Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session L1: Recent Advances in Density Functional Theory V
Sponsoring Units: DCP DCOMPChair: Aurora Pribram-Jones, University of California, Irvine
Room: 103/105
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L1.00001: Recent progress in density functional theory Invited Speaker: Donald Truhlar Ongoing work involves several areas of density functional theory: new methods for computing electronic excitation energies, including a new way to remove spin contamination in the spin-flip Tamm-Dancoff approximation and a configuration-interaction-corrected Tamm-Dancoff Approximation for treating conical intersections; new ways to treat open-shell states, including a reinterpreted broken-symmetry method and multi-configuration Kohn-Sham theory; a new exchange-correlation functional; new tests of density functional theory against databases for electronic transition energies and molecules and solids containing metal atoms; and applications. A selection of results will be presented. I am grateful to the following collaborators for contributions to the ongoing work: Boris Averkiev, Rebecca Carlson, Laura Fernandez, Laura Gagliardi, Chad Hoyer, Francesc Illas, Miho Isegawa, Shaohong Li, Giovanni Li Manni, Sijie Luo, Dongxia Ma, Remi Maurice, Rub\'{e}n Means-Pa\~{n}eda, Roberto Peverati, Nora Planas, Prasenjit Seal, Pragya Verma, Bo Wang, Xuefei Xu, Ke R. Yang, Haoyu Yu, Wenjing Zhang, and Jingjing Zheng. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L1.00002: Density-Decomposed Orbital-Free Density Functional Theory for Covalent Systems and Application to Li-Si alloys Junchao Xia, Emily Carter We propose a density decomposition scheme using a Wang-Govind-Carter (WGC)-based kinetic energy density functional (KEDF) to accurately and efficiently simulate covalent systems within orbital-free (OF) density functional theory (DFT). By using a local, density-dependent scale function, the total density is decomposed into a localized density within covalent bond regions and a flattened delocalized density, with the former described by semilocal KEDFs and the latter treated by the WGC KEDF. The new model predicts reasonable equilibrium volumes, bulk moduli, and phase ordering energies for various semiconductors compared to Kohn-Sham (KS) DFT benchmarks. The surface energy of Si(100) also agrees well with KSDFT. We further apply the model to study mechanical properties of Li-Si alloys, which have been recently recognized as a promising candidate for next-generation anodes of Li-ion batteries with outstanding capacity. We study multiple crystalline Li-Si alloys. The WGCD KEDF predicts accurate cell lattice vectors, equilibrium volumes, elastic moduli, electron densities, alloy formation and Li adsorption energies. Because of its quasilinear scaling, coupled with the level of accuracy shown here, OFDFT appears quite promising for large-scale simulation of such materials phenomena. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L1.00003: Density-Functional Theory of Thermal Transport F.G. Eich, A. Principi, M. Di Ventra, G. Vignale We have recently introduced a non-equilibrium density-functional theory of local temperature and associated energy density that is suitable for the study of thermoelectric phenomena from first principles [1]. This theory rests on a local temperature field coupled to the energy-density operator. Here we apply the theory to a simple two-terminal setup, in which the terminals are held at different temperatures. We show that our treatment becomes equivalent to the standard Landauer-B{\"u}ttiker formulation of thermal transport in the non-interacting limit. \\[4pt] [1] arXiv:1308.2311 [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L1.00004: Accurate and systematically improvable quantum embedding methods for complex systems Jason Goodpaster, Taylor Barnes, Frederick Manby, Thomas Miller We describe embedded density functional theory (e-DFT) methods that avoid approximations to the kinetic energy functional and provide a formally exact approach to performing electronic structure calculations in the e-DFT framework. This framework allows systems to be divided into smaller subsystems which can be treated at different levels of theory, with the intersubsystem potential calculated using our e-DFT protocol. We use this framework to develop robust wavefunction embedding methods. This allows for wavefunction calculations to be used in regions of large systems where DFT is known to perform poorly, such as van der Waals interactions and strongly correlated electrons. Through a systematic analysis of embedding errors, we determine the largest source of error from wavefunction-in-DFT embedding to be the evaluation of the approximate non-additive exchange-correlation functional. We suggest new algorithms to systematically reduce these errors. These improvements allow for embedding methods that accurately reproduce reference couple-cluster calculations for a series of chemical reactions. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L1.00005: Wavefunctions, Adiabatic Connections, and Universal Functionals for 1-Matrix Functional Theory Invited Speaker: Paul Ayers In Kohn-Sham density functional theory, a reference system of noninteracting electrons with the same density as the physical system is used as a zeroth-order approximation to the system. Using an adiabatic connection to the target system, one can then correct the Kohn-Sham approximation. In this talk, I will establish an analogous approach for the 1-electron reduced density matrix (DM1). The reference system now contains \textit{interacting} electrons, which we choose to describe with the Richardson Hamiltonian. The Richardson Hamiltonian includes the noninteracting-electrons limit (Kohn-Sham) and strictly-correlated-electrons limit (antisymmetrized geminal power). Any singlet-state DM1 can be reproduced by a Richardson Hamiltonian, and an adiabatic connection from the Richardson Hamiltonian to the target physical system provides a rigorous definition for the correlation functional in 1-density matrix functional theory (DM1FT). Preliminary numerical results are favorable. Because DM1FT treats both the strongly correlated and weakly correlated limits exactly, it seems to be a very promising avenue for research. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L1.00006: Semiclassical approach to the exchange energy from potential functional theory Attila Cangi, Peter Elliott, Stefano Pittalis, E.K.U. Gross, Kieron Burke Although Kohn-Sham (KS) density functional theory is being successfully and ever increasingly applied for computing the electronic structure of matter, there is a lack of a systematic procedure for deriving reliable approximations for its main ingredient -- the exchange-correlation (XC) energy functional. Potential functional theory[1,2,3] is an alternative approach that may provide a solution to this long-standing problem. In our line of research we had only considered potential functional approximations to the KS kinetic energy[4,5] so far. In this work, we (i) propose approximating the XC energy straight as a functional of the KS potential and (ii) derive a highly accurate potential functional approximation to the exchange energy for the simplest relevant model system using semiclassical techniques[6]. [1] W. Yang, P. W. Ayers, and Q. Wu, Phys. Rev. Lett. 92, 146404 (2004). [2] A. Cangi, D. Lee, P. Elliott, K. Burke, and E.K.U. Gross, Phys. Rev. Lett. 106, 236404 (2011). [3] A. Cangi, E.K.U. Gross, K. Burke, Phys. Rev. A (2013), accepted. [4] P. Elliott, D. Lee, A. Cangi, and K. Burke, Phys. Rev. Lett. 100, 256406 (2008). [5] A. Cangi, D. Lee, P. Elliott, and K. Burke, Phys. Rev. B 81, 235128 (2010). [6] A. Cangi, P. Elliott, S. Pittalis, E.K.U. Gross, K. Burke, submitted. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L1.00007: Three- to two-dimensional crossover in time-dependent density-functional theory Shahrzad Karimi, Carsten Ullrich Quasi-two-dimensional (2D) systems, such as an electron gas confined in a quantum well, are of great theoretical interest and practical importance. Earlier studies of the crossover from 3D to 2D in ground-state density-functional theory have shown that local and semilocal exchange-correlation functionals which are based on the 3D electron gas as reference system work well for wide quantum wells, but eventually break down as the true 2D limit is approached. We now consider the dynamical case and study the performance of various linear-response exchange kernels in time-dependent density-functional theory. We compare approximate local and orbital-dependent exchange kernels with time-dependent Hartree-Fock theory for n-doped quantum wells, and analyze their behavior for intersubband charge and spin plasmons as they cross over from the quasi-2D to the bulk plasmon regime. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L1.00008: Stationary state Kohn-Sham Theory: Modern algorithms breathe new life into an old theory Deniz Gunceler, Ravishankar Sundararaman, T.A. Arias In this talk, we will discuss stationary-state Kohn-Sham theory, an old (Phys. Rev. B 31, 6264-6272) but largely ignored idea that is recently undergoing revival. It is based on an in-principle exact scheme in which excited states are computed as the stationary states of the Hohenberg-Kohn functional. We will discuss the objections of Gaudoin and Burke (Phys. Rev. Lett. 93, 17), and also describe the computational difficulties which prevented this theory from becoming popular in the past, and present new algorithms for computing the predictions of this theory. The resulting technique has inherent computational advantages over TDDFT and GW, and results using semilocal functionals show great promise for molecules. However, the method as implemented exhibits large errors for solids. In this talk, we shall show that the origin of this behaviour is related to the fact that different errors dominate the solid and molecular cases, and we shall discuss prospects for improvement of the theory in the future. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L1.00009: Increasing the efficiency and accuracy of time-resolved electronic spectra calculations with on-the-fly ab initio quantum dynamics methods Jiri Vanicek Rigorous quantum-mechanical calculations of coherent ultrafast electronic spectra remain difficult. I will present several approaches developed in our group that increase the efficiency and accuracy of such calculations: First, we justified the feasibility of evaluating time-resolved spectra of large systems by proving that the number of trajectories needed for convergence of the semiclassical dephasing representation/phase averaging is independent of dimensionality. Recently, we further accelerated this approximation with a cellular scheme employing inverse Weierstrass transform and optimal scaling of the cell size. The accuracy of potential energy surfaces was increased by combining the dephasing representation with accurate on-the-fly ab initio electronic structure calculations, including nonadiabatic and spin-orbit couplings. Finally, the inherent semiclassical approximation was removed in the exact quantum Gaussian dephasing representation, in which semiclassical trajectories are replaced by communicating frozen Gaussian basis functions evolving classically with an average Hamiltonian. Among other examples I will present an on-the-fly ab initio semiclassical dynamics calculation of the dispersed time-resolved stimulated emission spectrum of the 54-dimensional azulene. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L1.00010: Exploring Nuclear Effects in the Dynamics of Nanomaterials with a Quantum Trajectory-Electronic Structure Approach Sophya Garashchuk A massively parallel, direct quantum molecular dynamics method is described. The method combines a quantum trajectory (QT) representation of the nuclear wavefunction discretized into an ensemble of trajectories with an electronic structure (ES) description of electrons, namely using the Density Functional Tight Binding (DFTB) theory. Quantum nuclear effects are included into the dynamics of the nuclei via quantum corrections to the classical forces. To reduce computational cost and increase numerical accuracy, the quantum corrections to dynamics resulting from localization of the nuclear wavefunction are computed approximately and included into selected degrees of freedom representing light particles where the quantum effects are expected to be the most pronounced. A massively parallel implementation, based on the Message Passing Interface allows for efficient simulations of ensembles of thousands of trajectories at once. The QTES-DFTB dynamics approach is employed to study the role of quantum nuclear effects on the interaction of hydrogen with a model graphene sheet, revealing that neglect of nuclear effects can lead to an overestimation of adsorption. [Preview Abstract] |
Session L2: Focus Session: Surface Chemistry and Catalysis V
Sponsoring Units: DCPChair: Jill Millstone, University of Pittsburgh
Room: 102
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L2.00001: Structural Characterization of Mg/Al hydrotalcite-like Compounds and their Thermal Stability Shuhua Zhang, Siyuan Yang, Cheng Wang, Weijun Liu, Xiaodan Gu, Wenjun Gan, Xiaoyu Xue Hydrotalcite-like compounds, repersented by the formula [M$^{2+}_{\mathrm{1-x}}$M$_{\mathrm{x}}^{3+}$(OH)$_{2}$]X$_{\mathrm{x/n}}^{\mathrm{n-}}$ $\cdot$ nH$_{2}$O (M$^{2+}=$ Ni$^{2+}$, Mg$^{2+}$, Cu$^{2+}$,etc; M$^{3+}=$Al$^{3+}$, Fe$^{3+}$, etc; X$^{\mathrm{n-}}=$CO$_{3}^{2-}$, NO$_{3}^{-}$, etc) possess the brucite-like layers [Mg(OH)$_{2}$] with positive charge and anionic compounds in the interlayer to form neutral materials. Catalytic effects to decompose NO$_{\mathrm{x}}$ from automobile exhaust were highly related with the difference of M$^{2+}$ and thermal stability because the catylists locate are about 200 $\sim$ 500${^\circ}$. In this paper, Mg-Al-Cu and Mg-Al-Ni hydrotalcite-like compounds were characterized by XRD and FT-IR spectra and the thermal stability were analyzed by TGA and DTA. Even though they both have the typical diffraction peaks of hydrotalcites, but their interlayer spaces are different. Some weak chemical bonds were observed to be formed in Mg-Al-Ni hydrotalcites by FT-IR. Mg-Al-Ni hydrotalcite-like compound degraded at lower temperature, by contrast, Mg-Al-Cu hydrotalcite has the better structural stablilty and thermal stability. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L2.00002: The Role of Cluster Size and Composition, the Nature of the Support / Interface on the Performance of gas Phase Heterogeneous Catalysts and Electrocatalysts Stefan Vajda In this paper, we will discuss the catalytic performance of with atomic precision size- and composition selected supported clusters consisting of a handful to several dozen atoms in 1) gas phase reactions of selective C$=$C bond activation, C-H bond breaking and CO oxidation; and 2) under electrochemical reactions, such as water splitting. Catalysts' performance will be evaluated as function of cluster size and composition, while the chemistry of the support material / interface will be used to fine-tune catalytic activity and selectivity. In situ X-ray techniques are used to monitor the size, shape and oxidation state of the catalyst under reaction conditions. As time will allow, results will be presented on the use of additives/modifiers which allow controlling the nature of the catalytic site under reaction conditions, and accordingly, its activity and selectivity. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L2.00003: Self-assembly of one-dimensional metal-organic nanoarchitectures on a surface Martina Capsoni, Agustin Schiffrin, Adam Shaw, Sarah Burke Supramolecular chemistry holds promise for designing low-dimensional nanostructures with predefined functionalities. In particular, the interface between metal-organic complexes and surfaces is relevant in applications such as photovoltaics, photocatalysis, molecular electronics, etc. The structural, chemical and electronic properties of these systems can be dramatically altered by the interaction with the underlying surface. It is therefore of great relevance to achieve morphological control of functional nano-assemblies on a substrate at the single molecule and atom level. Here, we investigate the \textit{in situ} coordination of bisterpyridine molecules with transition metal adatoms on Ag(111), by means of low-temperature scanning tunneling microscopy. The bare ligand adsorbs following specific orientations with respect to the substrate atomic lattice. Ordered supramolecular domains emerge via parallel adjacent non-covalent binding of the molecules. Coordination between deposited iron adatoms and terpyridine ligands is activated at room temperature, likely mediated by an intramolecular conformational change of the pyridine groups. The resulting self-assembled one-dimensional nanostructures are described. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L2.00004: Unraveling the Role of Metal-Support Interactions in Heterogeneous Catalysis Invited Speaker: J.R. Schmidt We examine the role of the metal-support interaction in modulating the activity and selectivity of oxide-supported metal nanoparticles, focusing specifically on the Fischer-Tropsch (FT) synthesis of ethanol (EtOH). Although it is well-known that oxide supports can play a non-innocent role in heterogeneous catalysis, a comprehensive and predictive picture of the role of such supports remains elusive. Using realistic computational models of supported nanoparticles, we decouple the electronic and geometric aspects of the metal-support interaction, and we show that the former can be largely understood in terms of charge transfer between support and nanoparticle. The resulting metal-support interactions induce significant changes in adsorbate binding energies, and thus significantly influence reaction thermodynamics and kinetics. For the specific case of FT, we show how our model can be used to understand the observed increase in EtOH selectivity when switching from silica to titania supports. More generally, we illustrate how these ideas can be used to crudely predict the influence of a support even in the absence of detailed calculations and provide a general framework for understanding the influence of various oxide supports on elementary association / dissociation reactions. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L2.00005: Controlling Surface-Mediated Interactions at the Organic Semiconductor / Metal Interface Oliver Monti, Nahid Ilyas, Rocio Cortes-Rodriguez, Percy Zahl, Peter Sutter We show by a combination of two-photon photoemission spectroscopy (2PPE) and low-temperature scanning tunneling microscopy (LT-STM) that surface-mediated interactions at the organic semiconductor / metal interface can be controlled by the molecular orientation on the surface. For the dipolar molecule boron subphthalocyanine chloride (dipole moment of 4.5 D) on Cu (111), nearest-neighbour distance distributions show radically different properties for the two different molecular orientations on the surface, ``Cl-up'' and ``Cl-down''. We are able to model the respective interaction potentials by a combination of Friedel oscillation-induced surface mediated interactions and van der Waals (Cl-up) vs. screened Coulomb interactions (Cl-down). These interaction modes, substantially different for the two molecular orientations, lead to completely different growth modes at higher coverages. They result from selective charge-transfer to Cl-down molecules, as corroborated by 2PPE. Our results suggest a pathway towards control of the interfacial electronic structure and molecular assembly at organic semiconductor / metal interfaces. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L2.00006: Trapping of Excited Electrons at the NaCl/Ag(100) Interface David Suich, Benjamin Caplins, Alex Shearer, Charles Harris Understanding metal/insulator systems, such as alkali halides on noble metals, and their properties are important for the fields of catalysis, devices, and the emerging field of nanoelectronics. To date, however, there remain few time-resolved studies of these systems. Time- and angle- resolved two-photon photoemission is used to study the dynamics of the electronic states at the NaCl/Ag(100) interface. We observe the n=1, 2, and 3 image potential states. Electrons in the n=1 state undergo a trapping with a high probability, as shown by a dynamic change in their energy. Momentum resolved measurements show this state is initially delocalized, but becomes localized within a few hundred femtoseconds. The source of trapping is believed to occur at step edges of the NaCl islands. Our studies correlate the coverage and temperature dependence with the dynamics and magnitude of electron trapping in this state. Qualitatively similar behavior has been observed for several other alkali halides on both the Ag(100) and Ag(111) surfaces, proving the generality of the phenomenon. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L2.00007: NO$_{x}$ Direct Decomposition: Potentially Enhanced Thermodynamics and Kinetics on Chemically Modified Ferroelectric Surfaces Arvin Kakekhani, Sohrab Ismail-Beigi NO$_{x}$ are regulated pollutants produced during automotive combustion. As part of an effort to design catalysts for NO$_{x}$ decomposition that operate in oxygen rich environment and permit greater fuel efficiency, we study chemistry of NO$_{x}$ on (001) ferroelectric surfaces. Changing the polarization at such surfaces modifies electronic properties and leads to switchable surface chemistry. Using first principles theory, our previous work has shown that addition of catalytic RuO$_{2}$ monolayer on ferroelectric PbTiO$_{3}$ surface makes direct decomposition of NO thermodynamically favorable for one polarization. Furthermore, the usual problem of blockage of catalytic sites by strong oxygen binding is overcome by flipping polarization that helps desorb the oxygen. We describe a thermodynamic cycle for direct NO decomposition followed by desorption of N$_{2}$ and O$_{2}$. We provide energy barriers and transition states for key steps of the cycle as well as describing their dependence on polarization direction. We end by pointing out how a switchable order parameter of substrate,in this case ferroelectric polarization, allows us to break away from some standard compromises for catalyst design(e.g. the Sabatier principle). This enlarges the set of potentially catalytic metals. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:24AM |
L2.00008: First principles analysis of metal and oxide-metal interfacial catalysis for hydrogen production Invited Speaker: Jeffrey Greeley Current and growing interest in the development of new catalytic materials for complex chemistries has challenged the methods traditionally employed by practitioners of computational catalysis. Explicit Density Functional Theory (DFT) analysis of all possible reaction pathways in biomass reaction networks, for example, is computationally prohibitive, and to make progress at a reasonable rate, strategies to accelerate the predictions made by DFT-based methods must be developed. In this talk, we will review some recent work in our group focusing on first principles analyses of the production of hydrogen from the decomposition of biomass-derived oxygenated hydrocarbons on heterogeneous catalytic surfaces. We will discuss, in particular, the development of accelerated DFT-based strategies to map the complex reaction networks associated with biomass decomposition at metal and oxide-metal interfaces, and we will show how these strategies can efficiently produce semi-quantitative predictions of activity and selectivity trends in hydrogen production on these surfaces. We will also briefly describe the development of reactivity trends for another chemical process that is relevant to biomass chemistry, the water-gas shift reaction, at metal-oxide interfaces, and will describe how bifunctional properties of these interfaces may promote this important chemistry. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L2.00009: Spectroscopic Analysis of Ion Concentration Profile at Electrode/Electrolyte Interface by Interferometry David Moore, Ravi Saraf Owing to the difference in Fermi levels at an electrode/electrolyte interface, ions form an electrical double layer (EDL) with ion concentrations well over 10-fold compared to bulk. The concentration profile of the EDL intrinsically affects the electrochemical reaction rates at the electrode, which is of great significance in many applications, such as batteries and biosensors. Conventionally, using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), the electrical properties of the EDL are represented as ``equivalent circuits'' consisting of the resistance to charge transfer (Rct), the double layer capacitance (Cdl) and a ``Warburg (constant phase) diffusion element'' that represents the long range diffusion of ions to the electrode. The translation to the well-understood physical structure can be lost as complicated effects are often lumped together. For example, the effect of subtle modification of the electrode surface by say, redox compounds, enzymes, or polymers is not directly measured, and must be inferred by capacitance changes. An interferometer method will be described to directly measure changes in concentration at the interface during redox process. This method in concert with CV or EIS performed concomitantly will lead to more information to model the diffuse layer for improved understanding of the kinetics of the reaction at different distances from the electrode. Applications to DNA and polymer adsorption binding will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L2.00010: Density functional theory simulation of hydrogen-bonding vibrational densities of states at the quartz (101)-water interface and its relation to dissolution in aqueous solutions of ions Mark DelloStritto, James Kubicki, Jorge Sofo Physical processes at aqueous interfaces are strongly dependent on structure at the interface, and for water/metal-oxide interfaces, the structure of the local H-bond network dominates the behavior at the interface. In particular, for silica it has been hypothesized that the increase in dissolution rate due to dissolved salts is due to a reorganization of the H-bond network by ions near the surface. This would explain the order of magnitude increase in dissolution rate despite the fact that the activation energy of the dissolution reaction does not change with the addition of salts. We investigate two hypotheses of the dissolution of SiO$_{2}$ in ionic solutions using ab-initio molecular dynamics simulations. These hypotheses are 1) that the presence of ions induces orientations in interfacial H$_{2}$O molecules which are preferential for proton transfer to bridging oxygen (BO) atoms, and 2) the presence of ions induces stronger H-bonding between terminal hydroxyl (TH) groups and BO atoms, allowing proton transfer. It is found that although elements of these hypotheses are true, the model structures produced by density functional theory simulations do not support the former as valid mechanisms of dissolution. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L2.00011: Directly Grafting Alkanethiol on Bare Si (111) by UV-assisted Photochemical Reaction Lo-Yueh Chang, Hung-Wei Shiu, Shangjr Gwo, Chia-Hao Chen Self-assembled monolayers (SAMs) are organic molecules that self-assembled and closely packed on substrate surface. The surface physic and chemical properties are dependent on the controllable tail of SAMs. Therefore, SAMs is attracting a lot of attention in bio-sensing, nano-manipulating, and microfluidic field. The alkanethiol on noble metal surface, such as gold and silver, is a well-known SAM system to understand the fundamental properties. However, alkanethiols grown on semiconductor surfaces was less systematically studied, especially on bare silicon surface, despite their prospective applications. To have in-depth understanding of such system, we tried to grow alkanethiol SAMs on hydrogen-terminated Si surface by UV-assisted photochemical reaction. The resulting monolayer was studied by means of water contact angle measurement, synchrotron radiation based X-ray photoemission spectroscopy, and polarization dependent near-edge X-ray absorption fine structure. The combined characterization probes revealed a hydrophobic ambient surface, and the n-alkanethiols were directly attached on Si through Si-S bond that formed a highly order monolayer to prevent the air oxidation and contamination. [Preview Abstract] |
Session L3: Focus Session: Solvation, Dynamics, and Reactivity in Complex Environments I
Sponsoring Units: DCPChair: Valeria Molinero, University of Utah
Room: 107
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L3.00001: Dynamic response in electric field-induced nanowetting in salt solution Dusan Bratko, Davide Vanzo, Alenka Luzar Electric field applied across hydrophobic nanopores can control wetting/dewetting transitions. This switching effect is of potential importance in applications from fluid flow control in nanofluidics to imbibition of nanoporous materials to surface energy absorption and storage. Dynamic response to the imposition or cessation of the field occurs at two stages characnterized by different timescales. Fast response, O(ps), involves the change in the effective surface tension, which takes place along with water polarization. Slower response, associated with wetting/dewetting transitions involves solution infiltration or expulsion, an activated process we show to be kinetically viable only in nanoscale pores. Using molecular dynamics simulations, we identify a window of conditions where O(ns) responses of the wetting/expulsion cycle can be secured for experimentally realizable fields, porosity and salinity of the solution. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L3.00002: First-principles investigation of boron incorporation into CRUD under Pressurized Water Reactor conditions Zs. Rak, C.J. O'Brien, D.W. Brenner CRUD (Chalk River Unidentified Deposit) is predominately a nickel-ferrite deposit on hot surfaces of nuclear fuel rods during reactor operation. The presence of CRUD modifies the core-coolant heat transfer and can induce localized corrosion on the cladding surface. Besides these unwanted effects boron, which is a neutron absorber, can accumulate within the CRUD, triggering shifts in the neutron flux and fluctuations in the reactor power level. Therefore, it is crucial to understand and predict the mechanisms by which B is trapped into the CRUD. As a first step, the incorporation of B defect into the crystal structure of NiFe$_{2}$O$_{4}$ has been investigated using the DFT framework. To obtain the formation energies of various interstitial and substitutional B-defects, theoretical results have been combined with experimental thermo-chemical data. Assuming solid-solid equilibrium conditions, the main factors that limit the incorporation of B are (i) the narrow stability domain of the host NiFe$_{2}$O$_{4}$ and (ii) the formation of ternary Fe-B-O and Ni-B-O compounds. The study also investigates the incorporation of B assuming solid-liquid equilibrium between NiFe$_{2}$O$_{4}$ and the surrounding aqueous solution under conditions of pressure, temperature, and pH characteristic to pressurized water reactors. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L3.00003: Atomistic Models for High-throughput Prediction of Hydration Free Energies Jianzhong Wu The classical density functional theory (DFT) is proposed as an efficient computational tool for high-throughput prediction of the solvation free energies of small molecules in liquid water at the ambient condition. With the solute molecules represented by the AMBER force field and the TIP3P model for the solvent, the new theoretical method predicts the hydration free energies of 500 neutral molecules with average unsigned errors of 0.96 kcal/mol and 1.04 kcal/mol in comparison with the experimental and simulation data, respectively. The DFT predictions are orders of magnitude faster than conventional molecular dynamics simulations and the theoretical performance can be further improved by taking into account the molecular flexibility of large solutes. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L3.00004: Collisions of Sodium Atoms with Liquid Glycerol: Insights into Na Atom Solvation and Ionization and the Reactions of Near-Interfacial Electrons Invited Speaker: Gilbert Nathanson Over the last 70 years, thousands of reactions between solvated electrons and dissolved species have been investigated in water and other protic solvents. Electrons born at the surface of the solvent, however, may react differently than those created within it. We have explored this interfacial reactivity by directing sodium atoms at the surface of liquid glycerol in vacuum. Gas-liquid scattering experiments show that electrons generated from the Na atoms produce hydrogen atoms and hydrogen molecules, hydroxide ions and water, and glycerol fragments. Remarkably, nearly half the hydrogen atoms created near the surface escape into vacuum before reacting with the solvent. Complementary ab initio molecular dynamics simulations of Na striking a 17-molecule glycerol cluster indicate that the glycerol hydroxyl groups reorient around the Na atom as it makes contact with the cluster and begins to ionize on the picosecond timescale. The experiments and simulations together indicate that Na-atom deposition provides a low-energy pathway for generating solvated electrons in the near-interfacial region of protic liquids. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L3.00005: Measurement of Ultraslow Rotational Dynamics of Probes in Imidazolium-Based Ionic Liquids Near and Below the Glass Transition Temperature: Studying the Role of Structural Heterogeneity on Dynamic Heterogeneity Kayla Mendoza, Rakhitha Udugama-Arachchilage, Fehmi Bardak, George Tamas, Edward Quitevis The dynamics of imidazolium-based ionic liquids were probed in the supercooled liquid regime by observing the fluorescence recovery after photobleaching of directionally oriented tetracene molecules. Spatial heterogeneity arises in ionic liquids containing a 1-alkyl-3-methylimidazolium cation for alkyl chain lengths equal to and exceeding four carbons; aggregation of the alkyl tails leads to the formation of non-polar domains, which increase in size with increasing alkyl chain length. Near the glass transition, supercooled liquids relax non-exponentially, and this non-exponentiality has been attributed to dynamic heterogeneity. The purpose of this study was to observe the role of structural heterogeneity on dynamic heterogeneity. The rotational dynamics of tetracene in 1-butyl-3-methylimidazolium bistriflate, 1, 3-dibutylimidazolium bistriflate, and 1-heptyl-3-methylimidazlium bistriflate were observed in the vicinity of their glass transition temperatures. From the weak dependence of the degree of non-exponentiality exhibited by the relaxation function on alky chain length and cation symmetry, it was concluded that structural heterogeneity does not play a strong role in determining dynamic heterogeneity. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L3.00006: CO2 capture in amine solutions from ab initio molecular dynamics Changru Ma, Fabio Pietrucci, Wanda Andreoni The most mature technology for post-combustion CO$_2$ capture exploits a cyclic process, in which CO$_2$ is selectively and reversibly absorbed in an amine solution, typically monoethanolamine(MEA) at 30\%wt concentration. Empirical efforts are ongoing worldwide to reduce the high energy penalty for amine regeneration and to increase the absorption rate. Computer simulations can help by providing new insights and the missing quantitative information. Using extensive large-scale Car-Parrinello molecular dynamics simulations, aided by accelerated sampling techniques, we have characterized the reactions leading to CO$_2$ capture in MEA 30\%wt solutions via the formation of the carbamate, and the subsequent CO$_2$ release. Deprotonation and CO$_2$ release turn out to be competitive for an intermediate zwitterion (free-energy barrier $\sim$10kcal/mol), with sizable entropic contribution, whereas CO$_2$ release from the carbamate has a much higher barrier ($\sim$50kcal/mol), mainly enthalpic and rather independent of temperature. An unprecedented characterization of structural and vibrational properties of the solution allows us to interpret recent experimental results. More results on other amines, allow us to rationalize their still unexplained better performance relative to MEA. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L3.00007: \textit{Ab initio} Molecular Dynamics Study of Carbon Dioxide Adsorption on the Ni Catalyst Surface Ji Il Choi, Yong-Hoon Kim The first-principles molecular dynamics simulations can provide insight into the surface reaction mechanisms by including thermodynamic environments and rigorously determining the transition states between reactants and products, which are not available from the static study of the species already adsorbed on the surface. In view of its importance in various energy applications, we here report on the first-principles molecular dynamics study of the chemical reaction of CO$_{\mathrm{2}}$ molecules on the representative atomicallyflat lowMiller-index Ni(111) surface. We adopted the DFT-D2 scheme to properly describe the van der Waals interactions, and considered various thermodynamic conditions including the temperature, pressure, and number of additional molecular species. To analyze the reaction mechanisms, in addition to the change of the electronic structure of CO$_{\mathrm{2}}$ upon adsorption on the Ni surface the energy barriers between the initial and final stages of CO$_{\mathrm{2}}$ deposition were calculated with and without the interactions with neighboring molecules. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:24AM |
L3.00008: Probing Intermolecular Interactions in Polycyclic Aromatic Hydrocarbons with 2D IR Spectroscopy Invited Speaker: Amber Krummel Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous in the environment and impact geochemical processes that are critical to sustainable energy resources. For example, asphaltenes exist naturally in geologic formations and their aggregates heavily impact the petroleum economy. Unfortunately, the chemical dynamics that drive asphaltene nanoaggregation processes are still poorly understood. Solvent dynamics and intermolecular interactions such as $\pi $-stacking interactions play integral roles in asphaltene nanoaggregation. Linear and nonlinear vibrational spectroscopy including two-dimensional infrared spectroscopy (2DIR), are well suited to explore these fundamental interactions. Teasing apart the vibrational characteristics in PAHs that model asphaltenic compounds represents an important step towards utilizing 2D IR spectroscopy to understand the intermolecular interactions that are prevalent in asphaltene nanoaggregation. A solar dye, N,N'-Dioctyl-3,4,9,10-perylenedicarboximide, is used in this work to model aphaltenes. Carbonyl and ring vibrations are used to probe the nanoaggregates of the model compounds. However, the characteristics of these normal modes change as a function of the size of the conjugated ring system. Thus, in order to fully understand the nature of these normal modes, we include a systematic study of a series of quinones. Our investigation employs a combination of 2DIR spectroscopy and electronic structure calculations to explore vibrational coupling in quinones and PAHs. We compare the calculated vibrational characteristics to those extracted from 2DIR spectra. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L3.00009: Freezing and Melting of Salt Hydrates Next to Solid Surfaces Probed by Infrared-Visible Sum Frequency Generation Spectroscopy Emmanuel Anim-Danso, Yu Zhang, Ali Dhinojwala Understanding the freezing of salt solution near solid surfaces is important in many scientific fields. Here, we have used sum frequency generation (SFG) spectroscopy to study the freezing of NaCl solution next to a sapphire substrate. During cooling we observe two transitions; the first transition corresponds to segregation of concentrated brine next to the sapphire surface as we cool the system down into the phase region where there is a coexistence of ice and brine. At this transition, the intensity of the ice-like peak decreases, suggesting the disruption of hydrogen bonding by sodium ions. The second transition corresponds to the formation of NaCl hydrates with abrupt changes in both SFG intensity and the sharpness of spectral peaks. The similarity in the position of the SFG peaks with those observed using IR and Raman spectroscopy indicates the formation of NaCl.2H$_2$O crystals next to the sapphire substrate. Freezing and melting of other hydrates will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L3.00010: Effects of Aqueous Solvation on the Photochemistry of Pyruvic Acid Allison Reed-Harris, Barbara Ervens, Richard Shoemaker, Rebecca Rapf, Jay Kroll, Elizabeth Griffith, Anne Monod, Veronica Vaida The role of organic compounds in atmospheric chemistry leading to aerosol formation is under investigation due to the necessity to understand the effects of aerosols on global climate change. It has recently been shown that important pathways in formation of organic aerosols are in aqueous environments where high molecular weight products are formed and can potentially contribute to atmospheric aerosol mass. This presentation describes the photochemistry of pyruvic acid in aqueous solutions representative of atmospheric fogs, clouds and wet aerosols. Solvation of pyruvic acid in water completely changes the photodissociation of this molecule compared to its photolysis in the gas phase. The reaction mechanism of pyruvic acid as a function of its environment and concentration will be presented along with the kinetics obtained in aqueous solution. The resulting first order rate constants will be presented to discuss the effect of water as a solvent in this chemistry. These results are input to atmospheric models to evaluate the atmospheric consequences of solvation of pyruvic acid on its atmospheric reactivity and its role in aerosol formation. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L3.00011: Collisions and Reactions of HNO$_{3}$ and N$_{2}$O$_{5}$ with Sea Spray Mimics Michael Shaloski, Timothy Bertram, Gilbert Nathanson Heterogeneous reactions occurring at the surface of sea spray aerosol (SSA) droplets can lead to changes in the chemical compositions of the droplet, the denitrification of the atmosphere, and the production of chlorine-containing gases. These processes ultimately influence both ozone and methane concentrations and air quality. We explore these reactions through gas-liquid scattering experiments in vacuum using salty and surfactant-coated glycerol (a low vapor pressure liquid) as a proxy for SSA. HNO$_{3}$ and N$_{2}$O$_{5}$ are atmospherically-relevant species that can dissociate and react at or near the surface of a protic liquid. In particular, N$_{2}$O$_{5}$ may react with the solvent to generate HNO$_{3}$ and glycerol nitrate and may react with near-interfacial Cl$^{-}$ to generate ClNO$_{2}$, Cl$_{2}$, and HONO. Our initial experiments will focus on reactions of DNO$_{3}$ to monitor the competition between HCl and HNO$_{3}$ formation and desorption. [Preview Abstract] |
Session L4: Focus Session: Quantum Spin Liquids
Sponsoring Units: GMAGChair: Alexander Chernyshev, University of California, Irvine
Room: 112/110
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L4.00001: Finite-temperature phase transition to a quantum spin liquid in a 3D Kitaev model Joji Nasu, Toshiyuki Kaji, Keisuke Matsuura, Masafumi Udagawa, Yukitoshi Motome The Kitaev model has recently attracted considerable attention due to the spin-liquid ground states. This model is defined on a honeycomb lattice, and is exactly solvable due to the Ising conserved quantities associated with each hexagon. In this study, we investigate the thermodynamic properties of a three-dimensional (3D) generalization of the Kitaev model defined on a hyperhoneycomb lattice, which was introduced in Ref. [1]. Although this model has spin-liquid ground states similar to the 2D model, the excited states are contrasting as they are described by Ising conserved quantities forming a loop-like structure on the pyrochlore lattice. We analyze this model in the limit where one of the inequivalent bonds is stronger than the others, where a classical Monte Carlo simulation is applicable [2]. As a result, we find a phase transition at a finite temperature between the gapped quantum spin liquid and paramagnet. This phase transition is of second order and belongs to the 3D Ising universality class. We provide a topological characterization of the phase transition in terms of a flux density. We also calculate the temperature dependence of the magnetic susceptibility. [1] S. Mandal and N. Surendran, Phys. Rev. B \textbf{79}, 024426 (2009). [2] J. Nasu et al., arXiv:1309.3068. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L4.00002: Density-Matrix Renormalization Group Study of Effective Spin Model for Na$_2$IrO$_3$ Kazuya Shinjo, Shigetoshi Sota, Takami Tohyama The Kitaev-Heisenberg honeycomb lattice model has recently been proposed to describe magnetic properties in A$_2$IrO$_3$(A=Na,Li). The model includes an isotropic Heisenberg term and strongly anisotropic Kitaev terms. The Kitaev terms give a spin-liquid ground state. With increasing the strength of Heisenberg coupling, the ground state first turns into a stripy antiferromagnetic phase and then into a Neel antiferromagnet. The x-ray and neutron scattering experiments indicate that the ground state of Na$_2$IrO$_3$ is most likely characterized by a zig-zag spin structure. However, this type of magnetic order cannot be theoretically explained by the Kitaev-Heisenberg model. It is necessary to introduce further neighbor Heisenberg couplings and/or trigonal distortion of the oxygen octahedra. We study an extended Kitaev-Heisenberg model including these additional terms, by using two-dimensional density-matrix renormalization group method. Calculating spin correlation functions, we find that the zigzag order appears in a parameter regime relevant to Na$_2$IrO$_3$. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L4.00003: Heisenberg-Kitaev model on the hyperhoneycomb lattice Eric Kin-Ho Lee, Robert Schaffer, Subhro Bhattacharjee, Yong Baek Kim Motivated by recent experiments on $\beta$-Li$_2$IrO$_3$, we study the phase diagram of the Heisenberg-Kitaev model on a three dimensional lattice of tri-coordinated Ir$^{4+}$, dubbed the hyperhoneycomb lattice. The lattice geometry of this material, along with Ir$^{4+}$ ions carrying $J_{eff}=1/2$ moments, suggests that the Heisenberg-Kitaev model may effectively capture the low energy spin physics of the system in the strong-coupling limit. Using a combination of semiclassical analysis, exact solution, and slave-fermion mean field theory, we find a spin-liquid and four different magnetically ordered phases---the Neel, the polarized ferromagnet, the skew-stripy, and the skew-zig-zag. The three dimensional $Z_2$ spin liquid, which extends over an extended parameter regime around the exactly solvable Kitaev point, has a gapless Majorana mode with a deformed Fermi-circle (co-dimensions, $d_c=2$). We discuss the effect of magnetic field and finite temperature on various phases that may be relevant for future experiments. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L4.00004: Magnetic Anisotropy of a 3-Dimensional Honeycomb Iridate Lattice Kimberly Modic We present magnetic anisotropy measurements of a 3-dimensional honeycomb iridate lattice. The large spin-orbit coupling and the edge-shared octahedra create the possibility for large spin-anisotropic Kitaev exchange. The structure preserves the connectivity of the honeycomb lattice indicating its potential as a spin-liquid candidate. A complete temperature and angular dependence of torque measurements provides evidence for highly spin-anisotropic exchange interactions. At high temperature, the geometry of the octahedral environment and the iridium g-factor anisotropy constrain the susceptibility. Upon lowering temperature, we unambiguously identify a reordering of the principle components of susceptibility. An order of magnitude increase in anisotropy with field orientation at low temperature highlights the strong orbital character of the coupling. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L4.00005: Kitaev's honeycomb model on a buckyball Paula Mellado, Olga Petrova, Oleg Tchernyshyov We study the effect of disclinations in the Kitaev's honeycomb model [1] by examining the effective tight binding hamiltonian of Majorana fermions on Buckminsterfullerene [2]. Disclinations are realized by the 12 pentagons which shape the buckyball. We found that the ground state of the system with isotropic nearest neighbor coupling $t_1$, corresponds to a uniform flux sector of the $Z_2$ gauge field, where hexagons are flux free and pentagons have the same fluxes. Inclusion of second neighbor couplings $t_2$, preserve the projective symmetries of the truncated icosahedron as long as fluxes through all plaquettes (triangles, pentagons, and hexagons) related by symmetries are the same. For $t_1/t_2$ smaller than $1/2$, the local density of states reorganizes suggesting that the zero energy Majorana modes localize at the disclinations. The robustness of this quantum state against noise is examined. \\[4pt] [1] A. Kitaev, Ann. Phys. 321, 2 (2006).\\[0pt] [2] Kroto, Harold W., et al., Nature 318.6042 (1985). [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L4.00006: Confinementr and deconfinement of topological excitations in Na$_{2}$IrO$_{3}$ Zhanybek Alpichshev, Fahad Mahmood, Gang Cao, Nuh Gedik Using phase sensitive heterodyne transient grating technique we establish that in the limit of low pumping fluences the optical response of Na-213 iridate system below the antiferromagnetic ordering temperature T$_{\mathrm{N}}$ is dominated by Hubbard excitons (HE). Unpaired single particle excitations (SE) constituting HE are strongly suppressed, appearing only above T$_{\mathrm{N}}$. We argue that this is due to the interplay between the frustrated Kitaev term in the Hamiltonian and the weak Heisenberg term responsible for the antiferromagnetic order below T$_{\mathrm{N}}$, which mediates an effective interaction between spin singlet SE excitations. This interaction grows linearly with distance resulting in a sudden increase of the exciton binding energy as the system enters the ordered state. This is a solid state realization of a phenomenon known in high energy physics as confinement. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L4.00007: Non-abelian anyons on dislocations in Kitaev's honeycomb spin liquid Invited Speaker: Olga Petrova Kitaev's honeycomb model [1] is an exactly solvable model of a quantum spin liquid. Its gapped phase exhibits $Z_2$ topological order and has low-energy excitations in the form of $Z_2$ fluxes (visons). Previous studies [2] have demonstrated that even trivial lattice defects such as vacancies induce free magnetic moments with peculiar properties. We show [3] that certain kinds of lattice dislocations and bond defects in this system carry even more exotic excitations: unpaired Majorana fermions. Each pair of such defects (known as twists [4]) gives rise to a non-local physical (complex) fermion mode made out of two Majorana (real) fermions connected by a $Z_2$ gauge string. Their interaction decays exponentially with the distance. The non-local fermion can be created or annihilated by winding a vortex around a dislocation. The vortex also changes its topological charge in this process. The model remains exactly solvable in the presence of such defects and reveals a crucial role of the emergent gauge field in the physics of Majorana modes. \\[4pt] [1] A. Kitaev, Ann. Phys. \textbf{321}, 2 (2006). \\[0pt] [2] A. J. Willans, J. T. Chalker, and R. Moessner, Phys. Rev. B \textbf{84}, 115146 (2011). \\[0pt] [3] O. Petrova, P. Mellado, and O. Tchernyshyov, Phys. Rev. B \textbf{88}, 140405 (2013). \\[0pt] [4] H. Bombin, Phys. Rev. Lett. \textbf{105}, 030403 (2010). [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L4.00008: Loops, sign structures and emergent Fermi statistics in three-dimensional quantum dimer models Yang Qi, Vsevolod Ivanov, Liang Fu We introduce and study three-dimensional quantum dimer models with positive resonance terms. We demonstrate that their ground state wavefunctions exhibit a nonlocal sign structure that can be exactly formulated in terms of loops, and as a direct consequence, monomer excitations obey Fermi statistics. The sign structure and Fermi statistics in these ``signful'' quantum dimer models can be naturally described by a novel parton construction, which becomes exact at the solvable point. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L4.00009: Magnetic Correlations in a Frustrated Ni$^{3+}$ - Based Spin 1/2 Honeycomb Lattice Kate Ross, John Roudebush, Daniel Pajerowski, Craig Brown, Jose Rodriguez, Collin Broholm, Robert Cava We have studied the magnetic properties, via thermodynamic probes and inelastic neutron scattering, of the new spin-1/2 honeycomb material Na$_{0.95}$Ni$_2$SbO$_6$ $\cdot$ 1.5D$_2$O [1]. This hydrated compound hosts well separated honeycomb layers of nickel ions in the unusual Ni$^{3+}$ oxidation state, which produces S=1/2 magnetic moments. While a Curie-Weiss temperature of -13K indicates overall anti-ferromagnetic interactions, specific heat and neutron scattering reveal the presence of ferromagnetic correlations with coherent spin excitations that build up gradually upon cooling below 10K. No transition to long range order is observed down to 2 K, as evidenced by specific heat and neutron scattering, although AC susceptibility measurements indicate a dramatic change in dynamics near 4.2K. The results indicate the presence of frustration arising from competing interactions between ions in the layers. This compound, along with potential isostructural analogs, opens a new route to study the phase diagram of spin 1/2 honeycomb lattice models with competing interactions. \\[4pt] [1] J.H. Roudebush and R.J. Cava. J. Solid State Chem, 204. 178-185 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L4.00010: Generic spin model for the honeycomb iridates beyond the Kitaev limit Jeffrey G. Rau, Eric Kin-Ho Lee, Hae-Young Kee Recently, realizations of Kitaev physics have been sought in the A$_2$IrO$_3$ family of honeycomb iridates, originating from oxygen-mediated exchange through edge-shared octahedra. However, for the $j_{\rm eff} = 1/2$ Mott insulator in these materials exchange from direct $d$-orbital overlap is relevant, and it was proposed that a Heisenberg term should be added to the Kitaev model. Here we provide the generic nearest-neighbour spin Hamiltonian when both oxygen-mediated and direct overlap are present, containing a bond dependent off-diagonal exchange in addition to Heisenberg and Kitaev terms. We analyze this complete model using a combination of classical techniques and exact diagonalization. Near the Kitaev limit we find new magnetic phases: 120${}^\circ$ and incommensurate spiral order, as well as extended regions of zigzag and stripy order. Possible applications to Na$_2$IrO$_3$ and Li$_2$IrO$_3$ are discussed. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L4.00011: Emergent magnetic scale in an exotic lithium iridate compound Nicholas Breznay, Tess Smidt, Kimberly Modic, Arkady Shekhter, Ross McDonald, James Analytis We study the low temperature magnetic properties of an exotic iridate compoud. Iridium-oxide materials show a range of interesting magnetic driven by their strong spin-orbit coupling and structural anisotropy, and may realize exotic magnetic behavior arising from Kitaev interactions. Newly synthesized single crystals exhibit a 3D structure and strongly anisotropic magnetic properties. We observe a kink in the low-temperature magnetization at a field $H^*$, corresponding to an induced moment of 0.2 $\mu_ B$. We will discuss the appearance and evolution of this new field scale, and its connections to magnetic order in this new family of materials. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L4.00012: Effective field theory, edge states and classification of symmetric Z2 spin liquids Yuan-Ming Lu, Ashvin Vishwanath Growing numerical evidence for gapped Z2 spin liquids in physically realistic spin models provides strong motivation for a deeper theoretical understanding of their properties. In particular the interplay of symmetry and topological order is known to lead to distinct phases of matter, symmetry enriched topological states, which differ in the action of symmetry on the topological excitations. In this work we present a Chern-Simons field theory description of symmetric spin liquids, which allows for a complete classification of these states as well as access to their physical properties such as edge states and quasiparticle quantum numbers. As an application we show that there are 6 distinct classes of Z2 spin liquids in the presence of a global Ising (Z2) symmetry. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L4.00013: Phases of the frustrated XY model on the honeycomb lattice Juan Carrasquilla, Andrea Di Ciolo, Federico Becca, Victor Galitski, Marcos Rigol We study the phase diagram of the frustrated XY model on the honeycomb lattice by using accurate correlated wave functions and variational Monte Carlo simulations. Our results suggest that a spin-liquid state is energetically favorable in the region of intermediate frustration, intervening between two magnetically ordered phases. We briefly discuss our results in the light of recent DMRG simulations where instead of a spin liquid, an unsual magnetically ordered state is found. [Preview Abstract] |
Session L6: Focus Session: Emergent Properties in Bulk Complex Oxides: Manganites
Sponsoring Units: GMAG DMPChair: Xia Hong, University of Nebraska-Lincoln
Room: 108
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L6.00001: Large lattice distortions associated with the magnetic transition in La0.7Sr0.3MnO3 Dmitry Reznik, Frank Weber, Oleg Prokhnenko, Dimitri Arguriou Colossal magnetoresistance (CMR) is associated with the phase transition from a metallic ferromagnetic to insulating paramagnetic phase, which can be controlled by an applied magnetic field. The insulating phase occurs due to trapping of the charge carriers by polaronic lattice distortions, which raise the resistivity. Theories based on local physics predict that the magnitude of the resistivity jump at Tc is determined by how much, on average, the amplitude of these distortions increases at the phase transition. Using neutron scattering, we measured the average distortion amplitude in La0.7Sr0.3MnO3. Surprisingly, its increase from below to above Tc is just as large as in other manganites, which have a much larger resistivity jump. This result suggests that the strength of CMR is determined not by the size of distortions, but by their cooperative nature specific to each compound. Existing theories need to be extended to include correlations between different unit cells to explain and predict the strength of CMR. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L6.00002: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L6.00003: Quantitative analysis of the diffuse scattering from a bilayer manganite S. Rosenkranz, J. Chen, D.J.P. Morris, S.N. Ancona, H. Zheng, J.F. Michell, M. Anitescu, R. Osborn, B.J. Campbell Magnetoresistance in manganese oxides is strongly enhanced by the presence of Jahn-Teller polarons and short range correlations between them. While previous investigations have shown the existence of both local lattice distortions and short-range order, a detailed quantitative description of the local structure, how it evolves as a function of temperature and doping, and how it affects the physical properties is still lacking. Here we present a detailed analysis of the diffuse scattering measured over a large range of temperatures from a bilayer manganite exhibiting colossal magnetoresistance. Focusing on the diffuse scattering around a single Bragg Peak and utilizing a point-defect approximation, we are able to derive a complete picture of the detailed temperature dependence of the Jahn-Teller distorted MnO$_6$ octahedra. These results further serve as a first test of the formalism for the quantitative analysis of the diffuse scattering over a large volume of reciprocal space, including both diffuse scattering from local defects close to Bragg Peaks, as well diffuse scattering due to short-range correlations. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L6.00004: Theoretical studies on the stability of metal-insulator coexistence in the absence of defects in perovskite manganites Keun Hyuk Ahn, Tsezar Seman, Turab Lookman, Alan R. Bishop We examine the stability of large metal-insulator domains in perovskite manganites in the absence of defects, using a model expressed in terms of symmetrized atomic-scale lattice distortion modes. Our results demonstrate that an intrinsic mechanism is responsible for the inhomogeneities in perovskite manganites, which involves long-range interactions between strain fields, the Peierls-Nabarro energy barrier, and complex energy landscapes with multiple metastable states. This is in contrast to an extrinsic mechanism such as chemical randomness or defects. We highlight experimental results which support the intrinsic mechanism rather than the extrinsic mechanism. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L6.00005: Spin Waves and Magnetic Correlations in the Multiferroic Sr0.56Ba0.44MnO3 Jeffrey Lynn, Daniel Pratt, James Mais, Omar Chmaissem, Bogdan Dabrowski Neutron diffraction and inelastic scattering measurements have been carried out on a polycrystalline sample of ferroelectric Sr$_{\mathrm{0.56}}$Ba$_{\mathrm{0.44}}$MnO$_{\mathrm{3}}$ (T$_{\mathrm{F\thinspace }}=$ 350 K) using the BT-7 and SPINS triple-axis spectrometers. The system orders antiferromagnetically at 197 K with an order parameter that varies smoothly with temperature. Inelastic measurements at base temperature reveal an energy gap of 4.6(5) meV, with a continuous distribution of magnetic scattering above the gap that exhibits a weak peak at 7.5 meV. The top of the magnon band was measured to be 43(1) meV. The data were modeled with a simple nearest-neighbor exchange$ J$ and the measured anisotropy gap, which was powder-averaged and fit to the data to yield an exchange constant $J=$4.8(2) meV. Above T$_{\mathrm{N}}$ strong correlations persist, consistent with the determined exchange as the scattering broadens in wave vector. Replace this text with your abstract body. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L6.00006: Doping influence of spin dynamics and magnetoelectric effect in hexagonal Y$_{0.7}$Lu$_{0.3}$MnO$_3$ Wei Tian, Guotai Tan, Liu Liu, Jinxing Zhang, Barry Winn, Tao Hong, Jaime Fernandez-Baca, Chenglin Zhang, Pengcheng Dai Inelastic neutron scattering experiments were performed to study spin waves and their correlation with the magnetoelectric effect in Y$_{0.7}$Lu$_{0.3}$MnO$_3$. The Mn trimerization distortion has been suggested to play a key role in determining the magnetic structure and the magnetoelectric effect in YMnO$_3$ and LuMnO$_3$. In Y$_{0.7}$Lu$_{0.3}$MnO$_3$, our INS study reveals a much smaller in-plane (hexagonal \textit{ab}-plane) anisotropy gap that coincides with a weaker in-plane dielectric anomaly at $T_N$. Since both the smaller in-plane anisotropy gap and the weaker in-plane dielectric anomaly are coupled to a weaker Mn trimerization distortion in Y$_{0.7}$Lu$_{0.3}$MnO$_3$ comparing to YMnO$_3$ and LuMnO$_3$, we conclude that the Mn trimerization is responsible for the magnetoelectric effect and multiferroic phenomenon in Y$_{1-x}$LuxMnO$_3$. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L6.00007: Ellipsometry studies of the optical phonons in hexagonal manganites $R$MnO$_{3}$ R. Basistyy, T.N. Stanislavchuk, M. Kotelyanskii, N. Lee, X. Wang, S-W. Cheong, G.L. Carr, A.A. Sirenko Optical properties of hexagonal multiferroic oxides $R$MnO$_{3}$, where $R=$ Ho, Er, Tm, Yb, and Lu, have been studied in the far-infrared spectral range between 10 and 4000 cm$^{-1}$ and temperatures between 1.5 K and 300 K. An advanced experimental technique of Muller matrix Spectroscopic Ellipsometry was used at the U4IR beamline of the National Synchrotron Light Source, Brookhaven National Lab. Spectra of the optical phonons will be presented in terms of the temperature dependencies of the phonon frequencies, their oscillator strength, anisotropy, and signatures of the spin-phonon interaction at the antiferromagnetic (AFM) phase transition. The spin-phonon interaction reveals itself as a non-Gr\"{u}neisen behavior of several phonon frequencies below $T_{\mathrm{N}}$ (Mn$^{3+})$. A decrease of the ionic radius for $R^{3+}$ ions between Ho$^{3+}$ and Lu$^{3+}$ resulted in a systematic increase of the optical phonon frequency. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L6.00008: Microwave imaging of charge-ordered phase coexistence in a layered manganite Eric Yue Ma, Ben Bryant, Yusuke Tokunaga, Zhi-Xun Shen Microwave impedance microscopy has been used to probe the co-existence of two charge- and orbital-ordered phases with slightly different in-plane conductivity in the layered manganite Pr(Sr$_{\mathrm{0.1}}$Ca$_{\mathrm{0.9}})$Mn$_{\mathrm{2}}$O$_{\mathrm{7}}$. Hysteretic, spatially inhomogeneous transitions between 1) the two charge-ordered phases and 2) the charge-ordered and disordered phases are observed, in which the transition temperature is affected by local strain induced by the presence of microscopic structural twin domains. In addition, conductivity contrast features are observed in the higher temperature, anti-ferroelectric charge ordered phase, which may represent anti-ferroelectric domain walls. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L6.00009: Magnetostructural coupling in the magnetodielectric Mn$_3$O$_4$ Moureen Kemei, Jaye Harada, Matthew Suchomel, Ram Seshadri At room temperature, the spinel Mn$_3$O$_4$ is distorted from cubic symmetry due to Jahn-Teller distortions of octahedral Mn$^{3+}$ and is described by the tetragonal spacegroup $I4_1/amd$. It undergoes ferrimagnetic ordering near 43$\,$K where anomalies in heat capacity and dielectric measurements are also observed. High-resolution variable-temperature synchrotron X-ray powder diffraction reveals a distortion of the long-range crystal structure at the onset of magnetic order. The structure of Mn$_3$O$_4$ below the magnetostructural ordering temperature is described by coexisting tetragonal and orthorhombic phases. A discontinuous change in lattice parameters at the magnetostructural ordering temperature illustrates that this system undergoes a first-order phase transition. Sharp changes in heat capacity at the magnetic ordering temperature are also consistent with a first-order phase transition. We present the complete crystallographic description of this important magnetodielectric spinel and suggest mechanisms behind the spin-driven lattice distortion. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L6.00010: Hybrid quasiparticles within the orthorhombic or hexagonal topology of RMO$_{3}$ (R$=$Nd,Pr,Tm,Er; M$=$Mn,Cr) under strong magnetic fields R. Sopracase, K. Holldack, L. del Campo, N.E. Massa, M.J. Mart\'Inez-Lope, J.A. Alonso We report on magnetoelectric quasiparticles that originate from electronic Coulomb and exchange correlations using a Bruker IFS125-HR interferometer at 0.5 cm$^{-1}$ resolution in the THz beamline of the electron storage ring BESSYII in Berlin. Orthorhombic NdMnO$_{3}$ and hexagonal TmMnO$_{3}$ have quasiparticles at energies of zone center magnons. In both cases, increasing the applied field, the $\sim$ 20 cm$^{-1}$ line matching the lowest energy magnon, has its intensity reduced sharply while bands associated in TmMnO$_{3}$ to magnon--acoustical phonon dispersion crossing and gap opening behave differently. The line at $\sim$ 48 cm$^{-1}$, the higher branch of the phonon gap, shows a Zeeman splitting-like behavior, while the lower branch at $\sim$ 31 cm$^{-1}$ has weak field dependences. The asymmetric envelope peaking at $\sim$ 35 cm$^{-1}$ in NdMnO$_{3}$ weakens, softens, and evolves at 8 T into two unresolved bands suggesting field induced TA$+$magnon coupling materializing a condition for a multiferroic state. Metastable orthorhombic ErMnO$_{3}$ has two bands at 5 K which resembles those of NdMnO$_{3}$. A remarkable 35 cm$^{-1}$ Zeeman splitting at 5 K in PrCrO$_{3}$ is tentatively associated to Cr$^{3+}$ electrons in a distorted polarizable p-d bond. ErCrO$_{3}$ shows such a feature at 50 cm$^{-1}$ as well additional zero field splitting at 8 and 9 cm$^{-1}$ in the spin reorientation phase. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L6.00011: Magnetoelectric orthorhombic and multiferroic hexagonal ErMnO$_{3}$: THz hybrid modes in RMnO$_{3}$(R)$=$ Nd, Er, Tm) N.E. Massa, V. Ta Phuoc, L. del Campo, D. De Sousa Meneses, P. Echegut, K. Holldack, M.J. Mart\'Inez-Lope, J.A. Alonso We report on far- and mid-infrared emission, reflection, and transmission spectra of metastable orthorhombic perovskite (Pbnm-T$_{\mathrm{N}}$ $\sim$ 42 K) and hexagonal (P6$_{3}$cm- T$_{\mathrm{N}}$ $\sim$ 84 K) ErMnO$_{3}$. The number of phonon modes remains constant from 300 K to 4 K. Magnetically disordered electrons in fluctuating orbitals lead to an ambient THz broad reflectivity band. On cooling toward T$_{\mathrm{N}}$ the electrons exhibit increasing charge and magnetic short-range correlations and condense into soft bands that harden at about T$_{\mathrm{N}}$ as magnetic order sets in. However, at difference of NdMnO$_{\mathrm{3}}$ and TmMnO$_{3}$ that show correlation with a gap opening in transverse acoustical phonon dispersion and spin order, hexagonal ErMnO$_{3}$ develops at 5 K a set of four strong hybridized modes centered $\sim$ 70 cm$^{-1}$ in addition to one peaking at $\sim$ 96 cm$^{-1}$ and another weaker at $\sim$ 44 cm$^{-1}$. Orthorhombic ErMnO$_{3}$ develops a 46 cm$^{-1}$ band and a very strong one at 91cm$^{-1}$ that seems to correlate to a weaker 98 cm$^{-1}$ phonon in the short-range only magnetic order environment. We conclude that Er$^{3+}$ paramagnetic fluctuations increases Mn spins frustration in both compounds being the disruption strongest in orthorhombic ErMnO$_{3}$ where the THz band and short range magnetic onset may be traced even at 150 K, probably, due to the increment in the Jahn-Teller distortion. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L6.00012: Phase Separation and Percolative Insulator-Metal Transition in LaMnO$_3$ Under Pressure: A Gutzwiller Variational Study Mohammad Sherafati, Sashi Satpathy We study the pressure-induced insulator-metal transition and phase separation in LaMnO$_3$ (LMO) using the Gutzwiller variational method. Being an insulator at ambient pressure, a long-debated question is whether LMO is a correlation-driven Mott insulator or a band insulator driven by the Jahn-Teller (JT) mechanism. Recent Raman measurements of LMO (Ref. 1) reveal a coexistence of domains of JT-distorted and undistorted octahedra between 3 and 34 GPa and indicate a critical threshold for the volume fraction of the latter domains as a herald of the metallic state. To explain these findings, we solve an extended Hubbard model including the JT distortions and cohesive energy as a function of volume for spinless $e_g$ electrons of Mn$^{3+}$ in paramagnetic LMO at room temperature. Our results clearly show a phase separation on both sides of the transition pressure ($P_c=32$ GPa) where domains of distorted (insulating) and undistorted (metallic) octahedra coexist supporting the percolative nature of the transition. Based on our work, the ground state under pressure is determined by the interplay of both Coulomb and JT interactions and it is predicted that the mixed-phase region extends well above $P_c$ into the metallic region. Ref. : 1) M. Baldini et al., PRL 106, 066402 (2011) [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L6.00013: Local orthorhombic distortion and enhanced susceptibility in LaNiO$_{3}$ paramagnet Bing Li, Shinichiro Yano, Despina Louca, Luke Marshall, Jian-Shi Zhou, John Goodenough, Mikhail Feygenson, Jorg Neuefeind The perovskite LaNiO$_3$ is metallic, and unlike other systems in this class of materials, it remains paramagnetic where only an enhancement in the magnetic susceptibility ($\chi$) is observed below 200 K. Other rare earth nickelates are antiferromagnetic with an enhancement of $\chi$ in paramagnetic metallic state. Using neutron powder diffraction and the pair density function analysis, it is observed that the temperature dependence of the local atomic structure cannot be reproduced assuming the average crystal symmetry which is rhombohedral with the $R\bar{3}c$ space group. With rising temperature, octahedral distortions involving displacements of oxygen set in, and the symmetry is reduced to $Pbnm$. In this symmetry, the equivalent O site in the $R\bar{3}c$ splits into two and can account for all the features observed in the local lattice. The structural changes occur gradually, between 100 and 200 K. The local Ni-O-Ni bond angles are reduced from 164.5 to 163.5 $^\circ$ during this transition. Such reduction of Ni-O-Ni bond angles may facilitate antiferromagnetic coupling and responsible for the temperature dependence of $\chi$ observed in LaNiO$_3$ below 200 K. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L6.00014: Origins of bad metal conductivity and the insulator-metal transition in the rare-earth nickelates ($R$NiO$_{3}$, $R =$ rare earth) Rafael Jaramillo, Sieu Ha, Daniel Silevitch, Shriram Ramanathan For most metals increasing temperature ($T)$ or disorder quickens electron scattering. This scattering time hypothesis informs the Drude model of electronic conductivity. However, for so-called bad metals with very low conductivity this hypothesis predicts scattering times so short as to conflict with Heisenberg's uncertainty principle. Bad metal conductivity has remained a puzzle since its discovery in the 1980s in high T superconductors. Here we introduce the rare-earth nickelates ($R$NiO$_{3}$, $R =$ rare earth) as a class of bad metals. We study SmNiO$_{3}$ thin films using infrared (IR) spectroscopy while varying $T$ and disorder. We show that the interaction between lattice distortions and Ni-O bond covalence explains both the bad metal conductivity and the insulator-metal transition (IMT) in the nickelates. It does so by shifting spectral weight over the large energy scale established by the Ni-O orbital interaction, thus enabling very low conductivity while preserving the Drude model and without violating the uncertainty principle. [Preview Abstract] |
Session L7: Focus Session: Dynamics and Symmetries of Magnetic Domain Walls
Sponsoring Units: GMAG DMPChair: Haifeng Ding, Nanjing University, China
Room: 106
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L7.00001: Chiral Magnetic Domain Wall Structure in Epitaxial Multilayers Invited Speaker: Yizheng Wu In magnetic ultrathin films, the common textbook picture distinguishes two canonical types of DWs: Bloch walls for perpendicularly magnetized films and N\'eel walls for in-plane magnetized films. It still remains an open question whether the Bloch wall should be necessarily the only type of DWs in perpendicularly magnetized ultrathin films although it has been the textbook example for a long time. In ultrathin film, the inversion symmetry broken at interface will induce Dzyaloshinskii-Moriya interaction (DMI). In this talk, we will show that the DMI at interface will induce the chiral N\'eel type domain wall in perpendicularly magnetized films. The spin structure in magnetic domain wall was identified in real-space at room temperature by spin-polarized low energy electron microscopy (SPLEEM). The chiral N\'eel-type domain wall was identified in the magnetic stripe domain phase in Fe/Ni/Cu(001), and the chirality can switch from the right-hand cycloid in Fe/Ni/Cu(001) to the left-hand cycloid in Ni/Fe/Cu(001), which indicates that the chirality is caused by the DMI mainly located at the Fe/Ni interface [1]. The chiral domain wall structure can also be observed in [Co/Ni]$_{n}$ multilayer grown on Pt(111) and Ir(111)[2]. We found that Pt(111) substrate can induce right-handed chirality, whereas Ir(111) substrate can induce left-handed chirality, moreover, the chirality of the DW evolves from right-handed to left-handed in [Co/Ni]$_{n}$ grown on Ir/Pt(111) by changing Ir thickness, and the DW near the transition point shows non-chiral Bloch-type. Our results prove that domain wall chirality together with the sign and strength of the DMI can be tuned through the interface engineering, which may enable more possibility for designing of new spintronics devices. This work was collaborated with G. Chen, J. Zhu, A. T. N'Diaye, T. P. Ma, H.Y. Kwon, C. Won, Y. Huo, J. Li, A. K. Schmid. \\[4pt] [1] G. Chen, et al., Phys. Rev. Lett. 110, 177204 (2013).\\[0pt] [2] G. Chen, et al., Nature Communication, 4,2671(2013). [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L7.00002: Spiral Spin Texture at Domain-Wall driven by Dzyaloshinskii-Moriya Interaction Minjun Lee, Jeonghoon Kwon, Sungmin Kim, Seong Joon Lim, Young Kuk Controlling an electron spin and its measurement play a crucial role in spintronics. Especially in nano-scale devices local probing capability is important. From this point of view, spin-polarized scanning tunneling microscopy is a suitable local probing method for revealing surface spin texture. Here, we report the observation of spiral spin texture at a domain-wall. Spiral spin texture is driven by Dzyaloshinskii-Moriya (DM) interaction, which is realized by spin-orbit coupling of electrons in an inversion-asymmetric crystal field. We chose Co/Pt(111) system to study the DM interaction. Because Pt is a substrate with strong spin-orbit coupling, and Co is a ferromagnetic material with out-of-plane spin direction. We grew Co islands on Pt(111) single crystal, and found the spiral spin texture at the magnetic domain wall using a spin-polarized scanning tunneling microscope. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L7.00003: Interaction of magnon current with a domain wall in an antiferromagnet Se Kwon Kim, Oleg Tchernyshyov, Yaroslav Tserkovnyak We study the dynamics of magnons in an easy-axis antiferromagnet in the presence of a domain wall (DW). As in a ferromagnet [1], magnons pass through a DW with a $\tanh{}$ profile with no back scattering. An important difference is that in an antiferromagnet a magnon can have spin $+1$ or $-1$ along the easy axis, whereas in a ferromagnet a magnon's spin is always opposite to the direction of magnetization. We find that magnons in an antiferromagnet pass through a $\tanh{}$ domain wall with their spin reversed and transfer two units of angular momentum to the DW. A magnon spin current can be generated by breaking the degeneracy of the two branches of spin waves in one domain, e.g., by irradiating it with circularly polarized microwaves. Uniform magnetization accumulated on the DW as a result of magnon spin inversion will cause the staggered magnetization to precess. We present a quantitative model incorporating magnon spin current in the equations of motion written in terms of the wall's collective coordinates [2]. The analytical results are confirmed by numerical simulations. \\ \\ \noindent $[$1$]$ P. Yan, X. S. Wang, and X. R. Wang, Phys. Rev. Lett. \textbf{107}, 177207 (2011). \\ \noindent $[$2$]$ E. G. Tveten \textit{et al.}, Phys. Rev. Lett. \textbf{110}, 127208 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L7.00004: Resonance in Magnetostatically Coupled Transverse Domain Walls Andrew Galkiewicz, Liam O'Brien, Paul Keatley, Russel Cowburn, Paul Crowell In a system of adjacent ferromagnetic nanowires, stray magnetic fields from a transverse domain wall (TDW) in one wire can give rise to an attractive interaction with a TDW in a separate wire. This has previously been shown to lead to an increase in the depinning fields for TDW propagation, and has also been predicted to lead to oscillatory motion should the two TDWs be separated laterally from equilibrium. Using time-resolved Kerr microscopy, we have observed the resonance associated with the TDW interaction. In addition, another resonance has been observed that we find is due to the pinning of the TDWs by the intrinsic edge roughness of the nanowires. The overall system dynamics are well described by a 1-D analytical model that incorporates both effects. Micromagnetics show that the energy scales of the intrinsic pinning and the inter-TDW coupling are similar and suggest that roughness should be accounted for in future dynamical investigations. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L7.00005: Domain Wall Trajectory Determined by its Fractional Topological Edge Defects Invited Speaker: Aakash Pushp The theory of topological defects has had a significant influence on the understanding of various physical phenomena ranging from superfluid Helium-3 to liquid crystals. Topological defects are general features in systems with broken symmetries such as head-to-head (HH) and tail-to-tail (TT) domain walls (DWs) in soft ferromagnetic nanowires (NWs). Such DWs are further composed of elementary topological bulk and edge defects with integer and fractional winding numbers, respectively; whose relative spatial arrangement determines the chirality of the DW. Understanding the influence of the DW structure on its motion is critical for both fundamental and technological reasons. In this talk, I will show how one can understand and control the trajectory of DWs in magnetic branched networks, composed of connected NWs, by a consideration of their fractional elementary topological defects and how they interact with those innate to the network. I will describe a simple and yet a highly reliable mechanism that we have developed for the injection of a DW of a given chirality into a NW and exploit it to show that it is the DW's chirality that determines which branch the DW follows at a symmetric Y-shaped magnetic junction, the fundamental building block of the network. Using these concepts, I'll unravel the microscopic origin of the one-dimensional (1D) nature of magnetization reversal of artificial spin ice systems that have been observed in the form of Dirac strings. This understanding will allow for the formation of more complex chiral magnetic orders by controllably generating and propagating several domain walls of specific chiralities into artificial spin ice structures to form defined lattices of Dirac strings. \\[4pt] Reference:\\[0pt] A. Pushp*, T. Phung*, C. Rettner, B. P. Hughes, S.-H. Yang, L. Thomas, S. S. P. Parkin, Nature Phys. 9, 505-511 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L7.00006: Single domain wall manipulation in curved nanowires using a mobile, local, circular field Madeline Shortt, Jessica Bickel, Mina Khan, Mark Tuominen, Katherine Aidala Ferromagnetic nanostructures present exciting physics with a range of potential applications in data storage devices, such as magnetoresistive random access memory (MRAM). These proposals require precise control and understanding of domain wall (DW) movement and interactions. We developed a technique that generates a local circular Oersted field at a precise location by applying current through the tip of the atomic force microscope (AFM). We previously used this technique to control DW motion in nanorings [1]. We extend this method to control individual DW movement in curved nanowires by placing the tip near a 180 DW at the vertex of a curved wire and generating a local field. In this way, we can examine the motion of domain walls through regions with different curvature and the effects of pinning. [1] T. Yang, N. R Pradhan, A Goldman, A. Licht, Y. Li, M T. Tuominen and K. E. Aidala, Applied Physics Letter, http://apl.aip.org/resource/1/applab/v98/i24/p242505$\backslash $\textunderscore s1 98, 242505, (2011) [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L7.00007: Observation of resonant modes of coupled domain walls Timothy Phung, Aakash Pushp, Charles Rettner, Brian Hughes, See-Hun Yang, Stuart S.P. Parkin Domain walls (DWs) in permalloy nanowires can be coupled together to form bound states (360$^{\circ}$ DWs) due to the repulsion arising from the interaction of the elementary topological defects of the DWs. Such repulsion prevents the annihilation that would otherwise occur due to magnetostatic interaction between the DWs. Here we demonstrate that the adjacent DWs mimic the response of coupled oscillators when driven by spin-polarized currents, and that their coupling can be tuned by applying a magnetic field to either push them closer or pull them apart. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L7.00008: Stabilization of Magnetic Antivortices and the role of Shape Anisotropy Martin Asmat-Uceda, Lin Li, Brian Shaw, Arabinda Haldar, Kristen Buchanan Magnetic vortices have attracted a great deal of interest in recent years due to their potential for applications such as data storage, microwave resonators, magnonic crystals, etc. Magnetic antivortices (AV) are expected to possess similarly interesting physical attributes; however, they have not been explored with the same intensity. The AV spin configuration may present some advantages over vortices, especially for channeling spin waves emitted from the dynamic core reversal and for de-coupling spin-transfer torque effects from parasitic Oersted fields. Currently only a few geometries have been identified that reliably promote the formation of an AV, thus limiting the study of their properties. We recently demonstrated a method to form AV's in pound-key-like structures made of Permalloy (Haldar et al. APL \textbf{102}, 112401, 2013). Here we investigate the dependence of the reliability of the AV formation on the details of the geometry of these structures. Magneto-optical Kerr effect (MOKE) hysteresis and magnetic force microscopy measurements show that the coercive field is also the nucleation field for the AV's. Micromagnetic simulations agree well with the experiments and highlight the role of shape anisotropy in the AV formation. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L7.00009: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L7.00010: Bloch points are sticky Oleg Tchernyshyov, Se Kwon Kim Bloch points are zero-dimensional topological defects in three-dimensional ferromagnets. A representative magnetic configuration is a hedgehog with magnetization pointing away from a center. The singular nature of a Bloch point's core leads to interesting and observable consequences [1]. A simple argument based on dimensional analysis shows that a magnetic lattice creates a periodic potential that can pin a Bloch point even if the lattice has no defects. The pinning force is of the order of the micromagnetic exchange constant, a few piconewtons in a typical ferromagnet. A domain wall in a cylindrical ferromagnetic wire with the diameter of a few tens of nanometers may contain a Bloch point. Such a domain wall will have a sizable depinning field, tens of oersteds. A Bloch point moving through an atomic lattice should emit electromagnetic waves at the frequency of a few hundred gigahertz. \\[4pt] [1] S. K. Kim and O. Tchernyshyov, Phys. Rev. B \textbf{88}, 174402 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L7.00011: In-situ magnetizing interlayer exchange coupled ferromagnetic discs with pinning exchange bias Sheng Zhang, Charudatta Phatak, Amanda Petford-Long, Olle Heinonen Multilayer structures consisting of both interlayer exchange coupling between two ferromagnetic layers separated by a nonmagnetic spacer and exchange bias from an antiferromagnetic layer on top were patterned into 1 micron diameter discs using focused ion beam lithography. The initial domains of the top ferromagnetic film set a linear exchange bias in the adjacent antiferromagnetic layer, causing a bias at the nucleation field of vortex structures in the in-situ magnetization experiments using Lorentz TEM when applying magnetic field in the positive and negative directions. We also observe unexpected vortex core shifting at some specific field during the in-situ magnetization experiment, possibly due to local pinning site and magnetization reversal of the pinned ferromagnetic disc layer. Micromagnetic simulations were performed to understand the magnetization reversal behavior on both ferromagnetic disc layers. [Preview Abstract] |
Session L8: Focus Session: Spin-Dependent Phenomena in Semiconductors: Spin-Orbit Coupling in 1D and 2D Systems
Sponsoring Units: GMAG DMP FIAPChair: Jean Heremans, Virginia Polytechnic Institute and State University
Room: 104
