Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session A1: Recent Advances in Density Functional Theory I
Sponsoring Units: DCP DCOMPChair: Weitao Yang, Duke University
Room: 103/105
Monday, March 3, 2014 8:00AM - 8:36AM |
A1.00001: Strong Correlation in Density-Functional Theory Invited Speaker: Axel Becke What do fractional occupancies mean in Kohn-Sham Density-Functional Theory (KS-DFT)? Can we model configuration mixing in KS-DFT? Can molecular bonds be dissociated using spin-restricted KS orbitals? These questions will be addressed in the context of recent work on a strongly-correlated, exact-exchange based (or Hartree-Fock based) correlation functional of the author. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A1.00002: Strong correlation in Kohn-Sham DFT Francesc Malet Giralt, Andr\'e Mirtschink, Jonas Cremon, Christian Mendl, Klaas Giesbertz, Stephanie Reimann, Paola Gori-Giorgi The knowledge on the strong-interacting limit of density functional theory can be used to construct exchange- correlation functionals able to address strongly-correlated systems without introducing any symmetry breaking. We report calculations on semiconductor nanostructures and one-dimensional models for chemical systems, showing that this approach yields quantitatively good results in both the weakly- and the strongly-correlated regimes, with a numerical cost much lower than the traditional wavefunction methods. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A1.00003: Local Correction to Reduce Delocalization Errors in Approximate Density Functionals Chen Li, Xiao Zheng, Weitao Yang We develop a local correction scheme to reduce delocalization errors in approximate density functionals. A concept of local fractional electron distribution is proposed and corresponding local functions are designed to evaluate its magnitude. Following our previous idea of linearizing each of the nonlinear components in Kohn-Sham density functional, we impose a local linearity condition rather than a global condition. By building our correction functionals in terms of our local functions, we can largely reduce the error in systems that present local fractional electron distribution but no global fractional charge. Our results show that the dissociation curves of diatomic molecules as well as dimer cations can be largely improved. Furthermore, the non-physical ionic product of dissociated molecules by traditional density functionals can be corrected to neutral atoms. We believe the analogous problems in charge transfer systems that are inaccessible for traditional density funcitonals can be correctly handled by our correction functional, and the well-known delocalization error in large extent improved. In addition, our correction scheme maintains the computational efficiency of traditional DFT, enabling it to be applicable to large scale systems. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A1.00004: Density-driven delocalization error for a model solvated electron system Stephen Dale, Alberto Otero-de-la-Roza, Erin Johnson Electrides are a unique class of ionic solids in which the anions are replaced by a lone electron localized within a crystal void. Theoretical modeling of these systems is possible using DFT methods. However, delocalization error inherent in common density-functional approximations increases the complexity of this study. To investigate delocalization error effects, we propose a simplified electride model, known in solvated electron chemistry as the Kevan structure. This model localizes an electron within a void formed by six radially-oriented, octahedrally-arranged water molecules. The Kevan structure is then coupled with atoms of various electronegativities, and the resulting complex is used to test different density functionals. We show that fractional charges caused by delocalization error have a significant impact on the electron density of the Kevan structure. Finally, we use results for the Kevan structure to rationalize the calculated band gaps for the actual electrides. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:48AM |
A1.00005: Gedanken Densities and Lower Bounds in Density Functional Theory Invited Speaker: John P. Perdew A gedanken density is not a real one but one imagined in the construction of density functional approximations. The uniform electron gas is the original gedanken density, but we will be concerned here with two others: (1) the ground-state density of one electron in the presence of a nonuniform periodic potential , in which the reduced density gradient $s=\left| {\nabla n} \right|/[2(3\pi^{2})^{1/3}n^{4/3}$diverges almost everywhere as the volume tends to infinity. This density was used in the construction [1] of a generalized gradient approximation (GGA): To satisfy the general Lieb-Oxford lower bound [2] on the exchange-correlation energy for all possible densities, the exchange enhancement factor $F_{x} \equiv \varepsilon _{x}^{approx} /\varepsilon_{x}^{unif} $ in the large-$s$ limit for a spin-unpolarized density must be less than or equal to 1.804. (2) a two-electron spherical ground-state density in which $s$ takes the same arbitrary positive value wherever the density is non-zero [3]. This density can be used to show that, to satisfy the tight Lieb-Oxford bound on the exchange energy of a two-electron density for every possible such density, $F_{x} $ for such a density (and probably for every density) must be less than 1.174. The local spin density approximation (LSDA) for exchange ($F_{x} =1)$ satisfies this tight bound, but standard GGA's and meta-GGA's do not. A talk by Jianwei Sun will present what may be the first beyond-LSDA approximation to satisfy this strong new constraint. \\[4pt] [1] J.P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. \textit{77,} 3815 (1996).\\[0pt] [2] E.H. Lieb and S. Oxford, Int. J. Quantum Chem. \textit{19}, 427 (1981).\\[0pt] [3] J.P. Perdew, J. Sun, A. Ruzsinszky, and K. Burke, in preparation. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A1.00006: First beyond-LSDA density functional satisfying a tight lower bound for exchange Jianwei Sun, John Perdew, Adrienn Ruzsinszky Universal constraints of density functional theory (DFT) play major roles in approximating its exchange-correlation energy ($E_{\mbox{xc}} )$. One of the prominent constraints is the Lieb-Oxford bound: $E_{xc}^{exact} [N]\ge \lambda_{xc} [N]E_{x}^{LDA} [N]$, where LDA stands for local density approximation, N is the electron number of systems, and $\lambda_{xc} [N]$ increases with N with an upper bound of 2.275. For ground-state 1-e systems, the above inequality reduces to $E_{x}^{exact} [N\mbox{=1}]\ge \lambda_{xc} [N\mbox{=1}]E_{x}^{LDA} [N\mbox{=1}]$ with a tight bound $\lambda_{xc} [N\mbox{=1}]=$1.48, shedding light on the exchange energy. Our recent study (John P. Perdew's talk) shows that, to avoid violating the tight bound for any possible 1-e densities, a semilocal functional should respect it locally. We further conjecture for exchange energies that $E_{x}^{exact} [N]\ge \gamma_{x} [N]E_{x}^{L\mbox{S}DA} [N]$ with$\gamma_{x} [N]$ decreasing with N and $\gamma_{x} [N=1]=\gamma_{x} [N=2]=\lambda _{xc} [N\mbox{=1}]$/2$^{1/3} =$1.174. Here, local spin density approximation (LSDA) is used as the reference since the exchange has a well-defined spin-scaling relation. Based on the tight Lieb-Oxford bound and the conjecture, we present a simple meta-generalized gradient approximation (MGGA) for exchange that interpolates different LSDAs for N$=$1 and uniform electron gas (N $\to$ infinity), respectively, and delivers excellent exchange energies for atoms. When combined with a modified PBE correlation, the MGGA yields good binding energies for molecules and lattice constants for solids. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A1.00007: Band gaps with approximate density functionals: the derivative discontinuity revealed from ensemble considerations Eli Kraisler, Leeor Kronik The band gap is a central property of solids. Unfortunately, this quantity is not generally equal to the Kohn-Sham band gap of density functional theory (DFT), even in principle. The two band gaps differ precisely by the derivative discontinuity. Popular approximate functionals are thought to be devoid of a derivative discontinuity, thereby eliminating their usefulness for gap prediction. Here we show that all exchange-correlation functionals possess a derivative discontinuity, which arises naturally from the application of ensemble considerations within DFT. The approach requires no empiricism and involves no approximations beyond the choice of the exchange-correlation functional. Furthermore, the derivative discontinuity can be expressed in closed form using quantities obtained in the course of a standard DFT calculation of the neutral system, allowing for band gap calculations in periodic systems. The approach is demonstrated by calculations of the band gap for eleven representative insulators and semiconductors, using the ensemble approach with the local density approximation. We find that the derivative discontinuity revealed by this approach accounts for a significant part of the overall band gap and its inclusion reduces the error in band gap prediction from 50\% to 10\%. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A1.00008: Gap renormalization of molecular crystals from density-functional theory Sivan Refaely-Abramson, Sahar Sharifzadeh, Manish Jain, Roi Baer, Jeffrey B. Neaton, Leeor Kronik Fundamental gap renormalization due to electronic polarization is a basic phenomenon in molecular crystals. Despite its ubiquity and importance, all conventional approaches within density-functional theory completely fail to capture it, even qualitatively. Here, we present a new screened range-separated hybrid functional, which, through judicious introduction of the scalar dielectric constant, quantitatively captures polarization-induced gap renormalization, as demonstrated on the prototypical organic molecular crystals of benzene, pentacene, and C60. This functional is predictive, as it contains system-specific adjustable parameters that are determined from first principles, rather than from empirical considerations [Phys. Rev. B 88, 081204(R) (2013)]. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A1.00009: Local Density Approximation Exchange-correlation Free-energy Functional Valentin Karasiev, Travis Sjostrom, James Dufty, S.B. Trickey Restricted path integral Monte-Carlo (RPIMC) simulation data for the homogeneous electron gas at finite temperatures [1] are used to fit the exchange-correlation free energy as a function of the density and temperature. Together with a new finite-$T$ spin-polarization interpolation, this provides the local spin density approximation (LSDA) for the exchange-correlation free-energy functional required by finite-$T$ density functional theory. We discuss and compare different methods of fitting to the RPIMC data. The new function reproduces the RPIMC data in the fitting range of Wigner-Seitz radius and temperature, satisfies correct high-density, low- and high-$T$ asymptotic limits and is applicable beyond the range of fitting data.\\[4pt] [1] Phys. Rev. Lett. \textbf{110}, 146405 (2013). [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A1.00010: Constrained Parmeterization of Reduced Density Approximation of Kinetic Energy Functionals Debajit Chakraborty, Samuel Trickey, Valentin Karasiev Evaluation of forces in ab initio MD is greatly accelerated by orbital-free DFT, especially at finite temperature [1]. The recent achievement of a fully non-empirical constraint-based generalized gradient (GGA) functional for the Kohn-Sham KE $T_s[n]$ [2] brings to light the inherent limitations of GGAs. This motivates inclusion of higher-order derivatives in the form of reduced derivative approximation (RDA)[3] functionals. That, in turn, requires new functional forms and design criteria. RDA functionals are constrained further to produce a positive-definite, non-singular Pauli potential. We focus on designing a non-empirical constraint-based meta-GGA[3-5] functional with certain combinations of higher-order derivatives which avoid nuclear-site singularities to a specified order of gradient expansion. Here we report progress on this agenda.\\[4pt] [1] Phys.\ Rev.\ B \textbf{86}, 115101 (2012);\\[0pt] [2] Phys.\ Rev.\ B \textbf{88}, 161108(R) (2013);\\[0pt] [3] Phys.\ Rev.\ B \textbf{80}, 245120 (2009);\\[0pt] [4] Phys.\ Rev.\ B \textbf{75}, 155109 (2007);\\[0pt] [5] Nuc.\ Phys.\ A \textbf{445} 263 (1985) [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A1.00011: Modeling Electron Correlation Using Geminal Hybrid Methods Brett Cagg, Vitaly Rassolov Two approaches to dynamic correlation correction for a variationally optimized, spin-unrestricted, multireference wavefunction based on strongly orthogonal, two electron geminals are presented. The first is a simple density functional based approach using standard correlation functionals rescaled empirically to reduce the correlation double counting error (DCE) inherent in all multireference DFT approaches. The second uses a two electron correlation operator to correlate only intergeminal, mean-field type, interactions within the wavefunction and effectively eliminates DCE. The performance of each is examined by geometric optimization and dissociation energy prediction of 38 diatomic molecules at two different basis sets. [Preview Abstract] |
Session A2: Focus Session: Surface Chemistry and Catalysis I
Sponsoring Units: DCPChair: Graeme Henkelman, University of Texas - Austin
Room: 102
Monday, March 3, 2014 8:00AM - 8:36AM |
A2.00001: Surface Chemistry of PdO(101) Invited Speaker: Jason Weaver The formation of palladium oxide (PdO) is thought to be responsible for the exceptional activity of supported Pd catalysts toward the complete oxidation of alkanes under oxygen-rich conditions. In this talk, I will discuss our investigations of the surface chemical properties of a PdO(101) thin film, focusing particularly on the adsorption and selective activation of alkanes. We find that $n$-alkanes adsorb relatively strongly on the PdO(101) surface by forming $\sigma $-complexes along rows of coordinatively-unsaturated Pd atoms, and that this adsorbed state acts as the precursor for initial C-H bond cleavage. I will discuss characteristics of the binding and activation of alkane $\sigma $-complexes on PdO(101) as determined from both experiment and density functional theory calculations. I will also discuss elementary processes involved in adsorbate oxidation on PdO(101) and make comparisons with the chemical reactivity of other late transition metal oxides. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A2.00002: Inelastic neutron scattering (INS) studies of hydrogen spillover on pure and Pd decorated metal oxides John Z. Larese, Sourav Adak, Nicholas Strange, Tilo Seydel, Chuck Sumner, Luke Daemen Recent INS and quasielastic neutron scattering (QENS) measurements of the interaction of H2 with pure and metal decorated metal oxides(MOs) will be discussed. These materials find widespread use as energy materials e.g.as oxidation and hydrogenation catalysts. These studies are aimed at revealing the microscopic details of the process(es) that underlie ``hydrogen spillover'' to identify what, if any, role it plays in the catalytic cycle. Hydrogen spillover refers to the diffusion of hydrogen from a surface capable of disassociating H2, onto an adjoining surface. This diffusing hydrogen may possess an electron capable of pairing with an unpaired free radical electron on an adjacent surface. Many catalysts consist of nm sized metal clusters supported on high surface area MOs and many catalytic reactions involve hydrogen. Recent INS observations show surface OH formation on MOs supports occurs only when the metal catalyst is present even at low temperatures. Spectral signatures in both the rotational and vibrational portions of the INS signals underscore this behavior. QENS data establishes that translational diffusion is significant at T\textless 40 K. Behavior on various support materials will be highlighted. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A2.00003: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 9:00AM - 9:12AM |
A2.00004: Simple Molecules Adsorption Studies on Highly Epitaxial -Pure Phase- Delafossite CuFeO$_{2}$ Thin Films Alejandro Cabrera, Piero Ferrari, Toyanath Joshi, Pavel Borisov, David Lederman Carbon dioxide (CO$_{2})$ and hydrogen (H2) adsorption studies on CuFeO$_{2}$ thin films grown on Al$_{2}$O$_{3}$ (00.1) substrates were performed in ultrahigh vacuum using thermal programmed desorption (TPD). Growth of pure phase Delafossite CuFeO$_{2}$ thin films on Al$_{2}$O$_{3}$ (00.1) substrates by pulsed laser deposition was systematically investigated as a function of growth temperature and oxygen pressure. CO$_{2}$ and H$_{2}$ TPD were performed on CuFeO$_{2}$ -grown at 600$^{\circ}$C and in 0.1mTorr pressure- indicating chemisorption of both gases on the oxide surface. TPD with a temperature ramp of 50 K/s showed a CO$_{2}$ peak at 573 K and H$_{2}$ peak at 823 K. The chemisorption of CO$_{2}$ and H$_{2}$ on the CuFeO$_{2}$ surface is relevant to the potential use of this material in photocatalytic applications for H$_{2}$ production and/or CO$_{2}$ conversion. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A2.00005: Computational Database of Metal-Oxide Surface Reactivities for Catalyst Design Michal Bajdich, Aleksandra Vojvodic, Jens K. N{\O}rskov We study surface reactivity of low index facets of MO, MO2 and ABO3 oxide groups using small atoms and molecules (O, OH, CO, NO, CH3, NH3). The computed database of adsorption and activation energies will be used to identify possible correlations with other quantities such as surface energies or electronic structure in order to establish scaling relations for future high-throughput screening efforts. A comparison will be made between DFT functionals of various levels of accuracy, e.g., GGA, GGA+U, GGA+vdW and GGA-hybrid, meta-GGA and hybrid meta-GGA, and compared to available experiments. This effort is part of the ``Predictive Theory of Transition Metal Oxide Catalysis'' funded through the DOE Materials Genome Project. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A2.00006: Density functional theory investigation of NO$_2$ and SO$_2$ adsorption on isolated and anatase-supported BaO clusters Mustafa Tek, Hande Ustunel, Daniele Toffoli BaO is the most commonly used storage component in NO$_x$ storage and reduction catalysts (NSR). TiO$_2$ has recently been suggested by several authors as a promising support material, with increased sulphur tolerance when compared with traditional supports such as $\gamma$-Al$_2$O$_3$. The optimization of NSR catalysts requires knowledge of the interaction between the storage and support components. In this talk, we present a DFT investigation of the electronic and structural properties of NO$_2$ and SO$_2$ adsorption on isolated and anatase-supported (BaO)$_n$ (n=1,2,4,6,8,9) clusters. Generally, supported BaO clusters are found to display better tolerance towards sulphur poisoning compared to both bare BaO (100) surface and supported BaO overlayers. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A2.00007: \textit{In-situ} NAP XPS studies of dissociative water adsorption on GaAs(100) surfaces Sylwia Ptasinska, Xueqiang Zhang In current semiconductor-based technology it is important to design and fabricate new materials in order to achieve specific well-defined properties and functionalities. Before such systems can be applied they first need to be understood, refined and controlled. Therefore, a basic knowledge about molecule/semiconductor surface interfaces is essential. In the present work dissociative water adsorption on the GaAs(100) surface is monitored using X-ray Photoelectron Spectroscopy (XPS) performed \textit{in situ} under near ambient conditions. Firstly, the crystal surface is exposed to water vapor pressures ranging from UHV to 0.5 kPa. At elevated pressures an increase of oxygenation and hydroxylation of Ga surface atoms has been observed in the Ga2p XPS spectra. Moreover, intense signals obtained from molecularly adsorbed water molecules or water molecules adsorbed \textit{via} hydrogen bond to surface OH groups have been also observed in the O1s spectra. Finally, the crystal surface is annealed up to 700 K at water vapor pressure of 0.01 kPa, which leads to desorption of physisorbed water molecules and further increase of surface oxidation. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:24AM |
A2.00008: Fundamental mechanistic studies in formic acid decomposition on transition metal surfaces Invited Speaker: Manos Mavrikakis Formic acid (HCOOH) is a simple molecule that is an abundant product of biomass processing and can serve as an internal source of hydrogen for oxygen removal and upgrading of biomass to chemicals and fuels. In addition, HCOOH can be used as a fuel for low temperature direct fuel cells. We present a systematic study of the HCOOH decomposition reaction mechanism starting from first-principles and including reactivity experiments and microkinetic modeling. In particular, periodic self-consistent Density Functional Theory (DFT) calculations are performed to determine the stability of reactive intermediates and activation energy barriers of elementary steps. Pre-exponential factors are determined from vibrational frequency calculations. Mean-field microkinetic models are developed and calculated reaction rates and reaction orders are then compared with experimentally measured ones. These comparisons provide useful insights on the nature of the active site, most-abundant surface intermediates as a function of reaction conditions and feed composition. Trends across metals on the fundamental atomic-scale level up to selectivity trends will be discussed. Finally, we identify from first-principles alloy surfaces, which may possess better catalytic properties for selective dehydrogenation of HCOOH than monometallic surfaces, thereby guiding synthesis towards promising novel catalytic materials. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A2.00009: Autocatalytic dissociation of water at stepped transition metal surfaces Rengin Pekoez, Swenja Woerner, Luca M. Ghiringhelli, Davide Donadio By means of density functional theory calculations, we investigate the adsorption and dissociation of water clusters on flat and stepped surfaces of several transition metals: Rh, Ir, Pd, Pt, and Ru. We find that water binds preferentially to the edge of the steps than to terrace sites, so that isolated clusters or one-dimensional water wires can be isolated by differential desorption. The enhanced reactivity of metal atoms at the step edge and the cooperative effect of hydrogen bonding enhance the chances of partial dissociation of water clusters on stepped surfaces. For example, water dissociation on Pt and Ir surface turns from endothermic at terraces to exothermic at steps. The interpretation of water dissociation is achieved by analyzing changes in the electronic structure of both water and metals, especially focusing on the interaction between the lone-pair electrons of water and the d-band of the metals [1]. The shift in the energetics of water dissociation at steps is expected to play a prominent role in catalysis and fuel cells reactions, as the density of steps at surfaces could be an additional parameter to design more efficient anode materials or catalytic substrates. \\[4pt] [1] D. Donadio, L.M. Ghiringhelli, and L. Delle Site, J. Am. Chem. Soc. 134, 19217 (2012). [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A2.00010: Trimethyltin Mediated Formation of Covalent Gold-Carbon Bonds Arunabh Batra, G. Kladnik, J. Meisner, M. Steigerwald, C. Nuckolls, D. Cvetko, A. Morgante, L. Venkataraman Covalent Au-C bond formation via trimethyltin (SnMe$_{\mathrm{3}})$ precursors has been hypothesized based on recent single-molecule conductance measurements$^{\mathrm{1}}$. Here, we provide spectroscopic evidence for the formation of Au-C bonds using a trimethylbenzyltin precursor on Au(110) and Au(111) surfaces. From X-Ray photoemission spectroscopy (XPS), we find that the precursor molecule cleaves on both surfaces at temperatures as low as 200K. As substrate temperature rises to 300K, shifts in the C1s and Sn3d XPS spectra indicate the formation of Au-Benzyl and Au-SnMe$_{\mathrm{3}}$ moieties on the surface. Near-edge X-Ray photoemission spectroscopy (NEXAFS) of the Au-Benzyl system on Au(110) shows a new unoccupied state near E$_{\mathrm{Fermi}}$ accompanied by a broadened lowest unoccupied molecular orbital (LUMO). That these features are missing in Au(111) suggests that the Au-C bond formation occurs preferentially on under-coordinated gold surfaces. Lastly, we use core-hole-clock resonant photoemission to understand the dynamics of charge transfer from this broadened LUMO to the underlying Au substrate, and find evidence for sub-femtosecond charge transfer [1] JACS 133, 17160--17163 (2011) [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A2.00011: Epicatalysis: Bending the third principle of catalysis Daniel Sheehan A standard principle of traditional catalysis -- that a catalyst cannot alter the final thermodynamic equilibrium of a reaction -- can fail in low-pressure, heterogeneous gas-surface reactions [1]. Kinetic theory for this {\em epicatalysis} is presented, and two well-documented experimental examples are shown: surface ionized plasmas and hydrogen dissociation on refractory metals. This phenomenon should be observable over a wide range of temperatures and pressures, and for a broad spectrum of heterogeneous reactions. By transcending some constraints of equilibrium thermodynamics, epicatalysis might provide new control parameters and synthetic routes for reactions, and enable product streams boosted in thermochemical energy or desirable species. Recent experiments involving hydrogen dissociation on tungsten and rhenium indicate that steady-state nonequilibria can be be maintained between competing epicatalysts within a single blackbody cavity, challenging thermodynamic expectations.\\[4pt] [1] Sheehan, D.P., Phys. Rev. E 88, 032125 (2013).\\[0pt] [2] Sheehan, D.P., D.J. Mallin, J.T. Garamella, and W.F. Sheehan, Found. Phys., in review (2013). [Preview Abstract] |
Session A3: Undergraduate Research - Society of Physics Students I
Chair: Toni Sauncy, Society of Physics Students - American Institute of PhysicsRoom: 107
Monday, March 3, 2014 8:00AM - 8:12AM |
A3.00001: Macroscopic Quantum Mechanics, Tunnelling, and Classical Gravity Deborah C. Good, Marie A.P. McLain, Lincoln D. Carr Macroscopic quantum mechanics is an active area of experimental research, which could benefit from understanding the effects of gravitational interactions in tunnelling. The Schr\"{o}dinger-Newton equation is one method for describing Newtonian gravitational interactions in quantum mechanics. While the Schr\"{o}dinger-Newton equation has been thoroughly described for the single-particle case, there are still open questions in the many-body case. Therefore, we investigate semi-classical solutions to the Schr\"{o}dinger-Newton equation for the many-body quantum tunnelling case using a variational-WKB method. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A3.00002: Modeling the Expansion and Collapse of Shell-Shaped Bose-Einstein Condensates Lydia Shannon, Courtney Lannert Bose-Einstein condensates, produced when atomic gases are cooled to near absolute zero, offer a macroscopic way to view the quantum mechanical world. In order to measure properties of these condensates, the cooled gas must be released from a potential trap and allowed to expand. We explore the three-dimensional system of a shell shaped BEC by applying a recent numerical method for solving the Gross-Pitaevskii equation, to study the properties of the condensate's expansion and collapse. Upon release of the BEC into a harmonic trap (inner collapse only), we observe self-interference fringes and central mass accumulation within the system, taking into account the interactions of atoms in the condensate. By manipulating the parameters of the trap, we also study spherically symmetric collective modes with properties that are distinct from that of a filled, spherical condensate. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A3.00003: Entanglement in ground and excited states of gapped fermion systems and their relationship with fermi surface and thermodynamic equilibrium properties Michelle Storms, Rajiv Singh We study bipartite entanglement entropies in the ground and excited states of model fermion systems, where a staggered potential, $\mu_s$, induces a gap in the spectrum. Ground state entanglement entropies satisfy the ``area law,'' and the ``area-law'' coefficient is found to diverge as a logarithm of the staggered potential, when the system has an extended Fermi surface at $\mu_s=0$. On the square-lattice, we show that the coefficient of the logarithmic divergence depends on the fermi surface geometry and its orientation with respect to the real-space interface between subsystems and is related to the Widom conjecture as enunciated by Gioev and Klich (Phys. Rev. Lett. 96, 100503 (2006)). For point Fermi surfaces in two-dimension, the ``area-law'' coefficient stays finite as $\mu_s\to 0$. The von Neumann entanglement entropy associated with the excited states follows a ``volume law'' and allows us to calculate an entropy density function $s_{V}(e)$, which is substantially different from the thermodynamic entropy density function $s_{T}(e)$ when the lattice is bipartitioned into two equal subsystems, but approaches the thermodynamic entropy density as the fraction of sites in the larger subsystem, that is integrated out, approaches unity. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A3.00004: Quantum phase transitions in the underscreened pseudogap Kondo model Jaimie Stephens, Kevin Ingersent The Kondo effect is the collective screening of the spin of a magnetic impurity atom by the electrons in a nonmagnetic host metal, a fundamental problem in many-body physics. This work addresses a variant in which an impurity spin-1 can only be partially screened by the spin-1/2 conduction electrons of the host. In particular, we study the pseudogap version of this underscreened Kondo model, where the conduction-electron density of states vanishes like $|E - E_F|^r$ at the Fermi energy $E = E_F$. This problem, of current interest in connection with the behavior of impurities in graphene, features a continuous quantum (zero-absolute-temperature) phase transition between underscreened-Kondo and non-Kondo ground states that occurs at a critical value of the impurity-band exchange coupling. We have used the numerical renormalization- group method to study the critical properties in the vicinity of the transition. Various critical exponents, which have a nontrivial dependence on the density of states exponent r, obey the hyperscaling relations expected at an interacting quantum critical point that cannot be described by any simple (mean-field) theory. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A3.00005: Quantum Theoretical Study of KCl and LiCl Clusters Ted Koetter, Ajit Hira, Justin Salazar, Danelle Jaramillo This research focuses on the theoretical study of molecular clusters to examine the chemical properties of small K$_{\mathrm{n}}$Cl$_{\mathrm{n}}$ and Li$_{\mathrm{n}}$Cl$_{\mathrm{n}}$ clusters (n $=$ 2 - 20). The potentially important role of these molecular species in biochemical and medicinal processes is well known. This work applies the hybrid ab initio methods of quantum chemistry to derive the different alkali-halide (M$_{\mathrm{n}}$H$_{\mathrm{n}})$ geometries. Of particular interest is the competition between hexagonal ring geometries and rock salt structures. Electronic energies, rotational constants, dipole moments, and vibrational frequencies for these geometries are calculated. Magic numbers for cluster stability are identified and are related to the property of cluster compactness. Mapping of the singlet, triplet, and quintet, potential energy surfaces is performed. Calculations were performed to examine the interactions of these clusters with some atoms and molecules of biological interest, including O, O2, and Fe. Potential design of new medicinal drugs is explored. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A3.00006: Density functional calculation of superatomic molecular orbitals in C60: First truly converged results on a real grid mesh Kyle Drake, Jason Bonacum, Guo-ping Zhang The molecular structure of Buckminster fullerene (C60) allows for electron delocalization in all of the pi-bonding electrons of the molecule. This coupled with the symmetry of the molecule allows for the formation of super-atomic molecular orbitals (SAMOs) similar to those observed in aluminum clusters. The SAMOs behave as if the molecule that they belong to is a single atom. We compute the eigenstates of C60 compulationally using density functional theory (DFT) and a grid mesh. Using larger radii also allows us to accurately describe SAMOs and test the convergence of our data. The results are interesting because for the first time, we can show the true converged super atomic orbitals in C60. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A3.00007: DFT Calculations of Carbon Monoxide Adsorbed on Pt and Ru Surfaces Nestor Navarro, Andres Salgado, Julian Velazquez, Nicholas Dimakis, Eugene Smotkin The effect of carbon monoxide surface coverage on Platinum and Ruthenium surfaces has been studied using density functional theory (DFT) on periodic structures. DFT shows that as CO coverage increases the adsorbate internal bond strengthens as verified by the corresponding stretching frequency upshifts. Moreover, increased surface coverage reduces the CO adsorption energy in agreement with prior repots. These results are correlated with changes in the hybrid adsorbate-substrate orbitals, their polarizations within the CO molecule unit, and changes in the CO dipole moment. Here, we establish a theoretical framework based on the $\pi $-attraction and $\sigma $-repulsion mechanism to explain the behavior of the CO on these surfaces at different coverages. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A3.00008: Studying the Toroidal Dipole Moment within Metamaterials Aaron Mohammed, Khagendra Bhattari, Jiangfeng Zhou Recently, a toroidal dipole moment was demonstrated by using metamaterials in the classical electrodynamic system, which behaves with a number of unusual electromagnetic properties. In this project, we are particularly interested in optimizing metamaterial design for enhancing the toroidal moment, which could be used in potential applications like low-threshold plasmonic lasing or biosensing. Through numerical simulations, a number of toroidal metamaterial designs, which are made up of planar split ring resonators (SRRs), are studied and the toroidal moment of each design is calculated. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A3.00009: Optimal Vaccination For A Probabilistic Epidemic Edwin Yuan, Sean Stromberg, Jean Carlson In epidemiology, herd immunity is the well-known idea that by vaccinating a sufficient fraction of a susceptible population, one lowers the basic reproduction number of the pathogen below one, and thereby prevents an epidemic. A natural conclusion from this is that given two identical populations, and enough vaccine to induce herd immunity in only one, we can prevent the greatest number of people from infection by inequitably distributing vaccine to completely protect one population while leaving the other much more relatively susceptible. This heuristic has been verified by simulation of the standard deterministic SIR epidemic. We show, however, that when stochasticity is introduced to the system, or more specifically when there is now a significant probability that an epidemic will not develop independently, it is counter-intuitively optimal to distribute vaccine more equally and not induce herd immunity in either population. There is thus a regime where the purely deterministic SIR model is a poor predictor of the optimal vaccination scheme. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A3.00010: Effects of Memantine and Oleocanthal on Alzheimer's Disease Mariyam Houston, Jason Bonacum, Guoping Zhang Alzheimer's Disease (AD) is a neurodegenerative disorder characterized by accumulation of neuritic plaques composed of amyloid-$\beta $ (A$\beta )$ proteins and neurofibrillary tangles composed of tau proteins. Although there is no known cure for AD, the symptoms can be treated with a drug called memantine. Memantine acts an NMDAR antagonist by inhibiting the action of the NMDA receptor. Recently, Oleocanthal, a phenolic molecule that is found in extra virgin olive oil, has been linked to reduced risk of AD. Though the mechanism by which Oleocanthal plays in reducing the risk of AD is not completely understood, recent studies have shown that Oleocanthal somehow inhibits the formation of the neurofibrillary tangles and reduces the formation of A$\beta $ senile plaques. Our first-principles calculation, based on Gaussian03 program, shows that in the M2 segment, memantine binds to serine, but ketamine binds to glycine. This may explain their different effects, despite the fact that they are both NMDAR antagonists. Using the same method, we also investigate how Oleocanthal binds to the peptides by comparing the relative energies of each of the structures. Our results may help better understand the mechanism by which Oleocanthal decreases the chances of developing AD. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A3.00011: Self-organized criticality of protein folding simulations using AMBER parameters Yura Sim, Joelle Murray Self-organized criticality is a framework that can be used to describe many natural processes, ranging from avalanches to forest fires. These processes exhibit power-law characteristics and scale invariance. Self-organized critical systems have yet to be applied to protein folding and its identification as such may be useful to understanding protein behavior. A dynamical simulation was constructed using AMBER energy parameters and evidence of self-organized criticality was investigated. Furthermore, the features of self-organized criticality were used to explore the development of protein structures within the simulation. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A3.00012: How They (Should Have) Built the Pyramids Gregory Gallagher, Joseph West, Kevin Waters A novel ``polygon method'' is proposed for moving large stone blocks. The method is implemented by the attachment of rods of analytically chosen radii to the block by means of rope. The chosen rods are placed on each side of the square-prism block in order to transform the square prism into a prism of higher order polygon, i.e. octagon, dodecagon etc. Experimental results are presented and compared to other methods proposed by the authors, including a dragging method and a rail method which includes the idea of dragging the block on rails made from arbitrarily chosen rod-shaped ``tracks,'' and to independent work by another group which utilized wooden attachments providing a cylindrical shape. It is found that the polygon method when used on small scale stone blocks across level open ground has an equivalent of a coefficient of friction order of 0.1. For full scale pyramid blocks, the wooden ``rods'' would need to be of order 30 cm in diameter, certainly within reason, given the diameter of wooden masts used on ships in that region during the relevant time period in Egypt. This project also inspired a ``spin-off'' project in which the behavior or rolling polygons is investigated and preliminary data is presented. [Preview Abstract] |
Session A4: Focus Session: Emergent Properties in Bulk Complex Oxides: Iridates I
Sponsoring Units: GMAG DMPChair: L. Andrew Wray, SLAC/Lawrence Berkeley National Laboratory
Room: 112/110
Monday, March 3, 2014 8:00AM - 8:12AM |
A4.00001: Electronic structure of Rh and Ru doped Sr$_2$IrO$_4$ Shalinee Chikara, Gilberto Fabbris, Jasminka Terzic, Tongfei Qi, Kamal Butrouna, Larissa Veiga, Narcizo Souza Neto, Gang Cao, Daniel Haskel Sr$_2$IrO$_4$ is a spin-orbit interaction(SOI) assisted insulator. It has been proposed that the weaker SOI in the $4d$-substituted Sr$_2$Ir$_{\mathrm{1-x}}$(Ru, Rh)$_{\mathrm{x}}$O$_4$ closes the insulating gap, rendering it a paramagnetic metal. Rh(${4d}^5$) is isoelectronic to Ir(${5d}^5$) whereas Ru(${4d}^4$) has one less electron in the $4d$-band. The AFM-I/PM-M transition takes place at lower $x$ for Ru than Rh, presumably due to the effect of hole doping. X-ray absorption near edge structure (XANES) and x-ray magnetic circular dichroism (XMCD) measurements at the Ir $ L_{2,3}$ edges show that $\langle\mathbf{L}.\mathbf{S} \rangle $ is non-zero and independent of $x$. This is indicative of a strong local $5d$ spin orbit interaction that is rather insensitive to the $4d$ doping. In contrast, measurements at the $ L_{2,3}$ edges of Ru and Rh show $\langle\mathbf{L}.\mathbf{S} \rangle \approx 0 $ for all $x$. The results point to the importance of local $4d/5d - 2p$ hybridization as opposed to $4d-5d$ band formation in the Rh and Ru doped Sr$_2$IrO$_4$. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A4.00002: Iridate compound produces extraordinarily high coercive magnetic field Vivien Zapf, Craig Topping, Jae-Wook Kim, Eun-Deok Mun, Paul Goddard, Saman Ghannadzadeh, Xuan Luo, Sang-Wook Cheong, John Singleton We present a data on an iridate compound that shows an extraordinarily large magnetic hysteresis loop. The coercive magnetic field exceeds 40 Tesla in single-crystal samples. The hysteresis coexists with a linear background, and the total remanent magnetization is about half a Bohr magneton. We will discuss the emergence of these properties from the interplay of spin-orbit coupling, magnetic exchange and possible frustration. The single crystalline material exhibits a magnetic hysteresis loop for one orientation of the magnetic field and a smooth linear increase in the magnetization with field for the other. Measurements were conducted in 65 T short-pulse magnets and the 60 Tesla shaped-pulse magnet at the National High Magnetic Field Lab in Los Alamos. We do not observe any dependence of the magnetic hysteresis on magnetic field sweep rate. Compounds containing Ir4$+$ have attracted attention recently due to strong spin-orbit coupling that competes with crystal-electric field and exchange interactions. This competition can result in non-Hund's-rule ground states with unusual properties. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A4.00003: Magnetic and structural behaviors in (Sr$_{\mathrm{1-x}}$La$_{\mathrm{x}})_{2}$IrO$_{4}$ Xiang Chen, Tom Hogan, Chetan Dhital, Zhensong Ren, Mani Pokharel, Mengliang Yao, Cyril Opeil, Stephen Wilson There has been a considerable amount of interest recently in exploring the compound Sr$_{2}$IrO$_{4}$ due to its similarity to the high-T$_{\mathrm{c}}$ superconducting parent compound La$_{2}$CuO$_{4}$ and the possible realization of a parallel unconventional superconducting state by electron-doping. There has since been renewed effort attempting to doped electrons into prototypical spin-orbit driven Mott phases such as Sr$_{2}$IrO$_{4}$ (Sr-214), where La-substitution within (Sr$_{\mathrm{1-x}}$La$_{\mathrm{x}})_{2}$IrO$_{4}$ remains one of the more promising avenues. Here we present a combined transport, magnetization, and diffraction study revisiting the mechanism and effect of doping in this compound. We will focus on how the evolution of known structural and electronic order parameters in the parent Sr$_{2}$IrO$_{4}$ evolve upon La-substitution. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A4.00004: Electronic Structure and Magnetism of Ir based Oxides Indra Dasgupta, Swarup Panda We have investigated in details the electronic structure of several Ir based oxides where in addition to crystal field and Coulomb repulsion, the spin-orbit coupling (SOC) plays an important role. We shall first consider two Ir based oxides with 4+ (d$^{5}$) charge state of Ir, namely the insulating double perovskite Sr$_{2}$CeIrO$_{6}$ and the metallic rutile IrO$_{2}$, and examine the validity of the novel spin-orbital entangled J$_{eff}$=1/2 states for the description of their electronic structure. In particular, explore in details whether the J$_{eff}$=1/2 state survives for the itinerant metallic IrO$_{2}$. Finally we shall also present our electronic structure calculations on 6H perovskite type iridates where different charge state of Ir (5+, 4.5+, and 4+) may be realized. We show in addition to SOC, the strong intra-dimer hopping play a crucial role for the magnetic ground state and the insulating property of these systems. We shall compare our results with available experiments. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A4.00005: Anisotropic magnetoresistance in Sr$_{2}$IrO$_{4}$ C. Wang, H. Seinige, G. Cao, J.-S. Zhou, J.B. Goodenough, M. Tsoi We report the first measurements of the point-contact magnetoresistance (MR) of antiferromagnetic semiconductor Sr$_{2}$IrO$_{4}$. The point-contact technique allows to probe very small volumes associated with point contacts and, therefore, looks for electronic transport on a microscopic scale. Point-contact measurements with single crystals of Sr$_{2}$IrO$_{4}$ were intended to see if the additional local resistance associated with a small contact area between a sharpened Cu tip and the antiferromagnet shows MR such as that seen in bulk crystals. The Sr$_{2}$IrO$_{4}$ crystals were grown by the flux method. Point-contact measurements at liquid nitrogen temperature revealed large MRs (up to 8{\%}) for modest magnetic fields (250 mT) applied within IrO$_{2}$ (ab) plane. The angular dependence of MR shows a crossover from four-fold to two-fold symmetry with an increasing magnetic field which may be tentatively attributed to the field-induced changes of antiferromagnetic order within IrO$_{2}$ planes. The observed MR can be potentially used to sense the antiferromagnetic order in spintronic applications. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A4.00006: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 9:12AM - 9:48AM |
A4.00007: Electronic Phase Separation and Magnetic Phase Behavior in the Ru-doped Spin-Orbit Mott Insulator Sr$_3$Ir$_2$O$_7$ Invited Speaker: Chetan Dhital Iridium-based 5d transition metal oxides host rather unusual electronic/magnetic ground states due to strong interplay between electronic correlation, lattice structure and spin-orbit interactions. Out of the many oxides containing iridium, the Ruddelsden-Popper (RP) series [Sr$_{n+1}$Ir$_n$O$_{3n+1}$] oxides are some of the most interesting systems to study both from the point of view of physics as well as from potential applications. Sr$_3$Ir$_2$O$_7$ (n=2) and Sr$_2$IrO$_4$ (n=1) are two representative candidates of this series. One way of experiencing the strength and relevance of electronic correlation in any condensed matter system is by doping charge carriers. The presence of electronic correlations in the host system determines the fate of the dopant and hence stabilizes a new electronic/magnetic ground state. I will discuss about importance of electronic correlations in one such doped system Sr$_3$ (Ir$_{1-x}$Ru$_x$)$_2$O$_7$ using combined neutron scattering, electric transport and magnetization techniques. Our findings demonstrate that correlation effects felt by carriers introduced within in a $5d$ Mott phase remain robust enough to drive electron localization, a key ingredient in emergent phenomena such as high temperature superconductivity and enhanced ferroic behavior.\\[4pt] [1] Dhital, Chetan, et al. ``Spin ordering and electronic texture in the bilayer iridate Sr$_3$Ir$_2$O$_7$.'' \textit{Physical Review B} 86.10 (2012): 100401.\\[0pt] [2] Dhital, Chetan, et al. ``Neutron scattering study of correlated phase behavior in Sr$_2$IrO$_4$.'' \textit{Physical Review B} 87.14 (2013): 144405.\\[0pt] [3] Dhital, Chetan, et al. ``Electronic phase separation in the doped spin-orbit driven Mott phase of Sr$_3$(Ir$_{1-x}$Ru$_x$)$_2$O$_7$.'' \textit{arXiv preprint arXiv:1311.0783} (2013). [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A4.00008: Evolution of Magnetism in Single-Crystal Honeycomb Iridates Jasminka Terzic, T.F. Qi, L. Li, V.S. Cao, S.J. Yuan, M. Tovar, G. Murthy, R.K. Kaul, G. Cao We report the successful synthesis of single-crystals of the layered iridate, (Na$_{1-x}$Li$_x$)$_2$IrO$_3$, 0 $\le$ x $\le$ 0.90, and a thorough study of its structural, magnetic, thermal and transport properties. The new compound allows a controlled interpolation between Na$_2$IrO$_3$ and Li$_2$IrO$_3$, while maintaining the novel quantum magnetism of the honeycomb Ir4$+$ planes. The measured phase diagram demonstrates a suppression of the Neel temperature at an intermediate x indicating that the magnetic order in Na$_2$IrO$_3$ and Li$_2$IrO$_3$ are distinct. X-ray data shows that for x$=$0.70 when the Neel temperature is suppressed the most, the honeycomb structure is least distorted, suggesting at this intermediate doping that the material is closest to the spin liquid that has been sought after in Na$_2$IrO$_3$ and Li$_2$IrO$_3$. By analyzing our magnetic data with a single-ion theoretical model we also show that the trigonal splitting, on the Ir4$+$ ions changes sign from Na$_2$IrO$_3$ to Li$_2$IrO$_3$. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A4.00009: Novel magnetism of Ir5$+$ ions in the double perovskite Sr2YIrO6 Gang Cao, T.F. Qi, La Li, J. Terzic, S.J. Yuan, L.E. DeLong, G. Murthy, R.K. Kaul We synthesize and study single crystals of a new double-perovskite Sr2YIrO6. Despite two strongly unfavorable conditions for magnetic order, namely, pentavalent Ir5$+$(5d4) ions which are anticipated to have J$=$0 singlet ground states in the strong spin-orbit coupling (SOC) limit, and geometric frustration in a face centered cubic structure formed by the Ir5$+$ ions, we observe this iridate to undergo a novel magnetic transition at temperatures below 1.3 K. We provide compelling experimental and theoretical evidence that the origin of magnetism is in an unusual interplay between strong non-cubic crystal fields, local exchange interactions and ``intermediate-strength'' SOC. Sr2YIrO6 provides a rare example of the failed dominance of SOC in the iridates. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A4.00010: Hallmarks of Metal Insulator transition in Doped Sr$_{2}$IrO$_{4}$ Yue Cao, Qiang Wang, Rajendra Dhaka, Justin Waugh, Theodore Reber, Haoxiang Li, Stephen Parham, Xiaoqing Zhou, Seung Ryong Park, Tongfei Qi, Oleksandr Korneta, Nicholas Plumb, Aaron Bostwick, Eli Rotenberg, Jonathan Denlinger, Michael Hermele, Gang Cao, Daniel Dessau How Mott insulators acquire metallicity upon the introduction of extra carriers lies at the heart of correlated electron physics. The evolution of the electronic structure and low energy dynamics in the ultra-low doped region where the Mottness begins to break down is a critical place to study this physics. We report ARPES studies of the Rh and La doped Sr$_{2}$IrO$_{4}$ and show the appearance and evolution of a pseudogap and Fermi arcs. Further more we present evidence how the Mott gap breaks down with a profound change in the band structure. The experimental results in the doped iridates resemble those observed in the cuprate systems, which are prototype Mott insulators, and suggest we could establish a series of signatures that occur in the metal insulator transition. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A4.00011: Destruction of J$_{\mathrm{eff}}=$1/2 Mott Phase by A-site doping in (Sr$_{\mathrm{1-x}}$La$_{\mathrm{x}})_{3}$Ir$_{2}$O$_{7}$ Tom Hogan, Chetan Dhital, Zahra Yamani, Cyril Opeil, Stephen Wilson Recent theoretical progress in describing the insulating behavior of the n$=$1 and n$=$2 Ruddlesden-Popper (RP) series iridates (Sr$_{2}$IrO$_{4}$ and Sr$_{3}$Ir$_{2}$O$_{7})$ has proposed a novel J$_{\mathrm{eff}}=$1/2 Mott ground state. This new Mott phase theoretically arises from the splitting of the Ir 5d orbitals by the crystal field combined with further splitting of the t$_{\mathrm{2g}}$ manifold by relativistic spin-orbit coupling which then combines with a modest U to form a Mott insulating phase. Our group's previous work in Ir-site substitution of Sr$_{3}$Ir$_{2}$O$_{7}$ (Sr-327) has revealed a rich interplay of correlated effects, presenting a strong argument that correlation physics plays a dominant role in the ground state of this material. While doping the transition metal B-site with Ru$^{4+}$ induces a percolative metal-insulator transition (MIT) with doped-holes remaining localized, it is known that the A-site doping of electrons results in a very abrupt MIT realized with only a small percentage level of doping ($\sim$ 3{\%}). Here we present a combined transport and diffraction study exploring the evolution of electronic and structural properties in electron-doped (Sr$_{\mathrm{1-x}}$La$_{\mathrm{x}})_{3}$Ir$_{2}$O$_{7}$ as it traverses the MIT in its electronic phase diagram. Elastic neutron scattering experiments alongside DC magnetization and electronic transport data will be presented exploring the ordering temperature, moment size, and the extent of any structural distortions present, as a function of dopant concentration. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A4.00012: Influence of non-magnetic dilution in honeycomb-lattice iridates A$_2$IrO$_3$ (A=Na,Li) Soham Manni, Philipp Gegenwart Honeycomb-lattice iridates A$_2$IrO$_3$ (A= Na,Li) display a spin-orbit Mott insulating state [1,2] and have been proposed as experimental realizations for the Kitaev-Heisenberg(KH) model[1] or a novel kind of quasi-molecular orbital(QMO) system [3]. Recently it has been proposed, that dilution of the Ir$^{4+}$ moments could be used to investigate the importance of next neighbor interactions (HK model) versus further next neighbor interactions (J1-J2-J3 model)[4]. We have synthesized A$_2$(Ir$_{1-x}$Ti$_x$)O$_3$ single- and polycrystals for the Na and Li system, respectively and investigated their magnetic and thermodynamic properties. Even very low Ti$^{4+}$ substitution leads to spin glassy behavior and spin glass temperature (T$_g$) is steeply suppressed towards the percolation threshold. This confirms that frustrated nearest-neighbor interactions are the most important factor to set up the magnetism in A$_2$IrO$_3$.\\[4pt] [1] Y.Singh et.al. - PRL 108, 127203 (2012).\\[0pt] [2] H. Gretarsson et.al. - PRL 110, 076402 (2013).\\[0pt] [3] I. Mazin et.al. - PRL 109, 197201 (2012).\\[0pt] [4] Eric C. Andrade et.al. - arxiv 1309.2951. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A4.00013: Evolution of magnetic structure in the square lattice iridate Sr$_{2}$Ir$_{\mathrm{1-x}}$Rh$_{\mathrm{x}}$O$_{4}$ Feng Ye, Songxue Chi, Masaaki Matsuda, Xiaoping Wang, Christina Hoffmann, Bryan Chakoumakos, Jaime Fernandez-Baca, Tongfei Qi, Gang Cao 5$d$ based iridates have continuously provides a fertile playground for the studies of novel physics driven by spin-orbit interaction (SOI) that rigorously competes with other relevant energies, particularly the on-site Coulomb interaction U. Using single crystal neutron diffraction and polarized neutron scattering analysis, we have investigated the evolution of spin and crystal structures in the doped Sr$_{2}$Ir$_{\mathrm{1-x}}$Rh$_{\mathrm{x}}$O$_{4}$(0$\le $x$\le $0.20). The parent Sr$_{2}$IrO$_{\mathrm{4}}$ shows canted antiferromagnetic structure with spin lies in the basal plane. The spin orientation closely follows the rotation of the IrO$_{6}$ octahedra with total ordered moment of 0.21 $\mu_{\mathrm{B}}$/Ir. A small amount of Rh ions doped at the Ir sites drastically reduces the magnetic transition and modifies the spin configuration. The neutron scattering results provide experimental insights into the magnetic and crystal structure crucial to the understanding this prototype iridates. [Preview Abstract] |
Session A6: Focus Session: Spin-Dependent Physics in Carbon-Based Materials I
Sponsoring Units: GMAG DMPChair: Roland Kawakami, Ohio State University
Room: 108
Monday, March 3, 2014 8:00AM - 8:12AM |
A6.00001: Spin injection into Pt-polymers with large spin-orbit coupling Dali Sun, Ryan McLaughlin, Gene Siegel, Ashutosh Tiwari, Z. Valy Vardeny Organic spintronics has entered a new era of devices that integrate organic light-emitting diodes (OLED) in organic spin valve (OSV) geometry (dubbed bipolar organic spin valve, or spin-OLED), for actively manipulating the device electroluminescence via the spin alignment of two ferromagnetic electrodes (\textit{Science} \textbf{337}, 204-209, 2012; \textit{Appl. Phys. Lett}. 103, 042411, 2013). Organic semiconductors that contain heavy metal elements have been widely used as phosphorescent dopants in white-OLEDs. However such active materials are detrimental for OSV operation due to their large spin-orbit coupling (SOC) that may limit the spin diffusion length and thus spin-OLED based on organics with large SOC is a challenge. We report the successful fabrication of OSVs based on pi-conjugated polymers which contain intrachain Platinum atoms (dubbed Pt-polymers). Spin injection into the Pt-polymers is investigated by the giant magnetoresistance (GMR) effect as a function of bias voltage, temperature and polymer layer thickness. From the GMR bias voltage dependence we infer that the ``impendence mismatch'' between ferromagnetic electrodes and Pt-polymer may be suppressed due to the large SOC. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A6.00002: Interface control of spin transport in magnetic tunnel junctions with MgO\textbackslash Cu-Phthalicyanine hybrid barrier Yu Jeong Bae, Nyun Jong Lee, Tae Hee Kim, Andrew Pratt, Yasushi Yamauchi In this work, systematic investigation of interface electronic properties in Fe(001)\textbackslash MgO(001)\textbackslash Cu-Phthalocyanine (CuPc) and Fe(001)\textbackslash CuPc was carried out by using spin polarized metastable He de-excitation spectroscopy (SP-MDS) technique. The electronic structure related to the absorption geometry of CuPc on the Fe (001) and MgO(001) was carefully explored. Differences in the spin resolved density of states were observed as a function of CuPc thickness. The clear evidence of spin-polarized organic spinterface appears even at room temperature in ultra-thin (< 2 nm) CuPc films on the epitaxially grown Fe(001)\textbackslash MgO(001) bilayer. These findings have significant implications for understanding of spin injection from a ferromagnetic layer into an organic semiconductor (OSC), and highlight the importance of adsorption geometry and interfacial exchange coupling in the process of spin injection. This is demonstrated in measurements of the spin transport of Fe\textbackslash MgO(001)\textbackslash CuPc\textbackslash Co tunnel junctions. For the MgO\textbackslash CuPC hybrid barrier, high magnetoresistance value ($> 100$\%) and rather small value ($\sim$ 10\%) were measured at 77 K and 300 K, respectively. Our results provide significant new insights into the phenomenon of spin injection into an OSC and the operation of molecular spintronic devices. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A6.00003: Spin Filtering in Graphene Magnetic Tunnel Junctions Enrique D. Cobas, Adam L. Friedman, Olaf M. J. van 't Erve, Berend T. Jonker We present experimental measurements of spin filtering across ferromagnet-graphene-ferromagnet tunnel junctions. These junctions are predicted to yield nearly 100{\%} spin-polarized charge currents [1,2] and were previously shown to sustain spin-polarized tunnel currents at room temperature [3]. In this work, high-quality multi-layer graphene was synthesized directly on crystalline (111) close-packed ferromagnetic thin films by chemical vapor deposition. All deposition and patterning steps employed standard, wafer-scale photolithography, deposition and ion milling techniques. The charge transport and spin transport across the junctions were measured in a four-probe geometry as a function of applied magnetic field and temperature ranging from 5K to 500K. The signature of minority-pass spin filtering with a low-resistance anti-parallel state is evident throughout the temperature range studied. [1] Karpan et al., Phys. Rev. Lett. 99, 176602, 2007 [2] Yazyev and Pasquarello, Phys. Rev. B. 80, 035408, 2009 [3] Cobas et al., Nano Letters 12, 3000, 2012. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A6.00004: Self-assembly of monolayers on SFMO for fabrication of molecular magnetic tunnel junctions Patrick Truitt, Hailong Wang, Fengyuan Yang, Ezekiel Johnston-Halperin Half-metallic oxides, such as La$_{0.7}$Sr$_{0.3}$MnO$_3$ (LSMO) and Sr$_2$FeMoO$_6$ (SFMO), are highly spin polarized and air stable, making them attractive as spin injectors for organic and molecular spintronics. Recently, it was demonstrated that self-assembled monolayers (SAMs) of alkylphosphonic acids can be grafted onto LSMO while maintaining LSMO`s spin polarization. However, due to its relatively low Curie temperature, the magnetoresistance of devices based on LSMO is severely curtailed at room temperature. In contrast, SFMO has a $T_C > 400$ K. As a first step in incorporating this material in a molecular magnetic tunnel junction, we show that it also supports alkylphosphonic SAMs. Epitaxial SFMO films are grown on STO via off-axis sputtering and have a room temperature magnetic moment per formula unit of about 1.2 $\mu_B$ and a Fe:Mo stoichiometry ratio of 0.9:1.0, determined by RBS. The quality and structure of SAMS grafted on these films is interrogated through methods including contact angle measurements, AFM, and FTIR spectroscopy. Progress towards fully realized spin polarized tunnel junctions and implications for room temperature molecular spintronics will be discussed. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A6.00005: Quantum Fluctuations of Local Magnetoresistance in Organic Spin Valves Mikhail Raikh, Robert Roundy, Demitry Nemirovsky, Victor Kagalovsky Aside from interfacial effects, the performance of organic spin valves is limited by the spin memory loss in course of electron transport between the magnetized electrodes. One of the most prominent mechanisms of this loss is the spin precession in the random hyperfine fields of nuclei. We assume that the electron transport is due to incoherent multi-step tunneling. Then the precession takes place while electron ``waits'' for the subsequent tunneling step. While the spatial coherence of electron is lost after a single step, the spin evolution remains absolutely coherent all the way between the electrodes. As a result, the {\em amplitudes} of subsequent spin rotation interfere with each other. We demonstrate that this interference leads to a wide spread in the {\em local} values of tunnel magnetoresistance (TMR). Moreover, if {\em on average} the TMR is positive, the portion of the surface area where the TMR is negative is appreciable. We calculate analytically and numerically the distribution of local TMR as a function of the spin-valve thickness. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A6.00006: Noise spectroscopy of magnetic tunnel junctions with organic barriers Farkhad Aliev, Juan Pedro Cascales, Jhen-Yong Hong, Minn-Tsong Lin Understanding the details of spin and charge transport through organic barriers remains one of the main challenges in organic spintronics. Here we present low frequency noise studies in magnetic tunnel junctions with thin (2-5nm) organic PTCDA barriers in the tunnelling regime, investigated at temperatures of under 1K up to 300K. Shot noise measurements show a superpoissonian contribution at low biases giving rise to a Fano factor of around 1.5-2. We tentatively link the enhanced shot noise with electron bunching induced by inelastic interaction with collective low frequency vibration modes of the molecules. On the other hand, the bias dependence of 1/f noise studied up to 350mV reveals reproducible anomalies which could be linked with excitations induced by inelastic tunnelling, due to individual vibrational modes of higher frequency of the PTCDA molecules. The dependence of the shot and 1/f noise with the magnetic alignment of the electrodes will also be discussed [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A6.00007: Disorder induced spin coherence in polyfluorene thin film semiconductors Richard G. Miller, Kipp van Schooten, Hans Malissa, David P. Waters, John M. Lupton, Christoph Boehme Charge carrier spins in polymeric organic semiconductors significantly influence magneto-optoelectronic properties of these materials [1]. In particular, spin relaxation times influence magnetoresistance and electroluminescence. We have studied the role of structural and electronic disorder in polaron spin-relaxation times. As a model polymer, we used polyfluorene, which can exist in two distinct morphologies: an amorphous (glassy) and an ordered (beta) phase [2]. The phases can be controlled in thin films by preparation parameters and verified by photoluminescence spectroscopy. We conducted pulsed electrically detected magnetic resonance (pEDMR) measurements to determine spin-dephasing times by transient current measurements under bipolar charge carrier injection conditions and a forward bias. The measurements showed that, contrary to intuition, spin-dephasing times increase with material disorder. We attribute this behavior to a reduction in hyperfine field strength for carriers in the glassy phase due to increased structural disorder in the hydrogenated side chains, leading to longer spin coherence times.\\[4pt] [1] C. Boehme, J.M. Lupton, Nature Nano. 8, 612 (2013).\\[0pt] [2] A. Khan et al. Phys. Rev. B 69, 085201 (2004). [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A6.00008: Suppression of the Hanle effect in organic spintronic devices Zhi-Gang Yu We study carrier spin transport under a transverse magnetic field in organic structures [1]. In organics, carriers are localized polarons and charge transport is via polaron hopping. Spin transport, however, can utilize the exchange coupling between localized polarons, which can be much faster than polaron hopping and rapidly increases with the carrier density. Consequently, a much stronger magnetic field is needed to modify spin polarization and observe the Hanle effect than estimated from the carrier mobility, which can help understand recent Hanle measurements in organic spin valves. The exchange-induced spin transport also greatly mitigates the conductivity mismatch between ferromagnets and organics, enabling spin injection into organics. [1] Z. G. Yu, Phys. Rev. Lett. 111, 016601 (2013). [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A6.00009: Simultaneous electrical and optical detection of magnetic resonance in MEH-PPV Marzieh Kavand, Doug Baird, Kipp van Schooten, Hans Malissa, Rachel Baarda, John M. Lupton, Christoph Boehme While it is established that spin Pauli blockade controlled s=1/2-pair transitions are dominant spin-dependent transitions [1] at room temperature in poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), there is controversy whether these pairs are unipolar or bipolar [1]. Spin-dependent processes can be observed with electrically (conductivity) and optically (photoluminescence) detected magnetic resonance. The former is sensitive to unipolar and bipolar processes [1] while the latter is sensitive only to bipolar charge carrier recombination [2]. Here, we present experiments on MEH-PPV organic light emitting diodes where the transient current and electroluminescence response to a pulsed magnetic resonance excitation of charge carriers is measured by detection of both observables on the same device at the same time. The measurements were made at various temperatures and injection (bias) conditions. Correlations between the dynamics of electrically and optically detected signals under these various conditions allows to discrimination between spin-dependent processes which affect one of the two observables only and those that affect both.\\[4pt] [1] C. Boehme, J.M. Lupton, Nature Nanotechnol.8 (9), 612-615 (2013).\\[0pt] [2] S.-Y. Lee et al. J. Am. Chem. Soc.133, 072019 (2011). [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A6.00010: Hyperfine spin interactions between polarons and nuclei in organic light emitting diodes: Magneto-EL measurements S.A. Crooker, M.R. Kelley, N. Martinez, W. Nie, A.D. Mohite, D.L. Smith, S. Tretiak, P.P. Ruden Considerable attention in recent years has focused on the effects of applied magnetic fields on the conductance, photocurrent, electroluminescence (EL), and photoluminescence of nominally nonmagnetic organic semiconductor materials and devices. These magnetic field effects have proven useful in revealing the underlying physical mechanisms and relevant spin interactions that influence the electrical and optical properties in these organic systems (e.g., hyperfine coupling, exchange interactions, and spin-orbit coupling). Here we study the field-dependent properties of organic light-emitting diode (OLEDs) based on MTDATA/LiF/Bphen layered structures, in which exciplex recombination at the interface dominates the EL spectra. Small applied magnetic fields ($\sim$10 mT) are found to boost the net EL yield by up to 10\%, due to a suppression of the mixing between singlet and triplet polaron pairs which, in turn, arises from hyperfine spin coupling of the polarons to the underlying nuclei of the host molecules. We discuss the dependence of these field-induced effects on the LiF barrier thickness, device bias, and on the orientation of the applied magnetic field, as well as the mechanisms responsible. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A6.00011: Electrical detection of nuclear spins in organic light-emitting diodes H. Malissa, M. Kavand, D.P. Waters, J.M. Lupton, Z.V. Vardeny, B. Saam, C. Boehme We present pulsed combined electrically detected electron paramagnetic and nuclear magnetic resonance experiments on MEH-PPV OLEDs. Spin dynamics in these structures are governed by hyperfine interactions between charge carriers and the surrounding hydrogen nuclei, which are abundant in these materials. Hyperfine coupling has been observed by monitoring the device current during coherent spin excitation [1]. Electron spin echoes (ESEs) are detected by applying one additional readout pulse at the time of echo formation [2]. This allows for the application of high-resolution spectroscopy based on ESE detection, such as electron spin echo envelope modulation (ESEEM) and electron nuclear double resonance (ENDOR) available for electrical detection schemes. We conduct electrically detected ESEEM [3] and ENDOR [4] experiments and show how hyperfine interactions in MEH-PPV with and without deuterated polymer side groups can be observed by device current measurements.\\[4pt] [1] D. R. McCamey et al., Phys. Rev. Lett. 104, 017601 (2010).\\[0pt] [2] W. J. Baker et al., Phys. Rev. Lett. 108, 267601 (2012).\\[0pt] [3] M. Fehr et al., Phys. Rev. B 84, 193202 (2011).\\[0pt] [4] F. Hoehne et al., Phys. Rev. Lett. 106, 187601 (2011). [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A6.00012: Tunable Magneto-conductance and Magneto-electroluminescence in Polymer Light-Emitting Electrochemical Planar Devices Rugang Geng, Nicholas Mayhew, Tho Nguyen We report first time studies of magneto-conductance (MC) and magneto-electroluminescence (MEL) in polymer light-emitting electrochemical \textit{planar devices} using ``super-yellow'' poly-(phenylene vynilene), SY-PPV. We observed consistent negative MC while MEL changes sign to positive when electroluminescence quantum efficiency increases (ELQE). At optimal ELQE, the MC has a much narrower width than MEL indicating that MC and MEL do not share a common origin. However, MC reverses and has the same width as MEL when exposed to a threshold laser power depending on the applied voltage. In addition, MC reduces its magnitude when the device current increases at constant illumination power. We discuss the results in the context of the existing models. We show that the e-h pair model can explain the positive MEL and MC while the negative MC can be explained by the bipolaron model. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A6.00013: Magneto-photocurrent in organic photovoltaic cells; the effect of short-lived charge transfer states Eitan Ehrenfreund, A. Devir-Wolfman, B. Khachatryan, B. Gautam, N. Tessler, Z.V. Vardeny The spin degrees of freedom are responsible for the magnetic field effects in organic devices at low magnetic fields. The MFE is formed via a variety of spin-mixing mechanisms, such as the hyperfine (typical strength: B$_{\mathrm{hf}}$\textless 0.003 T), triplet-polaron or triplet-triplet (B$_{\mathrm{trip}}$\textless 0.1 T) interactions, that limit the response by their respective strength. We report on magneto-photocurrent (MPC) response of bulk hetero-junction organic photovoltaic cells in an extended field range B$=$0.00005 - 8 Tesla, and found that spin mixing mechanisms are still operative even at the highest fields. In fact, the response MPC(B) can be divided into three main regions, each with a different sign: sharp response that increases with B up to B$_{1}$ $\sim$ 0.04 T; broad response that decreases with B in the range from B$_{1}$ to B$_{2}$ $\sim$ 0.3-0.7 T; and even broader response that increases above B$_{2}$; this response does not saturate even at 8.5 T. We attribute the latter MPC component to short-lived charge transfer excitons (CTE) where spin-mixing is caused by the difference of the donor/acceptor g factors; a mechanism that is increasingly more effective at high magnetic field. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A6.00014: Optical Detection of Injected Spin Aligned Carriers in Organic Semiconductors by Sagnac Interferometry Ryan McLaughlin, Zhiheng Liu, Dali Sun, Z. Valy Vardeny Conventionally, spin-aligned carrier injection into organic semiconductors has been investigated by Giant magneto-resistance (GMR), which is inherently difficult to interpret due to the large number of artifacts in organic spintronic devices. Optical detection of spin injection would allow for the direct characterization of spin-polarized carriers without the need for spin-analyzer layers, but has been considered difficult to achieve due to the small spin-orbit coupling in organic semiconductors. A Magneto-Optic Kerr Effect (MOKE) sensitive Sagnac Interferometer offers a robust, highly sensitive approach for detection of spin polarization in semiconductors with weak spin-orbit coupling. We have successfully constructed a Sagnac Interferometer having $\sim$50 nano-radian resolution for the change in polarization angle. Here we describe several experiments using the Sagnac to study spin injection in a variety of organic and inorganic spintronic junctions and devices, by measuring the Kerr rotation induced by the spin aligned carriers, to unambiguously demonstrate the injection of spin-polarized current from the ferromagnetic electrode. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A6.00015: Testing the spin-dependent polaron-pair transition model for low–temperature PEDOT:PSS through electrically detected spin-Rabi beat detuning Douglas L. Baird, Kipp J. van Schooten, Rachel Baarda, John M. Lupton, Christoph Boehme Poly[styrenesulfonate] doped poly[3,4-ethylenedioxythiophene] (PEDOT:PSS), which is a well-known organic metal at room temperature, exhibits a very distinct spin-dependent transition at low temperatures ($<$70K). We have studied this with pulsed electrically detected magnetic resonance spectroscopy which revealed a two-spin s=1/2 pair recombination process with very weakly spin-spin coupled pairs that are exposed to very weak hyperfine fields. This is in contrast to strong hyperfine fields reported for similar mechanisms in other materials [1]. In absence of hyperfine fields, the detuning behavior of spin-Rabi oscillation controlled electronic transition rates as predicted by Rajevac et al. [2] can be tested. This theory predicts that the Rabi-beat frequency approaches twice the detuning (= difference between excitation frequency and the Larmor frequency of the spins), in contrast to the Rabi nutation frequency, which approaches the detuning frequency. Our electrically detected spin-Rabi beat oscillation measurements as a function of the detuning experimentally confirm these predictions with very high precision.\\[0pt] [1] W. J. Baker et al., Phys. Rev. Lett. 108, 267601 (2012).\\[0pt] [2] V. Rajevac et al., Phys. Rev. B 74, 245206 (2006). [Preview Abstract] |
Session A7: Focus Session: Patterned Magnetic Nanostructures
Sponsoring Units: GMAG DMPChair: Olle Heinonen, Argonne National Laboratory
Room: 106
Monday, March 3, 2014 8:00AM - 8:36AM |
A7.00001: Controlled Magnetic Reversal and Frustration in Artificial Quasicrystals Invited Speaker: Vinayak Bhat Recent studies of ferromagnetic (FM) antidot arrays have been restricted to simple periodic lattices (square, triangular, etc.). We have fabricated artificial FM quasicrystals (AFQ), which are \textit{\textbf{aperiodic}} antidot lattices that are self-similar, retain definite rotational symmetry, and consist of a multiply-connected network of permalloy film segments. We focus on Penrose P2 tilings (P2T) constructed from film segments of two lengths (d$_{\mathrm{1}}=$ 810 nm $-$1618 nm, d$_{\mathrm{2}} =$ 500 nm$-1$ $\mu$ m), width W $\approx $ 100 nm, and thickness t $=$ 25 nm [1]. Static and dynamic magnetizations were studied using DC magnetometry, broadband (BB) FMR, and micromagnetic simulations (MS). Reproducible ``knee'' anomalies observed in the hysteretic, low-field DC magnetization M(H,T) signal a series of abrupt transitions between ordered magnetization textures, concluding in a smooth evolution into a saturated state. Numerous FMR mode signatures quantitatively reproduce in opposite DC field sweeps in the near-saturated regime, which suggests pinning of the magnetization parallel to the AD edges and confinement of domain walls at P2T vertices control segment polarization and reversal. Novel ``asymmetric'' modes, defined by their presence on only one side of the field origin in a given sweep, are observed only in the reversal regime, and accompany knee anomalies in M(H,T). MS agree with experimental DC hysteresis loops and FMR spectra, and indicate that systematic control of magnetic reversal and domain wall motion can be achieved via tiling design, offering a new paradigm of \textbf{magnonic quasicrystals}. AFQ also behave as novel artificial spin ice systems that exhibit non-stochastic switching due to their aperiodicity and inequivalent pattern vertices. MS indicate pinned Dirac monopoles and confined magnetic avalanches exist in AFQ. \\[4pt] [1] V. S. Bhat \textit{et al.}, Phys. Rev. Lett. \textbf{111}, 077201 (2013). [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A7.00002: Size-tuned Highly-ordered Magnetic Nanodot Arrays via ALD-Assisted Block Copolymer Nanolithography Srinivas Polisetty, Chun-Hao Lin, Wayne L. Gladfelter, Marc H. Hillmyer, Chris Leighton Block copolymer nanolithography of large-area well-ordered magnetic nanostructures is now possible \textit{via} a variety of approaches and holds considerable appeal for fundamental science and for bit patterned recording media. Here, we demonstrate a non-lift-off damascene-type approach [1] combined with low temperature atomic layer deposition (ALD) of a conformal ZnO layer to provide size-controlled magnetic nanodots. Perpendicularly-aligned nonporous templates were achieved by solvent annealing polystyrene-$b$-polylactide (PS-PLA) films. Low temperature ALD was then used to conformally coat the template with a ZnO layer of variable thickness to systematically reduce the pore diameter. Our damascene-type non-lift-off process [1] was then used to synthesize Ni$_{\mathrm{80}}$Fe$_{\mathrm{20}}$ dot arrays from such templates, achieving tunable dot diameters (6-30 nm) and controlled dot height (by Ar milling time). Magnetic measurements were used as a probe of island volume, good agreement being obtained between simple calculations, imaging, and blocking temperature measurements. The results demonstrate a simple route to size control from a fixed polymer template, enabling detailed studies of separation-dependent inter-dot magnetic interactions for example. \\[4pt] [1] Baruth, \textit{et al.}, \textit{ACS Appl. Mater. Interfaces} \textbf{3}, 3472 (2011). [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A7.00003: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 9:00AM - 9:12AM |
A7.00004: Concave nanomagnetic triangles: size and shape effect on the anisotropy and magnetization switching Alexander Kozhanov, Terence Fisher, Alexander Anferov, Igor Eremin, Ivan Vasilyevskiy Single domain nanomagnets are essential for magnetic memory and non-volatile logic applications. Recently a non-volatile logic device based on the triangular nanomagnet was proposed. Dependent on the triangle's shape and dimensions ``Y'' or ``buckle'' magnetization alignment ground states are defined by configurational anisotropy. Triangle shape distortions such as corner rounding results in preferable ``buckle'' ground state not favorable for nonvolatile logic applications. In this work we investigate the effect of triangle dimensions and shape on the configurational anisotropy and magnetization ground state profile. 50$\mu $m $\times$ 50$\mu $m arrays of 50-500nm aside equilateral permalloy triangles capped with Al layer were fabricated. Arrays of triangles with different corner radius, amount of concavity and vertex extrusion were fabricated. Field modulated MOKE technique was used to characterize triangle anisotropy. Micromagnetic simulations accompanied the experimental results and were used to investigate the ground states magnetization alignment, energy profile and switching dynamics. We demonstrate that triangle shape variations can be used to effectively manipulate its anisotropy profile, ground state stability and switching times. We map the ``Y'' and ``buckle'' triangle ground state diagram in the concavity-corner rounding-vortex extrusion parameter space. The investigated triangle shapes are assessed for the non-volatile logic applications. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:48AM |
A7.00005: Collective spin waves in reconfigurable artificial crystals and magnonic meta-materials Invited Speaker: Dirk Grundler Periodically nanopatterned ferromagnets have generated great interest in the research field of magnonics in that they support spin-wave (SW) nanochannels, allow for multi-directional emission of short-wavelength SWs via the grating coupler effect and form artificial crystals for SWs (magnons) in the GHz frequency regime. Allowed SW minibands and forbidden frequency gaps are not just tailored by the geometrical and material parameters, but reflect decisively the periodic order of the nanomagnets' remanent magnetization. Thereby a further degree of freedom is offered for controlling wave phenomena in solids compared to photonics and plasmonics. We investigated such so-called reconfigurable magnonic crystals (MCs) consisting of a one-dimensional (1D) array of permalloy nanostripes that allow one to vary the Brillouin zone boundaries, forbidden frequency gaps and number of SW minibands in one-and-the same device. When excited by a microwave antenna, an unexpected metamaterial property was found in that both reciprocal and nonreciprocal SW excitation occurred depending on the parallel and antiparallel alignment of magnetic moments in neighboring stripes. Such excitation characteristics are not found in natural materials. Switching an individual stripe from parallel to antiparallel magnetization in an otherwise saturated 1D MC modified the transmitted SW amplitude considerably offering SW control on the nanoscale. Combined with the grating coupler effect, periodically nanopatterned ferromagnets are expected to provide interesting building blocks for magnonic applications aiming at transmitting and processing information at microwave frequencies with spin waves. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A7.00006: Effects of dipolar interactions in magnetic nanoparticle systems Sergiu Ruta, Ondrej Hovorka, Roy Chantrell Understanding the effects of magnetostatic interactions in magnetic nanoparticle systems is of importance in magnetic recording, biomedical applications such as in hyperthermia cancer treatment, or for sensing approaches in biology and chemistry, for example. In this talk we discuss the macroscopic and microscopic effects of dipole-dipole interactions in three-dimensional assemblies of magnetic nanoparticles in various spatial arrangements, including the BCC, FCC, or randomized lattices. Our study is based on the kinetic Monte-Carlo modelling and concentrates on exploring the effect of the particle arrangement, distributions of particle volumes and anisotropy axes, and the role of thermal effects on the overall behaviour of hysteresis loops, ZFC/FC temperature scans and the magnetization decay data computed during the relaxation to equilibrium. In the case of the FCC lattice we find a counter-intuitive effect where increasing the interaction strength enhances/suppresses the hysteresis loop coercivity at high/low temperatures. The analysis of the domain pattern formation and pair correlation functions suggests for the observed behaviour to be a result of the phenomenon of frustration. We also discuss the possibility of observing the super-ferromagnetic phases on similar syste [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A7.00007: Effect of dipolar nanoparticle interaction on transverse magnetic susceptibility: particle pair model Elizabeth Plowman, Ondrej Hovorka, Gennady Friedman Determining nanoparticle dipolar interactions from experimental measurement of magnetic moments is a classical inverse problem in magnetism. It is important in a variety of applications including magnetic information storage and Magnetic Particle Imaging (MPI). Historically, magnetic moment relaxation has been used to characterize system parameters including dipolar interactions. However, the results are sensitive to particle size distribution. We demonstrate that dipolar coupling strength in a nanoparticle-pair can be determined from transverse magnetic susceptibility, a readily measured parameter. Moreover, we demonstrate that this method is insensitive to particle size, rendering it more robust for real-world experiments. We present both analytical and numerical models for transient and steady-state transverse magnetic susceptibility and resulting interaction strength of our two-particle system. In the analytical model master equation is employed. The particles are assumed to be immobile and the set of possible states is discrete. In the numerical models both master equation and Landau-Lifshitz-Gilbert dynamics are employed. In these models random particle anisotropy directions are taken into account. The results of each model are compared. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A7.00008: The role of geometrical symmetry on thermally activated processes in clusters of interacting dipolar moments Ondrej Hovorka, Joe Barker, Gary Friedman, Roy Chantrell Thermally activated magnetization decay is studied in ensembles of clusters of interacting dipolar moments by applying the master-equation formalism, as a model of thermal relaxation in systems of interacting single-domain ferromagnetic nanoparticles. Solving the associated master-equation reveals a breakdown of the energy barrier picture depending on the geometrical symmetry of structures. Deviations are most pronounced for reduced symmetry and result in a strong interaction dependence of relaxation rates on the memory of initialization of an ensemble. Developed is a simple two-state system description of an ensemble, which accounts for the observed anomalies. These results follow from a semi-analytical treatment, and are fully supported by kinetic Monte-Carlo simulations. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A7.00009: Synthesis of Fe Nanowires via a Novel Approach Kinjal H. Gandha, Kevin Elkins, J. Ping Liu Iron nanowires with high magnetization and high coercive force were fabricated via reduction of as-synthesized $\alpha $-FeOOH nanowires. Thermal treatment is used to facilitate subsequent phase transformation from the precursor to $\alpha $-Fe phase. Increasing reduction time and temperature leads to agglomeration and sintering of the nanowires. By using fluid bed technique and by adjusting the reaction temperature, time and gas component in the process of heat treatment, $\alpha $-Fe nanowires with length of 200nm$-$300nm and diameter of 20nm$-$30nm were prepared. The iron nanowires have a coercive force of 628 Oe and saturation magnetization of 197 emu/g at room temperature. This novel process is effective to produce iron nanowires with well controlled morphology and composition. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A7.00010: Energy minimization to optimize information lifetime in arrays of nanowires Eugenio Vogel, Eduardo Cisternas Magnetic nanowires trapped in the alumina membrane used to produce them can be used to store information at the nanoscale (symbols, barcodes, etc.). This is achieved by inscribing a ferromagnetic domain over the randomly oriented magnetizations as left by the fabrication process. This is achieved by a powerful magnetic tip which is able to overturn wire magnetization of sectors with a few wire diameters across. As the tip is withdrawn the ferromagnetic symbols prevail and wires in the sector interact repulsively so the overall energy is increased. This is a factor of instability for the stored information. In the present paper we investigate ways of minimizing this repulsive energy but still preserving the information stored by the original symbol at its original scale. The inscription of an opposite ferromagnetic band is a possible technique to minimize the repulsive energy (JMMM 337 (2013) 74-78). Application of this stabilization technique to different symbols is discussed. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A7.00011: Structural and Magnetic Characterizations of Y$_{\mathrm{x}}$Co$_{\mathrm{y}}$ Nanowires Bishnu Dahal, Keshab Sapkota, Rajendra Dulal, Parshu Gyawali, Ian L. Pegg, John Philip Nanowires of YxCo$_{\mathrm{y}}$ (Y$_{2}$Co$_{17}$, YCo$_{3}$ and YCo$_{5})$ are grown using electrospinning technique and by annealing at high temperature. The size of the nanowires varies from 80 -- 300 nm in diameter. Structural analyses show that Y$_{2}$Co$_{17}$ exhibits rhombohedral crystal structure while YCo$_{5}$ displays hexagonal crystal structure. The as-grown nanowires are polycrystalline in nature with an average grain size of 40 nm. YCo$_{3}$ nanowires are amorphous in nature. All the Y$_{\mathrm{x}}$Co$_{\mathrm{y}}$ nanowires are found to be strong ferromagnetic materials as reported in the bulk system. The observed coercivity of the Y$_{\mathrm{x}}$Co$_{\mathrm{y}}$ nanowires is low, typically around 500 Oe in comparison to the large coercivity observed in YCo nanoparictles [Preview Abstract] |
Session A8: Focus Session: Magnetodynamics: Generation and Application
Sponsoring Units: GMAGChair: Jeffrey Grossman, Massachusetts Institute of Technology
Room: 104
Monday, March 3, 2014 8:00AM - 8:36AM |
