Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session W1: Focus Session: Charge & Energy Transfer IV
Sponsoring Units: DCPChair: Carlos Silva, University of Montreal
Room: 103/105
Thursday, March 6, 2014 2:30PM - 2:42PM |
W1.00001: Carrier dynamics in colloidal indium arsenide quantum dots in the weak excitation limit Austin Spencer, William Peters, Vivek Tiwari, Byungmoon Cho, Nathan Neale, David Jonas The dynamics of photo-excited carriers in colloidal indium arsenide (InAs) quantum dots are characterized by degenerate pump--probe spectroscopy at 1.5 times the band gap. This material is of particular interest due to reports of efficient multiple exciton generation and its potential application in third-generation photovoltaic devices. Use of a sample renewal technique based on laser beam scanning enables long resampling times ($>$0.5 s) with minimal spatial overlap between successive laser shots thereby minimizing repetitive excitation. Pump--probe transients at a range of excitation probabilities are reported, from 2.6\%, where signal from biexcitons is small (1.9\%), to 36\%, where the biexcitons contribute 45\% of the signal. These transients are well described by a tri-exponential fit which includes time constants of approximately 1 ps, 16 ps, and 750 ps tentatively attributed to carrier cooling, multi-exciton recombination, and single exciton recombination respectively. By an excitation probability of 10\%, biexciton dynamics are detectable and continue to grow in magnitude as the excitation probability increases. The pump power dependence of the signal at 20 ps, which deviates from linearity at an excitation probability of 10\%, reflects biexciton recombination. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W1.00002: Photoinduced Electron Transfer to Engineered Surface Traps in CdSe Nanocrystals Marco Califano, Haiming Zhu, Ye Yang, Kim Hyeon-Deuk, Nianhui Song, Youwei Wang, Wenqing Zhang, Oleg Prezhdo, Tianquan Lian Quantum confined nanomaterials, such as semiconductor nanocrystals (NCs), have emerged in the past decade as a new class of materials for solar energy conversion. An appropriate model for describing photoinduced charge transfer in these systems is, however, still lacking. Recently we observed that the rate of photoinduced electron transfer from CdSe NCs to molecular acceptors increased with decreasing NC size (and increasing driving force) exhibiting a lack of Marcus inverted regime behaviour over an apparent driving force range of ~ 0-1.3 V. Our atomistic semiempirical pseudopotential calculations show that an Auger assisted ET mechanism, in which the transfer of the electron is coupled to the excitation of the hole, can circumvent the unfavourable Frank-Condon overlap (that is a signature of inter- or intra- molecular electron transfer) in the Marcus inverted regime, reproducing our observed ET rates with remarkable accuracy. We conclude that electron transfer from quantum dots differs from electron transfer originating from both molecules and bulk semiconductors. It proceeds via a novel Auger-assisted pathway which we believe is available to most excitonic nanomaterials. This new finding will have a major impact on the design of next generation solar energy harvesting devices. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W1.00003: Electron - acoustic phonon coupling in colloidal lead sulfide quantum dots Byungmoon Cho, Vivek Tiwari, Austin Spencer, Dmitry Baranov, Samuel Park, David Jonas Lead chalcogenide quantum dots (QDs) with bandgaps in the shortwave infrared are candidate materials for next generation photovoltaics exceeding the Shockley-Queisser limit. Despite ongoing controversy, multiple exciton generation (MEG) in QDs offers potential for improved photovoltaic efficiency. Hot carriers from high energy photoexcitation dissipate excess energy via coupled phonons; this is detrimental to MEG. The electron-phonon coupling (EPC) magnitude, partitioning among modes and dependence on the size/shape are poorly understood. We performed degenerate femtosecond pump-probe spectroscopy to investigate Auger recombination dynamics, a reverse process of MEG. We observe a quantum beat due to coherent acoustic phonons in femtosecond pump-probe signals from oleate capped colloidal lead sulfide QDs in toluene. A 3.4 ps period oscillation decays with 4.6 ps damping constant in 8 nm diameter dots; the amplitude increases linearly with pump energy and modulation is weaker than reported in smaller dots. An elastic continuum model for acoustic phonon frequency vs. dot diameter suggests a not yet understood quantitative discrepancy with prior work. These relaxation processes have important implications for QD photovoltaics. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W1.00004: Engineered semiconductor nanocrystals with enhanced carrier multiplication yields Invited Speaker: Victor Klimov Carrier multiplication (CM) is a process whereby absorption of a single photon results in multiple electron-hole pairs (excitons). This process could benefit a number of solar-energy conversion technologies, most notably photocatalysis and photovoltaics. This presentation overviews recent progress in understanding the CM process in semiconductor nanocrystals, motivated by an outstanding challenge in this field - the lack of capability to predict the CM performance of nanocrystals based on their known photophysical properties or documented parameters of parental bulk solids. Here, we present a possible solution to this problem by showing that, using biexciton Auger lifetimes and intraband relaxation rates inferred from ultrafast spectroscopic studies, we can rationalize relative changes in CM yields as a function of nanocrystal composition, size and shape. Further, guided by this model, we demonstrate a two-fold enhancement in multiexciton yields in PbSe nanorods vs. quantum dots attributed to enhanced Coulomb interactions. We also explore the control of competing intra-band cooling for increasing multiexciton production. Specifically, we design a new type of hetero-structured PbSe/CdSe quantum dots with reduced rates of intra-band relaxation and demonstrate a four-fold boost in the multiexciton yield. These studies provide useful guidelines for future efforts to achieve the ultimate, energy-conservation-defined CM efficiencies. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W1.00005: Characterization and 2D FT electronic spectra of colloidal PbSe nanocrystals Dmitry Baranov, Samuel Park, Byungmoon Cho, David Jonas In the last decade, colloidal lead selenide nanocrystals (PbSe NCs) have been actively studied by ultrafast laser spectroscopy techniques due to interesting electronic structure and rich carrier dynamics. Theoretical studies of small PbSe NCs by Zunger group predicted intervalley splitting of the electron and hole ground states which are degenerate in the bulk. Recent theoretical work by Goupalov group predicted that magnitude of the splitting depends on the structure of NC's core and presence of surface shell. These predictions and abundant reports of sample to sample variation in observed photophysics of PbSe NCs point towards the need for a better relation of sample's structure and composition to results of spectroscopic measurements. In this work, we investigate well-defined colloidal PbSe NCs by 2D FT electronic spectroscopy in the short-wave infrared region. Samples of oleate-capped PbSe colloidal NCs (3.3 nm average diameter, 1.09 eV band gap) have uniform shapes with a size distribution varying by less than half of bulk PbSe lattice constant. Studied samples of PbSe NCs were prepared following a protocol which has been reported to produce Pb-rich NCs, suitable for testing predictions about the intervalley splitting. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W1.00006: Single Molecule Spectroscopy and Scanning Probe Microscopy to Investigate Excited State Energy Transport in Quantum Dot Higher Order Structures Alan Van Orden, Martin Gelfand, Duncan Ryan, Kevin Whitcomb Single molecule fluorescence spectroscopy and scanning probe microscopy have been used to investigate small isolated clusters of CdSe/ZnS nanocrystalline quantum dots dispersed on insulating, conducting, and semiconducting surfaces. The aggregated quantum dots exhibit excited state energy transfer and charge transport which affects the time dependent autocorrelation of the photoluminescence (PL) emission intensity, photon counting statistics, blinking statistics, and PL lifetime, as observed by single molecule fluorescence spectroscopy. The structural arrangement of the nanocrystals and the electron transfer between the quantum dots and substrate can be investigated using atomic force microscopy, transmission electron microscopy, and scanning tunneling microscopy. These combined experiments provide novel perspectives on energy and electron transport in quantum dot higher order structures and the effects of structural arrangements, substrates, and attached ligands. These insights will enhance the development of technological applications of quantum dots, including bioimaging, display technology, and alternative energy technology. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W1.00007: Breaking the symmetry of a PbSe quantum dot by a hot electron-hole pair Minh-Tuan Trinh, Xiaoyang Zhu Optical excitation of semiconductor nanocrystals or quantum dots (QDs) near the bandgap is now well-understood, but the same cannot be said about excitation significantly above the bandgap. Here, we report an observation of symmetry breaking imposed by a hot electron-hole pair from above-gap excitation in PbSe QDs using ultrafast pump-probe spectroscopy. The breaking of symmetry results in a modification of optical dipole selection rules, as well as the broadening and redshift of dipole-allowed transitions, during the picosecond lifetime of the hot carriers. The observations can be interpreted as a transient Stark effect resulting from the bulk-like behavior of the hot electron-hole pair. At a short time scale right after excitation, the hot electron and hole can be viewed as independent carriers that generate a net transient internal electric field which breaks the symmetry of the QD. By varying the excitation energy we show that the symmetry breaking effect increases with excitation energy and disappears at the bandgap excitation. Such a breaking of symmetry via transient Stark effect should be of general significance to the understanding of QD photophysics above the bandgap. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W1.00008: Multiexciton Generation in Nanocrystals and Nanorods Invited Speaker: Eran Rabani Multiexciton generation (MEG) is a process where several excitons are generated upon the absorption of a single photon in semiconductors. This process enjoys great technological ramifications for solar cells and other light harvesting technologies. For example, it is expected that the more charge carriers created shortly after the photon is absorbed, the larger fraction of the photon energy can successfully be converted into electricity, thus increasing the device efficiency. Strict selection rules and competing processes in the bulk allows MEG at energies of five times the band gap. It was suggested that nanocrystals, where quantum confinement effects are important, may exhibit MEG at lower values of (typically 2 to 3 times the band gap). Indeed, MEG in nanocrystals has been reported recently for several systems, showing that the threshold was size and band-gap independent. However, more recent studies have questioned the high efficiency of MEG in nanocrystals. In this talk we will discuss the process of MEG in semiconducting nanocrystals (NCs) and nanorods (NRs). A general theoretical framework will be presented and the limits of indirect absorption and impact ionization will be derived. The role of composition material, size, geometry and energy on the MEG efficiencies will be explored using a stochastic approach to calculate MEG with a numerical effort that scales linearly with system size. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W1.00009: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W1.00010: Hierarchical Equation of Motion Investigation of Decoherence and Relaxation Dynamics in Nonequilibrium Transport through Interacting Quantum Dots Rainer Hartle, Guy Cohen, David R. Reichman, Andrew J. Millis A recently developed hierarchical quantum master equation approach [1,2] is used to investigate nonequilibrium electron transport through an interacting double quantum dot system in the regime where the inter-dot coupling is weaker than the coupling to the electrodes. The corresponding eigenstates provide tunneling paths that may interfere constructively or destructively, depending on the energy of the tunneling electrons [3]. Electron-electron interactions are shown to quench these interference effects in bias-voltage dependent ways, leading, in particular, to negative differential resistance, population inversion and an enhanced broadening of resonances in the respective transport characteristics [2]. Relaxation times are found to be very long, and to be correlated with very slow dynamics of the inter-dot coherences (off diagonal density matrix elements). The ability of the hierarchical quantum master equation approach to access very long time scales is crucial for the study of this physics. [1] JCP 128, 234703 (2008) [2] arXiv:1309.1170 (2013) [3] PRB 87, 085422 (2013) [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W1.00011: Studying the Photoexcitation Quenching Mechanism in Quantum Dot-Nitroxide Radical Hybrid Sytems Poulami Dutta, Jeffrey Sayen, Remi Beaulac The study of energy/electron transfer processes in inorganic/organic complexes is an active area of research, with applications in fields ranging from energy conversion to chemical catalysis. A really interesting variation of this is the use of colloidal semiconductor nanostructures as one of the donor and/or acceptor units; since this area has remained quite unexplored. In the present work, nitronyl- and imino-nitroxide organic free radicals have been synthesized and coupled to colloidal II-VI quantum dots (QDs). The nature of the donor-acceptor interactions in these QD-radical hybrids has been investigated using spectroscopic techniques such as ground state UV-Vis absorption, steady-state and time-resolved photoluminescence(PL) spectroscopy. In all cases, these radicals quench the Luminescence intensity from the photo-excited QDs through an efficient non-radiative process. The excited-state quenching rate constant is highly dependent on parameters like QD size and composition, the identity and concentration of the organic free radical, and the nature of the medium surrounding the QDs. The quenching rates can be further correlated to the surface-binding sites on the QDs and can also be tuned by modifying the nature of the QD surface by growing different QD hetero-structures. [Preview Abstract] |
Session W2: Focus Session: Solvation, Dynamics, and Reactivity in Complex Environments IV
Sponsoring Units: DCPChair: Ward Thompson, University of Kansas
Room: 102
Thursday, March 6, 2014 2:30PM - 3:06PM |