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L8.00001: Spin transport on parallel coupled nanowires with Rashba spin-orbit interaction Mariama Rebello Sousa Dias, Victor Lopez-Richard, Gilmar Marques, Sergio Ulloa The nature of various electron and spin transport mechanisms can be unveiled by exploring the properties of parallel coupled nanowires with Rashba spin-orbit interaction (SOI). Studies of a directional coupler proved the modulation of quantum transport through the proximity of waveguides. The overall control of charge and even spin flux in this system appears promising for spintronics, as well as in hybrid devices that include superconducting or magnetic materials nearby. In this work, we have studied the spin transport properties of parallel coupled nanowires, with an electric field applied in the mixing region, using a transfer matrix formalism. In this configuration, a Rashba SOI is generated, which breaks the spin degeneracy. Moreover, various configurations of gate voltages, applied on the wire structure, are considered. Under this configuration we are able to analyze the modulation of the spin transport through the combination of SOI and system dimensions. The combination of SOI and gate voltages allows a modulation of the polarization, when the measured spin is projected along the direction of the Rashba spin-orbit field. We will discuss how this polarization depends on structure features and explain how to use this effect to control the spin flux. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L8.00002: Large spin-orbit quantum interference effects in a dual-gated InSb nanowire device Ilse van Weperen, Debbie Eeltink, Brian Tarasinski, Michael Wimmer, Sebastien Plissard, Erik Bakkers, Leo Kouwenhoven InSb nanowires are the material of choice for one-dimensional topological superconducting systems. One of their favorable properties is their strong spin-orbit interaction (SOI). Measurements of the SOI strength in InSb nanowires in an open system, relevant to topology experiments, are however lacking. We therefore study the SOI in InSb nanowires in a dual-gated InSb nanowire device by means of low field magnetoconductance measurements. At a temperature of 4 K we observe a large amplitude ($\sim$0.2 e$^{2}$/h) SOI quantum interference effect. The large quantum correction to the conductance indicates a strong SOI and a long phase coherence length. We observe a crossover between positive magnetoconductance at low conductance and negative magnetoconductance at larger conductance. We examine the tunability of SOI quantum interference effects at constant conductance. Surprisingly, SOI quantum interference effects do not depend on the orientation of the magnetic field w.r.t. the nanowire. We employ simulations of the coherent backscattering trajectories in a nanowire to elucidate this isotropic response. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L8.00003: Dynamics of a localized spin excitation close to the spin-helix regime Gian Salis, Matthias Walser, Patrick Altmann, Christian Reichl, Werner Wegscheider The time evolution of a local spin excitation in a (001)-confined two-dimensional electron gas subjected to Rashba and Dresselhaus spin-orbit interactions of similar strength is investigated theoretically and compared with experimental data. Specifically, the consequences of a finite spatial extension of the initial spin polarization are studied for non-balanced Rashba and Dresselhaus terms and for finite cubic Dresselhaus spin-orbit interaction. We show that the initial out-of-plane spin polarization evolves into a helical spin pattern with a wave number that gradually approaches the value $q_0$ of the persistent spin helix mode. In addition to an exponential decay of the spin polarization that is proportional to both the spin-orbit imbalance and the cubic Dresselhaus term, the finite width $w$ of the spin excitation reduces the spin polarization by a factor that approaches $\exp(-q_0^2w^2/2)$ at longer times. This result bridges the gap between the formation of a long-lived helical spin mode and a spatially homogeneous spin decay described by the Dyakonov-Perel mechanism. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L8.00004: Determination of the Dresselhaus spin-orbit interaction in a (110)-oriented GaAs quantum well Yuansen Chen, Stefan Faelt, Werner Wegscheider, Gian Salis The Dresselhaus spin-orbit (SO) interaction is studied in a $(110)$-oriented and symmetrically doped GaAs quantum well (QW) by means of time-resolved Kerr rotation with a magnetic field applied along an oblique angle. The nonzero averaged SO field is obtained by introducing a DC current through the QW to shift the Fermi circle of the electron gas. By monitoring the change of the electron Larmor precession frequency induced by the current, we can determine both the magnitude and the direction of the Dresselhaus SO field. In agreement with the theoretical expectation, we find the SO field to be out-of-plane and to linearly increase with a current applied along the $[1\overline{1}0]$ direction. A negligible SO field is observed for a current along the $[001]$ direction. The vector sum of the SO field and the in-plane component of an external magnetic field leads to an observable tilting of the spin precession axis. The unidirectional SO field is expected to support a persistent spin helix state in the QW. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L8.00005: Ferroelectricity and Rashba-type band splittings in metal halides Minsung Kim, Jino Im, Arthur Freeman, Jisoon Ihm, Hosub Jin In this study, we investigate Rashba-type band splittings in metal halides. We use a minimal tight-binding model and first principles calculations based on density functional theory to understand the electronic structures of the materials. We find that different types of Rashba bands occur in the conduction and valence band edges in terms of the angular momentum textures. Also, the characteristics of the band splittings will be discussed in connection with the ferroelectric property. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L8.00006: Renormalization group study of interaction effects in Rashba-type tight-binding models Giulio Schober, Manfred Salmhofer The interplay between Rashba-type spin splitting and electron-electron interactions is studied using the functional renormalization group. Our starting point is an effective tight-binding model for the spin-split lowest conduction bands of BiTeI. The giant spin splitting of bulk energy states in this semiconductor arises as a consequence of strong atomic spin-orbit coupling and the noncentrosymmetric crystal structure. By successively integrating out high-energy degrees of freedom we investigate the competition between Fermi liquid instabilities focusing on unconventional superconductivity. The vertex function is efficiently parametrized in an N-patch scheme which takes into account the most relevant momenta on the Fermi surface and realizes a flow of effective interactions with broken SU(2) symmetry. Abstracting from the concrete material, we further study a class of minimal tight-binding models on the hexagonal Bravais lattice with Rashba-type dispersions near several time-reversal invariant momenta in the first Brioullin zone. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L8.00007: Influence of CoFe nanopillars on spin coherence in an InGaAs quantum well Yao Zhang, J.J. Heremans An array of ferromagnetic (Co$_{0.6}$Fe$_{0.4}$) nanopillars was fabricated on the surface of an InGaAs/InAlAs heterostructure. The interactions between the magnetic moments of the nanopillars and two-dimensional electrons in the In$ _{0.53} $Ga$ _{0.47} $As quantum well are experimentally studied by low-temperature antilocalization measurements. The presence of ferromagnetic nanopillars increases the spin-orbit scattering rate, interpreted as due to the spatially varying nanopillar magnetic field. At the quantum well, also an appreciable average fringing field $\sim$ 35 G normal to the surface is generated by the large saturation magnetization of the nanopillars. Numerical values show a good correspondence between the analysis of the experimental antilocalization data and calculations from a simple micromagnetic model. The measurements further show an increase in mobility due to surface metal coverage. Consistently, non-magnetic coverage is observed to decrease the spin-orbit scattering rate, as expected for increased Coulombic screening under the Elliott-Yafet spin-decoherence mechanism. The analysis also shows the inelastic scattering rate increasing as temperature increases, consistent with the Nyquist mechanism. The work is supported by DOE DE-FG02-08ER46532. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L8.00008: Interplay between spin-orbit coupling and many body effects in spin-flip waves in CdMnTe quantum wells Mehul Dixit, Carsten A. Ullrich We present a numerical study of spin-flip wave dispersions in a spin-polarized electron gas in an n-type dilute magnetic semiconductor heterostructure, using time-dependent density-functional response theory. The system under study is an asymmetrically modulation-doped CdMnTe quantum well with an in-plane magnetic field. Rashba and Dresselhaus spin-orbit coupling induces a wavevector-dependent spin splitting in the conduction bands. We demonstrate a striking organization and enhancement of the spin-orbit fields acting on the collective spin-flip wave due to an interplay with electronic many-body effects. Our results agree with recent inelastic light scattering experiments. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L8.00009: Current-induced spin polarization in anisotropic spin-orbit fields Benjamin Norman, Christopher Trowbridge, David Awschalom, Vanessa Sih Current-induced spin polarization is a phenomenon in which carrier spins are oriented when subjected to current flow. As an all-electrical means of aligning spins it has the potential to be useful for spintronics applications. However, the mechanism that produces this spin polarization remains an open question. Measurements are taken on strained In$_{0.04}$Ga$_{0.96}$As exhibiting a spin-orbit interaction that is anisotropic in momentum\footnote{B. M. Norman, C. J. Trowbridge, D. D. Awschalom, and V. Sih, ``Current-induced spin polarization in anisotropic spin-orbit fields,'' submitted.}. We pattern contacts such that electrical conduction can be oriented along any in-plane direction. By varying the electron current direction we can continuously tune between the spin-orbit field extrema. We show that, contrary to expectation, the magnitude of the current-induced spin polarization decreases with increasing spin-orbit field. Furthermore, we find that the steady-state in-plane spin polarization is not along the direction of the spin-orbit field, except in the cases that the spin-orbit field is along an eigenvector of the spin relaxation tensor. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L8.00010: Inverse Edelstein Effect Ka Shen, Giovanni Vignale, Roberto Raimondi In this work, we provide a precise microscopic definition of the recently observed ``Inverse Edelstein Effect", in which a non-equilibrium spin accumulation in the plane of a two-dimensional (interfacial) electron gas drives an electric current perpendicular to its own direction. The drift-diffusion equations that govern the effect are presented and applied to the interpretation of the experiments. By taking into account the Rashba spin-orbit interaction, we show how the inverse Edelstein effect arises as a combination of the $z$-spin current flowing along the $y$-direction due to an non-equilibrium $S^y$ polarization and of the inverse spin Hall effect mechanism which yields, in turn, a $x$-flowing charge current. Explicit results have been shown in the diffusive regime where we have used the theoretical framework of the $SU(2)$ formulation for linear-in-momentum spin-orbit coupling. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L8.00011: Casimir effect in spin-orbit-coupled materials Andrew Allocca, Justin Wilson, Victor Galitski We propose the Casimir effect as a general method to observe Lifshitz transitions in two-dimensional electron systems. The concept is demonstrated with a planar semiconductor system with Rashba spin-orbit coupling and an applied Zeeman field. We calculate the Casimir force between the semiconductor and a plane parallel metallic plate at fixed separation as a function of the Zeeman splitting in the semiconductor. We find that as the Zeeman energy increases, the vanishing of a Fermi surface in either the upper or lower band of the Rashba system is indicated by a kink in the Casimir force. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L8.00012: Hidden spin polarization in inversion-symmetric bulk crystals Qihang Liu, Xiuwen Zhang, Jun-Wei Luo, Arthur Freeman, Alex Zunger Spin-orbit coupling (SOC) can induce spin polarization in nonmagnetic 3D crystals when the inversion symmetry is broken, as manifested by the bulk Rashba (R-1) and Dresselhaus (D-1) SOC effects. Here we note that these spin polarization effects originate fundamentally from atomic site asymmetries, producing site dipole field (DF) or being site inversion asymmetry (IA), rather than from the well-known bulk space-group crystal asymmetry. In non-centrosymmetric crystals where bulk inversion symmetry is absent, the local atomic polarizations due to site DF and site IA add up to create the bulk R-1 and D-1 net polarization effects, respectively. On the other hand, in centrosymetric crystals with sectors $\alpha $ and $\beta $ that form together inversion partners, the\textbf{\textit{ total}} spin is zero in every twofold-degenerate energy bands (termed spin degenerate). Yet we find that \textbf{\textit{local spin polarization}} can exist on each asymmetric sector, leading to ``R-2'' or ``D-2'' spin polarizations respectively. These local spin polarizations from asymmetric sectors $\alpha $and \quad $\beta $ compensated each other, forced by the bulk inversion symmetry. We demonstrate such remarkable R-2 and D-2 polarizations in some specific centrosymmetric crystals by first-principles calculations. This understanding leads to the recognition that a previously overlooked hidden form of spin polarization should exist in a much broader class of 3D bulk solids that own global inversion symmetry, and thus open the possibility to provide new routines for manipulating electron spins. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L8.00013: Spin-Orbit Effect on Dirac Cone in Selenium and Tellurium under Pressure Motoaki Hirayama, Shoji Ishibashi, Takashi Miyake We study the electronic structures of the group-VI elements, Se and Te, from first-principles in the local spin density approximation with the GW self-energy correction included. Both Se and Te are gapful at the ambient pressure. We use the Quantum MAterials Simulator (QMAS) package for the calculation with the spin-orbit interaction [1], and the full-potential linear muffin-tin orbitals method for the calculation of the non-relativistic self-energy. The calculated band gap is in excellent agreement with experiments, where the spin-orbit interaction substantially reduces the gap. The materials undergo an insulator-to-metal transition under pressure. In the metallic phase, at a certain pressure, two conducting states appear at around the H point, and they cross each other near the Fermi level. If the spin-orbit interaction is neglected, the states have linear dispersion in the vicinity of the crossing point, forming Dirac cone. The band crossing is protected even in the presence of spin-orbit interaction by the helical structure with the threefold symmetry. The spin structure at the H-point is peculiar: The spins for all non-degenerate states are confined in the ab plane, and points to the radial direction. [1] http://www.qmas.jp/ [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L8.00014: Microscopic Origin of the 0.7-Anomaly in Quantum Point Contacts Jan Heyder, Florian Bauer, Jan von Delft Quantum point contacts (QPCs) are short one-dimensional constrictions, usually patterned in a two-dimensional electron system, e.g. by applying voltages to local gates. The linear conductance of a point contact is quantized in units of $G_Q = 2e^2/h$. In addition, measured conductance curves exhibit an unexpected shoulder around $0.7 G_Q$. In this regime quantities like electrical and thermal conductance, noise and thermo-power have anomalous behavior. These phenomena are collectively known as the ``0.7-anomaly'' in QPCs, and their origin is still subject to controversial discussions. We offer a detailed microscopic explanation for the 0.7-anomaly. Its origin is a smeared van Hove singularity in the local density of states at the bottom of the lowest one-dimensional subband of the point contact, which causes an anomalous enhancement in the Hartree potential barrier, magnetic spin susceptibility and inelastic scattering rate. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L8.00015: The 0.7-anomaly in quantum point contacts with spin-orbit coupling Olga Goulko, Florian Bauer, Jan Heyder, Jan von Delft In addition to plateaus at integer values of $G_0 = 2e^2/h$, the linear conductance of a quantum point contact shows an anomalous shoulder at around $0.7G_0$. We study how the shape of this 0.7-anomaly is influenced by spin-orbit effects. We discuss both non-interacting and interacting systems at zero temperature, the latter of which we study using a functional renormalization group approach. In the presence of an external magnetic field, spin-orbit effects can significantly influence the shape of the 0.7-anomaly by mimicking and enhancing the effect of Coulomb interactions via changing the relative heights of the peaks of the local densities of states in the two spin states. We provide a detailed microscopic explanation of this effect and propose a setup for an experimental realization. [Preview Abstract] |
Session L10: Focus Session: Single Molecule Studies of Enzymes
Sponsoring Units: DBIOChair: Jennifer Ross
Room: 201
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L10.00001: Identifying Single Molecule Dynamics in Real Time via Time-Resolved Coherent Anti-Stokes Raman Scattering Steven Yampolsky, Dmitry Fishman, Shirshendu Dey, Eero Hulkko, Mayukh Banik, Eric Potma, V. Ara Apkarian By enhancing the local response of a single molecule with a dipolar nano-antenna, the vibrational dynamics at the single molecule limit can be measured in real time, by Time-Resolved Coherent Anti-Stokes Raman Scattering (TR-CARS). Such measurements involve the preparation and subsequent probing of vibrational wavepackets on the ground electronic state. In contrast to ensemble measurements, the vibrational coherence of a single molecule is not subject to dephasing; it exhibits dynamic phase and amplitude noise due to the~collapse of the wavepacket upon measurement. Continuous measurements of the amplitude noise distribution as a function of phase delay, allows the complete reconstruction of the state of the system. Under ambient conditions, repeated measurements of a single molecule coherence reduces to a statistical state of a system coupled to the thermal bath. The signature of the statistical state of a single molecule is characteristic, distinct from that of two, or three, or many; and this can be directly demonstrated through measurements. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L10.00002: Impact of the Diffusion of Microtubule-Associated Protein EB1 on Kinesin Translocation \textit{in Vitro} Benjamin Lopez, Megan Valentine Using the slowly hydrolyzable GTP analog GMPCPP, we polymerize microtubules that recapitulate the end binding behavior of EB1 along their entire length, and investigate the impact of EB1 on kinesin translocation. Through direct observation of single molecules of EB1 fused to GFP, we find that EB1 diffuses along the microtubule lattice, and that the presence of taxol affects the rate of diffusion. To test whether EB1 presence and diffusion has an effect on kinesin-driven cargo transport, we observe quantum dot labeled kinesins walking on microtubules assembled with GMPCPP and taxol and coated with EB1. We find that the addition of EB1 significantly reduces kinesin speed compared to the no EB1 condition, but when microtubules stabilized by both taxol and GMPCPP are used, the speed reduction is nearly abolished. Our data suggest a new possible mechanism for the regulation of kinesin function by EB1 in which kinesin speed is directly modulated through the interference of EB1 diffusion. Our results also raise important questions about the effects of taxol on microtubule-MAP interactions. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L10.00003: Acceleration of molecular complex dissociation by occlusion of rapid rebinding Thayaparan Paramanathan, Daniel Reeves, Larry Friedman, Jane Kondev, Jeff Gelles Molecular complexes in biology are held together by non-covalent interactions. Explicit consideration of molecular diffusion suggests that the dissociation kinetics of these systems are not adequately explained by simple distinctions between ``bound'' and ``free'' states of its molecular components. We formulated physical models that describe the effect of competitor molecules on the dissociation of a complex. The models show that competitor acceleration of complex dissociation by occluding rapid re-association is a natural feature of a molecular competition that could play a significant role in a wide variety of biological regulatory processes. We use single-molecule fluorescence colocalization experiments on a model complex to test this prediction and show that the effect is observed in biologically relevant ranges of competitor concentration. The results also demonstrate that single-molecule colocalization experiments can accurately measure dissociation rates despite their limited spatiotemporal resolution. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L10.00004: Single molecule analysis of cytoplasmic dynein motility Invited Speaker: Ahmet Yildiz Cytoplasmic dynein is a homodimeric AAA$+$ motor that transports a multitude of cargos towards the microtubule (MT) minus end. The mechanism of dynein motility remains unclear, due to its large size (2.6 MDa) and the complexity of its structure. By tracking the stepping motion of both heads at nanometer resolution, we observed that dynein heads move independently along the MT, in contrast to hand over hand movement of kinesins and myosin. Stepping behavior of the heads varies as a function of interhead separation and establishing the basis of high variability in dynein step size. By engineering the mechanical and catalytic properties of the dynein motor domain, we show that a rigid linkage between monomers and dimerization between N-terminal tail domains are not essential for processive movement. Instead, dynein processivity minimally requires the linker domain of one active monomer to be attached to an inert MT tether retaining only the MT-binding domain. The release of a dynein monomer from the MT can be mediated either by nucleotide binding or external load. Nucleotide dependent release is inhibited by the tension on the linker domain at high interhead separations. Tension dependent release is highly asymmetric, with faster release towards the minus-end. Reversing the asymmetry of the MT binding interface results in plus end directed motility, even though the force was generated by the dynein motor activity. On the basis of these measurements, we propose a model that describes the basis of dynein processivity, directionality and force generation. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L10.00005: Single-molecule comparison of DNA Pol I activity with native and analog nucleotides Osman Gul, Tivoli Olsen, Yongki Choi, Brad Corso, Gregory Weiss, Philip Collins DNA polymerases are critical enzymes for DNA replication, and because of their complex catalytic cycle they are excellent targets for investigation by single-molecule experimental techniques. Recently, we studied the Klenow fragment (KF) of DNA polymerase I using a label-free, electronic technique involving single KF molecules attached to carbon nanotube transistors [1]. The electronic technique allowed long-duration monitoring of a single KF molecule while processing thousands of template strands. Processivity of up to 42 nucleotide bases was directly observed, and statistical analysis of the recordings determined key kinetic parameters for the enzyme's open and closed conformations. Subsequently, we have used the same technique to compare the incorporation of canonical nucleotides like dATP to analogs like 1-thio-2'-dATP. The analog had almost no affect on duration of the closed conformation, during which the nucleotide is incorporated. On the other hand, the analog increased the rate-limiting duration of the open conformation by almost 40{\%}. We propose that the thiolated analog interferes with KF's recognition and binding, two key steps that determine its ensemble turnover rate. [1] T. J. Olsen, et. al., JACS 135, 7855 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L10.00006: Harmonic force spectroscopy reveals a force-velocity curve from a single human beta cardiac myosin motor Jongmin Sung, Suman Nag, Christian Vestergaard, Kim Mortensen, Henrik Flyvbjerg, James Spudich A muscle contracts rapidly under low load, but slowly under high load. Its molecular mechanisms remain to be elucidated, however. During contraction, myosins in thick filaments interact with actin in thin filaments in the sarcomere, cycling between a strongly bound (force producing) state and a weakly bound (relaxed) state. Huxley et al. have previously proposed that the transition from the strong to the weak interaction can be modulated by a load. We use a new method we call ``harmonic force spectroscopy'' to extract a load-velocity curve from a single human beta cardiac myosin II motor. With a dual-beam optical trap, we hold an actin dumbbell over a myosin molecule anchored to the microscope stage that oscillates sinusoidally. Upon binding, the motor experiences an oscillatory load with a mean that is directed forward or backward, depending on binding location We find that the bound time at saturating [ATP] is exponentially correlated with the mean load, which is explained by Arrhenius transition theory. With a stroke size measurement, we obtained a load-velocity curve from a single myosin. We compare the curves for wild-type motors with mutants that cause hypertrophic cardiomyopathies, to understand the effects on the contractile cycle [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L10.00007: Force-Manipulation Single-Molecule Spectroscopy Studies of Enzymatic Dynamics H. Peter Lu, Yufan He, Maolin Lu, Jin Cao, Qing Guo Subtle conformational changes play a crucial role in protein functions, especially in enzymatic reactions involving complex substrate-enzyme interactions and chemical reactions. We applied AFM-enhanced and magnetic tweezers-correlated single-molecule spectroscopy to study the mechanisms and dynamics of enzymatic reactions involved with kinase and lysozyme proteins. Enzymatic reaction turnovers and the associated structure changes of individual protein molecules were observed simultaneously in real-time by single-molecule FRET detections. Our single-molecule spectroscopy measurements of enzymatic conformational dynamics have revealed time bunching effect and intermittent coherence in conformational state change dynamics involving in enzymatic reaction cycles. The coherent conformational state dynamics suggests that the enzymatic catalysis involves a multi-step conformational motion along the coordinates of substrate-enzyme complex formation and product releasing. Our results support a multiple-conformational state model, being consistent with a complementary conformation selection and induced-fit enzymatic loop-gated conformational change mechanism in substrate-enzyme active complex formation. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:24AM |
L10.00008: Molecular motors are stymied by microtubule lattice defects Invited Speaker: Michael Gramlich The microtubule surface provides the tracks that molecular motors use to transport cargo throughout the cell. Much like any surface lattice, the microtubule surface may have surface defects such as dislocations or step edges caused by missing tubulin dimers or shifts in the number of protofilaments, respectively. It is an open question as to how microtubule lattice defects affect molecular motors walking along microtubule surfaces. We used the kinesin-1 motor that walks along a single protofilament and has a short step size of only 8 nm to test how lattice defects affect transport. We created microtubule lattice defects by end-to-end annealing microtubules with different protofilament numbers and differential fluorescence labeling, creating a transition in microtubule radius at the annealed site that is directly visualizable. Surprisingly, we observed that kinesin-1 motors are significantly inhibited by protofilament shift defects. GFP-tagged kinesin-1 motors detach at the defect site during at least 70{\%} of encounters with the defect. We find end-to-end annealed microtubules without the additional change in protofilament number at the defect site inhibit at least 50{\%} of kinesin-1 motors at the defect, suggesting that the process of end-to-end annealing creates defects within the lattice. Our results imply that defects within the microtubule lattice can inhibit motility, and must be corrected. Our work sheds light on the biological importance of removing and correcting lattice defects, an activity known to occur by multiple methods in cells. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L10.00009: Interplay between Velocity and Travel Distance of Kinesin-based Transport in the Presence of Tau Jing Xu, Stephen King, Maryse Lapierre-Landry, Brian Nemec Although the disease-relevant microtubule-associated protein tau is known to severely inhibit kinesin-based transport in vitro, potential mechanisms for reversing this detrimental effect to maintain healthy transport in cells remain unknown. Here we report the unambiguous up-regulation of multiple-kinesin travel distance despite the presence of tau, via decreased single-kinesin velocity. Intriguingly, the presence of tau also modestly reduced velocity in multiple-kinesin transport. Our stochastic simulations indicate that the tau-mediated reduction in single-kinesin travel is sufficient for the observed reduction in multiple-kinesin velocity. Taken together, our observations suggest that single-kinesin velocity is a promising experimental handle for tuning the effect of tau on multiple-kinesin travel distance, and uncover a previously unexplored role of tau for inhibiting multiple-kinesin velocity via reducing single-kinesin travel distance. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L10.00010: Molecular motors and the 2$^{\mathrm{nd}}$ law of thermodynamics Zhisong Wang Molecular motors from biology and nanotechnology often operate on chemical energy of fuel molecules in an isothermal environment, unlike macroscopic heat engines that draw energy from a heat flow between two temperatures. Nevertheless, isothermal molecular motors are still subject to the 2$^{\mathrm{nd}}$ law of thermodynamics in a fundamental way: their directional motion must cost a finite amount of energy other than the environmental heat even though no work is done; otherwise the 2$^{\mathrm{nd}}$ law would be violated. Hence the 2$^{\mathrm{nd}}$ law requires a finite energy price for pure direction of molecular motors. But what is the lowest price of direction allowed by the 2$^{\mathrm{nd}}$ law? And how does the 2$^{\mathrm{nd}}$ law-decreed price of direction limit performance of molecular motors? In the talk, I shall present our theoretical study of the 2$^{\mathrm{nd}}$ law-molecular motor link on basis of the accumulated biomotor phenomenology, and also introduce our experimental effort to develop biomimetic DNA bipedal nanomotors following the mechanistic guidelines out of the theoretical study. [Main contents of this talk are from references: J. Chem. Phys. 139, 035105 (2013); Phys. Rev. E 88, 022703 (2013); Phys. Rev. Lett. 109, 238104 (2012)] [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L10.00011: A propagating ATPase gradient drives transport of surface-confined cellular cargo Anthony Vecchiarelli, Keir Neuman, Kiyoshi Mizuuchi The process of DNA segregation is of central importance for all organisms. Although eukaryotic mitosis is relatively well established, the most common mechanism employed for bacterial DNA segregation has been unclear. ParA ATPases form dynamic patterns on the bacterial nucleoid, to spatially organize plasmids, chromosomes and other large cellular cargo, but the force generating mechanism has been a source of controversy and debate. A dominant view proposes that ParA-mediated transport and cargo positioning occurs via a filament-based mechanism that resembles eukaryotic mitosis. Here we present direct evidence against such models. Our cell-free reconstitution supports a non-filament-based mode of transport that may be as widely found in nature as actin filaments and microtubules. [Preview Abstract] |
Session L11: Focus Session: Physics of Proteins II
Sponsoring Units: DBIO DPOLYChair: Corey O'Hern, Yale University
Room: 203
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L11.00001: Thermal stability and folding kinetics analysis of intrinsically disordered protein, securin Chia-Ching Chang, Hsueh-Liang Chu, Li-Ping Ho Lacking a stable tertiary structure, intrinsically disordered proteins (IDPs) possess particular functions in cell regulation, signaling, and controlling pathways. The study of their unique structure features, thermal stabilities, and folding kinetics is intriguing. In this study, an identified IDP, securin, was used as a model protein. By using a quasi-static five-step (on-path) folding process, the function of securin was restored and analyzed by isothermal titration calorimetry. Fluorescence spectroscopy and particle size analysis indicated that securin possessed a compact hydrophobic core and particle size. The glass transition of securin was characterized using differential scanning microcalorimetry. Furthermore, the folding/unfolding rates (k$_{\mathrm{obs}}$) of securin were undetectable, implying that the folding/unfolding rate is very fast and that the conformation of securin is sensitive to solvent environment change. Therefore, securin may fold properly under specific physiological conditions. In summary, the thermal glass transition behavior and undetectable k$_{\mathrm{obs}}$ of folding/unfolding reactions may be two of the indices of IDP. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L11.00002: Predicting the conformational preferences of proteins using a physics-based free energy method Arijit Roy, Alberto Perez, Justin Maccallum, Ken A. Dill Protein molecules often undergo conformational changes. In order to get insights about the forces that drive such changes, it would be useful to have a method that computes the per-residue contributions to the conversion free energy. Here, we will describe the ``Confine-Convert-Release'' (CCR) method which is applicable to large conformational changes of proteins. CCR correctly predicts the stable states of several ``chameleon'' sequences that have previously been challenging for molecular simulations. CCR can often discriminate better from worse predictions of native protein models in CASP. We will show how the total conversion free energies can be parsed into per-residue free-energy components. Such parsing gives insights into which amino acids are most responsible for given transformations. For example, we are able to ``reverse-engineer'' the known design principles of the chameleon proteins. This opens up the possibility for systematic improvements in structure-prediction scoring functions, in the design of protein conformational switches, and in interpreting protein mechanisms at the amino-acid level. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L11.00003: Investigating the mechanisms leading to protein aggregation Ruth McNamara, Jennifer J. McManus The formation of protein aggregates is a feature of several diseases and is a problem during the manufacture of biopharmaceutical and protein based food products. During processing, stability may become compromised leading to the condensation of proteins to form non-native aggregates. The aim of this work is to induce aggregation on model proteins by the imposition of a particular stress to evaluate the extent of aggregation and to assess the degree of structural change to the protein. Aggregation of two proteins, lysozyme and bovine serum albumin has been induced by several mechanisms. Using various techniques (electrophoresis, HPLC, spectroscopic analysis, and microscopic techniques) both the level of aggregation extent of protein unfolding has been investigated for a range of solution conditions. Our results show that the amount of aggregation depends strongly on the mechanism by which non-native aggregation proceeds, and within each mechanism, solution conditions are an important factor. With the exception of aggregation by self-association (which is concentration dependent), the appearance of aggregation is driven by structural changes induced by the applied stress (heat, chemical denaturant, oxidation or contact with a surface). [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L11.00004: Novel insights into protein signaling by high-resolution structural biology Invited Speaker: Ilme Schlichting |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L11.00005: Bioinformatic prediction and in vivo validation of residue-residue interactions in human proteins Daniel Jordan, Erica Davis, Nicholas Katsanis, Shamil Sunyaev Identifying residue-residue interactions in protein molecules is important for understanding both protein structure and function in the context of evolutionary dynamics and medical genetics. Such interactions can be difficult to predict using existing empirical or physical potentials, especially when residues are far from each other in sequence space. Using a multiple sequence alignment of 46 diverse vertebrate species we explore the space of allowed sequences for orthologous protein families. Amino acid changes that are known to damage protein function allow us to identify specific changes that are likely to have interacting partners. We fit the parameters of the continuous-time Markov process used in the alignment to conclude that these interactions are primarily pairwise, rather than higher order. Candidates for sites under pairwise epistasis are predicted, which can then be tested by experiment. We report the results of an initial round of \emph{in vivo} experiments in a zebrafish model that verify the presence of multiple pairwise interactions predicted by our model. These experimentally validated interactions are novel, distant in sequence, and are not readily explained by known biochemical or biophysical features. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L11.00006: Study on the Dynamics of Influenza Hemagglutinin Based on Energy Landscape Theory Xingcheng Lin, Nathanial Eddy, Jeffrey Noel, Paul Whitford, Jianpeng Ma, Jose Onuchic Hemagglutinin (HA2), a homotrimeric influenza surface protein crucial for membrane fusion, undergoes an drastic structural rearrangement during viral invasion of the host. X-ray crystallography shows that the pre- and post-fusion configurations have largely disparate secondary, tertiary and quaternary structures. Simulations allow us to explore the time-dependent high resolution structural information and function of HA2 dynamics. Here we use an approach based on energy landscape theory that combines the native information from both the starting and end points. Our simulation shows two key events in the conformational transition of HA2: The extension of its fusion peptides away from the viral membrane and the melting of its globular C-terminal portion. The similar timescale and a kinetic competition between these two events lead to two main pathways and generic kinetic intermediates during this transition. Through considering the biological context of HA, we test perturbations of the baseline model that are useful in understanding the robustness of our predictions and how they translate into the function of HA. The all-atom explicit solvent simulation is performed and convince the cracking phenomenon at the start of this protein dynamics. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L11.00007: Investigation of protein fluctuations via Anisotropic Network Model and Molecular Dynamics Osman B. Okan, Deniz Turgut, Aravind Rammohan, Angel E. Garcia, Rahmi Ozisik We use Anisotropic Network Model (ANM) and compare its protein fluctuation predictions against molecular dynamics (MD) simulations and experimental findings for 210 globular proteins. The ANM results are analyzed using bond orientational order (BOO) parameters. We show that BOO parameters could be reformulated as a sum of contact density and geometrical (distribution of contacts in space) components. This reformulation of BOO makes it possible to investigate the role of each individual component separately, and identify cut--off ranges where each component dominates protein fluctuations. Our results indicate that the widely accepted correlation between mean squared displacements (MSDs) and inverse contact density is valid for ANM within the cut-off range of 10--15 {\AA}. We show that the two components of the BOO dominate protein fluctuations at different length scales: contact density at small length scales and geometric distribution of residues at length scales comparable to the protein size. It is also shown that the relationship between MSD and contact density is firmly rooted in BOO, and is rendered possible with a unique distribution of residues that nullifies the average geometric component's contribution to the BOO within the 10$-$15 {\AA} cut-off.\textunderscore \textunderscore [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L11.00008: Ensemble Activation of G-Protein$-$Coupled Receptors Revealed by Small-Angle Neutron Scattering Xiang-qiang Chu, Suchithranga Perera, Utsab Shrestha, Udeep Chawla, Andrey Struts, Shuo Qian, Michael Brown Rhodopsin is a G-protein$-$coupled receptor (GPCR) involved in visual light perception and occurs naturally in a membrane lipid environment. Rhodopsin photoactivation yields \textit{cis-trans} isomerization of retinal giving equilibrium between inactive Meta-I and active Meta-II states. Does photoactivation lead to a single Meta-II conformation, or do substates exist as described by an ensemble-activation mechanism (EAM)? We use small-angle neutron scattering (SANS) to investigate conformational changes in rhodopsin-detergent and rhodopsin-lipid complexes upon photoactivation. Meta-I state is stabilized in CHAPS-solubilized rhodopsin, while Meta-II is trapped in DDM-solubilized rhodopsin. SANS data are acquired from 80{\%} D$_{2}$O solutions and at contrast-matching points for both DDM and CHAPS samples. Our experiments demonstrate that for detergent-solubilized rhodopsin, SANS with contrast variation can detect structural differences between the rhodopsin dark-state, Meta-I, Meta-II, and ligand-free opsin states. Dark-state rhodopsin has more conformational flexibility in DDM micelles compared to CHAPS, which is consistent with an ensemble of activated Meta-II states. Furthermore, time-resolved SANS enables study of the time-dependent structural transitions between Meta-I and Meta-II, which is crucial to understanding the ensemble-based activation. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L11.00009: Conformational entropic maps of functional coupling domains in GPCR activation: A case study with beta2 adrenergic receptor Fan Liu, Ravinder Abrol, William Goddard III, Dennis Dougherty Entropic effect in GPCR activation is poorly understood. Based on the recent solved structures, researchers in the GPCR structural biology field have proposed several ``local activating switches'' that consisted of a few number of conserved residues, but have long ignored the collective dynamical effect (conformational entropy) of a domain comprised of an ensemble of residues. A new paradigm has been proposed recently that a GPCR can be viewed as a composition of several functional coupling domains, each of which undergoes order-to-disorder or disorder-to-order transitions upon activation. Here we identified and studied these functional coupling domains by comparing the local entropy changes of each residue between the inactive and active states of the $\beta $2 adrenergic receptor from computational simulation. We found that agonist and G-protein binding increases the heterogeneity of the entropy distribution in the receptor. This new activation paradigm and computational entropy analysis scheme provides novel ways to design functionally modified mutant and identify new allosteric sites for GPCRs. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L11.00010: A theory for protein dynamics: Global anisotropy and a normal mode approach to local complexity Jeremy Copperman, Pablo Romano, Marina Guenza We propose a novel Langevin equation description for the dynamics of biological macromolecules by projecting the solvent and all atomic degrees of freedom onto a set of coarse-grained sites at the single residue level. We utilize a multi-scale approach where molecular dynamic simulations are performed to obtain equilibrium structural correlations input to a modified Rouse-Zimm description which can be solved analytically. The normal mode solution provides a minimal basis set to account for important properties of biological polymers such as the anisotropic global structure, and internal motion on a complex free-energy surface. This multi-scale modeling method predicts the dynamics of both global rotational diffusion and constrained internal motion from the picosecond to the nanosecond regime, and is quantitative when compared to both simulation trajectory and NMR relaxation times. Utilizing non-equilibrium sampling techniques and an explicit treatment of the free-energy barriers in the mode coordinates, the model is extended to include biologically important fluctuations in the microsecond regime, such as bubble and fork formation in nucleic acids, and protein domain motion. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 11:00AM |
L11.00011: TBD Invited Speaker: Dongping Zhong |
Session L12: Invited Session: The Physics of Cell Division
Sponsoring Units: DBIOChair: M. Betterton, University of Colorado Boulder
Room: 205
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L12.00001: Microtubule Depolymerization as a Driver for Chromosome Motion Invited Speaker: Richard McIntosh Microtubules (MTs) are rigid polymers of the protein, tubulin, which function as intracellular struts. They are also tracks along which motor enzymes can run, carrying cargo to specific cellular locations. Most MTs are dynamic; they assemble and disassemble rapidly, particularly during cell division when the cell forms the ``mitotic spindle,'' a machine that organizes the duplicated chromosomes into a planar disk, then pulls the duplicate copies apart, moving them to opposite ends of the cell. This process is necessary for the daughter cells to have a full complement of DNA. The mitotic spindle is a labile framework that exerts several kinds of forces on the chromosomes to move them in well organized ways. It contains many motor enzymes that contribute to spindle formation, but genetic evidence shows that the motors that attach to chromosomes and might contribute to chromosome motion are dispensable for normal mitosis. Apparently MT dynamics can also serve as a motor and is an important source of force for chromosome motion. We have studied this process and find that MTs can be coupled to a load by specific spindle proteins so that MT depolymerization can exert substantial force. With the yeast protein, Dam1, a single MT can generate 30 pN, about 5-fold more than is generated by a motor enzyme like kinesin or myosin. The resulting motions are processive, so a depolymerizing MT can carry its load for many micrometers. However, Dam1 is found only in fungi. We have therefore sought other proteins that can serve as analogous couplers. Several MT-dependent motor enzymes can do the job in ways that do not require ATP, their normal source of energy. Some non-motor MT-associated proteins will also work, e.g., the kinetochore proteins NDC80 and CENP-F. Data will be presented that show the strengths and weaknesses of each coupler, allowing some generalization about how the mitotic machinery works. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L12.00002: Microtubules search for chromosomes by pivoting around the spindle pole Invited Speaker: Nenad Pavin During cell division, proper segregation of genetic material between the two daughter cells requires that the spindle microtubules attach to the chromosomes via kinetochores, protein complexes on the chromosome. The central question, how microtubules find kinetochores, is still under debate. We observed in fission yeast that kinetochores are captured by microtubules pivoting around the spindle pole body, instead of growing towards the kinetochores. By introducing a theoretical model, we show that the observed angular movement of microtubules is sufficient to explain the process of kinetochore capture. Our theory predicts that the speed of the capture process depends mainly on how fast microtubules pivot. We confirmed this prediction experimentally by speeding up and slowing down microtubule pivoting. Thus, microtubules explore space by pivoting, as they search for intracellular targets such as kinetochores.\\[4pt] In collaboration with Iva Tolic-Norrelykke, Max Planck Institute of Molecular Cell Biology and Genetics. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L12.00003: A minimal model for kinetochore-microtubule dynamics Invited Speaker: Andrea Liu During mitosis, chromosome pairs align at the center of a bipolar microtubule (MT) spindle and oscillate as MTs attaching them to the cell poles polymerize and depolymerize. The cell fixes misaligned pairs by a tension-sensing mechanism. Pairs later separate as shrinking MTs pull each chromosome toward its respective cell pole. We present a minimal model for these processes based on properties of MT kinetics. We apply the measured tension-dependence of single MT kinetics [1] to a stochastic many MT model, which we solve numerically and with master equations. We find that the force-velocity curve for the single chromosome system is bistable and hysteretic. Above some threshold load, tension fluctuations induce MTs to spontaneously switch from a pulling state into a growing, pushing state. To recover pulling from the pushing state, the load must be reduced far below the threshold. This leads to oscillations in the two-chromosome system. Our minimal model quantitatively captures several aspects of kinetochore dynamics observed experimentally. \\[4pt] [1] Akiyoshi et al. (2010) Nature 468, 576. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:24AM |
L12.00004: An engineer's understanding of kinetochore motility and signaling Invited Speaker: Ajit Joglekar The kinetochore is a macromolecular motor that couples chromosome movement to microtubule polymerization and depolymerization. It is also a mechanochemical signaling hub. A kinetochore that lacks microtubule attachment generates a biochemical signal to arrest the cell cycle. Both kinetochore functions require numerous copies of $\sim$ 8 proteins and protein complexes. A cohesive explanation of how multiple kinetochore components cooperate to achieve motility and signaling remains elusive. I will describe on-going ``architecture-function'' analysis in my lab that applies an engineer's perspective to study the machine that is the kinetochore. This analysis is based on the definition of the protein architecture of the kinetochore using known protein structures, copy numbers in the kinetochore, average positions, and distributions. This architecture enables us to assign specific functions to each kinetochore component in generating movement. The architecture also reveals the molecular mechanism of kinetochore signaling embedded within the kinetochore. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 11:00AM |
L12.00005: Positioning of Microtubule organizing centers (MTOC) in 3D confinement Invited Speaker: Sophie Roth Important functions of eucaryotic cells, like motility or division, depend sensitively on cytoskeletal mechanics and organization. In particular, microtubules (MTs) are dynamic polymers that can move and position organelles such as their MTOC by pushing, pulling or sliding [1]. How the shape and size of cells, as well as the presence of pushing and/or pulling forces influence this positioning is in many cases still unclear. To assess the influence of confinement on MTOC positioning, we reconstruct a dynamic microtubule cytoskeleton in vitro, inside 3D water in oil emulsion droplets. We study the positioning of centrosomes, from which microtubules are nucleated, that exert pushing and/or dynein- mediated-pulling forces when reaching the cortex. We show that the central position of one centrosome confined in a spherical droplet is drastically destabilized by pushing forces, while pulling forces tend to center the centrosome. Interestingly, when two centrosomes are present, pushing forces will lead the centrosomes to take a stable position at opposite sides of the droplet. When both pushing and pulling forces are present, two centrosomes adopt an equilibrium position balancing the centering effect of the cortical pulling forces and the repulsion effect of the two centrosomes. Summarizing, we show that very simple systems, involving only microtubule dynamics, confinement, pushing and pulling forces can lead to self-organized patterns that are biologically relevant. In particular, we reproduce a ‘mitotic spindle’ like organization with just these components. This sets the stage for a better understanding of the function of additional components of natural mitotic spindles such as mitotic motors and crosslinkers that we plan to add to our system. \\[4pt] [1] Tolic-Norrelykke I, \textbf{2008} Push-me-pull-you: how microtubules organize the cell interior \textit{Eur Biophys J} 37: 1271-1278 [Preview Abstract] |
Session L13: Focus Session: Fe Based Superconductors-Pairing Symmetry
Chair: Peter Hirschfield, Unversity of FloridaRoom: 207
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L13.00001: Superconducting gap in LiFeAs from three-dimensional spin-fluctuation pairing calculations Yan Wang, Andreas Kreisel, Peter Hirschfeld, Volodymyr Zabolotnyy, Sergey Borisenko, Bernd B\"uchner, Thomas Maier, Douglas Scalapino The lack of nesting of Fermi-surface sheets in the Fe-based superconductor LiFeAs, with a $T_c$ of 18 K, has led to questions as to whether the origin of superconductivity in this material might be different from other Fe-based superconductors. Here we present calculations of the superconducting gap and pairing in the random-phase approximation using Fermi surfaces derived from ARPES. The gaps obtained are qualitatively different from previous 2D theoretical works and in good agreement with ARPES on the main Fermi-surface pockets. We analyze the contributions to the pairing vertex thus obtained and show that the scattering processes between electron and hole pockets still dominate the pairing as in other Fe-based superconductors despite the lack of nesting, leading to gaps with anisotropic $s_\pm$ structure. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L13.00002: Orbital fluctuation mediated S$_{++}$ wave state based on the SC-VC$_{\Sigma}$ method -- Impact of vertex correction for the gap equation Seiichiro Onari, Hiroshi Kontani We develop the theory of superconductivity by introducing the vertex correction (VC). Although the Coulomb interaction violates Migdal's theorem, the VC for the gap equation have usually been neglected for simplicity. However, the VC is inevitable to satisfy Ward identity. Here, we show that S$_{++}$ wave state mediated by the orbital fluctuation is favored by the VC for the gap equation. Previously, we have shown that the orbital fluctuations are strongly enhanced due to the spin-orbital mode-coupling described by the VC for the irreducible susceptibility. The structural phase transition and softening of shear modulus C$_{66}$ are naturally explained by the orbital fluctuations. In this study, both the VC and the self-energy ($\Sigma$) are obtained self-consistently (SC-VC$_{\Sigma}$ method). By using the SC-VC$_{\Sigma}$ method, we aim to understand whole phase diagram including the superconducting state in H-doped LaFeAsO and LiFeAs. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L13.00003: Effects of magnetic flux in a loop formed by an s-wave superconductor and an $\mathrm{s_{\pm}}$ superconductor Rosa Rodriguez-Mota, Tami Pereg-Barnea Identifying the correct order parameter structure of the iron based superconductors will provide insight into the pairing mechanism in these materials. Due to the multi-orbital band structure of these materials and the proximity of the superconducting phase to an anti-ferromagnetic phase, most theories favor magnetic fluctuations as the pairing mechanism and an order parameter with the so-called $\mathrm{s_{\pm}}$ symmetry. However, it is experimentally challenging to distinguish the $\mathrm{s_{\pm}}$ symmetry from conventional s-wave symmetry; thus, the $\mathrm{s_{\pm}}$ structure remains unconfirmed. In 2010, Chen \textit{et al} showed evidence of integer and half integer flux quantum transitions in an $\mathrm{Nb}$-$\mathrm{NdFeAsO_{0.88}F_{0.12}}$ loop excited by electromagnetic pulses [1]. We present a theoretical study of the effects of magnetic flux in a superconducting s/$\mathrm{s_{\pm}}$ loop inspired by these results. Our findings are in agreement with preliminary results of a phenomenological Ginzburg Landau model [2], and help clarify the relation between the transitions observed in the experiment and the $\mathrm{s_{\pm}}$ symmetry.\\[4pt] [1] C.-T. Chen \textit{et al}, Nature Physics 6, 260 (2010).\\[0pt] [2] Berg, Lindner, Pereg-Barnea, Phys. Rev. Lett. 106, 1470. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L13.00004: Topological defects in s$+$is and d$+$id superconductors Egor Babaev, Julien Garaud Recently arguments were advanced that various compounds can have s$+$is or d$+$id superconductivity. I will discuss topological defects which can arise in such superconducting states, their properties and experimental signatures. The allowed defects: vortices, domain walls and Skyrmions all have distinct magnetic features so their observation can be a confirmation of the s$+$is or d$+$id nature of superconducting states. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L13.00005: Orbital-Parity Selective Superconducting Pairing Structures of Fe-based Superconductors under Glide Symmetry Chiahui Lin, Chung-Pin Chou, Wei-Guo Yin, Wei Ku We show that the superconductivity in Fe-based superconductors consists of zero and finite momentum $(\pi,\pi,0)$ Cooper pairs with the same and different parities of the Fe $3d$ orbitals respectively. The former develops the distinct gap structures for each orbital parity, and the latter is characteristic of spin singlet, spacial oddness and time reversal symmetry breaking. This originates from the unit cell containing two Fe atoms and two anions of staggered positioning with respect to the Fe square lattice. The in-plane translation is turned into glide translation, which dictates orbital-parity selective quasiparticles. Such novel pairing structures explain the unusual gap angular modulation on the hole pockets in recent ARPES and STS experiments. Work supported by DOE DE-AC02-98CH10886 and Chinese Academy of Engineering Physics and Ministry of Science and Technology. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L13.00006: On the internal $d$-wave structure of s$^{\pm}$ pairs in Iron-based Superconductors Tze Tzen Ong, Piers Coleman A key issue in understanding the high temperature iron-based superconductors concerns the mechanism by which the paired electrons minimize their strong mutual Coulomb repulsion. Whereas electronically paired superconductors generally avoid the Coulomb interaction through the formation of higher angular momentum pairs, iron based superconductors involve s-wave (s$^{\pm}$) pairs with zero angular momentum. By taking account of the orbital degrees of freedom of the iron atoms, here we show that the s$^{\pm}$ pairs in these materials possess hidden d-wave symmetry, forming orbital triplets in which the the d-wave angular momentum of the pairs is compensated by the internal angular momentum of the orbitals. The recent observation of a gap with octahedral structure in KFe$_{2}$As$_{2}$ materials can be understood as a transition to a ``high spin'' configuration of the d-wave orbital triplets, through the alignment of the two angular momentum components of the pair. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L13.00007: Similarities, diferences and the pairing interaction of the Fe-based and cuprate superconductors Invited Speaker: Douglas Scalapino The undoped multi-orbital, multi-Fermi surface Fe-based superconductors exhibit metallic antiferromagnetism while the undoped cuprates are insulating Mott antiferromagnets which when optimally doped have one large Fermi surface. Nevertheless, both systems exhibit a neutron spin resonance in the superconducting state providing evidence of an unconventional sign changing superconducting gap which appears in proximity or coexisting with antiferromagentism. Here we will examine what the similarities and differences of these two classes of materials tell us about the pairing mechanism. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L13.00008: Symmetry measurements of the order parameter of BaFe$_{2}$As$_{2}$ superconductors Juan Atkinson, Dale Van Harlingen, Paul Canfield Since the discovery of the Fe-pnictide superconductors, extensive efforts have been directed toward understanding the symmetry and mechanism of the superconducting pairing. Extended s-wave models, predominantly the s$\pm$ model, are predicted by many theories, but a definitive experimental verification has been elusive. In this case, the phase-sensitive Josephson interferometry measurements that have been effective in determining the symmetry of many superconductors do not give a definitive signature. As an alternative approach, we are searching for proximity-induced structure in the density-of-states of an s-wave superconductor proximity-coupled to Co-doped BaFe$_2$As$_2$ superconductive crystals. Unique features are predicted to arise for s$\pm$ pairing that allow this symmetry to be distinguished from other order parameter forms such as s++ or d-wave. Observation of this structure would provide both magnitude and phase information about the multigap structure of the BaFe$_2$As$_2$ crystal. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L13.00009: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L13.00010: Time-reversal symmetry broken metallic states in multiband superconductors Troels Arnfred Bojesen, Egor Babaev, Asle Sudbo The recent discovery of so-called multiband superconductors, like the iron pnictides, has spurred a surge in interest for superconductors with several bands crossing the Fermi level. The reason for this is that frustration in interband couplings may lead to a broken $Z_2$ (``time reversal'') symmetry in addition to the ``ordinary'' breaking of the $\mathrm{U}(1)$ symmetry in single band superconductors, opening up for the possibility of new forms of topological excitations and interesting new physics. We have investigated phase diagrams and phase transitions of $\mathrm{U}(1)\times Z_2$ superconductors in 2D and 3D beyond mean-field approximation, using large-scale Monte Carlo simulations. In addition to the superfluid $\mathrm{U}(1)\times Z_2$ and $\mathrm{U}(1)$ broken states, we find, in a certain parameter regime, a new, non-superfluid (metallic) $Z_2$ broken (but $\mathrm{U}(1)$ symmetric) state where time-reversal symmetry is spontaneously broken. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L13.00011: Improper s-wave symmetry for the electronic pairing in iron-based superconductors by first-principles calculations Michele Casula, Sandro Sorella By means of space-group symmetry arguments, we argue that the electronic pairing in iron-based high temperature superconductors shows a structure which is a linear combination of planar s-wave and d-wave symmetry channels, both preserving the 3-dimensional $A_{1g}$ irreducible representation of the corresponding crystal point-group. We demonstrate that the s- and d-wave channels are determined by the parity under reflection of the electronic orbitals through the iron planes, and by improper rotations around the iron sites. We provide evidence of these general properties by performing accurate quantum Monte Carlo ab-initio calculations of the pairing function for a FeSe lattice. In order to achieve a higher resolution in momentum space we introduce a BCS model that faithfully describes our QMC variational pairing function. This allows us to provide a k-resolved image of the pairing function, and show that non-isotropic contributions in the BCS gap function are related to the improper s-wave symmetry. Our theory can rationalize and explain a series of contradictory experimental findings, such as the observation of twofold symmetry in the FeSe superconducting phase and the $s$-to-$d$-wave gap transition in BaFe$_2$As$_2$ under K doping. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L13.00012: Field Induced Superlattice Modulation in Hole-Doped Iron-Pnictide Superconductors Hong-Yi Chen, C.S. Ting Based upon a phenomenological model with competing spin-density-wave and extended s$\pm$ pairing superconductivity, the vortex states in $Ba_{1-x}K_xFe_2As_2$ are investigated by solving Bogoliubov-de Gennes equations. Our results for the optimally doped compound with slightly induced SDW at the center of the vortex are in agreement with STM experiments. We also propose that in the underdoped compound the field induced superlattice modulation. The emergence of the superlattice modulation with period $12a_0$ results in a band reconstruction around the Fermi energy. We found out that the reconstructed band has particle-hole symmetry. The symmetric particle-hole band causes a zero bias resonance peak. The band reconstruction can be also used to explain the absence of the peak at the vortex center in $Ba(Fe_{1-x}Co_x)_2As_2$ electron doped pnictide. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L13.00013: Zero energy Andreev Bound states in odd parity pairing superconductors Chandan Setty, Jiangping Hu We study the properties of zero energy Andreev bound states in SNS junctions. The superconductors on either sides of the normal metal are assumed to have two sub-lattices with either odd or even parity pairing within each sub-lattice. In addition, we add a uniform even parity pairing between the two sub-lattices and study its effect on the Andreev zero energy bound state. In general, we find that the even parity pairing tends to weaken/destroy the zero bias peak (ZBP). We point out the relevance of our results to a recently proposed superconducting ground state in Iron based superconductors (FeSCs) [1]. \\[4pt] [1] Jiangping Hu PRX, 3, 031004 (2013) [Preview Abstract] |
Session L14: Invited Session: Understanding Ion Containing Polymer Systems using Computer Simulations
Sponsoring Units: DPOLY DCOMPChair: Gary Grest, Sandia National Laboratories
Room: 301-303
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L14.00001: Complexation of Oppositely Charged Polyelctrolytes and Diblock Polyampholytes Invited Speaker: Michael Rubinstein The conformational properties of both symmetric and asymmetric flexible diblock polyampholytes and oppositely charged polyelectrolytes are investigated by molecular dynamics simulations and scaling theory. The electrostatically driven coil-globule transition of a symmetric diblock polyampholyte consist of three regimes identified with increasing electrostatic interaction strength: the folding regime, the weak association regime dominated by the fluctuation-induced attractions between oppositely charged sections of the chains, and the ion binding regime that starts with direct binding of oppositely charged monomers (dipole formation), followed by a cascade of multipole formation leading to multiplets analogous to those found in ionomers. In asymmetric block polyampholytes we find the globule to tadpole transition with the increase of charge asymmetry. In the weak association regime this transition is controlled by the balance of net charge and surface tension of the complex and characterized by the ratio of the difference in the number of electrostatic ``blobs'' between oppositely charged blocks and one third power of the total number of electrostatic blobs. We find the maximum overcharging of the complexes formed by either asymmetric diblock polyampholytes or by pairs of oppositely charged polyelectrolytes is by 50{\%} independent of system parameters. We use scaling theory to estimate the average size of the complex and the electrostatic correlation length as functions of chains length, strength of electrostatic interactions, charge fractions, and solvent quality. The theoretically predicted scaling laws of these conformational properties are in good agreement with our simulation results. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L14.00002: Where Scattering and Computations Meet: Structure and Dynamics of Ionic Co-Polymers Invited Speaker: Dvora Perahia Ion transporting polymers constitute vital components in clean energy generation and storage devices including electrolytic media in fuel cells and ion conducting separators in batteries. While different polymers are currently in use, achieving controlled ion transport and storage ability while retaining mechanical and chemical stability remains a challenge: under the conditions which optimize the transport and storage for specific application, either mechanical or chemical stabilities are compromised. Designing block-co-polymers with ion transporting blocks bound to blocks that enhance mechanical and chemical stability would mitigate the challenge. Tailoring block copolymers with blocks that exhibit various desired properties, results in a new set of open questions that pertain to new complex materials including defining the phase diagram and understanding the interfacial regions of the muliblocks. Here we present the first molecular-level computational insight of the behavior of a pentablock, A-B-C-B-A, co-polymer that consists of an A block of poly(t-butyl-styrene), a B block of ethylene-r-propylene and a C block of a randomly sulfonated styrene, in solution in comparison with neutron scattering data. Neutron studies have shown that in hydrophobic solvents this pentablock forms elongated micelles in dilute solutions where the ionic block dominates the solution structure. These studies provide ensemble average of structure and properties. The computational studies provided further molecular-level insight. Here we will discuss the interrelations between scattering results and computational studies to provide remarkable understanding of a complex ionic system. Pathways to advance this molecular understanding to an actual membrane will be then discussed. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L14.00003: Atomistic Simulations of Aggregation in Ionomer Melts Invited Speaker: Amalie L. Frischknecht Ionomers, polymers containing a small fraction of covalently bound ionic groups, are of interest as possible electrolytes in batteries. A single-ion conducting polymer electrolyte would be safer and have higher efficiency than currently-used liquid electrolytes. However, to date ionomers do not have sufficiently high conductivities for practical application, most likely because the ions tend to form aggregates, leading to slow ion transport. An understanding of the relationships between ionomer chemistry, morphology, and ion transport is needed to design ionomers with improved conductivity. To provide insight into the ionic aggregate morphology, we have performed molecular dynamics simulations of a series of polyethylene-based model ionomer melts, in which the spacing between functional groups is precisely controlled. We vary the counterion type, the neutralization level, and the length of the spacer. The simulations provide new insights into the shape, size and composition of ionic aggregates. In particular, we observe a wide variety of aggregate morphologies, ranging from small spherical aggregates to string-like shapes and large percolated networks. The structure factors calculated from simulation agree well with X-ray scattering data. Depending on the morphology, the simulation and experimental scattering curves can be well fit with either a modified hard sphere or a modified hard cylinder model. These fits, combined with the simulation data, provide the first (indirect) experimental evidence of string-like aggregate morphologies in ionomer melts. We speculate that stringy, percolated aggregates may enhance ionic conduction. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:24AM |
L14.00004: Molecular Dynamics Simulations of Polyelectrolyte Solutions Invited Speaker: Andrey Dobrynin Polyelectrolytes are polymers with ionizable groups. In polar solvents, these groups dissociate releasing counterions into solution and leaving uncompensated charges on the polymer backbone. Examples of polyelectrolytes include biopolymers such as DNA and RNA, and synthetic polymers such as poly(styrene sulfonate) and poly(acrylic acids). In this talk I will discuss recent molecular dynamics simulations of static and dynamic properties of polyelectrolyte solutions. These simulations show that in dilute and semidilute polyelectrolyte solutions the electrostatic induced chain persistence length scales with the solution ionic strength as $I^{\mathrm{-1/2}}$. This dependence of the chain persistence length is due to counterion condensation on the polymer backbone. In dilute polyelectrolyte solutions the chain size decreases with increasing the salt concentration as \textit{R $\sim$ I}$^{-1/5}$. This is in agreement with the scaling of the chain persistence length on the solution ionic strength, $l_{p}$\textit{ $\sim$ I}$^{-1/2}$. In semidilute solution regime at low salt concentrations the chain size decreases with increasing polymer concentration, \textit{R $\sim$ c}$_{p}^{-1/4}$. While at high salt concentrations one observes a weaker dependence of the chain size on the solution ionic strength, \textit{R $\sim$ I}$^{-1/8}$. Analysis of the simulation data throughout the studied salt and polymer concentration ranges shows that there exist general scaling relations between multiple quantities $X(I)$ in salt solutions and corresponding quantities $X(I_{0})$ in salt-free solutions, $X(I)=X(I_{0})(I/I_{0})^{\beta }$. The exponent $\beta =$ -1/2 for chain persistence length $l_{p}, \beta =$ 1/4 for solution correlation length, $\beta =$ -1/5 and $\beta =$ -1/8 for chain size $R$ in dilute and semidilute solution regimes respectively. Furthermore, the analysis of the spectrum and of the relaxation times of Rouse modes confirms existence of the single length scale (correlation length) that controls both static and dynamic properties of semidilute polyelectrolyte solutions. These findings confirm predictions of the scaling model of polyelectrolyte solutions. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 11:00AM |
L14.00005: Effective interactions and aggregation of rodlike polyelectrolytes Invited Speaker: Erik Luijten Rodlike polyelectrolytes are known to exhibit various aggregation phenomena, assembling into structures that range from bundles to rafts. Such self-assembly is important in numerous biological and synthetic applications. Here, I provide an overview of various important aspects of such phenomena. In particular, I will highlight computational work on the free-energy landscape of various rod configurations. Furthermore, the role of many-body effects will be discussed. First, ionic excluded-volume effects lead to correlations, which can become particularly important in the dense environment within an aggregate. Second, induced polarization charges arise from the dielectric mismatch between the polyelectrolyte and the surrounding solvent. The latter are rarely taken into account, owing to the computational complexity of solving the bound charges, but can significantly alter the electrostatic interactions that are responsible for aggregation in the first place. New, efficient techniques now make it possible to incorporate these effects in standard molecular dynamics or Monte Carlo simulations. Using these techniques, I examine the role of both types of many-body effects on bundle configuration and stability. [Preview Abstract] |
Session L15: Focus Session: Inferring Physical Models from Noisy Biological Data
Sponsoring Units: DBIOChair: Steve Presse, University of California, San Francisco
Room: 304
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L15.00001: Path-Integral Statistical Learning of Continuous Stochastic Dynamics from single-molecule FRET data Invited Speaker: Jhih-Wei Chu |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L15.00002: Techniques for Statistically Scrutinizing Stochastic Model Assumptions Using a Single Noisily Measured Trajectory Christopher Calderon The increased spatial and temporal resolution afforded by recent single-molecule experiments has inspired researchers to consider new techniques for quantifying molecular-level kinetics. Many researchers have contributed methods for improving the quality of estimators characterizing single-molecule kinetics, however techniques for checking the consistency of implicit distributional assumptions behind an assumed stochastic against a single experimental trajectory are under-developed. In this talk, likelihood-based goodness-of-fit testing and other model-based hypotheses tests accounting for the complexities of single-molecule trajectory analysis (heterogeneity, transient kinetic regime shifts, measurement noise, etc.) are discussed. Utility of the testing procedures are demonstrated on (i) single particle tracking (SPT) experiments characterizing mRNA motion in the cytoplasm of yeast cells and (ii) protein kinetics in the primary cilium of mammalian cells. In both cases, the testing procedures facilitated the discovery of new kinetic signatures of molecular motor facilitated transport not accounted for in traditional SPT models. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L15.00003: Inferring cardiac phase response curve in vivo Arkady Pikovsky, Bjoern Kralemann, Matthias Fruehwirth, Michael Rosenblum, Thomas Kenner, Jochen Schaefer, Maximilian Moser Characterizing properties of biological oscillators with phase response cirves (PRC) is one of main theoretical tools in neuroscience, cardio-respiratory physiology, and chronobiology. We present a technique that allows the extraction of the PRC from a non-invasive observation of a system consisting of two interacting oscillators, in this case heartbeat and respiration, in its natural environment and under free-running conditions. We use this method to obtain the phase coupling functions describing cardio-respiratory interactions and the phase response curve of 17 healthy humans. We show at which phase the cardiac beat is susceptible to respiratory drive and extract the respiratory-related component of heart rate variability. This non-invasive method of bivariate data analysis for the determination of phase response curves of coupled oscillators may find application in other biological and physical systems. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:36AM |
L15.00004: Form-Function Relationship in E. coli Chemotaxis Invited Speaker: Jayajit Das Cell-to-cell variations in protein abundance in clonal cell populations are ubiquitous in living systems. Because protein composition determines responses in individual cells, it stands to reason that the variations themselves are subject to selective pressures. However, the functional role of these cell-to-cell differences is not well understood. One way to tackle questions regarding relationships between form and function is to perturb the form (e.g., change the protein abundances) and observe the resulting changes in some function. We take on the form-function relationship from the inverse perspective, asking instead what specific constraints on cell-to-cell variations in protein abundance are imposed by a given functional phenotype [1]. We develop a maximum entropy based approach to posing questions of this type and illustrate the method by application to the well-characterized chemotactic response in {\it Escherichia coli}. We find that full determination of observed cell-to-cell variations in protein abundances is not inherent in chemotaxis itself but, in fact, appears to be jointly imposed by the chemotaxis program in conjunction with other factors (e.g., the protein synthesis machinery and/or additional non-chemotactic cell functions, such as cell metabolism). These results illustrate the power of maximum entropy as a tool for the investigation of relationships between biological form and function. \\[4pt] [1] Sayak Mukherjee, Sang-Cheol Seok, Veronica J. Vieland, and Jayajit Das, Proceedings of National Academy of Sciences {\bf 110} 18531 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L15.00005: Inferring the Spatiotemporal DNA Replication Program from Noisy Biological Data John Bechhoefer, Antoine Baker We generalize a stochastic model of DNA replication to the case where replication-origin-initiation rates vary locally along the genome and with time. Using this generalized model, we address the inverse problem of inferring initiation rates from experimental data concerning replication in cell populations. Previous work based on curve fitting depended on arbitrarily chosen functional forms for the initiation rate, with free parameters that were constrained by the data. We introduce a model-free, non-parametric method of inference that is based on Gaussian process regression. The method replaces specific assumptions about the functional form of initiation rate with more general prior expectations about the smoothness of variation of this rate, along the genome and in time. Using this inference method, we show that we can recover with high precision simulated replication schemes with data that are typical of current experiments. The method of Gaussian process regression can be profitably applied to a wide range of physical and biological problems. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L15.00006: Distribution of population averaged observables in stochastic gene expression Bhaswati Bhattacharyya, Ziya Kalay Observation of phenotypic diversity in a population of genetically identical cells is often linked to the stochastic nature of chemical reactions involved in gene regulatory networks. We investigate the distribution of population averaged gene expression levels as a function of population, or sample size for several stochastic gene expression models to find out to what extent population averaged quantities reflect the underlying mechanism of gene expression. We consider three basic gene regulation networks corresponding to transcription with and without gene state switching and translation. Using analytical expressions for the probability generating function (pgf) of observables and Large Deviation Theory, we calculate the distribution of population averaged mRNA and protein levels as a function of model parameters and population size. We validate our results using stochastic simulations also report exact results on the asymptotic properties of population averages which show qualitative differences for different models. We calculate the skewness and coefficient of variance for pgfs to estimate the sample size required for population average that contains information about gene expression models. This is relevant to experiments where a large number of data points are unavailable. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L15.00007: Multi-Scale Modeling to Improve Single-Molecule, Single-Cell Experiments Brian Munsky, Douglas Shepherd Single-cell, single-molecule experiments are producing an unprecedented amount of data to capture the dynamics of biological systems. When integrated with computational models, observations of spatial, temporal and stochastic fluctuations can yield powerful quantitative insight. We concentrate on experiments that localize and count individual molecules of mRNA. These high precision experiments have large imaging and computational processing costs, and we explore how improved computational analyses can dramatically reduce overall data requirements. In particular, we show how analyses of spatial, temporal and stochastic fluctuations can significantly enhance parameter estimation results for small, noisy data sets. We also show how full probability distribution analyses can constrain parameters with far less data than bulk analyses or statistical moment closures. Finally, we discuss how a systematic modeling progression from simple to more complex analyses can reduce total computational costs by orders of magnitude. We illustrate our approach using single-molecule, spatial mRNA measurements of Interleukin 1-alpha mRNA induction in human THP1 cells following stimulation. Our approach could improve the effectiveness of single-molecule gene regulation analyses for many other process. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L15.00008: Outcome prediction in a mathematical model of immune response to infection Manuel Mai, Kun Wang, Michael Kirby, Mark D. Shattuck, Corey S. O'Hern In clinical settings, it is of great importance to diagnose patients in the shortest amount of time and with the highest achievable accuracy. Current open questions concerning the modeling of the host response to infection include: How many measurements and with what frequency are needed to diagnose patients with a given accuracy? What is the effect of patient variation on the prediction accuracy? We employ machine-learning techniques to predict disease outcomes from data generated from a set of ordinary differential equations (ODE) used to model the immune response to infection. ODE models have the advantage that we can generate an unlimited amount of data, and we can easily simulate patient differences by varying model parameters. We explore the dependence of the prediction accuracy on data sets generated from the sets of ODEs as a function of the number of and spacing between measurements, number of measured variables, and the size of the patient variability. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L15.00009: Information content and cross-talk in biological signal transduction: An information theory study Ashok Prasad, Samanthe Lyons Biological cells respond to chemical cues provided by extra-cellular chemical signals, but many of these chemical signals and the pathways they activate interfere and overlap with one another. How well cells can distinguish between interfering extra-cellular signals is thus an important question in cellular signal transduction. Here we use information theory with stochastic simulations of networks to address the question of what happens to total information content when signals interfere. We find that both total information transmitted by the biological pathway, as well as its theoretical capacity to discriminate between overlapping signals, are relatively insensitive to cross-talk between the extracellular signals, until significantly high levels of cross-talk have been reached. This robustness of information content against cross-talk requires that the average amplitude of the signals are large. We predict that smaller systems, as exemplified by simple phosphorylation relays (two-component systems) in bacteria, should be significantly much less robust against cross-talk. Our results suggest that mammalian signal transduction can tolerate a high amount of cross-talk without degrading information content, while smaller bacterial systems cannot. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L15.00010: Network topological analysis reveals the functional cohesiveness for the newly discovered links by Yeast 2 Hybrid approach Susan Ghiassian, Sam Pevzner, Thomas Rolland, Murat Tassan, Albert Laszlo Barabasi, Mark Vidal Protein-protein interaction maps and interactomes are the blueprint of Network Medicine and systems biology and are being experimentally studied by different groups. Despite the wide usage of Literature Curated Interactome (LCI), these sources are biased towards different parameters such as highly studied proteins. Yeast two hybrid method is a high throughput experimental setup which screens proteins in an unbiased fashion. Current knowledge of protein interactions is far from complete. In fact the previous offered data from Y2H method (2005), is estimated to offer only 5\% of all potential protein interactions. Currently this coverage has increased to 20\% of what is known as reference HI In this work we study the topological properties of Y2H protein-protein interactions network with LCI and show although they both agree on some properties, LCI shows a clear unbiased nature of interaction selections. Most importantly, we assess the properties of PPI as it evolves with increasing the coverage. We show that, the newly discovered interactions tend to connect proteins that have been closer than average in the previous PPI release. reinforcing the modular structure of PPI. Furthermore, we show, some unseen effects on PPI (as opposed to LCI) can be explained by its incompleteness. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L15.00011: Spatio-temporal dynamcis of a cell signal cascade with negative feedback Jose Luis Maya Bernal, Guillermo Ramirez-Santiago We studied the spatio-temporal dynamics of a system of reactio-diffusion equations that models a cell signal transduction pathway with six cycles and negative feedback. The basic cycle consists of the phosphorylation-dephosphorylation of two antagonic proteins. We found two regimes of saturation of the enzimatic reaction in the kinetic parameters space and determined the conditions for the signal propagation in the steady state. The trajectories for which transduction occurs are defined in terms of the ratio of the enzimatic activities. We found that in spite of the negative feedback the cell signal cascade behaves as an amplifier and produces phosphoprotein concentration gradients within the cell. This model behaves also as a noise filter and as a switch. [Preview Abstract] |
Session L16: Focus Session: Extreme Mechanics: Origami and Structural Metamaterials
Sponsoring Units: GSNP DPOLYChair: Jose Bico, ESPCI
Room: 401
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L16.00001: Capillary Aggregation of Nanofilaments into Superstructures Invited Speaker: Michael De Volder Over the past decades, methods have been developed to coat surfaces with high aspect ratio nanofilaments for applications including supercapacitors, solar cells, superhydrophobic surfaces, and biomimetic adhesives. Importantly, these nanofilaments often come in contact with wet environments during their synthesis, post-treatment, or in their final application. Because high aspect ratio nanofilaments have a very low stiffness, they can easily be manipulated by capillary interactions. Upon drying for instance, capillary forces can collapse nanofilaments into random aggregates, which is typically an unwanted effect. However, new studies show that by understanding and controlling capillary aggregation, it is possible to fabricate complex and robust architectures in a scalable manner [1]. Besides providing an overview of the above developments, this talk will focus on a process we are developing towards the capillary aggregation of vertically aligned carbon nanotubes ``forests.'' We found capillary aggregation to be an effective method for increasing nanotube packing density, but also to form complex nanotube superstructures. These were for instance integrated in microsensors and other MEMS devices. This talk is based on joint research with AJ Hart, S Tawfick, SJ Park and D Copic. \\[4pt] [1] Engineering Hierarchical Nanostructures by Elastocapillary Self-Assembly, M De Volder, AJ Hart, Angewandte Chemie, 52 (9), 2412-2425 [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L16.00002: Bad origami Scott Waitukaitis Origami research often assumes rigid plates connected by free hinges, but interesting and useful behaviors emerge if these assumptions are relaxed. I will show how breaking the rules of flat and rigid folding, $i.e.$ making ``bad origami,'' can lead to structures that exhibit mechanical rigidity and bistability. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L16.00003: 3D Buckligami: Digital Matter Martin Van Hecke, Koen de Reus, Bastiaan Florijn, Corentin Coulais We present a class of elastic structures which exhibit collective buckling in 3D, and create these by a 3D printing/moulding technique. Our structures consist of cubic lattice of anisotropic unit cells, and we show that their mechanical properties are programmable via the orientation of these unit cells. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L16.00004: Metallurgy of Miura-ori: lattice theory for inhomogeneous deformations of origami tessellations Arthur Evans, Jesse Silverberg, Lauren McLeod, Itai Cohen, Christian Santangelo In nature, as well as in art, one often encounters thin materials that have been deformed by their environment or their creator into complex folded states; examples include the folds of the endoplasmic reticulum, the villi in the intestinal tract, and tessellated patterns in the ancient Japanese art of origami. One (engineering) advantage of creating a folded structure is that the geometric constraints associated with creasing imbues the construction with exotic mechanical properties, such as generating a material with a negative Poisson's ratio. Materials exhibiting novel behavior of this type, arising from the special properties of the unit cell, are generally classified as metamaterials. In this talk I consider a mechanical metamaterial known as Miura-ori, an origami tessellation pattern that displays soft modes and crystallographic defects not accounted for by a purely geometric theory of an infinitely thin material. I will discuss a method for deriving how inhomogeneous deformations arise from bending within Miura-ori, and show that this leads to a natural coherence length over which the inhomogeneity decays. Additionally, I will show how the modular nature of origami unit cells lends additional richness to the mechanical properties associated with deformation. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L16.00005: Mechanics of Miura-ori Origami Lattice Defects Jesse Silverberg, Lauren McLeod, Arthur Evans, Jessica Ginepro, Christian Santangelo, Thomas Hull, Itai Cohen The mechanical properties of origami-inspired materials show remarkable potential for responsive, tunable next-generation materials. For example, the Miura-ori fold is predicted to have negative Poisson ratio and anisotropic compressive properties. Using a custom mechanical testing device and 3D laser profilometry, we investigate the moduli and the role of curvature in setting these material properties. Because defects are known to dramatically alter the bulk properties in other periodic materials, we introduce defects into the folding pattern to investigate their effects on the macroscopic mechanical properties. Interestingly, we find that a single defect increases the overall material stiffness, but the introduction of a second defect in the opposite direction can cancel out the first, tending to restore the original material properties. Moreover, these defect pairs can be arranged to form edge dislocations, grain boundaries, and many other topological configurations familiar from the study of crystallographic lattice defects. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L16.00006: Cutting and Folding for Tunable Materials Properties Pablo Damasceno, Paul Dodd, Terry Shyu, Matthew Shlian, Max Shtein, Nicholas Kotov, Sharon Glotzer Despite the small set of building blocks used for their assembly, naturally occurring materials such as proteins show remarkable diversity in their mechanical properties ranging from something resembling rubber--low stiffness, high resilience and extensibility--to silk--high stiffness and strength. Moreover, their self-folding properties inspire the design of structures capable of tunable reconfiguration. Motivated by such versatility, we report on simulations and experiments for the design of nanocomposites sheets whose mechanical properties can be made tunable via ``secondary structures'' patterning. Our simulations reveal the main cutting features needed to obtain desired material extensibility. Additionally, we study how similar sheets could self-fold into their desired ``native'' structure via stochastic forces. Our results open the possibilities for manufacture of flexible and reconfigurable materials with targeted strength and extensibility. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L16.00007: Non-dissipative shapable sheet Naomi Oppenheimer, Thomas Witten A sheet of paper that has been crumpled and flattened retains some amount of shapability that a bare, uncrumpled, sheet does not have: when deformed by external forces, it retains the deformed shape after the forces are removed. Using a frustrated two dimensional lattice of springs, we show that such shapability can be attained in a non-dissipative system. Numerical investigations suggest an extensive number of bistable energy minima using several variants of this scheme. The numerical sheet can be bent into a nearly-closed cylinder that holds its shape. We verify that the deformed shape is locally stable and compare its bending modulus in the deformed state with that in the initial flat state. We investigate the threshold for non-elastic deformation using various kinds of forcing. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L16.00008: Folding by Design Paul Dodd, Pablo Damasceno, Sharon Glotzer A form of self-assembly, ``self-folding'' presents an alternative approach to the creation of reconfigurable, responsive materials with applications ranging from robotics to drug design. However, the complexity of interactions present in biological and engineered systems that undergo folding makes it challenging to isolate the main factors controlling their assembly and dis-assembly. Here we use computer simulations of simple, minimalistic self-foldable structures and investigate their stochastic folding process. By dynamically accessing all the states that lead to, or inhibit, successful folding, we show that the mechanisms by which general stochastic systems can achieve their ``native'' structures can be identified and used to design rules for optimized folding propensity. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L16.00009: Auto-origami with defects: modeling blueprinted liquid crystal polymer networks Robin Selinger, Andrew Konya, Vianney Gimenez-Pinto Coupling between topological defects and curvature plays an important role in morphology and microstructural evolution of soft matter with orientational order. We explore this coupling in photoresponsive liquid crystal polymer networks (LCN), which deform under illumination by shrinking along the nematic director and expanding in orthogonal directions. If a non-uniform director field is imposed when a sample is cross-linked, known as ``blueprinting,'' illumination induces non-uniform strain, causing the sample to change shape. The 3-D director field thus encodes a complex deformation, a form of programmed auto-origami. Topological defects in the director field induce an initially flat sample to deform out-of-plane, forming structures with Gaussian curvature. Using 3-D finite element elastodynamics simulation studies, we model the actuation of a photoresponsive LCN containing high order topological defects (from $+$10 to -10) and defect arrays, and compare to recent experiments by McConney et al [1]. We also model blueprinted structures with a striped pattern of twisted domains which form tear-drop shaped accordion folds, and compare to experiments by de Haan et al [2]. Finally, we compare the physics of defect-curvature coupling in LCN with that in other materials such as lipid membranes.\\[4pt] [1] DOI: 10.1002/adma.201301891\\[0pt] [2] DOI:~10.1002/adfm.201302568 [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L16.00010: Measuring mechanical properties of thin hydrogel sheets by elasto-capillary origami. Jinhye Bae, Ryan Hayward Characterizing the mechanical properties of soft elastic materials is critical for understanding their fundamental behaviors, as well as for their use in applications as biomaterials and stimuli-responsive devices. However, quantitative measurements of soft materials, especially those with micro-scale dimensions, is challenging using conventional methods. We take advantage of the recently developed understanding of the elasto-capillary deformation of thin sheets under conditions where interfacial tension is comparable to elastic bending energy, as a means to characterize the elastic properties of micro-scale gel sheets. We first calibrate the method by studying the relationship between the minimum encapsulation length (L$_{crit})$ and the elasto-capillary length (L$_{ec})$ using commercial polymer films with known thickness and modulus, and then apply it photo-crosslinked temperature-responsive hydrogel sheets over a range of temperatures. We anticipate that surface tension will provide a versatile probe for characterizing the properties of soft materials on the micro-scale. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L16.00011: Auto-Origami and Soft Programmable Transformers: Simulation Studies of Liquid Crystal Elastomers and Swelling Polymer Gels Andrew Konya, Christian Santangelo, Robin Selinger When the underlying microstructure of an actuatable material varies in space, simple sheets can transform into complex shapes. Using nonlinear finite element elastodynamic simulations, we explore the design space of two such materials: liquid crystal elastomers and swelling polymer gels. Liquid crystal elastomers (LCE) undergo shape transformations induced by stimuli such as heating/cooling or illumination; complex deformations may be programmed by ``blueprinting'' a non-uniform director field in the sample when the polymer is cross-linked. Similarly, swellable gels can undergo shape change when they are swollen anisotropically as programmed by recently developed halftone gel lithography techniques. For each of these materials we design and test programmable motifs which give rise to complex deformation trajectories including folded structures, soft swimmers, apertures that open and close, bas relief patterns, and other shape transformations inspired by art and nature. In order to accommodate the large computational needs required to model these materials, our 3-d nonlinear finite element elastodynamics simulation algorithm is implemented in CUDA, running on a single GPU-enabled workstation. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L16.00012: Targeting Fold Stiffness to Design Enhanced Origami Structures Philip Buskohl, Giorgio Bazzan, Andrew Abbott, Michael Durstock, Richard Vaia Structures with adaptive geometry are increasingly of interest for actuation, sensing and packaging applications. Origami structures, by definition, can ``shape-shift'' between multiple geometric configurations that are predefined by a pattern of folds. Plastic deformation and local failure at the fold lines transform an originally homogenous material into a grid with locally tailored mechanical properties that bias the response of the overall structure to external loading. Typically, origami structures focus on uniformly stiff fold lines with rigid facets. In this study, we discuss how localized variations in stiffness can influence global properties, including energy budget to transition from flat to folded structure, the preferred path through configuration space, and the final mechanical response of the folded architecture. A simple, bi-stable origami fold pattern is laser machined into polypropylene sheets of different compliance and the critical load of the transition is measured. We model the structure as a truss with bar elongation, folding, and facet bending in order to predict ways to enhance or mitigate the critical load. Targeting local folding properties to modify global performance directly extends to the analysis of more complex architectures. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L16.00013: Tunable Helical Origami Zi Chen, Eric Dai, Huang Zheng Origami, the Japanese art of paper folding, is traditionally viewed as an amusing pastime and medium of artistic expression. However, in recent years, origami has begun to inspire innovations in science and engineering. For example, K. Miura led the study of a paper folding pattern in regards to deployment of solar panels to outer space, resulting in more efficient packing and unpacking of the solar panels into tightly constrained spaces. In this work, we study the geometric and mechanical properties of a twisting origami pattern. The pattern created by the fold exhibits several interesting properties, including rigid foldibility, and finely tunable helical coiling, with control over pitch, radius, and handedness of the helix. In addition, the pattern closely mimics the twist buckling patterns shown by thin materials, for example, a mobius strip. In our work, we relate the six parameters of the twisting origami pattern to generate a fully tunable graphical model of the fold. In addition, we demonstrate that the morphogenesis of such folding pattern can be modeled through finite element analysis. We hope our research into the diagonal fold brings insight into the potential scientific and engineering applications of origami and spark further research into how the traditional paper art can be applied as a simple, inexpensive model for complex problems. [Preview Abstract] |
