\footnote{C. R. Kmety \textit{et al.}, Phys. Rev. B \textbf{62} (2000) 5576.} [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G16.00013: Quantum critical dynamics in the one-dimensional spin chain compound copper pyrazine dinitrate probed by NMR spectroscopy Hannes Kuehne, A.P. Reyes, P.L. Kuhns, A.A. Zvyagin, S. Grossjohann, W. Brenig, M. Guenther, H.-H. Klauss, C.P. Landee, M.M. Turnbull The metalorganic compound copper pyrazine dinitrate is known to be one of the best realizations of the antiferromagnetic $S = 1/2$ Heisenberg chain model with a comparatively small nearest neighbor exchange constant $J/k_B$ = 10.7 K. The zero temperature saturation field $B_c$ = 14.6 T corresponds to a quantum critical point (QCP), where the system is driven from a Luttinger liquid state to ferromagnetic polarization. With an emphasis on the vicinity of the QCP, a comprehensive comparison of our experimental findings from $^{13}$C NMR spectroscopy with both numerical (quantum Monte Carlo) and analytical (conformal field theory) approaches is presented. In particular, this comparison reveals a well-defined maximum of $1/T_1$ $(B,T)$ \textit{below} $B_c$ as the signature of essential one-dimensional spin-spin interactions in the Luttinger liquid regime. [Preview Abstract] |
Session G17: Focus Session: Improper Ferroelectrics
Sponsoring Units: DMP GMAGChair: Taner Yildirim, NIST Center for Neutron Research
Room: 319
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G17.00001: Coupling of Magnetic and Ferroelectric Order Parameters in Improper Ferroelectrics Invited Speaker: A. Brooks Harris This talk concerns systems for which the onset of incommensurate magnetic order induces ferroelectricity. I review how Landau theory [1,2] provided a convenient phenomenological explanation of this phenomenon. In the simplest and most frequent scenario, as the temperature is lowered, one first induces collinear incommensurate magetic order. At a lower temperature transition, transverse magnetic components appear and these two different symmetry magnetic order parameters combine to induce ferroelectricity via a trilinear magnetoelectric coupling. I will present several examples of this mechanism, subsequently discussed by Mostovoy[3] within a model of spiral magnetic order. Landau theory also explains [4] a contrasting scenario in which ferroelectric and magnetic can order within a single phase transition as in RbFe(MoO$_4$)$_2$, whose magnetic spiral contradicts the Mostovoy construction, but which Kaplan[5] has subsequently shown to be consistent with a more complete symmetry analysis of microscopic interactions. Other more exotic higher order magnetoelectric couplings, not easily accessible to an analysis of microscopic interactions, are also possible, especially in the presence of nonuniform magnetic order. I close with a few remarks on microscopic models for magnetically induced ferroelectricity. \\[4pt] [1] G. Lawes {\it et al.} PRL 95, 087205 (2005).\newline [2] M. Kenzelmann {\it et al.}, PRL {\bf 95}, 087206 (2005).\newline [3] Mostotovy, PRL {\bf 96}, 067201 (2006).\newline [4] M. Kenzelmann {\it et al.}, PRL {\bf 98}, 267205 (2007).\newline [5] T. Kaplan {\it et al}, PRB {\bf 83}, 174432 (2011) [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G17.00002: A route to high polarization multiferroics Priya Mahadevan, Hirak Chandra, Kapil Gupta, Ashis Nandy Large ferroelectric polarizations are usually seen in $d^0$ ferroelectrics, while those with a finite d-electron count usually have a polarization which is two orders of magnitude smaller. The route then to high polarization multiferroics, seems quite obvious - examine if we can stabilize the $d^0$ type distortions in finite d-electron systems. The way we went about this was to dope carriers into BaTiO$_3$ and examine if ferroelectricity survived. Considering the example of V doping in BaTiO$_3$, we found that ferroelectricity was strongly stabilized, much stronger that in the undoped limit. Microscopic modeling coupled with ab-initio calculations revealed that part of the stability of the ferroelectric distortions about the V site emerged from first-order Jahn-Teller effects. The dilute doping limit was used to identify some design principles and helped us to design new multiferroics. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G17.00003: Exchange Constants from Combined Light and Neutron Scattering Experiments: Application to Magnetoelectric LiMnPO$_{4}$ Cesar J. Calderon Filho, Paulo F. Gomes, Ali F. Garc\'{I}a-Flores, Gaston E. Barberis, David Vaknin, Eduardo Granado Two-magnon Raman scattering is observed in magnetoelectric LiMnPO$_4$, carrying quantitative information on the magnetic interactions between local Mn$^{2+}$ moments. A simulated annealing fitting procedure using these Raman data combined with magnon dispersion curves from neutron diffraction is demonstrated to greatly improve the accuracy and reliability of the determined exchange constants up to at least fifth-nearest neighbors. First-nearest neighbor interactions are shown to be largely dominant in LiMnPO$_4$, ruling out magnetic frustration as a relevant ingredient for this material. This methodology may be instrumental to investigate other magnetoelectric and multiferroic materials as well as superconductors at the border of magnetism, where knowledge of exchange constants without ambiguity is important to pin down the relevant physics. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G17.00004: A Model of Magnetic Phase Diagrams of Monoclinic Multiferroics CuO and MnWO$_4$ Guy Quirion, R. Villarreal, M.L. Plumer, M. Poirier, T. Usui, T. Kimura A mean-field Landau-type free energy model developed using symmetry arguments is used to investigate the magnetic field - temperature phase diagrams of monoclinic multiferroics such as CuO and MnWO$_4$. Our analysis supports the necessity of having an intermediate collinear phase between the paramagnetic and magnetoelectric spin spiral phases. The numerical predictions agree well with the experimental phase diagram of CuO (\textbf{H}$\parallel$\textbf{b}) determined recently by high resolution ultrasonic velocity measurments [1] which reveal a new transition at T$_{N3}=230.0$~K associated with collinear ordering, just above the spiral phase at T$_{N2}=229.5$~K. The model also reproduces the magnetic phase diagrams reported for MnWO$_4$ with the applied field along the three principal axes [2] and elucidates the nature of newly identified high-field phases. \\[4pt] [1] R. Villarreal et al., PRL \textbf{109}, 167206 (2012).\\[0pt] [2] H. Mitamura et al., J. Phys. Soc. Japan \textbf{81}, 054705 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G17.00005: Muon spectroscopy as a probe for multiferroic materials Carlos Aristizabal, Alan Drew, Donna Arnold, Finlay Morrison, Laura Nuccio, Viswanathan Mohandoss, Andrei Rotaru, Nicola Morley, Francis Pratt, Sean Giblin, Michael Carpenter Multiferroic magnetoelectrics are materials that exhibit both, ferromagnetic and ferroelectric ordering in the same phase. Thus, they have a spontaneous magnetization that can be manipulated with an applied magnetic field, a spontaneous ferroelectric polarization that can be switched by an applied electric field, and in some cases, there exist some form of coupling between the two order parameters. Such coupling is of great technological importance as it offers the possibility of new multifunctional devices such as transducers, actuators, sensors and memories [1]. Muon spectroscopy (MS) [2] has shown itself to be an extremely versatile and powerful probe of magnetic properties of materials as well as a flexible technique in terms of experimental set up to be able to show magnetic behaviour under an applied electric field. By means of MS and other complementary techniques, I will present, in an entirely new tetragonal tungsten bronze (TTB) class of multiferroic material, a direct coupling in the form of an internal magnetic field that varies hysteretically with an applied electric field. [1] N. A. Spaldin et al., Science 309, 391 (2005) [2] S. J. Blundell, Contemp. Phys. 40, 175, (1999). [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G17.00006: Magneto--optical properties of complex oxides Peng Chen, Brian Holinsworth, Kenneth O'Neal, Tanea Brinzari, Janice Musfeldt, Nara Lee, Luo Xuan, Sang Cheong, Nyrissa Rogado, Robert Cava, Yaqi Wang, Bernd Lorenz, Steve McGill We investigated the magneto-optical properties of $\alpha$-Fe$_2$O$_3$, frustrated system Ni$_3$V$_2$O$_8$, and rare earth indium oxides like DyInO$_3$ in order to understand the interplay between charge and magnetism. We discovered that hematite appears more red in applied magnetic field than in zero field conditions, an effect that is amplified by the presence of the spin flop transition. Furthermore, magnetic field aligns the spins into fully polarized state and induces optical band gap change in Ni$_3$V$_2$O$_8$. As a consequence, Ni$_3$V$_2$O$_8$ appears more green in 35 T. $\it{f}$ electron excitations in DyInO$_3$ changes dramatically in applied magnetic field because of enormous spin-orbit coupling effect in the rare earth elements. These findings advance our understanding of spin-charge coupling and motivate spectroscopic work on other functional materials under extreme conditions. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G17.00007: Effects of rare earth ion size on the stability of the coherent Jahn-Teller distortions in undoped perovskite manganites K.H. Ahn, T.F. Seman, T. Lookman, A. Saxena, A.R. Bishop, P.B. Littlewood We present a theoretical study on the relation between the size of the rare earth ion, often known as chemical pressure, and the stability of the coherent Jahn-Teller distortions in undoped perovskite manganites. Using a Keating model expressed in terms of atomic scale symmetry modes for a simplified two- dimensional model, we show that there exists a coupling between the uniform shear distortion and the staggered buckling distortion within the Jahn-Teller energy term. It is found that this coupling provides a mechanism by which the coherent Jahn-Teller distortion is more stabilized by smaller rare earth ion. We analyze the appearance of the uniform shear distortion below the Jahn-Teller ordering temperature, estimate the Jahn-Teller ordering temperature and its variation among LaMnO3, PrMnO3, and NdMnO3 and obtain the relations between distortions. We find good agreement between theoretical results and experimental data. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G17.00008: Magnetic Coupling in the multiferroic hexagonal ErMnO$_{3}$ Huibo Cao, Jun Zhao, Tao Hong, Jie Ma, Bryan Chakoumakos Hexagonal ErMnO$_{3}$ is one of the rare earth manganites RMnO$_{3}$ and has attracted renewed interest due to its multiferroic properties. Understanding the coupling between spin, charge, and lattice degrees of freedom is crucial to explore and design strong magnetic-ferroelectric coupled materials. We measured the crystal and magnetic structures of ErMnO$_{3}$ at selected temperatures and magnetic fields by single crystal neutron diffraction. Combined with planned inelastic neutron scattering measurements, the magnetic-magnetic and magnetic-lattice interactions will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G17.00009: Vortex interactions and formation of vortex networks in hexagonal YMnO$_3$ Sergey Artyukhin, Karin M. Rabe, David Vanderbilt, Maxim Mostovoy Multiferroic materials with their coexisting magnetic and ferroelectric orders are of pressing interest for spintronics and information storage technology. In hexagonal manganites there is an additional order, structural trimerization, which strongly interacts with both charge and spin degrees of freedom [1,2]. This results in the clamping of structural, ferroelectric and antiferromagnetic domain walls and gives rise to the appearance of multiferroic vortices [3,4,2]. Motivated by the recent experiments of the group of S-W. Cheong visualizing vortex networks formed in YMnO$_3$ at different cooling rates, we use Landau-type theory and electronic structure calculations to study vortex network formation and interpret experimental observations. Our results emphasize the importance of strains for the understanding of vortex interactions in this material.\newline [1] C.J. Fennie, K.M. Rabe, PRB 72, 100103 (2005)\newline [2] S. Artyukhin et al., arXiv:1204.4126\newline [3] T. Choi et al., Nature Materials 9, 253 (2010)\newline [4] M. Fiebig et al., Nature 419, 818 (2002) [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G17.00010: Interplay of octahedral distortions in electronic and structural phase transitions in $ABO_{3}$ perovskites Prasanna V. Balachandran, James M. Rondinelli In this work, we investigate group-subgroup relationships afforded to $ABO_{3}$ perovskites from combinations of $BO_{6}$ distortions -- bond stretching and bond angle rotations -- with the objective of identifying new pathways for tuning their properties through electron-lattice interactions. Using nickelate and bismuthate perovskite compounds as a template, we decompose their low-symmetry structures into orthonormal symmetry-breaking lattice modes of the parent cubic space group. Statistical analysis of mode decomposition data uncovers previously unappreciated relationships between microscopic octahedral distortion modes and macroscopic physical properties. Finally, we propose novel crystal engineering strategies to study perovskites near phase boundaries that are otherwise extremely difficult to probe experimentally. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G17.00011: Cation Ordering in Layered Nickelates Brittany Nelson-Cheeseman, Hua Zhou, Antonio Cammarata, Jason Hoffman, Prasanna Balachandran, James Rondinelli, Anand Bhattacharya The single layer Ruddlesden-Popper nickelates present a model system to understand how the effects of digital dopant cation ordering may affect the properties of 2-dimensional conducting sheets. We investigate the effects of aliovalent A-site cation order on LaSrNiO$_{4}$ films. Using molecular beam epitaxy, we interleave full layers of SrO and LaO in a series of chemically equivalent films, varying the pattern of SrO and LaO layers relative to the NiO$_{2}$ layers. Through synchrotron surface x-ray diffraction and Coherant Bragg Rod Analysis (COBRA), we directly investigate the A-site cation order and the resulting atomic displacements for each ordering pattern. We correlate these results with theoretical calculations and transport measurements of the layered nickelate films. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G17.00012: Total energy calculations of correlated electron compounds: theory and application to rare earth nickelates Hyowon Park, Andrew Millis, Chris Marianetti We use density functional theory (DFT) plus dynamical mean field theory (DMFT) method, along with DFT+U and Hartree-Fock methods to compute the electronic energy as a function of crystal structure for rare earth nickelates. We show that full charge self-consistency can be essential for obtaining qualitative agreement with experiment and that the choice of double counting correction has an important effect on the energy. Furthermore, the precise definition (projector vs Wannier) of the correlated d-orbitals has a minimal effect. We show that charge self-consistent DFT+DMFT, as opposed to DFT+U, is critical to describing the magnetic-insulator to paramagnetic-metal phase boundary in the rare earth nickelate phase diagram. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G17.00013: Neutron Scattering in Multiferroics Ba$_{2}$CoGe$_{2}$O$_{7}$ Minoru Soda, Takatsugu Masuda, Masashige Matsumoto, Severian Gvasaliya, Martin Mansson, Andrey Zheludev Ba$_{2}$CoGe$_{2}$O$_{7}$ having the noncentrosymmetric crystal structure shows a staggered antiferromagnetic structure in the (001) plane below $T_{N}$=6.7 K. In the magnetically ordered state, a ferroelectric polarization is observed even at a magnetic field $H$=0, and largely enhanced under $H$. In Ba$_{2}$CoGe$_{2}$O$_{7}$, Murakawa and co-workers have shown that the ferroelectricity is induced by the spin-dependent $d$-$p$ hybridization mechanism. Furthermore, the 4 meV excitation, which is an electric-active mode through the coupling between spin and electric-dipole, was observed in the electromagnetic wave absorption. In the present study, the neutron scattering measurements were carried out in Ba$_{2}$CoGe$_{2}$O$_{7}$ under the magnetic field. We found one acoustic and two optical modes in zero field, which are reasonably reproduced by the extended spin wave theory. Furthermore, our result indicates that the anisotropy of the magnetic moments also connects with the multiferroic property of Ba$_{2}$CoGe$_{2}$O$_{7}$. [Preview Abstract] |
Session G18: Two Dimensional Topological Insulators II: Graphene and Related Materials
Sponsoring Units: DCMPChair: Shaffique Adam, Yale University
Room: 320
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G18.00001: Is graphene on the edge of being a topological insulator? Jose Gonzalez We show that, at sufficiently large strength of the long-range Coulomb interaction, a mass term breaking parity (so-called Haldane mass) is dynamically generated in the many-body theory of Dirac fermions describing the graphene layer. While the tendency towards a conventional excitonic instability is stronger than for the dynamical breakdown of parity at spatial dimension D greater than 2, we find that the situation is reversed at D = 2. The need to regularize the many-body theory in a gauge-invariant manner (taking the limit D = 2 from below) is what leads to the dominance of the parity-breaking pattern in graphene. We compute the critical coupling for the generation of a parity-breaking mass from the finite radius of convergence of the ladder series supplemented with electron self-energy corrections, finding a value quite close to the effective interaction strength for graphene in vacuum after including Fermi velocity renormalization and static RPA screening of the Coulomb interaction. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G18.00002: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G18.00003: Excahnge and correlation energy of electrons dressed with circularly-polarized light in graphene and three-dimensional topological insulators Andrii Iurov, Godfrey Gumbs We have formulated a theory for investigating the conditions which are required to achieve entangled states of electrons on graphene and three-dimensional (3D) topological insulators. We consider the quantum entanglement of spins by calculating the exchange energy. A gap is opened up at the Fermi level between the valence and conduction bands at zero doping when graphene as well as 3D topological insulators are irradiated with circularly-polarized light. This energy band gap is dependent on the intensity and frequency of the applied electromagnetic field. The electron-photon coupling also gives rise to a unique energy dispersion of the dressed states which is different from either graphene or the conventional two-dimensional electron gas (2DEG). In our calculations, we obtain the dynamical polarization function for imaginary frequencies. The polarization function is determined by both the energy dispersion and the overlap of pseudo-spin wave functions. The correlation energy is calculated in the random phase approximation (RPA). The application of the derived results to quantum computation will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G18.00004: Stabilizing topological phases in graphene via random adsorption Jiang Jiang, Zhenhua Qiao, Haiwen Liu, Junren Shi, Qian Niu We study the possibility of realizing topological phases in graphene with randomly distributed adsorbates. When graphene is subjected to periodically distributed adatoms, the enhanced spin-orbit couplings can result in various topological phases. However, at certain adatom coverages, the intervalley scattering renders the system a trivial insulator. Using both finite-size scaling method and transport calculation, we show that when the adatom distribution becomes random, the intervalley scattering is weakened, but other quantities (e.g. spin-orbit couplings, and exchange field) are not affected. This finding points out that the topological states are graphene-favored ground states in the presence of randomly distributed adtoms. \\[4pt] Hua Jiang, Zhenhua Qiao, Haiwen Liu, Junren Shi and Qian Niu, Phys. Rev. Lett. \textbf{109}, 116803 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G18.00005: Bound States of Conical Singularities in Graphene-Based Topological Insulators Andreas Ruegg, Chungwei Lin We investigate the electronic structure induced by wedge-disclinations (conical singularities) in a honeycomb lattice model realizing Chern numbers $\gamma=\pm 1$. We establish a correspondence between the bound state of (i) an isolated $\Phi_0/2$-flux, (ii) an isolated pentagon $(n=1)$ or heptagon $(n=-1)$ defect with an external flux of magnitude $n\gamma \Phi_0/4$ through the center and (iii) an isolated square or octagon defect without external flux, where $\Phi_0=h/e$ is the flux quantum. Due to the above correspondence, the existence of isolated electronic states bound to the disclinations is robust against various perturbations. These results are also generalized to graphene-based time-reversal invariant topological insulators. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G18.00006: Topological kink states at the tilt boundary in gated multi-layer graphene Eun-Ah Kim, Abolhassan Vaezi, Yufeng Liang, Darryl Ngai, Li Yang Search for new realization of symmetry protected topological states with protected edge states is an active area of research. We show that a tilt boundary in gapped multi-layer graphene supports topologically protected gapless kink states. We investigate such kink states from two perspectives: the microscopic perspective of tight-binding model and an ab-initio calculation on bilayer, and the perspective of symmetry protected topological (SPT) states for general multi-layer. We show that the bilayer tilt boundary supports gapless kink states that are undeterred by strain concentrated at the boundary. Further we establish the kink states as concrete examples of edge states of {\it time-reversal symmetric} $Z$-type SPT, protected by $T$ and two $U(1)$ symmetries in the absence of inter-valley mixing. Recent observations of such boundaries in multi-layer samples suggest that transport through these topological kink states might explain the long standing puzzle of sub-gap conductance. We discuss possible topological phase transitions upon breaking subset of symmetries from SPT perspective. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G18.00007: Topological Proximity Effects in Graphene Nanoribbon Heterostructures Gufeng Zhang, Xiaoguang Li, Guangfen Wu, Jie Wang, Dimitrie Culcer, Efthimios Kaxiras, Zhenyu Zhang Topological insulators (TI) are bulk insulators that possess robust chiral conducting states along their interfaces with normal insulators. A tremendous research effort has recently been devoted to TI-based heterostructures, in which conventional proximity effects give rise to many exotic physical phenomena. Here we establish the potential existence of ``topological proximity effect'' at the interface of a topological graphene nanoribbon (GNR) and a normal GNR. Specifically, we show that the location of the topological edge states exhibits versatile tunability as a function of the interface orientation, as well as the strengths of the interface coupling and spin-orbit coupling in the normal GNR. For zigzag and bearded GNRs, the topological edge state can be tuned to be either at the interface or outer edge of the normal ribbon. For armchair GNR, the potential location of the topological edge state can be further enriched to be at the edge of or within the normal ribbon, at the interface, or diving into the topological GNR. We also discuss potential experimental realization of the predicted topological proximity effects, which may pave the way for integrating the salient functionality of TI and graphene in future device applications. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G18.00008: Designer quantum spin Hall phase transition in molecular graphene Pouyan Ghaemi, Sarang Gopalakrishnan, Taylor Hughes Graphene was the first material predicted to be a time-reversal-invariant topological insulator; however, the insulating gap is immeasurably small owing to the weakness of spin-orbit interactions in graphene. A recent experiment demonstrated that designer honeycomb lattices with graphene-like ``Dirac'' band structures can be engineered by depositing a regular array of carbon monoxide atoms on a metallic substrate. Here, we argue that by growing such designer lattices on metals or semiconductors with strong spin-orbit interactions, one can realize an analog of graphene with strong intrinsic spin-orbit coupling, and hence a highly controllable two-dimensional topological insulator. We estimate the range of substrate parameters for which the topological phase is achievable, and consider the experimental feasibility of some candidate substrates. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G18.00009: Topological Classification of Crystalline Insulators with Point Group Symmetry Di Xiao, Priyamvada Jadaun, Qian Niu, Sanjay Banerjee We show that in crystalline insulators point group symmetry alone gives rise to a topological classification based on the quantization of electric polarization. Using $C_3$ rotational symmetry as an example, we first prove that the polarization is quantized and can only take three inequivalent values. Therefore, a $Z_3$ topological classification exists. A concrete tight-binding model is derived to demonstrate the $Z_3$ topological phase transition. Using first-principles calculations, we identify graphene on BN substrate as a possible candidate to realize the $Z_3$ topological states. To complete our analysis we extend the classification of band structures to all 17 two-dimensional space groups. This work will contribute to a complete theory of symmetry conserved topological phases and also elucidate topological properties of graphene like systems. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G18.00010: Surface band topology of Ge on Ag(111) Athanasios Dimoulas, Evangelos Golias, Evangelia Xenogiannopoulou, Dimitra Tsoutsou, Polixronis Tsipas, Sigiava Giamini While compelling evidence for silicene on Ag (111) has been recently published [1], the existence of germanene remains elusive. We have performed MBE growth of (sub) monolayer Ge on single crystal Ag (111) substrates, supported by DFT calculations, with the aim to obtain germanene. RHEED data indicate a $\left( {\sqrt 3 \times \sqrt 3 } \right)R30^{0}$ superstructure, while \textit{in-situ} ARPES reveals a rich surface band structure consisting of linearly, highly dispersive cone-like features with hexagonal and snow-flake warping clearly imaged in the constant energy contour plots k$_{\mathrm{x}}$-k$_{\mathrm{y}}$. Unlike the case of graphene-like 2D crystals where Dirac cones are expected at the K-points, here the cone-like features appear at the center ($\Gamma$ points) of the surface Brillouin zone similar to what is observed in topological insulators. This suggests the possibility to witness a non-trivial surface band topology triggered by intrinsic spin-orbit coupling as predicted [2] for 2D honeycomb Ge lattices or by strong Ge and Ag p orbital hybridization in an ordered surface alloy Ag$_{2}$Ge.\\[4pt] [1] P. Vogt et al., PRL 108, 155501 (2012);\\[0pt] [2] C.C-Liu et al., PRL 107, 076802 (2011) [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G18.00011: Polarization-driven topological insulator transition in a GaN/InN/GaN quantum well M.S. Miao, Q. Yan, C.G. Van de Walle, W.K. Lou, L.L. Li, K. Chang Topological insulators (TIs), a new state of quantum matter, have recently attracted significant attention, both for their fundamental research interest and for their potential device applications. Although many families of TI materials have been found, the realization of TI in conventional semiconductors remains elusive, mainly due to their sizable gaps and small spin-orbit interactions (SOI). Based on advanced first-principles calculations combined with an effective low-energy k$\cdot$p Hamiltonian, we show that the intrinsic polarization of materials can be utilized to simultaneously reduce the energy gap and enhance the SOI, driving the system to a TI state. The proposed system consists of ultrathin InN layers embedded into GaN, a layer structure that is experimentally achievable. We found that the TI transition happens at GaN/InN/GaN quantum well with 3 to 4 InN atomic layers. Since polarization fields occur in many materials, a similar mechanism may apply to other systems as well. Our approach may pave the way toward integrating controllable TIs with conventional semiconductor devices. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G18.00012: Electronic Structure calculations in a 2D SixGe1-x alloy under an applied electric field Jos\'e Eduardo Padilha, Renato B. Pontes, Leandro Seixas, Ant\^onio J.R. da Silva, Adalberto Fazzio The recent advances and promises in nanoscience and nanotechnology have been focused on hexagonal materials, mainly on carbon-based nanostructures. Recently, new candidates have been raised, where the greatest efforts are devoted to a new hexagonal and buckled material made of silicon, named Silicene. This new material presents an energy gap due to spin-orbit interaction of approximately 1.5 meV, where the measurement of quantum spin Hall effect(QSHE) can be made experimentally. Some investigations also show that the QSHE in 2D low-buckled hexagonal structures of germanium is present. Since the similarities, and at the same time the differences, between Si and Ge, over the years, have motivated a lot of investigations in these materials. In this work we performed systematic investigations on the electronic structure and band topology in both ordered and disordered SixGe1-x alloys monolayer with 2D honeycomb geometry by first-principles calculations. We show that an applied electric field can tune the gap size for both alloys. However, as a function of electric field, the disordered alloy presents a W-shaped behavior, similarly to the pure Si or Ge, whereas for the ordered alloy a V-shaped behavior is observed. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G18.00013: Reflection from surface step defect in topological insulator nanofilm Thakshila M. Herath, Prabath Hewageegana, Vadim M. Apalkov Ultrathin topological insulator nanofilm with a step-like defect, which divides two regions of nanofilm with different thicknesses, is considered. Electron, propagating along the nanofilm surface, is reflected from the step. We calculate the reflectance of such electron for different parameters of the nanofilm and different parameters of the defect. We demonstrate that such system has an interesting property. Namely, the incident electron wave not only produces the reflected and transmitted waves, but also generates the mode, localized at the step-like defect. Such mode results in an enhancement of the electron density at the defect by $\sim$20\%. The strength of such enhancement depends on the parameters of the nanofilm and the height of the step. [Preview Abstract] |
Session G19: URu2Si2 Hidden Order and other U-based Systems
Sponsoring Units: DCMPChair: Jason Jeffries, Lawrence Livermore National Lab
Room: 321
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G19.00001: Precursor Hidden Order Fluctuations in URu$_2$Si$_2$ Peter Riseborough It has been proposed that the Hidden Order phase in URu$_2$Si$_2$ is a combined spin-orbit density wave, which is stabilized by the effect of the spin-flip part of the inter-orbital Hund's rule exchange. The transition involved the nesting of bands with different orbital characters and results in a partial gapping of the Fermi-surface. Above the transition temperature, the system exhibits combined spin and orbital fluctuations whose lifetimes and amplitudes increase as the temperature is reduced towards the critical temperature. These fluctuations produces hot-spots on the Fermi-surface, modifies the electronic structures as precursor to the opening of the gap. We examine the dependence of precritical fluctuations on the transition temperature. As the critical temperature is reduced to zero, it is found that the nature of the transition changes from second-order to first-order. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G19.00002: Charge-$2e$ Skyrmion condensate in a hidden order state Chen-Hsuan Hsu, Sudip Chakravarty A higher angular momentum ($l=2$) $d$-density wave, a mixed triplet and a singlet, interestingly, admits skyrmionic textures. The Skyrmions carry charge $2e$ and can condense into a spin-singlet $s$-wave superconducting state. In addition, a charge current can be induced by a time-dependent inhomogeneous spin texture, leading to quantized charge pumping. The quantum phase transition between this mixed triplet $d$-density wave and skyrmionic superconducting condensate likely leads to deconfined quantum critical points. We suggest connections of this exotic state to electronic materials that are strongly correlated, such as the heavy fermion URu$_2$Si$_2$. At the very least, we provide a concrete example in which topological order and broken symmetry are intertwined, which can give rise to non-BCS superconductivity. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G19.00003: Evidence for an orbital moment in the superconducting state of URu$_2$Si$_2$ Gang Li, Qiu Zhang, Daniel Rhodes, Bin Zheng, Pallab Goswami, P. Tobash, Filip Ronning, Joe D. Thompson, Eric D. Bauer, Luis Balicas URu$_2$Si$_2$ was suggested to be a chiral \emph{d}-wave superconductor with a $k_z (k_x \pm ik_y)$ orbital component for the Cooper pair wave-function. This state breaks time-reversal symmetry due to the orbital moment associated with this pair wave-function. Here, we report torque magnetometry in URu$_2$Si$_2$ at high fields and very low temperatures revealing a change in the sign of the magnetic hysteresis for $H \rightarrow H_{c2}$, and for angles $15^{\circ}$ away from the \emph{ab}-plane, i.e. from a clear diamagnetic response dominated by the pinning of vortices to a state with a much smaller but paramagnetic-like hysteretic response which \emph{disappears} at $H_{c2}$. If diamagnetism results from screening super-currents, we conclude that this hysteretic paramagnetic response must result from super-currents circulating in the opposite sense which generate an effective moment as expected for a chiral superconductor. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G19.00004: Global k-space perspective of temperature-dependent U f-states in URu$_{2}$Si$_{2}$ J.D. Denlinger, L. Dudy, J.-S. Kang, J.W. Allen, N.P. Butch, M.B. Maple In recent years, high-resolution angle-resolved photoemission (ARPES) measurements [1] have identified a narrow band of $f$-states close to the Fermi level in URu$_{2}$Si$_{2}$ whose temperature dependent spectral weight and/or energy shifts correlate to the hidden order transition at 17.5K. These $f$-states have been observed close to normal emission at a few select photon energies of $\sim$ 6, 21 and 30 eV corresponding to momentum space locations close to Z, $\Gamma $ and Z points respectively. We attempt to provide a more global $k$-space context for the presence of such $f$-states and their relation to the bulk Fermi surface topology using synchrotron-based wide-angle and photon energy-dependent ARPES mapping of the electronic structure. In addition, x-ray polarization and small-spot spatial dependences are exploited to assist identification of these narrow-band $f$-states and their relation to specific U- or Si-terminations of the cleaved surface.\\[4pt] [1] A.F. Santander-Syro, Nat. Phys. 2009; R. Yoshida, Phys. Rev. B 2010; G.L. Dakovski, Phys. Rev. B 2011. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G19.00005: Formation of coherent heavy fermion states at the hidden order transition in URu$_2$Si$_2$, as seen by ARPES Shouvik Chatterjee, Jan Trinckauf, Torben Hanke, Daniel Shai, John Harter, Travis Williams, Graeme Luke, Jochen Geck, Kyle Shen We present high-resolution angle-resolved photoemission (ARPES) spectra that allow us to delineate the evolution of the low energy electronic structure of the heavy-fermion superconductor URu$_2$Si$_2$ across the hidden order (HO) transition. By employing a range of excitation photon energies, we are able to disentangle various features in the electronic structure which, to date, have not been clearly identified. In contrast to the conventional Kondo lattice scenario, we find that precisely at T$_{\mathrm{HO}}$, the low energy electronic structure changes due to hybridization from incoherent and localized f states to a coherent heavy fermion liquid. We also observe a sharp drop in the scattering rate upon cooling through T$_{\mathrm{HO}}$, suggesting that the large scattering rate is caused by fluctuations in the order parameter. Our findings place clear constraints on the possible theoretical models for the HO state while clarifying a few of the apparently inconsistent observations of the previous ARPES measurements. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G19.00006: Heavy fermion phases probed by temperature dependent tunneling spectroscopy Ana Maldonado, Isabel Guillam\'on, Jose Gabriel Rodrigo, Hermann Suderow, Sebasti\'an Vieira, Dai Aoki, Jacques Flouquet Heavy fermions offer a rich physical phenomenology at very low temperatures, exhibiting different phase transitions on cooling that determine their electronic properties. Their ground states cover many electronic interactions, such as Kondo effect, superconducting or long range magnetic ones and, eventually, their coexistence. Thus, exploring the local electronic properties of these systems using scanning tunneling microscopy/spectroscopy (STM/S) at different temperatures is essential. In this communication, tunneling spectroscopy measurements using a superconducting tip of Al in the superconducting phase of URu$_{2}$Si$_{2}$\footnote{A. Maldonado et al., \textit{Phys. Rev. B} $\mathbf{85}$, 214512 (2012)} and using one of Au in the paramagnetic and antiferromagnetic phases of, respectively, CeRu$_{2}$Si$_{2}$ and CeRh$_{2}$Si$_{2}$\footnote{A. Maldonado et al., Accepted in \textit{J. Phys.: Condens. Matter}} will be discussed. The features found in the tunneling spectroscopy of each compound at 0.15K and their respective thermal evolution reflect the formation of different electronic ground states. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G19.00007: From hidden order to magnetic order: Optical conductivity reveals new behavior in URu$_2$Si$_2$ Jesse Hall, Noravee Kanchanavatee, Marc Janoschek, Kevin Huang, Nicholas Butch, Brian Maple, Thomas Timusk As a new generation of experimental techniques is brought to bear against the heavy-Fermion compound URu$_2$Si$_2$, striking new details about the electronic structure changes at the mysterious hidden order (HO) transition are becoming clear. Far infrared optical conductivity measurements were performed on oriented samples of URu$_2$Si$_2$ doped with both Fe and Re. While Re-doping pushes the material towards ferromagnetism, Fe-doping substitutes for hydrostatic pressure and enhances the temperature of the HO transition slightly before pushing the material into antiferromagnetism. Optical conductivity measurements have revealed new information about the charge dynamics at the transition, and how these evolve with doping. Both the structure and energy of the gap are altered as the material is pushed towards magnetic ordering. Comparison is made between the gap seen in optical conductivity and the charge gaps seen in scanning tunneling spectroscopy and ARPES, as well as the gaps in the magnetic excitation spectrum seen in neutron scattering. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G19.00008: NMR Evidence for psuedogap in URu$_2$Si$_2$ Kent Shirer, Adam Dioguardi, John Crocker, Nicholas apRoberts-Warren, Abigail Shockley, Ching Lin, David Nisson, Jason Cooley, Brian Maple, Jason Haraldsen, Matthias Graf, Nicholas Curro We report $^{29}$Si NMR measurements in single crystals and aligned powders of URu$_2$Si$_2$ in the hidden order and paramagnetic phases. In the paramagnetic phase, the spin lattice relaxation data reveal evidence of spin fluctuations of U moments. Furthermore, we find evidence for partial suppression of density of states below 30 K. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G19.00009: Spin Correlations in the Different Phases of URu$_2$Si$_2$ Travis J. Williams, H. Barath, Z. Yamani, J.A. Rodriguez-Rivera, J.B. Leao, J.D. Garrett, G.M. Luke, W.J.L. Buyers, C. Broholm We report a neutron scattering study of the magnetic excitation spectrum in three temperature and pressure driven phases of URu$_2$Si$_2$: the paramagnetic, `hidden order' and antiferromagnetic phases. The experiment was conducted using the novel neutron scattering spectrometer MACS at the NIST Center for Neutron Research[1]. Wide-angle detector coverage offers comprehensive scattering data covering an entire plane in momentum space with excellent energy resolution. The ambient pressure data show a magnetic excitation spectrum characteristic of Fermi surface nesting in the paramagnetic phase and the development of a gap in the excitation spectrum upon cooling through the T$_C$~=~17.5~K phase transition. The efficiency of MACS allowed a comprehensive data set in the high-pressure phase. Throughout the (H0L) scattering plane we find qualitatively similar excitations as in the hidden order phase though with a substantial reduction in the overall spectral weight and an upward shift in energy. These data should allow a critical evaluation of recent theoretical work to understand the small and large moment phases of URu$_2$Si$_2$. [1] J.A. Rodriguez-Rivera, Meas. Sci. Technol. 19, 034023 (2008). [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G19.00010: Phonon Behavior in the Hidden Order state of the Heavy Fermion Superconductor URu$_2$Si$_2$ Dillon Gardner, Craig Bonnoit, Travis Williams, Graeme Luke, Young Lee The heavy fermion compound URu$_2$Si$_2$ has generated much interest after the initial discovery of coexisting superconductivity and magnetism. Subsequent measurements revealed a phase transition at T=17.5 K into what is referred to as the ``hidden order'' state. The order parameter of this state remains unknown. Anomalous behavior in both the lattice component of thermal conductivity and thermal expansion parameters suggest that the phonons may also exhibit anomalous behavior that can shed light on the nature of the Hidden Order. We present inelastic X ray scattering measurements of lattice dynamics in both the hidden order phase and high temperature phase. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G19.00011: Anisotropic phonon softening in URu$_{2}$Si$_{2}$ Nicholas Butch, Michael Manley, Jason Jeffries, Marc Janoschek, Kevin Huang, Brian Maple, Jeffrey Lynn We studied the low-energy phonons of URu$_{2}$Si$_{2}$ via inelastic neutron scattering. At the wave-vectors associated with magnetic excitations, the phonons show surprisingly little modification. However, we find important temperature and direction dependence of the phonons in the basal plane. Possible ramifications for the symmetry of the hidden order will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G19.00012: High-magnetic field magnetostriction and thermal expansion in URu$_2$Si$_2$ V.F. Correa, S. Francoual, M. Jaime, N. Harrison, A. Lacerda, T.P. Murphy, E.C. Palm, S.W. Tozer, P.A. Sharma, J.A. Mydosh We report high magnetic field (up to $\mu_0$H = 45 T) $c$-axis thermal expansion and magnetostriction experiments on URu$_2$Si$_2$ single crystals. The sample length change $\Delta$L$_c$(T$_{HO}$)/L$_c$ associated with the transition to the ``hidden order'' phase becomes increasingly discontinous as the magnetic field is raised above 25 T. The re-entrant ordered phase III is clearly observed in both the thermal expansion $\Delta$L$_c$(T)/L$_c$ and magnetostriction $\Delta$L$_c$(B)/L$_c$ above 36 T, in good agreement with previous results. The sample length is also discontinous at the boundaries of this phase, mainly at the upper boundary. A change in the sign of the coefficient of thermal-expansion is observed at the metamagnetic transition (B$_M$ ? 38 T) which is likely related to the existence of a quantum critical end point. See V.F. Correa et al., Phys. Rev. Lett. (in the press). [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G19.00013: Revealing the electronic structure of USb$_{2}$ using femtosecond optical pulses Jingbo Qi, Tomasz Durakiewicz, E. Bauer, R. Baumbach, K. Gofryk, T. Klimczuk, P. Riseborough, Antoinette Taylor, Rohit Prasankumar USb$_{2}$ is a very interesting moderately heavy system, as it displays dispersive 5f bands as well as the first example of a clear kink structure in f-electron systems. This material also exhibits a renormalized zone-centered hole-like band, driven by boson-mediated interband scattering processes. Employing ultrafast optical spectroscopy, we explored the nature of the boson participating in this band renormalization, and explicitly characterized the gap structures near the Fermi surface in USb$_{2}$ for the first time. Our results reveal new physical properties of this material, which have not previously been unveiled by other experimental methods. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G19.00014: Non-Fermi vs. Inhomogeneous-Fermi Liquid behaviour in UCu$_4$Ni in the context of the Kondo Disorder Model Ariana Valdez, Oscar Bernal, G.R. Stewart, J.S. Kim UCu$_4$Ni is a site-disordered material with diverging thermodynamic and anomalous transport properties. Local nuclear magnetic resonance (NMR) experiments in combination with bulk magnetic susceptibility $\chi$ measurements performed on the same samples indicate that the low-temperature divergence of $\chi$ might be due in part to the presence of paramagnetic impurities. In this contribution, we describe the magnetization in terms of a Kondo disorder model and extract a set of parameters of the distribution of Kondo temperatures, which indicate that the low temperature side of the distribution does not have sufficient area to accommodate a non-Fermi liquid divergence. We use the same parameters to subsequently calculate the specific heat $C$ and to extrapolate to low temperatures, which allows us to compare with the known divergence of the magnetic contribution to $C/T$ below 10~K. We discuss to what extent the physics of this material is that of a non-Fermi liquid as opposed to an inhomogeneous Fermi fluid. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G19.00015: The Effects of Grinding on the Magnetic Susceptibility of UCu$_{3.95}$Ni$_{1.05}$ Carlos Sanchez, Carmen Quen, Edith Soto, Oscar Bernal, G.R. Stewart The effects of grinding on the magnetic susceptibility of UCu$_{3.95}$Ni$_{1.05}$ were studied in order to understand magnetization measurements in this material. Substantial information was recovered from these experiments, which were done at temperatures ranging from 3K to 300K and magnetic fields from 500 Oe to 4.75 kOe. For instance, a new and unexpected ferromagnetic (FM) phase transition was found at about 150~K in both ingot and powder samples. Similarly the magnetic properties of the powder seem to differ slightly from the ingot's. The powder's magnetic susceptibility $\chi_{\rm pwd}$ appears greater than the ingot's $\chi_{\rm ing}$ at all temperatures measured, with the difference $\Delta\chi=\chi_{\rm pwd}-\chi_{\rm ing}$ increasing with decreasing temperature. We analyze the observed $\Delta\chi$ in terms of two potential sources: impurities added to the powder during the grinding process and the effects of sample geometries in combination with the presence of a second (FM) phase in the studied material. We discuss how the measured differences might affect the study of the physics of this non-Fermi liquid/quantum critical compound. [Preview Abstract] |
Session G20: Focus Session: Metamaterials - Nanoparticles and Nanoparticle Arrays
Sponsoring Units: DMPChair: Shun Shang Lo, University of Notre Dame
Room: 322