W2.00001: Imaging and controlling intracellular reactions: Lysosome transport as a function of diameter and the intracellular synthesis of conducting polymers Invited Speaker: Christine Payne Eukaryotic cells are the ultimate complex environment with intracellular chemical reactions regulated by the local cellular environment. For example, reactants are sequestered into specific organelles to control local concentration and pH, motor proteins transport reactants within the cell, and intracellular vesicles undergo fusion to bring reactants together. Current research in the Payne Lab in the School of Chemistry and Biochemistry at Georgia Tech is aimed at understanding and utilizing this complex environment to control intracellular chemical reactions. This will be illustrated using two examples, intracellular transport as a function of organelle diameter and the intracellular synthesis of conducting polymers. Using single particle tracking fluorescence microscopy, we measured the intracellular transport of lysosomes, membrane-bound organelles, as a function of diameter as they underwent transport in living cells. Both ATP-dependent active transport and diffusion were examined. As expected, diffusion scales with the diameter of the lysosome. However, active transport is unaffected suggesting that motor proteins are insensitive to cytosolic drag. In a second example, we utilize intracellular complexity, specifically the distinct micro-environments of different organelles, to carry out chemical reactions. We show that catalase, found in the peroxisomes of cells, can be used to catalyze the polymerization of the conducting polymer PEDOT:PSS. More importantly, we have found that a range of iron-containing biomolecules are suitable catalysts with different iron-containing biomolecules leading to different polymer properties. These experiments illustrate the advantage of intracellular complexity for the synthesis of novel materials. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W2.00002: Myosin-II sets the optimal response time scale of chemotactic amoeba Hsin-Fang Hsu, Christian Westendorf, Marco Tarantola, Eberhard Bodenschatz, Carsten Beta The response dynamics of the actin cytoskeleton to external chemical stimuli plays a fundamental role in numerous cellular functions. One of the key players that governs the dynamics of the actin network is the motor protein myosin-II. Here we investigate the role of myosin-II in the response of the actin system to external stimuli. We used a microfluidic device in combination with a photoactivatable chemoattractant to apply stimuli to individual cells with high temporal resolution. We directly compare the actin dynamics in Dictyostelium discodelium wild type (WT) cells to a knockout mutant that is deficient in myosin-II (MNL). Similar to the WT a small population of MNL cells showed self-sustained oscillations even in absence of external stimuli. The actin response of MNL cells to a short pulse of chemoattractant resembles WT during the first 15 sec but is significantly delayed afterward. The amplitude of the dominant peak in the power spectrum from the response time series of MNL cells to periodic stimuli with varying period showed a clear resonance peak at a forcing period of 36 sec, which is significantly delayed as compared to the resonance at 20 sec found for the WT. This shift indicates an important role of myosin-II in setting the response time scale of motile amoeba. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W2.00003: Fluorescent Dendrimer Nanoconjugates as Advanced Probes for Biological Imaging Daniel Reilly, Sung Hoon Kim, John A. Katzenellenbogen, Charles M. Schroeder Recent advances in fluorescence microscopy have enabled improvements in spatial resolution for biological imaging. However, there is a strong need for development of advanced fluorescent probes to enable a molecular-scale understanding of biological events. In this work, we report the development of a new class of probes for fluorescence imaging based on dye-conjugated dendrimer nanoconjugates. We utilize molecular-scale dendritic scaffolds as fluorescent probes, thereby enabling conjugation of multiple dyes and linkers to the scaffold periphery. In particular, we use polyamidoamine dendrimers as molecular scaffolds, wherein dye conjugation can be varied over a wide range. Single molecule fluorescence imaging shows that dendrimer nanoconjugates are far brighter than single fluorophores, resulting in increased localization precision. In addition, we further developed a new set of remarkably photostable probes by conjugating photoprotective triplet state quenchers directly onto the dendritic scaffold. We observe large increases in the photobleaching times compared to single dyes and reduced transient dark states (blinking). Overall, we believe that these new probes will allow for single molecule imaging over long time scales, enabling new vistas in biological imaging. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W2.00004: Preferential melting of secondary structures during protein unfolding in different solvents: Competition between hydrophobic solvation and hydrogen bonding Biman Bagchi, Susmita Roy, Rikhia Ghosh Aqueous binary mixtures such as water-DMSO, water-urea, and water-ethanol are known to serve as denaturants of a host of proteins, although the detailed mechanism is often not known. Here we combine studies on several proteins in multiple binary mixtures to obtain a unified understanding of the phenomenon. We compare with experiments to support the simulation findings. The proteins considered include (i) chicken villin head piece (HP-36), (ii) immunoglobulin binding protein G (GB1), (iii) myoglobin and (iv) lysozyme. We find that for amphiphilic solvents like DMSO, the hydrophobic groups and the strong hydrogen bonding ability of the \textgreater S$=$O oxygen atom act together to facilitate the unfolding. However, the hydrophilic solvents like urea, due to the presence of more hydrophilic ends (C$=$O and two NH$_{\mathrm{2}})$ has a high propensity of forming hydrogen bonds with the side-chain residues and backbone of beta-sheet than the same of alpha helix. Such diversity among the unfolding pathways of a given protein in different chemical environments is especially characterized by the preferential solvation of a particular secondary structure. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W2.00005: The $gp41_{659-671}$ HIV-1 antibody epitope: a structurally challenging small peptide Yuan Zhang, Celeste Sagui We present the results of extensive Molecular Dynamics (MD) simulations of the tridecapeptide corresponding to residues 659-671 of the envelope glycoprotein gp41 of HIV-1, which spans the 2F5 monoclonal antibody epitope ELDKWA. The most recent AMBER force fields ff99SB and ff12SB in both implicit and explicit solvents have been used for a cumulative time longer than 7.2 $\mu s$. We have analyzed the conformational ensembles of the peptide both with and without applied tensile restraints, and found that: (1) The amount of helical populations is important in aqueous solution, but this structure forms part of a flexible conformational ensemble with a rugged free energy landscape with shallow minima, which agrees well with the bulk of the experimental observations; (2) our results are more consistent with the experimental results than those from previous simulations; (3) under uniaxial tension, the disordered peptide first becomes fully helical before melting into turns, loops and $3_{10}$-helices. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W2.00006: Solvation dynamics and enzyme catalysis in a designed enzyme undergoing directed evolution Carl Schreck, Teresa Head-Gordon We explore whether catalysis of a de novo designed enzyme-substrate complex is correlated to necessary solvent fluctuations to induce a chemical reaction. By studying a designed KEMP Eliminase as it goes through rounds of directed evolution to improve it's catalytic activity, we have found that catalytic activity correlates with an increase in density and structure of water near the active site. This suggests fluctuations in the solvation water near the active site couple to fluctuations in KEMP Eliminase to facilitate the catalytic process. To flesh this idea out, we are studying the progression of vibrational properties and cooperative fluctuations of solvation water by simulating the terahertz observable. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W2.00007: Environmental Affects on Surfactin Studied Using Multidimensional Infrared Spectroscopy Jacob Nite, Amber Krummel Surfactin, a cyclic lipopeptide produced by Bacillus subtilis, is a pore forming toxin that has been studied in the literature extensively. It is known to exist in two different conformations, S1 and S2, which are thought to relate to surfactin's pore forming ability. The vibrational characteristics of surfactin have been studied using linear infrared spectroscopy as well as two-dimensional infrared spectroscopy in different environments. The environments probed were specifically chosen to mimic surfactin in an aqueous environment as well as a lipid membrane environment. The vibrational spectra were interpreted using transitional dipole coupling to relate the coupling evident in the data to the structural conformers obtained from NMR data. These measurements have been used to link the structural characteristics of surfactin to different solvent environments to gain insight into surfactin's pore forming ability mechanisms. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W2.00008: Effect of surfaces in modulating protein folding mechanisms Invited Speaker: Joan Shea Protein-surface interactions are ubiquitous in the crowded cytosol, where proteins encounter a variety of surfaces, ranging from membranes surfaces, to the surfaces presented by chaperone molecules. Protein-surface interactions are also at the heart of a number of emerging technologies, including protein micro-arrays, biosensors and biomaterials. The effect of surfaces on protein structure and stability can vary substantially depending on the chemical composition of the surface. In this talk, I will present detailed atomistic simulations of the folding of a small beta-sheet protein in the presence of graphite and titanium oxide surfaces. The role of water-mediated and direct protein-surface interactions in governing protein conformations will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W2.00009: Solvation in Surfactant Films at the Water-Air Interface Russell Perkins, Rebecca Rapf, Elizabeth Griffith, Veronica Vaida Surfactant films at water-air interfaces are used as models for environmental systems and biological membranes. This presentation describes surface sensitive experiments probing solvation of phenylalanine aggregates in surfactant films used as model membranes. Solvation in this complex environment is different from solvation in the bulk aqueous phase, with interesting changes to chemical and morphological structure of these surface films. Consequences of the changes induced in model membranes by the solvation of biological aggregates will be discussed using results of molecular dynamics simulations. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W2.00010: Temperature Dependent Solvation Dynamics of the Chromophore Environment in the Far-Red Fluorescent Protein mPlum Chola Regmi, Prem Chapagain, Bernard Gerstman We used molecular dynamics (MD) simulations to investigate the solvation dynamics of the chromophore environment in the far red fluorescent protein mPlum. Low temperature experiments on mPlum show a reduced Stokes shift compared to the room temperature. This suggests that the flexibility of the chromophore environment is related to the large Stokes shift in the red fluorescent protein mPlum. We performed MD simulations at various temperatures and systematically explored the protein-chromophore hydrogen bond pattern as well as the dynamics of the internal water molecules near the chromophore. We also investigated the dynamics of the hydrogen bond formed between residues E16 and I65, which is considered to play an important role in the observed large red shift in the emission spectrum. We quantify the hydrogen bonding pattern as a function of temperature to show how the Stokes shift in mPlum is correlated to the E16-I65 hydrogen bond dynamics. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W2.00011: QTES-DFTB dynamics study on the effect of substrate motion on quantum proton transfer in soybean lipoxygenase-1 James Mazzuca, Sophya Garashchuk, Jacek Jakowski It has been shown that the proton transfer in the enzymatic active site of soybean lipoxygenase-1 (SLO-1) occurs largely by a quantum tunneling mechanism. This study examined the role of local substrate vibrations on this proton tunneling reaction. We employ an approximate quantum trajectory (QT) dynamics method with linear quantum force. The electronic structure (ES) was calculated on-the-fly with a density functional tight binding (DFTB) method. This QTES-DFTB method scales linearly with number of trajectories, and the calculation of the quantum force is a small addition to the overall cost of trajectory dynamics. The active site was represented as a 44-atom system. Quantum effects were included only for the transferring proton, and substrate nuclei were treated classically. The effect of substrate vibrations was evaluated by freezing or relaxing the substrate nuclei. Trajectory calculations were performed at several temperatures ranging from 250-350 K, and rate constants were calculated through the quantum mechanical flux operator which depends on time-dependent correlation functions. It was found that the substrate motion reliably increases the rate constants, as well as the P/D kinetic isotope effect, by approximately 10\% across all temperatures examined. [Preview Abstract] |
Session W3: Multi-cellular Processes and Development
Sponsoring Units: DBIOChair: Shane Hutson, Vanderbilt University
Room: 107
Thursday, March 6, 2014 2:30PM - 2:42PM |
