Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Y1: Focus Session: Solvation, Dynamics, and Reactivity in Complex Environments V
Sponsoring Units: DCPChair: Amber Krummel, Colorado State University
Room: 103/105
Friday, March 7, 2014 8:00AM - 8:36AM |
Y1.00001: Weighted Random Mixing and Exact Finite Lattice Descriptions of Molecular Aggregation Equilibria Invited Speaker: Dor Ben-Amotz Entropic and energetic contributions to a broad class of molecular aggregation and self-assembly processes are described by performing a mean field Boltzmann average over aggregate size distributions pertaining to an idealized random mixture. Predictions obtained using the resulting weighted random mixing (WRM) model are compared with exact finite lattice and fluid molecular dynamics simulation results for systems in which each aggregate resembles a central molecule with multiple ligand binding sites. Good agreement between the exact and WRM results is found for systems with interaction energies of various magnitudes (and signs), both in the large and small cohesive interaction energy regimes (or at low and high temperature, respectively). The latter two regimes are separated by a critical point on either side of which qualitatively different aggregation behavior is predicted and observed. More specifically, both the WRM model and exact finite lattice aggregation results reveal that when half the ligand binding sites are filled, the corresponding aggregate size distributions are bimodal below and unimodal above the corresponding critical temperature, whose value depends on the ligand-ligand interaction energy, but is independent of the binding energy of each ligand to the central molecule. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y1.00002: Rotational-Rate Heterogeneity in Polymers on the Picosecond and Second Timescales through 2D Kinetics Mark Berg, Sachin Dev Verma, David Vanden Bout In a polymer, the anisotropy decay of a small solute is nonexponential, i.e. it has rate dispersion. This dispersion could reflect heterogeneity in the local structure and dynamics of the polymer. On the other hand, homogeneous mechanisms are also possible: anisotropic local structure, multiple, independent rotational processes, or a relaxation hierarchy. Two-dimensional (2D) kinetics separate heterogeneous and homogeneous relaxation by monitoring dynamics over two time periods before ensemble averaging. Far from the glass transition, subnanosecond rotation in PDMS has been measured by MUPPETS (multiple population-period transient spectroscopy), a nonequilibrium technique. These experiments are the first to use polarization in MUPPETS to make 2D anisotropy measurements. Near the glass transition, rotation on the seconds timescales in poly(cyclohexyl acrylate) has been analyzed with 2D correlation functions calculated from single-molecule trajectories [\textit{J. Phys. Chem. B}, \textbf{113} 2253 (2009)]. This method is an equilibrium approach to 2D kinetics. By averaging over the entire ensemble, the known problems of subensemble averaging are avoided. Both measurements indicate that rotational-rate dispersion in polymers is primarily due to heterogeneous local environments. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y1.00003: Computer simulation study of structure and dynamics of supercooled water in silica nanopores Nicholas Kuon, Branka Ladanyi In narrow hydrophilic pores, interactions with pore walls and confinement dimensions allow water to remain liquid well below the normal freezing point. We investigate the properties of nanoconfined supercooled water by means of molecular simulation. The focus of our study is confinement in approximately cylindrical silica pores, with diameters in the 20-40 {\AA} range, a model for MCM-41 materials. We use Gibbs-ensemble Monte Carlo method to determine water density in the pores in equilibrium with the bulk and molecular dynamics simulation to study the properties of confined water [1]. We study the translational and rotational mobilities of molecules in different interfacial layers and the effects on water dynamics of interfacial hydrogen bonding. We make contact with quasi-elastic neutron scattering experiments on supercooled water in MCM-14 silica pores by calculating and analyzing self-intermediate scattering functions of water hydrogens. [1] A. A. Milischuk and B. M. Ladanyi, J. Chem. Phys. \textbf{135}, 174709 (2011). [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y1.00004: Electrolyte Structure near Electrode Interfaces in Lithium-Ion Batteries Vincenzo Lordi, Mitchell Ong, Osvalds Verners, Adri van Duin, Erik Draeger, John Pask The performance of lithium-ion secondary batteries (LIBs) is strongly tied to electrochemistry and ionic transport near the electrode-electrolyte interface. Changes in ion solvation near the interface affect ion conductivity and also are associated with the formation and evolution of solid-electrolyte interphase (SEI) layers, which impede transport but also passivate the interface. Thus, understanding these effects is critical to optimizing battery performance. Here we present molecular dynamics (MD) simulations of typical organic liquid LIB electrolytes in contact with graphite electrodes to understand differences in molecular structure and solvation near the interface compared to the bulk electrolyte. Results for different graphite terminations are presented. We compare the results of density-functional based MD to the empirical reactive forcefield ReaxFF and the non-reactive, non-polarizable COMPASS forcefield. Notable differences in the predictive power of each of these techniques are discussed. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:48AM |
Y1.00005: Femtosecond optical force microscopy Invited Speaker: Eric Potma Investigating the nonlinear optical properties of individual nanoscale objects, including single molecules, requires exquisitely sensitive tools. Although several optical microscopy approaches have demonstrated single molecule sensitivity, the acquisition of the nonlinear optical response from individual objects with femtosecond resolution has remained a challenge. In this presentation, we will discuss a new type of microscopy, femtosecond optical force microscopy, which is designed to sensitively probe ultrafast excitation dynamics at the nanoscale. Optical force microscopy detects the molecular response after optical manipulation through minute changes in the force between an atomically sharp tip and the molecule. This approach achieves spectroscopy with femtosecond time resolution and 10 nm spatial resolution. We will highlight the principles of this technique and outline several applications in molecular spectroscopy, including measurements sensitive to excited state dynamics (pump-probe) and experiments that probe ground state vibrational dynamics (stimulated Raman). [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y1.00006: Spatio-spectral infrared vibrational nano-imaging of intermolecular coupling Benjamin Pollard, Eric Muller, Markus Raschke Molecular self-assembly, the function of biomembranes, and the performance of organic solar cells rely on molecular interactions on the nanoscale. The understanding and design of such heterogeneous functional soft matter has long been impeded by a lack of spectroscopic tools with sufficient nanometer spatial resolution, attomolar sensitivity, and intermolecular spectroscopic specificity. We implement vibrational scattering-scanning near-field optical microscopy ($s$-SNOM) in a multi-spectral modality to investigate the structure-function relationship in PS-$b$-PMMA block copolymers. Using a vibrational resonance as a sensitive reporter of local structure, coupling, and dynamics, we resolve spectral Stark shifts and line broadening correlated with molecular-scale morphologies. By creating images of solvatochromic vibrational shifts we discriminate local variations in electric fields between nanoscale bulk and interface regions, with quantitative agreement to dielectric continuum models. This new nano-chemometric ability to directly resolve nanoscale morphology and associated intermolecular interactions can form a basis for the systematic control of functionality in multicomponent soft matter systems. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y1.00007: Synchrotron Infrared Nano-Spectroscopy Eric Muller, Benjamin Pollard, Hans Bechtel, Michael Martin, Markus Raschke Heterogeneity underlies many fundamental physical processes and biological functions, and characterizing or ultimately controlling these requires spectroscopic imaging with simultaneous nanometer spatial resolution and sensitivity to chemical structure and composition. In ultrahigh resolution spectromicroscopies, however, spectroscopic sensitivity and spatial resolution often oppose one another. We overcome this limitation with scattering scanning near-field optical microscopy using synchrotron infrared radiation. In this method, the tip of an atomic force microscope acts as an optical antenna, localizing broadband synchrotron infrared radiation with high irradiance and low noise, enabling tip-limited imaging at $\le$40 nm resolution. Optical heterodyne amplified, Fourier-transform detection enables rapid spectral acquisition, spanning 700-5000cm$^{-1}$, with zeptomole (10$^{-21}$) sensitivity. Synchrotron infrared nano-spectroscopy (SINS) is broadly applicable, which we demonstrate through investigations of surface phonon polaritons, biominerals and proteins. Finally, we show preliminary results incorporating advanced optical-antenna designs, with the goal of single molecule infrared spectroscopy. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y1.00008: Crystallization of supercooled liquids Takashi Odagaki, Yuuna Shikuya We investigate the crystallization process on the basis of the free energy landscape (FEL) approach to non-equilibrium systems. In this approach, the crystallization time is given by the first passage time of the representative point arriving at the crystalline basin in the FEL. We devise an efficient method to obtain the first passage time exploiting a specific boundary condition. Applying this formalism to a model system, we show that the first passage time is determined by two competing effects; one is the difference in the free energy of the initial and the final basins, and the other is the slow relaxation. As the temperature is reduced, the former accelerates the crystallization and the latter retards it. We show that these competing effects give rise to the typical nose-shape form of the time-temperature transformation curve and that the retardation of the crystallization is related to the mean waiting time of the jump motion. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 11:00AM |
Y1.00009: Single Molecule Approaches to Studying Heterogeneity in Molecular Supercooled Liquids Invited Speaker: Laura Kaufman Supercooled liquids display behaviors consistent with the presence of heterogeneous dynamics. We investigate the length scales over which such heterogeneities exist and the time scales over which they persist using single molecule (SM) fluorescence microscopy. In previous work, multiple perylene diimide (PDI) probes were employed to investigate whether probe properties affected breadth of heterogeneity reported in the fragile supercooled liquid ortho-terphenyl (OTP) as well as in less fragile supercooled glycerol. In both cases, the fastest rotating probes reported the greatest breadth of heterogeneity in the host, regardless of physical probe size, suggesting slow probes were averaging over dynamic changes in the environment in time. Here, we introduce a new set of BODIPY-core based probes that are both smaller and more quickly rotating in OTP than the PDI probes. These probes show qualitatively different behavior than the PDI probes, reporting more spatial and temporal heterogeneity than previously studied probes. The newly employed probes open the door to studying the full range of consequences of dynamic heterogeneity in supercooled liquids on the molecular length scale. [Preview Abstract] |
Session Y2: Focus Session: Charge & Energy Transfer V
Sponsoring Units: DCPChair: Tim Lian, Emory University
Room: 102
Friday, March 7, 2014 8:00AM - 8:12AM |
Y2.00001: Bipolar Charge Transport Properties of Poly(imide-thienyl(thienylenevinylene)) Evan Lafalce, Xiaomei Jiang, Cheng Zhang The charge transport properties of $\pi $--conjugated polymers are of interest from a fundamental perspective and also because they are a limiting factor for many optoelectronic device applications. In this work, we study the charge carrier mobility and recombination in Poly(imide-thienyl(thienylenevinylene)) (imide-PTV), a novel PTV derivative with imide side group. The electron and hole mobility are determined separately through the Space Charge Limited Current (SCLC) analysis of single carrier diodes. These devices are fabricated using interfacial layers that provide carrier selective contacts. A mobility asymmetry factor of approximately 20 that favors hole transport is observed, with the hole mobility of the order of 10$^{-5}$ [cm$^2$/V*s]. Similar results are obtained from the analysis of the intensity dependence of photoconductivity. Complimentary analysis of the ambipolar carrier mobility through carrier extraction under linearly increasing voltage (CELIV) and double injection transient techniques are also presented. The effects of carrier recombination and trapping are discussed. We conclude that the hole transport is not the limiting factor for power conversion efficiency of photovoltaic device based on imide-PTV and PCBM. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y2.00002: A State Representation Approach for Atomistic Time-Dependent Transport Calculations in Molecular Junctions Tamar Zelovich, Leeor Kronik, Oded Hod A new method for simulating electron dynamics in open quantum systems out of equilibrium, motivated by the intuitive and practical nature of the damped Liouville von-Neumann equation approach of S\'anchez et al. [J. Chem Phys, 124, 214708 (2006)], is presented. The new approach is based on a transformation of the Hamiltonian matrix from an atomistic to a state representation of the molecular junction. This allows us to define the bias voltage across the system uniquely while maintaining a proper thermal distribution within the lead models. Furthermore, it allows us to investigate time-dependent effects in non-linear and multi-lead configurations. We investigate the degree of conservation of exact conditions such as the N-representability of the density matrix and suggest ways to remedy the violation of Pauli's exclusion principle. We believe that the new approach offers a practical and physically sound route for performing atomistic time-dependent transport calculations in realistic models of molecular electronics junctions. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y2.00003: Femtosecond Nonlinear Optical Studies of Radiationless Decay in Carotenoids and in the Peridinin--Chlorophyll $a$ Protein Soumen Ghosh, Michael Bishop, Jenny Jo Mueller, Nolan Shepherd, Warren Beck, Harry Frank Femtosecond transient-grating spectroscopy with optical heterodyne detection was employed to observe the time evolution of the absorption and dispersion components of the third-order nonlinear optical signal following resonant excitation of the S$_{2}$ (1B$_{u}^{+})$ states of $\beta ${\-}carotene in benzonitrile and peridinin in methanol. The absorption and dispersion components exhibit distinct time profiles owing to the population of dark intermediate states. An initial intermediate is populated on an ultrashort (\textless 30~fs) time scale in both carotenoids owing to the onset of torsional distortions on the S$_{2}$-state potential surface. The time-resolved transient-grating spectra obtained for peridinin in the peridinin--chlorophyll $a$ protein from \textit{Amphidinium carterae} indicate that the intermediate is formed even more rapidly than in solution. This finding suggests that the twisted conformation of the peridinin chromophore is controlled in the binding site so as to optimize energy transfer to chlorophyll $a$ by enhancing the formation of an intramolecular charge-transfer character. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 9:12AM |
Y2.00004: Energy and Charge Transfer in Dinuclear Ru-based Complexes Invited Speaker: Valeria Kleiman In this work, the excited state dynamics of a series of dinuclear compounds combining Ru based cromophores with M$=$Ru(II), Fe(II), Fe(III), Cr(III) are explored. Ru-$\mu $-NC-M dimers are good candidates to investigate the competition between electron and energy transfer in arrays of chromophores. The presence of a $\mu $-NC bridge affords a strong coupling between the moieties without providing acceptor states that might act as electron traps. Polypyridyl Ru based compounds play an important role on light-harvesting antennas for energy conversion. With proper knowledge of the excited state dynamics, multinuclear arrays of chromophores can be developed. Our studies focus on (i) energy/electron transfer from the Ru(II) to a 2$^{\mathrm{nd}}$ M center through the cyanide bridge, and (ii) geometry changes due to the exchange of one of the Ru(II) polypiridyl ligands . Broadband ultrafast spectroscopy shows excited state dynamics in the psec time regime. These dynamics depend strongly on the nature of the acceptor and the orientation of the ligand involved in the photoinduced transition. Hence, the competition between energy and electron transfer across the bridge is modulated by the selective choice of the secondary M center. We conclude that transition metals from the 3$^{\mathrm{rd}}$ row are good candidates for longer arrays since their lack of low-lying MC states precludes thermal deactivation.\\[4pt] Work done in collaboration with Shiori Yamazaki and Jaired Tate, University of Florida; Alejandro Cadranel and Luis Baraldo, Universidad de Buenos Aires. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y2.00005: Optical Excitations in Cycloparaphenylene Molecules of Various Sizes Lyudmyla Adamska, Iffat Nayyar, Anna Swan, Steven Doorn, Sergei Tretiak Cycloparaphenylene ([$n$]CPP) molecule can be imagined as $n$ benzene molecules connected in a periodic chain. [$n$]CPPs with even number of links have alternating dihedral angles of $+$/- 34 degrees, whereas odd-numbered [$n$]CPPs cannot adopt such a high symmetry configuration, so they have a ``defect'': one of the rings is connected to its neighbors by dihedral angles of about 20 degrees. This ``defect'' plays a role of a localization site for an exciton. In this work we show that in [$n$]CPPs with $n$\textgreater 8 the exciton is localized on 5-6 rings, which strongly reduce their dihedral angles, while preserving the ground state geometry on the rest of the rings. This occurs both in odd-numbered and, surprisingly, in even-numbered [$n$]CPPs. We use electronic structure theory to address the spatial extent/properties of electronic wavefunctions and resulting electronic functionalities in [$n$]CPP molecular chromophores. Localization of excitonic states due to electron-phonon coupling in cycloparaphenylenes invalidates Condon approximation and breaks optical selection rules, making these materials to be efficient emitters. The effect of solvent is also discussed. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y2.00006: Signature of Nonadiabatic Coupling in Excited-State Vibrational Modes Tammie Nelson, Miguel Soler, Adrian Roitberg, Sergei Tretiak, Sebastian Fernandez-Alberti Using analytical excited-state gradients, vibrational normal modes are obtained at the minimum of the electronic excited-state potential energy surfaces for a set of extended conjugated molecules. In regions of strong coupling, the contribution to the forces in the direction of the corresponding non-adiabatic (NA) coupling vector (i.e., the Pechuckas force) is the dominant driving force for nuclear motion and should be reflected in the specific adiabatic excited-state equilibrium normal modes (ES-ENMs) responsible for the coupling. Specifically, the projection of the NA coupling vector on the basis on ES-ENMs with a significant agreement with a single ES-ENM indicates an effective decoupled direction for NA energy transfer. The influence of the nonadiabatic coupling on the excited-state equilibrium normal modes is revealed as a unique highest frequency adiabatic vibrational mode that overlaps with the coupling vector. Comparison with vibrational modes computed in a locally diabatic representation demonstrates that the effect of nonadiabaticity is confined to only a few modes. Such an approach is encouraging as it suggests that the nonadiabatic character of a system may be detected spectroscopically by identifying these unique high frequency modes. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y2.00007: Non-Adiabatic, Multi-State Ring-Polymer Molecular Dynamics Franziska Bell, Artur Menzeleev, Thomas Miller III Ring-polymer molecular dynamics (RPMD) has been shown to be a promising method for studying mechanisms and rates in large systems which require the inclusion of quantum effects, such as zero-point energies and tunneling. Examples involve electron and/or proton transfer reactions in enzymes and artificial catalysts. However, the traditional formulation of RPMD has several shortcomings: (i) it is restricted to migrations of only one distinguishable electron, (ii) it cannot describe photophysical processes, and (iii) it cannot be used in conjunction with potential energy surfaces obtained from electronic structure methods. Here I present a parameter-free extension of the RPMD method that addresses these issues and allows for the direct simulation of non-adiabatic processes involving many-electron wavefunctions without prior assumptions of the reaction mechanism. The new approach is demonstrated to provide a quantitative description of electron-transfer reaction rates and mechanisms throughout (i) the normal and inverted regimes and (ii) the weak- and strong-coupling regimes. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y2.00008: Exploring the Vibrational Stark Effect in Fullerene and Derivatives from First Principles Peter Doak, Yajing Li, Douglas Natelson, Leeor Kronik, Jeffrey Neaton Fullerene (C60) and its derivatives have played a central role in molecular and organic electronics, where its electron affinity and high symmetry result in key functionality. Understanding the impact of local fields on C60 properties in situ is of considerable interest, and here we determine how electric fields alter vibrational modes via the vibrational Stark effect. Using density functional theory-based finite-difference approach, we calculate the shifts in mode energy and symmetry in electric fields in gas-phase C60, PCBM, and other derivatives of fullerene. We examine the effect of high and low symmetry electronic field orientations, symmetry breaking functionalization, and doping on the the magnitude and mode-dependence of the vibrational Stark effect. The implications for fullerene-based materials under device conditions is discussed. This work is supported by DOE and computational resources were provided by NERSC. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y2.00009: Excitation Energy Transfer in Peridinin-Chlorophyll $a$-Protein Complex Using F\"{o}rster Resonance Energy Transfer William Bricker, Cynthia Lo Peridinin-Chlorophyll $a$-Protein (PCP) is a trimeric light-harvesting protein complex containing peridinin and chlorophyll $a$ pigment molecules, and the excitation energy transfer (EET) efficiency within this system is estimated at close to 90{\%}. To study the EET in PCP, we use the F\"{o}rster Resonance Energy Transfer (FRET) model, assuming that energy transfer occurs incoherently. We calculate the FRET coulombic coupling and spectral overlap terms using multi-reference configuration interaction (MR-CI) with semi-empirical basis sets in MOPAC. The two dominant EET pathways within PCP are from the S$_{\mathrm{2}}$ state of peridinin to the Q$_{\mathrm{x}}$ band of chlorophyll $a$, and from the S$_{\mathrm{1}}$ state of peridinin to the Q$_{\mathrm{y}}$ band of chlorophyll $a$. In peridinin, absorption in the visible spectrum is due to the strongly allowed S$_{\mathrm{0}}$ to S$_{\mathrm{2}}$ transition, while the S$_{\mathrm{0}}$ to S$_{\mathrm{1}}$ transition is optically forbidden and has double excitation character. To more accurately describe the FRET coulombic coupling term, we are working on an extension to the MOPAC source code, which will provide transition densities for all transitions. Our EET study of PCP using FRET reveals the interplay of pigment geometry and environment on the EET rates and efficiencies within the complex. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y2.00010: Electric field manipulation of magnetoresistance in a single molecular spin-valve device Kamal Dhungana, Ranjit Pati Manipulation of spin transport in a molecular spin-valve device using external electric field is a challenging as well as an exciting task from both fundamental and technological points of view. The weak spin-orbit and hyperfine interactions in organic molecules make them potential candidates for spin conserved tunneling. Tunable spin transport properties in single molecular junctions have recently been demonstrated using spin polarized scanning tunneling microscope. Here, we model a molecular spin-valve device by attaching an organic molecule between two ferromagnetic electrodes. A single-particle many-body Green's function approach together with unrestricted density functional theory is employed to ca [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y2.00011: Electron transfer in a two-level system within a Cole-Davidson vitreous bath Mark Ratner, Mehdi Zarea, Michael Wasielewski We study electron transfer (ET) in a two level quantum system coupled to a glassy viscous bath. The bath is modeled by the Cole-Davidson (CD) spectral density. The ET in this model is compared to the ET in a normal Drude-Debye (DD) model. It is shown that at low temperatures and when the coupling to the bath is weak, the viscous bath preserves the quantum coherence for a longer time. However in the strong coupling regime, the tunneling rate is higher in the CD. In the classical high temperature limit the difference between the CD and DD models is negligible. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y2.00012: Exact factorization of the electron-nuclear wave function: General theory and applications Federica Agostini, Ali Abedi, Seung Kyu Min, Yasumitsu Suzuki, E.K.U. Gross The exact factorization of the molecular wave function [1, 2] to the product of an electronic factor, parametrically depending on nuclear positions, and a nuclear wave function is presented. This starting point is used to decomposed the time dependent Schr\"odinger equation into two equations, that generate the evolution of the electronic and nuclear components. In this formulation, time dependent scalar and vector potentials mediate the coupling between the two sets of degrees of freedom, in a formally exact way. They represent what is usually referred to as electronic ``back-reaction'' on the nuclei. In a model system for non-adiabatic electron transfer, we investigate the properties of the potentials [3] and we analyze the classical approximation [4] of nuclear dynamics, in comparison to exact dynamics. This last point will lead to the development of a practical scheme to deal with non-adiabatic dynamics in the mixed quantum-classical approximation. \\[4pt] [1] A. Abedi, N.T. Maitra and E.K.U. Gross, Phys. Rev. Lett. 105 (2010)\\[0pt] [2] A. Abedi, N.T. Maitra and E.K.U. Gross, J. Chem. Phys. 137 (2012)\\[0pt] [3] A. Abedi, F. Agostini, Y. Suzuki and E.K.U. Gross, Phys. Rev. Lett. 110 (2013)\\[0pt] [4] F. Agostini, A. Abedi, Y. Suzuki and E.K.U. Gross, accepted in Mol. Phys. DOI:10.1080/00268976.2013.84373 [Preview Abstract] |
Session Y4: Focus Session: Frustrated and Low-Dimensional Magnets
Sponsoring Units: GMAGChair: Matthew Stone, Oak Ridge National Laboratory
Room: 112/110
Friday, March 7, 2014 8:00AM - 8:12AM |
Y4.00001: Resonating Valence Bond states for low dimensional S=1 antiferromagnets Zheng-Xin Liu, Yi Zhou, Tai-Kai Ng We study $S=1$ spin liquid states in low dimensions. We show that the resonating-valence-bond (RVB) picture of $S=1/2$ spin liquid state can be generalized to $S=1$ case. For $S=1$ system, a many-body singlet (with even site number) can be decomposed into superposition of products of two-body singlets. In other words, the product states of two-body singlets, called the singlet pair states (SPSs), are over complete to span the Hilbert space of many-body singlets. Furthermore, we generalized fermionic representation and the corresponding mean field theory and Gutzwiller projected stats to $S=1$ models. We applied our theory to study 1D anti-ferromagnetic bilinear-biquadratic model and show that both the ground states (including the phase transition point) and the excited states can be understood excellently well within the framework. Our method can be applied to 2D $S=1$ antiferromagnets. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y4.00002: A new S $=$ 1/2 frustrated ferromagnetic chain with random exchange Christopher Landee, Susan Herringer, Mark Turnbull, Jordi Ribas, Juan Novoa We report on a new frustrated ferromagnetic Heisenberg chain, CuCl$_{2}$(2-Cl-3-Mepy) in which the Cu(II) ions are linked into well-isolated chains via bichloride bridges. The pyridine ligand is two-site disordered, leading to three unique magnetic superexchange pathways occurring at random throughout the chains. Magnetic susceptibility measurements (1.8 -- 300 K) show dominant ferromagnetic interactions; a maximum in the $\chi $T product is observed near 30 K, but no maximum in $\chi $ occurs. \textit{DFT} calculations of the exchange strengths predict ferromagnet exchange strengths of 81(5), 55(5), and 23(3) K occurring at random, but with nearly constant next-nearest-neighbor interactions of -13(2) K. We note that within uncertainties the ratios of the nnn interaction to the smaller two ferromagnetic interactions exceed or equals the quantum critical ratio of 0.25 at which the ground state of the system changes. Comparison of the experimental $\chi $T data to simulations of random configurations will be presented. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y4.00003: Excitations of quasi-one-dimensional field-induced spin density wave and nematic phases Oleg Starykh, Leon Balents We study the excitation spectrum and dynamical response functions for several quasi-one-dimensional spin systems in magnetic fields without dipolar spin order transverse to the field. This includes both nematic phases, which harbor ``hidden'' breaking of spin-rotation symmetry about the field direction and have been argued to occur in high fields in certain frustrated chain systems with competing ferromagnetic and antiferromagnetic interactions, and spin density wave states, in which spin-rotation symmetry is truly unbroken. Using bosonization, field theory, and exact results on the integrable sine-Gordon model, we establish the collective mode structure of these states, and show how they can be distinguished experimentally. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y4.00004: Dzyaloshinskii-Moriya induced inter-chain frustration in a new spin-chain compound Wolfram Lorenz, Manuel Haelg, Kirill Yu. Povarov, Andrey Zheludev We present a first study of the Dzyaloshinski-Moriya spin-chain material K$_2$CuSO$_4$Br$_2$, highlighting the peculiar frustration of inter-chain couplings between nearest neighbor chains in this compound. Bulk magnetization and specific heat data are found well consistent with spin-chain models. Inelastic neutron scattering data support the pronounced one-dimensionality of the compound. In detail, intra-chain exchange amounts to $20.4$ K, yet magnetic long-range-order sets in only at $100$ mK. An extraordinary magnetic phase diagram is observed, which may be attributed to inter-chain frustration of helical intra-chain correlations. The relevant Dzyaloshinskii-Moriya exchange is evidenced by ESR-data. Funding by SNSF division 2 is acknowledged. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y4.00005: Spontaneously magnetized Tomonaga-Luttinger liquid in frustrated quantum antiferromagnets Shunsuke Furuya, Thierry Giamarchi Spontaneous symmetry breaking and Nambu-Goldstone bosons (NGB) going with it are fundamental for a wide range of physical systems. NGB which governs low-energy physics is affected by dimensionality. In 1D quantum antiferromagnet, although the true long-range antiferromagnetic order is absent, we can consider a counterpart of the NGB of the antiferromagnetic order, that is, the Tomonaga-Luttinger liquid (TLL). Here TLL is related to the short-range antiferromagnetic order and described by a critical relativistic field theory. This nature of TLL is consistent with a general statement of NGB in higher dimensions that the NGB originating from the antiferromagnetic (ferromagnetic) order has a relativistic (non-relativistic) dispersion relation. In this context, it would be very surprising to find a TLL structure compatible with ferromagnetic order. In this talk, I will present a theory of spontaneously magnetized TLL which is realized in quasi-1D geometrically frustrated quantum antiferromagnets. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y4.00006: Anisotropic magneto-thermal transport in the frustrated spin ladder BiCu$_{2}$PO$_{6}$ B.-G. Jeon, B. Koteswararao, G.J. Shu, F.C. Chou, K.H. Kim We report the thermal conductivity ($\kappa )$ of $S=$1/2 frustrated two-leg ladder BiCu$_{2}$PO$_{6}$\textbf{. }In this material, an enhanced heat transport along the leg direction (\textbf{b}-axis) is observed above 10 K. This is in good accordance with the previous observation of dispersive, anisotropic magnetic excitation along the reciprocal \textbf{b}-axis [1], which suggests the presence of anisotropic magnetic heat transport along the leg direction. The suppression of the $\kappa (T)$ and its magnetic field dependence are also observed around 15 K which probably leads to the clear double-peak feature in the $\kappa (T)$ at about 6 K and 60 K. Based on the Debye model, this can be interpreted by the resonant scattering between phonon and magnetic energy levels which affects the phonon heat transport.\\[4pt] [1] K. W. Plumb \textit{et al}., Phys. Rev. B \textbf{88}, 024402 (2013). [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y4.00007: Successive magnetic-phase transitions in a frustrated two-leg spin ladder Takanori Sugimoto, Michiyasu Mori, Takami Tohyama, Sadamichi Maekawa Recently, successive phase transitions induced by magnetic field have been observed in a frustrated two-leg spin ladder BiCu$_2$PO$_6$, in which the frustration is introduced by the next-nearest-neighbor antiferromagnetic exchange interaction in the leg direction. First, we calculate the magnetic-field dependence of magnetization by using density-matrix renormalization-group method. In this calculation, we find a phase transition induced by strong frustration, which does not appear in a non-frustrated two-leg spin ladder. The phase transition emerges as a jump of susceptibility with unsaturated finite magnetization. We also investigate the origin of the phase transition by using the bond-operator mean-field approximation. We conclude that the phase transition is the same type as the Lifshitz transition. Our study is useful to analyze experimental data of BiCu$_2$PO$_6$. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y4.00008: Magnetic Ordering In SrHo$_{2}$O$_{4}$ Jiajia Wen, Seyed Koohpayeh, Tyrel McQueen, Wei Tian, Haifeng Li, Jiaqiang Yan, Ovidiu Garlea, Collin Broholm We report the experimental observation of unusual magnetic ordering in a frustrated magnet SrHo$_{2}$O$_{4}$. The Ho$^{3+}$ ions in this material form a lattice consisting of edge-sharing triangles making chains extending along the c direction. The chains in turn form a honeycomb-like structure in the a-b plane. Despite a Curie Weiss temperature of -16.9 K, magnetic long range ordering (LRO) occurs at the much lower Ne\'{e}l temperature (T$_{\mathrm{N}})$ of 0.7 K. Above T$_{\mathrm{N}}$, single crystal neutron scattering experiments carried out on MACS(NCNR) shows diffusive patterns of elastic scattering in momentum space, revealing anisotropic short-range correlation with extended correlations along the chain direction. Single crystal diffraction measurement down to 0.3 K at HB1A(ORNL) shows coexistance of a long range ordering component and a short range ordering one below T$_{\mathrm{N}}$ . A proper partition of the magnetic lattice allows excellent modeling of the experimental data, and show that an interplay of frustration, crystal field effects, and low dimensionality lay behind the deeply suppressed and unusual partial order in this system. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y4.00009: Evidence for SrHo$_2$O$_4$ and SrDy$_2$O$_4$ as model 1D J$_1$-J$_2$ zig-zag chain materials Amy Poole, Vladimir Pomjakushin, Anne-Christine Uldry, Bobby Prevost, Alexandre Desilets-Benoit, Andrea Bianchi, Britt Hansen, Robert Cava, Michel Kenzelmann This presentation will focus on the Ho and Dy members of the recently discovered Sr$R_2$O$_4$ family of frustrated rare-earth ($R$) materials. Despite the $R$ sites forming a honeycomb in the $ab$ plane, we demonstrate that the physics is dominated by 1D correlations in the $c$-direction and that these can be mapped onto the Ising $J_1$-$J_2$ spin chain model. Three-dimensional magnetic order is suppressed from $\theta_w$ = -16.9 K to T$_N$ = 0.66 K in SrHo$_2$O$_4$ and is not observed to 50 mK in SrDy$_2$O$_4$. Our neutron powder diffraction measurements indicate that prior to the 3D order in the SrHo$_2$O$_4$ compound a 1D magnetically correlated state exists and that a similar state is found at the lowest measured temperatures in SrDy$_2$O$_4$. Fits to the diffuse data allow us to identify the 1D nature of the material and the key structural motifs are found from the data collected in the long-range ordered phase of SrHo$_2$O$_4$. Point charge calculations are fitted to the inelastic neutron scattering data and identify a large Ising anisotropy. The combination of neutron scattering and novel modelling techniques has allowed us to unambiguously determine the magnetic structure, identify the key interactions in the system and understand the 1D nature of the system. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y4.00010: Global phase diagram of the stacked frustrated triangular Ising system in a transverse field: a quantum Monte Carlo study Han Ma, Jie Lou, Alexei Tsvelik, Naoki Kawashima, Yan Chen The global phase diagram of the stacked frustrated triangular Ising magnet in a transverse field is obtained by using continuous time quantum Monte Carlo method. As the inter-plane interaction is strengthened, an first-order transition from a ferrimagnetic phase with two equivalent sublattices (FR2) to a partially disordered antiferrimagnetic phase (AF) occurs at small transverse field. In the quasi-one dimensional case, i.e. antiferromagnetically coupled transverse Ising chains, which corresponds to the realistic material CoNb2O6, our simulation reveals the existence of the low-field FR2 phase. In contrast, in the quasi-two dimensional limit, i.e. weakly coupled triangular Ising magnet, upon increasing the transverse magnetic field, the FR2 and AF phases successively appear in the order. In the vicinity of the ordered-disordered phase transition, the nature of phases can hardly be identified within our computational ability. At large transverse field, the paramagnetic phase trivially appears. Future experiments on CoNb2O6 at low temperature are expected to evidence the different magnetic patterns of this frustrated magnet based on our results in the quasi-one dimensional limit. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y4.00011: Low temperature magnetic ordering in the frustrated zigzag ladder system BaNd$_{2}$O$_{4}$ A.A. Aczel, L. Li, J.-Q. Yan, F. Weickert, V.S. Zapf, M. Jaime, L. Civale, R. Movshovich, V. Keppens, D. Mandrus The AR$_{2}$O$_{4}$ family (R $=$ rare earth) have recently been attracting interest as a new series of frustrated magnets, with the magnetic R atoms forming zigzag chains running along the c-axis. We have investigated polycrystalline BaNd$_{2}$O$_{4}$ with a combination of low temperature magnetization, heat capacity, and neutron diffraction measurements. This material has a Curie-Weiss temperature of -24 K, while our zero field heat capacity measurements indicate a magnetic transition of only 1.7 K, indicative of a high magnetic frustration index. Combined magnetization and neutron diffraction data show evidence for a complex, canted antiferromagnetic ground state with a propagation vector of (0 0.5 0.5) and the spins lying in the ac-plane. Furthermore, low temperature magnetization and heat capacity measurements as a function of applied field reveal that the order can be completely suppressed in an applied field of only 3.5 T. Direct comparison of these results to previous work on SrR$_{2}$O$_{4}$ shows that there is a rich diversity of magnetic behavior in this family of frustrated magnets, likely due to a competition between single ion anisotropy, dipole-dipole interactions, and exchange interactions. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y4.00012: Variational study of the $J_1-J_2$ Antiferromagnetic Heisenberg model on square lattice using correlated valence bond state Ling Wang, Olexei Motrunich We propose a variational ansatz in the context of resonating valence bond state for the $J_1-J_2$ Antiferromagnetic model on square lattice in the strongly frustrated regime. The ansatz modifies the nearest neighbor valence bond state by considering short range bond-bond correlations and their local resonances. The correlated bond-bond pairs have their local sign factor, which can produce the correct sign structure (nodal structure) as can be checked on $4\times 4$ and $6\times 6$ torus. The variational energy (up to size $10\times 10$) is highly competitive with the recent DMRG study on torus and the slave fermion variational Monte Carlo study with two step lanczos projection. We will discuss phases in the intermediate coupling region. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y4.00013: Reentrant Berezinskii-Kosterlitz-Thouless Transitions in a Triangular Ising Thin Film Yoshitomo Kamiya, Shi-Zeng Lin, Gia-Wei Chern, Cristian Batista We study the triangular lattice Ising model with a finite number of vertically stacked layers and demonstrate a low temperature reentrance of one or two Berezinskii-Kosterlitz-Thouless transitions, which results in an extended disordered regime down to T $=$ 0. This regime exhibits a novel and peculiar low-temperature thermodynamics such as the enhanced short-range magnetic correlation as temperature is increased. Numerical results are complemented with the derivation of an effective dimer theory that quantitatively describes the low temperature physics. Qualitative features of the global phase diagram are obtained by mapping the classical spin model into the single-layer quantum Ising model. (Reference: Shi-Zeng Lin, Yoshitomo Kamiya, Gia-Wei Chern, and Cristian Batista, \underline {arXiv:1310.3468}) [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y4.00014: Simulation of the magnetism of the 2D XY spin system in finite magnetic field Chuntai Shi, Sungho Han, Clare Yu Experiments implicate surface spins as the source of flux and inductance noise in SQUIDs. There is experimental evidence that interactions between these surface spins cannot be ignored. We investigate the effect of applying an external magnetic field on an interacting surface spin system. As a candidate model of the surface spins, we present Monte Carlo simulations of the classical 2D XY spin model on a square lattice. We monitor the time evolution of the susceptibility of the system after an external magnetic field is applied. We also look at the local magnetic field at a site produced by the neighboring spins. We monitor how the distribution of local fields changes after the external field is applied. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y4.00015: High magnetic field phases of the $J_1$-$J_2$-$J_3$ triangular antiferromagnet Giacomo Marmorini, Tsutomu Momoi We present a thorough study of the $J_1$-$J_2$-$J_3$ triangular lattice antiferromagnet close to the saturation field, where the magnetic structure is determined by the condensation of magnons. We focus on the case of ferromagnetic $J_1$, that is particularly rich because frustration effects can allow for magnons of different (commensurate or incommensurate) wave-vectors to condense simultaneously. Our calculation includes an interlayer coupling $J_0$, that can be taken as small as $10^{-4}$ (in units of $J_1$), in which case the system is nearly two-dimensional. Besides the well-known spiral and fan phases, we find a new double-$q$ phase (superposition of two modes), dubbed ``01'' phase, whose features (including a new type of multiferroic behavior) can be seen as intermediate between the two. Furthermore, in some regions of the parameter space, we show that a dilute gas of magnon can not be stable and phase separation (corresponding to a magnetization jump) is expected. Related to this, we discuss the presence of quantum tricritical points. In the $J_1$-$J_2$ model ($J_3=0$) bound states of two and three magnons may also appear, but it is an open issue whether or not they form a stable condensate and then give rise to nematic or octupolar order. [Preview Abstract] |
Session Y6: Focus Session: Magnetic Oxide Thin Films and Heterostructures: Oxide Films and Nanoparticles
Sponsoring Units: DMP GMAGChair: Jiun-Haw Chu, Lawrence Berkeley National Laboratory
Room: 108
Friday, March 7, 2014 8:00AM - 8:12AM |
Y6.00001: Linear magnetoresistance of hetroepitaxial thin films of pyrochlore iridates Bi$_2$Ir$_2$O$_7$ Jiun-Haw Chu, Scott Riggs, Maxwell Shapiro, Jian Liu, Claudy Ryan Serero, Di Yi, Matthew Melissa, S.J. Suresha, Carlos Frontera, Ashvin Vishwanath, Xavi Marti, Ian Fisher, R. Ramesh We report on the discovery of linear magnetoresistance in hetroepitaxial thin films of the pyrochlore iridates Bi$_2$Ir$_2$O$_7$. The magnetoresistance of Bi$_2$Ir$_2$O$_7$ shows a highly isotropic, linear field dependence at T = 1.6K, but gradually evolves towards a quadratic field dependence as temperature increases. By interfacing the Bi$_2$Ir$_2$O$_7$ with pyrochlore spin ice compound Dy$_2$Ti$_2$O$_7$, the magnetoresistance at sub-kelvin temperatures shows pronounce anisotropy with a complex field dependence. We argued that these unusual magnetotransport behaviors cannot be explained by disorder induced quantum correction, but might be related to the magnetism of Bi$_2$Ir$_2$O$_7$. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y6.00002: Reversal of lattice, electronic structure, and magnetism in epitaxial SrCoO$_{x}$ thin films H. Jeen, W.S. Choi, J.H. Lee, V.R. Cooper, H.N. Lee, S.S.A. Seo, K.M. Rabe SrCoO$_{x}$ ($x =$ 2.5 -- 3.0, SCO) is an ideal material to study the role of oxygen content for electronic structure and magnetism, since SCO has two distinct topotactic phases: the antiferromagnetic insulating brownmillerite SrCoO$_{2.5}$ and the ferromagnetic metallic perovskite SrCoO$_{3}$. In this presentation, we report direct observation of a reversible lattice and electronic structure evolution in SrCoO$_{x}$ epitaxial thin films as well as different magnetic and electronic ground states between the topotactic phases.\footnote{W. S. Choi \textit{et al.}, Phys. Rev. Lett. \textbf{111}, 097401 (2013).} By magnetization measurements, optical absorption, and transport measurements drastically different electronic and magnetic ground states are found in the epitaxially grown SrCoO$_{2.5}$ and SrCoO$_{3}$ thin films by pulsed laser epitaxy. First-principles calculations confirm substantial, which originate from the modification in the Co valence states and crystallographic structures. By real-time spectroscopic ellipsometry, the two electronically and magnetically different phases can be reversibly changed by changing the ambient pressure at greatly reduced temperatures. Our finding provides an important pathway to understanding the novel oxygen-content-dependent phase transition uniquely found in multivalent transition metal oxides. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y6.00003: Local Structure in Magnetically Phase Separated Perovskite SrCoO$_{3-y}$ Z.H. Zhu, F.J. Rueckert, J.I. Budnick, W.A. Hines, B.O. Wells, Ch. Niedermayer, B. Dabrowski Magnetic phase separation has recently been found in the oxygen deficient perovskite SrCoOx (2.88$\le $x$\le $3). Samples with appropriate oxygen concentration show two component magnetic behavior while maintaining a single crystallographic phase. The two magnetic phases match those found in SrCoO$_{2.88}$ and SrCoO$_{3}$ with Tc $=$ 220 K and 280 K, respectively. Muon Spin Rotation ($\mu$ SR) has been used to explore the local spin structures and phase behavior of these cobaltates. The data reveal the possible existence of spatially separated magnetic region and two true phase transitions. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y6.00004: Metal-insulator transition with ferrimagnetic order in epitaxial thin films of spinel NiCo$_{2}$O$_{4}$ Punam Silwal, Ludi Miao, Ilan Stern, Xiaolan Zhou, Jin Hu, Leonard Spinu, Dae Ho Kim, Diyar Talbayev Spinel NiCo$_{2}$O$_{4}$ is attractive for various technological applications but is less studied partly because of the unavailability of NiCo$_{2}$O$_{4}$ single crystal or epitaxial thin film. We have grown high-quality crystalline epitaxial NiCo$_{2}$O$_{4}$ thin films on MgAl$_{2}$O$_{4}$ (001) substrates. The systematic investigation of the films grown at various temperatures reveals a strong correlation between the structural, magnetic, and electrical transport properties. The low-temperature grown films show metallic behavior with strong ferrimagnetic ordering while the high temperature grown films are insulating with suppressed magnetic order. In addition, these films show excellent transport and magnetic properties down to 2 unit-cell thickness. Our study of temperature- and growth-condition dependent optical conductivity provides further insight in the carrier transport of these films. We observed coherent band-like transport in both low- and high temperature grown films, whereas only thermally activated hopping conductivity was reported in previous studies. The confirmation of coherent band like transport provides a basis for further improving NiCo$_{2}$O$_{4}$ for the application as transparent conducting oxide. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y6.00005: Magnetic phase diagram of thin film La$_{2- x}$Sr$_ x$CuO$_4$ studied by low energy muon spin rotation E. Stilp, B.M. Wojek, H. Keller, A. Suter, T. Prokscha, H. Luetkens, E. Morenzoni, A. Gozar, G. Logvenov, I. Bozovic The magnetic phase diagram of La$_{2- x}$Sr$_ x$CuO$_4$ thin film samples grown on SrLaAlO$_4$ has been determined by low-energy muon spin rotation. The obtained phase diagram shows the same features as that one of the bulk, but the transition temperatures are drastically shifted. In the antiferromagnetic phase the Neel temperatures $T_{\rm N}$ are strongly reduced compared to the bulk material and no spin freezing was observed at low temperatures. In the disordered magnetic phase ($x \geq 0.02$) the transition temperature $T_{\rm g}$ is enhanced. It is concluded that the main reason for the pronounced differences between the magnetic phase diagrams of thin film and bulk samples is strain induced disorder in the thin films. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y6.00006: Spin-orbital separation in the anisotropic ladder system CaCu$_2$O$_3$ Valentina Bisogni, Krzysztof Wohlfeld, Satoshi Nishimoto, Claude Monney, Jan Trinckauf, Kejin Zhou, Roberto Kraus, Thorsten Schmitt, Jeroen van den Brink, Jochen Geck Recently, resonant inelastic X-ray scattering (RIXS) on the 1D spin system Sr$_2$CuO$_3$ has revealed an unprecedented dispersion of orbital excitations [Nature 485, 82 (2012)]. This result has been interpreted as the fractionalization of spin and orbital degree of freedom from the elementary electron, hallmark of one dimensional physics as the previously observed spin-charge separation [Nature Phys. 2, 397 (2006)]. How these phenomena carry over into higher dimensions remains currently unclear. To clarify this point, we studied the spin and orbital excitations of the anisotropic ladder CaCu$_2$O$_3$, which realizes a first step towards 2D correlated electron systems. Combining high-resolution RIXS experiments with theoretical model calculations we show that spin-orbital fractionalization indeed occurs in CaCu$_2$O$_3$ and prevails beyond the strict 1D limit [arXiv:1310.8346]. We also establish that such a fractionalization is far more robust than the spin-charge separation. The main reasons behind this are the intrinsic 1D orbital dynamics and the fact that the spinons are faster than the orbitons but slower than the holons. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y6.00007: Continuous wave terahertz spectroscopy of Sr$_{2}$CrReO$_{6}$ thin films at cryogenic temperatures and in high magnetic fields D.R. Daughton, R. Higgins, S. Yano, C.H. Du, A.J. Hauser, R. Adur, J.M. Lucy, H.L. Wang, D.V. Pelekhov, E. Johnston-Halperin, F.Y. Yang, P.C. Hammel Temperature and magnetic field dependent terahertz spectroscopies have proven useful in characterizing and manipulating the structural, charge, and magnet ordering in complex oxide systems. THz transmission measurements on epitaxial thins films of the double-perovskite ferrimagnet Sr$_{2}$CrReO$_{6}$ (SCRO) were performed with a novel continuous-wave terahertz transmission spectrometer operating from 5 K to 300 K and with fields up to 9 T. Temperature-dependent changes in the film conductivity manifest as strong variations in the Fabry-Perot interference patterns from the supporting substrate. Indicative of variable-range hopping transport in the films, we find the conductivity varies with the THz frequency (f) as f$^{s}$ with s$\sim $0.8 at 5 K. Depending on the handedness of the incident THz source, magnetic fields in excess of 4 T enhance or suppress the THz transmission of the SCRO films by $\sim $8{\%}. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y6.00008: The Electric, Magnetic, and Optical Characterization of Permalloy Oxide Grown by Dual-Ion Beam Sputtering Maclyn Compton, Elizabeth LeBlanc, Wilhelmus Geerts, Nelson Simpson, Michael Robinson Permalloy (Ni$_{80}$Fe$_{20})$ is a commonly used soft magnetic material in magnetic reading heads. Its magnetic properties do not depend on stress, a parameter difficult to control in thin film devices. Permalloy Oxide (PyO) on the other hand, has a high resistivity (\textgreater 4$\cdot$10$^{3} \Omega $ cm), is anti-ferromagnetic and has recently been shown to strongly enhance the performance of lateral spin valve devices. Historically, the oxidation of permalloy has been seen as a defect that should be avoided by appropriate encapsulation and very little is known on its electric and optical properties. We deposited thin PyO films by Dual Ion Beam Sputtering (DIBS) at room temperature on various substrates. Van der Pauw and Hall measurements were carried out from 77K to 400K and at magnetic fields up to 9T in order to determine its electronic bandgap, resistivity, free carrier concentration, and its mobility. The dielectric properties and defects were studied using a CV-setup and an impedance analyzer. Magnetic measurements were conducted on a Quantum Design PPMS VSM to determine the state of oxidation. Optical properties were measured by a M2000 Woollam variable angle spectroscopic ellipsometer. These properties were used to determine film thickness, bandgap and the optical constants of PyO. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y6.00009: Rashba Spin-Orbit Anisotropy and the Electric Field Control of Magnetism Jun'ichi Ieda, Stewart E. Barnes, Sadamichi Maekawa The control of the magnetism of ultra-thin ferromagnetic layers using an electric field would lead to many technologically important applications. To date, while it is usually assumed the changes in the magnetic anisotropy, leading to such a control, arises from surface charge doping of the magnetic layer, a number of key experiments cannot be understood within such a scenario. Much studied is the fact that, for non-magnetic metals or semi-conductors, a large surface electric field gives rise to a Rashba spin-orbit coupling which leads to a spin-splitting of the conduction electrons. Here we develop a simple analytic theory for the existence and electrical control of the magnetic anisotropy based upon the Rashba spin-orbit interaction and the Stoner model of magnetism. We show that the competition between the Rashba spin-orbit fields and the exchange interaction leads to a very large magnetic anisotropy arising from the internal electric fields which exist at, e.g., ferromagnetic/metal and ferromagnetic/oxide insulator interfaces but modified by the addition of an applied electric field. This different path to an electrically induced anisotropy energy can explain the electric field, thickness, and material dependence reported in many experiments. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y6.00010: Magnetic and transport signatures of Rashba spin-orbit coupling on the Kondo lattice model in two dimensional clusters Jose Riera Motivated by emergent phenomena at oxide surfaces and heterostructures, particularly those involving transition metal oxides with perovskite crystal structure such as LaTiO$_3$/SrTiO$_3$, we examine the Kondo lattice model in the presence of a Rashba spin-orbit coupling (RSOC). Using an array of numerical techniques, under the assumption that the electrons on localized orbitals may be treated as classical continuum spins, we compute various charge, spin and transport properties on square clusters and on ladders at zero and finite temperatures. The main goal is to determine magnetic and transport signatures due to the RSOC. The same model can be used to study at an effective level the combined effect on magnetic and transport properties of Rashba and ferromagnetic moments, such as the ones present at LMnO$_3$/SrMnO$_3$ interfaces. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y6.00011: Magnetization and Hysteresis of Dilute Magnetic-Oxide Nanoparticles Ralph Skomski, B. Balamurugan, D.J. Sellmyer Real-structure imperfections in dilute magnetic oxides tend to create small concentrations of local magnetic moments that are coupled by fairly long-range exchange interactions, mediated by p-electrons. The robustness of these interactions is caused by the strong overlap of the p orbitals, as contrasted to the much weaker interatomic exchange involving iron-series 3d electrons. The net exchange between defect moments can be positive or negative, which gives rise to spin structures with very small net moments. Similarly, the moments exhibit magnetocrystalline anisotropy, reinforced by electron hopping to and from 3d states and generally undergoing some random-anuisotropy averaging. Since the coercivity scales as 2\textit{K}$_1$/\textit{M} and \textit{M} is small, this creates pronounced and --- in thin films --- strongly anisotropic hysteresis loops. In finite systems with \textit{N} moments, both \textit{K}$_1$ and \textit{M} are reduced by a factor of order \textit{N}$^{1/2}$ due to random anisotropy and moment compensation, respectively, so that that typical coercivities are comparable to bulk magnets. Thermal activation readily randomizes the net moment of small oxide particles, so that the moment is easier to measure in compacted or aggregated particle ensembles. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y6.00012: Computational Nano-materials Design of Dynamically Created New Functional Ordered Oxide Nano-superstructures by Spinodal Nano-Decomposition: Design vs. Experimental Realizations Masayoshi Seike, Tetsuya Fukushima, Kazunori Sato, Hiroshi Katayama-Yoshida Based on ab initio electronic structure calculation and multi-scale simulation, we discuss the design of magnetic mechanism and the self-organized Spinodal Nano-Decomposition in dilute magnetic oxides in MgO, SrO, BaO, CaO, ZnO, NiO , and Re-RAM with d0 ferromagnetism. By controlling the dimensionality (2D and 3D) of the crystal growth, crystal growth speed, substrate temperatures, and seeding in the self-organized nanostructure formation, we design the shape controlled quantum-dot (Dairiseki-Phase) and quantum nanowire (Konbu-Phase), and the new functionality such a Re-RAM, and high-blocking temperature in super-para-magnetism. We compare our recent computational nano-materials design data with the recent available experimental verifications.\\[4pt] [1] K. Sato et al., Rev. of Mod. Phys., 82, (2010) 1633.\\[0pt] [2] M. Toyoda, et al., Physica B 376, (2006) 647.\\[0pt] [3] Nguyen Dang Vu, et al., Appl. Phys. Express, 4, (2011) 015203.\\[0pt] [4] K. Kenmochi, et al., J. Phys. Soc. Jpn, 73, (2004) 2952.\\[0pt] [5] M. Seike et al., Jpn. J. Appl. Phys.50 (2011) 090204.; ibid 51 (2012) 050201.\\[0pt] [6] K. Oka et al., J. Am. Chem.Soc. 134 (2012) 2535. [Preview Abstract] |
Session Y7: Focus Session: Magnetic Structures: Novel Mechanisms and Experimental Exploration
Sponsoring Units: GMAGChair: Aakash Pushp, IBM Research
Room: 106
Friday, March 7, 2014 8:00AM - 8:12AM |
Y7.00001: Towards magnetic 3D x-ray imaging Peter Fischer, R. Streubel, M.-Y. Im, D. Parkinson, J.-I. Hong, O.G. Schmidt, D. Makarov Mesoscale phenomena in magnetism will add essential parameters to improve speed, size and energy efficiency of spin driven devices. Multidimensional visualization techniques will be crucial to achieve mesoscience goals. Magnetic tomography is of large interest to understand e.g. interfaces in magnetic multilayers, the inner structure of magnetic nanocrystals, nanowires or the functionality of artificial 3D magnetic nanostructures. We have developed tomographic capabilities with magnetic full-field soft X-ray microscopy combining X-MCD as element specific magnetic contrast mechanism, high spatial and temporal resolution due to the Fresnel zone plate optics [1]. At beamline 6.1.2 at the ALS (Berkeley CA) a new rotation stage allows recording an angular series (up to 360 deg) of high precision 2D projection images. Applying state-of-the-art reconstruction algorithms it is possible to retrieve the full 3D structure. We will present results on prototypic rolled-up Ni [2] and Co/Pt tubes and glass capillaries coated with magnetic films and compare to other 3D imaging approaches e.g. in electron microscopy [3]. \\[4pt] [1] P. Fischer, Mat. Sci {\&} Eng. R72 81 (2011)\\[0pt] [2] R. Streubel et al., doi:10.1002/adma.201303003.\\[0pt] [3] C. Phatak et al, Ultramicroscopy 109 264 (2009) [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y7.00002: Magnetic contrast tuning with nonlinear magneto-plasmonics Wei Zheng, Aubrey T. Hanbicki, Berend T. Jonker, Gunter Lupke Magneto-plasmonics describes systems where plasmonic and ferromagnetic properties coexist. The nonlinear-optical magnetic second-harmonic generation (MSHG) technique is extremely sensitive to subtle modifications of the spin-polarized electronic structure of transition metal surfaces, the same region where surface plasmons (SP) are present. This technique, which builds a direct link between plasmonics and the magneto-optical effect, is called nonlinear magneto-plasmonics. We will present results of experiments that show that not only can the MSHG signal be enhanced by SPs in an attenuated total reflection (ATR) condition, but also that the magnetic contrast can be tuned by the angle-of-incidence. Furthermore, the magnetic contrasts of transverse and longitudinal MSHG display opposite trends. The tuning effect originates from the change of relative phase between magnetic and non-magnetic MSHG components. This new effect enhances the sensing of magnetic switching which has potential usage in quaternary magnetic storage systems and bio-chemical sensors due to its very high surface sensitivity. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y7.00003: Faraday Rotation Spectrum of Bismuth-Doped Rare-Earth Iron Garnets for Magneto-Optic Sensor Applications Mannix Shinn, Dong Ho Wu, Anthony Garzarella, Rongjia Tao Iron garnet Faraday rotators are a promising sensor material for measuring magnetic fields. The rotator's field sensitivity increases inversely with wavelength and beam path, but so does the insertion loss. We wish to optimize sensor sensitivity by studying the transmission coefficient and Verdet constant over a spectrum from 0.4 to 2 um in samples of bismuth-doped rare-earth iron garnet. Data for two different gallium doped samples will be presented, including data of other magnetic field dependent effects that were observed. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y7.00004: Material dependence of magnetic hysteresis in single nanometer-scale ferromagnetic particles Patrick Gartland, Wenchao Jiang, Dragomir Davidovic The characteristics of ferromagnetic particles change remarkably when their size approaches the nanometer scale and below. We conduct single-electron tunneling experiments to study the breakdown of magnetic hysteresis in single particles 2-5 nm in diameter, made of Co, Fe, Ni, and Py=Ni$_{0.8}$Fe$_{0.2}$. At $T=4.2$K and at mK-temperature, we observe a dramatic difference in magnetic hysteresis among these metals: All of the Co and Fe particles, but only 4\% of the Ni particles exhibit magnetic hysteresis. The tunneling spectra of Ni particles at mK-Temperature display evidence of ferromagnetism, despite the absence of hysteresis. We will present recent experimental data that will shed light on the possible mechanisms driving this strong suppression of hysteresis in Ni. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y7.00005: Assembly of Magnetic Nanoclusters Balamurugan Balasubramanian, Ralph Skomski, Bhaskar Das, George Hadjipanayis, David Sellmyer Nanostructured Co or Fe-rich magnetic materials are of interest for a wide range of applications because of their novel structures and spin phenomena [1, 2]. In this presentation, the synthesis and stability of nanoparticles of (Co-Fe):X alloys (X $=$ Hf, Sm, Si) having unusual crystal structures will be discussed. The nanoparticles are produced using a single step-process in a cluster-deposition system and are smaller than 10 nm with an rms standard deviation of $\sigma $/$d \le $ 0.15. In particular, Co-rich nanoparticles such as HfCo$_{\mathrm{7}}$ and SmCo$_{5}$ exhibit high magnetocrystalline anisotropies ($K_{1}$ \textgreater 10 Mergs/cm$^{3})$ and saturation magnetic polarizations ($J_{s}$ \textgreater 10 kG). The nanoscale effects on the magnetism including spin structure, magnetic polarization, and other intrinsic properties, and the potential of the nanostructures for various applications will be presented. \\[4pt] [1] B. Balamurugan, B. Das, V.R. Shah, R. Skomski, X.Z. Li, and D.J. Sellmyer, \textit{Appl. Phys. Lett.} 101, 122407 (2012).\\[0pt] [2] Advanced Magnetic Nanostructures, Eds. D.J. Sellmyer and R. Skomski. Springer: New York, 2001. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y7.00006: Structure and magnetism of nanostructured Zr$_{2}$Co$_{11}$ Bhaskar Das, Balamurugan Balasubramanian, Wenyong Zhang, Ralph Skomski, David Sellmyer Recently nanostructured Zr$_{2}$Co$_{11}$-based alloys crystallizing in the rhombohederal structure have emerged as novel magnetic material with an appreciable magnetocrystalline anisotropy constant ($K_{1}\approx $ 20 Mergs/cm$^{3}$), a high saturation magnetic polarization ($J_{s} \ge $ 10 kG), and a high Curie temperature ($T_{c} \approx $ 783 K) [1, 2]. The nanostructured Zr$_{2}$Co$_{11}$ films were fabricated using cluster-deposited nanoparticles of smaller than 10 nm as building blocks. The nanoscale effect on structure and room-temperature magnetic properties was investigated by comparing those of melt-spun bulk alloys. In addition, the magnetic properties at elevated temperatures also will be discussed and this will provide a further insight to understand the magnetism of Zr$_{2}$Co$_{11}$ nanostructures and explore the possibility of using them for high-temperature applications.\\[4pt] [1] B. Balamurugan, B. Das, R. Skomski, W. Y. Zhang and D. J. Sellmyer, \textit{Adv. Mater. }25, 6089 (2013) \\[0pt] [2] B. Balamurugan, B. Das, R. Skomski, W. Y. Zhang and D. J. Sellmyer, \textit{J. Phys.: Condens. Matter.} (in press). [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y7.00007: Insight into the Slater-Pauling behavior of Permalloy-Cu alloys Ronny Knut, Justin Shaw, Hans Nembach, Patrik Grychtol, Emrah Turgut, Dmitriy Zusin, Henry Kapteyn, Margaret Murnane, Dario Arena, Erna Delczeg, Olle Eriksson, Olof Karis, Tom Silva Magnetic 3d transition alloys are of great importance, but a rigorous understanding of the magnetization for these systems remains elusive. While it is well-known that the average magnetic moment of 3d transition metal alloys obey Slater-Pauling, the oft-cited basis for our understanding of this ``law'' is suspect: The high-valence side of Slater-Pauling was originally explained by Slater via rigid-band theory (RBT), with the primary effect to be the filling of the minority d-band with valence electrons upon alloying. However, many ab-initio calculations do not support RBT since alloying affects the band-structure. We used X-ray magnetic circular dichroism to test the veracity of RBT as a model for Slater-Pauling behavior for Permalloy-Cu alloys, (Ni0.8Fe0.2)xCu1-x. We find that our data agrees with Slater-Pauling regarding average magnetic moments. Also, the dilution of Permalloy by Cu does result in some charge transfer from Cu to Ni/Fe, in qualitative agreement with RBT, but the charge transfer is inadequate to explain the dependence of spin moment on Cu concentration. Of equal importance is the decrease of exchange splitting due to the reduced number of magnetic neighbors. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y7.00008: Experimental Monocrystalline Micromagnetics: A Vortex Spin Topology with Cubic Anisotropy in YIG Lance C. Parsons, Joseph E. Losby, Fatemeh Fani Sani, Dylan T. Grandmont, Zhu Diao, Tayyaba Firdous, Douglas Vick, Wayne K. Hiebert, Mark R. Freeman The detailed magnetostatic characterization of an individual, single-crystalline yttrium iron garnet micromagnetic disk is reported. The crystalline orientation is such that a (111) direction of the cubic crystal structure is perpendicular to the disk surface. An easy axis is thus aligned with the core of the magnetic vortex state. The 600 nm-thick, 600 nm-radius disk is transferred to a nanomechanical torsional resonator for characterization by torque magnetometry. The experimental results show a pristine, Barkhausen-free low field response of the vortex magnetization to in-plane field. For angular measurements of magnetic hysteresis as a function of the in-plane direction of applied magnetic field, it is observed that the field strengths at which the vortex annihilation transition occurs are significantly less sensitive to magnetic anisotropy than are the nucleation fields. Micromagnetic simulation results show a rich, topologically stable structure owing to the disk thickness and monocrystalline nature. The comprehensive magnetostatic measurements yield an incisive determination of the degree to which ideal micromagnetic response has been approached in the fabricated disk, and of the role of magnetocrystalline anisotropy on vortex behavior and topological spin structure. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y7.00009: Determination of the electronic state of Er in sputtered AlN:Er films by magnetic measurements V. Narang, M.S. Seehra, D. Korakakis The optoelectronic [1] and piezoelectric [2] properties of AlN:Er thin films for device applications have been of great recent interest. The magnitude of optical activity depends on local crystalline environments of Er [3]. Here we focus on the electronic state of Er in AlN:Er (1.6 at.\%) films prepared by reactive magnetron sputtering on Si substrate. X-ray diffraction of the films shows that Er doping expands the lattice and XPS studies confirm the presence of Er. To determine if Er is present as Er metal, Er$_{2}$O$_{3}$ or Er$^{3+}$ substituting for Al$^{3+}$, magnetization was measured vs. temperature (2 K to 300 K) in H = 1kOe and data is found to fit the Curie law with a magnetic moment ${\mu}$ = 4.85 ${\mu}$$_{B}$ per Er, in good agreement with expected value for Er$^{3+}$ substituting for Al$^{3+}$ in AlN [4]. The presence of Er$_{2}$O$_{3}$ and Er metal is ruled out since magnetic transitions expected for Er$_{2}$O$_{3}$ (Er metal) at 3.4 K ({$\sim$}30 K) are not observed, thus establishing that Er substitutes for Al as Er$^{3+}$ in the AlN:Er films.\\[4pt] [1] A.R. Zanatta et al, J. Appl. Phys., 98, 093514 (2005);\\[0pt] [2] V. Narang et al, MRS Symp. Proc. 1519 (2013);\\[0pt] [3] R.G. Wilson et al, Appl. Phys. Lett., 66, 992 (1994);\\[0pt] [4] S. Yang, et al, J. Appl. Phys., 105, 023714 (2009) [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y7.00010: Control of magnetization alignment in magnetic nanocontacts Gavin D. Scott Transport measurements together with finite element micromagnetic simulations are used to investigate the magnetoresistance response of permalloy break junction devices. Ferromagnetic nanogap and point contact structures may be used to study the interplay between Kondo correlations and magnetic excitations by tuning the source and drain contact magnetization configuration. However, the magnetization of the leads is not trivially related to the precise arrangement of domain walls at the nanocontact tip region most relevant to transport. The shape anisotropy of elliptical electrodes together with improved fabrication techniques lead to competition between exchange and magnetostatic energies that may result in desirable source-drain magnetization alignments free of vortex states. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y7.00011: Magnetic structures of the anisotropic Dirac metals AMnBi$_{2}$ (A $=$ Ca, Sr) Yanfeng Guo, Andrew Princep, Pascal Manuel, Dimitry Khalyavin, Andrew Boothroyd Magnetism is potentially important in the Dirac materials AMnBi$_{2}$ (A $=$ Sr and Ca) because long-range magnetic order of the Mn spins provides an additional periodic potential that could influence the Fermi surface and hence the behavior of the Dirac fermions. We report powder and single crystal neutron diffacraction measurements of the magnetic order in AMnBi$_{2}$ (A $=$ Sr and Ca), two layered manganese pnictides with anisotropic Dirac fermions on a Bi square net. Both compounds are found to order in k $=$ 0 antiferromagnetic structures, with ordered Mn moments at T $=$ 10 K of approximately 3.8 $\mu_{\mathrm{B}}$ aligned along the c axis. The magnetic structures are N\'{e}el-type within the Mn-Bi layers, consistent with density functional theory predictions, but the interlayer ordering is different in the two materials, being antiferromagnetic in SrMnBi$_{2}$ and ferromagnetic in CaMnBi$_{2}$. This allows a mean-field coupling of the magnetic order to the Dirac fermions in CaMnBi$_{2}$, but not in SrMnBi$_{2}$. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y7.00012: Orbital order and Hund's rule frustration in Kondo lattices Kazushi Aoyama, Leonid Isaev, Indranil Paul, Ilya Vekhter We analyze a microscopic origin of the Kondo effect-assisted orbital order in heavy-fermion materials. By studying the periodic two-orbital Anderson model with two local electrons, we show that frustration of Hund's rule coupling due to the Kondo effect leads to an incommensurate spiral orbital and magnetic order, which exists only inside the Kondo screened (heavy-electron) phase. This spiral state can be observed in neutron and resonant X-ray scattering measurements in ${\rm U}$- and ${\rm Pr}$-based heavy-fermion compounds, and realized in cold atomic gases, e.g. fermionic ${}^{173}{\rm Yb}$. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y7.00013: Toroidal ordering in metals: band shift, magnetotransport, and magnetoelectric effect Satoru Hayami, Hiroaki Kusunose, Yukitoshi Motome The electromagnetic effect, interplay between electronic and magnetic properties, is one of the most interesting issues in condensed matter physics. Recently, it has been studied intensively in magnetic insulators without spatial-inversion and time-reversal symmetries. Especially, a toroidal moment defined by a vector product of magnetization and electronic polarization has attracted interest because it leads to intriguing phenomena, such as a linear electromagnetic effect and nonreciprocal directional dichroism. It, however, has not been fully understood how such toroidal ordering affects the electronic structure and transport property in metallic systems. In order to clarify this issue, we investigate a microscopic model for locally noncentrosymmetric systems. Starting from an $s$-$p$ four-band tight-binding model with local inversion symmetry breaking, we derive an effective model with the antisymmetric spin-orbit coupling. By analyzing the model at the mean-field level, we find that a ferroic ordering of a microscopic toroidal moment acts as an effective gauge field, which leads to a center-of-mass momentum shift in the band structure. Furthermore, within the linear response theory, we show that toroidal ordering induces anomalous magnetotransport and magnetoelectric effects. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y7.00014: Stability of the helical spin mode in RKKY systems with disorder Younghyun Kim, Meng Cheng, Bela Bauer, Roman Lutchyn Motivated by the experimental proposals[1-4] trying to realize topological superconductivity and Majorana zero-energy modes in RKKY systems, we study magnetic properties of one-dimensional spin chains consisting of localized magnetic moments coupled by itinerant electrons via the Rudermann-Kittel-Kasuya-Yosida (RKKY)-type interaction. As a source of itinerant electrons, we consider one- and two-dimensional electron gas with spin-orbit coupling. We obtain the phase diagram of the system in the presence of Rashba spin-orbit coupling in a clean limit, and identify the stability regions for the helical spin mode. We then examine the stability of this mode using diagrammatic techniques and Monte Carlo in presence of disorder. [1] S. Nadj-Perge, I. K. Drozdov, B. A. Bernevig, and Ali Yazdani, Phys. Rev. B 88, 020407(R) (2013) [2] Jelena Klinovaja, Peter Stano, Ali Yazdani, and Daniel Loss, Phys. Rev. Lett. 111, 186805 (2013) [3] M. M. Vazifeh and M. Franz, Phys. Rev. Lett. 111, 206802 (2013) [4] Bernd Braunecker and Pascal Simon, Phys. Rev. Lett. 111, 147202 (2013) [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y7.00015: Helimagnetism in Cr doped FeGe Yuen Yiu, Nirmal Ghimire, Lisa Debeer-Schmitt, Michael McGuire, Arnab Banerjee, Ken Littrell, David Mandrus, Stephen Nagler We have studied helimagnetism in Fe(1-x)Cr(x)Ge via bulk magnetic measurements and Small Angle Neutron Scattering (SANS). Cubic FeGe exhibits helimagnetism below Tc = 276 K. This transition is suppressed by Cr doping, and cannot be detected above 4K for x = 0.4 or greater. SANS measurements for samples with x = 0.1, 0.2, and 0.3 shows clear evidence for helimagnetic structures with characteristic periods of several hundred angstroms. The x = 0.2 and 0.3 samples show a hump in the susceptibility accompanied by an anomaly in the SANS signal at a temperature below the onset of helimagnetic order. [Preview Abstract] |