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G20.00001: Rare earth doped upconverting particles for different photonic applications Madhab Pokhrel, Ajith Kumar Gangadharan, Dhiraj Kumar Sardar Trivalent rare earth ions especially erbium (Er$^{3+})$ and ytterbium (Yb$^{3+})$ co-doped in various host nanoparticles are known for their extraordinary spectroscopic properties. A thorough optical characterization including the absolute upconversion quantum yield (QY) measurement is of critical importance in evaluating their potential for various photonic applications. In this paper, we will be presenting a measured absolute upconversion QYs for Yb$^{3+}$ and Er$^{3+}$ doped in La$_{2}$O$_{2}$S under 980 and 1550 nm excitation at various power densities. Comparison of absolute QYs for different concentrations of Yb$^{3+}$ and Er$^{3+}$ doped in La$_{2}$O$_{2}$S will be made for all the upconversion emissions with respect to reported most efficient upconverting phosphor NaYF$_{4}$ doped with 20{\%} Yb$^{3+}$ and 2{\%} Er$^{3+}$. Furthermore, applications of these phosphors in different areas such as bio-imaging, solar cell, security, etc. will be explored depending on the measured absolute upconversion quantum yields. In addition, preliminary results on in vitro imaging using upconverting nanoparticles as a contrast agent will be reported. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G20.00002: Characterization and light emission from Erbium Oxide Nanoparticles Muhammad Maqbool, Lynda Wilkinson, Iftikhar Ahmad The present work reports light emission from Erbium Oxide nanoparticles. The nanoparticles, with 43 nm diameter, were obtained in the form of nanopowder with 99.9{\%} purity. These nanoparticles were characterized for their light emission under a 532 nm Nd:YAG laser excitation. A Photoluminescence (PL) system was used to detect fluorescence emission from the nanoparticles. The PL system consisted of Pixis brand CCD camera with a range of 300 to 2000 nm. The Erbium Oxide nanoparticles were also mixed in distilled water to obtain spectrum. Two emission peaks were observed at 554 nm and 813 nm. The green emission at 554nm was obtained as a result of $^4$I$_{15/2} \to $ $^4$S$_{3/2}$ transition, and the near infrared emission from $^4$I$_{15/2} \to $ $^4$I$_{13/2}$ transition. The process was also repeated in vacuum and it was found that the green emission enhances tremendously when the nanoparticles are excited in vacuum. This enhanced luminescence from the Erbium Oxide nanoparticles shows their potential importance in the optical devices and Biomedical applications. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G20.00003: The Vibrational and Photoluminescence Properties of TiO$_2$ Nanoparticles Reacted with Eu$^{3+}$ Ions under Hydrothermal Conditions L. Farris, H. Yan, P. McCart, R. Mayanovic TiO$_{2}$ has been shown to be an effective material for environmental purification and photocatalysis. The catalytic activity of TiO$_{2}$ nanoparticles (NPs) is enhanced due to the increase in the ratio of surface area to volume at the nano-scale. The enhancement of catalytic activity is further increased by the modification of the surface due to the adsorption of transition-metal ions on TiO$_{2}$ NPs. The reactivity of Eu$^{3+}$ ions with anatase TiO$_{2}$ nanoparticles under various pH and pressure-temperature (P-T) conditions in aqueous fluids has been investigated. A hydrothermal reactor was used to modify the surface of the TiO$_{2}$ nanoparticles with Eu$^{3+}$ ions in aqueous fluids at high P-T conditions. The Eu-reacted and untreated TiO$_{2}$ NPs were examined using XRD, SEM, and Raman and photoluminescence spectroscopy. The modifications of the vibrational and photoluminescence properties of the TiO$_{2}$ NPs due to the surface-adsorption of Eu$^{3+}$ ions are discussed. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G20.00004: Optical Spectroscopy of Single Gold Nanoparticles Invited Speaker: Michel Orrit Compared to electron microscopy or to scanning probe microscopy, the optical selection of individual nanoparticles in a far-field microscope provides non-invasive probing of deep layers and commands a wide range of time-resolved and frequency-resolved techniques. Optical signals provide unique insights into the dynamics of nano-objects and of their surroundings [1]. I shall illustrate applications of single-nanoparticle optics with recent topics from our group. i) We study single gold nanoparticles by photothermal and pump-probe microscopy [2]. These experiments can be done in an optical trap, where a single nanorod orients along the trapping polarization, and studied the acoustic damping of gold nanoparticles. ii) Photothermal microscopy opens the study of non-fluorescent absorbers, down to single-molecule sensitivity [3]. Combining photothermal contrast with photoluminescence, we can measure the luminescence quantum yield on a single-particle basis. Moreover, the high signal-to-noise ratio opens up uses of individual gold nanoparticles for local plasmonic and chemical probing, down to single-protein level [4]. iii) Gold nanorods generate strong field enhancements near their tips. By matching the rod's aspect ratio to a dye's fluorescence and excitation spectra, we could observe thousand-fold enhancements for the fluorescence of single Crystal Violet molecules [5]. Gold nanorods can produce local fields as high as those of bow-tie antennas, thanks to their narrow plasmon resonance, but they are much easier to synthesize, functionalize and disperse in solution than lithographically made nanostructures. Acknowledgement : The work presented was done over the last 7 years by F. Kulzer, M. Lippitz, A. Tchebotareva, A. Gaiduk, P. Zijlstra, S. Khatua, M. A. van Dijk, P. V. Ruijgrok, M. Yorulmaz, HF. Yuan, and N. Verhart in the author's group.\\[4pt] [1] F. Kulzer et al., Angew. Chem. \textbf{49} (2010) 854.\\[0pt] [2] A. L. Tchebotareva et al., Laser Photon. Rev. \textbf{4} (2010) 581-597.\\[0pt] [3] A. Gaiduk et al. Science \textbf{330} (2010) 353\\[0pt] [4] P. Zijlstra et al., Nature Nanotech. \textbf{7} (2012) 379.\\[0pt] [5] HF. Yuan et al., Angew. Chem., in press (2013). [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G20.00005: Physical property control of nanoparticles for effective light-energy use Sungsook Ahn, Sung Yong Jung, Sang Joon Lee Up-to-now only limited materials are useful for solar energy harvesting, which makes the expansion of available photoactive materials important. In this point of view, physical property control is one of the reasonable solutions rather than creation of new materials. In this study, as a representative light-responsive metal nanoparticle (NP), gold NPs of a fixed size (average diameter of 20 nm) are surface-activated in pH-controlled aqueous solutions or chemically cross-linked, followed by electron-beam treatment. Chemical-interlinking of NPs behaves like a polymerization, generating characteristic structures (Fractal dimension). The absorbance at UV-vis and THz regions are significantly modified depending on the surface-modification and controlled cluster structures of NPs. Electron beams of different doses are applied to change the surface energy of NPs forming a specific surface layer (proximity length) and the structural modification of NP clusters. This changes the absorption energy band toward shorter-wavelength UV-vis light, benefiting solar energy harvesting. This study contributes to fundamental understanding on nanoparticle technology and provides general information for new metamaterial design for effective light energy use. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G20.00006: Size and shape dependence of electronic and optical excitations in TiO$_2$ nanocrystals Kopinjol Baishya, Serdar Ogut We present results for the electronic structures, quasi-particle gaps, and the absorption spectra of TiO$_2$ nanocrystals of both rutile and anatase phases with various shapes, sizes, and surfaces exposed. We study the size and shape dependences of these electronic and optical properties, computed both within time-dependent density functional theory and many-body perturbation methods such as the GW-BSE, using appropriately passivated nanocrystals to mimic bulk termination. Surface effects are examined by using nanocrystals of various sizes with particular surfaces, such as (110) in rutile and (101) in anatase phases, exposed. We interpret the resulting optical absorption spectra of these nanocrystals in terms of the bulk spectra and compare them with predictions from classical Mie-Gans theory. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G20.00007: Pattern curvature to control pore shape and its ordering Guiduk Yu, Kyusoon Shin Triangular pore in inverse-hexagonal packing was fabricated by anodizing Al with convex pattern in hexagonal packing. The convexly patterned Al was prepared \textit{via} replication of the concave structure formed in self-assembled anodized aluminum oxide (AAO). Self-assembled AAO without pre-patterning produces hexagonal packing circular pores. Exploitation of the inversed structure was found to create well-defined triangular pores in inverse-hexagonal packing. Anisotropic pore feature was discussed to come from the alternating distance between the pits and the curvature of the pattern. Also, by controlling the topography of the convex pattern around pits, we investigated the effect of pattern topography on pore initiation. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G20.00008: Patterning of GaAs and Si substrates using self-organized Al$_{2}$O$_{3}$ templates and epitaxial growth of GaAs nanostructures Archana Kumari, John Hatch, Jaesuk Kwon, Xin Zhang, Everett Fraser, Chae Hyun Kim, Hao Zeng, Hong Luo Reactive ion etching is used with Al$_{2}$O$_{3}$ templates to pattern SiO$_{2}$ films deposited on GaAs and Si substrates. The technique allows nanopatterning of substrates without photo or e-beam lithography. The SiO$_{2}$ film pattern consists of holes of about 80 nm diameter with a pitch of about 100 nm. GaAs nanostructures are grown on the patterned substrates by molecular beam epitaxy. The observed arrays of nanostructures closely follow the patterns on SiO$_{2}$. Several types of structures are observed depending on the growth conditions, including pillars with flat hexagonal tops and pyramidal triangular tops. Characterization of the structures will be discussed. This work was supported by NSF DMR1006286. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G20.00009: Nanostructures Using Anodic Aluminum Oxide Ilya Valmianski, Carlos M. Monton, Juan Pereiro, Ali C. Basaran, Ivan K. Schuller We present two fabrication methods for asymmetric mesoscopic dot arrays over macroscopic areas using anodic aluminum oxide templates. In the first approach, metal is deposited at 45$^{\mathrm{o}}$~to the template axis to partially close the pores and produce an elliptical shadow-mask. In the second approach, now underway, nanoimprint lithography on a polymer intermediary layer is followed by reactive ion etching to generate asymmetric pore seeds. Both these techniques are quantified by an analysis of the lateral morphology and lattice of the pores or dots using scanning electron microscopy and a newly developed MATLAB based code (available for free download at http://ischuller.ucsd.edu). The code automatically provides a segmentation of the measured area and the statistics of morphological properties such as area, diameter, and eccentricity, as well as the lattice properties such as number of nearest neighbors, and unbiased angular and radial two point correlation functions. Furthermore, novel user defined statistics can be easily obtained. We will additionally present several applications of these methods to superconducting, ferromagnetic, and organic nanostructures. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G20.00010: Spectroscopic ellipsometric studies of randomly distributed plasmonic Gallium nanoparticles Yang Yang, Tong-Ho Kim, Neset Akozbek, April Brown, Henry Everitt Ultraviolet surfaced-enhanced Raman scattering (UV-SERS) was recently observed using randomly distributed Gallium nanoparticles (Ga NPs) deposited on sapphire by molecular beam expitaxy at room temperature. Atomic force and scanning electron microscopies revealed that the radii of the hemispheroid NPs follow unimodal or bimodal pseudo Gaussian distributions whose mean diameters increase with increasing Ga dosage (i.e. growth time). Variable angle spectroscopic ellipsometric measurements were then performed on Ga NP ensembles to explore the correlation between the polarimetric optical response and the local morphology. An effective medium composed of single or double Lorentzian oscillators was found to reproduce the optical response of Ga NP ensembles with resonance frequencies that decrease monotonically with increasing NP size. In addition, a strong depolarization response was observed for near-normal incidence. Interestingly, the sample for which the depolarization peak was closest to the 325nm laser excitation wavelength was the sample with the highest SERS enhancement factor. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G20.00011: Optical Properties of Focused Ion Beam-Induced Plasmonic Ga Nanoparticle Arrays on Compound Semiconductor Surfaces Myungkoo Kang, Jia-Hung Wu, Timothy Saucer, Ali Al-Heji, Jieun Lee, Vanessa Sih, Rachel Goldman Recently, metallic nanoparticles (NPs) on semiconductor surfaces have enabled the generation of surface plasmon resonances (SPR) which are promising for enhanced light emission, highly-efficient solar cell, ultra-sensitive biosensors, and negative refractive index metamaterials. Ion sputtering-induced surface pattern formation has the potential to become a cost-effective method for achieving metallic NP arrays. Here, we report optical properties of focused ion beam (FIB)-induced plasmonic Ga NP arrays on compound semiconductor surfaces. To date, we have examined SPR energy of FIB-induced Ga NP arrays. The SPR energies increase with decreasing NP or chain diameter, due to particle diameter-dependent dipole interactions within the metallic NPs. We have utilized SPR of FIB-induced Ga NPs for the enhancement of GaAs photoluminescence (PL) efficiency. The maximum PL enhancement occurs for the Ga NP diameter predicted to exhibit a SPR energy corresponding to the GaAs donor-acceptor pair emission energy. When the SPR energy matches the energy of the free carriers in GaAs, it is transferred to the Ga NPs, inducing an enhancement of the spontaneous emission rate. These results suggest that FIB-induced Ga NPs can be a promising alternative plasmonic material. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G20.00012: Engineering structured light with Vogel spiral arrays of nanoparticles Nate Lawrence, Jacob Trevino, Luca Dal Negro We present a general analytical model for light scattering by arbitrary Vogel spiral arrays of circular apertures uniformly illuminated at normal incidence. This model suffices to unveil the fundamental mathematical structure of their complex Fraunhofer diffraction patterns and enables the engineering of optical beams carrying multiple values of orbital angular momentum (OAM). By performing analytical Fourier-Hankel decomposition of spiral arrays and far field patterns, we rigorously demonstrate the ability to encode specific numerical sequences onto the OAM values of diffracted optical beams. In particular, we show that these OAM values are determined by the rational approximations of the continued fraction expansions of the irrational angles utilized to generate Vogel spirals. Finally, we experimentally demonstrate structured light carrying multiple values of OAM in the far-field scattering region of Vogel spiral arrays of metallic nanoparticles. Using Fourier-Hankel mode decomposition analysis and interferometric reconstruction of the complex amplitude of scattered waves, we show the ability to encode well-defined numerical sequences, determined by the aperiodic spiral geometry, into azimuthal OAM values, in excellent agreement with analytical scattering theory. The generation of sequences of OAM values by light scattering from engineered aperiodic surfaces is relevant to a number of device applications for secure optical communication, classical and quantum cryptography. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G20.00013: Bandgap analysis and emission enhancement from Aperiodic Vogel Spiral Arrays of dielectric nanopillars Jacob Trevino, Gary Walsh, Luca Dal Negro We report on an experimental and theoretical investigation of the structural and photonic mode properties of Vogel spiral arrays of dielectric cylinders in air. We have designed and fabricated hydrogenated amorphous silicon (aSi:H) golden angle spiral nanopillar arrays with localized bandedge modes at the emission wavelength of a commercial near-infrared (NIR) laser dye. Variable-angle reflectance measurements are utilized to experimentally investigate the photon dispersion diagram of spiral arrays and to locate photonic bandgaps. Experimental results are found to be in good agreement with rigorous coupled-wave analysis (RCWA) calculations. These findings offer the opportunity to create novel photonic devices that leverage radially localized and isotropic bandedge modes to enhance light-matter coupling, such as optical sensors, light sources, concentrators, and broadband optical couplers. [Preview Abstract] |
Session G21: Focus Session: Theories of Electric, Elastic, Magnetic and Cross-coupling Terms in Ferroic Lattices
Sponsoring Units: DMPChair: Philippe Ghosez, Universite de Liege
Room: 323
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G21.00001: Converse Piezoelectricity Michael Springborg, Bernard Kirtman Piezoelectricity results from a coupling between responses to mechanical and electric perturbations and leads to changes in the polarization due to strain or stress or, alternatively, the occurrence of strain as a function of an applied external, electrostatic field (i.e., converse piezoelectricity). Theoretical studies of those properties for extended systems require accordingly that their dipole moment or polarization can be calculated. However, whereas the definition of the operator for the dipole moment for any finite system is trivial, it is only within the last 2 decades that the expressions for the equivalent operator in the independent-particle approximation for the infinite and periodic system have been presented. Here, we demonstrate that the so called branch dependence of the polarization for the infinite, periodic system is related to physical observables in contrast to what often is assumed. This is related to the finding that converse piezoelectric properties depend both on the surfaces of the samples of interest even for samples with size well above the thermodynamic limit. However, we shall demonstrate that these properties can be calculated without explicitly taking the surfaces into account. Both the foundations and results for real system shall be presented. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G21.00002: Are Polarization and Magnetization really Bulk Properties? Raffaello Bianco, Raffaele Resta Microscopic understanding of P and of (orbital) M was achieved only recently; the modern theories express both as BZ integrals. Since k-space is an artificial construct, all bulk properties must be embedded in the ground state density matrix $\rho({\bf r,r'})$, ``nearsighted'' in insulators, independently of the boundary conditions, either periodic (PBCs) or open (OBCs). A basic tenet of the modern theory is that the bulk electron distribution determines P only modulo a ``quantum'': therefore P is not independent of the boundaries. Instead M is not affected by any quantum indeterminacy and an expression in terms of the bulk $\rho$ is not ruled out: we explicitly find such expression. In a finite homogeneous sample, within OBCs, the macroscopic magnetization is cast as a function of the bulk $\rho({\bf r,r'})$. Remarkably, our approach applies even to topological (Chern) insulators, where M explicitly depends on the chemical potential. The boundary currents contribute to M, but even their contribution is ``bulk'' in the above sense; the value of M is robust and cannot be altered by acting on the boundaries only. Instead, P can be varied (by a quantum) by acting on the boundaries only. Simulations performed on a 2d model Hamiltonian within OBCs demonstrate our approach. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G21.00003: Noncollinear magnetism and single-ion anisotropy in multiferroic perovskites Eric Bousquet, Carlo Weingart, Nicola Spaldin The link between the crystal distortions of the perovskite structure and the magnetic exchange interaction (J), the single-ion anisotropy (SIA), and the Dzyaloshinsky-Moriya (DM) interaction are investigated by means of density-functional calculations in AFeO$_3$ systems. We explore the effect of the crystal distortions (Antiferrodistortive-AFD and Ferroelectric) as well as the effect of the A-cation chemistry on the 3 magnetic properties, J, SIA and DM. Our analysis shows a never explored but possible switching of the weak ferromagnetism in the $R3c$ phase of BiFeO$_3$ through the competition of the SIA shapes induced by the AFD and the ferroelectric distortions. We also found that-in spite of the d$^5$ electronic configuration of Fe$^{3+}$, the SIA is very large in some structures and is surprisingly strongly sensitive to the chemistry of the A-site cation of the ABO$_3$ perovskite. To clarify the origin of this unexpected effect, we analyze the crystal field splitting by means of Wannier functions. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G21.00004: First-principles theory of flexoelectricity Invited Speaker: David Vanderbilt Flexoelectricity is the linear response of polarization to a strain gradient. Because strain gradients break inversion symmetry, flexoelectricity occurs in all insulating crystals. The flexoelectric effect is negligible on conventional length scales, but it can become very strong at the nanoscale where large strain gradients can significantly affect the functional properties of dielectric thin films and superlattices. Previous theories have tended to focus either on the lattice [1-3] or the electronic (i.e., frozen-ion) [4-5] contribution, and have involved some approximations or limitations. Here we develop a general first-principles theory of the flexoelectric tensor, formulated in such a way that the tensor elements can be computed directly in the context of density-functional calculations. Special attention will be paid to several subtleties, including surface contributions, pseudopotential dependence, the calculation of transverse components, fixed $E$ vs.\ fixed $D$ boundary conditions, and a degree of non-uniqueness that is present for some strain gradients. We introduce several practical supercell-based methods for calculating the flexoelectric coefficients from first principles, and demonstrate them by computing the coefficients for a variety of insulating materials.(Work done in collaboration with Jiawang Hong. Supported by ONR N00014-12-1-1035.) \\[6pt][1] A.K. Tagantsev, Phys. Rev. B {\bf 34}, 5883 (1986). \\[0pt][2] R. Maranganti and P. Sharma, Phys. Rev. B {\bf 80},054109 (2009). \\[0pt][3] I. Ponomareva, A.K. Tagantsev, and L. Bellaiche, Phys. Rev. B {\bf 85}, 104101 (2012). \\[0pt][4] R. Resta, Phys. Rev. Lett. {\bf 105}, 127601 (2010). \\[0pt][5] J. Hong and D. Vanderbilt, Phys. Rev. B {\bf 84}, 180101 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G21.00005: First-principles calculations of flexoelectric coefficients Jiawang Hong, David Vanderbilt Flexoelectricity, which is the linear response of polarization to a strain gradient, can have a significant effect on the functional properties of dielectric thin films, superlattices and nanostructures. Despite growing experimental interest, there have been relatively few theoretical studies of flexoelectricity, especially in the context of first-principles calculations. In this talk, we present a complete theory of both the electronic (or ``frozen-ion'')\footnote{J. Hong and D. Vanderbilt, Phys. Rev. B, {\bf 84} 180101(R) (2011).} and lattice contributions to flexoelectricity, and demonstrate a supercell method for calculating the flexoelectric coefficients using first-principles density-functional methods. Results are presented for cubic materials including CsCl and SrTiO$_3$. In order to obtain all the elements of the flexoelectric tensor, transverse as well as longitudinal, we carry out calculations on supercells extended along different orientations (e.g., [110] as well as [100]), taking special care to carry out conversions between objects calculated under fixed E or fixed D electric boundary conditions in different parts of the procedure. In this way, all the elements of both the electronic and lattice contributions to the flexoelectric tensor are determined. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G21.00006: Linear Magnetoelectric Effect by Orbital Magnetism Andrea Scaramucci, Eric Bousquet, Michael Fechner, Maxim Mostovoy, Nicola Spaldin The linear magnetoelectric effect is the linear induction of a static magnetization (electric polarization) by an applied static electric (magnetic) field. Using symmetry analysis and ab initio calculations we show that, in addition to mechanisms involving magnetic moments of spins, such an effect can originate from the response of orbital magnetic moment to polar distortions induced by an applied electric field. Considering LiFePO$_4$ as model compound, we show that spin-orbit coupling partially lifts the quenching of 3d orbitals and causes small orbital magnetic moments at the magnetic ions sites. An applied electric field modifies the sizes of these orbital magnetic moments and results in a net magnetization. Furthermore, we discuss the link between this mechanism and the electric field dependence of magnetocrystalline anisotropy. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G21.00007: Longitudinal magnetoelectric susceptibility of Cr2O3: First-principles calculations using the converse approach Sai Mu, A. L. Wysocki, K.D. Belashchenko Temperature-dependent longitudinal magnetoelectric (ME) susceptibility of Cr2O3 is calculated as a response of the magnetization M to the applied electric field E. The ionic displacements are found using the calculated force constant matrix and Born effective charges. The exchange parameters are calculated using total energy calculations for different spin configurations on the perturbed lattice, and the magnetization is evaluated using the pair cluster approximation to the quantum spin-3/2 Hamiltonian. When similar approximations are used, the results agree with the inverse approach of Mostovoy et al. [Phys. Rev. Lett. 105, 087202 (2010)]. The electronic contribution is found to be approximately 40{\%} of the ionic contribution and opposite in sign to it. The ME susceptibility is found to depend strongly on the choice of the Hubbard U parameter, increasing as U is increased. On the other hand, the ME response is only weakly depressed by the inclusion of intersite spin correlations within the pair cluster approximation. The methodology developed here can facilitate the search for new materials with desirable ME properties. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G21.00008: A theoretical study of the dynamical magnetic charge tensor in crystalline Cr$_2$O$_3$ Meng Ye, David Vanderbilt Magnetoelectric (ME) materials are of fundamental interest and are investigated for their broad potential for technological applications. The search for, and eventually the theoretical design of, materials with large ME couplings present challenging issues. First-principles methods have only recently been developed to calculate the full ME response tensor $\alpha$ including both electronic and ionic (i.e., lattice-mediated) contributions.\footnote{A. Malashevich et al., Phys. Rev. B, {\bf 86}, 094430 (2012).} In several materials, the dominant contribution to the ME response has been shown to be the ionic term $\alpha_{\rm ion}$, which is proportional to both the Born dynamical electric charge $Z^{\rm e}$ and its analogue, the dynamical magnetic charge $Z^{\rm m}$.\footnote{J. \'{I}\~{n}iguez, Phys. Rev. Lett. {\bf 101}, 117201 (2008).} Here we present a theoretical study whose ultimate goal is to understand the mechanisms that would enhance the magnetic charge $Z^{\rm m}$. Using first-principles density-functional methods within a relativistic framework with the inclusion of the spin-orbit interaction, we calculate the atomic magnetic charge tensors $Z^{\rm m}$ for both Cr and O atoms in Cr$_2$O$_3$, and discuss how these contribute to the ME response in this material. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G21.00009: Ferroelectricity induced by interatomic magnetic exchange interaction Chungang Duan, Xiangang Wan, Hang-Chen Ding, Sergey Y. Savrasov Multiferroics, where two or more ferroic order parameters coexist, is one of the hottest fields in condensed matter physics and materials science. To search multiferroics, currently most researches are focused on frustrated magnets, which usually have complicated magnetic structure and low magnetic ordering temperature. Here, we argue that actually simple interatomic magnetic exchange interaction already contains a driving force for ferroelectricity, thus providing a new microscopic mechanism for the coexistence and strong coupling between ferroelectricity and magnetism. We demonstrate this mechanism by showing that even the simplest antiferromagnetic (AFM) insulator MnO, can display a magnetically induced ferroelectricity under a biaxial strain. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G21.00010: Domain wall fluctuations in ferroelectrics Richard Brierley, Peter Littlewood Ferroelectric domain walls typically have 90- or 180-degree orientations due to the long-range constraints of dipolar and ferroelastic interactions. We calculate the excitation spectrum for deviations from ideal flat walls in these orientations. In the presence of ferroelastic interactions, fluctuations in the polarization orientation must be matched by changes in local strain. The finite acoustic phonon velocity implies a retarded response of the strain fields. This retardation produces a gap as $k\to 0$, limiting the domain wall motion. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G21.00011: Theory of Relaxor Ferroelectric Electrocalorics Gian Guzman-Verri, Peter Littlewood Conventional perovskite ferroelectrics are the material of choice in many modern day technologies such as capacitive energy storage devices, infrared sensors, and random access memories. Conventional ferroelectrics, however, have not been exploited in cooling applications mainly because their narrow region of critical fluctuations of polarization results in a small electrocaloric effect (a few miliKelvin per volt). Relaxor ferroelectrics, on the other hand, exhibit a broad region of critical fluctuations which makes them promising candidates for large electrocalorics. In this talk, we present a theoretical study of electrocalorics in relaxor ferroelectrics. We compute isothermal changes in entropy and adiabatic changes in temperature within a model of polarizable unit cells with local short-range forces, dipolar forces, and compositional disorder. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G21.00012: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G21.00013: Dynamical mean field studies on mid-gap states of SrTiO$_3$ Chungwei Lin, Alexander A. Demkov We study the mid-gap states obtained by photon luminescence of SrTiO$_3$ using dynamical mean field theory. The 2.4 eV peak observed in the SrTiO$_3$ luminescence experiment is attributed to the strong electron-optical phonon coupling when an electron is excited from oxygen 2p bands to titanium 3d bands, and is conventionally modeled by Franck-Condon model which contains only one fermion and one phonon field. Here we extend this model to a realistic lattice described by the tight-binding approximation, using dynamical mean field theory with exact diagonalization solver. We found the main features of Franck-Condon model preserve. The effects of correlation on oxygens will be discussed. [Preview Abstract] |
Session G22: Electronic Phenomena of Nanostructures
Sponsoring Units: DCMPChair: Nancy Sandler, Ohio University
Room: 324
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G22.00001: Fermi liquid nature of the ground state of multiple-quantum dots in parallel Manas Kulkarni, Robert Konik We argue through a combination of 1/N diagrammatic expansion, slave boson mean field theory and the Bethe ansatz that the ground state of multiple quantum dots arranged in parallel is a singlet Fermi-liquid ground state. This conclusion is arrived at by showing the validity of Friedel Sum Rule (a fingerprint of Fermi-liquid physics) and finding that impurity entropy vanishes in the limit of zero temperature (singlet). Our conclusion is in contradiction to other studies that predict a non-Fermi liquid ground state. We discuss possible reasons for this discrepancy. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G22.00002: Coupled collective modes in electronic systems of different dimensionalities Ben Yu-Kuang Hu, Euyheon Hwang, Sankar Das Sarma We consider electronic collective modes in coupled systems in which the individual components have different dimensionalities. Many-body diagramnatic techniques are used to derive formal results for the screened intra- and inter-system Coulomb interaction. We specifically investigate the case of a quasi-one-dimensional quantum wire in close proximity to a two-dimensional electron gas. We evaluate the screened intra- and inter-system Coulomb interaction within the random phase approximation, and find the existence of modes which have hybrid properties characteristic of both one- and two-dimensional systems. We also investigate the spatial dependence of the coupled 1-d + 2-d collective modes within the two-dimensional electron gas, and show that the coupled modes within that layer vary from being purely two-dimensional in character far away from the quantum wire to being strongly hybridized close to the wire. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G22.00003: Influence of Rashba spin-orbit interactions on the Kondo effect Arturo Wong, Kevin Ingersent, Mahdi Zarea, Sergio Ulloa, Nancy Sandler Recent studies [1] have pointed out that the thermodynamics of the Kondo effect are essentially unaltered by the presence of Rashba spin-orbit interactions in a host two-dimensional electron gas. However, it has also been proposed [2] that the presence of bulk Rashba interactions induces a coupling between a magnetic impurity and conduction electrons with nonzero orbital angular momentum about the impurity site. In this work we revist this problem using the numerical renormalization group. In agreement with previous studies, we find only minor changes in the Kondo temperature scale when the Rashba coupling is increased at fixed Fermi energy. However, for fixed band filling, increasing the spin-orbit coupling can move the Fermi energy near to a Van Hove singularity in the effective density of states, leading to an exponential enhancement of the Kondo scale. Static spin correlations confirm that the impurity couples to conduction channels of nonzero orbital angular momentum. We also explore the effects of a magnetic field applied in the plane of the host system.\\[4pt] [1] J. Malecki, J. Stat. Phys. 129, 741 (2007); R. \u{Z}itko and J. Bon\u{c}a, Phys. Rev. B 84, 193411 (2011). \newline [2] M. Zarea, S. Ulloa and N. Sandler, Phys. Rev. Lett. 108, 046601 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G22.00004: Spin-polarized conductance in double quantum dots: Interplay of Kondo, Zeeman and orbital effects Luis Dias da Silva, Edson Vernek, Kevin Ingersent, Nancy Sandler, Sergio Ulloa We study the effect of an external magnetic field in the Kondo regime of a double-quantum-dot system in which a strongly correlated dot is coupled to a noninteracting dot that is also connected to external leads. In zero field, the spectral function of the hanging dot has previously been shown to exhibit a split-peak structure near the Fermi level due to ``Kondo resonance filtering'' by the noninteracting dot. We show, using the numerical renormalization group, that application of a magnetic field leads to a subtle interplay between electronic interference, Kondo physics, and Zeeman splitting with nontrivial consequences for the spectral and transport properties. The value of the correlated-dot spectral function at the Fermi level exhibits a nonuniversal field dependence that can be explained using a generalized Friedel sum rule for a Kondo system with energy-dependent hybridization. By tuning gate voltages and the magnetic field, one can achieve complete spin polarization of the linear conductance between the leads, raising the prospect of applications of the device as a highly tunable spin filter. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G22.00005: Controlling entanglement and spin-correlations in double quantum dots with electrical currents in the non-equilibrium regime C. A. Busser, F. Heidrich-Meisner We study the non-equilibrium dynamics in a parallel double-quantum dot structure induced by a large bias voltage. By applying both a magnetic flux and a voltage, it is possible to generate spin-spin-correlations between the two quantum dots. The sign and absolute value of these correlations can be controlled by changing the bias voltage. Using a canonical transformation we argue that the mechanism that drives the spin-spin correlations can be understood in terms of an effective Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction that is mediated by the current. Our study is based on the Anderson-impurity model and we use time-dependent density matrix renormalization group simulations to obtain currents and spin-correlations in the non-equilibrium regime. We also perform quench in the Hamiltonian to prove the stability of the entangled state. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G22.00006: SU(4) Kondo effect in a double quantum dot Andrew Keller, Sami Amasha, Ileana Rau, Lucas Peeters, Jordan Katine, Hadas Shtrikman, David Goldhaber-Gordon Lateral quantum dots are highly tunable experimental systems ideal for exploring the interplay of spin and charge correlations. We present studies of a parallel-coupled double quantum dot system in a GaAs/AlGaAs heterostructure. In the limit of negligible inter-dot tunneling, the conductance through both dots is enhanced at inter-dot orbital degeneracies, where the energy for an electron to be on either dot is the same. We show how at four-fold orbital and spin degeneracies, signatures in the zero-bias conductance, the temperature dependence, and the bias spectroscopy suggest an SU(4) Kondo effect may be realized, combining spin and pseudospin. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G22.00007: What Is Measured in the Scanning Gate Microscopy of a Quantum Point Contact? Steven Tomsovic, Rodolfo A. Jalabert, Wojciech Szewc, Dietmar Weinmann The conductance change due to a local perturbation in a phase-coherent nanostructure is calculated. The general expressions to first and second order in the perturbation are applied to the scanning gate microscopy of a two-dimensional electron gas containing a quantum point contact. The first-order correction depends on two scattering states with electrons incoming from opposite leads and is suppressed on a conductance plateau; it is significant in the step regions. On the plateaus, the dominant second-order term likewise depends on scattering states incoming from both sides. It is always negative, exhibits fringes, and has a spatial decay consistent with experiments. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G22.00008: Understanding Nanocontacts with atomic precision Carlos Sabater, Maria Jose Caturla, JuanJose Palacios, Carlos Untiedt Measuring the variations of the conductance indentation experiments between two electrodes, we can obtain information on the changes in the atomic structure of the contact. We have analysed the Jump-to-Contact(JC) phenomenon which can observed as the first contact when the two metals approach each other. Moreover, we have studied the Jump-out-of-contact(JOC) phenomenon which is the last contact before breaking the two electrodes. Secondly, as we further approach the two electrodes and when the indentation depth is limited to a certain value of conductance, almost the exact behaviour in the evolution of the conductance can be obtained for hundreds of cycles of formation and rupture. That is, the same sequence of atomic configurations was followed. Both processes are rationalized using MD simulations together with DFT transport calculations, which show: a) the most probable atomic configurations in the first atomic contact following the JC or JOC processes; b) that after repeated indentations the two metallic electrodes are shaped into tips of a reproducible structure formed through a mechanical annealing process. These results improve our understanding of atomic-sized contacts and the evolution of their structural characteristics. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G22.00009: Emergent Localization from Many-Body Physics in Clean Quantum Point Contacts Caspar H. van der Wal, M.J. Iqbal, E.J. Koop, J.B. Dekker Dekker, J.P. de Jong, J.H.M. van der Velde, D. Reuter, A.D. Wieck, R. Aguado, Y. Meir Quantized conductance in quantum point contacts (QPCs) is the signature of control over electron transport at the nanoscale. As a function of channel width the conductance then increases in steps of $G_0=2e^2/h$. However, experiments often show an additional feature with a conductance plateau near $0.7G_0$, known as the 0.7 anomaly. This has been studied since 1995 but its full understanding is still an open problem. Spontaneous localization due to many-body effects in open QPCs, and the associated Kondo effect, has emerged as a promising theory for the 0.7 anomaly [1]. This theory work predicted that the many-body effects should for certain QPC geometries not give a single localized state but a pair of localized states, but signatures of this were till now not reported. For the first time, we have fabricated length-tunable QPCs in clean semiconductors and we discovered a periodic modulation of the 0.7 anomaly as a function of length. This modulation correlates with signatures for single and paired quasi-localized states, in the form of single- and two-impurity Kondo physics. Our results demonstrate that Friedel oscillations and emergent impurity states from many-body physics are fundamental to these phenomena. [1] T. Rejec and Y. Meir, Nature 442, 900 (2006). [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G22.00010: Electrolyte gating of gold point contacts Trevor Petach, Menyoung Lee, David Goldhaber-Gordon Gold point contacts are fabricated in-situ by electromigration in an ionic liquid bath. These contacts are shown to be stable at room temperature at conductances as small as 50 G$_0$. By electrolyte gating the contacts using a counter electrode in the ionic liquid, conductance changes of 25$\%$ are observed, corresponding to accumulation of more than one electron per gold surface atom. Double step chronocoulometry and x-ray reflectometry suggest that ion ordering in the ionic liquid near the gold interface are consistent with the observed changes in conductance. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G22.00011: Electron energy spectra in two dimensional quantum rings consisting of two nanoelements Avag Sahakyan, Ruzan Movsesyan, Armen Kocharian Electron spectrum and ground state properties in two dimensional confined quantum rings with R$_{1,2}$ radiuses consisting of the two different (materials) nanoelements divided by two sectored finite size quantum wells with various potentials and spanning angles, is studied in the presence of transverse magnetic field. The calculated wave function shows oscillations along the radial direction which are progressing by approaching to the internal radius of the ring, R$_2$. Situation here is similar to the problem of fall of the particle on the attractive center. However, these oscillations are interrupted on the internal ring boundary by providing the new ground state which is sensitive to the change of magnetic flux. For shallow energy levels some energy states are undergoing changes controlled by magnetic field accompanied with the persistent current and abrupt phase transitions. Magnetization and magnetic susceptibility show characteristic two dimensional anomalous behaviors different from one found in one dimensional ring. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G22.00012: Lateral quantization of two-dimensional electron states by embedded Ag nanocrystals Chris Van Haesendonck, Koen Schouteden We show that quantization of image-potential state (IS)electrons \emph{above} the surface of nanostructures can be experimentally achieved by Ag nanocrystals that appear as stacking fault tetrahedrons (SFTs) at Ag(111) surfaces. By means of cryogenic scanning tunneling spectroscopy the $n = 1$ IS of the Ag(111) surface is revealed to split up in discrete energy levels, which is accompanied by the formation of pronounced standing wave patterns that directly reflect the eigenstates of the SFT surface. The IS confinement behavior is compared to that of the surface state electrons \emph{in} the SFT surface and can be directly linked to the particle-in-a-box model. ISs provide a novel playground for investigating quantum size effects and defect induced scattering \emph{above} nanostructured surfaces. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G22.00013: MoS$_2$ grain-boundary: First-principles investigations Duy Le, Talat S. Rahman We present results of our first-principles electronic structure investigations, using the spin-polarized density-functional-theory, of the electronic and geometric structures of various models of grain-boundaries formed between different MoS$_2$ domains when grown as a single layer. From analysis of electronic band structures, we find, in all considered models, that the grain-boundaries exhibit metallic behavior. More interestingly, we find signatures of magnetism in the grain-boundary formed between two sulfur edges with 0\% sulfur coverage. Details analysis of the geometric structures lead us to the conclusion that certain grain-boundaries undergo $(2\times 1)$ reconstructions. We provide full details of the electronic and spin density states and change redistribution at the domain boundaries. We make contact with recent experimental observations and discuess the modifications in the characteristics for MoS$_2$ grown on Cu(111) [1]. \\[4pt] [1] D. Kim \textit{et al}, Langmuir \textbf{27}, 11650 (2011) and unpublished results. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G22.00014: Ab initio Simulations of charge transfer properties at Quantum Dot/TiO$_{2}$ Interface in Quantum Dot-Sensitized Solar Cells Xukai Xin, Rana Biswas, Zhiqun Lin Quantum dot-sensitized solar cells (QDSSCs) have emerged as a very promising solar architecture for next generation photovoltaics. The QDSSCs exhibit a remarkably fast electron transfer from the quantum dot (QD) donor to the TiO$_{2}$ acceptor with size quantization properties that allows for the modulation of QD band gaps to control the photoresponse and photoconversion efficiency of QDSSCs. To understand the mechanisms that underpin this rapid charge transfer, the electronic properties of CdSe and PbSe QDs on the TiO$_{2}$ substrate were simulated using a rigorous ab initio method. In contrast to the plane wave approaches, this method capitalized on localized orbital basis set that is computationally less intensive, and provides excellent electronic structure of the constituent systems. We consider QDs grown on TiO$_{2\, }$without functional ligands passivating the QD surface. We find a remarkable set of electron bridging states between QDs and TiO$_{2}$ occurring via the strong bonding between the conduction bands of QDs and TiO$_{2}$. Such bridging states account for the fast adiabatic charge transfer from the QD donor to the TiO$_{2}$ acceptor, and may be a general feature for other strongly coupled donor/acceptor systems and nanostructured semiconductor interfaces. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G22.00015: Effect of hydrogen passivation on the electronic structure of ionic semiconductor nanostructures Huixiong Deng, Shu-Shen Li, Jingbo Li, Su-Huai Wei In theoretical studies of thin film and nanostructured semiconductors, pseudohydrogen (PH) is widely used to passivate the surface dangling bonds. Based on these calculations, it is often believed that nanostructured semiconductors, due to quantum confinement, have a larger band gap than their bulk counterparts. Using first- principles band structure theory calculation and comparing systematically the differences between PH-passivated and real-hydrogen--passivated (RH-passivated) semiconductor surfaces and nanocrystals, we show that, unlike PH passivation that always increases the band gap with respect to the bulk value, RH passivation of the nanostructured semiconductors can either increase or decrease the band gap, depending on the ionicity of the nanocompounds. The difference between PH and RH passivations decreases when the covalency of the semiconductor increases and can be explained using a band coupling model. This observation greatly increases the tunability of nanostructured semiconductor properties, especially for wide-gap ionic semiconductors. [Preview Abstract] |
Session G23: Focus Session: Dopants and Defects in Semiconductors V
Sponsoring Units: DMPChair: John Lyons, University of California at Santa Barbara
Room: 325
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G23.00001: Modeling of Threading Dislocation Core Fields and of Point Defects in GaN Jennifer R. Snively, Stefan C. Badescu Point defects and dislocations in GaN are involved in failure mechanisms of GaN-based electronic devices. Compared to bulk material, the electronic states and diffusivities of point defects are modified by dislocation elastic fields. For accurate descriptions atomistic calculations have to take into account both the long (Volterra) and the short-range components of the latter. We present an atomistic picture of defect energy levels and diffusion barriers for vacancies, interstitials and impurities next to threading dislocations in GaN. We include the dislocation core field derived from stress calculations using periodic supercells. We show that this increases significantly the point defect-dislocation interaction by comparison to the Volterra field and that the diffusion energy barriers are changed by as much as 50{\%}. The dependence of charged energy levels on the Fermi level is also modified on many lattice spacings away from the dislocations. We discuss in more detail the charged N vacancies and the Ga and Al interstitials. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G23.00002: Carrier-Induced Lattice Instability in Amorphous Oxide Semiconductors Yong-Sung Kim, Ho-Hyun Nahm, Dae-Hwan Kim Amorphous oxide semiconductors (AOS's) have high electron mobility even in amorphous phase. The AOS-based thin film transistors (TFTs) are nowadays intensively pursued to be adopted into high-resolution flat-panel displays. However, a facing bottleneck of the AOS-based TFT display applications is the instability problem under bias and illumination stress conditions. Especially, by negative bias and illumination stress (NBIS) or only by illumination stress (IS), the threshold voltage of the AOS TFTs is largely negative-shifted. In this work, we study the instability mechanism of the AOS's based on first-principles calculations. The valence band tail states of the AOS's are found to be characterized by the O-O pp$\sigma $* anti-bonding state. The excited localized-holes thus give lattice instability and form O-O bonds through the pp$\sigma $-hybridized interaction. The pp$\sigma $* level is heightened up into the conduction bands along with the O-O bond formation, and two electrons left from the created O-vacancy (V$_{\mathrm{O}})$ occupy the delocalized conduction band states. The O-O and V$_{\mathrm{O}}$ complex (a peroxide defect) is found to be a meta-stable donor defect and suggested as an origin of the NBIS and IS instabilities in AOS TFTs. Based on the suggested mechanism, we propose a direction to improve the stability of AOS thin films with optimizing the cation compositions. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G23.00003: Shuffle-Glide Transition of Dislocations in Silicon Zhi Li, Nithin Mathew, Catalin Picu Dislocation motion in diamond cubic Si can take place either on the shuffle or glide set of \textbraceleft 111\textbraceright planes. It is commonly accepted that shuffle planes are active at low temperatures and high applied stresses, while the glide planes become active at high temperatures and lower stresses. The transition of dislocations from one plane to the other is still a matter of debate, with some authors suggesting that such transition is impossible, and others proposing intermediate metastable states and transition barriers. In this work we show a mechanism by which shuffle dislocations may move to the glide plane without any intermediate state and evaluate the activation barrier (and activation volume) for the transition. We also support the previously observed sensitivity of dislocation mobility in the shuffle plane to the stress acting normal to the glide plane, and link this sensitivity to the nature of the gamma surface. The role of the normal stress in the shuffle-glide transition is also discussed. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G23.00004: Role of defects in resistive switching TiO$_{2}$- and SrTiO$_{3}$-based devices Invited Speaker: Marek Skowronski Oxide-based resistive switching devices hold promise of being the next generation of non-volatile high density memory. While small size, fast switching and long retention time have been demonstrated, many questions remain pertaining the switching mechanism. In particular, the role of point and extended defects in resistance switching remains to be elucidated. Most proposed interpretations invoke oxygen vacancy redistribution as the origin of the resistance change. However, the measurements of the vacancy drift in the Schottky barriers on SrTiO$_{3}$ indicated that mobility at room temperature is eight orders of magnitude too low to account for the reported switching times. This difficulty could be alleviated by faster vacancy motion along the dislocation lines and/or local Joule heating during switching. Careful mapping of the dislocation distribution before and after switching in nanoscale devices did not find a good correlation between dislocations and the I-V characteristics. Lateral SrTiO$_{3}$ devices with no dislocations have similar switching behavior to the ones fabricated on high dislocation density materials. The only correlation present was the generation of dislocations in devices with dissipated power level above 10 mW. The apparent mechanism is the thermal stress driven plastic deformation. While the Joule heating could speed up the defect motion, it could not explain the long retention times. This is frequently interpreted as due to formation of oxygen-deficient phases in TiO$_{2}$-based memristors. Transmission Electron Microscopy analysis of nanoscale vertical devices has revealed numerous physical changes with the extent strongly dependent on the level of dissipated power. Optimized device structures that switch with the power dissipation below 2 mW exhibited very limited degree of crystallization and no new phases. Many of the initially posed questions remain unanswered. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G23.00005: Oxygen vacancies at the surface of SrTiO$_3$ thin films Alexandre R. Silva, Gustavo M. Dalpian The 2-D electron gas at the interface between LaAlO$_3$ (LAO) and SrTiO$_3$ (STO), two band insulators, has been the subject of intense research owing to the fact that this interface can show metallic, superconducting, and magnetic effects, properties that are absent in the bulk counterparts. The metallic behavior has also been observed at the STO surface, without the need of the oxides' interface. Although the reason of this behavior is not well defined, there are three hypotheses for this: the polar catastrophe; the oxygen vacancies produced in the experiment, and cations intermixing. In this work, first principles calculations based on the density functional theory and using hybrid functionals were performed to reveal the atomic and the electronic structure of vacancies at the (001) surface of STO films. We have analyzed both the TiO$_2$ and SrO-terminated surfaces. For pure surfaces, we observed atomic relaxations up to the 5$^{th}$ atomic layer. The surface band structure of ideal STO slabs shows that the STO thin films are insulating in both terminations, but insert surface levels in the gap of bulk STO. Defective STO slabs are observed to be metallic, and we observe a strong tendency for the oxygen vacancies to migrate into the surface. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G23.00006: Persistent photoconductivity in strontium titanate Matthew McCluskey, Marianne Tarun Strontium titanate (SrTiO$_{\mathrm{3}})$ is often used as a substrate for oxide thin films such as high-temperature superconductors. It has the perovskite structure and an indirect band gap of 3.25 eV. Our prior work showed that hydrogen impurities form a defect complex that contains two hydrogen atoms. The complex was tentatively attributed to a passivated strontium vacancy. Alternatively, it could be a partially passivated titanium vacancy. In order to suppress strontium vacancies (and create titanium vacancies), we annealed samples in an evacuated ampoule with SrO powder. These samples show unexpected behavior. After illuminating with light (405 nm, 3.06 eV), the free-electron concentration increases significantly. After the light is turned off, the high conductivity persists at room temperature. We tentatively attribute this effect to the excitation of an electron from a titanium vacancy into the conduction band, with a high barrier for recapture. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G23.00007: A hybrid density functional study on energetics of native defects in anatase $\rm TiO_2$ Adisak Boonchun, Pakpoom Reunchan, Naoto Umezawa, Jinhua Ye The energetics and electronic structures of native defects in anatase $\rm TiO_2$ have been studied by means of hybrid density-functional calculations. Here, we show that oxygen vacancy ($V_{\rm O}$) and titanium interstitial ($\rm Ti_\textit{i}$) are both shallow donors and are likely to form with a substantial concentration in an oxygen poor condition. While titanium vacancies ($V_{\rm Ti}$) is a deep acceptor. The charge neutrality showed that Fermi level is pinned at the conduction band minimum in good agreement with the common observations of n-type conductivity in a reduced $\rm TiO_2$. Self-trap hole ($\rm O_O$) states are localized at oxygen anions. On the other hand the self-trapped electron ($\rm Ti_{Ti}$) cannot be produced in the bulk. Although, $\rm Ti_\textit{i}$ is the strongest candidate for the unintentional $n$-type conductivity owning to its low formation energy, we show that the post-growth of $V_{\rm O}$ in anatase is also possible under annealing temperature and pressure. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G23.00008: DFT-based first-principle calculation of the carrier activation ratio in the F-doped anatase TiO$_2$ and the thermodynamic analysis of the formation of TiOF$_2$ phase Hideyuki Kamisaka, Nanako Mizuguchi, Koichi Yamashita, Tetsuya Hasegawa The F-doped anatase TiO$_2$ (FTO) could be an alternative transparent conductive oxide, but the experimentalists have reported low carrier activation ratio of 20 - 30{\%}, and that the formation of TiOF$_2$ hampers its production when using the PLD method. We investigated this system using the standard DFT-based band structure method. The PBE functional was adapted with the Hubburd $+$U terms. The value of $+$U parameters was adjusted to meet the generalized Koopman's theorem (gKT). We found that the formation energy of F$_{\mathrm{O}}$ (F dopant substituting O) and F$_{\mathrm{O}}^{+}$ is quite close to each other, and the two crosses when the Fermi level is slightly above the conduction band minimum. Combining the Burstein-Moss effect and this crossing of the formation energies, a simple statistical analysis was made. The calculated activation ratio was about 10{\%} - 32{\%}, which agreed with the experimental data. The free energies of bulk TiO$_2$, 3{\%} FTO, 6{\%} FTO and TiOF$_2$ were compared using the DFT result. Contributions from the distribution entropy of the anions, lattice vibrations, free energy of conductive electrons, and the entropy from the spin state of trapped electrons was considered with relevant approximations. We found that the free energy of TiO$_2$, 3{\%} FTO, and TiOF$_2$ come close to each other under the condition of T$=$800K and P(O$_2)=$10$^{-5}$ Torr, which coincides the experimental report. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G23.00009: Origins of Persistent Photoconductivity in GaAsN Alloys R.L. Field III, Y.Q. Wang, C. Kurdak, R.S. Goldman In GaAs$_{\mathrm{1-x}}$N$_{x}$ alloys, we observe significant persistent photoconductivity (PPC) at cryogenic temperatures for $x$ \textgreater\ 0.006, with the PPC strength increasing with increasing $x$ and decreasing upon rapid-thermal annealing (RTA). Since the RTA-induced suppression is accompanied by a reduction of the interstitial N fraction, the N-induced donor state is likely associated with N pairs. PPC is attributed to the promotion of carriers from a ground N-pair state to the conduction band edge, inducing modifications in the N-pair molecular bond configuration. When illumination is terminated, an energy barrier hinders the return of carriers to the N-pair induced complex. With the addition of thermal energy, the original N-pair configuration is restored and the N-pair induced complex is then able to accept carriers. We use PPC at cryogenic temperatures to go through a metal-insulator transition in GaAsN by increasing the carrier density with illumination. For different illumination durations we determine the minimum metallic conductivity, giving us the critical carrier density, $n_{c}$, at the transition point. We then determine the effective mass, $m*$, using the Mott criterion $n_{c}^{1/3}a_{H} = $ 0.26 where $a_{H} = $ (4$\pi \varepsilon $h$^{2})$/($e^{2}m$*) is the Bohr radius. We use PPC to induce a metal-insulator transition in GaAsN. We will discuss the effective mass as a function of N concentration and compare to the predictions of the band anticrossing model. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G23.00010: Defect Related Magnetism and Conduction in As-grown and Annealed Pulsed Laser Deposited SnO$_2$:Co Thin Films Gratiela M. Stoian, P.A. Stampe, R.J. Kennedy, E. Lochner, Y. Xin, S. von Molnar Magnetic semiconductor SnO$_{2}$:Co films were grown on r-cut sapphire substrates via Pulsed Laser Deposition from a doped target with a nominal Co concentration of 5 at.{\%}. To study the role of oxygen vacancies and other defects in tuning the ferromagnetic (FM) and electrical properties of these materials, films were deposited at different growth rates, temperatures and oxygen pressures. In addition, some films were annealed at various conditions. Magnetometry data show that films grown at optimal conditions are FM at room temperature with a saturation magnetization of 20 emu/cm$^{3}$. The moment per unit area varies linearly with the film thickness, suggesting the magnetism in our materials is a volume property. Magnetization decreases monotonically with the growth rate. A transition from a semiconducting, magnetic material to an insulating, non-magnetic material was observed below a film thickness of 50 {\AA}. Annealing films grown at a higher than optimal deposition rate under the same conditions used for their growth, led to an initial rapid increase in the magnetization followed by constant magnetization after further annealing. We also report on the temperature dependence of the electro-magnetic properties. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G23.00011: Exploring nanoscale fluctuations and ferroelectric phase stabilization in S doped PbTe thermoelectrics Kevin Knox, Emil Bozin, Christos Malliakas, Mercouri Kanatzidis, Simon Billinge PbTe is one of the most important commercial thermoelectric materials for applications above room temperature. A paraelectric phase of fluctuating ferroelectric-like Pb structural dipoles emerges in PbTe at elevated temperatures, although it adopts an average rock-salt structure at all temperatures. These intrinsic nanoscale fluctuations are believed to improve the thermoelectric properties of PbTe by limiting the lattice thermal conductivity. Additionally, alloying and chemical substitution in PbTe appreciably improve the thermoelectric figure of merit, as is the case in PbTe$_{1-x}$S$_x$. However, the exact mechanism for this enhancement is not well understood. It has been shown that PbTe$_{1-x}$S$_x$ exhibits a peak in resistivity at a doping dependent temperature. By analogy with Ge doped PbTe, this anomalous resistivity may be the signature of a ferroelectric phase stabilization. In this talk, we explore this possibility by characterizing the average and the local structure of PbTe$_{1-x}$S$_x$ as a function of temperature and doping using a neutron based atomic pair distribution function (PDF) approach. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G23.00012: H$_2$ exposure induced structural and electrical modulation in MoS$_2$ Min Park, Sung Jin Chang, Hu Young Jung, Seung Jae Baek, Yongseok Jun, Byung Hoon Kim, Yung Woo Park We have demonstrated the structural modulation at the edge of MoS$_2$ due to H$_2$ exposure with spatially resolved Raman spectroscopy and the electrical characteristics of few-layer MoS$_2$ with respect to H$_2$ pressure from vacuum to 20 bars at 295 K \textless\ $T$ \textless\ 350 K. Upon H$_2$ exposure, the significant change of the edge in E$_{\mathrm{2g}}$ mode was observed. The conductance increases and threshold voltage ($V_{th})$ shifts toward a negative gate voltage region, indicating n-type doping. These behaviors are enhanced by high temperature and long exposure time ($t)$. The results reveal the creation of vacancy at the edge sites of MoS$_2$ in H$_2$ atmosphere causing the enhancement of $n$-type doping due to increase of metallic region. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G23.00013: Discovery of Burstein-Moss shift in Re-doped MoS$_2$ nanoparticles Qi Sun, Lena Yadgarov, Rita Rosentsveig, Gotthard Seifert, Reshef Tenne, Janice Musfeldt We investigated the optical properties of Re-doped MoS$_2$ nanoparticles and compared our findings with the pristine and bulk analogs. Our measurements reveal that confinement softens the exciton positions and reduces spin-orbit coupling whereas doping has the opposite effect. We model the doping-induced exciton blue shift in terms of the Burstein-Moss effect. These findings are important for understanding doping and finite length scale effects in model nanoscale materials. [Preview Abstract] |
