Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session S1: Focus Session: Charge & Energy Transfer III
Sponsoring Units: DCPChair: Martin Wolf, Fritz-Haber-Institute
Room: 103/105
Thursday, March 6, 2014 8:00AM - 8:12AM |
S1.00001: First-principles calculation of photo-induced electron transfer rate constants in phthalocyanine-C$_{60}$ organic photovoltaic materials: Beyond Marcus theory Myeong H. Lee, Barry D. Dunietz, Eitan Geva Classical Marcus theory is commonly adopted in solvent-mediated charge transfer (CT) process to obtain the CT rate constant, but it can become questionable when the intramolecular vibrational modes dominate the CT process as in OPV devices because Marcus theory treats these modes classically and therefore nuclear tunneling is not accounted for. We present a computational scheme to obtain the electron transfer rate constant beyond classical Marcus theory. Within this approach, the nuclear vibrational modes are treated quantum-mechanically and a short-time approximation is avoided. Ab initio calculations are used to obtain the basic parameters needed for calculating the electron transfer rate constant. We apply our methodology to phthalocyanine(H$_2$PC)-C$_{60}$ organic photovoltaic system where one C$_{60}$ acceptor and one or two H$_2$PC donors are included to model the donor-acceptor interface configuration. We obtain the electron transfer and recombination rate constants for all accessible charge transfer (CT) states, from which the CT exciton dynamics is determined by employing a master equation. The role of higher lying excited states in CT exciton dynamics is discussed. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S1.00002: Exciton dissociation at phthalocyanine-C$_{60}$ interfaces S.W. Robey, G.J. Dutton Exciton dissociation and charge transfer processes occurring within 10's of nanometers of donor-acceptor interfaces are critical for the performance of organic photovoltaic (OPV) structures. We investigated fundamental issues of exciton dissociation near prototypical donor-acceptor interface using time-resolved two-photon photoemission (TR-2PPE). Phthalocyanine (Pc)-C$_{60}$ interfaces with known structures were formed using organic molecular beam epitaxy. Pc $\pi \to \pi $* (Q-band) transitions were created by a sub-picosecond pump pulse, producing a population of singlet (S$_{1})$ Pc excitons. The dynamics of this population were then probed via photoemission by a time-delayed UV pulse. For CuPc$\backslash $C$_{60}$ interfaces, the dynamics for excitons created far from the interface were modeled with a combination of vibrational or intraband relaxation plus intersystem crossing (ISC) to triplet levels. Relaxation leads predominantly to triplet (T$_{1})$ exciton levels on timescales of $\approx $ 1-2 ps. The decay dynamics of S$_{1}$ excitons excited in the CuPc layer adjacent to C$_{60}$ were increased due to the additional channel leading to exciton dissociation, occurring with a rate of $\approx $ 7 x 10 $^{12}$ sec$^{-1}$. However, excitons that relax to T$_{1}$ levels at the interface dissociate with a rate $\approx $ 500 to 1000 times slower, providing a picture of the energy dependence of exciton dissociation at this interface. The dependence of exciton dissociation versus Pc thickness at analogous H$_{2}$Pc $\backslash $C$_{60}$ interfaces will also be presented. The results indicate that, for this interface, exciton dissociation is much faster for the interfacial layer with dissociation from the 2$^{\mathrm{nd}}$, and subsequent layers of H$_{2}$Pc, reduced by at least a factor of 10. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S1.00003: Passive exciton gating via directed energy transfer in organic photovoltaic cells S. Matthew Menke, Tyler K. Mullenbach, Russell J. Holmes We present a Kinetic Monte Carlo method to model exciton diffusion across non-reflective, non-quenching, exciton permeable interfaces where standard analytical and numerical solutions to the exciton diffusion equation are not available. The combination of energy transfer rates and separately measured natural lifetimes allows for the modeling of exciton diffusion across a wide range of inhomogeneous landscapes. This model is successfully applied to quantitatively account for the photocurrent enhancements present in dilute donor organic photovoltaic (OPV) devices incorporating the archetypical electron donor boron subphthalocyanine chloride (SubPc), the wide energy gap host material p-bis(triphenylsilyl)benzene (UGH2), and fullerene C60. Furthermore, this modeling is extended to provide an optimization route for advanced, cascade OPVs where excitons are transferred between layers before reaching the acceptor material. Analysis of exciton motion in these architectures also reveals how inhomogeneous energy transfer landscapes can lead to directed exciton motion, thus deviating from the typical diffusive or sub-diffusive behavior. The implications for directed exciton motion are then discussed in terms of passive exciton gating towards the ultimate goal of finely controlling the migration of energy in these devices as well as the broader field of organic optoelectronic devices. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S1.00004: Direct observation of ultrafast long-range charge separation at polymer:fullerene heterojunctions Invited Speaker: Carlos Silva In polymeric semiconductors, charge carriers are polarons, which means that the excess charge deforms the molecular structure of the polymer chain that hosts it. This effect results in distinctive signatures in the vibrational modes of the polymer. We probe polaron photogeneration dynamics at polymer:fullerene heterojunctions by monitoring its time-resolved resonance-Raman spectrum following ultrafast photoexcitation. We conclude that polarons emerge within 200\,fs, which is nearly two orders of magnitude faster than exciton localisation in the neat polymer film. Surprisingly, further vibrational evolution on $\la 50$-ps timescales is modest, indicating that the polymer conformation hosting nascent polarons is not significantly different from that in equilibrium. This suggests that charges are free from their mutual Coulomb potential, under which vibrational dynamics would report charge-pair relaxation. Our work addresses current debates on the photocarrier generation mechanism at organic semiconductor heterojunctions, and is, to our knowledge, the first direct probe of molecular conformation dynamics during this fundamentally important process in these materials. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S1.00005: Exploring the Influence of Dynamic Disorder on Excitons in Solid Pentacene Zhiping Wang, Sahar Sharifzadeh, Peter Doak, Zhenfei Lu, Jeffrey Neaton A complete understanding of the spectroscopic and charge transport properties of organic semiconductors requires knowledge of the role of thermal fluctuations and dynamic disorder. We present a first-principles theoretical study aimed at understanding the degree to which dynamic disorder at room temperature results in energy level broadening and excited-state localization within bulk crystalline pentacene. Ab initio molecular dynamics simulations are well-equilibrated for 7-9 ps and tens of thousands of structural snapshots, taken at 0.5 fs intervals, provide input for many-body perturbation theory within the GW approximation and Bethe-Salpeter equation (BSE) approach. The GW-corrected density of states, including thousands of snapshots, indicates that thermal fluctuations significantly broaden the valence and conduction states by \textgreater 0.2 eV. Additionally, we investigate the nature and energy of the lowest energy singlet and triplet excitons, computed for a set of uncorrelated and energetically preferred structures. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S1.00006: Singlet fission in reduced dimensions of crystals Paul Teichen, Joel Eaves In some molecular systems the decay of an initially excited singlet into two independent triplets, a process called singlet fission, is highly efficient. Organic crystals are among the most promising candidates for increasing yields in next-generation photovoltaics. Although excitons are known to exist in reduced dimensions of crystals the role of dimensionality in the entanglement of two triplets born out of singlet fission remains unclear. We develop a quantum lattice model for singlet fission to examine the role of quantum entanglement and exciton delocalization. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S1.00007: Fission of Entangled Spins: An Electronic Structure Perspective Xintian Feng, Anatoliy Luzanov, Anna Krylov Electronic structure aspects of singlet fission process are discussed. Correlated adiabatic wave functions of the bright singlet and dark multiexciton states of tetracene and pentacene dimers are analyzed in terms of their character (excitonic, charge-resonance, multiexciton). At short interfragment separation (3.5-4.0 angstroms), both multiexcitonic and singly-excited singlet states have noticeable charge-resonance contributions that fall off quickly at longer distances. Non-adiabatic couplings between the states are discussed. The limitations of diabatic framework in the context of singlet fission are explained. Based on the Cauchy-Schwarz inequality, we propose using the norm of one-particle transition density matrix, $||\gamma||$, as a proxy for couplings. The analysis of $||\gamma||$ and state characters reveals that the couplings between the multiexciton and singly-excited states depend strongly on the weights of charge-resonance configurations in these states. To characterize energetics relevant to triplets separation step, we consider multiexciton binding energy (E$_b$) defined as the difference between the quintet and singlet multiexciton states. The effect of fragment orientation on the couplings and E$_b$ is analyzed. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:24AM |
S1.00008: Exciton fission, dissociation and transport in organic conjugated materials: Modeling insights Invited Speaker: David Beljonne Electronic excited states in conjugated organic materials involve an admixture of localized (Frenkel-like) and charge-transfer (CT) excitations. We will first review some recent modeling work showing that this mixed Frenkel-CT character steers the Davydov splitting, mediates singlet fission and prompts triplet energy migration in oligoacenes crystals. Ultrafast and efficient charge separation occurs at interfaces between properly designed molecular donors and acceptors, despite the large electron-hole Coulomb binding energy. In a second part of the talk, we will describe the various mechanisms for such a dissociation process and assess them from atomistic simulations based on a combination of force-field, quantum-chemical and model Hamiltonian calculations. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S1.00009: First Principles Calculations of Conformational and Electronic Properties of PTB7 Ram Bhatta, David Perry, Mesfin Tsige The thieno[3,4-b]thiophene-altbenzodithiophene copolymer (PTB7) is a promising electron donor in organic photovoltaic (OPV) devices with a power conversion efficiency (PCE) of about 9 percent. Further enhancement of the PCE is required for the practical realization and successful commercialization, which, in turn relies on the core understanding of structure-property relationships in OPV materials. Here, we present large-scale density functional calculations of the torsional and electronic properties of PTB7 oligomers. These first principles results include the chain length dependence of the torsional potential, the nearest neighbor torsional coupling, the band gap and the electronic conjugation length. PTB7 was found to have weaker nearest-neighbor torsional coupling, a lower band gap and a longer conjugation length compared to the other conjugated polymers like polythiophene and poly(3-alkylthiophene). These results help to explain the relative efficiency of OPV devices in which PTB7 is the electron donor. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S1.00010: Understanding the Role of Orientational Heterogeneity on Photophysical Properties of Organic Polycrystalline Films Sahar Sharifzadeh, Cathy Wong, Hao Wu, Naomi Ginsberg, Leeor Kronik, Jeffrey Neaton Organic semiconductors are a highly tunable class of optically active materials that are promising as next-generation photovoltaics. Utilizing these materials for efficient solar energy conversion relies on an understanding of the connection between their excited-state electronic structure and their solid-state morphology. While many organic materials have varying degrees of disorder, crystalline films with long-range order provide an opportunity to understand many fundamental physical properties relevant to solar energy conversion. Here, we use a combined theoretical and experimental approach to investigate the nature of low-energy excitons and their dynamics within 6,13-bis(triisopropylsilylethynyl)-pentacene (TIPS-Pen) polycrystalline films. First-principles many-body perturbation theory and optical absorption spectroscopy on ordered domains reveal multiple low-energy absorption peaks that are composed of delocalized excitonic states. Further, we examine the nature of excitons in grains of different relative orientations and at grain boundaries, and discuss implications for their dynamics as measured by spatially-resolved transient absorption spectroscopy. This work was supported by DOE; computational resources provided by NERSC. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S1.00011: Theoretical Modeling and Design of Organic Semiconductors with High Carrier Mobility Xiao Ma, Changgua Zhen, John Kieffer Charge transport in organic materials can be quite different from that in inorganic materials. The weak van der Waals interaction between organic molecules invalidates the band model used widely in inorganic materials. We have applied a multiscale hopping model based on Fermi's golden rule to study the carrier mobility in a pentacene single crystal structure. The pentacene single crystal adopts a herringbone stacking, which strongly limits the $\pi $-orbital overlap between neighboring molecules, resulting in poor charge carrier transfer and long-range mobility. To improve the charge transport performance of pentacene-related organic materials, we functionalize the pentacene with polyhedral oligomeric silsesquioxanes (POSS) cages to induce a parallel configuration. A higher theoretical carrier mobility is predicted based on using a combination of molecular dynamics, density functional theory calculations and kinetic Monte Carlo simulations. Accordingly, simulations constitute a cost-efficient means to derive design principle for materials with improved transport properties to be used in photovoltaic devices. [Preview Abstract] |
Session S2: Focus Session: Solvation, Dynamics, and Reactivity in Complex Environments III
Sponsoring Units: DCPChair: Christine Payne, Georgia Institute of Technology
Room: 102
Thursday, March 6, 2014 8:00AM - 8:12AM |
S2.00001: Time-dependent density-functional theory for real-time electronic dynamics on material surfaces Rulin Wang, Dong Hou, Xiao Zheng The real-time electronic dynamics on material surfaces is critically important to a variety of applications. However, numerical simulations are rather challenging for conventional first-principles methods such as the time-dependent density-functional theory (TDDFT). To solve this problem, we extend the applicability of TDDFT to open electronic systems[\newblock Phys. Rev. B {\bf 75}, 195127 (2007)]. The dissipative system-environment interactions are treated by a hierarchical equations of motion (HEOM) approach. The combined TDDFT--HEOM method, along with a \emph{k}-sampling scheme [J. Chem. Phys. {\bf 132}, 114703 (2010)] for calculating the spectral function of a two-dimensional system, is applied to simulate real-time electronic dynamics on material surfaces. Two prototypical scenarios are exemplified [Phys. Rev. B (accepted) (2013)]: the relaxation of an excess electron on a graphene surface, and the electron transfer across the molecule-graphene interface. These two examples accentuate the fundamental importance and usefulness of an open-system TDDFT approach, and they also provide some insights into the characteristic features of temporal electron evolution and dissipation on surfaces of bulk materials. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S2.00002: The electrostatic double layer of Pt/water interfaces from first principles molecular dynamics Clotilde Cucinotta, Pietro Ballone, Stefano Sanvito The formation of the electrostatic double layer is the most basic phenomenon taking place at electrified interfaces. However, even in the relatively simple case of a Pt/water interface, none of the current theoretical approaches provides a realistic microscopic view of this double layer, accounting for electronic, polarization and solvent re-organization effects. Here we provide for the first time a comprehensive description of the electrostatic double layer of a Pt-water interface, based on ab initio computations, including charge polarization effects at both sides of the interface, explicit solvent and its rearrangements upon changing the electrode polarization. This interface has been modeled with up to 1000 atoms. A simple, fully dissociated salt in solution has been explicitly included. Varying the relative number of cations and anions provides a way to control the charge on the electrode, controlling, in turn, the applied potential. The proposed approach allows to provide a detailed description of the structure of the Pt/water double layer reproducing the localization of electric field and potential energy drop within a microscopic distance from the metal surface. An a posteriori calibration of the relation between charge and potential is performed, analyzing the potential energy profile vs. the distance from the electrode for any given charge, providing for the first time a realistic ab initio determination of the interface capacitance and the point of zero charge. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S2.00003: Venturing into the kinetics and mechanism of nanoconfined solid-state reactions: Trimerization of sodium dicyanamide in nanopores Benjamin Yancey, Sergey Vyazovkin This study represents the first attempt to determine the effect of nanoconfinement on the kinetics and mechanism of solid-state reactions. FTIR, NMR, and DSC were employed to analyze the thermally initiated trimerization of sodium dicyanamide (NaC$_{2}$N$_{3}$) to sodium tricyanomelaminate (Na$_{3}$C$_{6}$N$_{9}$) in bulk and organically modified nanopores. The trimerization occurred at a decelerated rate as evidenced by an increase in reaction temperature as measured by DSC. Nanoconfinement did not cause apparent changes in the reaction mechanism as the products of the reaction were the same in bulk and in nanopores. Kinetic analysis linked the deceleration to a dramatic decrease (several orders of magnitude) in the pre-exponential factor. This effect is especially significant in view of previous studies on nanoconfined liquid state reactions in which the effect is opposite: considerable acceleration due to an increase in the pre-exponential factor. We propose that the difference arises respectively from disordering of the solid and ordering of the liquid reaction media. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S2.00004: Water adsorption and proton conduction in metal-organic frameworks: Insights from molecular simulations Invited Speaker: Francesco Paesani Metal-organic frameworks (MOFs) are a relatively new class of porous materials that hold great potential for a wide range of applications in chemistry, materials science, and nanoengineering. Compared to other porous materials such as zeolites, MOF properties are highly tunable. In particular, it has been shown that both size and shape of the MOF pores can be rationally designed for specific applications. For example, the ability to modify the framework properties with respect to hydrophilicity/hydrophobicity and acidity/basicity can enable the direct control of proton conduction through carrier molecules adsorbed inside the pores. Here, I report on our current efforts aimed at providing a molecular-level characterization of water-mediated proton conduction through the MOF pores. Particular emphasis will be put on correlation between proton conduction and both structural and chemical properties of the frameworks as well as on the dynamical behavior of water confined in the MOF pores. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S2.00005: Simulating and Modeling Transport Through Atomically Thin Membranes Joseph Ostrowski, Joel Eaves The world is running out of clean portable water. The efficacy of water desalination technologies using porous materials is a balance between membrane selectivity and solute throughput. These properties are just starting to be understood on the nanoscale, but in the limit of atomically thin membranes it is unclear whether one can apply typical continuous time random walk models. Depending on the size of the pore and thickness of the membrane, mass transport can range from single stochastic passage events to continuous flow describable by the usual hydrodynamic equations. We present a study of mass transport through membranes of various pore geometries using reverse nonequilibrium simulations, and analyze transport rates using stochastic master equations. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S2.00006: Theoretical analysis on ion transport through polymer networks in electrochemical capacitors Jos W. Zwanikken, Yufei Jing, Vikram Jadhao, Charles E. Sing, Niels Boon, Monica Olvera de la Cruz The development of predictive methods for deformable electronics calls for an equally composite theoretical foundation that unites traditionally separated fields. We are pioneering theoretical methods that unite polymer physics with liquid state theory, and develop a dynamical algorithm for inhomogeneous polarizable media between capacitor plates. By a quantitative study of the local molecular correlations we can explain the macroscopic behavior and the induced (non-equilibrium) potentials of mean force between the ions, the supporting medium, and the electrodes. Several timescales are found that correspond to different relaxation processes, related to ion diffusion, double layer formation, and the elastic response of the network. The application of an alternating current reveals a complex frequency-dependent response, by which the relative importance of the different underlying processes can be tuned. Typical non-equilibrium forces, generated by the applied field, are found to arise between regions with sharp gradients in the molecular structure or supporting background. The results may inform experimental efforts on noise reduction in soft capacitors, and suggest new functionality based on frequency-dependent non-equilibrium forces. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S2.00007: Direct simulation of proton-coupled electron transfer reaction dynamics and mechanisms Joshua S. Kretchmer, Thomas F. Miller III Proton-coupled electron transfer (PCET) reactions, in which both an electron and an associated proton undergo reactive transfer, play an important role in many chemical and biological systems. Due to the complexity of this class of reactions, a variety of different mechanisms fall under the umbrella of PCET. However, the physical driving forces that determine the preferred mechanism in a given system still remain poorly understood. Towards this end, we extend ring polymer molecular dynamics (RPMD), a path-integral quantum dynamics method, to enable the direct simulation and characterization of PCET reaction dynamics in both fully atomistic and system-bath models of organometallic catalysts. In addition to providing validation for the simulation method via extensive comparison with existing PCET rate theories, we analyze the RPMD trajectories to investigate the competition between the concerted and sequential reaction mechanisms for PCET, elucidating the large role of the solvent in controlling the preferred mechanism. We further employ RPMD to determine the kinetics and mechanistic features of concerted PCET reactions across different regimes of electronic and vibrational coupling, providing evidence for a new and distinct PCET reaction mechanism. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:24AM |
S2.00008: Wetting of hydrophobic and nanostructured surfaces Invited Speaker: Alenka Luzar Understanding wetting phenomena on nanostructured and nanopatterned surfaces is important in materials science and biology. The talk will highlight some of our recent progress on nanowetting of surfaces with topological and chemical heterogeneities using molecular modeling. Examples will include electric, pressure, or chemistry induced dynamic transitions of water on superhydrophobic surfaces to achieve reversible switching between contrasting wetting states. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S2.00009: Memory in 2D FT Spectra of Quantum Dots Samuel Park, Dmitry Baranov, Byungmoon Cho, Trevor Courtney, David Jonas We have used the first femtosecond 2DFT spectrometer in the short-wave infrared to record 2DFT spectra of the polar dye IR26 in dichloroethane. The 2DFT spectra of IR26 at early mixing times shows a diagonally elongated positive peak, which reflects the strong correlation between excitation and detection frequencies. The peak also has a slight shift above the diagonal and an off-diagonal negative region that is indicative of vibrational and solvent frequency memory (the finite timescale for frequency shifts from inertial solvation). Nearly all correlation is gone at long mixing times and the 2D spectra approach a product lineshape. We also measured the first 2DFT spectra of oleate-capped colloidal PbSe quantum dots in tetrachloroethylene in the short-wave infrared region. These measure a bi-exciton binding energy that is consistent with prior spectrally resolved pump-probe experiments. Most interestingly, certain similarities between the 2DFT spectra of IR26 and quantum dots at early mixing times points towards coherent phonon/solvent memory; calculations by Prezhdo and co-workers both predicted phonon memory in quantum dots and indicated that it affects carrier relaxation. The analysis and implications of these results will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S2.00010: First-principles study of single water interactions with theLaMnO$_3$ surface in the presence of defects, Sr substitution and varied surface morphologies Chris Billman, Hai-ping Cheng Lanthanum manganite (LaMnO$_3$) has been shown to have tremendous catalytic activity for the oxygen reduction reaction (OER) and oxygen evolution reaction (ORR) and is cheaper than other catalytic materials (Suntivich, Jin et al. 2011 Nature Chemistry 3, 546). Previous work studying ORR and OER indicates that water plays an important role in the intermediate reactions, however very little research has been done on the interaction between water and the LaMnO$_3$ surface (Wang Yan, et al. 2013. Journal Phys Chem C 5, 2106). Using density functional theory calculations, we examine the details of water adsorption and dissociation on a perfect and defective LaMnO$_3$ surfaces. We find that oxygen vacancies cause a strong preference for water dissociation on the surface but that the interaction is largely robust in the presence of strontium (Sr) substitutions. We also explore the dependence of interaction on structural parameters with a few different surface morphologies. Our results provide insights to the catalytic function of LaMnO$_3$ in both ORR and OER applications. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S2.00011: Nucleus-Coupled Electron Transfer Mechanism for TiO2-Catalyzed Water Splitting Yiyang Sun, Michael Lucking, Damien West, Shengbai Zhang Using first-principles calculations employing explicit interface of TiO$_2$ crystal and liquid water, we reveal the microscopic mechanism of the oxygen evolution reaction (OER). It is found that, during the formation of an O--O species, such as HO--OH and O--OH, an occupied molecular orbital with anti-bonding character evolves from the valence band and pops up all the way into the conduction band of TiO$_2$. This occupied high-energy orbital results in a high reaction barrier making the OER forbidden in the dark. The presence of photoholes depletes this anti-bonding orbital, which significantly reduces the reaction energy and determines the reaction barrier in the rate-limting step. A novel reaction mechanism, termed necleus-coupled electron transfer (NCET), emerges from this study. In this mechanism, the oxidation of a pair of hydroxyl groups, which is an electron transfer reaction, is enabled by the movement of the nuclei (i.e., the two O atoms moving towards O-O bond formation) that pushes the $reactive$ orbital (the $\sigma^{*}_{2p}$ orbital in the present case) to become the $frontier$ orbital (i.e., above the valence band maximum of TiO). Based on the NCET mechanism, we identify a reaction pathway of the OER that exhibits a kinetic barrier surmountable at room temperature. [Preview Abstract] |
Session S3: Focus Session: Intracellular Organization
Sponsoring Units: DBIOChair: Margaret Gardel, The University of Chicago
Room: 107
Thursday, March 6, 2014 8:00AM - 8:12AM |
S3.00001: Self-organized spatiotemporal patterns of PIP$_{3}$ and PTEN during spontaneous cell polarization Fabian Knoch, Marco Tarantola, Wouter-Jan Rappel, Eberhard Bodenschatz During spontaneous cell polarization of Dictyostelium discoideum cells, PIP3 (phosphatidylinositol (3,4,5)-triphoshpate) and PTEN (phosphatase tensin homolog) have been identified as key signaling molecules, which govern the process of polarization in a self-organized manner. Gerisch et al. have shown that randomly triggered excitable PIP3 waves regulate the anti-correlated PTEN concentration. Here we show that this requires a switch-like dynamics of the overall membrane bound PTEN concentration in combination with two species of PTEN differing in their dephosphorylation rates. A quantitative modeling with a coupled reaction-diffusion system shows excellent agreement with experimental results and predicts a ratio $\sigma $ of dephosphorylation rates acting on PIP3 of $\sigma \approx $ 80 $-$ 100. Our quantitative analysis suggests that surface-attached cell membrane spanning PIP3 waves are necessary for resetting the global actin network. This is evidenced by the experimentally observed delay between polarization-cycles also quantitatively captured by our analysis. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S3.00002: Dynamics of myosin II organization into cortical contractile networks and fibers Wei Nie, Ming-tzo Wei, Daniel Ou-Yang, Sabrina Jedlicka, Dimitrios Vavylonis The morphology of adhered cells critically depends on the formation of a contractile meshwork of parallel and cross-linked stress fibers along the contacting surface. The motor activity and mini-filament assembly of non-muscle myosin II is an important component of cell-level cytoskeletal remodeling during mechanosensing. To monitor the dynamics of myosin II, we used confocal microscopy to image cultured HeLa cells that stably express myosin regulatory light chain tagged with GFP (MRLC-GFP). MRLC-GFP was monitored in time-lapse movies at steady state and during the response of cells to varying concentrations of blebbistatin which disrupts actomyosin stress fibers. Using image correlation spectroscopy analysis, we quantified the kinetics of disassembly and reassembly of actomyosin networks and compared them to studies by other groups. This analysis suggested that the following processes contribute to the assembly of cortical actomyosin into fibers: random myosin mini-filament assembly and disassembly along the cortex; myosin mini-filament aligning and contraction; stabilization of cortical myosin upon increasing contractile tension. We developed simple numerical simulations that include those processes. The results of simulations of cells at steady state and in response to blebbistatin capture some of the main features observed in the experiments. This study provides a framework to help interpret how different cortical myosin remodeling kinetics may contribute to different cell shape and rigidity depending on substrate stiffness. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S3.00003: Brownian dynamics simulation of fission yeast mitotic spindle formation Christopher Edelmaier, Robert A. Blackwell, Oliver M. Sweezy, Matthew A. Glaser, Meredith D. Betterton The mitotic spindle segregates chromosomes during mitosis. The dynamics that establish bipolar spindle formation are not well understood. We have developed a computational model of fission-yeast mitotic spindle formation using Brownian dynamics and kinetic Monte Carlo methods. Our model includes rigid, dynamic microtubules, a spherical nuclear envelope, spindle pole bodies anchored in the nuclear envelope, and crosslinkers and crosslinking motor proteins. Crosslinkers and crosslinking motor proteins attach and detach in a grand canonical ensemble, and exert forces and torques on the attached microtubules. We have modeled increased affinity for crosslinking motor attachment to antiparallel microtubule pairs, and stabilization of microtubules in the interpolar bundle. We study parameters controlling the stability of the interpolar bundle and assembly of a bipolar spindle from initially adjacent spindle-pole bodies. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S3.00004: Membrane tension regulates clathrin-coated pit dynamics Invited Speaker: Allen Liu Intracellular organization depends on close communication between the extracellular environment and a network of cytoskeleton filaments. The interactions between cytoskeletal filaments and the plasma membrane lead to changes in membrane tension that in turns help regulate biological processes. Endocytosis is thought to be stimulated by low membrane tension and the removal of membrane increases membrane tension. While it is appreciated that the opposing effects of exocytosis and endocytosis have on keeping plasma membrane tension to a set point, it is not clear how membrane tension affects the dynamics of clathrin-coated pits (CCPs), the individual functional units of clathrin-mediated endocytosis. Furthermore, although it was recently shown that actin dynamics counteracts membrane tension during CCP formation, it is not clear what roles plasma membrane tension plays during CCP initiation. Based on the notion that plasma membrane tension is increased when the membrane area increases during cell spreading, we designed micro-patterned surfaces of different sizes to control the cell spreading sizes. Total internal reflection fluorescence microscopy of living cells and high content image analysis were used to quantify the dynamics of CCPs. We found that there is an increased proportion of CCPs with short (\textless 20s) lifetime for cells on larger patterns. Interestingly, cells on larger patterns have higher CCP initiation density, an effect unexpected based on the conventional view of decreasing endocytosis with increasing membrane tension. Furthermore, by analyzing the intensity profiles of CCPs that were longer-lived, we found CCP intensity decreases with increasing cell size, indicating that the CCPs are smaller with increasing membrane tension. Finally, disruption of actin dynamics significantly increased the number of short-lived CCPs, but also decreased CCP initiation rate. Together, our study reveals new mechanistic insights into how plasma membrane tension regulates the dynamics of CCPs. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S3.00005: Probing the Effects of Cargo Loads in Single-Molecule Kinesin Mechanochemistry B.D. Jacobson, S.J. Koch, S.R. Atlas The influence of cargo loading on the dynamics of motor proteins such as kinesin is key to understanding fundamental aspects of their kinetic cycles and mechanochemistry. Kinetic models offer insight into these complex processes which occur on time scales up to seconds, and coupled with experimental data, they are a powerful tool in generating an increasingly fine-grained understanding of the chemical and mechanical mechanisms involved in kinesin procession, as well as the prospect of direct coupling to atomistic-scale simulations [1]. Here we present a kinetic model of single-molecule kinesin to study the effects of external forces due to intracellular cargo transport on chemical and mechanical rate constants. We use a simulated annealing algorithm that optimizes rate constants to fit published experimental data on kinesin speed and processivity, and kinetic Monte Carlo to compare predicted values with independent experimental measurements. We also discuss the application of sensitivity analysis to provide additional insight into the critical transitions and states of the processing protein under load.\\ \noindent[1] B. D. Jacobson, L. J. Herskowitz, S. J. Koch and S. R. Atlas, Investigation of kinesin processivity via simulated annealing, Biophys. J., submitted (2013). [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S3.00006: Deciphering Neurofilament Motility in live cells Christopher Johnson, Peter Jung Neurofilaments are the most abundant cytoskeletal elements of mature neuronal axons. They are assembled in the axon and responsible for regulating the axon's diameter. These filaments are transported in a characteristic stop-and-go fashion along microtubule tracks toward the nerve terminal driven by the motor proteins, kinesin and dynein.~ To explore the mechanisms underlying the observed stop-and-go transport, we devise a computational model~in which kinesin and dynein---coupled by respective force generation (tug-of-war)--- are attached to the neurofilament cargo.~ The resulting movement and kinetic characteristics are compared with highly time-resolved kymograph recordings.~ This comparison determines whether the observed kinetics are consistent with a tug-of-war model, and also renders an estimate of how many motors are attached to the cargo during transport. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 10:12AM |
S3.00007: \textit{In vivo} control mechanisms of motor-cargo movement on microtubules Invited Speaker: Shermali Gunawardena Within axons, molecular motors transport essential components required for neuronal growth and viability. Although many levels of regulation must exist for proper anterograde and retrograde transport of vital proteins, little is known about these mechanisms. Previous work suggested that the amyloid precursor protein (APP) functions as a kinesin-1 receptor during transport. However, how APP vesicle motility is regulated is unclear. Using genetics and \textit{in vivo} imaging in \textit{Drosophila} we showed that reduction of presenilin (PS) substantially increased anterograde and retrograde APP vesicle velocities. Strikingly, PS deficiency had no effect on an unrelated cargo vesicle containing synaptotagmin, which is powered by a different kinesin motor. Increased PS-mediated velocities required functional kinesin-1 and dynein motors. We also found that these PS-mediated effects on motor protein function were mediated via a pathway that involves glycogen synthase kinase-3$\beta $ (GSK-3$\beta )$. PS genetically interacted with GSK-3$\beta $ in an activity dependent manner. Excess of active GSK-3$\beta $ perturbed transport by causing axonal blockages, which were enhanced by reduction of kinesin-1 or dynein, while excess of non-functional GSK-3$\beta $ had no effect. Strikingly, GSK-3$\beta $-activity dependent transport defects were enhanced by reduction of PS. Collectively, our findings suggest that PS and GSK-3$\beta $ are required for normal motor protein function, and we propose a model in which PS likely regulates GSK-3$\beta $ activity during transport. These findings have important implications for our understanding of the complex regulatory machinery that must exist \textit{in vivo} and how this system is coordinated during vesicle motility on microtubules. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S3.00008: Mechanical guidance through cell-cell and cell-surface contact during multicellular streaming Chenlu Wang, Meghan Driscoll, Satyandra K. Gupta, Carole Parent, Wolfgang Losert During collective cell migration, mechanical forces arise from the extracellular matrix (ECM) through cell-surface contact and from other cells through cell-cell contact. These forces regulate the motion of migrating cell groups. To determine how these mechanical interactions balance during cell migration, we measured the shape dynamics of Dictyostelium discoideum cells at the multicellular streaming stage. We found that cells can coordinate their motion by synchronizing protrusion waves that travel along their membranes when they form proper cell-cell adhesion and cell-surface adhesion. In addition, our experiments on live actin labeled cells show that intracellular actin polymerization actively responds to the change of cell-cell/surface adhesion and helps to stabilize multicellular migration streams. Our finding suggests that the coordination of motion between neighboring cells in collective migration requires a balance between cell-cell adhesion and cell-surface adhesion, and that the cell cytoskeleton plays an important role in this balance. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S3.00009: Model of Exploratory Search for Mating Partners by Fission Yeast Daniel Hurwitz, Felipe Bendezu, Sophie Martin, Dimitrios Vavylonis During conditions of nitrogen starvation, the model eukaryote \textit{S. pombe} (fission yeast) undergoes sexual sporulation. Because fission yeast are non-motile, contact between opposite mating types during spore formation is accomplished by polarizing growth, via the Rho GTP-ase Cdc42, in each mating type towards the selected mate, a process known as shmooing. Recent findings showed that cells pick one of their neighboring compatible mates by randomizing the position of the Cdc42 complex about the cell membrane, such that the complex is stabilized near areas of high concentration of the opposite mating type pheromone. We developed Monte Carlo simulations to model partner finding in populations of mating cells and in small cell clusters. We assume that pheromones are secreted at the site of Cdc42 accumulation and that the Cdc42 dwell time increases in response to increasing pheromone concentration. We measured the number of cells that succeed in successful reciprocal pairing, the number of cells that were unable to find a partner, and the number of cells that picked a partner already engaged with another cell. For optimal cell pairing, we find the pheromone concentration decay length is around 1 micron, of order the cell size. We show that non-linear response of Cdc42 dwell time to pheromone concentration improves the number of successful pairs for a given spatial cell distribution. We discuss how these results compare to non-exploratory pairing mechanisms. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S3.00010: Centrosomes are autocatalytic droplets of pericentriolar material organized by centrioles David Zwicker, Markus Decker, Steffen Jaensch, Anthony A. Hyman, Frank J\"ulicher We propose a physical description of the centrosome, a membrane-less organelle involved in cell division. In our model, centrosome material occurs in a soluble form in the cytosol and a form that tends to undergo phase separation from the cytosol. We find that an autocatalytic chemical transition between these forms accounts for the temporal evolution observed in experiments. Interestingly, the nucleation of centrosomes can be controlled by an enzymatic activity of the centrioles, which are present at the core of all centrosomes. This non-equilibrium feature also allows for multiple stable centrosomes, a situation which is unstable in equilibrium phase separation. Our theory explains the growth dynamics of centrosomes for all cell sizes down to the eight-cell stage of the \textit{C. elegans} embryo. It also accounts for data acquired in experiments with aberrant numbers of centrosomes and altered cell volumes. Furthermore, our model can describe unequal centrosome sizes observed in cells with disturbed centrioles. Our example suggests a general picture of the organization of membrane-less organelles. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S3.00011: Quantifying the Dynamic Interactions Between a Clathrin-Coated Pit and Cargo Molecules Aubrey Weigel, Michael Tamkun, Diego Krapf Clathrin-mediated endocytosis is a major pathway of internalization of cargo in eukaryotic cells. This process involves the recruitment of cargo molecules into a growing clathrin-coated pit (CCP). However, cargo-CCP interactions are difficult to study because CCPs display a large degree of lifetime heterogeneity and the interactions with cargo molecules evolve over time. We use single-molecule total internal reflection fluorescence (TIRF) microscopy, in combination with automatic detection and tracking algorithms, to directly visualize the recruitment of individual voltage-gated potassium channels into forming CCPs in living cells. Contrary to widespread ideas, cargo often escapes from a pit before abortive CCP termination or endocytic vesicle production. By measuring tens of thousands of capturing events, we build the distribution of capture times and the times that cargo remains confined to a CCP. An analytical stochastic model is developed and compared to the measured distributions. Due to the dynamic nature of the pit, the model is non-Markovian and it displays long-tail power law statistics. Our findings identify one source of the large heterogeneities observed in CCP maturation and provide a mechanism for the anomalous diffusion of proteins in the plasma membrane. [Preview Abstract] |
Session S4: Focus Session: Kagome Antiferromagnets I
Sponsoring Units: GMAGChair: Oleg Tchernyshyov, Johns Hopkins University
Room: 112/110
Thursday, March 6, 2014 8:00AM - 8:12AM |
S4.00001: Experimental Studies on Single Crystal Samples of Spin Liquid Materials Tian-Heng Han, Young Lee, John Schlueter, Thomas Rosenbaum, Eric Isaacs Frustrated antiferromagnetism on spin lattices with triangular geometries receives increasing attention due to the promise of RVB spin liquids. I will discuss about recent thermodynamic and scattering studies on highly frustrated magnets, such as triangular kappa-(ET)$_{2}$Cu$_{2}$(CN)$_{3}$ and kagome ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$. It was only until recent years that large crystal samples have been successfully grown for leading spin liquid candidates. Latest experimental studies will be introduced. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S4.00002: Microscopic magnetic modeling for the spin-$\frac12$ kagome compound $[$NH$_4]_2[$C$_7$H$_{14}$N$][$V$_7$O$_6$F$_{18}]$ Oleg Janson, Alexander A. Tsirlin, Ioannis Rousochatzakis, Helge Rosner, Raivo Stern In the recently synthesised compound $[$NH$_4]_2[$C$_7$H$_{14}$N$][$V$_7$O$_6$F$_{18}]$, magnetic $S$=$\frac12$ V$^{4+}$ atoms form an ideal kagome lattice $[$1$]$. Very recent $\mu$SR studies indicate the emergence of a gapless spin liquid state $[$2$]$. Using density functional theory calculations, we address the microscopic magnetic model of this interesting compound. We show that its peculiar symmetry gives rise to two inequivalent nearest-neighbor couplings. The behavior of the resulting spin model is studied using exact diagonalization and compared to the experiments. \\ $[$1$]$ F.H.Aidoudi~\textsl{et~al.}, Nature~Chem.~\textbf{3}, 810 (2011). \\ $[$2$]$ L.Clark~\textsl{et~al.}, Phys.~Rev.~Lett.~\textbf{110}, 207208 (2013). [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S4.00003: Negative quantum renormalization of excitation energies in the distorted kagome lattice antiferromagnet Cs$_2$Cu$_3$SnF$_{12}$ K. Matan, T. Ono, Y. Nambu, T. J. Sato, H. Tanaka Magnetic excitations in the distorted kagome lattice antiferromagnet Cs$_2$Cu$_3$SnF$_{12}$ were studied using neutron scattering. At room temperature, Cs$_2$Cu$_3$SnF$_{12}$ crystalizes in the hexagonal R$\bar{3}m$ space group with the lattice parameters $a = 7.142(4)$ {\AA} and $c=20.381(14)$ {\AA}. The $S=1/2$ Cu$^{2+}$ ions form a perfect kagome lattice. The system undergoes the structural transition at $T_s = 185$ K, doubling the in-plane lattice parameter $a$, and magnetic transition to the N\'{e}el state at $T_N = 20$ K. Spin-wave excitations in the ordered state can be qualitatively described by linear spin-wave theory (LSWT). However, the exchange interactions extracted from the spin-wave data are renormalized by a factor of 0.6 from those calculated by LSWT, almost irrespective of the momentum transfer. This inadequacy of LSWT is attributed to quantum effects and provides evidence of negative quantum renormalization of excitation energies in the kagome magnet. Recent results from a high-intensity pulsed neutron scattering experiment, which show the absence of high-energy spin-wave modes, will also be discussed. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S4.00004: Tensor Renormalization of Quantum Many-Body Systems using Projected Entangled Simplex States T. Xiang, Z.Y. Xie, J. Chen, J.F. Yu, X. Kong, B. Normand We propose a new class of tensor-network states, which we name projected entangled simplex states (PESS), for studying the ground-state properties of quantum lattice models. These states extend the pair-correlation basis of projected entangled pair states (PEPS) to a simplex. PESS are an exact representation of the simplex solid states and provide an efficient trial wave function that satisfies the area law of entanglement entropy. We introduce a simple update method for evaluating the PESS wave function based on imaginary-time evolution and the higher-order singular-value decomposition of tensors. By applying this method to the spin-1/2 antiferromagnetic Heisenberg model on the kagome lattice, we obtain an accurate result for the ground-state energy, $e_0 = - 0.4388(1) J$, which sets the lowest upper bound yet achieved for this quantity. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S4.00005: Neutron Scattering and Thermodynamic Studies of a Flat Mode in an S=$\frac{1}{2}$ Kagome Ferromagnet Robin Chisnell, Danna Freedman, Joel Helton, Deepak Singh, Chris Stock, Franz Demmel, Robert Bewley, Daniel Nocera, Young Lee Systems with flat bands provide macroscopic degeneracy that allows for the emergence of interesting strongly correlated phenomena such as the fractional quantum Hall effect. Hopping models on geometrically frustrated lattices with spin-orbit interactions predict the existence of flat, topologically nontrivial bands. Experimental realizations of these systems have proved challenging, as the flat band is often distorted by additional interactions. Cu(1,3-bdc) is a hybrid organometallic compound featuring S=$\frac{1}{2}$ Cu$^{2+}$ ions on a kagome lattice. The magnetic moments order ferromagnetically below T=1.8K. We present neutron scattering and thermodynamic measurements of Cu(1,3-bdc) and note the emergence of a flat magnon band in the ordered phase. The presence of a small Dzaloshinskii-Moriya(DM) interaction along with an applied magnetic field perpendicular to the kagome plane creates a gap between the flat band and lower energy dispersive band. The DM interaction also gives two of the magnon bands, including the flat band, a non-zero Chern number. We explore possible topological properties of these bands. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S4.00006: Chern-Simons theory for frustrated Heisenberg spins on Kagome Lattice Krishna Kumar, Kai Sun, Eduardo Fradkin There has been a lot of renewed interest in frustrated spin systems on Kagome lattices especially with the discovery of materials like volborthite and herbertsmithite. In the presence of an external magnetic field (or at fractional fillings), these systems can give rise to magnetization plateaus. Numerous studies indicate the existence of a m=1/3 plateau on the Kagome lattice. Here, we look at the problem of anti-ferromagnetic Heisenberg spins using a Jordan-Wigner transformation that maps the spins onto a problem of fermions coupled to a Chern-Simons gauge field. This method has been used successfully to study unfrustrated systems like the square lattice. At a mean-field level the above ideas have also been applied to frustrated systems. However, fluctuations are generally strong in these models and can affect the mean-field physics. We report a method to rigorously extend the Chern-Simon's term to frustrated lattices like the Kagome lattice. We discuss the different phases that arise at the mean-field level from this theory focusing specifically on the case of 1/3-filling, which gives rise to a magnetization plateau and is a topological phase. Finally, we will also comment on the implications of our model in the case of 1/2-filling. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S4.00007: Chiral Spin Liquids Invited Speaker: Laura Messio Frustrated spin lattices are theoretically and experimentally challenging systems in which many fascinating phases exist. On bidimensional lattices, unordered phases (i.e. that are neither N\'eel ordered nor break any other Hamiltonian symmetry) can survive until zero temperature, giving rise to the so-called spin liquid phases. They are related to superconductivity, quantum computing, spintronics... Two such phases were recently identified in experiments on the magnetic compounds Herbertsmithite and Kapellasite. But how to classify all the different spin liquids? How to distinguish several phases without any order parameter? The answer lies in quantities called fluxes, defined on lattice loops. Depending on the Hamiltonian symmetries and on the lattice, only some patterns of fluxes are possible, as was explained by Wen in 2002 with the use of group theory. When only the time reversal symmetry is broken, the phase is a chiral spin-liquid. In that case, new patterns of fluxes are allowed as they can be non trivial (i.e. different from 0 or $\pi$). They are obtained by extending the projective symmetry group approach of Wen. Some spin liquids have a parent classical state, sharing similar flux patterns. This state can be seen as a classical spin liquid. It has specific symmetry properties and is called a regular state. A chiral spin liquid leads to a chiral classical state. Combined with this semi-classical approach, the projective symmetry group theory extended to chiral states has led until now to the identification of two interesting chiral spin liquids. The first one is a new candidate for the kagome antiferromagnet ground state and the second one partially explains the experimental results obtained on Kapellasite. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S4.00008: The nature of the quantum spin-liquid state in Herbertsmithite Matthias Punk, Debanjan Chowdhury, Subir Sachdev Recent neutron scattering experiments on the layered spin-1/2 kagome lattice antiferromagnet Herbertsmithite revealed the first signature of fractionalized excitations in a quantum spin liquid state. The precise nature of this state remains unclear, however. Mean-field models of gapped as well as gapless spin liquids exhibit sharp features in the dynamic structure factor, none of which have been observed in experiment. We are going to show that several of the experimentally observed details can be explained by the presence of topological vortex excitations in a gapped Z2 spin liquid. These so called vison excitations form almost flat bands on the kagome lattice and act as a momentum sink for the spin-carrying excitations probed by neutron scattering. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S4.00009: Symmetry breaking Schwinger Boson Mean Field Theory solutions on Kagome Shivam Ghosh, Christopher L. Henley Schwinger Boson Mean Field theory (SBMFT) is a powerful technique for describing both quantum disordered and symmetry broken phases of Heisenberg spins as a function of spin length $\kappa=2S$. Previous applications of SBMFT have been to study \emph{symmetric} SL's which preserve lattice and time reversal symmetries (TRS). The \emph{assumption} of a symmetric ground state reduces the number of mean field variables simplifying search for SL saddle points. We go beyond the manifold of \emph{symmetric} SL's on the kagome lattice and using an optimization \footnote{G.Misguich, PRB 86, 245132 (2012)} technique search for solutions that may \emph{spontaneously} break lattice and TRS. An exhaustive search for saddle points on a $4\times4$ lattice shows that the lowest energy solutions have zero flux ($[0hex]$) through hexagons in agreement with the Greedy Boson theorem \footnote{O. Tchernyshyov et al. EPL, 73, 278 (2006)} However, amongst the manifold of $[0hex]$ solutions we find a state \emph{lower} in energy than Sachdev's uniform $Q_{1}=-Q_{2}$ state, extending up to $\kappa=0.3$, which \emph{spontaneously} breaks lattice symmetry and differs from uniform solution in flux patterns through length eight loops . We also characterize other (higher in energy) \emph{chiral} saddle points [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S4.00010: Study of vison-spinon bound states on the kagome lattice Junping Shao, Shivam Ghosh, Gil-Young Cho, Michael Lawler We search for low-energy vison-spinon bound states on the kagome lattice. We do this by applying an optimization algorithm to a bosonic spin liquid state with a well separated pair of visons inserted. The resulting wavefunction reveals that the low energy eigen-modes correspond to bound spinon states localized around the visons. We study these modes and their symmetry properties. Our results provide evidence supporting the low energy effective theories of Z2 spin liquids whose bosonic spinons, fermonic spinons and visions are characterized by projective symmetry groups consistent with the expected fusion rules and duality relations. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S4.00011: Phase diagram of an easy-axis Kagome antiferromagnet under a magnetic field Xavier Plat, Fabien Alet, Sylvain Capponi, Pierre Pujol, Keisuke Totsuka We present a quantum Monte-Carlo (QMC) study of a spin-1/2 XXZ model, with second and third-neighbour terms, under a magnetic field on the Kagome lattice. This model, introduced in the zero field case by Balents, Fisher and Girvin [1], exhibits, in the easy-axis limit, a topological gapped Z2 phase with fractional excitation [2-4]. When adding a magnetic field, other gapped incompressible phases are stable for magnetizations 1/3 and 2/3 of its saturation value. Using state-of-the-art measurements, including recently developped tools to compute the topological entropy, we investigate the nature of these ground-states. Finally, we make some connection between these microscopic models and effective constrained models (such as quantum loop model or quantum dimer model respectively), which allow to provide a better understanding of the physical properties. \\[4pt] [1] L. Balents, M. P. A. Fisher, and S. M. Girvin, Phys. Rev. B {\bf 65}, 224412 \\[0pt] [2] S. V. Isakov, Y. B. Kim, and A. Paramekanti, Phys. Rev. Lett. {\bf 97}, 207204 (2006) \\[0pt] [3] S. V. Isakov, M. B. Hastings, and R. G. Melko, Nat. Phys. {\bf 7}, 772 (2011) \\[0pt] [4] S. V. Isakov, R. G. Melko, and M. B. Hastings, Science {\bf 335}, 193 (2012) [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S4.00012: Magnetic Field Driven Phase Transitions in S = $\frac{1}{2}$ Kagome Lattice Antiferromagnet ZnCU$_{3}$(OH)$_{6}$Cl$_{2}$ Lu Li, T. Asaba, T. Han, B.J. Lawson, F. Yu, C. Tinsman, Z. Xiang, G. Li, Y.S. Lee Herbertsmithite ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ is a kagome lattice antiferromagnet with 1/2 spin and has been demonstrated to be a likely candidate of spin liquid by recent neutron scattering measurements. The high magnetic field response of the kagome lattice sample is hard to separate from the magnetic signals from Cu impurities sitting between the kagome planes. To separate these two contributions, we measured the magnetization of a single crystalline ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ using torque magnetometry at temperatures from 20mK to 15K in intense magnetic field as high as 31 T. Below 2 K, several phase transitions are observed in field near 8 T - 16 T, and the transition fields do not show significant dependence on the temperature in the range of 20 mK $\leq T \leq$ 2 K. Moreover, the transition fields are independent of the magnetic field orientation. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S4.00013: $^{17}$O Single Crystal NMR Study on S $=1/2$ Kagome Lattice ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ Mingxuan Fu, Takashi Imai, Tianheng Han, Young. S. Lee Herbersmithite ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ is known to be a promising candidate material hosting a quantum spin liquid ground state. The recent success in single crystal growth of ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ as well as the discovery of a continuum of spinon excitations using inelastic neutron scattering\footnote{T. H. Han \textit{et al}., Nature {\bf 492}, 406(2012)} have opened a new chapter in the study of highly frustrated magnetism. However, the mechanism behind the realization of the non-magnetic ground state in ZnCu$_{3}$(OH)$_{6}$Cl$_{2}$ remains controversial, mainly due to the difficulty in understanding the role of defects in its physical properties. Through single-crystal $^{17}$O NMR study, we identified multiple O sites with distinct local magnetic environments. The behavior of local spin susceptibility and spin dynamics observed at these O sites provide invaluable insights into the nature of defects and their potential influence on the kagome spin lattice.\footnote{M. Fu, T. Imai \textit{et al}., in preparation. Also see T. Imai. \textit{et al}., Phys. Rev. B {\bf 84}, 020411(R) (2011); Phys. Rev. Lett. {\bf 100}, 077203 (2008)} [Preview Abstract] |
Session S6: Focus Session: Magnetic Oxide Thin Films and Heterostructures: Interface Effects
Sponsoring Units: DMP GMAGChair: Jason Hoffman, National Institute of Standards and Technology
Room: 108
Thursday, March 6, 2014 8:00AM - 8:36AM |
S6.00001: An Emergent Spin-Filter at the interface between Ferromagnetic and Insulating Layered Oxides Invited Speaker: Yaohua Liu Complex oxide heterostructures are of keen interest because modified bonding at the interfaces can give rise to fundamentally new phenomena and valuable functionalities. Particularly, an induced magnetization is widely observed at epitaxial interfaces between layered transition-metal oxides; however, much less effort has been spent on investigating how it affects the charge transport properties. To this end, we have studied magnetic tunneling junctions consisting of ferromagnetic manganite La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (LCMO) and insulating cuprate PrBa$_{2}$Cu$_{3}$O$_{7}$ (PBCO). Contrary to the typically observed steady increase of the tunnel magnetoresistance with decreasing temperature, this system exhibits an anomalous decrease at low temperatures. Polarized neutron reflectometry (PNR) and x-ray magnetic circular dichroism (XMCD) studies on LCMO/PBCO/LCMO trilayers show that the saturation magnetization of the LCMO contacts increase as the temperature decreases. In other words, degradation of the ferromagnetic contacts is ruled out as a cause. Interestingly, there exists induced net Cu moments, which indicates that the spin degeneracy of the conduction band of the PBCO barrier is lifted and thus the barrier becomes spin selective. Our calculations, within the Wentzel-Kramers-Brillouin approximation, show that the complex temperature dependence can arise from a competition between the high positive spin polarization of the manganite electrodes and a negative spin-filter effect from the interfacial Cu magnetization [1]. This work illustrates that the interface-induced magnetization in layered oxide heterostructures can have non-trivial effects on the macroscopic transport properties. Work performed in collaboration with FA Cuellar, Z Sefrioui, C Leon, J Santamaria (Universidad Complutense de Madrid), JW Freeland, SGE te Velthuis (ANL) and MR Fitzsimmons (LANL). \\[4pt] [1] Yaohua Liu, FA Cuellar, Z Sefrioui, JW Freeland, MR Fitzsimmons, C Leon, J Santamaria, SGE te Velthuis, ``An emergent spin-filter at the interface between ferromagnetic and insulating layered oxides,'' Phys. Rev. Lett. \textit{in press} (2013). [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S6.00002: Charge transfer at YBa$_{2}$Cu$_{3}$O$_{7}$/La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ interface Jiunn-Yuan Lin, Vu-Thanh Tra, Ying-Hao Chu In this paper, the ferromagnetic (F) /La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/superconducting (S) YBa$_{2}$Cu$_{3}$O$_{7}$ heterostructures of two distinct interfaces with atomically precise interface control have been fabricated to explore the coupling between these two functional layers. A new mechanism of charge transfer in these heterostructures was identified and confirmed by the results of the first principle calculations. This charge transfer, in addition to the previously considered F/S proximate effect, is critical to to the superconductivity and magnetism in these heterostructures. Direct observation of the charge transfer by x-ray absorption spectroscopy is presented. The results from resonant x-ray scattering is likely to be discussed for both type of interfaces. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S6.00003: Non-Collinear Spin Structures in LaNiO$_3$/La$_{2/3}$Sr$_{1/3}$MnO$_3$ Superlattices Jason Hoffman, Brian Kirby, Anand Bhattacharya The exchange coupling between magnetic layers separated by non-magnetic spacers can give rise to spin structures that are distinct from those observed in the bulk constituents. In this work, we investigate a non-collinear spin arrangement in superlattices containing paramagnetic LaNiO$_3$ and ferromagnetic La$_{2/3}$Sr$_{1/3}$MnO$_3$. We use molecular beam epitaxy, to fabricate a series of (LaNiO$_3$)$_n$/(La$_{2/3}$Sr$_{1/3}$MnO$_3$)$_9$ superlattices on (001) SrTiO$_3$ and LSAT substrates, where $n$ is varied between 1 and 9 unit cells. The total thickness of the superlattices is kept constant at 60 nm by varying the number of superlattice repetitions. The magnetic structure of the superlattices was investigated as a function of temperature and in-plane magnetic field using polarized neutron reflectometry. We find the magnetization of neighboring La$_{2/3}$Sr$_{1/3}$MnO$_3$ layers to be non-collinear at low fields due to an antiferromagnetic interlayer exchange coupling, which persists to temperatures above 250 K. We discuss underlying mechanisms for the observed behavior and possible applications to oxide-based magnetoresistive devices. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:36AM |
S6.00004: Spin structure in an interfacially-coupled epitaxial ferromagnetic oxide heterostructure Invited Speaker: Xianglin Ke We report the spin structure of an exchange-biased ferromagnetic oxide heterostructure, La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ / SrRuO$_{3}$, through magnetization and polarized neutron reflectometry measurements. We reveal that the magnetization reversal process of the La$_{0.67}$Sr$_{0.33}$MnO$_{3}$ biased layer critically depends on the frozen-in spin structure of the SrRuO$_{3}$ biasing layer during the cooling process. Furthermore, we observe unexpected double-shifted hysteresis loops of the biased layer that originates from the formation of lateral 180$^{\circ}$ magnetic domains within the biasing layer, a new mechanism not found in conventional exchange-bias systems [1]. The effects of the thus-formed spin structure on the magnetotransport properties will be presented as well. This work was done in collaboration with L. J. Belenky, V. Lauter, H. Ambaye, C. W. Bark, C. B. Eom, M. S. Rzchowski, J. Smith, and M. Zhu. \\[4pt] [1] X. Ke et al, Phys. Rev. Lett. \textbf{110}. 237201 (2013). [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S6.00005: Magnetic ordering temperatures at oxide interface LaAlO$_3$/SrTiO$_3$ Tomoya Asaba, Gang Li, Ben Lawson, Fan Yu, Ziji Xiang, Colin Tinsman, Harold Hwang, Jochen Mannhart, Lu Li A number of recent experiments demonstrate the existence of magnetic ordering at the conductive oxide interface LaAlO$_{3}$/SrTiO$_{3}$ (LAO/STO). Understanding the origin of this magnetism requires determination of the magnetic state at elevated temperature. In this study we carried out torque magnetometry measurements to track the magnetic transition temperatures in the interface samples with different LAO thickness. The magnetic ordering temperature is found to vary greatly as the thickness of LAO changes. . Our results suggest that the growth condition such as LAO thickness affects the magnetic coupling of the interface magnetic moments. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S6.00006: Electronic and Magnetic Properties of Ultrathin SrRuO3 (111) Film on SrTiO3 Bongjae Kim, B.I. Min We have investigated electronic and magnetic properties of ultrathin SrRuO$_{3}$ (SRO) film grown on (111) SrTiO$_{3}$ substrate using the {\it ab initio} electronic structure calculations. Ru-terminated SRO (111) film suffers from strong surface atomic relaxations, while SrO$_{3}$-terminated one preserves the surface structure of ideal perovskites. Both Ru- and SrO$_{3}$-terminated SRO (111) film show unexpected interlayer antiferromagnetic (AFM) structure at the surface, but with different characters and mechanisms. The AFM structure for the former results from the large surface atomic relaxation, whereas that for the latter results from the truncated film effect. Interestingly, for the SrO$_{3}$-termination case, the half-metallic nature emerges despite the interlayer AFM structure. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S6.00007: Control of electrical and magnetic properties of Mn thin film on BaTiO3 Anh Tuan Duong, Yooleemi Shin, Van Quang Nguyen, Duc Dung Dang, Sunglae Cho Bulk Mn material is one of transition metals that has been well known as an antiferromagnetic material due to an anti-parallel spin alignment with negative exchange integral. However, theory predicted that the magnetic properties of Mn can be transited to ferromagnetic with the expansion in volume following Hund's rule. A current active research topic is electric field controlled magnetism. To accomplish this goal, a way is to use multiferroic material. Epitaxial Mn thin film has successfully been grown on BaTiO3 substrate by using molecular beam epitaxy (MBE). We could control the degree of a structural deformation of Mn thin film using unique four different crystal structures of BaTiO3 below 400 K. We observed three jumps at 185, 290, and 390 K in temperature dependent electrical resistivity, corresponding to the temperatures of structural phase transitions in BaTiO3. The modification of magnetism from antiferromagnetism to ferrimagnetism in Mn film was observed. We also observed two jumps at 290 and 365K in the temperature dependent magnetization. The calculated magnetic moment was 0.66 $\mu$B/Mn at 320K. These results indicate the possibility of the tuning of electrical and magnetic properties from antiferromagnetic to ferrimagnetic or vice versa in Mn film by modulating strain. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S6.00008: Ferroelectric and Magnetic SrTiCoO3 films on Silicon and Niobium-doped SrTiO3 substrates Mehmet Onbasli, Andy Cruz, T. Goto, Caroline Ross Perovskites hold great potential for fundamental studies of structure-multiferroicity relationship as well as technological applications such as multi-level memories. We demonstrate multiferroic behavior of Cobalt-substituted SrTiO3 (STCo) films on Silicon and on Niobium-doped SrTiO3 substrates (Nb:STO). STCo films were grown on Si, silicon-on-insulator, Nb:STO, 3 $\mu $m thick SiO2 coated Si, and pure STO substrates using pulsed laser deposition under different oxygen pressures (1, 3, 6 $\mu $Torr, 1.6 mTorr). The film composition is SrTi0.70Co0.30O3-$\delta $, as confirmed by $\omega $-2$\theta $ scans of x-ray difractometer. Magnetic hysteresis loops indicate that the films have out-of-plane easy axis with anisotropy field of several kOe, which is attributed to magnetoelastic anisotropy. Saturation magnetizations of 0.9, 0.3, 0.5 and 0.2 $\mu $B/Co ion were obtained for samples grown on Nb:STO under oxygen pressures 1, 3, 6 $\mu $Torr, 1.6 mTorr, respectively. Ferroelectric saturation polarizations of 67 to 118 $\mu $C/cm2 and resistivities between 1e6 to 1e9 $\Omega \cdot$ cm were obtained for STCo on Nb:STO and on Silicon. The origin of the magnetic and ferroelectric properties will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S6.00009: Interface states in CoFe$_{2}$O$_{4}$ spin-filter tunnel junctions Pavel Lukashev, J.D. Burton, Alexander Smogunov, Julian Velev, Evgeny Tsymbal Spin-filter tunneling is a promising way to generate highly spin-polarized current, a key component for spintronics applications. In this work we explore the tunneling conductance across the spin-filter material CoFe$_{2}$O$_{4}$ interfaced with Au electrodes, a geometry which provides nearly perfect lattice matching at the CoFe$_{2}$O$_{4}$/Au(001) interface.\footnote{P. Lukashev, et al., Phys. Rev. B, \textbf{88}, 134430 (2013).} Using density functional theory calculations we demonstrate that interface states play a decisive role in controlling the transport spin polarization in this tunnel junction. For a realistic CoFe$_{2}$O$_{4}$ barrier thickness, we predict a tunneling spin polarization of about $-$60{\%}. We show that this value is lower than what is expected based solely on considerations of the spin-polarized band structure of CoFe$_{2}$O$_{4}$, and therefore that these interface states can play a detrimental role. We argue that this is a rather general feature of ferrimagnetic ferrites and could make an important impact on spin-filter tunneling applications. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S6.00010: Spin-orbit coupling and the ultimate limit for spin-polarized tunneling from half-metallic electrodes J.D. Burton, Evgeny Y. Tsymbal Half-metallic materials, i.e. metals that have free carriers only in one spin channel, should act as ideal materials for spin-polarized transport applications. In magnetic tunnel junctions with identical half-metallic electrodes, for example, there would in principle be zero tunneling transmission (infinite resistance) when the magnetization of the electrodes are aligned anti-parallel, making the tunneling magnetoresistance (TMR) ratio infinite. In practice, however, it is thought that this idealized case can only hope to work at zero temperature and when the electrodes are in a truly mono-domain configuration: effects which are generally very difficult to minimize. Also, however, one factor that can never be suppressed is the mixing of the spin-polarized carriers induced by spin-orbit-coupling (SOC). We will present results of density functional calculations on idealized magnetic tunnel junctions with La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3}}$ (LSMO) electrodes and SrTiO$_{\mathrm{3}}$ (STO) tunneling barrier. In the absence of SOC, LSMO is predicted to be a half-metal having Fermi-level density of states only for majority spins, and an electronic gap for the minority spin-channel. Indeed, transport calculations based on a generalized scattering approach predict an infinite TMR effect in LSMO/STO/LSMO junctions. The inclusion of SOC into the calculations, however, opens a channel for transmission through the barrier in the anti-parallel magnetic configuration leading to a large, yet finite, TMR ratio. With all other spin-flip mechanisms suppressed, this represents the ultimate limit for TMR in idealized junctions. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S6.00011: Spin polarization at the interface of LaMnO3 and Si heterostructure Huiping Zhu, Guoping Zhang, Xiaoshan Wu Spin injection at a ferromagnet- semiconductor interface is one of the promising ways to add new functionality to the conventional devices. Here we study the spin injection from ferromagnetic LaMnO3 to semiconductor Si using first-principles calculations. We use two different methods: (1) Changing the distance between Si and LaMnO3 layers and (2) introducing the dimerization at the first Si layer. We find that when we reduce the distance between Si and LaMnO3 layer, both the total spin moment and the spin polarization at the Fermi-level change. There is a general trend that a stronger spin moment corresponds to a weaker spin polarization, but they do not follow exactly this trend at each distance. For the Si dimerization case, when the distance between the two Si atoms is reduced, the total spin moment increases whereas the spin polarization at the Fermi-level is not. Our results indicate that Si atoms with a smaller total spin moment may have a stronger spin polarization at the Fermi-level, and vice versa. Since the electrons at the Fermi-level play a key role in transport, the spin polarization at the Fermi-level, therefore, is more important than the total spin moment for spin injection. [Preview Abstract] |
Session S7: Focus Session: Low-D Quantum Spins II
Sponsoring Units: GMAGChair: Tom Lancaster, Durham University
Room: 106
Thursday, March 6, 2014 8:00AM - 8:12AM |
S7.00001: Haldane gap evolution in quasi-one-dimensional spin-1 Heisenberg antiferromagnets Keola Wierschem, Pinaki Sengupta We study a spatially anisotropic spin-1 antiferromagnetic Heisenberg model on simple cubic lattices that is equivalent to a system of coupled chains with interchain coupling $J$ and intrachain coupling set to unity. In the limit of uncoupled chains ($J=0$), the ground state is known to be gapped as per the Haldane conjecture. As the coupling is turned on, this gapped phase persists up to a critical value $J_c$ beyond which there is a quantum phase transition to the gapless N\'{e}el state with long range magnetic order. Using the stochastic series expansion quantum Monte Carlo method, we accurately determine $J_c$ and calculate the string order parameter in the gapped phase for $0 |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S7.00002: S=2 quasi-one-dimensional spin waves in CrCl2 Matthew Stone, Georg Ehlers, Garrett Granroth We examine the magnetic excitation spectrum in the $S=2$ Heisenberg antiferromagnet CrCl$_2$. Inelastic neutron scattering measurements on powder samples are able to determine the significant exchange interactions in this system. A large anisotropy gap is observed in the spectrum below the N\'{e}el temperature and the ratio of the two largest exchange constants is $J_c / J_b = 9.1 \pm 2.2$. However, no sign of a gapped quantum spin liquid excitation was found in the paramagnetic phase. The research was performed at Oak Ridge National Laboratory's Spallation Neutron Source and was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S7.00003: Antiferromagnetic Ordering of Mn(III)F(salen) M.W. Meisel, Tong Wang, S.E. Brown, M. Botko, E. \v{C}i\v{z}m\'ar, O.N. Risset, D.R. Talham Due to a report suggesting Mn(III)F(salen), salen = H$_{14}$C$_{16}$N$_2$O$_2$, is an $S = 2$ Haldane system with $J/k_B = 50$~K and no long-range order down to 2~K based on standard magnetometry studies,\footnote{T.~Birk \emph{et al.}, Inorg.~Chem.~{\bf50} (2011) 5312.} specific heat and NMR measurements were performed. Using small single crystals, specific heat studies revealed the presence of an anomaly near 23~K, and this response was robust in fields up to 9~T. The $^1$H NMR results performed on a single crystal in 1~T revealed a sharp transition characteristic of antiferromagnetic ordering at 22.5~K. Measuring the magnetic response of the same single crystal in a commercial magnetometer reveals the presence of a subtle feature, near 23~K, that is not resolved with as-grown, randomlly oriented microcrystalline samples. These findings provide insight into the results obtained in torque magnetometry, EPR, and neutron scattering data.\footnote{J.-H.~Park \emph{et al.}, Acta Phys.~Pol.~A, in press.} [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S7.00004: Molecular-based 2D S $=$ 1/2 Heisenberg Antiferromagnetic Layers and Ladders Invited Speaker: Christopher Landee Low dimensional Quantum Heisenberg Antiferromagnets (QHAF) have long provided materials [1] with which to examine the influence of dimensionality and exchange anisotropy on critical behavior of cooperative systems. Molecular-based QHAF have provided materials with low exchange strengths ($\approx $ 10 K), facilitating examination of the compounds up to the saturation fields using current facilities. This presentation will provide an overview of recent developments of two classes of molecular magnets: 2D QHAF and spin ladders. Recent specific heat studies of Cu(pz)$_{2}$(ClO$_{4})_{2}$ in fields up to 45 tesla have determined the (H,T) phase diagram for this quasi-2D QHAF; the results will be compared to the results of QMC simulations of the diagram as a function of the intralayer exchange $J$, the interlayer exchange $J'$, and the XY-exchange anisotropy parameter. Developments in the study of spin ladders include the discovery of Luttinger liquid behavior for two molecular-based spin layers: the strong-rung ladder BPCB, (piperidinium)$_{2}$CuBr$_{4}$ [2] and the strong-rail ladder DIMPY, (2,3-dimethypyridinium)$_{2}$CuBr$_{4}$ [3]. The properties of a new, isotropic spin ladder will be reported. \\[4pt] [1] L. J. de Jongh and A. R. Miedema, Adv. Phys. \textbf{50}, 947 (2001).\\[0pt] [2] M. Klanj\v{s}ek \textit{et al}, Phys. Rev. Lett. \textbf{101}, 137207 (2008); Ch. R\"{u}egg \textit{et al}, Phys. Rev. Lett. \textbf{101}, 247207 (2008).\\[0pt] [3] K. Ninios \textit{et al}, Phys. Rev. Lett. \textbf{108}, 097201 (2012); D. Schmidiger \textit{et al}, Phys. Rev. Lett. \textbf{108}, 167201 (2012). [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S7.00005: $\mu^+$SR study of spin dynamics in the one dimensional Heisenberg antiferromagnet Cu(pyz)(NO$_3$)$_2$ Fan Xiao, Tom Lancaster, Rob Williams, Johannes Moeller, Stephen Blundell, Francis Pratt, Peter Baker, Jamie Manson We present the results of longitudinal-field muon spin relaxation ($\mu^+$SR) measurements on the one-dimensional quantum Heisenberg antiferromagnet (1DQHAF) Cu(pyz)(NO$_3$)$_2$ (pyz=pyrazine). The intrachain coupling strength $J/k_{\mathrm{B}}$ in this compound is 10.6 K and the ordering temperature $T_{\mathrm{N}}$ is 0.11 K. Spin dynamics were studied with $\mu^+$SR using applied longitudinal fields at two temperatures ($T$=0.33 K and 1.4 K) between $T_{\mathrm{N}}$ and $J/k_{\mathrm{B}}$. For $B>5$ mT, the nuclear contribution to the relaxation rate is quenched and the data can be fitted to an exponential decay along with a background correction. The relaxation rate $\lambda$ was found to follow a $\lambda\propto B^{-1/2}$ power law between 10 mT and 100 mT at both temperatures, suggesting diffusive spin transport of excitations in this 1DQHAF. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S7.00006: Spin Pseudogap in Ni-Doped SrCuO$_2$ Gediminas Simutis, Martin Mansson, Severian Gvasaliya, Alexander Chernyshev, Ashwin Mohan, Surjeet Singh, Christian Hess, Bernd B\"{u}chner, Andrei Savici, Alexander Kolesnikov, Andrea Piovano, Toby Perring, Igor Zaliznyak, Andrey Zheludev Effect of spin-1 impurities on the spectrum of an archetypical Heisenberg antiferromagnetic spin-$\frac{1}{2}$ chain SrCuO$_2$ is studied by inelastic neutron scattering [1]. We find that a spin pseudogap appears in the spectrum upon introduction of the impurities. We show that the pseudogap is a generic feature of quantum spin chains with dilute defects. This allows us to express the dynamic structure factor in a universal scaling form even for the system with fragmented chains. A simple model based on chain fragmentation shows good quantitative agreement with the experimental data for a broad temperature range.\\[4pt][1] Simutis et al, PRL 111, 067204 (2013) [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S7.00007: Interplay of covalency and correlations in the edge shared spin 1/2 $A_{3}T_{2}$O$_{4}$ chain compounds ($A$ = Na, K; $T$ = Cu, Ni) Deepa Kasinathan, Klaus Koepernik, Helge Rosner Na$_{3}$Cu$_{2}$O$_{4}$, K$_{3}$Cu$_{2}$O$_{4}$ and K$_{3}$Ni$_{2}$O$_{4}$ belong to a new class of quasi-1D insulating cuprates which feature strongly buckled, one-dimensional $^{1}_{\infty}$CuO$_{2}$ ribbon-like chains consisting of edge-sharing CuO$_{4}$ plaquettes. Structural analysis of the metal-oxygen bond lengths and thermodynamic measurements[1,2,3] imply that these systems are intrinsically charge ordered ($\ldots$ (Ni/Cu)$^{2+}$-(Ni/Cu)$^{3+}$-(Ni/Cu)$^{2+}$-(Ni/Cu)$^{3+}\ldots$) and show dominant antiferromagnetic interactions. No electronic structure analysis of these systems exist to date. Using density functional theory based calculations (LDA, Wannier functions, LDA+$U$), we analyze the microscopic origin of the magentic interactions in these systems. The main interaction along the chains are the second neighbor superexchanges. Nonetheless, a careful analysis of the first neighbor interaction between the magnetic (Cu$^{2+}$/Ni$^{3+}$) cation and the non-magnetic cation (Cu$^{3+}$/Ni$^{2+}$) is necessary. We report on the interplay of covalency, crystal field splitting and correlations in these systems. [1] Z. Anorg. Allg.Chem. vol. 462, 92 (1980). [2] J. Solid State Chem. vol. 178, 3708 (2005). [3] Z. Anorg. Allg. Chem. vol. 637, 1101 (2011). [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S7.00008: Localization of Spinons in Random Majumdar-Ghosh Chains Guillaume Roux, Arthur Lavarelo We study the effect of disorder on frustrated dimerized spin-1/2 chains at the Majumdar-Ghosh point. Using variational methods and density-matrix renormalization group approaches, we identify two localization mechanisms for spinons which are the deconfined fractional elementary excitations of these chains. The first one belongs to the Anderson localization class and dominates at the random Majumdar-Ghosh point. There, spinons remain gapped and localize in Lifshitz states whose localization length is analytically obtained. The other mechanism is a random confinement mechanism which induces an effective interaction between spinons and brings the initially gapped antiferromagnetic chain into a gapless and partially polarized phase for arbitrarily small disorder. This Imry-Ma mechanism induces domains which statistics is analyzed. Last, the connection to the real-space renormalization group method suited for the strong disorder limit is discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S7.00009: Generation of chiral solitons in antiferromagnetic chains by a quantum quench Barbara Bravo, Ariel Dobry, Diego Mastrogiuseppe, Claudio Gazza In classical nonlinear physics, solitons are peculiar solutions which can be characterized by constant velocity and shape. In a recent paper [1], pursuing the understanding of the extension of the soliton concept to the quantum regime, the easy-axis ferromagnetic $XXZ$ model was chosen to analyze the evolution of a localized wave packet. It was shown that, besides the delocalization due to the uncertainty principle, they are in qualitative agreement with their classical counterparts. Following the objective of deciphering the quantum soliton term, we analyze the time evolution of a magnetic excitation in a spin-$\frac12$ antiferromagnetic Heisenberg chain after a quantum quench. By a modulation of the magnetic exchange, we prepare a static soliton of total spin $\frac{1}{2}$ as an initial state. Using bosonization and a time dependent density matrix renormalization group algorithm, we show that the excitation evolves to a state composed of two counter-propagating chiral states, which interfere to yield $<\!\!S^z\!\!> = \frac14$ for each mode. These dynamically generated states remain considerably stable in time. We propose spin-Peierls materials and ultracold-atom systems as experimental scenarios to conduct and observe this mechanism. [1] Phys. Rev. B 85, 184433 (2012). [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S7.00010: Theoretical study of the electronic and magnetic properties of $\beta$-TeVO$_4$ Andres Saul, Guillaume Radtke The $\beta$ phase of this compound can be described by zigzag chains formed by VO$_5$ distorted square pyramids sharing corners. This oxide, with V$^{4+}$ ions as magnetic centers, can be thus seen as a realization of a quasi-one-dimensional Heisenberg S=1/2 Hamiltonian. The corner-sharing of the VO$_5$ pyramids could lead to the prediction of AFM nearest neighbor interactions mediated by a weak super-exchange mechanism opening the possibility of complex magnetic properties due to competing next nearest-neighbors or inter-chain interactions. In this work we have studied its electronic and magnetic properties using density functional calculations. In particular, we evaluated the magnetic couplings on the basis of broken-symmetry formalism. We have performed extensive calculations comparing the results of the standard GGA (PBE) functional to the hybrid PBE0 functional and two different GGA+U implementations (SIC and AMF). The overall picture that arises from our calculations is of a frustrated AFM system with small FM nearest neigbors interactions but larger AFM nearest neighbors couplings. We discuss our results in the framework of the Kugel-Khomskii model using a projection of the electronic structure in localized Wannier functions. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S7.00011: Incommensurate dynamic correlations and continuum scattering in BiCu$_2$PO$_6$ Kemp Plumb, G.J. Shu, G.E. Granroth, A.T. Savici, Zahra Yamani, M. Matsuda, F.C. Chou, Young-June Kim We report comprehensive inelastic neutron scattering measurements on single crystals of the frustrated two-leg ladder BiCu$_2$PO$_6$, whose ground state is a spin liquid phase with no static magnetic correlations down to 5 K. A combination of triple-axis and time-of-flight experiments were performed to explore magnetic excitations over a broad range of phase space. Operation of the instruments in a high resolution configuration enabled a detailed measurement of the dynamical structure factor over many Brillouin zones; revealing an extremely rich and highly unusual magnetic excitation spectrum. Two branches of steeply dispersing long-lived spin excitations are observed with gaps of 1.90(9) meV and 3.95(8) meV. Significant frustrating next-nearest-neighbor interactions along the ladder-leg drive the minimum of each excitation branch to incommensurate wavevectors 0.574$\pi$ and 0.553$\pi$ for the lower and upper energy branches respectively. Intriguingly, the spin excitations merge into a broad continuum near the top of each excitation band which persists to an upper boundary of 40 meV. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S7.00012: Full magnetic dispersion relation in the frustrated quasi-1D ferromagnet Ca$_2$Y$_2$Cu$_5$O$_{10}$ M. Matsuda, J. Ma, V. O. Garlea, S. Nishimoto, S.-L. Drechsler, R. O. Kuzian, T. Ito, H. Yamaguchi, K. Oka Ca$_2$Y$_2$Cu$_5$O$_{10}$ consists of edge-sharing CuO$_2$ chains, in which Cu$^{2+}$ ions carry spin 1/2. The nearest-neighbor ($J_1$) and the next-nearest-neighbor interaction ($J_2$) are ferromagnetic and antiferromagnetic, respectively. For the $J_1$-$J_2$ model the theory predicts that when the ratio $\alpha$(=$|J_2/J_1|$) becomes larger than 0.25, the ground state becomes a spiral state. For the aforementioned compound, Kuzian et al. determined $\alpha$ to be 0.19, which is close to the critical value [1]. However, the parameters were fitted using the observed data up to $\sim$10 meV, above which the magnetic excitations were found to be broadened [2]. In order to determine the overall dispersion relation, we performed inelastic neutron scattering experiments using the HYSPEC neutron spectrometer at the SNS. We succeeded in observing the full magnetic dispersion that extends up to $\sim$55 meV. As previously observed, the magnetic excitations appeared to almost vanish at $\sim$11.5 meV. We also found another noticeable gap-like behavior at $\sim$28 meV. We re-evaluate the essential exchange coupling parameters and discuss the origin of gap-like regions in the spin-wave dispersion. [1] R. O. Kuzian et al., PRL109, 117207 (2012). [2] M. Matsuda et al., PRB63, 180403 (2001). [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S7.00013: A hierarchy of ``meson" bound state excitations in the 1D ferromagnetic Ising chain CoNb$_2$O$_6$ Christopher Morris, Seyed Koopayeh, Anirban Ghosh, Oleg Tchernyshyov, Tyrel M. McQueen, N. Peter Armitage, Rolando Vald\'es Aguilar, Jason Krizan, Robert J. Cava The quantum magnet CoNb$_2$O$_6$ was recently demonstrated to be an excellent realization of the one-dimensional ferromagnetic Ising spin chain. Low energy spin-flip excitations in the chains were recently observed via inelastic neutron scattering.\footnote{R. Coldea, \textit{et al}, Science \textbf{327}, 177 (2010)} The energy spectrum of these excitations was shown to have a interesting energy scaling governed by symmetries of the E8 exceptional Lie group. Here, time-domain terahertz spectroscopy (TDTS) is used to investigate these optically active spin flip excitations in CoNb$_2$O$_6$. A series of nine spin flip bound states is observed, whose energies can be modeled exceedingly well by the Airy function solutions to a 1D Schr{\"o}dinger equation. Additionally, a novel bound state of excitations on neighboring chains is observed just below the onset of a two particle continuum. [Preview Abstract] |