W3.00001: On Growth and Form of the Zebrafish Gut Microbiome Matthew Jemielita, Michael Taormina, Annah Rolig, Adam Burns, Jennifer Hampton, Karen Guillemin, Raghuveer Parthasarathy The vertebrate gut is home to a diverse microbial community whose composition has a strong influence on the development and health of the host organism. Researchers can identify the members of the microbiota, yet little is known about the spatial and temporal dynamics of these microbial communities, including the mechanisms guiding their nucleation, growth, and interactions. We address these issues using the larval zebrafish (\textit{Danio rerio}) as a model organism, which are raised microbe-free and then inoculated with controlled compositions of fluorophore-expressing bacteria. Live imaging using light sheet fluorescence microscopy enables visualization of the gut's entire microbial population over the first 24 hours of colonization. Image analysis allows us to quantify microbial populations that range from a few individuals to tens of thousands of microbes, and analyze the structure and growth kinetics of gut bacterial communities. We find that genetically-identical microbes can show surprisingly different growth rates and colonization abilities depending on their order of arrival. This demonstrates that knowing only the constituents of the gut community is insufficient to determine their dynamics; rather, the history of colonization matters. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W3.00002: Wave Propagation in Expanding Cell Layers Kazage J Christophe Utuje, Shiladitya Banerjee, M. Cristina Marchetti The coordinated migration of groups of cells drives important biological processes, such as wound healing and morphogenesis. In this talk we present a minimal continuum model of an expanding cell monolayer coupling elastic deformations to myosin-based activity in the cells. The myosin-driven contractile activity is quantified by the chemical potential difference for the process of ATP hydrolysis by myosin motors. A new ingredient of the model is a feedback of the local strain rate of the monolayer on contractility that naturally yields a mechanism for viscoelasticity of the cellular medium. By combining analytics and numerics we show that this simple model reproduces qualitatively many experimental findings, including the build-up of contractile stresses at the center of the cell monolayer, and the existence of traveling mechanical waves that control spreading dynamics and stress propagation in the cell monolayer. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W3.00003: Assessing ODE models of tumor growth Hana Dobrovolny, Hana Jaafari, Michael Ellis Mathematical models are often used to study and optimize treatment of cancer. In order to accurately predict the efficacy of a particular treatment, the model must correctly describe tumor growth. Over the years, several differential equation models of tumor growth have been proposed and independently fit to experimental data sets. While all the models provide reasonable fits to tumor growth data, the models have never been confronted with the same experimental data to determine whether any of the models provides a more accurate description of tumor growth. We collected tumor growth data from the literature and fit the various tumor growth models to the data to determine which model best describes tumor growth. Our results indicate that no single model can capture the variety of growth behavior captured in experiments. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W3.00004: Villification of the gut Tuomas Tallinen, Amy E. Shyer, Clifford J. Tabin, L. Mahadevan The villi of the human and chick gut are formed in similar stepwise progressions, wherein the mesenchyme and attached epithelium first fold into longitudinal ridges, then a zigzag pattern, and lastly individual villi. We combine biological manipulations and quantitative modeling to show that these steps of villification depend on the sequential differentiation of the distinct smooth muscle layers of the gut, which restrict the expansion of the growing endoderm and mesenchyme, generating compressive stresses that lead to their buckling and folding. Our computational model incorporates measured elastic properties and growth rates in the developing gut, recapitulating the morphological patterns seen during villification in a variety of species. Our study provides a mechanical basis for the genesis of these epithelial protrusions that are essential for providing sufficient surface area for nutrient absorption. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W3.00005: Energy barriers and cell migration in confluent tissues Dapeng Bi, J.H. Lopez, J.M. Schwarz, M. Lisa Manning Biological processes such as embryogensis, tumorigenesis and wound healing require cells to move within a tissue. While the migration of single cells has been extensively studied, it has remained unclear how single cell properties control migration through a confluent tissue. We develop numerical and theoretical models to calculate energy barriers to cell rearrangements, which govern cell motility. In contrast to sheared foams where energy barriers are power-law distributed, energy barriers in tissues are exponentially distributed and depend systematically on the cell's number of neighbors. Using simple extensions of `trap' and `Soft Glassy Rheology' models, we demonstrate that these energy barrier distributions give rise to glassy behavior and use the models to make testable predictions for two-time correlation functions and caging times. We incorporate these ideas into a continuum model that combines glassy rheology with active polarization to better understand collective migration in epithelial sheets. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W3.00006: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W3.00007: Mechanical analysis of a heat-shock induced developmental defect Sarah M. Crews, W. Tyler McCleery, M. Shane Hutson Embryonic development in~\textit{Drosophila}~is a complex process involving coordinated movements of mechanically interacting tissues. Perturbing this system with a transient heat shock can result in a number of developmental defects. In particular, a heat shock applied during the earliest morphogenetic movements of gastrulation can lead to apparent recovery, but then subsequent~morphogenetic failure 5-6 hours later during~germ band retraction. The process of germ band retraction requires an intact amnioserosa -- a single layered extra-embryonic epithelial tissue -- and heat shock at gastrulation can induce the later opening of~holes in the amnioserosa. These holes are highly correlated with failures of germ band retraction. These holes could be caused by a combination of~mechanical weakness in the amnioserosa~or~local increases in mechanical~stress.~ Here, we assess the role of mechanical stress using confocal imaging to compare cell and tissue morphology in the amnioserosa of normal and heat-shocked embryos and laser hole drilling to map the stress field around the times and locations at which heat-shock induced holes open.~ [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W3.00008: Predictive modeling of multicellular structure formation by using Cellular Particle Dynamics simulations Matthew McCune, Ashkan Shafiee, Gabor Forgacs, Ioan Kosztin Cellular Particle Dynamics (CPD) is an effective computational method for describing and predicting the time evolution of biomechanical relaxation processes of multicellular systems. A typical example is the fusion of spheroidal bioink particles during post bioprinting structure formation. In CPD cells are modeled as an ensemble of cellular particles (CPs) that interact via short-range contact interactions, characterized by an attractive (adhesive interaction) and a repulsive (excluded volume interaction) component. The time evolution of the spatial conformation of the multicellular system is determined by following the trajectories of all CPs through integration of their equations of motion. CPD was successfully applied to describe and predict the fusion of 3D tissue construct involving identical spherical aggregates. Here, we demonstrate that CPD can also predict tissue formation involving uneven spherical aggregates whose volumes decrease during the fusion process. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W3.00009: Charting the Vasculome: Uncovering the Principles of Vascular Organization Jacob Oppenheim, Marcelo Magnasco The efficient distribution of resources in any system requires a carefully designed architecture that is both space filling and efficient. While the principles of such networks are beginning to be uncovered in plants, they remain poorly elucidated in the case of higher animals. We have developed a high-throughput, easily implemented method of mapping vascular networks in mammalian tissue. By combining high resolution, rapid fluorescence blockface imaging with serial sectioning, we are able to map the vasculature of the rat liver at a resolution of 10 microns, revealing the structure above the level of the capillaries, constituting the largest vascular dataset yet assembled. We have developed algorithms for the efficient three-dimensional reconstruction from two-dimensional images, allowing skeletonization and investigation of its geometry and topology. We are able to calculate the scaling properties of these networks as well as the frequency of loops at each level. Using sophisticated topological tools, we are beginning to elucidate the principles of their organization. Ultimately, a greater understanding of vasculature is necessary for the success of efforts in synthetic and regenerative biology along with the better understanding of the growth and development of cancers. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W3.00010: Characterizing Loopy Biological Distribution Networks in Three Dimensions Carl Modes, Eleni Katifori, Marcelo Magnasco In order to develop useful predictive models for vascular or other biological distribution networks that include the effects of network architecture, development, and topology some set of tools must be chosen to characterize vasculature in a physically relevant, mathematically compact way. Few such tools are extant. To address this issue we have generalized the existing two dimensional leaf venation characterization to a fully three dimensional setting, from whence it may be brought to bear on many problems in human and mammalian vasculature, particularly where that vasculature is extremely complex, as in the organs. The new algorithm rests on the abstraction of the physical `tiling' from the two dimensional case to an effective, statistical tiling of an abstract surface that the network may be thought to sit in. Generically these abstract surfaces are richer than the flat plane and as a result there are now two families of fundamental units that may aggregate upon cutting weakest links -- the plaquettes of the tiling and the longer `topological' cycles associated with the abstract surface. Upon sequential removal of these weakest links, two characterizing trees emerge that condense most of the relevant information from the full network. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W3.00011: Fundamental limits to the precision of multicellular sensing Andrew Mugler, Matthew D. Brennan, Andre Levchenko, Ilya Nemenman Recent experiments suggest that connected cells detect shallower chemical gradients together than alone, and that this enhanced sensitivity is lost when cell-to-cell communication is blocked. Here we derive the fundamental limits to the precision of gradient sensing by a chain of communicating cells. We show using linear response theory how precision is limited by the external diffusion of signaling molecules, by the exchange of messenger molecules between cells, and by the number of cells in the chain. We discuss how our predictions could be compared with ongoing experiments. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W3.00012: Noise Sensitivity in Force-inference Techniques David N. Mashburn, M. Shane Hutson, Jim H. Veldhuis, G. Wayne Brodland Forward finite-element modeling of developmental processes has vastly improved our understanding of morphogenesis; however, determining the model parameters necessary to reproduce in vivo behaviors typically requires either computationally-intensive parameter searches or somewhat arbitrary user selections. To bypass these difficulties, we previously developed an inverse technique called Video Force Microscopy (VFM) that takes positional information from microscopy data and infers the forces necessary to reproduce the observed dynamics. Applying VFM to the output of a forward model produces exact results, but the over-determined solutions for any real data will generally have a nonzero residual error. Applying VFM to real images requires converting the image into a segmented mesh of polygonal cells, a process in which the image resolution creates inherent positional noise and the choice of mesh nodes influences the angles used in the force balance equations. We have investigated the robustness and quality of VFM solutions by analyzing sensitivity to both the unavoidable positional noise and addition/removal of mesh nodes. We have also evaluated the accuracy of both the residual and the matrix condition numbers as predictors of the true error (as measured against a gold standard). [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W3.00013: A Mechanical and Biochemical Model of Intimal Atherosclerotic Lesions Pak-Wing Fok, Rebecca Vandiver We investigate a 1D axisymmetric model of intimal hyperplasia using hyperelasticity theory. Our model incorporates growth of the intima due to cell proliferation which in turn is driven by the release of a cytokine such as Platelet-Derived Growth Factor (PDGF). The main novelty of our model is that the growth rate is tied to local stresses and the local concentration of PDGF. The resulting system is a triple free boundary problem with different regions of the vessel wall having different homeostatic stress, depending on the local PDGF concentration. This system is coupled to force-balance equations that yield distributions for the stress and deformation. We find that rapid intimal thickening coupled to a quiescent media puts the intima in a state of compression and results in slow time scales of evolution. Our results are compared with intima-media thickness measurements of carotid arteries from previous clinical studies. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W3.00014: The Mechanics of Cell Intercalation Madhav Mani, Boris Shraiman, Thomas Lecuit Cell-intercalation involves the cytoskeleton-driven exchange of cellular neighbors. Developmental cues produce directional biases in the pattern of neighbor-exchanges, resulting in the alteration of tissue shape -- morphogenesis. Focusing on cell-intercalation during early fly development, I will address both static and dynamical aspects. A quantitative correspondence is drawn between cytoskeletal levels, stresses and geometry. This construction of a constitutive law, relies on a novel image analysis tool that infers mechanical features of the cellular lattice from live imaging (from the Lecuit Lab, Marseilles). Building on our understanding of these static aspects, we construct a phenomenological, and physically-motivated, model for cytoskeletal remodeling based on temporal correlation analyses. This model predicts the qualitative phases of junctional states, insights into the T1 event that mediates intercalation, and several of the collective properties of cell-intercalation that have remained unaddressed so far -- we go on to validate these predictions. We conclude with introducing the idea that tissue-wide anisotropies, central to morphogenesis and patterning in the embryo, can emerge as a consequences of the collective aspects of mechanical interactions. [Preview Abstract] |
Session W4: Focus Session: Domain Dynamics
Sponsoring Units: GMAGChair: John Cumings, University of Maryland
Room: 112/110
Thursday, March 6, 2014 2:30PM - 2:42PM |