Session G24: Focus Session: Computational Studies of Interactions between Electromagnetic Fields and Materials II
Sponsoring Units: DCOMPChair: kalman varga, Vanderbilt
Room: 326
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G24.00001: Plasmon enhanced light harvesting 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. Another important but still relatively unexplored 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. This process is a quantum mechanical effect: the decay of plasmon quanta into electron-hole pairs. I will discuss how plasmon induced hot electrons can be used in various 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 a photovoltaic device; and to induce local doping of a nearby graphene sheet. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G24.00002: Ab Initio Simulation of Fragmentation in Polyatomic Molecules by Short Intense Laser Pulses Arthur Russakoff, Sergiy Bubin, Kalman Varga We study ionization and fragmentation of polyatomic molecules induced by short intense laser pulses by performing ab initio simulations within the formalism of Time Dependent Density Functional Theory. Within this formalism we investigate intra-molecular electron dynamics during a fragmentation reaction on a pre-chemistry time-scale. The time-scale of the dynamics bridges the time-domain of sub-femtoseconds, on which the electrons move, and that of the much slower motion of the heavier ions (e.g. carbon ions), which proceeds on a time-scale of tens to hundreds of femtoseconds. The kinetic energy spectrum of the fragments and the charge state of the molecule prior to fragmentation are calculated and compared to experiment. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G24.00003: Laser-Induced High Harmonic Generations in Nano-Graphene Molecules Mingqiang Gu, Guoping Zhang, Xiaoshan Wu Nano graphene molecules is a promising material for the non-linear optical devices. We performed a first-principles calculation on Graphene Molecules. Two distinct signals are noticed: the integer higher-order harmonic generation (HHG) and the intrinsic emissions. Due to the small gap between HOMO and LUMO of graphene molecule, the HHG can be generated for the infrared laser pulse with the photon energy ranging from 20 meV to 1 eV. The intrinsic emission corresponds to the electron excitation between eigen states. They can be generated using a relatively low intensity laser pulse (0.05 eV/\AA) through the multiphoton process. Moreover, these signals are very sensitive to the molecule size and the hydrogen passivation. They can be the fingerprints for detecting the product in fabrication. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G24.00004: Surface Integral Formulations for the Design of Plasmonic Nanostructures Carlo Forestiere, Giovanni Iadarola, Guglielmo Rubinacci, Antonello Tamburrino, Luca Dal Negro, Giovanni Miano Numerical formulations based on surface integral equations (SIEs) provide an accurate and efficient framework for the solution of the electromagnetic scattering problem by three-dimensional plasmonic nanostructures in the frequency domain. In this work, we present a unified description of SIE formulations with both singular and nonsingular kernel and we study their accuracy in solving the scattering problem by metallic nanoparticles with spherical and nonspherical shape. In fact, the accuracy of the numerical solution, especially in the near zone, is of great importance in the analysis and design of plasmonic nanostructures, whose operation critically depends on the manipulation of electromagnetic hot spots. Four formulation types are considered: the N-combined region integral equations, the T-combined region integral equations, the combined field integral equations and the null field integral equations. A detailed comparison between their numerical solutions obtained for several nanoparticle shapes is performed by examining convergence rate and accuracy in both the far and near zone of the scatterer as a function of the number of degrees of freedom. A rigorous analysis of SIE formulations can have a high impact on the engineering of numerous nano-scale optical devices. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G24.00005: Dynamics of irradiation: from molecules to nano-objects and from material science to biology Eric Suraud, Phuong Mai Dinh, Paul-Gerhard Reinhard We discuss microscopic mechanisms of irradiation in clusters and molecules considering the case of isolated molecules/clusters [1] and/or in an environment [2]. We use Time Dependent Density Functional Theory (for electrons) coupled to Molecular Dynamics (for ions) and follow explicitly in time irradiation and response of the system. Examples are taken from free metal clusters, fullerenes, molecules of biological interest and clusters deposited on a surface or embedded in a matrix [3,4]. We analyse in particular properties of emitted electrons (photo electron spectra, angular distributions...) which provide a key tool of analysis of properties of irradiated clusters and molecules [5]. We also discuss pump and probe scenarios (opening the road to manipulation at the molecular scale) with help of dedicated laser pulses, exploring in particular very short times scales down towards the attosecond domain. \\[4pt] [1] F. Calvayrac et al, Phys. Reports 337(2000)493\\[0pt] [2] P. M. Dinh et al, Phys. Reports 485 (2009) 43\\[0pt] [3] Z.P. Wang et al, Int. J. Mass Spect. 285 (2009) 1430\\[0pt] [4] U. F. NdongmuoTaffoti et al, Eur. Phys. J. D 58 (2010) 131\\[0pt] [5] Th. Fennel et al, Rev. Mod. Phys. 82 (2010) 1 [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G24.00006: Time-dependent transition density matrix for visualizing charge-transfer excitations in photoexcited organic donor-acceptor systems Yonghui Li, Carsten Ullrich The time-dependent transition density matrix (TDM) is a useful tool to visualize and interpret the induced charges and electron-hole coherences of excitonic processes in large molecules. Combined with time-dependent density functional theory on a real-space grid (as implemented in the octopus code), the TDM is a computationally viable visualization tool for optical excitation processes in molecules. It provides real-time maps of particles and holes which gives information on excitations, in particular those that have charge-transfer character, that cannot be obtained from the density alone. Some illustration of the TDM and comparison with standard density difference plots will be shown for photoexcited organic donor-acceptor molecules. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G24.00007: Density functional studies of plasmons, hybridizations and electron diffractions in carbon fullerene nanomaterials Himadri Chakraborty, Lamine Madjet Quantized plasma waves in carbon valence electron clouds driven by photon or charged particle fields create plasmon resonances in the ionization of fullerene nanomaterials [1]. If the materials have composite structures, like nested fullerenes (buckyonions) or fullerenes endohedrally doped by an atom (endofullerenes), then plasmonic motions dynamically hybridize, leading to spectacular effects in the emission spectra [2,3]. Further, for fast ejected electrons, diffraction type modulations in the momentum space of emission intensities enrich the ionization process which offer an unusual spectroscopic route to image the charge cloud geometry [4,5]. Using a time-dependent local density functional methodology, but smearing the ionic core into a jellium, we recently completed some studies of such processes for fullerene nanomaterials. Results have shown good agreements with measurements. [1] Madjet et al., J. Phys. B 41, 105101 (2008); [2] McCune et al., J. Phys. B Fast Track Comm. 44, 241002 (2011); [3] Madjet et al., Phys. Rev. Lett. 99, 243003 (2007); [4] Patel et al., J. Phys. B Fast Track Comm. 44, 191001 (2011); [5] Ruedel et al., Phys. Rev. Lett. 89, 125503 (2002). [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G24.00008: Non-Linear Optical Response Simulations for Strongly Corellated Hybrid Carbon Nanotube Systems Areg Meliksetyan, Igor Bondarev, Maxim Gelin Hybrid carbon nanotube systems, nanotubes containing extrinsic atomic type species (dopants) such as semiconductor quantum dots, extrinsic atoms, or ions, are promising candidates for the development of the new generation of tunable nanooptoelectronic devices -- both application oriented, e.g., photovoltaic devices of improved light-harvesting efficiency, and devices for use in fundamental research. Here, we simulate non-linear optical response signals for a pair of spatially separated two-level dipole emitters (to model the dopants above) in the regime where they are coupled strongly to a low-energy surface plasmon resonance of a metallic carbon nanotube. Such a coupling makes them entangled [1], and we show that the cross-peaks in 2D photon-echo spectra are indicative of the bipartite entanglement being present in the system [2]. We simulate various experimental conditions and formulate practical recommendations for the reliable experimental observation of this unique quantum phenomenon of relevance to the solid-state quantum information science.\\[4pt] [1] I.V. Bondarev, J. Comp. Theor. Nanosci. 7, 1673 (2010).\\[0pt] [2] M.F. Gelin, I.V. Bondarev, A.V. Meliksetyan, Chem. Phys., at print. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G24.00009: Dynamical density matrix renormalization group study of non-linear optical response of one-dimensional strongly correlated electron system Shigetoshi Sota, Seiji Yunoki, Takami Tohyama We studied the third-order non-linear optical response of one-dimensional Mott insulators by using the dynamical density matrix renormalization group method. We employed an one-dimensional extended Hubbard model which corresponds to the one-dimensional Mott insulators. Also, we introduced a Holstein-type electron-phonon interaction which is important for understanding the optical response in the one-dimensional Mott insulators. We calculated the non-linear optical response using the parameters corresponding to Sr$_2$CuO$_3$ which is known as a kind of the one-dimensional Mott insulators. Our calculated results show a relatively large effect of the electron-phonon interaction on the calculated third-order non-linear optical response. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G24.00010: Real-time TDDFT calculations of core-hole spectral functions J.J. Kas, J.J. Rehr, A.J. Lee, F.D. Vila Core-hole response is important in a variety of x-ray spectra, including x-ray absorption, resonant and non-resonant inelastic x-ray scattering, and x-ray photo-electron spectroscopy, but has usually been treated within the adiabatic approximation. Here we explore the dynamic response of valence electrons to the sudden appearance of a deep core-hole using real time time dependent density functional theory (RT-TDDFT). The core-hole is treated as a transient time dependent potential which excites the valence electrons, as in the edge-singularity theory of Nozieres and De Dominicis. RT-TDDFT provides an efficient approach for treating response to time-dependent external fields including interactions among the valence electrons, which has recently been applied to calculations of optical and x-ray spectra.\footnote{A. J. Lee, F. D. Vila, and J. J. Rehr, PRB {\bf 86} 115107} Here we generalize this approach to explore the role of the strength and localization of the core-hole potential and its effects on the spectral function and various x-ray spectra, together with comparisons to the adiabatic approximation. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G24.00011: Quantal Density Functional Theory (QDFT) in the Presence of an Electromagnetic Field Xiao-Yin Pan, Viraht Sahni We derive the QDFT equations of electrons in an external time-dependent field $\cal{E} ({\bf{r}} t) = -$ {\boldmath $\nabla$} $ v ({\bf{r}} t)$ and in the presence of an electromagnetic field characterized by the magnetic induction ${\bf{B}} ({\bf{r}} t) =$ {\boldmath $\nabla$} $\times {\bf{A}} ({\bf{r}} t)$ and electric field ${\bf{E}} ({\bf{r}}t) = -$ {\boldmath $\nabla$} $\Phi ({\bf{r}}t) - (1/c) \partial {\bf{A}} ({\bf{r}}t)/\partial t$. The QDFT is comprised of the mapping from this system to one of noninteracting fermions with the same density $\rho ({\bf{r}}t)$ and physical current density ${\bf{j}} ({\bf{r}}t)$. The mapping is in terms of `classical' fields representative of the different electron correlations that must be accounted for. On deriving the `quantal Newtonian' second law for the interacting and model systems, we obtain the local electron-interaction potential $v_{ee} ({\bf{r}} t)$ of the latter to be the work done in a conservative effective field $\cal{F}^{\mathrm{eff}} ({\bf{r}} t)$. The components of $\cal{F}^{\mathrm{eff}} ({\bf{r}} t)$ are representative of correlations due to the Pauli exclusion principle and Coulomb repulsion and the Correlation-(Kinetic, Current Density, Electric, and Magnetic) effects. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G24.00012: Basic Variables in the Presence of a Magnetostatic Field Viraht Sahni, Xiao-Yin Pan We present our recent understanding of the issue of what properties constitute the basic variables in quantum mechanics for electrons in the presence of external electrostatic ${\cal{E}} ({\bf{r}}) = -$ {\boldmath $\nabla$} $v ({\bf{r}})$ and magnetostatic ${\bf{B}} ({\bf{r}}) =$ {\boldmath $\nabla$} $\times {\bf{A}} ({\bf{r}})$ fields. In this case, the relationship between the potentials $\{v, {\bf{A}} \}$ and the ground state wave function $\Psi$ can be many-to-one. We discuss our prior work\footnote{Pan and Sahni, Int. J. Quantum Chem. 110, 2833 (2010); J. Phys. Chem. Solids. 73, 630 (2012).} in which we claimed that the basic variables are the ground state density $\rho ({\bf{r}})$ and physical current density ${\bf{j}} ({\bf{r}})$. We prove here more fully this to be the case for the nondegenerate ground state for which $\Psi$ is real. The proof explicitly accounts for the many-to-one relationship between $\{v, {\bf{A}} \}$ and $\Psi$. We also draw parallels between our work on the density and physical current density functional theory and those of the Hohenberg-Kohn and Percus-Levy-Lieb definitions of density functional theory. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G24.00013: The magnetization of periodic solids from time-dependent current-density-functional theory Arjan Berger, Nathaniel Raimbault, Paul de Boeij, Pina Romaniello The evaluation of the macroscopic magnetization of solids is problematic when periodic boundary conditions are used because surface effects are artificially removed. This poses a problem unless surface effects can be reformulated in terms of bulk quantities. For example, in case of the macroscopic polarization one can express the contribution of the charge density accumulated at the surface in terms of the bulk current density through the continuity equation. Therefore one can work in the framework of time-dependent current-density functional theory to efficiently calculate the macroscopic polarization [1,2]. In this presentation we will show how also the magnetization can be described within this framework. \\[4pt] [1] F. Kootstra, P. L. de Boeij, and J. G. Snijders, J. Chem. Phys. 112, 6517 (2000).\\[0pt] [2] J. A. Berger, P. Romaniello, R. van Leeuwen, and P. L. de Boeij , Phys. Rev. B 74, 245117 (2006) [Preview Abstract] |
Session G25: Itinerant Photons, Squeezed States, and Entanglement
Sponsoring Units: GQIChair: Lev Bishop, University of Maryland
Room: 327
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G25.00001: Qubit-Photon Entanglement and Hong-Ou-Mandel Interference with Propagating Microwaves Christopher Eichler, Christian Lang, Johannes Fink, Joonas Govenius, Lars Steffen, Stefan Filipp, Andreas Wallraff, Matthew Woolley, Alexandre Blais Itinerant microwave photons offer an attractive carrier of quantum information in superconducting circuits. However, until recently it remained challenging to measure photon statistics and coherence properties of microwave fields beyond the Gaussian level -- mainly due to the absence of efficient detectors in this frequency range. Here, we present the on-demand generation and efficient characterization of microwave radiation and its entanglement with stationary qubits. Based on novel tomography techniques and low noise parametric amplification we are able to resolve all relevant quantum correlations between the propagating field and the superconducting qubit to demonstrate entanglement with high fidelity [1,2]. We have also created entangled microwave fields traveling in two spatially separated modes. Making use of the two-photon interference at a microwave beamsplitter we are able to prepare propagating NOON-type states, which we fully characterize by measuring the joint photon statistics of the two modes. The possibility to synthesize, guide and detect entanglement correlations between itinerant microwave photons and stationary qubits put microwave based quantum network experiments within reach. [1] arXiv:1209.0441v1 [2] C. Eichler et al., Phys. Rev. A 86, 032106 (2012) [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G25.00002: Catching Microwave Photons in a Superconducing Resonator with Tunable Coupling James Wenner, Yi Yin, Yu Chen, R. Barends, B. Chiaro, J. Kelly, M. Mariantoni, A. Megrant, J. Mutus, C. Neill, S. Ohya, D. Sank, T. White, A.N. Cleland, John M. Martinis When transferring a quantum state from a freely propagating mode to a resonator, reflections must be minimized to avoid energy loss. Performing this transfer with high fidelity requires tunable coupling. We experimentally studied a 50 Ohm transmission line with tunable coupling to a 6GHz superconducting coplanar waveguide resonator, which in turn is capacitively coupled to a phase qubit for calibration. We classically drove the resonator while measuring the reflected and captured signals using a HEMT amplifier. Following theory by Korotkov (PRB 84, 014510, 2011), we find that the photon capture efficiency is maximized with an exponentially increasing drive; further improvements come from varying pulse duration and dynamic coupling. With these techniques, we reduce reflections so that presently over 80\% of the pulse energy is captured by the resonator. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G25.00003: Radiative decay of a superconducting qubit in squeezed vacuum S.J. Weber, K.W. Murch, K.M. Beck, E. Ginossar, I. Siddiqi When the conventional vacuum fluctuations of the electromagnetic environment are replaced by the asymmetric, reduced fluctuations associated with squeezed vacuum, the radiative properties of an atom are predicted to be dramatically altered. We present measurements of the transverse and longitudinal decay rates of a superconducting qubit that couples predominantly to a continuum of squeezed electromagnetic vacuum. We use a lumped element Josephson parametric amplifier to squeeze vacuum fluctuations by up to 10dB with a bandwidth of 20 MHz. The amplifier output is directly coupled to a transmon qubit in a microwave cavity. We observe a dependence of the transverse decay rate on the relative angle between the squeezed axis and the qubit. In particular, at certain angles, we observe an improvement in the qubit T2 time above its nominal value. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G25.00004: Lossless, coherent Josephson three-wave combiner Baleegh Abdo, Katrina Sliwa, Flavius Schackert, Nicolas Bergeal, Michael Hatridge, Luigi Frunzio, Douglas Stone, Michel Devoret We designed and operated a three-wave beam-splitter/combiner, based on Josephson parametric converters, which performs frequency conversion without introducing losses and thus adding no noise to the processed signal. We in particular show that the unitary signal-idler scattering parameters of the device can be fully modulated in-situ by varying the intensity and phase of the pump tone feeding the system. By operating the device as a 50/50 beam-combiner, we interfere coherently two input coherent microwave beams with different frequencies and demonstrate that the resulting interference fringes generated by the relative phase of the pump is in agreement with theoretical predictions. Potential applications of the device include quantum information transduction and realization of an ultra-sensitive interferometer with controllable feedback. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G25.00005: Extracting Past-Future Vacuum Correlations Using Circuit QED Borja Peropadre, Carlos Sabin, Marco del Rey, Eduardo Martin-Martinez In this work we propose a realistic circuit QED experiment to test the extraction of past-future vacuum entanglement to a pair of superconducting qubits. A qubit P --for past-- interacts with a quantum field along an open transmission line for an interval $T_{on}$ and then, after a time-lapse $T_{off}$ of no interaction, a second qubit F --for future-- starts interacting for a time $T_{on}$ in a symmetric fashion. After this protocol, past-future quantum correlations will have transferred to the qubits, even if the qubits do not coexist at the same time. We show that this experiment can be realized with current technology and discuss its utility as a possible implementation of a quantum memory. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G25.00006: Observation of the squeezed state of microwave photon by resolving the even-number Fock states in circuit QED Kyungsun Moon We theoretically propose an elegant way to detect the microwave parametric down conversion in the circuit QED system. The qubit energy splitting E$_{01}$ is tuned to be quite close to the fundamental frequency $\omega _{1\, }$of the microwave photon and the frequency of the pump beam is chosen to be $\omega_{2}$. We place the qubit at the two-thirds away from the center of the central resonator, which will make the capacitive coupling to the third harmonic mode to be negligible. Since the qubit acts as an optical coupler in our system, one may expect that the following process a$_{2}^{+}$a$_{1}^{+}$a$_{3}$ may appear and compete with the squeezing process a$_{1}^{+}$a$_{1}^{+}$a$_{2}$, which will seriously degrade the quality of squeezing by mixing into the channel. Since the coupling to the third harmonic mode is negligible for our system, we expect instead to observe the clear squeezing of the microwave photon with frequency $\omega _{1}$. We calculate the absorption spectrum of the qubit, which is experimentally measurable and will clearly reveal the squeezed states as the coherent superposition of the even-number Fock states. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G25.00007: Generation of Nonclassical States of Microwave Radiation via Single Photon Detection Emily Pritchett, Luke Govia, Frank Wilhelm We describe the creation of nonclassical states of microwave radiation via ideal dichotomic single photon detection, i.e., a detector that only indicates presence or absence of photons. Ideally, such a detector has a back action in the form of the subtraction operator. Using the non-linearity of this back action, it is possible to create nonclassical states of microwave radiation, including squeezed and cat-like states, starting from a coherent state. We discuss the applicability of this protocol to current experimental designs of Josephson Photomultipliers (JPMs). [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G25.00008: Path Entanglement of Continuous-Variable Quantum Microwaves E. P. Menzel, F. Deppe, P. Eder, L. Zhong, M. Haeberlein, A. Baust, E. Hoffmann, A. Marx, R. Gross, R. Di Candia, E. Solano, D. Ballester, M. Ihmig, K. Inomata, T. Yamamoto, Y. Nakamura Entanglement is a quantum mechanical phenomenon playing a key role in quantum communication and information processing protocols. Here, we report on frequency-degenerate entanglement between continuous-variable quantum microwaves propagating along two separated paths. In our experiment, we combine a squeezed and a vacuum state via a beam splitter. Overcoming the challenges imposed by the low photon energies in the microwave regime, we reconstruct the squeezed state and, independently from this, detect and quantify the produced entanglement via correlation measurements (E.~P.~Menzel {\it et al.}, arXiv:1210.4413). Our work paves the way towards quantum communication and teleportation with continuous variables in the microwave regime.\\ \noindent This work is supported by SFB~631, German Excellence Initiative via NIM, EU projects SOLID, CCQED and PROMISCE, MEXT Kakenhi ``Quantum Cybernetics'', JSPS FIRST Program, the NICT Commissioned Research, EPSRC EP/H050434/1, Basque Government IT472-10, and Spanish MICINN FIS2009-12773-C02-01. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G25.00009: Cooper Pair Transistor Embedded in a dc-Biased High-Q Microwave Cavity Juliang Li, Fei Chen, Joel Stettenheim, A.J. Sirois, R.W. Simmonds, M.P. Blencowe, A.J. Rimberg A Cooper pair transistor (CPT) is directly coupled to a high-Q microwave cavity, which allows introduction of a dc bias to the CPT without significantly degrading the cavity Q. In the subgap region of the CPT, the dc bias generates a tunable oscillating current through the CPT via the ac Josephson effect. Evidence of such self-oscillations has been observed as current peaks in our dc measurements, which are in good agreement with calculated cavity modes, and indicate the strong coupling between the CPT and the cavity. Tunneling Cooper pairs can both emit photons into and absorb photons from microwave cavity modes. Photons emitted into the cavity are also directly probed and are in good agreement with dc measurements. Recent experimental results including the importance of careful filtering of the DC bias lines will be discussed. This work is supported by the NSF, AFOSR and DARPA. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G25.00010: Photon Emission from a Self-Oscillating Cavity-Embedded Cooper Pair Transistor Fei Chen, Juliang Li, Joel Stettenheim, A. J. Sirois, R. W. Simmonds, M. P. Blencowe, A. J. Rimberg A strongly non-linear superconducting device consisting of a Cooper pair transistor embedded in a dc voltage biased microwave cavity is investigated. The cavity-embedded Cooper pair transistor (CCPT) is driven via the ac Josephson effect by an applied dc bias and exhibits self-oscillation without an external ac drive. Photon emission arising from both sequential tunneling and cotunnelling processes of Cooper pairs has been observed. We have characterized the measured photon field by heterodyne quadrature detection and have reconstructed its quasi-probability distribution by implementing an iterative procedure for maximum-likelihood estimation of its density matrix. The CCPT offers an interesting system for studying nonlinear quantum dynamics and the quantum-to-classical transition. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G25.00011: The Quantum-Classical Correspondence for a Self-Oscillating Cavity-Embedded Cooper Pair Transistor System Erind Brahimi, Fei Chen, Juliang Li, Joel Stettenheim, Andrew Armour, Alex Rimberg, Miles Blencowe We provide a theoretical model for our recent experiment involving a dc voltage biased Cooper pair transistor (CPT) that strongly drives a high quality factor microwave cavity via the ac Josephson effect. Depending on the tunable dc voltage bias, the model shows that the CPT can generate a range of non-trivial cavity quantum states involving large average microwave photon number. Using a Floquet basis approach to solving for the quantum dynamics and a Wigner function representation of the system state, we compare some of the model photon state predictions with experiment. The good agreement validates the low noise, dc biased cavity-CPT system for exploring the quantum-classical correspondence in strongly nonlinear, macroscopic systems. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G25.00012: Shaping the Spontaneous Emission Pulse from a Superconducting Qubit Srikanth Srinivasan, Yanbing Liu, Gengyan Zhang, Terri Yu, Jay Gambetta, Steven Girvin, Andrew Houck We report on measurements of spontaneous emission in a circuit quantum electrodynamics system. A superconducting qubit with tunable coupling to a coplanar waveguide cavity is operated in a regime where the qubit relaxation time, and consequently the spontaneous emission rate, is dominated by the interaction strength. This fast control knob on the coupling strength is used to shape the emitted single photon's wavepacket. The independent control over the coupling allows the dressed qubit frequency to remain truly constant during the emission. The wavepacket shape becomes important in experiments where quantum information needs to be transported between various nodes in a quantum network. The transfer can happen with a very high fidelity if the wavepacket is time-symmetric, since emission by the source and absorption by the destination become time reversed processes. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G25.00013: Emergent Non-Adiabatic Wavefunctions for Strongly Dissipative Qubits Soumya Bera, Serge Florens, Harold Baranger, Nicolas Roch, Ahsan Nazir, Alex W. Chin We show that a qubit strongly interacting with its environment leads to highly entangled states with emerging non-adiabatic features (Schrodinger-cat-like states of the environment). The model we consider is a two-level-system (qubit) coupled to a continuum of quantum oscillators (bosons), which can be realized, for instance, by a superconducting qubit coupled to a transmission line of photons. We show that the joint system is remarkably well described by a generalized variational coherent state ansatz, an ansatz which is justified by comparing with exact quantum tomography of the states found through Numerical Renormalization Group (NRG) calculations. Our coherent state ansatz includes not only the well-known polaronic contributions but also non-adiabatic anti-polaron contributions; these later contributions are critical for an accurate description of the strong coupling regime. We calculate the entanglement entropy of the qubit plus a single bosonic mode with the rest of the system; this joint entropy peaks for a bosonic mode around the Kondo scale, an effect due to the anti-polaronic contribution. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G25.00014: Coherence and indistinguishability of single electron wavepackets emitted by independent sources Gwendal Feve, Erwann Bocquillon, Vincent Freulon, Jean-Marc Berroir, Pascal Degiovanni, Bernard Pla\c{c}ais, Antonella Cavanna, Yong Jin Using two independent on-demand electron sources [1], the emission of two single-electron wavepackets is triggered at different inputs of an electronic beamsplitter. Whereas classical particles would be randomly partitioned by the splitter, we observe two-particle interferences resulting from quantum exchange in this electronic analog [2,3] of the optical Hong-Ou-Mandel [4] experiment. Both electrons, emitted in indistinguishable wavepackets with synchronized arrival time on the splitter, exit in different outputs as recorded by the low frequency current noise. Full random partitioning is recovered when the arrival of one electron is delayed with respect to the other. This two-electron interference experiment demonstrates the possibility to generate on-demand coherent and indistinguishable single-electron wavepackets for quantum information processing in quantum conductors. [1] G. F\`{e}ve et al., Science \textbf{316}, 1169 (2007). [2] Ol'khovskaya et al., Physical Review Letters \textbf{101}, 166802 (2008). [3] T. Jonckheere et al., Phys. Rev. B \textbf{86}, 125425 (2012) [4] C. K. Hong et al., Physical Review Letters, \textbf{59}, 2044 (1987). [Preview Abstract] |
Session G26: Quantum Characterization, Verification, and Validation II
Sponsoring Units: GQIChair: Robin Blume-Kohout, Sandia National Laboratories
Room: 328
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G26.00001: Quantum process verification methods and applications to superconducting qubits Invited Speaker: Jay Gambetta Determining how well a quantum gate is implemented on a quantum device is of fundamental importance. Such a characterization allows a direct comparison between different architectures for computation as well as an understanding of the performance of the building blocks of a quantum computer. In this talk I will show that the standard approach of process tomography is grossly inaccurate in the case where the states and measurement operators used to interrogate the system are generated by gates that have some systematic error, a situation all but unavoidable in any practical setting. I will then present some recent proposals with experimental implementations that are resilient to this type of noise. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G26.00002: Quantum State Tomography of Spin Qubits Invited Speaker: Oliver Dial Quantitative and accurate state tomography is becoming increasingly necessary to establish gate fidelities, entanglement measures, and optimize the increasingly complex gate sequences needed to perform experiments. In singlet-triplet spin qubits, to perform state tomography single-qubit rotations are used to map different axes of the Bloch sphere to the singlet-triplet axis, followed by projective measurement onto the singlet-triplet axis. The two nominally orthogonal rotations needed are provided by two physically distinct mechanisms: magnetic field gradients and exchange rotations. The complex interplay between these mechanisms, noise sources, and pulse distortions make it difficult to accurately predict the angle and axis of rotations from first principles, leading to a circular problem: how can one calibrate tomographic rotations without any calibrated tomography? We describe and experimentally demonstrate a method which, using minimal assumptions, makes it possible to detect and correct for both axis errors in tomography and losses during rotations associated with state tomography. Unlike conventional tomography tuning schemes, this technique is not iterative, allowing it be used to post-correct data with minimal overhead and effort. The technique is easily adaptable to other implementations of qubits, and should be of value wherever accurate tomography is needed but tuning up a complete set of ideal rotations is unnecessary. Finally, we will discuss the influence of non-Markovian noise on state tomography and possible approaches to circumvent state estimation errors arising thereof. Together these techniques allow us to perform state tomography with unprecedented precision in spin qubits. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G26.00003: Compressed Sensing Quantum Process Tomography of Superconducting Qubit Gates Andrey Rodionov, Alexander N. Korotkov, Robert L. Kosut, Matteo Mariantoni, Daniel Sank, James Wenner, John M. Martinis We characterize the quantum gates based on superconducting phase qubits using the Quantum Process Tomography (QPT) with strongly reduced set of initial states and/or measurement configurations. This is done by applying the Compressed Sensing (CS) method to estimate the process matrix $\chi$. Using experimental data for 2-qubit controlled-Z gate, we show that the CS-QPT method gives an estimate of the $\chi$-matrix with reasonably high fidelity, compared with full QPT. The method works well even when the amount of used data is so small, that the standard QPT would have an underdetermined system of equations. The CS-QPT is also applied to the analysis of a three-qubit Toffoli gate with numerically added noise. Similarly, we show that the method works reasonably well for a strongly reduced set of data, including the underdetermined case. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G26.00004: Xmons: Transmon qubits for a scalable architecture Rami Barends, J. Kelly, D. Sank, J. Bochmann, B. Campbell, Y. Chen, B. Chiaro, E. Jeffrey, M. Mariantoni, A. Megrant, J. Mutus, C. Neill, P. O'Malley, S. Ohya, P. Roushan, A. Vainsencher, J. Wenner, T. White, A.N. Cleland, J.M. Martinis We have developed a new type of transmon qubit, the Xmon, which shows long coherence, allows for straightforward coupling to multiple elements, and has a low parasitic coupling. The Xmon is UCSB's building block for a superconducting multiqubit processor. The Xmon easily couples to four elements and is dispersively read out, making it compatible for use in a surface code quantum processor. At present, we are experimentally testing multiqubit chips for demonstrating single and two qubit state preparation and gates with high fidelity. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G26.00005: Benchmarking gates in a qubit-bus-qubit tunable transmon architecture Julian Kelly, R. Barends, J. Bochmann, B. Campbell, Y. Chen, B. Chiaro, E. Jeffrey, M. Mariantoni, A. Megrant, J. Mutus, C. Neill, P. O'Malley, S. Ohya, P. Roushan, D. Sank, A. Vainsencher, J. Wenner, T. White, A.N. Cleland, J.M. Martinis Using a newly developed frequency tunable transmon qubit (``Xmon''), we are beginning to construct the fundamental gates and architecture for a quantum computer. We show experimental data for gates in a qubit-bus-qubit configuration. We quantify the fidelity of a set of single qubit gates with both randomized benchmarking and tomography. We also investigate the fast swap style cPhase gate [Strauch PRL 2003], where the control qubit is swapped into the bus and interacts dispersively with the target qubit, as a fundamental two-qubit interaction. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G26.00006: Characterization of addressability by simultaneous randomized benchmarking John Smolin The control and handling of errors arising from cross-talk and unwanted interactions in multi-qubit systems is an important issue in quantum information processing architectures. We introduce a benchmarking protocol that provides information about the amount of addressability present in the system and implement it on coupled superconducting qubits. The protocol consists of randomized benchmarking each qubit individually and then simultaneously, and the amount of addressability is related to the difference of the average gate fidelities of those experiments. We present the results on two similar superconducting transmon qubits with different amounts of cross-talk and unwanted interactions, which agree with predictions based on simple models for the amount of residual coupling. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G26.00007: Implementation of a Robust Tomography Toolbox Colm Ryan, Blake Johnson, Marcus Da Silva, Shelby Kimmel, Thomas Ohki Recent advances in coherence times and control techniques have dramatically improved gate fidelities in superconducting qubits. Already, estimates of these small errors are dominated by errors in the state preparation and measurment pulses of quantum process tomography. Randomized benchmarking (RB) provides a way to isolate gate errors, but only for estimating the fidelity of Clifford operations. Here we implement several extensions to RB that provide more detailed information about specific gates while maintaining the key RB advantage of being robust to state and measurement errors. We will show: interleaved benchmarking results to characterize the average fidelity of specific gates; simultaneous benchmarking to characterize addressabilty errors with multiple qubits; and robust tomography results that show a full unital characterization of a trace preserving operation. Taken together these provide a full suite of characterization tools useful to any quantum experimentalist. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G26.00008: Robust Tomography using Randomized Benchmarking Marcus Silva, Shelby Kimmel, Blake Johnson, Colm Ryan, Thomas Ohki Conventional randomized benchmarking (RB) can be used to estimate the fidelity of Clifford operations in a manner that is robust against preparation and measurement errors --- thus allowing for a more accurate and relevant characterization of the average error in Clifford gates compared to standard tomography protocols. Interleaved RB (IRB) extends this result to the extraction of error rates for individual Clifford gates. In this talk we will show how to combine multiple IRB experiments to extract all information about the unital part of {\em any} trace preserving quantum process. Consequently, one can compute the average fidelity to {\em any} unitary, not just the Clifford group, with tighter bounds than IRB. Moreover, the additional information can be used to design improvements in control. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G26.00009: Experimental realization of non-abelian geometric gates with a superconducting three-level system Abdufarrukh Abdumalikov, J. M. Fink, K. Juliusson, M. Pechal, S. Berger, A. Wallraff, S. Filipp Geometric gates hold promise to provide the building blocks for robust quantum computation. In our experiments, we use a superconducting three-level system (transmon) to realize non-adiabatic non-abelian geometric gates. As computational basis we choose the ground and second excited states, while the first excited state acts as an ancilla state. The gates are realized by applying two resonant drives between the transmon levels. During the geometric gate ration of the amplitudes of the two drive tone is kept constant. Different gates are obtained for different ratio of the drive tones. We implement a Hadamard, a $NOT$ and a phase gates with the fidelities of $95\%$, $98\%$, and $97\%$ as determined by full process tomography and maximum likelihood methods. We explicitly show the non-abelian nature of gates by applying two non-commuting gates in alternating order. The demonstrated holonomic gates are not exclusive to superconducting quantum devices, but can also be applied to other three level systems with similar energy level structure. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G26.00010: Implementation of a five-cavity / four-qubit 3D circuit QED system Douglas McClure, Chad Rigetti, Jay Gambetta, Stefano Poletto, Erik Lucero, Mark Ketchen, Matthias Steffen Surface code error correction schemes, which have emerged as a guiding paradigm for the development of small prototype quantum processors, have a natural implementation on a skew square 2D lattice of cavities and qubits. We describe the experimental realization of a modular segment containing a unit cell of this lattice in a device consisting of five 3D waveguide cavities and four superconducting transmon qubits. In this system, we demonstrate high-fidelity one- and two-qubit gates with low crosstalk. Moreover, this device provides an extensible framework for tests of protocols needed for error correction in much larger systems. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G26.00011: Quantum lost property: A possible operational meaning for the Hilbert-Schmidt product Matthew Pusey, Terry Rudolph Minimum error state discrimination between two mixed states $\rho$ and $\sigma$ can be aided by the receipt of ``classical side information'' specifying which states from some convex decompositions of $\rho$ and $\sigma$ apply in each run. I will quantify this phenomena by the average trace distance, and give lower and upper bounds on this quantity as functions of $\rho$ and $\sigma$. The lower bound is simply the trace distance between $\rho$ and $\sigma$, trivially seen to be tight. The upper bound is $\sqrt{1 - {\rm tr}(\rho\sigma)}$, and we conjecture that this is also tight. I will show how to reformulate this conjecture in terms of the existence of a pair of ``unbiased decompositions'', which may be of independent interest. Time permitting, I will outline the evidence for this conjecture. Based on http://arxiv.org/abs/1208.2550 [Preview Abstract] |
Session G27: Quantum Error Correction and Quantum Control
Sponsoring Units: GQIChair: Leonid Pryadko, University of Californa, Riverside
Room: 329
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G27.00001: Towards Fault-Tolerant Dynamical Decoupling Gregory Quiroz, Daniel Lidar Dynamical Decoupling (DD) is a error suppression technique which combats decoherence by applying strong and fast pulses to a quantum system to effectively average system-environment interactions. Although many DD constructions have been designed which exhibit suppression of interactions to high orders in time-dependent perturbation theory, this result is predominately in the ideal pulse limit as DD effectiveness degrades significantly in the presence of additional errors generated by faulty pulses. Here, we present a decoupling scheme which provides robustness to certain forms of pulse errors and utilizes concatenation to attain high order error suppression. Using numerical simulations, we convey the advantages of this scheme over additional robust DD constructions and provide evidence for the possibility of arbitrary order error suppression in the presence of pulse errors. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G27.00002: Fault-Tolerant Storage of Quantum Information by Large Block Codes Ching-Yi Lai, Todd Brun An important issue in the implementation of a quantum computer is to protect quantum information from decoherence. Concatenated quantum codes and topological quantum codes are extensively studied for fault-tolerant quantum computation. However, there is not much research on large block codes in any fault-tolerant scheme. Here we propose a method for storage of quantum information by a large block code, which has a high code rate and high distance. To access or protect the quantum information stored in a large block code requires only the fault-tolerant implementation of the gates from the Clifford group. We derive the lifetime of the quantum information stored in a large block code by CSS code construction. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G27.00003: Progress in analytical investigations of the achievement of fault tolerance in quantum computing Gerald Gilbert, Yaakov Weinstein We describe progress made in understanding and assuring fault tolerance in quantum computation. We introduce and explore analytical techniques for explicitly determining the logical state of a quantum computer undergoing dynamical evolution according to an arbitrary quantum algorithm. We carry out detailed analyses of the effects of errors, paying special attention to the general case of non-equiprobable errors, i.e., the important and realistic situation in which the probabilities for sigma\textunderscore x, sigma\textunderscore y and sigma\textunderscore z errors are not necessarily the same (sigma\textunderscore x, sigma\textunderscore y and sigma\textunderscore z are the Pauli operators). [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G27.00004: Simulating the Transverse Ising Model on a Quantum Computer: Error Correction with the Surface Code Hao You, Michael Geller, Phillip Stancil We estimate the resource requirements for the quantum simulation of the ground state energy of the one-dimensional quantum transverse Ising model (TIM), based on the surface code implementation of a fault-tolerant quantum computer. The surface code approach has one of the highest known tolerable error rates ($\sim$1\%) which makes it currently one of the most practical quantum computing schemes. Compared to results of the same model using the concatenated Steane code, the current results indicate that the simulation time is comparable but the number of physical qubits for the surface code is 1-2 orders of magnitude larger than that of the concatenation code. Considering that the error threshold requirements of the surface code is four orders of magnitude higher than the concatenation code, building a quantum computer with a surface code implementation appears more promising given current physical hardware capabilities. We would like to acknowledge valuable discussions with Joydip Ghosh, Matteo Mariantoni, Andrew Sornborger, James Whitfield and Zhongyuan Zhou. This work was supported by the National Science Foundation through grant CDI 1029764. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G27.00005: Multi-Run Quantum Error Correction in Coupled Electron-Nuclear Systems Robabeh Rahimi Darabad, Daniel K. Park, Jonathan Baugh, Raymond Laflamme It has been a milestone in realizing quantum computing, to enhance our control over physical systems so that making quantum processors performing accurately and precisely in presence of environmental noise. For practical uses, quantum error correction should be employed in multi-run cycles in order to keep the encoded qubit, that is carrying the information, safe from noise. We have been working towards implementing multi-run quantum error correction in molecular systems that involve electron and nuclear spins. Electron spins of a molecular sample are used for pumping up the nuclear spin polarizations, in addition to addressing and manipulating the nuclear spins. The required experimental conditions for having access to refreshable ancilla qubits are very much enhanced by a careful design of the molecular sample. We report the progress and prospects towards overcoming the experimental challenges in terms of sample preparation; irradiation imposed free electron samples, free radical molecular spin systems, and triplet state photoexcitable co-crystal samples. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G27.00006: Resilience of topological error-correction codes to concurrent qubit and measurement errors Ruben S. Andrist, Hector Bombin, Miguel Angel Martin-Delgado, Helmut G. Katzgraber Topologically-protected quantum computing schemes avert decoherence by storing quantum information in nonlocal degrees of freedom while actively correcting for local errors. To date, the effects of individual error sources, such as, for example, bit flips, phase flips, or measurement errors have been studied. A more realistic assessment of the error stability is given by studying the combination of different error sources, such as bit flips and measurement errors. So far this has only been accomplished under the assumption that both bit-flip and measurement errors occur with the same probability [New J. Phys. 13, 083006 (2011)]. Here we study in detail the interplay between bit-flip and measurement errors, and analyze the resilience of topological error-correction codes to concurrent, nonsymetric bit flips and measurement errors. The error threshold is determined by mapping the problem onto classical, disordered lattice gauge theories, that are investigated using large-scale Monte Carlo simulations and improved estimators for systems with local gauge symmetries. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G27.00007: Quantum error correction with soft-pulse dynamically corrected gates with always-on qubit couplings on bipartite lattices Amrit De, Leonid P. Pryadko We suggest a scalable implementation of a universal set of high fidelity quantum gates on a bipartite lattice with always-on Ising couplings using dynamical decoupling (DD) sequences with second-order self-refocusing pulses. In addition to decoupling the unwanted parts of the inter-qubit interaction, the constructed gates also protect the qubits against low-frequency phase noise. This allows heterogeneous concatenation of DD and quantum error correction. We illustrate the technique by simulating the encoding/decoding and repeated ancilla based measurements for 4- and 5-qubit quantum error detecting/correcting codes on a spin chain and on a star graph. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G27.00008: Protecting OAM states of light from the decoherence effects of a turbuent atmosphere Jose Raul Gonzalez Alonso, Todd Brun While there are many advantages to using the polarization of photons to encode quantum information, a major disadvantage is that the limited dimension of the Hilbert space that describes the polarization state allows only the encoding of one qubit per photon. However, if one uses the the orbital angular momentum (OAM) of photons then the Hilbert space that describes the OAM state of a photon is infinite dimensional. Thus, it is possible to encode more than one qubit per photon. This advantage can be exploited in quantum key distribution (QKD) and in quantum secure direct communications. However, unlike the polarization of a photon, the OAM is prone to the decoherence effects produced by interactions with a turbulent atmosphere. In this work, we derive an expression for these decohering effects, and numerically simulate them to find a Kraus error map. We then theoretically demonstrate encoding and information recovery methods that could mitigate such unwanted effects. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G27.00009: Unitary Transformations in a Large Hilbert Space Brian Anderson, Hector Sosa Martinez, Aaron Smith, Carlos Riofrio, Charlie Baldwin, Ivan Deutsch, Poul Jessen Quantum systems with Hilbert space dimension greater than two (qudits) provide an alternative to qubits as carriers of quantum information, and may prove advantageous for quantum information tasks if good laboratory tools for qudit manipulation and readout can be developed. We have implemented a protocol for arbitrary unitary transformations in the 16 dimensional hyperfine ground manifold of Cesium 133 atoms, using phase modulated rf and microwave magnetic fields to drive the atomic evolution. Our phase modulation waveforms are designed numerically using a variant of the highly efficient GRAPE algorithm. The fidelity of the resulting transformations is verified experimentally through randomized benchmarking, which indicates an average fidelity better than 97\% across a sample of random unitaries. Our toolbox for quantum control is in principle applicable for a broad class of physical systems, such as large spins or anharmonic oscillators. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G27.00010: Resonant Microwave Control of a Symmetric Exchange-Only Spin Qubit J. Medford, J. Beil, J. M. Taylor, H. Lu, A. C. Gossard, C. M. Marcus We demonstrate two-axis control of an exchange-only spin qubit in a GaAs triple quantum dot using a resonant microwave excitation. The qubit is operated in a regime where two separate exchange interactions are active simultaneously, suppressing leakage out of the qubit subspace and providing some immunity to charge noise. Spectroscopic probes of the qubit reveal that the resonance frequency can be adjusted between 100 MHz and 1.5 GHz with a voltage applied to the middle quantum dot. We find a coherence time $T_2\sim20~ \mu$s for a 64 pulse Carr-Purcell-Meiboom-Gill dynamical decoupling sequence. Finally, analysis of the coherence time for multiple sequences reveals a power spectrum $S(\omega)\sim \omega^{-0.9}$, which suggests that the fluctuating Overhauser fields are not the dominant source of dephasing in this system. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G27.00011: Decay of the rotating-frame spin echo and its application to sensing the local environment of a NV center Vagharsh Mkhitaryan, Xiao-Xuan Huang, Viatcheslav Dobrovitski We study a NV electron spin subjected to a strong driving field, which reverses its sign with the period $\tau$ (multi-pulse Solomon echo), and analyze the rotating-frame echo decay at long times (large number of reversals). The form and the rate of the echo decay is calculated analytically and numerically, by modelling the decohering spin environment as a magnetic noise. For short $\tau$ the decay is strongly suppressed, being of the 4th order in $\tau$ (vs. 3rd order in the regular Carr-Purcell decoupling, and 2nd order in the standard continuous-wave decoupling). This ensures exceptional decoupling stability with respect to the slow fluctuations of the external magnetic field. Moreover, we find that the decay rate depends non-monotonically on the correlation time of the environment, decreasing for both very fast and very slow spin baths. Using these results, we demonstrate how the multi-pulse version of the Solomon echo can be harnessed to sense and analyze in detail the local spin environment of the NV center. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G27.00012: Sudden Decoherence Transitions for Quantum Discord Hyungjun Lim, Robert joynt We formulate the computation of quantum discord in terms of the generalized Bloch vector, focusing on the case of 2 qubits. This provides useful insights on the time evolution of quantum coherence for the open stem, particularly the comparison of entanglement and discord. We introduce a numerical method for calculating quantum discord for a special class of multipartite states. In agreement with previous work in low-dimensional cases (L. Mazzola et al., Phys. Rev. Lett. 104, 200401 (2010), we find situations in which there is a sudden transition from classical to quantum decoherence characterized by the discord remaining relatively robust (classical decoherence) until a certain point from where it begins to decay quickly whereas the classical correlation decays more slowly (quantum decoherence). We propose a general condition to observe this phenomenon. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G27.00013: Correlation Dynamics of Qubit-Qutrit Systems in a Classical Dephasing Environment Goktug Karpat, Baris Cakmak, Zafer Gedik We study the time evolution of classical and quantum correlations for hybrid qubit-qutrit systems in independent dephasing environments. Our discussion involves a comparative analysis of the Markovian dynamics of negativity, quantum discord, geometric measure of quantum discord and classical correlation. In the presence of multilocal dephasing noise, we demonstrate the phenomenon of frozen quantum discord for qubit-qutrit states. We show that geometric discord can also get frozen for a class of separable states in this case. On the other hand, when only the qutrit is under the action of a dephasing channel, we observe that the partial coherence left in the system might enable quantum discord to remain invariant throughout the whole dynamics even though the entanglement in the qubit-qutrit state disappears in a finite time interval. [Preview Abstract] |