A8.00001: Spin transfer torque excited spin-waves in metal--magnetic insulator bilayer Invited Speaker: Yan Zhou We develop a self-consistent theory for current-induced spin-wave excitations in normal metal-magnetic insulator bilayer structures. We compute the spin-wave dispersion and dissipation, including dipolar and exchange interactions in the magnet, the spin diffusion in the normal metal, as well as the surface anisotropy, spin-transfer torque, and spin pumping at the interface. We find that (1) the spin-transfer torque and spin pumping affect the surface modes more than the bulk modes; (2) spin pumping inhibits high-frequency spin-wave modes, thereby redshifting the excitation spectrum; (3) easy-axis surface anisotropy induces a new type of surface spin wave, which reduces the excitation threshold current and greatly enhances the excitation power. We propose that the magnetic insulator surface can be engineered to create spin-wave circuits utilizing surface spinwaves as information carriers. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A8.00002: Phonon Diodes and Transistors from Magneto-acoustics Sophia Sklan, Jeffrey Grossman The creation of non-reciprocal phononic systems holds the promise of allowing computers that would process thermal or acoustic (rather than electronic) signals. By sculpting the magnetic field applied to magneto-acoustic materials (which couple phonons to a magnetic field, typically due to effects like magnon-phonon coupling in yttrium iron garnet), phonons can be used for information processing in analogy with photonic computing. Using a combination of analytic and numerical techniques, we demonstrate designs for diodes (isolators) and transistors that are independent of their conventional, electronic formulation. We analyze the experimental feasibility of these systems, including the sensitivity of the circuits to likely systematic and random errors. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A8.00003: Ultra low-power hybrid spintronics-straintronics clocked with Surface Acoustic Waves (SAW) Mohammad Salehi Fashami, Supriyo Bandyopadhyay, Jayasimha Atulasimha The study of magnetization dynamics in magnetostrictive materials triggered with surface acoustic waves (SAWs) is of great interest not only from a fundamental point of view, but also for potential applications in energy efficient nanomagnetic computing. In this presentation, we model magnetization dynamics in dipole coupled arrays of nanomagnets clocked by acoustic waves. Specifically, this theoretical work demonstrates the feasibility of sequential logic devices such as flip-flops by showing that NAND gates and information propagation with cross-over of nanomagnet ``wires'' can be implemented and synchronously clocked with surface acoustic waves. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A8.00004: Surface Acoustic Wave induced magnetization change in Perpendicular anisotropy Co/Pt multilayers Uday Singh, Shireen Adenwalla We have demonstrated the ability of focused surface acoustic waves (FSAW) to control the magnetization direction of Co/Pt multilayers microstructure (4 x 5 $\mu$m) with perpendicular anisotropy. The strain wave generated by the FSAW results in large values of periodic compressive and tensile strain at the focal spot. The magnetoelasticity of Co results in changes in the magnetization easy axis with strain. To switch the magnetization from out of plane to in-plane requires tensile strain of more than 1{\%}. These large strains are obtained using annular interdigital transducers (AIDT) fabricated on 128$^{\circ}$ Y-Cut LiNbO$_{3}$, with a fundamental resonance frequency of 87.95MHz. We have mapped the strain distribution at the focal center using optical reflectivity and a knife edge, which selects for reflections above the specular edge. An array of Co/Pt multilayers was patterned at the focal center using e-beam lithography. In the region of highest strain, we observe magnetization changes in the Co/Pt multilayers excited by FSAW. We will discuss both dc and rf measurements of the changes in magnetization. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A8.00005: Dipole-exchange modes in transversely magnetized ferromagnetic stripes Rodrigo Arias, Zheng Duan, Ilya Krivorotov We present a theory of dipole-exchange modes in transversely magnetized ferromagnetic stripes of rectangular cross sections: a comparison is made with experimental results on Permalloy stripes. The model applies to very thin stripes (of the order of the exchange length): the magnetization is considered uniform over their thickness, and we consider modes of long wavelength along the longitudinal direction of the stripes. An applied magnetic field saturates the stripes along the transverse direction, and we also consider the effect of the exchange and dipolar fields. Under these assumptions we obtain the frequencies and shapes of the modes either considering free or pinned boundary conditions. We obtain good agreement with measurements of the frequency spectra in Permalloy nano wires of several rectangular cross sections: this happens for modes with appreciable amplitude throughout the samples. There is frequency disagreement for edge modes due to limitations of the model, since the effects of roughness, corners and imperfections at the edges of the samples are quite relevant in this case. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A8.00006: Ultra-low-energy analog straintronics using multiferroic composites Kuntal Roy Multiferroic devices, i.e., a magnetostrictive nanomagnet strain-coupled with a piezoelectric layer, are promising as binary switches for ultra-low-energy digital computing in beyond Moore's law era [Roy, K. Appl. Phys. Lett. \underline {103}, 173110 (2013), Roy, K. et al. Appl. Phys. Lett. \underline {99}, 063108 (2011), Phys. Rev. B \underline {83}, 224412 (2011), Scientific Reports (Nature Publishing Group) \underline {3}, 3038 (2013), J. Appl. Phys. \underline {112}, 023914 (2012)]. We show here that such multiferroic devices, apart from performing digital computation, can be also utilized for analog computing purposes, e.g., voltage amplification, filter etc. The analog computing capability is conceived by considering that magnetization's mean orientation shifts gradually although nanomagnet's potential minima changes abruptly. Using tunneling magnetoresistance (TMR) measurement, a continuous output voltage while varying the input voltage can be produced. Stochastic Landau-Lifshitz-Gilbert (LLG) equation in the presence of room-temperature (300 K) thermal fluctuations is solved to demonstrate the analog computing capability of such multiferroic devices. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A8.00007: Efficient room-temperature Spin Hall nano-oscillator Andrei Zholud, Sergei Urazhdin \newline Spin current injected into a ferromagnet exerts a spin torque on the magnetization, modifying its dynamical damping. Complete compensation of damping by spin current can result in magnetization auto-oscillations, as was demonstrated for in-plane point-contact spin Hall oscillator devices utilizing Pt spin Hall material as a source of spin current and permalloy (Py) as active magnetic layer [1]. Electronic spectroscopy has demonstrated microwave generation by oscillations of magnetization at cryogenic temperatures, but this microwave generation decreases with increasing temperature and disappears at room temperature[2]. We will describe a new device geometry that decouples spin transport from the magnetic configuration by separate patterning of the spin Hall Pt layer and the active Py layer. We demonstrate that this device geometry can operate at smaller driving dc currents for microwave generation that persists up to room temperature. We discuss the physical mechanisms that affect the temperature- and geometry-dependent performance of spin Hall nano-oscillators.\newline [1] V. Demidov, S. Urazhdin and S.O. Demokritov, Nature Mater. 9, 984 (2010) [2] R.H. Liu, W.L. Lim, and S. Urazhdin, Phys. Rev. Lett. 110, 147601 (2013) [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A8.00008: Ferromagnetic cross junction based spin wave logic device Alexander Kozhanov Spin wave based signal processing/logic devices have long history of development and exploration. Typically the spin wave phase is used to encode the input information. Spin wave interference is used to produce the device output in form of the spin wave amplitude. Electronic amplitude-to-phase signal converter is required to build a logic gate capable of providing necessary fan-out. In case of destructive interference the phase information is lost and a ``new'' wave should be excited at the next logic stage. In this work we demonstrate the spin wave interference in ferromagnetic CoTaZr cross and propose a spin wave logic device based on this structure. Two neighboring arms of the cross serve as the device inputs. For the certain input wave phase offsets the interference is constructive in one output arm of the cross while destructive in another and vice versa thus resulting in a phase controlled spin wave switching. The output waves in the cross arms have different phase offsets dependent on the input wave phase offset. By merging the spin waves scattered into the cross output arms the device output is formed with a wave phase following the OR/NOR logic operation. We model local spin wave scattering in the cross center and discuss the effect of the local spin wave modes in the cross junction on the proposed device operation. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A8.00009: Experiments on straintronic nanomagnetic logic with two-state elliptical and four-state diamond and concave magnetostrictive nanomagnets Noel D'Souza, Mohammad Salehi Fashami, Supriyo Bandyopadhyay, Jayasimha Atulasimha Experimental work on strain-induced magnetization switching of single-domain magnetostrictive nanomagnets grown on a bulk \textless 001\textgreater PMN-PT substrate is demonstrated through Magnetic Force Microscopy (MFM) studies. Low-moment MFM probes are used in order to minimize tip-induced magnetization switching of the nanomagnets. Voltages are applied along the length of the PMN-PT substrate ($d_{33}$ mode) to generate the required strain in the magnetostrictive nanomagnet. Domain switching is then investigated in uniaxial (two-state) i) isolated, ii) dipole-coupled, and iii) an array of nanomagnets to implement NAND logic. Subsequent theoretical studies focus on four-state magnetostrictive nanomagnets (diamond- and concave-shaped). The magnetization characteristics of these shapes, particularly the switching coherence, are examined for various criteria (size, concavity depth, thickness, etc.) with the conclusion that concave nanomagnets are the ideal shape for coherent and reliable magnetization switching in future magnetoelectric devices. Experimental results of magnetic field- and stress-induced switching in these concave nanomagnets on a bulk PMN-PT substrate are also presented. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A8.00010: Large-area patterned substrates for micromagnetic actuation of superparamagnetic microbeads Minae Ouk, Geoffrey Beach Superparamagnetic microbeads (SBs) are widely used to capture biological entities in a fluid environment. Chip-based magnetic actuation provides a means to transport SBs in lab-on-a-chip technologies. This is usually accomplished using the stray field from patterned magnetic microstructures [1], or domain walls in magnetic nanowires [2]. However, lithographic patterning over a large area is costly and impractical using conventional techniques such as electron beam lithography. Here we use a simple floating-transfer technique [3] for large-area self-assembly of polystyrene microspheres on a Si wafer to produce lithographic masks texturing a substrate. Hexagonal patterns are used as lift-off and etching masks to create magnetic dot and anti-dot arrays in CoFe thin films, with a size and spacing that can be tuned via sphere diameter and RIE etch time. Using a rotating magnetic fields, we show that these magnetically-patterned substrates can transport SBs across large distances on the wafer surface, opening the possibility to augment or replace microfluidic actuation for long distance transport. [1] B. Yellen, et al., Lab Chip, 7, 1681 (2007) [2] E. Rapoport and G. S. D. Beach, APL 100, 082401 (2012) [3] X. Ye and L. Qi, Nano Today 6, 608 (2011) [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A8.00011: Spin gated transistors for reprogrammable logic Chiara Ciccarelli, Fernando Gonzalez-Zalba, Andrew Irvine, Richard Campion, Liviu Zarbo, Brian Gallagher, Andrew Ferguson, Tomas Jungwirth, Joerg Wunderlich In spin-orbit coupled magnetic materials the chemical potential depends on the orientation of the magnetisation [1,2]. By making the gate of a field effect transistor magnetic, it is possible to tune the channel conductance not only electrically but also magnetically. We show that these magnetic transistor can be used to realise non-volatile reprogrammable Boolean logic. The non-volatile reconfigurable capability resides in the magnetization-dependent band structure of the magnetic stack. A change in magnetization orientation produces a change in the electrochemical potential, which induces a charge accumulation in the correspondent gate electrode. This is readily sensed by a field-effect device such as standard field-effect transistors or more exotic single-electron transistors. We propose circuits for low power consumption applications that can be magnetically switched between NAND and OR logic functions and between NOR and AND logic functions. [1] J. Wunderlich et al., PRL 97, 077201 (2006) [2] C. Ciccarelli, et al., Appl. Phys. Lett. 101, 122411(2012) [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A8.00012: Electric-field modulation of the phase shift for spin waves Tianyu Liu, Xufeng Zhang, Hong Tang, Michael E. Flatt\'{e} An electric field has been predicted to manipulate the phase of spin waves in yttrium iron garnet (YIG) through the spin-orbit interaction, which couples the electric field with the gradient of the magnetization [1,2]. We have observed an electric-field-dependent phase shift in the propagation of surface spin waves in a YIG waveguide. In addition to the spin-orbit effect there is a stronger effect on the phase shift due to the change of the magnetization of the YIG due to the applied electric field (a magnetoelectric effect). The contributions of the two effects can be distinguished by varying the direction of the electric field relative to the YIG magnetization. \\[4pt] [1] T. Liu and G. Vignale, Phys. Rev. Lett. 106, 247203 (2011).\\[0pt] [2] T. Liu and G. Vignale, Journal of Applied Physics 111, 083907-083907-6 (2012). [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A8.00013: Fast Deterministic Bipolar Switching in Orthogonal Spin Torque Devices via the Control of the Relative Spin Polarizations Junbo Park, Daniel C. Ralph, Robert A. Buhrman We model 100 ps pulse switching dynamics of orthogonal spin transfer (OST) devices that employ an out-of-plane polarizer (OPP) and an in-plane polarizer (IPP). Simulation results indicate that increasing the spin polarization ratio, $C_{P\, }= \quad P_{IPP}/P_{OPP}$, results in deterministic switching of the free layer without over-rotation (360 degree rotation). By using spin torque asymmetry to realize an enhanced effective $P_{IPP}$, we experimentally demonstrate this behavior in OST devices. Modeling predicts that decreasing the effective demagnetization field can substantially reduce the minimum $C_{P}$ required to attain deterministic bipolar switching, while retaining low critical switching current, $I_{p\, }=$ 500 $\mu $A. [Preview Abstract] |
Session A10: Focus Session: Mechanics of Cells and Biological Networks I
Sponsoring Units: DBIO DPOLY GSNPChair: Daniel Blair, Georgetown University
Room: 201
Monday, March 3, 2014 8:00AM - 8:36AM |
A10.00001: Mechanics of composite cytoskeletal and extracellular networks Invited Speaker: Moumita Das Living cells sense and respond to mechanical forces in their surroundings. This mechanical response is mainly due to the cell cytoskeleton, and its interaction with the extracellular matrix (ECM). The cell cytoskeleton is a composite polymeric scaffold made of many different types of protein filaments and crosslinking proteins. Two major filament systems in the cytoskeleton are actin filaments (F-actin) and microtubules (MTs). Actin filaments are semiflexible, while the much stiffer MTs behave as rigid rods. I shall discuss theories that help understand how the direct coupling to the surrounding F-actin matrix allows intracellular MTs to bear large compressive forces and controls the range of force transmission along the MTs, and how the MTs not only enhance the stiffness of the cell cytoskeleton, but can also dramatically endow an initially nearly incompressible F-actin matrix with enhanced compressibility relative to its shear compliance. A second source of compositeness in the cytoskeleton is the presences of different types of crosslinkers that can interact cooperatively leading to enhanced mechanical rigidity and tunable response. Like the cytoskeleton, the ECM is also a polymeric composite. It is primarily composed of a mesh of fibrous proteins, mainly stiff collagen filaments, and a comparatively flexible gel of proteoglycans and hyaluronan. I shall discuss a model that shows how the interplay between the collagen network and the background elastic gel leads to a mechanically robust ECM. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A10.00002: Modeling the role of nuclear mechanics in determining cell shape and motility through microfluidic channels Jake Shechter, Kara Maki, Moumita Das Cell mechanics and migration through tight spaces are critical to life processes such as immune response and fertilization, in several diseases, and in diagnostics and drug delivery. For example, breast cancer cells have been shown to deform more easily and transit more rapidly through microfluidic channels than healthy breast cells. In this computational biophysics project, we simulate a cell moving through a microfluidic channel. We calculate the deformation energy of a model cell, which includes contributions from the cell cytoskeleton and the cell nucleus. We study how the model cell deforms in response to external forces, focusing on the deformability of the cell as it squeezes into and through a microfluidic channel and how the nucleus plays a part in this. Recent experiments suggest that the nucleus can be up to an order of magnitude stiffer than the rest of the cell and our results may provide insights into how the nucleus influences cell mechanics and migration. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A10.00003: Dynamic contact guidance of migrating cells Wolfgang Losert, Xiaoyu Sun, Can Guven, Meghan Driscoll, John Fourkas We investigate the effects of nanotopographical surfaces on the cell migration and cell shape dynamics of the amoeba Dictyostelium discoideum. Amoeboid motion exhibits significant contact guidance along surfaces with nanoscale ridges or grooves. We show quantitatively that nanoridges spaced 1.5 $\mu $m apart exhibit the greatest contact guidance efficiency. Using principal component analysis, we characterize the dynamics of the cell shape modulated by the coupling between the cell membrane and ridges. We show that motion parallel to the ridges is enhanced, while the turning, at the largest spatial scales, is suppressed. Since protrusion dynamics are principally governed by actin dynamics, we imaged the actin polymerization of cells on ridges. We found that actin polymerization occurs preferentially along nanoridges in a ``monorail'' like fashion. The ridges then provide us with a tool to study actin dynamics in an effectively reduced dimensional system. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A10.00004: Fc-receptor induced cell spreading during frustrated phagocytosis in J774A.1 macrophages Daniel Kovari, Jennifer Curtis, Wenbin Wei Phagocytosis is the process where by cells engulf foreign particles. It is the primary mechanism through which macrophages and neutrophils (white blood cells) eliminate pathogens and debris from the body. The behavior is the result of a cascade of chemical and mechanical cues, which result in the actin-driven expansion of the cell's membrane around its target. For macrophages undergoing Fc-mediated phagocytosis, we show that above a minimum threshold the spreading rate and maximum cell-target contact area are independent of the target's opsonin density. Qualitatively, macrophage phagocytic spreading is similar to the spreading of other cell types (e.g. fibroblasts, lymphocytes, and Dict.d.). Early spreading is most likely the result of ``passive'' alignment of the cell to the target surface. This is followed by an active expansion period driven by actin. Finally upon reaching a maximum contact area, typically 2-3 times the size of ``non-activated'' cells, macrophages often undergo a period of rapid contraction not reported in other cell types. We hypothesize that this, as yet unexplained, transition may be specific to the chemical and mechanical machinery associated with phagocytosis. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A10.00005: Silk Electrogel Rheology A.P. Tabatabai, J.S. Urbach, D.L. Blair, D.L. Kaplan We present experimental results on the rheology on electrogels derived from aqueous solutions of reconstituted \textit{Bombyx Mori} silk fibroin protein. Through electrochemistry, the silk protein solution develops local pH changes resulting in the assembly of protein into a weak gel. We determine the physical properties of the electrogels by performing rheology and observe that they exhibit the characteristics of a crosslinked biopolymer network. Interestingly, we find that these silk gels exhibit linear elasticity over a range of up to two orders of magnitude larger than most crosslinked biopolymer networks. Moreover, the nonlinear rheology exhibits a strain-stiffening behavior that is fundamentally different than the strain-stiffening observed in crosslinked biopolymers. Through rheological techniques we aim to understand this distinctive material that cannot be explained by current polymeric models. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A10.00006: Nonlinear intracellular elasticity controlled by myosin-generated fluctuating stress Ming-Tzo Wei, H. Daniel Ou-Yang The mechanics of biological cells are governed by a network of cytoskeletal filaments and molecular motors forming a dynamic mechanical entity. It has been found that local elasticity of in vitro active polymer networks, a synthesized cytoskeletal network, increase as a result of myosin-generated stresses. It is unknown this also holds in the local intracellular stress. We study the intracellular stress by the combination of the approaches of active and passive microrheology to measure the myosin-generated fluctuating stress and intracellular elasticity. Our experimental data show an increase in the fluctuations of the cellular elasticity with increasing motor-generated fluctuating local stress inside living cells. In addition, we found a direct correlation between the mean intracellular elasticity and steady-state intracellular stress. Our study provides a link between in vitro active polymer networks and in vivo cell experiments. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A10.00007: Structure-function relations in cartilage under shear: Does fiber organization matter? Moumita Das, Jesse Silverberg, Aliyah Barrett, Poul Peterson, Lawrence Bonassar, Itai Cohen Confocal elastography have enabled spatially resolved measurements of soft biological tissues such as articular cartilage (AC). With this technique it was discovered that the AC shear modulus has a compliant region near the tissue surface that is 10-100 times smaller than the bulk. This region also dissipates $\sim$ 90{\%} of the energy absorbed during shear, suggesting a functional role protecting the underlying tissue. Though the mechanical properties have depth-dependent trends that parallel the stereotypical collagen fiber organization, we explore this observation with structural, compositional, and shear mechanical data. We show the fiber-reinforced interpretation of the collagen network is inconsistent with experiments at small strains. Instead, we find the shear modulus strongly correlates with cartilage matrix density leading to the result that a 50{\%} variation in matrix density leads to a 10,000{\%} variation in shear modulus. We interpret these results in terms of a biopolymer rheology model that is known to produce such trends. This scaling arises from a second-order mechanical phase transition known as rigidity percolation, and with the inclusion of a reinforcing medium to more closely mimic cartilage, the empirical trends are reproduced. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:24AM |
A10.00008: Strain stiffening and stress heterogeneities in sheared collagen networks Invited Speaker: Jeffrey Urbach Disordered networks of stiff or semi-flexible filaments display unusual mechanical properties, including dramatic stiffening when sheared, but little is known about the spatial distribution of stresses. This talk will introduce the technique of \textit{Boundary Stress Microscopy}, which adapts the approach of traction force microscopy to rheological measurements in order to quantify the non-uniform surface stresses in sheared soft materials. Our results on networks of the biopolymer collagen, a major component of the extracellular matrix, show stress variations over length scales much larger than the network mesh size. We find that the heterogeneity increases with strain stiffening, with stresses at high strains exceeding average stresses by an order of magnitude. The strain stiffening behavior over a wide range of mesh sizes can be parameterized by a single characteristic strain and associated stress, which describes both the strain stiffening regime and network yielding. The characteristic stress is approximately proportional to network density, but the peak stress at both the characteristic strain and at yielding are remarkably insensitive to concentration. These results show the power of Boundary Stress Microscopy to reveal the nature of stress propagation in disordered soft materials, which is critical for understanding many important mechanical properties, including the ultimate strength of a material and the nature of appropriate microscopic constitutive equations. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A10.00009: Longitudinal fluctuations and higher cumulants of biopolymers in the strong stretching limit Lipeng Lai, Jianshu Cao Biopolymers, such as actins and spectrins, are important constituents in cytoskeletons. Previous studies revealed that the mechanical properties of cytoskeletons, which are essential to the functions of living organisms, are largely dictated by the elastic properties of individual polymers. Here we studied the fluctuations of individual biopolymers when they are strongly stretched with very small transverse deformations. Based on the Worm-like chain (WLC) model, general formulae for the fluctuations and higher cumulants of the end-to-end distance along the stretching direction (longitudinal) are obtained when the energy of the semi-flexible chains retains a quadratic form (e.g., when the polymer is subject to a point force at the end or a constant plug flow with the other end fixed). Our results are consistent with previous theoretical and experimental work. Besides providing additional criteria to check the region of validity of the WLC model, the results may also provide more insights into the study of the elastic properties of polymers and cytoskeletal networks. Furthermore, our results can also be generalized to other situations when the polymers are rod-like. A good example is the actin network, where the actin segments are stiff due to their large persistence length. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A10.00010: Universal Crossover Dynamics of a Semi-Flexible Polymer in Two Dimensions Aniket Bhattacharya, Aiqun Huang, Ramesh Adhikari, Kurt Binder We present a unified scaling theory for the dynamics of monomers for dilute solutions of semiflexible polymers under good solvent conditions in the free draining limit. Our theory encompasses the well-known regime of mean square displacements (MSDs) of stiff chains growing like $t^{3/4}$ with time (R. Granek, J. Phys. II (Paris) {\bf 7}, 1767 (1997); E. Farge and A. C. Maggs, Macromolecules {\bf 26}, 5041 (1993)) due to bending motions, and the Rouse regime $t^{2 \nu / (1+ 2\nu)}$ where $\nu$ is the Flory exponent describing the radius $R$ of a swollen flexible coil. We identify how the prefactors of these laws scale with the persistence length $\ell_p$, and show that a crossover from stiff to flexible behavior occurs at a MSD of order $\ell^2_p$ (at a time proportional to $\ell^3_p$), a second crossover (to diffusive motion) occurs when the MSD is of order $R^2$. We also provide compelling evidence for the theory by carrying out large scale Molecular Dynamics simulations in $d=2$ dimensions. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A10.00011: Conformations and Transverse Fluctuations of a Semi-Flexible Chain in Two Dimensions Aiqun Huang, Aniket Bhattacharya, Kurt Binder We study conformations and transverse fluctuations of a semi-flexible polymer using Langevin Dynamics simulation in two dimensions(2D). By showing that the end-to-end distance $\langle R_N^2 \rangle $ for a semiflexible chain characterized by its contour length $L$ and the persistence length $\ell_p$ follows the scaling relation $\langle R_N^2 \rangle \sim L^{1.5}\ell_p^{0.5}$, as proposed by Schaefer {\em et al.} and Nakanishi, we verify the absence of the Gaussian regime, thus disprove the validity of the worm-like chain (WLC) theory in 2D. We also verify that the bond autocorrelation function exhibits a power law $\langle \vec{b}_i\cdot \vec{b}_{i+s} \rangle \sim s^{-\beta}$ instead of an exponential decay as predicted by the WLC model. We further show that the normalized transverse fluctuations $\sqrt{\langle l_{\bot}^2\rangle}/L$ for the semiflexible chains of different persistence length and contour length collapse onto the same master plot as a function of $L/\ell_p$, which exhibits $\sqrt{\langle l_{\bot}^2\rangle}/L \sim (L/\ell_p)^{0.5}$ and $\sqrt{\langle l_{\bot}^2\rangle}/L \sim (L/\ell_p)^{-0.25}$ at two extreme limits $L/\ell_p \rightarrow 0$ and $L/\ell_p \rightarrow \infty$, respectively and exhibits a maximum for $L/\ell_p \sim 1.0$. [Preview Abstract] |
Session A11: Focus Session: Bacterial Biophysics I
Sponsoring Units: DBIOChair: Gerard Wong, University of California, Los Angeles
Room: 203
Monday, March 3, 2014 8:00AM - 8:36AM |
A11.00001: Bacterial surface adaptation Invited Speaker: Andrew Utada Biofilms are structured multi-cellular communities that are fundamental to the biology and ecology of bacteria. Parasitic bacterial biofilms can cause lethal infections and biofouling, but commensal bacterial biofilms, such as those found in the gut, can break down otherwise indigestible plant polysaccharides and allow us to enjoy vegetables. The first step in biofilm formation, adaptation to life on a surface, requires a working knowledge of low Reynolds number fluid physics, and the coordination of biochemical signaling, polysaccharide production, and molecular motility motors. These crucial early stages of biofilm formation are at present poorly understood. By adapting methods from soft matter physics, we dissect bacterial social behavior at the single cell level for several prototypical bacterial species, including \textit{Pseudomonas aeruginosa and Vibrio cholerae}. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A11.00002: Filaments in curved flow: Rapid formation of \textit{Staphylococcus aureus} biofilm streamers Min Young Kim, Knut Drescher, On Shun Pak, Bonnie L. Bassler, Howard A. Stone Biofilms are surface-associated conglomerates of bacteria that are highly resistant to antibiotics. These bacterial communities can cause chronic infections in humans by colonizing, for example, medical implants, heart valves, or lungs. \textit{Staphylococcus aureus,} a notorious human pathogen, causes some of the most common biofilm-related infections. Despite the clinical importance of \textit{S. aureus} biofilms, it remains mostly unknown how physical effects, in particular flow, and surface structure influence biofilm dynamics. Here we use model microfluidic systems to investigate how environmental factors, such as surface geometry, surface chemistry, and fluid flow affect biofilm development in \textit{S. aureus. }We discovered that \textit{S. aureus} rapidly forms flow-induced, filamentous biofilm streamers, and furthermore if surfaces are coated with human blood plasma, streamers appear within minutes and clog the channels more rapidly than if the channels are uncoated. To understand how biofilm streamer filaments reorient in curved flow to bridge the distances between corners, we developed a mathematical model based on resistive force theory and slender filaments. Understanding physical aspects of biofilm formation in \textit{S. aureus} may lead to new approaches for interrupting biofilm formation of this pathogen. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A11.00003: A bacterial swimming strategy with two alternating speeds of propagation Matthias Theves, Johannes Taktikos, Vasily Zaburdaev, Holger Stark, Carsten Beta We used microfluidics together with high-speed video microscopy to acquire large data sets of swimming trajectories of \textit{Pseudomonas putida}, a bacterium with multiple polar flagella known for its ability to degrade aromatic hydrocarbons. The motion of cells in the bulk fluid is dominated by periods of persistent displacement along a straight line (runs) and sharp reorientation events (turns). The distribution of turning angles is bimodal with a dominating peak around 180 degrees and a minor peak around zero degrees. During the majority of turns, the cell reverses its swimming direction and the corresponding trajectories resemble a zig-zag pattern. Our analysis revealed that upon a reversal, the cell systematically changes its swimming speed by a factor of two on average. Based on the experimentally observed values for rotational diffusion and average runtime we developed a run-reverse random walk model with two distinct swimming speeds, which successfully recovers the mean square displacement and in an extended version also the observed negative dip in the directional autocorrelation. Our model demonstrates that by alternating between two swimming speeds, the cell explores its environment more efficiently than a cell swimming at a constant intermediate speed. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A11.00004: Bacterial navigation in chemical and nonchemical environments Bo Hu, Yuhai Tu Navigation of cells to the optimal environmental niches is critical for their survival and growth. E. coli cells, for example, can detect various chemicals and move up or down those chemical gradients (i.e., chemotaxis). Using the same signaling machinery, they can also sense other external factors such as pH and temperature and navigate from both sides toward some intermediate levels of those stimuli. This mode of precision sensing is more sophisticated than the (unidirectional) chemotaxis strategy and requires distinctive molecular mechanisms. To understand different bacterial taxis behaviors, we develop a theoretical model which incorporates microscopic signaling events in individual cells into macroscopic population dynamics. We find that the equilibrium population distribution is governed by an effective potential, the landscape of which depends on the external environment (chemical stimuli, pH, and temperature). We uncover the key conditions for various taxis behaviors and directly connects the cellular taxis performances with the underlying molecular parameters. This approach is used to examine and predict how background attractants and downstream temperature effects influence the performance and stability of thermotaxis, which can be tested in future experiments. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A11.00005: Mechanical Evolution of Bacterial Films at Oil-Water Interfaces Daniel Allan, Liana Vaccari, Jian Sheng, Robert Leheny, Kathleen Stebe Bacteria can assemble at the interface between oil and water to form films that strongly affect the mechanical properties of the interface. In comparison with biofilms on solid substrates, such biofilm formation at fluid-fluid interfaces has been the subject of relatively little study. The microstructure of the films, which can include not only packings of bacteria but macromolecular surfactants secreted by the bacteria and the remains of dead bacteria, resembles a quasi-two-dimensional colloidal suspension in a polymer solution. We have characterized the mechanical response of bacterial films at oil-aqueous interfaces during their formation via passive microrheology and pendant drop imaging. With increasing age, the films undergo a transition from a viscous to an elastic interfacial shear rheology and eventually acquire a bending rigidity. These findings will be discussed in terms of viscoelstic models and in the context of the active nature of the bacteria in the films and in the adjoining aqueous suspension. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A11.00006: Role of surface properties in bacterial attachment Jacinta Conrad, Sumedha Sharma Bacterial biofilms foul a wide range of engineered surfaces, from pipelines to membranes to biomedical implants, and lead to deleterious costs for industry and for human health. Designing strategies to reduce bacterial fouling requires fundamental understanding of mechanisms by which bacteria attach to surfaces. We investigate the attachment of \textit{Escherichia coli} on silanized glass surfaces during flow through a linear channel at flow rates of 0.1--1 mL/min using confocal microscopy. We deposit self-assembled monolayers of organosilanes on glass and track the position and orientation of bacteria deposited on these surfaces during flow using high-throughput image processing algorithms. Here, we report differences in deposition rate and surface-tethered motion of cells as a function of surface charge and surface energy, suggesting that attachment of bacteria on these engineered surfaces is dominated by different physical mechanisms. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A11.00007: CdiGMP signaling at early stages of biofilm formation in \textit{Pseudomonas Aeruginosa} Kun Zhao, Maxsim Gibiansky, Wujing Xian, Andrew Utada, Gerard Wong Biofilm communities on surfaces constitute an important physiological state of bacteria. CdiGMP is a secondary messenger that has recently emerged as a master regulator of biofilm behavior. It has been shown that cdiGMP can affect bacterial adhesion, motility and exopolysaccharides production, which are important in regulating biofilm formation. However, at a single cell level, the details of how cdiGMP regulate bacterial behavior are largely unknown. Here we examine the dynamics of intracellular cdiGMP levels at early stages of biofilm in \textit{Pseudomonas Aeruginosa}, by using cell tracking techniques. We show that cells with different cdiGMP levels play different roles in the microcolony development at early stages of biofilm. The correlation between Psl and cdiGMP levels is also investigated. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A11.00008: Theoretical and Experimental Study of Bacterial Colony Growth in 3D Xinxian Shao, Andrew Mugler, Ilya Nemenman Bacterial cells growing in liquid culture have been well studied and modeled. However, in nature, bacteria often grow as biofilms or colonies in physically structured habitats. A comprehensive model for population growth in such conditions has not yet been developed. Based on the well-established theory for bacterial growth in liquid culture, we develop a model for colony growth in 3D in which a homogeneous colony of cells locally consume a diffusing nutrient. We predict that colony growth is initially exponential, as in liquid culture, but quickly slows to sub-exponential after nutrient is locally depleted. This prediction is consistent with our experiments performed with E. coli in soft agar. Our model provides a baseline to which studies of complex growth process, such as such as spatially and phenotypically heterogeneous colonies, must be compared. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A11.00009: EPS forces in Bacillus subtilis biofilms Wenbo Zhang, Thomas Angelini, Shih-Ming Tsai, Ryan Nixon Bacteria have evolved to congregate in complex communities known as biofilms. The structure that holds a biofilm together is a matrix called extracellular polymeric substance (EPS). It has been observed in previous studies that EPS up-regulation occurs when the nutrient levels fall below a threshold concentration; this increase in EPS concentration produces an osmotic pressure that forces the colony to spread outward. This osmotic pressure may drive nutrient uptake, but the stresses generated by the EPS matrix has never been measured. Here we present measurements of the forces exerted by a biofilm on its supporting substrate and on its fluid nutrients. In our experiments, we use a technique analogous to traction force microscopy to measure strain in agar nutrient substrates imposed by Bacillus subtilis biofilms. By running additional test to measure the permeability and elastic modulus of the agar, we can estimate the pressure generated by the biofilm. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A11.00010: The Spatial-Temporal Evolution of the Interface Between Growing {\em E.~coli} Colonies Ue-yu Pen, Dan Sigal, William Ryu An \textit{Escherichia coli} colony is a popular model used to study the physical interactions of a multicellular system. However, the development of the interface between two interacting colonies has not been well studied. In this work, we tracked the development and interaction of two cellular colonies formed from single founder cells. We observed that the colony-colony interface exhibited a range of roughening, sometimes producing a linear interface (zero roughening) and other times producing a highly sinuous interface (increased roughening). Using time-lapse microscopy, we captured images of a number of interacting colonies and quantified the evolution of their interface and show that it is highly correlated with a number of factors such as colony distance, growth rate, and age. To connect the microscopic details of the spatial orientation of cells to the macroscopic roughening, we simulated growing colonies and found that the orientation of the cells at the interface plays an important role in the roughening of the interface. Initially cells are highly aligned along the interface, but as time progresses, the cell alignment becomes more anisotropic, and it is the level of anisotropy that is highly correlated with the interface roughening. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A11.00011: Spatiotemporal evolution of bacterial biofilm colonies James Wilking, Stephan Koehler, Naveen Sinha, Agnese Seminara, Michael Brenner, David Weitz Many bacteria on earth live in surface-attached communities known as biofilms. Gene expression in a biofilm is typically varied, resulting in a variety of phenotypes within a single film. These phenotypes play a critical role in biofilm physiology and development. We use time-resolved, wide-field fluorescence microscopy to image triple-labeled fluorescent Bacillus Subtilis colonies grown on agar to determine in a non-invasive fashion the evolving phenotypes. We infer their transition rates from the resulting spatiotemporal maps of gene expression. Moreover, we correlate these transition rates with local measurements of nutrient concentration to determine the influence of extracellular signals on gene expression. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A11.00012: Spatio-temporal Kinetics of Nontypeable \textit{Haemophilus influenzae }(NTHi) Biofilms Aleya Dhanji, Lucia Rosas, William Ray, Ciriyam Jayaprakash, Lauren Bakaletz, Jayajit Das Bacteria can form complex spatial structures known as biofilms. Biofilm formation is frequently associated with chronic infections due to the greatly enhanced antibiotic resistance of resident bacteria. However, our understanding of the role of basic processes, such as bacteria replication and resource consumption, in controlling the development and temporal change of the spatial structure remains rudimentary. Here, we examine the growth of cultured biofilms by the opportunistic pathogen NTHi. Through spatial information extracted from confocal microscopy images, we quantitatively characterize the biofilm structure as it evolves over time. We find that the equal-time height-height pair correlation function decreases with distance and scales with time for small length scales. Furthermore, both the surface roughness and the correlation length perpendicular to the surface growth direction increase with time initially and then decrease. We construct a spatially resolved agent based model beginning with the simplest possible case of a single bacteria species Fisher-Kolmogorov-Petrovsky-Piscounov equation. We show that it cannot describe the observed spatio-temporal behavior and suggest an improved two-species model that better captures the dynamics of the NTHi system. [Preview Abstract] |
Session A12: Invited Session: Phase Trasitions in Biology
Sponsoring Units: DBIOChair: Cliff Brangwynne, Princeton University
Room: 205
Monday, March 3, 2014 8:00AM - 8:36AM |
A12.00001: Epidemics, networks, and percolation Invited Speaker: Mark Newman Diseases spread over networks of physical contact between individuals, and the structure of these networks, along with the biological properties of diseases, dictate patterns of disease spread, risk of infection, sizes of outbreaks, and many other epidemiological quantities. With the help of epidemiological models and a little network theory, this talk will discuss the wide range of behaviors that can appear in epidemiological systems and particularly the many different phase transitions that can occur when one or more diseases spread through a single population. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 9:12AM |
A12.00002: Critical behavior in networks of real neurons Invited Speaker: Gasper Tkacik The patterns of joint activity in a population of retinal ganglion cells encode the complete information about the visual world, and thus place limits on what could be learned about the environment by the brain. We analyze the recorded simultaneous activity of more than a hundred such neurons from an interacting population responding to naturalistic stimuli, at the single spike level, by constructing accurate maximum entropy models for the distribution of network activity states. This -- essentially an ``inverse spin glass'' -- construction reveals strong frustration in the pairwise couplings between the neurons that results in a rugged energy landscape with many local extrema; strong collective interactions in subgroups of neurons despite weak individual pairwise correlations; and a joint distribution of activity that has an extremely wide dynamic range characterized by a zipf-like power law, strong deviations from ``typicality,'' and a number of signatures of critical behavior. We hypothesize that this tuning to a critical operating point might be a dynamic property of the system and suggest experiments to test this hypothesis. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:48AM |
A12.00003: Cooperation, cheating, and collapse in biological populations Invited Speaker: Jeff Gore Natural populations can collapse suddenly in response to small changes in environmental conditions, and recovery from such a collapse can be difficult. We have used laboratory microbial ecosystems to directly measure theoretically proposed early warning signals of impending population collapse. Yeast cooperatively break down the sugar sucrose, meaning that below a critical size the population cannot sustain itself. We have demonstrated experimentally that changes in the fluctuations of the population size can serve as an early warning signal that the population is close to collapse. The cooperative nature of yeast growth on sucrose suggests that the population may be susceptible to ``cheater'' cells, which do not contribute to the public good and instead merely take advantage of the cooperative cells. We confirm this possibility experimentally and find that such social parasitism decreases the resilience of the population. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:24AM |
A12.00004: Organizing the bacterial chromosome for division Invited Speaker: Chase Broedersz The chromosome is highly organized in space in many bacteria, although the origin and function of this organization remain unclear. This organization is further complicated by the necessity for chromosome replication and segregation. Partitioning proteins of the ParABS system mediate chromosomal and plasmid segregation in a variety of bacteria. This segregation machinery includes a large ParB-DNA complex consisting of roughly 1000 ParB dimers, which localizes around one or a few centromere-like \textit{parS} sites near the origin of replication. Despite the apparent simplicity of this segregation machinery as compared to eukaryotic segregations systems, puzzles remain: In particular, what is the nature of interactions among DNA-bound ParB proteins, and how do these determine the organizational and functional properties of the ParB-DNA partitioning complex? A crucial aspect of this question is whether ParB spreads along the DNA to form a filamentous protein-DNA complex with a 1D character, or rather assembles to form a 3D complex on the DNA. Furthermore, it remains unclear how the presence of only one or even a few \textit{parS} sites can lead to robust formation and localization of such a large protein-DNA complex. We developed a simple model for interacting proteins on DNA, and found that a combination of 1D spreading bonds and a 3D bridging bond between ParB proteins constitutes the minimal model for condensation of a 3D ParB-DNA complex. These combined interactions provide an effective surface tension that prevents fragmentation of the ParB-DNA complex. Thus, ParB spreads to form multiple 1D domains on the DNA, connected in 3D by bridging interactions to assemble into a 3D ParB-DNA condensate. Importantly, this model accounts for recent experiments on ParB-induced gene-silencing and the effect of a DNA ``roadblock'' on ParB localization. Furthermore, our model provides a simple mechanism to explain how a single \textit{parS} site is both necessary and sufficient for the formation and localization of the ParB-DNA complex. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 11:00AM |
A12.00005: Critical composition fluctuations in artificial and cell-derived lipid membranes Invited Speaker: Aurelia Honerkamp-Smith Cell plasma membranes contain a mixture of lipid types which can segregate into coexisting liquids, a thermodynamic phenomenon which may contribute to biological functions. Simplified, artificial three-component lipid vesicles can be prepared which display a critical miscibility transition near room temperature. We found that such vesicles exhibit concentration fluctuations whose size, composition, and timescales vary consistently with critical exponents for two-dimensional conserved order parameter systems. However, the critical miscibility transition is also observed in vesicles formed directly from the membranes of living cells, despite their more complex composition and the presence of membrane proteins. I will describe our critical fluctuation measurements and also review a variety of more recent work by other researchers. Proximity to a critical point alters the spatial distribution and aggregation tendencies of proteins, and makes lipid mixtures more susceptible to domain formation by protein-mediated interactions, such as adhesion zones. Recent work suggests that critical temperature depression may also be relevant to the mechanism of anaesthetic action. [Preview Abstract] |
Session A13: Focus Session: Fe based superconductors-Magnetism and Nematicity
Sponsoring Units: DMPChair: Wei-Cheng Lee, University of Illinois
Room: 207
Monday, March 3, 2014 8:00AM - 8:12AM |
A13.00001: Magnetic order without tetragonal symmetry-breaking in the iron pnictides Xiaoyu Wang, Rafael Fernandes In most iron pnictides, the magnetic state is orthorhombic, displaying domains of magnetic stripes with ordering vectors $Q_1=(\pi,0)$ or $Q_2=(0,\pi)$. However, some recent experiments on Mn and Na doped Ba-122 found evidence for magnetic order at $Q_1$ and $Q_2$ without tetragonal symmetry breaking. Such a state corresponds to a spin configuration $S(r)=M_1 e^{iQ_1\cdot r}+M_2e^{iQ_2\cdot r}$ with $|M_1|=|M_2|$, in contrast to the stripe case where either $M_1$ or $M_2$ vanish. Here we discuss possible microscopic mechanisms responsible for this unusual order and its manifestations in the electronic and spin-wave spectra, focusing on the Mn doped compound. We show that the coupling between the itinerant Fe electrons and the Neel fluctuations arising from local Mn moments can give rise to a tetragonal-symmetric magnetic state with $M_1\parallel M_2$ -- a non-uniform state that induces checkerboard charge order. In contrast, the state with $M_1\perp M_2$ is non-collinear and gives rise to peculiar spin-wave modes. These characteristic features can be used to unambiguously identify the magnetic states, without relying on the absence of orthorhombic distortion. The existence of such states implies that tetragonal symmetry breaking is a consequence, not a cause, of magnetism. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A13.00002: Itinerant scenario of magnetism and superconductivity in iron-based superconductors Yu-Zhong Zhang, Ming-Cui Ding, Hai-Qing Lin We will show in this talk that magnetic and superconducting phases of iron-based superconductors can be systematically understood from itinerant weak coupling limit except for the K-doped iron selenides where Fe vacancy order plays dominant roles, FeTe where excess Fe in the interstitial is responsible for the unique bicollinear antiferromagnetic order, and LaFePO where superconducting state at low temperature comes out of competitions of two instabilities between ($\pi$,$\pi$) and ($0$,$0$) which show tendency towards collinear antiferromagnetic state and N\'{e}el ordered anferromagnetic or ferromagnetic state, respectively. We also exhibits that Fermi surface nesting is not a necessary condition for the itinerant magnetism in multi-orbital systems. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A13.00003: Longitudinal spin excitations and magnetic anisotropy in antiferromagnetically ordered BaFe$_2$As$_2$ Yuan Li, Chong Wang, Rui Zhang, Huiqian Luo, Fa Wang, Pengcheng Dai, Louis-Pierre Regnault In the iron-based superconductors, there is an outstanding debate on the microscopic origin of the magnetism, whether it arises from local moments or itinerant electrons with Fermi-surface nesting. To answer this question, we performed a spin-polarized inelastic neutron scattering study of spin waves in the antiferromagnetically ordered state of BaFe$_2$As$_2$. Three distinct excitation components are identified, with spins fluctuating along the $c$-axis, perpendicular to the ordering direction in the $ab$-plane, and parallel to the ordering direction. While the first two ``transverse'' components can be described by a linear spin-wave theory with magnetic anisotropy and inter-layer coupling, the third ``longitudinal'' component is generically incompatible with the local moment picture. It points towards a contribution of itinerant electrons to the magnetism already in the parent compound of this family of Fe-based superconductors. (arXiv:1309.7553) [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A13.00004: Interplay between accidental gap nodes and nematic order in iron-based superconductors Jian Kang, Alexander Kemper, Rafael Fernandes In some iron-based materials, long-range nematic order coexists with superconductivity -- either via a spontaneous tetragonal symmetry-breaking taking place at temperatures above $T_{c}$ or via application of a small external uniaxial strain to detwin the sample. Here we discuss the impact of nematic order on the anisotropic properties of the superconducting state, focusing on the particular case where accidental nodes are present at the electron pockets. Using both a 5-orbital tight-binding model and a phenomenological 3-band model, we investigate how the $d_{xz}$-$d_{yz}$ orbital order triggered by nematic order affects the magnetically-mediated pairing interaction and the gap structure. We find that proximity between $s$ and $d$ superconducting instabilities enhances the effects of nematic order, which may even lift the nodes in one of the electron pockets. We also compute thermodynamic properties in the nematic- superconducting state, such as the penetration depth and the thermal conductivity, and discuss its experimental manifestations. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A13.00005: Double magnetic resonance and spin anisotropy in Fe-based superconductors due to static and fluctuating antiferromagnetic orders Weicheng Lv, Adriana Moreo, Elbio Dagotto Motivated by recent neutron scattering experiments in Fe-based superconductors, we study how the magnetic resonance in the superconducting state is affected by the simultaneous presence of either static or fluctuating magnetic orders using the random phase approximation. We find that for the underdoped materials with coexisting superconducting and antiferromagnetic orders, spin rotational symmetry is explicitly broken at the ordering momentum $\mathbf{Q}_1 = (\pi,0)$. Only the longitudinal susceptibility exhibits the resonance mode, whereas a spin-wave Goldstone mode develops in the transverse component. Meanwhile, at the frustrated momentum $\mathbf{Q}_2 = (0,\pi)$, the susceptibility becomes isotropic in spin space and the magnetic resonance exists for both components. Furthermore, the resonance energies at $\mathbf{Q}_1$ and $\mathbf{Q}_2$ have distinct scales, which provides a natural explanation for the recently observed double resonance peaks. In addition, we show that near optimal doping the existence of strong magnetic fluctuations, which are modelled here via a Gaussian mode, can still induce the spin anisotropy in the magnetic susceptibility. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A13.00006: Nature of magnetic excitations in the electron-doped superconductor BaFe$_{2-x}$Ni$_x$As$_2$ Huiqian Luo, Xingye Lu, Meng Wang, Pengcheng Dai Inelastic neutron scattering experiments are extensively carried out on electron doped BaFe$_{2-x}$Ni$_x$As$_2$ single crystals. The effect of electron doping was found to modify spin waves in the parent compound below $\sim $100 meV and induce a neutron spin resonance at the commensurate AF ordering wave vector that couples with superconductivity. Our careful temperature dependent study of the resonance reveals that the resonance suddenly changes its Q width below Tc and disperses with increasing energy. Upon further electron doping, the resonance becomes weaker and transversely incommensurate at all energies, while spin excitations above $\sim $100 meV are still not much affected. Together with RPA calculation, we conclude that the low energy spin excitations are more likely dominated by itinerant magnetism originating from Fermi surface nesting. In the heavily electron doping x$=$0.3, the low energy spin excitations are totally gapped out below 50 meV. The whole spin spectrum reveal that the low-energy spin excitation coupling with itinerant electron is important for superconductivity, even though the high-energy spin excitations are weakly doping dependent. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:48AM |
A13.00007: Magnetically driven nematicity in the iron-pnictide superconductors Invited Speaker: Joerg Schmalian While the existence of nematic order in iron-based superconductors is now a well-established experimental fact, its origin remains controversial. In this talk we discuss the physical motivation and experimental implications of Ising nematic order caused by magnetic fluctuations. We demonstrate how emergent nematic order and nematic fluctuations are consistent with the overall phase diagram as well as numerous properties of both the normal and superconducting states of the iron pnictides. Due to its magnetic origin, nematic order enhances the strength of magnetic fluctuations and induces a highly anisotropic fluctuation spectrum, leaving distinctive signatures that affect elastic and transport properties, neutron scattering experiments and the NMR spin-lattice relaxation rate. In particular we show that scaling between magnetic and lattice fluctuations provides strong evidence for a magnetically-driven nematicity in the iron-pnictide superconductors. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A13.00008: The effect of magnetic order on the superconducting gap in the co-existence phase of Fe-pnictides Alberto Hinojosa Alvarado, Andrey Chubukov We study the structure and symmetry of the superconducting gap in the presence of spin density wave (SDW) order in iron-based superconductors. We show that SDW order generally induces a spin-triplet component of the gap, in addition to the conventional spin singlet. We further show that, in some range of temperatures below $T_c$, the phases of superconducting order parameters on different reconstructed Fermi surfaces differ by an amount other than 0 or $\pi$, i.e., superconductivity directly reflects the breaking of time-reversal symmetry by SDW order. We specifically consider co-existing SDW and superconducting orders in a model with circular hole pockets and elliptic electronic pockets and present analytical results for the phase diagram and the structure of the superconducting gap at various temperatures. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A13.00009: Resistivity Anisotropy and Novel Impurity-Induced States in Fe-based Superconductors Brian Andersen, Maria Gastiasoro, Peter Hirschfeld We investigate emergent impurity-induced states arising from point-like scatterers in the spin-density wave (SDW) phase of iron-based superconductors within a microscopic five-band model [1]. Independent of the details of the band-structure and disorder potential, it is shown how stable magnetic (pi,pi) unidirectional nematogens are formed locally by the impurities. Interestingly, these nematogens exhibit a dimer structure in the electronic density, are directed along the antiferromagnetic a-axis, and have typical lengths of 10 lattice constants in excellent agreement with recent scanning tunnelling experiments [2]. These electronic dimers provide a natural explanation of the dopant-induced transport anisotropy found e.g. in the 122 iron pnictides [3]. We also study the extension of the (pi,0) SDW state above the putative Neel transition temperature T$_{\mathrm{N}}$ by addition of magnetic impurities. This study is relevant for recent neutron scattering studies showing induced magnetic high-temperature phases for sufficient amounts of Mn substitution in 122 materials [4]. Below T$_{\mathrm{N}}$ neutron studies have found enhanced (pi,pi) scattering which also can be reproduced within our scenario [5]. \\[4pt] [1] M. N. Gastiasoro \textit{et al.}, arXiv:1307.4913 (2013).\\[0pt] [2] M. P. Allan \textit{et al.,} Nat. Phys. \textbf{9}, 220 (2013).\\[0pt] [3] S. Ishida \textit{et al.,} Phys. Rev. Lett. \textbf{101}, 207001 (2013).\\[0pt] [4] D. S. Inosov \textit{et al., }Phys. Rev. B \textbf{87}, 224425 (2013).\\[0pt] [5] G. S. Tucker \textit{et al., }Phys. Rev. B \textbf{86}, 020503 (2012). [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A13.00010: The interfacial effects on the spin density wave in FeSe/SrTiO3 thin film Hai-Yuan Cao, Shiyong Tan, Hongjun Xiang, D.L. Feng, Xin-Gao Gong Recently, the signs of both superconducting transition temperature beyond 60 K and spin density wave (SDW) have been observed in FeSe thin film on SrTiO3 substrate, which suggests a strong interplay between superconductivity and magnetism. With the first-principles calculations, we find that the substrate-induced tensile strain tends to stabilize the SDW state in FeSe thin film by enhancing of the next-nearest-neighbor superexchange antiferromagnetic interaction bridged through Se atoms. On the other hand, we find that when there are oxygen vacancies in the substrate, the significant charge transfer from the substrate to the first FeSe layer would suppress the magnetic order there, and thus the high-temperature superconductivity could occur. In addition, the stability of the SDW is lowered when FeSe is on a defect-free STO substrate due to the redistribution of charges among the Fe 3d-orbitals. Normally, heavy electron doping would kill superconductivity as it suppresses the spin fluctuations as well, but the expanded lattice constants in this system enhance the magnetism and thus preserve the superconductivity. Our results build a foundation for the further exploration of the superconductivity and magnetism in this novel superconducting interface. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A13.00011: Scaling between magnetic and lattice/nematic fluctuations in iron pnictides Rafael Fernandes, Anna B\"ohmer, Christoph Meingast, J\"org Schmalian The origin of the tetragonal-to-orthorhombic transition in the iron pnictides, and its relationship to the magnetically ordered state, remains a subject of intense debate, with potential implications to the mechanism behind the unconventional superconducting state. Here we investigate the coupling between these two normal-state instabilities -- magnetic and structural -- by comparing their corresponding fluctuations in the tetragonal paramagnetic phase of Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$. We find for several doping compositions a robust scaling relation between shear modulus data -- which probes the orthorhombic lattice fluctuations -- and NMR spin-lattice relaxation rate data -- which probes magnetic fluctuations. We explain this scaling using a theoretical model where the tetragonal symmetry breaking is triggered by an electronic nematic transition that emerges from degenerate magnetic fluctuations. Therefore, our results provide strong evidence that the structural transition in the iron pnictides is magnetically-driven. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A13.00012: Effect of SDW order fluctuations on the specific heat jump in iron pnictides Dushko Kuzmanovski, Alex Levchenko, Maxim Khodas, Maxim Vavilov A conjecture about existence of a quantum critical point beneath a superconducting dome has recently attracted attention to the properties of iron-based pnictide superconductors (FeSC) near optimal doping. Spin-density wave fluctuations in the vicinity of the critical point are expected to significantly affect thermodynamics properties of FeSC, including magnetic penetration depth, effective mass, and specific heat. We study the effect of thermal fluctuations of the SDW order on the specific heat jump at the onset of superconducting transition in the iron-based superconductors (FeSCs) based on the minimal two-band model. We find that, beyond mean-field level, the discontinuity of $\Delta C/T_c$ at the tetra-critical point (the end point of the coexistence phase) transforms into a sharp peak. We demonstrate that specific heat jump scales not simply logarithmically with $x - x_c$, as expected for the quantum critical behavior, but it acquires an even more singular power-law dependence. We fit to the experimental data from P. Walmsley $\mathit{et \ al}$., Phys. Rev. Lett. $\mathbf{110}$, 257002 (2013) including this additional term, and the increased goodness of fit suggests significant importance of the latter effect. [Preview Abstract] |
Session A14: Invited Session: Industrial Applications of Olefin Block Copolymers
Sponsoring Units: DPOLYChair: Brent Neal, Milliken and Company
Room: 301-303
Monday, March 3, 2014 8:00AM - 8:36AM |
A14.00001: (Electro)Mechanical Properties of Olefinic Block Copolymers Invited Speaker: Richard Spontak Conventional styrenic triblock copolymers (SBCs) swollen with a midblock-selective oil have been previously shown to exhibit excellent electromechanical properties as dielectric elastomers. In this class of electroactive polymers, compliant electrodes applied as active areas to opposing surfaces of an elastomer attract each other, and thus compress the elastomer due to the onset of a Maxwell stress, upon application of an external electric field. This isochoric process is accompanied by an increase in lateral area, which yields the electroactuation strain (measuring beyond 300{\%} in SBC systems). Performance parameters such as the Maxwell stress, transverse strain, dielectric breakdown, energy density and electromechanical efficiency are determined directly from the applied electric field and resulting electroactuation strain. In this study, the same principle used to evaluate SBC systems is extended to olefinic block copolymers (OBCs), which can be described as randomly-coupled multiblock copolymers that consist of crystallizable polyethylene hard segments and rubbery poly(ethylene-co-octene) soft segments. Considerations governing the development of a methodology to fabricate electroresponsive OBC systems are first discussed for several OBCs differing in composition and bulk properties. Evidence of electroactuation in selectively-solvated OBC systems is presented and performance metrics measured therefrom are quantitatively compared with dielectric elastomers derived from SBC and related materials. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 9:12AM |
A14.00002: Thermoplastic Adhesives based on polyolefin and olefinic copolymers Invited Speaker: Rituparna Paul H.B. Fuller has been a leading global industrial adhesive manufacturer for over 125 years. It is a company with a rich history of consistently delivering adhesive innovations for enhancing product performance in the market place. H.B. Fuller technologies/products find application in several markets including packaging, personal hygiene and nonwovens, durable assembly and electronics. In this presentation, H. B. Fuller's technology innovation journey will be shared with emphasis on groundbreaking technologies/products based on polyolefin and olefin copolymers. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:48AM |
A14.00003: The versatility in morphology and physical properties offered by chain shuttled olefin block copolymers Invited Speaker: Jeffrey Weinhold Chain shuttling catalysis enables the production of olefin block copolymers (OBCs) with a wide range of block compositions. Unique morphology and property combinations can be achieved with highly crystalline hard blocks and low crystallinity or fully amorphous soft blocks. The effect of the amount of comonomer in the soft blocks on phase behavior, morphology and properties will be the focus of this presentation. In one class of materials, the soft blocks contain just enough octene comonomer to give elastic behavior but, unlike a random copolymer-based olefin elastomer, the soft segments are held together by thick crystals formed by the hard blocks. In addition to strengthening the network, these crystals provide temperature resistance and, by solidifying at higher temperature, they allow faster product fabrication. Increasing the soft block's octene content yields the next class of materials which have improved compatibility with polypropylene. This property allows the formation of fine, uniformly-dispersed OBC elastomer particles in PP. Since the impact strength of toughened PP increases as the particle size is reduced, a lower amount of elastomer is required to achieve an application's target for toughness. The direct benefit of lower elastomer loading is an increase in modulus, which enables lightweighting in applications. With further increases in the soft block's octene content, the incompatibility between the hard and soft blocks becomes large enough to cause the OBCs to form ordered melt morphologies. In the solid state, the alternating crystalline and amorphous regions have surprisingly large domain spacings and, due to the difference in refractive index between the domains, the periodicity results in a partial photonic band gap for frequencies in the visible spectrum. Comparisons to the morphology of monodisperse block copolymers and the predictions of theories will be presented. Also, the results of an extension to strong segregation theory will be shown, providing greater insight into the behavior of these polydisperse block copolymers. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:24AM |
A14.00004: TBD Invited Speaker: Miriam Rafailovitch . [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 11:00AM |
A14.00005: Panel Discussion on Industrial Research for Graduate Students and Postdoctoral Researchers Invited Speaker: Brent Neal The session organizers invite all industrial researchers and those who might be interested in a career in industrial research for an open discussion on careers in industry. The topics covered will be flexible based on audience interest, but are expected to include career paths, resume building, networking, interviewing, and transitioning out of academia. [Preview Abstract] |
Session A15: Focus Session: Physics of Sensory Neuroscience
Sponsoring Units: DBIORoom: 304
Monday, March 3, 2014 8:00AM - 8:36AM |
A15.00001: Space, Time, Neural Oscillations and Memory Invited Speaker: Mayank R. Mehta All animals move in space and keep track of time. Hence, they must have a clear percept of space-time. Unlike many sensory processing, space-time is abstract concepts because they can neither be directly felt nor readily controlled. How does the brain, or the ensemble of neurons, create a perception of space-time? This has puzzled scientists for a long time. Research in the past few decades have revealed that individual neurons in a few key brain regions, especially the hippocampus and entorhinal cortex, fire selectively as a function of the subject’s position in space and as a function of elapsed time. In fact, the spatial activity pattern of specific neurons forms a hexagonal lattice. The biophysical mechanisms governing the neural maps of space and time have remained elusive. A primary difficulty has been that when animals walk in the real world, stimuli from various modalities, e.g. sound, light, smell, texture etc., all change at the same time and these changes are difficult to measure, let alone control, precisely. Hence, we have developed a noninvasive, immersive and multisensory virtual reality system where the hardware and software transform the movements of a rat into the evolution of complex stimuli surrounding him to form an audio-visual space. Using this apparatus we have measured the activities of thousands of individual neurons. We then developed analysis techniques to decipher the spatio-temporal activity patterns buried in these neural ensembles, and related the emergent neural dynamics to spatial behavior in the virtual world. Finally we have developed computational models that can capture the emergent neural dynamics to reveal the biophysical mechanisms governing the emergent neural dynamics. This has revealed surprising findings which I will discuss. Specifically, we find that neural oscillations are crucial for perceiving space-time. Further, just as in physical world, spatial representation is relative not absolute. These findings open up unprecedented experimental and theoretical avenues for understanding how neural ensembles perceive space-time and guide complex behavior. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A15.00002: Transformation of stimulus correlations by the retina Jason Prentice, Kristina Simmons, Gasper Tkacik, Jan Homann, Heather Yee, Stephanie Palmer, Phillip Nelson, Vijay Balasubramanian Correlations in the responses of sensory neurons seem to waste neural resources, but can carry cues about structured stimuli and help the brain correct for response errors. To assess how the retina negotiates this tradeoff, we measured simultaneous responses from many retinal ganglion cells presented with natural and artificial stimuli that varied in correlation structure. Responding to spatio-temporally structured stimuli such as natural movies, pairs of ganglion cells were more correlated than in response to white noise checkerboards, but were much less correlated than predicted by a non-adapting functional model of retinal response. Meanwhile, responding to stimuli with purely spatial correlations, pairs of ganglion cells showed increased correlations consistent with a static, non-adapting receptive field and nonlinearity. We found that in response to spatio- temporally correlated stimuli, ganglion cells had faster temporal kernels and tended to have stronger surrounds. These properties of individual cells, along with gain changes that opposed changes in effective contrast at the ganglion cell input, largely explained the pattern of correlations across stimuli. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A15.00003: Dynamics of modularity of neural activity in the brain during development Michael Deem, Man Chen Theory suggests that more modular systems can have better response functions at short times. This theory suggests that greater cognitive performance may be achieved for more modular neural activity, and that modularity of neural activity may, therefore, likely increase with development in children. We study the relationship between age and modularity of brain neural activity in developing children. The value of modularity calculated from fMRI data is observed to increase during childhood development and peak in young adulthood. We interpret these results as evidence of selection for plasticity in the cognitive function of the human brain. We present a model to illustrate how modularity can provide greater cognitive performance at short times and enhance fast, low-level, automatic cognitive processes. Conversely, high-level, effortful, conscious cognitive processes may not benefit from modularity. We use quasispecies theory to predict how the average modularity evolves with age, given a fitness function extracted from the model. We suggest further experiments exploring the effect of modularity on cognitive performance and suggest that modularity may be a potential biomarker for injury, rehabilitation, or disease. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:36AM |
A15.00004: Sensory Coding in Oscillatory Peripheral Receptors Invited Speaker: Alexander Neiman Rhythmical activity have been observed in several types of peripheral sensory receptors, e.g. in senses of hearing, balance and electroreception. We use two examples of spontaneously oscillating peripheral sensory receptors: bullfrog saccular hair cells and electroreceptors of paddlefish, to discuss how oscillations emerge, how these sensors may utilize oscillations to optimize their sensitivity and information processing. In the hair cell system oscillations occur on two very different levels: first, the mechano-sensory hair bundle itself can undergo spontaneous mechanical oscillations and second, self-sustained voltage oscillations across the membrane of the hair cell have been documented. Modelling show that interaction of these two compartment results in enhanced sensitivity to periodic mechanical stimuli. The second example, a single peripheral electroreceptor, is a complex system comprised of several thousands of sensory epithelial cells innervated by a few primary sensory neurons. It embeds two distinct oscillators: one residing in a population of epithelial cells, synaptically coupled to another oscillator residing in a branched myelinated afferent axon. We show how neuronal oscillations emerge in a complex network of excitable nodes. We further demonstrate that epithelial oscillations results in extended serial correlations of neruonal discharges enhancing coding of external stimuli. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A15.00005: Chimera States in Neural Oscillators Sonya Bahar, Tera Glaze Chimera states have recently been explored both theoretically and experimentally, in various coupled nonlinear oscillators, ranging from phase-oscillator models to coupled chemical reactions. In a chimera state, both coherent and incoherent (or synchronized and desynchronized) states occur simultaneously in populations of identical oscillators. We investigate chimera behavior in a population of neural oscillators using the Huber-Braun model, a Hodgkin-Huxley-like model originally developed to characterize the temperature-dependent bursting behavior of mammalian cold receptors. One population of neurons is allowed to synchronize, with each neuron receiving input from all the others in its group (global within-group coupling). Subsequently, a second population of identical neurons is placed under an identical global within-group coupling, and the two populations are also coupled to each other (between-group coupling). For certain values of the coupling constants, the neurons in the two populations exhibit radically different synchronization behavior. We will discuss the range of chimera activity in the model, and discuss its implications for actual neural activity, such as unihemispheric sleep. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A15.00006: Why Internally Coupled Ears (ICE) Work Well J. Leo van Hemmen Many vertebrates, such as frogs and lizards, have an air-filled cavity between left and right eardrum, i.e., internally coupled ears (ICE). Depending on source direction, internal time (iTD) and level (iLD) difference as experienced by the animal's auditory system may greatly exceed [C. Vossen et al., JASA 128 (2010) 909--918] the external, or interaural, time and level difference (ITD and ILD). Sensory processing only encodes iTD and iLD. We present an extension of ICE theory so as to elucidate the underlying physics. First, the membrane properties of the eardrum explain why for low frequencies iTD dominates whereas iLD does so for higher frequencies. Second, the plateau of iTD $=\gamma$ ITD for constant $1 < \gamma < 5$ and variable input frequency $< \nu_{\circ}$ follows; e.g., for the Tockay gecko $\nu_{\circ} \approx 1.5$ kHz. Third, we use a sectorial instead of circular membrane to quantify the effect of the extracolumella embedded in the tympanum and connecting with the cochlea. The main parameters can be adjusted so that the model is species independent. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:36AM |
A15.00007: Using the structure of natural scenes and sounds to predict neural response properties in the brain Invited Speaker: Michael DeWeese The natural scenes and sounds we encounter in the world are highly structured. The fact that animals and humans are so efficient at processing these sensory signals compared with the latest algorithms running on the fastest modern computers suggests that our brains can exploit this structure. We have developed a sparse mathematical representation of speech that minimizes the number of active model neurons needed to represent typical speech sounds. The model learns several well-known acoustic features of speech such as harmonic stacks, formants, onsets and terminations, but we also find more exotic structures in the spectrogra representation of sound such as localized checkerboard patterns and frequency-modulated excitatory subregions flanked by suppressive sidebands. Moreover, several of these novel features resemble neuronal receptive fields reported in the Inferior Colliculus (IC), as well as auditory thalamus (MGBv) and primary auditory cortex (A1), and our model neurons exhibit the same tradeoff in spectrotemporal resolution as has been observed in IC. To our knowledge, this is the first demonstration that receptive fields of neurons in the ascending mammalian auditory pathway beyond the auditory nerve can be predicted based on coding principles and the statistical properties of recorded sounds. We have also developed a biologically-inspired neural network model of primary visual cortex (V1) that can learn a sparse representation of natural scenes using spiking neurons and strictly local plasticity rules. The representation learned by our model is in good agreement with measured receptive fields in V1, demonstrating that sparse sensory coding can be achieved in a realistic biological setting. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A15.00008: Microstate description of stable chaos in networks of spiking neurons Maximilian Puelma Touzel, Monteforte Michael, Fred Wolf Dynamic instabilities have been proposed to explain the decorrelation of stimulus-driven activity observed in sensory areas such as the olfactory bulb, but are sensitive to noise. Simple neuron models coupled through inhibition can nevertheless exhibit a negative maximum Lyapunov exponent, despite displaying irregular and asynchronous (AI) activity and having an exponential instability to finite-sized perturbations above a critical strength that scales with the size, density and activity of the circuit [1]. This stable chaos, a phenomenon first found in coupled-map lattices, produces a large, finite set of locally-attracting, yet mutually-repelling AI spike sequences ideally suited for discrete, high-dimensional coding. We analyze the effects of finite-sized perturbations on the spiking microstate and reveal the mechanism underlying the stable chaos. From this, we can analytically derive the aforementioned scaling relations and estimate the critical value of previously observed transitions to conventional chaos. This work highlights the features of intra-neuron dynamics and inter-neuron coupling that generate this phase space structure, which might serve as an attractor reservoir that downstream networks can use to decode sensory input.\\[4pt] [1] Monteforte, M. \& Wolf, F., PRX 2, 1(2012). [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A15.00009: ERP Energy and Cognitive Activity Correlates Michael Jay Schillaci, Jennifer M.C. Vendemia We propose a novel analysis approach for high-density event related scalp potential (ERP) data where the integrated channel-power is used to attain an energy density functional state for channel-clusters of neurophysiological significance. The method is applied to data recorded during a two-stimulus, directed lie paradigm and shows that deceptive responses emit between 8\% and 10\% less power. A time course analysis of these cognitive activity measures over posterior and anterior regions of the cortex suggests that neocortical interactions, reflecting the differing workload demands during executive and semantic processes, take about 50\% longer for the case of deception. These results suggest that the proposed method may provide a useful tool for the analysis of ERP correlates of high-order cognitive functioning. We also report on a possible equivalence between the energy functional distribution and near-infrared signatures that have been measured with other modalities. [Preview Abstract] |
Session A16: Focus Session: Continuum Description of Discrete Materials
Sponsoring Units: GSNPChair: Kenneth Kamrin, Massachusetts Institute of Technology
Room: 401
Monday, March 3, 2014 8:00AM - 8:12AM |
A16.00001: Bipotential continuum models for granular mechanics Joe Goddard Most currently popular continuum models for granular media are special cases of a generalized Maxwell fluid model, which describes the evolution of stress and internal variables such as granular particle fraction and fabric,in terms of imposed strain rate. It is shown how such models can be obtained from two scalar potentials, a standard elastic free energy and a ``dissipation potential" given rigorously by the mathematical theory of Edelen. This allows for a relatively easy derivation of properly invariant continuum models for granular media and fluid-particle suspensions within a thermodynamically consistent framework. The resulting continuum models encompass all the prominent regimes of granular flow, ranging from the quasi-static to rapidly sheared, and are readily extended to include higher-gradient or Cosserat effects. Models involving stress diffusion, such as that proposed recently by Kamrin and Koval ({\it PRL} {\bf 108} 178301), provide an alternative approach that is mentioned in passing. This paper provides a brief overview of a forthcoming review articles by the speaker ({\it The Princeton Companion to Applied Mathematics}, and {\it Appl. Mech. Rev.},in the press, 2013). [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A16.00002: Grain fragmentation in sheared granular flow: weakening effects, energy dissipation, and strain localization Charles K.C. Lieou, Ahmed E. Elbanna, Jean M. Carlson We describe the shear flow of a disordered granular material subject to grain fracture using the shear-transformation-zone (STZ) theory of amorphous plasticity adapted to systems with a hard-core inter-particle interaction. To this end, we develop the equations of motion for this system within a statistical-thermodynamic framework analogous to that used in the analysis of molecular glasses. For hard-core systems, the amount of internal, configurational disorder is characterized by the compactivity $X = \partial V / \partial S_C$, where $V$ and $S_C$ are respectively the volume and configurational entropy. Grain breakage is described by a constitutive equation for the temporal evolution of a characteristic grain size $a$, based on fracture mechanics. We show that grain breakage is a weakening mechanism, significantly lowering the flow stress at large strain rates, if the material is rate-strengthening in character. We show in addition that if the granular material is sufficiently aged, spatial inhomogeneity in configurational disorder results in strain localization. We also show that grain splitting contributes significantly to comminution at small shear strains, while grain abrasion becomes dominant at large shear displacements. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A16.00003: A granular-continuum model of channelization in sedimentary layers by sub-surface flow Vikrant Yadav, Arshad Kudrolli We discuss experiments where channels form in a quasi-two dimensional bed of consolidated granular particles by fluid flow. A continuum three phase model was developed recently [A. Mahadevan, A.V. Orpe, A. Kudrolli, and L. Mahadevan, EPL, 2012] which shows that channels can develop from small differences in packing in an otherwise homogeneous medium which leads to increased porosity and nonlinear feedback. To build on this model, an erodible porous medium composed of millimeter scale grains and Bentonite clay was prepared in a Hele-Shaw cell. The cohesive strength between the grains is directly proportional to the amount of clay binder. When water is pumped through this porous medium, the binder dissolves and loose beads are advected out of the erodible medium, and an initially uniform flow of water through the porous medium gets localized into channels over time. We will discuss the measured integrated rates of erosion as well as the statistical development of heterogeneity and comparison with the three-phase model as a function of binding strength and consolidation of the medium. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A16.00004: Rate and age-dependence of shear yield stress in loose sphere packings Greg Farrell, Narayanan Menon Frictional packings of hard spheres can be stable in loose volume fractions well below random close packing. We study the stability of these solids to shear perturbations in the little-studied regime close to the random loose packing limit [1]. We present experimental data on the shear yield stress as a function of rate and age in sedimented loose packings of frictional, non-cohesive, PMMA spheres. The yield stress is found to depend on both the rate of strain and age of the packing since last breakage, both to approximately the positive one-third power. The regime of elastic response at finite strain-rate is insensitive to the viscosity of the interstitial fluid. With this common choice of materials and preparation conditions, no rate-independent elastic regime was seen, even at the smallest strains experimentally achieved. [1] Farrell GR, Martini KM, and Menon N, \emph{Soft Matter}, \textbf{6}, 2925 (2010). [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A16.00005: Continuum modeling of diffusion and dispersion in dense granular flows Ivan C. Christov, Howard A. Stone Continuum modeling of granular flows remains a challenge of modern statistical physics. Granular materials do not perform Brownian motion, yet diffusion and shear dispersion can be observed in such systems when agitation causes inelastic collisions between particles. In a number of canonical flow regimes (e.g., in a rotating container or down an incline), granular materials can behave like fluids. We formulate and solve the granular counterparts to two basic fluid mechanics problems: diffusion of a pulse and shear dispersion of a pulse for dense granular materials in rapid flow. We provide a theory to account for the concentration-dependent diffusivity of bidisperse granular mixtures, and we give an asymptotic argument for the self-similar behavior of such a diffusion process for which an exact self-similar analytical solution does not exist. For shear dispersion, we show that the effective dispersivity of the depth-averaged concentration of the dispersing powder varies as the P\'eclet number squared, as in classical Taylor--Aris dispersion of molecular solutes. The calculation is extended to generic shear profiles, showing a significant enhancement for convex profiles due to the shear-rate dependence of the diffusivity of granular materials. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A16.00006: Connecting the behavior of granular layers on inclined planes to the nonlocal fluidity model Ken Kamrin, David Henann Recently, a grain-size-sensitive rheology for granular flow has been proposed based on the nonlocal fluidity concept. While primarily intended to describe the effect that grain size has on developed flow fields, this talk will show how the same framework also explains the Hstop phenomenon commonly observed in thin granular layers on inclined planes, in which thinner layers appear to be stronger than thicker ones. Moreover, the experimental phase diagram for flow vs no-flow of a layer of glass beads in this geometry is well-predicted using the same modeling parameters that describe the steady flow of those beads in split-bottom cells and other geometries. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A16.00007: Collisional Diffusion of Granular Materials: From Creep to Rapid Flow Paul Umbanhowar, Yi Fan, Julio Ottino, Richard Lueptow The diffusion of granular material is driven by random collisions between particles and quantified by the diffusion coefficient, $D$. We computationally study the dependence of $D$ on local shear rate, $\dot{\gamma}$, from the dense flow regime to the creep flow regime in open and closed heap flows. Measurements of $D$ obtained for both geometries, monodisperse and bidisperse systems, various flow rates, and at different streamwise positions collapse onto a single curve when plotted vs.\ $\dot{\gamma}\bar{d}^2,$ where $\bar{d}$ is the local mean particle diameter. In the dense flow regime, where $\dot{\gamma}$ is larger, $D$ is proportional to $\dot{\gamma}\bar{d}^2$, similar to previous studies. However, in the creep flow regime, where $\dot{\gamma}$ is smaller, $D$ is independent of $\dot{\gamma}.$ The solids fraction and velocity fluctuations are also constant in this regime. Further study of the effect of gravity on $D$ shows that it determines the transition between rate-dependent and rate-independent regimes and controls the value of $D$ in the creep regime. These results demonstrate that the shear rate is not the relevant time scale in the creeping flow regime. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A16.00008: Microscopic Order Parameter for Shear Anisotropy for Systems near Shear Jamming Robert Behringer, Dong Wang, Jie Ren, Joshua Dijksman Sheared granular systems at packing fractions between $\phi_S \le \phi \leq \phi_J$ can exist in states with zero and nonzero stress. When a system, prepared in a stress-free states in this density range, is sheared, it exhibits first fragile, then shear jammed states, both having high stress and fabric anisotropy. The onset of shear jammed states resembles an order-disorder transition. In recent work, we showed that the order appears in a force space (Bi et al. PRL 2013). Here, we identify an order parameter associated with individual particles, making it possible to construct spatial correlations. We identify local (particle-scale) order with $\Gamma$, the deviatoric part of the force-moment tensor. This is a real symmetric, traceless matrix characterized by two coefficients, a and b, such that $\Gamma = aU_1+bU_2$, and where $U_1$ is diagonal with elements $\pm1$, and $U_2$ has 0's on the diagonal, and 1 for the off-diagonal elements. The $U_i$'s are orthogonal under an appropriate scalar product. Then, $(a,b)$ provides a vector particle-scale order parameter. $\Gamma$ is additive over all particles, and is analogous to the magnetization in a spin system. Also, particles with orthogonal shear stresses now correspond to anti-parallel vectors. We use this representation to investi [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A16.00009: Shear-induced rigidity in athermal materials Bulbul Chakraborty, Sumantra Sarkar In this talk, we present a minimal model of rigidity and plastic failure in solids whose rigidity emerges directly as a result of applied stresses. Examples include shear-jamming (SJ) in dry grains and discontinuous shear thickening (DST) of dense non-Brownian suspensions. Both SJ and DST states are examples of non-equilibrium, self-assembled structures that have evolved to support the load that created them. These are strongly-interacting systems where the interactions arise primarily from the strict constraints of force and torque balance at the local and global scales. Our model is based on a reciprocal-space picture that strictly enforces the local and global constraints, and is, therefore, best suited to capturing the strong correlations in these non-equilibrium systems. The reciprocal space is a tiling whose edges represent contact forces, and whose faces represent grains. A separation of scale between force fluctuations and displacements of grains\footnote{ Sumantra Sarkar et al, Phys. Rev. Lett. 111, 068301 (2013)} is used to represent the positional disorder as quenched randomness on variables in the reciprocal space. Comparing theoretical results to experiments, we will argue that the packing fraction controls the strength of the quenched disorder. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:24AM |
A16.00010: Granular Materials by Design Invited Speaker: Heinrich Jaeger Granular materials are large amorphous aggregates of discrete, individually solid particles. One of the key issues has long been how to link particle-level properties in a predictive manner to the behavior of the aggregate as a whole. In particular, the shape of particles has been recognized as important factor, with smooth spherical shapes known to behave quite differently from angular or faceted ones. However, except for a small set of simple convex shapes, very little detailed knowledge exists that allows one to predict aggregate mechanical response from individual particle properties. Furthermore, for actually designing a granular material, the inverse problem needs to be solved: for a given desired overall mechanical response, the task becomes finding the appropriate particle-level properties. This talk discusses recent experiments on a wide range of convex and non-convex particle shapes in an effort to provide a baseline for modeling the effect of non-sphericity on parameters such as the effective Young`s modulus or yield stress of a granular material. It also discusses a new approach to tackle the inverse problem by bringing concepts from artificial evolution to granular materials design, making it possible to find with high efficiency the shapes best adapted to a given goal. These results have general applicability and open up wide-ranging opportunities for materials optimization and discovery. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A16.00011: Random Organization of Suspensions: Geometry versus Hydrodynamics Emmanouela Filippidi, Alexandre Franceschini, Paul Chaikin, David Pine Suspensions of athermal spheres at moderate volume fractions (0.2-0.4) under slow periodic strain undergo a phase transition from an absorbing to an active state despite the low Reynolds number regime of the flow imposed. In the absorbing state, the particles return to their original positions after every cycle, while in the active steady state, they appear diffusive. To explain the scaling near the transition and explore its universality class, we propose to replace the spherical particle with an effective particle whose shape depends on strain. We experimentally measure the particle pair correlation and the time evolution of the rheology and the stress-strain curves. The pair correlation is compared to the one expected for our effective particle and the time evolution curves are compared to theoretical existing models. While the geometrical approach of the effective particle captures the main physics of the system, it overestimates the effects. The consideration of hydrodynamics seems essential in understanding the finer details and the stresses in the suspension. Together, reductionist geometrical approach and detailed hydrodynamics provide a more complete picture in understanding the observed critical phase transition. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A16.00012: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 10:48AM - 11:00AM |
A16.00013: Continuum modeling of mechanically-induced creep using the nonlocal fluidity model David Henann, Ken Kamrin Recently, the nonlocal fluidity model applied to granular materials has successfully been used to predict the size of flow features in a wide variety of flow configurations, including all variations of the split-bottom cell as well as other geometries. A related problem in granular flow is that of mechanically-induced creep, in which shear deformation in one region of a granular medium fluidizes quiescent regions far from the sheared zone. This enables creep deformation when a force is applied in the quiescent region through an intruder such as a cylindrical or spherical probe. In this talk, we show that the nonlocal fluidity model is capable of describing this phenomenology. Specifically, we explore the creep of a rod in an annular Couette cell and show that the model captures all salient features observed in experiments. [Preview Abstract] |