Session L17: Focus Session: Friction and Adhesion
Sponsoring Units: GSNPChair: Gianpietro Moras, Fraunhofer IWM
Room: 402
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L17.00001: Probing locally the onset of slippage at a model multi-contact interface Victor Romero, Elie Wandersman, George Debr\'egeas, Alexis Prevost We investigate the interfacial dynamics in a frictional joint between a micro-patterned elastomer and a smooth glass slide in the stick-slip regime. A novel technique is developed to decorate the surface of PDMS blocks with thousands of spherical caps (100 $\mu$m in curvature) whose positions and heights are controlled at $\mu$m scale. Such samples are rubbed against bare glass slides while the macroscopic normal and shear loads are monitored. The use of model spherical asperities provides a direct access to the local normal and shear stress within the frictional joint through optical tracking of the micro-contacts area and in-plane displacement. This method allows us to evidence the existence of one or more slip waves propagating inward from the contact edge right before the onset of slip events. The wave front is found to propagate normally to the iso-pressure contour lines at a velocity proportional to the macroscopic (imposed) shear rate. A simple quasi-static model of the multi-contact interface is derived that qualitatively accounts for the observed dynamics of these slip precursors. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L17.00002: Depinning transition and 2D superlubricity in incommensurate colloidal monolayers Davide Mandelli, Andrea Vanossi, Nicola Manini, Erio Tosatti Colloidal monolayers sliding over periodic corrugated potential are highly tunable systems allowing to visualize the dynamics between crystalline surfaces [1]. Based on molecular dynamics, Vanossi and coworkers [2] reproduced the main experimental results and explored the potential impact of colloid sliding in nanotribology. The degree of interface commensurability was found to play a major role in determining the frictional properties, the static friction force Fs becoming vanishingly small in incommensurate geometries for weak corrugation U0.Lead by this result,here we systematically investigate the possibility to observe a 2D Aubry-like transition [3] from a superlubric state to a pinned state for increasing U0. By using a reliable protocol, we generate annealed configurations at different values of U0 for an underdense monolayer. We find Fs to be vanishingly small up to a critical corrugation Uc coinciding with an abrupt structural transition in the ground state configuration. Similarly to what is observed in the Frenkel Kontorova model,this transition is characterized by a significant decrease in the number of particles sampling regions near the maxima of the substrate potential.\\[4pt] [1] T. Bohlein, Nat. Mat.,11,126; [2] A. Vanossi, PNAS USA,109,16426; [3] S. Aubry, Phys.D,8,381. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L17.00003: Does rotational melting make molecular surfaces more slippery? Andrea Benassi, Carlo Pignedoli, Daniele Passerone, Andrea Vanossi, Erio Tosatti Crystals made up of spherical, weakly interacting molecules generally exhibit a phase transition between a low temperature ordered phase and a plastic phase, where the rotational order is thermally lost. In C$_{60}$ fullerene, the transition takes place at T$_r$=260K in bulk, initiating at a lower temperature at a (111)surface. We explore by MD simulations whether a slider should experience a change of friction on that surface in correspondence with the phase transition. Modeling the slider as a C$_{60}$ flake attached to a sliding tip, we obtain a response dependent on the orientation and the angular compliance of the flake. An orientation angle commensurate with the C$_{60}$ surface yields a large adhesion and friction, both dropping by only about 20\% at the plastic transition. An incommensurate angle yields both adhesion and friction a factor 2 smaller and relatively unaffected by the transition. Finally, a sliding flake with an incommensurate angle but a compliant orientation offers the possibility of a very different sliding behavior, remaining incommensurate with very low adhesion/friction above T$_r$, but jumping to a commensurate angle with high adhesion/friction below T$_r$. This third possibility might have been realized in the AFM experiment by Liang et al.(PRL 2003). [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L17.00004: Tribo-induced melting transitions and internal friction at magnetic and nonmagnetic asperity contacts Jacqueline Krim, Liming Pan, Keeley Stevens We report a study of tribo-induced nanoscale surface melting mechanisms that employs a combined QCM-STM technique [1] for a range of Au and Au-Ni alloys with varying compositional percentages and phases [2]. A transition from solid-solid to solid-``liquid like'' contact was observed for most samples at sufficiently high asperity sliding speeds. Pure gold, solid-solution and two-phase Au-Ni (20 at.{\%} Ni) alloys were compared [3]. Samples with 5-20{\%} nickel alloyed with gold were deposited as a homogenous solid-solution or as a two-phase FCC solid through the modification of annealing procedures. The solid solution is known to be paramagnetic for concentrations below 35{\%} Ni while the two phase solid maintains domains of ferromagnetism within bulk gold. A ``flexing'' effect associated with the application of an external magnetic field on the two-phase alloy samples illuminates physical mechanisms that correlate with the observed tribo-induced melting temperatures [4].\\[4pt] [1] B. D. Dawson, S. M. Lee, and J. Krim, Phys. Rev. Lett. 103, 205502 (2009).\\[0pt] [2] L. Pan, Ph.D. Thesis, North Carolina State University (2011).\\[0pt] [3] Zhenyin Yang; Lichtenwalner, D.J.; Morris, A.S.; Krim, J.; Kingon, A.I, Journal of Microelectromechanical Systems, April 2009, Volume: 18 Issue:2, 287-295.\\[0pt] [4] K. Stevens, L. Pan and J. Krim, (2014) submitted [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L17.00005: Non-intrusive measurements of frictional forces between micro-spheres and flat surfaces Wei-Hsun Lin, Chiara Daraio We report a novel, optical pump-probe experimental setup to study micro-friction phenomena between micro-particles and a flat surface. We present a case study of stainless steel microspheres, of diameter near 250$\mu$m, in contact with different surfaces of variable roughness. In these experiments, the contact area between the particles and the substrates is only a few nanometers wide. To excite the particles, we deliver an impulse using a pulsed, high-power laser. The reaction force resulting from the surface ablation induced by the laser imparts a controlled initial velocity to the target particle. This initial velocity can be varied between 10$^{-5}$ to 1 m/s. We investigate the vibrating and rolling motions of the micro-particles by detecting their velocity and displacement with a laser vibrometer and a high-speed microscope camera. We calculate the effective Hamaker constant from the vibrating motion of a particle, and study its relation to the substrate's surface roughness. We analyze the relation between rolling friction and the minimum momentum required to break surface bonding forces. This non-contact and non-intrusive technique could be employed to study a variety of contact and tribology problems at the microscale. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L17.00006: Exploring field effects on ionic liquid boundary lubrication Rosario Capozza, Andrea Benassi, Andrea Vanossi, Erio Tosatti Ionic liquids, organic salts that are liquid at room temperature, are of great physical as well as of technological interest. Their adhesion properties to solid surfaces under pressure suggests their use as boundary lubricants. One potentially interesting feature would be the possibility that electrical charging of the solid plates or more generally an applied static or dynamic electric field could modify the nearby perpendicular and parallel ordering of ions, and in turn also modify the sliding friction. While these effects have just begun to be pursued by experimental groups, we have undertaken molecular dynamics simulations aimed at exploring some of these questions. Preliminary results obtained using very simple molten salt boundary lubrication models will be presented and discussed. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L17.00007: Friction from formation and rupture of molecular contacts Invited Speaker: Michael Urbakh Frictional motion plays a central role in diverse systems and phenomena that span vast ranges of scales, from the nanometer contacts inherent in micro- and nanomachines and biological molecular motors to the geophysical scales characteristic for earthquakes. Despite the practical and fundamental importance of friction and the growing efforts in the field, many key aspects of dynamics of friction are still not well understood. One of the main difficulties in understanding and predicting frictional response is the complexity of highly non-equilibrium processes going on in any tribological contact which include detachment and re-attachment of multiple microscopic contacts (bonds) between the surfaces in relative motion while still in contact. In this lecture I will discuss microscopic models which establish relationships between the dynamics of formation and rupture of individual contacts and frictional phenomena. First, I will introduce a phenomenological model that describes friction through thermally activated rupture and formation of molecular contacts. Then, I will focus on a microscopic model that includes the effect of thermally activated jumps of the surface atoms between the sliding surfaces on nanoscopic friction. I will show that the proposed models explain a nonmonotonic dependence of friction on temperature, which has been observed in recent friction force microscopy experiments for different material classes. These models offer a new conceptual framework to describe the dynamics of nanoscale friction. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L17.00008: Effect of Surface Roughness on Nanoparticle Adhesion Zhen Cao, Andrey Dobrynin, Jan-Michael Carrillo, Andrew Oyer, Mark Stevens We study effect of surface roughness on adhesion of soft nanoparticles. Using molecular dynamics simulations we obtained deformation of nanoparticles and their effective contact area with substrates as a function of nanoparticle crosslinking density, surface energy, work of adhesion, and surface roughness. We modeled adhesion of nanoparticles on substrates with periodic patterns (1-D stripes and 2-D square lattice of posts) and with random height distribution. Our simulations show that the JKR-like model can be applied to describe adhesion of strongly crosslinked large nanoparticles, while for the weakly crosslinked nanoparticles, that undergo large deformations, the change of surface energy should be included to account for nanoparticle shape deformation. We propose a simple scaling model which shows that equilibrium shape of nanoparticle is a result of fine interplay between nanoparticle surface energy, elastic energy and work of adhesion to the substrate. The predictions of the scaling model are in a very good agreement with simulation results. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L17.00009: When are rough surfaces sticky? Lars Pastewka, Mark Robbins Van-der Waals interactions operate between all surfaces and are strong enough to hold 1000kg per square centimeter. Yet, few surfaces are adhesive. This discrepancy between atomic and macroscopic forces is due to roughness and has been dubbed the adhesion paradox. To quantify this behavior, we carried out molecular statics and continuum simulations of the contact area, stiffness and adhesion between rigid, randomly rough surfaces and elastic substrates. The surfaces are self-affine with Hurst exponent 0.3 to 0.8 and different short and long wavelength cutoffs. The rms surface slope and the range and strength of the adhesive potential are also varied. For parameters typical of most solids, the effect of adhesion decreases as the ratio of long to short wavelength cutoff increases. In particular, the pull-off force decreases to zero and the area of contact A becomes linear in the applied load L. A simple scaling argument is developed that describes the increase in the ratio A/L with increasing adhesion and a corresponding increase in the contact stiffness. The argument predicts a crossover to finite contact area at zero load when surfaces are exceptionally smooth or the ratio of surface tension to bulk modulus is unusually large, as for elastomers. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L17.00010: Contact of rough surfaces at the atomic scale Tristan Sharp, Lars Pastewka, Mark O. Robbins Roughness on solid surfaces typically causes contacting solids to remain microscopically separated, with important consequences for sealing, adhesion, and friction. Continuum theory and continuum simulations of rough contact have greatly contributed to a statistical understanding of these processes. However, continuum treatments neglect atomic-scale geometry at the surface, thereby making an uncontrolled approximation. Here, we perform molecular dynamics simulations of rough surfaces to test the consequences of atomic-scale features. We focus on atomic plasticity and atomic steps that form terraces on rough surfaces of crystalline solids. We find that the atomic features treated here do not dramatically alter the large-scale solid deformations predicted by continuum calculations. However, different behavior emerges at small scales. Continuum treatments underestimate the number of atoms with very low and very high stress. Step edges concentrate stress and change the small-scale morphology of contact patches. A new statistical quantity, the characteristic ratio of step height to step width, is found to be useful when extending continuum theory to treat atomic-scale steps. These results are discussed in context of the recent scaling theory of Persson. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L17.00011: Friction in the peeling test Suomi Ponce, Jose Bico, Benoit Roman Peeling tests are commonly used to probe adhesives. We are interested in the adhesion of soft elastomers on rigid substrates through van der Waals interactions. Such elastomers are prone to slide when sheared and exhibit a characteristic friction stress. We show how the classical relation from Kendall fails to predict the peeling force for such systems for low values of the peeling angle (the actual force being largely underestimated). We propose an implement to Kendall's approach that accounts for friction. In the limit of zero angle this description provides a maximum force proportional to the adhered area in agreement with our experimental observations. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L17.00012: Strong dynamical effects during stick-slip adhesive peeling Marie-Julie Dalbe, Stephane Santucci, Loic Vanel, Pierre-Philippe Cortet We consider the classical problem of the stick-slip dynamics observed when peeling an adhesive tape at a constant velocity. From fast imaging recordings, we extract the dependencies of the stick and slip phases durations with the imposed peeling velocity and peeled ribbon length. Predictions of Maugis and Barquins [in Adhesion 12, edited by K.W. Allen, Elsevier ASP, London, 1988, pp. 205-222] based on a quasistatic assumption succeed to describe quantitatively our measurements of the stick phase duration. Such model however fails to predict the full stick-slip cycle duration, revealing strong dynamical effects during the slip phase.\\[4pt] Dalbe et al., Soft Matter, (2013), DOI : 10.1039/c3sm51918j. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L17.00013: Stick-slip friction and ageing in Velcro$^{\mbox{\textregistered }}$ Lisa Mariani, Paul Angiolillo The mesoscopic hook and loop system of Velcro$^{\mbox{\textregistered }}$ provides a model of stick-slip friction that exhibits behavior reminiscent of results seen in nanoscale model systems. The friction is linearly dependent on contact area and independent of driving velocity. Morever, there is a power law dependence of the friction on loading, with exponent between 1/4 and 1/3. Furthermore, the evolution of stick-slip to more smooth sliding, as controlled by contact area, is also noted. These transition predictions follow power law profiles, as well, with respect to increasing contact area. Thus, the hook-and-loop system shows to be a good mesoscale model system of stick-slip friction and provides a link between nanoscale and macroscale friction. Through an investigation into the ageing of the hooks in the system, the data suggests that the hooks age during the shearing regime and take a characteristic time to return to initial attachment strength. Additionally, there does not appear to be a significant affect of ageing on the kinetic friction experienced by the system. [Preview Abstract] |
Session L18: Vesicles and Membranes
Sponsoring Units: DCMP GSNPChair: E Lyman, University of Delaware
Room: 403
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L18.00001: Mechanism of Cationic Nanoparticles and Cell-Penetrating Peptides Direct Translocate Across Cell Membranes Jiaqi Lin, Alfredo Alexander-Katz Cationic Nanoparticles (NPs) and cell-penetrating peptides (CPPs) are known effective intracellular delivery agents. These positively charged particles can bypass traditional endocytosis route to enter the cytosol, which is known as direct translocation. However, mechanism of direct translocation of both NPs and CPPs is not well understood. Using Coarse-grained (CG) molecular dynamics simulation, we found that gold nanoparticles (AuNPs) as well as HIV-1 Tat peptides can translocate across model biological membranes through nanoscale holes under a transmembrane (TM) potential. After the translocation, the TM is strongly weakened and the holes gradually reseal themselves, while the NPs/CPPs roam freely in the ``intracellular region.'' Both size and shape of the NPs/ CPPs are found to be a determine factor of their translocation behaviour, and the relationship between direct translocation and endocytosis is also discussed. The results provided here establish fundamental rules of direct translocation entry of NPs/CPPs, which may guide the rational design of cationic intracellular nanocarriers. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L18.00002: Hydrophobic matching between melittin and phosphocholine lipid bilayers having different thicknesses William Heller, Shuo Qian The lipid bilayer of the cellular membrane is more than a simple medium that houses proteins with specific function. Instead, it is an elastic medium that plays an active role in the function of the membrane and that both drives the function of membrane proteins and alters its properties in response to their presence. The conceptual simplicity of membrane active peptides makes them attractive model systems for studying membrane-protein interactions. Melittin, a 27 amino acid cationic peptide having a helix-hinge-helix motif, is one of the most extensively studied examples. Small-angle neutron scattering (SANS) measurements of melittin associated with lipid bilayer vesicles having different hydrocarbon thicknesses showed that the bilayer thickness stretches to match the thickness of the peptide in a manner consistent with a rigid, extended melittin having its helical axis oriented parallel to the bilayer normal. This behavior is surprising considering the helix-hinge-helix motif of the peptide and in contrast to studies indicating that transmembrane helices tilt with respect to the bilayer normal to accommodate differences in hydrophobic thicknesses. Possible sources of the discrepancy will be discussed and explored. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L18.00003: The Role of Osmotically-induced Tension in Binding of N-BAR to Lipid Vesicles Anthony D. Dinsmore, Jaime B. Hutchison, Derek A. Wood, Robert M. Weis We measured the binding affinity of a curvature-sensing protein domain (N-BAR) as a function of applied membrane tension while the membrane curvature was held nearly constant. We focus on the N-BAR domain of Drosophila amphiphysin, which participates in a range of key cell functions including synaptic vesicle endocytosis. We monitored N-BAR binding on unilamellar vesicles composed of 90 mol{\%} DOPC and 10 mol{\%} PIP. Controlled tension was applied by osmotic stress. We found that the bound fraction of N-BAR was enhanced by a factor 6.5 when the tension increased from zero to 2.6 mN/m. This tension-induced response can be explained by the hydrophobic insertion mechanism with a hydrophobic domain area that is consistent with known structure. These results suggest that membrane strain might explain the previously reported curvature affinity of N-BAR. This work was supported by the National Science Foundation through grant DMR-0907195. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L18.00004: The Molecular Structure of the Liquid Ordered Phase Edward Lyman Molecular dynamics simulations reveal substructures within the liquid-ordered phase of lipid bilayers. These substructures, identified in a 10 $\mu $sec all-atom trajectory of liquid-ordered/liquid-disordered coexistence (L$_{\mathrm{o}}$/L$_{\mathrm{d}})$, are composed of saturated hydrocarbon chains packed with local hexagonal order, and separated by interstitial regions enriched in cholesterol and unsaturated chains. Lipid hydrocarbon chain order parameters calculated from the L$_{\mathrm{o}}$ phase are in excellent agreement with $^{\mathrm{2}}$H NMR measurements; the local hexagonal packing is also consistent with $^{\mathrm{1}}$H-MAS NMR spectra of the L$_{\mathrm{o}}$ phase, NMR diffusion experiments, and small angle X-ray- and neutron scattering. The balance of cholesterol-rich to local hexagonal order is proposed to control the partitioning of membrane components into the L$_{\mathrm{o}}$ regions. The latter have been frequently associated with formation of so-called rafts, platforms in the plasma membranes of cells that facilitate interaction between components of signaling pathways. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L18.00005: Why Hydrophilic Water can Permeate Hydrophobic Interior of Lipid Membranes Baofu Qiao, Monica Olvera de la Cruz Water molecules as well as some small molecules have long been found to be able to diffuse across lipid membranes. Such permeation is of significant biological and biotechnological importance. For instance, the permeation of water across lipid membrane plays a important role in regulating ionic concentrations inside of cells. Such water permeation without the assistance of proteins embedded in membranes has been found to be a energetically unfavorable process. We, for the first time, explicitly depict the driving force for such an energetically unfavorable process. Atomistic molecular dynamics simulations are employed to investigate water diffusion in both liquid-crystalline and ordered gel phases of membranes containing zwitterionic DPPC or anionic DLPS lipid. The membrane conformation is calculated to have a critical role in water permeation, regardless of the type of lipid. The fluctuations in the potential energy are found to have a significant, if not the exclusive, role in the transportation of water across lipid membranes. Our results are also informative for the diffusion of small molecules of CO$_2$, O$_2$ and drug molecules, the absence of diffusion of ions, and the diffusion of water into the hydrophobic pores of carbon nanotubes. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L18.00006: Dynamic Morphologies of Microscale Droplet Interface Bilayers Charles Collier, Jonathan Boreyko, Prachya Mruetusatorn, Stephen Sarles, Douglas Hayes Droplet interface bilayers (DIBs) are a powerful platform for studying the dynamics of synthetic cellular membranes; however, very little has been done to exploit the unique dynamical features of DIBs. Here, we generate microscale droplet interface bilayers ($\mu $DIBs) by bringing together femtoliter-volume water droplets in a microfluidic oil channel. By varying the initial conditions of the system, we identify three distinct classes of dynamic morphology. \textit{(1) Buckling and Fission}: \quad When forming $\mu $DIBs using lipids initially in the oil, lipids in the shrinking monolayers continually pair together and slide into the bilayer to conserve their mass. As the bilayer continues to grow, it becomes confined, buckles, and eventually fissions one or more vesicles. \textit{(2) Uniform Shrinking}: When using lipids initially in the aqueous phase to form $\mu $DIBs, lipids uniformly transfer from the monolayers and bilayer into vesicles contained inside the water droplets. \textit{(3) Stretching and Unzipping}: Finally, when the droplets are pinned to the wall(s) of the microfluidic channel, the droplets become stretched during evaporation, culminating in the unzipping of the bilayer and droplet separation. These findings offer a better understanding of the dynamics of coupled lipid interfaces. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L18.00007: Tube formation in fluid membranes Tao Zhang, Rastko Sknepnek, Jennifer Schwarz, Mark Bowick Consider a point force pulling on a fluid membrane. As the magnitude of the force increases, there is a first-order shape transition from nontubular to tubular with a force barrier in between. Motivated by tube formation in endocytosis in yeast, we generalize this problem by including additional force components and steric interactions. Both new ingredients are a consequence of the underlying actin cytoskeletal network, which exerts active forces on the cell membrane to deform it into a tube. We study this generalized problem using variational and Monte Carlo methods in order to quantify endocytosis in yeast. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L18.00008: Effect of micellar topology on shear rheology Simon Rogers, Michelle Calabrese, Norman Wagner Micellar topology affects the nonlinear shear rheology of self-assembled surfactant solutions. Segmental alignment of wormlike micelles (WLMs) exhibiting varying degrees of branching was investigated under shear in the flow-gradient and flow-vorticity planes using new small angle neutron scattering (SANS) sample environments. The degree of branching in mixed cationic/anionic surfactant (CTAT/SDBS) WLMs is controlled via the addition of the hydrotropic salt sodium tosylate. Shear-induced segmental alignment of the micelles is characterized by spatially-resolved flow-small angle neutron scattering (flow-SANS). Our ability to resolve structural projections in both planes provides insight to branch behavior and kinematics under shear flows. Local segmental orientation and alignment in the flow-gradient plane is a non-monotonic function of branching level and radial position. Alignment in the flow-gradient plane is significantly higher than that observed in the flow-vorticity plane, suggesting that branches may simultaneously migrate into the vorticity direction and inhibit spatially localized flows. Shear and normal stresses calculated from micellar alignment using the stress-SANS law are favorably compared with rheological measurements under identical conditions. The results provide evidence for the effects of micellar topology on the nonlinear shear rheology of WLM solutions. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L18.00009: Lipid Biomembrane in Ionic Liquids Brian Yoo, Benxin Jing, Jindal Shah, Ed Maginn, Y. Elaine Zhu Ionic liquids (ILs) have been recently explored as new ``green'' chemicals in several chemical and biomedical processes. In our pursuit of understanding their toxicities towards aquatic and terrestrial organisms, we have examined the IL interaction with lipid bilayers as model cell membranes. Experimentally by fluorescence microscopy, we have directly observed the disruption of lipid bilayer by added ILs. Depending on the concentration, alkyl chain length, and anion hydrophobicity of ILs, the interaction of ILs with lipid bilayers leads to the formation of micelles, fibrils, and multi-lamellar vesicles for IL-lipid complexes. By MD computer simulations, we have confirmed the insertion of ILs into lipid bilayers to modify the spatial organization of lipids in the membrane. The combined experimental and simulation results correlate well with the bioassay results of IL-induced suppression in bacteria growth, thereby suggesting a possible mechanism behind the IL toxicity. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L18.00010: Ca$^{2+}$ induced changes in PIP$_{2}$ containing membranes at physiological concentrations Martin Forstner, Adolphe Badiambile, Ian McCabe Phosphoinositides (PIPs) play a crucial role in many cellular processes such as calcium release, exocytosis or endocytosis. In order to specifically regulate these functions PIPs must segregate in pools at the plasma membrane. A possible mechanism that could induce and regulate such organization of phosphoinositides is their interaction with bivalent cations. Using Langmuir monolayers, we investigated the effect of calcium and magnesium on the surface pressure-area/lipid isotherm of monolayer of phosphatidylinositol, phosphatidylinositol bisphosphate, dioleoylphosphatidylglycerol and palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine. The observed decrease of area per lipid, i.e. the increase in aggregation, is mostly dependent on the lipid's head group charge but ion specific. In addition, we discuss changes in free energy and compressibility of these monolayer-ion systems. Furthermore, a series of experiments were conducted on supported lipid bilayers containing physiological quantities of PIP$_{2}$. Fluorescence correlation spectroscopy was used to study the response of the PIP$_{2}$ to changes [Ca$^{2+}$ ]. As Ca$^{2+}$ concentration increases, the FCS indicates that PIP$_{2}$ goes from a freely diffusing single species to a multiple species system. The diffusion rates of the additional species decrease with increasing [Ca$^{2+}$], thus indicating increasing aggregate sizes with increasing, but physiological relevant Ca$^{2+}$ concentrations. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L18.00011: Neutron Scattering Function for Branched Worm-Like Micelles Gregory Beaucage, Karsten Vogtt Micellar solutions can display a wide range of phase structure as a function of counter ion content, surfactant concentration, and the presence of ternary components. Under some conditions extended cylindrical structures are produced that display chain persistence and a scaling regime reminiscent of polymers coils. These worm-like micelles (WLMs) can form branched, chain structures, for instance at relatively high salt concentrations. The rheology of these branched WLMs is strongly dependent on migration of the branch points, and the dynamics of branch formation and removal. A scattering model that can quantify the branching density, branch length, branch functionality and the hyperbranch (branch-on-branch) content of these polymer-like structures will be presented. Data from several WLM systems will be explore using this new model. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L18.00012: Fragmentation and wetting of spherical micelles in confined flow Mona Habibi, Colin Denniston, Mikko Karttunen We use coarse-grained molecular-dynamics (MD) simulations to study the structural and dynamical properties of surfactant micelles under Poiseuille-like flow in a nano-confined geometry. The effect of flow, confinement, and wetting on spherical micelles of sodium dodecyl sulfate (SDS) is explored when the micelle is forced through a channel slightly smaller than its equilibrium size. Inside the channel, the micelle may fragment into smaller micelles. We demonstrate that in addition to the flow rate, the wettability of the channel surface dictates whether the micelle fragments and determines the size of daughter micelles. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L18.00013: Structure and flow properties of micelle-nanoparticle solutions from Molecular Dynamics simulations Radhakrishna Sureshkumar, Subas Dhakal, Abhinanden Sambasivam In aqueous media, cationic surfactant molecules spontaneously self-assemble into diverse morphologies depending upon temperature, surfactant concentration and solution ionic strength. Spherical, cylindrical and long ($\sim $ microns) flexible wormlike structures with or without branches with distinct rheological properties are observed. Inclusion of nanoparticles (NPs) provides additional means to manipulate structure and create active ``nano-fluids'' that respond to optical, magnetic or electrical stimuli. We study self-assembly, dynamics and rheology of such fluids using coarse-grained Molecular Dynamics simulations in presence of explicit solvent and salt. Specifically, we will discuss the mechanisms underlying fascinating phenomenology observed experimentally such as the pronounced non-monotonic dependence of the zero shear viscosity on salt/NP concentration, shear-induced structure formation, and isotropic to nematic transitions. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L18.00014: Nonperturbative Renormalization Group Approach to Polymerized Membranes Karim Essafi, Jean-Philippe Kownacki, Dominique Mouhanna Membranes or membrane-like materials play an important role in many fields ranging from biology to physics. These systems form a very rich domain in statistical physics. The interplay between geometry and thermal fluctuations lead to exciting phases such flat, tubular and disordered flat phases. Roughly speaking, membranes can be divided into two group: fluid membranes in which the molecules are free to diffuse and thus no shear modulus. On the other hand, in polymerized membranes the connectivity is fixed which leads to elastic forces. This difference between fluid and polymerized membranes leads to a difference in their critical behaviour. For instance, fluid membranes are always crumpled, whereas polymerized membranes exhibit a phase transition between a crumpled phase and a flat phase. In this talk, I will focus only on polymerized phantom, {\it i.e.} non-self-avoiding, membranes. The critical behaviour of both isotropic and anisotropic polymerized membranes are studied using a nonperturbative renormalization group approach (NPRG). This allows for the investigation of the phase transitions and the low temperature flat phase in any internal dimension $D$ and embedding $d$. Interestingly, graphene behaves just as a polymerized membrane in its flat phase. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L18.00015: Soap film as a 2D system: Diffusion and flow fields Skanda Vivek, Eric Weeks \\ We use microrheology to measure the 2D (interfacial) viscosity of soap films. Microrheology uses the diffusivity of tracer particles suspended in the soap film to infer viscosity. Our tracer particles are colloids of diameters d = 0.10 and 0.18 microns. We measure the interfacial viscosity of soap films ranging in thickness from 0.1 to 3 microns. The thickness of these films is measured using the infrared absorbance of the water based soap films. From film thickness, viscosity of the fluid used to make the film and particle diffusivity, we can infer the interfacial viscosity due to the surfactant layers at the film/air interfaces. We find positive constant interfacial viscosities for thin films (h/d $<$ 5), within error. For thicker films, we find negative viscosities, indicating 3D effects begin to play a role, as air stresses become less important. The transition from 2D to 3D properties as a function of h/d is sharp at about h/d=6. Additionally, we measure larger length scale flow fields from correlated particle motions and find good agreement with what is expected from the theory of 2D fluids for all our films. In conclusion, single particle diffusion shows a sharp transition away from 2D like behavior as h/d increases, but the long-range flow fields still act as 2D. [Preview Abstract] |
Session L20: Focus Session: Dynamics of Glassy Polymers under Nanoscale Confinement I
Sponsoring Units: DPOLYChair: Connie Roth, Emory University
Room: 405
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L20.00001: Elastic modulus and surface tension of a polyurethane rubber in nanometer thick films Meiyu Zhai, Gregory McKenna Estane is a kind of polyurethane with thermodynamically incompatible hard and soft segments. In this study the macro and micro properties of Estane have been characterized and compared. The viscoelastic properties of this material in bulk scale have been determined using dynamic rheometry. Time-temperature superposition was found to be applicable for this material, and a master curve was successfully constructed from the dynamic shear responses of G'($\omega )$ and G''($\omega )$. Also a novel nano bubble inflation method was used to obtain the creep compliance of the Estane ultrathin films and the results show stiffening in the rubbery region for the Estane over thicknesses ranging from 110nm to 22nm. The dependence of the rubbery stiffening on film thickness is studied and the relative influences of nano confinement and surface tension effect are analyzed using both a direct stress strain analysis and an energy balance method for the membrane. The contributions of surface tension and nano confinement are considered separately. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L20.00002: Effect of Microstructural Order on Plasticity in Thin PS-P2VP Films Bekele Gurmessa, Andrew B. Croll We report the results of an experimental investigation of the onset of plastic deformation in polystyrene-b-poly (2-vinylpyridine) (PS-P2VP) thin films. PS-P2VP is a glass-forming diblock copolymer which serves as a model material for the study of the effects of microstructure on mechanical response due to the similarities of the mechanical properties (glass transition temperature, entanglement molecular weight, and bulk elastic modulus) of each block. In particular, we measure the onset of plasticity using an elastic instability technique to locally bend and locally impart a tensile stress in a thin film that is subsequently examined for damage. Similar to our earlier results from experiments conducted on homopolymer polystyrene, we show that failure in PS-P2VP is initiated at extremely low strain and that the failure strain is influenced by thin film confinement effects. For the first time, we show that the onset of plasticity increases as a sample is annealed from a disorganized, as-cast state to that of a film with a well ordered internal microstructure. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L20.00003: Theory of Activated Relaxation in Nanoscale Confined Liquids Stephen Mirigian, Kenneth Schweizer We extend the recently developed Elastically Cooperative Nonlinear Langevin Equation(ECNLE) theory of activated relaxation in supercooled liquids to treat the case of geometrically confined liquids. Generically, confinement of supercooled liquids leads to a speeding up of the dynamics(with a consequent depression of the glass transition temperature) extending on the order of tens of molecular diameters away from a free surface. At present, this behavior is not theoretically well understood. Our theory interprets the speed up in dynamics in terms of two coupled effects. First, a direct surface effect, extending two to three molecular diameters from a free surface, and related to a local rearrangement of molecules with a single cage. The second is a longer ranged ``confinement'' effect, extending tens of molecular diameters from a free surface and related to the long range elastic penalty necessary for a local rearrangement. The theory allows for the calculation of relaxation time and T$_{g}$ profiles within a given geometry and first principles calculations of relevant length scales. Comparison to both dynamic and pseudo-thermodynamic measurements shows reasonable agreement to experiment with no adjustable parameters. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L20.00004: Dynamics and mechanical properties of glassy polymers under cylindrical confinement Amit Shavit, Robert Riggleman Even after two decades of active research surrounding glassy polymers under confinement, we still lack a complete understanding of the changes in mechanical properties as a bulk material is confined. Understanding the properties of glassy polymers in confinement is relevant not only for our fundamental understanding, but also for applications in semiconductor manufacturing and fabrication of novel materials. Here, we used molecular dynamics simulations to investigate dynamical and mechanical properties of glass-forming polymers in bulk and pillar geometries. We examine how the free surface influences the dynamics locally in the film, and we show that the dynamics in the surface are several orders of magnitude faster than in the bulk, which is similar to the enhancement we see in thin-films. Finally, we show that the mechanical properties of the bulk differ significantly from the pillar, and that the path to the glassy state has significant consequences for the overall material properties. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L20.00005: Role of free surface and interface effects on viscoelastic properties of ultrathin polystyrene films Heedong Yoon, Gregory McKenna The surface properties of 20 nm polystyrene (PS) films with different under layer substrates were investigated by employing a silica particle embedment method. The under layer substrates used were PS, poly(2-vinyl pyridine) (P2VP), and poly(methyl methacrylate) (PMMA) with thicknesses ranging from 17 nm to 350 nm. The apparent particle height change was monitored through the use of Atomic Force Microscopy at different experimental temperatures ranging from T$_{\mathrm{g}}$-10 to T$_{\mathrm{g}}+$10 $^{\mathrm{o}}$C. The Hutcheson and McKenna model [Phys. Rev. Lett. \textbf{94}, 076103 (2005)] was applied to the particle embedment depth to obtain the surface rheological temperatures. The results showed that the 20 nm top layer of PS films soften below T$_{\mathrm{g}}$ and stiffen above T$_{\mathrm{g}}$ for the different types of under layer substrates. The rheological temperatures of the 20 nm surface layer PS films were independent of under layer PS thickness in the range from 17 nm to 350 nm. Furthermore, the rheological temperature of the top layer of PS film also showed that different types of under layer substrates such as PS, P2VP, and PMMA can slightly alter the T$_{\mathrm{g}}$ of the top layer of the PS film, but the change was less than 10 K. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L20.00006: Molecular weight dependence of surface flow near the bulk glass transition temperature Yu Chai, Thomas Salez, Michael Benzaquen, Elie Raphael, James A. Forrest We present the study on molecular weight dependent sub-T$_{\mathrm{g}}$ surface dynamics of polymer thin films by using the Nano-step experiment [McGraw et al. Soft Matter 7, 7832 (2011)]. By varying the molecular weight, we are able to probe the surface dynamics of the free surface below T$_{\mathrm{g}}$ with the polymer size comparable to the surface depth. In particular, we define and use a correlation function to compare measured and calculated profiles to analyze the transition from the bulk flow to flow restricted to the surface region. Surprisingly, even for the polymers with M$_{\mathrm{w}}=$22,000 surface flow is still observed below the bulk T$_{\mathrm{g}}$ value. A numerical simulation of random walk is used to find the fraction of polymer of which all of the polymer segments are located in the free surface region. The simulation results indicate that there are still a significant fraction of polymer molecules where all segments are in the near free surface region. These molecules can undergo flow consistent with the experimental results. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L20.00007: Viscosity of Polymer Nanometer Films Invited Speaker: Ophelia Tsui Ample studies have shown that the $T_g$ of polymer films can change visibly when the film thickness ($h$) is decreased below 100 nm. It implies that the viscosity ($\eta$) of the films can differ notably from the bulk. In this talk, I shall discuss the viscosity measurement performed on two systems - polystyrene and poly(methal methacrylate) on silicon oxide (PS/SiO$_x$ and PMMA/SiO$_x$) where the $T_g$ decreases and increases with decreasing $h$, respectively. At low molecular weight ($M_w$), $\eta$ of both systems can be described by a layer model. For PS/SiO$_x$, a two-layer model assuming the films to comprise a mobile layer on top of a bulklike layer is sufficient. For PMMA/SiO$_x$, a three-layer model including an additional slow substrate layer is needed. For PS/SiO$_x$, the layer model is also able to describe the data at high $M_w$. However, the $M_w$-dependence displays a qualitative change when the polymer size exceeds the estimated thickness of the surface mobile layer. Beyond that, the surface chains are partially embedded in the bulklike layer so cannot facilitate enhanced transport as in the low-$M_w$ films. We hypothesize that the surface mobile layer operates in the low-$M_w$ films, but a confinement effect in the high-$M_w$ films in lowering $\eta$. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L20.00008: Surface Dynamics and Structures of Swollen Polymer Brushes Mark Foster, Liang Sun, Bulent Akgun, Suresh Narayanan, Jim Browning The surface height fluctuations of a film of untethered linear polystyrene (PS) chains can be well described by a continuum hydrodynamic theory of overdamped capillary waves. When the chains in a film are tethered to the substrate to form a ``dry'', densely grafted PS brush, the surface fluctuations are dramatically suppressed so that they are no longer observable in the time and scattering vector windows available for X-ray Photon Correlation Spectroscopy (XPCS). Here, surface fluctuations of PS brushes highly swollen in toluene vapor are investigated using XPCS and the structures of these swollen polymer brushes are investigated using Neutron Reflectivity (NR). Surface fluctuations of densely grafted PS brushes are still strongly hindered and not observable even if the brush is substantially swollen in a good solvent vapor. When there is a condensed liquid toluene layer on top of the brush, the surface fluctuations are observed with a relaxation time orders of magnitude larger than that of a thick film of pure toluene. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L20.00009: Solvent-polymer thin films drying below Tg Didier Long, Gregoire Julien, Elian Masnada We propose a model for describing the dynamics of polymer-solvents systems close to the glass transition. We extend to these systems the facilitation mechanism due to free volume diffusion proposed by Merabia and Long (EPJ E 2002; J. Chem. Phys. 2006, Chen et al 2009) to the case of polymer-solvent systems. Our model is solved on a 2D lattice with a spatial resolution corresponding to the scale of dynamical heterogeneities. It allows to describe how a solvent penetrates a glassy film (case II diffusion, Kramer et al 1988). Regarding the process of film drying, we show that films up to a few hundred of nm thick can be almost completely dried in an accessible experimental time scale, even at temperatures 100 K below the glass transition temperature of the pure polymer as shown experimentally by Catherine Allain and co-workers (2002, 2003). The thinner the films, the shorter the drying time, allowing to obtain far from equilibrium polymer films a few tens of nanometer thick in a state very different from bulk glassy polymers. For thicker films, a glassy crust a few hundred of nanometers thick appear and the subsequent evaporation of solvent slows down progressively over very long time scales. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L20.00010: Dynamics of bound polymer layers in CO2 Naisheng Jiang, Levent Sendogdular, Mani Sen, Maya K. Endoh, Tadanori Koga, Bulent Akgun, Michael Dimitriou, Sushil Satija Recently, there has been growing interest in bound polymer layers (BPLs) on planar solids due to their strong influence on the physical and mechanical properties of confined polymeric materials. It is known that BPLs are immobile (in air) even at temperature far above the bulk glass transition temperature. Here, we used CO$_{2}$ as a plasticizer for polystyrene (PS) bound layers ($\sim$10 nm in thickness) formed on planar silicon (Si) substrates. By using high pressure neutron reflectivity, we studied the swelling behavior of the BPL and the interdiffusion process for bilayers of the bottom BPL and an deuterated PS overlayer (about 50 nm-thick) in CO$_{2}$ at T = 36$^{\circ}$C. The results have clearly shown that CO$_{2}$ swells the BPL and induces the interdiffusion of polymer chains across the BPL/overlayer interface. We further studied the interdiffusion processes as a function of molecular weights of the overlayer, allowing us to highlight the unique interdiffusion process relative to the bulk. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L20.00011: Confinement for Thin Film on Substrates with Different Geometric Curvatures Jie Xu, Jiao Chen, Gi Xue Molecular chain conformation in thin polymer film on substrates with different geometric curvature was examined using fluorescence non-radiative energy transfer (NRET) spectroscopy. We find that thin film on concave substrate exihibits significant differences in vitrification behavior, in both magnitude and thickness dependence, from the planar film. NRET measured a more compact morphology, while dynamical scanning calorimetry detected an increased glass transition temperature (Tg) for the concave thin film, with respect to bulk film. In contrast to planar film where properties are thickness dependent, polymer concave film shows that its conformation and Tg are solely dependent on curvature radius. Surprisingly, these properties converted back to the bulk values when the substrate was removed, indicating the crucial importance of interaction imposed by the concave hard wall. These spectroscopic data matched perfectly the calorimetric results and provided a new implication to understanding geometric confinement on dynamics. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L20.00012: The effect of nanoconfinement on network topology and thermo-mechanical properties of glassy polymers dynamically reacted using MD simulation Katherine Sebeck, John Kieffer The interaction of polymers with an ordered surface affects the thermo-mechanical properties of polymer matrix composite and nanoconfined polymeric materials. Atomistic simulations of glassy polymers such as epoxides offer unique insights to the network topology of such structures. A series of epoxide network structures using a bifunctional epoxide and polyfunctional aliphatic amines have been generated using a dynamic polymerization simulation technique. This allows for natural evolution of the network topology in both bulk and confined structures. The nature of structural organization at the surface of confined structures will be analyzed as a function of distance from the interface using various metrics including spatial distribution functions. These structural characteristics are then correlated with dynamical properties such as Tg and modulus, comparing nanoconfined networks to bulk systems. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L20.00013: A Simple Model of Dynamic Heterogeneity: Connection with Experimental Results Jane Lipson, Nicholas Tito, Scott Milner We have developed the Limited Mobility (LM) model to study dynamic heterogeneity in a system that exhibits kinetic arrest, i.e. a glass transition. In recent work we have investigated the approach to the bulk transition from above and below, as well as the effects of perturbations on the transition. Results in the latter area have included looking at buried slabs of different mobility than the surround, as well as studies on a supported film. In this talk we will focus on characterizing sample mobility in the bulk, via measurement of the diffusion constant of mobile material, D, as well as in a film, via characterization of mobility fronts. We find that as the bulk glass transition is approached the LM model exhibits the same kind of deviation from Stokes-Einstein behaviour as is observed in experiment and other model studies.~ In the film the LM model shows a time-dependent growth of the mobility front that scales with the same D that characterizes mobility in analogous bulk samples; this has also been seen experimentally in glass-forming liquids. These results will be discussed, in addition to others that help connect the LM model with data on real systems. [Preview Abstract] |
Session L21: Focus Session: Polymers for Energy Storage and Conversion II - Photovoltaics
Sponsoring Units: DPOLY GERAChair: Brian Collins, National Institute of Standards and Technology
Room: 406
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L21.00001: Expanded Morphological Paradigm of Polymeric Solar Cells: Contributions by Soft X-ray Methods Invited Speaker: Harald Ade The complex three dimensional morphology of polymeric/organic donor:fullerene bulk heterojunction solar cells and the structure of the discrete and recently inferred dispersed interfaces are critical to performance, yet have been very difficult to study due to a paucity of adequate characterization methods. Recently developed soft X-ray microscopy and scattering tools and methods can provide new avenues and contribute substantially to infer the number of phases present in a device, determine the minimum fullerene content in mixed domains and to provide a quantitative statistical measurement of the composition variations and size distribution. This contributed to the realization that mixed domains are prevalent and rather than just being detrimental can have important beneficial contributions for charge generation and charge transport as such mixed domains represent a special form of hierarchical structures (E.g. [1-3]). Furthermore, polarized x-ray scattering can reveal preferential orientation of the donor polymer/small molecule (edge-on or face-on) relative to the fullerene aggregate interface. Such ordering has previously not been observed nor controlled in fullerene-based solar cells and is shown here to be a critical factor for high performance in a number of systems.\\[4pt] [1] Collins, B. A. \textit{et al.} \textit{Nat. Mater.} \textbf{11}, 1--8 (2012).\\[0pt] [2] Collins, B. A.\textit{ et al.} \textit{Adv. Energy Mater.} \textbf{3}, 65-74 (2013).\\[0pt] [3] Ma, W.\textit{ et al.} \textit{Adv. Energy Mater.}, \textbf{3}, 864 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L21.00002: Understanding the Role of Additives in Improving the Performance of Polymer:Fullerene Bulk Heterojunction Solar Cells Wei Chen Solar cells based on the polymer:fullerene bulk heterojunction (BHJ) represent one of the most promising technologies for next-generation solar energy conversion due to their low-cost and scalability. In the last fifteen years, research efforts have led to organic photovoltaic (OPV) devices with power conversion efficiencies (PCEs) $\sim$ 12{\%}, but these values are still insufficient for the devices to become widely marketable. To further improve solar cell performance, a thorough understanding of the complex processing-structure-performance relationships in OPV devices is required. Recently, the use of processing additives have been proved to be one of the most effective methods to tune the nanomorphology of polymer:fullerene active layer, as the incorporation of a small percentage of solvent additives results in a nearly doubling of device efficiency. However, the physics behind these improved performances by processing additives still remains unclear. In this work, by taking advantage of resonant soft x-ray scattering (RSoXS) and energy-filtered transmission electron microscopy (EFTEM), we have determined that the solvent additives induce the change in the formation mechanism of polymer:fullerene nanomorphologies in the process of film casting. Progress established in the course of these studies on structural and morphological characterizations will serve as the foundation for further improving the efficiency of polymer solar cells to realize their large-scale commercial use. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L21.00003: High molecular weight insulating polymers can improve the performance of molecular solar cells Ye Huang, Wen Wen, Edward Kramer, Guillermo Bazan Solution-processed molecular semiconductors for the fabrication of solar cells have emerged as a competitive alternative to their conjugated polymer counterparts, primarily because such materials systems exhibit no batch-to-batch variability, can be purified to a greater extent and offer precisely defined chemical structures. Highest power conversion efficiencies (PCEs) have been achieved through a combination of molecular design and the application of processing methods that optimize the bulk heterojunction (BHJ) morphology. However, one finds that the methods used for controlling structural order, for example the use of high boiling point solvent additives, have been inspired by examination of the conjugated polymer literature. It stands to reason that a different class of morphology modifiers should be sought that address challenges unique to molecular films, including difficulties in obtaining thicker films and avoiding the dewetting of active photovoltaic layers. Here we show that the addition of small quantities of high molecular weight polystyrene (PS) is a very simple to use and economically viable additive that improves PCE. Remarkably, the PS spontaneously accumulates away from the electrodes as separate domains that do not interfere with charge extraction and collection or with the arrangement of the donor and acceptor domains in the BHJ blend. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L21.00004: Suppressing intermolecular charge recombination in photovoltaics through conjugated block copolymer architectures Hao Kuang, Enrique Gomez, Michael Janik Block copolymers have the potential to control the interfacial and mesoscopic morphology of the active layer of organic photovoltaics and consequently enhance device performance. For example, the self-assembly of conjugated block copolymers into periodic microstructures with nanometer length scales could facilitate exciton dissociation by creating large amounts of donor-acceptor interfaces. Furthermore, the interfacial structure may strongly affect charge transfer processes. Using Density Functional Theory, we have examined charge transfer rates in model interfaces of poly(3-hexylthiophene)$-$block$-$poly-((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(thiophen-5-yl)-2,1,3-benzothiadiazole]-2$\prime $,2?-diyl) donor-acceptor block copolymers which yield 3{\%} efficient devices when incorporated into solar cells. Our results demonstrate that intermolecular charge recombination can depend on the interfacial breadth, where sharp interfaces (ca. 1 nm) suppress intermolecular charge recombination by orders of magnitude. Furthermore, we compare intramolecular and intermolecular charge transfer rates in donor-acceptor block copolymers through Constrained Density Functional Theory calculations. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L21.00005: Solvent Annealing in Selective Solvents: A Novel Method to Tune the Morphology of Low Band Gap Polymer:Bis-Fullerene Heterojunctions Mark Dadmun, Huipeng Chen, Yu-Che Hsiao, Bin Hu One of the most important challenges facing our society is the development of technologies for renewable energy conversion. Polymeric bulk-heterojunction (BHJ) photovoltaics, based on conjugated polymers and fullerenes, are an economically viable option for low cost renewable power generation. The most promising conjugated polymer:fullerene active layers in organic photovoltaics now utilize low band-gap (LBG) copolymers. Unfortunately, for most of these LBG devices, the as-cast film is not usually optimal, and there are few further treatment available after film deposition to optimize the morphology. To address this problem, we have exploited the selective solubility of the LBG:fullerene nanocomposite components to direct the assembly of these mixtures by annealing in the vapor of a selective solvent. Our recent work demonstrates that annealing in a solvent that is selective to the fullerene forms a sample with fullerene aggregation, while annealing in a solvent vapor that is selective to the polymer forms a thin film with polymer precipitation. There is also a direct correlation between the resultant morphology and OPV performance, increasing PCE by 190{\%}. These results indicate that solvent annealing and solvent choice provides a unique tool to precisely tune the morphology of CP:Fullerene BHJ systems, optimizing the morphology and performance of the active layer. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L21.00006: Using Molecular Simulations to Link Chemical and Physical Features of Conjugated Polymers and Fullerene Derivatives to Bulk Heterojunction Morphology for Organic Photovoltaics Hilary Marsh, Eric Jankowski, Arthi Jayaraman The morphology of blends of conjugated polymers (electron donors) and fullerene derivatives (electron acceptors) strongly affects the charge transport, charge separation and the overall efficiency of organic photovoltaic devices. In this talk we will present coarse-grained molecular simulation studies to understand how molecular-level features such as alkyl side chain length, alkyl side chain spacing along thiophene polymer backbone and fullerene functionalization (and in turn miscibility with the conjugated polymer) affect the blend morphology. Our coarse-grained models are validated by reproducing neat polymer (without acceptors) morphologies observed in experiments, such as lamellae and hexagonally packed cylinders. Furthermore, for blends of conjugated polymers and fullerene derivatives, this work shows how conjugated polymer architecture and acceptor miscibility can be tuned to obtain new blend morphologies with features that are known to enhance efficiency of organic solar cells. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L21.00007: Phase Behavior of Polymer Blends for Organic Photovoltaic Applications Jillian Emerson, Eric Furst, Thomas Epps, III Polymer blends offer a promising and economically-viable route to creating organic photovoltaic (OPV) devices, as blends can form bicontinuous domains via spinodal decomposition. Understanding the phase behavior of conjugated polymer blends commonly used in OPVs is vital to producing more efficient devices. In this work, we determined the Flory-Huggins solvent--polymer and polymer--polymer interaction parameters for a model system of poly(3-hexylthiophene) (P3HT) and polystyrene (PS) through solvent vapor swelling of thin polymer films. From these interaction parameters, we constructed a polymer/polymer/solvent phase diagram. The phase diagram was validated experimentally with solution-based transmission measurements of PS/P3HT. This work highlights a method to determine the phase behavior in polymer/polymer/solvent blends that can be extended to other combinations of macromolecules relevant to organic photovoltaics, composites, and other materials systems. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L21.00008: Solvent-Polarity-Induced Active Layer Morphology Control in Crystalline Diketopyrrolopyrrole-Based Low Band Gap Polymer Photovoltaics Sunzida Ferdous, Feng Liu, Dong Wang, Thomas Russell The effects of various processing solvents on the morphology of diketopyrrolopyrrole (DPP)-based low band gap polymer (PDPPBT) and phenyl-C71-butyric acid methyl ester (PC71BM) blends are studied. The quality of the processing solvents was varied systematically using a mixture of a non-aromatic polar primary solvent with high boiling point secondary solvents of increasing polarities. An unfavorable solvent-PC71BM interaction affects the growth process of polymer crystallites inside the blend. When non-aromatic polar solvent was used, large PC71BM aggregates were formed that increase in size with the addition of non-polar secondary solvents. When polar solvents were instead used as the secondary solvents, the size scales of the aggregates decrease markedly, creating a percolated fibrillar network. Power conversion efficiencies of 0.03{\%} to 5{\%} are obtained, depending on the solvent system used. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L21.00009: Synthesis, Morphology, and Optoelectronic Properties of All-Conjugated Block Copolymers Kendall Smith, Rafael Verduzco, Yen-Hao Lin, Jorge Mok Recent work has demonstrated the potential of all-conjugated block copolymers for solution-processed photovoltaic devices, with power conversion efficiencies near 3{\%}. However, optoelectronic properties and structure-property-processing relationships are poorly understood for this class of materials. Here, we present systematic studies on the processing, morphology, and optoelectronic properties of model all-conjugated block copolymer systems. All-conjugated block copolymer poly(3-dodecylthiophene)-\textit{block}-poly(9,9-dioctylfluorene) (P3DDT-$b$-PF) exhibit simultaneous crystallization of both blocks but no clear evidence of microphase segregation. By contrast, under solvent annealing, poly(3-hexylthiophene) --$b$--poly(9,9-dioctylfluorene) (P3HT-$b$-PF) exhibit lamellar ordering, evidenced by multiple reflections under GIWAXS and GISAXS analysis, including an in-plane reflection indicative of strong $\pi $-$\pi $ stacking for both P3HT and PF blocks. The characteristic lamellar domain spacing (4.2 nm) is found to be independent of block ratio or total molecular weight. Optoelectronic measurements and photovoltaic device results are presented for all-conjugated block copolymers that incorporate ambipolar PFTBT polymer block and high-performance $p$-type PTB7 polymer. These results provide guidelines for optimizing the morphology of all-conjugated block copolymers through materials design and processing. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L21.00010: Improving the performance of All-Polymer Solar Cells Yan Jin, Fei Yu, Vikram Kuppa We find that the power conversion efficiency (PCE) of photovoltaic devices based on conjugated polymer blends is dramatically improved by the addition of small quantities of pristine -unmodified- graphene in the active layer. The graphene is obtained by solvent exfoliation from graphite, and is spin-coated along with the conjugated polymer blend from solution to make cells. The PCE as well as short circuit current (Jsc) show an approximately three-fold increase with increasing graphene concentration. The incorporation of graphene changes the recombination mechanism in such cells from monomolecular (geminate) to bimolecular (non-geminate) recombination, as revealed from current-light intensity studies. In contrast to neat devices, the addition of graphene leads to an increase in the thickness of the active layer, which also influences performance. These investigations reveal three major effects of graphene on polymer blend solar cells: the incorporation of graphene (i) enhances exciton dissociation, (ii) increases the charge transport, and (iii) modifies the polymer morphology. The results demonstrate the potential for graphene in improving OPV performance by addressing poor charge mobilities, which are a fundamental drawback of OPV cells. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L21.00011: Role of Domain Size and Phase Purity on Charge Carrier Density, Mobility and Recombination in P3HT:PC$_{61}$BM Devices Bingyuan Huang, Jojo Amonoo, Anton Li, Chelsea Chen, Peter Green From an experimental perspective, understanding the interrelationships between the morphological structure, transport properties and device performance remains an important question. We designed and fabricated active material morphologies that possess dissimilar domain sizes/phase purities using different processing strategies: organic solvent casting, supercritical carbon dioxide (scCO$_{2})$ processing and thermal annealing. The short circuit currents of the as-cast samples, J$_{\mathrm{as-cast}}$, were appreciably lower than those in the scCO$_{2}$ processed samples, J$_{\mathrm{scCO2}}$, and the thermally annealed samples, J$_{\mathrm{thermal}}$. While J$_{\mathrm{scCO2}}$ $\sim$ J$_{\mathrm{thermal}}$, the initial carrier densities in the scCO$_{2}$ processed samples, n(0)$_{\mathrm{scCO2}}$, and the carrier recombination coefficients,~$\alpha _{\mathrm{scCO2}}$, were significantly higher than those in the thermally annealed samples (n(0)$_{\mathrm{scCO2}}$ $\sim$ 5n(0)$_{\mathrm{thermal}}$; $\alpha_{\mathrm{scCO2}}$ $\sim$ 2$\alpha_{\mathrm{thermal}})$. It is also shown that while J$_{\mathrm{scCO2}}$ $\sim$ 3J$_{\mathrm{as-cast}}$, the n(0)$_{\mathrm{scCO2}}$ $\sim$ n(0)$_{\mathrm{as-cast}}$, yet $\alpha_{\mathrm{scCO2}}$\textgreater $\alpha_{\mathrm{as-cast}}$. These observations are reconciled on the basis of details of the morphologies of these systems. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L21.00012: Morphology Development During Deposition in OPV Low Band Gap Polymer:Bis-Fullerene Heterojunctions: Effect of a Second Solvent Huipeng Chen, Yu-Che Hsiao, Bin Hu, Mark Dadmun Polymer based bulk-heterojunction solar cells, based on blends of conjugated polymers and fullerenes are one potential option for low cost renewable power generation. One way to improve power conversion efficiency (PCE) of this cell is to increase the open-circuit voltage (V$_{\mathrm{oc}})$. It has been reported that replacing PCBM with bis-adduct fullerenes (i.e. ICBA) significantly improves V$_{\mathrm{oc}}$ and PCE in P3HT device. However, for the most promising low band-gap polymer (LBP) system, replacing PCBM with ICBA gives very poor short-circuit current (J$_{\mathrm{sc}})$ and PCE although V$_{\mathrm{oc\thinspace }}$is significantly improved. As J$_{\mathrm{sc}}$ and PCE strongly depend on the morphology, we therefore tried to optimize the morphology of as-cast LBP/ICBA mixture by adding a second solvent with varying solubility to LBP and ICBA to the deposition solution. The results show that there is no change of LBP ordering by adding the second solvent regardless of its solubility. The morphology of all the as-cast samples is then determined by neutron scattering. A homogenous dispersion of ICBA in LBP is found in the sample where the second solvent is selective to LBP, giving poor PCE. Aggregates of ICBA are formed in those samples where the second solvent is selective to ICBA. The resultant morphology improves PCE by up to 246{\%}. A quantitative analysis of neutron data shows that the interfacial area between ICBA aggregates and LBP/ICBA mixed phase is improved in these samples, which appears to facilitate charge transport and reduce the recombination of free charge carriers. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L21.00013: Modifying growth of perylene diimide nanocrystals with poly(3-hexyl thiophene) as additives Laju Bu, Ryan Hayward The shape, size, and crystallinity of organic semiconductors play vital roles in their applications in optoelectronics. Various methods to control crystallization of organic semiconductors, including thermal/solvent annealing, addition of poor solvents, and chemical structure modification, have been applied to improve the performance of organic photovoltaics. While soluble additives controlled crystallization are commonly found in biomineralization, pharmaceutics, and food science, they have rarely been applied to organic semiconductors. Here, we show that a p-type polymer, P3HT, serves as a soluble additive in crystallization of a n-type semiconductor, perylene diimide (PDI), by preferentially adsorbing on lateral crystal faces, which reduce lateral growth of PDI crystals relative to longitudinal growth, yielding extended 1-D nanofibers. Upon subsequent crystallization of P3HT, the PDI nanofibers serve as efficient nucleation sties, resulting in shish-kebab like p/n heterostuctures. Using ultrasound to enhance nucleation of PDI crystals, variations in P3HT molecular weight and concentration, and sonication temperature, allow PDI nanocrystal size and uniformity to be tuned. The uniform PDI nanocrystals can act as seeds to crystallize additional PDI to get segmented nanocrystals. [Preview Abstract] |
Session L22: Focus Session: Directed Assembly of Hybrid Materials I - Crystallization and Multicomponent Systems
Sponsoring Units: DPOLYChair: Christopher Li, Drexel University
Room: 407
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L22.00001: Manipulating the morphologies and lamellar orientations of substrate-supported polyester films using end-grafted poly(methacrylate) brushes Ya-Ting Hsieh, Eamor M. Woo, Atsushi Takahara, Yuji Higaki Crystallization of polymeric materials on solid substrates has technological and scientific importance in applications such as coatings, electronic devices and solar cells. Crystalline morphologies and orientations of polymer near the polymer/substrate interface can be greatly altered by tuning the specific interactions between polymer and substrate. In this talk, we will show the effect of end-grafted poly(methacrylate) brushes in controlling the spherulitic morphologies and lamellar assembly patterns of thin polyesters films on glass substrate. Poly(methyl methacrylate) (PMMA) and poly(benzyl methacrylate) (PBzMA) brushes were grafted on glass surface using surface-initiated atom transfer radical polymerization method. The crystalline morphologies and lamellar orientations of polyesters on the brush-grafted substrate were then investigated using polarized optical microscopy (POM) and atomic force microscopy (AFM). The results clearly showed that the spherulitic morphologies of polyesters are strongly depending on interaction strength between polyesters and brushes. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L22.00002: Crystallization in sequence-defined peptoid diblock copolymers induced by microphase separation Jing Sun, Nitash Balsara, Ronald Zuckermann Atomic level synthetic control over a polymer's chemical structure is desired for tuning the microphase separation and other properties of crystalline block copolymers. In order to explore the impact of side chain structure on crystallization behavior, we designed a series of chemically-defined, highly monodisperse peptoid diblock copolymers poly-N-2-(2-(2-methoxyethoxy)ethoxy)ethylglycine-block-poly-N-decylglycine (pNte-b-pNdc) with volume fraction of pNte (?Nte) values ranging from 0.28 to 0.71 and a polydisperisty indices ? 1.0003. Both monomers have nearly identical molecular volumes but the pNte block is amorphous while the pNdc block is crystalline. We demonstrate that all the block copolypeptoids self-assemble into lamellar microphases driven by crystallization of the pNdc block. To our knowledge, there are no previous reports of crystallization of a polymer chain induced by microphase separation. These investigations show that polypeptoids provide a unique platform for examining the intertwined roles of side chain organization on the thermodynamic properties of diblock copolymers. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L22.00003: Polymer Crystallization at Liquid-Liquid Interface Christopher Li, Wenda Wang, Hao Qi, Ziyin Huang Curved space is incommensurate with typical ordered structures with three-dimensional translational symmetry. However, upon assembly, soft matter, including colloids, amphiphiles, and block copolymers, often form structures depicting curved surface/interface. On the other hand, twisted and curved crystals are often observed in crystalline polymers. Various mechanisms have been proposed for these non-flat crystalline morphologies. In this presentation, we will discuss the recent development of crystallization at flat and curved liquid/liquid (L/L) interface. We show that structure, morphology and chain folding behaviors are strongly affected by L/L interfacial energy and polymer chain ends. Polymer crystallization behavior at L/L interface will be compared with solution and bulk crystallization. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L22.00004: Kinetics of nucleation and crystallization of poly(epsilon-caprolactone) - multiwalled carbon nanotube composites Invited Speaker: Christoph Schick The nucleation efficiency of multi-wall carbon nano-tubes (MWCNT) in poly($\varepsilon $-caprolactone) (PCL), as an example, was tested for a wide range of temperatures and cooling rates and compared to the efficiency of homogeneously formed nuclei. The temperature range below the maximum of crystallization rate is generally not accessible for non-isothermal cooling experiments because the sample becomes amorphous at the needed cooling rates. Isothermal experiments after fast quenches extend the temperature range down to and below the glass transition. The employed differential fast scanning calorimeter (DFSC) allows cooling at rates up to 100,000 K/s and precise adjustment and control of isothermal conditions in the time range from 10$^{-4}$ to 10$^{4}$ s and longer. As shown in previous work, heterogeneous crystal nucleation dominates at low supercooling, revealing a significant dependence of crystallization rate on MWCNT concentration. Nevertheless, no saturation of the nucleation activity at a MWCNT loading of 0.2 to 0.5 wt{\%} as seen in slow DSC experiments was observed at the much higher cooling rates employed here. At high supercooling, where homogeneous nucleation is prevalent, the addition of MWCNT does not enhance neither reduce the crystallization rate. At the temperature of maximum homogeneous nucleation rate, formation of homogeneous nuclei always dominates crystallization. \\[4pt] E. Zhuravlev, J.W.P. Schmelzer, B. Wunderlich, C. Schick, Kinetics of nucleation and crystallization in poly(epsilon caprolactone) (PCL) Polymer, 52 (2011) 1983-1997. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L22.00005: Two-dimensional folded chain crystals composed of a single isotactic poly(methyl methacrylate) chain observed by atomic force microscopy Jiro Kumaki, Takahiro Anzai We successfully visualized crystallization behavior of a single isolated polymer chain at a molecular level by atomic force microscopy (AFM). Previously, we found that isotactic poly(methyl methacrylate) (it-PMMA) formed two-dimensional folded chain crystals upon compression of its Langmuir monolayer on a water surface, and the molecular images of the crystals deposited on mica were clearly visualized by AFM (Kumaki, et al. JACS 2005, 127, 5788; J. Phys. Chem. B 2013, 117, 5594). In the present study, a high-molecular-weight it-PMMA was diluted in a monolayer of an it-PMMA oligomer which cannot crystallize due to the low molecular weight. At a low surface pressure, isolated amorphous chains of the high-molecular-weight it-PMMA solubilized in the oligomer monolayer were observed. On compression, the isolated chains converted to crystals composed of a single chain. Detailed AFM observations of the crystals indicated that the crystalline nuclei preferably formed at the ends of the chains, and the size of the nuclei was almost independent on the molecular weight of the it-PMMA in a wide range. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L22.00006: Structure Formation of Spinning Polymer Fibers Studied by Monte Carlo Simulations Wenbing Hu, Qi Liu We performed dynamic Monte Carlo simulations of lattice polymer solutions to investigate the solidification process in a fluid filament after extruded from a spinneret into a coagulation bath. We observed skin-core structure formation under the interplay of phase separation and polymer crystallization. We found that a radial temperature gradient dominates the formation of a highly oriented solid skin, while a radial influx of non-solvent dominates the formation of a concentrated core. The processing parameters can adjust and even eliminate the skin-core structure in the fibers. Our molecular-level observations facilitate a better understanding of the fiber processing for the industry to make high quality fibers. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L22.00007: Precise Tetrahedral Giant Molecules Based on Polyhedral Oligosilsesquioxane (POSS) Nano-atoms Mingjun Huang, Chih-Hao Hsu, Shan Mei, Wen-bin Zhang, Stephen Z.D. Cheng The assembly of building blocks with specific shape and symmetry in 3D space is a long-lasting topic in scientific research. If ``nano-atoms'' are placed on the apexes of a rigid polyhedron linker to form a larger faceted giant molecule, such molecules would amplify the symmetry of the linkers and result in giant polyhedra molecules. When four POSS cages are linked to the apex of a tetrahedron, we obtain a giant tetrahedron. Depending on the linkers, it can be a semi-rigid or a rigid giant polyhedron. An interesting approach is to utilize the sp3-carbon or adamantane core to introduce the Td symmetry, and utilize ``click reaction'' to connect four hydrophobic isobutyl-POSS (BPOSS) at four corners. Our preliminary results show that the giant tetrahedron Tetra-4BPOSS forms an interdigitated diamondoid structure. In these giant polyhedra, we can use different ``nano-atoms'' with different functional groups, which may also act as an additional factor to affect the final ordered structures. The progresses of our research lead to three hydrophobic and one hydrophilic HPOSS (HPOSS represents seven hydroxyl group functionalized POSS), and two hydrophobic BPOSS and two hydrophilic HPOSS. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:24AM |
L22.00008: Novel Polymer Ferroelectric Behavior via Crystal Isomorphism and Nanoconfinement Effect Invited Speaker: Lei Zhu Despite comprehensive understanding of novel ferroelectric [i.e., relaxor ferroelectric (RFE) and antiferroelectric (AFE)] behaviors for ceramics, RFE and double hysteresis loop (DHL) behaviors have just emerged for ferroelectric crystalline polymers since the past 15 years. A number of applications such as electrostriction, electric energy storage, and electrocaloric cooling have been realized by utilizing these novel ferroelectric properties. However, the fundamental understanding is still lacking. In this invited talk, we intend to unravel the basic physics behind these novel ferroelectric behaviors via systematic studies of poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)]-based terpolymers and e-beam irradiated copolymers. It is found that both crystal internal structure and crystal-amorphous interaction are important for achieving the RFE and DHL behaviors. For the crystal internal structure effect, friction-free dipole switching and nanodomain formation by pinning the polymer chains are essential, and they can be achieved via the mechanism of crystal repeating unit isomorphism. Physical pinning [e.g., in P(VDF-TrFE)-based terpolymers] induces a reversible RFE$\leftrightarrow $FE phase transition and thus the DHL behavior, whereas chemical pinning [e.g., in e-beam irradiated P(VDF-TrFE)] results in the RFE behavior. Finally, the crystal-amorphous interaction (or the nanoconfinement effect) results in a competition between the polarization and depolarization local fields. When the depolarization field becomes stronger than the polarization field, a DHL behavior can also be observed. Obviously, the physics is different from ceramics and can be largely attributed to the long chain nature of semicrystalline ferroelectric polymers. This understanding will help us design new ferroelectric polymers with improved electroactive properties and/or better applications. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L22.00009: When Do Semi-crystalline Polymer Fold during Crystallization? Toshikazu Miyoshi, You-Lee Hong Semi-crystalline polymers are crystallized as folded chains in thin lamellae of ca. 5-20 nm from random coils in the melt and solution states. Lauritzen--Hoffman theory implied the crystallization process is dominated by sequential stem deposition on the growth front. Conversely, Allegra proposed a bundle model in which aggregates of 10-20 stems are produced by folding in the pre-stage of crystallization. The pre-folded chains are kinetically deposited on the growth front and thus determine the morphology at different crystallization temperatures. The folded chains preserve their own chain-folding directions, numbers, and fractions as a function of concentrations and supercooling, which would provide detailed chain-folding mechanism. We recently developed a new strategy using $^{\mathrm{13}}$C-$^{\mathrm{13}}$C double-quantum NMR to investigate chain-trajectory of $^{\mathrm{13}}$C selectively labeled polymer in bulk crystals. Here, we report how re-entrance sites, fraction, and number of folded chains of \textit{isotactic} poly(1-butene) in form III single crystals depends on supercooling conditions and solvent effects. On the basis of molecular level structures, we will discuss about chain-folding process of $i$PB1 in dilute solutions. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L22.00010: Chain-Folding Structures of a Semi-crystalline Polymer in Bulk and Single Crystals Elucidated by 13C--13C Double Quantum NMR You-lee Hong, Toshikazu Miyoshi Semi-crystalline polymers are crystallized as folded chains in thin lamellae of ca. 5-20 nm from random coils in the melt and solution states. However, understanding of detailed chain-folding structure and crystallization mechanism are still challenging issue due to various experimental limitations. We recently developed a new strategy using $^{13}$C--$^{13}$C double-quantum (DQ) NMR with selectively $^{13}$C isotope labeled \textit{isotactic} poly(1-butene) form I to investigate chain-trajectory in solution and melt grown crystals at various \textit{Tc}s. This new method can determine the re-entrance sites, the successive folding number ($n)$, and the fractions ($F)$ of chain-folding in a wide Tc range. In melt grown crystals at \textit{Tc} $=$ 95 $^{\circ}$C, a comparison of experimental and simulated DQ efficiency determined that the polymer chains alternatively change chain-folding directions and the stems tightly pack via intramolecular interactions, and the fraction (F) of adjacent re-entry structure ranges from 70{\%} at n $=$ 4 to 100{\%} at mixed structures of n $=$ 1 and 2. Furthermore, DQ efficiency is independent of Tc in bulk crystals. This means chain-folding do not change in a wide Tcs. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L22.00011: Torsional Tapping atomic force microscopy for molecular resolution imaging of semicrystalline polymers Jamie Hobbs, Nic Mullin, Rebecca Savage Torsional tapping atomic force microscopy (TTAFM) provides a considerable improvement in signal-to-noise when compared with conventional AFM imaging approaches. This enables the routine use of ultra-sharp whisker tips and leads to true molecular resolution imaging in the crystalline and crystal-amorphous interface zones in semi-crystalline samples. Peak-to-peak resolution below 0.4 nm is obtainable even on topographically rough samples. Here we will present the result of recent studies showing the molecule by molecule chain structure of various polymer samples including polyethylene and polypropylene, showing how chain conformation within the crystal and at the crystal-amorphous and crystal-air interface is influenced by processing conditions. Of particular interest are observations of the roughness of the crystal fold surface at the nanometer level even on samples that have been annealed for long times. It is also clear that the crystal surface that is presented is not always dominated by the chain like nature of the molecules, but in some cases can have a more complex character that might strongly influence how the process of crystallization should be modelled. Data on the chain level internal structure of bulk samples as revealed by cryo-microtoming, will also be discussed. [Preview Abstract] |
Session L23: Invited Session: Industrial Physics Forum: Physics and Industrial Applications of Optoelectronics
Sponsoring Units: FIAPChair: Robert Hickernell, National Institute of Standards and Technology, Steven S. Rosenblum, Corning, Inc.