Session G28: Statistical Mechanics of Social Systems
Sponsoring Units: GSNPChair: Stefan Boettcher, Emory University
Room: 336
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G28.00001: Characterizing production and consumption in Physics Qian Zhang, Fabio Ciulla, Bruno Goncalves, Nicola Perra, Alessandro Vespignani We analyze the entire database of publications in the American Physical Society and generate longitudinal (50 years) citation networks at two different geographical levels. We define the knowledge diffusion proxy and Scientific Production Ranking algorithms to capture the complex nature of citation networks, and to provide a global view of spatial distributions of production and consumption of knowledge in Physics as well as its temporal evolution. Using the knowledge diffusion proxy we identify the key actors in producing and consuming knowledge in Physics as a function of time. The ranking results from the Scientific Production Ranking algorithm allow us to characterize the top countries/cities in the world for Physical sciences. Among all the results, we find that in 50 years major states and cities in US stably rank on the top and have been main knowledge producers, whereas the major European countries, Japan and Russia have greatly improved their their ranking since 1990. Interestingly, we notice that China and Spain as well as major cities in those countries have gradually become major knowledge consumers in the last two decades. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G28.00002: The possible role of resource requirements and academic career-choice risk on gender differences in publication rate and impact Xiaohan Zeng, Jordi Duch, Marta Sales-Pardo, Filippo Radicchi, Shayna Otis, Teresa Woodruff, Luis Amaral Many studies demonstrate that there is still a significant gender bias, especially at higher career levels, in many areas including science, technology, engineering, and mathematics (STEM). We investigated field-dependent, gender-specific effects of the selective pressures individuals experience as they pursue a career in academia within seven STEM disciplines. We built a unique database that comprises 437,787 publications authored by 4,292 faculty members at top United States research universities. Our analyses reveal that gender differences in publication rate and impact are discipline-specific. Our results also support two hypotheses. First, the widely-reported lower publication rates of female faculty are correlated with the amount of research resources typically needed in the discipline considered, and thus may be explained by the lower level of institutional support historically received by females. Second, in disciplines where pursuing an academic position incurs greater career risk, female faculty tend to have a greater fraction of higher impact publications than males. Our findings have significant, field-specific, policy implications for achieving diversity at the faculty level within the STEM disciplines. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G28.00003: Analysis of Science and Technology Trend Based on Word Usage in Digitized Books Jinhyuk Yun, Pan-Jun Kim, Hawoong Jeong Throughout mankind's history, forecasting and predicting future has been a long-lasting interest to our society. Many fortune-tellers have tried to forecast the future by ``divine'' items. Sci-fi writers have also imagined what the future would look like. However most of them have been illogical and unscientific. Meanwhile, scientists have also attempted to discover future trend of science. Many researchers have used quantitative models to study how new ideas are used and spread. Besides the modeling works, in the early 21st century, the rise of data science has provided another prospect of forecasting future. However many studies have focused on very limited set of period or age, due to the limitations of dataset. Hence, many questions still remained unanswered. Fortunately, Google released a new dataset named ``Google N-Gram Dataset.'' This dataset provides us with 5 million words worth of literature dating from 1520 to 2008, and this is nearly 4\% of publications ever printed. With this new time-varying dataset, we studied the spread and development of technologies by searching ``Science and Technology'' related words from 1800 to 2000. By statistical analysis, some general scaling laws were discovered. And finally, we determined factors that strongly affect the lifecycle of a word. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G28.00004: The Knowledge Economy Bruno Gon\c{c}alves, Nicola Perra, Fabio Ciulla, Qian Zhang, Alessandro Vespignani Although the study of scientific and citation networks is well developed, the way in which ideas and concepts flow between scientific groups scattered around the world is still an open problem. We take a first step in this direction by using the citation patterns over the course of decades to shed light on how areas and fields in the general area of physics have evolved both temporally and geographically. By geocoding the affiliations associated with each article published by the APS journals to the country level, and by borrowing concepts from the field of economics and international trade we can explore how ideas produced in one country are exported, through citations, to other countries. An objective way of ranking countries based on their contributions to the overall scientific effort is also proposed as well as a map of how the different subfields of Physics are related to each other. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G28.00005: Power Law Distributions of Patents as Indicators of Innovation Dion O'Neale, Shaun Hendy The total number of patents produced by a country (or the number of patents produced per capita) is often used as an indicator for innovation. Such figures however give an overly simplistic measure of innovation within a country. Here we present evidence that the distribution of patents amongst applicants within many countries is well-fitted to a power law distribution with exponents that vary between 1.66 (Japan) and 2.37 (Poland). We suggest that this exponent is a useful new metric for studying innovation. Using simulations based on simple preferential attachment-type rules that generate power laws, we find we can explain some of the variation in exponents between countries, with countries that have larger numbers of patents per applicant generally exhibiting smaller exponents in both the simulated and actual data. Similarly we find that the exponents for most countries are inversely correlated with other indicators of innovation, such as research and development intensity or the ubiquity of export baskets. This suggests that in more advanced economies, which tend to have smaller values of the exponent, a greater proportion of the total number of patents are filed by large companies than in less advanced countries. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G28.00006: Discrete Lognormal Model as an Unbiased Quantitative Measure of Scientific Performance Based on Empirical Citation Data Joao Moreira, Xiaohan Zeng, Luis Amaral Assessing the career performance of scientists has become essential to modern science. Bibliometric indicators, like the h-index are becoming more and more decisive in evaluating grants and approving publication of articles. However, many of the more used indicators can be manipulated or falsified by publishing with very prolific researchers or self-citing papers with a certain number of citations, for instance. Accounting for these factors is possible but it introduces unwanted complexity that drives us further from the purpose of the indicator: to represent in a clear way the prestige and importance of a given scientist. Here we try to overcome this challenge. We used Thompson Reuter's Web of Science database and analyzed all the papers published until 2000 by $\sim$1500 researchers in the top 30 departments of seven scientific fields. We find that over 97\% of them have a citation distribution that is consistent with a discrete lognormal model. This suggests that our model can be used to accurately predict the performance of a researcher. Furthermore, this predictor does not depend on the individual number of publications and is not easily ``gamed'' on. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G28.00007: Information theory in econophysics: stock market and retirement funds Eugenio Vogel, G. Saravia, J. Astete, J. D\'Iaz, R. Erribarren, F. Riadi Information theory can help to recognize magnetic phase transitions, what can be seen as a way to recognize different regimes. This is achieved by means of zippers specifically designed to compact data in a meaningful way at is the case for compressor wlzip [1]. In the present contribution we first apply wlzip to the Chilean stock market interpreting the compression rates for the files storing the minute variation of the IPSA indicator. Agitated days yield poor compression rates while calm days yield high compressibility. We then correlate this behavior to the value of the five retirement funds related to the Chilean economy. It is found that the covariance between the profitability of the retirement funds and the compressibility of the IPSA values of previous day is high for those funds investing in risky stocks. Surprisingly, there seems to be no great difference among the three riskier funds contrary to what could be expected from the limitations on the portfolio composition established by the laws that regulate this market. \\[4pt] [1] E.E. Vogel, G. Saravia, L.V. Cortez, Physica A 391 (2012) 1591. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G28.00008: DebtRank a centrality measure for financial systems and beyond Guido Caldarelli, Stefano Battiston, Michelangelo Puliga, Rahul Kaushik, Paolo Tasca Use of network theory made possible to measure quantitatively many features of social and technological systems. In this spirit, inspired by traditional measures of centrality we introduce DebtRank a novel measure of systemic impact. We that we intend the risk of default of a large portion of the financial system, depends on the network of financial exposures among institutions. As an application, we analyse a new and unique dataset on the USD 1.2 trillion FED emergency loans program to global financial institutions during 2008--2010. We find that a group of 22 institutions, which received most of the funds, form a strongly connected graph where each of the nodes becomes systemically important at the peak of the crisis. Moreover, a systemic default could have been triggered even by small dispersed shocks. Other application to different systems are also presented. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G28.00009: ABSTRACT WITHDRAWN |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G28.00010: Cascading failures in Europe's transmission system Andrea Asztalos, Sameet Sreenivasan, Boleslaw Szymanski, G. Korniss Cascading failures constitute an important vulnerability of infrastructure networks, hence understanding the origin and propagation of these failures is of great interest. To this end, we study cascades of overload failures within the framework of the cascade model introduced by Motter and Lai (2002) applied for flows that have a distributed character. We investigate numerically the properties of these failures in the high-voltage European electric power transmission system from 2002 (Zhou and Bialek, 2005). The network consists of 1254 nodes (substations) specified by geographical locations, and 1812 links (transmission lines) that are assumed to be undirected. We find that assigning excess capacities in proportion to initial loads does not significantly mitigate cascading failures. Moreover, increasing the fractional excess capacity does not yield monotonically increasing gains. Using a simple model of spatial network - random geometric graph - we investigate methods beyond the proportional excess capacity allocation in order to improve the gains in mitigating such failures. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G28.00011: Cascade Failures in Power Grids with Distributed Generation Antonio Scala, Sakshi Pahwa, Caterina Scoglio Power grids are nowadays experiencing a transformation due to the introduction of Distributed Generation based on Renewable Sources. At difference with classical Distributed Generation, where local power sources mitigate anomalous user consumption peaks, Renewable Sources introduce in the grid intrinsically erratic power inputs. By introducing a simple schematic (but realistic) model for power grids with stochastic distributed generation, we study the effects of erratic sources on the robustness of several IEEE power grid test networks with up to $2 \times 10^3$ buses. We find that increasing the penetration of erratic sources causes the grid to fail with a sharp transition. We compare such results with the case of failures caused by the natural increasing power demand. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G28.00012: Distribution of Betweenness in Networks Failing by Overload Mark Tuchman, Gilad Barach, Sergey Buldyrev, Gabriel Cwilich We study the Motter and Lai [1] model of cascading failures based on the betweenness centrality of the nodes, for a random regular network. After removing a fraction of the nodes, we study the size of the giant component at the end of the cascade of failures, as a function of the fraction of the nodes that survived the initial attack. We find that the type of transition through which the network disintegrates changes from first order to second order as the maximum capacity of the nodes increases. We examine the distribution of the betweenness of the nodes in the vicinity of the critical fraction of initial surviving nodes, and we look at the distribution at different stages of the cascade. We explore the disintegration of the network when the size of the initial attack approaches the percolation threshold of the network. We compare our results with an analytical ansatz of the role of subcomponents of the network nearly isolated from the giant component.\\[4pt] [1] A. Motter, Y. Lai, ``Cascade-based attacks on complex networks,'' \textit{Phys. Rev. E} \textbf{66}, 065102(R) (2002) [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G28.00013: Cascading Failures in Networks with Proximate Dependent Nodes Yosef Kornbluth, Steven Lowinger, Gabriel Cwilich, Sergey Buldyrev We study a system composed of two identical, random regular, interdependent networks. When a fraction of nodes in the first network are eliminated by failure or attack, further nodes that become isolated or lose their dependent node fail in turn, initiating a process of cascading failures. In contrast to previous models, these networks are constructed such that interdependent nodes are no more than a set distance away, with the distance defined by the number of intervening nodes. We find that as the maximum distance and the degree of connectivity increase, the disintegration of the system shifts from being similar to that of a single network to resembling the failure found in other models of interdependent networks. As the distance and degree increase, the collapse at the critical threshold changes from a second-order transition to a first-order one. The critical threshold monotonically increases as the distance increases. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G28.00014: Comparative advantage between traditional and smart navigation systems Jeongkyu Shin, Pan-Jun Kim, Seunghwan Kim The smart navigation system that refers to real-time traffic data is believed to be superior to traditional navigation systems. To verify this belief, we created an agent-based traffic model and examined the effect of changing market share of the traditional shortest-travel-time algorithm based navigation and the smart navigation system. We tested our model on the grid and actual metropolitan road network structures. The result reveals that the traditional navigation system have better performance than the smart one as the market share of the smart navigation system exceeds a critical value, which is contrary to conventional expectation. We suggest that the superiority inversion between agent groups is strongly related to the traffic weight function form, and is general. We also found that the relationship of market share, traffic flow density and travel time is determined by the combination of congestion avoidance behavior of the smartly navigated agents and the inefficiency of shortest-travel-time based navigated agents. Our results can be interpreted with the minority game and extended to the diverse topics of opinion dynamics. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G28.00015: The Cancho i Ferrer - Sol\'e model does not explain Zipf's Law Ronald Dickman, Nicholas Moloney We examine the cost-minimization problem posed by Ferrer i Cancho and Sol\'e in their information-theory based communication model [1], proposed in efforts to explain Zipf's Law (that is a power-law frequency-rank relation for words in written texts). Using a simple inequality, we obtain the exact minimum-cost solution as a function of the parameter $\lambda$, as obtained previously via other methods [2-4]. ($\lambda$ defines the relative weights of speaker's and listener's costs.) We show that at the phase transition, the minimum-cost solutions do not correspond to a power law except for a vanishingly small subset, even if we impose the additional condition of equal costs to speaker and listener. Finally we consider the model at finite temperature using mean-field theory and entropic Monte Carlo simulation, and find a line of {\it discontinuous} phase transitions in the $\lambda$-$T$ plane. The simulations yield no evidence of a power-law frequency-rank distribution. \noindent 1. R. Ferrer i Cancho and R.~V. Sol\'e, PNAS {\bf 100}, 788 (2003).\\ 2. R. Ferrer i Cancho and A. D\'{\i}az-Guilera, J. Stat. Mech.: Theory Exp. (2007) P06009.\\ 3. A. Trosso, Master's thesis, 2008, University of Turin, Italy.\\ 4. M. Prokopenko et al., J. Stat. Mech. (2010) P11025. [Preview Abstract] |
Session G29: FIAP Prize Session
Sponsoring Units: FIAPChair: Mark Bernius, Dow Chemical Company
Room: 337
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G29.00001: George E. Pake Prize Lecture: Crystalline Silicon Photovoltaics: Accelerating to Grid Parity Invited Speaker: Mark Pinto Lost in recent headlines about solar company failures, reduced government support and depressed stock valuations is the fact that photovoltaic (PV) systems continue to be installed at an extremely healthy rate -- a ten-fold increase between 2007 and 2012, to a cumulative 100GWp of installations worldwide. The primary factor behind this remarkable growth has been cost reduction at the installed system level afforded by manufacturing and technology improvements to the crystalline silicon (c-Si) PV cell. In fact in the past 2 years, c-Si module cost learning curves have accelerated over their historical norms as a function of both volume and time, and as a result c-Si PV has reached parity with conventional forms of electricity in 20$+$ countries worldwide. In this presentation future c-Si technology paths will be reviewed along with market implications, leading to the projection that between 2015 and 2020, c-Si based PV electricity will be cost-effectively delivered to \textgreater 95{\%} of the world's population. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G29.00002: Prize for Industrial Applications of Physics Lecture: A physicist in Business Invited Speaker: John Woollam In the 1980s I inherited a famous ellipsometry laboratory. To speed up data acquisition and analysis I associated myself with creative scientists and engineers. We started a company which grew. Together we rapidly improved acquisition speed, accuracy, precision, spectral range, and types of applications. Yet, a business is much more than technology. In this talk I outline how a high-tech business functions, and illustrate the role of physicists and engineers in making a company successful. It is fast-paced, exciting, and enormously gratifying to provide quality instruments for researchers and industry. [Preview Abstract] |
Session G30: Self-Assembly
Sponsoring Units: DCMPChair: Bulent Akgun, National Institute of Standards and Technology
Room: 338
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G30.00001: Is a hierarchical dynamics the best route to the self-assembly of a hierarchical structure? Thomas Haxton, Stephen Whitelam Mimicking nature's ability to assemble functional hierarchical materials will require understanding how to promote the self-assembly of structure on multiple lengthscales while avoiding kinetic traps. We use computer simulation to study the self-assembly of a simple hierarchical structure, a square lattice whose repeat unit is a tetramer. Although the target material is organized hierarchically, it self-assembles most reliably when its assembly pathway consists of the sequential addition of monomers to a single structure. Hierarchical assembly pathways via dimer and tetramer intermediates result in lower yield, because these intermediates tend to associate in ways incompatible with the target structure. In addition, assembly via tetramers results in the formation of incomplete building blocks (trimers) that cannot combine to form the target crystal. We use analytic theory to relate assembly pathways to the underlying thermodynamics, identifying two principles for optimal assembly: 1) make the free energy gap between the target phase and the most stable fluid phase comparable to the thermal energy, and 2) ensure that no other dense phases (liquids or close-packed solids of monomers or oligomers) or fluids of incomplete building blocks fall within this gap. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G30.00002: Optimized assembly and steady-state length-scale control in dissipative systems of photo-switchable colloids Antonio Osorio-Vivanco, Monica Olvera de la Cruz, Sharon Glotzer Photo-switchable nanoparticles, such as those developed by Wei et al.,\footnote{Y.H. Wei, S. B. Han, J. Kim, S. L. Soh and B. A. Grzybowski, J. Am. Chem. Soc., 2010, 132, 11018-11020.} can be assembled into a broad range of structures using light exposure as a control parameter. Jha et al.\footnote{P.k. Jha, V. Kuzovkov, B.A. Grzybowski, and M. Olvera del la Cruz, Soft Matter, 2012, 8, 227-234} explored the evolution of these structures using kinetic Monte Carlo simulations. In this work, we build on these studies using Molecular Dynamics with a Langevin thermostat to, by judicious choice of exposure parameters that control the dissipative nature of the system, engineer and optimize the self-assembly pathways as well as control the length scales of the steady-state structures. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G30.00003: Self organization of exotic oil-in-oil phases driven by tunable electrohydrodynamics Anand Yethiraj, Atul Varshney, Shankar Ghosh, S. Bhattacharya The tuning of electrostatic interactions has helped to elucidate when coherent crystalline structures or incoherent amorphous structures form in colloidal systems. However, there is little understanding of self-organization in situations where hydrodynamic interactions are also present. We present a minimal two-component oil-in-oil model system where we can control the strength and length scale of the electrohydrodynamic interactions by tuning the amplitude and frequency of the imposed electric field. As a function of the hydrodynamic length scale, we observe a rich phenomenology of exotic structure and dynamics, from incoherent cloud-like structures and chaotic droplet dynamics, to polyhedral droplet phases, to coherent droplet arrays.\\[4pt] Reference: A. Varshney et al., Scientific Reports 2, 738 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G30.00004: Novel Behavior in Self-Assembled Superparamagnetic Nanoparticle Monolayers at the Air-Water Interface Jacob Stanley, Leandra Boucheron, Yeling Dai, Binhua Lin, Mati Meron, Oleg Shpyrko Iron oxide nanoparticles, coated with an oleic acid ligand, have been found to form self-assembled monolayers when deposited at the air-water interface. Even for low particle densities these particles aggregate into hexagonally close-packed islands which merge into a uniform layer at higher densities. Using Grazing Incidence Small Angle X-Ray Scattering (GISAXS) we were able to measure the first through fifth order diffraction peaks. By analyzing the positions and shapes of these peaks we investigated the in-plane structure of these monolayers and characterized how the structure changes as a function of compression in a Langmuir-Blodgett trough. Since iron oxide nanoparticles are known to be super-paramagnetic, we sought to investigate the role magnetic effects may have on the interparticle interactions and ordering within the film. We performed Grazing Incidence Diffraction (GID) measurements on the film while varying an external magnetic field. We will discuss the results of our findings. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G30.00005: Structure and dynamics of self-assembly Henrik van Lengerich, Richard James We investigate structures that are composed of many identical building blocks. Of particular interest are equilibrium structures where every building block sees the same environment - we call these ``objective structures''. For example, carbon nanotubes and virus capsids are both objective structures. The dynamics of assembly is investigated through experiments and simulations. The experiment consist of a macro-scale shaker containing identical neutrally buoyant magnetic particles. Simulations model the translation and rotation of particles using Langevin dynamics. This kind of modelling is applicable to both our experiment and to molecular assembly. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G30.00006: How to Maximize Self-Assembly in Free Surface Films by Resonant Wavelength Excitations Sandra Troian, Nan Liu Application of an external force probe on self-assembly processes in thin liquid films can offer significant insight into the fundamental dynamics of pattern formation. Less appreciated is the fact that modulation of such forces can induce resonant excitation effects in linearly unstable systems. While temporal modulation is rather common, there has been less emphasis on spatial forcing as a method for corralling emergent structure formation; such studies have also been strictly limited to 2D. In this talk, we call attention to a novel 3D hydrodynamic instability in nanoscale films whose free surface is exposed to a large uniform thermal gradient. Such films spontaneously develop arrays of nanopillars whose uniformity is often compromised by nanoscale inhomogeneities in film thickness, temperature and surface defects. In this talk we focus on resonant wavelength excitations induced by spatial modulation of the external thermal field near the linear stability point. Linear stability, weakly nonlinear analysis and simulations of the full nonlinear interface equation demonstrate the existence of a spatial coherence regime leading to more rapid growth and denser packing of perfectly uniform arrays, of significance to recent advances in lithographic patterning. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G30.00007: Analysis of pattern formation in systems with competing range interactions Vyacheslav R. Misko, Haijun Zhao, Francois M. Peeters Pattern formation is governed by competing interaction. Examples include: Langmuir monolayers, colloids and gels, ferrofluids, magnetic garnet thin films, type-I superconductors, the pasta phase in neutron stars, etc. We analyzed pattern formation and identified various morphologies in a system of particles interacting through a non-monotonic potential with a competing range interaction characterized by a repulsive core ($r < r_{c}$) and an attractive tail ($r > r_{c}$), using molecular-dynamics simulations [1]. Depending on parameters, the interaction potential models the inter-particle interaction in various physical systems ranging from atoms, molecules and colloids to vortices in superconductors. We constructed a ``morphology diagram'' in the plane ``critical radius $r_{c}$ $-$ density $n$'' and proposed a new approach to characterize the patterns. Namely, we elaborated a set of quantitative criteria in order to identify the different pattern types, using the radial distribution function (RDF), the local density function and the occupation factor. We also discuss the dynamics of the obtained patterns [2]. \\[4pt] [1] H. J. Zhao, V. R. Misko, and F. M. Peeters, New Journal of Physics {\bf 14}, 063032 (2012). \\[0pt] [2] H. J. Zhao, V. R. Misko, and F. M. Peeters, submitted (2012). [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G30.00008: Crystallographic Tailoring: Self-Assembling Complex Crystals Through Building Block Design Pablo F. Damasceno, Michael Engel, Sharon C. Glotzer A primary challenge for the development of bulk, scalable and high yield materials with interesting properties is the limited number of structures that can be obtained via self-assembly of nano and micrometer sized particles. To increase this variability, several suggestions have been proposed among which the exploration of new anisotropic building blocks have received much attention. Here we present the results of a systematic and extensive computational study of hard polyhedral particles [1,2] and their subsequent assembly into a diverse range of complex structures. Our results show that 1) by utilizing more complex, anisotropically designed building blocks new structures can be self-organized purely from entropy maximization principles and, 2) a predictive criteria for assembly can be formulated, allowing for specific choices of building blocks given a target structure to be self-assemble. [1] Pablo F. Damasceno, Michael Engel {\&} Sharon C. Glotzer. ACS NANO (2012). [2] Pablo F. Damasceno, Michael Engel {\&} Sharon C. Glotzer. SCIENCE (2012). [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G30.00009: Design Rules for the Self-Assembly of Voronoi Particles Benjamin Schultz, Pablo Damasceno, Michael Engel, Sharon Glotzer Recent theoretical advances have developed methodologies for predicting the assembly of hard, polyhedral particles. In this work, we use the Voronoi tessellation to generate polyhedral shapes that form space-filling superlattices that are isostructural to well-known atomic crystals. We focus on the assembly of these polyhedra into crystalline superlattices with orientational and positional order. Analogous to potentials designed to stabilize crystals at zero temperature, these particles are designed to stabilize the space-filling tiling at infinite pressure. We study a set of these particles in simulation and characterize how their symmetry and other geometric features affect their assembly characteristics at finite pressure. We calculate the relative stability of competing structures for several shapes that do not assemble their target structure and discuss how features of the shape affect this stability. From our conclusions, we demonstrate how to move beyond the concept of Voronoi tessellation for the design of hard polyhedral particles targeted for self-assembly. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G30.00010: Self-assembly of binary space-tessellating compounds Mihir Khadilkar, Umang Agarwal, Fernando Escobedo Self-assembly of polyhedral nanoparticles and their mixtures has been a topic of interest in both experimental and simulation studies due to its potential to help engineer novel materials. Hard-core mixtures that tessellate space are particularly interesting since they are expected to form entropy-driven high-pressure ordered structures. Using Monte Carlo simulations, we study three such binary tessellating mixtures; namely, cuboctahedra + octahedra (Mixture 1), octahedra + tetrahedra (Mixture 2), and truncated cubes + octahedra (Mixture 3). We see that upon gradual compression of the isotropic system, Mixtures 1 and 2 form a metastable, glassy disordered phase while Mixture 3 demixes into a disordered phase and an unusual `semi-crystalline' phase where truncated cubes form a cubic lattice while the octahedra remain disordered occupying interstitial pockets. While our results identify some relations between properties of individual species and their mixtures, they also illustrate the potential of tessellating mixtures as designable materials that can lead to novel equilibrium phases or serve as entropic glass formers. Preliminary results on non-tessellating binary mixtures will also be briefly discussed to provide a broader context of the results for the tessellating cases. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G30.00011: Targeted self-assembly of complex lattices and meta materials from isotropic interactions Oskar Lindgren, Erik Edlund, Martin Nilsson Jacobi I will present an analytical method for designing isotropic interactions causing particles to self-assemble into complex lattices. The method is direct as opposed to previous trial and error schemes where the interactions are modified and tested until the desired pattern self-assembles. Since a naive implementation of the design scheme generally yields interaction potentials too complicated to implement experimentally, we provide a systematical simplification scheme to minimize the interaction potentials' complexity without changing which pattern is produced by the self-assembly process. We also prove that our suggested simplification scheme is optimal. The method has been tested using simulated systems and proven to work for a wide range of patterns, ranging from chiral 2D surfaces to 3D diamond-like crystals. The recent improvements in simplicity for the designed potentials makes experimental realization feasible. The interactions can also be designed so that the self-organizing systems obtain different material properties like directional sound propagation or stealth-like properties via the diffraction pattern. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G30.00012: Rheology of Self-Assembling Colloidal Chains Kazem V. Edmond, Stefano Sacanna, Zachary D. Forbes, Andrew D. Hollingsworth, David J. Pine We probe the rheology of self-assembling chains of ``pacman'' particles using a Zimm viscometer, a modified Couette apparatus. Pacman particles are microscopic spherical particles specially designed to have a spherical indentation on their surface. In the presence of a depletant, overlap between the indentation and another particle's surface maximizes the excluded volume between the two interacting particles, resulting in a selective attraction between them. Careful tuning of the interaction strength in a suspension of particles induces the formation of long chains. Shearing this material can twist, stretch, and break the chains, causing the material to exhibit unique rheological properties. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G30.00013: Shaping Colloids for Self-Assembly Stefano Sacanna, Gi-Ra Yi, David Pine The creation of a new material often starts from the design of its constituent building blocks at a smaller scale. From macromolecules to colloidal architectures, to granular systems, the interactions between basic units of matter can dictate the macroscopic behavior of the resulting engineered material and even regulate its genesis. Information can be imparted to the building units by altering their physical and chemical properties. In particular, the shape of building blocks plays a fundamental role at the colloidal scale, as it can govern the self-organization of particles into hierarchical structures and ultimately into the desired material. Herein we report a simple and general approach to generate an entire zoo of new anisotropic colloids. Our method is based on a controlled deformation of multiphase colloidal particles that can be selectively liquified, polymerized, dissolved and functionalized in bulk. We further demonstrate control over the particle functionalization and coating by realizing patchy and Janus colloids. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G30.00014: Probing transition pathways of self-assembled colloidal clusters Rebecca W. Perry, Miranda Holmes-Cerfon, Michael P. Brenner, Vinothan N. Manoharan Clusters of colloidal particles bound by weak interactions explore rich energy landscapes characterized by a few minima and many higher-energy, non-rigid configurations. To investigate how such systems transit through their energy landscapes, we designed a two-dimensional system that lends itself to simple observations with brightfield video microscopy. In our aqueous system, a short-range depletion interaction strongly confines the diffusion of the spherical polystyrene colloids to a shallow volume close to a glass cover slip. The same depletion interaction provides reversible bonds between the spheres. Analyzing time series of clusters of 3, 4, and 6 spheres allows us to compare the free energy of rigid configurations to that of the transition states and to measure the kinetics of the transitions. Combining experimental measurements of the kinetics with a recent theory using a geometrical approach for calculating energy landscapes leads to a new understanding of how hydrodynamics effect transitions rates between energy minima. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G30.00015: Optical assembly of thermodynamically stable colloidal clusters mediated by depletion Bhaskar Jyoti Krishnatreya, Stefano Sacanna, Kazem Edmond, David Pine, David G. Grier Colloidal particles with complementary shapes can self-organize into composite structures under the influence of entropic attractions mediated by depletion. What structures can form is governed by the colloidal components' shapes. The structures' stability can be tuned by adjusting the strength of the depletion attraction. Even when a particular colloidal cluster configuration is thermodynamically stable, achieving the stable structure typically involves substantial kinetic barriers. We overcome these kinetic barriers by assembling geometrically organized colloidal clusters using holographic optical tweezers in three dimensions. Once formed, the structures are stable and undergo three-dimensional shape fluctuations that can be measured with video microscopy. [Preview Abstract] |
Session G31: Padden Award Symposium
Sponsoring Units: DPOLYChair: Nitash Balsara, University of California at Berkeley
Room: 339
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G31.00001: Scaling Reversible Adhesion in Synthetic and Biological Systems Michael Bartlett, Duncan Irschick, Alfred Crosby High capacity, easy release polymer adhesives, as demonstrated by a gecko's toe, present unique opportunities for synthetic design. However, without a framework that connects biological and synthetic adhesives from basic nanoscopic features to macroscopic systems, synthetic mimics have failed to perform favorably at large length scales. Starting from an energy balance, we develop a scaling approach to understand unstable interfacial fracture over multiple length scales. The simple theory reveals that reversibly adhesive polymers do not rely upon fibrillar features but require contradicting attributes: maximum compliance normal to the substrate and minimum compliance in the loading direction. We use this counterintuitive criterion to create reversible, easy release adhesives at macroscopic sizes (100 cm$^{2}$) with unprecedented force capacities on the order of 3000 N. Importantly, we achieve this without fibrillar features, supporting our predictions and emphasizing the importance of subsurface anatomy in biological adhesive systems. Our theory describes adhesive force capacity as a function of material properties and geometry and is supported by over 1000 experiments, spanning both synthetic and biological adhesives, with agreement over 14 orders of magnitude in adhesive force. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G31.00002: Cavitation in block copolymer modified epoxy Carmelo Declet-Perez, Lorraine Francis, Frank Bates Today, brittleness in epoxy networks limits most commercial applications. Significant toughness can be imparted by adding small amounts of micelle forming block copolymers (BCP) without compromising critical properties such as high use temperature and modulus. Curing the network locks in the self-assembled BCP micellar structures formed in the monomer resin providing control of the resulting morphology. Despite significant research over the last decade, a complete description of the parameters influencing toughness in block copolymer modified epoxies is still lacking. In this presentation we compare the ultimate mechanical behavior of epoxies modified with spherical micelle forming BCP's containing rubbery and glassy cores using real-time in-situ small-angle X-ray scattering (SAXS) performed during tensile deformation. Striking differences in the 2D SAXS patterns were documented for epoxies modified with rubbery (PEP) versus glassy (PS) micelle cores. Rubbery cores dilate by 100{\%} in volume upon specimen yielding, while the glassy micelle cores deform at approximately constant volume. These results provide direct evidence of a cavitation mediated mechanism for toughness in block copolymer modified epoxies. We further interpret characteristic butterfly features in the 2D SAXS patterns in terms of epoxy network deformation. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G31.00003: Polymer Welding and Self-healing: Strength Through Entanglements Ting Ge, Mark O. Robbins, Dvora Perahia, Gary S. Grest Polymer interfaces are crucial in determining the mechanical strength of many systems. A common means of welding joints or self-healing cracks is to apply heat and allow polymers to interdiffuse. As the microscopic mechanism of interface strengthening is difficult to isolate experimentally, we probe the molecular origins of interfacial strength using large scale molecular simulations of welding and self-healing of cut systems. Systems are heated well above the glass temperature $T_{g} $ and then quenched below $T_{g} $ for mechanical testing. The interfacial strength is characterized by the maximum shear stress $\sigma_{\max } $ before failure. As strength grows, the dominant failure mode changes from chain pullout at the interface to chain scission, as in the bulk. In all simulations, $\sigma_{\max } $ saturates long before polymers diffuse by their own size. Bulk strength is observed for miscible welds, while strength is suppressed for cut systems due to short chain segments that remain near the interface. Entanglements are tracked using the Primitive Path Analysis. We find that the bulk response is not fully recovered until the density of entanglements at the interface reaches the bulk value. Moreover, the increase of $\sigma_{\max } $ before saturation is proportional to the number of interfacial entanglements between chains from opposite sides, which correlates linearly with the interdiffusion depth. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G31.00004: Magnetically aligned polymer-nanowire composites for solar energy harvesting Pawel Majewski, Candice Pelligra, Chinedum Osuji We present a solution-based approach of producing aligned arrays of ZnO nanowire-polythiophene composites for photovoltaic applications. We employ a two-step hierarchical self-assembly to maximize the efficiency of electron and hole transport in the system. First, we coat the wires with the polymer utilizing nanowire surface-directed crystallization and alignment of the polymer backbones along the long axes of the wires, then we employ magnetic fields to direct the assembly of the composites into the ordered arrays. We present quantitative SAXS data taken in-situ during the alignment process addressing the influence of paramagnetic doping level of ZnO and the magnetic field strength on the quality of the alignment. We compare the electrical conductivity of the aligned arrays of the composites to non-aligned ones and discuss the possible degree of conductivity enhancement upon the alignment in this and in analogous systems. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G31.00005: Self-similarity and energy dissipation in stepped polymer films Joshua McGraw, Thomas Salez, Oliver Baeumchen, Elie Raphael, Kari Dalnoki-Veress We have recently learned how to prepare polymer films whose only feature is a step in the height profile. In the melt, Laplace pressure drives a flow that levels the topography, with the excess energy of the height step being dissipated by viscosity. It has been observed that the profiles are self-similar in time for a variety of molecular weights and geometries. Given the surface tension, this simple observation allows a precise measurement of the viscosity by comparison with numerical solutions of the thin film equation. It is also possible to derive a master expression for the time dependence of the excess surface energy as a function of the material properties and film geometry. Thus, all geometries and molecular weights fall on a single temporal curve. The material parameter allowing this collapse is the capillary velocity -- the ratio of the surface tension to the viscosity. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G31.00006: The Consequence of Donor-acceptor Miscibility on Charge Transport and Photovoltaic Device Performance Kiarash Vakhshouri, Derek Kozub, Chenchen Wang, Alberto Salleo, Enrique Gomez Recent energy-filtered transmission electron microscopy studies revealed that amorphous mixed phases are ubiquitous within mesostructured polythiophene/fullerene mixtures. The role of mixing within nanophases on charge transport of organic semiconductor mixtures, however, is not fully understood. Through the combination of Flory-Huggins theory and energy-filtered transmission electron microscopy, we have estimated the miscibility limit of polythiophene/fullerene blends. We have also demonstrated the interplay between miscibility and percolation to describe field-effect mobilities as a measure of the conductive pathways present in a model organic semiconductor mixture (amorphous polythiophene/fullerene blends). Our studies reveal that the miscibility of the components strongly affects electron transport within amorphous blends. Immiscibility promotes efficient electron transport by promoting percolating pathways within organic semiconductor mixtures. However, strongly immiscible systems would readily phase separate into large domains, preventing efficient charge separation in organic photovoltaics. Consequently, an optimum degree of miscibility between donor/acceptor mixtures exists for the application of such mixtures to organic solar cells. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G31.00007: Theory of Polymers in Poor Solvent: Phase Equilibrium, Nucleation Behavior and Globule-to-Coil Transition Rui Wang, Zhen-Gang Wang We study the phase equilibrium and nucleation behavior of polymers in poor solvent by accounting for the large, localized fluctuations in the form of single-chain globules and multi-chain clusters. The density profile and free energy of the globule and clusters are obtained by self-consistent-field theory, which is then used in the dilute solution thermodynamics to investigate the cluster size distribution, solubility limit, as well as nucleation in the supersaturated state. Our results show that the solubility of the polymer in the dilute side of the solution is enhanced by several orders of magnitude relative to the prediction of the Flory-Huggins (F-H) theory, which scales with the chain length to the 2/3 power rather than a linear power as predicted from the F-H theory. In the supersaturated state, we work out an effective spinodal where the nucleation barrier to phase separation via growth of the clusters becomes comparable to the thermal energy. For a given supersaturation, we find that the nucleation barrier is quadratic in the chain length, suggesting a much slower precipitation rate for longer polymer chains. Tracking the density profile of the globule with decreasing $\chi$, we find the critical $\chi$ for the globule-to-coil transition of an infinitely long chain. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G31.00008: Dramatic role of fragility in determining the magnitude of T$_{\mathrm{g}}$ perturbations to ultrathin film layers and near-infinitely dilute blend components Christopher Evans, John Torkelson Using fluorescence, we measure the glass transition temperatures (T$_{\mathrm{g}})$ of ultrathin (11-14 nm) polystyrene (PS, bulk T$_{\mathrm{g}} =$ 103 $^{\circ}$C) layers which can be tuned over $\sim$ 80 $^{\circ}$C when sandwiched between two bulk neighboring layers of poly(4-vinyl pyridine) (P4VP), polycarbonate, poly(vinyl chloride) (PVC) or poly(tert-butyl acrylate). Between P4VP, an ultrathin PS layer has its dynamics slaved and reports the T$_{\mathrm{g}}$ of bulk P4VP. In contrast, an ultrathin PS layer is weakly perturbed (T$_{\mathrm{g}} =$ 97 $^{\circ}$C) when placed between PVC. These perturbations to the PS T$_{\mathrm{g}}$ become evident even for layers 10s of nanometers in thickness. Additionally, binary blends were prepared with 0.1 wt{\%} PS components surrounded by the same neighboring polymers as in the trilayers. The T$_{\mathrm{g}}$ reported by an ultrathin PS layer and a 0.1 wt{\%} PS blend component are the same for a given polymer pair indicating that the T$_{\mathrm{g}}$ perturbations in these two systems arise from a common physical origin. The strength of perturbations to PS correlate with the fragility of the neighboring domain in both blends and multilayers indicating that it is a key variable in determining the strength of T$_{\mathrm{g}}$-confinement effects. Fragility also tracks with the magnitude of T$_{\mathrm{g}}$-confinement effects observed in single layer polymer films supported on silicon wafers. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G31.00009: Confined Crystallization in Poly(3-alkylthiophene)-containing Diblock Copolymers Victor Ho, Rachel Segalman Organic optoelectronic device active layers require optimization of both the crystalline structure and the morphology at the nanometer length scale. These can be controlled simultaneously with a block copolymer in which one component is a crystalline conjugated polymer such as poly(3-alkylthiophene) (P3AT). While self-assembly of these systems requires balancing the driving forces of crystallization and self-assembly, in many systems, crystallinity dominates resulting in significant distortion or destruction of the melt phase structure. However, we show that judicious selection of the alkyl side chain in P3ATs results in melting transitions which can be controlled over a range of 150 C, and when incorporated into a block copolymer, these depressed melting transitions lead to regions of phase space for which the strength of segregation is sufficiently high at crystallization to allow for self-assembly. Phases such as crystalline majority-phase hexagonally-packed cylinders and lamellae are observed, and importantly the crystallinity of the conjugated polymer is retained in these confined geometries. [Preview Abstract] |
Session G32: Focus Session: Polymer Nanocomposites: Active Particles
Sponsoring Units: DPOLYChair: Russell Gorga, North Carolina State University
Room: 340