Session A17: Focus Session: Strong Correlations in Systems Far from Equilibrium I
Sponsoring Units: GSNPChair: Michel Pleimling, Virginia Polytechnic Institute and State University
Room: 402
Monday, March 3, 2014 8:00AM - 8:12AM |
A17.00001: Localization Protected Quantum Order Rahul Nandkishore, David Huse, Shivaji Sondhi, Vadim Oganesyan, Arijeet Pal Closed quantum systems with quenched randomness exhibit many-body localized regimes wherein they do not equilibrate even though prepared with macroscopic amounts of energy above their ground states. We show that such localized systems can order in that individual many-body eigenstates can break symmetries or display topological order in the infinite volume limit. Indeed, isolated localized quantum systems can order even at energy densities where the corresponding thermally equilibrated system is disordered, i.e.: localization protects order. In addition, localized systems can move between ordered and disordered localized phases via non-thermodynamic transitions in the properties of the many-body eigenstates. We give evidence that such transitions may proceed via localized critical points. We note that localization provides protection against decoherence that may allow experimental manipulation of macroscopic quantum states. We also identify a `spectral transition' involving a sharp change in the spectral statistics of the many-body Hamiltonian. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A17.00002: Many-body Localization and Symmetry Protected Topological Order Vedika Khemani, Anushya Chandran, C.R. Laumann, S.L. Sondhi Recent work shows that highly excited many-body localized eigenstates can exhibit broken symmetries and topological order, including in dimensions where such order would be forbidden in equilibrium. We extend this analysis to discrete symmetry protected order via the explicit examples of the Haldane phase of one dimensional spin chains and the topological Ising paramagnet in two dimensions. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A17.00003: Signatures of electron-magnon interaction in charge and spin current in magnetic tunnel junctions: A nonequilibrium many-body perturbation theory approach Farzad Mahfouzi, Branislav Nikolic We develop a numerically exact scheme for resumming certain classes of Feynman diagrams in the perturbation expansion for the electron and magnon self-energies of the nonequilibrium Green function (NEGF) formalism applied to electron-magnon (e-m) interacting system driven out of equilibrium by finite bias voltage. This is then employed to understand the effect of inelastic e-m scattering on current-voltage ({\em I--V)} characteristics of F/I/F (F-ferromagnet; I-insulating barrier) magnetic tunnel junctions (MTJs). For this purpose, we evaluate self-consistently Fock diagram for the electron self-energy which ensures charge current conservation (i.e., sum of charge currents in all leads must be zero), as well as electron-hole polarization bubble diagram for magnon self-energy, where respective GF lines within these diagrams are the fully interacting ones. Furthermore we present the formulation to calculate the Fano factor in correlated systems out of equilibrium and then investigate the effect of e-m coupling on noise in MTJs. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A17.00004: Propagation of wave pakets in 1d spin systems - ballistic, diffusive, many-body localized? Christoph Karrasch, Fabian Heidrich-Meisner, Jens Bardarson, Frank Pollmann, Joel Moore We study the propagation of local spin wave pakets in one-dimensional XXZ spin chains in presence of disorder. We employ a time-dependent finite-temperature density matrix renormalization group algorithm. For clean chains, the spin density spreads ballistically in the Luttinger liquid phase and diffusively in the gapped phase. We investigate the interplay of interactions and disorder and discuss how (and if) metallic and many-body localized phases manifest in this setup. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A17.00005: Double-pulse deexcitations in a one-dimensional strongly correlated system Takami Tohyama, Hantao Lu, Janez Bonca We investigate the ultrafast optical response of the one-dimensional half-filled extended Hubbard model exposed to two successive laser pulses [1]. By using the time-dependent Lanczos method, we find that following the first pulse, the excitation and deexcitation process between the ground state and excitonic states can be precisely controlled by the relative temporaldisplacement of the pulses. The underlying physics can be understood in terms of a modified Rabi model. Our simulations clearly demonstrate the controllability of ultrafast transition between excited and deexcited phases in strongly correlated electron systems.\\[4pt] [1] H. Lu, J. Bonca, and T. Tohyama, EPL {\bf 103}, 57005 (2013). [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A17.00006: Driven-dissipative Bose-Einstein condensation: perturbative field-theoretic renormalization group approach Sebastian Diehl, Uwe C. Tauber The universal critical behavior of the driven-dissipative non-equilibrium Bose condensation transition is investigated employing the field-theoretic renormalization group method. Such criticality may be realized in broad ranges of driven open systems on the interface of quantum optics and many-body physics, from exciton-polariton condensates to cold atomic gases. The starting point is a noisy and dissipative Gross-Pitaevski equation corresponding to a complex valued Landau-Ginzburg functional, which captures the near critical non-equilibrium dynamics, and generalizes Model A for classical relaxational dynamics with non-conserved order parameter. We confirm and further develop the physical picture previously established by means of a functional renormalization group study of this system. Complementing this earlier numerical analysis, we analytically compute the static and dynamical critical exponents at the condensation transition to lowest non-trivial order in the dimensional $\epsilon$ expansion about the upper critical dimension $d_c = 4$, and establish the emergence of a novel universal scaling exponent associated with the non-equilibrium drive. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:48AM |
A17.00007: Exciton reactions on carbon nanotubes: an experimental testbed for critical dynamics Invited Speaker: Jeremy Allam The one-dimensional coalescing random walk ($X+X\to X)$ is a paradigmatic reaction-diffusion system due to both its exact solvability [1,2] and the experimental observation of nonclassical kinetics at asymptotically long times [3]. The solvability rests on the anticommutative property of intersecting trajectories of particles that react \textit{instantly} and at \textit{short range}: however, the validity of these assumptions in real systems has not previously been tested by experiment. We have shown that exciton-exciton recombination (fusion) on carbon nanotubes provides a platform for quantitative studies of critical kinetics in a simple non-equilibrium system [4]. Under high excitation density we observed a crossover in the exciton density $n$ between regimes of classical $(n\propto t^{-1})$ and anomalous $(n\propto t^{-1/2})$ scale invariance as predicted by renormalization group [5] and approximate [1] theories, arising from a finite reaction probability of $\approx 0.2$ per encounter. At long times the exciton population per nanotube exponentially approaches unity (i.e. a finite size effect), allowing calibration of the exciton density and hence a demonstration of universality extending over both classical and critical regimes. Under low excitation, the early kinetics followed a Smoluchowski-Noyes form ${dn} \mathord{\left/ {\vphantom {{dn} {dt}}} \right. \kern-\nulldelimiterspace} {dt}\propto n^{2}t^{-1/2}$ rather than the asymptotic ${dn} \mathord{\left/ {\vphantom {{dn} {dt}}} \right. \kern-\nulldelimiterspace} {dt}\propto n^{3}$, providing direct evidence for the spatial self-ordering that precedes critical scale invariance. We studied the re-emergence of microscopic detail at the classical-nonclassical crossover, which is abrupt and nonmonotonic due to competition between temporal and spatial averaging of critical fluctuations (i.e. finite reaction rate and range). It appears that real-world experiments will require more complete descriptions of the interactions than is available in existing models.\\[4pt] [1] D. Ben-Avraham and S. Havlin, \textit{Diffusion and Reactions in Fractals and Disordered Systems} (Cambridge University Press, Cambridge, 2000).\\[0pt] [2] G. M. Sch\"{u}tz, Phase Transitions and Critical Phenomena 19, 1 (2001).\\[0pt] [3] J. Prasad and R. Kopelman, Phys. Rev. Lett. 59, 2103 (1987).\\[0pt] [4] J. Allam et al., Phys. Rev. Lett. 111, 197401 (2013).\\[0pt] [5] U. C. T\"{a}uber et al., \textit{J. Phys. A: Math. Gen.} 38, R79--R131 (2005). [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A17.00008: On local temperatures near absolute zero in nonequilibrium quantum systems Abhay Shastry, Justin Bergfield, Charles Stafford The local temperature of a quantum conductor with source at finite temperature and drain at or near absolute zero is investigated, a problem outside the scope of linear response theory. The local temperature is defined by the measurement of a floating thermoelectric probe. It is shown that cold spots with local temperatures near absolute zero exist within the system, and the applicability of the third law of thermodynamics is investigated. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A17.00009: Critical exponents describing non-stationary $1/f$ noise for intermittent quantum dots Sanaz Sadegh, Eli Barkai, Diego Krapf Semiconductor quantum dots (QDs) exhibit bright fluorescence, but this emission switches randomly between ``on'' and ``off'' states that are distributed according to universal power laws. This scale-free dynamics is responsible for weak ergodicity breaking and non-stationarity. Such stochastic processes yield a power spectrum of the form $S(f)=A/f^{\beta}$. Power spectrum analysis is a superior method for studying the properties of QD emission because it does not depend on the arbitrary determination of a threshold, typically used in the discrimination between ``on'' and ``off'' states. Recently, intriguing predictions have been made about the power spectrum aging properties and the role of finite measurement time. To test these predictions, we study the emission power spectra from 1200 QDs at room temperature. We find that five exponents are needed to describe the power spectrum properties, namely spectral exponent, power spectrum aging, cutoff frequency, zero frequency spectrum, and total power. We also compare our results with numerical simulations and explain observed discrepancies based on the combined action of Gaussian noise and the truncation of the ``on''-time distribution. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A17.00010: Many-body localization in the quantum random energy model Chris Laumann, Arijeet Pal The quantum random energy model is a canonical toy model for a quantum spin glass with a well known phase diagram. We show that the model exhibits a many-body localization-delocalization transition at finite energy density which significantly alters the interpretation of the statistical ``frozen'' phase at lower temperature in isolated quantum systems. The transition manifests in many-body level statistics as well as the long time dynamics of on-site observables. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A17.00011: The effects of magnetic field and temperature quenches on non-equilibrium relaxation properties of vortex lines in type-II superconductors Hiba Assi, Ulrich Dobramysl, Michel Pleimling, Uwe T\"{a}uber Technological applications of type-II superconductors require a deep understanding of the dynamics of vortex matter in these complex materials. We model vortices in the London limit as interacting elastic lines, and simulate their dynamics employing a Langevin molecular dynamics (LMD) algorithm. This LMD algorithm is utilized to investigate the non-equilibrium relaxation properties of interacting lines, subject to randomly-placed point or correlated columnar pinning sites, by studying various two-time correlation functions. We consider experimentally-motivated initial conditions by applying quenches in the system temperature or the magnetic field, which is realized by suddenly adding or removing vortex lines from the system. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A17.00012: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 10:48AM - 11:00AM |
A17.00013: A General Model for Brownian Vortexes Henrique Moyses, Ross Bauer, David Grier Brownian Vortexes are a class of non-equilibrium steady states that arise from the motion of Brownian particles in non-conservative force fields. At non-zero temperature the non-conservative part of the force bias the particles' fluctuations into probability currents, which due to the conservation of probability should feature closed loops. Previous studies have shown that Brownian Vortexes comprise a distinct class of stochastic processes whose direction and topology of the developed flux can be tuned by changing the temperature of the system. Here we present a general model for Brownian Vortexes that is based on a perturbation theory scheme of the Fokker - Planck equation to get the probability distribution and non-equilibrium steady state flux of such system. This generalized model features the temperature induced probability flux reversal and topological changes characteristic of this stochastic system in the case where the non-conservative part of the force is small compared to the conservative one. We further compare the theoretically predicted results with numerical simulations and propose an experimental test system based on the motion of colloidal particles in optical traps. [Preview Abstract] |
Session A18: Soft Glassy Materials
Sponsoring Units: DCMP GSNP DPOLYChair: Gary Hunter, New York University
Room: 403
Monday, March 3, 2014 8:00AM - 8:12AM |
A18.00001: A Long-Lived Oscillatory Space-Time Correlation Function of Two Dimensional Colloids Jeongmin Kim, Bong June Sung Diffusion of a colloid in solution has drawn significant attention for a century. A well-known behavior of the colloid is called Brownian motion : the particle displacement probability distribution (PDPD) is Gaussian and the mean-square displacement (MSD) is linear with time. However, recent simulation and experimental studies revealed the heterogeneous dynamics of colloids near glass transitions or in complex environments such as entangled actin, PDPD exhibited the exponential tail at a large length instead of being Gaussian at all length scales. More interestingly, PDPD is still exponential even when MSD was still linear with time. It requires a refreshing insight on the colloidal diffusion in the complex environments. In this work, we study heterogeneous dynamics of two dimensional (2D) colloids using molecular dynamics simulations. Unlike in three dimensions, 2D solids do not follow the Lindemann melting criterion. The Kosterlitz-Thouless-Halperin-Nelson-Young theory predicts two-step phase transitions with an intermediate phase, \textit{the hexatic phase} between isotropic liquids and solids. Near solid-hexatic transition, PDPD shows interesting oscillatory behavior between a central Gaussian part and an exponential tail. Until 12 times longer than translational relaxation time, the oscillatory behavior still persists even after entering the Fickian regime. We also show that multi-layered kinetic clusters account for heterogeneous dynamics of 2D colloids with the long-lived anomalous oscillatory PDPD. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A18.00002: Categorizing Dense Attractive 2D Colloidal Packings using Vibrational Modes and Local Structure Matthew Lohr, Tim Still, Kevin Aptowicz, Ye Xu, Matthew Gratale, Arjun Yodh In this work, we investigate the microscopic dynamics of quasi-2D dense attractive colloidal systems. We confine bidisperse polystyrene spheres between glass coverslips in a suspension of water and 2,6-lutidine; as we increase the temperature of the sample into a critical regime, lutidine wets the colloids, creating a strong attractive interaction (greater than 4kT). We track the particle locations via bright-field video microscopy and analyze the dynamics of packings at various packing fractions. Subsequent calculations of the vibrational modes of the systems demonstrate a hallmark of ``glassy'' vs. ``gel-like'' behavior at low frequencies. Specifically, we observe a sudden increase in the density of low-frequency modes with decreasing packing fraction. These modes appear to be coupled to collective motion of large groups of particles. Additionally, there is evidence that this change in vibrational behavior is tied to a packing's local structure. By investigating the correlation between collective vibrations and local packing, we take a significant step towards delineating ``gel'' and ``attractive glass'' states in attractive, dense disordered systems. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A18.00003: Microscopic and mechanical roles of disorder in a jammed polycrystal Nathan Keim, Paulo Arratia We present experiments on the relationship between the microscopic structure of a polycrystalline 2D solid and its response to deformation. The material is a monolayer of mutually repulsive particles adsorbed at an oil-water interface, for which we simultaneously measure bulk mechanical response (oscillatory shear rheology) and image the motion of many individual particles. Crystallinity is varied through changes to materials and preparation. We investigate the role of less-ordered regions (e.g. grain boundaries) in the system's response to deformation, including the locations of particle rearrangements as deformation amplitude is increased. We consider the extent to which even a small amount of disorder makes elastic deformation, energy dissipation, and yielding behaviors similar to those of highly disordered materials. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A18.00004: Flowing properties of quasi-2D emulsions in Couette geometry Carlos Orellana, Eric Weeks We study the flow of dense emulsion in a quasi-two-dimensional Couette geometry. Our samples are oil-in-water emulsions confined between two close-spaced parallel plates, so that the droplets are deformed into pancake shapes.~By means of microscopy, we measure the droplet positions and their deformation,~which is related to the stress on the individual droplet. In this system without static coulomb friction, we observe a continuous transition from affine motion to topological rearrangements that span the whole system as the area fraction is increased. We compare our results to granular experiments and simulations. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A18.00005: Equilibrium and non-equilibrium aggregation in two dimensional systems with competing interactions Mahesh Bandi, Tamoghna Das A two dimensional system of mono-disperse particles with competing short range attraction and long range repulsion is numerically investigated. Keeping the competing interaction strength fixed at low temperature and density, a dynamical transition from an equilibrium to a non-equilibrium state can be achieved by tuning the repulsion length alone. This is accompanied by a structural transition from non-compact (equilibrium) to compact (non-equilibrium) aggregates. Whereas strong bonding is responsible for non-compact cluster formation, caging dynamics in compact clusters result in non-exponential relaxation characteristic of glassy behaviour. With increasing temperature, the non-equilibrium aggregation gives way to an ergodic liquid. With increasing density, the system undergoes a geometric transition into a percolating gel state, independent of temperature and repulsion length. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A18.00006: Experimental signature of the self-caging in quiescent colloidal glasses Minh Triet Dang, Sanne Loenen, Katharine Jensen, Rojman Zargar, Daniel Bonn, Peter Schall Glasses have liquid-like structure, but solid-like properties. Here, we use colloidal glasses to directly visualize particle configurations in glasses and supercooled liquids. In hard-sphere systems, the particle configurations provide a unique route to the free energy of the system, determined by geometry only. We determine the free volumes of the particles, and directly relate to their free energy changes. We observe two different length scales of free volume distribution in space and a long-range correlation of local free energy of colloidal glasses. This is the first experimental signature self-caging of colloidal glasses, which indicates the first-order phase transition in glasses. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A18.00007: Soft Spots in Aging Colloidal Glasses Moyosore Odunsi, Craig Maloney, Eric Weeks We track the movement of particles in colloidal glasses using confocal microscopy. Colloidal glasses are out of equilibrium and so they age: the motion of the particles slows down over time. The aging is initiated by stirring the sample. In our aging samples, we examine the motion of the particles on small time scales to infer positions of soft spots (localized regions of low-frequency vibrational modes). We then look at correlations between these soft spots and subsequent particle rearrangements on larger time scales. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A18.00008: 3D confinement effect on diffusive behaviors of dense colloidal suspensions Bo Zhang, Xiang Cheng We design an experimental system to investigate three-dimensional (3D) confinement effect on dense colloidal suspensions. By solidifying the aqueous phase of an oil-in-water emulsion, we achieve a 3D confinement with no-slip boundary conditions. Fast confocal microscopy is used to image dynamics of colloidal particles at different volume fractions and confinement lengths. We systematically measure particles' mean square displacement (MSD) and the system's overlap factor proposed in the random first order transition theory. Based on these measurements, a confinement ``phase diagram'' is constructed. We find a strong confinement effect for suspensions at moderate volume fractions with the confinement length smaller than 10 particle diameters. Finally, we also compare our results with other 3D confined systems with different boundary conditions including systems with slip boundaries and with fixed particle boundaries. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A18.00009: Mechanical response of a colloidal glass undergoing repeated local perturbation Tim Still, Ye Xu, Kevin Aptowicz, Arjun Yodh If an amorphous solid is deformed beyond a certain threshold, it undergoes rearrangements on a microscopic level. Often these rearrangements are irreversible and the glassy material finds a new minimum in the energy landscape. However, if the glass is repeatedly perturbed with a moderate cyclical deformation, the mechanical response of the glass can evolve from irreversible to reversible. In our experiments, we utilize colloidal particles with strong thermophoretic properties and local laser heating to generate singular and periodic local non-homogeneous perturbations in quasi-two-dimensional colloidal glasses. The individual particles are soft and deformable, and the elasticity of the material induces mechanical recovery when laser heating ceases. Optical microscopy and particle tracking allow us to follow the path of each individual particle and determine the reversibility and affinity of the mechanical response on a single particle level. This enables us to investigate the microscopic mechanisms of energy dissipation in model glasses and sheds light on the onset of mechanical failure in disordered materials. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A18.00010: Stretched-exponential relaxation in sheared non-Brownian suspensions Joseph Paulsen, Sidney Nagel Relaxations in glasses are often approximated by a stretched exponential. Many models to explain this behavior posit a heterogeneous spread of single exponential processes with a wide distribution of relaxation times. Here, we study the approximately stretched-exponential relaxation that we find in a model, developed by Cort\'{e} et al.~[1], of sheared non-Brownian suspensions. Using a one-dimensional version with a variety of interaction rules [2], we investigate how the wide spectrum of relaxation timescales originates from density fluctuations in the initial (random) configuration of particles. Our theoretical arguments are in good agreement with numerical simulations and reveal a functional form for the relaxation that is distinct from, but well-approximated by, a stretched exponential. \\[4pt] [1] L. Cort\'{e}, P. M. Chaikin, J. P. Gollub, and D. J. Pine, Nature Phys. 4, 420 (2008). \newline [2] N. C. Keim, J. D. Paulsen, and S. R. Nagel, Phys. Rev. E 88, 032306 (2013). [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A18.00011: Yielding of glasses -- a dynamic first-order transition? Peter Schall, Triet Dang, Bernd Struth, Dmitry Denisov We use a new combination of x-ray scattering and rheology to elucidate the yielding of glasses. By combining dynamic structure factor measurements with oscillatory rheology, we can resolve structural changes during the yielding of colloidal glasses. Surprisingly, we find a sharp symmetry change in the structure factor upon yielding, signaling a first order transition of the glass. This symmetry change is accompanied by a sharp change of fluctuations from non-Gaussian to Gaussian distributions of the scattered intensity. We interpret these observations as a new dynamically induced first order transition from a solid- to a liquid-like state of the glass. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A18.00012: Emergence of cooperativity in plasticity of soft glassy materials J\'er\^ome Crassous, Antoine Le Bouil, Axelle Amon, Sean McNamara The elastic coupling between plastic events is generally invoked to interpret plastic properties and failure of amorphous soft glassy materials. We report an experiment where we observe that plastic zones form structures of growing size as the system approaches failure. For this we impose a homogeneous stress on a granular material, and measure local deformations for very small strain increments using a light scattering setup. We observe non-homogeneous strains that appear as small line segments of mesoscopic size that lengthen as the system approaches failure. The line segments have a well defined orientation clearly distinct from macroscopic shear band that appears at failure. The presence and the orientation of those localized deformations may be understood by considering how a localized plastic reorganizations redistribute stresses in a surrounding continuous elastic medium. The mesostructure of the plastic deformation before failure and the presence of plastic events are confirmed by numerical simulations. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A18.00013: Two step yielding of core-shell microgels: An investigation of ``hard'' and ``soft'' cage yielding mechanisms via Rheo-SANS Javoris Hollingsworth, Zhi Zhou, Song Hong, Guangmin Wei, He Cheng, Charles Han The yielding mechanism of hybrid microgels composed of a polystyrene (PS) core and thermosensitive poly(N-isopropylacrylamide) (PNIPAM) shell were studied via small-angle neutron scattering with rheological measurements (rheo-SANS). While the PS core attributes to the hard sphere properties of the microgels, the soft sphere properties are due to their PNIPAM shell; softness increases as a function of temperature and decreases with PNIPAM shell thickness. By varying the core-shell ratio or temperature, a series of particles ranging from near-hard sphere to typical soft microgels were obtained. Generally, the first yielding event occurs when short range interactions (bonds between interconnected or local clusters) are broken, whereas the second yielding event is due to the breaking of long range interactions (nearest-neighbor ``cages''). According to the results, near-hard sphere suspensions exhibit single-step yielding; however, suspensions at intermediate core-shell ratios display two-step yielding characteristics. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A18.00014: Effects of particle softness on shear thickening of microgel suspensions He Cheng, Zhi Zhou, Charles Han A series of microgel particles composed of a polystyrene (PS) core and thermo-sensitive poly (N-isopropylacrylamide) (PNIPAM) shell with different shell thicknesses were investigated to elucidate the effect of microgel softness on its shear thickening behavior. Since the softness of the microgels increases with decreasing temperature through the volume phase transition effect of PNIPAM shell, the measured softness parameter, n, which is derived from the Zwanzig-Mountain equation, was used to measure and describe the combined influences of temperature and shell thickness. According to our results, the softness parameter is able to estimate the shear thickening behavior of microgel suspensions at least semi-quantitatively. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A18.00015: ABSTRACT WITHDRAWN |
Session A19: Focus Session: Theory and Simulations of Macromolecules I - Self Consistent Field Theory
Sponsoring Units: DPOLYChair: Roland Faller, University of California, Davis
Room: 404
Monday, March 3, 2014 8:00AM - 8:12AM |
A19.00001: Fluctuating Field-Theoretic Polymer Simulations of Multispecies Melts and Composites Kris Delaney, Wei Li, Dominik Duechs, Glenn Fredrickson We discuss computational strategies for conducting efficient and stable beyond-mean-field simulations of complex multi-species block polymer melts and composites, with composition fluctuations included through complex Langevin sampling. Our framework is applicable to assemblies of polymer chains of a variety of architectures with interactions introduced through a matrix of Flory-Huggins parameters. We demonstrate the stability, efficiency, and accuracy of a multi-species exchange mapping, and apply the method to the study of fluctuation-induced microphase structures in blends and composites. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A19.00002: Theoretical Basis of Monte-Carlo Field Theoretic Simulations David Morse, Mark Matsen, Pawel Stasiak Monte-Carlo field theoretic simulations (MC-FTS) of incompressible polymer models rely on a rather poorly understood approximation to the complex-Langevin field theoretic simulation (CL-FTS) method of Fredrickson and coworkers, but yield results that appear to be both surprisingly accurate and much more easily interpreted than the results of CL-FTS simulations. Specifically, two of us (Stasiak and Matsen) have shown [1] that results of CL-FT simulations exhibit a simple dependence on spatial resolution (grid-size) that can analytically corrected for to obtain results that are independent of grid spacing, in which the relationship between the effective $\chi$ parameter and the simulation input parameter is given by a simple analytic formula. We give theoretical analysis that explains the accuracy of the method, the nature of its errors, and the reasons for this simple dependence on spatial resolution. We show that a much more complicated dependence on spatial resolution or interaction range is expected in full CL-FT simulations. Our analysis suggests a simple modification of the CL-FT method that should improve its accuracy without effecting its efficiency or other favorable properties. [1] P. Stasiak and M.W. Matsen, {\it Macromolecules} {\bf 46}, 8037 (2013) [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A19.00003: Directing the self-assembly of copolymers into a metastable complex network phase via a deep and rapid quench Marcus Mueller, De-Wen Sun The free-energy landscape of self-assembling copolymer systems is characterized by a multitude of metastable minima. Using particle-based simulations of a soft, coarse-grained model we explore opportunities to reproducibly direct the spontaneous ordering of these self-assembling systems into a metastable complex network morphology -- specifically, Schoen's I-WP periodic minimal surface -- starting from a highly unstable state that is generated by a rapid expansion. This process-controlled self-assembly provides an alternative to fine-tuning molecular architecture or blending for fabricating complex network structures. Comparing our particle-based simulation results to recently developed free-energy techniques we critically assess their ability to predict spontaneous formation and highlight the importance of non-equilibrium molecular conformations in the starting state and the local conservation of density. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A19.00004: A massively parallel space-time formulation for SCFT David Ackerman, Baskar Ganapathysubramanian We present a massively parallel, scalable Self Consistent Field Theory framework for modeling multi block copolymers. This is based on a finite-element based real-space implementation - which enables investigating complex, non-periodic domains - integrated into a space-time formulation. A space time formulation allows the implementation of a posteriori error analysis to ensure rigorous error bounds on the propagator. The space-time formulation increases the computation problem size but dramatically enhances the scalability of the problem. We show scaling up to 45,000 processors. The system remains tractable through the use of high order integration schemes which allow a coarser chain model while retaining accuracy of lower order schemes. This framework is applied to rod-coil diblock copolymers utilizing a worm-like chain model. Results of this modeling on complex surfaces (spheres, tori) are presented. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A19.00005: Short Polymer Modeling using Self-Consistent Integral Equation Method Yeongyoon Kim, So Jung Park, Jaeup Kim Self-consistent field theory (SCFT) is an excellent mean field theoretical tool for predicting the morphologies of polymer based materials. In the standard SCFT, the polymer is modeled as a Gaussian chain which is suitable for a polymer of high molecular weight, but not necessarily for a polymer of low molecular weight. In order to overcome this limitation, Matsen and coworkers have recently developed SCFT of discrete polymer chains in which one polymer is modeled as finite number of beads joined by freely jointed bonds of fixed length. In their model, the diffusion equation of the canonical SCFT is replaced by an iterative integral equation, and the full spectral method is used for the production of the phase diagram of short block copolymers. In this study, for the finite length chain problem, we apply pseudospectral method which is the most efficient numerical scheme to solve the iterative integral equation. We use this new numerical method to investigate two different types of polymer bonds: spring-beads model and freely-jointed chain model. By comparing these results with those of the Gaussian chain model, the influences on the morphologies of diblock copolymer melts due to the chain length and the type of bonds are examined. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A19.00006: Liquid-state integral equations via the self-consistent field approach Issei Nakamura, Zhen-Gang Wang We develop liquid-state integral equations via the self-consistent field approach. Taking electrolytes and van der waals fluids as examples, we provide a generic procedure to bridge the longstanding gap between the self-consistent field theory and other multi-scale theories such as the density functional theory and the integral equation theory, and thus a major improvement on the statistical field theory. Our new self-consistent filed theory simultaneously accounts for many important features in soft matters, the molecular interactions at an atomic scale that are expressed beyond a mean-field form, the equation of state, the liquid-vapor phase coexistence, and the oscillatory behavior of the pair distribution function in a liquid phase and the monotonic decay of the pair distribution function in a gas phase. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A19.00007: Effective potentials for multiscale representations of polymer melts Marina Guenza, James McCarty, Jeremy Copperman, Anthony Clark Numerically optimized reduced descriptions of macromolecular liquids often present thermodynamic inconsistency with atomistic level descriptions even if the total correlation function, i.e. the structure, appears to be in agreement. We present an analytical expression for the effective potential between a pair of coarse-grained units, for a polymer liquid where each chain is represented as a collection of interpenetrating soft coarse-grained units, with a variable number of units, $n_b$, of size $N_b$. The potential is characterized by a long tail, slowly decaying with characteristic scaling exponent of $N_b^{1/4}$. This general result applies to any coarse-grained model of polymer melts with units larger than the persistence length. It is our contention that with a reasonable molecular model along with the correct parameters, both structural and thermodynamic properties can be simultaneously preserved in coarse-graining, at variable length of the unit size, without the need of recurring to any numerical re-optimization scheme. The effect of the potential on the structural and dynamical properties of polymer melts will be illustrated. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A19.00008: Coherent States Formulation of Polymer Field Theory Xingkun Man, Kris Delaney, Michael Villet, Henri Orland, Glenn Fredrickson We introduce a stable and efficient complex Langevin scheme to enable the first direct numerical simulations of the coheret-states (CS) formulation of polymer field theory. In contrast with Edwards' well known auxiliary-field framework, the CS formulation does not contain an embedded nonlinear, non-local, implicit functional of the auxiliary fields and the action of the field theory has a fully explicit, semi-local and finite-order polynomial character. We present the route for deriving CS canonical ensemble theories, and a method for studying asymptotically long polymer chains with composition fluctuations fully included using a simplified field theory in the ground-state-dominance approximation. The formalism is potentially applicable for conducting systematic coarse-graining and numerical renormalization-group studies [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A19.00009: Semi-flexible polymer brush confined in a nanoslit: A high performance single chain in mean field simulation study Jiuzhou Tang, Xinghua Zhang, Dadong Yan We develop a single chain in mean field simulation method based on worm-like chain model for investigating the compression effect of semi-flexible polymer brush. For the commonly used self-consistent field theory (SCFT) based on the Gaussian chain model, the compression of the polymer brush leads to an isotropic deformation of the chain. However, for the case of high grafting density, the nematic phase will be formed even in a flexible brush. SCFT with gaussian chain model cannot provide any prediction on this property. Therefore, all the effects from the compression of nematic phase were totally ignored in the previous theoretical studies. In present work, the response of nematic polymer brush to the compression along the nematic axis is studied by applying a high performance single chain in mean field simulation. Our results predict that for the semi-flexible polymer chain under compression, the nematic order director of polymer brush reorient, leading to a XY-model-like lateral rotational symmetry breaking. The quasistatic analysis of the compressing and the relaxing of the confinement indicates that this symmetry breaking corresponds to a first order phase transition. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A19.00010: Exponential Time Differencing Methods for Numerical Self-Consistent Field Theory Yi-Xin Liu, Hong-Dong Zhang We present a fast and accurate numerical method for self-consistent field theory (SCFT) studies of polymer systems. For polymers in bulk, periodic boundary conditions are used. For confined polymers, the confining walls with or without preferential interactions with polymers, are modeled by appropriate non-periodic boundary conditions, which avoids the use of surface field terms and the mask technique in a conventional approach. Then the modified diffusion equations subject to these boundary conditions are solved by an exponential time differencing method with Fourier collocation and Chebyshev collocation for periodic and non-periodic boundary conditions, respectively. It exhibits fourth-order accuracy in time and spectral accuracy in space. The performance of this method is examined in comparison with the operator splitting pseudospectral methods. Numerical experiments show that the time differencing method is more efficient than the operator splitting methods in high accuracy SCFT calculations. Applications of this method to polymer brushes, block copolymers both in bulk and under confinement are demonstrated. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A19.00011: DPD with effective pair potential from integral equation theory of molecular liquids Alexander Kobryn, Dragan Nikoli\'{c}, Olga Lyubimova, Sergey Gusarov, Andriy Kovalenko A coarsening method of soft matter systems in solution is presented, in which the coarse grained (CG) force field is determined based on the statistical mechanical, integral equation theory of molecular liquids in interaction site representation, also known as reference interaction site model (RISM). Coarse graining is accomplished by a structure-matching procedure for solute CG beads without solvent that reproduces the corresponding distribution of all-atom solute in solvent obtained from RISM. Termed as an effective pair potential, the introduced potential of interaction between CG beads includes the effect of solvent and is used in dissipative particle dynamics (DPD) instead of the conservative force potential defined heuristically. It enables high flexibility in specifying the composition of solute CG beads and allows excluding solvent from explicit consideration in DPD. The suggested CG molecular model has been tested computationally and is shown to be a useful tool in investigating both structural and dynamic properties of polymer solutions and a promising platform for studies of macromolecular, supramolecular, and biomolecular systems in solution that require thermodynamic consistency, high accuracy, and computational efficiency. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A19.00012: Coarse-grain Tunable Dissipative Particle Dynamics: Droplet Dynamics in Micro- and Nano-emulsions Arman Boromand, Joao Maia Due to the multiscale phenomena in droplet dynamics from single droplets to highly concentrated emulsions (HCE) on one hand and complexity of the problem on the other hand, there is a need for a mesoscale simulation technique to capture the right underlying physics in these systems. This makes Dissipative Particle Dynamics (DPD) a suitable candidate, since it is capable of capturing microscopic phenomena and provide comparison to macroscopic simulations and experiments, within a reasonable calculation time compared to Molecular Dynamics (MD). In this presentation, we focus on the interplay between droplet size and the stress level in shear flows for three different combinations: Newtonian/non-Newtonian droplet in Newtonian/non-Newtonian matrix. The geometrical changes in these three cases will be compared to macroscopic models and experimental results and the validity of this mesoscopic simulation will be discussed. In addition, the dependency of surface tension to droplet size in the case of Newtonian droplets in Newtonian matrix is shown and the effect of this term on the dynamics of nanoscopic droplets is discussed. For the case of non-Newtonian droplets, their dynamics is studied for polymer chains with different molecular weight and chain characteristics. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 11:00AM |
A19.00013: Self-consistent field theory of wormlike chains and its applications in polymer physics Invited Speaker: Jeff Z.Y. Chen In recent years, a whole body of knowledge has been built on the structural and conformational properties of polymers, based on the Gaussian description of polymer statistics. Whether we are dealing with a single polymer or spatially inhomogeneous polymer systems, the solution of the self-consistent field theory has been one of the central focuses of theoretical studies. A wormlike-chain model, which differs from a Gaussian-chain model by using a bond-bending energy rather than monomer-monomer stretching energy, is more suitable for dealing with semiflexible polymers; a self-consistent field theory can be built for systems where both spatial inhomogeneity and orientational ordering need simultaneously considered. The wormlike chain model captures a number of physical features of a polymer system that go beyond those described by a Gaussian-chain model. A wormlike-chain based self-consistent field theory can be used to study the structural properties within the length scale smaller than the persistence length and the length scale where the polymer is strongly extended to an almost fully stretched conformation. As well, such a theory can be used to study a system where the orientational properties of polymer segments are important such as a liquid-crystal system. In this talk, we review the progress in solving the self-consistent field theory of wormlike chains for various physical problems and specifically discuss two recent examples of the applications of the theory: (a) the influence of chain rigidity on the phase diagram of AB diblock copolymers [Y. Jiang and J. Z. Y. Chen, Phys. Rev. Lett. \textbf{110}, 138305 (2013); Phys. Rev. E \textbf{88}, 042603 (2013) ]; and (b) liquid-crystal defect structures in confined geometry [J. Z. Y. Chen, Soft Matter \textbf{9}, 10921 (2013)]. As well, we address the question that under what physical conditions, the self-consistent field theory of wormlike chains recovers the theory of Gaussian chains. [Preview Abstract] |
Session A20: Focus Session: Microfluidics and Nanofluidics I - The Physics of Confined Fluids
Sponsoring Units: DPOLY DFD GSNPChair: German Drazer, Rutgers University
Room: 405
Monday, March 3, 2014 8:00AM - 8:12AM |
A20.00001: Processing Cyclic Peptide-polymer Conjugates in Block Copolymer Thin Films for Sub-nm Porous Membranes Chen Zhang, Ting Xu Porous thin films containing subnanometer channels oriented normal to the surface exhibit unique transport and separation properties and can serve as selective membranes for separation. Inspired by natural protein channels, we have developed an approach using cyclic peptide nanotubes (CPNs) embedded in polymeric matrix to mimic the transport of natural channels. The co-assembly of polymer-covered CPNs in a block copolymer (BCP) thin film requires the synchronization of two self-assembly processes, namely the microphase separation of BCP and the nanotube growth of CP-polymer conjugates. We systematically investigated the co-assembly of isolated CP-poly(ethylene glycol) (CP-PEG) conjugates and polystyrene-b-poly (methyl methacrylate) (PS-b-PMMA) in thin films as a function of CP-PEG loading (f$_{CP-PEG}$) and solvent-polymer interactions. We find that there is a strong dependence of the co-assembly process on f$_{CP-PEG}$ due to thermodynamic limit of incorporating one CPN in one PMMA microdomain, as well as the kinetic pathway in which favorable PEG-solvent interaction helps to disperse CPNs and thus lowers the activation energy barrier of the system. This study presents critical insights in guided assemblies of functional building blocks within nanoscopic frameworks. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A20.00002: Tunable water desalination across Graphene Oxide Frameworks Adrien Nicolai, Vincent Meunier ``Water, water, everywhere, nor any drop to drink.'' wrote Samuel Taylor Coleridge in 1798. Today's scientific advances in water desalination promise to change the second part of the sentence into ``and every drop to drink,'' by transforming sea water into fresh water and quench the thirst of 1.2B people facing shortages of water. To achieve this, the design of nanoporous materials with high water permeability and coupled with high salt rejection capacity is crucial. Graphene Oxide Frameworks (GOF) materials are a class of porous materials consisting of layers of graphene oxide sheets interconnected by linear boronic acid linkers. Water desalination across GOF is studied using classical Molecular Dynamics simulations. We used quantum mechanically obtained boron-related force field parameters to study the diffusion of water molecules inside bulk GOF. Properties, such as the self-diffusion coefficient of water molecules increases linearly with linker concentration $n$. Further, the desalination performance of GOF membranes reveals that the water permeability of GOF is several orders of magnitude higher than conventional membranes and an high water permeability can be coupled with a 100{\%} efficiency of salt rejection by choosing the appropriate concentration $n$ and thickness $h$. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A20.00003: Quantized Water Transport: Ideal Desalination through Graphyne-4 Membrane Chongqin Zhu, Hui Li, Xiao Cheng Zeng, E.G. Wang, Sheng Meng The shortage of clean and fresh water is one of most pervasive problems afflicting human being's life in the world. Desalination is one viable solution to produce clean water, since 98{\%} of the available water in the form of salty water. Using molecular dynamics simulations, we demonstrate that graphyne sheet exhibits promising potential for nanoscale desalination to achieve both high water permeability and salt rejection rate. In addition, Graphyne sheets also are mechanically robust with high tolerance to deformation. Especially, $\gamma $-graphyne-4 has the best performance with 100{\%} slat rejection and an unprecedented water permeability of $\sim$ 13L/cm2/day/MPa. 3 orders of magnitude higher than prevailing commercial membranes based on reverse osmosis, and $\sim$ 10 times higher than the state-of-the-art nanoporous graphene. Strikingly, water permeability across graphyne exhibits unexpected nonlinear dependence on the pore area. This counter-intuitive behavior is attributed to the quantized nature of water flow at the nanoscale, which has wide implications in controlling nanoscale water transport and designing highly effective membrane. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A20.00004: Electro-Induced Dewetting and Concomitant Ionic Current Avalanche in Nanopores Xikai Jiang, Jingsong Huang, Bobby Sumpter, Rui Qiao Electrically driven ionic transport of room-temperature ionic liquids (RTILs) through nanopores is studied by molecular dynamics simulations. It is observed that a gradual dewetting transition occurs in nanopores originally wetted by RTILs if the applied voltage is increased, and meanwhile the ionic current through the system increases sharply. These phenomena originate from the solvent-free nature of RTILs in which the ions' mobility increases sharply when their concentration decreases and are contrary to the transport of conventional electrolytes through nanopores. The results also show that the amplification of ionic current is possible by manipulating the properties of the nanopore and RTILs and is especially pronounced in charged nanopores. The results highlight the unique physics of nonequilibrium transport of RTILs in confined geometries and point to potential experimental approaches for manipulating ionic transport in nanopores, which can benefit diverse techniques including nanofluidic circuitry and nanopore analytics. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A20.00005: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 9:00AM - 9:12AM |
A20.00006: Green-Kubo relation and hydrodynamic tails of friction at solid/liquid interfaces Kai Huang, Izabela Szlufarska Understanding boundary conditions at the liquid/solid (L/S) interface has been a subject of many scientific investigations. It also has important implications for design of materials for such applications as micro-/nanofluidics. Design of functionalized surfaces and interfaces with optimized friction and slip properties is hindered by existing challenges in measuring these properties either in experiments or in simulations. Here, we have developed a Green-Kubo (GK) relation that enables accurate calculations of friction at L/S interfaces directly from equilibrium molecular dynamics (EMD) simulations and that provides a pathway to bypass the time scale limitations of typical non-equilibrium molecular dynamics (NEMD) simulations. The theory has been validated for a number of different of interfaces and it is demonstrated that the L/S slip is an intrinsic property of an interface. Because of the high numerical efficiency of our method, it opens up new opportunities for computational design of functionalized surfaces for L/S applications. Details of the friction correlation function also permit a full analysis of the time-dependent and frequency-dependent friction in a dynamic system. At the hydrodynamic time scale, the memory kernel of the friction coefficient exhibits an algebraic decay, which leads to a -3/2 power long time tail in the velocity autocorrelation function of fluid particles near a wall. This behavior differs from the predictions of previous theoretical and simulation results, which employed no-slip boundary conditions. Our findings provide new insights into understanding the dynamics of interfacial colloids and nano-particle flow in liquids. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A20.00007: The interplay between apparent viscosity, wettability, and slip in nanoconfined water Elisa Riedo, Deborah Ortiz, Hsiang-Chih Chiu, Suenne Kim Understanding and manipulating fluids at the nanoscale is a matter of growing scientific and technological interest. Here we show that the viscous shear forces in nanoconfined water can be orders of magnitudes larger than in bulk water if the confining surfaces are hydrophilic, whereas they greatly decrease when the surfaces are increasingly hydrophobic. This decrease of viscous forces is quantitatively explained with a simple model that includes the slip velocity at the water surface interface. The same effect is observed in the energy dissipated by a tip vibrating in water perpendicularly to a surface. Comparison of the experimental data with the model shows that interfacial viscous forces and compressive dissipation in nanoconfined water can decrease up to two orders of magnitude due to slippage. These results offer a new understanding of interfacial fluids, which can be used to control flow at the nanoscale. NATURE COMMUNICATIONS (2013) \textbar DOI: 10.1038/ncomms3482 [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A20.00008: Unsteady electrokinetic microfluidics with hydrodynamic slippage effect Myung-Suk Chun, Yoona Yang The nature of low Reynolds number flows and confined spaces inherent in microscale or extended nanoscale channels imply the significant influence of solid wall boundaries. We investigate the unsteady pulsatile electrokinetic flows by extending our previous simulations concerning electrokinetic microfluidics. The body force originated from between the nonlinear Poisson--Boltzmann field and the flow-induced electric field is employed in Navier--Stokes equation, and Nernst--Planck equation in connection with the net current conservation is further coupled. Our explicit model allows one to quantify the effects of time delay, oscillating frequency, and conductance of the Stern layer, considering the fluid slippage at hydrophobic surfaces and the electric double layer interaction. This presentation reports new results regarding the implication of pressure pulsations toward realizing mechanical to electrical energy transfer with high conversion efficiency. A combined role of the fluid slippage and conductance of channel wall is examined to obtain possible enhancements of streamwise velocity and streaming potential, with taking advantage of pulsating pressure field. Note that our framework can serve as a useful basis for micro/nanofluidics design. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A20.00009: Effects of ionic strength on nonlinear electrophoretic mobility of fd virus in solid-state nanopore Wang Miao, Liping Liu, Anna Lu, Hongwen Wu, Prerna Sharma, Zvonimir Dogic, Xinsheng Ling We report an experimental study of electrophoretic mobility of rod-like \textit{fd} virus in solid-state nanopores. It is found that the velocity $v$ is a nonlinear function of the electric filed E, and can be described by $v = \mu ^{\mathrm{(1)}}$E $+ \mu^{(3)}$E$^{3}$. In addition to the linear Smoluchowski term, there is a second term with cubic dependence on E which has been described as a Stotz-Wien effect caused by the polarization of the Debye counter ion cloud. Here we report a study of this nonlinear electrophoresis effect under different salt concentrations. We found that at low ionic strength, the cubic mobility term becomes less pronounced. The origin of this observation will be discussed. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A20.00010: Water confinement in three different substances Sahar Mirshamsi, Hai-Ping Cheng Confined water in nano-pores of different materials appears in geological, physical, industrial and biological systems. Confined water demonstrates significantly different behavior than bulk water, which has motivated researchers to study the effects of confinement on structural and dynamical properties of water. We study the confinement of water in silica, carbon nanotubes, and gold nano-pores and compare the effect of these different materials on the properties of water. Compared to bulk water viscosity, we find that the viscosity of water increases in silica nano-pores but decreases when confined in carbon nanotubes. Increasing water density inside the silica nano-pores further increases water viscosity. Finally, we discuss how the diffusion coefficient of water and its density profile changes due to confinement. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A20.00011: On the Strong Localization and Rapid Time Scales of Superheating and Vapor Nucleation in Nanopores Edlyn Levine, Gaku Nagashima, David Hoogerheide, Michael Burns, Jene Golovchenko Extreme localized superheating and homogeneous vapor nucleation have recently been demonstrated in thin, solid state nanopores. Electrolytic solution present within the pore is superheated to well above its boiling point by ohmic heating from ionic current driven through the pore. Continued heating of the metastable liquid can eventually lead to explosive nucleation of a vapor bubble in the pore. Here we report on the consistency of theoretical predictions with experimental results concerning the thermal, spatial and temporal scales involved. Calculations demonstrate that extreme spatial localization of the temperature distribution is achieved in the nanopore heating experiments. Our results indicate that the liquid at the center of the pore can be rapidly superheated to several hundred degrees kelvin above the boiling point within tens of microseconds. The temperature within the pore is shown to increase by about 100K from the edge to the center of a 60nm radius pore. This degree of localization strongly indicates that vapor nucleation is homogeneous due to the high temperature dependence of the kinetic nucleation rate. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A20.00012: Observation of Superheating and Single Bubble Nucleation in Thin, Solid State Nanopores Gaku Nagashima, Edlyn Levine, David Hoogerheide, Michael Burns, Jene Golovchenko We demonstrate localized and extreme superheating, and homogeneous single bubble nucleation in a nanopore in a thin silicon nitride membrane immersed in an electrolyte solution. The high temperatures are achieved by Joule heating from a highly focused ionic current induced to flow through the pore by a modest voltage bias applied across the membrane. The superheating of the electrolyte is observed by monitoring a change in electrical conductance of the system which increases with temperature. The high temperatures can lead to transient explosive vapor bubble nucleation in the pore. The nucleation event is detected both electronically and optically. The bubble nucleation event is highly deterministic and reproducible. Optical transmission experiments indicate that the bubble nucleation is homogeneous, occurring near the pore center. These phenomena have been observed in pores down to 60 nm in radius. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A20.00013: Escape of water molecular from Carbon Nanotubes Jiaxi Li, Wenfeng Li, Jianwei Zhang Understanding and controlling the transport of water molecules through nanopores have attracted great interest due to potential applications for designing novel nanofluidic devices, machines and sensors. In this work, we theoretically investigate the effects of an external nonuniform electric field on the escape of water molecules through single-walled carbon nanotubes (SWNTs) by using of molecular dynamics (MD) simulations. When polar water molecules are placed in the gradient electric field, the electric force is experienced that can drive the water molecules. Molecular dynamics simulations show that the escape probability of water obeys the Boltzmann distribution. Our results show that energy barrier delta E is independent of temperature which indicates that it is a single-barrier system. From the MD results statistics, the key parameters could be determined such that the relationship between energy barrier delta E and diameter of SWNTs and nozzle distance of the charge r would be revealed that could deepen our current theoretical understanding on transport of water molecular inside SWNTs with the nonuniform electric field. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A20.00014: Multiscale transport concept for nanoporous materials incorporating microstructure and interface properties at nanoconfinement Arturas Ziemys, Miljan Milosevic, Milos Kojic Transport theories based on the continuum hypothesis may not be appropriate, especially in case of diffusion, due to surface effects at nanoscale. Our computational and experimental findings, supported by studies elsewhere, revealed the necessity to account for interface and confinement effects. Thermodynamic aspects were established that might be responsible for reduced diffusivity at interface; more specifically -- due to entropy-enthalpy compensation and cage-breaking processes. The thickness of liquid with altered diffusivity at solid-liquid interface depends on material and diffusing molecule nature and properties. We have developed a concept and computational model to bridge those molecular effects within nanoconfinement with transport at macro scale for systems where interface dominates over other properties (e.g. nanochannels, nanopores, polymers). The concept was validated against molecular transport through nanochannels and polymers. Novel parameters are introduced that determine diffusion regime and kinetics within the nanoscale confined fluids. New diffusion transport characteristics are established when nanochannel confining dimension approaches sizes of diffusing molecules, determining bounds of the non-Fickian transport regimes. The developed multiscale method could be used to study material transport and optimize nanoporous materials for biomedical and industrial applications. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A20.00015: Continuum-based multiscale approach to predict the structure and theromdynamic properties of confined fluids S.Y. Mashayak, N.R. Aluru We present a continuum-based theory to predict the structure and thermodynamic properties of fluids confined in multiple length-scales, ranging from few Angstroms to micron length channels. In this work, we introduce a free energy functional for classical DFT (cDFT) based on the empirical potential-based quasi-continuum theory (EQT). EQT is a simple and fast approach to predict the inhomogeneous density and potential profiles of confined fluids, and the results from EQT compare well with MD simulations. Using the density and potential profiles from EQT, we construct a grand potential functional for cDFT. EQT-cDFT based grand potential can be used to predict various thermodynamic properties of confined fluids. Here, we demonstrate applicability of the EQT-cDFT approach by simulating water confined inside slit-like channels of graphene at various thermodynamic states and channel widths. Using EQT-cDFT approach, we calculate the structure and thermodynamic properties of confined water, such as density profiles, adsorption, PMF profiles, surface tension, local pressure profiles, and solvation forces. It is found that the EQT-cDFT results compare well with the reference water MD simulation results. [Preview Abstract] |
Session A21: Focus Session: Soft Nanoparticles, Block Copolymer Micelles, and Polymersomes I
Sponsoring Units: DPOLYChair: Darrin Pochan, University of Delaware
Room: 406
Monday, March 3, 2014 8:00AM - 8:36AM |
A21.00001: Solution Self-Assembly of Globular Protein-Polymer Conjugate Block Copolymers Invited Speaker: Bradley Olsen Controlling the nanostructured self-assembly of globular proteins and enzymes can significantly advance the applications of soft materials as catalysts, sensors, and medical materials. However, the incorporation of globular proteins as one block in the block copolymer introduces changes in chain shape, chain entropy, and specific interactions that significantly impact the thermodynamics of self-assembly. Here, we explore the self-assembly of model globular protein-polymer block copolymers in concentrated solutions to form nanostructured materials. A phase diagram as a function of concentration and temperature for a model material mCherry-poly(N-isopropylacrylamide) (PNIPAM) is asymmetric, showing hexagonal cylinders for coil fractions less than 0.5 and a lamellar ordering for coil fractions greater than 0.5, divided by a narrow region of hexagonally perforated lamellae. Order-order transitions as a function of temperature are driven by the thermoresponsive desolvation of PNIPAM. Surprisingly, the materials exhibit reentrant order-disorder transition behavior, such that the conjugate block copolymers are disordered at both low and high concentrations but well-ordered at intermediate concentrations. Changing the polymer chemistry to monomers with different types of hydrogen bonding results in significant changes in the self-assembly, including the observation of a cubic phase that shows the same scattering pattern as the gyroid phase observed in traditional block copolymers. The choice of polymer also has a strong impact on the order-disorder transition concentration, demonstrating that the polymer-protein interaction plays a significant role in governing self-assembly in solution. Consistent with this effect, the order-disorder transition concentration is minimized in symmetric conjugates. Changing the protein from mCherry to myoglobin results in a reduction in ordering, suggesting that the regularity of the protein shape is important. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A21.00002: Tethered Nanoparticle$-$Polymer Composites: Phase behavior and rheology Rahul Mangal, Lynden A. Archer Polymer nanocomposites with particle radius (a) approaching the radius of gyration (R$_{g})$ of entangled host polymer have been reported to exhibit an unusual negative reinforcement effect, which leads to an anomalous reduction in relative an anomalous reduction in relative viscosity at low particle loadings ($\varphi )$. This so-called Non-Einsteinian flow behavior is understood to be sensitive to the dispersion state of particles in host polymer. We studied suspensions of SiO$_{2}$ nanoparticles tethered with polethylene glycol (PEG) in polymethylmethacralate (PMMA) with molecular weights (Mw) from 17 KDa to 280 KDa. Due to strong enthalpic interactions between PEG and PMMA ($\chi =$ -0.65), nanoparticles are expected to be well-dispersed, independent of Mw of PMMA. Using small angle x-ray scattering measurements we show that the phase stability of suspensions depends on Mw of the tethered PEG, host PMMA, and $\varphi $. Particles functionalized with low molecular weight PEG aggregate at low $\varphi $, but disperse at high $\varphi $. In contrast, nanoparticles functionalized with higher molecular weight PEG are well dispersed for host chain lengths (P) to tethered chain length (N), (P/N), is as high as 160. The stability boundary of these suspensions extends well beyond expectations for nanocomposites based on tethered PEG chains suspended in PEG. Through in-depth analysis of rheology and x-ray photon correlation spectra we explore the fundamental origins of non-Einsteinian flow behavior. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A21.00003: Effect of Protein Surface Potential on Globular Protein-Polymer Block Copolymer Self-Assembly Christopher Lam, Minkyu Kim, Carla Thomas, Dongsook Chang, Gabriel Sanoja, Chimdimma Okwara, Bradley Olsen The effects of protein surface potential on the self-assembly of protein-polymer block copolymers are investigated in globular proteins with a controlled shape through two approaches: (1) the self-assembly of the two structurally homologous proteins mCherry and EGFP conjugated to poly(N-isoprpopylacrylamide) (PNIPAM) and (2) bioconjugates containing mutants of mCherryS131C are prepared to specifically alter the electrostatic patchiness of the protein. Despite the large difference in amino acid sequence between mCherry and EGFP, identical phases are observed in concentrated solution at low temperatures and in bulk. At high temperatures above the thermoresponsive transition temperature, differences in micellar stability are observed at low concentrations, and different phases are observed between conjugates at high concentrations. Similarly, conjugates of four mCherryS131C variants with changes to their electrostatic surface patchiness showed minimal change in the concentrated solution phase behavior. Measurements of protein/polymer miscibility, protein second virial coefficients, and zeta potential indicate that coarse-grained interactions are able to largely capture the relevant physics for soluble, monomeric globular protein-polymer conjugate self-assembly. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A21.00004: Equilibrium Structure and Miscibility of Soft Nanoparticles in Polymer Melts Debapriya Banerjee, Kenneth Schweizer Crosslinked polymeric nanoparticles of tunable softness are modeled statistically using particle form factors obtained from scattering experiments. The model yields effective interactions between two fluctuating particles, and one fuzzy particle and a monomer. Using these effective interactions, microscopic PRISM integral equation theory is employed to study the structure and miscibility of soft nanogels in homopolymer melt. In the dilute particle limit, under chemistry-matched conditions, the monomer-particle pair correlations exhibit increasing polymer penetration in the nanogels with increasing surface fuzziness leading to improved dispersion of the particles, contrary to the depletion attraction induced between hard spheres by non-adsorbing polymers. However, beyond certain fuzziness, the polymers are excluded from the surface and the particles tend to ``self-bridge'' leading to aggregation. Miscibility of soft nanogels turns out to be a non-monotonic function of both particle softness and size. Increasing the matrix degree of polymerization tends to destabilize the system. In the non-dilute-particle limit, the many-body effects of the particles on the structure are studied and different qualitative trends are predicted depending on the particle softness. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A21.00005: Thiol-Functionalized Gold-Nanoscale Organic Hybrid Materials-Attractive to Soft glasses Akanksha Agrawal, Lynden Archer We report on the flow properties of self-suspended nanoparticles based on gold nanoparticles densely grafted with polyethylene glycol methyl ether thiol(PEG) chains. We studied the effect of temperature, olume fraction and polymer chain length on the transition from attractive glass to soft glassy flow behavior. Gold nanoparticles densely grafted with short PEG-thiol chains(MW 800,2kDa and 6kDa)are shown to form self-suspended systems over a range of polymer grafting densities and particle volume fractions,$\varphi $.Transmission electron and atomic force microscopy measurements reveal that the particles are uniformly dispersed. Oscillatory shear measurements performed on low $\varphi $ systems show a two-step yielding behavior reflecting bond breaking and cage breaking transitions at the nanoscale; both characteristics of soft glassy materials dominated by attractive forces. With increased temperature a transition to one-step yielding and subsequently back to two-step yielding is observed. At high $\varphi $ a single yielding transition and soft glassy flow behavior are observed. We employ SAXS, vibration spectroscopy, thermal analysis, and rheology to interrogate the configuration state of the tethered chains and particle-particle interactions in detail. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A21.00006: A quick and simple route to form soft Janus colloids Chris Sosa, Rodney Priestley, Robert Prud'homme Janus colloids, i.e., particles with two chemically distinct compartments or ``faces,'' are of significant scientific interest as they could serve as the enabling material for self-organizing superstructures and functional nanodevices. The internally segregated structures present in Janus particles are not only beneficial for self-assembly applications, but are also attractive from a more fundamental scientific perspective for the insight they can provide on hybrid material interfaces. Here, we present a novel, one-step nano-precipitation process for the formation of soft Janus colloids composed of two compositionally distinct and surface-active polymer domains. In particular, this approach allows for the fabrication of Janus particles from both homopolymers and block co-polymers, generates phase-separated Janus structures on extremely fast timescales, and provides excellent scalability. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A21.00007: Phase behavior of star-shaped polystyrene-block-poly(methyl methacrylate) copolymer Sangshin Jang, Hong Chul Moon, Dusik Bae, Jongheon Kwak, Youngmin Lee, WonBo Lee, Jin Kon Kim Phase behavior of star-shaped 18-arm polystyrene-block-poly(methyl methacrylate) copolymers ((PS-b-PMMA)$_{18}$) with various volume fraction of PS block (f$_{PS}$) was investigated via transmission electron microscopy and small angle X-ray scattering. (PS-b-PMMA)$_{18}$ was synthesized by atom transfer radical polymerization from $\alpha$-cyclodextrin ($\alpha$-CD) having 18 functional groups for the initiation. We also prepared corresponding linear PS-b-PMMAs by cutting the ester groups connecting $\alpha$-CD and PS chains in (PS-b-PMMA)$_{18}$ through the hydrolysis. The microdomains of (PS-b-PMMA)$_{18}$ changed from body-centered cubic spheres, hexagonally packed cylinders, perforated lamellae, and lamellae with decreasing fPS from 0.7 to 0.2. Interestingly, (PS-b-PMMA)n with f$_{PS}$ of 0.23 showed highly asymmetric lamellar microdomains, while corresponding linear PS-b-PMMA with the same volume fraction exhibited spherical microdomains. Thus, the microdomains are highly affected by the molecular architecture of block copolymer. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A21.00008: Synthesis and characterization of A2B Miktoarm Star Copolymers Composed of Regioregular Poly(3-hexylthiophene) and Poly(methyl methacrylate) containing rigid core Jicheol Park, Hong Chul Moon, Jin Kon Kim The $\pi$-$\pi$ interaction between P3HT arms in Miktoarm star copolymer composed of P3HT and PMMA ((P3HT)2PMMA) was strong enough to arrange two P3HT backbone chains in (P3HT)$_2$PMMA to stack one by one along the nanofibril axis on thin film. This is because of very small $\pi$-$\pi$ stacking distance between P3HT backbones compared with the size of the core. Here, we report a facile synthesis of (P3HT)$_2$PMMA containing rigid core. First, we synthesized coupled P3HT having 1,3-diazidobenzylaldehyde at the center using click reaction between mono-ethynyl P3HT and 1,3-diazidobenzylaldehyde. The aldehyde at center in coupled P3HT was replaced by ethynyl group using Grignard reaction with ethynyl magnesiumbromide (P3HT-core-P3HT). Then, copper(I)-catalyzed Huisgen 1,3-dipolar cycloaddition click reaction between P3HT-core-P3HT and PMMA-N$_3$ was performed to synthesize (P3HT)$_2$PMMA. The optical property and self-assembly of (P3HT)$_2$PMMA was investigated. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A21.00009: Polyelectrolyte Microcapsules: Ion Distributions from a Poisson-Boltzmann Model Qiyun Tang, Alan R. Denton, Damith Rozairo, Andrew B. Croll Recent experiments have shown that polystyrene-polyacrylic-acid-polystyrene (PS-PAA-PS) triblock copolymers in a solvent mixture of water and toluene can self-assemble into spherical microcapsules. Suspended in water, the microcapsules have a toluene core surrounded by an elastomer triblock shell. The longer, hydrophilic PAA blocks remain near the outer surface of the shell, becoming charged through dissociation of OH functional groups in water, while the shorter, hydrophobic PS blocks form a networked (glass or gel) structure. Within a mean-field Poisson-Boltzmann theory, we model these polyelectrolyte microcapsules as spherical charged shells, assuming different dielectric constants inside and outside the capsule. By numerically solving the nonlinear Poisson-Boltzmann equation, we calculate the radial distribution of anions and cations and the osmotic pressure within the shell as a function of salt concentration. Our predictions, which can be tested by comparison with experiments, may guide the design of microcapsules for practical applications, such as drug delivery. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A21.00010: Synchrotron Radiation Investigation in Epoxy Resin Modified with Polysiloxane System Wenjun Gan, Weizhen Li, Jindian Ding, Xiaodan Gu, Cheng Wang Epoxy resins are one of the most important classes of thermosetting polymers. Epoxy resin modified with polysiloxane is expected that the siloxane moiety may exert its qualities of thermal stability, impact toughness and surface-modification properties. Our group tried to introduce polysiloxane into epoxy resin by blending diglycidyl-ether of bisphenol-A with epoxypropoxypropyl terminated polydimethyl-siloxane and polyetherimide-siloxane in different proportion. These polysiloxane modified epoxy resins have been investigated using a combination of small- and wide angle X-ray scatterings (SAXS and WAXS) and scanning transmission soft X-ray microscopy (STXM). Nano- to micro-scale domain size, distribution and chemical composition were observed with spatial and spectroscopic sensitivities offered by both hard and soft x-ray scattering/microscopy. In-situ SAXS experiments were performed to understand the mechanism of microphase separation and dynamics of nanostructure evolution. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A21.00011: Association of Multi-Chain Pentablock Ionomers in Solutions: A Molecular Dynamics Simulation Study Dipak Aryal, Dvora Perahia, Thusitha Etampawala, Gary Grest Ionic block copolymers in solutions are of interest due to their fascinating ability to self-assemble into a variety of ordered microscopic structures such as ionic domains and hydrocarbon domains. These polymers show unique properties such as chemical and mechanical stability that arise from incompatibility between individual blocks, proton conductivity, ion transportability, and hydrophilicity. Using molecular dynamics simulations we have studied the association of multi-chain pentablock copolymers (A-B-C-B-A) in a 1:1 mixture of cyclohexane and heptane (mutual solvent), and in water at 300K and 500K. The center block consists of randomly sulfonated polystyrene connected to a flexible poly (ethylene-r-propylene) bridge and end caped with poly (t-butyl styrene). We found that the pentablock in mutual solvent forms micelles in solutions with the sulfonated polystyrene in the core and chains of swollen flexible poly (ethylene-r-propylene) and poly (t-butyl styrene) in the corona. In water, the micelle remains quasi-spherical with the ionic groups located on the outer surface at both temperatures. These results are good agreement with those obtained from small angle neutron scattering (SANS). [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A21.00012: Structure and Conformation of Ionic Conjugated Polymers: Polydots Naresh Osti, Thusitha Etampawala, Sidath Wijesinghe, Dvora Perahia Conjugated polymers confining into nano dimension form long-lived highly luminescent tunable organic particles of having enormous potential for intracellular imaging and drug delivery. Even though the chains are not in their thermodynamically stable conformation, the poly-dots remain stable over long period of times. Incorporation of ionic groups into conjugated polymers introduces a configuration control factor that impacts their conformation and their applications as luminescent probes. The current work investigates the structure and stability of poly-dots of di-alkoxy para polyphenyleneethynylene (PPE) conjugated polymer substituted with carboxylate side chain. Our small angle neutron scattering (SANS) studies have shown that ionic PPE forms spherical poly-dots in water. Ionic Poly-dots remain stable up to a temperature of 800C compare to neutral conjugated polymer poly dots. These polymer dots were allowed to assemble at a solid surface and observed by AFM which showed the nano aggregates of different sizes that assembled in different ways depending on the concentration and molecular parameters of the ionic PPEs used. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A21.00013: Giant surfactants of poly(ethylene oxide)-$b$-polystyrene-(molecular nanoparticle): nanoparticle-driven self-assembly with sub-10-nm nanostructures in thin films Chih-Hao Hsu, Zhiwei Lin, Xue-Hui Dong, I-Fan Hsieh, Stephen Z.D. Cheng Giant surfactants are built upon precisely attaching shape- and volume-persistent molecular nanoparticles (MNP) to polymeric flexible tails. The unique class of self-assembling materials, giant surfactants, has been demonstrated to form self-assembled ordered nanostructures, and their self-assembly behaviors are remarkably sensitive to primary chemical structures. In this work, two sets of giant surfactants with functionalized MNP attached to diblock copolymer tails were studied in thin films. Carboxylic acid-functionalized [60]fullerene (AC$_{60})$ tethered with PEO-$b$-PS (PEO-PS-AC$_{60})$ represents an ABA' (hydrophilic-hydrophobic-hydrophilic) giant surfactant, and fluoro-functionalized polyhedral oligomeric silsesquioxane (FPOSS) tethered with PEO-$b$-PS (PEO-PS-FPOSS) represents an ABC (hydrophilic-hydrophobic-omniphobic) one. The dissimilar chemical natures of the MNPs result in different arrangement of MNPs in self-assembled structures, the dispersion of AC$_{60}$ in PEO domain and the single domain of FPOSS. Moreover, the chemically bonded MNPs could induce the originally disordered small molecular PEO-$b$-PS to form ordered cylindrical and lamellar structure, as evidenced by TEM and GISAXS, leading to sub-10-nm nanostructures of copolymer in the thin film state. [Preview Abstract] |
Session A22: Surfaces, Interfaces and Polymeric Thin Films
Sponsoring Units: DPOLYChair: Joshua Sangoro, University of Tennessee-Knoxville
Room: 407
Monday, March 3, 2014 8:00AM - 8:12AM |
A22.00001: The effect of block-copolymer structures on the polymeric liquid-liquid interface: Molecular Dynamic Study Jiho Ryu, Won Bo Lee, Bumjoon Kim The change of free energy caused by different morphology of surfactants (block- and grafted-copolymers) in the biphasic system, is investigated by molecular dynamic simulations. We studied two different structures of surfactants. Type 1 is a diblock-copolymer surfactant composed with 60 monomers(30 A beads and 30 B beads). Type 2 is a grafted-copolymer surfactant of which two side chains composed of, respectively, 15 B monomers, are attached to main back bone chain composed of 30 A monomers. All simulations were performed in the NVT ensemble at 373K. Free energy are computed by thermodynamic integration from the coupled state to the uncoupled state where the surfactant does not interact with the biphasic system. In addition, we discuss various effects such as stiffness of polymers. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A22.00002: Relaxation and Intermediate Asymptotics of a Surface Perturbation in a Viscous Film Oliver B\"aumchen, Michael Benzaquen, Thomas Salez, Joshua D. McGraw, Matilda Backholm, Paul Fowler, Elie Rapha\"el, Kari Dalnoki-Veress The surface of a thin liquid film with nonconstant curvature flattens as a result of capillary forces. While this leveling process is driven by local curvature gradients, the global boundary conditions greatly influence the dynamics. Here, we study the evolution of rectangular trenches in a polystyrene nanofilm. We report on full agreement between theory and experiments for the capillary-driven flow and resulting time dependent height profiles, a crossover in the power-law dependence of the viscous energy dissipation as a function of time as the trench evolution transitions from two noninteracting to interacting steps, and the convergence of the profiles to a universal self-similar attractor that is given by the Green's function of the linear operator describing the dimensionless linearized thin film equation. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A22.00003: Filling up of a cylindrical hole in a viscous film Matilda Backholm, Michael Benzaquen, Thomas Salez, Elie Raphael, Kari Dalnoki-Veress A small cylindrical hole is a naturally occurring surface perturbation in viscous films. The flow dynamics of the hole is relevant for industrial applications, but more generally also important for the understanding of relaxation processes in thin films. Here, the capillary levelling of cylindrical holes in viscous polystyrene films was studied using atomic force microscopy and analytical scaling arguments. The relaxation of holes of various sizes was shown to consist of two different time regimes: an early regime where opposing sides of the hole do not interact, and a late regime where the hole is filling up. All theoretically derived scaling laws as well as the long-term solution of the intermediate asymptotic regime were shown to be in excellent agreement with experiments. In short, the system presented here provides an ideal sample geometry with which to probe flow on the nano-scale. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A22.00004: The Rayleigh-Plateau Instability on a Fiber Revisited - Influence of the Hydrodynamic Boundary Condition Sabrina Haefner, Oliver Baeumchen, Michael Benzaquen, Thomas Salez, Robert Peters, Joshua D. McGraw, Elie Raphael, Karin Jacobs, Kari Dalnoki-Veress The Rayleigh-Plateau Instability (RPI) of a liquid column underlies a variety of hydrodynamic phenomena that can be observed in everyday life. In the classical case of a free liquid column, linear perturbation theory predicts characteristic rise-times and wavelengths. However, the description of a liquid layer on a fiber requires the consideration of the solid/liquid interface in addition to the free interface. In this study, we revisit the RPI of a viscous liquid layer on a solid fiber by varying the hydrodynamic boundary condition at the fiber/liquid interface. The rise of the amplitudes of the surface undulations is precisely tracked and the growth rate of the instability is determined for the different slip boundary conditions and compared to the theoretical models. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A22.00005: Influence of Slip on the Rayleigh-Plateau Rim Instability in Dewetting Viscous Films Ralf Blossey, Oliver Baeumchen, Ludovic Marquant, Sabrina Haefner, Andreas M\"unch, Dirk Peschka, Barbara Wagner, Karin Jacobs A viscous film that retracts from a solid substrate develops a characteristic fluid rim at its receding edge due to mass conservation. In the course of this dewetting process the rim becomes unstable via an instability of Rayleigh-Plateau type. An important difference exists between this classic instability of a liquid column and the rim instability in the liquid film as the growth of the rim is continuously fueled by the receding film. We explain how the development and macroscopic morphology of the rim instability are controlled by the slip of the film on the substrate. Numerical calculations of a single thin-film model capture quantitatively the characteristics of the evolution of the rim observed in our experiments. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A22.00006: Controlling Marangoni induced instabilities in spin-cast polymer films: How to prepare uniform films Paul Fowler, C\'{e}line Ruscher, James Forrest, Kari Dalnoki-Veress In both research and industrial settings spin coating is extensively used to prepare thin polymer films of reproducible thickness. Normally spin coating produces highly uniform films, however under certain conditions the spin coating process results in films with non-uniform surface morphologies. Although the spin coating process has been extensively studied, the origin of these morphologies is not fully understood and the formation of non-uniform spincast films remains a practical problem. Here we report on experiments indicating that the formation of surface instabilities during spin coating is dependent on temperature. Furthermore, we find that non-uniformities in the film thickness can be entirely avoided simply by changing the spin coating temperature. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A22.00007: Time and temperature dependent wrinkling of stiff thin films on shape memory polymers Yu Wang, Kai Yu, Jerry Qi, Jianliang Xiao Shape memory polymers (SMPs) can remember two or more distinct shapes, and therefore can have a lot of potential applications. We here present combined experimental and theoretical studies on the wrinkling of stiff thin films on SMPs. Experimental results show well-defined, wavy profiles of the thin films. Time and temperature dependent wrinkle formation and evolution were observed. Finite element simulations accounting for the thermomechanical behavior of SMPs were used to study wrinkling of thin films on SMPs, which show good agreement with experiments. This study can have important implications in surface engineering, stretchable electronics and advanced manufacturing. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A22.00008: Frictional Response of Molecularly Thin Liquid Polymer Films Subject to Constant Shear Stress Charles Tschirhart, Sandra Troian Measurements of the frictional response of nanoscale viscous films are typically obtained using the surface force apparatus in which a fluid layer is confined between smooth solid substrates approaching at constant speed or force. The squeezing pressure causes lateral flow from which the shear viscosity can be deduced. Under these conditions however, molecularly thin films tend to solidify wholly or partially and estimates of the shear viscosity can exceed those in macroscale films by many orders of magnitude. This problem can be avoided altogether by examining the response of an initially flat, supported, free surface film subject to comparable values of surface shear stress by application of an external inert gas stream. This method was first conceived by Derjaguin in 1944; more recent studies by Mate et al. at IBM Almaden on complex polymeric systems have uncovered fluid layering and other interesting behaviors. The only drawback is that this alternative technique requires an accurate model for interface distortion. We report on ellipsometric measurements of ultrathin polymeric films in efforts to determine whether the usual interface equations for free surface films based purely on continuum models can be properly extended to nanoscale films. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A22.00009: Inducing surface morphologies in polymer films through exposure to non-solvents Chad Daley, Zin Tun, James Forrest Non-solvents are generally considered to have no lasting effect on polymer materials and are commonly employed in the production or processing of thin film polymer samples. Through a combination of atomic force microscopy and neutron reflectivity experiments we show that some non-solvents have the ability to drastically alter a film's surface morphology on the nanometer scale. An explanation for the structuring process is presented and reinforced through theoretical considerations of surface chains. These results suggest that caution should be exercised when making use of non-solvents wherever nanoscale surface properties are of importance. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A22.00010: Single-Molecule Tracking of Polymer Surface Diffusion Michael Skaug, Joshua Mabry, Daniel Schwartz The mobility of polymers adsorbed on a solid surface is important in thin film formation, adhesion phenomena and biosensing applications, but it is still poorly understood. We used single-molecule fluorescence experiments to follow the motion of isolated polyethylene glycol chains adsorbed at a hydrophobic solid-aqueous interface. We found that molecules moved on the surface via a continuous time random walk mechanism, where periods of immobilization were punctuated by flights through the bulk liquid. The dependence of surface mobility on molecular weight suggested that surface-adsorbed polymers maintained effectively three-dimensional surface conformations. These results indicate that polymer surface diffusion, rather than occurring in the two dimensions of the interface, is dominated by a three-dimensional mechanism that leads to large surface displacements and significant bulk-surface coupling. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A22.00011: Diffusion of polyelectrolyte chains within layer-by-layer films: effect of film stratification Victor Selin, Aliaksandr Zhuk, John F. Ankner, Svetlana Sukhishvili We compare the molecular weight dependence of the diffusion of polyelectrolyte chains within polyelectrolyte multilayer films with a different degree of internal layer intermixing. Linearly and ``exponentially'' grown films were prepared by the layer-by-layer (LbL) technique using poly(methacrylic acid) (PMAA) as a polyanion and quaternized poly-2-(dimethylamino)ethyl methacrylate (QPDMAEMA) as a polycation. Diffusion of polyelectrolyte chains in directions parallel and perpendicular to the film surface was measured using fluorescence recovery after photobleaching (FRAP) and neutron reflectometry (NR), respectively. We find that in solutions of 0.2-0.4~M~NaCl, lateral chain diffusion is enhanced in the exponential regime. More importantly, the scaling of the center-of-mass diffusion of polyelectrolyte chains with polymer molecular weight changed from a $D \sim M^{-1}$ dependence in the linear regime to a stronger dependence for the exponential regime, where polymer chains were stronger intermixed. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A22.00012: Nano-rheometry near the free-surface in polystyrene Kurt M. Schreiter, James A. Forrest Recent work has suggested enhanced mobility at the free-surface of glassy polymers. Traditional rheometers are incompatible with free surfaces so they cannot be used to confirm this. Lateral force spectroscopy has failed to observe deviations from bulk behaviour because the time scales required are too fast. We describe a technique for observing near surface dynamics in the free surface of soft matter systems that overcomes these limitations. Data collected from polystyrene films shows two simultaneous and separate relaxation behaviours. One of these is consistent with a bulk glassy material and the other indicates enhanced dynamics. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A22.00013: Hydrogen-bond Dynamics at The Interface Between Water and Oxidized Atactic Polystyrene Selemon Bekele, Mesfin Tsige Hydrogen bonding is very critical to a wide range of systems, from the existence of liquid water at room temperature to the structure of DNA (double helix) and many other biomolecules. The presence of an interface is expected to significantly change the structure and dynamics of the hydrogen bonded network as compared to the situation in the bulk. Understanding the strength and dynamics of hydrogen bonds at surfaces and interfaces has thus stimulated a large and growing body of experimental and theoretical work in recent years. Using all-atom molecular dynamics simulations we have studied the dynamics of hydrogen-bond (H-bond) between water and oxidized atactic polystyrene (aPS). The number of hydrogen bonds between water molecules and oxidized polystyrene is found to monotonically increase with oxygen concentration on the aPs surface. The life-time of this H-bond and the frequency of its formation have also been investigated as a function of oxygen concentration and the results will be presented. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A22.00014: Binding kinetics of lock-key colloids: surface diffusion enhancement of the rate of specific binding Laura Colon-Melendez, Daniel J. Beltran-Villegas, Greg van Anders, Jun Liu, Matthew Spellings, Stefano Sacanna, David J. Pine, Sharon C. Glotzer, Ronald G. Larson, Michael J. Solomon The kinetics of anisotropic particle assembly are expected to be slow due to specific directional interactions between the assembly building blocks. We investigate the lock-and-key colloidal system (Sacanna et al, Nature 464, 575-578 (2010)), to identify and understand the mechanisms that lead to specific lock-key pair binding. For lock pockets of a particular shape, we experimentally identify the importance of nonspecific lock-key binding as a pathway to specific lock-key pair formation. In this pathway, key particles can diffuse on the surface of the lock and bind specifically to the dimple of the lock. We find that this mechanism can be more important to specific bond formation than the direct binding mechanism. We model the surface diffusion mechanism as a mean first-passage time problem. Using an anisotropic interaction potential between a lock and key particle pair (van Anders et al, arXiv:1309.1187), we compare Stokesian dynamics simulations of lock and key binding to the experiments. We propose that nonspecific interactions can play an important role in accelerating anisotropic particle assembly. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A22.00015: Importing super-resolution imaging into nanoscale puzzles of materials dynamics John King, Chi Hang Boyce Tsang, William Wilson, Steve Granick A limitation of the exciting recent advances in sub-diffraction microscopy is that they focus on imaging rather than dynamical changes. We are engaged in extending this technique beyond the usual biological applications to address materials problems instead. To this end, we employ stimulated emission depletion (STED) microscopy, which relies on selectively turning off fluorescence emitters through stimulated emission, allowing only a small subset of emitters to be detected, such that the excitation spot size can be downsized to tens of nanometers. By coupling the STED excitation scheme to fluorescence correlation spectroscopy (FCS), diffusive processes are studied with nanoscale resolution. Here, we demonstrate the benefits of such experimental capabilities in a diverse range of complex systems, ranging from the diffusion of nano-objects in crowded 3D environments to the study of polymer diffusion on 2D surfaces. [Preview Abstract] |