Session S8: Focus Session: Spin-Dependent Transport, Tunneling, and Spin Torque
Sponsoring Units: GMAGChair: Kui Gong, McGill University
Room: 104
Thursday, March 6, 2014 8:00AM - 8:12AM |
S8.00001: Novel Spin-dependent tunneling magnetoresistance of Fe/O/NaCl/O/Fe Kui Gong, Lei Zhang, Dongping Liu, Hong Guo We propose and theoretically investigate an very attractive novel magnetic tunnel junction (MTJ) Fe(001)/O/NaCl(001)/O/Fe(001) for spintronics. Due to the presence of the single p(1$\times$1)O layer between Fe electrode and NaCl insulator, the interfacial strain can be full released. Therefore, area perfectly ordered NaCl can be grow on top of Fe electrode. Since the unit cell of Fe crystal in [001] direction has two layers, there are two different kinds of contact interface between Fe electrode and NaCl insulator, i.e., the translational and mirror symmetry configurations. According to our ab initio total energy calculation, both of them are experimentally accessible. For the translational symmetry configuration, the tunneling magnetoresistance (TMR) ratio of Fermi energy is in the magnitude of 500\%. More interestingly, for the mirror symmetry configuration, the TMR ration will drastically increase to 5500\%. Different from the role of MgO barrier in well known Fe/MgO/Fe MTJ, the evanescent state with the $\Delta_5$ symmetry dominates the transmission of the majority spin electrons through the NaCl barrier. By studying the scattering states and the complex band structure of NaCl insulator, we systematically understand the transport properties of Fe/O/NaCl/O/Fe MTJ. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S8.00002: Interface structure of CoFeB/MgO magnetic tunnel junctions from hard x-ray photoelectron spectroscopy S. Mukherjee, D.D. Sarma, B. Pal, R. Knut, J. {\AA}kerman, S. Thiess, W. Drube, M. Gorgoi, A. Sahoo, P. Anilkumar, J. Perssons, O. Karis Present sensors in hard drives rely on tunnel magnetoresistance (TMR) in CoFeB/MgO/CoFeB structures. The device fabrication has been refined to meet strict demands. Despite this, fundamental understanding of the optimization process, i.e. post-annealing, is missing. In particular, boron diffusion has been suggested to be integral to the creation of a textured CoFe alloy with boron diffused either into the MgO tunnel barrier, forming boron oxides, or into a seed layer. Such diffusion would thus indirectly be essential for a large MR in the device. We have used hard x-ray photoelectron spectroscopy (HAXPES), to investigate a series of CoFeB/MgO/CoFeB structures. By systematically studying the modifications of chemical state of various constituents for different structures and post-annealing conditions, we are able to provide a detailed geometric interpretation of how elements diffuse and modify the structure. In particular we show that at the annealing temperatures required for achieving optimal MR, boron diffusion is limited to a very thin (sub-nm) region at the interface and does not progress beyond this point. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S8.00003: Recent Advances in Magnetic Tunnel Junction Materials and Stack for Thermally-Assisted Magnetic Random Access Memory Anthony Annunziata, Philip Trouilloud, Sebastien Bandiera, Stephen Brown, Michael Gaidis, Erwan Gapihan, Eugene O'Sullivan, Nathan Marchack, Daniel Worledge We report magnetic and electrical characterization measurements of sheet films and nanopillar magnetic tunnel junction devices useful for a new type of Magnetic Random Access Memory that is capable of operating at high ambient temperatures (greater than 125 C) and of surviving the high process temperatures used in silicon chip manufacturing. For unpatterned sheet film stacks, we report measurements of the magnetization versus applied field and temperature, antiferromagnet blocking temperature, and tunneling magnetoresistance. For patterned nanopillar devices in the size range of 80 - 200 nm, we report measurements of the tunneling magnetoresistance, depinning voltage and temperature, and sense and storage layer reversal fields. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S8.00004: Magnified Spin-Motive Forces in MRAM Magnetic Tunnel Junctions Stewart Barnes In the Slonczewski 2005 theory [1] for spin-torque-transfer (STT) of a magnetic tunnel junction (MTJ) the tunnelling magneto resistance (TMR) and Gilbert damping parameter $\alpha$ are of key importance. However the observed critical voltage from the switching of STT-MRAM implies a $\alpha$ ten times that measured by ferromagnetic resonance (FMR). In addition the TMR is strongly voltage dependent while the STT effect is not. This along with the weak dependence of the critical current on switching direction are inconsistent with the tunnelling model and have never been properly explained. Here will be described a circuit model based upon SU(2) theory for a MTJ for which the basic SMF of about 10$\mu$V is magnified to a 200mV shift between the parallel P and anti-parallel AP branches of the IV characteristic. It is implied that the TMR has for origin an SMF. [1] J. C. Slonczewski Phys, Rev. B71, 024411 (2005) [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S8.00005: Spin Transfer Torque in Spin Filter Tunnel Junctions Christian Ortiz Pauyac, Alan Kalitsov, Aurelien Manchon, Mair Chshiev STT in MTJs is well known for its potential spin electronic applications. However, recently a new class of MTJs based on spin filtering across magnetic insulators (SFTJ) has been attracting much attention since in such MTJs electrons with a certain spin orientation tunnel much more efficiently. In this structure, STT remains to be addressed and clarified. Here we present a systematic study of its angular and voltage bias dependences consisting of one or two FM layers separated by a magnetic insulator (MI). The calculations were performed within the tight-binding model using NEGF technique in the framework of Keldysh formalism. We predict that STT is higher in magnitude compared to regular MTJs, which strongly depends in the relative directions of the magnetic states of the free layer (FM2) and MI. Namely, in case of parallel orientation of MI and FM2 moments in a FM1|MI|FM2 structure, the system behaves as a regular MTJ with a modest increase of STT magnitude. However, as the angle between MI and FM2 moments increases, the field-like torque becomes three orders of magnitude higher than the Slonczewski component and oscillates with bias as band-filling increases. This may have practical implications. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S8.00006: Spin-transfer torque in antiferromagnetic and ferrimagnetic tunnel junctions Pablo Merodio, Alan Kalitsov, Helene Bea, Vincent Baltz, Mairbek Chshiev Spin Transfer Torque (STT) and Giant-Magnetoresistance in ferromagnets (F) are the two essential underlying phenomena in modern spintronics. These effects have also been predicted to occur in nanostructures comprising only normal and antiferromagnetic materials. Therefore, antiferromagnets (AF) could potentially be used in place of F in future spintronic applications. We present a theoretical study of STT and Tunnelling Magnetoresistance (TMR) in AF and ferrimagnet (FI) based tunnel junctions, where two magnetic metal electrodes with at least one of them being FI or AF are separated by a thin nonmagnetic insulating barrier. We found that electronic structure parameters such as Fermi energy and exchange splitting of the FI and AF leads strongly influence STT and TMR properties including their bias dependence. In particular, STT spatial distribution within the leads shows nontrivial behavior which can be explained in terms of interplay between exchange splittings of the two AF or FI sublattices. Such insights will be of importance for optimizing current induced magnetization reversal phenomena. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S8.00007: Spin torques between ferromagnetic and compensated antiferromagnetic layers Adrian Popescu, Khartik Prakia, Paul Haney The current induced torques between a ferromagnetic layer and a compensated antiferromagnetic layer of various symmetries are considered. The general conditions under which these current induced torques can stabilize the out-of-plane configuration of the ferromagnet are provided, along with numerical results for specific models. The effects of phase breaking scattering and their experimental implications are also discussed. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S8.00008: Theory of Intrinsic Spin Torque Due to Interface Spin-Orbit Coupling Alan Kalitsov, Mairbek Chshiev, William Butler, Oleg Mryasov The effect of intrinsic spin torque due to spin-orbit coupling (SOC) at the interface between thin ferromagnetic film and non-magnetic metal has attracted significant fundamental and applied research interest [1]. We report quantum theory of SOC driven spin torque (SOT) within the Rashba model of SOC and two-band tight binding (TB) Hamiltonian including s-d exchange interactions ($J$). We employ the non-equilibrium Green Function formalism and find that SOT to the first order in SOC has symmetry consistent with the earlier quasi-classical diffusive theory [2]. An obvious benefit of the proposed approach is the expression for the SOT given in terms of TB parameters which enables a physically transparent analysis of the dependencies of SOT on material specific parameters such as Rashba SOC constant, hopping integral, Fermi level and $J$. On the basis of analytical and numerical results we discuss trends in strength of SOT and its correlation with the Spin Hall conductivity.\\[4pt] [1] I. M. Miron \textit{et al}., Nature \textbf{476}, 189 (2011).\\[0pt] [2] A. Manchon and S. Zhang, Phys. Rev. B \textbf{78}, 212405 (2008). [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S8.00009: Current-induced magnetization switching of a three terminal perpendicular magnetic tunnel junction by spin-orbit torque Murat Cubukcu, Marc Drouard, Olivier Boulle, Kevin Garello, Ioan Mihai Miron, Juergen Langer, Berthold Ocker, Pietro Gambardella, Gilles Gaudin A current flowing in the plane of a magnetic multilayer with structural inversion asymmetry, such as Pt/Co/AlO$_{\mathrm{x}}$, creates a torque on the magnetization [1]. This torque is due to the strong spin-orbit interaction present in such multilayers and can lead to fast magnetization reversal with a low writing energy [2]\textbf{. }We will present the first proof of concept of a perpendicular spin-orbit torque magnetic random access memory (SOT-MRAM) cell composed of a Ta/FeCoB/MgO/FeCoB magnetic tunnel junction. The basic write and read operations, i.e., the magnetization reversal by current injection in the Ta track and its detection using the high TMR signal, are demonstrated [3]. Our results open a path for the development of a novel class of three terminal MRAM combining fast, reliable and low energy writing. [1] I. M. Miron et al. Nature 476, 189 (2011) [2] K. Garello et al \underline {arXiv:1310.5586 }(2013) [3] M. Cubukcu et al., \underline {arXiv:1310.8235} (2013) [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S8.00010: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S8.00011: Magnetoresistive anomaly in amorphous GdFeCo thin films Nattawut Anuniwat, Xiaopu Li, Joseph Poon, Jiwei Lu Spin valves generally consist of two ferromagnetic layers sandwiching a thin non-magnetic layer. High and Low resistance states can be obtained depending upon the relative magnetization alignment of the ferromagnetic layers. Recently, unexpected spin-valves like magneto-resistance has been observed in disordered ferrimagnetic crystalline material [1] and antiferromagnetic-based tunnel junction [2]. Here, we demonstrate spin-valve-like magnetoresistance in amorphous ferrimagnetic thin films. The amorphous GdFeCo films were deposited by rf magneton sputtering with the thickness $\sim$ 60 nm. The as-deposited film exhibited low saturation moment as $M_{S}$ $\sim$ 80 emu/cm$^{3}$. The compensation temperature is also observed near room temperature. The magneto-transport properties are performed on patterned Hall bar as a function of temperature ranging from 50K to 400K. The anomalous Hall resistance exhibits the same compensation temperature as magnetic moment. The asymmetric magnetoresistance reverse polarity at temperature below the compensation point indicating the different scattering mechanism than the anomalous Hall effect. We also demonstrate the bi-stable magneto-resistance with the absence of external H fields as a function of temperature up to 400K. The possible origins of this asymmetric MR will also be discussed.\\[4pt] [1] S. Singh, et al., Phys Rev Lett \textbf{109}, 246601 (2012).\\[0pt] [2] B. G. Park, et al., Nat Mater \textbf{10}, 347 (2011). [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S8.00012: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S8.00013: Role of spin polarization in FM/Al/FM trilayer film at low temperature Ning Lu, Richard Webb Measurements of electronic transport in diffusive FM/normal metal/FM trilayer film are performed at temperature ranging from 2K to 300K to determine the behavior of the spin polarized current in normal metal under the influence of quantum phase coherence and spin-orbital interaction. Ten samples of Hall bar with length of 200 micron and width of 20 micron are fabricated through e-beam lithography followed by e-gun evaporation of Ni$_{0.8}$Fe$_{0.2}$, aluminum and Ni$_{0.8}$Fe$_{0.2}$ with different thickness (5nm to 45nm) in vacuum. At low temperature of 4.2K, coherent backscattering, Rashba spin-orbital interaction and spin flip scattering of conduction electrons contribute to magnetoresistance at low field. Quantitative analysis of magnetoresistance shows transition between weak localization and weak anti-localization for samples with different thickness ratio, which indicates the spin polarization actually affects the phase coherence length and spin-orbital scattering length. However, at temperature between 50K and 300K, only the spin polarization dominates the magnetoresistance. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S8.00014: Ferroelectrically controlled organic spin valves with tunable magnetoresistance Mei Fang, Dali Sun, Xiaoshan Xu, Lu Jiang, Hangwen Guo, Ho Nyung Lee, Paul C. Snijders, T.Z. Ward, Zheng Gai, X.-G Zhang, Jian Shen, Lifeng Yin, Yanmei Wang, Wenting Yang Organic spin valves (OSV) with tunable magnetoresistance (MR) will promote organic spintronics for many potential applications. In this work, a novel ferroelectrically (FE) controlled organic spin valve (FE-OSV) was successfully fabricated by inserting a thin PbZr$_{0.2}$Ti$_{0.8}$O$_{3}$(PZT) buffer layer between the ferromagnetic bottom La$_{0.67}$Sr$_{0.33}$MnO$_{3}$(LSMO) electrode and the organic Alq$_{3}$ layer. The magnitude of MR values in these FE-OSV strongly depends on the history of the bias voltage applied, giving rise to a strong hysteretic behavior of MR vs. V. Moreover, the sign of MR in the FE-OSV can be electrically switched when the electric polarization of PZT layer is reversed. Both behaviors are not observed in the devices without the FE layer. These new findings are attributed to the tunability of the electric dipole moment of the PZT layer, which can actively shift the relative energy level between Alq$_{3}$ and LSMO and thence alter the spin injection. [Preview Abstract] |
Session S10: Focus Session: Confined Nucleic Acids I
Sponsoring Units: DBIOChair: Robert Riehn, North Carolina State University
Room: 201
Thursday, March 6, 2014 8:00AM - 8:12AM |
S10.00001: Regimes of DNA confined in a nanochannel Liang Dai, Patrick Doyle Scaling regimes for polymers confined to tubular channels are well established when the channel cross-sectional dimension is either very small (Odjik regime) or large (classic de Gennes regime) relative to the polymer Kuhn length. In the literature, there is no clear consensus regarding the intermediate region and if subregimes even exist to connect these two classic bounding regimes. The confluence of emerging single DNA mapping technologies and a resurged interest in the fundamental properties of confined polymers has led to extensive research in this area using DNA as a model system. Due to the DNA molecule's properties and limitations of nanofabrication, most experiments are performed in this intermediate regime with channel dimensions of a few Kuhn lengths. Here we use simulations and theory to reconcile conflicting theories and show that there are indeed extended de Gennes, partial alignment and hairpin regimes located between the two classic regimes. Simulations results for both chain extension and free energy support the existence of these regimes. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S10.00002: Modeling the Relaxation Time of DNA Confined in a Nanochannel Yanwei Wang, Douglas R. Tree, Kevin D. Dorfman Using a mapping between a dumbbell model and fine-grained Monte Carlo simulations, we have computed the relaxation time of $\lambda$-DNA in a high ionic strength buffer confined in a nanochannel (Tree {\it et al.}, {\it Biomicrofluidics} {\bf 2013}, {\it 7}, 054118). The relaxation time thus obtained agrees quantitatively with experimental data (Reisner {\it et al.}, {\it PRL} {\bf 2005}, {\it 94}, 196101) using only a single $O(1)$ fitting parameter to account for the uncertainty in model parameters. In addition to validating our mapping, this agreement supports our previous estimates of the friction coefficient of DNA confined in a nanochannel (Tree {\it et al.}, {\it PRL} {\bf 2012}, {\it 108}, 228105), which have been difficult to validate due to the lack of direct experimental data. Furthermore, our calculation shows that as the channel size passes below $\sim$100 nm (or roughly the Kuhn length of DNA) there is a dramatic drop in the relaxation time. Inasmuch as the chain friction rises with decreasing channel size, the reduction in the relaxation time can be solely attributed to the sharp decline in the fluctuations of the chain extension. Practically, the low variance in the observed DNA extension in such small channels has important implications for genome mapping. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S10.00003: Topological events in single molecules of long genomic DNA confined in nanochannels Jeffrey Reifenberger, Kevin Dorfman, Han Cao ct- We present a rapid genome-wide analysis method based on new NanoChannel Array technology (Irys$^{\mathrm{TM}}$ System) that confines and linearizes extremely long DNA molecules (100 to 1,000 kilobases) for direct image analysis at tens to hundred of gigabases per run. Genomic DNA is stained with YOYO and labeled specifically at the `GCTCTTC' sequence with fluorescent dyes allowing each molecule to be uniquely patterned and mapped to its corresponding reference. This high-throughput platform automates the imaging of such barcoded patterns on genomic DNA to identify wide spread structural variations in a genome. Here we describe a method to rule out possible topologically altered molecules in linear confinement by identifying possible topological events through a T-test looking for spikes in the fluorescence of the YOYO stained DNA backbone. These events are confirmed through aligning the marked individual molecules to a standard reference and measuring a distance differential between labels surrounding the suspected topological event compared to the reference. Such events could be flagged to distinguish from true structural variations. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S10.00004: Segregation and chain extension of overlapping semiflexible macromolecules in channel Peter Cifra, Dusan Racko Decrease of overall chain extension in channel by local backfolding together with an increased extension of sequences running parallel have been reported to complicate linearization experiments under moderate confinements. Less known related effect occurs in two overlapping chains in channel. Investigation of overlap and segregation of polymer coils in channel was extended here relative to previous studies from flexible to semiflexible chains. Results are based on simulation of confinement free energy of a chain and on direct simulation of coil segregation process. For confinement free energy we confirm the predicted opposite trend with increasing chain stiffness for the weak and strong confinement regimes. Results of two different approaches are consistent, in agreement with theoretical analysis and indicate a stronger segregation tendency of flexible chains relative to semiflexible chains, both in its extent and dynamics. Mutual excluded volume between confined chains leads to extension of overlapping chains along channel and this effect is stronger for flexible chains but weak for stiffer macromolecules such as DNA. Support from Slovak Res. and Develop. Agency (SRDA-0451-11) is acknowledged. D. Racko, P. Cifra, J. Chem. Phys. 138, 184904 (2013) [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S10.00005: Linearization of DNA by a squeezing flow in a tunable nanoscale tube: a simulation study Minsub Han, Byoung Choul Kim, Toshiki Matsuoka, Shuichi Takayama Deoxyribose nucleic acid(DNA) is the biomaterial for storage of genetic information of all living organisms. The linearization of DNA is an initial step in one of the important methods to probe the vital information in biological and clinical settings. Squeezing the solution in flexible nanoscale channel proved to be a highly effective method for fully linearizing DNA (Toshiki et al. Nano Lett 2012). The detailed physical basis of the process is studied by using dissipative particle dynamics simulation, whose results corresponds to the lambda DNA in the nanoscale PDMS channel in the experiment. The squeezing process typically consists of a large degree of elongation by the advective flow, which is followed by recoiling back and adjusting to the narrower confinement. Strong gradient in advection and nanoscale confinement are thus the major thrust for the stretching in the process. The degree of the linearization also depends on the initial position relative to the center in the axial direction as well as the contour length. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S10.00006: Measuring DNA Confinement and Excluded Volume Parameters: Scaling with confinement and ionic strength Alexander Klotz, Lyndon Duong, Laurence Coursol, Walter Reisner Using nanofluidic devices for genomic mapping requires an understanding of the underlying polymer physics of confined DNA. Despite many years of study, there are still aspects that are poorly understood, including the role that excluded volume and semiflexibility play under confinement. Here, a hybrid nanofluidic device consisting of a narrow slit embedded with a lattice of square pits was used to study confined DNA. At equilibrium, molecules tend to occupy one or more pits. The partitioning of molecular contour between the pits and the slit is dependent on maximizing entropy by removing contour from the highly confining slit while reducing excess free energy due to excluded volume interactions from increased concentration in the pit. Measurements of the average number of occupied pits as a function of pit dimension, slit height, and ionic strength serves as a probe of the underlying polymer physics. In particular, the free energy of slit-like confinement and the effective molecular width were measured across a range of slit heights and ionic strengths. It was found that effective width scales with ionic strength according to Stigter's charged rod theory, and that the Chen-Sullivan interpolation formula for the slit-like energy of confinement describes the data well for narrow slits. Unexpected scaling of the confinement free energy with ionic strength indicates that excluded volume effects are relevant for confined DNA. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S10.00007: Dynamics of large DNA confined to nanoslits Invited Speaker: Patrick Doyle Microfabricated platforms present a model system to study the conformation and dynamics of DNA in reduced geometries. Slit and tube-like geometries have been widely studied. Slits confine DNA to a quasi-2D or so-called Hele-Shaw geometry. The role of slit confinement on polymer dynamics is not straightforward due to the lack of an intrinsic hydrodynamic screening length. In this talk I will discuss our recent work in understanding DNA conformation and dynamics in slits. We make use of both single molecule experiments and large-scale molecular simulations. This aforementioned work was performed in a good solvent. I will next explore the role of confinement on the coil-globule transition. We show that for modestly poor solvents, the collapse process has two stages and that the duration of both stages is significantly affected by confinement. Our results suggest that the primary effect is not hydrodynamic in nature, but more related to a modification of the free energy landscape. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S10.00008: On the Odijk regime in nanoslits Wesley Reinhart, Douglas Tree, Kevin Dorfman The physics of polymer confinement has long been a topic of interest as a fundamental problem in soft matter physics. In biaxial confinement such as a tube or channel, there is a well-documented crossover from a regime dominated by flexible polymer blobs to a regime dominated by stiff deflection segments as the confinement length scale becomes much smaller than the persistence length. However, several fundamental questions remain concerning the exact nature of the strongly confined regime in uniaxial confinement in a slit, where the slit height is much smaller than the persistence length of the semiflexible polymer. We have investigated this problem by an off-lattice implementation of a chain-growth Monte Carlo algorithm, the pruned-enriched Rosenbluth method. Using our numerical results, we confirm that there is indeed a regime dominated by deflection segments in slits, and we provide a simple interpretation of the polymer size in this regime. Furthermore, we investigate previous claims in the literature regarding the effect of excluded volume on strongly confined polymers, where we find a connection between the slit geometry and a semiflexible polymer confined to a plane. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S10.00009: Topological and metric properties of linear and circular DNA chains in nano-slits and nano-channels Enzo Orlandini, Cristian Micheletti Motivated by recent advancements in single DNA molecule experiments, based on nanofluidic devices, we investigate numerically the metric and topological properties of a modelof open and circular DNA chains confined inside nano-slits and nano-channles. The results reveal an interesting characterization of the metric crossover behaviour in terms of the abundance, type and length of occuring knots. In particular we find that the knotting probability is nonmonotonic for increasing confinement and can be largely enhanced or suppressed, compared to the bulk case, by simply varying the slit or channel trasversal dimension. The observed knot population consists of knots that are far simpler than for DNA chains in spherical (i.e. cavities or capsids) confinement. These results suggest that nanoslits and nanochannels can be properly designed to produce open DNA chains hosting simple knots or to sieve DNA rings according to their knotted state. Finally we discuss the implications that the presence of knots may have on the dynamical properties of confined DNA chains such as chain elongation, injection/ejection processes and entanglement relaxation. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S10.00010: Convex Lens-Induced Nanoscale Templating Daniel Berard, Francois Michaud, Christopher McFaul, Sara Mahsid, Walter Reisner, Sabrina Leslie We demonstrate a new platform, ``Convex Lens-Induced Nanoscale Templating'' (CLINT), for dynamic manipulation and trapping of single DNA molecules. In the CLINT technique, the curved surface of a convex lens is used to deform a flexible coverslip above a substrate containing embedded nanotopography, creating a nanoscale gap that can be adjusted during an experiment to confine molecules within the embedded nanostructures. Critically, CLINT has the capability of actively transforming a macroscale flow-cell into a nanofluidic device without need for high-temperature direct bonding, leading to ease of sample loading and greater accessibility of the surface. Moreover, as DNA molecules present in the gap will be driven into the embedded topography from above, CLINT eliminates the need for the high pressures or electric fields necessitated by direct bonded nanofluidic devices for loading DNA in the confined structures. To demonstrate the versatility of CLINT, we confine DNA to nanogroove structures, demonstrating DNA nanochannel-based stretching. Using ionic strengths that are in line with typical biological buffers, we have successfully extended DNA in sub 30nm nanochannels, achieving high stretching (90\%) that is in good agreement with Odijk deflection theory. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S10.00011: Fluctuations of DNA Mobility in Nanofluidic Entropic Traps Lingling Wu, Stephen Levy We studied the mobility of DNA molecules driven by an electric field through a nanofluidic device containing a periodic array of deep and shallow regions termed entropic traps. Since the depth of the shallow region is smaller than the DNA equilibrium size, DNA molecules are trapped for a characteristic time and must deform themselves to traverse the boundary between deep and shallow regions. Consistent with previous experimental results, we observed a nonlinear relationship between mobility and electric field strength and that longer DNA molecules have higher mobility. In repeated measurements under seemingly identical conditions we measured fluctuations in the mobility significantly larger than expected from statistical variation. The variation was most pronounced for lower electric field strengths where the trapping time is considerable relative to the drift time. To determine the origin of these fluctuations, we investigated the dependence of the mobility on several variables: DNA concentration, ionic strength, fluorescent dye staining ratio, ionic current, and electroosmotic flow. The mobility fluctuations were not strongly correlated with these variables within the ranges in which they were varied. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S10.00012: Hindered DNA coordinated motion in nanochannels Yeng-Long Chen, Yu-Hui Lin, Dmytro Luzhbin We investigate the relaxation dynamics of long DNA molecules confined in micro- and nano-channels with Brownian dynamics simulations. Prior experiment by Reisner et al. found that the stretch fluctuation correlation time ($t_{relax}$) of DNA molecules in nanochannels increases as the channel height ($H$) decreases for $H$ greater than the DNA Kuhn length ($\sigma_k$), and $t_{relax}$ decreases as $H$ decreases for $H < \sigma_k$. Our simulations capture this behavior, and quantitatively agree with the experimental results within the error bars. The scaling-law dependence of $t_{relax}$ on $H$ in different regimes is verified. Rouse mode analysis of the chain relaxation mechanism further shows that segmental relaxation on length scale longer than $\sigma_k$ are hindered and the dynamics of segments shorter than $H$ dominate the chain relaxation processes. We also find that the inclusion of intra-chain hydrodynamic interactions affect segmental relaxation. The implications for DNA translocation through nanopores and nanochannels are discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S10.00013: Experimental Test of the Wormlike Chain Model for Confined DNA Damini Gupta, Julian Sheats, Abhiram Muralidhar, Jeremy J. Miller, Derek E. Huang, Kevin D. Dorfman, Walter Reisner We present experimental validation of a wormlike osculating-sphere model for double-stranded DNA. Single molecule fluorescence imaging of $\lambda $-DNA (48,500 base pairs) in rectangular nanochannel confinement was performed to measure the ensemble of molecule extensions in a low ionic strength buffer. The mean and variance of the extension furnish the parameters of the effective Hookean spring description of the chain extension. The geometric means of the channel widths range from 100 nm to 316 nm, thereby probing the transition between Odijk and de Gennes regimes in nanochannel sizes ranging from approximately 1.5 to 5 times the DNA persistence length. We found a steep drop in the variance of the extension as the channel size decreased in these channels. Our experimental results are in strong quantitative agreement with Pruned-Enriched Rosenbluth Method simulations using the wormlike osculating-sphere model in rectangular channels, where the only fitting parameter is the stained DNA contour length.~ [Preview Abstract] |
Session S11: Focus Session: Physics of Proteins III
Sponsoring Units: DBIO DPOLYChair: Andrea Markelz, SUNY Buffalo
Room: 203
Thursday, March 6, 2014 8:00AM - 8:12AM |
S11.00001: What is the Origin of Internal Friction in Unfolded Proteins? Garegin Papoian, Ignacia Echeverria The unfolded state is being increasingly recognized as critical to many biological processes. There are many proteins that are found only transiently in the unfolded state, eventually folding into globular structures. Other proteins, called intrinsically disordered proteins (IDPs), may be unfolded even when carrying out important biological functions. Despite its ubiquitousness, the unfolded ensemble is not fully understood. In this work, we have investigated the origin of friction for the unfolded proteins undergoing conformational diffusion. We used extensive all-atom molecular dynamics simulations to study the dynamics of the unfolded cold-shock protein (CSP) from Thermotoga maritima at different solvent viscosities and at different denaturant concentrations. We systematically analyzed the reconfiguration dynamics of relevant structural features such as dihedral angle rotations, hydrogen bonds and hydrophobic contacts forming and breaking. The results of our calculations are broadly consistent with the corresponding experimental measurements. Our findings have important implications for the folding kinetics of proteins, especially under physiological conditions. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S11.00002: Photoinduced Localized Unfolding of Tubulin Dimers Bound to a Water Soluble Porphyrin and the Search for Binding Location Using Docking Simulations Guided by a Combination of Resonance Raman Spectroscopy and Density Functional Theory Brady McMicken, Lorenzo Brancaleon, Robert Thomas The ability to modify the conformation of a protein by controlling its partial unfolding may have practical applications such as inhibiting its function. One method of locally unfolding a protein involves the use of a photosensitizer non-covalently bound to a protein which triggers photochemical reactions upon irradiation leading to protein conformational changes. We investigate the photoinduced conformational changes of tubulin mediated by a bound water soluble porphyrin which acts as a photosensitizer. Also of interest is how conformational changes of tubulin affect its function such as forming microtubules and the mechanism responsible for the structural change. To better understand the conformational change we must find the binding location between the porphyrin and protein. Density functional theory calculations will be combined with Resonance Raman spectroscopy to correlate the changes in vibrational modes of the porphyrin with changes in its physical structure upon binding to tubulin. This will allow us to determine the distorted conformation of the porphyrin when bound to tubulin which will subsequently be used in docking simulations to find the most likely binding configuration. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S11.00003: Structure and DNA-binding of meiosis-specific protein Hop2 Donghua Zhou, Hem Moktan, Roberto Pezza Here we report structure elucidation of the DNA binding domain of homologous pairing protein 2 (Hop2), which is important to gene diversity when sperms and eggs are produced. Together with another protein Mnd1, Hop2 enhances the strand invasion activity of recombinase Dmc1 by over 30 times, facilitating proper synapsis of homologous chromosomes. However, the structural and biochemical bases for the function of Hop2 and Mnd1 have not been well understood. As a first step toward such understanding, we recently solved the structure for the N-terminus of Hop2 (1-84) using solution NMR. This fragment shows a typical winged-head conformation with recognized DNA binding activity. DNA interacting sites were then investigated by chemical shift perturbations in a titration experiment. Information of these sites was used to guide protein-DNA docking with MD simulation, revealing that helix 3 is stably lodged in the DNA major groove and that wing 1 (connecting strands 2 and 3) transiently comes in contact with the minor groove in nanosecond time scale. Mutagenesis analysis further confirmed the DNA binding sites in this fragment of the protein. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S11.00004: Metal ion coupled protein folding and allosteric motions Invited Speaker: Wei Wang Many proteins need the help of cofactors for their successful folding and functioning. Metal ions, i.e., Zn$^{2+}$, Ca$^{2+}$, and Mg$^{2+}$ etc., are typical biological cofactors. Binding of metal ions can reshape the energy landscapes of proteins, thereby modifying the folding and allosteric motions. For example, such binding may make the intrinsically disordered proteins have funneled energy landscapes, consequently, ensures their spontaneous folding. In addition, the binding may activate certain biological processes by inducing related conformational changes of regulation proteins. However, how the local interactions involving the metal ion binding can induce the global conformational motions of proteins remains elusive. Investigating such question requires multiple models with different details, including quantum mechanics, atomistic models, and coarse grained models. In our recent work, we have been developing such multiscale methods which can reasonably model the metal ion binding induced charge transfer, protonation/deprotonation, and large conformational motions of proteins. With such multiscale model, we elucidated the zinc-binding induced folding mechanism of classical zinc finger and the calcium-binding induced dynamic symmetry breaking in the allosteric motions of calmodulin [1,2]. In addition, we studied the coupling of folding, calcium binding and allosteric motions of calmodulin domains. In this talk, I will introduce the above progresses on the metal ion coupled protein folding and allosteric motions. \\[4pt] [1] Li WF, Zhang J, Wang J, and Wang W, Metal-Coupled Folding of Cys2His2 Zinc-Finger, J. Am. Chem. Soc. 130 (2008) 892-900;\\[0pt] [2] Tan C, Li WF, Wang W, and Thirumalai D, Dynamic symmetry breaking during allosteric transitions in Calmodulin is driven by quantized dehydration of Ca2$+$ ions, (2013) submitted. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S11.00005: Direct Probing of Solvent Accessibility and Mobility at the Binding Interface of Polymerase (Dpo4)-DNA Complex Yangzhong Qin, Dongping Zhong Water plays essential structural and dynamical roles in protein-DNA recognition through contributing to enthalpic or entropic stabilization of binding complex and by mediating intermolecular interactions and fluctuations for biological function. These interfacial water molecules are confined in nanospace but mostly highly mobile. Here, we report our studies of interfacial water dynamics in the binary and ternary complexes of a polymerase (Dpo4) with DNA and an incoming nucleotide using a site-specific tryptophan probe with femtosecond resolution. By systematic comparison of the interfacial water motions and local sidechain fluctuations in the apo, binary and ternary states of Dpo4, we observed that the DNA binding interface and active site is dynamically solvent accessible and the interfacial water dynamics are slightly slow but similar to the surface hydration water fluctuations on picosecond time scales. Our MD simulations also show the binding interface full of water molecules and nonspecific weak interactions with protein and DNA. Such a fluid binding interface facilitates the polymerase sliding on DNA for fast translocation while the spacious and mobile hydrated active site contributes to the low fidelity of the lesion-bypass Y-family DNA polymerase. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S11.00006: Accessible surface area of proteins from purely sequence information and the importance of global features Eshel Faraggi, Yaoqi Zhou, Andrzej Kloczkowski We present a new approach for predicting the accessible surface area of proteins. The novelty of this approach lies in not using residue mutation profiles generated by multiple sequence alignments as descriptive inputs. Rather, sequential window information and the global monomer and dimer compositions of the chain are used. We find that much of the lost accuracy due to the elimination of evolutionary information is recouped by the use of global features. Furthermore, this new predictor produces similar results for proteins with or without sequence homologs deposited in the Protein Data Bank, and hence shows generalizability. Finally, these predictions are obtained in a small fraction (1/1000) of the time required to run mutation profile based prediction. All these factors indicate the possible usability of this work in de-novo protein structure prediction and in de-novo protein design using iterative searches. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S11.00007: Excited State Electronic Structure of Fluorescent Proteins Revealed by Two-Dimensional Double Quantum Coherence Spectroscopy Patrick Konold Red fluorescent proteins (RFPs) are nearly ideal probes for monitoring subcellular processes with extremely high spatial and temporal precision. Modern derivatives with increased brightness are sought to further enhance imaging applicability, however, photostability issues represent a universal obstacle towards RFP development. In this work, we employed Two-Dimensional Double Quantum Coherence (2D2Q) spectroscopy to probe the excited state electronic structure of mKate, a widely used RFP. Our results help explain the excited state absorption contributions observed in spectrally resolved transient grating measurements that ultimately relate to excited state photophysics thought to modulate the dark state conversion and irreversible photobleaching processes leading to poor brightness. Moreover, we contrast results across a panel of point mutants of the S158 residue and find a connection between chromophore-sidechain interactions and the position of energy levels in the doubly excited manifold. Such observations highlight the role of the protein environment in tuning excited state photophysics and may provide a clue for engineering more photostable RFPs. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S11.00008: Spectroscopic Monitoring of Proton Transfer in Green Fluorescent Protein J. Timothy Sage, Mannis O'Brien, Bridget Salna, Benabbas Abdelkrim, Paul M. Champion, Jasper van Thor Vibrational spectroscopy is an ideal probe for proton transfer in biological molecules because of its sensitivity to the motion of protons, which are difficult to track with more direct structural methods such as X-ray crystallography. Previous time-resolved infrared measurements provided direct experimental evidence for Glu 222 as the excited state proton acceptor following excitation of green fluorescent protein (GFP). Here, we use infrared cryospectroscopy to characterize a low quantum yield photochemical channel that leads to decarboxylation of Glu 222 coupled with proton transfer to complete the methyl group on the resulting $\alpha$-aminobutyric acid residue. The \emph{irreversible} nature of this process allows us to obtain infrared data at much higher sensitivity and over an extended frequency range. Difference spectra recorded over the full 1000-4000 cm$^{-1}$ range at 100 K probe perturbations of internal water molecules and nearby amino acids as well as the chromophore. We identify vibrational frequencies that probe hydrogen bonding along the ``proton wire'' that connects the chromophore to Glu 222. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S11.00009: Correlated Protein Motion Measurements of Dihydrofolate Reductase Crystals Mengyang Xu, Katherine Niessen, James Pace, Vivian Cody, Andrea Markelz We report the first direct measurements of the long range structural vibrational modes in dihydrofolate reductase (DHFR). DHFR is a universal housekeeping enzyme that catalyzes the reduction of 7,8-dihydrofolate to 5,6,7,8-tetra-hydrofolate, with the aid of coenzyme nicotinamide adenine dinucleotide phosphate (NADPH). This crucial enzymatic role as the target for anti-cancer [methotrexate (MTX)], and other clinically useful drugs, has made DHFR a long-standing target of enzymological studies. The terahertz (THz) frequency range (5-100 cm$^{-1}$), corresponds to global correlated protein motions. In our lab we have developed Crystal Anisotropy Terahertz Microscopy (CATM), which directly measures these large scale intra-molecular protein vibrations, by removing the relaxational background of the solvent and residue side chain librational motions. We demonstrate narrowband features in the anisotropic absorbance for mouse DHFR with the ligand binding of NADPH and MTX single crystals as well as Escherichia coli DHFR with the ligand binding of NADPH and MTX single crystals. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S11.00010: Structure-based simulations of kinesin motor domain for the study and characterization of its different microtubule and ligand-binding states Srirupa Chakraborty, Wenjun Zheng Kinesins are molecular motors acting as enzyme-based nanomachines that transport intracellular cargo along microtubules (MT). To obtain a detailed structural and energetic picture of the various conformations of the kinesin motor domain, we built atomistic models using available crystal structures, homology modeling and flexible fitting into cryo-electron microscopy (EM) maps. These models depict the various biochemical states of the kinesin head, such as - with the neck-linker docked and undocked in the MT-free state, and the different nucleotide (ADP, ATP and APO) bound states in the kinesin-MT complex. Here we perform molecular dynamics simulation techniques and large-scale computational probing of differences in these states, by an exhaustive search of interactions that differ between them, identify key residues in the active site and binding interface, and investigate the binding free-energy between kinesin and MT, and kinesin and ligand to compare with experimentally obtained results. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 11:00AM |
S11.00011: Allosteric Ligand Binding and Anisotropic Energy Flow in Albumin Invited Speaker: Brian Dyer Protein allostery usually involves propagation of local structural changes through the protein to a remote site. Coupling of structural changes at remote sites is thought to occur through anisotropic energy transport, but the nature of this process is poorly understood. We have studied the relationship between allosteric interactions of remote ligand binding sites of the protein and energy flow through the structure of bovine serum albumin (BSA). We applied ultrafast infrared spectroscopy to probe the flow of energy through the protein backbone following excitation of a heater dye, a metalloporphyrin or malachite green, bound to different binding sites in the protein. We observe ballistic flow through the protein structure following input of thermal energy into the flexible ligand binding sites. We also observe anisotropic heat flow through the structure, without local heating of the rigid helix bundles that connect these sites. We will discuss the implications of this efficient energy transport mechanism with regard to the allosteric propagation of binding energy through the connecting helix structures. [Preview Abstract] |
Session S12: Invited Session: Impacts of Physics Research on the Economy
Sponsoring Units: FPSChair: Pushpalatha Bhat, Fermi National Accelerator Laboratory
Room: 205
Thursday, March 6, 2014 8:00AM - 8:36AM |
S12.00001: Physics for Knowledge and Economic Growth Invited Speaker: Peter Littlewood |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S12.00002: The Effects of Physics R\&D on the European Economy Invited Speaker: David Lee |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S12.00003: Lasers and their Economic Impact in the United States Invited Speaker: Thomas Baer The laser was first proposed in the late 1950s and first demonstrated in 1960. It is one of several primary innovations that were a direct result of the ``quantum technology revolution'' which took place during the twentieth century and which superseded the industrial revolution. Over the past 50 years the laser has been integrated fully into our economy and daily lives. I will describe how lasers are an integral part of our communications, manufacturing, medical, defense, and energy infrastructures. I will also illustrate how the laser was a direct consequence of crucial government and private investment in basic research. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:24AM |
S12.00004: Applications of Superconductivity and Impact on U.S. Economy Invited Speaker: Venkat Selvamanickam In the past few decades, low temperature superconducting wires (niobium-titanium) have enabled multibillion dollar industries such as magnetic resonance imaging and nuclear magnetic resonance spectroscopy which otherwise would not have been possible. High temperature superconductors (HTS) hold the promise of impacting even a larger market in diverse applications such as energy, health, military, telecommunication, transportation and research. HTS tapes are now being manufactured in quantities of few hundred kilometers annually with current carrying capacity of about 300 times that of copper wire of the same cross section. Power transmission cables up to few kilometers in length made with HTS tapes have already been inserted in the power grid world-wide. In the past few of years, tremendous advancements have occurred in nanoscale defect engineering in these thin film superconducting tapes that has led to a doubling of critical current performance in high magnetic fields and operating temperatures of interest for various applications. Technologies developed in this area have been successfully inserted in production HTS tapes by industry. With the availability of such high performance HTS tapes, a number of coil-based applications are now being aggressively pursued by several institutions. HTS coils enable power devices with high power density with significant weight, size and power benefits. Energy storage, generation, use, transformation and transmission applications as well as magnetic applications such as magnetic shields, plasma confinement, and ultra-high field magnets are becoming possible with the availability of high-performance HTS tapes. An overview of the development and use of superconductors in electric power and magnetic applications will be provided in this presentation. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 11:00AM |
S12.00005: Panel Discussion |
Session S13: Focus Session: Fe Based Superconductors-DFT and Pressure Effect
Chair: Rong Yu, Renmin University of ChinaRoom: 207
Thursday, March 6, 2014 8:00AM - 8:12AM |
S13.00001: Lifshitz Transitions in 122-Pnictides Under Pressure Khandker Quader, Michael Widom The issue of Lifshitz transitions [1] in solids has been of considerable interest over several decades. These occur due to topological change(s) of the Fermi surface caused by external pressure or chemical substitution. Resulting anomalies in lattice parameters, density of states near the Fermi energy, elastic properties, and electron dynamics manifest in thermodynamic and transport properties that may be observed experimentally. At sufficiently high pressures, the 122 pnictides, AFe$_{2}$As$_{2}$ (A = Ca, Sr, Ba), display transition from a tetragonal phase (T) to a ``collapsed'' phase (cT). Based on our T = 0 first principles total energy density functional theory calculations as a function of pressure, we propose that the observed T-cT transitions result from T= 0 K Lifshitz transitions. Our results for energy band dispersions and spectra, c- and a-axis lattice parameters, and elastic constants over a wide range of hydrostatic pressure support our view. \\[4pt] [1] I. M. Lifshitz, Sov. Phys. JETP 11, 1130 (1960) [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S13.00002: First-principles investigation of the effect of pressure on CaFe$_2$As$_2$ and Pr-doped Ca$_{1-x}$Pr$_{x}$Fe$_ {2}$ As$_ {2}$ Tanju Gurel, A. V. Lukoyanov, Esra Erturk, Guven Akcay, Resul Eryigit, V. I. Anisimov In a recent study (arXiv:1310.3842), superconductivity has been observed at critical temperature ~51 K under pressure 1.9 GPa for rare-earth doped Ca$_{0.86}$Pr$_{0.14}$Fe$_2$As$_2$ which is the highest T$_c$ reported in the class of 1-2-2 iron-based superconductors. Motivated by this, we present density functional theory calculations on iron-based pnictide undoped CaFe$_2$As$_2$ and Pr-doped Ca$_{1-x}$Pr$_x$Fe$_2$As$_2$ ($x$=0.25 and 0.125). The calculations have been carried out using plane-waves and pseudopotential approach within generalized gradient approximation (GGA) and also within GGA+U in order to investigate the influence of correlation effects. The effect of pressure on crystal structure, magnetic order, and electronic structure are investigated for both undoped and Pr-doped structures for comparison and discussed with experimental findings. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S13.00003: Ab initio study on rare-earth iron-pnictides $R$FeAsO ($R=$Pr, Nd, Sm, Gd) in low-temperature \emph{Cmma} phase Resul Eryigit, Tanju Gurel, Esra Erturk, A.V. Lukoyanov, Guven Akcay, V.I. Anisimov We present density functional theory calculations on iron-based pnictides $R$FeAsO ($R=$Pr, Nd, Sm, Gd). The calculations have been carried out using plane-waves and projector augmented wave (PAW) pseudopotential approach. Structural, magnetic and electronic properties are studied within generalized gradient approximation (GGA) and also within GGA+U in order to investigate the influence of electron correlation effects. Low-temperature $Cmma$ structure is fully optimized by GGA considering both non-magnetic and magnetic cells. We have found that spin-polarized structure improves the agreement with experiments on equilibrium lattice parameters, particularly $c$ lattice parameter and Fe-As bond-lengths. Electronic band structure, total density of states, and spin-dependent orbital-resolved density of states are also analyzed in the frameworks of GGA and GGA+U and discussed. For all materials, by including on-site Coulomb correction, rare earth 4f states move away from the Fermi level and the Fermi level features of the systems are found to be mostly defined by the 3d electron-electron correlations in Fe. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S13.00004: Density functional theory for plasmon-assisted superconductivity Ryosuke Akashi, Ryotaro Arita The predictive calculation of superconducting transition temperatures (Tc) is a fascinating but extremely difficult problem in the field of superconductivity. For a conventional phonon-induced superconducting mechanism, an accurate predictive scheme to calculate Tc is established by the recent progress in the density functional theory for superconductors (SCDFT) [Lueders et al., PRB 72, 024545 (2005); Marques et al., PRB 72, 024546 (2005)]; the current SCDFT-based scheme systematically reproduces Tc observed by experiments in the conventional systems such as niobium and MgB2, with discrepancies no more than a few kelvin. However, further extensions including other mechanisms are essential to treat more general materials. Recently, we extended the SCDFT-based scheme to include a plasmon mechanism of superconductivity [Akashi and Arita, PRL 111, 057006 (2013)]. The plasmon mechanism, which has been considered solely in rather dilute electron systems, is also expected to be relevant in a wider range of materials because it can cooperate with the conventional phonon mechanism. Our extended scheme enables us to evaluate the effects on Tc of the plasmon and phonon mechanisms on equal footing. In the talk, we present recent applications to elemental metals. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S13.00005: Pressure-driven magnetic and structural transitions in the 122-pnictides Michael Widom, Khandker Quader Pnictides of the family AFe2As2, where A is an alkali earth element, exhibit several phase transitions in their structure and magnetic order as functions of applied pressure. We employ density functional theory total energy calculations at T=0K to model these transitions for the entire set of alkali earths (A=Ca, Sr, Ba, Ra) which form the 122 family. Three distinct types of transition occur: an enthalpic transition [1] in which the striped antiferromagnetic orthorhombic (OR-AFM) phase swaps thermodynamic stability with a competing tetragonal phase; a magnetic transition in which the OR-AFM phase loses its magnetism and orthorhombicity; a lattice parameter anomaly in which the tetragonal c-axis collapses. We identify this last transition as a ``Lifshitz transition'' caused by a change in Fermi surface topology. Depending on the element A, the tetragonal state exhibiting the Lifthitz transition might be metastable (A=Ca) or stable (A=Sr, Ba and Ra). \\[4pt] [1] M. Widom and K. Quader, Phys. Rev. B 88 (2013) 045117 [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S13.00006: Direct Detection of Interstitial Oxygen and its Electronic Structure in Superconducting Fe$_{\mathrm{1+y}}$TeO$_{\mathrm{x}}$ Thin-Films Jian-Min Zuo, Hefei Hu, Mao Zheng, Can Zhang, Laura Greene, James Eckstein, Ji-Hwan Kwon Fe$_{\mathrm{1+y}}$Te thin films become a superconductor when doped with oxygen, in which the interstitially incorporated oxygen plays crucial role for emergence of superconductivity. In this study, we investigated atomic and electronic structure of the oxygen-doped Fe$_{\mathrm{1+y}}$Te thin films using electron energy loss spectroscopy (EELS) and first principles calculation based on density functional theory (DFT). Atomic-resolution EELS reveals interstitial oxygen position next to the Fe layer, four fold hollow site, which location is consistent with DFT calculation result showing that the experimentally-found structure is the most energetically stable interstitial site. The relaxed geometry of oxygen-incorporated FeTe through DFT calculation shows drastic structural distortions in the planar structure of FeTe layer, which is associated with large amount of magnetic moment fluctuation in the Fe atoms with the bicollinear antiferromagnetic configuration. The detailed analysis including the charge density and electronic structures of the oxygen-doped FeTe will be presented. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S13.00007: Correlations and effects of pressure in Fe-pnictides Invited Speaker: Roser Valenti In this talk we will explore the effects of correlations and pressure in Fe-based superconductors by considering a combination of density functional theory calculations and dynamical mean field theory and compare our results with recent ARPES and de Haas van Alphen experiments. We will discuss the importance of orbital-selective correlations in the 111 (LiFeAs, LiFeP) and 122 families (BaFe$_2$As$_2$,CaFe$_2$As$_2$,KFe$_2$As$_2$) [1,2] and indicate how the topology of the Fermi surface, specially in KFe2As2, is influenced by these effects. In this context, we will show why MgFeGe, an isostructural and isoelectronic system to LiFeAs, doesn't superconduct [3]. In the case of the 122 systems, we will predict and analyze changes in the electronic and magnetic properties under hydrostatic, tensile and compressive pressure and will discuss our results in relation to (i) superconductivity, (ii) magnetism and (iii) the mechanisms involved in the detwinning process of an orthorhombic iron-pnictide crystal a [4].\\[4pt] [1] J. Ferber, H. O. Jeschke, R. Valenti, Phys. Rev. Lett 109, 236403 (2012).\\[0pt] [2] D. Guterding, S. Backes, H.O. Jeschke, R. Valenti, in preparation.\\[0pt] [3] H.O. Jeschke, I. I. Mazin, and R. Valenti, Phys. Rev. B (RC) 87, 241105 (2013).\\[0pt] [4] M. Tomic, H. O. Jeschke, R. M. Fernandes, R. Valenti, Phys. Rev B 87, 174503 (2013). [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S13.00008: Superconductivity from repulsion in LiFeAs: novel $s$-wave symmetry and potential time-reversal symmetry breaking Ilya Eremin, Felix Ahn, Johannes Knolle, Vladimir Zabolotnyy, S. Borisenko, Roderich Moessner, Bernd B\"uchner, Andrey Chubukov Using the ten orbital tight-binding model, derived from the ab-initio LDA calculations and fitted to the ARPES experiments, we analyze the structure of the superconducting gap in LiFeAs. We treat superconductivity as quasi-2D and decompose the pairing interaction for various $k_z$ cuts into $s-$ and $d$-wave components. Analyzing the leading superconducting instabilities we find that in addition to the conventional $s^{+-}$-wave superconducting order parameter where the gap changes sign between electron and hole pockets LiFeAs possesses another instability where the superconducting gap also changes sign between two smaller inner hole pockets. This occurs due to relatively large repulsion between these two small pockets and also relatively weak interaction between outer and inner hole pockets. The sizes of the gaps on the inner hole pockets is larger than the average value of the superconducting gap on the outer hole pockets and electron pockets which agrees with experimental data. Depending on the input parameters this gap structure is either a leading instability in the s-wave channel or a subleading one to the usual $s^{+-}$. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S13.00009: Effect of uniaxial strain on structural and magnetic phase transitions in Ba (Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$As$_{2}$(0$\le $x$\le $0.04) Chetan Dhital, Zahra Yamani, Zhensong Ren, Tom Hogan, Masa Matsuda, Stephen Wilson Most of the parent compounds of iron based superconductors have either simultaneous or nearly simultaneous structural and magnetic transitions from the high temperature paramagnetic tetragonal phase to the low temperature orthorhombic antiferromagnetic phase. Different measurement probes either directly/indirectly using uniaxial strain (dc transport, optical, thermodynamic and spectroscopic) indicate the presence of a high-temperature electronically anisotropic, nematic phase in both doped and undoped iron pnictide compounds which persists well beyond the nominal magnetic and structural transition temperatures. Here we will discuss the influence of uniaxial strain on the magnetic and structural transition temperatures across a series of Ba (Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$ As$_{2}$ (0$\le $x$\le $0.04) single crystals studied via elastic neutron diffraction. We will present in detail how the effect of strain field varies as function of concentration of cobalt and discuss the relevance of our result to the previously observed electronic anisotropies. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S13.00010: Do Se vacancies electron dope monolayer FeSe? Tom Berlijn, Hai-Ping Cheng, P.J. Hirschfeld, Wei Ku Following the discovery of the potentially very high temperature superconductivity in monolayer FeSe we investigate [1] the doping effect of Se vacancies in these materials. We find that Se vacancies pull a vacancy centered orbital below the Fermi energy that absorbs most of the doped electrons. Furthermore we find that the disorder induced broadening causes an effective hole doping. The surprising net result is that in terms of the Fe-$d$ bands Se vacancies behave like hole dopants rather than electron dopants. Our results exclude Se vacancies as the origin of the large electron pockets measured by angle resolved photoemission spectroscopy. TB was supported by DOE CMCSN and as a Wigner Fellow at the Oak Ridge National Laboratory.\\[4pt] [1] T. Berlijn, H.-P. Cheng, P. J. Hirschfeld, and W. Ku, arXiv:1307.0140. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S13.00011: Phonon spectrum of SrFe$_{2}$As$_{2}$ determined using multizone phonon refinement D. Parshall, R. Heid, J.L. Niedziela, Th. Wolf, M.B. Stone, D.L. Abernathy, D. Reznik The ferropnictide superconductors exhibit a sensitive interplay between the lattice and magnetic degrees of freedom, including a number of phonon modes are much softer than predicted by nonmagnetic calculations using density functional theory (DFT). However, it is not known what effect, if any, the long-range magnetic order has on phonon frequencies above 23~meV, where several phonon branches are very closely spaced in energy and it is challenging to isolate them from each other. We measured these phonons using inelastic time-of-flight neutron scattering in $\approx$ 40~Brillouin zones, and developed a technique to determine their frequencies. We find this method capable of determining phonon energies to better than 0.1~meV accuracy, and that the DFT calculations using the experimental structure yield qualitatively correct energies and eigenvectors. We do not find any effect of the magnetic transition on these phonons. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S13.00012: Space group symmetry, spin-orbit coupling, and the low-energy effective Hamiltonian for iron-based superconductors Vladimir Cvetkovic, Oskar Vafek Iron-based superconductors are multiband semimetals with competing instabilities. This motivates us to use the method of invariants when constructing a low-energy effective theory for these materials. In the construction we use the space group which, being non-symmorphic, leads to peculiar consequences at the Brillouin zone corner, precisely where the low-energy states reside. Our model displays good agreement with the multiband tight-binding models. The spin-orbit coupling, significant in iron, is easily incorporated in our model. We predict its consequences. The nodal spin-density wave (SDW) is proven to be unstable toward any finite spin-orbit coupling. Both colinear or coplanar SDW are shown to induce magnetic moments on pnictogen atoms. The quasiparticle dispersion in the presence of an s-wave spin singlet superconducting order is studied. In the absence of spin-orbit coupling, our minimal model yields isotropic gaps on bothhole Fermi surfaces. The gap structure on the electron Fermi surfaces is determined by the ratio of pairing parameters. The presence of spin-orbit interaction results in the gap anisotropy on the hole Fermi surfaces and a qualitative change of the gap structure on the electron Fermi surfaces. [1] V. Cvetkovic and O. Vafek, Phys. Rev. 88, 134510 (2013). [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S13.00013: Quasi-particle spectrum around vortex core states in iron-selenide superconductors Qianen Wang, Fuchun Zhang We study electronic structure of vortex core states of iron-selenide superconductors based on a three-orbital model by solving the Bogoliubov-de Gennes equation self-consistently. The absence of hole pocket at Gamma point and multi-orbital band structure are two special features of FeSe superconductors. We calculate quasi-particle wavefunctions and local density of states of the vortex core states for isotropic s-wave, anisotropic s-wave, and $d_{x^2-y^2}$-wave pairing symmetries, respectively. It turns out that the orbital-resolved vortex core states in different paring symmetries manifest themselves as distinguishable structures due to different behavior of the qusi-particle wavefunctions. [Preview Abstract] |
Session S14: Invited Session: Dynamics of Polymers at Interfaces and in Confinement
Sponsoring Units: DPOLY DFDChair: Oliver Baeumchen, Max Planck Institute for Dynamics and Self-Organization
Room: 301-303
Thursday, March 6, 2014 8:00AM - 8:36AM |
S14.00001: Polymer dynamics and stress transmission at polymer interfaces Invited Speaker: Frederic Restagno End tethered polymer chains have been recognized to be excellent adhesion promoters at polymer or are known to be able to modify the friction at interfaces. In this talk, we will present 3 related sets of experiments on the dynamics of polymer close to an interface. We will present results of polymer chains in contact with a melt, in contact with an elastomer and in contact with a solvent. In a first part, we will present neutron reflectivity experiments characterizing the density profiles and dynamics of interdigitation between H-PS brushes, in contact with d-PS thick layers, heated above their glass transition temperature. This interdigitated brushes will be shared and disentangled from the melt leading to a large slip at the interface. The density profiles of the sheared brushed will be presented. In a second part we will present some friction measurement at a PDMS chains -- PDMS elastomer interface and we will show that the grafted chains penetration and dynamics allows to control the friction at such elastomer interfaces. Finally we shall present some measurements of the mechanical response of the swollen PDMS chains using nanorheology experiments and we will evidence the role of the confining surfaces in the measurements. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S14.00002: Welding and healing of polymer interfaces: Connecting structure, dynamics and strength Invited Speaker: Mark Robbins Applying heat to polymer interfaces is a common means of welding polymer components or healing cracks in polymers. Once chains have diffused by their radius of gyration, the properties of the interface should be indistinguishable from those of the bulk. In practice, welds can achieve bulk strength at much shorter times. The mechanism of strength growth is difficult to determine with experiments, because they cannot directly access the evolution of molecular configurations and entanglements. Large-scale simulations were used to follow the dynamics of interdiffusion at welds and cracks and the associated changes in density and molecular conformations.\footnote{T. Ge, G. S. Grest and M. O. Robbins, ACS Macro Letters 2, 882-886 (2013) T. Ge, F. Pierce, D. Perahia, G. S. Grest and M. O. Robbins, Phys. Rev. Lett. 110, 098301 (2013).} The evolution of entanglements was tracked using Primitive Path Analysis and shown to be directly related to the mechanical strength under shear and tensile loading. As in experiment, the maximum shear strength $\sigma_{max}$ of a homopolymer interface rises as a power of welding time $t$ and then saturates at the bulk value. Simulations show that $\sigma_{max}$ is proportional to the areal density of interfacial entanglements at short times and saturates when chains have formed 2-3 entanglements across the interface. Enthalpy limits interdiffusion across heteropolymer interfaces, and there is a corresponding reduction in interfacial entanglements and mechanical strength. A minimum loop length of order the entanglement length must diffuse across the interface to form entanglements. Cracks are more complicated because of the presence of short segments produced during fracture. Segments that are too strong to confer bulk strength, but longer than the entanglement length, remain near the interface for long time intervals. This leads to a plateau in strength that is below the bulk value. Crazes form under tensile loading. A low interfacial entanglement density can stabilize craze formation and significantly enhance the fracture energy, but the bulk fracture energy is recovered at about the same time as bulk shear strength. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S14.00003: Submicron flow of polymer solutions: slippage reduction due to confinement Invited Speaker: Hugues Bodiguel Managing flows of polymer solutions through micro- and nanochannels is important for many applications in various fields, including energy conversion processes, nanotechnologies and flows in porous media. Even above the micron scale, violation of the no-slip boundary condition has been recognized to be very important, especially for high molecular weight polymers for which slip length up to tens of micrometers have been reported. As a consequence, pressure-driven flow of polymer solutions in channels of the order of one micrometer and less should be dominated by slippage. Using micro- and nanofabrication methods to control the geometry and fluorescence photobleaching based velocimetry technique, we developed an experimental approach to investigate flows of polymer solutions in slits of thickness as low a 150 nanometers. In non-adsorbing conditions, and for various polymer solutions (polyacrylamide or polystyrene at different concentrations), our results unambiguously show that the resistance to the flow decreases when the channel height decreases below the micron scale. Besides, the apparent slippage of the solutions is characterized macroscopically on similar surfaces. Though slippage can explain qualitatively the effective viscosity reduction, quantitative comparison fails. The reduction of the effective viscosity is significantly smaller than one could have expected knowing the slippage properties. This shows that the slip length is reduced below the micron scale. This effect is stronger when decreasing the polymer length or when lowering the concentration. Interpretations of these results are based on chain migration due to hydrodynamics interactions, which would explain on the one hand the large macroscopic slip lengths and on the other hand their reduction in confinement. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:24AM |
S14.00004: The instabilities of a polymer sheet floating at a fluid interface Invited Speaker: Narayanan Menon The beautiful patterns seen on thin floating polymer sheets have led to a new and broadened understanding of the instabilities of an elastic sheet under tension. I will briefly review this progress, which includes identification of a dimensionless number -- the bendability -- that demarcates regimes in which the wrinkling instability of the sheet may either be successfully described by conventional post-buckling theory or requires an entirely different scheme of calculation in which the bending energy is negligible. This new understanding throws into relief new puzzles associated with the dynamics of the pattern growth, and with the transition from the wrinkled state to a crumpled state. I will also describe the new opportunities opened up by phenomena at high bendability. These include measurements of surface energies and contact angles on a deformable substrate, a new method for studying the modulus and extensional rheology of a thin polymer film, and techniques for modification of surface properties of a fluid interface. I thank NSF DMR 12-0778 and the NSFon Polymers at UMass Amherst DMR 08-20506 My thanks to J. Huang, H. King, K.B. Toga, T.P. Russell for collaborations on the experiments and to B. Davidovitch, E. Cerda and R. Schroll for theoretical collaborations. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 11:00AM |
S14.00005: Nanoscale Confinement in Single-Layer and Multilayer Supported Polymer Films: Effects on Glass Transition Temperature and Surface Capillary Wave Dynamics near the Glass Transition Invited Speaker: John Torkelson A number of studies have reported major differences in the effects of confinement on the glass transition temperature, Tg, of polymers as determined by (pseudo-)thermodynamic methods and on cooperative segmental dynamics as probed by techniques such as dielectric spectroscopy. While substantial Tg-confinement effects are often observed, the effects on cooperative mobility are often muted or absent. Here, we describe studies employing single-layer films and multilayer films of immiscible polymers in which both Tg and dynamics, related to surface capillary wave relaxation characterized by x-ray photon correlation spectroscopy, are strongly affected by confinement and neighboring polymer layer species. Regarding Tg, we show that a key parameter governing the effect of confinement is polymer fragility -- that of the polymer being characterized for Tg in single-layer films and that of the neighboring layer for multilayer films. Similarly, at temperature near Tg, surface capillary wave dynamics of a top layer of a bilayer film can be strongly affected by the neighboring underlayer, with underlayer modulus and confinement itself being important factors governing the dynamics. Both factors are negligible at Tg $+$ 40 K in the case of polystyrene top layers, demonstrating the importance of temperature in tuning the effects of confinement and substrates on dynamics. [Preview Abstract] |
Session S15: Focus Session: Active Soft Matter III - Soft, Self-Propelled Particles
Sponsoring Units: DPOLY DBIO GSNPChair: Juan Aragones, Massachusetts Institute of Technology
Room: 304
Thursday, March 6, 2014 8:00AM - 8:12AM |