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G32.00001: Magnetic Field Driven Alignment of Cobalt Nanoparticles and Directional Strengthening Effect in Polystyrene Matrix Nanocomposites Hongyi Yuan, Jeffrey Pyun, Alamgir Karim Nanocomposite thin films of Polystyrene (PS) and PS-coated cobalt (Co) nanoparticles were prepared by solution-mixing and flow-coating. Ferromagnetic Co nanoparticles were either randomly dispersed or aligned in 1-D by applying a weak magnetic field during the flow-coating process. AFM and TEM images show nano-chain formation by self-assembly of the Co nanoparticles in the concentration range of 2-10 wt{\%} relative to PS in the presence of magnetic field. The technique of Strain-Induced Elastic Buckling Instability for Mechanical Measurements (SIEBIMM) was employed to determine the elastic moduli of neat PS and PS / Co nanocomposite thin films, which were calculated from the buckling patterns generated by applying and releasing tensile stresses. Strengthening effect was found in nanocomposite thin films depending on the alignment direction of the dispersed Co nanoparticles. The effect of shape and concentration of nanoparticles on the elastic modulus of nanocomposite thin films will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G32.00002: Magnetic field gradient driven self-assembly of superparamagnetic nanoparticles using programmable magnetically-recorded templates L. Ye, B. Qi, T.G. Lawton, O.T. Mefford, C. Rinaldi, S. Garzon, T.M. Crawford Using the enormous magnetic field gradients (100 MT/m @ z$=$20 nm) present near the surface of magnetic recording media, we demonstrate the fabrication of diffraction gratings with lines consisting entirely of magnetic nanoparticles assembled from a colloidal fluid onto a disk drive medium, followed by transfer to a flexible and transparent polymer thin film. These nanomanufactured gratings have line spacings programmed with commercial magnetic recording and are inherently concave with radii of curvature controlled by varying the polymer film thickness. The diffracted intensity increases non-monotonically with the length of time the colloidal fluid remains on the disk surface. In addition to comparing longitudinal and perpendicular magnetic recording, a combination of spectral diffraction efficiency measurements, magnetometry, scanning electron microscopy and inductively coupled plasma atomic emmission spectroscopy of these gratings are employed to understand colloidal nanoparticle dynamics in this extreme gradient limit. Such experiments are necessary to optimize nanoparticle assembly and obtain uniform patterned features. This low-cost and sustainable approach to nanomanufacturing could enable low-cost, high-quality diffraction gratings as well as more complex polymer nanocomposite materials assembled with single-nanometer precision. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G32.00003: Self-assembly and Photo-patterning in Polymer-fullerene Nanocomposite Thin Films Him Cheng Wong, Anthony Higgins, Andrew Wildes, Jack Douglas, Joao Cabral We report the directed self assembly of fullerenes in polymer thin films. The fullerenes are found to assemble spontaneously into spinodally coordinated clusters upon thermal annealing. The process yields well-defined structures, ranging from sparse heterogeneous nucleation to dense spinodal-like morphologies with tuneable characteristic spatial frequency and amplitude which coarsen with time, following well-defined scaling laws [1]. Mapping of this self assembly process utilized both real and reciprocal space techniques: optical and scanning force microscopy and neutron reflectivity. With external fields: light exposure and substrate surface energy, we demonstrate further tuneability over nanocomposite thin film morphology and substantial improvement on ultrathin film stability. By modulating the external fields on nanocomposite film with photomask, followed by thermal annealing, the film morphology and stability can be directed into various patterns, including a prototype polymer-fullerene circuit [2]. These results provide insights into fullerene self assembly in polymers and underscore their photoactive nature, an effect of great interest in the performance and stability of organic photovoltaics (OPV). [1] Wong H C and Cabral J T 2010 Phys. Rev. Lett. 105 038301 and 2011 Macromolecules 44 4530. [2] Wong H C, Higgins A M, Wildes A, Douglas J F, Cabral J T 2012 Adv. Mater. In Press. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G32.00004: Co-assembly of Nanorods and Photosensitive Polymer Blends Ya Liu, Olga Kuksenok, Anna Balazs Using computational modeling, we establish means of controlling structure formation in nanocomposites comprising nanorods and a photosensitive binary blend. The complex cooperative interactions in the system include the preferential wetting between the rods and one of the phases in the blend, steric repulsion between the coated rods and the response of the binary blend to light. Namely, under uniform illumination, the binary mixture undergoes both phase separation and a reversible chemical reaction, leading to a morphology resembling that of a microphase-separated diblock copolymer. When a second, higher intensity light source is rastered over the sample, the binary blend and the nanorods co-assemble into regular, periodically ordered structures. In particular, the system displays an essentially defect-free lamellar morphology, with the nanorods localized in the energetically favorable domains. By varying the speed at which the secondary light is rastered over the sample, we can control the directional alignment of rods within the blend. Our approach provides an effective route for achieving morphological control of both the polymeric components and nanoparticles, providing an effective means of tailoring the properties and ultimate performance of the composites. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G32.00005: Anisotropic Thermal Processing of Polymer Nanocomposites via the Photothermal Effect of Gold Nanorods J.R. Bochinski, S. Maity, L.I. Clarke, K.A. Kozek, W. Wu, J.B. Tracy Embedding metal nanoparticles within polymeric materials enables spatially-selective, in-situ thermal polymer processing [1,2]. When irradiating such a nanocomposite with light resonant with the particle's surface plasmon resonance, the photothermal effect efficiently transforms the energy into localized heat. Utilizing anisotropically-shaped particles enables further heating control based on the polarization sensitivity of the light-particle interaction. Photothermal heating from oriented gold nanorods selectively heats polymeric nanofibers by melting fibers lying only along a chosen direction while leaving the remaining material largely unaffected [3]. Fluorescence-based temperature-sensing measurements confirms heating in selected fibers and its absence in counter-aligned fibers. Such facile thermal processing of a specified subset of a sample, while the remainder is unchanged cannot be achieved through conventional heating. Results on spatially-selective heating and nanoscale temperature measurements within polymer systems doped with active nanoparticles will be discussed.\\[4pt] [1] S. Maity et al., \textit{Polymer} \textbf{52}, 1674 (2011).\\[0pt] [2] S. Maity et al., \textit{Adv. Funct. Mat.} (in press) (2012).\\[0pt] [3] S. Maity et al., \textit{Part. \& Part. Syst. Char.} (in press) (2012). [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G32.00006: Responsive and Hybrid Nanostructures through Self-Assembly of Polymeric Macroions, Inorganic Nanoclusters and Dyes Franziska Groehn, Jasmin Duering, Daniel Moldenhauer Recently we have introduced a novel type of self-assembled ``nano-objects'' in solution: From the association of macroions and multivalent counterions well-defined and stable structures in the shape of spheres, rod, rings, hollow spheres and networks can form in solution. Using light-addressable counterions, it is possible to switch the particle size through UV irradiation. Building blocks can be of organic or inorganic nature: Using gold or cadmium sulphide nanoclusters results in hybrid assemblies which also functionally combine nanoparticle and dye. Thermodynamic studies in combination with a detailed structural characterization yield insight into driving forces and structural control in the self-assembly process. Crucial is the delicate interplay of ionic, $\pi -\pi $, and Hamaker interaction. The concept is particularly attractive, as it relies on general physical effects - that is the combination of different non-covalent interactions - and hence is very versatile. Great potential of the structures presented lies in areas such as catalysis and energy conversion. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G32.00007: Utilizing Matrix-Filler Interactions in the Design of Stimuli-Responsive, Mechanically-Adaptive Electrospun Composites Nandula Wanasekara, David Stone, Gary Wnek, LaShanda Korley A new class of all-organic, stimuli-responsive and mechanically-adaptive electrospun nanocomposites, which have the ability to alter their stiffness upon hydration, were developed. These materials were fabricated by incorporating an electrospun mat of poly(vinyl alcohol) (PVA) as the filler in a polymeric matrix consisting of either poly(vinyl acetate) (PVAc) or ethylene oxide-epicholorohydrin copolymer (EO-EPI). The incorporation of high stiffness, high aspect ratio PVA filler mat significantly enhanced the tensile storage modulus of EO-EPI based composites, while modulus enhancement was only noticed above the glass transition for PVAc-based composites. Composite materials based on a rubbery EO-EPI host polymer and PVA filler exhibit an irreversible reduction by a factor of 12 of the tensile modulus upon hydration. In contrast, composites comprised of PVAc show a reversible reduction of modulus by a factor of 280 upon water uptake. The mechanical morphing of the electrospun composites is the result of the filler crystallinity, and matrix-filler interactions facilitated by the surface hydroxyl groups of the PVA filler. The choice of polymer matrix and electrospun nanofiber fillers allow control of matrix-filler interactions in a new series of all-organic composites to achieve desired stimuli-responsiveness and mechanical-adaptability upon exposure to various stimuli. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G32.00008: Structure and Transport Anomalies in Soft Colloids Samanvaya Srivastava, Lynden Archer We present structure, dynamics and rheology measurements for model nanoparticle suspensions comprising of silica nanoparticles, densely grafted with oligomeric polyethylene glycol (PEG) chains and suspended in similar PEG oligomers. Small angle X-ray scattering reveals anomalous structural trends wherein the particle-particle correlations are found to decrease as the particle volume fraction rises beyond the point of particle overlap. Upon further increase in the particle loading, investigation of the particle dynamics through X-ray photon correlation spectroscopy points towards an unusual speeding up of the nanoparticles. Analogous ``cascade of anomalies'' are observed in systems including complex molecular fluids like water and silica as well as in systems interacting via soft repulsive potentials, and similar forces are expected to lead to the origin of these anomalous trends in all the cases. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G32.00009: Simulations of Nanoparticle Ordering in Polymer Brush/Solvent Mixtures Gary S. Grest, Shengfeng Cheng, Mark J. Stevens Organizing nanoparticles into a desired super-structure is crucial for their technological applications. We present molecular dynamics simulations of the assembly of nanoparticles during the evaporation of solvent from 3-component mixtures of nanoparticles and solvent in contact with an end-grafted polymer brush. The organization of nanoparticles strongly depends on their interaction with polymer chains. For relatively weak attraction between the nanoparticles and brush, the nanoparticles straddle the brush surface and form an ordered lattice. For a strong attraction between the nanoparticles and polymer, the nanoparticles are engulfed inside the brush and the packing quality diminishes, because the lateral diffusion of the nanoparticles is suppressed. The opposite trend is observed in the case in which the polymer chains are not grafted to a substrate. In this case, a layer of nanoparticles is entrapped in the concentrated polymer film at the interface and assemble into a close-packed hexagonal lattice for strong mutual attraction, while for weak interactions the nanoparticles are mostly dispersed in the relatively solvent-rich solution below the interface and remain disordered. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G32.00010: Tailoring Surface Roughness by Grafting Nanoparticles to Random Copolymer Films Matthew Caporizzo, Rami Ezzibdeh, Russell Composto The effect of random copolymer composition on surface attachment and sinking of amine functionalized silica nanoparticles (d$=$45 nm) is investigated. Films of poly(styrene-ran-tert-butyl acrylate) (StBA) with 37{\%} tBA are converted to poly(S-ran-acrylic acid) (SAA) by annealing for 15h at temperatures ranging from 135C to 200C. The conversion of the tBA ranges from under 10{\%} to 100{\%} and is monitored by ellipsometry and ATR-FTIR. At complete conversion (25 wt{\%} AA), SAA forms nano-phase separated domains that result in particle aggregation within AA rich domains. At lower AA conversion, a disordered polymer morphology leads to grafting sites which are randomly distributed. NPs graft from nearly a complete monolayer to multilayers depending the percent of AA. Both the rate of NP attachment and the maximum loading of NPs into the film scale with the fraction of AA; this behavior is attributed to a reduction in the energetic barrier for the particle to sink into the film with increased swelling (more hydrophilic). A particularly attractive outcome of this systematic study is that optically transparent films with controlled roughness can be routinely prepared. Such films are of interest for investigating biomolecular adsorption and superhydrophobic, clear, non-fouling coatings. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G32.00011: Photothermally-induced rotation of gold nanorods within a polymer matrix to probe local nanocomposite properties Somsubhra Maity, Laura Clarke, Jason Bochinski The photothermal effect of gold nanorods embedded in polymer thin films produces localized heat depending upon the relative orientation of the rod and incident light field polarization. Simultaneous application of electric and light fields enables creation of thin films having aligned nanorods from those with initially randomly-oriented particles, as well as subsequent manipulation of rod orientation within the material environment. This is due to local melting of the polymer in the immediate vicinity of the particles which facilitates particle re-orientation. Conversely, solely under sufficient resonant light irradiation, initially aligned nanorods tend to randomize their orientation when the local environment melts. The rotational dynamics of the rods (i.e., alignment fidelity and rotation speed) depends on the polymer melt viscosity and thus directly reflects the local temperature around the rods which may vary significantly from the bulk temperature: conveniently, both rod orientation and bulk temperature can be simultaneously determined using optical methods. Thus, this combined approach provides both an in situ post-fabrication technique to manipulate alignment of rods and a tool to probe local temperature in polymer nanocomposites. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 2:03PM |
G32.00012: Polymer Nanocomposite Films: Dispersion of Polymer Grafted Nanorods and Optical Properties Invited Speaker: Russell Composto The thermodynamic factors that affect the dispersion of polymer-brush grafted gold nanorods (NR) in polymer matrix films have been studied by experiment and theory. When brush and matrix have a favorable interaction, such as poly(ethylene oxide) (PEO)-NR/ poly(methyl methacrylate) (PMMA) and polystyrene (PS)-NR / poly(2,6-dimethyl-p-phenylene oxide) (PPO), nanorods are uniformly dispersed. For PEO-NRs in PMMA, the NRs are regularly spaced and well dispersed, independent of the ratio of the degree of polymerization of the matrix (P) to that of the brush (N), namely P/N. As the NR volume fraction increases, the local orientation of the nanorods increases, whereas the macroscopic orientation remains isotropic. When the brush and matrix are similar (i.e., PS-NR / PS and PEO-NR / PEO), the nanorods randomly disperse for P/N \textless\ 2 (i.e., wet brush), but align side-by-side in aggregates for P/N \textgreater\ 2. UV-visible spectroscopy and discrete dipole approximation (DDA) calculations demonstrate that surface plasmon coupling leads to a blue shift in the longitudinal surface plasmon resonance (LSPR) as P/N increases. For P/N \textgreater\ 2, self-consistent field theory (SCFT) calculations and Monte Carlo (MC) simulations indicate that nanorod aggregation is caused by depletion-attraction forces. Starting with a dry brush system, namely, a PS matrix where P/N $=$ 30, these attractive forces can be mediated by adding a compatibilizing agent (e.g., PPO) that drives the NRs to disperse. Finally, dry and wet brush behavior is observed for NR aspect ratios varying from 2.5 to 7. However, compared at the same volume fraction, long rods for the dry case exhibit much better local order than lower aspect ratio nanorods, suggesting that long rods may exhibit nematic-like ordering at higher loadings. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G32.00013: Self-assembly of defect-free particle monolayers on flexible films Md.Shahadat Hossain, Bhavin Dalal, Sathishkumar Gurupatham, Ian Fischer, Pushpendra Singh, Nadine Aubry We have recently shown that the capillarity-based process for self-assembling particle monolayers on fluid-liquid interfaces can be improved by applying an electric field in the direction normal to the interface. The electric field gives rise to repulsive dipole-dipole forces amongst the particles causing them to move apart, and thus allowing them to move freely without blocking one another. The latter is important in the formation of virtually defect-free monolayers with long-range order. In this talk, we present a technique for freezing these expanded monolayers onto the surface of a flexible thin film. The technique involves assembling the monolayer on the interface between a UV-curable resin and a fluid which can be air or another liquid, and then curing the resin by applying UV light. The monolayer becomes embedded on the surface of the solidified resin film. [Preview Abstract] |
Session G33: Focus Session: Organic Electronics and Photonics - Theoretical Photophysics and Excited State Dynamics
Sponsoring Units: DMPChair: Richard Lunt, Michigan State University
Room: 341
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G33.00001: Polaritons in Organic Microcavities: The Effect of Phonons on the Dicke Model Justyna Cwik, Jonathan Keeling We study the effect of vibrational excitations on the condensation of polaritons. Recently, a lot of attention has been focused on microcavities based on organic semiconducting materials since, unlike their inorganic counterpart, they provide a suitable environment for the formation of a room temperature Bose-Einstein condensate. In order to model such materials we add terms to the usual Dicke Hamiltonian to account for the coupling of each two-level system to vibrational excitations (phonons). A mean field treatment, at zero temperature, gives us insights into the phase diagram of the Hamiltonian. In particular, we discuss the origin of the first order phase transition between two superradiant states which occurs as the coupling between the phonons and two-level systems is varied. An extension of the mean field treatment leads to the discussion of the equilibrium luminescence spectrum in the presence of phonons. We also present the way in which these results are modified at finite temperature. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G33.00002: Estimating the Magnitude of Exciton Delocalization in Regioregular P3HT through Computational Modeling and Transient Absorption Spectroscopy Michael Heiber, Ali Dhinojwala Exciton delocalization has been shown to have a potentially strong impact on the performance of organic solar cells. However, very few attempts have been made to estimate the magnitude of exciton delocalization in common semiconducting polymers. We show how the magnitude of exciton delocalization can be extracted from two types of femtosecond transient absorption spectroscopy experiments using computational modeling tools. By fitting exciton delocalization models to previously published experimental data, we extract two separate estimates of the magnitude of exciton delocalization in regioregular poly(3-hexylthiophene) (P3HT). First, fitting exciton-exciton annihilation behavior in pristine P3HT films leads to an estimation of the exciton delocalization radius of 1.6$\pm$0.25 nm. Second, dynamic Monte Carlo modeling of the exciton dissociation dynamics for a P3HT:PCBM blend film results in a second approximation of the exciton delocalization radius of 1.9$\pm$0.6 nm. These estimates are significantly smaller than previously published values and provide strong evidence for less delocalization than used in previous device models. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G33.00003: Influence of \textit{trans} and \textit{cis } defects on the localization of charged excitations in $\pi $-conjugated organic polymers Iffat Nayyar, Enrique Batista, Sergei Tretiak, Avadh Saxena, Darryl Smith, Richard Martin Optoelectronic devices with $\pi $-conjugated polymers are in demand for use in light-emitting diodes (LED), solar cells and lasers. A recent study predicted differences in the response of the hyperfine field by polaronic species in organic LEDs. The improved fluorescence exhibited by different isomeric forms of PPV derivatives in these devices motivated us to investigate the influence of various conformational defects of \textit{trans} and \textit{cis} nature on the energetics and localization of positive (P$^{\mathrm{+}})$ and negative (P$^{\mathrm{-}})$ polarons using density functional theory. We observe the P$^{\mathrm{+}}$ and P$^{\mathrm{-}}$ states are highly sensitive on the structural conformation and atomic charge distributions. The P$^{\mathrm{-}}$ state is observed to be more localized than P$^{\mathrm{+\thinspace }}$in consistent with recent experiments when the polarization effects are included. These defects not only break the particle-hole symmetry but demonstrate higher charge-carrier mobilities for holes than electrons. This helps in tuning the charge-transport and photo-physical properties of organic materials by understanding their structure-property correlations for technological innovations. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G33.00004: The role of exciton diffusion in the Forster-type energy transfer in hybrid organic-inorganic nanocomposites Burak Guzelturk, Pedro Ludwig Hernandez Martinez, Donus Tuncel, Hilmi Volkan Demir The role of exciton diffusion in the Forster-type energy transfer in hybrid organic-inorganic nanocomposite is essential for devices applications. To understand the underlying interplay between the exciton transfer and exciton diffusion, we investigate the temperature dependent nonradiative energy transfer (NRET) in polymer-quantum dot (QDs) nanocomposites at high and low QD loading levels. For the low QD loading, the diffusion coefficient (D) is estimated to be greater than 1000 nm2/ns and the diffusion length (LD) is approximately 13 nm at room temperature. However, significant modifications of D and LD are observed in the case of high QD loading, where D is estimated to be 150 nm2/ns and LD is smaller than 5 nm. This suppression is attributed to the increased rates of NRET from the polymer to the QDs, with a smaller effective donor-acceptor separation at high QD loadings. In summary, the exciton diffusion plays a critical role in the resulting exciton dynamics of such polymer-QD nanocomposites, and the experimental evidence and supporting theoretical model suggest that the exciton diffusion is weak at the high loading levels when the exciton transfer dominates. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G33.00005: First-principles simulations of exciton diffusion in organic semiconductors Xu Zhang, Zi Li, Gang Lu Exciton diffusion is of great importance to the performance of organic optoelectronic devices, including organic photovoltaics and solid-state lighting. The ability to control exciton diffusion in organic semiconductors is crucial to the design of efficient optoelectronic devices. However, such ability can only be achieved through a fundamental understanding of exciton diffusion mechanism. We have proposed a first-principles based frame work that can predict exciton dynamics in organic semiconductors.The framework is based on time-dependent density functional theory to provide the energy and many-body wave functions of excitons. Nonadiabatic \textit{ab initio} molecular dynamics is used to calculate phonon-assisted transition rates between localized exciton states. Using Monte Carlo simulations, we determine exciton diffusion length, lifetime, diffusivity, and harvesting efficiency in poly(3-hexylthiophene) polymers at different temperatures, and the results agree very well with corresponding experimental values. We find that exciton diffusion is primarily determined by the density of states of low-energy excitons; a widely speculated diffusion mechanism has been confirmed and elucidatedby the simulations. Some general guidelines for designing more efficient organic solar cells can be gleaned from the simulation results [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G33.00006: Relating Crystal Structure and the Charge-Transfer Nature of Excitons in Pentacene from First Principles Sahar Sharifzadeh, Pierre Darancet, Leeor Kronik, Jeffrey Neaton The nature of low energy optical excitations within pentacene has been the subject of many experimental and theoretical studies, with much disagreement as to the degree of their charge-transfer character. Here, we use many-body perturbation theory to study singlet excitons within different solid phases of pentacene and demonstrate that inter-molecular interactions lead to delocalized, charge-transfer-like excitations in the bulk crystalline phase. Using the Bethe-Salpeter two-particle correlation function, we demonstrate that the interplay between intermolecular hybridization, local exchange interactions, and attractive electron/hole interactions controls the nature of the exciton. Additionally, we explore simple models to understand and predict the nature of the excitonic wavefunction, in particular whether it has charge-transfer character. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G33.00007: Exploring the correlation between molecular conformation and optoelectronic properties of conjugated polymers : side-chain versus main-chain electron acceptors Yu-Chen Huang, Ching-I Huang Polythiophene derivatives have been shown among the most promising materials for solar cell application because of their high charge mobility and light absorption. In the mostly studied, a recombination process often occurs, which is mainly due to the fact that the mobility of hole is much lower than that of electron. Hence, research about conjugated polymers containing donor-accepter pairs (such as PT-TPD) becomes quite popular because these materials have narrow band-gaps. Interestingly, these experimental studies have indicated a much more complex correlation between the optoelectronic properties and molecular conformation for polymers with acceptor units on either main or side chain. However, the effects associated with the molecular packing on the resultant chain conformation behavior and thereafter the optoelectronic properties have not been systematically discussed. In order to clarify the effects of the molecular conformation as well as the optoelectronic properties, we employ molecular dynamics and quantum mechanical methods to examine PBTTPD molecules with acceptor unit (TPD) on either main or side chain [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G33.00008: Band structure of polyethylene from many-body perturbation theory Ariel Biller, Sahar Sharifzadeh, Lior Segev, Sohrab Ismail-Beigi, Jeffrey B. Neaton, Leeor Kronik The electronic structure of polyethylene is an important benchmark and the infinite chain limit for the electronic properties of many molecules, monolayers, and oligomers. Therefore, the band structure of the ideal, one-dimensional polyethylene chain has been extensively researched, from both the experimental and the theoretical viewpoints. Despite this extensive effort, to the best of our knowledge agreement between theoretical calculations and the electronic structure obtained from photoelectron spectroscopy could only be obtained using artificial shifting and ``stretching'' of the computed data. Here, we present a quantitative quasi-particle band-structure for polyethylene using many-body perturbation theory. The approach is employed within the $G_{0}W_{0}$ approximation, based on a starting point calculated within the generalized gradient approximation to density functional theory. We compare our calculated band-structure to angle resolved photoemission spectroscopy measurements for various long saturated carbohydrates, demonstrate a much improved agreement with experiment, and discuss remaining discrepancies and their possible origins within both theory and experiment. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G33.00009: Ideal Energy-Level Alignment at the ZnO/P3HT Photovoltaic Interface Keian Noori, Feliciano Giustino Despite the significant progress made during the past decade, hybrid organic-inorganic photovoltaic devices comprising P3HT and ZnO still suffer from low short-circuit currents and moderate open-circuit voltages. These barriers call for a detailed examination of the atomic-scale physics underlying the energy-level alignment at the ZnO/P3HT interface, which is of critical importance if we are to understand what is the maximum ideal open-circuit voltage for this class of solar cell. Here we present the results of a first-principles study [1] on large model interfaces between ZnO and P3HT. Using a combination of density-functional theory (DFT) and post-DFT methods based on hybrid functionals, we analyze the atomic structure and energetics of the semiconductor/polymer interface, as well as the interfacial energy-level alignment. We explore the effect of charge transfer on the ideal open-circuit voltage and identify a failure in the standard electron affinity rule. We determine a maximum ideal open-circuit voltage of $\sim$2 V, which suggests that there is significant room for enhancing the performance of ZnO/P3HT solar cells by optimizing the interface at the nanoscale. \\[4pt] [1] K. Noori, F. Giustino, Adv. Funct. Mater. DOI:10.1002/adfm.201201478 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G33.00010: Interactions between linear organic chromophores: an improved line-dipole approximation Jean-Christophe Denis, Stefan Schumacher, Ian Galbraith Modelling accurately the interactions between chromophores is key for realistic simulations of the dynamics of exciton transfer and annihilation in organic semiconductor films. In the framework of F\"orster theory, it is required to calculate the interaction matrix elements for all relative orientations and separations of chromophores. Therefore a fast and robust approximation is necessary to simulate extended multi-chromophoric systems. From this perspective, using the line-dipole approximation is a very natural approach. However, by a comparative study of the dipole approximation with quantum chemistry (TD-DFT) we demonstrate that the usual line-dipole theory, while successful for short molecules, does not describe well the interactions of longer molecules, where separations are smaller than the interacting chromophores - a limit typically reached in polymer films. As an alternative, we propose an improved way of distributing the sub-dipole moments within a line. This approach remains simple enough to be used in large-scale calculations, while the agreement with the quantum chemistry is significantly improved for all relative orientations. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G33.00011: Identifying molecular features that maximize the second hyperpolarizability Christopher Burke, Timothy Atherton, Joseph Lesnefsky, Rolfe Petschek Designing materials with high nonlinear optical properties is of importance for a variety of applications ranging from optical switching to chemical sensing. A key figure of merit is the intrinsic molecular second hyperpolarizability $\gamma_{int}$, a dimensionless quantity which measures how close a molecule's second hyperpolarizability is to the theoretical maximum. By modeling a molecule as a one dimensional linear piecewise potential, $\gamma_{int}$ was optimized with respect to the shape of the potential. The number of parameters needed to describe the potential was varied. Searches were carried out for extrema in both the positive and negative directions, finding optimum potentials with $\gamma_{int}$ of 0.60 and -0.15. The optimum potentials possess parity symmetry and are specified by a very small number of parameters due to our simple and well chosen representation. Based on the shape of the optimized potentials, we use these results to suggest possible routes for synthesizing molecules with high $\gamma_{int}$. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G33.00012: Photoexcitation and Photochemical Stability of Organic Photovoltaic Materials from First Principles Na Sai, Kevin Leung The development of high efficiency organic photovoltaics (OPV) has recently become enabled by the synthesis of new conjugated polymers with low band gap that allow light absorption over a broader range of the spectrum. Stability of these new polymers, a key requirement for commercialization, has not yet received sufficient attention. Here, we report first-principles theoretical modeling of photo-induced degradation of OPV polymers carried out using ab-initio density functional theory (DFT). We report photooxidation routes and reaction products for reactive species including superoxide oxygen anions and hydroxyl groups interacting with the standard workhorse OPV polymer, poly(3-hexyl-thiophene) (P3HT). We discuss theoretical issues and challenges affecting the modeling such reactions in OPV polymers. We also discuss the application of theoretical methods to low-band-gap polymers, and in particular, the effect of the chemical substitution on the photoexcitation properties of these new polymers. Sandia National Laboratories is a multiprogram laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Deparment of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G33.00013: Understanding the influence of solvent field and fluctuations on the stability of photo-induced charge-separated state in molecular triad D. Balamurugan, Adelia Aquino, Hans Lischka, Francis Dios, Lionel Flores, Margaret Cheung Molecular triad composed of fullerene, porphyrin, and carotene is an artificial analogue of natural photosynthetic system and is considered for applications in solar energy conversion because of its ability to produce long-lived photo-induced charge separated state. The goal of the present multiscale simulation is to understand how the stability of photo-induced charge-separated state in molecular triad is influenced by a polar organic solvent, namely tetrahydrofuran (THF). The multiscale approach is based on combined quantum, classical molecular dynamics, and statistical physics calculations. The quantum chemical calculations were performed on the triad using the second order algebraic diagrammatic perturbation and time-dependent density functional theory. Molecular dynamics simulations were performed on triad in a box of THF solvent with the replica exchange method. The two methods on different length and time scales are bridged through an important sampling technique. We have analyzed the free energy landscape, structural fluctuations, and the long- range electrostatic interactions between triad and solvent molecules. The results suggest that the polarity and re-organization of the solvent is critical in stabilization of charge-separated state in triad. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G33.00014: Quantum dynamics simulations of interfacial charge-transfer in organic dye-sensitized solar cells Luis G.C. Rego, R. da Silva, D.A. Hoff We describe a novel time-dependent quantum-mechanics/molecular-mechanics method for studying electron transfer in dye sensitized semiconductor interfaces, that takes into account the interacting electron-hole quantum dynamics, the underlying nuclear fluctuations and solvation dynamics. We provide a comprehensive investigation of the quantum dynamics, the electronic and the structural properties of prototypical D-$\pi$-A organic dyes sensitizing the TiO2 anatase surface, both in vacuum and solvated by liquid acetonitrile. The organic dyes are comprised of an electron donating moiety and an anchoring acceptor moiety, conjugated by thiophene bridges. Although interfacial electron transfer is very efficient, it is demonstrated that the coupling between the photoexcited electron and the hole delays the electron injection. Simulations demonstrate that the solvent screens the dye from the surface, narrowing the absorption peaks and delaying the electron injection. We have also studied several aspects that are relevant for the recombination process, such as the role played by surface defects and the interaction of redox species with the TiO2 surface, and the effect of additives. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G33.00015: First principles modeling of panchromatic dyes for solar cells applications. Rosa Di Felice, Arrigo Calzolari, Rui Dong, Marco Buongiorno Nardelli The state-of-the-art dye in Gr\"atzel solar cells, N719, exhibits a total solar-to-electric conversion efficiency of 11.2\%. However, it severely lacks absorption in the red and the near infrared regions of the electromagnetic spectrum, which represent more than 70\% of the solar radiation spectrum. Using calculations from first principles in the time-dependent domain, we have studied the electronic and optical response of a novel class of panchromatic sensitizers that can harvest solar energy efficiently across the visible and near infrared regions, which have been recently synthesized [A. El-Shafei, M. Hussain, A. Atiq, A. Islam, and L. Han, J. Mater. Chem. {\bf 22}, 24048 (2012)]. Our calculations show that, by tuning the properties of antenna groups, one can achieve a substantial improvement of the optical properties. [Preview Abstract] |
Session G34: Polymer Blends
Sponsoring Units: DPOLYChair: Julie Albert, North Carolina State University
Room: 342
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G34.00001: An unusual route to develop poly(lactic acid) based materials with deformation-recovery properties Sahas Rathi, David Ng, E. Bryan Coughlin, Shaw Hsu, Charles Golub, Gerald Ling, Mike Tzivanis A novel method based on co-crystallizing polymer blends was developed to obtain Poly(lactic acid) (PLA) based materials with deformation recovery properties. Two sets of blends were studied. One based on the PDLA-soft polymer-PDLA triblock copolymer and PLLA, where D and L refer to the two chiral isomers of PLA, while the other was based on homopolymer blends of PDLA/soft polymer/PLLA having identical chemical composition. The mechanical properties and morphological features of the two sets of blends were completely different. The triblock copolymer/ PLLA blends gave rise to flexible, tough semicrystalline materials while the corresponding homopolymer blends exhibited very low strains at break and high dissipative/dampening properties. The drastically different stereocomplex crystallization kinetics in the two sets of blends led to interspherulitic segregation of the amorphous chains in the triblock blends while intraspherulitic segregation occurred in the homopolymer blends. The presence of significant connectivity between the stereocomplex crystallites formed, in the triblock copolymer/ PLLA blends, was important for the deformation shape recovery characteristics observed. In addition, it was found that the use of ether-ester based plasticizers significantly reduced the glass transition temperature and enhanced the recovery property of the triblock copolymer based PLA blends. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G34.00002: Nanoporous polystyrene samples through the selective removal of low-Mw component in PS/PS blend samples James Forrest, Chad Daley, Sonia Zhang, Sharon Yang, Stefan Idziak We present here a novel technique for producing low density nanoporous polystyrene samples. The method hinges upon the ability to selectively dissolve away the low-M$_w$ component from blend samples which consist of high- and low-M$_w$ atactic polystyrenes with drastically different M$_w$'s. Given the chemical similarity between the two components it is possible to prepare blend samples while avoiding microscopic phase separation. Removal of the low-M$_w$ component then leaves behind a sample with nanoscopic voids on the order of 10's of nm. This is in contrast to porous polymer materials prepared through the removal of chemically distinct polymer species, where larger scale pores are the end result. Tuning of the initial fraction of the low-M$_w$ component allows for variation in the density of the porous material; ellipsometric measurements indicate samples with densities lower than 0.5 g/cm$^3$. Characterization of the samples using ellipsometry, AFM, and X-ray diffraction will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G34.00003: Effect of critical molecular weight of PEO in epoxy/EPO blends as characterized by advanced DSC and solid-state NMR Xiaoliang Wang, Shoudong Lu, Pingchuan Sun, Gi Xue The differential scanning calorimetry (DSC) and solid state NMR have been used to systematically study the length scale of the miscibility and local dynamics of the epoxy resin/poly(ethylene oxide) (ER/PEO) blends with different PEO molecular weight. By DSC, we found that the diffusion behavior of PEO with different Mw is an important factor in controlling these behaviors upon curing. We further employed two-dimensional 13C-\textbraceleft 1H\textbraceright PISEMA NMR experiment to elucidate the possible weak interaction and detailed local dynamics in ER/PEO blends. The CH2O group of PEO forms hydrogen bond with hydroxyl proton of cured-ER ether group, and its local dynamics frozen by such interaction. Our finding indicates that molecular weight (Mw) of PEO is a crucial factor in controlling the miscibility, chain dynamics and hydrogen bonding interaction in these blends. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G34.00004: Effect of Supercritical Carbon Dioxide on Polymer Blend Miscibility Nicholas Young, Sebnem Inceoglu, Andrew Jackson, Stephane Costeaux, Nitash Balsara Supercritical fluids have been investigated as environmentally benign solvents for the processing of polymers on the industrial scale. In this work, we study the effect of supercritical carbon dioxide (scCO$_{\mathrm{2}})$ on the phase behavior of a blend of a random copolymer and a homopolymer. Styrene-acrylonitrile copolymer (SAN) and poly(methyl methacrylate) (PMMA) are known to display lower critical solution temperature-type phase behavior, undergoing a transition from a homogeneous mixture to a phase-separated blend upon heating. Depending on certain parameters such as SAN composition (w$_{\mathrm{AN}})$ and blend fraction ($\phi_{\mathrm{SAN}})$, the miscibility window for the two polymers can be tuned over a significant temperature range by introducing scCO$_{\mathrm{2}}$ into the system. Using small angle neutron scattering, the thermodynamic interactions between SAN and PMMA as described by the Flory-Huggins parameter $\chi $ are shown to be strongly dependent on scCO$_{\mathrm{2}}$ activity. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G34.00005: Microphase-Separated Structures of Gold Nanoparticle Grafted with Two Immiscible Polymers Daisuke Kawaguchi, Tatsuhiro Nakano, Yushu Matsushita It is important to control structures of organic-inorganic hybrid materials to make functional devices. If gold-nanoparticle (AuNP) grafted with two immiscible polymers can self-assemble into microphase-separated structures, it can be expected that AuNP arranges on their own microphase-separated interface in nanometer scale. In this study, we prepared AuNP grafted with polyisoprene (PI) and polystyrene (PS) which were immiscible polymers and investigated their microphase-separated structures by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). The AuNP complexes form various microphase-separated structures such as lamellar, cylindrical and spherical structures with changing polymeric composition. The TEM image and SAXS profile for the AuNP complexes for the symmetric composition shows that PS and PI form lamellar structures and the AuNPs are forced into the PS/PI interface. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G34.00006: Improving the Compatibility between Polystyrene and Polybutadiene by Adding Silica Nanoparticles Yuping Xie, Damien Maillard, Sanat Kumar, Brandon Cash, Brian Benicewicz The compatibility between polystyrene (PS) and polybutadiene (PB) was improved by adding bare silica or PS-grafted silica nanoparticles. The grafting density varies from 0.01 chains/nm$^{2}$ to 0.10 chains/nm$^{2}$. Thin sections are obtained by cryomicrotome at -140 $^{\circ}$C for TEM analysis. Without adding nanoparticles, bulk phase separation occurs for the PS-PB blend, although a few droplets of PS are found presumably due to the viscoelastic phase separation. When the grafting density is less than 0.05 chains/nm$^{2}$, the particles are found to partition between the PS-PB interface and the continuous PS phase. However, when the grafting density is greater than or equal to 0.05 chains/nm$^{2}$, the particles are found to locate only in the dispersed PS phase, and the size of the PS phases decreases with increasing grafting density. Phase inversion also occurs at 70 wt{\%} of PS when the grafting density is fixed at 0.10 chains/nm$^{2}$. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G34.00007: Application of Self-consistent Field Theory to Compressible Polymer Blends: $\chi $, interfacial tension, and anomaly Junhan Cho The self-consistent field (SCF) theory, which was first developed by Helfand, is generalized to compressible polymer blends in order to investigate pressure dependence of interfacial behavior for those systems. A statistical mechanical off-lattice equation-of-state model is incorporated with the formalism and proper SCF equations for saddle points are presented. Taking typical blends as our model system, the relationship between effective Flory-Huggins parameter and interfacial tension is considered on a temperature-pressure window. Anomaly in those physical properties regarding their pressure dependence is discussed. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G34.00008: Morphology and Rheology of the Phase-separating Polybutadiene /Polyisoprene Blend under Small Amplitude Oscillatory Shear Xia Dong, Fasheng Zou, Dujin Wang, Charles C. Han We are mainly focusing on the late stage of phase separation process where the two phases have reach their equilibrium compositions and the droplet dimension or interface area is the key factor in influencing the dynamic moduli. Two kinds of phase-separating structure evolutions of the PB/LPI blend have been investigated. For the near-critical and symmetric blend LPI50, the co-continuous phase-separating structures are observed and lead to a power law behavior of the dynamic storage modulus at low frequencies. With the growth of the co-continuous structure, the storage modulus at low frequency decreases dramatically. For the off-critical and asymmetric blend LPI70, the droplet/matrix two-phase structures appear and result in a rather complex elastic behavior at the mediate and low frequency region. It is observed that with the droplet size increases, the storage modulus at the mediate frequencies generally decreases while the storage modulus at the low frequencies usually increases. Besides, the platform and terminal moduli at a given frequency can be scalable with the phase separation time and the characteristic relaxation time and domain size of the droplets can be obtained by rheology. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G34.00009: Effective Mixing of UHMWPE with Polyethylene: Rheological, Mechanical and Crystallization Behavior of Novel Blends Made by Solid-State Shear Pulverization Mirian Diop, John Torkelson In comparison with conventional polyolefins, ultrahigh molecular weight polyethylene (UHMWPE) possesses outstanding mechanical properties, including impact strength, making it highly desirable for applications ranging from body armor to implants. Unfortunately, UHMWPE comes with a downside: an ultrahigh melt viscosity that renders common melt processes useless for making products from UHMWPE. Attempts to overcome this problem by blending UHMWPE with polyethylene (PE) by conventional melt mixing have been unsuccessful because of the enormous viscosity mismatch and have led to suspensions of UHMWPE particles within a PE matrix. Here, we show the utility of solid-state shear pulverization (SSSP) to effectively and intimately mix UHMWPE/PE blends. Oscillatory shear rheology of blends containing up to 20 wt{\%} UHMWPE shows both the major impact of the UHMWPE fraction in strongly modifying the low shear rate flow behavior and the very muted effect of that fraction on the high shear rate flow behavior. The latter effect indicates that such blends can be processed by melt extrusion and injection molding. Differential scanning calorimetry supports the presence of co-crystallization in these blends. Mechanical properties of these blends, including impact strength, will also be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G34.00010: Assessing the Strength Enhancement of Heterogeneous Networks of Miscible Polymer Blends Carl Giller, Mike Roland At the typical crosslink densities of elastomers, the failure properties vary inversely with mechanical stiffness, so that compounding entails a compromise between stiffness and strength. Our approach to circumvent this conventional limitation is by forming networks of two polymers that: (i) are thermodynamically miscible, whereby the chemical composition is uniform on the segmental level; and (ii) have markedly different reactivities for network formation. The resulting elastomer consists of one highly crosslinked component and one that is lightly or uncrosslinked. This disparity in crosslinking causes their respective contributions to the network mechanical response to differ diametrically. Earlier results showed some success with this approach for thermally crosslinked blends of 1,2-polybutadiene (PVE) and polyisoprene (PI), as well as ethylene-propylene copolymer (EPM) and ethylene-propylene-diene random terpolymer (EPDM), taking advantage of their differing reactivities to sulfur. In this work we demonstrate the miscibility of polyisobutylene (PIB) with butyl rubber (BR) (a copolymer of PIB and polyisoprene) and show that networks in which only the BR is crosslinked possess greater tensile strengths than neat BR over the same range of moduli. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G34.00011: Investigation of Flame Retardancy, Mechanical Properties, and Bicompatibility of Polystyrene Blends Liudi Zhang, Seongchan Pack, Miriam Rafailovich Our research focused on thermal, mechanical properties, and cytotoxicity of Polystyrene system. Brominated Polystyrene was incorporated to replace halogenated Flame Retardant in Polystyrene blends. We have previously shown that ditallow functionalized clays could become nearly universal class of compatiblizers [si-2006]. Here we show that a new type of surface with Resorcinol bis (biphenyl phosphate) (RDP) could achieve the same goal. We demonstrate the strong compatibilization on this highly immiscible system. Furthermore, we show that this system also works well, when a flame retardant Antimony Trioxide (AO) is added. Tensile test, dynamic mechanical analysis, and UL-94 flame test were applied to investigate this system. We found that the amount of AO used could be minimized by adding RDP clay, which could also increase some mechanical properties that Cloisite 20A clay couldn't. Besides, we evaluated the cytotoxicity of Cloiste 20A and RDP clay. These clays were tested both within PS blends and as a monolayer film. Langmuir-Blodgett trough and atomic force microscopy were used to make and check monolayer clay. Confocal laser scanning microscopy was used to assess cell morphology. The results showed RDP clay has potential for biomaterial applications. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G34.00012: Avalanches of dewetting holes in viscoelastic phase separation Changqian Yu, Sung Chul Bae, Steve Granick Textbook ideas fails regarding phase separation of polymer solutions, because of viscoelastic effects. Here with fluorescence microscopy we visualize in real time this process. Quasi two dimensional polymer solutions of polystyrene near the critical concentration are confined between non-wetting surfaces. Apart from a double phase separation induced by rapid hydrodynamic coarsening, we observe novel avalanched dewetting of solvent-enriched holes, not only in the polymer-enriched phase but also near the walls. Strikingly, this occurs at the late stage of the spinodal decomposition. These dewetting holes govern pattern evolution of the phase-separated polymer network. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G34.00013: Coalescence of Pickering emulsion droplets induced by electric-field Guo Chen, Peng Tan, Shuyu Chen, Jiping Huang, Weijia Wen, Lei Xu Combining high-speed photography with electric current measurement, we investigate the coalescence of Pickering emulsion droplets. Under high enough electric field, the originally-stable droplets coalesce via two distinct approaches: normal coalescence and abnormal coalescence. In the normal coalescence, a liquid bridge grows continuously and merges two droplets together, similar to the classical picture. In the abnormal coalescence, however, the bridge fails to grow indefinitely; instead it breaks up spontaneously due to the geometric constraint from particle shells. Such connecting-then-breaking cycles repeat multiple times, until a stable connection is established. In depth analysis indicates that the defect size in particle shells determines the exact merging behaviors: when the defects are larger than a critical size, normal coalescence will show up; while abnormal coalescence will appear for smaller defects. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G34.00014: Arrested of coalescence of emulsion droplets of arbitrary size Badel L. Mbanga, Christopher Burke, Donald W. Blair, Timothy J. Atherton With applications ranging from food products to cosmetics via targeted drug delivery systems, structured anisotropic colloids provide an efficient way to control the structure, properties and functions of emulsions. When two fluid emulsion droplets are brought in contact, a reduction of the interfacial tension drives their coalescence into a larger droplet of the same total volume and reduced exposed area. This coalescence can be partially or totally hindered by the presence of nano or micron-size particles that coat the interface as in Pickering emulsions. We investigate numerically the dependance of the mechanical stability of these arrested shapes on the particles size, their shape anisotropy, their polydispersity, their interaction with the solvent, and the particle-particle interactions. We discuss structural shape changes that can be induced by tuning the particles interactions after arrest occurs, and provide design parameters for the relevant experiments. [Preview Abstract] |