W4.00001: Thermal effects in artificial spin ice Jasper Drisko, Stephen Daunheimer, John Cumings Frustrated systems, typically characterized by competing interactions that cannot all be simultaneously satisfied, are ubiquitous in nature and display many rich phenomena and novel physics. Artificial spin ices (ASIs), arrays of lithographically patterned Ising-like single-domain magnetic nanostructures, are highly tunable systems that have proven to be a novel method for studying the effects of frustration and associated phenomena. Recently, thermal activation of ASI systems has been demonstrated [1, 2], introducing the spontaneous reversal of individual magnets and allowing for new explorations of novel phase transitions and phenomena using these systems. We have fabricated ASI samples made from thin film FePd$_{\mathrm{3}}$, which possesses simultaneously a high magnetic moment and a relatively low Curie temperature, and we investigate them using Lorentz Transmission Electron Microscopy. We observe thermally activated reversal of individual magnets when they are heated close to the Curie temperature of the film. Our analysis of Kagome spin ice arrays reveals signatures of competing interactions between vortex formation on the edges of the structures and magnetic charge ordering within the theoretically predicted Kagome ice-II state [3]. These observations are consistent with the emergent frustration of these systems. \\[4pt] [1] S. Zhang et al., \textit{Nature }\textbf{500}, 553-557 (2013) \\[0pt] [2] A. Farhan et al., \textit{Nature Phys}. \textbf{9}, 375-382 (2013) \\[0pt] [3] G.-W. Chern et al., \textit{Phys. Rev. Lett. }\textbf{106}, 207202 (2011) [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W4.00002: Role of Cu in Intrinsic Exchange Bias in FeMn Igor V. Roshchin, Pavel N. Lapa, Dogan Kaya, E. Skoropata, J. Van Lierop, H. Belliveau, P. Jayathilaka, Casey W. Miller We report studies of the role of Cu in the exchange bias (EB) in FeMn. The multilayers of Ta(50 {\AA})/[FeMn(50$-$450 {\AA})/Cu(50 {\AA} )]$_{10}$/Ta(50 {\AA}) exhibit EB while Ta(50 {\AA})/[FeMn(50$-$450 {\AA})/Ta(50 {\AA})]$_{10}$ show no EB. This ``\textit{intrinsic}'' EB occurs between pinned and unpinned uncompensated magnetization (UM) in FeMn. The unpinned UM is distributed uniformly throughout FeMn [1]. Since the magnitude of H$_{e}$ scales with the inverse thickness of FeMn, from Malozemoff's model for the bilayer EB systems, the pinned magnetization should be located near the interface of FeMn. This is consistent with the required for the EB presence of Cu. To test if Cu diffuses into FeMn, M\"{o}ssbauer spectroscopy is performed using the naturally occurring $^{57}$Fe in FeMn. The spectra of the samples consisting of one FeMn layer (50 and 150 {\AA}) with and without Cu show two components: One corresponds to the ``bulk'' FeMn, while the other is attributed to Fe-rich areas of FeMn. These areas are likely to be the source of the unpinned UM. No measurable difference in these spectra for the samples with and without Cu indicates that Cu is unlikely to diffuse into FeMn, contrary to the previously proposed hypothesis [1].\\[4pt] [1] D. Kaya \textit{et al.}, J. Appl. Phys., \textbf{113}, 17D717 (2013). \newline [2] A. P. Malozemoff, Phys. Rev. B \textbf{35}, 3679 (1987), \textit{ibid}., \textbf{37}, 7673 (1988). [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W4.00003: Anomalous magnetization dynamics in artificial spin ice S.K. Mishra, V.S. Bhat, D.H. Parks, J.T. Lee, X. Shi, L.E. DeLong, S.D. Kevan, S. Roy Modern nanotechnology permits one to mimic bulk spin ice crystals with ordered arrays of ferromagnetic nano-islands, which constitute a method for designing fully controlled model systems for studies of frustrated magnetic interactions in two dimensions. Thermal fluctuations within a highly degenerate ground state excite topological magnetic charges, but the nature of the field-dependent equilibrium states and their magnetic dynamics has remained elusive. Here we present results of time dependent, coherent speckle intensity and X-ray correlation spectroscopy (XPCS) in a diffraction geometry, which yields information concerning the dynamics of emergent topological charge defects in a square artificial spin ice. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W4.00004: Simulations of magnetic reversal in continuously distorted artificial spin ice lattices Barry Farmer, Vinayak Bhat, Justin Woods, J. Todd Hastings, Lance De Long Artificial spin ice (ASI) systems consist of lithographically patterned ferromagnetic segments that behave as Ising spins. The honeycomb lattice is an ASI analogue of the Kagom\'{e} spin ice lattice found in bulk pyrochlore crystals. We have developed a method to continuously distort the honeycomb lattice such that the pattern vertex spacings follow a Fibonacci chain sequence. The distortions break the rotational symmetry of the honeycomb lattice and alter the segment orientations and lengths such that all vertices retain three-fold coordination, but are no longer equivalent. We have performed micromagnetic simulations (OOMMF) of magnetization reversal for many samples having different strengths of distortion, and found the kinetics of magnetic reversal to be dramatically slowed, and avalanches (sequential switching of neighboring segments) shortened by only small deviations from perfect honeycomb symmetry. The coercivity increases as the distortion is strengthened, which is consistent with the retarded reversal. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W4.00005: Study of dipolar interaction between nano-disks Megha Chadha, Stephanie K. Walton, Katharina Zeissler, David M. Burn, Solveig Felton, Lesley F. Cohen, Will R. Branford Ferromagnetic nano-dot arrays are interesting for data storage applications, but as the density of disks becomes high the dipolar interactions between disks become strong. In this work we study lithographically prepared arrays of densely packed single domain perm-alloy nano-disks where the dipolar correlations are significant. We study the collective magnetic array properties for different array geometries and varying disk separation and explore the effect of magnetic frustration in these systems. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W4.00006: Control and influence of domain wall chirality in Artificial Spin Ice Stephanie Walton, Katharina Zeissler, Sam Ladak, Dan Read, Tolek Tyliszczak, Lesley Cohen, Will Branford Artificial Spin Ice, comprising ferromagnetic nanobars arranged in a honeycomb geometry, is a directly imageable frustrated system which has demonstrated rich Physics. Its magnetic reversal is mediated by domain wall propagation in the presence of external magnetic fields. These domain walls carry magnetic charge and have a distinct structure or ``chirality,'' namely up or down in the transverse domain wall regime or clockwise or anticlockwise in the vortex domain wall regime. In this talk, both experimental Scanning Transmission X-ray Microscopy and micromagnetic simulations which suggest that the domain wall performs a non-random walk through Artificial Spin Ice due to its chirality are presented. In addition, the role of Walker Breakdown in both the transverse and vortex domain wall regimes is discussed. Furthermore, modes of controlling and measuring domain wall chirality are explored. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W4.00007: Electrical transport measurements on honeycomb artificial spin ice. Katharina Zeissler, Megha Chadha, Lesley Cohen, Will Branford Artificial spin ice is a macroscopic playground for magnetically frustrated systems. We have previously shown that in a cobalt honeycomb artificial spin ice composed of 1 micron long nanowires there are unusual features in the magnetotransport below 50K. Here we explore the low temperature transport of equivalent artificial spin ice structures fabricated from permalloy. We discuss the extent to which the phenomenon is generic to the honeycomb artificial spin ice geometry and the effect of changing the constituent material on the onset temperature and the magnitude of the magnetotransport effect. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W4.00008: Electronic Transport Study of Connected Artificial Kagome Spin Ice D.W. Rench, B.L. Le, P.E. Lammert, R. Misra, V.H. Crespi, N. Samarth, P. Schiffer We present experimental and computational results of magnetotransport in connected ferromagnetic nanowire arrays (connected artificial spin ice). We probed the artificial kagome spin ice lattice using AC transport techniques as a function of applied magnetic field strength and angle and compared these results to calculated transport properties based on OOMMF computational modeling. We find that many of the transport properties observed experimentally can be described in a simple manner using the Anistropic Magnetoresistance (AMR) model for individual nanowires and then calculating the net resistance using classical circuit analogues. Supported by the US Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division under grant number DE-SC0005313. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W4.00009: Dynamic Magneto-Optical Kerr Imaging of Perpendicular Anisotropy Artificial Spin Ice Geometries Robert Fraleigh, Paul Lammert, Vin Crespi, Nitin Samarth, Ian Gilbert, Peter Schiffer We present a spatially resolved magneto-optical Kerr imaging study on the magnetization reversal, as a function of applied field, of patterned arrays of perpendicular anisotropy single domain islands. Patterns are made of large collections of CoPt multilayer islands with frustrated (Kagome, triangular) and unfrustrated (square, hexagonal) geometries. Field induced switching is imaged with a Kerr imaging apparatus equipped with an objective lens that allows for diffraction limited spatial resolution as low as 250nm and imaging acquisition as fast as 12 frames/second. The magnetization reversal process is probed by varying lattice spacing, geometry, and artificial defects in the patterned arrays. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W4.00010: Edge mode spectroscopy and imaging for film edge properties in magnetic nanostructures Invited Speaker: Robert McMichael Lithography is an act of violence. Often, films are almost entirely obliterated by patterning, leaving only nanostructures behind with film edges that have borne the brunt of the damage, edges that carry with them the scars of energetic ion bombardment, reactive ions, liftoff and exposure to ambient conditions. In this talk, I will present a variation on ferromagnetic resonance force microscopy that can provide insight into the magnetic properties of film edges in magnetic nanostructures. The method relies on the non-uniformity of the magnetic field in patterned-film nanostructures that are magnetized in-plane, specifically, the low-field regions that form near where the magnetization is directed normal to the edge. In these regions, localized precession forms as trapped spin wave modes, and the resonance condition of these modes serves as an indicator of the edge properties. I will present modeling and measurements on a 500~nm diameter, 25~nm thick Permalloy disk to illustrate the method. Micromagnetic modeling of this disk predicts a main mode that is nearly uniform across the sample and three localized edge modes with higher resonance fields. The spectra measured with various tip positions and mode imaging are consistent with the modeling results. In addition to a strong center mode, three distinct edge modes are observed when the tip is near the disk edge. For a symmetric disk, the modeling predicts that the edge mode resonances are identical on the two opposite edges. However, the measured edge mode resonances on opposite edges of the disk are detected at different resonance fields, suggesting inhomogeneity of the edge properties. By rotating the applied field, we control the position of the localized edge mode along the edge of the disk and confirm that the edge mode resonance field has a strong angular dependence, showing that edge mode properties can vary significantly in a nominally circular disk. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W4.00011: FMR Study of an Eightfold Artificial Quasicrystal Lance De Long, Vinayak Bhat, Joseph Sklenar, Barry Farmer, Justin Woods, John Ketterson, Todd Hastings We have performed DC magnetization, and broad-band and narrowband FMR measurements on eightfold-rotationally-symmetric artificial quasicrystals. Permalloy films of thickness 25 nm were patterned with 1$^{\mathrm{st}}$ and 4$^{\mathrm{th}}$ generation \textbf{\textit{Ammann tilings}} (AT) [1] using standard electron beam lithography. The AT can be viewed as an antidot lattice of squares and rhombi whose edges are film segments of length 1000 nm (7 $\mu $m), and width 130 nm (910 nm), respectively, in 4$^{\mathrm{th}}$ (1$^{\mathrm{st}})$ generation AT. In spite of clear DC magnetization hysteresis in the low-field regime, we observed remarkably sharp and reproducible FMR spectra (including both the low-field-reversal and the saturated regimes) that strongly reflected the geometry of the AT. The applied DC field \textbf{H} could be oriented in-plane at an angle $\varphi $ with respect to a AT reference axis. Our FMR spectra exhibit the expected eight-fold symmetry of the AT for experimentally accessible RF frequencies (7 to 18.5 GHz). Static and dynamic micromagnetic simulations were in good agreement with our experimental FMR spectra. \\[4pt] [1] B. Gr\"{u}nbaum and G. C. Shephard, \textit{Tilings and Patterns} (Freemann, New York, 1986). [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W4.00012: Simultaneously generated spin waves in the magnetostatic backward volume wave and magnetostatic surface wave configuration in a cross-like structure Jason Liu, Grant Riley, Ferran Macia, Andrew Kent, Kristen Buchanan Spin waves, or magnons, in laterally confined microstrips have attracted a great deal of attention recently due to their potential for magnonic logic applications. Previous experimental work on spin wave propagation in metallic magnetic nanowires has focused on the magnetostatic surface wave (MSSW) configuration where the static magnetic field is applied in-plane, perpendicular to the nanowire, because they can be excited relatively easily by an antenna. Spin wave propagation in the less studied magnetostatic backward volume wave (MSBVW) configuration where the magnetization direction is along the nanowire is, however, also of interest because spin waves can propagate without the need for an external field in this geometry. In this work, micro-Brillouin light scattering (micro-BLS) was used to investigate the generation of propagating spin waves in a cross-like Permalloy structure that allows for simultaneous excitation of MSSW in one of the wires and MSBVW in the other. Micro-BLS measurements were conducted as a function of applied field and pumping frequency to probe the efficiency of the generation of the two types of spin waves. Two dimensional spatial profiles were obtained to explore possible interference of the two types of spin waves at the center of the cross-like structure. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W4.00013: Lattice Symmetry Breaking of Spin Wave Propagation in Two-Dimensional Magnonic Crystals Glade Sietsema, Michael E. Flatt\'e We solve the Landau-Lifshitz-Gilbert equation for spin waves in a two-dimensional magnonic crystal using the plane wave expansion method[1]. In doing this we have found that the inclusion of the dipolar field in the LLG equation results in the dispersion relations and linewidths having a lower symmetry than the crystal latice. The magnitude of this symmetry breaking is determined by the strength of the dipolar field relative to the exchange field. Adjusting the crystal parameters can change the relative strength of these fields, thereby allowing this effect to be enhanced or reduced. We have also calculated the Green's functions for this system, which show highly directional propagation of the spin waves depending on the excitation frequency.\\[4pt] [1]arXiv:1111.2506 [Preview Abstract] |