Session Y8: Focus Session: Spin-Dependent Phenomena in Semiconductors: Spin and Noise in Quantum Dots
Sponsoring Units: GMAG DMP FIAPChair: George Kioseoglou, University of Crete, Greece
Room: 104
Friday, March 7, 2014 8:00AM - 8:12AM |
Y8.00001: Nonequilibrium Spin Noise Spectroscopy Nikolai Sinitsyn, Yuri Pershin, Valery Slipko, Fuxiang Li Spin Noise Spectroscopy (SNS) is an experimental approach to obtain correlators of mesoscopic spin fluctuations in time by purely optical means. We explore the information that this technique can provide when it is applied to a weakly non-equilibrium regime when an electric current is driven through a sample by an electric field. We find that the noise power spectrum of conducting electrons experiences a shift, which is proportional to the strength of the spin-orbit coupling for electrons moving along the electric field direction. We propose applications of this effect to measurements of spin orbit coupling anisotropy and separation of spin noise of conducting and localized electrons. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y8.00002: Using spin fluctuation correlations to reveal homogeneous linewidths within (In,Ga)As quantum dot ensembles: Two-color spin noise spectroscopy Luyi Yang, Scott Crooker, Philipp Glasenapp, Alex Greilich, Manfred Bayer, Dmitri Yakovlev ``Spin noise spectroscopy'' is a powerful optical technique for passively probing the spin dynamics of electrons and holes that is based on measuring their intrinsic spin fluctuations while in thermal equilibrium. This approach is guaranteed by the fluctuation-dissipation theorem. Here, we use the \textit{correlation properties} of spin fluctuations to reveal the underlying homogeneous linewidth of (In,Ga)As quantum dots (QDs) in an otherwise strongly inhomogeneously-broadened ensemble. When two narrowband probe lasers are tuned in wavelength far from each other, each is sensitive only to spin fluctuations from those QDs that are spectrally close to that laser. Therefore the detected spin noise signals from each laser are uncorrelated. In contrast, when the two lasers have exactly the same wavelength, then they are sensitive to the same QDs and the spin noise signals are perfectly correlated. By measuring the degree of correlation as a function of laser detuning, we reveal the homogeneous linewidth of the QDs even in the presence of a strong inhomogeneous broadening. This information is otherwise inaccessible by conventional linear optical spectroscopic techniques. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y8.00003: Nodal ``ground states'' and orbital textures in semiconductor quantum dots Jeongsu Lee, Karel Vyborny, Jong Han, Igor Zutic Unlike the common expectation, theoretical calculations in quantum wires and quantum dots have predicted hole ground state wavefunctions with a node [1-2] that are often associated with the formation of dark excitons [3]. The inversion of the energy level ordering between nodeless (S-like) and nodal (P-like) wavefunction states occurs due to various factors, e.g., confinement size and strength, choice of a material, and spin-orbit interaction. However, the existence of the nodal ground states has been debated and even viewed merely as an artifact of a k\textbullet p model [4]. Using complementary approaches of both k\textbullet p and tight-binding models, further supported by an effective Hamiltonian for a continuum model, we reveal that the nodal ground states in quantum dots are not limited to a specific theoretical model. Remarkably, the emergence of the nodal ground states can be attributed to the formation of the orbital vortex textures that minimizes ``divergence''. We discuss how our findings and the studies of orbital textures could be also relevant for different materials systems. [1] K. V\'{y}born\'{y} et al., PRB 85, 155312 (2012) [2] A. Bagga et al., PRB 74, 035341 (2006); P. Horodysk\'{a} et al., PRB 81, 045301 (2010); J. Xia and J. Li, PRB 60, 11540 (1999); M. P. Persson and H. Q. Xu, PRB 73, 125346 (2006). [3] M. Nirmal, et al., PRL 75, 3728 (1995); Al. L. Efros, et al., PRB 54, 4843 (1996). [4] L. W. Wang et al., APL 76, 339 (2000) [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 9:12AM |
Y8.00004: Charge and spin noise from semiconductor quantum dots Invited Speaker: Richard J. Warburton Improving the quantum coherence of solid-state systems that mimic two-level atoms, for instance spin qubits or single-photon emitters using semiconductor quantum dots, involves dealing with the noise inherent to the device. Charge noise results in a fluctuating electric field, spin noise in a fluctuating magnetic field at the location of the qubit, and both can lead to dephasing and decoherence of optical and spin states. We investigate noise in an ultrapure semiconductor device using a minimally invasive, ultrasensitive local probe: resonance fluorescence from a single quantum dot. We distinguish between charge noise and spin noise through a crucial difference in their optical signatures. Noise spectra for both electric and magnetic fields are derived from 0.1 Hz to 100 kHz. The charge noise dominates at low frequencies, spin noise at high frequencies. The noise falls rapidly with increasing frequency, allowing us to demonstrate transform-limited quantum-dot optical linewidths by operating the device above 50 kHz. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y8.00005: Spin-polarized conductance in double quantum dots with ferromagnetic contacts Ireneusz Weymann, Krzysztof Wojcik We study the transport properties of double quantum dots in T-shape geometry strongly coupled to external ferromagnetic contacts. The analysis is performed with the aid of the numerical renormalization group method, which allows us to study the behavior of respective spectral functions and the linear conductance through the system in the full parameter space of the model. The considered device enables a unique possibility to explore the interplay of the Fano and Kondo effects with ferromagnetic-contact induced exchange field. We show that the presence of gate-tunable exchange field leads to strong dependence of the spin polarization of conductance on the position of the dot levels. By tuning the level of the decoupled dot, the conductance may become fully spin polarized. Moreover, when changing the dot level positions, one can also tune the sign of the spin polarization. The increased spin polarization of the conductance is a consequence of a subtle interplay between the interference effects, the Kondo effect and the exchange field. Double quantum dots with ferromagnetic contacts can be thus considered as efficient spin current sources, where the degree of spin polarization can be tuned by purely electrical means, without the necessity to apply external magnetic field. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y8.00006: Single-shot readout of electron high-spin states in a quantum dot coupled to quantum Hall edge states Haruki Kiyama, Akira Oiwa, Seigo Tarucha The ability to prepare and probe an electron spin in a quantum dot (QD) is indispensable for spintronics and quantum information processing. Spin-resolved quantum Hall edge states (SRESs) are expected to be applied for such applications, since their spatial separation by two-dimensional electron gas (2DEG) edge potential provides spin-dependent tunnel coupling with QDs. However, the spin filtering efficiencies reported previously have not been high enough for spin injection and detection. In this work, we firstly enhanced the efficiency of the spin filtering by the electrical tuning of 2DEG potential landscape. Larger separation of SRESs are obtained by making the change of 2DEG potential near the tunnel barrier more gradual. Secondly, using the highly efficient spin filtering, we demonstrated single-shot readout of electron spins in a QD. The maximum visibility of two-electron spin readout reached to 94{\%}. This is the highest value among reports in GaAs-based QDs. Subsequently we applied this scheme to measure the dynamics of the multi-electron high-spin states. We find that the spin relaxation rate of the S$=$3/2 or S$=$2 high-spin excited states to the S$=$1/2 or S$=$1 ground spin states are about 10 times faster than that of S$=$0 first excited state. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y8.00007: Emerging Modified Transverse-Field Ising Model On A Hydrogenated Silicon Surface Burkhard Ritter, Kevin Beach Advances in the precise placement of dangling bonds on a hydrogenated silicon surface open the prospect of manufacturing large scale quantum dot arrays. Small clusters of specifically arranged quantum dots comprise a system of bistable, interacting cells. Starting from an extended Hubbard model and using a set of controlled Hilbert space truncations, we show that such a system of quantum dot cells can be mapped to a modified transverse-field Ising model with long-ranged interactions. Each cell is described by a pseudo-spin. Because we control cell orientation and placement, we can construct a wide range of structures, with ferromagnetic and antiferromagnetic chains as simple examples. The Ising-like model is amenable to stochastic series expansion Monte Carlo, allowing the simulation and characterization of large systems. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y8.00008: Semiconductor Hall magnetometers for magnetic measurement of (In,Cr)As quantum dots Joon-Il Kim, T. Guan, S. von Molnar, P. Xiong, S.L. Wang, H.L. Wang, J.H. Zhao Recently, SQUID magnetometry measurements of MBE-grown self-assembled (In,Cr)As QDs showed magnetic hysteresis indicating possible existence of ferromagnetic ordering above 300 K [1]. However, the temperature dependence of the remnant magnetization did not follow the standard Brillouin-like behavior, and the interpretation of the data and elucidation of the origin of the ferromagnetism in the QDs have been hindered by the large ensemble-averaged measurement. Measurements on small clusters or even individual QDs would facilitate a direct correlation of the measured magnetic properties with their structural and chemical characteristics, possibly enabling a definitive understanding of the origin of the ferromagnetism in the diluted magnetic semiconductor QDs. Towards this goal, we have fabricated integrated micro-Hall magnetometers based on high-mobility GaAs/AlGaAs 2DEG in order to facilitate static and dynamic magnetic measurements of the QDs via the Hall gradiometry technique. Integrated structures of (In,Cr)As QDs on top of a GaAs/AlGaAs heterostructure were grown entirely in situ by MBE. Micro-Hall magnetometer devices with six Hall-crosses were fabricated using photolithography and wet chemical etching. Using carefully calibrated selective chemical etching, all QDs were removed except those on three of the Hall-crosses so as to enable gradiometry measurement. Results of on-going measurements will be discussed.\\[4pt] [1] H. J. Meng et al., Euro. Phys. Lett. 84, 58007 (2008). [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y8.00009: Overhauser effect in spin blockaded double quantum dots-the case of dual hysteresis Siddharth Buddhiraju, Bhaskaran Muralidharan In the spin blockade transport regime across GaAs double quantum dots (DQD), experiments [1] revealed that the hyperfine interaction with host nuclei can have profound consequences on the electron-spin dynamics. One of which, is the observation of bistablity and flat-topped behavior in the current versus applied DC magnetic-field $I(B_{dc} )$characteristics. In this talk, we will first explain the essence of this flat-topped hysteretic behavior using a simple six-state model that captures the multiple-feedback mechanisms that are involved. We will then consider a more detailed model that elucidates the role of the physical parameter space of the DQD set up and a feedback mechanism involving the difference Overhauser field caused by the two separate nuclear spin baths of the DQD set up. [1] K. Ono and S. Tarucha, Phys Rev Lett., 92, 256803 (2004). [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y8.00010: A theory on the self-sustained current oscillations in spin-blockaded quantum dots Xiangrong Wang, Bin Hu Based on the experimental fact that the self-sustained current oscillations (SSCO) in spin-blockaded double quantum dots is closely associated with the dynamically polarized nuclear spins, we consider the possible scenario that the SSCO in the spin-blockaded double quantum dots is the manifestation of the periodic motion of dynamical nuclear spin polarization (along a limit cycle) under an external magnetic field and a spin-transfer torque. Based on the Landau-Lifshitz-Gilbert equation, it is shown that a sequence of semistable limit cycle, Hopf, and homoclinic bifurcations occur as the external field is tuned. Although the fundamental time scale is nanoseconds for electron tunneling and micro seconds for nuclear spin dynamics under the external field or Overhauser fields, the divergent period near the homoclinic bifurcation explains well why the period in experiments can be many orders of magnitude longer than all microscopic time scales. Some predictions associate with the theory may also be tested experimentally. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y8.00011: Parallel single qubit gates in distant semiconductor quantum dots using engineered optical pulses Angela Gamouras, Reuble Mathew, Sabine Freisem, Dennis Deppe, Kimberley Hall Semiconductor quantum dots are promising for the development of a scalable system of qubits as such a platform would benefit from established semiconductor fabrication capabilities. Here we report the demonstration of simultaneous high-fidelity $\pi$ and 2$\pi$ single qubit gates on excitons in two uncoupled self-assembled quantum dots within the micron-scale control laser focal spot by engineering the phase of the broad-bandwidth femtosecond control pulse. The pulse phase is engineered using optimal quantum control (OQC), which has been applied to the optimization of quantum gates in atomic and molecular systems in recent years [1] and is extended here to gate optimization in a system of solid state qubits. The deterministic control of two distant, uncoupled qubits we have achieved constitutes a step towards scaling of semiconductor-based quantum computing platforms, and may enable the development of small quantum simulators based on complex instruction set quantum computing using semiconductor quantum dots [2]. [1] Campbell et al. Phys. Rev. Lett. 105, 090502 (2010); Kirchmair et al., New J. Phys. 11, 023002 (2009); Amitay et al., Chem. Phys. Lett. 359, 8 (2002). [2] Sanders et al., Phys. Rev. A 59, 1098 (1999). [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y8.00012: Spin Ordering and Fluctuations in Magnetic Quantum Dots James Pientka, Rafal Oszwaldowski, Igor Zutic, Jong Han, Andre Petukhov The presence of magnetic impurities in quantum dots can lead to unconventional ordering in carrier and impurity spins [1-3]. While, due to its simplicity, it is tempting to use the mean field approximation for such systems, we explain critical problems of that description and an essential role of spin fluctuations. We consider two different situations of singly and doubly occupied magnetic dots that correspond to the formation of magnetic polarons [4,5] and bipolarons [1,2]. The two cases reveal qualitatively different manifestations of finite size effects on spin ordering and the removal of spurious phase transitions at the mean field level. Using both analytical and Monte Carlo methods, we elucidate the interplay of spin ordering and fluctuations in these systems. \\[4pt] [1] J. M. Pientka, R. Oszwaldowski, A. G. Petukhov, J. E. Han, and I. Zutic, Phys. Rev. B. 86, 161403(R) (2012).\\[0pt] [2] R. Oszwaldowski, I. Zutic, and A. G. Petukhov, Phys. Rev. Lett. 106, 177201 (2011).\\[0pt] [3] R. Oszwaldowski, P. Stano, A. G. Petukhov, and I. Zutic, Phys. Rev. B. 86, 201408(R) (2012).\\[0pt] [4] R. Beaulac et al., Science 325, 973 (2009).\\[0pt] [5] I. R. Sellers, et al., Phys. Rev. B 82, 195320 (2010). [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y8.00013: Ultrafast adiabatic rapid passage in single InAs quantum dots Reuble Mathew, Eric Dilcher, Angela Gamouras, Sabine Freisem, Dennis Deppe, Kimberley Hall Fast quantum gates are important in quantum information processing because operations must be performed within the coherence time of the qubit. Faster single-qubit operations are also important for specific applications such as decoherence control via dynamical decoupling [1], entanglement operations to generate cluster states [2], and probabilistic gates [3]. Here we report an experimental demonstration of optical state inversion of the p-shell exciton in a single InGaAs quantum dot via adiabatic rapid passage (ARP) using a 1.1 ps optical pulse. Population inversion via ARP using frequency swept pulses is robust against experimental instabilities associated with the control pulse and variations in the light-matter coupling parameters. In contrast to previous work [4], shorter optical control pulses provide a 13.5 fold improvement in operation speed and a 200 fold reduction in the required chirp. We find that a chirp of 55,000 fs$^2$, corresponding to a pulse width of 1.1 ps, is sufficient to achieve ARP. [1] Viola \textit{et al.}, PRA 58, 2733 (1998), [2] Barrett \textit{et al.}, PRA 71, 060310(R) (2005), [3] Olmschenk \textit{et al.}, Science 323, 486 (2009), [4] Wu \textit{et al.}, PRL 106, 067401 (2011), Simon \textit{et al.}, PRL 106, 1666801 (2011) [Preview Abstract] |
Session Y10: Emerging Biophysical Techniques
Sponsoring Units: DBIOChair: Wolfgang Losert
Room: 201
Friday, March 7, 2014 8:00AM - 8:12AM |
Y10.00001: Two-color super-resolution imaging of dendritic spines of hippocampal neurons using a custom STED microscope Stephanie Meyer, Kevin Woolfrey, Baris Ozbay, Diego Restrepo, Mark Dell'Acqua, Emily Gibson We built a 2-color STED microscope and imaged dendritic spines in mouse hippocampal neurons at sub-diffraction limit resolution. The microscope is designed similar to one developed by Johanna B\"uckers, et. al. (Opt. Exp. 2011) in the lab of Dr. Stefan Hell. The STED microscope images at Atto590/Atto647N wavelengths and is capable of doing so simultaneously. We characterized the resolution of the system by imaging 40nm fluorescent beads as $\sim$58nm (Atto590) and $\sim$44 nm (Atto647N). The microscope is part of the UC Denver Advanced Light Microscopy Core, primarily for use by neuroscientists. We then performed 2-color STED imaging on hippocampal neurons immuno-labeled at PSD-95 (a postsynaptic density marker) along with either the GluA1-subunit of the AMPA-type glutamate receptor or the signaling scaffold protein AKAP150 in order to visualize nm-scale compartmentalization of these proteins within single postsynaptic dendritic spines. Importantly, for both GluA1 and AKAP150, STED imaging visualized sub-diffraction dimension clusters in spines located at both synaptic (overlapping or proximal to PSD-95) and extrasynaptic locations. In the future 2-color STED imaging should be useful for studying changes in the localization of these proteins during synaptic plasticity. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y10.00002: Single-cell Genomics using Droplet-based Microfluidics Anindita Basu, Evan Macosko, Alex Shalek, Steven McCarroll, Aviv Regev, Dave Weitz We develop a system to profile the transcriptome of mammalian cells in isolation using reverse emulsion droplet-based microfluidic techniques. This is accomplished by (a) encapsulating and lysing one cell per emulsion droplet, and (b) uniquely barcoding the RNA contents from each cell using unique DNA-barcoded microgel beads. This enables us to study the transcriptional behavior of a large number of cells at single-cell resolution. We then use these techniques to study transcriptional responses of isolated immune cells to precisely controlled chemical and pathological stimuli provided in the emulsion droplet. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y10.00003: Sensitize hydration shells of ions by the dynamics of water with terahertz spectroscopy Deepu George, John Rahmani, Vinh Nguyen Comparison of the relaxation dynamics of water in bulk state to that in a confined state is of significant importance to the study of interaction of biomolecules with its environment. In relation to this, the ability of terahertz dielectric spectroscopy to probe intermolecular dynamics has been explored in the past decade to look at the dynamics of water molecules which forms a hydration shell around proteins. The change in dynamics of water when its molecules interact with different types of solute molecules forms the basis of water- bio-molecular interaction. There have been several studies in the past looking at the effects of ion interactions with water molecules. In this study we have employed a vector network analyzer based terahertz dielectric spectrometer operating over the frequency range from 0.5 GHz to 1.1 THz to examine the water dynamics in several alkali metal chloride solutions. The terahertz dielectric response of these solutions as a function of concentration as well as the ion size has been studied. We have confirmed that for all these solutions the dynamics can be best described by a three Debye relaxation process of water. The relaxation times does not seem to depend on salt concentrations but on the other hand strength of relaxation modes is dependent on the molarity. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y10.00004: Co-regulation of cell behavior by electromagnetic stimulus and extracellular environment Toloo Taghian, Abdul Sheikh, Daria Narmoneva, Andrei Kogan Chronic wounds do not effectively respond to pharmacological treatments because of insufficient blood supply (Impaired angiogenesis) in the wound. Developing non-pharmacological treatments requires application of advanced technology to control natural cell signals to trigger desired cell responses. Application of external electric field (EF) has been shown to enhance angiogenesis through manipulation of naturally-generated EF in the ionic environment surrounding cells and across the cell membrane; however biophysical mechanisms of cell responses to EF remain unknown. EF-cell interactions may be affected by both the distribution of the induced EF within the cell and the properties of the extracellular matrix (ECM), which is known to regulate cell response to the external stimuli. We have developed a combined theoretical-experimental approach to study EF-cell interactions. Our theoretical 3D interaction model provides spatial distribution of the induced EF in cell and extracellular space and predicts a frequency specific cell response to EF. Experimentally measured responses of cells to EF including growth factor expression and capillary morphogenesis confirm this prediction. We show that natural versus synthetic ECM can differentially mediate cell response to EF. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y10.00005: Superparamagnetic nanoplatforms for theragnostic applications: a structural investigation Irena Milosevic, Laurence Motte, Marie-Louise Saboungi, Bachir Aoun, Tao Li, Chengjun Sun, Yang Ren Magnetic nanoplatforms are being developed for use in bioassays, diagnosis, therapy and nano-organocatalysis. The nanoparticle has two essential roles: to act as a probe owing to its specific magnetic properties and to carry on its surface antitumoral molecules, precursor groups for the covalent coupling of biological recognition molecules, or small organic catalysts such as amino acids and alkaloids. The nanoplatforms consist of a superparamagnetic iron oxide core and different coatings for surface passivation and stabilization. We report recent results obtained at the Advanced Photon Source on three kinds of nanoplatforms, differing in their coating molecules: shape and size determination by small-angle X-ray scattering, distribution of valences and chemical environments of the iron ions deduced from X-ray absorption near-edge structure measurements, and atomic structures determined by x-ray diffraction. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y10.00006: High resolution electron microscopy and spectroscopy of ferritin in thin window liquid cells Canhui Wang, Qiao Qiao, Tolou Shokuhfar, Robert Klie In-situ transmission electron microscopy (TEM) has seen a dramatic increase in interest in recent years with the commercial development of liquid and gas stages. High-resolution TEM characterization of samples in a liquid environment remains limited by radiation damage and loss of resolution due to the thick window-layers required by the in-situ stages. We introduce thin-window static-liquid cells that enable sample imaging with atomic resolution and electron energy-loss (EEL) spectroscopy with 1.3 nm resolution. Using this approach, atomic and electronic structures of biological samples such as ferritin is studied via in-situ transmission electron microscopy experiments. Ferritin in solution is encapsulated using the static liquid cells with reduced window thickness. The integrity of the thin window liquid cell is maintained by controlling the electron dose rate. Radiation damage of samples, such as liquid water and protein, is quantitatively studied to allow precision control of radiation damage level within the liquid cells. Biochemical reactions, such as valence change of the iron in a functioning ferritin, is observed and will be quantified. Relevant biochemical activity: the release and uptake of Fe atoms through the channels of ferritin protein shell is also imaged at atomic resolution. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y10.00007: Multiphoton-generated localized electron plasma for membrane permeability modification in single cells T. Merritt, M. LeBlanc, J. McMillan, J. Westwood, G.A. Khodaparast Successful incorporation of a specific macromolecule into a single cell would be ideal for characterizing trafficking dynamics through plasmodesmata or for studying intracellular localizations. Here, we demonstrate NIR femtosecond laser-mediated infiltration of a membrane impermeable dextran-conjugated dye into living cells of Arabidopsis thaliana seedling stems. Based on the reactions of fluorescing vacuoles of transgenic cells and artificial cell walls comprised of nanocellulose, laser intensity and exposure time were adjusted to avoid deleterious effects. Using these plant-tailored laser parameters, cells were injected with the fluorophores and long-term dye retention was observed, all while preserving vital cell functions. This method is ideal for studies concerning cell-to-cell interactions and potentially paves the way for introducing transgenes to specific cells. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y10.00008: Comparative study on the biodegradation and biocompatibility of silicate bioceramic coatings on biodegradable magnesium alloy as biodegradable biomaterial M. Razavi, M.H. Fathi, O. Savabi, S.M. Razavi, B. Hashemibeni, M. Yazdimamaghani, D. Vashaee, L. Tayebi Many clinical cases as well as in vivo and in vitro assessments have demonstrated that magnesium alloys possess good biocompatibility. Unfortunately, magnesium and its alloys degrade too quickly in physiological media. In order to improve the biodegradation resistance and biocompatibility of a biodegradable magnesium alloy, we have prepared three types of coating include diopside (CaMgSi2O6), akermanite (Ca2MgSi2O6) and bredigite (Ca7MgSi4O16) coating on AZ91 magnesium alloy through a micro-arc oxidation (MAO) and electrophoretic deposition (EPD) method. In this research, the biodegradation and biocompatibility behavior of samples were evaluated in vitro and in vivo. The in vitro analysis was performed by cytocompatibility and MTT-assay and the in vivo test was conducted on the implantation of samples in the greater trochanter of adult rabbits. The results showed that diopside coating has the best bone regeneration and bredigite has the best biodegradation resistance compared to others. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y10.00009: Mechanics and optics of stretchable microlenses for artificial compound eye camera Zhengwei Li, Jianliang Xiao Due to the wide-angle field of view, low aberrations, high acuity to motion and infinite depth of field, insect eye-inspired imaging devices have attracted more and more interest. Recently, researchers have developed an imaging device that resembles the structure and functions of insects' apposition eyes. Elastomeric microlens array that can be mechanically stretched to very large extent without deteriorating the optics is critical to this development. The stretchable microlens array is composed of a number of hemispherical microlenses each sitting on top of a pedestal and connected through a continuous elastomeric film. Here we present our study on mechanical and optical aspects of stretchable microlens. Our results show that proper designs of the hemispherical microlens, pedestal and film are critically important to meet both mechanical and optical requirements simultaneously. Our study can have important implications in not only the design of artificial compound eye cameras, but also other developments that require stretchable optical elements. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y10.00010: Systems with High Diffusivity Contrast: Treatment of Stochastic Force Matters Zheng Ma, Gary W. Slater It has been suggested, based on computer simulations, that systems containing regions with drastically different diffusivity could be used for controlled drug delivery. However, these studies neglect the fact that for particles diffusing in inhomogeneous media, the particular interpretation of the stochastic force has a significant impact. We present systematic investigations of several such systems using Lattice Monte-Carlo (LMC) methods based on Ito, Stratonovich and isothermal calculus. We find that even for moderate diffusivity contrast ($\sim 100$), different calculi predict distinct distributions of particles among regions. Results of previous work that implicitly use Ito calculus (without physical justification) crucially rely on particles accumulating in the low diffusivity medium, which is not observed for all choices of calculi. We argue that a proper choice of calculus, depending on the microscopic origin of the diffusivity contrast, must be made before any convincing conclusion can be drawn about what might constitute a promising candidate system for controlled drug delivery. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y10.00011: Photothermal Mid-Infrared Microscopy: a new tool for hyperspectral chemical imaging Alket Mertiri, Mi Hong, Michelle Sander, Shyamsunder Erramilli We describe a method for label free microscopy in the mid-infrared region of the electromagnetic spectrum based on the photothermal effect. A Quantum Cascade Laser (QCL) tuned to an infrared active vibrational molecular normal mode is used as the pump laser. A low-phase noise Erbium-doped fiber (EDF) laser (1.5$\mu$m) is used as the probe. We demonstrate the method using a patterned image target with liquid crystal 4-cyano-4'-octylbiphenyl (8CB) as the mid-infrared absorber. The QCL is tuned across the C-H scissoring band, with a peak absorption at 1607cm$^{-1}$. Absorption of the modulated pump beam results in a change in the dielectric function and the refractive index at the probe beam frequency. The resultant scatter of the probe is observed in heterodyne lock-in detection. The combination of heterodyne detection, high brightness mid-infrared QCLs and low-phase noise stable EDF lasers provides an ultra-sensitive method for obtaining mid-infrared microscope images using short-wavelength optical detectors, whose performance far exceeds those of cryogenically cooled broadband mid-infrared detectors. The method provides a powerful new tool for hyperspectral label-free mid-infrared imaging. [Preview Abstract] |
Session Y11: Focus Session: Physics of Proteins IV
Sponsoring Units: DBIO DPOLYChair: Timothy Sage, Northeastern University
Room: 203
Friday, March 7, 2014 8:00AM - 8:12AM |
Y11.00001: Beating the Heat: Fast Scanning Melts Beta Sheet Crystals Peggy Cebe, Xiao Hu, David Kaplan, Evgeny Zhuravlev, Andreas Wurm, Daniella Arbeiter, Christoph Schick Beta-pleated-sheet crystals are among the most stable of protein secondary structures, and are responsible for the remarkable physical properties of many fibrous proteins, such as silk. Previous thinking was that beta-pleated-sheet crystals in the dry solid state would not melt upon input of heat energy alone. Indeed, at conventional heating rates ($\sim$1-50 $^{\circ}$C/min), silk exhibits its glass transition ($\sim$175 $^{\circ}$C), followed by cold crystallization, and then by immediate thermal degradation beginning at about 225 $^{\circ}$C. Here we demonstrate that beta-pleated-sheet crystals can melt directly from the solid state to become random coils, helices, and turns. We use fast scanning chip calorimetry at 2,000 K/s to avoid thermal degradation, and report the first reversible thermal melting of protein beta-pleated-sheet crystals, exemplified by silk fibroin. The similarity between thermal melting behavior of lamellar crystals of synthetic polymers and beta-pleated-sheet crystals is confirmed. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y11.00002: Sterically allowed configuration space for amino acid dipeptides Diego Caballero, Jukka Maatta, Maria Sammalkorpi, Corey O'Hern, Lynne Regan Despite recent improvements in computational methods for protein design, we still lack a quantitative, predictive understanding of the intrinsic propensities for amino acids to be in particular backbone or side-chain conformations. This question has remained unsettled for years because of the discrepancies between different experimental approaches. To address it, I performed all-atom hard-sphere simulations of hydrophobic residues with stereo-chemical constraints and non-attractive steric interactions between non-bonded atoms for ALA, ILE, LEU and VAL dipeptide mimetics. For these hard-sphere MD simulations, I show that transitions between $\alpha$-helix and $\beta$-sheet structures only occur when the bond angle $\tau(N-C_{\alpha}-C)>110^{\circ}$, and the probability distribution of bond angles for structures in the `bridge' region of $\phi$-$\psi$ space is shifted to larger angles compared to that in other regions. In contrast, the relevant bond-angle distributions obtained from most molecular dynamics packages are broader and shifter to larger values. I encounter similar correlations between bond angles and side-chain dihedral angles. The success of these studies is an argument for re-incorporating local stereochemical constraints into computational protein design methodology. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y11.00003: Generation of protein-like structures via simple rules imposed on a cubic lattice Rahmi Ozisik, Deniz Turgut, Osman B. Okan, Aravind Rammohan, Angel E. Garcia In the current study, protein-like coarse-grained structures are generated by a simple set of rules on simple cubic lattice (SCL). The coarse-graining was based on individual amino acids. Detailed analysis of the average structure of 210 real proteins' radial distribution function (RDF) and number of neighbors as a function of cut-off distance suggest that SCL is an appropriate choice of lattice. Three simple rules were imposed to generate protein-like structures: finite size (presence of a molecular surface), random inclusion of voids, and a simple connectivity of remaining lattice sites. The set of on-lattice points (that mimic residues of a protein) satisfy many structural characteristics of real proteins. These on-lattice structures were subsequently relaxed either by random moves or by a combined Reverse Monte Carlo/Simulated Annealing (RMC/SA) algorithm that used the average RDF of proteins as its target function. The on-lattice and relaxed structures' characteristics were also analyzed via bond orientational order and graph theory. The results showed that although relaxation algorithms improved the structural characteristics of the generated structures, the improvement over the on-lattice structures are minimal. Based on various structural properties, our results indicate that the artificially generated structures closely resemble real proteins coarse-grained at the residue level.\textunderscore [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 9:12AM |
Y11.00004: Monitoring Single-Molecule Protein Dynamics with a Carbon Nanotube Transistor Invited Speaker: Philip G. Collins Nanoscale electronic devices like field-effect transistors have long promised to provide sensitive, label-free detection of biomolecules. Single-walled carbon nanotubes press this concept further by not just detecting molecules but also monitoring their dynamics in real time. Recent measurements have demonstrated this premise by monitoring the single-molecule processivity of three different enzymes: lysozyme [1], protein Kinase A [2], and the Klenow fragment of DNA polymerase I [3]. With all three enzymes, single molecules tethered to nanotube transistors were electronically monitored for 10 or more minutes, allowing us to directly observe a range of activity including rare transitions to chemically inactive and hyperactive conformations. The high bandwidth of the nanotube transistors further allow every individual chemical event to be clearly resolved, providing excellent statistics from tens of thousands of turnovers by a single enzyme. Initial success with three different enzymes indicates the generality and attractiveness of the nanotube devices as a new tool to complement other single-molecule techniques. Research on transduction mechanisms provides the design rules necessary to further generalize this architecture and apply it to other proteins [4]. The purposeful incorporation of just one amino acid is sufficient to fabricate effective, single molecule sensors from a wide range of enzymes or proteins.\\[4pt] [1] Y. Choi et. al., Science 335, 319 (2012); Y. Choi et. al., J. Am. Chem. Soc. 134, 2032 (2012).\\[0pt] [2] P. C. Sims et. al., JACS 135, 7861 (2013).\\[0pt] [3] T. J. Olsen et. al., JACS 135, 7855 (2013).\\[0pt] [4] Y. Choi et. al., Nano Lett. 13, 625 (2013). [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y11.00005: Novel computational methods to design protein-protein interactions Alice Qinhua Zhou, Corey O'Hern, Lynne Regan Despite the abundance of structural data, we still cannot accurately predict the structural and energetic changes resulting from mutations at protein interfaces. The inadequacy of current computational approaches to the analysis and design of protein-protein interactions has hampered the development of novel therapeutic and diagnostic agents. In this work, we apply a simple physical model that includes only a minimal set of geometrical constraints, excluded volume, and attractive van der Waals interactions to 1) rank the binding affinity of mutants of tetratricopeptide repeat proteins with their cognate peptides, 2) rank the energetics of binding of small designed proteins to the hydrophobic stem region of the influenza hemagglutinin protein, and 3) predict the stability of T4 lysozyme and staphylococcal nuclease mutants. This work will not only lead to a fundamental understanding of protein-protein interactions, but also to the development of efficient computational methods to rationally design protein interfaces with tunable specificity and affinity, and numerous applications in biomedicine. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y11.00006: Structural basis underlying the metallic-like conductivity of microbial nanowires Nikhil Malvankar, Madeline Vargas, Mark Tuominen, Derek Lovley Microbial nanowires are electrically conductive proteinaceous pili nanofilaments secreted by \textit{Geobacter sulfurreducens}. In contrast to current biochemical understanding that proteins are insulators, \textit{G. sulfurreducens} pili show organic metallic-like conductivity [1]. Pili also enable direct exchange of electrons among \textit{Geobacter} co-cultures [2]. Site-directed mutagenesis studies revealed that aromatic amino acids confer conductivity to pili [3]. In order to develop a structural understanding of the pili to probe the conduction mechanism at a molecular level, we employed three complementary structural methods -- X-ray microdiffraction using synchrotron radiation, rocking curve X-ray diffraction, and electron diffraction. Studies performed with all these three methods revealed a 3.2 {\AA} periodic spacing in wild-type \textit{G. sulfurreducens} pili, expected for metal-like conductivity and a lack of such spacing in genetically modified non-conductive pili. Notably, both the peak intensity and the conductivity increased 100-fold with lowering the pH from pH 10.5 to pH 2, demonstrating a structure-function correlation in pili. We also reconstructed the three dimensional tertiary structure of pili with homology modeling, which further suggested the 3.2 {\AA} spacing among aromatics associated with metal-like conductivity. \\[4pt] [1] \textit{Nature Nanotechnology}, 6, 573 (2011)\\[0pt] [2] \textit{Science}, 330, 1413 (2010)\\[0pt] [3] \textit{mBio} 4:e00105-13 (2013) [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y11.00007: Relaxation dynamics of proteins Martin Wolf, Rudolf Gulich, Peter Lunkenheimer, Alois Loidl We provide broadband dielectric spectra on aqueous lysozyme solutions of various concentrations and analyze the three dispersion regions commonly found. The beta-dispersion, occurring in the frequency range around 10 MHz and the gamma-dispersion arising around 20 GHz can be attributed to the rotation of the polar protein molecules in the aqueous medium and the reorientational motion of the free water molecules, respectively. The nature of the third relaxation (delta-relaxation) around 100 MHz, which is often ascribed to the motion of protein-bound water molecules, is not yet fully understood and the hydration-shell dynamics of biomolecules is an ongoing field of research [1-3]. Additional insight can be gained by analyzing the subzero temperature spectra, where the beta- and gamma-dispersions, which partly superimpose the delta-relaxation for temperatures above 273 K, disappear due to the freezing of the bulk water. In contrast, the water molecules in the protein hydration shell are known to remain in the liquid state well below the freezing point. This allows to investigate the delta-relaxation in an extended temperature range and to shed new light on the hydration-shell dynamics of biomolecules.\\[4pt] [1] W. Doster, S. Cusack, and W. Petry, Nature \textbf{337}, 754 (1989).\\[0pt] [2] M. Vogel, Phys. Rev. Lett. \textbf{101}, 225701 (2008).\\[0pt] [3] A. Benedetto, Biophys. Chem. \textbf{182}, 16 (2013). [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y11.00008: Protein dynamics, solvation, and quasielastic scattering Paul Fenimore Quasielastic M\"ossbauer and neutron scattering (QES) have been used to measure protein dynamics for about 50 years. These low energy transfer spectra show two prominent features: a sharp elastic line and a broad quasielastic band. Current theory assumes that the elastic line and the quasielastic band are independent features of the spectrum, caused by motions in the sample. Current practice extracts information about dynamics from the spectra by assuming specific models with a few parameters that are determined by data fitting. We claim that this approach is flawed; it is based on questionable assumptions and has no predictive power. We propose a model where the elastic line and the broad band are one inhomogeneous spectrum of shifted, sharp natural-width lines. The model makes predictions of QES lineshapes and elastic fractions for M\"ossbauer and neutron scattering. Essential features of this description include: (i) QES lineshape and elastic fraction are sensitive to protein vibrations, and fluctuations slaved to the hydration shell and bulk solvent. (ii) Independently measured dielectric fluctuation spectra predict the QES lineshape. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y11.00009: Why the observed mean square motional displacement depends on wave vector Q Derya Vural, Henry R. Glyde The motional mean square displacement (MSD) of hydrogen (H) in proteins is extensively measured using neutron scattering techniques. The MSD increases rapidly with temperature near room temperatures and a large MSD is often associated with protein function. One shortcoming of these measurements is that the observed MSD depends on the wave vector (Q) of the neutron data used to obtain the MSD. This dependence is often attributed to use of the Gaussian approximation made to the scattering function in the analysis of the data. To test this we have simulated the protein lysozyme and calculated the intermediate scattering function (ISF), both the full ISF and the ISF in the Gaussian approximation. We find that the MSD extracted in the usual way was the same and still Q dependent in both cases. Also, direct calculation of the terms beyond the Gaussian approximation shows these terms are small. Rather, we find that the apparent Q dependence of the MSD arises from the ``dynamical diversity'' of the H in lysozyme. Specifically, if the ISF of an individual H in the protein is calculated and the MSD extracted in the usual way, then the MSD is independent of Q. This Q dependence arises from ignoring the dynamical diversity in the data analysis. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y11.00010: Continuum model of non-conformational allosteric regulation Michael S. Dimitriyev, Paul M. Goldbart, T.C.B. McLeish Allosteric regulation of proteins, in which the activity of one binding site on a protein is modified by the binding of a small ligand elsewhere on the protein, is traditionally understood as the result of conformational changes. It is now known that allostery is not always conformational: it may be attributed to an alteration of the thermal motion of the protein about an unchanged mean shape. We present a simple model in which the addition of a small ligand alters the thermal fluctuations about the equilibrium configuration of a continuum linear elastic caricature of a protein, and the attached ligand is treated as a small, localized shape perturbation. To determine the change in fluctuations, we develop a perturbation expansion for the change in the elastic fluctuation correlator due to the shape perturbation. We apply this scheme to a simple binding model, and calculate the change in binding energy due to the presence of a ligand. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y11.00011: Minimal Mechanochemical Model for the Processivity of Myosin VI Yubo Yang, Ian Lowe, Riina Tehver Myosin VI is an ATPase responsible for force generation in cells. It dimerizes upon actin binding, and is proposed to walk along the actin filament. Single headed reaction mechanism of myosin VI is well understood but much of its walking mechanism remains unclear. We aim to construct a minimum model for the myosin VI walking mechanism and explore the minimal requirements for processivity. We constructed a kinetic model for the stepping mechanism of Myosin VI using minimum assumptions. The kinetics of the myosin VI dimer is modeled as a three state linear reaction network with reaction rates extracted from relevant experiments. The time limiting step in in-vitro experiments (low APT concentration) is the diffusion of detached head. In this process the myosin dimer is modeled as a tethered polymer with a flexible joint at the dimerization site. The relevance of this polymer model is checked with coarse-grained simulation. We found that the motor maintains processivity for a wide range of kinetic parameters, however long persistence length for the lever arm is crucial for processivity especially under resistive load. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y11.00012: Comparison of Side-Chain Motion of Calbindin D-9k in Its Four Calcium Binding States by Molecular Dynamics Simulation Mahendra Thapa, Mark Rance Calbindin D-9k,a small single domain protein found predominantly in tissues involved in the uptake and transport of calcium, consists of a single pair of a helix-loop-helix motif (called EF-hand) that binds calcium with the ligands provided by the loop residues and helical residues immediately adjacent to the loop. It exits in four calcium binding states: a doubly loaded state (a state with a calcium atom in each of its two binding sites), two singly loaded states (a state with calcium in its first binding site only and a state with calcium in its second binding site) and an apo-state (a state with no calcium atom). Experiments have shown that calcium binding occurs in a positive cooperative fashion. This fact is also supported by computational studies on dynamics of backbone of the protein.Studies of the methyl side chain dynamics of the doubly loaded state of the protein by molecular dynamics simulation further enhances the point. To further investigate by computation, the molecular dynamics simulation approach has been used to study the side chain dynamics of all four calcium binding states of the protein. In the study, the different kinds of force fields, especially the AMBER (a molecular dynamics simulation suit) force fields, and different kinds of water models are employed in the GPU environment. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y11.00013: Lipid-Mediated Activation of G-Protein-Coupled Receptors in Membranes Michael F. Brown, Udeep Chawla, Suchithranga M.D.C. Perera, Andrey V. Struts The role of lipid-protein interactions in membrane function is an important question in the field of lipid membrane biophysics. Lipid effects on G-protein-coupled receptors (GPCRs) are revealed by UV-visible and FTIR spectroscopic studies of rhodopsin [1]. During rhodopsin light activation, the photoreactive 11-\textit{cis}-retinylidene chromophore is isomerized to all-\textit{trans} leading to an equilibrium between the inactive Meta-I and active Meta-II states. Modulation of the metarhodopsin equilibrium depends on the polar head groups and acyl chain composition of the membrane lipids. A flexible surface model (FSM) describes elastic coupling of the membrane bilayer to the conformational energetics of rhodopsin. According to the FSM, membrane lipids whose spontaneous curvature stabilizes the activated state within the membrane are involved in regulating protein function. The new biomembrane model explains the effects of bilayer thickness, nonlamellar-forming lipids, and osmotic stress on protein function. An ensemble-mediated activation mechanism is proposed for rhodopsin in a natural membrane lipid environment. Bulk water is involved in the activation of rhodopsin-like GPCRs in membranes [2]. Membrane proteins and membrane-bound peptides are affected by curvature forces due to elastic deformation of the bilayer, thus giving a new paradigm for membrane lipid-protein interactions in biophysics.\\[4pt] [1] M. F. Brown (2012) \textit{Biochemistry} \textbf{51}, 9782.\\[0pt] [2] A. V. Struts et al. (2011)~\textit{PNAS}$~$\textbf{108}, 8263. [Preview Abstract] |
Session Y12: Invited Session: Novel Modeling Approaches to Cell Motility
Sponsoring Units: DCMP GSNPChair: Lev Tsimring, University of California, San Diego
Room: 205
Friday, March 7, 2014 8:00AM - 8:36AM |
Y12.00001: Coupling actin flow, adhesion, and morphology in a computational cell motility model Invited Speaker: Herbert Levine Eukaryotic cells crawl by means of the coordinated spatiotemporal dynamics of an active polymer gel, consisting of actin, myosin and regulators thereof. Motility is necessarily coupled to shape, as the force generating mechanisms such as polymerization-based protrusions interact with the elasticity of the cell membrane and thereby determine the cell morphology. We have introduced a ``phase-field'' model of crawling cells, utilizing a mathematical approach originally developed for morphology problems arising in the field of liquid-solid phase transitions. Our model can be used to explain the pattern of traction forces applied to the substrate as well as some recent observations concerning oscillatory instabilities of cells moving on one-dimensional fiber tracks. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 9:12AM |
Y12.00002: Modeling crawling cell movement on soft engineered substrates Invited Speaker: Igor Aronson Self-propelled motion, emerging spontaneously or in response to external cues, is a hallmark of living organisms. Systems of self-propelled synthetic particles are also relevant for multiple applications, from targeted drug delivery to the design of self-healing materials. Self-propulsion relies on the force transfer to the surrounding. While self-propelled swimming in the bulk of liquids is fairly well characterized, many open questions remain in our understanding of self-propelled motion along substrates, such as in the case of crawling cells or related biomimetic objects. How is the force transfer organized and how does it interplay with the deformability of the moving object and the substrate? How do the spatially dependent traction distribution and adhesion dynamics give rise to complex cell behavior? How can we engineer a specific cell response on synthetic compliant substrates? Here we present a phase-field model for a crawling cell by incorporating locally resolved traction forces and substrate deformations. The model captures the generic structure of the traction force distribution and faithfully reproduces experimental observations, like the response of a cell on a gradient in substrate elasticity (durotaxis). It also exhibits complex modes of cell movement such as ``bipedal'' motion. Our work may guide experiments on cell traction force microscopy and substrate-based cell sorting and can be helpful for the design of biomimetic ``crawlers'' and active and reconfigurable self-healing materials. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:48AM |
Y12.00003: Cell Motility Resulting form Spontaneous Polymerization Waves Invited Speaker: Karsten Kruse The crawling of living cells on solid substrates is often driven by the actin cytoskeleton, a network of structurally polar filamentous proteins that is intrinsically driven by the hydrolysis of ATP. How cells organize their actin network during crawling is still poorly understood. A possible general mechanism underlying actin organization has been offered by the observation of spontaneous actin polymerization waves in various different cell types. We use a theoretical approach to investigate the possible role of spontaneous actin waves on cell crawling. To this end, we develop a meanfield framework for studying spatiotemporal aspects of actin assembly dynamics, which helped to identify possible origins of self-organized actin waves. The impact of these waves on cell crawling is then investigated by using a phase-field approach to confine the actin network to a cellular domain. We find that spontaneous actin waves can lead to directional or amoeboidal crawling. In the latter case, the cell performs a random walk. Within our deterministic framework, this behavior is due to complex spiral waves inside the cell. Finally, we compare the seemingly random motion of our model cells to the dynamics of cells of the human immune system. These cells patrol the body in search for infected cells and we discuss possible implications of our theory for the search process' efficiency. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:24AM |
Y12.00004: A modular view of directed cell migration Invited Speaker: Pablo Iglesias Chemotaxis, the directed migration of cells in response to external chemical gradients, involves the coordinated action of separable but interrelated processes: motility, gradient sensing, and polarization. We have previously argued that separate ``modules'' give rise to these processes individually. In this talk I will describe a computational model in which the different modules are implemented in terms of reaction-diffusion equations. The central module is an excitable network. This module links to an idling cytoskeletal oscillator. In the absence of chemical stimuli, the excitable network can generate the signals that give rise to random migration. The response to combinations of uniform stimuli and gradients is mediated by a local excitation, global inhibition (LEGI) module that biases the direction in which excitability is directed. A polarization module linked to the excitable network through the cytoskeleton allows unstimulated cells to move persistently and, for cells in gradients, to gradually acquire distinct sensitivity between front and back. Migration and the accompanying changes in cellular morphology are simulated using a mechanical model of the cell implemented in the level set framework. [Preview Abstract] |
Session Y13: Focus Session: Fe Based Superconductors-Correlation Effects
Sponsoring Units: DMPChair: Rafael Fernandes, University of Minnesota
Room: 207
Friday, March 7, 2014 8:00AM - 8:12AM |
Y13.00001: A DFT+DMFT Investigation on Electron-Phonon Coupling in FeSe Subhasish Mandal, R.E. Cohen, K. Haule The dramatic increase of superconducting temperature with external pressure in undoped FeSe has opened up a new route to investigate the mechanism of superconductivity. Using the self-consistent density functional theory-dynamical mean field theory (DFT-DMFT) method for paramagnetic FeSe as a function of compression, we find that there is greatly enhanced coupling between some correlated electron states between $\Gamma$ and Z points and the $A_{1g}$ lattice distortion. Except at high pressure, the maximum deformation potential in DFT for this mode is insensitive to increasing pressure, whereas the corresponding DFT-DMFT maximum deformation potential shows the same behavior with pressure as the experimental $T_c$, which first increases and then decreases with pressure. The Fermi surface average of the deformation potential in DFT-DMFT method can increase up to 50\% when compared to standard DFT [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y13.00002: Spin dynamics and a novel orbital-antiphase pairing symmetry in iron-based superconductors Zhiping Yin, Kristjan Haule, Gabriel Kotliar We use first-principles many-body method, including ab initio determined two-particle vertex function, to study the spin dynamics and superconducting pairing symmetry in a large number of Fe-based superconductors. In Fe compounds with high transition temperature, we find both the dispersive high-energy spin excitations, and very strong low energy commensurate or nearly commensurate spin response, suggesting that these low energy spin excitations play the dominate role in cooper pairing. We find three closely competing types of pairing symmetries, which take a very simple form in the space of active Fe $3d$ orbitals, and differ only in the relative quantum mechanical phase of the $xz$, $yz$ and $xy$ orbital contributions. The extensively discussed s$^{+-}$ symmetry appears when contributions from all orbitals have equal sign, while the opposite sign in $xz$ and $yz$ orbitals leads to the $d$ wave symmetry. A novel orbital antiphase $s^{+-}$ symmetry emerges when $xy$ orbital has opposite sign to $xz$ and $yz$ orbitals. We propose that this orbital-antiphase pairing symmetry explains the puzzling variation of the experimentally observed superconducting gaps on all the Fermi surfaces of LiFeAs. This novel symmetry of the order parameter may be realized in other Fe superconductors. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y13.00003: In-plane anisotropic resistivity in the antiferromagnetic phase of Fe-based superconductors Koudai Sugimoto, Peter Prelov\v{s}ek, Eiji Kaneshita, Takami Tohyama The parent compound of Fe-based superconductors has a stripy-ordered antiferromagnetic phase showing in-plane anisotropy of resistivity. Recent experiments have suggested that impurities such as Co substituted for Fe play a crucial role in the anisotropy. We start with a five-orbital Hubbard model with mean-field approximation. We examine the anisotropy of resistivity in the antiferromagnetic phase by applying the memory-function approach treating the isotropic nonmagnetic impurities. Near undoped region, where the Drude weight gives anisotropy opposite to experimental observation, the memory-function approach yields a proper anisotropic behavior: The resistivity in the antiferromagnetically ordered direction is smaller than that in the ferromagnetic direction and the anisotropy reverses when holes are introduced. The origin of the anisotropy can be understood from the interplay of impurity scattering and the character of Fermi surface. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y13.00004: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y13.00005: Theory of disorder-limited transport anisotropy in Fe-based superconductors Peter Hirschfeld, Yan Wang, Indranil Paul The unusual temperature dependence of the resistivity and its in-plane anisotropy observed in the Fe-based superconducting materials, particularly BaFe$_2$As$_2$, has been a longstanding puzzle. Here we construct a crude phenomenological model of the scattering of electrons from dopant induced nematic impurity states which explains many qualitative features of these experiments. Within this model, the high-temperature transport anisotropy near the magnetic and structural transitions is due almost entirely to the disordered nematogens, while below $T_N$. Fermi surface reconstruction competes with this effect. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y13.00006: Collective excitations in AFe2Se2 superconductors: application to Raman Maxim Khodas, Andrey Chubukov We present studies of Raman scattering in A$_x$Fe$_{2-y}$Se$_2$ (A=K,Rb,Cs) superconductors with only electron pockets. The pairing symmetry consistent with both ARPES and neutron scattering is hybridization induced $s^{+-}$ state. Such order parameter changes sign between the hybridized pockets. The peak in calculated $B_{2g}$ Raman intensity signifies the in-gap charge excitation of a $d$-wave symmetry. A single $B_{2g}$ mode emerges as a result of strong mixing between the $d$-wave superconducting fluctuations (Bardasis-Schrieffer modes) and the charge nematic fluctuations. Increase in pocket ellipticity promotes $d$-wave ordering. As a result the $B_{2g}$ Raman mode softens, and becomes critical at the transition to an $s+id$ state with broken time reversal symmetry. The reported $B_{2g}$ excitation corresponds to the in-gap feature in experimentally observed Raman spectra. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:48AM |
Y13.00007: The iron pnictides and chalcogenides, a DMFT perspective Invited Speaker: Kristjan Haule The complex multi-band nature of iron pnictides and chalchogenides makes the interplay of superconductivity with spin and orbital dynamics very intriguing, leading to very material dependent magnetic excitations, and pairing symmetries. We use the first-principles Dynamical Mean Field method, including ab-initio determined two-particle vertex function, to study the spin dynamics and superconducting pairing symmetry in a large number of iron-based superconductors. In iron compounds with high transition temperature, we find both the dispersive high-energy spin excitations, and very strong low energy commensurate or nearly commensurate spin response, suggesting that these low energy spin excitations play the dominat role in cooper pairing. We find three closely competing types of pairing symmetries, which take a very simple form in the space of active iron $3d$ orbitals, and differ only in the relative quantum mechanical phase of the $xz$, $yz$ and $xy$ orbital contributions. The extensively discussed s$^{+-}$ symmetry appears when contributions from all orbitals have equal sign, while the opposite sign in $xz$ and $yz$ orbitals leads to the $d$ wave symmetry. A novel orbital antiphase $s^{+-}$ symmetry emerges when $xy$ orbital has opposite sign to $xz$ and $yz$ orbitals. We propose that this orbital-antiphase pairing symmetry explains the puzzling variation of the experimentally observed superconducting gaps on all the Fermi surfaces of LiFeAs. This novel symmetry of the order parameter may be realized in other iron superconductors. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y13.00008: Magnetic impurities in the two-band model for Fe-based superconductors M.M. Korshunov, O.V. Dolgov, A.A. Golubov, D.V. Efremov Superconductors with different gap symmetries behave differently being subject to the disorder. It is especially important to determining this exact behavior in the Fe-based materials where both the order parameter symmetry and the mechanism of superconductivity are unknown. Here we analyze how the magnetic disorder affects the low-energy properties of the two-band $s_\pm$ and $s_{++}$ models. In a general case, $T_c$ is suppressed approximately following the Abrikosov-Gor'kov trend. There are, however, few exceptional cases with the saturation of $T_c$ for the finite amount of impurities: 1) $s_\pm$ superconductor with the purely interband impurity scattering potential or with the unitary impurities, 2) $s_{++}$ state with the purely interband scattering. We show that the latter unusual behavior is due to the $s_{++} \to s_\pm$ transition. Similar to the case of non-magnetic impurities in a two-band superconductor, the transition occurs depending on the sign of the average coupling constant $\langle \lambda \rangle$. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y13.00009: Magnetic Domain Walls Induced by Twin Boundaries in Low Doped Fe-pnictides Bo Li, Jian Li, Kevin Bassler, C.S. Ting Inspired by experimental observations of the enhancement of superconductivity at the twin-boundary (TB) in slightly electron doped Ba(Ca)(FeAs)$_2$ where a strong $2\times1$ antiferromagnetic (AF) collinear order is in presence, we investigate theoretically the effects of TBs on the complex interplay between magnetism and superconductivity using a minimum phenomenological two-orbital model. The spatial distributions of the magnetic, superconducting and charge density orders near two different types of TBs are calculated. Each of the TBs has two different orientations. We find that the first type TBs, which corresponds to a $90^\circ$ lattice rotation in the $a-b$ plane, enable magnetic domain walls (DWs) to be pinned at them, and that superconductivity is enhanced at such TBs or DWs. This result is consistent with experiments for a TB with an orientation of $45^\circ$ from the x-axis. Contrastingly, we predict that superconductivity is suppressed at the second type of TBs which correspond to an asymmetrical placement of As atoms on the opposite sides of the TB. Furthermore, the lattice-mismatch effect across the TBs is investigated. The comparison of our results with the observations from the nuclear-magnetic-resonance (NMR) experiments is also discussed. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y13.00010: Disorder effects in a multiband superconductor in the presence of competing order : Implications for underdoped pnictides Vivek Mishra In unconventional superconductors superconductivity emerges at the onset of a magnetic order, and in many cases with a co-existing region of superconductivity and magnetism in the phase diagram. Here I consider the effect of disorder in a multiband superconductor appropriate for ferro-pnictide superconductors. I consider both interband and intraband scattering for a two band model within a self consistent T-matrix approximation. I calculate the effect of disorder on the critical temperature and on the low energy excitation spectrum in the superconducting state with different possible order parameters. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y13.00011: Localization Effects of Cu Substitutions in FeSe: an Improved Green's Function Decoupling Approach Yang Liu, Yun Song We study the substitution effects of Cu in FeSe system by using a Green's function decoupling method based on an extended Hubbard-I approximation. A three-band Hubbard Hamiltonian with randomly distributed impurities is employed to describe the iron superconductor Fe$_{1-x}$Cu$_{x}$Se. The parameters of the three-band model are determined by fitting the Fermi structures with the results of experiments. With the increasing of Cu substitution, the nesting between hole-like and electron-like Fermi pockets is destroyed completely. We also find that Cu substitution can introduce strong localization effects and lead the system to have a metal- insulator transition. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y13.00012: Energy-dependent modulations in the local density of states in the under-doped and optimally doped NaFe$_{1-x}$Co$_{x}$As Lihua Pan, C.S. Ting Motivated by recent scanning tunneling microscopy experiment investigating the quasiparticle interference (QPI) patterns in NaFe$_{1-x}$Co$_{x}$As, we investigate the energy-dependent modulation of local density of states induced by a weak defect using the first-order T-matrix approximation. In the under-doped sample with spin-density-wave ordering, the electrons disperse along the antiferromagneitc direction but remain static along the ferromagnetic direction. The optimally doped sample exhibits square-like QPI patterns. The corresponding QPI in $\textbf{k}$ space is also presented. These numerical results exhibit essential features as those measured by the experiment. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y13.00013: Phase diagram with doping dependence in phosphorous-doped iron-based superconductor and magnetic order study Yuanyuan Zhao, Yuan-Yen Tai, C.S. Ting Recent experiments discover the isovalent doping can induce superconductivity in a unique iron-based superconductor, BaFe2(As1-xPx)2, through substitution of phosphorous (P) for arsenic (As). While the phosphorous (P-) doping is often considered not to alter the occupation of Fe-3d bands, surprisingly, it has the similar phase diagram just like the heterovalent doped cases: with the P-doping, the magnetic order is suppressed and the superconductivity emerges. Here we theoretically give a possible explanation of the phase diagram based on a recent minimal two-orbital tight-binding model. Moreover, through the spin susceptibility calculation, we try to study the effect of the hopping parameters on the magnetic order. [Preview Abstract] |
Session Y14: Invited Session: Dynamics of Polymer Nanocomposites
Sponsoring Units: DPOLY DBIOChair: Russell Composto, University of Pennsylvania
Room: 301-303
Friday, March 7, 2014 8:00AM - 8:36AM |
Y14.00001: Dynamics of nanoparticles in models of soft and hard porous media Invited Speaker: Jacinta Conrad The transport properties of nanoparticles in complex porous media impact the processing of polymer and hydrogel nanocomposites. In the limit of strong confinement, in which the size of nanoparticles is comparable to typical length scales within the complex confining medium, the local mechanisms that influence nanoparticle transport remain poorly understood. I will describe experiments in which we use optical microscopy to probe the diffusive and transport properties of particles of size 200-400 nm in models of hard and soft porous media. As models of hard porous media, we fabricate arrays of nanoposts that are arranged in a square lattice; as models of soft porous media we formulate aqueous soultions of hydrolyzed polyacrylade over a wide range of dilute and semi-dilute concentrations. In both quiescent diffusion and in flow-driven transport through hard media, we generally find that the dynamics of the nanoparticles become increasingly slowed and stretched as the particles are more strongly confined. Strong confinement leads to deviations from the dispersion behaviors expected for small solute molecules. In soft media, we find that the local viscosity experienced by the nanoparticles systematically underestimates the zero-shear rate viscosity measured using bulk rheology, which we attribute to coupling between particle and polymer dynamics. I will discuss these results and their implications for nanocomposite processing as well as for other applications that require confined transport of nanomaterials in complex media. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 9:12AM |
Y14.00002: X-ray photon correlation spectroscopy studies of nanoparticle motion in glassy polymer melts and entangled polymer solutions Invited Speaker: Robert Leheny Microrheology, in which colloids suspended in a complex fluid probe the mechanical environment, can provide unique information on the microscopic length scales characterizing the fluid's hierarchical structure. We describe x-ray photon correlation spectroscopy (XPCS) studies tracking the nanometer-scale motion of dilute suspensions of gold nanoparticles in low-molecular-weight polystyrene melts and in high-molecular-weight polystyrene solutions. In the melts, the high-temperature nanoparticle dynamics are diffusive with a rate that tracks the melt viscosity. Close to the glass transition, a hyper-diffusive process that we identify with heterogeneous strain in the melts supersedes the diffusion. Following a quench, the hyper-diffusive dynamics display characteristics of aging. Similar slow, heterogeneous strain has been observed in a range of soft glassy materials such as colloidal gels and emulsions. The apparently universal nature of the phenomenon hence provides a link between the microscopic processes of aging in hard and soft glassy systems. In contrast, the nanoparticle motion in the high-molecular-weight solutions reveals qualitatively different behavior. Over displacements from nanometers to tens of nanometers, the particles undergo anomalous subdiffusion in which the particle mean-squared displacement grows as a power law in time with power-law exponent in the range 0.3 to 0.5 depending on solution conditions. Scaling behavior of the nanoparticle mobility with respect to temperature and polymer concentration and molecular weight indicates that the subdiffusive motion results from the temporal evolution of the entanglement mesh in the immediate vicinity of the particles. The results thus provide novel microscopic characterizations of the structural dynamics in the melts and entangled solutions and more broadly demonstrate the ability of XPCS-based microrheology to interrogate the nanoscale mechanical behavior of polymer materials. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:48AM |
Y14.00003: Microscopic Theories of Diffusion, Tube Localization and Slow Relaxation in Polymer Nanocomposites Invited Speaker: Kenneth Schweizer Dynamics in polymer nanocomposites is rich and complex but poorly understood due to the presence of multiple length scales, excluded volume effects and other factors. We have developed new statistical mechanical theories at the level of forces for particle and polymer motion in flexible and rigid polymers. This talk presents an overview, including quantitative comparisons to simulations and experiments. First, by combining Brownian motion, polymer physics and mode coupling ideas, a self-consistent theory for the non-hydrodynamic diffusion of a spherical nanoparticle in melts has been constructed. Three competing mechanisms are predicted: sieving-like diffusion through unentangled regions, reptation-driven constraint release in entangled melts, and activated hopping through entanglement meshes. The controlling mechanism depends on particle size, tube diameter and entanglement density. The approach can also treat soft fillers, nonspherical particles, adsorption, solutions and networks. Second, a self-consistent microscopic theory for the slow dynamics of a needle fluid in a matrix of static spheres has been developed which exactly enforces inter-needle topological uncrossability and needle- sphere impenetrability constraints at the two-body level. The rich dependences of the effective tube diameter and anisotropic diffusion constants on filler-needle aspect ratio, polymer concentration and particle volume fraction has been established. Due to steric blocking of longitudinal motion by obstacles, a literal localization transition is predicted that is controlled by the particle to tube diameter ratio. For a restricted window of parameter space, needles are predicted to diffuse via a ``renormalized'' reptation dynamics where compression of the tube and suppression of longitudinal diffusivity enter in a manner that depends on all system variables. Generalization of the approach to treat mobile fillers, flexible chains and nonrandom microstructure is possible. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:24AM |