S15.00001: Surprises in the nonequilibrium self-organization of active Janus particles Jie Zhang, Jing Yan, Steve Granick As a minimal model of natural active matter and nonequilibrium system, self-propelled colloidal particles are excellent for the study of collective behavior that is common in biological systems spanning from mesoscopic to macroscopic scales, such as swarming, pattern formation and anomalous fluctuations. Here we use induced-charge electrophoretic (ICEP) Janus particles to show experimentally that self-propelled active colloidal particles can display macroscopic phase aggregation despite only particle-particle repulsion in this system. The evolution of the active dense phase grows with time following a power law of 1/2. Strikingly, this agrees with the attachment-detachment coarsening mechanism of phase separation systems at thermodynamic equilibrium. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S15.00002: Self-propelled Janus particles in an asymmetric channel: Effects of rectification and autonomous pumping Vyacheslav R. Misko, Pulak K. Ghosh, Fabio Marchesoni, Franco Nori Using numerical simulations, Brownian transport of self-propelled overdamped microswimmers (i.e., Janus particles) in a two-dimensional periodically compartmentalized asymmetric channel has been investigated for different compartment geometries, boundary collisional dynamics, and particle rotational diffusion [1]. The resulting time-correlated active Brownian motion is subject to rectification in the presence of spatial asymmetry. We demonstrate that ratcheting of Janus particles is much stronger than for ordinary thermal potential ratchets and thus experimentally accessible. In particular, we show that autonomous pumping of a large mixture of passive particles can be induced by just adding a small fraction of self-propelled Janus particles. \\[4pt] [1] Pulak K. Ghosh, Vyacheslav R. Misko, Fabio Marchesoni, and Franco Nori, Phys. Rev. Lett. {\bf 110}, 268301 (2013). [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S15.00003: Territory Covered by $N$ Self-Propelled Brownian Agents in 2 dimensions Francisco J. Sevilla, Luis Alberto G\'omez Nava We consider the problem of the territory covered by $N$ non-interacting self-propelled Brownian agents where self-propulsion is modeled by a non-linear friction term in the Langevin-like equations of motion for each agent. Our study generalizes, to a continuous time and space description, the well known problem of the territory explored by $N$ Random Walkers [1]. Numerical and analytical approaches are presented to exhibit the effects of self-propulsion on the many independent agents exploring two dimensional homogenous regions. Our results may have a wide range of applications in a variaty of non-equilibrium systems. \\[4pt] [1] L Hernan et al. Territory covered by $N$ diffusing particles. Nature 355, 423-426 (1992). [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S15.00004: Self-propelling microswimmer made of a bi-faced hydrogel Alexander Alexeev, Svetoslav V. Nikolov, Peter Yeh We use dissipative particle dynamics to design a new self-propelling microswimmer. Our microswimmer features a simple design and represents an X-shaped gel layer. The gel has two distinct sides or facets: one facet is responsive and swells when an external stimulus is applied, whereas the other facet is passive. We show that when an external stimulus is applied periodically, the swimmer propels itself unidirectionally in a highly viscous fluid. The propulsion is associated with periodic shape changes. When the stimulus is applied, the responsive facet swells, inducing internal stresses in the gel causing both lateral expansion and bending of the microswimmer. When the external stimulus is removed, the responsive layer contracts, and elastic forces cause the microswimmer to straighten and to recover the initial shape. The combination of the sequential expansion, bending, contraction, and straightening produces a time irreversible motion pattern leading to net propulsion at low Reynolds number. We examine how the swimming speed can be enhanced by selecting material properties and geometry of the swimmer. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S15.00005: Phase separation of biphasic mixture of active Janus colloids Jing Yan, Ming Han, Erik Luijten, Steve Granick Recently there is a surge of interest in the phase behavior of active matter in which building blocks display self-propelling motion. Although much has been known from theory and simulation, experimental examples are very rare. Specifically, the epitomic problem of a binary mixture of active matter defies any experiment or theory so far. Here we present an experimental realization of binary mixture of particles, which only acquires activity when they collisionally interact with the opposite kind. We used a system in which the only difference in the two particles is the phase in their cyclic motion, precluding any artifact due to difference in interparticle potential. We observe phenomena strikingly similar to spinodal decomposition of molecular system, in addition to new features due to the nonequilibrium nature of the system. We derived a general, effective Flory-Huggins theory for spinodal decomposition of bicomponent active system, and rationalized the 1/3 power law growth of the domain size in regions where thermodynamic analogy is valid. The system also presents a plethora of nonequilibrium phenomena such as critical fluctuation, lane formation, and dynamic absorbing state in different parameter space. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S15.00006: Optimal hydrodynamic synchronization of colloidal rotors Pietro Cicuta, Jurij Kotar, Nicolas Bruot, Luke Debono, Stuart Box, Stephen Simpson, David Phillips, Simon Hanna Synchronization of driven oscillators is a key in flow generation in artificial and biological systems at the micro-scale. Flow can be driven efficiently by filaments undergoing periodic motion. These filaments are typically colloidal-scale and semi-flexible, and thus have many conformational degrees of freedom and are subject to thermal noise; in the case of biological cilia, they are driven in a complex fashion by internal molecular motors that induce bending. The question of synchronization is thus best addressed by simpler systems, such as individual driven spheres, in which the multiple degrees of freedom are coarse-grained into a few control parameters which can be tuned and understood theoretically, and in which the hydrodynamic interaction is readily described. The system of `rotors' is considered here: spheres are driven along predefined trajectories, with a given force law. In this model it is possible to address quantitatively the conditions for hydrodynamic synchronization. Previous theoretical work pointed to the importance of two factors: modulation of the driving force around the orbit, or the deformability of the trajectory. We show via experiments, numerical simulations and theory that both factors are to be considered, and at play in biological systems. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S15.00007: Mode-coupling theory for the glassy dynamics of self-propelled particles Grzegorz Szamel, Elijah Flenner, Ludovic Berthier We examine glassy dynamics of self-propelled particles. The self-propulsion is modeled as a random force that evolves according to the Ornstein-Uhlenbeck process. Starting from the microscopic description of the dynamics, we derive an effective many-particle diffusion equation describing the time evolution of the probability density of the particles' positions. Next, we assume pair-wise additivity of the effective many-particle interaction and use the standard procedure to derive mode-coupling equations for the time-dependence of density fluctuations. The most important consequence of the self-propulsion is the replacement of the equilibrium structure factor by the self-propulsion-dependent steady state structure factor. To test the theory, we use steady state structure factors obtained from computer simulations of self-propelled particles. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S15.00008: Accelerated Self-Replication under Non-Equilibrium, Periodic Energy Delivery Rui Zhang, Monica Olvera de la Cruz Self-replication is a remarkable phenomenon in nature that has fascinated scientists for decades. In a self-replicating system, the original units are attracted to a template, which induce their binding. In equilibrium, the energy required to disassemble the newly assembled copy from the mother template is supplied by thermal energy. The possibility of optimizing self-replication is explored by controlling the frequency at which energy is supplied to the system. A model system inspired by a class of light switchable colloids is considered where light is used to control the interactions. Conditions under which self-replication can be significantly more effective under non-equilibrium, cyclic energy delivery than under equilibrium constant energy conditions are identified. Optimal self-replication does not require constant energy expenditure. Instead, the proper timing at which energy is delivered to the system is an essential controllable parameter to induce high replication rates. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S15.00009: Dynamics and Jamming for Run-and-Tumble Swimmers in the Presence of Quenched Disorder Cynthia Olson Reichhardt, Charles Reichhardt We consider run-and-tumble swimmers in two dimensions that interact with each other and with a random array of obstacles. In the absence of obstacles, there is a well-defined transition to a cluster or living crystal state for increasing density and run length. We apply a directional drift force to the particles such that they would move at an average drift velocity in the absence of obstacles. In the presence of quenched disorder, the average drift velocity initially increases with increasing run time before reaching a maximum and then decreasing for increasing run time. We correlate the regime where the drift velocity decreases with the onset of the clustering phase. We also show that for increasing density, the obstacles induce an active matter jamming phase, and that the density at which the jamming occurs decreases for increasing run length. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S15.00010: Microscopic walkers in concentrated colloidal monolayers: Oscillators, rollers and spinners Juan Luis Aragones, Joshua Steimel, Alfredo Alexander-Katz We have studied the dynamical behavior of paramagnetic particles under a rotational magnetic field in concentrated passive colloidal monolayers, and their effects on the dynamics and structure of the monolayer. Depending on the direction of the applied rotating magnetic field, paramagnetic particles will rotate parallel or perpendicular to the substrate plane, generating two types of active particles: \textit{rollers}, whose angular momentum is converted to translational motion through the force of friction between the particles and substrate, and \textit{spinners}, which rotate parallel to the substrate. Additionally, \textit{oscillators} can be created from \textit{rollers} by applying oscillating magnetic fields. We have carried out experiments and simulations to analyze the dynamics of these active particles in dense colloidal monolayers. The non-equilibrium nature of these systems confers quite interesting behavior; we observed activity-induced phase separation and local vortex formation around \textit{rollers} and \textit{spinners} due to the fluid media. These vortices interact between them creating patterns and cooperative movements. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S15.00011: Microscopic Tribotactic Walkers Joshua Steimel, Juan Aragones, Alfredo Alexander-Katz The translational motion of a rotating object near a surface is strongly dependent on the friction between the object and the surface. The process of friction is inherently directional and the friction coefficient can be anisotropic even in the absence of a net friction coefficient gradient. This is macroscopically observed in the ordering motif of some animal hair or scales and a microscopic analog can be imagined where the friction coefficient is determined by the strength and density of reversible bonds between a rotating object and the substrate. For high friction coefficients most of the rotational motion is converted into translational motion; conversely for low friction coefficients the object primarily rotates in place. We exploited this property to design and test a new class of motile system that displays tribotaxis, which is the process by which an object detects differences in the local friction coefficient and moves accordingly either to regions of higher or lower friction. These synthetic tribotactic microscopic walkers, composed of a pair of functionalized superparamagnetic beads, detect gradients in the spatial friction coefficient and migrate towards high friction areas when actuated in a random fashion. The effective friction between the walkers and the substrate is controlled by the local density of active receptors in the substrate. The tribotactic walkers also displayed trapping in high friction areas where the density of free receptors is higher. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S15.00012: Swimming bacteria in liquid crystal Andrey Sokolov, Shuang Zhou, Igor Aranson, Oleg Lavrentovich Dynamics of swimming bacteria can be very complex due to the interaction between the bacteria and the fluid, especially when the suspending fluid is non-Newtonian. Placement of swimming bacteria in lyotropic liquid crystal produces a new class of active materials by combining features of two seemingly incompatible constituents: self-propelled live bacteria and ordered liquid crystals. Here we present fundamentally new phenomena caused by the coupling between direction of bacterial swimming, bacteria-triggered flows and director orientations. Locomotion of bacteria may locally reduce the degree of order in liquid crystal or even trigger nematic-isotropic phase transition. Microscopic flows generated by bacterial flagella disturb director orientation. Emerged birefringence patterns allow direct optical observation and quantitative characterization of flagella dynamics. At high concentration of bacteria we observed the emergence of self-organized periodic texture caused by bacteria swimming. Our work sheds new light on self-organization in hybrid bio-mechanical systems and can lead to valuable biomedical applications. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S15.00013: ``Casimir effect'' with active swimmers Dipanjan Ray, Lena Lopatina, Cynthia Olson Reichhardt, Charles Reichhardt In recent years, active matter has increasingly found applications in nanoengineering.\footnote{R. DiLeonardo et al, Proc. Natl. Acad. Sci. U.S.A. 107, 9541 (2010); B. Kaehr and J. B. Shear, Lab on a Chip, 9, 2632 (2009).} Here we show using molecular dynamics simulations that the natural motion of ``run-and-tumble'' bacteria will push together two parallel walls arranged in a Casimir geometry. This effect is robust as long as the wall separation is comparable to or smaller than the bacterial run-length, so that the bacterial motion is not Brownian on the length scale of the walls. The magnitude of the attractive force between the walls exhibits an unusual exponential dependence on the wall separation. The attraction arises from a depleted concentration of bacteria in the region between the plates; this is caused by the tendency of the bacteria to slide along the walls, which breaks time-reversal symmetry and allows a density difference to develop. The same mechanism was used recently to explain bacterial rectification.\footnote{M. B. Wan et al, Phys. Rev. Lett., 101, 018102 (2008); J. Tailleur and M. E. Cates, Europhys. Lett. 86, 60002 (2009).} The inclusion of steric interactions between the bacteria reduces the attraction between the plates but does not eliminate it. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S15.00014: Confined dynamics of non-aligning self-propelled particles in the small box limit Yaouen Fily, Aparna Baskaran, Michael Hagan Recent years have brought the realization that even the simplest of active particle models can exhibit rich behavior. Here we study the confined dynamics of non-aligning self-propelled particles when the size of the confining box is small compared with the distance traveled by a particle before its orientation decorrelates. Using a combination of analytical and computational tools, we characterize the inhomogeneities of the density for a class of box shapes. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S15.00015: Swimming against the flow - An orientational disorder to order transition Chih-kuan Tung, Florencia Ardon, Alyssa G. Fiore, Lian Hu, Susan S. Suarez, Mingming Wu Micro-organisms often need to swim against fluid flows for their survival. In native state, mammalian sperm swim against a flow to reach the egg. Using bull sperm as a model system, we studied the impact of fluid flows on sperm swimming behavior. Interestingly, we find that a directional swimming pattern emerges as the fluid flow rate exceeds a critical value. Using the average directional vector, $<$$S_{x}$$>$ or $<$$S_{y}$$>$ of the sperm head, as an order parameter, and fluid flow rate (along $x$-axis) as a control parameter, we find that $<$$S_{x}$$>$ or $<$$S_{y}$$>$ increases continuously with the increase of flow rate above the onset point, following a power law with an exponent close to 0.5. We will discuss the sources of this transition, and implications in both physics and biology. [Preview Abstract] |
Session S16: Extreme Mechanics: Morigami, Metamaterials, and Elasticity
Sponsoring Units: GSNP DPOLYChair: Gregory Grason, University of Massachusetts-Amherst
Room: 401
Thursday, March 6, 2014 8:00AM - 8:12AM |
S16.00001: Surface patterning by using plastic deformation Atsushi Takei, Lihua Jin, Hiroyuki Fujita We presents a method of surface patterning using plastic deformation. Localized deformation pattern is formed on a surface of a bi-layer system composed of elastic substrate and plastic thin film. With the stretch beyond the yield stress of the film, the film is deformed plastically, and the mismatch of the lengths between the film and the substrate is induced at the release of the stretch. Consequently, the mismatch induces buckling on the surface. With the stretch $\lambda_0$ > 1.5, the deformation of the surface is localized unlike conventional wrinkle patterns. The localized deformations of the bi-layer system both in one-dimension and in two-dimension are analyzed through experiments and simulations. Besides the theoretical aspect, we present that our method achieves functional surfaces such as a hydrophobic surface in a simple manner, and also present that our method can be used for surface patterning of a wide variety of geometry such as a flat plane, fiber and micro -channel. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S16.00002: Programmable Mechanical Metamaterials: BiHolar Networks Bastiaan Florijn, Corentin Coulais, Martin van Hecke We probe the mechanics of BiHolar metamaterials, 2D elastic media with a square lattice of circular holes of two different sizes. Biaxial loading of these BiHolar structures leads to a wealth of mechanical responses, including mechanically switchable hysteresis and memory effects. We show that we can program the mechanical response with the loading force and the hole size ratios [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S16.00003: Dynamics of Geometrically Reconfigurable 1D and 2D Magneto-Elastic Lattices Marshall Schaeffer, Massimo Ruzzene Periodic structures are presented that exhibit multistability due to the nonlinearities of magneto-elastic interactions and structure geometry. The multistability of these structures affords them the ability to adapt their properties though geometric reconfiguration, bringing about changes in stiffness and Poisson's ratio, and introducing anisotropy. These changes in structural properties cause drastic changes in wave propagation, which is of interest for mechanical wave control. The dynamic transformation of one-dimensional (1D) and two-dimensional (2D) lattices between stable states are studied through nonlinear numerical simulations. The analysis is conducted using a lumped mass system of magnetic particles. The structures studied include hexagonal, re-entrant, and kagome lattices. Changes in plane wave propagation properties are predicted by applying Bloch theorem to lattice unit cells with linearized interactions. Results from Bloch analysis are then verified through direct numerical simulations. The propagation of plane waves in these lattices before and after topological changes is compared, and large differences are evident. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S16.00004: Complex ordered patterns in mechanical instability induced geometrically frustrated triangular cellular structures Sung Kang, Sicong Shan, Andrej Kosmrlj, Wim Noorduin, Samuel Shian, James Weaver, David Clarke, Katia Bertoldi Geometrical frustration arises when a local order cannot propagate throughout the space due to geometrical constraints. It plays a major role in many natural and synthetic systems including water ice, spin ice, and metallic glasses. All of these geometrically frustrated systems are degenerate and tend to form disordered ground-state configurations. Here, we report a theoretical and experimental study on the behavior of buckling-induced geometrically frustrated triangular cellular structures. To our surprise, we find that mechanical instabilities induce complex ordered patterns with tunability. For structures with low porosity, an ordered symmetric pattern emerges, which shows striking correlations with the ideal spin solid. In contrast, for high porosity systems, an ordered chiral pattern forms with a new spin configuration. Our analysis using a spin-like model reveals that the connected geometry of the cellular structure plays a crucial role in the formation of ordered states in this system. Since in our study geometrical frustration is induced by a mechanical instability that is scale-independent, our findings can be extended to different materials, stimuli, and length scales, providing a general strategy to study and visualize the physics of frustration. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S16.00005: Negative post-buckling stiffness of meta-beams Corentin Coulais, Johannes Overvelde, Katia Bertoldi, Martin van Hecke We study the mechanical response of meta-materials whose building blocks undergo buckling. Euler elastica theory describes buckling of slender beams and predicts a positive post-buckling stiffness. Here, we demonstrate experimentally, numerically and theoretically that this limit breaks down when beams become non-slender and that the post-buckling stiffness eventually becomes negative. We further show that the poisson ratio can play the role of an additional design parameter and demonstrate experimentally and numerically that the mechanical response of auxetic meta-beams can indeed become unstable. This paves the way to a new generation of elastic switches, that can be triggered by simple uni-axial experiments. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S16.00006: Instability and Wave Propagation in Structured 3D Composites Narges Kaynia, Nicholas X. Fang, Mary C. Boyce Many structured composites found in nature possess undulating and wrinkled interfacial layers that regulate mechanical, chemical, acoustic, adhesive, thermal, electrical and optical functions of the material. This research focused on the complex instability and wrinkling pattern arising in 3D structured composites and the effect of the buckling pattern on the overall structural response. The 3D structured composites consisted of stiffer plates supported by soft matrix on both sides. Compression beyond the critical strain led to complex buckling patterns in the initially straight plates. The motivation of our work is to elaborate the formation of a system of prescribed periodic scatterers (metamaterials) due to buckling, and their effect to interfere wave propagation through the metamaterial structures. Such metamaterials made from elastomers enable large reversible deformation and, as a result, significant changes of the wave propagation properties. We developed analytical and finite element models to capture various aspects of the instability mechanism. Mechanical experiments were designed to further explore the modeling results. The ability to actively alter the 3D composite structure can enable on-demand tunability of many different functions, such as active control of wave propagation to create band-gaps and waveguides. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S16.00007: Capillary Origami with a Twist Timothy Farmer, James Bird Often, when a liquid drop contacts a solid, the droplet deforms to minimize surface energy. For sufficiently thin solids, the solid can instead minimize the combined surface and elastic energy by wrapping around the drop. This mechanism has been used to direct the 3-dimensional self-assembly of 2-dimensional sheets, in a process often referred to as capillary origami. Past experiments have shown that a variety of bending modes can exist for a droplet wetting a thin elastic sheet. However, these studies have only considered interactions between materials with uniform properties and are thus limited to symmetric deformations. In this talk, we present results for asymmetric deformations obtained by controlling these elastocapillary interactions with a pattern of surface chemistries. Our results demonstrate that spontaneous twist can be initiated in a body through a combination of surface chemistry and capillarity. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S16.00008: Shape Selection in Chiral Ribbons - From Seed Pods to Supramolecular Assemblies Hillel Aharoni, Shahaf Armon, Eran Sharon We provide a geometric-mechanical model for calculating equilibrium configurations of chemical systems that self-assemble into chiral ribbon structures. The model is based on incompatible elasticity and uses dimensionless parameters to determine the equilibrium configurations. As such, it provides universal curves for the shape and energy of self-assembled ribbons. We provide quantitative predictions for the twist-to-helical transition, which was observed experimentally in many systems. In addition, we predict bi-stability of wide ribbons and also show how geometrical frustration can cause arrest of ribbon widening. Finally, we show that the model's predictions provide explanations for experimental observations in different chemical systems. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S16.00009: Metric Description of Defects in Amorphous Elastic Materials Michael Moshe, Eran Sharon, Raz Kupferman We suggest a description for dislocations, using a torsion-free Riemannian manifold equipped with a reference metric. This metric expresses the local equilibrium geometry within the material. In this description, dislocations are singularities in the intrinsic curvature structure. The model is not based on a crystalline structure; therefore it can describe dislocations even in amorphous materials. We provide explicit expression for edge dislocation, which is a dipole of curvature. Apparently, higher multipoles of curvature can be used to describe plastic deformations in amorphous materials. The model is supported with experimental results. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S16.00010: ABSTRACT MOVED TO M31.00004 |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S16.00011: Changing shape of elastic shells via electrostatic interactions Vikram Jadhao, Creighton Thomas, Monica Olvera de la Cruz Shape plays a key role in the design of synthetic structures such as biomimetic red blood cells, metallic nanocontainers and colloidal building blocks for self-assembly. It is therefore crucial to enhance our current capabilities to synthesize membranes of desired shapes with precision and provide a simple procedure to induce shape modifications. We show that Coulomb interactions can be used as a tool for designing and manipulating shapes of soft elastic shells at the nanoscale. We investigate the minimal-energy conformations of charged, elastic nanoshells subject to the constraint of fixed total volume for a wide range of electrostatic and elastic parameters. We find that the shape of the shell changes when we decrease the electrolyte concentration in the surrounding environment or increase the total charge on the shell surface. We obtain a variety of smooth shapes that include ellipsoids, discs, and bowls. A discussion on the possible origins of these shapes and related procedures to induce shape deformations is also provided. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S16.00012: Buckling of liquid crystal elastomers in confined geometries Thanh-Son Nguyen, Andrew Konya, Robin Selinger, Jonathan Selinger Liquid crystal elastomers (LCEs) are materials that combine the orientational order of liquid crystals with the elastic properties of polymer networks. Whenever the liquid-crystal order changes (by heating, cooling, or other stimuli), the shape of the polymer network changes. If the liquid-crystal director is nonuniform, then the polymer network is generally frustrated---i.e. the local director favors a certain local strain, but these strains are incompatible; they do not fit together to fill up space. As a result, the shape can become very complex, and it can only be calculated by numerical methods. In order to understand the phenomenon of frustration in LCEs, we consider simple systems where the liquid-crystal director is uniform but frustration is introduced by confinement, so that the sample cannot extend along the director. As the induced strain passes a critical threshold, the system releases part of the frustration by buckling. The simplicity of the system allows us to evaluate several properties of the buckling process analytically, including the threshold strain and the instability wavelength. The analytic results are compared with numerical finite-element simulations of the same geometries, and with related studies of other elastic sheets. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S16.00013: Mechanical properties of warped membranes Andrej Kosmrlj, Kechao Xiao, James C. Weaver, Joost J. Vlassak, David R. Nelson We explore how a frozen background metric affects the mechanical properties of solid planar membranes at zero temperature. Our focus is a special class of ``warped membranes'' with a preferred random height profile characterized by random Gaussian variables $h(q)$ in Fourier space with zero mean and variance $< |h(q)|^2 > \sim q^{-m}$. Using statistical physics tools to treat this quenched random disorder, we find that in the linear response regime, similar to thermally fluctuating polymerized membranes, an increasing scale-dependent effective bending rigidity, while the Young and the shear moduli are reduced. Compared to flat plates of the same thickness $t$, the bending rigidity of warped membranes is increased by a factor $\sim h_v/t$ while the in-plane elastic moduli are reduced by $\sim t/h_v$, where $h_v =\sqrt{ < |h(x)|^2 > }$ describes the frozen height fluctuations. Interestingly, $h_v$ is system size dependent for warped membranes characterized with $m>2$. Numerical results show good agreement with theoretical predictions, which are now being tested experimentally, where warped membranes are prepared with 3D printers. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S16.00014: The Influence of Order and Disorder on Buckling 2D granular layers Andrew B. Croll, Bekele Gurmessa, Antoinette Tordisillas, David Carey, Jingyu Shi The buckling of thin films has recently received considerable attention in both the materials and the continuum elasticity communities. To the former, elastic instabilities form a platform for the mechanical measurement of material properties under increasing degrees of confinement. To the latter, instabilities represent a testing ground for advanced elastic theory. Buckling is also of considerable importance in the evolution of granular systems, which often show deformations that resemble those of continua. Previously, we documented several differences between continuum theory and discrete elasticity in a discrete model of a thin film experimentally constructed from a well ordered (hexagonally packed) layer of colloid scale particles. Here we consider how the structure of the 2D layer influences the buckling process. In particular, we examine the details of how a complex, disordered (glassy) 2D layer resting on soft foundations responds to in-plane compressive stress. We show how the fundamental buckling lengthscale remains identical to that of ordered layers, despite considerable heterogeneity in the motion of the particles. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S16.00015: The Buckling-Fracture Transition in Non-Euclidean Plates Eran Sharon, Yael Klein Non-Euclidean Plates (NEP) are thin elastic plates, in which lateral equilibrium distances of the material are described by a non-Euclidean reference metric. Previous studies showed that such plates buckle spontaneously -- while free of external constraints. In the thin limit the geometry of the buckled configurations approaches the reference metric. In this talk we show the existence of a new, \textit{buckling to fracture}, transition in these plates. Depending on the parameters of the system, NEP might undergo fracture instability instead, or together with, buckling instability. We propose the scaling of this transition and verify it experimentally. Our observations lead us to propose an intrinsic geometrical description of fracture, which is consistent with, but different from Linear Elastic Fracture Mechanics. [Preview Abstract] |
Session S17: Avalanches and Rearrangements during Shear
Chair: Karen Daniels, North Carolina State UniversityRoom: 402
Thursday, March 6, 2014 8:00AM - 8:12AM |
S17.00001: Slip avalanches in granular systems under shear Dmitry Denisov, Karin Dahmen, Peter Schall We study the evolution of slowly sheared granular systems deforming via discrete strain bursts (slips). The granular sample consisting of 10$^{\mathrm{5}}$ hard spheres (mm-size) is subjected to applied shear and studied with the combination of two techniques -- precise stress-strain measurements and laser sheet imaging. Fluctuations in the stress-strain profile correspond to internal slip avalanches leading abrupt reconstructions in the system due to the shear. The magnitude of the fluctuations is taken as the size of the avalanche events. The power-law distribution of the slip sizes signifies the existence of the dynamically critical state in granular samples under the shear. Laser sheet imaging allows us to visualize each individual slip event, estimate its spatial distribution together with local strain change and connect it to the global fluctuation in the stress-strain curve. Such unique combination of the techniques and analysis lead us to comprehensive understanding of slip avalanches. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S17.00002: On the stability of amorphous solid Jie Lin, Alaa Saade, Edan Lerner, Alberto Rosso, Matthieu Wyart The plasticity of amorphous material occurs via local plastic rearrangements, shear transformation zones(STZ). The elastic coupling between STZs can generate large-scale avalanches of plastic events. We study the stability condition of amorphous solid toward extensive avalanches. We argue that stability is controlled by the distribution $P(x)$ of the local stress increase $x$ that would lead to an instability. In particular stability requres that $P(x)\sim x^{\theta}$ where $\theta$ satisfies a lower bound. To investigate this, we use a elasto-plastic model based on two ingredients: local plastic events above microscopic stress, and the non-local elastic stress release generated by these plastic events. For a class of models of lond range interaction, $\theta$ is found to lie near saturation. For quadrupole interaction, the model yields $\theta\approx 0.6$ in 2D, and $\theta\approx 0.4$ in 3D. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S17.00003: Laboratory Earthquakes: Granular Friction and Scaling Robert Ecke, Drew Geller, Sergiy Gerashchenko, Scott Backhaus Geological processes drive shear motion between tectonic plates over 10-100 km. The rupture gap, of order meters, contains granular matter - fault gouge - produced by the grinding motion of the plates over millennia. The complex behavior of natural earthquakes and the difficulty in making {\it in situ} measurements, has led to laboratory experiments that allow more control. We describe a laboratory experiment to model the physics of earthquakes that involves the interaction of continuum and granular behavior around a fault. Two photo-elastic plates confine about 3000 bi-disperse rods in a gap with a length-to-width ratio 50. The plates are held rigidly along their outer edges with one held fixed while the other is driven at constant speed at strain rates of $10^{-5}$/s. We measure strains from the motions of small spheres on the plate surface, stresses from plate photo-elastic response, and granular motion using particle tracking. We determine the dependence of the friction and the moment distribution of the system on the normal force. The moment distribution scales with a power law close to -1.5. There is an increasing probability for large events with a non-random recurrence time at higher normal force. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S17.00004: Stress avalanches in sheared granular materials Somayeh Farhadi, Karin Dahmen Granular systems, subject to external shear stress, deform plastically at the yield point, where stress is released in the form of avalanches. The sizes(strength) of stress avalanches show a broad range in magnitude and demonstrate scaling properties. Here, we perform MD simulation to study stress avalanches of granular materials. The systems are dense packings of both 2D and 3D Hertzian spheres (close to their jamming points). Both micro-structure of failure as well as global stress are measured throughout gradual shear steps. Finally, we compare mean-field predictions of an existing model of failure, with our simulation data. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S17.00005: Record-Breaking Avalanches in Nonlinear Threshold Systems Robert Shcherbakov Record-breaking avalanches generated by the dynamics of several driven non-linear threshold models are studied. Such systems are characterized by intermittent behaviour, where slow build-up of energy is punctuated by an abrupt release of energy through avalanche events which usually follow scale invariant statistics. From the simulations of these systems it is possible to extract sequences of record-breaking avalanches, where each subsequent record-breaking event is larger in magnitude than all previous events. In the present work, several cellular automata are analyzed among them the sandpile model, Manna model, Olami-Feder-Christensen (OFC) model, and the forest-fire model to investigate the record-breaking statistics of model avalanches which exhibit temporal and spatial correlations (Shcherbakov et al., PRE 87, 2013, 052811). Several statistical measures of record-breaking events are derived analytically and confirmed through numerical simulations. It is found that the statistics of record-breaking avalanches for the above cellular automata exhibit behaviour different from that observed for i.i.d. random variables which in turn can be used to characterize complex their spatio-temporal dynamics. The most pronounced deviations are observed in the case of the OFC model. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S17.00006: Scaling Analysis And Tuning Parameters For Avalanches On A Slowly-Driven Conical Bead Pile Susan Lehman, Lilianna Christman, Paroma Palchoudhuri, D.T. Jacobs We report the results of our investigation of the dynamic behavior of a 3D conical beadpile composed of 3~mm steel beads. Beads are added to the pile by dropping them onto the apex one at a time; avalanches are measured through changes in pile mass. We have previously shown that the avalanche size distribution generally follows a power law relation for beads dropped onto the pile apex from a low drop height; for higher drop heights or beads dropped over a larger region, the distribution deviates from a power law due to a reduction in the number of larger avalanches. We are now tuning the critical behavior of the system through the addition of cohesion from a uniform magnetic field, and we find an increase in the probability of very large avalanches and decreases in the mid-size avalanches. Similar distributions have been observed previously by other researchers in conical piles of sand, suggesting a possibility that cohesion may have been a factor. All our distributions without cohesion show universality by collapsing onto a common curve in a scaling analysis; so far no scaling has been found in the system with cohesion. The distribution of the time between avalanche events of various size has also been analyzed and shown to depend on both drop height and cohesion strength. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S17.00007: Unexplained voltage signals from granular materials Troy Shinbrot, Karen Daniels, Chris Marone, Theo Tsiu Powders and grains exhibit unpredictable jamming-to-flow transitions that manifest themselves on geophysical scales in catastrophic slip events such as landslides and earthquakes, and on laboratory/industrial scales in profound processing difficulties. Over the past few years, insight into these transitions has been provided by new evidence that slip events may accompanied, or even preceded, by electrical effects. In this talk, we describe three independent experiments using organic powders, polymeric disks, and glass particles, all of which generate unexplained voltage signals. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S17.00008: Listening to Anticracks in Laboratory Earthquakes under the True Triaxial 3D Remote Stress Field H.O. Ghaffari, M.H.B. Nasseri, R. Paul Young A real deformation in the earth produces 3D stress as well as displacements on polymodal fault sets. Here we present the results of the multi-stationary acoustic waveforms from the orthorhombic faulting patterns in sandstone under 3D-polyaxial stress fields. Based on the analysis of over 104 rupture fronts and using the functional acoustic network theory, we show that generally waveforms from true triaxial tests carry shorter rapid slip phase (8-10 $\mu$s), implying the controlling role of the intermediate remote-stress field on mesoscopic faulting which is explained with inducing irregular micro-cracking. Furthermore, we extract failure criterion in network's phase space per each occurred failure of heterogeneity/asperity, confirming the macroscale failure measures in true triaxial tests. Our results suggest that boundary conditions can drastically change the regime of ruptures in laboratory earthquakes by inducing a sort of anti-crack like ruptures. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S17.00009: Unjamming and jamming transitions of granular avalanches Jie Zhang, Ziwei Wang Study of the jamming transitions of granular materials has become an active field of research in recent years. A closely related inverse process is the unjamming transition, where granular systems may suddenly lose rigidity and start to flow freely. Understanding such a process is of crucial implication towards the understanding of natural disasters such as snow avalanches, landslides and earthquakes. Recent work by Banigan and colleagues (Nature Physics 2013) has provided a new perspective in the study of unjamming and jamming transitions by applying nonlinear dynamical methods. To test their proposition experimentally, we have designed a rotating drum filled with bidisperse photo-elastic disks to create particle avalanches. In unjamming transition, Lyapunov vector and velocity fields are indeed strongly correlated in spatial domain, whereas in jamming transition no such a strong correlation is observed. The Lyapunov exponents are positive in unjamming transition and negative in jamming transition. In addition, the total stress variation, kinetic energy, and non-affine motion of particles all show strong correlations in the time domain during avalanches. Their spatial correlations have also been analyzed. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S17.00010: Correlations in Particle Displacements and Plastic Deformation in Sheared Amorphous Solids Mark O. Robbins, K. Michael Salerno We present results from molecular-dynamics simulations of model disordered solids under quasi-static, steady-state shear in two and three dimensions. Plastic deformation occurs through intermittent ``avalanches'' of local rearrangements. As in other slowly-driven systems from magnets to geologic faults, avalanches exhibit critical scaling behavior. Particle motion during avalanche events leads to local yielding and plastic strain. Local strain statistics for individual avalanche events will be discussed. Over many avalanche events long-range spatial correlations form in the particle displacement and strain fields. These correlations are seen most visibly in the power spectra of local measures of particle motion, S(q). One result of these correlations is system-size dependent effective particle ``diffusion,'' with particle mean-square displacement that is linear in the applied strain. Results for three different particle damping regimes will be compared. Results from two and three dimensions will also be presented and compared. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S17.00011: Structure and deformation in compressed disordered packings Jennifer Rieser, Wenbin Li, Ju Li, Jerry Gollub, Douglas Durian How the local structural configuration influences large-scale deformation in disordered materials is not known. We explore this relationship in two-dimensional disordered granular packings under uniaxial compression. The two-dimensionality of the system allows for direct observation of all particle dynamics during the compression of a pillar. The grains can be cohesive, with an attraction governed by tunable capillary forces that are induced through an interstitial fluid. Topological quantities derived from a generalized Voronoi diagram as well as the resulting triangulation are used to characterize local structure within the packing. Dynamics are characterized by local deformations to the triangulation as well as the local non-affine motion. We investigate correlations between these structural and dynamical measures, and we observe that holes tend to develop in regions of high strain. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S17.00012: Shear deformations in foam: will it T1? Merlijn van Deen, Vera Janssen, Alexander Siemens, Martin van Hecke When dry foams are sheared, energy is dissipated in localized, plastic, T1 events, where particles swap neighbors. In wet foams, the picture is different. We have experimentally probed rearrangements in wet packings of bubbles in a bi-axial shear cell. We show that the volume fractions at which T1's dominate is limited, and show the rich behavior that ensues closer to the jamming transition. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S17.00013: Avalanches of Rearrangements in 2D Emulsion Hopper Flow Xia Hong, Kenneth Desmond, Dandan Chen, Eric Weeks We conduct experiments with a quasi-two-dimensional binary emulsion flowing through a hopper. Our samples are oil-in-water emulsions confined between two close-spaced parallel plates, so that the droplets are deformed into pancake shapes. In this system, there is only viscous friction and no static friction between droplets. By imaging the droplets during flow, we observed T1 events, which are topological rearrangement events when droplets exchange neighbors. Avalanche like flow behavior has been found by controlling the flow velocity and area fraction. We study the statistics of rearrangements as a function of the control parameters and observe a fairly smooth transition from avalanche-dominated flow to continuous flow. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S17.00014: Structural perturbations in granular beds due to shear-flow-driven, fluvial erosion Julia Salevan, Mark Shattuck, Corey O'Hern, Nicholas Ouellette The complex interactions between granular media and flowing fluid play a principal role in shaping landscapes via erosion. Despite a large body of work in granular materials and in large scale topographical changes of granular beds due to fluid flow, the detailed physical mechanisms that underlie the coupling between hydrodynamic shear and internal rearrangement remain poorly understood. To address these questions, we perform experimental studies of shear flow across granular beds and monitor changes in the structural properties of the granular packing. We pay particular attention to the extent of perturbations of the packing as a function of depth within the bed and examine the effects of varied fluid flow regimes and time scales on bed rearrangements. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S17.00015: Synthesis and Mechanical Response of Colloidal Micropillars Daniel Strickland, Lei Zhang, Yun-Ru Huang, Daeyeon Lee, Daniel Gianola We present a new approach for studying the uniaxial compressive behavior of colloidal micropillars as a function of structural order/disorder, pillar and colloid dimension, and interparticle interaction. By varying the polydispersity of the particles, ordered packing may be promoted or suppressed, leading to the formation of crystalline or amorphous pillars. Pillars composed of nanometer scale particles develop cracks during drying, while pillars composed of micron scale particles dry crack-free. We subject the pillars, with diameters ranging from 300$\mu $m to 1mm, to uniaxial compression experiments using a custom-built micromechanical testing apparatus. In pillars with pre-existing cracks, compression activates the macroscopic defects, leading to fracture and stochastic mechanical response as a result of the flaw distribution. Pillars that dry crack-free fail by shear bands that develop near the punch face. While macroscopically identical, pillar-to-pillar mechanical response varies significantly. We rationalize the difference in behavior as a result of varying structure and environmental conditions. Specifically, the level of atmospheric humidity significantly affects particle-particle cohesion and friction, resulting in dramatically different mechanical response. We discuss the results in the context of underlying particle rearrangements leading to mesoscopic shear localization and examine comparisons with atomic disordered systems such as metallic glasses. [Preview Abstract] |
Session S18: Disordered and Glassy Systems II
Sponsoring Units: DCMP GSNPChair: Sindee Simon, Texas Tech University
Room: 403
Thursday, March 6, 2014 8:00AM - 8:12AM |
S18.00001: Density of states of Frenkel excitons in strongly disordered two-dimensional systems Robert Siemann, Abdelkrim Boukahil We present the calculation of the density of states of Frenkel excitons in strongly disordered two-dimensional systems. A random distribution of transition frequencies with variance $\sigma ^{\mathrm{2}}$ characterizes the disorder. The Coherent Potential Approximation (CPA) calculations show a strong dependence of the density of states (DOS) on the disorder parameter $\sigma $. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S18.00002: Effects of disorder on the optical properties of Frenkel excitons Abdelkrim Boukahil, Robert Siemann The Coherent Potential Approximation (CPA) is used to study the effects of disorder on the absorption line shapes of Frenkel excitons in one-, two-, and three-dimensional systems. A Gaussian distribution of transition frequencies with rms width $\sigma $ was used. Several values of the disorder parameter $\sigma $. The CPA results show that short tails on the high-energy side of the peaks are $\sigma $ dependent, and long tails on the low-energy side of the peaks are independent of the disorder parameter $\sigma $. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S18.00003: Depolarized Photon Correlation Spectroscopic Study of the Glass-Forming Liquid Cumene at Very High Pressures Kevin Lyon, Tim Ransom, William Oliver In recent years full-spectrum analysis of light-scattering data has been utilized to explore the liquid-glass transition at variable temperatures and ambient pressure. We have developed methods for doing depolarized photon correlation spectroscopy (PCS) in the diamond anvil cell in order to probe directly the structural relaxation time of glass-forming liquids at very high pressures. Here we present results for liquid cumene at 25~C between 1 bar and pressures approaching the room-temperature glass transition at 2.1~GPa. Data along higher-temperature isotherms will also be presented. Methods for minimizing any undesired heterodyne component in the collected light as well as the use of the longitudinal modes of the Brillouin spectrum to aid in the acquisition and spatial filtering of the scattered light will be discussed. Intensity-intensity correlation data were found to be well represented by the KWW equation with a nearly constant stretching parameter of ${\rm g}=$ 0.66 for 25~C. Furthermore, the relaxation time as a function of pressure is described will using a modified VTF expression: (P)$=_{\mathrm{0}}$exp\textbraceleft DP/(P$_{\mathrm{0}}$-P)\textbraceright , with values of $_{\mathrm{0}} =$ 11.9 ps, D $=$ 18.6, and P$_{\mathrm{0}} =$ 3.4 GPa at T $=$ 25 $^{\circ}$C. Thus, (P) has been obtained at 25 $^{\circ}$C for Cumene over seven decades from about a microsecond to several seconds and is found to be in excellent agreement with previously determined values for the alpha relaxation at lower pressures obtained from Brillouin data [G. Li, \textit{et al.}, Phys. Rev. Lett. \textbf{74}, 2280 (1995)]. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S18.00004: Glass-Forming Liquids under Extreme Conditions William Oliver, Kevin Lyon, Tim Ransom The nature of glass-forming liquids and the glass transition remain incompletely understood despite intense effort over many years. Though important contributions to our understanding of viscous liquids and glasses at high pressure have been made during this time, the overwhelming majority of studies have consisted of temperature-dependent studies at 1 bar. Recent experimental advances have begun to change this situation in important new ways [see, e.g., A.A. Pronin \textit{et al.}, JETP Letters \textbf{92}, 479 (2010)]. Glass-forming liquids can be exposed to record high pressures of several GPa with the diamond anvil cell (DAC); however, sample volumes are tiny (nanoliters) and the DAC is most amenable to optical techniques. Recent methods for probing glass-forming systems in the DAC will be highlighted in this presentation including direct measurement of Tg(P), the combination of depolarized Brillouin and photon correlation spectroscopies to measure the alpha relaxation time as a function of pressure from picoseconds to many seconds, and lastly, in the spirit of recent temperature dependent studies at one bar [see, e.g., Zhang \textit{et al}., Phys. Rev. E \textbf{70}, 011502 (2004)], we can now carry out full spectrum analyses in which depolarized backscattering with forward scattering spectra are combined in a self-consistent way to determine the significance of things such as rotation-translation coupling. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S18.00005: High Pressure Light Scattering Study of Relaxation in the Glass Former Cumene Tim Ransom, Kevin Lyon, William Oliver To understand relaxation dynamics in glassy systems, a light scattering study on Cumene has been carried out in a diamond anvil cell (DAC) at pressures from 0.2 GPa to 2.5 GPa isothermally at 75 $^{\circ}$C. Polarized and depolarized spectra were taken in both near-backscattering and equal-angle 60$^{\circ}$ forward-scattering geometries at several free spectral ranges from 0.5 GHz to 300 GHz. Depolarized backscattering spectra are converted into susceptibility featuring the evolution of the $\alpha $-relaxation peak, yielding structural relaxation times $\tau _{\alpha}$ from 10 ps to 1 ns. We have also developed photon correlation spectroscopy (PCS) in a DAC, giving $\tau_{\alpha}$ from $\sim$ 1 $\mu$s to 1 s. We fit $\tau_{\alpha}$ over these many decades with a modified VFTH equation $\tau _{\alpha} = \tau_{0}$exp[DP/(P$_{0}$-P)] giving parameters $\tau _{0}=$ 9.2 ps, D$=$ 17.5, and P$_{\mathrm{0}}=$ 4.5 GPa at 75$^{\circ}$C. After the $\alpha $-relaxation peak moves into lower frequencies (P $\sim$ 1 GPa), we observe the emergence of the $\beta $-relaxation minimum region. We fit the $\beta $-minimum to a power law scaling form $\chi $''($\omega )=$b($\omega $/$\omega _{\mathrm{min}}$)$^a$ $+$ a($\omega_{\mathrm{min}}$/$\omega$)$^b$. Polarized backscattering and forward scattering gives frequency shift $\omega_{\mathrm{B}}$ and linewidth $\Gamma _{\mathrm{B}}$ values of the longitudinal acoustic modes at two different q. We observe that the usual peak in linewidths does not coincide with $\omega_{\mathrm{B}}\tau_{alpha} \approx $ 1, indicating that the longitudinal acoustic modes do not couple with structural relaxation. Tansverse acoustic modes also appear in the depolarized forward scattering spectra. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S18.00006: Two-State Dynamics in Liquids and Glass on a Picosecond Timescale Marcus Cicerone, Miaochan Zhi, Juan dePablo We present results from neutron scattering, atomistic MD simulations, and optical Kerr effect spectroscopy (OKE) to demonstrate that liquids and glasses exhibit two dynamic states at short times. We provide evidence that the two dynamic states arise from molecules that are either tightly caged or loosely caged on a ps timescale. This heterogeneous motion is associated with hopping at low temperature, but the two-state scenario persists well above the melting point, and also contributes significantly to transport at the higher temperatures. Using concepts derived from this model we are able to quantitatively predict self-diffusion of small molecule glassformers well into the supercooled regime. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S18.00007: Fast Scanning Calorimetry Studies of Glassy Films of Toluene Deepanjan Bhattacharya, Vladislav Sadtchenko Fast scanning calorimetry was used to prepare and characterize micron thick vapor-deposited and liquid-cooled films of toluene on a thin filament. At temperatures above and below standard glass transition temperature (Tg) of toluene, the vapor-deposited films were prepared by physical vapor deposition at deposition rates of approximately 15 nm/s and the liquid-cooled films were prepared by quenching of liquid, 10 K above Tg, at a rate of approximately 5 K/s. It was found that vapor-deposited films have lower enthalpy and higher kinetic stability than liquid-cooled films even at temperatures approaching slightly above Tg. The most stable vapor-deposited films were prepared at temperatures 5 K below Tg. The kinetic stability of this film increased by about 4 K when the deposition rate was lowered from 40 nm/s to about 0.5 nm/s. A negligible change in the kinetic stability of this film was observed as long as the thickness was above 200- 300 nm range. The nature of the substrate had negligible impact on the phase's kinetic stability. We will report the results of our FSC studies and compare them with those of other contemporary findings. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S18.00008: Thermophysical and Rheological Properties of Imidazolium-Based Ionic Liquids: The Effect of Aliphatic versus Aromatic Functionality Ran Tao, Lianjie Xue, George Tamas, Edward Quitevis, Sindee Simon As a material class, ionic liquids possess attractive properties and have a wide range of potential uses. In this work, a series of imidazolium-based ionic liquids with the same carbon number varying from aliphatic to aromatic functionalities are investigated. The effects of cation symmetry and larger aromatic polycyclic functionality are studied. The thermal properties, including the glass transition temperature, melting temperature, and decomposition temperature, are characterized, and the density and the ionic conductivity are measured as a function of temperature. Rheological studies are performed using both steady-state and dynamic shear modes. The Cox-Merz relationship between the steady shear viscosity and the dynamic viscosity is examined. The temperature dependence of viscosity is described by the Vogel-Fulcher-Tammann equation and the dynamic fragility is calculated for each ionic liquid and compared to the fragility obtained from calorimetry. Master curves of dynamic shear responses are also constructed and will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S18.00009: Atomistic study of rejuvenation of amorphous metals via thermal loading Masato Wakeda, Junji Saida, Ju Li, Shigenobu Ogata Rejuvenation is the structural excitation of amorphous system accompanied by enthalpy and free volume rise, and it is one of the promising approaches for improving the deformability of amorphous metals, which usually exhibit macroscopic brittle fracture. However, methods for controlling the rejuvenation and feasibility conditions of the rejuvenation remain unclear because of few experimental evidences and lack of clear knowledge of nonequilibrium glass properties. In this study, we investigate a method to control the rejuvenation through thermal loading and the feasibility conditions of the thermal rejuvenation. Using molecular dynamics techniques, we constructed an amorphous alloy model via melt-quenching process, and then conducted annealing and quenching processes. It is observed that thermal rejuvenation occurs via a thermal loading process of annealing at temperatures above a critical value and subsequent quenching at a cooling rate that is higher than that of the initial melt-quenching process. The level of rejuvenation increases with increasing annealing temperature and quenching rate. We discuss the background nature of rejuvenation and potential application of thermal rejuvenation to control the mechanical properties of amorphous metals. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S18.00010: Cooperativity in glassy dynamics investigated by higher-harmonic dielectric spectroscopy Thomas Bauer, Peter Lunkenheimer, Alois Loidl In recent years, due to experimental advances initiated by hole burning experiments, nonlinear dielectric spectroscopy has gained increasing interest in the field of glass-forming matter. For example, refining the technique of high-field permittivity measurements, we found a surprising lack of nonlinearity in the so-called excess wing region, that could not be accessed by this method before [1]. In the present contribution, we report new, detailed measurements of the third-order nonlinear dielectric susceptibility $\chi_{3}$ of four glass-forming liquids for a broad temperature range [2]. We find a significant hump in $\chi_{3}(\nu )$, from which we deduce the number of correlated molecules N$_{corr}$. We detect a continuous increase of N$_{corr}$ on approaching the glass-transition temperature. Comparing these results with the temperature-dependent apparent energy barriers in these systems, our experiments finally prove the old notion that intermolecular correlations of glassy systems are responsible for the non-canonical temperature development of glassy dynamics. [1] Th. Bauer, P. Lunkenheimer, S. Kastner, A. Loidl, Phys. Rev. Lett. \textbf{110}, 107603 (2013) [2] Th. Bauer, P. Lunkenheimer, A. Loidl, Phys. Rev. Lett., in press (arXiv:1306.4630) [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S18.00011: Plasticity of amorphous carbon Julian von Lautz, Michael Moseler, Lars Pastewka We use molecular dynamics simulations to probe the plastic response of representative bulk volumes of amorphous carbon at densities from 2.0 g cm$^{\mathrm{-3}}$ to 3.3 g cm$^{\mathrm{-3}}$ in simple and triaxial shear. After an initial elastic response the samples yield with only little strain hardening or softening. Individual plastic events in this network forming glass are strikingly similar to those observed for bulk metallic glasses: We find that plasticity is carried by fundamental rearrangements of regions of around 100 atoms, the shear transformation zone. In the simple shear geometry, those events coalesce to form a shear-band on longer time scales. During plastic deformation, the material changes its hybridization by transforming sp$^{\mathrm{3}}$ carbon atoms to sp$^{\mathrm{2}}$. We provide evidence that this transformation of the structural state occurs before the material yields, hence weakening the material. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S18.00012: Anomalous Temperature Dependence and Isotope Effect in the Structural Dynamics of Deeply Supercooled Water Alexander L. Agapov, Alexander I. Kolesnikov, Vladimir N. Novikov, Ranko Richert, Alexei P. Sokolov Despite simple chemical structure and its importance in our life, water remains one of the most puzzling liquids.\footnote{Angell, C. A. \textit{Science} 319, 582-587 (2008).} Combining neutron scattering and dielectric spectroscopy we show that quantum fluctuations have a pronounced effect on dynamics in deeply supercooled water. Dielectric measurements revealed that water has an anomalously weak temperature dependence of structural dynamics close to $T_{g} \approx $ 136K with unphysical low fragility index $m \approx $ 14. Additionally, we observed an anomalously large isotope shift of $T_{g}$ between H$_{2}$O and D$_{2}$O, $\Delta T_{g}$ $\sim$ 8-10K, in a strong contrast to the isotope effect on $T_{g}$ observed in other hydrogen bonding liquids. The observed anomalous behavior is consistent with the recently suggested idea of quantum zero-point vibrations affecting dynamics of supercooled water.\footnote{Novikov, V. N. {\&} Sokolov, A. P. \textit{Phys. Rev. Lett.} 110, 065701 (2013).} We speculate that the apparent fragile-to-strong crossover in dynamics of water can be ascribed to quantum effects dominating structural relaxation at low temperatures. These results have significant implications for our understanding of water dynamics and its peculiar behavior at low temperatures. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S18.00013: Liquid-liquid coexistence and crystallization in supercooled ST2 water Fausto Martelli, Jeremy Palmer, Pablo Debenedetti, Roberto Car We have computed the free energy landscape of ST2 water in the supercooled regime (228.6 K and 2.4 kbar) using several state-of-the-art computational techniques, including umbrella sampling and metadynamics. Such results conclusively demonstrate coexistence between two liquid phases, a high-density liquid (HDL) and a low-density liquid (HDL), which are metastable with respect to cubic ice. We show that the three phases have distinct structural features characterized by the local structure index and ring statistics. We also find that ice nucleation, should it occur, does so from the low-density liquid. Interestingly, we find that the number of 6-member rings increases monotonically along the path from HDL to LDL, while non-monotonic behavior is observed near the saddle point along the LDL-ice Ic path. This behavior indicates a complex re-arrangement of the H-bond network, followed by progressive crystallization. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S18.00014: Coherent neutron scattering and collective dynamics on mesoscale Vladimir Novikov, Kenneth Schweizer, Alexei Sokolov By combining and extending, a variety of theoretical concepts for the dynamics of liquids in the supercooled regime, we formulate a simple analytic model for the temperature and wavevector dependent collective density fluctuation relaxation time that is measurable using coherent dynamic neutron scattering. Comparison with experiments on the ionic glass-forming liquid CKN in the lightly supercooled regime suggests the model captures the key physics in both the local cage and mesoscopic regimes, including the unusual wavevector dependence of the collective structural relaxation time. The model is consistent with the idea that the decoupling between diffusion and viscosity is reflected in a different temperature dependence of the collective relaxation time at intermediate wavevectors and near the main (cage) peak of the static structure factor. More generally, our analysis provides support for the ideas that decoupling information and growing dynamic length scales can be at least qualitatively deduced by analyzing the collective relaxation time as a function of temperature and wavevector, and that there is a strong link between dynamic heterogeneity phenomena at the single and many particle level. Though very simple, the model can be applied to other systems, such as molecular liquids. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S18.00015: A genetic algorithm to determine metastable MS1 phase of the Al-Sm system Zhuo Ye, Feng Zhang, Yang Sun, Manh Cuong Nguyen, Mikhail Mendelev, Matthew Kramer, Cai-Zhuang Wang, Kai-Ming Ho An efficient genetic algorithm (GA) was used to determine a metastable Al$_{60}$Sm$_{11}$ phase [termed MS1 in Mater. Sci. Eng. A179--A180, 600 (1994)] that evolves during rapid solidification of an amorphous melt-spun Al-10{\%}Sm alloy. The MS1 phase is of particular interest as it is the 1st observed phase during devitrification and is believed to possess a strong connection to the undercooled liquids. It also presents a severe challenge to theoretical crystal structure prediction methods since it 1) has a big unit cell with a $\sim$1.4 nm, 2) is metastable and not necessarily the ground state, and 3) contains site-occupancy and anti-site defects. A GA combined with experimental characterization of phase transitions and Rietveld refinements provides the necessary identification of the MS1 crystal structure. Calculated X-ray diffraction patterns of the MS1 phase match perfectly with experiments. Interestingly, the MS1 phase shares the same motif as undercooled Al-10{\%}Sm liquids. The topological connection between undercooled liquid and crystal structures is worth further investigation, to understand how the topological order in the starting amorphous phase correlates with phase selection during devitrification. [Preview Abstract] |
Session S19: Focus Session: Thin Films of Block Copolymers and Hybrid Materials I - Solvent Vapor Annealing
Sponsoring Units: DPOLYChair: Mark Stoykovich, Chemical and Biological Engineering, University of Colorado
Room: 404
Thursday, March 6, 2014 8:00AM - 8:12AM |
S19.00001: Self-assembly kinetics in Symmetric Diblock Copolymer Thin Films during solvent assisted thermal treatments Michele Perego, Federico Ferrarese Lupi, Monica Ceresoli, Tommaso J. Giammaria, Gabriele Seguini, Diego Antonioli, Valentina Gianotti, Katia Sparnacci, Michele Laus, Luca Boarino Block copolymer (BCP) microphase separation and ordering by thermal annealing is often a challenge because of its slow kinetic. Towards the objective of rapid processing and accessing desired nanostructures, in this study we propose and discuss an alternative approach based on the use of a Rapid Thermal Processing (RTP) system that allows self-organizing symmetric polystyrene-b- poly(methyl methacrylate) (PS-b-PMMA) thin films in few seconds, taking advantage of the residual amount of solvent present in the film after the spinning process. Distinct ordered morphologies, coexisting along the sample thickness, can be obtained in PS-b-PMMA samples with the formation of lamellae laying over a hexagonal pattern of PMMA cylinders embedded in the PS matrix and perpendicularly oriented with respect to the substrate. The thermal evolution of the entrapped solvent and the dynamics and morphological stabilization of the coexisting phases are described and the intimate mechanism of the self-assembly process are discussed and fully elucidated. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S19.00002: Thin Film Morphologies of Bulk-Gyroid Polystyrene-block-Polydimethylsiloxane under Solvent Vapor Annealing Wubin Bai, Adam Hannon, Kevin Gotrik, Hong Kyoon Choi, Karim Aissou, George Liontos, Konstantinos Ntetsikas, Alfredo Alexander-Katz, Apostolos Avgeropoulos, Caroline Ross Thin film morphologies of a 75.5 kg/mol polystyrene-block-polydimethylsiloxane (PS-PDMS) diblock copolymer (SD75) subject to solvent vapor annealing are described. Thin films were spin-cast from 1{\%} solution of SD75 in cyclohexane and annealed in cosolvent vapors consisting of mixed toluene and heptane vapors. The PS-PDMS has a double-gyroid morphology in bulk, but as a thin film the morphology consists of spheres, cylinders, perforated lamellae or gyroids, depending on the film thickness, its commensurability with the microdomain period, and the ratio of toluene:heptane vapors used for the solvent annealing process. The morphologies are described by self-consistent field theory simulations. Thin film structures with excellent long-range order were produced, which are promising for nano patterning applications. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S19.00003: Design, construction, and testing a purpose-built climate-controlled solvent vapor annealing chamber for guided self-assembly of block polymer thin films Ryan Gnabasik, Rustin Haase, Andrew Baruth Despite its efficacy to produce well-ordered, periodic nanostructures, the intricate role multiple parameters play in solvent vapor annealing has not been fully established. In solvent vapor annealing a thin polymer film is exposed to the vapors of a solvent(s) thus forming a swollen and mobile layer to direct the self-assembly process at the nanoscale. Recent developments in both theory and experiment have directly identified critical parameters, but controlling them in any systematic way has proven non-trivial. These identified parameters include vapor pressure, solvent concentration in the film, and, critically, the solvent evaporation rate. To explore their role, a purpose-built solvent vapor annealing chamber was designed and constructed. The all-metal chamber is inert to solvent exposure and pneumatically actuated valves allow for precision timing in the introduction and withdrawal of solvent vapor. Furthermore, the mass flow controlled inlet, chamber pressure gauges, \textit{in situ} spectral reflectance-based thickness monitoring, and high precision micrometer relief valve, give real-time monitoring and control during the annealing and evaporation phases. Using atomic force microscopy to image the annealed films, we are able to map out the parameter space for a series of polystyrene-$b$-polylactide ($M_{n} \quad =$ 75 kg/mol and $f_{PLA} \quad =$ 0.28) block polymer thin films with an intrinsic cylindrical morphology and identify their role in directed assembly. Funded by Creighton University Summer Research Grant. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S19.00004: Block copolymer alignment by shear induced during solvent vapor annealing with a crosslinked elastomer capping layer Invited Speaker: Bryan Vogt The long range alignment of block copolymers (BCPs) is generally accomplished through application of a gradient shear force or by topographical or chemical cues patterned into the substrate. These techniques require lithographic patterning, specialty substrates or custom built equipment to achieve the alignment, which limits the broad academic application of aligned BCPs. One technique to improve the large range ordering of BCPs is solvent vapor annealing (SVA), which exposes the BCP film to a controlled atmosphere of solvent vapor to swell the BCP and provide significant enhancements in the chain mobility. Here, we discuss a minor modification of the SVA process; a thin piece of crosslinked poly(dimethyl siloxane) (PDMS) is placed on top of the BCP film before SVA. Exposure to organic solvent vapors causes the PDMS to swell, while the solvent also plasticizes the BCP film. Removal of the solvent induces a shear to the BCP film as the PDMS shrinks back to its initial dimensions. The shape of the PDMS cap determines the anisotropy in the stress applied on deswelling that aligns and orients the BCP domains. Polystyrene-block-polyisoprene-block-polystyrene (SIS) is utilized as a model system to illustrate how the processing parameters impact the orientation as determined by both grazing incidence small angle x-ray scattering (GISAXS) and atomic force microscopy (AFM). Quantification of the alignment by Herman's orientational parameter (S) illustrates high degree of alignment (S$=$0.95) is possible through appropriate selection of processing conditions. This SVA-based alignment method provides a relatively simple method to orient BCP films within general SVA processing protocols. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S19.00005: Optimization of long-range order in solvent-annealed polystyrene-$b$-polylactide block polymer thin films for nanolithography A. Baruth, M. Seo, C.-H. Lin, K. Walster, A. Shankar, M.A. Hillmyer, C. Leighton We demonstrate long-range order in solvent-annealed polystyrene-$b$-polylactide block polymer thin films for nanolithographic applications. This is accomplished \textit{via} climate-controlled solvent vapor annealing, \textit{in situ} solvent concentration measurements, and small angle x-ray scattering. By connecting the properties of swollen and dried films, we identify ``best practices'' for solvent-annealing, including that exposing block polymer films to a neutral solvent concentration just below the identified (\textit{via }x-ray scattering) order-disorder transition, at low pressures, with fast solvent evaporation rates, will consistently yield large lateral correlation lengths (\textgreater 6.9 $\mu $m) of hexagonally-packed cylinders that span the entire thickness of the film with center-to-center spacing ranging from 43 -- 59 nm. The resultant films have sufficient fidelity for pattern transfer to an inorganic material, as evidenced by patterning of Ni metal nanodots using a damascene-type approach. We argue that our results can be qualitatively understood by analogy to thermal annealing of a single-component solid, where annealing just below the melting point leads to optimal recrystallization. Such reliability, combined with recently developed pattern-transfer techniques, places this cheap and rapid method of nanolithography in competition with conventional lithography schemes. Funded by NSF MRSEC and Creighton University Summer Research Award. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S19.00006: Unidirectional alignment of block copolymer templated porous films using solvent vapor annealing with soft shear Zhe Qiang, Kevin Cavicchi, Bryan Vogt Porous films templated by block copolymers (BCPs) have been extensively investigated due to their potential numerous applications such as sorbents and nanolithography. However, in many cases, their performance critically depends on their nanostructural alignment and orientation. Achieving unidirectional alignment of these nanostructures over macroscopic dimensions is still challenging especially for BCPs with very high $\chi $ and Tg. Here, we illustrate a new method based on solvent vapor annealing with soft shear (SVA-SS), where a crosslinked poly(dimethylsiloxane) (PDMS) cap is simply adhered to the polymer films during SVA, to fabricate macroscopically aligned cylindrical structured mesoporous films using poly(styrene-block-N,N,-dimethyl-n-octadecylamine p-styrenesulfonate) (PS-b-PSS-DMODA) as the soft-template and phenolic resin as the precursor. The evolution of structures through the SVA-SS, thermal annealing and carbonization is determined by grazing incidence small angle x-ray scattering (GISAXS) and atomic force microscopy (AFM). Highly ordered mesoporous carbon films with S\textgreater 0.8 can be obtained by this method. Potential applicability of this method to nanostructures besides cylinders, such as spheres and gyroid will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S19.00007: Highly enhanced dynamics of microdomains ordering by solvent vapor annealing of thin block copolymer films on polymer network supports Larisa Tsarkova, Anja Stenbock-Fermor, Alexander B\"{o}ker, Armin Knoll We studied the solvent driven ordering dynamics of block copolymer films supported by a densely cross-linked organic hard mask (HM) designed for lithographic fabrication. We found that the ordering of microphase separated domains on the HM layer proceeds significantly faster as compared to similar films on silicon wafers. This leads to a pronounced enhancement of the dynamics of both the terrace-formation as well as the long-range lateral ordering of the microdomains. The effect is independent on the chemical structure and volume composition of the studied block copolymers (cylinder-/ lamella-forming). Importantly, enhanced ordering is achieved even at a reduced degree of swelling corresponding to an intermediate to strong segregation regime, when similar films on conventional substrate show very limited ordering. In-situ ellipsometric measurements of the swollen films revealed an insignificant increase by 1-3 vol. \% in the solvent up-take by HM-supported films. Therefore we attribute the enhanced dynamics to reduced interactions at the block copolymer/HM-support interface. Apart from immediate technological impact in block copolymer-assisted nanolithography, our findings convey novel insight into effects of molecular architecture on polymer-solvent interactions. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S19.00008: Direct Immersion Annealing (DIA) of Block Copolymer Thin Film Arvind Modi, Alamgir Karim Solvent Vapor Annealing (SVA) methodologies of block copolymer (BCP) films have demonstrated excellent potential for control of nanostructures and morphologies. However, SVA designs require sophisticated instrumentation, and fine control of system parameters in batch processing mode which is relatively complex and limits its feasibility. We developed a faster and robust solvent immersion strategy for microphase separation and nanostructure control of as-cast BCP thin films with minimal sophistication. Our Direct Immersion Annealing (DIA) method requires immersion in a mixture of non-solvent and good solvent (for BCP) for annealing. A non-solvent component prevents dissolution of the film resting on substrate while a good solvent percolates through the film, plasticizes it, and shifts glass-transition below room temperature leading to microphase separation and ordering. Our study of PS-PMMA system demonstrates that a robust control over thin film ordering and transient swelling could be achieved through a fine control of solubility parameter of solvent mixture and temperature with no dead-time. Further, we exhibit the utility of DIA for alignment of BCP domains on topographically patterned substrates. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S19.00009: Dynamical SCFT Simulations of Solvent Annealed Thin Films Sean Paradiso, Kris Delaney, Hector Ceniceros, Carlos Garcia-Cervera, Glenn Fredrickson Block copolymer thin films are ideal candidates for a broad range of technologies including rejection layers for ultrafiltration membranes, proton-exchange membranes in solar cells, optically active coatings, and lithographic masks for bit patterning storage media. Optimizing the performance of these materials often hinges on tuning the orientation and long-range order of the film's internal nanostructure. In response, solvent annealing techniques have been developed for their promise to afford additional flexibility in tuning thin film morphology, but pronounced processing history dependence and a dizzying parameter space have resulted in slow progress towards developing clear design rules for solvent annealing systems. In this talk, we will report recent theoretical progress in understanding the self assembly dynamics relevant to solvent-annealed and solution-cast block copolymer films. Emphasis will be placed on evaporation-induced ordering trends in both the slow and fast drying regimes for cylinder-forming block copolymers from initially ordered and disordered films, along with the role solvent selectivity plays in the ordering dynamics. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S19.00010: Solvent-Assisted Self-Assembly of Block Copolymer Films: A Simulation Approach Su-Mi Hur, Gurdaman S. Khaira, Paul Nealey, Marcus M\"uller, Juan J. de Pablo Solvent annealing has been shown to provide an effective means for controlling the self assembly in block copolymer films; it also provides opportunities to create structures that cannot be achieved by thermal annealing. The intrinsic non-equilibrium nature of these processes presents challenges to their theoretical understanding. We have developed an efficient simulation tool for modeling the solvent annealing of block copolymer films that enables study of the evolution of microstructure and the transformations between various microphases in response to film swelling and solvent evaporation. We study the effect of process and thermodynamical variables such as solvent pressure, molecular weight and segregation force, on the self assembled structure of block copolymer thin films. And we identify conditions that lead to a defect-free copolymer morphology. We also discuss the effects of relative time scales of solvent evaporation, diffusion of solvent and of polymer chains on the self-assembly of block copolymer thin films. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S19.00011: An in-situ Study of Kinetics of Rapid Self- assembly in Lamellar Forming Poly (styrene-b- lactic acid) (PS-b-PLA) Block Copolymer during Microwave Annealing Parvaneh Mokarian-Tabari, Cian Cummins, Michael A. Morris This work exploits the effect of microwave annealing on kinetics of pattern formation for lamellar PS-$b$-PLA film. A well-ordered pattern lamellar PS-$b$-PLA is formed on UV/ozone treated Si in less than one minute upon exposure to microwave energy in presence of THF. To understand the interaction of polymers with microwave radiation, we carried out an \textit{in-situ }temperature measurement of the Si substrate during the annealing. Our \textit{in-situ} experiment shows neither Si nor PS-$b$-PLA go through dramatic temperature rise during exposure to microwave energy. We suggest the dopant level in our Si is not high enough to activate the microwave absorption. Also, the high frequency of the electromagnetic field does not allow polar substances like PLA enough time to oscillate. We believe THF which is a polar liquid contribute significantly to the rapid self-assembly of the film. The vapor pressure of THF rises from 19.8 kPa to 70 kPa (at 55 $^{\circ}$C) within few seconds. The high pressure plasticizes the polymers. The highly mobilized chains phase separate quickly due to high-$\chi $ parameter. The results are compared with conventional thermal annealing method. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S19.00012: An in situ GISAXS study of BCP thin films during annealing in selective solvent vapor: Solvent removal effects in films of different initial thickness Ilja Gunkel, Xiaodan Gu, Alexander Hexemer, Thomas Russell Solvent vapor annealing is a rapid and effective means to achieve well-ordered structures in block copolymer (BCP) thin films. The underlying physical mechanisms however are ill understood and systematic studies of the annealing process are scarce. Here, we used grazing-incidence small-angle x-ray scattering (GISAXS) to investigate the ordering of BCP microdomains as solvent vapor was added or removed. We studied polystyrene-block-poly(4-vinyl pyridine) (PS-b-P4VP) BCP thin films of different initial thickness ranging from a few ten to a few hundred nanometers during annealing in THF vapor, a selective solvent for PS. While the degree of lateral order of the BCP microdomains in the swollen state was found to be exceptional for all film thicknesses, the packing of microdomains was found to depend on the initial film thickness and the amount of swelling. The effect of solvent removal on the degree of lateral order was studied by deswelling films of different thickness at different removal rates. Here, we observed a substantial deterioration of lateral order of microdomains that is significantly stronger than in comparable deswelling studies of BCP thin films in neutral solvent vapors. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S19.00013: An \textit{in situ} grazing incidence x-ray scattering study of block copolymer thin films during solvent vapor annealing Xiaodan Gu, Ilja Gunkel, Alexander Hexemer, Thomas Russell Although solvent vapor annealing (SVA) has been widely applied to block copolymer (BCP) thin films to obtain well-ordered microdomains, the mechanism of enhancing lateral order is not well understood. Here, we used real time \textit{in situ} grazing-incidence small-angle x-ray scattering (\textit{in situ~}GISAXS) to study the self-assembly of PS-b-P2VP BCP BCPs with different molecular weights thin films in THF vapor, a near neutral solvent for both blocks. Both swelling and deswelling behavior of BCP thin films were examined. The extent of swelling$~$and the solvent removal rate not only affect the domain spacing of BCPs but also dictate the extent of lateral ordering of the BCP microdomains. Larger grains were observed at higher values of the swelling ratio (close to disordering). To preserve the maximal lateral ordering of the microdomains in the swollen state, the fastest solvent removal rate is required to freeze in the ordered microdomain structure of the swollen BCP film. [Preview Abstract] |