Session A23: Invited Session: Phase Transitions in Classical and Quantum Lattice Models
Sponsoring Units: DCOMP GSNPChair: Anders Sandvik, Boston University
Room: 505-507
Monday, March 3, 2014 8:00AM - 8:36AM |
A23.00001: Aneesur Rahman Prize: The Inverse Ising Problem Invited Speaker: Robert Swendsen Many methods are available for carrying out computer simulations of a model Hamiltonian to obtain thermodynamic information by generating a set of configurations. The inverse problem consists of recreating the parameters of the Hamiltonian, given a set of configurations. The problem arises in a variety of contexts, and there has been much interest recently in the inverse Ising problem, in which the configurations consist of Ising spins. I will discuss an efficient method for solving the problem and what it can tell us about the Sherrington-Kirkpatrick model. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 9:12AM |
A23.00002: Deconfined quantum criticality in two-dimensional bipartite SU(N) anti-ferromagnets Invited Speaker: Ribhu Kaul I will give an overview of unbiased numerical work on the N\'eel- valence bond solid (VBS) phase transition in $d=2$ anti-ferromagnets. This progress has been possible due to the discovery of a new class of Hamiltonians of SU($N$) spins that are free of the sign problem of quantum Monte Carlo. I will show through extensive numerical studies of the quantum phase transition on a variety of bipartite systems: square, rectangular, honeycomb and square bilayer, for a number of values of $N$ ($2 \leq N \leq 10$), that a close to complete picture of an unusual ``deconfined critical point'' has emerged. Significantly, no direct evidence for first order behavior has been found on the largest simulations with $256\times 256$ spins, the crucial role of Berry phases at the critical point has been verified, strong evidence for non-compact CP$^{N-1}$ universality is evident for a range of $N$ values, the inferred ``dangerous'' (ir)relevance of lattice anisotropy at the critical point is consistent with various limiting analytic calculations on the CP$^{N-1}$ field theory and close to the critical point dramatic signatures of the emergent photon excitation have been detected in VBS correlation functions. I will conclude with some open theoretical issues that remain to be resolved and possible experimental realizations. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:48AM |
A23.00003: Deconfined Quantum Criticality and Phase Transitions in 3D Classical Loop Models Invited Speaker: John Chalker I will talk about the statistical physics of a family of three-dimensional lattice models for completely-packed loops that have transitions between phases of two types: one in which there are only short loops, and another in which some loops are extended. The models can be viewed as lattice discretisations of $CP^{n-1}$ sigma models in 3D. Alternatively, they can be seen as quantum $SU(n)$ quantum magnets in (2+1)D. In this case, the phase with long loops is a Neel phase, the phase with only short loops is a valence bond phase, and the models are closely related to loop algorithms developed for quantum Monte Carlo simulations. Depending on the design of the model, the short loop phase is either unique (representing a valence bond liquid) or spontaneously breaks a spatial symmetry (representing a valence bond solid). The transition from the Neel phase to the valence bond solid is a candidate deconfined critical point and the loop model gives access to this transition via Monte Carlo simulations. I will present results from large-scale simulations of the transition. \\[4pt] [1] Adam Nahum, J. T. Chalker, P. Serna, M. Ortuno, and A. M. Somoza, Phys. Rev. Lett. {\bf 107} 110601 (2011).\\[0pt] [2] Adam Nahum, J. T. Chalker, P. Serna, M. Ortuno, and A. M. Somoza, Phys. Rev. B {\bf 88}, 134411 (2013).\\[0pt] [3] Adam Nahum, J. T. Chalker, P. Serna, M. Ortuno, and A. M. Somoza, in preparation. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:24AM |
A23.00004: Quantum phase diagrams and phase transitions in frustrated two-dimensional Heisenberg models Invited Speaker: Donna Sheng The quantum spin liquid is an emergent state of matter, which has attracted a lot of recent attention. I will review recent numerical progress based on the density matrix renormalization calculations in identifying gapped spin liquid in two-dimensional frustrated spin systems. I will first focus on extended model with Heisenberg exchange couplings on kagome lattice and demonstrate a topological state with fractionalized spinon and emergent gauge field clearly shown in numerical simulations. I will present concrete results on the quantum phase diagram of the extended kagome Heisenberg model, and compare that with the phase diagrams of the square and honeycomb lattice models with the dominant plaquette valence bond phase in nonmagnetic region. I will discuss numerical effort and theoretical challenge in fully pinning down the nature of the gapped topological phase, and also the nature of the quantum phase transitions in these Heisenberg systems. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 11:00AM |
A23.00005: Mott Transitions of Correlated Dirac Fermions from SU(2) to SU($N$) Invited Speaker: Thomas C. Lang The rise of graphene and topological insulators has sparked countless investigations of interacting electrons on the honeycomb lattice. We present recent advances in the study of the evolution from the weak-coupling semimetal into the strong-coupling, insulating regime by means of unbiased quantum Monte Carlo simulations of the Hubbard and related models on the honeycomb lattice at half filling. Employing a novel approach to quantum phase transitions, we perform non-equilibrium imaginary time quenches of the Hubbard model in (zero temperature) projective quantum Monte Carlo simulations. This allows us to efficiently access order parameters on a finite size lattice for a wide range of coupling values in a single run. We extract reliable estimates for the scaling properties and critical exponents of the semimetal-insulator quantum phase transition. Furthermore, we investigate the extension of the Hubbard model with an explicit SU($N$)-symmetric, Heisenberg-like nearest-neighbor flavor exchange interaction. From the large-$N$ regime down to the SU(6) case, the insulating state is found to be a columnar valence bond crystal, with a direct transition to the semimetal at weak, finite coupling, in agreement with the mean-field result in the large-$N$ limit. At SU(4) however, the insulator exhibits a subtly different valence bond crystal structure, stabilized by resonating valence bond plaquettes. Furthermore, we discuss the new possibility to efficiently access the Renyi entanglement entropy within the auxiliary field quantum Monte Carlo algorithm. Using the example of a correlated topological insulator we present the development of the stable computation of higher order Renyi entropies, in order to access the entanglement spectrum. [Preview Abstract] |
Session A24: Focus Session: Materials for Electrochemical Energy Storage: Characterization and Computation Methods
Sponsoring Units: DMP GERA DCOMPChair: Brandon Wood, Lawrence Livermore National Laboratory
Room: 504
Monday, March 3, 2014 8:00AM - 8:36AM |
A24.00001: Electron microscopy characterization of Li-based cathode materials for battery applications Invited Speaker: Patrick Phillips While recent advances in energy storage have revolutionized the consumer electronics market, we still lack an understanding of the aging effects that currently limit the batteries' lifetime and energy storage capacity. Aberration-corrected scanning transmission electron microscopy (STEM) is becoming one of the most promising characterization tools to study the effects of repeated charging/discharging cycles on electrode aging in Li-ion battery materials, due to the wide-range of techniques available on advanced STEM instruments, including the direct imaging of both heavy and light elements, energy-dispersive X-ray and electron energy loss (EEL) spectroscopies and a variety of in-situ methods. This talk will focus on the structural and chemical characterization of Li-ion battery electrode materials, both in a pristine and lithiated/delithiated state. We will examine the nucleation of structural transitions with STEM experiments utilizing various imaging modes, including annular dark field and bright field imaging, and in-situ electro-chemical experiments using the open-cell design. These results will be combined with EELS measurements and first principle density functional theory calculations to elucidate the role of the observed transitions on the material's structural stability.\\[4pt] In collaboration with Robert Klie, University of Illinois at Chicago; Hakim Iddir, Roy Benedek, and Daniel Abraham, Chemical Sciences and Engineering Division, Argonne National Laboratory. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A24.00002: Finite temperature effects on the X-ray absorption spectra of energy related materials Tod Pascal, David Prendergast We elucidate the role of room-temperature-induced instantaneous structural distortions in the Li K-edge X-ray absorption spectra (XAS) of crystalline LiF, Li$_{2}$SO$_{4}$, Li$_{2}$O, Li$_{3}$N and Li$_{2}$CO$_{3}$ using high resolution X-ray Raman spectroscopy (XRS) measurements and first-principles density functional theory calculations within the eXcited electron and Core Hole (XCH) approach. Based on thermodynamic sampling via \textit{ab-initio} molecular dynamics (MD) simulations, we find calculated XAS in much better agreement with experiment than those computed using the rigid crystal structure alone. We show that local instantaneous distortion of the atomic lattice perturbs the symmetry of the Li 1$s$ core-excited-state electronic structure, broadening spectral line-shapes and, in some cases, producing additional spectral features. This work was conducted within the Batteries for Advanced Transportation Technologies (BATT) Program, supported by the U.S. Department of Energy Vehicle Technologies Program under Contract No. DE-AC02-05CH11231. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A24.00003: Soft X-ray Spectroscopy for Understanding the Cycling Mechanism of Novel Lithium-ion Batteries Ruimin Qiao, Robert Kostecki, Ivan Lucas, Kristin Persson, Wei Chen, Hong Li, Rui Wang, Wanli Yang Energy and environment are two major concerns of the modern world. Transition to the sustainable clean energy globally in the future, however, depends on the development of next generation electrical energy storage systems. Among the energy storage techniques considered at present, rechargeable lithium-ion batteries, which are ubiquitous in today's portable electronic devices and now enable the electric vehicles, remain promising to facilitate the use of renewable energy on a large scale. For such application, transformational changes in battery technologies are critically needed, which require a fundamental understanding of the complex, interrelated physical and chemical processes between electrode materials and electrolytes Soft x-ray absorption spectroscopy(sXAS) is a powerful tool to probe the chemical species and the electronic states with elemental sensitivity. This presentation will discuss examples on using sXAS to study battery materials for both fundamental understanding and practical developments. We will showcase how sXAS fingerprints the battery operation by detecting the evolving electron states. Recent results on SEIs and Li-rich cathode materials will be discussed. Our results offer important information for improving Li batteries. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A24.00004: First-principles investigation of the structural changes in Li-rich cathode composites Hemant Dixit, Wu Zhou, Jagjit Nanda, Juan-Carlos Idrobo, Valentino Cooper Lithium ion batteries have high energy densities and are widely used in consumer electronics. However, it is essential to improve their power rate and cycle life for long-term usage. Cathode materials containing Li-excess layered oxide compounds, $x$Li$_2$MnO$_3$(1-$x$)LiMO$_2$, (where M=Mn, Co, Ni and $x$= 0.2-0.7) have two times higher capacities than the conventional cathode material but during cycling a decrease in energy density and a concomitant development of a low voltage plateau are often observed. Furthermore, recent experimental studies have observed the formation and clustering of the anti-site defects near the surface. Thus a detailed understanding of the structural changes at the atomic scale of these Li-rich composites is essential to establish the correlation between the structural and electrochemical property. We present first-principles density functional theory study of the structural and electronic properties in Li-rich cathode composites. These cathode composites are modelled as solid solutions of the LiMnO$_2$ (R$\bar{3}$m) and Li$_2$MnO$_3$ (C$_2$m) phases. We discuss the stability of the proposed model, the diffusion energy barriers of Li$^+$ ions calculated using nudged-elastic band method and the formation energies of the antisite defects. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A24.00005: Atomistic simulation studies of nanostructured Li2MnO3 Phuti Ngoepe, Thi Sayle, Dean Sayle The structures of the lithium-rich layered materials are basically derived from the layered rock-salt a- NaFeO2 type structure with space group R-3m. Nano-sized crystalline cathode material, Li2MnO3, was prepared by single step hydrothermal reaction [1]. The prepared materials delivered a high electrochemical reversible capacity charged/discharged between 2.0-4.3V, which indicates their promising future potential. In the current study simulated amorphisation recrystallisation method [2] is used to nucleate and crystallise ternary a nanoparticle of Li2MnO3, which is an end member compound of composites. The generated structure is characterized and the Li layer is found to accommodate some Mn ions. The latter is explained in terms of heats of formation deduced from DFT calculations. Microstructural features and transport properties are presented and a possible origin of the electrochemical activity is discussed. [1] G. R. Liu, S.C. Zhang, X. X. Lu, X. Wei, Proceedings Int. Conf. Nanomaterials: Applications and Properties, Vol. \textbf{2, }No 2, 02PCN25(3pp) (2013). [2] P.E.Ngoepe, R.R. Maphanga and D.C. Sayle, (2013), ``Towards the Nanoscale'', Chapter 9 in Computational Approaches to Energy Materials, pp 261-290, edited by C.R.A. Catlow, A. Sokol and A. Welsch, John Wiley and Sons Ltd. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A24.00006: In Situ Tomographic Profiling of Ag$_{2}$VP$_{2}$O$_{8}$ Li-Ion Batteries using Energy Dispersive X-ray Diffraction Kevin Kirshenbaum, David Bock, Amy Marschilok, Zhong Zhong, Kenneth Takeuchi, Esther Takeuchi Bimetallic cathodes for use in Li-ion batteries have been studied in recent years as they may provide multiple electron reduction, yielding both high capacity and high current on discharge. In this study, we investigate the progress of the reaction of Ag$_{2}$VP$_{2}$O$_{8}$ on discharge in a lithium anode cell using in-situ energy dispersive x-ray diffraction at beamline X17B1 at NSLS I. By measuring diffraction patterns in 20 $\mu$m segments through the cathode as a function of depth of discharge we are able to produce tomographic images of discharged cells. After analyzing the resulting spectra, we were able to observe the presence and relative intensity of Ag metal formed in the cathode upon discharge shedding light on the mechanisms limiting performance. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A24.00007: Hybrid functional studies of defects in layered transition metal oxides Khang Hoang, Michelle Johannes Layered oxides LiMO$_{2}$ (M is a transition metal) have been studied extensively for Li-ion battery cathodes. It is known that defects have strong impact on the electrochemical performance. A detailed understanding of native point defects in LiMO$_{2}$ is however still lacking, thus hindering rational design of more complex materials for battery applications. In fact, first-principles defect calculations in LiMO$_{2}$ are quite challenging because standard density functional theory (DFT) calculations using the generalized gradient approximation (GGA) of the exchange-correlation functional fail to reproduce the correct physics. The GGA+U extension can produce reasonable results, but the transferability of U across the compounds is limited. In this talk, we present our DFT studies of defects in LiMO$_{2}$ (M=Co, Ni) using the Heyd-Scuseria-Ernzerhof (HSE) screened hybrid functional. The dominant point defects will be identified and compared with experiment; and their impact on the structural stability and the charge (electronic and ionic) and mass transport will be addressed. We will also discuss possible shortcomings of the HSE functional in the study of these electron-correlated materials. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:24AM |
A24.00008: Li-ion energy storage of two-dimensional ``MXene'' transition metal carbides Invited Speaker: Paul Kent A new class of two-dimensional early transition metal carbides and carbonitrides, the so-called MXenes, has been synthesized by extracting the ``A'' element from MAX phases. Experiments have demonstrated that MXenes (Ti$_2$C, V$_2$C, Nb$_2$C, Ti$_3$C$_2$...) are promising anode materials for lithium ion batteries, delivering high storage capacity and good rate performance. However, the mechanism of Li-ion storage on MXene surfaces is not clear, with counterintuitive differences in predicted vs measured capacities, and large differences between exfoliated and delaminated samples. I will discuss how a strong collaboration between theory and a range of experimental characterization methods, including x-ray adsorption spectroscopy and inelastic neutron scattering, is able to provide a including for the highest measured Li capacities.\\[4pt] In collaboration with Yu Xie, Alexander Kolesnikov, Oak Ridge National Laboratory; Xiquan Yu, Kyung-Wan Nam, Xiao-Qing Yang, Brookhaven National Laboratory; Michael Naguib, Vadym Mochalin, Michel Barsoum, and Yury Gogotsi, Department of Materials Science and Engineering, and A.J. Drexel Nanotechnology Insititute, Drexel University. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A24.00009: Post-test analysis of lithium-ion battery materials at Argonne National Laboratory Javier Bareno, Nancy Dietz-Rago, Ira Bloom Electrochemical performance is often limited by surface and interfacial reactions at the electrodes. However, routine handling of samples can alter the very surfaces that are the object of study. Our approach combines standardized testing of batteries with sample harvesting under inert atmosphere conditions. Cells of different formats are disassembled inside an Argon glove box with controlled water and oxygen concentrations below 2 ppm. Cell components are characterized \textit{in situ}, guaranteeing that observed changes in physicochemical state are due to electrochemical operation, rather than sample manipulation. We employ a complementary set of spectroscopic, microscopic, electrochemical and metallographic characterization to obtain a complete picture of cell degradation mechanisms. The resulting information about observed degradation mechanisms is provided to materials developers, both academic and industrial, to suggest new strategies and speed up the Research {\&} Development cycle of Li-ion and related technologies. This talk will describe Argonne's post-test analysis laboratory, with an emphasis on capabilities and opportunities for collaboration. Cell disassembly, sample harvesting procedures and recent results will be discussed. [Preview Abstract] |
Session A25: Focus Session: Organic Electronics and Photonics - Electronic Processes at Interfaces
Sponsoring Units: DMP DPOLYChair: Vitaly Podzorov, Rutgers University
Room: 503
Monday, March 3, 2014 8:00AM - 8:36AM |
A25.00001: Interface Energetics and Chemical Doping of Organic Electronic Materials Invited Speaker: Antoine Kahn The energetics of organic semiconductors and their interfaces are central to the performance of organic thin film devices. The relative positions of charge transport states across the many interfaces of multi-layer OLEDs, OPV cells and OFETs determine in great part the efficiency and lifetime of these devices. New experiments are presented here, that look in detail at the position of these transport states and associated gap states and electronic traps that tail into the energy gap of organic molecular (e.g. pentacene) or polymer (P3HT, PBDTTT-C) semiconductors, and which directly affect carrier mobility in these materials. Disorder, sometime caused by simple exposure to an inert gas, impurities and defects are at the origin of these electronic gap states. Recent efforts in chemical doping in organic semiconductors aimed at mitigating the impact of electronic gap states are described. An overview of the reducing or oxidizing power of several n- and p-type dopants for vacuum- or solution-processed films, and their effect on the electronic structure and conductivity of both vacuum- and solution-processed organic semiconductor films is given. Finally, the filling (compensation) of active gap states via doping is investigated on the electron-transport materials C$_{\mathrm{60}}$ and P(NDI$_{\mathrm{2}}$OD-T$_{\mathrm{2}})$, and the hole-transport polymer PBDTTT-C. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A25.00002: Efficient Density Functional Approximation for Electronic Properties of Conjugated Systems Mar\'ilia J. Caldas, Jos{\'e} Maximiano Pinheiro Jr, Volker Blum, Patrick Rinke There is on-going discussion about reliable prediction of electronic properties of conjugated oligomers and polymers, such as ionization potential IP and energy gap. Several exchange-correlation (XC) functionals are being used by the density functional theory community, with different success for different properties. In this work we follow a recent proposal [1]: a fraction $\alpha$ of exact exchange is added to the semi-local PBE XC [2] aiming consistency, for a given property, with the results obtained by many-body perturbation theory within the G0W0 approximation. We focus the IP, taken as the negative of the highest occupied molecular orbital energy. We choose $\alpha$ from a study of the prototype family trans-acetylene, and apply this same $\alpha$ to a set of oligomers for which there is experimental data available (acenes, phenylenes and others). Our results indicate we can have excellent estimates, within 0,2eV mean ave. dev. from the experimental values, better than through complete $E_{N-1}-E_N$ calculations from the starting PBE functional. We also obtain good estimates for the electrical gap and orbital energies close to the band edge.\\[4pt] [1] V. Atalla, M. Yoon, F. Caruso, P. Rinke, M. Scheffler PRB 88, 165122 (2013).\\[0pt] [2] J.P. Perdew, K. Burke, M. Ernzerhof PRL 77, 3865 (1996). [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A25.00003: Surface-enhanced Raman spectroscopic studies of the Au-pentacene interface: a combined experimental and theoretical investigation Suchismita Guha, Danish Adil A large enhancement in the Raman intensity due to surface-enhanced Raman scattering (SERS) is observed from pentacene when probed through the Au contact in organic field-effect transistor (OFET) structures. The SERS spectrum is shown to exhibit a high sensitivity to disorder introduced in the pentacene film by Au atoms. The Raman signature of the metal-semiconductor interface in pentacene OFETs is calculated within density-functional theory by explicitly considering the Au-pentacene interaction. The observed enhancement in the 1380 cm$^{-1}$ and the 1560 cm $^{-1}$ regions of the experimental Raman spectrum of pentacene is successfully modeled by Au-pentacene complexes, giving insights into the nature of disorder in the pentacene sp$^2$ network. Raman maps across the pentacene-Au interface provide a powerful visualization tool for correlating the device performance, namely changes in the threshold voltages upon bias stress, to structural changes of the molecule. Unlike high-operating voltage OFETs, low-operating voltage OFETs show no change in the SERS spectra before and after the application of a bias stress, concurrent with no degradation in their threshold voltage. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A25.00004: Subphthalocyanine on C$_{70}$ Contact Layer Structure and Properties from First Principles Calculations John Kieffer, Hossein Hashemi, Xiao Ma, Michael Waters, Steven Morris, Max Shtein Boron subphthalocyanine (SubPc) is a promising donor material for organic photovoltaics, having one of the highest reported open circuit voltages among bilayer OPVs when coupled with C$_{60}$. Recently, C$_{70}$ has attracted attention as a substitute for C$_{60}$, largely due to a broader optical absorption spectrum, which leads to a higher current at relatively high voltages. The structure and electronic properties of SubPc derivatives on C$_{70}$-fullerene were explored using density functional theory (DFT) calculations with added Van der Waals interactions. Total-energy calculations were used to elucidate the initial adsorption derivatives on low index surfaces of C$_{70}$. The dependence of the electronic and optical excitations on the interface morphology is studied within the Green's-function GW and Bethe-Salpeter approaches. Insights gained from these calculations, and how they can be used to improve device efficiency, are discussed. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A25.00005: The formation of conductive polymer chains from Biphenyl-4,4'-dithiol (BPDT) molecules on rough Ag surfaces Ruqian Wu, V. Ara Apkarian, Yanning Zhang Single-molecular electronics, which exploits novel physical and chemical properties of organic molecules, has attracted much attention in the last decade. Many experimental and theoretical efforts have been made in manipulating high-quality organic polymers, understanding electron transport properties and developing electronic devices. Our experiments show that self-assembled monolayer (SAM) of Biphenyl-4,4'-dithiol (BPDT) can readily form on roughened surfaces of elemental silver, instead of a flat surface. To understand ``why so,'' we performed systematic density functional studies on the structural, energetic and electronic characteristics of both isolated BPDT molecules and BPDT on Ag(111) surfaces, with the inclusion of van der Waals correction in DFT. The formation of S-Ag-S linkage makes the molecule chain metallic, different from the insulating feature of S-S linkage. The adsorption of BDPT on roughened Ag surface is energetically preferred compared to that on flat surface. Moreover, the Ag adatom makes BPDT molecules attractive to each other on Ag(111) surface, crucial for the formation of polymer chains. Our joint theoretical and experimental results indicate the feasibility of fabricating conductive organo-silver polymer sheets. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A25.00006: Current focussing in organic semiconductors due to high local field, inhomogeneous trap distributions, and fibrous morphologies Kanokkorn Pimcharoen, Phillip Duxbury Charge transport in organic devices is a key factor controlling device performance and as a means for characterizing devices. We have developed a fully three dimensional device simulation tool enabling treatment of inhomogeneous systems including c-AFM tip geometry, spatially varying trap distributions, and fibrous morphologies. The model and simulation procedures will be described and current focussing in three cases will be presented (i) high voltage at a c-AFM tip, (ii) inhomogeneous trap distributions and (iii) fibrous morphologies. Inhomogeneous trap distributions contribute to current focusing in both device and tip geometries and in both cases transport preferentially follow low trap pathways. In fibrous systems where the fibers have a low trap density, current flow concentrates on pathways where the low trap fibers occupy a higher fraction of the total path length. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A25.00007: Graphene-Based Polymer Bulk Heterojunction Solar Cells Fei Yu, Vikram Kuppa We propose and demonstrate BHJs that utilize pristine graphene in order to facilitate exciton dissociation and charge transfer in polymeric solar cells. Devices based on P3HT:PCBM:graphene were fabricated on patterned ITO glass, and the effect of graphene on performance was investigated. Various device parameters including short-circuit current density, open-circuit voltage, fill factor, power conversion efficiency, and external quantum efficiency are compared with traditional BHJs. Results are discussed in the context of the morphology of the active layer, and the distribution and orientation of graphene platelets, as characterized by GIXRD, and neutron reflectometry. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A25.00008: Ferromagnetic-organic interfacial states detected by transient conductivity and their role on low voltage current injection in organic spinvalves Hongtao Zhang, Theo Kreouzis, William Gillin, Alan Drew Recently, there has been an increasing interest in utilising organic materials as spin transport layers as they have long spin-coherence times due to low spin-orbit and hyperfine coupling present in these materials [1]. Whilst there has been considerable research into organic spinvalves, there is a fundamental unsolved problem of how spin injection occurs. All organic spinvalves have been found to operate best at very low voltages, in the order of millivolts, where there should be no carrier injection. In this work we investigate the role of hybrid interface states (HINTS) between a ferromagnetic contact (FM) and an organic semiconductor (OSC). Using transient conductivity measurements on a variety of devices, the presence of these HINTS in a real device but only in the presence of a FM contact. We then consider the consequences that these filled HINTS will have on the electrical properties of devices. We argue that the filling of these HINTS introduces a large electric field at the FM-OSC interface, which causes an effect analogous to ``band-bending'' in conventional semiconductors. This explains the Ohmic injection seen in organic spinvalves which results in hole injection even at low (mV) applied voltages. \\[4pt] [1] Dediu, V. A., Nat. Mat. 8, 707-716 (2009). [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A25.00009: Controlling organic magnetoresistance via interface engineering C.A. Richter, H.-J. Jang, S.J. Pookpanratana, J.I. Basham, C.A. Hacker, O.A. Kirillov, R.J. Kline, O.D. Jurchescu, D.J. Gundlach We present the results of experiments in which we manipulate organic magnetoresistance (OMAR) in devices based on Alq3 (tris-(8-hydroxyquinoline) aluminum) and TPD (N,N$\prime $-Bis(3-methylphenyl)-N,N$\prime $-diphenylbenzidine) by adding a self-assembled monolayer (SAM). The results of OMAR measurements on this OLED-like architecture are correlated with impedance spectroscopy results to elucidate charge carrier transport and accumulation. We observe competing OMAR mechanisms in these devices, the relative strength of which can be tuned by adding SAMs at electrode interfaces. To determine how the interfacial and structural properties of these organic devices effect the OMAR, we obtained a complete picture of the interfacial, topological, and crystalline properties of these devices by performing UPS (Ultraviolet Photoelectron Spectroscopy), XPS (X-ray PS), XRD (X-ray diffraction), and AFM (atomic force microscopy). To verify our understanding of how interfacial changes affect OMAR, we characterized simple Alq3-only devices: one with a SAM and one without it. Despite having the same current density at room temperature, the latter shows a negative MR while the former displays a positive MR. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A25.00010: Influence of Interactions between Excited States on Magnetic Field Effects in Organic Semiconducting Materials Lei He, Bin Hu, Mingxing Li, Augustine Urbas The magnetic field effects in organic semiconducting materials are essentially determined by spin-exchange interaction and hyperfine interaction within individual intermolecular excited states. Intermolecular excited states can inevitably experience interactions between them due to their spatially extended wavefunctions. This interaction can be involved in the development of magnetic field effects, but this important issue has not been discussed. We study the influence of interactions between intermolecular excited states on magnetic field effects by using magneto-photoluminescence based on well-controlled organic composite containing N,N-dimethylaniline and pyrene in liquid state. We find that the interactions between intermolecular excited states can cause a line-shape narrowing in magneto-photoluminescence. The line-shape narrowing indicates that the interactions between the intermolecular excited states can decrease the force-constant of magnetic field-dependent singlet-triplet intersystem crossing within individual intermolecular excited states. Our studies show that the interactions between the excited states can occur through three different regimes, namely long-range Coulomb interaction, mid-range spin-orbital interaction, and short-range spin interaction, and consequently influence the spin-conserving and spin-dephasing processes within individual intermolecular excited states in the development of magnetic field effects in organic semiconducting materials. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A25.00011: Engineering hybrid polymer/metal-oxide interfaces by self-assembled molecular interlayers Alessandro Mattoni Hybrid organic heterojunctions are of great technological interest as both optically active layers as well as hole blocking interfaces in organic or hybrid solar cells. Despite the potential of combining processable organic polymers with inorganic components, they have not yet demonstrated high efficiencies. promising approach towards more efficient systems consists in engineering the interface by self-assembled molecular interlayers that can selectively affect the interactions of the donor and acceptor components. a combination of molecular dynamics and electronic structure calculations [1] we study thermodynamic and optoelectronic properties of polymer/metaloxide interfaces in presence of several molecular interlayers such as metal-organic macrocyclic complexes [2,4] or pyridine derivatives [1]. The theoretical results are tested on specifically designed hybrid solar cells providing evidence of impressive enhancement of interface properties.\\[4pt] [1] M. I. Saba, et al. J. Phys. Chem. C 115, 9651--9655 (2011).\\[0pt] [2] C. Melis et al. ACS Nano 5 9639 (2011).\\[0pt] [3] E. Canesi et al.,Energy Environ. Sci. 5 9068 (2012).\\[0pt] [4] G. Mattioli et al. Submitted (2013)/ [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A25.00012: Molecular Ordering of Poly(3-hexylthiophene) on Self-Assembled Monolayers Yeneneh Yimer, Mesfin Tsige The molecular ordering of semiconducting polymers such as Poly(3-hexylthiophene) (P3HT) at surfaces and interfaces has significant influence on the performance of organic solar cell devices. The charge-carrier transport and the charge collection at the electrodes strongly depend on the molecular ordering of P3HT at interfaces. Molecular ordering of P3HT can be tuned by varying the substrate surface chemistry and film processing conditions. Using all-atom molecular dynamics simulations and validated force field parameters, we have investigated the molecular ordering of P3HT on self-assembled monolayers (SAMs) of n-alkanethiols with varying end-functional groups and spacer length. In this study we elucidate the dependence of the molecular ordering of P3HT (edge-on or face-on conformation) on the surface chemistry of SAMs. Moreover, we investigated the effect of solvent on the molecular ordering of P3HT on SAMs surfaces. Understanding the correlation between P3HT morphology and surface chemistry will help in designing P3HT-based solar devices with better efficiency. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A25.00013: Charge injection across a metal-organic interface suppressed by thermal diffusion Carlos Monton, Thomas Saerbeck, Ilya Valmianski, Ivan K. Schuller Considerable progress has been made in developing metallophthalocyanine devices, although details of the underlying mechanisms in electrical transport are not fully understood. More importantly, few studies have explored their performance at realistic working temperatures, well above room temperature. In this work we explore the performance of Co-phthalocyanine (CoPc) vertical capacitive devices up to 460K. We find that the ohmic conductance is irreversibly suppressed by orders of magnitude when the devices are heated above 340 K. Detailed structural and transport studies imply that the changes in the conductance are due to diffusion of the top Pd electrode into the CoPc layer. This leads to a decrease in Pd electrode work function, which increases the potential barrier for hole injection. These results have a direct impact on technological applications since the instabilities of metallic-organic capacitive devices occur at operational temperatures typical for electronic (350K to 400K). [Preview Abstract] |
Session A26: Materials at High Pressure: Methods and Superconductors
Sponsoring Units: DCOMP DMP GSCCMChair: Christopher Seagle, Sandia National Laboratories
Room: 502
Monday, March 3, 2014 8:00AM - 8:12AM |
A26.00001: Improvements on the Murnaghan Equation of State Michael Mehl Formulas for interpolating the equation of state of a material are useful to consolidate both experimental and computational data. A good equation of state can reduce the computational effort needed to determine the equilibrium energy and to prediction phase transitions. The Murnaghan equation of state, which starts from the simple approximation that the bulk modulus of a system is linear in the pressure, is popular because it yields analytic expressions for V(P), P(V), E(V), and H(P) = E + P V. However, it has unphysical behavior as the bulk modulus approaches zero, predicting this to occur only at infinite volume, and at high pressure, where it misses the softening of B'(P). This paper presents a simple, analytic modification of B(P) which properly accounts for both regimes, and which still yields analytic behavior of the volume, pressure, energy, and enthalpy. The accuracy of the resulting equation of state is compared to the Murnaghan, Birch, and ``Universal'' equation of state for both simple and complex systems. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A26.00002: Towards an accurate dissociative potential for water Omololu Akin-Ojo Most models of water describe the molecule as rigid, i.e., with fixed bond angles and bond lengths, or as flexible in which the bond angles and bond lengths vary but the chemical bonds cannot be broken. In this work we present our progress in the development of a water model which allows for the breaking and formation of chemical bonds. The force field was obtained by fitting {\it ab initio} (not DFT) energies, forces, and molecular properties. The ability of the model to predict properties of water at ambient and extreme conditions will be presented. We will also report on the modeling of small clusters of water using the dissociative force field. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A26.00003: Emergence of charge density wave domain walls above the superconducting dome in TiSe$_2$ Y.I. Joe, X.M. Chen, P. Ghaemi, K.D. Finkelstein, G.A. de la Pe\~na, Y. Gan, J.C.T. Lee, S. Yuan, J. Geck, G.J. MacDougall, T.C. Chiang, S.L. Cooper, E. Fradkin, P. Abbamonte Superconductivity (SC) in so-called ``unconventional superconductors'' is nearly always found in the vicinity of another ordered state, such as antiferromagnetism, charge density wave (CDW), or stripe order. This suggests a fundamental connection between SC and fluctuations in some other order parameter. 1$T$-TiSe$_2$ is a prototypical CDW material in the transition-metal dichalcogenide family and was previously shown to exhibit SC when the CDW is suppressed by hydrostatic pressure or intercalation of Cu atoms. Here, we present detailed high pressure x-ray scattering study on 1$T$-TiSe$_2$. We found that the CDW phase of 1$T$-TiSe$_2$ is completely suppressed on the application of hydrostatic pressure and established the existence of a quantum critical point (QCP). Unexpectedly, we observed a weakly first order, incommensurate CDW phase, suggesting the presence of a Lifshitz tricritical point somewhere above the superconducting dome. Our study suggests that SC in 1$T$-TiSe$_2$ may not be directly connected to the QCP of the CDW order, but to the formation of CDW domain wall. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A26.00004: High-pressure neutron scattering of Prussian blue analogue magnets D.M. Pajerowski, S.E. Conklin, J. Leao Pressure sensitive magnetism is known to be useful in sensors, and while applications tend to use metallic alloys, molecule based magnets (MBMs) have been shown to have large inverse magnetostrictive (IMS) response. A promising group of MBMs are the Prussian blue analogues (PBAs), in which magnetic ordering can be tuned by external stimuli such as light, electric field, and pressure. Two IMS active PBAs are KFe$_{\mathrm{3}}$[Cr(CN)$_{\mathrm{6}}$]$_{\mathrm{2}}$ (Fe-Cr) and KNi$_{\mathrm{3}}$[Cr(CN)$_{\mathrm{6}}$]$_{\mathrm{2}}$ (Ni-Cr), and there are open questions about the details of the observed effects. Presently, it is believed that under applied pressure, Fe-Cr undergoes a linkage isomerism (LI) that changes carbon coordination of the CN from Cr to Fe, resulting in a change in magnetic configuration of the Fe cation from high-spin (HS) S $=$ 4 to low-spin (LS) S $=$ 0, thus reducing the observed magnetization. On the other hand, Ni-Cr is thought to undergo random spin-canting due to either bond-deformation or LI. We utilize neutron diffraction to test these theories. Polarized beam experiments are also performed to test a contrary hypothesis of domain wall movement providing the pressure sensitive magnetism. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A26.00005: P-V-T equation of state of SiC-3C: implications for primary pressure scale Kirill Zhuravlev, Alexander F. Goncharov, Sergey Tkachev, Przemyslaw Dera, Vitali Prakapenka We present a new primary pressure scale based on concomitant measurements of the density and elastic parameters of the single crystal samples of cubic silicon carbide (3C-SiC) under quasi-hydrostatic pressures up to 65 GPa and 773 K. The established pressure scale has precision of 2{\%}--4{\%} up to 65 GPa and will allow more accurate pressure determination in that range than the previously used pressure scales. We also report x-ray diffraction data and Raman spectroscopy on 3C-SiC up to 75 GPa. We determined the P-V-T equation of state (EOS) of 3C-SiC and pressure and temperature dependencies of the zone-center phonons, elastic tensor, and mode Gruneisen parameters. Cubic SiC lattice was found to be stable up to 75 GPa, but there is a tendency for destabilization above 40 GPa, based on softening of a transverse sound velocity. We proposed corrections to the existing ruby and neon pressure scales, and~also calibrated cubic SiC as an optical pressure~marker using Raman spectroscopy. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A26.00006: Mercury Fluorides under high pressure: Hg as a pressure-induced transition metal Jorge Botana, Xiaoli Wang, Dadong Yang, Haiqing Ling, Yangming Ma, Mao-Sheng Miao Hg has recently been found experimentally to be capable of forming a chemical compound, HgF$_4$, where it behaves as a transition metal, with an oxidation number of IV, but this molecule is very short lived. In this work we present theoretical evidence obtained through \textit{ab initio} calculations that higher oxidation states than II can be stabilized in crystalline form for Hg, under extreme pressure. We have performed a structural search and optimization by means of Particle Swarm Optimization and Density Functional Theory for the crystalline series of HgF$_n$ (n=3,4,5,6), and then used those data to draw the phase diagram of the equilibrium among those stoichiometries and HgF$_2$ and F$_2$. We have found that from $0$ to $38$ $GPa$ only the mixture of HgF$_2$ and F$_2$ phases is thermodynamically stable. HgF$_3$ and HgF$_4$ have been found to be thermodynamically stable in different pressure ranges (from $73$ $GPa$ to at least $500$ $GPa$ and from $38$ $GPa$ to $200$ $GPa$, respectively). We have also found that the HgF$_3$ crystal shows a very interesting band structure that suggests it could be a transparent conductor. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A26.00007: Electrical resistivity study of Magnetite under high pressure Takaki Muramatsu, Viktor Struzhkin, Lev Gasparov Magnetite is known as one of the oldest magnetic materials and crystallizes in the inversed spinel structure. At about 120 K magnetite undergoes a structural phase transition called Verway transition where electrical resistivity abruptly increases with decreasing temperature. Pressure effects of Verway transition studied by magnetic susceptibility and electrical resistivity by several groups revealed Verway transition decreased with pressure and the precise pressure effects depend on the pressure condition i.e., pressure transmitting media. In this work, electrical resistivity measurements were made to revisit the property of magnetite under pressure. Both metallization observed in precedent work using cubic anvil press and the higher pressure properties beyond metallization are examined by diamond anvil cell. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A26.00008: Observation of antiferromagnetic order collapse in the pressurized insulator LaMnPO Jing Guo, Jack Simonson, Liling Sun, Qi Wu, Peiwen Guo, Chao Zhang, Dachun Gu, Gabriel Kotliar, Meigan Aronson, Zhongxian Zhao The emergence of superconductivity in the iron pnictide or cuprate high temperature superconductors usually accompanies the suppression of a long-ranged antiferromagnetic (AFM) order state in a corresponding parent compound by doping or pressurizing. A great deal of effort by doping has been made to find superconductivity in Mn-based compounds, which are thought to bridge the gap between the two families of high temperature superconductors, but the AFM order was not successfully suppressed. Here we report the first observations of the pressure-induced elimination of long-ranged AFM order at $\sim$ 34 GPa and a crossover from an AFM insulating to an AFM metallic state at $\sim$ 20 GPa in LaMnPO single crystals that are iso-structural to the LaFeAsO superconductor by \textit{in-situ} high pressure resistance and \textit{ac} susceptibility measurements. These findings are of importance to explore potential superconductivity in Mn-based compounds and to shed new light on the underlying mechanism of high temperature superconductivity. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A26.00009: Study electron-phonon interactions under high pressures by ultrafast time-resolved spectroscopy Xiaojing Tan, Alexander Goncharov, Viktor Struzhkin, Xiaojia Chen We study the electron-phonon interactions in Al under high pressures by ultrafast time-resolved spectroscopy. We observe dramatic change of the pump-probe signals with pressure between 3.0 and 7.5 GPa: a fast decay in order of picosecond, followed by a bump that evolves with pressure. The change can be explained as competitions between electron-phonon coupling, hot electron diffusion and non-Fermi distribution of hot electrons. It's the decrease of electron-phonon coupling with pressure that cause hot electron diffusion play the main role for the fast decay, and the change of the non-Fermi electrons distribution results the evolution of the bump with pressure. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A26.00010: Fermi-surface reconstruction under pressure in the cuprate superconductors YBCO and Nd-LSCO Sophie Dufour-Beaus\'ejour, Olivier Cyr-Choini\`ere, Ga\"el Grissonnanche, Marcin Matusiak, Fazel Fallah Tafti, Elena Hassinger, Samuel Ren\'e de Cotret, Nicolas Doiron-Leyraud, Louis Taillefer, Brad Ramshaw, Ruixing Liang, Doug Bonn, Walter Hardy, Jianshi Zhou, John Goodenough, David Graf It is well established by now that the Fermi surface of hole-doped cuprates such as YBCO [1], Eu-LSCO [2], Nd-LSCO [3] and Hg1201 [4] undergoes a reconstruction caused by the emergence of charge order at low temperature. Here we show how the process of Fermi-surface reconstruction evolves as a function of applied hydrostatic pressure in both YBCO and Nd-LSCO, via measurements of the resistivity and Hall coefficient as a function of temperature and magnetic field. \\[4pt] [1] D. LeBoeuf \textit{et al}., Physical Review B \textbf{83}, 054506 (2011). \\[0pt] [2] F. Lalibert\'{e} \textit{et al}., Nature Communications \textbf{2}, 432 (2011). \\[0pt] [3] R. Daou \textit{et al}., Nature Physics \textbf{5}, 31 (2009). \\[0pt] [4] N. Doiron-Leyraud \textit{et al}., Physical Review X \textbf{3}, 021019 (2013). [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A26.00011: High Pressure study of Ba$_{1-x}$Na$_x$Ti$_2$Sb$_2$O with x = 0, 0.10, and 0.15 Melissa Gooch, Phuong Doan, Zhongjia Tang, Bernd Lorenz, Arnold Guloy, Ching Wu Paul Chu Here we report a systematic study of the effects of pressure on the resistivity for the superconducting and spin/charge density wave (SDW/CDW) transitions of Ba$_{1-x}$Na$_x$Ti$_2$Sb$_2$O (x = 0, 0.10, and 0.15). With increasing pressure no measurable change is observed for the SDW/CDW transition temperature (T$_S$) for x = 0.15; however, for x = 0 and 0.10 a decrease of the SDW/CDW transition temperature T$_S$ is observed. With respect to the superconducting transition temperature T$_c$, the effects of pressure effect on the three samples are different. The T$_c$ of BaTi$_2$Sb$_2$O increases linearly from 1.2 K to ~ 2.9 K at 16.1 kbars. In contrast, T$_c$ of Ba$_{0.90}$Na$_{0.10}$Ti$_2$Sb$_2$O only initially increases to 4.2 K and then saturates at higher pressure values. For Ba$_{0.85}$Na$_{0.15}$Ti$_2$Sb$_2$O, T$_c$ continuously decreases with increasing pressure. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A26.00012: Large enhancement of $T_{c}$ of Sr$_{2}$RuO$_{4}$ under uni-axial strain Daniel O. Brodsky, Clifford W. Hicks, Edward A. Yelland, Alexandra S. Gibbs, Jan A.N. Bruin, Mark E. Barber, Stephen D. Edkins, Keigo Nishimura, Shingo Yonezawa, Yoshiteru Maeno, Andrew P. Mackenzie We present AC magnetic susceptibility data taken on samples of the spin-triplet superconductor Sr$_{2}$RuO$_{4}$ under uni-axial strain. To do this, we built a probe that enables us to vary the strain applied to our samples continuously from compression to tension, whilst at cryogenic temperatures. We found that $T_{c}$ changes dramatically with in-plane strain: strain along the crystallographic [100] direction leads to a strong strain-symmetric response of $T_{c}$, which is pushed up from 1.35 K to 1.9 K for 0.23{\%} strain. Conversely, the response along the [110] direction is weak and mostly linear in strain. We discuss these results in the context of the predicted p$_{x} + i$p$_{y}$ topological order parameter. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A26.00013: The Strain Derivatives of $T_c$ in HgBa$_2$CuO$_{4+\delta}$: CuO$_2$ Plane Alone Is Not Enough Shibing Wang, Jianbo Zhang, Xiao-Jia Chen, Viktor Struzhkin, Wojciech Tabis, Neven Barisic, Mun Chan, Chelsey Dorow, Xudong Zhao, Martin Greven, Wendy Mao, Ted Geballe The strain derivatives of $T_c$ along the $a$ and $c$ axes have been determined for HgBa$_2$CuO$_{4+\delta}$ (Hg1201), the simplest monolayer cuprate with the highest $T_c$ of all monolayer cuprates ($T_c$ = 97 K at optimal doping). The underdoped compound with the initial $T_c$ of 65~K has been studied as a function of pressure up to 20 GPa by magnetic susceptibility and X-ray diffraction (XRD). The observed linear increase in $T_c$ with pressure is the same as previously been found for the optimally-doped compound. The above results have enabled the investigation of the origins of the significantly different $T_c$ values of optimally doped Hg1201 and the well-studied compound La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO), the latter value of $T_c$ = 40 K being only about 40\% of the former. Hg1201 can have almost identical CuO$_6$ octahedra as LSCO if specifically strained. When the apical and in-plane CuO$_2$ distances are the same for the two compounds, a large discrepancy in their $T_c$ remains. Differences in crystal structures and interactions involving the Hg-O charge reservoir layers of Hg1201 may be responsible for the different $T_c$ values exhibited by the two compounds. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A26.00014: A DFT investigation of pressure effects in the infinite-layer ACuO$ _{2} $ cuprate superconductor for A=\{Mg, Ca, Sr, Ba\} Ben Mallett, Nicola Gaston, James Storey, Grant Williams, Alan Kaiser, Jeffery Tallon We use density functional theory to investigate external-pressure and ``internal-pressure'' effects in the infinite-layer cuprate ACuO$_2$ for A=\{Mg, Ca, Sr, Ba\}, where internal-pressure is induced by ion-size substitution. Where these materials have been synthesised we find good agreement between our calculated structural parameters and the experimental ones. We find that these non-hydrostatic pressure-effects can have a significant effect on the superconducting energy gap via modifications to the electronic dispersion. Furthermore, pressure alters the dispersion independently of how it is applied (internal or external) so that the superconducting energy gap correlates with the unit-cell volume. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A26.00015: Pressure-induced insulator to metal transitions in potential 3D topological insulators Ag$_{2}$Se and Ag$_{2}$Te Zhao Zhao, Shibing Wang, Artem Oganov, Pengcheng Chen, Haijun Zhang, Zhenxian Liu, Wendy Mao Silver chalcogenides Ag$_{2}$Se and Ag$_{2}$Te are non-magnetic compounds exhibiting interesting physics like linear magnetoresistance and dimensionality tunable band gap. To explore their behaviors under high pressure, we performed synchrotron X-ray diffraction and infrared experiments combined with evolutionary algorithm structure predictions and \textit{ab-initio} band structure calculations. For Ag$_{2}$Se, the unusual increase of phase I's band gap and topologically nontrivial features of its band structure support $\beta -$Ag$_{2}$Se as a potential 3D topological insulator. The bulk insulating phase I first transforms to a bulk metallic phase II with 2.4 percent volume drop, marked by the appearance of the distinctive Se(Ag1)-Ag2-Se(Ag1) triple layers stacking pattern. At higher pressure, a transition from the phase II to a completely metallic phase III was observed. And for Ag$_{2}$Te, our study shows that the bulk insulating phase I first transforms into a semi-metallic phase II with the perseverance of its topologically non-trivial nature, and then to a bulk metallic phase III. Our study highlights pressure's role in tuning the electronic structures of Ag$_{2}$Se and Ag$_{2}$Te. [Preview Abstract] |
Session A27: Electronic Structure Methods I
Sponsoring Units: DCOMPChair: Brandon Cook, Oak Ridge National Laboratory
Room: 501
Monday, March 3, 2014 8:00AM - 8:12AM |
A27.00001: Selectively Localized and Symmetric Wannier Functions Runzhi Wang, Emanuel Lazar, Hyowon Park, Andrew Millis, Chris Marianetti The method of Marzari and Vanderbilt for computing maximally localized Wannier functions (MLWF) is widely used to represent localized orbitals in periodic materials. However the standard MLWF method minimizes the global spread of all orbitals in a preselected energy window. In many methods for strongly correlated electronic systems, including the density functional plus dynamical mean field method, one wishes to localize one particular class of orbitals (ie. a transition metal d orbital) without regard for the localization of the other states (eg. oxygen p and s). In addition, guaranteed preservation of pre-specficied point symmetry is desirable. Here we present an approach to this problem and demonstrate its implementation in model systems and real materials. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A27.00002: Failure of hybrid functionals to predict band gaps of materials at nanoscale Xinquan Wang, Zhigang Wu It is well known that density functional theory (DFT) within LDA/GGA severely underestimates band gaps in semiconductors and insulators due to the lack of derivative discontinuity in exchange-correlation, and hybrid functionals have been widely employed to improve band-gap calculations within the framework of DFT. In this work we show that hybrid functionals are not reliable in predicting band gap for nanostructures by comparing the hybrid functionals results of Si nanowires with those obtained using the many-body perturbation theory within the GW approximation. The hybrid functionals give a worse band-gap scaling law than that of LDA/GGA and their success in bulk materials is largely fortuitous, because they cannot correctly describe the response to the variation in screening. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A27.00003: DFT properties of Quasi-One-Dimensional Nanostructured Materials John Mintmire, Junwen Li, Daniel Gunlycke, Carter White Over the past several years we have made substantial progress in developing an approach for density-functional electronic structure calculations on quasi-one-dimensional nanostructures with helical symmetry. In this talk we discuss the application of these first-principles methods using Gaussian basis sets for calculating the electronic band structure of periodic graphitic nanostructures such as carbon nanotubes and graphene nanoribbons In particular we discuss how chemical effects at the edges of saturated graphene nanoribbons can cause ribbons to twist and form three-dimensional helical structures. Our calculations show that F-terminated armchair ribbons twist into helices, unlike flat H-terminated strips. Twisting ribbons of either termination couple the conduction and valence bands, resulting in band gap modulation. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A27.00004: A Self-consistent Mixing Parameter Scheme for Hybrid Functionals Applied to Periodic Systems Jonathan Skone, Marco Govoni, Giulia Galli We present a self-consistent scheme for determining the optimal fraction of exact exchange ($\alpha$) for hybrid functionals applied to condensed phase systems. It has been previously shown that the optimal mixing parameter is related to the inverse of the dielectric constant in solids, which in turn is related to the statically screened exchange term in the electronic self-energy within the GW approximation. We use this relationship to evaluate $\alpha$ self-consistently so as to obtain a mixing-parameter that is independent of the (arbitrary) choice of the initial fraction of exact-exchange. Our self-consistent scheme (sc-EXX) does not rely on any empirical parameters and is straightforward to apply to semiconducting and insulating, periodic systems. We show that for a variety of solids the sc-EXX scheme yields macroscopic dielectric constants in excellent agreement with experiment and provides considerable improvement in quasi-particle gaps over other non-empirical hybrid functionals with fixed exact exchange (e.g. PBE0). Furthermore, this approach provides an affordable way of capturing the static screening effects in a self-consistent manner, thus providing a superior starting point for GW calculations that include full dynamical screening. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A27.00005: Charged excitations in extended nanostructures from Koopmans-compliant functionals Nicolas Poilvert, Ismaila Dabo Koopmans-compliant (K) functionals aim to restore the piecewise linearity of approximate density-functional theory (DFT) functionals, generalizing ideas first introduced in the case of DFT+U functionals, but not restricted to predefined atomic orbitals. K functionals enable one to recover meaningful energy levels, which can thus be interpreted as charged excitation energies. Although it has been shown that K calculations yield energy levels and cross sections in excellent agreement with photoelectron spectroscopies, applications to crystalline materials have been lacking. Here, we report on recent progress in describing the band structures of periodic systems within K approximations. Our approach proceeds by analyzing the response of the electron density upon charged excitation in the limit of increasingly large systems. This analysis underscores important differences between conventional DFT approximations and their K counterparts, and enables us to generalize K functionals to extended systems. Validation of this approach is provided by the accurate description of $sp^{2}$-bonded carbon nanostructures. In the process, we highlight the performance of common DFT approximate functionals in capturing charged excitations in materials, provided that electronic localization is enforced [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A27.00006: Hybrid functional and quasiparticle calculations of the Schottky barrier height at TiN/HfO$_{2}$ interface Young Jun Oh, Alex Taekyung Lee, Hyeon-Kyun Noh, K.J. Chang In high-k/metal gate transistors, it is important to control the metal work function such that it should be close to the valence and conduction band edges of Si in $p$- and $n$-channel devices, respectively. The Schottky barrier height (SBH) is affected by composition of metal gate, impurity, and deposition process. In theoretical studies, using the local density functional approximation, the SBH is severely underestimated because of the underestimation of the dielectric band gap. In this work, we perform both hybrid functional and quasiparticle calculations to improve the band gap and effective work function in TiN/HfO$_{2}$ interface. We consider two types of TiN/HfO$_{2}$ interface structures, which consist of either Ti-O or N-Hf interface bonds. Depending on the type of interface bonds, the SBH differs by 0.36 eV. In the many-body perturbation theory, the \textit{GW}$_{0}$ approach, which employs the self-consistent Green's function and the full frequency-dependent dielectric function, greatly improves the agreement of the SBH with experiments. We discuss the effects of the self-consistency and the plasmon-pole approximation on the SBH. On the other hand, with the hybrid functional, the SBH is overestimated due to the larger downward shift of the valence band edge of HfO$_{2}$. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A27.00007: Unfolding the Berry curvature of supercell calculations Raffaello Bianco, Raffaele Resta, Ivo Souza Unfolding band structures of supercell calculations has become a valuable tool for visualizing the influence of point impurities on the electronic states in crystals. In the same spirit, we introduce a procedure which maps the $k$-space Berry curvature of the occupied states from the small BZ of a supercell onto the normal BZ of the perfect (or virtual) crystal. As an application, we analyze the $k$-space distribution of the unfolded curvature of bcc Fe$_{1-x}$Co$_x$ ordered alloys, to better understand the influence of alloying on the anomalous Hall conductivity. Comparing with the ordinary curvature calculated in the virtual-crystal approximation, we find that the lowering of translational symmetry by the Co ``impurities'' introduces ``extrinsic'' contributions, which correlate with changes in the spectral function near the Fermi surface. In particular, the unfolded curvature displays additional sharp peaks associated with low-energy \textit{pseudovertical} transitions. These occur in regions of $k$-space where two unfolded bands, which in the virtual crystal would be separated in $k$-space (and therefore would not jointly contribute to its Berry curvature), lie on either side of the Fermi level and are coupled by the impurity potential. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A27.00008: Relaxation of atomic orbitals in a plane-wave basis set Jose Luis Martins, Carlos L. Reis We investigate a first-principles calculations scheme that uses a small or even minimal atomic orbital basis-set which is expanded in plane-waves and is subsequently augmented by a simple relaxation procedure in that same plane-wave basis set. Our results show good agreement between the standard pseudopotential plane-wave method and the novel hybrid methodology. The proposed method is simple to implement in existing plane-wave computer programs and leads to substantial gains in computation speed while maintaining reasonable accuracy. We show results for some test cases, including local-density band-structures of silicon and graphite and radial distribution functions of liquid silicon. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A27.00009: Consistent treatment of charged systems within periodic boundary conditions: The Projector Augmented-Wave and pseudopotential methods revisited Jean-Paul Crocombette, Fabien Bruneval, Xavier Gonze, Boris Dorado, Marc Torrent, Francois Jollet The \textit{ab initio} calculation of charged defect properties in solids is not straightforward because of the delicate interplay between the long-range Coulomb interaction and the periodic boundary conditions. We derive the Projector Augmented-Wave (PAW) energy and hamiltonian with a special care on the potentials from Coulomb interaction. By explicitly treating the background compensation charge, we find a new term in the total energy of charged cells and in the potential. We show that this background term is needed to accurately reproduce all-electron calculations of the formation energy of a charged defect. In particular, the previous PAW expressions were spuriously sensitive to the pseudization conditions and this artifact is removed by the background term. This PAW derivation also provides insights into the norm-conserving pseudopotential framework. We propose then an alternative definition for the total energy of charged cells and for the potential within this framework. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A27.00010: A finite-size supercell correction scheme for charged defects in one-dimensional systems: Application to impurities in silicon nanowires Sunghyun Kim, Ji-Sang Park, K.J. Chang The formation energies of defects in solids are important to determine their stability and charge transition levels. In first-principles calculations for charged defects, supercells subject to periodic boundary conditions are commonly used. However, this approach suffers from spurious interactions between the defect and its image charges. Due to the long-ranged Coulomb interaction, a very large supercell is inevitable to obtain the numerical convergence. To overcome this problem, several finite-size supercell corrections have been proposed for bulk solids and two-dimensional systems. In this work, we propose a new finite-size correction scheme for charged defects in one-dimensional systems, where the medium is surrounded by vacuum in radial directions. The energy correction is obtained by solving the Poisson equation with the macroscopic dielectric constant. We show that the macroscopic dielectric constant and the defect charge distribution can be derived from the electrostatic potential in first-principles calculations. We test our scheme for charged B and P impurities in silicon nanowires. We find that the corrected formation energies converge rapidly with either increasing of the wire length or increasing of the vacuum pad, providing reliable charge transition levels. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A27.00011: Optimized Gaussian Basis Sets for Plane-Wave Compatible Calculations Jin Cheng, Florian Libisch, Mohan Chen, Emily Carter The Wu-Yang optimized effective potential method (WY-OEP) is becoming widely used in embedding theories to get the exact kinetic energy potential for a given density. Our group implemented this scheme in the plane-wave code (ABINIT) and showed that it performs well for potential functional embedding on the density functional theory (DFT)/DFT level. To extend this embedding scheme and the WY-OEP method to correlated-wavefunction (CW)/DFT embedding, it is necessary to perform a WY-OEP calculation with a CW density. However, the incompleteness of Gaussian basis sets used in CW calculations causes numerical instabilities and leads to unphysical behavior in the kinetic energy potential. We propose a method to construct a basis set that systematically approaches the plane-wave basis density while retaining the quality of the CW. By doing so, basis set incompatibility has been eliminated. Test calculations show that good agreement for the density can be reached and the CW calculations give reasonable results. Furthermore, the WY-OEP has been performed with densities from a variety of CWs. The densities are well-reproduced and the kinetic energy potential is free of unphysical behavior, boding well for such potential-functional-embedded CW calculations. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A27.00012: Shirley Reduced Basis DFT: plane-wave generality and accuracy at reduced computational cost Maxwell Hutchinson, David Prendergast The Shirley Reduced Basis (SRB) provides a means for performing density functional theory electronic structure calculations with plane-wave accuracy and generality in a basis of significantly reduced size. The SRB is comprised of linear combinations of periodic Bloch functions sampled coarsely over the Brillouin zone (BZ) and selected for maximal information content using proper orthogonal decomposition [E. Shirley, Phys. Rev. B 54, 464 (1996)]. A basis produced from only order 10 samples, lying on the BZ boundary, is able to reproduce energies and stresses to sub meV and kbar accuracy, respectively, with order 10 basis functions per electronic band. Unlike other electronic structure bases of similar sizes, the SRB is adaptive and automatic, making no model assumptions beyond the use of pseudopotentials. We provide the first self-consistent implementation of this approach, enabling both relaxations and molecular dynamics. We demonstrate the usefulness of the method on a variety of physical systems, from crystalline solids to reduced dimensional systems under periodic boundary conditions, realizing order of magnitude performance improvements while kept within physically relevant error tolerances. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A27.00013: Relativistic Optimized Norm-Conserving Vanderbilt Pseudopotentials D.R. Hamann Two-projector fully non-local pseudopotentials obeying the generalized norm-conserving condition\footnote{D. Vanderbilt, Phys. Rev. B \textbf{41}, 7892 (1990).} and incorporating systematic convergence optimization\footnote{A. M. Rappe\textit{ et al.}, Phys. Rev. B \textbf{41}, 1227 (1990).} have been shown to accurately reproduce all-electron results with high computational efficiency.\footnote{D. R. Hamann, Phys. Rev. B \textbf{88}, 085117 (2013).} The generalized norm-conservation theorem guarantees exact reproduction of all-electron norms, radial log-derivatives, and first energy derivatives of radial log derivatives at several energies, as well as the hermiticity of the non-local pseudopotential operator. This theorem is exact only for non-relativistic all-electron wave functions.\footnote{Vanderbilt} Averaging out small asymmetries of the non-local operators generated using scalar-relativistic Schr\"{o}dinger equation solutions preserves agreement of these quantities to order 10$^{-4}$, and yields excellent results for solids.\footnote{Hamann} I show that fully-relativistic Dirac-equation solutions can be treated in the same manner, with comparably small errors. Spin-orbit band splittings as well as other properties of several solids calculated with these pseudopotentials will be compared to fully-relativistic all-electron results. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A27.00014: Visualizing the Kohn-Sham kinetic energy density in molecules Antonio C. Cancio, Aeryk Kuna In recent years, driven by applications at high temperature and large system size, interest has turned to the construction of orbital-free density functionals, modeling the kinetic energy solely as a functional of the electron density and its derivatives. We visualize the Kohn-Sham kinetic energy density (KED) for the AE6 test set of molecules commonly used to test density functional performance for atomization energies. Calculations are performed using the ABINIT plane-wave code with over-converged cutoffs and simulation cell sizes to produce as accurate results as possible within a pseudopotential approximation. The orbital-dependent KED is compared to simple orbital-free models such as the Thomas-Fermi and von-Weiszacker KED's and to a sophisticated metaGGA-level functional proposed by Perdew and Constantin (PC). All models fail to reproduce the Kohn-Sham KED reasonably in high density regions -- covalent and polar bonds and valence lone-pairs. In particular, the PC model actually disimproves on the simpler gradient expansion model in these regions. A simple fix is proposed for the PC functional, substantially modifying its behavior for regions of high values of the Laplacian of the density and low density gradient. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A27.00015: An investigation of the generalized phase shifts and the integrated density of states for full-potential single site scattering G. Malcolm Stocks, Yang Wang, J. Sam Faulkner In the conventional multiple scattering theory approach to {\em ab initio} electronic structure calculations, the integrated density of states (IDOS) is determined by taking the imaginary part of the Green function integrated along an energy contour. In this presentation, we show a numerically more reliable approach that uses an analytical expression for the IDOS, derived from the Krein's theorem. We compare both approaches, Krein's theorem versus the Green function method, in single site cases (e.g., Cu, Al, Mo). And we discuss the concept of generalized phase shifts, which are the diagonal elements of a unitary transformation of the S-matrix for the full-potential single site scattering, and show their applications in the determination of the IDOS. [Preview Abstract] |
Session A28: Focus Session: Superconducting Qubits: Design & Tunable Coupling
Sponsoring Units: GQIChair: Douglas McClure, IBM
Room: 601
Monday, March 3, 2014 8:00AM - 8:36AM |
A28.00001: Noise spectroscopy and decoherence mitigation during free and driven evolution Invited Speaker: William Oliver Gate operations in a quantum information processor are generally realized by tailoring specific periods of free and driven evolution of a quantum system. Unwanted environmental noise, which may be distinct during these two periods, acts to decohere the system and increase the gate error rate. In this talk, we review our work on noise spectroscopy of superconducting qubits (persistent-current qubits, transmons) undergoing both free and driven evolution, and we present dynamical decoupling methods that can mitigate coherent errors in both cases. We discuss these results in the context of our present work and future directions. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A28.00002: Engineering Robust Superconducting Qubits Nate Earnest, Thomas Yu, Yao Lu, David Mckay, Jay Lawrence, Jens Koch, David Schuster The coherence times of superconducting qubits have advanced dramatically within the last few years. ~This has been primarily achieved by improving the microwave environment and reducing materials loss. ~Though the qubit parameters have changed significantly over the years, the qubit circuit has remained in the form of a LCJ circuit (flux,charge, and phase qubits). ~Recently, more sophisticated circuits, such as the 0-pi circuit proposed by Brooks et al.[1] offer the possibility of enhanced protection from dephasing and relaxation. In this talk, we will discuss the implementation of such circuits with realizable parameters, and present preliminary experimental results. [1] Brooks et al. PRA 87, 052306 (2013) [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A28.00003: Systematic Design of Tuned Transmon Qubits David Abraham, Jay M. Gambetta, Jerry M. Chow, Srikanth Srinivasan, Matthias Steffen We demonstrate a systematic method for designing two-dimensional superconducting transmon qubits with highly controllable and reproducible properties, including anharmonicity, resonant frequency and the ratio Ej/Ec. The main source of variation in these qubit properties is shown to be due to spreads in the critical current of the Josephson junction connecting the transmon capacitor pads. This technique is illustrated in a series of qubits with a range of properties, culminating in a design which accurately meets the desired operating point for multiqubit operation, and in addition obtains coherence times 2x higher than previously obtained, using conventional materials and fabrication methods. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A28.00004: Coherence properties of a capacitively-shunt flux qubit Jeffrey Birenbaum, Adam Sears, Christopher Nugroho, Ted Gudmundsen, Paul Welander, Jonilyn Yoder, Archana Kamal, Simon Gustavsson, Jamie Kerman, William Oliver, John Clarke Coherence times for typical flux qubits have plateaued at $5-10 ~\mu$s for $T_1$ and $1-3 ~\mu$s for $T_{Ramsey}$. To achieve longer coherence times we study capacitively-shunted flux qubits using high-Q capacitors to individually shunt all four Josephson junctions (JJs). The additional shunt capacitance moves $90+\%$ of the qubit energy from the lossy capacitance of the JJs into the high-Q shunts while preserving an anharmonicity greater than $100\%$ and maintaining $f_{01} < f_{12}$. The band structure is also flattened providing moderately decreased sensitivity to flux noise. Using high-quality MBE aluminum [1] we fabricate a capacitively-shunted flux qubit inductively coupled to a lumped-element readout resonator. The qubit junctions are deposited via aluminum e-beam evaporation using a bridgeless mask. We characterize the influence of qubit design parameters such as capacitance and geometry on the coherence time of the device. \newline [1] Megrant, \textit{et al}. APL \textbf{100}, 113510 (2012) [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A28.00005: Design and measurement of improved capacitively-shunted flux qubits Adam Sears, Jeffrey Birenbaum, David Hover, Theodore Gudmundsen, Andrew Kerman, Paul Welander, Jonilyn L. Yoder, Simon Gustavsson, Xiaoyue Jin, Archana Kamal, John Clarke, William Oliver The addition of a capacitive or inductive shunt across one of the junctions can alter the coherence properties of a classic flux or RF-SQUID qubit. We have studied the performance of capacitively shunted flux qubits fabricated with MBE aluminum[1], starting from a 2D coplanar waveguide geometry used in similar high-performance transmon qubits, and measured dispersively. We will detail the importance of design parameters that preserve the flux qubit's anharmonicity and discuss conclusions about materials quality based on calculations of the participation of junction, dielectric, and superconductor components. This research was funded in part by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA); and by the Assistant Secretary of Defense for Research \& Engineering under Air Force Contract number FA8721-05-C-0002. All statements of fact, opinion or conclusions contained herein are those of the authors and should not be construed as representing the official views or policies of IARPA, the ODNI, or the U.S. Government \\[4pt] [1] Megrant et al., APL 100, 113510 (2012). [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A28.00006: Superconducting qubits with adjustable coupling, Part I: Architecture Yu Chen, C. Neill, P. Roushan, R. Barends, B. Campbell, B. Chiaro, Z. Chen, A. Dunsworth, I. Hoi, E. Jeffrey, J. Mutus, A. Megrant, P. O'Malley, C. Quintana, D. Sank, J. Wenner, T. White, J. Kelly, A.N. Cleland, J.M. Martinis Building a practical quantum computer requires a scalable architecture suitable for large numbers of qubits. A major challenge is to achieve on-demand qubit-qubit interaction, such that turning the coupling off allows isolated single-qubit operations and turning the coupling on allows multi-qubit operations. By combining the high coherence Xmon qubits with an adjustable inductance, we have developed a new qubit architecture called g-mon, which has a tunable qubit-qubit interaction from 10 MHz to -50 MHz. We achieved nanosecond control of the coupling from positive to negative through zero, allowing for a high on/off ratio exceeding 1000. With additional advantages such as high modularity and moderate-distance compatibility, the g-mon architecture provide a potential scalable approch for future quantum computers. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A28.00007: Superconducting qubits with adjustable coupling, Part II: Fast two-qubit gates Charles Neill, Yu Chen, Pedram Roushan, Rami Barends, Brooks Campbell, Zijun Chen, Ben Chiaro, Andrew Dunsworth, IoChun Hoi, Evan Jeffrey, Julian Kelly, Anthony Megrant, Josh Mutus, Peter O'Malley, Chris Quintana, Daniel Sank, Jim Wenner, Ted White, Andrew Cleland, John Martinis The g-mon architecture combines high coherence Xmon qubits with fast tunable coupling. In this work, we demonstrate the advantages of tunable coupling to high fidelity single and two-qubit gates. By suppressing the qubit-qubit interaction, we are able to achieve high-fidelity simultaneous single qubit operations without the need for substantial detuning. Turning on the qubit-qubit interaction allows for a fast two-qubit controlled Z with gate times less than 30 ns. By eliminating the frequency crowding issues associated with static coupling and achieving two-qubit gate times approaching that of single qubit operations, the g-mon architecture is a promising system for scalable quantum computation. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A28.00008: Superconducting qubits with adjustable coupling, Part III: Simulating topological properties of quantum systems Pedram Roushan, C. Neill, Y. Chen, M. Kolodrubetz, R. Barends, I. Hoi, E. Jeffrey, J.Y. Mutus, B. Campbell, Z. Chen, B. Chiaro, A. Dunsworth, J. Kelly, A. Megrant, P. O'Malley, C. Quintana, D. Sank, T. White, J. Wenner, A. Polkovnikov, A. Cleland, J. Martinis The g-mon architecture with its adjustable qubit-qubit coupling makes a promising candidate for building a quantum simulator. Here, we demonstrate the versatility of this system to simulate the topological properties of interacting Hamiltonians. So far, experimental studies of topological invariants in condensed matter systems have been limited to transport measurements. Recently, it was proposed [1] that the topological properties of Hamiltonians can be inferred from quantum dynamics. The Berry curvature, a quantity that reflects the geometrical properties of the eigenstates, can emerge as the non-adiabatic response to the rate of change of an external parameter. Using superconducting g-mon qubits, we measure the Berry curvature for various eigenstates of the Hamiltonian of the system. We will discuss the robustness of the measured Chern numbers, by showing their path independence in the parameter space. \\[4pt] [1] Gritsev and Polkovnikov, PNAS, 109, 6457 (2012). [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A28.00009: Tunable coupling between two superconducting resonators F. Deppe, F. Wulschner, A. Baust, E. Hoffmann, E.P. Menzel, A. Marx, R. Gross, E. Solano, D. Zueco, J.-J. Garcia Ripoll During the last decade, tremendous progress has been made towards quantum computation and quantum simulation with superconducting circuits. In such architectures, the controlled exchange of information between two superconducting transmission line resonators via a tunable coupling is a useful tool. Here, we present experimental progress on such devices. Specifically, the coupling is mediated either by a superconducting flux qubit or by an RF~SQUID. Our results allow us to analyze the tunable coupling in frequency and time domain. We acknowledge support from: the DFG via SFB~631; the German excellence initiative via NIM; the EU projects \mbox{CCQED}, PROMISCE, SCALEQIT; Spanish MINECO FIS2009-12773-C02-01, FIS2011-25167, FIS2012-36673-C03-02; UPV/EHU UFI 11/55; Basque Government IT472-10. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A28.00010: Tunable-cavity QED with phase qubits Jed D. Whittaker, Fabio da Silva, Michael Shane Allman, Florent Lecocq, Katarina Cicak, Adam Sirois, John Teufel, Jose Aumentado, Raymond W. Simmonds We describe a tunable-cavity QED architecture with an rf SQUID phase qubit inductively coupled to a single-mode, resonant cavity with a tunable frequency that allows for both tunneling and dispersive measurements. Dispersive measurement is well characterized by a three-level model, strongly dependent on qubit anharmonicity, qubit-cavity coupling and detuning. The tunable cavity frequency provides dynamic control over the coupling strength and qubit-cavity detuning helping to minimize Purcell losses and cavity-induced dephasing during qubit operation. The maximum decay time $T_1 = 1.5\,\mu\rm{s}$ is limited by dielectric losses from a design geometry similar to planar transmon qubits. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A28.00011: A Simple, Rapid, and Accurate Method to Calculate Coupling in Coplanar Superconducting Qubit Circuits B. Chiaro, R. Barends, B. Campbell, Y. Chen, Z. Chen, A. Dunsworth, E. Jeffrey, J. Kelly, M. Mariantoni, A. Megrant, J. Mutus, C. Neill, P. O'Malley, C. Quintana, P. Roushan, D. Sank, A. Vainsencher, J. Wenner, T. White, A.N. Cleland, J.M. Martinis A critical design consideration for quantum circuits is the coupling between constituent elements. Both capacitive and inductive coupling can be accurately calculated through numerical simulations with commercial software. However, this approach can be slow and obscures the underlying physics, motivating the development of an analytic theory. The case of coupling between electrodes embedded in a groundplane is particularly interesting to the planar superconducting qubit community. As circuits in this field become more complex, notably in the UCSB multi-Xmon experiments, it is essential to understand the nature of electrode interactions and calculate them rapidly and accurately. I will show how to calculate electrode couplings with a simple integral method and compare its predictions with experimental data and Sonnet software simulations of coupling in planar circuits used for quantum computing. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A28.00012: Protected Josephson Rhombi Chains Matthew Bell, Josh Paramanandam, Lev Ioffe, Michael Gershenson We have studied the low-energy excitations in a minimalistic protected Josephson circuit which contains two basic elements (rhombi) characterized by the $\pi$ periodicity of the Josephson energy. The novel design of these elements, which reduces their sensitivity to the offset charge fluctuations, has been employed. We have observed that the lifetime $T_{1}$ of the first excited state of this quantum circuit in the protected regime is increased by up to $70\mu$s, a factor of $\sim$100 longer than that in the unprotected state. The decay quality factor $\omega_{01}T_{1}$ of this qubit exceeds $10^{6}$. Our results are in agreement with theoretical expectations; they demonstrate the feasibility of symmetry protection in rhombi-based qubits fabricated with existing technology. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A28.00013: Controlling discrete and continuous symmetries in ``superradian'' phase transitions Alexandre Baksic, Cristiano Ciuti The Dicke model describing the interaction of a single-mode boson field to an ensemble of two-level systems is an important paradigm in quantum optics. In particular, the physics of the ``superradiant phase transition'' is the subject of a vigorous research activity. Recently, we explored a model describing a collection of two-level systems, each one coupled to both quadratures of a boson mode [1]. We show that by tuning the two quadrature coupling constants it is possible to control the symmetries of the system, with the possibility of having a U(1)-symmetry even in presence of non-rotating wave (anti-resonant) coupling terms, which are relevant in the ultrastrong coupling regime. We determine the rich phase diagram of such model and show the appearance of Goldstone and amplitude modes. We also show an example of circuit QED configuration where those effects can be observed, by coupling both capacitively and inductively a Josephson junction artificial atom to a superconducting resonator.\\[4pt] [1] A. Baksic and C. Ciuti, arXiv:1310.3780 (2013). [Preview Abstract] |
Session A29: Driven, Dissipative Many-body Systems
Sponsoring Units: DAMOPChair: Daniel Grief, ETH Zurich
Room: 603
Monday, March 3, 2014 8:00AM - 8:12AM |