Room: 505-507
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L23.00001: The National Academies' Report on Optics and Photonics: The Road to a National Photonics Initiative Invited Speaker: Alan Willner This presentation will highlight aspects of the recent report from the U.S. National Academies on Optics and Photonics. Enabling science and technology issues were discussed, as well as the past and future impact on the economy. A key recommendation of the study is the formation of a National Photonics Initiative, which has started taking shape with the crucial backing of the major professional societies. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L23.00002: Nanophotonic phenomena in systems of macroscopic sizes Invited Speaker: Marin Soljacic Nanophotonic techniques provide uncprecedented opportunities for controlling behavior of light. However, to make these techniques useful for many applications of interest (e.g. energy applications) one has to have the ability to implement nanophotonic techniques in systems of large sizes. I will present some promising novel nanophotonic phenomena, as well as some fabrication techniques to implement them on large scales. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L23.00003: Exploring the nano-world with bright soft x-ray laser beams Invited Speaker: Carmen Menoni The generation of bright soft x-ray (SXR) laser beams on a table-top is opening new opportunities for visualizing and altering the nano-world. Compact plasma-based SXR lasers operating at wavelengths between 10 nm and 50 nm are enabling the implementation of ultra-high resolution microscopes, chemical imaging tools, and defect-tolerant nano-patterning tools. SXR laser-based microscopes can image objects with 30 nm resolution with a single laser shot. Such ``flash'' illumination makes it possible to image dynamic phenomena at the nanoscale. We are also combining SXR laser-induced nano-ablation with mass spectrometry to image chemical composition in three dimensions at the nanoscale. The application of this technique to map the composition of metallic, dielectrics, and organic samples will be described. High average power beams of SXR laser light also make it possible to print arrays of nanostructures defect-free using the Talbot effect. The coherent SXR laser illumination of masks with arrays of arbitrary nano-patterns will allow, for example, the printing of plasmonic structures, arrays of nano-antennas and two-dimensional photonic crystals. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:24AM |
L23.00004: Hybrid III-V Silicon Lasers Invited Speaker: John Bowers Abstract: A number of important breakthroughs in the past decade have focused attention on Si as a photonic platform. We review here recent progress in this field, focusing on efforts to make lasers, amplifiers, modulators and photodetectors on or in silicon. We also describe optimum quantum well design and distributed feedback cavity design to reduce the threshold and increase the efficiency and power output. The impact active silicon photonic integrated circuits could have on interconnects, telecommunications and on silicon electronics is reviewed. Biography: John Bowers holds the Fred Kavli Chair in Nanotechnology, and is the Director of the Institute for Energy Efficiency and a Professor in the Departments of Electrical and Computer Engineering and Materials at UCSB. He is a cofounder of Aurrion, Aerius Photonics and Calient Networks. Dr. Bowers received his M.S. and Ph.D. degrees from Stanford University and worked for AT{\&}T Bell Laboratories and Honeywell before joining UC Santa Barbara. Dr. Bowers is a member of the National Academy of Engineering and a fellow of the IEEE, OSA and the American Physical Society. He is a recipient of the OSA/IEEE Tyndall Award, the OSA Holonyak Prize, the IEEE LEOS William Streifer Award and the South Coast Business and Technology Entrepreneur of the Year Award. He and coworkers received the EE Times Annual Creativity in Electronics (ACE) Award for Most Promising Technology for the hybrid silicon laser in 2007. Bowers' research is primarily in optoelectronics and photonic integrated circuits. He has published ten book chapters, 600 journal papers, 900 conference papers and has received 54 patents. He has published 180 invited papers and conference papers, and given 16 plenary talks at conferences. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 11:00AM |
L23.00005: Laser Material Processing in Manufacturing Invited Speaker: Marshall Jones This presentation will address some of the past, present, and potential uses of lasers for material processing in manufacturing. Laser processing includes welding, drilling, cutting, cladding, etc. The U.S. was the hot bed for initial uses of lasers for material processing in the past with Europe, especially Germany, presently leading the way. The future laser processing leader may still be Germany. Selected uses, past and present, of lasers within GE will also be highlighted as seen in such business units as Aviation, Lighting, Power and Water, Healthcare, and Transportation. [Preview Abstract] |
Session L24: Focus Session: NanoPV Novel Photophysics and Transport I
Sponsoring Units: GERAChair: Richard Wiener, Research Corporation for Science Advancement
Room: 504
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L24.00001: Mapping Energy Flow with Ultrafast Optical Spectroscopy Invited Speaker: Vanessa Huxter Ultrafast electronic spectroscopy connects the spatial, temporal and dynamic landscapes of complex systems. These connections are essential to our understanding of structure-function relationships and energy transport through materials. Using two-dimensional electronic spectroscopy (2DES), we can generate correlation maps that relate the initial absorptive interaction with the signal emission, allowing us to follow the flow of energy through a system via the vibrational and electronic coherences and populations. Using 2DES to study synthetic and natural photosynthetic pigments provides insight into the advantages of particular molecular architectures that are ubiquitous in nature, optimizing efficient energy transfer and demonstrating the importance of static disorder and vibrational coupling. The critical role of vibrations in these pigments is mirrored in the response of the nitrogen vacancy centers in diamond (NV-diamond) quantum material system. 2DES studies of NV-diamond reveal an array of coherent nuclear vibrations coupled to the electronic state. The effect of the vibrational coherences on the dynamics of the NV-diamond system may provide a route to increased efficiency of energy transport in nanostructured solar cells. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L24.00002: Energy level modification in lead sulfide quantum dot photovoltaics through ligand exchange Patrick Brown, Donghun Kim, Richard Lunt, Moungi Bawendi, Jeffrey Grossman, Vladimir Bulovic The electronic properties of lead sulfide colloidal quantum dots (PbS QDs) can be controlled through modification of QD size and surface chemistry. Novel surface passivation techniques involving organic or inorganic ligands have contributed to a rapid rise in the efficiency of QD photovoltaics, yet the influence of ligand-induced surface dipoles on PbS QD energy levels and photovoltaic device operation is not yet completely understood. Here, the valence band energies of PbS QDs treated with twelve different ligands are measured using ultraviolet photoelectron spectroscopy (UPS), and a valence band shift of up to 0.75 eV is observed between different ligand treatments. Atomistic simulations of ligand binding to pristine PbS(100) and PbS(111) slabs qualitatively reproduce the measured energy level shifts. 1,2-benzenedithiol and 1,3-benzendithiol treatments, which result in valence band energies differing by $\sim$ 0.2 eV, are employed for PbS QDs in three different solar cell architectures, and changes in device performance are correlated with the measured energy level shift. These findings complement the known bandgap-tunability of colloidal QDs and highlight an additional level of control over the electronic properties of PbS QDs. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L24.00003: Light Harvesting in Functionalized SiQD Assemblies via Spatially Separated Excitons Huashan Li, Zhigang Wu, Tianlei Zhou, Alan Sellinger, Mark Lusk Silicon quantum dots (SiQDs) with diameters less than 5 nm are particularly attractive for photovoltaic applications [1,2], but their optical gap is too large to match the solar spectrum. Although recent progress on solution processing techniques provides more opportunities for functionalizing SiQD, nontrivial absorption under 3 eV has yet to be achieved [3]. The absorption of photons through the direct generation of spatially separated excitons at dot-ligand interfaces may be a promising strategy for overcoming this challenge. We consider the idea computationally and show that it is indeed possible to capture photons of much lower energy using very small SiQD. The key is to establish a type-II energy level alignment in conjunction with strong electronic coupling between the dot and ligand. Our analysis indicates that conjugated vinyl bonds to common organic ligands satisfy both of these conditions. In principle, this allows the optical gap of SiQD to be tuned to arbitrarily small values independent of their size. For the prototype system of 2.6 nm SiQDs, we predict that triphenylamine (TPA) termination will result in a 0.47 eV redshift of the optical gap along with a boost of absorption intensity near the band edge, a result consistent with our experimental realization of the system. We will also discuss the results of a computational analysis of the robustness of the absorption spectrum against oxidation and extra alkyl ligands within this new paradigm. [1]Lin, Z. et al., ACS Nano 6, 4029, 2012. [2] Li, H. et al., ACS Nano 6, 9690, 2012. [3] Dung, M. X. et al., Chem. Asian J. 8, 653, 2013. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L24.00004: Electronic and Optical Properties of Novel Phases of Silicon and Silicon-Based Derivatives Chin Shen Ong, Sangkook Choi, Steven Louie The vast majority of solar cells in the market today are made from crystalline silicon in the diamond-cubic phase. Nonetheless, diamond-cubic Si has an intrinsic disadvantage: it has an indirect band gap with a large energy difference between the direct gap and the indirect gap. In this work, we perform a careful study of the electronic and optical properties of a newly discovered cubic-Si$_{20} $ phase of Si that is found to sport a direct band gap. In addition, other silicon-based derivatives have also been discovered and found to be thermodynamically metastable. We carry out \textit{ab initio} GW and GW-BSE calculations for the quasiparticle excitations and optical spectra, respectively, of these new phases of silicon and silicon-based derivatives. 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 DOE at Lawrence Berkeley National Laboratory's NERSC facility and the NSF through XSEDE resources at NICS. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L24.00005: ``New'' energy states lead to phonon-less optoelectronic properties in nanostructured silicon Vivek Singh, Yixuan Yu, Brian Korgel, Prashant Nagpal Silicon is arguably one of the most important technological material for electronic applications. However, indirect bandgap of silicon semiconductor has prevented optoelectronic applications due to phonon assistance required for photon light absorption/emission. Here we show, that previously unexplored surface states in nanostructured silicon can couple with quantum-confined energy levels, leading to phonon-less exciton-recombination and photoluminescence. We demonstrate size dependence (2.4 - 8.3 nm) of this coupling observed in small uniform silicon nanocrystallites, or quantum-dots, by direct measurements of their electronic density of states and low temperature measurements. To enhance the optical absorption of the these silicon quantum-dots, we utilize generation of resonant surface plasmon polariton waves, which leads to several fold increase in observed spectrally-resolved photocurrent near the quantum-confined bandedge states. Therefore, these enhanced light emission and absorption enhancement can have important implications for applications of nanostructured silicon for optoelectronic applications in photovoltaics and LEDs. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L24.00006: Quantum confined nanocrystalline silicon Tianyuan Guan, Chito Kendrick, San Theingi, Luigi Bagolini, Kory Riskey, Lauren Vitti, Grant Klafehn, Craig Taylor, Mark Lusk, Brain Gorman, Reuben Collins, Jeremy Fields, Pauls Stradins Quantum confined (QC) semiconductors have drawn much attention in photovoltaics due to their tunable optoelectronic properties and potential for efficiency improvements. Here, we report a study of nanocrystalline silicon (nc-Si:H), consisting of silicon nano-particles (SiNPs) embedded in hydrogenated amorphous silicon (a-Si:H) matrix. Films were grown by depositing the SiNPs and a-Si:H sequentially from separate plasma reactors in a common deposition chamber. Several characterizations were used to ensure the material had low defect density and that the SiNPs were highly crystalline and well within the QC regime. Optical properties of hybrid SiNP/a-Si:H films were explored using visible to near infrared photoluminescence (PL). At low temperature, PL revealed two primary emission features, one from conventional a-Si:H $\sim$ 1.3 eV and a second peak which can be attributed to recombination in SiNPs. The energy of this peak is higher than the bulk c-Si bandgap ($\sim$ 1.2 eV), and with decreasing SiNP size, it increases to $\sim$ 1.7 eV. This quantum confinement effect agrees with Density Functional Theory predictions. In addition, we also see that the PL peak for SiNPs surrounded by a-Si:H shifts to lower energy relative to the isolated SiNPs. This shift is also consistent with the modeling results which show that surrounding SiNPs with a-Si:H leads to a softening of the confinement barrier and a redshift in the optical gap. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L24.00007: PECVD Environmental Effects on Silicon Nanoparticle Size and Quality Grant Klafehn, Chito Kendrick, Tianyuan Guan, San Theingi, Kory Riskey, Lauren Vitti, Luigi Bagolini, Mark Lusk, Brian Gorman, Craig Taylor, Reuben Collins, Jeremy Fields, Paul Stradins Silicon based nanoparticles (SiNPs) have recently been of great interest to the PV community because of their unique properties compared to their bulk constituents. By decreasing a nanoparticle's (NP) size below its exciton Bohr radius, its band gap can be increased relative to the bulk. This talk will discuss fundamental variables involved in defining and controlling plasma-grown SiNP size and quality. A quartz tube with a RF electrode ring is used to create a plasma in an argon-silane mixture to grow the SiNPs. Their quality and size can be changed by varying the reactor pressure, gas flow, and thus the resulting residence time. They are then characterized by Raman, PL, ESR, XRD, and TEM, and then mapped to a phase diagram with respect to pressure and flow. Higher residence times of 10 ms resulted in highly crystalline, 7 nm SiNPs. Residence times of 2 ms create 4 nm particles, while below 2 ms will result in highly defective material, even though the PL exhibits peaks at 1.6 eV. These parameters will be discussed, including how each variable affects the resultant SiNP size, quality. Also included will be a discussion about additive gasses and their additional effects on SiNP characteristics. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L24.00008: Study of band-edge optical absorption of silicon nanoparticles using photothermal deflection spectroscopy San Theingi, Chito Kendrick, Tianyuan Guan, Lauren Vitti, Grant Klafehn, Luigi Bagolini, Mark Lusk, Brian Gorman, Paul Stradins, Craig Taylor, Reuben Collins Silicon nanoparticles (SiNPs) are a promising optoelectronic material with unique properties such as a size tunable bandgap, sensitivity to surface termination, and efficient optical emission. Here, we present an optical absorption study of size varied, free standing SiNPs films using photothermal deflection spectroscopy (PDS). In general, it is difficult to directly observe the absorption threshold in SiNPs because of silicon's low absorption coefficient. PDS, which directly measures the optical absorption of materials through the generated heat, is known for its extremely high sensitivity. The SiNPs are grown using a plasma process and deposited as films on quartz substrates. Different amounts of SF$_{6}$ gas are introduced into the process gas to control the size of these SiNPs. Photoluminescence measurements show a strong blue shift in emission with increased SF$_{6}$ flow. PDS measurements allow a corresponding blue shift in the band edge absorption which is attributable to quantum confinement to be observed. In addition, PDS measurements also allow us to probe the defect level of our material, and the size distribution of SiNPs in our sample. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L24.00009: The influence of Auger recombination on the performance of quantum-dot light-emitting diodes Jeffrey Pietryga, Wan Ki Bae, Young-Shin Park, Istvan Robel, Victor Klimov Colloidal quantum dots are the subject of intense research as fluorophores for light-emitting diodes (LEDs) due to properties such as spectrally narrow, tunable emission and facile processibility via solution-based methods. Continued improvement of LEDs based on quantum dots is restricted by an incomplete understanding of the physics underlying current performance limitations. More specifically, little is known about the influence of multi-carrier processes on overall LED efficiency, and on the reduction of efficiency at high currents (known as efficiency roll-off, or droop). Here, we present an investigation of this issue involving studies that correlate the excited state dynamics of structurally engineered quantum dots with their emissive performance within LEDs. We find that because of significant charging of quantum dots with extra electrons, multi-carrier Auger recombination greatly impacts both LED efficiency and the onset of efficiency roll-off at high currents. We conclude by examining two specific approaches for mitigating this problem using heterostructured quantum dots that either suppress Auger recombination, or that directly address the problem of charge-injection imbalance. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L24.00010: Nanoscale engineering of efficient photovoltaic conversion in quantum dot media Sergeev Andrei, Li Yanshu, Vagidov Nizami, Mitin Vladimir, Sablon Kimberly, Oktyabrsky Serge, Yakimov Michael The main problem of photovoltaic nanomaterials for high efficiency conversion is enhanced recombination of photocarriers. Selective doping of quantum dot (QD) media allows for control of three-dimensional potential profile and adds more functionality and scalability to photovoltaic materials and structures. Optimization of the nanoscale barriers and reduction of wetting layer in a QD medium substantially suppress recombination processes and enhance ittersubband transitions, which provide electron extraction from QDs. We report that the optimized 1-$\mu $m InAs/GaAs QD media placed in 3-$\mu $m base GaAs p-n junction increases the short circuit current from 22.0 mA/cm$^{\mathrm{2}}$ to 28 mA/cm$^{\mathrm{2}}$. Spectral analysis of conversion processes shows that the IR sub-bangap photons and hot electrons created by high energy photons provide comparable contributions to photovoltaic conversion via charged QDs. The reduction of the wetting layer, which otherwise accumulates electrons, increases extraction of electrons from QDs due to interaction with hot electrons created by high energy photons. Nanoscale engineering of electron processes by charging of QDs provides wide possibilities for further suppression of recombination and thermalization losses in QD photovoltaic devices. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L24.00011: Exciton shelves for charge and energy transport in third-generation quantum-dot devices Samuel Goodman, Vivek Singh, Hyunwoo Noh, Josep Casamada, Anushree Chatterjee, Jennifer Cha, Prashant Nagpal Quantum dots are semiconductor nanocrystallites with size-dependent quantum-confined energy levels. While they have been intensively investigated to utilize hot-carriers for photovoltaic applications, to bridge the mismatch between incident solar photons and finite bandgap of semiconductor photocells, efficient charge or exciton transport in quantum-dot films has proven challenging. Here we show development of new coupled conjugated molecular wires with ``exciton shelves'', or different energy levels, matched with the multiple energy levels of quantum dots. Using single nanoparticle and ensemble device measurements we show successful extraction and transport of both bandedge and high-energy charge carriers, and energy transport of excitons. We demonstrate using measurements of electronic density of states, that careful matching of energy states of quantum-dot with molecular wires is important, and any mismatch can generate midgap states leading to charge recombination and reduced efficiency. Therefore, these exciton-shelves and quantum dots can lead to development of next-generation photovoltaic and photodetection devices using simultaneous transport of bandedge and hot-carriers or energy transport of excitons in these nanostructured solution-processed films. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L24.00012: Silicon nanowire arrays with passivated axial p-i-n junctions for photovoltaic applications Peng Zhang, Pei Liu, Alexander Zaslavsky, Domenico Pacifici, Jong-Yoon Ha, Sergiy Krylyuk, Albert Davydov Metal catalyst-assisted vapor-liquid-solid mechanism can be used to grow large areas of nanowires (NWs) with compositional and doping control in either axial or core-shell geometries. Here, we report on vertical arrays of Si axial $p$-$i$-$n$ oxide-passivated NWs that were 12 microns long with a 4 micron intrinsic section. The NW arrays were planarized using SU-8 photoresist, followed by reactive ion etching to expose the NW tips. Top $n$-contact was realized by sputter deposition of a 200 nm IZO layer. The $p$-contact was made by backside metallization of the $p$-Si substrate. Under AM 1.5 illumination, unpassivated NW arrays exhibited an open-circuit voltage, $V_{\mathrm{OC}}$ of 170 mV, a short-circuit current density $J_{\mathrm{SC}}$ \textgreater 3.7 mA/cm$^{2}$ (with uncertainty due to the unknown fraction of properly contacted NWs), and a fill factor of 28.9{\%}. After the passivation, $V_{\mathrm{OC}}$, $J_{\mathrm{SC}}$ and FF increased to 250 mV, \textgreater 9.2 mA/cm$^{2}$ and 35.7{\%}, respectively. The measured normal reflectance was around 6{\%} over the 400--1000 nm spectral range, whereas the diffuse reflectance was around 20{\%} over the same range, indicating strong light scattering and absorption by the NWs. The photovoltaic performance of passivated single NWs and NW arrays were compared using a 532 nm laser with a power density of about 10 W/cm$^{\mathrm{2}}$. Higher values of $V_{\mathrm{OC}}$ and FF obtained for the latter are explained by light trapping in the NW arrays. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L24.00013: Multiple Exciton Generation in Silicon QD arrays Andrei Kryjevski, Dmitri Kilin We use Density Functional Theory (DFT) combined with the many body perturbation theory to calculate multiple exciton generation (MEG) in several semiconductor nanosystems. Hydrogen-passivated $Si_{29}H_{36}$ quantum dots (QDs) with crystalline and amorphous core structures, the quasi one dimensional (1-D) arrays constructed from these QDs, as well as crystalline and amorphous Si nanowires have been studied. Quantum efficiency, the average number of excitons created by a single photon, has been calculated in these nanoparticles to the leading order in the screened Coulomb interaction. Amorphous nanostructures are predicted to have more effective carrier multiplication. [Preview Abstract] |
Session L25: Focus Session: Thermoelectric Materials
Sponsoring Units: GERA DMPChair: Eric Toberer, Colorado School of Mines
Room: 503
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L25.00001: High throughput search for thermoelectric materials. Computational stability, transport and doping properties Invited Speaker: Georg K.H. Madsen Thermoelectric materials can be utilized for an efficient conversion of waste heat to electric power. While thermoelectric properties of known compounds can be rationalized and predicted using only the structure as an input [1], it turns out that a large number of semiconductor structures show potential for favorable thermoelectric properties [2]. This leaves the feasibility of achieving the optimal doping [3] and a low thermal conductivity as key bottlenecks in discovering new thermoelectric materials. In this talk I will discuss simple procedures to screen for these properties and illustrate this by the discovery of an industrially relevant thermoelectric material.\\[4pt] [1] L. Bjerg, G. K. H. Madsen, B. B. Iversen, Chem. Mater. 2011, 23, 390 \\[0pt] [2] I. Opahle, A. Parma, E. J. McEniry, R. Drautz, G. K H Madsen, New J. Phys., 2013, 15, 105010 \\ [0pt] [3] L. Bjerg, G. K. H. Madsen, B. B. Iversen, Chem. Mater. 2012, 23, 390 [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L25.00002: Thermoelectric Power Factor Enhancement in Tetrahedrites Xu Lu, Donald Morelli, Yongsheng Zhang, Chris Wolverton We report a strategy for power factor enhancement of the thermoelectric properties of Cu$_{12}$Sb$_{4}$S$_{13}$ tetrahedrites. Our previous strategy to improve the figure of merit in tetrahedrites was to reduce the electronic thermal conductivity at the expense of reducing the power factor by replacing monovalent Cu with divalent Zn or Fe. Here, we substitute S with Se, which is isovalent with S and therefore does not induce a doping effect. However, we observe a reduction in electronic resistivity in Cu$_{12}$Sb$_{4}$S$_{13-x}$Se$_{x}$ without affecting the thermopower, which leads to at least a 20{\%} enhancement in power factor. Furthermore, the substitution of S with Se causes a reduction in the lattice thermal conductivity via a solid solution effect, keeping the total thermal conductivity unchanged. Density Functional Theory (DFT) calculations indicate a narrowing of the band gap in Cu$_{12}$Sb$_{4}$Se$_{13\, }$relative to the sulfide; however, DFT also shows that the pure selenide is not thermodynamically stable. But Cu$_{12}$Sb$_{4}$S$_{13-x}$Se$_{x}$ single phase materials may be synthesized up to at least x $=$ 3. We believe this strategy will introduce additional degenerate energy levels near the top of valence band. Further studies should be performed to investigate the optimal Se concentration and its effect on figure of merit. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L25.00003: Structurally complex Zintl compounds for high temperature thermoelectric power generation Alexandra Zevalkink, Gregory Pomrehn, Zachary Gibbs, Jeffrey Snyder Zintl phases, characterized by covalently-bonded substructures surrounded by highly electropositive cations, exhibit many of the characteristics desired for thermoelectric applications. Recently, we demonstrated promising thermoelectric performance ($zT$ values between 0.4 and 0.9) in a class of Zintl antimonides that share a common structural motif: anionic moieties resembling infinite chains of linked tetrahedra. These compounds ($A_5M_2$Sb$_6$ and $A_3M$Sb$_3$ compounds where $A$ = Ca or Sr and $M$ = Al, Ga and In) crystallize as four distinct, but closely related chain-forming structure types. Their large unit cells lead to exceptionally low lattice thermal conductivity due to the containment of heat in low velocity optical phonon modes. Here, we show that chemical substitutions on the $A$ and $M$ sites can be used to control the electronic and thermal transport properties and optimize the thermoelectric figure of merit. Doping with alio-valent elements allows for rational control of the carrier concentration, while isoelectronic substitutions can be used to fine-tune the intrinsic properties. A combination of Density Functional calculations and classical transport models was used to explain the experimentally observed transport properties of these compounds. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L25.00004: Crystal structure and thermoelectric properties of kuramite Cu$_{3}$Sn(Se,S)$_{4}$ with cation disorder Yosuke Goto, Yoichi Kamihara, Masanori Matoba Ternary or quaternary compounds Cu$_{2}$--$T_{M}$--A--\textit{Ch}$_{4}$ ($T_{M}$; transition metal, A; group 14 or 15 elements, \textit{Ch}; chalcogen) are promising $p$--type thermoelectric materials because of their heavy but still conducting valence band, which is composed of Cu 3$d$ and Ch 3$p$ orbitals. Lattice thermal conductivity should be suppressed by cation disorder, however, coexistence of transition metals such as Cu and Zn on quaternary compounds complicate the understanding of the details of cation disorder by means of conventional X-ray diffraction. In this work, We demonstrate the crystal structure and thermoelectric properties of kuramite Cu$_{3}$Sn(Se,S)$_{4}$. Structural analysis revealed that polycrystalline samples crystallize in tetragonal $I$--42$m$ space group. The 2$a$ site was occupied by Cu only, while 2$b$ and 4$d$ sites were occupied by Cu and Sn partial disorder. Both electrical conductivity ($\sigma )$ and Seebeck coefficient ($S)$ were increased with substitution of Se for S, resulting $\sigma =$ 5.68 $\times$ 10$^{2}$ Scm$^{-1}$ and $S =$ 114 $\mu $VK$^{-1}$ at 623 K, respectively. At the conference, we will also report the alloy effect on thermal conductivity of Cu$_{3}$Sn(Se,S)$_{4}$ solid solution. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L25.00005: Carrier concentration optimization and Band convergence of Mg$_{2}$Si$_{\mathrm{1-x}}$Sn$_{\mathrm{x}}$ thermoelectric materials Xinfeng Tang, Wei Liu, Xianli Su, Ctirad Uher In our research, the optimization of electron concentration and the tuning of conduction band structure are explored in order to push it for application in power generation. Systematical experiments indicate that the appropriate over-stoichiometry of Mg content is beneficial to the adjustment of electron concentration in n-type Mg$_{2}$Si$_{\mathrm{1-x}}$Sn$_{\mathrm{x}}$ solid solutions. Moreover, the electron concentration of Mg$_{2}$Si$_{\mathrm{1-x}}$Sn$_{\mathrm{x}}$ is validly controlled by doping Sb or Bi with the combination of proper over-stoichiometry of Mg. First-principles calculations of the band structure and plentiful experimental researches proved that the bottom of the conduction band of Mg$_{2}$Si$_{\mathrm{1-x}}$Sn$_{\mathrm{x}}$ is characterized a double band structure, including a heavy band and a light band which converged in energy with the increase of the Sn/Si ratio and further degenerated at x $=$ 0.65 $\sim $ 0.68. Consequently, the convergence and degeneration of two conduction bands give rise to remarkable elevation on the carrier effective mass and Seebeck coefficient without a detrimental effect on the carrier mobility, and therefore lead to a largely enhanced power factor. Due to the optimization of both the electron concentration and conduction band structure, n-type Mg$_{2}$Si$_{\mathrm{1-x}}$Sn$_{\mathrm{x}}$, being provided with electron concentration in the range of 1.6 $\times$ 10$^{20}$ $\sim $ 2.5 $\times$ 10$^{\mathrm{20}}$ cm$^{-3}$ and x $=$ 0.6 $\sim $ 0.7, show the highest \textit{ZT} values of 1.3 at around 725 K and average \textit{ZT} values about 1.0 within 500 $\sim $ 800 K. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L25.00006: Electronic structure of defects in Mg$_2$Si and Cu$_3$SbSe$_4$ and their thermoelectric significance S.D. Mahanti, Dat Do Defects play an important role in the thermoelectric properties of narrow band gap semiconductors. Recently, Mg$_2$Si and its solid solutions with Mg$_2$Sn have been found to be excellent n-type thermoelectrics and have been studied extensively due to its unique feature called conduction band convergence [Liu et al., PRL 108, 166601 (2012)]. In this talk we will discuss the physics of defects in Mg$_2$Si and explore the possibilities of improving its thermoelectric properties by co-doping, using first principle calculation and supercell model. In addition, we will also discuss some of our results using the same approach on the nature of defects in another important thermoelectric system Cu$_3$SbSe$_4$ where the lone pairs of Sb control the nature of states near the band gap [Dat Do et al., J. Phys.: Condens. Matter 24, 415502 (2012)]. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L25.00007: Optimally doped hybridization gap semiconductor FeGa$_{3}$ as potential thermoelectric alloy* Vijayabarathi Ponnambalam, Donald T. Morelli FeGa$_{3}$, a hybridization gap semiconductor with a band gap of $\sim$ 0.5 eV can be a potential thermoelectric material if optimally doped. Due to the involvement of d-band in the transport, high Seebeck coefficient is a possibility. To achieve the optimum doping level, Mn, Co and Zn containing FeGa$_{3}$ alloys are being prepared either via the flux or solid state reaction method. Phase characterization will be carried out. Electrical and transport properties including resistivity, Seebeck and Hall coefficients and thermal conductivity will be measured over a wide temperature range of 80- 1000 K. These results will be presented and the potential of these compositions as thermoelectrics will be discussed.\\[4pt] *This work was supported as part of the Center for Revolutionary Materials for Solid State Energy Conversion, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001054. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L25.00008: Alkaline earth lead and tin compounds $Ae_2$Pb, $Ae_2$Sn, $Ae$=Ca,Sr,Ba, as thermoelectric materials David Parker, David Singh We present a detailed theoretical study of three alkaline earth compounds Ca$_2$Pb, Sr$_2$Pb and Ba$_{2}$Pb, which have undergone little previous study, calculating electronic band structures and Boltzmann transport and bulk moduli using density functional theory. We also study the corresponding tin compounds Ca$_2$Sn, Sr$_{2}$Sn and Ba$_{2}$Sn. We find that these are all narrow band gap semiconductors with an electronic structure favorable for thermoelectric performance, with substantial thermopowers for the lead compounds at temperature ranges from 300 to 800 K. For the lead compounds, we further find very low calculated bulk moduli - roughly half of the values for the lead chalcogenides, suggestive of soft phonons and hence low lattice thermal conductivity. All these facts indicate that these materials merit experimental investigation as potential high performance thermoelectrics. We find good potential for thermoelectric performance in the environmentally friendly stannide materials, particularly at high temperature. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L25.00009: High-temperature Thermoelectric Properties of Ag$_{2}$Se$_{0.5}$Te$_{0.5}$ Fivos Drymiotis, Tristan Day, David Brown, Nicholas Heinz, G. Jeffrey Snyder We will be presenting the high-temperature thermoelectric properties of Ag$_{2}$Se$_{0.5}$Te$_{0.5}$. This particular alloy displays very low thermal conductivity and competitive thermoelectric performance. Specifically, in the temperature region from 520 K to 620 K we observe non-hysteretic behavior between the heating and cooling curves and zT values ranging from 1.2 to 0.8. Higher zT values are observed at lower temperatures on cooling. Our results suggest that this alloy is a competitive thermoelectric material for intermediate temperature power generation applications. The authors would like to thank the U.S. Air Force Office of Scientific Research for supporting this work. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L25.00010: Skutterudite Derivatives: A Fundamental Investigation of New Materials with Potential for Thermoelectric Applications Kaya Wei, Yongkwan Dong, George Nolas Thermoelectric devices allow for the direct conversion of heat into electricity as well as solid-state refrigeration. Skutterudites continue to be of great interest for power generation applications. For example, when atoms are placed into the interstitial cages of these open-structured materials, the lattice thermal conductivity can be substantially reduced compared with that of unfilled skutterudites. Recently we began a fundamental investigation of new compounds with a modified skutterudite structure. Fundamental studies on the synthesis and low temperature transport properties of unfilled and partially filled rhombohedrally modified skutterudite derivatives will be presented. Along with Reitveld refinement, the structure and stoichiometry of those compositions as well as their transport properties will be discussed. This work aims to further the fundamental investigation of new skutterudites, while continuing the research on these materials towards thermoelectric power generation applications. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L25.00011: Synthesis and thermoelectric property of Ca and In-doped n-type Bi$_{85}$Sb$_{15}$ alloy Kamal Kadel, Wenzhi Li, Giri Joshi, Zhifeng Ren In the present work we investigated the thermo-electric properties of undoped Bi$_{85}$Sb$_{15}$ and different Ca-doped Bi$_{85}$Sb$_{15}$Ca$_{x}$ (x$=$0.5, 2, and 5) and In-doped Bi$_{85}$Sb$_{15}$In$_{\mathrm{x}} $(x$=$0.5, 2) alloys synthesized via arc-melting first and followed by ball milling and hot pressing. Effect of different Ca and In doping levels on transport properties of Bi$_{85}$Sb$_{15}$ alloys has been investigated. It is found that thermal conductivity decreases with increasing Ca and decreasing In. Electrical transport measurements show that power factor increases with doping level of Ca up to Bi$_{85}$Sb$_{15}$Ca$_{2}$ and then decreases yielding the maximum power factor of 3.8 $\times$ 10$^{-3}$ Wm$^{-1}$K$^{-2}$ and zT of 0.39 at room temperature for Bi$_{85}$Sb$_{15}$Ca$_{2}$. For indium doping, power factor decreases with doping level from 0.5 to 2, yielding the maximum zT value of 0.37 at room temperature for Bi$_{85}$Sb$_{15}$In$_{0.5}$. In this work, calcium doping in Bi$_{85}$Sb$_{15}$ alloy is found to yield better thermoelectric property than indium doping. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L25.00012: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L25.00013: Small-polaron transport and thermoelectric properties of the misfit-layer composite (BiSe)$_{109}$TaSe$_{2}/$TaSe$_{2}$ Jin-hee Kim, Yoo Jang Song, Jong-Soo Rhyee, Bong-Seo Kim, Su-Dong Park, Hyeung Jin Lee, Jae-Wook Shin We studied the thermoelectric properties of the composite of misfit-layered compounds (BiSe)$_{109}$TaSe$_{2}$ and TaSe$_{2}$. The x-ray diffraction pattern on the cross-sectional plane of the sintered body shows a preferred orientation of the (00$l)$ direction for (BiSe)$_{109}$TaSe$_{2}/$TaSe$_{2}$ indicating anisotropic alignment during hot pressing. Because of the crystallographic alignment, the temperature-dependent electrical resistivity $\rho (T )$, Seebeck coefficient $S(T )$, and the thermal conductivity $\kappa (T)$ exhibit in-plane and out-of-plane anisotropic transport behavior. The Seebeck coefficient is very low because of the coexistence of electron and hole mixing, as confirmed by the two-carrier model. The lattice thermal conductivity $\kappa_{L} $of the covalent bonding layer (in-plane) is lower than those of the layer with van der Waals bonding (out-of-plane) implying the existence of a charge density wave along the in-plane. We observed a sign anomaly of the positive Hall coefficient $R_{H}$ and negative Seebeck coefficient $S$. According to Holstein's small-polaron model, the sign anomaly may come from the odd number of small-polaron hopping sites. [Preview Abstract] |
Session L26: Focus Session: Materials in Extremes: High-Strain-Rate Phenomena I
Sponsoring Units: GSCCM DCOMP DMPChair: Dawn Flicker, Sandia National Laboratories
Room: 502
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L26.00001: Plasticity induced by pre-existing defects during high strain-rate loading Invited Speaker: Eduardo Bringa High strain-rate deformation of metals has been typically studied for perfect monocrystals. Computational advances now allow more realistic simulations of materials including defects, which lower the Hugoniot Elastic Limit, and lead to microstructures differing from the ones from perfect monocrystals. As pre-existing defects one can consider vacancy clusters, dislocation loops, grain boundaries, etc. New analysis tools allow analysis of dislocation densities and twin fractions, for both f.c.c. and b.c.c. metals. Recent results for defective single crystal Ta [Tramontina et al.., High Energy Den. Phys. 10, 9 (2014), and Ruestes et al., Scripta Mat. 68, 818 (2013)], and for polycrystalline b.c.c metals [Tang et al., Mat. Sci. Eng. A 580, 414 (2013), and Gunkelmann et al., Phys. Rev. B 86, 144111 (2012)] will be highlighted, alongside new results for nanocrystalline Cu, Ta, Fe, and Zr [Ruestes et al., Scripta Mat. 71, 9 (2014)]. This work has been carried out in collaboration with D. Tramontina, C. Ruestes, E. Millan, J. Rodriguez-Nieva, M.A. Meyers, Y. Tang, H. Urbassek, N. Gunkelmann, A. Stukowski, M. Ruda, G. Bertolino, D. Farkas, A. Caro, J. Hawreliak, B. Remington, R. Rudd, P. Erhart, R. Ravelo, T. Germann, N. Park, M. Suggit, S. Michalik, A. Higginbotham and J. Wark. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L26.00002: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L26.00003: Anomalous plasticity in defect-mediated phase transformations Punam Ghimire, R. Ravelo, T.C. Germann Large-scale molecular dynamics simulations of shocked wave propagation in metallic single crystals exhibit high elastic limits and are ideally suited for investigating the role defect nucleation and multiplication play on the kinetics of phase transformations. Here we report on the morphology and kinetics of shocked-induced phase transformations in Aluminum single crystals. The atomic interactions were modeled utilizing various embedded atom method (EAM) models of Aluminum, with most models exhibiting an artificial fcc$\to$bcc phase transformation in the 25-30 GPa range. For cases where plastic deformation precedes the phase transformation, anomalous defect structures atypical of plastic deformation in bcc lattices nucleate early on but anneal out with time. In all cases, the defect-mediated phase transitions proceed at faster rates than defect-free ones. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L26.00004: High velocity sliding at polycrystalline ductile metal interfaces J.E. Hammerberg, J.L. Milhans, R. Ravelo, T.C. Germann We present the results of large scale 3-dimensional NonEquilibrium Molecular Dynamics (NEMD) simulations for Al-Al and Al-Ta interfaces for sliding velocities in the range 20-4000 m/s at pressures of 15 GPa. System sizes include 8 M, 26 M and 138 M atoms for times to 40 ns. We discuss polycrystalline samples with initial grain sizes of 13 nm and 20 nm. For velocities above a size dependent critical velocity, v$_{c}$, the frictional force per unit area agrees with single crystal simulations. For velocities below v$_{c}$, the polycrystalline interfaces evolve to a new steady state grain size distribution characterized by very large plastic deformation with larger grain sizes, time dependent coarsening and refinement, a graded size distribution in the direction normal to the sliding interface, and significantly larger frictional forces per unit area compared to similar single crystal sliding interfaces. We also find that for the Al-Ta interface the frictional properties are determined by the weaker Al material. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L26.00005: A new dynamic method for determining the frictional force between ductile metals at high velocities and compressions E.N. Loomis, J.C. Cooley, J.E. Hammerberg, G.T. Gray III, C.A. Bronkhorst We present a new dynamic method for determining the frictional force between ductile metals at ns and $\mu$s time scales under shock loading conditions. The method uses laser driven plate impacts at the LANL TRIDENT Laser Facility to launch a shock wave into a target consisting of a central cylindrical plate of Be and an outer ring of Cu. The Be/Cu interface is at a 6 degree angle to the shock direction. The interface behavior is diagnosed using line-imaging velocity interferometry (line-VISAR) and surface imaging displacement interferometry (TIDI) in the region of the interface on the target rear surface (away from the impact). The TIDI diagnostic gives surface information with a 600 $\mu$m x 600 $\mu$m field of view and out of plane displacement information with 10s of nm sensitivity using gated, fast framing cameras. Using these diagnostics we extract the surface profile near the interface and from numerical continuum materials dynamics simulations determine the interfacial frictional force and its velocity and pressure dependence. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L26.00006: Sound speed measurements in shock compressed tantalum Robert Scharff, Paulo Rigg, Robert Hixson Shock compression experiments were performed on tantalum to determine the longitudinal sound speed on the Hugoniot from 36 to 105~GPa. Tantalum samples were impacted directly on to lithium fluoride windows at velocities ranging from 2.5 to 5.0~km/s and the resulting particle velocity profiles at the sample/window interface were recorded using optical velocimetry techniques. The time of arrival of the rarefaction wave from the back surface of the tantalum sample was then used to determine the longitudinal sound speed at the corresponding impact stress. In contrast to recently reported work, we see no evidence of a phase transition in the tantalum in this stress range. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L26.00007: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 9:48AM - 10:24AM |
L26.00008: Plastic flow and lattice dynamics experiments on shock and ramp loaded solid-state samples at extreme pressures and strain rates Invited Speaker: Bruce Remington Experiments are being done on high power lasers, such as the Omega laser at LLE and the Janus and NIF lasers at LLNL, to probe the solid-state plastic response of materials to high pressure (50-500 GPa), and very high strain rate deformation (1.e6 - 1.e10 1/s). Two classes of experiments will be described. Dynamic Laue diffraction experiments with a time resolution of $\sim$0.1 ns have been developed to probe the microscopic lattice response of single crystal samples to a strong shock. In particular, the time scale for the onset of plasticity and the rate of the 1D to 3D lattice relaxation are a direct measure of how rapidly dislocations can be generated and transported on sub-nanosecond time scales (lattice kinetics). Macroscopic plastic flows at high pressure and strain rate can be generated that span a few tens of nanoseconds by using the Rayleigh-Taylor or Richtmyer-Meshkov fluid instabilities. Results from both classes of experiments will be compared with simulations using various models of flow stress (strength), a multi-scale model for bcc strength, and with analytic theory, where possible. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L26.00009: A Backward Characteristics Method for Impact Test Haibo Xu, Hao Pan The laser velocity interferometer has becoming an important method to study the dynamic response of materials under shock loading. A backward characteristics method considers the interaction between incident waves and reflects waves. This method can give more reasonable analysis results from the interface/freesurface velocity of the sample under test. The mechanical variables under adiabatic releasing can be obtained. Comparing with the simulationof the impact test of Tantalum, the sound speed vs. particle velocity, stress vs. Volume strain calculated by the backward characteristics method are according with the results gived by the hydrodynamics code. The backward characteristics method is used to analysis the reverse impact tests of Tin and more wealthy and important information about phase transiton under shock loading, yield strength and adiabatic releasing path is obtained. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L26.00010: The effect of moisture content on the dynamic fragmentation of wet sand at high strain rates Kun Xue A comprehensive model is established to account for the instability onset of rapidly expanding granular shells subject to the explosion loadings generated by the detonation of the central explosives. The moisture content strongly influences the shock interactions in the wet particle beds and the ensuing evolvement of the granular compacts. A material model for granular materials which can account for the degree of saturation was incorporated into a non-linear dynamic simulation program to investigate the moisture of effect on the shock responses of wet granular materials. In conjunction with our instability model, the predicted instability diameters of the expanding dry/wet granular shells are in a good agreement with the experimental results. Particularly the postponed instability onset of the wet granular shell found both experimentally and analytically can largely be attributed to the significantly greater kinetic energy obtained by wet particles thanks to less energy of shock wave consumed in compacting the granular materials. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L26.00011: ABSTRACT WITHDRAWN |
Session L27: Electronic Structure Methods III
Sponsoring Units: DCOMPChair: James Shepherd, Rice University
Room: 501
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L27.00001: Probing the electronic structure of liquid water with many-body perturbation theory Tuan Anh Pham, Cui Zhang, Eric Schwegler, Giulia Galli We present a first-principles investigation of the electronic structure of liquid water based on many-body perturbation theory (MBPT), within the G$_0$W$_0$ approximation. The liquid quasiparticle band gap and the position of its valence band maximum and conduction band minimum with respect to vacuum were computed and it is shown that the use of MBPT is crucial to obtain results that are in good agreement with experiment. We found that the level of theory chosen to generate molecular dynamics trajectories may substantially affect the electronic structure of the liquid, in particular, the relative position of its band edges and redox potentials. Our results represent an essential step in establishing a predictive framework for computing the relative position of water redox potentials and the band edges of semiconductors and insulators.\\[4pt] [1] T. Anh Pham, C. Zhang, E. Schwegler and G. Galli (submitted). [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L27.00002: Hybrid DFT First-Principles Study of the Properties of Water Martin Schlipf, Francois Gygi Water plays a crucial role in many chemical and biological reactions. Hence, an accurate description of the properties of water is imperative for a detailed understanding of these reactions. In recent years, hybrid density functionals such as PBE0 and HSE06 have improved the accuracy of DFT calculations for many insulators and semiconductors, as well as for aqueous solutions[1]. However, the evaluation of the Hartree Fock (HF) exchange energy makes these functionals computationally very demanding. We present a comparison of the structural and electronic properties of water obtained using the PBE0 and HSE06 density functionals. Simulations were performed using the Qbox code[2], and a recursive bisection scheme[3] reducing the computational cost of HF integrals. We discuss the effect of this approximation on the accuracy and the computation time, and compare our results to related work[4-5]. \\{} [1] C. Zhang, {\it et al.}, J. Chem. Phys. {\bf 138}, 181102 (2013).\\{} [2] http://eslab.ucdavis.edu/software/qbox \\{} [3] F. Gygi and I. Duchemin, J. Chem. Theory Comput. {\bf 9}, 582 (2012).\\{} [4] T. Todorova, {\it et al.}, J. Phys. Chem. B {\bf 110}, 3685 (2006).\\{} [5] M. Guidon, {\it et al.}, J. Chem. Phys. {\bf 128}, 214104 (2008). [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L27.00003: Accurate calculation of the x-ray absorption spectrum of water via the GW/Bethe-Salpeter equation Keith Gilmore, John Vinson, Josh Kas, Fernando Vila, John Rehr We calculate x-ray absorption spectra (XAS) of water within the OCEAN code, which combines plane-wave, pseudopotential electronic structure, PAW transition elements, GW self-energy corrections, and the NIST BSE solver [1]. Due to the computational demands of this approach, our initial XAS calculations were limited to 17 molecule super cells [2]. This lead to unphysical, size dependent effects in the calculated spectra. To treat larger systems, we extended the OCEAN interface to support well-parallelized codes such as QuantumESPRESSO. We also implemented an efficient interpolation scheme of Shirley. We applied this large-scale GW/BSE approach to 64 molecule unit cell structures of water obtained from classical DFT/MD and PIMD simulations [3]. In concurrence with previous work [4], we find the calculated spectrum both qualitatively and quantitatively reproduces the experimental features. The agreement implies that structures based on PIMD, which are similar to the traditional distorted tetrahedral view, are consistent with experimental observations. \\[4pt] [1] J. Vinson et al., PRB 83, 115106 (2011); J. Vinson and J.J. Rehr, PRB 86, 195135 (2012). [2] J. Vinson et al., PRB 85, 045101 (2012). [3] J.A. Morrone and R. Car, PRL 101, 017801 (2008). [4] L. Kong et al., PRB 86, 134203 (2012). [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L27.00004: Sum frequency generation spectra of ice surfaces from first principles simulations Quan Wan, Francois Gygi, Giulia Galli The morphology and structure of ice surfaces play an important role in a variety of chemical and physical processes. Yet a detailed knowledge of the structural properties of ice surfaces at the molecular level is still lacking. Sum frequency generation (SFG) spectroscopy is a promising technique to address this problem, due to its high interface sensitivity. Here we present first principles simulations of SFG spectra of ice surfaces obtained by \textit{ab initio} molecular dynamics, combined with density functional perturbation theory (DFPT), as implemented in the Qbox Code. We computed SFG signals from classical time correlation functions of the system's dipole moment and polarizability tensor, evaluated by using maximally localized Wannier functions and DFPT. By projecting the total SFG intensities onto each molecules, we analyzed the stretching band region of the SFG spectra and identified intra- and inter-bilayer modes. This analysis was then used to shed light on whether the ice surface is proton ordered or disordered. In addition to ice, we will also discuss simulations of SFG signals obtained for solid-water interfaces using the same \textit{ab initio} method. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L27.00005: Energy Level Alignment at Aqueous GaN and ZnO Interfaces Mark S. Hybertsen, Neerav Kharche, James T. Muckerman Electronic energy level alignment at semiconductor-electrolyte interfaces is fundamental to electrochemical activity. Motivated in particular by the search for new materials that can be more efficient for photocatalysis, we develop a first principles method to calculate this alignment at aqueous interfaces and demonstrate it for the specific case of non-polar GaN and ZnO interfaces with water. In the first step, density functional theory (DFT) based molecular dynamics is used to sample the physical interface structure and to evaluate the electrostatic potential step at the interface. In the second step, the GW approach is used to evaluate the reference electronic energy level separately in the bulk semiconductor (valence band edge energy) and in bulk water (the 1b$_{1}$ energy level), relative to the internal electrostatic energy reference. Use of the GW approach naturally corrects for errors inherent in the use of Kohn-Sham energy eigenvalues to approximate the electronic excitation energies in each material. With this predicted interface alignment, specific redox levels in water, with potentials known relative to the 1b$_{1}$ level, can then be compared to the semiconductor band edge positions. Our results will be discussed in the context of experiments in which photoexcited GaN and ZnO drive the hydrogen evolution reaction. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L27.00006: Structural and electronic properties of NaCl dissolved in water Alex P. Gaiduk, Francois Gygi, Giulia Galli We carried out \emph{ab initio} molecular dynamics simulations of a 1 M NaCl aqueous solution with the \emph{Qbox} code, using the hybrid functional PBE0 and the bisection technique introduced in Ref.~[1] to compute the Hartree--Fock exchange. We performed both NVT and NVE simulations. We found that the position of the Cl$^-$ and Na$^+$ energy levels is considerably improved compared to the one obtained using the PBE functional, and that the anion highest occupied orbital [2] is unaffected by the presence of the sodium counterion. We also found that the average properties obtained in the NVE ensemble and those computed in the NVT ensemble with the Bussi--Donadio--Parrinello thermostat [3] are the same, within the statistical error bars of our simulations.\\[4pt] [1] F. Gygi and I. Duchemin, \emph{J.~Chem.\ Theory\ Comput.} \textbf{9}, 582 (2013).\\[0pt] [2] C. Zhang, T. Anh Pham, F. Gygi, and G. Galli, \emph{J.~Chem.\ Phys.} \textbf{138}, 181102 (2013).\\[0pt] [3] G. Bussi, D. Donadio, and M. Parrinello, \emph{J.~Chem.\ Phys.} \textbf{126}, 014101 (2007). [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L27.00007: \textit{Ab-Initio} Calculations of the Electronic Properties of Boron Nitride Anthony Stewart, Bethuel Khamala, Daniel Hart, Diola Bagayoko The potential of Boron Nitride (BN) in nanotechnology is tremendous. BN in its bulk form has a wide band gap with excellent thermal and chemical stability. BN structures can be tailored using various techniques in order to obtain desired materials properties. The State-of-the-art Proton Exchange Membrane Fuel Cell (PEMFCs) technology exploits graphitized carbon as a support for platinum-type catalysts.~ However, some forms of carbon are susceptible to long-term durability issues such as corrosion which is a detriment to fuel cell performance and viability. Novel non-carbon supports such as BN may provide a pathway for addressing the durability and performance issues associated with carbon support materials. We present preliminary theoretical studies, using an linear combination of atomic orbital (LCAO) quantum chemistry package from Ames Laboratory, of the electronic properties of this potentially important material. Our calculated band gap of 6.48 eV for the cubic structure, obtained with an LDA potential and the BZW-EF method, is in agreement with experiment. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L27.00008: Ab-initio atomic level stresses in Cu-Zr systems Madhusudan Ojha, Don M. Nicholson, Takeshi Egami In our recent studies [D. M. Nicholson, Madhusudan Ojha and Takeshi Egami, J. Phys.: Condens. Matter 25 435505 (2013)] we have calculated ab-initio atomic level stresses in the simple B2 Cu-Zr system, Cu50Zr50 liquid and glass and have found tremendous atomic level stress in the B2 structure due to strong bonding between Cu and Zr and significantly smaller atomic level stresses in liquid and glass due to reduced chemical order. We have extended our studies to additional structures and stoichiometries. On the basis of these results we discuss the relationship between short-range order, bonding, electronic density of states and atomic level stress. We are searching for an explanation of the unique position of Zr as a promotor of glass forming ability. We report the differences in atomic level stress, bonding, and density of states when Ti, Y, and Nb replace Zr on fixed structures. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L27.00009: Ab initio DFT calculations of vibrational properties S.M. Story, F.D. Vila, J.J. Kas, J.J. Rehr Vibrational properties such as EXAFS and crystallographic Debye-Waller factors, vibrational free energies, phonon self-energies, and phonon contributions to the electron spectral function, are key to understanding many aspects of materials beyond ground state electronic structure. Thus, their simulation using first principles methods is of particular importance. Many of these vibrational properties can be calculated from the dynamical matrix and electron-phonon coupling coefficients obtained from DFT calculations. Here we present a code DMVP [1] that calculates these properties from the output of electronic structure codes such as ABINIT, Gaussian, Quantum Espresso and VASP. Our modular interfacing tool AI2PS allows us to translate the different outputs into a DMVP compatible format and generate vibrational properties in an automated way [2]. Finally, we present some current applications that take advantage of the modular form of AI2PS to extend its capabilities to the calculation of coefficients of thermal expansion and other properties of interest such as infrared spectra. \\[4pt] [1] F. D. Vila et al., Phys. Rev. B {\bf 76}, 014301 (2007). \\[0pt] [2] J.J. Rehr et al., C. R. Physique {\bf 10} (2009). [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L27.00010: End-group Influence on the Frontier Molecular Orbital Reorganization in Molecular Junctions --- Effect on Thermopower Janakiraman Balachandran, Pramod Reddy, Barry Dunietz, Vikram Gavini The frontier molecular orbital (FMO) reorganization and in turn on the thermopower of the aromatic molecules trapped between metal electrodes (aka molecular junctions) depends on two effects namely (1) the stabilization effect -- due to the physical presence of the metal electrode atoms and (2) change in e-e interactions -- due to end-group mediated charge transfer. The stabilization effect always reduces the FMO energies. The charge transfer effect increases the FMO energies in charge-gaining molecules, which in turn opposes the stabilization effect resulting in a small overall shift. However, the charge transfer effect decreases the FMO energies in charge-losing molecules, which in turn complements the stabilization effect resulting in a large overall downward shift. This hypothesis is validated by delineating the shifts due to stabilization and charge-transfer effects independently. Further we also demonstrate the generality of the hypothesis by applying it on a wide range of aromatic molecules with different length and end-groups. Finally, we also present computationally efficient strategies, based on the proposed mechanism, to quantitatively compute the FMO reorganization which in turn has potential for high throughput analysis of molecular junctions. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L27.00011: Momentum-dependent local ansatz approach to correlated electrons Yoshiro Kakehashi, Sumal Chandra, M. Atiqur R. Patoary Variational approach has been a simple and useful tool to describe the ground state of correlated electrons in solids, and many methods as well as wavefunctions for electron correlations have been proposed. Although the numerical methods such as the variational Monte-Carlo can quantitatively describe the correlations in the low dimensional system, the description of the real 3D system based on the wavefunction method has not yet been established well. We present here the momentum-dependent local ansatz approach (MLA) to the correlated electron system. The wavefunction consists of the two-particle excitation operators with momentum-dependent variational parameters, which are projected onto local orbitals, and a hybrid wavefunction which interpolates between the Hartree-Fock and the Hubbard alloy-analogy wavefunctions. On the basis of the numerical calculations in infinite dimensions, we demonstrate that the analytic MLA improves the Gutzwiller method in both the weak and strong interaction regimes, and that the MLA is applicable to the realistic systems with use of the 1st-principles LDA+U Hamiltonian. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L27.00012: Effects of local atomic environment and atomic long range order on magnetism in Fe-Al alloys Sergii Khmelevskyi Using first-principle Local Self-Consistent Green Function (LSGF) method and Local Spin Density Approximation we have studied local environment effects on magnetism of Fe in FeAl alloys near and at 1:1 atomic composition. The long range atomic order (LRO) in the alloys has been varied from complete disorder (A1) to full order (B2). We have derived the dependence of the Fe magnetic moments on local atomic environment up to second nearest neighbor (NN) shell, which is found to exhibit a highly not-trivial behavior. In order to explain a sharp difference in the magnetic behavior of ordered and disordered Fe-Al alloys we calculate the inter-atomic magnetic exchange interactions and their dependence on the state of the atomic LRO using conventional Lichtenstein formalism. We have found strong antiferromagnetic interactions between Fe atoms on long distance NN shells of underlying bcc lattice, which compete in size with ferromagnetic one on the first NN shells. The magnitude of these interactions is strongly depending on the state of the atomic LRO and this effect is the reason for onset of few different spin-glass regimes in the partially ordered Fe-AL alloys. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L27.00013: Applications of DFT to Lanthanides and Precious Metal Complexes Alex Balboa, Margaret Hurley, Amanda Jenkins Density functional theory is widely used for computational characterization of novel materials. While study of materials containing the lighter elements is commonplace, the application of these methods to the bottom of the periodic table, including the lanthanides and the heavier precious metals such as Osmium, requires careful validation. Here we present results of recent quantum mechanical studies to characterize Lanthanide/Graphene Materials and assess the suitability of DFT for these systems. Additionally, we will present recent work on similar application of DFT to characterize Os bipridine complexes. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L27.00014: First-Principles Calculations of Magnetic Properties of MnBi doped with Co Chandani N. Nandadasa, Vivek Dikshith, Sungho Kim, Seong-Gon Kim, Jihoon Park, Yang-Ki Hong First principles total-energy calculations were performed to investigate the magnetic and electronic properties of MnBi doped with Co. We used Density Functional Theory (DFT) within the generalized gradient approximation (GGA) with Projector Augmented Wave (PAW) potentials. We found that when MnBi was doped with Co,the magnetization increased as the concentration of Co increased. We also calculated magnetic anisotropy energy (MAE) and magnetic anisotropy constant ($K_u$) of MnBi before and after doping Co. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L27.00015: Effect of Orbital Differentiation on Magnetism in the 111 iron-based superconductors Ming-Cui Ding, Yu-Zhong Zhang Though similar lattice and electronic structures were found in MgFeGe, LiFeAs and NaFeAs, distinct behaviors at low temperature were reported from experiments. While MgFeGe is nonmagnetic and non-superconducting down to $2$~K, LiFeAs is a good superconductor with T$_c=18$~K, which are in sharp contrast to NaFeAs where a magnetically driven structural phase transition above the superconducting transition is detected. By calculating orbitally resolved Pauli susceptibility, we conclude that the occurrence of magnetism and superconductivity in these materials can be well interpreted from the weak coupling limit provided orbital degrees of freedom are considered.The stronger magnetic instability appearing in the d$_{x^2-y^2}$ orbital is responsible for the occurrence of weak magnetism in NaFeAs, compared to the superconducting LiFeAs, while featureless q dependent magnetic instability is responsible for the nonmagnetism in MgFeGe. [Preview Abstract] |
Session L28: Tutorial for Authors and Referees
Sponsoring Units: APSRoom: 601
Wednesday, March 5, 2014 8:00AM - 9:30AM |
L28.00001: Tutorial for Authors and Referees Editors from \textit{Physical Review Letters} and \textit{Physical Review} will provide information and tips for less experienced referees and authors. Topics for discussion will include advice on how to write good manuscripts, similarities and differences in writing referee reports for PRL and PR, and other ways in which authors, referees, and editors can work together productively. Following a short presentation from the editors, there will be a moderated discussion. A light breakfast of bagels, pastries, coffee, and tea will be served. [Preview Abstract] |
Session L29: Electronic and Valleytronic Properties of Graphene
Chair: Michael Fogler, University of California, San DiegoRoom: 603
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L29.00001: Magnetic field confinement in graphene with Gaussian deformations Martin Schneider, Daiara Faria, Silvia Viola Kusminskiy, Nancy Sandler It has been proposed that graphene holds the potential for novel transport properties under the combined effect of deformations and external fields. Strain produced by deformations results in a pseudomagnetic field that substantially modifies the real space density of states. Analogously, external magnetic fields provide a controllable mechanism to confine states in graphene. To investigate how strain and magnetic fields combine to produce peculiar electronic properties, we study a model for graphene in the presence of an out-of-plane deformation, in the continuum limit. In particular, we focus on a Gaussian height profile that produces an inhomogeneous pseudomagnetic field with trigonal symmetry. We address the question of confinement of electrons due to this deformation, using a scattering formalism based on the Dirac equation description of graphene. Our results reveal a space dependent enhancement of the local density of states as the deformation is introduced. In analogy with the Landau level formation and confined states produced by constant magnetic fields, we discuss the formation of local Landau levels and confined states by the deformation and how the two combined effects affect the transport properties of the system. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L29.00002: Trapping electrons in graphene in a rotating saddle Johan Nilsson We consider particle motion in rotating saddle-shaped potentials. It is known that such rotating potentials can generate bounded motion for particles with a parabolic dispersion law through the combination of potential, centrifugal and Coriolis forces in the rotating frame. When applied to massless Dirac particles, for example electrons in graphene, such a potential is shown to lead to eigenstates that are spatially localized near the center of the saddle at certain energies. Although other states also exist at these energies, they have non-overlapping support in the oscillator basis, which tend to give the localized states a substantial life-time also when imperfections are present. Reference: J. Nilsson, PRL 111, 100403 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L29.00003: Variation of electronic and magnetic properties of bilayer zigzag graphene nanoribbons by sliding and electric field Ramazan Tugrul Senger, Mehmet Yagmurcukardes Structural, electronic and magnetic properties of bilayer zigzag graphene nanoribbons (BZGNR) are studied using density functional theory methods. We find that ground state stacking geometry of the layers depends on the width of BZGNR. Energy bandgap , edge-localized magnetic moments and the magnetic ordering are all modified by mechanical sliding of the layers and/or by external applied electric fields. These effects can be utilized in design of electro-mechanical and magneto-mechanical nano devices. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L29.00004: Tunable Quantum Temperature Oscillations in Graphene and Carbon Nanoribbons Justin Bergfield, Mark Ratner, Charles Stafford, Massimiliano Di Ventra We investigate the local electron temperature distribution in carbon nanoribbon (CNR) and graphene junctions subject to an applied thermal gradient. Using a realistic model of a scanning thermal microscope, we predict quantum temperature oscillations whose wavelength is related to that of Friedel oscillations but is not directly related to the local density of states. Experimentally, this wavelength can be tuned over several orders of magnitude by gating/doping, bringing quantum temperature oscillations within reach of the spatial resolution of existing measurement techniques. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L29.00005: Electron-phonon vertex correction and Migdal's theorem in monolayer graphene Bitan Roy, Jaydeep Sau Corrections to the electron-phonon vertex in three dimensional Fermi liquid system scales as the ratio of the electric to the ionic mass, and are therefore negligible. This outcome is often referred as Migdal's theorem. In this talk we will briefly review the applicability of the Migdal's theorem for non-relativistic Fermi liquids in two and one spatial dimensions. In th later part of the talk we will concentrate on the electron-phonon vertex corrections for quasi-relativistic Dirac fermions in graphene. We here consider take into account only the acoustic phonon and its coupling with Dirac fermions. We will present the relevance of the electron-phonon vertex corrections, which otherwise depends only the ratio of the velocity of acoustic phonon and the Fermi velocity. If time permits, relevance of the electron-phonon vertex function in bilayer graphene will also be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L29.00006: Tunneling through graphene and topological insulators in presence of pn junction: transport properties and device prospects Redwan Sajjad, K.M. Masum Habib, Frank Tseng, Avik Ghosh We emphasize the role of pn junction (PNJ) in graphene electrical transport. In the ballistic regime, the resistance depends upon two key factors -- length to width aspect ratio and the PNJ formed between the doped region of graphene under metal contact. In the diffusive limit, these remain the deciding factors for the minimum conductivity. The PNJ allows us to demonstrate Klein tunneling - by either creating a PNJ electrostatically within the device or through the nature of Fabry-Perot oscillation between the two contacts. We then discuss the details of electron transport - the nature of peak device resistance, minimum contact resistance achievable with commonly used metals, effects such as electron hole asymmetry and negative differential resistance -- all being affected by the multiple PNJs formed near the contacts. We then show that PNJ acts as a filter for pseudo-spins in graphene and how this can be manipulated for gate modulation of resistance. The existence of a Dirac cone on the surface of a topological insulator has the potential of similar filtering action but for real spins instead of pseudo-spins. We adopt Non-Equilibrium Green's Function (NEGF) formalism and compare results with recent transport measurements. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L29.00007: Valley Polarization and Transport in Dual Gated Bilayer Graphene Patrick Maher, Kenji Watanabe, Takashi Taniguchi, Philip Kim The low energy band structures of graphene and its bilayer contain a valley degeneracy due to the two inequivalent points in the Brillouin zone. In bilayer graphene, this degree of freedom can be experimentally controlled through the breaking of layer symmetry by a transverse electric field. Notably, this can open a band gap at charge neutrality. Additionally, breaking of layer symmetry can give rise to broken symmetry quantum Hall states, and there are predictions that it can be used to create topological kink states. We report on transport measurements of ultra high quality dual-gated bilayer graphene samples encapsulated in hexagonal boron nitride. Our fabrication method involves no direct exposure of the graphene to resist, resulting in exceptionally low-disorder. In a magnetic field, we observe tunable symmetry-broken quantum Hall states. In addition, through the use of aligned split top and bottom gates, we can study transport along the one dimensional boundary between electric fields of opposite polarity. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L29.00008: VOI-based valley filters and valley valves in bilayer graphene quantum wires Ning-Yuan Lue, George Yu-Shu Wu Electrons in graphene have an inherent valley degree of freedom called valley pseudospin. Based on the valley-orbit interaction(VOI) in gapped graphene[1], we propose a valley filter/valve in gapped bilayer graphene(BLG)-based quantum wire device, and it's consistent with planar processing and is operated with electric gates producing an in-plane electric field transverse to the wire. The device consists of a quantum wire patterned in BLG by electrical gates, with the vicinity of the quantum wire being oxidized (or implanted with a line of point defects parallel to the wire). The transverse electric field produces a Rashba-type splitting in the valley subbands, and the oxidation (or defects) opens a pseudogap at the point where the two subbands cross. Valley polarization is generated when placing the Fermi level inside the pseudogap. We discuss the pseudogap, the valley polarization, and their dependence on the strength of the electric field and the distance between the oxidized region and the quantum wire. When the electric field is reversed, opposite valley polarity is attained. Therefore, the proposed valley filter can also be put together to form a valley valve. [1] Wu et al., PRB 84, 195463(2011); PRB 88, 125422(2013); Lee et al., PRB 86, 165411(2012). [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L29.00009: The effect of Landau level mixing on the fractional quantum Hall effect in spin and valley polarized graphene Michael Peterson, Chetan Nayak The fractional quantum Hall effect (FQHE) in graphene presents many theoretical and experimental challenges and is not yet fully understood even in the lowest Landau level (LL). Besides spin and valley degrees of freedom being important, LL mixing is also important since it does not depend on the strength of the magnetic field (in contrast to the FQHE in semiconductors, i.e., parabolic bands) but instead depends on the dielectric of the substrate or lack thereof. Recently, we have produced an effective Hamiltonian for the FQHE in graphene that incorporates the effects of LL mixing. As a first step, we numerically study the FQHE in spin and valley polarized graphene in both the lowest and first excited LL while fully incorporating LL mixing. We find the interesting results that the lowest LL of graphene is nearly identical to that of semiconductors, even in the presence of LL mixing, and the anti-Pfaffian is stabilized in the half-filled first LL under moderate LL mixing. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L29.00010: Spin and valley skyrmions in Landau levels $\left\vert N\right\vert \geq 1$ of graphene and bilayer graphene Rene Cote, Wenchen Luo In a two-dimensional electron gas (2DEG), skyrmions are the lowest-energy charged excitations at filling factor $\nu =1$ while, for the chiral 2DEG in graphene, valley and spin skyrmions can exist up to Landau level $\left\vert N\right\vert =3$ $[1]$. In this talk, we discuss the excitation energy of spin and valley skyrmions in Landau level $\left\vert N\right\vert \geq 1$ in both graphene and bilayer graphene. In graphene, we consider a finite Zeeman term in order to compute the range of Zeeman coupling for which skyrmions are the lowest-energy charged excitations. We also show how the excitation energy is modified when screening is considered $[2]$. In bilayer graphene, we first derive the phase diagram of the chiral 2DEG at integer filling of the quartet of states in Landau levels $\left\vert N\right\vert \geq 1$ and show how a finite potential difference applied between the two layers can control the spin and pseudospin polarizations. We then compute the excitation energy of valley and spin skyrmions by using an anisotropic $\sigma $ model derived from the Hartree-Fock Hamiltonian and adding screening corrections.\\[4pt] [1] Kun Yang, S. Das Sarma, and A. H. MacDonald, Phys. Rev. B 74,075423 (2006).\\[0pt] [2] Wenchen Luo and R. Cote, Phys. Rev. B 88, 115417(2013). [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L29.00011: Reading valley-hybridization and universal symmetry of graphene with mesoscopic conductance fluctuations Vidya Kochat, Atindra Nath Pal, Arindam Ghosh In graphene, the K and K' valleys act as spin-like entities, and can form the basis of valley-based electronics, having applications ranging from valley-based quantum computation, to valley filters or polarizers. The valleys hybridize to form new quantum states, such as the valley singlet and triplets, that lead to anti-localized quantum transport, non-locality and flavour Hall effect. Here we demonstrate a direct route for reading and manipulating the valley coherent states of disordered graphene by measuring the mesoscopic conductance fluctuations. We observe that the conductance fluctuations in graphene at low temperatures are reduced by a factor of four at high carrier densities, due to the gapping out of valley triplet states by short-range disorder. We also show that this results in a gate-tunable universal symmetry class, which is yet another unique and fundamental feature of the 2D honeycomb lattice of graphene. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L29.00012: Valley-dependent resonant inelastic transmission through a time-modulated region in graphene C.S. Chu, L. Chang, T.L. Liu Valley-dependent transmission is one of the key physical characteristics for valleytronics. In this work, we focus upon the valley-dependent nature of the quantum transport through a time-modulated-potential region in graphene, when the incident flow is collimated, with a given group velocity direction. Of particular interest is the interplay between the resonant sideband process and the trigonal-warping. The former causes transmission dip-structures which condition of occurrence is determined by sideband processes to a relevant band edge. The latter causes the relevant band-edge energy to become valley-dependent. The relevant band is a fixed-$k_{y}$ projection of the graphene energy band, where $k_{y}$ (along the time-modulate region interface) is conserved in the transmission. The valley polarization$ P$ in the transmission, for valley-unpolarized incident collimated beam, is calculated. Based on our understanding on the above valley-dependent nature, ways to optimize $P$ will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L29.00013: The Rashba Type Splitting of Valley Pseudospins in Gapped Graphene Quantum Wires Yen-Chun Chen, Ning-Yuan Lue, Miao-Lin Lin, George, Yu-Shu Wu A semiconductor with a strong spin-orbit interaction (SOI) exhibits pronounced Rashba spin splitting when subject to an electric field. Although gapped graphene is a semiconductor with an extremely weak SOI, a spin-like electron degree of freedom called valley pseudospin, in association with the doubly degenerate energy band valleys at Dirac points (K and K'), exists in graphene [1] and interacts with the orbital degree of freedom via the so-called valley-orbit interaction (VOI) [2]. In the presence of an in-plane electric field, the VOI induces the pseudospin splitting similar to the Rashba spin splitting. Here, we report our recent numerical study of Rashba type splitting of valley pseudopsins in gapped (monolayer and bilayer) graphene quantum wires subject to in-plane transverse electric fields. \\[4pt] [1] K. S. Novoselov et al. Science \textbf{306},666(2004); A. H. Casto Neto et al. Rev. Mod. Phys. \textbf{81},109(2009); A. Rycerz et al. Nature Phys.\textbf{3},172-175(2007); \\[0pt] [2] G. Y. Wu et al. PRB \textbf{84},195463(2011); M. K. Lee PRB \textbf{86},165411(2011); G. Y. Wu et al. PRB\textbf{ 88},125422(2013) [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L29.00014: Valley-Spin Polarization in the Magneto-Optical Conductivity of Silicene and Other Buckled Honeycomb Lattices Calvin Tabert, Elisabeth Nicol The successful isolation of graphene made the field of two-dimensional (2D) crystals a reality. Recently, increasing attention has begun to focus on other 2D honeycomb systems such as those that map onto a low-energy Kane-Mele type Hamiltonian. These systems have an intrinsic band gap due to spin-orbit coupling. One such material is silicene, the silicon equivalent of graphene. Here, the silicon atoms form a buckled honeycomb lattice. This vertical buckling creates the possibility for a tunable band gap when an electric field is applied. As the electric field is varied, the system is predicted to transition between a topological insulator and a band insulator[1,2]. We show[3,4] that when this system is subjected to a magnetic field, we retain a Landau level (LL) spectrum similar to that of gapped graphene; however, the application of an electric field spin-splits the LLs at a given valley. By varying the electric field strength, one can elucidate signatures of the two insulating regimes. It is also possible to optically excite charge carriers of definite valley-spin polarization. [1]N.D. Drummond et al., PRB 85, 075423 (2012) [2]M. Ezawa, NJP 14, 033003 (2012) [3]C.J. Tabert and E.J. Nicol, PRL 110, 197402 (2013) [4]C.J. Tabert and E.J. Nicol PRB 88, 085434 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L29.00015: Disordered Dirac Fermions in Topological Superconductors and Artificial Graphene: Level Statistics, Chalker Scaling, and Quantum Hall Metal Yang-Zhi Chou, Matthew Foster We study a single Dirac fermion in 2D, subject to static vector potential disorder. This model describes the minimal surface state of a topological superconductor with time reversal and spin U(1) symmetries. Most of the zero-energy attributes are already known; in particular, the model is critically delocalized with multifractal wavefunctions. We numerically investigate various properties, especially those related to the finite energy states. The energy levels in the vicinity of zero energy (chiral point) show universal statistics while the multifractal spectra of the zero-energy wavefunction and the dynamical critical exponent reveal non-analyticity at the freezing transition. The two-wavefunction correlations in the chiral region show quantum critical scaling, even in the case of strong disorder. Moreover, we confirm that the finite energy states are delocalized, and their multifractal spectra are consistent with the integer quantum Hall plateau transition. Our model can also be possibly realized in the artificial materials like molecular graphene. [Preview Abstract] |