Y14.00004: Rotational diffusion in polymer nanocomposites as probed by anisotropic particles Invited Speaker: Laura Clarke Metal nanoparticles strongly absorb specific wavelengths of light with no (or only a very weak) radiative relaxation by which to release this energy. As a result, the absorbed energy is efficiently converted to local heat (a photothermal effect). With an effective cross-section of up to 10 times its physical size, each particle acts as a ``super-sized'' absorber even when embedded within a transparent material environment such as a polymer, resulting in dramatic heating originating at the particles. Thus, with spatially-uniform illumination, one can metaphorically reach inside a polymer nanocomposite and apply heat to pre-selected subsets (e.g., causing them to dramatically change properties due to actuation, cross-linking, crystallization, or chemical reaction) without heating the sample surface or strongly affecting the remainder of the material. By utilizing optically-accessible additives including the particles themselves, the thermal gradient from the particle outward can be experimentally determined. In particular, rotational diffusion of anisotropic particles can be used to measure the temperature at the nanoparticle, which is the warmest point in a polymeric film or nanofiber under photothermal heating. Conversely, the same technique can be utilized to measure polymer dynamics in nanocomposites in the immediate vicinity of the particle.\\[4pt] [1] S. Maity et al., \textit{Polymer} \textbf{52}, 1674 (2011).\\[0pt] [2] S. Maity et al., \textit{Adv. Funct. Mater} \textbf{22}, 5259 (2012).\\[0pt] [3] S. Maity et al., \textit{Part. \& Part. Sys. Char} \textbf{30}, 193 (2013). [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 11:00AM |
Y14.00005: Polymer Diffusion in the Presence of Nanoparticles Invited Speaker: Karen Winey The center-of-mass diffusion of polymers within a polymer melt proceeds by the mechanism of reptation wherein the polymer is confined to a tube that is defined by neighboring entanglements and moves along its contour. Polymer diffusion is perturbed when the melt contains nanoparticles that are comparable in size to the radius of gyration (Rg) of the polymers. Within this talk, we will present tracer diffusion coefficients (D) results for three types of nanocomposite: spherical nanoparticles with surface functionalization, spherical nanoparticles with brushes, and cylindrical nanoparticles (aspect ratio $=$ 5 to 50). When functionalized spherical nanoparticles have neutral or attractive interactions with the polymer matrix, a monotonic decrease in the diffusion coefficient is observed across a wide range of polymer molecular weight, nanoparticle size, and nanoparticle concentration. These data collapse onto a master curve when plotted as D normalized by the diffusion coefficient into a neat homopolymer (D/Do) versus our confinement parameter defined as the interparticle distance divided by 2Rg (ID/2Rg). Polymer diffusion in systems with grafted spherical nanoparticles exhibit the same D/Do versus ID/2Rg, when ID accounts for the extent to which the tracer polymer penetrates the polymer brush. For various cylindrical nanoparticles D/Do versus nanoparticle concentration exhibits a minimum when 2Rg is both larger than the nanoparticle diameter and smaller than the nanoparticle length. Complimentary molecular dynamics simulations and neutron scattering results will also be presented. [Preview Abstract] |
Session Y15: Stellar & General Fluid Dynamics
Sponsoring Units: DFDChair: Xifan Wu, Temple University
Room: 304
Friday, March 7, 2014 8:00AM - 8:12AM |
Y15.00001: Toward Connecting Core-Collapse Supernova Explosions with Observations of their Supernova Remnants Timothy Handy, Tomasz Plewa, Artur Gawryszczak We study the process of collapse of a massive star and the following explosion process until the formation of a young supernova remnant in a single simulation. These new models are critically evaluated against a database of core-collapse supernovae (ccSNe) explosion models obtained with a standard supernova code. We develop a multiphysics hydrocode capable of accounting for physics from before collapse occurs until the supernova remnant phase. This enables ccSNe studies with a single code without the need of remapping or transferring data between multiple codes. The code uses a new algorithm to account for the effects of neutrino-matter interaction in the collapsing stellar core. The algorithm uses ray-casting in three dimensions and enables performing collapse and explosion simulations on AMR meshes, including non-radial discretizations. Heating due to radioactive decay, and magnetization of the ejecta are included in the model. The asymmetry of the explosion continues to play a role well beyond the shock breakout phase. In particular, the lateral momentum deposited in the process of shock revival helps shape the supernova ejecta. Another important contributing factor shaping the ejecta is due to radioactive decay of nucleosynthetic products of the explosion. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y15.00002: The Sun is Condensed Matter and has a Real Surface Pierre-Marie Robitaille The idea that the Sun was a gaseous in nature was born from 1858-65. At that time, a group of men, including Herbert Spencer, Father Angelo Secchi, Warren de la Rue, Balfour Stewart, and Benjamin Loewy, advanced that the Sun was a ball of gas. In 1865, Herv\'{e} Faye was the first to argue that the solar surface was merely an illusion. Dismissing all signs to the contrary, solar physics has promoted this idea to the present day, as manifested by the Standard Solar Model. In this work, overwhelming observational evidence will be presented that the Sun does indeed possess a distinct surface (see P.M. Robitaille, Forty Lines of Evidence for Condensed Matter --- \underline {The Sun on Trial}: Liquid Metallic Hydrogen as a Solar Building Block, Progress in Physics, 2013, v. 4, 90-143). Our telescopes and satellites are sampling real structures on the surface of the Sun. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y15.00003: Branching of Tsunami Waves Henri-Philippe Degueldre, Jakob Metzger, Ragnar Fleischmann, Theo Geisel Branched flow is a universal phenomenon occuring in particle or wave flows propagating through weakly scattering, correlated, random media. Even for very weak disorder in the medium, it can lead to extremely strong fluctuations in the wave intensity. We show how tsunami waves are affected by branching. We model the tsunamis propagating over the ocean floor with its complex height fluctuations by the linearized shallow water wave equations with random bathymetries. We calculate the typical distance from the source at which the strongest wave fluctuations occur as a function of the statistical properties of the bathymetry. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y15.00004: Imaging heat transfer processes in a fluid with temperature sensitive paint Jun Huang, Tianshu Liu, Weili Luo The temperature profile inside a fluid was imaged by temperature sensitive paint in a quasi one-dimensional cell, where temperature gradients were established by heating on one side of the sample and cooling on the other. Similar experiment was performed on colloids consisting nanoparticles suspended in solvent. The change of the profile for different heat-transfer processes as functions of time will be discussed. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y15.00005: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y15.00006: Adjoint problem in duct acoustics and its reciprocity to forward problem by the Time Domain Wave Packet method Ibrahim Kocaogul, Fang Hu, Xiaodong Li Radiation of acoustic waves at all frequencies can be obtained by Time Domain Wave Packet (TDWP) method in a single time domain computation. Other benefit of the TDWP method is that it makes possible the separation of acoustic and instability wave in the shear flow. The TDWP method is also particularly useful for computations in the ducted or waveguide environments where incident wave modes can be imposed cleanly without a potentially long transient period. The adjoint equations for the linearized Euler equations are formulated for the Cartesian coordinates. Analytical solution for adjoint equations is derived by using Green's function in 2D and 3D. The derivation of reciprocal relations is presented for closed and open ducts. The adjoint equations are then solved numerically in reversed time by the TDWP method. Reciprocal relation between the duct mode amplitudes and far field point sources in the presence of the exhaust shear flow is computed and confirmed numerically. Applications of the adjoint problem to closed and open ducts are also presented. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y15.00007: Ab initio theory of X-ray emission spectral of liquid water Charles Swartz, Santra Biswajit, Robert DiStasio, Xifan Wu X-ray emission spectral (XES) has recently been established as a powerful experimental tool in detecting the local H-bond network of liquid water. In the current work, we have developed an {\it ab initio} scheme for calculating the XES spectra of water. Based on the equilibrium trajectories generated by {\it ab initio} molecular dynamics, we perform additional atomic dynamics in the presence of a core-hole within the time scale corresponding to the experimental core-hole lifetime. The XES spectra is then determined by computing the transition matrix based on many body perturbation theory within the GW approximation. It is found that both the core-hole dynamics and the ensemble average of the local chemical environment, due to fluctuations of the H-bond network, are crucial in obtaining a physically meaningful XES spectra of liquid water. [Preview Abstract] |
Session Y16: Complex Networks and their Applications I
Sponsoring Units: GSNPChair: Kevin Bassler
Room: 401
Friday, March 7, 2014 8:00AM - 8:12AM |
Y16.00001: Targeted Control of Complex Networks Jianxi Gao, Yangyu Liu, Raissa M. D'Souza, Albert-Laszlo Barabasi Network controllability is typically formulated as the ability to drive an entire network from any initial state to any desired final state in finite time, using a minimum number of inputs. However, in many circumstances it is neither feasible nor necessary to control the entire network. This prompts us to explore how to efficiently control a subset of nodes in a network, i.e., ``targeted controllability.'' We develop an alternate ``k-walk'' theory based on the fact that a single node can control a set of nodes provided the path length from the control node to each target node is unique. For the general case, we develop a greedy algorithm, based on k-walk theory, to identify the approximate minimal set of necessary driver nodes. We demonstrate that partial controllability has fundamentally different features when compared to full controllability. For example, we find that degree heterogeneous networks can be partially controllable with higher efficiency than degree homogeneous networks. Moreover we show that the structures of many real-world networks are highly efficient for targeted control. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y16.00002: Energy Spectrum of Complex Systems Control Georgios Tsekenis, Baruch Barzel, Yang-Yu Liu, Jean-Jacques Slotine, Albert-Laszlo Barabasi Understanding how to control a complex network is a fundamental scientific question with a wealth of potential applications in man-made and natural systems. When controlling a complex system one has to steer it from one point of its state space to another by an appropriate set of inputs. The state space complex systems operate in is high-dimensional with dimensionality equal to the system's size. The energy of control across directions span several decades of orders of magnitude exhibiting an extremely wide range. The control energy spectrum is dominated by a fat-tail that is largely independent of the properties of the network. As a result control is easy in most directions while it is very hard in few of them. Naturally the energy of control decreases by increasing the number of driver nodes. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y16.00003: Cascading Failures Due to Multiple Causes in Interdependent Networks Yosef Kornbluth, Sergey Buldyrev In recent years, several models of network failure have been introduced. Some of these models are based on overload, in which increased traffic destroys nodes, while others are based on partial isolation, in which a node needs several functional neighbors to survive. In these systems, failure of a small fraction of nodes can cause a cascade of failures which may completely destroy the network. The majority of these models are studied in single networks. However, many real-world systems are comprised of multiple interdependent networks. Recent studies based on the concept of mutual percolation show that these systems are much more vulnerable than a single network. We numerically and analytically investigate how multiple causes of failure simultaneously acting in a system of interdependent networks affect their vulnerability. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y16.00004: Fractional dynamics of complex networks Malgorzata Turalska, Bruce J. West The relation between the behavior of a single element and the global dynamics of its host network is an open problem in the science of complex networks. Typically one attempts to infer the global dynamics by combining the behavior of single elements within the system, following a bottom-up approach. Here we address an inverse problem. We show that for a generic model within the Ising universality class it is possible to construct a description of the dynamics of an individual element, given the information about the network's global behavior. We demonstrate that the individual trajectory response to the collective motion of the network is described by a linear fractional differential equation, whose analytic solution is the Mittag-Leffler function. This solution is obtained through a subordination procedure without the necessity of linearizing the underlying dynamics, that is, the solution retains the influence of the nonlinear network dynamics on the individual. Moreover the solutions to the fractional equation of motion suggest a new direction for designing control mechanisms for complex networks. The implications of this new perspective are explored by introducing a control signal into a small number of network elements and analyzing the subsequent change in the network dynamics. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y16.00005: Random Regular Networks with Distance-limited Interdependent Links Steven Lowinger, Yosef Kornbluth, Gabriel Cwilich, Sergey Buldyrev We study the mutual percolation of a system composed of two interdependent random regular networks. We introduce a notion of distance, $d$, to explore the effects of the proximity of interdependent nodes on the cascade of failures after an initial attack. The nature of the transition through which the networks disintegrate depends on the parameters of the system, which are the degree of the nodes and the maximum distance between interdependent nodes. As the distance and degree increase, the collapse at the critical threshold changes from a second-order transition to a first-order one. The critical threshold monotonically increases with distance. We find a transitional case, in which a novel type of phase transition appears. The case $d=$1 can be completely solved analytically and it maps into a discrete version of the R\'{e}nyi parking problem [1]. \\[4pt] [1] A. R\'{e}nyi, Publ. Math. Inst. Hung. Acad. Sci. 3, 109-127(1958). [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y16.00006: n-tangle: A network comparison method Lazaros K. Gallos, Nina H. Fefferman The ability to compare systems has always been a strong driving force in science. Network analysis has allowed researchers across fields to quantify and characterize numerous patterns of interactions among individuals. The comparison of different networks, though, still remains a puzzling problem. Here, we introduce the n-tangle method to directly compare two networks for structural similarity, based on the distribution of edge density in network subgraphs. The crux of this method is to capture how many affiliations we expect to find when we isolate any given size of connected sub-group. We demonstrate that this method can efficiently introduce comparative analysis into network science and opens the road for many new applications. Our approach avoids inherent constraints of other methods and can be applied to networks of different size and structure. Our method can be expanded to study a multitude of additional properties, such as network classification, changes during time evolution, convergence of growth models, and detection of structural changes during damage. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y16.00007: Symmetry in Critical Random Boolean Networks Dynamics Kevin E. Bassler, Shabnam Hossein Using Boolean networks as prototypical examples, the role of symmetry in the dynamics of heterogeneous complex systems is explored. We show that symmetry of the dynamics, especially in critical states, is a controlling feature that can be used to both greatly simplify analysis and to characterize different types of dynamics. Symmetry in Boolean networks is found by determining the frequency at which the various Boolean output functions occur. Classes of functions occur at the same frequency. These classes are orbits of the controlling symmetry group. We find the nature of the symmetry that controls the dynamics of critical random Boolean networks by determining the frequency of output functions utilized by nodes that remain active on dynamical attractors. This symmetry preserves canalization, a form of network robustness. We compare it to a different symmetry known to control the dynamics of an evolutionary process that allows Boolean networks to organize into a critical state. Our results demonstrate the usefulness and power of using symmetry to characterize complex network dynamics, and introduce a novel approach to the analysis of heterogeneous complex systems. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y16.00008: Critical Behavior in a Class of Heterogeneous Complex Systems Shabnam Hossein, Florian Greil, Kevin E. Bassler Dynamical critical behavior of a prototypical heterogeneous complex system, random Boolean networks, is studied. Using analytical arguments, we show that the networks, at the boundary between their frozen and chaotic dynamical phases, display universal critical behavior in their attractor period distribution, which has the functional form of a decaying power-law. Using a known result that nodes relevant to the dynamics on attractors at criticality can be divided into separate components, we analyze the structure of these relevant components and how their dynamics combine to find the distribution of attractor periods. This is accomplished by mapping the problem to the enumeration of binary Lyndon words. We show that the attractor period distribution becomes scale-free in the large network limit with a decay described by a critical exponent of 1. Results of numerical simulations that support this finding, but that also show that substantial finite-size corrections exist, will also be presented. The universal nature of this behavior is demonstrated by comparison to that of the evolved critical state achieved through the playing of an adaptive game that selects for diversity of node behavior. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y16.00009: Eigenvalue Separation in the Laplacian Spectra of Random Geometric Graphs Amy Nyberg, Kevin E. Bassler The graph Laplacian spectra of networks are important for characterizing both their structural and dynamical properties. We investigate the spectra of random geometric graphs (RGGs), which are prototypical examples of networks with strong correlations and describe networks whose nodes have a random physical location and are connected to other nodes that are within a threshold distance. RGGs model transportation grids, wireless networks, and biological processes. The spectrum consists of two parts, a discrete part consisting of a collection of integer valued delta function peaks centered about the average degree and a continuous part that exhibits the phenomenon of eigenvalue separation. We find that the number of eigenvalues in each separated peak depends on the dimension and boundary conditions of the embedding space of the RGG. Partly because of the bounds it places on connectivity, the smallest nonzero eigenvalue, or algebraic connectivity, is of particular interest. We find that in the regime of separation, the algebraic connectivity varies as a power of the average degree. This power depends on the dimension of the embedding space. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y16.00010: Laplacian Spectra of Random Hyperbolic Geometric Networks Florian Greil, Kevin E. Bassler Random geometric networks embedded in hyperbolic 2D space have been suggested as models of social networks where the spatial metric implements an interplay between popularity and similarity. [F. Papadopulos et. al., Nature {\bf 489}, 589 (2012)] We show that the eigenvalue spectrum of their combinatorial Laplacian matrix can be employed as a useful tool to understand the structural and dynamical properties of these networks. Features of the spectrum, including eigenvalue separation and localization, are associated with specific network properties. These properties are studied as a function of average node connectivity and curvature of the embedding space. For large networks, a transition in the properties of the network is found as the curvature varies. This transition is indicated by the development of power-law spectra at high curvature. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y16.00011: Extracting connectivity of networks from dynamics of nodes Emily S.C. Ching, P.-Y. Lai, C.Y. Leung The knowledge of how the different nodes of a network interact or connect with one another is crucial for the understanding of the collective behavior and the functionality of a network. We have recently developed a method that extracts network connectivity using only measurements of the dynamics of the nodes for bidirectional networks with uniform coupling strength. Our method is built upon a noise-induced relation between the Laplacian matrix $L_{ij}$ of the network and the pseudoinverse of the dynamical covariance matrix $C_{ij}^+$: $L_{ij}=\sigma^2/(2g)C_{ij}^+ $, where $\sigma$ is the noise amplitude and $g$ the coupling strength. This relation is exact for consensus dynamics. The extraction of connectivity is based on the separation of $r_{ij}\equiv C_{ij}^+/C_{ii}^+$, for each node $i$, into two groups depending on whether node $j$ is connected to node $i$ or not. Such a separation is guaranteed by the above relation, and has been found to exist even in networks with nonlinear dynamics. Using examples of different networks and dynamics, we have demonstrated that our method can give accurate local connectivity information as well as global network properties of the degree distribution and eigenvalue spectrum of the adjacency matrix for a wide range of $\sigma$ and $g$. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y16.00012: Distributions of Betweenness in Cascades of Overload Failure in Random Regular Networks Gilad Barach, Mark Tuchman, Gabriel Cwilich, Sergey Buldyrev We study the Motter and Lai [1] model of cascading failures of a network by overload based on the betweenness centrality of the nodes, for the case of a random regular network. ~We study numerically by several means~the disintegration of the network as a function of the fraction~$p$~of the nodes that survive an initial random attack: the size of the final giant component, the number of cascade stages, and the distribution of the betweenness of the nodes for different stages of the cascade. ~We find that the nature of the transition through which the network disintegrates changes from first order to second order as the tolerance increases. After this large drop, in which a substantial part of the network disintegrates, we find that the size of the final~giant component does not decrease monotonically when increasing~the size of the initial attack~\textit{(1-p)}, but rather presents a series of maxima and minima as a function of~$p$. [1] Cascade-based attacks on complex networks, Phys. Rev. \textbf{E 66}, 065102(R) (2002) [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y16.00013: Controlling Contagion Processes in Time-Varying Networks Nicola Perra, Suyu Liu, Marton Karsai, Alessandro Vespignani The vast majority of strategies aimed at controlling contagion processes on networks considers the connectivity pattern of the system as either quenched or annealed. However, in the real world many networks are highly dynamical and evolve in time concurrently to the contagion process. Here, we derive an analytical framework for the study of control strategies specifically devised for time-varying networks. We consider the removal/immunization of individual nodes according the their activity in the network and develop a block variable mean-field approach that allows the derivation of the equations describing the evolution of the contagion process concurrently to the network dynamic. We derive the critical immunization threshold and assess the effectiveness of the control strategies. Finally, we validate the theoretical picture by simulating numerically the information spreading process and control strategies in both synthetic networks and a large-scale, real-world mobile telephone call dataset. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y16.00014: Cascades in the Threshold Model with Multiple Initiators and Heterogeneous Threshold Values P. Karampourniotis, S. Sreenivasan, B.K. Szymanski, G. Korniss The threshold model (TM) is a classical opinion diffusion model, under which a node adopts an opinion only when its threshold is lower than the fraction of its neighbors already possessing that opinion. The TM has been thoroughly investigated for uniform thresholds with small sizes ($< 0.01)$ of initially active nodes (initiators) (Watts, 2002) and with multiple initiators (Singh, 2013). However, a model with uniform threshold does not capture the complex nature of social influencing when multiple initiators are present. We find that for sufficiently large spread in the threshold distribution, the tipping point in the social influencing process disappears and crosses over to a smooth transition governed by the size of initiators. Specifically, we study cascades in the TM when nodes are assigned a threshold value drawn from the Normal Distribution with varying mean threshold $\varphi $ and standard deviation $\sigma $. We analyze both synthetic and empirical networks using different sizes of initiators. We observe a non-monotonic change in the cascade size for varying $\sigma $ that for small initiator sizes follows Watts Cascade Condition. In addition, we find that, unlike the case of uniform thresholds, for large enough $\sigma $, a critical initiator size beyond which cascades become global ceases to exist. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y16.00015: Stability of Dominating Sets in Complex Networks against Random and Targeted Attacks F. Molnar, N. Derzsy, B.K. Szymanski, G. Korniss Minimum dominating sets (MDS) are involved in efficiently controlling and monitoring many social and technological networks. However, MDS influence over the entire network may be significantly reduced when some MDS nodes are disabled due to random breakdowns or targeted attacks against nodes in the network. We investigate the stability of domination in scale-free networks in such scenarios. We define stability as the fraction of nodes in the network that are still dominated after some nodes have been removed, either randomly, or by targeting the highest-degree nodes. We find that although the MDS is the most cost-efficient solution (requiring the least number of nodes) for reaching every node in an undamaged network, it is also very sensitive to damage. Further, we investigate alternative methods for finding dominating sets that are less efficient (more costly) than MDS but provide better stability. Finally we construct an algorithm based on greedy node selection that allows us to precisely control the balance between domination stability and cost, to achieve any desired stability at minimum cost, or the best possible stability at any given cost. Analysis of our method shows moderate improvement of domination cost efficiency against random breakdowns, but substantial improvements against targeted attacks. [Preview Abstract] |
Session Y17: Flow Near Jamming
Room: 402
Friday, March 7, 2014 8:00AM - 8:12AM |
Y17.00001: Finite size analysis of zero-temperature jamming transition under applied shear stress Hao Liu, XiaoYi Xie, Ning Xu We generate jammed packings of frictionless spheres under constant shear stress by minimizing an enthalpy-like energy. At fixed volume fraction and shear stress, we enumerate jammed states out of a large number of independent minimizations. The yield stress is defined as the shear stress at which the probability of finding jammed states is 50{\%}. We find that the yield stress for three-dimensional systems with harmonic repulsion satisfies the finite size scaling, which implies a diverging length scale approaching the unjamming transition at zero temperature and shear stress. Interestingly, the same length scale is exhibited as well in finite size scaling of typical quantities concerned in the study of jamming at zero shear stress, including the potential energy, pressure, coordination number, and shear modulus. This consistency indicates that the length scale found here is robust and universal for three-dimensional systems with harmonic repulsion. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y17.00002: Pinning Susceptibility Near the Jamming Transition Samer Nashed, Amy Graves, Carl Goodrich, Elliot Padgett, Andrea Liu The study of jamming in the presence of pinned obstacles is of both practical and theoretical interest. In simulations of soft, bidisperse disks and spheres, we pin a small fraction, $n_f$ of particles prior to the equilibration process. The presence of pinned particles is known to lower the critical packing fraction, $\phi_J$, for jamming. Further, around this threshold there is a peak in a quantity which we have termed the ``pinning susceptibility'': $\chi_P = \lim_{n_f \rightarrow 0} \frac{\partial P_J (\phi, n_f)}{\partial n_f}$. In the thermodynamic limit, we have posited that $\chi_P \propto |\Delta \phi |^{-\gamma_P} $. Finite-size scaling calculations, involving careful fits of $P_J$ to logistic sigmoidal functions, yield a value for the critical exponent, $\gamma_P$. This new exponent is proposed to be independent of inter-particle potential. Its dependence on dimensionality (2 vs. 3 dimensions) will be discussed. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y17.00003: Soft(er) solids: Strain softening near jamming Brian Tighe, Julia Boschan, Ell\'ak Somfai Many solids become softer when sheared beyond a threshold strain. The strain softening crossover signals the breakdown of linear superposition and the onset of strain dependent elastic moduli. Using simulations of soft spheres close to their jamming transition, we probe the softening regime to characterize its strain and pressure dependence. We identify a threshold strain that vanishes at unjamming, indicating that marginal solids are easily driven into the softening regime and that softening, rather than linear response, is likely to be observed in experiments. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y17.00004: On the route to shear jamming, are fragile states real? Ling Zhang, Jie Zheng, Jie Zhang Shear jammed states have been discovered in recent experiments (Zhang et al Granular Matter 2010, Zhang et al Soft Matter 2010, and Bi et al Nature 2011). Due to the existence of friction between the system and the third dimension, it is unclear whether a fragile state would still exist along the route of shear jamming if the friction were completely eliminated. In a novel apparatus developed recently at SJTU, the friction is successfully eliminated by letting the particles float on the surface of a shallow water layer, revealing more details of the route of shear-jamming. Using high-precision force-gauge and simple-beam apparatus, we are able to measure small forces of three orders of magnitude below the limit of the photo-elastic resolution between particles and boundaries. In this talk, we are going to report the recent progress towards the understanding of the nature of the fragile states. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y17.00005: Dynamics of Unjammed Emulsions Rodrigo Guerra, Thomas Kodger, David Weitz Light scattering and NMR densitometry measurements of quiescent emulsions have shown that amorphous packings of soft, repulsive droplets unjam at osmotic pressures $10^5$ times larger than the typical droplet thermal energy density: $\frac{3\,k_B T}{4\pi R^3}$. This transition corresponds to the pressure at which the thermal fluctuations of individual droplet positions match the yield strain of the packing and drive the fluidization of the material. We use confocal microscopy to investigate the microscopic dynamics of this fluid-like phase and find them to be fundamentally different from those of conventional glass-forming liquids; cage-breaking dynamics are not evident from droplet mean squared displacements and the effective viscosity of the emulsion, though $10^5$ larger than the background fluid, appears largely insensitive to the confining pressure. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y17.00006: Rheology of Soft Colloids Near Rigidity Onset: Critical Scaling, Thermal and Non-thermal Responses Ye Xu, Anindita Basu, Tim Still, Paulo Arratia, Zexin Zhang, Kerstin Nordstrom, Jerry Gollub, Douglas Durian, Arjun Yodh We study the rheological behavior of colloidal suspensions composed of soft sub-micron-size hydrogel particles across the liquid-solid transition. Specifically, steady-state and frequency-dependent rheometric measurements of three-dimensional mono- and bi-disperse colloidal suspensions are carried out as a function of volume fraction. We found the shear stress versus strain-rate curves exhibit very similar critical scaling features characteristic of jamming transition reported in microfluidic experiments [1] and simulation [2,3]. On the other hand, the observed stresses and shear rates near rigidity onset differ significantly in suspensions with different particle size and stiffness. We understand the difference by normalizing the measured stress and strain-rate data by thermal stress and time scales, as suggested by recent simulation work [2,3]. In this context, the normalized data in our systems reside in a regime wherein thermal effects are important, though suspension rheology across the full range of microgel particle experiments appear to exhibit both thermal and athermal mechanisms. [1] K. N. Nordstrom, et al., Phys. Rev. Lett., 2010. [2] A. Ikeda, et al. Phys. Rev. Lett., 2012. [3] A. Ikeda, et al. Soft Matter, 2013. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y17.00007: Pressure Distribution and Critical Exponent in Statically Jammed and Shear-Driven Frictionless Disks Stephen Teitel, Daniel V{\aa}gberg, Yegang Wu, Peter Olsson We numerically study the distributions of global pressure that are found in ensembles of statically jammed and quasistatically sheared systems of bidisperse, frictionless, disks at fixed packing fraction $\phi$ in two dimensions. We use these distributions to address the question of how pressure increases as $\phi$ increases above the jamming point $\phi_J$, $p\sim |\phi - \phi_J|^y$. For statically jammed ensembles, our results are consistent with the exponent $y$ being simply related to the power law of the interparticle soft-core interaction. For sheared systems, however, the value of $y$ is consistent with a non-trivial value, as found previously in rheological simulations. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y17.00008: Effect of friction on shear jamming Dong Wang, Jie Ren, Joshua Dijksman, Robert Behringer Shear Jamming of granular materials was first found for systems of frictional disks, with a static friction coefficients $\mu_s \simeq 0.6$. Jamming by shear is obtained by starting from a zero-stress state with a packing fraction $\phi_S \leq \phi \leq \phi_J$ between $\phi_J$ (isotropic jamming) and a lowest $\phi_S$ for shear jamming. This phenomenon is associated with strong anisotropy in stress and the contact network in the form of ``force chains,'' which are stabilized and/or enhanced by the presence of friction. We address experimentally how reducing friction affects shear jamming by using either teflon disks of teflon wrapped photoelastic particles. The teflon disks were placed in a wall driven 2D shear apparatus, in which we can probe shear stresses mechanically. Teflon-wrapped disks were placed in a bottom driven 2D shear apparatus (Ren et al., PRL 2013). Both apparatuses provide uniform simple shear. In all low-$\mu$ experiments, the shear jamming occurred, as observed through stress increases on the packing. However, the low-$\mu$ differences observed for $\phi_J - \phi_S$ were smaller than for higher friction particles. Ongoing work is studying systems using hydrogel disks, which have a lower friction coefficient than teflon. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y17.00009: Dissipation and Rheology of Sheared Soft-Core Frictionless Disks Daniel V\aa gberg, Peter Olsson, S. Teitel We use numerical simulations to investigate the effect of different dissipative models on the shearing rheology of massive soft-core frictionless disks in two dimensions. We show that the presence of Newtonian (overdamped) vs Bagnoldian (inertial) rheology is related to the formation of large connected clusters of disks, and that sharp transitions may exist between the two as system parameters vary. In the limit of strongly inelastic collisions, we find that rheological curves collapse to a well-defined limit when plotted against an appropriate dimensionless strain rate. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y17.00010: High-velocity drag friction in granular media near the jamming point Yuka Takehara, Ko Okumura Drag friction that acts on a disk in a two-dimensional granular medium is studied at high packing fractions in a closed horizontal cell used in Ref. [1]. We concentrate on a high-velocity region, in which the dynamic component of the force, obtained as average of strongly fluctuating force, clearly scales with velocity squared. We change the packing fraction to experimentally access the rheology near the jamming point and we find that the dynamic force and fluctuation of the force tend to diverge as the packing fraction approaches the jamming point. This is in contrast with the case of soft colloids in which the stress is finite at the jamming point. In addition, we develop a simple theory, which takes into account a collective collision around the disk and is equipped with a length scale diverging towards the jamming point. This theory explains well the experimental data. Unexpectedly, the static component of the force, the total force and the fluctuation of the total force also diverge towards the jamming point, with virtually the same exponent.\\[4pt] [1] Y.Takehara, S. Fujimoto and K. Okumura, High-velocity drag friction in dense granular media, \textit{EPL}, \textbf {92} (2010) 44003. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y17.00011: Velocity fluctuations in hopper flow near the clogging transition Charles Thomas, Douglas Durian Dynamic arrest in granular systems continues to elude a comprehensive description. We consider granular flow from a hopper as a quintessential example of a system which can spontaneously evolve from a freely flowing state to a jammed state. With a large enough opening of size D, grains flow out freely. When D is smaller, however, grains flow for a period and then stop, and the entire hopper has clogged. A critical opening size Dc is defined as the smallest D for which the flow will never clog, and marks the clogging transition. We systematically investigate the grain motion in a quasi-2D hopper as a function of D when D > Dc. Using a high-speed camera, we track the particles and find their instantaneous velocities. We report on the fluctuations of these particle velocities relative to their time-averaged velocity. In other systems, this has been seen to grow on approach to jamming. Additionally, we describe the time scales associated with the intermittency of the flow. Diverging time scales are also a key characteristic of a system near jamming. Furthermore, a clog can be considered an intermittent event of indefinite duration. The similarities and contrasts between the clogging and the jamming transitions will further illuminate systems which undergo dynamic arrest. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y17.00012: Flow of Weakly Vibrated Granular Media Geert Wortel, Olivier Dauchot, Martin van Hecke We experimentally study the response of a granular material that is subjected to vibration and shear -- a combination that leads to very rich behavior. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y17.00013: The Role of Anisotropy in Hopper Flows Audrey Melville, Yaqi Hou, Junyao Tang, Joshua Dijksman, Robert Behringer In this work, we examine granular flows in a quasi-two-dimensional hopper. We use two high-speed cameras to record granular flows composed of photoelastic disks. Our dual camera approach provides synchronized particle tracking data and the force response of each particle. The photoelastic measurements allow us to extract a measure for the local anisotropy of the stress field. Using this data, we probe the relationship between the local flow dynamics with local measurements of the stress anisotropy, particle density, and pressure in the system. Current work includes correlating these quantities in the context of a shear jamming picture. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y17.00014: Evolutionarily designing the least obstructive hopper - with application on 3D printing Guo-Jie Gao, Corey O'Hern, Shigenobu Ogata Placing an obstacle near an orifice of a granular hopper has been shown to facilitate the gravitational granular flow through the orifice by a factor of 100 [I. Zuriguel et al., Phys. Rev. Lett. 107, 278001 (2011)]. Using multiple obstacles, we want to further clarify the physics behind this phenomenon, and study if this approach can further improve the control of the granular flow rate. We develop molecular dynamics (MD) simulations to study the discharging of frictionless grains, and figure out the best design of placing obstacles that discharges densely stored grains most efficiently using an evolutionary procedure that progressively exhausts all possible placements of multiple obstacles including the one recovering the original design in the cited literature. We emphasize the impact of applying our results to 3D printing that makes solid objects of virtually any shape using granular materials. [Preview Abstract] |
Session Y18: Self- and Directed Assembly
Sponsoring Units: GSNP DCMPChair: Tom Truskett, University of Texas at Austin
Room: 403
Friday, March 7, 2014 8:00AM - 8:12AM |
Y18.00001: Directing Colloidal Structure Using a Quench-Disordered Large Mesh Polymer Gel Ryan Jadrich, Kenneth Schweizer The use of a quench disordered template, such as a large mesh gel composed of long rigid rod polymers, may provide a powerful tool to mediate inter-particle interactions, structure, thermodynamics and properties of colloidal/nanoparticle suspensions. We employ the Replicated Reference Interaction Site Model integral equation approach to study a model system composed of a quenched rod or fiber gel immersed in a spherical colloid fluid. The theory predicts a sharp wetting-like transition with increasing colloid-fiber attractions accompanied by strong thermodynamic and colloid packing changes. By increasing the colloid-colloid attractions at constant colloid-fiber interactions, greatly enhanced adsorption onto the gel network and a well-defined state of maximum adsorption is predicted. This phenomenon suggests a strategy for avoiding macrophase separation and instead achieving a sharp, cusp-like transition driven by large density fluctuations of order the long rod length. The possibility of exploiting these phenomena to create responsive and functional colloidal assemblies that can be switched between electrically conductive and non-conductive states is explored. The approach can be generalized to nonspherical colloids, both chemically homogeneous and Janus. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y18.00002: Patterned Substrates to Direct Self-Assembly of Particle Monolayers Mark Ferraro, Thomas Truskett, Roger Bonnecaze As current lithographic techniques approach practical engineering limits for resolution, directed self-assembly of nanoparticles becomes an attractive scalable nanomanufacturing process for creating ordered arrays of particles at a variety of length scales that could be used both as patterning agents and functional materials. However the roles of interparticle forces and external fields on directed self-assembly of particles is not well understood. In this presentation density functional theory (DFT) and Monte Carlo (MC) simulations are used to explore the use of larger scale patterned substrates to drive smaller scale directed self-assembly of particle monolayers. Square patterned substrates with varying energy barriers at length scales N-fold the final desired particle pitch are considered (N\textgreater 1). Ranges of N, bulk density and patterned substrate field strength are identified that disrupt the entropically favored hexagonally close-packed lattice and promote square lattice formation for hard-spheres. Monte Carlo simulations are then employed to verify the predictions from DFT and to further analyze the self-assembly process. These DFT and MC results are used to discuss and define the energetically and kinetically accessible spaces for non-hexagonal lattice formation. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y18.00003: Shape-sensitive crystallization in colloidal superball fluids Laura Rossi, Stefano Sacanna, Vishal Soni, Paul Chaikin, David Pine, Albert Philipse, William Irvine Uniform colloidal silica superballs crystallize into a variety of ordered phases when depletant objects induce attraction between the colloids. The differences in these entropy-driven self-assembled structures are driven by minute deviations of the particle shape and are uniquely determined by an interplay between the size of depletants and superballs. Tuning this ratio allows to smooth the deviation in particle shape, allowing the observation of different depletion-stabilized crystalline structures for the same superball fluid. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y18.00004: Elasto-capillary interactions between solid spheres at smectic thin films Mohamed A. Gharbi, Randall D. Kamien, Shu Yang, Kathleen J. Stebe Colloidal particles organize spontaneously at fluid interfaces owing to a variety of interactions to form well organized structures that can be exploited to synthesize advanced materials. While the physics of colloidal assembly at isotropic interfaces is well understood, the mechanisms that govern interactions between particles at liquid crystal interfaces are not yet clearly established. In particular, smectic liquid crystal films offer important degrees of freedom that can be used to direct particles into new structures. In this work, we report the behavior of solid micrometric beads with homeotropic anchoring confined at interfaces of thin smectic films. We study the interactions and self-assembly of these particles in both supported and free standing films. When particles are captured in thin membranes, they induce distortions of the smectic interface to satisfy wetting properties at particle boundaries, leading to capillary interactions. These forces compete with elastic ones induced by the distortion of the smectic layers. The resulting potential drives assembly of the spheres into new different structures in a self-assembly process. Recent progress in understanding the mechanism of particle self-organization is presented. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y18.00005: Size-dependent scaling and ordering in nanoparticle island self-assembly Jacques Amar, Chakra Joshi, Yunsic Shim, Terry Bigioni While there are a number of similarities between nanoparticle (NP) island self-assembly at a liquid-air interface during drop-drying and epitaxial growth there are also some important differences. Here we present experimental results for the dependence of the island density, island-size distribution, and capture-zone distribution on coverage, deposition flux, and NP diameter which we then compare with epitaxial growth models. Our results indicate that, due to the increase in the strength of the short-range attraction between NPs with increasing NP size, the critical island-size decreases with increasing NP size. However, we also find deviations from epitaxial growth models for small NPs which indicate that additional effects may play a role. We also present results for the ordering of large NP islands which indicate the existence of long-range repulsive interactions. One possible mechanism for such an interaction is the existence of a small net dipole moment on each NP which occurs as a result of an asymmetry in the distribution of attached thiols. Consistent with this mechanism, we find good agreement between experimental results for the nearest-neighbor distribution between islands and simulations which include dipole repulsion. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y18.00006: Kinetic Monte Carlo simulations of nanoparticle island growth Bradley Hubartt, Jacques Amar In recent studies of the self-assembly of dodecanethiol-coated Au nanoparticle (NP) islands dissolved in toluene, the island-size distribution (ISD) was found to be quite sharp - despite the fact that island diffusion and coalescence are expected to lead to a broad ISD - while the island density was found to be anomalously low. In order to understand this, we have first used molecular dynamics (MD) simulations of a simplified model of islands adsorbed at an interface in order to study the dependence of the island diffusion coefficient on island-size. In order to understand island stability, we have also carried out additional MD simulations using an effective potential which takes into account the van der Waals and ligand-ligand interactions, which indicate that the rate of dimer break-up is surprisingly large. By including these results in kinetic Monte Carlo simulations we have obtained reasonable agreement with experiment. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y18.00007: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y18.00008: New Experimental Determination of Topology for Two-Dimensional Nanostructured Polymers Peter Beaucage, Gregory Beaucage In recent years, considerable interest in the unique structure and properties of graphene has expanded to include a wide variety of other inorganic and organic two-dimensional materials including molybdenum disulfide, tungsten oxide, polyfullerene networks, synthetic two-dimensional polymers, and biological membranes, among others. We have previously developed a fractal scaling model to describe the structure of a crumpled two-dimensional sheet in solution and applied it to characterization of crumpling behavior of graphene in solution, with the capability to statistically quantify the mole fraction degree of crumpling and lead to the effective projected area of the structure, which is useful for the Helfrich dynamic models. Recent efforts have focused on applying this model to characterization of a wide variety of polymeric materials, ultimately moving toward \textit{in situ} methods for elucidation of structure-processing-property relationships in 2D polymers and other materials. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y18.00009: Optical magnetic shift of a directed assembly of CdSe/ZnS quantum dots and Fe$_{3}$O$_{4}$ nanopaticles in soft matter at low magnetic fields Jose Amaral, A.L. Rodarte, J. Wan, M.T. Quint, M. Scheibner, S. Ghosh We are investigating the ensemble behavior of magnetic nanoparticles (MNPs) and CdSe/ZnS quantum dots (QDs) when dispersed in an electro-optically active liquid crystalline (LC) matrix. The directed assembly of NPs in the matrix is driven by the temperature-induced transition of the LC from the isotropic to the nematic phase as the NPs are mostly expelled into the isotropic regions, finally ending up clustered around LC defect points when the transition is complete. Using high-resolution scanning magneto-optical Kerr effect (MOKE), we characterize the spatial distribution and magnetic behavior of Fe$_{3}$O$_{4}$ MNPs in a room temperature nematic LC, 5CB. Our results show a two-fold intensity increase of QD photoluminescence (PL) intensity with applied fields lower than 200 G. We speculate this increase is due to a reorientation of LC molecules at the edge of the NP clusters causing QDs to coalesce toward the center of the cluster. This work was funded by NSF. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y18.00010: Heterotypic Self-Assembly of Type-I and Type-III Collagens Esma Eryilmaz, Winfried Teizer, Wonmuk Hwang Collagen fibrils, the main constituents of the extracellular matrix, are ``biological alloys'' that contain many additive molecules for fine-tuning the dynamical and biological properties. A representative example is the type-I collagen fibril, the most abundant among the 28 collagen types, which also contains type-III collagen. We perform atomic force microscopy (AFM) to elucidate the co-assembly of these two important members into heterotypic fibrils on mica surfaces. Time-lapse AFM imaging of samples at different ratios of type-I and type-III collagen molecules revealed that type-III assembles and nucleates fibrils slower than type-I. Furthermore, in the type-I/III mixture, nucleation appeared to be enhanced, resulting in formation of more fibrils compared to cases with either type-I or III only. We discuss possible mechanisms for the enhanced fibril nucleation in the co-assembly process of the two molecules that differ slightly in physical properties. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y18.00011: Emergence of limit-periodic structure without matching rules Catherine Marcoux, Travis Byington, Joshua Socolar We study the emergence of nonperiodic order in a tiling model based on a certain 2D hexagonal prototile with nearest-neighbor interactions. The model is closely related to the Taylor-Socolar tiling model~[1], but with a simpler Hamiltonian with a degenerate class of ground states that includes both periodic and limit-periodic structures. We present the results of a lattice Monte Carlo simulation of the orientational degrees of freedom of a system of the prototiles. We find that the limit-periodic structure emerges from a sufficiently slow quench through the same infinite sequence of second-order phase transitions observed in the full Taylor-Socolar model. A related 3D model with a simple cubic prototile exhibits similar behavior, but with first-order transitions and a more complex set of limit-periodic ground states.\\[4pt] [1] T. W. Byington and J. E. S. Socolar, {\it Phys. Rev. Lett.} {\bf108}, 045701 (2012). [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y18.00012: Why (almost) all bundles are chiral Zachary V. Kost-Smith, Robert A. Blackwell, Matthew A. Glaser We examine the self assembly of bundles of achiral hard rods with distributed, short-range attractive interactions. We show that in the majority of cases the equilibrium state of the bundle is chiral, with a double twist structure. We use biased Monte Carlo techniques and cell theory to compute the free energy as a function of an appropriately defined twist order parameter, and show that the formation of spontaneously chiral bundles is driven by maximization of orientational entropy. The finite curvature of the bundle boundary permits {\em orientational escape}, in which the circumferential angular range of motion of the rods is maximized for some finite average tilt. We map out the phase diagram of bundles in terms of the density, the ratio of rod length to bundle radius, $L/R$, and rod aspect ratio, $L/D$, and find transitions between untwisted, weakly twisted, and strongly twisted states. This work helps explain the common observation of twisted macroscopic bundles, and may provide insight into observations of twist in self-assembled membranes of colloidal rods.\footnote{Reconfigurable self-assembly through chiral control of interfacial tension, Nature, 481:348-351, Jan 2012} [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y18.00013: Critical adsorption and colloidal interaction in multi-component liquids Sharmine Alam, Ashis Mukhopadhyay We studied critical adsorption on colloidal nanoparticles in binary liquid mixture of 2,6 lutidine $+$ water by using fluorescence correlation spectroscopy (FCS). Our results indicated that the adsorbed film thickness is of the order of correlation length associated with concentration fluctuations. The excess adsorption per unit area increases following a power law in reduced temperature with an exponent of -1, which is the mean-field value for the bulk susceptibility exponent. The measurements at higher particle volume fractions, where phenomena such as the particle-particle interaction, self-assembly, ternary phase separation become important will be presented. [Preview Abstract] |
Session Y19: Polymer Blends
Sponsoring Units: DPOLYChair: Hengxi Yang, University of Michigan
Room: 404
Friday, March 7, 2014 8:00AM - 8:12AM |
Y19.00001: What Drives Blend Miscibility? Ronald White, Jane Lipson With no mixture data available, can one predict phase behavior in polymeric systems based on pure component information only? Due to the very weak entropic drive for large molecules to mix, predicting and understanding miscibility behavior is indeed very difficult. However, while not perfect, some \textit{a priori} insight is attainable when pure component properties are analyzed within the framework of a theoretical model. A theory provides a platform, allowing one to define quantities and other measures that may not always be directly measurable, but, are physically appealing and insightful none-the-less. Are there properties that can explain for example, why a polymer like polyisobutylene (PIB) exhibits such different phase behavior compared to other polyolefins? Applying our simple lattice-based equation of state, we have recently analyzed a large number of different polymers. In this talk we will present insights from trends and patterns we have observed. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y19.00002: Miscibility of Polymers in Supercritical Carbon Dioxide Jeffrey DeFelice, Jane Lipson We have developed a simple model that allows us to correlate underlying thermodynamic behavior with trends in miscibility, which we have applied to mixtures of polymers and supercritical carbon dioxide (scCO$_{2})$. scCO$_{2}$ is considered a ``green'' solvent, making it an attractive choice over familiar organic solvents. Experimental cloud point investigations have determined the miscibility of a diverse array of polymers in scCO$_{2}$. Properties of these polymers such as fluorination, alkyl group size, and molecular weight have a strong effect on mixture miscibility. Although polymer/scCO$_{2}$ mixtures have been modeled with some success in the past, the ability of an equation of state (EOS) to make accurate predictions has yet to be demonstrated. We have used a simple EOS to study several of these mixtures. We draw insight from the trends observed via our parameterization of pure component experimental data and discuss how the use of pure component information, alone, leads us to predictions about mixture behavior. This will ultimately aid in our understanding of what is controlling polymer miscibility in scCO$_{2}$. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y19.00003: Effect of supercritical carbon dioxide on the thermodynamics of miscible polymer blends Nicholas Young, Sebnem Inceoglu, Andrew Jackson, St\'ephane Costeux, Nitash Balsara The design of environmentally-benign polymer processing techniques is an area of growing interest, motivated by the desire to reduce the emission of volatile organic compounds. Recently, supercritical carbon dioxide (scCO$_{2})$ has gained traction as a viable candidate for various processes as either a polymer solvent or diluent. To elucidate the impact of scCO$_{2}$ on polymer miscibility, the phase behavior and thermodynamic interactions of multicomponent mixtures comprising scCO$_{2}$, styrene-acrylonitrile copolymer (SAN), and poly(methyl methacrylate) (PMMA) were studied by small angle neutron scattering. Application of the Random Phase Approximation and Flory-Huggins Theory allowed quantitative analysis of scattering profiles to obtain the dependence of pairwise interaction parameters on scCO$_{2}$ activity. The location of the spinodal boundary was found to have a non-trivial dependence on scCO$_{2}$ processing conditions which can be interpreted in the context of balancing interaction strengths. The presence of scCO$_{2}$ was shown to disrupt the miscibility of SAN-PMMA induced by intramolecular repulsion, and decrease the accessible demixing temperature by over 130 $^{\circ}$C. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y19.00004: Composition Dependency of the Flory-Huggins $\chi $ Parameter in Isotopic Polymer Blends Travis Russell, Brian Edwards, Bamin Khomami Flory-Huggins Theory has been the basis for understanding polymer solvent and blended polymer thermodynamics for much of the last 50 years. Within this theory, a parameter ($\chi$) was developed to account for the energy of dispersion between distinct components. Thin film self-assembly of block copolymers and polymer melts depends critically on this parameter, and in application, $\chi$ has generally been assumed to be independent of the concentrations of individual components in the system. However, Small Angle Neutron Scattering data on isotopic polymer blends, such as polyethylene and deuterated polyethylene, have shown a parabolic concentration dependency for $\chi$. In order to better understand the nature of $\chi$ and develop more accurate morphological data for polymer systems, an investigation of this concentration dependency was undertaken from both structural ($\chi$S) and thermodynamic ($\chi$T) theories. Structural calculations for $\chi$S were based on the Random Phase Approximation of de Gennes, and thermodynamic information was obtained through integration of the free energy with $\chi$T defined using original Flory-Huggins Theory. Comparison of the two theories revealed that while both $\chi$S and $\chi$T possess a composition dependence, it is not the same. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y19.00005: Controlling Morphology in Coatings Made from Polyolefin Dispersions Jodi Mecca, Jeffrey Wilbur, Rick Lundgard, Sean Tang, Bernhard Kainz Semi-crystalline polymers have excellent mechanical properties, thermal stability, and chemical resistance that would be attractive for coating applications, but those same properties make it impractical to efficiently deposit these materials using solvent-based or 100{\%} solid approaches. Instead, aqueous formulations have been developed based on polyolefins dispersed as micron-scale particles. The macroscopic properties of coatings made from these materials are strongly dependent on coating morphology, which in turn is governed by the interactions between component polymers, the curing chemistry and the curing process. We will discuss thermodynamic and kinetic approaches to control the morphology and macroscopic properties of coatings based on polyolefins. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y19.00006: Tuning the Miscibility of Polystyrene / Poly(vinyl methyl ether) Blends with Electric Fields Annika Kriisa, Connie Roth Application of electric fields seem experimentally simple, as they can be switched on and off instantly and effortlessly. Nevertheless the influence of electric fields on the phase separation temperature Ts in small molecules and polymeric mixtures is not yet well understood. Available theoretical calculations use thermodynamic arguments for adding an electrostatic free energy term to the total free energy of mixing and predict changes in Ts due to external electric fields that are much smaller than what most experimental results report. To date, neither theory or experiments have found a clear consensus on whether uniform electric fields enhance mixing or demixing. As only a few experimental results have been published over the past several decades with typically only small shifts in Ts, more experiments with unambiguously large shifts in Ts are needed to better understand this effect. Using a fluorescence technique we have developed for measuring the phase separation temperature Ts of polystyrene (PS) / poly(vinyl methyl ether) (PVME) blends [J. Polym. Sci., Part B 2012, 50, 250-256], we investigate the change in Ts due to the presence of electric fields. We show that electric fields strongly enhance mixing in PS/PVME polymer blends. For example, for a 50/50 PS/PVME blend composition, Ts is increased by over 10 K for electric fields of 18 kV/mm. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y19.00007: Efficacy of Different Block Copolymers in Facilitating Microemulsion Phases in Polymer Blend Systems Gunja Pandav, Venkat Ganesan Polymeric microemulsions are formed in a narrow range of phase diagram when a blend of immiscible homopolymers is compatibilized by copolymers. In this study, we consider the ternary blend system of A and B homopolymers mixed with block copolymers containing A and B segments, and probe the efficacy of different copolymer configurations in promoting the formation of microemulsion phases. Specifically, we consider: (a) Monodisperse diblock copolymers; (b) Diblock copolymers with bidisperse molecular weights (MW); (c) Block copolymers having MW polydispersity in one of the blocks; (d) Diblock copolymers having monodisperse MW but bidispersity in average composition; and (e) Gradient copolymers exhibiting a linear variation in the average composition. Using single chain in mean field simulations effected in two dimensions, we probe the onset of formation and the width of the bicontinuous microemulsion channel in the ternary phase diagram of homopolymer blended with compatibilizer. We rationalize our results by explicitly quantifying the interfacial activity and the influence of fluctuation effects in the respective copolymer systems. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y19.00008: Effective Blending of Ultrahigh Molecular Weight Polyethylene with High-Density Polyethylene via Solid-State Shear Pulverization (SSSP) Mirian Diop, John Torkelson Compared with conventional polyolefins, ultrahigh molecular weight polyethylene (UHMWPE) possesses outstanding mechanical properties, including impact strength and crack resistance, that make it it highly desirable for applications ranging from body armor to implants. Unfortunately, UHMWPE has an ultrahigh melt viscosity that renders common melt processes ineffective for making products from UHMWPE. Attempts to overcome this problem by blending UHMWPE with polyethylene (PE) by conventional melt mixing have been unsuccessful because of the enormous viscosity mismatch between blend components and have led to large suspensions of UHMWPE particles within a PE matrix. Here, we show the utility of solid-state shear pulverization (SSSP) in achieving effectively and intimately mixed UHMWPE/PE blends. For blends with up to 50 wt{\%} UHMWPE we observe only slight increases in viscosity ($\eta )$ at high shear rates but major increases in $\eta $ with increasing UHMWPE content at low shear rates. Using extensional rheology, we confirm the strain hardening behavior of SSSP blends. Additionally, shear rheology and differential scanning calorimetry data indicate that the degree of mixing between UHMWPE and HDPE domains can be increased dramatically with subsequent passes of SSSP and single screw extrusion. Finally, blends prepared via SSSP show dramatic increases in impact strength; e.g., for a 30/70 wt{\%} UHMWPE/HDPE blend, impact strength increases by about 300 {\%} (relative to the parent neat HDPE). [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y19.00009: Component Dynamics and the Corresponding Compositional Heterogeneity in Bulk and Thin Film Miscible Polymer Blends Hengxi Yang, Peter Green Miscible polymer blends are known to be compositional heterogeneous, due to self-concentration and thermally driven compositional fluctuation. In this work we investigate the segmental dynamics of poly(vinyl methyl ether) (PVME) in miscible polymer blends of polystyrene (PS) and PVME, using broadband dielectric spectroscopy, and manifest the correspondence between the component dynamics and the compositional heterogeneity in miscible blends. A single $\alpha $-relaxation is observed at high temperatures, $T$, obeying Vogel-Fulcher relation, whereas two separate relaxations exist at low $T$. One relaxation, slower and exhibiting a strong $T$-dependence, is associated with an average local composition with smaller PVME fraction. The other relaxation, known as $\alpha '$-relaxation, is weakly $T$-dependent and Arrhenius-like at low $T$; it reflects the PVME-rich domains within the confines of glassy PS-rich domains. In PVME/PS thin films confined between aluminum (Al) substrates, an additional relaxation process, due to PVME chains that preferentially segregated to Al interfaces, emerges. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y19.00010: Biodegradability and mechanical properties of poly(butylene succinate) composites with finely dispersed hydrophilic poly(acrylic acid) Sawako Mizuno, Atsushi Hotta Biodegradability and mechanical properties of aliphatic poly(butylene succinate) (PBS) films with finely dispersed hydrophilic poly(acrylic acid) (PAA) were investigated. First, 3.5 wt{\%} of PAA was chemically grafted onto the surface of the PBS films (surface-grafted PBS) by photo grafting polymerization, and then the grafted PAA was homogeneously and finely dispersed into PBS by dissolving the surface-grafted PBS into chloroform before mixing and drying to get solid PAA-dispersed PBS. Degradation of these modified PBS was investigated using gel permeation chromatography (GPC) and tensile testing. According to the GPC results, it was found that the PAA-dispersed PBS had intermediate biodegradability with the intermediate water intake, and the reaction constant of PAA-dispersed PBS was in between those of untreated PBS and surface-grafted PBS, in fact 25{\%} higher and 17{\%} lower, respectively. The experimental results presented that the biodegradability of PBS could be well controlled by the dispersion of PAA, possibly leading to the widespread use of PBS for biodegradable polymers. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y19.00011: Block copolymer toughened epoxy: Theory and experiment Carmelo Declet-Perez, Lorraine Francis, Frank Bates We have recently combined small angle x-ray scattering and tensile experiments to follow real-time deformation of block copolymer nanostructures in order to understand toughness enhancement in block copolymer modified epoxies. Our experiments provided direct evidence of internal cavitation in rubbery nanodomains. In this presentation we show that our observations are consistent with the predictions from an energy balance-based cavitation criteria recently modified by Bucknall and Paul [Polymer \textbf{50}, 5539 (2009) \& Polymer \textbf{54}, 320 (2013)]. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y19.00012: Collective dynamic response of bound polymer chains to nanofillers in a good solvent Tad Koga, Naisheng Jiang, Maya Endoh, Tomomi Masui, Hiroyuki Kishimoto, Takashi Taniguchi, Michihiro Nagao As proposed initially by Stickney and Falb, a bound polymer covers the surface of filler particles with a stable layer of macromolecules via van der Walls interactions and is thus resistant to dissolution even in a good solvent. The most thorough experimental and theoretical studies on bound polymer layers (BPLs) have been carried out for carbon black (CB)-filled rubber systems. However, a molecular scale description of real chain conformations/dynamics within such a very thin BPL (typically 1-5 nm in thickness) remains unsolved due to the lack of methods capable of providing high-resolution structural information. Here we present small-angle neutron scattering and neutron spin-echo spectroscopy results for bound polybutadiene (PB, M$_{w}$ = 38,000) chains to the CB surface in toluene. To label the bound layer for the neutron scattering experiments, deuterated toluene, which has the nearly same scattering length density as that of CB, was used. We will highlight the unique collective dynamic response of the bound polymer chains in the good solvent. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y19.00013: Fluctuation/Correlation Effects on the Phase Behavior of Incompressible Polymer Blends Quantified by Fast Lattice Monte Carlo Simulations Pengfei Zhang, Qiang Wang Fast lattice Monte Carlo (FLMC) simulations [Q. Wang, \textbf{Soft Matter 5}, 4564 (2009); \textbf{6}, 6206 (2010)] with multiple occupancy of lattice sites and Kronecker $\delta $-function interactions give orders of magnitude faster/better sampling of configuration space for many-chain systems than conventional lattice MC simulations with the self- and mutual- avoiding walk and nearest-neighbor interactions. Adapting the cooperative motion algorithm to a lattice with multiple occupancy, we studied incompressible and symmetric binary polymer blends using FLMC simulations in a semi-grand canonical ensemble with replica exchange and multiple histogram reweighting, and performed finite-size scaling analysis of our simulation results. Comparing the critical point and binodal curve obtained from FLMC simulations with the predictions from the corresponding Flory-Huggins (FH) and Gaussian-fluctuation (GF) theories, all based on the same model system and thus without any parameter-fitting, we unambiguously quantified the effects of fluctuations/correlations neglected in FH theory and treated approximately in GF theory. [Preview Abstract] |
Session Y20: Semi Crystalline Polymers
Sponsoring Units: DPOLYChair: Kari Dalnoki-Veress, McMaster University
Room: 405
Friday, March 7, 2014 8:00AM - 8:12AM |