A29.00001: Extrapolation of the long time behavior of a quasithermal field driven quantum system Herbert Fotso, Jim Freericks We study the relaxation of an isolated interacting heavy-light Fermi-Fermi mixture after it is placed under the influence of an artificial constant electric field. In the regime where the dynamics of the system are characterized by a monotonic evolution through quasi-thermal states and respects the fluctuation dissipation theorem throughout its relaxation, we use the monotonic increase of the effective temperature to infer the two-time self-energy and extrapolate it at long times. This allows us to calculate, in the Dynamical Mean Field Theory framework, momentum dependent quantities at longer times than would otherwise be accessible. Such an approach makes it possible to observe the real-time approach to the steady state with a very low computational cost. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A29.00002: Beyond Planck-Einstein quanta: Crossover from frequency driven to amplitude driven excitation in a nonequilibrium many-body system James Freericks, Wen Shen, Tom Devereaux Planck introduced the idea of light quanta to calculate the spectrum for black body radiation, which was employed by Einstein to explain the photoelectric effect. Later, Kubo and Greenwood derived the linear response of a quantum system to an applied external field, and found that the energy available for excitation was determined by the frequency of the driving field as given by the Planck-Einstein relation. As the magnitude of the driving field is further increased into the nonlinear regime, one expects to see multiphoton processes and then for there to be a crossover from frequency-driven excitation of the quantum system to amplitude-driven excitation. Here we use the exact quantum solution of ultracold spinless fermions in a double-well optical lattice driven by an artificial pulsed electric field to show generically how such a crossover occurs. We find that the behavior is quite complex due to excitation and de-excitation processes, so that it is no longer true that tunneling is optimized when the field amplitude is the highest. When the field amplitude becomes very large, there is a novel quantum oscillatory behavior in the excitation spectroscopy that appears to describe a new regime for quantum phenomena. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A29.00003: Dissipation as a resource for atomic binding and crystallization Mikhail Lemeshko, Johannes Otterbach, Hendrik Weimer The formation of molecules and supramolecular structures results from bonding by conservative forces acting among electrons and nuclei and giving rise to equilibrium configurations defined by minima of the interaction potential. Here we show that bonding can also occur by the non-conservative forces responsible for interaction-induced coherent population trapping. The bound state arises in a dissipative process and manifests itself as a stationary state at a preordained interatomic distance. Remarkably, such a dissipative bonding is present even when the interactions among the atoms are purely repulsive. The dissipative bound states can be created and studied spectroscopically in present-day experiments with ultracold atoms or molecules and can potentially serve for cooling strongly interacting quantum gases [1]. An extension of this technique to a many-particle system (Bose-Einstein Condensate of Rydberg-dressed atoms) allows to observe long-range ordered crystalline structures emerging due to dissipation [2]. \\[4pt] [1] M. Lemeshko, H. Weimer, ``Dissipative binding of atoms by non-conservative forces'' Nature Communications 4, 2230 (2013)\\[0pt] [2] Johannes Otterbach, Mikhail Lemeshko, ``Long-Range Order Induced by Dissipation,'' arXiv:1308.5905 [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A29.00004: Quantum quench dynamics in the presence of a thermal bath Smitha Vishveshwara, Aashish Clerk, Peter Nalbach We explore the dynamics of a system driven through a quantum critical point by quenching its associated Hamiltonian at a specific rate in the presence of dissipation due to a thermal bath. In contrast to the quantum version of the well-known Kibble-Zurek mechanism in the absence of the bath, we discuss the enhancement of post-quench defect production due to thermal excitations. We argue that the degree of enhancement depends on an interplay between the out-of-equilibrium dynamics determined by the quench rate and finite temperature, and that it respects a scaling form related to these two quantities. We demonstrate our arguments within the specific context of the transverse Ising system in the presence of a global bath. Our approach is based on the physics of a Landau-Zener system coupled to a dissipative bath and it allows us to extend our analyses to a broad class of systems of differing dimensions and universality classes. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A29.00005: Coherence and a quench dynamics of a dissipative quantum system: renormalization group and dynamical phases Oleksiy Kashuba We study dissipation in a small quantum system coupled to an environment held in thermodynamic equilibrium. The relaxation dynamics of a system subjected to an abrupt quench in the parameters of the underlying Hamiltonian was investigated using two complementary renormalization group approaches. The methods were applied to the Ohmic spin-boson model close to the coherent-to-incoherent transition. In particular, the role of non-Markovian memory and the spin-boson coupling strength in the pre- and post-quench behavior is investigated. Additionally, we revealed several ``phases'' of the relaxation dynamics distinguished by the discrimination of the damping at long and intermediate time scale. Surprisingly, elevated temperature can render the system ``more coherent'' by inducing a transition from the partially coherent to the coherent regime. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A29.00006: Fluorescence spectrum of thermally driven array of QED cavities Mykola Bordyuh, Marco Schiro, Camille Aron, Baris Oztop, Hakan Tureci Developments in cavity QED technology allow us to engineer strong interactions between photons and atoms and therefore create possibilities to use light-matter systems as quantum simulators of many-body quantum systems. Quantum Phase Transition (QPT) of photons in arrays of coupled cavities described by models of closed systems such as the Jaynes-Cummings-Hubbard and the Rabi-Hubbard models (RHM) [1-8] have been studied extensively during last few years [1-8]. Our aim is to describe more realistic situations in which the system is open to an environment. We consider the RHM in which photons can leak out of the cavities. Based on the generalized input-output formalism, we show that measuring the fluorescence spectrum of the leaked photons gives us information about the system and the nature of the QPT.\\[4pt] [1] A. D. Greentree et al., Nature Phys. {\bf 466}, 856 (2006)\\[0pt] [2] M. J. Hartmann, F. G. S. L. Brandao, and M. B. Plenio, Nature Phys. {\bf 462}, 849 (2006).\\[0pt] [3] A. Tomadin, V. Giovannetti, R. Fazio, D. Gerace, I. Carusotto, Hakan E. T\"ureci, A. Imamoglu Phys. Rev. A {\bf 81}, 061801(R) (2010).\\[0pt] [4] S. Schmidt and G. Blatter Phys. Rev. Lett. {\bf 103}, 086493 (2009). [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A29.00007: Quantum phases of the Rabi lattice in the dispersive regime Guanyu Zhu, Sebastian Schmidt, Jens Koch Photon-based strongly correlated lattice models like the Jaynes-Cummings and Rabi lattices differ from their more conventional relatives like the Bose-Hubbard model by the presence of an additional tunable parameter: the frequency detuning between the pseudo-spin degree of freedom and the harmonic mode frequency on each site. Whenever this detuning is large compared to relevant coupling strengths, the system is said to be in the dispersive regime. The physics of this regime is well-understood at the level of a single Jaynes-Cummings or Rabi site, and can be realized in circuit-QED architecture. Here, we extend the theoretical description of the dispersive regime to lattices with many sites, for both strong and ultra-strong coupling. We discuss the nature and spatial range of the resulting qubit-qubit and photon-photon coupling. In the ultra-strong coupling regime, we demonstrate the emergence of the paramagnetic-to-ferromagnetic phase transition of photon-dressed qubits in the negative detuning regime, and the photon-pairing and vacuum squeezing in the positive detuning regime. We illustrate our results by exact diagonalization of the Rabi dimer. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A29.00008: Pattern Formation and Strong Nonlinear Interactions in Exciton-Polariton Condensates Li Ge, Ani Nersisyan, Baris Oztop, Hakan Tureci Exciton-polaritons generated by light-induced potentials can spontaneously condense into macroscopic quantum states that display nontrivial spatial and temporal density modulation. While these patterns and their dynamics can be reproduced through the solution of the generalized Gross-Pitaevskii equation, a predictive theory of their thresholds, oscillation frequencies, and multi-pattern interactions has so far been lacking. Here we represent such an approach based on current-carrying quasi-modes of the non-Hermitian potential induced by the pump. The presented theory allows us to capture the patterns formed in the steady-state directly and account for nonlinearities exactly. We find a simple but powerful expression for thresholds of condensation and the associated frequencies of oscillations, quantifying the contribution of particle formation, leakage, and interactions. We also show that the evolution of the condensate with increasing pump strength is strongly geometry dependent and can display contrasting features such as enhancement or reduction of the spatial localization of the condensate. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A29.00009: Time-reversal symmetric expansion of the time evolution operator of open quantum systems Naomichi Hatano, Gonzalo Ordonez We here consider open quantum systems of the tight-binding model, specifically a tight-binding chain with a scatter in the center. We succeeded in deriving a new expansion of the time evolution operator only with respect to the states of point spectra (bound, anti-bound, resonant and anti-resonant states), without the background integral over the continuum spectrum of scattering states. Since the expansion has no arbitrariness of the integration contour upon including decaying states, the expansion is perfectly symmetric under the time reversal. Among the expansion terms, the decaying resonant states naturally survive when we consider the time evolution from an initial condition, while the growing anti-resonant states naturally survive when we consider the time evolution to a terminal condition. This clearly shows that the emergence of the arrow of time is due to the choice of initial or terminal conditions, that it is not embedded in the time evolution itself. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A29.00010: Perturbative approach to open circuit QED systems Andy C.Y. Li, Francesco Petruccione, Jens Koch Perturbation theory (PT) is a powerful and commonly used tool in the investigation of closed quantum systems. In the context of open quantum systems, PT based on the Markovian quantum master equation is much less developed. The investigation of open systems mostly relies on exact diagonalization of the Liouville superoperator or quantum trajectories. In this approach, the system size is rather limited by current computational capabilities. Analogous to closed-system PT, we develop a PT suitable for open quantum systems. The proposed method is useful in the analytical understanding of open systems as well as in the numerical calculation of system observables, which would otherwise be impractical. This enables us to investigate a variety of open circuit QED systems, including the open Jaynes-Cummings lattice model. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A29.00011: Splitting time dependence in Lindblad master equation Gehad Sadiek, E.I. Lashin We consider the Markovian master equation in the Lindblad form. We assume an ansatz for the solution of the equation in the form of a time-dependent density matrix of the same form as the solution of the Hamiltonian but with time-dependent coefficients. We show that applying this ansatz one can find the solution of the master equation by solving a system of coupled differential equations of the coefficients utilizing the known time-evolved wave function driven by the Hamiltonian only. This approach splits the time dependence problem in the master equation into two parts, one carried by the wave function of the Hamiltonian and the other by the coefficients of the density matrix, which significantly simplifies the evaluation process. As an example we apply this approach to the problem of a system of qubits coupled to a Lindblad environment and demonstrate how powerful it is treating the problem. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A29.00012: Quantum Interference between independent environments in open quantum systems Ching-Kit Chan, Guin-Dar Lin, Susanne Yelin, Mikhail Lukin When a general quantum system interacts with multiple environments, the environmental effects are usually treated in an additive manner in the master equation. This assumption becomes questionable for non-Markovian environments that have finite memory times. Here, we show that quantum interferences between independent environments exist and can qualitatively modify the dynamics of the reduced physical system. We illustrate this effect with examples of atomic systems coupled to structured reservoirs, and discuss its origin in general using a non-equilibrium diagrammatic technique. The consequential decoherence dynamics cannot be captured by an additive master equation. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A29.00013: Observation of a Dissipation-Induced Classical to Quantum Transition James Raftery, Darius Sadri, Sebastian Schmidt, Hakan T\"ureci, Andrew Houck The emergence of non-trivial structure in many-body physics has been a central topic of research bearing on many branches of science. Important recent work has explored the nonequilibrium quantum dynamics of closed many-body systems. With the rapid technological advances in solid state quantum optics, it is now possible to experimentally study strongly correlated photons, and to build model systems whose open nature gives rise to rich emergent behavior. We report the experimental observation of a novel dissipation driven dynamical localization transition of strongly correlated photons in an extended superconducting circuit. Interaction with an environment has been argued to provide a mechanism for the emergence of classical behavior from a quantum system. Surprisingly, homodyne measurements reveal the observed localization transition to be from a regime of classical oscillations into a macroscopically self-trapped state manifesting revivals, a fundamentally quantum phenomenon. This experiment also demonstrates a new class of scalable quantum simulators with well controlled coherent and dissipative dynamics suited to the study of quantum many-body phenomena out of equilibrium. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A29.00014: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 10:48AM - 11:00AM |
A29.00015: Symmetries and conserved quantities in open quantum systems Victor V. Albert, Zaki Leghtas, Robert Schoelkopf, Michel Devoret, Mazyar Mirrahimi, Liang Jiang Recent developments in quantum engineering and quantum information highlight the need to better understand the steady-state behavior of open systems whose steady state is dependent on initial conditions. This work analyzes the structure and response to perturbations of the steady-state subspace of time-independent Lindblad master equations. We discuss the role of symmetries and conserved quantities in Lindblad systems and show that the effect of the environment in the infinite-time limit can be tracked exactly for arbitrary initial state and without knowledge of dynamics at intermediate time [1]. Applying this approach, we analytically determine the steady state for driven single-mode two-photon absorption and calculate that dephasing is exponentially suppressed to first order in such a system [2]. \newline [1] V.V. Albert and Liang Jiang, arXiv:1310.1523 \newline [2] M. Mirrahimi {\it et al.} (in preparation) [Preview Abstract] |
Session A30: Focus Session: Graphene Devices: Fabrication, Characterization and Modeling: Plasmons
Sponsoring Units: DMPChair: Dmitri Basov, University of California, San Diego
Room: 605
Monday, March 3, 2014 8:00AM - 8:12AM |
A30.00001: Efficient nonlinear generation of surface plasmons in graphene and topological insulators Alexey Belyanin, Xianghan Yao, Mikhail Tokman Two-dimensional materials with massless Dirac electrons such as graphene and topological insulators (TIs) support surface plasmon modes with a number of peculiar properties making them an attractive alternative to metal plasmonics. In this theoretical work we show that a coherent surface plasmon mode guided by graphene or a TI surface can be excited with high efficiency through the second-order nonlinear process of difference frequency generation (DFG). Although graphene is an isotropic medium for low-energy electron excitations, the second-order nonlinear susceptibility becomes non-zero when its spatial dispersion is taken into account. In this case the anisotropy is induced by the in-plane wave vectors of obliquely incident or in-plane propagating electromagnetic waves. The dispersion curves of surface plasmons strongly deviate from the photon dispersion already at terahertz (THz) frequencies, leading to a tight vertical confinement and large in-plane wave vector which can be matched to the sum of the photon wave vectors at mid- or even near-IR frequencies. This enables phase-matched DFG of THz plasmons with counter-propagating mid-infrared pump fields. The DFG process can reach efficiencies of 0.01/W and is broadly tunable by gating or varying an angle of incidence. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A30.00002: Measurement of THz radiation from DC heated graphene Jiayue Tong, Martin Muthee, Jun Yan, K. Sigfrid Yngvesson High mobility, tunable broadband optical response, and robust room temperature plasmon excitations have made graphene a promising candidate for THz applications. In this talk, I will discuss our studies of an antenna-coupled graphene THz source on a SiO$_{2}$/Si substrate, coupled through a silicon lens. Our experiments show that graphene samples coupled to a resonant double patch antenna emit strong radiation at about 2 - 2.5 THz. We then characterize the gate dependent THz radiation from heated graphene by simulating the antenna performance and comparing simulation results with experimental measurements. Our work paves the way for making practically useful graphene-based THz sources. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A30.00003: Enhanced control of THz optical conductivity in graphene via optimal extraordinary optical transmission electrode design Sara Arezoomandan, Berardi Sensale-Rodriguez Terahertz (THz) technology has recently arisen much attention for a wide range of applications. One of the major challenges in semiconductor-based THz devices is how to enhance and, therefore more efficiently tune, the material optical conductivity. Graphene due to its extraordinary properties has been extensively used for reconfigurable THz devices. By gating graphene, one can control its electrical thus THz properties. Several gating mechanisms have been proposed, including: Si-substrate, ion-gel, and extraordinary optical transmission (EOT) electrode. However, the optical conductivity swings in CVD graphene are typically limited to the 0.15-1.0mS range. Here we propose a simple device structure consisting of EOT electrode-gated graphene that can effectively achieve an enhancement of its effective optical conductivity swing, e.g. 3.8X effective conductivity enhancement at 2THz and 0.9mS. We accomplished this by optimizing the width/spacing of the metal stripes as well as the separation between graphene and the EOT electrode. The proposed structure does not require the addition of other epitaxially stacked electromagnetic structures (such as additional periodic metallic structures). This configuration shows promise as the active element in low-cost compact THz optoelectronics. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A30.00004: Plasmon anomaly in the dynamical optical conductivity of graphene Kostyantyn Kechedzhi We theoretically consider the manifestation of plasmon collective modes in the frequency dependence of the optical conductivity of disordered graphene. We generalize the equation of motion formalism for Dirac electrons in graphene. We show that the presence of the plasmon pole in the dynamical dielectric function of graphene results in the screening effect of graphene electron gas failing at plasmon frequency. As a result of frequency dependent screening the effective strength of charged impurities varies with frequency. This results in the frequency dependent scattering rate. We predict a characteristic broad feature in the frequency dependence of the optical conductivity of graphene appearing at intermediate frequencies, i.e. larger than the disorder broadening of electron states but smaller than the Fermi energy. We predict that this feature could be observable in graphene on $\mathrm{hBN/SiO_{2}/Si}$ substrate with a relatively thick Boron nitride layer of order $10\mathrm{nm}.$ [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A30.00005: The lifetime of Dirac plasmons in graphene Alessandro Principi, Giovanni Vignale, Matteo Carrega, Marco Polini Dirac plasmons in a doped graphene sheet have recently been shown to enable confinement of light to ultrasmall volumes. In this work we calculate the intrinsic lifetime of a Dirac plasmon in a doped graphene sheet by analyzing the role of electron-electron interactions beyond the random phase approximation. The damping mechanism at work is intrinsic since it operates also in disorder-free samples and in the absence of lattice vibrations. We demonstrate that graphene's sublattice-pseudospin degree of freedom suppresses intrinsic plasmon losses with respect to those that occur in ordinary two-dimensional electron liquids. We relate our findings to a microscopic calculation of the homogeneous dynamical conductivity at energies below the single-particle absorption threshold. Finally, we compute the impact of disorder on Dirac plasmon losses and then show that a very reasonable concentration of charged impurities yields a plasmon damping rate which is in good agreement with s-SNOM experimental results. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A30.00006: Electrostatically Controlled Graphene Thermocouple Patrick Herring, Allen Hsu, Nathaniel Gabor, Yong Cheol Shin, Jing Kong, Tomas Palacios, Pablo Jarillo-Herrero Graphene has a broad-band optical absorption ranging from the visible ($\lambda $\textless 532 nm) all the way to the far-infrared ($\lambda $\textgreater 10$\mu$m). Additionally, graphene's optical phonon energy and electrostatically tunable Fermi energy are in the mid-infrared energy range. Together, determining these properties could enable a new generation of carbon-based infrared photodetectors. Electrostatically gated p-n junctions have demonstrated photocurrents in near-IR measurements (850nm), generated primarily through photo-thermoelectric effects. By fabricating electrostatically controlled p-n junctions using chemically vapor grown graphene, we determine the photoresponse mechanism to be primarily thermoelectric in nature at mid-infrared wavelengths and strongly influenced by substrate interactions. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A30.00007: Effect of magnetic field on the plasmons and electron self-energy of gapped graphene Andrii Iurov, Godfrey Gumbs, Danhong Huang The plasmon modes in gapped graphene are calculated in the presence of a uniform perpendicular magnetic field ${\bf B}$. The gap may be produced by external influences on the Dirac cone and include symmetry-breaking perturbations such as multi-layer epitaxially grown graphene, circularly polarized light and an underlying substrate. While one expects the gap to make graphene behave more like conventional 2DEG, we demonstrate the important differences in the plasma excitations and self-energy brought about through the interplay the presence of magnetic field and the symmetry-breaking perturbation. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A30.00008: Graphene Plasmo-Mechanical Structures and Devices Parinita Nene, Jared Strait, Weimin Chan, Christina Manolatou, Paul McEuen, Farhan Rana The manipulation of microstructures with optical forces has generated tremendous interest in recent years. In most cases, the light forces on a dielectric material are induced via the polarization of the medium. The idea of manipulating atomic membranes, like graphene, and related devices with light forces is extremely attractive. But graphene has a small dielectric constant represented by the imaginary part of the optical conductivity. In contrast, in graphene plasmonic structures, the real part of the conductivity is extremely large at the plasmon resonance frequencies. This large current response results in extremely large plasmonic forces between adjacent graphene plasmonic resonators under light illumination. The forces are a result of the charge density associated with the plasmon oscillations and can reach force density values larger than a micro-Newton/micron in plasmonic resonators, such as graphene strips and discs. These plasmonic forces can be tailored to be attractive or repulsive depending on the plasmon mode symmetries. We present results from theoretical and computational models for plasmonic forces, and show several examples of structures and devices that can be manipulated with plasmonic forces, and demonstrate that plasmo-mechanics can be a very effective tool in manipulating and transducing graphene micro- and nano-structures. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A30.00009: Characterization of the plasmon mode of graphene/LaAlO$_3$/SrTiO$_3$ system Weitao Dai, Sangwoo Ryu, Chang-Beom Eom, Cheng Cen Engineering graphene's properties in nanoscale with minimum material degradation is an outstanding challenge in graphene based technologies. Here we present a method targeting at on-demand tuning of 2D plasmon in graphene based on the integration of graphene and a novel complex oxide heterostructure. The recent development of complex oxides has raised the prospect for new classes of electronic devices. In particular, researchers have discovered a high-mobility two-dimensional electron gas forming at the interface between LaAlO$_3$ (LAO) and SrTiO$_3$ (STO). More interestingly, in samples with 3-unit-cell LAO film grown on STO substrate, a biased conducting atomic force microscope probe can locally and reversibly controls the interfacial metal-insulator transition. The close coupling of graphene with these programmable interfacial nanostructures in graphene/LAO/STO heterostructures presents numerous device opportunities. Samples with contacts addressing graphene and oxide interface separately are fabricated. Transport experiments are performed to study the carrier coupling in such hybrid bilayer conducting system. We also report the investigation of plasmonic properties using a variable temperature near field scanning optical microscope. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A30.00010: Electron-phonon coupling in bilayer and single-layer graphene at sub-Kelvin temperatures Chris McKitterick, Heli Vora, Xu Du, Michael Rooks, Daniel Prober Graphene has been proposed by many groups as a detector of terahertz photons$^{1,2,3}$, due to its very small heat capacity and predicted low thermal conductance. We present Johnson noise thermometry measurements of single and bilayer graphene samples fabricated at Stony Brook University and at Yale University. These measurements probe the graphene electron-phonon coupling at sub-Kelvin temperatures. The devices are fabricated with superconducting contacts (NbN at Stony Brook, Al and Nb at Yale) to confine the hot electrons in the graphene device, diminishing the contribution of electron out-diffusion in cooling the electron system. By using commercially-available CVD-grown graphene for some samples, we can define large area sections, allowing us to emphasize the thermal conductance due to electron-phonon coupling. These measurements allow for performance estimates for using similar graphene devices to detect terahertz photons.\\\\ $^1$C. B. McKitterick, D. E. Prober, B. S. Karasik, Journal of Applied Physics 113, 044512 (2013).\\ $^2$H. Vora, P. Kumaravadivel, B. Nielsen, X. Du, Applied Physics Letters 100, 153507 (2012).\\ $^3$K. Fong, K. Schwab, Physical Review X 2, 1 (2012). [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A30.00011: Propagation and guiding of exciton polaritons in a patterned microcavity with an embedded graphene layer German Kolmakov, Oleg Berman, Roman Kezerashvili We consider propagation of a Bose-Einstein condensate (BEC) of exciton polaritons formed in a high-quality microcavity with an embedded quantum well or a gapped graphene layer. We study the effects of patterning of the microcavity by using materials with different electric or optical properties that create the potential landscape for the polaritons in the microcavity plane. The landscapes that enable one to guide the polariton BEC propagation and deliver the polaritons to a desired location are discussed. The possibility to govern the polariton propagation in a microcavity with the embedded graphene by dynamically changing the band gap in the graphene layer by means of an external electric field is considered. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A30.00012: Tuning electron-phonon interactions in graphene and its bilayer for sensitive bolometry Heli Vora, Bent Nielsen, Xu Du Graphene's weak electron-phonon coupling and small electronic heat capacity are of considerable advantage for achieving highly sensitive bolometers and fast single photon detectors. Minimizing electron phonon coupling in graphene can be utilized for designing state-of-the-art bolometers. For such purpose, it is important to understand electron-phonon interaction and its dependence on temperature, Fermi energy, disorder and number of layers experimentally. In particular, single and bilayer graphene are expected to show opposite Fermi energy dependence of electron phonon coupling constant. We study graphene-superconductor tunnel junctions, where the superconducting contacts effectively confine the hot electrons inside the graphene absorber, allowing access to phonon cooling regime at low temperatures. We show results on the temperature and doping dependence of electron-phonon coupling in graphene and its bilayer. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A30.00013: Electro-Optical Plasmonic Switch Based On Graphene Suk-Young Park, Kyungsun Moon We have studied an electro-optical plasmonic waveguide, which controls the transmission of incident light by switching the coupling of the surface plasmon polariton (SPP) localized on graphene. It has been previously shown that the propagation length of the SPP localized on the copper surface can be effectively reduced by a factor of two or three by applying external bias potential. In our study, we have demonstrated that the propagation length of the SPP localized on graphene can be dramatically reduced by a factor of ten or so and the wavelength of SPP can be reduced by several hundredths of the incident light as well. This may help develop a nano-scale plasmonic switch. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A30.00014: Probing the graphene-plasmon interaction via ultrafast pump-probe studies of CVD graphene-metal nanostructures Adam Gilbertson, Tyler Roschuk, Dominic Moseley, Themis Sidiropoulos, Yan Francescato, Vincenzo Giannini, Stefan Maier, Rupert Oulton, Lesley Cohen Graphene exhibits ultrafast broadband absorption that has attracted considerable interest for optoelectronic applications. A major hurdle for real applications is the low optical absorption (2.3{\%}) of graphene, limited by its atomic thickness. A promising approach is to integrate graphene with metal nanostructures that concentrate light into nanoscopic volumes, promoting stronger absorption effects.\footnote{T. J. Echtermeyer, et al., Nature Communications 2, (2011).} Here we present progress towards integration of nanostructured metals with commercially available CVD graphene into photoconductive device architectures. Through fs pump-probe measurements conducted at 300K, hot carrier dynamics in these graphene-plasmonic systems have been studied. We will discuss both the spectral and polarization dependent dynamic response of these systems and the impact of the nanostructures on the resulting photoconductive device performance. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A30.00015: Three-dimensional graphene photonic crystal Duanni Huang, Xiaoyin Xiao, Ronen Polsky, David Bruce Burckel, Ting Luk, Igal Brener, Danhong Huang, Wei Pan We perform finite element method (FEM) simulations on a three-dimensional, multi-layer graphene structure patterned by interferometric lithography. The structure shows periodicity and face centered cubic (fcc) symmetry and a lattice constant of 1.7 microns. Initial simulations predict a photonic bandgap centered on a wavelength of 2.5 microns for high dielectric contrast ($\varepsilon_{\mathrm{r\thinspace }}$\textgreater 16). Further simulations modeling graphene as a dispersive material find evidence of surface plasmon activity. We believe the structure shows promise as a 3-D photonic crystal with a tunable bandgap by utilizing the ability to modify the Fermi level and plasma frequency of the material. Such a device may have applications in quantum sensing. Current efforts on this topic focus on experimental verification of the bandgap as well as a deeper understanding of the interaction between electrical and photonic mechanisms within the structure. [Preview Abstract] |
Session A31: Focus Session: Computational Discovery and Design of New Materials I
Sponsoring Units: DMP DCOMPChair: Bruce Harmon, Iowa State University
Room: 607
Monday, March 3, 2014 8:00AM - 8:36AM |
A31.00001: Materials Discovery via CALYPSO Methodology Invited Speaker: Yanming Ma Materials design has been the subject of topical interests in materials and physical sciences for long. Atomistic structures of materials occupy a central and often critical role, when establishing a correspondence between materials performance and their basic compositions. Theoretical prediction of atomistic structures of materials with the only given information of chemical compositions becomes crucially important, but it is extremely difficult as it basically involves in classifying a huge number of energy minima on the lattice energy surface. To tackle the problems, we have developed an efficient CALYPSO (Crystal structural AnLYsis by Particle Swarm Optimization) approach [1-2] for structure prediction from scratch based on particle swarm optimization algorithm by taking the advantage of swarm intelligence and the spirit of structures smart learning. The method has been coded into CALYPSO software (http://www.calypso.cn) which is free for academic use. Currently, CALYPSO method is able to predict structures of three-dimensional crystals, isolated clusters or molecules [3], surface reconstructions [4], and two-dimensional layers [5]. The applications of CALYPSO into purposed materials design of layered materials [6], high-pressure superconductors [7], and superhard materials [8] were successfully made. Our design of superhard materials [8] introduced a useful scheme, where the hardness value has been employed as the fitness function. This strategy might also be applicable into design of materials with other desired functional properties (e.g., thermoelectric figure of merit, topological Z2 number, etc.). For such a structural design, a well-understood structure to property formulation is required, by which functional properties of materials can be easily acquired at given structures. An emergent application is seen on design of photocatalyst materials.\\[4pt] [1] Y. Wang, J. Lv, L.Zhu, and Y. Ma, Phys. Rev. B, 2010, 82, 094116.\\[0pt] [2] Y. Wang, J. Lv, L.Zhu, and Y. Ma, Comput. Phys. Commun. 183, 2063 (2012).\\[0pt] [3] J. Lv, Y. Wang, L.Zhu, and Y. Ma, J. Chem. Phys. 137, 084104 (2012).\\[0pt] [4] S. Lu, Y. Wang, H. Liu, M. Miao, and Y. Ma, Nature Commun. (in review).\\[0pt] [5] Y. Wang, et al., J. Chem. Phys. 137, 224108 (2012).\\[0pt] [6] X. Luo, et al., J. Am. Chem. Soc. 133, 16285 (2011).\\[0pt] [7] H. Wang, J. S. Tse, K. Tanaka, T. Iitaka, and Y. Ma, Proc. Natl. Acad. Sci. USA, 2012, 109, 6463-6466.\\[0pt] [8] X. Zhang, et al., J. Chem. Phys. 138, 114101 (2013). [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A31.00002: New Stable Phases in the Re-B System: A First-principles Study Xin Zhao, Manh Cuong Nguyen, Cai-Zhuang Wang, Kai-Ming Ho We studied rhenium borides using genetic algorithm in combination with first-principles calculations and revealed several new stable phases in the Re-B system. The structures obtained from our genetic algorithm search are energetically much superior to those proposed in the literature. Two new phases of Re2B were found to be thermodynamically stable at different pressures, which possibly explains the recent experimental observations (Solid State Sciences 25 85-92 (2013)). ReB is stable against decomposition reactions below 10 GPa and ReB3 is stable above 22 GPa. A C2/m structure was discovered for ReB4 to have lower energy than the R-3m structure reported earlier (J. Alloys Compd. 573 20-26 (2013)). Elastic properties from first-principles calculations indicate that the structures we report in this work are mechanically stable and promising targets as new ultra-hard materials. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A31.00003: Prediction and confirmation of new MB4 crystal structures (M=Cr,Fe,Mn) Abram Van Der Geest, Aleksey Kolmogorov The family of 3$d$ transition metals tetraborides has recently attracted a lot of interest due to the materials' unusual structural, mechanical, and superconducting properties [1-6]. We overview our computational work on the determination of their ground state structures and show that all the predictions have been confirmed by experimental groups. Namely, the true ground state of the known CrB$_4$ and MnB$_4$ compounds have been determined to be new orthorhombic and monoclinic structures, as predicted by a combination of high-throughput and evolutionary searches [3,6,7]. The proposed brand-new FeB$_4$ superconducting compound [3] has been synthesized by our colleagues and shown to be a superhard superconductor [4,5]. We discuss the possibility of raising the material's superconducting critical temperature by doping. [1] A. N. Kolmogorov, S. Shah, et al., Phys. Rev. Lett., 105, 217003. [2] A. F. Bialon, T. Hammerschmidt, et al., Appl. Phys. Lett., 98 081901. [3] H. Niu, J. Wang, et al., Phys. Rev. B, 85, 144116. [4] H. Gou, N. Dubrovinskaia, et al. Phys. Rev. Lett., 111, 157002. [5] Filip Ronnig and John L. Sarrao, Physics 6, 109 (2013). [6] A.G. Van Der Geest and A. N. Kolmogorov, ArXiv: http://xxx.lanl.gov/abs/1310.4157 . [7] MAISE http://maise-guide.org . [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A31.00004: Systematic Phase Diagram of LiSi and LiAl compounds from Minima Hopping Method Aldo Romero, Miguel Marques, Silvana Botti, Rafael Sarmiento-P\'erez, Irais Valencia-Jaime, Max Amsler, Stefan Goedecker We performed an extensive theoretical exploration of the structural phase diagram of LiSi and LiAl alloys through global structural prediction. These compounds have very interesting properties. For example, LiSi alloys have been considered for high energy density anodes for future rechargeable battery technology, while LiAl alloys are expected to have applications in the field of structural components due to its light weight and maleability. The global structural prediction was performed with the minima hopping method. In this method the low energy structures are obtained by solving a set of dynamical equations of motion that allows efficient visits to local minima on the Born Oppenheimer surface. We found very good agreement between our simulations and previously reported stoichiometries. Moreover, we were able to identify several novel thermodynamically stable compositions that have not been previously synthesized. The ground-state structures were further characterized both structurally and electronically. Our calculations show that global structural prediction is a very powerful tool to predict new thermodynamically stable materials, and that it consistently outperforms other methods commonly used. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A31.00005: Computational design of three-dimensional metallic boron nitride Qian Wang, Shunhong Zhang, Yoshiyuki Kawazoe, Puru Jena Based on comprehensive calculations, we have shown that the 3D BN structures composed of interlocking BN hexagons are metallic and dynamically stable. These newly designed 3D BN structures (T-BxNx, x$=$4n-1, n$=$1, 2, 3...) are hybrid systems with one B and one N atom in sp3 hybridization and (4n-2) sp2-bonded B and N atoms respectively per unit cell. The sp3 bonded B (N) atom binds to its surrounding four sp2 bonded N (B) atoms forming the 3D backbone and is responsible for stability. The sp2 bonded B atoms, on the other hand, play the key role in rendering the conducting network and metallicity. This special geometrical feature results in a unique property: unlike previously reported functionalized c-BN thin film whose metallicity stems from strong inbuilt polarization, the metallicity in 3D T-BxNx is intrinsic and comes from the delocalized electrons distributed around the B sites. The metallicity exhibited in the studied structures opens new door for BN materials with potential applications in electron transport, metal-free catalysis and electronic devices. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A31.00006: Design of novel solar thermal fuels with high-throughput ab initio simulations Yun Liu, Jeffrey Grossman Solar thermal fuels (STF) store the energy of sunlight, which can then be released later in the form of heat, offering an emission-free and renewable solution for both solar energy conversion and storage. However, this approach is currently limited by the lack of low-cost materials with high energy density and high stability. Previously we have predicted a new class of functional materials that have the potential to address these challenges. Recently, we have developed an ab initio high-throughput computational approach to accelerate the design process and allow for searches over a broad class of materials. The high-throughput screening algorithm we have developed can run through large numbers of molecules composed of earth-abundant elements, and identifies possible metastable structures of a given material. Corresponding isomerization enthalpies associated with the metastable structures are then computed. Using this high-throughput simulation approach, we have discovered molecular structures with high isomerization enthalpies that have the potential to be new candidates for high-energy density STF. We have also discovered physical design principles to guide further STF materials design through the correlation between isomerization enthalpy and structural properties. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A31.00007: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 9:48AM - 10:00AM |
A31.00008: A Computational Method for Materials Design of Interfaces Jakub Kaminski, Christian Ratsch, Sadasivan Shankar In the present work we propose a novel computational approach to explore the broad configurational space of possible interfaces formed from known crystal structures to find new hetrostructure materials with potentially interesting properties. In the series of subsequent steps with increasing complexity and accuracy, the vast number of possible combinations is narrowed down to a limited set of the most promising and chemically compatible candidates. This systematic screening encompasses (i) establishing the geometrical compatibility along multiple crystallographic orientations of two (or more) materials, (ii) simple functions eliminating configurations with unfavorable interatomic steric conflicts, (iii) application of empirical and semi-empirical potentials estimating approximate energetics and structures, (iv) use of DFT based quantum-chemical methods to ascertain the final optimal geometry and stability of the interface in question. We also demonstrate the flexibility and efficiency of our approach depending on the size of the investigated structures and size of the search space. The representative results from our search protocol will be presented for selected materials including semiconductors, transition metal systems, and oxides. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A31.00009: Using machine learning to drive computational discovery in self-organizing material systems Carolyn Phillips, Gregory Voth In a complex self-organizing system, small changes in the interactions between the system's components can result in different emergent macrostructures or macrobehavior. In chemical engineering and material science, such spontaneously self-assembling systems, using polymers, nanoscale or colloidal-scale particles, and DNA are an attractive way to create materials that are precisely engineered. Computer simulations of such systems are a powerful tool for discovery. However, as the rate at which data can be amassed continues to accelerate, the pace of discovery becomes limited not by the rate at which data can be generated, but can be analyzed. We consider this problem from two ends, using a model particle system that self-assembles simple and complex crystals. First, we show how the ordered states can be discovered in a large data set of simulation results by using a hierarchy of pattern analysis techniques including shape matching and machine learning algorithms. Second, we introduce a learning algorithm, inspired by adaptive mesh refinement, that guides the deployment of computational experiments. This algorithm densely searches the space of the degrees of freedom for a self-organizing system, while targeting certain features, thus, gathering more information for less computational effort. New algorithmic techniques, such as these, for managing the growing volume of simulation data catalyze advancing computational power to be a tool for discovery. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A31.00010: Engineering of nanomaterials by following the flow of structural information guided by targeted outcome Vladan Mlinar A fundamental understanding of materials over multi-length scales -- with the aim of designing novel nanomaterials and breakthroughs in modern technology is still pending. On the several-atom scale, it has been possible to explore the range of geometrically possible structures and predict new materials that have targeted physical/mechanical properties. However, the question of using those materials in real sizes and ``real world'' applications is still open. For larger systems, the structure of a material is represented by so-called structural ``motifs'' such as composition profile, shape, confining potential, or representative volume elements. Here, I will present a methodology based on generalized information theory, where information is conceived in terms of uncertainty. I will demonstrate a mathematical formalism of how to (i) track the loss of structural information between the atomistic description of the structure and description via structural motifs, and (ii) develop a procedure to find the structural motifs responsible for controlling a targeted physical property. To illustrate validity of the approach, I will discuss the design of nanomaterials for intermediate-band solar cells, and how to engineer optimized nanomaterials that can exceed the light-trapping limit. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A31.00011: First-principles configurational thermodynamics of alloyed nanoparticles with adsorbates Lin-Lin Wang, Teck L. Tan, Duane D. Johnson Transition-metal, alloyed nanoparticles (NPs) are key components in current and emerging energy technologies because they are found to improve catalytic activity and selectivity for many energy-conversion processes. However, thermodynamic investigations of the compositional profile of alloyed nanoparticles, which determines their catalytic properties, have been limited mostly to NP core-shell preference without the presence of adsorbates. Here, by extending cluster expansion methods to treat both alloyed nanoparticles and adsorbates, we study the configurational thermodynamics of bimetallic NPs under chemically reactive conditions, using databases from density functional theory calculations. With a few examples, we show that such simulations can provide information needed for rational design of NP catalysts. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A31.00012: Vibrational, magnetic and many-electron effects in quantitative theory for accelerated materials design Igor A. Abrikosov, Alena V. Ponomareva, Anton Yu. Nikonov, Andrey I. Dmitriev, Svetlana A. Barannikova, Marcus Ekholm, Bjorn Alling, Peter Steneteg, Olle Hellman We discuss a need to develop modern theory for accelerated materials design, significantly reducing number of approximations in calculations and explicitly taking into account conditions at which materials operate when used in technological applications. We illustrate the need to explicitly account for vibrational, magnetic and many-electron effects by considering several examples, including calculations of phase diagrams for Zr-based alloys, simulations of order-disorder phase transition in Fe-Ni permalloy [1], and estimations of decomposition trends and elastic properties of transition metal nitrides [2-4]. We demonstrate that in this way the accuracy of theoretical predictions can be made comparable to or exceeding the experimental accuracy, significantly increasing usefulness of the theory for the materials design. \\[4pt] [1] M. Ekholm, \textit{et al.}, Phys. Rev. Lett. \textbf{105}, 167208 (2010).\\[0pt] [2] B. Alling, \textit{et al.}, Appl. Phys. Lett. \textbf{102}, 031910 (2013).\\[0pt] [3] P. Steneteg, \textit{et al.}, Phys. Rev. B \textbf{85}, 144404 (2012). \\[0pt] [4] P. Steneteg, \textit{et al.}, Phys. Rev. B \textbf{87}, 094114 (2013). [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A31.00013: Non-uniquely defined ground states of a neutral and anionic gold cluster and methods for generating stationary point databases Bastian Schaefer, Rhitankar Pal, Maximilian Amsler, Ali Sadeghi, Xiao Cheng Zeng, Lai-Sheng Wang, Volker Blum, Stefan Goedecker Using the Minima Hopping structure prediction method at the density functional level, we found new low energy minima for mid-sized neutral and singly charged anionic gold clusters. We demonstrate that the local- density and a generalized gradient approximation of the exchange-correlation functional predict different structural motifs. For both the anionic and the neutral system there exist a vast number of structurally different isomers within a small energy window above the putative global minima. Consequently, no uniquely defined ground states are expected to exist for these systems at finite temperatures. For the anionic system we present a disconnectivity graph that has been build completely at the density functional level. The transition states used to build this disconnectivity graph are complete enough in order to predict that the anionic system could have a fluxional shell, which may implicate catalytic activity for this cluster. We also discuss methods to generate stationary point databases required for the generation of disconnectivity graphs. [Preview Abstract] |
Session A32: Invited Session: Simulation of Materials: Quantum Computing Meets Classical Computing
Sponsoring Units: GQI DCOMPChair: Alejanndro Perdomo-Ortiz, NASA
Room: 708-712
Monday, March 3, 2014 8:00AM - 8:36AM |
A32.00001: Nicholas Metropolis Award for Outstanding Doctoral Thesis Work in Computational Physics: Quantum many-body physics of ultracold molecules in optical lattices: models and simulation methods Invited Speaker: Michael Wall Experimental progress in generating and manipulating synthetic quantum systems, such as ultracold atoms and molecules in optical lattices, has revolutionized our understanding of quantum many-body phenomena and posed new challenges for modern numerical techniques. Ultracold molecules, in particular, feature long-range dipole-dipole interactions and a complex and selectively accessible internal structure of rotational and hyperfine states, leading to many-body models with long range interactions and many internal degrees of freedom. Additionally, the many-body physics of ultracold molecules is often probed far from equilibrium, and so algorithms which simulate quantum many-body dynamics are essential. Numerical methods which are to have significant impact in the design and understanding of such synthetic quantum materials must be able to adapt to a variety of different interactions, physical degrees of freedom, and out-of-equilibrium dynamical protocols. Matrix product state (MPS)-based methods, such as the density-matrix renormalization group (DMRG), have become the de facto standard for strongly interacting low-dimensional systems. Moreover, the flexibility of MPS-based methods makes them ideally suited both to generic, open source implementation as well as to studies of the quantum many-body dynamics of ultracold molecules. After introducing MPSs and variational algorithms using MPSs generally, I will discuss my own research using MPSs for many-body dynamics of long-range interacting systems. In addition, I will describe two open source implementations of MPS-based algorithms in which I was involved, as well as educational materials designed to help undergraduates and graduates perform research in computational quantum many-body physics using a variety of numerical methods including exact diagonalization and static and dynamic variational MPS methods. Finally, I will mention present research on ultracold molecules in optical lattices, such as the exploration of many-body physics with polyatomic molecules, and the next generation of open source matrix product state codes. This work was performed in the research group of Prof. Lincoln D. Carr. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 9:12AM |
A32.00002: Quantum Control nd Measurement of Spins in Cold Atomic Gases Invited Speaker: Ivan Deutsch Spins are natural carriers of quantum information given their long coherence time and our ability to precisely control and measure them with magneto-optical fields. Spins in cold atomic gases provide a pristine environment for such quantum control and measurement, and thus this system can act as a test-bed for the development of quantum simulators. I will discuss the progress my group has made in collaboration with Prof. Jessen, University of Arizona, to develop the toolbox for this test-bed. Through its interactions with rf and microwave magnetic fields, whose waveforms are designed through optimal control techniques, we can implement arbitrary unitary control on the internal hyperfine spins of cesium atoms, a 16 dimensional Hilbert space (isomorphic to 4 qubits). Control of the collective spin of the ensemble of many atoms is performed via the mutual coupling of the atomic ensemble to a mode of the electromagnetic field that acts as a quantum data bus for entangling atoms with one another. Internal spin control can be used to enhance the entangling power of the atom-photon interface. Finally, both projective and weak-continuous measurements can be performed to tomograhically reconstruct quantum states and processes. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:48AM |
A32.00003: Simulating quantum chemistry Invited Speaker: Alan Aspuru-Guzik |
Monday, March 3, 2014 9:48AM - 10:24AM |
A32.00004: Quantum Simulation with Superconducting Circuits Invited Speaker: Andrew Houck Superconducting circuits and circuit quantum electrodynamics provide an excellent toolbox for non-equilibrium quantum simulation. In circuit QED, the strong interaction of light with a single qubit can lead to strong qubit-mediated photon-photon interactions. Recent theoretical proposals have predicted phase transitions in arrays of these cavities, demonstrating that complex matter-like phenomena can emerge with such interacting photons. Due to inevitable photon dissipation and the ease of adding photons through driving, these systems are fundamentally open and a useful tool for studying non-equilibrium physics. I will discuss recent experimental and theoretical progress towards realization of these non-equilibrium quantum simulators, including a localization-delocalization crossover in a pair of coupled cavities and preliminary measurements of large cavity arrays. I will show a variety of available measurements in these systems, including transport, photon number statistics, and a scanned local quantum probe. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 11:00AM |
A32.00005: Extensions of density functional theory approaches to treating quantum phenomena and quantum entanglement Invited Speaker: Angel Rubio |
Session A33: Focus Session: Quantum Foundations: Quantum Theory Meets Relativity
Sponsoring Units: GQIChair: Jonathan Oppenheim, University College London
Room: 706
Monday, March 3, 2014 8:00AM - 8:36AM |
A33.00001: Quantum computation versus firewalls Invited Speaker: Patrick Hayden |
Monday, March 3, 2014 8:36AM - 8:48AM |
A33.00002: Quantum capacity of quantum black holes Chris Adami, Kamil Bradler The fate of quantum entanglement interacting with a black hole has been an enduring mystery, not the least because standard curved space field theory does not address the interaction of black holes with matter. We discuss an effective Hamiltonian of matter interacting with a black hole that has a precise analogue in quantum optics and correctly reproduces both spontaneous and stimulated Hawking radiation with grey-body factors. We calculate the quantum capacity of this channel in the limit of perfect absorption, as well as in the limit of a perfectly reflecting black hole (a white hole). We find that the white hole is an optimal quantum cloner, and is isomorphic to the Unruh channel with positive quantum capacity. The complementary channel (across the horizon) is entanglement-breaking with zero capacity, avoiding a violation of the quantum no-cloning theorem. The black hole channel on the contrary has vanishing capacity, while its complement has positive capacity instead. Thus, quantum states can be reconstructed faithfully behind the black hole horizon, but not outside. This work sheds new light on black hole complementarity because it shows that black holes can both reflect and absorb quantum states without violating the no-cloning theorem, and makes quantum firewalls obsolete. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A33.00003: Direct detection of classically undetectable dark matter through quantum decoherence C. Jess Riedel Although various pieces of indirect evidence about the nature of dark matter have been collected, its direct detection has eluded experimental searches despite extensive effort. If the mass of dark matter is below 1 MeV, it is essentially imperceptible to conventional detection methods because negligible energy is transferred to nuclei during collisions. Here I propose directly detecting dark matter through the quantum decoherence it causes rather than its classical effects such as recoil or ionization. I show that quantum spatial superpositions are sensitive to low-mass dark matter which is inaccessible to classical techniques. This provides new independent motivation for matter interferometry with large masses, especially on spaceborne platforms. The apparent dark matter wind we experience as the Sun travels through the Milky Way ensures interferometers and related devices are directional detectors, and so are able to provide unmistakable evidence that decoherence has galactic origins. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A33.00004: Difficult Requirements for a Gravitational Wave Mission using Atom Interferometry Peter L. Bender A PRL paper by Graham, Hogan, Kasevich, and Rajendran in April, 2013 suggested gravitational wave observations in space using single photon transitions on highly forbidden optical lines for atom interferometry measurements. The main example given was based on use of the 698 nm optical clock transition in Sr-87, a 1000 km baseline, and large momentum transfer laser pulse sequences producing 2400 state transitions for a given atom over a 100 s observation period. A specific scenario for such a mission is needed in order to permit evaluation of the requirements. As a stop-gap, a laser power of 30 W, square laser pulses, 1 m diam. transmitting telescopes, and operation of 4 concurrent pairs of atom interferometers are being assumed. Based on these assumptions, the atom cloud temperature requirement would be below 0.1 pK, and the number of atoms required per cloud would be extremely high. Such a mission would be much more complex than a laser interferometry mission with better overall sensitivity, such as the extensively studied LISA mission or the recently proposed evolved-LISA (eLISA) mission. A LISA Pathfinder mission is scheduled for launch in 2015, funded mainly by ESA . A gravitational wave observation theme is being considered by ESA as part of their Cosmic Vision Programme. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A33.00005: Local two-level PT symmetric system violates no-signalling condition Yi-Chan Lee, Min-Hsiu Hsieh, Steven Flammia, Ray-Kuang Lee We examine $\mathcal{PT}$ symmetric quantum theory by considering a composite physical system. The parties of this composite system are spatially separated and each hold half of a part of a maximally entangled state. According to the transition rule between Hermitian quantum systems and $\mathcal{PT}$ symmetric quantum systems which is used in previous literature, the existence of a local $\mathcal{PT}$ symmetric quantum system will cause a violation of the non-signalling condition. Our results reveal that either the transition rules need to be modified or $\mathcal{PT}$ symmetric quantum theory is not a local theory. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A33.00006: Inferring causal structure: a quantum advantage Katja Ried, Robert Spekkens The problem of inferring causal relations from observed correlations is central to science, and extensive study has yielded both important conceptual insights and widely used practical applications. Yet some of the simplest questions are impossible to answer classically: for instance, if one observes correlations between two variables (such as taking a new medical treatment and the subject's recovery), does this show a direct causal influence, or is it due to some hidden common cause? We develop a framework for quantum causal inference, and show how quantum theory provides a unique advantage in this decision problem. The key insight is that certain quantum correlations can only arise from specific causal structures, whereas pairs of classical variables can exhibit any pattern of correlation regardless of whether they have a common cause or a direct-cause relation. For example, suppose one measures the same Pauli observable on two qubits. If they share a common cause, such as being prepared in an entangled state, then one never finds perfect (positive) correlations in every basis, whereas perfect anticorrelations are possible (if one prepares the singlet state). Conversely, if a channel connects the qubits, hence a direct causal influence, perfect anticorrelations are impossible. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A33.00007: Trapping light by mimicking gravitational lensing Hui Liu, Chong Sheng, Shining Zhu, Dentcho Genov One of the most fascinating predictions of the theory of general relativity is the effect of gravitational lensing, the bending of light in close proximity to massive stellar objects. Recently, artificial optical materials have been proposed to study the various aspects of curved spacetimes, including light trapping and Hawking's radiation. However, the development of experiments 'toy' models that simulate gravitational lensing in curved spacetimes remains a challenge, especially for visible light. Here, by utilizing a microstructured optical waveguide around a microsphere, we propose to mimic curved spacetimes caused by gravity, with high precision. We experimentally demonstrate both far-field gravitational lensing effects and the critical phenomenon in close proximity to the photon sphere of astrophysical objects under hydrostatic equilibrium. The proposed microstructured waveguide can be used as an omnidirectional absorber, with potential light harvesting and microcavity applications. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A33.00008: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 10:00AM - 10:12AM |
A33.00009: The Dirac Equation in Curved Space-Time and the Nonrelativistic Limit Jonathan Noble, Ulrich Jentschura The Foldy-Wouthuysen transformation is applied to a number of generalized Dirac particles in curved space-times. The Dirac-Schwarzschild Hamiltonian is covariantly coupled to a central gravitational field (black hole) within general relativity. We identify the averaged trajectory (after the elimination of the zitterbewegung which proceeds at the velocity of light). The transformed Hamiltonian is much easier to understand as it clearly displays the gravitational correction terms. These include terms describing the kinetic corrections to the gravitational coupling, a Darwin (zitterbewegung) term, and a spin orbit coupling term. Additionally, we apply the transformation to the transition current, and find a gravitational kinetic correction as well as gravitational corrections to the magnetic coupling. We also obtain results for a few other phenomenologically interesting generalized Dirac Hamiltonians, such as those describing a Dirac particle in a non-inertial frame. Finally, we discuss the possible pitfalls which one can encounter when preforming the transformation, including the ``chiral'' method which has some elegant analytic properties, but does break the fundamental symmetries of the original Hamiltonian, as well as change the physical interpretation. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A33.00010: Complex Teichm\"{u}ller Space below the Planck Length for the Interpretation of Quantum Mechanics Friedwardt Winterberg As Newton's mysterious action at a distance law of gravity was explained as a Riemannian geometry by Einstein, it is proposed that the likewise mysterious non-local quantum mechanics is explained by the analytic continuation below the Planck length into a complex Teichm\"{u}ller space. Newton's theory worked extremely well, as does quantum mechanics, but no satisfactory explanation has been given for quantum mechanics. In one space dimension, sufficient to explain the EPR paradox, the Teichm\"{u}ller space is reduced to a space of complex Riemann surfaces. Einstein's curved space-time theory of gravity was confirmed by a tiny departure from Newton's theory in the motion of the planet Mercury, and an experiment is proposed to demonstrate the possible existence of a Teichm\"{u}ller space below the Planck length. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A33.00011: Origin of Dynamical Quantum Non-locality Cesar E. Pachon, Leonardo A. Pachon Non-locality is one of the hallmarks of quantum mechanics and is responsible for paradigmatic features such as entanglement and the Aharonov-Bohm effect. Non-locality comes in two ``flavours'': a \emph{kinematic non-locality}-- arising from the structure of the Hilbert space-- and a \emph{dynamical non-locality}-- arising from the quantum equations of motion--. Kinematic non-locality is unable to induce any change in the probability distributions, so that the ``action-at-a-distance'' cannot manifest. Conversely, dynamical non-locality does create explicit changes in probability, though in a ``causality-preserving'' manner. The origin of non-locality of quantum measurements and its relations to the fundamental postulates of quantum mechanics, such as the uncertainty principle, have been only recently elucidated. Here we trace the origin of dynamical non-locality to the superposition principle. This relation allows us to establish and identify how the uncertainty and the superposition principles determine the non-local character of the outcome of a quantum measurement. Being based on group theoretical and path integral formulations, our formulation admits immediate generalizations and extensions to to, e.g., quantum field theory. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A33.00012: Experimental unconditionally secure bit commitment Yang Liu, Yuan Cao, Marcos Curty, Sheng-Kai Liao, Jian Wang, Ke Cui, Yu-Huai Li, Ze-Hong Lin, Qi-Chao Sun, Dong-Dong Li, Hong-Fei Zhang, Yong Zhao, Teng-Yun Chen, Cheng-Zhi Peng, Qiang Zhang, Adan Cabello, Jian-Wei Pan Quantum physics allows unconditionally secure communication between parties that trust each other. However, when they do not trust each other such as in the bit commitment, quantum physics is not enough to guarantee security. Only when relativistic causality constraints combined, the unconditional secure bit commitment becomes feasible. Here we experimentally implement a quantum bit commitment with relativistic constraints that offers unconditional security. The commitment is made through quantum measurements in two quantum key distribution systems in which the results are transmitted via free-space optical communication to two agents separated with more than $20$ km. Bits are successfully committed with less than $5.68\times10^{-2}$ cheating probability. This provides an experimental proof of unconditional secure bit commitment and demonstrates the feasibility of relativistic quantum communication. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A33.00013: No Drama Quantum Electrodynamics? Andrey Akhmeteli Is it possible to offer a ``no drama'' quantum electrodynamics, as simple (in principle) as classical electrodynamics -- a theory described by a system of partial differential equations (PDE) in 3+1 dimensions, but reproducing unitary evolution of a quantum field theory in the Fock space? The following results suggest an affirmative answer: 1. The scalar field can be algebraically eliminated from scalar electrodynamics. 2. After introduction of a complex 4-potential (producing the same electromagnetic field (EMF) as the standard real 4-potential), the spinor field can be algebraically eliminated from spinor electrodynamics. 3. The resulting theories describe independent evolution of EMF and can be embedded into quantum field theories. Another fundamental result: in a general case, the Dirac equation is equivalent to a 4th order PDE for just one component, which can be made real by a gauge transform. Issues related to the Bell theorem and the connection with Barut's self-field electrodynamics are discussed. A. Akhmeteli, Int'l Journal of Quantum Information, Vol. 9, Suppl., 17-26 (2011) A. Akhmeteli, Journal of Mathematical Physics, Vol. 52, 082303 (2011) A. Akhmeteli, quant-ph/1111.4630 A. Akhmeteli, European Physical Journal C, Vol. 73, 2371 (2013) (open access) [Preview Abstract] |
Session A34: Focus Session: AMO Quantum Information Processing and Superconducting Qubits: Exploring Interactions of Photons and Qubits
Sponsoring Units: GQI DAMOPChair: Kenneth Brown, Georgia Institute of Technology
Room: 704
Monday, March 3, 2014 8:00AM - 8:36AM |
A34.00001: Coupling ions and photons via an optical cavity Invited Speaker: Tracy Northup Trapped ions are a key experimental platform for quantum computing and simulation, while photons can transport quantum information over long distances. Optical cavities provide a coherent interface between these two quantum systems, an essential ingredient for future quantum networks. I will describe a cavity-mediated, bichromatic Raman transition using trapped calcium ions which allows us to connect the ions' electronic states with the polarization states of cavity photons. This transition is the basis for two protocols: the transfer of a quantum state from an ion onto a photon, and the generation of ion-photon entanglement. Furthermore, if two ions are coupled to the cavity mode, they can be entangled with one another, an event heralded by the detection of two orthogonally polarized cavity photons. Such entanglement could provide a robust link between remote ion-based quantum computers, and within a single cavity, enables enhanced quantum memories and the generation of photonic cluster states. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A34.00002: Addressing cavity finesse degradation in ion-cavity systems Tailin Wu, Molu Shi, Isaac Chuang High finesse optical cavities are an essential part for achieving strong coupling between single ions and single photons, which offers a strong experimental platform in the pursuit of efficient light-matter interactions. However, degradation of cavity finesse has been repeatedly observed in ultra-high vacuum (UHV) systems, especially in the blue and ultraviolet part of the spectrum. One hypothesis attributes this finesse degradation to oxygen depletion from the mirror top layer coating. We have investigated this decay behavior of optical cavities of different top layer coatings, with resonance at 422nm in UHV conditions. In this talk, I will present our studies of finesse decay with and without baking, and recovery in oxygen at different temperature settings. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A34.00003: Development of a high-Q superconducting microwave resonator for coupling to trapped laser-cooled atoms Jared Hertzberg, Kristen Voigt, Zaeill Kim, Jonathan Hoffman, Jeff Grover, Jongmin Lee, Pablo Solano, Rangga Budoyo, Cody Ballard, James Anderson, Chris Lobb, Luis Orozco, Steven Rolston, Frederick Wellstood We present progress towards a hybrid quantum system in which microwave quanta may be exchanged between a superconducting qubit and laser-trapped atoms via a magnetic dipole interaction. In initial experiments, we seek to couple a thin-film superconducting LC resonator cooled to 20 mK to the 6.835 GHz hyperfine splitting in an ensemble of optically trapped 87Rb atoms.[1] The atoms will be trapped in the evanescent optical field on the waist of a tapered 500-nm-diameter optical fiber which is moved to within a few microns of the inductor in the LC resonator. Rayleigh scattered light from defects in the optical fiber will impinge on the superconducting device. We describe the resulting effects of absorbed photons and how to minimize optical effects as well as results on positioning the resonator relative to the optical fiber within a dilution refrigerator. [1] Z. Kim et al., AIP ADVANCES 1, 042107 (2011). [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A34.00004: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 9:12AM - 9:24AM |
A34.00005: Designing the Environmental Structure for a Giant Artificial Atom Anton Frisk Kockum, Martin Gustafsson, Thomas Aref, Per Delsing, Goran Johansson In traditional quantum optics, where the interaction between atoms and light at optical frequencies is studied, the atoms can be approximated as point-like compared to the wavelength of the light. So far, this relation has also been true for artificial atoms made out of superconducting circuits or quantum dots, interacting with microwave radiation. However, recent and ongoing experiments using surface acoustic waves show that one can couple a single artificial atom to a bosonic field at several points wavelengths apart. In this work, we theoretically study this type of system. We find that the multiple coupling points give rise to a frequency dependence for the coupling strength between the atom and its environment, and also for the Lamb shift of the atom. The frequency dependence can be designed, since it is given by the discrete Fourier transform of the coupling point coordinates. We discuss a number of possible applications for this phenomenon, including tunable coupling, single-atom lasing, and other effects that can be achieved by designing the relative coupling strengths of different transitions in a multi-level atom. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A34.00006: Dynamics of superconducting qubits in open transmission lines Garcia-Ripoll Juan Jose, David Zueco, Diego Porras, Borja Peropadre The time and space resolved dynamics of a superconducting qubit with an Ohmic coupling to propagating 1D photons is studied, from weak coupling to the ultrastrong coupling regime (USC). A nonperturbative study based on Matrix Product States (MPS) shows the following results [1]: (i) The ground state of the combined systems contains excitations of both the qubit and the surrounding bosonic field. (ii) An initially excited qubit equilibrates through spontaneous emission to a state, which under certain conditions, is locally close to that ground state, both in the qubit and the field. (iii) The resonances of the combined qubit-photon system match those of the spontaneous emission process and also the predictions of the adiabatic renormalisation [2]. These results set the foundations for future studies and engineering of the interactions between superconducting qubits and propagating photons, as well as the design of photon-photon interactions based on artificial materials built from these qubits. \\[4pt] [1] B. Peropadre, D. Zueco, D. Porras, J. J. G. R., arXiv:1307.3870\\[0pt] [2] A.\ J. Leggett \textit{et al}, Rev. Mod. Phys. 59, 1, (1987) [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A34.00007: Microwave-controlled generation of shaped photons in circuit QED Marek Pechal, Christopher Eichler, Sina Zeytinoglu, Simon Berger, Andreas Wallraff, Stefan Filipp Techniques for quantum information transfer using photons propagating between distant qubits are often based on the ability to engineer the shape of the emitted photon waveform. For instance, a shape symmetric in time enables a reversal of the emission process leading to efficient reabsorption of the photon by the target qubit [1]. Here, we demonstrate the generation of shaped microwave photons in a superconducting circuit QED system consisting of a standard transmon circuit coupled to a transmission line resonator [2]. We make use of the multi-level structure of the transmon and employ a tunable Raman transition induced by a modulated microwave signal to emit a single shaped photon. This technique known from quantum optics allows us to produce symmetric photons with controllable amplitude and phase using microwave control signals only. The method is easy to implement in standard circuit QED systems because it does not rely on specialized circuit elements [3,4] to tune the transmon-photon coupling. \\[4pt] [1] J.~I.~Cirac {\em et al.}, Phys.~Rev.~Lett.~{\bf 78}, 3221 (1997).\newline [2] M.~Pechal {\em et al.}, arXiv:1308.4094.\newline [3] Y.~Yin {\em et al.}, Phys.~Rev.~Lett.~{\bf 110}, 107001 (2013).\newline [4] S.~J.~Srinivasan {\em et al.}, arXiv:1308.3471. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A34.00008: Tunable coupling in atom-mirror system I.-C. Hoi, L. Tornberg, A.F. Kockum, A. Pourkabirian, G. Johansson, C.M. Wilson, P. Delsing We embedded an artificial atom, a superconducting transmon qubit, at a distance from the end of a transmission line. The distance between the qubit and the end (mirror) determines the electromagnetic (EM) environment coupled to the qubit. By tuning the transition wavelength of the qubit, we can control the coupling. In particular, we show that when the qubit stays a node of the EM field, the coupling is completely off. For finite coupling, we investigate the coherent scattering properties of the system, including both the time dynamic response and the steady state behavior. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A34.00009: Hybrid quantum magnetism in circuit-QED: from spin-photon waves to many-body spectroscopy Alejandro Bermudez, Andreas Kurcz, Juan Jose Garcia-Ripoll We introduce a model of quantum magnetism induced by the non-perturbative exchange of microwave photons between distant superconducting qubits. By interconnecting qubits and cavities, we obtain a spin-boson lattice model that exhibits a quantum phase transition where both qubits and cavities spontaneously polarise. We present a many-body ansatz that captures this phenomenon all the way, from a the perturbative dispersive regime where photons can be traced out, to the non-perturbative ultra-strong coupling regime where photons must be treated on the same footing as qubits. Our ansatz also reproduces the low-energy excitations, which are described by hybridised spin-photon quasiparticles, and can be probed spectroscopically from transmission experiments in circuit-QED, as shown by simulating a possible experiment by Matrix-Product-State methods. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A34.00010: Tuning the Dispersive Coupling Rate and Suppressing the Photon Shot Noise in a Superconducting Qubit Gengyan Zhang, Srikanth Srinivasan, Yanbing Liu, Andrew Houck We report on measurements of the dispersive coupling between a microwave cavity and a tunable coupling qubit (TCQ). By operating the TCQ in the straddling regime, we can tune $\chi$, the cavity pull of the qubit, from a few MHz to close to zero while maintaining a constant qubit frequency. The vanishing $\chi$ leads to the suppression of the photon shot noise, which is one of the sources of qubit decoherence. Readout of the qubit at small $\chi$ is achieved by transferring the qubit state to a higher energy level. Experimental results of tunable $\chi$ and its impact on qubit coherence will be presented. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A34.00011: Observation of superradiance in a small ensemble of artificial atoms J.A. Mlynek, A.A. Abdumalikov Jr, C. Eichler, A. Wallraff Superradiant effects can be efficiently addressed in an experimental setting where the atomic linewidth $\gamma$ is small compared to the coupling rate $g$ and the cavity linewidth $\kappa$. We have realized this parameter regime, known as the bad cavity limit, in a circuit QED architecture, consisting of a coplanar wavequide resonator and two transmon qubits. One main advantage of introducing only a small number of two-level systems is the possibility to prepare well defined initial states by exploiting full quantum control of the individual atom states. For the set of initial states presented here the obtained responses are clearly non-exponential and show strong indication of a correlated behavior. Preparing both qubits in superposition states with relative phase $\phi$ and measuring the amplitude of the emitted signal indicates that the effect arises in an underlying phase locking mechanism. We have further observed the suppression of the emission from one excited qubit due to the presence of a second qubit in its ground state. The coherence properties of the emitted radiation have been analyzed by measuring high order photon correlations and are in good agreement with the theoretical statistics. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A34.00012: Parametric interactions in circuit-QED Michael DeFeo, Manuel Castellanos-Beltran, Adam Sirois, Leonardo Ranzani, Florent Lecocq, Raymond Simmonds, John Teufel, Jose Aumentado The circuit-QED architecture is a versatile platform for implementing quantum optics at microwave frequencies. Incorporating additional nonlinear coupling elements between linear modes in this architecture provides a mechanism to drive parametric interactions. These interactions are a powerful set of tools that can be used to synthesize complex quantum states, generate entanglement and enhance measurement. We will discuss experimental results utilizing parametric interactions to generate and study quantum states in superconducting circuits. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A34.00013: The Forgotten Quantum Number: The Radial Modes of Laguerre-Gauss Beams Anton Zeilinger, William Plick, Mario Krenn, Sven Ramelow The orbital angular momentum quantum number of Laguerre-Gauss beams has received an explosively increasing amount of attention over the past twenty years. However, often overlooked is the so-called radial number of these beams. We present a quantum-mechanical operator formalism of this ``forgotten" quantum number. We place an emphasis on the detailed understanding of the physical interpretation of this formalism, including it's connection to concrete physical observables and conjugate variables. We then draw some connections between this new formalism and the effect the radial number has on beam stability with possible application to quantum communication. [Preview Abstract] |
Session A35: Spinor Gases
Sponsoring Units: DAMOPRoom: 702
Monday, March 3, 2014 8:00AM - 8:12AM |
A35.00001: Fate of Topology in Spin-1 Spinor Bose-Einstein Condensate Yun-Tak Oh, Panjin Kim, Jin-Hong Park, Jung Hoon Han One of the excitements generated by the cold atom systems is the possibility to realize varied topological phases stemming from multi-component nature of the condensate. Popular examples are the antiferromagnetic (AFM) and the ferromagnetic (FM) phases in the three-component atomic condensate with effective spin-1. It follows, from consideration of homotopy, that different sorts of topological defects will be stable in each manifold. Countering such common perceptions, here we show on the basis of a new wave function decomposition scheme that there is no physical parameter regime wherein the temporal dynamics of spin-1 condensate can be described solely within AFM or FM manifold. Initial state of definite topological number prepared entirely within one particular phase must immediately evolve into a mixed state. Accordingly, the very notion of topology and topological stability within the sub-manifold of AFM or FM become invalid. Numerical simulations of the Gross-Pitaevskii equation confirms our claim. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A35.00002: Heteronuclear coherent spinor dynamics in an ultracold spin-1 mixture Dajun Wang, Xiaoke Li, Bing Zhu, Fudong Wang, Xiaodong He, Jun Chen, Mingyang Guo Ultracold spinor gas has been a subject of great interest in quantum gas research for many years. So far, however, all the experimental studies are carried out with a single atomic species, mostly either $^{23}$Na or $^{87}$Rb atom. Only very recently, it has been proposed theoretically that spinor dynamics can also exist in heteronuclear spin-1 mixtures. To explore this, we have prepared an optically trapped ultracold mixture of spin-1 $^{23}$Na and $^{87}$Rb atoms. With well controlled initial spin populations and magnetic fields, we have observed rapid spin population and magnetization oscillations for both Na and Rb due to heteronuclear spin-spin interactions. Following this first demonstration, we believe that rich heteronuclear spinor physics can be studied in the future. We are supported by RGC Hong Kong (grant nos. CUHK 403111 and CUHK 404712). [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A35.00003: Nambu-Goldstone modes in segregated Bose-Einstein condensates Hiromitsu Takeuchi, Kenichi Kasamatsu Nambu-Goldstone modes in immiscible two-component Bose-Einstein condensates are studied theoretically.$^1$ In a uniform system, a flat domain wall is stabilized and then the translational invariance normal to the wall is spontaneously broken in addition to the breaking of two U(1) symmetries in the presence of two complex order parameters. We clarify the properties of the low-energy excitations and identify that there exist only two Nambu-Goldstone modes: an in-phase phonon with a linear dispersion and a ripplon with a fractional dispersion. The ripplon in the low-energy limit is considered as a linear combination of a relative rotation of phases of order parameters and a transverse shift of the domain wall. The signature of the characteristic dispersion can be verified in segregated condensates in a harmonic potential. \\ \\ 1 Hiromitsu Takeuchi and Kenichi Kasamatsu, Phys. Rev. A {\bf 88}, 043612 (2013) [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A35.00004: Magnetic resonance spectroscopy by magnetic field modulation for spinor Bose condensates Akiyuki Tokuno, Shun Uchino Spinor Bose-Einstein condensates in the presence of magnetic fields exhibit nontrivial spin orders caused by peculiar effects to atoms such as spin dependent interactions and quadratic Zeeman splitting. For such a system, one of the challenges is to develop measurement techniques to capture complicated spin orders. As an attempt for this issue we theoretically study resonance phenomena induced by magnetic fields.[1] Assuming a dynamically modulated magnetic field, we formulate the experimentally measurable energy absorption rate (EAR) for spin-F interacting bosons within linear response theory. The EAR spectrum is found to be described by a new type of spin correlation function: the autocorrelation of a quadratic Zeeman term. In addition, in order to test whether the states with different spinor order can be specified from the viewpoint of such a spectral feature, we consider spin-1 Bose condensate, and calculate the EAR spectrum of the ordered states by using Bogoliubov theory. As a result the spectrum in each phase is found to show individual characteristic behavior, and this spectroscopy is expected to have possibility to specify various magnetic states in other systems. [1] A. Tokuno and S. Uchino, Phys. Rev. A 87, 061604 (2013). [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A35.00005: Rotating $SU(2)$ Bose-Einstein condensates Peder Galteland, Asle Sudbo The topological excitations inherent in Bose-Einstein condensates are important elements of both superconductors and superfluids, and are even relevant in cosmology and high energy physics. Condensates with multiple components and intercomponent couplings open up new possibilities for novel vortex physics, and which have been studied numerically and realized experimentally. We have studied a uniformly frustrated $2$-component Ginzburg-Landau theory with amplitude fluctuations and density-density interactions included, through the use of Metropolis Monte Carlo techniques. We have explored the ground states as a function of rotational frequency, and inter- and intra-component coupling strength. It was found that the model exhibits both hexagonal lattices of co-centered vortices, and square lattices of interpenetrating vortices. These lattices exhibit a first order melting transition. The special case of an $SU(2)$ symmetric potential was also explored. With this additional symmetry, dimer vortex configurations, strong staggering of the amplitude fields and massive degeneracy of the ground states appear. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A35.00006: 10 dB of Spin Squeezing via Measurement -- a Useful Entanglement Resource Kevin Cox, Justin Bohnet, Matthew Norcia, Joshua Weiner, Zilong Chen, James Thompson We report results from an experiment to generate and directly observe 10.2(6) dB of spin squeezing using a quantum non-demolition measurement (QND), the most directly observed spin squeezing in an atomic ensemble to date. The squeezing is generated by measuring state populations through an optical cavity on a closed optical transition in an ensemble of $5\ast {10}^{5} \quad^{87}$Rb atoms. Such a scheme can be applied to optical lattice clocks using Sr and Yb. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A35.00007: Realizing SU(N) magnets in thermal alkaline-earth gases Michael Beverland, Alexey Gorshkov, Ana Maria Rey, Gorjan Alagic We show that thermal fermionic alkaline-earth atoms in flat-bottom traps allow one to implement a spin model displaying two symmetries: the symmetry that swaps atoms occupying different vibrational levels of the trap and the SU(N) symmetry associated with N nuclear spin states. The high symmetry allows us to analytically calculate the full spectrum, the eigenstates, and the dynamics. Armed with such a solid understanding, we show how this system can be used to generate entangled states usable for Heisenberg limited metrology (e.g. clocks), to make measurements useful for quantum information processing, and to understand spin diffusion in SU(N) systems. The best news is that this highly symmetric spin model should be readily realizable even when the vibrational levels are occupied according to a high-temperature thermal or a non-thermal distribution. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A35.00008: Classification of Spin Ordering of Fermions with Large Spin Biao Huang, Tin-Lun Ho Cold atom research provides a unique opportunity for studying the physics of high spin particles, as most bosonic atoms have non-zero spin, and most fermionic atoms have spin larger than 1/2. While there have been many experiments of large spin bosons, there are few experimental studies of large spin fermions. Here, we present a general scheme for classifying the spin ordering of fermions with arbitrary spins by studying the symmetry of their single particle density matrices. Our scheme is based on the Majorana representation of spins, which provides a geometric representation of the ordering in spin space. It readily concludes that there are no spin orders with tetrahedron symmetry. We have also used mean field theory to illustrate the emergence of various type of spin ordering. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A35.00009: Dynamical scalability and control of totally connected spin networks across quantum phase transitions \'Oscar L. Acevedo, Luis Quiroga, Ferney J. Rodr\'Iguez, Neil F. Johnson Dynamical quantum phase crossings of spin networks have recently received increased attention thanks to their relation to adiabatic quantum computing, and their feasible realizations using ultra-cold atomic and molecular systems with a highly tunable degree of connectivity. Dynamical scaling of spatially distributed systems like Ising models have been widely studied, and successfully related to well-known theories like the Kibble-Zurek mechanism. The case of totally connected networks such as the Dicke Model and Lipkin-Meshkov-Glick Model, however, is known to exhibit a breakdown of these frameworks. Our analysis overcomes the lack of spatial correlation structure by developing a general approach which (i) is valid regardless the connectivity of the system, (ii) goes beyond critical exponents, and (iii) provides a time-resolved picture of dynamical scaling. By treating these models as a method for macroscopic quantum control of their subsystems, we have found microscopic signatures of the dynamical scaling as well as instances of dynamical enhancement of distinctive quantum properties such as entanglement and coherence. Our results yield novel prescriptions for the fields of quantum simulations and quantum control, and deepen our fundamental understanding of phase transitions. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A35.00010: Competing exotic quantum phases of spin-1/2 ultra-cold lattice bosons with extended spin interactions Chia-Chen Chang, Valery Rousseau, Richard T. Scalettar, George Batrouni Rapid progress in pure optical trapping techniques makes it possible now to create degenerate Bose gases with spin degrees of freedom. Systems such as ${}^{87}$Rb or ${}^{23}$Na in the $F=1$ hyperfine state offer a unique platform for studying the interplay of superfluidity and magnetism, phases resulting from macroscopic quantum coherence and symmetry breaking respectively. Motivated by these experimental developments, we study ground state phases of a two-component spinor Bose gas loaded on an optical lattice. The system is described effectively by the Bose-Hubbard Hamiltonian with onsite and extended spin-spin interactions. Using mean-field theory and quantum Monte Carlo simulations, we map out the phase diagram of the system. A rich variety of phases is identified, including antiferromagnetic (AF) Mott insulators, ferromagnetic or AF superfluids, and supersolids. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A35.00011: Fluctuations in a Spin Chain and the Entanglement Hamiltonian Ari Turner, Eugene Demler How are quantum fluctuations and thermal fluctuations different in many-body systems? I will compare the variance of the fluctuations of spin in a segment of a spin chain in the ground state and at a finite temperature, showing that fluctuations in the ground state are much more correlated than in the thermal state. The full distribution function of spin can also be determined, and is non-Gaussian. These effects could possibly be measured in a chain of sodium atoms in an optical lattice. The method involves mapping the system to an imaginary thermal system called the ``entanglement Hamiltonian.'' Measuring the ground state fluctuations of the spin chain gives an indirect way of measuring the entanglement Hamiltonian. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A35.00012: Enhanced Mobility in a two-species ultracold atomic system Derek Lee, Mike Gunn, Nicola Wilkin We consider the mobility of two species of interacting atoms in an optical lattice. The behaviour of the system is radically modified as a function of the relative detuning of the optical lattice to resonances of the two atomic species. We will discuss the polaronic and pairing physics in such systems. We will also present proposals for possible experimental realizations of our theoretical model using Feshbach resonances. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A35.00013: Improving the Hubbard-Lattice Gauge Theory correspondence on optical lattices Yannick Meurice, Alexei Bozavov, Yuzhi Liu, Chen-Yen Lai, Shan-Wen Tsai There exists a strong coupling equivalence between the Hubbard model with repulsive on site interactions and SU(2) lattice gauge theory with one fermion. The correspondence holds at lowest nontrivial order in degenerate perturbation theory, but fails to reproduce the plaquette interactions of the gauge theory. We discuss modifications that can be implemented experimentally on optical lattices and could improve this situation. This includes bipartite lattices with s and p orbitals (plaquette currents) and dipolar molecules in an external field (long range dipole interactions). We discuss recent numerical calculations based on determinantal Monte Carlo and aimed at testing improvement ideas obtained from mean field theory or strong coupling arguments. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A35.00014: ABSTRACT MOVED TO M35.00015 |
Monday, March 3, 2014 10:48AM - 11:00AM |
A35.00015: Magnetic properties of ultracold fermions in multilayered Lieb lattices Kazuto Noda, Kensuke Inaba, Makoto Yamashita The recent experimental development in atomic physics enables us to realize novel many-body systems using optical lattices. We study the magnetic properties of cold fermions in multilayered Lieb lattices, which are the ideal model systems for investigating the flat-band ferromagnetism. Our dynamical mean-field results of bilayer, trilayer, and several multilayers elucidate that finite magnetization at the surface layers in the odd-layered lattices emerges even in the infinitesimal small interaction region. This is a striking feature of the flat-band ferromagnetism in multilayered systems as a consequence of the Lieb theorem. We also discuss how this phenomenon appears in the infinite-layered (three dimensional) system. [Preview Abstract] |
Session A36: Focus Session: Semiconductor Qubits: Optically Addressed Impurities & Quantum Dots
Sponsoring Units: GQIChair: Sophia Economou, Naval Research Laboratory
Room: 703
Monday, March 3, 2014 8:00AM - 8:12AM |