Session W6: Focus Session: Emergent Properties in Bulk Complex Oxides: Strongly Spin-Orbit Coupled Systems
Sponsoring Units: DMPChair: Mark Dean, Brookhaven National Laboratory
Room: 108
Thursday, March 6, 2014 2:30PM - 2:42PM |
W6.00001: Novel Magnetism from the Spin-Orbit-Coupling Strength Alternation Weiguo Yin, X. Liu, A.M. Tsvelik, M.P.M. Dean, M.H. Upton, Jungho Kim, D. Casa, A. Said, T. Gog, T.F. Qi, G. Cao, J.P. Hill We show that bringing close two magnetic ions with strong and weak spin-orbit coupling, respectively, can yield strong ferromagnetic anisotropy from antiferromagnetic superexchange. We applied this novel exchange anisotropy generating mechanism to explain the unique magnetism of the copper-iridium oxide Sr$_3$CuIrO$_6$ containing chains of alternating Cu(II) and Ir(IV) ions. The calculated large-gap spin excitation spectrum agrees well with the Ir $L_3$ edge resonant inelastic x-ray scattering experiment. Our findings point to novel magnetic behavior to be expected in mixed $3d-5d$ transition-metal systems via exchange pathways that are absent in pure $3d$ or $5d$ systems. Reference: W.-G. Yin et al., PRL 111, 057202 (2013). [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W6.00002: J$_{\mathrm{eff}}=$1/2 Mott spin-orbit insulating state close to the cubic limit in Ca$_{4}$IrO$_{6}$ S. Calder, G.-X. Cao, S. Okamoto, J.W. Kim, V.R. Cooper, Z. Gai, B.C. Sales, M.D. Lumsden, D. Mandrus, A.D. Christianson The J$_{\mathrm{eff}}=$1/2 Mott spin-orbit insulating state is manifested in systems with large cubic crystal field splitting and spin-orbit coupling that are comparable to the on-site Coulomb interaction, U. 5d transition metal oxides host parameters in this regime and recently strong evidence for this state in Sr$_{2}$IrO$_{4}$, and additional iridates, has been presented. All the candidates, however, have distorted octahedra, such as the elongation along the c-axis in Sr$_{2}$IrO$_{4}$, and consequently a non-cubic local crystal field environment. Consequently the materials form a mixed J$_{\mathrm{eff}}=$1/2,3/2 ground state. The lack of a material with an unmixed J$_{\mathrm{eff}}=$1/2 has impacted the development and testing of robust models of this novel insulating and magnetic state. We present neutron diffraction, resonant x-ray scattering and DFT calculations that not only reveal Ca$_{4}$IrO$_{6}$ is a new candidate J$_{\mathrm{eff}}=$1/2 material with long-range magnetic order, but furthermore resides close to the required cubic limit. Both our experimental and theoretical investigation indicate Ca$_{4}$IrO$_{6}$ is uniquely positioned to act as a canonical system to investigate of the J$_{\mathrm{eff}}=$1/2 state. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W6.00003: Quantum Critical Phases in Strong Spin-Orbit coupling Systems: Application to Pyrochlore Iridates Eun Gook Moon, Cenke Xu, Yong Baek Kim, Leon Balents We study quantum critical phases in strong spin-orbit coupling systems which are protected by underlying symmetry and topology. Three semi-metallic stable phases are considered, and the absence of the energy gap and density of states at the Fermi level induces non-trivial screening effect. Thus, the Ground states of the phases receive characteristic corrections from the long range Coulomb interaction. The standard renormalization group method is used to investigate properties of the ground states. Considering symmetry breaking terms, we obtain phase diagrams, and generic features of the quantum critical theories between the phases are discussed. We apply our theory to pyrochlore iridates and implication of the presence of the quantum critical phases is discussed. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W6.00004: Strongly spin-orbit coupled Mott insulators Invited Speaker: Hidenori Takagi |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W6.00005: Enhancement of spin-orbit interaction by electron correlation Hiroki Isobe, Naoto Nagaosa We discuss the interplay between relativistic spin-orbit interaction (SOI) and electron correlation, and report the enhancement of the effective strength of SOI by electron correlation. Here we consider a two-site model of $t_{2g}$ orbitals to show that there are cases where the SOI is effectively enhanced by Hund's coupling. Since the strong SOI usually requires heavy and rare elements, it is desirable to realize a strongly spin-orbit coupled electronic system with lighter and abundant elements, especially from the viewpoint of applications. This finding of the effective enhancement of SOI by electron correlation will pave the way for the realization of such systems. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W6.00006: Anisotropic magnetic interactions in 5d iridium oxides by many-body quantum chemistry calculations Vamshi M. Katukuri, Satoshi Nishimoto, Viktor Yushankhai, Ioannis Rousochatzakis, Liviu Hozoi, Jeroen van den Brink Ir 5d$^5$ oxides are being actively studied due to the realization of novel spin-orbit coupled j$_{eff}\approx$1/2 ground states. One remarkable feature in these compounds is the highly anisotropic magnetic interactions, orders of magnitude stronger than in 3d oxides. We address the nature of the anisotropic exchange in the 2D honeycomb (Na/Li)$_2$IrO$_3$ ((Na/Li)213) and square-lattice (Sr/Ba)$_2$IrO$_4$ ((Sr/Ba)213) iridates, by ab initio multireference configuration-interaction calculations on large embedded clusters. For Na213 we find that the Kitaev term is ferromagnetic and defines the dominant energy scale while the nearest-neighbor Heisenberg contribution is antiferromagnetic. Although Li213 is structurally similar, we predict quite different set of interaction parameters in Li213. We further analyze the magnetic order and the essential differences between these two materials by exact diagonalization and density-matrix renormalization-group calculations that additionally include 2nd and 3rd neighbor couplings. Sizable symmetric anisotropic interactions are also computed for Ba214. From the ab initio data, the relevant in-plane spin model for Ba214 turns out to be a Heisenberg-compass effective model. We finally discuss the Dzyaloshinskii-Moriya exchange in Sr214. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W6.00007: Tunnelling into the twisted Mott insulator Sr2IrO4 with atomic resolution Armin Ansary, John Nichols, Noah Bray-Ali, Gang Cao, Kwok-Wai Ng We studied the single-layered iridate Sr$_2$IrO$_4$ with a scanning tunneling microscope. The finite low temperature conductance enables the electronic structure of this antiferromagnetic Mott insulator to be measured by tunneling spectroscopy. We imaged the topography of freshly cleaved surfaces and measured differential tunneling conductance at cryogenic temperatures. We found the Mott gap in the tunneling density of states to be 2$\Delta$ = 615 meV. Within the Mott gap, additional shoulders are observed which are interpreted as inelastic loss features due to magnons. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W6.00008: Topological and magnetic phases in the honeycomb Iridates Stephan Rachel Iridates are amongst the most interesting complex oxide materials. The non-interacting band structure of the honeycomb Iridates has been claimed to feature the quantum spin Hall effect due to large spin orbit coupling. The true materials exhibit considerable Coulomb interactions leading to different types of magnetic order (e.g., zig-zag or spiral order). Here we show how one can obtain such magnetic phases by combining topological band structure and local Coulomb interactions into a topological Hubbard model which we analyze in detail. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W6.00009: Effective J=1/2 insulating state in Ruddlesden-Popper iridates: An LDA+DMFT study Hongbin Zhang, Kristjan Haule, David Vanderbilt Using {\it ab-initio} methods, we\footnote{H. Zhang, K. Haule and D. Vanderbilt, arXiv:1308.4471 (2013).} investigate the metal-insulator transition across the Ruddlesden-Popper (RP) iridates and explore the robustness of the effective $J\!=\!1/2$ insulating state\footnote{B.J. Kim {\it et al.}, Phys. Rev. Lett. {\bf 101}, 076402 (2008).} against band effects due to itineracy, tetragonal distortion, octahedral rotation and Coulomb interaction. The electronic structures we obtained are in good agreement with recent ARPES measurements.\footnote[3]{B.M. Wojek, {\it et al.}, J. Phys.: Condens. Matter {\bf 24}, 415602 (2012).}$^,$\footnote[4]{Q. Wang, {\it et al.}, Phys. Rev. B {\bf 87}, 245109 (2013).}$^,$\footnote[5]{Y. Nie, P. King, and K. Shen, private communication.} We predict the effects of epitaxial strain on the optical conductivity, magnetic moments and effective $J\!=\!1/2$ ground-state wave functions in the RP series. We demonstrate that the deviation from the ideal effective $J\!=\!1/2$ state is negligible at short time scales for both Sr$_2$IrO$_4$ and Sr$_3$Ir$_2$O$_7$, while it becomes quite significant for Sr$_3$Ir$_2$O$_7$ at long times and low energy, leading to a reconciliation of previous contradictory experimental results. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W6.00010: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W6.00011: Spin orbit coupled magnetism and transport in double perovskites Arun Paramekanti, Ashley Cook We consider a model of the double perovskite Ba$_2$FeReO$_6$, a room temperature ferrimagnet with correlated and spin-orbit coupled Re t$_{2g}$ electrons moving in the background of Fe moments stabilized by Hund's coupling. We show that for such 3d/5d double perovskites, strong correlations on the 5d-element (Re) are essential in driving a half-metallic ground state. Incorporating both strong spin-orbit coupling and the Hubbard repulsion on Re, we uncover (i) Weyl nodes in the band structure, (ii) a significant anomalous Hall effect with hole doping, and (iii) a large spin polarization at the Fermi level. We also obtain a semi-quantitative understanding of (i) the saturation magnetization of Ba$_2$FeReO$_6$, (ii) X-ray magnetic circular dichroism data indicating a significant orbital magnetization, and (iii) the tetragonal distortion accompanying ferrimagnetic order. Strong correlations also lead to local moment formation on Re, and the calculated dynamic spin structure within a local moment picture is in good agreement with neutron scattering experiments. We will also discuss generalizations to other 3d/5d oxides. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W6.00012: Spin waves in half-metallic double perovskite magnets via Dyson's method Kelly Reidy, Katherine Jones-Smith, Harsh Mathur A new method for treating strongly correlated electronic matter is applied to an idealized model of double exchange magnetism and to the half-metallic double perovskite $Sr_{2}FeMoO_{6}$, a conducting material that is magnetically ordered at room temperature and potentially useful for spintronics. Our method [1] is a supersymmetric generalization of Dyson's analysis of spin waves in ferromagnets wherein quasiparticles are governed by a non-Hermitian Hamiltonian. Using this method, we obtain the spin wave dispersion relation for the idealized model and for the double perovskite material. We also reformulate these problems using a supersymmetric generalization of Schwinger boson mean-field theory. The results will be compared to a semi-classical exact diagonalization Monte-Carlo analysis [2], and broader implications of the method of non-Hermitian quasiparticles will be discussed. [1] Katherine Jones-Smith, Phil. Trans. Roy. Soc. A28, 371 (2012). [2] O. Erten et al., Phys. Rev. Lett. 107, 257201 (2011) [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W6.00013: Noncollinear magnetic order in quadruple perovskite LaMn$_3$V$_4$O$_{12}$ Masayuki Toyoda, Kunihiko Yamauchi, Tamio Oguchi The microscopic mechanism of noncollinear magnetic order in quadruple perovskite LaMn$_3$V$_4$O$_{12}$ has been investigated by first-principles density-functional theory calculations. Unlike other provskite-type manganites, Mn ions are located in the A'-sites that are surrounded by four oxygens with square-planar geometry, whereas the octahedrally coordinated B-sites are occupied by magnetically inactive V ions. In order to understand the magnetic interactions between the Mn spins, magnetic exchange coupling constants are estimated by mapping the numerically calculated energies with constraints on spin orientations onto an effective Heisenberg model. The antiferromagnetic coupling between the 2nd-nearest neighbors is found to satbilize the noncollinaer magnetic order which is relatively stronger in the present compound than in other G-type antiferromagnetic quadruple perovskites such as YMn$_3$Al$_4$O$_{12}$ and LaMn$_3$Cr$_4$O$_{12}$. We will also discuss on possibility of the ferroelectricity in the present compound in analogy with CaMn$_7$O$_{12}$, which is a multiferroic quadruple perovskite with noncollinear magnetic order. [Preview Abstract] |
Session W7: Focus Session: Molecule-Based Magnets
Sponsoring Units: GMAGChair: Mark Meisel, University of Florida
Room: 106
Thursday, March 6, 2014 2:30PM - 2:42PM |