Y20.00001: Nanoscale alignment of interfacial crystallites and effects on electrical properties on oCVD PEDOT polymer Asli Ugur, Ferhat Katmis, Kripa K. Varanasi, Karen K. Gleason The precise mechanism of charge transport in conducting polymers is not yet fully understood. It can be influenced by multiple factors including, long- and short-range ordering of the polymeric chains, degree of crystallinity, crystallite size, morphology, and defects. Here we demonstrate that the chain orientation of poly (3,4-ethylenedioxythiophene) (PEDOT) can be controlled by oxidative chemical vapor deposition (oCVD) and the controlled orientation is used to understand the efficient transport direction to obtain high electrical conductivity. The parallel polymer backbone to the substrate resulted in higher conductivities compared to the chains that are perpendicularly oriented with respect to the substrate, where the conductivity is measured both in in-plane and out of plane directions. As film thickness decreases, the electrical conductivity reveals a remarkable improvement, up to $\sim$3000 S/cm by using high substrate deposition temperatures. We have correlated the electrical properties with structural features, e.g. the interface, density and domain sizes of the polymer films by using X-ray and electron based diffraction measurements. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y20.00002: Manipulation of P3AT Crystallization Behavior Bryan S. Beckingham, Victor Ho, Rachel A. Segalman While the commonly studied P3HT has a final melting transition approaching that of its thermal degradation; by judicious substitution of the alkyl side chain the melting transition can be controlled over a range of approximately 150C. Specifically, P3EHT has a melting transition that occurs between 35 and 85C, well below potential degradation temperatures, making P3EHT a model system for examining crystallization in P3ATs. Interestingly, the observed melting endotherms of P3ATs are multimodal in nature such that upon isothermal crystallization of P3EHT, three distinct peaks are observed by differential scanning calorimetry upon heating. Here, we resolve the lowest temperature feature to be a result of a rigid amorphous fraction forming at long times and high relative crystallinity. Moreover, we demonstrate that the two more distinct higher temperature features are a consequence of a melt-recrystallization process. Lastly, we explore how by understanding these processes the initial crystallization conditions and subsequent thermal treatments can be used to manipulate the crystal populations and thereby the properties of P3EHT. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y20.00003: Fabrication of a Conjugated Polymer on Surfaces Studied by Electron Microscopy Hiroshi Jinnai, Takeshi HIguchi, Daiki Murakami, Mitsuo Suga, Atsushi Takahara A controlled ``in-situ'' nano-patterning is highly demanded in various fields of nano-technology. A nano-rod array oriented perpendicular to the substrate is one of the typical nano-structures useful in energy, biomimetic and memory applications. We here proposed a novel nano-patterning method to synthesize polymers by irradiation of electron beam into monomer liquid under atmospheric pressure and concurrently to pattern nano-rod array by scanning the beam. A polymer that can be synthesized by this method includes the poly(3-hexylthiophene), one of the most frequently used conjugated polymers in an organic photovoltaic device. The ``Atmospheric Scanning Electron Microscope (ASEM),'' which enables us to observe morphologies and dynamic phenomena in liquid under atmospheric pressure, was used for fabricating nano-rod structures in this study. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y20.00004: Formation and Characterization of Lyotropic Liquid Crystal Phase in Poly(3-hexylthiophene) Solutions Nabil Kleinhenz, Karthik Nayani, Sourav Chatterjee, Xujun Zhang, Jung Ok Park, Mohan Srinivasarao, Paul Russo, Elsa Reichmanis We have studied poly(3-hexylthiophene) (P3HT) as a model~$\pi $-conjugated polymer for organic semiconductor applications and have investigated the formation of a lyotropic liquid crystalline phase that might be a potential precursory to well-ordered thins film for device applications. Through various processing techniques including the use of mixed solvent systems, slow solution cooling, aging, and capillary flow, we have observed persistent birefringence and alignment of P3HT~in solution. Raman spectroscopy of these solutions in capillaries displays anisotropy in their Raman spectra. UV-Vis spectroscopy was employed to understand the intermolecular interactions that give rise to alignment, and dynamic light scattering was used to quantify the dimensions of P3HT aggregates.~ [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y20.00005: Controlling the Thermal and Optoelectronic Properties in Poly(3-alkylthiophene) Random Copolymers Victor Ho, Hoi Ng, Rachel Segalman Although thermal annealing is a common technique for changing the crystalline texture of polymer-based active layers, the high thermal transition temperatures of many conjugated polymers, such as poly(3-alkylthiophenes) (P3ATs), prevents precise control over the morphology and optoelectronic properties studied at room temperature. In the past we have shown that substitution of the aliphatic side chain is an effective method to alter thermal transitions over a range of 150C while still retaining similar crystalline texture in homopolymer thin films. In this talk random copolymerization of P3ATs results in cocrystallization of repeat units over all the entire range of compositions into unit cells which are related to one of the two parent homopolymers. In turn, the optical absorption edge closely matches that of either of the two homopolymers while the melting temperatures gradually transition between that of the pure components. This presents a synthetically convenient approach to controlling the melting transition of conjugated polymers without detrimentally affecting the optical absorption. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y20.00006: Quantifying Order in Poly(3-hexylthiophene) Chad Snyder, Ryan Nieuwendaal, Dean DeLongchamp, Christine Luscombe, Prakash Sista, Shane Boyd While poly(3-hexylthiophene) (P3HT) is one of the most studied polymers in organic electronics, it remains one of the most challenging in terms of quantitative measures of its order, e.g., crystallinity. To address this challenge, we prepared a series of highly regioregular P3HT fractions ranging from 3.3 kg/mol to 23 kg/mol. Using this series plus a high molar mass (62 kg/mol) commercial material, we compare different metrics for order in P3HT via calorimetry, solid state NMR, and x-ray diffraction. We reconcile the results of our work with those of recent studies on oligomeric (3-hexylthiophenes). One challenges of quantifying low molar mass P3HT samples via DSC is a thermal fractionation effect due to varying chain lengths. We quantify these effects in our molar mass series, and a clear crossover region from extended chain crystals to chain folded crystals is identified through the thermal fractionation process. New values for the enthalpy of fusion of high molar mass P3HT and its equilibrium melting temperature are established through our work. Another result of our research is the validation of high heating rate DSC methods for quantifying crystallinity in P3HT samples with device relevant film thicknesses. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y20.00007: Counit Inclusion in Hydrogenated Polynorbornene Copolymer Crystals Adam Burns, Michael Showak, Andrew Stella, Richard Register Crystallization in poly(A-\textit{co}-B) random copolymers, where homopolymer A is crystalline but B is not, is dictated by the degree to which crystals of A can include B units. Typically, B units are strongly excluded from the A crystals, drastically reducing the degree of crystallinity w$_{c}$ and crystal thickness t$_{c}$ even at modest comonomer contents. However, in some cases, B units can be incorporated into the crystals as defects, significantly diminishing the counits' impact on w$_{c}$ and t$_{c}$. The extent and consequences of counit inclusion have been investigated in hydrogenated polynorbornene (hPN) with alkylnorbornene counits, synthesized by living ring-opening metathesis polymerization followed by hydrogenation. In the case of 5-hexylnorbornene (HxN) counits, a steep decline in w$_{c}$ and t$_{c}$ with counit content is found, indicative of strong exclusion. In contrast, when the counits are 5-methylnorbornene (MeN), extensive inclusion of MeN units into the crystals is observed. hP(N-\textit{co}-MeN) copolymers maintain appreciable crystallinity above 30 mol{\%} MeN, and the dependence of the melting point T$_{m}$ on t$_{c}$ tracks that of the hPN homopolymer. Four times as much MeN as HxN (molar basis) is required to produce a comparable drop in w$_{c}$. Therefore, copolymerization with MeN can be used to tune T$_{m}$ without drastically reducing w$_{c}$. Additionally, hPN exhibits a polymorphic transition to a rotationally disordered (RD) crystal at temperature T$_{cc\, }$\textless T$_{m}$. Incorporation of comonomers increases the disparity between T$_{m}$ and T$_{cc}$, indicating that thin crystals stabilize the RD phase. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y20.00008: Curvature-directed crystallization of polymer dielectrics in nanopores Dariya Reid, Bridget Ehlinger, Lin Shao, Jodie Lutkenhaus When a polymer is constricted in geometries smaller than its unperturbed molecular size its properties may greatly differ from the bulk state. The effect of cylindrical confinement on crystallization of isotactic polypropylene (iPP) and polycarbonate (PC) was studied. Polymer nanowires were prepared by melt-wetting into nanoporous templates of varying diameter (15 -- 200 nm). X-ray diffraction (XRD) studies reveal that iPP crystallizes into the $\alpha $-phase and preferentially orients along the long axis of the pore. Using differential scanning calorimetry (DSC) it is shown that iPP transitions from hetero to homogeneous nucleation as the pore diameter decreases. The isothermal crystallization kinetics is analyzed using Avrami analysis and a progression, with time, into primarily 1D crystallization is presented. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y20.00009: SAXS/WAXS studies of flow-induced crystallization of poly(1-butene) in shear flow Binbin Luo, Wesley Burghardt Flow-induced crystallization of poly(1-butene) was studied in shear flow, produced using a Linkam shear cell modified to allow x-ray access for in situ synchrotron x-ray scattering measurements. After loading in the the shear cell, samples were first heated well into the melt, and then cooled to a crystallization temperature selected such that negligible quiescent crystallization would occur on reasonable time scales. A short burst of shear flow was then applied at various rates, after which simultaneous wide- and small-angle x-ray scattering (SAXS and WAXS) data were collected to study the resulting accelerated crystallization kinetics, as well as the morphology of the resulting crystallites (e.g. degree of crystallite orientation). SAXS and WAXS data provided generally self-consistent measures of the extent of crystallization, although WAXS data consistently reported a higher degree of crystallite orientation than SAXS. Average crystallite orientation was found to decrease over the course of crystallization. The impact of both deformation rate and total applied strain on the crystallization process were examined. The sample was also studied under similar flow conditions using (i) turbidity and (ii) linear viscoelasticity as probes of the developing crystallinity. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y20.00010: Extensional Flow Induced Crystallization of Polyethylene David Nicholson, C. Rebecca Locker, Andy Tsou, Gregory Rutledge The majority of manufactured polyethylene is used in films mostly through the blown film fabrication process where extensional flow induced crystallization is a critical component in affecting the development of crystalline morphology and amorphous topology. In order to optimize the blown film performance, it is critical to understand the mechanism of extensional flow induced crystallization of polyethylene. Model high density polyethylene with a $M_n$ of 20,000 g/mol and a PDI (polydispersity) of 2 and lower were synthesized by organometallic catalysts. Extensional flow induced crystallization of these materials was measured using the SER (Sentmanat Extensional Rheometer) either at a given rate with varying temperatures or vice versa. A continuum model was applied to analyze the flow induced crystallization data. All samples after extensional flow were quenched in ice water and the resulting morphology was characterized using SAXS and WAXS. The extensional rate was found to be effective in modifying morphology whereas the temperature was not; neither temperature nor strain rate affected the final film crystallinity. With an increase in extensional rate, crystallites became thinner and narrower with potentially higher connectivity which could lead to higher toughness. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y20.00011: SAXS/WAXS studies of flow-induced crystallization of poly(1-butene) in uniaxial extensional flow Erica McCready, Wesley Burghardt We report studies of flow-induced crystallization of poly(1-butene) in uniaxial extensional flow. Flow was produced using an SER extensional flow fixture housed in a custom built convection oven designed to provide x-ray access for in situ studies of polymer structure using synchrotron x-ray scattering techniques. Samples were loaded into the SER fixture, heated well into the melt, and then cooled to a temperature at which quiescent crystallization would be prohibitively slow. A short interval of uniaxial extensional flow was then applied, after which simultaneous wide- and small-angle x-ray scattering (SAXS and WAXS) patterns were collected to study the phase transformation kinetics and morphology of the subsequent accelerated crystallization. The degree of crystallite orientation was generally found to decrease over the course of the crystallization. WAXS measurements yielded systematically higher degrees of crystallite orientation than SAXS. Both SAXS and WAXS gave generally consistent results for the extent of crystallization, although the SAXS invariant showed a decrease at long times that is not mirrored in the WAXS data. The impact of both deformation rate and total applied strain on the crystallization process were examined. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y20.00012: Probing the interlamellar amorphous phase in semicrystalline polyolefins using vapor flow and neutron scattering Amanda McDermott, Chad Snyder, Ronald Jones Measuring equilibrium swelling as a function of solvent vapor activity is an established method of simultaneously measuring the polymer-solvent interaction parameter and the properties of crosslinks---or in semicrystalline polymers, tie-chains, which strongly impact mechanical properties. Gravimetric experiments do not differentiate between uptake by extralamellar and interlamellar amorphous material and determine amorphous layer swelling by treating the permeant and amorphous phase as incompressible fluids. Measurements of swelling and solvent uptake that are \textit{independent} of one another and \textit{specific} to interlamellar amorphous material could enhance understanding of mechanical properties, barrier properties, and solvent processing. Small-angle neutron scattering combined with \textit{in situ} swelling by deuterated solvent vapor fulfills this requirement. A shift in the long-period peak wavevector indicates swelling, while peak intensity independently indicates interlamellar solvent concentration. Preliminary results suggest that interlamellar polymer-solvent energetic interactions may be affected by mesophases with chain orientation and mobility different from the bulk. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y20.00013: An Integrated Ultrafast Scanning Calorimetric and Micro Raman Spectroscopic Investigation of Polymer Crystallization Dongshan Zhou, Lai Wei, Jing Jiang, Gi Xue, Xiaoliang Wang Ultrafast differential scanning calorimetry (UFDSC) with scanning rate up to 1,000,000 K/s has already been used to study the kinetics of crystallization and phase transition of some polymers and liquid crystal. Recently, we developed stage type UFDSC (ST-UFDSC) with comparable controlled heating and cooling rates. ST-UFDSC enables sample treatment and measurement integrated with microstructural characterization. As an example, we investigated the Raman spectroscopy of PET at different crystallization stage obtained by programed rapid cooling and heating processes. Although the Raman spectroscopy is not acquired during rapid heat treatments, the structure is assumed to remain by ultrafast quench below the glass transition temperature, when the Raman spectroscopy is collected. We expect that the combination technique can be also used to investigate dynamic relaxation behaviors of metastable states obtained by ultrafast heat treatments. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y20.00014: Molecular mechanism of viscoelasticity in aligned polyethylene Ali Hammad, Hikmatyar Hasan, Thomas Swinburne, Stefano Del Rosso, Lorenzo Iannucci, Adrian Sutton Aligned polyethylene is used in industrial and medical applications due to its low density and high tensile strength. Extensive experimental work has been done to determine its mechanical properties, notably its viscoelasticity. However, the molecular processes that underlie these macroscopic properties are poorly understood. We develop a united atom model of aligned chains, in which intermolecular interactions are modelled by a Lennard-Jones potential, and the elastic energy within chains is modelled with harmonic springs. Using this simple model, we demonstrate the nucleation of solitons from chain ends, as one molecular chain is stretched with respect to another, and how load is transferred between chains in disregistry by intermolecular interactions. We develop an equation of motion for the movement of solitons along molecular chains, allowing us to replace a collection of aligned chains with a gas of solitons. Although solitons have been invoked to account for dielectric relaxation in crystalline regions of polyethylene, we believe this may be the first time they are discussed in the context of mechanical properties of aligned polyethylene. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y20.00015: Holographic reconstruction from electron diffraction patterns: true atom images of thousands of atoms Carsten Westphal, Tobias Luehr After its discovery in the early 70ies of the last century x-ray photo-electron diffraction (XPD) has been very successfully applied for the characterization of crystalline systems and adsorbate structures later. The emitted electron wave contains the full spatial information of the atoms' arrangement around the emitter atom. However, a holographic reconstruction yielding a 3-dimensional image of the investigated structure was with the exception of a very few special cases rarely successful. In most cases, the reconstruction contained strong image distortions due to the strong anisotropic scattering characteristics in the electron-atom interaction. Here, we present a new approach from angle-resolved diffraction patterns recorded at electron kinetic energies above 10 keV for the first time. The new reconstruction scheme is a direct method for revealing the crystal structure without any further information. We present spatial images of different crystal systems showing thousands of atoms at their correct location. [Preview Abstract] |
Session Y21: Elastic Instabilities and Pattern Formation
Sponsoring Units: DPOLY GSNP DFDChair: Andrew Croll, North Dakota State University
Room: 406
Friday, March 7, 2014 8:00AM - 8:12AM |
Y21.00001: Combined Bending, Stretching, and Wrinkling of Thin Sheets Katia Bertoldi, Michael Taylor, Benny Davidovitch Thin elastic sheets develop surface undulations, or wrinkles, in the presence of small compressive strain. In recent years, interest in thin sheets has greatly increased due to their relevance in a wide array of applications such as biological tissues, integrated circuits and solar sails. As a result, wrinkling has recently attracted considerable attention among engineers, physicists and biologists. Although the most basic buckling instability of uniaxially compressed plates was understood by Euler more than two centuries ago, recent experiments and simulations have shown significant deviations from predictions. Motivated by this puzzle we investigate wrinkling in a thin sheet under axisymmetric loading conditions and systematically compare numerical and analytical solutions. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y21.00002: The role of substrate pre-stretch on post-wrinkling bifurcations Anesia Auguste, Lihua Jin, Zhigang Suo, Ryan Hayward Wrinkles in compressed elastic bilayers, resulting from a balance between the bending energy of a stiff skin layer and the stretching energy of a softer substrate, have been applied in a variety of contexts including to change the wetting, optical, and adhesive properties of surfaces. Previous work has shown that at large compression, wrinkles transition into sub-harmonic modes and eventually form ridges or self-contacting folds due to the non-linearity of the substrate elasticity. However, our understanding of how pre-stretch of the substrate affects period doubling and other post-wrinkling bifurcations remains incomplete. We have performed a combined experimental and numerical study wherein the strain state in each layer can be independently varied. We find shifts in the critical strain for post-wrinkling bifurcation and, at high substrate pre-compression, the emergence of ``chaotic'' patterns with irregular spacings between the troughs that grow in amplitude. Our findings highlight the critical importance of substrate pre-stretch in determining the nature of post-wrinkling bifurcation modes. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y21.00003: Viscoelastic instability and detachment folds in soft elastomer interfaces Koushik Viswanathan, Anirban Mahato, Srinivasan Chandrasekar Physical contacts between a soft elastomer surface and a hard glassy polymer are largely governed by adhesion at the interface. Under application of sufficiently large tangential stress, relative motion occurs at the interface and compressive and tensile stresses develop at the leading and trailing edges of the contact respectively. This kinematic condition leads to a viscoelastic instability at the leading edge causing the elastomer surface to buckle and readhere --- a process governed both by the viscoelastic relaxation time of the elastomer and the sliding velocity $v$. Above a critical velocity $v_c$, detachment folds form ahead of the indenter and propagate through the contact region at velocities much greater than $v$. These are commonly referred to as Schallamach waves, after their discoverer, and are considered to be precursors to failure in soft materials. While their onset can be justified using linear elasticity, not much is known about their subsequent propagation. We present high-speed images of a glass-PDMS contact, and use front-tracking to estimate surface strains and the variation of wave velocity and generation frequency with $v$. A model for the dynamics of the wavefronts is also discussed, showing the onset and propagation of the instability. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y21.00004: Shape transitions in soft spheres regulated by elasticity Craig Fogle, Amy Rowat, Alex Levine, Joseph Rudnick Soft core shell structures abound in nature. Examples of these structures, comprised of a thin outer membrane bounding an elastic core, include raisins, gel-filled vesicles, and a variety of membrane-bound organelles in the cell. We study the elasticity-driven morphological transitions of spherical core shell structures when either their surface area is increased or their interior volume is decreased. We demonstrate a transition, which is related to the Euler buckling, from the spherical initial shape to a lower symmetry one. We discuss the dependence of the critical excess surface area (relative to that of a bounding sphere) for buckling, the internal stresses in the core, and the symmetry of the buckled state on the elastic parameters of the system. We compare these predictions to a variety of observed morphological transitions in hard and soft materials, and discuss extensions of this work to growing viscoelastic media. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y21.00005: Elastic instabilities in a model cerebral cortex David Mayett, Oksana Manyuhina, J.M. Schwarz Soft and biological systems exhibit elastic instabilities, such as buckling, folding and wrinkling, in the presence of external loads, growth, or both. The modeling of such systems calls for a continuum approach to account for the interplay between local elastic stresses and global growth profiles. It is this interplay that can lead to non-trivial geometries. We propose a model of the cerebral cortex, described as an anisotropic multi-layered material with two basic components (white matter and grey matter) undergoing differential growth. We explore the nature of buckling instabilities, assuming a compatibility between the growth and geometric deformation, by solving a nonlinear variational problem with a free interface. We expect that this simplified approach, based on a combination of geometry and elasticity, could give insight into the formation and splitting of folds observed during the development of the cerebral cortex. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y21.00006: Interaction of two cracks in plastic sheets Stephane Santucci, Marie-Julie Dalbe, Juha Koivisto, Loic Vanel, Osvanny Ramos, Mikko Alava We study experimentally the interaction of two cracks in a plastic sheet submitted to uniaxial stress at a constant imposed velocity. We can observe a repulsive regime, where the cracks deviate with an angle, which depends on the geometrical parameters as well as on the material studied. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y21.00007: Hierarchically UVO patterned elastomeric and thermoplastic structures Ying Chen, Manish Kulkarni, Allan Marshall, Alamgir Karim We demonstrate a simple yet versatile method to fabricate tunable hierarchical micro-nanostructures on flexible Poly(dimethylsiloxane) (PDMS) elastomer and thermoplastic polymer surface by a two-step process. Nanoscale patterned PDMS was obtained by imprinting compact disc (CD)/digital video disc (DVD) patterns. The second micro pattern was superposed by selective densification of PDMS by exposing to ultraviolet-ozone radiation (UVO) through micro-patterned TEM grid as a mask. The nanoscale patterns are preserved through UVO exposure step leading to formation of deep hierarchical patterns, so that for a 19 um square mesh, the micro pattern has a depth of 600nm with 6h PDMS UVO exposure time. This simple method can be promoted to fabricate hierarchical structures of thermoplastic materials (such as polystyrene), from which the mechanism of capillary imprinting and thermal stability of hierarchical patterns are investigated. This study is potentially important to various applications ranging from biomimetic scaffolds to solar cell. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y21.00008: Axi-symmetric patterns of active polar filaments on spherical and composite surfaces Pragya Srivastava, Madan Rao Experiments performed on Fission Yeast cells of cylindrical and spherical shapes, rod-shaped bacteria and reconstituted cylindrical liposomes suggest the influence of cell geometry on patterning of cortical actin. A theoretical model based on active hydrodynamic description of cortical actin that includes curvature-orientation coupling predicts spontaneous formation of acto-myosin rings, cables and nodes on cylindrical and spherical geometries [P. Srivastava et al, PRL {\bf 110}, 168104(2013)]. Stability and dynamics of these patterns is also affected by the cellular shape and has been observed in experiments performed on Fission Yeast cells of spherical shape. Motivated by this, we study the stability and dynamics of axi-symmetric patterns of active polar filaments on the surfaces of spherical, saddle shaped and conical geometry and classify the stable steady state patterns on these surfaces. Based on the analysis of the fluorescence images of Myosin-II during ring slippage we propose a simple mechanical model for ring-sliding based on force balance and make quantitative comparison with the experiments performed on Fission Yeast cells. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y21.00009: Tuning Surface Wettability Using Single Layered and Hierarchically Ordered Arrays of Spherical Colloidal Particles Ali Dhinojwala, Ila Badge, Sarang Bhawalkar, Li Jia A control over wetting properties of a surface can be achieved by tuning surface roughness and surface chemistry. In this study, we formed single level and dual hierarchical roughness with hexagonal non-contiguously close packed (HNCP) patterns of spherical particles using colloidal lithography. Surface chemistry was controlled using plasma-enhanced chemical vapour deposition (PECVD). A hexagonal unit cell model, which is representative of the HNCP pattern, was used to predict the contact angles. The predictions of this model were in good agreement with experimentally measured contact angles. The systematic thermodynamic analysis of wetting properties is important when using structured surfaces at different hydrostatic pressures, relative humidity, temperature fluctuations or prolonged exposure to water. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y21.00010: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y21.00011: Shape Programming through Hierarchic Crystallization of Semicrystalline Elastomers Qiaoxi Li, Jing Zhou, Sara Turner, Valerie Ashby, Jan-Michael Carrillo, Andrey Dobrynin, Sergei Sheiko Hierarchic organization of semi-crystalline morphology has proved to be key to encoding different shapes at different stages of the crystallization process. We have studied shape transformations as a new tool to gain insights of a crystallization process and then translated the hierarchic crystallization into programmable shape transformations. Reversible transitions between multiple shapes has been achieved through partial melting of a crystalline scaffold, leaving a latent template, which inverts shape recovery by steering crystallization along kinetically preferred pathways replicating the scaffold. A composite model has been applied to interpret the relationship between shape, elastic modulus and crystallinity of semi-crystalline elastomers, assuming morphological transition between isolated crystallites, clusters, and percolated scaffold. [Preview Abstract] |
Session Y23: Invited Session: Quantum Bath Engineering with Superconducting Circuits
Sponsoring Units: DCMP GQIChair: Alexandre Blais, Universite de Sherbrooke
Room: 505-507
Friday, March 7, 2014 8:00AM - 8:36AM |
Y23.00001: Taking Control of Superconducting Qubits Invited Speaker: Irfan Siddiqi One of the fundamental challenges in quantum information processing is to sustain coherence over a time interval practical for performing a computation or simulation. Until recently, boosting coherence has involved hardware development to minimize coupling to a dissipative environment which typically transforms a quantum superposition into a classical state. In the domain of superconducting circuits, the development of robust quantum-noise-limited microwave amplifiers and quantum bits with lifetimes in excess of 100 microseconds has enabled the use of bath engineering to actively suppress decoherence. In particular, we have been able to tailor the dissipative environment, either via measurement or control pulses, to stabilize quantum superposition states and coherent oscillations indefinitely, generate entanglement, and maintain a pure quantum state by real-time tracking. Future directions for improving measurement efficiency and architectures for on-chip measurement in a multi-qubit setting will also be discussed. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 9:12AM |
Y23.00002: Autonomously stabilized entanglement between two superconducting qubits Invited Speaker: Shyam Shankar Quantum error-correction codes are designed to protect an arbitrary state of a multi-qubit register against decoherence-induced errors, but their implementation is an outstanding challenge for the development of large-scale quantum computers. A first step is to stabilize a non-equilibrium state of a simple quantum system such as a qubit or a cavity mode, in the presence of decoherence. Several groups have recently accomplished this goal using measurement-based feedback schemes. A next step is to prepare and stabilize a state of a composite system. Here we demonstrate the stabilization of an entangled Bell state of a quantum register of two superconducting qubits for an arbitrary time. Our result [1] is achieved by an autonomous feedback scheme which combines continuous drives along with a specifically engineered coupling between the two-qubit register and a dissipative bath. Similar bath engineering techniques have recently been used for qubit reset, single qubit state stabilization, as well as for the creation and stabilization of states of multipartite quantum systems. Unlike conventional, measurement-based schemes, an autonomous approach which uses engineered dissipation to counteract decoherence, obviates the need for a complicated external feedback loop to correct errors. Instead the feedback loop is built into the Hamiltonian such that the steady state of the system in the presence of drives and dissipation is a Bell state, an essential building-block for quantum information processing. Such autonomous schemes, which are broadly applicable to a variety of physical systems, will be an essential tool for the implementation of quantum-error correction.\\[4pt] [1] \texttt{http://dx.doi.org/10.1038/nature12802} [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:48AM |
Y23.00003: Dissipation engineering in a coherent feedback electromechanical network Invited Speaker: Joseph Kerckhoff Modern superconducting microwave circuit experiments often consist of a quantum circuit under study, followed by a quantum-limited microwave amplifier. The subfield of quantum electromechanics, in which the quantum circuit is a mechanical resonator coupled to a microwave resonator, is no exception. However, a simple modification of the cables between these devices turns this open-loop, serial network into a fully-cryogenic, coherent feedback network. In effect, this easy-to-build network becomes a brand new kind of device, with useful and novel dynamics. Applied to an electromechanical context, the microwave and electromechanical dissipation is greatly modified through these closed loop dynamics, leading to dynamically tunable and phase-sensitive decay. We experimentally demonstrate that the microwave decay rate may be modulated by at least a factor of 10 at a rate greater than $10^4$ times the mechanical response rate. Similarly, the mechanical state can be dynamically squeezed and unsqueezed. While we have only investigated dynamics in the classical regime, we expect analogous behavior in the quantum regime. Finally, this approach is suitable for both 3D and planar architectures. I will describe my observations of this network and the general utility of networks of modular quantum circuits to dissipation engineering. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:24AM |
Y23.00004: Perfect squeezing by damping modulation in circuit quantum electrodynamics Invited Speaker: Nicolas Didier Dissipation-driven quantum state engineering uses the environment to steer the state of quantum systems and preserve quantum coherence in the steady state. We theoretically show that modulating the damping rate of a microwave resonator generates a new squeezing mechanism that creates a vacuum squeezed state of arbitrary squeezing strength, thereby allowing perfect squeezing. Given the recent experimental realizations in circuit QED of a microwave resonator with a tunable damping rate, superconducting circuits are an ideal playground to implement this technique. By dispersively coupling a qubit to the microwave resonator, it is possible to obtain qubit-state dependent squeezing. Moreover, when two qubits are coupled to the resonator, damping modulation can be used to produce entanglement between the qubits. Preprint: arXiv:1307.5311. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 11:00AM |
Y23.00005: Optomechanical entanglement via reservoir engineering Invited Speaker: Yingdan Wang A mechanical resonator could serve as an ideal system for transferring quantum states and mediating interactions between very different kinds of photons. To this end, recent experiments have realized three-mode optomechanical systems, where a single mechanical resonator simultaneously interacts with both an optical and a microwave cavity. In this talk I will discuss different strategies which use reservoir engineering in such a system as a powerful tool to generate robust, stationary entanglement between the two cavity fields. By manipulating the mechanical resonator to effectively cool delocalized Bogoliubov modes, we find that large intracavity entanglement can be achieved [1], at a level which is well above the maximum achievable via a coherent two-mode interaction. We have also analyzed the entanglement of the output fields of the two cavities. While there are significant differences from the intra-cavity fields, we again find that with proper parameter choices, large amounts of entanglement can be achieved. While the emphasis is on optomechanics, our results can also be applied directly to other 3-mode bosonic systems (e.g., as could be realized with superconducting microwave circuits). \\[4pt] [1] Ying-Dan Wang and A. A. Clerk, Phys. Rev. Lett. 110, 253601 (2013). [Preview Abstract] |
Session Y24: Instrumentation and Measurements
Sponsoring Units: GIMSChair: Ricardo Jimenez-Matinez, National Institute of Standards and Technology
Room: 504
Friday, March 7, 2014 8:00AM - 8:12AM |
Y24.00001: Use of Atomic Layer Deposition to create homogeneous SRXF/STXM standards Nicholas Becker, Jeffrey Klug, Steve Sutton, Anna Butterworth, Andrew Westphal, John Zasadzinski, Thomas Proslier The use of Standard Reference Materials (SRM) from the National Institute of Standards and Technology (NIST) for quantitative analysis of chemical composition when analyzing samples using Synchrotron based X-Ray Florescence (SR-XRF) and Scanning Transmission X-Ray Microscopy (STXM) is common. However, these standards can suffer from inhomogeneity in chemical composition and often require further corrections to obtain quantitative results. This inhomogeneity can negatively effect the reproducibility of measurements as well as the quantitative measure itself, and the introduction of assumptions for calculations can further limit reliability. Atomic Layer Deposition (ALD) is a deposition technique known for producing uniform, conformal films of a wide range of compounds on nearly any substrate material. These traits make it an ideal deposition method for producing thin films to replace the NIST standards and create SRM on a wide range of relevant substrates. Utilizing Rutherford Backscattering, STXM, and SR-XRF we will present data proving ALD is capable of producing films that are homogenous over scales ranging from 100$\mu$m to 1nm on TEM windows. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y24.00002: Experimental Platform for Studying Thermoelectric Properties in Vacuum Gaps and Molecular Junctions Wonho Jeong, Youngsang Kim, Kyeongtae Kim, Woochul Lee, Pramod Reddy Electromigrated break junction (EBJ) based molecular devices have enabled many research groups to study nanoscale charge transport. Although EBJ devices have been extensively used due to the advantages of a three terminal configuration in tuning the electronic structure, it has not been possible to use them to study thermoelectric properties. This is because creating temperature differentials across the nanogap of EBJs is technically challenging. In order to overcome this experimental limitation, we carefully designed and created a new experimental platform (EBJIH, EBJ with integrated heater) that enables us to study thermoelectric properties in vacuum gaps and molecular junctions. To prove that temperature differentials can be established in these three terminal devices, we performed nanometer resolution thermal imaging using scanning thermal microscopy under UHV conditions. The results clearly show that temperature differentials can indeed be established in the devices. Further, we have used these devices to study the thermoelectric properties of vacuum gaps between gold electrodes and found that the thermoelectric properties were very sensitive to gap dimensions. We are also currently adopting this platform to study thermoelectric properties in molecular junctions. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y24.00003: Temperature Dependent Behavior of Near Field Radiative Heat Transfer Robert Joachim We have designed and implemented an apparatus capable of measuring near field radiative heat transfer (NFRT) from room temperature down to cryogenic temperatures in vacuum. Utilizing a bimaterial cantilever with a 20 $\mu $m glass sphere attached to the end in the pendulum geometry as a thermal detector and an optical fiber interferometer as a displacement detector we were able to measure the heat flux between a substrate and the glass sphere. The apparatus was sensitive enough to measure displacements of 1 nm and heat fluxes of 50 pW. NFRT was observed at temperatures ranging from 300K to 100K and at displacements down to 100nm. These measurements were performed for various combinations of Si, SiO$_{2}$ and sapphire. The thermodynamic formulation of Lifshitz's theory for attraction between dielectrics [1] predicts that NFRT will scale as T$^{2}$ while far field radiative transfer will scale as T$^{4}$ and that the crossover between these two regimes will occur at a distance given by (1/2$\pi )(\hbar $c/k$_{b}$T). Our data confirms these predictions. \\[4pt] [1] J. Loomis and H. Maris. Phys Rev. B. 50, 18517 (1994). [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y24.00004: Nanoscale volumetric chemical imaging by soft x-ray laser ablation mass spectrometry Ilya Kuznetsov, Jorge Filevich, Mark Woolston, Gerald Gasper, David Carlton, Weilun Chao, Erik Anderson, Elliot Bernstein, Dean Crick, Jorge Rocca, Carmen Menoni Mass Spectrometry Imaging (MSI) has played an important role in the direct examination of the chemical composition of complex inorganic and organic samples. Typically a visible/ultraviolet laser is used to ablate the sample and create ions that when detected enables the identification of molecular composition. We report the use of soft x-ray (SXR) lasers in the implementation of a novel laser ablation mass spectrometry (XLAMS) nanoprobe that can probe chemical composition from sample regions of a few attoliters volume and with high sensitivity. The concept exploits: i) high focusability, ii) low penetration depth and iii) high photo-ionization efficiency of the 46.9 nm wavelength SXR laser light. In this work we demonstrate the capabilities of XLAMS to realize chemical contrast imaging with $\sim$ 140 nm lateral and $\sim$ 50 nm depth resolution and high sensitivity. The high lateral and depth resolution and high sensitivity of XLAMS imaging method offer great potential for composition imaging of nanofilms and nanostructures and imaging the chemical distribution of dopants and trace elements. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y24.00005: A Scanning, All-Fiber Sagnac Interferometer for High Resolution Magneto-Optic Measurements at 820 nm Alexander Fried, Aharon Kapitulnik, Martin Fejer The Sagnac Interferometer, has historically been used for detecting non-reciprocal phenomena, such as rotation. Here we demonstrate a method by which the technique is used as a high resolution method for measuring the Magneto-Optical Polar Kerr effect--a direct indicator of magnetism. Previous designs have incorporated free-space components which are bulky and difficult to align. We improve upon this technique by using strictly fiber-optic coupled optical components and demonstrate operation at a new wavelength, 820 nm, with which we can achieve better than 1 $\mu$rad resolution. Mounting the system on a piezo-electric scanner allows us to acquire diffraction limited images with 1.5 $\mu$m spatial resolution. We also provide extensive discussion on the details and of the SI's construction. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y24.00006: Measuring shear modulus of individual fibers Herbert Behlow, Deepika Saini, Luciana Oliviera, Malcolm Skove, Apparao Rao Fiber technology has advanced to new heights enabling tailored mechanical properties. For reliable fiber applications their mechanical properties must be well characterized at the individual fiber level. Unlike the tensile modulus, which can be well studied in a single fiber, the present indirect and dynamic methods of measuring the shear properties of fibers suffer from various disadvantages such as the interaction between fibers and the influence of damping. In this talk, we introduce a quasi-static method to directly measure the shear modulus of a single micron-sized fiber. Our simple and inexpensive setup yields a shear modulus of 16 and 2 GPa for a single IM7 carbon fiber and a Kevlar fiber, respectively. Furthermore, our setup is also capable of measuring the creep, hysteresis and the torsion coefficient, and examples of these will be presented. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y24.00007: Tuning to fast changing phenomena with real-time digital processing Fedor Balakirev A new crop of computationally-intensive digital signal detection techniques brought to light the need for speedier data processing approaches, where conventional data acquisition techniques fall short. We review recent advances in real-time solutions which enable sophisticated fast-feedback detection schemes with sub-microsecond tuning to rapidly changing physical phenomena. The apparatus is particularly suitable for pulsed magnetic field measurements of superconducting critical currents and high-frequency oscillatory signals, among others. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y24.00008: Nuclear Magnetic Resonance Gyroscope Michael Larsen, Robert Griffith, Michael Bulatowicz The navigation grade micro Nuclear Magnetic Resonance Gyroscope (micro-NMRG) being developed by the Northrop Grumman Corporation (NGC) has concluded the fourth and final phase of the DARPA Navigation Grade Integrated Micro Gyro (NGIMG) program. Traditional MEMS gyros utilize springs as an inherent part of the sensing mechanism, leading to bias and scale factor sensitivity to acceleration and vibration. As a result, they have not met performance expectations in real world environments and to date have been limited to tactical grade applications. The Nuclear Magnetic Resonance Gyroscope (NMRG) utilizes the fixed precession rate of a nuclear spin in a constant magnetic field as an inertial reference for determining rotation. The nuclear spin precession rate sensitivity to acceleration and vibration is negligible for most applications. Therefore, the application of new micro and batch fabrication methods to NMRG technology holds great promise for navigation grade performance in a low cost and compact gyro. This presentation will describe the operational principles, design basics, and demonstrated performance of the NMRG including an overview of the NGC designs developed and demonstrated in the DARPA gyro development program. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y24.00009: Resistive sensitivity functions for van der Pauw astroid and rounded crosses and cloverleafs Daniel Koon, Ole Hansen We have calculated the sensitivity of van der Pauw resistances to local resistive variations for circular, square and astroid discs of infinitesimal thickness, as well as for the families of rounded crosses and cloverleafs, as a function of specimen parameters, using the direct formulas of our recent paper (Koon \textit{et al}. 2013 \textit{J. Appl. Phys. }\textbf{114} 163710) applied to ``reciprocally dual geometries'' (swapped Dirichlet and Neumann boundary conditions) described by Mare\v{s}~\textit{et al. }(2012~\textit{Meas. Sci. Technol.}~\textbf{23}~045004). These results show that (a) the product of any such sensitivity function times differential area, and thus (b) the ratio of any two sensitivities, is invariant under conformal mapping, allowing for the pointwise determination of the conformal mapping function. The family of rounded crosses, which is bounded in parameter space by the square, the astroid and an ``infinitesimally thin'' cross, seems to represent the best geometry for focusing transport measurements on the center of the specimen while minimizing errors due to edge- or contact-effects. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y24.00010: Squeeze Flow of Yield Stress Fluids David Pelot, Alexander Yarin The squeeze flow of yield stress materials are investigated using a non-invasive optical technique. In the experiments, cylindrically-shaped samples of Carbopol solutions and Bentonite dispersions are rapidly compressed between two transparent plates using a constant force and the instantaneous cross-sectional area is recorded as a function of time using a high speed CCD camera. Furthermore, visualization of the boundary reveals that the no-slip condition holds. In addition, shear experiments are conducted using parallel-plate and vane viscometers. The material exhibits first a fast stage of squeezing in which the normal stresses dominate and viscosity plays the main role. Then, the second (slow) stage sets in where the material exhibits a slow deformation dominated by yield stress. At the end, the deformation process is arrested by yield stress. The material response is attributed to the Bingham-like or Herschel-Bulkley-like rheological behavior. Squeeze flow is developed into a convenient and simple tool for studying yield stress materials. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y24.00011: Developing high-resolution carbon-13 and silicon-29 MRI of solids in sedimentary rocks Robert Blum, Sean Barrett, Ravinath Viswanathan, Yi-Qiao Song Mapping pore structure and flow properties of sedimentary rock is directly relevant to current challenges in geophysics like carbon sequestration and oil/gas exploration. Such applications require detailed information about both structure and composition of porous rocks. However, existing scanning methods tend to be limited to gathering one or the other type of information. MRI could be used to measure both composition and structure simultaneously, but conventional MRI in such systems, which targets the proton signal of interstitial fluid, is severely limited by signal losses due to magnetic susceptibility inhomogeneity. Our lab has recently made advances in obtaining high spatial resolution (sub-400 $\mu$m)$^3$ three-dimensional $^{31}$P MRI of bone through use of the quadratic echo line-narrowing sequence (1). In this talk, we describe our current work applying these methods to sedimentary rock, targeting the isotopes $^{13}$C and $^{29}$Si. We describe the results of characterization of limestone and shale samples, and we discuss our progress with producing MRI of these systems. (1) M. Frey, et al. \textit{PNAS} \textbf{109}: 5190 (2012) [Preview Abstract] |
Session Y25: Focus Session: Thermoelectrics - Organic and Nanomaterials
Sponsoring Units: DMP GERA FIAPChair: Lakshmi Krishna, Colorado School of Mines
Room: 503
Friday, March 7, 2014 8:00AM - 8:12AM |
Y25.00001: Designing $\pi$-stacked molecular structures to be thermal insulators but electric conductors Gediminas Kirsanskas, Qian Li, Martin Leijnse, Gemma Solomon, Karsten Flensberg We show that $\pi$-stacked molecular structures in transport junctions can be designed to have a reduced thermal phonon conductance, while maintaining a high electric conductivity. The relevant contribution to the phonon thermal conductance, up to room temperature, comes from the center of mass motion of the molecules. Therefore, we propose a molecular design consisting of two large masses coupled to each other and to the leads. By having a small coupling (spring constant) between the masses, it is possible to reduce the phonon thermal conductance. This can be achieved by a $\pi$-stacking of the molecules. For the proposed model, the effects of mass' asymmetry, coupling asymmetry, and coupling strength are also examined. The resulting heat conductance is compared with the situation when the molecule is modeled as a single mass. The effective coupling strengths (spring constants) for the simplified model are extracted from density functional theory calculations. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y25.00002: Lorenz number of conducting PEDOT:PSS Xiaojia Wang, Nelson Coates, Rachel Segalman, David Cahill The electronic thermal conductivity is related to the electrical conductivity through the Wiedemann-Franz law (WFL), which predicts that the ratio of the electronic thermal conductivity to the electrical conductivity is proportional to the absolute temperature. The WFL has been validated for various materials; however, deviations may arise under certain circumstances, in which the relaxation times for the electrical and thermal processes are not identical. In this work, we investigate the Lorenz number, the proportionality factor in the WFL, of conjugated polymers. We prepare samples made of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) with tunable electrical conductivity. The in-plane electrical resistivity is characterized with setups of both 4-point probe and Van der Pauw configurations. To determine the thermal conductivity along the same direction as that for the electrical resistivity, we measure the through-plane thermal conductivity of the cross section of PEDOT:PSS using time-domain thermoreflectance. The effects of anisotropy and inhomogeneity on the thermal conductivity of PEDOT:PSS are also examined. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y25.00003: Effect of Alkyl Ligand Size on Thermoelectric Properties of Gold Nanocrystal Arrays William Chang, Boris Russ, Jeffrey Urban, Rachel Segalman Traditional thermoelectric materials suffer from low efficiencies due to inverse coupling of the Seebeck coefficient and electrical conductivity, which limits the power factor. Decoupling of these two physical properties represents an exciting opportunity, and has previously been demonstrated in molecular junctions. Using molecular junction design principles for guidance, we designed gold nanocrystal arrays with varying alkyl linkers. We demonstrate that the conductivity of these nanocrystal arrays follows a conventional tunneling model, where the length between nanoparticles dictates conductance. Interestingly, the Seebeck coefficients are not explained by single molecule tunneling junction theory. Metal ligand charge transfer theory, in conjunction with optical spectroscopy, is used to explain thin film charge transport. We compare these macroscale thin film transport properties to single molecule electronic transmission measurements reported in previous studies. This result will lend further insight into how molecular junctions and nanocrystal arrays can be integrated for materials with higher power factors. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y25.00004: Developing P-type Nanocomposites for Optimized Thermoelectrics A.D. Avery, K.S. Mistry, B.L. Zink, M.L. Olsen, P.A. Parilla, J.L. Blackburn, A.J. Ferguson Nanocomposites constructed of conducting polymers with organic inclusions such as single-walled carbon nanotubes are promising candidates for materials where the thermal and electrical transport properties can be decoupled, with the aim of realizing more efficient organic thermoelectric composites. Successful realization of high-performance organic thermoelectric devices requires a detailed fundamental understanding of the factors governing thermal and electrical transport through these materials. Additionally, in reduced geometries, many of these materials are expected to be anisotropic, necessitating the ability to measure these properties in the sample plane. In this talk, we describe our suspended membrane technique for directly measuring the in-plane thermal and electrical transport in the same sample, and present results for several different thin films. We present our approach to developing p-type materials with tunable transport behavior, through fabrication of composites consisting of single-walled carbon nanotubes (SWCNTs) dispersed in a polymer matrix. Finally, we discuss post-fabrication treatments of the SWCNT thin films and the benefits offered by nanostructuring these architectures to optimize the thermoelectric dimensionless figure-of-merit, ZT. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y25.00005: Tailoring thermopower of single-molecular junctions by temperature-induced surface reconstruction Chiung-Yuan Lin, Bailey Hsu, Yau-Shian Hsieh, Yu-Chang Chen Recent experiments revealed that surface reconstruction occurs at around 300-400K in the interface of C$_{60}$ adsorbed on Cu(111) substrate by scanning tunneling microscope techniques. To understand effects of such reconstruction on thermopower, we investigate the Seebeck coefficients of C$_{60}$ single-molecular junctions without and with surface reconstruction as a function of temperature at different tip-to-molecule heights from first-principles. Our calculations show that surface reconstruction can enhance or suppress Seebeck coefficients according to junctions at different tip heights. We further observe that the Seebeck coefficient of the junction at $d =$3.4{\AA} may change from p- to n-type under surface reconstruction. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y25.00006: Predicting the Influence of Secondary Phases on the Thermoelectric Performance in Cobalt Containing Spinels (ACo$_2$O$_4$; A=\{Co,Ni,Zn\}) Terence Musho, Anveeksh Koneru, David Mebane A promising new compositional space comprised of cobalt containing spinels bound by three end-members, Co$_2$(Co)O$_4$, Co$_2$(Ni)O$_4$ and Co$_2$(Zn)O$_4$, has brought to issue the presence of secondary phases (CoO, NiO, ZnO). These secondary phases are a result of not achieving a complete solid solution across the compositional space. It is hypothesized that under controlled fabrication the geometry and dispersion of these secondary phase can be leveraged to not only limit phonon transport but possibly increase electrical transport resulting in enhanced thermoelectric performance. To understand the influence of these secondary phases, a computational model has been developed that relies on a two dimensional non-equilibrium Green's function (NEGF) formalism to predict both the electrical and thermal contributions to the overall thermoelectric performance. This presentation will discuss the electrical and thermal transport models and approaches taken to incorporate dissipative carrier mechanisms into the quantum models. In addition, computational results predicting the optimal geometry and spacing of the secondary phases will be discussed. In closing, remarks will be made on how these models are currently being integrated into a high-throughput framework for materials discovery. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y25.00007: Large Enhancements in Thermopower and Electrical Conductivity in Nano-structured Half-Heusler Alloys Alexander Page, Anton van der Ven, Pierre Poudeu, Ctirad Uher Recent improvements have often been made to thermoelectric materials by adding nano-structures in order to scatter heat carrying phonons, however, the reduction in thermal conductivity is accompanied by large drops in the electrical conductivity caused by mobility reductions. In this work we show that Half-Heusler (HH) alloys can be combined with nano-scale Full-Heusler (FH) inclusions to simultaneously improve the power factor and reduce thermal conductivity. HH structures are of the form MNiSn and MCoSb (M$=$ Ti, Zr, or Hf) and the FH counterparts are created by filling the vacancies on the Ni or Co planes respectively, giving MNi$_{\mathrm{2}}$Sn and MCo$_{\mathrm{2}}$Sb. Experimental results show FH nano-inclusions were coherently integrated into the matrix HH material resulting in enhanced ZT which is attributed to energy filtering effects that occur at the HH-FH grain boundaries as well as moderate reductions in thermal conductivity by nano-inclusion phonon scattering. \textit{Ab Initio} calculations, in combination with a cluster expansion, are used to test the stability of FH structures in HH matrix and create thermodynamic pseudo-binary phase diagram for MNiSn-MNi2Sn compositions, elucidating the possibilities for future approaches to enhance ZT. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y25.00008: Fabrication and Thermoelectric Properties of Bulk Si$_{0.8}$Ge$_{0.2}$-FeSi$_{2}$ Nanocomposite Amin Nozariasbmarz, Mohamed AbuDakka, Lobat Tayebi, Daryoosh Vashaee We report enhancement of thermoelectric figure of merit (ZT) in bulk nanocomposites of n-type Si$_{0.8}$Ge$_{0.2}$-FeSi$_{2}$. The nanocomposite material was prepared via rapid sintering of the mixed powder of Si$_{0.8}$Ge$_{0.2}$ and FeSi$_{2}$ in a die under axial pressure. The thermoelectric properties of the samples were measured versus temperature. A remarkable reduction in thermal conductivity was observed while the thermoelectric power factor (Seebeck coefficient squared times the electrical conductivity) was maintained compared to the corresponding properties of the crystalline Si$_{0.8}$Ge$_{0.2}$. As a result the ZT increased to about 1.2 at 950 C, which is 20{\%} more than that of the n-type crystalline silicon germanium. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y25.00009: Nanoscale thermoelectric properties of fs-laser induced nanotracks on Sb$_{2}$Te$_{3}$ Jenna Walrath, Yen-Hsiang Lin, Yuwei Li, Vladimir Stoica, Lynn Endicott, Kevin Pipe, Ctirad Uher, Roy Clarke, Rachel Goldman Antimony telluride (Sb$_{2}$Te$_{3}$) is a canonical material for thermoelectric applications. It was recently shown that $\sim$20 nm diameter Sb$_{2}$Te$_{3}$ nanowires, fabricated by the vapor-liquid-solid method, exhibit $\sim$20{\%} enhancement in the Seebeck coefficient, S, in comparison to that of the bulk [1]. In addition, nanotrack formation was recently induced by fs-laser irradiation of Sb$_{2}$Te$_{3}$ [2]. Here, we report on the nanoscale thermoelectric properties of such fs-laser induced nanotracks on Sb$_{2}$Te$_{3}$ using scanning tunneling spectroscopy (STS) to probe the local density of states near the surface, and scanning thermoelectric microscopy (SThEM) to probe the local Seebeck coefficient just below the surface [3]. In the pristine (nanotrack) regions of Sb$_{2}$Te$_{3}$, STS reveals a bandgap of $\sim$0.3 eV (\textgreater 1 eV), suggesting the presence of an insulating surface layer in the irradiated regions. However, SThEM shows similar thermovoltages across both the pristine and nanotrack regions, presumably due to the buried regions of Sb$_{2}$Te$_{3}$. These data suggest that the nanotracks are buried beneath an insulating surface layer, consistent with our recent transmission electron microscopy observations. \\[4pt] [1] Y.M. Zuev, J.S. Lee, C. Galloy, H. Park, P. Kim, Nano Lett., \textbf{10}, 3037 (2010).\\[0pt] [2] Y. Li, V. A. Stoica, L. Endicott, G. Wang, H. Sun, K.P. Pipe, C. Uher, R. Clarke, Appl. Phys. Lett. \textbf{99}, 121903 (2011).\\[0pt] [3] J.C. Walrath, Y.H. Lin, K.P. Pipe, R.S. Goldman, Appl. Phys. Lett., in press (2013). [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y25.00010: Enhancement of thermoelectric figure of merit of nanostructured FeSb$_{2}$ by adding Cu nanoparticles Machhindra Koirala, Huaizhou Zhao, Mani Pokharel, Shuo Chen, Cyril Opeil, Gang Chen, Zhifeng Ren We present the enhancement of thermoelectric properties of FeSb$_{2}$ through modulation doping by Cu nanoparticles. Since, FeSb$_{2}$ and Cu have matched work function, the electrical conductivity of this Kondo-like system can be increased dramatically without affecting Seebeck coefficient. The optimized nanocomposite FeSb$_{2}$Cu$_{0.045}$ has enhancement of power factor by 90{\%} compared to pure nanostructured FeSb$_{2}$. The further reduction of thermal conductivity from FeSb$_{2}$/Cu interface gives the total enhancement of figure of merit (ZT) by 110{\%}. This strategy has been widely used on other semiconductors to improve ZT. Our result demonstrates that the potential of the modulation doping technique can also be extended to Kondo insulator systems. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y25.00011: Predicting Phononic and Thermal Properties of Nanocrsytal Superlattice Structures Using Atomistic Models Mehdi Zanjani, Jennifer Lukes We use fully atomistic models with MD simulations to predict phononic and thermal properties of nanocrystal superlattices (NCSLs). NCSLs are formed by assembly of nanocrystals into organized structures with interesting and tunable properties. They present new phononic behaviors by combining dissimilar materials structured on the nanometer scale. The small thermal conductivity of these materials makes them promising candidates for thermoelectric applications as well. We have calculated phonon dispersion curves of NCSLs by generalizing the lattice dynamics methods. We also calculated thermal conductivity of these materials using 3-D equilibrium MD simulations and the Green-Kubo method. Atomistic models along with MD simulations provide a complement to experiments for understanding the behavior of NCSLs, and help us modify the design of these structures to achieve better phononic and thermal properties. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y25.00012: Thermoelectric Study of Copper Selenide Mengliang Yao, Weishu Liu, Zhifeng Ren, Cyril Opeil Nanostructuring has been shown to be an effective approach in reducing lattice thermal conductivity and improving the figure of merit of thermoelectric materials. Copper selenide is a layered structure material, which has a low thermal conductivity and p-type Seebeck coefficient at low temperatures. We have evaluated several hot-pressed, nanostructured copper selenide samples with different dopants for their thermoelectric properties. The phenomenon of the charge-density wave observed in the nanocomposite, resistivity, Seebeck, thermal conductivity and carrier mobility will be discussed. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y25.00013: Thermoelectric Properties of Nanostructured CeAl$_{3}$ Mani Pokharel, Tulashi Dahal, Zhifeng Ren, Cyril Opeil Past investigations into the heavy fermion compound CeAl$_{3}$ reveal a complex low-temperature physics resulting from the strong hybridization of localized 4f states with delocalized conduction electrons. This phenomenon gives rise to unusual electronic, thermal, and magnetic properties. We investigate the low-temperature thermoelectric properties of this strongly correlated system for its potential application as a $p$-type Peltier cooling element. In our work, nanostructured samples of CeAl$_{3}$ have been prepared using dc hot-press method and evaluated for their thermoelectric properties. Effects of different hot-pressing temperatures on the nanostructure and the thermoelectric properties will be discussed. Our results on CeAl$_{3}$ will be compared with our previous work on CeCu$_{6}$. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y25.00014: Electronic structure of Zr-Ni-Sn systems: the role of nanostructures and clustering in Half-Heusler and Heusler limits Dat Do, S.D. Mahanti Half-Heusler and Heusler compounds have been of great interest for several decades for thermoelectric, magnetic, half-metallic and many other interesting properties. Among these systems, Zr-Ni-Sn compounds are interesting thermoelectrics which can go from semiconducting half-Heusler (HH) limit, ZrNiSn, to metallic Heusler limit (FH), ZrNi2Sn. Recently Makogo et al. [J. Am. Chem. Soc. 133, 18843 (2011)] found that dramatic improvement in the thermoelectric power factor of HH can be achieved by putting excess Ni into the system. This was attributed to an energy filtering mechanism due to the formation of FH nanostructures in the HH matrix. Using density functional theory we have investigated clustering and nanostructure formation in HH$_{1-x}$FH$_x$ systems near the HH and FH ends. These results and the effects of nanostructures on electronic structure and thermoelectric properties will be discussed in this talk. [Preview Abstract] |
Session Y26: Focus Session: Modeling of Rare Events
Sponsoring Units: DCOMPChair: Amit Samanta, Princeton University
Room: 502
Friday, March 7, 2014 8:00AM - 8:36AM |
Y26.00001: Computer Simulation of Membrane Permeation by Milestoning Invited Speaker: Ron Elber Atomically detailed molecular dynamics trajectories in conjunction with Milestoning are used to analyze the different contributions of coarse variables to the permeation process of a small peptide (N-acetyl-L-tryptophanamide, NATA) through a 1,2-dioleoyl-\textit{sn}-glycero-3-phosphocholine (DOPC) membrane. Milestoning is a theory and algorithm that exploits the use of short trajectories between interfaces in phase space (milestones) to compute equilibrium and long time behavior. The permeation process takes hours, which makes it appropriate for a Milestoning study. Reasonable agreement between experiment and simulation is obtained. The peptide reverses its overall orientation as it permeates through the biological bilayer. The large change in orientation is investigated explicitly but is shown to impact the free energy landscape and permeation time only moderately. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 9:12AM |
Y26.00002: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 9:12AM - 9:48AM |
Y26.00003: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 9:48AM - 10:24AM |
Y26.00004: Computing Rates of Small Molecule Diffusion Through Protein Channels Using Markovian Milestoning Invited Speaker: Cameron Abrams Measuring diffusion rates of ligands plays a key role in understanding the kinetic processes inside proteins. For example, although many molecular simulation studies have reported free energy barriers to infer rates for CO diffusion in myoglobin (Mb), they typically do not include direct calculation of diffusion rates because of the long simulation times needed to infer these rates with statistical accuracy. We show in this talk how to apply Markovian milestoning along minimum free-energy pathways to calculate diffusion rates of CO inside Mb. In Markovian milestoning, one partitions a suitable reaction coordinate space into regions and performs restrained molecular dynamics in each region to accumulate kinetic statistics that, when assembled across regions, provides an estimate of the mean first-passage time between states. The mean escape time for CO directly from the so-called distal pocket (DP) through the histidine gate (HG) is estimated at about 24 ns, confirming the importance of this portal for CO. But Mb is known to contain several internal cavities, and cavity-to-cavity diffusion rates are also computed and used to build a complete kinetic network as a Markov state model. Within this framework, the effective mean time of escape to the solvent through HG increases to 30 ns. Our results suggest that carrier protein structure may have evolved under pressure to modulate dissolved gas release rates using a network of ligand-accessible cavities. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y26.00005: Study of methane hydrate nucleation by accelerated molecular simulation Yuanfei Bi, Tianshu Li Recently clathrate hydrates have gained increasing attention due to their significance in energy, environment, safety, and gas transportation. The formation of such important compounds remains elusive, as a molecular level understanding of the nucleation mechanism is still missing. To gain such understanding, we combined forward flux sampling method with molecular dynamics, to simulate the nucleation process of methane hydrate. In particular, we have developed an effective order parameter that allows calculating hydrate nucleation rate explicitly for the first time. The order parameter is constructed based on the topological analysis of the tetrahedral network, and is capable of efficiently distinguishing hydrate from ice and liquid water. Employing this approach, we conducted molecular simulation under different thermodynamics conditions. Ensembles of nucleation pathways, containing both crystalline and amorphous hydrate nuclei, were obtained and analyzed under different conditions. In particular, pressure was found to significantly affect hydrate nucleation rate and pathways. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y26.00006: Sampling saddle points on the free energy surface Amit Samanta |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y26.00007: Three-Dimensional self-learning kinetic Monte Carlo model for arbitrary surfaces Andreas Latz, Lothar Brendel, Dietrich E. Wolf The self-learning kinetic Monte Carlo (SLKMC) method combines the calculation of transition rates from a realistic potential with the efficiency of a rate catalog, using a pattern recognition scheme. Originally limited to twodimensional systems with one specific surface orientation, we recently extended the method to three dimensions and arbitrarily shaped surfaces. We showed that by setting up an initial database, the concomitant huge increase of rate calculations on the fly can be decreased significantly. The model is applied to the homoepitaxial growth of Ag on Ag(111) at low temperatures and the drift of voids and islands due to electromigration. [Preview Abstract] |
Session Y27: Quantum Many-Body Systems III
Sponsoring Units: DCOMPChair: Zach Pozun, University of Pittsburgh
Room: 501
Friday, March 7, 2014 8:00AM - 8:12AM |
Y27.00001: Full-counting statistics and phase transition in an open quantum system of non-interacting electrons Mariya Medvedyeva, Stefan Kehrein We develop a method for calculating the full-counting statistics for a non-interacting fermionic system coupled to memory-less reservoirs. The evolution of the system is described by the Lindblad equation. We introduce the counting field in the Lindblad equation which yields the generating function and allows us to obtain all cumulants of the charge transport. In a uniform system the cumulants of order k are independent of the system size for systems longer than k+1 sites. The counting statistics from the Lindblad approach does not take into account the interference in the reservoirs which gives a decreased value of noise in comparison to the Green function approach which describes phase coherent leads. The two methods yield the same value for the current, which is due to current conservation. The Fano factors are different (and linearly related) and allow us to distinguish between memory-less and phase coherent reservoirs. We also consider the influence of dissipation along the chain allowing for both tunneling into and out of the chain along its length. Infinitesimally small dissipation along the chain induces a quantum phase transition which manifests itself as a discontinuity in transport properties and entropy. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y27.00002: Quantum Monte Carlo studies of Shannon-Renyi entropies and participation spectra in interacting spin systems David J. Luitz, Fabien Alet, Nicolas Laflorencie Shannon-Renyi entropies are measures of the participation of basis states in a wave function. Previous work for one dimensional systems showed that they exhibit a subleading scaling behavior with system size that contains universal information, such as e.g. the Luttinger Liquid parameter. Here, we introduce quantum Monte Carlo schemes to calculate these quantities and the related participation spectra for unfrustrated quantum many body systems in any dimension and apply them to interacting spin systems. Our results demonstrate the universality of subleading scaling terms for different kinds of phase transitions with a spontaneous breaking of discrete or continuous symmetries and at quantum critical points. Aditionally, we also discuss the signature of quantum phase transitions in the participation spectra of subsystems. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y27.00003: An ab initio many-body approach to understanding magnetism in the unconventional superconductor, FeSe Brian Busemeyer, Lucas Wagner We report on the progress of many-body ab initio fixed-node diffusion Monte Carlo (FM-DMC) calculations performed on the unconventional superconductor FeSe. The exact nature of the pairing mechanism in unconventional superconductors is still controversial; however, the fact that these materials demonstrate antiferromagnetism near the conditions necessary for superconductivity suggests some combination of lattice and magnetic interactions may be responsible. FN-DMC has been shown to obtain high accuracy on a number of strongly correlated materials, and so is particularly well-suited to study the correlations that give rise to superconductivity. We perform FN-DMC calculations on FeSe to determine the energetic orderings of the low-lying magnetic states, and investigate the dominant correlations between the single particle states. FeSe also demonstrates a pressure dependent transition temperatures, hence, we also investigate the pressure dependence of the energy and dominant correlations in the various magnetic states. We expect our results will shed light on the nature of magnetism in the iron-based superconductors, and possibly its relationship with superconductivity. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y27.00004: Quantum Monte Carlo Studies of Zinc-Porphyrin and Molybdenum dimer Adem Halil Kulahlioglu, Lubos Mitas We present fixed-node diffusion Monte Carlo (FN-DMC) studies focused on the calculation of vertical excitation energy in Q band corresponding to the excitation between the singlet ground-state ($1^{1}A_{g}$) and the lowest-lying singlet excited state ($1^{1}E_{u}$) of Zinc-Porphyrin (ZnP) molecule and the binding energy of the ground-state Molybdenum dimer (Mo$_2$). In the ZnP study, several trial wave functions for the excited state such as CIS, TDDFT and others were tested. We have obtained a very good agreement both with experiments and with high accuracy basis set correlated wave function calculations. The calculations show that the studied excitation is not well described by single-reference trial wave functions. In the Mo$_2$ study, the bias introduced by the fixed-node approximation with single-reference trial function is significant so we attempt to reduce it by means of the selected Configuration Interaction (selected-CI) technique. The single-particle orbitals that appear to lead to the lowest fixed-node error were generated by the hybrid version of the TPSS meta-GGA (TPSSh) functional. In this way, we have obtained significant improvements over the results from the single-reference trial function. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y27.00005: Simulations of quantum spin decoherence and spin diffusion using coherent-state representation Viatcheslav Dobrovitski Understanding non-equilibrium spin dynamics and spin diffusion in open quantum systems is of fundamental importance. It is also essential e.g. for NMR characterization of materials based on the spin diffusion between different $^{13}$C sites. However, exact numerical modeling of large 3-D spin systems with arbitrary-range couplings is exponentially difficult, and approximate methods for such systems are actively sought. We consider the approach based on the coherent-state P-representation for the density matrix of the many-spin system [1], as applied to spin decoherence and spin diffusion in the presence of the spin bath. We consider both model spin systems (such as the central spin problem), and realistic NMR experiments (spin diffusion in graphite bilayers and in organic molecular crystals). The approximate modeling results are compared with the exact simulations performed on the systems of 20-25 spins. We determine the features of the system which justify the use of the P-representation modeling, and demonstrate that this approach is applicable to a wide range of situations important for quantum information processing and NMR experiments.\\ \lbrack 1\rbrack K. Al-Hassanieh et al., Phys. Rev. Lett. 97, 037204 (2006); V. V. Dobrovitski et al., Phys. Rev. Lett. 102, 23760 (2009) [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y27.00006: Matrix product states for anyonic systems and efficient simulation of dynamics Sukhbinder Singh, Robert Pfeifer, Guifre Vidal, Gavin Brennen Anyons are exotic quasiparticles that exhibit non-trivial exchange statistics and arise as low lying excitations of topological phases of matter. Many-body systems of anyons offer a realm of new physics to explore that depends on their topological properties. The formalism of Matrix Product States [1] (MPS) has led to significant advances in the study of quantum many-body systems with local degrees of freedom such as spins or bosons. The MPS also forms the basis of the highly successful ``time-evolving block decimation'' [2] (TEBD) algorithm, which can be used to efficiently simulate dynamics of 1D systems. I will describe how to extend the MPS formalism and the TEBD algorithm to study lattice systems of anyons, which carry non-local degrees of freedom. I will also present supporting simulation results for chains of interacting anyons, including results for an anyonic Hubbard-type model [3] that give insight into the transport properties of anyons. \\[4pt] [1] S. Ostlund {\&} S. Rommer, PRL 75, 3537 (1995)\\[0pt] [2] G. Vidal, PRL 91, 147902 (2003)\\[0pt] [3] L. Lehman, V. Zatloukal, et al. PRL 106, 230404 (2011) [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y27.00007: SU(3) classical representation of quantum dynamics of interacting spins Shainen Davidson The Wigner-Weyl representation of quantum mechanics allows quantum operators to be represented as functions over phase space variables. In this representation, the Wigner function plays the role of a phase space probability distribution, although it can be negative due to quantum mechanics. The Truncated Wigner Approximation (TWA) is a semi-classical approach, where the dynamics are approximated using the classical dynamics of phase space variables averaged over the Wigner function. We can use this formalism to study spin dynamics as well; however, if there are any terms not linear in spin operators, the dynamics are not exact. In the case of spin one systems, we can linearize a single spin Hamiltonian by recasting it in terms of $SU(3)$ operators, where now we have eight operators instead of the usual three. Thus with TWA we can study the quantum dynamics of arbitrary spin one systems using eight ``classical'' spin variables per site, and the local TWA will be exact. I will discuss implications of this approach to interacting spin one systems and to the Bose-Hubbard model. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y27.00008: Eigenvalue degeneracy relations for a fully connected isotropic spin network Mark Coffey, Grant Allen We present and indicate the proofs of identities for the eigenvalue degeneracy of a fully connected spin network with isotropic spin coupling. Such a network has application to quantum information processing, especially for solid-state implementations, and in fact the qubit case with anisotropic coupling has been recently realized. One set of proofs and other relations for the case of qubits is given in the context of hypergeometric summation. We then generalize to arbitrary spin, using combinatorial arguments. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y27.00009: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y27.00010: Spectral gaps of AKLT Hamiltonians using Tensor Network methods Artur Garcia-Saez, Valentin Murg, Tzu-Chieh Wei Using exact diagonalization and tensor network techniques we compute the gap for the AKLT Hamiltonian in 1D and 2D spatial dimensions. Tensor Network methods are used to extract physical properties directly in the thermodynamic limit, and we support these results using finite-size scalings from exact diagonalization. Studying the AKLT Hamiltonian perturbed by an external field, we show how to obtain an accurate value of the gap of the original AKLT Hamiltonian from the field value at which the ground state verifies $e_0<0$, which is a quantum critical point. With the Tensor Network Renormalization Group methods we provide direct evidence of a finite gap in the thermodynamic limit for the AKLT models in the 1D chain and 2D hexagonal and square lattices. This method can be applied generally to Hamiltonians with rotational symmetry, and we also show results beyond the AKLT model. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y27.00011: Matrix product state formulation of frequency-space dynamics at finite temperatures Salvatore R. Manmana, Alexander C. Tiegel, Andreas Honecker We consider finite temperature properties of dynamical spectral functions of $S=1/2$ XXZ chains with Dzyaloshinskii-Moriya (DM) interactions in magnetic fields and analyze the effect of these symmetry breaking interactions on the nature of the spectral functions by comparing to results obtained for systems without DM interactions. This is achieved by extending matrix product state approaches working at finite temperatures to compute dynamical spectral functions in the frequency domain. We provide proof of principle results for the computation of experimentally relevant quantities like line shapes in neutron or light-scattering experiments. Based on our results, we provide an outlook for further improvements and developments of finite temperature approaches to dynamical spectral functions. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y27.00012: Nonequilibrium dynamical mean-field study of the nonthermal fixed point in the Hubbard model Naoto Tsuji, Martin Eckstein, Philipp Werner A fundamental question of whether and how an isolated quantum many-body system thermalizes has been posed and attracted broad interest since its ideal realization using cold atomic gases. In particular, it has been indicated by various theoretical studies that the system does not immediately thermalize but often shows ``prethermalization'' as a quasi-stationary state, where local observables quickly arrive at the thermal values while the full momentum distribution stays nonthermal for long time. Here we study the thermalization process for the fermionic Hubbard model in the presence of the antiferromagnetic long-range order [1][2]. Time evolution is obtained by the nonequilibrium dynamical mean-field theory. Due to classical fluctuations, prethermalization is prevented, and the transient dynamics is governed by a nonthermal fixed point, which we discuss belongs to a universality class distinct from the conventional Ginzburg-Landau theory. [1] N. Tsuji, M. Eckstein, P. Werner, Phys. Rev. Lett. 110, 136404 (2013). [2] N. Tsuji, P. Werner, Phys. Rev. B 88, 165115 (2013). [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y27.00013: Unexpected z-direction Ising antiferromagnetic order in a frustrated spin-1/2 J 1-J 2 XY model on the honeycomb lattice Zhenyue Zhu, David Huse, Steven White Using the density matrix renormalization group (DMRG) on wide cylinders, we study the phase diagram of the spin-1/2 XY model on the honeycomb lattice, with first-neighbor ($J_1 = 1$) and frustrating second-neighbor ($J_2>0$) interactions. For the intermediate frustration regime $0.22 < J_2 < 0.36$, we find a surprising antiferromagnetic Ising phase, with ordered moments pointing along the z axis, despite the absence of any $S_zS_z$ interactions in the Hamiltonian. Surrounding this phase as a function of $J_2$ are antiferromagnetic phases with the moments pointing in the $x-y$ plane for small $J_2$ and a close competition between an $x-y$ plane magnetic collinear phase and a dimer phase for large values of $J_2$. We do not find any spin liquid phases in this model. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y27.00014: Photo-induced topological phase transitions in the Hubbard model on honeycomb lattice Takahiro Mikami, Naoto Tsuji, Hideo Aoki ``Floquet topological states'' as first proposed by Oka and Aoki [1] are attracting much attention, where Dirac electrons in circularly polarized ac-fields undergo a nonequilibrium transition to topological stationery states with a photo-induced Hall effect in zero magnetic field. Such a transition has indeed been observed recently by a detection of the Floquet band structure on the surface of a topological insulator [2]. In equilibrium, on the other hand, electron correlation has been suggested to produce rich phases on honeycomb lattice. Hence it should be interesting to study what effects the electron correlation can exert on the photo-induced topological transitions. This has motivated us to study of the Hubbard model on honeycomb lattice in circularly polarized ac-fields. For this we have implemented the Floquet DMFT method[3]. We have indeed obtained a novel phase diagram, where most notably (i) there is a lobe structure between the topological and Mott phases and also (ii) a topological-topological transition with a change in the Chern number. \\[4pt] [1] T. Oka and H. Aoki, Phys. Rev. B 79, 081406(R) (2009)\\[0pt] [2] Y. H. Wang, et al., Science 342, 453 (2013)\\[0pt] [3] N. Tsuji, et al., Phys. Rev. Lett. 103, 047403 (2009) [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y27.00015: Classification of spin liquids in materials with strong spin-orbit coupling Johannes Reuther, Shu-Ping Lee, Jason Alicea The investigation of spin liquids is a fascinating field in condensed matter physics that is increasingly motivated by experiments. Exhaustive classifications of spin liquids have been carried out in several systems, particularly when full SU(2) spin-rotation symmetry is present. Systematic studies that explore strongly spin-orbit-coupled magnetic compounds (for which there are many experimental examples) are, however, relatively scarce. We report on a classification of $Z_2$ spin liquids on the square lattice when SU(2) spin symmetry is maximally lifted. Using projective symmetry group methods, we find that, surprisingly, the lifting of spin symmetry yields vastly more spin liquid states compared to SU(2)-invariant systems. Many of these spin liquids possess gapless edge states protected by lattice symmetries and, hence, constitute magnetic analogues of topological crystalline superconductors. [Preview Abstract] |