Session S20: Polymer Glasses
Sponsoring Units: DPOLYChair: Rodney Priestly
Room: 405
Thursday, March 6, 2014 8:00AM - 8:12AM |
S20.00001: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S20.00002: Segmental mobility measured during constant strain rate deformation of poly(methyl methacrylate) glasses Kelly Christison, Benjamin Bending, Josh Ricci, Mark Ediger We have measured segmental mobility in poly(methyl methacrylate) glasses during constant strain rate deformation using a dye reorientation method. At 19 K below the glass transition temperature and for strain rates between 5.5x10$^{\mathrm{-6}}$ and 1.5x10$^{\mathrm{-4}}$ s$^{\mathrm{-1}}$, mobility increases as yield is approached, after which, it remains constant. In the post-yield regime, higher strain rates are found to be correlated with higher values of mobility. These results are consistent with the simulations of Riggleman et al. and the theory of Chen and Schweizer. On a log-log plot of mobility versus strain rate, our data falls on two parallel lines with slopes of -1. Data associated with high strain rates falls on a line consistent with the theory of Chen and Schweizer. Low strain rate data falls on a separate line shifted toward lower mobility. To our knowledge, this behavior is not predicted by existing simulations or modeling approaches. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S20.00003: Multi-step loading/unloading experiments that challenge constitutive models of glassy polymers James Caruthers, Grigori Medvedev The mechanical response of glassy polymers depends on the thermal and deformational history, where the resulting relaxation phenomenon remains a significant challenge for constitutive modeling. For strain controlled experiments the stress response is measured during loading/unloading ramps and a constant strain. By judiciously combining the basic steps, a set of multi-step experiments have been designed to challenge existing constitutive models for glassy polymers. A particular example is the ``stress memory'' experiment, i.e. loading through yield, unloading to zero stress, and holding at final strain, where the subsequent evolution of the stress exhibits an overshoot. The observed dependence of the overshoot on the loading strain rate cannot be explained by the models where the relaxation time is a function of stress or strain. Another discriminating multi-step history experiment involves strain accumulation to test the common assumption that the phenomenon of strain hardening is caused by a purely elastic contribution to stress. Experimental results will be presented for a low Tg epoxy system, and the data will be used to critically analyze the predictions of both traditional viscoelastic/viscoplastic constitutive models and a recently developed Stochastic Constitutive Model. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S20.00004: Challenges in predicting non-linear creep and recovery in glassy polymers Grigori Medvedev, James Caruthers The phenomenon of non-linear creep of amorphous polymeric glasses is difficult to predict using the traditional viscoelastic and viscoplastic constitutive frameworks, where two features present a particular challenge: (i) the tertiary stage of the creep and (ii) the recovery from large creep upon removal of the load. Representative examples of these two nonlinear responses will be shown for lightly cross-linked PMMA and an epoxy material, where the creep and recovery behavior has been studied as a function of temperature and aging time. The acceleration of creep during the tertiary stage is not caused by damage since the original dimensions of a cross-linked sample are fully recoverable by annealing above Tg. The assumption that the relaxation time is a function of strain runs into qualitative problems when predicting multi-step constant strain rate loading experiments. Recovery from creep as predicted by the constitutive models where the relaxation time depends on the deformation history is too abrupt compared to the experiment - this known as the ``accelerated aging'' problem. A recently developed Stochastic Constitutive Model that acknowledges dynamic heterogeneity in the glass state naturally predicts both the tertiary creep and the smooth recovery from creep. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S20.00005: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S20.00006: Stress relaxation behavior of polymer glasses in uniaxial extension Panpan Lin, Shiwang Cheng, Jianning Liu, Shi-Qing Wang Ductile polymers can undergo large tensile extension upon mechanical precondition (i.e. milling and melt-stretching). In the post-yield regime the tensile stress can still grow with the extension partially because of the elastic energy buildup as the chain tension grows from the stretching of the chain network [1]. To learn more about the nature of the mechanical stress, we carried out a series of stress relaxation experiments of both milled and melt-stretched PC. We found rescaling behavior, i.e., the stress relaxation is faster from a faster tensile extension by exactly the same amount. In other words, for an extension made at a cross-head speed of V$_{1}$, the stress relaxation occurs on a time scale of t$_{1}$. Then the stress relaxation from an extension produced at V$_{2}$ \textgreater V$_{1}$ occurs on a time scale of t$_{1}$(V$_{1}$/V$_{2})$. This is true for a range of nearly five orders of magnitude in V. We have studied this surprising scaling law as a function of the precondition. \\[4pt] [1] ``Strain hardening in homogeneous deformation of polymer glasses,'' P. P. Lin \textit{ et al.}, Phys. Rev. Lett., under review. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S20.00007: The effect of normal stress on the rheology of sub-micron thick polymer melt Janet Wong, Aleks Ponjavic The rheology of sub-micron thick polymer melt confined and sheared between a sphere and a flat surface (resulting in a circular point contact) is examined by obtaining the local through-thickness flow profiles of the melt. The effect of normal stress exerted to the melt is investigated. The rheology of the melt and the mechanical response of the fluid system are then correlated. The possibility of rheological heterogeneity within the confined melt is also explored. It is observed that behaviour of the confined melt is insensitive to the range of shear rate tested. Normal stress exerted, on the other hand, influences the rheology of the confined melt significantly. A critical normal stress exists below which Couette-like flow profiles are observed. Above the critical normal stress, the flow profiles signify plug-flow. This can be due to pressure-induced polymer glass transition. The existence of a critical stress is confirmed by the variation of local flow profiles within the point contact that closely resembles the normal pressure distribution in the contact. While a switch in flow behaviour occurs at a critical normal stress, the corresponding change in mechanical response in terms of measured friction forces is only marginally. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S20.00008: Scalar softness field correlates to molecular rearrangements for a thermal polymer glass Anton Smessaert, J\"{o}rg Rottler A fundamental challenge in the field of amorphous materials is to understand the structural causes for the spatial distribution of plastic events. Recent studies suggest that the low frequency vibrational modes encode information about structurally weak regions. Such ``soft spots'' were shown to strongly correlate to molecular rearrangements for an athermal amorphous solid in 2D~[1]. Building on these ideas, we construct a scalar ``softness field'' from a weighted superposition of low frequency modes and we show that this field identifies regions in which particles undergo rearrangements. We test the predictive strength of the field computationally for a 3D polymer glass model in a quiescent state at several temperatures. Rearrangements are identified as particle hops using a previously introduced detection algorithm~[2]. We find that hops are clustered in regions of large softness, and present a quantitative analysis of the correlation. The autocorrelation of the field shows that the soft regions are long lived compared to the timescales of the rearrangements. Furthermore, we find that particles hop preferentially along soft directions that are predicted by the softness field.\\[4pt] [1] M.L. Manning \& A.J. Liu,PRL 107,108302(2011)\\[0pt] [2] A. Smessaert \& J. Rottler,PRE 88, 022314(2013) [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S20.00009: Pressure-induced rheological transition of polymer melt Luca di Mare, Janet Wong Experiments have recently shown that a critical normal stress exists where the flow of a polymer melt under shear transits from Couette flow to plug flow as the normal stress exerted onto the melt increases. It has been conjectured that the observation is related to pressure-induced glass transition of the polymer melt. Experimentally this is challenging to verify. Hence MD simulations are carried out to elucidate the origin of such transition. The simulation consists of model polymer chains being sheared between two hard walls under isothermal conditions. The conformation, the density distribution, the dynamics of the chains, the viscosity of the melts, and the through-thickness velocity profiles are simulated by varying the shear velocity, the molecular weight of the chains and its distribution, and the normal stress of the system. The simulated results are then compared with experimental observations. Preliminary results show that the through-thickness viscosity of the melt is heterogeneous under high normal stress conditions. This can result in non-linear velocity profiles resemble experimental findings. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S20.00010: Developing a molecular picture for polymer glasses under large deformation Shi-Qing Wang, Shiwang Cheng, Panpan Wang Polymer glasses differ from most other types of glassy materials because they can be ductile under tensile extension. Remarkably, a ductile polymer can turn brittle and vice versa. For example, upon cooling, the glass changes from ductile to brittle at a temperature known as the brittle-ductile transition temperature (BDT). Aging causes the ductile glass to be brittle. Mechanical ``rejuvenation'' or pressurization brings a brittle glass into a ductile state. Finally, one glass can be ductile 100 degrees below T$_{\mathrm{g}}$ while another polymer is already brittle even just 10 degree below T$_{\mathrm{g}}$. Polystyrene and bisphenol A polycarbonate are at the two extremes in the family of polymer glasses. How to rationale such a wide range of behavior in terms of a molecular picture has been a challenging task. What is the role of ``chain entanglement''? Since many of the procedures including the temperature change do not alter the ``chain entanglement'', it is clearly insufficient to explain the nature of the BDT in terms of the entanglement density. Our work attempts to answer the question of what then is the role of chain networking. We have formulated a molecular picture that presents a unifying and coherent explanation for all the known phenomenology concerning the BDT and condition for crazing. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S20.00011: Enhancing polymer T$_{\mathrm{g}}$ and tuning mechanical properties with stiff molecular additives Jayachandra Hari Mangalara, David Simmons Small-molecule additives are commonly employed to alter glass formation, mechanical, and transport properties of polymers. For example, plasticizers are used to suppress polymer T$_{\mathrm{g}}$ and soften the glassy state, while antiplasticizers, which stiffen the glassy state of a polymer while suppressing its T$_{\mathrm{g}}$, are employed to enhance protein and tissue preservation. Recent advances in the understanding of additives' effects on glass formation suggest that additional combinations of temperature-dependent alterations to properties including T$_{\mathrm{g}}$, viscosity, and glassy moduli can be obtained via rational selection of additive properties. Here we employ coarse-grained molecular dynamics simulations to study the effect of introducing a stiff molecular additive to an unentangled polymer melt. Results indicate that, in contrast to plasticizer and classical antiplasticizer additives, these stiff molecular additives enhance the T$_{\mathrm{g}}$ of the matrix polymer. We further examine the impact of these additives on glassy moduli and yield stress of the polymer. These results highlight the importance of additive stiffness as a design parameter enabling more rational control of glass formation behavior. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S20.00012: Disorder-driven glass transition of polymers Alessio Zaccone, Eugene Terentjev The mechanical response of solids depends on temperature because the way atoms and molecules respond collectively to deformation is affected at various levels by thermal motion. This is a fundamental problem of solid state science and plays a crucial role in materials science. In glasses the vanishing of shear rigidity upon increasing temperature is the reverse process of the glass transition. It remains poorly understood due to the disorder leading to nontrivial (nonaffine) components in the atomic displacements. Our theory explains the basic mechanism of the melting transition of amorphous (disordered) solids in terms of the lattice energy lost to this nonaffine motion, compared to which thermal vibrations turn out to play only a negligible role. The theory is in good agreement with classic data on melting of amorphous polymers (for which no alternative theory can be found in the literature) and offers new opportunities in materials science. Ref: A. Zaccone \& E.M. Terentjev, Phys. Rev. Lett. 110, 178002 (2013). [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S20.00013: Exploring chain tension in cold drawing of polymer glasses Shiwang Cheng, Panpan Lin, Mesfin Tsige, Shi-Qing Wang Ductile polymer glasses can undergo large tensile extension (cold draw) to double its original length either homogeneously or through necking. The corresponding tensile stress is typically much higher than the rubbery elastic modulus. Apart from the plastic component, there is also an energetic contribution to the mechanical stress. The origin of this elastic stress appears to arise from the existence of a chain network. The elastic yielding phenomenon [1] indicates that significant chain tension builds up during the cold drawing. Atomistic molecular dynamics simulation is carried out to delineate the nature of the chain tension and explore the suggestion of bond distortion in deformation of polymeric glasses. In a simple model to mimic a polymer glass with sufficient chain networking, we found evidence for the bond distortion that grows with the degree of extension. \\[4pt] [1] ``Elastic yielding in cold drawn polymer glasses well below the glass transition temperature,'' S. W. Cheng and S. Q. Wang, \textit{Phys. Rev. Lett. }\textbf{110}, 065506 (2013). [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S20.00014: Nonlinear mechanics of thermoreversibly associating dendrimer glasses Arvind Srikanth, Robert S. Hoy, Berend C. Rinderspacher, Jan W. Andzelm The integration of thermoreversibly associating groups into polymers produces a wide variety of complex behavior arising from the finite lifetime of the ``sticky,'' thermoreversible bonds. Using hybrid molecular dynamics / Monte Carlo simulations, we characterize the nonlinear mechanical properties of associating trivalent dendrimer network glasses with a focus on their energy dissipation properties. Various combinations of sticky bond (SB) strength and kinetics are employed. The toughness (work to fracture) of these systems displays a surprising deformation-protocol dependence; different association parameters optimize different properties. In particular, ``strong, slow'' SBs optimize strength, while ``weak, fast'' SBs optimize ductility via self-healing during deformation. We relate these observations to breaking, reformation, and partner switching of SBs during deformation. These studies point the way to creating associating-polymer glasses with tailorable mechanical properties. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S20.00015: Measuring the nanoscale properties of laser-deposited glassy polymer nanodroplets Kimberly Shepard, Craig Arnold, Rodney Priestley Glassy polymer nanodroplets are fabricated via the Matrix Assisted Pulsed Laser Evaporation (MAPLE) technique using short deposition times. At longer deposition times, the nanodroplets act as nanoscale building blocks, forming nanostructured bulk films with thickness on the order of microns. These nanostructured glassy films exhibit unique properties, including 40{\%} reduced density along with a 40K increase in the glass transition temperature compared with glasses prepared by cooling from the liquid state. Indirect experimental study of the thermal properties of the nanoscale features has indicated that the stability of the bulk film may be a result of the nanostructure. Here, we directly measure the properties of the nanoscale building blocks and connect the results to observations about the global film properties. Heated atomic force microscopy is used to measure the volume of individual nanodroplets as they are heated in situ. MAPLE-deposited droplets exhibit large excess volumes and enhanced thermal stability compared with similarly-sized droplets prepared from polymer nanoparticles. We discuss this behavior in the context of the MAPLE process of nanodroplet formation. [Preview Abstract] |
Session S21: Focus Session: Polymer Nanocomposites I - Active Particles and Dynamics
Sponsoring Units: DPOLY GSNPChair: Nigel Clarke, University of Sheffield
Room: 406
Thursday, March 6, 2014 8:00AM - 8:36AM |
S21.00001: Engineering polymer-fullerene thin films and solar cells with external fields Invited Speaker: Joao Cabral Trace amounts of nanoparticles, including fullerenes, can impart stability to thin polymer films against dewetting by the combined effects of pinning the contact lines of dewetting holes and by effectively altering the polymer-substrate interaction. Polymer nanocomposite (meta)stable thin films can yield well-defined morphologies from uniform to spinodal-like, via spontaneous polymer-nanoparticle phase separation and crystallization. Confinement breaks the structural isotropy and generally causes (partial) segregation of components orthogonally to the film surface. Surface energy patterning can thus modulate composition and morphology, both in plane and normal to the surface. Further, UV-visible, and even background, light exposure, in both solutions and melts, is shown to tune the solution stucture and morphology of dewetting and phase separating polymer-fullerene thin films. Neutron reflectivity allows us to locate the various constituents within the film. We find a coupling of fullerene photo-sensitivity and both self-assembly processes which results in controlled pattern formation, and we illustrate the potential with a model polymer-fullerene circuit pattern. We then translate this approach into the directed assembly of energy harvesting bulk heterojunctions thin films. Indeed, a key challenge to the commercialization of organic solar cells remains the achievement of morphological stability, particularly under thermal stress conditions. The directed assembly a blend polymer:PC$_{60}$BM solar cells via a simple light processing step results in a 10-100 fold increase in device thermal stability and, under certain conditions, enhanced device performance. The enhanced stability is linked to the light-induced oligomerisation of PC$_{60}$BM that effectively hinders diffusion and crystallization in blends. This effect appears to be general and promises to be an effective and cost-effective strategy to optimize fullerene-based solar cell performance. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S21.00002: Spatial temperature mapping in polymer nanocomposites due to ultrafast photothermal heating of gold nanorods Somsubhra Maity, Colin Curtis, Wei-Chen Wu, Chao Xu, Joseph Tracy, Kenan Gundogdu, Jason Bochinski, Laura Clarke In pulsed laser irradiance, extremely high peak powers (low average powers) can be attained due to short bursts of energy. This property can be exploited for photothermal heating of polymers using gold nanorods in which the incident radiation can be efficiently converted into heat in short pulses. This leads to extreme localization of heat energy which does not affect the global polymer temperature significantly. In this work, we describe the effect of using pulsed laser to generate photothermal heat within polymer matrices doped with gold nanorods, and novel optical techniques to determine the corresponding temperature distribution. The rotation of the nanorods are studied to monitor the temperature of the polymer melt immediately surrounding the nanorods and the polarized fluorescence of probe molecules* are used to determine the temperatures of concentric volumes of polymer far away from the nanorods. The experimental techniques discussed provide simple tools to monitor the ensemble behavior of the nanorods and map the temperature distribution due to pulsed heating. The pulsed photothermal effect enables nanoscale thermal manipulations without altering the bulk temperature or morphology of the polymer. *S. Maity et al. Adv. Funct. Mater. 22, 5259 (2012) [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S21.00003: Photothermal heating and mechanical properties of Au/PEO and Ag/PEO nanocomposites Merve Seyhan, Katherine Rickard, U. Ecem Yarar, Deniz Rende, Nihat Baysal, Rahmi Ozisik, Seyda Bucak In the current study, the photothermal effect of gold (Au) and silver (Ag) nanoparticles in poly(ethylene oxide) is investigated. Both Au and Ag nanoparticles were synthesized in-house and were characterized by dynamic light scattering, UV-Visible spectroscopy and transmission electron microscopy experiments. The average size of the Au and Ag nanoparticles was found to be on average 8.9 and 8.4 nm, respectively. The Au/PEO and Ag/PEO nanocomposites containing 0.01--2{\%} nanoparticles (by weight) were prepared via solution mixing. Mechanical and thermo-mechanical properties were investigated by static and dynamic tests. The results indicate that the Young's modulus increases with increasing nanoparticle concentration, however, the modulus values reached a plateau at high concentrations. Both nanocomposites were heated via laser radiation at appropriate wavelengths and via traditional heating (using a heating stage). The temperature variations were measured through Raman spectroscopy experiments and by correlating Raman and traditional heating experiments. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S21.00004: Annealing polymer nanofibrous nanocomposite mats via photothermal heating: effects on overall crystallinity, morphology, and mechanical properties Russell Gorga, Laura Clarke, Jason Bochinski, Vidya Viswanath, Somsubhra Maity Metal nanoparticles embedded within polymeric systems can be made to act as localized heat sources thereby aiding in-situ polymer processing. This is made possible by the surface plasmon resonance mediated photothermal effect of metal nanoparticles, wherein incident light absorbed by the nanoparticle generates a non-equilibrium electron distribution which subsequently transfers this energy into the surrounding medium, resulting in a temperature increase in the immediate region around the particle. Here we demonstrate this effect in polyethylene oxide-gold nanoparticle electrospun nanofibrous mats, which have been annealed at temperatures above the glass transition. A non-contact temperature measurement technique utilizing embedded fluorophores (perylene) has been used to monitor the average temperature within samples. The effect of annealing methods (conventional and photothermal) and annealing conditions (temperature and time) on the fiber morphology, overall crystallinity, and mechanical properties is discussed. In conclusion we demonstrate that the specificity of plasmonic heating coupled with the inside-outside approach of annealing presents a unique tool to improve crystallinity, and therefore mechanical properties, of the polymer mats while maintaining the unique nanofibrous morphologies. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S21.00005: Harnessing Interfacially-Active Nanorods to Regenerate Severed Polymer Gels Xin Yong, Olga Kuksenok, Krzysztof Matyjaszewski, Anna Balazs With newly developed computational approaches, we design a nanocomposite that enables self-regeneration of the gel matrix when a significant portion of the material is severed. The cut instigates the dynamic cascade of cooperative events leading to the re-growth. Specifically, functionalized nanorods localize at the new interface and initiate Atom Transfer Radical Polymerization with monomers and cross-linkers in the outer solution. The reaction propagates to form a new cross-linked gel, which can be tuned to resemble the uncut material. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S21.00006: Translational and Rotational Motion of Nanocrystals in Rubber Yuya Shinohara, Akira Watanabe, Hiroyuki Kishimoto, Yoshiyuki Amemiya We present the observation of translational and rotational dynamics of carbon-black nanocrystals in styrene-butadien rubber using coherent X-ray scattering. X-ray photon correlation spectroscopy (XPCS) exploits the partial coherence of X-rays to provide the information of microscopic dynamics. In diffracted X-ray tracking (DXT) measurement, the motion of diffraction spots from single nanocrystals is monitored to track their rotational motion. A combination of XPCS and DXT reveals the detailed translational and rotational motion of nanocrystals in a medium. Experimentally XPCS requires a monochromatic beam whereas DXT requires a wide energy range to increase the probability of diffraction spots being on the Ewald sphere shells. This experimental incompatibility can be overcome by using an intense pink beam X-ray that is available using a helical undulator at synchrotron facilities. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S21.00007: Directed Assembly of Polymeric Films Filled with Gold Nanoparticles Ren Zhang, Gurpreet Singh, Michael Bockstaller, Alamgir Karim Incorporation of nanoparticles (NPs) into polymer matrices has been explored extensively as an efficient way to fabricate novel functional materials, such as photonic bandgap materials, nanostructured solar cells, and high-density magnetic storage media. Towards that end, it is essential to disperse NPs in a well-controlled manner. We applied our unique dynamic thermal field processing method to gold nanopaticles with PS corona (AuNPs) into PS-b-PMMA cylinder forming block copolymer (c-BCP) thin films, and observed a sharp transition from vertical to horizontal cylinder orientation with AuNP loading fraction increasing. This transition is attributed to enrichment of AuNPs at the substrate side and favorable interaction of PMMA chains with gold cores. Furthermore, we investigated the dynamics of phase separation behavior of AuNP filled PMMA films as a function of time. It is intriguing that the homogeneous one-phase distribution of AuNPs transformed into a two-phase state upon thermal annealing accompanied with film surface undulating. Moreover, the phase separation phenomenon was effectively suppressed when confined with an elastomeric overlayer, thus leading to excellent dispersion. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S21.00008: Dynamic gold nanoparticle, polymer-based composites Millicent Firestone, Ann Junghans, Steven Hayden, Jaroslaw Majeski Artificial polymer-based biomembranes may serve as a foundational architecture for the integration and spatial organization of metal nanoparticles forming functional nanocomposites. Nonionic triblock copolymer (PEO-PPO-PEO), lipid-based gels, containing Au nanoparticles (NPs) can be prepared by either external doping of the preformed nanoparticles or by in-situ reduction of Au $^{3+}$. Neutron reflectivity on quartz supported thin films of the Au NP --doped polymer-based biomembranes was used to determine the location of the Au. The nanoparticles were found to preferentially reside within the ethylene oxide chains located at the interface of the bulk water channels and the amphiphile bilayers. The embedded Au nanoparticles can act as localized heat sinks, inducing changes in the polymer conformation. The collective, thermally-triggered expansion and contraction of the EO chains modulate the mesophase structure of the gels. Synchrotron X-ray scattering (SAXS) was used to monitor mesophase structure as a function of both temperature and photo-irradiation. These studies represent a first step towards designingexternally-responsive polymer-nanoparticle composites. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S21.00009: Self-Assembly of Supramolecular Nanocomposite in Thin Film: A Kinetic Study Joseph Kao, Kari Thorkelsson, Peter Bai, Ting Xu The comprehensive studies on the thermodynamics in block copolymer-based nanocomposites have paved the way for hierarchically structured materials with unique collective properties. We extend the investigation to the assembly kinetics to gain further control over the 3D spatial organization of nanoparticles (NPs) in thin films of supramolecular nanocomposites. Our studies reveal that, by simply controlling the solvent fraction (f$_{s}$) in the film during solvent annealing, the thermodynamic driving force for defect elimination, the chain mobility, and the activation energy for interdomain diffusion can be modulated to tailor 3D NP assemblies in thin films. At a low f$_{s}$, the anisotropy in the local diffusion coefficient results in NP arrays normal to the surface. As f$_{s}$ reaches an optimal value, the solvent and the small molecules effectively reduce the activation energy for interdomain diffusion and the T$_{g}$ of the supramolecule. This leads to rapid formation of highly ordered 3D NP arrays in seconds. The nanocomposite eventually undergoes an order-disorder transition as f$_{s}$ increases substantially. The fundamental insights gained from these studies lay the foundation for the rational design of functional nanocomposites with tunable macroscopic properties. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S21.00010: Segmental dynamics and atomistic motions in PMMA/SWNT composites Rana Ashkar, Mansour AbdulBaki, Madhusudan Tyagi, Antonio Faraone, Paul Butler, Ramanan Krishnamoorti The addition of single wall nanotubes (SWNT) to polymers has been repeatedly shown to have a significant impact on the macroscopic properties of the host polymer, including enhanced mechanical properties and shifts in the glass-transition temperatures, $T_{g} $. These properties usually result from collective structural and dynamical interactions of the polymer chains in the composite. Here, we investigate the effect of nanotubes on the polymer dynamics in PMMA composites with 10 wt{\%} SWNT. Neutron spin echo (NSE) and backscattering (BS) are used in probing local polymer dynamics in deuterated samples and atomistic hydrogen motions in hydrogenated samples, respectively. NSE data, collected at Q-values corresponding to inter-chain correlations and at $T > T_{g} $, indicate an order of magnitude increase in the chain relaxation time in the SWNT composite relative to pure PMMA. BS data support this observation and show suppressed atomistic motions in the composite in the same temperature range. A peculiarly opposite trend, however, is observed below $T_{g} $, indicating that the presence of SWNTs promotes polymer mobility in the glassy state, in contrast with previous reports on PMMA/C60 composites which exhibit suppressed mobility at all temperatures below and above $T_{g} $. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S21.00011: Fast Electromechanical Response in Liquid Crystal Elastomer Nanocomposites Rafael Verduzco, Aditya Agrawal, Jeff Jacot, Tomi Adetiba Liquid crystal elastomers (LCEs) combine the elasticity of polymer networks with the fluidity and responsiveness of liquid crystals. LCEs can respond to a variety of external stimuli -- heat, light, electric and magnetic fields -- with large and reversible shape-changes. However, the response can be slow and/or require large external fields. Here, we present our recent work with LCE bilayers and LCE composite materials that demonstrates LCEs can respond quickly and with 3-D shape changes. Nematic LCE bilayers are prepared by depositing a PS film on top of a nematic LCE, and the bilayers exhibit reversible wrinkling, folding, and curling with temperature. The shape change of LCE bilayers is quantitatively predicted using finite-element modeling. Next, we show that a fast response to an electric field is achieved in nematic LCE composites. While typical nematic LCEs are relatively unresponsive to electric fields, LCE composites with 2 wt {\%} carbon black can reversibly contract and expand in response to a 40 V electric field. The response time ( 0.1 -- 10 Hz) and amplitude of shape change (1 -- 20 {\%}) depends on the external field and carbon black content. These composites may be useful for biomedical applications, such as substrates for dynamic cell culture and biocompatible scaffolds for heart tissue regeneration. Neonatal rat ventricular myocytes remain viable on LCE-carbon black bilayer substrates, and aligned myocyte cell sheets were successfully grown on LCE-composite bilayers. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S21.00012: Dynamics of responsive polypeptide composite particle suspended in a liquid crystal matrix and jamming Cornelia Rosu, Lu Zou, Chanjoong Kim, Paul S. Russo The emerging field of polypeptide composite particles, PCPs, has received an increased interest in the last years because of many opportunities open to a variety of applications. A PCP made of a silica core and a homopolypeptide shell resembles unique properties that cannot be achieved by the core or soft polymer shell alone. PCPs are responsive to external stimuli (e.g. light, electric and magnetic field) by incorporating fluorescent dyes and magnetic nuggets inside the silica core. Beside the responsive core, the polypeptide shell undergoes conformational transitions as a function of pH, temperature and solvent. Magnetic PCPs coated with a sparse polypeptide corona can be used as platforms to study colloidal self-assembly under jamming conditions. They are also reliable models to investigate the dynamics of complex fluids consisting of PCPs suspended in a liquid crystal matrix. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S21.00013: Untangling colloidal caging in entangled PEG solutions Subhalakshmi Kumar, Tsang Chi Hang Boyce, Steve Granick Using fluorescence microscopy, we record motion of colloids of size intermediate between correlation length of a polymer solution and size of a polymer molecule in entangled regime. The analysis of trajectory points of colloids show a transition from ``caged'' localization to diffusive randomization. The size and time spent in each individual cage is quantified using several statistical methods to give a distribution that is remarkably well-behaved and whose averages are consistent with values obtained from ensemble-average methods of trajectory analyses. [Preview Abstract] |
Session S22: Charged and Ion-Containing Polymers
Sponsoring Units: DPOLYChair: Yossef Elabd, Drexel University
Room: 407
Thursday, March 6, 2014 8:00AM - 8:12AM |
S22.00001: Chain Dynamics and Layering within Spin-Assisted versus Dip-Assisted Polyelectrolyte Multilayer Assemblies Aliaksandr Zhuk, Victor Selin, John F. Ankner, Svetlana Sukhishvili We report on the effect of deposition technique on polyelectrolyte (PE) chain dynamics within layer-by-layer (LbL) films, and on the stability of these films in salt solutions. Spin-assisted LbL (SA-LbL) films demonstrated a higher degree of film stratification, as well as greater salt stability, compared to dip-coated assemblies. Lateral and vertical polyelectrolyte diffusion of PE chains within LbL films, measured by fluorescence recovery after photobleaching (FRAP) and neutron reflectometry (NR), respectively, revealed a much higher degree of anisotropy for SA-LbL films as compared to dip-coated multilayers, with diffusion coefficients $\sim$ 10$^{6}$ larger in parallel to the substrate versus those in the perpendicular direction. The data suggest that the degree of PE chain flattening and film stratification significantly affects PE chain dynamics and film behavior in salt solutions. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S22.00002: Temperature-triggered transformations in shape of layer-by-layer microtubes in aqueous media Choonghyun Sung, Ajay Vidyasagar, Katelin Hearn, Jodie Lutkenhaus Nano- and microstructured layer-by-layer (LbL) assemblies have been of considerable interest for various applications. In particular, one-dimensional LbL microtubes have garnered interest for their ability to shrink or swell in response to changes in pH. Temperature has also been known to trigger transformations in shape. In this presentation, we report on the thermal behavior of LbL microtubes of poly(allylamine hydrochloride) (PAH) and poly(acrylic acid) (PAA). PAH/PAA LbL microtubes were prepared using polycarbonate membranes as porous templates. The thermal behavior of both freely released microtubes and template-bound microtubes was investigated in aqueous media as a function of temperature and time using confocal laser scanning microscopy and scanning electron microscopy. When free microtube suspensions were incubated at high temperatures, the microtubes became shorter and ellipsoid in shape. In contrast, the template-bound microtubes showed periodic voids on the surface. In both cases, pronounced transformations occurred above the hydrated glass transition temperature of the PAH/PAA LbL microtube. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S22.00003: Dynamics of Precise Ethylene-Acrylic Acid Copolymers and Ionomers Using Dielectric Spectroscopy James Runt, U Hyeok Choi, Hanqing Masser, C. Francisco Buitrago, L. Robert Middleton, Karen Winey, Joseph Cordaro, Amalie Frischknecht This investigation focuses on a molecular-level understanding of the dynamics of novel copolymers, consisting of monodisperse ethylene sequences between very precisely spaced acrylic acid or ionic functionality. Incorporating ions in precise acid copolymers(via neutralization of a portion of the acid functionality) results in significant changes in the association state of the acid and ionic groups, as well as polymer and ion dynamics. The dynamics of these materials were explored over a wide temperature and frequency range using dielectric spectroscopy. Acid copolymers exhibit two local relaxations in the glassy state and a segmental relaxation above T$_{\mathrm{g}}$. In addition, two slower relaxations above T$_{\mathrm{g}}$ were observed in ionomers, and their origin will be discussed in the presentation. For example, the highest temperature process is assigned to Maxwell-Wagner-Sillars (MWS) interfacial polarization, associated with the microphase separated structure. A transition in the MWS relaxation frequency and strength is observed around the melting point of copolymers with sufficiently long ethylene sequences, suggesting that the MWS process in these materials is strongly correlated with crystallinity of the ethylene backbone. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S22.00004: Plastic Deformation and Morphological Evolution of Precise Acid Copolymers L. Robert Middleton, Jason Azoulay, Dustin Murtagh, Joseph Cordaro, Karen Winey Acid- and ion-containing polymers have specific interactions that produce complex and hierarchical morphologies that provide remarkable mechanical properties. Historically, correlating the hierarchical structure and the mechanical properties of these polymers has been challenging due to the random arrangements of the polar groups along the backbone, ex situ characterization and the difficulty in deconvolution the effects of crystalline and amorphous regions along with secondary interactions between polymer chains. We address these challenges through in situ deformation of precise acid copolymers and relate the structural evolution to bulk properties by considering a series of copolymers with 9, 15 or 21 carbons between acid groups. Simultaneous synchrotron X-ray scattering and room temperature uniaxial tensile experiments of these precise acid copolymers were conducted. The different deformation mechanisms are compared and the microstructural evolution during deformation is discussed. For example, the liquid-like distribution of acid aggregates within the bulk copolymer transitions into a layered structure concurrent to a dramatic increase in tensile strength. Overall, we evaluate the effect and control of introducing acid groups on mechanical deformation of the bulk copolymers. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S22.00005: Morphology and Water Uptake in Block Copolymer Electrolyte Membranes Xi Chen, David Wong, Sergey Yakovlev, Keith Beers, Nitash Balsara Polymer electrolyte membranes (PEMs) consisting of proton-conducting hydrophilic channels and a mechanically-strong hydrophobic matrix are attractive due to their wide clean energy applications. In an effort to understand the fundamentals of proton transport in PEMs, we fabricated a series of non-porous and mesoporous sulfonated poly(styrene-b-ethylene-b-styrene) (S-SES) copolymer membranes. We examine the effects of porosity and humidity level on the morphology of S-SES membranes. The relationship between morphology and water uptake of the membranes at different humidity levels are established. We show that by controlling the porosity of the membranes, we are able to optimize the water uptake of the membranes at desired humidity levels to maximize proton conductivity. Furthermore, we show the in situ morphology change of the membranes from fully hydrated state to dry state in a drying experiment. Morphology and water content of the membranes as a function of time are examined. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S22.00006: Morphology and Ionic Conductivity of Humidity-Responsive Polymerized Ionic Liquid Block Copolymers Sharon Sharick, Kelly Meek, Yuesheng Ye, Yossef A. Elabd, Karen I. Winey We present the ionic conductivity and morphology of humidity-responsive polymerized ionic liquid block copolymers (PIL BCPs), poly(methyl methacrylate-$b$-1-[2-(methacryloyloxy)ethyl]-3-butylimidazolium-X), where X is a bromide (Br) or hydroxide (OH) anion, as a function of relative humidity (RH), temperature, and PIL composition ($\phi_{PIL})$. PIL BCPs were characterized by in situ small-angle X-ray scattering and electrochemical impedance spectroscopy. These PIL BCPs have microphase separated morphologies and long-range order increases as $\phi_{PIL}$ increases. Notably, ionic conductivity increases 3 to 4 orders of magnitude when RH increases from 30 to 90 percent. When $\phi_{PIL}$ is greater than 0.37, BCP ionic conductivity approaches or exceeds that of the homopolymer, suggesting that the dynamics in PIL microdomains mimic the homopolymer and long-range order aids ion transport. Moreover, over 60 percent of the BCP is nonconductive without a penalty in ion transport. When $\phi_{PIL}$ is less than 0.37, BCP conductivity is 1 to 2 orders of magnitude less than the homopolymer and non-conductive PMMA segments dominate ion transport, as expected. Ionic conductivities at 80 $^{^{\circ}}$C, 90 percent RH, are 7.6 mS/cm for the Br-containing BCP with $\phi_{PIL} \quad =$ 0.53 and 25.0 mS/cm for the OH-containing BCP with $\phi_{PIL} \quad =$ 0.50. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S22.00007: Conformation of cellulose based polyelectrolyte NaCMC in solution: Effect of concentration and solvent quality Carlos Lopez, Joao Cabral, Peter Graham, Ralph Colby We report small angle neutron scattering (SANS) experiments on the crossover between semidilute entangled and concentrated regimes as well as the effect of a non-solvent in the conformation of sodium carboxy methyl cellulose (NaCMC), a semi-flexible polyelectrolyte. We investigate solvation and extract the correlation length dependence on concentration, solvent quality and salt content, up to eventual peak disappearance. While a sharp change is observed in rheological measurements at the crossover to the concentrated regime, only a small change or no change in the scaling of the correlation length is measured by SANS, depending on solvophobicity. The addition of a non solvent causes an increase in the low wavenumber (q) range but has little or no effect on the magnitude of the correlation length, in contrast to conductivity measurements which show a significant amount of counterion condensation. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S22.00008: A new insight into the counterion distribution of charged polymer brushes Xiao Chu, Guangming Liu, Jiang Zhao Counterions distribution of a number of polyeletrolyte brushes are investigated using a combination of methods as QCM-D, ellipsomety and single molecule fluorescence microscopy. The experiemtnal evidence show that, as the salt level increases, counterions penetrate into the brushes, bringing about enhanced swelling and mechanical response of the brushes. The results have privided a new picture about counterion distribution of charged polymer brushes. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S22.00009: Macroion induced dehydration of weak polyelectrolyte brushes Zhongli Zheng, Y. Elaine Zhu The interaction of macroions, including polyelectrolytes, DNAs, and proteins, with polymer and cellular surfaces is critically related to many biomolecular activities, such as protein adsorption and DNA hybridization at probe surfaces. In an experimental approach to examine the macroion electrostatic interaction with a polymer surface while minimizing the long-debated hydrophobic interaction, we study the interaction of molybdenum-based inorganic polyoxometalate (POM) nanoclusters carrying 42 negative charges as model hydrophilic macroions with surface-tethered poly-2-vinylpyridine (P2VP) brushes immersed in aqueous solutions. By AFM, QCM, and contact goniometer, we have observed the collapse of P2VP chains by adding POM macroions at a constant pH. Surprisingly, added POM macroions can cause the shift of swollen-to-collapse transition pH to a lower value, in contrast to the shift to high pH value by adding simple monovalent salts. At sufficiently high POM concentration, a stable POM-P2VP composite layer, showing little dependence on solution pH and additional salts, can be formed, suggesting a simple route to construct meso-porous polymer membranes. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S22.00010: Self-assembly of polyelectrolyte surfactant complexes using large scale MD simulation Monojoy Goswami, Bobby Sumpter Polyelectrolytes (PE) and surfactants are known to form interesting structures with varied properties in aqueous solutions. The morphological details of the PE-surfactant complexes depend on a combination of polymer backbone, electrostatic interactions and hydrophobic interactions. We study the self-assembly of cationic PE and anionic surfactants complexes in dilute condition. The importance of such complexes of PE with oppositely charged surfactants can be found in biological systems, such as immobilization of enzymes in polyelectrolyte complexes or nonspecific association of DNA with protein. Many useful properties of PE surfactant complexes come from the highly ordered structures of surfactant self-assembly inside the PE aggregate which has applications in industry. We do large scale molecular dynamics simulation using LAMMPS to understand the structure and dynamics of PE-surfactant systems. Our investigation shows highly ordered pearl-necklace structures that have been observed experimentally in biological systems. We investigate many different properties of PE-surfactant complexation for different parameter ranges that are useful for pharmaceutical, engineering and biological applications. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S22.00011: Surface tension of polyelectrolyte coacervates Jian Qin, Dimitrios Priftis, Robert Farina, Sarah Perry, Lorraine Leon, Jonathan Whitmer, Kyle Hoffman, Matthew Tirrell, Juan J. de Pablo Stoichiometric solutions of polycations and polyanions can phase separate, resulting in the coexistence of a supernatant phase and a polymer-rich complex phase. The complex phase may be liquid-like or solid-like, depending on the ionic strength and the temperature. Liquid-like complexes, known as ``coacervates'', retain a large amount of water, up to 70-80\% by weight, and exhibit an ultra-low interfacial tension with the coexisting supernatant phase (smaller than the water surface tension by three orders of magnitude). Previous experiments have observed that this interfacial tension decreases with the amount of salt, and vanishes near a critical salt concentration according to a $3/2$ power of the salt undersaturation. In this work we derive analytical expressions for the interfacial tension in both the low and high charge density limits. For solutions with added salts, we provide explicit expressions for the interfacial tension near the critical salt concentration and explain the $3/2$ power dependence. Our results are shown to be in good agreement with experiment. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S22.00012: Multiscale simulation of complex coacervates Kyle Q. Hoffmann, Jonathan K. Whitmer, Jian Qin, Dimitris Priftis, Sarah Perry, Lorraine Leon, Matthew Kade, Matthew Tirrell, Juan J. de Pablo Aqueous solutions of polymers having opposite charge can separate into a coacervate phase and a supernatant water phase.The conditions leading to such behavior, including chain lenght, ionization fraction, ionic strength, molecular structure, and temperature are poorly understood. Though thermodynamic models of this phase separation exist, they offer little descriptive power for the mechanism of complex coacervation, and the internal structure of the coacervate and precipitate phases. Here we use atomic-level and coarse-grained representations of polypeptides to study features of the phase diagram, scaling relations, and microstructure of complex coacervates, comparing results to experimental data and model calculations. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S22.00013: Polyelectrolyte (PE) induced interactions between Charged and zwitterionic Colloids Victor Pryamitsyn, Venkat Ganesan A numerical self-consistent field (SCF) theory approach was developed for studying mixture of polyelectrolytes with charged and uncharged nanoparticles. Such an approach was used to analyze within the mean-field limit the polyelectrolyte-mediated effective interactions between the particles. The system considered allows for the local PE and particle charges to be defined by the local concentration of ionizable on groups on the particles and polyelectrolytes, ambient conditions like pH and the local electrostatic potential. Calculation of the free energy of a system of one, two and three particles in the polyelectrolyte solution allowdd us to calculate the particle insertion free energy, two and three body particle-particle interactions as a function of the properties of solution, polymer-particle interactions and the particle size. For the situation involving acidic PE and a base type positively charged particles, the PE mediated particle-particle interaction is purely repulsive for the larger particle-particle distances at low polymer concentrations. At short-particle particle distances and/or higher polyelectrolyte concentrations the particle-particle interaction becomes a depletion-type attraction. For Zwitterionic positively chaged paticles particles we have found a a range [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S22.00014: Dispersing Functionalized Nanoparticles in PEO-based Single Ion Conductors Michael O'Reilly, Karen Winey Lithium single-ion conductors have the potential to reach high lithium transference numbers and high viscosities, but demonstrate poor ion transport properties. Ion mobility is inversely related to structural rigidity, so the highest ionic conductivity is usually achieved by ionomers with the most liquid-like properties. Solid nanofillers designed to enhance ion dynamics at the particle-polymer interface may improve the viscosity of an ionomer without arresting ion mobility. We demonstrate how silica nanoparticles are functionalized for favorable and unfavorable interactions with a sulfonated PEO-based ionomer matrix. We find that nanoparticle surface chemistry and loading fraction have implications on thermal properties, nanoparticle dispersion, viscosity, and lithium conduction. For functional groups designed for favorable ionomer interactions but poor nanoparticle dispersion, the viscosity of the nanocomposite may be altered significantly while negligibly affecting ionic conductivity even at high volume fractions of non-ionic filler. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S22.00015: Dramatic Changes in Polyelectrolyte Blend Phase Behavior due to Charge Correlations Monica Olvera de la Cruz, Jos Zwanikken, Charles Sing Polymer blends are typically used to impart a unique combination of properties to a material using a blend of two or more common homopolymers. Designing such systems requires knowledge of the mixing thermodynamics of the two polymers being blended; if one species is charged (e.g. an ionomer), then the thermodynamics is highly dependent on both the tendency of the system to mix (via the ubiquitous Flory $\chi$-parameter) and the electrostatics of the charged polymer backbone and the corresponding counterions. While mean-field theories treat the former well, the latter is difficult in low dielectric constants due to the inadequacy of perturbation theories in describing highly-correlated charged structures. We demonstrate that a new hybrid liquid state integral equation-self consistent field theory (LS-SCFT) calculation can provide articulation of both local charge correlations as well as macroscopic thermodynamics, and show that these correlations can profoundly affect polyelectrolyte blend phase behavior. Ultimately, even polymers that mix at all temperatures ($\chi N= 0$) can be driven to phase separate upon inclusion of charges. [Preview Abstract] |
Session S23: Invited Session: Industrial Physics Forum: Frontiers of Nanomaterials and Interfaces
Sponsoring Units: FIAPChair: Ichiro Takeuchi, University of Maryland, Luigi Colombo, Texas Instruments Incorporated
Room: 505-507
Thursday, March 6, 2014 8:00AM - 8:36AM |
S23.00001: Novel Carbons as Electrodes for Electrical Energy Storage Invited Speaker: Rodney S. Ruoff In this talk I will speculate about directions for carbon materials as the electrode(s) in EES systems such as ultracapacitors and Li ion batteries. Perhaps the penultimate electrode material for ultracapacitors (based on charge storage by electrical double layer capacitance, EDLC) would be a ``negative curvature carbon'' (NCC, akin to the Schwartzite structures) with atom thick walls, and possibly substitutionally doped with, e.g., N atoms in case the all-carbon structure were limited by quantum (i.e., intrinsic) capacitance. Such an NCC would have a distribution of pore sizes that would likely (for optimal performance) span ``mesoscale'' and ``microscale'' pores, which in the parlance of porous materials means pores ``above 2-3 nanometers'' and pores ``below about 2 nanometers,'' respectively. Making such materials offers exciting challenges for materials chemists/synthetic chemists, and to date only the ``basic'' Schwarzite structures (ideal crystals studied by DFT with periodic boundary conditions and relatively simple unit cells) have been modeled in terms of properties such as their electronic states and in some cases, potential as all carbon ferromagnets. I identified the NCCs as candidates for EES for ultracapacitors, in a paper published in Science in 2011 with coauthors. We made an aperiodic carbon that had atom thick walls and surface areas as high as 3200 m2/g, along with ``good'' powder electrical conductivity, high carbon content, and apparently close to 100\% trivalently bonded carbon in the walls of this very porous carbon. We have learned in one set of experiments, as published in Energy and Environmental Science, that doping with N atoms can increase the EDLC, which we suggest could be a consequence of limiting quantum capacitance in the all-carbon analogue. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S23.00002: Nanocellulose as Material Building Block for Energy and Flexible Electronics Invited Speaker: Liangbing Hu In this talk, I will discuss the fabrications, properties and device applications of functional nanostructured paper based on nanocellulose. Nanostructures with tunable optical, electrical, ionic and mechanical properties will be discussed. Lab-scale demonstration devices, including low-cost Na-ion batteries, microbial fuel cells, solar cells, transparent transistors, actuators and touch screens will be briefly mentioned. These studies show that nanocellulose is a promising green material for electronics and energy devices. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S23.00003: Directed Self-Assembly of III-V Semiconductor Nanowire and 2D Atomic Crystal Nanosheet Arrays for Advanced Nanoelectronic Devices Invited Speaker: Theresa Mayer A variety of advanced materials and structures are being explored for next-generation ultra-low-power nanoelectronic devices to augment the capabilities provided by Si-based complementary logic. Interband tunneling field effect (TFET) transistors are particularly attractive because of their sub-60 mV/dec subthreshold swing (SS) and high current drive capabilities. This talk will provide an overview of recent progress to integrate abrupt, axially doped InGaAs nanowire TFET arrays and 2D atomic crystal nanosheets onto Si substrates using electric-field directed self-assembly. This strategy has enabled fabrication of the first lateral p$^{\mathrm{+}}$-i-n$^{\mathrm{+}}$ InGaAs nanowire TFETs with up to ten parallel aligned wires to study the effect of aggressive scaling on device figures of merit. Arrays of micron-scale, few-layer 2D layered group IV-monochalcogenide and transition metal dichalcogenide crystals are also being assembled for subsequent Hall and field-effect mobility measurements. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:24AM |
S23.00004: Materials 3.0 - Nanomaterials and The Next Revolution in Materials Invited Speaker: Sadasivan Shankar Materials have played a central role during all advances in human civilization as far back as recorded histories exist. It is possible to characterize the application of materials in technologies as three distinct eras. In the first era, during the industrial revolution materials were mainly used for structural and functional applications. In the second era which includes the information technology revolution, material properties were exploited by integrating them in structures and combining different materials in a systematic manner. In the next era, we indicate that materials application will enter the next era in which size will be used to design materials with targeted properties. In this, for the advent of so-called ``smart'' materials, nano dimensions (between atomic and macrostructures) where both properties and synthesis will lead to may new applications, where differences between devices and materials will disappear. ``Nanomaterials'' will have newer properties because of many new phases the ability to manufacture the using nanotechnology. However, this will also pose challenges in terms of modeling and characterization given the complex nature of the materials and also due to increasing effects of interfaces of these materials. We will outline with examples from multiple industries. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 11:00AM |
S23.00005: How Events at the Nano/Bio Interface Determine Good and Adverse Biological Outcomes Invited Speaker: Andre Nel We have come to recognize that much of biology is executed at the nanoscale level, therefore providing a rational approach to using discovery about the structure and function of engineered nanomaterials (ENMs) at the nano/bio interface for interrogation of disease, diagnosis, treatment, and imaging at levels of sophistication not possible before. Moreover, the behavior of ENM's at the nano/bio interface also constitutes the basis for hazard generation and is therefore key for understanding the safety assessment and safer design of nanomaterials. In this overview, I will discuss how discovery at the molecular, cellular, organ and systemic nano/bio interfaces has helped us to make progress in the fields of nanomedicine and nanotoxicology. I will explain how the physicochemical properties of nanomaterials relate to nanoscale interactions at the membrane, intracellular organelles, tissues and organs in response to exposure to a variety of commercial ENMs as well as for therapeutic nanocarriers. I will delineate how the use of high throughput screening to establish structure-activity relationships can be used for the design of improved nanocarriers for cancer treatment as well as hazard and risk ranking of large categories of commercial ENMs on their way to the marketplace. [Preview Abstract] |
Session S24: Detectors, Sensors, and Transducers
Sponsoring Units: GIMSChair: Rob Duncan, University of Missouri
Room: 504
Thursday, March 6, 2014 8:00AM - 8:12AM |
S24.00001: Fabrication and Characterization of a Nanocoax-Based Electrochemical Sensor Binod Rizal, Michelle M. Archibald, Jeffrey R. Naughton, Timothy Connolly, Stephen C. Shepard, Michael J. Burns, Thomas C. Chiles, Michael J. Naughton We used an imprint lithography process to fabricate three dimensional electrochemical sensors comprising arrays of vertically-oriented coaxial electrodes, with the coax cores and shields serving as working and counter electrodes, respectively, and with nanoscale separation gaps.\footnote{B. Rizal, M. M. Archibald, T. Connolly, S. Shepard, M. J. Burns, T. C. Chiles, and M. J. Naughton, Anal.Chem. 85, 10040 (2013).} Arrays of devices with different electrode gaps (coax annuli) were prepared, yielding increasing sensitivity with decreasing annulus thickness. A coax-based sensor with a 100 nm annulus was found to have sensitivity ~100 times greater than that of a conventional planar sensor control, which had millimeter-scale electrode gap spacing. We suggest that this enhancement is due to an increase in the diffusion of molecules between electrodes, which improves the current per unit surface area compared to the planar device. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S24.00002: Geometric Studies of Shunt and Lead Orientation in EEC Devices F.M. Werner, S.A. Solin Electric field sensors are ubiquitous in modern technology, from field effect transistors (FETs) in circuit boards to point-of-care testing (POCT) devices used in detecting the presence of specific protein markers in blood. The transport properties of these devices are limited by two general categories: intrinsic material properties and extrinsic geometric effects. Devices with a maximum electric field resolution of 3.05V/cm were previously reported [1-2]. The metal semiconductor hybrid (MSH) devices are constructed by forming a Schottky interface between a mesa of nGaAs and Ti, while four ohmic leads surround the perimeter of the mesa and are used for four point resistance measurements. These devices exhibit extraordinary electroconductance (EEC) and make it possible to correlate measured four point resistance to changes in the local electric field. While maximizing the EEC response by optimizing the intrinsic material properties has been theoretically investigated [2], we present a phenomenological study of the impact of lead orientation and shunt geometry in the sensing capabilities of these devices. Ref [1] Yun Wang, et al, Appl. Phys. Lett. 92, 262106 (2008). [2] A.K.M. Newaz, et al, Phys Rev B. 79, 195308 (2009). [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S24.00003: Fabrication and Characterization of a Novel Nanodendrite-based Electrochemical Sensor for the Detection of Disease Biomarkers Timothy Connolly, Michelle M. Archibald, Nathan T. Nesbitt, Matthew Rossi, Jennifer A. Glover, Michael J. Burns, Michael J. Naughton, Thomas C. Chiles Technologies to detect early stage cancer would provide significant benefit to cancer disease patients. Clinical measurement of biomarkers offers the promise of a noninvasive and cost effective screening for early stage detection. We are currently developing a novel 3-dimensional nanopillar dendrite biosensor array for the detection of human cancer biomarkers ($e.g.$ CA-125 for early-stage ovarian cancer) in serum and other fluids. Here, we describe a nanoscale 3D architecture that can afford molecular detection at room temperature. We report our efforts on the development of an all-electronic, ambient temperature, rapid-response dendritic biosensor fabricated by directed electrochemical nanowire assembly (DENA) that achieves molecular-scale sensitivity for protein biomarker based detection. Each sensor is a vertically-oriented nanodendritic array where an electrochemical signal is detected from the oxidation of the redox end-product of an enzyme-linked immunosorbent assay (ELISA). Our results demonstrate the feasibility of using the present nanodendritic array structure as a sensitive device to detect a range of proteins of interest, including disease biomarkers. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S24.00004: Shear piezoelectric coefficients of PZT, LiNbO$_3$ and PMN-PT at cryogenic temperatures Syed Bukhari, Md Islam, John Beamish, Ariel Haziot Piezoelectric transducers are widely used as sensitive detectors of stress and to generate nanometer scale displacements. However, their piezoelectric coefficients often decrease substantially at cryogenic temperatures, limiting their performance in, e.g., low temperature STMs. We have recently used PZT shear transducers to measure the elastic modulus of solid $^4$He at very low strains and to plastically deform the helium at high strains. From our elastic measurements, we inferred a shear piezoelectric coefficient d$_{15} = $1.0x10$^{-10}$ m/V at temperatures below 1 K. This is about 6 times smaller than the room temperature value for PZT and comparable to d$_{15}$ for single crystal LiNbO${_3}$ transducers (7x10$^{-11}$ m/V). We have developed a capacitive technique and have directly measured the temperature dependence of d$_{15}$ for ceramic (PZT) and single crystal (LiNbO$_3$ and PMN-PT) shear transducers. PMN-PT has an extremely large d$_{15}$ at room temperature (4x10$^{-9}$ m/V) but it decreases rapidly below 100 K. LiNbO$_3$ has the smallest room temperature d$_{15}$, but it is nearly temperature-independent. At 4 K, the three types of transducers have similar piezoelectric shear coefficients. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S24.00005: Characterizing Random Telegraph Frequency Noise in a Micromechanical Oscillator Fengpei Sun, Jie Zou, Zakhar Maizelis, Ho Bun Chan We perform a comprehensive study of the effect of random telegraph frequency noise(RTFN) on a micromechanical torsional oscillator. A sinusoidal driving voltage is applied to one electrode of the oscillator to excite its torsional vibration. Telegraph noise is applied to the other electrode so that the eigenfrequency of the oscillator randomly jumps back and forth between two states. This arrangement resembles a mechanical oscillator dispersively coupled to a classical or quantum two-level system. As the jumping rate of the eigenfrequency is increased, the two peaks in the spectrum of the time-averaged vibration amplitude merge into a single peak, displaying spectral broadening followed by motional narrowing. Furthermore, we analyze the ratios of the moments of the complex vibration amplitude to the powers of the averaged complex amplitude as a function of the driving frequency. If RTFN is absent, the ratios are equal to one; otherwise they deviate from one near resonance and approach to one far off resonance. The shape of the spectra depends strongly on the characteristics of RTFN and this dependence remains valid even in the presence of strong thermal or detector noise. Our results are in good agreement with theoretical predictions. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S24.00006: Discovery of higher order modes in a cylindrical reentrant-ring cavity resonator for high sensitivity displacement measurements Michael Tobar, Yaohui Fan, Zhengyu Zhang, Natalia Carvalho, Jean-Michel Le Floch, Qing Shan A microwave reentrant cavity transducer is a highly sensitive transducer, which has been developed in the past for many precision applications, including gravitational wave detection, high sensitivity optomechnics and investigating the dynamic Casimir effect. Such systems may be used for displacement measurements, sideband cooling, amplification of mechanical motion and investigating quantum behavior of mechanical resonators. The key component of the reentrant transducer is a narrow-gap superconducting reentrant cavity, which has achieve high displacement sensitivity and electrical Q-factor at low temperatures. Rigorous analysis of the properties of resonant modes in such a structure comprising of a post and ring is undertaken and verified experimentally. For the first time we show the existence of higher order reentrant cavity modes, with a significantly better displacement sensitivity compared to the common fundamental mode in a reentrant cylindrical cavity with just a single post. Thus, this type of cavity has the potential to operate as a highly sensitive transducer for a variety of precision measurement applications. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S24.00007: Terahertz heterodyne detection with high-Tc superconducting Josephson junctions Maxime Malnou, Cheryl Feuillet-Palma, Alan Luo, Thomas Wolf, Christian Ulysse, Pascal Fevbre, Jerome Lesueur, Nicolas Bergeal The terahertz region of the electromagnetic spectrum [0.3-10THz] has, so far, not been exploited fully due to the lack of suitable sources and detectors. Indeed, THz frequency lies between the frequency range of traditional electronics and photonics where the existing technology cannot be simply extended. Superconductor-insulator-superconductor Niobium tunnel junctions that are currently used as mixing element in heterodyne receivers are intrinsically limited in frequency by the energy gap of Nb and operate only at low temperature (4.2K). An alternative to these devices consists of using High-Tc superconducting receivers. Over the past years, we have developed a new approach based on ion irradiation to make Josephson nano-junctions with YBa2Cu3O7 thin films [1,2]. In this talk we will present the fabrication process we developed and a study of the high-frequency mixing properties of such junctions from 20GHz to 400 GHz [3]. Finally, we will present the ongoing work to build an integrated heterodyne receiver that operates with an on-chip Josephson local oscillator.\\[4pt] [1] N. Bergeal et al. Appl. Phys. Lett. 87, 102502 (2005).\\[0pt] [2] N. Bergeal et al. J. Appl. Phys. 102, 083903 (2007).\\[0pt] [3] M. Malnou et al. Appl. Phys. Lett. 101, 233505 (2012). [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S24.00008: Direct X-ray detection with hybrid solar cells based on organolead halide perovskites Hardeep Singh Gill, Bassem Elshahat, Erno Sajo, Jayant Kumar, Akshay Kokil, Piotr Zygmanski, Lian Li, Ravi Mosurkal Organolead halide perovskite materials are attracting considerable interest due to their exceptional opto-electronic properties, such as, high charge carrier mobilities, high exciton diffusion length, high extinction coefficients and broad-band absorption. These interesting properties have enabled their application in high performance hybrid photovoltaic devices. The high Z value of their constituents also makes these materials efficient for absorbing X-rays. Here we will present on the efficient use of hybrid solar cells based on organolead perovskite materials as X-ray detectors. Hybrid solar cells based on CH$_{3}$NH$_{3}$PbI$_{3}$ were fabricated using facile processing techniques on patterned indium tin oxide coated glass substrates. The solar cells typically had a planar configuration of ITO/CH$_{3}$NH$_{3}$PbI$_{3}$/P3HT/Ag. High sensitivity for X-rays due to high Z value, larger carrier mobility and better charge collection was observed. Detecting X-rays with energies relevant to medical oncology applications opens up the potential for diagnostic imaging applications. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S24.00009: Bright Cathodoluminescent Thin Films for Scanning Nano-Optical Excitation and Imaging Craig Hetherington, D.M. Kaz, C.G. Bischak, H.H. Howard, X. Marti, J.D. Clarkson, C. Adamo, D.G. Schlom, R. Ramesh, S. Aloni, D.F. Ogletree, N.S. Ginsberg Demand for visualizing nanoscale dynamics in biological and advanced materials continues to drive the development of sub-diffraction optical probes. While many strategies employ scanning tips for this purpose, we instead exploit a focused electron beam to create scannable nanoscale optical excitations in an epitaxially grown thin-film of cerium-doped yttrium aluminum perovskite (YAP:Ce), whose cathodoluminescence response is bright, robust, and spatially resolved to 18 nm. We also demonstrate lithographic patterning of the film's luminescence at the nanoscale. In combination with our observation of resonant energy transfer (RET) from the film to nearby fluorophores, preliminary results after exposing both faces of the scintillating film point toward a powerful means of fast, bright, non-invasive near-field optical microscopy without the complication of mechanical scanning. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S24.00010: Semiconducting boron carbide polymers devices for neutron detection Elena Echeverria, Frank L. Pasquale, Robinson James, Juan A. Col\'on Santana, Shireen Adenwalla, Jeffry A. Kelber, Peter A. Dowben Boron carbide materials, with aromatic compounds included, prove to be effective materials as solid state neutron detector detectors. The I-V characteristic curves for these heterojunction diodes with silicon show that these modified boron carbides, in the presence of these linking groups such as 1,4-diaminobenzene (DAB) and pyridine, are p-type. Cadmium was used as shield to discriminate between neutron-induced signals and thermal neutrons, and thermal neutron capture is evident, while gamma detection was not realized. Neutron detection signals for these heterojunction diode were observed, a measurable zero bias current noted, even without complete electron-hole collection. This again illustrates that boron carbide devices can be considered a neutron voltaic [1]. \\[4pt] [1] N. Hong, J. Mullins, K. Foreman, S. Adenwalla, J. Phys. D-Appl. Phys. 43 (2010) 275101 [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S24.00011: PTIS (Photo-Thermal Ionization Spectroscopy) and its application in HPGe purification and crystal growth Yutong Guan, Jayesh Govani, Gang Yang, Guojian Wang, Chaoyang Jiang, Dongming Mei Detector fabrication requires high pure Germanium crystal with impurity level of $\sim$ 1010/cm3. To reach such a low impurity level, it's important to identify the impurity and trace its source during zone refining and crystal growth. PTIS (Photo-thermal ionization spectroscopy) is the combination of Fourier Transform Infrared Spectroscopy and photo-thermal ionization of shallow impurities (acceptors and donors). Working with JASCO, we have developed a PTIS at USD. With a PTIS in house, we identify the major impurities, boron, aluminum and phosphor, in HPGe. The feedback is provided to control the parameters and procedure for the zone refining and crystal growth. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S24.00012: Atomic Force Microscope using Length-Extension Resonator, revealing an atomic resolution Sungmin Kim, Myungchul Oh, Hyung Joon Cho, SangJun Jeon, Minjun Lee, Beomyong Hwang, Seong Joon Lim, Young Kuk Various molecular images with an sub-angstrom have been reported using an Atomic Force Microscope (AFM) force-sensed with a qPlus tuning fork. Length-Extension Resonator (LER) is alternative way to achieve the same goal. An LER has usually a higher resonance frequency and higher Q value that result in an order of magnitude higher sensitivity than a q-plus sensor. The noise can be reduced from the frequency dependence of 1/$\surd $f. In this study we used an LER with the resonance frequency of $\sim$ 630 KHz. We designed and fabricated a cryogenic temperature current preamplifier having an 1 MHz bandwidth. This AFM was operated at 4.5 K under ultrahigh vacuum. Various molecular images will be presented in this presentation. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S24.00013: MOVED TO M23.002 |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S24.00014: Variable Temperature Setup for Scanning Electron Microscopy in Liquids and Atmospheric Pressure Gaseous Environments Ahmed Al-Asadi, Jie Zhang, Jianbo Li, Lauraine Denault, Radislav Potyrailo, Andrei Kolmakov A thermoelectric cooling / heating setup for commercial Quantomix QX WETSEM scanning electron microscopy environmental cells was designed and tested. This addition allows extending ambient pressure \textit{in situ} studies to be conducted in a wide temperature range both in liquid and gaseous environments. Instead of cooling/heating the entire body of QX-WETCELL, ultrathin polyimide electron transparent membrane window supported by metal mesh on the top of the cell has been used as an agent for heat transfer to/ from the Pelltier element. A butterfly wing of \textit{Morph sulkowskyi} has been used as a model object in the QX-WETCELL's chamber due to its unique micro/nanostructure and peculiar wettability behavior. The dynamics of the water desorption, condensation and freezing processes were observed complementary using both optical microscopy and Scanning Electron Microscopy \textit{in} \textit{vivo}. The observations revel that the initial droplet formation were most likely taking place on the top of the wing ridges due to the waxy component of its surface. In addition, The SEM observation showed that the high intensity electron beam can heat the butterfly wing locally delaying the water condensation and freezing processes. [Preview Abstract] |
Session S25: Focus Session: Thermoelectrics - Phonons and Heat Conduction II
Sponsoring Units: DMP GERA FIAPChair: Jonathan Malen, Carnagie Mellon University
Room: 503
Thursday, March 6, 2014 8:00AM - 8:36AM |