Session G35: Focus Session: Search for New Superconductors I
Sponsoring Units: DMPChair: Horst Rogalla, University of Colorado
Room: 343
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G35.00001: Superconductivity and New Compounds in the Bi-O-S System David Wallace, Kathryn Arpino, W. Adam Phelan, Ken Livi, Che Seaborne, Andrew Scott, Tyrel McQueen Recent reports of superconductivity in Bi$_4$O$_4$S$_3$ and LaO$_{1-x}$F$_x$BiS$_2$ have stimulated interest in a potentially new family of layered superconductors based on BiS$_2$ units. The most interesting structural feature of the reported crystal structure of Bi$_4$O$_4$S$_3$ is that it contains both reduced sulfides (S$^{2-}$) and oxidized sulfates (S$^{6+}$) within the same compound. However, the scattering factors of oxygen and sulfur relative to bismuth make the precise structure of Bi$_4$O$_4$S$_3$ difficult to determine by x-ray diffraction, and limit the ability to compare with theoretical predictions of electronically driven structural distortions. Here we report the results of studies on the structure and physical properties of compounds in the Bi-O-S system through electron diffraction, high resolution transmission electron microscopy, synchrotron x-ray diffraction, and IR spectroscopy, including the discovery of two new ternary Bi-O-S phases. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G35.00002: Phonon-mediated superconductivity in electrostatically and chemically doped single-layer MoS2 Yizhi Ge, Amy Y. Liu MoS$_2$ is a semiconductor with a layered structure that can be exfoliated to make few-layer and single-layer samples. Superconductivity has recently been reported in electrostatically doped few-layer MoS$_2$ samples, with a transition temperature above 9 K, which is higher than the maximum T$_c$ found in intercalated bulk MoS$_2$. Here we report a density functional theory study of electron-phonon coupling in doped single-layer MoS$_2$. With electrostatic n-type doping at levels comparable to those achieved in MoS$_2$ field-effect transistors, the electron-phonon coupling constant is calculated to be consistent with a superconducting T$_c$ of 5-10 K. While deposition of alkali atoms on the surface also introduces carriers into the conduction band, we find that in some cases, it creates significant changes in the electronic structure, leading to a weaker interaction between electrons and phonons. The dependence of the electron-phonon interaction on carrier concentration, for both n-type and p-type doping, will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G35.00003: Superconductivity in Nb$_3$Pd$_{0.75}$Se$_7$ Daniel Rhodes, Qiu Zhang, Bin Zheng, Gang Li, Andhika Kiswandhi, Tiglet Besara, Theo Siegrist, Luis Balicas Here, we report the discovery of superconductivity in the transition metal chalcogenide Nb$_3$Pd$_{0.75}$S$_7$ with a transition temperature $T_c = 1.9 $ K. In extremely thin, needle like single crystals we observe upper critical fields $H_{c2}^b(T \rightarrow 0$ K) $\simeq 14 $ T for fields directed along the needle axis, or the crystallographic \emph{b}-axis. This value is 4 times larger than the expected weak coupling Pauli limiting field. For fields applied along two directions perpendicular to the \emph{b}-axis, we observe considerably smaller but anisotropic upper critical fields. For fields along and perpendicular to the $b$-axis we observe a temperature-dependent anisotropy $\gamma = H_{c2}^b/H_{c2}^{\perp b}$ as large as 6 (as $T \rightarrow T_c$). This behavior suggests that this compound is a multi-band superconductor. The low symmetry of its crystallographic structure implying low electronic dimensionality, coupled to multi-band behavior and very high upper critical fields, suggests an unconventional superconducting ground-state. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G35.00004: The Search for Higher Temperature Superconductors: Two Case Studies Invited Speaker: Malcolm Beasley The recent confluence of optimism in the prospects for higher temperature superconductivity and the documented need for new higher temperature superconductors (if electric power applications above liquid nitrogen temperatures are to be possible) has simulated several focused programs in the search new and improved high-Tc superconductors. In this talk, we review these motivating factors and present the results of two case studies. The first is the study of the high Tc bismuthate superconductors to understand the mechanism of their superconductivity and the factors governing Tc. We find that the bismuthates are moderately correlated materials with a dynamically enhanced electron-phonon interaction that exhibit dimorphism and a sensitivity of Tc to disorder. The second is the study of Cu/CuO interfaces (for which evidence of trace high temperature superconductivity has been reported) at which we find a new proximity effect in which antiferromagnetism is induced into a metal (Cu) by proximity to a charge transfer insulator (CuO). [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G35.00005: Electronic properties of $\alpha $-FeSi$_{2}$ -- single crystal study Wojciech Miiller, Jan Tomczak, Jack Simonson, Greg Smith, Meigan Aronson The discovery of high temperature superconductivity (HTS) in Fe pnictides has simulated a lot of work in field of Fe-based materials. We focus on the tetragonal (high-temperature) form of the iron disilicide, which crystal structure resembles one of the HTS, LiFeAs ($T_{sc}=$18 K). Single crystals of $\alpha $-FeSi$_{2}$ with Fe$_{0.83}$Si$_{2}$ composition were grown and magnetic, transport and heat capacity studies were performed in consistent way. Magnetic susceptibility of $\alpha $-FeSi$_{2}$ increases in a linear fashion with increasing temperature, as was commonly observed among Fe HTS. In a contrast to superconducting pnictides, where $\chi (T)$ $\sim$ $T$ is associated to antiferromagnetic fluctuations, in $\alpha $-FeSi$_{2}$ this behavior is rather related to the electronic structure of this metal. In Fe-based HTS proximity of the SDW instability seems to be crucial for the emergence of superconducting state -- in $\alpha $-FeSi$_{2}$ the experimental data do not find evidence for any strong electronic correlations. Our LDA and DMFT calculations results find low density of states, supporting weakness of correlations and suggest electronic configuration of Fe close to d$^{6}$. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G35.00006: Magnetic Excitations in LaMnPO Daniel McNally, Jack Simonson, Greg Smith, Jeff Lynn, Yang Zhao, Meigan Aronson We performed inelastic neutron scattering experiments on LaMnPO at the BT-7 triple-axis spectrometer at NIST Center for Neutron Research. LaMnPO is an insulating pnictide compound and is antiferromagnetically ordered below T$_{N}$ = 375 K. Constant energy scans were performed above T$_{N}$, and revealed spin-spin correlations in the paramagnetic state with characteristic wavevector Q = 1.6 \AA$^{-1}$, near the antiferromagnetic ordering wavevector Q$_{AFM}$ = 1.55 \AA$^{-1}$. We performed constant wavevector scans above and below T$_{N}$ and these show there is a q-dependent and temperature-dependent energy gap in the magnetic excitations that vanishes at T$_{N}$ = 375 K. Constant energy scans below T$_{N}$ show the peak in the magnetic excitations does not change up to a measured energy transfer of 15 meV, suggesting exchange interactions are quite strong. The magnetic excitations in LaMnPO are similar to those observed in the parent compounds of the iron pnictide superconductors. Research supported by a National Security Science and Engineering Faculty Fellowship by the AFOSR [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G35.00007: Pressurized LaMnPO: antiferromagnetic insulator to magnetic metal J.W. Simonson, M. Pezzoli, J. Guo, J. Liu, L.L. Sun, G. Kotliar, M.C. Aronson It is felt that high temperature superconductivity stems from proximity to an electron delocalization transition, such as the metal-insulator transitions exhibited by the cuprates or the antiferromagnetic transitions of the iron pnictides. We subjected the manganese pnictide LaMnPO to hydrostatic pressures up to 43 GPa, measured x-ray diffraction patterns, and solved the crystal structures at various pressures. We then performed LSDA electronic structure calculations using the observed lattice constants and atomic parameters to obtain the magnitudes of the insulating gap and the ordered state magnetic moment. While the calculations found the gap to close near 10 GPa, the magnetic moment persisted until a structural collapse at 31 GPa. These results imply that the metal-insulator transition and the antiferromagnetic transition are separated within the pressure phase diagram of LaMnPO. We discuss these results in light of the inherent differences between Mott-like and Hund's-like systems. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G35.00008: Complex magnetic properties in multilayer rare earth oxypnictides Jiakui Wang, 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 $\sim$ 2.1 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] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G35.00009: Angle Resolved Photoemission Spectroscopy Study on Layered Oxy-pnictide BaTi2As2O Haichao Xu, Min Xu, Qingqin Ge, Rui Peng, Yan Zhang, Xiangfeng Wang, Xianhui Chen, M. Arita, K. Shimada, Donglai Feng Recently, superconductivity has been discovered in Ba$_{1-x}$Na$_x$Ti$_2$Sb$_2$O, a titanium-based oxy-pnictide with an anti-CuO$_2$ type Ti$_2$O plane and a CDW/SDW anomaly at 54K. The isostructured BaTi$_2$As$_2$O, where signs of CDW/SDW at 200K has been observed, could be viewed as one of the parent compounds of this new family of superconductors. Here we report the Angle Resolved Photoemission Spectroscopy Study on BaTi$_2$As$_2$O. Parallel sections were found in Fermi surface structure, indicating possible nesting condition. The orbital character of bands supports the important role of Ti-Ti direct interaction. No abrupt change was observed at the critical temperature; however, spectral weight change takes place at wide energy scale both above and below the critical temperature, revealing the strong electron-lattice coupling effect in this system. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G35.00010: Investigation of the normal and superconducting states of Ba$_x$Na$_{1-x}$Ti$_2$Sb$_2$O (0 $\geq$ x $\geq$ 0.33) : a pnictide oxide compound with hole doped titanium-oxygen layers M. Gooch, B. Lorenz, P. Doan, Z.J. Tang, J. Tapp, A. M\"oller, A.M. Guloy, D. Pratt, J. Lynn, C.W. Chu The interest in layered transition metal oxides/pnictides was re-ignited by the discovery of the iron pnictides; 2 such as examples are, Na$_2$Ti$_2$Pn$_2$O and BaTi$_2$As$_2$O. Both compounds are comprised of a layered structure and exhibit a SDW/CDW, similar to the iron pnictide parent compounds. It is well established that by suppressing the SDW, superconductivity emerges in pnictides; therefore, can a similar approach be used for these titanium based pnictide oxides? To date, the lowering of the critical temperature for the SDW/CDW has been reported, but no superconductivity was seen for BaTi$_2$As$_2$O. We report the effects of hole doping on BaTi$_2$Sb$_2$O and its influence on the SDW and superconducting states. Initial findings from neutron scattering will also be discussed. Our parent compound, which is similar to the BaTi$_2$As$_2$O in structure, shows a SDW/CDW at 57 K. A systematic lowering of the critical temperature is seen for the SDW/CDW with increased doping. In addition, the superconducting temperature increases up to 6 K. The phase diagram as a function of doping is derived from the normal and superconducting states of the system. The lowering of the critical temperature of the SDW/CDW seems to be the key for the emergence of superconductivity. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G35.00011: Crystal structure and superconducvitiy in BaPbO$_3$/BaBiO$_3$ thin films G.W.J. Hassink, K. Munakata, R.H. Hammond, M.R. Beasley Thin bilayers of BaPbO$_3$ and BaBiO$_3$ were grown on SrTiO$_3$ by e-beam evaporation in the hope of testing the proximity effect route to high T$_{\mathrm{c}}$ superconductivity suggested by Kivelson et al [Phys.Rev.B 78, 094509]. X-ray diffraction measurements show that the bilayers are single-phase, but fully relaxed. Depth-profiling by XPS showed that for a deposition temperature of 500 $^{\circ}$C there is a gradual intermixing of Pb and Bi in the top BaPbO$_3$ layer. This could result in a superconducting Ba(Pb,Bi)O$_3$ film, but XRD points to well-resolved layers. Superconductivity in these films is BCS-like, with $\xi_{GL}(0)$ $\sim$ 10 nm comparable to bulk values. However, the superconductivity was not primarily correlated with the Bi content as determined from surface XPS scans, but by the crystal structure. The superconducting films consistently have a larger unit cell volume, mostly due to larger in-plane lattice constants. This increase coincides with a higher Ba/Pb elemental ratios, which in literature has been linked to the occurrence of the tetragonal form of Ba(Pb,Bi)O$_3$ [Sol.State.Comm. 60, 897-900]. This larger unit cell may result in a lower tilt angle of the oxygen octahedra, which has a positive influence on the superconductivity [Phys.Rev.B 83, 174512]. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G35.00012: Multigap Superconductivity at 5.4 K in $\beta$-Bi$_2$Pd Yoshinori Imai, Fuyuki Nabeshima, Taiki Yoshinaka, Kosuke Miyatani, Atsutaka Maeda, Ryusuke Kondo, Seiki Komiya, Ichiro Tsukada We report the superconducting properties of new multigap superconductor Bi$_2$Pd($\beta$-Bi$_2$Pd; space group: $I4/mmm$)[1]. $\beta$-Bi$_2$Pd single crystals were grown via a melt-growth method. The temperature dependences of the electrical resistivity and the magnetic susceptibility reveal that the superconducting transition occurs at 5.4 K in the $\beta$-Bi$_2$Pd single crystal. This value is greater than the value of 4.25 K reported in the previous paper [2]. Here, it is interesting to note that the $T_{\mathrm{c}}$ of $\beta$-Bi$_2$Pd reported here is almost the same as that of Pd-intercalated Bi$_2$Te$_3$ with a very small superconducting volume fraction ($<1 \%$) in ref. [3], where the possibility that the topological insulator Bi$_2$Te$_3$ can be made into an SC by Pd intercalation between the Bi$_2$Te$_3$ layers is argued. In addition, the temperature dependences of the upper critical magnetic field and the specific heat suggest that $\beta$-Bi$_2$Pd is a multiple-band/multiple-gap superconductor. \\[4pt] [1] Y. Imai \textit{et al.}, J. Phys. Soc. Jpn. 81 (2012) 113708. (arXiv: 1207.5905.)\\[0pt] [2] N.~E. Alekseevski \textit{et al.}, Zh. Eksp. Teor. Fiz. 27 (1954) 125.\\[0pt] [3] Y.~S. Hor \textit{et al.}, J. Phys. Chem. Sol. 72 (2011) 572. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G35.00013: Different doping from apical and planar oxygen vacancies in Ba$_{2}$CuO$_{4-\delta}$ and La$_{2}$CuO$_{4-\delta}$ Thomas Jarlborg, Bernardo Barbiellini, Robert Markiewicz, Arun Bansil First principles band-structure calculations for large supercells of Ba$_{2}$CuO$_{4-\delta}$ and La$_{2}$CuO$_{4-\delta}$ with different distributions and concentrations of oxygen vacancies show that the effective doping on copper sites strongly depends on where the vacancy is located. A vacancy within the Cu layer produces a weak electron doping effect while a vacancy located at an apical oxygen site acts as a stronger electron dopant on the copper layers and gradually brings the electronic structure close to that of La$_{2-x}$Sr$_x$CuO$_{4}$. These effects are very robust and only depend marginally on lattice distortions. Our results show that deoxygenation can reduce the effect of traditional La/Sr or La/Nd substitutions. Our study clearly identifies location of the dopant in the crystal structure as an important factor in doping of the cuprate planes. [Preview Abstract] |
Session G36: Superconductivity: Transport Properties
Sponsoring Units: DCMPChair: Zhuan Xu, Zhejiang University, China
Room: 344
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G36.00001: c-axis resistivity, pseudogap, superconductivity and Widom line in doped Mott insulators Giovanni Sordi, Patrick Semon, K. Haule, A.-M. S. Tremblay Layered doped Mott insulators, such as the cuprates, show unusual temperature dependence of the resistivity. We calculate the c-axis resistivity $\rho_c$ for the two-dimensional Hubbard model within plaquette cellular dynamical mean-field theory. The temperature and doping dependencies of $\rho_c$ are controlled by the first-order transition between pseudogap and correlated metal phases from which superconductivity can emerge. On the large doping side of the transition $\rho_c(T)$ is metallic, while on the low-doping side $\rho_c(T)$ changes from metallic to semi-conducting behavior with decreasing $T$. As a function of doping, the jump in $\rho_c$ across the first-order transition evolves into a sharp crossover at higher temperatures. This crossover coincides with the pseudogap temperature $T^*$ in the single-particle density of states, the spin susceptibility and other observables. Such coincidence in crossovers is expected along the continuation of the first-order transition into the supercritical regime, called the Widom line. This implies that not only the dynamic and the thermodynamic properties but also the DC transport in the normal state are governed by the hidden first-order transition. Refs: G. Sordi et al, Sci. Rep. 2, 547 (2012); G.Sordi et al, arXiv:1211.1702 (2012) [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G36.00002: C-Axis Conductivity of a Layered Superconductor in a Transverse Magnetic Field Shimul Akhanjee, Robert Konik We study the temperature and field dependence of Josephson pair tunneling between parallel superconducting films in the presence of a transverse magnetic field, modeled as a 2+1 dimensional XY model, transformed under the Villain duality. The magnetic field-induced diamagnetism is treated using a variational scheme developed by Benfatto et. al (2007) and the conductivity is described in terms of correlations between quantum phase slip events. We find that the universal point contact conductivity is modified by characteristic power laws. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G36.00003: Low-frequency Electronic Transport Noise in La$_{2-x}$Ba$_x$CuO$_4$ Nanowires Adam Weis, Yizhou Xin, Dale Van Harlingen In the pseudogap regime, high temperature superconductors often exhibit electronic structure, such as charge stripes. Charge stripes pinned to disorder have been predicted to contribute to low-frequency resistance fluctuations when sample dimensions are comparable to the size of stripe domains (Carlson, 2006). We are extending our previous studies of resistance fluctuations in YBa$_2$Cu$_3$O$_{7-\delta}$ (Bonetti, 2004; Caplan, 2010) to thin films of La-based cuprates expected to have a more stable stripe phase, particularly in the regime near 1/8-filling. We present measurements of the low-frequency electronic transport in La$_{2-x}$Ba$_x$CuO$_4$ nanowires fabricated by pulsed laser deposition and lithographic techniques. We discuss temperature dependence of the power spectral density and its relevance to correlated electron phases above T$_c$. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G36.00004: Hall effect and ghost critical field in disordered superconducting films Nicholas Breznay, Aharon Kapitulnik abstract-We observe a peak in the Hall resistance ocurring at a magnetic field H* in superconducting disordered thin films. Below the zero-field transition temperature Tc0, H* exactly tracks the upper critical field, Hc2, all the way to zero temperature. Near Tc0, H* becomes vanishingly small, while above Tc0 the peak again scales to higher fields as the temperature is further increased. Companion measurements of the fluctuation magnetoconductivity at temperatures above Tc0 allow precise and indpendent determination of the 'ghost critical field' Hgc, the field scale for suppression of superconducting fluctuations above Tc0. We find that H* and Hgc are distinct quantities but show similar scaling above Tc0, and compare these results to similar findings in studies of the Nernst effect in thin films [A. Pourret et al. Phys. Rev. B 76, 214504, (2007)] and high-temperature superconductors [J. Chang et al. Nature Physics 8, 751 (2012)]. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G36.00005: Comparing the specific heat to the cyclotron mass in two dopings of YBCO in the underdoped regime Jonathon Kemper, Scott C. Riggs, O. Vafek, A. Migliori, J.B. Betts, B.J. Ramshaw, R.D. McDonald, F.F. Balakirev, R. Liang, D.A. Bonn, G.S. Boebinger Two underdoped High T$_{\mathrm{c}}$ superconductors, YBa$_{2}$Cu$_{3}$O$_{6.51}$ and YBa$_{2}$Cu$_{3}$O$_{6.56}$, show finite electronic specific heat even in vanishing magnetic field and temperature as well as quantum magneto-oscillations at high magnetic~ fields. Previous results show the high-field electronic specific heat up to 45 T to be a sum of contributions from superconducting vortices and quantum magneto-oscillations, the latter a signature of an un-gapped Fermi surface. The vortex contribution appears as a smooth square-root field dependence of the Sommerfeld coefficient that shows no sign of diminished superconductivity up to 45 T. We present new results which allow a comparison between the cyclotron mass from high field experiments to the zero field Sommerfeld coefficient in the same crystal. The discussion will include implications of the new results on the interpretation of the previous data. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G36.00006: Exfoliated Bi2Sr2CaCu2O8+x thin flakes for electronic transport experiments Menyoung Lee, Michael Neumann, David Goldhaber-Gordon, Luke Sandilands, Kenneth Burch, Zhijun Xu, Alina Yang, Genda Gu Bismuth-based cuprates are the model high-temperature superconductor of choice for experimental probes that are spectroscopic and sensitive to the surface (STM, ARPES), while studies of transport properties have typically focused on rare-earth element-based compounds. We will first discuss preparation methods for and characterization of tape-exfoliated single crystal thin (few to tens nm) flakes of the Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ compound, in particular focusing on protocols designed to produce exposed conducting surfaces that are atomically smooth over several microns, and addressing the factors that influence the surface morphology and Raman scattering properties of BSCCO thin flakes. In addition, data from electronic transport measurements, aimed at observing a modulation of critical temperature and an insulator to superconductor transition as a function of hole density in the compound, will be presented. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G36.00007: Nonlinear transport properties of La$_2$CuO$_4$/ La$_{\mathrm{2-x}}$Sr$_{\mathrm{x}}$ CuO$_4$ heterostructures in the resistive state Bo Wen, Roman Yakobov, M.P. Sarachik, Sergey Vitkalov, A. Bollinger, I. Bozovic, A. Sergeev We report measurements of the nonlinear transport properties of oxide heterojunctions La$_{2}$CuO$_{4}$/La$_{\mathrm{2-x}}$Sr$_{\mathrm{x}}$CuO$_{4}$ in the vicinity of the superconducting transition. The transition occurs over a wide temperature range (7-15K) and shifts to lower temperatures in the presence of a magnetic field, as expected. Strongly nonlinear behavior is observed for the $V-I $characteristic. At low bias currents the nonlinearity has a non-thermal origin close to the transition temperature and is strongly sensitive to magnetic fields. Above the middle of the superconducting transition the nonlinear behavior is consistent instead with electron heating with a value of electron-phonon thermal conductance of $\sim$ 10$^{-6}$ W/K per square micron, which is significantly smaller than the thermal conductance of Nb and NbN ultrathin superconducting films. Our results indicate that this novel low-dimensional superconducting material shows great promise for substantial enhancement of direct detection and wide band mixing of radiation. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G36.00008: Tuning the static spin stripe phase and supercoductivity in La$_{2-x}$Ba$_{x}$CuO$_{4}$ ($x$ = 1/8) by hydrostatic pressure Zurab Guguchia, Alexander Shengelaya, Alexander Maisuradze, Giorgi Ghambashidze, Ekaterina Pomjakushina, Kazimierz Conder, Hugo Keller Muon spin rotation (${\mu}$SR) and magnetization measurements were performed in stripe-stabilized La$_{1.875}$Ba$_{0.125}$CuO$_{4}$ as a function of pressure up to $p$ ${\simeq}$ 2.25 GPa. At ambient pressure this system exhibits static spin stripe order below $T{\rm so}$ ${\approx}$ 30 K. Zero-field ${\mu}$SR experiments indicate that the volume fraction ${\omega}$ of static spin-stripe order significantly decreases with increasing $p$, while the size of the ordered moment and the ordering temperature remain constant. The magnetization measurements show that the sample exhibits a tiny superconducting (SC) volume fraction at ambient pressure. However, by the application of pressure the SC phase volume increases proportionally to the non-magnetic volume fraction (1-${\omega}$). These results indicate that in La$_{1.875}$Ba$_{0.125}$CuO$_{4}$ magnetism and superconductivity occur in mutually exclusive spatial regions. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G36.00009: Spin fluctuations on the verge of Mott localization Himadri Barman, N.S. Vidhyadhiraja We investigate the effects of local, transverse spin fluctuations on transport and thermodynamic quantities in the proximity of a paramagnetic Mott transition. Low temperature Fermi liquid coherence is seen to cross over to universal power laws in resistivity, optical conductivity and specific heat at higher temperatures and frequencies. Striking agreement with the normal phase properties of LSCO and BSSCO series of high temperature superconductors (HTS) is found. We conclude that the anomalous properties of HTS above the superconducting dome originate from spin fluctuation scattering concomitant to Mott localization. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G36.00010: Resistivity of Sr$_2$ RuO$_4$ under uniaxial stress Daniel Brodsky, Clifford Hicks, Andrew P. Mackenzie We report high precision resistivity measurements on single crystals of Sr$_2$RuO$_4$ under in-plane uniaxial stress. A specially built probe enables the stress to be varied continuously whilst the sample is at low temperature. The needle-like shape of the samples ensures that the strain is homogeneous in the region probed. We compare results for different directions of applied stress relative to the crystal axes. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G36.00011: Circuit influence on electron transport in hybrid superconductor---normal-metal nanostructures Vladimir Bubanja, Shuichi Iwabuchi We study the effects of the circuit impedance on transport properties of hybrid structures consisting of a superconductor tunnel-coupled to two normal-metal electrodes. At subgap conditions (i.e. at low voltages and temperatures with respect to the superconducting energy gap) the dominant transport mechanism of Cooper pairs is by Andreev reflection. We derive the analytic expressions for the direct and crossed Andreev current, and the elastic cotunneling current. The results can be used for improving the accuracy of hybrid devices employed in electrical metrology and for noise measurements in quantum information processing. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G36.00012: Micro-mechanical and Structural Properties and Activation Energy Calculation of Nd$_{2}$O$_{3}$ Added Bi$_{2}$Sr$_{2}$Ca$_{1}$Cu$_{2}$O$_{y}$ Superconducting System Ozgur Ozturk, Elif Asikuzun, Murat Coskunyurek, Seydanur Kaya, Mustafa Yilmazlar, Gurcan Yildirim, Cabir Terzioglu Nd added Bi-2212 superconducting samples with x$=$0, 0.001, 0.005, 0.01, 0.05 and 0.1 were prepared by conventional solid state reaction method and annealed at 840$^{\circ}$C for 72 h. For the comparison, an undoped sample was produced to the same conditions. The effects of Nd addition on structural and micromechanical properties were systematically investigated. The volume fraction, lattice parameters, crystal structure and grain size of the samples were characterized using the X-ray diffractometer and Scanning Electron Microscope. In addition, this study includes determination of the activation energy of Nd in the Bi-2212 system using the magnetoresistivity measurements. And also, we were investigated the mechanical properties for all samples using the Vickers microhardness measurements. Microhardness values of the samples decrease with increasing adding and applied load. The Vickers hardness of the samples studied, exhibits the typical indentation size effect (ISE). [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G36.00013: Analysis of Indentation Size Effect (ISE) Behavior in Low-Load Vickers Microhardness Testing of (Sm123)$_{\mathbf{1-x}}$(Nd123)$_{\mathbf{x}}$ Superconductor System Sukru Celik, Ozgur Ozturk, Elvan Co\c{s}kun, Elif Asikuzu, Kemal Ozturk, Cabir Terzioglu Indentation size effect (\textit{ISE}) for (Sm123)$_{1-x}$(Nd123)$_{x}$ superconducting samples which were fabricated by the solid state reaction technique for values of x$=$0.00, 0.05, 0.10, 0.20, and 0.30 was investigated by analyzing the theoretical models. When the experimental data of a number of single crystals which have the different crystal structure and different chemical bonding inside the polycrystallined samples were analyzed with the \textit{ISE} models, the sample encountering with resistance and elastic deformation was observed as well as plastic deformation. The microhardness values on different surfaces of materials were calculated by using Meyer Law, \textit{PSR} model, MPSR model, \textit{EDP} (Elastic / Plastic Deformation model) model and the Hays-Kendall (\textit{HK}) approach. The results showed that the Hays- Kendall approach was determined as the most successful model. Furthermore, XRD and SEM measurements were analyzed for superconducting properties of (Sm123)$_{1-x}$(Nd123)$_{x}$ superconductor system. The results showed that while Nd123 concentration is increasing, microhardness values at the minimum load and averaged plateau region of load. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G36.00014: Investigation of Indentation Size Effect (ISE) of Bi$_2$Sr$_2$CaNd$_{\mathrm{x}}$Cu$_2$O$_{\mathrm{y}}$ Superconducting System using Vickers Microhardness Test Method Elif Asikuzun, Ozgur Ozturk, Seydanur Kaya, Murat Coskunyurek, Nevin Soylu, Ahmet Varilci, Cabir Terzioglu In this work, the effects of Nd doping on the structural and mechanical properties of the samples were analyzed. Nd$_{2}$O$_{3}$ doped Bi-2212 superconductors were obtained using solid state reaction method. Microhardness measurements were made to analyze the mechanical properties. XRD and SEM measurements were done for determination of crystal structure and surface morphology and calculation of the lattice parameters. The Vickers microhardness was calculated for undoped and doped samples. The experimental results of the microhardness measurements were analyzed using Kick's Law, PSR (proportional specimen resistance), modified PRS (MPSR) and Hays--Kendall (HK) approach. The microhardness values of the samples decreased with increasing Nd doping and applied load. Finally, the Hays-Kendall \textit{(HK)} approach was determined as the most successful model describing the mechanical properties of our samples. [Preview Abstract] |
Session G37: Focus Session: Fe-based Superconductors: RPA and Beyond/Gap Structure
Sponsoring Units: DMP DCOMPChair: Peter Hirchfeld, Univ. Florida
Room: 345/346
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G37.00001: Development of orbital and spin fluctuations in Fe-based superconductors based on the self-consistent vertex correction (SC-VC) method Invited Speaker: Hiroshi Kontani To achieve unified understanding of the whole phase diagram of Fe-based superconductors, we analyze the multiorbital Hubbard model going beyond the random phase approximation (RPA). The 2nd-order non-magnetic structure transition at $T_{\mbox{S}} (>T_{\mbox{N}} )$, nematic order as well as large softening of shear modulus $C_{66} $ indicate the strong orbital fluctuations in the normal state. However, only the spin fluctuations develop within the RPA. To resolve this discrepancy, we develop the self-consistent vertex correction (SC-VC) method beyond the RPA, and find the mutual development of orbital and spin fluctuations due to the Aslamazov-Larkin VC, which describes the Kugel-Khomskii type spin-orbital coupling [1]. We find that (i) both the antiferro-orbital and ferro-orbital ($=$nematic) fluctuations develop for $J/U > 0.17$ by including the self-energy correction ($=$SC-V$\Sigma $ method): Both fluctuations contribute to the s-wave superconductivity, and the nematic fluctuations are the origin of the structure transition and the softening of $C_{66} $. (ii) The coexistence of orbital and spin fluctuations can induce the loop-shape nodes on the electron-pockets in BaFe$_{2}$(As,P)$_{2}$, as well as (impurity-induced) smooth $s_{\pm } \to s_{++} $crossover with high $T_{\mathrm{c}}$ [2,3]. Also, the horizontal node on the $z^{2}$-orbitlal hole-pocket predicted by RPA is filled by the inter-orbital fluctuations due to the VC, consistently with laser ARPES and other bulk experiments of 122 compounds. (iii) The same orbital nematic fluctuations are obtained in a simple two-orbital model for Sr$_{3}$Ru$_{2}$O$_{7}$, not only by the SC-VC method [4] but also by the two-dimensional RG method [5]. Therefore, the VC is expected to be the origin of novel orbital physics in various multioritital $d$- and $f$-electron systems.\\[4pt] [1] S. Onari and H. Kontani, PRL \textbf{109}, 137001 (2012).\\[0pt] [2] H. Kontani and S. Onari, PRL \textbf{104}, 157001 (2010).\\[0pt] [3] S. Onari and H. Kontani, PRL \textbf{103}, 177001 (2010).\\[0pt] [4] Y. Ohno, M. Tsuchiizu, S. Onari, and H. Kontani, arXiv:1209.3629.\\[0pt] [5] M. Tsuchiizu, S. Onari, and H. Kontani, arXiv:1209.3664. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G37.00002: Theoretical Study of 3D Superconducting Gap Structure in Iron Based Superconductors Tetusro Saito, Seiichiro Onari, Hiroshi Kontani The mechanism and symmetry of the superconducting (SC) gap in Fe-based superconductors have been studied actively, and both the spin fluctuation-mediated $s_{\pm}$-wave SC state and orbital fluctuation-mediated $s_{++}$-wave SC state had been proposed. To obtain important information on the pairing mechanism, we analyze the Eliashberg gap equation using the 3-dimensional 10-orbital model. When we perform the RPA by considering only the Coulomb interaction, only the spin fluctuations develop, and the SC gap of $z^2$-orbital dominant part on the hole pockets is almost zero. The resultant horizontal node is inconsistent with several measurements. However, the orbital fluctuations develop by introducing the quadrupole interaction $g$ (due to the vertex correction) and it is found that (i) the horizontal node disappears and (ii) the crossover from $s_{\pm}$-state to $s_{++}$-state is realized. During the crossover, we obtained the loop-node structures on the electron pockets, which are actually observed by ARPES measurements in BaFe$_2$(As,P)$_2$. We expect that optimally doped BaFe$_2$(As,P)$_2$ is in the crossover regime between s$_{++}$-state and s$_{\pm}$-state. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G37.00003: Pressure dependence of critical temperature of bulk FeSe from spin fluctuation theory Peter Hirschfeld, Andreas Kreisel, Yan Wang, Milan Tomic, Harald Jeschke, Anthony Jacko, Roser Valenti, Thomas Maier, Douglas Scalapino The critical temperature of the 8K superconductor FeSe is extremely sensitive to pressure, rising to a maximum of 40K at about 10GPa [1]. We test the ability of the current generation of fluctuation exchange pairing theories to account for this effect, by downfolding the density functional theory electronic structure for each pressure to a tight binding model. The Fermi surface found in such a procedure is then used with fixed Hubbard parameters to determine the pairing strength using the random phase approximation for the spin singlet pairing vertex. We find that the evolution of the Fermi surface captured by such an approach is alone not sufficient to explain the observed pressure dependence, and discuss alternative approaches.\newline [1] S. Margadonna, \textit{et al.}, Phys. Rev. B \textbf{80}, 064506 (2009); S. Medvedev, \textit{et al.}, Nat. Mater. \textbf{8}, 630 (2009). [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G37.00004: Effect of realistic finite-size impurities on Tc in Fe-based superconductors Youichi Yamakawa, Seiichiro Onari, Hiroshi Kontani Recently, the phase diagram of LaFeAsO$_{1-x}$H$_x$ is reported and two-dome structure of superconducting state, first dome for $x<0.2$ with $T_c^{\rm max}=29$K and second dome for $0.2< x<0.5$ with $T_c^{\rm max}=36$K, has attract great attention[1]. To clarify the origin of the second superconducting dome, we construct tight-binding models for each doping level $x$ and investigate the spin and orbital fluctuations based on the random phase approximation. We fined that the nesting between electron-hole Fermi surfaces is monotonically weakened with $x$ and spin density wave order with momentum ${\bf q}=(\pi,\pi)$ disappears. In the over-doped regime for $x>0.2$, however, the nesting between electron-electron Fermi surfaces increases, and an incommensurate spin density wave order emerges. The orbital order also shows a re-entrant phase diagram. The spin and orbital fluctuations due to the incommensurate nesting would then be the origin of the second superconducting dome reported in the H-over-doped LaFeAsO. The obtained electronic states for $x=0.5$ are very similar to that for KFe$_2$Se$_2$[2], which is a heavily electron doped system(0.5 electron/Fe). [1] S. Iimura, {\it et al.}, Nat. Commumn. {\bf 3}, 943 (2012). [2] T. Saito, {\it et al.}, Phys. Rev. B {\bf 83}, 140512 (2011). [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G37.00005: Magnetic degeneracy and C$_4$ symmetric magnetic phase in iron-based superconductors Ilya Eremin, Andrey V. Chubukov We analyze the magnetic phase diagram of iron pnictides by going beyond Ginzburg-Landau expansion and solving full non-linear equation for magnetic order parameter within itinerant model with hole pockets centered around (0,0) and electron pockets centered at ($\pi $,0) and (0, $\pi )$ in the unfolded Brillouin zone. We extend our previous analysis of the itinerant model to higher carrier concentrations when Fermi surface nesting is weaker, and find that the phase diagram is richer than previously anticipated. We show that, in addition to stripe SDW order which breaks C$_{4}$ rotational symmetry, there exists, in some range of parameters, a different type of SDW order which preserves C$_{4}$ symmetry. The order parameter in this new phase is equally weighted combination of SDW components with wavevectors Q$_{\mathrm{X}}=(\pi $,0) and Q$_{\mathrm{Y}}=$(0,$\pi )$. The new phase emerges from the stripe phase via a second transition at T \textless\ T$_{\mathrm{N}}$. Such a phase is highly unlikely in the orbital scenario for magnetism in iron-based superconductors, because in this scenario the breaking of the symmetry between Q$_{\mathrm{X}}$ and Q$_{\mathrm{Y}}$ is the pre-condition for the magnetic transition. We discuss recent experiments in which the second low-T magnetic phase which does not break the symmetry between Q$_{\mathrm{X}}$ and Q$_{\mathrm{Y}}$ has been detected and argue that its observation is a strong indication that the magnetic order is of magnetic rather than of orbital origin. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G37.00006: Pairing strength and gap functions in multiband superconductors: 3D effects Andreas Kreisel, Yan Wang, Peter Hirschfeld, Thomas Maier, Douglas Scalapino We examine the superconducting pairing symmetry in Fe-based superconductors using spin-fluctuation pairing theory. It has been shown in multi-orbital models that the different matrix elements of the pairing vertex are essential in determining the symmetry. In our approach we perform a 10-orbital spin-fluctuation calculation to account for the full matrix elements and the 3 dimensional character of the bandstructure which is most important in the systems under consideration (LiFeAs and K$_{\mathrm{x}}$Fe$_{\mathrm{2-y}}$Se$_2$). Our approach contains both, the deviations from tube-like Fermi surface that also allows different pairing strengths in the z-direction, and the hybridization of the Fermi surface. Starting from the tight-binding Hamiltonian corresponding to the real crystal cell, we find several competing 3D gap structures and compare with ARPES experiments. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G37.00007: Spin fluctuation theory of pairing in AFe$_2$As$_2$ Thomas Maier, Yan Wang, Andreas Kreisel, Peter Hirschfeld, Douglas Scalapino The absence of Fermi surface hole pockets in the alkaline iron selenides has challenged the usual spin fluctuation arguments leading to the most popular s+- superconducting gap structure in the closely related iron pnictide superconductors. Thus they provide a new venue to study the nature of pairing in a system with only electron pockets. Here, we present the results of spin fluctuation calculations of the pairing interaction based on realistic descriptions of the bandstructure of the iron selenides. In particular, we will discuss the predictions of these studies with regard to the gap structure in the absence of hole pockets, its consequences for the magnetic neutron scattering spectrum, and their evolution with doping. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G37.00008: Phase diagram of doped BaFe$_2$As$_2$ superconductor under broken C$_4$ symmetry Yuan-Yen Tai, Jian-Xin Zhu, Matthias J. Graf, C.S. Ting We developed a minimal multi-orbital tight-binding model with realistic hopping parameters that breaks the symmetry of the point group by lowering it from C$_4$ to D$_{2d}$. The model accurately describes the Fermi surface evolution of the electron, BaFe$_{2-x}$Co$_x$As$_2$, and hole, Ba$_{1-y}$K$_y$Fe$_2$As$_2$, doped compounds. Since in this class of materials the competing superconductivity and co-linear antiferromagnetism rely on the evolution of the Fermi surface with doping, we investigated the phase diagram with a mean-field t-U-V Bogoliubov-de Gennes equation. Our results match the experimental electron-doped phase diagram. Furthermore, the model is in reasonable agreement with the experimental hole-doped part with only one set of t, U and V parameters. The self-consistently calculated superconducting order parameter exhibits s+/-d pairing symmetry in the entire doping range. It is the subtle result of competing interactions in the multi-orbital mean-field Hamiltonian based on the broken C$_4$ symmetry and might be observable in STM and ARPES experiments. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G37.00009: Hidden ($\pi$,0) instability as an itinerant origin of bicollinear antiferromagnetism in Fe$_{1+x}$Te Yu-Zhong Zhang, Ming-Cui Ding, Hai-Qing Lin By calculating orbitally resolved Pauli susceptibilities within maximally localized Wannier orbital basis transformed from first principles band structures, we find that magnetism in Fe$_{1+x}$Te still has its itinerant origin even without Fermi surface nesting, provide orbital modulation of particle-hole excitations are considered. This leads to strong magnetic instabilities at wave vector ($\pi$,0)/(0,$\pi$) in d$_{xz}$/d$_{yz}$ orbitals that are responsible for bicollinear antiferromagnetic state as extra electrons donated from excess Fe are considered. Magnetic exchange coupling between excess Fe and in-plane Fe further stabilizes the bicollinear antiferromagnetic order. Our results reveal that magnetism and superconductivity in iron chalcogenides may have different orbital origin, as Pauli susceptibilities of different orbitals evolves differently as a function of concentration of excess Fe and height of chalcogen atom measured from iron plane. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G37.00010: Fluctuation of Valley Density Wave in Iron Pnictides Jian Kang, Zlatko Tesanovic We studied the fluctuations within the U(n)*U(n) [1] theory, which was developed to explain the magnetic and structural transitions in the parent compound of iron pnictides. The self-energy of the fermion contains singularity in low energy scale. It behaves similar to marginal Fermi liquid theory and depends on n. The optical conductivity and spin lattice relaxation time are calculated and compared with some experiment on ``pseudogap'' in iron pnictides. More experiments are proposed to provide a direct view our U(4)*U(4) theory being assembled as one moves from low to high energies. \\[4pt] [1] J. Kang and Z. Tesanovic, Phys. Rev. B 83, 020505(R) (2011). [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 2:15PM |
G37.00011: Superconducting energy gap and nodes in the doped BaFe2As2 system Invited Speaker: Shik Shin Angle resolved photoemission spectroscopy (ARPES) is very powerful to know the solid state properties. We have developed low-temperature high-resolution laser-based ARPES system and recently achieved the highest energy resolution of $\sim$ 100 $\mu $eV and the lowest sample temperature of $\sim$ 1.0 K. We would like to show our recent results of superconducting-gap measurements on the iron-based superconductors by laser-ARPES, mainly for Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ [1-3]. Little Fermi-surface dependent superconducting gap sizes are found for the Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ that has the maximum Tc around 40K [1]. Inter-orbital interaction is important as well as intra-orbital interaction. On the other hand, KFe$_{2}$As$_{2}$ is an extremely hole-doped compound in Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$ system and no longer has electron Fermi surfaces. Regardless of this, KFe$_{2}$As$_{2}$ still exhibits superconductivity with Tc of 3.4 K and the existence of nodes in its superconducting gap has been suggested by the several transport measurements. Our ultrahigh-resolution laser ARPES [2] unveils that KFe$_{2}$As$_{2}$ is a nodal s-wave superconductor with highly unusual FS-selective multi-gap structure: a nodeless gap on the inner FS, an unconventional gap with octet-line nodes on the middle FS, and an almost-zero gap on the outer FS. This gap structure may arise from the frustration between competing pairing interactions on the hole FSs causing the eightfold sign reversal. Our results suggest that the A1g superconducting symmetry is universal in iron-pnictides, in spite of the variety of gap functions.\\[4pt] [1] Shimojima \textit{et al.}, Science \textbf{332} (2011) 564.\\[0pt] [2] Okazaki \textit{et al.}, Science \textbf{337} 1314 (2012).\\[0pt] [3] Malaeb \textit{et al}., Phys. Rev.B\textbf{86} (2012) 165117. [Preview Abstract] |
Session G39: Matter at Extreme Conditions: Experiment
Sponsoring Units: GSCCM DCOMP DMPChair: Renata Wentzcovitch, University of Minnesota
Room: 348
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G39.00001: Powder X-ray diffraction of dynamically-compressed tantalum and lead in the terapascale pressure regime Amy Lazicki, Jon Eggert, Ryan Rygg, Federica Coppari, Dayne Fratanduono, Dave Braun, Gilbert Collins We will present advances in powder x-ray diffraction methods for measuring crystal structure in the Terapascale pressure regime on laser ramp-compressed solids, and will show results for dynamically compressed tantalum up to 750 GPa and lead up to 600 GPa. Both of these systems show signatures of high pressure phase transitions not yet seen in static high pressure studies. We will discuss the possible effects of temperature and kinetics on high pressure phase transitions in ramp-compressed materials. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G39.00002: Kinetics studies across the melting line of metals using \textit{dynamic}-DAC Jing-Yin Chen, Zsolt Jenei, Hyunchae Cynn, Magnus Lipp, William Evans We utilize the time-resolved synchrotron x-ray diffraction and \textit{in-situ }optical spectroscopy to study the dynamic properties of several metals across the melting lines under different compression rates at different temperatures. The dynamic properties of metals across the pressure-induced liquid-solid transitions, such as the nucleation time and the mechanism of recrystallization are lacking. Time scales for metal nucleation and growth rates are challenging to obtain. X-ray diffraction under rapid compression will provide unique insight to understand the melting and crystallization mechanisms. In addition, the dynamical pressure changes can dramatically influence the microstructure and even phase boundaries, further affecting the properties of metals. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory in part under contract W-7405-Eng-48 and in part under Contract DE-AC52-07NA27344. This work was funded by the Laboratory Directed Research and Development Program at LLNL under project tracking code 11-ERD-046. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G39.00003: Experimental Measurement of Speeds of Sound in Liquid Carbon Monoxide and Development of High-Pressure, High-Temperature Equations of State Joseph Zaug, Jeffrey Carter, Sorin Bastea, Laurence Fried We report the adiabatic sound speeds for liquid carbon monoxide along two isotherms, from 0.17 to 2.13 GPa at 297 K and from 0.31 to 3.2 GPa at 600 K. The carbon monoxide was confined in a resistively heated diamond-anvil cell and the sound speed measurements were conducted \textit{in situ} using a recently reported variant of the photoacoustic light scattering effect. The measured sound speeds were then used to parameterize a polarized exponential-6 intermolecular potential for carbon monoxide. P$\rho $T thermodynamic states, sound speeds, and shock Hugoniots are calculated using the newly parameterized intermolecular potential and compared to previously reported experimental results. Additionally, we present an analytical equation of state for carbon monoxide that was generated by fitting to a grid of calculated P$\rho $T states over a range of 0.1-10 GPa and 150-2000 K. * This work was performed under the auspices of the U.S. Department of Energy jointly by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G39.00004: More than doubled ambient superconducting transition in a heavily compressed aromatic hydrocarbon Xiao-Jia Chen, Takaki Muramatsu, Wenge Yang, Viktor Struzhkin Struzhkin, Ho-kwang Mao, Qingzhen Huang, Zhen-Xing Qin, X.F. Wang, J.J. Ying, P. Cheng, Z.J. Xiang, X.H. Chen Exploring superconductivity at higher transition temperatures $T_{\mathrm{c}}$s in light elements such as hydrogen and carbon and their organic compounds has long been an attractive issue. Cation-doped aromatic hydrocarbons have been discovered to be superconductive with an increasing $T_{\mathrm{c}}$ by adding more hydrocarbon rings. Here we present a discovery of an enhancement of $T_{\mathrm{c}}$ from the ambient 4.8 K to 12.2 K in compressed Ba$_{\mathrm{1.5}}$Phenanthrene by magnetic susceptibility measurements up to 61 GPa. In contrast to the existence of superconductivity within a very narrow pressure range in fullerides, we find that this organic compound maintains superconductivity at more than doubled ambient $T_{\mathrm{c}}$ even at 61 GPa. A phase transition in the region between 3.0 and 5.4 GPa and an orientational disorder at around 28 GPa are identified using synchrotron X-ray diffraction technique. A nice correction between $T_{\mathrm{c}}$ and the angle between two crystal axes indicates the essential role of electronic correlations. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G39.00005: Stable External Heating of Diamond Anvil Cell: Examples and Issues Hyunchae Cynn, Zsolt Jenei, Jesse Smith, Changyong Park, Hans-Peter Liermann, William Evans While laser heating has been applied to successfully study materials at extreme conditions, external heating also has been extensively developed and applied for material studies at moderate temperature below $\sim$1000 K at high pressures. We have tested various external heating methods to accomplish stable heating at high pressures. Experimental measurements using two mini coil heaters at 900 K and 580 K to 100 GPa and 185 GPa, respectively and isobaric heating at 95 GPa up to 1000 K will be presented. New measurements using a graphite gasket heater will be compared along with internal heating methods. We will present comparison among different external heating methods and different temperature measurements using various examples. HP-CAT is supported by CIW, CDAC, UNLV, and LLNL through funding from DOE-NNSA, DOE-BES, and NSF. The APS is supported by DOE-BES under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G39.00006: Homoepitaxial Boron Doped Diamond Anvils as Heating Elements in a Diamond Anvil Cell Jeffrey Montgomery, Gopi Samudrala, Spencer Smith, Georgiy Tsoi, Yogesh Vohra, Samuel Weir Recent advances in designer-diamond technology have allowed for the use of electrically and thermally conducting homoepitaxially-grown layers of boron-doped diamond (grown at 1200 $^{\circ}$C with a 2{\%} mixture of CH$_{4}$ in H, resulting in extremely high doping levels $\sim$ 10$^{20}$/cm$^{3})$ to be used as heating elements in a diamond anvil cell (DAC). These diamonds allow for precise control of the temperature inside of the diamond anvil itself, particularly when coupled with a cryostat. Furthermore, the unmatched thermally conducting nature of diamond ensures that no significant lateral gradient in temperature occurs across the culet area. Since a thermocouple can easily be attached anywhere on the diamond surface, we can also measure diamond temperatures directly. With two such heaters, one can raise sample temperatures uniformly, or with any desired gradient along the pressure axis while preserving optical access. In our continuing set of benchmark experiments, we use two newly created matching heater anvils with 500$\mu $m culets to analyze the various fluorescence emission lines of ruby microspheres, which show more complicated behavior than traditional ruby chips. We also report on the temperature dependence of the high-pressure Raman modes of paracetamol (C$_{8}$H$_{9}$NO$_{2})$ up to 20 GPa. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G39.00007: Phase transition dynamics in high-pressure VO$_{2}$ Wen-Pin Hsieh, Mariano Trigo, Zhao Zhao, David A. Reis, Wendy L. Mao Vanadium dioxide VO$_{2}$ is a prototypical strongly correlated material which presents an insulator-metal transition at both ambient and high pressures. We use synchrotron X-ray diffraction combined with a diamond anvil cell to determine the pressure-temperature phase diagram of VO$_{2}$. We also use ultrafast coherent phonon spectroscopy to study its phase transition dynamics at high pressure. We find that, in contrast with ambient pressure experiments where strong photoexcitation promptly changes the lattice potential symmetry, at pressures as high as P$=$11 GPa the coherent phonons are still observed upon the photo-driven phase transition to the metallic state. The mechanism of the phase transition dynamics will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G39.00008: Vanadium and V-Ti alloys at high pressure Zsolt Jenei, Hyunchae Cynn, William J. Evans, Simon MacLeod, Stanislav Sinogeikin, Yue Meng Experimental studies of vanadium found that during compression it undergoes a phase transition from the low pressure body centered cubic crystal structure to a rhombohedral phase at 65 GPa when compressed under quasihydrostatic conditions (PRB 83, 054101). Theoretical studies are in reasonable agreement with the transition pressure and predict that upon further compression above 200 GPa the bcc phase becomes stable again. The latest study (PRL 103, 235501) predicts that alloying vanadium with small amounts of the neighboring elements can increase or decrease the stability of the bcc phase relative to the rhombohedral phase. We performed powder x-ray diffraction experiments in diamond anvil cell of pure vanadium and V-Ti alloys at ambient temperature to very high pressures. We will discuss our results, including the equation of state and the stability of the rhombohedral phase at high pressures. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G39.00009: Nuclear magnetic resonance at pressures of up to 10.1 GPa detects an electronic topological transition in aluminum metal J\"urgen Haase, Thomas Meissner, Swee K. Goh, Manuel Richter, Klaus Koepernik, Helmut Eschrig We present high sensitivity $^{27}$Al nuclear magnetic resonance (NMR) measurements on metallic aluminum under high pressures of up to 10.1 GPa. The measured Knight shift and spin-lattice relaxation rate indicate an unexpected negative curvature in the pressure dependence of the electronic density of states (DOS) that violates a free electron behavior. Based on a careful analysis of the Fermiology of aluminum metal with numerical LDA calculations we attribute the observed change in the DOS to a pressure induced electronic topological transition. We discuss an unexpected increase of the NMR linewidth above 4.2 GPa that is not in agreement with the metal's cubic symmetry. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G39.00010: Ultrasonic Investigation of Cerium under High Pressure Magnus Lipp, Zsolt Jenei, Hyunchae Cynn, Chantel Aracne-Ruddle, William Evans, Yoshio Kono, Curtis Kenney-Benson, Changyong Park The contribution of the lattice to the famous volume collapse transition in cerium is re-evaluated using a unique combination of several techniques available at sector 16 BMB / HPCAT. These eliminate any indirect /iterative procedures employed previously: Energy dispersive X-ray scattering provides the pressure of the sample (as well as quality control about the state of the sample), X-ray radiography delivers a shadow image allowing a precise length measurement and the ultrasound pulse overlap method gives the transit time of the longitudinal and transverse pulses. Our preliminary analysis indicates a larger contribution by the lattice as previously thought. This work was performed under the auspices of the US DOE by LLNL under Contract DE-AC52-07NA27344. The X-ray studies were performed at HPCAT (Sector 16), APS/ANL. HPCAT is supported by CIW, CDAC, UNLV and LLNL through funding from~DOE-NNSA, DOE-BES and NSF. APS is supported by DOE-BES, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G39.00011: High Pressure Crystalline Structure and Resistance of Vanadium Dioxide to 13.5 GPa Nathaniel Brady, Kannatassen Appavoo, Jeffery Montgomery, Yogesh Vohra, Richard Haglund, David Hilton We have investigated the insulator-to-metal transition in thin film vanadium dioxide as a function of pressure at ambient temperature using a designer diamond anvil cell (DAC). Four-point probe resistance measurements show a monotonic decrease over the entire pressure range studied with no significant discontinuity. High-pressure X-ray diffraction measurements observe an $\mathrm{M_1}$ ($\mathrm{P2_1/c}$) phase at 0 GPa, an $\mathrm{M_2}$ (C2/m) phase from 0.8 GPa to 1.1 GPa, and a reentrant $\mathrm{M_1}$ phase from 1.1 GPa to 13.5 GPa. Crystal refinement above 1.1 GPa shows a monotonically decreasing $a$, $b$ and $c$ lattice constants and a minimum in the monoclinic angle, $\beta$, near 8.5$\pm$0.5 GPa. The atomic positions show that the first V-V nearest neighbor distance ($d$) decreases over the entire pressure range, the second nearest neighbor distance ($s$) increases until 5 GPa after which it is constant with $s$$\approx$$f$$\approx$3.2 {\AA}. The next most closely spaced V-V distance ($f$), which corresponds to V atoms in different unit cells, is approximately constant across the entire pressure range measured. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G39.00012: Unraveling Convoluted Structural and Electronic Transitions in SnTe at High Pressure Dan Zhou, Quan Li, Yanming Ma, Qiliang Cui, Changfeng Chen The longstanding uncertainty in high-pressure structural evolution of SnTe has greatly impeded the understanding of its complex electronic properties. Here we unravel the convoluted high-pressure phase transitions of SnTe using angle-dispersive synchrotron x-ray diffraction combined with first-principles structural search. We identify three coexisting intermediate phases of Pnma, Cmcm, and GeS type structure and establish the corresponding phase boundaries. We further unveil the intricate pressure-driven evolution of the energetics, kinetics and lattice dynamics of SnTe to elucidate its distinct phase-transition mechanisms. Subsequent electronic band calculations reveal pressure-induced metallization, superconductivity and topological phase transition in SnTe. These findings resolve structures and predict intriguing properties of SnTe, which have broad implications for other IV-VI semiconductors that likely harbor similar novel high-pressure phases and properties. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G39.00013: Compression of HgCr$_2$S$_4$ and HgCr$_2$Se$_4$ spinels I. Efthymiopoulos, A. Yaresko, V. Tsurkan, J. Deisenhofer, A. Loidl, C. Park, Y. Wang The family of ACr$_2$X$_4$ spinels constitutes a prototype system for studying magnetism in solids [1]. More recently, members of this series were found to exhibit multiferroicity [2]. The origin of the ferroic properties is unknown; the role of the structure, however, appears to be important [3]. Given the strong interplay between structural and ferroic properties in these systems, structural tuning by pressure can provide valuable hints for multiferroicity. We have performed high-pressure structural investigations on the multiferroic HgCr$_2$S$_4$ and the HgCr$_2$Se$_4$ compounds. HgCr$_2$S$_4$ exhibits three structural transitions: the starting cubic phase adopts a tetragonal structure at 20 GPa, at 27 GPa an orthorhombic distortion occurs, and a third transition takes place above 37 GPa. As for HgCr$_2$Se$_4$, our studies detect a structural transition at 14 GPa, near the theoretically predicted band gap closure [4]. We discuss the possible mechanisms for the observed phase transitions for both Cr-spinels.\\[4pt] [1] T. Rudolf \textit{et al}., N. J. Phys. 9, 27 (2007) and refs. therein\\[0pt] [2] S. Weber \textit{et al}., PRL 96, 157202 (2006)\\[0pt] [3] V. Gnezdilov \textit{et al}., PRB 84, 045106 (2011)\\[0pt] [4] S. Guo \textit{et al}., JPCM 24, 045502 (2012) [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G39.00014: Electron-phonon interaction of GaAs nanowires under pressure Wei Zhou, Jian-Bo Zhang, Xin-Hua Li, YuQi Wang, XiaoJia Chen, Alexander Goncharov We present resonant Raman scattering (RRS) investigation of wurtzite and zinc-blende phase GaAs nanowires under hydrostatic pressure up to 30 GPa. The Raman spectra are excited by 532 nm and 488 nm laser lines. High order longitudinal optical modes 2LO and 3LO are observed under the resonant conditions. Pressure dependence of band gap of WZ and ZB nanowires has been obtained from the corresponding resonant pressures, and band gap of WZ nanowires is found to be larger than that of ZB nanowires. When applying pressure at 21 GPa, Raman signals of WZ and ZB phases disappear, manifesting phase change to a high-pressure phase. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G39.00015: Response of Aluminum under ramp Compression to Mbar Mu Li, Jianheng Zhao, Chengwei Sun, Hongping Zhang, Feng Wang, Guanghua Chen, Hua Shu Laser-Produced X-ray drive is an important tool for ramp compression to very high pressure. Its application was often limited by the length of rise pulse, the peak pressure was not higher than 400GPa for metal steps. A new method was developed using heavy reservoir film, that can absorb high energy Au M-band x rays generated within the halfraum which otherwise could preheat the step sample. Meanwhile, heavy reservoir can also produce higher pressure peak and longer rise time.Results from this reservoir shot (4.5Mbar) at the SG-III prototype are presented. Al/LiF interface velocities versus time for multiple sample thicknesses were measured and converted to p-v relations using backward integration. [Preview Abstract] |