Session L30: Graphene Devices: Fabrication, Characterization and Modeling: Nanomechanics
Sponsoring Units: DMPChair: Cory Dean, City College of New York
Room: 605
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L30.00001: Mechanical Properties of Graphene on Surfaces Having Patterned Pyramid Arrays Stephen Gill, Henry Hinnefeld, William Swanson, Nadya Mason There is wide interest in the science and applications of strain-engineering graphene's physical properties. To explore the mechanical behavior of graphene under strain with triangular symmetry, we deposited graphene on fabricated arrays of pyramid-shaped protrusions that were patterned on both polydimethylsiloxane (PDMS) and SiO$_{\mathrm{2}}$ surfaces. Using atomic force microscopy (AFM), we studied the morphological and adhesion changes of graphene on pyramid arrays having different spacing. The strain was also examined using Raman spectroscopy. We show that the mechanical properties of graphene vary with the spacing of pyramids in an array. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L30.00002: Unconventional superconducting quantum interference in a suspended graphene resonator Monica Allen, Daniyar Nurgaliev, Anton Akhmerov, Amir Yacoby In a coherent electron cavity, quantum interference of electron waves replaces classical diffusion as a key feature of electronic transport. Here we report novel behavior that emerges by coupling superconducting reservoirs to a Fabry-Perot resonator in bilayer graphene. In this device, a pair of superconducting electrodes is coupled to a suspended graphene membrane and defines a ballistic cavity between the two graphene-electrode interfaces. Tuning the Fermi wavelength in the cavity with a gate electrode moves the system on and off resonance, thus inducing an oscillatory critical current whose period satisfies the Fabry-Perot interference conditions. By varying the magnetic flux through the junction, we explore the rich interplay between superconducting quantum interference and resonant cavity states and demonstrate a non-trivial correspondence between the supercurrent and normal state resistance. To describe our findings, we use a numerical model based on the tight-binding approach and Landauer-Buttiker scattering formalism. These results constitute a departure from the conventional Josephson effect in graphene and motivate exploration of new effects at the intersection of superconductivity and optics-like phenomena. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L30.00003: Tuning graphene properties by shear Andres Concha, Shengfeng Cheng, Lucian Covaci, L. Mahadevan Graphene being the thinnest possible membrane made out of carbon atoms is prone to deformations under slight external forcing. Here, we take advantage of this proneness to deformations to manipulate transport properties of graphene ribbons. We analyze the effect on conductance and LDOS of the spontaneous pattern produced when a wide ribbon is subject to shear. The deformation of the ribbon produces pseudo-magnetic fields, scalar potentials, and Fermi velocity renormalization resulting in the modification of transmission properties without the need of an external gate potential. Our proposal paves the way for producing electronic waveguides by using an elastic instability that spans from the nano to macro-scales. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L30.00004: Graphene Nanoelectromechanical Systems as Stochastic-Frequency Oscillators Tengfei Miao, Peng Wang, Brian Standley, Marc Bockrath We measure the quality factor $Q$ of electrically-driven few-layer graphene drumhead resonators, providing an experimental demonstration that $Q$ $\sim$ $1/T$, where $T$ is the temperature. We develop a model that includes intermodal coupling and tensioned graphene resonators, yielding good quantitative agreement to experiment. Because the resonators are atomically thin, out-of-plane fluctuations are large. As a result $Q$ is mainly determined by stochastic frequency broadening rather than frictional damping, in analogy to nuclear magnetic resonance. Additionally, at larger drives the resonance linewidth is enhanced by nonlinear damping, in qualitative agreement with recent theory of damping by radiation of in-plane phonons. Parametric amplification produced by periodic thermal expansion from the ac drive voltage yields an anomalously large linewidth at the largest drives. Our results contribute towards a general framework for understanding the mechanisms of dissipation and spectral line broadening in atomically thin membrane resonators. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L30.00005: Focused Ion Beam patterning of suspended graphene for cantilever and kirigami devices Peter Rose, Pinshane Huang, Melina Blees, Arthur Barnard, David Muller, Paul McEuen We have developed techniques that use a Focused Ion Beam (FIB) to cut and manipulate suspended graphene. Using a dual-beam FIB, we can make cuts with a resolution of tens of nanometers, manipulate and pick up finished devices using a micromanipulator, and remove device and micromanipulator from the vacuum chamber. Remarkably, we have demonstrated that singly clamped graphene cantilevers can be fabricated reliably and are robust enough to be freely manipulated in air. This gives us the potential to perform novel electrostatic and mechanical measurements of graphene. Using the FIB's direct writing capabilities, we are also able to cut out more complex shapes, drawing inspiration from kirigami, the art of paper cutting. Using specific cuts, we can create soft in-plane springs, which might be used to study tension. This exploration of the fabrication and manipulation of graphene in three dimensions is a promising new avenue toward harnessing graphene's unique properties, and also holds promise for other 2D materials. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L30.00006: Intrinsic mode-coupling and thermalization in nanomechanical graphene drums Daniel Midtvedt, Zenan Qi, Alexander Croy, Harold S. Park, Andreas Isacsson Nanomechanical graphene resonators display strong nonlinear behavior, which leads to coupling between normal modes. This coupling allows for intermodal energy-transfer, which facilitates the redistribution of energy initially localized in a single mode. Further, the mode-coupling intrinsically limits the quality factor of the device. We study the mode-coupling in a circular graphene resonator using molecular dynamics and continuum mechanics. Mimicking a ring-down setup, the fundamental mode is excited with a given energy, and the time-evolution of this energy is computed. At $T>0$, we find a relaxation rate independent of system size and proportional to $T^*/\epsilon_{\rm pre}^2$, where $T^*$ is the effective temperature and $\epsilon_{\rm pre}$ is the pre-strain of the system \footnote{D. Midtvedt, Z. Qi, A. Croy, H. S. Park, A. Isacsson,arXiv:1309.1622}. At low temperatures, the system enters a metastable state where only very few low-frequency modes are excited, the life-time of which increases exponentially with decreasing excitation energy. This is similar to what is seen in the much studied Fermi-Pasta-Ulam (FPU) problem. We make a detailed comparison between the dynamics of a graphene drum and the FPU system, and propose to use graphene drums as test beds for FPU physics. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L30.00007: Mechanical Nonlinearity in Graphene Resonators Isaac Storch, Robert Barton, Roberto De Alba, Vivekananda Adiga, Harold Craighead, Jeevak Parpia, Paul McEuen Electromechanical resonators made from suspended graphene sheets show promise for many potential applications, including mass sensing, optomechanics, and tunable radio frequency electronics.[1,2,3] However, fundamental properties of these resonators, such as the strongly temperature dependent resonant frequency and quality factor, have yet to be understood. Measurements of mechanical nonlinearity can provide additional information about the properties of graphene membranes, including the modulus for stretching.[2,4] Here, we present careful studies of the nonlinear response of fully-clamped graphene resonators. We measure the coefficients of the cubic (Duffing) nonlinearity and nonlinear damping terms. We also discuss how these terms compare to expectations from elastic and entropic theories. These measurements increase our physical understanding of the mechanics of atomically thin membranes, and can help improve the performance of these novel electromechanical devices. [1] J. S. Bunch, et al., Science 315, 490 (2007) [2] C. Chen, et al., Nature Nanotechnology 4, 861-867 (2009) [3] R. A. Barton, et al., Nano Letters 12, 4681--4686 (2012) [4] A. Eichler, et al., Nature Nanotechnology 6, 339-342 (2011) [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L30.00008: Fabrication and Characterization of Graphene Nano-Mechanical Oscillators Shonali Dhingra, Brian D'Urso Copper foil is the most commonly used substrate for chemical vapor deposition (CVD) growth of graphene, despite the impact of its surface roughness and polycrystalline structure on the resulting graphene. We instead grow graphene grown on large-domain thick ultra-flat copper discs, using LPCVD and APCVD. Compared to copper foil, graphene grown on these thick ultra-flat copper substrates by APCVD results in 50 times smoother graphene on copper. The grown graphene is transferred from copper using Poly (methyl methacrylate) (PMMA), and is patterned into nano-mechanical oscillators (NMO) of different geometrical shapes, using deep-UV lithography of PMMA. A study of the phase noise in the resonant frequency of the NMO, exhibits the characteristic `1/f' noise, which seems to depend on the number of layers of graphene. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L30.00009: Mirror buckling of freestanding graphene membranes induced by local heating due to a scanning tunneling microscope tip J.K. Schoelz, M. Neek Amal, P. Xu, S.D. Barber, M.L. Ackerman, P.M. Thibado, A. Sadeghi, F.M. Peeters Scanning tunneling microscopy has been an invaluable tool in the study of graphene at the atomic scale. Several STM groups have managed to obtain atomic scale images of freestanding graphene membranes providing insight into the behavior of the stabilized ripple geometry. However, we found that the interaction between the STM tip and the freestanding graphene sample may induce additional effects. By varying the tunneling parameters, we can tune the position of the sample, in either a smooth or step like fashion. These phenomena were investigated by STM experiments, continuum elasticity theory and large scale molecular dynamics simulations. These results confirm that by increasing the tip bias, the electrostatic attraction between the tip and sample increases. When applied on a concave surface, this can result in mirror buckling which leads to a large scale movement of the sample. Interestingly, due in part to the negative coefficient of thermal expansion of graphene, buckling transitions can also be induced through local heating of the surface using the STM tip. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:24AM |
L30.00010: Graphene Nanomechanics Invited Speaker: Jim Hone |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L30.00011: Graphene Kirigami Melina Blees, Peter Rose, Arthur Barnard, Samantha Roberts, Paul L. McEuen We have developed a powerful new approach to working with graphene by applying the principles of kirigami, the sculptural art of paper cutting. We have release graphene from the surface, allowing us to treat it like a sheet of atom-thick paper. Working in water, we can pull the graphene along the surface or peel it up entirely. Combining this technique with lithographic patterning, we have created a variety of graphene kirigami devices including three-dimensional structures and resilient, atomically-thin hinges. We have also created soft in-plane springs by patterning a series of cuts into the graphene. The spring constants of these devices depend on the pattern of cuts, so the patterned graphene becomes an adjustable mechanical metamaterial. With possible spring constants ranging from 1 N/m to 10$^{\mathrm{-9}}$ N/m, these springs could be used as sensitive force measurement devices. Such kirigami patterning techniques could also be applied to flexible and stretchable electronics, including soft electrodes for biological experiments. This unusual way of interacting with graphene opens up a world of potential applications that we are just beginning to explore. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L30.00012: Parametric stabilization of levitated graphene microparticles Pavel Nagornykh, Bruce Kane After graphene was discovered research on it grew rapidly because of a variety of possible applications arising from its unique properties. Since graphene is strongly affected by its environment, it is desirable to decouple graphene from any kind of substrate when studying it. One of the ways to achieve this is to levitate charged graphene microparticles in a quadrupole trap [1]. Though graphene particles can stay in such trap for weeks under low vacuum conditions ($\sim$1 mTorr), their trapping time is significantly reduced down to a few hours when trap chamber is pumped to ultra-high vacuum ($\sim$10$^{-8}$ Torr). In this talk we investigate the possibility of increasing trapping time of such particles as well as for their cooling by means of parametric feedback similar to the feedback scheme used for cooling down laser-trapped nanoparticles [2]. In our system, motion of the graphene is used to provide a feedback signal which in turn is used to modulate a trap frequency at twice the frequency of graphene oscillations. Current progress and possible improvements and applications of such feedback scheme are discussed in the talk. \\[0pt][1] B. Kane, Phys. Rev. B \textbf{82}, 115441 (2010) \\[0pt][2] J. Gieseler et al., Phys. Rev. Lett. \textbf{109}, 103603 (2012) [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L30.00013: Measuring the Strength of Single Crystal and Polycrystalline Graphene Haider Rasool, Colin Ophus, William Klug, Alex Zettl, James Gimzewski The mechanical properties of materials depend strongly on their crystallinity. In our work, we measure the yield strength of suspended single crystal and bicrystal graphene membranes fabricated from chemical vapor deposition grown graphene. Membranes are characterized structurally by transmission electron microscopy and mechanically tested using atomic force microscopy. A single crystal diamond tip with a large indentation radius is used to measure the intrinsic strength of suspended membranes for mechanical measurements. Single crystal membranes prepared by chemical vapor deposition retain strengths that are comparable to previous results of single crystal membranes prepared by mechanical exfoliation. Bicrystal grain boundary membranes with large mismatch angles have enhanced strengths when compared to their low angle counterparts. These boundaries show strengths that are comparable to single crystal graphene. To investigate this enhanced strength, we use aberration corrected high resolution transmission electron microscopy to map the atomic scale strain fields in suspended graphene. The enhanced strength of large angle bicrystal membranes is attributed to the presence of low atomic-scale strain at the boundaries. [Preview Abstract] |
Session L31: Focus Session: Computational Discovery and Design of New Materials III
Sponsoring Units: DMP DCOMPChair: Hongjun Xiang, Fudan University
Room: 607
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L31.00001: Mechanical and electronic properties of pristine and Ni-doped Si, Ge, and Sn sheets Aaditya Manjanath, Vijay Kumar, Abhishek Singh Silicene, a graphene analogue of silicon, has been generating immense interest due to its potential for applications in miniaturized devices. Unlike planar graphene, silicene prefers a buckled structure. Here we explore the possibility of stabilizing a planar form of silicene by Ni doping using first principles density functional theory based calculations. It is found that planar as well as buckled structure is stable for the Ni doped silicene, but the buckled sheet has slightly lower total energy. The planar silicene sheet has unstable phonon modes. A comparative study of the mechanical properties reveals that the in-plane stiffness of both the pristine and the doped planar silicene is higher compared to that of the buckled silicene. This suggests that planar silicene is mechanically more robust. Electronic structure calculations of the planar and buckled Ni-doped silicene show that the energy bands at the Dirac point transform from linear behavior to parabolic dispersion. Furthermore, we extend our study to Ge and Sn sheets that are also stable and the trends of comparable mechanical stability of the planar and buckled phases remain the same. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L31.00002: Single-layer honeycomb like structure of silica Seymur Cahangirov, V. Ongun Ozcelik, Salim Ciraci Silica or SiO$_2$, the main constituent of earth's rocks has several 3D complex crystalline and amorphous phases, but it does not have a graphite like layered structure in 3D. Our theoretical analysis and numerical calculations from the first-principles predict that silica can have stable, suspended, single-layer honeycomb like allotrope, h$\alpha$-silica (silicatene), which can be viewed to be derived from the oxidation of silicene and it has intriguing atomic structure with re-entrant angles in hexagons. It is a wide band gap semiconductor, which attains remarkable electromechanical properties showing geometrical changes under external electric field. In particular, it is an auxetic nanomaterial with negative Poisson's ratio and has high piezoelectric coefficient. Coverage of foreign adatoms can attribute new functionalities to h$\alpha$-silica such that by oxidation it turns into to a wide band gap insulator like the parent quartz. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L31.00003: First-principles prediction of the structural, electronic and vibrational properties for t-silicene Eduardo Cifuentes-Quintal, Romeo de Coss With the synthesis of graphene and the discovery of its amazing properties, the research and the design for new 2D-materials began. In this line, silicene (silicon analogous to graphene) becomes very attractive because most of the current technology is already based on silicon. Therefore, the discovery of new silicon based 2D-materials with different properties could be helpful. In this work we have studied the vibrational stability for a new two-dimensional silicon allotrope based on buckled tetrarings (T-silicene). Our results were obtained within the framework of the density functional perturbation theory, using the plane-wave pseudopotential method, and the GGA-PBE96 exchange-correlation functional. We found that, in analogy to hexagonal Silicene, plane and buckled T-silicene are energetically stables. However, plane T-silicene shows imaginary phonon frequencies, and therefore is vibrationally unstable. Thus, buckled T-silicene is vibrationally stable and presents metallic character. More interestingly, the electronic structure shows that the energy bands crossing the Fermi level have a linear behavior with the wave vector. We will present a detailed analysis of this feature. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L31.00004: Computational discovery of new structures using the minima hopping method Invited Speaker: Stefan Goedecker Theoretical structure prediction methods can find a huge number of possible low energy structures of materials. I will present some basic principles for locating them efficiently and show how these principles are exploited in the minima hopping method. I will next survey some of our applications to various materials. I will present our studies of several hydrogen storage materials for which we found numerous hitherto unknown structures, computer generated silicon allotropes that have promising applications for photovoltaic applications and summarize our search for stable fullerene like structures beyond carbon. I will also address the question of whether theoretically found materials can be synthesized in practice and single out features of the potential energy landscape that facilitate the synthesis. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L31.00005: Novel Be-intercalated Hexagonal Boron Layers Structure of BeB2 Kai-Ming Ho, Manh Cuong Nguyen, Xin Zhao, Cai-Zhuang Wang Using genetic algorithm method and first-principle calculations, we performed searches for low-energy crystal structures of BeB2. We found a new family of structures, where the B atoms form hexagonal layers intercalated by Be atoms. The lowest-energy structure has formation energy of -99.47 meV/atom with 4 formula units in the unit cell, which is much more stable than the models proposed before. The formation energies of structures in the new structure family can be well described by a Ising-like model with ``anti-ferromagnetic'' coupling between the displacements of Be atoms from the mid-plane between two B layers. We also performed phonon calculation as well as electronic band structure calculation to verify the stability and investigate the electronic properties of the newly found ground-state structure. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L31.00006: Prediction of a reconstructed $\alpha$-boron (111) surface by the minima hopping method Maximilian Amsler, Stefan Goedecker, Silvana Botti, Miguel A.L. Marques Boron exhibits an impressive structural variety and immense efforts have recently been made to explore boron structures of low dimensionality, such as boron fullerenes, two-dimensional boron sheets or boron nanotubes which are theoretically predicted to exhibit superior electronic properties compared to their carbon analogues. By performing an extensive and systematic \textit{ab initio} structural search for the (111) surface of $\alpha$-boron (111) using the minima hopping structure prediction method we found very strong reconstructions that lead to two-dimensional surface layers. The topmost layer of these low energy reconstructions is a conductive, nearly perfectly planar boron sheet. If exfoliation was experimentally possible, promising precursors for a large variety of boron nano-structures such as single walled boron nanotubes and boron fullerenes could be obtained. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L31.00007: The ferromagnetic properties of MnX2 Monolayers Qiang Sun Since the successful synthesis of graphene, tremendous efforts have been devoted to two-dimensional monolayers such as boron nitride (BN), silicene and MoS2. These 2D materials exhibit a large variety of physical and chemical properties, but they are intrinsically nonmagnetic in their pristine forms. In order to explore the applications in spin-related devices, considerable efforts have been made to study ferromagnetic monolayers. We have systematically studied the electronic and magnetic properties of the MnX2 (X$=$O, S, Se) monolayers, and found that they display intrinsic ferromagnetism with high Curie temperatures.. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L31.00008: Aligning the band edges of Si surfaces with water redox potentials: a first-principles study Eric Schwegler, Tuan Anh Pham, Donghwa Lee, Giulia Galli We present first-principles calculations of the alignment between the band edges of several Si surfaces with water redox potentials using many-body perturbation theory, coupled with \textit{ab initio} molecular dynamics simulations. Our results show that surface functionalization strongly influences the electronic properties of the interface, and indicate that a favourable alignment of band edges and water redox potentials for water splitting applications may be achieved by engineering the surface termination of the Si-based photo-electrodes. In addition, we found that in the case of hydrophilic Si surfaces, the use of simple computational schemes that neglect the detailed microscopic structure of the interfacial water layer may lead to substantial errors in predicting the alignment between the solid band edges and water redox potentials. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L31.00009: Stability and Electronic Properties of Two-Dimensional Silicene and Germanene on Graphene Chih-Piao Chuu, Yongmao Cai, C.-M. Wei, M.-Y. Chou Recently, there have been experimental attempts to synthesize silicene, a two-dimensional (2D) graphene-like form of silicon on metal surfaces such as Ag(111) and Ir(0001). The possibility of preparing silicene on ZrB2 thin films grown on silicon wafers has also been reported. This suggests new perspectives for the applications of massless fermions in materials that are compatible with Si-based electronics. It is expected that many of the unique electronic properties of graphene can also be realized in this new 2D system. However, the interaction between the 2D silicon structure and the metal substrate is found to be quite strong, leading to distortion in the adlayer and consequently the disappearance of the Dirac cone. Therefore, finding a suitable substrate that interacts with silicene weakly and preserves the sublattice symmetry is of ultimate importance. We have performed first-principles calculations of silicene and germanene on graphene in order to understand the effect of substrate interaction on the physical properties of these systems. Of particular interest is the induced change in the electronic structure, the modification of the Fermi velocity, the gap opening, the charge doping from the substrate, and the stability of the combined system. The energetics of forming the 2D silicone structure on a substrate is carefully evaluated in comparison with possible three-dimensional cluster structures. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L31.00010: Prediction of novel single-layer materials for device applications Benjamin C. Revard, William W. Tipton, Richard G. Hennig Single-layer materials represent a new materials class with potentially transformative properties for applications in nanoelectronics and solar energy harvesting. With the goal to discover novel 2D materials with unusual compositions and structures, we have developed a grand-canonical evolutionary algorithm for two-dimensional materials. Here we present the details of the algorithm and our initial results. Using both empirical and first principles total energy methods in the evolutionary algorithm, we show that the method can successfully identify known structures of 2D materials such as graphene and graphane. We currently apply the approach to a number of other promising candidate systems and will report the findings. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L31.00011: A molecular dynamics study on the structural and electronic properties of two-dimensional icosahedral B12 cluster based structures Cherno Baba Kah, M. Yu, C.S. Jayanthi, S.Y. Wu Our previous study on one-dimensional icosahedral B12 cluster ($\alpha $-B12) based chain [Bulletin of APS Annual Meeting, p265 (2013)] and ring structures has prompted us to study the two-dimensional (2D) $\alpha $-B12 based structures. Recently, we have carried out a systematic molecular dynamics study on the structural stabilities and electronic properties of the 2D $\alpha $-B12 based structures using the SCED-LCAO method [PRB 74, 15540 (2006)]. We have considered several types of symmetry for these 2D structures such as $\delta $3, $\delta $4, $\delta $6 (flat triangular), and $\alpha $' types. We have found that the optimized structures are energetically in the order of $\delta $6 \textless $\alpha $' \textless $\delta $3 \textless $\delta$4 which is different from the energy order of $\alpha $'\textless $\delta $6 \textless $\delta $4 \textless $\delta $3 found in the 2D boron monolayer sheets [ACS Nano 6, 7443 (2012)]. A detailed discussion of this study will be presented. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L31.00012: Temperature Effects on Optical Spectra of Monolayer MoS2 Li Yang We present the effects of temperature on the electronic structure and optical spectra of monolayer MoS2. Newly measured optical absorption and photoluminescence spectra reveal substantial frequency shifts of exciton peaks as monolayer MoS2 is cooled from 300 K to 4 K. First-principles simulations using the GW-Bethe Salpeter Equation approach satisfactorily reproduce these frequency shifts by incorporating the thermal expansion. Studying these temperature effects in monolayer MoS2 is crucial for rectifying the results of room-temperature experiments with the previous predictions of zero-temperature-limit simulations. Additionally, we show that tracking the frequency shifts in the exciton peak of optical spectra may serve as a convenient way of estimating thermal expansion coefficients in two-dimensional chalcogenides. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L31.00013: Magnetization and magnetic anisotropy of 3d adatoms on a MoS$_{2}$ monolayer Marcio Costa, Jun Hu, Ruqian Wu MoS$_{2}$ is layered semiconductor that goes under a transition from indirect (bulk - 1.2 eV) to a direct (monolayer - 1.8 eV) gap. MoS$_{2}$ monolayer has been drawing attention due to its peculiar transport properties, with mobilities of 200 cm$^{2}$ V$^{-1}$ s$^{-1}$ at room temperature. Using Density Functional Calculations, we studied the adsorption of transition metal adatoms on the MoS$_{2}$ monolayer. The adsorption energies of Mn, Fe, Co and Ni on MoS2 monolayer were calculated over different sites. We also determined their magneto crystalline anisotropy (MCA) energies, for the purpose of using these systems in spintronic devices. To manipulate the magnetic properties, the effect of coadsorption of Bi and other elements were also investigated. [Preview Abstract] |
Session L32: Invited Session: Magnetism in Quantum Gasses
Sponsoring Units: DAMOPRoom: 708-712
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L32.00001: In situ observation of strongly interacting ferromagnetic domains in a shaken optical lattice Invited Speaker: Cheng Chin . [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L32.00002: Short-range Quantum Magnetism of Ultracold Fermions in an Optical Lattice Invited Speaker: Daniel Greif Quantum magnetism describes quantum many-body states with spins coupled by exchange interactions. At low temperatures this leads to short- and long-range magnetic ordering, which is for example the case in spin-liquids, valence-bond solids and antiferromagnets.\\ We report on the observation of magnetic spin correlations on neighboring sites of a Fermi gas in an optical lattice. The key to obtaining and detecting the short-range magnetic order is an entropy redistribution technique in a tunable-geometry optical lattice. We load a low-temperature two-component gas with repulsive interactions into either a dimerized or anisotropic simple cubic lattice. The correlations manifest as an excess number of singlets as compared to triplets consisting of two atoms with opposite spins. For the anisotropic lattice, we determine the transverse spin correlator from the singlet-triplet imbalance and observe antiferromagnetic correlations along one spatial axis. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L32.00003: Direct observation of interacting Magnons in optical lattices Invited Speaker: Christian Gross The quantum simulation of spinful many-body systems with ultracold atoms in optical lattices promises novel insight into fundamental aspects of magnetism. Here we report on the direct observation of coherent Magnon propagation after a local spin flip in the isotropic Heisenberg regime. Using our quantum gas microscope we track the position of the flipped spins during their propagation in the bath of opposite spins. When the local quantum quench is realized by flipping two adjacent spins the subsequent dynamics shows clear signatures of a stable Two-Magnon bound state propagating through the lattice. We extract the propagation velocity of the bound state and find slower dynamics due to the larger effective mass of the compound object. Tuning the system from the Mott insulating into the superfluid regime, we observe polaronic features in the non-equilibrium dynamics of a single spin impurity. Our results show the potential of local manipulation and detection for the study of correlations in magnetic quantum systems. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:24AM |
L32.00004: Realizing a Kondo-correlated state with ultracold atoms Invited Speaker: Johannes Bauer We propose a novel realization of Kondo physics with ultracold atomic gases. It is based on a Fermi sea of two different hyperfine states of one atom species forming bound states with a different species, which is spatially confined in a trapping potential. We show that different situations displaying Kondo physics can be realized when Feshbach resonances between the species are tuned by a magnetic field and the trapping frequency is varied. We illustrate that a mixture of ${}^{40}$K and ${}^{23}$Na atoms can be used to generate a Kondo correlated state and that momentum resolved radio frequency spectroscopy can provide unambiguous signatures of the formation of Kondo resonances at the Fermi energy. We discuss how tools of atomic physics can be used to investigate open questions for Kondo physics, such as the extension of the Kondo screening cloud. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 11:00AM |
L32.00005: Many-body quantum quench in an atomic one-dimensional Ising chain Invited Speaker: Hanns-Christoph Naegerl Quantum tunneling is one of the most fundamental processes in nature. Single particle hopping of ultracold atoms in optical lattices changes its character dramatically when the ensemble is prepared in strongly correlated quantum phases due to atom-atom interactions. Correlated hopping in a Mott-insulating chain of bosons that is tilted to the Mott gap has recently been employed to study long-range order in the 1D transvers Ising model [1,2]. We study correlated tunneling dynamics for an ensemble of tilted 1D Mott chains after a sudden quench to the vicinity of the Ising paramagnetic to antiferromagnetic phase transition point [3]. The quench results in coherent oscillations for the orientation of effective Ising spins, detected via oscillations in the number of doubly occupied lattice sites. We characterize the quench by varying the system parameters. We report significant modification of the tunneling rate induced by interactions and show clear evidence for collective effects in the oscillatory response. We observe higher-order many-body tunneling processes over up to five lattice sites when the tilt per site is tuned to integer fractions of the Mott gap. Second- and third-order tunneling shows up in the transient response after the quench, from which we extract the characteristic scaling in accordance with perturbation theory and numerical simulations. In a second set of experiments we study the response of an ensemble of 1D superfluids in the Bose-Hubbard regime when subject to a tilt [4]. For large values of the tilt, we observe interaction-induced coherent decay and matter-wave quantum phase revivals of the Bloch oscillating ensemble. We analyze the revival period dependence on interactions by means of a Feshbach resonance. When reducing the value of the tilt, we observe the disappearance of the quasi-periodic phase revival signature towards an irreversible decay of Bloch oscillations, indicating the transition from regular to quantum chaotic dynamics.\\[4pt] [1] J. Simon et al., Nature 472, 307 (2011)\\[0pt] [2] S. Sachdev et al., Phys. Rev. B 66, 075128 (2002)\\[0pt] [3] F. Meinert et al., Phys. Rev. Lett. 111, 053003 (2013)\\[0pt] [4] F. Meinert et al., arXiv:1309.4045 (2013) [Preview Abstract] |
Session L34: Superconducting Qubits: Two-level Systems & Decoherence
Sponsoring Units: GQIChair: Jerry Chow, IBM
Room: 704
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L34.00001: Anomalous Behavior of High Quality Factor Planar Superconducting Resonators Anthony Megrant, Zijun Chen, Ben Chiaro, Andrew Dunsworth, Chris Quintana, Brooks Campbell, Julian Kelly, Rami Barends, Yu Chen, Evan Jeffrey, Josh Mutus, Charles Neill, Peter O'Malley, Daniel Sank, Amit Vainsencher, Jim Wenner, Ted White, Jorg Bochmann, IoChun Hoi, Christopher Palmstrom, John Martinis, Andrew Cleland Superconducting coplanar waveguide resonators have proven to be invaluable tools in studying some of the decoherence mechanisms found in superconducting qubits. Surface two-level states tend to dominate decoherence at temperatures below ~ Tc/10 and at very low microwave powers, assuming loss through other channels (e.g. quasiparticles, vortices, and radiation loss) has been mitigated through proper shielding and design. I will present recent measurements of resonators whose behavior diverges significantly from the standard two-level state model at low temperatures and low excitation energies, resulting in startling behavior of the internal quality factor. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L34.00002: Microscopic origin of dissipative two-level systems in Al$_2$O$_3$ Luke Gordon, Hazem A. Farsakh, Anderson Janotti, Chris G. Van de Walle Resonant absorption of two-level systems (TLS) poses a serious limitation to the performance of superconducting qubit devices for quantum computing. Experiments indicate that the TLS is associated with defects in the dielectric layers of the device. However, the nature of these defects has yet to be established. Using hybrid functional calculations, we investigate possible defects in Al$_2$O$_3$ that can act as sources of resonant absorption. Hydrogen is a ubiquitous impurity, and easily incorporates in the interstitial sites (H$_i$) in Al$_2$O$_3$. In the positive charge state, H$_i$ is bonded to one oxygen atom, but also interacts with a secondary oxygen atom. At particular O-O distances, close to those found in amorphous Al$_2$O$_3$ or near the Al$_2$O$_3$/Al interface, the H atom is effectively in a double well. We calculate the three-dimensional potential energy surface (PES) for the H atom in a so-called ``coincidence configuration,'' which allows for tunneling between two equivalent positions. Using the calculated PES, we solve the Schr\"odinger equation for the tunneling proton and determine the tunneling frequency. We find that H$_i$ gives rise to resonant absorption in the range of 10-100 GHz, in agreement with experimental observations. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L34.00003: Individual two level fluctuators in the tunneling conductance of Al/AlO$_{x}$/Al Josephson junctions for superconducting qubits Christopher Nugroho, Vladimir Orlyanchik, Dale Van Harlingen Two level system (TLS) defects in AlO$_{x}$ tunnel barriers can lead to low-frequency $1/f$ critical current noise and losses in coherent superconducting circuits. Understanding the nature of these defects and how to eliminate them are critical in order to achieve ultra-long coherence times. We present measurements of the tunneling conductance of ultrasmall, $A<(100~\textrm{nm})^{2}$, Al/AlO$_{x}$/Al shadow evaporated junctions. The tunneling conductance of these junctions exhibits several isolated TLSs, which permitted the detailed analysis of the individual switching rates and behavior of the TLSs. We have studied the thermal activation behavior of these TLSs, and in some cases observe a crossover into quantum-limited tunneling at lower temperatures. Tracking the TLS switching rates as a function of the applied voltage bias provides an estimate of the TLS charge dipole moment. In some quantum tunneling limited TLSs we have observed a non-equilibrium enhancement of the switching rates that cannot be explained by simple dissipative heating of the TLSs. Further investigations into these TLS defects may lead to the identification of their physical origins and strategies to eliminate them. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L34.00004: Cavity QED of individual two-level systems observed in insulating films Bahman Sarabi, Aruna N. Ramanayaka, Alexander L. Burin, Frederick C. Wellstood, Kevin D. Osborn Previous work shows that individual two-level systems (TLS) can be observed in alumina tunnel barriers and used as quantum memory. We investigate the TLS within insulating films using superconducting microwave resonators of different electric-field volumes. The insulating film, made of hydrogenated amorphous silicon nitride, is orders of magnitude thicker than tunneling barriers and its volume is many orders of magnitude larger than that of qubit junction barriers where individual TLS are routinely observed. In the largest-volume resonators we observe bulk dielectric behavior of TLS. In the smallest-volume resonators, strong coupling to the TLS is observed and explained by cavity QED. When two resonances are observed, the data are fit to quantum theory, showing that the strongly coupled TLS have coherence times on the order of microseconds. Furthermore, the power dependence of transmission in the TLS-cavity hybrid system is measured, which shows clear saturation of the TLS near a single photon. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L34.00005: Efficient characterization of spurious two-level systems in superconducting qubits under non-ideal conditions Markku P.V. Stenberg, Yuval R. Sanders, Frank K. Wilhelm The presence of spurious two-level systems (TLSs) is a long-standing problem in superconducting qubits. We present a characterization method that is able to determine both the TLS frequency $\omega$ and its coupling strength $g$ with the qubit efficiently. With the method, the mean squared error of the estimates decreases exponentially with the number of measurement shots in contrast to power-law scalings exhibited by the conventional methods. Significantly, our method also works in the presence of decoherence and measurement errors. This is accomplished by applying Bayesian inference in a feedback algorithm that updates the measurement setup based on the previous measurement outcomes while data is being collected. Surprisingly, we find that it is usually possible to characterize $\omega$ and $g$ with high precision with only some hundreds of measurement shots - even if the same set of measurements does not allow establishing highly precise expectation values for a quantum state. In addition to TLSs, our method can also be used to precisely characterize stripline resonators. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L34.00006: Stimulated microwave photon transfer through dielectric two-level systems Yaniv Rosen, Moe Khalil, Kevin Osborn Two level systems (TLS) in dielectrics are a major source of energy loss for superconducting circuits at milli-kelvin temperatures. We will show measurements taken with a bias-bridge resonator circuit, which allows the simultaneous application of an electric dc field, while measuring the loss tangent of applied microwave fields. Previous measurements with this device show that 10$^{\mathrm{9}}$ TLS can be tuned through the microwave resonance and the loss of these can be changed due to rapid passage of the TLS. We extend these studies by concurrently applying multiple microwave frequencies, and explore the possible transfer of photons from one frequency to the other using stimulated processes with the TLS and resonator fields. These experiments show that the background of disordered TLS, usually thought of as deleterious, may be controlled in potentially useful ways. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L34.00007: Photon Correlations in a Waveguide Coupled to Multiple Two- and Three- Level Systems Yao-Lung L. Fang, Huaixiu Zheng, Harold U. Baranger We study photon-photon correlations in a waveguide strongly coupled to multiple qubits (up to 10) described by either two-level systems (2LS) or three-level systems (3LS). The calculated second-order correlation function $g_2(t)$ for this ``waveguide QED'' system has rich structure that is sensitive to the frequency of the incident photons and the separation between the qubits, arising from the interference among photons scattered multiple times by the qubits. In the multiple 2LS case [1], transmitted and reflected photons can both be bunched initially and then oscillate between bunched and anti-bunched for a long time interval that increases as the number of impurities, N, increases (up to 10). For 3LS qubits, when operating at the peak of electromagnetically induced transparency (EIT), the N=2 case generates bunched photons persisting for a long time, comparable to that in the N=10 2LS case. To probe the interplay between the time-delay inherent in the 3LS under EIT conditions and the 2LS-produced correlations, we study the hybrid structures 3LS-2LS-3LS and 2LS-3LS-2LS. [1] For first results see Y.-L. L. Fang, H. Zheng, and H. U. Baranger, arXiv/1308.6551 [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L34.00008: An efficient method for studying low-frequency two-state fluctuators Fei Yan, Simon Gustavsson, Xiaoyue Jin, Archana Kamal, Terry Orlando, William Oliver We propose a driven-evolution-based pulse sequence as an efficient tool to study low-frequency random telegraph noise. The sequence originates from the two-dimensional chemical exchange experiment in NMR, but dramatically reduces measuring time with a one-dimensional modification. The sequence is also more sensitive to weak fluctuators than the dynamical-decoupling-type sequences. By applying the sequence to a qubit, the existence of a two-state fluctuator is characterized by an oscillating signal, whose frequency and amplitude correspond to the fluctuator's strength and correlation time respectively. The method opens a way to investigate noise in the quasistatic regime, which cannot be resolved by conventional coherence-characterization methods. The pulse sequence can be used to study phenomena in Josephon-junction qubits such as quasiparticle tunneling. The Lincoln Laboratory portion of this work was sponsored by the Assistant Secretary of Defense for Research \& Engineering under Air Force Contract number FA8721-05-C-0002. Opinions, interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United States Government. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L34.00009: Optically Activated Two-Level Systems in a Thin-Film Superconducting Microwave Resonator R.P. Budoyo, J.B. Hertzberg, C.J. Ballard, K.D. Voigt, J.R. Anderson, C.J. Lobb, F.C. Wellstood We have fabricated an isolated thin-film superconducting Al lumped-element resonator (resonant frequency 6.72 GHz) on a sapphire substrate and mounted it inside an Al 3d cavity (TE101 mode frequency 7.50 GHz). The thin-film resonator is very weakly coupled to the microwave drive line with $Q_e \approx 5 \times 10^9$. We illuminated the resonator with 780 nm light from an optical fiber and measured the internal loss in the resonator and its dependence on applied optical and rf powers at temperatures as low as 20 mK. With no applied optical power, the resonator reaches an internal quality factor $Q_i \approx 2 \times 10^6$ at high rf photon numbers. Our measurements show that the applied optical power causes an increase in loss due to an apparent increased contribution from two-level systems as well as the expected increase from quasiparticles. We discuss our results and possible mechanisms for the optical activation of two-level systems. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L34.00010: Discrete Two-Level Systems and Two-Level Fluctuators in a Superconducting Microwave Resonator K.D. Voigt, J.B. Hertzberg, Z. Kim, A. Choudhary, J.R. Anderson, C.J. Lobb, F.C. Wellstood We measure the effect of two-level systems on a thin-film superconducting Al microwave resonator at 6.83 GHz that is weakly coupled to an on-chip transmission line [1]. The device is intended for coupling to the hyperfine splitting of trapped $^{\mathrm{87}}$Rb atoms. At 12 mK the internal quality factor at low microwave power is typically 100,000. Applying a dc voltage to the transmission line leads to reproducible shifts of up to 6 kHz in the resonance frequency. These shifts are more pronounced at lower RF power, suggesting that discrete charged two-level systems in the sapphire substrate or surface Al oxide are responsible, and that the dc voltage shifts the transition energy of the two-level systems. We also see evidence for thermally activated two- level fluctuators which can be turned on and off by the applied dc voltage. We discuss our results and the characteristics of the underlying two-level systems and two-level fluctuators.\\[4pt] [1] Z. Kim et al., AIP ADVANCES 1, 042107 (2011). [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L34.00011: Etch Effects on Surface loss in High Quality Aluminum on Silicon Superconducting Coplanar Resonators Andrew Dunsworth, Anthony Megrant, Rami Barends, Yu Chen, IoChun Hoi, Evan Jeffrey, Josh Mutus, Pedram Roushan, Brooks Campbell, Zijun Chen, Ben Chiaro, Julian Kelly, Charles Neill, Peter O'Malley, Chris Quintana, Daniel Sank, Amit Vainsencher, Jim Wenner, Ted White, Andrew Cleland, John Martinis Superconducting coplanar resonators are a powerful tool for studying capacitive loss from two level states (TLS's) in superconducting qubits. We have found evidence that standard processing of aluminum on sapphire superconducting devices leaves behind $\approx$2 nm organic residues which can contribute to loss at the Q$> 10^{6}$ level that we are presently working with. Removing these residues is possible on a silicon substrate as it allows various sidewall etchings and profilings via chemical and physical etches. I will present recent Q factor measurements of aluminum on silicon resonators that were defined through a variety of etching conditions. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L34.00012: Lift-Off Processing and Superconducting Circuit Coherence C.M. Quintana, A. Megrant, A. Dunsworth, Zijun Chen, B. Chiaro, R. Barends, B. Campbell, Yu Chen, E. Jeffrey, J. Kelly, J.Y. Mutus, C. Neill, P.J.J. O'Malley, P. Roushan, D. Sank, J. Wenner, T.C. White, A.N. Cleland, John M. Martinis As superconducting circuit coherence continues to increase, careful attention must be paid to device fabrication techniques. Substantial evidence points to dielectric loss from two-level state defects in thin amorphous interfacial regions as a limiting relaxation mechanism for superconducting qubits. Transmon qubits have traditionally been fabricated using lift-off aluminum deposited together with their Josephson junctions; however, improved coherence times have recently been found in transmons which use lift-off metal for only a small fraction of the qubit. To better understand this improvement and predict any remaining limits imposed by the incorporation of lift-off, we characterize the increased loss found in coplanar waveguide resonators formed with lift-off metal. We vary surface treatment such as oxygen ashing and ion milling, and study the effects of double-angle evaporation, e-beam resist residue, and surface roughness on resonator quality factors. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L34.00013: Hydrogen-free amorphous silicon with no tunneling states Xiao Liu, Daniel Queen, Thomas Metcalf, Julie Karel, Frances Hellman The ubiquitous low-energy excitations, known as the two-level tunneling systems (TLS), are one of the universal phenomena of amorphous solids. These excitations dominate the acoustic, dielectric, and thermal properties of structurally disordered solids. One exception has been a type of hydrogenated amorphous silicon (a-Si:H) with 1 at.{\%} hydrogen. Using low temperature elastic measurements of electron-beam evaporated amorphous silicon (a-Si), a model monatomic amorphous system, we show that TLS are also diminished in this system as the films become denser and more structurally ordered. Our results demonstrate that TLS are not intrinsic to the glassy state but instead reside in low density regions of the amorphous network. This work obviates the role hydrogen was previously thought to play in removing TLS in a-Si:H and favors an ideal four-fold covalently bonded amorphous structure as the cause for the disappearance of TLS. Our result supports the notion that a-Si can be made a ``perfect glass'' with ``crystal-like'' properties, thus offering an encouraging opportunity to use it as a simple crystal alternative in applications, such as in modern quantum devices where TLS are the source of dissipation, decoherence and 1/f noise. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L34.00014: Noise study of insulating films within superconducting LC resonators A.N. Ramanayaka, B. Sarabi, K.D. Osborn Two-level systems (TLS) in amorphous dielectrics, known to be a major source of decoherence in superconducting qubits, are also known to cause low-frequency phase noise in resonating superconducting circuits. Here we will report on an effort to characterize this noise using microwave LC resonators fabricated with a trilayer capacitor containing a deposited silicon nitride dielectric film containing TLS, sandwiched by superconducting electrodes. The resonators are probed at frequencies of approximately 6 GHz and at temperatures of 10-200 mK. The noise dependence on temperature, microwave power, and dielectric volume will be discussed in the context of standard tunneling model of two level systems and newer models. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L34.00015: Overcoming charge noise decoherence by photon-assisted pair-breaking in a charge qubit Sebastian de Graaf, Juha Lepp\"akangas, Astghik Adamyan, Andrey Danilov, Tobias Lindstr\"om, Mikael Fogelstr\"om, G\"oran Johansson, Sergey Kubatkin We report on recent measurements [1] of a charge qubit, a Cooper-pair box, coupled to a high-Q microwave cavity in the strong driving regime. This we model using a dressed state formalism, and we find evidence for a process that involves energy transfer corresponding to a large number ($\sim$14) of photons. This energy is sufficient to break a Cooper-pair on the island, and it results in a new relaxation channel for the qubit. Specifically, this relaxation resets the qubit into a charge state determined by the static bias conditions, resulting in a sudden population inversion around each dressed state degeneracy point. At low driving strengths, decoherence is governed by charge noise in the environment, while in the discovered strong driving regime the relaxation rate due to pair-breaking can overcome the environmental charge relaxation rate. This results in a regime that is especially attractive for charge sensing since the qubit response becomes immune to non-equilibrium quasiparticle poisoning and less susceptible to its charge noise environment. \\[4pt] [1] S. E. de Graaf et al. PRL 111, 137002 (2013); J. Lepp\"{a}kangas et al., J. Phys. B, 46, 224019 [Preview Abstract] |
Session L35: Focus Session: Quantum Computing Architectures and Algorithms: Quantum Error Correction
Sponsoring Units: GQIRoom: 702
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L35.00001: Fibre bundle framework for quantum fault tolerance Lucy Liuxuan Zhang, Daniel Gottesman We introduce a differential geometric framework for describing families of quantum error-correcting codes and for understanding quantum fault tolerance. In particular, we use fibre bundles and a natural projectively flat connection thereon to study the transformation of codewords under unitary fault-tolerant evolutions. We'll explain how the fault-tolerant logical operations are given by the monodromy group for the bundles with projectively flat connection, which is always discrete. We will discuss the construction of the said bundles for two examples of fault-tolerant families of operations, the string operators in the toric code and the qudit transversal gates. This framework unifies topological fault tolerance and fault tolerance based on transversal gates, and is expected to apply for all unitary quantum fault-tolerant protocols. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L35.00002: Magic-state encoder and magic teleportation: Efficient fault-tolerant non-Clifford gates with concatenated quantum codes Hayato Goto, Satoshi Nakamura, Mamiko Kujiraoka, Kouichi Ichimura In fault-tolerant quantum computation, Clifford operations, e.g., controlled-NOT gates, can be efficiently implemented in a fault-tolerant manner. However, non-Clifford gates such as the $T$ gate $\pi/8$ rotation) and the Toffoli gate are difficult to implement efficiently. A standard approach to non-Clifford gates is ``magic state distillation,'' which can provide high-fidelity magic states using more low-fidelity magic states. Thus, reliable non-Clifford gates can be performed with the high-fidelity magic states and reliable Clifford operations. However, the resource overhead for magic state distillation is much larger than those for Clifford gates. To solve this problem, here we propose a new approach: magic-state encoder. This can be applied to concatenated quantum codes with a property that Hadamard gates can be implemented transversally. The magic-state encoder encodes (not distills) a high-fidelity level-$(l+1)$ encoded magic state with low-fidelity level-$l$ encoded magic states. As a result, non-Clifford gates (here we focus on the $T$ gate) can be performed with an overhead comparable to Clifford gates. By performing the $T$ gate by teleportation with an entangled state generated with a magic state, which we call ``magic teleportation,'' further improvement is possible. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L35.00003: Quantum compiling with low overhead Guillaume Duclos-Cianci, David Poulin I will present a scheme to compile complex quantum gates that uses significantly fewer resources than existing schemes. In standard fault-tolerant protocols, a magic state is distilled from noisy resources, and copies of this magic state are then assembled into produced complex gates using the Solovay-Kitaev theorem or variants thereof. In our approach, we instead directly distill magic states associated to complex gates from noisy resources, leading to a reduction of the compiling overhead of several orders of magnitude. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L35.00004: The overhead of fault-tolerant quantum computing Invited Speaker: Daniel Gottesman The threshold theorem for fault tolerance tells us that it is possible to build arbitrarily large reliable quantum computers provided the error rate per physical gate or time step is below some threshold value. Most research on the threshold theorem so far has gone into optimizing the tolerable error rate under various assumptions, with other considerations being secondary. However, for the foreseeable future, the number of qubits may be an even greater restriction than error rates. The overhead, the ratio of physical qubits to logical qubits, determines how expensive (in qubits) a fault-tolerant computation is. Earlier results on fault tolerance used a large overhead which grows (albeit slowly) with the size of the computation. I show that it is possible in principle to do fault-tolerant quantum computation with low overhead, and with the overhead constant in the size of the computation. The result depends on recent progress on quantum low-density parity check codes. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L35.00005: Extremal Optimization for estimation of the error threshold in topological subsystem codes at $\mathbf{T=0}$ Jorge E. Mill\'an-Otoya, Stefan Boettcher Quantum decoherence is a problem that arises in implementations of quantum computing proposals. Topological subsystem codes (\emph{TSC}) have been suggested as a way to overcome decoherence. These offer a higher optimal error tolerance when compared to typical error-correcting algorithms. A TSC has been translated into a planar Ising spin-glass with constrained bimodal three-spin couplings. This spin-glass has been considered at finite temperature to determine the phase boundary between the unstable phase and the stable phase, where error recovery is possible.\footnote{R. S. Andrist et al., \emph{Optimal error correction in topological subsystem codes}, Phys. Rev. A., \textbf{85}, 050302(R) (2012)} We approach the study of the error threshold problem by exploring ground states of this spin-glass with the Extremal Optimization algorithm (\emph{EO}).\footnote{S. Boettcher et al., \emph{Optimization with extremal dynamics}, Phys. Rev. Lett., \textbf{86}, 5211 (2001)} EO has proven to be a effective heuristic to explore ground state configurations of glassy spin-systems.\footnote{S. Boettcher, \emph{Stiffness of the Edwards-Anderson model in all dimensions}, Phys. Rev. Lett., \textbf{95}, 197205 (2005)} [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L35.00006: Error-thresholds for qudit-based topological quantum memories Ruben S. Andrist, James R. Wootton, Helmut G. Katzgraber Extending the quantum computing paradigm from qubits to higher-dimensional quantum systems allows for increased channel capacity and a more efficient implementation of quantum gates. However, to perform reliable computations an efficient error-correction scheme adapted for these multi-level quantum systems is needed. A promising approach is via topological quantum error correction, where stability to external noise is achieved by encoding quantum information in non-local degrees of freedom. A key figure of merit is the error threshold which quantifies the fraction of physical qudits that can be damaged before logical information is lost. Here we analyze the resilience of generalized topological memories built from d-level quantum systems (qudits) to bit-flip errors. The error threshold is determined by mapping the quantum setup to a classical Potts-like model with bond disorder, which is then investigated numerically using large-scale Monte Carlo simulations. Our results show that topological error correction with qutrits exhibits an improved error threshold in comparison to qubit-based systems. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L35.00007: The intrinsic error thresholds of the surface code with correlated errors Pejman Jouzdani, Eduardo Mucciolo, Eduardo Novais We study how the resilience of the surface code to decoherence is affected by the presence of a bosonic bath. The surface code experiences an effective dynamics due to the coupling to a bosonic bath that correlates the qubits of the code. The range of the effective induced qubit-qubit interaction depends on parameters related to the bath correlation functions. We show hat different ranges set different intrinsic bounds on the fidelity of the code. These bounds appear to be independent of the stochastic error probabilities frequently studied in the literature and to be merely a consequence of the induced dynamics by the bath. We introduce a new definition of stabilizers based on logical operators that allows us to efficiently implement a Metropolis algorithm to determine the intrinsic upper bounds to the error threshold. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:24AM |
L35.00008: Using concatenated quantum codes for universal fault-tolerant quantum gates Invited Speaker: Tomas Jochym-O'Connor Quantum error correction and fault-tolerance are essential for large scale quantum information processing tasks. A standard method for implementing a logical fault-tolerant gate is by applying the gate transversally, that is without coupling qubits within an encoded codeblock. However, Eastin and Knill [Phys. Rev. Lett. 102, 110502 (2009)] proved that it is impossible to have a set of universal transversal gates for a given quantum error correcting code. In this work, we present sufficient conditions to obtain a set of universal fault-tolerant quantum gates by concatenating two quantum error correcting codes. Namely, the concatenation scheme does not require the preparation of special ancillary states in order to obtain universality, unlike schemes such as magic state distillation. The construction exploits the transversality of different sets of gates for the given codes, protecting for the non-transversal gates in one code by implementing these logical gates using transversal gates in the second code. The full distance of the concatenated code is sacrificed to protect against low-weight arbitrary errors, ensuring fault-tolerance. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L35.00009: Error correction with machine learning: one man's syndrome measurement is another man's treasure Joshua Combes, Hans Briegel, Carlton Caves, Christopher Cesare, Christopher Ferrie, Gerard Milburn, Markus Tiersch Syndrome measurements that are made in quantum error correction contains more information than is typically used. We show using the data from syndrome measurements (that one has to do anyway) the following: (1) a channel can be dynamically estimated; (2) in some situations the information gathered from the estimation can be used to permanently correct away part of the channel; and (3) can allow us to perform hypothesis testing to determine if the errors are correlated or if the error rate exceeds the ``expected worst case''. The unifying theme to these topics is making use of all of the information in the data collected from syndrome measurements with a machine learning and control algorithms. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L35.00010: Finite-size scaling of the decoherence time of the Toric Code in contact with a thermal reservoir C. Daniel Freeman, CM Herdman, Dylan Gorman, Birgitta Whaley We present an analysis of the finite-size scaling of the decoherence time of a topological qubit in contact with a thermal bath. While the relaxation time of the toric code at finite temperature in the thermodynamic limit has a system size independent bound, we find nontrivial finite-size scaling of the decoherence time in the low temperature crossover regime on a finite lattice. Using a continuous-time Monte Carlo method, we explicitly compute the low temperature nonequilibrium dynamics of the toric code on finite lattices. We demonstrate how this nontrivial finite-size scaling is governed by the scaling of topologically nontrivial 2D classical random walks. As this finite temperature scaling competes with the scaling of the robustness to unitary perturbations, this analysis may elucidate the scaling of decoherence times of possible physical realizations of topological qubits. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L35.00011: Spin glass reflection of quantum error correcting codes Alexey Kovalev, Leonid Pryadko We study the decoding transition for quantum error correcting codes with the help of a mapping to random-bond Wegner spin models. Known families of quantum low density parity check (LDPC) codes lead to unexplored earlier generally non-local Wegner models with rich phase diagrams that include ordered, disordered, and spin glass phases. The decoding transition corresponds to a transition from the ordered phase by proliferation of extended defects which generalize the notion of domain walls to non-local spin models. In recently discovered quantum LDPC code families with finite rates the number of distinct classes of such extended defects is exponentially large, corresponding to extensive ground state entropy of these codes. Here, the transition can be driven by the entropy of the extended defects, a mechanism distinct from that in the local spin models where the number of defect types (domain walls) is always finite. We construct numerically phase diagrams for models corresponding to several families of quantum LDPC codes. We formulate similar mapping to random bond Wegner models for the case of errors in syndrome measurements, and find several examples of code families with highest fault-tolerant thresholds. [Preview Abstract] |
Session L36: Focus Session: Semiconductor Qubits: Relaxation, Decoherence, and Realistic Devices
Sponsoring Units: GQIChair: Charlie Tahan, LPS
Room: 703
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L36.00001: Long spin coherence in a strong spin-orbit qubit Invited Speaker: Andrew Higginbotham We measure long spin coherence and strong spin-orbit coupling in Ge/Si nanowire quantum dots. Spin coherence is measured by examining the dephasing of singlet-correlated spins separated between two quantum dots, with each quantum dot occupied by several holes. Spin-orbit coupling is measured by examining the statistical properties of Coulomb blockade peak heights. The measured spin dephasing and spin-orbit coupling suggest that a spin-orbit qubit formed in a Ge/Si nanowire may have an unprecedentedly high manipulation fidelity. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L36.00002: Electron Spin Coherence Times in Si/SiGe Quantum Dots R.M. Jock, Jianhua He, A.M. Tyryshkin, S.A. Lyon, C.-H. Lee, S.-H. Huang, C.W. Liu Single electron spin states in silicon have shown a great deal of promise as qubits due to their long spin relaxation (T1) and coherence (T2) times. Recent results exhibit a T2 of 250 us for electrons confined in Si/SiGe quantum dots at 350 mK. These experiments used conventional X-band (10 GHz) pulsed Electron Spin Resonance on a large area (3.5 mm x 20 mm), dual-gated, undoped Si/SiGe heterostructure quantum dots. These dots are induced in a natural Si quantum well by e-beam defined gates having a lithographic radius of 150 nm and pitch of 700 nm. The relatively large size of these dots led to closely spaced energy levels and long T2's could only be measured at sub-Kelvin temperatures. At 2K confined electrons displayed a 3 us T2, which is comparable to that of 2D electrons at that temperature. Decreasing the quantum dot size increases the electron confinement and reduces the effects of valley-splitting and spin-orbit coupling on the electron spin coherence times. We will report results on dots with 80 nm lithographic radii and a 375 nm pitch. This device displays an extended electron coherence time of 30 us at 2K, suggesting tighter confinement of electrons. Further measurements at lower temperatures are in progress. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L36.00003: Electron spins at metal-oxide-silicon (MOS) interfaces J.-S. Kim, R.M. Jock, A.M. Tyryshkin, S.A. Lyon Single electron spins confined in lithographically defined quantum dots in silicon demonstrate long coherence times and are an attractive candidate for qubits and quantum computing applications. Confining electrons at the interface of a metal-oxide-silicon (MOS) structure, as opposed to other Si-based heterostructures, allows for smaller quantum dots by bringing the 2-dimensional electron gas closer to the confining gates. In order to use these electrons as qubits they must be individually confined in quantum dots, but defects at the oxide-silicon interface can lead to unintended electron trapping. The density and depth of these states are functions of the oxide quality and device processing conditions. As such, we have tailored our fabrication process to avoid any high energy processes after the final high temperature anneal. In this work we will characterize the density of trap states in a large area MOS device using electron spin resonance techniques and will present work towards the fabrication of MOS quantum dots. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L36.00004: Spin Relaxation due to Charge Noise in Si Quantum Dot with Valley Splitting Peihao Huang, Xuedong Hu We study the relaxation of an electron spin qubit in a Si quantum dot due to charge noise. In particular, we clarify how the presence of the conduction band valleys influences the spin relaxation. In single-valley semiconductor quantum dots, spin relaxation is through the mixing of spin and envelope orbital states via spin-orbit interaction. In Si, the relaxation could also be through the mixing of spin and valley states. We find that this additional spin relaxation channel, via spin-valley mixing and charge noise, is indeed important for an electron spin in a Si quantum dot. By considering both spin-valley and intra-valley spin-orbit mixings and the charge noise in a Si device, we find that spin relaxation rate peaks at the hot spot, where the Zeeman splitting matches the valley splitting. Furthermore, because of a weaker field-dependence, the spin relaxation rate due to charge noise could dominate over phonon noise at low magnetic fields, which fits well with recent experiments. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L36.00005: Prediction of Dephasing Rates of Si/SiO$_2$ Singlet-Triplet Qubits due to Charge and Spin Defects Neil M. Zimmerman, Dimitrie Culcer Previous theoretical studies of dephasing rates due to time-dependent fluctuations of defects in Si have mostly used ``model'' defects, without reference to the body of knowledge concerning such defects. In this talk, we will present theoretical predictions of the dephasing rates of singlet-triplet qubits in quantum dots at the Si/SiO$_2$ interface, using properties of the known classes of defects in this material system; these defects have been studied intensively for many years in the microelectronics industry, and thus there is a fair amount of knowledge known about them. We set up a theoretical framework aimed at enabling experiment to efficiently identify the most deleterious defects, and complement it with the knowledge of defects. We relate the dephasing rates $\Gamma_\phi$ due to various classes of defects to experimentally measurable parameters such as charge dipole moment, spin dipole moment and fluctuator switching times. Perhaps surprisingly, we find that for spin qubits charge fluctuators are more efficient in causing dephasing than spin fluctuators. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L36.00006: Atomistic analysis of valley-orbit hybrid states and inter-dot tunnel rates in a Si double quantum dot Rifat Ferdous, Rajib Rahman, Gerhard Klimeck Silicon quantum dots are promising candidates for solid-state quantum computing due to the long spin coherence times in silicon, arising from small spin-orbit interaction and a nearly spin free host lattice. However, the conduction band valley degeneracy adds an additional degree of freedom to the electronic structure, complicating the encoding and operation of qubits. Although the valley and the orbital indices can be uniquely identified in an ideal silicon quantum dot, atomic-scale disorder mixes valley and orbital states in realistic dots. Such valley-orbit hybridization, strongly influences the inter-dot tunnel rates.Using a full-band atomistic tight-binding method, we analyze the effect of atomic-scale interface disorder in a silicon double quantum dot. Fourier transform of the tight-binding wavefunctions helps to analyze the effect of disorder on valley-orbit hybridization. We also calculate and compare inter-dot inter-valley and intra-valley tunneling, in the presence of realistic disorder, such as interface tilt, surface roughness, alloy disorder, and interface charges. The method provides a useful way to compute electronic states in realistically disordered systems without any posteriori fitting parameters. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L36.00007: Spin-valley physics in realistic silicon quantum dots Rusko Ruskov, Charles Tahan Silicon quantum dots are leading approach for solid-state quantum bits. However, one must contend with new physics due to the multi-valley nature of silicon. At a Si heterostructure interface the valley degeneracy is lifted and the different valley subspaces of the confined electron spin configurations do not interact. When, however, the valley states are brought at resonance in the presence of a non-ideal interface, spin-valley mixing can occur via spin-orbit coupling. Within the same theoretical framework, we can successfully describe the spin relaxation processes in non-ideal quantum dots [e.g., relaxation ``hot spots'' in C. H. Yang, A. Rossi, R. Ruskov, N. S. Lai, F. A. Mohiyaddin, S. Lee, C. Tahan, G. Klimeck, A. Morello, and A. S. Dzurak, Nature Comm. 4, 2069, (2013)] and a new electron spin resonance (ESR) anticrossing splitting in a double quantum dot transport experiment [X. Hao, R. Ruskov, M. Xiao, C. Tahan, and H. W. Jiang, work in preparation]. Understanding the spin-valley physics of inelastic tunneling is critical to a proper understanding of the transport through double quantum dots, with or without an ESR drive field. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L36.00008: Anisotropic spin-echo dynamics: Maximizing purity for hole spins in quantum dots William Coish, Xiaoya Judy Wang, Stefano Chesi We theoretically study spin-echo dynamics for a central spin qubit coupled anisotropically to a spin bath. Our main focus is on hole spins in quantum dots, with an anisotropic hyperfine coupling to nuclear spins. Through direct application of a systematic Magnus expansion, we analyze the purity of the spin qubit. The purity can characterize non-classical correlations between the spin qubit and bath and provides a figure-of-merit for preserving an ancilla qubit in some initial state. Interestingly, we show that the purity can be preserved to a greater degree by `parking' the spin qubit in a superposition of Zeeman eigenstates, rather than allowing it to align along an applied magnetic field. The procedure reported here provides a general strategy for preserving ancilla qubits in the presence of anisotropic interactions. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L36.00009: The decoherence of exchange-only qubits in triple quantum dots Jo-Tzu Hung, Jianjia Fei, Mark Friesen, Xuedong Hu We study decoherence of a three-electron-spin qubit in a linear triple quantum dot (TQD) by hyperfine interaction. The qubit is encoded in the ($S=1/2$, $S_{z}=1/2$) subspace, and can be fully controlled electrically via exchange interactions $J_{12}$ and $J_{23}$ between the electron spins. We clarify how hyperfine interaction dephases the qubit by constructing effective Hamiltonians and presenting estimates of free evolution and Hahn echo decay for such qubit in a GaAs TQD. When the three electron spins are uniformly coupled, i.e., $J_{12}=J_{23}$, the two states of our qubit are the eigenstates. We find that the qubit decoherence is of order of single-spin decoherence ($T^{*}_{2}\sim 10$ ns, $T_{2}$ on the scale of $\mu$s). On the other hand, a difference between $J_{12}$ and $J_{23}$ requires one to diagonalize the qubit space to obtain an appropriate eigenbasis. Alternatively, the qubit can be viewed as undergoing a rotation. We find that the decoherence rates in the new basis are not significantly modified when comparing them with those in the $J_{12}=J_{23}$ case. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L36.00010: Microscopic models for charge dephasing in semiconductor qubits F\'elix Beaudoin, William A. Coish Charge noise is a ubiquitous source of dephasing in solid-state qubits. In typical models seeking to explain this decoherence mechanism, the charge qubit is dipole-coupled to two-level charge fluctuators distributed in the host material, at interfaces or in oxide layers. Here, we consider various microscopic mechanisms causing fluctuations in the environmental two-level systems, and study the charge qubit's coherence properties in each scenario. In light of recent experimental results reported with semiconductor qubits, we identify which noise mechanism reasonably dominates, and make testable predictions for future experiments. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L36.00011: Effect of Nuclear Quadrupole Moments on Electron Spin Coherence in Semiconductor Quantum Dots Erik Welander, Evgeny Chekhovich, Alexander Tartakovskii, Guido Burkard We theoretically investigate the influence of the fluctuating Overhauser field on the spin of an electron confined to a quantum dot. The fluctuations arise from nuclear spin being exchanged between different nuclei via the nuclear magnetic dipole coupling. We focus on the role of the nuclear interaction from electric quadrupole moments (QPM), which generally cause a reduction in internuclear spin transfer efficiency. By dividing the nuclear problem into subcells we are able to describe $10^4 - 10^5$ nuclei, which are realistic numbers for a quantum dot. The effects on the electron spin coherence time are studied by modeling an electron spin echo experiment. We find that the QPM cause an increase in the electron spin coherence time and that an inhomogeneous distribution, where different nuclei have different QPM, causes an even larger increase than a homogeneous distribution. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L36.00012: Slow Exciton Spin Relaxation in Single Self-Assembled In$_{1-x}$Ga$_x$As/GaAs Quantum Dots Lixin He, Hai Wei, G.-C. Guo We calculate the acoustic phonon-assisted exciton spin relaxation in single self-assembled In$_{1-x}$Ga$_x$As/GaAs quantum dots using an atomic empirical pseudopotential method. We show that the transition from bright to dark exciton states is induced by Coulomb correlation effects. The exciton spin relaxation time obtained from sophisticated configuration interaction calculations is approximately 15--55 $\mu$s in pure InAs/GaAs QDs and even longer in alloy dots. These results is more than three orders of magnitudes longer than previous theoretical and experimental results (a few ns), but agree with more recent experiments that suggest that excitons have long spin relaxation times ($>$ 1 $\mu$s). [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L36.00013: Decoherence of a Driven Qubit Jun Jing, Peihao Huang, Xuedong Hu We study the relaxation of a field-driven qubit. In particular, we find that driving, whether on resonance or off resonance, alters the qubit relaxation rate, allowing both a blue and a red sideband contribution. Depending on the reservoir spectral density and its frequency dependence, the qubit relaxation rate could either be accelerated or reduced. We apply our general theory to the example of an electron spin qubit that is driven by an electric field via electrically driven spin resonance (EDSR), and analyze how spin relaxation induced by charge noise during EDSR varies as a function of driving frequency, driving magnitude, spin-orbit coupling strengths, noise spectrum, and the applied field. [Preview Abstract] |