A36.00001: Cavity QED in a Quantum Dot Molecule Coupled to a Photonic Crystal Cavity Patrick Vora, Samuel Carter, Chul Soo Kim, Mijin Kim, Timothy Sweeney, Lily Yang, Peter Brereton, Allan Bracker, Daniel Gammon Semiconductor quantum dots (QDs) are a promising system for quantum information. InAs QDs grown within a GaAs diode heterostructure can be charged with a single electron, thereby serving as a spin qubit, and can easily be integrated with photonic circuits. An alternative qubit is the quantum dot molecule (QDM), a pair of QDs separated by a tunnel barrier. QDMs can be charged with two electrons that form spin singlet and triplet ground states which are less susceptible to nuclear spin effects. Furthermore, QDMs allow radiative recombination between carriers localized on different QDs. These interdot transitions can be tuned over a large range with applied voltage making them attractive as single photon sources. We have demonstrated coupling between a QDM and a two-dimensional photonic crystal cavity. A number of novel cavity-QED phenomena have been observed such as Purcell enhancement of interdot transitions, cavity-assisted Raman scattering, Autler-Townes splitting, and a cavity-induced AC Stark effect in a solid state $\Lambda$ system. These results have important implications for highly tunable single photon sources and also the development of a quantum network within a photonic crystal structure. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A36.00002: Complete quantum control of an exciton qubit bound to an isoelectronic center in GaAs Gabriel Ethier-Majcher, Philippe St-Jean, Gianluca Boso, Alberto Tosi, Sebastien Francoeur Various schemes of quantum information processing rely on interconnection of matter qubits via optical photons, flying qubits. To achieve scalable and robust quantum computing and networking within those schemes, matter qubits must present high optical homogeneity and strong coupling to photons. Here, coherent optical manipulation of excitons bound to single isoelectronic centers formed from a pair of nitrogen isovalent impurities in GaAs is demonstrated. Using a time-gated technique, resonant fluorescence of the exciton is measured and the power dependence of the fluorescence shows Rabi rotations from which a dipole moment as high as 55 D can be extracted. Interestingly, excitation induced dephasing, a phenomenon lowering the fidelity of exciton gating in quantum dots, is strongly reduced in our system. The complete quantum control of the qubit is demonstrated through Ramsey interferometry. The coherence time of the exciton reaches 115 ps. Our results show that isoelectronic centers combine the strong dipole moments of quantum dots and the optical homogeneity and predictability of atomic systems such as NV centers in diamond, establishing them as promising candidates for quantum information processing. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A36.00003: Universal control and error correction in multi-qubit spin registers in diamond Tim Hugo Taminiau, Julia Cramer, Toeno van der Sar, Viatcheslav V. Dobrovitski, Ronald Hanson Quantum registers of nuclear spins coupled to electron spins of individual solid-state defects are a promising platform for quantum information processing. Pioneering experiments selected defects with favourably located nuclear spins having particularly strong hyperfine couplings. For progress towards large-scale applications, larger and deterministically available nuclear registers are highly desirable. Here we present universal control over multi-qubit spin registers by harnessing abundant weakly coupled nuclear spins [1,2]. We use the electron spin of a nitrogen-vacancy centre in diamond to selectively initialize, control and read out carbon-13 spins in the surrounding spin bath and construct high-fidelity single- and two-qubit gates [2]. We exploit these new capabilities to implement a three-qubit quantum-error-correction protocol and demonstrate the robustness of the encoded state against applied errors. These results transform weakly coupled nuclear spins from a source of decoherence into a reliable resource, paving the way towards extended quantum networks and surface-code quantum computing based on multi-qubit nodes. \\[4pt] [1] T. H. Taminiau et al., Phys. Rev. Lett. 109, 137602 (2012)\\[0pt] [2] T. H. Taminiau et al., arXiv:1309.5452 (2013) [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 9:12AM |
A36.00004: Cavity-stimulated Raman emission from a single quantum dot spin Invited Speaker: Timothy Sweeney The integration of solid state quantum emitters into photonic structures represents a scalable path to developing quantum information technologies. Unfortunately, solid state emitters suffer from spectral inhomogeneity, due to spectral wandering of a single emitter or intrinsic dissimilarity between different emitters. In this talk I will present a means to overcome spectral inhomogeneity with a $\Lambda $-type solid-state emitter that is coupled to an optical cavity. For this demonstration we used a charge controlled InAs/GaAs quantum dot that is coupled to a photonic crystal cavity [1]. We exploit a cavity-stimulated Raman process in this $\Lambda $-type system, which allows for the emission of a quantum dot to be detuned from its optical transition by at least 125 GHz [2]. This process not only overcomes spectral inhomogeneity but also can enable an efficient, tunable source of indistinguishable photons and deterministic entanglement of distant spin qubits in a photonic crystal quantum network. \\[4pt] [1] S. G. Carter, et. al., \textit{Quantum control of a spin qubit coupled to a photonic crystal cavity}, Nat. Photonics, vol. 7, no. 4, pp. 329--334, Mar. 2013. \\[4pt] [2] T. M. Sweeney, et. al., \textit{Cavity-stimulated Raman emission from a single quantum dot spin}, (submitted Nov. 2013). [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A36.00005: High-selectivity detection of single nuclear spins using rotary echo on a nitrogen-vacancy center in diamond Vagharsh Mkhitaryan, Viatcheslav Dobrovitski The properties of the nitrogen-vacancy (NV) centers in diamond make them an excellent tool for nanoscale spin detection and sensing, capable of detecting individual nuclear spins located 0.5-1 nm away [1]. However, the selectivity of the current methods is limited. We show that the rotating-frame control of the NV center's electron spin can improve the sensing selectivity 10-1000 times in comparison with the existing methods. We employ periodically changing Rabi driving (multiple rotary echo [2]) with a precisely chosen period, corresponding to the precession of the given nuclear spin. The rotary echo decouples the NV center from most nuclear spins, efficiently protecting coherence. At the same time, the given nuclear spin, whose precession fits a stringent resonance condition, does not decouple, and can be detected by its decohering impact on the NV spin. We evaluate the resolution and sensitivity of this detection scheme analytically, and verify the results by numerical simulations. \\[4pt] [1] T. H. Taminiau et al., Phys. Rev. Lett. 109, 137602 (2012), S. Kolkowitz et al., Phys. Rev. Lett. 109, 137601 (2012), N. Zhao et al., Nat. Nanotechnol. 7, 657 (2012), P. London et al., Phys. Rev. Lett. 111, 067601 (2013). \\[0pt] [2] I. Solomon, Phys. Rev. Lett. 2, 301 (1959). [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A36.00006: Measurement of the Berry Phase in a Single Solid-State Spin Qubit Kai Zhang, N.M. Nusran, B.R. Slezak, M.V. Gurudev Dutt Geometric phases in quantum mechanics have a long history and may offer some advantages in quantum information processing techniques, e.g. geometric phases are intrinsically robust to fluctuations in control parameters. We demonstrate a controlled way of accumulating geometric phase by Berry's method in a single Nitrogen-Vacancy (NV) center in diamond lattice. We perform state tomography measurement to confirm this Berry phase and we find no evidence for geometric dephasing in our system. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A36.00007: Dual-Channel Lock-in Magnetometry with a Single Spin in Diamond N.M. Nusran, M.V. Gurudev Dutt Diamond spin probes are promising candidates for nanoscale magnetometry and magnetic imaging. Although dynamic decoupling (DD) technique with the spin can lead to high sensitivity ($\sim nT / \sqrt{Hz}$) , certain limitations exist in the standard sensing approach for AC magnetic fields: i) a trade-off between the sensitivity and the dynamic range and ii) constraints on the AC magnetic field phase. We present an experimental scheme that incorporates DD and phase estimation algorithms to address these problems. We achieve nearly decoherence-limited sensitivity over a wide dynamic range, and we also demonstrate unambiguous reconstruction of the amplitude and phase of the magnetic field. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A36.00008: All-optical spin manipulation methods for a solid-state defect spin C.G. Yale, F.J. Heremans, D.J. Christle, D.D. Awschalom, L.C. Bassett, B.B. Buckley, G. Burkard The nitrogen-vacancy (NV) center in diamond is an optically-addressable defect spin with promising applications in quantum information processing and metrology. Here we discuss all-optical methods of dynamically manipulating the spin state of the NV center by exploiting coherent interactions with light at temperatures below 10 K. We study the spin dynamics of the NV center using coherent pulses of light, and achieve rotations of the spin state at sub-nanosecond timescales\footnote{L.C. Bassett*, F.J. Heremans*, D.J. Christle, C.G. Yale, G. Burkard, B.B. Buckley, and D.D. Awschalom, \emph{in preparation} }. With ultrafast pump-probe spectroscopy and by tuning the excited-state spin Hamiltonian with a magnetic field, we demonstrate arbitrary-axis spin rotations and controlled unitary evolution within the full spin-triplet manifold. These experiments also complement recent work demonstrating optical spin control using coherent dark states\footnote{C.G. Yale*, B.B. Buckley*, D.J. Christle, G. Burkard, F.J. Heremans, L.C. Bassett, and D.D. Awschalom, \emph{PNAS} \textbf{110}, 7595 (2013).}. These all-optical techniques provide a probe for decoherence and a pathway toward integrating spin qubits and photonic networks. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A36.00009: Optical studies of ultrafast orbital dynamics of a single spin in diamond F.J. Heremans, D.J. Christle, C.G. Yale, D.D. Awschalom, L.C. Bassett, B.B. Buckley, G. Burkard The nitrogen-vacancy (NV) center in diamond shows great potential as an optically addressable solid-state spin for use in quantum information and metrology. At low temperature ($T < 10$ K) the NV center's orbital-doublet, spin-triplet excited state becomes stable and optically coherent with the ground state. Here we use ultrafast optical pump-probe techniques coupled with optical polarization selection rules to investigate coherent orbital dynamics of the NV center's excited state\footnote{L. C. Bassett*, F. J. Heremans*, D. J. Christle, C. G. Yale, G. Burkard, B. B. Buckley, and D. D. Awschalom, \textit{in preparation}.}. The experiments reveal dynamics which occur on nanosecond down to femtosecond timescales due to the interplay amongst these three orbital levels. These techniques enable all-optical control of the NV center's spin state and could provide a probe to investigate orbital decoherence and phonon interactions in the NV center and other defect systems. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A36.00010: Toward Deterministic Implantation of Nitrogen Vacancy Centers in Bulk Diamond Crystals T.O. Brundage, Z. Atkins, S. Sangtawesin, J.R. Petta Over the last decade, research investigating the room temperature stability, coherence, and optical manipulation of spin states of the nitrogen vacancy (NV) center in diamond has made it a strong candidate for applications in magnetometry and quantum information processing. As research progresses and we begin to investigate the dynamics and scalability of multiple NV systems, the ability to place NV centers deterministically in the host material with high accuracy is critical. Here we implement a simple fabrication method for NV implantation. We expose and develop small dots in PMMA using an electron-beam lithography tool. Unexposed PMMA serves as a mask for 20 keV nitrogen-15 implantation. The implanted sample is then cleaned in a boiling mixture of nitric, sulfuric, and perchloric acid. Annealing at 850$^\circ$ for 2 hours allows vacancies to diffuse next to implanted nitrogen atoms, forming NV centers with an efficiency of a few percent. SRIM simulations provide nitrogen ion distribution within our diamond substrate and PMMA mask as functions of implantation energy. Thus, after balancing implantation parameters and exposure hole cross-sections, NV center placement can be achieved with accuracy limited by the precision of available electron-beam lithography equipment. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A36.00011: Room Temperature Optically-Detected Magnetic Resonance of Silicon Vacancies in SiC Samuel G. Carter, Evan R. Glaser, Brad D. Weaver Single vacancies and vacancy pairs in silicon carbide (SiC) have shown strong potential as quantum bits (qubits) due to demonstrations of spin coherence at room temperature and the maturity of semiconductor device processing in this material system. The majority of work so far on the Si vacancy has still been at low temperatures and high magnetic fields with electron paramagnetic resonance detection, which are not well-suited for many applications. Here, we demonstrate room temperature optically detected magnetic resonance (ODMR) of the Si vacancy in SiC for a series of relatively low magnetic fields. At these fields, there are changes in the ODMR signal due to various effects including the crossing of different spin states. We measure the excitation wavelength dependence and time-dependence of the optical process that orients and detects the spin state, perform microwave pulse control of the spins showing Rabi oscillations, and measure the emission lifetime of the defect to be 6 ns. These results provide a better understanding of the properties of this system and the conditions under which the spin states of the vacancy can be controlled. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A36.00012: Photonic engineering of defect qubit systems in silicon carbide G. Calusine, A. Politi, D.D. Awschalom The recent discovery of electronic states in silicon carbide (SiC) exhibiting properties similar to the negatively charged nitrogen vacancy center in diamond has opened up the possibility of scalable integration of defect qubits into solid state devices that can be fabricated on the wafer scale [1,2]. One form of SiC termed ``3C'' grows as a high quality thin film on silicon making it a promising platform for incorporating these defect systems into three dimensional device architectures such as photonics and micro- and nano-mechanical devices. We present the results of our recent progress towards incorporating optically active defects states in 3C SiC into high Q, small mode volume planar photonic crystal optical cavities. We demonstrate an optimized process for producing modified H1 and L3 cavities with Q's as high as 2700. Additionally, we utilize a combination of resonant scattering spectroscopy techniques and FDTD simulations to study the coupling of defect optical transitions to cavity modes and intrinsic cavity properties. \\[4pt] [1] W. F. Koehl et al., Nature 479, 84-87 (2011)\\[0pt] [2] A. L. Falk et al., Nat. Commun. 4, 1819 (2013) [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A36.00013: Electrically driven spin resonance in silicon carbide color centers Paul Klimov, Abram Falk, Bob Buckley, David Awschalom We demonstrate that the spin of optically addressable point defects can be coherently controlled with AC electric fields [1]. Based on magnetic-dipole forbidden spin transitions, this scheme enables spatially confined spin control, the imaging of GHz-frequency resonant electric fields, and the characterization of defect spin multiplicity. Our results are based on the QL1 defect in 6H-SiC, which is one of many newly appreciated paramagnetic defects in SiC that can be optically addressed and exhibit long spin coherence times. Our methods apply generally to optically addressable spin systems in many semiconductors, including the nitrogen-vacancy center in diamond. Since electric fields are readily confined on nanometer scales, electrically driven spin resonance offers a viable route towards scalable quantum control of electron spins in a dense network.\\[4pt] [1] P. V. Klimov et al., arXiv:1310.4844 (2013). [Preview Abstract] |
Session A37: Focus Session: Graphene on SiC: Growth, Structure and Properties
Sponsoring Units: DMPChair: Thushari Jayasekera, Southern Illinois University Carbondale
Room: 705/707
Monday, March 3, 2014 8:00AM - 8:12AM |
A37.00001: STM Study of Sidewall Graphene Nanoribbons on SiC(0001) Yuntao Li, David B. Torrance, M. Tien Hoang, Meredith S. Nevius, Edward H. Conrad, Phillip N. First Graphene nanoribbons grown on SiC sidewall nanofacets have shown interesting transport and electronic structure. We use scanning tunneling microscopy and spectroscopy (STM/STS) to explore their local atomic and electronic structure. Nanoribbon formation is found to depend critically on nanofacet orientation, nanofacet density and growth conditions. Under some conditions, nanoribbons grow predominantly on the nanofacet, under others, they can be induced to grow only at the edges of nanofacets. Significant electronic density-of-states features, resolved by STS, are determined by the different nanoribbon configurations. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A37.00002: Wide-gap Semiconducting Graphene from Nitrogen-seeded SiC Feng Wang, G. Liu, S. Rothwell, M. Nevius, A. Locatelli, T. Mentes, A. Sala, I. Rodriguez, A. Retana, A. Tejeda, A. Taleb-Ibrahimi, L. Fieldman, P. Cohen, E. Conrad We demonstrate a new approach to produce semiconducting graphene that uses surface nitrogen-seeded SiC substrates to grow graphene. The surface nitrogen atoms pin the graphene to the SiC. The starting material is a sub-monolayer of N produced by NO annealing the SiC surface at 1175C. The oxide is then removed chemically to leave $\sim$0.5ML of N that is stable at the graphene growth temperature. Graphene is grown at 1400C by CCS(confinement controlled sublimation) method. Post growth studies with LEED, ARPES, LEEM, PEEM, micro-ARPES and STM show that this N-graphene is continuous but rippled. No nitrogen defect is included in the graphene film. The most important finding is that both ARPES and PEEM show that the N-graphene has a finite bandgap $\sim$0.5-0.7eV depend on graphene thickness. The origin of the band gap is not yet understood although there are strong experimental reasons to suspect strain gradients play an important role. We will also show that the SiC/nitrogen surface can be pre-patterned to high resolution prior to graphene growth. Post growth, the graphene film becomes a periodic N-graphene/normal-graphene modulated structure. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A37.00003: Unraveling the (3$\times$3)-SiC($\bar{1} \bar{1} \bar{1}$) reconstruction and its role as an interface structure Lydia Nemec, Florian Lazarevic, Patrick Rinke, Volker Blum, Matthias Scheffler To refine the growth quality of epitaxial graphene on the C-side of SiC and improving the resulting electronic character of these films, it is important to understand the atomic and electronic-structure of the interface. A phase mixture of different surface phases is observed just when surface graphitization first sets in. However, the atomic structure of some of the competing surface phases as well as of the SiC-graphene interface is unknown. We performed a density functional theory study on the C-side of the polar SiC($\bar{1} \bar{1} \bar{1}$) surface using the all-electron numeric atom-centered basis function code FHI-aims. The formation energy of different reconstructions and model systems for the interface is presented within the thermodynamically allowed range. The surface energies of the known (2$\times$2) phase is compared with several structural models of the (3$\times$3) phase proposed in the literature. Inorian comparison all the previously suggested (3$\times$3) models are higher in energy than the known (2$\times$2) phase. We present a new model for the (3$\times$3) reconstruction. Its formation energy crosses that of the (2$\times$2) phase just at the carbon rich limit of the chemical potential, which explains the observed phase mixture. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A37.00004: Amorphous carbon for structured step bunching during graphene growth on SiC James Palmer, Jan Kunc, Yike Hu, John Hankinson, Zelei Guo, Claire Berger, Walt de Heer Structured growth of high quality graphene is necessary for technological development of carbon based materials. Specifically, control of the bunching and placement of surface steps under epitaxial graphene on SiC is an important consideration for graphene device production. We demonstrate lithographically patterned evaporated amorphous carbon as a method to pin SiC surface steps. Evaporated amorphous carbon is an ideal step-flow barrier on SiC due to its chemical compatibility with graphene growth and its structural stability at high temperatures, as well as its patternability. The amorphous carbon is deposited in vacuum on SiC prior to graphene growth. In the graphene furnace at temperatures above 1200$^{\circ}$C, mobile SiC steps accumulate at these amorphous carbon barriers, forming an aligned step free region for graphene growth at temperatures above 1330$^{\circ}$C. AFM imaging and Raman spectroscopy support the formation of quality step-free graphene sheets grown on SiC with the step morphology aligned to the carbon grid. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A37.00005: Oxynitride and Silicates at Epitaxial Graphene on SiC (0001) Hansika Sirikumara, Jaime Bohorquez, Thushari Jayasekera Epitaxial graphene, the sp$^{2}$-hybridized network of carbon grown on another material is one way of creating large-scale graphene. Intercalated oxygen at the interface has shown to saturate the Si dangling bonds, and is a promising way of tuning the charge density in epitaxial graphene on SiC [1]. It would be interesting to investigate how oxy-nitrides and silicates at the SiC/graphene interface can change the electronic properties of the graphene layer. Based on the first principles density functional theory calculations, we discuss the electronic and structural properties of epitaxial graphene on SiC with Si$_{2}$O$_{5}$ and SiON layers at the interface. \\[4pt] [1] C. Mathieu, et al, Phys. Rev. B 86, 035435 (2012) [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A37.00006: Effect of Oxygen on Sublimation Growth of Graphene on C-face SiC Zachary Robinson, Glenn Jernigan, Konrad Bussmann, Marc Currie, Rachael Myers-Ward, Virginia Wheeler, Luke Nyakiti, Satoshi Oida, James Hannon, Chip Eddy, D. Kurt Gaskill Graphene grown on Si-face SiC has demonstrated improved thickness uniformity when formed in an argon environment. For C-face growth, expected to yield graphene with superior electronic properties, similar progress has not yet been achieved. A systematic study of C-face SiC surface preparation and graphene growth in an argon environment has been carried out in a high temperature chemical vapor deposition system modified for low pressure sublimation. For all growth conditions investigated, the resulting graphene films were found to have non-uniform thickness. Further, x-ray photoelectron spectroscopy (XPS) measurements reveal significant amounts of oxygen on the surface, which has been suggested to cause the non-uniformity [1]. Thus, a sample was transferred to an ultra-high vacuum (UHV) system equipped with in situ XPS, where a UHV anneal of 1200$^{\circ}$C was shown to be necessary to desorb the oxygen. Post-anneal exposure to atmospheric conditions resulted in the return of only 20\% of the original oxygen concentration, suggesting that a robust oxide may be present during growth. Preliminary low energy electron micscoscopy results confirm that trace amounts of oxygen significantly affects the graphene growth process. \\[4pt] [1] Phys. Rev. B 82, 235406 (2010) [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:48AM |
A37.00007: STM Properties and Manipulation of Epitaxial Graphene Invited Speaker: Paul Thibado Epitaxial graphene grown on SiC has been identified as one of the most likely avenues to graphene-based electronics. Understanding how morphology affects electronic properties is therefore important. In our work, epitaxial graphene was grown on the polar and non-polar a-, m-, and r-crystallographic oriented surfaces of SiC, and was investigated using scanning tunneling microscopy (STM). Bunched nano-ridges ten times smaller than previously recorded were observed throughout the surface. A new STM technique called electrostatic-manipulation scanning tunneling microscopy (EM-STM) was performed to modify the morphology of the nano-ridges. By modeling the electrostatics involved in the EM-STM measurement, we estimate that a force of 5 nN and energy of 10 eV was required to alter the local interfacial bonding. At the atomic scale, STM images of Moire patterns reveal low-angle, twisted bi-layer graphene, grain boundaries, and an apparent lattice constant dilation. We will show that this dilation is due to the STM tip electrostatically dragging the graphene surface. Collaborators: P. Xu, D. Qi, M.L. Ackerman, S.D. Barber, J.K. Schoelz, and J. Thompson, Department of Physics, University of Arkansas, Fayetteville, AR, 72701, USA; V.D. Wheelr, R.L. Myers-Ward, C.R. Eddy, Jr., and D.K. Gaskill, U.S. Naval Research Laboratory, Washington, DC 20375, USA; and L.O. Nyakiti, Texas A\&M University.\\[4pt] Financial Support: P.X. and P.M.T. gratefully acknowledge the financial support of ONR under grant N00014-10-1-0181 and NSF under grant DMR-0855358. Work at the U.S. Naval Research Laboratory is supported by the Office of Naval Research. L.O.N. gratefully acknowledges postdoctoral fellowship support through the ASEE. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A37.00008: Transport in chemically gated graphene p-n junctions Christoph Tegenkamp, Jens Baringhaus, Alexander St\"ohr, Ulrich Starke The chirality of charge carriers in graphene allows them to get through potential barriers without any reflection (known as Klein tunneling). To study this effect the fabrication of well-defined p-n junctions is necessary. We use the intercalation of Ge to convert the buffer layer on the SiC(0001) surface into graphene with local p-type or ntype doping depending on the local Ge coverage. The buffer layer is initially patterned using optical lithography, to fabricate isolated n-p, npn and pnp-structures. The n- and p-type doping (340 meV, -290 meV) is confirmed by STS which also reveals very narrow p-n junctions with a length below 5 nm. The corresponding electric fields are as high as $10^6 V/cm$ and therefore significantly higher than those induced by field effects, providing a perfect environment to study Klein tunneling. Transport experiments are carried out by means of a 4-tip STM system,on n-p-n as well as p-n-p structures. Their resistance was found to be strongly dependent on temperature and the inner barrier length. While short barriers ($<$ 200 nm) appear almost transparent, the resistance increases rapidly for barrier widths exceeding the coherence length ($>$ 600 nm). [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A37.00009: Asymmetric Electron Transport Induced by Friedel Oscillations at Monolayer-Bilayer Heterojunctions of Epitaxial Graphene Kendal Clark, X.-G. Zhang, Gong Gu, Guowei He, Randall Feenstra, An-Ping Li We report asymmetric electron transport upon bias polarity reversal at individual monolayer-bilayer (ML-BL) boundaries in epitaxial graphene on SiC (0001), revealed by multi-probe scanning tunneling potentiometry. A greater voltage drop is observed when the current flows from ML to BL graphene than in the reverse direction, and the difference remains nearly unchanged when bias exceeds a threshold. This is not a typical nonlinear conductance due to electron transmission through an asymmetric potential. Rather, it indicates the opening of an energy gap at the Fermi energy. Our theoretical analysis finds that Friedel charge oscillation opens a gap for electrons with wave vectors perpendicular to the boundary. The Friedel gaps are different on the ML and BL sides, which can shift under bias and lead to asymmetric transport upon reversing the bias polarity. A quantitative agreement is seen between experiment and theory on both the sign and the magnitude of the asymmetry. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A37.00010: Selective Growth of Graphene by Pulsed Laser Annealing Ion Implanted SiC Kara Berke, Xiaotie Wang, Nick Rudawski, Dinesh Venkatachalam, Joel Fridmann, Brent Gila, Fan Ren, Rob Elliman, Arthur Hebard, Bill Appleton We report a method for site-selective graphene growth on SiC for direct nano-scale patterning of graphene. Crystalline SiC was implanted with Si and C ions to amorphize the sample surface, then subjected to pulsed laser annealing (PLA); graphene growth occurred only where ions were implanted. PLA parameters including the fluence, number of pulses, and annealing environment were investigated to optimize the growth process. Our previous work involving Au, Cu, and Ge implants in SiC suggested that both the implanted species and surface amorphization affect graphene growth. In this work, we show that surface amorphization alone, without the presence of foreign ionic species, can be used with PLA to create site-selective graphene growth on SiC. Samples were characterized using Raman spectroscopy and cross-sectional transmission electron microscopy. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A37.00011: Tip-induced Coulomb gap in scanning tunneling microscopy experiment on graphene Yue Zhao, Jungseok Chae, Suyong Jung, Cory Dean, Lei Wang, Yuanda Gao, James Hone, Joao Rodrigues, Shaffique Adam, Takashi Taniguchi, Kenji Watanabe, Kenneth Shepard, Andrea Young, Philip Kim, Nikolai Zhitenev, Joseph Stroscio Graphene is a two-dimensional-electron-gas (2DEG) system exposed at the surface, which allows scanning tunneling microscopy (STM) to investigate the electron-electron interactions associated with the Dirac nature on a local scale, with a variety of tuning knobs, such as carrier density, spatially varying disorder potential, and applied magnetic field. However, the electron-electron interaction in graphene is sensitive to the disorder details. Moreover, a tip induced potential can significantly complicate the interpretation of details in the tunneling spectra. Utilizing high mobility graphene devices with low residual disorder, we can minimize the effect of local potential fluctuation, to better understand the role of tip-induced potentials in the measurement. We report the observation of large energy gaps and modification to Landau level (LLs) spectra, which are due to the spatially inhomogeneous density profile caused by tip gating. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A37.00012: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 10:48AM - 11:00AM |
A37.00013: ABSTRACT WITHDRAWN |
Session A38: Invited Session: Bringing Newcomers into the Physics Community - The Importance of Growth and Community Support
Sponsoring Units: FEdChair: Angela Little, University of California, Berkeley
Room: 709/711
Monday, March 3, 2014 8:00AM - 8:36AM |
A38.00001: Developing mindful, collaborative, and resilient physics students through regular reflection and empathetic feedback Invited Speaker: Dimitri Dounas-Frazer Low retention in the sciences is due in part to students' perceptions of grading practices as harsh and of faculty as unapproachable. Improving retention of science students therefore requires the creation of educational spaces where students feel better supported in their development as learners. To this end, we are piloting a system that facilitates regular student reflection and personalized instructor feedback to support students in becoming mindful, collaborative, and resilient scientists. Students choose one of four topics to guide their reflections, and instructor responses aim to acknowledge and empathize with students' difficulties, recognize their efforts to improve, and provide them with additional resources whenever appropriate. In addition to fostering a supportive learning environment, this system further acts as a vehicle for continual formative assessment, enabling instructors to modify the learning environment to respond to students' needs in real time. In this talk, we report preliminary results on how regular reflection and feedback shape students' experiences in a physics course and how students' reflections evolve over time. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 9:12AM |
A38.00002: Building Bridges to Belonging: Mindsets that Reduce Stereotype Threat and Increase Participation, Achievement, and Learning in STEM Invited Speaker: Catherine Good Ability-impugning stereotypes have been implicated in race and gender gaps in students' STEM achievement, aspirations, and learning, a phenomenon known as stereotype threat. Research-based interventions to help students overcome the impact of stereotype threat include shaping their mindsets about learning and achievement. In particular, combating the culture of talent in STEM by encouraging students to view intelligence as a malleable quality rather than a fixed trait has been shown to reduce race and gender gaps in achievement. Furthermore, fostering students' sense of belonging—their feelings of being an accepted member of an academic community whose contributions are valued—has been linked to increased achievement and motivation, especially when those feelings of belonging are based on effort and engagement rather than underlying ability. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:48AM |
A38.00003: Enlightened Searches for Talent are Needed to Bring Newcomers into Physics Invited Speaker: Casey W. Miller The National Academies have suggested that increasing diversity in STEM will be critical to the future competitiveness of the US in these areas [1], and the leadership of both the NSF [2] and the APS is taking this seriously. Physics and Astronomy programs grant, on average, only one PhD every 5 and 10 years, respectively, to members of underrepresented groups [3]. We are therefore not surprisingly the least diverse of the sciences [4]. In this talk, I will discuss several opportunities that may help our community move toward meeting these goals. The most universally applicable regard perturbing graduate admissions policies and practices [5], and employing key features of successful Bridge Programs into graduate programs [6]. For the former, we need to reevaluate the use of the GRE exams [7], and develop and implement more enlightened searches for talent. \\[4pt] [1] ``Expanding Underrepresented Minority Participation: America's Science and Technology Talent at the Crossroads,'' The National Acadamies Press (2011);\\[0pt] [2] Joan Ferrini-Mundy, ``Driven by Diversity,'' Science \textbf{340}, 278 (2013).\\[0pt] [3] Stassun, K.G., ``Building Bridges to Diversity'', Mercury, \textbf{34}, 3 (2005).\\[0pt] [4] http://www.aps.org/programs/education/statistics/minoritydegrees.cfm\\[0pt] [5] Casey W. Miller, ``Admissions Criteria and Diversity in Graduate School,''APS News, The Back Page, February 2013. http://www.aps.org/publications/apsnews/201302/backpage.cfm\\[0pt] [6] Stassun, K.G., Sturm, S., Holley-Bockelmann, K., Burger, A., Ernst, D., {\&} Webb, D., Am. J. Phys. \textbf{79}, 374 (2011).\\[0pt] [7] http://www.hispanicphysicists.org/news/GREandDiversity.html [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:24AM |
A38.00004: Supporting the Physics Identity and Practices of Both University and K-12 Students through Outreach Invited Speaker: Katie Hinko Many physics departments seek to bring newcomers of all ages into the field through the facilitation of outreach programs. Through careful design, such informal learning environments act as hybrid spaces wherein public audiences and physicists build and redefine physics community together. Participation in outreach has the potential to reinforce students' identification with physics, reshape their view of the field, and hone their scientific and communication practices. Through interactions, participants negotiate roles, engage in authentic physics practices and provide feedback to each other. In this talk, I will discuss ways that outreach can affect the perceptions and practices of physics novices (K-12 students) and apprentices (undergraduate and graduate students) with regard to the physics community. I will also present findings from studies of undergraduates, graduate students and children who participate in an afterschool physics program sponsored by the University of Colorado Boulder and JILA. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 11:00AM |
A38.00005: Panel Discussion: Common Themes Across ``Bringing Newcomers Into The Physics Community'' Invited Speaker: Angela Little I will be facilitating a discussion between the audience and the four speakers in this session: Dimitri Dounas-Frazer, Catherine Good, Casey Miller, and Katie Hinko. They will all be speaking on the same general topic of supporting newcomers to the physics community at critical transition points but come from a set of diverse contexts and perspectives. Their work spans a wide age range of STEM students and they approach their work through many different lenses: as physics faculty, program directors, education and psychology researchers, and combinations thereof. Broad themes across these contexts and perspectives will be explored such as the role of growth mindset, community, and professional development. [Preview Abstract] |
Session A39: Invited Session: Order from Disorder in Superfluid 3He in Aerogel
Sponsoring Units: DCMPChair: William Halperin, Northwestern University
Room: Mile High Ballroom 2A-3A
Monday, March 3, 2014 8:00AM - 8:36AM |
A39.00001: Superfluid $^3$He in ``nematically ordered'' aerogel Invited Speaker: Vladimir Dmitriev Liquid $^3$He immersed in aerogel allows investigation of the influence of impurities on unconventional superfluidity. In most of such experiments silica aerogels are used. These aerogels consist of thin strands which form a ``wisp.'' Although it is established that superfluid phases of $^3$He in silica aerogels (A-like and B-like) have the same order parameters as A and B phases of bulk $^3$He, many new phenomena were observed. In particular, it was found that global anisotropy of aerogel (e.g. caused by squeezing or stretching) can orient the order parameter. Depending on prehistory and on the type of the anisotropy the A-like phase may be homogeneous or in a state with random orbital part of the order parameter. Theory predicts that a large stretching anisotropy may even influence the order parameter structure: polar phase (or A phase with polar distortion), which are not realized in bulk $^3$He, may become more favorable than pure A phase [1]. Large stretching anisotropy is hardly achievable in silica aerogel. Therefore in experiments described in the talk we used a new type of aerogel, consisting of Al$_2$O$_3\cdot$H$_2$O strands which are parallel to each other [2], i.e. this aerogel may be considered as infinitely stretched. We found that the superfluid phase diagram of $^3$He in such ``nematically ordered'' aerogel is different from the case of $^3$He in silica aerogel and that both observed A and B phases have large polar distortion. This distortion is larger at low pressures and grows on warming. There are indications that a pure polar phase appears near the superfluid transition temperature [3]. Recent results will be also presented.\\[4pt] [1] K. Aoyama and R. Ikeda, Phys.Rev.{\bf B}, {\bf 73}, 060504 (2006).\\[0pt] [2] R.Sh. Askhadullin, P.N. Martynov, P.A. Yudintsev et al., J. Phys.: Conf. Ser. {\bf 98}, 072012 (2008).\\[0pt] [3] R.Sh. Askhadullin, V.V. Dmitriev, D.A. Krasnikhin et al., JETP Lett., {\bf 95}, 326 (2012). [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 9:12AM |
A39.00002: Half - Quantum Vortices in the polar phase of He-3 in nematic aerogel Invited Speaker: Vladimir Mineev Unlike to superfluid He-4 the superfluid He-3A support the existence of vortices with half quantum of circulation as well as single quantum vortices. The singular single quanta vortices as well as nonsingular vortices with 2 quanta of circulation have been revealed in rotating He-3A. However, the half quantum vortices in open geometry always possess an extra energy due to spin-orbit coupling leading to formation of domain wall at distances larger than dipole length from the vortex axis. Fortunately the same magnetic dipole-dipole interaction does not prevent the existence of half-quantum vortices in the polar phase of superfluid He-3 that can be realized in peculiar porous media ``nematically ordered'' aerogel. Here we discuss this exotic possibility. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:48AM |
A39.00003: Engineering the glass phase of superfluid $^3$He-A with disorder Invited Speaker: J.I.A. Li It is established theoretically that an ordered state with continuous symmetry is inherently unstable to arbitrarily small amounts of disorder [1, 2]. Based on this idea it was predicted [3] that $^3$He-A in high porosity aerogel would become a superfluid glass, provided the aerogel has no global anisotropy that breaks the continuous symmetry of the system. We report here our nuclear magnetic resonance (NMR) measurements on $^3$He in an aerogel sample, characterized to be extremely homogeneous and isotropic. When the superfluid state is generated by cooling from the normal state, the long range orientational order of the intrinsic superfluid orbital angular momentum is destroyed, confirming the existence of a superfluid glass of $^3$He-A[3]. In this disordered glass state, the NMR response of the superfluid state vanishes and the order parameter structure of the superfluid is completely hidden, a behavior of potential significance for understanding exotic superconductors such as URu$_2$Si$_2$. In contrast, $^3$He-A generated by warming from superfluid $^3$He-B has perfect long-range orientational order, providing a mechanism for switching off this effect. Furthermore, by uniaxial compression of $\approx 20\%$ on the same sample we introduce uniform global anisotropy into this aerogel which breaks the 3-D continuous symmetry and restores perfect orientational order along the compression axis. However, in the plane perpendicular to the compression axis, the remaining 2-dimensional continuous symmetry gives rise to an in-plane glass state. \\[4pt] [1] Larkin, A. I. JETP Lett. 31, 784-786 (1970).\\[0pt] [2] Imry, Y. and Ma, S. Phys. Rev. Lett. 35, 1399-1401 (1975).\\[0pt] [3] Volovik, G. E. JETP Lett. 63, 301-304 (1996). [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:24AM |
A39.00004: Signatures of Superfluid Phases in Torsion Pendulum Experiments on He-3 Confined in Uniaxially Compressed Silica Aerogel Invited Speaker: Nikolay Zhelev Embedding superfluid He-3 into the random matrix of porous aerogel has proven to be a practical way to introduce disorder in the otherwise absolutely pure system. ``Dirty'' He-3 confined in aerogel exhibits markedly different properties compared to the bulk fluid. I present data from an experiment in which a deliberate anisotropy has been induced in the aerogel sample. A 98\% open silica aerogel, grown in a cell within the head of a torsion pendulum, is compressed by 10\% along the pendulum's axis. Through observing the pendulum's period shift and dissipation ($Q^{-1}$), we map out the modification of the superfluid phase diagram by anisotropic disorder [1]. Data for $Q^{-1}$ of the pendulum in both the superfluid phases cannot be fully explained by the existing theoretical framework, and as such should motivate new models for the interaction of the superfluid and the aerogel network [2]. \\[4pt]This experiment was done in collaboration with R.G. Bennett, E.N. Smith, J. Pollanen, W.P. Halperin and J.M. Parpia. \\[4pt][1] R. G. Bennett et al., Phys. Rev. Lett. \textbf{107}, 235504 (2011). \\[0pt][2] N. Zhelev et al., arXiv:1308.4724 [cond-mat.supr-con]. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 11:00AM |
A39.00005: Ultrasonic measurements of normal and superfluid He-3 in high porosity aerogel Invited Speaker: Yoonseok Lee Ultrasound spectroscopy and nuclear magnetic resonance have been proven to be the most valuable spectroscopic tools in the study of superfluid $^{3}$He. These experimental methods provide complementary information on the orbital and spin structure of the Cooper pairs. In particular, the rich spectrum of the order parameter collective modes, a direct consequence of the exotic broken symmetry in the superfluid phases, have been mapped out by ultrasonic spectroscopic techniques. Aerogel possesses a unique structure, whose topology is at the antipode of conventional porous media such as Vycor glass and metallic sinters. High porosity aerogel presents additional scattering channel that substantially changes the ultrasonic behavior in both normal and superfluid phase of $^{3}$He. For example, in the normal fluid the classic first to zero sound crossover is effectively prohibited due to the residual elastic scattering from aerogel. However, the hydrodynamic-Knudsen crossover arises owing to the unique structure and the widely varying inelastic mean free path in $^{3}$He. In superfluid, no signatures of the order parameter collective modes were observed but the gapless superfluidity has been clearly verified through ultrasound measurements. In this paper, we will present the experimental results obtained in the past decade using ultrasonic techniques. [Preview Abstract] |
Session A40: Invited Session: Interplay Between Geometry, Organization and Function of Fluid Membranes
Sponsoring Units: DCMP GMAGChair: Martin Forstner, Syracuse University
Room: Mile High Ballroom 2B-3B
Monday, March 3, 2014 8:00AM - 8:36AM |
A40.00001: Faceted structures in liquid crystalline vesicles Invited Speaker: Mark Bowick The shape of liquid-crystalline vesicles, molecularly thin membrane sacs enclosing a finite volume, is determined by the competition between liquid-crystalline deformations on a surface to be determined and the bending energy of the surface in the ambient bulk. We discuss this problem in two limits: stiff (high bending rigidity compared to Frank modulus) and floppy (low bending energy compared to Frank modulus). The solution in the floppy limit is quite remarkable: it is the surface of a regular tetrahedron with topological defects at the vertices. Thus floppy liquid crystalline vesicles, which have no translational order, are sharp faceted structures more commonly found in hard crystalline materials. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 9:12AM |
A40.00002: Membrane Bending by Protein Crowding Invited Speaker: Jeanne Stachowiak From endosomes and synaptic vesicles to the cristae of the mitochondria and the annulus of the nuclear pore, highly curved membranes are fundamental to the structure and physiology of living cells. The established view is that specific families of proteins are able to bend membranes by binding to them. For example, inherently curved proteins are thought to impose their structure on the membrane surface, while membrane-binding proteins with hydrophobic motifs are thought to insert into the membrane like wedges, driving curvature. However, computational models have recently revealed that these mechanisms would require specialized membrane-bending proteins to occupy nearly 100{\%} of a curved membrane surface, an improbable physiological situation given the immense density and diversity of membrane-bound proteins, and the low expression levels of these specialized proteins within curved regions of the membrane. \textit{How then does curvature arise within the complex and crowded environment of cellular membranes? } Our recent work using proteins involved in clathrin-mediated endocytosis, as well as engineered protein-lipid interactions, has suggested a \underline {\textit{new hypothesis}} - that \textit{lateral pressure generated by collisions between membrane-bound proteins can drive membrane bending}. Specifically, by correlating membrane bending with quantitative optical measurements of protein density on synthetic membrane surfaces and simple physical models of collisions among membrane-bound proteins, we have demonstrated that protein-protein steric interactions can drive membrane curvature. These findings suggest that a simple imbalance in the concentration of membrane-bound proteins across a membrane surface can drive a membrane to bend, providing an efficient mechanism by which essentially any protein can contribute to shaping membranes. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:48AM |
A40.00003: Membrane shape instabilities induced by BAR domain proteins Invited Speaker: Tobias Baumgart Membrane curvature has developed into a forefront of membrane biophysics. Numerous proteins involved in membrane curvature sensing and membrane curvature generation have recently been discovered, including proteins containing the crescent-shaped BAR domain as membrane binding and shaping module. Accordingly, the structure determination of these proteins and their multimeric complexes is increasingly well-understood. Substantially less understood, however, are thermodynamic and kinetic aspects and the detailed mechanisms of how these proteins interact with membranes in a curvature-dependent manner. New experimental approaches need to be combined with established techniques to be able to fill in these missing details. Here we use model membrane systems in combination with a variety of biophysical techniques to characterize mechanistic aspects of BAR domain protein function. This includes a characterization of membrane curvature sensing and membrane generation. We also establish kinetic and thermodynamic aspects of BAR protein dimerization in solution, and investigate kinetic aspects of membrane binding. We present two new approaches to investigate membrane shape instabilities and demonstrate that membrane shape instabilities can be controlled by protein binding and lateral membrane tension. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:24AM |
A40.00004: Peptides that influence membrane topology Invited Speaker: Gerard C.L. Wong We examine the mechanism of a range of polypeptides that influence membrane topology, including antimicrobial peptides, cell penetrating peptides, viral fusion peptides, and apoptosis proteins, and show how a combination of geometry, coordination chemistry, and soft matter physics can be used to approach a unified understanding. We will also show how such peptides can impact biomedical problems such as auto-immune diseases (psoriasis, lupus), infectious diseases (viral and bacterial infections), and mitochondrial pathologies (under-regulated apoptosis leads to neurodegenerative diseases whereas over-regulated apoptosis leads to cancer.) [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 11:00AM |
A40.00005: Measuring membrane rigidity and viscosity: New methods, and new insights Invited Speaker: Raghuveer Parthasarathy Lipid membranes are remarkable materials: flexible, two-dimensional fluids whose physical properties guide cellular function. Bending rigidity and viscosity are two of the key mechanical parameters that characterize membranes. Both, however, are challenging to measure. I describe improvements in experimental techniques to quantify the bending modulus and the two-dimensional viscosity of lipid membranes. First, I show that using selective plane illumination microscopy (SPIM, also known as light sheet fluorescence microscopy) to image the thermal fluctuations of freely suspended giant lipid vesicles enables straightforward measurements of membrane rigidity, and also provides insights into changes in rigidity induced by cargo trafficking proteins. Second, I show that tracking both the rotational and translational diffusion of membrane-anchored tracer particles allows quantification of membrane viscosity, measurement of the effective radii of the tracers, and assessment of theoretical models of membrane hydrodynamics. Surprisingly, we find a wide distribution of effective tracer sizes, due presumably to a wide variety of couplings to the membrane. I also provide an example of protein-mediated changes in lipid viscosity. [Preview Abstract] |
Session A41: Focus Session: Multiferroics and Magnetoelectrics
Sponsoring Units: DMP DCOMPChair: Manuel Bibes, Unite Mixte de Physique CNRS
Room: Mile High Ballroom 3C
Monday, March 3, 2014 8:00AM - 8:12AM |
A41.00001: Systematic investigation of the growth, structure, and ferroic properties of strained epitaxial Ni$_{\mathrm{1-x}}$Ti$_{\mathrm{1-y}}$O$_{3}$ thin films with multiferroic potential T. Varga, T.C. Droubay, M.E. Bowden, S.A. Stephens, S. Manandhar, V. Shutthanandan, R.J. Colby, B.C. Kabius, E. Apra, S.A. Chambers Ferroelectrically induced weak ferromagnetism has been predicted a few years back in perovskite MTiO$_{3}$ (M$=$Fe,Mn,Ni). We set out to stabilize this metastable perovskite structure by growing NiTiO$_{3}$ epitaxially on different substrates in an attempt to achieve the multiferroic properties in these compounds. Epitaxial Ni$_{\mathrm{1-x}}$Ti$_{\mathrm{1-y}}$O$_{3}$ films of different thicknesses were deposited on Al$_{2}$O$_{3}$, Fe$_{2}$O$_{3}$/Al$_{2}$O$_{3}$, and LiNbO$_{3}$ substrates by pulsed laser deposition at different temperatures, and characterized using several techniques. The effect of substrate choice, film thickness, deposition temperature, and film stoichiometry on lattice strain, film structure, and physical properties was investigated. Our structural data from x-ray diffraction, electron microscopy, and x-ray absorption spectroscopy, suggest that the predicted perovskite structure was made. Our physical property characterization showing lattice polarization, ferromagnetism, and a likely coupling between the ferroic order parameters indicate that \textit{R3c} NiTiO$_{3}$ with potential multiferroic properties has been synthesized. Lattice strain from mismatch has a marked effect on the structure of the films. Film stoichiometry and the choice of substrate were found to affect the observed ferroic properties. These results suggest that the properties of the films can be controlled by the choice of substrate and film stoichiometry. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A41.00002: Engineered phase competition in $A$-site-ordered manganites $R$BaMn$_2$O$_6$ ($R$=Y and rare earth elements) from first principles Jiangang He, Craig J. Fennie ($A/A^\prime$)MnO$_3$ manganites for which the $A$-site cations order in layers, e.g., $R$BaMn$_2$O$_6$ ($R$=Y and rare earth elements) show higher charge and orbital ordering temperatures as compared with $A$-site disordered manganites. The degree of MnO$_6$ octahedra rotation, and therefore the Mn-O-Mn angle and Mn-O bond length, in $R$BaMn$_2$O$_6$ varies strongly with the ionic size of the rare earth ion. In fact $R$BaMn$_2$O$_6$ spans from a ferromagnetic metal ($R$=La) to an $A$-type antiferromagnetic metal ($R$=Pr and Nd), to a CE-type charge/orbital-ordered insulator ($R$=Sm-Y). The tuning of the electronic and magnetic ground states coincides with changes in the rotation patters and structural transitions from tetragonal ($R$=La-Nd), to orthorhombic ($R$=Sm-Gd), to monoclinic ($R$=Tb-Y) as the radius of $R$ decreasing, reflecting the competition among charge, orbital, magnetic, Jahn-Teller, and lattice degrees of freedom. In this talk we present the epitaxial phase diagram calculated from first-principles for these $A/A^\prime$ layered manganites and discuss the possibility of using an electric-field to control the competition among these phases via octahedral rotation induced ferroelectricity. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 9:00AM |
A41.00003: Spectroscopic signatures of domain walls in multiferroic ErMnO$_3$ Invited Speaker: Janice Musfeldt We investigated the spectroscopic response of stripe- and vortex-containing ErMnO$_3$ in order to uncover the dynamic signatures of the domain walls. We quantify Born effective charge and polarization differences using the lattice behavior, analyze the local rare earth environment from the f-manifold excitations, and reveal how shifts in the charge transfer excitations impact the band gap. These findings are unified with a discussion of hybridization and domain wall density effects. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A41.00004: Magnetic-field-induced spin flop transition and magnetoelectric effect in Ca$_{2}$Fe$_{\mathrm{2-x}}$Al$_{\mathrm{x}}$O$_{5}$ Nobuyuki Abe, Taka-hisa Arima, Nguyen Khanh, Takahiko Sasaki Ca$_{2}$Fe$_{\mathrm{2-x}}$Al$_{\mathrm{x}}$O$_{5}$ compounds with x \textgreater\ 0.5 have the same crystal structure as brownmillerite, where (Fe,Al)O$_{6}$ octahedron layers and (Fe,Al)O$_{4}$ tetrahedron layers alternately stacks. The space group is orthorhombic Ibm2, which allows the presence of spontaneous polarization along the c-axis. These materials also exhibit the antiferromagnetic transition at the 350K $\sim$ 570K. We have investigated the magnetoelectric effect of single crystals. In a magnetic field applied along the spin easy axis, a metamagnetic transition is observed to accompany an anomaly of the electric polarization and the dielectric constant. The anomalies can be ascribed to a noncollinear spin arrangement in the domain walls between two magnetic phases and/or the spin direction dependent modulation of the metal-ligand hybridization. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A41.00005: Multiferroic Pr$_2$Ti$_2$O$_7$: A candidate material to search for the electric dipole moment of the electron Maribel Nunez Valdez, Nicola Spaldin We use density functional theory (DFT) to explore the suitability of the A$_n$B$_n$O$_{3n+2}$ perovskite oxides [1] as materials for searching for the electric dipole moment (eEDM). The experimental search for the eEDM is of interest as its observation would confirm the violation of charge-parity (CP) symmetry in the Universe. Experiments involving electric-field-correlated measurements in solids are promising. In particular, multiferroic Eu$_{0.5}$Ba$_{0.5}$TiO$_3$, which was designed specifically to search for the eEDM, set an improved limit compared with previous solid-state searches [2], but suffered from hysteretic heating [3]. Here we show that the A$_n$B$_n$O$_{3n+2}$ layered perovskites ($n=4$, A=Pr,Gd and B=Ti) have an alternative mechanism for ferroelectricity plus appropriate magnetic interactions, suggesting that they are suitable candidates for an eEDM search. [1] F. Lichtenberg, A. Herrnberger, K. Wiedenmann, Prog. Solid State Chem. \textbf{36} (2008). [2] K.Z. Rushchanskii, S. Kamba, V. Goian, \textit{et al.}, Nature Mater. \textbf{9}, 649 (2010). [3] S. Eckel, A.O. Sushkov, and S.K. Lamoreaux, Phys. Rev. Lett. \textbf{109}, 193003, (2012). [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A41.00006: High temperature ferrielectricity and ferrimagnetism in LnACrOsO$_{6}$ by design Hena Das, Saurabh Ghosh, Martha Greenblatt, Tanusri Saha-Dasgupta, Craig Fennie Despite intense efforts over the last decade, there are surprisingly few multiferroics in which a net magnetization coexists with a switchable polarization at room temperature. Since magnetism tends to be the harder problem, one approach to solve this challenge is to start with a material that is magnetically ordered at room temperature and drive it ferroelectric. In this regard, the double perovskite Sr$_{2}$CrOsO$_{6}$ is a promising candidate; it is ferromagnetic and insulating with a $T_{\mathrm{c}} =$ 725 K, the highest known $T_{\mathrm{c}}$ of any magnetic insulating oxide with appreciable uncompensated magnetic moment. Here we discuss our first-principles study of the ferroic properties of as not yet synthesized 3$d$-5$d$ double perovskites, LnACrOsO$_{6}$ (Ln $=$ La, Y, Ce-Lu; A $=$ Na, K). We identify polar compounds that have moderate polarization switching barriers and display ferrimagnetism that is expected to persist above room temperature. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A41.00007: Probing spin fluctuations in the paramagnetic phase of EuTi$O_{3}$ by muon spin rotation techniques Zurab Guguchia, Hugo Keller, Alexander Shengelaya, Jurgen Kohler, Annette Bussmann-Holder The muon spin rotation (${\mu}$SR) technique was used to search for theoretically predicted spin fluctuations in EuTi$O_{3}$ (ETO) deep in the paramagnetic phase. ETO is a perovskite with cubic structure above $T_{S}$=282 K, followed by a tetragonal phase below $T_{S}$ and shows antiferromagnetic (AFM) ordering at $T_{N}$=5.7 K. A strong spin-lattice coupling exists at low temperatures. Even though it is not apparent that this spin-lattice coupling continues to high temperatures, model calculations predict a strong paramagnon-phonon coupling at elevated temperatures. In order to test these predictions, ${\mu}$SR studies on ETO have been performed at temperatures above and below $T_{S}$. While the AFM phase is clearly observed in the ${\mu}$SR signal, a finite signal remains also in the paramagnetic phase, following closely the temperature dependence of the zone boundary soft mode. This unusual finding demonstrates that spin fluctuations are present deep in the paramagnetic phase and are tied to the soft zone boundary mode. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A41.00008: Large dynamical magnetic charges driven by exchange striction Meng Ye, David Vanderbilt Magnetoelectric (ME) materials are of fundamental interest and are investigated for their broad potential applications. First-principles methods have only recently been developed to calculate the full ME response tensor $\alpha$ including both electronic and ionic contributions.\footnote{A. Malashevich et al., Phys. Rev. B, {\bf 86}, 094430 (2012).} In several materials, the dominant contribution to the ME response has been shown to be the ionic term $\alpha_{\rm ion}$, which is proportional to both the Born charge $Z^{\rm e}$ and its analogue, the dynamical magnetic charge $Z^{\rm m}$.\footnote{J. \'{I}\~{n}iguez, Phys. Rev. Lett. {\bf 101}, 117201 (2008).} Here we present a theoretical study of mechanisms that could enhance the magnetic charge $Z^{\rm m}$. The KITP\-ite structure is reported with large ME response arising from exchange striction and spin frustration.\footnote{K. Delaney et al., Phys. Rev. Lett., {\bf 102}, 157203 (2009).} Using first-principles density-functional methods, we calculate the atomic $Z^{\rm m}$ tensors in KITP\-ite and conclude that even when SOC is completely absent, the exchange striction acting on the non-collinear spin structure induces much larger magnetic charges than in the case when $Z^{\rm m}$ is driven by SOC as in $\rm{Cr_2O_3}$. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A41.00009: Correlation between bulk magnetoelectricity and boundary magnetization in Cr$_{2}$O$_{3}$ Junlei Wang, Christian Binek Boundary magnetization is a roughness insensitive net magnetization. It emerges at the surface or interface of a magnetoelectric antiferromagnet in a single-domain state and has been utilized in voltage controlled spintronic system for potential ultra-low power application based on exchange bias system with Cr$_{2}$O$_{3}$. Previous work has lacked to demonstrate the direct relation between the bulk spin structure and the boundary magnetization. In this work, we use magneto-optical Faraday effect to observe boundary magnetization and correlate it with the bulk magnetoelectric response of a Cr$_{2}$O$_{3}$ single crystal on an applied electric field, $E$. Our method discriminates the $E$- dependent bulk Faraday rotation, $\theta $, from the stationary boundary magnetization. To this end we investigate $\theta $ vs. $E$ in two distinct antiferromagnetic single-domain states which are prepared via magnetoelectric annealing. Temperature dependence of the boundary magnetization, $m_{BM} \propto \Theta (E =$ 0), as well as the corresponding bulk magnetoelectric susceptibility, $\alpha \propto $ d$\Theta $/d$E$, is obtained from separate investigations of $\theta $ vs. $E $for the two single domain states. Our magneto-optical setup uses a near-infrared laser so that transmission loss is admissible for our sample of 500 $\mu $m thickness. We utilize lock-in and compensation techniques to maximize measurement precision and to enable absolute Faraday rotation measurement which is gauged with respect to magnetization. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A41.00010: Electromagnons: Electrically active spin excitations in multiferroics Stanislav Kamba, Veronica Goian, Filip Kadlec, Christelle Kadlec, Premysl Vanek, Martin Kempa, Marti Gich In some multiferroics spin wave can be excited by electric component of elmg. radiation and such excitations activated by dynamic magnetoelectric coupling are called electromagnons. We will discuss mechanism of electromagnon activation in the THz spectra of three different compounds: In the multiferroic TbMnO$_{3}$, the ferroelectricity is induced by inverse Dzyaloshinskii-Moriya interaction, but two electromagnons are activated by the magnetostriction. Second example is CaMn$_{7}$O$_{12}$, whose polarization is the highest among all spin-induced ferroelectrics. In this material we observed three electromagnons, whose frequencies correspond to maxima of magnon density of states, so they should correspond to magnons from Brillouin zone boundary. Finally we will demonstrate that electromagnons are not limited to spin-induced ferroelectrics. We have observed an electromagnon in nanograin ceramics of epsilon-Fe$_{2}$O$_{3}$. This material is below 490 K a pyroelectric ferrimagnet and the electromagnon activates in the THz spectra only below 110 K, when the magnetic structure becomes incommensurately modulated. We will show how by combining infrared, THz and inelastic neutron scattering experiments, the electromagnons can be discerned from magnons or phonons. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A41.00011: Multiferroicity in Cu$_{2}$OSeO$_{3}$? Eugen Ruff, Stephan Krohns, Helmuth Berger, Peter Lunkenheimer, Alois Loidl Topological spin textures in solids are in the focus for applications in future spin-electronic technology, like high-density magnetic storage devices. Prominent materials are metallic alloys with B20 structure, such as MnSi [1], where skyrmions, vortex-like objects of nanometer scale, have been experimentally detected. In these materials, it is well known that low currents can drive skyrmion switching. In contrast, the discovery of magnetoelectric skyrmions in an insulating chiral-lattice magnet Cu$_{2}$OSeO$_{3}$ leads to another promising route to electric control [2]. This system is suggested to carry a local electric dipole, which implies that the skyrmions should be controllable by the external electric field without losses due to joule heating. Here we provide a thorough analysis of the magnetic and polar phases, using SQUID and pyrocurrent measurements. In order to investigate the possible ferroelectric properties of Cu$_{2}$OSeO$_{3}$, we have performed dielectric spectroscopy in various magnetic fields in a broad frequency range below 70 K. Combining all these different techniques, we address the question whether Cu$_{2}$OSeO$_{3}$ is magnetoelectric or multiferroic.\\[4pt] [1] S.M\"{u}hlbauer \textit{et al}., Science \textbf{323}, 915 (2009).\\[0pt] [2] S.Seki \textit{et al}., Science \textbf{336}, 198 (2012). [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A41.00012: Probing the Origin of Large Magnetic Field coupled Electric Polarization in the RAl$_{3}$(BO$_{3}$)$_{4}$ system Han Zhang, Tian Yu, Trevor Tyson, Christine Nelson, Leonard Bezmaternykh The multiferroic system RAl$_{3}$(BO$_{3})_{4}$ (R$=$rare earth) is known to exhibit a strong coupling of the magnetic field to the electrical polarization. To understand the origin of this behavior, detailed structural studies on single crystals and powders derived from crystals were conducted. The structure as a function of temperature, magnetic field and pressure was explored. The results are compared with magnetic field dependent electric polarization and heat capacity measurements. This work is supported by DOE Grants DE-FG02-07ER46402 (NJIT). [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A41.00013: Growth and Characterization of the Multiferroic Barium Transition Metal Fluorides Ba$M$F$_{4}$ Trent Johnson, Pavel Borisov, David Lederman We have investigated the temperature dependent growth, as well as the magnetic and ferroelectric properties of thin films of the isostructural compounds Ba$M$F$_{4}$, where $M=$Fe, Co, Ni. The films were grown by molecular beam epitaxy to thicknesses of 50 or 100 nm on single crystal Al$_{2}$O$_{3}$ (0001) substrates. X-ray diffraction shows that this family of films grow epitaxially in the (010) orientation, but are twinned in the plane, with three domain orientations rotated by 120$^{\circ}$ relative to one another. Measurements of the remanent hysteresis via interdigitated electrodes show that the compounds $M=$Co, and Ni are ferroelectric, but no switching behavior was observed in the Fe system at electric fields up to 400 kV/cm. Measurements of the field-cooled and zero-field-cooled magnetic moment confirm the existence of low temperature magnetic behavior. [Preview Abstract] |
Session A42: Focus Session: Non-equilibrium Effects in Topological Insulators
Sponsoring Units: DMPChair: Tom Devereaux, SLAC National Accelerator Laboratory
Room: Mile High Ballroom 4A
Monday, March 3, 2014 8:00AM - 8:12AM |
A42.00001: Time-resolved terahertz dynamics in thin films of the topological insulator Bi$_{2}$Se$_3$ Rolando Vald\'es Aguilar, J. Qi, A.J. Taylor, D.A. Yarotski, R.P. Prasankumar, M. Brahlek, N. Bansal, S. Oh Experiments at terahertz frequencies (1 THz $\sim$ 4 meV) in thin films of Bi$_2$Se$_3$ have provided evidence of the surface response, and give a picture of relatively mobile surface carriers with a bulk response that makes a small contribution to the THz conductivity. In this report we use optically pumped time-resolved THz spectroscopy at low temperature to distinguish the bulk and surface contribution on thin films of Bi$_2$Se$_3$ of several thicknesses. We find that for very thin films, where pure 2D behavior is expected, the optical pump induces a change in the 2D transport scattering rate which decays in a time-scale of 20 picoseconds. For thicker films, we see an additional contribution that increases the conductivity and scales with the increase in both the film thickness and the fluence of the pump beam. This contribution has much faster rise and decay times of approximately 5 ps, as well as a much larger scattering rate than the previously identified surface term. The different dynamics between surface and bulk electrons close to the Fermi energy evidenced in this study indicate a decoupling of surface and bulk carriers at low temperature, and present the possibility of accessing long-lived surface photo-carriers for optoelectronic applications. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A42.00002: Observation of Floquet-Bloch states on the surface of a topological insulator Yihua Wang, Hadar Steinberg, Pablo Jarillo-Herrero, Nuh Gedik The unique electronic properties of the surface electrons in a topological insulator are protected by time-reversal symmetry. Circularly polarized light naturally breaks time-reversal symmetry, which may lead to an exotic surface quantum Hall state. Using time- and angle-resolved photoemission spectroscopy, we show that an intense ultrashort mid-infrared pulse with energy below the bulk band gap hybridizes with the surface Dirac fermions of a topological insulator to form Floquet-Bloch bands. These photon dressed surface bands exhibit polarization-dependent band gaps at avoided crossings. Circularly polarized photons induce an additional gap at the Dirac point, which is a signature of broken time-reversal symmetry on the surface. These observations establish the Floquet-Bloch bands in solids and pave the way for optical manipulation of topological quantum states of matter. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A42.00003: Tunable Floquet Majorana Modes in Coupled Quantum Dots Yantao Li, Arijit Kundu, Fan Zhong, Babak Seradjeh We study theoretically the appearance of Floquet Majorana fermions in a double quantum dot system coupled by a superconducting lead and driven by separate AC potentials. We argue that the system could be fine tuned controllably in the expanded parameter space of the drive frequency, amplitude, and phase difference across the two dots. While these Majorana fermions are not topologically protected, the all-electric, highly tunable setup could provide a realistic system for observing the exotic physics associated with Majorana fermions. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A42.00004: Ultrafast transient decoupling and multi-phonon effects in driven electron-phonon systems Alexander Kemper, Michael Sentef, Brian Moritz, James Freericks, Thomas Devereaux Pump-probe experiments have become an increasingly important tool in studying the interaction between electrons and bosons in condensed matter. Here we discuss some of the transient effects that occur during the pumping process using the non-equilibrium Keldysh technique to numerically solve the equations of motion for a strongly coupled electron-phonon system. The scattering of spectral weight while the pump is on decouples the electrons from the phonons, leading to a transient weakening of the electron-phonon spectral features known as kinks. We further note that higher order kinks in the spectra are more readily visible in the time domain by observing changes in the time-resolved ARPES spectra. Finally, we revisit the question of time-resolved relaxation dynamics and highlight the effects of the transients spectral weight re-arrangement there. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A42.00005: Photo-induced topological phase transition in graphene studied by exact simulation of pump-probe photoemission spectroscopy Michael Sentef, Alexander Kemper, Brian Moritz, James Freericks, Thomas Devereaux The idea of inducing a nontrivial topological band structure using circularly polarized light was triggered by the observation that in a steady ``Floquet'' state, periodically driven Dirac fermions can be mapped [1] to the Haldane model for a quantum Hall state without Landau levels [2]. A recent observation of Floquet-Bloch states on the surface of a spin-orbit driven topological insulator and a surface state energy gap opened by time-reversal symmetry breaking [3] poses the question how a topological phase transition occurs in real time on ultrashort time scales. We use a well developed Keldysh Green function technique [4] to compute the exact time evolution of tight-binding electrons on the honeycomb lattice coupled to realistic short laser pulses. The time- and angle-resolved photoemission response reveals a photo-induced topological phase transition with energy gaps $>$ 100 meV at the Dirac point that should be observable experimentally. [1] T.~Oka and H.~Aoki, Phys. Rev. B 79, 081406 (2009); T.~Kitagawa et.~al., Phys. Rev. B 82, 235114 (2010); N.~H.~Lindner et.~al., Nature Physics 7, 490-495 (2011). [2] F.~D.~M.~Haldane, Phys. Rev. Lett. 61, 2015-2018 (1988). [3] Y.~H.~Wang et.~al., Science 342, 453 (2013). [4] M.~Sentef et.~al., arXiv:1212.4841 (Phys. Rev. X 2013). [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A42.00006: Polarization-controlled picosecond currents in topological insulators Alexander Holleitner, Christoph Karnetzky, Helmut Karl, Christoph Kastl Controlling spin currents in topological insulators may lead to applications in future spintronic devices [1]. Here, we show that surface currents in Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$ can be controlled by circularly polarized light on a time-scale of a picosecond with a fidelity near unity even at room temperature. We reveal the temporal interplay of such ultrafast spin currents with photo-induced thermoelectric and drift currents in optoelectronic circuits [2]. [1] C. Kastl, et al, APL 101, 251110 (2012). [2] C. Kastl et al. under review (2013). [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A42.00007: Nonequilibrium Spectroscopy of Topological Edge Liquids Alex Levchenko, Stanislav Apostolov We develop theory for the energy and spatially resolved tunneling spectroscopy of the topological quantum spin Hall helical states driven out of equilibrium. When helical liquid is constrained between two superconducting reservoirs transport at the edge is governed by the multiple Andreev reflections. The resulting distribution functions of the edge channels exhibit multiple discontinuities at the subgap energies with the periodicity of an applied voltage. The combined effect of interactions and disorder leads to the inelastic backscattering processes mixing different helicity modes thus causing smearing of these singularities. If equilibration is strong then distribution functions of the edge channels collapse into a single Fermi-like function with an effective temperature determined by the superconducting gap, applied voltage and interaction parameter. We conclude that mapping out nonequilibrium distribution function in the experiments may provide valuable information about the relevant perturbations that spoil ballistic edge transport. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A42.00008: Time evolution of wave-packets in topological insulators Poliana H. Penteado, Sebastian Duque Mesa, Gerson J. Ferreira, J. Carlos Egues Topological insulators (TIs) are a fantastic new class of materials that have gapless helical surface (3D TIs) or edge (2D TIs) states embedded within the bulk gap of its host material. This unique property rises from an interface between materials with topologically inequivalent sets of bands structures, i.e. gaps with different signs. Here we investigate the time-evolution of wave-packets in TIs. Within the Dirac equation, the interference between eigenstates from positive and negative energy bands leads to the relativistic oscillatory behavior well known as Zitterbewegung. It was recently discussed the time evolution of the guiding center of a wave-packet in TIs converging towards the edge states. Here we show a more detailed discussion of the evolution of the full wave-packet and its behavior regarding the collision with the edges of the system. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A42.00009: Steady States of Floquet Topological Insulators in the presence of Electron-Phonon Interaction Karthik Seetharam, Netanel Lindner, Gil Refael Floquet topological insulators (FTI) employ a well chosen periodic drive to induce a non-equilibrium topological state in an otherwise trivial semiconductor system. By using a periodic drive, time translation symmetry is partially broken and the topological features are captured by the resulting quasienergy bands defined modulo the drive frequency. When considering a solid state system with electron-phonon interaction, the inevitable contribution of relaxation processes involving multi-photon transitions (``Umklapp''-type processes in quasienergy) leads to unique steady states, which differ from a Fermi-Dirac distribution expected in the absence of such processes. Understanding these steady states is crucial for calculating transport properties of the driven system. Using kinetic equations we study the evolution of the quantum state of the FTI in the presence of electron-phonon interaction, obtain a description of the steady state of the driven system, and study the transport properties of the FTI. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A42.00010: Ultrafast Dynamics of Bi1.5Sb0.5Te1.8Se1.2 Topological Insulator Liang Cheng, Chi-Sin Tang, Saritha Krishnankutty Nair, Bin Xia, Lan Wang, Jian-Xin Zhu, Ee Min Elbert Chia Bi1.5Sb0.5Te1.8Se1.2~(BSTS) is a type of topological insulator, which is an insulator in bulk but surface states are gapless. In this work, we took optical pump-probe data on BSTS crystal to analyze the dynamics of phonons and charge carriers. The ultrafast dynamics were obtained as a function of temperature ranging from 10K to 300K, as well as fluence ranging from 1 $\mu $J/cm2 to 10 $\mu $J/cm2. In additional to the coherent optical phonon mode found in other topological insulators, acoustic phonon mode was observed in our experiment. We also observed phonon softening and the temperature dependence of carrier lifetime in BSTS. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A42.00011: Towards Phononic Topological Insulators Pai Wang, Katia Bertoldi Recent studies in optics have shown that the concept of topological insulators can be extended to band theories of classical waves and bosonic systems. Here, we present some design considerations in realization and observation of topological edge states for phonons. The goal is to achieve topologically protected one-way propagation of surface acoustic / elastic waves against back-scattering and localization due to defects and disorders by utilizing phononic crystals, which have micro-structures with periodicity comparable to the wavelength of the propagating elastic waves. Both theoretical and practical challenges in creating non-reciprocal elastic media will be discussed. Possible candidates include temporal modulation of phononic crystals, coupled wave guides, chiral local resonators, artificial magneto-acoustic effects and asymmetric body forces induced by external fields. These symmetry breaking mechanisms can potentially lead to the phononic analogue of electronic quantum hall effect. The robustness of reflection-immune unidirectional elastic wave has promising applications in surface acoustic wave (SAW) devices that are widely used in modern telecommunication, geophysics as well as micro-fluidics. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A42.00012: Dielectric Screening of Surface States in a Topological Insulator J.P.F. LeBlanc, J.P. Carbotte Hexagonal warping provides an anisotropy to the dispersion curves of the helical Dirac fermions that exist at the surface of a topological insulator. We show how modifications to the Dirac spectrum by inclusion of hexagonal warping, as well as a Schr\"odinger and gap term modify the polarization function of the surface states. We derive in the long wavelength limit the plasmon dispersion and show that it obtains a weak dependence on the direction of scattering momentum, q. Further, we show numerically the plasmon dispersions at large q and find considerable directional anisotropy of the plasmon bands in comparison to the pure Dirac plasmons. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A42.00013: Observation of resonance in second harmonic generation measurements of topologically insulating Bi2Se3 thin films Mikel Holcomb, Yuri Glinka, Sercan Babakiray, Trent Johnson, Alan Bristow, David Lederman Second harmonic generation is an ideal probe of topological insulator surface states due to its sensitivity to space inversion symmetry breaking, which naturally occurs at a material's surface. We measured the angular dependence of second harmonic intensity for s-s, p-p, s-p and p-s polarization configurations of the incoming and outgoing light in films ranging in thickness from 6 to 40 nm of the topological insulator Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$. We assign each of these angular arrangements to the symmetry of specific Se and Bi atomic layers near the surface of the material. Exploiting this information, we separate the bulk and surface crystal structure contributions. Modelling the response requires use of a conventional second-order nonlinear term and a third-order electric-field induced SHG term. The latter dominates the thickness dependence showing a strong peak at about 10 nm. We apply appropriate models to explain this behavior and will discuss the resonance-like feature observed within a small thickness range and its implications. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A42.00014: Topological polaritons from non-topological quantum-wells in an optical cavity Torsten Karzig, Netanel Lindner, Gil Refael We study the formation of topological polaritons from semiconductor excitons coupled to cavity photons. Oscillating classical electromagnetic fields can turn a trivial band structure into a Floquet topological insulator. In a similar spirit cavity photons can induce topology when coupling to otherwise trivial excitons. We discuss the necessary ingredients to lend the polaritons a non-trivial topology through a ``winding'' coupling of the excitons to the photons. One hallmark signature of the topological polaritons are chiral edge modes which allow for unidirectional photon propagation as a part of the exciton-photon edge mode. [Preview Abstract] |
Session A43: Defects in Topological Insulators
Sponsoring Units: DCMPChair: Ruihua He, Boston College
Room: Mile High Ballroom 4B
Monday, March 3, 2014 8:00AM - 8:12AM |
A43.00001: ``Holographic'' treatment of surface disorder in topological insulators Kun Woo Kim, Roger Mong, Marcel Franz, Gil Refael What is the effect of surface-only disorder on the electronic states of a 3d TI? The layers in the clean bulk parallel to surface probe the surface impurities as they hop in and out of the surface layer. A recursive treatment of the impurity effects is made possible through successive elimination of the lattice layer by layer. This leads to non-linear renormalization group flow of an effective surface impurity potential. We found an exact mapping between the recursion relation and Schrodinger equation along the layers, therefore the modified self energy due to surface impurity could be simply obtained from the transfer matrix method. As a concrete example of 2d topological insulator, we found the exact expression of on-layer self energy for a clean system and an asymptotic expression that captures a general behavior of layers deep in the bulk. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A43.00002: Absence of levitation and annihilation at the topological phase transition of a disordered one-dimensional model in class AIII Ian Mondragon-Shem, Juntao Song, Taylor L Hughes, Emil Prodan We study the disorder-induced topological phase transition of a one-dimensional model belonging to class AIII of the Altland-Zirnbauer classification of fermions. To characterize the topological state, we derive a covariant real-space representation of the integer invariant. Using this invariant, we show that the system remains topological even after all the single particle states of the system become localized and the energy spectrum becomes gapless. For a critical disorder strength which we compute analytically, there emerges a delocalized state at zero energy where the topological invariant changes value and the nontrivial ground state transforms into a trivial one. This type of topological phase transition is fundamentally different from the levitation and annihilation paradigm that is found in higher-dimensional systems e.g. the quantum Hall state. In order to understand this type of phase transition, we map the system to a spin-1/2 model which provides an insightful real-space picture of the underlying physics near the critical point. EP and JTS were supported by U.S. NSF grants DMS 1066045, DMR-1056168, NSFC grant 11204065 and RFDPHEC grant. A2013205168. TLH and IM-S were supported by ONR Grant No. N0014-12-1-0935. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A43.00003: Bulk-Defect Correspondence in Particle-Hole Symmetric Insulators and Semimetals Andreas Ruegg, Fernando de Juan, Dung-Hai Lee Lattices with a basis can host crystallographic defects which share the same topological charge (e.g. the Burgers vector $\vec b$ of a dislocation) but differ in their microscopic structure of the core. We demonstrate that in insulators with particle-hole symmetry and an odd number of orbitals per site, the microscopic details drastically affect the electronic structure: modifications can create or annihilate non-trivial bound states with an associated fractional charge. We show that this observation is related to the behavior of end modes of a dimerized chain and discuss how the end or defect states are predicted from topological invariants in these more complicated cases. Furthermore, using explicit examples on the honeycomb lattice, we explain how bound states in vacancies, dislocations and disclinations are related to each other and to edge modes and how similar features arise in nodal semimetals such as graphene. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A43.00004: Topological Invariants for Disordered Systems: Analysis and Computation Juntao Song, Emil Prodan Non-Commutative Geometry enables one to formulate topological invariants for aperiodic systems, in particular, for Disordered Topological Insulator from different symmetry classes with or without magnetic fields. Examples of such invariants, to be discussed in this talk, are the non-commutative Chern numbers, non-commutative winding numbers, electric polarization of systems from certain symmetry classes and the magneto-electric response of strong topological insulators. We show that these non-commutative formulas provide the basis for some of the most efficient and accurate algorithms for computing topological invariants in the presence of strong disorder. Using explicit calculations, we demonstrated that, in many instances, we obtain quantization of the invariants with machine precision, even when the Fermi level is in dense localized spectrum (i.e. not in spectral gap). Phase diagrams computed with these algorithms will be presented, for models from various symmetry classes. Acknowledgement: This research was supported from U.S. NSF grants DMS-1066045, DMR-1056168 and DMS-1160962 and NSFC grant 11204065 and RFDPHEC grant. A2013205168. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A43.00005: Disorder induced Floquet Topological Insulators Paraj Bhattacharjee, Netanel Lindner, Mikael Rechtsman, Gil Refael We investigate the possibility of realizing a disorder induced topological state in two dimensional periodically driven systems. This phenomenon is akin to the topological Anderson insulator (TAI) in equilibrium systems. We focus on graphene band structures, where in the presence of the driving electromagnetic field, but in the absence of disorder, the system starts off in a trivial state due to the presence of a sublattice potential. We show that by adding on-site disorder a topological state is induced in this system. We numerically compute the average Bott index (the analog of the Chern number for disordered systems) to show that starting from a trivial phase, topological behavior can be observed at finite disorder strength. In the topological phase, we detect chiral edge states by a numerical time evolution of wavepackets at the edge of the system. We propose an experimental set-up in photonic lattices to observe this phenomenon. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A43.00006: Soliton Defects in One-dimensional Topological Three-band Hamiltonian Gyungchoon Go, Kyeong Tae Kang, Jung Hoon Han Defect formation in the one-dimensional topological three-band model is examined within both lattice and continuum models. Classic results of Jackiw-Rebbi and Rice-Mele for the soliton charge is generalized to the three-band model. The presence of the central flat band in the three-band model makes the soliton charge as a function of energy behave in a qualitatively different way from the two-band Dirac model case. Quantum field-theoretical calculation of Goldstone and Wilczek is also generalized to the three-band model to obtain the soliton charge. Diamond-chain lattice is shown to be an ideal structure to host a topological three-band structure. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A43.00007: Phases of a one dimensional chain of topological twist defects Abhishek Roy, Jeffrey Teo, Xiao Chen A topological twist defect acts on a system containing abelian anyons by permuting anyon labels in a manner that preserves their braiding properties. We investigate a one dimensional chain of twist defects. The Hamiltonian consists of Wilson loop operators, each enclosing a pair of neighbouring defects. We explore both gapped and gapless phases. For the former, we use anyon pumping to classify the ground states. For the latter, we present numerical evidence for the central charge for various values of the coupling constants. We extend the above results from twofold defects (which are similar to $Z_k$ parafermions) to threefold defects introduced by us earlier in an exactly solvable lattice model [1]. \\[4pt] [1] Unconventional Fusion and Braiding of Topological Defects in a Lattice Model. Jeffrey C.Y. Teo, Abhishek Roy, Xiao Chen arXiv:1306.1538 [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A43.00008: Quantum dot as a magnetic impurity in a helical edge: a source of resistance weakly dependent on temperature Jukka Vayrynen, Moshe Goldstein, Yuval Gefen, Leonid Glazman The bulk of a doped two-dimensional topological insulator may accommodate spontaneously-formed quantum dots (charge puddles). We show that a Coulomb blockaded quantum dot hosting an odd number of electrons acts as a magnetic impurity effective in backscattering of electron moving along the helical edge. The exchange interaction between the dot and the edge, derived from a microscopic Hamiltonian, is anisotropic in general. The exchange anisotropy makes the dot spin an efficient backscatterer. The resulting negative correction to the helical edge conductance may exhibit a broad plateau in its temperature dependence. Being averaged over the Fermi level position, the correction to the ideal conductance becomes logarithmic in temperature. The effect of external magnetic field on transport is also discussed, and a connection to recent experiments is made. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A43.00009: Weak antilocalisation in topological insulators Xintao Bi, Ewelina Hankiewicz, Dimitrie Culcer Topological insulators (TI) have changed our understanding of insulating behaviour. They are insulators in the bulk but conducting along their surfaces due to spin-orbit interaction. Much of the recent research focuses on overcoming the \textit{transport bottleneck}, the fact that surface state transport is overwhelmed by bulk transport stemming from unintentional doping. The key to overcoming this bottleneck is identifying unambiguous signatures of surface state transport. This talk will discuss one such signature, which is manifest in the coherent backscattering of electrons. Due to strong spin-orbit coupling in TI one expects to observe weak antilocalisation rather than weak localisation, meaning that coherent backscattering increases the electrical conductivity. The features of this effect, however, are rather subtle, because in TI the impurities have strong spin-orbit coupling as well. I will show that spin-orbit coupled impurities introduce an additional time scale, which is expected to be shorter than the dephasing time, and the resulting conductivity has a \textit{logarithmic dependence} on the carrier density, a behaviour hitherto unknown in 2D electron systems. The result we predict is observable experimentally and would provide a smoking gun test of surface transport. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A43.00010: Anderson Localization in Disordered Systems with Competing Channels Hongyi Xie In a variety of physical contexts, for example, exciton-polaritons and field-effect transistors based on bi- or trilayer graphene, the situation arises that two or more propagating channels with different transport properties are coupled together and modifying each other's properties. One could ask what happens to the localization properties when a less localized lattice is coupled to a more localized one? Will the less localized one dominate the localization of the system or the more localized? The qualitative answer to this question depends on the dimensionality of the system. Correspondingly, we exactly solved the Anderson models on a two-leg ladder and on a two-layer Bethe lattice. In one dimension, the localization lengths of two coupled chains are of the order of the localization length of the more localized chain under resonance conditions. On the Bethe lattices, the less disordered lattice is not affected much by the more disordered lattice in the presence of coupling. These trends are believed to be persistent in high dimensions. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A43.00011: The Even and Odd Chern Numbers for Disordered Topological Insulators Emil Prodan The $K^0(M)$ group classifies the projectors and the $K^{-1}(M)$ classifies the unitaries defined over a manifold $M$. The even and the odd Chern numbers assign integers to the topological classes from $K^0(M)$ and $K^{-1}(M)$, respectively. If $M$ is the Brillouin torus in various dimensions, the even and the odd Chern numbers become the classifying invariants for the A and AIII symmetry classes of Topological Insulators, respectively. For arbitrary (even/odd) dimension, we recently showed that these two invariants can be defined in the presence of strong disorder. Inspired by the Non-Commutative Geometry program, we were able to demonstrate that both invariants remain quantized and non-fluctuating as long as the Fermi level resides in a region of localized spectrum. The most direct consequence of this result is that {\it all} topological phases from A and AIII symmetry classes are surrounded by phase-boundaries harboring extended states. Summary of these results and phase diagrams of various disordered models from the A and AIII symmetry classes will be presented. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A43.00012: Quantum dot in Topological Insulator Nanofilm: energy spectra and optical transitions Thakshila Herath, Prabath Hewageegana, Vadym Apalkov We introduce a quantum dot in topological insulator nanofilm as a bump at a surface of nanofilm. Such quantum dot can localize an electron if the size of the dot is large enough, $>$ 5 nm. The quantum dot in topological insulator nanofilm has two types of states, corresponding to ``conduction'' and ``valence'' bands of topological insulator nanofilm. We study the energy and optical (intraband and interband) spectra of such defined quantum dots and their dependence on the dot parameters. Both intraband and interband optical transitions have the same selection rules. While the interband absorption spectra have multi-peak structure, the intraband spectra has one strong peak and a few weak high frequency satellites. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A43.00013: Dephasing effect on backscattering of helical surface states in 3D topological insulators Haiwen Liu, Hua Jiang, Qing-feng Sun, X.C. Xie We analyze the dephasing effect on the backscattering behavior of the helical surface states in 3D topological insulators. Considering the dilute non-magnetic impurities condition, we calculate the second-order scattering amplitude and the backscattering cross-section for both short-range and long-range scattering potentials. Our results indicate the combination effect of dephasing and scattering can cause backscattering in the helical SS, although one of them can not alone. In specific, the long-range Coulomb potential can cause extremely large backscattering when energy is close to the Dirac point. This large backscattering can lead to the anomalous ``gap-like'' features observed in recent experiments [$Nat. Phys. {\bf7}, 840 (2011)$]. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A43.00014: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 10:48AM - 11:00AM |