W7.00001: A DFT+DMFT study of spin crossover molecules Jia Chen, Chris Marianetti, Andrew Millis Recent studies have shown promise in applying dynamical mean-field theory (DMFT) based methods to molecular systems. Molecules with transition metals are an ideal target system as they may be complex enough to preclude a treatment via accurate quantum chemical methods, yet exhibit sophisticated many-body behaviour which eludes density functional theory (DFT). Here we apply DFT+DMFT to the molecular complex Fe(phen)2(NCS)2 which exhibits a spin crossover transition as a function of temperature. First, we show that DFT overestimates the stability of low spin state and hence yields an excessively large crossover temperature, in agreement with existing DFT studies. Subsequently, we present both DFT+U and DFT+DFMT to show the effect of treating local correlations. When using the standard double-counting and an on-site Coulomb repulsion computed within linear response, we show that both methods excessively favour the high-spin state. Alternate approaches to the methodology are discussed to remedy this deficiency. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W7.00002: Photoinduced Magnetism in Heterostructures of a Hofmann-like Framework and a Prussian Blue Analogue M.K. Peprah, P.A. Quintero, T.V. Brinzari, M.W. Meisel, B. Hosterman, M. Sendova, C.R. Gros, D.R. Talham Heterostructured films of the Prussian blue analogue (PBA), K${}_{k}$Ni[Cr(CN)${}_{6}$]${}_{l}$$\cdot$nH${}_{2}$O, and the 3D Hofmann-like Fe(azpy)[Pt(CN)${}_{4}$]$\cdot$mH${}_{2}$O (azpy = 4,4$^\prime$-azopyridine) spin crossover system have been studied by magnetometry and Raman spectroscopy. The magnetization of the ferromagnetic NiCr-PBA, $T_c$~$\approx$ 70~K, can be altered by white light irradiation when coupled with the photoactive Fe-Pt framework. The effect is attributed to interface strain that develops when cooling from room temperature. This lattice distortion is relaxed when irradiation causes the Fe(II) to experience a low spin $(S=0)$ to high spin $(S=2)$ transition at $T\leq 50$~K. This work extends our recent photo-controlled magnetic heterostructures studies\footnote{D. M. Pajerowski \textit{et al.}, J. Am. Chem. Soc. \textbf{132} (2010) 4058; M. F. Dumont \textit{et al.}, Inorg. Chem. \textbf{50} (2011) 4295.} to include Fe(II) spin crossover complexes. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W7.00003: Transport Properties of Films of Cobalt Iron Prussian Blue Analogues Pedro A. Quintero, Mark W. Meisel, Olivia N. Risset, Daniel R. Talham The magnetic and transport properties of films of the bistable, photomagnetic, cobalt iron Prussian blue analogue A$_j$Co$_k$[Fe(CN)$_6$] (A = Na, K, Rb)\footnote{O. Sato \emph{et al}., Science \textbf{272} (1996) 704; O. Sato \emph{et al}., J. Am. Chem. Soc. \textbf{126} (2004) 13176.} on ITO and FTO substrates as a function of temperature \mbox{(100 - 300 K)} and under white light irradition have been studied. The magnetic data show the charge transfer induced spin transition (CTIST) between \mbox{230 - 280 K.} The DC and AC transport measurements also show a transition between different conduction states in the same temperature \mbox{region.} Specifically, the AC data (1 kHz) reveals a smooth hysteresis, which is similar to the response observed in the magnetic data. Upon irradiation with white light, swithching is observed between the two conduction states at low temperature \mbox{T $\sim$ 120 K}. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W7.00004: Tuning spatial exchange and single-ion anisotropies in low-dimensional Ni(II) quantum magnets Invited Speaker: Jamie Manson Spatial exchange anisotropy is key to determining $J$'/$J$ and $T_{N}$/$J$ critical ratios and unveiling $B$/$T$ phase diagrams in low-dimensional quantum magnets. Systematic design strategies have enabled us to synthesize a series of model $S=$1 Ni(II) systems whereby this anisotropy, in combination with the single-ion anisotropy, can be tuned by adjusting the nature of the coordinating ligands. For example, the coordination polymers [Ni$L_{x}$(pyz)$_{2}$]$Y$ (pyz $=$ pyrazine; $L=$ HF$_{2}$, $x=$ 1, $Y=$ PF$_{6}$, SbF$_{6}$; $L=$ Cl, Br, I, $x=$ 2, $Y=$ nil), possess 2D [Ni(pyz)$_{2}$]$^{2+}$ square lattices that are spaced apart by bridging or non-bridging $L$ anions such that 1.7 $\le T_{N}\le $ 12 K depending on the magnitude of $J$'. Chemical substitution of pyz for other organic ligands leads to quasi-1D [Ni(HF$_{2})$(3-Clpy)$_{4}$]BF$_{4}$ (Clpy $=$ chloropyridine) and the 2D Kagome lattice [Ni(H$_{3}$F$_{4})$(3-Fpy)$_{4}$]SbF$_{6}$ (Fpy $=$ fluoropyridine) which contain HF$_{2}^{-}$ or H$_{3}$F$_{4}^{-}$ bridges, respectively. Furthermore, the inherent flexibility of strong F\textbullet \textbullet \textbullet H\textbullet \textbullet \textbullet F and O-H\textbullet \textbullet \textbullet F bonds also renders them highly sensitive to external stimuli such as high pressure. Time permitting, these examples and others will be presented. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W7.00005: Magnetism in a family of $S=$ 1 square lattice antiferromagnets, Ni$X_{2}$(pyz)$_{2}$ ($X=$ Cl, Br, NCS; pyz $=$ pyrazine) J. Liu, J.L. Manson, T.J. Woods, K.E. Carreiro, H.E. Tran, S.H. Lapidus, K.H. Stone, P.W. Stephens, Y. Kohama, J.S. Moeller, F.L. Pratt, P.J. Baker, T. Lancaster, A. Ardavan, S.J. Blundell, J. Singleton, P.A. Goddard The crystal structures of Ni$X_{2}$(pyz)$_{2}$ ($X=$ Cl, Br and NCS, henceforth Ni-Cl, Ni-Br and Ni-NCS, respectively), were determined at 298 K from synchrotron powder X-ray diffraction data. All three compounds consist of two-dimensional (2D) [Ni(pyz)$_{2}$]$^{2+}$ square lattices spaced by $X$ ligands, resulting a staggered stacking fashion of 2D layers. Long-range antiferromagnetic order occurs below 1.5 (Ni-Cl) and 1.9 K (Ni-Br and Ni-NCS) as determined by heat capacity and Muon-spin relaxation. The single-ion anisotropy and $g$ factor of Ni-Br and Ni-NCS were measured by electron spin resonance where no zero-field splitting was found. The magnetism of Ni-NCS is interpreted by the 3D simple cubic Heisenberg model with the Ni-pyz-Ni interaction $J_{pyz}=$ 0.70 K. A good overall agreement was found between the pulsed field magnetization data, magnetic susceptibility and $T_{N}$ for Ni-NCS. Ni-Cl and N-Br are characterized as quasi-2D antiferromagnets with the interlayer magnetic coupling significantly suppressed by varying the $X$ ligand. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W7.00006: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W7.00007: Evolution of magnetic interactions in a pressure-induced Jahn-Teller driven magnetic dimensionality switch Johannes Moeller, Saman Ghannadzadeh, Paul Goddard, Tom Lancaster, Fan Xiao, Stephen Blundell, Alexander Maisuradze, Rustem Khasanov, Jamie Manson, Stan Tozer, David Graf, John Schlueter Much of the research in molecular magnetism focusses on low-dimensional magnetic systems. Here we discuss the interesting possibility of controlling the magnetic dimensionality in a molecular magnet by driving the system through a quantum critical point using applied pressure. We present the results of muon-spin relaxation measurements and high-field magnetisation experiments on the coordination polymer CuF$_2$(H$_2$O)$_2$(pyrazine) in pressures up to 22.5 kbar that demonstrate a transition from a quasi-two-dimensional to a quasi-one-dimensional antiferromagnetic phase driven by a rotation of the Jahn-Teller axis at 9.1 kbar. Antiferromagnetic ordering is observed in both regimes. The dimensionality switch is accompanied by a halving of the primary magnetic exchange energy $J$ and a fivefold decrease in the ordering temperature $T_{\mathrm{N}}$. Density-functional theory calculations of the spin density and muon sites are used to complement the experimental data. Part of this work is published in S. Ghannadzadeh et al., Phys. Rev. B 87, 241102 (R). [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W7.00008: Pressure-Induced Magnetic Crossover Driven by Hydrogen Bonding in CuF$_{2}$(H$_{2}$O)$_{2}$(3-chloropyridine) Kenneth O'Neal, Tatiana Brinzaria, Joshua Wright, Janice Musfeldt, Santanab Giri, Qian Wang, Puru Jena, John Schlueter, Zhenxian Liu We combined high pressure vibrational spectroscopy with complementary dynamics calculations to investigate the 0.8 GPa pressure driven magnetic crossover in CuF$_{2}$(H$_{2}$O)$_{2}$(3-chloropyridine). Our work reveals that compression forces the 3-chloropyridine ring closer to the H$_{2}$O ligands, resulting in the formation of --Cl$^{...}$H$_2$O hydrogen bonds. These new intermolecular hydrogen bonds act as ancillary superexchange pathways between copper centers, introducing a third dimension to the hydrogen bonding network and triggering the antiferromagnetic to ferromagnetic crossover. These findings are important for understanding magnetoelastic coupling in quantum magnets and other multifunctional materials in which hydrogen bond formation facilitates magnetic switching. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W7.00009: Optical and Raman Study of $\alpha$-RuCl$_3$ Luke Sandilands, Anjan Reijnders, Yao Tian, Kemp Plumb, Jennifer Sears, Vijay Venkataraman, Hae-Young Kee, Young-June Kim, Kenneth Burch We report on the optical and Raman spectroscopy of the layered honeycomb magnet $\alpha$-RuCl$_3$. Anomalies in the phonon spectrum reveal a structural phase transition near 150K and we also also observe a series of Ru $d-d$ excitations in the range 0.1 to 3.5 eV . We compare our measurements with LDA calculations of the electronic structure and discuss the possibility of $J_{eff}$ physics in this compound. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W7.00010: Single-Crystal Growth of the Low-Dimensional Antiferromagnet NiTa$_2$O$_6$ Aaron Schye, Sueli Masunaga, J.J. Neumeier Single crystals of the low-dimensional antiferromagnet NiTa$_2$O$_6$ were prepared via the floating zone method and chemical vapor transport with TeCl$_4$ as the transport agent. X-ray powder diffraction was used to verify the sample purity and the single crystals were oriented using Laue diffraction. The results of magnetic susceptibility, specific heat, and thermal expansion measurements will be presented. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W7.00011: Transition from short-range to long-range antiferromagnetic order in single-crystalline CoSb$_2$O$_6$ Aaron B. Christian, A. Rebello, M.G. Smith, J.J. Neumeier Single-crystalline CoSb$_2$O$_6$ has been grown via chemical vapor transport. A broad peak in the magnetic susceptibility indicates short-range antiferromagnetic order. Anisotropy among the peak positions suggests a ratio of interchain coupling constants along the \textit{a}- and \textit{c}-axes of $J_a/J_c\sim 1.4$. A second order phase transition to long-range antiferromagnetic order is observed at 13.4 K as an abrupt change in slope. Heat capacity measurements suggest that local 1D magnetic order begins upon cooling below $\sim 70$ K. Thermal expansion measurements also exhibit anisotropic behavior that is affected by the short-range order. Unlike that of the \textit{a}-axis, the thermal expansion coefficient, $\mu$, for the \textit{c}-axis becomes negative below $\sim 70$ K due to anharmonic lattice vibrations resulting from the formation of short-range 1D antiferromagnetic order. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W7.00012: Growth and Transverse Field Muon Spin Rotation of Cobalt Niobate Timothy Munsie, Anna Millington, Casey Marjerrison, Teresa Medina, Murray Wilson, Edwin Kermarrec, Lian Liu, Hanna Dabkowska, Yasutomo Uemura, Travis Williams, Graeme Luke Cobalt niobate, CoNb$_{2}$O$_{6}$, is a material whose spins, when in a transverse field, act like the theoretical ideal 1D-Ising model. This occurs due to the magnetic spins aligning highly anisotropically along the Co$^{2+}$ chains. Because of this unique structure and material performance, the creation and characterization of this material is of both experimental and theoretical interest. The research we will present is a detailing of changes in the characteristics of the growth of the material utilizing the optical floating zone crystal growth method compared to previous growth parameters and an examination of this material in a moderately high transverse field using the technique of muon spin rotation ($\mu$SR). We have determined that the quality of crystals created by the floating zone are highly dependent on the growth parameters utilized (original ceramic shape and rotation rate) and dictate the speed at which the growth can be performed. Transverse Field $\mu$SR shows a gradual but significant change to the magnetic structure of the material below 5 K. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W7.00013: $\mu$SR Investigation of Transition Metal Doped IrTe$_2$ Murray Wilson, Jiaqiang Yan, David Mandrus, Timothy Munsie, Teresa Medina, Graeme Luke Materials with strong spin orbit coupling such as IrTe$_2$ frequently exhibit interesting magnetic and electronic properties. In particular, this material exhibits a structural phase transition at 270~K into a charge density wave state, which is supressed when the parent compound is doped with transition metals such as Pd, Fe, Pt, Ni, Mn, Cu, or Co. As this transition is supressed, superconductivity or magnetic order appears at low temperatures, depending on the metal dopant. These low temperature properties are of interest as the high spin-orbit coupling raises the possibility of topological superconducting states or exotic magnetic order. However, despite significant interest in recent years, the relationship between magnetism and superconductivity in this material system has not yet been well established. In this talk, we present an investigation of the properties of transition metal doped IrTe$_2$ by $\mu$SR which confirms spin glass behavior for Fe doping and probes the low temperature magnetic states for other transition metal dopants. [Preview Abstract] |
Session W8: Focus Session: Ferromagnetic Resonance and Damping
Sponsoring Units: GMAGChair: Hans Nembach, University of Colorado Boulder
Room: 104
Thursday, March 6, 2014 2:30PM - 2:42PM |
W8.00001: Correlation of spin-wave mode structure and shape imperfections in individual Ni80Fe20 nanomagnets via heterodyne magneto-optic microwave microscopy Hans Nembach, Justin Shaw, Carl Boone, Robert McMichael, Tom Silva It was recently shown that modes localized at the edges are sensitive to presumed defects. We measured localized spin-wave modes of individual Ni80Fe20 nanomagnets (NMs) with sizes ranging from 100 nm to 400 nm via heterodyne magneto-optical microwave microscopy. Comparison of field-swept spectra with micromagnetic simulations allows for identification of the observed spin-wave modes. One of the modes, the ``center-mode'', extends throughout the NM. The lowest order (highest resonance field) ``end-modes'' are localized at the ends of the nanomagnet. As such, it is expected that the end modes are more susceptible to edge defects. Spectra from nominally identical nanomagnets show that the resonance fields of the two end-modes vary substantially between nanomagnets.. We measured the lateral shape of the NMs with scanning electron microscopy, and then used the measured shapes to simulate the mode-spectra, but shape distortions cannot explain the observed mode distortions. Sidewall angle, re-deposition, and mill-induced edge-damage might also be important to accurately model end-mode distortions. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W8.00002: Splitting of spin-wave modes in magnetic nanostructures under axial symmetry violation Martha Pardavi-Horvath, Olena Tartakivska, Olga Salyuk During the last two decades the spin wave dynamics in magnetic nanostructures is a topic of intensive study. However, the analytical theoretical description of spin waves in such confined structures is a complicated problem. Due to the inhomogeneity of the internal demagnetizing field, it is possible to find exact eigenfunctions for only selected cases with the simplest geometries. For practical applications the direction of the external magnetic field may deviate from the symmetry axis. Additional spin-wave modes can be the source of magnetic noise. Therefore, it is important to investigate the evolution of the spin wave spectra in the case of symmetry violation. In this work the evolution of spin wave spectra of submicron circular dots and cylindrical nanowires have been studied for the case when the magnetic field deviates from the symmetrical (parallel to the normal) direction. It is shown that for such geometry the symmetry violation leads to a splitting of spin-wave modes, and that the number of the split peaks depends on the mode number. A quantitative description of the spectra is given using a simple perturbation theory. The role of boundary conditions (pinned, mixed or free) is discussed. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W8.00003: Wavenumber dependent Gilbert damping in metallic ferromagnets Yi Li, William Bailey New terms to the dynamical equation of magnetization motion, associated with spin transport, have been reported over the past several years. Each newly identified term is thought to possess both a real and an imaginary effective field leading to fieldlike and dampinglike torques on magnetization. Here we show that three metallic ferromagnets possess an imaginary effective-field term which mirrors the well-known real effective-field term associated with exchange in spin waves. Using perpendicular standing spin wave resonance between 2-26 GHz, we evaluate the magnitude of the finite-wavenumber ($k$) dependent Gilbert damping of the uniform mode ($\alpha_u$) and the first spin wave mode ($\alpha_s$) in three typical ferromagnets, Ni$_{79}$Fe$_{21}$, Co, and Co$_{40}$Fe$_{40}$B$_{20}$. By taking the difference of $\alpha _s$ and $\alpha _u$ and excluding the eddy current damping $\alpha_E$ ($\Delta\alpha_k=\alpha_s-\alpha_u+\alpha_E$), we find the presence of a $k^2$ term, as $\Delta\alpha_k=\Delta\alpha_0+A_{k}\cdot k^2$ in all three metals. We interpret the new term as the continuum analog of spin pumping, predicted recently, and show that its magnitude, $A_{k}$=0.07-0.1 nm$^2$, is consistent with transverse spin relaxation lengths (1-3 nm) as measured by conventional spin pumping. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W8.00004: Nonlinear ferromagnetic resonance shift in nanostructures Feng Guo, Lyuba Belova, Robert McMichael In dynamic magnetic systems, various experiments have shown that the ferromagnetic resonance frequency can shift up or down with increasing driving power in the nonlinear regime. The resonance shift is important in understanding nonlinear physics in nanomagnets and for applications of spin-torque oscillators. Here, we present a systematic study on the sign of the nonlinear coefficient, i.e. the direction of the resonance field/frequency shift. We use ferromagnetic resonance force microscopy (FMRFM) to measure the ferromagnetic resonance of a series of submicron NiFe ellipses with varying aspect ratios. We find the sign of the resonance shift is determined by both the applied field and the anisotropy field. Our measurement and micromagnetic modeling results are in qualitative agreement with a macro-spin analysis developed by Slavin and Tiberkevich [1]. However, both measurement and modeling results exhibit values of the nonlinear coefficient that are more positive (meaning that the resonance tends to shift toward low field direction) than are predicted by the macrospin model. We attribute the difference to the non-uniformity of the precession modes in the ellipses. By analogy with standing spin waves, we show that nonuniform precession tends to increase the nonlinear frequency coefficient through a magnetostatic mechanism. [1] A. Slavin and V. Tiberkevich, IEEE Trans. Mag., 45, 1875 (2009). V. Tiberkevicha, I. Krivorotovb, G. Gerhartc and A. Slavin, J. Magn. Magn. Mater., 321 (2009) L53 [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W8.00005: Ferromagnetic Resonance in a Height Modulated Permalloy Film Joseph Sklenar, Seongjae Lee, Phillip Tucciarone, Rok-Jun Lee, Daniel Tice, Ivan Nevirkovets, Olle Heinonen, John Ketterson We have performed ferromagnetic resonance experiments on permalloy films that are deposited on a colloidal crystal template. The colloidal crystal substrates we used consisted of polystyrene spheres that were hexagonally close packed with sphere diameter varying between 100-300 nm. On a single substrate only one sphere diameter was used. When sputtered onto the colloidal crystal the permalloy film is no longer uniform and obtains a periodic height modulated perturbation from the underlying spheres. When performing FMR experiments we varied the in-plane magnetic field and observed two main anisotropic modes with an angular dependence obeying an expected six-fold symmetry from the underlying perturbations. To explain the origin of these modes we will also present micromagnetic simulations. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W8.00006: Damping of perpendicular standing spin waves via VNA-FMR in sputtered Py/Ta films Thomas Silva, Martin Schoen, Hans Nembach, Justin Shaw, Carl Boone, Mathias Weiler, Mikhail Kostylev Bar'yakhtar first proposed that non-local damping processes $\propto \nabla ^{2}m$ are intrinsic in systems with exchange splitting. A recent theory from Tserkovnyak, \textit{et al.}, substantiates Bar'yakhtar's claim with quantitative estimates based upon s-d exchange and diffusive spin transport models. We measured mode-dependent damping in 10-nm-thick nanomagnets. Data were in qualitative agreement with Bar'yakhtar/Tserkovnyak theory, though the magnitude of the effect was far greater than expected, suggestive that spin-orbit/interface effects are important (PRL 110, 117201). To test the theory further, we measured field-swept spectra of perpendicular standing spin waves up to wavenumber $k=2\times 10^{6}$ cm$^{-1}$ ($\approx 30$ GHz) in Permalloy/Ta films with Py thicknesses from 50 nm to 200 nm by use of vector-network-analyzer FMR. The spectra are fitted simultaneously with multiple complex susceptibilities to account for inter-mode interference. To account for eddy currents, results are compared to a 1-d electrodynamic model that solves the Maxwell and Landau-Lifshitz equations. The damping data shows no clear trend with $k$, indicating that nonlocal effects are too small to observe for bulk spin waves over the accessible range in $k$-space. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 4:18PM |
W8.00007: Detection of microwave phase variation in nanometer-scale magnetic heterostructures Invited Speaker: Cheng Cheng The internal phase profile of electromagnetic (EM) radiation determines many functional properties of metal, oxide, or semiconductor heterostructures. In magnetic heterostructures, emerging spin electronic phenomena depend strongly upon the phase profile of the magnetic field $\tilde{H}$ at gigahertz frequencies. Here we demonstrate nanometer-scale, layer-resolved detection of EM phase through the rf magnetic field $\tilde{H}_{\textrm{rf}}$ in magnetic heterostructures. Time-resolved x-ray magnetic circular dichroism reveals the local phase of $\tilde{H_{\textrm{rf}}}$ acting on individual magnetizations $\tilde{M_{i}}$ through the susceptibility as $\tilde{M}=\tilde{\chi}\tilde{H}_{\textrm{rf}}$. An unexpectedly large phase variation, $\sim$ 40$^{\circ}$, is detected across spin-valve trilayers driven at 3 GHz. The results have implications for the identification of novel effects in spintronics and suggest general possibilities for EM phase profile measurement in heterostructures. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W8.00008: Amplitude and phase of perpendicular standing spin waves via VNA-FMR in sputtered Py/Ta films Martin A. Schoen, Hans T. Nembach, Mathias Weiler, Justin M. Shaw, Carl T. Boone, Mikhail Kostylev, Thomas J. Silva The first observation of perpendicular standing spin waves via ferromagnetic resonance in thin films dates back to the mid `50s. However, phase-sensitive, broad-band FMR methods have only recently been developed with sufficient signal-to-noise to examine the dependence of spin-wave phase on mode index. We measured field-swept complex spectra of perpendicular standing spin waves (PSSWs) up to wavenumber 2x10$^{6}$ cm$^{-1}$ (30 GHz) in Py/Ta films of different thickness with vector-network-analyzer FMR with coplanar waveguide excitation. The spectra are fitted simultaneously with multiple complex susceptibilities to account for inter-mode interference. The spin-wave stiffness constant D and the pinning-parameter $\varepsilon $ are determined from a quadratic fit of the exchange field to the mode number. For all film thickness, $\varepsilon $ is indicative of weak pinning. Mode amplitude vs. excitation direction points to a dead layer at the Py/Ta interface. We observe strong inter-mode phase variations, especially for thinner (\textless 100 nm) films. Results are compared to a 1-d electrodynamic model that simultaneously solves the Maxwell and Landau-Lifshitz equations. The observed phase shifts are not expected if substrate conductivity is ignored. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W8.00009: Phase-sensitive detection of both inductive and non-inductive ac voltages in ferromagnetic resonance Mathias Weiler, Justin M. Shaw, Hans T. Nembach, Martin A. Schoen, Carl T. Boone, Thomas J. Silva Spin pumping causes significant damping in ultrathin ferromagnetic/normal metal (NM) multilayers via spin-current generation of both dc and ac character in the NM system. While the nonlinear dc component has been investigated in detail by utilization of the inverse spin Hall effect (iSHE) in NMs, much less is known about the linear ac component that is presumably much larger in the small-excitation limit. We measured generated ac voltages in a wide variety of Permalloy/NM multilayers via vector-network-analyzer ferromagnetic resonance. We employ a custom, impedance-matched, broadband microwave coupler that features a ferromagnetic thin film reference resonator to accurately compare ac voltage amplitudes and phases between varieties of multilayers. By use of the fact that inductive and ac iSHE signals are phase-shifted by $\pi$/2, we find that inductive signals are major contributors in all investigated samples. It is only by comparison of the phase and amplitude of the recorded ac voltages between multiple samples that we can extract the non-inductive contributions due to spin-currents. Voltages due to the ac iSHE in Permalloy(10nm)/platinum(5nm) bilayers are weaker than inductive signals, in agreement with calculations based upon recent theoretical predictions. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W8.00010: Ferrimagnetic resonance in thin film organic-based magnets Howard Yu, Rohan Adur, Yu Lu, Megan Harberts, Arthur J. Epstein, P. Chris Hammel, Ezekiel Johnston-Halperin Recent advances in spintronics suggest that the high frequency response of ferromagnetic materials is an attractive path to generating pure spin currents. Further, experiments in inorganic systems indicate that the linewidth of the ferromagnetic resonance (FMR) is an important metric for FMR driven spin injection. Here we perform magnetic resonance measurements on the organic-based ferrimagnetic semiconductor V[TCNE]$_{\mathrm{x\sim 2}}$, consisting of vanadium ions in a network of organic linking molecules. We observe a single resonance with an extremely sharp linewidth, on the order of 1 Oe (yttrium iron garnet, YIG, has a comparable linewidth). Previous studies of V[TCNE]$_{\mathrm{x\sim 2}}$ show similar linewidths but with many peaks in the spectrum, indicating that our results represent a significant improvement in sample homogeneity. Finally, we also demonstrate the ability to manipulate the magnetic properties through chemical modification of the organic linker, yielding thin films of V[MeTCEC] and V[EtTCEC]. These studies demonstrate the potential for high frequency all-organic spintronic and magnetoelectronic devices. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W8.00011: Yttrium Iron Garnet Thick Films Formed by the Aerosol Deposition Method for Microwave Inductors Scooter Johnson, Harvey Newman, E.R. Glaser, Shu-Fan Cheng, Marko Tadjer, Fritz Kub, Charles Eddy, Jr. We have employed the aerosol deposition method (ADM) to direct-write 40 $\mu$m-thick polycrystalline films of yttrium iron garnet (YIG, Y$_3$Fe$_5$O$_{12}$) at room temperature onto patterned gold inductors on sapphire substrates at a deposition rate of 1--3 $\mu$m/min as a first step toward integration into microwave magnetic circuits. A challenge to integrating magnetic films into current semiconductor technology is the high-temperature regime (900--1400$^\circ$C) at which conventional ferrite preparation takes place. The ability of the ADM to form dense, thick films at room temperature makes this a promising approach for integrated magnetics where low-temperature deposition and thick films are required. The ADM YIG film has an rms roughness of 3--4 $\mu$m, is comprised of nano-crystalline grains with a density 50\% of the theoretical value. XRD patterns of the as-deposited film and starting powder indicate a polycrystalline single-phase film. In-plane VSM and FMR measurements reveal a saturation of 22 emu/g, coercivity of 27 Oe, and linewidth of 360 Oe. Early measurements of air-filled and YIG-filled gold inductors between 0.01--10 GHz indicate an improved inductance of nearly a factor of 2 at low frequency. At higher frequency, resonance effects diminish this improvement. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W8.00012: Magnon excitation and decay in Ferromagnetic Insulator/metal multilayers Tao Liu, jiaxi Li, Jianwei Zhang We studied magnon excitation in a Ferromagnetic Insulator(FI) layer(such as YIG), which connected two Ferromagnetic/normal metal multilayers in two sides. In our modeling, we adopted self-consistent spin dependent Boltzmann equations in metal layers and magnon Boltzmann equation in FI layer. When applying an in-plane current in first FM layer, a transverse spin current was generated due to Anomalous Hall effect(AHE), after crossing normal metal layer, this transverse spin current will produce magnon excitation at N/FI interface. With carrying spin information, magnon excitations in FI can eventually excite a new spin current at second F/N interface. Although the FI cannot support any spin current propagation across it, but spin polarization information was passed through FI with propagation of magnon. Finally, the transverse spin current in second FM layer can also generate another in-plane spin current by AHE. The magnon excitation in FI layer is dominated by the interfacial interaction at Normal/FI boundary. Our results show the magnon in FI layer have decay behaviors to low energy model. We also showed that when applying a magnetic field on FI layer, spin current in final FM layer can be manipulated by varying magnon excitation. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W8.00013: Resonant damping in $Fe_{3} O_{4} $/Ag below the Verwey Transition Michael Sinko, Daniel Stanley, Michael Pechan, P.B. Jayathilaka, Casey Miller The temperature dependence of damping in epitaxial $Fe_{3} O_{4} $/Ag bilayers grown on [001] MgO substrates was investigated as a function of Ag thickness. The $Fe_{3} O_{4} $ layers were fixed at 350 nm thick, while the Ag thicknesses ranged from 0 to 500 nm. The epitaxial nature of the films was verified by in plane XRD of the $Fe_{3} O_{4} $ (311) and (220) directions. Ferromagnetic resonance (FMR) measurements at 9.2 GHz were carried out with the sample film normal to the applied magnetic field and at temperatures ranging from 30 to 295 K. All samples exhibited easy-plane anisotropy consistent with thin-film shape effects and a sample magnetization of approximately 400 $emu/cm^{3}$. Room temperature resonance line-widths were largely independent of Ag layer thickness and remained fairly constant with decreasing temperature until the Verwey transition ($T_{V} \simeq 110 K$), below which damping increases dramatically for all samples. Of particular note is the influence of the Ag layer thickness on the low temperature damping, wherein a peak in magnitude is observed at approximately 50 nm of Ag. This unexpected damping resonance will be discussed in terms of spin pumping into the Ag in conjunction with the changing $Fe_{3} O_{4} $magnetodynamics associated with the Verwey transition. [Preview Abstract] |
Session W10: Focus Session: Physics of Behavior II
Sponsoring Units: DBIOChair: Joshua Shaevitz, Princeton University
Room: 201
Thursday, March 6, 2014 2:30PM - 3:06PM |