Session L37: Focus Session: Vacancy and Grain Boundary Effects in Graphene
Sponsoring Units: DMPChair: Kurt Gaskill, U.S. Naval Research Laboratory
Room: 705/707
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L37.00001: Electrical Properties of Overlapped Graphene Grain Boundaries Probed by Raman Spectroscopy Rahul Rao, Neal Pirce, Avetik Harutyunyan The effect of grain boundaries and wrinkles on the electrical properties of polycrystalline graphene is pronounced. Here we investigate the stitching between grains of polycrystalline graphene, specifically, overlapping of layers at the boundaries, grown by chemical vapor deposition (CVD) and subsequently doped by the oxidized Cu substrate. We analyze overlapped regions between 60 -- 220 nm wide via Raman spectroscopy, which reveals their structure to be AB--stacked bilayers. The Raman spectra from the overlapped regions are distinctively different from bilayer graphene and exhibit splitting of the G band peak. The degree of splitting, peak widths, and peak intensities depend on the width of the overlapped layer. We attribute these features to inhomogeneous doping by charge carriers (holes) across the overlapped grain boundaries via the oxidized Cu substrate. As a result, the Fermi level at the overlapped grain boundaries lies between 0.3 and 0.4 eV below the charge neutrality point. The dependence of charge distribution on the width of overlapping of grain boundaries may have strong implications for the growth of large-area graphene with enhanced conductivity. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L37.00002: Vacancy vacancy interactions in graphene using first principles calculations Priya Francis, Chiranjib Majumder, S.V. Ghaisas We employ first principles calculations to study the defect induced magnetism on monolayer graphene sheet. Removal of single, double and triple carbon atoms from a 6 $\times$ 6 monolayer graphene sheet gives magnetic moment (MM) comparable with those reported in the literature. For single vacancy defect, MM value lies in between 1.4 and 1.6 $\mu_B$, depending on the type of lattice atoms ($\alpha$ or $\beta$). For triple vacancy, MM is 1.04 $\mu_B$. Detailed study of double vacancy in graphene layer with a removal of similar and different type of lattice atoms gives interesting results in terms of magnetic moment as well as in spin density distributions. For double vacancy our study gives magnetic moment values 0 and 3 $\mu_B$, depending on the position and lattice type of the removed atoms. Our study reveals that removal of near by same lattice type atoms ($\alpha$ type) also leads to 0 magnetic moment, same as that of the removal of different lattice type atoms ($\alpha$ and $\beta$, irrespective of the distances). Here the distance between the removed two $\alpha$ type atoms is 2.46 \AA. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L37.00003: Measuring the charge of a defect in graphene using atomic force microscopy Lawrence Hudy, Ying Liu, Michael Weinert, Lian Li Graphene exhibits linear dispersion at the Dirac point, leading to novel properties that can be further tailored by the introduction of defects into the honeycomb lattice. In this work, we created vacancies on epitaxial graphene/SiC(0001) using N and Ar plasma, and studied the atomic structure of these defects using non-contact atomic force microscopy with a Q-plus sensor and density functional theory (DFT) calculations. We also determined charges carried by the vacancy defects by local contact potential measurements. These results and comparisons with DFT calculations will be discussed at the meeting. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L37.00004: Defect-mediated coupling between graphene monolayer and hexagonal boron nitride sheet Sohee Park, Gunn Kim Hexagonal boron nitride (hBN) offers an ideal substrate for graphene-based devices and promises to deliver higher device performances. A monolayer of graphene will be weakly coupled to a hBN substrate, but defects between the layers or within the hBN substrate are likely to enhance this coupling. We thus employed first-principles calculations to determine what effect such defects would have on the structural and electronic properties of graphene. We show in our paper that a boron (nitrogen) monovacancy in the hBN layer gives rise to p-doped (n-doped) graphene and that metal atom impurities may increase the energy of the Fermi level of graphene. We also demonstrate that the presence of monovacancies and some impurity atoms could induce residual scattering. However, we found that small triangular defect in hBN are unlikely to result in significant changes in the electronic transport of graphene. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L37.00005: Gate-induced edge-state gap opening in an irradiated graphene nano-ribbon Li Chang, C.S. Chu Graphene undergoes topological phase transition when irradiated by a circularly polarized light. Dispersive helical edge states in both zigzag and armchair graphene nano-ribbons (ZGNR/AGNR) are the signatures. It is of interest to drive the system out-of the edge-state regime by electrical means. To this end, we propose a gate-potential configuration that covers part of the ribbon. For the case of ZGNR, the gate potential is shown to tune the degree of hybridization between edge states formed at opposite edges of the ribbon, leading eventually to edge-state gap opening. For the case of AGNR, the gate potential is less effective. The physics behind our findings will be discussed and supported by both numerical and analytical analysis. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L37.00006: Electron Beam Irradiation Modulated Graphene Electronic Devices Yangbo Zhou, Robert O'Connell, Daniel Fox, Hongzhou Zhang We report controllable graphene doping using focused electron beams. Graphene field-effect-transistor (FET) devices with tunable electronic properties have been fabricated in a Scanning Electron Microscope (SEM). The doping state of graphene can be varied from electron to hole type at different electron beam energies (0.5keV to 30keV). The graphene FET devices exhibit high carrier concentrations ($\sim$ 5x10$^{12}$cm$^{-2})$, high mobility ($\sim$ 5x10$^{3}$cm$^{2}$/V$\cdot$s) and remain quite stable in vacuum. It is found that substrate charging and the induced internal electrical field is responsible for the doping effect. This enables a high spatial resolved, non-destructive and tunable manipulation for graphene electronic device prototyping. Graphene devices based on multi-doped p-n junctions and fine super-lattice structures have also been demonstrated. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L37.00007: Graphene grain boundary resistivity revealed by scanning tunneling potentiometry Corentin Durand, Kendal W. Clark, Xiaoguang Zhang, Ivan V. Vlassiouk, An-Ping Li All large-scale graphene films contain extended topological defects dividing graphene into domains or grains. Here, we study grain boundary (GB) resistivity in CVD graphene on Cu subsequently transferred to a SiO2 substrate. By using a scanning tunneling potentiometry (STP) setup with a cryogenic four-probe STM, the spatial variation of the local electrochemical potential is resolved across individual GBs on a graphene surface in the presence of a current [1]. The 2D distributions of electric field and conductivity were then numerically extracted by solving conduction equations. The derived conductivity of individual grains was compared to that measured with microscopic four-probe STM method to provide a model-independent determination of conductivity map for specific type of defect in graphene. The resistance of a GB is found to change with the width of the disordered transition region between adjacent grains. A quantitative modeling of boundary resistance reveals the increased electron Fermi wave vector within the boundary region, possibly due to boundary induced charge density variation. [1] K. W. Clark et al. ACS Nano 2013, 7, 7956 [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L37.00008: Symmetry protected low energy electronic states in graphene grain boundaries Madeleine Phillips, E.J. Mele We study graphene grain boundaries with the goal of predicting whether they support symmetry protected low energy electronic states localized at the boundary. Starting from a structural model for a grain boundary treated in the short range tight binding limit, we provide an algorithm for generating a chiral operator that anticommutes with the full Hamiltonian. For grain boundaries with nonbipartite lattice structures this operator is highly nonlocal. We identify two classes of grain boundaries: those that admit this chiral representation and those that do not. In the former case a zero energy electronic state is required by symmetry. We compare our results with electronic structures computed for various grain boundaries presented in the literature. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L37.00009: Spanning graphene to carbon-nitride: A 2-D semiconductor alloy system of carbon and nitrogen Joel Therrien, Yancen Li, Daniel Schmidt With the explosion of materials that form 2-D structures in the past few years, there have been a much more diverse ecosystem of combinations of characteristics to explore. Yet with the majority of materials investigated, the properties are fixed according to the composition of the material. Ideally, one wishes to have a tunable system similar to the semiconductor alloy systems, such as Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As. There have been some theoretical studies of transition metal dichalogenides, none have been reported experimentally as of this writing. The tertianary alloy of BCN has been synthesized, however it was found that the boron had the tendency to cause phase segregation of the material into domains of graphene and boron nitride. Here we will report on the synthesis of non-phase seperated carbon-nitrogen 2D alloys ranging from graphene (E$_{\mathrm{g}}=$0 eV) to carbon-nitride, or melon, (E$_{\mathrm{g}}=$2.7 eV). We will report on synthesis methods and a summary of relevant electronic and material properties of selected alloys. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L37.00010: Topological Aspects of Charge-Carrier Transmission across Grain Boundaries in Graphene Oleg V. Yazyev, Fernando Gargiulo Dislocations and grain boundaries are intrinsic topological defects of large-scale polycrystalline samples of graphene. These structural irregularities have been shown to strongly affect electronic transport in this material. Here, we report a systematic investigation of the transmission of charge carriers across the grain-boundary defects in polycrystalline graphene by means of the Landauer-B\"uttiker formalism within the tight-binding approximation. Calculations reveal a strong suppression of transmission at low energies upon decreasing the density of dislocations with the smallest Burgers vector ${\mathbf{b}}=(1,0)$. The observed transport anomaly is explained from the point of view of resonant back-scattering due to localized states of topological origin. These states are related to the gauge field associated with all dislocations characterized by ${\mathbf{b}}=(n,m)$ with $n-m\neq3q$ ($q\in Z$). Our work identifies an important source of charge-carrier scattering caused by the topological defects present in large-area graphene samples produced by chemical vapor deposition. \\[4pt] Reference: F. Gargiulo and O. V. Yazyev, arXiv:1307.6746. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L37.00011: Tunable Anderson Localization in Hydrogenated Graphene Based on the Electric Field Effect: First-Principles Study Joongoo Kang, Su-Huai Wei We present a mechanism for reversible switching of the Anderson localization (AL) of electrons in hydrogenated graphene through modulation of the H coverage on graphene by external electric fields. The main idea is to exploit the unique acid-base chemistry (i.e., proton transfer reaction) between NH$_{\mathrm{3}}$ gas and hydrogenated graphene, which can be controlled by applying perpendicular electric fields. The proposed field-induced control of disorder in hydrogenated graphene not only has scientific merits in a systematic study of AL of electrons in grapheme but can also lead to new insight into the development of a new type of transistor based on reversible on/off switching of AL. Furthermore, the reversible and effective tuning of the H coverage on graphene should be useful for tailoring material properties of weakly hydrogenated graphene. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L37.00012: Drift of Dirac points in defected graphene Marc Dvorak, Zhigang Wu Graphene's remarkable electronic properties are due to isotropic hopping between nearest-neighbor carbon atoms on a honeycomb lattice. If anisotropic hopping is introduced, Dirac points move in reciprocal space away from \textbf{K} and \textbf{K}'. In this work, we investigate the effect of periodic defects on the electronic structure of graphene using both analytic theory and numerical \textit{ab initio} computations. Our tight-binding model suggests that if the defect has a preferred direction, or anisotropy, the Dirac points move in reciprocal space. Analytic predictions for the magnitude and direction of drift are in excellent agreement with \textit{ab initio} calculations. In addition, we show that a semimetal-to-insulator transition occurs when the Dirac points drift onto certain high symmetry points of the supercell Brillouin zone. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L37.00013: Exchange coupling between localized defect states in graphene nanoflakes Matthias Droth, Guido Burkard Graphene nanoflakes are interesting because electrons are naturally confined in these quasi zero-dimensional structures, thus eluding the need for a bandgap. Defects inside the graphene lattice lead to localized states and the spins of two such localized states may be used for spintronics. We perform a tight-binding description on the entire system and, by virtue of a Schrieffer-Wolff-transformation on the bonding and antibonding states, we extract the coupling strength between the localized states. The coupling strength allows us to estimate the exchange coupling, which governs the dynamics of singlet-triplet spintronics. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L37.00014: Spatially-resolved thermopower of graphene: role of boundary and defects Jewook Park, Guowei He, Randall Feenstra, An-Ping Li We show a spatially-resolved thermopower of graphene on epitaxial graphene of SiC using a scanning tunneling microscopy (STM) method. A temperature difference between the tip and sample induces a thermovoltage, and it reflects a local variation of thermopower with atomic resolution. The epitaxial graphene shows a high thermoelectric power of 42 uV/K with a large change (9.6uV/K) at the monolayer-bilayer boundary. Since the thermovoltage is proportional to the logarithmic derivative of the local density of the state of sample, the thermovoltage map is sufficiently sensitive to distinguish electronic properties at the boundaries and defects. For instance, a thermovoltage map discloses a Fermi level shift toward the Dirac point near the step edge and long-wavelength Friedel oscillations of the bilayer terrace adjacent to the step. As a result, the thermopower distribution measurement with STM allows probing of the electronic, thermoelectric, and structural properties down to the individual defect level [1]. [1] Jewook Park et al., Nano Lett., 13 3269 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L37.00015: Thermal and electrical conductivity of defective graphene: From grain boundaries to haeckelite Zhen Zhu, Zacharias G. Fthenakis, David Tomanek We study the effect of structural defects on the electronic and thermal conductivity of graphene from first-principles calcluations. After optimizing defective structures using density functional theory, we describe ballistic charge transport using the non-equilibrium Green's function formalism and thermal transport using non-equilibrium molecular dynamics simulations. We find that both the electrical conductance $G$ and thermal conductivity $\lambda$ depend sensitively on the nature, concentration and arrangement of 5-7 and 5-8 defects, which may form grain boundaries in the honeycomb lattice of graphene or, at large concentrations, convert it to haeckelite. Lines of defects in graphene turn both $\sigma$ and $\lambda$ anisotropic. In a defective structure of graphene nanoribbons interconnected by haeckelite strips, the electrical conductance $G$ increases, whereas the thermal conductivity is quenched by up to 1-2 orders of magnitude, mainly due to the reduced phonon mean free path. We conclude that defects play a profound role in the electrical and thermal transport of graphene. [Preview Abstract] |
Session L38: Invited Session: History of the Communication of Science to the Public
Sponsoring Units: FHP FOEPChair: Brian Schwartz, City University of New York
Room: 709/711
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L38.00001: The Establishment of Science Communication for the Public at the Royal Institution Invited Speaker: Frank Burnet The Royal Institution was founded in 1799, the same year Napoleon came to power by coup d’état in France. Britain had good reason to fear invasion by their old enemy and was at the time very dependent on food imports. The initiative to create the RI was led by Joseph Banks, the then President of the Royal Society of London and Benjamin Thompson, an American who fled to England after picking the losing side in the War of Independence, and became amongst many other things the Bavarian Army Minister and a Count of the Holy Roman Empire. The mission of the RI was to be: ``Diffusing the knowledge and facilitating the general introduction of useful mechanical inventions and improvements, and for teaching by courses of philosophical lectures and experiments, the application of science to the common purposes of life.'' Much of its early activity was directed towards promoting innovation in the field of agriculture and the majority of its founding ``Proprietors'' were wealthy landowners. The teaching part of the Mission was led for over fifty years by two of the greatest scientists of their time Humphry Davy and Michael Faraday. Both of whom played a key role in adding ``blue sky'' as well as applied research to the RIs activities. This presentation will seek to combine an historical account of the RI with reflection on the perspective this provides on current initiatives at the science, innovation and society interfaces. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L38.00002: Displaying Science: The Exhibits Revolution in Science and Natural History Museums, 1900-1990 Invited Speaker: Karen Rader Once defined primarily by their collections, by the end of the twentieth century, American natural history and science museums had become institutions defined largely by their displays. This talk will use life science and physics exhibits to illustrate how and why this transformation occurred. Efforts to modernize displays shaped and were themselves shaped by new institutional roles and identities for museums in twentieth-century science education and in American culture. Drawing on a forthcoming co-authored book (``Life on Display,'' U. Chicago, 2014) this talk will reveal the controversies that accompanied exhibition building, chronicling how and why curators, designers, and educators worked with and against one another to build displays intended to communicate new ideas about topics like evolution, animal behavior, and radiation to the American public. It explains that scientists were extraordinarily invested in the success of museums' displays and saw display as an integral element of their own public outreach work and research agendas. In turn, rapidly professionalizing exhibit designers were periodic participants in the research process, supplementing and sometimes prompting research projects through the displays they built. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L38.00003: The Role of Living History in the Communication of Science to the Public Invited Speaker: Susan Marie Frontczak |
Wednesday, March 5, 2014 9:48AM - 10:24AM |
L38.00004: The Historical Role of the New York Times in the Communication of Science to the Public Invited Speaker: Dennis Overbye |
Wednesday, March 5, 2014 10:24AM - 11:00AM |
L38.00005: The Future of the New Media in the Communication of Science Invited Speaker: Joseph Hanson New media, that which is based around social networks, ubiquitous consumer technology, and today's near-universal access to information, has transformed the way that science is communicated to the scientist and non-scientist alike. We may be in the midst of mankind's greatest shift in information consumption and distribution since the invention of the printing press. Or maybe not. The problem with predicting the future is that it's very hard, and unless you're Isaac Asimov, it's very easy to be wrong. When one predicts the future regarding the internet, that risk becomes almost a certainty. Still, we can apply lessons learned from the near and distant history of science communication to put today's new media evolution into perspective, and to give us clues as to where social media, digital journalism, open access, and online education will lead science communication in years to come. Most importantly, it remains to be seen whether this new media evolution will translate into a shift in how science is viewed by citizens and their policymakers. [Preview Abstract] |
Session L39: Invited Session: Interplay between Charge Order and Superconductivity in Cuprates
Sponsoring Units: DCMPChair: Steven Kivelson, Stanford University
Room: Mile High Ballroom 2A-3A
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L39.00001: CDW in cuprates: insights from inelastic photon scattering Invited Speaker: Matthieu Le Tacon In the search for the mechanism of HTSC, intense research has been focused on the evolution of the spin excitation spectrum on doping from the AF insulating to the superconducting state of the cuprates. We used RIXS to show that~a large family of superconductors (YBa2Cu3O6$+$x) exhibits damped spin~excitations with dispersions and spectral weights closely similar to those of magnons in undoped cuprates [1,2]. In addition to magnetic excitations, RIXS is also sensitive to charge. The greatly enhanced sensitivity of the scattering signal to the valence electron system led us to the discovery of a fluctuating charge density wave competing with the superconducting order at low doping levels [3-5]. Using high resolution inelastic x-ray scattering, we observe a central peak analogous to those observed in conventional CDW systems and attributed to pining of CDW nano-domains on defects. The study of low energy phonons with wavevectors near the CDW ordering vector, also revealed extremely large superconductivity induced lineshape renormalizations as well as anomalous normal state broadening. This provides important insights regarding the long-standing debate of the role of the electron-phonon interaction, a major factor influencing the competition between collective instabilities in correlated-electron materials [6]. Finally I will show how unambiguous signatures of the CDW can be detected using more conventional inelastic scattering of visible light (Raman scattering) [7]. \\[4pt] [1] M. Le Tacon\textit{ et al.}, Nat. Phys. \textbf{7}, 725 (2011).\\[0pt] [2] M. Le Tacon\textit{ et al.}, Phys. Rev. B. \textbf{88}, R020501 (2013).\\[0pt] [3] G. Ghiringhelli\textit{ et al.}, Science \textbf{337}, 821 (2012).\\[0pt] [4] A. J. Achkar\textit{ et al.}, Phys. Rev. Lett. \textbf{109}, 167001 (2012).\\[0pt] [5] S. Blanco-Canosa\textit{ et al.}, Phys. Rev. Lett. \textbf{110}, 187001 (2013).\\[0pt] [6] M. Le Tacon\textit{ et al.}, Nat. Phys. \textit{in press} (2013).\\[0pt] [7] M. Bakr\textit{ et al.}, submitted (2013). [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L39.00002: Competition between superconductivity and charge order in YBCO via X-ray diffraction Invited Speaker: Johan Chang Recently, charge order has been probed by NMR [1], x-ray diffraction [2-5] and ultra sound [6] experiments in the cuprate system YBCO. In this talk, the most recent hard x-ray experiments [2-3] in high magnetic fields will be discussed. Consequences of competition between superconductivity and charge density wave order will be demonstrated. It will for example be shown how direct measurements of the upper critical field, H$_{\mathrm{c2}}$, reveal a 20-fold drop of the superconducting condensation energy as a result of this competition [7]. Connections to the Fermi-surface reconstruction detected by quantum oscillations will also be discussed.\\[4pt] [1] T. Wu \textit{et al.}, Nature 477, 191 (2011).\\[0pt] [2] E. Blackburn \textit{et al}., Physical Review Letters \textbf{110}, 137004 (2013).\\[0pt] [3] J. Chang \textit{et al}., Nature Physics \textbf{8}, 871 (2012).\\[0pt] [4] S. Blanco-Canosa \textit{et al}., Physical Review Letters \textbf{110}, 187001 (2013).\\[0pt] [5] G. Ghiringhelli \textit{et al}., Science \textbf{337}, 821 (2012).\\[0pt] [6] D. LeBoeuf \textit{et al}., Nature Physics \textbf{9}, 79 (2013).\\[0pt] [7] G. Grissonnanche \textit{et al}., arXiv:1303.3856 [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L39.00003: Ubiquitous Interplay between Charge Ordering and High-Temperature Superconductivity in Cuprates Invited Speaker: Eduardo H. da Silva Neto In this talk, we will report on scanning tunneling microscopy (STM) and resonant elastic x-ray scattering measurements that are used to establish the formation of charge ordering in the high-temperature superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{\mathrm{8+x}}$. Depending on the hole concentration, the charge ordering in this system occurs with the same period as those in Y-based or La-based cuprates, but also displays the analogous competition with superconductivity. These results indicate the universality of charge organization competing with superconductivity across different families of cuprates. Our spectroscopic STM measurements demonstrate that this charge ordering leaves a distinct signature in its energy-dependence, which allows us to distinguish the charge order from impurity-induced quasiparticle interference, and to connect it to the physics of a doped Mott insulator [1]. Finally, we will comment on recent claims of electronic nematicity in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{\mathrm{8+x}}$ from STM studies. We show that anisotropic STM tip structures can induce energy-dependent features in spectroscopic maps on different correlated electron systems (cuprates and heavy-fermions) that can be misidentified as signatures of a nematic phase. Our findings show that such experimental features, which can be reproduced by a simple toy model calculation, can be understood as a generic tunneling interference phenomenon within an STM junction [2]. Work done in collaboration with: P. Aynajian, A. Frano, R. Comin, E. Schierle, E. Weschke, A. Gyenis, J. Wen, J. Schneeloch, Z. Xu, R. Baumbach, E. D. Bauer, J. Mydosh, S. Ono, G. Gu, M. Le Tacon, and A. Yazdani \\[4pt] [1] E.H. da Silva Neto et al. Science (2013).\\[0pt] [2] E.H. da Silva Neto et al. PRB 87, 161117(R) (2013).\\[0pt] [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:24AM |
L39.00004: Charge order phase transition in an underdoped cuprate: a sound velocity study Invited Speaker: david le boeuf The recent discovery of charge order in underdoped YBCO has attracted a remarkable wealth of attention [T. Wu et al. Nature 477 191 (2011)]. Not only does it bring a natural explanation for the dramatic change in the Fermi surface observed with quantum oscillation, but recent theories also suggest it might be a fundamental ingredient for the formation of the pseudogap phase [K. B. Efetov et al. Nature Physics 9 442 (2013)]. Hence, the exact role of this charge order in the phase diagram of high-T$_{\mathrm{c}}$ cuprates must be elucidated. Here we use sound velocity, a simple yet very sensitive probe of phase transition, to study this charge order in underdoped YBCO [D. LeBoeuf et al. Nature Physics 9 79 (2013)]. I will show that sound velocity is a unique thermodynamic probe that allows to determine the (B,T) phase diagram of this charge order. This diagram contains information about key aspects of the interplay between charge order and superconductivity, such as competition. I will also show that using tensor properties of elastic constants one can elaborate about the symmetry of this charge order. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 11:00AM |
L39.00005: Electronic ordering and disorder effects in the pseudogap state of YBa$_{2}$Cu$_{3}$O$_{y}$ Invited Speaker: Marc-Henri Julien We report NMR measurements in the normal state of underdoped YBa$_{2}$Cu$_{3}$O$_{y}$. While unambiguous indication of charge-density-wave ordering is found in the pseudogap state, we interpret this as short range CDW order nucleated around native defects. We discuss the connections of this result to the initial evidence of CDW order in YBa$_{2}$Cu$_{3}$O$_{y}$ from NMR in high magnetic fields, to the more recent X-ray scattering data in the normal state as well as to a wider body of experimental results which have been considered to characterize the pseudogap state. [Preview Abstract] |
Session L40: Invited Session: New Horizons for Magnetism and Competing Phases in Heavy Fermions
Sponsoring Units: DCMPChair: Paul Canfield, Ames Laboratory
Room: Mile High Ballroom 2B-3B
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L40.00001: Tuning magnetism by Kondo effect and frustration Invited Speaker: Hilbert v.L\"{o}hneysen Heavy-fermion systems are an ideal playground for studying the quantum phase transition (QPT) between paramagnetic and magnetically ordered ground states arising from the competition between Kondo and RKKY interactions [1]. Two different routes have been identified by various experiments, i. e., the more traditional spin-density-wave (SDW) [2] and the Kondo-breakdown [3] approaches. However, up to now an \textit{a-priori} assignment of a given system to these different routes has not been possible. Yet another route to quantum criticality not included in the above approaches might be geometric frustration of magnetic moments, a route well known for insulating magnets with competing interactions [4]. First experiments on metallic systems have recently been conducted. In the canonical partially frustrated antiferromagnetic system CePd$_{\mathrm{1-x}}$Ni$_{\mathrm{x}}$Al, the N\'{e}el temperature $T_{\mathrm{N}}(x)$ decreases, with $T_{\mathrm{N}}\to $ 0 at the critical concentration $x_{c}\approx $ 0.144. The low-temperature specific heat $C(T)$ evolves toward $C$/$T\alpha $ ln($T_{\mathrm{0}}$/$T)$ for $x\to x_{c}$ [5]. The unusual $T$ dependence of $C$/$T$ is compatible with the Hertz-Millis-Moriya (HMM) scenario of quantum criticality [2] if the quantum-critical fluctuations are two-dimensional in nature. Here two-dimensionality might arise from antiferromagnetic planes that are effectively decoupled by the frustrated Ce atoms in between. An exciting possibility is that the planes of frustrated Ce moments form a two-dimensional spin liquid. In the prototypical heavy-fermion system CeCu$_{\mathrm{6-x}}$Au$_{\mathrm{x}}$ the experiments by Schr\"{o}der et al.[6] provided the initial evidence of local quantum criticality. While concentration and pressure tuning of the quantum phase transition (QPT) are described by this scenario, magnetic-field tuning the QPT is in line with the SDW scenario [7]. Elastic neutron scattering experiments on CeCu$_{\mathrm{5.5}}$Au$_{\mathrm{0.5}}$ under hydrostatic pressure $p$ [8] show that at $p=$ 8 kbar, $T_{\mathrm{N}}$ and the magnetic propagation vector attain almost the values of CeCu$_{\mathrm{5.7}}$Au$_{\mathrm{0.3}}$. This $x-p$ analogy away from the QPT is highly remarkable since the ambient-pressure magnetic structures for $x=$ 0.3 and 0.5 are quite different. These results give clues to a general ($x$,$p$,$B)$ phase diagram at $T=$ 0 and might explain the existence of different universality classes. \\[4pt] [1] H. v. L\"{o}hneysen et al., Rev. Mod. Phys. \textbf{79}, 1015 (2007).\\[0pt] [2] J. A. Hertz, Phys. Rev. B \textbf{14}, 1165 (1976); A. J. Millis, Phys. Rev. B \textbf{48}, 7113 (1993); T. Moriya and T. Takamoto, J. Phys. Soc. Jpn. \textbf{64}, 960 (1995).\\[0pt] [3] Q. Si et al., Nature \textbf{413}, 804 (2001).\\[0pt] [4] B. Keimer and S. Sachdev, Physics Today \textbf{64} (2), 29 (2011).\\[0pt] [5] V. Fritsch et al., arXive 1301.6062, submitted for publication (2013).\\[0pt] [6] A. Schr\"{o}der et al., Nature \textbf{407}, 351-355 (2000).\\[0pt] [7] O. Stockert et al., Phys. Rev. Lett. \textbf{99}, 237203 (2007).\\[0pt] [8] A. Hamann et al., Phys. Rev. Lett. \textbf{110}, 096404 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L40.00002: Magnetic field tuned quantum criticality of heavy fermion system YbPtBi Invited Speaker: Eundeok Mun Quantum criticality triggers an emergence of new quantum phase of matters due to the critical behavior of quantum fluctuations. Heavy fermion (HF) compounds have provided the cleanest evidence for the quantum phase transition. The face-centered cubic YbPtBi is one of the few frustrated stoichiometric Yb-based HF compounds. Measurements of magnetic field and temperature dependent resistivity, specific heat, thermal expansion, Hall effect, and thermoelectric power indicate that the antiferromagnetic (AFM) order ($T_{\mathrm{N}}$ $\sim$ 0.4 K) can be suppressed by applied magnetic field of $H_{\mathrm{c}}$ $\sim$ 4 kOe. In the $H$-$T$ phase diagram of YbPtBi, three regimes of its low temperature states emerges: (I) AFM state, characterized by spin density wave (SDW) like feature, which can be suppressed to $T =$ 0 by the relatively small magnetic field of $H_{\mathrm{c}}$ $\sim$ 4 kOe, (II) field induced anomalous state in which the electrical resistivity follows $\rho (T)$ $\sim$ $T^{1.5}$ between $H_{\mathrm{c}}$ and $\sim$ 8 kOe, and (III) Fermi liquid (FL) state in which $\rho (T)$ $\sim$ $T^{2}$ for $H$ \textgreater 8 kOe. Regions I and II are separated at $T =$ 0 by what appears to be a quantum critical point. Whereas region III appears to be a FL associated with the hybridized 4$f$ states of Yb, region II may be a manifestation of a spin liquid state. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L40.00003: Scaling of the magnetic Gr\"{u}neisen ratio near quantum critical point Invited Speaker: Yoshi Tokiwa The magnetic Gr\"{u}neisen ratio $\Gamma_{\mathrm{H}}=$(1/T)dT/dH is the most sensitive probe of quantum criticality. Its divergence signals the underlying instability. We have studied quantum criticality in the frustrated Kondo lattice system YbAgGe and the heavy fermion superconductor CeCoIn$_{\mathrm{5}}$ by high-precision magnetocaloric effect measurements. In the former, NFL behavior appears around a metamagnetic spin-flop transition between two symmetry broken phases. Previously, it was unclear how the two ordered phases are related to the NFL state. Here, we propose a novel quantum bicritical point (QBCP) scenario, which is distinct from either quantum critical end point or ordinary QCPs with single symmetry broken phase. The observed scaling behavior of $\Gamma_{\mathrm{H}}$ and its characteristic asymmetry across the critical field are consistent with a QBCP scenario. We also report a possible violation of Wiedemann-Franz law at the QBCP in YbAgGe. In CeCoIn$_{\mathrm{5}}$ indications of a quantum critical field hidden inside the superconducting (SC) phase have been extensively debated. We show $\Gamma_{\mathrm{H}}$ data and scaling analysis in the normal state, which surprisingly suggests a zero-field QCP. Anomalous behaviors of $\Gamma_{\mathrm{H}}$ and specific heat within the SC state further support this conclusion.\\[4pt] Work done in collaboration with Markus Garst, Institute for Theoretical Physics, University of Cologne; Jinkui Dong, I. Physical Institute, University of Goettingen; Sergey Bud'ko, Ames Laboratory; Eric Bauer; Los Alamos National Laboratory; Paul Canfield, Ames Laboratory; and Philipp Gegenwart, I. Physical Institute, University of Goettingen. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:24AM |
L40.00004: Quantum Critical Behavior in Heavy-Fermion Iron Oxypnictide Ce(Ru$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})$PO Invited Speaker: Kenji Ishida Quantum phase transition in itinerant ferromagnets is one of the major topics in a strongly correlated electron system, since it has been suggested to be always first order when the ferromagnetic (FM) order is suppressed by pressure or chemical doping [1]. In order to obtain universal features of the FM quantum criticality, we have studied the two-dimensional heavy-fermion (HF) system Ce(Ru$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})$PO from microscopic $^{31}$P-NMR measurements [2-4]. A HF ferromagnet CeRuPO turns into a HF paramagnet by an isovalent Fe substitution for Ru. We found that Ce(Ru$_{0.15}$Fe$_{0.85})$PO shows critical fluctuations down to $\sim$ 0.3 K, as well as the continuous suppression of Curie temperature and the ordered moments by the Fe substitution. These experimental results suggest the presence of a FM quantum critical point (QCP) at around x $=$ 0.86, which is a rare example among itinerant ferromagnets. In addition, we point out that the critical behaviors in Ce(Ru$_{0.15}$Fe$_{0.85})$PO share a similarity with those in YbRh$_{2}$Si$_{2}$ [5], where the local criticality of f electrons has been discussed [6]. We reveal that Ce(Ru$_{\mathrm{1-x}}$Fe$_{\mathrm{x}})$PO is a new system to study FM quantum criticality in HF compound. \\[4pt] [1] D. Belitz, T. R. Kirkpatrick, and J. Rollb\"uhler, Phys. Rev. Lett. \textbf{94}, 247205 (2005).\\[4pt] [2] S. Kitagawa\textit{ et al.}, Phys. Rev. Lett. \textbf{107,} 277002 (2011).\\[0pt] [3] S. Kitagawa \textit{et al.}, K. Ishida, T. Nakamura, M. Matoba, and Y. Kamihara, Phys. Rev. Lett. \textbf{109}, 227004 (2012).\\[0pt] [4] S. Kitagawa \textit{et al.}, J. Phys. Soc. Jpn. \textbf{82}, 033704 (2013). \\[0pt] [5] K. Ishida \textit{et al.}, Phys. Rev. Lett. \textbf{89}, 107202 (2002).\\[0pt] [6] Q. Si, S. Rabello, K. Ingersent, and J. L. Smith, Nature (London) \textbf{413}, 804 (2001). [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 11:00AM |
L40.00005: Tricritical point and wing structure in the phase diagram of UGe$_2$ Invited Speaker: Valentin Taufour Among the numerous reports on quantum criticality, studies on ferromagnets are less common than studies on antiferromagnetic compounds. This is surprising since the paramagnetic to ferromagnetic transition is a textbook example of second order transition and there are several examples where the ferromagnetic transition can be tuned to zero temperature by applied pressure, chemical doping or magnetic field. However, it seems that the transition becomes first order at a tricritical point before being fully suppressed, changing the quantum critical point to a first order quantum phase transition. I will present the case of the superconducting ferromagnet UGe$_2$. In this material, we experimentally located the tricritical point in the temperature-pressure phase diagram. By applying magnetic field, the critical end point, which corresponds to the tricritical point at zero field, can be located leading to a wing-structure in the temperature-pressure-magnetic field phase diagram. The suppression of the critical end point to zero temperature leads to a new kind of quantum criticality: a quantum critical end point. The case of UGe$_2$ will be compared with other ferromagnets, in particular LaCr$_{1-x}$V$_x$Ge$_3$. The work on UGe$_2$ was performed at CEA Grenoble, France with D. Aoki, G. Knebel, H. Kotegawa, L. Malone, I. Sheikin and J. Flouquet. The work on other compounds is performed at my present institution Ames Laboratory, Iowa State University, Ames, Iowa, U.S.A. with U. Kaluarachchi, X. Lin, S. K. Kim, S. L Bud'ko and P. C. Canfield supported by AFOSR-MURI grant FA9550-09-1-0603. \\[4pt] [1] Tricritical point and wing structure in the itinerant ferromagnet UGe$_2$ V. Taufour, D. Aoki, G. Knebel, and J. Flouquet Physical Review Letters 105, 217201 (2010) \\[0pt] [2] Evolution toward Quantum Critical End Point in UGe$_2$ H. Kotegawa, V. Taufour, D. Aoki, G. Knebel, and J. Flouquet Journal of the Physical Society of Japan Vol. 80, No. 8, 083703 (2011) \\[0pt] [3] Ferromagnetic Quantum Critical Endpoint in UCoAl D. Aoki, T. Combier, V. Taufour, T. D. Matsuda, G. Knebel, H. Kotegawa, and J. Flouquet Journal of the Physical Society of Japan Vol. 80, No. 9, 094711 (2011) \\[0pt] [4] Suppression of ferromagnetism in the LaV$_x$Cr$_{1-x}$Ge$_3$ system Lin, Xiao and Taufour, Valentin and Bud'ko, Sergey L. and Canfield, Paul C. Phys. Rev. B 88 094405 (2013) [Preview Abstract] |
Session L41: Focus Session: Local Ionic Dynamics and Domain Walls in Oxides
Sponsoring Units: DMPChair: Lane Martin, University of Illinois
Room: Mile High Ballroom 3C
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L41.00001: Probing Local Ionic Dynamics in Functional Oxides: From Nanometer to Atomic Scale Invited Speaker: Sergei Kalinin Vacancy-mediated electrochemical reactions in oxides underpin multiple applications ranging from electroresistive memories, to chemical sensors to energy conversion systems such as fuel cells. Understanding the functionality in these systems requires probing reversible (oxygen reduction/evolution reaction) and irreversible (cathode degradation and activation, formation of conductive filaments) electrochemical processes. In this talk, I summarize recent advances in probing and controlling these transformations locally on nanometer level using scanning probe microscopy. The localized tip concentrates the electric field in the nanometer scale volume of material, inducing local transition. Measured simultaneously electromechanical response (piezoresponse) or current (conductive AFM) provides the information on the bias-induced changes in material. Here, I illustrate how these methods can be extended to study local electrochemical transformations, including vacancy dynamics in oxides such as titanates, La$_{x}$Sr$_{\mathrm{1-x}}$CoO$_{3}$, BiFeO$_{3}$, and Y$_{x}$Zr$_{\mathrm{1-x}}$O$_{2}$. The formation of electromechanical hysteresis loops and their bias-, temperature- and environment dependences provide insight into local electrochemical mechanisms. In materials such as lanthanum-strontium cobaltite, mapping both reversible vacancy motion and vacancy ordering and static deformation is possible, and can be corroborated by \textit{post mortem} STEM/EELS studies. In ceria, a broad gamut of electrochemical behaviors is observed as a function of temperature and humidity. The possible strategies for elucidation ionic motion at the electroactive interfaces in oxides using high-resolution electron microscopy and combined ex-situ and in-situ STEM-SPM studies are discussed. In the second part of the talk, probing electrochemical phenomena on in-situ grown surfaces with atomic resolution is illustrated. I present an approach based on the multivariate statistical analysis of the coordination spheres of individual atoms to reveal preferential structures and symmetries. The relevant statistical techniques including k-means clustering, principal component analysis, and Baesian unmixing are briefly intriduced. This approach is illustrated for several systems, including chemical phase identification, mapping ferroic variants, and probing topological and structural defects, and provides real space view on surface atomic processes. Research supported (SVK) by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division and partially performed at the Center for Nanophase Materials Sciences (AK, SJ), a DOE-BES user facility. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L41.00002: Charge carrier trapping into mobile, ionic defects in nanoporous ultra-low-k dielectric materials Joel Plawsky, Juan Borja, Toh-Ming Lu, William Gill Reliability and robustness of low-k materials for advanced interconnects has become a major challenge for the continuous down-scaling of silicon semiconductor devices. Metal catalyzed time dependent breakdown (TDDB) is a major force preventing the integration of sub-32nm process technology nodes. We investigate how ionic species can become trapping centers (mobile defects) for charge carriers. A mechanism for describing and quantifying the trapping of charge carriers into mobile ions under bias and temperature stress is presented and experimentally investigated. The dynamics of trapping into ionic centers are severely impacted by temperature and species mass transport. After extended bias and temperature stress, the magnitude of charge trapping into ionic centers decreases asymptotically. Various processes such as the reduction of ionic species, moisture outgassing, and the inhibition of ionic drift via the distortion of local fields were investigated as possible cause for the reduction in charge trapping. Simulations suggest that built-in fields reduce the effect of an externally applied field in directing ionic drift, which can lead to the inhibition of the trapping mechanism. In addition, conduction mechanisms are investigated for reactive and inert electrodes. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L41.00003: Resistive Switching and Temperature-dependent Transport in HfOx-based Resistive Memory Devices Seyoung Kim, Chiyui Ahn, Tayfun Gokmen, Oliver Dial, Mark Ritter Resistive switching phenomenon in transition metal oxide materials has been studied intensively as a candidate technology for future non-volatile memory applications and electronic synapse devices. Here, we demonstrate an HfOx-based resistive memory device with rare earth metal contact in which the device resistance can be modulated with applied voltage and current. Repeatable and self-compliance switching as well as high yield and device-to-device uniformity are achieved in our devices. To understand the conduction mechanism, we perform transport measurement in multiple devices at different resistance states (initial, low and high resistance states) by probing current as a function of applied voltage at temperatures from 40K to 350K. We find that temperature insensitive tunneling conduction dominates at low temperature, while thermally activated conduction is observed at high temperature. Trap-assisted tunneling and Poole-Frenkel mechanisms are accounted for the characteristics found in different regimes. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L41.00004: Study of the Filament Evolution in TiO$_2$ Resistive Switching Device by Schottky Junction Analysis Trithep Devakul, Badih A. Assaf, Pegah M. Hosseinpour, Laura H. Lewis, Don Heiman Resistive switching in TiO$_2$-based metal-dielectric-metal devices is thought to be driven by the dynamics and evolution of oxygen deficient conducting filaments through the bulk of TiO$_2$ [1]. We present an analysis of the resistive switching characteristics of Ti/TiO$_2$/Au devices grown by anodizing Ti on Si and Ti foil. The device is SET and RESET by increasing the switching voltage in small steps. Current-voltage data is obtained at low voltages at each step and analyzed using the Simmons model for thermionic emission of electrons over an energy barrier. The energy barrier consists of an insulating TiO$_2$ barrier sandwiched between an electrode and an oxygen deficient TiO$_x$ filament. The IV fits yield information about the height and width of the energy barrier. In the low resistance state, we find that the barrier width becomes wider, but this is overcome by a lower barrier height. The observed results can be explained by a model in which a field-driven migration of oxygen vacancies [2] modulates the Schottky barrier height and width. \\[4pt][1] D-H. Kwon et al, \emph{Nature Nanotechnology} {\bf 5}, 148-153 (2010) \\[0pt][2] K. J. Yoon et al, \emph{Nanotechnology} {\bf 23}, 185202 (2012) [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L41.00005: Surface potential modification of molecular beam epitaxially grown SrTiO$_{3-\delta}$/Si(001) measured by Kelvin Force Probe Microscopy Ryan Cottier, Alexander Currie, Nikoleta Theodoropoulou SrTiO$_{3}$ (STO) films have been grown by molecular beam epitaxy on p-Si(001), n-Si(001), and STO(001) substrates. The STO/Si films were of high crystalline quality as determined by x-ray diffraction (XRD) and TEM and ranged in thickness from 3.6 to 60 nm as measured by x-ray reflectivity (XRR). The partial pressure of oxygen (O$_{2})$ was varied during growth to induce oxygen vacancies within the STO structure. Through additional XRD and magnetotransport studies, we estimate that for the lowest O$_{2}$ pressure the oxygen deficiency is $\delta =$0.02. The surface potential of the films was modified through the use of a conducting atomic force microscopy (AFM) tip by scanning regions of the STO surface in contact mode with a DC bias on the tip (referred to as `writing'). Regions were written with either positive or negative voltage and then analyzed by Kelvin Force Probe Microscopy (KFPM). Following this writing mode, KFPM revealed a retained surface potential of the same polarity used in writing. The ability of the films to be written and read through this method depended on the growth O$_{2}$ partial pressure with higher O$_{2}$ pressures demonstrating weaker surface potential modification. The results agree with other studies regarding the drift and diffusion of charged O$_{2}$ vacancies in STO. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L41.00006: Effect of antisite-like defect in ferroelectricity of SrTiO$_{3}$ films Fang Yang, Zhenzhong Yang, Lin Gu, Jiandong Guo Ferroelectricity in thin films of nominally nonferroelectric materials such as SrTiO$_{3}$ has been widely studied. It is known that some extrinsic factors such as strain [M. P. Warusawithana et al. Science 324, 367 (2009)] and defect [H. W. Jang et al., Phys. Rev. Lett. 104, 197601 (2010), M. Choi et al., Phys. Rev. Lett. 103, 185502 (2009)] can result in the ferroelectricity of SrTiO$_{3}$ thin films. The SrTiO$_{3}$ thin films with ferroelectricity were prepared on Si (001) substrates by oxide molecular beam epitaxy. The energy dispersive x-ray spectroscopy (EDX) mapping measurement results demonstrate Sr diffuses to the interface of SrTiO$_{3}$ and Si. The cross sectional high-resolution transmission electron microscopy (HRTEM) results show that there are interstitial Ti atoms in the unit cells. The off-centered Ti from the Sr site along [100] or [110] direction can be regarded as a polar defect pair composed of a Sr vacancy and an interstitial Ti. It is predicted that Ti antisitelike defects in SrTiO$_{3}$ are responsible for the ferroelectricity . Such antisitelike defects observed in SrTiO$_{3}$ films are considered as the origin of the ferroelectricity. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L41.00007: Main photorefractive Fe center in KNbO$_{3}$ Sergey Basun, Dean Evans KNbO$_{3}$ crystals have been attracting a lot of interest in nonlinear optics. In iron-doped samples, a dramatic increase in photorefractive sensitivity and speed can be achieved. Despite the variety of Fe centers known in KNbO$_{3}$:Fe, the Fe center responsible for the photorefractive effect has not been previously identified. Correlated EPR and optical studies of the as-grown and reduced samples shed light on the nature of the main photorefractive center in KNbO$_{3}$:Fe -- it is Fe[Nb]-V$_{\mathrm{O}}$, Fe on the Nb site next to oxygen vacancy. Fe[Nb] centers that are commonly considered as the cause of photorefraction in KNbO$_{3}$:Fe are only of secondary significance. Free electrons are provided through photoionization of Fe$^{2+}$[Nb]-V$_{\mathrm{O}}$ with photon energies higher than 0.85 eV, Fe$^{3+}$[Nb]-V$_{\mathrm{O}}$ centers serve as electron traps. Concentration of the Fe[Nb]-V$_{\mathrm{O}}$ centers is quite comparable to that of Fe[Nb], but in the as-grown samples they are only present in the form of Fe$^{3+}$[Nb]-V$_{\mathrm{O}}$. Reduction of the samples takes almost no effect on Fe$^{3+}$[Nb], but it appreciably converts Fe$^{3+}$[Nb]-V$_{\mathrm{O}}$ to Fe$^{2+}$[Nb]-V$_{\mathrm{O}}$, which gives rise to the concentration of the photo-electron donors and dramatically improves photorefractive performance. Locations of the Fe$^{2+/3+}$[Nb]-V$_{\mathrm{O}}$ and Fe$^{3+/4+}$[Nb] centers in the bandgap will be presented. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L41.00008: Structure and switching of in-plane ferroelectric nano-domains in strained Pb(x)Sr(1-x)TiO(3) thin films Sylvia Matzen, Oleksiy Nesterov, Gijsbert Rispens, Jeroen Heuver, Beatriz Noheda, Michael Biegalski, Hans Christen Understanding and controlling domain formation in nanoscale ferroelectrics is interesting from a fundamental point of view and of great technological importance. Increasing miniaturization allows creating complex domain structures, offering novel functionalities that could be particularly useful for the development of nanoelectronic devices. While most studies in thin films focus on domain patterns with up/down polarization for ferroelectric memories, domain structures with purely in-plane polarization have not been much investigated. However, such structures are potentially useful in optical devices or to avoid depolarization fields in ultra-thin films, as long as the domains can be addressed and switched. We use a combination of compositional substitutions and epitaxial growth on a substrate in order to tune the domain configuration. The substitution of Pb by Sr in Pb$_{\mathrm{x}}$Sr$_{\mathrm{1-x}}$TiO$_{\mathrm{3}}$ thin films grown epitaxially on (110)-DyScO$_{\mathrm{3}}$, stabilizes a domain structure with purely in-plane polarization. In this work, we show that it is possible to stabilize and control a complex domain architecture at two different length scales, yielding periodic ferroelectric nano-domains with purely in-plane polarization. Most importantly, these in-plane domains can be switched by a scanning probe. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L41.00009: Complex structural behaviors of domain walls revealed from first principles Jorge Iniguez, Jacek C. Wojdel, Oswaldo Dieguez We have used a variety of theoretical techniques, including accurate first-principles methods and model potentials allowing for large-scale simulations, to investigate the structural and lattice-dynamical behavior of the domain walls in several ferroic perovskite oxides. In this talk I will review the most striking effects our work has revealed, which range from emerging orders confined at the walls to novel forms of polytypism. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:48AM |
L41.00010: Charged domain walls in ferroelectrics Invited Speaker: Tomas Sluka Solid interfaces including compositionally homogeneous ferroic domain walls (DWs) display uniquely distorted electronic structures and ionic displacements. Their intrinsic properties may therefore be fundamentally different from those of their parent matrices. Indeed, phenomena like semiconductor-metal transition, the quantum Hall effect, magnetoresistance and superconductivity were discovered at hetero-interfaces between transition metal oxides and elevated photoactivity and conductivity were reported at (multi-) ferroic DWs. Unlike hetero-interfaces, the DWs provide ``perfect'' structure by nature and can be written, displaced, and erased inside a material monolith of functioning devices. Theory predicts the existence of charged DWs which seemingly violate electrostatic compatibility due to head-to-head and tail-to-tail polarization discontinuity, but are stable because bound polarization charge is compensated by mobile charge carriers including quasi-two-dimensional electron gas. This talk will introduce current theory, engineering, control and characteristics of charged DWs, which are mobile, extremely wide and exhibit steady metallic-like conductivity up to $10^9$ times that of the insulating bulk. [Preview Abstract] |
Session L42: Focus Session: Topological Insulators: Spin Textures
Chair: Zahid Hasan, Princeton UniversityRoom: Mile High Ballroom 4A
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L42.00001: Mapping the Spin texture of topological insulators Invited Speaker: Alessandra Lanzara The helical spin texture of surface electrons in topological insulators has attracted a great deal of interest in the past few years. In this talk I will present new results obtained by using an innovative ultra-high efficiency spin-resolved photoemission spectrometer to map the spin texture of Bi$_2$Se$_3$ topological insulator throughout the entire momentum space. We discover a surprising property of these surface electrons, e.g. that the spin polarization of the resulting photoelectrons can be fully manipulated by light in three dimensions. The evolution of spin texture as a function of the light polarization is also studied. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L42.00002: Hedgehog spin texture and competing orders on the surface of strained topological crystalline insulators Cheng-Yi Huang, Wei-Feng Tsai, Yung Jui Wang, Hsin Lin, Arun Bansil We discuss spin reorientation phenomena, which may or may not yield gap formation, on the surface of topological crystalline insulators Pb$_{\mathrm{1-x}}$Sn$_{\mathrm{x}}$(Te, Se) under various applied strains. The low-energy surface electrons on the (001) surface behave like massless Dirac particles with four Dirac points centered along the intersection of the mirror (\textit{xz} or \textit{yz}) plane and the surface plane. We use a four-band k.p model, which captures the spin and orbital texture of the surface states around surface X (or Y) point up to the energy around the Lifshitz transition, and systematically study effects of the applied strain. In contrast to the case without any strain, where the absence of the out-of-the-plane spin component is guaranteed by both the mirror and the time-reversal symmetries, we find that without time-reversal symmetry breaking, the hedgehog-like spin textures associated with a gap formation can be induced by the strain only, breaking the \textit{xz} mirror symmetry. The other cases cannot induce a gap at Dirac points. We also investigate interaction-driven competing orders under the strain and obtain a phase diagram at the mean-field level to reveal the possible novel surface states in the system. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L42.00003: Laser-based spin- and angle-resolved photoemission spectroscopy for rapid, high-resolution measurements Kenneth Gotlieb, Aaron Bostwick, Zahid Hussain, Alessandra Lanzara, Christopher Jozwiak A unique spin-and angle-resolved photoemission spectrometer (spin-ARPES) is coupled with a 6 eV laser to achieve unprecedented measurements of near-EF physics in topological insulators and Rashba systems. The pairing of the spin-ARPES system with the laser allows for energy and angular resolutions never before seen in a spin-ARPES experiment. ~Most importantly, the high efficiency of the system and high photon flux of the laser make measurements very rapid, permitting exploration of a large experimental phase space. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L42.00004: Tunable spin helical Dirac quasiparticles in HgTe surfaces Chang Liu, Guang Bian, Su-Yang Xu, Ilya Belopolski, Hsin Lin, Christian Matt, Irek Miotkowski, Nasser Alidoust, Madhab Neupane, Robert S. Markiewicz, Arun Bansil, Vladimir N. Strocov, Mark Bissen, Alexei V. Fedorov, Taichi Okuda, Yong P. Chen, M. Zahid Hasan We show with photoemission spectroscopy that bulk HgTe is a topologically nontrivial material, possessing a Dirac-cone surface state with clear, unmodulated, left-right imbalanced spin polarization and circular dichroism. This topological surface state maintains its surface character even within the bulk continuum due to topological protection, in drastic contrast with ordinary solid where a surface band usually extends into the bulk and loses its surface character when it degenerates in energy with a bulk state. The Dirac transport regime of HgTe, where the topological surface state is fully exposed and free from influences of the bulk bands, can be easily reached by alkali metal deposition onto the surface. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L42.00005: Photoemission of Bi$_2$Se$_3$ with Circularly Polarized Light: Probe of Spin Polarization or Means for Spin Manipulation? O. Rader, J. S\'anchez-Barriga, A. Varykhalov, J. Braun, S.-Y. Xu, N. Alidoust, O. Kornilov, J. Min\'ar, K. Hummer, G. Springholz, G. Bauer, R. Schumann, L.V. Yashina, H. Ebert, M.Z. Hasan, G. Bauer Topological insulators are characterized by Dirac cone surface states with spins aligned in the surface plane and perpendicular to their momenta. Recent theoretical and experimental work implied that this specific spin texture should enable control of photoelectron spins by circularly polarized light. However, these reports questioned the so far accepted interpretation of spin-resolved photoelectron spectroscopy. We solve this puzzle and show that vacuum ultraviolet photons (50-70 eV) with linear or circular polarization probe indeed the initial state spin texture of Bi$_2$Se$_3$ while circularly polarized 6 eV low energy photons flip the electron spins out of plane and reverse their spin polarization. Our photoemission calculations, considering the interplay between the varying probing depth, dipole selection rules and spin-dependent scattering effects involving initial and final states explain these findings, and reveal proper conditions for light-induced spin manipulation for future applications in opto-spintronic devices. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L42.00006: Photoelectron spin-polarization control in the topological insulator Bi$_2$Se$_3$ Zhihuai Zhu, C.N. Veenstra, S. Zhdanovich, M.P. Schneider, G. Levy, T. Okuda, K. Miyamoto, S.-Y. Zhu, P. Syers, N.P. Butch, J. Paglione, M.W. Haverkort, I.S. Elfimov, A. Damascelli We study Bi$_2$Se$_3$ by angle-resolved photoemission spectroscopy (ARPES) and density functional theory. We find that the topological surface state (TSS) is characterized by a layer-dependent entangled spin-orbital texture, which becomes apparent through photoelectron interference effects in ARPES. This explains the discrepancy between the spin polarization obtained in spin-ARPES--ranging from 20\% to 85\%--and the 100\% value assumed in phenomenological models [1]. We demonstrate how to probe the intrinsic spin texture of TSS by spin-ARPES, and continuously manipulate the spin polarization of photoelectrons and photocurrents all the way from 0 to +/-100\% by an appropriate choice of photon energy, polarization, and angle of incidence [2]. As illustrated by a minimal two-atomic-layer model, photoelectron spin-polarization control is generically achievable in systems with a layer-dependent entangled spin-orbital texture as a direct manifestation of dipole selection rules, photoelectron interference, and TSS complex structure [2].\\[4pt] [1] Z.-H. Zhu et al., Phys. Rev. Lett. 110, 216401 (2013)\\[0pt] [2] Z.-H. Zhu et al., submitted to Phys. Rev. Lett. (2013) [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L42.00007: Observation of a collective mode of the helical liquid on the surface of Bi$_{2}$Se$_{3}$ Anshul Kogar, Sean D. Vig, Alexander Thaler, Man-Hong Wong, Tai-Chang Chiang, Gregory J. MacDougall, Luc Venema, Peter Abbamonte The helical Dirac band structure at the surface of three-dimensional topological insulators has been theoretically predicted to give rise to unusual surface collective modes. Of particular interest is the ``spin-plasmon,'' a coupled collective excitation involving both the spin and charge degrees of freedom. In this talk, I will present data suggesting that we have observed this excitation on the surface of Bi2Se3 using angle-resolved inelastic electron scattering from the surface. In our study, we have grown samples of different dopings and shown, using angle-resolved photoemission, that we can suppress the Fermi Level into the bulk band gap. The evolution from a bulk-band free-carrier surface plasmon into a Dirac band surface spin-plasmon has been observed as a function of doping. The dependence of the spin-plasmon on momentum transfer as well as time will also be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L42.00008: Orbit- and Atom-Resolved Spin Textures of Intrinsic, Extrinsic and Hybridized Dirac Cone States Lin Miao, Zhengfei Wang, Mengyu Yao, Fengfeng Zhu, J. Hugo Dil, Chunlei Gao, Canhua Liu, Feng Liu, Dong Qian, Jinfeng Jia Combining first-principles calculations and spin- and angle-resolved photoemission spectroscopy measurements, we identify the helical spin textures for three different Dirac cone states in the interfaced systems of a 2D topological insulator (TI) of Bi(111) bilayer and a 3D TI Bi$_{2}$Se$_{3}$ or Bi$_{2}$Te$_{3}$. The spin texture is found to be the same for the intrinsic Dirac cone of Bi$_{2}$Te$_{3}$ or Bi$_{2}$Se$_{3}$ surface state, the extrinsic Dirac cone of Bi bilayer state induced by Rashba effect, and the hybridized Dirac cone between the former two states. Further orbit- and atom-resolved analysis shows that $S$ and $P_{z}$ orbits have the conventional helical spin texture; $P_{\mathrm{x}}$ and$ P_{\mathrm{y}}$ orbits show individually radial spin component, while the sum of the two shows a total in-plane helical spins. The orbit-dependent spin structure is a signature property of spin-orbit coupling, independent of topology. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L42.00009: Spin-resolved Andreev levels and parity crossings in hybrid superconductor-semiconductor nanowires Ramon Aguado, Eduardo Lee, Xiaocheng Jiang, Manuel Houzet, Charles Lieber, Silvano De Franceschi I will present measurements and theory of the Zeeman effect on the Andreev levels of a semiconductor quantum dot, based on an InAs nanowire, with large electron g-factors strongly coupled to a conventional superconductor with large critical field. This material combination allows spin degeneracy to be lifted without destroying superconductivity. When the system is in a spin singlet state, a spin-split Andreev level crossing the Fermi energy results in a quantum phase transition to a spin-polarized state, implying a change in the fermionic parity of the system. This crossing manifests itself as a finite-field, zero-bias conductance anomaly [1] whose properties resemble those expected for Majorana modes in a topological superconductor [2-3]. While this resemblance is understood without evoking topological superconductivity, the observed parity transitions could be regarded as precursors of Majorana modes in the long-wire limit [4]. \\[4pt] [1] E. J. H. Lee, X. Jiang, M. Houzet, R.Aguado, C. M. Lieber, and S. De Franceschi, Nature Nanotechnology, in press (2013).\\[0pt] [2] V. Mourik, et al, Science 336, 1003-1007 (2012).\\[0pt] [3] A. Das, et al. Nature Phys. 8, 887-895 (2012).\\[0pt] [4] T. D. Stanescu, et al, Phys. Rev. B 87, 094518 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L42.00010: Orbital-Selective Spin Texture and its Manipulation in a Topological Insulator Zhuojin Xie, Shaolong He, Chaoyu Chen, Ya Feng, Hemian Yi, Aiji Liang, Lin Zhao, Daixiang Mou, Junfeng He, Yingying Peng, Xu Liu, Yan Liu, Guodong Liu, Xiaoli Dong, Li Yu, Jun Zhang, Shenjin Zhang, Zhimin Wang, Fengfeng Zhang, Feng Yang, Qinjun Peng, Ziaoyang Wang, Chuangtian Chen, Zuyan Xu, Xingjiang Zhou Topological insulators represent a new quantum state of matter, possessing a unique electronic structure and spin texture. In the Dirac surface state, the spin is locked with the crystal momentum. Here we report a new phenomenon of the spin texture locking with the orbital texture in a TI Bi2Se3. The laser-based SARPES can directly measure the spin texture of both the upper and lower Dirac cones in Bi2Se3 surface state under different light polarizations. An unexpected spin texture is revealed for the first time in the s-polarization geometry that the upper Dirac cone exhibits the same spin chirality with the lower one, while the opposite spin chirality is observed for the upper and lower Dirac cones in the p-polarization geometry. Because different orbitals and their coupled spin texture are selectively probed by using variable light polarizations, these results constitute strong evidence of the orbital-dependent spin texture in Bi2Se3. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L42.00011: The aging effect in topological insulator Bi2Se3 Kyungwha Park, Christophe De Beule, Bart Partoens Topological insulators (TIs) have attracted a lot of interest due to their topologically protected surface states, as well as exotic proximity-induced phenomena. Since the first experimental data of TIs, angle-resolved photoemission spectra (ARPES) showed that the electronic structure of the topological surface states significantly changes with time after cleavage. The origin and underlying mechanism of this aging effect are still under debate, despite its importance. Here we present our study of the evolution of the surface Dirac cone for Bi2Se3 films upon asymmetric potassium (K) adsorption, using density-functional theory and a tight-binding model. We find that the K adatoms induce short-ranged downward band bending within 2-3 nm from the surface, due to charge transfer from the K to the TI. Our findings are in contrast to earlier proposals in the literature. As the charge transfer increases, we also find that a new Dirac cone, localized slightly deeper into the TI than the original one, appears at the K-adsorbed surface, arising from strong Rashba-split conduction-band states. Our results suggest possible reinterpretations of experiments because the new Dirac cone might have been observed in ARPES measurements instead of the original one that appears just after cleavage. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L42.00012: Evidence for coexistence of Rashba and Dresselhaus effect on semiconductor Wonsig Jung, S.H. Jo, B.Y. Kim, M. Leandersson, B. Thiagarajan, J.S. Hong, J.H. Shim, Changyoung Kim We have performed preliminary circular dichroism angle-resolved photoemission spectroscopy (ARPES) experiments on InSb. Our results show very strong circular dichroism (CD) signal, indicating probable existence of orbital angular momentum (OAM). Non-zero OAM in zincblend semiconductor can appear when there is an inversion symmetry breaking (IBS) in the bulk and on the surface. We find that the dichroism has momentum and band dependence. The CD modulations can be the evidence for coexistence of Rashba and Dresselhaus effect on semiconductor. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L42.00013: Unusual Surface Termination of the Topological Insulator Bi2Se3 Andrew Hewitt, Jingying Wang, Jonathon Boltersdorf, Tianshuia Guan, Paul Maggard, Daniel Dougherty The strong three-dimensional topological insulator has been heralded as a new state of matter with metallic topological surface states (TSS's) of interest for spintronic applications. Recent experiments of the strong 3-D TI Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ have shown surprising discrepancies about whether the atomic termination of Bi2Se3 is either Se or Bi with evidence for both types. We have observed both metallic Bi and Se terminated surfaces on single crystals of Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ synthesized by established growth techniques and purchased from commercial suppliers. X-ray Photoelectron Spectroscopy reveals little oxidation for air-cleaves in either the Bi4f or the Se3d core levels. However, a low binding energy component for the Bi4fs is often observed which is indicative of Bi-Bi bonds near the surface. This termination is observed in approximately 50{\%} of air-cleaved samples that have been stored in air, whereas samples stored in rough vacuum have a lower tendency ($\sim$ 10{\%}) to have a Bi-terminated surface. Angle-Resolved Photoelectron Spectroscopy shows unusual valence band structure for Bi-terminated samples which upon annealing revert to a similar band structure for Se-terminated surfaces. Understanding the surface of these materials is essential for interpreting transport measurements on cleaved single crystals. [Preview Abstract] |
Session L43: Metals I
Sponsoring Units: DCMPChair: David Singh, Oak Ridge National Laboratory
Room: Mile High Ballroom 4B
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L43.00001: Metal-based photocathode materials able to sustain high currents Zhaozhu Li, Kaida Yang, Jose Riso, Rosa Lukaszew Existing photocathode technology may not meet the various requirements for long photocathode lifetime, high current and repetition rate, high polarization and/or low emittance that are required for next generation light sources and nuclear physics accelerator capabilities, particularly for electron ion colliders (EIC). Specifically, next-generation light sources will need MHz repetition rates with high charge, high energy, low emittance, and a very high repetition rate while new EIC proposals stipulate hundreds of mA of current. Metallic photocathodes offer several advantages over present semiconductor photocathodes for these stringent requirements but also exhibit low QE. Coupling to the surface Plasmon polariton (SPP) modes on the metal surface offers an ideal solution to decrease the optical penetration depth and reduce the metal reflectivity thus leading to higher QE. We will present our results exploring metallic photocathode performance by enabling Surface Plasmon Polariton excitation as well as the use of adequate over-layers since it has been shown that this can also reduce the work function thus enhancing QE. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L43.00002: Majorana Fermion Induced Non-local Current Correlations in Spin-orbit Coupled Superconducting Wires Jie Liu, Fu-Chun Zhang, K.T. Law The observation of zero bias conductance peaks in semiconductor wire-superconductor heterostructures has generated great interest, and there is a hot debate on whether the observation is associated with Majorana Fermions (MFs) or other effects which enhance local Andreev reflections. In this work, we study the transport of a normal lead/semiconductor wire-superconductor heterostructure /normal lead junction. We show that when MF end states from the two ends of the wire are strongly coupled, the MF end states can suppress local Andreev reflections and strongly enhance crossed Andreev reflections (CARs), in which an electron from one lead is reflected as a hole in a different lead. In the CAR dominated regime, the current-current correlations between the two leads are strongly enhanced. Moreover, the Fano factor of a normal lead, which is the ratio of the shot noise to the average current, is reduced from 2e to e. Since the CAR associated effects are non-local effects and they cannot be induced by processes which enhance local Andreev reflections, therefore, the measurement of Fano factors and current-current correlations of the normal leads can be used to identify MFs. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L43.00003: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L43.00004: Launching low energy surface plasmons in purple gold (AuAl$_{2}$) Panupon Samaimongkol, Hans D. Robinson The intermetallic compound AuAl$_{2}$ is sometimes known as ``purple gold'' due to its intense purple color. It was long assumed this color originated from an interband absorption transition, but, as has recently be pointed out [1], it is related to a surface plasmon resonance which exists in this material at a lower energy than in any of the pure metals (including Au, Ag, Cu, etc.). Thin films of purple gold can readily be prepared by layered evaporation of Au and Al in the proper~ratios, followed by annealing at moderately elevated temperatures. Fabricating AuAl$_{2}$ films on the hypotenuse of high index glass prisms, we have been able to launch surface plasmons in this material in the Kretschmann configuration, from which we directly extract the dispersion relation of surface plasmons, in good agreement with predictions from previous measurements of its dielectric function. Surface plasmon sensing results using AuAl$_{2}$ will also be presented. Finally, we will discuss the possible applications for purple gold in nanoparticle form, where it has the property of being an excellent light absorber across the entire visible spectrum.\\[4pt] [1] Keast, V.J., et al., Appl. Phys. Lett.~99, 111908~(2011). [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L43.00005: Infrared optical conductivity for Ni$_{1-x}$Pt$_{x}$ alloys and Ni$_{1-x}$Pt$_{x}$Si monosilicides Lina Abdallah, S. Zollner, C. Lavoie, A. Ozcan, M. Raymond We used infrared spectroscopic ellipsometry to measure the optical constants of Ni$_{1-x}$Pt$_{x}$ alloys and Ni$_{1-x}$Pt$_{x}$Si monosilicides in the infrared region (200-6000 wave numbers). Nickel platinum alloys (up to 30\% Pt) were deposited on top of a thick layer of thermal oxide. Similar alloys were deposited on top of silicon and were annealed at 500 $^{\circ}$ for 30 seconds to create monosilicides. The Pt composition dependence of the optical conductivity of the unreacted metal and monosilicide was investigated. We also studied the thickness dependence of Ni1-xPtx alloys. Four different fitting techniques were applied to our data using a point-by-point fit as well as an oscillator fit where we found a two carrier effect in the dielectric function of the unreacted metal. The data obtained from the IR region was combined with previous data in the visible spectrum to get more comprehensive optical constants for both unreacted metal and reacted silicide. Results indicate that optical conductivity is higher for thicker samples. Furthermore SiO$_{2}$ has a strong absorption peak at 1040 wave numbers caused by bond stretching vibration. This absorption peak appears in the ellipsometric data of the thinner films and is absent in thicker ones. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L43.00006: Anomalous Kondo transport in a single-electron transistor driven by microwave field Zhan Cao, Cheng Chen, Fu-Zhou Chen, Hong-Gang Luo The Kondo transport in a single-electron transistor continues to provide unexpected physics due to the interplay between magnetic field and microwave applied, as shown in a recent experiment(B. Hemingway et al., arXiv:1304.0037). For a given microwave frequency, the Kondo differential conductance shows an anomalous magnetic field dependence, and a very sharp peak is observed for certain field applied. Additionally, the microwave frequency is found to be larger of about one order than the corresponding Zeeman energy. These two features are not understood in the current theory. Here we propose a phenomenological mechanism to explain these observations. When both magnetic field and microwave are applied in the SET, if the frequency matches the (renormalized) Zeeman energy, it is assumed that the microwave is able to induce spin-ip in the single-electron transistor, which leads to two consequences. One is the dot level shifts down and the other is the renormalization of the Zeeman energy. This picture can not only explain qualitatively the main findings in the experiment but also further stimulate the related experimental study of the Kondo transport. Additional microwave modulation may provide a novel way to explore the functional of the SET in nanotechnology and quantum information processing. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L43.00007: Kondo effect of an adatom in graphene and metallic surface Hong-Gang Luo I will present the Kondo effect of a single magnetic adatom on the surface of graphene[1] and metal[2]. The unique linear dispersion relation near the Dirac points in graphene favors magnetic moment[3], which simply means that the Kondo resonance can be observed in a more wider parameter region than in the metallic host. Our work indicated that the Kondo resonance, whenever the chemical potential is tuned away from the Dirac points, indeed can form ranged from the Kondo regime, to the mixed valence, even to the empty orbital regime defined in the conventional metal host. Correspondingly, the Kondo resonance can exhibit as a sharp peak, a peak-dip or an anti-resonance in different regimes. These lineshapes result from the Fano resonance [4] due to the significant background leaded by dramatically broadening of the impurity level in graphene. The scanning tunneling microscopy (STM) spectra of the adatom are also showed and have obvious particle-hole asymmetry when the chemical potential is tuned by the gate voltages applied to the graphene. References: [1] L. Li et al., New J. Phys. 15, 053018 (2013). [2] H.-G. Luo et al., Phys. Rev. Lett. 92, 256602(2004); \textit{ibid}., 96, 019702 (2006). [3] B. Uchoa et al., Phys. Rev. Lett. 101, 026805(2008). [4] U. Fano, Phys. Rev. 124, 1866 (1961). [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L43.00008: Low temperature thermal conductivity of alloys used in cryogenic coaxial cables Akihiro Kushino, Soichi Kasai We have developed thin seamless coaxial cables applied for readout in low temperature experiments below liquid helium temperature. Stainless steel employed as the center and outer electrical conductors of the coaxial cable has adequately low thermal conductivity compared to pure metals and can be used when heat penetration into low temperature stages through cables should be lowered however it has large electrical resistivity which can disturb sensitive measurements. Superconducting NbTi alloy has good performance with rather low thermal conductivity and high electrical conductivity. Meanwhile coaxial cables using normal conducting copper alloys such as cupro-nickel, brass, beryllium-copper, phosphor-bronze are advantageous with their good electrical, thermal and cost performances. We investigated thermal conductivity of such alloys after the drawing process into coaxial cables, and compared to expected values without drawing. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L43.00009: Doping dependence of structural transitions in LaCu$_{6-x}$Ag$_x$ L. Poudel, M. Koehler, M. McGuire, V. Keppens, C. Dela Cruz, D. Mandrus, A. Christianson CeCu$_{6-x}$Au$_x$ is a well known heavy fermion system that exhibits a quantum critical point (QCP) at x $\simeq$ 0.1. One interesting feature of the CeCu$_{6-x}$Au$_x$ system is the doping dependence of the structural phase transition. The end member CeCu$_6$ undergoes a structural transition from an orthorhombic to a monoclinic phase at 230 K. The transition temperatures drop linearly with Au concentration until the transition is suppressed at x $\simeq$ 0.1. This is the same composition where the antiferromagnetic quantum critical point occurs. We study the related system, LaCu$_{6-x}$Ag$_x$, in order to determine the behavior of structural transition as is it suppressed without the complicating influence of magnetism. In analogy with the CeCu$_{6-x}$Au$_x$ system, LaCu$_6$ displays a structural transition from an orthorhombic to a monoclinic phase at 460 K, which is suppressed by Ag doping. The suppression of the transition temperature occurs in conjunction with an expansion on of the lattice. Both the transition temperature and the monoclinic order parameter $(ac Cos\beta)^2$ scale linearly with Ag doping. Extrapolation of either the transition temperature or the monoclinic order parameter indicates that the structural transition is suppressed completely for x $\simeq$0.225 [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L43.00010: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L43.00011: The Role of Lattice Dynamics on The Thermal Properties of Cu-Ni Alloys Berk Onat, Sondan Durukanoglu We have investigated Cu-Ni alloys with both disorder and order phases in fcc structures to analyze the effect of temperature dependent vibrational thermodynamical properties. The interactions between the atoms in the model systems are defined using an EAM type potential, specifically developed for Cu-Ni alloys. Vibrational thermodynamic functions are determined within the harmonic approximation of lattice dynamics and the vibrational densities of states are calculated using real space Green's function technique. In addition, through ab-initio calculations we have estimated the electronic contributions to set the ground for a comparative discussion. Our results show that the overall characteristics of thermodynamic functions of Cu-Ni alloys of varying concentrations are governed by the lattice vibrations. We will present our results for free energy, heat capacity and entropy of ordered/disordered Cu-Ni alloys with the experimental findings and discuss the electronic, anharmonic and lattice dynamic contributions. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L43.00012: Defect Interaction in Iron and Iron-based Alloys Haixuan Xu, G. Malcolm Stocks, Roger Stoller Magnetism has a profound influence on the defect properties in iron and iron-based alloys. For instance, it has been shown from first principles calculations that the helium interstitial occupies the tetrahedral site instead of octahedral site in contrast to all previous work that neglected the magnetic effects. In this study, we explore the effects of magnetism on the defect interaction, primarily interstitial-type defects, in bcc iron and Fe-Cr systems. The magnetic moment change during the interaction of two 1/2 \textless 111\textgreater interstitial loops in bcc iron was calculated using the \textit{ab initio} locally self-consistent multiple-scattering (LSMS) method and a significant fluctuation was observed. Adding Cr significantly modifies the magnetic structure of the defects and defect interactions. In addition, the effects of magnetism on the defect energetics are evaluated. This study provides useful insights on whether magnetism can be used as a effective means to manipulate the defect evolution in iron-based structural alloys. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L43.00013: Prediction of A2 to B2 Phase Transition in the High Entropy Alloy Mo-Nb-Ta-W William Huhn, Michael Widom In this talk we show that an effective Hamiltonian fit with first principles calculations predicts an order/disorder transition occurs in the high entropy alloy Mo-Nb-Ta-W. Using the Alloy Theoretic Automated Toolset, we find T=0K enthalpies of formation for all binaries containing Mo, Nb, Ta, and W, and in particular we find the stable structures for binaries at equiatomic concentrations are close in energy to the associated B2 structure, suggesting that at intermediate temperatures a B2 phase is stabilized in Mo-Nb-Ta-W. Our ``hybrid Monte Carlo/molecular dynamics'' results for the Mo-Nb-Ta-W system are analyzed to identify certain preferred chemical bonding types. A mean field free energy model incorporating nearest neighbor bonds will be presented, allowing us to predict the mechanism of the order/disorder transition. We find the temperature evolution of the system is driven by strong Mo-Ta bonding. Comparison of the free energy model and our MC/MD results suggest the existence of additional low-temperature phase transitions in the system likely ending with phase segregation into binary phases. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L43.00014: Predictors of the stability of high entropy alloys: is the entropy of mixing sufficient? M. Claudia Troparevsky, James R. Morris, Paul Kent, G. Malcolm Stocks High entropy alloys (HEAs) have attracted extensive attention due to their remarkable combination of strength, ductility, thermal stability, corrosion and wear resistance. However, little is known about why these alloys are stable in a single-phase solid solution or how to predict which combinations of elements will form a single phase HEA. Here, we present density functional theory calculations of the heat of formation of several HEAs in an effort to assess the role of the entropy of mixing in the stability of these alloys. The systems studied here include both single-phase and multi-phase alloys. The heats of formation show no significant differences, regardless of their single or multi phase formation, and no trends that could explain the stability of the single phase materials. Moreover, all of the calculated heats of formation are positive. These findings indicate that the entropy of mixing is insufficient to explain the unique stability of these alloys, and highlights the need for new criteria to explain the formation of single-phase solid solutions. We also discuss the minimum energy structures of several FCC and BCC alloys as well as their relative phase stability. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L43.00015: The interaction of Cr and Ni solute atoms with core of screw and edge dislocation in bcc Fe Yuri Osetsky, Odbadrakh Khorgolkhuu, German Samolyuk, Don Nicholson, Roger Stoller, Malcolm Stocks Mobility of dislocations controls the plasticity in metals. Density functional theory (DFT) is an effective tool in providing {\it ab initio} information on the energetic and magnetic properties of defects including dislocations and its interaction with other defects. We present DFT calculations on atomic properties of $1/2<111>$ screw and $1/2<111>(110)$ and $1/2<111>(112)$ edge dislocations in Fe-Cr/Ni system. The periodic quadrupole approach was applied to model the core dislocation structure, core interaction with Cr/Ni solute atoms. The size of supercell changes from 130 atoms for screw to 1800 atoms for edge dislocations. We investigated sensitivity of the binding energy of impurity atoms with a dislocation to lattice relaxation and size of modeling supercell. It was demonstrated that magnetic moment of solute atoms is ordered in the same direction as that of Fe matrix atoms for the case of Ni and in the opposite direction for Cr. Binding energy was found to be very sensitive to magnetic ordering. [Preview Abstract] |