A43.00015: Magnetic interaction and magnetic fluctuations in topological insulators with ordered and disordered magnetic adatoms Maia G. Vergniory, Levan Chotorlishvili, Arthur Ernst, Vitali Dugaev, Andreas Komnik, Mijail Otrokov, Evgueni Chulkov, Jamal Beradkar Using a first-principles Green's function approach we study magnetic properties of the magnetic binary topological insulators Bi$_2$Se$_3$, Bi$_2$Te$_23$ and Sb$_2$Te$_3$ doped with 3d transition metals. We analyze the magnetic phase for each dopant, the exchange interaction, the Curie temperature and the Bloch spectral function. Furthermore, we observe that the interaction of magnons with surface electrons essentially renormalizes the electron energy spectrum. The renormalized spectrum is nonlinear and can be characterized by a negative effective mass of electrons and holes for any k point different from 0. The electron velocity near the Dirac point depends on the electron-magnon coupling. [Preview Abstract] |
Session A44: Focus Session: Defects in Semiconductors: PV Materials
Sponsoring Units: DMP FIAPChair: Shiyou Chen, China Eastern Normal University
Room: Mile High Ballroom 4C
Monday, March 3, 2014 8:00AM - 8:36AM |
A44.00001: Band alignment of zinc-blende and chalcopyrite semiconductors: Effects of misfit dislocations Invited Speaker: Fumiyasu Oba The band offset is a key quantity that largely determines electrical transport across the heterointerfaces in electronic devices and photovoltaic cells. Its accurate determination has therefore been one of the central issues in computational materials science. The band offset by nature depends on the atomistic and electronic structure of heterointerfaces. Assuming such dependences to be weak at interfaces composed of structurally and chemically similar materials, a band alignment diagram, where relevant materials are aligned using a common reference level, carries information of the band offsets. Quantities such as branch points from bulk calculations and ionization potentials from surface calculations, as well as band offsets explicitly obtained from interface calculations, have been used for the alignment, but the effects of misfit dislocations at semicoherent interfaces have been neglected. In this talk I will revisit the band alignment of zinc-blende and chalcopyrite semiconductors using semilocal and hybrid density functional calculations [1-4]. In particular, the effects of misfit dislocations on the band offsets are discussed for selected zinc-blende heterointerfaces via explicitly treating edge dislocation arrays in the calculations [1]. The variation in the electrostatic potential associated with the presence of misfit dislocations is found to be localized around the dislocation cores. The misfit dislocations typically affect the band offsets by only about 0.1 eV at a distance of 1 nm from the interfaces.\\[4pt] [1] Y. Hinuma, F. Oba, and I. Tanaka, Phys. Rev. B 88, 075319 (2013).\\[0pt] [2] Y. Hinuma, F. Oba, Y. Kumagai, and I. Tanaka, Phys. Rev. B 88, 035305 (2013).\\[0pt] [3] Y. Hinuma, F. Oba, Y. Nose, and I. Tanaka, J. Appl. Phys. 114, 043718 (2013).\\[0pt] [4] Y. Hinuma, F. Oba, Y. Kumagai, and I. Tanaka, Phys. Rev. B 86, 245433 (2012). [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A44.00002: Stacking faults and lamellar twins with intrinsic point defects in poly-crystalline CdTe analyzed by density functional theory Christopher Buurma, Maria Chan, Tadas Pauluaskas, Robert Klie, Sivalingam Sivananthan Polycrystalline CdTe is a prominent photovoltaic material with proven industry success. To develop the next generation of thin film CdTe solar cells, higher open-circuit voltages and longer minority carrier lifetimes must be achieved. Playing a major role in doping, defect migration, recombination, and current transport are grain boundaries and other extended defects within grains of poly-crystalline CdTe. Commonly observed with STEM in CdTe are twins and stacking faults that extend throughout the entire grain. These twins can appear as lamellar repeating twins, or as single column stacking faults occurring in repetition near that of a Wurtzite structure. In this talk, we will use first principles density functional theory to investigate the thermodynamics and electronic structures such structures observed in STEM. The interaction energetics between adjacent twins and sets of twins are investigated. We will also investigate the likelihood of formation of neutral and charged native point defects in and near these extended defect structures. Binding energies of multiple point defects near such structures are also revealed. Implications towards PV efficiencies are discussed. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A44.00003: Origin of reduced efficiency in high Ga concentration Cu(In,Ga)Se$_{2}$ solar cell S.-H. Wei, B. Huang, H. Deng, M.A. Contreras, R. Noufi, S. Chen, L.W. Wang CuInSe$_{2}$ (CIS) is one of the most attractive thin-film materials for solar cells. It is well know that alloying Ga into CIS forming Cu(In,Ga)Se$_{2}$ (CIGS) alloy is crucial to achieve the high efficiency, but adding too much Ga will lead to a decline of the solar cell efficiency. The exact origin of this puzzling phenomenon is currently still under debate. Using first-principles method, we have systemically studied the structural and electronic properties of CIGS alloys. Our phase diagram calculations suggest that increasing growth temperature may not be a critical factor in enhancing the cell performance of CIGS under equilibrium growth condition. On the other hand, our defect calculations identify that high concentration of antisite defects M$_{Cu} $(M$=$In, Ga) rather than anion defects are the key deep-trap centers in CIGS. The more the Ga concentration in CIGS, the more harmful the deep-trap is. Self-compensation in CIGS, which forms 2V$_{Cu}+$M$_{Cu} $defect complexes, is found to be beneficial to quench the deep-trap levels induced by M$_{Cu}$ in CIGS, especially at low Ga concentration. Unfortunately, the density of isolated M$_{Cu}$ is quite high and cannot be largely converted into 2V$_{Cu}+$M$_{Cu}$ complexes under thermal equilibrium condition. Thus, nonequilibrium growth conditions or low growth temperature that can suppress the formation of the deep-trap centers M$_{Cu}$ may be necessary for improving the efficiency of CIGS solar cells with high Ga concentrations. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A44.00004: Defect Segregations at Grain Boundaries of CuInSe2 and Cu2ZnSnSe4 and Its Impact on Photovoltaic Performance Wanjian Yin, Yelong Wu, Su-Huai Wei, Rommel Noufi, Mowafak Al-Jassim, Yanfa Yan Grain boundaries (GBs) in absorber layers of polycrystalline thin-film solar cells play important roles in cell performance. In this presentation, we will review our recent results of density functional theory (DFT) study on the GB properties in solar cell materials including CuInSe2 (CIS) and Cu2ZnSnSe4 (CZTSe). We found that intrinsic GBs in these semiconductors are detrimental, probably due to the formation of deep gap states caused by wrong bonds. However, intrinsic defects and some extrinsic impurities have the tendency to segregate to grain boundaries. The segregations lead to two major effects: (1) passivating the deep defect states in the band gap by breaking or weakening the wrong bonding at GBs and (2) creating neutral hole barriers. The existence of Na$^{\mathrm{+}}_{\mathrm{i}}$ further induces the band bending and increases the hole barrier. Our results suggest benign GB properties in CIS. We further propose approaches to engineer GBs in CZTSe to improve its cell performance. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A44.00005: Stability and Electronic Structure of $Cu_{2} ZnSnS_{4} $ Surfaces: a First-Principles Study Peng Xu, Shiyou Chen, Bing Huang, Hong-Jun Xiang, Xin-Gao Gong, Su-Huai Wei Through the surface energy first-principles calculations, we studied the possible surface structures of the frequently observed cation-terminated (112) and anion-terminated ($\overline {112} )$ surfaces in various sample grown conditions. We found that the polar surfaces are stabilized by the charge-compensating defects, such as vacancies ($V_{Cu} $,$V_{Zn} )$, antisites ($Zn_{Cu} $,$Zn_{Sn} $ , $Sn_{Zn} )$ and defect clusters ($Cu_{Zn} +Cu_{Sn} $,$2Zn_{Cu} +V_{Sn} $ ). In stoichiometric single-phase CZTS samples, Cu-enriched defects are favored on (112) surfaces and Cu--depleted defects are favored on ($\overline {112}$) surfaces, while in non-stoichiometric samples grown under Cu poor and Zn rich conditions, both surfaces favor the Cu-depleted defects, which explains the observed Cu-deficiency on the surfaces of the synthesized CZTS thin films. The electronic structure analysis shows that Cu-enriched surfaces produce detrimental states in the band gap, while Cu-depleted surfaces produce no gap states and are thus benign to the solar cell performance. The calculated surface properties are consistent with experimental observation that Cu-poor and Zn-rich CZTS solar cells have higher efficiency. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A44.00006: Understanding the effects of dopant impurities on quaternary chalcogenide system properties by investigating and modeling local vibrational modes and Raman lineshapes Prashant Sarswat, Michael Free Cu$_{\mathrm{2}}$ZnSnS$_{\mathrm{x}}$Se$_{\mathrm{4-x}}$ (CZTSSe) has gained attention as a p-type absorber layer due to its attractive properties such as optimum band gap, high absorption coefficient, and use of low cost elements. However, impurities in CZTSSe produce detrimental effects, which limit the device performance. Phonon dispersion in most of the semiconductors is found to be susceptible to the pairing between atoms within the lattice. Hence, a change in phonon dispersion can be used to investigate the effects of foreign impurities on such pairing. Thus a series of experiments were conducted to investigate the effect of free holes on the optical phonons of doped CZTSSe system as well as to evaluate asymmetry in the Raman lineshape. When irradiated with photons, doped CZTS possibly produces a continuum of inter-valence band electronic excitations, which can envelop the Raman-active phonon energy. Such overlap between the electronic continuum and discrete state can cause interference effects in CZTSSe. It was observed that Raman lineshape becomes more asymmetric, wider, and shifts towards lower frequency when laser power density increased. All these observations were found for Raman A mode as well as E (TO, LO) mode for doped CZTSSe samples. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A44.00007: Defect formation enthalpy of Cu2ZnSnS4 and Cu2ZnSnSe4 revisited: ab initio insights into the limitations of CZTS technology Julien Vidal, Pawel Zawadzki, Vladan Stevanovic, Stephan Lany The defect physics of earth abundant quaternary compound Cu2ZnSnS4 (CZTS) and Cu2ZnSnSe4 (CZTSe) is extremely complex not only because of the many competing phases but also because of the many possible cationic substitutions. Previous theoretical studies have indicated that CZT(S,Se) has a quite high hole concentration originating from intrinsic defects such as Cu vacancies and Cu-on-Zn antisites. In this study, we have carried out state-of-the-art defect calculations including thermochemical corrections to the phase diagram and specific correction to the formation enthalpy of shallow defects. The latter was found to be a critical point in the analysis of intrinsic defects in both CZTS and CZTSe. Indeed, and at variance with previous studies, our GW-corrected ab initio defect calculation reveals that both Cu-on-Zn and Zn-on-Cu antisites have comparable formation enthalpy, which results in the pinning of the Fermi level in the mid-band gap region. The latter has two important consequences: the relatively low carrier concentration in CZT(S,Se) and the limitation of the open circuit voltage. It is also found that higher carrier concentrations are achievable under growth conditions where CZT(S,Se) is only marginally stable and may decompose into binary or ternary competing phases. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A44.00008: Origins of varying carrier concentration in Cu$_2$SnS$_3$ photovoltaic absorbers Lauryn Baranowski, Pawel Zawadzki, Stephan Lany, William Tumas, David Ginley, Eric Toberer, Andriy Zakutayev Within the Cu-Sn-S family of earth abundant photovoltaic absorbers, the Cu$_2$SnS$_3$ phase is predicted to be the most promising absorber material [P. Zawadzki, et al.]. To date there has been limited synthetic work on the Cu$_2$SnS$_3$ phase, particularly the carrier concentration. In this study, we develop an understanding of the effects of RF sputtering growth conditions on the hole concentrations of Cu$_2$SnS$_3$ absorber films, and use these results to identify the underlying causes of the observed variations in carrier concentration. Two effects are identified that control the carrier concentration in Cu$_2$SnS$_3$ films. The first effect, which occurs during Cu-rich growth, is isostructural alloying with a metallic Cu$_3$SnS$_4$ phase, which gives rise to hole concentrations above 10$^{19}$ cm$^{-3}$. The second effect is that, when the Cu$_2$SnS$_3$ films are grown under Sn-rich conditions, varying the S chemical potential during film deposition gives 10$^{18}$-10$^{19}$ cm$^{-3}$ holes. This variation in carrier concentration with S chemical potential can be explained by a Cu vacancy defect model. Understanding the origins of the varying doping density in Cu$_2$SnS$_3$ films allows for targeted growth to achieve desired carrier concentrations for device integration. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A44.00009: Observation of Coulomb repulsion between Cu intercalants in Cu$_{\mathrm{x}}$Bi$_{2}$Se$_{3}$ Christopher Mann, Damien West, Ireneusz Miotkowski, Yong Chen, Shengbai Zhang, Chih-Kang Shih Using scanning tunneling microscopy and \textit{ab initio }simulations, we have identified several configurations for Cu-dopants in Cu$_{\mathrm{x}}$Bi$_{2}$Se$_{3}$, with Cu intercalants being the most abundant. Through statistical analysis, we show strong short-range repulsive interactions between Cu intercalants. At intermediate range (\textgreater 5nm), the pair distribution function shows oscillatory structure along the \textless 1 0 -1\textgreater\ directions which appears to be influenced by different diffusion barriers along the \textless 1 0 -1\textgreater\ and \textless 2 -1 -1\textgreater\ directions. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A44.00010: Interface effects on calculated defect levels for oxide defects Arthur Edwards, Hugh Barnaby, Peter Schultz, Andrew Pineda Density functional theory (DFT) has had impressive recent success predicting defect levels in insulators and semiconductors [Schultz and von Lillienfeld, 2009]. Such success requires care in accounting for long-range electrostatic effects. Recently, Komsa and Pasquarello have started to address this problem in systems with interfaces. We report a multiscale technique for calculating electrostatic energies for charged defects in oxide of the metal-oxide-silicon (MOS) system, but where account is taken of substrate doping density, oxide thickness, and gate bias. We use device modeling to calculate electric fields for a point charge a fixed distance from the interface, and used the field to numerically calculate the long-range electrostatic interactions. We find, for example, that defect levels in the oxide do depend on both the magnitude and the polarity the substrate doping density. Furthermore, below 20 {\AA}, oxide thickness also has significant effects. So, transferring results directly from bulk calculations leads to inaccuracies up to 0.5 eV-- half of the silicon band gap. We will present trends in defect levels as a function of device parameters. We show that these results explain previous experimental results, and we comment on their potential impact on models for NBTI. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A44.00011: A comprehensive ab initio study of doping in bulk ZnO with group V elements Guido Petretto, Fabien Bruneval Zinc-oxyde, despite being a promising candidate for several electronic applications, up to now has provided several challenges to the scientific community, both from an experimental and theoretical point of view [1]. In fact, a reliable p-type doping still has not been achieved and standard density functional theory (DFT) calculations has often provided unsatisfactory results and failed to help in the search for better configurations to obtain such property. To solve the band gap underestimation problem we have made use of the HSE hybrid functional[2], tuning the admixing parameter to match the experimental band gap. Within this framework, we extensively studied the formation and transition energies of group V elements related defects. These include simple substitutional defects X$_\textrm{O}$, X$_\textrm{Zn}$ (X=N, P, As, Sb) and complexes of the form X$_\textrm{Zn}$-2V$_\textrm{Zn}$ and X$_\textrm{Zn}$-V$_\textrm{Zn}$. The stability of these complexes is also addressed. We show that it is unlikely to obtain good acceptor states from these elements due to deep transition energies and the presence of donor-like defects. [1]Avrutin, V. et al., Proceedings of the IEEE, 98, 1269 (2010) [2]Heyd, J. et al., Journal of chemical physics 118, 8207 (2003) [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A44.00012: Identifying microscopic mechanisms for hole traps in nitride heterostructures John Lyons, Luke Gordon, Anderson Janotti, Chris G. Van de Walle Some recent designs of nitride semiconductor devices employ heterostructures (such as N-face high-electron-mobility transistors) in which the electronic Fermi level is established near the valence-band maximum due to the influence of polarization fields. In many of these heterostructures, the presence of hole-trapping centers is thought to adversely affect device performance. This behavior has been observed in many different types of devices, and its physical origin remains unknown. Using first-principles calculations based on a hybrid functional, we investigate possible origins for this phenomenon. We explore both intrinsic defect candidates as well as impurities. With Schr\"{o}dinger-Poisson simulations, we then investigate how the behavior of these species and their spatial distribution within the heterostructure layers is reflected in the performance of nitride semiconductor devices. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A44.00013: Electronic and optical properties of GaSb:N from first principles Priyamvada Jadaun, Hari Nair, Vincenzo Lordi, Seth Bank, Sanjay Banerjee We present an ab-initio study of dilute nitride III-Vs, focusing on dilute nitride GaSb (GaSb:N). GaSb:N displays promise towards realization of optoelectronic devices accessing the mid-infrared wavelength regime. Theoretical and experimental results on its electronic and optical properties are however few. To address this, we present a first principles, density functional theory study using the hybrid HSE06 exchange-correlation functional of GaSb doped with 1.6\% nitrogen. We conduct a comparative study on GaAs:N, also with 1.6\% nitrogen mole fraction, and find that GaSb:N has a smaller band gap and displays more band gap bowing than GaAs:N. In addition we examine the orbital character of the bands, finding the lowest conduction band to be quasi-delocalized, with a large N-3s contribution. At high concentrations, the N atoms interact via the host matrix, forming a dispersive band of their own which governs optoelectronic properties and dominates band gap bowing. While this band drives the optical and electronic properties of GaSb:N, its physics is not captured by traditional models for dilute-nitrides. We thus propose that a complete theory of dilute-nitrides should incorporate orbital character examination, especially at high N concentrations. [Preview Abstract] |
Session A45: Quantum Hall Effect: Bilayers and Microwave Induced Resistance Oscillations
Sponsoring Units: FIAPChair: Michael Zudov, University of Minnesota
Room: Mile High Ballroom 4D
Monday, March 3, 2014 8:00AM - 8:12AM |
A45.00001: Tunneling at $\nu_T=1$ in a bilayer quantum Hall exciton condensate D. Nandi, T. Khaire, A.D.K. Finck, J.P. Eisenstein, L.N. Pfeiffer, K.W. West Closely-spaced bilayer quantum Hall systems at total filling factor $\nu_T=1$ exhibit spontaneous interlayer phase coherence. This phase coherence, which is tantamount to excitonic Bose condensation, is most dramatically revealed via interlayer tunneling measurements.In the condensed phase the tunneling current-voltage ($IV$) characteristic of this semiconductor system strongly resembles the dc Josephson effect observed in superconducting tunnel junctions. Here we report on a detailed study of this phenomenon. We find the maximum, or critical tunneling current $I_c$ to be a well-defined global property of the macroscopic tunnel junction, insensitive to external circuit elements and the precise contact configuration used to observe it. Interestingly, the temperature dependence of $I_c$ displays an unexpected scaling behavior. At the lowest temperatures the slope of the ``supercurrent'' branch of the tunneling $IV$ curve, while extremely large, remains finite. Careful measurements in this regime suggest that dissipative processes arising from in-plane exciton transport limit the maximum tunneling conductance. Finally, comparisons of the experimentally observed $IV$ with recent theoretical predictions will be discussed. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A45.00002: Effect of perpendicular electric fields on quantum Hall stripe phases J. Pollanen, J.P. Eisenstein, L.N. Pfeiffer, K.W. West High quality two dimensional electron systems (2DES) in GaAs can exhibit large transport anisotropies near half filling of excited Landau levels [1,2] associated with the emergence of collective electron states possessing broken rotational symmetry in the plane of the 2DES. These states, known as the stripe phases, appear to be among the first known examples of purely electronic nematic liquid crystals. Experiments show that the orientation of the stripes is keyed to the crystallographic axes of the GaAs host lattice. Identification of the native symmetry-breaking potential remains an active area of interest, with strain and spin-orbit mechanisms recently proposed [3,4] as being responsible. Noting that both strain and spin-orbit effects can be altered by the application of a perpendicular electric field, we have performed magneto-transport experiments on narrow (20nm) GaAs quantum wells equipped with front and backside electrostatic gates. These gates allow us to study the effect, at constant 2D electron density, of perpendicular electric fields on the various quantum Hall stripe phases. [1]M.P. Lilly et al. PRL 82, 394 (1999) [2]R.R. Du et al. Solid State Commun. 109, 389 (1999) [3]S.P. Kovudayur et al. PRL. 106, 016804 (2011) [4]I. Sodemann and A.H. MacDonald, arXiv:1302.3896 [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A45.00003: Bilayer mapping of the paired quantum Hall state in the half-filled second Landau level Jae-Seung Jeong, Kwon Park The fractional quantum Hall effect observed in the half-filled second Landau level is one of the most fascinating phenomena in condensed matter physics due to the possibility of emergent pairing with quasiparticle excitations satisfying non-Abelian statistics. The leading theory for the paired quantum Hall state in the half-filled second Landau level is based on the Moore-Read Pfaffian wave function, which is intimately connected with the Halperin (331) wave function for the bilayer quantum Hall system in the sense that the former is obtained via antisymmetrization of the spatial part of the latter. Motivated by this intriguing connection, we investigate a generalized mapping between the bilayer and the paired quantum Hall state at half filling via exact diagonalization in the torus geometry. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A45.00004: Observation of microwave-induced resistance oscillations in high-mobility 2D hole gas in sGe/SiGe quantum wells Q.A. Ebner, P.D. Martin, Q. Shi, M.A. Zudov, O.A. Mironov, R.J.H. Morris, D.R. Leadley Microwave-induced resistance oscillations (MIRO) have been extensively studied for more than a decade but, until now, have remained unique to GaAs/AlGaAs-based 2D electron systems. In this talk we report on the observation of MIRO in a very different setting, a 2D hole gas hosted in strained Ge/SiGe quantum wells. These findings demonstrate that MIRO is a universal phenomenon and that microwave photoresistance can be utilized to probe the energy spectrum and the correlation effects of 2D holes in Ge/SiGe quantum wells. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A45.00005: Effect of direct current on giant negative magnetoresistance in two-dimensional electron systems Q. Shi, P.D. Martin, Q.A. Ebner, M.A. Zudov, L.N. Pfeiffer, K.W. West We report on a giant negative magnetoresistance in a 200 micron-wide Hall bar fabricated from GaAs/AlGaAs quantum well. Comparison with theory shows that magnetoresistance is much stronger than one could expect from either electron-electron interaction or classical memory effects due to sharp and smooth disorder. In this talk we systematically examine the effect of direct current and compare our findings with temperature dependence. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A45.00006: Photoresistance of two-dimensional electron gas at sub-Terahertz frequencies P.D. Martin, M.A. Zudov, J.D. Watson, M.J. Manfra, L.N. Pfeiffer, K.W. West Extending experiments on photoresistance of ultra-high mobility 2DES to higher radiation frequencies allows to enter the regime of strong Shubnikov-de Haas oscillations (SdHO), which remains largely unexplored. This talk reports on low-temperature photoresistance measurements using frequencies from 0.2 to 0.4 THz. At higher radiation intensity, we observe a series of very strong and narrow peaks which occur near the cyclotron resonance. At lower intensities, strong peaks disappear and the data reveal a suppression of SdHO near the cyclotron resonance, and to a lesser extent, near its harmonics. These findings will be compared to existing theoretical predictions. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A45.00007: Subharmonics of microwave induced resistance oscillations in the presence of high electric fields S. Chakraborty, A.T. Hatke, L.W. Engel, M. Manfra, J. Watson, M.P. Lilly, J. Reno We investigate the photoresistance of a two-dimensional electron system to high power microwave radiation, using a Hall bar within the slot of a coplanar waveguide (CPW) capable of electric fields in excess of 100 V/cm. The contacts of the Hall bar were screened within the ground plane of the CPW. Our measurements focus on longitudinal transport at magnetic fields larger than that of the cyclotron resonance at the frequency applied to the CPW. We observe a series of subharmonic resonances that can have amplitude in excess of the cyclotron resonance photoresistance as well as different power and temperature dependence. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A45.00008: Giant negative magnetoresistance in irradiated two-dimensional electron systems M.A. Zudov, Q. Shi, P.D. Martin, Q.A. Ebner, J.L. Reno, L.N. Pfeiffer, K.W. West Several recent magnetotransport studies in high-mobility two-dimensional electron systems reported very strong negative magnetoresistance whose origin remains unclear. In an attempt to advance our understanding of this phenomenon, we have performed measurements on microwave-irradiated GaAs/AlGaAs heterostructures and quantum wells exhibiting giant magnetoresistance. We have found that microwave photoresistance is usually positive over a wide range of magnetic fields indicating that negative magnetoresistance is suppressed by microwave radiation. This suppression, however, is too strong to be attributed solely to radiation-induced heating of electrons. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A45.00009: Imaging 2D Electron Density Variations in High Mobility AlGaAs/GaAs Systems Jerry Lee, Ken West, Kirk Baldwin, Loren Pfeiffer, Lara Fernandes Lavelli, Aron Pinczuk We demonstrate two different techniques to measure the local 2D electron densities of high mobility AlGaAs/GaAs systems on the micron scale. We used micro-photoluminescence imaging to look for 2D density variations on the 50 micron scale, as well as local magneto-transport measurements to look for 2D density and mobility variations on the scale of several hundred microns. Our results suggest that the 2D electron systems indeed have local 2D densities and mobilities that vary from their corresponding mean values. Spatial maps suggest that the origin of these variations is likely due to variations in the MBE layer thicknesses across the wafer, as well as by fixed charge sites that we believe are located within the GaAs substrate or at the substrate-MBE interface. Our findings suggest that the current limits on the 2D electron mobility could be raised by devising methods to negate the effects of these fixed charges. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A45.00010: Combined study of microwave-power-dependence and linear-polarization-dependence of the microwave-radiation-induced magnetoresistance oscillations Tianyu Ye, Han-Chun Liu, Ramesh Mani, Werner Wegscheider Microwave radiation induced magnetoresistance oscillations (MRIMOs) represent an interesting electrical property of the high mobility two dimensional electron gas (2DEG) at low temperatures in a perpendicular magnetic field and under microwave excitation. Some questions under discussion in this topic include: (a) whether MRIMOs' amplitudes grow linearly with the microwave power and (b) how the MRIMO amplitudes change with the rotation of the microwave polarization with respect to the sample. In this study, we utilize swept microwave power and continuously changed linear polarized microwave polarization angle as two variables in four-terminal low-frequency lock-in magnetoresistance measurements of the 2DEG samples. The results show that amplitude of MRIMOs varies non-linearly with the microwave power. Also, the microwave polarization dependence measurements show that MRIMOs depend sensitively on the polarization angle of the linearly polarized microwaves, while the oscillatory magnetoresistance follows a cosine square function of the polarization angle. We provide a simple model that conveys our understanding of our observations. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A45.00011: Size-dependent giant-magnetoresistance in millimeter scale GaAs/AlGaAs 2D electron devices R.G. Mani, A. Kriisa, W. Wegscheider This study examines a ``bell-shape'' negative Giant Magneto-Resistance (GMR) that grows in magnitude with decreasing temperatures in $mm$-wide devices fabricated from the high-mobility GaAs/AlGaAs 2-Dimensional Electron System (2DES). Experiments show that the span of this magnetoresistance on the magnetic-field-axis increases with decreasing device width, $W$, while there is no concurrent Hall resistance, $R_{xy}$, correction. A multi-conduction model, including negative diagonal-conductivity, and non-vanishing off-diagonal conductivity, reproduces experimental observations. The results suggest that boundary scattering in the $mm$-wide 2DES with $mm$-scale electron mean-free-paths might be responsible for the observed ``non-ohmic'' size-dependent negative GMR [1]\\[4pt] [1] R. G. Mani, A. Kriisa, and W. Wegscheider, Sci. Rep. 3, 2747 (2013). [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A45.00012: Study of the phase-shift in the linear-polarization-angle-dependence of the microwave radiation-induced magnetoresistance oscillations in the GaAs/AlGaAs system Han-Chun Liu, Tianyu Ye, R.G. Mani, W. Wegscheiger Transport studies of microwave- and terahertz-induced magneto-resistance oscillations (MTIMRO) identified novel photo-excited zero-resistance states in the GaAs/AlGaAs two-dimensional electron system system. Some theories based on the premise of linear-polarization-insensitivity have been developed for the MRIMRO. Some studies have shown, however, a strong linear polarization sensitivity of MTIMRO [1,2] using new experimental methods. In addition, Ramanayaka \textit{et al.} [2] has observed that using fitting formula, R$_{\mathrm{xx}}(\theta )=$A $\pm$ Ccos$^{2}(\theta $-$\theta_{0})$, to sinusoidal variation of diagonal resistance, R$_{\mathrm{xx}}$, with polarization angle $\theta $, the extracted phase shift, $\theta_{0}$, depends on radiation frequency, magnetic field $B$, sign of $B$ [2]. Here, in addition to those mentioned factors, we investigate the dependence of the phase shift $\theta_{0}$ in the linear-polarization-angle-dependence upon sample geometry. \\[4pt] [1] R. G. Mani \textit{et al.}, Phys. Rev. B 84, 085308 (2011). \\[0pt] [2] A. N. Ramanayaka \textit{et al.}, Phys. Rev. B 85, 205315 (2012). [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A45.00013: Degeneracy and Effective Mass in the Valence Band of Two-Dimensional (100)-GaAs Quantum Well Systems Vinicio Tarquini, Talbot Knighton, Zhe Wu, Jian Huang, Loren Pfeifer, Ken West Quantum Hall measurement of two-dimensional high-mobility ($\mu\sim 2\times$ 10$^6$ cm$^2$/(V$\cdot$s)) hole systems confined in a 20 nm wide (100)-GaAs quantum well have been performed for charge densities between $4-5\times$ 10$^{10}$ cm$^{-2}$ in a temperature range of 10-160 mK. The Fourier analysis of the Shubnikov-de Haas (SdH) oscillations of the magnetoresistance vs. the inverse of the magnetic field $1/B_{\bot}$ reveals a single peak, indicating a degenerate heavy hole (HH) band. The corresponding hole density $p=(e/h)\cdot f$ agrees with the Hall measurement result within $3\%$. The HH degeneracy is understood through the diminishing Rashba spin-orbit interaction due to the low charge density and the nearly symmetric confinement. SdH oscillations fitted for 0.1 T $\leq B_{\bot} \leq 0.25 $ T to the Dingle parameters yield an effective mass ($m^*$) between 0.39 $m_e$ and 0.51 $m_e$ that increases moderately with increasing magnetic field and charge density, in very good agreement with previous cyclotron resonance measurements. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A45.00014: Cyclotron mass and g-factor of high mobility holes in SiGe/Ge/SiGe in tilted magnetic field A. Suslov, I. Drichko, V. Malysh, I. Smirnov, L. Golub, S. Tarasenko, O. Mironov, M. Kummer, H. von Kanel Complex ac conductivity of a high quality single quantum well p-GeSi/Ge/GeSi (p=6$\cdot$10$^{11}$cm$^{-2}$) is measured using the surface acoustic wave technique at frequencies 30 and 85 MHz in magnetic fields of up to 18 T in the temperature range 0.3 - 5.8 K. In minima of the conductivity oscillations with small filling factors in integer quantum hall regime the ac conductivity is of the hopping nature and is described within the ``two-site'' model. In tilted fields for odd filling factors 3 and 5 increase of conductivity in the minima of the oscillations is due to effect of the in-plane field component on the g-factor. The same in-plane component causes rising of the cyclotron effective mass and damping of the oscillation magnitudes at even filling factors larger than 8. [Preview Abstract] |
Session A46: Bilayer Graphene: Stacking, Transport and Magnetics
Sponsoring Units: DCMPChair: Jun Zhu, Pennsylvania State University
Room: Mile High Ballroom 4E
Monday, March 3, 2014 8:00AM - 8:12AM |
A46.00001: Spontaneous Domain Walls in Bilayer Graphene Xiao Li, Fan Zhang, Qian Niu, Allan MacDonald Intrinsic bilayer graphene is susceptible to a family of gapped broken symmetry states in which each spin-valley flavor spontaneously polarizes between layers. These states are close in energy and can coexist above a critical temperature, separated by domain walls (DW), the collective and topological excitations. Our simulation further reveals three important facts. (i) The critical temperature of DW nucleation is smaller than the homogenous mean-field estimation and determines the phase transition. (ii) A Ginzburg-Landau theory can be built to characterize the DWs. (iii) Each DW leads to a surprising nonlinear transverse pseudospin response, which can be explained by the presence of zero modes propagating along the DW. We discuss the rich classification of these spontaneous DWs based on their distinct zero modes which may form unusual Luttinger liquids. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A46.00002: Critical Behavior of Four-Terminal Junctions of Bilayer Graphene Domain Walls Benjamin Wieder, Fan Zhang, Charles Kane Bilayer graphene in a perpendicular electric field can host domain walls between regions of reversed field direction or interlayer stacking. The gapless modes propagating along these domain walls, while not strictly topological, nevertheless have interesting physical properties, including valley-momentum locking. A junction where four domain walls meet forms the analogue of a quantum point contact. We study theoretically the critical behavior of this junction near the pinch-off transition, which is controlled by a non-trivial quantum critical point. At low temperatures, the transition sharpens and the conductance is described by a universal scaling function, which we compute. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A46.00003: Disorder-tuned selection of ordered state in bilayer graphene Junhua Zhang, Rahul Nandkishore, Enrico Rossi The nature of the symmetry-broken state driven by interaction in bilayer graphene (BLG) has attracted a lot of interest. Theoretical studies predict various possible ordered phases as the candidate for the ground state of BLG. To identify what instability is the most favorable in BLG, a number of experiments have been performed by several groups. However, there is no consensus: some experiments show evidence for a fully gapped state while others seem more consistent with a nematic state. By exploring the influence of disorder on a variety of competing ordered states, we find that the pair breaking effect due to disorder varies among the candidate phases, giving rise to different amount of suppression on the mean-field transition temperatures. This suggests a simple and natural scenario to resolve the discrepancy between experimental observations. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A46.00004: Electronic transmission through AB-BA domain boundary in bilayer graphene Mikito Koshino We study the electron transmission through the domain boundary on bilayer graphene separating AB and BA stacking regions, which was recently found in the experiment. We calculate the electron transmission probability as a function of the electron energy and the incident angle, for several specific boundary structures. The transmission strongly depends on the crystallographic direction of the boundary and also on the atomic configuration inside. At the low energy, the boundary is either insulating or highly transparent depending on the structure. In insulating cases, the transmission sharply rises when the Fermi energy is increased to a certain level, suggesting that the electric current through the boundary can be controlled by the field effect. The boundary parallel to the zigzag direction generally have different transmission properties between the two different valleys, and this enables to generate the valley polarized current in a certain configuration. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A46.00005: Electron-electron interaction induced effective mass suppression in bilayer graphene Jing Li, Ke Zou, Adam Stabile, Donald Seiwell, Jun Zhu The effective mass of carriers m* captures fundamental properties of a material. In a two-dimensional electron system, the electron-electron (e-e) interaction manifests in the renormalization of m*. Extending previous studies[1] to lower carrier densities, where the interaction effect is expected to be stronger, we present precision measurements of the electron and hole effective mass m$_e$* and m$_h$* in high-quality ($\mu\sim$30,000cm$^2$/Vs) hexagonal boron nitride supported bilayer graphene using temperature-dependent Shubnikov-de Hass oscillations. Our measurements probe carrier densities down to 2$\times$10$^{11}$/cm$^2$. Comparison to tight-binding bands and previous data shows excellent agreement at carrier densities above 5$\times$10$^{11}$/cm$^2$, where m$_e$* and m$_h$* can be well described by a renormalized Fermi velocity of v$_F$= 1.11$\times$10$^6$m/s. At lower carrier densities, m$_h$*continuously decreases from the tight-binding band value, reaching m$_h$*=0.0234m$_e$ at n= 2$\times$10$^{11}$/cm$^2$. This corresponds to a suppression of 30\% and an increased v$_F$=1.37$\times$10$^6$m/s. The deviation is much smaller for electrons. We compare our results with theory and discuss its implications. [1] K. Zou, X. Hong, and J. Zhu, Phys. Rev. B 84, 085408 (2011). [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A46.00006: Electric-field-dependent electronic structure of graphene bilayer: from the Bernal stacking to the unconventional orthorhombic stacking Gunn Kim, Changwon Park, Mina Yoon In this presentation, we report the electronic properties of bilayer graphene structures with various stackings, which can be formed, for instance, during the structural transition from graphite-to-diamond at high pressure, or at boundaries of stacking domains or at diamond surfaces. We performed ab initio calculations and the Wannier interpolations for accurate two-dimensional band structure with extremely dense (1600$\times$1600) $k$-point grid. Using tight-binding parameters obtained from maximally localized Wanneir function analysis, we also constructed the effective Hamiltonian for the graphene bilayer with various stacking. The overall electronic structures can be described by the relative shift and the coupling of two Dirac cones, depending on their stacking geometry. Our results reveal that external electric field is another parameter to control the electronic properties of the bilayer-graphene. In particular, the external fields significantly enhance the coupling of two Dirac cones, which result in additional or new van Hove singularities near the Fermi level. We compared the electronic structure of the orthorhombic stacking with those of AA and AB stackings. Our study may provide a deeper understanding of sliding effects of multilayer graphene. This work was supported by the Priority Research Center Program (2011-0018395) and the Basic Science Research Program through MEST/NRF (2013R1A1A2009131). This research was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A46.00007: Electronic Transport in Twisted Bilayer Graphene Superlattices Jason Luo, Javier Sanchez-Yamagishi, Sang Hyun Choi, Kenji Watanabe, Pablo Jarillo-Herrero Twisted bilayer graphene is the ultimate limit of a bilayer 2DEG, where two graphene layers are stacked with an interlayer distance of only 0.34nm. The interlayer tunnel coupling can be continuously tuned by twisting the two layers, leading to different physics in the small and large twist angle limits. At small twist angles, the two layers form a large superlattice unit cell and hybridization of the layers leads to low-energy van Hove singularities in the electronic spectrum. We report transport measurements of high-quality twisted bilayer graphene in the low twist angle regime where interlayer interactions have drastic effects on electronic properties. We demonstrate that in this regime, where the magnetic length scale is comparable to the superlattice constant, we can observe Hofstadter's butterfly physics at low magnetic fields. At zero magnetic field, we also observe a strong departure from the typical monolayer graphene transport properties. We discuss the possible roles that interlayer tunneling and electron-electron interactions play in our observed phenomena as well as the effect of interlayer electric field. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A46.00008: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 9:36AM - 9:48AM |
A46.00009: Magnetic breakdown in twisted bilayer graphene Chi-Ken Lu, H.A. Fertig Recently, superlattice patterns on graphene sheets have become available in both twisted bilayer systems and graphene on BN substrates. These systems are interesting due to the recent demonstration that in magnetic fields they host an observable Hofstadter spectrum, and that (for TBG) they host low energy saddle points(SPs) in their spectra which can lead to interesting many-body instabilities. We consider the role that these SPs play in weak magnetic fields, where the crossover from Landau level behavior to Hofstadter behavior in the spectrum is controlled by them. This phenomenon is a realization of magnetic breakdown, in which semiclassical trajectories change their topology when the energy passes through such SPs. In the TBG, we find that a description fully incorporating the magnetic symmetries is only possible if one doubles the Brillouin zone. This leads to a multiplicity of semiclassical quantization conditions on orbits above the saddle points, beyond the usual interior flux quantization, allowing the richness of the Hofstadter spectrum to become apparent at these higher energies. Possible experimental implications, including cyclotron resonance, magnetic susceptibility, and creation of open orbits (with accompanying metal-insulator transition) will be discussed. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A46.00010: Tunneling of dressed electrons in bilayer graphene Dipendra Dahal, Godfrey Gumbs, Andrii Iurov, Danhong Huang Closed-form analytic expressions have been derived for electron dressed states, resulting from the interaction between Dirac electrons in bilayer graphene and circularly polarized light. The dressed states in bilayer graphene exhibit some novel properties, such as left-right polarization asymmetry, which are absent in monolayer-layer graphene. We have investigated the dressed state tunneling through a square electrostatic potential barrier and present a detailed analysis of the way in which the physics involving the special type of zero-transmission Klein paradox in bilayer graphene is modified in the presence of strong electron-photon coupling. Additionally, we have investigated and compared various electron-tunneling behaviors in the presence of a finite-width magnetic barrier for both monolayer and bilayer graphene. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A46.00011: Bound state energy of a Coulomb impurity in gapped bilayer graphene: ``Hydrogen atom with a Mexican hat'' Brian Skinner, Boris Shklovskii, Mikhail Voloshin Application of a perpendicular electric field induces a band gap in bilayer graphene, and it also creates a ``Mexican hat'' structure in the dispersion relation. This structure has unusual implications for the hydrogen-like bound state of an electron to a Coulomb impurity. We calculate the ground state energy of this hydrogen-like state as a function of the applied interlayer voltage and the effective fine structure constant. Unlike in the normal hydrogen atom, the resulting wavefunction has many nodes of density even in the ground state. Further, the electron state undergoes ``atomic collapse'' into the Dirac continuum both at small and large voltage. Our results have important implications for the pursuit of a robust, tunable band gap in bilayer graphene. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A46.00012: Effect of long-range disorder on competing orders in bilayer graphene Martin Rodriguez-Vega, Christopher Triola, Junhua Zhang, Enrico Rossi Two general classes of spontaneously broken symmetry phases have been proposed for bilayer graphene: a gapped phase and a nematic phase. Some experiments suggest the establishment of a nematic phase whereas others suggest the presence of a gapped phase. In this talk I will present the results of our theoretical study of the effect of long-range disorder on the conditions for the establishment of a nematic or a gapped phase in bilayer graphene. In particular I will discuss the effect of the disorder-induced carrier density inhomogeneities on the properties and robustness of each phase. I will then discuss the relevance of our results for the current experiments. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A46.00013: Possible deconfined critical transition in bilayer graphene Junhyun Lee, Subir Sachdev Deconfined criticality describes a quantum transition between two ordered states with unrelated symmetry, which is not allowed in the Landau-Ginzburg-Wilson framework [1]. Although many numerical studies have shown evidence for deconfined criticality, it has not yet been observed in experiments. We point out that the conductivity measurements in suspended bilayer graphene [2] could imply the presence of such a transition. The phase transition is between two insulating phases and occurs in a finite magnetic field when we tune the electric field, both fields perpendicular to the graphene plane. We argue that in the strong coupling limit, the effective spin Hamiltonian of bilayer graphene suggests a ``N\'{e}el to a kekul\'{e} valence bond solid'' transition. We also present the study of zero-energy states in the VBS vortex near the critical point.\\[4pt] [1] T. Senthil et al, Science 303, 1490 (2004).\\[0pt] [2] R. T. Weitz et al, Science 330, 812 (2010). [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A46.00014: Magnetic and Kohn-Luttinger instabilities near a Van Hove singularity: monolayer versus twisted bilayer graphene Jose Gonzalez We report on the many-body instabilities of electrons interacting near Van Hove singularities arising in monolayer and twisted bilayer graphene. It is found that a pairing instability must be dominant over the tendency to magnetic order as the Fermi level is tuned to the Van Hove singularity in the conduction band of graphene. As a result of the extended character of the saddle points in the dispersion, the pairing of the electrons takes place preferentially in a channel of f-wave symmetry, with an order parameter vanishing at the position of the saddle points along the Fermi line. In the case of the twisted bilayers, the dispersion has instead its symmetry reduced down to the $C_3v$ group and, most importantly, it leads to susceptibilities that diverge at the saddle points but are integrable along the Fermi line. This implies that a ferromagnetic instability becomes dominant in the twisted graphene bilayers near the Van Hove singularity, with a strength which is amplified as the lowest subband of the electron system becomes flatter for decreasing twist angle. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A46.00015: Tunable magnetic order in bilayer graphene Jin-Hua Sun, Dong-Hui Xu, Yi Zhou, Fu-Chun Zhang Layered antiferromagnetic spin density wave (LAF) state is one of the plausible ground state of charge neutral Bernal stacked bilayer graphene. Additionally, bilayer system offers a freedom of inducing a shift in the electrochemical potential to two graphene layers, which is proposed to induce half-metallicity into the system. In this talk, we will report the theoretical results on the effect of the electric field on the magnetic order in bilayer grapheme by using mean-field theory and determinant quantum Monte Carlo method. In neutral bilayer graphene, the ground state has layered antiferromagnetism at~weak electric field and undergoes a quantum phase transition to a charge order or possibly different type magnetic ordered states at a high electric field. [Preview Abstract] |
Session A47: Superconducting Devices and Applications
Sponsoring Units: DCMPChair: Neda Foroozani, Washington University
Room: Mile High Ballroom 4F
Monday, March 3, 2014 8:00AM - 8:12AM |
A47.00001: Thickness dependence of superconducting properties in NbN thin films Matthew Burton, Douglas Beringer, Melissa Beebe, Elizabeth Visosky, David Brantley, Shaan Sharma, Kaida Yang, Ale Lukaszew Thin film NbN is a promising material currently researched for improvements in superconducting radio frequency (SRF) technology and applications. At present, bulk niobium SRF accelerating cavities suffer from a fundamental upper limit in maximally sustained accelerating gradients; however, a scheme involving multi-layered superstructures consisting of superconducting-insulating-superconducting (SIS) layers has been proposed to overcome this fundamental material limit of 50 MV/m [1]. The SIS multi-layer paradigm is reliant upon implementing a thin shielding material with a suitably high Hc1 which may prevent early field penetration in a bulk material layer and consequently delay the high field breakdown. It has been predicted that for thin superconducting films --- thickness less than the London penetration depth ($\sim$200 nm in the case of NbN) --- the lower critical field Hc1 will be enhanced with decreasing thickness. Thus, NbN thin films with a high Hc1 value are prime candidates for such SIS structures. Here we present our study on the structure and superconducting properties of a series of epitaxial NbN thin films and correlate the effects of film thickness on the lower critical field, Hc1. \\[4pt] [1] A. Gurevich, Appl. Phys. Lett., 88, 012511 (2006). [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A47.00002: Quality factors of plasma resonances in Josephson junction chains Thomas Wei{\ss}l, Bruno K\"{u}ng, \'{E}tienne Dumur, Alexey Feofanov, C\'{e}cile Naud, Olivier Buisson, Wiebke Guichard One dimensional Josephson junction arrays (1DJJA's) have found application in a variety of superconducting circuits. They are used as tunable inductors in microwave resonators, as non-linear elements in parametric amplifiers and large inductors in quantum bits. 1DJJA's show internal resonances that are caused by the finite self capacitance of the superconducting islands. The self-capacitance couples the plasma resonances of the individual junctions leading to plasma modes extended over the entire array[1]. We present microwave measurements of plasma modes of a chain containing 200 squids with $E_j/E_c \approx 10$. Three of the plasma modes can be accessed directly in our experiment. By two-tone spectroscopy we observe the 14 lowest modes[2]. We observe quality factors that are strongly power dependent down to low signal levels. This power dependence is analyzed taking into account the low critical current of our arrays. The signal observed in the two-tone detection is higher than one would expect for a cross-Kerr coupling. We show that this sensitivity is due to the fact that the quality factor of a mode depends on the number of photons in all other modes in the array. [1] N. A. Masluk et al., Phys. Rev. Lett. 109, 137002 (2012). [2] T. Wei{\ss}l et al. (in preparation). [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A47.00003: Ginzburg-Landau modeling of Nano-SQUIDs John Kirtley, Dibyendu Hazra, Klaus Hasselbach, Olivier Buisson NanoSQUIDs are micron-sized Superconducting Quantum Interference Devices with narrow (50 nm) sized constrictions as weak links. They are used for, e.g., studying switching dynamics in magnetic nanoparticles and high spatial resolution magnetic microscopy. When the constriction dimensions become comparable to or larger than the superconducting coherence length, the current-phase relations become non-sinusoidal, reducing the flux modulation depth and increasing the thermally activated flux noise. We have numerically solved the Ginzburg-Landau (GL) equations for the nanoSQUID geometry to obtain current-phase relations, the dependence of critical current on magnetic flux, and the thermally activated escape rates. We predict NanoSQUIDs with short coherence lengths to have critical current distribution widths, and therefore flux noises, proportional to T$^{1/2}$, as opposed to tunnel junction SQUIDs, which are proportional to T$^{2/3}$. Our GL simulations predict that the ultimate noise performance of Al nanoSQUIDs, with their longer coherence lengths, should be better than Nb nanoSQUIDs, with suspended bridge Al/Nb nanoSQUIDs intermediate between the two. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A47.00004: Batch fabrication of nano-SQUIDs for the single spin detection Lei Chen, Xixi Liu, Zhen Wang, Xiaoming Xie The superconducting quantum interference device (SQUID) is well known as a super sensitive spin detector. The minimal detectable spin number of a SQUID is proportional to the size of its superconducting loop. Hence, nano-SQUIDs consisted of two constricted junctions are possible to be scaled down towards the single spin detection. Here, we are going to present our current research progress on a top-down batch fabrication process of NbN nano-SQUIDs. Ultra-thin NbN film of high quality can be grown on MgO substrate epitaxially. The spin sensitivity of such thin film nano-SQUID can be further enhanced by coupling spins directly to the constricted junctions. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A47.00005: A tunable microstrip SQUID amplifier for the Axion Dark Matter eXperiment (ADMX) Sean O'Kelley, Elan Weingarten, Michael Mueck, Jorn Hansen, John Clarke We present a microstrip SQUID amplifier (MSA) with an octave of tunability for use in the ADMX collaboration. The axion dark matter candidate is detected via conversion to a microwave photon stimulated by an apparatus consisting of an 8 tesla magnet and a cryogenically cooled high-Q tunable microwave cavity. The microwave photon frequency is a function of the unknown axion mass, so the detector must scan over a broad frequency range. An MSA is constructed by flux-coupling a resonant microstrip to a resistively-shunted SQUID biased into the voltage state. We demonstrate a gain exceeding 20 dB with a tunability of nearly one octave from 415 MHz to 800 MHz. Tunability is achieved by terminating the microstrip with a low inductance GaAs varactor that operates at cryogenic temperatures, allowing a variable reflected phase of nearly 0 to $\pi $ at the end of the microstrip, and thus a standing wave tunable from nearly $\lambda $/2 to $\lambda $/4. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A47.00006: Fabrication and study of ultrathin MgB$_{2}$ films for hot electron bolometer applications Matthaeus Wolak, Teng Tan, Daniel Cunnane, Boris Karasik, Xiaoxing Xi Hot electron bolometers (HEBs) based on superconducting thin films have already been demonstrated and successfully employed in the past. Magnesium diboride (MgB$_{2}$) has a potential to replace the currently used materials due to its higher critical temperature (39 K) and shorter intrinsic electron-phonon relaxation time. The high $T_{c}$ of MgB$_{2}$ could lead to advanced HEBs with higher operating temperatures, while the short relaxation time could help achieve a higher intermediate frequency bandwidth. In order to fabricate MgB$_{2}$ based HEBs, high-quality thin films with thicknesses of about 10 nm are required. We fabricated ultrathin MgB$_{2}$ films of 10 nm and less in our lab using the hybrid physical chemical vapor deposition (HPCVD) technique. The fabrication, characterization, and feasibility of these films for hot electron bolometer applications will be presented. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A47.00007: Electron heating in superconducting cuprate heterostructures and its application for advanced sensing Andrei Sergeev, Bo Wen, Roman Yakobov, Sergey Vitkalov, Boris Karasik Low electron density in superconducting LaSrCuO heterostructures containing quasi-two dimensional CuO layers leads to strong reduction of the interaction between electrons and thermal phonons and simultaneously to substantial enhancement of the electron-electron interaction. This hierarchy of kinetic processes provides very effective quasiparticle multiplication and slow quasiparticle relaxation and recombination. Strong heating of quasiparticles in the superconducting and resistive states makes these superconducting nanomaterials to be very attractive for various sensing applications based on electron heating. These nanostructures allow for the managing of quasiparticle relaxation rate from low values determined by the electron-phonon relaxation to high values in short devices with out-diffusion electron cooling. Therefore, LSCO heterostructures are very interesting for applications in sensitive resistive detectors, kinetic inductance detectors, and wideband mixers. We experimentally determined key material parameters, design corresponding sensors and evaluated their parameters. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A47.00008: MgB$_{2}$ thin films for SRF application Teng Tan, Matthaeus Wolak, Narendra Acharya, Ke Chen, Xiaoxing Xi Superconducting RF (SRF) cavities are usually fabricated from bulk niobium (Nb), a material which is thoroughly studied and approaching its limits. Magnesium diboride (MgB$_{2})$ has a higher $T_{c}$ of 39 K, a lower residual resistivity of \textless 0.1 $\mu\Omega$ cm (at 42 K), and a higher thermodynamic critical field $H_{c}$ value comparing with Nb. These properties imply that a MgB$_{2}$-coated SRF cavity would work at a higher temperature with a lower energy dissipation. However, the lower critical field $H_{c1}$ of MgB$_{2}$ is low and vortex dissipation above $H_{c1}$ can lead to degradation of the quality factor and a low RF breakdown field. In this work, we report an enhancement of $H_{c1}$ in both c-axis oriented epitaxial and polycrystalline MgB$_{2}$ thin films. The value of $H_{c1}$ (5 K) was increased from 60 mT in a 300 nm-thick MgB$_{2}$ film to 180 mT when the MgB$_{2}$ layer thickness was 100 nm in both cases. The microwave properties of the MgB$_{2}$ films were characterized as well by using the dielectric resonator technique. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A47.00009: Doping optimization for ultra-high quality factor superconducting niobium cavities for particle acceleration Alexander Vostrikov, Alexander Romanenko, Anna Grassellino, Young-Kee Kim Increasing quality factor of the fundamental mode in superconducting radio frequency (SRF) niobium cavities is vital for development of the future particle accelerator facilities, i.e. LCLS-II, Project X, ERLs, and ADS for nuclear energy and waste transmutation, since it directly affects the dissipated power in cavity walls. It has been discovered that doping of certain concentration of nitrogen into the surface of superconducting niobium significantly improves the quality factor of SRF cavities. We report the results of the nitrogen doping optimization guided by diffusion model and present two surface treatment procedures that allow achieving optimal value of nitrogen concentration at the surface of cavity: one with electropolishing required, another one without it. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A47.00010: Tunneling rates of electron pumping in the R-SINIS transistor Vladimir Bubanja We consider the influence of the electromagnetic fluctuations on the transport properties of a hybrid single electron transistor, consisting of superconducting electrodes and a normal-metal island, when operated as a turnstile. We derive the analytic expressions for the rates near the thresholds of single electron tunneling, Andreev reflection, and Cooper-pair--electron cotunneling processes. These results show that the dissipative on-chip impedance suppresses the rates of the undesirable higher-order tunneling processes much stronger than the single electron tunneling which can therefore be utilized to increase the accuracy of such a device in quantum metrological applications. \\[4pt] [1] J.P. Pekola, J.J. Vartiainen, M. M\"{o}tt\"{o}nen, O.-P. Saira, M. Meschke, D.V. Averin, Nat. Phys. 4, 120 (2008).\\[0pt] [2] D.V. Averin, J.P. Pekola, Phys. Rev. Lett. 101, 066801 (2008).\\[0pt] [3] V. Bubanja, Phys. Rev. B 83, 195312 (2011).\\[0pt] [4] V. Bubanja, submitted. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A47.00011: Novel Schemes for Cuprate Nanodevice Fabrication Nick Litombe, Anthony Bollinger, Ivan Bozovic, Jenny Hoffman We have fabricated La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO) nanodevices using high resolution electron beam writing coupled with photolithographic techniques. We characterized the patterned films with electrical transport measurements using a variable temperature cryostat. We report on the search for additional, non-deleterious fabrication schemes to yield pristine and small cross-section cuprate nanodevices. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A47.00012: Superconducting Memristors Massimiliano Di Ventra, Sebastiano Peotta In his original work Josephson [Phys. Lett. 1, 251 (1962)] predicted that a phase-dependent conductance should be present in superconductor tunnel junctions. This effect attracted considerable attention in the past but is difficult to detect, mainly because it is hard to single it out from the background pair current. Here, we propose to isolate it by using a two-junction interferometer where the junctions have the same critical currents but different conductances. The pair current is completely suppressed when the magnetic flux in the loop is half of a flux quantum and the device is characterized by a pure phase-dependent conductance. According to the theory of nonlinear circuit elements this is in fact an ideal voltage-controlled memristor. Possible applications of this memristive device are memories and neuromorphic computing within the framework of ultrafast and low-energy superconducting digital circuits. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A47.00013: A terahertz imaging system using high $T_{c}$ superconducting oscillators fabricated from the Bi2212 single crystals T. Kashiwagi, K. Nakade, Y. Saiwai, H. Minami, T. Kitamura, C. Watanabe, K. Ishida, S. Sekimoto, K. Asanuma, T. Yasui, Y. Shibano, K. Kadowaki, M. Tsujimoto, T. Yamamoto, B. Markovic, J. Mirkovic We have developed a terahertz (THz) oscillator based on high $T_{c}$ superconductor of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$(Bi2212) single crystals\footnote{L. Ozyuzer \textit{et al.}, Science \textbf{318} (2007) 1291.} and have succeeded in developing 30$\mu $W level of output power,\footnote{S. Sekimoto \textit{et al.}, Appl. Phys. Lett. \textbf{130} (2013) 023703.} which is continuous, monochromatic as well as stable at frequencies between 0.3 $\sim$ 1.0 THz.\footnote{T. Kashiwagi \textit{et al.}, Jpn. J. Appl. Phys. \textbf{51} (2012) 010113.} Recently, for the purpose of application use of our THz oscillator, we have developed the reflection type of the imaging system in addition to the transmission imaging system reported previously.\footnote{M. Tsujimoto \textit{et al.}, J. Appl. Phys. \textbf{111} (2012) 123111.} We will show the details of the system and the images obtained here as practical example and compared those with previous results. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A47.00014: SLUG Microwave Amplifiers for Scalable Superconducting Qubit Readout Shaojiang Zhu, David Hover, Guilhem Ribeill, Ted Thorbeck, Robert McDermott We describe a phase-insensitive microwave linear amplifier based on the Superconducting Low-inductance Undulatory Galvanometer (SLUG). The amplifier is well suited to the high fidelity quantum nondemolition measurement of superconducting qubits in a circuit quantum electrodynamics architecture. The amplifier has achieved instantaneous bandwidth greater than 400 MHz and system added noise of order one quantum in the GHz frequency range; moreover, the SLUG -1 dB compression point is around -95 dBm, about two orders of magnitude higher than that achieved with typical Josephson parametric amplifiers. We describe efforts to increase instantaneous bandwidth toward 1 GHz and discuss prospects for simultaneous measurement of multiple superconducting qubits using frequency-domain multiplexing with a broadband SLUG amplifier. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A47.00015: Temperature and Current Bias Dependence of All-MgB$_{2}$ RSFQ Toggle Flip Flop Circuits Elias Galan, Daniel Cunnane, Ke Chen, Xiaoxing Xi We have fabricated and tested Rapid Single Flux Quantum Toggle Flip Flop (TFF) Circuits using self-shunted MgB$_{2}$/MgO/MgB$_{2}$ Josephson Junctions with a single ground layer to reduce parasitic inductance. The MgB$_{2}$ film and electrodes were deposited using HPCVD, and the MgO barrier was deposited using DC reactive sputtering. We highlight the circuits' operation dependence on current bias and temperature which show operation from 8 K to 33 K. The highest attained operating speed is 240 GHz at 10 K. These results demonstrate the versatile temperature range and speed of MgB$_{2}$ based circuits and devices. [Preview Abstract] |
Session A48: Focus Session: Spin-Dependent Phenomena in Semiconductors: Spin Injection and Transport in Semiconductors
Sponsoring Units: GMAG DMP FIAPChair: Hanan Dery, University of Rochester
Room: Mile High Ballroom 1A
Monday, March 3, 2014 8:00AM - 8:36AM |
A48.00001: Spin-pumping-induced spin transport in Si and graphene at room temperature Invited Speaker: Masashi Shiraishi Spin transport in Si is one of the quite significant research targets in semiconductor spintronics, since Si is expected to possess long spin coherence because of its lattice inversion symmetry and spin transistors using Si can be a potential beyond CMOS device. By now, much effort has been paid to realize room temperature spin transport in n-type and p-type Si, however there was no report on it in p-type Si. Here, our recent success on spin transport in p-type Si at room temperature [1] by using spin pumping is presented. Spin pumping is well known as a potential method for spin injection into materials with a large spin-orbit coupling, resulting in successful conversion from a pure spin current to a charge current [2]. Simultaneously, spin pumping is also potential for generating spin-wave spin current in YIG [3]. Now, we used this attractive method for generating a conventional pure spin current and for transporting spin angular momentum in solids [1,4,5]. A number of control experiments for p-Si spin devices corroborated our claim, and the spin coherence at room temperature was estimated to be ca. 120 ps in the simplest model. This method can be used in graphene [4] and Al [5], and they will be also introduced in the presentation. \\[4pt] [1] E. Shikoh, M. Shiraishi et al., Phys. Rev. Lett. 110, 127201 (2013).\\[0pt] [2] E. Saitoh et al., Appl. Phys. Lett. 88, 182509 (2006).\\[0pt] [3] Y. Kajiwara, E. Saitoh et al., Nature 464, 262 (2010).\\[0pt] [4] Z. Tang, M. Shiraishi et al., Phys. Rev. B87, 140401(R) (2013).\\[0pt] [5] Y. Kitamura, M. Shiraishi et al., Sci. Reports 3, 1739 (2013). [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A48.00002: Origin of three-terminal Hanle-type signals in low-temperature ferromagnet-silicon structures with direct Schottky contacts Lan Qing, Hanan Dery, Yuichiro Ando, Shinya Yamada, Kenji Kasahara, Kohei Masaki, Masanobu Miyao, Kohei Hamaya, Kentarou Sawano We analyze three-terminal electrical Hanle-type measurements in CoFe/Si devices. We show that at low temperatures there exists a Lorentzian-like dependence of the voltage signal on external magnetic field that does not correspond to the spin lifetime. The signal stems from spin-dependent scattering of electrons by neutral impurities in the bulk Si channel. The measured signal amplitude is explained by exchange interactions between free (injected) and localized electrons, while the ``Lorentzian" width by exchange between localized electrons on adjacent impurities. The theory reproduces the observed dependencies on temperature and injected current density. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A48.00003: Mobility and spin lifetime enhancement in thin silicon films by shear strain Dmitri Osintsev, Viktor Sverdlov, Siegfried Selberherr We investigate numerically the spin lifetime and mobility enhancement in (001) silicon films. Surface roughness and electron-phonon scattering is taken into account. To find the wave functions and scattering matrix elements we use the ${\mathbf{k\cdot p}}$ Hamiltonian with spin-orbit interaction for the relevant [001] valleys [1]. Knowing the wave functions at the center of the two-dimensional Brillouin zone is sufficient for mobility calculations. When shear strain increases the [110] mobility is enhanced due to the transport mass lowering and the usually ignored wave functions' dependence and the corresponding matrix elements' reduction. For spin relaxation calculations the in-plane momentum dependence of the subband wave functions due to spin-orbit coupling responsible for spin admixture must be preserved. This significantly increases demands for computational resources and requires extensive code parallelization. The spin lifetime is mostly determined by the spin-flip processes between the opposite [001] valleys strongly coupled by the effective spin-orbit interaction. Shear strain mitigates this coupling by lifting the valley degeneracy. This results in a strong increase of the spin lifetime with shear strain.1.P.Li and H.Dery, {\it Phys.Rev.Lett.}{\bf 107}, 107203 (2011). [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A48.00004: Real time electrical detection of coherent spin oscillations in silicon Hans Huebl, Felix Hoehne, Christian Huck, Martin S. Brandt In this presentation we demonstrate that the bandwidth of pulsed electrically detected magnetic resonance (EDMR) can be increased to at least 80 MHz using a radio frequency-reflectometry scheme based on a tank circuit and homodyne detection. Using this technique, we measure Rabi oscillations of phosphorus donors and Si/SiO2 interface states in real time during a resonant microwave pulse. We find that the observed signal is in quantitative agreement with simulations based on rate equations modeling the recombination dynamics of the spin system under study. The increased bandwidth demonstrated opens the way to study faster spin-dependent transport processes and could therefore significantly broaden the range of spin systems studied by EDMR. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A48.00005: Spin Relaxation Theory in Amorphous Silicon and Germanium Nicholas Harmon, Michael E. Flatt\'e Research into spintronic devices using amorphous inorganic semiconductors has seen little attention despite the surge of interest in amorphous organic spintronics. In many ways the two materials are similar - for instance hopping transport is observed in both for certain regimes. Amorphous semiconductors such as silicon and germanium offer advantages such as the ability to greatly reduce and control hyperfine field effects by the process of hydrogenation, and considerably higher mobilities. We present a theory of spin relaxation in amorphous semiconductors based on the theory of a continuous-time random walk, and obtain analytic results in several regimes. We also calculate the spin relaxation with a Monte Carlo simulation. We find that the spin-orbit coupling is the primary limit to long spin lifetimes in amorphous silicon and germanium. The theory we introduce is very general and can also be applied to amorphous organic semiconductors. We compare our results for amorphous inorganic and amorphous organic materials. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 10:00AM |
A48.00006: A graphene solution to conductivity mismatch: spin injection from ferromagnetic metal/graphene tunnel contacts into silicon Invited Speaker: Olaf van 't Erve New paradigms for spin-based devices, such as spin-FETs and reconfigurable logic, have been proposed and modeled. These devices rely on electron spin being injected, transported, manipulated and detected in a semiconductor channel. This work is the first demonstration on how a single layer of graphene can be used as a low resistance tunnel barrier solution for electrical spin injection into Silicon at room temperature. We will show that a FM metal / monolayer graphene contact serves as a spin-polarized tunnel barrier which successfully circumvents the classic metal / semiconductor conductivity mismatch issue for electrical spin injection. We demonstrate electrical injection and detection of spin accumulation in Si above room temperature, and show that the corresponding spin lifetimes correlate with the Si carrier concentration, confirming that the spin accumulation measured occurs in the Si and not in interface trap states. An ideal tunnel barrier should exhibit several key material characteristics: a uniform and planar habit with well-controlled thickness, minimal defect / trapped charge density, a low resistance-area product for minimal power consumption, and compatibility with both the FM metal and semiconductor, insuring minimal diffusion to/from the surrounding materials at temperatures required for device processing. Graphene, offers all of the above, while preserving spin injection properties, making it a compelling solution to the conductivity mismatch for spin injection into Si. Although Graphene is very conductive in plane, it exhibits poor conductivity perpendicular to the plane. Its sp$^{\mathrm{2}}$ bonding results in a highly uniform, defect free layer, which is chemically inert, thermally robust, and essentially impervious to diffusion. The use of a single monolayer of graphene at the Si interface provides a much lower \textit{RA} product than any film of an oxide thick enough to prevent pinholes (1 nm). Our results identify a new route to low resistance-area product spin-polarized contacts, a crucial requirement enabling future semiconductor spintronic devices, which rely upon two-terminal magnetoresistance, including spin-based transistors, logic and memory. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A48.00007: Efficient spin injection in Co$_{2}$Mn$_{x}$Fe$_{1-x}$Si/GaAs heterostructures Kevin Christie, Chad Geppert, Lee Wienkes, Sahil Patel, Chris Palmstr{\O}m, Paul Crowell Several Heusler alloys that are well lattice-matched to the In$_y$Ga$_{1-y}$As family of semiconductors are also candidates for half-metallic ferromagnets. We investigate here their potential for generating near unity spin polarizations in a semiconductor channel. We report on all-electrical measurements of the spin transport properties of epitaxial Co$_{2}$Mn$_{1-x}$Fe$_x$Si / $n$-GaAs heterostructures. The FM/$n$-GaAs interface is degenerately doped to form a narrow Schottky barrier as in previous work on Fe-based devices. The heterostructures were patterned into lateral spin-valve devices, and spin accumulation has been detected at temperatures up to 200 K using both spin-valve and Hanle techniques over a contact separation of 10 $\mu$m. In Co$_2$MnSi devices, a spin splitting of the chemical potential on the order of the Fermi energy (over 2 mV) is observed at 30 K. This is the largest spin accumulation observed to date in a FM/III-V system. We observe a change in sign of the spin accumulation at high Fe concentrations. The connection of this sign inversion to either the bulk of the ferromagnet or interfacial band structure is being investigated. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A48.00008: Electrical Spin Injection and Detection in Mn$_{5}$Ge$_{3}$/Ge/Mn$_{5}$Ge$_{3}$ Nanowire Transistors Jianshi Tang, Chiu-Yen Wang, Kang L. Wang, Lih-Juann Chen We report the electrical spin injection and detection in Ge nanowire transistors with single-crystalline Mn$_{5}$Ge$_{3}$ as the ferromagnetic source/drain contacts. The magnetoresistance (MR) of the Mn$_{5}$Ge$_{3}$/Ge/Mn$_{5}$Ge$_{3}$ nanowire transistor was found to be largely affected by the applied bias. Specifically, negative and hysteretic MR curves were observed under a large current bias from 2 K up to 50 K, clearly indicating successful spin injection into the Ge nanowire. In addition, the MR amplitude was found to exponentially decay with the Ge channel length. The fitting of MR further revealed a spin diffusion length of about 480 nm and a spin lifetime exceeding 244 ps at 10 K in $p$-type Ge nanowires, which are much larger than those reported for bulk $p$-type Ge. Our study of the spin transport in the Ge nanowire transistor points to a possible realization of spin-based transistors, and it may also open up new opportunities to create novel nanowire-based spintronic devices. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A48.00009: Model for the Spin Seebeck Effect in InSb in a Magnetic Field Nicholas Pike, David Stroud The spin Seebeck effect is the generation of a voltage due to spin currents in the presence of a temperature gradient. We have developed a theory for this effect in the semiconductor InSb in a magnetic field. We consider spin-$1/2$ electrons in the conduction band of InSb with a temperature gradient parallel to the applied magnetic field. A Boltzmann equation approach leads to a spin current parallel to the field and proportional to the temperature gradient. The spin-orbit interaction induces a canting of the electronic spin which produces an electric field perpendicular to the temperature gradient via the inverse spin Hall effect. This effect is measured in experiments as the spin Seebeck coefficient [1]. We find that the spin current exhibits oscillations as a function of magnetic field which arise when the Fermi energy crosses the bottom of a Landau band. These oscillations resemble those seen in measurements of the spin Seebeck coefficient in the semiconductor InSb. \\[4pt] [1] C.M. Jaworski, et al. Nature {\bf 487}, 210-213 (2012). [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A48.00010: Spin injection in LaAlO$_{3}$/SrTiO$_{3}$ heterostructures Adrian Swartz, Satoshi Harashima, Yanwu Xie, Bongju Kim, Takashi Tachikawa, Christopher Bell, Yasuyuki Hikita, Harold Hwang There are new device opportunities at the interface of complex oxide heterostructures due to the interplay of charge, orbital, and spin degrees of freedom. A model system is the low dimensional conducting layer generated at the interface of LaAlO$_{3}$ and SrTiO$_{3}$, which has demonstrated high mobilities and tunable carrier densities. However, little has been explored towards employing these high mobility interfaces as spin transport channels. Such conducting interfaces could be practical routes for realizing efficient spin transistors in which spin manipulation functionality could be epitaxially incorporated. First, spin injection, a key requirement of the spin transistor, must be explored. Here, we report our investigations of spin injection into the LaAlO$_{3}$/SrTiO$_{3}$ interface in a three-terminal geometry. Complex oxide films are grown by pulsed laser deposition and patterned into devices through lithography and hard-mask techniques. Using Hanle spin precession, we have observed spin lifetimes in the range of 80 - 100 ps. Notably, the devices exhibit unusual bias dependence in the Hanle signal and high field magnetoresistance. These results provide a building block in the field of oxide-based spintronics. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A48.00011: A differential spin current detection scheme Bryan Hemingway, Ian Appelbaum We offer an alternative scheme to detect spin polarization of conduction electrons injected into a nonmagnetic metal or degeneratively doped semiconductor using transport to two oppositely polarized ferromagnetic metal contacts. We show that, as in the well-known spin injection problem, detection efficiency can be amplified by the addition of spin-selective tunneling barriers. Considering the appropriate geometry and achievable injection rates, we estimate that the differential current can be as high as 1-10?nA for reasonable design parameters. We will also discuss the realization of this detection scheme in laboratory set-ups. [Preview Abstract] |
Session A49: Nanostructures: Interaction Effects
Sponsoring Units: DCMPChair: Michael Scheibner, University of California, Merced
Room: Mile High Ballroom 1C
Monday, March 3, 2014 8:00AM - 8:12AM |
A49.00001: Elementary Electronic Excitations of quantum wells probed by resonant Raman scattering Virgilio Anjos, Alison Arantes, Maria Jose Bell Electron-electron interactions and quantization may be investigated by means of Raman scattering. Its selection rules on the incoming and outgoing light polarizations allows one to study the intersubband charge- and spin-density excitations. The first gives information about the collective charge-density excitations raised by Coulombian interactions. The latter only collective spin-density excitations due to exchange-correlation effects are present. When the incoming laser light matches an optical gap of the host semiconductor, the electron gas presents also excitations whose energies turn out to be close to the bare electronic transitions of the conduction subbands of the semiconductor. By this reason such excitations are called single-particle excitations. In this work, we study the intersubband excitations of modulation-doped GaAs-AlGaAs quantum-wells where the incoming laser light is resonant with the split-off gap of the GaAs. From the theoretical point of view, we show that the intersubband single-particles excitations are actually \textit{coherent} collective excitations and that physically, a direct correspondence between the resonant Raman scattering and the formation of superconducting state in the BCS theory of normal metals exists. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A49.00002: Collective modes in coupled electronic systems of different dimensionalities Ben Yu-Kuang Hu, E.H. Hwang We consider electronic collective modes in coupled systems in which the individual components have different dimensionalities. Many-body diagramnatic techniques are used to derive formal results for the screened intra- and inter-system Coulomb interaction. We evaluate the screened intra- and inter-system Coulomb interaction within the random phase approximation. We investigate the spatial dependence of the coupled 1-d + 2-d collective modes within the two-dimensional electron gas, and show that the coupled modes within that layer vary from being purely two-dimensional in character far away from the quantum wire to being strongly hybridized close to the wire. We existence of modes which have hybrid properties characteristic of both one- and two-dimensional systems. We also find that in certain circumstances, the coupling between the one- and two-dimensional plasmons causes the one-dimensional plasmons modes experience significant damping and essentially disappear. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A49.00003: The edge-state mediated collective transport in a network of quantum dot array W.Y. Wei, K.T. Lin, D.C. Ling, C.C. Chi, J.C. Chen We report the edge-state mediated transport property of a one-dimensional quantum dot array, consisting of six quantum dots defined by the surface gating technique in the two-dimensional electron gas formed at the interface of the GaAs/AlGaAs heterostructure. The conductance G in high magnetic field (B) exhibits a series of dip structures on the last quantized plateau, and a series of Coulomb blockade peaks before the conductance channel is closed by biasing the gate voltage to a sufficiently negative value. The dips in conductance evolve with B and reveal a pronounced charging effect, which are gradually smeared with increasing temperature. The Coulomb blockade diamonds in the differential conductance spectrum show nested features distinctly different from what are observed in conventional quantum dot systems. After careful data analysis and theoretical modeling, our results suggest that our sample, which although meant to be a serial quantum dot array by design, actually behaves like a parallel dot array under certain specific B and gate voltage. A novel collective quantum transport in the network of quantum dot array mediated by the edge states is responsible for the observed phenomena. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A49.00004: Synthetic helical liquid in a quantum wire Mariana Malard, George Japaridze, Henrik Johannesson We show that the combination of a Dresselhaus interaction and a spatially periodic Rashba interaction leads to the formation of a helical liquid in a quantum wire when the electron-electron interaction is weakly screened. The effect is sustained by a helicity-dependent effective band gap which depends on the size of the Dresselhaus and Rashba spin-orbit couplings. We propose a design for a semiconductor device in which the helical liquid can be realized and probed experimentally. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A49.00005: Correlation between Tc and roughness in Nb/Mo superlattices Juan Pereiro, Thomas Saerbeck, Ivan K. Schuller We have studied the superconducting properties of Nb/Mo superlattices grown by RF sputtering at different temperatures. Both Mo and Nb are elemental superconductors, with opposite behavior of the critical temperature as a function of disorder. The critical temperature of Nb decreases as it disorders, while the critical temperature of Molybdenum increases as it becomes amorphous. In superlattices the disorder is imposed by the growth process within each layer and by the period of the structure. We varied the superlattice period between 2 nm and 45 nm and control the intrinsic disorder by the substrate temperature. The samples were characterized by X-Ray reflectivity, X-Ray diffraction, electrical transport, and magnetization measurements. The behavior of the critical temperature as a function of the period shows two different regimes, depending on whether the crystallite size is imposed by the structure or by the growth temperature, $i.e.$ if the grains are larger or smaller than the period of the structure. Furthermore, we will show a correlation between the critical temperature and the interface roughness. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A49.00006: Non-linear Conductance Study of Electron Correlation Effects in Asymmetric Quantum Point Contacts Hao Zhang, Phillip Wu, Albert Chang Both the linear and non-linear(dI/dV) conductance of highly asymmetric quantum point contacts (QPCs) show evidence of quasi-bound states formation \footnote{P. Wu, P. Li, H. Zhang, A. M. Chang, Phy. Rev. B, 85, 085305(2012)} and Kondo-related physics.\footnote{H. Zhang, P. Wu , A. M. Chang, Phy. Rev. B, 88, 075311(2013)} . The non-linear conductance of highly asymmetric QPCs shows additional peaks near zero bias below the first quantized conductance level ($2e^2/h$) at low temperature (down to 25 mK). We have studied the evolution of these extra peaks by tuning the gate voltages at different temperature and different in plane magnetic field. By investigating the evolution of these extra peaks, which can not be fully understood by conventional theory, we explore the possible connections with electron correlation and spin correlated physics. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A49.00007: Interplay between the Kondo, Rashba, and Zeeman effects Arturo Wong, Kevin Ingersent, Nancy Sandler, Sergio Ulloa Motivated by proposed optical experiments on semiconductor nanostructures, we investigate the properties of a magnetic impurity in a two-dimensional electron gas with strong Rashba spin-orbit interactions when the system is subjected to an effective magnetic field $B$ that couples only to the host spins. Even in the absence of spin-orbit coupling, this problem departs from the well-studied Kondo physics in a field that couples to the impurity and possibly also to the conduction band. Through a combination of perturbative and numerical renormalization-group analysis, we show that the effect of the magnetic field can be subsumed into a spin-splitting of the impurity level. The impurity magnetization is found to be a universal function of $\Gamma B / F T_K$, where $\Gamma$ is the hybridization width of the impurity level, $T_K$ is the Kondo temperature in the absence of the field, and $F$ is a function of $E_R$ and of energy scales associated with the impurity. This behavior contrasts with the standard Kondo effect where $T_K$ alone sets the scale for the magnetic-field-induced destruction of the Kondo effect. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A49.00008: Multichannel Numerical Renormalization Group study of the Anderson Hamiltonian with multiple impurities James Stokes, Robert Konik Using the Numerical Renormalization Group (NRG), the low energy sector of the Anderson Hamiltonian with two impurities in parallel has been previously argued to be consistent with an underscreened spin-1 Kondo effect (R. Zitko and J. Bonca, Phys. Rev. B 76, 241305 (2007); Logan et al., Phys. Rev. B 80, 125117 (2009)). Bethe Ansatz and slave boson calculations have given the ground state as a singlet (M. Kulkarni and R. M. Konik, Phys. Rev. B 83, 245121 (2011)). As an attempt to understand these differences, we have developed a modified NRG routine that takes into account the multiple channels arising from the logarithmic discretization of the Fermi sea. This could conceivably allow for more complicated screening processes suggested by the Bethe ansatz computations. Results of studies using this code for various numbers of impurities and channels will be presented and discussed in relationship to these conflicting views. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A49.00009: Decoherence of an entangled states of a strongly-correlated double quantum dot structure through tunneling processes C.A. B\"usser, F. Heidrich-Meisner The entanglement of the spin state of two quantum dots is investigated out of equilibrium. First, we prepare a two-dot system in a perfect singlet state at time $t=0$. For $t>0$, one of the dots is tunnel-coupled to leads, including a finite voltage. Using the time-dependent density matrix renormalization group method, we study the time evolution of the spin correlations and the concurrence as a function of time since electrons hopping on and off the tunnel-coupled dot lead to decoherence. We observe that the spin correlation between the dots decays exponentially determining a decoherence rate. A similar rate can be defined for the concurrence. We study the dependence of these rates on voltage, tunnel coupling, and Coulomb repulsion and compare our numerical results to a master-equation approach derived for the weak-coupling limit. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A49.00010: Discovery of supercoupling between heavy-hole and light-hole in self-assembled quantum dots Jun-Wei Luo, Gabriel Bester, Alex Zunger The mixing of quantum states is a fundamental principle of quantum mechanics. In the case of a diatomic molecule, the eigenstates of atom A mix with the ones of atom B to form molecular orbitals with a mixing inversely proportional to the energy separation between the respective eigenvalues. This fundamental result of quantum mechanics leads to the expectation that states that are well separated in energy will tend to retain their own character and avoid mixing. By studying self-assembled quantum dots, often denoted as ``artificial atoms", we show that heavy-hole (HH) states can significantly mix with light-hole (LH) states, despite the fact that they are energetically well separated, through supercoupling --- a coupling meditated by intermediate states (as in superexchange). This new interband coupling mechanism explains light-hole mixing, which is the key quantity for the use of quantum dots in quantum information and quantum optical schemes, such as for the generation of entangled photon pairs, the decoherence of hole states, the optical polarization anisotropy and the preparation of qubits. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A49.00011: Resolution effects on the current measurement in a resonant level model Yasuhiro Yamada, Masatoshi Imada Current measurements have attracted much attention in studies on understanding the intrinsic information of nanoscale systems. Here, we theoretically study the influence of the smallest detectable change in the measurement, i.e. resolution, on the outcomes of the measurements, using an extension of the full counting statistics for a resonant level model. It is shown that the limited resolution of current measurement gives rise to a positive excess noise, which leads to a violation of the Johnson-Nyquist relation naively expected between the measured conductance and the measured current noise. The deviation from the Johnson-Nyquist relation exhibits universal single-parameter scaling with the nondimensional scaling variable $S_0/S_M$ where $S_0$ is the intrinsic noise and $S_M$ represents the characteristic noise determined from the measurement process. In addition, our findings offer an explanation for anomalous enhancement of noise temperature observed in Johnson noise thermometry. [1] Y. Yamada and M. Imada, arXiv:1307.7535. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A49.00012: Visualization of Percolation Path in a Two-Dimensional Array of Nanoparticle Exhibiting Room Temperature Single Electron Effect Jason Kee Yang Ong, Jennifer Kane, Ravi Saraf The conductance of a two-dimensional (2D) metal nanoparticle array is sensitive to local charging of nanoparticles at a single electron level, which leads to a threshold bias, VT caused by Coulomb blockade along the percolation path. As a result, the current flowing through the array of nanoparticles does not obey Ohm's-law. Generally, cryogenic temperatures are required to observe a robust VT. It is theorized that the charge centers posing the Coulomb blockade on the percolation path are fixed and independent of external bias. With the self-assembly of 1D nanoparticle necklaces into 2D array, it was possible to observe VT at room temperature due to the high topological constraint on the percolation path. Along with the success of nanofabrication of nanoparticle necklace array on a substrate using a combined technique of soft-lithography and electron beam writing, a single electron device with a single percolation pathway was tailored where the ``opening'' of the conduction path was directly visualized using field-emission Scanning Electron Microscopy (FESEM) as the Coulomb blockade due to the quenched charge distribution was progressively overcome. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A49.00013: Effect of Substrate Annealing and Seeding on ZnO Nanowires Synthesized Using a Hydrothermal Method Orlando Lopez, Ashley Tucker, Kimberly Singh, Spencer Mamer, Huizhong Xu ZnO nanowires have been extensively studied due to their remarkable mechanical, thermodynamic, electrical and optical properties. Amongst the various ZnO nanowire synthesis methods, the hydrothermal growth method is quite attractive due to its simplicity and tolerable growth conditions. In this work, we apply the hydrothermal method to grow ZnO nanowires on gold-coated glass substrates and study how different pre-growth treatment of the substrates affects the morphology, distribution, and dimensions of the ZnO nanowires. We have found that pre-growth annealing of the substrate at temperatures above 250 $^{\circ}$C is required to have vertically aligned nanowires. Our results have also revealed that the nanowire density and dimension are dramatically different for substrates pre-seeded with ZnO nanoparticles and unseeded substrates. The ZnO nanoparticle seeds play an important role in providing nucleation sites that are much smaller than the critical size of precipitation out of the solution, resulting in nanowires of smaller dimensions for pre-seeded substrates. The dependence of the nanowire dimensions on the precursor concentration for both pre-seed and unseeded samples is also studied and discussed. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A49.00014: Eshelby Twist and Magic Helical Zinc Oxide Nanowires and Nanotubes Traian Dumitrica, Evgeniya Akatyeva Twisted zinc oxide nanowires and nanotubes were recently synthesized by screw-dislocation growth. We show theoretically that once their diameter increases above a critical size of the order of a few atomic spacings, the existence of these structures can be rationalized in terms of the energetics of surfaces and veritable Eshelby's twist linear elasticity mechanics supplemented by a nonlinear core term. For Burgers vector larger than the minimum allowed one, a twisted nanotube with well-defined thickness, rather than a nanowire, is the most stable nanostructure. Results are assistive for designing ultrathin nanostructures made out of nonlayered materials [1]. \\[4pt] [1] E. Akatyeva and T. Dumitrica, Phys. Rev. Lett. 109, 035501 (2012). [Preview Abstract] |