S25.00001: On Minority Carrier Scattering for Thermoelectrics Invited Speaker: Jihui Yang Most of the past studies on thermoelectric materials have been focused on majority carriers and lattice phonons in heavily doped semiconductors. In this talk I will show that minority carriers, however, could have a significant impact on both electrical and thermal transport, especially at elevated temperatures. I will also describe means of improving thermoelectric performance of heavily doped semiconductors via selective minority carrier scattering. These results offer insights for understanding experimental findings and optimizing thermoelectric properties of narrow band-gap semiconductors.\\[4pt] In collaboration with Shanyu Wang, Jiong Yang, and Trevor Toll, Department of Materials Science and EngineeringUniversity of Washington, Seattle, WA, USA. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S25.00002: Low Cost Advanced Thermoelectric (TE) Technology for Automotive Waste Heat Recovery G.P. Meisner Low cost, fully integrated TE generators (TEGs) to recover waste heat from vehicle exhaust will reduce transportation sector energy consumption and emissions. TEGs will be the first application of high-temperature TE materials for high-volume use and establish new industrial sectors with scaled up production capability of TEG materials and components. We will create a potential supply chain for practical automotive TEGs and identify manufacturing and assembly processes for large scale production of TEG materials and components. Our work focusses on several innovative R{\&}D paths: (1) enhanced TE material performance by doping and compositional tuning, (2) optimized TE material fabrication and processing to reduce thermal conductivity and improve fracture strength, (3) high volume production for successful skutterudite commercialization, (4) new material, nanostructure, and nanoscale approaches to reduce thermal interface and electrical contact resistances, (5) innovative heat exchangers for high efficiency heat flows and optimum temperature profiles despite highly variable exhaust gas operating conditions, (6) new modeling and simulation tools, and (7) inexpensive materials for thermal insulation and coatings for TE encapsulation. Recent results will be presented. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S25.00003: High-Temperature, High-Concentration Solar Thermoelectric Generators Emily Warren, Lauryn Baranowski, Michele Olsen, Paul Ndione, Judy Netter, Alan Goodrich, Matthew Gray, Philip Parilla, David Ginley, Eric Toberer Solar thermoelectric generators (STEGs) powered with concentrated solar energy have potential for use as primary energy converters or as topping-cycles for more conventional concentrated solar power (CSP) technologies. Modeling based on current record modules from JPL suggests thermoelectric efficiencies of 18$\%$ could be experimentally expected with a temperature gradient of 1000 $-$ 100$^\circ$C. Integrating these state-of-the-art TEGs with a concentrating solar receiver requires simultaneous optimization of optical, thermal, and thermoelectric systems. This talk will discuss the modeling, design, and experimental testing of STEG devices under concentrated sunlight. We have developed a model that combines thermal circuit modeling with optical ray tracing to design selective absorber coatings and cavities to minimize radiation losses from the system. We have fabricated selective absorber coatings and demonstrated that these selective absorber films can minimize blackbody radiation losses at high temperature and are stable after thermal cycling to 1000$^\circ$C. On-sun testing of STEG devices and thermal simulators is ongoing and preliminary results will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S25.00004: Rattler behavior in As skutterudites and oxy-skutterudites Frank Bridges, Brad Car, Mikaela Hoffman-Stapleton, Trevor Keiber, Logan Sutton, M. Brian Maple We report EXAFS measurements for the series CeX$_4$As$_{12}$ (X = Fe, Ru, Os) and NdCu$_3$Ru$_4$O$_{12}$ as a function of temperature for most elements in the structure. In each case the rare earth atom is a ``rattler'' atom, with a low Einstein temperature while the skutterudite cage structure is relatively stiff. From temperature dependencies of the correlated Debye model for the cage atoms, one can estimate the effective spring constant for various atom pairs. We also find for the oxy-skutterudites that the planar CuO$_4$ sub-structure is very stiff, and likely vibrates as a rigid unit. We compare the behavior of the As-skutterudites with other skutterudites and with the oxy-skutterudites, and discuss in terms of the rigid cage model. The second neighbor pair Ce-X for the As-skutterudites is softer than expected while for the oxy-skutterudites the second neighbor Nd-Ru pair is stiffer than the nearest neighbor Nd-O pair. Models are need to explore this behavior. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S25.00005: Modeling correlated motion in thermoelectric skutterudite materials Trevor Keiber, Frank Bridges Filled skutterudite compounds, LnT$_{4}$X$_{12}$ (Ln=rare earth; T=Fe,Ru,Os; X=P,As,Sb), have previously been modeled using a rigid cage approximation for the ``rattling'' rare earth atom. The large thermal broadening with temperature of the rattler can be fit using an Einstein model. Recent measurements of the second neighbor Ln-T peaks show an unusually large thermal broadening suggesting motion of the cage of atoms. To incorporate these results we developed three and four mass spring models to give the acoustic and optical phonon mode spectra. For the simplest three mass model we identify the low energy optical mode as the rattling mode. This rattling mode is likely coupled to the acoustic mode, and responsible for the low thermal conductivity of the skutterudite compound. We extend this model to four atoms to describe the CuO$_{4}$ rings in oxy-skutterudites and the X$_{4}$ rings in LnT$_{4}$X$_{12}$. This talk provides a model for the experimental results of the previous presentation. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S25.00006: Effect of triple fillers on thermoelectric properties of p-type skutterudites Tulashi Dahal, Qing Jie, Zhifeng Ren Experiments were carried out to investigate the effect of triple fillers on the thermoelectric properties of p type skutterudites. We have synthesized the samples by hot pressing nano powders made by ball milling annealed ingots of Ca$_{\mathrm{x}}$Ce$_{\mathrm{y}}$Nd$_{\mathrm{y}}$Fe$_{3.5}$Co$_{0.5}$Sb$_{12}$. By tuning the concentration of Ca, Ce, and Nd, we have achieved a lower thermal conductivity $\sim$ 2 W m$^{-1}$ K$^{-1}$ at room temperature and $\sim$ 2.6 W m$^{-1}$ K$^{-1}$ at 530 $^{\circ}$C), leading to a peak ZT of about 1.1 at 475 $^{\circ}$C. The observed lower thermal conductivity can be attributed due to a broad range of phonon scattering due to multiple fillers. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S25.00007: Doping CoSb3 p-type with Al substitution for Sb Michael J. Adams, Michele D. Nielsen, Joseph P. Heremans Skutterudites such as CoSb$_{3}$ are compounds composed of group IX-B atoms (Co, Rh, and Ir) forming a simple cubic structure, and group V-A$_{3}$ pnictide atoms (primarily Sb and As) forming rings inside 6 of every 8 cubes. The remaining cubes remain empty. A common method for reducing thermal conductivity is to introduce impurity atoms such as rare-earths in the cubes that act as rattlers. P-type doping of CoSb$_{3}$ has led to some advances in zT, but the p-type material remains less performing than the n-type material due to the fact that the valence band, dominated by Sb levels, has a low effective mass. A promising method for improving p-type properties is to introduce an effective resonant level into the energy levels occupied by the light hole band, thereby increasing the Seebeck coefficient without strongly effecting other transport properties. A first attempt using Sn substitution was not successful. Here we try various concentrations of Al substituted at Sb sites to generate a resonant level. Material properties are measured and compared with a calculated Pisarenko relation, where thermopower is plotted as a function of hole concentration. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S25.00008: Systematic Studies on Anharmonicity of Rattling Phonons in Type I Clathrates by Low Temperature Heat Capacity Measurements Katsumi Tanigaki, Jiazhen Wu, Yoichi Tanabe, Satoshi Heguri, Hidekazu Shiimotani Clathrates are featured by cage-like polyhedral hosts mainly composed of the IV$^{th}$ group elements of Si, Ge, or Sn and alkali metal or alkaline-earth metal elements can be accommodated inside as a guest atom. One of the most intriguing issues in clathrates is their outstanding high thermoelectric performances thanks to the low thermal conductivity. Being irrespective of good electric conductivity $\sigma$, the guest atom motions provide a low-energy lying less-dispersive phonons and can greatly suppress thermal conductivity $\kappa$. This makes clathrates close to the concept of ``phonon glass electron crystal: PGEC'' and useful in thermoelectric materials from the viewpoint of the figure of merit. In the present study, we show that the local phonon anharmonicity indicated by the tunneling-term of the endohedral atoms ($\alpha$T) and the itinerant-electron term ($\gamma_{e}$T), both of which show T-linear dependences in specific heat C$_{p}$, can successfully be separated by employing single crystals with various carrier concentrations in a wide range of temperture experimennts. The factors affecting on the phonon anharmonicity as well as the strength of electron-phonon interactions will be discussed based on our recent experiments. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S25.00009: Computational Study of the Vibrational, Thermal and Transport Properties of the Type II Tin Clathrate Compounds Cs$_{\mathrm{x}}$Sn$_{136}$ (x $=$ 12, 16, 20) Dong Xue, Charles Myles The Type II clathrates A$_{\mathrm{x}}$B$_{136}$ (A $=$ alkali atom; B $=$ Si, Ge, Sn) are interesting because of their low lattice thermal conductivity and thermoelectric properties. Their low thermal conductivity is due to the low-frequency vibrational modes produced by the alkali guests in the host lattice cages. Heat transport theory predicts that such modes will scatter with the acoustic phonons of the host, reducing the lattice thermal conductivity. To understand this effect for the Type II Sn clathrates, we have performed a first principles computational study of the vibrational, thermal, and transport properties of Cs$_{\mathrm{x}}$Sn$_{136}$ (x $=$ 12,16,20). Our calculations use the VASP and PHONOPY codes to calculate the vibrational modes. We present results for the phonon modes, the heat capacity, and the Gruneisen parameter in Cs$_{\mathrm{x}}$Sn$_{136}$. Our calculated Cs modes are in the range 8-15 cm$^{-1}$ and we find that these frequencies decrease as x increases. Our results for the vibrational contribution to the heat capacity predict that it weakly depends on x, and that it increases smoothly with temperature T, approaching the Dulong-Petit value at T $=$ 600 K. Our calculations of the Gruneisen parameter predict that it has a weak x dependence, it increases with increasing T for 160 K $\le $ T $\le $ 300 K and it has a weak dependence on T for 300 K $\le $ T $\le $ 550 K. We have used these results to calculate the lattice thermal conductivity in Cs$_{\mathrm{x}}$Sn$_{136}$ within the kinetic theory approximation. The results of these calculations are also presented and discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S25.00010: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S25.00011: Reduction of thermal conductivity in silicon phononic metamaterials William Jones, Axel Scherer, Slobodan Mitrovic We explore the limits of thermal conductivity reduction through phononic design of single crystal silicon membranes by direct measurement via thermal bridge method. Phononic metamaterials with nanoscale critical dimensions can modify the dispersion for heat carrying phonons via Brillouin-zone folding. In monolithic crystalline thin films, thermal conductivity can be further reduced by employing a superstructure of these patterned regions. We hypothesize that this approach can reduce the thermal conductivity due to phonon reflection. We also discuss the potential of these structures to enable a true phonon-glass, electron-crystal material and push the limits of thermoelectric cooling/heat conversion efficiency. We predict that the high electrical conductivity of doped single crystal silicon will endow this phononic metamaterial with a high ZT characteristic. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S25.00012: Thermal transport in nanostructured silicon membranes Sanghamitra Neogi, Luiz F.C. Pereira, Davide Donadio The recent focus in thermal management in nanostructures and energy harvesting using thermolectric devices has motivated the interest towards understanding the role of phononic thermal transport in these nanoscale materials. One way to obtain thermoelectric systems with improved efficiency is to engineer nanostructured semiconductors, so as to reduce the thermal conductivity of the crystalline materials while preserving their electronic properties [1]. Our study is driven towards understanding the nature of phononic thermal transport in nanostructured silicon membranes. We use harmonic lattice dynamics and classical molecular dynamics to compute the phonon transport properties in Si membranes, with thickness up to ~20 nm. We show that dimensionality reduction has a significant effect on the vibrational properties and leads to a 4-fold reduction in the thermal conductivity of the membranes. Combining dimensional reduction with surface modification, we obtain a reduction in the thermal conductivity of the membranes to a factor of 20 with respect to the bulk, implying a 20-fold enhancement of ZT at room temperature. Such figures make nanostructured silicon membranes viable materials for thermoelectric units. [1] M. S. Dresselhaus et al, Adv. Mater., 22, 3970 (2010). [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S25.00013: Probing Large-Wavevector Phonons in Strain-Relief Patterned Silicon/Silicon Germanium Heterostructure Nanomembranes Kyle McElhinny, Gokul Gopalakrishnan, Don Savage, Max Lagally, Martin Holt, Paul Evans Freestanding Si/SiGe nanomembranes have promising thermal properties as a result of the ability to decouple the electronic and thermal transport. Challenges in fabrication of freestanding Si/SiGe nanomembranes arise due to buckling in reaction to stresses generated by the lattice mismatch between the Si and SiGe layers. This results in an equilibrium state where the elastic energy is minimized through a buckling distortion. We demonstrate that the strain and curvature of these membranes is reduced by nearly an order of magnitude through the strain-relief patterning of the buckled membrane. X-ray thermal diffuse scattering (TDS) studies of these membranes evaluate the effect of confinement on the phonon dispersion of the Si/SiGe heterostructure by probing the populations of acoustic phonons at wavevectors spanning the Brillouin zone. A comparison between the x-ray TDS intensity distributions of Si and Si/SiGe heterostructure nanomembranes demonstrates the importance of fabricating SiGe nanostructures with reduced strain and curvature. Results of these experiments show deviations in TDS intensity compared to bulk and Si nanomembranes. In Si nanomembranes these deviations have previously indicated a softening of 1-2 meV at large wavevectors. [Preview Abstract] |
Session S26: Focus Session: Materials in Extremes
Sponsoring Units: GSCCM DCOMP DMPChair: Marc Cawkwell, Los Alamos National Laboratory
Room: 502
Thursday, March 6, 2014 8:00AM - 8:12AM |
S26.00001: Unusual magnetic fields of Uranus and Neptune William Nellis Voyager 2 measured spatial distributions of the magnetic fields of Uranus and Neptune (U/N) in the 1980s. Prior to Voyager 2 known planetary magnetic fields were dipolar with dipole axis aligned closely with the axis of rotation. Surprisingly, the fields of U/N are non-dipolar and non-axisymetric. If those field geometries are force-fit to dipoles, the dipole axes are tilted $\sim$45 deg. from the axes of rotation and off-centered by 30\% of planet radii. Stanley and Bloxham developed a 3D thin-shell dynamo model that matches measured field geometries, assuming fluid metal at radii below the inner radius of the thin shell is stably stratified [1]. Pressures and temperatures exceed $\sim$300 GPa and several 1000 K in that region. Consideration of measured electrical conductivities of metallic fluid H, N, O and of ionic water and SU (a representative Ice mixture) up to 180 GPa, a theoretical prediction of metallization of water at 300 GPa and several 1000 K, condensed matter physics of electrical conduction in disordered systems, and likely mutual solubilities suggests it is reasonable to expect stable stratification in the deep interiors of U/N, as assumed by Stanley and Bloxham. \\[4pt] [1] S. Stanley and J. Bloxham, Nature 248, 151 (2004). [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S26.00002: Structure at the bottom of an accreted neutron star crust, and at the top of a magnetized crust Tyler Engstrom, Noah Yoder, Vincent Crespi, Benjamin Owen, James Brannick, Xiaozhe Hu Neutron star crusts play a role in a growing list of observable phenomena. These include cooling and thermal structure of the star, gravitational wave emission, and quasi-periodic oscillations in the tails of magnetar flares. Below neutron drip density $4\times 10^{11}$ g/cc, an accreted crust contains a variety of nuclear species embedded in a relativistic, degenerate electron gas. We model interactions with Yukawa pair potentials, and carry out extensive structure searches using a genetic algorithm. The search results are used to calculate equilibrium phase diagrams for representative ternary systems. Pulsars are magnetic neutron stars with surface fields $\sim 10^{12}$ gauss. The outermost several meters of pulsar crust is a good candidate for description with the magnetic Thomas-Fermi model. We introduce a novel domain decomposition method for solving the nonlinear, periodized version of this model, and calculate the single-component phase diagram, equation of state, and other properties. Connections to astrophysical observables will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S26.00003: Nonlocal orbital-free density functional theory for warm dense matter Travis Sjostrom Accurate simulations of warm dense matter remain challenging in current research, while being motivated further as recent experiments probe more accurately into this regime. While the \textit{de facto} standard is quantum molecular dynamics using Kohn-Sham DFT, this methods scales significantly with temperature due to the orbital dependence. From the other side, the orbital-free Thomas-Fermi approximation works well for hot dense systems, but loses accuracy at lower temperatures. Recently developed nonlocal orbital-free functionals for the noninteracting free energy [Phys. Rev. B 88, 195103], which show near Kohn-Sham accuracy for broad ranges of temperature and density are presented. The application of which are detailed in regards to pseudopotentials and molecular dynamics for various systems. Comparisons with local orbital-free methods as well as orbital-dependent Kohn-Sham calculations, including accuracy and computational cost are made. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S26.00004: Optical Response of Warm Dense Matter Using Real-Time Electron Dynamics Andrew Baczewski, Luke Shulenburger, Michael Desjarlais, Rudolph Magyar The extreme temperatures and solid-like densities in warm dense matter present a unique challenge for theory, wherein neither conventional models from condensed matter nor plasma physics capture all of the relevant phenomenology. While Kubo-Greenwood DFT calculations have proven capable of reproducing optical properties of WDM, they require a significant number of virtual orbitals to reach convergence due to their perturbative nature. Real-time TDDFT presents a complementary framework with a number of computationally favorable properties, including reduced cost complexity and better scalability, and has been used to reproduce the optical response of finite and ordered extended systems. We will describe the use of Ehrenfest-TDDFT to evolve coupled electron-nuclear dynamics in WDM systems, and the subsequent evaluation of optical response functions from the real-time electron dynamics. The advantages and disadvantages of this approach will be discussed relative to the current state-of-the-art. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S26.00005: \textit{Ab initio} calculations of the electron momentum distribution function for ordered and disordered warm dense matter (WDM) E. Klevak, B.A. Mattern, J.J. Kas, J.J. Rehr, G.T. Seidler We report new calculations of the electron momentum distribution $n(p)$ for ordered and disordered materials of interest for warm dense matter research. The central role of the electron-ion interaction and the need to orthogonalize the valence-electron and core-electron wave functions has often been ignored in the interpretation of x-ray Thomson scattering studies of WDM.\footnote{Mattern, B.~A. et al. arXiv:\textbf{1308.2990} (2013)} This has led to substantial uncertainty in the inferred temperatures and ionization states in laser-shock generated dense plasmas. Real space Green's function calculations as a function of density and disorder are used to evaluate the possibility of a broadly applicable universal rescaling of the free-electron $n(p)$ by an effective volume and effective temperature to approximate the effects of valence-core orthogonalization. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S26.00006: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S26.00007: Controlling shock wave propagation in individual nanoplasmas: experiment and hydrodynamic simulations Daniel Hickstein, Wei Xiong, Franklin Dollar, Jennifer Ellis, Ellen Keister, Chengyuan Ding, Henry Kapteyn, Margaret Murnane, Jim Gaffney, Mark Foord, Stephen Libby, Brett Palm, Jose Jimenez, George Petrov By coupling a velocity-map-imaging spectrometer with a nanoparticle aerosol source, we present the first observations of individual nanoscale plasmas (nanoplasmas) generated from isolated nanoparticles. We show that short (40 fs) infrared (800 nm) laser pulses at a relatively low intensity (10$^{\mathrm{14}}$ W/cm$^{\mathrm{2}})$ are capable of driving shock waves in the expanding nanoplasma, providing a new method for studying shock physics in a relatively unexplored regime of dense, low-temperature, nanoplasmas. We demonstrate control of the shock waves by using a 400-nm heating pulse to pre-expand the plasma on a picosecond timescale, providing a significant enhancement in the intensity of the shock wave. Numerical hydrodynamic calculations using the HYDRA software reveal the mechanism for shock formation and suggest how the energy and intensity of the shocks can be tailored by adjusting the laser parameters. In addition, we generate nanoplasmas from various dielectric and conducting nanomaterials, and demonstrate that the direction of ion ejection can be controlled by changing the geometric shape of metal nanostructures. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S26.00008: Atomistic simulation of systems driven through phase transitions by hot electron distributions Xukun Xiang, Jenni Portman, Faran Zhou, Chong-yu Ruan, Frederique Pellemoine, Don Morelli, Phillip Duxbury A variety of pump-probe experiments are emerging to monitor the ultrafast structural response of materials. Typically a hot electron distribution is generated by an ultrafast laser pulse or by high energy particle beams, such as swift heavy ions. The hot electron distribution then thermalizes relatively quickly, on timescales in the 100fs range, while the lattice response is slower. Structural probes such as ultrafast electron diffraction or ultrafast x-ray diffraction, are able to image the structural response typically on timescales of 100fs to nanoseconds. In this presentation we discuss the results of simulations to elucidate this ultrafast structural response when materials are driven through a phase transition. Results for titanium, graphite and phase-change materials (such as Ge$_2$Sb$_2$Se$_5$) will be presented. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S26.00009: Propagation in atmosphere of ablated material from femtosecond laser machining of fused silica Trevor Bowman, Brian Canfield, Lloyd Davis Femtosecond laser pulses provide a means to machine structures with small heat-affected areas through highly non-linear mechanisms that enable direct writing of nanoscale features, which can be applied for fabricating a range of devices, including micro-optics and micro-fluidics. A single, tightly focused ultrashort pulse induces extreme conditions on sub-picosecond time-scales and forms a region of expanding plasma beyond the focal region. This plasma, which typically limits the depth of the nanoscale features to create shallow craters, results in the ejection of micro/nano-particles. The generation and use of these particles have a large range of applications in nanotechnology. We have studied the propagation, in atmosphere, of micro/nano particles ejected using single pulses from a 100 fs, 800 nm laser tightly focused with either a line or spot profile near the back surface of a fused silica substrate. The substrate was translated such that a fresh portion was ablated with each pulse. Time-gated images of the ejected material were taken using an intensified charged coupled device camera with additional illumination along the axial direction. Physical mechanisms and experimental results to date will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S26.00010: Exceptionally high aspect ratio micromachining with single femtosecond laser pulses Brian K. Canfield, Trevor S. Bowman, Alexander Terekhov, Lino Costa, Deepak Rajput, William H. Hofmeister, Lloyd M. Davis Traditional microchannel laser machining techniques involve overlapping focal spots from many laser pulses by scanning the substrate. However, this procedure is both time-consuming and allows thermal and mechanical damage to accumulate, degrading the quality of the channel profile and surrounding substrate. We have developed an alternate means of machining a very long microchannel in fused silica with a single pulse, using combinations of cylindrical lenses and an aspheric lens to reshape a near-Gaussian beam into a tight line focus. For microfluidic applications, channels should possess near diffraction-limited cross-sections just a few microns deep while being up to 2 mm long. However, depending on the pulse energy, the extremely high peak fluences can induce nonlinear effects such as filamentation and self-focusing. These effects mayproduce unexpected features, including beam-path bifurcations and multiple foci that sometimes blend into exceptionally deep channel profiles. We demonstrate microchannels that range from 5 microns to more than 30 microns deep but are only about 1 micron wide along the entire channel length. We explore the underlying extreme physical processes that might yield such extraordinary results. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S26.00011: Efficient ab-initio thermodynamic calculations at high pressure and temperature Hugh Wilson Prediction of solubility properties and phase diagrams under conditions of high temperature and pressure requires the computation of the Gibbs free energies of materials, a property not directly accessible from molecular dynamics trajectories. Two-step coupling constant integration methods have previously achieved success in the computation of free energies of fluid, solid, and superionic phases of materials by connecting the ab-initio system of interest to a non-interacting reference system via a series of thermodynamic integration steps. These methods, however, require a series of time-consuming and computationally awkward integrations over molecular dynamics trajectories, limiting the utility of the method. Here we propose and demonstrate a method for more efficiently carrying out the same thermodynamic integration without the need for separate molecular dynamics runs, and show how it may be used to carry out the integration up to an order of magnitude more efficiently, in a massively parallel manner, and without the need for code modification. Applications of thermodynamic integration including core solubility in Jupiter and Saturn, and superionic-to-superionic phase transitions in Uranus and Neptune, will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S26.00012: The confinement effect of inert materials on insensitive high explosives Yutao Sun, Ming Yu, Li Tang The paper aims at investing the confinement effect of inert materials on insensitive high explosives by means of shock polar curve and phenomenological reaction model. The confinement types are categorized by the shock polar theory, which built on the leading shock wave based on the detonation ZND model. If the sonic velocity of the confinement material is less than the CJ velocity of an explosive, the shock polar theory can be utilized. In general, there are several types of interactions that give a ?match? of the pressure and streamline-deflection across the interface between IHE and confinement material. A two-dimensional Lagrangian hydrodynamic method with three-term Lee-Tarver rate law is used to numerically simulate all types of confinement interactions. The important character of confinement material include: compressibility, thickness, the representative assembled layers, such as bakelite-iron and iron-beryllium. An improved detonation model is established to simulate the pre-compression effect on unreact explosive. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S26.00013: Effect of slow energy releasing on divergent detonation of Insensitive High Explosives Xiaomian Hu, Hao Pan, Yong Huang, Zihui Wu There exists a slow energy releasing (SER) process in the slow reaction zone located behind the detonation wave due to the carbon cluster in the detonation products of Insensitive High Explosives (IHEs), and the process will affect the divergent detonation wave's propagation and the driving process of the explosives. To study the potential effect, a new artificial burn model including the SER process based on the programmed burn model is proposed in the paper. Quasi-steady analysis of the new model indicates that the nonlinearity of the detonation speed as a function of front curvature owes to the significant change of the reaction rate and the reaction zone length at the sonic state. What's more, in simulating the detonation of IHE JB-9014, the new model including the slow reaction can predict a slower jump-off velocity, in good agreement with the result of the test. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S26.00014: Study on the Mechanism of the Deflagration to Detonation Transition Process of Explosive Yangjun Ying, Xiaomian Hu, Lan Wei In this paper we presented a numerical study of the mechanisms of the deflagration to detonation transition (DDT) process of explosives to assess its thermal stability. We treated the modeling system as a mixture of solid explosives and gaseous reaction products. We utilized a one-dimensional two-phase flow modeling approach with space-time conservation element and solution element (CE/SE) method. Simulation results show a plug area of high density with relatively slow chemical reactions, whose forward boundary is the fast running shock wave, and rearward boundary is the burning wave.We identified a criterion of steady detonation through a detailed analysis of the characteristics of the reaction process: steady detonation occurs at locations where different physical quantities, such as pressure, density, temperature and velocity, reach peak values simultaneously.We also simulated the high temperature DDT tube experiments of HMX-based high explosive. We found good agreement between the simulation results of detonation velocity and run length determined by the above criterion and the experimental results. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S26.00015: PETN, RDX, HMX, TATB: band gap dependence on pressure under hydrostatic compression from DFT with GW and vdW corrections Andrei Mukhanov, Alexei Yanilkin In the middle of 1990s Gilman (Gilman J. J. 1995 Phil. Mag. B, 71:6, 1057) proposed the idea that explosives transit from insulator to conductor state with following adiabatic expansion of free electrons in shockwave. One of the reasons of such a behavior of electrons is narrowing or disappearing of the fundamental band gap in explosive single crystal. It is well known that similar behavior can be simulated by DFT. But there is a severe problem of lowering the value of gap by DFT. So for quantitative prediction of narrowing of gap under pressure it is necessary to use more complicated methods like GW. From first principle calculations we determined elastic moduli for ideal crystals of PETN, RDX, HMX, and TATB. Accounting for those moduli we simulated the 0 K isotherms for hydrostatic compression of single crystal. Due to the essential role of van der Waals interaction in such materials the vdW corrections to DFT in Grimme's form was used. We obtained the dependencies for band gap on pressure under hydrostatic compression. Our preliminary results on GW calculations show that for TATB at initial uncompressed volume we have the value of gap twice a bigger in GW than in DFT. [Preview Abstract] |
Session S27: Quantum Many-Body Systems I
Sponsoring Units: DCOMPChair: Robert DiStasio Jr., Princeton University
Room: 501
Thursday, March 6, 2014 8:00AM - 8:12AM |
S27.00001: Path Integral Monte Carlo Methods for Fermions Ethan Ethan, Jonathan DuBois, David Ceperley In general, Quantum Monte Carlo methods suffer from a sign problem when simulating fermionic systems. This causes the efficiency of a simulation to decrease exponentially with the number of particles and inverse temperature. To circumvent this issue, a nodal constraint is often implemented, restricting the Monte Carlo procedure from sampling paths that cause the many-body density matrix to change sign. Unfortunately, this high-dimensional nodal surface is not a priori known unless the system is exactly solvable, resulting in uncontrolled errors. We will discuss two possible routes to extend the applicability of finite-temperatue path integral Monte Carlo. First we extend the regime where signful simulations are possible through a novel permutation sampling scheme. Afterwards, we discuss a method to variationally improve the nodal surface by minimizing a free energy during simulation. Applications of these methods will include both free and interacting electron gases, concluding with discussion concerning extension to inhomogeneous systems. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S27.00002: Exact Dynamics via Poisson Process: a unifying Monte Carlo paradigm James Gubernatis A common computational task is solving a set of ordinary differential equations (o.d.e.'s). A little known theorem says that the solution of any set of o.d.e.'s is exactly solved by the expectation value over a set of arbitary Poisson processes of a particular function of the elements of the matrix that defines the o.d.e.'s. The theorem thus provides a new starting point to develop real and imaginary-time continous-time solvers for quantum Monte Carlo algorithms, and several simple observations enable various quantum Monte Carlo techniques and variance reduction methods to transfer to a new context. I will state the theorem, note a transformation to a very simple computational scheme, and illustrate the use of some techniques from the directed-loop algorithm in context of the wavefunction Monte Carlo method that is used to solve the Lindblad master equation for the dynamics of open quantum systems. I will end by noting that as the theorem does not depend on the source of the o.d.e.'s coming from quantum mechanics, it also enables the transfer of continuous-time methods from quantum Monte Carlo to the simulation of various classical equations of motion heretofore only solved deterministically. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S27.00003: Algorithmic differentiation of diffusion Monte Carlo Tom Poole, Matthew Foulkes, James Spencer, Peter Haynes Algorithmic differentiation (AD) [1] is a programming technique for the efficient evaluation of the derivatives of a computed function. This approach proceeds via the application of the chain rule to the lines of source code that constitute the mathematical operation of a computer program, allowing access to the derivatives of functions that lack an algebraic representation. Another important element of the AD method is that the ``reverse mode'' of operation yields the derivative of a function output with respect to all inputs, simultaneously, in a small multiple of the computational cost of evaluating the underlying function in isolation. These features make this method particularly applicable to the diffusion Monte Carlo (DMC) algorithm where, despite a number of recent advances in the area, total energy derivatives have remained problematic. Here we present results illustrating accurate DMC energy derivatives with respect to both the input wave function parameters and the nuclear positions, with the former enabling DMC wave function optimization and the latter facilitating DMC molecular dynamics simulations.\\[4pt] [1] A. Griewank and A. Walther, Evaluating Derivatives: Principles and Techniques of Algorithmic Differentiation, 2nd ed. (SIAM, Philadelphia IL, 2008). [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S27.00004: Advances in the application of diffusion Monte Carlo to solids L. Shulenburger, T.R. Mattsson The need for high fidelity electronic structure calculations has catalyzed an explosion in the development of new techniques. Improvements in DFT functionals, many body perturbation theory and dynamical mean field theory are starting to make significant headway towards reaching the accuracy required for a true predictive capability. One technique that is undergoing a resurgence is diffusion Monte Carlo (DMC). The early calculations with this method were of unquestionable accuracy (providing a valuable reference for DFT functionals) but were largely limited to model systems because of their high computational cost. Algorithmic advances and improvements in computer power have reached the point where this is no longer an insurmountable obstacle. In this talk I will present a broad study of DMC applied to condensed matter (arXiv:1310.1047). We have shown excellent agreement for the bulk modulus and lattice constant of solids exhibiting several different types of binding, including ionic, covalent and van der Waals. We will discuss both the opportunities for application of this method as well as opportunities for further theoretical improvements. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S27.00005: Diffusion Monte Carlo calculations of solids using transcorrelated trial wave functions Yoshiyuki Yamamoto, Ryo Maezono, Masayuki Ochi, Shinji Tsuneyuki Diffusion Monte Carlo (DMC) method is an \textit{ab initio} wave-function theory that can treat correlated quantum systems to high accuracy within reasonable computational time, and enables us to calculate large systems such as solids. For electronic systems, DMC suffers from the fermion-sign problem, and in order to avoid it we have to use the fixed-node approximation. The amount of fixed-node error depends on the quality of the nodal structure of the trial wave function that we prepare in advance. A promising trial wave function is that of transcorrelated (TC) method, which is one of the wave-function theories. In this method, wave functions are approximated as the Slater-Jastrow form and the orbitals in the Slater determinant are relaxed by solving one-electron equations of similarity-transformed Hamiltonian. The nodal structure of the Slater-Jastrow wave function is determined by its determinantal part, so we can optimize the nodal structure of the Slater-Jastrow wave function by TC method. In this talk, we will present the fixed-node DMC energies of solids using TC trial wave functions and compare the energies with those using trial wave functions constructed from density functional theory and Hartree-Fock method. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S27.00006: Construction of low energy effective Hamiltonians from Ab Initio Quantum Monte Carlo Hitesh Changlani, Lucas Wagner Solving the first principles quantum many-body Schroedinger equation can result in very high accuracy, but physical insight and generalization of the result can be hard to obtain. On the other hand, low energy effective model Hamiltonians often contain the essential physics of the problem, but may not provide sufficient accuracy needed to understand the properties of real materials. To connect the two approaches, we present a framework for obtaining low energy effective Hamiltonians from ab initio Quantum Monte Carlo calculations for molecular and extended systems. As a demonstration of the method, we focus on a few representative strongly correlated materials. We fit the Hamiltonian parameters to best reproduce the two body density matrix of the ground state obtained from ab initio calculations. We assess the accuracy of the resultant model, by comparing excited state properties to the original ab initio result. Such effective Hamiltonians are advantageous in reducing the computational complexity of the many-electron problem, and once generated, can be used for larger scale calculations using techniques designed for discrete systems. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S27.00007: Flat histogram diagrammatic Monte Carlo method: Motion of a hole in a columnar antiferromagnet Nikolaos Diamantis, Efstratios Manousakis We will present a version of the diagrammatic Monte Carlo (Diag-MC) method in which we incorporate the flat histogram principle and we term the improved version ``Flat Histogram Diagrammatic Monte Carlo'' method. We demonstrate the superiority of the method over the standard Diag-MC in extracting the long-imaginary-time behavior of the Green's function, without incorporating any {\it a priori} knowledge about this function, by applying the technique to the polaron problem. We have also applied the technique to the motion of a hole inside a $J_1$-$J_2$ quantum antiferromagnet with columnar order. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S27.00008: Approximations beyond the initiator approach for ameliorating the sign problem Adam Holmes, Cyrus Umrigar, Bryan Clark Full CI Quantum Monte Carlo [1-2] is a computationally expensive method that projects out the ground state in a given basis without resorting to the fixed-node approximation. Instead, it has an initiator bias, which disappears as one approaches the infinite walker limit. While the semistochastic improvement on the method increases the efficiency by about three orders of magnitude [3], converging the ground state energy with respect to the walker population can still require prohibitively many walkers, not only as the number of electrons is increased, but even as the size of the basis is increased for a fixed number of electrons. We therefore investigate other approaches to ameliorating the sign problem, e.g., fixed-node and partial-node approximations [4], and compare the tradeoffs between accuracy and efficiency. [1] George H. Booth, Alex JW Thom and Ali Alavi, J. Chem. Phys. {\bf 131}, 054106 (2009). [2] Cleland, Deidre, George H. Booth and Ali Alavi, J. Chem. Phys. {\bf 132}, 041103 (2010). [3] F. R. Petruzielo, A. A. Holmes, Hitesh J. Changlani, M. P. Nightingale and C. J. Umrigar, Phys. Rev. Lett. {\bf 109}, 30201 (2012). [4] M. Kolodrubetz and B. K. Clark. Phys. Rev. B {\bf 86}, 075109 (2012). [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S27.00009: Spins as variables in electronic structure quantum Monte Carlo calculations Lubos Mitas, Minyi Zhu, Shi Guo Current electronic structure quantum Monte Carlo (QMC) methods keep particle spins static in configurations that correspond to spin-space symmetries of calculated states. Here we present a generalization of the QMC approaches for treating fermionic spin degrees of freedom as variables. The developed method possesses two key properties that make it suitable for high accuracy calculations of real systems. First, the spinors entering the trial function are kept intact during the imaginary time evolution. Second, the approach has the zero variance property pointwise for arbitrary configurations of spatial and appropriately chosen spin coordinates. The spin coordinates are overcomplete and therefore can be smoothly evolved in the imaginary time propagation. The method is illustrated on molecules and atomic excitations of heavy elements with spin-orbit interactions and on 2D electron gas with the Rashba interaction. The performance of the method is similar to the commonly used static spin calculations in several aspects such as achieved accuracy and energy fluctuations. The corresponding wave functions have lower symmetries and therefore can exhibit potentially stronger multi-reference character as is observed in some cases. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S27.00010: Monte Carlo simulations of two-dimensional fermion models with string bond tensor-network states Jeong-Pil Song, R.T. Clay We present computational results using the string-bond tensor network ansatz for Fermionic lattice models in two dimensions. We use quantum Monte Carlo to calculate ground state quantities combined with stochastic optimization to optimize the matrix elements of matrix-product state strings. We apply the approach to a two-dimensional spinless fermion model with nearest-neighbor Coulomb repulsion under periodic boundary conditions. We test the numerical accuracy and convergence with matrix size D of the method with comparisons with the free fermion system, exact diagonalization results, and density matrix renormalization group results. The phase boundary between low entangled charge ordered and highly entangled metallic phases can be determined using finite size scaling of charge structure factor in the thermodynamic limit. Since this stochastic approach does not suffer from a fermion sign problem, it can handle frustrations and be applied to the Hubbard models with periodic boundaries in two dimensions. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S27.00011: Symmetry-projected Hartree-Fock wave functions in quantum Monte Carlo calculations Hao Shi, Carlos Hoyos, Rayner Rodriguez, Gustavo Scuseria, Shiwei Zhang Symmetry-projected Hartree-Fock wave functions provide an ansatz which accounts for static correlations while preserving symmetry. We implement such wave functions in constrained path (CP) auxiliary-field quantum Monte Carlo (AFQMC) calculations as the trial wave function. Unlike usual multi-determinant trial wave functions obtained from a configuration interaction picture, the computational cost of this class of trial wave functions can be made to scale as a low power with system size. A systematic test is carried out in the two-dimensional Hubbard model on the accuracy of the approach. It is found that the accuracy of the calculated ground-state energy increases as more symmetries are restored, while the statistical variance decreases. We find that wave functions with space group and spin symmetry significantly reduce the CP systematic error in AFQMC compared to simple Hartree-Fock trial wave functions. Essentially all the correlation energy is recovered by the AFQMC when the fully symmetry-projected trial wave function is used. Correlation functions are accurately predicted. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S27.00012: Downfolding calculations in solids by auxiliary-field quantum Monte Carlo Fengjie Ma, Wirawan Purwanto, Shiwei Zhang, Henry Krakauer We present a recent development in {\it ab initio} auxiliary-field quantum Monte Carlo (AFQMC) \footnote{S.~\ Zhang and H.~\ Krakauer, Phys. Rev. Lett. {\bf 90}, 136401 (2003)} calculations of solid systems using downfolded Hamiltonians. For a given system, the many-body downfolded Hamiltonian is expressed with respect to a truncated basis set of Kohn-Sham orbitals, which are obtained from a high-quality density-functional calculation. This approach allows many-body calculations to treat a much simpler Hamiltonian while retaining material-specific properties. Typical size of the basis set is more than an order of magnitude smaller than the original (the number of plane-waves), leading to large savings in AFQMC computation. The Hamiltonians are systematically improvable and allow one to dial, in principle, between the simplest model and the full Hamiltonian. As a by-product of this approach, pseudopotential errors can essentially be eliminated \footnote{W.~\ Purwanto, S.~\ Zhang, H.~\ Krakauer, J. Chem. Theory Comput. {\bf 9}, 4825 (2013)}. The method is demonstrated by calculating the lattice constant and bulk modulus of solids, including classic semiconductors (Si and diamond), an ionic insulator (NaCl), and metallic systems (Na and Al). [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S27.00013: Possibility of Deconfined Criticality in SU($N$) Heisenberg Models at Small $N$ Kenji Harada, Takafumi Suzuki, Tsuyoshi Okubo, Haruhiko Matsuo, Jie Lou, Hiroshi Watanabe, Synge Todo, Naoki Kawashima To examine the validity of the scenario of the deconfined critical phenomena[1], we carry out quantum Monte Carlo simulation for the SU($N$) generalization of the Heisenberg model with four-body and six-body interactions[2]. The quantum phase transition between the SU($N$) N\'eel and valence-bond solid phases is characterized for $N=2,3,$ and $4$ on the square and honeycomb lattices. While finite-size scaling analysis works well up to the maximum lattice size ($L=256$) and indicates the continuous nature of the phase transition, a clear systematic change towards the first-order transition is observed in the estimates of the critical exponent $y \equiv 1/\nu$ as the system size increases. We discuss the details of finite-size scaling analysis. [1] T. Senthil, A. Vishwanath, L. Balentz, S. Sachdev and M.P.A. Fisher, Science 303 (2004). [2] K. Harada, T. Suzuki, T. Okubo, H. Matsuo, J. Lou, H. Watanabe, S. Todo, and N. Kawashima, arXiv:1307.0501. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S27.00014: Phase diagram of easy-plane deformations of SU(N) magnets Jonathan Demidio, Ribhu K. Kaul We consider Hamiltonians of SU($N$) quantum magnets with easy-plane deformations, leaving a U(1) rotation symmetry about each of the $N-1$ diagonal generators and a discrete $Z_{N}$ symmetry. For $N=2$ our model reduces to the XY model and can hence be considered as a larger-$N$ generalization of this well-studied model. We present numerical data from quantum Monte Carlo simulations which allows us to map the phase diagram of these models as a function of $N$, including both two-spin and four-spin interactions. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S27.00015: Quantum Monte Carlo Gap Analysis of Neel-VBS Phase Transition Hidemaro Suwa, Anders Sandvik We have developed a generalized moment method for calculating excitation gaps in finite-temperature and ground-state projector quantum Monte Carlo simulations. We show analytically that this estimator is unbiased in the low-temperature or long-projection-length limit. Not only the first gap but also the second gap for each quantum number can be calculated without any systematic errors. As a demonstration, we have applied this approach to the Neel--valence-bond-solid transition of the two-dimensional J-Q spin model. The transition point was successfully obtained from the singlet-triplet level crossing. Interestingly, the size-scaling of the crossing point is the same as in the one-dimensional case ($1/L^2$, $L$ being the system length). [Preview Abstract] |
Session S32: Invited Session: Topological Quantum Information and Phases of Matter
Sponsoring Units: GQI DCMPChair: Parsa Bonderson, Microsoft Corporation
Room: 708-712
Thursday, March 6, 2014 8:00AM - 8:36AM |
S32.00001: The search for Majorana zero-energy modes in solid-state systems Invited Speaker: Roman Lutchyn The search for topological phases of matter has become an active and exciting pursuit in condensed matter physics. Among the notable recent developments in this direction are the discoveries of topological insulators and superconductors. In this talk, I will focus on topological superconductors and discuss how one can engineer non-trivial superconductivity in the laboratory at the interface of a conventional s-wave superconductor and a semiconductor with a spin-orbit interaction. I will show that the topological superconducting state emerging at the interface supports Majorana zero-energy modes. The defects carrying these modes are Ising anyons and obey unconventional (non-Abelian) exchange statistics. The unique properties of Majoranas can be exploited for implementing fault-tolerant quantum computation schemes that are inherently decoherence-free. I will conclude this talk by reviewing recent experimental efforts in realizing and detecting Majorana zero-energy modes in one-dimensional nanowires. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S32.00002: Universal topological quantum computation from a superconductor/Abelian quantum Hall heterostructure Invited Speaker: Roger Mong Non-Abelian anyons promise to reveal spectacular features of quantum mechanics that could ultimately provide the foundation for a decoherence-free quantum computer. A key breakthrough in the pursuit of these exotic particles originated from Read and Green's observation that the Moore-Read quantum Hall state and a (relatively simple) two-dimensional $p+ip$ superconductor both support so-called Ising non-Abelian anyons. Here we establish a similar correspondence between the $Z_3$ Read-Rezayi quantum Hall state and a novel two-dimensional superconductor in which charge-$2e$ Cooper pairs are built from fractionalized quasiparticles. In particular, both phases harbor Fibonacci anyons that---unlike Ising anyons---allow for universal topological quantum computation solely through braiding. Using a variant of Teo and Kane's construction of non-Abelian phases from weakly coupled chains, we provide a blueprint for such a superconductor using Abelian quantum Hall states interlaced with an array of superconducting islands. These results imply that one can, in principle, combine well-understood and widely available phases of matter to realize non-Abelian anyons with universal braid statistics. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S32.00003: Lattice quantum codes and exotic topological phases of matter Invited Speaker: Jeongwan Haah Is it possible to build a ``hard disk drive'' for quantum information? The quantum coherence time in a usual thermal system is fundamentally limited by the inverse Boltzmann factor $\exp[\Delta/kT]$, where $\Delta$ is the energy scale of the system. This limitation is not enhanced even with a conventional topologically ordered system in three or lower dimensions. Here, a new three-dimensional spin model is presented that shows a qualitatively different behavior. It can be viewed as a quantum error correcting code, and is thus exactly solvable. The ground states are locally indistinguishable, for which it may be called topologically ordered. However, the model only admits immobile pointlike excitations, and the immobility is not affected by small perturbations of the Hamiltonian. The degeneracy of the ground state, though also insensitive to perturbations, is a complicated number-theoretic function of the system size. Under real-space renormalization group transformations, the system bifurcates into multiple noninteracting copies of itself. Similarities and differences of the model in comparison to Wegner's Ising gauge theory will be explained. When quantum information is encoded into a ground state of this model and subjected to thermal errors, the errors remain easily correctable for a long time without any active intervention, because a macroscopic energy barrier due to the immobility of excitations keeps the errors well localized. As a result, stored quantum information can be retrieved faithfully for a memory time $\exp[(\Delta/kT)^2]$. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:24AM |
S32.00004: Genons and twist defects Invited Speaker: Xiao-Liang Qi |
Thursday, March 6, 2014 10:24AM - 11:00AM |
S32.00005: Topological Phases and Surface States with Strong Interactions Invited Speaker: Ashvin Vishwanath Previous work on topological insulators and superconductors has been largely based on free fermions with topological ``band'' structures. We will discuss qualitatively new phenomena that arise with strong interactions, where one cannot invoke a band structure. First, we point out examples of new topological phases with protected edge modes that only appear in the presence of interactions. Next, in contrast to conventional wisdom which held that 3D Topological Insulators and superconductors must be associated with gapless, metallic surface states if the symmetries are preserved, we argue that the 2D surface \textit{can} in fact acquire a gap while remaining fully symmetric if it develops topological order. That is, if the surface state contains excitations with fractional statistics, like in a fractional Quantum Hall state. Interestingly, in some cases the surface states must contain particles with non-Abelian statistics. Finally, we discuss how interactions can modify the classification of free fermion topological phases in 3D. In particular, using surface topological order as a tool, we show that the integer classification of topological superconductors in 3D (class DIII, with time reversal symmetry) is actually reduced to a Z$_{16}$ classification. [Preview Abstract] |
Session S33: Focus Session: Artificial Gauge Fields and Systems with Long Range Interactions II
Sponsoring Units: DAMOPChair: Chuangwei Zhang, University of Texas, Dallas
Room: 706
Thursday, March 6, 2014 8:00AM - 8:12AM |
S33.00001: Realization of the Harper Hamiltonian with Artificial Gauge Fields in Optical Lattices Hirokazu Miyake, Georgios Siviloglou, Colin Kennedy, William Cody Burton, Wolfgang Ketterle Systems of charged particles in magnetic fields have led to many discoveries in science--such as the integer and fractional quantum Hall effects--and have become important paradigms of quantum many-body physics. We have proposed and implemented a scheme which realizes the Harper Hamiltonian, a lattice model for charged particles in magnetic fields, whose energy spectrum is the fractal Hofstadter butterfly. We experimentally realize this Hamiltonian for ultracold, charge neutral bosonic particles of $^{87}$Rb in a two-dimensional optical lattice by creating an artificial gauge field using laser-assisted tunneling and a potential energy gradient provided by gravity. Laser-assisted tunneling processes are characterized by studying the expansion of the atoms in the lattice. Furthermore, this scheme can be extended to realize spin-orbit coupling and the spin Hall effect for neutral atoms in optical lattices by modifying the motion of atoms in a spin-dependent way by laser recoil and Zeeman shifts created with a magnetic field gradient. Major advantages of our scheme are that it does not rely on near-resonant laser light to couple different spin states and should work even for fermionic particles. Our work is a step towards studying novel topological phenomena with ultracold atoms. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S33.00002: Synthetic gauge fields in quantum gases of dysprosium Hui Zhai, Benjamin Lev To the toolbox of quantum gas-based many-body physics, highly magnetic atoms offer large, possibly non-perturbative, long-range dipolar interactions concomitant with extraordinarily large SU(2) spinors and novel atomic structure. We report on our recent proposal [1] to create a diversity of exotic quantum many-body phases--non-Abelian quantum magnets, high-spin quantum Hall states--using the unusual properties of dysprosium under the influence of large light-induced gauge fields, both Abelian and non-Abelian, to generate large synthetic magnetic fields and spin-orbit coupling. We will describe recent experimental progress as well as new results on the collisional properties of quantum dipolar Bose and Fermi gases of Dy, recently produced in our laboratory for the first time [2,3], including Feshbach resonance spectra [4]. \\[4pt] [1] X. Cui, B. Lian, T.-L. Ho, B. Lev, and H. Zhai, Synthetic Gauge Field with Highly Magnetic Lanthanide Atoms, PRA 88, 011601(R) (2013). \\[0pt] [2] M. Lu, N. Burdick, S. Youn, and B. Lev, A Strongly Dipolar Bose-Einstein Condensate of Dysprosium, PRL 107, 190401 (2011). \\[0pt] [3] M. Lu, N. Burdick, and B. Lev, Quantum Degenerate Dipolar Fermi Gas, PRL, 108, 215301 (2012).\\[0pt] [4] N. Burdick, K. Baumann, M. Lu, and B. Lev, to be published. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S33.00003: Bosons with Artificial Gauge Fields and Mott Physics on the Honeycomb Lattice Ivana Vidanovic, Alexandru Petrescu, Karyn Le Hur, Walter Hofstetter We study bosons in the tight-binding model on the honeycomb lattice introduced by Haldane. We analyze the ground state topology and quasiparticle properties in the Mott phase by applying bosonic dynamical mean field theory, strong-coupling perturbation theory, exact diagonalization and numerical evaluations of sample Hall conductivity. The phase diagram also contains two different superfluid phases. The quasiparticle dynamics, number fluctuations, and local currents are measurable in cold atom experiments. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S33.00004: Topological states in photonic systems Mohammad Hafezi, Sunil Mittal, Prabin Adhikari, Jingyun Fan, Alan Migdall, Jacob Taylor Topological features -- global properties which are not discernible locally -- emerge in systems from magnets to fractional quantum Hall systems. The best known examples are quantum Hall effects, where insensitivity to local properties manifests itself as conductance through edge states that is insensitive to defects and disorder. In this talk, I demonstrate how similar physics can be observed for photons; specifically, how various quantum Hall Hamiltonians can be simulated with linear optical elements. I report the first observation of topological photonic edge state using silicon-on-insulator technology. Furthermore, I discuss the prospect of measuring integer topological invariants, the addition of optical non-linearity and the possibility of implementing fractional quantum Hall states of photons, in both optical and circuit-QED systems. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S33.00005: Site-resolved detection of current fluctuations Florian Marquardt, Stefan Kessler Two recent developments have significantly expanded the toolbox for ultracold atoms in optical lattices: Site-resolved single-atom detection and the generation of artificial gauge fields. We propose a scheme for site-resolved detection of local current operators [arXiv:1309.3890]. This will allow to measure spatial correlations in fluctuating current patterns and the full counting statistics of local currents. We illustrate the possibilities via numerical simulations for interacting systems of ultracold atoms, with and without an artificial magnetic field. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S33.00006: Ultracold atoms in a cavity: synthetic gauge fields and cavity-mediated long-range interactions Farokh Mivehvar, David Feder The collective coupling of ultracold neutral atoms to electromagnetic fields in cavity QED results in cavity-mediated long-range atom-atom interactions, paving the way for the realization of strongly correlated states and collective phenomena. That said, quantum Hall and topological insulator states are not directly accessible in these environments because they require the coupling of the particles' center-of-mass motion to external magnetic fields and to internal spin degrees of freedom, respectively. In this work, we show that coupling three-level atoms to two counter-propagating ring-cavity modes in the $\Lambda$ scheme can give rise to synthetic spin-orbit interactions and large synthetic magnetic fields. In the presence of an additional optical lattice, the Hamiltonian in the weak-coupling regime corresponds to an effective spin-orbit coupled Hubbard model for the atoms in the first Bloch band, including a variety of long-range atom-atom interactions. The eigenstates of this model are explored for various choices of the parameters. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S33.00007: Experimental investigation of spin-orbit coupled BECs Invited Speaker: Peter Engels Ultracold atomic gases provide a powerful tool to study the physics of artificial gauge fields and complex Hamiltonians. In this context, the implementation of spin-orbit coupling is an advancement that is currently met with great interest, both theoretically and experimentally. In our lab we have implemented spin-orbit coupling by using a Raman dressing scheme. Our recent experiments with spin-orbit coupled Bose-Einstein Condensates (BECs) include the observation of quantum quench dynamics and Zitterbewegung, upper spin-orbit band dynamics, and an analogy for the Dicke type phase transition. Furthermore, in our experiments we have studied the physics arising from a combination of spin-orbit coupling and a moving optical lattice: by measuring atom loss due to modulational instability in a system composed of a spin-orbit coupled BEC loaded into a moving optical lattice, the dispersion relation of this system can be investigated. This investigation of the band structure is corroborated by matching theoretical results. I will report on the current status of our ongoing investigations. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S33.00008: Evidence of Dirac Monopoles in a Spin-1 Bose-Einstein Condensate Michael Ray, Emmi Ruokokoski, Saugat Kandel, Mikko M\"{o}tt\"{o}nen, David Hall Isolated magnetic poles (monopoles) have not yet been observed, although there are good theoretical reasons for thinking that they may exist --- and profound implications if they do. The first successful theoretical description of a magnetic monopole consistent with quantum mechanics was formulated by Dirac [1], but may be applied more generally to quantum-mechanical systems in the presence of gauge potentials. We describe the successful experimental creation of Dirac monopoles in a \emph{synthetic} magnetic field in the context of a dilute-gas Bose-Einstein condensate. The existence of a monopole is inferred from direct observations of a vortex line that terminates inside the condensate, which evidence is supported by excellent agreement between experiment and numerical simulations. [1] P.A.M. Dirac, Proc. R. Soc. Lond. A \textbf{133}, 60 (1932). [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S33.00009: Spin-orbit coupling induced FFLO-like superfluidity and skyrmion-like polarization textures in trapped Fermi gases Menderes Iskin We study the interplay between the Zeeman field and spin-orbit coupling (SOC) in harmonically trapped Fermi gases loaded into a two-dimensional single-band tight-binding optical lattice. Using the Bogoliubov-de Gennes theory, we find that the Zeeman field combined with a Rashba SOC gives rise to $(i)$ Fulde-Ferrell-like superfluidity and $(ii)$ skyrmion-like polarization textures near the edges of the system. The effects of interaction, temperature, SOC anisotropy and Zeeman field anisotropy on the superfluid ground state and polarization textures will also be discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S33.00010: Spin Transport in Spin Orbit Coupled Bose Einstein Condensates Robert Niffenegger, Abraham Olson, Chuan-Hsun Li, Yong Chen We study spin transport induced by synthetic spin-dependent electric fields in spin-orbit coupled (SOC) Bose Einstein Condensates (BECs). The 1D SOC is created with counter propagating Raman lasers which couple hyperfine spins ($m_F=-1$ and 0, of F=1) and momentum states of $^{87}Rb$, allowing us to engineer spin dependent vector potentials. Quickly lowering the Raman laser intensity (spin-orbit Raman coupling) splits the spin vector potentials in opposite directions and applies opposite synthetic electric fields to the two dressed spin BECs. We allow them to oscillate in opposite directions within the optical trap (exhibiting a spin dipole mode) and measure their momentum after time of flight. The oscillations damp when the spin BECs collide and the damping increases as the Raman coupling is increased, possibly related to the Raman coupling dressing and increasing the effective spin interactions. Over longer time scales, thermalization accompanies the damping of the bare spins' oscillations. However, with Raman coupling, the overdamped dressed spins' oscillations are accompanied by rich excitations in the BEC but less thermalization. Our experiments may provide new insights for understanding and controlling spin transport and spin decoherence in atomtronic or spintronic devices. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S33.00011: Quantum quasicrystals and other exotic states of spin-orbit coupled dipolar bosons Sarang Gopalakrishnan, Ryan Wilson, Brandon Anderson, Benjamin Lev, Charles Clark, Ivar Martin, Eugene Demler We study dipolar Bose gases in which the bosons experience a Rashba spin-orbit coupling. We show that the degenerate dispersion minimum due to the spin-orbit coupling, combined with the long-range dipolar interaction, can stabilize a rich phase diagram including a number of exotic phases, such as a quantum quasicrystal [1] (in the quasi-2D limit) and a meron state [2] (in the 3D limit), as one tunes the strength of the dipolar interaction and the spin-orbit coupling. We discuss specific level schemes for exploring this phase diagram using ultracold dysprosium. [1] S. Gopalakrishnan, I. Martin, and E.A. Demler, Phys. Rev. Lett. 111, 185304 (2013) [2] R.M. Wilson, B.M. Anderson, and C.W. Clark, Phys. Rev. Lett. 111, 185303 (2013) [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S33.00012: Observing Topological Chiral Orders in 2D Optical Lattices without Spin-orbit Coupling Xiong-Jun Liu, Zheng-Xin Liu, K.T. Law, W. Vincent Liu, T.K. Ng We propose to observe topological chiral orders with cold atoms without spin-orbit coupling in a two-dimensional optical lattice directly based on the recent experiments which use Raman beams to induce the hopping between nearest-neighbor sites. In the simplest case with s-orbital model, the chiral Chern insulating phases are predicted in the single-particle regime. Moreover, by considering a spin-1/2 system, we predict that the chiral spin liquid phase may exist in the interacting regime. This work proposes realistic cold atom platforms to observe topological chiral orders in the experiment. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S33.00013: Magnetically generated spin-orbit coupling for ultracold atoms Brandon Anderson, Ian Spielman, Gediminas Juzeli\={u}nas We present a new technique for producing two and three dimensional Rashba-type spin-orbit coupling for ultra cold atoms without involving light. The method relies on a sequence of pulsed inhomogeneous magnetic fields imprinting suitable phase gradients on the atoms. For sufficiently short pulse durations, the time-averaged Hamiltonian well approximates the Rashba Hamiltonian. Higher order corrections to the energy spectrum are calculated exactly for spin-1/2 and perturbatively for higher spins. The pulse sequence does not modify the form of rotationally symmetric atom atom interactions. Finally, we present a straightforward implementation of this pulse sequence on an atom-chip. [Preview Abstract] |
Session S34: Focus Session: AMO Quantum Information Processing: Photons and Atoms
Sponsoring Units: GQI DAMOPChair: Grant Biedermann, Sandia National Laboratories
Room: 704
Thursday, March 6, 2014 8:00AM - 8:36AM |
S34.00001: Classical Computers Very Likely Can Not Efficiently Simulate Multimode Linear Optical Interferometers with Arbitrary Fock-State Inputs-An Elementary Argument Invited Speaker: Jonathan Dowling Aaronson and Arkhipov recently used computational complexity theory to argue that classical computers very likely cannot efficiently simulate linear, multimode, quantum optical interferometers with arbitrary Fock state inputs [S. Aaronson and A. Arkhipov, arXiv:1011.3245]. Here we present an elementary argument that utilizes only techniques from quantum optics. We explicitly construct the Hilbert space for such an interferometer and show that that its dimension scales exponentially with all the physical resources. We then link the simulation of the device to the computationally hard problem of computing the permanent of a matrix. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S34.00002: Efficient separation of the orbital angular momentum eigenstates of light Mehul Malik, Mohammad Mirhosseini, Zhimin Shi, Robert Boyd The orbital angular momentum (OAM) modes of light show great promise as a means to extend quantum communication and computation into the high-dimensional regime. OAM modes reside in a discrete, unbounded state space and have the potential to dramatically increase the information capacity of QKD systems. Furthermore, the use a large alphabet increases the tolerance of a QKD system to eavesdropping attacks. A key capability for the use of OAM modes in communication is the ability to efficiently sort single photons based on their OAM content. Here we show an experimental technique that uses two optical transformations in order to do this. The first transformation, demonstrated by Berkhout et al. in 2010, employs a Cartesian to Log-polar transformation to map the azimuthal phase profile of an OAM mode to a tilted planar wavefront, whose tilt is proportional to the OAM quantum number. The second transformation creates seven adjacent copies of the transformed plane-wave mode, resulting in a mode with a larger size as well as a larger phase ramp. The transformed modes are then focused by a lens to spots with greater than 92\% separation efficiency (97\% in theory). We use a similar technique to sort modes in the angular basis, which is mutually unbiased with respect to the OAM basis. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S34.00003: Measurement- and comparison-based sizes of Schr\"{o}dinger cat states of light Tyler Volkoff We extend several measurement-based definitions of effective ``cat-size" to coherent state superpositions with branches composed of either single coherent states or tensor products of coherent states. These effective cat-size measures depend on determining the maximal quantum distinguishability of certain states associated with the superposition state: e.g., in one measure, the maximal distinguishability of the branches of the superposition is considered as in quantum binary decision theory; in another measure, the maximal distinguishability of the initial superposition and its image after a one-parameter evolution generated by a local Hermitian operator is of interest. The cat-size scaling with the number of modes and mode intensity (i.e., photon number) is compared to the scaling derived directly from the Wigner function of the superposition and to that estimated experimentally from decoherence. We also apply earlier comparison-based methods for determining macroscopic superposition size that require a reference GHZ state. The case of a hierarchical Schr\"{o}dinger cat state with branches composed of smaller superpositions is also analyzed from a measurement-based perspective. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S34.00004: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S34.00005: Method for Sorting Photon Orbital Angular Momentum States by Pattern Decomposition Jennifer Lumbres, David Van Buren, Susan Terebey In addition to the photon spin responsible for the two polarization states, photons possess an orbital angular momentum (OAM) with values that are signed integer multiples of h-bar and travel in a helical shape. We present a table-top spectroscopy experiment to generate, manipulate, and measure OAM states of photons from a laser. We create multiple beams with different OAM content using computer generated fork holograms implemented in 35mm film slides. After overlapping the beams into one combined beam, we use multipoint interferometer apertures to interrogate this beam and generate interference patterns on an imaging detector. Since the different OAM states are orthogonal, these patterns sum. A decomposition of the summed pattern is performed using a simple sorting algorithm which retrieves the intensities of each of the original OAM beams. We show several examples of OAM content retrieval via our method.~ This research seeks to perform OAM spectroscopy of natural light sources such as direct and scattered sunlight. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S34.00006: Storing single-photons in microcavities arrays Imran M. Mirza, S.J. van Enk, H.J. Kimble Coupling light to arrays of microcavities is one of the most promising avenues to store/delay classical light pulses [F. Krauss, Nat. Phot. 2, 448-450 (2008)]. However, from the perspective of benefiting quantum communication protocols, the same ideas in principle can be extended down to the single-photon (quantum) level as well. Particularly, for the purposes of entanglement purification and quantum repeaters a reliable storage of single photons is needed. We consider in our work [I. M. Mirza, S. Van Enk, H. Kimble JOSA B, 30,10 (2013)] cavities that are coupled through an optical fiber which is assumed to be forming a Markovian bath. For this study two powerful open quantum system techniques, Input-Output theory for cascaded quantum systems and the Quantum Trajectory approach are used in combination. For the confirmation of photon delays the Time-Dependent Spectrum of such a single photon is obtained. Interestingly this leads to a hole-burning effect showing that only certain frequency components in the single photon wavepackets are stored inside the cavities and hence are delayed in time. Since on-demand production of single photons is not an easy task we include in our description the actual generation of the single photon by assuming a single emitter in one the resonators. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S34.00007: Quantum teleportation in artificial photosystems: possibilities and limitations Mehdi Zarea, Raanan Carmieli, Mark Ratner, Michael Wasielewski The possibility of performing quantum teleportation (QT) in a molecular system consisting of an unknown radical spin and a singlet-coupled acceptor-donor pair is studied. The recombination of radical-acceptor pair to its ground state is spin-selective. Here we show that in the presence of exchange interaction between the acceptor and donor, the spin-selective recombination acts as the Bell state measurement. The spin-recombination and the exchange interaction derive the initial quantum state to one of the four Bell states; as a result the spin state of the radical is teleported to the donor spin. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:24AM |
S34.00008: Robust and Addressable Control of Atomic Qubits and Qudits Invited Speaker: Poul Jessen The standard paradigm for quantum computation and simulation with neutral atoms assumes that constituent atoms can be used as individually addressable qubits. To accomplish this in optical lattices with sub-micron atom separation, we have developed a resonance addressing scheme that combines a position dependent light shift of the qubit transition with resonant microwave ($\mu $w) pulses. In a proof-of-principle experiment, we show that numerically optimized composite pulses can implement quantum gates on Cs qubits at targeted lattice sites, with minimal cross-talk to neighboring sites and significant robustness against uncertainty in the atom position. Coherence is verified through two-pulse experiments, and the average gate fidelity is measured to be 95$+$/-3{\%} [1]. Because most atoms have more than two accessible levels, one might also consider if the existing toolbox for qubit control can be extended to $d$-level systems (qudits). Over the past several years we have used the 16-dimensional ground hyperfine manifold of cold, untrapped Cs atoms as an experimental testbed for such work. Driving the atoms with a combination of phase modulated radio frequency (rf) and $\mu $w magnetic fields, we use numerical optimization techniques to design control waveforms (rf and $\mu $w phases as function of time) that accomplish a wide range of control tasks, from quantum state-to-state maps [2] to full unitary transformations, with average fidelities that vary from \textgreater 99{\%} for the former to $\sim$ 98{\%} for the latter. We further show that tools for inhomogeneous control and dynamical decoupling can be generalized to qudits, allowing transformations that are robust to static as well as dynamic perturbations, and thus in principle compatible with optical traps and the resonance addressing scheme demonstrated for qubits. \\[4pt] [1] J. H. Lee et al., Nature Comm. \textbf{4}, Article no. 2027 (2013), doi:10.1038/ncomms3027.\\[0pt] [2] A. Smith et al, Phys. Rev. Lett. \textbf{111}, 170502 (2013). [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S34.00009: Generation of high-fidelity spin entanglement by controlling Wannier orbitals of ultracold atoms in an optical lattice Yuuki Tokunaga, Kensuke Inaba, Kiyoshi Tamaki, Kazuhiro Igeta, Makoto Yamashita We propose a method for generating high-fidelity multipartite spin entanglement of ultracold atoms in an optical lattice within short operation time, which is suitable for measurement-based quantum computation. To produce the desired spin entanglement, we propose to actively utilize the extra degrees of freedom (DOFs) included in the Hubbard Hamiltonian of atoms, such as, (pseudo)charge and orbital DOFs, which are usually neglected in the perturbative spin interaction. Our active control of the Wannier orbital DOF allows us to overcome the fundamental difficulty of simultaneous achievement of high fidelity, short operation time, and scalability due to the fact that enhancing interaction for short operation time breaks the perturbative condition and intrinsically induces unwanted correlations among spin and the extra DOFs. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S34.00010: Quantum Information with Rydberg atoms: Role of dissipation and decoherence Durga Bhaktavatsala Rao Dasari, Klaus Molmer Originally inhomegeneities, decoherence and decay of the atomic systems were minimized in quantum computing proposals so that their effects would not disturb the ideal unitary evolution of the system. Recent works, however, suggest a quite opposite strategy, where inhomegeneities are created on purpose and and the system is driven on resonance with short lived states such that it dephases and decays to robust steady states. By suitable use of the interactions, these states can be selected, e.g., as entangled states or states encoding the outcome of a quantum computation. We investigate the coherent effects induced by dissipation and decoherence in neutral atom based quantum computing proposals, for creating robust entangled states and long distance gates. We also show that these incoherent effects can also be helpful for deterministic loading of optical traps with single atoms and to reliably store and emit single photons. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S34.00011: Maximally entangled states in a Bose-Hubbard trimer Sebastian Reyes, Luis Morales-Molina, Miguel Orszag We study the generation of entanglement for interacting cold atoms in a three-site Bose-Hubbard ring. We propose a scheme by which maximally entangled states (MES) between two distinct atomic species can be prepared. Depending on the choice of experimental parameters, we demonstrate that it is possible to obtain different types of MES. Furthermore, we show that these MES are highly protected against experimental noise, making them good candidates for potential applications. [Preview Abstract] |
Session S35: Vortices, Solitons, and Driven Systems
Sponsoring Units: DAMOPChair: Tom Purdy, JILA
Room: 702
Thursday, March 6, 2014 8:00AM - 8:12AM |