Session Y32: Invited Session: Recent Developments in the Kibble-Zurek Problem
Sponsoring Units: DCMPRoom: 708-712
Friday, March 7, 2014 8:00AM - 8:36AM |
Y32.00001: Universality of Phase Transition Dynamics: Topological Defects from Symmetry Breaking Invited Speaker: Wojciech Zurek As a result of the critical slowing down (the divergence of the relaxation time near the critical point) the dynamics of the non-equilibrium second order phase transition ceases to be adiabatic in the vicinity of the critical point. This results in a local choice of the broken symmetry of the order parameter, and can lead to the formation of topological defects. The Kibble-Zurek mechanism uses equilibrium scalings of the relaxation time and healing length in the vicinity of the critical point to describe the associated non-equilibrium dynamics of symmetry breaking and to estimate the density of topological defects as a function of the quench rate through the transition. Originally developed for classical phase transitions, it has been by now extended to quantum phase transitions (where local symmetry breaking is seeded by quantum rather than classical---e.g., thermal---fluctuations). During recent years, several new experiments investigating formation of defects in phase transitions induced by a quench both in classical and quantum mechanical systems were carried out, and more are on the way. At the same time, some established results were called into question [1]. I will review [2,3] Kibble-Zurek mechanism focusing in particular on this recent surge of activity, and suggest possible directions for further progress. \\[4pt] [1] Zurek, W. H., Topological relics of symmetry breaking: winding numbers and scaling tilts from random vortex-antivortex pairs, JOURNAL OF PHYSICS-CONDENSED MATTER Volume: 25 Issue: 40 Article Number: 404209 (2013). \\[0pt] [2] del Campo, A.; Kibble, T. W. B.; Zurek, W. H., Causality and non-equilibrium second-order phase transitions in inhomogeneous systems, JOURNAL OF PHYSICS-CONDENSED MATTER Volume: 25 Issue: 40 Article Number: 404210 (2013). \\[0pt] [3] del Campo, A.; Zurek, W. H., Universality of Phase Transition Dynamics: Topological Defects from Symmetry Breaking, arXiv:1310.1600 (2013). [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 9:12AM |
Y32.00002: Spontaneous creation of Kibble-Zurek solitons in a Bose-Einstein condensate Invited Speaker: Gabriele Ferrari The Kibble-Zurek mechanism (KZM) describes the spontaneous formation of defects in systems that cross a second-order phase transition at a finite rate. The mechanism was first proposed in the context of cosmology to explain how, during the expansion of the early Universe, the rapid cooling below a critical temperature induced a cosmological phase transition resulting in the creation of domain structures. In fact, the KZM is ubiquitous in nature and regards both classical and quantum phase transitions. Experimental evidences have been observed in superfluid $^3$He, in superconducting films and rings and in ion chains. Bose-Einstein condensation in trapped dilute gases has been considered as an ideal platform for the KZM as the system is extremely clean, controllable and particularly suitable for the investigation of effects arising from the spatial inhomogeneities induced by the confinement. Quantized vortices produced in a pancake-shaped condensate by a fast quench across the transition temperature have been already observed, but their limited statistics prevented a test of the KZM scaling. The KZM has been studied across the quantum superfluid to Mott insulator transition with atomic gases trapped in optical lattices. Here we report on the observation of solitons resulting from phase defects of the order parameter, spontaneously created in an elongated Bose-Einstein condensate of sodium atoms. We show that the number of solitons in the final condensate grows according to a power-law as a function of the rate at which the transition is crossed, consistent with the expectations of the KZM, and provide the first indication of the KZM scaling with the sonic horizon. We support our observations by comparing the estimated speed of the transition front in the gas to the speed of the sonic causal horizon, showing that solitons are produced in a regime of inhomogeneous Kibble-Zurek mechanism. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:48AM |
Y32.00003: Universality and Dynamic Localization in Kibble-Zurek of the Quantum Ising Model Invited Speaker: Michael Kolodrubetz Recent work has suggested that the Kibble-Zurek mechanism can be re-interpreted as a theory of critical scaling out of equilibrium. While this has been shown for some classical and integrable models, in this work we demonstrate one crucial aspect of critical scaling theory: universality. We solve for the time-dependent finite-size scaling functions of the 1D transverse-field Ising chain during a linear-in-time ramp of the field through the quantum critical point. We then simulate Mott-insulating bosons in a tilted potential, an experimentally-studied system in the same equilibrium universality class, and demonstrate that universality holds for the dynamics as well. We find qualitatively athermal features of the scaling functions, such as negative spin correlations, and show that they should be robustly observable within present cold atom experiments. In addition, we discuss recent results in which the Ising model is extended by imbuing the magnetic field with dynamics. We predict using Kibble-Zurek scaling and numerically confirm that the magnetic field ramp is dynamically arrested at the quantum critical point. Extensions of this theory indicate that dynamic localization near critical points should be omnipresent in nature, and may have implications in particle physics as a possible mechanism for giving the Higgs boson a light mass. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:24AM |
Y32.00004: Direct observation of the proliferation of ferroelectric loop domains and vortex-antivortex pairs Invited Speaker: Seung Chae |
Friday, March 7, 2014 10:24AM - 11:00AM |
Y32.00005: Kibble-Zurek Scaling and String-Net Coarsening in Topologically Ordered Systems Invited Speaker: Vedika Khemani The classic formulation of the KZ mechanism involves phase transitions with broken symmetries. This work considers instead the the non-equilibrium dynamics of topologically ordered systems driven across a continuous phase transition into proximate phases with no, or reduced, topological order. This is interesting both as a non-trivial extension of the KZ ideas but also on account of the interest in topological phases in the theory of correlated electron systems. The work shows that the dynamics exhibits scaling in the spirit of Kibble and Zurek but now without the presence of symmetry breaking and a local order parameter. The late stages of the process are seen to exhibit a slow, coarsening dynamics for the string-net that underlies the physics of the topological phase, a potentially interesting signature of topological order. The work discusses phase transitions involving both abelian and non-abelian topological order. [Preview Abstract] |
Session Y33: Fundamental Issues in Quantum Theory
Sponsoring Units: GQIChair: Chris Ferrie, University of New Mexico
Room: 706
Friday, March 7, 2014 8:00AM - 8:12AM |
Y33.00001: Backward traverse on Lattices seems to be easier than forward traverse Richard Kriske If one were to look at a Feynman Diagram as edges and nodes, and a great group of these Diagrams would comprise the Non-Laplacian Statistics that are used in complete Calculations of Events, such as the penetration of light into a group of stacked atoms, then there are a large number of missing diagrams that are of a strange sort. That being those diagrams that come to the end of a calculation and then back up in time to a previous node and start down a parallel path. It turns out that photons that back up in time can travel down parallel paths, can do so more easily than photons that move foward in time from that node. The reason that this appears to be so, is that forward motion in time for photons is a two-step phenomena because of the Non-Laplacian Statistics. The backward path to the previous node is already known. There is another method that nature could use to travel parallel paths and that is to simply have a totally original path, with nothing to do with the known nodes and edges. This author suggests that there is a hidden structure to time that creates easier paths backward in time for photons than forward in time, so that contrary to current thinking most photons travel backward in time easily, then forward in time along parallel paths. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y33.00002: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y33.00003: On Energy and Momentum in Contemporary Physics Peter Sujak This paper analyzes the quantities of energy and momentum in the definitional relationship of classical mechanics and relativistic mechanics, in the de Broglie momentum hypothesis and in the Klein-Gordon, Dirac and Schrodinger equation. The results of analysis shows that $\lambda $ designated in the de Broglie hypothesis $\lambda =h/mv$ as the wave of matter with rest state value $\lambda =\infty $ must be connected with a real dimension of a particle with rest state value $\lambda =l_{o} =h/m_{o} c$ and that on this basis we can come to the fundamental equations of quantum mechanics that are the Klein-Gordon, Dirac and Schrodinger equation without the necessity of the wave functions. Energies in relativistic mechanics as $mc^{2}$,$mvc$, and $m_{o} c^{2}$, and energy of a photon $h\nu $ do not represent quantities of energies, but quantity of momentums intentionally multiplied by $c$, so $mc\cdot c$, $mv\cdot c$, $m_{o} c\cdot c$, $h\nu /c\cdot c$ and merely the dimension of such quantities equals in dimension the quantity of energy. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y33.00004: Derivatives, Lagrangians, Points, Strings, and Balls Alfred Phillips Jr. Constrained by the uncertainty principle, the speed limit for information transfer, the equivalence of mass and energy, and gravity, we speculate as to whether or not we need new mathematics and/ or new physical insights to resolve the 120 order-of-magnitude difference in the vacuum energy of quantum theory and general relativity. We speculate on the specific subject objects, given the aforementioned constraints. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y33.00005: The Travelling Wave Group -- 5 Departures from Dirac's Principles Antony J. Bourdillon The Traveling Wave Group (TWG) for a free particle is written, $\psi = A(X^{2}/2\sigma^{2}+X)$. Here, $X=i(kx-\omega t)$, $\sigma $ is an experimental initial value, with $A $a normalizing constant dependent on it, while $\omega $ is the mean angular frequency, and \textbf{\textit{k}} the mean wave vector. Unlike Dirac's unstable wave packet; the TWG is stable. From it, the following are derived: the Uncertainty Principle [1]; Planck's law; the de Broglie hypothesis; phase velocity; pseudo mass M' [2]; conservation of M'PT [3]; 5-dimensional space; mass as a local excess of energy over momentum [4]; an explanation for entanglement at a distance, etc.\\[4pt] [1] Bourdillon, A.J., \textit{J. Mod. Phys. }\textbf{3} 290-296 (2012), DOI 10.4236/jmp.2012.33041 (open source).\\[0pt] [2] Bourdillon, A.J.,\textit{ J. Mod. Phys. }\textbf{4} 705-711 (2013), DOI 10.4236/jmp.2013.46097 (open source).\\[0pt] [3] Bourdillon, A.J., A travelling wave group III, conservation of M'PT, submitted to \textit{Phys. Rev. {\&} Res. Int.}(open source).\\[0pt] [4] Bourdillon, A.J., A traveling wave group and consequences, \textit{2013 Annual meeting of the CA-NV section of the APS.} [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y33.00006: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y33.00007: The quantal algebra and the principle of complementarity Samir Lipovaca We will derive the quantal algebra based on the general complementary concepts platform where the anticommutator is understood as a complementary concept of a commutator. An obvious property of a commutator is that it is antisymmetrical. It reminds us of the antisymmetrical tensor of the electromagnetic field. We recall that homogeneous pair of Maxwell's equations can be written as a single tensor equation. If we replace the tensor in this equation by the commutator we arrive at the Jacobi identity which is the first defining identity of the quantal algebra. If we replace commutators by anticommutators in the Jacobi identity, obviously there is no possibility of terms cancelation due to only addition. A subtraction between two successive terms leads to the second defining identity of the quantal algebra. There is always some relationship between complementary concepts. Guided by this observation we seek a relation between the commutator and the anticommutator and arrive at the third defining identity of the quantal algebra. Identities with the unit element are immediate consequences of the commutator and anticommutator definitions. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y33.00008: Photon Frequency Shifts by its Spin and Hubble Galaxy Red Shift vs Distance Sang Boo Nam A new mechanism for the photon frequency shifts by its spin, occurring from the inertial frame to the non-inertial (rotating) frame, is discussed. The photon spin one is shown with the frequency shifts by the Maxwell equations, without quantization of the photon field. The shifts are found to be varying with the photon path length, distance between its source and its observer. With the rotation of our galaxy, they account for the Hubble galaxy red shift vs distance and red shifts via supernovae, and blue shifts via galaxies. The sunlight red and blue shifts by its spin, are predicted, with the earth-self rotation. A mechanical (rotation) scheme is given for determination of a particle spin. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y33.00009: A new observation of a cosmic microwave background radiation supports my idea that a balance universe model between stellar matter and dark matter is like Chinese TaiJi Model Dayong Cao Einstein's equation gives a balance model of the universe. However it is not a steady model. As we know, there is a large amount of dark matter around stars and galaxies. This dark matter structure is very special. A new balance model of the universe, includes the dark matter, needed to be looked for. The stellar matter is a positive of Einstein's model. The dark matter is a negative of Einstein's model by ``mass-energy coordinate'' system because it has a space-time center. http://meetings.aps.org/link/BAPS.2010.SES.FC.9, Both of them build up a Dynamic Steady Balance Universal TaiJi Model without cosmological constant. http://meetings.aps.org/link/BAPS.2010.DNP.FE.9, So the map of CMB is like the Chinese TaiJi map. And there may be a ``hot spot'' across ``Axis of Evil'' and opposite to the ``cold spot.'' http://www.dailymail.co.uk/sciencetech/article-2430415, So there is a negative black-body radiation. A positive black-body radiation and a negative black-body radiation also builds up a balance model such as TaiJi Model. http://meetings.aps.org/link/BAPS.2012.APR.K1.78, The paper supposes that dark negative heat never spontaneously flows from a hot substance to a cold substance in dark matter system. The negative second law of thermodynamics and the second law of thermodynamics build up a balance model such as TaiJi Model too. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y33.00010: The Misapplication of Probability Theory in Quantum Mechanics Ronald Racicot This article is a revision of two papers submitted to the APS in the past two and a half years. In these papers, arguments and proofs are summarized for the following: \begin{enumerate} \item The wrong conclusion by EPR that Quantum Mechanics is incomplete, perhaps requiring the addition of ``hidden variables'' for completion. Theorems that assume such ``hidden variables,'' such as Bell's theorem, are also wrong. \item Quantum entanglement is not a realizable physical phenomenon and is based entirely on assuming a probability superposition model for quantum spin. Such a model directly violates conservation of angular momentum. \item Simultaneous multiple-paths followed by a quantum particle traveling through space also cannot possibly exist. Besides violating Noether's theorem, the multiple-paths theory is based solely on probability calculations. Probability calculations by themselves cannot possibly represent simultaneous physically real events. \end{enumerate} None of the reviews of the submitted papers actually refuted the arguments and evidence that was presented. These analyses should therefore be carefully evaluated since the conclusions reached have such important impact in quantum mechanics and quantum information theory. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y33.00011: A Local Realistic Reconciliation of the EPR Paradox Bryan Sanctuary The exact violation of Bell's Inequalities is obtained with a local realistic model for spin. The model treats one particle that comprises a quantum ensemble and simulates the EPR data one coincidence at a time as a product state. Such a spin is represented by operators $\sigma_{x},i\sigma _{y},\sigma_{z}$ in its body frame rather than the usual set of $\sigma _{X},\sigma_{Y},\sigma_{Z}$ in the laboratory frame. This model, assumed valid in the absence of a measuring probe, contains both quantum polarizations and coherences. Each carries half the EPR correlation, but only half can be measured using coincidence techniques. The model further predicts the filter angles that maximize the spin correlation in EPR experiments. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y33.00012: Relativistic Nonlocality and the EPR Paradox Thomas Chamberlain Denial of a privileged coordinate system, or aether, in formulating the electro-mechanical function of systems in motion was Einstein's profound contribution resulting in his relativity theory in 1905. His correct rejection of an empirically meaningful aether was made emphatically clear by the constant, isotropic light-speed stipulation. However, many laboratory experiments of the Michelson-Morley kind, before and after 1905, have established Lorentz-invariant photon round-trip time between opposing mirrors as more fundamental. On this more essential basis an alternative clock synchronization model and convention is defined which implicitly retains the Lorentz transformation. Anisotropic photon velocity is an integral feature of this model. In one (of two) limits, photon flight between mirrors becomes unbounded in the at-rest coordinate system, thereby advancing the prospect of instantaneous affect---but not instantaneous finite-distance information exchange. Prospective/heuristic resolution of the apparent paradox between special-relativity required locality and quantum mechanics non-locality under Bell's inequality theorem is addressed. [Preview Abstract] |
Session Y34: Nano/Optomechanics for Quantum Information Processing
Sponsoring Units: GQIRoom: 704
Friday, March 7, 2014 8:00AM - 8:12AM |
Y34.00001: Quantum back-action evading measurement of micro-mechanical motion Junho Suh, Aaron Weinstein, Chan U Lei, Emma Wollman, Keith Schwab Quantum mechanics imposes unavoidable finite back-action in measuring a mechanical resonator's position and places limits on its ultimate sensitivity, which is well known as the standard quantum limit (SQL). However, if the detector couples to only a single quadrature of motion, it is possible to place this quantum back-action in the uncoupled quadrature, realizing sensitivity below SQL. We demonstrate this back-action evading measurement using a micro-electromechanical device tightly coupled to a superconducting microwave resonator. We observe classical and quantum back-action from microwave photons, and demonstrate that the measurement back-action is 9dB lower than that from microwave shot noise. The measurement imprecision reaches 2dB smaller than the zero-point fluctuation level at the same time, showing the detector noise product five times from the quantum limit. We expect further improvements of this technique would provide a route to the generation of quantum squeezed states of motion, highly desirable for precision measurement of force and quantum engineering applications. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y34.00002: Manipulating a qubit through the backaction of sequential partial measurements and real-time feedback Cristian Bonato, Machiel Blok, Matthew Markham, Daniel Twitchen, Viatcheslav Dobrovitski, Ronald Hanson Quantum measurements not only extract information from a system but also alter its state. Although the outcome of the measurement is probabilistic, the backaction imparted on the measured system is accurately described by quantum theory. Therefore, quantum measurements can be exploited for manipulating quantum systems without the need for control fields. We demonstrate measurement-only state manipulation on a nuclear spin qubit in diamond by adaptive partial measurements. We implement the partial measurement via tunable correlation with an electron ancilla qubit and subsequent ancilla readout. We vary the measurement strength to observe controlled wavefunction collapse and find post-selected quantum weak values beyond 10. By combining a novel quantum non-demolition readout on the ancilla with real-time adaptation of the measurement strength, we realize steering of the nuclear spin to a target state by measurements alone. Besides being of fundamental interest, adaptive measurements can improve metrology applications and are key to measurement-based quantum computing. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y34.00003: Narrow spectral peaks induced by phase noise in modulated oscillators Mark Dykman, Yaxing Zhang, J. Moser, A. Eichler, A. Bachtold We show that frequency noise leads to additional peaks in the power spectra of modulated vibrational systems. We also provide experimental evidence of the occurrence of such peaks in suspended carbon nanotubes. The peaks are shown to emerge even for linear vibrations, in which case their parameters are independent of the thermal noise that accompanies relaxation. They can be thought of as a result of weakly inelastic scattering of the modulating field due to frequency noise. The peaks are centered near the modulation frequency and near the oscillator eigenfrequency, with strengths that depend on the noise spectrum. The peak near the modulation frequency is determined by the low-frequency part of the noise spectrum and can be much narrower than the peak in the oscillator absorption spectrum. We also show that the vibration nonlinearity can lead to a characteristic extra structure in the power spectrum in the presence of modulation. The modulation-induced spectral peaks are not only a direct indicator of frequency fluctuations, but they also provide information about the fluctuation intensity and spectrum thus enabling full characterization of the fluctuations. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y34.00004: The role of broken potential symmetry for nanomechanical resonators Alexander Eichler, Joel Moser, Mark Dykman, Adrian Bachtold Vibrational modes in nanomechanical systems as well as in nonlinear microwave cavities can have broken inversion symmetry, which can significantly affect the mode dynamics. We demonstrate a technique that allows us to reveal the symmetry breaking and to study its manifestation in linear and nonlinear resonant response. We study vibrational modes of carbon nanotubes, where the symmetry breaking is associated with the nanotube bending. We find that symmetry breaking leads to spectral broadening of mechanical resonances, and to an apparent quality factor that drops below 100 at room temperature. The low quality factor at room temperature is a striking feature of nanotube resonators whose origin has remained elusive for many years. Our results shed light on the pivotal role played by symmetry breaking in the dynamics of carbon nanotube mechanical resonators (to be published in Nature Communications). [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y34.00005: Two-mode squeezed states in cavity optomechanics via single-mode reservoir engineering Matthew Woolley, Aashish Clerk The generation and verification of a macroscopic, all-mechanical entangled state is a major goal and (at present) outstanding task in the study of mechanical systems in the quantum regime. The canonical continuous-variable entangled state is the two-mode squeezed state. Here we describe how to prepare and detect a highly-pure, all-mechanical two-mode squeezed state in an optomechanical system via coupling to only one (rather than two [1,2]) cavity mode(s). The approach taken may be viewed as a perturbation of a two-mode back-action-evading measurement [3], and generalizes an earlier proposal for single-mode mechanical squeezing [4].\\[4pt] [1] Y.-D. Wang and A. A. Clerk, Phys. Rev. Lett. 110, 253601 (2012).\\[0pt] [2] H. Tan, G. Li, and P. Meystre, Phys. Rev. A 87, 033829 (2013).\\[0pt] [3] M. J. Woolley and A. A. Clerk, Phys. Rev. A 87, 063846 (2013).\\[0pt] [4] A. Kronwald, F. Marquardt, and A. A. Clerk, arXiv:1307.5309. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y34.00006: Room-temperature ultra-sensitive mass spectrometer via dynamic decoupling Nan Zhao, Zhang-qi Yin We propose an ultra-sensitive mass spectrometer based on a coupled quantum-bit-oscillator system. Under dynamical decoupling control of the quantum bit (qubit), the qubit coherence exhibits a comb structure in time domain. The time-comb structure enables high precision measurement of oscillator frequency, which can be used as an ultra-sensitive mass spectrometer. Surprisingly, in ideal case, the sensitivity of the proposed mass spectrometer, which scales with the temperature $T$ as $T^{-1/2}$, has better performance in higher temperature. While taking into account qubit and oscillator decay, we show that the optimal sensitivity is independent on environmental temperature $T$. With present technology on solid state spin qubit and high-quality optomechanical system, our proposal is feasible to realize an ultra-sensitive mass spectrometer in room temperature. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y34.00007: High-Q Bulk Acoustic Quartz Resonators and Application to Hybrid Quantum Systems Michael Tobar, Maxim Goryachev, Daniel Creedon, Serge Galliou Latest results on cryogenically cooled Balk Acoustic Wave quartz resonators will be presented. We demonstrate the ability of such devices to have quality factors approaching $10^{10}$ and frequencies approaching $1$ GHz. The corresponding $Q \times f$ products make these devices several orders of magnitude better than any other mechanical system cooled to near the ground state. Such results are achieved for very-high overtones (up to 227th) of the longitudinally polarized phonons, such high overtones have never been observed previously. We discuss the basic requirements to achieve the extremely high quality factor regimes in acoustic devices by describing the main sources of losses. This includes material (two-level system, thermoelastic, Landau-Rumer losses), surface scattering, acoustic version of Raleigh scattering, clamping (phonon tunneling to the environment) loss mechanisms. Several types of BAW quartz resonators are compared. Finally, we discuss a range of applications of extremely low-loss acoustic cavities and how the very narrow bandwidths of the cavities (of orders of tens of $mHz$) can be incorporated into hybrid quantum systems. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y34.00008: Strong acoustic coupling to a superconducting qubit Martin Gustafsson, Thomas Aref, Anton Frisk Kockum, Maria Ekstr\"om, G\"oran Johansson, Per Delsing Micromechanical resonators can be used to store quantum information, as shown in several recent experiments. These resonators typically have the form of membranes or beams, and phonons are localized to their vibrational eigenmodes. We present a different kind of mechanical quantum device, where \emph{propagating} phonons serve as carriers for quantum information. At the core of our device is a superconducting qubit, designed to couple to Surface Acoustic Waves (SAW) in the underlying substrate through the piezoelectric effect. This type of coupling can be very strong, and in our case exceeds the coupling to any external electromagnetic modes. The acoustic waves propagate freely on the surface of the substrate, and we use a remote electro-acoustic transducer to address the qubit acoustically and listen to its emission of phonons. This presentation focuses on the basic properties of our acoustic quantum system, and we include experimental data that demonstrate the quantized coupling between the qubit and the propagating acoustic waves. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y34.00009: Acoustic equivalents of experiments from quantum optics Thomas Aref, Martin Gustafsson, Anton Kockum, Maria Ekstr\"om, G\"oran Johansson, Per Delsing On-chip quantum optics at microwave frequencies is a recent development, where solid-state qubits couple to itinerant photons in a one-dimensional electrical transmission line. We have demonstrated a new electromechanical hybrid, where a superconducting qubit couples to Surface Acoustic Waves (SAW), which propagate freely on the surface of piezoelectric microchip. This allows us to perform direct equivalents of quantum-optical experiments, with acoustic phonons taking over the role of photons as carriers of quantum information. We can address the qubit both electrically and with SAW, and listen to its quantized acoustic emission with an on-chip mechanical transducer. Our experiments are done at microwave frequencies, and compared to electromagnetic signals, the acoustic waves propagate orders of magnitude slower and have correspondingly shorter wavelengths. This, along with the potential for very strong coupling via the piezoelectric effect, opens for phononic exploration of quantum regimes that are difficult or impossible to reach with photons. In this talk, we present data from acoustic quantum experiments, with a focus on the prospective future role of propagating phonons in quantum informatics. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y34.00010: Control and measurement of an optomechanical system using a superconducting qubit Florent Lecocq, John Teufel, Michael Allman, Katarina Cicak, Fabio Da Silva, Adam Sirois, Jed Whittaker, Jose Aumentado, Raymond Simmonds In cavity optomechanics one can use photons to manipulate and measure the mechanical motion of a macroscopic object. With these techniques, ground state cooling of a mechanical resonator [1] and coherent transfer between a state of light and mechanical motion have been demonstrated [2]. So far these experiments have been using Gaussian resources, and therefore are limited to the observation of Gaussian states. I will discuss recent experiments that use an artificial atom as a non-linear resource for cavity optomechanics. The device consists of a superconducting phase qubit coupled to a lumped element microwave cavity, whose capacitance is formed by a mechanically compliant vacuum-gap capacitor. The motion of the mechanical resonator is encoded in the intra-cavity microwave field. The cavity can thus mediate an interaction between the qubit and the mechanical resonator, enabling preparation and readout of non-classical states of motion. In this talk I will show how we use the qubit to measure of the time evolution of the photon distribution in the microwave cavity, allowing us to infer the phonon distribution in the mechanical resonator. [1] Teufel et al, Nature 475, 359 (2011) [2] Palomaki et al, Nature 495, 210 (2013) [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y34.00011: Entangling Mechanical Motion with Microwave Fields Tauno Palomaki, John Teufel, Raymond Simmonds, Konrad Lehnert We demonstrate entanglement between the motion of a mechanical oscillator and a propagating microwave field. The mechanical oscillator is coupled to a microwave resonator such that by applying a pump we can realize either a beam-splitter or parametric down-conversation interaction. We exploit both interactions to create two microwave pulses that are sufficiently correlated to be in an inseparable state. As the second pulse encodes the state of the mechanical oscillator, the first microwave pulses was consequently entangled with the mechanical oscillator. This result further demonstrates the potential for mechanical oscillators to both store and generate quantum mechanically useful states. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y34.00012: Nonclassical State Revealed in a Fully Classical Harmonic System Wayne Huang In 1947, the measurement of Lamb and Retherford on the hyperfine spectrum of the hydrogen atom gave the first experimental evidence of the electromagnetic vacuum field. The interaction between matter and the vacuum field has since become an important topic in fundamental quantum electrodynamics. In this presentation, I would like to first discuss the excitation spectrum of a classical harmonic oscillator immersed in the vacuum field. Both our numerical simulation and perturbation analysis indicate that such a classical system exhibits the same excitation spectrum as its quantum counterpart. Then, I would like to show preliminary results on realizing the ``non-classical states'' within such a classical scheme. Namely, upon excitation the classical harmonic oscillator in the vacuum field displays interesting dynamical properties that are analogous to a coherent state, a squeezed state, and a Schrodinger cat state of a quantized light field. The intriguing connection between the classical harmonic system and the quantized light field may find application in the generation of nonclassical light using nano/optomechanical systems. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y34.00013: Mechanically mediated amplification of microwave fields at the quantum limit John Teufel, Florent Lecocq, Raymond Simmonds, Jose Aumentado In cavity opto- and electro-mechanical devices, the parametric interaction between the electromagnetic and mechanical modes can strongly modify both the motional degree of freedom and the light field emerging from the cavity. For example, by driving the cavity at the sum frequency of the two modes, one naturally amplifies both the light and the motion. Unfortunately, in this method of operation, the finite linewidth and temperature of the mechanical mode limit the gain-bandwidth product and the added noise, respectively. Here we use a microwave optomechanical circuit to demonstrate experimentally a novel form of parametric amplification that goes beyond these traditional limits. In order to quantify the ideality of the microwave amplification, we integrate a normal-metal tunnel junction as an in situ, calibrated noise source. In this way, we demonstrate parametric gain in excess of 80 dB and show that the amplification process adds only the minimum noise required by quantum mechanics. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y34.00014: Nanomechanical quantum many-body phonon-qubit systems for quantum simulators Oney Soykal, Charles Tahan We investigate nanomechanical systems consisting of arrays of coupled phonon cavities each including an impurity qubit in silicon. These experimentally feasible architectures can exhibit quantum many-body phase transitions, e.g. Mott insulator and superfluid states, due to a strong phonon-phonon interaction, and are suitable in the pursuit of quantum simulators. We investigate driven dissipative non-equilibrium systems at zero and non-zero temperatures. These quantum many-body phonon systems can be implemented using either on-chip nano mechanical systems in silicon or DBR heterostructures in silicon-germanium. We examine the experimental procedures to detect these states and show that temperature and driving field (write/read-out) play a critical role in achieving these phonon superfluid and insulator states. These many-body cavity phonon/qubit systems with strong phonon-phonon interactions can be used in forming truly quantum many-body mechanical states for quantum simulators as well as to complement other nano/optomechanical systems. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y34.00015: An Optomechanical Transducer for Microwave to Optical Quantum State Transfer A. Vainsencher, J. Bochmann, G. Peairs, K.J. Satzinger, D.D. Awschalom, A.N. Cleland Recent experiments have demonstrated that macroscopic optomechanical systems can be operated in the quantum regime\footnote{Safavi-Naeini et al. Phys. Rev. Lett. \textbf{108}, 033602 (2012)}\footnote{Teufel et al. \textit{Nature} \textbf{475}, 359 (2011)}\footnote{Chan et al. \textit{Nature} \textbf{478}, 89 (2011)}. Such systems offer a wide range of possibilities for new applications, potentially enabling coupling between disparate quantum systems. In this talk, we will describe our approach to using an optomechanical system as a microwave to optical transducer, with the eventual goal of coupling superconducting quantum bits to a light field. Our implementation uses an optomechanical crystal made of aluminum nitride, a strong piezoelectric. This choice of design and material offers the necessary optomechanical and electromechanical coupling rates that should make quantum state transfer possible. We will present recent results for our transducer concept, including classical operation\footnote{Bochmann, Vainsencher et al. \textit{Nature Physics} \textbf{9}, 712 (2013)}, design improvements, and cryogenic operation. [Preview Abstract] |
Session Y35: Thermalization and Many-Body Localization
Sponsoring Units: DAMOPChair: Andrew Daley, University of Pittsburgh
Room: 702
Friday, March 7, 2014 8:00AM - 8:12AM |
Y35.00001: Role of the Initial State in the Nonequilibrium Quantum Dynamics of Many-Body Systems Lea F. Santos, Eduardo J. Torres-Herrera We show that the dynamics of isolated many-body quantum systems after a quench depends on the interplay between the initial state and the Hamiltonian dictating the evolution. The systems considered are in the nonperturbative regime. The relaxation process is controlled by the width of the energy distribution of the initial state and may be very similar for both chaotic and integrable Hamiltonians. Our analytical expression for the fidelity decay displays excellent agreement with our numerical results. This decay is Gaussian and may persist until saturation. We also provide analytical expressions that describe very well the initial evolution of the Shannon entropy and of few-body observables. The analyses are developed for deterministic one-dimensional systems and initial states of interest to current experiments with cold atoms in optical lattices. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y35.00002: On thermalization in a nonintegrable quantum system: who thermalizes? Hyungwon Kim, Mari Carmen Banuls, David Huse, Ignacio Cirac We study properties of local operators with a nonintegrable Hamiltonian. We look for the cases where non-thermal (nonequilibrium) behaviors may be persistent even in the long time limit. First, we consider eigenstates which do not obey the Eigenstate Thermalization Hypothesis (ETH) in a finite size system. We show that the expectation values of local observable of these ``outliers'' converge to the scenario of ETH as we increase the system size. Next, we construct a local operator that gives the smallest value of commutator with the Hamiltonian. As we increase the range of the operator, the commutator quickly decreases with the range. This may imply the existence of local operators which may take fairly long to thermalize. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y35.00003: Finite-size scaling of eigenstate thermalization Wouter Beugeling, Roderich Moessner, Masud Haque According to the eigenstate thermalization hypothesis (ETH), even isolated quantum systems can thermalize because the eigenstate-to-eigenstate fluctuations of typical observables vanish in the limit of large systems. Since isolated systems are by nature finite, the finite-size scaling of such fluctuations is a central aspect of the ETH. We propose that for generic non-integrable systems these fluctuations scale with a universal power law in the dimension of the Hilbert space. We present extensive multiple-system numerical evidence for this scaling law and provide supporting arguments. We also show how the scaling changes when approaching integrability. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y35.00004: Relaxation towards negative absolute temperature states Stephan Mandt, Adrian Feiguin, Salvatore Manmana Motivated by the recent experimental observation of negative absolute temperature states with ultracold atoms in optical lattices, [Braun et al., Science 339 52 (2013)], we discuss the formation of these states. More specifically, we consider the relaxation after a sudden inversion of the external parabolic confining potential. First, previous numerical simulation results of a semiclassical Boltzmann equation for the case of fermions will be discussed, which show a surprisingly slow equilibration due to the diffusive rearrangement of the local kinetic energies in the inhomogeneous system. We then focus on the integrable system of one-dimensional hard-core bosons. Here, we provide convincing numerical evidence for the relaxation to a generalized Gibbs ensemble at negative absolute temperature, a notion we define in this context. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y35.00005: Thermalization timescales in a 1d Hubbard model with slightly broken integrability Fabian Biebl, Stefan Kehrein Understanding relaxation in quantum systems is essential to determine whether an experimental setup can be described by equilibrium concepts. For example integrable systems do not thermalize, but develop into non-thermal steady states. By slightly breaking integrability, thermalization of such non-thermal (prethermalized) states becomes possible. An important question is to identify the corresponding timescale for thermalization due to the breaking of integrability. We investigate this question for a fermionic Hubbard chain. The integrability breaking term is a small next to nearest neighbor hopping term [1,2]. The thermalization timescale is extracted from the quantum Boltzmann equation and depends strongly on temperature.\\[4pt] [1] M. L. R. F\"urst et al., Phys. Rev. E 86, 031122 (2012).\\[0pt] [2] M. L. R. F\"urst et al., Phys. Rev. E 88, 012108 (2013). [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y35.00006: Relaxation and thermalization of isolated quantum many-body systems after a local quench Eduardo J. Torres-Herrera, Lea F. Santos A single on-site defect in the middle of a one-dimensional spin-1/2 XXZ model is enough to break its integrability. By quenching the excess energy of the defect, we investigate the relaxation process for various initial states and the viability of thermalization. Changing the defect energy is equivalent to weakly perturbing the system, which prevents the initial state (projected into the energy eigenbasis) from achieving a Gaussian shape; it has instead a Breit-Wigner form. We show that in this scenario, the relaxation process is slower and the role of the Eigenstate Thermalization Hypothesis becomes more prominent. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y35.00007: Exact analysis of prethermalization of a coherently split one-dimensional Bose gas Eriko Kaminishi, Tatsuhiko Ikeda, Takashi Mori, Masahito Ueda We theoretically study the prethermalization dynamics of a coherently split one-dimensional Bose gas by using the Bethe ansatz method. Prethermalization is a relaxation process to a quasi-stationary state before reaching the true equilibrium state. The concept of prethermalization is important for understanding the fundamental aspects of quantum statistical mechanics such as ``equilibration'' and ``relaxation'' in isolated quantum many-body systems. Prethermalization and its connection to integrability in one-dimensional quantum systems have been intensively studied by both experiments and theories. For instance, M. Gring et al. recently observed the evolution of a rapidly and coherently split one-dimensional Bose gas for large numbers of particles and compare the evolution of the system to the prediction of the Tomonaga-Luttinger liquid (TLL) theory. Here we employ the Bethe ansatz method and precisely analyze the prethermalization process over a long-time scale beyond the TLL prediction. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y35.00008: Random matrix study of the time scale of thermalization after a quantum quench Tatsuhiko Ikeda, Yu Watanabe Thermalization in isolated quantum systems has been theoretically predicted and actually observed in experiments by using, for example, the cold atoms. However, the time scale, which the theories of thermalization require as a sufficient condition, is exponentially large in the number of particles and thus too large compared with that observed in experiments. We study thermalization after a quantum quench which is described by random matrices, in particular the sparse Gaussian Unitary Ensemble, and show that the time scale is given by $\tau_{\rm eq} =\hbar/[2\sigma(H)]$, where $\sigma(H)$ is the energy fluctuation of the initial state. Since the energy fluctuation grows only polynomially in the number of particles, this time scale can be regarded as more realistic one than the sufficient condition mentioned above. We also conduct numerical simulations of quantum quenches in the hard-core Bose-Hubbard model to validate the result in physically realistic situations. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y35.00009: Equilibration and Generalized GGE in Tonks Girardeau Regime Garry Goldstein, Natan Andrei We study the nonequilibrium properties of the 1-D Lieb-Liniger model in the infinite repulsion Tonks-Girardeau regime, Introducing a new version of the Yudson representation applicable to finite sized systems and appropriately taking the infinite volume limit we are able to study the equilibration of the Lieb-Liniger gas in the thermodynamic limit. We provide a formalism to compute various correlation functions for highly non-equilibrium initial states. In the Tonks Girardeua limit we are able to find explicit analytic expressions for the long time limit of the expectation of the density, density density and related correlation functions. We show that the gas equilibrates to a steady state from arbitrary initial states with ``smooth'' correlation functions. For nearly translationally invariant states the gas equilibrates to a diagonal ensemble which we show is equivalent to a generalized version of the GGE for sufficiently simple correlation functions, which in particular include density density correlations. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y35.00010: Characterizing a conducting-to-nonconducting transition in an inhomogeneous Hubbard model out of equilibrium via tDMRG simulations Daniel Gruss, Chih-Chun Chien, Massimiliano Di Ventra, Michael Zwolak The study of time-dependent, many-body transport phenomena is increasingly within reach of ultra-cold atom experiments. These systems not only allow experimental emulation of solid state systems, but allow us to probe the dynamics of transport at a previously unreachable level of detail. We will discuss computational results for the dynamics of fermionic transport in optical lattices that emulate an inhomogeneous Hubbard model. We demonstrate that this system displays a many-body, nonequilibrium conducting to nonconducting transition that depends on the interaction strength and filling.\footnote{New J. Phys. 15 063026} We characterize the transition by deconstructing the dynamical behavior of the fermionic density. We will also discuss these results in the context of present-day cold atom experiments. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y35.00011: Non-equilibrium dynamics and state preparation in bilayer optical lattices Stephan Langer, Andrew J. Daley We study dynamical schemes to obtain low entropy ground states of strongly interacting many body systems. The focus of our work is on ultra-cold Bose and Fermi gases in bilayer optical lattice systems with separately tunable interlayer coupling, energy offset between the layers and repulsive interactions. The case of two coupled one-dimensional chains is treated in a numerically exact manner using the adaptive time-dependent density matrix renormalization group which allows us to study the change of offset and interlayer coupling in real time. We identify parameter regimes where the ground state of the coupled system in the limit of small interlayer coupling consists of a Mott insulator in one layer and a superfluid/metallic state in the other layer can serve as an entropy reservoir. We then investigate the time-dependent dynamics of this system, studying entropy transfer between layers and the emergence of characteristic many-body correlations as we change the layer offset energy and coupling strength. In addition to applications as a preparation scheme for fully interacting Mott-insulator states, feasible with available experimental techniques, the investigated protocols could be easily adapted to also allow for a controlled preparation of highly excited states. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y35.00012: Nonequilibrium Dynamics beyond the Mean Field Approximation Ingo Homrighausen, Stefan Kehrein Mean field type approximations are one of the most accessible methods to study the complexity of quantum many body systems out of equilibrium. However, the validity of such approximations has to be examined in each case. Building on Ref. [1] we investigate three different quantum many particle models on finite fully connected lattices: the transverse field Ising model, the Bose-Hubbard model and the Jaynes Cummings model. In particular, we explore the nonequilibrium dynamics of the order parameter and its variance after a quantum quench. The most intriguing observation is that all three models exhibit the same universal behavior: For quenches within the ordered phase, the variance of the order parameter shows a quasiperiodic breathing behavior. The local maxima of this breathing increase in time whereas the local minima decrease. Applying a semiclassical expansion, we explain these findings and argue why the observations are generic. We also discuss the time scale of validity of our analysis by comparing to numerically exact data. [1] B. Sciolla, G. Biroli, J. Stat. Mech. (2011) P11003. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y35.00013: Dynamical quantum phase transitions in random spin chains Ronen Vosk, Ehud Altman Quantum systems can exhibit a great deal of universality at low temperature due to the structure of ground states and the critical points separating distinct states. On the other hand, quantum time evolution of the same systems involves all energies and it is therefore thought to be much harder, if at all possible, to have sharp transitions in the dynamics. In this paper we show that phase transitions characterized by universal singularities do occur in the time evolution of random spin chains. The sharpness of the transitions and integrity of the phases owes to many-body localization, which prevents thermalization in these systems. Using a renormalization group approach, we solve the time evolution of random Ising spin chains with generic interactions starting from initial states of arbitrary energy. As a function of the Hamiltonian parameters, the system is tuned through a dynamical transition, similar to the ground state critical point, at which the local spin correlations establish true long range temporal order. As in ground state quantum phase transitions, the dynamical transition has unique signatures in the entanglemenent properties of the system. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y35.00014: Entanglement properties after a partial measurement: a numerical study of excited states in Hubbard-like models James R. Garrison, Ryan V. Mishmash, Tarun Grover, Matthew P.A. Fisher Our growing understanding of entanglement in condensed matter systems continues to provide incredible insight into characterizing phases of matter. Recently, progress in many-body localization (MBL) has revealed a deep connection between the entanglement properties of finite energy density eigenstates and whether or not the state is thermalized. Inspired by developments in MBL as well as a desire to identify and characterize a proposed ``quantum disentangled liquid,'' we have performed exact diagonalization studies on one-dimensional Hubbard-like models. Specifically, we begin with an excited energy eigenstate, perform a partial measurement (e.g., measure the total spin on each site), and study the properties of the resulting wave function. By numerically studying small systems, we can gain insights into whether spin and charge thermalize independently, and develop intuition which may one day guide experiments on cold atom systems. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y35.00015: Many-body Localization with Dipoles Norman Yao, Chris Laumann, Sarang Gopalakrishnan, Michael Knap, Markus Mueller, Eugene Demler, Mikhail Lukin Statistical mechanics is the framework that connects thermodynamics to the microscopic world. It hinges on the assumption of equilibration; when equilibration fails, so does much of our understanding. In isolated quantum systems, this breakdown is captured by the phenomenon known as many-body localization. I will briefly introduce the basic phenomena of many-body localization and review its theoretical status. To date, none of these phenomena has been observed in an experimental system, in part because of the isolation required to avoid thermalization. I will consider several dipolar systems which we believe to be ideal platforms for the realization of MBL phases and for investigating the associated delocalization phase transition. The power law of the dipolar interaction immediately raises the question: can localization in real space persist in the presence of such long-range interactions? I will review and extend several arguments producing criteria for localization in the presence of power laws and present small-scale numerics regarding the MBL transition in several of the proposed dipolar systems. [Preview Abstract] |
Session Y36: Optical Cavites and Optomechanics
Sponsoring Units: DAMOPChair: Kaden Hazzerd, JILA
Room: 703
Friday, March 7, 2014 8:00AM - 8:12AM |
Y36.00001: Dynamics of a ``Nearly Lightless'' Laser Joshua Weiner, Justin Bohnet, Kevin Cox, Matthew Norcia, James Thompson Bad-cavity (superradiant) lasers using highly forbidden atomic transitions are expected to achieve coherence lengths on the order of the earth-sun distance, potentially improving optical atomic clocks and other precision measurements. We have realized a proof-of-principle cold-atom Raman laser operating deep in the superradiant regime, where the effective atomic linewidth is much narrower than the cavity linewidth. Here we present experimental studies of active and passive sensing of external fields with a superradiant laser, relaxation oscillations, and phase synchronization between two spatially distinct ensembles emitting into a single optical cavity. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y36.00002: Cavity cooling of free nanoparticles in high vacuum Peter Asenbaum, Stefan Kuhn, Ugur Sezer, Stefan Nimmrichter, Markus Arndt Cavity cooling has been successfully applied to single atoms, ions and atomic ensembles. It is however, most indispensable for larger and more complex particles, where direct laser cooling techniques are not applicable. We demonstrate cavity cooling of a silicon nanoparticle with a reduction of the transverse kinetic energy by a factor of over 30 [Asenbaum, P. et al. Nat. Commun. 4:2743]. Utilizing a pulsed laser we create and launch silicon nanoparticles beneath a high finesse cavity in high vacuum environment. While the particles transit through the intense cavity field the transverse velocity is reduced. By detecting the scattered light from the particle we can trace its movement in real time. Advancing this technique will be crucial to enable quantum coherence experiments with nanoparticles. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y36.00003: Opto-mechanics with sub-wavelength grating-membranes Haitan Xu, Utku Kemiktarak, Corey Stambaugh, Mathieu Durand, John Lawall, Jacob Taylor We fabricate highly reflective sub-wavelength grating membranes using stoichiometric silicon nitride. We achieve a grating reflectivity of 99.6\% with a membrane mechanical frequency of $\sim$1 MHz. We integrate the grating-membrane into a Fabry-Perot cavity and investigate its opto-mechanical properties. We also consider the prospect of using them for three mode opto-mechanics experiments where the two optical cavity modes are coupled through a mechanical mode. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y36.00004: Quantum nonlinearity near optomechanical instabilities Xunnong Xu, Michael Gullans, Jacob Taylor We show that is possible to realize significant optomechanical nonlinearities at the few quanta level in strongly driven two-mode optomechanical systems. In particular, as the strength of the driving laser increases the energy of one of the optomechanical normal modes approaches zero and the associated harmonic oscillator length becomes large, which leads to an enhanced nonlinear coupling between this mode and the driven mode. This enhances the intrinsic nonlinearity of the optomechanical coupling by an amount scaling with sidebands resolution. We show that this could be measured in two-photon correlations when the system is in the side-band resolved regime with relatively large single-photon optomechanical coupling. These conditions are within the reach of current devices and especially of optomechanical photonic/phononic crystals. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y36.00005: Classical oscillators for understanding quantum descriptions of mechanical systems Chiao-Hsuan Wang, Jacob Taylor Optomechanics has been successfully applied to systems involving wide range of scales from as small as $10^{-21}$ g for atomic level objects like cold atoms to as large as $10^3$ g for macroscopic scale systems like LIGO project. As the size of the mechanical object getting larger, more degrees of freedom come in and the quantum harmonic oscillation treatment of optomechanics becomes questionable. We propose models to show that spring-like classical oscillators may occur at large scale, and they describe methods for distinguishing between quantum harmonic oscillations and other oscillatory behavior. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y36.00006: Quantum synchronization of a driven self-sustained oscillator Christoph Bruder, Andreas Nunnenkamp, Stefan Walter Synchronization is a universal phenomenon that is important both in fundamental studies and in technical applications. Here we investigate synchronization in the simplest quantum-mechanical scenario possible, i.e., a quantum-mechanical self-sustained oscillator coupled to an external harmonic drive [1]. Using the power spectrum we analyze synchronization in terms of frequency entrainment and frequency locking in close analogy to the classical case. We show that there is a step-like crossover to a synchronized state as a function of the driving strength. In contrast to the classical case, there is a finite threshold value in driving. Quantum noise reduces the synchronized region and leads to a deviation from strict frequency locking. [1] S. Walter, A. Nunnenkamp, and C. Bruder, arXiv:1307.7044 [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y36.00007: Oscillator tunneling dynamics in the Rabi model Elinor Irish, Julio Gea-Banacloche The familiar Rabi model (or single-mode spin-boson model), comprising a two-level system coupled to a quantum harmonic oscillator, continues to produce rich and surprising physics when the coupling strength becomes comparable to the individual subsystem frequencies. We construct approximate solutions for the regime in which the oscillator frequency is small compared to that of the two-level system and the coupling strength matches or exceeds the oscillator frequency. Relating our fully quantum calculation to a previous semi-classical approximation, we find that the dynamics of the oscillator can be considered to a good approximation as that of a particle tunneling in a classical double-well potential, despite the fundamentally entangled nature of the joint system. We assess the prospects for observation of oscillator tunneling in the context of nano- or micro-mechanical experiments and find that it should be possible if suitably high coupling strengths can be engineered. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y36.00008: Linear and nonlinear optomechanics in a cryogenic membrane-in-the-middle system Donghun Lee, Mitchell Underwood, David Mason, Alexey Shkarin, Scott Hoch, Jack Harris In cavity optomechanics, linear optomechanical interactions have been used to readout and cool the motion of mechanical oscillators, while nonlinear interactions have been proposed to study quantum non-demolition measurements of mechanical oscillators and the production of non-Gaussian mechanical states. A membrane-in-the-middle system can provide both types of interactions. In this talk, we will present recent results measured in both linear and nonlinear interaction regimes with a membrane-in-the-middle system operating at 500 mK. Linear coupling in this device enables us to cool the mechanical mode of a SiN membrane at 705 kHz to roughly one phonon. During the cooling measurement, we also observed strong asymmetry between the mechanical sidebands, in agreement with the phonon number inferred from other measurements. We also measured nonlinear optomechanics, in particular the quadratic interaction. With a simple theoretical model, we systematically characterized the classical dynamics arising from this quadratic optomechanical interaction. We expect that by combining quadratic coupling with resolved-sideband laser cooling, this device will be able to explore the aforementioned quantum phenomena. We gracefully acknowledge financial support from AFOSR (No. FA9550-90-1-0484). [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y36.00009: Pattern formation in the synchronization dynamics of arrays of optomechanical oscillators Steven Habraken, Roland Lauter, Christian Brendel, Max Ludwig, Florian Marquardt We consider two-dimensional arrays of coupled optomechanical cells, each of which consists of a laser-driven optical cavity interacting with a mechanical (vibrational) mode. The mechanical modes can be driven in self-sustained oscillations. We study the collective classical non-linear dynamics of the phases of these oscillations, which is described by the well-studied Kuramoto model and optomechanical extensions thereof [1]. The model parameters can be tuned by the laser drives. We focus on pattern formation and find that, depending on the parameters, the phases may or may not synchronize in a stationary configuration of vortex-antivortex pairs. We identify a relevant length scale and find hysteresis associated to the synchronization transition. For some model parameters, this length scale becomes comparable to the lattice spacing, in which case the phase configurations develop structure on smaller and smaller scales and eventually settle into random patterns. Besides, we address the stability and time evolution of binary patterns in which all oscillators are initialized to phases of $0$ or $\pi$.\\ \\ $[1]$ G. Heinrich, M. Ludwig, J. Qian, B. Kubala and F. Marquardt, Phys. Rev. Lett. 107, 043603 (2011). [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y36.00010: Sideband Raman Cooling of Optical Phonons in Semiconductors Jun Zhang, Leong Chuan Kwek, Qihua Xiong Last century has witnessed a tremendous success of laser cooling technology from trapped atomic ions to solid-state optical refrigeration[1,2]. As one of the laser cooling techniques, sideband Raman cooling plays an important role in quantum ground state preparation, coherent quantum-state manipulation and quantum phenomena study. However, those studies still limited in trapped atomic ions and cavity optomechanics, which need be cooled it below than 0.1 Kelvin even tens of nano-Kelvin due to very low frequency of phonons from several kHz to GHz. Here we report sideband Raman cooling and heating experiments of longitudinal optical phonon (LOP) with a 6.23 THz in semiconductor ZnTe nano-ribbons[3]. By using of red-sideband laser, we cool the LOP from 225 to 55 Kelvin, corresponding to an average occupation number reduced from 0.36 to 0.005. We also observe a LOPs heating from 230 to 384 Kelvin with a blue-sideband pumping. Our experiment opens a possibility of all solid state quantum applications using semiconductor optical phonon mediated coupling at room temperature. [1] arxiv, 1303.0733v1 (2013); [2] Nature, 493, 504 (2013); [3] J. Zhang, et. al, sideband Raman cooling of optical phonon in semiconductors, (prepared) [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y36.00011: Squeezing of a mechanical resonator Emma Wollman, Chan U Lei, Aaron Weinstein, Junho Suh, Keith Schwab It is well-known that quantum mechanics places limits on the minimum energy of a harmonic oscillator via the ever-present zero point fluctuations of the quantum ground state. Through squeezing, however, it is possible to decrease the noise of a single motional quadrature below the zero point level. While squeezing below the quantum noise level has been achieved with light, squeezing of the motion of a mechanical resonator below its zero-point fluctuations has yet to be realized. A recent proposal by Kronwald, Marquardt, and Clerk (1) suggests a method of squeezing a single quadrature of the mechanics more than 3dB below the level of its zero point fluctuations. Such squeezing is achievable even if the resonator starts in a thermal state with occupation well above the ground state. We present phase-dependent measurements showing squeezing of mechanics approaching the quantum limit.\\ \\ (1) A. Kronwald, F. Marquardt, and A.A. Clerk. arXiv:1307.5309 (2013). [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y36.00012: Trampoline Resonator Fabrication for Tests of Quantum Mechanics at High Mass Matthew Weaver, Brian Pepper, Petro Sonin, Hedwig Eerkens, Frank Buters, Sven de Man, Dirk Bouwmeester There has been much interest recently in optomechanical devices that can reach the ground state. Two requirements for achieving ground state cooling are high optical finesse in the cavity and high mechanical quality factor. We present a set of trampoline resonator devices using high stress silicon nitride and superpolishing of mirrors with sufficient finesse (as high as 60,000) and quality factor (as high as 480,000) for ground state cooling in a dilution refrigerator. These devices have a higher mass, between 80 and 100 ng, and lower frequency, between 200 and 500 kHz, than other devices that have been cooled to the ground state, enabling tests of quantum mechanics at a larger mass scale. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y36.00013: Time-Continuous Bell Measurements Sebastian G. Hofer, Denis V. Vasilyev, Markus Aspelmeyer, Klemens Hammerer We combine the concept of Bell measurements, in which two systems are projected into a maximally entangled state, with the concept of continuous measurements, which concerns the evolution of a continuously monitored quantum system. For such time-continuous Bell measurements we derive the corresponding stochastic Schr\"odinger equations, as well as the unconditional feedback master equations. Our results apply to a wide range of physical systems, and are easily adapted to describe an arbitrary number of systems and measurements. Time-continuous Bell measurements therefore provide a versatile tool for the control of complex quantum systems and networks. As examples we show show that (i) two two-level systems can be deterministically entangled via homodyne detection, tolerating photon loss up to 50\%, and (ii) a quantum state of light can be continuously teleported to a mechanical oscillator, which works under the same conditions as are required for optomechanical ground state cooling. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y36.00014: Real-Space Tailoring of the Electron-Phonon Coupling in Ultra-Clean Nanotube Mechanical Resonators Avishai Benyamini, Assaf Hamo, Silvia Viola Kusminskiy, Felix von Oppen, Shahal Ilani The coupling between electrons and phonons is at the heart of many fundamental phenomena in nature. Despite tremendous advances in controlling electrons or phonons in engineered nanosystems, the control over their coupling is still widely lacking. Here we demonstrate the ability to fully tailor electron-phonon interactions using a new class of suspended carbon nanotube devices, in which we can form highly-tunable single and double quantum dots at arbitrary locations along a nanotube mechanical resonator. We find that electron-phonon coupling can be turned on and off by controlling the position of a quantum dot along the resonator. Using double quantum dots we structure the interactions in real-space to couple specific electronic and phononic modes. This tailored coupling allows measurement of the phonons' spatial parity and imaging of their mode shapes. Finally, we demonstrate coupling between phonons and internal electrons in an isolated system, decoupled from the random environment of the electronic leads, a crucial step towards fully-engineered quantum-coherent electron-phonon systems. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y36.00015: Detection of the mechanical motion of a carbon nanotube resonator by an adjacent nanotube single-electron-transistor Assaf Hamo, Avishai Binyamini, Shahal Ilani, Felix von Oppen In recent years the detection of nano-mechanical motion of carbon nanotubes has made substantial progress. This enabled the measurement of mechanical coupling to single electrons, improved mass detection to the level of an individual proton, and improved force detection to extremely tiny forces. In all these experiments the nanotube was used both as the mechanical resonator and the detector of its own motion, requiring the nanotube to be in a conducting state and reducing the detection sensitivity due to back-action and mechanical nonlinearities. Here, we demonstrate a detection scheme using a separate detector based on a second nanotube single-electron-transistor, eliminating these limitations. The separation of the detector and the mechanical resonator in our system opens the way to investigation of new nano-mechanical phenomena, inaccessible to date. [Preview Abstract] |