Session L44: Focus Session: Defects in Semiconductors: Nano Materials
Sponsoring Units: DMP FIAPChair: Jun-Wei Luo, National Renewable Energy Laboratory
Room: Mile High Ballroom 4C
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L44.00001: Dopant binding energies in P-doped Ge[110] nanowires using a real-space pseudopotential approach Alex J. Lee, James R. Chelikowsky, Tzu-Liang Chan We apply a real-space pseudopotential formalism for charged one-dimensional periodic systems to examine the binding energies of P dopants in Ge$[110]$ nanowires with varying periodicities and diameters. Binding energies calculated by density functional quasiparticle energies of the neutral dopant are severely underestimated whereas those calculated by quasiparticle energies of the ionized defect are overestimated. We found the best method for determining binding energies is to adopt a composite approach that evaluates the total energy difference between charged and neutral systems for the ionization energy of the P dopant, but uses the quasiparticle energy for the electron affinity of the pure Ge nanowire. Our formalism offers a simple density functional method for calculating dopant binding energies of small nanowire systems without the use of computationally intensive many-body perturbation theory calculations. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L44.00002: Donor Wave Functions Delocalization in Silicon Nanowires Alberto Debernardi, Guido Petretto, Marco Fanciulli The localization of the donor electron wave function can be of key importance in various silicon application, since it determines the interactions between neighboring donors and influences the charge density close to the donor atom. This is important in light of applications like nuclear spin qubits [1] or for determining the critical density of metal-insulator transitions [2]. In particular the delocalization is a critical feature when dealing with nanostructures, where the confinement induces a squeezing of the donor wave function. Using ab-initio calculations, we have studied the delocalization of the donor electron wave function along the axis of a nanowire with different orientations for P and Se donors[3]. We show that the shape and delocalization is greatly influenced by the orientation of the nanowire and that it considerably larger for [011] oriented nanowires, compared to [001] and [111] orientations. We also demonstrate that its value can be controlled by applying a compressive or tensile uniaxial strain. We also show the effect of the delocalization on the hyperfine parameters. [1] B. E. Kane, Nature 393, 133 (1998) [2] D. Belitz, T. R. Kirkpatrick, Rev. Mod. Phys. 66, 261 (1994) [3] G. Petretto, A. Debernardi, M. Fanciulli, Nano Letters 13, 4963 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L44.00003: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L44.00004: Efficient $n$-type doping of zinc-blende III-V semiconductor nanowires Lucas V. Besteiro, Luis Tortajada, J. Souto, L.J. Gallego, James R. Chelikowsky, M.M.G. Alemany We demonstrate that it is preferable to dope III-V semiconductor nanowires by $n$-type anion substitution as opposed to cation substitution. Specifically, we show the dopability of zinc-blende nanowires is more efficient when the dopants are placed at the anion site as quantified by formation energies and the stabilization of \textit{DX}-like defect centers. The comparison with previous work on $n-$type III-V semiconductor nanocrystals also allows to determine the role of dimensionality and quantum confinement on doping characteristics of materials. Our results are based on first-principles calculations of InP nanowires by using the PARSEC code. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L44.00005: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L44.00006: First-principles method to study defect properties in semiconductor nanostructures Bart Partoens, Mozhgan Amini, Bob Schoeters, Rolando Saniz, Dirk Lamoen The standard theoretical approach to examine the deep or shallow nature of defects in bulk crystals is through first-principles calculations of their (neutral and charged) formation energies. The character of a defect in a nanostructure might differ from its character in the bulk material and may vary with its position in the nanostructure. However, the standard method cannot be transferred directly to nanostructures. In calculations for a charged defect, a uniform background charge is considered. While this is well-defined for bulk calculations, the total energy of a charged nanostructure depends on the vacuum width. Therefore, total energies of charged nanostructures cannot be used to calculate defect formation energies. Here we propose a solution to this problem and present a first-principles method to determine formation energies for defects in different charge states in a nanostructure, together with the transition levels. As example, we focus on $V_O$ in ZnO slabs and Si$_{\mbox{Ga}}$ in GaAs slabs. Their preferential position as function of the distance to the surface is determined, together with the evolution of their optical and thermal ionization energies. This new method allows to study the character of a wide range of intrinsic and extrinsic defects in nanostructures. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L44.00007: Effects of dopants on the band structure of quantum dots Robert Meulenberg, Joshua Wright, Stuart Lawson Understanding the role that chemical dopants play in modifying the properties of quantum dots (QDs) has been an active field of research for the last decade. In this presentation, we will discuss our efforts towards investigating the effects of copper doping in CdSe QDs. Extended x-ray absorption fine structure (EXAFS) spectroscopy measurements provide conclusive evidence for substitutional doping of Cu in the CdSe lattice. EXAFS suggests the local coordination environment is reduced, likely due to surface doping. Both x-ray absorption near edge structure spectroscopy (XANES) and theoretical modeling are used to examine effects of hybridization on the conduction band minimum (CBM) in doped CdSe quantum dots (QDs). Experimentally, Cd $M_3$-edge XANES provides evidence for a lowering of the CB minimum for Cu doped CdSe QDs that is dependent on Cu concentration. Theoretical modeling suggests the effects of hybridization between Cu and Cd atoms in the QD can explain our experimental results. The hybridization effect leads to active emissive states below the CBM resulting in tunable near-infrared photoluminescence. Our work shows that a simple chemical model can provide a predictive tool towards probing the effects of hybridization on the CB levels in QDs. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L44.00008: Size-dependent properties of Ga- and Al-doped zinc oxide nanocrystals N. Scott Bobbitt, Minjung Kim, Noa Marom, Na Sai, James R. Chelikowsky The feasibility of introducing n-type dopants into ZnO, a popular semiconductor for many photovoltaic and optoelectronic applications, suggests the possibility of controlling the carrier concentration in ZnO nanocrystal systems. We use a real-space pseudopotential method constructed within density functional theory to examine the role of Ga and Al dopants in ZnO nanocrystals in the size regime of 0.8 - 1.5 nm. Nanocrystals with both wurtzite and zincblende structures are examined. We find that while the dopants affect the width of the gap slightly, the highest occupied dopant states are nearly degenerate with the lowest empty state of the undoped nanocrystal. We find the spatial distribution of the dopant state quite similar to the lowest empty state, i.e., they are both localized on the central atom of the nanocrystal. We find that the defect formation energy decreases with increasing particle size, suggesting a less favorable formation energy for smaller nanocrystals. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L44.00009: Strong green emission from ZnO-MgO nanocomposite and its origin Ramachandra Reddy A, Sowri Babu K, Mallika AN, Venugopal Reddy K ZnO-MgO nanocomposite was prepared through a simple sol-gel method. The effect of high thermal annealing on photoluminescence of ZnO-MgO nanocomposite was studied. PL of ZnO showed only a sharp and intense UV emission positioned at 396 nm when annealed at 600 $^{\circ}$C. ZnO-MgO nanocomposite also exhibited same emission peak with enhanced intensity at the same temperature i.e. at 600 $^{\circ}$C. But, as the temperature increased from 600 $^{\circ}$C to 900 $^{\circ}$C an intense green emission positioned at 503 nm was observed with monotonous increase in its intensity. But further increase in temperature to 1000 $^{\circ}$C decreases the intensity of green emission. XRD results demonstrated that strain increased with increase of temperature till 900 $^{\circ}$C and decreased at 1000 $^{\circ}$C. Moreover, intensity of the diffraction peak corresponding to MgO phase was decreased gradually with temperature. It was also found that intensity of green emission depended on concentration of MgO in the sample. By combining the XRD and PL results, it can be concluded that the huge enhancement in the green PL intensity is due to the increase in oxygen vacancies due to the formation of highly dislocated region at the interface of ZnO and MgO due to the large lattice mismatch between them. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L44.00010: Understanding polarity in semiconductor nanorods with linear-scaling density-functional theory simulations Peter Haynes, Philip Avraam, Nicholas Hine, Paul Tangney Binary polar semiconductors with the wurtzite structure have been observed to exhibit large dipole moments along $[0001]$. To explore the origin of these dipole moments, we use a linear-scaling density-functional theory code [1] to perform first-principles calculations of entire wurtzite GaAs nanorods consisting of several thousand atoms. We find that both the direction and magnitude of the dipole moment of a nanorod, and the electric field, depend sensitively on how its surfaces are terminated and not strongly on the spontaneous polarization of the underlying lattice [2]. We show that our calculations can be explained in terms of a pinning of the Fermi level at the polar surfaces that fixes the potential difference across the nanorod, and that this effect can have a determining influence on the polarity of nanorods, with consequences for the way a nanorod responds to changes in its surface chemistry, the scaling of its dipole moment with its size, and the dependence of polarity on its composition [3]. We discuss the implications of these results for tuning nanocrystal properties, and for their growth and assembly. \\[4pt] [1] J.~Chem.~Phys. 122, 084119 (2005).\\[0pt] [2] Phys.~Rev.~B 83, 241402(R) (2011).\\[0pt] [3] Phys.~Rev.~B 85, 115404 (2012) [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L44.00011: The electronic and structural properties of SnO$_{2}$ nanoparticles doped with antimony and fluorine Minjung Kim, Noa Marom, Scotty Bobbitt, James R. Chelikowsky Transparent conducting oxide (TCO) materials are important owing to their broad industrial applications such as optoelectronic devices and photovoltaics. The most widely used TCO material is In-doped tin oxide (ITO), but In is not abundant in nature. Sb- and F-doped tin oxide nanoparticles are considered as a good candidate of ITO as they have been successfully synthesized. The electronic properties of these nanoparticles depend on the impurity species, the nanoparticle size and shape. We present electronic structure calculations on Sb- and F-doped tin oxide nanoparticles by employing real-space pseudopotential calculations based on density functional theory. We examine the impurity formation energies and electron binding energies with respect to the size of the nanoparticle and the location of the impurity site. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L44.00012: The Quantum Pinch Effect in Semiconducting Quantum Wires M.S. Kushwaha A two-component, cylindrical, quasi-one-dimensional quantum plasma subjected to a {\em radial} confining harmonic potential and an applied magnetic field in the symmetric gauge is investigated. It is demonstrated that such a system as can be realized in semiconducting quantum wires offers an excellent medium for observing the quantum pinch effect at low temperatures. An exact analytical solution of the problem allows us to make significant observations: surprisingly, in contrast to the classical pinch effect, the particle density as well as the current density display a {\em determinable} maximum before attaining a minimum at the surface of the quantum wire. The effect will persist as long as the equilibrium pair density is sustained. Therefore, the technological promise that emerges is the route to the precise electronic devices that will control the particle beams at the nanoscale. We will shed light on the observability of the quantum pinch effect [Appl. Phys. Lett. {\bf 103}, 173116 (2013)]. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L44.00013: Describing Nanomaterials: A Uniform Description System John Rumble, Steve Freiman, Clayton Teague Products involving nanomaterials are growing rapidly and nanoparticles also occur naturally. Materials, scientists, engineers, health officials, and regulators have realized they need a common description system. Led by CODATA and VAMAS, a Uniform Description System (UDS) for nanomaterials is being developed to meet the requirements of a broad range of scientific and technical disciplines and different user communities. The goal of the CODATA/VAMAS effort is the creation of a complete set of descriptors that can be used by all communities, e.g., materials, physics, chemistry, agricultural, medical, etc., interested in nanomaterials. The description system must be relevant to researchers, manufacturers of nanomaterials, materials selectors, and regulators. The purpose of the UDS for materials on the nanoscale is twofold: Uniqueness and Equivalency. The first step in the development of the UDS has been the creation of a Framework that will be used by the different communities to guide in the selection of descriptors relevant to their needs. This talk is a brief description of the draft of such a Framework, and how the framework will be translated into a robust description system with input from many scientific communities including physics. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L44.00014: Optical and structural properties of (III-V)$_{x}$(IV)$_{5-2x}$ alloys Jos\'e Men\'endez, Patrick Sims, Liying Jiang, John Kouvetakis A novel class of (III-V)-IV semiconductor alloys was recently introduced by our group. The alloys are designed to incorporate entire (IV)$_{3}$-V-III tetrahedral building blocks formed in the gas phase by reactions of (V)-(IV-H$_{3}$)$_{3}$ molecules with group-III atomic beams. This structure leads to the highest possible concentration of isolated III-V pairs in a group-IV matrix. Thick, highly crystalline films have been grown on Si and Ge substrates using the group-III elements In and Al, the group-V elements N, P, and As, and the group-IV elements Si and Ge. Results from an array of structural and optical characterization probes will be compared with theoretically proposed structures and predicted optical properties for these new alloys. The existence of III-V compounds with lattice constants very similar to those of elemental Si and Ge implies that the corresponding (III-V)$_{x}$(IV)$_{5-2x}$ alloy will have a tunable electronic structure at a fixed lattice constant, a property that may find applications in areas such as photovoltaics. [Preview Abstract] |
Session L45: Semiconductor Electronic Structure: Theory & Spectra I
Sponsoring Units: FIAPChair: Gael Nardin, NIST/University of Colorado
Room: Mile High Ballroom 4D
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L45.00001: Differences in the surface electronic structure of Ge(001) and Si(001) from angle-resolved photoemission spectroscopy and \textit{ab-initio} theory Richard C. Hatch, Hosung Seo, Patrick Ponath, Miri Choi, Agham B. Posadas, Alexander A. Demkov Using high-resolution angle-resolved photoemission spectroscopy (ARPES) we compare the surface electronic structure of both Ge(001) and Si(001) surfaces. Unlike previous ARPES experiments, where the Ge(001) surfaces were prepared using cycles of ion sputtering and annealing, our Ge(001) surfaces were prepared using a combination of wet etching and oxygen plasma cleaning. This new technique has the advantage that it avoids the incomplete healing of surface roughening associated with sputtering and annealing cycles. The ARPES data show that the dimer-derived surface state that determines the charge neutrality level, and thus the Schottky barrier height in Si, is actually a surface resonance in Ge, and the highest occupied state is a bulk state. In order to avoid theory predicting an overlap of the valence and conduction bands, we employed first-principles, hybrid density functional theory (DFT). This theory effectively explains the presence of a number of photoemission features in both Si and Ge. We found it is necessary to incorporate spin-orbit interaction in the hybrid DFT calculations for Ge in order to model ARPES data, and we found a spin-orbit splitting of 0.28 eV both experimentally and theoretically. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L45.00002: Differences in the surface electronic structure of Ge(001) and Si(001) from hybrid density functional theory and angle-resolved photoemission spectroscopy Hosung Seo, Richard C. Hatch, Patrick Ponath, Miri Choi, Agham B. Posadas, Alexander A. Demkov Even with renewed interest in Ge as a competitor to Si in field effect transistors, several key features of the surface electronic structure of Ge(001) have remained controversial. Notably, the character of the valence band top of Ge(001) has been heavily debated. Using first-principles hybrid density functional theory and angle-resolved photoemission spectroscopy, we unambiguously establish the critical differences between the electronic structure of the Si and Ge (001) surfaces. In order to avoid the problems associated with the band gap underestimation in LDA and GGA, we utilized the screened Hartree-Fock hybrid exchange correlation density functional due to Heyd, Scuseria, and Ernzerhof (HSE06). We explicitly show that the surface state that determines the charge neutrality level, and thus the Schottky barrier height in Si, is actually a surface resonance in Ge. Our results strongly suggest that the recently observed strong Fermi level pining in Ge/metal junctions comes from the evanescent states. Additionally, using surface resonance calculations and bulk HSE06 calculations with the spin-orbit coupling, we identify the origin of a number of highly debated ARPES features for Ge(001) and Si(001). [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L45.00003: Optoelectronics, Theory and Defect Physics of Zn-IV Nitride Semiconductors Prineha Narang, Shiyou Chen, Aashrita Mangu, Jason Cooper, Sheraz Gul, Junko Yano, Lin-Wang Wang, Nathan Lewis, Harry Atwater ZnSn$_{\mathrm{x}}$Ge$_{\mathrm{1-x}}$N$_{\mathrm{2}}$ alloys with optical band gaps ranging from 2-3.1eV can be tuned to span a large portion of the solar spectrum, and could therefore be a viable earth-abundant light absorber and replacement for InGaN in nitride optoelectronic devices. They exhibit local order as demonstrated via X-ray absorption fine structure spectroscopy (EXAFS) and a linear relationship between the (002) peak position and composition in XRD studies. The bowing parameter is 0.29 eV for the measured band gaps of ZnSn$_{\mathrm{1-x}}$Ge$_{\mathrm{x}}$N$_{\mathrm{2}}$, significantly smaller than that of In$_{\mathrm{1-x}}$Ga$_{\mathrm{x}}$N, indicating that the ZnSn$_{\mathrm{1-x}}$Ge$_{\mathrm{x}}$N$_{\mathrm{2}}$ alloy band gaps can be tuned almost linearly by controlling the Sn/Ge composition. In this presentation we show theoretical studies of the optoelectronic behavior and defect physics of Zn(Sn,Ge)N$_{\mathrm{2\thinspace }}$series and experimental investigations via X-ray absorption and emission spectroscopy to probe the conduction and valence-band partial density of states. Band structure calculations from different methods will be shown in comparison with the experimental optical properties. Resonant inelastic scattering studies of the Zn(Sn,Ge)N$_{\mathrm{2}}$ lattice will be presented with their carrier dynamics obtained from pump-probe spectroscopy. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L45.00004: Quantum Electron-Hole Droplets in Gallium Arsenide Quantum Wells Andrew Hunter, Hebin Li, Steven Cundiff, Martin Mootz, Mackillo Kira, Stephan Koch In a solid, the choice of appropriate quasiparticles greatly simplifies our understanding of the system. For example, (quasi)electrons allow one to disregard the interaction between an electron and the macroscopic number of ionic potentials in a solid, and instead treat the system as a free quasielectron with an effective mass. Similarly, we improve our understanding of the complex electronic many-body system in a solid if we can identify the stable many-body quasiparticles of the system, such as excitons, biexcitons, and trions. We present experimental and theoretical evidence for the existence of a new quasiparticle that we call a quantum droplet, a charge-neutral bound state of a few electrons and holes. Unlike the macroscopic electron-hole droplets observed in indirect-gap semiconductors, quantum droplets contain only a small number of particles, leading to quantization of binding energy, but with a two-particle correlation function characteristic of a liquid. Using transient-absorption spectroscopy with ultrafast pulses, we show that we can create quantum droplets in gallium arsenide quantum wells. Projection onto a correlated quantum optical state allows us to separate the effects of the droplet state from other multiple-particle states, such as biexcitons. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L45.00005: Auger recombination in sodium iodide Andrew McAllister, Emmanouil Kioupakis, Daniel \r{A}berg, Andr\'e Schleife Scintillators are an important tool used to detect high energy radiation - both in the interest of national security and in medicine. However, scintillator detectors currently suffer from lower energy resolutions than expected from basic counting statistics. This has been attributed to non-proportional light yield compared to incoming radiation, but the specific mechanism for this non-proportionality has not been identified. Auger recombination is a non-radiative process that could be contributing to the non-proportionality of scintillating materials. Auger recombination comes in two types - direct and phonon-assisted. We have used first-principles calculations to study Auger recombination in sodium iodide, a well characterized scintillating material. Our findings indicate that phonon-assisted Auger recombination is stronger in sodium iodide than direct Auger recombination. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L45.00006: \textit{Ab-initio} Calculations of Electronic Properties of AlP, GaP and InP Yuriy Malozovsky, Azizjon Saliev, Lashaunda Franklin, Chinedu Ekuma, Guang-Lin Zhao, Diola Bagayoko We present results from \textit{ab-initio}, self consistent local density approximation (LDA) calculations of electronic and related properties of zinc blende aluminum, gallium and indium phosphides (AlP, GaP {\&} InP). We employed a local density approximation (LDA) potential and implemented the linear combination of atomic orbitals (LCAO) formalism. This implementation followed the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF). Our calculated, indirect band gap of 2.56 eV for AlP, and of 2.14 eV for GaP, from $\Gamma $ to X, are in excellent agreement with experimental values. Our calculated direct band gap of 1.40 eV, at $\Gamma $ -point for InP is also in excellent agreement with experimental value. We also report calculated electron and hole effective masses for AlP, GaP and InP and total (DOS) and partial (pDOS) densities of states. This research is funded in part by the National Science Foundation (NSF) and the Louisiana Board of Regents, through LASiGMA [Award Nos. EPS- 1003897, NSF (2010-15)-RII-SUBR] and NSF HRD-1002541, the US Department of Energy -- National, Nuclear Security Administration (NNSA) (Award No. DE-NA0001861), LaSPACE, and LONI-SUBR. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L45.00007: \textit{Ab-initio} Calculations of Electronic Properties of Boron Phosphide (BP) John Ejembi, Lashaunda Franklin, Yuriy Malozovsky, Diola Bagayoko We present results from \textit{ab-initio}, self consistent local density approximation (LDA) calculations of electronic and related properties of zinc blende boron phosphide (BP). We employed a local density approximation (LDA) potential and implemented the linear combination of atomic orbitals (LCAO) formalism. This implementation followed the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF). We discuss our preliminary results for the indirect band gap, from $\Gamma $ to X, of Boron Phosphide. We also report calculated electron and hole effective masses for Boron Phosphide and total (DOS) and partial (pDOS) density of states. Acknowledgments: This research is funded in part by the National Science Foundation (NSF) and the Louisiana Board of Regents, through LASiGMA [Award Nos. EPS- 1003897, NSF (2010-15)-RII-SUBR] and NSF HRD-1002541, the US Department of Energy -- National, Nuclear Security Administration (NNSA) (Award No. DE-NA0001861), LaSPACE, and LONI-SUBR. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L45.00008: \textit{Ab-initio} Calculations of Accurate Electronic Properties of Wurzite AlN Ifeanyi Nwigboji, Yuriy Malozovsky, Diola Bagayoko We present results from \textit{ab-initio}, self consistent local density approximation (LDA) calculations of electronic and related properties of wurtzite Aluminum Nitride (w-AlN). Our non-relativistic computations employed the Ceperley and Alder LDA potential and the linear combination of atomic orbital (LCAO) formalism. The implementation of the LCAO formalism followed the Bagayoko, Zhao, and Williams' method as enhanced by Ekuma and Franklin (BZW-EF). The BZW-EF method verifiably obtains the minima of the occupied energies; these minima provide the most variationally and physically valid density functional theory (DFT) description of the ground states of materials under study. Our preliminary results for w-AlN show that w-AlN has a direct band gap of 5.82 eV at the $\Gamma $ point. The preliminary energy bands were obtained with a basis set comprising 48 functions. None of the several, larger basis sets tested to date led to occupied energies lower than those obtained with the above 48. While most previous LDA calculations are 2 eV smaller or more than the experimental value of 5.9 eV that is in excellent agreement with our finding, considering the typical experimental uncertainty of 0.2 eV for absorption measurements on AlN. We also discuss our calculated density of states (DOS) and partial densities of states (pDOS). [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L45.00009: Density Functional Theory Revisited: The Mathematical and Physical Conditions for the Physical Content of the Eigenvalues Diola Bagayoko, Lashounda Franklin, Yuriy Malozovsky, Bethuel Khamala, Chinedu Ekuma, Yacouba Diakite, Azizjon Saliev We briefly recall the derivation of density functional theory (DFT) and of its local density approximation (LDA). From this derivation, we show that eigenvalues resulting from self consistent DFT calculations utilizing a single input basis set do not necessarily have much physical content. We subsequently present \textit{the necessary conditions for obtaining eigenvalues with a physical content, for the ground state and low energy excited states.} These conditions include the verifiable attainment of the minima of the occupied energies, on the one hand, and the avoidance of a mathematical artifact stemming from the Rayleigh theorem, on the other. We show a few new results, obtained with DFT potentials, that agree very well with corresponding experimental ones. These results include band gaps, effective masses, and structural properties of selected semiconductors. Our calculations utilized the Bagayoko, Zhao, and Williams (BZW) method as enhanced by Ekuma and Franklin (BZW-EF). The distinctive feature of the method includes its strict adherence to the necessary conditions described noted above. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L45.00010: Electron-Phonon Renormalization of Band Gap of BN and Si nanowires Andrew Franson We compute the electron-phonon renormalization of band gap of BN with three different crystal structures from first-principles using the linear-response theory and the Allen-Heine theory. We find that the zero-point renormalization of band gap in BN depends strongly on crystal structures, varying from -222 meV to -434 meV. We also calculate the temperature-dependent band gaps and electronic band structures of BN. In addition, we investigate the quantum confinement effect on electron-phonon renormalization by comparing band gap shifts due to electron-phonon coupling in bulk Si and Si nanowires. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L45.00011: Effect of Interface Roughness in Ultra-Thin Semiconductor Quantum Wells Yu Song, Rajaram Bhat, Chung-En Zah, Claire Gmachl Ultra-thin, few monolayer semiconductor quantum well (QW) structures are extensively used in optoelectronic devices, especially when the effective mass is high ($\geq 0.1 m_e$). Traditionally, interfaces roughness in QWs are either ignored, or treated as a 2D scattering potential ideally localized on the interface plane. This treatment is valid when roughness is small compared to the layer thickness. But in situations of ultra-thin QWs, a more systematic model is needed. In this work, we model the potential associated with the interface roughness as a 3D function with dependence on the actual interface position. With the help of Green's function we show that this potential, when averaged in-plane, produces an effective grading potential out of the plane which significantly alters the energy spectrum. This effect is reaffirmed by the experimental results from the measurement of intersubband (ISB) optical transitions in III-Nitride thin QWs. The general expression of the scattering matrix element for carrier transport is also derived, which requires full 3D calculation and significantly differs from the traditional treatment. The scattering lifetimes are calculated for the example of III-nitride ISB devices, and the results are being compared to that from the traditional formulas. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L45.00012: Intermediate band in type-II silicon clathrate with Cu/Ag guest atoms ZhaoHui Huang, HuaShan Li, ZhiGang Wu, Mark T. Lusk We investigate the structural and electronic properties of type-II clathrate with guests of Cu and Ag atoms, and our first-principles calculations demonstrate that an intermediate band (IB) would exist in the originally forbidden gap if one or two isolated Cu or Ag atoms located in a cage. These IBs have nearly ideal energy separations to VBM and CBM of the host clathrate, thus they would be useful for making highly-efficient solar cells. However, Cu and Ag atoms tend to form clusters larger than two atoms, which lead to a heavily doped semiconductor instead of generating useful IBs. We will discuss possible approaches to overcome this severe problem. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L45.00013: Valence Band Alignment at (111) / (0001) ScN/SiC and ScN/GaN Interfaces as Determined by Photoemission Sean King, Robert Nemanich, Robert Davis Scandium nitride (ScN) is a transition metal nitride material that over the past decade has garnered significant interest for nano-electronic, spin-tronic, optoelectronic, electro-acoustic, and thermoelectric applications. This is due to the reasonably close lattice matching exhibited between the (111) plane of ScN (0.3139 nm) and the (111) / (0001) planes of SiC and GaN (0.3073 and 0.3189 nm respectively). For these specific applications, the valence and conduction band alignment of ScN to SiC and GaN will play a significant role. In this regard, we have utilized x-ray photoelectron spectroscopy (XPS) to investigate the growth and interfacial valence band alignment for gas-source molecular beam epitaxy (GSMBE) of ScN on (111) 3C-SiC / (0001) 6H-SiC substrates. Using a detailed analysis of the attenuation of the Si2p core level from multiple ScN growths and XPS measurements, we find that ScN grows on (111) 3C-SiC in a layer by layer fashion. UPS measurements (Figure 1) show the ScN valence band to be 1.6-2.1 eV below the system Fermi level indicating a minimum band gap on this order. Detailed XPS/UPS measurements indicate the ScN/3C-SiC valence band offset is small ($\le $ 0.3 eV). Additional measurements for GSMBE GaN on ScN show a larger interfacial valence band discontinuity of $\sim $ 0.8 eV. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L45.00014: Spectral Description of Multi-Photon Processes in Quantized Many-Electron Systems Based on a Reduced-Density-Matrix Approach Verne Jacobs, Alex Kutana The frequency-dependent transition rates for multi-photon processes in quantized many-electron systems are evaluated using a reduced-density-matrix approach. We provide a fundamental foundation for systematic spectral simulations for atomic, molecular, and solid-state systems. A perturbation expansion of the frequency-domain Liouville-space self-energy operator is employed in detailed evaluations of the spectral-line widths and shifts in the isolated-line and short-memory-time (Markov) approximations. The lowest-order contributions associated with environmental electron-photon and electron-phonon interactions are systematically taken into account. Our description is directly applicable to electromagnetic processes in a wide variety of semiconductor, photochemical, and biological systems, without premature approximations. In particular, our approach can be applied to investigate optical phenomena involving electrons in both bulk and nanoscale semiconductor materials entirely from first principles, using the density functional formalism and existing electronic structure codes. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L45.00015: Strain-engineered Surface Transport in Si(001): Complete Isolation of the Surface State via Tensile Strain Miao Zhou, Zheng Liu, Zhengfei Wang, Zhaoqiang Bai, Yuanping Feng, Max G. Lagally, Feng Liu When silicon channel layer grows increasingly thinner in microelectronics, surface conductance becomes increasingly dominant, opening an opportunity to use surface states for novel devices. By combining density functional theory, non-equilibrium Green's function formulism, and effective-Hamiltonian approaches, we demonstrate strain-engineered surface transport in Si(001), with the complete isolation of the Si surface states from the bulk bands. Our results show that sufficient tensile strain can effectively remove the overlap between the surface valence state and the bulk valence band, because of the drastically different deformation potentials. Isolation of the surface valence state is possible with a tensile strain of $\sim$1.5\%, a value that is accessible experimentally. Quantum transport simulations of a chemical sensing device based on strained Si(001) surface confirm the isolation of dominating surface conductance, giving rise to an enhanced molecular sensitivity. Our results show the promise for combining surface engineering with strain engineering to further our ability to manipulate surface states for quantum information processing and surface-state based devices. [Preview Abstract] |
Session L46: Heavy Fermions and Non-Fermi Liquids: Theory
Sponsoring Units: DCMPChair: Pallab Goswami, National High Magnetic Field Laboratory
Room: Mile High Ballroom 4E
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L46.00001: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L46.00002: Emergence of heavy quasiparticles from a massless Fermi sea: Optical conductivity Hyun-Yong Lee, Stefan Kettemann We study the density of states and the optical conductivity of a Kondo lattice which is immersed in a massless Dirac Fermi sea, as characterized by a linear dispersion relation. As a result of the hybridization $V$ with the $f$-electron levels, the pseudo-gap in the conduction band becomes duplicated and is shifted both into the upper and the lower quasiparticle band. Furthermore, we find that due to the linear dispersion of the Dirac fermions, the Kondo insulator gap is observable in the optical conductivity in contrast to the Kondo lattice system in a conventional conduction band, and the resulting gap[$\Delta_{\rm gap}(T)$] depends on temperature. The reason is that the Kondo insulator gap is an indirect gap in conventional Kondo lattices, while it becomes a direct gap in the Dirac Fermi Sea. We find that the optical conductivity attains two peaks and is vanishing exactly at $2bV$ where $b$ is a condensation of slave boson depending on temperature. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L46.00003: Topological defects of N\'eel order and Kondo singlet formation for Kondo-Heisenberg model on a honeycomb lattice Qimiao Si, Pallab Goswami Heavy fermion systems represent a prototypical setting to study magnetic quantum phase transitions. In this context, we study the spin one-half Kondo-Heisenberg model on a honeycomb lattice at half filling [1]. The problem is approached from the Kondo destroyed, antiferromagnetically ordered insulating phase. We describe the local moments in terms of a coarse grained quantum non-linear sigma model, and show that the skyrmion defects of the antiferromagnetic order parameter host a number of competing order parameters. In addition to the spin Peierls, charge and current density wave order parameters, we identify for the first time Kondo singlets as the competing dual orders of the antiferromagnetism, which can be related to each other via generalized chiral transformations of the underlying fermions. We also show that the conduction electrons acquire a Berry phase through their coupling to the hedgehog configurations of the N\'eel order, which cancels the Berry phase of the local moments. Our results demonstrate the competition between the Kondo-singlet formation and spin-Peierls order when the antiferromagnetic order is suppressed, thereby shedding new light on the global phase diagram of heavy fermion systems at zero temperature. \\[4pt] [1] P. Goswami and Q. Si, arXiv: 1309.0501 [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L46.00004: Finite $f$-Electron Bandwidth in a Heavy Fermion Model Axel Euverte, Simone Chiesa, Richard Scalettar, George Batrouni Determinant Quantum Monte Carlo is used to study the effect of non-zero hopping $t_f$ in the localized $f$-band of the periodic Anderson model in two dimensions. We show that a remnant of the band insulator to metal line at $U_f$ = 0 persists in the interacting system, manifesting itself as a maximal tendency toward antiferromagnetic correlations at low temperature. In this optimal $t_f$ region, short- and long-range spin correlations develop at similar temperatures in stark contrast with the more common scenario where short range correlations are stronger and develop at higher temperature. The effect that finite $t_f$ has on Kondo screening is investigated by considering the evolution of the local density of states for selected $t_f$ as a function of $V$. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L46.00005: Global phase diagram of heavy fermion metals: Insights from an Ising-anisotropic Kondo lattice model tuned by a transverse magnetic field Emilian Marius Nica, Lili deng, Kevin Ingersent, Jian-Xin Zhu, Qimiao Si Quantum criticality in heavy fermion metals involves the interplay between quantum fluctuations within the local moments and those associated with the Kondo interaction. The resulting global phase diagram [1,2] has provided a means to categorize heavy-fermion quantum critical points [3] and motivated the study of materials with tunable quantum fluctuations [4]. It can be theoretically characterized within an Extended Dynamical Mean-Field Theory (EDMFT). Towards this goal, we studied an Ising-anisotropic Bose-Fermi Kondo model with a local transverse field [2]. We found a line of critical points separating a Kondo screened phase and a local moment phase. We present preliminary results for the EDMFT study of an Ising-anisotropic Kondo lattice model tuned by a transverse magnetic field. In addition, we discuss the implications of the line of critical points for the global phase diagram.\\[4pt] [1] Q. Si, Phys. Status Solidi B247, 476 (2010); Physica B378, 23 (2006) [2] E.M. Nica et al, PRB 88, 014414 (2013) [3] S. Friedemann et al, Nat. Phys. 5, 465 (2009). [4] J. Custers et al, Nat. Mater. 11, 189 (2012); M. S. Kim {\&} M. C. Aronson, PRL 110, 017201 (2013); V. Fritsch et al, arXiv:1301.6062. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L46.00006: Un-Fermi Liquids: Unparticles in Strongly Correlated Electron Matter Brandon Langley, Philip Phillips, Jimmy Hutasoit Since any non-trivial infrared dynamics in strongly correlated electron matter must be controlled by a critical fixed point, we argue that the form of the single-particle propagator can be deduced simply by imposing scale invariance. As a consequence, the unparticle picture proposed by Georgi[1] is the natural candidate to describe such dynamics. Unparticle stuff is scale-invariant matter with no particular mass. Scale invariance dictates that the propagator has an algebraic form which can admit zeros and hence is a candidate to explain the ubiquitous pseudogap state of the cuprates. The non-perturbative electronic state formed out of unparticles we refer to as an un-Fermi liquid. We show that the underlying action of the continuous mass formulation of unparticles can be recast as an action in anti de Sitter space which serves as the generating functional for the propagator. We find that this mapping fixes the scaling dimension of the unparticle to be $d_U=d/2+\sqrt{d^2+4}/2$ and ensures that the corresponding propagator has zeros with $d$ the spacetime dimension of the unparticle field. [1] H. Georgi, Phys. Rev. Lett. 98, 221601 (2007) [2] B.W. Langley, P.W. Phillips, J.A. Hutasoit, Phys. Rev. B 88, 115129 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L46.00007: Critical charge fluctuations in a pseudogap Anderson model Tathagata Chowdhury, Kevin Ingersent Experiments on heavy-fermion $\beta$-YbAlB$_4$ raise the possibility of critical destruction of the Kondo effect in a mixed-valence system. We consider a toy model of this phenomenon: the particle-hole asymmetric Anderson model with a pseudogapped density of states $\rho(\epsilon) \propto |\epsilon-\epsilon_F|^r$ where $\epsilon_F$ is the Fermi energy. The model exhibits a critical spin response at a quantum phase transition separating a Kondo phase from a non-Kondo (local-moment) phase, where the Kondo energy scale is driven continuously to zero on approach from the Kondo side [1]. This Kondo-destruction transition has recently been shown, for certain values of $r$, to be accompanied by a divergence of the charge susceptibility coming from either phase [2]. Here we present a systematic numerical renormalization-group study of the charge response as a function of $r$. The charge fluctuations are described by critical exponents that show nontrivial $r$ dependence. Over a range of $r$ values, these exponents satisfy hyperscaling equations consistent with a scaling anzatz for the critical free energy at an interacting quantum phase transition. [1] K. Ingersent and Q. Si, Phys. Rev. Lett. 89, 076403 (2002). [2] J. H. Pixley et al., Phys. Rev. Lett. 109, 086403 (2012). [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L46.00008: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L46.00009: Fermi energy and dispersion anomalies in a bad metal Wenhu Xu, Kristjan Haule, Gabriel Kotliar The transport measurements in strongly correlated metals often reveal a vanishing Fermi liquid temperature. It is unexpectedly smaller than the effective Fermi energy indicated by spectroscopic measurements. We attribute this dichotomy to the strong temperature dependence and asymmetry in quasiparticle renormalization near Fermi surface. The quasiparticles hold as well-defined excitations up to much higher energy than the Fermi liquid scale implied by transport. Furthermore, the asymmetry leads to incoherent spectral weight only for quasiparticles near Fermi surface, thus the discontinuity in dispersion rises as a natural consequence. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L46.00010: Ubiquity of Linear Resistivity at Intermediate Temperature in Strongly Correlated Metals Greg Boyd, V. Zlatic, Jim Freericks Correlated metals display transport behavior that differs from what is commonly seen in ordinary metals (Fermi-liquids). One of the most salient features is a resistivity that is linear in temperature over decades in temperature and rises to well above the Ioffe-Regel limit (where the mean-free path is less than a lattice spacing). Using an exact representation of the Kubo linear response, we show that a linear resistivity naturally occurs in a minimal model that includes only hopping and correlation. We expect this to be common to many systems at an incoherent intermediate-temperature state, above the Fermi coherence scale. We verify the analytic arguments with exact calculations for Falicov-Kimball model which is solved with dynamical mean-field theory. Similar features have also been seen in Hubbard models, which can be approximated by the Falicov-Kimball model. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L46.00011: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L46.00012: Quantum Monte Carlo investigation of Knight shift anomaly in Periodic Anderson model Mi Jiang, Nicholas Curro, Richard Scalettar We report a Determinant Quantum Monte Carlo investigation of the Knight shift anomaly observed in nuclear magnetic resonance (NMR) of heavy fermion materials. As opposed to normal Fermi liquids, the Knight shift in heavy fermion materials deviates from the total susceptibility $\chi$ below a crossover temperature $T^{\ast}$. This deviation is believed to originate in the different temperature dependence of the conduction electron and local moment components of the total susceptibility $\chi$. Here we quantify the behavior of $\chi_{cc}(T), \chi_{cf}(T),$ and $\chi_{ff}(T)$ in the framework of periodic Anderson model (PAM), focussing on the evolution with different degree of conduction electron-local moment hybridization. These results confirm several predictions of the two-fluid theory of the Knight shift anomaly, including the demonstration of a universal logarithmic divergence of the contribution of the heavy electrons to the Knight shift. This universal behavior, which occurs with decreasing temperature below $T^{\ast}$ in the paramagnetic state, agrees well with experimental findings, and indicates that different heavy fermion materials exhibit a common scaling, differing only in the coherence temperature scale, $T^{\ast}$. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L46.00013: Supersymmetric SP(N) spin representation for heavy fermion systems Aline Ramires, Piers Coleman In heavy fermion materials, the character of the spin correlations change radically between the antiferromagnetic and fermi liquid region of the phase diagram. In the latter, the spin behaves as a bosonic object, condensing into magnetic order, which we traditionally describe using a Holstein-Primakoff or Schwinger bosons, whereas in the the fermi liquid phase, the spin binds to the conduction electrons to form a composite heavy fermion, usually described by Abriskosov fermions. Past work [1] developed a supersymmetric representation of SU(N) spin operators. Here we analyze the supersymmetry of SP(N) symplectic spin operators, which provides us with the capability of studying antiferromagnetic and superconducting order. As a warm up problem, we show how this formalism can be applied to a two site Kondo model, coupled via a Heisenberg coupling. [1] P. Coleman, C. Pepin and A. Tsvelik, Phys. Rev. B 62, 3852 (2000). [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L46.00014: Low temperature phases of periodic Anderson model with electron-phonon correlated conduction band Enzhi Li, Peng Zhang, Ka-Ming Tam, Juana Moreno, Mark Jarrell We study a periodic Anderson model with the conduction electrons coupled to phonons. It has been shown by using the dynamical mean field theory that the model contains two disordered phases, the Kondo singlet phase and the local moment phase. In the hybridization--temperature plane, they are separated by a first order phase transition line which terminates at a second order phase transition point. At low enough temperature the entropy in the Kondo singlet phase is quenched by Fermi liquid formation, while the local moment phase will have residual entropy unless it is quenched by ordering. In this talk, we discuss this ordering by constructing the lattice susceptibilities from dynamical mean field theory. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L46.00015: Charge Kondo effect in a triple quantum dot Gwangsu Yoo, Jinhong Park, S.S.-B. Lee, H.-S. Sim We predict that the charge Kondo effect appears in a triangular triple quantum dot. The system has two-fold degenerate ground-state charge configurations, interdot Coulomb interactions, lead-dot electron tunnelings, but no interdot electron tunneling. We show, using bosonization and refermionization, that the system is described by the {\em anisotropic} Kondo model. The anisotropy can be tuned by changing lead-dot electron tunneling strength, which allows one to experimentally explore the transition between the ferromagnetic non-Fermi liquid and antiferromagnetic Kondo phases in the Kondo phase diagram. Using numerical renormalization group method, we demonstrate that the transition is manifested in electron conductances through the dot. [Preview Abstract] |
Session L47: Metal-Insulator and Other Electronic Phase Transitions: Experiment II
Sponsoring Units: DCMPChair: Rajeswari Kolagani, Towson University
Room: Mile High Ballroom 4F
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L47.00001: Hysteretic melting transition of a soliton lattice in IrTe$_2$ Weida Wu, Pin-Jui Hsu, Tobias Mauerer, Matthias Vogt, Matthias Bode, J.J. Yang, Yoon Seok Oh, S-W. Cheong We report on the observation of the hysteretic transition of a commensurate charge modulation in IrTe$_2$ from transport and scanning tunneling microscopy (STM) studies. Below the transition ($T_{\rm C} \approx 275$ K on cooling) a $q = 1/5$ charge modulation was observed, which is consistent with previous studies [1,2]. Additional modulations [$q_n = (3n+2)^{-1}$] appear below a second transition at $T_{\rm S}\approx 180$ K on cooling. The coexistence of various modulations persist up to $T_{\rm C}$ on warming. The atomic structures of charge modulations and the temperature dependent STM studies suggest that 1/5 modulation is a periodic soliton lattice which partially melts below $T_{\rm S}$ on cooling. Our results provide compelling evidence that the ground state of IrTe$_2$ is a commensurate 1/6 charge modulation, which originates from periodic dimerization of Te atoms visualized by atomically resolved STM images [3]. [1] Yang, {\it et al.}, Phys. Rev. Lett. 108, 116402 (2012). [2] Oh, {\it et al.}, Phys. Rev. Lett. 110, 127209 (2013). [3] Hsu, {\it et al.}, arXiv:1311.3015, (2013). [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L47.00002: Electronic signatures of dimerization in IrTe$_{2}$ Jixia Dai, Weida Wu, Yoon Seok Oh, S.-W. Cheong, J.J. Yang Recently, the mysterious phase transition around T$_{c} \quad \approx $ 260 K in IrTe$_{2}$ has been intensively studied. A structural supermodulation with q$=$1/5 was identified below T$_{c}$. A variety of microscopic mechanisms have been proposed to account for this transition, including charge-density wave due to Fermi surface nesting, Te p-orbital driven structure instability, anionic depolymerization, ionic dimerization, and so on. However, there has not been an unified picture on the nature of this transition. To address this issue, we have performed low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) experiments on IrTe$_{2}$ and IrTe$_{2-x}$Se$_{x}$. Our STM data clearly shows a strong bias dependence in both topography and local density of states (STS) maps. High resolution spectroscopic data further confirms the stripe-like electronic states modulation, which provides insight to the ionic dimerization revealed by X-ray diffraction. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L47.00003: A series of modulated states in the dimerized IrTe$_2$ V. Kiryukhin, L.G. Pascut, S.-W. Cheong, M.J. Gutmann, J.J. Yang IrTe$_2$ is a layered compound exhibiting stripes of dimerized Ir ions. With decreasing temperature, a series of structures with different periodicities is observed. These structures are distinguished by the arrangement of the dimerized stripes, as found using x-ray diffraction. The structural transitions appear to be driven by the Ir dimerization and Ir-Ir bonding in this compound. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L47.00004: Dimerization-Induced Cross-Layer Quasi-Two-Dimensionality in Metallic Iridate IrTe$_{2}$ Gheorghe-Lucian Pascut, Kristjan Haule, Matthias J. Gutmann, Sarah A. Barnett, Alessandro Bombardi, Sergey Artyukhin, Turan Birol, David Vanderbilt, Junjie Yang, Sang-Wook Cheong, Valery Kiryukhin IrTe$_{2}$ a layered chalcogenide metal composed of stacked layers of IrTe$_{6}$ octahedra, has recently received a lot of interest due to: (1) the structural phase transition ($T_{S}=$280 K) to a modulated structure characterized by the wave vector \textbf{\textit{q}}$_{\mathbf{0}}=$(1/5, 0, 1/5) and (2) the superconducting properties of the Pd/Pt-doped/intercalated samples. Using the crystal structure obtained from single crystal X-ray diffraction and first principle calculations we show that the mechanism for the structural phase transition is driven by the Ir dimerization and bonding, contrary to mechanisms proposed before based on: orbital-driven Peierls instability, crystal field splitting the Te $p$ orbitals and depolymerization of the inter-layer Te bonds. In this talk I will describe the Ir/Te dimers and the electronic structure calculations which reveal an intriguing quasi-two-dimensional electronic state, with planes of reduced density of states cutting diagonally through the Ir and Te layers. These planes are formed by the dimers exhibiting a signature of covalent bonding character development. The role of the electronic correlations and spin orbit coupling will be also discussed. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L47.00005: Metal-insulator transition in SrTi$_{\mathrm{1-x}}$V$_{\mathrm{x}}$O$_{3}$ thin films Man Gu, Stuart Wolf, Jiwei Lu Epitaxial SrTi$_{\mathrm{1-x}}$V$_{\mathrm{x}}$O$_{3}$ (0 $\le $ x $\le $ 1) thin films with thicknesses of $\sim$ 16 nm were grown on (001)-oriented LSAT substrates using the pulsed electron-beam deposition technique. The transport study revealed a temperature driven metal-insulator transition (MIT) at 95 K for the film with x $=$ 0.67. The films with higher vanadium concentration (x \textgreater 0.67) were metallic, and the electrical resistivity followed the T$^{2}$ law corresponding to a Fermi liquid system. In the insulating region of x \textless 0.67, the temperature dependence of electrical resistivity for the x $=$ 0.5 and 0.33 films can be scaled with Mott's variable range hopping model. The possible mechanism behind the observed MIT might be associated the interplay between electron-electron interactions and disorder-induced localization. The Ti$^{4+}$ ion substitution introduces Anderson-localized states as well as lattice distortions that result in a reduction in the effective 3d bandwidth W. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L47.00006: Epitaxial growth of Ruddlesden-Popper La$_{n+1}$Ni$_{n}$O$_{3n+1}$ series using reactive molecular-beam epitaxy June Hyuk Lee, I-Cheng Tung, Jarrett Moyer, Guangfu Luo, Seo Hyoung Chang, Dane Morgan, Hawoong Hong, Peter Schiffer, Dillon Fong, John Freeland We report the growth of single crystalline La$_{n+1}$Ni$_{n}$O$_{3n+1}$ epitaxial thin films using reactive molecular-beam epitaxy. Ruddlesden-Popper La$_{n+1}$Ni$_{n}$O$_{3n+1}$ compounds, consisting of LaO$^{+}$ and NiO$_{2}$$^{-}$ layers, have been considered a potential candidate for solid-oxide fuel cell cathodes and thermoelectrics. However, the growth of higher order La$_{n+1}$Ni$_{n}$O$_{3n+1}$ single crystals has not been possible so far. We utilize synchrotron x-ray diffraction at the Advanced Photon Source during layer?by?layer deposition together with density functional theory calculations to understand how LaO$^{+}$ and NiO$_{2}$$^{-}$ oxide layers re-arrange dynamically during growth. Using this layer re-arrangement, epitaxial La$_{2}$NiO$_{4}$, La$_{3}$Ni$_{2}$O$_{7}$, and La$_{4}$Ni$_{3}$O$_{10}$ films on (001)-oriented SrTiO$_{3}$ have been synthesized with the proper nickel valance state and structure. Here we will discuss the connection between structure and electrical transport properties. Work at the APS, Argonne is supported by the U.S. Department of Energy, Office of Science, and Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L47.00007: Understanding Metal-Insulator transitions in ultra-thin films of LaNiO$_{3}$ Jayakanth Ravichandran, Philip D.C. King, Darrell G. Schlom, Kyle M. Shen, Philip Kim LaNiO$_{3}$ (LNO) is a bulk paramagnetic metal and a member of the family of RENiO$_{3}$ Nickelates (RE = Rare Earth Metals), which is on the verge of the metal-insulator transition. Ultra-thin films of LNO has been studied extensively in the past and due to its sensitivity to disorder, the true nature of the metal-insulator transition in these films have been hard to decipher. We grow high quality ultra-thin films of LNO using reactive molecular beam epitaxy (MBE) and use a combination of ionic liquid gating and magneto-transport measurements to understand the nature and tunability of metal-insulator transition as a function of thickness for LNO. The underlying mechanisms for the transition are discussed in the framework of standard transport models. These results are discussed in the light of other Mott insulators such as Sr$_{2}$IrO$_{4}$, where we have performed similar measurements around the insulating state. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L47.00008: Photoinduced insulator-to-metal dynamics in strained NdNiO3 ultrathin films Richard D. Averitt, Elsa Abreu, Jingdi Zhang, Derek Meyers, Kun Geng, Jak Chakhalian Epitaxial rare-earth nickelate thin films are ideally suited for the study and control of electronic correlations associated with the insulator-to-metal phase transition since strain can be precisely controlled. We investigate picosecond conductivity dynamics of strained NdNiO3 utilizing time-domain terahertz spectroscopy. Starting from the insulating phase, photoexcitation results in a conductivity increase towards the metallic state. The dynamics consist of a fast ~1ps increase accompanied by a slower increase occurring over tens of picoseconds. These results will be compared to those obtained on the vanadates where the slow rise time is ascribed to nucleation and growth of the metallic phase. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L47.00009: Renormalization of the nickelate phase diagram in strained thin films Ankit Disa, Divine Kumah, Joseph Ngai, Fred Walker, Charles Ahn, Eliot Specht, Dario Arena As a result of strong electron-lattice coupling, the bulk electronic phase diagrams of correlated oxides can be modified in epitaxial thin films using reduced dimensionality and substrate-induced strain. Taking advantage of the unique features of these materials, such as correlation-driven magnetic and metal-insulator transitions, requires a systematic understanding of how the thin film phase diagram differs from the bulk. Here, we explore the phase diagram of thin films of rare-earth nickelates, $R$NiO$_3$, which in the bulk exhibit a systematic dependence of the transition temperature, $T_{MI}$, with $R$. Studying solid solutions of NdNiO$_3$ and LaNiO$_3$ (Nd$_y$La$_{1-y}$NiO$_3$ with $0 \leq y\leq 1$) under compressive epitaxial strain, we observe a consistent renormalization of $T_{MI}$ to lower temperatures. By examining the physical and electronic structure of the films using synchrotron x-ray diffraction and absorption spectroscopy, we determine that the renormalization is due to an enhanced Ni-O overlap as a result of coherent compressive strain. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L47.00010: Magnetotransport of NdNiO$_{3}$ thin films Adam Hauser, Evgeny Mikheev, Nelson Moreno, Tyler Cain, Jinwoo Hwang, Jack Zhang, Susanne Stemmer The Hall coefficient of epitaxial NdNiO$_{3}$ films is evaluated in a wide range of temperatures, from the metallic into the insulating phase. It is shown that for temperatures for which metallic and insulating regions co-exist, the Hall coefficient must be corrected for the time-dependence in the longitudinal resistance, which is due to a slow evolution of metallic and insulating domains. The positive Hall and negative Seebeck coefficients, respectively, in the metallic phase are characteristic for two bands participating in the transport. We report on magnetoresistance measurements at low temperature and interpret them in terms of the specific magnetic ordering in these films, as a function of epitaxial film strain. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L47.00011: Tunneling spectroscopy in Mott insulators, LaNiO$_{3}$ and NdNiO$_{3}$: Pseudo gaps and real gaps S. James Allen, Adam Hauser, Evgeny Mikheev, James Kally, Alex Kozhanov, Daniel Ouellette, Susanne Stemmer To explore the low lying excitations of prototypical charge transfer Mott insulators, we fabricated 4-terminal tunnel junctions and measured the temperature dependent tunnel conductance, along with the sheet resistivity, for LaNiO$_{3}$ a metal, and NdNiO$_{3}$ which underwent a metal insulator transition at 125 K. Films were deposited by rf magnetron sputtering on LaAlO$_{3}$ substrates. At low temperatures the tunneling conductance in LaNiO$_{3}$ develops a pronounced pseudogap 20 meV wide. NdNiO$_{3}$ exhibits a pseudo gap above the transition temperature. Just below the transition, the tunnel conductance at 0 bias is strongly suppressed but enhanced at larger bias voltages, signaling a redistribution of the quasi-particle density states as the system enters the insulating phase. At the lowest temperatures the tunnel conductance is suppressed by 4-5 orders of magnitude and a well developed gap appears - 25 meV wide. Comparisons are made with extant optical conductivity as well as recent theories based on Fermi surface instabilities.- SungBin Lee, Ru Chen, and Leon Balents, ``Landau Theory of Charge and Spin Ordering in the Nickelates,'' Phys. Rev. Lett. \textbf{106}, 016405 (2011). [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L47.00012: Electronic properties of nickelates across the metal-insulator transition: a high-resolution ARPES study R.S. Dhaka, Z. Ristic, N.C. Plumb, W. Kong, M. Medarde, M. Shi, L. Patthey, M. Radovic, J. Mesot The metal-insulator transition (MIT) in rare earth nickelate thin films has started to be a focus of the research in condensed matter physics [1]. Transport, magnetization and neutron scattering studies have shown a temperature driven MIT and magnetism in strongly correlated RNiO3 bulk samples [2] as well as in thin films [1]. In these systems it is believed that strong electron-electron correlations play an important role in MIT phenomena [3]. However, no direct information of the momentum resolved electronic structure across the MIT has been provided so far. Here, by combining in-situ PLD and high-resolution angle-resolved photoemission study, we report the band structure and Fermi surface (FS) of NdNiO3 thin films across the temperature driven MIT. In the metallic phase, we prove the existence of electron and hole FS pockets at the center and corner of the Brillouin zone (BZ), respectively. These FS pockets show strongly three-dimensional nature along the c-axis. Upon cooling across the MIT, we observe transfer of the spectral weight from near the Fermi level to higher binding energy in the entire BZ. Our results demonstrate the loss of coherent quasiparticle spectral weight associated with strong electron correlations involved in the MIT. [1] J. Liu et al., Nature Comm 4, 2714 (2013); R. Scherwitzl, et al., Advanced Materials 22, 5517 (2010). [2] M. Medarde, JPCM 9, 1679 (1997). [3] M. Imada, et al., Rev. Mod. Phys. 70, 1039 (1998). [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L47.00013: Specific Heat of Single Crystalline Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ Carlos Sanchez, Victor Aguilar, Oscar Bernal, Guo-meng Zhao Substantial studies of magnetization and specific heat on Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ have demonstrated the existence of a charge ordering (CO), ferromagnetic (FM), antiferromagnetic (AFM) transitions. In this work, the specific heat of two single crystalline Nd$_{0.5}$Sr$_{0.5}$MnO$_{3}$ samples, one containing $^{16}$O and the other highly concentrated $^{18}$O, was measured as a function of temperature, from 3K to 300K, at both zero and 50 kOe applied field. Measurements were done using a Quantum Design Physical Property Measurement System (PPMS) with Specific Heat option. The FM transition was found to depend on the isotope mass, which seems to agree with previous works. The CO transitions was observed as a sharp peak at the CO temperature (T$_{co}$), which seems to depend strongly on field, oxygen isotope mass, and thermal cycling history. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L47.00014: Nematicity in charge and orbital ordered structure: a new description of phase transitions in La$_{0.33}$Ca$_{0.67}$MnO$_{3}$ Jing Tao, W.G. Yin, Y. Zhu, K. Sun Doped manganites have a well-known unidirectional superlattice modulation at low temperatures, although the origin of the modulation is still under debate. The phase transition of the modulation in this compound has been characterized by the superlattice reflections and the transition temperature was determined when the modulation becomes long-range. Here we report a new description of the phase transition in La$_{0.33}$Ca$_{0.67}$MnO$_{3}$ from the aspect of symmetry by measuring anisotropy based on transmission electron microscopy results. Instead of one phase transition, we found that the electronic structures undergo smectic, nematic and isotropic behaviors upon warming. Comparing to previous characterizations of the phase transition in La$_{0.33}$Ca$_{0.67}$MnO$_{3}$, this symmetry measurement enables a better unification between electronic structure and other properties such as the crystal lattice variation. Moreover, we directly observed the creation of dislocation pairs in the smectic phase, which is consistent with the dislocation-proliferation mechanism predicted by the nematicity theory in correlated systems. The defect observations also suggest the charge and orbital ordering nature of the modulation in La$_{0.33}$Ca$_{0.67}$MnO$_{3}$. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L47.00015: Charge disproportionation without charge transfer in the rare-earth nickelates as a possible mechanism for the metal-insulator transition Steven Johnston, Anamitra Mukherjee, Ilya Elfimov, Mona Berciu, George Sawatzky We study a model for the metal-insulator (MI) transition in the rare-earth nickelates RNiO$_3$, based upon a negative charge transfer energy and coupling to a rock-salt like lattice distortion of the NiO$_6$ octahedra. Using exact diagonalization and the Hartree-Fock approximation we demonstrate that electrons couple strongly to these distortions. For small distortions the system is metallic, with ground state of predominantly $d^8 \, ligand$ character, where $ligand$ denotes a ligand hole. For sufficiently large distortions ($\delta d_{\rm Ni-O} \sim 0.05 - 0.10$ {\AA}), however, a gap opens at the Fermi energy as the system enters a periodically distorted state alternating along the three crystallographic axes, with $(d^8 \, ligand^2)_{S=0}(d^8)_{S=1}$ character, where $S$ is the total spin. Thus the MI transition may be viewed as being driven by an internal volume ``collapse'' where the NiO$_6$ octahedra with two ligand holes shrink around their central Ni, while the remaining octahedra expand accordingly, resulting in the superstructure observed in x-ray diffraction in the insulating phase. This insulating state is an example of charge ordering achieved without any actual movement of the charge, similar to that reported in a prior DMFT study. [Preview Abstract] |
Session L48: Invited Session: Light-matter Interaction in Valleytronic Materials and Topological Insulators
Sponsoring Units: DCMPRoom: Mile High Ballroom 1A-1B
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L48.00001: (1) Majorana fermions in pinned vortices; (2) Manipulating and probing Majorana fermions using superconducting circuits; and (3) Controlling a nanowire spin-orbit qubit via electric-dipole spin resonance Invited Speaker: Franco Nori We study [1,2] a heterostructure which consists of a topological insulator and a superconductor with a hole. This system supports a robust Majorana fermion state bound to the vortex core. We study the possibility of using scanning tunneling spectroscopy (i) to detect the Majorana fermion in this setup and (ii) to study excited states bound to the vortex core. The Majorana fermion manifests itself as an $H$-dependent zero-bias anomaly of the tunneling conductance. The excited states spectrum differs from the spectrum of a typical Abrikosov vortex, providing additional indirect confirmation of the Majorana state observation. We also study [3] how to manipulate and probe Majorana fermions using super-conducting circuits. In [4] we consider a semiconductor nanowire quantum dot with strong spin-orbit coupling (SOC), which can be used to achieve a spin-orbit qubit. In contrast to a spin qubit, the spin-orbit qubit can respond to an external ac electric field, i.e., electric-dipole spin resonance. We develop a theory [4] that can apply in the strong SOC regime. We find that there is an optimal SOC strength $\eta_{\mathrm{opt}}=\surd $2/2, where the Rabi frequency induced by the ac electric field becomes maximal. Also, we show that both the level spacing and the Rabi frequency of the spin-orbit qubit have periodic responses to the direction of the external static magnetic field. These responses can be used to determine the SOC in the nanowire. \\[4pt] [1] A.L. Rakhmanov, A.V. Rozhkov, F. Nori, \textit{Majorana Fermions in Pinned Vortices}, Phys. Rev. B \textbf{84}, 075141 (2011).\\[0pt] [2] R.S. Akzyanov, A.V. Rozhkov, A.L. Rakhmanov, F. Nori, \textit{Tunneling Spectrum of a Pinned Vortex with a Robust Majorana State}, arXiv:1307.0923.\\[0pt] [3] J.Q. You, Z.D. Wang, W. Zhang, F. Nori, \textit{Manipulating and probing Majorana fermions using superconducting circuits }(2011). arXiv:1108.3712\\[0pt] [4] R. Li, J.Q. You, C.P. Sun, F. Nori, \textit{Controlling a Nanowire Spin-Orbit Qubit via Electric-Dipole Spin Resonance}, Phys. Rev. Lett. \textbf{111}, 086805 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L48.00002: Engineering vacuum and thermal fluctuations with metamaterials Invited Speaker: Zubin Jacob In 1987, the search for a medium that expels vacuum fluctuations in a prescribed bandwidth and rigorously forbids spontaneous emission led to the concept of the photonic crystal. Here, we argue that the search for the opposite effect: enhancing vacuum and thermal fluctuations inside a medium within a prescribed bandwidth can be accomplished by an artificial medium known as a hyperbolic metamaterial. We will present the fluctuational electrodynamics of such media with hyperbolic dispersion and show that they exhibit broadband super-planckian thermal emission in the near-field. We will also present the quantum nanophotonics of such media where the enhanced vacuum fluctuations within the medium leads to a broadband Purcell effect. Finally, we will present associated effects in such artificial media such as optical topological transitions which make it viable to experimentally detect the signatures of these predicted effects. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:48AM |
L48.00003: Topological valleytronics in 2D Transition Metal Dichalcogenides Semiconductors Invited Speaker: Di Xiao In many crystals the Bloch bands have inequivalent and well separated energy extrema in the momentum space, known as valleys. The valley index constitutes a well-defined discrete degree of freedom for low-energy carriers that may be used to encode information. This has led to the concept of valleytronics, a new type of electronics based on manipulating the valley index of carriers. In the first part of this talk, I will describe a general scheme based on inversion symmetry breaking to control the valley index, using graphene and monolayers of MoS2 as an example. In particular, the valley Hall effect and valley-dependent optical selection will be discussed. In the second part, I will discuss the Berry phase effect on excitons formation and dynamics. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:24AM |
L48.00004: Polaron-like nature of massive Dirac fermions in valleytronic materials and topological insulators Invited Speaker: Zhou Li In this talk I will investigate the interplay of curvature modifications and spin-orbit interaction. As is well known, band curvature modifications can origin from electron-phonon interaction or other distortions, for example, the cubic or even higher in momentum warping term and the quadratic in momentum classical term, both of which modify drastically the transport properties (optical and magneto-optical, for example) of Dirac fermions in a topological insulator. A not so well known fact is that Berry curvature will also be modified by electron-phonon interaction and this may change the topology and dichroism of the system. Strong coupling theory of small polarons will be revisited in the presence of spin-orbit interaction and wave functions obtained there will be useful to construct low energy effective theory from the strong coupling limit. Phonon structures can be identified in many experiments, for example, STM, ARPES, Raman spectra, inelastic neutron scattering and so on. We have provided results from theoretical investigations for the first two experiments. In a recent work we have studied the optical conductivity in the presence of three terms, which are cubic, quadratic and linear in momentum, and find the interband optical conductivity will vanish when a SU(2) symmetry is recovered. This can be verified in both semiconductors and cold atoms, although the energy scale of these two systems differs by at least 1000000 times. \\[4pt] [1] Phys. Rev. B \textbf{87}, 155416 (2013).\\[0pt] [2] Phys. Rev. B \textbf{88}, 045414 (2013).\\[0pt] [3] Phys. Rev. B \textbf{88}, 045417 (2013).\\[0pt] [4] Phys. Rev. B \textbf{88}, 195133 (2013).\\[0pt] [5] Scientific Reports \textbf{3}, 02828 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 11:00AM |
L48.00005: Enhanced Valley Splitting for Quantum Electronics in Silicon Invited Speaker: Andre Saraiva Silicon is a placid environment for quantum degrees of freedom with long spin and valley coherence times [1]. A natural drawback is that the same features that protect the quantum state from its environment also hamper its control with external fields. Indeed, engineered nanostructures typically lead to sub-meV splittings between valley states [2], hindering the implementation of both spin [1] and valley [3] based quantum devices. We will discuss the microscopic theory of valley splitting [2,4], presenting three schemes to control valleys on a scale higher than 1 meV: a) in a quantum well, the adoption of a barrier constituted of a layered heterostructure might lead to constructive reflection if the layer thicknesses match the electron wavelength, in analogy with a Bragg mirror [5]; b) the disparity between the high valley splitting in a impurity donor potential and the low splitting in a Si/Insulator interface may be harnessed controlling the tunneling between these two states, so that the valley splitting may be controlled digitally [6]; c) intrinsic Tamm/Shockley interface states might strongly hybridize with conduction states, leading to a much enhanced valley splitting[4], and its contribution to the 2DEG ground state may be experimentally identified [7]. We argue that this effect is responsible for the enhanced splitting in Si/BOX interfaces [8]. \\[4pt] [1] F. Zwanenburg et al., Rev. Mod. Phys. \textbf{85}, 961 (2013).\\[0pt] [2] A Saraiva, M. J. Calder\'{o}n, Xuedong Hu, S. Das Sarma and Belita Koiller, PRB \textbf{80}, 081305 (2009).\\[0pt] [3] D. Culcer, A. L. Saraiva, Belita Koiller, Xuedong Hu, and S. Das Sarma, PRL \textbf{108}, 126804 (2012).\\[0pt] [4] A. Saraiva, Belita Koiller and M. Friesen, Phys. Rev. B~\textbf{82}, 245314 (2010).\\[0pt] [5] L. Zhang, J.-W. Luo, A Saraiva, Belita Koiller, Alex Zunger, Nature Comm. \textbf{4}, 2396 (2013).\\[0pt] [6] A. Baena, A. L. Saraiva, Belita Koiller, and M. J. Calder\'{o}n, PRB~86, 035317 (2012).\\[0pt] [7] A. Dusko, A. Saraiva and Belita Koiller, arXiv:1310.6878 (2013).\\[0pt] [8] K. Takashina, Y. Ono, A. Fujiwara, Y. Takahashi and Y. Hirayama, \textit{PRL} \textbf{96, }236801 (2006). [Preview Abstract] |