S35.00001: Josephson junction in the double-well potential with a fast-oscillating barrier Aydin Cem Keser, Juraj Radic, Victor Galitski We present an analysis of the Bose gas in a double-well potential with a fast-oscillating barrier. We study the Floquet spectrum of the system and find the effective time-independent Hamiltonian where the tunneling coefficient gets modified due to the periodic driving. The system realizes a Josephson junction with a high control of the tunneling coefficient (the coefficient can now change sign, which is impossible in the stationary double-well potential). We connect the corresponding Josephson equations with equations of motion for Kapitsa's pendulum and study the ways to dynamically stabilize certain states of the system. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S35.00002: Finite--temperature effects in rotational hysteresis of ring Bose--Einstein condensates N. Murray, C. Lanier, M. Edwards, Y.-H. Wang, C.W. Clark, S. Eckel, F. Jendrzejewski, G.K. Campbell A ring Bose--Einstein condensate (BEC) with zero circulation ($m=0$) stirred by a barrier will eventually jump to an $m=1$ state when stirred faster than a certain critical speed, $\Omega_{c}^{+}$. A ring BEC with $m=1$ will drop to $m=0$ when stirred at a critical speed, $\Omega_{c}^{-}$, which is lower than $\Omega_{c}^{+}$. The loop areas, $\Omega_{c}^{+}-\Omega_{c}^{-}$, of this hysteretic response of the BEC to stirring predicted by zero--temperature Gross--Pitaevskii equation (GPE) disagreed significantly with the results of a recent experiment. In the work reported here, we simulated this experiment with the phenomenologically damped GPE, [S. Choi, S. A. Morgan, and K. Burnett, Phys.\ Rev.\ A {\bf 57}, 4057 (1999)], and with the Zaremba--Nikuni--Griffin (ZNG) theory. The ZNG theory can account for finite--T, non--equilibrium dynamics. We compare the results of these simulations with the experimental data. The simulations show that a vortex/antivortex pair forms in the barrier region during the stirring and that this drives the hysteresis. We also show how the presence of an interacting, thermal cloud affects the dynamics of these pairs. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S35.00003: Persistent Currents in Bosonic Mixtures in the Ring Geometry Zhigang Wu, Eugene Zaremba We investigate the possibility of bosonic mixtures supporting persistent currents in the ring geometry. Our analysis is based on an approach developed by F. Bloch which focuses on the ground state energy of the condensate as a function of its angular momentum $L$, the so-called yrast spectrum. According to this approach, persistent currents are stable if the energy exhibits a local minimum at some non-zero angular momentum. We extend Bloch's analysis to a two-component mixture containing $N_A$ atoms of species $A$ and $N_B$ atoms of species $B$, with masses $M_A$ and $M_B$, respectively. For the special case of $M_A=M_B$ and equal interaction strengths between all the species, we use analytic soliton solutions of a two-component Bose gas in the ring geometry to analyze the mean-field yrast spectrum of the system. We find that the spectrum exhibits a surprisingly rich structure as a result of an intricate interplay of interparticle interactions and population imbalance. We discuss the implication of these results in regard to the possibility of persistent currents at higher angular momenta. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S35.00004: Oscillation and Instability of a Soliton in Superfluid Atomic Gas Liangsheng Zhang, David Huse We use superfluid hydrodynamics and force equations to phenomenologically investigate the oscillation of a soliton in harmonic trap and the ``snake'' instability of a soliton in a uniform background. The results obtained are functions of missing mass $m_s$ which characterizes the missing number of atoms inside the soliton and a ``mobility'' parameter $C$ which determines the relation between the soliton velocity and the phase difference across it to leading order. It is found that by making $|m_s|$ and $C$ small, the soliton will have a slower oscillation and tend to be more stable, as is seen in recent MIT experiment on the unitary Fermi gas [T. Yefsah, A. T. Sommer, M. J. H. Ku, L. W. Cheuk, W. Ji, W. S. Bakr, and M. W. Zwierlein, Nature 499, 426 (2013)]. We also use the hydrodynamic equations with perturbation theory to approximately solve Gross Pitaevskii equation and then use the solution to test our hydrodynamic approach to oscillation and instability in the case of Bose Einstein condensation with weak interactions. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S35.00005: Bright-like dark solitons and current-phase characteristics of superfluid Bose mixtures near the first-order Mott transition Ippei Danshita, Daisuke Yamamoto, Yasuyuki Kato We consider a superfluid phase of binary Bose mixtures in an optical lattice. It is well known that the superfluid-Mott insulator transition in this system is of first order when the filling factor is even and the inter-species repulsion is smaller than but close to the intra-species repulsion. We show that in the vicinity of the first-order boundaries to the Mott insulators the superfluid order parameters obey the nonlinear Schr\"{o}dinger equation (NLSE) with not only cubic but also quintic nonlinearity. We analytically solve the cubic-quintic NLSE to obtain soliton solutions. In particular, when the superfluid state changes from a ground state to a metastable one, a standard dark soliton turns into a bright-like dark soliton, which has a non-vanishing density dip and no $\pi$ phase kink even in the case of a standing soliton. In the presence of a potential barrier, we find the critical barrier strength above which there is no superfluid solution and unconventional current-phase characteristics, owing to the bright-like dark soliton. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S35.00006: Emergence of Reflectionless Scattering from Linearizations of Physically Relevant Integrable PDEs around Solitons Andrew Koller, Zaijong Hwang, Maxim Olshanii We present four examples of integrable partial differential equations (PDEs) of mathematical physics, that when linearized around a localized stationary solution, exhibit scattering without reflection---at {\it all} energies. Starting from the most well-known and the most empirically relevant phenomenon of the transparency of one-dimensional bright bosonic solitons to Bogoliubov excitations, we proceed to the sine-Gordon, Korteweg-de Vries, and Liouville's equations whose stationary solitons also support our assertion. The proposed connection between integrability and reflectionless scattering seems to span two distinct integrability mechanisms: Lax pairing in the first three cases, and a nonlinear differential map to a linear PDE in the last one. We argue that the transparency shown by linearized integrable PDEs is necessary to ensure that they can support the transparency of stationary solitons at the level of the original nonlinear PDE. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S35.00007: Topological solitons in scalar field theory Aliaksei Halavanau Over last 40 years the topological solitons, the localized, lump-like, finite-energy field configuration which appear in non-linear theories in various space-time dimensions have been intensively studied in various frameworks. We present a numerical study of the process of the kink-antikink collisions in three one-dimensional potential models, such as $\phi^4$ (double well), coupled $\phi^4$ and $\phi^6$ (triple well). We also take into consideration the case of real scalar field in 3 spacial dimensions, where there are simple theories from the Skyrme family with soliton solutions. Different types of field configurations are discussed. Our results reveal new types of soliton solutions in coupled $\phi^4$ model along with new high charge and massive configurations in Faddeev-Skyrme model. Extensive study of $\phi^4$ potential is presented. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S35.00008: Knotting of vortex tangle in three-dimensional random waves Alexander Taylor, Mark Dennis Quantised vortices are fundamental to the description of disordered 3D complex scalar fields such as turbulent superfluids or BECs, but also a wide range of other phenomena including optical volume speckle, the quantum eigenfunctions of chaotic 3D cavities, and liquid crystal phases. These systems all exhibit statistically random large scale vortex tangles that are difficult to describe analytically, but certain properties appear universal despite the physically different origin of complexity. We track vortex tangle in numerical simulations of the random wave model of chaotic eigenfunctions [1], where the waves are linear, but the zeros themselves are very nonlinear features forming a dense tangle of filaments whose geometry and topology we analyse numerically. We observe that while many standard quantities reveal only a common statistical scaling on the large scale, the topology - particularly the occurrence of knots in vortex loops - discriminates between tangles with different origins. In fact, knotting is surprisingly rare when compared to standard random walk models. \newline [1] M V Berry and M R Dennis, \emph{Proc R Soc A} \textbf{456}, 2059-79 (2000) [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S35.00009: Fractional Vortices in $J = 2$ Condensates David Ferguson, James Sauls We consider the possible ground-states and topologically stable line defects in BCS and BEC condensates with total spin $J=2$, including spinor BECs, as well BCS condensates with total angular momentum $J=2$. For cold Fermi gases it may be possible to realize $^{1}D_{2}$ or $^{3}P_{2}$ condensates of BCS pairs described by a symmetric and traceless matrix, $A_{\mu\nu}$, for the $2J+1=5$ complex amplitudes that transform as a rank 2 tensor under joint spin and orbital rotations. Condensates with $J=2$ have a rich phase diagram. We discuss the residual symmetry and fundamental group of $J=2$ condensates exhibiting \emph{complex, bi-axial} order, $A_{\mu\nu}=\Delta\,e^{i\varphi} \left[u_{\mu}u_{\nu}+\epsilon v_{\mu} v_{\nu}+\epsilon^2 w_{\mu} w_{\nu}\right]$, where $\epsilon=e^{i\,2\pi/3}$ and $u,v,w$ are an othogonal triad. This remarkable phase has tetrahedral point symmetry and is described by a non-abelian fundamental group $\pi_{1}(G/H)$. We classify the topologically stable line defects and show that conventional $U(1)$ phase vortices can dissociate into \emph{fractional} vortices with $2 \pi/3$ phase winding combined with tetrahedral rotations, indexed by the conjugacy classes of the non-abelian isotropy subgroup $H$, and consider associated fermionic bound states. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S35.00010: Rotational properties of a rapidly rotating two-component Bose gas Elife Karabulut, Francesc Malet, Georgios Kavoulakis, Stephanie Reimann One of the hallmarks of a superfluid is its response to rotation. Bose-Einstein condensates (BECs) of ultra-cold atoms are ideal systems for exploring this problem. In such systems, the presence and properties of the quantized vortex states are strongly influenced by the form of the confinement. Several experimental and theoretical studies report that confining potentials rising more steeply than quadratically introduce many novel phases, where the picture becomes more interesting in the case of a multi-component BEC. We investigate the rotational properties of a two-component BEC confined in an anharmonic trapping potential using both numerical and analytic methods. More specifically, with the use of a variational approach we derive analytically the phase diagram of the system as a function of the rotational frequency of the trap and of the coupling constant for sufficiently weak values of the anharmonicity and of the coupling. The more general structure of the phase diagram is investigated numerically. We compare our results with the ones of (i) a single-component BEC confined in an anharmonic potential and (ii) a two-component BEC, which is confined in a harmonic trapping potential. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S35.00011: Annular Bose metal from interacting Rashba Bosons Ashvin Vishwanath, Andreas Ruegg There has been much recent interest in realizing ultracold atoms with spin-orbit coupling. Here we study bosons in 2$+$1 dimensions with Rashba spin-orbit interactions. The dispersion minimum occurs along a circle in momentum space that frustrates Bose condensation and raises the possibility of a novel phase. Here we propose a`Bose metal' ground state, for which we construct a wavefunction and evaluate its properties using Variational Monte Carlo calculations. We show that this is an uncondensed state with a hidden Fermi surface and a ring like momentum distribution function -- hence the name annular Bose metal. We also discuss the competition with an ordinary Bose condensate and related states. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S35.00012: Imaging and manipulating effective ferromagnetism in a shaken optical Colin Parker, Li-Chung Ha, Karina Jim{\'e}nez-Garc{\'i}a, Cheng Chin Recentely, we have developed a powerful lattice shaking technique to introduce long-range itinerant ferromagnetic order in cold atomic gases, using only one atomic internal state[1]. By using near-resonant lattice shaking we can engineer a band with two minima, which we label as spin-up and spin-down. Here we extend this technique to shaking in two directions. The resulting band has four minima and thus permits four types of domains, allowing for new possibilities such as ``Y'' or ``X'' type boundaries. I will discuss the prospects for mapping this domain structure, as well as for using a superlattice to manipulate domains. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S35.00013: Non-adiabatic dynamics of strongly paired fermions across a Feshbach resonance Maxim Dzero, Emil Yuzbashyan, Victor Gurarie We present a theory of far-from-equilibrium degenerate Fermi gas interacting through a diatomic Feshbach resonance. The basis of our theory is a two-channel model which describes strongly interacting fermionic and bosonic (molecular) degrees of freedom. We employ integrability of the two-channel model to describe the limiting dynamics of the pairing amplitude as well as the steady state wave function for sudden changes of detuning frequency across the BCS-BEC crossover. In collisionless regime, on a time scale larger then the order parameter relaxation time condensate reaches a steady state. We find the following three steady states for an arbitrary strength of the perturbation: (i) gapless steady state; (ii) steady state with constant value of the order parameter; (iii) steady state with the periodic order parameter and determine the asymptotic behavior of the order parameter in each of these regimes exactly. We also discuss the features of the superfluid steady-state dynamics which would allow experimental verifications of our results. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S35.00014: Quantum kinetic description of attractive two-component fermions from the weak-coupling Fermi liquid to the strong-coupling Bose liquid regime Mehrtash Babadi, Eugene Demler We derive a set of quantum kinetic equations governing the non-equilibrium dynamics of two-component fermions with short-range attractive interactions from the leading order large-N expansion of the effective action of an Sp(N)-symmetric Fermi gas. The derived kinetic equations reduce to the Boltzmann equation describing the evolution of the occupation of fermionic quasiparticles and long-lived composite bosons in the weak- and strong-coupling limits, respectively, while providing a smooth interpolation of the two limits for the intermediate pairing pseudogap regime. The obtained formalism successfully explains the findings of a recent experiment with two-dimensional ultracold Fermi gases. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S35.00015: Ferromagnetic response of a ``high-temperature'' quantum antiferromagnet Xin Wang We study the antiferromagnetic phase of the ionic Hubbard model at finite temperature using dynamical mean-field theory. We find that the ionic potential plays a dual role in determining the antiferromagnetic order. A small ionic potential (compared to the Hubbard repulsion) increases the super-exchange coupling, thereby implying an increase of the Neel temperature of the system, which should facilitate observation of antiferromagnetic ordering experimentally. On the other hand, for large ionic potential, the antiferromagnetic ordering is killed and the system becomes a charge density wave with electron occupancies alternating between 0 and 2. This novel way of degrading antiferromagnetism leads to spin polarization of the low energy single particle density of states. The dynamic response of the system thus mimics ferromagnetic behavior, although the system is still an antiferromagnet in terms of the static spin order [1].\\[4pt] [1] X. Wang, R. Sensarma, and S. Das Sarma, arXiv:1308.1091 [Preview Abstract] |
Session S36: Focus Session: Semiconductor Qubits: Device, Control, and Measurement System Engineering
Sponsoring Units: GQIChair: Matt Borselli, HRL
Room: 703
Thursday, March 6, 2014 8:00AM - 8:36AM |
S36.00001: Quantum hardware Invited Speaker: David Reilly |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S36.00002: SiGe HBT cryogenic preamplification for higher bandwidth donor spin read-out Matthew Curry, Stephen Carr, Greg Ten-Eyck, Joel Wendt, Tammy Pluym, Michael Lilly, Malcolm Carroll Single-shot read-out of a donor spin can be performed using the response of a single-electron-transistor (SET). This technique can produce relatively large changes in current, on the order of 1 (nA), to distinguish between the spin states. Despite the relatively large signal, the read-out time resolution has been limited to approximately 100 (kHz) of bandwidth because of noise. Cryogenic pre-amplification has been shown to extend the response of certain detection circuits to shorter time resolution and thus higher bandwidth. We examine a SiGe HBT circuit configuration for cryogenic preamplification, which has potential advantages over commonly used HEMT configurations. Here we present 4 (K) measurements of a circuit consisting of a Silicon-SET inline with a Heterojunction-Bipolar-Transistor (HBT). We compare the measured bandwidth with and without the HBT inline and find that at higher frequencies the signal-to-noise-ratio (SNR) with the HBT inline exceeds the SNR without the HBT inline. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. The work was supported by the Sandia National Laboratories Directed Research and Development Program. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S36.00003: Optimal post-processing for a generic single-shot qubit readout Benjamin D'Anjou, William A. Coish We analyze three different post-processing methods applied to a single-shot qubit readout: the average-signal (boxcar filter), peak-signal, and maximum-likelihood methods. In contrast to previous work, we account for a stochastic turn-on time $t_i$ associated with the leading edge of a pulse signaling one of the qubit states. This model is relevant to spin-qubit readouts based on spin-to-charge conversion and would be generically reached in the limit of large signal-to-noise ratio $r$ for several other physical systems, including fluorescence-based readouts of ion-trap qubits and nitrogen-vacancy center spins. We find that the peak-signal method outperforms the boxcar filter significantly when $t_i$ is stochastic, but is only marginally better for deterministic $t_i$. We generalize the theoretically optimal maximum-likelihood method to stochastic $t_i$ and show numerically that a stochastic turn-on time $t_i$ will always result in a larger single-shot error rate. Based on this observation, we propose a general strategy to improve the quality of single-shot readouts by forcing $t_i$ to be deterministic. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S36.00004: Electron transport in double quantum dots: Pauli spin blockade and an ultrasmall magnetic field effect Jeroen Danon We consider electron transport in a double quantum dot tuned to the Pauli spin blockade regime. We revisit the role of the random nuclear fields in the two dots and develop a theory going beyond the usual master-equation approach. This allows us to take into account a dark state that forms when the average effective field in the two dots is zero. We show that this small-field dark state can survive averaging over the random fields, most noticeably at intermediate interdot tunnel coupling strength. Besides deepening our understanding of the electron dynamics in double quantum dots, our results might help explaining a so-called ultrasmall magnetic field effect observed in some organic semiconductors. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S36.00005: Field-effect-induced two-dimensional electron gas utilizing modulation doping for improved ohmic contacts Sumit Mondal, Geoff Gardner, John Watson, Micheal J. Manfra Recently there has been a significant interest in the use of GaAs-based quantum dots for spin qubits. Progress is hindered by the presence of charge noise in modulation doped heterostructures where fluctuations occurring in the remote ionized dopant layer couple to the qubit. In this work we demonstrate the experimental realization of a new field effect transistor (FET) device where the active channel region is locally devoid of the silicon doping layer and hence precludes the possibility of charge fluctuations on ionized dopants causing instability. The underlying heterostructure was grown by molecular beam epitaxy and is designed with an etch-stop between the silicon delta-doping layer and single interface GaAs/AlGaAs heterojunction that facilitates removal of the modulation doping at precise locations defined by lithography. The resulting 2DEG is induced by a field-effect and the density is tunable in a wide range of 6X10$^{\mathrm{10}}$ cm$^{\mathrm{-2}}$ to 2.7X10$^{\mathrm{11}}$ cm$^{\mathrm{-2}}$. The design, fabrication, and operation of these devices along with low temperature (T $=$ 0.3K) transport data is presented. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S36.00006: Accumulation-Only Device Architecture for Si/SiGe Quantum Dots K. Wang, D. Zajac, T. Hazard, C. Payette, J.R. Petta Silicon is one of the most promising candidates for ultra coherent quantum bits due to its relatively weak spin-orbit coupling and the absence of nuclear spin in its naturally abundant isotope [1]. High quality charge and spin qubits have been demonstrated with a dual-gate device geometry [1] [2]. Due to the larger effective mass of electrons in Si, it is desirable to have a more tightly confined quantum dot to increase the orbital level spacings. Here we demonstrate a new silicon quantum dot device architecture. The quantum dot and potential barriers are individually formed by corresponding accumulation gates, potentially allowing more precise control over electron occupation and tunnel coupling. The gate geometry can also be scaled up to create multiple quantum dot devices. [1] B. W. Maune \emph{et al.}, Nature \textbf{481}, 344 (2012). [2] K. Wang \emph{et al.}, Phys. Rev. Lett. \textbf{111}, 046801 (2013). [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S36.00007: Si/SiGe quadruple quantum dots with direct barrier gates Daniel Ward, John Gamble, Ryan Foote, Donald Savage, Max Lagally, Susan Coppersmith, Mark Eriksson We have fabricated a quadruple quantum dot in a Si/SiGe heterostructure with the aim of demonstrating a two-qubit quantum gate. This device makes use of direct barrier gates, in which individual gates are placed directly over the quantum dots and tunnel barriers. This design enables rational control of both energies and tunnel rates in coupled quantum dots. In this talk we discuss the design, fabrication, and initial characterization of the device. This work was supported in part by ARO (W911NF-12-0607), NSF (DMR-1206915), and the United States Department of Defense. The views and conclusions contained in this document are those of the authors and should not be interpreted as representing the official policies, either expressly or implied, of the US Government. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S36.00008: Design considerations for multielectron double quantum dot qubits in silicon Erik Nielsen, Edwin Barnes, Jason Kestner Solid state double quantum dot (DQD) spin qubits can be created by confining two electrons to a DQD potential. We present results showing the viability and potential advantages of creating a DQD spin qubit with greater than two electrons, and which suggest that silicon devices which could realize these advantages are experimentally possible. Our analysis of a six-electron DQD uses full configuration interaction methods and shows an isolated qubit space in regimes which 3D quantum device simulations indicate are accessible experimentally. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S36.00009: Charge Offset Stability in Si Single Electron Devices with Al Gates M.D. Stewart, Jr., Chih-Hwan Yang, Nai Shyan Lai, Wee Han Lim, Andrew Dzurak, Neil Zimmerman The charge offset drift (time stability) is an important real-world issue in single electron devices (SEDs). For use as current standards for electrical metrology, we require time stability over long periods of time. For use as qubits, we require time stability for device integration and because, on short timescales, the charge offset drift can contribute to dephasing. Recently, workers have shown excellent qubit performance using aluminum gates on bulk Si wafers [1]. We report on the charge offset drift in these devices: the value (0.15 $e$) is intermediate between that of Al/AlO$_x$/Al tunnel junctions (greater than 1 $e$) and Si SEDs defined with Si gates (0.01 $e$). This range of values suggests that defects in the AlO$_x$ are the main cause of the charge offset drift instability. \\[4pt] [1] J. J. Pla, K. Tan, J. Dehollain,W. Lim, J. Morton, D. Jamieson, A. Dzurak, and A. Morello, Nature 489, 541 (2012). [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S36.00010: Epitaxial growth of$^{28}$Si enriched \textit{in situ} to 99.9998{\%} for quantum information Kevin Dwyer, Joshua Pomeroy, David Simons In support of quantum information devices, we epitaxially deposit \textgreater 100 nm $^{28}$Si films enriched \textit{in situ} to \textgreater 99.9998 {\%} isotope fraction at high temperature. Using our silicon enrichment ion beam deposition source, we explore electrical and structural properties of our $^{28}$Si films using \textit{in situ }reflection high energy electron diffraction (RHEED), transmission electron microscopy (TEM) and electrical measurements including capacitance--voltage profiling. Secondary ion mass spectrometry (SIMS) is used to show $^{28}$Si films have residual $^{29}$Si isotope fractions \textless 1 ppm (40 times less than previously reported $^{28}$Si sources). We also demonstrate the ability to produce isotope heterostructures with applications including $^{28}$Si/$^{28}$Si$^{74}$Ge quantum wells. $^{28}$Si is a critical material for quantum computing as removal of $^{29}$Si spins means qubits such as phosphorous atoms can have nuclear coherence (T$_{2})$ times of minutes even up to room temperature and can be addressed optically due to hyperfine transitions not normally resolvable in natural Si. Despite these advantages, $^{28}$Si is quite scarce making it clear that an alternate source such as the one we demonstrate is needed. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S36.00011: Cavity Exciton-Polariton mediated, Single-Shot Quantum Non-Demolition measurement of a Quantum Dot Electron Spin Shruti Puri, Peter McMahon, Yoshihisa Yamamoto The quantum non-demolition (QND) measurement of a single electron spin is of great importance in measurement-based quantum computing schemes. The current single-shot readout demonstrations exhibit substantial spin-flip backaction. We propose a QND readout scheme for quantum dot (QD) electron spins in Faraday geometry, which differs from previous proposals and implementations in that it relies on a novel physical mechanism: the spin-dependent Coulomb exchange interaction between a QD spin and optically-excited quantum well (QW) microcavity exciton-polaritons. The Coulomb exchange interaction causes a spin-dependent shift in the resonance energy of the polarized polaritons, thus causing the phase and intensity response of left circularly polarized light to be different to that of the right circularly polarized light. As a result the QD electron's spin can be inferred from the response to a linearly polarized probe. We show that by a careful design of the system, any spin-flip backaction can be eliminated and a QND measurement of the QD electron spin can be performed within a few 10's of nanoseconds with fidelity 99:95\%. This improves upon current optical QD spin readout techniques across multiple metrics, including fidelity, speed and scalability. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S36.00012: Simulation of electrical control of a solid-state flying qubit Seyed Mostafa Akrami Solid-state approaches to quantum information technology are attractive because they are scalable. The coherent transport of quantum information over large distances, as required for a practical quantum computer, has been demonstrated by coupling solid-state qubits to photons. However, there have been no demonstrations to date of techniques that can coherently transfer scalable qubits and perform quantum operations on them at the same time. The resulting so-called flying qubits are attractive because they allow for control over qubit separation and non-local entanglement with static gate voltages, which is a significant advantage over other solid-state qubits in confined systems for integration of quantum circuits. Here a numerical investigation has been performed over the transportation of coherent electrons. The simulation has been done in order to model a quantum point contact (QPC), tunnel-coupled wire, Aharonov-Bohm ring and finally a complete system for control of a solid-state flying qubit by means of Landauer-Buttiker formalism. The flying qubit state is defined by the presence of a travelling electron in either channel of the wire, and can be controlled without a magnetic field. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S36.00013: Dynamical spin-spin coupling of quantum dots Vahram Grigoryan, Jiang Xiao We carried out a nested Schrieffer-Wolff transformation of an Anderson two-impurity Hamiltonian to study the spin-spin coupling between two dynamical quantum dots under the influence of rotating transverse magnetic field. As a result of the rotating field, we predict a novel Ising type spin-spin coupling mechanism between quantum dots, whose strength is tunable via the magnitude of the rotating field. Due to its dynamical origin, this new coupling mechanism is qualitatively different from the all existing static couplings such as RKKY, while the strength could be comparable to the strength of the RKKY coupling. The dynamical coupling with the intristic RKKY coupling enables to construct a four level system of maximally entangled Bell states in a controllable manner. [Preview Abstract] |
Session S37: Focus Session: Carbon Nanotubes: Diameter, Wall & Chirality Control
Sponsoring Units: DMPRoom: 705/707
Thursday, March 6, 2014 8:00AM - 8:36AM |
S37.00001: Recent studies on double wall and triple wall carbon nanotubes Invited Speaker: Mildred Dresselhaus Recent progress is reported in studying 1) metallicity effects in double wall and triple wall carbon nanotubes, and 2) self healing effects in bundles due to ruptures in current flow due to Joule heating in carbon nanotube bundles. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S37.00002: Diameter-Selective Alignment of Carbon Nanotubes on Si (001) Stepped Surfaces Batnyam Enkhtaivan, Masahide Yoshimura, Jun-Ichi Iwata, Atsushi Oshiyama The necessity of aligning carbon nanotube (CNT) raises important questions of whether the alignment is energetically feasible and of whether the electronic properties of CNTs are modified on the substrate surface. We report total-energy electronic-structure calculations based on the DFT that provide stable adsorption sites, structural characteristics, and energy bands of CNTs adsorbed on the Si(001) stepped surfaces. We choose (5,5), (9,9) and (13,13) armchair CNTs with the diameters of 6.8 {\AA}, 12.2 {\AA} and 17.6 {\AA} and explore all the possible adsorption sites either on the terrace or at step edges. We find that the (9,9) CNT is most favorably adsorbed at the edge of the double-layer step D$_{B}$ along the $<$110$>$ direction, whereas the (5,5) and (13,13) CNTs favor the terrace site where the CNTs are perpendicular to the Si dimer rows. This finding is indicative of the diameter-selective self-organized alignment of CNTs by exploiting the Si surface steps. We also find that the electronic structure of each CNT is modified upon adsorption depending on the adsorption site and the diameter of the CNTs. In particular, the (9,9) CNT at the most stable step edge site becomes semiconducting and also an interesting flat band appears at Fermi level. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S37.00003: Chirality, Metallicity, and Transition Dependent Asymmetries in Resonance Raman Excitation Profiles of Chirality-Enriched Carbon Nanotubes Stephen Doorn, Juan Duque, Hagen Telg, Erik Haroz, Xiaomin Tu, Ming Zheng Access to carbon nanotube samples enriched in single chiralities allows the observation of new photophysical behaviors obscured or difficult to demonstrate in mixed-chirality ensembles. Recent examples include the observation of strongly asymmetric G-band excitation profiles resulting from non-Condon effects$^{1}$ and the unambiguous demonstration of Raman interference effects.$^{2}$ We present here our most recent results demonstrating the generality of the non-Condon behavior to include metallic species (specifically several armchair chiralities). Additionally, the E$_{ii}$ dependence in non-Condon behavior with excitations from E$_{11}$ thru E$_{44}$ for both RBM and G modes will be discussed. 1. J.G. Duque, et. al., ACS Nano, 5, 5233 (2011). 2. J.G. Duque, et. al., Phys. Rev. Lett. 108, 117404 (2012). [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S37.00004: Resonance Raman Spectroscopy of Single-Wall Carbon Nanotubes Separated via Aqueous Two-Phase Extraction J.R. Simpson, J.A. Fagan, A.R. Hight Walker We report Resonance Raman Spectroscopy (RRS) measurements of single-wall carbon nanotube (SWCNT) samples dispersed in aqueous solutions via surfactant wrapping and separated using aqueous two-phase extraction (ATPE) into chirality-enriched semiconducting and metallic SWCNT species. ATPE provides a rapid, robust, and remarkably tunable separation technique that allows isolation of high-purity, individual SWCNT chiralities via modification of the surfactant environment. We report RRS measurements of individual SWCNT species of various chiral index including, armchair and zigzag metals. Raman provides a powerful technique to quantify the metallic SWCNTs in ATPE fractions separated for metallicity. We measure Raman spectra over a wide range of excitation wavelengths from 457\,nm to 850\,nm using a series of discrete and continuously tunable laser sources coupled to a triple-grating spectrometer with a liquid-nitrogen-cooled detector. The spectra reveal Raman-active vibrational modes, including the low-frequency radial breathing mode (RBM) and higher-order modes. SWCNT chiral vectors are determined from the Raman spectra, specifically the RBM frequencies and corresponding energy excitation profiles, together with input from theoretical models. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S37.00005: Chirality Separation of Single-Wall Carbon Nanotubes using Aqueous Two-Phase Extraction Jeffrey Fagan Aqueous two-phase extraction (ATPE) was recently demonstrated to enable the separation of individual species of single-wall carbon nanotubes (SWCNTs) across the separated phases. In this presentation I will describe the use of a dextran - polyethylene glycol aqueous two-phase system along with a separation scheme of varying surfactant concentrations to enable isolation at high purity of specific small diameter SWCNT species. Separation by ATPE is rapid and robust, with a remarkable tunability that allows isolation of most single nanotube chiralities at high purity. Choice of surfactant(s), temperature, polymer concentrations, and the addition of small molecule salts can all be used to tune the exact partitioning of single SWCNT species between the two phases. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S37.00006: Chirally - Selective Growth of Single Walled Carbon Nanotube on Fe$_{13}$ Nanocatalyst Anteneh Tefera, Mogus Mochena Controlled growth of single - walled carbon nanotubes with desired chiral indices remains the holy grail of single walled carbon nanotube synthesis. We performed ab initio molecular dynamics calculation of the nucleation and early stage growth of (5,0) SWCNT in the low temperature range where the nanocatalyst is a solid. We show that a zigzag formation of carbon atoms is possible when the surface of the pentagonal pyramid of Fe$_{13}$ icosahedron is exposed to ambient carbon atoms or carbon atoms and dimers or a ring of ten carbon atoms. The possibility of anomalous cap formation resulting from competing repulsive and attractive forces is presented. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S37.00007: Why do carbon nanotubes grow chiral? Evgeni Penev, Vasilii Artyukhov, Boris Yakobson Carbon nanotubes (CNT) hold enormous technological promise. It can turn into reality only if one can control in a practical way the CNT chirality---the geometric feature of the tubular carbon topology that governs the CNT electronic properties. Experimental efforts over the last decade have consistently revealed a puzzling strong preference towards specific chiral CNT grown via catalytic chemical vapor deposition, challenging any existing hypotheses and turning chirality control even more elusive. Here we investigate the roles of different factors in shaping the chirality distribution of CNT yield, including nanotube-catalyst interface [1], the energetic landscape of CNT caps [2], or growth kinetics [3], building upon our ``Nanoreactor'' framework developed for graphene synthesis [4]. Our theory shows promise in explaining narrow chirality distributions seen in multiple recent experimental studies.\\[4pt] [1] Y. Liu, A. Dobrinsky, B. I. Yakobson, Phys. Rev. Lett. \textbf{105}, 235502 (2010).\newline [2] E. S. Penev, V. I. Artyukhov, and B. I. Yakobson (submitted).\newline [3] F. Ding, A. R. Harutyunyan, B. I. Yakobson, Proc. Natl. Acad. Sci. U.S.A. \textbf{106}, 2506 (2009).\newline [4] V. I. Artyukhov, Y. Liu, B. I. Yakobson, Proc. Natl. Acad. Sci. U.S. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S37.00008: Energy landscape of carbon nanotube caps: lack of intrinsic chirality bias and consequences for selective growth Vasilii Artyukhov, Evgeni Penev, Boris Yakobson In the initial stages of carbon nanotube (CNT) growth, a fixed pattern of six pentagons encodes what unique $(n,m)$ chirality a nascent CNT would inherit and can be viewed as its ``inorganic gene''. We shall present the results from a large-scale computational effort designed to establish a quantitative structure--property (intrinsic elastic energy) relation for a set of more than 4500~caps, including all isomers that obey the isolated pentagon rule, corresponding to tube diameters $d \leq 1$~nm. Our analysis shows that the energy scale associated with the CNT caps is small, compared to that of the CNT/catalyst interface. Such a flat energy landscape is irrelevant to chiral selectivity and lends further credibility to interface-controlled scenarios for selective growth of single-walled CNTs. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S37.00009: Chirality-Dependent Vapor-Phase Epitaxial Growth and Termination of Single-Wall Carbon Nanotubes Bilu Liu, Jia Liu, Chongwu Zhou Chirality-pure single-wall carbon nanotubes are highly desired for both fundamental study and many of their technological applications. Recently, we have shown that chirality-pure short nanotubes can be used as seeds for vapor-phase epitaxial cloning growth, opening up a new route toward chirality-controlled carbon nanotube synthesis. Nevertheless, the yield of vapor-phase epitaxial growth is rather limited at the present stage, due to the lack of mechanistic understanding of the process. Here we report chirality-dependent growth kinetics and termination mechanism for the vapor-phase epitaxial growth of seven single- chirality nanotubes of (9, 1), (6, 5), (8, 3), (7, 6), (10, 2), (6, 6), and (7, 7), covering near zigzag, medium chiral angle, and near armchair semiconductors, as well as armchair metallic nanotubes. Our results reveal that the growth rates of nanotubes increase with their chiral angles while the active lifetimes of the growth hold opposite trend. Consequently, the chirality distribution of a nanotube ensemble is jointly determined by both growth rates and lifetimes. These results correlate nanotube structures and properties with their growth behaviors and deepen our understanding of chirality-controlled growth of nanotubes. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S37.00010: A Molecular Dynamics Study of Single-Walled Carbon Nanotubes (SWCNTs) Dispersed in Bile Salt Surfactants Frederick Phelan Jr., Huai Sun Single-walled carbon nanotubes (SWNCTs) are materials with structural, electronic and optical properties that make them attractive for a myriad of advanced technology applications. A practical barrier to their use is that SWCNT synthesis techniques produce heterogeneous mixtures of varying lengths and chirality, whereas applications generally require tubes with narrow size distributions and individual type. Most separation techniques currently in use to obtain monodisperse tube fractions rely on dispersion of these materials in aqueous solution using surfactants. The dispersion process results in a mixture of colloidal structures in which individual tubes are dispersed and contained in a surfactant shell. Understanding the structure and properties of the SWCNT-surfactant complex at the molecular level, and how this is affected by chirality, is key to understanding and improving separations processes. In this study, we use molecular dynamics (MD) simulations to study the structure~and properties of SWCNT-surfactant colloidal complexes. We tested a number of methods and protocols in order to build an accurate model for simulating SWCNT systems for a variety of bile salt surfactants as well as anionic co-surfactants, components that are widely used and important in experimental separation studies at NIST. The custom force field parameters used here will be stored in WebFF, a Web-hosted smart force-field repository for polymeric and organic materials being developed at NIST for the Materials Genome Initiative. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S37.00011: Chiral Selection of Single-Wall Carbon Nanotubes in Murine Organs John Heddleston, Ashwin Bhirde, Zhe Wang, Constantine Khripin, Jeffrey Fagan, Ming Zheng, Xiaoyuan Chen, Angela Hight Walker Single-wall carbon nanotubes (SWCNTs) have garnered significant interest as innovative tools for biomedical applications. They are being used for a variety of purposes, e.g. to deliver drug payloads, monitor cellular activity, or as in vivo imaging tools. However our current understanding of how SWCNTs behave in biological systems is limited. In this work we use Raman spectroscopy to measure the radial breathing modes (RBMs) of SWCNTs in murine organs following intravenous administration. We identify RBMs in multiple homogenized organs and can additionally measure the less SWCNT-specific carbon Raman peaks in others. Further, we quantify significant changes in the relative contribution of different SWCNTs chiralities to the overall RBM distribution. We observe this change in SWCNTs with a smaller diameter (7.6 A) but not in larger diameter tubes (14 A). These data are among the first to measure RBMs in organs and suggest that chiral selection can occur in biological systems with susceptibility for selection dependent on SWCNT diameter. [Preview Abstract] |
Session S38: Invited Session: Assessment Issues in Physics Education
Sponsoring Units: FEdChair: Eric Brewe, Florida International University
Room: 709/711
Thursday, March 6, 2014 8:00AM - 8:36AM |
S38.00001: Foundations for Conceptualizing Assessment in Your Course Invited Speaker: Eugenia Etkina In this talk I will discuss how to think about assessment as a part of student learning experiences and of faculty professional development. Specifically, how to set the goals of the class and how to collect evidence that the students achieved the goals and how to use this evidence to change what the students and you will do later. These three seemingly simple steps, called Assessment For Learning (AFT), might dramatically change your approach to assessment and possibly to what you do in your classroom in general. I will provide examples of setting goals, developing assessment questions and altering existing instructional approaches for conceptual understanding, quantitative problem solving, and experimental abilities. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S38.00002: Research-based assessment instruments: Design, validation and interpretation Invited Speaker: Wendy Adams This presentation describes the process for creating and validating a research-based assessment instrument that measures the effectiveness of instruction. These assessments are designed to measure how well instruction causes students to think like experts. Although, the primary goal is not to obtain a comprehensive assessment of student learning; rather it is to provide formative assessment of teaching. Specific examples of instruments for physics conceptual understanding and student perceptions of the discipline of physics will be described, including the extent and limitations of how results can be interpreted. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S38.00003: Colorado Learning about Science Survey for Experimental Physics (E-CLASS) Invited Speaker: Heather Lewandowski In response to national calls to better align physics laboratory courses with the way physicists engage in research, we have developed an epistemology and expectations survey to assess how students perceive the nature of physics experiments in the contexts of laboratory courses and the professional research laboratory. The E-CLASS evaluates students' shifts in epistemology and affect at the beginning and end of a semester. Also, at the end of the semester, the E-CLASS assesses students' reflections on their course's expectations for earning a good grade. By basing survey statements on widely embraced learning goals and common critiques of teaching labs, the E-CLASS serves as an assessment tool for lab courses across the undergraduate curriculum and as a tool for PER research. We present the development, evidence of validation, and initial formative assessment results from a sample that includes 45 classes at 20 institutions. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:24AM |
S38.00004: Using research-based assessment to improve teaching in your classroom and department: New resources on the PER User's Guide Invited Speaker: Adrian Madsen Often physics faculty want to know how their students are doing compared to other ``students like mine.'' As part of the PER User's Guide (http://perusersguide.org), we are developing a national database of research validated assessment results (like the Force Concept Inventory) and an accompanying data explorer. Here faculty can securely upload their students' anonymized assessment results and compare them to students from peer institutions, the national dataset or before and after a change in teaching method. ``One-click analysis'' allows faculty members to visualize their data and view statistics such as gain scores and statistical significance and then download a report of the results. Results can be used to improve teaching, to make a case for more resources, for accreditation reports, or for promotion and tenure. Additionally, we are developing guides to these research validated assessments including background, guidelines for administration, and access to the test itself. We will showcase our new online system and provide information about how you can use it. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 11:00AM |
S38.00005: TBA Invited Speaker: Melissa Dancy |
Session S39: Invited Session: Artificial Spin Ice and Artificial Frustrated Systems: Desiging Topology, Controlling Frustration, Engineering Emergence
Sponsoring Units: DCMP GMAGChair: Cristiano Nisoli, Los Alamos National Laboratory
Room: Mile High Ballroom 2A-3A
Thursday, March 6, 2014 8:00AM - 8:36AM |
S39.00001: Magnetic Charge Organization and Screening in Thermalized Artificial Spin Ice Invited Speaker: Ian Gilbert Artificial spin ice is a material-by-design in which interacting single-domain ferromagnetic nanoislands are used to model Ising spins in frustrated spin systems. Artificial spin ice has proved a useful system in which to directly probe the physics of geometrical frustration, allowing us to better understand materials such as spin ice. Recently, several new experimental techniques have been developed that allow effective thermalization of artificial spin ice [1-3]. Given the intense interest in magnetic monopole excitations in spin ice materials and artificial spin ice's success in modeling these materials, it should not come as a surprise that interesting monopole physics emerges here as well. The first experimental investigation of thermalized artificial square spin ice determined that the system's monopole-like excitations obeyed a Boltzmann distribution and also found evidence for monopole-antimonopole interactions [1]. Further experiments have implicated these monopole excitations in the growth of ground state domains [2]. Our recent study of artificial kagome spin ice [3], whose odd-coordinated vertices always possess a net magnetic charge, has revealed a theoretically-predicted magnetic charge ordering transition which has not been previously observed experimentally. We have also investigated the details of magnetic charge interactions in lattices of mixed coordination number. This work was done in collaboration with Sheng Zhang, Cristiano Nisoli, Gia-Wei Chern, Michael Erickson, Liam O'Brien, Chris Leighton, Paul Lammert, Vincent Crespi, and Peter Schiffer. \\[4pt] [1] J.P$.$ Morgan et al., Nature Phys. 7, 75 (2011).\\[0pt] [2] A. Farhan et al., Phys. Rev. Lett. 111, 057204 (2013).\\[0pt] [3] S. Zhang, et al., Nature 500, 553 (2013). [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S39.00002: Thermally active two dimensional artificial spin-ice systems: experiment and simulation Invited Speaker: Peter Derlet Recently it has been possible to fabricate two dimensional arrays of interacting nano-magnetics which are thermally active within the time-frame of a photoemission electron microscopy (PEEM) experiment. Employing X-ray magnetic circular dichroism, such a local experimental probe can image the changing magnetic state of finite kagome and square lattice systems. Both equilibrium and non-equilibrium conditions have been considered revealing non-trivial dynamics which for the case of the kagome system depends strongly on the underlying magnetic frustration. To give insight into the observed dynamics, monte carlo and kinetic monte carlo methods are performed using a simple Ising-like Hamiltonian. This talk will discuss the origins of such an Ising-like Hamiltonian and its application to specific experiments. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S39.00003: Collective Properties of Nanomagnet Arrays; Electric and Magnetic Currents in Artificial Spin Ice Invited Speaker: Will Branford I will discuss arrays of single domain nanomagnets. The shape of each nanomagnets controls the magnetic anisotropy and the elements are closely spaced so dipolar interactions are important. Lattices are chosen such that the geometry prevents all dipole interactions from being satisfied. The building block of such frustrated lattices is the equilateral triangle because it cannot support simple antiparallel ordering. A two dimensional array of corner sharing triangles is known as the kagome lattice and a three-dimensional array of corner sharing tetrahedral is known as pyrochlore. Magnetic pyrochlore chemical compounds (spin ices) have recently attracted much attention with the observation of emergent magnetic monopoles, but they have limitations as model frustrated systems: tuning the lattice parameter by chemical doping tends to break the symmetry, specific defects cannot be engineered and the spins cannot be directly imaged. The use of frustrated artificial nanostructures overcomes these problems through the tremendous versatility in array fabrication and compatibility with a suite of magnetic imaging techniques. Here I will show direct magnetic imaging studies of monopole defects [1-2] and magnetic charge flow. [3-4] The magnetic charge is carried by transverse domain walls and the chirality of the domain wall is found to control the direction of propagation. In addition to magnetic imaging studies of the magnetization state, I will also present magnetoresistance and Hall effect measurements. These techniques probe the array as a whole and can be very sensitive to the details of the spin structure. A change in symmetry in the Hall response of connected honeycomb nanostructures is observed at low temperatures indicating a collective response of the array of nanomagnets. [5] \\[4pt] [1] S. Ladak, D. E. Read, G. K. Perkins, L. F. Cohen {\&} W. R. Branford. Nature Physics 6, 359, (2010).\\[0pt] [2] S. Ladak, D. Read, T. Tyliszczak, W. R. Branford {\&} L. F. Cohen. New Journal of Physics 13, 023023, (2011).\\[0pt] [3] S. Ladak, S. K. Walton, K. Zeissler, T. Tyliszczak et al. New Journal of Physics 14, 045010, (2012).\\[0pt] [4] K. Zeissler, S. K. Walton, S. Ladak, D. E. Read et al. Sci. Rep. 3, 01252, (2013).\\[0pt] [5] W. R. Branford, S. Ladak, D. E. Read, K. Zeissler {\&} L. F. Cohen. Science 335, 1597, (2012). [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:24AM |
S39.00004: Resonant dynamics of topological magnetic structures Invited Speaker: Olle Heinonen A variety of topological magnetic structures have recently been observed and discussed in atomic structures. Examples are spin ices in pyrochlore lattices [1], or skyrmion lattices [2] in helical magnets, such as MnSi. Underlying these structures are competing interactions, which cannot all be simultaneously minimized. Patterned magnetic nanostructures can be engineered to have competing interactions that give rise to frustration, which can enable the formation of topological magnetic structures on the nanoscale and at room temperatures that can rather conveniently be observed [3-5]. In addition to interesting ground states or metastable states, the resonant dynamics of topological structures can be very interesting and different from the dynamics of the non-topological states [6-8]. This leads to the possibility of changing the resonant dynamics in magnetic system rather dramatically both in in frequency and space by small variations in a control parameter. In this introductory talk of the symposium, I will give examples of such states and the ensuing dynamics, and discuss possible future directions and applications. \\[4pt]Argonne National Laboratory is a US DOE Science Laboratory operated under contract no. DE-AC02-06CH11357 by UChicago Argonne, LLC. \\[4pt] [1] M. J. Harris et al, Phys. Rev. Lett., {\bf79}, 2554 (1997).\newline [2] S. M\"uhlbauer et al., Science {\bf323}, 915 (2009). \newline [3] R.F. Wang et al.,, Nature {\bf439}, 303 (2006).\newline [4] L. Sun et al., Phys. Rev. Lett. {\bf110}, 167201 (2013).\newline [5] C. Phatak et al., Phys. Rev. B {\bf83}, 174431 (2011).\newline [6] S. Gliga et al, Phys. Rev. Lett. {\bf110}, 117205,(2013).\newline [7] I. Makhfudz, B. Krueger, and O. Tchernyshyov, Phys. Rev. Lett. {\bf109}, 217201 (2012).\newline [8] Y.Y Dai et al., Phys. Rev. B {\bf88}, 054403 (2013). [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 11:00AM |
S39.00005: Two-dimensional artificial skyrmion crystals stabilized by nano-patterning Invited Speaker: Haifeng Ding The skyrmion crystal is a new material of current interest. It carries a topological charge and a Berry phase in real space and is anticipated to produce unconventional spin-electronic phenomena, such as the topological Hall effect and to exhibit spectacular dynamic properties. Technologically, skyrmion crystal may be exploited as a new class of spintronic material due to its unusual response to an electric charge current and spin current. A skyrmion crystal typically arises from helical spin structures induced by the Dzyaloshinskii--Moriya (DM) interaction. Experimentally, what has impeded its property exploration is that it is only to be found in few systems and within a narrow temperature and magnetic field range. In this talk, we present a practical design of a 2D skyrmion crystal, which completely by-passing the need for strong (or, indeed, any) DM interaction. The methodology is demonstrated with micromagnetic simulations and the computed skyrmion number per unit cell. The created skyrmion crystal has a robost working regime including room temperature, much broader than that for DM-driven skyrmion crystals. The method can dramatically widen the scope of the properties exploration and practical applications of the skyrmion crystal. In addition, from a more general point of view, previous experimental and theoretical studies of systems with DM interactions have already shown amply that the DM interaction is not sufficient for the spontaneous formation of a skyrmion crystal all by itself, since many systems with DM interaction do not display skyrmion-crystal self-assembly. Our method demonstrates that the DM interaction is not necessary either. [Preview Abstract] |
Session S40: Invited Session: Spin Current and Magnetization Dynamics--Pure Spin Current Generation and Transport
Sponsoring Units: GMAGChair: Paul Crowell, University of Minnesota
Room: Mile High Ballroom 2B-3B
Thursday, March 6, 2014 8:00AM - 8:36AM |
S40.00001: The Spin Hall Effect, Spin Currents and Spin Orbit Torques in Ferromagnetic/Normal Metal Multilayer Nanostructures Invited Speaker: Robert Buhrman In the spin Hall effect (SHE) the passage of a charge current through a non-ferromagnetic metal (NM) film generates a transverse pure spin current that when it impinges onto an adjacent ferromagnetic (FM) film will exert both a damping-like torque and a field-like torque on the FM, with the former arising from the absorption of the transverse component of the incident spin current and the latter due to spin rotation during the reflection of a portion of the incident spin current. Certain NMs (e.g. Pt, Ta, and W) have been found to exhibit a strong SHE and the damping-like torque that can be exerted in this manner on thin film magnetic materials has significant potential for spintronics in that it has been demonstrated to be capable of reversibly switching the magnetization direction of both in-plane and out-of-plane magnetized nanomagnets, to induce persistent microwave magnetic oscillations, and to facilitate the high-speed manipulation of domain walls in magnetic nanostrips. I will report some recent results from our SHE studies, including investigations into the fundamental role that the interfacial spin-mixing conductance plays in determining the effectiveness of the SHE for exerting strong anti-damping spin torques on the adjacent ferromagnet, and experiments which demonstrate that both the damping-like torque and a strong field-like torque can arise from the ``bulk'' SHE. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S40.00002: Enhanced pure spin current emission by spin-lattice coupling Invited Speaker: Sergej Demokritov |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S40.00003: \textit{g}-Factor Anisotropy Driven Spin Relaxation in Germanium Invited Speaker: Pengke Li In semiconductors possessing more than a single conduction band valley, g-factor anisotropy opens a new channel of electron spin relaxation. This unusual mechanism arises in a magnetic field because the effective Zeeman field is tilted along the valley axis, and is randomized when electrons undergo intervalley scattering. This fluctuation depolarizes electron spins [1], similar to the Dyakonov-Perel mechanism in noncentrosymmetric semiconductors where spin relaxation is driven by a wavevector dependent magnetic field. We study the unique nature of g-factor anisotropy spin relaxation by spin transport measurements from long-distance germanium devices in a magnetic field aligned to the initial spin orientation [2]. The confluence of electron-phonon scattering (leading to Elliott-Yafet spin flips) and this previously unobserved physics enables the extraction of spin lifetime solely from spin-valve measurements. We find spin lifetimes in Ge up to several hundreds of ns at low temperature, far beyond any other available experimental results. Electric field and magnetic field are used to manipulate the spin signal by accelerating the spin polarized electrons and generating carrier heating, or by inducing Hanle spin precession. \\[4pt] [1] J.-N. Chazalviel, J. Phys. Chem. Solids 36, 387 (1975)\\[0pt] [2] Pengke Li, Jing Li, Lan Qing, Hanan Dery, and Ian Appelbaum, Phys. Rev. Lett. (2013) [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:24AM |
S40.00004: Exchange Coupling with Exponential Decay in Y$_3$Fe$_5$O$_{12}$/Barrier/Pt Heterostructures Invited Speaker: Yong Pu Understanding the mechanism of spin pumping is essential for advancing this exciting field and realizing potential applications of pure spin currents. It is believed that exchange interaction between the ferromagnet and nonmagnetic material is responsible for this phenomenon. We have grown high-crystalline quality Y$_3$Fe$_5$O$_{12}$ epitaxial thin films by off-axis sputtering and observed millivolt–level inverse spin Hall effect (ISHE) voltages in Y$_3$Fe$_5$O$_{12}$/Pt bilayer excited by an FMR cavity. By inserting an insulating barrier between Y$_3$Fe$_5$O$_{12}$ and Pt, we detect an exponential decay of the ISHE voltages over three orders of magnitude for four different barrier materials, including SrTiO$_3$, Sr$_2$GaTaO$_6$, Sr$_2$CrNbO$_6$, and Si. Exponential decay lengths of 0.16, 0.19, and 0.23 nm are extracted for Sr$_2$GaTaO$_6$, SrTiO$_3$, and Sr$_2$CrNbO$_6$ with band gaps of 2.36, 3.40, and 4.91 eV, respectively. The exponential dependence of spin pumping on barrier thicknesses can be explained by quantum tunneling of the conduction electrons in Pt through the barrier and coupling with the precessing magnetization of Y$_3$Fe$_5$O$_{12}$ through exchange interaction to acquire spin polarization. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 11:00AM |
S40.00005: Issues related to YIG spintronics - thin film growth, spin pumping efficiency, and spin current generation Invited Speaker: Mingzhong Wu If a magnetic field is applied to a magnetic material, the field produces a torque on the magnetization of the material and drives it to precess. This precession is similar to the motion of a spinning top where the gravitational field produces a torque, instead of the magnetic field. It turns out that magnetization precession in yttrium iron garnets (YIG) decays slower than in any other known magnetic materials. This fact gives rise to the recent birth of a new paradigm in the discipline of spintronics -- ``spintronics using YIG.'' This talk will touch on several topics related to YIG spintronics. The first part will demonstrate the feasibility of the use of pulsed laser deposition and magnetron sputtering to grow low-damping, nanometer-thick YIG films. The second part will address the efficiency of spin angular momentum transfer across YIG/normal metal interfaces. The last part will report on the use of YIG thin films to produce pure spin currents; Detailed discussions will be provided on the comparison between spin current generations using traveling spin waves and uniform ferromagnetic resonance modes, the field dependence of spin current generation, and spin current enhancement in YIG/Pt structures via the use of a thin Cu spacer. [Preview Abstract] |
Session S41: Focus Session: Hexagonal Manganites and Ferrites and Perovskite Stannates
Sponsoring Units: DMP DCOMPChair: Hiroyuki Takenaka, University of Pennsylvania
Room: Mile High Ballroom 3C
Thursday, March 6, 2014 8:00AM - 8:12AM |
S41.00001: Temperature Dependent Properties of E-Phase Perovskite ScMnO$_{3}$ Haiyan Chen, Tian Yu, Trevor Tyson, A.M. Milinda Abeykoon, Keun Ahn, Yusheng Chen Perovskite~E-type ScMnO$_{3}$ was synthesized under high temperature and pressure. Combined local and long range structural measurements were conducted using XAFS, PDF and single crystal XRD methods. The local structure of different ion sites was explored with x-ray absorption spectroscopy in low temperature. The detailed changes in structure on crossing into the magnetically ordered region are explored and compared with DFT electric polarization calculations. The accuracy of DFT models to assess the electric polarization components (structural and electronic) is discussed. This work is supported by DOE Grant DE-FG02-07ER46402. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S41.00002: Local Structural Changes at Low Temperature in Hexagonal InMnO$_{3}$ Tian Yu, Peng Gao, Tao Wu, Trevor Tyson, Xingguo Hong, Yusheng Chen, Roger Lalancette Single crystal structural, electric polarization and heat capacity measurements on the hexagonal InMnO$_{3}$ have revealed that this small R ion material is ferroelectric at room temperature. In addition, temperature dependent electrical polarization measurements down to $\sim$ 10K were conducted. In parallel, structural measurements on single crystal and powders were carried out. Evidence is found for local distortions which are strongly enhanced as temperature decreases. The connection between the structural changes and the bulk polarization is explored. Strong spin lattice coupling is evidenced in two temperature regions (near $\sim$ 120 and 40 K). This work is supported by DOE Grant DE-FG02-07ER46402. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S41.00003: $Z_2\times Z_3$ vortex density in hexagonal manganites and the Kibble-Zurek mechanism Xueyun Wang, Sang-Wook Cheong Hexagonal $REMnO_3 (RE=Ho, Er, Tm, Yb, Lu)$ is an improper ferroelectric where the size mismatch between RE layers and Mn-O layers induces a simultaneous ferroelectric-trimerization structural phase transition. Six types of ferroelectric-trimerization domains form the so-called $Z_2\times Z_3$ vortices. We have carefully examined how the $Z_2\times Z_3$ vortex density changes with the cooling rate across the transition temperature, and compared the result with the so-called Kibble-Zurek mechanism, which is relevant to the formation of cosmological defects such as cosmological strings. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S41.00004: Structural domain walls in polar hexagonal manganites Invited Speaker: Yu Kumagai The domain structure in the multiferroic hexagonal manganites is currently intensely investigated, motivated by the observation of intriguing sixfold topological defects at their meeting points [Choi, T. et al,. Nature Mater. 9, 253 (2010).] and nanoscale electrical conductivity at the domain walls [Wu, W. et al., Phys. Rev. Lett. 108, 077203 (2012).; Meier, D. et al., Nature Mater. 11, 284 (2012).], as well as reports of coupling between ferroelectricity, magnetism and structural antiphase domains [Geng, Y. et al., Nano Lett. 12, 6055 (2012).]. The detailed structure of the domain walls, as well as the origin of such couplings, however, was previously not fully understood. In the present study, we have used first-principles density functional theory to calculate the structure and properties of the low-energy structural domain walls in the hexagonal manganites [Kumagai, Y. and Spaldin, N. A., Nature Commun. 4, 1540 (2013).]. We find that the lowest energy domain walls are atomically sharp, with \textbraceleft 210\textbraceright orientation, explaining the orientation of recently observed stripe domains and suggesting their topological protection [Chae, S. C. et al., Phys. Rev. Lett. 108, 167603 (2012).]. We also explain why ferroelectric domain walls are always simultaneously antiphase walls, propose a mechanism for ferroelectric switching through domain-wall motion, and suggest an atomistic structure for the cores of the sixfold topological defects. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S41.00005: Crystal Structure and Physical Properties of Multiferroic TmMnO$_{3}$ single crystals Yuqin Zhang, Tian Yu, Trevor Tyson We present here single crystal diffraction and local structure results of hexagonal and orthorhombic TmMnO$_{3}$ single crystals. Combined with specific heat and electric polarization properties across the N\'{e}el temperature, we further explore the close correlation between ferroelectric behavior at low temperature and local and long range structure change in these two different crystal structures. This work is supported by DOE Grants DE-FG02-07ER46402 (NJIT). [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S41.00006: Fluctuation-enhanced magnetoelectric effect in hexagonal manganites Yanan Geng, Hena Das, A.L. Wysocki, Xueyun Wang, S-W. Cheong, M. Mostovoy, Craig J. Fennie, Weida Wu Intensive studies have been focused on enhancing magnetoelectric (ME) effect ever since Dzyaloshinskii and Astrov's seminal works on linear ME effect in Cr$_{\mathrm{2}}$O$_{\mathrm{3}}$. The coupling between the magnetic and electric dipoles in multiferroic and magnetoelectric materials holds promise of conceptually new electronic devices. Herein, we report on the Magnetoelectric Force Microscopy (MeFM) studies on the multiferroic hexagonal manganites. The direct visualization of the ME domains with topological vortex pattern provides compelling evidence for the mechanism of lattice-mediated ME response. Furthermore, our MeFM results reveal a diverging magnetoelectric effect in the vicinity of a tri-critical point, suggesting a possibility to enhance ME effects by harnessing critical fluctuations. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S41.00007: Mode Coupling between the Nonpolar K$_{3}$ and Polar $\Gamma _{2}^{-}$ Phonons as the Ferroelectricity Origin of Multiferroic h-LuMnO$_{3}$ Seungwoo Song, Seungyang Heo, Hyun Myung Jang LuMnO$_{3}$ is expected to show the highest stability towards the hexagonal phase among 15 different lanthanide-based manganites. Currently, the most puzzling problem associated with the hexagonal LuMnO$_{3}$ (h-LMO) is the observed large temperature-gap between the structural phase transition to the polar P6$_{3}$cm phase at $\sim$ 1290 K and the emergence of the spontaneous polarization at a substantially reduced temperature, $\sim$ 750 K. This anomalous temperature-gap has also been observed in h-YMnO$_{3}$. To resolve this puzzling issue, we have carried out density-functional theory calculations and found that the structural phase transition to the polar P6$_{3}$ cm phase from the nonpolar P63/mmc phase is mediated by the freezing-in of the zone-boundary K$_{3}$ phonon in h-LMO. However, the spontaneous ferroelectric polarization does not appear until the amplitude of K$_{3}$ phonon becomes a certain critical value above which the coupling of the polar $\Gamma_{2}^{-}$ mode with the nonpolar K$_{3}$ mode is practically turned on. This mode-coupling-induced polarization, thus, elucidates the above puzzle. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S41.00008: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S41.00009: Large photoconductivity in transparent perovskite semiconductor BaSnO3 Jisung Park, Useong Kim, Hoonmin Kim, Kookrin Char Photoconductivity of perovskite oxide such as SrTiO3 (STO) has been intensively studied because of its large potential for applications such as UV detector and optical devices. We have measured the photoconductivity of transparent perovskite semiconductor BaSnO3 (BSO) which has started to attract a large attention due to its high electron mobility and thermal stability. Measured photoconductivity of BSO is 1000 times higher than that of STO, although BSO and STO have the same perovskite structure and similar band gap. Epitaxial thin films of BSO and STO were made by pulsed laser deposition on a large bandgap substrates such as sapphire and MgO. The spectral responses measured by a monochromator have peaks around the band gap of each film, which is a clear evidence that electron-hole pair generation is the main mechanism of photoconductivity of both materials. The high mobility of BSO, which is two orders of magnitude larger than that of STO at room temperature, should be partially responsible for the higher photoconductivity. In addition, the small difference between the direct gap and indirect gap of BSO may make band to band transition easier. The results suggest that BSO can be used as more effective UV detector and optical devices than STO. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S41.00010: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S41.00011: Field Effect in transparent perovskite semiconductor BaSnO3 Useong Kim, Chulkwon Kim, Chanjong Ju, Woongjae Lee, Kee Hoon Kim, Kookrin Char A field effect transistor made entirely of perovskite materials has great potentials since it can be easily integrated in future devices controlling various properties. Recently, perovskite La-doped BaSnO$_{3}$ (BLSO) were reported to possess high electron mobility and thermal stability. We fabricated a field effect transistor structure on SrTiO$_{3}$ substrates using BLSO as a channel layer and lattice-matched LaInO$_{3}$(LIO) as a gate dielectric. We have measured the dielectric properties of the epitaxial LIO, such as the dielectric constant and the breakdown field. Using this gate oxide, we obtained the conductivity modulation by a factor of 5 in the channel with the electric field of 1.3 MV/cm. The magnitude of the field effect was found to be consistent with the materials parameters of BLSO and LIO. During the modulation process, the leakage current between the gate and channel was about 4 orders of magnitude lower than the channel current. The field effect mobility in the channel was calculated as 13 cm$^{2}$/Vs, which is comparable to the mobility of BLSO films in our previous study and indicates that the conductivity modulation was caused by modulation of carriers. Our field effect results suggest that the BLSO/LIO interface seems ideal, devoid of any charge trap or extra scattering. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S41.00012: pn junctions based on a single transparent perovskite semiconductor BaSnO3 Hoon Min Kim, Useong Kim, Chulkwon Park, Hyukwoo Kwon, Woongjae Lee, Tai Hoon Kim, Kee Hoon Kim, Kookrin Char Successful p doping of transparent oxide semiconductor will further increase its potential, especially in the area of optoelectronic applications. We will report our efforts to dope the BaSnO3 (BSO) with K by pulsed laser deposition. Although the K doped BSO exhibits rather high resistivity at room temperature, its conductivity increases dramatically at higher temperatures. Furthermore, the conductivity decreases when a small amount of oxygen was removed from the film, consistent with the behavior of p type doped oxides. We have fabricated pn junctions by using K doped BSO as a p type and La doped BSO as an n type material. I\textunderscore V characteristics of these devices show the typical rectifying behavior of pn junctions. We will present the analysis of the junction properties from the temperature dependent measurement of their electrical properties, which shows that the I\textunderscore V characteristics are consistent with the material parameters such as the carrier concentration, the mobility, and the bandgap. Our demonstration of pn junctions based on a single transparent perovskite semiconductor further enhances the potential of BSO system with high mobility and stability. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S41.00013: Structural, Magnetic and Transport Study on SrSn$_{\mathrm{1-x}}$Ru$_{\mathrm{x}}$O$_{3}$ system HyukWoo Kwon, Juyeon Shin, Kookrin Char SrSnO$_{3}$ is a diamagnetic material with a wide band gap. A theoretical calculation predicts that small Ru doped SrSnO$_{3}$ can be a dilute magnetic semiconductor (DMS) material. We have epitaxially grown the SrSn$_{\mathrm{1-x}}$Ru$_{\mathrm{x}}$O$_{3}$ (0 $\le $ x $\le $ 0.3) system by the pulsed laser deposition X-ray diffraction measurements show that films maintain a single phase over the doping range and lattice constants of the system decrease monotonously as the doping increases Transport measurements show that the films are semiconducting and their resistivities dramatically decrease as the Ru doping increases. On the other hand, Hall measurement data shows that the conduction of this system is mediated by hole carriers, which is closely related to the p-type conduction in SrRuO$_{3}$, and its corresponding mobility values vary from 0.1 $\sim$ 30 cm$^{2}$/V$\cdot$s, depending on the doping rate. Magnetic measurement data will be presented to investigate its ferromagnetism due to the doped Ru 4d character. In light of the electrical and magnetic property of this system, SrSn$_{\mathrm{1-x}}$Ru$_{\mathrm{x}}$O$_{3}$ system can be a promising material system for the field of spintronics and optoelectronics. [Preview Abstract] |
Session S42: Focus Session: New Topological Materials
Sponsoring Units: DMPChair: Genda Gu, Brookhaven National Laboratory
Room: Mile High Ballroom 4A
Thursday, March 6, 2014 8:00AM - 8:12AM |
S42.00001: Two-dimensional topological insulator molecular networks: dependence on structure, symmetry, and composition Liang Z. Tan, Steven G. Louie 2D molecular networks can be fabricated from a wide variety of molecular building blocks, arranged in many different configurations. Interactions between neighboring molecular building blocks result in the formation of new 2D materials. Examples of 2D organic topological insulators, that contain molecular building blocks and heavy elements arranged in a hexagonal lattice, have been recently proposed by Feng Liu and coworkers (Nano Lett., 13, 2842 (2013)). In this work, we present a systematic study of the design space of 2D molecular network topological insulators, elucidating the role of structure, symmetry, and composition of the networks. We show that the magnitude and presence of spin-orbit gaps in the electronic band structure is strongly dependent on the symmetry properties and arrangement of the individual components of the molecular lattice. We present general rules to maximize the magnitude of spin-orbit gaps and perform ab-initio calculations on promising structures derived from these guidelines. This work was supported by National Science Foundation Grant No. DMR10-1006184, the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Computational resources have been provided by the NSF through XSEDE resources at NICS. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S42.00002: A large-energy-gap oxide topological insulator based on the superconductor BaBiO3 Binghai Yan, Martin Jansen, Claudia Felser Topological insulators are a new class of quantum materials that are characterized by robust topological surface states (TSSs) inside the bulk-insulating gap, which hold great po- tential for applications in quantum information and spintronics as well as thermoelectrics. One major obstacle is the relatively small size of the bulk bandgap, which is typically around 0.3eV for the known topological insulator materials. Here we demonstrate through ab initio calculations that a known superconductor BaBiO3 (BBO) with a Tc of nearly 30 K emerges as a topological insulator in the electron-doped region. BBO exhibits a large topological energy gap of 0.7 eV, inside which a Dirac type of TSSs exists. As the first oxide topological insulator, BBO is naturally sta- ble against surface oxidization and degradation, distinct from chalcogenide topological insulators. An extra advantage of BBO lies in its ability to serve as an interface between TSSs and superconductors to realize Majorana fermions for future applications in quantum computation. Reference: B. Yan, M. Jansen, C. Feler, Nature Physics (2013) doi:10.1038/nphys2762 (arXiv:1308.2303) [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S42.00003: Emergent topological phenomena in thin films of pyrochlore iridates Bohm Jung Yang, Naoto Nagaosa With the recent development of thin film and artificial superstructure growth technique, it is possible to fabricate a system, moothly connecting the two-dimensions (2D) and three-dimensions (3D). In this work we unveil the dimensional crossover of emergent topological phenomena. In particular, by focusing on the thin film of pyrochlore iridate antiferromagnets grown along the [111] direction, we demonstrate that it can show giant anomalous Hall conductance, which is as large as the Hall conductance of 3D quantum Hall insulators, even though there is no Hall effect in 3D bulk material. In addition, we show the emergence of a genuine new topological phase, dubbed the anti-Chern insulator, which is realized only in thin films. This shows that the thin film of topological materials is a new platform to search unexplored novel topological phenomena. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S42.00004: Bulk Dirac Points in Distorted Spinels Julia Steinberg, Steve Young, Saad Zaheer, Charles Kane, Eugene Mele, Andrew Rappe A Dirac point is characterized by four degenerate states that disperse linearly with momentum around a single point $bk$ in the Brillouin zone. The resulting low energy theory is pseudorelativistic. A well-known example in two dimensions is graphene, which has a Fermi surface consisting exclusively of Dirac points that are responsible for many of its exotic properties. We report on an analogous Dirac-like Fermi surface in three-dimensional bulk materials in a distorted spinel structure on the basis of density functional theory (DFT) as well as tight-binding theory. The four examples we provide in this paper are BiZnSiO$_{4}$, BiCaSiO$_{4}$, BiMgSiO$_{4}$, and BiAlInO$_{4}$. A necessary characteristic of these structures is that they contain a Bi lattice which forms a hierarchy of chain-like substructures, with consequences for both fundamental understanding and materials design. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S42.00005: Zeeman Field-``Rotated'' Transitions for Surface Chern Insulators E.J. Mele, Fan Zhang, Xiao Li, Ji Feng, C.L. Kane Mirror symmetric surfaces of a topological crystalline insulator (e.g. SnTe) host even number of Dirac surface states. A surface Zeeman field generically gaps these states leading to a quantized anomalous Hall effect. Varying the direction of Zeeman field induces transitions between different surface insulating states with any two Chern numbers between -4 and 4. In the crystal frame the phase boundaries occur for field orientations which are great circles with (111)-like normals on a sphere. [arXiv:1309.7682] [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S42.00006: Topological Phase Transition in Antimony Man-Hong Wong, Guang Bian, Caizhi Xu, Thomas Miller, Tai-Chang Chiang Spin-orbit coupling (SOC) is believed to cause the parity exchange that drives normal band insulators into the topological regime. Changing the strength of the effective SOC can also induce quantum phase transitions in materials. We performed a first-principles calculation to elucidate the quantum phase transition from a topologically trivial to nontrivial system in a 15-bilayer Sb film. We increased the k-space sampling relative to previous studies and varied the effective SOC in order to observe the changes in the bulk band gap and topological surface states. A transition from a metal to a semimetal is observed as the SOC is tuned from 0\% to 100\%. At a SOC value near 300\%, a transition from a nontrivial topological semimetal to a topological insulator occurs. Varying the effective SOC strength can be realized experimentally by alloy substitution with elements in the same column in the periodic table. Increasing the effective SOC of the Sb film to values above 100\% is a model of the Bi$_{1-x}$Sb$_{x}$ alloy, the first three-dimensional topological insulator. Further studies using this method on different systems may lead to the discovery of new topological insulators. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S42.00007: Predicting Organic Topological Insulators in Organometallic Lattices Invited Speaker: Feng Liu Topological insulators (TIs) are a recently discovered class of materials having insulating bulk electronic states but conducting boundary states distinguished by nontrivial topology. So far, several generations of TIs have been theoretically predicted and experimentally confirmed, all based on inorganic materials. In this talk, I will present our recent study of a family of two-dimensional organic TIs made of organometallic lattices [1-4], based on first-principles calculations and tight-binding model analyses. Designed by assembling molecular building blocks of organometallic compounds with strong spin-orbit coupling into a hexagonal and Kagome lattices, these new classes of organic topological materials are shown to exhibit nontrivial topological edge states in both Dirac bands [1,4] and flat Chen bands (so-called fractional Chern insulator) [2,4], which are robust against significant lattice strain. Realization of half metallic state and anomalous quantum Hall effect in magnetic organic TIs with the inclusion of transition metal elements will also be discussed [3]. We envision that organic topological materials will greatly broaden the scientific scope and technological impact of emerging topological materials. \\[4pt] [1] Z. F. Wang, Zheng Liu and Feng Liu, ``Organic topological insulators in organometallic lattices,'' Nature Commun. 4, 1471 (2013).\\[0pt] [2] Z. Liu, Z. F. Wang, J.-W. Mei, Y. Wu and Feng Liu, ``Flat Chern Band in a Two-Dimensional Organometallic Framework,'' Phys. Rev. Lett. 110, 106804 (2013). \\[0pt] [3] Z. F. Wang, Z. Liu and Feng Liu, ``Quantum anomalous Hall effect in 2D organic topological insulator,'' Phys. Rev. Lett. 110, 196801 (2013). \\[0pt] [4] Z. F. Wang, N. Su and Feng Liu, ``Prediction of a Two-Dimensional Organic Topological Insulator,'' Nano Letters, 13, 2842 (2013). [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S42.00008: Probing the Topological Phase Transition via Density Oscillations in Silicene and Two-Dimensional Germanium Jianhui Zhou, Hao-Ran Chang, Hui Zhang, Yugui Yao, Di Xiao The quantum spin Hall effect (QSHE) has attracted much attention from both theoretical and experimental aspects. First principles calculations predict that low-buckled silicene and two dimensional (2D) germanium are the promising candidates for QSHE. We theoretically investigated two kinds of density oscillations: the Friedel oscillations and collective excitation in the silicene and 2D germanium within random phase approximation. We found that the tunable spin-valley constraint band structure could lead to some exotic properties in the two phenomena. Based on the exact analytical and numerical results, we demonstrated that the beating structure of screened potential as well as the undamped plasmon mode can be taken as a probe of topological phase transition from a band insulator to a topological insulator in silicene and 2D germanium. Our proposal could establish the connection between the topological phase transition and the density oscillations that can be accessed by a variety of experimental techniques. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S42.00009: Edge-states engineering of bismuth bilayer nanoribbons using first principles calculation Kyung-Hwan Jin, Seung-Hoon Jhi Study of topological insulator (TI) is recently showing remarkable progress in both theory and experiment, particularly in finding three dimensional materials. Two dimensional TI (quantum spin Hall) materials, on the other hand, have comparatively fewer examples. As such, theoretical predictions of single Bi (111) bilayers to be TI draw great attention from experiment. We investigate the edge states of quantum spin-Hall phase Bi (111) bilayer nano-ribbons (BNRs) using first-principles calculations. In contrast to the case of unsaturated atomic edges, we observe very well-defined helical edge states with linear energy dispersion when the edge atoms are passivated by chemicals such as H, NO2 or -OH. Our calculations show that the Fermi velocity and spin texture of the edge states in the BNRs is very sensitive to the kind of chemicals. We demonstrate that BNRs can be used as spin-current valves to rectify the spin-polarized electric currents via the edge states by selective passivation. Further, we examine the electronic transport properties of BNR with impurities in particular configurations. Our results provide a practical way of utilizing two-dimensional topological insulator Bi bilayer for spintronic devices. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S42.00010: Anomalous Edge Transport in the Quantum Anomalous Hall State Shou-Cheng Zhang, Jing Wang, Biao Lian, Haijun Zhang We predict by first-principles calculations that thin films of a Cr-doped (Bi,Sb)2Te3 magnetic topological insulator have gapless nonchiral edge states coexisting with the chiral edge state. Such gapless nonchiral states are not immune to backscattering, which would explain dissipative transport in the quantum anomalous Hall (QAH) state observed in this system experimentally. Here, we study the edge transport with both chiral and nonchiral states by the Landauer-Buttiker formalism and find that the longitudinal resistance is nonzero, whereas Hall resistance is quantized to h/e$^{2}$. In particular, the longitudinal resistance can be greatly reduced by adding an extra floating probe even if it is not used, while the Hall resistance remains at the quantized value. We propose several transport experiments to detect the dissipative nonchiral edge channels. These results will facilitate the realization of pure dissipationless transport of QAH states in magnetic topological insulators. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S42.00011: Large-gap Quantum Spin Hall Insulators in Tin Films Yong Xu, Binghai Yan, Hai-Jun Zhang, Jing Wang, Gang Xu, Peizhe Tang, Wenhui Duan, Shou-Cheng Zhang The search of large-gap quantum spin Hall (QSH) insulators and effective approaches to tune QSH states is important for both fundamental and practical interests. Based on first-principles calculations we find two-dimensional tin films in a honeycomb lattice, which we call ``Stanene'' structures, are QSH insulators with sizable bulk gaps of 0.3 eV, sufficiently large for practical applications at room temperature. These QSH states can be effectively tuned by chemical functionalization and by external strain. The mechanism for the QSH effect in this system is band inversion at the $\Gamma$ point, similar to the case of HgTe quantum well. Possible experimental realization of the Stanene structure will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S42.00012: Correlation-induced phase transitions in (111) bilayers of perovskite transition-metal oxides Satoshi Okamoto, W. Zhu, Y. Nomura, R. Arita, D. Xiao, N. Nagaosa We investigate the correlation-induced Mott, magnetic and topological phase transitions in (111) bilayers of perovskite transition-metal oxides LaAuO$_3$ and SrIrO$_3$ for which the previous density functional theory (DFT) calculations predicted topological insulating states. Using the dynamical-mean-field theory with DFT band structure and realistic Coulomb interactions, LaAuO$_3$ bilayer is shown to be far away from a Mott insulating regime, and a topological-insulating state is robust. On the other hand, SrIrO$_3$ bilayer is on the verge of an orbital-selective topological Mott transition. We also study the effect of magnetism in these systems. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S42.00013: Topological Insulator Proximity Effect: Emergence and Detection of Perfectly Conducting Channels Viktor Krueckl, Sven Essert, Klaus Richter We show that the proximity of a two-dimensional topological insulator can be employed to induce a channel with a perfect transmission eigenvalue hosted in the bulk of a conducting material with extended states. The perfectly conducting state inherits its topological protection from the adjacent topological insulator, and can be distinguished from a conventional edge state by its signatures in magneto transport. We present how these states are formed using band structure calculations of a model system and numerical calculations based on HgTe heterostructures. Furthermore, we propose two experimental configurations, which are able to verify the induction of a perfectly conducting channel in the localized and the diffusive regime by transport and shot noise signatures. [Preview Abstract] |