Session Y37: Carbon Nanotube Optical Properties
Sponsoring Units: DCMPChair: Feng Wang, University of California, Berkeley
Room: 705/707
Friday, March 7, 2014 8:00AM - 8:12AM |
Y37.00001: Electron temperature dependence of the electron-phonon coupling strength in double-wall carbon nanotubes Ioannis Chatzakis We apply Time-Resolved Two-Photon Photoemission spectroscopy to probe the electron-phonon (e-ph) coupling strength in double-wall carbon nanotubes. The e-ph energy transfer rate G(T$_{\mathrm{e}}$, T$_{\mathrm{l}})$ from the electronic system to the lattice depends linearly on the electron (T$_{\mathrm{e}})$ and lattice (T$_{\mathrm{l}})$ temperatures for T$_{\mathrm{e}}$ \textgreater $\Theta_{\mathrm{Debye}}$. Moreover, we numerically solved the Two-Temperature Model. We found: (i) a T$_{\mathrm{e}}$ decay with a 3.5 ps time constant and no significant change in T$_{\mathrm{l}}$; (ii) an e-ph coupling factor of 2 $\times$ 10$^{16}$ W/m$^{3}$; (iii) a mass-enhancement parameter, $\lambda $, of (5.4 $\pm$ 0.9) $\times$ 10$^{-4}$; and (iv) a decay time of the electron energy density to the lattice of 1.34 $\times$ 0.85 ps. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y37.00002: Possibility for exciton Bose-Einstein condensation in carbon nanotubes Igor Bondarev, Areg Meliksetyan We demonstrate theoretically a possibility for exciton Bose-Einstein condensation (BEC) in individual small-diameter ($\sim $1-2 nm) semiconducting carbon nanotubes [1]. The effect occurs under the exciton-interband plasmon coupling controlled by an external electrostatic field applied perpendicular to the nanotube axis. The effect requires fields $\sim $1 V/nm and temperatures below 100 K that are experimentally accessible. The effect offers a testing ground for fundamentals of condensed matter physics in one dimension and opens up perspectives to develop tunable highly coherent polarized light source with carbon nanotubes. Possibilities for achieving BEC in 1D and 2D systems are theoretically demonstrated in the presence of an extra confinement potential [2]. We show that the strongly correlated exciton-plasmon system in a carbon nanotube presents such a special case. We find the critical BEC temperature and the condensate fraction as functions of temperature and electrostatic field applied. We discuss how the effect can be observed experimentally.\\[4pt] [1] I.V.Bondarev, A.V.Meliksetyan, arXiv1304.2804 (submitted to PRB);\\[0pt] [2] V.Bagnato, D.Kleppner, PRA 44, 7439; W.-S.Dai, M.Xie, PRA 67, 027601. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y37.00003: Band Gap Renormalization in Semiconducting Carbon Nanotubes Nicholas Lanzillo, Neerav Kharche, Saroj Nayak We use first-principles density functional theory (DFT) in conjunction with the GW Approximation to show that the presence of a dielectric substrate is found to alter the electronic properties of semiconducting carbon nanotubes. In particular, a substrate-induced polarization effect stabilizes the correlation energy in both the valence band maximum and the conduction band minimum, resulting in a decrease in the electronic band gap. This effect is due to non-local dielectric screening which can be described accurately through the GW Approximation but is not captured in DFT alone. While similar band gap renormalization effects have been observed for isolated molecules and even for two-dimensional materials on substrates, this is the first prediction of such an effect in a strictly one-dimensional geometry. We find that the magnitude of the band gap reduction is on the order of 0.5 eV when deposited on a hexagonal boron nitride substrate. This type of band gap modulation is of great importance in developing future opto-electronic devices. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y37.00004: Spontaneous exciton dissociation in carbon nanotubes Masahiro Yoshida, Yusuke Kumamoto, Akihiro Ishii, Akio Yokoyama, Takashi Shimada, Yuichiro K. Kato Simultaneous photoluminescence and photocurrent measurements on individual single-walled carbon nanotubes reveal spontaneous dissociation of excitons into free electron-hole pairs.\footnote{Y. Kumamoto \textit{et al.}, arXiv:1307.5159 (2013).} A simple model is constructed to consistently describe the excitation power and voltage dependence of the photoluminescence and photocurrent. Using this model, we find that a significant fraction of excitons are dissociating before recombination. Furthermore, the combination of optical and electrical signals also allows for extraction of the absorption cross section and the oscillator strength. Our observations explain the reasons for photoconductivity measurements in single-walled carbon nanotubes being straightforward despite the large exciton binding energies. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y37.00005: Binding energy of the trion complex in carbon nanotubes Areg Meliksetyan, Igor Bondarev We derive an analytical expression for the binding energy of the trion complex (charged exciton) in small diameter ($\sim $1nm) carbon nanotubes. We use the (asymptotically exact) Landau-Herring approach [1,2] that was previously implemented by one of us (Ref.[3]) to evaluate the biexciton binding energy in carbon nanotubes. Within this approach, we find the universal asymptotic relationship between the trion, biexciton and exciton binding energies in the same carbon nanotube. Particularly, the trion binding energy we obtained is estimated to be greater than the corresponding biexciton binding energy by a factor $\sim $1.5 for carbon nanotubes with diameters $\sim $1nm, which reasonably agrees with the latest non-linear optical spectroscopy measurements reported in Refs.[4] and [5] (1.46 for the (6,5) nanotube and 1.42 for the (9,7) nanotube, respectively).\\[4pt] [1] L.D.Landau, E.M.Lifshitz, Quantum Mechanics (Pergamon, Oxford, 1991);\\[0pt] [2] C.Herring, Rev. Mod. Phys. 34, 631;\\[0pt] [3] I.V.Bondarev, PRB 83, 153409;\\[0pt] [4] B.Yuma et al., PRB 87, 205412;\\[0pt] [5] L.Colombier et al., PRL 109, 197402. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y37.00006: Photoluminescence Imaging of Oxygen Doped Individual Single-Walled Carbon Nanotubes Sibel Ebru Yalcin, Hisato Yamaguchi, Charudatta Galande, Jared J. Crochet, Aditya D. Mohite, Gautam Gupta, Xuedan Ma, Han Htoon, Stephen K. Doorn Semiconducting single-walled carbon nanotubes (SWNTs) are attractive candidates for near-IR optoelectronic applications. But they show low fluorescence quantum yield. Recent oxygen doping studies have shown that the quantum yield of the excitons can be enhanced by an order of magnitude due to the formation of local 0D sites on the SWNT surface. However, these studies have been limited to ensemble measurements. Understanding the dopant site, exciton migration and trapping dynamics on individual SWNTs is critical for controllably tuning the photo-physical behavior. We have studied ozonated individual (6,5) nanotubes as a function of progressive ozonation. We spatially resolved the pristine and doped state using visible and NIR sensitive cameras. We demonstrate PL imaging as a probe of the emission dynamics as a function of dopant concentration. The spectral studies show the red-shifted emission in the PL of the NTs due to the ozonated site. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y37.00007: Charge Generation in Photoexcited Large-Diameter Semiconducting Single-Walled Carbon Nanotube/Fullerene Blends Kevin Mistry, Bryon Larson, Nikos Kopidakis, Garry Rumbles, Jeffrey Blackburn Semiconducting single-walled carbon nanotubes (s-SWCNTs) have a number of extraordinary electrical and optical properties including high charge mobilities and tunable band gaps that make them appealing for FETs and PV devices. Using narrow chiral distributions of large-diameter (d \textgreater 1.2 nm) s-SWCNTs can be beneficial to these devices through increased carrier mobility and reduced trapping due to energetic differences between different chiralities. Additionally, much of the visible and near-IR region of the solar spectrum can be covered by appropriately tuning the diameter range of these s-SWCNTs along with careful selection of the fullerene acceptor material. Time-resolved microwave conductivity (TRMC) was used to explore charge separation in such s-SWCNT:fullerene (donor:acceptor) blends. TRMC allows for sensitive monitoring of charge generation and decay in response to photoexcitation. We will report on charge carrier lifetime dynamics and changes to free carrier yield in blends using carefully tuned combinations of SWCNT diameters and fullerene acceptors. Furthermore, we will discuss how these results can be used to design enhanced s-SWCNT:fullerene active layers. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y37.00008: Accessing the Dark Exciton States in Semiconducting Single-Walled Carbon Nanotubes with Terahertz Pulses Liang Luo, Ioannis Chatzakis, Jigang Wang Singled-walled carbon nanotubes (SWNTs) represent a model system to systematically investigate correlated charge excitation in 1-D limits. One of the most outstanding issues both in fundamental nanotube physics and for their technological development is to detect and understand optically-forbidden, dark collective states. Thus far supporting evidence of dark states has been demonstrated in static magneto-optics and light scattering. However, the unique internal transitions from dark excitonic ground states and their dynamic evolution remain highly elusive. We report our investigation of this problem using optical pump, terahertz probe spectroscopy of (6,5) and (7,5) SWNTs. We measure transient THz conductivity from 0.5-2.5 THz (2-10.5 meV) at low temperature down to 5 K with resonant and off-resonant excitation at the E$_{22}$ transitions of (6,5) and (7,5) nanotubes. These results reveal, for the first time, dynamics of lowest dark excitons and density-dependent renormalization of these many-particle states. The internal-excitonic spectroscopy with THz pulses represents a fundamentally new spectroscopy tools to study dark excitons and shine new lights on the correlation physics of excitonic ground states. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y37.00009: Photoluminescence microscopy on air-suspended carbon nanotubes coupled to photonic crystal nanobeam cavities R. Miura, S. Imamura, T. Shimada, R. Ohta, S. Iwamoto, Y. Arakawa, Y.K. Kato Because carbon nanotubes are room-temperature telecom-band emitters and can be grown on silicon substrates, they are ideal for coupling to silicon photonic cavities.\footnote{R. Watahiki et al., Appl. Phys. Lett. 101, 141124 (2012).}$ ^{,}$\footnote{S. Imamura et al., Appl. Phys. Lett. 102, 161102 (2013).} In particular, as-grown air-suspended carbon nanotubes show excellent optical properties, but cavity modes with large fields in the air are needed in order to achieve efficient coupling. Here we investigate individual air-suspended nanotubes coupled to photonic crystal nanobeam cavities. We utilize cavities that confine air-band modes which have large fields in the air. Dielectric mode cavities are also prepared for comparison. We fabricate the devices from silicon-on-insulator substrates by using electron beam lithography and dry etching to form the nanobeam structure. The buried oxide layer is removed by wet etching, and carbon nanotubes are grown onto the cavities by chemical vapor deposition. We perform photoluminescence imaging and excitation spectroscopy to find the positions of the nanotubes and identify their chiralities. For both types of devices, cavity modes with quality factors of $\sim$3000 are observed within the nanotube emission peak. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y37.00010: Alternating gate-voltage effects on photoluminescence of air-suspended carbon nanotubes M. Jiang, Y. Kumamoto, A. Ishii, M. Yoshida, Y.K. Kato Gate voltages cause quenching of photoluminescence in carbon nanotubes through phase-space filling and doping-induced exciton relaxation.\footnote{S. Yasukochi et al., Phys. Rev. B 84, 121409(R) (2011).} In this work, we apply square-wave voltages to partially gated nanotubes and find that such quenching can be eliminated at high frequencies. The devices are fabricated on a silicon-on-insulator substrate and we start by etching trenches through the top silicon layer into the buried oxide. The top silicon layer is thermally oxidized for use as the gate and we form an electrode on one side of the trench. From catalyst particles placed on the electrode, nanotubes are grown over the trench onto the gate. For square-wave voltages at low frequencies, photoluminescence quenching occurs as expected. When the frequency becomes higher, we observe that emission increases linearly and saturates above a threshold frequency. Time-averaging of the voltage cannot explain such an increase, as it also occurs when offset voltages are added to the square-wave. Furthermore, the threshold frequency increases as the excitation laser power is turned up. These observations could be explained by a model in which photocarriers are stored by the gate fields and voltage switching induces light emission. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y37.00011: Diffusion-related exciton decay processes in air-suspended single-walled carbon nanotubes studied by photoluminescence microscopy A. Ishii, M. Yoshida, Y.K. Kato In carbon nanotubes, exciton diffusion causes complex photoluminescence properties through end quenching and exciton-exciton annihilation. In order to clarify the effects of these processes in air-suspended carbon nanotubes, where they are isolated from the surroundings, we perform photoluminescence measurements on over a hundred individual nanotubes. Nanotube length dependence is investigated by measuring emission from nanotubes suspended over trenches with various widths\footnote{S. Moritsubo \textit{et al.}, Phys. Rev. Lett. \textbf{104}, 247402 (2010).} and excitation power dependence is also investigated on each nanotube. We analyze the results by calculating the effects of end quenching as a function of the tube length using a first-passage approach.\footnote{M. D. Anderson \textit{et al.}, Phys. Rev. B \textbf{88}, 045420 (2013).} At low excitation powers where the exciton-exciton annihilation is negligible, this model gives intrinsic exciton diffusion lengths and relative values of photoluminescence action cross section. For higher excitation powers, Monte Carlo simulations are used to quantitatively evaluate the exciton-exciton annihilation rates and spatial profiles of the exciton density. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y37.00012: Atomic and Excitonic Stability in Dirac Materials: A White Dwarf Perspective Kirill Velizhanin Dirac materials - systems where the low-energy spectrum of electronic excitations can be understood via solving the Dirac equation - draw a great amount of attention of the scientific community lately due to their enormous application potential and interesting basic physics. Examples of such materials include carbon nanotubes, graphene and, more recently, single-layer transition metal dichalcogenides. One surprising application of Dirac materials is their use as a platform to simulate various atomic and high-energy physics ``on a chip.'' For example, graphene has been recently used to ``mimic'' an atomic collapse of superheavy atoms [Y. Wang et al, Science, 340, 734, 2013]. In this talk I will discuss an unexpected similarity between atomic and excitonic collapse in Dirac materials and the limit of stability of such exotic astrophysical objects as degenerate stars (e.g., white dwarfs, neutron stars). Various aspects of this similarity, e.g., an application of the concept of the Chandrasekhar limit to the exciton stability in transition metal dichalcogenides, will be discussed. [Preview Abstract] |
Session Y38: Invited Session: Quantitative Biomedical Imaging
Sponsoring Units: GIMSChair: Ron Goldfarb, National Institute of Standards and Technology, Boulder
Room: 709/711
Friday, March 7, 2014 8:00AM - 8:36AM |
Y38.00001: Quantitative Magnetic Resonance Imaging and Phantom Development Invited Speaker: Kathryn Keenan Magnetic Resonance Imaging (MRI) uses strong magnetic fields and radiofrequency pulses to produce images of proton locations and properties. Image contrast reflects relative density of excited water protons, differences in relaxation times of water protons due to surrounding structure, and the sequence of RF pulses used to excite the water protons. MRI can be used to quantitatively measure longitudinal (T1) and transverse (T2) spin relaxation times, measure tissue volumes, track motion of water molecules (flow/diffusion), measure temperature, assess susceptibility differences, create maps of tissue electrical properties, etc. This talk will focus on quantitative measurement of relaxation times, diffusion and electrical properties. Diffusion MRI varies the homogeneous magnetic field using an initial gradient, followed by a refocusing gradient with the same magnitude with opposite direction: protons begin to precess at different rates, depending on the applied gradient, and will disperse. The refocusing gradient cannot refocus spins that have moved between gradient pulses, and the apparent proton diffusion can be calculated from the signal attenuation. Typically, gradient pulses are applied in three orthogonal directions to calculate a bulk diffusion coefficient. Tissue electrical properties can be mapped by measuring the complex RF transmit and receive fields (B1$+$, B1-). New methods estimate local electrical conductivity from \textit{in vivo} B1$+$ phase measurements based on the homogeneous Helmholtz equation. Quantitative relaxation measurements, diffusion and electrical properties can distinguish healthy tissue from malignant tumor from benign tumor or identify the time of a particular event, e.g. a stroke. In this talk, I will describe how the NIST system, diffusion, and breast phantoms help validate these important measurements. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 9:12AM |
Y38.00002: Quantitative Ultrasound: Transition from the Laboratory to the Clinic Invited Speaker: Timothy Hall There is a long history of development and testing of quantitative methods in medical ultrasound. From the initial attempts to scan breasts with ultrasound in the early 1950's, there was a simultaneous attempt to classify tissue as benign or malignant based on the appearance of the echo signal on an oscilloscope. Since that time, there has been substantial improvement in the ultrasound systems used, the models to describe wave propagation in random media, the methods of signal detection theory, and the combination of those models and methods into parameter estimation techniques. One particularly useful measure in ultrasonics is the acoustic differential scattering cross section per unit volume in the special case of the 180$^{\circ}$ (as occurs in pulse-echo ultrasound imaging) which is known as the backscatter coefficient. The backscatter coefficient, and parameters derived from it, can be used to objectively measure quantities that are used clinically to subjectively describe ultrasound images. For example, the ``echogenicity'' (relative ultrasound image brightness) of the renal cortex is commonly compared to that of the liver. Investigating the possibility of liver disease, it is assumed the renal cortex echogenicity is normal. Investigating the kidney, it is assumed the liver echogenicity is normal. Objective measures of backscatter remove these assumptions. There is a 30-year history of accurate estimates of acoustic backscatter coefficients with laboratory systems. Twenty years ago that ability was extended to clinical imaging systems with array transducers. Recent studies involving multiple laboratories and a variety of clinical imaging systems has demonstrated system-independent estimates of acoustic backscatter coefficients in well-characterized media (agreement within about 1.5dB over about a 1-decade frequency range). Advancements that made this possible, transition of this and similar capabilities into medical practice and the prospects for quantitative image-based biomarkers will be discussed. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:48AM |
Y38.00003: High Resolution Peripheral Quantitative Computed Tomography for Assessment of Bone Quality Invited Speaker: Galateia Kazakia The study of bone quality is motivated by the high morbidity, mortality, and societal cost of skeletal fractures. Over 10 million people are diagnosed with osteoporosis in the US alone, suffering 1.5 million osteoporotic fractures and costing the health care system over {\$}17 billion annually. Accurate assessment of fracture risk is necessary to ensure that pharmacological and other interventions are appropriately administered. Currently, areal bone mineral density (aBMD) based on 2D dual-energy X-ray absorptiometry (DXA) is used to determine osteoporotic status and predict fracture risk. Though aBMD is a significant predictor of fracture risk, it does not completely explain bone strength or fracture incidence. The major limitation of aBMD is the lack of 3D information, which is necessary to distinguish between cortical and trabecular bone and to quantify bone geometry and microarchitecture. High resolution peripheral quantitative computed tomography (HR-pQCT) enables in vivo assessment of volumetric BMD within specific bone compartments as well as quantification of geometric and microarchitectural measures of bone quality. HR-pQCT studies have documented that trabecular bone microstructure alterations are associated with fracture risk independent of aBMD.... Cortical bone microstructure -- specifically porosity -- is a major determinant of strength, stiffness, and fracture toughness of cortical tissue and may further explain the aBMD-independent effect of age on bone fragility and fracture risk. The application of finite element analysis (FEA) to HR-pQCT data permits estimation of patient-specific bone strength, shown to be associated with fracture incidence independent of aBMD. This talk will describe the HR-pQCT scanner, established metrics of bone quality derived from HR-pQCT data, and novel analyses of bone quality currently in development. Cross-sectional and longitudinal HR-pQCT studies investigating the impact of aging, disease, injury, gender, race, and therapeutics on bone quality will be discussed. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:24AM |
Y38.00004: Quantitative Magnetic Resonance Thermometry and Its Use with MR-Guided Focused Ultrasound Invited Speaker: Kim Pauly Focused ultrasound (FUS) uses a large area array, typically outside the body, that is geometrically or electronically focused to a point deep in the body. Such focusing provides amplification of the ultrasound intensity, thereby allowing heating of tissue to the point of coagulation at the focus, without damage to the intervening tissue. Guidance of FUS treatments deep in the body can be done quantitatively with magnetic resonance (MR) thermometry, termed MRgFUS. The physics behind MR thermometry lie in the changes in hydrogen bonding with temperature. As tissue temperature rises, hydrogen bonds break, allowing the return of the electron cloud to shield water protons, reducing the magnetic field seen by the protons, and the resonant frequency. The change in resonant frequency is -0.01 ppm per degree C and is the same for all aqueous tissues. The result of the shift in proton resonant frequency is seen in the phase of gradient echo images. Subtraction of the phase of images acquired before and during heating allows the removal of background phase from other sources, yielding quantitative temperature maps. Temperature standard deviations less than 1 degree C are readily achievable and thermal dose maps are easily calculated. Thermal dose is found from a conversion of the whole temperature-time curve to an equivalent number of minutes at 43 degrees C. A thermal dose of 240 minutes is often taken as the threshold for tissue damage. MR thermometry is complicated by the motion of the target tissue and/or motion of other organs such as occurs during respiration. More sophisticated algorithms than the simple baseline subtraction take advantage of the facts that motion can be repetitive (in the case of respiratory motion) and/or the fact that the focal region in MRgFUS is small, allowing for extraction of the heat from the phase profile without subtraction of a background phase. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 11:00AM |
Y38.00005: Development of Traceable Phantoms for Improved Image Quantification in Positron Emission Tomography Invited Speaker: Brian Zimmerman Clinical trials for new drugs increasingly rely on imaging data to monitor patient response to the therapy being studied. In the case of radiopharmaceutical applications, imaging data are also used to estimate organ and tumor doses in order to arrive at the optimal dosage for safe and effective treatment. Positron Emission Tomography (PET) is one of the most commonly used imaging modalities for these types of applications. In large, multicenter trials it is crucial to minimize as much as possible the variability that arises due to use of different types of scanners and other instrumentation so that the biological response can be more readily evaluated. This can be achieved by ensuring that all the instruments are calibrated to a common standard and that their performance is continuously monitored throughout the trial. Maintaining links to a single standard also enables the comparability of data acquired on a heterogeneous collection of instruments in different clinical settings. As the standards laboratory for the United States, the National Institute of Standards and Technology (NIST) has been developing a suite of phantoms having traceable activity content to enable scanner calibration and performance testing. The configurations range from small solid cylindrical sources having volumes from 1 mL to 23 mL to large cylinders having a total volume of 9 L. The phantoms are constructed with $^{\mathrm{68}}$Ge as a long-lived substitute for the more clinically useful radionuclide $^{\mathrm{18}}$F. The contained activity values are traceable to the national standard for $^{\mathrm{68}}$Ge and are also linked to the standard for $^{\mathrm{18}}$F through a careful series of comparisons. The techniques that have been developed are being applied to a variety of new phantom configurations using different radionuclides. Image-based additive manufacturing techniques are also being investigated to create fillable phantoms having irregular shapes which can better mimic actual organs and tumors while still maintaining traceability back to primary standards for radioactivity. This talk will describe the methods used to construct, calibrate, and characterize the phantoms, focusing on the preservation of the traceability link to the primary standards of the radionuclides used. The on-going development of specialized traceable phantoms for specific organ dosimetry applications and imaging physics studies will also be discussed. [Preview Abstract] |
Session Y39: Invited Session: Emergent States Driven by Spin-Orbit Coupling
Sponsoring Units: DCMPChair: Tanmoy Das, Los Alamos National Laboratory
Room: Mile High Ballroom 2A-3A
Friday, March 7, 2014 8:00AM - 8:36AM |
Y39.00001: Novel 2D electron gases at the surface of transition-metal oxides: role of topology and spin-orbit coupling Invited Speaker: Andr\'es F. Santander-Syro Transition-metal oxides (TMOs) are correlated-electron systems with remarkable properties, such as high-temperature superconductivity or large magnetoresistance. The realization of two-dimensional electron gases (2DEGs) at surfaces or interfaces of TMOs, a field of current active research, is crucial for harnessing the functionalities of these materials for future applications. Additionally, these 2DEGs offer the possibility to explore new physics emerging from the combined effects of electron correlations and low-dimensional confinement. Recently, we discovered that a 2DEG can be simply realized at the vacuum-cleaved surface of SrTiO$_{\mathrm{3}}$, a transparent, insulating TMO with a gap of 3.5 eV. We directly imaged its multiple heavy and light subbands using angle-resolved photoemission spectroscopy [A. F. Santander-Syro \textit{et al}., Nature \textbf{469}, 189 (2011)]. In this talk, I will show that one can also create and tailor 2DEGs in other TMO surfaces, opening vast possibilities for the study of correlations in low dimensions in materials showing diverse functionalities. I will first discuss the specific case of KTaO$_{\mathrm{3}}$, a wide-gap insulator with a spin-orbit coupling 30 times larger than in SrTiO$_{\mathrm{3}}$. I will show that quasi-2D confinement in this system results in comparable scales for the Fermi energy, the subband splitting, and the spin-orbit coupling, leading to a complete reconstruction of the orbital symmetries and band masses [A. F. Santander-Syro \textit{et al}., Phys. Rev. B \textbf{86}, 121107(R) (2012)]. Then, I will show that by choosing various surface terminations of different symmetries one can modify the electronic structure of the 2DEGs at the surface of TMOs [C. Bareille \textit{et al}., submitted (2013); T. R\"{o}del \textit{et al}., submitted (2013)]. All these results demonstrate that, in TMOs, the strong correlations, together with the electron confinement and the surface-lattice symmetry, can lead to novel states at the surface that are not simple extensions of the bulk bands. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 9:12AM |
Y39.00002: Topological Insulators, Semi-Metals and Superconductors From First Principles Electronic Structure Calculations Invited Speaker: Sergey Savrasov Using first-principles electronic structure calculations we investigate novel phases that emerge from the interplay of electron correlations, strong spin-orbit coupling and electron-phonon interactions. We first [1] focus on describing the topological semimetal, a three-dimensional phase of a magnetic solid, and argue that it may be realized in a class of pyrochlore iridates (such as Y2Ir2O7) based on calculations using the LDA $+$ U method. This state is a three-dimensional analog of graphene with linearly dispersing excitations and provides a condensed-matter realization of Weyl fermions that obeys a two-component Dirac equation. It also exhibits remarkable topological properties manifested by surface states in the form of Fermi arcs, which are impossible to realize in purely two-dimensional band structures. We second [2] predict that osmium compounds such as CaOs2O4 and SrOs2O4 can be stabilized in the geometrically frustrated spinel crystal structure. They show ferromagnetic order in a reasonable range of the on-site Coulomb correlation U and exotic electronic properties, in particular, a large magnetoelectric coupling characteristic of axion electrodynamics. Finally, the issue of topological superconductivity and the possibility of the odd pairing will be discussed in Cu doped Bi2Te3 materials where electron-phonon coupling constant is calculated for various pairing symmetries using density functional linear response approach [3].\\[4pt] [1] Xiangang Wan, Ari Turner, Ashvin Vishwanath, Sergey Y. Savrasov, \textit{Phys. Rev. }B \textbf{83}, 205101 (2011);\\[0pt] [2] Xiangang Wan, Ashvin Vishwanath, and Sergey Y. Savrasov, \textit{Phys. Rev Lett.} \textbf{108}, 146601 (2012);\\[0pt] [3] Xiangang Wan, and Sergey Y. Savrasov, arXiv:1308.5615. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:48AM |
Y39.00003: Role of local orbital angular momentum in the electronic structure under inversion symmetry breaking Invited Speaker: Changyoung Kim Orbital angular momentum (OAM), usually ignored in solids (OAM quenching), is found to play an important role in the electronic structure for a broad range of materials [1,2]. It will be shown how one can detect existence of OAM by using circular dichroism angle resolved photoemission (CD-ARPES). CD-ARPES is used to study a topological insulator Bi2Se3. The result reveals that not only spins but also OAM forms chiral structure, and the energy scale is mostly determined by the interaction of asymmetric charge distribution and electric field. This observation is different from the conventional understanding of the Rashba effect and suggests that we must consider a new effective Hamiltonian based on OAM. The new Hamiltonian along with crystal field and atomic spin-orbit coupling (SOC) should determine the surface electronic structures. We further investigated surface states with various atomic SOC strengths to study such mechanism. It is shown experimentally and theoretically that, as the atomic SOC strength decreases, the OAM in different Rashba bands change from anti-parallel to parallel configuration. The split energy in such case is found to come from atomic SOC (parallel and anti-parallel alignment of the spin to OAM). Local OAM is found to play an important role in the electronic structure for a wide range of materials with inversion symmetry breaking. We will show some examples where OAM is quite strong, such as semi-conductors with zinc blende structure and transition metal oxide surface states.\\[4pt] [1] S. R. Park, Phys. Rev. Lett.,108, 046805 (2012).\\[0pt] [2] S. R. Park, Phys. Rev. Lett.,107, 0156803 (2011). [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:24AM |
Y39.00004: Emergence of a Robust Dirac Cone from Rashba-Split Surface States on a Topological Semimetal Invited Speaker: Anjan Soumyanarayanan Topological materials (TMs) host protected surface states that emerge from strong spin orbit coupling in the bulk and on the surface. While their remarkable properties have generated much interest, previous studies have shown nanoscale variations in their surface state properties [1,2], prompting the development of a nanoscale band structure probe. Here we report the simultaneous observation and quantitative reconciliation of Landau quantization and quasiparticle interference phenomena on the topological semimetal Sb, which we employ to reconstruct its multi-component surface state band structure[3]. We thereby establish the technique of \textbf{band structure tunneling microscopy (BSTM)}, and utilize it to elucidate the relationship between bulk conductivity and surface state robustness, and to quantify essential metrics for spintronics applications. Meanwhile, our Landau quantization results help us visualize the evolution of quasiparticle behavior from massive Rashba to massless Dirac character, and determine the surface state $g$-factor. \\[2ex] [1] H. Beidenkopf \emph{et al.}, Nature Physics 7, 939 (2011)\\[0ex] [2] Y. Okada \emph{et al.}, Nature Communications 3, 1158 (2012)\\[0ex] [3] A. Soumyanarayanan \emph{et al.}, arxiv:1311.1758 (2013) [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 11:00AM |
Y39.00005: Spin orbit density wave: A non-magnetic phase of two-dimensional electron gas Invited Speaker: Tanmoy Das We propose and formulate an interaction induced staggered spin-orbit order as a new emergent phase of two-dimensional Fermi gases. We show that when some form of inherent spin splitting via Rashba-type spin-orbit coupling renders two helical Fermi surfaces to become significantly ``nested,'' a Fermi surface instability arises. To lift this degeneracy, a spontaneous symmetry-breaking spin-orbit density wave develops, causing a surprisingly large quasiparticle gapping with chiral electronic states. Since the time-reversal invariant spin-orbit density wave is associated with a condensation energy, quantified by the gap value, destroying such spin-orbit interaction costs sufficiently large magnetic field or temperature or dephasing time. The BiAg$_{\mathrm{2}}$ surface state is shown to be a representative system for realizing such novel spin-orbit interaction. We also apply this theory to LAO/STO interface, Iridates compunds, and in the enigmatic `hidden-order' phase in URu$_{\mathrm{2}}$Si$_{\mathrm{2}}$ T. Das. Phys. Rev. Lett. \textbf{109}, 246406 (2012). [Preview Abstract] |
Session Y40: Invited Session: New Views of Thermal Transport
Sponsoring Units: DCMPChair: Barry Zink, University of Denver
Room: Mile High Ballroom 2B-3B
Friday, March 7, 2014 8:00AM - 8:36AM |
Y40.00001: Heat under the microscope: towards a microscopic understanding of thermal transport in solids Invited Speaker: Austin Minnich The thermal conductivity of a solid is typically the observable quantity used to study heat conduction. While knowledge of thermal conductivity is important, a wealth of microscopic information remains obscured because the thermal conductivity represents an average over a thermal distribution. Recent efforts have demonstrated that this microscopic information, in the form of the phonon mean free path distribution, can in fact be measured directly by systematically observing the transition from the diffusive to the ballistic transport regimes. In this talk, I will describe our recent efforts to develop this thermal conductivity spectroscopy technique as a general tool, as well as how the technique is giving a detailed picture of phonon heat conduction in a variety of solids. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 9:12AM |
Y40.00002: Nanophononics at low temperature: manipulating heat at the nanoscale Invited Speaker: Olivier Bourgeois Nanophononics is an emerging field of condensed matter that deals with transport of thermal phonons at small length scales. When the section of a waveguide becomes smaller than the mean free path or the phonon wavelength, heat transfer are strongly affected. Here, I will present the results we obtained by ultra- sensitive measurements of thermal conductance of suspended nano-objects (nanowires and membranes) using the 3$\omega$ method. This experimental set-up allows the measurement of power as small as a fraction of femtoWatt (10$^{-15}$ Watt). These experiments show that the concepts of mean free path and dominant wavelength are crucial to understand the phonon thermal transport below 10K. The phonon transport, at this temperature, is well described by the Casimir-Ziman model used here to treat the data. The contribution of the thermal contact between a nanowire and the heat bath has been estimated to be close to one, thanks to the fact that the nanowire are made out of monolithic single crystal. Strong reduction of thermal conductance has been obtained in serpentine nanowire where the transport of ballistic phonons is blocked. Moreover, in corrugated silicon nanowire, we showed that the corrugations induce significant backscattering of phonon that severely reduces the mean free path, beating in some cases, the Casimir limit. These experiments demonstrate the ability to manipulate ballistic phonons by adjusting the geometry of thermal conductors, and hence manipulate heat transfer. Finally, the use of these new concepts of engineering ballistic phonons at the nanoscale allows considering the development of new nanostructured materials for thermoelectrics at room temperature, opening exciting prospects for future applications in the energy recovery. J.-S. Heron, T. Fournier, N. Mingo and O. Bourgeois, Nano Letters 9, 1861 (2009). J-S. Heron, C. Bera, T. Fournier, N. Mingo, and O. Bourgeois, Phys. Rev. B 82, 155458 (2010). C. Blanc, A. Rajabpour, S. Volz, T. Fournier, and O. Bourgeois, Appl. Phys. Lett. 103, 043109 (2013). [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:48AM |
Y40.00003: Engineering thermal conductance using a two-dimensional phononic crystal Invited Speaker: Ilari Maasilta Controlling thermal transport has become very relevant in recent years, in light of the strong push to develop novel energy harvesting techniques based on thermoelectricity, the need to improve the heat dissipation out of semiconductor devices, and the push to increase the sensitivity of bolometric radiation detectors. Traditionally, this control has been achieved by tuning the scattering of phonons by including various types of scattering centers in the material (nanoparticles, impurities etc.). Recently we have taken another approach and demonstrated that one can also use coherent bandstructure effects to control phonon thermal conductance, with the help of periodically nanostructured phononic crystals. Working at around 1 Kelvin where the wavelength of the dominant thermal phonons is more than two orders of magnitude longer than at room temperature, we have created phononic crystals with a period of 1 $\mu$m that strongly reduce the thermal conduction. In addition, we performed theoretical calculations that accurately determine the ballistic thermal conductance in a phononic crystal device, showing full quantitative agreement with the experiments. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:24AM |
Y40.00004: Thermal transport in amorphous nanostructures: the (enduring) role of low-energy phonons Invited Speaker: Jason Underwood Micromachined amorphous solid structures have proven to be ideal platforms for physicists to challenge their understanding of phonon transport. Such nanostructures have been exploited for early experimental demonstrations of the quantum of thermal conductance. These structures also serve important technological functions. Amorphous silicon nitride (SiN$_x$) nanostructures, in particular, are increasingly critical to the operation of state-of-the-art low temperature detector arrays. Achieving control over which phonon modes propagate in a given structure --- phononics --- is a major goal for engineering better thermoelectric materials, for regulating heat flow in ever-shrinking microprocessors, and for the developing field of caloritronics. At very low temperatures, it is generally accepted that phonons with energy much lower than the Debye energy (i.e., $\omega \ll 10^{13}$~Hz) dominate thermal transport. At room temperature, the preponderance of higher energy modes is usually reason enough to assume that the low energy modes do not contribute substantially to the overall thermal conductance. While generally true for crystals, the efficient scattering of high-energy phonons in amorphous solids means that the remaining low-energy modes may acquire comparably long mean free paths. Recent measurements of SiN$_x$ nanostructures strongly suggest that this bias in mean free paths leads to the result that low-energy phonons may contribute up to 50\% of the overall thermal conductance of the structure --- even at room temperature. After a brief review of thermal transport in the low-energy regime, I will discuss these results, as well as other recent experiments where low-energy phonons play an important role. [Preview Abstract] |
Session Y41: Topological Insulators Under Extreme Conditions - Theory
Sponsoring Units: DCMPChair: Xinjiang Zhou, Institute of Physics, Chinese Academy of Sciences
Room: Mile High Ballroom 3C
Friday, March 7, 2014 8:00AM - 8:12AM |
Y41.00001: Bulk-surface Correspondence and the Hofstadter Problem of SU(2) Landau Levels in 3D Lattices Yi Li We show that the continuum description of topological states in three-dimensional SU(2) Landau levels can be connected to topological Bloch states in three dimensions. We consider SU(2) Landau levels in a cubic lattice, which exhibits spin-orbit coupled surface states protected by the time-reversal symmetry. We show that the bulk topological properties can be obtained from the topology in the surface states. We also show a generalized Hofstadter problem in three dimensions. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y41.00002: Low-temperature conductivity and magnetoconductivity of two-dimensional Dirac fermions in topological insulators Hai-Zhou Lu Low-temperature electronic transport in topological insulators exhibits a dilemma. A negative cusp in weak-field magnetoconductivity is widely believed to be the signature of weak antilocalization (WAL) from the topological surface states. WAL is a quantum interference effect that enhances the conductivity with decreasing temperature at very low temperatures. A magnetic field can destroy WAL as well as the enhanced conductivity, giving rise to the negative magnetoconductivity showing that WAL used to be there. However, the conductivity in most experiments was observed to drop logarithmically with decreasing temperature, a behavior opposite to WAL. We model the surface and bulk states in topological insulators as massless and massive Dirac fermions, and derive the conductivity formula as a function of magnetic field and temperature, by taking into account the quantum interference and electron-electron interaction simultaneously. The formula reconciles the dilemma by explicitly clarifying that the WL-like temperature dependence of the conductivity is dominated by the interaction while the WAL-like magnetoconductivity is mainly contributed by the quantum interference. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y41.00003: Dyon condensation: an effective way to construct topological phases with symmetry Peng Ye, Xiao-Gang Wen In this work, we construct three-dimensional exotic phases of bosons with time-reversal symmetry and boson number conservation U(1) symmetry by means of \emph{fermionic projective construction}. We first construct an \emph{algebraic bosonic insulator} which is a symmetric bosonic state with an emergent U(1) gapless gauge field. We then obtain many gapped bosonic states that do not break the time-reversal symmetry and boson number conservation via proper \emph{dyon condensations}. We identify the constructed states by calculating the allowed electric and magnetic charges of their excitations, as well as the statistics and the symmetric transformation properties of those excitations. This allows us to show that our constructed states can be trivial SPT states (\emph{i.e.} trivial Mott insulators of bosons with symmetry), non-trivial SPT states (\emph{i.e.} bosonic topological insulators) and SET states (\emph{i.e.} fractional bosonic topological insulators). In non-trivial SPT states, the elementary monopole (carrying zero electric charge but unit magnetic charge) and elementary dyon (carrying both unit electric charge and unit magnetic charge) are fermionic and bosonic, respectively. In SET states, intrinsic excitations may carry fractional charge. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y41.00004: Melting transitions of quantum liquid crystalline order coexisting with electronic topological Chern insulators or other topological phases Onkar Parrikar, Gil Young Cho, Robert Leigh, Taylor Hughes Motivated by recent progress in understanding the interplay between the lattice and electronic topological phases, we consider quantum-melting transitions of liquid crystalline order that coexists with electronic topological phases. In certain classes of Chern band insulators, it has been previously demonstrated that there are topological Chern-Simons terms for local lattice deformations such as a Hall viscosity term. The Chern-Simons terms can induce non-trivial statistics for the topological lattice defects and furthermore dress the defects with certain symmetry quantum numbers. On the other hand, the melting transitions of such liquid-crystalline orders are driven by the condensation of lattice defects. Based on these observations, we show how the topological terms can change the nature of the proximate phases of the quantum liquid crystalline phases. We derive and study the effective field theories for the quantum phase transitions, and demonstrate some concrete examples. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y41.00005: Magnetoelectric Effect in Topological Insulator Films Beyond Linear Response Regime Oleg Tretiakov, Dashdeleg Baasanjav, Kentaro Nomura We study the response of topological insulator films to strong magnetic and electric fields beyond the linear response theory. As a model, we use three-dimensional lattice Wilson-Dirac Hamiltonian where we simultaneously introduce both magnetic field as Aharonov Bohm phase and electric field as potential energy depending on lattice coordinate. We compute the energy spectrum by numerically diagonalizing this Hamiltonian for electrons and obtain the quantized magnetoelectric polarizability. In addition, we find that the magnetoelectric effect vanishes as width of the film decreases, due to the hybridization of surface wavefunctions. Furthermore, by applying a gate voltage between the surfaces, we observe multiple quantized plateaus of $\theta$-term. We explain that the multiple quantization rule of $\theta$ is mainly determined by the physics of Landau level structures on the top and bottom surfaces of topological insulator, whereas the small deviations from the exact quantization are coming from the asymmetry of the surface wavefunctions in the bulk. We also show that the magnetoelectric effect persists even for strong bulk interactions with magnetic field or magnetic impurities. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y41.00006: Stability of surface states of general weak Z2 topological insulators and superconductors Takahiro Morimoto, Akira Furusaki A three-dimensional weak topological insulator (TI) is adiabatically connected to stacked layers of two-dimensional strong topological insulators and typically possesses two surface Dirac cones that can be gapped out without breaking the time-reversal symmetry. Unexpected strength of weak TIs has been pointed out by recent theoretical studies, showing that the surface Dirac fermions of weak TIs are not localized when the mean of the random potential is zero, as a consequence of the uniqueness of the dimerization mass term gapping out the surface Dirac cones. Motivated by these, we study the surface stability of weak $Z_2$ topological insulators and superconductors (TIs/TSCs) in the general Altland-Zirnbauer symmetry classes, considering representative Dirac Hamiltonians in various spatial dimensions. We show that we can always find a unique Dirac mass term that dimerizes stacked layers and gaps out surface Dirac fermions. The two dimerized gapped phases with different signs of the mass are distinguished by a $Z_2$ index. If we impose spatial uniformity of the randomness of the surface on average, then the gapless surface states are not localized because they are connected with the quantum critical point between the two $Z_2$-distinct dimerized phases. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y41.00007: A self-consistent study of the ferromagnetic ordering of magnetic adatoms on the surface of topological insulator Wei Qin, Zhenyu Zhang Ferromagnetically coupled magnetic adatoms on the surface of a three-dimensional topological insulator (TI) will induce a band gap by breaking time-reversal symmetry. The opened gap not only causes a lowering of the total energy of the band electrons, but also influences the magnetic coupling between the magnetic adatoms; in turn, variations in the magnetic coupling will affect the original collective magnetic states of the adatoms on the TI surface. We study \textit{self-consistently} the RKKY interactions between magnetic adatoms on a TI surface, mediated by massive Dirac electrons. Analytical expressions of RKKY interactions are presented, which contain the widely known Heisenberg-like, Ising-like, and DM-like terms [1]. Our results show that the self-consistent band gap will weaken the ferromagnetic couplings between the magnetic adatoms. Finally, we expand our study to the case that magnetic adatoms interact with the surface electrons of TI via anisotropic exchange couplings. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y41.00008: Weak-field Hall conductivity in a single Dirac cone Masaki Noro, Shuichi Murakami Weak-field Hall conductivity is calculated in a single Dirac cone within a self-consistent Born approximation. In this system, electronic orbital motion and Zeeman splitting term contribute to the Hall conductivity. The contribution from the electronic orbital motion agrees with the Hall conductivity in graphene apart from the factor of four coming from spin and valley degeneracy, where the electronic structure forms pseudo-spin Dirac cones. On the other hand, the contribution from the Zeeman splitting term, which is seen only in a single Dirac cone, shows unique behavior. The contribution from Zeeman splitting term is symmetric with respect to the Dirac point as a function of the Fermi energy, in contrast to the antisymmetric behavior of the orbital contribution. It does not depend so much on energy for high Fermi energy region, while it shows a dip around the Dirac point. Besides its singular behavior, we note that the size of this contribution is comparable with that of the electronic orbital motion when we apply experimentally reasonable parameters. Consequently, the total weak-field Hall conductivity is neither symmetric nor antisymmetric with respect to the Dirac point, We compare this result with experimental results on surfaces of topological insulators. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y41.00009: Calculation of Magnetoresistance of an Ideal Topological Insulator Using Boltzmann Transport Teoman Ozturk, Richard Field III, Yun Suk Eo, Steven Wolgast, Kai Sun, Cagliyan Kurdak The electrical conductivity of a topological surface state is expected to be enhanced due to the suppression of backscattering resulting from spin-momentum locking. We will present numerical calculation of this enhancement factor using Boltzmann transport for an ideal topological insulator with a single Dirac cone in an arbitrary magnetic field and discuss the manifestations of this for a Corbino sample placed in a tilted magnetic field. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y41.00010: Theoretical studies of surface states in three-dimensional topological-insulator thin films in a strong magnetic field. Anna Pertsova, Carlo M. Canali, Allan H. MacDonald The peculiar structure of the Landau levels (LLs) in topological insulators (TIs), in particular the existence of a field-independent (zeroth) LL, is a characteristic signature of the Dirac surface states. However, recently it has been shown that the hybridization between top and bottom surfaces in a 3D TI thin film may lead to a splitting of the zeroth LL and even to its absence in the ultra-thin film limit. We report on microscopic tight-binding modelling of Bi$_{2}$Se$_{3}$ thin films [1] in the presence of a strong magnetic field. We find that the zeroth LL is absent for thicknesses below 4QLs, in agreement with experiments. Calculations of the LL spectrum of a 5QL-thick slab reveal a strong asymmetry with respect to the Dirac point and a clear signature of the first LL, in good agreement with Dirac-Hamiltonian model calculations. The latter feature persists in a wide range of magnetic fields and involves an extended window of energies, including bulk states away from the Dirac point. We use our results to predict an interplay between the external magnetic field and gate-voltage dependence of the anomalous Hall effect that is characteristic of topological magnetic states.\\[4pt] [1] A.Pertsova and C.M.Canali, arXiv:1311.0691. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y41.00011: Magnetization fluctuations on the surface of a magnetically-doped topological insulator Dmitry Efimkin, Victor Galitski Ordering of magnetic impurities on the surface of a topological insulator gaps out the surface states and gives rise to anomalous quantum Hall effect, as demonstrated in recent experiments [1, 2]. Here we study theoretically fluctuation phenomena that occur in the vicinity of the ferromagnetic transition in such magnetically-doped topological insulators. We calculate the density of states of the electronic excitations and study transport properties in the fluctuation region. \\[4pt] [1] S.Y. Xu et al., Nature Phys. 8, 616 (2012). \\[0pt] [2] Y. L. Chen et al., Science 329, 659 (2010). [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y41.00012: Interplay between Mn-acceptor state and Dirac surface states in Mn-doped Bi$_{2}$Se$_{3}$ topological insulator M.R. Mahani, A. Pertsova, F. Islam, C.M. Canali Bi$_{2}$Se$_{3}$ is a 3D topological insulator (TI) exhibiting a single Dirac cone of surface states localized on the (111) surface. Magnetic impurities embedded in the surface of a TI may cause a breaking of time-reversal symmetry, opening a gap at the Dirac point that changes the topological character of the surface states. Substitutional Mn also introduces acceptor states in the bulk gap of the host material. These acceptor levels can be directly probed by scanning tunneling spectroscopy. However, the nature of these states and their interplay with the Dirac surface states has not been analyzed theoretically. Here we present results of DFT calculations investigating the properties of a single substitutional Mn and its associated acceptor state in Bi$_{2}$Se$_{3}$TI. In agreement with experiment we find that Mn impurities are in Mn$^{2+}$ valence state, with a magnetic moment close to 5 $\mu_{\mathrm{B}}$. The Mn-acceptor is predominantly of $p$ character and is localized mainly on the Mn and the nearest-neighbor Se atoms. Its electronic structure and spin-polarization are determined by the hybridization with the Mn $d$ levels, which is strongly affected by lattice relaxation and electronic correlations at the Mn site. We argue that magnetism and the topological character of Mn-doped Bi$_{2}$Se$_{3}$ is the result of this non-trivial interplay between acceptor and Dirac electron spins, and their coupling with the localized Mn magnetic moment. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y41.00013: Fractionalization of Faraday lines in generalized compact quantum electrodynamics models in three dimensions Olexei Motrunich, Scott Geraedts Motivated by ideas of fractionalization and topological order in bosonic models with short-range interactions, we consider similar phenomena in formal lattice gauge theory models. Specifically, we show that a compact quantum electrodynamics (CQED) in (3+1)D can have, besides familiar Coulomb and confined phases, additional unusual confined phases where excitations are quantum lines carrying fractions of the elementary unit of electric field strength. We construct a model that has $N$-tupled monopole condensation and realizes 1/N fractionalization of the quantum Faraday lines; this phase has another excitation which is a $Z_N$ particle that picks up a phase of $e^{i 2\pi/N}$ when going around the fractionalized electric field line excitation. Alternatively, we can introduce a conventional bosonic field and condense bound states of monopoles and bosons. This can lead to fractionalization of both Faraday lines and bosons, as well as a quantized transverse response. We compare and contrast with bosonic topological insulators in (3+1)D. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y41.00014: Physical observables in a model with topological structure and time-dependent perturbations Benjamin M. Fregoso, Jan P. Dahlhaus, Joel E. Moore, James K. Freericks We study a model of spin-full fermions on a lattice in a finite geometry that is acted upon by a time-dependent perturbation, e.g., an intense laser pulse, which induces non-trivial topological band structure. While it is possible for such time-dependent perturbations to modify the band structure, e.g, creating edge states or modifying the Chern number, it is far less clear under what conditions such topological effects can be observed in experimental settings. Two regimes are studied, the transient regime and the non-equilibrium steady state regime. We provide conditions under which physical observables carry signatures of the induced topological structure. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y41.00015: Chern and Majorana modes of quasiperiodic systems Gerardo Naumis, Indubala Satija In this work, we investigate the self-similar states found in quasiperiodic systems characterized by topological invariants--the Chern numbers. We show that the topology introduces a competing length in the self-similar band edge states transforming peaks into doublets of size equal to the Chern number [1]. This length intertwines with quasiperiodicity and introduces an intrinsic scale, producing Chern beats related to Friedel oscillations. An explanation based on Thouless equations for band edge modes of the Harper equation is provided to understand the Chern dressing of the fractal spectrum. Chern numbers also influence the zero-energy mode that, for quasiperiodic systems, is related to the Majorana modes: the remnant of the edge localized topological state that delocalizes at the onset to a topological transition. In superconducting wires, the exponentially decaying profile of the edge localized Majorana modes also encode fingerprints of the Chern states that reside in close proximity to zero energy. \\[4pt] [1] I. Satija, G.G. Naumis, ``Chern and Majorana modes in Quasicrystals,'' Phys. Rev. B 88, 054204 (2013). [Preview Abstract] |
Session Y42: Dielectrics: Optical and Bulk Properties
Sponsoring Units: DCMPChair: Ben White, University of California, San Diego
Room: Mile High Ballroom 4A
Friday, March 7, 2014 8:00AM - 8:12AM |
Y42.00001: High-Resolution Thermal Expansion Measurements of H$_{2}$O Ice David T.W. Buckingham, Sueli H. Masunaga, Forrest C. Gile, J.J. Neumeier Water is one of the most important substances in nature. Surprisingly, little detail is known about its thermal expasion due to the low-resolution of past measurements. The goal of this research is to measure the thermal expansion of single crystalline H$_{2}$O ice Ih with $\sim$$10^{4}$ times greater relative resolution than has previously been done. This presentation will discuss single-crystal growth and characterization, our high-resolution thermal expansion technique, some of the challenges we faced in carrying out the measurements, and briefly present preliminary measurements on the thermal expansion and heat capacity of polycrystalline ice. The thermal expansion in the vicinity of the glass transition at $\sim$110K [1] will be discussed. \\[4pt] [1] Suga, H., \textit{Thermochimica Acta}, \textbf{300}, pp. 117-126, 1997. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y42.00002: Transverse Light Localization in waveguide arrays with random absorption or amplification A. Basiri, Y. Bromberg, A. Yamilov, H. Cao, T. Kottos We investigate the possibility to induce transverse localization of light in an array of waveguides with randomness pertaining to the imaginary part of the dielectric constant. Although this new set-up is distinct from the traditional Anderson scenario, where localization emerges due to multiple scattering from a real random potential, we find that disordered amplification/attenuation can also lead to exponential localization. We quantify the degree of localization of the Floquet-Bloch modes of our system via their participation number, which is shown to satisfy a one-parameter scaling theory. The effects of this transverse localization of the normal modes in the paraxial beam propagation are theoretically predicted and confirmed by numerical experiments. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y42.00003: Attosecond spectroscopy of band-gap dynamics excited by the electric field of light Martin Schultze, Krupa Ramasesha, Daniel M. Neumark, Stephen R. Leone The basis of modern electronics and information processing is the control of the electric properties of semiconductors with microwave fields. Speeding up electronics requires extending this control to optical frequencies. We apply attosecond solid state spectroscopy to investigate and compare light field induced ultrafast carrier dynamics in a prototypical semiconductor (silicon) and dielectric (SiO$_{2})$. After excitation by a highly intense few-cycle visible laser pulse, a time-delayed extreme ultraviolet attosecond pulse centered around the Silicon L-edge transition maps the conduction band population and thus probes the unfolding electronic dynamics with sub femtosecond resolution. While the induced changes in SiO$_{2}$ appear only in the presence of the strong light field, the experiment on silicon measures a permanent population transfer into the conduction band triggered by the electric field of light as well as ultrafast renormalization of the band structure. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y42.00004: Small Polaron Conduction in Glasses: Established Physics and New Data Mark Henderson, Biprodas Dutta, Ian Pegg Electronic and optical phenomena in crystals and glasses can be usefully analyzed in the band-structure framework, which determines the ways electrons are allowed to behave inside these materials. Especially interesting is the wide array of phenomena that can be observed in glasses. Disorder in glasses leads to high rates of electron scattering, large band-gaps, and, for strong enough atomic interactions, the localization of electrons. Modes of conduction, such as site hopping, still exist for these trapped electrons; resistivity can be greatly changed depending on the environment created by the inclusion of various ions. In this presentation, I will begin with the band-structure model for crystals and then glasses. I will discuss the ways electrons propagate in these materials and how optical processes depend on the allowed energy states of those electrons. I will also explain how the mixing of different ions in glasses leads to striking changes in their properties. Finally, I will present hot off the alumina substrate data indicating the curious existence of trapped electron hopping in bismuth containing borosilicate glasses. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y42.00005: Leakage current mechanisms in PECVD-grown amorphous hydrogenated boron carbide thin films Bradley Nordell, Chris Keck, Justin Hurley, Thuong Nguyen, Sean King, Sudaunshu Purohit, Anthony Caruso, Michelle Paquette Thin-film amorphous hydrogenated boron carbide (a-BxC:Hy), grown by plasma-enhanced chemical vapor deposition (PECVD) from orthocarborane (C2B10H12), has emerged as a promising semi-insulating, moderately high bandgap (2--4 eV), p-type material for direct-conversion solid-state neutron detector and low-dielectric-constant (low-$\kappa )$ intra/interlayer dielectric (ILD) applications. Attaining a complete understanding of the electrical transport properties for amorphous semiconductors is challenging, but essential for material maturation and optimization. For the above applications, understanding leakage current mechanisms, in both the low and high field regimes, is particularly relevant. This contribution will shed light on the charge transport mechanisms in a-BxC:Hy and discuss the role played by Urbach energy and band gap. Current density (J) as a function of field (E) was measured for a range of films grown with different PECVD parameters, and the resulting J--E curves were analyzed. The band gap and Urbach energy were measured by fitting absorption coefficient data obtained from transmission UV-Vis spectroscopy in the Tauc and exponential regions. We will discuss how optimizing bandgap and Urbach energy can be used to improve leakage current. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y42.00006: Magnetic-Field Dependent IR Active Modes in CrOCl Li-Chun Tung, Haidong Zhou, Zhigang Jiang, Zhiqiang Li, Dmitry Smirnov CrOCl is a magnetoelastic material in which a correlation between an antiferromagnetic transition at 13.5K and a lattice structural distortion was discovered in the absence of the magnetic field. An applied magnetic field is known to induce a spin-flip transition at around 4T, while a correlation between this magnetic transition and the lattice order has not been observed. Greenish leaves-like CrOCl crystals are carefully laid upon the Scotch tape to create a film-like sample for IR transmittance and reflectance measurements at 4K in the magnetic field as high as 35T. Several sharp optical absorptions have been observed and they can be attributed to the IR-active phonon modes in CrOCl lattice. Among them, several modes exhibit a strong correlation to the spin-flip transition, implying a magnetic-field induced structural transition. Moreover, evolution of these modes agrees with the magnetic hysteresis of the spin-flip transition. Implications of the strong magnetoelastic coupling in CrOCl will be discussed in the presentation. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y42.00007: Ferroelectric nanoparticles and their use in disparate optical devices Dean Evans, Sergey Basun, Carl Liebig, Ighodalo Idehenre The fabrication [1] and ``harvesting'' [2] of stressed ferroelectric nanoparticles and the characterization of these materials will be discussed. Due to the induced surface stress in \textless 10 nm size nanoparticles, a strong spontaneous polarization is achieved (4-5 times greater than found in the bulk for the case of BaTiO$_{3})$ [3,4]. These materials have been characterized in both isotropic and anisotropic liquids. The benefits of using these nanoparticles have been demonstrated by a significant enhancement in the field sensitivity (display) and optical gain (hybrid photorefractive) liquid crystal systems.\\[4pt] [1] H. Atkuri, G. Cook, D. R. Evans, C.-I. Cheon, A. Glushchenko, V. Reshetnyak, Yu. Reznikov, J. West, K. Zhang, \textit{Journal of Optics A: Pure and Applied Optics}, \textbf{11,} 024006 (2009).\\[0pt] [2] G. Cook, J. L. Barnes, R. F. Ziolo, A. Ponce, V. Yu. Reshetnyak, A. Glushchenko, S. A. Basun, P. P. Banerjee, D. R. Evans, \textit{J. Appl. Phys. }\textbf{108,} 064309 (2010).\\[0pt] [3] S. A. Basun, G. Cook, V. Yu. Reshetnyak, A. V. Glushchenko, and D. R. Evans. \textit{Phys. Rev. B}, \textbf{84,} 024105 (2011).\\[0pt] [4] D. R. Evans, S. A. Basun, G. Cook, I. P. Pinkevych, and V. Yu. Reshetnyak, \textit{Phys. Rev. B}, \textbf{84,} 174111 (2011). [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y42.00008: Dielectric function of NiO and Si from 25 meV to 6 eV: What's the difference? S. Zollner, C.M. Nelson, T.I. Willett-Gies, L.S. Abdallah, A. Ghosh Using spectroscopic ellipsometry, we determined the dielectric function of bulk NiO and Si from 25 meV to 6 eV to compare their lattice dynamics and electronic structure. From 0.7 to 6.6 eV, we studied the temperature dependence of the dielectric function from 77 to 800 K. In the visible and UV spectral region, both materials have a remarkably similar dielectric function: Both materials are transparent in the near-infrared. A slow rise of the absorption with increasing photon energies throughout the visible is followed by a sharp peak at 3.4 eV (Si) and 3.8 eV (NiO). In Si, this peak is caused by transitions from the highest valence band to the lowest conduction band along the (111) direction of the Brillouin zone. In NiO, this peak is associated with the charge-transfer gap. In both materials, the peaks broaden and redshift with increasing temperature, due to electron phonon interactions. Many recent band structure calculations for NiO focus on this main charge-transfer absorption peak (3.8 eV) and ignore the absorption below the main peak. In Si, this absorption rises smoothly (due to indirect transitions), but several peaks appear in the visible absorption of NiO, which apparently do not depend on temperature. In the infrared, the lattice absorption of Si is small because of inversion symmetry. In NiO, we find strong TO phonon absorption, which is modified by the antiferromagnetic ordering. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y42.00009: Probing the mechanical properties of high-k dielectric nano-films by Brillouin light scattering study Jonathan Zizka, Jeffrey Bielefeld, Sean King, R. Sooryakumar As microelectronic transistors scale to smaller dimensions, device functionality suffers from current leakage. This problem can be overcome by using thicker gate materials with a high dielectric constant. SiO$_{\mathrm{2}}$ has been the material of choice, but becomes unsuitable due to its relatively low dielectric constant (k $=$ 3.9). Alternate materials, such as BN:H (k $=$ 5.7) and HfO$_{\mathrm{2}}$ (k $=$ 25) are promising choices to replace SiO$_{\mathrm{2}}$ to achieve the desired performance while preserving ultra-thin thickness (\textless 10 nm). Despite these promising features, one concern of including these materials, are their mechanical and thermal properties that could degrade device functionality. There is thus a growing need for non-destructive techniques to evaluate the mechanical properties of such laminar structures since traditional methods like nano-indentation are not effective at these dimensions. We report on Brillioun light scattering studies to determine the individual elastic constants and, thus the mechanical properties of BN:H and HfO$_{\mathrm{2}}$ high-k films with thicknesses as low as 24 nm. Young's modulus (E) and Poisson's ratio ($\nu )$ were determined by measuring the frequency dispersion of confined and traveling transverse and longitudinal acoustic waves as well as their associated light scattering intensities. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y42.00010: Refractive Index Variation of Zn-rich BaZnO Alloys Grown by Pulsed Laser Deposition Hamad Albrithen, Zeyad Alahmed, Ahmed Elnaggar, Essa Alsalmani, Anwar Alanazi, Ahmed Alyamani, Joselito Labis Ba$_{x}$Zn$_{1-x}$O alloys have been grown by pulsed laser deposition on sapphire(0001) substrates. Three concentration were investigated, $x =$ 0.05, 0.1, and 0.25 . The XRD of the films, all concentrations, did not exhibit significant peaks, indicating amorphous structures yet film of $x =$ 0.05 exhibited a very weak peak representing little crystallite within the amorphous surrounding. Spectroscopic Ellipsometry measurements were carried out to probe the optical properties of the films and the topography of their structures by an optical means. It was found that the addition of Ba to the ZnO film reduced the index of refraction for the $x =$ 0.05 and 0.1. However, when Ba doping was increased the index of refraction increased. Moreover, Ba-dpoed ZnO with $x =$ 0.05 and 0.1 barium had homogenous films while at $x =$ 0.25 the film incorporated voids, as indicated by the Elipsometric analysis as well. Funding is provided by Saudi National Plan for Science and Technology; the funding {\#} is 10-NAN1197-02. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y42.00011: Microscopic Insight into the Isosymmetric Jahn-Teller Bond Axis Reorientation in Na$_3$MnF$_6$ Nenian Charles, James Rondinelli Using first-principles density functional theory calculations, we investigate the hydrostatic pressure-induced spontaneous reorientation of the Jahn-Teller bond axis in the fluoride cryolite Na$_3$MnF$_6$. We find a first-order isosymmetric transition occurs between crystallographically equivalent monoclinic structures at approximately 2.15 GPa, consistent with earlier experimental studies. Analogous calculations for Na$_3$ScF$_6$ show no evidence of a transition up to 6.82 GPa. Mode crystallography analysis of the pressure-dependent structures in the vicinity of the transition reveals a clear evolution of the Jahn-Teller bond distortions in cooperation with an asymmetrical stretching of the equatorial fluorine atoms in the MnF$_6$ octahedral units. We identify a change in orbital occupancy of the e$_g$ manifold in the d$^4$ Jahn-Teller active Mn(III) to be responsible for the transition, which stabilizes one monoclinic P2$_1$/n variant over the other. From our results, we conjecture that the same transition may be accessible in epitaxially grown thin films of Na$_3$MnF$_6$ with a modest biaxial tensile strain. [Preview Abstract] |
Session Y43: Topological Insulators: Engineered Structures II
Sponsoring Units: DCMPChair: Madhab Neupane, Princeton University
Room: Mile High Ballroom 4B
Friday, March 7, 2014 8:00AM - 8:12AM |