Session L49: Focus Session: Spectrocopy, Magnetism, and Transport at LaAlO3/SrTiO3 Interfaces
Sponsoring Units: DMPChair: Jochen Mannhart, Max Planck Institute for Solid State Research
Room: Mile High Ballroom 1C
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L49.00001: Photoemission spectroscopy studies of buried complex oxide interfaces Invited Speaker: Claudia Cancellieri At the interface between complex oxides, unexpected electronic properties different from those of the constituent bulk materials can arise. A particularly interesting example is the appearance of 2-dimensional conductivity at the interface of the band insulators LaAlO$_3$ (LAO) and SrTiO$_3$ (STO) above a critical LAO thickness of 4 unit cells. Photoemission spectroscopy is a powerful technique which directly probes the electronic structure of materials and can thus provide important information for a better understanding of their properties. The interface of LAO/STO has been investigated by soft x-ray photoelectron spectroscopy for different layer thicknesses across the insulator-to-metal interface transition. We measured clear spectroscopic signatures of Ti$^{3+}$ signal at the Fermi level in fully oxygenated sample. Our results show that Ti$^{3+}$-related charge carriers are present only for conducting samples, and are confined to a few monolayers from the interface. No Fermi-edge signal could be detected for insulating samples below the critical thickness. Polarization-controlled synchrotron radiation was subsequently used to map the electronic structure of conducting interfaces in a resonant angle-resolved photoemission experiment. A strong dependence on the light polarization of the Fermi surface and band dispersions is demonstrated, highlighting the distinct Ti~3$d$ orbitals involved in 2D conduction. Samples with different doping levels were prepared and measured by photoemission, revealing different band occupancies and Fermi-surface shapes. A direct comparison between the photoemission measurements and advanced first-principle calculations carried out for different 3$d$-band fillings is presented in conjunction with the 2D carrier concentration obtained from transport measurements. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L49.00002: Direct k-space mapping of the electronic structure in oxide-oxide interfaces M. Sing, F. Pfaff, J. Gabel, L. Dudy, G. Berner, P. Schuetz, H. Fujiwara, A. Yamasaki, Y. Saitoh, A. Sekiyama, S. Suga, V. Rogalev, V. Strocov, Y.Z. Chen, N. Pryds, R. Claessen Novel quantum phases can form at oxide heterointerfaces. Famous is the 2D electron system (2DES) in LaAlO$_3$/SrTiO$_3$ (LAO/STO). Its origin has been related to electronic reconstruction (ER). There electrons are transferred to the interface to compensate the potential gradient due to the polar discontinuity. The novel Al$_2$O$_3$ (AO)/STO also exhibits a 2DES but with much higher mobility [1]. In contrast to LAO, AO is regarded to be non-polar [1]. Hence ER should not be at work. It is assumed that O vacancies (Ovac) at the STO side of the interface induce the 2DES. We have directly mapped the k-resolved electronic structure of the interface states by resonant soft x-ray photoemission [2]. While we find a dichotomy of mobile and trapped charge -- the latter being ascribed to Ovac --, in both systems, they also show remarkable differences regarding the proportion of mobile and trapped carriers, the electron dispersions and Fermi surfaces, shedding light on the different role of Ovac.\\[4pt] [1] Y.Z. Chen et al, Nat. Comm. 4, 1371 (2013)\\[0pt] [2] G. Berner et al, PRL 110, 247601 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L49.00003: Orbital Inversion and Carrier Confinement in SrTiO$_{3}$/LaAlO$_{3}$ Interfaces Grown on NdGaO$_{3}$ Substrates Mark Golden, E. Slooten, B. Shi, B. Zwartsenberg, Z. Huang, T. Venkatesan, A. Annandi, M. Gorgoi, F. Radu, R. Abrudan, P. Miedema, C. Schuessler, J. Goedkoop, D. Doennig, J. Munding, R. Pentcheva, A. Ariando We investigate the role of the SrTiO$_{3}$ thickness on the electronic properties of heterointerfaces comprised of NdGaO$_{3}$ (bulk) / SrTiO$_{3}$ (n uc) / LaAlO$_{3}$ (15 uc) using X-ray linear dichroism (XLD) in absorption, Hard X-ray Photoemission Spectroscopy (HAXPES) and DFT$+$U calculations. XLD shows an orbital inversion for \textit{all} STO thicknesses: the lowest energy $d$-orbitals are of \textit{xz/yz} character, unlike `regular' STO/LAO, in which the \textit{xy} orbitals are lowest in energy. For n\textless 6 transport shows the carriers to be localized, HAXPES data showing electronic confinement to the NGO/STO interface. For STO thickness $\ge $8 uc, only weak localization is seen in transport, the STO/LAO interface becomes populated and carrier concentrations approaching half an electron per unit cell are deduced from both Hall measurements and HAXPES. Data from layer-resolved DFT$+$U calculations on STO/LAO and STO/NGO superlattices, and from NGO/STO/LAO are presented and compared to the experimental data, thereby providing a complete picture of the orbital polarization in these systems. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L49.00004: Electronic Orbital Reconstruction at (110)-oriented LaAlO$_{3}$/SrTiO$_{3}$ interfaces Gervasi Herranz, David Pesquera, Mateusz Scigaj, Nico Dix, Florencio Sanchez, Josep Fontcuberta, Pierluigi Gargiani, Javier Herrero, Eric Pellegrin, Manuel Valvidares, Alessandro Barla, Ruben Weht About ten years ago, a two-dimensional gas (2DEG) was discovered at the interface between two insulators: SrTiO$_{3}$ (STO) and LaAlO$_{3}$ (LAO). Later on, superconductivity as well magnetism were reported, making the LAO/STO interface an extremely intriguing system. So far the research was essentially directed to the (001)-interface, along which a built-in electrostatic potential is thought to generate the 2DEGs. Recently, however, high-mobility 2DEGs have been discovered along other directions, including \textless 110\textgreater for which such a built-in potential was unexpected. Yet, a direct fingerprint of the distinctive nature of the electronic structure at the (110)- interface has not been provided. Here, based on X-ray linear dichroism (XLD) experiments we show explicitly the dissimilar hierarchy of the electronic states at the (001)- and (110)- interfaces. In particular, our XLD experiments demonstrate that the degeneracy is fully removed in the t$_{\mathrm{2g}}$ and the e$_{\mathrm{g}}$ levels. Contrary to (001)- interfaces --where the first accessible orbitals are dxy--, our DFT calculations show, in agreement with XLD, a very strong contribution of the dxz/dyz orbitals at the first available levels in energy at (110)-interfaces. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L49.00005: An X-ray magnetic circular dichroism study of the interface Magnetism in titanate Heterostructures Marco Salluzzo The 2D-electron system (2DES) created at the interface between LaAlO$_{3}$ and SrTiO$_{3}$ have attracted strong interest in recent years. This system shows an intriguing inversion the Ti3d bands hierarchy at the interface respect the bulk [1], and some reports even suggested coexistence between ferromagnetism and superconductivity [2]. By using x-ray magnetic circular dichroism we show that oxygen vacancies induce magnetic interfacial localized Ti3$+$ states, which couple to the 2DES, with a negative exchange interaction. The magnetic dichroism signal is quenched in standard LAO/STO interfaces annealed in high oxygen pressure after the deposition and showing a homogeneous superconducting ground state [3], suggesting a decisive role of oxygen vacancies in the magnetism of these oxide interfaces [4,5]. \\[4pt] [1] M. Salluzzo, et al. Phys. Rev. Lett. 102, 166804 (2009); M. Salluzzo, et al., Adv. Mater. 25, 2333 (2013).\\[0pt] [2] J.A. Bert, et al., Nature Physics \textbf{7}, 767 (2011).\\[0pt] [3] D. Stornaiuolo, et al., Appl. Phys. Lett. 101, 222601 (2012).\\[0pt] [4] N. Pavlenko, et al., Phys. Rev. B 85, 020407(R) (2012).\\[0pt] [5] M. Salluzzo, et al., Phys. Rev. Lett. 111, 087204 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L49.00006: Competition between Kondo screening and Magnetism in the LaAlO3/SrTiO3 Interface Jonathan Ruhman, Arjun Joshua, Shahal Ilani, Ehud Altman We present a theory of magnetic phenomena at LaAlO3/SrTiO3 interfaces, which includes coupling between the conduction bands and local magnetic moments originating from charge traps at the interface. Tuning the itinerant electron density drives transitions between a heavy Fermi liquid phase with screened moments and various magnetic states. The dependence of the magnetic phenomena on the electron density stems from competing magnetic interactions between the local moments and the different conduction bands. At low densities only the lowest conduction band, composed of the $d_{xy}$ orbitals of Ti, is occupied. Its antiferromagnetic interaction with the local moments leads to screening of the moments at a Kondo scale that increases with density. However, above a critical density the $d_{xz}/d_{yz}$ bands begin to populate. Their ferromagnetic interaction with the local moments competes with the antiferromagnetic interaction of the $d_{xy}$ band leading to eventual reduction of the Kondo scale with density. We explain the distinct magneto transport regimes seen in experiments as manifestations of the magnetic phase diagram computed from the model. We also present interpretation of previously unpublished data supporting the theoretical model. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L49.00007: Magnetism at Amorphous Oxide Interfaces Spencer Tomarken, Andrea Young, Sang Woon Lee, Roy Gordon, Raymond Ashoori Recent work has shown that a mobile interfacial electron gas is created when certain amorphous transition metal oxides are grown on strontium titanate. We report torque magnetometry and transport measurements on a series of STO-based oxide heterostructures with amorphous overlayers grown by atomic layer deposition. We observe in-plane ferromagnetic ordering that is qualitatively similar to results observed in crystalline LAO/STO samples. We will discuss the implications of these results on the origin of magnetism in both polar and amorphous oxide interfaces. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L49.00008: Room-Temperature Electronically-Controlled Ferromagnetism at the LaAlO$_3$/SrTiO$_3$ Interface Feng Bi, Mengchen Huang, Chung-Wung Bark, Sangwoo Ryu, Chang-Beom Eom, Patrick Irvin, Jeremy Levy Reports of emergent conductivity, superconductivity, and magnetism at oxide interfaces have helped to fuel intense interest in their rich physics and technological potential. We employ magnetic force microscopy to search for room-temperature magnetism in the well-studied LaAlO$_3$/SrTiO$_3$ system.\footnote{F. Bi, \textit{et al.}, arXiv:1307.5557} Using electrical top gating to deplete electrons from the oxide interface, we directly observe an in-plane ferromagnetic phase with sharply defined domain walls. Itinerant electrons, introduced by a top gate, align antiferromagnetically with the magnetization, at first screening and then destabilizing it as the conductive state is reached. Subsequent depletion of electrons results in a new, uncorrelated magnetic pattern. This newfound control over emergent magnetism at the interface between two non-magnetic oxides portends a number of important technological applications. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L49.00009: A suggestion for making the ferromagnetism at perovskite oxide interfaces robust Nirmal Ganguli, Paul Kelly LaAlO$_3|$SrTiO$_3$ heterostructures have received much attention following observations of ferromagnetism, superconductivity and of an insulator to metal transition at the interface between otherwise conventional band insulators. One of the challenges posed by recent observations is to understand how high mobility charge carriers and local magnetic moments can coexist at $n$-type interfaces where the lack of a detailed knowledge of the interface structure from experiment is a major impediment to understanding these physical properties. A more extensive first principles study of the ferromagnetically ordered state found for modest values of Hubbard $U$ in the presence of GdFeO$_3$-type octahedral tilts at the interface [1] suggests that it should be possible to make the interface ferromagnetism more robust by enhancing the octahedral tilts. We screened a number of oxide interfaces with first principles calculations and identified the LaAlO$_3|$CaTiO$_3$ (001) interface as the most promising candidate in the large charge transfer limit, owing to the large intrinsic tilt of TiO$_6$ octahedra in CaTiO$_3$.\\[4pt] [1] Z. Zhong and P. J. Kelly, Europhys. Lett. \textbf{84}, 27001 (2008) [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L49.00010: Band alignments at oxide interfaces and consequences for devices Chris Van de Walle, Lars Bjaalie, Burak Himmetoglu, Leigh Weston, Anderson Janotti Oxide heterostructures have been shown to exhibit unusual physics and hold the promise of novel electronic applications. We present a set of criteria to select and design interfaces, particularly those that can sustain a high-density two-dimensional electron gas (2DEG). We describe how first-principles calculations, based on hybrid density functional theory, can contribute to a qualitative and quantitative understanding, illustrated with the key issue of band alignment. Band offsets determine on which side of the interface the 2DEG will reside, as well as the degree of confinement. We present band alignments for a number of complex oxides, considering materials with different types of conduction-band character, polar or nonpolar character, and band insulators as well as Mott insulators. We suggest promising materials combinations that could lead to a 2DEG with optimized properties, such as high 2DEG densities and high electron mobilities. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L49.00011: Low Temperature Transport of LaAlO$_3$/SrTiO$_3$ interfaces Stefano Gariglio, Alexandre Fete, Danfeng Li, Daniela Stornaiuolo, Jean-Marc Triscone The conducting interface between the two band insulators LaAlO$_3$ and SrTiO$_3$ has drawn a large share of attention, as it presents a variety of exciting electronic properties that are tunable by an electric field [1]. At low temperatures, magnetotransport analysis has revealed a strong Rashba spin-orbit interaction originating from the breaking of inversion symmetry [2] and, in field effect devices, the ground state has been tuned from an insulating to a superconducting state. I will discuss these results in light of recent magnetotransport experiments in field-effect devices to probe the evolution across the phase diagram of the weak localization /weak anti-localization transport regime, its relation to the strength and anisotropy of the superconducting state. \\[4pt] [1] A. D. Caviglia \textit{et al.}, Nature \textbf{456}, 624 (2008).\\[0pt] [2] A. D. Caviglia \textit{et al.}, Phys. Rev. Lett. \textbf{104}, 126803 (2010); A. F\^ete \textit{et al.}, Phys. Rev. B \textbf{86}, 201105 (2012). [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L49.00012: Superconducting Interfaces between Artificially-Grown LaAlO$_3$ and SrTiO$_3$ Thin Films Danfeng Li, Stefano Gariglio, Claudia Cancellieri, Alexandre Fete, Daniela Stornaiuolo, Jean-Marc Triscone Realization of a fully metallic two-dimensional electron gas (2DEG) at the interface between artificially-grown LaAlO$_3$ and SrTiO$_3$ thin films has been an exciting challenge. Here we present for the first time the successful realization of a superconducting 2DEG at interfaces between artificially-grown LaAlO$_3$ and SrTiO$_3$ thin films. Our results highlight the importance of two factors - the growth temperature and the SrTiO$_3$ termination. We use local friction force microscopy and transport measurements to determine that in normal growth conditions the absence of a robust metallic state at low temperature in the artificially-grown LaAlO$_3$/SrTiO$_3$ interface is due to the nanoscale SrO segregation occurring on the SrTiO$_3$ film surface during the growth and the associated defects in the SrTiO$_3$ film. By adopting an extremely high SrTiO$_3$ growth temperature, we demonstrate a way to realize metallic, down to the lowest temperature, and superconducting 2DEG at interfaces between LaAlO$_3$ layers and artificially-grown SrTiO$_3$ thin films. This study paves the way to the realization of functional LaAlO$_3$/SrTiO$_3$ superlattices and/or artificial LaAlO$_3$/SrTiO$_3$ interfaces on other substrates. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L49.00013: Quantum oscillations and Hall plateaus at LaAlO$_{3}$/SrTiO$_{3}$ interface Yanwu Xie, Christopher Bell, Yasuyuki Hikita, Harold Y. Hwang In this work, we tuned the sheet carrier density and mobility of the quasi-two-dimensional electron gas (q2DEG) confined at the LaAlO$_{3}$/SrTiO$_{3}$ interface by surface control, and studied the magneto-transport behavior of the q2DEG. We observed a universal trend that mobility increases with decreasing sheet carrier density, with a maximum mobility of \textgreater 20,000 cm$^{2}$V$^{-1}$s$^{-1}$. In a low sheet carrier density regime, we observed well resolved Shubnikov-de Haas quantum oscillations in the longitudinal resistance, and a plateau-like structure in the Hall conductivity. The frequency of the quantum oscillations shows a clear transition with increasing magnetic field, with a high / low field frequency ratio close to 3. In addition, the Landau indices of the plateaus in the Hall conductivity data show spacing close to 4, in units of the quantum of conductance. These features can be understood by considering magnetic breakdown orbits and account for all of the carriers. [Preview Abstract] |
Session L50: Focus Session: Quantum Plasmonics, Metamaterials, and Nanocrystals
Chair: Yanwen Wu, University of South CarolinaRoom: Mile High Ballroom 1D
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L50.00001: Quantum Plasmonics and Nanoscale Gap Plasmons with graphene, semiconductors and molecules Invited Speaker: Jeremy Baumberg Coupling between plasmonic nano-components generates strongly red-shifted resonances combined with intense local field amplification on the nanoscale. We have combined plasmonics with soft materials to tune this interaction dynamically, accessing the strong coupling domain for gaps below 1nm, reliably made by bottom-up self-assembly. At these distances coupled dipoles cannot describe the response, and a better account comes from gap plasmons. Crucial is the extreme sensitivity to separation, and how quantum tunneling starts to play an influence that can be directly seen at room temperature in ambient conditions. We recently demonstrated how quantum plasmonics controls the very smallest space that light can be squeezed into. We also demonstrate the possibility to track few molecules using surface-enhanced CARS. A new generation of 2D semiconductors coupled to such nano-scale gaps utilizes a nanoparticle on mirror geometry. \\[4pt] [1] \textit{Nature} \textbf{491}, 574 (2012); Revealing the quantum regime in tunnelling plasmonics.\\[0pt] [2] \textit{Nano Letters} \textbf{10}, 1787 (2010); Actively-Tuned Plasmons on Elastomeric Au NP Dimers.\\[0pt] [3] \textit{ACS Nano} \textbf{5}, 3878 (2011); Precise sub-nm plasmonic junctions within Au NP assemblies.\\[0pt] [4] \textit{Nano Lett} doi:10.1021/nl4018463 (2013); Controlling Sub-nm Plasmonic Gaps using Graphene. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L50.00002: A single-wavefunction density functional approach to the plasmonic nanostructures in the extreme quantum limit Dafei Jin, Fan Wang, Nicholas Fang We have constructed a single-wavefunction density functional model, which can reproduce the key physical properties of silver, such as its work function, exchange-correlation energy, bulk and surface plasmon frequencies. We apply this model to the studies of silver thin films, nanowires, and silver-dielectric indefinite metamaterials, at the length scale from subnanometers to tens of nanometers. We find that the quantum kinetics of electrons in silver can cause a large nonlocal dependence and blueshift of surface plasmon frequency, when the plasmonic wavelength and the typical size of structures become smaller than 50 nm. Our calculated results can be used to explain the spectrum broadening phenomena observed in recent cathodoluminescence and electron energy loss spectroscopy experiments. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L50.00003: Photogeneration of hot plasmonic carriers with metal nanocrystals Alexander Govorov, Hui Zhang, Yurii Gun'ko We investigate the effect of plasmon-assisted carrier injection from metal nanocrystals to a semiconductor contact or to adsorbed molecules. We treat the problem of optically-driven metal nanocrystal using the quantum approach of equation of motion of density matrix. Energy distributions of optically-excited plasmonic carriers are very different in metal nanocrystals with large and small sizes. In large nanocrystals, most excited carriers have very small energies and the electron distribution resembles the case of a plasmon wave in bulk. For gold nanocrystal with smaller sizes (less than 20nm), the energy distribution of hot carriers becomes flat and has a large number of carriers with high energy. Therefore, smaller nanocrystals are preferable for injection of plasmonic carriers into semiconductors or into molecules on the surface. The physical reason for the above behavior is non-conservation of momentum in a nanocrystal. The geometry, type of metal, and orientation of the external electric field are important to obtain high quantum efficiencies of generation and injection of plasmonic electrons. The results obtained in this study can be used to design a variety of plasmonic nano-devices based on hot electron injection for photocatalysis, light-harvesting, and solar cells. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L50.00004: Excitation of plasmons in metallic nanostructures by hot electrons in an adjacent semiconductor Jiantao Kong, Chaobin Yang, Juan Merlo, Michael J. Burns, Michael J. Naughton, Krzysztof Kempa It has been shown in a simple model calculation that hot electrons excited in a semiconductor can emit plasmons in an adjacent metallic nanostructure at a very high rate, exceeding that of phonon emission [1]. This effect could provide a possible route to high photovoltaic energy conversion efficiency in a hot electron solar cell. Here, we study this process in specific nanostructures, toward maximizing the effect. In theoretical work, we employ the high fidelity, finite difference time domain (FDTD) simulation technique to study the optical response of the systems considered, combined with quantum mechanical calculation of the scattering rates. We will also discuss fabrication and near and far-field optical measurements of test samples.\\[4pt] [1] K. Kempa, \textit{Phys. Status Solidi RRL} \textbf{7}, 465 (2013). [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L50.00005: Attosecond Electro-Magnetic Forces Acting on Metal Nanospheres Induced By Relativistic Electrons M.J. Lagos, P.E. Batson, A. Reyes-Coronado, P.M. Echenique, J. Aizpurua Swift electron scattering near nanoscale materials provides information about light-matter behavior, including induced forces. We calculate time-dependent electromagnetic forces acting on 1-1.5 nm metal nanospheres induced by passing swift electrons, finding both impulse-like and oscillatory response forces. Initially, impulse-like forces are generated by a competition between attractive electric forces and repulsive magnetic forces, lasting a few attoseconds (5-10 as). Oscillatory, plasmonic response forces take place later in time, last a few femtoseconds (1- 5 fs), and apparently rely on photon emission by decay of the electron-induced surface plasmons. A comparison of the strength of these two forces suggests that the impulse-like behavior dominates the process, and can transfer significant linear momentum to the sphere. Our results advance understanding of the physics behind the observation of both attractive and repulsive behavior of gold nano-particles induced by electron beams in aberration-corrected electron microscopy. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L50.00006: Nanoscale Assemblies of Fluorescent, Few-Atom Silver Clusters Stacy Copp, Danielle Schultz, Nemanja Markesevic, Kira Gardner, Sumant Oemrawsingh, Dirk Bouwmeester, Elisabeth Gwinn Silver clusters with sizes small enough to display high fluorescence quantum yields can be stabilized by DNA. These clusters show evidence for rod-like structure [1], opening up possibilities for new functionalities based on structure-modulated near-field patterning and anisotropic polarization response. We develop DNA clamps to hold two silver clusters composed of 10 and 15 atoms in nanoscale proximity, while retaining the individual structure of each cluster [2]. Thermally modulated fluorescence resonance energy transfer (FRET) verifies assembly formation, with clusters held 5 - 6 nm apart, in the range of the best resolution that can be achieved in DNA scaffolds. The absence of spectral shifts in these dual-cluster FRET pairs, relative to the individual cluster spectra, shows that few-atom silver clusters of different sizes can be sufficiently stable to retain their structural integrity when held within a nanoscale DNA construct. [1] D. Schultz, \textit{et al.}, Adv. Mater. \textbf{25}, 2797 (2013) [2] D. Schultz, \textit{et al.}, ACS Nano, ASAP (DOI: 10.1021/nn4033097). [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L50.00007: Direct observation of resonant modes in circular cavities by LRM Juan Merlo, Fan Ye, Michael J. Burns, Michael J. Naughton The observation of plasmonic cavities has become an important topic, as a number of novel technologies are being conceived and developed with such systems.\footnote{M. Khajavikhan, A. Simic, M. Katz, J.H. Lee, B. Slutsky, A. Mizrahi, V. Lomakin, Y. Fainman, Nature, 482, 204 (2012).} We present the experimental observation of the resonant plasmon modes in circular cavities by using an alternative scheme of the leakage radiation microscope. The reported method is very simple to implement (wide-field, non-scanning) without sample requirements more than the patterned cavity. The calculation of the cavities' radii for specific excited modes is based on a simple drumhead model.\footnote{F. Ye, M.J. Burns, and M. J. Naughton, Nano Lett. 13, 519 (2013).} Numerical simulations confirm our observations and suggest that the detected field is related to the in-plane components of the modes in the cavity, an expected result when the leakage radiation microscopy is used. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L50.00008: Experimental observation of TM propagated modes in nanocoax structures Michael J. Naughton, Binod Rizal, Fan Ye, Michael J. Burns, Juan M. Merlo The nanoscale manipulation of light has become one of the most important research areas in the last years.\footnote{R. R. Oulton, V. J. Sorger, T. Zentgraf, R. M. Ma, C. Gladden, L.Dai, G. Bartal, X. Zhang, Nature 461, 629-632 (2009).} Several studies in nanoscale waveguides have been done and the coaxial waveguide is among the most promising due to its broadband properties.\footnote{D. Pozar, D. ``Microwave Engineering,'' 3rd. Edition. John Wiley and Sons, Inc. USA, 2005.} Here, we report the experimental observation of photonic and plasmonic transverse magnetic mode propagation in a nanocoax structure by use of leakage radiation microscopy and near-field scanning optical microscopy in the visible and near-infrared ranges of the electromagnetic spectrum. Numerical calculations are consistent with our experimental results and suggest that the propagated modes are mainly TM$_{10}$-like (plasmonic) and TM$_{11}$ (photonic) modes, confirming theoretical results previously reported.\footnote{ Peng Y., Wang W., Kempa K., Opt. Express. 3, 1758-1763 (2008).} [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L50.00009: Meta-Atom Interactions and Coherent Response in RF SQUID Metamaterials Melissa Trepanier, Daimeng Zhang, Oleg Mukhanov, Philipp Jung, Susanne Butz, Alexey Ustinov, Steven Anlage We have designed, fabricated, and measured RF SQUID (radio frequency superconducting quantum interference devices) metamaterials and demonstrated their extreme tunability with temperature, DC magnetic field, and rf current [1]. The SQUID metamaterial can be modelled as an array of weakly coupled oscillators with tunable resonant frequencies. An array of identical SQUIDs under identical conditions will have a coherent collective response regardless of the strength of the interactions between them. In the presence of disorder (nonuniform magnetic flux for instance) the individual SQUIDs in the array may or may not tune coherently. Since we are interested in metamaterial applications, the coherent response is desirable. In this talk we examine the conditions required for the SQUIDs to tune coherently, and compare to experimental data on tuning and nonlinearity in a variety of RF SQUID metamaterials.\\[4pt] [1] M. Trepanier*, Daimeng Zhang*, Oleg Mukhanov, Steven M. Anlage, Phys. Rev. X (in press), arXiv:1308.1410v2 [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L50.00010: Extremely nonlinear and switchable SQUID metamaterial Daimeng Zhang, Melissa Trepanier, Oleg Mukhanov, Philipp Jung, Susanne Butz, Alexey Ustinov, Steven Anlage We present experimental results on a superconducting metamaterial with remarkably nonlinear and switchable properties in the microwave range. The meta-atoms are RF Superconducting Quantum Interference Devices (SQUIDs), a superconducting loop interrupted by a single Josephson Junction. RF SQUIDs are similar to split-ring resonators except that the inductance is tunable due to the nonlinear Josephson inductance. This metamaterial has high tunability via DC magnetic field, temperature and applied RF power [1]. Here we focus on the nonlinearity in our metamaterial due to the Josephson effect. The intermodulation measurements show a highly nonlinear response from the metamaterial. In an RF power dependence experiment we observed hysteretic behavior in transmission which indicates the metamaterial is a nonlinear multi-state system. As a result, we can control the transmission by switching between metastable states via manipulating the applied RF power. We also observe a unique self-induced transparency of meta-atoms in a certain applied RF power range. This extremely nonlinear metamaterial has potential application for next-generation digital RF receiver systems. \\[4pt] [1] M. Trepanier*, D. Zhang*, O. Mukhanov, S.M. Anlage, Phys. Rev. X (in press), arXiv:1308.1410v2. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L50.00011: Band gap variations in ferritin-templated nanocrystals John Colton, Stephen Erickson, Trevor Smith, Richard Watt Ferritin is a 12 nm diameter protein shell with an 8 nm ``cage'' inside that can be used as a template for nanoparticle formation. The native particle is an iron oxide, ferrihydrite, but can be altered or replaced. We have used optical absorption spectroscopy to study the band gap of the ferrihydrite nanoparticles as they age (and become more crystalline), and as they respond to surface interactions with ions in solution. We will also present results of particle composition variations due to incorporation of oxo-anions into the interior of the nanoparticles and substitution of iron with other metals such as cobalt and manganese. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L50.00012: Band gap measurements and tunability of ferrihydrite nanocrystals Stephen Erickson, John Colton, Trevor Smith, Richard Watt Ferrihydrite nanocrystals occur naturally within the protein ferritin, a spherical shell with an 8nm wide interior. The nanocrystal core of ferritin can be removed and replaced with a variety of other minerals of a controlled size, allowing for potential to tune their band gap for a variety of applications. However, band gap measurements of even the native ferrihydrite have proven elusive, with reported values of the band gap in the literature ranging from 1.0-3.5 eV. We have resolved these discrepancies using the well-established method of optical absorption spectroscopy, finding evidence of an indirect gap of 2.14 eV and an onset of direct transitions occurring 3.05 eV. A defect-related mid-gap state also exists, with a binding energy of 0.22 eV. Furthermore, we have shown that the band gap can be tuned from (at least) 1.92 - 2.24 eV by controlling the size of the nanocrystals. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L50.00013: Multi-functional single electron device at room temperature Chieu Nguyen, Jason Kee Yang Ong, Ravi F. Saraf Smart designs of sub-wavelength structures enable observation of unusual properties of materials as in metamaterials. Typically, Coulomb blockade is observed in array of conducting particles at cryogenic temperature due to local charging of few particles by a single electron in the percolation path. We will report 1-D network of cemented Au nanoparticles in a multi-functional single electron device exhibiting Coulomb blockade at room temperature. The 1-D array is a self-assembled monolayer network spanning between electrodes 10-100$\mu$m apart. It is formed by first bridging the negatively charged 10nm Au NPs with positive ions (Cd2+or Fe3+) followed by cementing with reactive gas to form a robust 2-D network. The network array cemented with CdS and Iron oxide exhibits robust single electron effect at room temperature with electroluminescence (EL) or ferromagnetism, respectively. The nature of EL in this symmetric structure is explained in term of field induced ionization. The EL is specular where the spots are independent of bias magnitude. The magnetic array exhibits ``spin-valve'' behavior with Barkhausen effect. These unique nano materials, fully self-assembled where, properties can be tailored by varying the cement chemistry, have potential applications in solid state lighting. [Preview Abstract] |
Session L51: Focus Session: Beyond Graphene Devices: Function, Fabrication, and Characterization III
Sponsoring Units: DMPChair: Jing Shi, University of California, Riverside
Room: Mile High Ballroom 1E
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L51.00001: Electric-field tuning phonon in single layer WS2 Yiling Yu, Yifei Yu, Alper Gurarslan, Linyou Cao The physical properties of two-dimensional semiconductor materials play crucial role in realizing next generation electronic and opto-electronic devices. In this work, we observe a dramatic change in Raman spectrum for single layer WS2 under external electric field. The intensity of Raman peak will increase or decrease for different direction of bias voltages. This indicates we can tune the optical phonon behavior by external electric field and enable a strong electron-phonon coupling in the single layer WS2. Our results can provides new physical understanding to electron-phonon coupling to two dimensional material systems, and suggest a potential promising way to control thermal conductivity of layered materials through external electric field, which is very interesting to both basic physics and device applications. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L51.00002: Thermopower of graphene and the validity of Mott's formula Fereshte Ghahari, Takashi Taniguchi, Kenji Watanabe, Philip Kim Thermoelectric power (TEP) of graphene is previously measured in the disorder limited transport regime where the semiclassical Mott relation agrees with experimental data. In this presentation, we report the TEP measurement on graphene samples deposited on hexagonal boron nitride substrates where drastic suppression of disorder is achieved. Our results show that at high temperatures the measured thermopower deviates from Mott relation and this deviation is greater for higher mobility samples. We quantify this deviation in both degenerate and non-degenerate regime using Boltzmann transport theory considering different scattering mechanisms in the system. [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L51.00003: Ballistic Thermal Conductance in Layered Two-Dimensional Materials Zuanyi Li, Yizhou Liu, Yong Xu, Wenhui Duan, Eric Pop The thermal properties of two-dimensional (2D) materials like graphene, h-BN, MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ are uniquely anisotropic, including high in-plane but low out-of-plane thermal conductivity $\kappa $. Here we provide a comparative study of the ballistic limits of heat flow in these 2D layers and stacks. Based on full phonon dispersions from density functional theory, we calculate their in-plane and cross-plane ballistic thermal conductance per cross-sectional area, $G$. For a given material, monolayers and multilayers have similar in-plane $G$ above 100 K, but monolayers show higher $G$ at low temperature due to the contribution of flexural phonons. At 300 K, graphene has the highest $G$ $\sim$ 4.2 GWK$^{\mathrm{-1}}$m$^{\mathrm{-2}}$, about 20{\%} higher than h-BN and 5 times higher than MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$. Cross-plane values are about one order of magnitude lower than in-plane values due to weak van der Waals interactions. Based on the calculated $G$, we can obtain phonon mean free path, given diffusive $\kappa $. These results are important as they establish the length scales of the ballistic-diffusive transition of heat flow and the non-classical regime where $\kappa $ depends on the system size. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 9:12AM |
L51.00004: Thermal and Thermoelectric Transport in Two-Dimensional Materials and Devices beyond Graphene Invited Speaker: Li Shi Besides the switching speed and on-off ratio, the hot spot temperature is one important performance metric of novel electronic devices fabricated from two-dimensional (2D) materials beyond graphene. This performance metric depends sensitively on the largely unknown thermal transport properties of various 2D materials. In addition, it still remains a grand challenge to experimentally verify the theoretical predictions of enhanced thermoelectric figure of merit in 2D systems and by topologically protected surface states. Following our prior works on thermal transport measurements of graphene, we have recently studied thermal transport in few-layer h-BN, MoS2, and germanane, and the thermoelectric properties of bismuth telluride nanoplates. The results reveal that surface perturbation suppresses the in-plane lattice thermal conductivity of these 2D materials. The thickness needed for recovery to the bulk lattice thermal conductivity scales with the bulk phonon mean free path. In addition, we have observed decrease in both the electrical conductivity and thermal conductivity with decreasing thickness of bismuth telluride nanoplates. While the electrical conductivity is still within the bulk range, the thermal conductivity is reduced to below the bulk range for nanoplates thinner than 20 nm. These results are explained by the presence of surface band bending and diffuse surface scattering of electrons and phonons in the nanoplates, where pronounced n-type surface band bending can yield suppressed and even negative Seebeck coefficient in unintentionally p-type doped nanoplates. Sb doping and surface functionalization are employed in our works to tune the Fermi level and surface band bending and modify the thermoelectric properties. [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L51.00005: Thermoelectric performance in ultra-thin transition metal dichalcogenides Darshana Wickramaratne, Ferdows Zahid, Roger Lake The thermoelectric figure of merit, ZT, is calculated for one to four monolayers of MoS$_{2}$, MoSe$_{2}$, WS$_{2}$ and WSe$_{2}$. The maximum ZT in this family of materials occurs in bilayer MoSe$_{2}$. Its ZT value of 2.39 is a factor of 8 increase compared to that of the bulk at room temperature. The values for the power factors and ZT change non-monotonically as the film thicknesses are increased from a single monolayer up to four layers. In contrast to Bi$_{2}$Te$_{3}$, the peak value of ZT occurs at a thickness greater than a single monolayer for all 4 materials. The shape of the distribution of the valence band and the conduction band density of modes explains the enhanced thermoelectric performance that occurs for film thicknesses above a single monolayer. Ab-initio electronic structure calculations are used in a Landauer approach to calculate the thermoelectric transport coefficients. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L51.00006: First-principles Raman spectra of MoS$_2$, WS$_2$ and their heterostructures Liangbo Liang, Vincent Meunier MoS$_2$ and WS$_2$ are graphene-like layered structures that are considered as alternative and complement to graphene. Raman spectroscopy is a very powerful tool to study them. Despite the extensive experimental Raman study on MoS$_2$ and WS$_2$, it remains unclear how Raman intensities and especially intensity ratio of Raman modes E$_{2g}$ and A$_{1g}$ depend on the thickness. To clarify such issues, we carried out density functional theory calculations for both MoS$_2$ and WS$_2$ to simulate their Raman spectra and reveal the intrinsic thickness dependence of Raman intensities and intensity ratio. More importantly, we quantitatively analyzed the laser polarization effect on the intensity ratio and revealed its high sensitivity to laser polarization, which could explain the large discrepancy between measured intensity ratios by different groups. We also studied $\textit{ab initio}$ Raman spectra of MoS$_2$/WS$_2$ heterostructures up to four layers in every possible combinations and stacking orders. Each configuration is found to possess a unique Raman spectrum in both frequency and intensity that can be explained by changes in dielectric screening and interlayer interactions. Our findings serve as guidelines for the experimental identification of heterostructure configurations. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L51.00007: Nano-imaging and nano-spectroscopy of tunable surface phonon polaritons in hexagonal boron nitride Siyuan Dai, Zhe Fei, Qiong Ma, Aleksandr Rodin, Martin Wagner, Alexander McLeod, Mengkun Liu, Will Gannett, William Regan, Mark Thiemens, Gerardo Dominguez, Antonio Castro Neto, Alex Zettl, Fritz Keilmann, Pablo Jarillo-Herrero, Michael Fogler, Dimitri Basov Van der Waals crystals such as graphene, topological insulators, cuprate high-temperature superconductors, and many other layered structures reveal a rich variety of enigmatic electronic, photonic and magnetic properties. We report infrared (IR) nano-imaging of surface phonon polaritons in a prototypical van-der-Waals crystal: hexagonal boron nitride (hBN). In the setting of an antenna-based IR spectroscopic nanoscope, we accomplished launching, detecting, and real space imaging of the polaritonic waves. We were able to alter both the wavelength and the amplitude of such waves by varying the number of crystal layers in our specimens. We demonstrated a new nano-photonics method for mapping the polariton dispersion. The dispersion is shown to be governed by the crystal thickness according to a scaling law that persists down to a few monolayers. Our results point to novel functionalities of van-der-Waals crystals as reconfigurable nano-photonic materials. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L51.00008: Generation and Detection of Coherent THz Acoustic Phonons in Few Atomic Layer MoS2 Haining Wang, Changjian Zhang, Wei Min Chan, Sandip Tiwari, Rana Farhan We present, for the first time, results on the generation and detection of coherent THz acoustic phonon oscillations in few-monolayer MoS2 by ultrafast pump-probe technique. In $\sim$ 1nm thick Dichalcogenides, the lowest confined LA phonon modes in the out-of-plane direction can have frequencies approaching one THz. In our experiments, a pump pulse is used to excite these phonon modes through the Raman process. The refractive index of few-layer MoS2 is sensitive to the layer separation, and, therefore, the transmission of the probe pulse changes with the layer separation allowing us to observe coherent phonon oscillations in real time. The measured phonon frequencies, for different number of monolayers (from $\sim$3 to $\sim$100), agree well with analytical model based on the quantization of the bulk LA phonon dispersion in the out-of-plane direction and the interlayer force constant was extracted. Our data also allow us to extract phonon lifetimes and quality factors. The observed ultrafast dynamics of the photoexcited carriers also evolve with the number of monolayers as the electronic bandstructure evolves. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L51.00009: Layer-dependent electronic and vibrational properties of SnSe$_{2}$ and SnS$_{2}$ 2D materials Joseph Gonzalez, Rudy Schalf, Ivan Oleynik Layered metal chalcogenides possess a wide range of unique electronic properties, which are currently explored for applications as novel two-dimensional electronic materials. SnS$_{2}$ and SnSe$_{2}$ layered materials consist of covalently bonded S-Se-S (Sn-Se-Sn) sheets bonded together by weak van der Waals interactions. The atomic, electronic and vibrational properties of SnS$_{2}$ and SnSe$_{2}$ thin films are investigated using first-principles density functional theory. The evolution of the thickness-dependent band structure and Raman spectra are discussed, as well as the effects of strain and the influence of the substrate. The first-principles results are compared with available experimental data. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L51.00010: Light Generation and Harvesting in a Van der Waals Heterostructure Oriol L\'opez S\'anchez, Esther Alarcon Llado, Volodymyr Koman, Anna Fontcuberta i Morral, Aleksandra Radenovic, Andras Kis We report on the realization of light-emitting diodes based on heterojunctions with monolayer MoS$_{2}$. Careful interface engineering allows us to realize diodes showing rectification and light emission from the entire surface of the heterojunction. Electroluminescence spectra show clear signs of the A and B excitons and the A- trion resonance related to the optical transitions between the conduction and valence bands. Our pn diodes can also operate as solar cells with an external quantum efficiency higher than 2{\%}. Our work opens up the way to more sophisticated optoelectronic devices such as 2D LEDs and solar cells based on monolayer MoS$_{2}$. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L51.00011: Electronic and optical properties of monolayer and few-layer of distorted transition-metal dichalcogenides Pierre Darancet, Andrew J. Millis, Chris A. Marianetti Groups IV, V, and VI- transition-metal dichalcogenides (TMDC) are layered compounds exhibiting a wealth of competing phenomena, ranging from charge density waves (CDW) to Mott transitions. We present investigations using density functional theory (DFT) and DFT+U regarding the electronic structure and electronic correlations arising in distorted tantalum disulfide (TaS2). We show that the monolayer material is a Mott insulator while the bulk is a metal, in contradiction with much of the existing literature, which argues that the bulk material is a Mott insulator. Properties of the few layer system will also be presented.Finally, we will discuss the influence of these competing energy scales on the transport and optical properties of these materials. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L51.00012: Persistent Photoconductivity in Monolayer MoS$_{2}$ on Organic-molecule-functionalized Substrates Wei-Hua Wang, Yueh-Chun Wu, Shao-Yu Chen, Cheng-Hua Liu, Po-Hsun Ho, Chun-Wei Chen, Chi-Te Liang We demonstrate a giant persistent photoconductivity (PPC) effect in monolayer MoS$_{2}$ in which the photocurrent robustly persists after illumination has ceased. This PPC effect in monolayer MoS$_{2}$ on organic-molecule-functionalized substrates sustains up to room temperature and can be highly suppressed by applying source--drain/back-gate voltages to the transistors. Based on this persistency and controllability of the PPC effect, we achieve a room-temperature conductance bistability by utilizing optical and electrical pulses. The observed giant PPC effect in MoS$_{2}$ can be attributed to a large electron-capture barrier of trap states, which is estimated to be as high as 390 meV. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L51.00013: Enhanced Photoluminescence and Photocurrent Spectra in MoS2 under Ionic Liquid Gating Zhen Li, Shun Wen Chang, Stephen Cronin We report substantial improvements in the photoluminescence (PL) and photocurrent (PC) spectra of monolayer MoS2 field effect transistors taken under electrostatic and ionic liquid gating conditions. The photocurrent and photoluminescence spectra show good agreement with a dominant peak at 1.9eV. The magnitude of the photoluminescence and photocurrent can be increased significantly by Si back gating and ionic liquid gating due to the passivation of surface states and trapped charges that act as recombination centers and cause non-radiative recombination of photoinduced electron-hole pairs. Under ionic liquid gating, we observe an increase in the photoluminescence intensity by 300{\%}, while the linewidth decreases by 37{\%}. The photocurrent also doubles when passivated by the ionic liquid. The acute sensitivity of monolayer MoS2 to ionic liquid gating and passivation arises because of its high surface-to-volume ratio, which makes it especially sensitive to trapped charge and surface states. Under high gating conditions, we observe a slight decrease in the photoluminescence intensity, most likely due to Auger recombination. These results reveal that, in order for efficient optoelectronic devices to be made from monolayer MoS2, some passivation strategies must be employed to mitigate the issues associated with surface states. [Preview Abstract] |
Session L52: Focus Session: Superconductivity, Vortex Matter I
Sponsoring Units: DMPChair: Leonardo Civale, Los Alamos National Laboratory
Room: Mile High Ballroom 1F
Wednesday, March 5, 2014 8:00AM - 8:36AM |
L52.00001: Nonlinear dynamics of vortices in nano-structured superconductors Invited Speaker: Francois Peeters Nonlinear flux dynamics in superconductors is a prototype for many nonlinear phenomena occurring in different areas of physics. It is well-known that vortices in nano-structured superconductors with dimensions comparable to characteristic length scales behave very different from those in bulk materials, e.g. multi-quanta giant vortices and symmetry-induced vortex-antivortex pairs can nucleate which are impossible in bulk. I will give a few examples of the surprising behavior of vortex matter in different type of nano-structured superconducting films. Our analysis is based on the time-dependent Ginzburg-Landau theory. Where possible comparison with experiments.will be made. For example large resistance oscillations are found, different from the usual Little-Parks effect, that are due to current-excited moving vortices. Unusal field-induced increase in the critical current may be observed as a consequence of the nonlinear distribution of the current in a sample. In type-I superconductors even in the intermediate state the smallest building block turns out to be a flux quantum and the current driven nucleation of flux domains is discretized to a single fluxoid. Domains of opposite flux, when driven towards each other, annihilate through a discretized sequence of single vortex-antivortex pairs. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L52.00002: Superconducting Thin films with Periodic Ferromagnetic Nanostructures Wonbae Bang, K.D.D. Rathnayaka, I.F. Lyuksyutov, W. Teizer, D.G. Naugle Studies of the transport properties of superconducting Tin (Sn) thin films covered by periodic ferromagnetic nanostructures are reported. The Sn thin films and the periodic nanosized ferromagnetic configurations were patterned by electron-beam lithography and deposited by thermal quench condensation. A Germanium (Ge) layer was thermally evaporated at room temperature as an insulating barrier between the Sn thin films and the ferromagnetic structures. When a current was applied parallel to ferromagnetic stripes, the critical current and hysteresis exceeded the critical current perpendicular to the ferromagnetic stripes resulting in a strong anisotropy. We have observed that the critical currents show a matching field effect. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L52.00003: In-situ tunable vortex pinning with an array of ferromagnetic anti-dots Yong-Lei Wang, Zhili Xiao, Leo Ocola, Ralu Divan, George W. Crabtree, Wai-Kwong Kwok We investigated vortex pinning effects of a ferromagnetic antidot array in a superconducting film. A square antidot array of 30 nm thick permalloy (Py) was patterned onto a MoGe superconducting film with thickness of 100 nm. Although we found no evidence of vortex pinning enhancement by the pristine magnetic antidot array in perpendicular magnetic fields, we found that by applying an independently controlled in-plane magnetic field the magnetic antidot array can provide excellent vortex pinning, resulting in a tunable superconducting critical current enhancement. Through micromagnetic simulation and magnetic force microscopy imaging, we demonstrate that the tunable vortex pinning originates from spatially periodic stray field generated by the magnetic antidot array in the presence of an in-plane magnetic field. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L52.00004: Highly effective superconducting vortex pinning in conformal crystals S. Gu\'enon, Y.J. Rosen, Ali C. Basaran, Ivan K. Schuller In the last few years, the search for an artificial pinning center (APC) distribution that pins a vortex lattice in a superconducting thin film over a wide magnetic field range has attracted a lot of attention. Recently, a conformal crystal obtained by conformally mapping a hexagonal lattice was proposed [1]. We compared the magneto-transport measurements of a conformal crystal and a randomly diluted APC distribution with a triangular reference lattice. We discovered for both APC distributions that the magneto-resistance is significantly reduced in a magnetic field interval between the first matching fields of the triangular reference lattice. Moreover, in this interval, the magneto-resistance of the conformal crystal APC distribution is below the noise floor indicating highly effective vortex pinning over a wide magnetic field range.\\[4pt] [1] D. Ray et al., Phys. Rev. Lett. 110, 267001 (2013) [Preview Abstract] |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L52.00005: Ratchet effect in a conformal pinning array Boldizsar Janko, Dipanjan Ray, Cynthia Olson Reichhardt, Charles Reichhardt Pinning arrays where the pinning sites are located at the vertices of a conformally transformed hexagonal lattice, also known as conformal pinning arrays (CPA), have recently been shown to greatly enhance the critical current of type-II superconductors both in simulation and in experiment\footnote{D. Ray et al, Phys. Rev. Lett. 110, 267001 (2013); Y. L. Wang et al, Phys. Rev. B 87, 220501(R) (2013); S. Guenon et al, Appl. Phys. Lett. 102, 252602 (2013).}. Here we show using molecular dynamics simulations that the differing flux-flow resistance of the CPA in the forward and reverse directions causes it to function as a highly effective vortex ratchet. We drive the vortices using an applied external ac current, and we find that the resulting dc output voltage for the CPA ratchet is larger than that for a random pinning array with a pinning gradient\footnote{C. J. Olson et al, Phys. Rev. Lett. 87, 177002 (2001).} by up to an order of magnitude. The enhancement is robust over a wide range of vortex densities, temperatures, and ac drive amplitudes and frequencies. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L52.00006: Magnetic flux structures and superconducting structures in nano-sized polygon superconducting plates Masaru Kato, Osamu Sato Vortex structures in nano-sized superconductors under an external field have been studied for decades. It was shown that vortices in square superconducting plates are different form those in bulk superconductors. The vortices are affected by the shielding current at edges of superconductors. Therefore the vortex structure depends on the shape of the superconductor. We recently studied the vortex structures in a pentagon superconducting plate at low temperature and showed how vortex structures changes with increasing the magnetic field. These structures agree with the experiment by Ishida et al. [1]. In this study, we investigate the vortex structures around the transition temperature in various polygon superconducting plates under the external field. For this purpose we solve the Ginzburg-Landau (GL) equations, especially linearized GL equations, using the finite element method. We show the relation between shapes of superconductors and vortex structures and superconducting order parameter structures. [1] T. H. Huy, M. Kato, T. Ishida, Supercond. Sci. Technol. 26 (2013) 065001. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 10:12AM |
L52.00007: Tailored Large Critical Currents in Iron-based Superconductors Invited Speaker: Wai-Kwong Kwok Iron-based superconductors, with their relatively high superconducting transition temperatures, high critical currents and low anisotropy holds great potential for applications such as superconducting generators and motors where the magnetic field environment can greatly affect the critical current capacity. These materials may hold the key to developing an isotropic, high field, high critical current, high-temperature superconductor, one of the grand challenges of applied superconducting research. I will discuss our recent work on using various types of particle irradiation to elucidate the vortex pinning behavior of these materials and the remarkable enhancement of the critical current that can be achieved with no detrimental effect on the transition temperature. Furthermore, I will show that certain induced correlated disorder can lower the thermodynamic anisotropy of these superconductors. Finally, I will discuss the advantages of composite defects induced by compounded proton and heavy-ion irradiation in further enhancing the critical current at high magnetic fields. These results on iron-based superconductors will be compared with the performance of current state-of-the-art commercial YBCO coated conductors. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L52.00008: Huge Critical Current Density and Tailored Superconducting Anisotropy in SmFeAsO(0.8)F(0.15) by Low Density Columnar-Defect Incorporation U. Welp, L. Fang, Y. Jia, V. Mishra, C. Chaparro, V.K. Vlasko-Vlasov, A.E. Koshelev, G.W. Crabtree, S.F. Zhu, N.D. Zhigadlo, S. Katrych, J. Karpinski, W.K. Kwok SmFeAsO(0.8)F(0.15) is of great interest because it has the highest transition temperature of all the iron-based superconductors. We find that the introduction of a low density of correlated nano-scale defects enhances the critical current density up to 2 $\times$ 10$^{7}$A/cm$^{2}$ at 5 K without any suppression in the high superconducting transition temperature of 50 K and amounting to 20 {\%} of the theoretical depairing current density. We also observed a surprising reduction in the thermodynamic superconducting anisotropy from 8 to 4 upon irradiation. A model based on anisotropic electron scattering predicts that the superconducting anisotropy can be tailored via correlated defects in semi-metallic, fully gapped type II superconductors. - We acknowledge support by the Center for Emergent Superconductivity, an EFRC funded by the US DOE, Office of Basic Energy Sciences (LF, YJ, VM, AEK, WKK, GWC), by the DOE, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357 (CC, VKV, UW), by the EC Research Council project SuperIron (JK, SK), and by the Swiss National Science Foundation and the National Center of Competence in Research MaNEP (NDZ). [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L52.00009: Doubling the Critical Current Density of 2G-Coated Conductors through Proton Irradiation Maxime Leroux, Y. Jia, D.J. Miller, J.G. Wen, W.K. Kwok, U. Welp, M. Rupich, S. Fleshler, A. Malozemoff, A. Kayani, O. Ayala-Valenzuela, L. Civale The in-field performance of production-line 2nd generation high temperature superconducting cable can be substantially improved by post-fabrication irradiation with 4 MeV protons. A dose of $8.10^{16} p/cm^2$ nearly doubles the critical current in fields of 6 T // c at 27 K and more generally the suppression of Jc in magnetic field is reduced. A mixed pinning landscape composed of preexisting precipitates and twin boundaries and small, finely dispersed irradiation induced defects may account for the improved vortex pinning in high magnetic fields. Our current data-set indicates that there is significant head-room for further enhancements.This work was supported by the Center for Emergent Superconductivity, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (Y.J., M.L., W.K.K., U.W., O.A.V., L.C.) and by the Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC02- 06CH11357 (D.J.M., J.G.W.). Irradiations were carried out at the Western Michigan University accelerator laboratory. Microstructure was characterized in the Electron Microscopy Center at Argonne, supported by the Office of Science-Basic Energy Science. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L52.00010: Time-dependent Ginzburg-Landau equations and vortex dynamics simulations on GPUs Ivan Sadovskyy, Andreas Glatz Most energy applications of superconductivity, such as electric power transmission over superconducting cables or powerful magnets, require low energy dissipation in high-temperature superconductors. Restricting the mobility of the vortices carrying magnetic field in the superconducting material by pinning them with admixed inclusions or confining their motion geometrically can minimize dissipation. We present modern simulation results of the time-dependent Ginzburg-Landau equation for large-scale mesoscopic superconductors, like narrow superconducting strips and nano-patterned superconductors. In particular, we discuss the case of nano-scale extended pinning inclusions, whose geometry has a non-trivial influence on the current-voltage characteristics. The required large-scale simulations were made possible with recent GPU computing techniques. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L52.00011: Thermal and quantum fluctuations effects on the vortex matter in Fe-based superconductors with naturally-grown and engineered pinning landscapes Leonardo Civale, Oscar Ayala Valenzuela, Boris Maiorov, Jeehoon Kim Vortex pinning and dynamics in Fe-based superconductors is at least as complex as in oxide superconductors. Clean single crystals may have very simple pinning landscapes dominated by a single type of defects and low critical current density ($J$$_c$). In contrast, thin films frequently show much higher $J$$_c$ arising from mixed pinning landscapes containing both uncorrelated and correlated disorder. On top of these features in as-grown samples, the pinning landscape can be effectively engineered by irradiation or by addition of non-superconducting second phases. A somewhat surprising characteristic of the vortex matter in Fe-based superconductors is that it tends to show large fluctuations effects similar or even larger than oxide HTS, such as fast flux creep and extended liquid phases. This is the case even in compounds where simple estimates based on the value of the Ginzburg number would suggest that fluctuation effects should be much smaller. I will present studies of vortex matter in Fe-based superconductors with naturally-grown and engineered pinning landscapes, and discuss the influence of thermal and quantum fluctuations and the characteristics of the vortex liquid phases. [Preview Abstract] |
Session L53: Focus Session: Molecular Adsorption and Metal and Oxide Film Growth
Sponsoring Units: DMPChair: Shirley Chiang, University of California, Davis
Room: Mile High Ballroom 2C
Wednesday, March 5, 2014 8:00AM - 8:12AM |
L53.00001: Diffusion of anthracene derivatives on Cu(111) studied by STM and DFT Jonathan Wyrick, Ludwig Bartels, Theodore Einstein Substituted anthracenes have drawn attention due to their ability to diffuse uniaxially on a Cu(111) surface. We compare anthracene to three of its derivatives whose 9,10 hydrogens are replaced by elements of the chalcogen group that act as linkers binding the molecules to a Cu(111) substrate. DFT calculations shed light on STM imaging and diffusion studies on the three substituted species. We present an analysis of the DFT results in which energetic contributions to the diffusion barriers are partitioned among the Kohn-Sham orbitals, allowing us to make assignments as to how each orbital affects diffusion for each species and draw comparisons between them. [Preview Abstract] |
Wednesday, March 5, 2014 8:12AM - 8:24AM |
L53.00002: Highly Ordered Organic Molecular Thin Films on Silicon Studied by STM and LEED Sean Wagner, Pengpeng Zhang Achieving growth of long-range ordered organic molecular thin films on inorganic substrates continues to be a significant challenge for organic electronics applications. Here, we report the growth of highly ordered zinc phthalocyanine (ZnPc) thin films both in-plane and out-of-plane on the deactivated Si(111) surface by scanning tunneling microscopy (STM) and low energy electron diffraction (LEED). By adjusting the substrate temperature during deposition, the anisotropic step-flow growth mode can be accessed causing a reduction in the substrate symmetry which allows for the long-range in-plane ordering as well as the decrease of grain boundary density [1] [2]. Additionally, the ZnPc molecules are able to maintain a highly ordered configuration in multi-layers despite a gradual decrease in the molecule-substrate interaction, which is attributed to the strong interlayer $\pi$-$\pi$ interaction [2]. \newline \newline [1] S. R. Wagner, R. R. Lunt, and P. P. Zhang, \textit{Phys. Rev. Lett.} \textbf{110}, 086107 (2013).\newline [2] S. R. Wagner and P. P. Zhang, (Submitted). [Preview Abstract] |
Wednesday, March 5, 2014 8:24AM - 8:36AM |
L53.00003: Adsorption geometry of ZnTPP molecules on Au(111): self-assembly and surface interaction Charles Ruggieri, Sylvie Rangan, Robert Bartynski, Elena Galoppini The interaction between Zinc Tetraphenylporphyrin (ZnTPP) molecules and a Au(111) surface, from initial adsorption sites to monolayer organization, is investigated using scanning tunnel microscopy with a particular emphasis on registry of the overlayer and surface atomic structure. At low coverages ZnTPP decorates step edges. With further deposition, ZnTPP molecules form self-organized islands of flat-lying macrocycles having a well-defined registry with, and dimensions bounded by, the underlying Au(111) herringbone reconstruction. At monolayer coverage, the herringbone reconstruction persists, enabling the relationship between the geometry of the self-organized molecular layer and that of the Au(111) surface to be established. Surface annealing generates a more complex self-assembled structure characterized by Au step edges that strictly align with ZnTPP molecular rows. The underlying mechanisms for this behavior will be discussed. [Preview Abstract] |
Wednesday, March 5, 2014 8:36AM - 8:48AM |
L53.00004: Scanning Tunneling Microscopy Analysis of a Pentacene/Graphene/SiC(0001) system Andrew Yost, Ozgun Suzer, Joseph Smerdon, TeYu Chien, Jeffrey Guest A complete understanding of the structure of molecular assemblies, as well as an understanding of donor-acceptor interactions is crucial in the development of emergent molecular electronics technologies such as organic photovoltaics. The pentacene (C$_{22}$H$_{14})$ is a good electron donor in Pentacene-C60 system, which is a model system of an organic photovoltaic cell.. Here we present scanning tunneling microscopy studies of the pentacene(Pn) molecule on Graphene(G) that is epitaxially grown on SiC(0001). In addition to the morphologies reported in literature, several new structures of Pn on on G/SiC(0001) were observed with different periodicity and registry both in monolayer and bilayer coverages of molecules on the surface. Preliminary scanning tunneling spectroscopy of the molecular system is also discussed; well-isolated states and a large HOMO-LUMO gap indicate the Pn is weakly coupled to the grapheme and underlying substrate. [Preview Abstract] |
Wednesday, March 5, 2014 8:48AM - 9:00AM |
L53.00005: STM and DFT examination of self-assembled 5,6,7-trithiapentacene-13-one on vicinal gold (788) Amanda Larson, Jeremiah van Baren, Jeremy Kintigh, Jun Wang, Jian Ming Tang, Glen P. Miller, Karsten Pohl The novel pentacene derivative 5,6,7-trithiapentacene-13-one (TTPO) is a robust electron donor candidate for use in high temperature photovoltaic devices. STM imaging has revealed interesting nanoscale surface structures of TTPO molecular chains as well as an ordered self-assembled monolayer on 3.9nm wide gold (788) surface terraces. TTPO is a polar species of pentacene with centered oxygen and sulfur bridge substituents. It is along this sulfur bridge that TTPO arranges itself laterally with a small cant angle between the molecule and the gold surface. This lateral assembly varies from the common flat-lying and standing-up phases of pentacene on surfaces. Combining imaging with density functional theory calculations allows for classification of these self-assembled structures with particular interest being directed toward the interaction between TTPO and gold at this organic-metallic interface. Understanding the structure of organic-metal interfaces with molecular precision potentially allows for the tailoring of those interfaces in order to maximize charge carrier transport. [Preview Abstract] |
Wednesday, March 5, 2014 9:00AM - 9:12AM |
L53.00006: ABSTRACT WITHDRAWN |
Wednesday, March 5, 2014 9:12AM - 9:24AM |
L53.00007: Combination of Two Nanoscale Quantum Systems by Controlled Manipulations Yang Li, Andrew DiLullo, Brandon Fisher, Saw-Wai Hla Modifying properties of materials at the nanoscale is an important ability for bottom-up designs of new materials, nanodevices, which might lead to new applications in the future. One method to achieve this goal is to put two nanoscale systems together and then study how they influence the properties of each other. Scanning tunneling microscopes (STMs) are ideal tools for these manipulations. Here, we report one way to modify electronic properties of two nanoscale systems, vacancies and molecules, by a novel process of STM manipulation. By these manipulations, performed near 6 K, surface vacancies were controllably created on a noble metal surface. Molecules were selectively moved into the created vacancies. Scanning tunneling spectroscopy was used to measure the change of the electronic structures of this new vacancy and molecule complex. It was found that the energy spectrum of the vacancy-molecule complex was a combination of the vacancy electronic structure and signature molecular orbitals. This work demonstrates the controlled combination of two nanoscale quantum systems which resulted in definite overlap of the electronic states of the constituent parts. By this process it is possible to design and form a new class of nanoscale systems for future research. [Preview Abstract] |
Wednesday, March 5, 2014 9:24AM - 9:36AM |
L53.00008: Templated quasicrystalline molecular layers Joe Smerdon, Kirsty Young, Michael Lowe, Sanger Hars, Thakur Yadav, David Hesp, Vinod Dhanak, An-Pang Tsai, Hem Raj Sharma, Ronan McGrath Quasicrystals are materials with long range ordering but no periodicity. We report scanning tunneling microscopy (STM) observations of quasicrystalline molecular layers on five-fold quasicrystal surfaces. The molecules adopt positions and orientations on the surface consistent with the quasicrystalline ordering of the substrate. Carbon-60 adsorbs atop sufficiently-separated Fe atoms on icosahedral Al-Cu-Fe to form a unique quasicrystalline lattice whereas further C$_{60}$ molecules decorate remaining surface Fe atoms in a quasi-degenerate fashion. Pentacene (Pn) adsorbs at tenfold-symmetric points around surface-bisected rhombic triacontahedral clusters in icosahedral Ag-In-Yb. These systems constitute the first demonstrations of quasicrystalline molecular ordering on a template. [Preview Abstract] |
Wednesday, March 5, 2014 9:36AM - 9:48AM |
L53.00009: Spin dynamics and quantum states in 3d atomic chains on Cu3N-Cu(110) molecular network Oleg Stepanyuk, Dmitry Bazhanov, Valeri Stepanyuk Based on first-principle calculations we studied the magnetic state and exchange coupling of transition metal atomic chains of Mn, Fe and Co deposited on a self-corrugated Cu3N-Cu(110) molecular network. We considered various atomic sites for adsorption on the corrugated Cu3N layer. By calculating the ground state magnetic configurations it was shown, that the magnetic order, anisotropy and exchange coupling within atomic chains depend sensitively on their chemical composition and adsorption sites on Cu3N network. We have found that exchange coupling in nanowires could be ferromagnetic and anti-ferromagnetic depending on the position of the chain on the surface. The spin-dynamics is investigated by means of kinetic Monte Carlo method based on transition-state theory. Using ab-initio determined exchange parameters and spin moments we apply the irreducible tensor operator technique to evaluate the Heisenberg-Dirac-Van Vleck quantum spin Hamiltonian for calculation of magnetic susceptibility of atomic chains. Using this value as a macroscopic entanglement witness we demonstrate that in antiferromagnetic chains of different length the entanglement temperature can be as much as 30-40 K. [Preview Abstract] |
Wednesday, March 5, 2014 9:48AM - 10:00AM |
L53.00010: Spin Crossover Transition in Molecular Adsorbates Xin Zhang, Sumit Beniwal, Axel Enders, Peter A. Dowben, Tatiana Palamarciuc, Patrick Rosa, Jean-Fran\c{c}ois L\'etard, Jing Liu, Eduardo V. Lozada, Fernand Torres, Luis G. Rosa, Bernard Doudin The occupied and unoccupied electronic structure of ultra thin films of the spin crossover [Fe(H$_{\mathrm{2}}$B(pz)$_{\mathrm{2}})_{\mathrm{2}}$(bipy)] complex (with H$_{\mathrm{2}}$B(pz)$_{\mathrm{2}}=$bis(hydrido)bis(1H-pyrazol-1-yl)borate and bipy $=$ 2,2'-bipyridine) was investigated by ultraviolet photoelectron spectroscopy (UPS), inverse photoemission (IPES) and X-ray absorption spectroscopy (XAS). The XAS spectra clearly shows the change of iron L edge spectra associated with thermal induced spin crossover. Generally changes occurring for the iron coordination, across the spin crossover transition, are seen to be very similar. The spin crossover transition, and certainly the unoccupied electronic structure, is influenced by the polarization direction of molecular ferroelectric poly(vinylidene fluoride -- trifluoroethylene) substrates at temperatures in the vicinity of the thermally driven spin cross-over transition. Combining the STM studies with the thickness dependent IPES results of the molecular adsorbate on gold substrates, we understand that the molecular thin film spin-states may also be affected by thickness of the Fe(H$_{\mathrm{2}}$B(pz)$_{\mathrm{2}})_{\mathrm{2}}$(bipy) film. [Preview Abstract] |
Wednesday, March 5, 2014 10:00AM - 10:12AM |
L53.00011: Sub-molecular electronic structure of self-assembled metal-organic nano-chains on a noble metal surface Agustin Schiffrin, Martina Capsoni, Adam Shaw, Sarah Burke Complexes composed of organic ligands coordinated with transition metal atoms exhibit broad absorption bands from the ultraviolet to the near-infrared. These are the result of the intrinsic molecular electronic properties, which include intra-ligand excitations and metal-to-ligand charge transfer. When adsorbed on a surface, these compounds are relevant for photovoltaic applications. In order to ensure a hierarchical transfer of function from the nano- to the macro-scale, electronic characterization at the single molecule level is essential. We present a low-temperature scanning tunneling spectroscopy study on the local electronic structure of one-dimensional self-assembled metal-organic nanostructures formed on a noble metal surface. The nano-chains consist of terpyridine-based ligands coordinated with iron (Fe) adatoms. We map the local density of electronic states of the system with sub-molecular spatial resolution. Energy-broadened highest-occupied molecular orbitals are dominated by metal states, whereas sharp resonances above Fermi are mainly related to the organic moiety. Coordination between the ligand and Fe induces energy shifts and a break of spatial symmetry of the unoccupied states, pointing to an electron transfer from the metal atom to the terpyridine groups. [Preview Abstract] |
Wednesday, March 5, 2014 10:12AM - 10:24AM |
L53.00012: Induced Nanoscale Surface Vacancies and their Local Electronic Characteristics Andrew DiLullo, Yang Li, Danda Pani Acharya, Noboru Takeuchi, Saw-Wai Hla Nanoscale surface topological variations effect local electrochemical properties. We directly alter nanoscale surface corrugation by local probe manipulations using a scanning tunneling microscope and report here the procedure and resulting changes in surface electrochemistry. Tunneling resonances, found at certain probe-sample biases, are found by analysis of spatial height-differential mapping (dz/dV). These resonances result from field emission where the emitted electron has greater energy than the local surface potential at the probe lateral position. We extract, by fitting to Gundlach's equation, the tip work function, sample work function at probe position, and absolute tip height from the sample. The difference in the extracted work function at the surface vacancies and the surface terraces demonstrates a significantly altered electronic character. It is important to be able to understand nanoscale variations in the local work function, as this surface potential can play a large role in determining the outcome of attempted surface electrochemistry. [Preview Abstract] |
Wednesday, March 5, 2014 10:24AM - 10:36AM |
L53.00013: Mg(0001): Electronic structure features controlling the limit of and reactivity in the thin-film regime, stacking fault of Mg adislands and adatom self-diffusion Marisol Alcantara Ortigoza, Maral Aminpour, Talat S. Rahman We analyze the electronic structure of the Mg(0001) surface as a function of slab thickness to reveal the features that control chemical reactivity of films of less than 17 layers. The thickness dependence of the oxidation rate of Mg thin films is directly related to the in-plane-PLDOS(E) of the first- and second-layer atoms around the Fermi level. Regarding the origin of the stacking fault - which is attached to a Friedel-oscillation-driven charge density pocket at the fcc site, we find that the role of the charge-density pocket is that of strengthening the substrate bonds since adsorption at the fcc site charge is distributed among surface atoms enhancing their mutual binding. Charge-density analyses, however, are only indirect evidence that the stacking fault is caused by Mg Friedel oscillations. To strengthen our arguments, we thus test an additional material: Be(0001) -- another hcp sp- and nearly-free-electron metal that is also strongly influenced by Friedel oscillations. Comparison of Mg(0001) and Be(0001) shows that the charge density enhancement at the fcc site for Be(0001) is dramatically larger than that found for Mg(0001). Most importantly, Be(0001) provides more evidence that the stacking fault preference is driven by the Friedel oscillations. Namely, the Be monomer on Be(0001) not only also prefers the fcc stacking fault site than the hcp one but the stacking fault energy is strikingly large: 44 meV. [Preview Abstract] |
Wednesday, March 5, 2014 10:36AM - 10:48AM |
L53.00014: The Physical Character of the Au (001) Surface Reconstruction in the Presence of CO and O2 Andrew Loheac, Michael S. Pierce, Andi Barbour, Vladimir Komanicky, Chenhui Zhu, Hoydoo You The interaction of carbon monoxide and oxygen on Au (001) single crystal facets has been investigated using synchrotron based surface x-ray diffraction and scattering techniques. Preliminary experiments confirm the quasi-hexagonal surface reconstruction can be influenced by exposure to CO and O, and indicate that oxidation may be present. Subsequent surface x-ray scattering experiments included a residual gas analyzer (RGA) with isotopic CO to tag the chemical species. Both CO (by itself) and O (dissociated from molecular $\mathrm{O}_2$ by the x-rays) are capable of lifting the hexagonal surface reconstruction resulting in a disordered bulk truncated surface. A wide range of pressures (1 mTorr - 10 Torr) and temperatures (300 K - 900 K) have been explored. We have also adapted a system of coupled partial differential equations to model the absorption kinetics and surface reconstructions. [Preview Abstract] |
Wednesday, March 5, 2014 10:48AM - 11:00AM |
L53.00015: When is one layer complete? Using simultaneous in-situ RHEED and x-ray reflectivity to map layer-by-layer thin-film oxide growth M.C. Sullivan, M.J. Ward, H. Joress, A. Gutierrez-Llorente, A.E. White, A. Woll, J.D. Brock The most popular tool for characterizing \textit{in situ} layer-by-layer growth is Reflection High-Energy Electron Diffraction (RHEED). X-ray reflectivity can also be used to study layer-by-layer growth, as long as the incident angle of the x-rays is far from a Bragg peak. During layer-by-layer homoepitaxial growth, both the RHEED intensity and the reflected x-ray intensity will oscillate, and each complete oscillation indicates the addition of one layer of material. However, it is well documented, but not well understood, that the maxima in the RHEED intensity oscillations do not necessarily occur at the completion of a layer. In contrast, the maxima in the x-ray intensity oscillations do occur at the completion of a layer, thus the RHEED and x-ray oscillations are rarely in phase. We present our results on simultaneous \textit{in situ} x-ray reflectivity and RHEED during layer-by-layer growth of SrTiO$_3$ and discuss how to determine the completion of a layer for RHEED oscillations independent of the phase of the RHEED oscillation. [Preview Abstract] |
Session L54: Meet the APS Editors Coffee Break
Sponsoring Units: APSRoom: Exhibit Hall F
Wednesday, March 5, 2014 10:45AM - 11:30AM |
L54.00001: Meet the APS Editors Coffee Break The APS Journal Editors invite you to join them for conversation and light refreshments. The Editors will be available to answer questions, discuss your ideas, and listen to your comments about the journals. All are welcome to attend. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700