Session S43: Weyl Semimetals: Theory and Experiment
Sponsoring Units: DCMPChair: Steven Disseler, National Institute of Standards and Technology
Room: Mile High Ballroom 4B
Thursday, March 6, 2014 8:00AM - 8:12AM |
S43.00001: Weyl nodes in the bandstructure of bcc iron Ivo Souza, David Vanderbilt Weyl points in 3D bandstructures are receiving increasing attention in connection with topological states of matter. In addition, a controversial suggestion has recently appeared\footnote{Y. Chen \textit{et al.}, Phys. Rev. B \textbf{88}, 125110 (2013).} to the effect that isolated touching points between fully occupied and partially occupied bands, which act as monopole sources of Berry curvature, give a non-quantized, non-Fermi-surface contribution to the intrinsic anomalous Hall conductivity of ferromagnets. With these motivations, we carry out a systematic search for Weyl nodes in the bandstructure of bcc Fe using first-principles calculations. We trace the evolution of the Chern index of the fully occupied 2D bands in the $(k_x,k_y)$ plane as a function of $k_z$, and find several touching events taking place at high-symmetry points and lines in the 2D BZ. The amount of Chern-number transfer is analyzed in terms of the symmetry labels of the crossing bands.\footnote{C. Fang \textit{et al.}, Phys. Rev. Lett. \textbf{108}, 266802 (2012).} In addition to conical intersections with a topological charge $q=1$, we find quadratic touching points with $q=2$ that are protected by $C_4^z$ symmetry, as well as lines of degeneracy carrying a net topological charge. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S43.00002: Weyl semimetal emerging from LaBiTe$_3$-class topological insulators Jianpeng Liu, David Vanderbilt We study the topological-to-normal transition in LaBiTe$_3$ and LuBiTe$_3$ by tuning the strength of the spin-orbit coupling (SOC). For centrosymmetric 3D topological insulators (TIs), the strong $Z_2$ index can be changed only by an accidental band touching at an odd number of time-reversal invariant momenta in the Brillouin zone (BZ), achieved at some critical value of an external parameter $\lambda$. These band-touching points (BTPs) are ``Dirac-like,'' carrying zero chiral charge. For general noncentrosymmetric TIs, however, one expects to see a stable Weyl semimetal phase over some finite interval of $\lambda$. As $\lambda$ is varied, one expects first the appearance of $2(2n+1)$ Dirac-like BTPs in the BZ, which then split into pairs of Weyl points carrying opposite chiral charges. These BTPs then migrate in the BZ and finally annihilate after exchanging partners, leaving behind an inverted strong $Z_2$ index. Based on first-principles calculations, we predict that this phenomenon can be realized as the SOC is tuned in LaBiTe$_3$ and LuBiTe$_3$. We also construct a low-energy effective model to describe the topological phases in these materials. Preliminary results suggest that other interesting phases could be observed when a Zeeman field is applied. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S43.00003: Discovery of a Three-dimensional Topological Dirac Semimetal, Na3Bi Y.L. Chen, Z.K. Liu, B. Zhou, Y. Zhang, Z.J. Wang, H.M. Weng, D. Prabhakran, S.-K. Mo, Z.X. Shen, Z. Fang, X. Dai, Z. Hussain Three-dimensional (3D) topological Dirac semimetals (TDSs) represent a novel state of quantum matter that can be viewed as ``3D graphene''. In contrast to two-dimensional (2D) Dirac fermions in graphene or on the surface of 3D topological insulators, TDSs possess 3D Dirac fermions in the bulk. The TDS is also an important boundary state mediating numerous novel quantum states, such as topological insulators, Weyl semi-metals, Axion insulators and topological superconductors. By investigating the electronic structure of Na3Bi with angle resolved photoemission spectroscopy, we discovered 3D Dirac fermions with linear dispersions along all momentum directions for the first time. Furthermore, we demonstrated that the 3D Dirac fermions in Na3Bi were protected by the bulk crystal symmetry. Our results establish that Na3Bi is the first model system of 3D TDSs, which can also serve as an ideal platform for the systematic study of quantum phase transitions between rich novel topological quantum states. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S43.00004: Tunable optical activity as a probe of the chiral anomaly in Weyl semimetals Pavan Hosur, Xiaoliang Qi Weyl semimetals are a three dimensional gapless topological phase in which bands intersect at arbitrary points in the Brillouin zone. These points carry a topological quantum number known as the ``chirality'' and always appear in pairs of opposite chiralities. The notion of chirality leads to the ``chiral anomaly,'' according to which charge associated with a given chirality is not conserved in an electromagnetic field E$\cdot$B. Since Weyl nodes are separated in momentum space, it is difficult for ordinary real space probes to probe this anomaly. Here, we propose a technique to probe the chiral anomaly optically. In particular, we observe that an E·B field induces a form of optical activity known as gyrotropy, which is directly proportional to the chirality of the underlying Hamiltonian, in Weyl semimetals. This dynamically induced gyrotropy can then be seen in routine Faraday and Kerr effect experiments. We estimate typical sizes of these effects and find them to be within experimental reach. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S43.00005: Weyl Semimetal in Hg$_{1-x-y}$Cd$_x$ Mn$_y$ Te Daniel Bulmash, Chao-Xing Liu, Xiao-Liang Qi We study strained Hg$_{1-x-y}$Cd$_x$Mn$_y$Te in a magnetic field using a $\mathbf{k}\cdot\mathbf{p}$ model and predict that the system is a Weyl semimetal with two nodes in an experimentally reasonable region of the phase diagram. We also predict two signatures of the Weyl semimetal phase which arise from tunability of the Weyl node splitting. First, we find that the Hall conductivity is proportional to the average Mn ion spin and thus is strongly temperature dependent. Second, we find an unusual magnetic field angle dependence of the Hall conductivity; in particular, we predict a peak in $\sigma_{xy}$ as a function of field angle in the $xz$-plane and a finite $\sigma_{yz}$ as the $x$-component of the field goes to 0. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S43.00006: Anomalous domain-wall conductance in pyrochlore-type Nd$_{2}$Ir$_{2}$O$_{7}$ on the verge of metal-insulator transition Kentaro Ueda, Jun Fujioka, Youtarou Takahashi, Takehito Suzuki, Shintaro Ishiwata, Yasujiro Taguchi, Masashi Kawasaki, Yoshinori Tokura Pyrochlore iridates have attracted much attention since the interplay between electron correlation and strong spin-orbit coupling can lead to various topologically-nontrivial phases such as Weyl semimetal. The Weyl semimetal phase shows k-linear dispersing excitations as described by the Weyl equation in the three-dimensional bulk and remarkable edge states (Fermi arcs) at the surface or domain boundary. Recent theoretical studies have shown that such metallic edge modes can survive at the magnetic domain wall even in the fully-gapped bulk state subsequent to the pair-annihilation of Weyl fermions. In this study, we have investigated the charge transport and the low-energy charge dynamics originating from the magnetic domain walls in pyrochlore-type Nd$_{2}$Ir$_{2}$O$_{7}$, whose bulk is a fully-gapped antiferromangnetic insulator in vicinity to Weyl semimetal. We observed that the antiferromagnetic domain wall is metallic, despite the fully-gapped insulating state in the bulk by means of charge transport and optical measurements. We discuss the origin of such highly conductive magnetic domain wall in terms of edge states inherent to the Weyl semimetal. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S43.00007: Synthesis and Characterization of Epitaxial Nd2Ir2O7 Thin Films Fabricated with Off-Axis Magnetron Sputtering James Gallagher, Robert Williams, Sam Wagers, David McComb, Fengyuan Yang There has been a recent interest in the study of 5d transition metal oxides due to the large spin-orbit coupling. In particular, the pyrochlore iridates $A_2Ir_2O_7$ are a group of interesting materials with geometric frustration of magnetic moments and large spin-orbit coupling, allowing the possibility of Mott insulators, topological insulators and Weyl semimetals. $Nd_2Ir_2O_7$ is of particular interest because it is near the metal-insulator transition, making it a potential candidate for search of topological insulators and Weyl semimetals. We grow pure phase, fully epitaxial $Nd_2Ir_2O_7$ thin films using ultrahigh vacuum off-axis magnetron sputtering on yittrium-stabalized zirconia. X-ray diffraction verified that the pure phase epitaxial relationship of the film to the substrate. Scanning transmission electron microscopy (STEM) images reveal the pyrochlore ordering between Nd and Ir and epitaxial nature of the film. Transport measurements show that the films undergo a metal-insulator transition around 70 K, up from around 35 K in the bulk. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S43.00008: Observation of a topological 3D Dirac semimetal phase in high-mobility Cd$_{3}$As$_{2}$ M. Neupane, S.-Y. Xu, R. Sankar, N. Alidoust, G. Bian, Chang Liu, I. Belopolski, T.-R. Chang, H.-T. Jeng, H. Lin, A. Bansil, Fangcheng Chou, M.Z. Hasan Experimental identification of three-dimensional (3D) Dirac semimetals in solid state systems is critical for realizing exotic topological phenomena and quantum transport. Using high-resolution angle-resolved photoemission spectroscopy, we performed systematic electronic structure studies on well-known compound Cd$_{3}$As$_{2}$. For the first time, we observe a highly linear bulk Dirac cone located at the Brillouin zone center projected onto the (001) surface, which is consistent with a 3D Dirac semimetal phase in Cd$_{3}$As$_{2}$. Remarkably, an unusually high Dirac Fermion velocity is seen in samples where the mobility far exceeds 20,000 cm$^{2}$/V.s suggesting that Cd$_{3}$As$_{2}$ can be a promising candidate as a hypercone analog of graphene in many device-applications, which can also incorporate topological quantum phenomena in a large gap setting. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S43.00009: Shubnikov-de Haas Oscillations in 3D Dirac semimetal Cd3As2 Shih-Ting Guo, R. Sankar, Yung-Yu Chien, Horng-Tay Jeng, F.C. Chou, Wei-Li Lee Cadmium arsenide (Cd3As2) was known for its inverted band structure and ultra-high electron mobility. It has been theoretically predicted to exhibit a 3D Dirac semimetal phase containing degenerate Weyl nodes, which was recently confirmed by ARPES experiments. By conducting magneto-transport measurements in high quality single crystals of Cd3As2, we found a single frequency F $=$ 67 Tesla in the Shubnikov-de Haas (SdH) oscillations giving an effective mass m* $=$ 0.057 me with electron mobility as high as 99,500 cm$^{2}$/V-sec, which is consistent with the calculated 3D Dirac semimetal band. In a certain field orientation, we found a splitting of the SdH oscillation frequency in the FFT spectrum, which infers a possible lifting of the degeneracies. Detailed magnetotransport data in several doped and undoped single crystals of Cd3As2 will be presented and discussed. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S43.00010: Transport properties of Dirac semimetal Cd$_3$As$_2$ Tian Liang, Quinn Gibson, Jun Xiong, Minhao Liu, Maximilian Hirschberger, Robert Cava, Nai Phuan Ong The semimetal Cd$_3$As$_2$ has emerged as an attractive candidate for a Dirac semimetal. A recent LDA calculation reveals that, at the Fermi energy, it has two bulk Dirac nodes which straddle the $\Gamma$ point along the k$_z$ axis. The Dirac nodes were recently observed by ARPES. We have made extensive transport measurements of Cd$_3$As$_2$. Because of possible Cd vacancy disorder in the very large unit cell (160 atoms), the SdH oscillations reveal a quantum lifetime that is moderately damped. Despite the disorder, the observed resistivity $\rho$ in some crystals displays a RRR of 1000. At 4 K, the residual resistivity is anomalously low (30 n$\Omega$ cm). We estimate that the mobility exceeds 10$^6$ cm$^2$V$^{-1}$s$^{-1}$. A magnetic field H strongly increases $\rho$ by factors of 100 to 1000 at 10 Tesla. This giant magnetoresistance (MR) is highly anisotropic. The MR is largest when H is perpendicular to the axis (110) and minimal when H is $ \parallel $(110). We will discuss possible origins of this unusual anisotropic giant MR. We also discuss the possibility of detecting an enhanced longitudinal MR associated with charge pumping between Weyl nodes (the chiral anomaly). [1] Wang et al. arXiv:1305.6780 [2] Borisenko et al. arXiv:1309.7978 [3] Neupane et al. arXiv:1309.7892 [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S43.00011: Superconducting Proximity Effect in the 3D Dirac Semimetal Cd3As2 Jun Xiong, Quinn Gibson, Tian Liang, Robert J. Cava, Nai Phuan Ong Cd3As2 is a semimetal that is a candidate for a 3D Dirac semimetal. Angle-resolved photoemission has observed bulk, massless Dirac nodes whose Fermi velocity is in good agreement with the value determined by quantum oscillation measurements. A number of novel transport features have been identified. The system is protected from 2kF backscattering despite a short quantum lifetime. Theory predicts the existence of surface states with Fermi arcs. We search for evidence for the arcs using Nb pads to proximitize the surface electrons. We have fabricated a Nb-Cd3As2-Nb hybrid structure to study the Josephson effect. Given the long electronic mean-free-path in Cd3As2, we have made Josephson junctions with different lengths to study the coherent transport of cooper pairs in 3D Dirac semimetal. To reveal the exotic nature of 3D Dirac electrons, we have measured the dc I-V curves of the junction, in the presence of a weak magnetic field. Supported by NSF-MRSEC (DMR 0819860), Army Research Office (ARO W911NF-11- 1-0379) and DARPA under SPAWAR program (Grant N66001-11-1-4110). [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S43.00012: Electronic Indication of Three-Dimensional Dirac Cone in Cd3As2 from Angle-Resolved Photoemission H.M. Yi, C.Y. Chen, Y.G. Shi, Z.J. Wang, Z.J. Xie, Y. Feng, A.J. Liang, S.L. He, J.F. He, Y.Y. Peng, X. Liu, Y. Liu, L. Zhao, G.D. Liu, X.L. Dong, J. Zhang, Arita M, Shimada K, Namatame H, Taniguchi M, Z.Y Xu, C.T. Chen, X. Dai, Z. Fang, X.J. Zhou The narrow gap semiconductor, Cd3As2 is well known to have inverted band structure. It draws much attention recently because of its none-trivial properties which is predicted to be a three-dimensional Dirac semi-metal. Analogous to two-dimensional layered material graphene, Cd3As2 can be viewed as a 3D version of Dirac Fermion material whose bulk conduction and valence band contact only at discrete (Dirac) points in the Brillouin zone and disperse linearly in all directions around these critical points. Here we report direct observation of three-dimensional Dirac cones in Cd3As2 by using high resolution angle resolved photoemission spectroscopy(ARPES). Our ARPES results reveal the unique band structures for this topological 3D Dirac material that will provide key information in understanding and exploring exotic phenomenon in Cd3As2. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S43.00013: Atomic scale imaging and spectroscopic investigation of Cd$_{3}$As$_{2}$ with the STM Sangjun Jeon, Brian Zhou, Andras Gyenis, Quinn Gibson, Robert Cava, Ali Yazdani Cd$_{3}$As$_{2}$ is known for having high carrier mobility and inverted HgTe-type band structure and is theoretically expected to be a three-dimensional Dirac semimetal. Recently, ARPES measurements on this material show the band structure to have linear dispersions in three dimensions to form a three-dimensional Dirac conelike structure. Much remains to be understood about the nature of the electronic states in this compound. Here we probe with high spatial and energy resolution the electronic structure of Cd$_{3}$As$_{2}$ using STM measurements. Cd$_{3}$As$_{2}$ single crystals grown by Bridgman method were cleaved in UHV environment and investigated in cryogenic STM. We will report on various STM measurements to determine the atomic structure of the cleaved surfaces and use spectroscopic measurements to probe its unique bulk and surface electronic properties. Work supported by ARO-MURI. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S43.00014: STM Spectroscopic Mapping of Cd$_{3}$As$_{2}$ Andras Gyenis, Sangjun Jeon, Brian Zhou, Benjamin Feldman, Quinn Gibson, Robert Cava, Ali Yazdani Recently, theoretical studies and experimental findings suggest that the well-known semiconductor, cadmium arsenide (Cd$_{3}$As$_{2})$ is one of the realizations of the three-dimensional Dirac semimetal state of matter. This new topological phase has various exotic physical properties originating from the existence of the single pair of bulk Dirac points and the unusual Fermi arcs of the surface states. To investigate the unique electronic structure of Cd$_{3}$As$_{2}$, we perform scanning tunneling spectroscopy measurements on samples with different doping levels. Similar to STM experiments on other materials, spectroscopic mapping with the STM can be used to understand the role of the impurities on the surface and to examine scattering transitions, which helps to establish topological properties of the 3D Dirac state. Work supported by ARO-MURI [Preview Abstract] |
Session S44: Focus Session: Defects in Semiconductors: Defects Engineering
Sponsoring Units: FIAPChair: Andriy Zakutayev, National Renewable Energy Laboratory
Room: Mile High Ballroom 4C
Thursday, March 6, 2014 8:00AM - 8:12AM |
S44.00001: Bipolar Doping Control in Sputter-deposited Cu$_{3}$N Thin Films as a Function of Growth Conditions Angela Fioretti, Steven Christensen, David S. Ginley, Eric S. Toberer, Andriy Zakutayev Experimental evidence of Cu$_{3}$N defect-tolerance has been observed in that it can be doped either n-type or p-type based solely on growth conditions. In this presentation, the control of bipolar doping behavior as a function of growth conditions in Cu$_{3}$N is demonstrated, and hypotheses as to the underlying physics of this behavior are explored. Thin films of Cu$_{3}$N were deposited using reactive RF-magnetron sputtering. Growth temperature and target power density were varied respectively in two sets of experiments. For both sets, Hall effect and Seebeck coefficient measurements were used to characterize carrier type. Furthermore, NEXAFS measurements were performed to investigate the fundamental differences in structure that may give rise to Cu$_{3}$N bipolar doping. Cu$_{3}$N grown under conditions in which the activity of nitrogen was low exhibited n-type conductivity, while films grown under conditions in which the activity of nitrogen was high exhibited p-type conductivity. NEXAFS measurements revealed the presence of mixed Cu valence (both Cu$^{+1}$ and Cu$^{+2})$, and this discovery helped to shed light on the underlying physics behind Cu$_{3}$N bipolar doping behavior. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S44.00002: Effect of annealing on electronic carrier transport properties of gamma-irradiated AlGaN/GaN high electron mobility transistors Anupama Yadav, Casey Schwarz, Max Shatkhin, Luther Wang, Elena Flitsiyan, Leonid Chernyak, Lu Liu, Ya Hwang, Fan Ren, Stephen Pearton AlGaN/GaN High Electron Mobility Transistors were irradiated with $^{\mathrm{60}}$Co gamma-ray doses from 100Gy to 1000Gy, in order to analyze the effects of irradiation on the devices' transport properties. Temperature dependent Electron Beam Induced Current (EBIC) measurements, conducted on the devices before and after exposure to gamma-irradiation, allowed for the obtaining of activation energy related to radiation-induced defects due to nitrogen vacancies. Later, the devices were annealed at 200$^{\mathrm{o}}$ C for 25 minutes. All the measurements were performed again to study the effect of annealing on the gamma-irradiated devices. Annealing of gamma-irradiated transistors shows that partial recovery of device performance is possible at this temperature. DC current-voltage measurements were also conducted on the transistors to assess the impact of gamma-irradiation and annealing on transfer, gate and drain characteristics. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S44.00003: Electronic and Optical Properties of ScN and (Sc,Mn)N Thin Films Deposited by DC-Magnetron Sputtering Bivas Saha, Gururaj Naik, Vladimir Drachev, Alexandra Boltasseva, Ernesto Marinero, Timothy Sands Scandium Nitride (ScN) is a rocksalt semiconductor with an interesting electronic structure for optoelectronic and dilute magnetic semiconductor applications. We present detailed studies of the electronic transport and optical properties of ScN and its alloys with manganese nitride (MnN). Our results suggest (a) dilute manganese doping in ScN compensates for the high $n$-type carrier concentrations arising due to oxygen impurities, and (b) an $n$-type to $p$-type carrier type transition occurs at a composition between 5.8{\%} and 11{\%} Mn on Sc sites. In terms of its optical properties, our analysis clearly indicates direct and indirect bandgap absorption edges of ScN located at 2.04 eV and 1.18eV respectively. In addition to the direct gap absorption edge, (Sc,Mn)N samples also show Mn-defect induced electronic absorption. Photoluminescence measurements at room temperature from ScN films exhibit a yellowish-green emission corresponding to direct gap radiative recombination. Direct gap recombination is not expected given the smaller indirect gap. A possible role of high excitation intensities in suppressing relaxation and recombination across the indirect bandgap is suspected.. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S44.00004: Photonic Crystal Cavities in Cubic (3C) Silicon Carbide Marina Radulaski, Thomas Babinec, Sonia Buckley, Armand Rundquist, J Provine, Kassem AlAssaad, Gabriel Ferro, Jelena Vuckovic Silicon carbide (SiC) combines many of the outstanding material properties of other well-known optical and quantum optical materials, including strong optical nonlinearity, high Young's modulus, and a host of optically-active crystalline defects, in a single CMOS-compatible platform. For many applications in classical and quantum information processing, the material properties of the cubic silicon carbide polytype (3C-SiC) in particular are advantageous. We therefore present the design, fabrication, and characterization of high quality factor and small mode volume planar photonic crystal cavities in cubic 3C-SiC thin films (200 nm). We demonstrate cavity resonances across the infrared telecommunications band, with wavelengths from 1.25 - 1.6 $\mu $m. Finally, we highlight our progress developing higher Q/V nanobeam cavities, as well as extending this optical cavity platform towards integration with SiC color centers. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S44.00005: Energy transfer among isoelectronic dopants in GaP Theresa Christian, Kirstin Alberi, Brian Fluegel, Angelo Mascarenhas Although GaP is an indirect-bandgap material, it can also be an efficient light-emitter at visible wavelengths when isoelectronic impurities mediate radiative recombination via states within the bandgap. Since these states also provide a medium for energy transfer via exciton hopping among localized isoelectronic trap sites, the carrier dynamics in doped GaP are strongly dependent on the distribution and density of impurity species. We present spectroscopic data demonstrating the role of energy transfer among isoelectronic states in GaP via temperature-dependent and time-resolved photoluminescence. Research was supported by the U. S. Department of Energy, Basic Energy Sciences, Materials Sciences and Engineering Division under contract DE-AC36-08GO28308 and by the Department of Energy Office of Science Graduate Fellowship Program (DOE SCGF), made possible in part by the American Recovery and Reinvestment Act of 2009, administered by ORISE-ORAU under contract no. DE-AC05-06OR23100. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S44.00006: Amphoteric Doping of GaAsBi alloys with Silicon R.L. Field III, T. Jen, B. Yarlagadda, M. Luengo-Kovac, V. Sih, C. Kurdak, R.S. Goldman Due to the significant bandgap reduction associated with bismuth incorporation, dilute bismuthide semiconductor alloys have been proposed for high-efficiency optoelectronic devices. Although Si and Be are the most common dopants for n- and p-type doping of GaAs and related materials during MBE growth, their use in high quality structures has limitations. For example, while Be has a high active solubility in GaAs, it is also a fast diffuser in GaAs. In this work, Si is found to be an amphoteric dopant in GaAsBi by varying the As$_{\mathrm{4}}$/Ga beam equivalent pressure ratio, resulting in n-type (p-type) films due to Si entering group III (group V) sites. The hole mobility is found to decrease with Bi composition, an indication that Bi-related defects are the main source of scattering in p-type GaAsBi. Yet, the electron mobility appears independent of Bi composition, at least in the range of compositions that have been fabricated and measured. To date, we have achieved Bi incorporation in excess of 6{\%} Bi substituting for As, with electron mobilities as high as 2500 cm$^{\mathrm{2}}$/V-s for Si-doped (n $\approx $ 10$^{\mathrm{18}}$ cm$^{\mathrm{-3}})$ GaAsBi. Using Si provides an alternative to the traditional use of C and Be as p-type dopants. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S44.00007: Tunable threshold voltage via molecular doping of solution-processed organic field-effect transistors James Belasco, Swagat Mohapatra, Yadong Zhang, Stephen Barlow, Seth Marder, Antoine Kahn The threshold voltage, Vth, is a key parameter to control for proper circuit operation. We demonstrate the controlled tuning of Vth of solution processed, small molecule, organic field effect transistors (OFET) via molecular doping of the solution. A 1:1 blend solution containing the $\pi $-conjugated small molecule 6, 13-triisopropylsilylethynylpentacene (TIPS-pentacene) and polystyrene is used as the baseline solution for the OFETs. The organic p-dopant, molybdenum tris-[1-trifluoroethanoyl-2-trifluoromethylethane-1,2-dithiolene] [Mo(tfd-COCF3)3], a soluble version of Mo(tfd)3 [1], is added at various concentrations up to 0.3 wt{\%} to make bottom gate, bottom contact devices by spin coating on a SiO2 dielectric. IV-measurements on the resulting devices give baseline OFETs with an average mobility of 0.5 cm2/V.s and Vth of -1.5 V, while doped OFETs show the same average mobility with Vth shifted up to an average maximum of $+$2.5V. Overall, the various doping levels produce a gradual increase in the threshold voltage which we attribute in part to the filling of trap states that are known to exist in organic semiconductor films [2], and in part to effects related to the organic/dielectric interface. The direct correlation between Vth and doping concentration can be used to tune the threshold voltage in this system. [1] Y. Qi et. al. J. Am. Chem. Soc. 131, 12530 (2009) [2] H. Sirringhaus et. al. Adv. Mater. 21, 3859 (2009) [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S44.00008: Near-surface effects of transient oxidation and reduction on Nb-doped SrTiO3 epitaxial thin films C.F. Chang, Q.Y. Chen, P.V. Wadekar, O. Lozano, M.S. Wong, W.C. Hsieh, W.Y. Lin, H.H. Ko, Q.J. Lin, H.C. Huang, N.J. Ho, L.W. Tu, H.H. Liao, P.V. Chinta, W.K. Chu, H.W. Seo We studied the effects of transient oxidation and reduction of Nb-doped epitaxial thin films through variations of PAr and PO2. The samples were prepared by co-sputtering of Nb and SrTiO3 on LaAlO3 substrates. The Nb-content were varied from 0-33.7\%, as determined by PIXE. Contact resistance, sheet resistance, and optical properties are used to discriminate the effects. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S44.00009: Role of iron impurity complexes in degradation of GaN/AlGaN HEMTs Yevgeniy Puzyrev, Sokrates Pantelides GaN/AlGaN high electron mobility transistors (HEMTs) are leading candidates for power RF devices, but they suffer from reliability issues, in particular, a current collapse. Experiments have shown that the current collapse is correlated with the presence of a Tp1 trap in either the GaN substrate or at the surface with an energy level at about 0.55 eV below the GaN conduction band. Recent experiments demonstrated that the $E_{c}$-0.55eV level increases with the decrease of the distance from the channel to the \textit{Fe}-doped GaN. Another study found a correlation between threading dislocation density (TDD) and the concentration of $E_{c}$-0.55eV trap. Drastic decrease of $E_{c}$-0.55eV trap concentration is observed after hydrogenation of the samples. During OFF state stress, the population of the generated Tp1 trap is proportional to the square root of the stress time, suggesting Tp1 generation is correlated to the diffusion of a point defect. We present results of first-principle calculations and show that degradation occurs by the dehydrogenation of Fe and Fe-vacancy complexes. Using these results we analyze available experimental data and provide a comprehensive picture of the generation of the $E_{c}$-0.55eV trap level. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S44.00010: Modeling Quantum and Coulomb Effects in Nanoscale Enhancement-Mode Tri-Gate III-V MOSFETs Sameer Al-Sibiani, Khadija Khair, Shaikh Ahmed Because of limited benefits of strain engineering in extremely scaled silicon devices and lack of demonstration of a performance gain at the product level with nanowires, nanotubes, graphene, and other exotic channel materials, there is a strong motivation to continue device scaling using high-transport III-V (such as InGaAs and InAsSb) channel materials beyond the year 2020. However, there are several challenges with III-V MOSFETs prohibiting their use in high-performance and low-power logic applications. In this work, we investigate the performance of the tri-gate III-V FETs as compared to the planar counterpart, and show how quantum size quantization and random dopant fluctuations (RDF) affect the tri-gate FET characteristics and how to curb these issues. A 3-D fully \textit{atomistic} quantum-corrected Monte Carlo device simulator has been used in this work. Space-quantization effects have been accounted for via a \textit{parameter-free} effective potential scheme (and benchmarked against the NEGF approach in the ballistic limit). To treat full Coulomb (electron-ion and electron-electron) interactions, the simulator implements a real-space corrected Coulomb electron dynamics (ED) scheme. Also, the essential bandstructure parameters (bandgap, effective masses, and the density-of-states) have been computed using a 20-band nearest-neighbour \textit{sp}$^{3}d^{5}s^{\ast }$ tight-binding scheme. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S44.00011: Fabrication of Si:P delta-doped layers with varying doping densities Tingbin Lim, Byron Villis, Samadhan Patil, Steven Schofield, Neil Curson, Gabriel Aeppli We are developing a programme to fabricate atomic scale device structures of phosphorus atoms in a silicon substrate. The first step in this process is the fabrication of 2D Si:P delta-doped layers in silicon, which have recently also been theoretically studied in terms of electrical transport by Hwang and Das Sarma (E. H. Hwang and S. Das Sarma, \textit{Phys. Rev. B}, \textbf{87}, 125411). The Si:P delta-doped layers are expected to exhibit interesting behaviors when the density of the P atom doping is varied through the metal-insulator transition, as well as for the high ($\sim$ 10$^{14}$ per cm$^{2})$ and low (below 10$^{13})$ doping regimes. We are fabricating Si:P delta-doped layers of varying densities from around 6 x 10$^{12}$ to 2 x 10$^{14}$ P atoms per cm$^{2}$, which we will use to experimentally assess the theoretical findings of Hwang and Das Sarma. Details of the fabrication process will also be discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S44.00012: Changes in surface chemical structure of BF$_{3}$ plasma doped Si$_{0.7}$Ge$_{0.3}$ films Jinwon Ma, Woo-Jung Lee, Jungmin Bae, Seunghoon Oh, Jeonghun Kim, Yuseon Kang, Mann-Ho Cho, Dae-Hong Ko, Yongseo Ahn, Hyungsub Kim, Snag-Il Seo, Nam-Hun Kim Ultra shallow junctions were formed using BF$_{3}$ plasma doping process in Si$_{0.7}$Ge$_{0.3}$ films. The damaged Si$_{\mathrm{x}}$Ge$_{\mathrm{y}}$ layer of a few {\AA} was observed in near surface region of doped Si$_{0.7}$Ge$_{0.3}$ films, which increased especially the interfacial germanium oxide states. While the surface oxide layer of as-grown Si$_{0.7}$Ge$_{0.3}$ film was mainly composed of silicon oxide, the oxide layers of doped Si$_{0.7}$Ge$_{0.3}$ films were largely composed of germanium oxide. It is reported that the interfacial GeO$_{\mathrm{2-x}}$ states are related with the interfacial defect states. In the doped films, however, GeO$_{\mathrm{2-x}}$ states were decreased after rapid thermal annealing (RTA) process. In especially, after RTA of doped sample with process conditions of 300 W and 30 s, it is shown that the formation of interfacial defect states were significantly decreased, which was caused by the Ge-F bond generated on the SiGe surface. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S44.00013: Energetics and Diffusion of Gold in Bismuth Telluride Michael Shaughnessy, Norm Bartelt, Jonathan Zimmerman, Josh Sugar We investigate experimentally and theoretically the long-term chemical and morphological stability of Au contacts on Bi$_2$Te$_3$. Electron microscopy and energy dispersive spectroscopy experiments show that thermal annealing severely degrades the integrity of micron-thick Au films, eventually leading to their complete dissolution. To explain this result, we have used density functional theory to calculate defect formation energies and diffusion barriers of Au within Bi$_2$Te$_3$. We identify an interstitial binding site consistent with previous reports of (rapid) anisotropic diffusion of Au in Bi$_2$Te$_3$. We find, however, that substitutional Au has lower formation energies. We suggest that these substitutional defects may be active in our experiments and account for the relatively long time scale of the contact degradation. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S44.00014: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S44.00015: 1/f noise in micrometer-sized ultrathin ITO films Sheng-Shiuan Yeh, Wei-Ming Hsu, Jui-Kan Lee, Yao-Jen Lee, Juhn-Jong Lin By employing the ac bridge technique, we have measured the low frequency noises of micrometer-sized ultrathin indium tin oxide (ITO) films at room temperature to investigate the effect of post thermal annealing on the noise level. The noises in all the samples studied reveal an approximate $1/f$ form in the frequency range $f \approx$ 0.1--20 Hz. The microstructures and grain sizes of our films were altered by adjusting the thermal annealing conditions. An enhancement of the noise level was observed for those samples comprising smaller grains, where larger amounts of grain boundaries exist. This enhancement in the noise level may be ascribed to atomic diffusion along grain boundaries or dynamics of two-level-systems near the grain boundaries. [Preview Abstract] |
Session S45: Semiconductors: Thermodynamic & Transport Properties I
Sponsoring Units: FIAPChair: Lucas Lindsay, U.S. Naval Research Laboratory
Room: Mile High Ballroom 4D
Thursday, March 6, 2014 8:00AM - 8:12AM |
S45.00001: First principles study of lattice thermal conductivity and large isotope effect in materials David Broido, Lucas Lindsay, Tom Reinecke The isotope effect---the percent enhancement to a material's lattice thermal conductivity, $k$, with isotopic purification---depends on the interplay between phonon-isotope and phonon-phonon scattering. Diamond is known to have the largest measured room temperature (RT) isotope effect of any bulk crystal, achieving a $k$ enhancement of 50{\%}. Using an \textit{ab initio }Boltzmann transport equation approach, we have identified several other materials with far larger RT isotope effects [1]. In particular, we find that germanium carbide (GeC) and beryllium selenide (BeSe) have RT isotope effects of 450{\%}, almost an order of magnitude higher than that in diamond. Isotopic purification in these materials gives surprisingly high intrinsic RT $k$ values, over 1500 Wm$^{-1}$K$^{-1}$ for GeC and over 600 Wm$^{-1}$ K$^{-1}$ for BeSe, well above those of the best metals. These large values stem from fundamental material properties that give both enhanced phonon scattering by isotopes and weak anharmonic phonon-phonon scattering. The physical insights discussed in this work provide guidance for efficient manipulation of thermal transport properties of compound semiconductors through isotopic modification. \\[4pt] [1] L. Lindsay, D. A. Broido and T. L. Reinecke, Phys. Rev. B 88, 144306 (2013). [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S45.00002: First principles determination of ultra-high thermal conductivity fo Boron Arsenide: A competitor for diamond? Lucas Lindsay, David Broido, Tom Reinecke We have calculated the thermal conductivities ($k)$ of cubic III-V boron compounds using a predictive first principles approach. Boron Arsenide (BAs) is found to have a remarkable room temperature $k$ over 2000Wm$^{-1}$K$^{-1}$; this is comparable to those in diamond and graphite, which are the highest bulk values known. We trace this behavior in BAs to an interplay of certain basic vibrational properties that lie outside of the conventional guidelines in searching for high $k$ materials. We also find that cubic BN and BSb will have high $k$ with isotopic purification. This work provides new insight into the nature of thermal transport at a quantitative level and predicts a new ultra-high $k$ material of potential interest for passive cooling applications. \\[4pt] [1] L. Lindsay, D. A. Broido, and T. L. Reinecke, Phys. Rev. Lett. 111, 025901 (2013). [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S45.00003: May the character of the metal-insulator transition of disordered materials be determined by how one looks at it? Arnulf Moebius In a recent experiment, Siegrist et al. studied the metal-insulator transition (MIT) of phase-change materials [1]. They conclude that these substances exhibit a finite minimum metallic conductivity. The striking contrast to reports on other disordered substances motivates the present study of the influence of the MIT criterion used on the character of the MIT obtained [2]. First, we discuss inherent biases of various approaches to locating the MIT. Second, reanalyzing GeSb$_2$Te$_4$ data from [1], we show that this solid strongly resembles other disordered materials: The data may also be interpreted in terms of a continuous MIT. Checking the justification of these fits, however, uncovers data inconsistencies preventing an unambiguous interpretation. Third, comparing with previous experiments on crystalline Si:As, Si:P, Si:B, Ge:Ga, disordered Gd, and nano-granular Pt-C, we show that such an inconclusive behavior occurs frequently: The logarithmic temperature derivative of the conductivity highlights serious inconsistencies in the original interpretations in terms of a continuous MIT. Thus, the question for the character of the MIT of these materials has to be considered as yet open. [1] T. Siegrist et al., Nature Materials 10 (2011) 202. [2] A. Moebius, arxiv.org/abs/1308.1538. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S45.00004: Measuring Nanoscale Heat Transfer for Gold--(Gallium Oxide)--Gallium Nitride Interfaces as a Function Chester Szwejkowski, kai Sun, Costel Constantin, Ashutosh Giri, Christopher Saltonstall, Patrick Hopkins Gallium nitride (GaN) is considered the most important semiconductor after the discovery of Silicon. Understanding the properties of GaN is imperative in determining the utility and applicability of this class of materials to devices. We present results of time domain thermoreflectance (TDTR) measurements as a function of surface root mean square (RMS) roughness. We used commercially available 5mm x 5mm, single-side polished GaN (3-7 $\mu $m)/Sapphire (430 $\mu $m) substrates that have a Wurtzite crystal structure and are slightly n-type doped. The GaN substrates were annealed in the open atmosphere for 10 minutes (900-1000 $^{\circ}$C). This high-temperature treatment produced RMS values from 1-60 nm and growth of gallium oxide (GaO) as measured with an atomic force microscopy and transmission electron microscopy respectively. A gold film (80nm) was deposited on the GaN surface using electron beam physical vapor deposition which was verified using ellipsometry and profilometry. The TDTR measurements suggest that the thermal conductivity decays exponentially with RMS roughness and that there is a minimum value for thermal boundary conductance at a roughness of 15nm. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S45.00005: Thermionic charge transport in CMOS nano-transistors Andreas Betz, M. Fernando Gonzalez Zalba, Sylvain Barraud, Quentin Wilmart, Bernard Placais, David A. Williams We report on DC and microwave electrical transport measurements in silicon-on-insulator CMOS nano-transistors at low and room temperature. At low source-drain voltage, the DC current and AC response show signs of quantization with an additional dependence on back-gate bias. We attribute the quantization to Coulomb blockade resulting from barriers formed under the spacer regions of the chip. We show that at high bias transport occurs thermionic over the highest barrier: Transconductance traces obtained from microwave scattering parameter measurements can be accurately fitted by a thermionic model. From this we deduce the ratio of gate capacitance and quantum capacitance $C_g/C_q = C_{ox}/(C_{ox}+C_q)$, as well as the electron temperature $T_e$. We show that transport in our devices remains thermionic at high bias up to room temperature. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S45.00006: Lattice-matched heterostructure envelope functions and band-to-band transmission through broken-gap heterostructures Bart Soree, Maarten Van de Put, Wim Magnus, William Vandenberghe We developed an envelope function formalism capable of describing the electronic structure in lattice matched heterostructures. The formalism takes into account the different nature of the materials involved by using matrix elements in the basis formed by the solutions of their respective bulk Hamiltonian. A transformation between these basis sets has been devised to allow for expansion in one consistent and complete set. This transformation is described without full knowledge of the basis function, as this would defeat the purpose of the envelope function method. We employ only the known interband momentum matrix elements to obtain the transformation coefficients. With this method it is not only possible to describe the electronic structure in heterostructures in a more rigorous way, it is also possible to describe band-to-band transitions through these heterostructures. In particular, we studied the transmission coefficients through broken-gap heterostructure. A large discrepancy was found with the effective mass approach, which predicts full transmission at a certain energy. Our method correctly predicts additional reflections due to the interface betweeen the two materials. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S45.00007: An envelope function expansion of the Wigner transport equation Maarten Van de Put, Bart Soree, Wim Magnus The Wigner function approach to quantum transport is well suited for application to nanoscaled electronic devices. However, the Wigner-Liouville equation is often formulated within the framework of the effective mass approximation. As the envelope function formalism based on k.p theory offers a more accurate description of the band structure, we have expanded the electron field operators in the corresponding envelope functions and rederived the Wigner transport equation accordingly. We obtain a set of coupled envelope-Wigner functions which enable us also to treat band- to-band transitions (BTBT) within the Wigner formalism. This way, we can provide a rigorous quantum mechanical treatment of BTBT events in phase space. Finally, we have extended this approach to the classical Boltzmann transport equation which introduces BTBT by invoking additional coupling terms on top of the classical drift-diffusion instead of ad-hoc generation and recombination terms. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S45.00008: Mobility--Lifetime Measurements of Amorphous Hydrogenated Boron Carbide Using the Steady-State Photoconductivity Method Justin Hurley, Mahbube Siddiki, Christopher Keck, Bradley Nordell, Thuong Nguyen, Anthony Caruso, Michelle Caruso As a p-type semiconductor with a high band gap (\textgreater 2.5 eV) and high electrical resistivity (\textgreater 10\textasciicircum 12 $\Omega $\textbullet cm), ortho-carborane-based amorphous hydrogenated boron carbide (a-BxC:Hy), grown by plasma-enhanced chemical vapor deposition, is one of a handful of materials suitable for direct-conversion solid-state neutron detection. Traditionally, there has been minimal investigation into the boron carbide class of solids outside of its mechanical uses, and the basic knowledge of electrical transport properties needed to optimize a-BxC:Hy for detector applications is lacking. In particular, the mobility--lifetime product ($\mu \tau )$, a measure of the ability to extract and transport charges within a material, is an important figure of merit for detector devices. Herein we will describe our implementation of the steady-state photoconductivity method, which provides a straightforward determination of $\mu \tau $ in a-BxC:Hy films. Values of $\mu \tau $ as a function of wavelengths spanning the UV-Vis range have been determined for a range of a-BxC:Hy samples. We will describe how thin-film growth conditions can be adjusted to optimize $\mu \tau $. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S45.00009: Density functional calculations of multiphonon capture cross sections at defects in semiconductors Georgios D. Barmparis, Yevgeniy S. Puzyrev, X.-G. Zhang, Sokrates T. Pantelides The theory of electron capture cross sections by multiphonon processes in semiconductors has a long and controversial history. Here we present a comprehensive theory and describe its implementation for realistic calculations. The Born-Oppenheimer and the Frank-Condon approximations are employed. The transition probability of an incoming electron is written as a product of an instantaneous electronic transition in the initial defect configuration and the line shape function (LSF) that describes the multiphonon processes that lead to lattice relaxation. The electronic matrix elements are calculated using the Projector Augmented Wave (PAW) method which yields the true wave functions while still employing a plane-wave basis. The LSF is calculated by employing a Monte Carlo method and the real phonon modes of the defect, calculated using density functional theory in the PAW scheme. Initial results of the capture cross section for a prototype system, namely a triply hydrogenated vacancy in Si are presented. The results are relevant for modeling device degradation by hot electron effects. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S45.00010: Nonlinear magneto-transport in two dimensional electron system with anisotropic mobility William Mayer, Sergey Vitkalov, Alexey Bykov, Andrey Goran Nonlinear magnetotransport of 2D electrons in GaAs/AlAs heterostructures with anisotropic mobility $\mu$ placed in quantizing magnetic fields is studied in Hall bar geometry. It is found, that for an electric current flowing in the direction corresponding to the low mobility, the transition of electron systems in the state with zero differential resistance occurs at considerably smaller value of the electric current than for a current flowing in the direction of the high mobility. The obtained results indicate the importance of the anisotropy of the scattering potential for the electron quantal heating. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S45.00011: Two-dimensional charge transport in polycrystalline black phosphorus Xuxu Bai, Yanru Song, Bei Bao, Lilin Sun, Shun Wang, Ying Liu Black phosphorus, a narrow band gap semiconductor, is the only elemental layered material other than graphene. Theoretical calculations indicate the electronic state of single layer black phosphorus is different from that of the bulk, similar to graphene, which has attracted attentions in the condensed matter physics community. Here we report preliminary electrical transport measurements in bulk polycrystalline black phosphorus. Our results of the temperature dependence of resistivity reveal 2D variable range hopping transport behavior below 10 K. In the hopping regime, the magnetoresistance is negative at weak magnetic field, due to quantum interference of the hopping wave functions in two dimensions, and positive at stronger field. Hall effect and the anisotropy of the magnetoresistance will also be discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S45.00012: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S45.00013: Mobility Determination of Amorphous Hydrogenated Boron Carbide from Dark-Injection Space-Charge-Limited Current Method Christopher Keck, Bradley Nordell, Thuong Nguyen, Justin Hurley, Anthony Caruso, Michelle Paquette There has been particular interest in thin-film amorphous hydrogenated boron carbide (a-BxC:Hy) for solid-state direct-conversion neutron detection because it has a high cross-section for neutron capture and demonstrates a high electrical resistivity (on the order of 10$^{12}$ $\Omega \cdot$ cm). Rigorous studies into the electrical transport properties of the material are yet to be done. The experimental determination of the material's mobility is complicated by the fact that it is likely below the noise floor of conventional measurements such as the DC Hall Effect method, wherein the measured Hall Voltage is directly proportional to the mobility. One way to circumvent this problem is to utilize a drift mobility measurement technique such as the dark-injection space-charge-limited current (DI-SCLC) method, where the mobility is calculated using the transit time of the charge carrier; as transit time and mobility are inversely proportional, this method is ideal for low mobility materials. The implementation of the DI-SCLC method for mobility measurements in a-BxC:Hy will be described, and the relationship of mobility, resistivity, and carrier concentration as a function of thin-film growth conditions will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S45.00014: First-principles calculation of mobility in silicon Yuning Wu, X.-G. Zhang, Sokrates T. Pantelides We introduce a new first-principles method to calculate Coulomb-scattering-limited electron mobility in silicon. The lifetime of a Bloch state due to scattering can be interpreted as arising from an additional imaginary part of electron self-energy. By introducing an artificial imaginary potential, the electron self-energy can be extracted from the complex band structure of a periodic system while eliminating the interference effect due to multiple scattering between impurities. This allows an implementation using density functional theory within the Quantum-Espresso package. The calculated electron mobility agrees with the experimental data. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S45.00015: Accumulation mode GaAs/AlGaAs 2D electron system with independent control of the channel and contact resistance S.J. MacLeod, A.M. See, I. Farrer, D.A. Ritchie, A. Ludwig, A. Wieck, A. Hamilton Semiconductor-insulator-semiconductor FETs (SISFETs) are an attractive alternative to modulation doped (MD) GaAs/AlGaAs systems. GaAs SISFETs consist of epitaxially grown layers of GaAs then AlGaAs capped with a degenerately doped GaAs layer. The cap acts as an in situ, over-all top-gate which attracts carriers to the GaAs/AlGaAs interface. The MBE grown top-gate eliminates scattering and charge-noise from surface states and unlike Schottky gates, there is no strain between the gate and insulating AlGaAs layer due to similar thermal expansion rates. The absence of a doping layer improves carrier mobility ($\mu$) at low densities ($n_s$) since in shallow MD devices the doping layer creates an additional long-range random impurity potential. For this reason SISFET devices improve $\mu$ at low $n_s$. However in the low $n_s$ regime the high contact resistance dominates the device resistance, which can limit electrical transport measurements. We fabricate a GaAs 2D electron SISFET with dual-gate architecture to independently control the contact and channel resistance. We characterize our device using standard low-temperature electrical transport measurements. The 2D $n_s$ could be varied from $0.1-3\times 10^{11}\,cm^{-2}$ with a $\mu$ of up to $9\times 10^{6}\,cm^{2}V^{-1}s^{-1}$. [Preview Abstract] |
Session S46: Organic Conductors and Related Topics
Sponsoring Units: DCMPChair: Claude Bourbonnais, Universite de Sherbrooke
Room: Mile High Ballroom 4E
Thursday, March 6, 2014 8:00AM - 8:12AM |
S46.00001: The role of electron-phonon interaction in a magnetically driven mechanism for superconductivity Claude Bourbonnais, Hassan Bakrim We use the renormalization group method to examine the effect of phonon mediated interaction on d-wave superconductivity driven by spin fluctuations in a quasi-one-dimensional electron system. The influence of a tight-binding electron-phonon interaction on the spin-density-wave and d-wave superconducting instability lines is calculated alongside its effect on the amplitude of spin correlations in the normal phase for arbitrary phonon frequency and antinesting of the Fermi surface. We show the existence of a positive isotope effect for spin-density-wave and d-wave superconducting critical temperatures that scales with the antinesting distance from quantum critical point where the two instabilities merge. We also study the electron-phonon strengthening of spin fluctuations at the origin of extended quantum criticality in the metallic phase above superconductivity. The impact of our results on quasi-one-dimensional organic conductors like the Bechgaard salts where a Peierls distortion is absent and superconductivity emerges near a spin-density-wave state under pressure is emphasized. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S46.00002: Ultrafast phase transitions in (TMTTF)2AsF6 and alpha-(ET)2I3 driven by infrared 1.5 cycle CEP stabilized pulse Shinichiro Iwai, Takahiro Ishikawa, Tuto Sagae, Hirotake Itoh, Kaoru Yamamoto, Takahiko Sasaki Recent progress of several 10 fs laser enables us to capture the coherent dynamics of the correlated electrons leading to the photoinduced phase transition. In this study, coherent 18 fs oscillations of the correlated CO electrons were captured in the 1D chain salts (TMTTF)2AsF6 and in layered organic salts alpha-(ET)2I3 by using the 1.5 optical-cycle (7 fs carrier envelope phase (CEP) stabilized) IR(1.7 micron) pulse. We observed the build up time of 50 fs for the coherent oscillation in CO and metallic phases. In CO phase, photoinduced CO melting is driven by the electron oscillation in (TMTTF)2AsF6. In the metallic phase of alpha-(ET)2I3, the coherent oscillation of the correlated charge triggers the metal to insulator transition which decays within 100 fs. Such ultrafast metal to insulator transtion can be detected only for the strong excitation condition. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S46.00003: Understanding the interplay between charge, spin and phonons in the spectral properties of the 1D Hubbard-Holstein model Mohammad Soltanieh-ha, Alberto Nocera, Adrian Feiguin We present an analytical construction to calculate the spectral functions of the Hubbard-Holstein model in the limit of strong electron-phonon coupling, and in the limit of U $\rightarrow \infty$ . We argue that in this limit, the phonons only couple to the charge, and not the spin. The resulting spectral function can be understood as a convolution of three contributions, originating from the charge, the spin, and the phonons, in a similar fashion as the large U limit of the Hubbard chain. We support the analytical results with extensive Density Matrix Renormalization Group simulations. We recognize and interpret the signatures of the three contributions in the final spectrum and we discuss their experimental implications. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S46.00004: Raman scattering study of electric-field-induced change of charge distribution in BEDT-TTF dimer compounds Hideo Kishida, Yuma Hattori, Satoshi Iguchi, Takahiko Sasaki, Shinichiro Iwai, Hiromi Taniguchi Some BEDT-TTF dimer-Mott insulators show novel dielectric properties resulting from the positional degree of freedom for charge within a dimer. We have performed the microscopic Raman spectroscopy of a dimer-Mott insulator, ${\beta}'$-(BEDT-TTF)$_{2}$ICl$_{2}$. The charge sensitive vibrational mode is a single peak without electric field, while, by applying electric field beyond a threshold value, two side peaks appear on the low-wavenumber and high-wavenumber sides of the original position. This implies that an imbalance of charge within a dimer occurs. Moreover, we measured the electric-field dependence, the positional dependence and the temperature dependence of the electric-field-induced Raman signals. The positions of the side peaks are not so largely influenced by the electric-field intensity. The side-peak intensity shows a significant positional dependence. We will discuss the nature of the electric-field-induced change of the charge distribution in BEDT-TTF dimer-Mott insulators. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S46.00005: Ultrafast photo-excited dynamics in two-dimensional charge ordered systems Hiroshi Hashimoto, Hiroaki Matsueda, Hitoshi Seo, Sumio Ishihara Charge-order (CO) is one of the central subjects in strongly correlated electron system such as transition metal oxides and organic salts. Recently the photo-induced non-equilibrium states of the CO states have been studied in both theories and experiments intensively. The femto-second spectroscopies show a variety of exotic phenomena, for example ultra-fast insulator-metal transition accompanied by melting of CO. In order to clarify ultra-fast photo-induced phenomena in CO materials, we study theoretically a real-time dynamics of a two-dimensional spinless fermion model by using the exact-diagonalization method in finite size clusters. This is the simplest theoretical model to describe CO and its melting. We calculate the real time-dependence of the charge-correlation function and other several physical quantities. We find reduction of the charge correlation for an initial CO pattern, and emergence of the correlation for other types of COs', that is, a photo-induced CO phase transition. Transient stripe-type CO correlations strongly depend on the light polarization of the pump photon. We identify mechanism of these exotic photo-induced phenomena, and discuss its implications to other correlated electron model and real materials. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S46.00006: NMR studies of phase transition form a metallic state to a Dirac-electron state in the organic system, $\theta$-(BEDT-TTF)$_{2}$I$_{3}$ Kazuya Miyagawa, Michihiro Hirata, Kyohei Ishikawa, Tomotaka Taniguchi, Masafumi Tamura, Kazushi Kanoda The Dirac electron phase is realized in the bulk organic systems, $\theta$ and $\alpha$-(BEDT-TTF)$_{2}$I$_{3}$. The bulky nature of the system allows one to study the Dirac electrons in the spin degrees of freedom by means of NMR (K. Miyagawa et al. JPSJ {\textbf 79}, 063703 (2010)). Moreover, in $\theta$-(BEDT-TTF)$_{2}$I$_{3}$, the Dirac electron phase neighbors a metallic (superconducting) phase in a pressure-temperature phase diagram. To clarify how the Dirac phase emerges from the metallic state, we performed $^{13}$C NMR measurements for this material at ambient and under pressures. The angular dependence of NMR spectra demonstrates that all the molecules are equivalent (Hirata et al. PRB {\textbf 85}, 195146, (2012)). The temperature dependences of Knight shift and spin-lattice relaxation rate, 1/$T_{1}$, hold the Korringa relation, which signifies metallicity under pressures before the transition to the Dirac phase. However, after the system undergoes a transition to the Dirac electron state, the NMR spectral shape becomes complicated indicating a structural phase transition. The analysis of the angular dependence of the NMR spectra shows the molecular arrangement changes from theta to alpha type. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S46.00007: Effects of Charge Fluctuations on Massless Dirac Fermions in Organic Conductor $\alpha$-(BEDT-TTF)$_2$I$_3$ Taro Kanao, Hiroyasu Matsuura, Masao Ogata A quasi-two-dimensional organic conductor $\alpha$-(BEDT-TTF)$_2$I$_3$ has attracted much interest both for its charge ordering (CO) transition and for its remarkable transport properties at high pressure which appear when the transition suppressed. The latter has been revealed to be due to massless Dirac fermions (MDFs) at the Fermi energy, by measurements such as transport, NMR, and specific heat. Recently, the MDF phase has been re-examined, and some behaviors beyond non-interacting MDFs has been reported. In transport measurements, the resistivity shows a logarithmic increase at low temperatures. Also, NMR measurements show deviations from the non-interacting behaviors. The cause of these behaviors has not been clarified. Since the transition between the MDF phase and the CO phase is almost continuous, charge fluctuations are important there. As a cause of the behaviors above, we investigate effects of charge fluctuations using a minimal model for this system. We analyze this model by a self-consistent renormalization (SCR) theory, which can deal with effects of fluctuations precisely. On this basis, electric resistivity (or damping rate), specific heat, and one-particle density of states are calculated. Relevance to the experimental facts is discussed. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S46.00008: Phase Diagram of the 2D Fermionic Ring Exchange Only Model Katharine Hyatt, Bryan K. Clark, Matthew P.A. Fisher There has been significant interest in understanding non-Fermi liquid phases. Recently, DMRG studies on 2-leg ladders have suggested the presence of such a phase (the $d$-wave metal) in the $t-J-K$ model on a 2D square lattice, where $K$ is a nearest neighbor ring exchange term. The fermion sign problem generically prevents Monte Carlo studies of this model on larger systems. However, in the $t=J=0$ limit, the Hamiltonian becomes sign free. Using Green's function Monte Carlo, we investigate the phase diagram of this ring-exchange only fermionic model as a function of density. In this talk, we report our findings. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S46.00009: Polarization dependence of wide-range Raman scattering spectra in $\kappa $-(BEDT-TTF)$_{2}$Cu$_{2}$(CN)$_{3}$ Yuto Nakamura, Naoki Yoneyama, Takahiko Sasaki, Arao Nakamura, Hideo Kishida We measured Raman scattering spectra of $\kappa $-(BEDT-TTF)$_{2}$Cu$_{2}$(CN)$_{3}$, which is a quantum spin-liquid candidate, over a wavenumber range, 25-3200 cm$^{-1}$. This compound shows a relaxor-like dielectric response that could originate in the charge disproportionation within each BEDT-TTF dimer. The Raman spectra are composed of not only sharp vibrational modes but some broad structures. One broad component is observed below 700 cm$^{-1}$ in cross polarization configuration. It is assigned to a magnetic excitation and has a significant intensity at as low as 25 cm$^{-1}$ at 10 K, which results from a spin frustration effect. Moreover, some of the vibrational Raman lines below 120 cm$^{-1}$, which are assigned to intermolecular vibration, show a similar temperature dependence with the dielectric constant. This might indicate that the charge instability leading to the relaxor-like dielectric behaviors affects the Raman signals for molecular motions. We will discuss the magnetic excitation and the charge disproportionation in $\kappa $-(BEDT-TTF)$_{2}$Cu$_{2}$(CN)$_{3}$ in terms of wide-range Raman scattering spectra. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S46.00010: Effect of randomness on dielectric property in dimer-type organic salts Makoto Naka, Sumio Ishihara Electronic ferroelectricity is known as phenomena where electric polarization is attributed to charge order without inversion symmetry. Organic material kappa-(ET)$_{2}$Cu$_{2}$(CN)$_{3\, }$is a candidate of the electronic ferroelectric material. The ET molecule dimers are arranged on a two dimensional triangular lattice. Recently, a dielectric anomaly is experimentally observed around 30K. An origin of this anomaly is supposed to be electronic dipoles generated by asymmetric electronic charge distribution in the ET dimers. The observed broad peak structure and dispersion in the dielectric constant suggest that a relaxor-like state is realized in this material. Motivated by these experimental results, we study theoretically effects of randomness on dielectric properties in a dimer-Mott insulator. We analyze an low-energy effective model where electric dipoles inside the ET molecules interact with each other under random electric field. This model is analyzed by using the cluster mean-field approximation. With increasing the random field, phase transition occurs from a ferroelectric charge ordered phase to a charge glass phase. This transition is first- (second-) order in low (high) temperature. A tricritical point exists on the transition line. In the charge glass phase near the tricritical point, the dielectric susceptibility shows broad peak structure. We also find a spin-charge glass phase due to the random electric field and spin-charge coupling. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S46.00011: Flow equation approach to one-body and many-body localization Victor Quito, Paraj Bhattacharjee, David Pekker, Gil Refael We study one-body and many-body localization using the flow equation technique applied to spin-1/2 Hamiltonians. This technique, first introduced by Wegner, allows us to exact diagonalize interacting systems by solving a set of first-order differential equations for coupling constants. Besides, by the flow of individual operators we also compute physical properties, such as correlation and localization lengths, by looking at the flow of probability distributions of couplings in the Hilbert space. As a first example, we analyze the one-body localization problem written in terms of spins, the disordered XY model with a random transverse field. We compare the results obtained in the flow equation approach with the diagonalization in the fermionic language. For the many-body problem, we investigate the physical properties of the disordered XXZ Hamiltonian with a random transverse field in the z-direction. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S46.00012: Many body localization in a quantum Ising model: A numerical study Jonas Kjall, Jens Bardarson, Frank Pollmann Closed correlated quantum systems with disorder can experience many-body localization. These systems do not thermalize and the properties of the individual finite energy eigenstates become important. Recently, Huse et. al. concluded that eigenstates with broken symmetry order the quantum system, even at energy densities where the corresponding thermal system is disordered. We perform a detailed exact diagonalization study of a random Ising chain with a short ranged interaction between the excitations. We find signatures of the three predicted localized phases. One is paramagnetic and the two others have a broken Z2 symmetry with spin-glass order. These last two can further be distinguished by spectral properties. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S46.00013: Non-Landau damping of magnetic excitations in systems with localized and itinerant electrons Dmitri Efremov, Joseph Betouras, Andrey Chubukov We discuss the form of the damping of magnetic excitations in a metal near a ferromagnetic instability. The paramagnon theory predicts that the damping term should have the Landau form $\gamma(q,\omega) \propto \omega/v_F q$. However, the experiments on uranium metallic compounds UGe$_2$ and UCoGe showed non-Landau damping $\gamma (q,\omega) \propto \omega/\Gamma$, with $\Gamma = const$ for small $q$. It would violate the spin conservation in systems with one type of fermions. Recently it has been conjectured that this non-Landau damping can arise due to the presence of both localized and itinerant electrons in these materials, with ferromagnetism involving predominantly localized spins. We present microscopic analysis of the damping of near-critical localized excitations due to interaction with itinerant carriers. We show explicitly how the presence of two types of electrons breaks the cancelation between the contributions to $\Gamma $ from self-energy and vertex correction insertions into the spin polarization bubble. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S46.00014: Finite size scaling in crossover among random matrix ensembles describing interacting lattice systems Subroto Mukerjee, Ranjan Modak We study the crossover among different random matrix ensembles [Poissonian, Gaussian Orthogonal Ensemble (GOE), Gaussian Unitary Ensemble (GUE) and Gaussian Symplectic Ensemble (GSE)] realized in different microscopic models of interacting and disordered lattice systems.. We find that the perturbation causing the crossover among the different ensembles scales to zero with system size as a power law with an exponent that depends on the ensembles between which the crossover takes place. This exponent is independent of microscopic details of the perturbation. We also find that the crossover from the Poissonian ensemble to the other three is dominated by the Poissonian to GOE crossover which introduces level repulsion while the crossover from GOE to GUE or GOE to GSE associated with symmetry breaking introduces a subdominant contribution. We also conjecture that the exponent is dependent on whether the system contains interactions among the elementary degrees of freedom or not and is independent of the dimensionality of the system. [Preview Abstract] |
Session S47: Low Temperature Properties of He3
Sponsoring Units: DCMPChair: William Halperin, Northwestern University
Room: Mile High Ballroom 4F
Thursday, March 6, 2014 8:00AM - 8:12AM |
S47.00001: Engineering superfluid $^3$He phase stability with disorder J.I.A. Li, A.M. Zimmerman, J. Pollanen, C.A. Collett, W.J. Gannon, W.P. Halperin We report our NMR measurements on $^3$He superfluid in well-characterized aerogel samples, with anisotropy induced by uniaxial compression of $\approx 20\%$. By comparing with our previous work on the same sample without compression, we show that a critical field appears in the $T$-$H^2$ phase diagram induced by the aerogel anisotropy, providing clear evidence that anisotropic impurity scattering modifies the relative phase stability of different $p$-wave superfluid state, making the isotropic B-phase energetically more favorable than the magnetic field induced A-phase. Furthermore, we demonstrate that the 3-dimensional glass phase of $^3$He-A observed in the isotropic aerogel is suppressed by the anisotropic disorder, and the remaining 2-D continuous symmetry in the plane perpendicular to the strain axis gives rise to a 2-D glass phase of $^3$He-A, contrary to the expected 1-D alignment of the A-phase texture along the strain axis. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S47.00002: A Variable Path Length Cell for Transverse Acoustic Studies of Superfluid $^3$He C.A. Collett, J.I.A. Li, A.M. Zimmerman, W.J. Gannon, W.P. Halperin Transverse acoustic cavities have recently been shown to provide a probe for the surface bound states of superfluid $^3$He-$B$.$^1$ These states are predicted to have Majorana characteristics in the specular scattering limit. We have developed an acoustic cavity which allows continuous in-situ variation of the path length in order to more fully explore the surface states and to quantify the relative attenuation observed from bulk and surface helium. The variable path length cavity will also allow us to reduce the cavity length down to several microns, sufficient to search for propagating transverse sound modes in the normal state, as predicted by Landau.$^2$ We report the progress we have made in constructing and implementing this new sample cell. \par \medskip \noindent 1. J.P. Davis {\it et al.}, Nature Physics {\bf 4}, 571-575 (2008). \par \noindent 2. L.D. Landau, Sov. Phys. JETP {\bf 32}, 59 (1957). [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S47.00003: Enhanced Methanol Diffusion in Homogeneous Isotropic and Anisotropic Silica Aerogels Jeongseop Lee, A.M. Mounce, Sangwon Oh, A.M. Zimmerman, W.P. Halperin It has recently been shown that chiral superfluid $^3$He states can be stabilized using stretched, anisotropic, high porosity silica aerogel.\footnote{New Chiral Phases of Superfluid $^3$He Stabilized by Anisotropic Silica Aerogel, J. Pollanen, et al., Nature Physics 8, 317 (2012).} We present a novel approach to characterize the aerogel structure using nuclear magnetic resonance measurement of the enhanced diffusion of methanol vapor, similar to previous reports of diffusion of water in partially filled porous glass.\footnote{Enhanced Self-Diffusion of Water in Restricted Geometry, F. D'Orazio, et al., Phys. Rev. Lett. 63, 43 (1989).} The diffusion coefficient is determined by the molecular motion in the vapor phase in fast exchange with adsorbed phase. Consequently, the diffusion is enhanced by two orders of magnitude beyond that of the bulk fluid but is limited by the elastic mean free path $\lambda$ for ballistic molecular motion in the aerogel. The mean free paths in the presence of global anisotropy in a stretched (radially shrunken) aerogel, were found to be larger in the direction of strain by an amount consistent with the strain amplitude measured independently. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S47.00004: Torsion Pendulum Experiments with Superfluid $^3$He in ``Nematically Ordered'' Aerogel Nikolay Zhelev, Eric Smith, Abhilash Sebastian, Jeevak Parpia A new type of highly anisotropic alumina aerogel [1] is used to induce directional disorder in superfluid $^3$He. The aerogel sample consists of a network of long strands that have a preferred orientation (nematic order). It is placed in the head of a double torsion pendulum with the anisotropy axis oriented along the axis of the pendulum. We observe the frequency shift of the symmetric torsion mode of the pendulum in order to determine the superfluid fraction of the embedded $^3$He. The superfluid transition temperature of the fluid in the aerogel is measured to be very close to that of bulk $^3$He. However, in contrast to the bulk phase diagram, the region of stability of the Equal Spin Pairing (ESP) superfluid phase is enhanced on cooling. In addition, unlike the case of $^3$He in isotropic silica aerogel, the ESP phase reappears on warming. We compare our measurements to the NMR data reported in [2] and discuss the possible structure of the observed superfluid phases. \newline \newline [1] R.Sh. Askhadullin, et.al, J. Phys.: Conf. Ser. 98, 072012 (2008). \newline [2] R.Sh. Askhadullin, et.al, JETP Lett. 95, 326 (2012 [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S47.00005: Ultrasound Attenuation in Normal Fluid $^{3}$He in 98\% Aerogel: Knudsen-to-Hydrodynamic Crossover Yoonseok Lee, H.C. Choi, B.H. Moon, N. Masuhara, M.W. Meisel, H. Takeuchi, S. Higashitani, K. Nagai, N. Mulders Mass flow in porous media is a widely occurring phenomenon as in water flow in aquifers, blood flow in vessels, and petroleum flow through sandstones. However, the understanding of these phenomena is a challenging task. In particular, when the mean free path of the fluid particles exceeds the pore size, the hydrodynamic description breaks down and the fluid mass is carried by the Knudsen diffusion. The $^{3}$He-aerogel system offers an opportunity that allows a systematic investigation of a wide range flow phenomena from the hydrodynamic to Knudsen regime owing to the strongly temperature dependent mean free path in liquid $^{3}$He at low temperatures. In this paper, we present ultrasound attenuation measurements of liquid $^{3}$He in 98\% aerogel. The Knudsen-hydrodynamic crossover is clearly demonstrated in a drastic change in the temperature dependence in attenuation observed in this system. \\ H. Takeuchi {\it et al.,} Phys. Rev. Lett. {\bf 108}, 225307 (2012). [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S47.00006: 2D Larkin-Imry-Ma state of deformed ABM phase of superfluid $^3$He in ``ordered'' aerogel Vladimir Dmitriev, Andrey Senin, Alexey Yudin We report NMR studies of high temperature superfluid phase of $^3$He in so called ``ordered'' aerogel$^1$ which strands are almost parallel to each other. Previously, it was found that the NMR properties of this phase depend on whether it is obtained on cooling from the normal phase or on warming from the low temperature phase$^2$. These two types of high temperature phase (called as ESP1 and ESP2) correspond to Anderson-Brinkman-Morel (ABM) phase with large polar distortion and with orbital vector being in 2D Larkin-Imry-Ma (LIM) state. Here we present results which show that the observed difference in NMR signatures of the ESP1 and the ESP2 states is due to that the corresponding 2D LIM states can be anisotropic. In the ESP1 phase the anisotropy is absent or small, while in the ESP2 phase the anisotropy is large. NMR data have allowed us to estimate values of these anisotropies. \\[4pt] $^1$The aerogel sample was produced by R.Sh. Askhadullin, P.N. Martynov and A.A. Osipov (Leypunsky Institute for Physics and Power Engineering, Obninsk, Russia) \\[0pt] $^2$R.Sh. Askhadullin, V.V. Dmitriev, D.A. Krasnikhin, et al., {\it JETP Lett.} {\bf 95}, 326 (2012) [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S47.00007: Effect of Aerogel Anisotropy in Superfluid $^3$He-A A.M. Zimmerman, J.I.A. Li, J. Pollanen, C.A. Collett, W.J. Gannon, W.P. Halperin Two theories have been advanced to describe the effects of anisotropic impurity introduced by stretched silica aerogel on the orientation of the orbital angular momentum $\hat{l}$ in superfluid $^3$He-A. These theories disagree on whether the anisotropy will orient $\hat{l}$ perpendicular\footnote{G. E. Volovik, J. Low Temp. Phys. \textbf{150}, 453 (2008).} or parallel\footnote{J. A. Sauls, arXiv:1307.7656.} to the strain axis. In order to examine this question we have produced and characterized a homogeneous aerogel sample with uniaxial anisotropy introduced during growth, corresponding to stretching of the aerogel. These samples have been shown to stabilize two new chiral states;\footnote{J. Pollanen et al. Nature Physics, 8, 317-320 (2012).} the higher temperature state being the subject of the present study. Using pulsed NMR we have performed experiments on $^3$He-A imbibed in this sample in two orientations: strain parallel and perpendicular to the applied magnetic field. From the NMR frequency shifts as a function of tip angle and temperature, we find that the angular momentum $\hat{l}$ is oriented along the strain axis, providing evidence for the theory advanced by Sauls. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S47.00008: Topological mass current on a domain wall in superfluid $^3$He A-phase Yasumasa Tsutsumi At a surface of the superfluid $^3$He A-phase, the surface Andreev bound state accompanied with edge mass current emerges due to a topological phase transition. The topological phase transition also occurs at a domain wall in the A-phase, namely, an interface between the A-phases with the opposite directions of $l$-vector. In the bulk A-phase, the chiral state is characterized by a topological number $\nu=\pm 1$ whose sign depends on the direction of $l$-vector. Topological properties of the Andreev bound state at the domain wall depends on the difference of the topological numbers on each side of the domain wall. I report that the direction and amplitude of the topological mass current parallel to the domain wall depends on, however, not only the difference of the topological numbers but also the difference of phases. Then, we can devise the domain wall without the topological mass current by fixing the appropriate phase difference. I also show the phase dependence of energy loss at the domain wall based on the quasiclassical theory including quasiparticle's information in the Andreev bound state, which is essential for an understanding of the dynamics of the domain walls in the A-phase. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S47.00009: Superfluid phases of $^{3}$He in a periodic confined geometry Joshua Wiman, J.A. Sauls We report theoretical and computational results on the phases of superfluid $^3$He confined by
a two-dimensional periodic array of square boundaries (``posts'') with maximal
pair-breaking on the boundaries and translational invariance in the third
dimension.
We obtain a phase diagram by numerically minimizing the Ginzburg-Landau free energy functional. Results
are reported for the pressure range $P=0-34\,\mbox{bar}$, based on material parameters that include
strong-coupling corrections that account for the bulk $^3$He phase diagram, and for lattice periods, $L
\le 30\xi_{0}$ and post dimensions, $0.5\xi_{0}\le d < L$.