Y43.00001: Transport studies of a superconductor- InAs/GaSb bilayer junction Xiaoyan Shi, Wenlong Yu, Z. Jiang, J.F. Klem, W. Pan We fabricated a superconductor- semiconductor junction, by depositing a superconducting Ta film onto a band inverted InAs/GaSb bilayer. In this talk, we focus on electrical transport studies of this junction as a function of magnetic fields. At Zero magnetic field, the tunneling results show a zero bias conductance peak and this conductance peak survives in a field even up to 2~T. With further increasing magnetic field, the conductance peak eventually becomes a dip above 4~T. Finally, by tuning the front gate, we were able to measure the tunneling conductance when the InAs/GaSb bilayer is in the charge neutrality regime. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y43.00002: Quantum Hall effect and insulating state near the charge neutrality point in an InAs/GaSb quantum well Atindra Nath Pal, Fabrizio Nichele, Patrick Pietsch, Thomas Ihn, Klaus Ensslin, Christophe Charpentier, Werner Wegscheider We present transport measurements in a gated InAs/GaSb double quantum well (QW) sandwiched between two AlSb barriers. In this system a QW for electrons in InAs and a QW for holes in GaSb coexist next to each other and a hybridization gap is expected to occur. We can tune the transport from electrons to the holes by applying a top gate voltage. In presence of a perpendicular magnetic field, we observe well defined quantum Hall plateaus in both sides. Interestingly, at the charge neutrality point a strong increase in the longitudinal resistivity is observed with increasing perpendicular magnetic field, accompanied by the onset of a non-local resistance of similar magnitude. The co-existence of these two effects is described by a model of counter-propagating and dissipative quantum Hall edge channels, shorted by a residual bulk conductivity.\\[4pt] Reference: Fabrizio Nichele \textit{et al}., arXiv:1308.3128 (2013). Christophe Charpentier \textit{et al}., \textit{Appl. Phys. Lett.} 103, 112102 (2013). [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y43.00003: Low Temperature STM Experiments on Helical Edge States in InAs/GaSb Rui-Rui Du, Tingxin Li, Xiaoyang Mou, Lingjie Du, Gerald Sullivan Inverted InAs/GaSb quantum wells have been recently shown to be a 2D topological insulator hosting robust helical edge states. Attributing to the fact that the hybridized minigap in this system opens at a finite wavevector, the edge states here have a low Fermi velocity V$_F$, and consequently their transport properties may reveal interesting interaction effects. Moreover, the V$_F$ in this system can be continuously tuned by electrostatic gates, providing an experimental knob for tuning the interactions. We report work in progress for STM/STS measurements of edge states in the tunneling regime, where the edge states are exposed at the cleaved edge/UHV interface. Experiments are performed in a 400 mK STM/vector magnet system with in situ sample cleavage and thin film deposition capabilities. Ref. I. Knez, R.-R. Du and G. Sullivan, Phys. Rev. Lett. 107, 136603 (2011); L-.J. Du, I. Knez, G. Sullivan, R-.R. Du, ArXiv:1306.1925 (2013). [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y43.00004: Quantum Anomalous Hall Effect in Magnetically Doped InAs/GaSb Quantum Wells Qingze Wang, Xin Liu, Hai-Jun Zhang, Nitin Samarth, Shou-Cheng Zhang, Chao-Xing Liu The quantum anomalous Hall effect has recently been observed experimentally in thin films of Cr doped (Bi,Sb)$_2$Te$_3$ at low temperature ($\sim$ 30mK). In this work, we propose realizing the quantum anomalous Hall effect in more conventional diluted magnetic semiconductors with magnetically doped InAs/GaSb type II quantum wells. Based on a four band model, we find a large increase of the Curie temperature for ferromagnetism due to the band edge singularity in the inverted regime of InAs/GaSb quantum wells. Below the Curie temperature, the quantum anomalous Hall effect is confirmed by the direct calculation of Hall conductance. Remarkably, our calculation based on eight-band Kane model reveals a band gap induced by exchange coupling reaching 10meV. The high sample quality and strong exchange coupling make the magnetically doped InAs/GaSb quantum well a good candidate for the quantum anomalous Hall insulator at high temperature. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y43.00005: Superconducting proximity effect in inverted InAs/GaSb quantum well structures with Ta electrodes Wenlong Yu, Yuxuan Jiang, Chao Huan, Xunchi Chen, Samuel D. Hawkins, John F. Klem, Zhigang Jiang, Wei Pan We report on a systematic study of the proximity effect in top-gated InAs/GaSb quantum wells in contact with a superconducting Ta electrode. We find that the electronic transport across the InAs-Ta interface exhibits distinct zero-bias behavior, either a conductance (dI/dV) peak or dip, depending on the interfacial transparency. For a relatively resistive interface, we observe a dI/dV peak at zero bias, accompanied by two dI/dV dips at high bias voltages, consistent with previous works. When a transparent InAs-Ta interface is achieved, a zero-bias dV/dI dip appears with two coherent-peak-like features forming at bias voltages corresponding to the superconducting gap of Ta. The dI/dV spectra of the transparent InAs-Ta interface at different gate voltages can be fit well using the standard BTK model and the temperature dependence follows a BCS-like behavior. Our work demonstrates the possibility of achieving the highly transparent interfaces in InAs/GaSb hybrid structures, needed for studying the intriguing Andreev bound states in this two-dimensional topological system. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y43.00006: Imaging Current in Si-doped InAs/GaSb Quantum Wells Eric M. Spanton, Katja C. Nowack, Lingjie Du, Gerard Sullivan, Rui-Rui Du, Kathryn A. Moler Quantum spin hall (QSH) insulators are characterized by current-carrying edges in which single-particle elastic backscattering is forbidden, resulting in a theoretical conductance of e2/h per edge. Various theoretical mechanisms have been proposed to explain why, in devices with edges longer than several microns, the measured resistance is greater than expected. We used a scanning superconducting quantum interference device to image 2D current flow in inverted InAs/GaSb composite quantum wells with edges of tens of microns. We compared wells with Si doping at the InAs/GaSb interface (which acts to suppress residual bulk conductivity) to wells without doping. In the Si-doped samples, we observed that the majority of current flowed along the edge of the device when it was tuned into the bulk gap using a front gate. The current at the edges is consistent with an edge resistance that remained unchanged over a wide range of temperature and gate voltage, even in the presence of bulk conduction. These results set strong limits on candidate mechanisms for edge scattering. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y43.00007: Observation of Quantum Spin Hall States in InAs/GaSb Bilayers under Broken Time-Reversal Symmetry Lingjie Du, Ivan Knez, Gerard Sullivan, Rui-Rui Du Topological insulators (TIs) are a novel class of materials with nontrivial surface or edge states. Time-reversal symmetry (TRS) protected TIs are characterized by the Z2 topological invariant. The fate of the Z2 TIs under broken TRS is a fundamental question in understanding the physics of topological matter but remains largely unanswered. Here we show, a two-dimensional TI is realized in an inverted electron-hole bilayer engineered from InAs/GaSb semiconductors which retains robust helical liquid (HL) edge states under a strong magnetic field. Wide conductance plateaus of 2e2/h value are observed; they persist to 10T applied in-plane field before transitioning to a trivial semimetal. In a perpendicular field up to 35T, broken TRS leads to a spatial separation of the movers in Kramers pair and consequently the intra-pair backscattering phase space vanishes, i.e., the conductance increases from 2e2/h in strong fields manifesting chiral edge transport. We propose a phenomenological phases diagram, where inside the topological gap the HL transfers into a ``canned helical state'' driven by perpendicular fields. Our findings suggest that once established, the HL is remarkably resilient and only undergoes adiabatic deformation under TRS breaking. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y43.00008: InAs/GaSb quantum wells: quantum spin Hall effect and topological superconductivity Matthias Sitte, Karin Everschor-Sitte, Allan MacDonald In recent years, topological insulators (TIs) have attracted great attention as a new quantum state of matter. The first experimental 2D TIs were HgTe/CdTe quantum well heterostructures. Recently, another semiconducting system -- the InAs/GaSb quantum well heterostructure -- was shown to be a 2D TI as well. These semiconducting heterojunctions have many advantages compared to HgTe/CdTe systems, including continuously tunable band structure via electric fields and stronger proximity coupling to superconductors. Proximity coupling of a 2D TI and an ordinary superconductor gives rise to one-dimensional topological superconductivity which supports non-local excitations known as Majoranas that can be used for topologically protected quantum computing. We perform empirical tight-binding calculations on these systems, studying the topological phases and their properties. With this knowlegde, we then extend our theory to study the proximity effects when InAs/GaSb quantum wells are coupled to a superconductor. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y43.00009: Exciton condensation and quantum spin Hall effect in InAs/GaSb bilayers Dmitry Pikulin, Timo Hyart We study the phase diagram of the inverted InAs/GaSb bilayer quantum wells as a function of tunneling between the layers and spin-orbit coupling. For small tunneling amplitude between the layers, we find that the system is prone to formation of an $s$-wave exciton condensate topologically trivial phase. On the contrary, for large tunneling amplitude, we obtain a topologically non-trivial quantum spin Hall insulator phase with a $p$-wave exciton order parameter, which enhances the hybridization gap and supports edge transport. These topologically distinct insulators are separated by an insulating phase with spontaneously broken time-reversal symmetry. Close to the phase transition between the quantum spin Hall and time-reversal broken phases, the edge transport shows quantized conductance in small samples, whereas in long samples the mean free path associated with the backscattering at the edge is temperature independent, in agreement with recent experiments. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y43.00010: Localized States and Quantum Spin Hall Effect in Si-Doped InAs/GaSb Quantum Wells Dong-Hui Xu, Jin-Hua Gao, Chao-Xing Liu, Jin-Hua Sun, Fu-Chun Zhang, Yi Zhou We study localized in-gap states and quantum spin Hall effect in Si-doped InAs/GaSb quantum wells. We propose a model describing donor and/or acceptor impurities to describe Si dopants. This model shows in-gap bound states and wide conductance plateau with the quantized value $2e^{2}/h$ in light dopant concentration, consistent with recent experiments by Du et al.[arXiv: 1306.1925] We predict a conductance dip structure due to backward scattering in the region where the localization length $\xi $ is comparable with the sample width $L_{y} $ but much smaller than the sample length $L_{x} $. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y43.00011: Pyroelectric Control of Rashba Spin-Split States and Spin-Relaxation Times in a GaN/InN/GaN Topological Insulator Parijat Sengupta Strong spin-orbit coupling leading to band inversion in bulk is necessary for creation of topological insulator states (TI). Electric field can also be used to invert the band structure. Nitrides in wurtzite phase possess an internal electric field due to spontaneous and piezoelectric polarization which is sufficient to invert the band-ordering of a narrow-gap InN. A TI state exists in a thin-film of InN sandwiched between GaN layers. For a certain quantum well thickness, inversion of bands happen at a threshold value of the polarization field. Polarization fields are controlled by selecting a facet orientation of the quantum well layer determined by the dominant polarization mechanism. Additionally, at a finite k-vector, the Rashba-induced spin-splitting on the surface of this heterostructure is computed. The splitting under a first-order approximation is independent of k-vector and corresponds to the polarization field's contribution to the Rashba coefficient. Finally, the interplay of mechanisms that control spin-relaxation times is used to design a spin transistor. An enhancement in the lifetime of the spin-polarized states under certain growth conditions is observed due to mutual cancelation the Rashba and Dresselhaus splitting to suppress spin-relaxation. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y43.00012: Spin helical transport from topological surface states and Rashba 2DEG in topological insulators Jifa Tian, Jiuning Hu, Helin Cao, Isaac Childres, Ireneusz Miotkowski, Yong P. Chen Topological insulators are an unusual phase of quantum matter with nontrivial spin-momentum-locked gapless topological surface states (TSS) and strong spin-orbit coupled bulk states. Coexistence of parallel conduction channels makes revealing transport signatures of the spin-momentum helical locked TSS difficult. Here, we report the fabrication of spin valve devices from exfoliated topological insulator thin flakes, with two outside nonmagnetic contacts for injecting a DC current together with a middle ferromagnetic (FM) contact for spin detection. By applying an in-plane magnetic field along the easy axis of FM contact, we observe a striking asymmetric magnetoresistance (MR) with a clear hysteresis. Furthermore, the trend of the asymmetric MR can be reversed by reversing the direction of the DC current. Our result is consistent with the current induced spin polarization from TSS, giving the direct transport evidence of spin-momentum-locking of TSS. Furthermore, from more metallic samples due to bulk conduction, we observe a current induced spin polarization opposite to that of TSS but consistent with Rashba 2D electron gas (2DEG) coming from band bending near surface. Our demonstration of spin helical transport opens ways for novel TI-based spintronc devices. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y43.00013: Correlated zero-bias transport in graphene and 2D topological insulators nanostructures Andrea Droghetti, Ivan Rungger, Awadhesh Narayan, Stefano Sanvito In recent years, the Kondo effect in graphene [1] and 2D topological insulators (TI) [2] has attracted considerable interest. While an impurity spin in graphene interacts with the Dirac fermions of the lattice, an impurity on the edge of a 2D-TI interacts with the helical edge liquid. Here we first describe the electronic structure of several graphene and 2D-TI model nanostructures, which incorporate a correlated impurity. Then, by combining continuous time quantum Monte Carlo with the Green function transport theory, we discuss how the transport properties are affected by the Kondo effect. Finally, we highlight how the employed method can be combined with density functional theory in the Smeagol code [3] in order to include material specific properties.\\[4pt] [1] V.N.~Kotov {\it et al.}, Rev. Mod. Phys. \textbf{84}, 1067 (2012).\\[0pt] [2] F.~Goth {\it et al.}, Phys. Rev. B \textbf{88}, 075110 (2013).\\[0pt] [3] A.R.~Rocha {\it et al.}, Phys. Rev. B. \textbf{73}, 085414 (2006). [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y43.00014: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y43.00015: Magnetically Defined Qubits on 3D Topological Insulators Gerson J. Ferreira, Daniel Loss We explore potentials that break time-reversal symmetry to confine the surface states of 3D topological insulators into quantum wires and quantum dots. A magnetic domain wall on a ferromagnet insulator cap layer provides interfacial states predicted to show the quantum anomalous Hall effect. Here, we show that confinement can also occur at magnetic domain heterostructures, with states extended in the inner domain, as well as interfacial QAHE states at the surrounding domain walls. The proposed geometry allows the isolation of the wire and dot from spurious circumventing surface states. For the quantum dots, we find that highly spin-polarized quantized QAHE states at the dot edge constitute a promising candidate for quantum computing qubits. See [Ferreira and Loss, Phys. Rev. Lett. 111, 106802 (2013)]. [Preview Abstract] |
Session Y45: Graphene Terahertz Optics and Strain Engineering
Chair: Xinghan Cai, University of MarylandRoom: Mile High Ballroom 4D
Friday, March 7, 2014 8:00AM - 8:12AM |
Y45.00001: Strain-induced time-reversal odd superconductivity in graphene Vladimir Juricic, Bitan Roy I will discuss the possibility of realizing a time-reversal-symmetry breaking superconducting state that exhibits an $f+is$ pairing symmetry in strained graphene [1]. Although the underlying attractive interactions need to be sufficiently strong and comparable in pristine graphene to support such pairing state, I will argue that strain can be conducive for its formation even for weak interactions. I will show that quantum-critical behavior near the transition is controlled by a fermionic multicritical point, characterized by various critical exponents computed in the framework of an $\epsilon$-expansion near four spacetime dimensions. I will then discuss the scaling of the superconducting gap with the strain-induced axial pseudo-magnetic field. Furthermore, a vortex in this mixed superconducting state hosts a pair of Majorana fermions supporting a quartet of insulating and superconducting orders, among which quantum spin Hall topological insulator. Finally, I will mention some experimental signatures of this $f+is$ time-reversal odd superconductor. These findings suggest that strained graphene could provide a platform for the realization of exotic superconducting states of Dirac fermions.\\[4pt] [1] B. Roy and V. Juricic, arXiv: 1309.0507. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y45.00002: Strain engineering on graphene directly from discrete atomic positions Salvador Barraza-Lopez, Alejandro Pacheco Sanjuan, Zhengfei Wang, Mihajlo Vanevic Graphene's strain engineering can be considered as a ``multiscale'' theory where effective Dirac fermions (Quantum Mechanics) couple with the Classical Mechanics and Geometry of an elastic membrane. Since the seminal work by Suzuura and Ando, the coupling to mechanics and geometry has been given in terms of continuum elasticity theory in the harmonic regime and relies on Riemannian, continuum geometry (e.g., [1-4]). Given an atomistic conformation is known, we express the coupling among effective Dirac fermions and mechanics directly onto the atomistic lattice and without reference to a continuum media; i.e., we couple effective Dirac fermions with atomistic mechanics. The approach has a solid mathematical underpinning known as Discrete Differential Geometry (DDG) [5]. We will provide a number of specific insights from this atomic-originated framework [6,7]. 1. H. Suzuura and T. Ando, \underline {PRB 65, 235412 (2002)}; 2. V. M. Pereira and A. H. Castro Neto, \underline {PRL 103, 046801 (2009)}. 3. F. Guinea, and M. I. Katsnelson, and A. K. Geim, \underline {Nat. Phys. 6, 30 (2010)}. 4. M. A. H. Vozmediano, and M. I. Katsnelson, and F. Guinea, Phys. Rep. 496, 109 (2010); 5. A. I. Bobenko, P. Schroder, J. M. Sullivan, and G. M. Ziegler, eds., Discrete Differential Geometry. Springer. 1st Ed.; 6. \underline {J. V. Sloan}, \underline {A. A. Pacheco Sanjuan}, \underline {Z. Wang}, \underline {C. Horvath}, and S. Barraza-Lopez. PRB 87, 155436 (2013); 7. S. Barraza-Lopez, A. A. Pacheco-Sanjuan, Z. Wang, and M. Vanevic. Solid State Comm 166, 70 (2013). [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y45.00003: Suspended graphene device under strain Fen Guan, Bent Nielsen, Xu Du It has been theoretically proposed that strain can induce changes in band structure and electrical transport properties of graphene. While some evidences have been reported on spectroscopy measurements, transport study on strained graphene has been limited and is mostly based on non-suspended graphene, where the detrimental effect of the substrates may smear out the intrinsic response. To overcome this problem, we report the fabrication of suspended monolayer and bilayer graphene devices on flexible substrates. By bending the substrate and measuring the transport characteristics of graphene as a function of temperature and gate voltage, these devices allow study of the intrinsic properties of the materials under strain. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y45.00004: Manipulation of electronic states in triangular graphene quantum dots using optical selection rules Eleftheria Kavousanaki, Keshav Dani Triangular graphene quantum dots with zigzag edges have been known to exhibit half-filled Fermi edge states with a non-zero ground state magnetic moment. Using the tight binding model, we study the optical selection rules for these structures with and without an external magnetic field and demonstrate that only transitions between states of specific rotational symmetry are allowed in the case of excitation with circularly polarized light. Using these rules, we analyze the optical absorption spectra of quantum dots with either zigzag or armchair edges at zero and nonzero magnetic field, discuss their differences, and show that they allow for the manipulation of the pseudo magnetic properties of these dots using optical pulses. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y45.00005: Is silicon dioxide essential to make graphene visible? Case studies of graphene-substrate interaction Chia-Hao Chen, Hung Wei Shiu, Lo Yueh Lo Yueh, Hung-Ying Chen, Shangjr Gwo Making exfoliated graphene flakes visible is the key to successfully study the fundamental properties of graphene. Conventionally, this can be achieved by placing the graphene flakes on top of silicon substrate with 300 nm SiO2, but this silicon dioxide layer may cause substrate charging effect. We therefore started to ask ourselves, is silicon dioxide the only material to make graphene visible? Moreover, a recent study has successfully demonstrated a working GaN LEDs with CVD-synthesized multi-layer graphene as conduction electrodes. However, the energy coupling between graphene and GaN is still unclear. To fully utilize the advantage of graphene as transparent electrode, a further understanding of the electronic structure between graphene and the substrate is an urgent task. To answer those questions, we employed theoretical simulation using a model based on Fresnel's law, to calculate the optical contrast of single-layer graphene on various substrate structures. Based on the results, we grew those particular substrates to test the graphene visibility. The graphene flakes and thickness were verified by optical microscope and micro-Raman spectroscopy. The graphene-substrate interactions were then studied by scanning photoelectron microscopy. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y45.00006: Two-Dimensional Optoelectronic Graphene Nanoprobes for Neural Nerwork Tu Hong, Kristina Kitko, Rui Wang, Qi Zhang, Yaqiong Xu Brain is the most complex network created by nature, with billions of neurons connected by trillions of synapses through sophisticated wiring patterns and countless modulatory mechanisms. Current methods to study the neuronal process, either by electrophysiology or optical imaging, have significant limitations on throughput and sensitivity. Here, we use graphene, a monolayer of carbon atoms, as a two-dimensional nanoprobe for neural network. Scanning photocurrent measurement is applied to detect the local integration of electrical and chemical signals in mammalian neurons. Such interface between nanoscale electronic device and biological system provides not only ultra-high sensitivity, but also sub-millisecond temporal resolution, owing to the high carrier mobility of graphene. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y45.00007: Photo Induced Fluorescence Enhancement and Correlated FTIR of Single Layer Graphene Oxide Charudatta Galande, Sibel Ebru Yalcin, Akhilesh Singh, Gautam Gupta, Rajesh Kappera, Andrew M. Dattelbaum, Manish Chhowalla, Stephen K. Doorn, Pulickel M. Ajayan, Aditya D. Mohite Ultrafast recombinations of photo-excited electron-hole pairs and low absorption in graphene have prevented its use for several low light applications. However, graphene oxide (GO) is a wide band gap material with emission in the visible spectrum. For optoelectronic applications, it is desired to have a material with good optical absorption and electrical transport. We report the in-situ photo induced observation of functional groups in progressively reduced GO due to the presence of intercalated water. We perform correlated fluorescence and FTIR spectroscopy on an individual GO flake and we assign the formation of a specific functional group(s) to the observed increase in the fluorescence intensity. This provides insights into tuning the band structure of graphene via controlled oxidation for relevant applications. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y45.00008: Sensitive Room-Temperature Terahertz Detection via Photothermoelectric Effect in Graphene Xinghan Cai, Andrei Sushkov, Ryan Suess, Mohammad Jadidi, Gregory Jenkins, Luke Nyakiti, Rachael Myers-Ward, Virginia Wheeler, Charles Eddy, Jr., Jun Yan, D. Kurt Gaskill, Thomas Murphy, H. Dennis Drew, Michael Fuhrer Due to the weak electron-phonon coupling and strong electron-electron interaction in graphene, the thermoelectric effect provides a highly sensitive detection mechanism for heat absorbed in the electronic system. We present here a bi-metal contacted graphene thermoelectric THz photodetector with sensitivity exceeding 100 V/W at room temperature and noise equivalent power less than 100 pW/Hz$^{1/2}$, competitive with the best room-temperature THz detectors, while time-resolved measurements indicate our graphene detector is eight to nine orders of magnitude faster. We also measured the thermoelectric response to Joule heating, and compare to the thermoelectric response due to optical excitation in the near infrared and at THz frequencies. A simple model of the response, including contact asymmetries reproduces the qualitative features of the data. We also suggest that orders-of-magnitude sensitivity improvements are possible by using local gates to define graphene pn-junctions. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y45.00009: Photon Induced Dynamic THz Conductivity Change in Graphene Sufei Shi, Tom Tang, Bo Zeng, Long Ju, Feng Wang The linear dispersion relation in graphene gives rise to large and highly tunable conductivity at THz regime, which makes graphene a promising candidate for new optoelectronic devices. We use optical pump THz probe spectroscopy to investigate photon induced conductivity change in graphene in time domain, and show that the THz response sensitively depends on the initial doping of graphene. This study sheds light on the carrier relaxation in graphene after optical excitation and provides valuable information for designing future graphene-based opto-electronic device.~ [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y45.00010: Doping-dependent THz photoconductivity in large-area graphene Alex Frenzel, Chun Hung Lui, Yong Cheol Shin, Jing Kong, Nuh Gedik We have performed a systematic investigation of the transient terahertz photoconductivity of large-area CVD graphene following femtosecond optical excitation as a function of electrically-tuned carrier density. We observe a dramatic change in the transient response as the photoconductivity changes from positive to negative when the Fermi level is tuned from the charge neutrality point to the electron or hole doped regime. This effect is discussed within the context of the Drude model for free carriers, taking into account the elevated electron and phonon temperatures in photoexcited graphene. Our results demonstrate that previous conflicting measurements of terahertz photoconductivity in epitaxial and CVD graphene arise primarily from their different doping levels. Additionally, our measurements provide a link between ultrafast optical experiments and DC photocurrent measurements. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y45.00011: Manipulation of the polarization of Terahertz wave in subwavelength regime via hybridizing pseudo-spin polarized gaphenes and metal hole array Xiao Xiao, Che-Ting Chan, Weijia Wen In this presentation, we show that the subwavelength quarter wave plate and half wave plate in terahertz regime can be realized in a metal hole array (MHA) sandwiched by two stacks of pseudo-spin polarized graphenes (PSPGs). When the gaps of the two PSPGs are the same, the hybrid resonances in the system can convert the linearly polarized incident light into the circularly polarized transmitted light; On the other hand, when the gaps of the two PSPGs are opposite in sign, the polarization of the reflected light can be rotated by $90^{\circ}$. Interestingly, when the PSPGs is tuned in the quantum Hall regime, the fine structure constant can relate with the resonant frequency and the geometrical parameters of MHA directly. The rich properties of the system guarantee its potential applications in THz technologies. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y45.00012: Terahertz time-domain spectroscopy of large-area graphene on various substrates Iwao Kawayama, Makoto Ohshiro, Yuki Sano, Hironaru Murakami, James Allred, Minjie Wang, Junichiro Kono, Robert Vajtai, Pulickel Ajayan, Masayoshi Tonouchi The advent of large-area graphene samples has opened up tremendous new opportunities for terahertz and infrared optoelectronic devices as well as for fundamental studies of low-energy excitations in graphene in the terahertz frequency range. While there have been extensive studies on the strong influence of the supporting substrate and the local molecular environment on the optical and DC transport properties of graphene, no systematic studies exist on their effects on graphene's terahertz properties. In this study, we a terahertz time-domain spectroscopy study of large-area graphene mono-layers on various terahertz-transparent substrates (e.g., InAs, InP, GaAs, MgO, and polypropylene). We found that the terahertz optical conductivity spectrum shows qualitatively different behaviors, depending on the substrate, which can be understood through substrate-induced doping and defects as well as interaction with substrate phnons. In addition, we observed that the effects of adsorbed gas molecules on the terahertz conductivity also vary, depending on the substrate. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y45.00013: THz investigations of graphene-complex-oxide heterostructures Giriraj Jnawali, Lu Chen, Patrick Irvin, Jeremy Levy, Sangwoo Ryu, Chang-Beom Eom, Fereshte Ghahari, Jayakanth Ravichandran, Philip Kim The unique and multifaceted properties of graphene have fascinated scientists and engineers for a decade now. A new frontier in research concerns properties of graphene in the THz-IR region, where the collective excitation of graphene 2D electron gas (2DEG) into plasmonic waves has proven the salient feature.\footnote{L. Ju, \textit{et al.}, Nat. Nanotechnol. \textbf{6}, 630 (2011)} Complex oxide heterostructures (e.g., LaAlO$_{3}$/SrTiO$_{3}$, LAO/STO) also support a 2DEG with high carrier densities and expected plasmonic behavior. A unique feature of the LAO/STO system is the ability to control the electron density with nanoscale precision.\footnote{C. Cen, \textit{et al.}, Nat. Mater. \textbf{7}, 298 (2008)} In addition, a method for sourcing and detecting broadband THz emission from LAO/STO nanojunctions has been recently demonstrated.\footnote{Y. Ma, \textit{et al.}, Nano Lett. \textbf{13}, 2284 (2013)} Here we describe initial efforts to investigate the THz properties of graphene-complex oxide (GCO) heterostructures. We envision that the proposed graphene plasmonic devices in the GCO will help to lay the foundation for a host of powerful THz-IR technologies for signal processing, imaging, spectroscopy and chemical sensing. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y45.00014: Theoretical modeling of the terahertz response of ultrafast photoexcited charge carriers in graphene Avinash Rustagi, Christopher J. Stanton We have formulated a semi-classical model to capture the terahertz response of photoexcited charge carriers in graphene. The model involves the time evolution of the initial carrier distribution function excited by a femtosecond laser pulse by solving the Boltzmann equation within the relaxation time approximation in presence of an in-plane DC electric field. We solve for the time dependent average velocity using the distribution function obtained from the Boltzmann equation. The time derivative of this average velocity is proportional to the terahertz signal measured in experiments. We also consider the contribution of virtual carriers to the terahertz signal. This model can also be applied to systems with a gapped graphene-like dispersion. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y45.00015: Transport in graphene exposed to a strong electromagnetic field Sergey Syzranov, Yaroslav Rodionov, Kliment Kugel, Franco Nori We study quasiparticle dynamics in graphene exposed to a linearly-polarized electromagnetic wave of very large intensity. We demonstrate that low-energy transport in such system can be described by an effective time-independent Hamiltonian, characterized by multiple Dirac points in the first Brillouin zone. Around each Dirac point the spectrum is anisotropic: the velocity along the polarization of the radiation significantly exceeds the velocity in the perpendicular direction. Moreover, in some of the points the transverse velocity oscillates as a function of the radiation intensity. These features of the quasiparticle spectrum manifest themselves in the conductance of graphene-based junctions in the regime of strong irradiation. For instance, we find that the conductance of a graphene p-n junction depends on the polarization as $G(\theta)\propto|\sin\theta|^{3/2}$, where $\theta$ is the angle between the polarization and the p-n interface, and oscillates as a function of the radiation intensity. [Preview Abstract] |
Session Y46: Focus Session: Superconductivity, Vortex Matter-III; Unconventional Structures and Transport
Sponsoring Units: DCMPChair: Alexei Koshelev, Argonne National Laboratory
Room: Mile High Ballroom 4E
Friday, March 7, 2014 8:00AM - 8:12AM |
Y46.00001: Transition from Vortex-Antivortex Pairs to Single Vortices in 2D SNS Josephson Junction Arrays Malcolm Durkin, Serena Eley, Nadya Mason SNS Josephson Junction arrays represent a model system for studying vortices in 2D systems.~ Despite long term interest in zero magnetic field effects, such as the Berezinsky-Kosterlitz-Thouless transition, the field tuned transition between a state dominated by vortex-antivortex pairs and one dominated by single vortices remains largely unstudied. Often, evidence for such a transition is mischaracterized as a large suppression of the single vortex energy barrier at low fields. Here we demonstrate, via transport measurements, a finite magnetic field transition between vortex-antivortex pair and single vortex excitations in a 2D superconductor. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y46.00002: Fractional Josephson vortices in two-gap superconductor long Josephson junctions Ju Kim We investigated the phase dynamics of long Josephson junctions (LJJ) with two-gap superconductors in the broken time reversal symmetry state. In this LJJ, spatial phase textures (i-solitons) can be excited due to the presence of two condensates and the interband Joesphson effect between them. The presence of a spatial phase texture in each superconductor layer leads to a spatial variation of the critical current density between the superconductor layers. We find that this spatial dependence of the crtitical current density can self-generate magnetic flux in the insulator layer, resulting in Josephson vortices with fractional flux quanta. Similar to the situation in a $YBa_2Cu_3O_{7-x}$ superconductor film grain boundary [1], the fractionalization of a Josephson vortex arises as a response to either periodic or random excitation of i-solitions. This suggests that magnetic flux measurements may be used to probe i-soliton excitations in multi-gap superconductor LJJs. \\ \\ 1. R. Mint and I. Papiashvili, Phys. Rev. B \textbf{64}, 134501 (2001). [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y46.00003: Dynamic vortex in a narrow superconducting film with Josephson junction Alex Gurevich A vortex moving along a planar Josephson junction perpendicular to a narrow thin film strip is investigated. Exact solutions of the equations of nonlocal Josephson electrodynamics describing the dynamic structure of the vortex driven by time-dependent current across an overdamped junction are obtained. It is shown that this problem reduces to two coupled nonlinear differential equations for the vortex position and core length. Using these equations, the critical film width below which the vortex turns into a phase slip is calculated. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y46.00004: Fractional Vortices in Multi-Gap Superconductors Yen Lee Loh, Monica Kim, Ju H. Kim Novel topological defects, known as fractional vortices, can occur in thin films of multi-gap superconductors. We study two-gap and three-gap superconducting films within a classical Ginzburg-Landau description, using numerical simulations and analytic approximations. In two-gap superconducting films, we find that the interband Josephson coupling $J_{12}$ leads to an effective attraction between half-vortices, whereas the permeability parameter $\mu$ leads to an effective repulsion between half-vortices. We locate the phase boundary in $(J_{12}, \mu)$ space that marks the onset of spontaneous vortex fractionalization. We describe how the size of a fractional vortex increases as one goes deeper into the fractionalized phase. Our results suggest that coating a multi-gap superconducting film with a paramagnetic overlayer will enhance the tendency towards vortex fractionalization. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y46.00005: Phase diagrams of vortex matter with multiple length scale pair interaction in layered superconductors Qingyou Meng, Christopher Varney, Hans Fangohr, Egor Babaev Recently, Romero-Isart {\em et al.} [Phys. Rev. Lett. 111, 145304 (2013)] proposed a new way of trapping ultracold atoms using the magnetic field generated by a vortex lattice, where the lattice is generated by pinning the vortices. Here, we show that the same effect can be achieved with layered superconductors without pinning the vortices. We utilize Langevin dynamics to determine the ground state phase diagrams for pair potentials that describe the vortex physics of layered superconducting systems. We also present two zero temperature phase diagrams of vortex matter with three and four short length scale pair interaction. In the first phase diagram, there are 10 phases such as hexagonal, dimer, stripe, void, like kagom\'e, square, dimer hexagonal, honeycomb, glass and cluster phases. And in the second one, there are 8 phases such as hexagonal, dimer, stripe, honeycomb, kagom\'e, glass, void and cluster phases. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y46.00006: Zero-Magnetic-Field Phase-Decoherence Transition in Underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ Paul Baity, Xiaoyan Shi, Zhenzhong Shi, Dragana Popovi\'{c} The two key prerequisites for superconductivity are electron pairing and phase coherence of the pair wave-function. We present an electrical transport study on underdoped La$_{2-x}$Sr$_{x}$CuO$_4$ (LSCO) films ($x=0.07$ and $0.08$) that suggests that, in zero magnetic field ($H=0$), superconductivity is destroyed by thermal unbinding of vortex-antivortex phase fluctuations at a temperature $T_{BKT}$. In particular, current-voltage ($I-V$) curves follow a power law $V \propto I^{\alpha(T)}$ with $\alpha (T) \geq 3$ for $T\leq T_{BKT}$. In addition, the contribution of the superconducting fluctuations to the conductivity, $\Delta\sigma_{SCF}(T,H=0)$, obtained by extrapolating the measured magnetoresistance from the normal state at high enough $H$ and $T$, increases monotonically with decreasing $T$ and diverges exponentially at $T_{BKT}$. These results suggest that the $H=0$ superconducting transition, where the Ohmic resistivity also vanishes, is due to the loss of phase coherence and manifests itself as a Berezinskii-Kosterlitz-Thouless transition. Our findings agree well with other experiments on LSCO with higher doping. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y46.00007: Transport Properties of Thin Films of The Noncentrosymmetric Superconductors ZrRe$_{6}$ and Re$_{3}$W Mojammel Alam Khan, Daniel Lepkowski, Ahmad Us Saleheen, Joseph Prestigiacomo, Amar Karki, Rongying Jin, Tijiang Liu, Shane Stadler, Phil Adams, David Young Thin films ($\sim$50{\AA} - 500{\AA}) of ZrRe$_{6}$ and Re$_{3}$W were grown by pulse laser deposition from arc-melted targets. The dependence of the film's critical temperatures, as well as the film's upper critical field, was determined as a function of film thickness and compared to bulk samples. A 500{\AA} film of ZrRe$_{6}$ had a transition temperature near 6.5 K. The thermal conductivity and thermo-electric power of bulk samples were also measured. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y46.00008: Quenched Randomness, the Imry-Ma theorem, and Topology Thomas Proctor, Eugene Chudnovsky, Dmitry Garanin In 1975, Imry and Ma made the analytical prediction that an exchange model in $d$ dimensions under the influence of a weak random field of strength $h_r$ will have a correlation length of $R_f \propto h_r^{-\frac{2}{4-d}}$ . However, numerical results since then have not given strong support to this analytical result. In our numerical studies, we have found that models that support topological structures, such as vortices or skyrmions, show spin states that have hysteresis, are highly dependent on initial conditions, and do not follow Imry-Ma prediction. Meanwhile, models that do not support these topological structures follow the Imry-Ma prediction, implying that the Imry-Ma state is not reached because of topological effects. These findings have implications for random magnets and flux lattices. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y46.00009: Magnetoresistance, Hall effect, and Point Contact Tunneling Spectroscopy of Superconducting LiTi$_{2}$O$_{4}$ Thin Films X.H. Zhang, R. Suchoski, S. Maruyama, S. Yasui, J.M. Shin, Y.P. Jiang, R.L. Greene, I. Takeuchi, G. He, L. Shan, K. Jin Superconducting LiTi$_{2}$O$_{4}$ thin films with a transition temperature of 11 K have been epitaxially fabricated on MgAl$_{2}$O$_{4}$ substrates using pulsed laser deposition (PLD). Systematic studies of the transport properties and the tunneling spectroscopy of the films (t $\sim$ 180nm) have been performed. In the normal state, the Hall coefficient shows a nearly constant value with a positive sign over a broad temperature range, suggesting a single-band hole-like electronic transport. The magnetoresistance of the material shows an unexpected change in the sign at 50 K. Below this temperature, the resistance shows a conventional parabolic increase with field. However, above this temperature, an unusual negative magnetoresistance appears. In the superconducting state, an upper critical field of about 18 Tesla is found by both magnetotransport and point-contact tunneling spectroscopy (PCS). In addition, our PCS results suggest that the superconducting gap in LiTi$_{2}$O$_{4}$ is BCS-like. A possible cause of the unusual negative magnetoresistance will be discussed. Preliminary results on the field effect using ionic liquid gating will also be presented. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y46.00010: I-V characteristics of atomically thin superconducting $La_{2-x}Sr_xCuO_4$ films Scott Dietrich, William Mayer, Sergey Vitkalov, Andrey Sergeev I-V characteristics of $La_{2-x}Sr_xCuO_4$ thin films grown by Molecular Beam Epitaxy are studied in a wide range of temperatures in zero magnetic field. Strongly nonlinear response to applied electric current was observed near the superconducting state. At temperatures $T>T_{BKT}$ the observed characteristics have three distinct features: (1) at small currents voltage $V$ is proportional to the current $I$; (2) at higher currents a cubic dependence ($I \sim I^3$) is observed; (3) and even higher currents $V \sim I^\alpha$ with coefficient $\alpha(T)<$ 3, which decreases with the temperature increase. At $T=T_{BKT}$ the coefficient $\alpha=3$. Observed nonlinear response is in-line with Berezinsky--Kosterlitz--Thouless (BKT) theory. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y46.00011: $H_{c2}$ as a probe for multiband superconductivity in LAO/STO Jonathan Edge, Alexander Balatsky We investigate the temperature dependence of the upper critical field $H_{c 2} $ as a tool to probe the possible presence of multiband superconductivity at the interface of LAO/STO. The behaviour of $H_{c 2} $ can clearly indicate two-band superconductivity through its nontrivial temperature dependence. For the disorder scattering dominated two-dimensional LAO/STO interface we find a characteristic non-monotonic curvature of the $H_{c 2} (T) $. We also analyse the $H_{c 2} $ for multiband bulk STO and find similar behaviour. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y46.00012: Magnetotransport in extremely-high magnetic fields of La$_{2-x}$Sr$_{x}$CuO$_{4}$ with nearly continuous doping Zachary Stegen, Greg Boebinger, Fedor Balakirev, Albert Migliori, Jie Wu, Ivan Bozovic The doping dependence of the Hall effect and longitudinal magneto-resistance in the high-temperature superconductor La$_{2-x}$Sr$_{x}$CuO$_{4}$ are examined. Samples were grown using Combinatorial Molecular Beam Epitaxy (COMBE), which allows electrical transport measurements with a doping resolution of $\Delta x \approx 0.0002$. This is an increase in doping resolution of about 50-fold compared to typical magneto-resistance measurements. The experiments were performed in pulsed magnetic fields up to 57 T with the goal of investigating transport properties of the magnetically-induced normal state near optimum doping. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y46.00013: Force balance on two-dimensional superconductors with a single moving vortex Chun Kit Chung, Emiko Arahata, Yusuke Kato We study forces on two-dimensional superconductors with a single moving vortex based on a recent fully self-consistent calculation of DC conductivity in an $s$-wave superconductor (E. Arahata and Y. Kato, arXiv:1310.0566). By considering momentum balance of the whole liquid, we attempt to identify various contributions to the total transverse force on the vortex. This provides an estimation of the effective Magnus force based on the quasiclassical theory generalized by Kita [T. Kita, Phys. Rev. B, {\bf 64}, 054503 (2001)], which allows for the Hall effect in vortex states. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y46.00014: Intermediate state: a new look at an old story Vladimir Kozhevnikov, Rinke Wijngaarden, Jesse de Wit, Chris Van Haesendonck One of the central problems of superconductivity is magnetic structure of vortices and elicitation of microscopic parameters from parameters of the mixed state (MS) in type-II superconductors. Similar problem, i.e. magnetic structure of normal (N) domains and elicitation of the microscopic parameters from parameters of the intermediate state (IS) in type-I materials, is the longest standing problem of superconductivity advanced by Landau in 1930s. We will report on our recent study of the IS in a high purity indium films using magneto-optical imaging, and transport and magnetization measurements. The least expected observation is that the magnetic flux density in N-domains can be as small as nearly 40\% of the thermodynamic critical field $H_c$. This fact contradicts and hence overthrows a paradigm stating that the N-phase is unstable in the fields less than $H_c$. We will present a new theoretical model of the IS for the first time consistently addressing this and $all$ other properties of the IS. Moreover, our model, based on rigorous thermodynamics of the equilibrium flux structure, allows for quantitative determination of the domain-wall parameter and the coherence length. Possible impact of our model on the vortex structure will be discussed. [Preview Abstract] |
Session Y47: Theory of Strongly Correlated Superconductivity
Sponsoring Units: DCMPChair: Brian Moritz, SLAC National Accelerator Laboratory
Room: Mile High Ballroom 4F
Friday, March 7, 2014 8:00AM - 8:12AM |
Y47.00001: d-wave superconducting phase diagram of the two dimensional Hubbard model Andre Marie Tremblay, Giovanni Sordi, Patrick Semon Superconductivity and Mott insulating state intertwine in materials such as cuprates and organic conductors. We study the d-wave superconducting phase at finite temperature in the two-dimensional Hubbard model on the square lattice within cellular dynamical mean-field theory and continuous-time quantum Monte Carlo. The whole phase diagram as a function of temperature, doping and interaction strength shows that a transition directly to the superconducting state from a Mott insulator is possible at the cellular dynamical mean-field level, whether the transition is bandwidth or doping driven. The dynamical mean-field superconducting transition temperature $T_c^d$ does not scale with the superconducting order parameter when there is a normal-state pseudogap. $T_c^d$ corresponds to the local pair formation temperature observed in tunneling experiments and is distinct from the pseudogap temperature, suggesting that pseudogap and superconductivity are distinct phenomena. Refs: G. Sordi et al., PRB 041101 (2013), G. Sordi et al. PRL 108 2164101 (2012) [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y47.00002: Resilience of d-wave superconductivity to nearest-neighbor repulsion A.G.R. Day, D. Senechal, V. Bouliane, A.-M.S. Tremblay Many theoretical approaches find $d$-wave superconductivity in the one-band Hubbard model for high-temperature superconductors. At strong-coupling ($U\geq W$, where $U$ is the on-site repulsion and $W=8t$ the bandwidth) pairing is controlled by the exchange energy $J=4t^2/U$. One may then surmise, ignoring retardation effects, that near-neighbor Coulomb repulsion $V$ will destroy superconductivity when it becomes larger than $J$, a condition that is easily satisfied in cuprates for example. Using Cellular Dynamical Mean-Field theory with an exact diagonalization solver for the extended Hubbard model, we show that pairing {\it at strong coupling} is preserved, even when $V\gg J$, as long as $V \la U/2$. While at weak coupling $V$ always reduces the spin fluctuations and hence $d$-wave pairing, at strong coupling, in the underdoped regime, the increase of $J=4t^2/(U-V)$ caused by $V$ increases binding at low frequency while the pair-breaking effect of $V$ is pushed to high frequency. These two effects compensate in the underdoped regime, in the presence of a pseudogap. While the pseudogap competes with superconductivity, the proximity to the Mott transition that leads to the pseudogap, and retardation effects, protect $d$-wave superconductivity from $V$. PRB 87, 075123 (2013) [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y47.00003: Competition between antiferromagnetism and superconductivity -- a quantum Monte-Carlo study Da Wang, Yi Li, Congjun Wu The competition between antiferromagnetism (AFM) and superconductivity (SC) remains a challenging problem in condensed matter physics because of the lack of non-perturbative method to handle strong correlations. Quantum Monte Carlo (QMC) simulations often suffers from the notorious fermion sign problem. It has been found that in a multi-band Hubbard model, or, equivalently, a large-spin Hubbard model, the sign problem can be removed at arbitrary fillings in a parameter regime in which SC competes with AFM. We have performed QMC simulations to investigate the phase diagram as doping and interaction strength. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y47.00004: A fully gapped superconducting state in small cuprate islands MIkael Fogelstr\"om, David Gustafsson, Dmitri Golubev, Annica Black-Schaffer, Tord Claeson, Sergey Kubatkin, Thilo Bauch, Floriana Lombardi We present a spectroscopic technique, based on an high-Tc superconducting nanoscale device that allows an unprecedented energy resolution thanks to Coulomb blockade effects, a regime practically inaccessible earlier in these materials. We found that the energy required to add an extra electron depends on the parity (odd/even) of excess electrons on the island and increases with magnetic field. This is inconsistent with a pure $d_{x^2-y^2}$ wave symmetry and demonstrates a complex order parameter component that needs to be encompassed in any theoretical model for high-Tc superconductivity. To address this inconsistency, we investigate subdominant order parameters stabilizing at low temperatures in nano-scale high-T$_c$ cuprate islands. Using complementary quasi-classical and tight-binding Bogoliubov-de Gennes methods, we show on distinctly different properties dependent on the symmetry being $d_{x^2-y^2}+i s$ or $d_{x^2-y^2}+i d_{xy}$. We find that a surface-induced $d_{x^2-y^2}+i s$ phase creates a global spectroscopic gap which increases with applied magnetic field, consistent with experimental observation. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y47.00005: Fermi-surface-free Superconductivity near a Topological Transition in Cuprates Peter Mistark, Hasnain Hafiz, Robert Markiewicz, Arun Bansil The phase diagram for cuprates has grown from a simple superconducting dome to one which includes a pseudogap phase and other potential orders which cut the superconducting dome into distinct parts. Investigating the doping dependence of the Fermi surface (FS) for hole doping of an antiferromagnetic (AFM) plus superconducting (SC) system, we find a transition from a FS with nodal hole pockets to one which also includes antinodal electron pockets. The key experimental signature of this transition is that the antinodal spectral weight increases dramatically after the appearance of the electron pocket. Just preceding this transition superconductivity can take advantage of the density of states associated with the antinodal band above the FS, creating a superconducting gap in the absence of a band crossing the FS: fermi-surface-free superconductivity. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y47.00006: The Puzzle of Anomalous Isotope Effect in Zr, Nb$_{3}$Sn, and YBa$_{2}$Cu$_{3}$O$_{7}$ Guang-Lin Zhao Anomalously small isotope effect in some high and low T$_{c}$ superconductors such as Zr, Nb$_{3}$Sn, YBa$_{2}$Cu$_{3}$O$_{7}$ created a great challenge for understanding. To shed light on a clue to solve this puzzle, a new methodology was implemented by integrating first-principles calculations of electronic structures of the materials into the theory of many-body physics for superconductivity. The aim is to seek a unified methodology to calculate the electronic and superconducting properties of these materials. It is shown that the electronic structures of Zr, Nb$_{3}$Sn, YBa$_{2}$Cu$_{3}$O$_{7}$ are very complex. The electron densities of states around the Fermi level in Zr, Nb$_{3}$Sn, YBa$_{2}$Cu$_{3}$O$_{7}$ possess sharp variations that could have a significant contribution to the anomalous isotope effect in these superconductors. However, there still exist some differences between the calculated and experimental results that require further research work. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y47.00007: Dynamics of competing orders in two-dimensional metals with antiferromagnetic exchange interactions Wenbo Fu, Ling-Yan Hung, Subir Sachdev We study the dynamics of bond order parameters after a quantum quench in a two-dimensional square lattice model with nearest-neighbor exchange and repulsion, using an unrestricted time-dependent Hartree-Fock computation. The mean-field model can be constructed by a set of operators, including $d$-wave Cooper pair and particle-hole pair, which form a SU(4) algebra, and thus their equations of motion are closed. After the quench, we find enhanced oscillation amplitude of the $d$-wave charge order below superconducting critical temperature ($T_c$) as observed in recent experiments in $YBa_2Cu_3O_{6+x}$. We also observe a phase shift when crossing $T_c$ and temperature-dependent frequencies. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y47.00008: A DFT study of rocksalt proxy copper monochalcogenide structures -- Implications for possible high-Tc superconductivity P.M. Grant, R.H. Hammond We report findings derived from a series of DFT calculations on the structural stability and paramagnetic ground states of four idealized copper monochalcogenide (CuO, CuS, CuSe, CuTe) rocksalt structures. Note that none of these target compounds occur naturally, but can possibly be fabricated using ``forced epitaxy'' MBE methods, as has been done to grow CuO tetragonal rocksalt films 5-6 monolayers thick.\footnote{W. Siemons, et al., PRB 79, 195122 (2009), DOI: 10.1103/PhysRevB.79.195122.}$^,$\footnote{P. M. Grant, J. of Physics: CS 129, 012042 (2008), DOI: 10.1088/1742-6596/129/1/012042} Therefore, we treat all examples we report herein as proxies intended to explore candidate implications for possible future high-T$_{\mathrm{C}}$ materials. In particular, we find, as might be expected from the long accepted Van Vleck-Anderson-Hubbard formalism describing antiferromagnetic insulators, the Neel temperature scales upward roughly as the width of the spin-carrying bands near or adjacent to the Fermi level or energy gap. We conclude such trend might result in higher superconducting transition temperatures should this be mediated by carrier-spin excitation/fluctuation driven pairing scaled by T$_{\mathrm{N}}$. Finally, we briefly discuss synthetic paths to realizing actual embodiments of our proxy exercises. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y47.00009: The Microscopic Spin fluctuation theory of superconductivity in Spin Density Wave metals Wenya Rowe, Ilya Eremin, Astrid R{\O}mer, Brian Andersen, Peter Hirschfeld We revisit the problem of electron pairing by spin waves in the commensurate spin density wave ordered state, and generalize the existing theory to include situations with electron pockets, hole pockets, or both. We derive simple analytic forms and the leading instabilities for the fluctuation exchange pairing vertex in these cases. In general pairing arises both from transverse spin waves and from gapped longitudinal charge and spin fluctuations, they acts primarily within one type of pocket. Only the d-wave state in the electron doped case is robust in the limit of weak magnetism and doping. By contrast, in the hole-doped case, we find that the spin-singlet odd parity $p$-wave state allowed in the SDW represents the leading instability. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y47.00010: General Rule of Negative Effective Ueff System \& Materials Design of High-Tc Superconductors by ab initio Calculations Hiroshi Katayama-Yoshida, Akitaka Nakanishi, Hiroki Uede, Yuki Takawashi, Tetsuya Fukushima, Kazunori Sato Based upon ab initio electronic structure calculation, I will discuss the general rule of negative effective U system by (1) exchange-correlation-induced negative effective U caused by the stability of the exchange-correlation energy in Hund's rule with high-spin ground states of d5 configuration, and (2) charge-excitation-induced negative effective U caused by the stability of chemical bond in the closed-shell of s2, p6, and d10 configurations. I will show the calculated results of negative effective U systems such as hole-doped CuAlO2 and CuFeS2. Based on the total energy calculations of antiferromagnetic and ferromagnetic states, I will discuss the magnetic phase diagram and superconductivity upon hole doping. I also discuss the computational materials design method of high-Tc superconductors by ab initio calculation to go beyond LDA and multi-scale simulations. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y47.00011: Superconductivity in anisotropic ferromagnets near a transverse saturation field Ilya Vekhter, Kazushi Aoyama, Hiroaki Ikeda In the uranium compounds such as URhGe, UCoGe, and UGe2, superconductivity emerges inside ferromagnetic phases and often exhibits a reentrant behavior in a magnetic field. Motivated by this experimental observation, we consider a model for superconductivity in an anisotropic ferromagnet under transverse field. We derive the spectrum of critical magnetic excitations near the saturation field, derive the pairing interaction due to exchange of these spin fluctuations, and compute the transition temperature into the superconducting state. We compare our results with experiments on U-based ferromagnetic superconductors and with recent theoretical analyses. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y47.00012: Topological Odd-Parity Superconductivity Close to Type-II 2D Van Hove Singularities Hong Yao, Fan Yang We study unconventional superconductivity induced by weak repulsive interactions in 2D electronic systems at Van Hove singularity (VHS) where electronic density of states is logarithmically divergent. We define two types of VH saddle points. For type-I VH systems, weak repulsive interactions generically induce unconventional singlet pairing. However and more interestingly, for type-II VH systems renormalization group treatment shows that weak repulsive interactions favor triplet pairing (e.g. p-wave) when the Fermi surface has no good nesting. When such type-II VH systems respecting tetragonal or hexagonal point group symmetry, topological superconductivity (chiral p$+$ip or time reversal invariant Z2 p$+$ip pairing) will generally occur. We shall also discuss implications of this study to recently discovered BiS2-based superconductors and other superconducting materials that host type-II VH singularities in their Fermi surfaces. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y47.00013: Single-polaron properties for double-well electron-phonon coupling Clemens Adolphs, Mona Berciu We introduce a new model to describe electron-phonon coupling in systems such as one-dimensional intercalated chains or two-dimensional $\mathrm{CuO}_2$ planes, where symmetry dictates that the linear coupling term vanishes. We show that, under certain conditions, an additional charge carrier dynamically changes the local lattice potential from a harmonic well into a double well. We use the Momentum Average approximation to study the properties of this model in the single-polaron limit. A detailed analysis reveals that despite some qualitative similarities to the linear Holstein model, a renormalized Holstein model cannot account for all of the physics of the double-well model. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y47.00014: DMFT analysis of the superconductivity in the Holstein-Hubbard model -- Interplay of strong Coulomb interaction and electron-phonon coupling Yuta Murakami, Philipp Werner, Naoto Tsuji, Hideo Aoki Phonon-mediated superconductivity when, as in the alkali-doped fullerides and aromatic compounds, the Coulomb interaction, electron-phonon coupling and phonon frequencies are all comparable to the electronic band width poses an intriguing question. In order to obtain insights into the superconductivity in this regime, we have analyzed the Holstein-Hubbard model with the dynamical mean-field theory with a continuous-time quantum Monte Carlo impurity solver. We focus on the s-wave superconducting state when the Hubbard repulsion $U$, the phonon mediated attractive interaction $\lambda$ and the phonon energy ($\omega_0$) are comparable to the bandwidth. A particular interest is the effects of the retardation and the strong Coulomb interaction on the behavior of the transition temperature $T_C$, the superconductivity order parameter and gap in spectrum ($\Delta$). We find that the Tc-dome against $U_{\rm eff} = U-\lambda$ significantly deviates from that in the anti-adiabatic limit, and that an effective model in the polaron representation reproduces the effect of the retardation and the Coulomb interaction well even for $\omega_0$ smaller than the bandwidth. We also show an unusual isotope effect for fast phonons and deviation of $2\Delta/k_B T_C$ from BCS value. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y47.00015: Unconventional Superconductivity by Fermi Surface Mismatch: A Diagrammatic Monte Carlo Study Jan Gukelberger, Evgeny Kozik, Lode Pollet, Kris Van Houcke, Nikolay Prokof'ev, Boris Svistunov, Matthias Troyer The conventional BCS pairing mechanism for s-wave superconductivity relies on spin rotation symmetry ensuring coinciding Fermi surfaces for both spin species. We study attractively interacting fermions on a square lattice where this symmetry is broken by imposing either a spin imbalance or a spin-dependent hopping anisotropy. The resulting Fermi surface mismatch disfavours conventional superconductivity making room for new kinds of order such as inhomogeneous or triplet superconductivity. We present unbiased numeric results for the low temperature phase diagrams of these models obtained with Diagrammatic Monte Carlo, a new technique for correlated fermionic systems based on sampling Feynman diagrammatic series directly in the thermodynamic limit. [Preview Abstract] |
Session Y48: Invited Session: Spin Transport in Novel 2d Electronic Systems
Sponsoring Units: GMAG DCMPChair: Berend Jonker, Naval Research Laboratory
Room: Mile High Ballroom 1A-1B
Friday, March 7, 2014 8:00AM - 8:36AM |
Y48.00001: Topological Electronic Structures and Spintronics Applications for Silicene and Other Spin-Orbit Thin Films Invited Speaker: Hsin Lin While spin-orbit coupling plays a critical role in generating topologically insulating phases, it also provides a novel route for realizing spin-split states in nonmagnetic materials without the need for exchange coupling. Two-dimensional thin films with significant spin-orbit coupling strength enable potential applications for spintronics devices because the spin-splitting energy can be controlled by an external field (gating). Moreover, spin-orbit coupling can induce nontrivial topological phases, i.e. quantum spin Hall phases, which could harbor back-scattering-free spin-polarized current at the edge. Recently, we have shown via first-principles calculations that field-gated silicene possesses two gapped Dirac cones exhibiting nearly 100\% spin-polarization, situated at the corners of the Brillouin zone. Band gaps as well as the band topology can be tuned with an external electric field perpendicular to the plane, which breaks the inversion symmetry of the system due to the presence of buckling in the honeycomb structure. Using this fact, we propose a design for a silicene-based spin-filter that would enable the spin-polarization of an output current to be switched electrically, without the need to switch external magnetic fields. Our quantum transport calculations indicate that the proposed designs will be highly efficient (nearly 100\% spin polarization) and robust against weak disorder and edge imperfections. We also propose a Y-shaped spin/valley separator that produces spin-polarized current at two output terminals with opposite spins. Ge, Sn, and Pb counterparts of silicene are shown to have similar properties, but their larger spin-orbit coupling results in larger energy differences between the spin-split states making these materials better suited for room temperature applications. Other spin-orbit thin films will be discussed. Our investigations demonstrate that spin-orbit thin films present great potential for manipulating spin/valley degrees of freedom efficiently, moving us a step closer to realizing the dream of spintronics applications. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 9:12AM |
Y48.00002: Direct electrical detection of spin-momentum locking in the topological insulator Bi$_{2}$Se$_{3}$ Invited Speaker: Connie H. Li Topological insulators (TIs) are a new quantum state of matter [1] characterized by metallic surface states populated by massless Dirac fermions. TIs are expected to exhibit new behaviors and open horizons for science previously inaccessible with ``conventional'' materials. One of the most striking properties is that \textit{of spin-momentum locking} -- the spin of the TI surface state lies in-plane, and is locked at right angle to the carrier momentum. An unpolarized charge current should thus create a net spin polarization whose amplitude and orientation are controlled by the charge current. This remarkable property has been anticipated by theory [2], but never accessed in a simple transport structure. Here we show that a bias current indeed produces a net surface state spin polarization \textit{via} spin-momentum locking in molecular beam epitaxially grown Bi$_{2}$Se$_{3}$ films, and this polarization is directly manifested as a voltage on a ferromagnetic metal contact. This voltage is proportional to the projection of the TI spin polarization onto the contact magnetization, is determined by the direction and magnitude of the bias current, scales inversely with Bi$_{2}$Se$_{3}$ film thickness, and its sign is that expected from spin-momentum locking rather than a Rashba effect [3]. Similar data are obtained for structures with two different ferromagnet/tunnel barrier contacts, demonstrating that these behaviors are independent of the details of the detector contact. These results demonstrate direct electrical access to the TI surface state spin system and enable utilization of its remarkable properties for future technological applications.\\[4pt] [1] J. E. Moore, Nature \textbf{464}, 194 (2010); M. Z. Hasan et. al., Rev. Mod. Phys. \textbf{82,} 3045 (2010); L. Fu et. al., PRL \textbf{98}, 106803 (2007); D. Hsieh et. al., Nature \textbf{452}, 970 (2008).\\[0pt] [2] A. A. Burkov et. al. PRL \textbf{105}, 066802 (2010); D. Culcer et. al., PRB \textbf{82}, 155457 (2010); V. Yazyev et. al., PRL \textbf{105}, 266806 (2010).\\[0pt] [3] S. Hong et. al., PRB \textbf{86}, 085131 (2012). [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:48AM |
Y48.00003: Intrinsic limitations of spin transport in 2D membranes Invited Speaker: Yang Song Two dimensional membranes have become the playground for both theorists and experimentalists due to their unique intrinsic properties emerged from simple lattice structures. They are the new focus of spintronic applications. Therefore, it is important that we have a clear view of the relaxation processes in spin transport, limited by their intrinsic and symmetry structures. In this talk, we present our findings [1,2] by systematically applying group theory to the coupling of phonons and transport carriers in spin-dependent scattering. Scattering by phonon is amplified in 2D membranes due to its unique and populous flexural mode. Opposite spin coupling by one flexural phonon is allowed by symmetry, unlike the momentum scattering by higher-order two flexural phonons. Furthermore, we specifically discuss the ultrafast electron spin relaxation in single-layer transition metal dichalcogenides (SL-TMDs) [1]. The additional factor stems from the decoupling of tiny conduction band spin splitting and the large spin scattering constant. The former results from conduction band orbital orientation, while the latter comes from inter-band coupling and reflects the atomic SOC strength. We will present that the essential use of group theory (invariant quantities) elucidates various spin-dependent selection rules of electron/hole-phonon interaction, within and between all relevant band-valley edges. Multiple potential applications of the derived results can be explored in transport problems, such as the strain effects [2], spin Gunn effect, hot exciton dynamics [1], and the scattering angle and spin anisotropy dependence. We compare different 2D membranes (graphene, SL-TMD, silicene and germanene) from general consideration of the lattice and band-edge symmetries. \\[4pt] [1] Y. Song and H. Dery, Phys. Rev. Lett., 111, 026601 (2013).\\[0pt] [2] T. Cheiwchanchamnangij, W. Lambrecht, Y. Song and H. Dery, Phys. Rev. B, 88, 155404 (2013). [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:24AM |
Y48.00004: Layered Chalcogenides beyond Graphene: from Electronic Structure Evolution to the Spin Transport Invited Speaker: Hongtao Yuan Recent efforts on graphene-like atomic layer materials, aiming at novel electronic properties and quantum phenomena beyond graphene, have attracted much attention for potential electronics/spintronics applications. Compared to the weak spin-orbit-interaction (SOI) in graphene, metal chalcogenides MX$_{\mathrm{2}}$ have heavy 4d/5d elements with strong atomic SOI, providing a unique way for generating spin polarization based on valleytronics physics. Indeed, such a spin-polarized band structure has been demonstrated theoretically and supported by optical investigations. However, despite these exciting progresses, following two important issues in MX$_{\mathrm{2}}$ community remain elusive: 1. the quantitative band structure of MX$_{\mathrm{2}}$ compounds (where are the valleys -band maxima/minima- locating in the BZ) have not been experimentally confirmed. Especially for those cleaved ultrathin mono- and bi-layer flakes hosting most of recently-reported exotic phenomena at the 2D limit, the direct detection for band dispersion becomes of great importance for valleytronics. 2. Spin transports have seldom been reported even though such a strong SOI system can serve as an ideal platform for the spin polarization and spin transport. In this work, we started from the basic electronic structures of representative MX$_{\mathrm{2}}$, obtained by ARPES, and investigated both the band variation between these compounds and their band evolution from bulk to the monolayer limit. After having a systematic understanding on band structures, we reported a giant Zeeman-type spin-polarization generated and modulated by an external electric field in WSe$_{\mathrm{2}}$ electric-double-layer transistors. The non-magnetic approach for realizing such an intriguing spin splitting not only keeps the system time-reversally invariant but also suggests a new paradigm for manipulating the spin-degrees of freedom of electrons. Acknowledge the support from DoE, BES, Division of MSE under contract DE-AC02-76SF00515. [Preview Abstract] |
Session Y49: Focus Session: Oxide Tunnel Junctions, Metal-Ferroelectric Interfaces, A-site Ordering
Sponsoring Units: DMPChair: Jorge Iniguez, ICMAB, Spain
Room: Mile High Ballroom 1C
Friday, March 7, 2014 8:00AM - 8:36AM |