Session A50: Quantum Dots and Nanocrystals: Optical and Electronic Properties
Sponsoring Units: DCMPChair: Sam Carter, U.S. Naval Research Laboratory
Room: Mile High Ballroom 1D
Monday, March 3, 2014 8:00AM - 8:12AM |
A50.00001: Photoluminescence in arrays of doped semiconductor nanocrystals Tianran Chen, Konstantin Reich, Alexander Efros, Boris Shklovskii We study dependence of the quantum yield of photoluminescence of a dense, periodic array of semiconductor nanocrystals (NCs) on the level of doping and NC size. Electrons introduced to NCs via doping quench photoluminescence by Auger process, so that practically only NCs without electrons contribute to the photoluminescence. Computer simulation and analytical theory are used to find a fraction of such empty NCs as a function of the average number of donors per NC and NC size. For an array of small spherical NCs, the quantization gap between 1S and 1P levels leads to transfer of electrons from NCs with large number of donors to those without donors. As a result, empty NCs get extinct and photoluminescence is quenched abruptly at the average number of donors per NC close to 1.8. The relative intensity of photoluminescence is shown to correlate with the type of hopping conductivity of an array of NCs. Ref: http://arxiv.org/abs/1310.0849 [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A50.00002: Millikelvin magneto-photoluminescence of isoelectronic bound excitons in type-II quantum dot superlattices Igor Kuskovsky, Haojie Ji, Siddharth Dhomkar, Jonathan Ludwig, Dmitry Smirnov, Maria Tamargo Photoluminescence (PL) spectrum of Zn-Se-Te multilayer system grown via migration enhanced epitaxy with submonolayer insertion of Te, has been reported to demonstrate coexistence of the isoelectronic centers along with the type-II quantum dots (Gu, \textit{et al.}, Phys. Rev. B \textbf{71}, 045340 (2005)). Spectrally, the band edge emission, originating from the isoelectronic bound excitons (IBE), is observed as characteristic `sharp lines' and their phonon replicas, whereas the low energy side is dominated by spatially indirect, type-II excitons. The latter exhibit robust Aharanov-Bohm oscillations in the intensity of the magneto-PL up to 30 K, while no such effect was expected for the IBEs. Here we report a high resolution spectral analysis of the magneto-PL spectra of various samples with relatively low Te content measured at millikelvin temperatures. The analysis reveals additional features in magneto-PL intensity at specific magnetic fields that appear only in the spectral region dominated by the `sharp lines'. Although the precise origin of these distinctive peaks is still unknown, they are thought to be arising due to 2-dimensional confinement of IBEs. Supported by the National Science Foundation under Award No. DMR-1006050 [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A50.00003: Control over density of submonolayer type-II ZnTe/ZnSe quantum dots grown via migration enhanced epitaxy Siddharth Dhomkar, Haojie Ji, Bidisha Roy, Igor L. Kuskovsky, Alice Wang, Maria C. Tamargo For practical applications of self-assembled semiconductor quantum dots (QDs), it is important to control their density, distribution and size; parameters that remain essentially elusive in case of submonolayer QDs. Such QDs grown via migration enhanced epitaxy (MEE), form without the formation of wetting layers, an advantageous feature for practical applications. Here we present a study of submonolayer type-II ZnTe/ZnSe QDs by combining a series of characterization tools, to obtain precise estimates of the dot densities, and their size. Type-II ZnTe/ZnSe QDs are particularly interesting because of their relatively large valence and conduction band offsets which can be utilized to tune optical and electrical properties in unique ways. Specifically, we have employed low temperature photoluminescence to estimate QD thicknesses while QD radius has been accurately determined via excitonic Aharonov-Bohm effect. Secondary ion mass spectroscopy has been employed to obtain average Te concentration, which was used to calculate QD density. The results demonstrate that the QD density can be controlled relatively precisely by varying Te flux and number of MEE Te cycles. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A50.00004: Magneto-photoluminescence studies of optical Aharonov-Bohm effect in type-II ZnTe/ZnSe semiconductor heterstructure Y.H. Chang, C.H. Hsu, C.C. Huang, W.C. Chou, Y.W. Suen Although the absolute phase of a quantum state is not measurable, the relative phase of a coherent charged particle wave could be manipulated. Recently, the effect of the magnetic flux on the excitonic energy has received much attention. In this talk we'll present our magneto-photoluminescence studies on the optical properties of type-II ZnTe/ZnSe self-assemble QDs system. The ZnTe/ZnSe samples were grown by molecular beam epitaxy with ZnSe layer grown on the GaAs substrate first and then ZnTe was grown on top of ZnSe. The ZnTe layers used in this study has thickness of 2.0, and 2.5 monolayers, respectively. Magneto-photoluminescence experiment was performed at T$=$1.4 K with a 14 T superconducting magnet in conjunction with a 405-nm diode laser and a monochromator. Sharp and clean emission peaks in magneto-PL spectra was observed and oscillation on the peak energy of the photoluminescence spectra as a function of magnetic field were observed for both the 2.0 ML and 2.5 ML samples and are attributed to the optical Aharonov-Bohm effect. The AB period changes from 9T for 2.0ML sample to about 6T for 2.5ML sample, i.e., we observed different AB periods for samples with different quantum dot size of the same system. In addition, the effect of impurity and defect on the AB oscillation will also be discussed. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A50.00005: Optical Anisotropy in Type-II ZnTe/ZnSe Submonolayer Quantum Dots Haojie Ji, Siddharth Dhomkar, Maria Tamargo, Igor Kuskovsky Type-II semiconductor quantum dots (QDs) characterized by spatial separation of charge carriers are good candidates for photovoltaics and photon manipulation applications. Implementation of practical devices requires detail understandings of the QD morphology, the mechanism of strain relief and defect formation. Here we report our study of polarization dependent photoluminescence (PL) in type-II ZnTe/ZnSe submonolayer QD superlattices, grown by migration-enhanced epitaxy. We show that the PL does not depend on the polarization state of excitation and exhibits strong linear polarization, indicating strong anisotropy in this material. We spectrally analyze the degree of linear polarization in samples grown with different Te fluxes, spacer thicknesses and number of periods. Based on our observations, we propose several reasons for the optical anisotropy, focusing on the anisotropic shape of the QDs and the anisotropy at the interfaces in the superlattices. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A50.00006: Statistical Properties of Exciton Fine Structure Splittings and Polarization Angles in Quantum Dot Ensembles Ming Gong, B. Hofer, E. Zallo, R. Trotta, Junwei Luo, Alex Zunger, O.G. Schmidt, Chuanwei Zhang We propose an effective model to describe the statistical properties of exciton fine structure splitting (FSS) and polarization angle of quantum dot ensembles (QDEs). We derive the distributions of FSS and polarization angle for QDEs and show that their statistical features can be fully characterized using at most three independent measurable parameters. The effective model is confirmed using atomistic pseudopotential calculations as well as experimental measurements for several rather different QDEs. The model naturally addresses three fundamental questions that are frequently encountered in theories and experiments. The answers to these fundamental questions yield a completely new physical picture for understanding optical properties of QDEs. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A50.00007: Sources of optical transition broadening in room temperature CdSe/ZnS nanocrystal quantum dots Michael Wolf, Jesse Berezovsky To understand the origins of optical transition broadening in CdSe/ZnS nanocrystal quantum dots (NCQDs) at room temperature, we study the photoluminescence excitation (PLE) spectra of individual NCQDs. The PLE spectra from single NCQDs reveal broadening of the optical transitions and variations of the transition energies between NCQDs. The observed features in the spectra are identified by comparison to transition energies calculated using an 8-band effective mass model. We attribute the broadening to three effects: phonon broadening, spectral diffusion, and size inhomogeneity. The first two mechanisms contribute to the broadening of transitions in single NCQDs. The third mechanism contributes to ensemble broadening. The broadening caused by spectral diffusion and size inhomogeneity both depend on the sensitivity of each transition to variations in the confining potential, leading to linewidths that depend on the particular electron and hole states involved in the transition. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A50.00008: Controlling Auger Decay Rates of CdSe/CdS Nanocrystals via Core/Shell Interfacial Alloying Young-Shin Park, Wan Ki Bae, Lazaro Padilha, Jeffrey Pietryga, Victor Klimov We report single-dot spectroscopic studies to evaluate the effect of the core/shell interface (i.e., the shape of the confinement potential) on nonradiative Auger decay rates of CdSe/CdS quantum dots (QDs) that have either a sharp or a graded interface. Alloyed QDs with a graded potential are prepared by incorporating a CdSe$_{\mathrm{x}}$S$_{\mathrm{1-x}}$ alloy layer of a controlled composition and thickness between the core and the shell. In second-order intensity correlation (g$^{\mathrm{(2)}})$ measurements, we observed that the interfacial layer has a negligible effect on single-exciton dynamics, but leads to a systematic increase in the biexciton photoluminescence quantum yield ($Q_{\mathrm{BX}})$. We found that $Q_{\mathrm{BX}}$ of alloyed QDs can be up to $\sim$ 10 times higher than that of the reference QDs with a sharp interface. These results are further supported by independent measurements of biexciton dynamics that show a considerable elongation of biexciton lifetimes (to several ns) upon interfacial alloying. Finally, a statistical investigation of over 100 individual QDs shows that the CdS shell thickness has only a minor effect on $Q_{\mathrm{BX}}$. All of these findings point a significant role of the shape of the confinement potential in Auger recombination and should facilitate the development of ``Auger-recombination-free'' QDs. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A50.00009: Entangled Photon Pairs via Two-Photon Transitions in a Quantum Dot Molecule Cameron Jennings, Andrew Jacobs, Michael Scheibner We theoretically investigate the use of tunnel-coupled quantum dots as a source of entangled photon pairs. By preparing the system in a molecular biexciton state [a], with the charges of one exciton in separate dots, the anisotropic electron-hole exchange splitting is greatly reduced for the first transition in the resulting radiative biexciton cascade. While the splitting returns for the second (intradot) transition, polarization-entangled photon pairs can still be recovered [b]. The fidelity of such a process depends crucially on the excitation conditions; we consider various scenarios, from non-resonant incoherent to coherent two-photon excitation of the biexciton state. We simulate two-photon processes in this system and determine optimal parameters for entangled photon generation experimentally accessible by electrical control of the energy level structure in quantum dot molecules. [a] Scheibner et al., Phys. Rev. Lett. 99, 197402 (2007) [b] Scheibner et al., J. Opt. Soc. Am. B 29, A82 (2012) [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A50.00010: 0D- to 2D-transition of electronic states in SK quantum dots Cyprian Czarnocki, Mark Kerfoot, Allan Bracker, Daniel Gammon, Michael Scheibner Level anti-crossing spectroscopy (LACS) has demonstrated the capabilities of mapping the discrete electronic states of a nearby quantum dot by sequentially tunnel coupling another dot's ground state to the energy level ladder of the quantum dot of interest [1]. Here we expand this method and identify the dot to wetting layer transition, i.e., the transition from zero- to two-dimensional states. We find a wealth of states at elevated energies. The identification of these states takes into account the charge and spin configuration of the involved optical transitions. Thereby we identify cross-talk channels between quantum dot molecule states. We anticipate our findings to provide insights for the lateral coupling of neighboring dots and dot molecules through extended 2-D like states. \\[4pt] [1] M. Scheibner, M. Yakes, A. S. Bracker, I. V. Ponomarev, M. F. Doty, C. S. Hellberg, L. J. Whitman, T. L. Reinecke, D. Gammon, \textit{Optically mapping the electronic structure of coupled quantum dots}, Nature Phys. Lett. \textbf{4}, 291-295 (2008). [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A50.00011: Group velocity slowdown using phonon-induced transparencies in a quantum dot molecule Andrew Jacobs, Cameron Jennings, Mark Kerfoot, Michael Scheibner In a recent study we have demonstrated coherent, non-dissipative behavior of phonons due to optical excitation, which is revealed via optical transparency [1]. Using a single external driving field, the absorption of the molecule demonstrates a marked reduction as a Fano-type resonance of a spatially indirect exciton and direct polaron form a molecular polaron state. The phonon coherence contrasts the typical role of these particles as a channel for non-radiative state decay or pure state dephasing. The optical response of the system is indicative of a coherent phenomenon, similar to electromagnetically induced transparency. Here we investigate theoretically how this phonon coherence affects the optical response of a 3-level V-type system in a tunnel-coupled quantum dot molecule. From the properties of the molecular polaron, we are able to determine the slowdown factor of the driving field group velocity, as well as the dependence of the slowdown on system parameters such as polaron and exciton lifetimes, tunneling strength, and transition dipole moments. The presence of slow light suggests this system is suitable for use in quantum computational components such as optical storage or qubit logic gates. \\[4pt] [1] M. Kerfoot et. al. ``Optophononics with coupled quantum dots'' (submitted) [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A50.00012: Exchange interaction and the tunneling induced transparency in coupled quantum dots Halyne Borges, Augusto Alcalde, Sergio Ulloa Stacked semiconductor quantum dots coupled by tunneling are unique ``quantum molecule'' where it is possible to create a multilevel structure of excitonic states. This structure allows the investigation of quantum interference processes and their control via electric external fields. In this work, we investigate the optical response of a quantum molecule coherently driven by a polarized laser, considering the splitting in excitonic levels caused by isotropic and anisotropic exchange interactions. In our model we consider interdot transitions mediated by the the hole tunneling between states with the same total spin and, between bright and dark exciton states. Using realistic experimental parameters, we demonstrate that the excitonic states coupled by tunneling exhibit an enriched and controllable optical response. Our results show that through the appropriate control of the external electric field and light polarization, the tunneling coupling establishes an efficient destructive quantum interference path that creates a transparency window in the absorption spectra, whenever states of appropriate symmetry are mixed by the hole tunneling. We explore the relevant parameters space that would allows with the experiments. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A50.00013: Electronic states in InGaAs/GaAs asymmetric quantum rings Vivaldo Lopes-Oliveira, Victor Lopez-Richard, Sergio Ulloa Semiconductor quantum rings (QRs) have attracted a great deal of attention due to the interesting multiply-connected geometry they provide for charge carriers. The observation of Aharonov-Bohm [1] effects on the excitonic response in this geometry and the appearance of localized defects during the growth processes call for theoretical studies of the impact of defects on the optical response in magnetic fields. We present here systematic studies of asymmetry in QRs under magnetic fields within a k.p formalism. Different kinds of perturbations are studied, as the model used allows modulation of confinement and perturbation strength. The angular momentum hybridization is characterized for different field intensity and confinement/defect shape. The coupling of unperturbed and symmetric states defines the potential appearance of crossings and new anticrossings in the electronic structure as function of field and structural parameters. The electronic orbitals are contrasted with unperturbed states and the effects on the optical response for inter-subband transitions are discussed as signatures of symmetry breakings.\\[4pt] [1] M.D.Teodoro et al. PRL 104, 086401 (2010). [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A50.00014: Photoconductivity and free-carrier dynamics of silicon quantum dots Matthew Bergren, Peter Palomaki, Nathan Neale, Thomas Furtak, Matthew Beard Silicon quantum dots (QDs) have recently been investigated for use in novel optoelectronic devices such as LEDs and PV. Plasma synthesized SiQDs offer very good control of SiQD size, crystallinity and size distribution. These dots have reported PLQYs \textgreater 60{\%}, making them excellent candidates for LEDs or biomarkers. In PV applications, charges need to be extracted from films, while for LEDs charges are injected into the film, and thus information about their electrical properties is desired. Time-resolved terahertz spectroscopy is uniquely suited to investigate photoinduced charge generation and transport in nanoscale systems in that it can measure the sample's photoconductivity with sub-ps resolution. In this study, we present the complex-frequency dependent photoconductivity of isolated SiQDs for a range of diameters. The ultrafast dynamics show a fast decay followed by a longer lifetime. We attribute the rapid decay to hot-carriers relaxing to bound excitons within the first ps after excitation for the isolated dots. A comparison is made between the isolated dots and thin films composed of the same material, illustrating the importance of QD-QD electronic coupling to achieve charge transport in these films. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A50.00015: Carrier recombination in mid-wave infrared InAs/InAsSb superlattices Yigit Aytac, Benjamin Varberg Olson, Jin K. Kim, Eric A. Shaner, Sam D. Hawkins, John F. Klem, Michael E. Flatt\'e, Thomas F. Boggess Measurements of carrier recombination rates using a temperature-dependent time-resolved differential transmission technique are reported for mid-wave infrared $InAs/InAs_{1-x}Sb_{x}$ type-2 superlattices (T2SLs). By engineering the layer widths and antimony compositions a 16K band-gap of $\sim$ 238 meV was achieved for all five unintentionally doped T2SLs. Carrier recombination rates were determined for all five samples by fitting a rate equation model to the density and temperature dependent data. Minority-carrier lifetimes as long as 22$\mu s$ were measured at 14K, while lifetimes in excess of 2$\mu s$ were measured for all five samples at 200K. The minority-carrier lifetimes were observed to generally increase with increasing antimony content. While minority-carrier lifetimes are much longer than those observed in InAs/Ga(In)Sb T2SLs, Auger recombination processes were found to be more prominent in the Ga-free T2SLs. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under Contract No. DE-AC04-94AL85000. This research was funded by the U.S. Government. [Preview Abstract] |
Session A51: Focus Session: Beyond Graphene: Synthesis, Defects, Structure, and Properties I
Sponsoring Units: DMPChair: Linyou Cao, North Carolina State University
Room: Mile High Ballroom 1E
Monday, March 3, 2014 8:00AM - 8:36AM |
A51.00001: Exploring the Flatlands: Synthesis, Characterization and Engineering of Two-Dimensional Materials Invited Speaker: Jun Lou In this talk, we report the controlled vapor phase synthesis of MoS$_{2}$ atomic layers and elucidate a fundamental mechanism for the nucleation, growth, and grain boundary formation in its crystalline monolayers. The atomic structure and morphology of the grains and their boundaries in the polycrystalline molybdenum disulfide atomic layers are examined and first-principles calculations are applied to investigate their energy landscape. The electrical properties of the atomic layers are examined and the role of grain boundaries is evaluated. More importantly, if precise two-dimensional domains of graphene, h-BN and MoS$_{2}$ atomic layers can be seamlessly stitched together, in-plane heterostructures with interesting electronic applications could potentially be created. Here, we show that planar graphene/h-BN heterostructures can be formed by growing graphene in lithographically-patterned h-BN atomic layers. Our approach can create periodic arrangements of domains with size that ranging from tens of nanometers to millimeters. The resulting graphene/h-BN atomic layers can be peeled off from their growth substrate and transferred to various platforms including flexible substrate. Finally, we demonstrate how self-assembled monolayers with a variety of end termination chemistries can be utilized to tailor the physical properties of single-crystalline MoS$_{2}$ atomic-layers. Our data suggests that combined interface-related effects of charge transfer, built-in molecular polarities, varied densities of defects, and remote interfacial phonons strongly modify the electrical and optical properties of MoS$_{2}$, illustrating an engineering approach for local and universal property modulations in two-dimensional atomic-layers. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A51.00002: Molecular Beam Epitaxy of Layered Material Superlattices and Heterostructures Suresh Vishwanath, Xinyu Liu, Sergei Rouvimov, Jacek K. Furdyna, Debdeep Jena, Huili Grace Xing Stacking of various layered materials is being pursued widely to realize various devices and observe novel physics. Mostly, these have been limited to exfoliation and stacking either manually or in solution, where control on rotational alignment or order of stacking is lost. We have demonstrated molecular beam epitaxy (MBE) growth of Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/MoSe$_{\mathrm{2}}$ superlattice and Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$/MoSe$_{\mathrm{2}}$/SnSe$_{\mathrm{2}}$ heterostructure on sapphire. We have achieved a better control on the order of stacking and number of layers as compared to the solution technique. We have characterized these structures using RHEED, Raman spectroscopy, XPS, AFM, X-ray reflectometry, cross-section (cs) and in-plane (ip) TEM. The rotational alignment is dictated by thermodynamics and is understood using ip-TEM diffraction patterns. Layered growth and long range order is evident from the streaky RHEED pattern. Abrupt change in RHEED pattern, clear demarcation of boundary between layers seen using cs-TEM and observation of Raman peaks corresponding to all the layers suggest van-der-waals epitaxy. In our knowledge this is a first demonstration of as grown superlattices and heterostuctures involving transition metal dichalcogenides and is an important step towards the goal of stacking of 2D crystals like lego blocks. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A51.00003: Synthesis of large-scale MoS2 films using gas phase precursor Jinhwan Lee, Youngbin Lee, Jeong Ho Cho, Jong-Hyun Ahn, Changgu Lee We present layer-controlled synthesis of large-scale and uniform molybdenum disulfide films on insulating substrates. For the synthesis, we used gas phase sulfuric precursor and molybdenum metal source. By controlling the deposition thickness of the metal, we could vary the synthesized film thickness in the precision of number of layers. From the synthesis, 2, 4, 8, 12 layers were grown on 2-in scale Si/SiO2 and quartz substrates up to 8 cm with almost perfect uniformity over the entire area. Also on one substrate, films with different thicknesses were grown in separate areas with layer (or atomic)-level uniformity. AFM, TEM, XPS, and optical spectroscopy characterizations show that the films have high crystalline quality without a sigh of amorphous phase. The films synthesized on quartz substrate were transparent and the transparency depended linearly on the film thickness. We also fabricated arrays of field effect transistor device for electrical characterization. 90{\%} among the devices operated with functionality and the measured mobility was on the level of 0.1 cm$^{2}$/Vs and on/off ratio was 10$^{5}$. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A51.00004: Controlled physical vapor growth of WSe$_{2}$ and other MX$_{2}$ monolayers Paul Nguyen, Joe Finney, Chunming Huang, Pasqual Rivera, Sanfeng Wu, Genevieve Clark, Zaiyao Fei, Xiaodong Xu, David Cobden Although exfoliated monolayers of two-dimensional semiconductors such as WSe$_{2}$ show extraordinary and potentially useful optical properties, the ability to grow them in a controlled way will be critical for tuning their properties, incorporating dopants, and making devices on larger scales and with high yield. We are investigating their growth by physical vapor deposition on insulators such as silicon dioxide without catalyst, systematically varying the growth parameters (gas flow and type, sources, temperature, and substrate), with a focus on WSe$_{2}$ which has the smallest gap and strongest spin-orbit coupling of the MX$_{2}$s. While MoS$_{2}$ monolayers of high optical quality can easily be grown as triangular single crystals tens of microns in size using a simple MoS$_{2}$ source, WSe$_{2}$ proves to be much more sensitive to the growth parameters, as well as to air leaks and contamination of the furnace tube. Nevertheless we have reproducibly grown monolayer WSe$_{2}$ crystals up to 15 microns in size showing excellent optical properties using a WSe$_{2}$ source and pure hydrogen carrier gas. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A51.00005: Growth of Molybdenum disulfide by Mo foil and ammonium heptamolybdate Carl Naylor, Ganghee Han, Nicholas Kybert, Charlie Johnson Molybdenum disulfide (MoS$_{\mathrm{2}})$ is one of the latest semiconducting materials to show huge attention, due to its tunable band gap by controlling the number of layers and reasonable values of mobility. Indeed, its astonishing electrical properties combined with having a high on/off ratio for field effect transistors that is difficult to reach with graphene, make MoS$_{\mathrm{2}}$ a promising material for nanosensing and many other applications. Here we introduce two different growth techniques for MoS$_{\mathrm{2}}$. Molybdenum foil and ammonium heptamolybdate were used as molybdenum feedstock while we sublimated sulfur source from solid for both techniques. Crystallinity of MoS$_{\mathrm{2}}$ from both techniques was checked by optical microscope, atomic force microscope, Raman spectroscopy and Transmission electron microscopy. We believe that our techniques would be facile routes for MoS$_{\mathrm{2}}$ growth. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A51.00006: ABSTRACT WITHDRAWN |
Monday, March 3, 2014 9:36AM - 9:48AM |
A51.00007: Single-Crystal Growth and Optical Characterization of Large-Area Monolayer WS$_{2}$ Ting Yu Nowadays, two-dimensional (2D) transition metal dichalcogenides (TMDs) have caused a great deal of interest in view of their unique properties and potential novel applications. Beyond graphene, monolayers of WS$_{2}$ with a direct band gap are attracting for developing 2D field-effect transistor and visible light-emitting devices. Till now, controllable synthesis of high-quality single-crystal WS$_{2}$ monolayer is still very challenging but highly demanded. In this study, we have successfully grown large-area single-crystal monolayer WS$_{2}$ by use of a modified chemical vapor deposition system. Particularly, the photoluminescence (PL) of WS$_{2}$ has been investigated. Valley-selective circular dichroism and intense red emission demonstrate high quality of as-grown WS$_{2}$ monolayers. Besides, uniform and non-uniform PL distributions over various samples were analyzed to identify the intrinsic emission characters. The PL weakening and its blue shift are attributed to the as-grown structural defects and the defect-induced n-doping. The present work paves the pathway to prepare large-scale single-crystal 2D TMDs and highlights the promising optical performance of WS$_{2}$ for future optoelectronics. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A51.00008: Chemically-exfoliated single-layer MoS$_2$: stability, lattice dynamics and catalytic adsorption from first principles Matteo Calandra Chemically and mechanically exfoliated MoS$_2$ single-layer samples have substantially different properties. While mechanically exfoliated single-layers are mono-phase ( 1H polytype with Mo in trigonal prismatic coordination), the chemically exfoliated samples show coexistence of three different phases, 1H, 1T (Mo in octahedral coordination) and 1T$^{'}$ (a distorted $2\times 1$ 1T-superstructure). By using first-principles calculations, we investigate the energetics and the dynamical stability of the three phases. We show that the 1H phase is the most stable one, while the metallic 1T phase, strongly unstable, undergoes a phase transition towards a metastable and insulating 1T$^{'}$ structure composed of separated zig-zag chains. We calculate electronic structure, phonon dispersion, Raman frequencies and intensities for the 1T$^{'}$ structure. We provide a microscopical description of the J$_1$, J$_2$ and J$_3$ Raman features first detected more then $20$ years ago, but unexplained up to now. Finally, we show that H adsorbates, that are naturally present at the end of the chemical exfoliation process, stabilize the 1T$^{\prime}$ over the 1H one. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A51.00009: Fabrication of Atomically Layered Material Heterostructures of WSe2 and hBN Yafang Yang, Hugh Churchill, Britt Baugher, Javier Sanchez-Yamagishi, Pablo Jarillo-Herrero We discuss fabrication methods for hBN-WSe2-hBN heterostructures designed to create high quality and high mobility monolayer WSe2 devices by encapsulating the WSe2 in a relatively clean and impurity-free environment.~ We use a release polymer to pick up hBN and WSe2 from a SiO2 substrate, and transfer the stack onto another pre-cleaned hBN flake. In this way the WSe2 channel is protected from resist residue by hBN above and below, and thus stays pristine and clean.~ Various fabrication strategies will be discussed, including a comparison of MMA and PPC as release polymers.~ We characterize the performance of these devices with electrical transport measurements.~ [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A51.00010: Large-area growth of molybdenum disulphide monolayers for integrated photonics Y. Jia, T. Stanev, E. Lenferink, N.P. Stern Electronic devices based on single/few-layer transition-metal dichacogenide semiconductors heavily rely on mechanically exfoliated micro-flakes. Uncontrollable position and dimension are significant obstacles to integration of electronics and photonics using these layered two-dimensional materials. In this report, we grow continuous few-layer MoS$_2$ film on SiO$_2$/Si wafers using a cost-effective solution process and thermal decomposition. The number of the layers can be controlled by the spin-coating rate of the solution. Multi-layers can be controllably reduced layer-by-layer using an Ar-plasma etch. Compared with chemical vapor depositions which usually require temperature of 600-900 C, the low temperature of 450 C used here offers more flexibility in MoS$_2$ direct growth on other materials such as flexible plastic substrates. The good crystalline quality over area of $50 \times 50$ $\mu$m$^2$ and the controlled layer thickness enable broad applications of 2D semiconductor films to realizing integrated photonic devices. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A51.00011: Synthesis and Characterization of Liquid Phase Exfoliated Tungsten Disulphide (WS$_{2}$) Flakes Milinda Wasala, Sujoy Ghosh, Andrew Winchester, Logan Moore, Barbara Nichols, Madan Dubey, Saikat Talapatra We report on the synthesis of 2D thin flakes of WS$_{2}$ obtained from liquid phase exfoliation of their bulk powder. Temperature dependent conductivity measurement as well as photo response of thin films prepared from these flakes will be presented. Our preliminary data, studied with in the temperature range 320K \textless\ T \textless\ 25, indicates that under a constant laser powers of wavelength $=$ 658 nm, photocurrent (I$_{\mathrm{ph}}$) decrease with decreasing temperature and becomes temperature independent at low temperatures. Further, it was found that I$_{\mathrm{ph}}$ $\sim$ (laser intensity)$^{\gamma}$ with 0.5 \textless\ $\gamma $ \textless\ 1.0. These findings will be discussed under various available models related to photoconductivity in semiconductors. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A51.00012: Stability of MoS$_{\mathrm{2(1-x)}}$Se$_{\mathrm{2x}}$ alloy: insight from first-principles calculations Duy Le, Talat S. Rahman Two-dimensional transition metal dichalcogenide (TMD) alloy is an interesting class of TMD because the ability to tune continuously its bandgap opens many new possibilities for basic studies, device concepts, and fabrication of novel heterostructures. We will present phonon dispersions of MoS$_{\mathrm{2(1-x)}}$Se$_{\mathrm{2x}}$ alloy calculated using Density Functional Perturbation Theory (DFPT). The dispersions do not show any existence of instability modes for various Se concentrations ($x)$, attesting the stability of this TMD alloy. We will, in addition, show electronic and optical properties, including Raman spectra, of MoS$_{\mathrm{2(1-x)}}$Se$_{\mathrm{2x}}$ alloy as a function of Se concentrations and their comparison with available experimental data. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A51.00013: Phonons and related spectra in V$_{2}$O$_{5}$ bulk and monolayer(001) Churna Bhandari, Walter R.L. Lambrecht We study the phonons at the zone center for the layered material V$_2$O$_5$ using density functional perturbation theory. The mode frequencies and their calculated infra-red and Raman spectra are shown to be in good agreement with results from literature. We find better agreement with the experiment, using a pseudopotential that treats vanadium semicore states 3s and 3p as bands. We also study the changes between bulk and monolayer using the same method. We find significant changes in some phonon frequencies. In particular, we see the high frequency modes related to bond-stretching between vanadium and vanadyl-oxygen exhibit a blue shift while a few low-frequency modes show a red-shift. The interatomic force constants, separated in their long-range and short range components are used to analyze the origin of these shifts. We find that the blue shifts arise predominantly from a change in the long-range force constants which is due both to the change in dielectric screening and the change in the Born effective charges. [Preview Abstract] |
Session A52: Superconductor Insulator Transitions
Sponsoring Units: DCMPChair: Michael Osofsky, Naval Research Laboratory
Room: Mile High Ballroom 1F
Monday, March 3, 2014 8:00AM - 8:12AM |
A52.00001: Dynamical conductivity across the disorder-tuned superconductor-insulator transition Mason Swanson, Yen Lee Loh, Mohit Randeria, Nandini Trivedi We study the superconductor-insulator transition (SIT) in both clean and disordered systems by calculating the dynamical conductivity $\sigma (\omega )$ and the bosonic (pair) spectral function $P(\omega )$ using quantum Monte Carlo simulations. We identify characteristic energy scales in the superconducting and insulating phases that vanish at the transition due to enhanced quantum fluctuations, despite the persistence of a robust fermionic gap across the SIT [1]. While $\sigma (\omega )$ shows a energy scale for absorption associated with a Higgs (amplitude) mode in the clean superconductor, disorder leads to enhanced low frequency absorption in $\sigma (\omega )$ on both the superconducting and insulating side of the transition. Disorder also expands the quantum critical region, due to a change in the universality class, with an underlying $T =$ 0 critical point with a universal low-frequency conductivity $\sigma $* $\cong $ 0.5 (4e$^{2}$/h) [2]. \\[4pt] [1] K. Bouadim, Y.L. Loh, M. Randeria, and N. Trivedi, \textit{Nat. Phys.} 7 884 (2011)\\[0pt] [2] M. Swanson, Y.L. Loh, M. Randeria, and N. Trivedi\textit{, arXiv} 1310.1073 (2013) [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A52.00002: Divergence of Dynamical Conductivity at Certain Percolative Superconductor-Insulator Transitions Rajesh Dhakal, Yen Lee Loh, John Neis, Evan Moen Coarse-grained superconductor-insulator composites can be modeled as random inductor-capacitor (LC) networks, which exhibit percolative superconductor-insulator transitions (SITs). We use a simple and efficient algorithm to compute the dynamical conductivity $\sigma(\omega,p)$ of one type of LC network on large ($6144 \times 6144$) square lattices, where $\delta=p-p_c$ is the tuning parameter for the SIT [1]. We confirm that the conductivity obeys a scaling form near criticality, so that the characteristic frequency scales as $\Omega \propto \left|\delta\right|^{\nu z}$ with $\nu z \approx 1.91$, the superfluid stiffness scales as $\Upsilon \propto \left| \delta \right|^t$ with $t \approx 1.3$, and the electric susceptibility scales as $\chi_E \propto \left| \delta \right|^{-s}$ with $s = 2\nu z - t \approx 2.52$. In the insulating state, the low-frequency dissipative conductivity is exponentially small, whereas in the superconductor, it is linear in frequency. The sign of m $\sigma(\omega)$ at small $\omega$ changes across the SIT. Most importantly, right at the SIT, Re $\sigma(\omega) \propto \omega^{t/\nu z-1} \propto \omega^{-0.32}$, so that the quasi-dc conductivity $\sigma^*$ is infinite, in contrast with most other classical and quantum models of SITs. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A52.00003: Superfluid-insulator transition in a two-dimensional bond-disordered quantum rotor model Min-Chul Cha, Sung-Been Park We study the critical properties of the superfluid-insulator transition in a disordered two-dimensional quantum rotor model with random spatial bonds. Via worm-algorithm Monte Carlo calculations of superfluid density, compressibility, and correlation function, we find the dynamical critical exponent $z\approx 1.17$ and the correlation length critical exponent $1/\nu \approx 1.2$ at commensurate filling, and$z\approx 2.0$ and $1/\nu \approx 1.5$ at incommensurate filling. These exponents are not consistent with the Bose-glass-to-superfluid transition and suggest possibility of new disordered insulating phase. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A52.00004: New method for the controlled creation of sub-15 nm aluminum nanowires to probe the 1D superconductor-insulator transition Tyler Morgan-Wall, Hannah Hughes, Nik Hartman, Tyrell McQueen, Nina Markovic We have developed a new method for the creation of sub-15 nm aluminum nanostructures using a sodium bicarbonate solution. Using PMMA masks patterned with e-beam lithography, we can controllably etch lithographically-produced nanostructures while measuring their resistances in-situ using a 4-probe measurement. This technique allows for precise control over the final resistance and thus can be used to create a wide variety of nanodevices. In particular, this technique allows for the creation of nanowires to probe the superconductor-insulator transition in 1D. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A52.00005: Evolution of the Cooper Pair Insulator Phase in a-Bi Films Grown on Nanohoneycomb Substrates with Varying Surface Topography J.C. Joy, X. Zhang, C. Zhao, S.M. Hollen, J.M. Valles, Jr., G. Fernandes, J.M. Xu The Cooper Pair Insulator (CPI) phase has been observed in a variety of systems close to both the disorder and field tuned Superconductor to Insulator Transition (SIT) in two dimensions. A number of recent experimental and theoretical studies suggest that the CPI phase arises due to inhomogeneities in the superconducting coupling constant on the nanoscale. Anodized Aluminum Oxide (AAO) substrates provide a convenient experimental platform for studying the influence of inhomogeneity on the CPI state, as the substrates exhibit both a nanohoneycomb structure which allows flux periodic behavior to be measured, as well as a controllable morphology which permits control of the level of inhomogeneity present. We will discuss recent experiments and analyses which examine the behavior of the CPI phase as the level of inhomogeneity in the films is reduced. We will also examine the potential implications of this work in understanding the extent of the CPI phase in two-dimensional systems. This work was supported by the NSF through grants No. DMR-1307290 and DMR-0907357 and by the AFRL, the ONR, and the AFOSR. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A52.00006: The Role of Mesoscopic Disorder in Determining the Character of the Field-Induced Insulating regime of Amorphous Ultrathin Films J.J. Nelson, Yen-Hsiang Lin, Allan Goldman A series of quench-condensed amorphous Bismuth films of different thicknesses were shown to exhibit nonmonotonic magnetoresistances and Arrhenius conduction in the magnetic field induced insulating regime. Neither behavior is found in similar measurements carried out on amorphous Bismuth films grown on smooth, thin, amorphous Antimony underlayers. Arrhenius behavior is found for films grown on thicker Antimony underlayers. \textit{ Ex situ }Atomic Force Microscopy measurements of a series of Antimony films of different thicknesses showed that the mesoscopic scale roughness of the surface increased with increasing thickness. This suggests that film roughness plays the role of nucleating superconducting clusters through thickness variations in the subsequently deposited amorphous Bismuth layer. The properties of the insulating regime appear to depend upon the level of mesoscopic scale disorder. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A52.00007: Current Bias Induced Negative Magneto-Resistance in Superconducting Tantalum Thin Films Sun-gyu Park, Eunseong Kim Negative Magneto-Resistance (MR) of 2D superconducting thin films has received attentions because the decreasing resistance with increasing magnetic field cannot be simply understood by conventional superconductivity. This behavior was ascribed to localized bosons, indicating the existence of a Bose insulator (BI) phase[1-3]. We found negative MR within a range of dc current bias in tantalum thin films, whereas no negative MR appears without bias. We measured R$_{\mathrm{xx}}$ and R$_{\mathrm{xy}}$ simultaneously as functions of current bias and magnetic field and construct the phase diagram at T$=$0 limit. We found that the DC biased negative MR in Ta thin film shows substantially different characteristics from those of reported no biased negative MR. We also found that the induced BI can be understood by the vortex instability state [4, 5].\\[4pt] [1] M. A. Paalanen, A. F. Hebard, and R. R. Ruel, PRL 69, 1604 (1992).\\[0pt] [2] G. Sambandamurthy et al., PRL 92, 107005 (2004).\\[0pt] [3] Y. Zou, G. Refael, and J. Yoon, PRB 80, 180503 (2009).\\[0pt] [4] A. I. Larkin, and Y. N. Ovchinnikov, Sov. Phys.-JETP 41, 960 (1975).\\[0pt] [5] D. Y. Vodolazov, and F. M. Peeters, PRB 76, 014521 (2007). [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A52.00008: Probing superfluid rigidity in ultrathin 2D superconductor at microscopic, mesoscopic, and macroscopic length scales H.D. Nam, J.S. Kim, C.D. Zhang, J. Yong, T.R. Lemberger, P.A. Kratz, J.R. Kirtley, K.A. Moler, C.K. Shih Within the conventional picture, two-dimensional (2D) superconductivity is fragile because phase fluctuations disrupt the long-range order of Cooper pairs. Investigations on epitaxially grown conventional superconductors in the ultra-thin regime, however, revealed rather surprisingly robust superconductivity. Since the robust T$_{\mathrm{C}}$ at this extreme limit was observed primarily using scanning tunneling spectroscopy (STS), it has been suggested that while Cooper pairing remains robust, phase fluctuations can still destroy long range coherence, leading to a much more fragile superconductivity. This work is aimed at addressing this issue by probing superfluid rigidity in ultra-thin Pb films at microscopic, mesoscopic, and macroscopic length scales, using STS, scanning SQUID (SSM), and double coil mutual inductance method, respectively. All three methods yield very similar Tc, attesting the robustness of the supercurrent at macroscopic scale. We further discuss the underlying mechanism for the strong phase rigidity for ultra-thin Pb films at microscopic and macroscopic length scales. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A52.00009: Thickness Dependent Superconductor-Insulator Transition in K$_{0.33}$WO$_{3}$ Phillip Wu, Chris Hart, Katherine Luna, Ko Munakata, Theodore H. Geballe, Malcolm R. Beasley We observe a thickness dependent superconductor to insulator transition in K-doped tungsten bronze superconductors. Via a two-step deposition and post-annealing procedure, K-doped WO$_{3}$ films with reproducible transport properties are obtained. Reducing the film thickness by reducing the film deposition time results in a superconductor to insulator transition. Scanning electron microscopy (SEM) images show that KWO$_{\mathrm{3}}$ crystallites become both thinner and less connected as the deposition time is reduced. Suppression of the density of states at the Fermi level observed using point contact tunneling spectroscopy in the superconducting films demonstrates that disorder-induced increased Coulomb interactions are present. Using the theory of Belitz [1] for the reduction of Tc due to disorder, we can infer that the film with highest observed Tc has a relatively large disorder dependent electron-phonon interaction parameter $\tilde{\lambda} \sim$ 1.2. Understanding microscopically why certain films display higher Tc will aid in the search for the trace high Tc superconducting anomalies observed in lightly surface doped bronzes. This work supported by an AFOSR under DoD MURI grant FA9550-09-1-0583. \\[4pt] [1] D. Belitz. Phys. Rev. B 40, 111 (1989). [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A52.00010: Electronic disorder, spin glass and large magnetoresistance in FeSr$_{2}$Y$_{2-y}$Ce$_{y}$Cu$_{2}$O$_{8+x}$ Sebastian Sambale, Grant Williams, Jibu Stephan, Shen Chong We have successfully synthesized FeSr$_{2}$Y$_{2-y}$Ce$_{y}$Cu$_{2}$O$_{8+x}$ (Fe1222) with a wide range of Ce and oxygen concentrations. Fe1222 belongs to an interesting group of compounds that contains a 2D-like CuO$_{2}$ layer and an oxygen deficient FeO$_{x}$ layer. They are structural very similar to the well-studied superconducting and magnetically ordered RuSr$_{2}$R$_{2-x}$Ce$_{x}$Cu$_{2}$O$_{10-x}$ (Ru1222). However, we do not observe superconductivity in Fe1222 and there is a spin-glass transition with antiferromagnetic exchange interactions arising from the disordered FeO$_{x}$ layer at $\sim$25 K, which does not depend on the Ce or oxygen concentration. The electronic transport in the oxygen reduced samples is highly disordered and involves variable range hopping between localized states where there is a large negative magnetoresistance of $\sim$-22\% at 8 T. The oxygen saturated samples are highly conducting at room temperature and display weak localization at low temperatures. The absence of superconductivity may be due to pair-breaking by a small Fe fraction in the CuO$_{2}$ planes. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A52.00011: Textured electronic states of the triangular lattice Hubbard model and Na$_x$CoO$_2$ near $x=1/3$ Kun Jiang, Sen Zhou, Ziqiang Wang The interplay between geometric frustration and strong correlation is studied in the triangular lattice Hubbard model near electron doping $x=1/3$, in connection to the sodium cobaltates Na$_x$CoO$_2$. We found a mechanism of alleviated magnetic frustration via charge and spin inhomogeneity. At $x=1/3$, the uniform paramagnetic ground state for $U < U_{c1}$ transforms into a $\sqrt{3}\times\sqrt{3}$ spin-charge textured insulating state for $U > U_{c2}$ with antiferromagnetic order on the underlying unfrustrated honeycomb lattice. The transition region, $U_{c1} < U < U_{c2}$, shows several textured semi-metallic states with both collinear and noncollinear magnetic order. We obtain the phase diagram and show that the strongly correlated phases near $x=1/3$ corresponds to doping the ``1/3 state'' with excess carriers forming electron or hole Fermi surface pockets, and compare to experimental findings. We thus propose that the cobaltates near $x=1/3$ are in proximity to such ``hidden'' textured phases with spin and charge order and the enhanced electronic fluctuations can mediate the superconducting pairing interaction. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A52.00012: Non-flux avalanches in the tunneling density of states of a superconductor in a high Zeeman field Joseph Prestigiacomo, Philip Adams We report an ongoing experimental study of the effects of disorder and temperature on the glassy dynamics of the Zeeman-limited critical field transition in ultrathin Al films. We have measured the tunneling density of states of the films through the first-order parallel critical field transition. We find that films with sheet resistance of a few hundred ohms exhibit large avalanches on the superheating branch of the critical field hysteresis loop. In contrast, the transition back into the superconducting phase (i.e., along the supercooling branch) is always continuous. Similar avalanche behavior is also observed in transport. We will discuss what our results imply about nature of the superconducting order parameter in the regime where the Zeeman splitting is of the order of the superconducting gap energy. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A52.00013: Disorder-Driven Superconductor-Insulator Transition in $d$-Wave Superconductors Yun Song, Long He We study the superconductor-insulator transition (SIT) in $d$-wave superconductors. By means of the kernel polynomial method, the Bogoliubov-de Gennes equations are solved self-consistently, making it possible to observe fully the nanoscale spatial fluctuations of the superconducting order parameters. It is shown that Anderson localization can not entirely inhibit the occurrence of the local superconductivity in strongly-disordered $d$-wave superconductors. Separated by an insulating ``sea'' completely, a few isolated superconducting ``islands'' with significant enhancement of the local superconducting order parameters can survive across the SIT. The disorder-driven SIT, therefore, is a transition from a $d$-wave superconductor to a boson insulator which consists of localized Cooper pairs. Unlike an $s$-wave superconductor which presents a robust single-particle gap across the SIT, the optical conductivity of a $d$-wave superconductor reveals a gapless insulating phase, where the SIT can be detected by observing the disappearance of the Drude weight with the increasing disorder. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A52.00014: Liquid-gated superconductor-insulator transition in an electron-doped cuprate Shengwei Zeng, Zhen Huang, Nina Bao, Weiming Lv, Zhiqi Liu, T.S. Herng, K. Gopinadhan, Linke Jian, J. Ding, T. Venkatesan, Ariando Ariando Doping charge carriers will causes the change of cuprates from antiferromagnetic Mott insulators to high-$T_{c}$ superconductors. Continuous changing of carrier density is necessary to understand the nature of such phase transition, and thus, further our understanding of cuprate superconductors. Electric field-effect doping, especially with electronic double layer transistors (EDLT) configuration which use ionic liquids (ILs) and polymer electrolyte as the gate dielectrics, is a potential avenue for this investigation and it has been shown its effectiveness in inducing phase transition in strongly correlated electron system. Owing to EDLT, superconductor-to-insulator transition (SIT) has been observed in hole-doped cuprates La$_{2-x}$Sr$_{x}$CuO$_{4}$ and YBa$_{2}$Cu$_{3}$O$_{y}$. Here we use EDLT to tune the carrier density in electron-doped cuprates Pr$_{2-x}$Ce$_{x}$CuO$_{4}$ ultrathin films and cause the sample evolves from a superconducting state to an insulating state. This present results could be helpful to study SIT between electron- and hole-doped cuprates. [Preview Abstract] |
Monday, March 3, 2014 10:48AM - 11:00AM |
A52.00015: Influence of spatial disorder on the superconducting state of a 3D superconductor Carolina Parra, Francis Niestemski, Paula Giraldo-Gallo, Alex W. Contryman, Theodore H. Geballe, Ian R. Fisher, Hari C. Manoharan We present the first measurements of the local tunneling density of states on the three-dimensional superconductor $BaPb_{1-x}Bi_{x}O_{3}$ as a function of Bi doping. Scanning tunneling spectroscopy measurements are performed on a sequence of samples which exhibit a field-tuned superconductor-to-insulator (SIT) transition. Our study shows that gap variations in the superconducting (SC) state (as a sign of SC disorder level) increase when the system moves towards the SIT phase boundary, with spatial inhomogeneity comparable in size to the material's coherence length. We demonstrate that this highly inhomogeneous local gap size is always finite at every location, even for Bi concentration closest to the SIT, where local insulating behavior is expected and globally confirmed in transport experiments. Our results also suggest a method for increasing the critical temperature for this material by reducing its spatial disorder in the appropriate part of the phase diagram. [Preview Abstract] |
Session A53: Quantum Criticality and Fluctuations in Copper-oxide Superconductors
Sponsoring Units: DCMPChair: Xiao-Jia Chen, Carnegie Institution of Washington
Room: Mile High Ballroom 2C
Monday, March 3, 2014 8:00AM - 8:12AM |
A53.00001: Quantum criticality in high temperature superconducting cuprates Arkady Shekhter, Brad Ramshaw, Ross McDonald, Jon B. Betts, Fedor Balakirev, Scott Riggs, Ruixing Liang, Doug Bonn, Walter Hardy, Albert Migliori Anomalous transport behavior in near-optimally-doped high temperature superconducting cuprates has been linked to fluctuations associated with a quantum critical point. Using resonant ultrasound spectroscopy we find evidence for a phase boundary inside the superconducting dome in the temperature-doping phase diagram of YBCO cuprates. This suggests a quantum critical point near optimal doping that is hidden by superconductivity. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A53.00002: Weak phase stiffess and nature of the quantum critical point in underdoped cuprates Wei Ku, Yucel Yildirim We demonstrate that the zero-temperature superconducting phase diagram of underdoped cuprates can be quantitatively understood in the strong binding limit, using only the experimental spectral function of the ``normal'' pseudo-gap phase without any free parameter. In the prototypical (La$_{1-x}$Sr$_x$)$_2$CuO$_4$, a kinetics-driven $d$-wave superconductivity is obtained above the critical doping $\delta_c\sim 5.2\%$, below which complete loss of superfluidity results from local quantum fluctuation involving local $p$-wave pairs. Near the critical doping, a enormous mass enhancement of the local pairs is found responsible for the observed rapid decrease of phase stiffness. Finally, a striking mass divergence is predicted at $\delta_c$ that dictates the occurrence of the observed quantum critical point and the sudden suppression of the Nernst effects in the nearby region. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A53.00003: Conductivity near quantum criticality in two space dimensions Snir Gazit, Daniel Podolsky, Assa Auerbach, Daniel P. Arovas We study relativistic $U(1)$ field theories near the quantum critical point in two space dimensions [1]. We compute the dynamical optical conductivity by means of large scale Monte Carlo simulations and numerical analytic continuation. Our main focus is on the universal properties and their relation to the low energy excitation spectrum. In both phases, the spectral function exhibits a sharp rise above a threshold frequency corresponding to the Higgs mass [2] in the ordered phase and to twice the single particle gap in the disordered phase. We determine the high frequency critical conductivity to be $\sigma^*_{\rm c} = 0.3 (\pm 0.1) \times 4e^2/h$. In addition, we find an approximate charge-vortex duality that is reflected in the ratio of the imaginary conductivity on either side of the transition. Our results are relevant to recent experiments on the superfluid to Mott insulator transition in cold atomic optical lattices and to THz spectroscopy of the superconductor to insulator transition in superconducting thin films.[1] S. Gazit, D. Podolsky, A. Auerbach, and D. P. Arovas, arXiv:1309.1765 (2013) [2] S. Gazit, D. Podolsky, and A. Auerbach, Phys. Rev. Lett. 110, 140401 (2013) [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A53.00004: Quantum-critical point of charge-density-wave order under superconducting dome in hole-doped cuprates Yuxuan Wang, Andrey Chubukov We analyze, within the spin-fermion model, a charge-density-wave (CDW) order in hole-doped cuprates with incommensurate momenta $(\pm Q, 0)$ and $(0, \pm Q)$. We show that spin fluctuations mediate attractive interaction in the CDW channel at these momenta. Moreover, the enhancement of the CDW vertex is logarithmical. We solve for the onset of CDW order at the magnetic critical point (when fermionic self-energy cannot be neglected) and show that the corresponding critical temperature $T_{CDW}$ may in fact be larger than superconducting $T_c$. We further consider CDW instability at a finite magnetic correlation length $\xi$ and show that the logarithm is cut off by $\xi_{cr}$. As a result, $T_{CDW}$ monotonically decreases with decreasing $\xi$ and vanishes at some finite $\xi$. This gives rise to a quantum-critical point under the SC dome. As a consequence, the SC dome is divided into two regions, one in which only superconductivity is present, and the other in which CDW and superconducting orders co-exist. We show that a number of observed features of underdoped cuprates, including a non-monotonic dependence of $T_c$ on doping, can be explained by a competition between the two orders. We compute the fermionic spectral function and compare with recent ARPES results. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A53.00005: Measurement of diamagnetic signal on UD YBCO Fan Yu, Gang Li, Tomoya Asaba, Benjamin Lawson, Max Hirschberger, John Singleton, T. Lowe, B. Keimer, N.P. Ong, Lu Li The ortho-II phase under-doped (UD) YBa$_2$Cu$_3$O$_{6+\delta}$ has shown a number of interesting phenomena such as quantum oscillations and field-driven charge ordering. An open question is the fate of the superconducting fluctuation in the magnetic field beyond the vortex melting field. To answer the question, we carried out the capacitance based cantilever torque magnetometry measurements on the $T_c = 60$ K phase YBa$_2$Cu$_3$O$_{6+\delta}$ up magnetic fields as high as 56T. At $T$ as low as 1.5 K, the magnetization hysteresis ends at 30 T, marking the melting of the vortex solid phase. Nonlinear diamagnetic signal exists beyond the melting field and persists at field 56T. Our observation suggests that the superconducting fluctuation persists into extensive field, as the Cooper pairing survives well above the vortex solid melting field. [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A53.00006: Spontaneously Generated Inhomogeneous Phases via Holography Kubra Yeter, James Alsup, Eleftherios Papantonopoulos, George Siopsis We discuss a holographic model consisting of a $U(1)$ gauge field and a scalar field coupled to a charged AdS black hole under a spatially homogeneous chemical potential. By turning on a higher-derivative interaction term between the $U(1)$ gauge field and the scalar field, a spatially dependent profile of the scalar field is generated spontaneously. The critical temperature at which the transition to the inhomogeneous phase occurs is calculated for various values of the parameters of the system. By solving the equations of motion below the critical temperature we show that the dual gauge theory on the boundary spontaneously develops a spatially inhomogeneous charge density. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A53.00007: Indications of a Quantum Critical Point in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ Using a Local Kondo Effect Eduardo Calleja, Jixia Dai, Gerald Arnold, Genda Gu, Kyle McElroy A complete understanding of the complex phase diagrams that are present in high temperature superconductors remains elusive. While there is an overwhelming amount of experimental data on the existence and interplay of the phases present in high T$_{\mathrm{c}}$ superconductors from local probes, much of the existing data only looks at the charge degree of freedom of the material. By substituting Fe atoms for Cu atoms in the CuO plane of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ (Bi2212), we gain the ability to access the spin degree of freedom since the Fe atoms retain their magnetization below the superconducting transition temperature. This leads to a local Kondo effect which can be observed using Spectroscopic-Imaging Scanning Tunneling Microscopy (SI-STM) and the local Kondo temperature can be extracted from spectra via a theoretical model. We show that the examination of this local Kondo temperature across local and sample average doping leads to the observation of a change in the quasiparticle spin degree of freedom at a quantum critical point (QCP) with a nominal hole doping of roughly 0.22, in agreement with other probes. The observation of the QCP in Bi2212 with this new method to access the spin degree of freedom helps to unravel some of the mystery behind the complex phase diagram of Bi2212. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A53.00008: Nodal Fermi surface pocket approaching an optimal quantum critical point in YBCO Suchitra Sebastian, Beng Tan, Gilbert Lonzarich, Brad Ramshaw, Neil Harrison, Fedor Balakirev, Chuck Mielke, S. Sabok, B. Dabrowski, Ruixing Liang, Doug Bonn, Walter Hardy I present new quantum oscillation measurements over the entire underdoped regime in YBa$_2$Cu$_3$O$_{6+x}$ and YBa$_2$Cu$_4$O$_8$ using ultra-high magnetic fields to destroy superconductivity and access the normal ground state. A robust small nodal Fermi surface created by charge order is found to extend over the entire underdoped range, exhibiting quantum critical signatures approaching optimal doping. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A53.00009: Mott physics revealed in high temperature superconductors by resonant inelastic X-ray scattering experiments Ting-Kuo Lee, Cheng-Ju Lin In recent resonant inelastic X-ray scattering (RIXS) experiments, energy dispersions of measured spin-wave or paramagnon excitations of cuprates show no signs of softening up to 40\% hole doping or substantially hardening after only 15\% electron doping. In this talk the anomalous result is explained by a simple explanation based one the t-J model. It reveals the presence of the strong correlation of Mott physics in highly doped cuprates. Some predictions will be also presented. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A53.00010: Impact of chain layer oxygen disorder on charge density wave order in YBa$_2$Cu$_3$O$_{6+\delta}$ Andrew Achkar, Xiaopan Mao, Christopher McMahon, Ronny Sutarto, Feizhou He, Ruixing Liang, Doug Bonn, Walter Hardy, David Hawthorn Charge density wave (CDW) order in the CuO$_2$ planes of underdoped YBa$_2$Cu$_3$O$_{6+\delta}$ (YBCO) has been reported to coexist and compete with superconductivity. Here we investigate the sensitivity of the CDW order to oxygen disorder in the chain layer of YBCO using resonant soft x-ray scattering. We find that disordering the chains in o-V and o-VIII ordered YBCO$_{6.67}$ decreases the intensity of the CDW superlattice peak by a factor of $\sim2$, but unexpectedly has little effect on the incommensurability, correlation length or temperature dependence of the CDW peak. The same is true for o-III ordered YBCO$_{6.75}$, although the disordering has a smaller effect on the CDW peak intensity. The observed insensitivity of the incommensurability, correlation length and temperature dependence to chain layer oxygen disorder indicates that chain layer defects have a limited role in pinning the CDW order in the CuO$_2$ planes. We will discuss scenarios for the disorder effect, including a possible influence on CDW domain formation (CDW volume fraction) and the CDW order parameter. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A53.00011: Monte Carlo simulations of an O(6) theory for the pseudogap regime of the cuprate superconductors Lauren Hayward, David Hawthorn, Roger Melko, Subir Sachdev We present a theory that describes the pseudogap regime of the hole-doped cuprate superconductors by incorporating the competing effects of superconducting and charge density wave orders into 6-dimensional degrees of freedom on a 2-dimensional lattice [arXiv:1309.6639]. Using Monte Carlo simulations, we calculate the charge order correlations associated with this O(6) theory, and show that the results compare well with recent X-ray scattering experiments on hole-doped YBa$_2$Cu$_3$O$_{6.67}$. We compare our simulation data to large-N calculations for the theory, and also demonstrate that the charge order continues to increase with increasing temperature for a small temperature range above the superconducting transition. For temperatures above this transition, we study our theory's diamagnetic response to a magnetic field applied perpendicular to the plane. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A53.00012: Quantum fluctuations in charge glass manifested by the hysteresis of Hall resistivity Jie Wu, Anthony Bollinger, Yujie Sun, Ivan Bozovic The Hall effect studies on the underdoped La$_{\mathrm{2-x}}$Sr$_{\mathrm{x}}$CuO$_{\mathrm{4}}$ films grown by Molecular Beam epitaxy (MBE) show dramatic hysteretic behavior at temperatures below 1.5 K, in agreement with the hypothesis that the low temperature ground state is a Coulomb glass. The fluctuations in the Hall resistivity R$_{\mathrm{H}}$ reach several hundred percent and even change the sign of R$_{\mathrm{H}}$, thus making the Hall effect measurement a very sensitive probe of the glassy state. Using a continuous doping gradient (COMBE), we scanned the dependence of R$_{\mathrm{H}}$ on the doping level x in extremely fine steps (x \textless 0.0001), in order to precisely map out the phase boundary between the charge glass state and the superconducting state. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A53.00013: Exploring Quasiparticles in High-T$_c$ Cuprates Through Photoemission, Tunneling, and X-ray Scattering Experiments Emanuele Dalla Torre, Yang He, David Benjamin, Eugene Demler One of the key challenges in the field of high-temperature superconductivity is understanding the nature of fermionic quasiparticles. Experiments consistently demonstrate the existence of a second energy scale, distinct from the d-wave superconducting gap, that persists above the transition temperature into the ``pseudogap'' phase. One common class of models relates this energy scale to the quasiparticle gap due to a competing order, such as the incommensurate ``checkerboard'' order observed in scanning tunneling microscopy (STM) and resonant elastic X-ray scattering (REXS). In this paper we show that these experiments are better described by identifying the second energy scale with the inverse lifetime of quasiparticles. We develop a minimal phenomenological model that allows us to quantitatively describe STM and REXS experiments and compare them with angle-resolved photo-emission spectroscopy (ARPES). Our study refocuses questions about the nature of the pseudogap phase to the study of the origin of inelastic scattering. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A53.00014: Current and Temperature Dependence of Resistance Fluctuations in the Striped Phase of La$_{2-x}$Ba$_x$CuO$_4$ Adam Weis, Ethel Perez, Marek Mroczek, Yizhou Xin, Dale Van Harlingen The high-temperature superconductor La$_{2-x}$Ba$_x$CuO$_4$ is known for its unusual suppression of T$_c$ at x=1/8, accompanied by the emergence of a charge stripe phase. A dynamic stripe phase with local resistance anisotropy is expected to cause measurable resistance fluctuations in samples with small dimensions. We report measurements of the transport and noise in microscopic wires patterned from thin films of La$_{2-x}$Ba$_x$CuO$_4$ grown by pulsed laser deposition. We observe a sudden change in noise power spectral density at temperatures consistent with the charge ordering temperatures observed in scattering experiments. We present the evolution of resistance noise with temperature and bias current as a characterization of the strongly correlated state near x=1/8 doping. [Preview Abstract] |
Session A54: Focus Session: Cooperative Phenomena: Spin Waves and Excitations
Sponsoring Units: GMAGRoom: Mile High Ballroom 1B
Monday, March 3, 2014 8:00AM - 8:12AM |
A54.00001: Inelastic neutron scattering studies of YFeO$_3$ Steven Hahn, Andrey Podlesnyak, Randy Fishman, Garrett Granroth, Alexander Kolesnikov, Ekaterina Pomjakushina, Kazimierz Conder, Georg Ehlers Spin waves in the the rare earth orthorferrite YFeO$_3$ have been studied by inelastic neutron scattering and analyzed with a full four-sublattice model including contributions from both the weak ferromagnetic and hidden antiferromagnetic orders. Antiferromagnetic (AFM) exchange interactions of $J_1 = -4.23\pm0.08$ meV (nearest-neighbors only) or $J_1 = -4.77\pm0.08 $ meV and $J_2= -0.21\pm0.03$ meV lead to excellent fits for most branches at both low and high energies. An additional branch associated with the hidden antiferromagnetic order was observed. This work paves the way for studies of other materials in this class containing spin reorientation transitions and magnetic rare earth ions. [Preview Abstract] |
Monday, March 3, 2014 8:12AM - 8:24AM |
A54.00002: Magnetic structure and spin excitations in BaMn$_2$Bi$_2$ M.D. Lumsden, S. Calder, B. Saparov, H.B. Cao, J.L. Niedziela, A.S. Sefat, A.D. Christianson The magnetic structure and associated spin wave excitations of the recently synthesized BaMn$_2$Bi$_2$ have been studied using neutron scattering. BaMn$_2$Bi$_2$ exhibits the same ThCr$_2$Si$_2$ crystal structure as the 122 iron superconductors (AFe$_2$As$_2$). Single crystal neutron diffraction reveals that the ordered state below T$_N$ $\sim$ 390 K is consistent with G-type antiferromagnetic order and suggests the presence of a structural phase transition at 100 K. Inelastic neutron scattering reveals anisotropic spin waves characterized by a gap of 16 meV, in-plane excitations with a maximum energy of 55 meV and a c-axis dispersion extending to about 35 meV. The observed magnetic excitations are well described by a J$_1$-J$_2$-J$_c$ Heisenberg Hamiltonian and the relevant exchange interactions are extracted. The results will be compared to other related materials such as BaFe$_2$As$_2$ and BaMn$_2$As$_2$. [Preview Abstract] |
Monday, March 3, 2014 8:24AM - 8:36AM |
A54.00003: Impact of inter-ladder coupling in a coupled spin-1/2 two-leg ladder Tao Hong, K.P. Schmidt, K. Coester, F.F. Awwadi, M.M. Turnbull, Y. Qiu, J.A. Rodriguez, X. Ke, C. Aoyama, Y. Takano, H. Cao, W. Tian, J. Ma, R. Custelcean, H.D. Zhou, M. Matsuda We present the zero-field specific heat and neutron scattering studies on an $S=$1/2 Heisenberg antiferromagnet to understand the nature of its spin Hamiltonian. The system is magnetically ordered below $T_{\mathrm{N}}=$2.0(1) K. The conclusion that the system is best described as coupled two-leg spin-1/2 ladders is supported by the material structure, neutron scattering measurements, and theoretical calculation. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 8:48AM |
A54.00004: Observation of a gapless linear dispersion at quantum criticality in the Ising chain ferromagnet CoNb$_2$O$_6$ in transverse field Ivelisse Cabrera, Jordan D. Thompson, Radu Coldea, Dharmalingam Prabhakaran, Robert I. Bewley, Tatiana Guidi The Ising chain in transverse field is one of the canonical paradigms for a continuous field-driven quantum phase transition between spontaneous magnetic order and a quantum paramagnet. The mechanism driving this phase transition has long been predicted to involve the closing of the spin gap, or minimum excitation energy, at the quantum critical point, where a characteristic linear dispersion is expected at low energies. We report single-crystal neutron diffraction and inelastic neutron scattering measurements that unveil how the magnetic order and excitations evolve in the very close proximity of the quantum critical point in the quasi-1D Ising chain ferromagnet CoNb$_2$O$_6$. Near criticality, we observe an essentially gapless spectrum with an almost perfectly-linear dispersion along the chain direction. Below the critical field, the frustrated interchain couplings stabilize 3D incommensurate spin-density-wave order, as observed through diffraction measurements. To our knowledge, this is the first time that essentially-gapless, linearly dispersive excitations have been observed in the very close proximity of a transverse field-tuned quantum critical point. [Preview Abstract] |
Monday, March 3, 2014 8:48AM - 9:00AM |
A54.00005: Cooperative spin decoherence in finite spin chains Fernando Delgado, Joaquin Fernandez-Rossier Overcoming the problem of relaxation and decoherence of magnetic nanostructures is one of the mayor goals in magnetic data storage. Although spin chains with as few as 12 magnetic atoms have revealed stability in cryogenic conditions [1], understanding the mechanism leading to these effects is essential for the engineered of stable structures. Here we consider the problem of spin decoherence and relaxation of finite size quantum spin chains due to elastic and spin conserving interactions with an electron gas. Specifically, we consider how the decoherence ($T_2$) and relaxation ($T_1$) times between the two degenerate ground states of a chain of $N$ coupled spins compares with the one of an isolated spin in the same environment. We find that the spin decoherence time of Ising chains can be either enhanced or suppressed depending on the matching between the Fermi wavelength $2\pi/k_F$ and the inter-spin distance $a$. In particular, we find that depending on the values of $k_Fa$, it can show, for certain values that depends on the dimensionality of the electron gas, a cooperative enhancement proportional to $N^2$ of the decoherence, analogous to super radiance decay of atom ensembles, or a suppression. [1] S. Loth et al., Science 335, 196 (2012). [Preview Abstract] |
Monday, March 3, 2014 9:00AM - 9:12AM |
A54.00006: Magnon Hall effect in the Shastry Sutherland material Judit Romhanyi, R Ganesh We demonstrate that SrCu$_2$(BO$_3$)$_2$ (SCBO), the well known realization of the Shastry Sutherland (SS) model, in fact hosts the magnon Hall effect. The SS model has an exact dimer singlet ground state. However, the material SCBO has small Dzyaloshinskii Moriya (DM) interactions which admix a triplet component into the ground state. The resulting state has small magnetic moments and its lowest excitations are three gapped magnon modes, well described by bond wave theory. An applied magnetic field splits these modes and opens band gaps. Surprisingly, we are left with topological magnon bands with non-zero Chern numbers ($\pm 2$). Thus, SCBO supports protected magnonic edge modes and is a magnetic analogue of the integer quantum Hall effect. Ultimately, this topological character stems from the DM interactions which generate a Lorentz force for magnons. We discuss several interesting consequences and possible experimental probes. [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:24AM |
A54.00007: Experimental Determination of Spin Glass Lower Critical Dimension Samaresh Guchhait, Raymond Orbach Zero field cooled (ZFC) measurements on thin film Ge:Mn spin glass can explore the lower critical dimension $d_l$. The correlation length $\xi(t, T)$ is nucleated upon a rapid quench into the spin glass phase, and grows to the thickness of the film, $L$, resulting in a transition for dynamics from $d=3$ to $d=2$ at a crossover time $t_{co}$. Our experiments demonstrate that conventional ZFC dynamics vanish at $t=t_{co}$, but there remain spins within a length scale $\leq L$ for which $d=3$ dynamics remain. Because of the ultrametric distribution of states, the rise of the remaining ZFC magnetization exhibits an exponential time dependence determined by the highest barrier surmounted at $t_{co}$, $\Delta_{\rm max}(t_{co}, T)$. By carefully choosing a temperature region where the dynamics fall within experimental time scales, both regimes are observed. Further, there is a direct relationship between the magnitude of $\xi(t_{co}, T)$ and $\Delta_{\rm max}({t_{co}}, T)$. This relationship is satisfied, determining the parameters controlling the growth of $\xi(t, T)$ without arbitrary parameters. The existence of the crossover establishes that $2 < d_l < 3$ for spin glass dynamics, in agreement with theory for Ising (Franz {\it et al.}) and Heisenberg (Lee and Young) spin glasses. [Preview Abstract] |
Monday, March 3, 2014 9:24AM - 9:36AM |
A54.00008: The``Higgs'' amplitude mode in weak ferromagnetic metals Yi Zhang, Paulo Farinas, Kevin Bedell Using Ferromagnetic Fermi liquid theory, Bedell and Blagoev derived the collective low-energy excitations of a weak ferromagnet. They obtained the well-known magnon (Nambu-Goldstone) mode and found a new gapped mode that was never studied in weak ferromagnetic metals. In this Letter we have identified this mode as the Higgs boson (amplitude mode) of a ferromagnetic metal. This is identified as the Higgs since it can be show that it corresponds to a fluctuation of the amplitude of the order parameter. We use this model to describe the itinerant-electron ferromagnetic material MnSi. By fitting the model with the existing experimental results, we calculate the dynamical structure function and see well-defined peaks contributed from the magnon and the Higgs. From our estimates of the relative intensity of the Higgs amplitude mode we feel that it can be seen in neutron scattering experiments on MnSi. [Preview Abstract] |
Monday, March 3, 2014 9:36AM - 9:48AM |
A54.00009: Magnetic Excitations in MnV$_{2}$O$_{4}$ Studied by Inelastic Neutron Scattering Keisuke Matsuura, Amane Uehara, Yoichi Nii, Nobuyuki Abe, Hajime Sagayama, Taka-hisa Arima, Sungdae Ji, Ryoichi Kajimoto We focus on the dynamical structure of a spin-orbital coupled system MnV$_{2}$O$_{4}$, which crystallizes in the spinel structure. Each V$^{3+}$ ion with the 3d$^{2}$ configuration is surrounded by an oxygen octahedron. The orbital degree of freedom consequently exists in the t$_{\mathrm{2g}}$ states. Below T$_{\mathrm{oo}}=$53K, the t$_{\mathrm{2g}}$ orbitals are arranged in the layered antiferroic way. Simultaneously, non-collinear ferrimagnetic ordering takes place. In this spin-orbital correlated system, in addition to conventional spin waves, orbital waves and spin-orbital coupled excitations are expected to appear. A measurement of inelastic neutron scattering on single crystals of MnV$_{2}$O$_{4}$ was carried out at 5K using a Fermi-chopper type spectrometer 4SEASONS installed at BL01, J-PARC, Japan. The dispersion of the magnetic excitations at 8-9meV have been revealed, which was only rather vaguely observed in the previous study. We have performed spin-wave calculations based on the spin Hamiltonian and compared with the experimental results in order to identify the 8-9meV modes. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:00AM |
A54.00010: deHaas-vanAlphen study of the ungapped Fermi surface in the spin-density-wave system GdSi D.M. Silevitch, Yejun Feng, Nayoon Woo, A.V. Suslov, J.-Q. Yan, T.F. Rosenbaum In the rare earth-intermetallic GdSi, the nested Fermi surface of the itinerant electrons induces strong interactions between local moments at the nesting vector, and the ordered local moments in turn provide the necessary coupling for a spin- density wave to form among the itinerant electrons. We examine the Fermi surface in the magnetically ordered phase through deHaas-vanAlphen magnetization measurements. Ungapped portions of the Fermi surface, consisting of tubular structures and discrete pockets, are found to span less than 5\% of the cross-sectional area of the first Brillouin zone projected along the three principal axes. The effective masses of orbiting electrons in the different regions of the Fermi surface are determined through the temperature dependence of the oscillation amplitudes. We interpret the implications of these results for magnetoresistive properties and responsiveness to pressure. [Preview Abstract] |
Monday, March 3, 2014 10:00AM - 10:12AM |
A54.00011: Hidden one-dimensional order in a three-dimensional metal Yejun Feng, Jiyang Wang, A. Palmer, B. Mihaila, J.-Q. Yan, P.B. Littlewood, T.F. Rosenbaum The rare-earth intermetallic GdSi has a spin-density-wave ground state originating from a cooperative interaction between nested itinerant spins and RKKY exchange-ordered local moments. We probe directly the stability of the SDW under pressure, using non-resonant x-ray magnetic diffraction. The incommensurate antiferromagnetic state remains unchanged up to 16.4 GPa, even though the lattice contracts by 5{\%}! Band structure calculations show that the stability can be attributed to a persistently nested portion of the Fermi surface that grows increasingly one-dimensional under pressure. This cooperatively ordered itinerant and local spin ensemble is expected to provide a stable antiferromagnetic state in thin films, even with large lattice strain and lattice mismatch, and could be suitable for spin-valve and giant magnetoresistance device applications. [Preview Abstract] |
Monday, March 3, 2014 10:12AM - 10:24AM |
A54.00012: Role of domain wall fluctuations in non-Fermi-liquid behavior of metamagnets Vladimir Zyuzin, Alexander Yu. Zyuzin In this paper we study the resistivity temperature dependence of a three-dimensional metamagnet near the metamagnetic phase transition point. The phase transition is characterized by a phase separation of regions with high and low magnetization. We show that, in the case of weak pinning, the spin relaxation time of the domain wall, which separates the two phases, is much larger than that of the volume spin fluctuations. This opens a temperature range where resistivity temperature dependence is determined by scattering of conducting electrons by the domain wall fluctuations. We show that it leads to quasi-linear low temperature dependence of resistivity. [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 10:36AM |
A54.00013: Supersymmetric integrable perturbations on the lattice Liza Huijse, Christian Hagendorf, Thessa Fokkema We study a supersymmetric model that describes the multicritical point where a Kosterlitz-Thouless and Ising transition coincide. The model is integrable at the multicritical point (Fendley, Nienhuis, Schoutens, 2003). We expand this result by identifying a line in parameter space that intersects with the multicritical point for which the model is Bethe Ansatz solvable. We show that this is a lattice realization of a well-known supersymmetric integrable perturbation of the field theory describing the multicritical point. We discuss how supersymmetry manifests itself in the Bethe equations and the consequences of dynamical supersymmetry on scaling functions. [Preview Abstract] |
Monday, March 3, 2014 10:36AM - 10:48AM |
A54.00014: Complex Solitary Wave Dynamics, Pattern Formation, and Chaos in the Gain-Loss Nonlinear Schr\"odinger Equation Justin Anderson, Lincoln Carr, Mingzhong Wu Complex solitary wave dynamics, pattern formation and chaos are numerically studied in the context of spin wave envelopes in magnetic thin film active feedback rings and analogous driven damped nonlinear physical systems. Distinct dynamical behaviors of the gain-loss nonlinear Schr\"odinger equation were numerically identified during a parameter space exploration utilizing over 180\,000 core hours of simulation. Numerically identified dynamical behaviors include: spatially symmetric/asymmetric interactions of solitary wave peaks; dynamical pattern formation and recurrence, intermittency, steady state solutions and chaotically modulating bright soliton trains. Ten new dynamical behaviors, eight demonstrating long lifetimes, are predicted to be observable in experiments. [Preview Abstract] |
Session A55: Invited Session: Weyl and Dirac Semimetals: From Transport and Chiral Anomaly to Physical Realizations
Sponsoring Units: DCMPChair: Asvin Vishwanath, University of California, Berkeley
Room: Four Seasons Ballroom 1
Monday, March 3, 2014 8:00AM - 8:36AM |
A55.00001: Topological response in Weyl semimetals and metallic ferromagnets Invited Speaker: Anton Burkov Standard picture of a topologically-nontrivial phase of matter is an insulator with a bulk energy gap, but metallic surface states, protected by the bulk gap. Recent work has shown, however, that certain gapless systems may also be topologically nontrivial, in a precise and experimentally observable way. In this talk I will review our work on a class of such systems, in which the nontrivial topological properties arise from the existence of nondegenerate point band-touching nodes (Weyl nodes) in their electronic structure. Weyl nodes generally exist in any three-dimensional material with a broken time-reversal or inversion symmetry. Their effect is particularly striking, however, when the nodes coincide with the Fermi energy and no other states at the Fermi energy exist. Such ``Weyl semimetals'' have vanishing bulk density of states, but have gapless metallic surface states with an open (unlike in a regular two-dimensional metal) Fermi surface (``Fermi arc''). I will discuss our proposal to realize Weyl semimetal state in a heterostructure, consisting of alternating layers of topological and ordinary insulator, doped with magnetic impurities. I will further show that, apart from Weyl semimetals, even such ``ordinary'' materials as common metallic ferromagnets, in fact also possess Weyl nodes in the electronic structure, leading to a non-quantized contribution to their intrinsic anomalous Hall conductivity, which can not be attributed to the Fermi surface. [Preview Abstract] |
Monday, March 3, 2014 8:36AM - 9:12AM |
A55.00002: Probing the chiral anomaly and transport in Weyl semimetals Invited Speaker: Siddharth Parameswaran The topological nature of Weyl semimetals is reflected in the Adler-Bell-Jackiw anomaly, an unusual bulk response where applying parallel electric ($\mathbf{E}$) and magnetic ($\mathbf{B}$) fields pumps electrons between nodes of opposite chirality at a rate proportional to $\mathbf{E}\cdot\mathbf{B}$. We argue that this pumping is measurable via nonlocal transport experiments, in the limit of weak internode scattering. Such nonlocal transport vanishes when the injected current and magnetic field are orthogonal, and therefore serves as a test of the chiral anomaly. I will also comment on the possibility of observing similar physics in the three-dimensional Dirac semimetallic phase proposed to exist in Na$_3$Bi and Cd$_3$As$_2$, which have been the subject of recent photoemission and transport experiments. Reference: arXiv preprint 1306.1234 (2013). [Preview Abstract] |
Monday, March 3, 2014 9:12AM - 9:48AM |
A55.00003: Three Dimensional Dirac Semimetals Invited Speaker: Saad Zaheer Dirac points on the Fermi surface of two dimensional graphene are responsible for its unique electronic behavior. One can ask whether any three dimensional materials support similar pseudorelativistic physics in their bulk electronic spectra. This possibility has been investigated theoretically and is now supported by two successful experimental demonstrations reported during the last year. In this talk, I will summarize the various ways in which Dirac semimetals can be realized in three dimensions with primary focus on a specific theory developed on the basis of representations of crystal spacegroups. A three dimensional Dirac (Weyl) semimetal can appear in the presence (absence) of inversion symmetry by tuning parameters to the phase boundary separating a bulk insulating and a topological insulating phase. More generally, we find that specific rules governing crystal symmetry representations of electrons with spin lead to robust Dirac points at high symmetry points in the Brillouin zone. Combining these rules with microscopic considerations identifies six candidate Dirac semimetals. Another method towards engineering Dirac semimetals involves combining crystal symmetry and band inversion. Several candidate materials have been proposed utilizing this mechanism and one of the candidates has been successfully demonstrated as a Dirac semimetal in two independent experiments. [Preview Abstract] |
Monday, March 3, 2014 9:48AM - 10:24AM |
A55.00004: Topological Semimetals in Realistic Compounds Invited Speaker: Zhong Fang Topological semimetal, characterized by Weyl/Dirac nodes in the bulk and Fermi arcs on the surfaces, is a new state of three-dimensional (3D) quantum matters, different from the 3D topological insulators. Weyl nodes are stable topological objects, and can be viewed as effective magnetic monopoles in the 3D momentum space. Its time-reversal invariant version --- 3D Dirac node, however, consists of two copies of distinct Weyl nodes with opposite chirality, and requires additional symmetry protection, such as the crystal symmetry. Novel properties, such as the giant diamagnetism and the linear quantum magnetoresistance can be expected for such semimetal states. In this talk, I will first present a general description of topological semimetal states, and then discuss its possible material realizations based on the first-principles calculations. We will show two theoretical predictions, Na$_3$Bi and Cd$_3$As$_2$, where the 3D Dirac cones are experimentally observed recently. \\[4pt] [1] G. Xu, H. M. Weng, Z. J. Wang, X. Dai, Z. Fang, Physical Review Letters 107, 186806 (2011). \\[0pt] [2] Z. J. Wang, Y. Sun, X. Q. Chen, C. Franchini, G. Xu, H. M. Weng, X. Dai, Z. Fang, Phys. Rev. B 85, 195320 (2012).\\[0pt] [3] Z. J. Wang, H. M. Weng, Q. S. Wu, X. Dai, Z. Fang, Phys. Rev. B 88, 125427 (2013). [Preview Abstract] |
Monday, March 3, 2014 10:24AM - 11:00AM |
A55.00005: Possible Weyl state near the metal-insulator boundary in pyrochlore iridates Invited Speaker: Stephen Julian Despite a rapidly growing theoretical literature on Weyl semi-metallic states, such states are proving elusive in real materials. Promising candidates, initially proposed by Wan et al.[1] and Witczak-Krempa et al. [2], are the pyrochlore iridate systems R$_2$Ir$_2$O$_7$, where $R$ is a rare earth. In this talk I will review experimental evidence for unconventional normal states near the metal-insulator boundary in these systems, focusing on Eu$_2$Ir$_2$O$_7$, where we have carried out transport measurements under pressure [3]. In measurements up to 12 GPa, we found a peculiar insulator-to-metal transition near 7 GPa. Across this pressure range magnetic order -- a prerequisite for a Weyl state in the pyrochlore lattice -- seems to be relatively unaffected, with $T_N \simeq 100-120$ K at all pressures. The normal state above 7 GPa is unusual, having a negative temperature derivative of resistance. Magnetoresistance measurements at 10 GPa down to 100 mK suggest the existence of small Fermi pockets. These behaviors may be consistent with a Weyl semi-metallic state near the metal-insulator boundary. Further transport measurements that could help to establish this are currently under way, and will be briefly described. \\[4pt] [1] X. Wan, A. M. Turner, A. Vishwanath and S. Y. Savrasov, Phys. Rev. B vol. 83 (2011) 205101.\\[0pt] [2] W. Witczack-Krempa and Y. B. Kim, Phys. Rev. B vol. 85 (2012) 045124.\\[0pt] [3] F. F. Tafti, J. J. Ishikawa, A. McCollam, S. Nakatsuji and S. R. Julian, Phys. Rev. B vol. 85 (2012) 205014. [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