At all pressures we find a transition from the normal state to a periodic polar phase with
$T_{c1} |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S47.00010: Fermionic spectrum of superfluid phases of $^3$He under strong confinement J.A. Sauls, Hao Wu For superfluid $^3$He in confined geometries and films, the interplay between Fermions confined on opposing surfaces will in general modify the surface spectrum. We calculate the surface spectrum of a polar phase and the B phase confined between specular reflecting on both surfaces. We show that for polar phase the surface bound states will develop a band structure for any in plane momentum with a sub-gap separating the bound states and continuum states. The bandwidth is determined by the thickness of film. However for B-phase, the interplay between surface states does not change the energy spectrum, but only modulates their spectral weight. The wave function of the surface bound states at both surfaces are calculated. It is shown that the bound state energy disperses linearly with parallel momentum $p_{||}$ and even though the spatial part of the wave functions overlap, the Nambu spinors for surface states are orthogonal to each other. This leads to robustness of surface spectrum in highly confined $^3$He-B. We reported that the Nambu spinor at $z = 0$ describes a right-handed helical state, while the Nambu spinor at $z = D$ describes a left-handed helical state. They give rise to a spin currents that are opposite on the opposing surfaces. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S47.00011: A Quasiparticle Detector for Quantum Turbulence Imaging in Superfluid 3He-B Shaun Fisher, Ian Bradley, Marcel Clovevcko, Sean Ahlstrom, Ed Guise, Rich Haley, Steve Holt, George Pickett, Roch Schanon, Viktor Tsepelin, Andrew Woods We describe the development of a two-dimensional quasiparticle detector to visualise quantum turbulence in superfluid 3He-B at ultra-low temperatures. The detector consists of 25 pixels each containing a miniature quartz tuning fork. The damping on each fork provides a measure of the local quasiparticle flux. The detector is illuminated by a beam of ballistic quasiparticles generated from a near-by black-body radiator. Vortices have a large cross-section for Andreev reflecting ballistic quasiparticles at low temperatures. We generate a tangle of vortices (quantum turbulence) in the path of the beam using a vibrating wire resonator. The vortices cast a shadow onto the face of the detector due to the Andreev reflection. This allows us to image the vortex tangle and to investigate the tangle dynamics. We describe the detector and present some preliminary results. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S47.00012: Effects of surface roughness on non-uniform phases of superfluid $^3$He and spin-triplet models for Sr$_2$RuO$_4$ Anton Vorontsov, James Sauls We present theoretical and computational results for the spectrum of surface bound states of confined superfluid $^3$He and spin-triplet, odd-parity pairing theories of Sr$_2$RuO$_4$. The surface states, despite being related to the topological structure of the condensed state, are sensitive to surface disorder. We investigate effects of surface roughness on the physical properties of the boundary layer of several coherence lengths. We find that for confined $^3$He-A or chiral phases proposed for Sr$_2$RuO$_4$ the spatial profile of the edge current is significantly modified for atomically rough surfaces compared to that for specular surfaces. The boundary effect is strongly reflected in the ground-state angular momentum generated by the edge states. In thin films of superfluid $^3$He with rough surfaces the effect of surface scattering is expected to be even more important since surface states dominate the thermodynamic properties. For specular boundaries we predicted new phases with spontaneously broken time-reversal or translational symmetries should appear in films of $D \sim 10 \xi_0$. We report results for the phase diagram for specular, diffuse and maximal pair-breaking resulting from retro-reflecting boundaries. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S47.00013: Phase Diagram for $^3$He Films on Boron Nitride: NMR Studies Yibing Tang, Neil Sullivan Recent studies of the thermodynamic properties of $^3$He films on graphite [1] have revealed the existence of a previously undetected self-bound liquid phase at low density coverages. We report the results of NMR relaxation time studies for $^3$He adsorbed on hexagonal boron nitride designed to explore the dynamics of the adsorbed $^3$He atoms in order to identify the phase boundaries as a function of temperature. A steep thermally activated temperature dependence is observed at high temperatures ($T > 2.6$) K, followed by a linear dependence for $0.77 < T < 2.6$ K. The linear dependence is consistent with that expected for thermal diffusion in the self-bound liquid state.\\[4pt] [1] D. Sato, {\it et al.}, Phys. Rev. Lett. {\bf 109}, 235306 (2012). [Preview Abstract] |
Session S48: Invited Session: Multi-orbital Effects and Pairing Symmetry in Iron-Based Superconductors
Sponsoring Units: DCMPChair: Andrey Chubukov, University of Wisconsin-Madison
Room: Mile High Ballroom 1A-1B
Thursday, March 6, 2014 8:00AM - 8:36AM |
S48.00001: Impact of nematicity on the competing superconducting instabilities of the iron pnictides Invited Speaker: Rafael Fernandes Magnetic fluctuations have been proposed not only to give rise to unconventional pairing states in iron pnictides and chalcogenides, but also to be responsible for an emergent electronic nematic transition that breaks the tetragonal symmetry of the system down to orthorhombic. In this talk, we discuss the interplay between nematicity and superconductivity using both a phenomenological approach and a microscopic electronic model. When only the $s^{+-}$ superconducting instability is present (i.e. gaps with different signs on electron and hole pockets), nematic order competes with superconductivity [1], resulting in a suppression of $T_{c}$ and in a hardening of the shear modulus across the superconducting transition. However, this scenario changes dramatically when a competing d-wave superconducting instability is also present [2], as it has been suggested in several iron-based compounds. In this case, an unusual tri-linear coupling between the superconducting and nematic order parameters arises in the free energy, strongly impacting the phase diagram [3]. On the one hand, nematic order now leads to an increase of $T_{c}$, and the shear modulus is softened across the superconducting transition. On the other hand, nematic fluctuations promote an effective attraction between the $s^{+-}$ and d-wave states, favoring a mixed phase that does not break time-reversal symmetry, but instead spontaneously breaks the tetragonal symmetry of the system. Our findings offer a new perspective on how $T_{c}$ can be enhanced in the iron pnictides, and demonstrate that nematicity can be used as a diagnostic tool to probe exotic pairing states in these materials.\\[5pt] [1] R. M. Fernandes, S. Maiti, P. W\"olfle, and A. V. Chubukov, Phys. Rev. Lett. \textbf{111}, 057001 (2013).\\[0pt] [2] R. M. Fernandes and A. J. Millis, Phys. Rev. Lett. \textbf{110}, 117004 (2013).\\[0pt] [3] R. M. Fernandes and A. J. Millis, Phys. Rev. Lett. \textbf{111}, 127001 (2013). [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S48.00002: Study of the Nematic State of Pnictides using the Spin Fermion model with Spin, Orbital, and Lattice Degrees of Freedom Invited Speaker: Adriana Moreo The anisotropic behavior of the resistivity above the Ne\'el temperature in several iron-pnictides has been explained in terms of a nematic phase whose origin is currently under heated debate. In some scenarios the leading role is attributed to the magnetic degrees of freedom while in others the orbitals act as triggers, and the lattice is always assumed to be a follower. To analyze these issues a three-orbital ($xz$, $yz$, $xy$) Spin-Fermion model was studied via Monte Carlo simulations [1,2]. Our main result is that in order to reproduce the experiments, including a separation between the structural critical temperature ($T_S$) and the magnetic Ne\'el temperature ($T_N$) both the lattice-orbital and lattice-spin couplings are needed. In general, the Ne\'el temperature increases with the spin-lattice constant while the separation between the structural and the N\'eel transition temperatures is controlled by the orbital-lattice coupling [2,3]. Experimental results for the anisotropic behavior of the resistivity, the ARPES orbital spectral weight varying temperature, and the neutron scattering weights at ($\pi$,0) and (0,$\pi$) are captured by the numerical simulations [2]. Calculations of the nematic susceptibility, which is proportional to the elastorresistivity coefficient $m_{66}$, will be presented [3] and contrasted against experimental results by the Stanford group. \\[4pt] [1] S.Liang et al., Phys.Rev.Lett.109, 047001 (2012).\\[0pt] [2] S.Liang et al., Phys.Rev.Lett.111, 047004 (2013).\\[0pt] [3] S.Liang et al., in preparation. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S48.00003: ARPES studies of the superconducting gap in highly hole-doped (Ba,K)Fe2As2 Invited Speaker: Hong Ding |
Thursday, March 6, 2014 9:48AM - 10:24AM |
S48.00004: Pairing symmetry in strongly hole-doped iron-based superconductors Invited Speaker: Fazel Fallah Tafti The fabric of superconductivity in the multiband iron-based superconductors is woven out of inter-band and intra-band interactions. By tuning the relative strength of different pairing interactions via external parameters such as pressure we can tune the pairing symmetry of these multiband superconductors. I will present experimental evidence for a pressure induced change of pairing state in the fully hole-doped iron-based superconductor KFe2As2. Our main experimental finding is a sharp reversal in the pressure dependence of Tc at a critical pressure Pc $=$ 18 kbar [1]. Compared to previous reports on two separate superconducting domes in fully electron-doped chalcogenides, our discovery points to several novel aspects: (a) Pc is very low, meaning structural changes are negligible; (b) Tc remains finite through the transition, suggesting the phase transition is confined within the superconducting state; (c) No anomalies are observed in the normal state properties, ruling out the possibility of a Lifshitz transition; (d) The two superconducting states manifest a different sensitivity to disorder. These observations lead us to conclude that the sharp reversal of Tc at the critical pressure signals a phase transition between two different pairing symmetries in KFe2As2: a transition which leaves no traces in the normal state properties. Theoretical calculations formulate such a phase transition between different pairing states favored by different inelastic scattering processes [2]. We explore this hypothesis by tracing Tc versus inelastic scattering and demonstrate that below the critical pressure, Tc correlates with inelastic scattering but above the critical pressure, Tc anticorrelates with inelastic scattering. This is consistent with different channels of interactions giving rise to different pairing symmetries and pressure simply tunes the relative strength of these interactions. \\[4pt] [1] F. F. Tafti \textit{et al}., Nature Physics \textbf{9}, 349 (2013).\\[0pt] [2] R. M. Fernandes and A. J. Millis, Physical Review Letters \textbf{110}, 117004 (2013). [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 11:00AM |
S48.00005: Peculiarities under the Superconducting Dome in Iron Based Superconductors Invited Speaker: Saurabh Maiti Pairing symmetry in Fe based superconductors is a heavily investigated topic. Of the many materials across several families, the K-doped (hole doped) BaFe$_2$As$_2$ is one of the most investigated ones. Upon heavy hole doping this material undergoes a change in the Fermi Surface topology which opens up discussion on its effect on the symmetry and structure of the superconducting order parameter. I will highlight some of the important consequences this has in the context of KFe$_2$As$_2$ (the end member of the above family) such as nodal s-wave gap structure with higher harmonics and also a possibility of a superconducting state that spontaneously breaks time reversal symmetry. [Preview Abstract] |
Session S49: Focus Session: Oxide Surfaces, Oxygen Vacancies/Disorder, Doped SrTiO3
Sponsoring Units: DMPChair: Gervasi Herranz, Institut de Ciencia de Materials de Barcelona
Room: Mile High Ballroom 1C
Thursday, March 6, 2014 8:00AM - 8:12AM |
S49.00001: Electronic structure and thermodynamic stability of the SrTiO$_3$ (111) surface Nikhil Sivadas, Hemant Dixit, Valentino Cooper, Di Xiao We investigate the electronic structure and thermodynamic stability of the SrTiO$_3$ (111) surface using density functional theory. We observe that, for the Ti-terminated SrTiO$_3$ (111) surface, there is indeed some electronic reconstruction, resulting in the emergence of a metallic surface state. Polar distortions play a crucial role in screening the internal electric field; thereby reducing the amount of charge transferred between the surfaces. Our analysis emphasizes the failure of the simple nominal charge counting argument in thin STO (111) slabs, which ignores the effect of polar distortion. As expected, having a surface oxygen atom at the Ti termination can stabilize the system, eliminating any electronic reconstruction, making the system insulating. An analysis of the surface thermodynamic stability suggests that the Ti terminated (111) surface should be experimentally realizable. This surface may be useful for exploring the behavior of electrons in oxide (111) interfaces and may have implications for modern device applications. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S49.00002: Theoretical investigation of oxygen vacancy clustering at SrTiO$_3$ surfaces Juan Shen, Roser Valenti, Harald Jeschke Oxygen vacancies have been found, both in experiment and in theory, to create a twodimensional electron gas at SrTiO$_3$ surfaces. We use density functional theory to search for the most stable structural arrangements of multiple oxygen defects. We find that there is a tendency of oxygen defects to cluster. While individual oxygen defects occupy Ti $t_{2g}$ orbitals and contribute to a metallic state, additional defects closeby form localized states in the former SrTiO$_3$ bulk gap. We discuss the relevance of these findings for photoemission experiments. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S49.00003: Bandgap Controlling of Oxygen-Vacancy-Induced Two-Dimensional Electron Gas in SrTiO3 Zhiqi Liu, Wenlai Lu, Shengwei Zeng, Jiawen Deng, Zhen Huang, Changjian Li, M. Motapothula, Weiming Lu, Jianqiang Zhong, Ping Yang, Nina Bao, Wei Chen, Jingsheng Chen, Yuanping Feng, J.M.D. Coey, T. Venkatesan, Ariando Ariando Strongly correlated oxides are full of fascinating phenomena owing to their interacting lattice, charge, spin and orbital degrees of freedom. Here we report a large bandgap enhancement in SrTiO3 thin films due to their defective nature, which was found to significantly change the electronic and magnetic phases in the oxygen-vacancy-induced two-dimensional electron gas at the interface between amorphous LaAlO3 and SrTiO3. Density functional theory calculations show the possibility of the role of Sr/Ti antisite defects on the observed properties. This may open an attractive path to tailor electronic, magnetic and optical properties of SrTiO3-based oxide interface systems under intense focus in the oxide electronics community. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S49.00004: Infrared study of metallicity in vacuum annealed strontium titanate Peng Xu, T.J. Huffman, M.M. Qazilbash, In Hae Kwak, Amlan Biswas Widely employed as a substrate and as a dielectric layer in heterostructures, strontium titanate (STO) has been thoroughly studied. Metallicity in Nb-doped SrTiO$_{3}$ and at the interface of SrTiO$_{3}$/LaAlO$_{3}$ superlattices is also well known. In this work, we focus on the charge dynamics of vacuum annealed SrTiO$_{3-\delta}$ crystals which have metallic and atomically smooth surfaces. Far-field and near-field infrared measurements supported by spectroscopic ellipsometry have been carried out to provide insight into the emergence of metallicity due to oxygen deficiency in this insulator. Infrared reflectance and near-field optical microscopy are employed to obtain the dielectric function of SrTiO$_{3-\delta}$. This information is analyzed to extract the characteristics of the electron gas in the metallic layer. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S49.00005: Electron Correlation in Oxygen Vacancy in SrTiO$_3$ Chungwei Lin, Alexander A. Demkov Oxygen vacancies are an important type of defect in transition metal oxides. In SrTiO$_3$ they are believed to be the main donors in an otherwise intrinsic crystal. At the same time, a relatively deep gap state associated with the vacancy is widely reported. To explain this inconsistency we investigate the effect of electron correlation in an oxygen vacancy (OV) in SrTiO$_3$. When taking correlation into account, we find that the OV-induced localized level can at most trap one electron, while the second electron occupies the conduction band. Our results offer a natural explanation of how the OV in SrTiO$_3$ can produce a deep in-gap level (about 1 eV below the conduction band bottom) in photoemission, and at the same time be an electron donor. Our analysis implies an OV in SrTiO$_3$ should be fundamentally regarded as a magnetic impurity, whose deep level is always partially occupied due to the strong Coulomb repulsion. An OV-based Anderson impurity model is derived, and its implications are discussed. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S49.00006: Vacancy Ordering and Emergent Electronic Phenomena in SrTiO$_{3}$ P. Ganesh, Dilpuneet S. Aidhy, Valentino R. Cooper, Paul R.C. Kent Electronic complexity in oxides results from the many different types of atomic defects (substitutional, vacancy or interstitial), local strain effects, electrical boundary conditions and other symmetry lowering effects. Among these, oxygen vacancies are one of the most abundant types of defects and can potentially be controlled during oxide growth. We aim to understand what type of oxygen defect configurations form when they are atomically confined to two dimensions in an oxide material i.e. do they prefer to order or prefer disordered configurations, and what types of electronic reconstruction result? Specifically focusing on SrTiO$_{3}$, one of the constituents of 2DEG forming oxide heterostructures, we have performed extensive density functional theory and cluster expansion based calculations to study vacancy structures and the corresponding changes in the electronic structure. Possibilities for vacancy ordering in the bulk, at surfaces, and at interfaces will be discussed. Acknowledgement: A portion of the research was sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy. A portion of this research was conducted at the Center for Nanophase Material Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U. S. Department of Energy. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S49.00007: Emerging magnetism and electronic phase separation at titanate interfaces Natalia Pavlenko, Thilo Kopp, Jochen Mannhart The emergence of magnetism in otherwise nonmagnetic compounds and its underlying mechanisms have become the subject of intense research. Here we demonstrate that the nonmagnetic oxygen vacancies are responsible for an unconventional magnetic state common for titanate interfaces and surfaces. Using an effective multiorbital modelling, we find that the presence of localized vacancies leads to an interplay of ferromagnetic order in the itinerant t2g band and complex magnetic oscillations in the orbitally-reconstructed eg-band, which can be tuned by gate fields at oxide interfaces. The magnetic phase diagram includes highly fragmented regions of stable and phase-separated magnetic states forming beyond nonzero critical defect concentrations. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S49.00008: Dominant Electron Scattering Mechanisms in SrTiO$_{3}$ Amit Verma, Adam Kajdos, Tyler Cain, Susanne Stemmer, Debdeep Jena Transport studies in the complex oxide, SrTiO$_{3}$ (STO), have been carried out for many decades, but a clear consensus on dominant electron scattering mechanisms in action at different temperature and carrier concentration ranges is lacking. Recent progress in the growth of STO by low energetic deposition techniques like MBE has enabled STO thin films with carefully controlled doping and record high mobilities. Such control in turn enables a careful study of scattering mechanisms. In this work, hall mobility data from La-doped STO thin films grown by hybrid-MBE, have been analyzed and modeled considering various electron scattering mechanisms. By comparing theory to measured mobilities, we find that in addition to longitudinal optical phonon and ionized impurity scattering, a) acoustic phonon, and b) a $\sim$ 6meV transverse optical phonon deformation potential scattering mechanisms are necessary to explain the dependence of transport on temperature (2-300K) as the doping concentration is varied over two orders of magnitude (8x10$^{17}$ -- 2x10$^{20}$ cm$^{-3})$. We hope the understanding of scattering mechanisms acting in STO coming out of this work would seed ideas to improve the mobility in this important material. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S49.00009: Stripes of enhanced transition temperature in superconducting strontium titanate Hilary Noad, Katja Nowack, Eric Spanton, Hisashi Inoue, Minu Kim, Chris Bell, Yasuyuki Hikita, Harold Hwang, Kathryn Moler Strontium titanate (SrTiO$_3$) is used widely in heterostructures that are the subject of intense research, such as the LaAlO$_3$/SrTiO$_3$ interface and FeSe grown on SrTiO$_3$, yet the nature and mechanism of superconductivity in SrTiO$_3$ itself are not fully understood. We used a scanning superconducting quantum interference device susceptometer to map the superfluid density as a function of temperature in a 5.5 nm-thick slab of niobium-doped SrTiO$_3$ embedded in undoped SrTiO$_3$. We find that stripe-like regions of the sample remain superconducting to temperatures typically $\sim$40 mK higher than the transition temperature of featureless regions. We associate the stripes with tetragonal domains in SrTiO$_3$, showing that the orientation of the tetragonal c-axis may be important for tuning the critical temperature. These data may be useful for distinguishing models of superconductivity in SrTiO$_3$. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S49.00010: Transparent conducting oxides: A $\delta$-doped superlattice approach Valentino Cooper, Suyoun Lee, Sung Seok Seo, Jun Sung Kim, Woo Seok Choi, Satoshi Okamoto, Ho Nyung Lee Interfaces between dissimilar insulating oxides have been shown to exhibit intriguing phenomena such as metallic states, superconductivity and magnetism. Despite tremendous progress in understanding their origins, very little is known about how to control the conduction pathways and the distribution of charge carriers. Using first principles simulations we examine the effect of SrTiO$_3$ (STO) spacer layer thickness on the physical and chemical properties of La $\delta$-doped STO superlattices. In superlattices with relatively thin STO layers, we predict that three-dimensional conduction would occur due to appreciable overlap of the quantum mechanical wavefunctions between neighboring $\delta$-doped layers. Experimentally these superlattices remain highly transparent to visible light; a direct consequence of the appropriately large gap between the O 2$p$ and Ti $d$ states. These results highlight the potential for using superlattice thickness as a means for tuning the properties of oxide heterostructures with demonstrated importance for optoelectronic devices; providing a unique route for creating transparent conducting oxides. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S49.00011: Structural stability of highly strained oxide thin films in trilayers Yuyang Zhang, Sokrates T. Pantelides, Timothy J. Pennycook, Stephen J. Pennycook Thin layers of yttria-stabilized zirconia (Y$_{2}$O$_{3}$)$_{x}$(ZrO$_{2}$)$_{1-x}$ (YSZ) sandwiched between SrTiO$_{3}$ (STO) layers with $7\%$ lattice-mismatch strain are known to have colossal ionic conductivity at room temperature [1]. This phenomenon has been attributed to a disordering of the O sublattice [2, 3]. Here we report first-principle calculations, including checking the existence of negative-frequency phonon modes, that probe the stability of such highly-strained films. We find that, when matched to the STO layers, the strained YSZ cation sublattice remains ordered despite the total disorder of the O sublattice. The disordered anion sublattice will lower the energy of the system and lead to a stable configuration without negative-frequency phonon modes. [1] J. G. Barriocanal et al., Science 321, 676 (2008); [2] T. J. Pennycook et al., Phys.Rev.Lett. 104, 115901 (2010); [3] T. J. Pennycook et al., Eur. Phys. J. Appl. Phys. 54, 33507 (2011). [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S49.00012: Real-time Observation of Vo ordering dynamics in LaCoO3/STO superlattice Jae Hyuck Jang, Rohan Mishra, Young-Min Kim, Qian He, Liang Qiao, Michael D. Biegalski, Andrew R. Lupini, Sokrates T. Pantelides, Stephen J. Pennycook, Sergei V. Kalinin, Albina Y. Borisevich Properties of solid oxide fuel cell, catalysts etc. is dependent on the distribution and transport behavior of oxygen ions. In this study, we observe the dynamics of vacancy ordering in LaCoO3/SrTiO3 (LCO/STO) superlattice and LCO films using high angle annular dark field and annular bright field (ABF) STEM. Vo ordering was directly observed by tracking interatomic spacings, withs nucleation, propagation and interaction of different Vo nuclei demonstrated. Moreover, ABF images show that on 1-D (110) vacancy channels form in the depleted layers.In the case for superlattice, very small contribution of vacancy injection was observed. When this approach is applied to 15 u.c. LCO film, however, a sequence of different phases is observed, starting from disordered perovskite LaCoO3-x to a brownmillerite La3Co3O8-x to eventually brownmillerite La2Co2O5-x. Kinetics of the ordering and vacancy injection, as well as implications for beam-driven phase-transformation at an atomic scale, will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S49.00013: Oxygen vacancy equilibrium concentrations in strontium-doped lanthanum cobalt iron oxides Heng Luo, Xi Lin Formation of oxygen vacancies by introducing various mixed-valent cation dopants is a common procedure to improve the cathode performance in solid oxide fuel cells. A generic computational procedure is developed in this work to predict the oxygen vacancy equilibrium concentrations at experimentally relevant temperatures and oxygen partial pressures for both bulk and surface oxide phases. The calculations are based on the first-principles density functional theory and a constrained free-energy functional. Quantitative agreements are found by direct comparisons to the thermogravimetry measurements for various strontium-doped lanthanum cobalt iron oxides. Our results indicate that the oxygen vacancies are energetically stabilized at surfaces for all temperatures and all oxygen partial pressures, while such surface stabilization effects become stronger at higher temperatures and lower oxygen partial pressures. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S49.00014: Epitaxial SrCoOx oxygen sponge H.N. Lee, H. Jeen, W.S. Choi, M.D. Biegalski, D. Shin, M.F. Chisholm, C.M. Folkman, D.D. Fong, J.W. Freeland, I-C. Tung, H. Ohta Perovskite-based transition metal oxides have been actively developed as the replacements of noble metal-based electrodes in energy and environmental devices due to their high catalytic activity and ionic conductivity. However, the high thermodynamic barrier and the robust cation's oxidation state have limited the realization of fast catalysis and bulk diffusion at low temperature, which can reduce thermomechanical degradation in such devices. Here, we report a low-temperature reversible redox reaction in SrCoO$_{x}$ grown directly by pulsed laser epitaxy as one of two distinct crystalline phases, either the perovskite SrCoO$_{3-\delta}$ or the brownmillerite SrCoO$_{2.5}$.\footnote{H. Jeen \textit{et al.}, Nature Mater. \textbf{12}, 1057 (2013).} Based on real-time temperature dependent x-ray diffraction, we found that the distinct valence state in each phase can be reversibly switched at a remarkably reduced temperature (200 $\sim$ 300 $^{\circ}$C) in a considerably short time (\textless 1 min) without destroying the parent framework. Therefore, our results on low temperature reversible redox reactions provide valuable insight not only in understanding the structure-physical property relationship in multivalent oxides, but also for developing new strategies to avoid thermomechanical degradation in high temperature electrochemical devices, such as solid oxide fuel cells. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S49.00015: Oxygen-vacancy-induced polar behavior in (LaFeO$_{3}$)$_{2}$/(SrFeO$_{3}$) superlattices Rohan Mishra, Sokrates Pantelides, Young-Min Kim, Juan Salafranca, Stephen Pennycook, Albina Borisevich, Seong Keun Kim, Seohyoung Chang, Anand Bhattacharya, Jeffrey Eastman, Dillon Fong Complex oxides displaying ferroelectric and/or multiferroic behavior are of high fundamental and applied interest. In this work, for the first time, we show that it is possible to achieve polar order in a superlattice made up of two non-polar oxides by means of oxygen vacancy ordering. Using scanning transmission electron microscopy imaging, we show polar displacement of magnetic Fe ions in a superlattice of (LaFeO$_{3}$)$_{2}$/(SrFeO$_{3}$) grown on a SrTiO$_{3}$ substrate. Using density functional theory calculations, we systematically study the effect of epitaxial strain. octahedral rotations and surface terminations in the superlattice and find them to have negligible effect on the antiferroelectric displacements of the Fe ions lying in between SrO and LaO layers of the superlattice. Introduction of oxygen vacancies, on the other hand, triggers a polar displacement of the Fe ions. We confirm this important result using electron energy loss spectroscopy, which shows oxygen vacancy ordering in the region where polar displacements are observed and an absence of vacancy ordering outside of that area. Overall our results open up a new pathway to design new ferroelectrics and multiferroics. [Preview Abstract] |
Session S50: Focus Session: Mesoscopic Materials and Devices I
Sponsoring Units: DMPChair: John Sarrao, Los Alamos National Laboratory
Room: Mile High Ballroom 1D
Thursday, March 6, 2014 8:00AM - 8:36AM |
S50.00001: Mesoscale Science with High Energy X-ray Diffraction Microscopy at the Advanced Photon Source Invited Speaker: Robert Suter Spatially resolved diffraction of monochromatic high energy ($> 50$ keV) x-rays is used to map microstructural quantities inside of bulk polycrystalline materials. The non-destructive nature of High Energy Diffraction Microscopy (HEDM) measurements allows tracking of responses as samples undergo thermo-mechanical or other treatments. Volumes of the order of a cubic millimeter are probed with micron scale spatial resolution. Data sets allow direct comparisons to computational models of responses that frequently involve long-ranged, multi-grain interactions; such direct comparisons have only become possible with the development of HEDM and other high energy x-ray methods. Near-field measurements map the crystallographic orientation field within and between grains using a computational reconstruction method that simulates the experimental geometry and matches orientations in micron sized volume elements to experimental data containing projected grain images in large numbers of Bragg peaks. Far-field measurements yield elastic strain tensors through indexing schemes that sort observed diffraction peaks into sets associated with individual crystals and detect small radial motions in large numbers of such peaks. Combined measurements, facilitated by a new end station hutch at Advanced Photon Source beamline 1-ID, are mutually beneficial and result in accelerated data reduction. Further, absorption tomography yields density contrast that locates secondary phases, void clusters, and cracks, and tracks sample shape during deformation. A collaboration led by the Air Force Research Laboratory and including the Advanced Photon Source, Lawrence Livermore National Laboratory, Carnegie Mellon University, Petra-III, and Cornell University and CHESS is developing software and hardware for combined measurements. Examples of these capabilities include tracking of grain boundary migrations during thermal annealing, tensile deformation of zirconium, and combined measurements of nickel superalloys and a titanium alloy under tensile forces. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 9:12AM |
S50.00002: Adding magnetic functionalities to epitaxial graphene by self assembly on or below its surface Invited Speaker: Rodolfo Miranda We show how to add magnetic functionalities to graphene's set of extraordinary electronic, mechanical or optical properties. We will discuss such two examples: \begin{enumerate} \item \textit{Achieving long range magnetic order on a monolayer of TCNQ adsorbed on graphene /Ru(0001). } \end{enumerate} Cryogenic STM and Spectroscopy and DFT simulations show that isolated TCNQ molecules deposited on gr/Ru(0001) [1-3] acquire charge from the substrate and develop a sizeable magnetic moment, which is revealed by a prominent Kondo resonance. The self-assembled molecular monolayer develops spatially extended spin-split electronic bands with only the majority band filled, thus becoming a 2D organic magnet whose predicted spin alignment in the ground state is visualized by spin-polarized STM at 4.6 K [4]. The long range magnetic order is originated by the charge transfer from graphene to TCNQ (which creates the magnetic moments) \underline {plus} the self-assembly of the molecular adlayer on the graphene layer (which creates spin-polarized intermolecular bands where the added electrons partly delocalize). Examples will be shown where the adsorbed molecules accept charge and develop magnetic moments, but do nor form bands (F4-TCNQ on graphene/Ru(0001)), or where similar bands do form, but they are not populated, because there is no charge transfer to the molecules (TCNQ on gr/Ir(111)). ii) \textit{Introducing a giant spin-orbit interaction on graphene/Ir(111) by intercalation of Pb.} The intercalation of an ordered array of Pb atoms below graphene results in the appearance a series of equally spaced, sharp peaks in the differential conductance, as revealed by STS at 4.6 K. The vicinity of Pb enhances the, usually negligible, spin-orbit interaction of graphene. The spatial variation of the spin-orbit coupling creates a gauge field that acts as an pseudo magnetic field opening a gap, confining electrons and originating pseudo Landau levels [5].\\[4pt] [1] A.L. V\'{a}zquez de Parga et al, Phys. Rev. Lett. \underline {100}, 056807 (2008);\\[0pt] [2] B. Borca et al, Phys. Rev. Lett. \underline {105}, 036804 (2010);\\[0pt] [3] D. Stradi et al, Phys. Rev. Lett. \underline {106}, 186102 (2011);\\[0pt] [4] M. Garnica et al, Nature Physics \underline {9}, 368 (2013);\\[0pt] [5] F. Calleja et al, in preparation. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S50.00003: Parallelization of Thermochemical Nanolithography Jennifer E. Curtis, Keith Carroll, Xi Lu, Suenne Kim, Yang Gao, Hoe-Joon Kim, Suhas Somnath, Laura Polloni, Roman Sordan, WIlliam King, Elisa Riedo One of the most pressing technological challenges in the development of next generation nanoscale devices is the rapid, parallel, precise and robust fabrication of nanostructures. We demonstrate the possibility to parallelize thermochemical nanolithography (TCNL) by employing five nano-tips for the fabrication of luminescent polymer nanostructures and graphene-based nanoribbons. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S50.00004: Measurement of mesoscopic Si:P delta-doped devices fabricated by rapid STM hydrogen depassivation lithography via field-emission M. Rudolph, S.M. Carr, G. Subramania, G. Ten Eyck, J. Dominguez, M.P. Lilly, M.S. Carroll, E. Bussmann Recently, a method to fabricate nanoelectronic and quantum devices has been developed that utilizes scanning tunneling microscopy (STM) to place dopants (P) into Si with deterministic atomic-precision. Dopant placement is achieved via STM hydrogen depassivation lithography (HDL). Typically HDL is performed in a low-voltage tunneling mode where electrons desorb one H at a time, which requires extremely slow scan rates. Here, we introduce a high-voltage field-emission HDL, increasing patterning scan rate by an order of magnitude. Using the field-emission mode, we fabricated several HDL-patterned Si:P delta-doped devices, including a microscale multi-terminal Hall Effect device and a nanoscale quantum point contact. Low temperature transport measurements of the Hall device reveal a dopant density of 10$^{14}$ cm$^{-2}$, resistance of 2 k$\Omega $/square, and mobility of 30 cm$^{2}$/Vs. The quantum point contact showed a blockaded voltage range of 80 mV, comparable to other similar devices patterned using conventional HDL. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. The work was supported by the Sandia National Laboratories Directed Research and Development Program. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the U. S. Department of Energy under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S50.00005: Negative Differential Transconductance in Silicon CMOS Quantum Well Field Effect Transistors Clint Naquin, Mark Lee, Hal Edwards, Tathagata Chatterjee, Guru Mathur Quantum well (QW) devices are potentially useful as high-speed oscillators and sensors, as well as high-density memory and multi-state logic. Historically, these devices have been built using III-V heterostructures grown epitaxially in the vertical direction. Silicon CMOS field effect transistors (FETs) that incorporate QWs through the lateral confinement of a silicon inversion layer are of particular interest due to their capability for mass production and industrial scalability. We report on the observation of negative differential transconductance (NDTC) in a set of Si CMOS QW FETs fabricated using industrial 45 nm node processing. Measurements of drain current as a function of gate voltage from 5 K to room temperature were conducted, and local current maxima and minima were observed leading to negative differential transconductance. When voltage-biasing the body terminal, NDTC appears at temperatures as high as 218 K; however, for measurements taken with the body terminal current-biased, NDTC appears at higher temperatures with peak-to-valley ratios (PVR) greater than two. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S50.00006: Transport spectroscopy and modeling of a clean MOS point contact tunnel barrier Amir Shirkhorshidian, Nathaniel Bishop, Jason Dominguez, Robert Grubbs, Joel Wendt, Michael Lilly, Malcolm Carroll We present transport spectroscopy of non-implanted and antimony-implanted tunnel barriers formed in MOS split-gate structures at 4K. The non-implanted barrier shows no signs of resonant behavior while the Sb-implanted barrier shows resonances superimposed on the clean transport. We simulate the transmission through the clean barrier over the entire gate and bias range of the experiment using a phenomenological 1D-tunneling model that includes Fowler-Nordheim tunneling and Schottky barrier lowering to capture effects at high bias. The model is qualitatively similar to experiment when the barrier height has a quadratic dependence in contrast to a linear one, which can be a sign of 2D effects such as confinement perpendicular to the transport direction. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE, Office of Basic Energy Sciences user facility. This work was supported by the Sandia National Laboratories Directed Research and Development Program. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S50.00007: Current-current correlations in time domain for a tunnel junction in the quantum regime Karl Thibault, Christian Lupien, Bertrand Reulet We have measured the current fluctuations emitted by a tunnel junction with a very wide bandwidth, from 0.5 to 12 GHz, down to very low temperature T=35mK. This allowed us to perform the spectroscopy (i.e., measure the frequency dependence) of thermal noise (no dc bias, variable temperature), shot noise (low temperature, variable dc voltage bias) and photon-assisted noise (ac bias). Thanks to the very wide bandwidth of our measurement, we can deduce the current-current correlator in time domain. We observe the thermal decay of this correlator as well as its oscillations with a period h/eV, a direct consequence of the effect of the Pauli principle in quantum transport. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S50.00008: Evidence For Photon Pairing In The Photoassisted Shot Noise Of A Tunnel Junction Jean-Charles Forgues, Christian Lupien, Bertrand Reulet We report the observation of photon pairs in the photo-assisted shot noise of a tunnel junction in the quantum regime, $\hbar \omega \gg k_BT$. This was realised by measuring the correlation between the noise power generated by the junction at two different frequencies, 4.4 and 7.2 GHz, while driving the junction with an ac excitation of variable frequency. We observe clear correlations even when the mean photon number per measurement is smaller than one, a strong evidence for photons being emitted in pairs. These data are in good agreement with predictions based on the fourth cumulant of the current fluctuations generated by the junction. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S50.00009: An Experimental and Theoretical Investigation of Ultrasound Transmission in Bubbly PDMS Phononic Crystals Caleb Christianson, Saikat Mukhopadhyay, Wolfgang Sachse, Derek Stewart Phononic crystals are two- and three-dimensional structures with a periodic arrangement of two or more materials with different acoustic properties. Depending on the size, structure, and characteristics of the constituent materials, metamaterials with interesting acoustic properties can be formed. These crystals can be used to control the transmission of sound at selected frequencies, focus sound, or serve as waveguides. In this talk, we will focus on the transmission of ultrasonic waves through polydimethylsiloxane (PDMS) films with entrapped air bubbles. Two different theoretical models were used to predict ultrasonic transmission through air-PDMS crystals: (1) a simple scattering model for a series of partially reflective thin films and (2) the code MULTEL, which calculates the transmission using multiple scattering theory. A fabrication process was also developed to stack layers of the crystals with unprecedented alignment. We measured the ultrasonic transmission through the films using the ultrasonic through-transmission mode in a water bath and found an excellent agreement between the measured and calculated transmission. Additionally, we used these models to predict the performance of new phononic structures by scanning a large parameter space and showed how ultrasonic transmission through PDMS layers can be engineered by varying the dimensions, separation, and arrangement of air bubbles. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S50.00010: Kondo effect in ferromagnetic atomic scale junctions Pavlo Zolotavin, Patrick Wheeler, Douglas Natelson The Kondo effect is one of the hallmark manifestations of electron-electron interactions in solids. Interest to the Kondo effect was reignited recently in connection with quantum dots and molecular junctions. In particular, it was theoretically predicted that when a quantum dot is in the Kondo regime, simultaneous one- and two-quasiparticle scattering results in a universal average quasiparticle charge of (5/3)$e$ that could be measured by shot noise. Experiments in quantum dots and carbon nanotubes have indeed found enhanced noise in the Kondo regime. The abovementioned theoretical prediction and experimental verification were made for a weak-coupling limit, when Kondo temperature, $T_{K},$ amounts to only several degrees. Recently the presence of Kondo resonance with substantially larger $T_{K}$ was demonstrated in mechanical breakjunctions made from ferromagnetic metals. This discovery opens the possibility of testing the validity of the theoretical predictions in the strong coupling limit and in the presence of magnetically correlated electrodes. We will report the results of the ongoing shot noise measurements in ferromagnetic breakjunctions. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S50.00011: A magnetized quantum wire can act as an optical amplifier Manvir S. Kushwaha We focus on the magnetoplasmon excitations investigated in a quantum wire characterized by a confining harmonic potential and subjected to a perpendicular magnetic field. Essentially, we embark on the device aspects of the intersubband collective (magnetoroton) excitation which observes a negative group velocity between the maxon and the roton. The computation of the gain coefficient suggests an interesting and important application: the electronic device based on such magnetoroton modes becomes capable of amplifying a small optical signal of definite wavelength [J. Appl. Phys. {\bf 109}, 106102 (2011)]. [Preview Abstract] |
Session S51: Focus Session: Beyond Graphene Devices: Function, Fabrication, and Characterization VI
Sponsoring Units: DMPChair: Bernhard Urbaszek, CNRS Toulouse
Room: Mile High Ballroom 1E
Thursday, March 6, 2014 8:00AM - 8:12AM |
S51.00001: The impact of crystalline inhomogeneity on electrical transport and 1/$f$ noise in MoS$_{2}$ field effect transistor Subhamoy Ghatak, Sumanta Mukherjee, D.D. Sarma, Arindam Ghosh We show that both conductivity and low frequency 1/$f$ noise are strongly influenced by the presence of localized trap states in ultra-thinMoS$_{2}$ field effect transistor. The trap states not only create Coulomb scattering of charge carriers but also slowly exchange carrier with channel. The trap density is quantitatively calculated which turns out two orders of magnitude higher than the typical SiO$_{2}$ surface trap density. This suggests a structural origin of the trap states in MoS$_{2}$ films. The result was also supported by similar noise measurement on MoS$_{2}$ devices on trap free hexagonal boron nitride substrate. The origin of these states is also investigated by spectroscopic studies, which indicate a possible presence of metallic 1T-patches inside the major semiconducting 2H phase. [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S51.00002: Nanoscale friction for strain engineering: a case study of MoS$_2$ Jason Christopher, Alexander Kitt, Xuanye Wang, Anna Swan, Bennett Goldberg 2D materials are superior to 3D materials in their ability to withstand large deformations without failure and so large strains can be applied to engineer electrical and optical properties. To control precisely the location, magnitude and direction of a strain field it is critical to understand the friction between the 2D layer and supporting substrate since sliding alters the strain distribution. Here we use MoS$_2$ covered microchambers strain tuned by applying a variable external pressure that deflects the suspended membrane creating strain in both the suspended and supported regions. This allows us to determine the friction between mono, bi and tri layer MoS$_2$ and SiO$_2$ as well as discern the strain dependence of the band-gap and Gr\"{u}neissen parameters of MoS$_2$. The friction between MoS$_2$ and SiO$_2$ is compared with the friction between graphene and SiO$_2$. These results are essential for strain engineering applications of MoS$_2$ and to all 2D materials by establishing this method for measuring friction. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S51.00003: Temperature dependent optical properties and thermal conductivities of single- and few-layer MoS$_{2}$ Xian Zhang, Dezheng Sun, Gwan-Hyoung Lee, Yumeng You, Xu Cui, Tony Heinz, James Hone, Yilei Li The transition metal dichalcogenide, MoS$_{2}$ have shown unique optical and electrical properties, such as band structure transition, high mobility, and strong photoluminescence, in a monolayer form. Here we investigate the thermal transport properties in exfoliated monolayer and bilayer MoS$_{2}$. By measuring the Raman peak shift in response to laser heating, and the Raman peak shift with temperature variation, we obtain the room-temperature thermal conductivity and the interface conductance of about (75+34/-26)W/mK and (0.24+0.06/-0.06)MW/m$^2$K for supported monolayer MoS$_{2}$. And the thermal conductivity of the suspended monolayer MoS2 is around (81+12/-11)W/mK at room temperature, and (60+11/-9)W/mK at 500K. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S51.00004: Exciton dynamics in a single layer MoS2 Jonghwan Kim, Xiaoping Hong, Sufei Shi, Chenhao Jin, Yinghui Sun, Feng Wang In a low dimensional semiconductor, exciton plays a crucial role in the optical property. Recently, a single layer of MoS2 has attracted significant attention due to its unique excitonic features. For example, exciton in MoS2 is predicted to have order of magnitude larger binding energy than conventional direct band gap material. For deeper understanding on such properties, however, it is important to understand how exciton is formed and decays in time domain. Our work on exciton dynamics in MoS2 by pump probe spectroscopy will be presented with control of both power and wavelength. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S51.00005: Temperature Dependent Raman Studies on Liquid Phase Exfoliated MoS2 Mitchell Connolly, Andrew Winchester, Peter Hale, Keshav M. Dani, Saikat Talapatra We investigate on the temperature dependence of in-plane E12g and out-of-plane A1g Raman modes of few-layer MoS2 prepared using the liquid-phase exfoliation method. Structural characterization performed after exfoliation using transmission electron microscopy (TEM) indicate the resultant material contains small, submicron size, few-layer flakes. Raman measurements indicating material thickness of 6 layers or fewer were observed at room temperature using a 532 nm laser line (E12g$=$381.4 cm-1 and A1g$=$405.8 cm-1). The Raman peak shifts indicate a linear dependence on temperature within the range 293 K to 77 K. The measured temperature coefficients of E12g and A1g modes will be presented and compared with other similar experimental/theoretical data available. [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S51.00006: Temperature and Power Dependent Photoluminescent Spectroscopy of MoS$_2$ M. Watson, J.R. Simpson, R. Yan, H.G. Xing, X. Wu, T. Luo, S. Bertolazzi, J. Brivio, A. Kis, A.R. Hight Walker We report temperature and power dependent photoluminescence (PL) of molybdenum disulphide (mos). Mechanical exfoliation of mos, from bulk provides single-layer flakes which are then transferred either to sapphire substrates or suspended over holes in Si/Si$_3 $N$_4$. We measure temperature dependence from $\approx$100\,K to 400\,K and power dependence from $\approx$6\,$\mu$W to $\approx$7\,mW using an Argon laser at 514.5\,nm and a HeNe laser at 632.8$\,$nm. The PL spectrum exhibits a main exitonic peak(A) at $\approx$1.87\,eV which consist of both neutral excitons and charged trions (A- or A+) [1]. The A exciton peak and the A- exciton peak redshift and broaden with increasing temperature and power. Along with the A peak, we observe a lower energy bound exciton (BE) that is likely related to defects. The BE,a broad peak centred at $\approx$1.7\,eV, linearly redshifts and narrows with increasing power. The power dependence of both the main and bound peak saturates above 0.5\,mW. Raman temperature and power dependence will also be discussed [2]. \\[4pt] [1] KF. Mak et al. Nat. Mat 12,207(2013).\\[0pt] [2] R.Yan and J.R.Simpson, S. Bertolazzi and J. Brivio, M. Watson, X.Wu and A. Kis, T.Luo, H.G.Xing, A.R. Hight Walker, (submitted ACS Nano 2013) [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:48AM |
S51.00007: Photoemission Spectroscopy of 2D Electron Systems Invited Speaker: ZX Shen |
Thursday, March 6, 2014 9:48AM - 10:00AM |
S51.00008: Ultrafast Optical Spectroscopy of Excitons in Monolayer WSe$_{2}$ Grant Aivazian, Hongyi Yu, Sanfeng Wu, Aaron Jones, Nirmal Ghimire, Jiaqiang Yan, David Mandrus, David Cobden, Wang Yao, Xiaodong Xu Recently there has been tremendous interest in monolayer transition metal dichalcogenides due to their true two-dimensional nature and strong excitonic properties. Photoluminescence measurements have shown strong emission from both neutral and charged exciton species, as well as valley-selective optical excitation and even signatures of valley coherence. Here we report on the dynamic nature of these excitons by resonant optical excitation in monolayer tungsten diselenide. Using ultrafast degenerate pump/probe spectroscopy we probe the differential reflection signal as a function of excitation energy and time delay. We have developed a theory from which we are able to extract the excited exciton population and lifetimes. These measurements provided critical information for future excitonic devices involving tungsten diselenide and opens up future work investigating the rich two-dimensional physics inherent in this material. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S51.00009: Photoluminescence spectroscopy of monolayer MoSe$_2$ in magnetic fields David MacNeill, Colin Heikes, Zachary Anderson, Kin Fai Mak, Kathryn McGill, Jiwoong Park, Paul McEuen, Daniel Ralph Single layer transition metal dichalcogenides are direct gap semiconductors with unique luminescence properties, including large excitonic effects and coupling between photon handedness and the exciton valley degree of freedom. Furthermore, the luminescence spectra may change under magnetic field due to valley degeneracy breaking, the Zeeman effect and Landau level formation. Here we report measurements of photoluminescence spectra for monolayer MoSe$_2$ at temperatures ranging from 4.2K to 300K and in out-of-plane magnetic fields up to 7T. The measurements are performed using a scanning confocal microscope integrated with a superconducting magnet dewar, with light coupled in and out of the system via an optical fiber. We observe luminescence peaks from the neutral and charged exciton, and explore the evolution of the peak energies, linewidths and intensities as a function of applied field and gating. We will also discuss the magnetic field dependence of the photoluminescence handedness in the Faraday geometry and its implications. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S51.00010: Atomic Resolution Transmission Electron Microscopy of Defects in Hexagonal Boron Nitride and Graphene Ashley Gibb, Nasim Alem, Chengyu Song, Jim Ciston, Alex Zettl Monolayer sheets of sp2-bonded materials such as graphene and hexagonal boron nitride (h-BN) have been studied extensively due to their properties including high mechanical strength, thermal conductivity, stability, interesting electronic properties, and potential for integration into novel devices. Understanding the atomic scale structure of defects in these materials is important because defects can significantly affect the physical properties in these materials. In particular, understanding the dynamics of these defects explains much about the material's stability. We have synthesized h-BN and graphene using low pressure chemical vapor deposition and imaged defects using atomic resolution aberration corrected transmission electron microscopy. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S51.00011: Photocurrent spectroscopy of excitons in ultraclean two-dimensional semiconductors - Part I A.K.M. Newaz, A.R. Klots, Bin Wang, Sokrates Pantelides, Kirill Bolotin The intrinsic properties of a monolayer materials can be perturbed by substrate-related disorder. To decrease the amount of disorder in a representative 2D material, monolayer molybdenum disulfide (MoS$_2$), we have fabricated suspended field effect devices. Upon suspension, we have observed a tenfold increase in carrier field effect mobility. Further cleaning of suspended devices through thermal annealing renders them nearly insulating at small bias voltages, which is expected for a pristine semiconductor with its Fermi energy in the middle of the bandgap and precludes detailed electrical characterization. To probe the intrinsic properties further, we have conducted photocurrent (PC) spectroscopy measurements. In every measured device, we have observed the following universal features: (i) sharp peaks in PC at $\sim 1.9$eV and $\sim 2.1$eV attributable to the optical transitions due to band edge excitons; (ii) a rapid onset of PC above $\sim 2.5$eV peaked at $\sim 2.9$eV, which we attribute to an excitonic absorption due to the van Hove singularity of MoS$_2$. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S51.00012: Photocurrent spectroscopy of excitons in ultraclean two-dimensional semiconductors -- Part II Andrey Klots, A.K.M. Newaz, Bin Wang, Sokrates Pantelides, Kirill Bolotin We investigate excitonic physics in pristine suspended monolayer molybdenum disulfide (MoS$_{2})$ by means of low-temperature photocurrent spectroscopy. Measured photocurrent spectra exhibit a robust set of features, including peaks at $\sim$ 1.9, 2.1 and 2.9 eV. We interpret the peaks around 1.9 and 2.1 eV as due to optical absorption by direct band edge excitons of MoS$_{2}$ and ascribe the peak at 2.9 eV to an excitonic transition associated with the van Hove singularity of MoS$_{2}$. We interpret the nature and binding energy of these states using a combination of first-principles calculations and simple mathematical models. Furthermore, we use source-drain bias dependence of the photocurrent to investigate dissociation mechanisms of the excitons. Finally, we study the photocurrent response of bilayer and multilayer MoS$_{2}$ samples, as well as that of other transition metal dichalcogenides, such as MoSe$_{2}$ and WSe$_{2}$. Comparison of photocurrent spectra of these materials to that of monolayer MoS$_{2}$ allows us to investigate the effects of confinement and spin-orbit interaction. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S51.00013: Temperature Dependent Optical Spectroscopy of Defect States in Monolayer Molybdenum Disulphide Field Effect Transistors Changjian Zhang, Haining Wang, Wei Min Chan, Sandip Tiwari, Farhan Rana Understanding the properties of defects is critical for improving 2D metal dichalcogenide materials and devices, yet little work in this field has been reported. We present optical spectroscopy (photoluminescence and absorption) studies of defects states in monolayer MoS2 field effect devices at different temperature. At low temperatures, a very large defect peak around $\sim$ 1.7 eV is seen in the PL spectra but not in the absorption spectra. The PL intensity decreases exponentially with temperature and vanishes when the temperature exceeds $\sim$ 150K. The PL quantum efficiency becomes extremely large when a negative gate bias is applied. Our data suggests the presence of both bright and dark defect states that contribute to recombination, and that the occupation of these defect states is accompanied by lattice distortions. The competition in the recombination process between these two kinds of defect states explains the temperature dependence as well as the gate bias dependence of our data. We find that, occupation of the dark defect states is thermally activated and their occupation quenches the PL from the bright defects. We attribute the defect states to originate from sulfur or molybdenum vacancies or to the presence of oxygen atoms. [Preview Abstract] |
Session S52: Focus Session: Supercondcutivity, Vortex Matter II
Sponsoring Units: DMPChair: Alex Gurevich, Old Dominion University
Room: Mile High Ballroom 1F
Thursday, March 6, 2014 8:00AM - 8:36AM |
S52.00001: Fractionalizing the vortex lattice in multiband superconductors in the flux flow region Invited Speaker: Shi-Zeng Lin Because of the discovery of $\mathrm{MgB_2}$ and iron-based superconductors, multiband superconductors have attracted considerable attention recently. Multiband superconductors are not always straightforward extensions of the single-band counterpart, and novel features may arise. In multiband superconductors, electrons in different bands form distinct superfluid condensates, which are coupled to the same gauge field. Each condensate thus supports vortex excitation with fractional flux quantum. However the energy of a fractional vortex diverges logarithmically in the thermodynamic limit. In the ground state vortices in different bands are bounded and their normal cores are locked together to form a composite vortex with the standard integer quantum flux. It is interesting to ask whether the vortices in different condensates can decouple under certain conditions. In this talk, I will discuss the dissociation of the composite vortex lattice in the flux flow region when the disparity of superfluid density and coherence length between different bands is large. The fractional vortex lattice in different bands move with different velocities after the dissociation transition, and the dissociation transition shows up as an increase of flux flow resistivity. In the dissociated phase, the Shapiro steps are developed when an ac current is superimposed with a dc current. We also propose to stabilize the fractional vortices by periodic pinning arrays. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S52.00002: Investigation of vortex dynamics in type-II superconductors using a scanning SQUID-on-tip microscope Lior Embon, Yonathan Anahory, Denis Vasyukov, Jo Cuppens, Ella Lachman, Dorri Halbertal, Naren Hoovinakatte, Elad Yaakobi, Aviram Uri, Yuri Myasoedov, Michael Rappaport, Martin Huber, Eli Zeldov A novel scanning microscope based on a nanoSQUID which is fabricated on the apex of a sharp tip has been developed. This SQUID-on-tip (SOT) based system possesses record spin sensitivity, spatial resolution, and operable magnetic fields, combined with a geometry which allows nanoscale sample-probe distance using tuning fork based AFM feedback. Our SQUIDs can operate at liquid 4He temperatures in applied magnetic fields of up to 1T, be made as small as 50 nm and display an extremely low flux noise of 50 $n\Phi_0/\sqrt{Hz}$ which corresponds to a spin sensitivity better than $1\mu _B/\sqrt{Hz}$ [1]. Using these newly acquired capabilities we can now directly image vortices in Pb films over a wide range of fields while running currents through the sample to exert force on the vortices and to controllably drive them from a static state to ``flux creep'' and to a ``flux flow'' regime. \\[4pt] [1] D. Vasyukov, Y. Anahory, L. Embon, D. Halbertal, J. Cuppens, L. Neeman, A. Finkler, Y. Segev, Y. Myasoedov, M. L. Rappaport, M. E. Huber, and E. Zeldov, Nature Nanotech. 8, 639 (2013) [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S52.00003: Scanning SQUID-on-tip microscopy of vortex matter Yonathan Anahory, Lior Embon, Denis Vasyukov, Jo Cuppens, Ella Lachman, Dorri Halbertal, Elad Yaakobi, Aviram Uri, Yuri Myasoedov, Michael L. Rappaport, Martin E. Huber, Eli Zeldov We present a scanning nanoSQUID microscope with record spatial resolution, spin sensitivity, and operating magnetic fields for the study of vortex matter. The key element of the microscope is the SQUID-on-tip (SOT) device, which is fabricated by pulling a quartz tube into a sharp pipette, followed by three steps of thermal evaporation of a thin superconducting film onto the sides and the apex of the pipette. The devices operate at 4.2 K in applied fields of up to 1T and can be made with diameters down to 50 nm. The SQUIDs-on-tip display an extremely low flux noise of $\Phi_{\mathrm{n}}$ $=$ 50 n$\Phi_{\mathrm{0}}$/Hz$^{\mathrm{1/2}}$ and corresponding spin sensitivity of better than 1 $\mu_{\mathrm{B}}$/Hz$^{\mathrm{1/2}}$ [1], which is about two orders of magnitude improvement over any previous SQUID. Using this new tool we have investigated static and dynamic behavior of vortices in superconducting Pb films. By driving \textit{ac} and \textit{dc} transport current we can study vortex displacement and the vortex potential landscape with sub-atomic precision. [1] D. Vasyukov, Y. Anahory, L. Embon, D. Halbertal, J. Cuppens, L. Neeman, A. Finkler, Y. Segev, Y. Myasoedov, M. L. Rappaport, M. E. Huber, and E. Zeldov, Nature Nanotech. \textbf{8}, 639 (2013) [Preview Abstract] |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S52.00004: Critical Current of Layered Superconductor with Columnar Defects in Tilted Magnetic Field: A Numerical Study Jose Rodriguez The critical current of a layered (a-b plane) superconductor with perpendicular (c-axis) columnar defects in titled external magnetic field is determined by numerical simulations of the corresponding London model for vortex dynamics. Intra-layer vortex dynamics is computed in parallel by central processors units (CPU) assigned to each layer, while inter-layer vortex dynamics is computed by a message passing interface (MPI) between the dedicated CPU's. We find that the critical current versus the angle that the external magnetic field makes with the columnar defects shows a cusp maximum at zero. At fixed tilt angle, we also find that the critical current increases monotonically with increasing electronic mass anisotropy, $m_c/m_{ab}$, in a manner consistent with collective-pinning theory. Comparison with experimental determinations of the critical current in films of high-temperature superconductors with both natural (line dislocation) and artificial (nano-rod) columnar defects is made where possible. [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S52.00005: Influence of Domain Width on Vortex Nucleation in Superconductor/Ferromagnet Hybrid Structures S. Moore, J. Fedor, V. Novosad, J. Pearson, S.D. Bader, G. Karapetrov, M. Iavarone We have investigated the effects of spatially inhomogenous magnetic fields on vortex nucleation, domain wall superconductivity and reverse domain superconductivity in magnetically coupled superconductor/ferromagnet hybrid structures. Using low temperature scanning tunneling microscopy and spectroscopy (LT-STM/STS) we have studied Pb/[Co-Pd] systems with varying magnetic domain widths. Visualization of the underlying magnetic template structure is achieved through field dependent conductance maps. In the case of zero applied fields these maps reveal the absence of vortices below a threshold domain width. In those systems with insufficient domain width to support generation of vortices in zero applied fields, nucleation can be restored through the application of an external magnetic field. We also observe that the domain wall superconductivity is strongly affected by the ferromagnetic domain size. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 10:00AM |
S52.00006: Vortex pinning vs. superconducting wire network in nanostructured superconductors Invited Speaker: Jose L. Vicent Superconducting films with array of ordered defects allow studying effects which are governed by the interplay between lengths of the nanostructured sample and lengths related to physical parameters, as for example coherence length. When the coherence length and the separation between the defects are similar, the sample can mimic a superconducting wire network. In this situation, applied magnetic fields induce Little-Parks oscillations due to fluxoid quantization constraint. These L-P oscillations vanish when the coherence length is smaller than the ``stripe'' superconducting region between the defects. In superconducting films with array of nanodefects periodic oscillations can also be detected in resistance R(H), critical current I$_{c}$(H), magnetization M(H) and \textit{ac-}susceptibility $\chi_{ac}$(H) in a broader temperature range than the temperature interval where L-P oscillations are present. Vortex pinning mechanisms are the origin of these oscillations. These oscillations emerge due to matching effects between two lattices: the vortex lattice and the lattice of defects. These oscillations are detected in a broader temperature interval than the temperature interval where L-P oscillations are present. Worth to note that, due to the coherence length divergence at T$_{c}$, a crossover to wire network behavior is experimentally found always. Interestingly, both mechanisms coexist close to superconducting critical temperatures; i. e. in the temperature region where the sample mimics superconducting wire network. These overlapping effects can be experimentally separated and both origins can be discriminated. We have analyzed and single out, with magnetotransport measurements, both mechanisms: pinning and fluxoid quantization constraint in superconducting films with arrays of non-magnetic and magnetic dots. [Preview Abstract] |
Thursday, March 6, 2014 10:00AM - 10:12AM |
S52.00007: Magnetoresistance oscillations in single-crystal NbSe$_2$ nanowires designed for the study of vortex dynamics Shaun Mills, Neal Staley, Jacob Wisser, Chenyi Shen, Zhuan Xu, Ying Liu The dynamic behavior of Abrikosov vortices has been of long-standing interest, both from fundamental and application-based perspectives. While data on static configurations of vortices and the collective motion of a vortex lattice have been accumulating, studies of vortex dynamics in nanoscale samples are rare. We have pursued electrical transport measurements on devices made of single-crystal NbSe$_2$ flakes -- overcoming large challenges in the fabrication of these single-crystal nano-devices -- in order to detect the motion of individual Abrikosov vortices. We also carried out recalculations of vortex configurations in our devices in the London approximation to assist the designing of our experiments. By tuning the strength of an external magnetic field and sample geometry, individual vortices can be confined within our devices. We present our recent progress towards the controlled motion of individual Abrikosov vortices, along with transport measurements on NbSe$_2$ nanowires, the latter of which reveal unexpected magnetoresistance oscillations attributable to vortex related physics. [Preview Abstract] |
Thursday, March 6, 2014 10:12AM - 10:24AM |
S52.00008: Vortex Lattice Metastability and Power Law Dynamics in MgB$_2$ Catherine Rastovski, S.J. Kuhn, K. Smith, M.R. Eskildsen, L. DeBeer-Schmitt, C.D. Dewhurst, W.J. Gannon, N.D. Zhigadlo, J. Karpinski Previous small-angle neutron scattering (SANS) studies of the vortex lattice (VL) of MgB$_2$ with H $\parallel$ c found a triangular VL which undergoes a field-driven 30$^\circ$ reorientation transition, forming three distinct ground state phases. A high degree of metastability exists between the VL phases of MgB$_2$ that cannot be attributed to vortex pinning and may be a result of the jamming of VL domains [C.~Rastovski \emph{et al.}, Phys. Rev. Lett. {\bf 111}, 107002 (2013)]. To further investigate the effect of vortex motion on the metastable to ground state VL transition, we applied a small AC magnetic field parallel or perpendicular to the vortices to ``shake" the lattice. The metastable VL volume fraction decreased with a two-step power law dependence on the number of applied AC cycles. The slow and then fast power law decay of the metastable state may indicate first nucleation and then growth of ground state VL domains. [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S52.00009: Vortex Transport in Thickness-Modulated Granular Aluminum Films August DeMann, Sara Mueller, Stuart Field, Yaohua Liu, Daniel Reich The nature of superconducting vortices driven in a periodic potential has been the subject of much recent theoretical and experimental interest. We report here the results of transport studies of vortex dynamics in a periodic potential fabricated using a novel technique; this technique yields exceptionally smooth, nearly sinusoidal potentials that are ideal for the investigation of both static and dynamics vortex configurations. Our method starts with a glass substrate into which a periodic square-wave grating is fabricated by electron-beam lithography followed by a subsequent wet etch. The period of the gratings is $\approx 2\ \mu$m. A subsequent annealing step at 650 $^\circ$C smooths the grating into a sinusoidal profile. Finally, a low-pinning granular aluminum film, with $T_c \approx 1.7$ K, is evaporated onto this substrate at an angle with respect to the normal, leading to a superconducting film that also has a sinusoidal modulation in its thickness. We have performed experiments comparing the transport properties of these modulated films to flat films grown at the same time; the modulated films show clear signatures of an increased critical current and effects due to matching and commensurability. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S52.00010: Vortex Flipping in Superconductor-Ferromagnet Spin Valve Structures Edgar J. Patino, Marco Aprili, Mark Blamire, Yoshiteru Maeno We report in plane magnetization measurements on Ni/Nb/Ni/CoO and Co/Nb/Co/CoO spin valve structures with one of the ferromagnetic layers pinned by an antiferromagnetic layer. In samples with Ni, below the superconducting transition Tc, our results show strong evidence of vortex flipping driven by the ferromagnets magnetization. This is a direct consequence of proximity effect that leads to vortex supercurrents leakage into the ferromagnets. Here the polarized electron spins are subject to vortices magnetic field occasioning vortex flipping. Such novel mechanism has been made possible for the first time by fabrication of the F/S/F/AF multilayered spin valves with a thin-enough S layer to barely confine vortices inside as well as thin-enough F layers to align and control the magnetization within the plane. When Co is used there is no observation of vortex flipping effect. This is attributed to Co shorter coherence length. Interestingly instead a reduction in pinning field of about 400 Oe is observed when the Nb layer is in superconducting state. This effect cannot be explained in terms of vortex fields. In view of these facts any explanation must be directly related to proximity effect and thus a remarkable phenomenon that deserves further investigation. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S52.00011: Critical currents in superconducting films with array of magnetic and non-magnetic dots: Interstitial vs. trapped vortices Javier del Valle, Alicia Gomez, Elvira Gonzalez, Jose Vicent Arrays of magnetic Py dots and non-magnetic Cu dots have been embedded in Nb superconducting films by electron beam lithography and sputtering techniques. (I,V) curves have been measured in both systems. The critical current values extracted from the (I,V) curves show periodic maxima which are induced by the well-known matching effect between the vortex lattice and the array of nanodefects. The Nb film with non-magnetic Cu array shows an unexpected larger number of maxima than the Nb film with the magnetic Py array. Furthermore, comparison between critical currents in both samples shows a striking result, a crossover is measured increasing the applied magnetic fields. At low applied magnetic fields critical current values are higher in the magnetic (Py) dot sample than in the non-magnetic (Cu) dot sample, but increasing the applied field the opposite occurs, i. e. the critical current in Nb film with Cu dots is higher than the critical current in Nb film with Py dots. These data are analyzed taking into account the different behavior between interstitial vortices and vortices trapped in the pinning potential wells which are generated by the different arrays. [Preview Abstract] |
Session S53: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures I
Sponsoring Units: DMPChair: Matthew Gilbert, University of Illinois
Room: Mile High Ballroom 2C
Thursday, March 6, 2014 8:00AM - 8:12AM |
S53.00001: Screening in Non-Equilibrium Dissipative System Jiajun Li, Jong Han Effect of screening is one crucial property of interacting electrons. However, it is still not completely understood in non-equilibrium dissipative system, partly due to a lack of convenient theoretical tool. It is recently shown that a DC-driven lattice attached to fermionic reservoirs [1,2] reproduces major physical properties of real system, and is accessible by comprehensive theoretical study even in strong field and correlated electron region. In this presentation, we will show a study of electronic screening within this model. First of all, current distribution out of impurities will be shown in steady-state non-equilibrium. With parameters changing in the regimes of linear and high-field, DC current shows distinctly different patterns, reflecting the underlying interplay between quantum dissipation and non-equilibrium physics. In addition, the density-density correlation function is calculated and RPA is used to study dielectric screening. The electron-hole excitation spectrum will be presented, which indicates interesting physics while fermionic dissipation, Coulomb interaction and external field compete with each other. \\[4pt] [1] J. E. Han, Phys. Rev. B \textbf{87}, 085119 (2013)\\[0pt] [2] J. E. Han, J. Li, Phys. Rev. B \textbf{88}, 075113 (2013) [Preview Abstract] |
Thursday, March 6, 2014 8:12AM - 8:24AM |
S53.00002: Strong correlation and multi-phase solution in nonequilibrium lattice systems coupled to dissipation medium Jong Han, Jiajun Li, Camille Aron, Gabriel Kotliar How does a strongly correlated electronic solid evolve continuously out of equilibrium when an electric field is applied? While this question may seem deceptively simple, it requires rigorous understanding of dissipation. We formulate the nonequilibrium steady-state lattice coupled to fermion baths in the Coulomb gauge. We demonstrate that the Hubbard model solved using the iterative perturbation theory within the dynamical mean-field approximation recovers the DC conductivity independent of the Coulomb interaction in a very narrow linear response regime. Due to the singular dependence of the effective temperature on the damping in the steady-state~\footnote{J. E. Han and Jiajun Li, Phys. Rev. B {\bf 88}, 075113 (2013)}, systems with damping have dramatic field-dependent effect, very different from dissipationless systems. We conclude that the dominant physics in lattice nonequilibrium is not the field vs quasi-particle energy, but rather the Joule heat vs the quasi-particle energy. Furthermore, we show that, in the vicinity of the Mott-insulator transition, the solution supports mixed-phase state scenario which indicates that the electron transport in solids under high-field can be spatially inhomogeneous leading to filamentary conducting paths, as suggested by experiments. [Preview Abstract] |
Thursday, March 6, 2014 8:24AM - 8:36AM |
S53.00003: Metastable Dynamics of Topological Superfluids Moon Jip Park, Matthew Gilbert Superfluid states resulting from the condensation of indirectly bound excitons arising from the attractive Coulomb coupling of electrons in one layer and holes in another spatially segregated layer have attracted a great deal of interest from both a fundamental and applied perspective due to their remarkable interlayer transport properties. Within these systems, one of the most important quantities is the critical current, the maximum interlayer current that can flow before coherence is lost. While a great deal is known about the superfluid state prior to reaching critical current, very little is known about the fate of the system after critical current. In this talk, we study non-equilibrium response of a dipolar intersurface superfluid in a 3D time-reversal invariant topological insulator using a fully time-dependent formulation of the Kadanoff-Baym equations. We find that past critical current there exist different metastable regions of intersurface voltage characterized by distinct time-dependent responses. While we will discuss the resultant physics of the metastable states present beyond the critical current within topological insulators, the physics is broadly applicable to both graphene and quantum Hall systems. [Preview Abstract] |
Thursday, March 6, 2014 8:36AM - 8:48AM |
S53.00004: Crossover between the Hikami and spin-resolved band limits of weak anti-localization in two-dimensional electron gases Yasufumi Araki, Guru Khalsa, Allan H. MacDonald We investigate the quantum interference corrections to transport which lead to weak localization (WL) or weak anti-localization (WAL) for the case of spin-independent disorder scattering in two-dimensional electron gases with spin-orbit interactions of arbitrary strength. We formulate our theory in terms of microscopic linear response including multiple scattering by the disorder potential to derive the current-current response function when Rashba (or Dresselhaus) spin-orbit coupling is included in the electronic band structure. We analyze the crossover from the weak spin-orbit coupling limit in which spin-splitting of the bands is not resolved, to the strong spin-orbit coupling limit of clearly spin-split bands. In the weak and strong spin-orbit coupling limits we generally recover the well-known WL and WAL behavior first predicted by Hikami, Larkin and Nagaoka, although the degeneracy of spin triplet channels is lifted leading to a more complex crossover between the traditional WL and WAL limits. Our results can be summarized by a phase diagram in spin-orbit coupling strength and temperature (or the coherence length from inelastic scattering), with several regions separated by different crossover lines. [Preview Abstract] |
Thursday, March 6, 2014 8:48AM - 9:00AM |
S53.00005: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 9:00AM - 9:12AM |
S53.00006: Hanbury Brown --Twiss type exchange effects in a four terminal diffusive conductor Jayanta Sarkar, Antti Puska, Akira Hida, Maciej Weisner, Pertti Hakonen We have investigated current-current correlations in a mesoscopic four-terminal diffusive conductor, and performed an electronic equivalent of Hanbury Brown -Twiss (HBT) type of experiment. In the experiment, cross -spectrum noise between two terminals of a cross is measured in three different bias configurations and exchange correction factor $\Delta $S $=$ \textbar SC\textbar - \textbar SA\textbar - \textbar SB\textbar [1] is calculated from these measurements. In the non-interacting regime, we find an increase in $\Delta $S with bias from the thermal limit and with further increase in bias the $\Delta $S becomes bias independent [2]. In contrast to the diffusive cross, our measurements on metallic islands yielded strong HBT-type of correlations that agree with the expected intrinsic correlations in a four-terminal cavity coupled to reservoirs by weak tunneling contacts. Moreover, we find non-classical HBT contributions for the case of interacting electrons (hot electrons), where $\Delta $S is found to be negative.\\[4pt] [1] Ya. M. Blanter, M. B\"{u}ttiker, Phys. Rev. B \textbf{56}, 2127 (1997).\\[0pt] [2] E. V. Shukhorukov, D. Loss, Phys. Rev. B \textbf{59}, 13054 (1999). [Preview Abstract] |
Thursday, March 6, 2014 9:12AM - 9:24AM |
S53.00007: Magnetotransport across the artificially designed tilted grain boundaries Aiping Chen, Zhenxing Bi, Chen-Fong Tsai, Li Chen, Qing Su, Xinghang Zhang, Haiyan Wang Single-phase epitaxial La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) thin films with significantly enhanced low-field magnetoresistance (LFMR) properties are demonstrated in this work. The LSMO films on SrTiO$_{3}$ (001) substrates exhibit tilted and well-aligned nanocolumn structure achieved by pulsed laser oblique-angle deposition (PLOAD) followed by subsequent postannealing. The tilted aligned nanocolumnar (TAN) arrays have been achieved at relative high deposition angles ($\ge $30$^{\circ})$ and low deposition temperatures ($\le $450 $^{\circ}$C). More attractively, the tilted grain boundaries (GBs) can be systematically manipulated by the postannealing process and so can the LFMR values of the LSMO TAN films. These results demonstrate that the tilted nanocolumnar films achieved by PLOAD and the GB tailoring by postannealing may provide a new approach to control and manipulate the magnetotransport properties of single-phase manganite perovskite films for device applications that require large LFMR effects, high epitaxial quality, and low resistivity. [Preview Abstract] |
Thursday, March 6, 2014 9:24AM - 9:36AM |
S53.00008: Dragon Segments in Electron Transport through Nanostructures Mark Novotny In transport through nanostructures connected to two semi-infinite leads, the transmission probability ${\cal T}(E)$ as a function of the energy $E$ of the incoming electron enters the Landauer calculation of the electrical conductance. Ballistic propagation occurs in pure materials due to the lack of scattering, and consequently ${\cal T}(E)=1$. It is shown that there is a large class of strongly disordered quasi-1D segments that also have ${\cal T}(E)=1$. Such segments are called quantum dragon segments. Quantum dragon segments have a serpentine quasi-1D structure, and if present cannot be observed by electron transport since ${\cal T}(E)=1$. Dragon segments are only possible when there is correlated disorder, thereby overcoming the Anderson localization that is present in 1D systems with random disorder. Dragon segments are found by using an exact mapping for ${\cal T}(E)$ at the level of the single-band tight-binding model. The mapping is from the quasi-1D model of the nanostructure onto a 1D model. Presented examples of dragon segments will include: select single-walled carbon nanotubes, Bethe lattices, conjoined Bethe lattices with random hopping within each ring, and quasi-1D systems formed from rectangular or orthorhombic lattice slices. [Preview Abstract] |
Thursday, March 6, 2014 9:36AM - 9:48AM |
S53.00009: Theory of Plasmonic Waves on a Chain of Metallic Nanoparticles in a Liquid Crystalline Host David Stroud, Nicholas Pike Linearly polarized plasmonic waves can propagate along a chain of metallic particles, of sufficiently small diameter and spacing. We have calculated the dispersion relations for these plasmonic waves when the host is either a nematic or a cholesteric liquid crystal (NLC or CLC). An NLC is found to alter the dispersion relations of both transverse ($T$) and or longitudinal ($L$) waves significantly from those for an isotropic host. If the NLC director is perpendicular to the metallic chain, the doubly degenerate $T$ branch is split into two linearly polarized branches. Similar results are obtained for a CLC with twist axis parallel to the chain, except that the $T$ branches are elliptically polarized. When a magnetic field is applied parallel to the chain, the dispersion relations for the $T$ branches are no longer symmetric about $k=0$ and the chain may act as a one-way waveguide at certain frequencies. We present numerical examples assuming spherical metal particle with a Drude dielectric function. [Preview Abstract] |
Thursday, March 6, 2014 9:48AM - 10:24AM |
S53.00010: Electrical Contacts in Carbon Nanotube Transistors Invited Speaker: Vasili Perebeinos Electrical properties of low-dimensional devices are dominated by the contact resistance. For carbon nanotube field effect transistors (CNT-FETs), as for graphene and MoS$_{\mathrm{2}}$ transistors, the electrical contacts are a key factor limiting device performance. Contact resistance reflects a complex interplay of many factors. With advances in scaling, the contact resistance and transfer length are becoming even more critical. We have developed a general purpose CNT device simulator which is unique in including quantum-mechanical tunneling, both acoustic and optical-phonon scattering, as well as the crucial transfer of carriers between the CNT and metal contact. To illustrate the unique capabilities relative to existing approaches such as non-equilibrium Green's function (NEGF) formalism, we predict the scaling of on-state current with contact length. The behavior is surprising. The transfer length is roughly a factor of two shorter at a typical drain voltage than at low bias. This reflects the onset of optical-phonon scattering underneath the metal contact for a ballistic channel. As we change the nanotube diameter (i.e. bandgap) and metal workfunction a Schottky to ohmic crossover in device characteristics takes place. Although the on-state current varies continuously, the transfer characteristics reveal a relatively abrupt crossover from Schottky to ohmic contacts [1]. The typical high-performance devices fall surprisingly close to the crossover. Therefore, tunneling plays an important role even in this regime, so that current fails to saturate with gate voltage as was expected due to ``source exhaustion''.\\[4pt] [1] V. Perebeinos, J. Tersoff, W. Haensch, Phys. Rev. Lett. (in press). [Preview Abstract] |
Thursday, March 6, 2014 10:24AM - 10:36AM |
S53.00011: Exciton and electron transport in metal oxide and metal-insulator nanostructures Wayne Hess, Alan Joly, Matthew Halliday, Alexander Shluger, Peter Sushko Understanding the dynamics of electronic excited states in solids is essential to forming mechanistic models relevant to electron and energy transport in materials. Irradiation of materials by ultraviolet (UV) photons, produces energetic species such as holes and free electrons, that relax to form electron-hole pairs and excitons capable of driving surface and bulk reactions such as atomic desorption. Photostimulated desorption experiments, when combined with ab-initio calculations, can be used to develop models for exciton transport and subsequent excited state dynamics. We use pulsed UV lasers to excite specific surface and bulk states of nano-structured metal oxides and measure velocities of desorbed atoms under controlled conditions. By measuring O-atom kinetic energy distributions, as a function of laser frequency, we demonstrate exciton transport on the surface and in the bulk of metal-oxides. We further interrogate electron and exciton dynamics at the metal-insulator interface of thin CsBr films grown on Cu(100). Photoexcitation at 6.4 eV specifically excites the CsBr surface exciton which leads to desorption of neutral Br-atoms with hyperthermal kinetic energies. In dramatic contrast, we observe thermal energy Br atoms from CsBr grown on Cu. The hyperthermal desorption channel is entirely quenched for UV laser desorption of Br atoms even for thick (\textgreater 60 nm) CsBr films on Cu. Possible models for electron transport and exciton relaxation will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 10:36AM - 10:48AM |
S53.00012: Observation of an insulating to conducting transition in an artificial quantum dot lattice Neal Staley, Nirat Ray, Marc Kastner, Micah Hanson, Arthur Gossard In a single quantum dot, where the electron occupation is controlled by the interplay between the geometry, and thus charging energy of the dot, and the externally applied gate voltage it is possible to observe a mesoscopic analog to a neutral atom. If one were to create a lattice of these ``artificial atoms'' with sufficiently low disorder it would be possible to create an artificial solid with tunable properties. Electrical transport measurements on quantum dot lattices have thus far been dominated by disorder. We fabricated quantum dot lattices on GaAs using electron beam lithography and reactive ion etching to define the boundary of each dot, with the electron density controlled by a global top gate. For single quantum dots fabricated using this technique we observe ``Coulomb diamond'' features characteristic of single electron charging into the dot when the device is depleted into the few electron regime. For lattices however we observe a striking transition from a high resistance (low current) state to low resistance (high current) as a function of increasing source drain bias. This transition occurs over a large range in gate voltage, and temperature and could be an indication of collective phenomena occurring within these artificial quantum dot lattices. [Preview Abstract] |
Thursday, March 6, 2014 10:48AM - 11:00AM |
S53.00013: ABSTRACT WITHDRAWN |
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