Y49.00001: Enhancement of Tunneling Electroresistance in tunnel junctions using bilyer barriers with ferroelectric driven phase transition Invited Speaker: Qi Li Ferroelectric and Multiferroic tunnel junctions (magnetic tunnel junction with a ferroelectric barrier) have become one of the very promising approaches to new generation of multifunctional devices. A large tunneling electroresistance (TER) (the resistance on-off ratio) is very desirable for utilizing the device as a resistance switch or for signal processing. We have designed a bilayer tunneling barrier in which one layer is ferroelectric and the other layer is close to metal-insulator as well as ferromagnetic to antiferromagnetic phase transition with a goal to significantly change the barrier parameters and the interface state with the ferroelectric polarization reversal. The phase transition can occur when the ferroelectric polarization is reversed. In La$_{0.7}$Sr$_{0.3}$MnO$_{3}$/BaTiO$_{3}$/La$_{0.5}$Ca$_{0.5}$MnO$_{3}$/La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ tunnel junctions\footnote{Y. W. Yin, et al, \textit{Nature Materials} 12, 397 (2013).} where the La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ is the phase transition layer, this has resulted an increase of TER from 30{\%} (without the La$_{0.5}$Ca$_{0.5}$MnO$_{3})$ to 10,000{\%} (with the inserted layer). The mechanisms of such large increase of TER come from two sources: one is the metal to insulator transition of the La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ which effectively change the barrier width for the two polarization states and hence the tunneling current; and the other is the polarization driven magnetic reconstruction of La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ from ferromagnetic to antiferromagnetic state. The antiferromagnetic phase in the barrier acted as a spin valve for spin polarized tunneling current to significantly reduce the tunneling current. The details of the sample structures, electrical characterization, and the magneto transport studies will be presented and the results will also be compared with the first principles calculation. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y49.00002: Enhanced Tunneling Electroresistance by Interfacial Phase Transitions in Ultrathin Oxide Heterojunctions Lu Jiang, Woo seok Choi, Hyoungjeen Jeen, Shuai Dong, Yunseok Kim, Takeshi Egami, Ho Nyung Lee, Sergei V. Kalinin, Elbio Dagotto The ferroelectric (FE) control of electronic transport is one of the emerging technologies. Many previous studies in FE tunnel junctions (FTJs) exploited solely the differences in the electrostatic potential across the FTJs that are induced by changes in the FE polarization direction. In this work, by using ultrathin PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$/(La,Sr)MnO$_{3}$ heterojunctions, we present that in practice the junction current ratio between the two polarization states can be further enhanced when correlated electron oxides are used as electrodes, and that FTJs with nanometer thin layers can effectively produce a considerably large electroresistance ratio at room temperature. To understand these surprising results, we employed an additional control parameter, which is related to the crossing of electronic and magnetic phase boundaries of the correlated electron oxide. Our study highlights that the strong coupling between degrees of freedom across heterointerfaces could yield versatile and novel applications in oxide electronics. *The work was supported by the U.S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y49.00003: Electric control of tunnel magnetoresistance in oxide multiferroic tunnel junction J. Tornos, Liu Yaohua, G. Sanchez-Santolino, C. Munuera, S.G.E. te Velthuis, F. Mompean, M. Garcia-Hernandez, M. Varela, S.J. Pennycook, Z. Sefrioui, C. Leon, J. Santamaria Magnetic tunnel junctions with a ferroelectric barrier are systems amenable to control the spin dependent tunnel conductance by the electric field. We have investigated La0.7Sr0.3MnO3(LSMO)/BaTiO3(BTO)/LSMO tunnel junctions and, despite their symmetric structure, we have found very large tunnel electroresistance (TER) close to 1000{\%} at low temperatures. This is interpreted in terms of a variation of the effective barrier thickness due to a large modulation of electron charge at the BTO/LSMO interface that is induced by the switching of ferroelectric polarization in BTO. Moreover, for the orientation of ferroelectric polarization that leads to the larger conductance value, the bias and temperature dependence of the tunnel magnetoresistance (TMR) is consistent with a depolarization (spin filtering) of the tunneling current. This behavior might be related to the presence of an induced Ti magnetic moment in BTO interface, antiparallel to that of Mn in LSMO, as detected by XMCD measurements. Our results reveal the possibility to tune spin dependent transport by an electric field through the reversal of the ferroelectric polarization of the barrier. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y49.00004: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y49.00005: Effect of interface structure on Schottky barrier height in SrRuO$_{3}$/SrTiO$_{3}$ heterojunctions Vemulavada Sampath Kumar, Manish Niranjan Complex oxide heterostructures are highly promising for technological applications as they offer novel device concepts and functionalities. One of the fundamental parameters that influence the characteristics of the metal/oxide heterostructure is the Schottky barrier formed at the interface. The Schottky barrier height (SBH) is strongly influenced by the atomic structure of the interface and is of fundamental interest as an intrinsic property of the system. The SrRuO3/SrTiO3 (001) heterostructure is a prototypical system to study SBH at the oxide metal/dielectric interface. In recent years, the SRO has attracted a lot of attention as an electrode material for ultrathin ferroelectric films. Using \textit{ab-initio} calculations, we have studied the $p$-type SBH and its dependence on the interface structure in SRO/STO heterostructure. In addition, we have estimated the $p$-SBH using semi-empirical Metal-Induced-Gap-States (MIGS) model. In particular we have considered three types of interfaces: RuO2/SrO/TiO2, RuO2/BaO/TiO2 and MnO2/SrO/TiO2 the \textit{ab-initio} estimate of $p$-SBH comes out to be 1.27, 1.33 and 0.78 eV for respective interfaces. We find that semi-empirical MIGS model overestimate the p-SBH by $\sim$2 eV. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y49.00006: Ferroelectric control of spin injection across the ferromagnet/ferroelectric interface Xiaohui Liu, J.D. Burton, Evgeny Tsymbal Magnetoelectric coupling has become one of the most attractive fields in modern materials research due their promise to electrically control spintronics-based devices. Previous investigations have shown that at the ferromagnet/ferroelectric interface, magnetization could be tuned by the reversal of ferroelectric polarization. We had previously predicted that ferroelectric polarization reversal can control the nature of the resistive contact at the SrRuO3/n-BaTiO3 heterojunction interface, going from the Ohmic to Schottky regimes with reversal of ferroelectric polarization [1]. It is known, however, that SrRuO3 displays robust ferromagnetism below the Curie temperature of about 160K. In this work, using first-principles density functional calculations, we explore the effect of ferroelectric polarization of spin-polarized transmission across the SrRuO3/n-BaTiO3 interface. Our study reveals that the interface transmission is negatively spin-polarized, and that ferroelectric polarization reversal leads to a change in spin polarization from -65{\%} for the Ohmic contact to -98{\%} for the Schottky contact. This sizeable change in the spin polarization could provide an interesting non-volatile mechanism to electrically control spin injection into semiconductor-based spintronics devices.\\[4pt] [1] X. Liu, et al., Phys. Rev. B 88, 165139 (2013). [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y49.00007: Ab initio study of a symmetric SrRuO3/PbTiO3/SrRuO3 ferroelectric capacitor Simon Divilov, Judith Gabel, Matthew Dawber, Marivi Fernandez-Serra We performed a density functional study of the free standing capacitor (SrRuO$_3$)$_1$/(PbTiO$_3$)$_m$/(SrRuO$_3$)$_1$ using local density approximation and Hubbard U to study the effects of SrRuO$_3$ [100] surfaces on the bulk properties of PbTiO$_3$. In addition we analyze how the thickness, epitaxial strain and termination plane of PbTiO$_3$ modify its bulk behavior. We observe different rumpling patterns for both paraelectric (PE) and ferroelectric (FE) phases, based on the termination plane. For the FE phase, we observe oxygen octahedra tilting dominated by in phase and out of phase tilts around the [100] axis. In all our simulations the SrRuO$_3$ layers remain metallic, even those at the open surfaces.An analysis of Schottky barriers and coupling between magnetism and ferroelectricity will be presented. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y49.00008: Electronic transport properties of PbTiO$_{3}$/SrRuO$_{3}$ superlattices Hsiang-Chun Hsing, Sara Callori, Judith Gabel, Fen Guan, Marivi Fernandez Serra, Xu Du, Matthew Dawber First principles calculations on PbTiO$_{3}$/SrRuO$_{3}$ superlattices indicate that even when the SrRuO$_{3}$ layers in these structures are only a single unit cell thick they retain a metallic character. In the out of plane direction the resistivity of the structures are expected to depend on the thickness of the PbTiO$_{3}$ layers which act as ferroelectric tunneling barriers. We have successfully fabricated high quality specimens of these superlattices using off axis RF magnetron sputtering and here we report on their transport properties. In the out of plane direction, as well as showing ferroelectric polarization-field hysteresis loops, the samples reveal tunneling characteristics that confirm that the SrRuO$_{3}$ layers do indeed retain their metallicity in the experimental realization of these structures. In addition to studying the effect of changing the thickness of the PbTiO$_{3}$ layers in the superlattice we have examined the impact that the ferroelectric polarization and the compositionally broken inversion symmetry have on current-voltage characteristics. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y49.00009: Linear magnetoelectricity at room temperature in perovskite artificial superlattices Saurabh Ghosh, Hena Das, Craig J. Fennie The primary challenge in the field of multiferroics remains to identify materials that have a functional coupling between an electrical polarization and a magnetization, i.e., a magnetoelectric effect, at room temperature. Such materials may, for example, facilitate technologically important devices based on the electric field control of magnetism. Atomic scale heterostructures of transition metal ABO$_3$ perovskites are an ideal platform to realize designer properties and functionalities that don't exist in the bulk phase diagrams of the constituent materials. Here we take advantage of a recent direction in functional perovskites (where the combination of heterointerfaces with rotations/tilts of the BO$_6$ octahedra facilitate ferroelectric order) to create a new class of room temperature multiferroics in which ferroelectricity induces linear magnetoelectricity. We consider heterostructures of rare-earth orthoferrites of \textit{Pnma} perovskites, (LnFeO$_3$)$_1$/(Ln$^{\prime}$FeO$_3$)$_1$. Computed values of linear ME coefficients are found to be comparable to the prototype ME compound Cr$_2$O$_3$. Finally, we discuss the role of the Ln \textit{f}-states in the ME response. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y49.00010: Engineered spatial inversion symmetry breaking in an oxide hetero-structure built from isosymmetric room temperature magnetically ordered components John Claridge, Jonathan Alaria, Matthew Dyer, Matthew Rosseinsky, Pavel Borisov, Troy Manning, Serban Lepadatu, Markys Cain, Elena Mishina, Natalia Sherstyuck, N.A. Ilyin, Joke Hadermann, David Lederman The oxide heterostructure [(YFeO$_{3}$)$_{5}$(LaFeO$_{3}$)$_{5}$]$_{40}$,which is magnetically ordered and piezoelectric at room temperature, has been constructed from two weak ferromagnetic AFeO$_{3}$ perovskites with different A cations using RHEED-monitored pulsed laser deposition. The polarisation arises by combining ordering on the A site, imposed by the periodicity of the grown structure, with appropriate orientations of the octahedral tilting, according to simple symmetry-controlled rules. Magnetization and MOKE measurements show that the heterostructure's magnetic structure is similar to that of the individual components. Evidence of the polarity was obtained from second harmonic generation and piezoelectric force microscopy measurements. Modeling of the piezoresponse allows extraction of d$^{33}$ (approximately 10 pC/N) of the heterostructure, which is in agreement with DFT calculations. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y49.00011: Band gap engineering $via$ electrostatic chemical strain in cation ordered LaSrAlO$_4$ Prasanna V. Balachandran, James M. Rondinelli In this work, we employ density functional theory to examine a novel design route that employs A-site cation ordering to engineer the band gaps of (A,A$^\prime$)BO$_4$ Ruddlesden-Popper (RP) oxides. Using LaSrAlO$_4$ as a model material, we show that the band gap is highly sensitive to the A-site cation ordering ranging from 3-4.5 eV. When the [AlO$_2$]$^{-1}$ layers are interleaved between two chemically equivalent [LaO]$^{1+}$ or [SrO]$^{0+}$ layers, we obtain the smallest band gap with a reduction of $\sim$1 eV determined from the Heyd, Scuseria, and Ernzerhof (HSE) hybrid exchange-correlation functional. We relate the observed band gap reduction to the local bond distortions arising from electrostatic chemical strain induced changes to the O 2$p$ and La 5$d$ states in the valence and conduction bands, respectively. [Preview Abstract] |
Session Y50: Nanoparticle Plasmonics
Sponsoring Units: DCMPRoom: Mile High Ballroom 1D
Friday, March 7, 2014 8:00AM - 8:12AM |
Y50.00001: The Calculation of the Electronic Structure and Surface Plasmon for Semiconductor Quantum Dots Chin-Sheng Wu The surface conduction electrons of semiconductor quantum dots provide the collective excitations. The frequencies of emitted laser increase as the size of the quantum dots decrease. The size of the laser crystal can be controlled during synthesis so that the excitation and emission of the quantum dots are highly tunable. In order to understand their relation we have to find the electronic structure of the quantum dot first therefore the Kohn-Sham self- consistent method is used. The introduction of the electronic density directly into the macroscopic dielectric constant is used as a means of calculating the plasmon frequency of inhomogeneous electronic systems. Multi-step spatial dependent dielectric constant of quantum dot permits an estimate of the frequencies of these surface plasmon. The complete optical calculation requires the solution of Maxwell's equations and the usual boundary conditions. The most significant feature of these profiles for this calculation is the increase plasmon frequency with decreasing dot size. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y50.00002: Selective Plasmon-Exciton Coupling in Nonradiative Energy Transfer: Donor-Selective versus Acceptor-Selective Pedro Ludwig Hernandez-Martinez, Tuncay Ozel, Evren Mutlugun, Onur Akin, Sedat Nizamoglu, Ilkem Ozge Ozel, Qing Zhang, Qihua Xiong, Hilmi Volkan Demir We report selectively plasmon-mediated nonradiative energy transfer between quantum dot (QD) emitters interacting with each other via F\"orster-type resonance energy transfer (FRET) under controlled plasmon coupling either to only the donor QDs or to only the acceptor QDs. The comparative results of theoretical modelling of the donor- and acceptor selective plasmon-exciton coupling of nonradiative energy transfer is presented. Here, we demonstrate the ability to enable/disable the coupled plasmon-exciton formation distinctly at the donor site or at the acceptor site of our choice. In the case of donor-selective plasmon-exciton coupling, we observed a substantial shortening in the donor QD lifetime from 1.33 to 0.29 ns as a result of plasmon-coupling to the donors and the FRET-assisted exciton transfer from the donors to the acceptors. This enhances the acceptor emission by a factor of 1.93. In the complementary case, we observed a 2.70-fold emission enhancement in the acceptor QDs as a result of the combined effects of the acceptor plasmon coupling and the FRET-assisted exciton feeding. Our theoretical results are in good agreement with the systematic experimental characterization. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y50.00003: Plasmon Resonances and Size-Quantization Effects in Doped Semiconductor Nanocrystals Hui Zhang, Vikram Kulkarni, Emil Prodan, Peter Nordlander, Alexander O. Govorov Doped semiconductor nanocrystals represent a new type of quantum plasmonic material with optical resonances in the infrared spectral interval. These nanocrystals are fundamentally different from the metal nanoparticles because the electron density in a semiconductor can be tuned over a wide interval. Using the DFT-based time-dependent formalism, we computed the absorption spectra of doped quantum dots as a function of the number of carriers in a dot. The dynamic properties of doped quantum dots undergo an interesting transition from the size-quantization regime to the classical regime of plasmon oscillations. We demonstrate this quantum-to-classical transition for self-doped Copper Chalcogenides dots and for impurity-doped II-VI nanocrystals, and our simulations agree with the recent experiments well. The obtained results here can be used to predict and describe the optical properties of a broad class of semiconductor nanocrystals with quantum plasmonic resonances. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y50.00004: Enhancement of light emission from anthracene-doped polyphenylsiloxane glass films containing Ag nanoparticles Ryoko Shimada, Megumi Kimura, Naoki Tarutani, Masahide Takahashi, Sanjay Karna, Arup Neogi Metal-nanoparticles can induce the localized electric filed in the narrow inter-particle gap. This localization can significantly enhance light emission from fluorescent materials embedding metal nanoparticles. In this phenomenon, the important factors are optical absorption and emission. However, the mechanism of enhancement has not been fully elucidated. In this work, anthracene-doped polyphenylsiloxane (PPS) glass films containing Ag nanoparticles (AgNPs) were prepared for the characterization of enhanced photoluminescence properties. AgNPs of $\sim$ 30nm diameter were synthesized by the polyol process, and mixed in the anthracene-doped PPS glass film. The anthracene-doped PPS thin films of thickness $\sim$ 200 nm, with/without AgNPs, were prepared by spin-casting method. The photoluminescence (PL), measured for these films at room temperature, changed with the anthracene and/or AgNPs concentrations. In the optimum condition, the integrated PL intensity enhancement factor was found to exceed 50. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y50.00005: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y50.00006: Tuning surface plasmon resonances of Ag nanoparticles Dexin Kong, Liying Jiang, Jos\'e Men\'endez, Jeff Drucker The localized surface plasmon resonance (LSPR) of metallic nanoparticles can be tuned by varying their size, shape and dielectric environment. Using spectroscopic ellipsometry, we investigate the LSPR energy of epitaxial Ag islands grown atop Si(100) and conclude that it can be tuned from the near-UV to the near-IR. We use two island sizes, 25 nm and 100 nm. Subsequent to Ag island growth, we deposited 30 nm equivalent thickness layers of Si or TiO$_{2}$ onto selected samples, enabling characterization of the epitaxial Ag island LSPR energy as a function of size and dielectric environment. For the bare Ag nanoparticles, we found that 25 nm Ag islands only show the dipolar LSPR (around 3.2 eV), and that the dipolar LSPR of 100 nm particles is located around 3.0 eV. The sample of 100 nm Ag islands also shows the multi-pole LSPR and bulk plasmon resonance. For 25 nm particles, the TiO$_{2}$ layer redshifts the LSPR to about 2.0 eV and the Si layer further redshifts the LSPR peak to around 1.1 eV. The TiO$_{2}$ layer redshifts the plasmon peak of the 100 nm islands to about 1.7 eV, and the Si layer shifts it to near 1.4 eV. These resonance energies semi-quantitatively agree with a simple analytical estimate of the dipole plasmon resonance. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y50.00007: Effects of protein shell on properties of gold nanoparticles Anh Phan, Trinh X. Hoang, Dustin A. Tracy, Lilia M. Woods Optical properties and surface interactions between nanoparticles present opportunities for many novel applications. Protein-conjugated nanoparticles are of particular interest in regards to various medical applications. Theoretical investigations are presented of protein-coated gold nanoparticles using the Mie theory and the coupled dipole method. The Mie theory along with the absorption spectra can be used to quantitatively determine the number of protein bovine serum molecules that aggregate on the gold surfaces. The internal field of protein-conjugated gold nanoparticles remains constant for large wavelength of light due to screening from the protein shell. Effects from other nanoparticles significantly influence the peak position in the spectra. Our study shows the specific regimes in terms of optical characteristics where cascaded plasmon resonant field enhancement can be observed. Results for the maximum ratio of the internal field to the incident field is also obtained and discussed. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y50.00008: Synthesis and electronic and magnetic properties of size and shape tunable Indium Nitride nanoparticles Basudeb Chakraborty, Remi Beaulac The basis of III-V semiconductor's functionality which plays a fundamental role in many of the technologies transforming everyday life, arises from a combination of distinctive properties such as high carrier mobility, highly favorable optoelectronic properties. Though there are numerous reported schemes to synthesize high quality II-VI semiconductor nanomaterials, efficient synthetic method to produce highly crystalline, monodispersed colloidal III-V semiconductor nanomaterials is still a handful. Here, wurtzite indium nitride (InN) nanocrystals have been synthesized with narrow size distribution, good crystallinity and reasonable amount of emissivity via a solution route using commercially available, inexpensive and easy to handle precursors. Quantum confinement in these InN nanocrystals is demonstrated and the band-gap (0.69 eV in bulk) is quantitatively correlated to the size of the nanoparticles. The size, shape and dispersity of the nanoparticles can be tuned by controlling the molar ratio of substrates and surfactants and rate of addition of the reactants. These nanocrystals doped with transition metals such as Manganese (Mn), Cobalt (Co) are expected to influence the electronic structure and magnetic properties of the material. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y50.00009: Plasmonic Circular Dichroism of Chiral Nanoparticle Assemblies Zhiyuan Fan, Hui Zhang, Alexander Govorov Plasmonic circular dichroism(CD) of chiral metal nanoparticle(MNP) assemblies in the visible band results from dipolar and multipolar interaction between plasmons on MNPs. Both isotropic and anisotropic CD signals are extremely dimension-sensitive and strongly configuration-dependent. In this presentation, such geometry-dependence of plasmonic CD response will be analytically studied using an expansion of many-dipole interaction of the systems [1]. In the multipole regime, numerical simulations show new features of multipole plasmon interactions. One interesting observation is that a chiral equilateral tetramer made of 4 different NPs shows nearly zero CD response in the point dipole interaction regime but moderately strong CD response from multipole interaction of closely packed NP assemblies. Generally, CD signals of closely packed MNP assemblies are significantly enhanced and more sensitive to the geometric parameters. They can be used in many novel sensing applications as either solid-state or colloidal systems.\\[4pt] [1] Z. Fan, H. Zhang and A. O. Govorov, Optical Properties of Chiral Plasmonic Tetramers: Circular Dichroism and Multipole Effects, The Journal of Physical Chemistry C, 117 (28), 14770, 2013. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y50.00010: Ultra-Low-Intensity Magneto-Optical and Mechanical Effects in Metal Nanocolloids Matthew Moocarme, Jorge-Luis Dominguez-Juarez, Luat Vuong Magneto-plasmonics is a designation generally associated with ferromagnetic-plasmonic materials since such optical responses from non-magnetic materials alone are considered weak. Here, we theoretically analyse, numerically investigate, and experimentally show that there exists a magneto-optical switching behaviour in noble-metal nanocolloids. The response is observable at ultra-low illumination intensities <1 W/cm$^2$ with DC magnetic fields <1 mT. Polarization-dependent nonzero time-averaged plasmonic loops and vortex power flows subsequently produce significant torque on nanoparticles and nanoclusters via dipole-dipole interactions. This work provides a new framework for the dynamical interaction between light polarization, nano-surfaces and material magnetization. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y50.00011: F\"{o}rster-type Nonradiative Energy Transfer for Assemblies of Arrayed Nanostructures: Confinement Dimension vs. Stacking Dimension Hilmi Volkan Demir, Pedro Ludwig Hernandez Martinez, Alexander O. Govorov We report a theoretical framework of generalized theory for the F\"{o}rster-type NRET with mixed dimensionality in arrays. These include combinations of arrayed nanostructures made of nanoparticles (NPs) and nanowires (NWs) assemblies in one-dimension (1D), two-dimension (2D), and three-dimensions (3D) completing the framework for the transfer rates in all possible combinations of different confinement geometries and assembly architectures, we obtain a unified picture of NRET in assembled nanostructures arrays. We find that the generic NRET distance dependence is modified by arraying the nanostructures. For an acceptor NP the rate distance dependence changes from $\gamma \propto d^{-6}$ to $\gamma \propto d^{-5}$ when they are arranged in a 1D stack, and to $\gamma \propto d^{-4}$ when in a 2D array, and to $\gamma \propto d^{-3}$ when in a 3D array. Likewise, an acceptor NW changes its distance dependence from $\gamma \propto d^{-5}$ to $\gamma \propto d^{-4}$ when they are arranged in a 1D array and to $\gamma \propto d^{-3}$ when in a 2D array. These finding shows that the numbers of dimensions across which nanostructures are stacked is equally critical as the confinement dimension of the nanostructure in determining the NRET kinetics. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y50.00012: Spectral splitting with a plasmonic nanowire on silicon chip Ru-Wen Peng, Qing Hu, Di-Hu Xu, Yu Zhou, Ren-Hao Fan, Nicholas X. Fang, Qian-Jin Wang, Xian-Rong Huang, Mu Wang On-chip nanophotonics serves as the foundation for the new generation of information technology, but it is challenged by the diffraction limit of light. Here we demonstrate that by cascading nano-corrugation gratings with different periodicities on silver nanowires atop silicon, different colors can be spatially separated and chronologically released at different grating junctions. The released light frequency depends on the grating arrangement and corrugation periodicities. Hence the nanowire acts as a spectral splitter for sorting/demultiplexing photons at different nano-scale positions with a ten-femtosecond-level interval. Such nanowires can be constructed further into compact 2D networks or circuits. This research may provide a promising approach for realizing spatiotemporal-sensitive spectral splitting and optical signal processing on nanoscales, and for general integration of nanophotonics with microelectronics. Reference: Q. Hu, D. H. Xu, Y. Zhou, R. W. Peng, R. H. Fan, N. X. Fang, Q. J. Wang, X. R. Huang, and Mu Wang, Sci. Rep. 3, 3095 (2013). [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y50.00013: Electronic Bias and Debye Length Calculations across Solid-state Nanopores for Self-referencing Arrays Muhammad Usman Raza, Sajid Saleem, Waqas Ali, Samir M. Iqbal Solid-state nanopores have been used as sensors for many types of biological entities. One application is the detection of disease biomarkers from body fluids. This requires selectivity in nanopores as well as high throughput of analysis. Generally, single nanopore is used for measurements but for high throughput and self-referenced selectivity, multiple nanopores are required on the same chip. To exclude the effects of the ionic current flowing through one nanopore from the adjacent nanopores, effects of electronic bias and Debye length were calculated. The simulations showed optimal distances needed between the nanopores and their measurement electrodes. A number of parameters like nanopore diameter, distance of electrodes from nanopores and amplitude of bias voltage showed dramatic effect on the Debye length. The simulation results were compared with the experimental data. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y50.00014: Extended reflection coherent diffraction imaging of nanostructures on a tabletop Bosheng Zhang, Matthew Seaberg, Dennis Gardner, Elisabeth Shanblatt, Margaret Murnane, Henry Kapteyn, Daniel Adams We demonstrate the most general form of reflection-mode coherent diffraction imaging (CDI) that is applicable to non-isolated samples at high numerical aperture, by combining ptychography CDI with tilted plane correction. Tabletop high harmonic (HHG) beams at 30 nm with curved wavefronts are used to illuminate Ti nano-patterns on a Si substrate, at 45 degree incident angle. High fidelity images of the nanostructures are reconstructed, giving quantitative information for both the amplitude and phase (i.e. height to $\approx $1 nm precision), at a spatial resolution of $\approx $ 150 nm (limited by the geometry). The images compare favorably with both scanning electron and atomic force microscopies. Combined with our previous transmission-mode results, we have a general full-field, non-destructive, tabletop ultrafast microscope. In the future, we can improve the resolution using shorter wavelength HHG to image nanostructures with sub-10 nm spatial resolution and femtosecond time resolution, to capture ultrafast magnetic dynamics and heat transport at the nanoscale. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y50.00015: Keyhole reflection-mode coherent diffraction imaging of nano-patterned surfaces using a tabletop EUV source Elisabeth Shanblatt, Matthew Seaberg, Bosheng Zhang, Dennis Gardner, Margaret Murnane, Henry Kapyetn, Daniel Adams We demonstrate the first reflection-mode keyhole coherent diffraction imaging (CDI) of non-isolated samples from a single diffraction pattern. A tabletop high harmonic generation (HHG) beam at 30 nm with a curved wave-front is used to illuminate Ti nano-patterns on a Si substrate at 45 degree angle of incidence. The 30 nm illumination beam profile is first characterized using ptychograhic CDI. Keyhole CDI is then used to image the nano-sample. In contrast to ptychography CDI, keyhole CDI needs only one diffraction pattern, and therefore requires no scanning of the sample. This is a significant advantage for ultrafast pump-probe imaging of thermal or spin transport, allowing a sequence of time-delayed images of the same region to be easily acquired. Our technique opens the door for imaging dynamics in nanostructures with sub-10 nm spatial resolution and fs temporal resolution. [Preview Abstract] |
Session Y51: Focus Session: Beyond Graphene Devices: Function, Fabrication, and Characterization VIII
Sponsoring Units: DMPChair: Peide Ye, Purdue University
Room: Mile High Ballroom 1E
Friday, March 7, 2014 8:00AM - 8:12AM |
Y51.00001: Phosphorene: A New High-Mobility 2D Semiconductor Han Liu, Adam Neal, Zhen Zhu, David Tomanek, Peide Ye The rise of 2D crystals has opened various possibilities for future electrical and optical applications. MoS$_{\mathrm{2}}$ n-type transistors are showing great potential in ultra-scaled and low-power electronics. Here, we introduce phosphorene, a name we coined for 2D few-layer black phosphorus, a new 2D material with layered structure. We perform \textit{ab initio} band structure calculations and show that the fundamental band gap depends sensitively on the number of layers. We observe transport behavior, which shows a mobility variation in the 2D plane. High on-current of 194 mA/mm, high hole mobility up to 286 cm$^{\mathrm{2}}$/V$\cdot $s and on/off ratio up to 10$^{\mathrm{4}}$ was achieved with phosphorene transistors at room temperature. Schottky barrier height at the metal/phosphorene interface was also measured as a function of temperature. We demonstrate a CMOS inverter with combination to MoS$_{\mathrm{2}}$ NMOS transistors, which shows great potential for semiconducting 2D crystals in future electronic, optoelectronic and flexible electronic devices. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y51.00002: Photostability of thin exfoliated black phosphorus Alexandre Favron, S\'ebastien Francoeur, Richard Leonelli, Richard Martel In its bulk form, black phosphorus has a direct gap of about 0.3 eV. Because of its lamellar structure, similar to that of graphite, black phosphorus can be exfoliated down to a single monolayer. The interesting properties is the possible tuning of the energy gap in the Near-IR using control of the layer thickness, which is of great interesting to develop sensors and other Near-IR optoelectronic devices. Preliminary studies on thin exfoliated layers revealed a fast photo-induced oxidation of black phosphorus, in room condition with an excitation higher than 1.8 eV. Using Raman spectroscopy as a probe of the quality and integrity of exfoliated layers, we present in this talk the results of a dynamical study of the photo-oxidation process at room temperature in a controlled atmosphere with the presence of the oxygen-water redox couple. A photo-induced charge transfer from black phosphorus to the redox couple is found to be responsible of the fast deterioration of the structure. Finally, we present Raman and Photoluminescence results on un-oxidized thin-layers measured at low temperature using different passivation schemes. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y51.00003: Electronic transport and device properties of monolayer CVD MoS$_{2}$ Wenjuan Zhu, Tony Low, Yi-Hsien Lee, Han Wang, Damon B. Farmer, Jing Kong, Fengnian Xia, Phaedon Avouris The electronic transport and device properties of monolayer molybdenum disulphide (MoS$_{2})$ grown by chemical vapor deposition (CVD) are studied in this work. We show that these devices have the potential to suppress short channel effects, be aggressively down-scaled and have high critical breakdown electric field. These properties make them a compelling alternative to organic and other thin film materials. However, our study reveals that the electronic properties of these devices are at present, severely limited by the presence of a significant amount of band tail trapping states. Through capacitance and ac conductance measurements, we systematically quantify the density-of-states and response time of these states. Due to the large amount of trapped charges, the measured effective mobility also leads to a large underestimation of the true band mobility and the potential of the material. These exponentially distributed states further limit the device's subthreshold slope to 200meV/dec, regardless of the temperature. Continual engineering efforts on improving the sample quality are needed for its potential applications in flexible electronics, high resolution displays, photo-detection and energy harvesting. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y51.00004: Substrate Effect on Thin Layer MoSe$_{2}$ Field-Effect Transistors with Photo-Response Matthew Z. Bellus, Hui-Chun Chien, David L. Sicilian, Benjamin I. Weintrub, Jatinder Kumar, A. Davis St. Aubin, T.B. Hoffman, Y. Zhang, J.H. Edgar, Hsin-Ying Chiu The discovery of graphene has opened the gates for the study of layered semiconducting materials such as the transition metal dichalcogenides (TMDs), i.e. MoS$_{2}$, MoSe$_{2}$, WS$_{2}$, WSe$_{2}$. In addition, recent works have shown that hexagonal boron nitride (hBN) can act as an ideal substrate with electrical performance enhancement for graphene and possibly for other materials as well. In this study, we examine this substrate effect for MoSe$_{2}$ by comparing various material properties on both SiO$_{2}$ and hBN. Field-effect transistors (FETs) were fabricated on both substrates using mechanically exfoliated MoSe$_{2}$. Our FETs show n-type doping and strong gate modulation yielding I$_{on}$/I$_{off}$ ratios larger than 10$^{6}$ for both substrates. Using a 4-probe measurement we found a relatively high mobility on SiO$_{2}$ that was larger than previous reports, with a slight variation between substrates. Under illumination, devices on both substrates showed ``photo-doping'' effects that in some cases were very large and persistent, thought to be the persistent photoconductivity (PPC) effect. These initial results have shown promising characteristics in MoSe$_{2}$ for applications in electronics and optoelectronics as well as shown the effects that a substrate can play in device performance and material properties. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y51.00005: Electron/Hole-phonon scattering and intrinsic carrier mobility in 2D transition metal dichalcogenides (TMDs) Zhenghe Jin, Xiaodong Li, Byoung-Don Kong, Jeffrey Mullen, Ki Wook Kim We have investigated electron/hole-phonon scattering mechanism in 2D transition metal dichalcogenides using a first-principles approach. Specifically, 2D TMDs, i.e., monolayer MX$_{2}$ (M=Mo and W; X=S and Se) material are investigated. The scattering rates are calculated using Density Functional Theory (DFT) and Density Functional Perturbation Theory (DFPT) and intrinsic electron/hole-mobility is obtained though full band Monte Carlo carrier transport simulation. Then, the parameters for the deformation potential model are extracted from the first principle's transport studies for practical purposes. Our calculation reveals WS$_2$ has the largest mobility among the investigated TMDs. At room temperature, the electron mobility of WS$_2$ is 300 cm$^2$/Vs, which is smaller than that of usual bulk semiconductor. Contrary to this, the hole mobility of WS$_2$ turns out to be over than 800 cm$^2$/Vs, which is even higher than that of bulk silicon, which provides a good opportunity of high-performance pMOSFET. Our work examines the electronic transportation property of 2D TMD material from first-principles approach and demonstrates the importance of electron/hole-phonon scattering in those materials and provides optimal channel material for future field effect transistor. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y51.00006: Electrical Transport and Photoresponse of Field-Effect Transistors Based on Two-Dimensional Metal-Layered Materials Ming-Wei Lin, Ivan Kravchenko, Jason Fowlkes, Jiaqiang Yan, Xufan Li, Alexander Puretzky, Christopher Rouleau, David Mandrus, David Geohegan, Kai Xiao High performance field effect transistors based on exfoliated two-dimensional (2D) layered materials of transition metal dichalcongenides (TMDCs) such as MoS$_{2}$, WSe$_{2}$ and MoSe$_{2}$ have been demonstrated. The electrical transport measurements show that the mobility is associated with the thickness and temperature for mono- and few-layered 2D materials. Besides, these 2D materials are demonstrated highly sensitive to the light, providing the potential applications for photodetectors or optoelectronic devices. In addition, the thickness dependence of noise measurement for these 2D materials will also be discussed. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y51.00007: Dielectric and Conductivity Mapping of Few-Layer Metal Chalcogenides Keji Lai, Di Wu, Yingnan Liu, Yuan Ren, Min Lin, Hailin Peng, Ariel Ismach, Rudresh Ghosh, Rodney Ruoff A novel microwave impedance microscope was used to spatially map the local dielectric constant and conductivity of few-layered metal chalcogenides without the need of contact electrodes. For phase-change In$_{2}$Se$_{3}$ nanoplates grown on mica substrates, our results showed a sudden drop of permittivity from the bulk value for thicknesses below 5 layers and strong dielectric inhomogeneity around 4 and 5 layers. For CVD-grown MoS$_{2}$ flakes on SiO$_{2}$/Si wafers, we observed highly conductive localized regions within monolayer islands. These regions, which can be imaged by scanning electron microscopy and atomic force microscopy, show enhanced Raman signals and PL signal quenching. Continued imaging effort is expected to shed some light on the growth mechanism and electron physics of these quasi-2D chalcogenides. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y51.00008: A comparison of MoSe$_{2}$ field-effect transistors on SiO$_{2}$ and parylene-C substrates: possible surface polar phonon effects Bhim Chamlagain, Qing Li, Minghu Pan, Tugeng Hong, Hsuen-Jen Chuang, Meeghage Perera, Yong Xu, Di Xaio, Nirmal Ghimire, Jiaqiang Yan, David Mandrus, Zhixian Zhou We report the fabrication and electrical characterization of high quality MoSe$_{2}$ field-effect transistors fabricated on both SiO$_{2}$ and parylene-C substrates. Multilayer MoSe$_{2}$ on parylene-C shows a significantly higher room temperature mobility of 100 cm$^{2}$V$^{-1}$s$^{-1} -$ 160 cm$^{2}$V$^{-1}$s$^{-1}$ than that on SiO$_{2}$ ($\approx $50 cm$^{2}$V$^{-1}$s$^{-1})$. Our variable temperature transport measurements indicate that the mobility of MoSe$_{2}$ devices on both SiO$_{2}$ and parylene-C increases to $\approx $ 500 cm$^{2}$V$^{-1}$s$^{-1}$ as the temperature decreases to below 100 K, with the mobility of MoSe$_{2}$ on SiO$_{2}$ increasing more rapidly. We attribute the observed difference in mobility and its temperature dependence between MoSe$_{2}$ on SiO$_{2}$ and on parylene-C primarily to the surface polar optical phonon scattering in the SiO$_{2}$ substrate, which is absent in parylene-C. [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y51.00009: ABSTRACT WITHDRAWN |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y51.00010: Charge Scattering and Mobility in Atomically Thin Semiconductors Nan Ma, Debdeep Jena The electron transport properties in atomically thin semiconductors have attracted intense interest. In this work, we study the scattering mechanisms that limiting the mobility of such semiconductors. The effects of the dielectric environments are also evaluated. We find that high-K dielectrics increase the charged-impurity-limited mobility, but weaken the free-carrier screening. The strong remote optical phonon scattering from high-K dielectrics severely decrease the high-temperature mobility. From a comparative study of different scattering mechanisms, we find that all current reported measured mobilities (around 100 cm$^{\mathrm{2}}$/Vs) are dominated by charged impurity scattering. When the impurity densities are reduced, remote phonon scattering determines the room-temperature mobility upper-limits. The mobilities achieved till date are far below the intrinsic potential in these materials. The truly intrinsic mobility over 10,000 cm$^{\mathrm{2}}$/Vs at room temperature can only be achieved in ultraclean suspended samples. Among the commonly used dielectrics, AlN and BN offer the best compromise if a high mobility over 1000 cm$^{\mathrm{2}}$/Vs and a high gate capacitance are simultaneously desired, as is the case in field effect transistors. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y51.00011: Conductivity and Carrier Dynamics in Multilayer Molybdenum Disulphide (MoS$_{2})$ Measured by THz Time-Domain Spectroscopy Jared Strait, Parinita Nene, Farhan Rana We present results on the ultrafast carrier dynamics and the frequency-dependent conductivity of multilayer MoS$_{2}$ using optical-pump terahertz-probe spectroscopy with sub-ps time resolution. Measurements done at various temperatures reveal that the photoexcited conductivity is well-described by the Drude model, with a mobility of 300 cm$^{2}$/V-s at 300 K increasing to 5200 cm$^{2}$/V-s at 30 K. We find that the Drude scattering rate increases linearly with temperature, which we attribute to phonon-dominated scattering. Various time scales are observed in the dynamics of photoexcited carriers. Immediately after photoexcitation, the conductivity takes $\sim$ 1-2 ps to reach its maximum value, as carriers undergo intraband relaxation, and then decays as they recombine. During the first 100 ps after photoexcitation, we observe $\sim$ 1/ns recombination rates with a linear dependence on the carrier density. Recombination rates become smaller and independent of carrier density as time progresses. Complete transients can last over tens of ns. Carrier dynamics are found to be temperature dependent, becoming faster at higher temperatures. We will present physical models that explain our data. [Preview Abstract] |
Session Y52: Competing Orders and Phase Diagrams in Copper-oxide Superconductors
Sponsoring Units: DCMPChair: Hai-Hu Wen, Nanjing University
Room: Mile High Ballroom 1F
Friday, March 7, 2014 8:00AM - 8:12AM |
Y52.00001: Superconducting fluctuations and Fermi surface reconstruction in underdoped cuprates Shizhong Zhang, Sumilan Banerjee, Mohit Randeria Recent observation of quantum oscillations in underdoped Hg1201 has indicated that the small Fermi surface (FS) is an intrinsic property of the Copper-oxide plane [1]. The emergence of small FS requires significant FS reconstruction in a magnetic field $H$. Experiments have found signatures of both uni- and bi-directional charge orders that can lead to such FS reconstruction. For the bi-directional charge order, however, a large, and possibly unrealistic, value of charge ordering potential is necessary to compare favourably with experiments. We show that by taking into account static superconducting (SC) fluctuations the required charge ordering potential can be dramatically reduced, while reconciling the observed $\sqrt{H}$ dependence of the specific heat. We comment on why dissipation leads to static, as opposed to dynamical, SC fluctuations [2] at low-temperature and high field. [1] N. Barisic et al., Nature Physics, doi:10.1038/nphys2792 (2013). [2] S. Banerjee, S. Zhang and M. Randeria, Nature Comm. 4, 1700 (2013). [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y52.00002: Evidence for Competing Order in Underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ from Break Junction Tunneling Spectroscopy John Zasadzinski, Nicholas Groll, Chaoyue Cao, Mike Hinton, Thomas Proslier, Thomas Lemberger Superconductor-insulator-superconductor (SIS) break junction tunneling measurements of the low-temperature, single-electron gap parameter, $\Delta $, are reported on heavily underdoped thin films of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ (Bi2212). This extends previous, doping-dependent studies on bulk single crystals and the combined data reveal that for hole concentrations, $p$ \textless 0.11 there is an abrupt change in slope of $\Delta (p)$ along with the observation of extraordinarily large single-electron energy gaps ($\Delta $ $\sim$ 115 meV-135 meV). This underdoped region displays a corresponding drop in the superfluid density and distinctive changes in the shape of the electronic density of states (DOS). The shape of $\Delta $(T) near Tc (as measured by the loss of Josephson current) is inconsistent with single gap scenarios. The combined results signal that a competing order has emerged. The underdoped $\Delta $ values are close to the antiferromagnetic exchange energy, J, and the overall trends indicate that the quasiparticle gap in the DOS has evolved from primarily superconducting to primarily magnetic character. These results may be relevant for pseudogap phenomena in underdoped cuprates. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y52.00003: Charge ordering and Fermi-arc instability in underdoped cuprates Riccardo Comin, George Sawatzky, Andrea Damascelli, Alex Frano, Bernhard Keimer, Michael Yee, Jennifer Hoffman, Enrico Schierle, Eugen Weshcke, Ronny Sutarto, Feizhou He, Yoshiyuki Yoshida, Hiroshi Eisaki The underdoped cuprate pseudogap, and related ``Fermi-arc'' phenomenology, is one of the most remarkable phenomena in strongly correlated-electron systems. Despite evidence for various forms of electronic instabilities, a direct link to an underlying ordered phase is still mysterious. Here we report a combined investigation of one single cuprate family by real- and momentum-space, and surface and bulk probes - resonant X-ray scattering (REXS), scanning-tunnelling microscopy (STM), and angle-resolved photoemission spectroscopy (ARPES)--which individually had a profound impact on the understanding of high-Tc cuprate superconductors, but have so-far been analyzed and interpreted within different phenomenological frameworks--and never for the very same compound. By bringing together these techniques, and with the aid of calculations of the electronic response, we establish a precise, quantitative correspondence between the Fermi arc phenomenon seen in ARPES and charge ordering as observed by REXS and STM. These converging findings suggest the existence of a universal charge-ordered state in underdoped cuprates and reveal its connection to the pseudogap phase and related fermiology. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y52.00004: Quantum oscillations in a non-Fermi liquid cuprate pseudogap state Yan He, Peter Scherpelz, K. Levin We analyze the properties of quantum oscillations in a pseudogap (ie, non-Fermi liquid) state and, thereby, address recent experiments in the high-field regime of cuprate superconductors. We use a Gor'kov-based, Landau level model of the pseudogap state appropriate for very high fields, and find that a gapped state will generally display oscillations in this regime. This is due both to d-wave pairing and to the presence of gap inhomogeneities, reflecting a blurred vortex state [1]. We calculate the temperature dependence of these oscillations and show that for realistic cuprate parameters, these systems display behavior essentially indistinguishable from that of Fermi liquids [2]. [1] P. Scherpelz, D. Wulin, K. Levin and A. Rajagopal, PRA 87 063602 (2013). [2] P. Scherpelz, Y. He and K. Levin, arXiv:1310.2645. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y52.00005: NMR investigation of charge order in YBa$_{2}$Cu$_{3}$O$_{\mathrm{y}}$ in high magnetic fields M. Hirata, H. Mayaffre, S. Kraemer, M. Horvatic, C. Berthier, MH. Julien, P.L. Kuhns, A.P. Reyes, R. Liang, W.N. Hardy, D.A. Bonn Recent observation of the charge-density-wave (CDW) order or CDW fluctuations in underdoped YBa$_{2}$Cu$_{3}$O$_{\mathrm{y}}$ (YBCO) marks an important step in high-$T_{\mathrm{C}}$ research because it lends support to the idea that charge ordering is a generic instability of the pseudogap state. However, the relevance of these results to the understanding of superconductivity remains unclear. An important question is how charge ordering evolves as a function of doping across the phase diagram. Here, we expand our previous work [1] and report high-field $^{17}$O NMR evidence of charge order in YBCO with doping level $p = $0.09 and $p = $0.13. We discuss the evolution of the temperature and field scales characterizing the CDW transition.\\[4pt] [1] T. Wu \textit{et al.} \textit{Nat.} \textit{Commun.}\textbf{4}, 2113 (2013). [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y52.00006: Doping Dependence of Spin and Phonon Hybridization in $La_{2-x}Ba_{x}CuO_{4}$ Jerod Wagman, J.P. Carlo, G. Van Gastel, M.B. Stone, J.L. Niedziela, G.E. Granroth, A.I. Koleshnikov, L. DeBeer-Schmitt, A.T. Savici, Z. Yamani, Z. Tun, Y. Zhao, A.B. Kallin, E. Mazurek, H.A. Dabkowska, B.D. Gaulin 'Hour-glass' shaped dispersions of antiferromagnetic (AF) spin fluctuations are a robust feature common to many high temperature superconductors. In La-214 cuprates, these phenomena are well known to display a strong dependence on the concentration of holes that are introduced into the copper oxide planes by doping. Here, we present a series of neutron scattering measurements on single crystals of $La_{2-x}Ba_{x}CuO_{4}$ (LBCO), with $0 \leq x \leq 0.095$. This is a doping range that spans the phase diagram from insulating three dimensional commensurate AF to superconducting two dimensional incommensurate AF. Our measurements comprehensively map out the evolution of the spin excitations below $\sim$ 40 meV. In particular, we focus on the rich structures that arise at the many crossings of the highly dispersive spin excitations with the many phonon eigenvectors in this system. The nature of these structures are suggestive of spin-phonon hybridized modes, which seem to pervade the phase diagram of LBCO. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:24AM |
Y52.00007: Competing charge, spin, and superconducting orders in underdoped YBCO M. Huecker, E. Blackburn, D.A. Bonn, J. Chang, N.B. Christensen, E.M. Forgan, O. Gutowski, W.N. Hardy, S.M. Hayden, A.T. Holmes, R. Liang, D.S. Robinson, U. Ruett, M. v. Zimmermann High energy X-ray diffraction experiments on $\rm YBa_2Cu_3O_{y}$ were performed to explore the doping evolution of the recently discovered charge density wave (CDW) phase. The results show that the CDW phase exists at least for charge carrier concentrations of $0.078 < p < 0.132$. Hence, the lower bound is located in vicinity of the quantum critical point to spin density wave order. For all dopings CDW order sets in in the normal state, but is partially suppressed upon cooling below $T_c$. This clearly suggests a competition between the two states. The incommensurability of the CDW order decreases approximately linear with the hole concentration, which is in contrast to the increase observed for stripe order in La-based cuprates. A detailed comparison with the charge stripe order in $\rm La_{2- \it x}Ba_{\it x}CuO_{4}$ is presented. [Preview Abstract] |
Friday, March 7, 2014 9:24AM - 9:36AM |
Y52.00008: Microscopic investigation of the dopant oxygen distribution using $^{199}$Hg NMR in the high temperature superconductor HgBa$_2$CuO$_{4+\delta}$ Yizhou Xin, A.M. Mounce, Jeongseop Lee, Sangwon Oh, W.P. Halperin, A.P. Reyes, P.L. Kuhns, M.K. Chan, C. Dorrow, L. Ji, D. Xia, X. Zhao, M. Greven In the high temperature superconductor HgBa$_2$CuO$_{4+\delta}$, it has been determined that the dopant oxygen O$_\delta$ resides in the Hg-plane [1]. The systematic development of the $^{199}$Hg NMR spectrum as a function of O$_\delta$ content is presented. For high O$_\delta$, 4 different resonance peaks are observed. Three of the peaks follow a binomial distribution and correspond to 0, 1, and 2 O$_\delta$ nearest neighbors. The fourth peak persists down to low doping and may be indicative of a Hg vacancy nearest neighbor. This work was supported by the DOE BES under grants No. DE-FG02-05ER46248 and No. DE-SC0006858 and the NHMFL through the NSF and State of Florida. \\[4pt] [1] J. G. Correia \textit{et}. PRB \textbf{64}, 11769 (2000). [Preview Abstract] |
Friday, March 7, 2014 9:36AM - 9:48AM |
Y52.00009: Antiferromagnetic fluctuations and the pseudogap in the moderately underdoped high-temperature superconductor HgBa$_{2}$CuO$_{4+\delta}$ Mun Chan, C. Dorow, M. Veit, Y. Tang, Y. Ge, M. Greven, L. Mangin-Thro, Y. Sidis, P. Bourges, X. Zhao, P. Steffens, A. Christianson, D.L. Abernathy, J.T. Park The two most salient features of the magnetic excitation spectrum of the cuprate superconductors are the hourglass dispersion and the resonance mode. Our neutron scattering measurements demonstrate that both features are either absent or significantly weakened in moderately underdoped HgBa$_{2}$CuO$_{4+\delta}$ (Hg1201; T$_{\mathrm{c}}\approx $71 K; p$\approx $0.09). The magnetic spectrum is gapped below 27 meV, and is commensurate with the antiferromagnetic wave-vector above the gap. Above 60 meV, it disperses upward into a ring of scattering. This Y-shaped spectrum is reminiscent of that observed in the pseudogap of YBa$_{2}$Cu$_{3}$O$_{\mathrm{6+y}}$ (YBCO). Unlike YBCO, the antiferromagnetic fluctuations of Hg1201 are rather impervious to the onset of superconductivity at T$_{\mathrm{c}}$. Instead, a large peak in the susceptibility at about 53 meV onsets at the much higher pseudogap temperature T*. The dramatic increase of magnetic scattering below T* reveals a strong connection between magnetism and the pseudogap. The results for this structurally simple compound present a challenge for theoretical explanations that have focused largely on the hourglass dispersion and the resonance. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y52.00010: Antiferromagnetic fluctuations in the very underdoped high-temperature superconductor HgBa$_2$CuO$_{4+\delta}$ M. Veit, M.K. Chan, C. Dorow, T. Yang, G. Yang, M. Greven, L. Mangin-Thro, Y. Sidis, P. Bourges, X. Zhao, P. Steffens, A. Christianson, D.L. Abernathy, J.T. Park We report inelastic neutron scattering measurements of magnetic fluctuations over a large energy and momentum range in the high-temperature cuprate superconductor HgBa$_2$CuO$_{4+\delta}$ (Hg1201) at two low doping levels (UD45: T$\mathrm{_c}$$\approx$45K, p$\approx$0.058; UD55: T$\mathrm{_c}$$\approx$55K, p$\approx$0.063). In both samples, the ``hourglass'' dispersion, thought to be universal among the cuprates, is not observed. Instead, the antiferromagnetic spectrum is commensurate just above the magnetic gap ($\sim$10 meV in both samples) and disperses outwards into a ring of scattering above $\sim$50 meV. The magnetic resonance is prominently observed in UD45 (at $\sim$20meV), but is small or non-existent in higher-doped UD55. This result runs counter to the heretofore accepted notion that the resonance is most prominent in the compounds with the highest optimal T$\mathrm{_c}$. Additionally, we find that the previously reported Ising-like dispersionless excitations in optimal and moderately underdoped Hg1201 is no longer observed in UD45. We conclude that there exists a crossover near p$\sim$0.06 between two distinct regimes. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y52.00011: Cuprate Pseudogap loop currents, observed by muon probing C. Boekema, P. Sakkaris, A. Love, M. Tran, F. Owens, H. Sio, W.K. Dawson Pertaining to Varma's loop currents (LpI) in the cuprate pseudogap phase, [1] we have observed [2] in zero field weak $\mu $SR LpI signals for two GdBCO samples above and below their T$_{\mathrm{c}}$'s of 81 K (underdoped) and 93 K (optimal doped). The measured fields are about the predicted 100 Oe. The question is, can the muon probe these fields. Shekhter \textit{et al} [3] found theoretically that positive muons destroy Varma's loop currents. In contrast, by analyzing transverse field $\mu $SR GdBCO data using MaxEnt, [4] we confirm the muon probes in \textit{insulating} regions near the BaO layers (Balmer sites) and CuO chain layers (Lin sites). [5] Thus, muons do \textit{not} destroy Varma's loop currents in the CuO$_{\mathrm{2}}$ planes, and can precisely probe their fields. In conclusion, well below RT we have detected LpI-$\mu $SR signals for GdBCO with a probability of occurrence of $\sim$ 65 {\%}. [2] Our ME$\mu $SR optimal doped GdBCO results indicate, the QCP in the cuprate phase diagram is located near the endpoint of the superconductivity dome. A magnetic origin for cuprate superconductivity is plausible. Research is supported by RSCA-SJSU and AFC San Jose. [1] CM Varma PRL \textbf{83} (1999) 3538; ME Simon, CM Varma PRL \textbf{89} (2002) 247003. [2] C Boekema \textit{et al,} 493 (2013) 136; T Songatikamas \textit{et al,} J Supercond {\&} Nov Magn \textbf{23} (2010) 793. [3] A Shekhter \textit{et al,} PRL \textbf{101} (2008) 227004. [4] C Boekema, MC Browne, AIP Conf Proc {\#}1073 (2008) 260; JC Lee \textit{et al}, J Appl Phys \textbf{95} (2004) 6906. [5] WK Dawson \textit{et al}, J Appl Phys \textbf{64} (1988) 5809; Hpf Interactions \textbf{63} (1990) 219. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y52.00012: Persistent spin excitations in doped cuprates revealed by resonant inelastic light scattering C.J. Jia, E.A. Nowadnick, K. Wohlfeld, Y.F. Kung, C.-C. Chen, S. Johnston, T. Tohyama, B. Moritz, T.P. Devereaux How coherent quasiparticles emerge upon doping a quantum antiferromagnet is a key question in correlated materials, underlying an understanding of the cuprate phase diagram. Recent resonant inelastic x-ray scattering (RIXS) experiments in hole-doped cuprates measured high energy collective spin excitations that persist well into the overdoped regime and bear a striking resemblance to those found in the parent compound, challenging the perception that spin excitations should weaken with doping and have a diminishing effect on superconductivity. We show that RIXS at the Cu L3-edge indeed provides access to the spin dynamical structure factor once one considers the full influence of light polarization. Further we demonstrate that high-energy spin excitations do not correlate with the doping dependence of Tc, while low-energy excitations depend sensitively on doping and show a crossover from antiferromagnetic to ferromagnetic correlations. This suggests that although high-energy spin excitations persist well into the overdoped regime, they are marginal to pairing in cuprate superconductors. \\[4pt] [1] arXiv:1308.3717 [cond-mat.str-el] [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y52.00013: Broken symmetry in the cuprate phase diagram: Oxygen engineer- ing in electron doped cuprate superconductors and its impact on competing orders Yoshiharu Krockenberger, Hiroshi Irie, Osamu Matsumoto, Keitaro Yamagami, Masaya Mitsuhashi, Akio Tsukada, Michio Naito, Hideki Yamamoto In high temperature superconductors, superconductivity is induced by chemical doping. This relation is one of the most studied in solid state physics since superconducting transition temperatures beyond the boiling point of nitrogen have been realized. So far, only doping stabilizes the superconducting ground state by destruction of the long-range antiferromagnetic order of the 3$d^{9}$ Cu$^{2+}$ moments and alternatives for an increase of the superconducting transition temperature are in great demand. We show that the conventional mechanism of doping for the induction of superconductivity does not apply to a class of materials characterized by square-planar coordination of copper. We prepared thin films of Pr$_{2}$CuO$_{4}$, a strongly correlated material with a magnetically driven insulating ground state - and show that annealing drives the localized charge carriers into motion, leading to the emergence of a superconducting ground state. This non-local switching of the electronic states is achieved by the application of an elaborate annealing method. The superconducting transition appears at temperatures higher than that induced by doping. Our results demonstrate a conceptually new tuning parameter for the induction of superconductivity, extending the concept of doping control. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y52.00014: Evidence for Intertwining of Superconductivity and Antiferromagnetism in a Cuprate John Tranquada, Zhijun Xu, C. Stock, S.X. Chi, A.I. Kolesnikov, G.Y. Xu, G.D. Gu We have used inelastic neutron scattering to measure the low-energy, incommensurate antiferromagnetic spin excitations both above and below the superconducting transition temperature ($T_c = 32$~K) of La$_{1.905}$Ba$_{0.095}$CuO$_4$ [1]. While the magnetic excitations in optimally-doped cuprates typically show the development of a spin gap and magnetic resonance below $T_c$, our sample shows no such effect. Instead strong, gapless spin excitations coexist with bulk superconductivity. To understand this, we note that previous transport measurements have shown that the superconducting layers are decoupled by a magnetic field applied along the $c$-axis, resulting in a state with frustrated interlayer Josephson coupling, similar to LBCO with $x=1/8$, where it has been proposed that pair-density-wave superconductivity occurs. This suggests that, in a similar fashion, the spatially modulated antiferromagnetic correlations (which we see directly in the $x=0.095$ sample) are intertwined with a spatially modulated superconducting pair wave function. \\[4pt] [1] Z. J. Xu {\it et al.}, arXiv:1309.2718. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y52.00015: Fermi-arcs from angular fluctuations of a multi-component order parameter Debanjan Chowdhury, Subir Sachdev Angle resolved photoemission experiments on the underdoped cuprates have seen evidence for highly unusual gapless Fermi-arcs. They appear as disconnected surfaces above the superconducting transition temperature and below an onset temperature scale, which is the onset of the pseudogap as has been seen in a large number of probes. Over the past few years a lot of insight has been gained into the nature of the mysterious pseudogap state with the discovery of a fluctuating charge-ordered state with a finite correlation length. It has recently been suggested [1] that the pseudogap regime is described by the angular fluctuations of a multi-component order parameter. In this work, we study the effect of these fluctuations on the spectral function of the underlying fermions and show that the arcs occur naturally in the presence of fluctuating charge-order and superconducting correlations. We analyze the relative importance of these fluctuations as a function of temperature and make connections with recent photoemission experiments. [1] L. Hayward, D. Hawthorn, R. Melko and S. Sachdev, arXiv:1309.6639. [Preview Abstract] |
Session Y53: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures IV
Sponsoring Units: DMPChair: William Vandenberghe, University of Texas at Dallas
Room: Mile High Ballroom 2C
Friday, March 7, 2014 8:00AM - 8:12AM |
Y53.00001: In situ studies of transient photoconductivity in PbSe quantum dot solar cells Jianbo Gao, Weon-Kyu Koh, Nikolay Makarov, Jeffrey Pietryga, Victor Klimov PbSe quantum dot (QD) solar cells have attracted significant interest due to their band gap tunability, easy-processing and flexibility. Efficiencies have risen from 1{\%} just a few years ago to nearly 9{\%} today. Furthermore, the novel concept of multiple exciton generation (MEG) resulting from quantum confinement makes these materials scientifically interesting counterparts to bulk semiconductors. Recent observations of more than 100{\%} external quantum efficiency in PbSe QD solar cells confirm direct relevance of MEG to practical photovoltaics. However, in order to take full advantage of this effect, one needs a better understanding of photogeneration dynamics and carrier transport in QD solar cells. In this talk, we discuss a new technique for in situ measurements of transient photoconductivity with fast response time (\textless 50 ps) applied to study carrier transport and photogeneration dynamics in PbSe QD solar cells. These measurements complement traditional photoconductivity techniques such as time-resolved microwave conductivity and time-of-flight. Based on the analysis of temperature, excitation wavelength and electrical field dependence measurements, we derive parameters such as MEG efficiency, carrier lifetime, trap-free mobility and carrier emission rate from trap states. [Preview Abstract] |
Friday, March 7, 2014 8:12AM - 8:24AM |
Y53.00002: Highly Nonlinear Photocurrent and Efficient Charge Separation in Lead Sulfide Nanowire Field Effect Transistors Yiming Yang, Xingyue Peng, Dong Yu We present our scanning photocurrent microscopy (SPCM) study of lead sulfide (PbS) NW field effect transistors (FETs). PbS NWs were synthesized via chemical vapor deposition (CVD) method, with controlled ambipolar doping from 10$^{19}$ cm$^{-3}$ (n-type) to 10$^{18}$ cm$^{-3}$ (p-type) [1]. We have observed highly nonlinear photocurrent in single PbS NW FETs under high-intensity optical excitation. The spatially resolved photocurrent images obtained from SPCM showed complex patterns, indicating reversal of the photocurrent direction at high excitation intensity, attributable to the non-equilibrium band structure modulated by the photo-injected carriers. Our numerical simulations agree well with the experimental results, when considering the electric field created by the bulk metal contact. In addition, we have also achieved high charge separation efficiency at the Schottky contact to NWs. The wavelength dependent photocurrent can be understood from the absorption cross-section of the NW obtained by the Finite-difference time-domain (FDTD) simulation. [1] Yang, Y. M.; Li, J.; Wu, H. K.; Oh, E.; Yu, D. Nano Lett. 2012, 12, 5890-5896. [Preview Abstract] |
Friday, March 7, 2014 8:24AM - 8:36AM |
Y53.00003: Measuring charge transport in nanopatterned PbS colloidal quantum dots using charge sensing Nirat Ray, Neal E. Staley, Darcy D. Wanger, Moungi G. Bawendi, Marc A. Kastner Colloidal quantum dots (CQDs) can self-assemble from solution into close-packed arrays, where the motion of electrons is expected to be correlated due to long-range coulomb interactions. In order to study electron transport in these arrays, measurement of conductance around zero bias is required. Devices fabricated using CQDs, however, tend to be highly resistive (owing to large tunnel barriers from the organic ligands), and techniques to increase the conductance, such as annealing, often lead to large scale cracking. We nanopattern PbS CQDs, using electron beam lithography and a liftoff process, adjacent to a charge sensor. The patterning process helps to eliminate cracking, and improve packing of the dots. By performing a time resolved measurement of charge through the dots, using the sensor, we are able to measure conductance values as low as 10$^{\mathrm{-19\thinspace }}\Omega^{\mathrm{-1\thinspace }}$with a voltage bias of just 100mV. Our technique also allows us to map out the current voltage characteristics, even at low temperatures where the current becomes immeasurably small. We present the first transport measurements, near zero bias, on nanopatterned PbS quantum dots. [Preview Abstract] |
Friday, March 7, 2014 8:36AM - 8:48AM |
Y53.00004: Properties and Modeling of Graphene/CdSe Nanoparticle Film/Graphene Tunneling Device Structures Datong Zhang, Chenguang Lu, Philip Kim, Irving P. Herman We fabricated graphene/monolayer CdSe nanoparticle film/graphene sandwich device structures through a multi-step procedure. The monolayer CdSe nanoparticle film is formed on a liquid-air surface before transfer onto the bottom graphene layer that had been micro-exfoliated onto a 285 nm SiO$_{\mathrm{2}}$/Si substrate. The top graphene layer is transferred to the targeted area on the CdSe nanoparticle film via a dry transfer technique. Current-voltage measurements across the device suggest tunneling-type transport; the I-V curves are fit by tunneling models with an effective thin insulator with barrier height of about 1.6 eV and a tunneling distance of about 2.8 nm, which matches the nanoparticle dimension. In photoconductivity measurement, the source-drain current is greatly enhanced when the laser is on the junction area. The magnitude of the photocurrent is in agreement with that estimated using the nanoparticle absorption coefficient and laser intensity. [Preview Abstract] |
Friday, March 7, 2014 8:48AM - 9:00AM |
Y53.00005: Energy Level Alignment for Efficient Carrier Transport in PbS Nanoparticles Capped with Cross-linking Ligand Danylo Zherebetskyy, Marcus Scheele, David Hanifi, Yi Liu, Paul Alivisatos, Lin-Wang Wang Arrays of inorganic nanoparticle (NP) can be used for different applications from solar cell to LED. The connection between the NP by organic linker molecule and the resulting carrier transport is a major issue in such applications. We theoretically investigate the electronic coupling between the NP and tetrathiafulvalene-tetraacid (TTFTA) as a function of energy level alignment using multiscale theoretical approach. First, standard DFT calculations are applied to get the geometry of TTFTA on NP surface. Second, the correct band structure is obtained for the molecule and surface from GW formalism including relativistic effects. Third, a long range polarization effect due to the NP dielectric media is included. Fourth, the quantum confinement effect is added to the PbS 1S$_{\mathrm{h}}$ and 1S$_{\mathrm{e}}$ levels. Finally, charge transport rate between NP through the TTFTA cross-linking molecules is calculated under Marcus theory. The resonant alignment between molecular HOMO and 1Sh state of NP is observed for 9.8 nm NP. The alignment is confirmed experimentally using cyclic voltammetry and ambient pressure X-ray photoelectron spectroscopy. [Preview Abstract] |
Friday, March 7, 2014 9:00AM - 9:12AM |
Y53.00006: Charge transfer between a CdSe/CdS quantum rod and a tethered ferrocene molecule linwang Wang, Kartick Tarafder, Yogesh Surendranath, Jacob Olshansky, Paul Alivisatos Hole transfer between a CdSe/CdS core/shell semiconductor nanorod and a surface-ligated alkyl ferrocene is investigated by a combination of ab initio quantum chemistry calculations and experimental measurements. The calculated driving force for hole transfer corresponds well with electrochemical measurements of nanorods partially ligated by 6-ferrocenylhexanethiolate. The calculations and the experiment suggest that the hole transfer from the valence band maximum to ferrocene is through a direct coherent hopping, not through any intermediate steps, and this hopping is in the Marcus inverted region. The calculated rate of hole transfer is in line with the photoinduced hole transfer rate determined experimentally, and the calculated state energy alignment agrees excellently with the experiments. Together, the calculations suggest that holes may be extracted more efficiently from well-passivated nanocrystals by reducing the energetic driving force for hole transfer, thus minimizing energetic losses. [Preview Abstract] |
Friday, March 7, 2014 9:12AM - 9:48AM |
Y53.00007: Chemical Synthesis, Computational Modeling, and Surface Reactions of Silicon Nanotube Anodes and Silicate Cathodes for Lithium Ion Batteries Invited Speaker: Christopher Hinkle Nanostructured materials show significant promise in enhancing the performance and safety of Li-ion batteries at greatly reduced cost. We highlight certain classes of materials for next generation anodes, cathodes, and solid electrolytes in addition to interface reactions and show how advanced chemical spectroscopy and first principles modeling can be utilized to improve battery performance and stability. In this work, we utilize advanced materials characterization techniques (in-situ XPS and FTIR, Raman, AFM, XRD) to elucidate the chemical bonding, nanostructure, and electrochemical properties that lead to improved storage capabilities in these materials. We describe the recent progress in chemical synthesis methods of fabricating hydrogenated amorphous-Si nanotube anodes and tetrahedral transition metal silicate cathodes (Li$_{2}$MSiO$_{4})$, which may be well-suited for future technologies. Additionally, insight into the redox potentials and ionic and electronic conductivities has been investigated using first-principles modeling. Our findings suggest that high-voltage, multi-component Li$_{2}$MSiO$_{4}$ cathodes (M $=$ Fe, Mn, Ni) with high Mn content are strong candidates for future Li-ion batteries. Inorganic solid electrolytes are also discussed highlighting their potential for improved safety, increased ionic conductivities, and stability against adverse reactions with the electrodes. Finally, we illustrate the complexity of interfacial chemistry in these new materials and the need for advanced spectroscopic characterization to make progress on all aspects of electrode and electrolyte development. [Preview Abstract] |
Friday, March 7, 2014 9:48AM - 10:00AM |
Y53.00008: Dielectric relaxation studies of ion diffusion into low-k dielectrics Archana Raja, Thomas Shaw, Eric Liniger, Fen Chen, Alfred Grill, Juan Borja, Griselda Bonilla, Joel Plawsky, Tony Heinz, Robert Laibowitz High speed interconnects in advanced integrated circuits require ultra-low-k dielectrics to reduce the RC time constant. Reduction of the dielectric constant in these films is typically achieved via incorporation of nanopores in materials containing silicon, carbon, oxygen and hydrogen (SiCOH). Trap states build-up as dielectric breakdown is approached and increased leakage is observed. To understand the mechanism of breakdown we study nanoporous SiCOH films of k=2.4 to 2.7 primarily using dielectric relaxation. Dielectric films, in the thickness range of 40 nm, are incorporated into interwoven capacitor structures. To quantify dielectric relaxation in the pre-breakdown regime, capacitance and dielectric losses are determined as a function of frequency and temperature. Through these dielectric measurements, we have obtained activation energies in the range of 0.1-0.2 eV for humidified and annealed capacitors; and 0.9-1.2 eV for copper ion incursion into the dielectric. We also deduce a charge center density of 10$^{15}$/cm$^3$. Our measurements provide an estimate of the impurity content and changes in activation energy with annealing and other fabrication parameters. [Preview Abstract] |
Friday, March 7, 2014 10:00AM - 10:12AM |
Y53.00009: Focused helium and neon ion beam induced etching for advance EUV lithography and mask repair Rajendra Timilsina, Carlos Gongalez, Philip Rack The gas field ion microscope was used to investigate helium and neon ion beam induced etching (IBIE) of nickel as a candidate technique for extreme ultraviolet (EUV) lithography mask editing. No discernable nickel etching was observed for room temperature helium exposures at 16 and 30 keV in the range of 1x10$^{\mathrm{15}}$-1x10$^{\mathrm{18}}$ He$^{\mathrm{+}}$/cm$^{\mathrm{2}}$, however transmission electron microscopy (TEM) revealed subsurface damage to the underlying Mo-Si multilayer EUV mirror. Subsequently, neon beam induced etching at 30 keV was investigated over a similar dose range and successfully removed the entire 50 nm nickel top absorber film at a dose of approximately 3x10$^{\mathrm{17}}$ Ne$^{\mathrm{+}}$/cm$^{\mathrm{2}}$. TEM also revealed subsurface damage in the underlying Mo-Si multilayer. To further understand the helium and neon damage, we simulated the ion-solid interactions with our EnvizION Monte Carlo sputtering program which reasonably correlated the observed damage and bubble formation to the nuclear energy loss and the implanted inert gas concentration, respectively. A critical nuclear energy density loss of approximately 80 eV/nm$^{\mathrm{3}}$ and critical implant concentration of approximately 10$^{\mathrm{20}}$ atoms/cm$^{\mathrm{3}}$ have been calculated for damage generation in the multilayer structure. [Preview Abstract] |
Friday, March 7, 2014 10:12AM - 10:24AM |
Y53.00010: The role of interfacial effects on enhanced catalytic performance of TiO2-graphene nanocomposites Dinko Chakarov, Raja Sellappan Graphene-containining TiO2 nanocomposites have significantly higher photocatalytic activity compared to bare TiO2 films. The enhancement is result of improved transport and higher efficiency of the charge carries separation at carbon-TiO2 interface. These effects were assessed by comparison of six anatase-graphene structures, fabricated by different synthesizing techniques and referenced to the performance of TiO2--graphitic-carbon and TiO2--Au thin films. [Preview Abstract] |
Friday, March 7, 2014 10:24AM - 10:36AM |
Y53.00011: High Surface Area Dendrite Nanoelectrodes for Electrochemistry Nathan Nesbitt, Jennifer Glover, Saurabh Goyal, Svetoslav Simidjiysky, Michael Naughton Solution-based electrodeposition of metal using a low ion concentration, surface passivation agents, and/or electrochemical crystal conditioning has allowed for the formation of high surface area metal electrodes, useful for Raman spectroscopy and electrochemical sensors. Additionally, high frequency electrical oscillations have been used to electrically connect co-planar electrodes, a process called directed electrochemical nanowire assembly (DENA). These approaches aim to control the crystal face that metal atoms in solution will nucleate onto, thus causing anisotropic growth of metal crystals. However, DENA has not been used to create high surface area electrodes, and no study has been conducted on the effect of micron-scale surface topography on the initial nucleation of metal crystals on the electrode surface. When DENA is used to create a high surface area electrode, such a texture has a strong impact on the subsequent topography of the three dimensional dendritic structures by limiting the areal density of crystals on the electrode surface. Such structures both demonstrate unique physics concerning the nucleation of metal dendrites, and offer a unique and highly facile fabrication method of high surface area electrodes, useful for chemical and biological sensing. [Preview Abstract] |
Friday, March 7, 2014 10:36AM - 10:48AM |
Y53.00012: First-principles studies of conformation and solution effects on DNA transport Bikan Tan, Miroslav Hodak, Wenchang Lu, Jerry Bernholc The electrical conductivity of DNA molecules is of fundamental interest in the life sciences. We use first-principles techniques combined with molecular dynamical (MD) simulations to calculate transport properties of B-DNA connected to carbon nanotubes via alkane linkers. The quantum transport properties are calculated for over a hundred of snapshots recorded in MD trajectories. We discover that the DNA conformation and especially the overlaps between sequential guanine bases play a critical role in electron transport. DNA charge transport is indeed governed by charge delocalization with wavefunctions extent controlled by geometrical overlaps. Solvent atoms also affect the conductivity, with counterions decreasing the conductance by a factor of 2-3. In addition, we find that water molecules around the double helix screen the negatively-charged phosphate groups suppressing the conductance of DNA. Comparing transport properties of 4-base-pair (BP) with 10-BP DNA, we find weak distance dependence of the conductivity. Finally, we discuss the effect of sequence on DNA conductivity. [Preview Abstract] |
Friday, March 7, 2014 10:48AM - 11:00AM |
Y53.00013: Functionalization of hybrid organic-inorganic materials for highly efficient photovoltaic materials Levi Lentz, Alexie Kolpak Low mobility and high recombination rates limit the incident photon conversion efficiency (IPCE) of organic-based photovoltaics. In this work we employ first-principles density functional theory calculations to investigate hybrid organic-inorganic materials designed to directly ameliorate these issues. By constructing superlattices composed of 2D transition metal phosphate sheets separated by ordered regions of well-known organic dyes, we show that one can significantly decrease recombination and increase charge carrier mobilities relative to typical organic photovoltaic materials. ~We discuss how functionalization of the molecules in the organic region can be used to simultaneously enhance exciton separation and tune the organic-inorganic band alignment to encourage charge transfer into the inorganic regions, which can act as high-mobility charge carrier channels. ~Our results suggest that nanostructured hybrid materials could significantly improve IPCE over traditional organic photovoltaics. [Preview Abstract] |
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