Session G40: Spin-Orbit Coupling in Ultracold Atom Systems
Sponsoring Units: DAMOPChair: Carlos Sa De Melo, Georgia Institute of Technology
Room: 349
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G40.00001: Synthetic gauge fields for ultracold atoms Invited Speaker: Ross Williams Ultracold atoms represent a unique system in which to investigate quantum many-body physics with unprecedented experimental control. The properties of these systems can be tailored to realize model many-particle Hamiltonians, familiar from condensed matter physics, in their most pure and essential form. Magnetic fields, and gauge fields in general, play an important role in collective phenomena in electronic systems, leading to iconic phenomena such as the fractional quantum Hall effect. More complex, matrix valued, gauge fields can be used to describe spin-orbit coupling: itself an essential ingredient in many topological insulators, and in spintronic devices. Given the charge neutrality of ultracold atoms it is not immediately obvious how such physics could be explored in a cold atom context. In this talk I will describe the experimental techniques we use to engineer artificial gauge fields for ultracold neutral atoms using Raman transitions. I will also describe the latest results from the NIST group. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G40.00002: Search for Majorana fermions in Spin-Orbit Coupled Ultra-cold Fermi Gases Invited Speaker: Chuanwei Zhang Topological quantum matter has been an active research field in physics in the past three decades with numerous celebrated examples, including quantum Hall effect, chiral superconductor, topological insulator, etc. In topological materials, Majorana fermions, first envisioned by Majorana in 1935 to describe neutrinos, often emerge as topological quasiparticle excitations of the systems. Majorana fermions are intriguing because they can be construed as their own anti-particles and follow non-Abelian anyonic statistics under a pair-wise exchange of the many-particle wave function, unlike Dirac fermions where electrons and positrons (holes) are distinct. Although the emergence of Majorana fermions in any condensed matter or atomic system is by itself an extraordinary phenomenon, they have also come under a great deal of recent attention due to their potential use in fault tolerant quantum computation. Motivated by the recent experimental realization of spin-orbit coupling for cold atoms, in this talk, I will discuss the emergence of Majorana fermions in spin-orbit coupled Fermi cold atomic superfluids. I will talk about various experimental relevant issues for the observation of Majorana fermions in such cold atomic systems.\\[4pt] [1] C. Zhang, S. Tewari, R. Lutchyn, and S. Das Sarma, Phys. Rev. Lett. 101, 160401 (2008).\\[0pt] [2] M. Gong, S. Tewari, C. Zhang, Phys. Rev. Lett. 107, 195303 (2011).\\[0pt] [3] M. Gong, G. Chen, S. Jia, C. Zhang, Phys. Rev. Lett. 109, 105302 (2012)\\[0pt] [4] G. Chen, M. Gong, and C. Zhang, Phys. Rev. A 85, 013601 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G40.00003: Majorana fermions in one-dimensional spin-orbit coupled Fermi gases Ran Wei, Erich Mueller We theoretically study trapped one-dimensional Fermi gases in the presence of spin-orbit coupling induced by Raman lasers. The gas changes from a conventional (non-topological) superfluid to a topological superfluid as one increases the intensity of the Raman lasers above a critical chemical-potential dependent value. Solving the Bogoliubov-de Gennes equations self-consistently, we calculate the density of states in real and momentum space at finite temperatures. We study Majorana fermions (MFs) which appear at the boundaries between topologically trivial and topologically non-trivial regions. We linearize the trap near the location of a MF, finding an analytic expression for the localized MF wavefunction and the gap between the MF state and other edge states. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G40.00004: Phase diagram of 1D spin-orbit coupled Fermi gases in optical lattices Chunlei Qu, Ming Gong, Chuanwei Zhang We consider a one dimensional spin-orbit coupled Fermi gas in optical lattices with open boundary condition. This system belongs to the BDI symmetry class because the Hamiltonian can be made real when the Zeeman field is assumed to be along the z direction, thus the topological superfluid is characterized by $Z$, instead of $Z_2$ class. In the optical lattice system, each site admits at most two fermions. The system can host plenty of phases depending on the filling factor and the Zeeman field. At finite Zeeman field we observe a strong competition between the topological superfluid phase and the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase. The latter phase is more likely to be observed near the half filling. The spin-orbit coupling plays the role of enhancing the topological superfluid phase and suppressing the FFLO phase, which the Hartree shift plays an utterly opposite role. The possible observation of topological phase is also discussed in the presence of a harmonic trap. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G40.00005: Topological Quantum Phase Transition of Fermi Gases and its Detections in a Synthetic Non-Abelian Gauge Potential Fadi Sun, Xiao-Lu Yu, Jinwu Ye, Heng Fan, Wu-Ming Liu We investigate the topological quantum phase transition of Fermi gases trapped in a honeycomb lattice in the presence of a synthetic non-Abelian gauge potential. We develop a systematic fermionic effective field theory to describe a topological quantum phase transition tuned by the non-Abelian gauge potential and explore its various important experimental consequences. We obtain the critical exponents at zero temperature, dynamic compressibility, uniform compressibility, specific heat and Wilson ratio at finite temperatures. We analyze the effects of atom-atom interactions and possible disorders in generating the non-Abelian gauge fields. We also perform direct numerical calculations on the lattice scale and compare with the results achieved from the fermionic effective field theory. When discussing various feasible experimental detections of the topological quantum phase transition, we stress the important roles of the gauge invariance to distinguish gauge invariant quantities from non-gauge invariant ones. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G40.00006: Trapped Fermi gases with Rashba spin-orbit coupling Menderes Iskin We use the Bogoliubov-de Gennes formalism to analyze harmonically trapped Fermi gases with Rashba-type spin-orbit coupling in two dimensions. We consider both population-balanced and -imbalanced Fermi gases throughout the BCS-BEC evolution, and study the effects of spin-orbit coupling on the spontaneously induced countercirculating mass currents and the associated intrinsic angular momentum. In particular, we find that even a small spin-orbit coupling destabilizes Fulde-Ferrel-Larkin-Ovchinnikov (FFLO)-type spatially modulated superfluid phases as well as the phase-separated states against the polarized superfluid phase. We also show that the continuum of quasiparticle and quasihole excitation spectrum can be connected by zero, one or two discrete branches of interface modes, depending on the number of interfaces between a topologically trivial phase (e.g. locally unpolarized/low-polarized superfluid or spin-polarized normal) and a topologically nontrivial one (e.g. locally high-polarized superfluid) that may be present in a trapped system. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G40.00007: Superfluid transition temperature across the BCS-BEC crossover induced by a synthetic non-Abelian gauge field Jayanth P. Vyasanakere, Vijay B. Shenoy A non-Abelian gauge field that induces a spin-orbit coupling on the motion of fermions engenders a BCS-BEC crossover even for weakly attracting fermions. The transition temperature at large spin-orbit coupling is known to be determined by the mass of the emergent boson -- the rashbon. We obtain the transition temperature of the system as a function of the spin-orbit coupling by constructing and studying a Gaussian fluctuation (Nozieres-Schmitt-Rink) theory. These results will help guide the upcoming experiments on spin-orbit coupled fermions. In addition, this work suggests a route to enhance the transition temperature of a weakly attracting fermionic system by tuning the spin-orbit coupling. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G40.00008: Fulde-Ferrell-Larkin-Ovchinnikov Phases in Two-dimensional Spin-Orbit Coupled Degenerate Fermi gas Zhen Zheng, Ming Gong, Yichao Zhang, Xubo Zou, Chuanwei Zhang, Guangcan Guo We examine the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase in two dimensional spin-orbit coupled degenerate Fermi gas using mean-field theory at zero temperature. The FFLO phase has been greatly enhanced due to the deformation of the Fermi surface, which arise from the interplay between spin-orbit coupling and in-plane Zeeman field. The emergence of FFLO phase has been carefully examined from different angles, and the properties of the BCS superfluid, the FFLO phase and normal gas have also been studied. The in-plane Zeeman field break the rotation symmetry thus the eigenvalues no longer appear in pairs. The experimental signatures for the observation of FFLO phase is also discussed. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G40.00009: FFLO and Topological Superfluid Phases in 2D Spin-Orbit Coupling Fermionic Optical Lattices Yong Xu, Chunlei Qu, Ming Gong, Chuanwei Zhang We investigate the phase diagram of 2D spin-orbit coupled ultra-cold Fermi atoms confined in a square lattice. By numerically solving the corresponding Bogoliubov-de Gennes equation self-consistently, we show that a finite Zeeman field can induce Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) and /or topological superfluid phases (which support Majorana fermions) in the presence of spin-orbit coupling. We find that the perpendicular Zeeman field favors the topological superfluid phase, while the in-plane Zeeman field favors the FFLO state. A simple physical explanation for the above results is also provided. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G40.00010: Finite-Momentum Dimer Bound State in Spin-Orbit Coupled Fermi Gas Lin Dong, Lei Jiang, Hui Hu, Han Pu We investigate the two-body properties of a spin-1/2 Fermi gas subject to a spin-orbit coupling induced by laser fields. When attractive $s$-wave interaction between unlike spins is present, the system may form a dimer bound state. Surprisingly, under proper conditions, the bound state obtains finite center-of-mass momentum, whereas under the same condition but in the absence of the two-body interaction, the system has zero total momentum. This unusual result can be regarded as a consequence of the broken Galilean invariance by the spin-orbit coupling. Such a finite-momentum bound state will have profound effects on the many-body properties of the system. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G40.00011: Ultra-cold fermions in the flatland: evolution from BCS to Bose superfluidity in two-dimensions with spin-orbit and Zeeman fields Li Han, Carlos Sa de Melo We discuss the evolution from BCS to Bose superfluidity for ultracold fermions in two-dimensions and in the presence of simultaneous spin-orbit and Zeeman fields. We analyze several thermodynamic properties to characterize different superfluid phases including pressure, compressibility, induced polarization, and spin susceptibility. Furthermore, we compute the momentum distribution and construct topological invariants for each of the superfluid phases. [Preview Abstract] |
Tuesday, March 19, 2013 2:15PM - 2:27PM |
G40.00012: Flow induced superfluidty and other novel effects in spin orbit coupled fermionic quantum gases Vijay B. Shenoy Recent experiments on fermions with synthetic gauge fields produce systems with spin-orbit coupling, detuning and Zeeman fields. We show by theoretical considerations that such systems have many interesting features when the fermions experience a contact attraction. In particular, a flow (finite centre of mass momentum) produces a ``stronger'' superfluid. In addition, we show that such systems can be tuned to have very interesting normal states paving way for studying spin-orbit coupled Fermi liquids. [Preview Abstract] |
Session G41: Attosecond Physics and Optics
Sponsoring Units: DAMOPChair: Stephen Eckel, National Institute of Standards and Technology
Room: 350
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G41.00001: Simulated Photoelectron-Based Imaging of Localized Surface Plasmons with Attosecond Resolution James Prell, Lauren Borja, Andrey Gandman, Desire Whitmore, Daniel Neumark, Stephen Leone Simulations of proposed photoelectron streaking experiments in the presence of an oscillating plasmon field are presented. The results indicate that localized surface plasmon dephasing can be imaged with attosecond resolution using electron time-of-flight (TOF) or velocity map imaging (VMI) techniques. In the simulation, localized surface plasmons are excited in metal nanoparticles by a few-cycle infrared or visible laser pulse. Using time-delayed single, isolated attosecond x-ray pulses, electrons are photoemitted from the metallic nanoparticles and streaked by both the plasmon and laser electric fields. The effects of these two fields in the streaking spectra and images can be separated so that the temporal evolution of the plasmon electric field can be directly extracted. The plasmon electric field induces a broadening of the photoelectron speed distribution with an envelope directly proportional to that of the plasmon dipole moment. Plasmon-induced oscillation of the angular distribution in VMI is predicted to report the spatial distribution of the plasmon electric field for nanoparticles with high aspect ratios. The simulations indicate that these techniques can be used to map plasmon dynamics with unprecedented temporal resolution. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G41.00002: Metal nanofilm in strong ultrafast optical fields: subcycle Bloch oscillations Vadym Apalkov, Mark Stockman We predict theoretically that a metal nanofilm subjected to an ultrashort optical pulse of a high field amplitude $\sim 3$ V/\AA $~$ shows semimetal behavior. At such high pulse intensity, the reflectivity of metal nanofilm is greatly reduced, while the transmissivity and the optical field inside the metal are greatly increased. The temporal profiles of the optical fields are predicted to exhibit pronounced subcycle oscillations which are attributed to the Bloch oscillations and formation of the Wannier-Stark ladder of electronic states. These effects are promising for applications as nanoplasmonic modulators and field-effect transistors with petahertz bandwidth. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 12:15PM |
G41.00003: Attosecond Streaking Chronoscopy of Surfaces Invited Speaker: Joachim Burgdoerfer With the advent of sub-femtosecond ultrashort XUV pulses and of phase-stabilized IR pulses with sub-cycle time resolution, novel pathways have been opened up for studying time-resolved electronic quantum dynamics on the attosecond scale. These experiments pose challenges for theory: How do short pulses interact with matter? Which novel information can be extracted from time-resolved spectroscopies that cannot be gained from precision experiments in the spectral domain? In this talk we discuss attosecond chronoscopy by streaking photoelectron emission from solid surfaces. Experimental photoemission data reveal a time delay between conduction electrons and core electrons on the $\sim$50 attosecond scale. We show that the temporal information accessible for such a many-electron system in the condensed phase includes both the coherent wavepacket dynamics characterized by the Eisenbud-Wigner-Smith (EWS) time delay as well as decohering processes in transport and relaxation. Extensions to nanostructures will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G41.00004: Strong-Field Emission From High Aspect Ratio Si Emitter Arrays Phillip Keathley, Michael Swanwick, Alexander Sell, William Putnam, Stephen Guerrera, Luis Vel\'asquez-Garc\'Ia, Franz K\"artner We discuss photoelectron emission from an arrays of high aspect ratio, sharp Si emitters both experimentally and theoretically. The structures are prepared from highly doped single-crystal silicon having a pencil-like shape with end radii of curvature of around 10 nm. The tips were illuminated at a grazing incidence of roughly 84deg.with a laser pulse having a center wavelength of 800 nm, and a pulse duration of 35 fs from a regenerative amplifier system. Native oxide coated Si tips were characterized using a time of flight (TOF) electron energy spectrometer. An annealing process was observed, resulting in a red shift of the energy spectra along with an increased electron yield. Total current yield from samples having the oxide stripped were also studied. Apeak total emission of 0.68 pC/bunch, corresponding to around 1.5x10$^3$ electrons/tip/pulse was observed at a DC bias of 70 V . Both spectral and current characterization results are consistent with a stong-field photoemission process at the surface of the tip apex. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G41.00005: Laser Beam Shaping For Lithography on Inclined and Curved Surfaces Using a liquid crystal Spatial Light Modulator Javad R. Gatabi, Wilhelmus Geerts, Dan Tamir, Kumar Pandey An exposure tool for lithography on non-flat substrates that includes a real time photoresist thickness and surface topography monitor is under development at Texas State University. Exposure dose and focusing are corrected on curved parts of the sample using novel laser beam shaping techniques: two approaches using a Holoeye liquid crystal spatial light modulator (LC-SLM) are being investigated: (1) the implementation of multiple lenses with different focal lengths to split the beam into several parts and keeping each part in focus depending on sample topography; (2) the implementation of a tilted lens function resulting in a tilt of the image plane. Image quality is limited by quantization aberration, caused by the phase modulator's bit depth limitation, and pixelation aberration, caused by the modulator's pixel size. A statistical analysis on lenses with different focal lengths provides a detailed description of the mentioned aberrations. The image quality, i.e. resolution and contrast of both techniques, are determined from developed photoresist patterns on curved samples and compared to the theory. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G41.00006: Imaging the signals emitted by multiple sources originating from a turbid medium Gabriel Cwilich, Juan Jose Saenz, Luis Froufe Perez We studied the problem of spatially closely positioned sources which emit waves inside a turbid medium, through fluorescence or other mechanisms. While for many of the traditional imaging methods, including FRET, the disorder might impose an insurmountable obstacle for the detections of the sources, the interference of the waves in the case of multiple scattering, gives raise, due the coherent propagation of the signals at the mesoscopic scale , to important effects both in the correlations and the fluctuations of the intensity being detected at a point lying outside the medium. The information obtained that way can be used to monitor the displacement of the sources and their degree of coherence even at scales below the wavelength of the radiation being emitted. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:27PM |
G41.00007: Attosecond view of the photoelectric effect and optical-field-induced current in dielectrics Invited Speaker: Ralph Ernstorfer Fundamental electronic processes in condensed matter like electron transport on atomic length scales, the plasmonic response in metals or the dielectric response in insulators occur on attosecond time scales. In the first part of my talk, I discuss how a streak camera operating at optical frequencies provides a time-resolved view of the photoelectric effect [1]. Photoelectrons emitted from metal surfaces by an attosecond extreme ultraviolet laser pulse are time-stamped by a few-cycle visible/near-infrared laser pulse. This technique allows for measuring the relative emission time of valence and core electrons with a precision of tens of attoseconds, thereby addressing the intrinsic dynamics of the photoemission. I present recent studies of a free-electron metal [2] as well as of oxygen-covered tungsten single crystals. The origin of the observed attosecond delays in the emission of photoelectrons from different initial states is discussed. In the second part of the talk, I report on electric current in dielectrics induced and controlled by ultrashort optical fields [3]. For very short periods of time, electric fields exceeding 10 V/nm, i.e. fields significantly beyond the threshold for dc dielectric breakdown, can be applied to insulators. In this regime, insulators exhibit a highly nonlinear dielectric response, resulting in an increase in conductivity by many orders of magnitude. Applying 1.5-cycle laser pulses to unbiased metal-dielectric-metal nanogaps, we demonstrate the generation of directly measurable photocurrents whose magnitude and directionality can be controlled with the carrier-envelope phase of the laser pulse, i.e. by the shape of the laser electric field. Such currents can be switched on and off on sub-femtosecond timescales as evidenced by employing two cross-polarized and time-delayed pulses. The ultrafast field-controlled current generation in a dielectric nanostructure may represent a first step towards the realization of optical-field-controlled electronics.\\[4pt] References:\\[0pt] [1] A.L. Cavalieri et al., Nature 449, 1029 (2007).\\[0pt] [2] S. Neppl et al., Phys. Rev. Lett. 109, 087401 (2012). \\[0pt] [3] A. Schiffrin et al., Nature (2013), doi:10.1038/nature11567. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G41.00008: Nondiffracting accelerating wave packets beyond the paraxial limit Peng Zhang, Yi Hu, Tongcang Li, Drake Cannan, Xiaobo Yin, Roberto Morandotti, Zhigang Chen, Xiang Zhang Self-accelerating Airy wave packets have stimulated rapidly growing research interest in the past five years. However, optical Airy beams are inherently subjected to the paraxial limit. Here, we demonstrate both theoretically and experimentally linear and nonlinear self-accelerating beams propagating along circular trajectories beyond the paraxial approximation. Such nonparaxial accelerating beams are exact solutions of the Helmholtz equation. Furthermore, we introduce and demonstrate nonparaxial Mathieu and Weber accelerating beams, generalizing the concept of all previously found accelerating wave packets. We show that such beams bend into large angles along elliptical or parabolic trajectories but still retain nondiffracting and self-healing capabilities. The circular nonparaxial accelerating beams can be considered as a special case of the Mathieu accelerating beams, while an Airy beam is only a special case of the Weber beams at the paraxial limit. Not only do generalized nonparaxial accelerating wave packets open up many possibilities of beam engineering for applications, but the fundamental concept developed here can be applied to other linear wave systems in nature, ranging from electromagnetic and elastic waves to matter waves. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G41.00009: Mapping of focused Laguerre-Gauss beams Jose R. Rios Leite, Vasily Klimov, Martial Ducloy, Daniel Bloch We study the detection of propagating optical fields bearing axial symmetry in the situation of an extreme focusing, when the paraxial approximation no longer holds. The results, obtained by general arguments based upon the vectorial nature of electromagnetic fields, show the rapid spatial variations of fields with ``complicated'' spatial structure [1]. Laguerre-Gauss beam, notably beams bearing a l = 2 orbital angular momentum for which a magnetic field and a gradient of the electric field are present on axis have been examined in their behavior upon an atomic size light detector sensitive to quadrupole electric transitions as well as to magnetic dipole transitions. nd apply it to the case of a Laguerre-Gauss beam.We detail how the mapping of such a beam depends on the nature and on the specific orientation of the detector. We also show that the interplay of mixing of polarization and topological charge, respectively associated to spin and orbital momentum when the paraxial polarization holds, modifies the apparent size of the beam in the focal plane.\\[0pt] [1]. V. Klimov, M. Ducloy, D. Bloch and JR Rios Leite, Phys. Rev. A 85, 053834 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G41.00010: Photonic temperature sensor based on microring resonators Haitan Xu, Zeeshan Ahmed, Mohammad Hafezi, Jingyun Fan, Gregory Strouse, Alan Migdall, Jacob Taylor Temperature needs to be controlled accurately and precisely in various areas, yet it is one of the most inaccurately measured physical quantities. We consider a new measurement method for temperature using the thermal response of a microring resonator built using Silicon-on-Insulator. The temperature-dependence of the index refraction maps temperature to the resonance frequency of the resonator, which can be measured with higher precision. We study the resolution and accuracy of our device, as well as future challenges for this approach for temperature metrology. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G41.00011: A new bound on excess frequency noise in second harmonic generation in PPKTP at the $10^{-19}$ level David Yeaton-Massey, Rana Adhikari Several experiments at the forefront of precision metrology and frequency standards use optical harmonic generation in their experiments. These include iodine stabilized Nd:YAG lasers, optical frequency combs, measurement of optical frequency ratios, and precision atomic spectroscopy. We present an experimental bound on the relative frequency fluctuations introduced in the nonlinear second harmonic generation process using PPKTP to double a 1064nm Nd:YAG laser. We report a measured amplitude spectral density of frequency noise with total RMS frequency deviation of 3mHz and a minimum value of 20 $\mu$Hz/Hz$^{1/2}$ over 250 seconds with a measurement bandwidth of 128 Hz, corresponding to an Allan deviation of $10^{-19}$ at 20 seconds. [Preview Abstract] |
Session G42: Focus Session: Physics of Glasses and Viscous Liquids I
Sponsoring Units: DCPChair: Patrick Charbonneau, Duke University
Room: Hilton Baltimore Holiday Ballroom 3
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G42.00001: Glass Transition of Polymers near Their Free Surface Invited Speaker: Zahra Fakhraai Recent experiments have indicated that the relaxation dynamics near a free polymer surface may become fundamentally different from bulk $\alpha$ relaxation times. The dynamics lose many of the characteristics commonly associated with glasses. The dynamic properties lose their typical Vogel-Fulcher-Tammann (VFT) temperature dependence and take on an Arrhenius dependence. There is also evidence that the dynamic properties become more homogeneous near the free surface. Such direct measurements of the relaxation dynamics are rare and extremely difficult to perform on a wide range of polymeric systems. It has been shown that cooling rate-dependent glass transition temperature ($\mathrm{T_g}$) measurements can be used as an effective and simple method to estimate the relaxation dynamics of the free surface. The cooling rate is inversely proportional to the relaxation time of the film at the temperature at which the system falls out of equilibrium, $\mathrm{T_g}$. In thin polymer films, as the film thickness is decreased the dynamics of the film are affected more strongly by surface dynamics and therefore they provide a lower bound to the surface relaxation times. In thin polystyrene films measurements of $\mathrm{T_g}$ as a function of cooling rate indicate a clear onset of deviations from bulk properties at a temperature a few degrees above the bulk $\mathrm{T_g}$. We hypothesize that this could be due to either a new mode of relaxation that is exclusively available near the surface, or typical glassy dynamics that have faster time scales near the surface. In this study we investigate the effect of molecular weight and the polymer structure on the value of the onset temperature to verify whether the properties are consistent with one of these hypotheses. It is also observed that under certain conditions, where the dynamics of the free surface and the bulk relaxation dynamics are many decades apart, the system exhibits two distinct $\mathrm{T_g}$s associated with either bulk or surface relaxations. This data can be used to estimate the length scale of the surface dynamics and the length scale over which the effects penetrate into the bulk of the film. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G42.00002: Anharmonicity and Fragility of Protic Ionic Liquids Jenny Kim, Austen Angell, Kazuhide Ueno, Madhu Tyagi, Christopher Soles, Kevin Masser Supercooled liquids are often characterized by their fragility which is associated with physicochemical properties. However, the origin of fragility is still controversial. Superfragile liquid, decahydroisoquinoline (DHiQ) is chosen as a parent molecule to systematically investigate the relationship between anharmonicity and fragility of supercooled liquids. Earlier study by Ueno et al. (J. Phys. Chem. B 2012, 116) demonstrated that the protonation of DHiQ by different Bronsted acids results in the loss of superfragility. To understand the nature of fragile liquids, we conducted inelastic/quasielastic (IE/QE) neutron scattering measurements to examine low frequency vibrational dynamics (boson peak) and the relaxation behavior of DHiQ (high fragility) and DHiQ-based ionic liquids with intermediate (formate, Fm) and low (trifluoromethansulfonimide, TFSI) fragilities. With the protonation, molecular acids will be hydrogen-deficient and the scattering will be dominated by cation, [DHiQ$^{+}$]. This strategy simplifies our interpretation in terms of understanding the fitting result from IENS/QENS spectra. By protonating DHiQ with stronger acids, large shift in low frequency vibrational modes and smaller mean square displacements were examined at temperatures higher than Tg. We illustrate how the degree of protonation and ionicity plays a role in the loss in superfragility of DHiQ. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G42.00003: Acoustic properties in glycerol glass-former: Molecular dynamics simulation Remi Busselez, Thomas Pezeril Study of high-frequency collective dynamics around TeraHertz region in glass former has been a subject of intense investigations and debates over the past decade. In particular, the presence of the Boson peak characteristic of glassy material and its relation to other glass anomalies. Recently, experiments and simulations have underlined possible relation between Boson peak and transverse acoustic modes in glassy materials. In particular, simulations of simple Lennard Jones glass former have shown a relation between Ioffe-Regel criterion in transverse modes and Boson peak. We present here molecular dynamics simulation on high frequency dynamics of glycerol. In order to study mesoscopic order (0.5-5nm$^{-1}$), we made use of large simulation box containing 80000 atoms. Analysis of collective longitudinal and transverse acoustic modes shows striking similarities in comparison with simulation of Lennard-Jones particles. In particular, it seems that a connection may exist between Ioffe-Regel criterion for transverse modes and Bose Peak frequency. However,in our case we show that this connection may be related with structural correlation arising from molecular clusters. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G42.00004: Fast Scanning Calorimetry study of non-equilibrium relaxation in fragile organic liquids Vlad Sadtchenko, Deepanjan Bhattacharya, Liam O'Reilly Fast scanning calorimetry (FSC), capable of heating rates in excess of 1000000 K/s, was combined with vapor deposition technique to investigate non-equilibrium relaxation in micrometer thick viscous liquid films of several organic compounds (e.g.2-ethyl-1-hexanol, Toluene, and 1-propanol) under high vacuum conditions. Rapid heating of samples, vapor deposited at temperatures above their standard glass softening transition (Tg), resulted in observable endotherms which onset temperatures were strongly dependent on heating rate and the deposition temperature. Furthermore, all of the studied compounds were characterized by distinct critical deposition temperatures at which observation of endotherm became impossible. Based on the results of these studies, we have developed a simple model which makes it possible to infer the equilibrium enthalpy relaxation times for liquids from FSC data. We will discuss implications of these studies for contemporary models of non-equilibrium relaxation in glasses and supercooled liquids. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 1:03PM |
G42.00005: Elementary excitations and flow in the liquid Invited Speaker: Takeshi Egami A new mode of excitation is introduced to elucidate the dynamics in simple liquids at the atomic scale. Some properties of liquid defy easy explanations. For instance, in liquids phonons are overdamped with a very short lifetime. Nevertheless the Dulong-Petit law (C$_{\mathrm{V}}$ $\sim$ 3k$_{\mathrm{B}})$ is widely observed at high temperatures. As temperature is reduced the specific heat markedly increases in the supercooled state, only to drop down sharply at the glass transition. Viscosity shows an Arrhenian behavior at high temperatures, but increases rapidly toward the glass transition in the supercooled state. We suggest that these perplexing observations can be naturally explained in terms of the local configurational excitations (LCE's) which locally change the atomic connectivity by an atom losing or gaining one nearest neighbor. We show that the lifetime of LCE, $\tau_{LC}$, is equal to the Maxwell relaxation time, $\tau_{M}$, at temperatures above the crossover temperature, $T_{A}$. Above T$_{\mathrm{A}}$ the phonon mean-free path, $\xi = c_{T}\tau_{LC}$, where $c_{T}$ is the transverse sound velocity, becomes shorter than the interatomic distance, resulting in phonon localization. Therefore LCE's are the elementary excitations in the liquid. They are independent of each other above $T_{A}$, but below $T_{A}$ LCE's interact through phonon exchange, resulting in the rapid increase in $\tau _{M}$, culminating in the glass transition. LCE's are also the mechanism of flow at low temperature under strong shear stress. In this case, however, losing and gaining of the neighbors are strongly coupled, so that $\tau _{M} = \tau_{LC}$ /2 [1]. We also discuss dynamic heterogeneity in terms of LCE interactions. \\[4pt] [1] T. Iwashita and T. Egami, \textit{Phys. Rev. Lett.}, \textbf{108,} 196001 (2012). [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G42.00006: Ab-initio atomic level stresses in Cu-Zr crystal, liquid and glass phases Madhusudan Ojha, Don M. Nicholson, Takeshi Egami The Cu-Zr system provides interesting playground for the study of glass structure, stability, and formability and liquid dynamics. Glasses form over a wide range of concentrations while they compete against various intermetallic compounds. We have calculated from first-principles the atomic level stresses, a new tool to characterize materials, within the local approximation to Density Functional Theory (DFT) for Cu-Zr glasses and compounds from low temperature to 4500K. Comparisons between ordered crystalline compounds and liquids and glasses allow us to relate atomic level stress to relaxation of chemical short-range order and structural relaxation. The results are counter-intuitive at times; a smaller atom is under higher compressive pressure, whereas geometrically they should be under tension. Ab-initio calculations were done using Vienna Ab-initio Simulation Package (VASP) and Locally Self-consistent Multiple Scattering (LSMS) codes. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G42.00007: Computer Simulations of Non-Equilibrium Dynamics in Silica Christopher H. Gorman, Katharina Vollmayr-Lee, Horacio E. Castillo, Azita Parsaeian We present results from molecular dynamics computer simulations of aging silica (modeled by the BKS model). The system is equilibrated at $T_{\rm i}=5000$~K and quenched instantaneously to $T_{\rm f}=2500$~K. After a waiting time $t_{\rm w}$ we investigate the dynamics of the Si- and O-atoms as the system evolves over time t. Our simulations run long enough in order to observe the transition from out-of-equilibrium to equilibrium dynamics. We determine for our system the generalized incoherent intermediate scattering function $C(q,t_{\rm w},t_{\rm w}+t)$ and the dynamic susceptibility $\chi_4(q,t_{\rm w},t_{\rm w}+t)$ where $q$ corresponds to the wavevector. Curves corresponding to different waiting times $t_{\rm w}$ collapse on the scaling plot $\chi_4(q,t_{\rm w},t_{\rm w}+t)/\chi_4^{\rm max}(q,t_{\rm w})$ as a function of $\large (1-C(q,t_{\rm w},t_{\rm w}+t) \large )$, which agrees with a prediction from spin glass theory. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G42.00008: Persistent medium-range order and anomalous liquid properties of Al$_{1-x}$Cu$_{x}$ alloys Joongoo Kang, Junyi Zhu, Su-Huai Wei, Eric Schwegler, Yong-Hyun Kim The development of short-to-medium-range order in atomic arrangements--that is, the aggregation or packing of short-range order (SRO) atomic clusters--has generally been observed in noncrystalline solid systems such as metallic glasses. Whether such medium-range order (MRO) can exist in materials at well above their melting or glass-transition temperature, manifesting itself in some observable property such as a liquid--liquid transition, has been a long-standing important scientific challenge. Here, using \emph{ab initio} molecular dynamics simulations, we show that a novel, persistent MRO exists in liquid Al-Cu alloys, both in the nano- and bulk phases, near the composition of CuAl$_{3}$. In a sense, the MRO liquid lies in between glasses and normal liquids, and thus it exhibits anomalous liquid properties. Our \emph{ab initio} calculations provide a detailed atomistic description of the MRO as well as a microscopic explanation for its formation via a percolation-like transition. Interestingly, we find that the appearance of MRO in the liquid phase manifests itself in a substantially enhanced viscosity that is consistent with a previously unexplained experimental observation of a peak in the viscosity of Al-Cu alloys. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 2:15PM |
G42.00009: Transient Networks and Dense Colloidal Suspensions: From Viscous Flow to Elastic Instabilities Invited Speaker: Elisabeth Bouchaud In order to analyze the mechanical response of viscoelastic materials in highly non-linear regimes, we have designed a new kind of Hele-Shaw cell where both viscous liquids and soft elastic solids can be tested at a controlled loading rate. We first consider model Maxwell liquids -- characterized by a single relaxation time -- with the project of benchmarking the response of complex, glassy systems. We use several solutions of microemulsions connected by telechelic polymers. We show that these materials undergo instability in a broad range of loading rates. At low rates, this instability is shown to be of the viscous Saffman-Taylor type. At high rates, we observe a purely elastic bulk instability discovered recently in the context of soft elastomers. A microfluidic version of our cell makes it possible to study the response of colloidal suspensions. We use more or less concentrated PNIPA aqueous solutions for which temperature controls the volume fraction. Observations are interpreted in the light of our understanding of their viscoelastic properties. [Preview Abstract] |
Session G43: Focus Session: Motor dynamics---from Single Molecules to Cells II
Sponsoring Units: DCPChair: Sean Sun, Johns Hopkins University
Room: Hilton Baltimore Holiday Ballroom 2
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G43.00001: Operation mechanism of rotary molecular motor F$_1$ probed by single-molecule techniques Invited Speaker: Ryota Iino F$_1$ is a rotary motor protein. Three catalytic $\beta $-subunits in the stator $\alpha_{3}\beta_{3}$ ring are torque generators, and rotate the rotor $\gamma $-subunit by sequential and cooperative conformational changes coupled with adenosine triphosphate (ATP) hydrolysis reaction. F$_1$ shows remarkable performances such as rotation rate faster than 10,000 rpm, high reversibility and efficiency in chemo-mechanical energy conversion. I will introduce basic characteristics of F$_1$ revealed by single-molecule imaging and manipulation techniques based on optical microscopy and high-speed atomic force microscopy. I will also discuss the possible operation mechanism behind the F$_1$, along with structurally-related hexameric ATPases, also mentioning the possibility of generating hybrid molecular motors. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G43.00002: Simultaneous measurement of DNA motor protein conformation and activity with combined optical trap and single-molecule fluorescence Invited Speaker: Yann Chemla We present single-molecule measurements of Superfamily 1 UvrD helicase DNA unwinding that reveal directly how helicase stoichiometry and conformation regulate motor activity. Using a new instrument that combines high resolution optical tweezers with single-molecule fluorescence microscopy, we record DNA unwinding activity with base pair-scale resolution (via optical tweezers) simultaneously with helicase stoichiometry and conformation (via fluorescence). Quantifying the fluorescence signal from labeled UvrD, we observe that pairs of UvrD molecules are required for long distance unwinding but that individual molecules exhibit limited, non-processive unwinding activity. UvrD is also known to exhibit two different conformations, `closed' and `open', based on the orientation of its 2B regulatory domain. The function of these conformations has remained elusive. Measuring the fluorescence of FRET labeled proteins, we detect directly the conformation of the 2B domain of individual UvrD molecules during unwinding activity. We observe that UvrD is in the `closed' conformation during DNA unwinding but surprisingly switches to the `open' conformation upon reversal of helicase direction, i.e. when UvrD switches strands and translocates on the opposing strand with the DNA junction rezipping behind it. We hypothesize that the 2B domain acts as a conformational switch that controls DNA unwinding vs. re-annealing. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 1:03PM |
G43.00003: How Interactions Affect Multiple Kinesin Dynamics Invited Speaker: Anatoly Kolomeisky Intracellullar transport is supported by several classes of enzymatic molecules known as motor proteins. Cellular cargos are frequently transported by teams of motor proteins, and recent experimental and theoretical studies uncovered many features of such complex dynamics. Here we investigate theoretically the role of nonmechanical interactions between kinesin motor proteins and microtubules in the collective motion of motor proteins. Our analysis is based on stochastic model that explicitly takes into account all chemical and mechanical transitions. Nonmechanical interactions are assumed to affect kinesin mechanochemistry only when the motors are separated by less than 3 microtubule lattice sites, and it is shown that relatively weak interaction energies can have a significant effect on collective motor dynamics. In agreement with optical trapping experiments on structurally defined kinesin complexes, the model predicts that these effects primarily occur when cargos are transported against loads exceeding single-kinesin stalling forces. These results highlights the complex dynamics of multiple motor proteins in cellular transport phenomena. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:39PM |
G43.00004: Molecular mechanism of motion and force generation by cytoplasmic dynein Invited Speaker: Arne Gennerich Cytoplasmic dynein is an intricate microtubule (MT) motor with four AAA (ATPase associated with various cellular activities) ATPases per head domain. Dynein homodimers take hundreds of consecutive steps, during which the leading and trailing heads experience intramolecular tension in opposite directions. We hypothesize that this asymmetry may differentially regulate the MT-binding and ATPase functions in each head, thereby facilitating processive movement. Here, we elucidate the function of tension in regulating dynein-MT interactions, and dissect the roles of its multiple AAA subunits in effecting and modulating this behavior. Using optical tweezers to measure unbinding forces of single S. cerevisiae dynein heads in the absence of nucleotide, we show that intrinsic dynein-MT binding is significantly weaker under forward (MT-minus-end directed) tension than under rearward tension. Thus, forward tension likely promotes rear head detachment in the dimeric motor. The nucleotide states of specific AAA sites modify this intrinsic behavior. Mutational analysis shows that ATP binding to AAA1 substantially weakens MT binding. Moreover, ADP binding to AAA3 `locks' dynein in a previously undescribed, weak MT-binding state with a relatively symmetric response to tension. Interestingly, tension also affects nucleotide affinity: ADP affinity is lower under rearward than under forward load, suggesting that the front head preferentially releases ADP (likely from AAA3), perhaps driving a transition from an ADP state with relatively weak MT attachment to a strongly MT-attached, nucleotide-free state. Our analysis suggests that intramolecular tension is key to dynein motility, and highlights the importance of including multiple AAA ATPases in models for dynein mechanochemistry. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 2:15PM |
G43.00005: Motor-motor interactions in ensembles of muscle myosin: using theory to connect single molecule to ensemble measurements Invited Speaker: Sam Walcott Interactions between the proteins actin and myosin drive muscle contraction. Properties of a single myosin interacting with an actin filament are largely known, but a trillion myosins work together in muscle. We are interested in how single-molecule properties relate to ensemble function. Myosin's reaction rates depend on force, so ensemble models keep track of both molecular state and force on each molecule. These models make subtle predictions, e.g. that myosin, when part of an ensemble, moves actin faster than when isolated. This acceleration arises because forces between molecules speed reaction kinetics. Experiments support this prediction and allow parameter estimates. A model based on this analysis describes experiments from single molecule to ensemble. In vivo, actin is regulated by proteins that, when present, cause the binding of one myosin to speed the binding of its neighbors; binding becomes cooperative. Although such interactions preclude the mean field approximation, a set of linear ODEs describes these ensembles under simplified experimental conditions. In these experiments cooperativity is strong, with the binding of one molecule affecting ten neighbors on either side. We progress toward a description of myosin ensembles under physiological conditions. [Preview Abstract] |
Session G44: Multi-cellular Processes and Development
Sponsoring Units: DBIOChair: Karen Kasza, Sloan-Kettering Institute
Room: Hilton Baltimore Holiday Ballroom 1
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G44.00001: The Mechanics of Angiogenesis in Collagen Tubes Jolie Breaux, Abigail De La Pena, Melanie Suaris, Steven Zehnder, Thomas Angelini Cells in all types of tissue are sensitive to their mechanical environment. Understanding cell mechanics in tissue growth can lead to advancements in important medical applications, like technologies that enhance angiogenesis during wound healing. Great progress has been made in understanding the mechanics of angiogenesis with assays performed in flat bottomed culture dishes. Here we present results from an in vitro study of collective endothelial cell mechanics in a 3D culture system that mimics the geometry of a real endothelium. Human Aortic Endothelial Cells were grown inside of a collagen tube supported by a rigid cylindrical scaffold. We developed a time-lapse small angle light scattering method to directly measure the radial distribution of cells in the 3D matrix over time. Accompanying live-cell time-lapse microscopy was performed to monitor the cells' collective movement and organization. We find that the cells generate sufficient contractile force to detach the collagen matrix from the support scaffold while maintaining a macroscopic cylindrical arrangement, creating a fiber. Cell sensitivity to scaffold material properties, curvature, and symmetry will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G44.00002: Instabilities and topology changes in planar polarized epithelial sheets David Lubensky Epithelia--sheets of cells joined together by specialized junctional structures--are one of the basic building blocks of tissues and organs in animals. In many epithelia, rotational symmetry is broken and cells become polarized in a particular direction in the plane of the sheet. Here, we study the interplay between such planar cell polarity and the shape and packing of individual cells. Using general symmetry arguments and simple phenomenological models, we give a classification of the instabilities that can occur in such a coupled system. In particular, we show that two routes to chiral symmetry breaking are possible, both of which require that cells first become elongated along one axis. We also consider the evolution of the cell packing after an initial instability, including how planar polarity affects T1 topological transitions. We close with possible applications of these results to development in \textit{Drosophila} and in zebrafish. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G44.00003: Imaging the Population Dynamics of Bacterial Communities in the Zebrafish Gut Matthew Jemielita, Michael Taormina, Adam Burns, W. Zac Stephens, Jennifer Hampton, Karen Guillemin, Raghuveer Parthasarathy The vertebrate gut is home to a diverse microbial ecosystem whose composition has a strong influence on the development and health of the host organism. While researchers are increasingly able to identify the constituent members of the microbiome, very little is known about the spatial and temporal dynamics of commensal microbial communities, including the mechanisms by which communities nucleate, grow, and interact. We address these issues using a model organism: the larval zebrafish (Danio rerio) prepared microbe-free and inoculated with controlled compositions of fluorophore-expressing bacteria. Live imaging with light sheet fluorescence microscopy enables visualization of individual bacterial cells as well as growing colonies over the entire volume of the gut over periods up to 24 hours. We analyze the structure and dynamics of imaged bacterial communities, uncovering correlations between population size, growth rates, and the timing of inoculations that suggest the existence of active changes in the host environment induced by early bacterial exposure. Our data provide the first visualizations of gut microbiota development over an extended period of time in a vertebrate. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G44.00004: In-plane video force microscopy of morphogenesis in epithelia M. Shane Hutson, David Mashburn, Eric Copenhaver, W. Tyler McCleery, Jim Veldhuis, Steven Kim, G. Wayne Brodland Video force microscopy (VFM) is a technique that takes segmented time-lapse images as input and makes least-squares estimates for the cell-edge tensions and cell-internal pressures needed to drive observed changes in cell shape. VFM has previously been used to estimate the cell-level forces that drive invagination during Drosophila gastrulation. Doing so required time-lapse images containing entire cross-sections of the embryo. Here, we extend video force microscopy to in-plane images of epithelia -- including examples in which the images cover only a small region of a larger epithelium. This extension requires imposition of constraints on the average cell-internal pressure and the average stress external to the observed patch. We will demonstrate successful estimation of forces in exact models, as well as anomalous cases that prevent successful force estimation. We will then show applications of this technique for inferring the forces driving Drosophila germband retraction and wound healing. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G44.00005: Substrate properties affect collective cell motion Adrian Pegoraro, Ming Guo, Allen Ehrlicher, David Weitz When cells move collectively, cooperative motion, which is characterized by long range correlations in cell movement, is necessary for migration. This collective cell motion is influenced by cell-cell interactions as well as by cell-substrate coupling. Furthermore, on soft substrates it is possible for cells to mechanically couple over long distances through the substrate itself. By changing the properties of the substrate, it is possible to decouple some of these contributions and better understand the role they play in collective cell motion. We vary both the substrate stiffness and adhesion protein concentration and find changes in the collective cell motion of the cells despite only small differences in total cell density and average cell size in the confluent layers. We test these changes on polyacrylamide and PDMS substrates as well as on structured substrates made of PDMS posts that prevent mechanical coupling through the substrate while still allowing stiffness to be varied. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G44.00006: Modeling Excitable Systems Coupled Through External Medium Javad Noorbakhsh, Pankaj Mehta Excitable systems are stable dynamical systems in which any input beyond a threshold results in a significant output. This behavior is ubiquitous in nature and is seen in biological systems such as Dictyostelium discoideum amoeba and neurons to oscillatory chemical reactions. In this work we will focus on transition to oscillation in populations of excitable systems coupled through an external medium and will study their synchronization. We will describe a mechanism to tune the frequency of oscillations using an external input and will study the effects of stochasticity and inhomogeneity on the collective behavior of the system. Furthermore we will include diffusion into the dynamics of the external medium and will study formation of spatial patterns, their characteristics and their robustness to different factors. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G44.00007: Volumetric Measurements of Amnioserosa Cells in Developing Drosophila David Mashburn, Aroshan Jayasinghe, Shane Hutson The behavior of cells in tissue in developing Drosophila melanogaster has become increasingly clearer over the past few decades, in large part due to advances in imaging techniques, genetic markers, predictive modeling, and micromanipulation (notably laser microsurgery). We now know apical contractions in amnioserosa cells are a significant factor in large scale processes like germ band retraction and dorsal closure. Also, laser microsurgery induces cellular recoil that strongly mimics a 2D elastic sheet. Still, what we know about these processes comes entirely from the apical surface where the standard fluorescent markers like cadherin are located, but many open questions exist concerning the remaining ``dark'' portion of cells. Does cell volume remain constant during contraction or do cells leak? Also, what shape changes do cells undergo? Do they bulge, wedge, contract prismatically, or something else? By using a marker that labels the entire membrane of amnioserosa cells (Resille, 117) and adapting our watershed segmentation routines for 4D datasets, we have been able to quantify the entire volumetric region of cells in tissue through time and compare changes in apical area and volume. Preliminary results suggest a fairly constant volume over the course of a contraction cycle. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G44.00008: The mechanics of retinal detachment Tom Chou, Michael Siegel We present a model of the mechanical and fluid forces associated with exudative retinal detachments where the retinal photoreceptor cells separate typically from the underlying retinal pigment epithelium (RPE). By computing the total fluid volume flow arising from transretinal, vascular, and retinal pigment epithelium (RPE) pump currents, we determine the conditions under which the subretinal fluid pressure exceeds the maximum yield stress holding the retina and RPE together, giving rise to an irreversible, extended retinal delamination. We also investigate localized, blister-like retinal detachments by balancing mechanical tension in the retina with both the retina-RPE adhesion energy and the hydraulic pressure jump across the retina. For detachments induced by traction forces, we find a critical radius beyond which the blister is unstable to growth. Growth of a detached blister can also be driven by inflamed tissue within which {\it e.g.}, the hydraulic conductivities of the retina or choroid increase, the RPE pumps fail, or the adhesion properties change. We determine the parameter regimes in which the blister either becomes unstable to growth, remains stable and finite-sized, or shrinks, allowing possible healing. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G44.00009: Direct micro-mechanical measurements on \textit{C. elegans} Matilda Backholm, William S. Ryu, Kari Dalnoki-Veress The millimeter-sized nematode \textit{Caenorhabditis elegans} provides an excellent biophysical system for both static and dynamic biomechanical studies. The undulatory motion exhibited by this model organism as it crawls or swims through a medium is ubiquitous in nature at scales from microns to meters. A successful description of this form of locomotion requires knowledge of the material properties of the crawler, as well as its force output as it moves. Here we present an experimental technique with which the material properties and dynamics of \textit{C. elegans} can be directly probed. By using the deflection of a flexible micropipette, the bending stiffness of \textit{C. elegans} has been measured at all stages of its life cycle, as well as along the body of the adult worm. The mechanical properties of the worm are modelled as a viscoelastic material which provides new insights into its material properties. The forces exerted by the worm during undulatory motion are also discussed. Direct experimental characterization of this model organism provides guidance for theoretical treatments of undulatory locomotion in general. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G44.00010: Emission of sound from the mammalian inner ear Tobias Reichenbach, Aleksandra Stefanovic, Fumiaki Nin, A.J. Hudspeth The mammalian inner ear, or cochlea, not only acts as a detector of sound but can also produce tones itself. These otoacoustic emissions are a striking manifestation of the mechanical active process that sensitizes the cochlea and sharpens its frequency discrimination. It remains uncertain how these signals propagate back to the middle ear, from which they are emitted as sound. Although reverse propagation might occur through waves on the cochlear basilar membrane, experiments suggest the existence of a second component in otoacoustic emissions. We have combined theoretical and experimental studies to show that mechanical signals can also be transmitted by waves on Reissner's membrane, a second elastic structure within the cochea [1]. We have developed a theoretical description of wave propagation on the parallel Reissner's and basilar membranes and its role in the emission of distortion products. By scanning laser interferometry we have measured traveling waves on Reissner's membrane in the gerbil, guinea pig, and chinchilla. The results accord with the theory and thus support a role for Reissner's membrane in otoacoustic emission.\\[4pt] [1] T. Reichenbach, A. Stefanovic, F. Nin, A. J. Hudspeth, Waves on Reissner's membrane: a mechanism for the propagation of otoacous [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G44.00011: Biofilms suck: how bacteria beat the diffusion limit Thomas Angelini, Wenbo Zhang, Steven Zehnder, Jolie Breaux Multicellular behavior in bacterial biofilms is intimately tied to the production of an extracellular polysaccharide (EPS) matrix that encases the cells and provides physical integrity to the colony as a whole. Recent work in \textit{Bacillus subtilis} biofilms shows that a sudden increase in EPS production generates osmotic stresses that cause the biofilm to expand. Moreover, EPS production is triggered by a nutrient depletion gradient that develops in the biofilm due to diffusive mass transport limitations. These polymer physics based biofilm behaviors suggest that EPS production may have evolved in biofilms to beat the diffusion limit of nutrient transport into the colony, though no direct observation of nutrient transport has been observed previously. Here we measure the rate of nutrient transport into \textit{b. subtilis} biofilms and find that when EPS production is up-regulated, the polymer sucks fluid into the colony with a characteristic time dependence like that of pressure driven flow. Preliminary data and analysis will be presented. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G44.00012: Inhomogeneous DNA replication kinetics is associated with immune system response John Bechhoefer, Michel G. Gauthier, Paolo Norio In eukaryotic organisms, DNA replication is initiated at ``origins,'' launching ``forks'' that spread bidirectionally to replicate the genome. The distribution and firing rate of these origins and the fork progression velocity form the ``replication program.'' Previous models of DNA replication in eukaryotes have assumed firing rates and replication fork velocities to be homogeneous across the genome. But large variations in origin activity and fork velocity do occur. Here, we generalize our replication model to allow for arbitrary spatial variation of initiation rates and fork velocities in a given region of the genome. We derive and solve rate equations for the forks and replication probability, to obtain the mean-field replication program. After testing the model on simulations, we analyze the changes in replication program that occur during B cell development in the mouse. B cells play a major role in the adaptive immune system by producing the antibodies. We show that the process of cell differentiation is associated with a change in replication program, where the zones of high origin initiation rates located in the immunoglobulin heavy-chain locus shift their position as the locus prepares to undergo the recombination events responsible for generating antibody specificity. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G44.00013: Effects of Viscosity on the Gravi-kinesis Responses of Swimming {\it Paramecia} Studied Using Manetic Force Buoyancy Variation Ilyong Jung, James M. Valles Previous studies have shown that {\it paramecia} exhibit negative gravi-kinesis. They exert a stronger propulsive force when swimming up than when swimming down. This behavior is very surprising since it suggests they sense their tiny apparent weight of only $\sim$ 80pN. In an effort to understand the mechanism of this sensing, we are testing how the viscosity of the swimming medium influences their gravi-kinetic response. We employ the technique of magnetic force buoyancy variation to simulate different effective gravity levels on swimming {\it Paramecia}. We are analyzing their swimming response employing a phenomenological model that relates the parameters describing their helical trajectories to the beating of their cilia.\\ This work was supported by NSF PHY0750360 and at the NHMFL by NSF DMR-0084173 [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G44.00014: Fundamental role of bistability in optimal homeostatic control Guanyu Wang Bistability is a fundamental phenomenon in nature and has a number of fine properties. However, these properties are consequences of bistability at the physiological level, which do not explain why it had to emerge during evolution. Using optimal homeostasis as the first principle and Pontryagin's Maximum Principle as the optimization approach, I find that bistability emerges as an indispensable control mechanism. Because the mathematical model is general and the result is independent of parameters, it is likely that most biological systems use bistability to control homeostasis. Glucose homeostasis represents a good example. It turns out that bistability is the only solution to a dilemma in glucose homeostasis: high insulin efficiency is required for rapid plasma glucose clearance, whereas an insulin sparing state is required to guarantee the brain's safety during fasting. This new perspective can illuminate studies on the twin epidemics of obesity and diabetes and the corresponding intervening strategies. For example, overnutrition and sedentary lifestyle may represent sudden environmental changes that cause the lose of optimality, which may contribute to the marked rise of obesity and diabetes in our generation. [Preview Abstract] |
Tuesday, March 19, 2013 2:03PM - 2:15PM |
G44.00015: Time-Dependent Kinematics of Complex Human Structures Saami J. Shaibani The human body can be arranged in numerous geometrical configurations, including many interesting scenarios from the sport of gymnastics. One particularly challenging analytical example among these is the forward flip with maximum separation from the ground at the apex of the flight. The temporal aspects of this move involve the evaluation of multiple different positions during the trajectory, which adds significantly to the effort required. When a forward flip was executed during a football game [1], ready access to the recording [2] of this allowed a detailed kinematic examination to be performed. Careful application of highly intricate protocols [3] produces results which are consistent with similar athletic environments. The emphasis in this research is to transcend standard approaches elsewhere, which are severely limited to generic athletes and/or generic circumstances. Pedagogical benefits of the rigorous methodology adopted here are explored beyond what was introduced in a recent related study [4].\\[4pt] [1] Cardinals at Bengals on 24/12/2011\\[0pt] [2] via popular video-sharing website\\[0pt] [3] OUEL reports 1426/82 \& 1427/82, 1982\\[0pt] [4] http://aapt.org/AbstractSearch/FullAbstract.cfm?KeyID=20973, 2012. [Preview Abstract] |
Session G45: Focus Session: Physics of Protein Aggregation
Sponsoring Units: DBIO DPOLY DCPChair: Daniel Cox, UC Davis
Room: Hilton Baltimore Holiday Ballroom 4
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G45.00001: Single Molecule Visualization of Protein-DNA Complexes: Watching Machines at Work Invited Speaker: Stephen Kowalczykowski We can now watch individual proteins acting on single molecules of DNA. Such imaging provides unprecedented interrogation of fundamental biophysical processes. Visualization is achieved through the application of two complementary procedures. In one, single DNA molecules are attached to a polystyrene bead and are then captured by an optical trap. The DNA, a worm-like coil, is extended either by the force of solution flow in a micro-fabricated channel, or by capturing the opposite DNA end in a second optical trap. In the second procedure, DNA is attached by one end to a glass surface. The coiled DNA is elongated either by continuous solution flow or by subsequently tethering the opposite end to the surface. Protein action is visualized by fluorescent reporters: fluorescent dyes that bind double-stranded DNA (dsDNA), fluorescent biosensors for single-stranded DNA (ssDNA), or fluorescently-tagged proteins. Individual molecules are imaged using either epifluorescence microscopy or total internal reflection fluorescence (TIRF) microscopy. Using these approaches, we imaged the search for DNA sequence homology conducted by the RecA-ssDNA filament. The manner by which RecA protein finds a single homologous sequence in the genome had remained undefined for almost 30 years. Single-molecule imaging revealed that the search occurs through a mechanism termed ``intersegmental contact sampling,'' in which the randomly coiled structure of DNA is essential for reiterative sampling of DNA sequence identity: an example of parallel processing. In addition, the assembly of RecA filaments on single molecules of single-stranded DNA was visualized. Filament assembly requires nucleation of a protein dimer on DNA, and subsequent growth occurs via monomer addition. Furthermore, we discovered a class of proteins that catalyzed both nucleation and growth of filaments, revealing how the cell controls assembly of this protein-DNA complex. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G45.00002: Hydrogen Bonding Motifs in MutSaphla and their response to binding damaged DNA Lacra Negureanu, Freddie Salsbury Over the past decade, there has been a growing interest in studying the binding of damaged DNA to the MutSalpha protein complex. This protein complex, the Msh2/Msh6 complex in humans, is the initial complex that binds mismatched DNA and other DNA defects that occur during replication. This complex has also been shown to bind at least some types of damaged DNA. As a result of this interest, multiple studies have contrasted the interactions of MutSalpha with its normal mismatched substrate and with the interactions of MutsSalpha to DNA damaged by the chemotherapeutic cisplatin. To complement these studies, we examined the interaction between MutSalpha and DNA damaged by carboplatin via all-atom molecular dynamics simulations. These simulations provide evidence for different hydrogen bonding interactions at the protein/DNA and protein/protein interface. The hydrogen bonding motifs found are broadly similar to those found in binding to the adduct from cis-platin, but have distinct differences. These subtle differences may play a role in the way the different damages are signaled by MutS. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G45.00003: Parallel Verlet Neighbor List Algorithm for GPU-Optimized MD Simulations Samuel Cho How biomolecules fold and assemble into well-defined structures that correspond to cellular functions is a fundamental problem in biophysics. Molecular dynamics (MD) simulations provide a molecular-resolution physical description of the folding and assembly processes, but the computational demands of the algorithms restrict the size and the timescales one can simulate. In a recent study, we introduced a parallel neighbor list algorithm that was specifically optimized for MD simulations on GPUs. We now analyze the performance of our MD simulation code that incorporates the algorithm, and we observe that the force calculations and the evaluation of the neighbor list and pair lists constitute a majority of the overall execution time. The overall speedup of the GPU-optimized MD simulations as compared to the CPU-optimized version is N-dependent and $\sim$ 30x for the full 70s ribosome (10,219 beads). The pair and neighbor list evaluations have performance speedups of $\sim$ 25x and $\sim$ 55x, respectively. We then make direct comparisons with the performance of our MD simulation code with that of the SOP model implemented in the simulation code of HOOMD, a leading general particle dynamics simulation package that is specifically optimized for GPUs. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G45.00004: Coarse-grained Simulations of Protein-Protein Association: Energy Landscape on a Globe Sichun Yang Understanding how proteins interact and associate into large functional complexes is critical in revealing the molecular basis of virtually every biological process in a living cell. Here, a theoretical simulation pipeline using coarse-grained (CG) models with an efficient sampling method is presented from the studies of protein-protein association. A concept of ``energy globe'' is introduced and implemented via the projection of simulation data onto a three-dimensional globe specifying protein-protein orientations and interacting energies. This energy-globe approach has the key advantage of locating and identifying multiple stable conformations that are physically accessible on the energy landscape. Tests on several well-studied protein-protein complexes show that the crystal-like conformation is favorable on the energy landscape even if the landscape is relatively rugged with metastable conformations. Recent applications to CG simulations of nuclear hormone receptors, whose experimental structure are still lacking, have predicted multiple favorable conformations on their corresponding landscapes, thereby providing insight into the cross-talk mechanisms of functional domains in the hormone signaling. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G45.00005: Snyder-Robinson Syndrome: Rescuing the Disease-Causing Effect of G56S mutant by Small Molecule Binding Zhe Zhang, Virginie Martiny, David Lagorce, Emil Alexov, Maria Miteva Snyder-Robinson Syndrome (SRS) is an X-linked mental retardation disorder, which is caused by defects in a particular gene coding for the spermine synthase (SMS) protein. Among the missense mutations known to be disease-causing is the G56S, which is positioned at the interface of the SMS homo-dimer. Previous computational and experimental investigations have shown that G56S mutation destabilizes the homo-dimer and thus greatly reduces the SMS enzymatic activity. In this study, we explore the possibility of mitigating the effect of G56S mutation by binding small molecules to suitable pockets around the mutation site. It is done by combined efforts of molecular dynamics simulations and in silico screening. The binding of selected molecules was calculated to fully compensate the effect of the mutation and rescue the wild type dimer affinity. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G45.00006: Aggregation of concentrated monoclonal antibody solutions studied by rheology and neutron scattering Maria Monica Castellanos, Jai Pathak, Ralph Colby Protein solutions are studied using rheology and scattering techniques to investigate aggregation. Here we present a monoclonal antibody (mAb) that aggregates after incubation at 40 $^{\circ}$C (below its unfolding temperature), with a decrease in monomer purity of 6{\%} in 10 days. The mAb solution contains surfactant and behaves as a Newtonian fluid when reconstituted into solution from the lyophilized form (before incubation at 40 $^{\circ}$C). In contrast, mAb solutions incubated at 40 $^{\circ}$C for 1 month exhibit shear yielding in torsional bulk rheometers. Interfacial rheology reveals that interfacial properties are controlled by the surfactant, producing a negligible surface contribution to the bulk yield stress. These results provide evidence that protein aggregates formed in the bulk are responsible for the yield stress. Small-angle neutron scattering (SANS) measurements show an increase in intensity at low wavevectors (q \textless\ 4*10$^{-2}$ nm$^{-1})$ that we attribute to protein aggregation, and is not observed in solutions stored at 4 $^{\circ}$C for 3 days before the measurement. This work suggests a correlation between the aggregated state of the protein (stability) and the yield stress from rheology. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G45.00007: Enhancing Nucleation rates using Porous Silica Sathish Akella, Seth Fraden The role of nucleants in promoting protein crystal nucleation is an on-going field of research. Porous silica acts as heterogeneous nucleation centers and enhances nucleation rates. For the protein lysozyme there are multiple polymorphs and we demonstrate that porous silica preferentially increases one of the polymorphs. Preliminary studies are presented in which accurate nucleation rates for the different polymorphs as a function of nucleant concentration are obtained through optical microscopy studies of thousands of crystallization trials in identical water-in-oil emulsion drops produced using microfluidics. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G45.00008: Solvent-induced size reduction of self-assembled siRNA/copolymer nanoparticles Wei Qu, Juan Wu, Hai-Quan Mao, Erik Luijten Small interfering RNA (siRNA) therapeutics has a demonstrated potential for treating numerous liver diseases. However, traditional polycation vectors used for siRNA delivery typically produce siRNA-containing particles of large size ($>100$~nm), along with high cytotoxicity and low colloidal stability. Inspired by earlier work on nanoparticles for plasmid DNA delivery [1], we graft hydrophilic and biocompatible polyethylene glycol (PEG) blocks to the polycation vector to overcome these limitations. We find that the PEG-grafted polycations result in slightly larger particle size, even though the hydrophilic PEG blocks are expected to hinder the formation of larger aggregates. To explain this observation, we investigate siRNA/copolymer self-assembly via computer simulations of coarse-grained polymer and siRNA models. Our calculations suggest that hydrogen bonding between PEG and the polycation leads to the increased particle size, and that smaller particles can be obtained by inhibiting hydrogen bonding in such system. Subsequent experiments employing solvents of lower polarity indeed lead to particles with smaller size. \\[4pt] [1] X. Jiang \emph{et al.}, Adv. Mater., doi: 10.1002/adma.201202932 [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G45.00009: Using Nanoscale Substrate Curvature to Control the Dimerization of Surface-Bound Proteins Hilary Paulin, Martin Kurylowicz, Josh Mogyoros, Maximiliano Giuliani, John Dutcher The influence of surface geometry on adsorbed proteins offers new possibilities for controlling quaternary structure by manipulating protein-protein interactions at a surface, with applications that are relevant to protein aggregation, fibrillation, ligand binding and surface catalysis. To understand the effect of surface curvature on the structure of surface-bound proteins, we have used a combination of polystyrene (PS) nanoparticles (NPs) and ultrathin PS films to fabricate chemically pure, hydrophobic surfaces that have nanoscale curvature and are stable in aqueous buffer. We have used Single Molecule Force Spectroscopy (SMFS) to measure the detachment contour lengths$~(L_{c})$ for~beta-lactoglobulin (b-LG) and~alpha-lactalbumin (a-LA) adsorbed onto neighbouring regions of highly curved and flat PS surfaces, allowing us to compare these values~\textit{in situ}~on the same sample.~In general, we measure peaks in the $L_{c}$ distributions corresponding to monomers and dimers. As the curvature of the underlying surface is increased, the population of dimers decreases such that only monomers are observed for b-LG adsorbed onto 25 nm dia NPs. These results indicate that surface curvature provides a new method of manipulating protein-protein interactions and controlling the quaternary structure of adsorbed proteins. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G45.00010: Coarse-grained Molecular Dynamics Simulation of Calmodulin-target Interactions Pengzhi Zhang, Qian Wang, Swarnendu Tripathi, Margaret Cheung Calmodulin (CaM) is a ubiquitous small protein playing an important role in Ca$^{\mathrm{2+}}$ signaling in eukaryotic cells, which can bind and regulate hundreds of target enzymes in the presence of Ca$^{\mathrm{2+}}$. Although the binding process is known to be diffusion controlled, however, due to the flexibility of CaM, methodology that provides molecular insights on target binding and recognition. In this study, Brownian dynamics simulations were used to mimic the process Ca$^{\mathrm{2+}}$-bound CaM binds with two target peptides: CaMKI and CaMKII. Using an experimentally-analogous criterion of number of contacts between targets and a specific residue of CaM to define the encounter complexes and to calculate the association rates, we are able to reveal the molecular reason why CaM-CaMKI has twice the rate of CaM-CaMKII while the numbers of amino acids are similar. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 1:51PM |
G45.00011: Linear and Nonlinear Microrheology of Interfacial Protein Layers Daniel Allan, Daniel Firester, Victor Allard, Daniel Reich, Robert Leheny Proteins can adsorb to the air-water interface to form a robust layer. As protein accretes and a layer forms, we monitor the layer's shear rheology employing both passive and active microrheology. Measurements of the linear rheology, using multiple-particle-tracking techniques, show a transition from a viscous to elastic interface with increasing layer age. Active measurements of the nonlinear rheology, in which ferromagnetic nanowires at the interface rotate in response to magnetic torques, show that the protein layers behave quantitatively like a Hershel-Bulkley fluid. We interpret these observations in terms of mechanisms of layer formation and protein interactions at the interface. [Preview Abstract] |
Tuesday, March 19, 2013 1:51PM - 2:03PM |
G45.00012: Contributions of equilibrium and non-equilibrium clusters to viscosity in concentrated protein solutions Prasad Sarangapani, Steven Hudson, Jai Pathak, Kalman Migler Equilibrium and non-equilibrium clustering are ubiquitous phenomena in soft matter physics and are typically observed in systems ranging from colloidal suspensions to monoclonal antibodies (mAbs). Such phenomena are central to understanding and preventing irreversible aggregation in addition to controlling viscosity challenges related to formulation and drug delivery of protein therapeutics. Curiously, little work has been done in exploring the cluster size dependence of low-shear viscosity and intrinsic viscosity in protein solutions in a controlled manner. In this work, we carefully control cluster size of reversible and irreversible clusters formed by globular proteins or monoclonal antibodies over a concentration range of 2 mg/mL-500 mg/mL and pH from 3-9. We find a marked dependence of low-shear viscosity on cluster size using custom-designed silicon-based microfluidic viscometers. Measurements of cluster sizes using static light scattering reveal a correlation of low shear viscosity as well as intrinsic viscosity with the average cluster size. We model the composition dependence of viscosity for the case of equilibrium and non-equilibrium clusters using an adaptation of a model recently presented by Minton for protein mixtures. [Preview Abstract] |
Session G46: SPS Undergraduate V
Sponsoring Units: SPSChair: Kendra Redmond, American Institute of Physics
Room: Hilton Baltimore Holiday Ballroom 5
Tuesday, March 19, 2013 11:15AM - 11:27AM |
G46.00001: Reverse Micelle Synthesis of Gadolinium Nanoparticles R.H. Fukuda, M.M. Castro, P.-C. Ho, S. Attar, M. Golden, D. Margosan Nanotechnology is an area of great interest due to its variety of applications such as nano-medicine. The reverse micelle method has been used to synthesize Gd nanoparticles by our research group. Through this method, a surfactant protectively cages particles of Gd in the presence of polar methanol and nonpolar hexane. This method can control particle size by growth temperature and the molar ratio of polar solvent to surfactant. The Gd was reduced from its chloride compound by using sodium borohydride. The final products have been derived either through a method of liquid liquid extraction or filtration. Scanning electron microscopy (SEM) paired with energy dispersive x-ray spectroscopy (EDX) was used to examine the size, shape, and composition of the products. The size and shape were also examined using a Leica light microscope between SEM analyses. We found that liquid liquid extraction does not work in the solvent combination of methanol-hexane due to the instability of the reverse micelles. Additionally, the process of carbon coating SEM samples may have destroyed the reverse micelle structures. [Preview Abstract] |
Tuesday, March 19, 2013 11:27AM - 11:39AM |
G46.00002: Ferromagnetic Nanoparticles for Biomedical Applications Frank Holder, Cristina Iftode, Tabbetha Dobbins This work examines the cytotoxicity of barium hexaferrite to fibroblast (HEK-293) cells and also the response of barium hexaferrite to magnetic fields. Cytotoxicity is a great way for pharmacies to measure for toxic compounds. Cytotoxicity assays are widely used by the pharmaceutical industry to screen new compounds which may be introduced to the cells. Results show the cytotoxicity of nanoparticles of barium hexaferrite. We chose barium hexaferrite because it is a magnetic material---so it can be driven using an applied magnetic field. This would be useful in biomedical applications where these particles may be added to direct treatment to various parts of the body and across the cell wall membrane by an applied magnetic field. [Preview Abstract] |
Tuesday, March 19, 2013 11:39AM - 11:51AM |
G46.00003: Morphological, Thermal, and Magnetic Analysis of Ball-Milled $\gamma $-Fe$_2$O$_3$ and Fe$_3$O$_4$ Nanoparticles for Biomedical Application Philip Burnham, Georgia C. Papaefthymiou, Arthur Viescas, Calvin Li, Norman Dollahon Superparamagnetic iron oxide nanoparticles are promising agents for hyperthermia cancer treatment, because, when exposed to an alternating magnetic field, they impart heat to surrounding tissue. A comparison of $\gamma $-Fe$_2$O$_3$ and Fe$_3$O$_4$ nanoparticles for such application is presented. The particles were obtained via surfactant-assisted high energy ball-milling in a hexane/oleic acid carrier-fluid environment. Particles with diameters of 5 to 16 nm were prepared with mass ratios (oleic acid):($\gamma $-Fe$_2$O$_3)$ of 0:1, 1:5, 1:10 and 1:20, with milling times of 3, 6, 9, and 12 hours. TEM micrographs revealed spherical morphology and the effect of oleic acid shells. Optimal size distributions were obtained for high oleic acid contents. At room temperature, a reduced internal magnetic field $\sim$480 kOe) was recorded via M\"{o}ssbauer spectroscopy compared to bulk $\gamma $-Fe$_2$O$_3$ $\sim$500 kOe), due to magnetic relaxation; Fe$_3$O$_4$ particles produced similar results. For the $\gamma $-Fe$_2$O$_3$ and Fe$_3$O$_4$ nanoparticles with 20{\%} oleic acid by mass, comparative ZFC/FC magnetization (H$_{\mathrm{app}}=$ 200 Oe in temperature range from 2 to 400 K) and hysteresis loops (T $=$ 2 K and 300 K up to H$_{\mathrm{app}}=$6 kOe) were obtained. Thermal transport characteristics were verified by Specific Absorption Rate (SAR) measurements using an AC magnetic field ($f=$282 kHz). Differences and similarities in behavior will be discussed. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:03PM |
G46.00004: Multi-scale Size Distributions of Colloidal Gold Clusters Measured by Ultrasmall Angle X-ray Scattering (USAXS) and Dynamic Light Scattering (DLS) Ashli Nieves, Jan Ilavsky, Tabbetha Dobbins Gold colloids are of interest as: (1) catalysts for energy conversion and (2) absorption agents for laser photothermal therapy. This research examines the agglomerate sizes (using DLS) and primary particle sizes (using USAXS) for gold nanoparticles synthesized by trisodium citrate reduction of gold chloroauric acid (HAuCl4). USAXS data was collected at the Advanced Photon Source, beamline 15ID-D. Model fitting of the data show primary particle sizes of 7nm to 14nm formed. DLS results show these particles to aggregate into a bimodal set of clusters centered on approximately 20nm and approximately 200nm. Preliminary results aimed at effectively breaking apart these aggregates are presented. [Preview Abstract] |
Tuesday, March 19, 2013 12:03PM - 12:15PM |
G46.00005: Dynamical properties of colloids immersed in a uniform electric field at high densities Matthew Wozniak, Manuel Valera, Athula Herat In light of the recent interest in the control of colloidal systems, we have explored specific properties of electrically interacting colloidal particles. We explored the structural and dynamical characteristics of mono-disperse systems of colloidal particles that are affected by dipole-dipole interactions while immersed in a uniform electric field and compared with the outcomes that could occur if different sizes of particles are mixed. We used molecular dynamics simulations to study the systems. We present results for the diffusion coefficient and other dynamical properties in the high density regime. [Preview Abstract] |
Tuesday, March 19, 2013 12:15PM - 12:27PM |
G46.00006: Synthesis and Characterization of Mg-doped ZnO Nanorods for Biomedical Applications H. Gemar, N.C. Das, A. Wanekaya, R. Delong, K. Ghosh Nanomaterials research has become a major attraction in the field of advanced materials research in the area of Physics, Chemistry, and Materials Science. Bio-compatible and chemically stable metal nanoparticles have biomedical applications that includes drug delivery, cell and DNA separation, gene cloning, magnetic resonance imaging (MRI). This research is aimed at the fabrication and characterization of Mg-doped ZnO nanorods. Hydrothermal synthesis of undoped ZnO and Mg-doped ZnO nanorods is carried out using aqueous solutions of Zn(NO$_{3})_{2}$ .6H$_{2}$O, MgSO$_{4}$, and using NH$_{4}$OH as hydrolytic catalyst. Nanomaterials of different sizes and shapes were synthesized by varying the process parameters such as molarity (0.15M, 0.3M, 0.5M) and pH (8-11) of the precursors, growth temperature (130$^{\circ}$C), and annealing time during the hydrothermal Process. Structural, morphological, and optical properties are studied using various techniques such as XRD, SEM, UV-vis and PL spectroscopy. Detailed structural, and optical properties will be discussed in this presentation. This work is partially supported by National Cancer Institute (1 R15 CA139390-01). [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 12:39PM |
G46.00007: Study of Thermal Conductivity of Si Nanowires with micro-Raman Spectroscopy Bingqing Li, Kathryn F. Murphy, Daniel S. Gianola, X.M. Cheng Nanowires have played an increasingly important role in thermoelectric technology due to their high figure of merit ZT resulting from the reduced thermal conductivity, K, and good electrical conductivity. In this work, we report the measurement of K of individual silicon nanowires (SiNWs) by mapping Raman temperature profiles along the testing nanowires using a microelectromechanical system (MEMS) device and a micro-Raman system with a 530 nm laser beam. Thermal conductivity was measured as a function of uniaxial tensile stress applied to the SiNWs, which was varied from 0 to 1.2 GPa. The measured K results for the unstrained nanowires agree well with the predictions based on diffuse phonon boundary scattering. The dependence of SiNWs' thermal conductivity on engineering stress can provide significant information for nanowires fabrication. [Preview Abstract] |
Tuesday, March 19, 2013 12:39PM - 12:51PM |
G46.00008: Characterization of Carbon Nanotubes Synthesized Using Chemical Vapor Deposition Andrew Zeidell, Shawn Huston, Nathanael Cox, Brian Landi, Tonya Coffey, Phillip Russell, Brad Conrad Carbon Nanotubes were synthesized using a Chemical Vapor Deposition system with precursor Cyclopentadienyliron Dicarbonyl Dimer and were systematically characterized over a variety of growth conditions using several methods. Scanning Electron Microscopy (SEM) was used to investigate catalyst contamination, tube diameters, growth morphologies, and material alignment. Transmission Electron Microscopy (TEM) was employed to quantify nanotube wall crystallinity and sidewall defects. Raman Spectroscopy was used in conjunction with Thermo-Gravimetric analysis to ascertain the purity levels of each sample. Results are discussed in terms of related precursors and are used to evaluate the efficacy of the precursor and material quality. [Preview Abstract] |
Tuesday, March 19, 2013 12:51PM - 1:03PM |
G46.00009: Bi2Te3 Nanostructure Synthesis on Multiple Substrates Nicha Apichitsopa, Jerome T. Mlack, Nina Markovic The chalcogenide Bi2Te3 is a known and widely used thermoelectric material that has received renewed experimental interest due to the recent discovery of its topologically protected surface states. Nanodevices of this material are particularly interesting because of their high surface-to-volume ratio, which enhances surface-related transport properties by minimizing bulk contributions. Many synthesis processes for Bi2Te3 have been reported, such as as Au-catalyzed vapor-liquid-solid mechanism (VLS) and lithographically patterned galvanic displacement (LPGD). The VLS mechanism is much simpler than the highly-controlled LPGD; however, remnant of Au catalyst on the nanostructures can alter their electronic structure, resulting in modification of TI surface. We report the synthesis of Bi2Te3 nanostructures by VLS mechanism without using Au catalyst, which improves the quality of the nanostructures. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:15PM |
G46.00010: SAM surface domains of 1-mercaptoundecanoic acid and 1-dodecanethiol mixtures on Au(111) investigated via polarized probes Rose Pasquale, Reshani Senevirathne, Indrajith Senevirathne SAM (Self Assembled Monolayer) surfaces with --COOH terminus is bio active and therefore has many bioengineering applications. However complex devices patterned on surfaces require a deeper understanding of the surface domain architecture of SAMs with multi component mixtures of thiols. Varying concentration mixed solutions of 1-mercaptoundecanoic acid (hydrophilic -COOH end) and 1-dodecanethiol (hydrophobic --R), dissolved in 200 proof Ethanol with total 5mM concentration were prepared. These solutions were used in developing SAMs on clean flat Au(111) on mica. Resulting SAMs surfaces were investigated with regular and custom built positively and negatively polarized AFM (Atomic Force Microcopy) probes via contact, non contact and lateral force mode AFM with topography and phase imaging. Domains of distinct thiols were identified as selective self assembly on step edges and terraces. Surface roughness, corrugation and morphology at each domain were estimated. Total RMS surface roughness is estimated at $\sim$ 2.44nm for SAMs with 75{\%} 1-mercaptoundecanoic acid while for SAMs with 25{\%} 1-mercaptoundecanoic acid it is estimated at $\sim$ 2.68nm. [Preview Abstract] |
Tuesday, March 19, 2013 1:15PM - 1:27PM |
G46.00011: Characterization of Nanophosphors for Solid State Lighting Devices Grown by Microwave Plasma Assisted Deposition Process Jedidiah McCoy, Marek Merlak, Sarath Witanachchi Increasingly, greenhouse farming and urban agriculture are being looked at as a more efficient and more cost effective way to grow produce. Currently the lights used in greenhouses rely on light sources that emit a broad spectrum of light. However, only light at wavelengths around 460 nm (blue) and 670 nm (red) are absorbed by most plants for photosynthesis. Solid state lighting devices can be engineered to produce light to match the needs of the plant while reducing the energy cost. An investigation into the photoluminescence properties of the nanophosphor La$_2$O$_3$ doped with Bi was done in an effort to produce a phosphor emitting in blue wavelengths. The La$_2$O$_3$:Bi coatings were grown using a microwave plasma growth process. Microwave power and chamber pressure were varied to find the optimum synthesis conditions. Power was varied from 100Watts to 900Watts and chamber pressure was varied from 30Torr to 60Torr. The process utilized O$_2$ and CO$_2$ plasma. The nanophosphors were investigated by X-ray diffraction, electron microscopy, and photoluminescent spectroscopy. Photoluminescence was shown to be higher from samples synthesized in a CO$_2$ plasma. [Preview Abstract] |
Tuesday, March 19, 2013 1:27PM - 1:39PM |
G46.00012: Slip, Slide, or Roll? Mike Testa Using an atomic force microscope the research project, ``Slip, Slide, or Roll?'' investigates rolling and sliding friction on the nanoscale. The findings of this study may be used to develop improved mechanical lubricants and surfaces. Friction may seem like a simple idea that is familiar to everyone, yet scientific literature explaining what dictates the translational modes of nanoscale objects is surprisingly lacking. In the macroscopic world spherical objects energetically prefer rolling over sliding, for nanoscale objects this is not necessarily the case. We are testing the hypothesis that size, surface chemistry, and elastic modulus dictate whether spherical nanoscale objects will slide or roll when a lateral force is applied. In order to understand the conditions that cause nanoscale particles to transition between the two translational modes we precisely manipulate these variables and measure their effects. [Preview Abstract] |
Session G47: Invited Session: Elasticity and Plasticity Outside of Equilibrium: Modeling From Micro to Meso Scales
Sponsoring Units: GSNPChair: Zhi Feng Huang, Wayne State University
Room: Hilton Baltimore Holiday Ballroom 6
Tuesday, March 19, 2013 11:15AM - 11:51AM |
G47.00001: Dislocation dynamics, plasticity and avalanche statistics using the phase-field crystal model Invited Speaker: Luiza Angheluta The plastic deformation of stressed crystalline materials is characterized by intermittency and scaling behavior. The sudden strain bursts arise from collective interactions between depinned crystal defects such as dislocations. Recent experiments on sheared nanocrystals provide insights into the connection between the crystal plasticity and the mean field theory of the depinning transition, based on the similar power-law statistics of avalanche events. However, a complete theoretical formulation of this connection is still lacking, as are high quality numerical data. Phase field crystal modelling provides an efficient numerical approach to simulating the dynamics of dislocations in plastic flows at finite temperature. Dislocations are naturally created as defects in a periodic ground state that is being sheared, without any ad hoc creation and annihilation rules. These crystal defects interact and annihilate with one another, generating a collective effect of avalanches in the global plastic strain rate. We examine the statistics of plastic avalanches both at finite and zero temperatures, and find good agreement with the predictions of the mean field interface depinning theory. Moreover, we predict universal scaling forms for the extreme statistics of avalanches and universal relations between the power-law exponents of avalanche duration, size and extreme value. These results account for the observed power-law distribution of the maximum amplitudes in acoustic emission experiments of crystal plasticity, but are also broadly applicable to other systems in the mean-field interface depinning universality class, ranging from magnets to earthquakes. The work reported here was performed in collaboration with: Georgios Tsekenis, Michael LeBlanc, Patrick Y Chan, Jon Dantzig, Karin Dahmen, and Nigel Goldenfeld. [Preview Abstract] |
Tuesday, March 19, 2013 11:51AM - 12:27PM |
G47.00002: Connecting grain boundary properties to microstructural evolution in polycrystalline metals Invited Speaker: Elizabeth Holm Within the last decade, both computational and experimental methods have evolved to the point that large-scale surveys of grain boundary properties have become tractable. Such studies have provided new information and insight about boundary structure, energetics, motion mechanisms, and mobility on a scale that invites application to polycrystalline systems. However, the complex behavior revealed in these studies often generates as many questions as it answers. This presentation will review pertinent computational and experimental studies of grain boundary properties in FCC metals, concentrating on boundary energy and mobility. The goal will be to identify the microstructural signatures of boundary properties in polycrystalline grain boundary networks. Topics will include how boundary energy and mobility trends manifest in real microstructures; the effects of shear coupling on boundary motion in bicrystals and polycrystals; the significance of boundaries that move in a non-thermally-activated manner to low temperature grain growth; and the consequences of the thermal roughening transition on grain stagnation. In each case, individual grain boundary properties couple with the characteristics of the grain boundary network to generate diverse microstructural outcomes. [Preview Abstract] |
Tuesday, March 19, 2013 12:27PM - 1:03PM |
G47.00003: Modeling polycrystalline multiferroics materials Invited Speaker: Ken Elder Multifeorroics are materials that involve the coupling of elasticity, magnetization and polarization. The ability to turn mechanical energy into electric or magnetic energy has been exploited for many years in device applications. More recently there has been a great deal of interest in systems that contain all three properties so that the elastic coupling can be used to control polarization with magnetic fields or magnetization with electric fields. In this talk I would like to discuss the development of a phase field crystal model that incorporates all of the rich physics contained in polycrystalline multiferroic materials. To extend the use of this model to larger length scales an amplitude description will be presented. This description also provides a natural link to traditional continuum field theories of multiferroic materials. [Preview Abstract] |
Tuesday, March 19, 2013 1:03PM - 1:39PM |
G47.00004: Phase-Field Crystal Modeling of Polcrystalline Pattern Evolution in Hard and Soft Matter Invited Speaker: Alain Karma The phase-field crystal (PFC) model has attracted considerable attention during the past decade for its potential application to model the complex defect-mediated dynamics of hard and soft crystalline materials on diffusive time scales. The model is rooted in earlier models of non-equilibrium pattern formation (Swift-Hohenberg equation), and classical density functional theory that expresses the free-energy of a system as a functional of its density. This talk will discuss progress made to investigate the dynamics for both isolated grain boundaries and complex polycrystalline patterns under the driving forces of boundary curvature and applied stress. The results highlight fundamental differences between polycrystalline pattern evolution in soft matter, including colloid crystals and crystalline non-equilibrium patterns described by the standard PFC dynamics, and crystalline solids described by a reformulation of this dynamics presented in this talk. The results also pave the way for a unified theory of polycrystalline pattern evolution in hard and soft matter. [Preview Abstract] |
Tuesday, March 19, 2013 1:39PM - 2:15PM |
G47.00005: Understanding the Evolution of Microstructure: What is the Role of Molecular Dynamics? Invited Speaker: Stephen Foiles The microstructure of a material, as characterized for example by grain size, determines a wide range of materials properties such as strength, toughness, and corrosion resistance. Understanding how the microstructure influences properties and how to obtain a desired microstructure are some of the enduring central problems of materials science. This challenge is inherently multi-scale since the fundamental mechanisms by which microstructures change occur at the atomic scale while the network of interfaces is on a scale of microns and up. In this talk, the role of molecular dynamics (MD) simulations in understanding the evolution of microstructure will be examined. The successes and outstanding challenges of using MD simulations to determine the properties of grain boundaries, in particular free energy and mobility, will be described. Further, microstructures with nanoscale grains evolve in times accessible to MD simulation. The insights into grain growth and deformation that can be obtained from such simulations will be described. [Preview Abstract] |
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