W10.00001: Wild swarms of midges linger at the edge of an ordering phase transition Invited Speaker: Irene Giardina The most notable hallmark of collective behavior in biological systems is the emergence of order: individuals synchronize their state, giving the stunning impression that the group behaves as one. Birds flocks, fish schools and mammal herds are just a few common examples of polarized animal groups. Mating swarms of mosquitoes and midges, on the other hand, do not display global order and it is therefore unclear whether swarms are a true instance of collective behavior or a mere epiphenomenon of the independent response of each insect to an environmental stimulus. The crucial task for a group, however, is not simply to achieve an ordered state, but to respond collectively to the environmental stimuli. For this to happen, correlation must be large, namely individuals must be able to influence each other's behavioral changes on a group scale. In this work, we experimentally study wild swarms of midges and find that, despite the lack of collective order, swarms display strong correlation, comparable to that found in highly ordered groups of vertebrates. Correlation is orders of magnitude larger in natural swarms than in random systems, indicating the existence of large clusters of insects responding together. We also find that the total amount of correlation, i.e. the susceptibility, increases sharply with the swarm density, a distinctive mark of an incipient ordering phase transition. Swarms, however, live at the near-critical edge of this transition, never plunging in the ordered phase, suggesting that their size and density are tuned to maximize collective response. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W10.00002: Self Organized Sorting in Swarms Katherine Copenhagen, David Quint, Ajay Gopinathan Swarming behavior extends across multiple length scales in biology ranging from bacteria to whales. Natural swarms are affected by erratic, or dissenting behavior by individuals within the swarm who may display different types of behaviors than the rest of the swarm. This research investigates the introduction of heterogenous behavior amongst individuals within a swarm and their impact on swarm formation and robustness. We model swarms with a finite number of agents utilizing a velocity alignment interaction and a Lennard-Jones potential, which provides both cohesive and repulsive interactions between neighboring agents. Depending on the parameters governing the swarming interactions and the level of heterogeneity in behavior introduced, we found a variety of collective behavior including sharp transitions from swarming to non-swarming regimes and self organized sorting of individuals based on their types of behavior. Our research sheds light on the varied responses of swarms to internal dissent and suggests optimal strategies to tolerate errant individuals. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W10.00003: Congestion and communication in confined ant traffic Nick Gravish, Gregory Gold, Andrew Zangwill, Michael A.D. Goodisman, Daniel I. Goldman Many social animals move and communicate within confined spaces. In subterranean fire ants {\em Solenopsis invicta}, mobility within crowded nest tunnels is important for resource and information transport. Within confined tunnels, communication and traffic flow are at odds: trafficking ants communicate through tactile interactions while stopped, yet ants that stop to communicate impose physical obstacles on the traffic. We monitor the bi-directional flow of fire ant workers in laboratory tunnels of varied diameter $D$. The persistence time of communicating ant aggregations, $\tau$, increases approximately linearly with the number of participating ants, $n$. The sensitivity of traffic flow increases as $D$ decreases and diverges at a minimum diameter, $D_c$. A cellular automata model incorporating minimal traffic features---excluded volume and communication duration---reproduces features of the experiment. From the model we identify a competition between information transfer and the need to maintain jam-free traffic flow. We show that by balancing information transfer and traffic flow demands, an optimum group strategy exists which maximizes information throughput. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W10.00004: Extracting distinct behaviors from laboratory insect swarms James Puckett, Nicholas Ouellette Throughout nature, self-organized collective motion in animal groups produces rich and complex behaviors. Many modeling approaches have been proposed from continuum to discrete agent based models which are capable of emulating the behavior observed in flocks and swarms. Most models assume uniformity in the way individuals interact and discard differences between individuals and changes of behavior with time. While in many cases individual differences may average out in large groups of animals, this is not likely the case for small groups. By measuring trajectories and kinematics of individual Chironomids in laboratory mating swarms, we assess the dynamics of individual behavior and discuss the impact of our results on current models. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W10.00005: Mapping the structure of animal behavior Gordon Berman, Daniel Choi, William Bialek, Joshua Shaevitz Most animals possess the ability to actuate a vast diversity of movements, ostensibly constrained only by morphology and physics. In practice, however, a frequent assumption in behavioral science is that most of an animal's activities can be described in terms of a small set of stereotyped motifs. Here we introduce a method for mapping the behavioral space of organisms, relying only upon the underlying structure of postural movement data to organize and classify behaviors. Applying our method to movies of size closely-related species of freely-behaving fruit flies, we find a wide variety of non-stereotyped and stereo-typed behaviors, spanning a wide range of time scales. We observe subtle behavioral differences between these species, identifying the some of the effects of phylogenic history on behavior. Moreover, we find that the transitions between the observed behaviors display a hierarchical syntax, with similar behaviors likely to transition between each other, but with a long time scale of memory. These results suggest potential mechanisms for the evolution of behavior and for the neural control of movements. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W10.00006: Predator pursuit strategies: how do falcons and hawks chase prey? Suzanne Amador Kane, Marjon Zamani, Andrew Fulton, Lee Rosenthal This study reports on experiments on falcons, goshawks and red-tailed hawks wearing miniature videocameras mounted on their backs or heads while pursuing flying or ground-based prey. Videos of hunts recorded by the raptors were analyzed to determine apparent prey positions on their visual fields during pursuits. These video data then were interpreted using computer simulations of pursuit steering laws observed in insects and mammals. A comparison of the empirical and modeling data indicates that falcons use cues due to the apparent motion of prey on the falcon's visual field to track and capture flying prey via a form of motion camouflage. The falcons also were found to maintain their prey's image at visual angles consistent with using their shallow fovea. Results for goshawks and red-tailed hawks were analyzed for a comparative study of how pursuits of ground-based prey by accipeters and buteos differ from those used by falcons chasing flying prey. These results should prove relevant for understanding the coevolution of pursuit and evasion, as well as the development of computer models of predation on flocks,and the integration of sensory and locomotion systems in biomimetic robots. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W10.00007: Physics of \textit{C. elegans }search Maria Panlilio, Frederic Bartumeus, William Ryu Movement is a fundamental feature of life. Organisms must search for prey, avoid predators, or explore new habitats. Using methods from statistical physics we seek to elucidate the behavioral strategies governing \textit{C. elegans} searches and their effects on both ecological and evolutionary timescales. Here we ask: how does the search strategy change under starvation? Animal movement studies are often hindered by difficult observations over large spatiotemporal scales, unknown environmental conditions, and complex behavioral descriptions. \textit{C. elegans} is a powerful model system for overcoming such challenges. Machine vision technologies capture high-resolution images of individuals crawling through a large isotropic environment. Trajectories are reconstructed from the images and behavioral reorientation events are automatically flagged. We find that short-term directional persistence initially increases with time away from food. We also quantify local and global spatial searching scales, which are modulated at least in part by the dynamics of one distinct behavior. Since other reorientation types are known to be suppressed under starvation, we propose that the long-term behavioral strategy acts as a compensatory mechanism to prevent both under and oversampling of the environment. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W10.00008: The dynamics of the thermal memory of \textit{C.~elegans} William Ryu, Konstantine Palanski, Frederic Bartumeus, Ilya Nemenman \textit{C.~elegans} has the capacity to learn associatively. For example, \textit{C.~elegans} associates temperature with food and performs thermotaxis towards this temperature when placed on a spatial thermal gradient. However, very little is understood how \textit{C.~elegans} acquires this thermal memory. We have developed a novel droplet-based microfluidic assay to measure the dynamics of the thermal memory of \textit{C.~elegans}. Individual animals are placed in an array of microdroplets on a slide, and a linear temperature gradient of 0.5 deg/cm is applied to the array. By measuring the swimming motions of \textit{C.~elegans} in the droplets, we show that they can perform thermotaxis. By calculating an index of this taxis behavior over time, we quantify the worm's thermal memory and measure its dynamics when the animals are exposed to different conditions of feeding and starvation. Over a time scale of hours, we find that the thermal preference of wild-type worms decays and will actually become inverted and that mutations in the insulin signaling pathway perturb the dynamics. This biphasic conditional association can be explained with a reinforcement learning model with independent reinforcement and avoidance pathways with distinct time scales. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W10.00009: Swimming Eigenworms Frank Van Bussel, Zeina Khan, Mizanur Rahman, Siva Vanapalli, Jerzy Blawzdziewicz The nematode C. Elegans is a much studied organism, with a fully mapped genome, cell structure, and nervous system; however, aspects of its behavior have yet to be elucidated, particularly with respect to motility under various conditions. Recently the ``Eigenworm'' technique has emerged as a promising avenue of exploration: via principle component analysis it has been shown that the state space of a healthy crawling worm is low dimensional, in that its shape can be well described by a linear combination of just four eigenmodes. So far, use of this methodology with swimming worms has been somewhat tentative, though medical research such as drug screening is commonly done with nematodes in fluid environments e.g. well plates. Here we give initial results for healthy worms swimming in liquids of varying viscosity. The main result is that at the low viscosities (M9 buffer solution) the state space is even lower dimensional than that for the crawling worm, with only two significant eigenmodes; and that as viscosity increases so does the number of modes needed for an adequate shape description. As well, the shapes of the eigenmodes undergo significant transitions across the range of viscosities looked at. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W10.00010: A quantifiably complete repertoire of C. elegans locomotion Andre Brown, Roland Schwarz, Robyn Branicky, William Schafer Visible phenotypes have played a critical role in understanding the molecular basis of behaviour in model organisms. However, most current descriptions of behaviour are based on manually identified events or a limited set of quantitative parameters. Here we report an extension of the concept of behavioural motifs to exhaustively catalogue C. elegans locomotion and derive a repertoire that is quantifiably complete. A repertoire learned for spontaneous behaviour in wild-type worms can be used to fit data from mutants or worms in different environmental conditions and provides a sensitive measure of phenotypic similarity. Repertoire comparison can also be used to assess inter-individual variation and the compositionality of behaviour, that is, the extent to which behavioural adaptation involves the creation of novel repertoire elements or the reuse of existing elements in novel sequences. Repertoire derivation is general, so that given a representation of posture, our approach will apply to other organisms. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W10.00011: A Quantitative Model of Motility Reveals Low-Dimensional Variation in Exploratory Behavior Across Multiple Nematode Species Stephen Helms, Leon Avery, Greg Stephens, Tom Shimizu Animal behavior emerges from many layers of biological organization---from molecular signaling pathways and neuronal networks to mechanical outputs of muscles. In principle, the large number of interconnected variables at each of these layers could imply dynamics that are complex and hard to control or even tinker with. Yet, for organisms to survive in a competitive, ever-changing environment, behavior must readily adapt. We applied quantitative modeling to identify important aspects of behavior in chromadorean nematodes ranging from the lab strain \textit{C. elegans} N2 to wild strains and distant species. We revealed subtle yet important features such as speed control and heavy-tailed directional changes. We found that the parameters describing this behavioral model varied among individuals and across species in a correlated way that is consistent with a trade-off between exploratory and exploitative behavior. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W10.00012: Insects traversing grass-like vertical compliant beams Chen Li, Ronald Fearing, Robert Full Small running animals encounter many challenging terrains. These terrains can be filled with 3D, multi-component obstacles. Here, we study cockroaches (\textit{Blaberus discoidalis}) moving through grass-like vertical compliant beams during escape. We created an apparatus to control and vary geometric parameters and mechanical properties of model grass including height, width, thickness, lateral and fore-aft spacings, angle, number of layers, stiffness, and damping. We observed a suite of novel locomotor behaviors not previously described on simpler 2D ground. When model grass height was \textgreater 2 $\times$ body length and lateral spacing was \textless 0.5 $\times$ body width, the animal primarily (probability $P =$ 50{\%}) rolled its body onto its side to rapidly (time $t =$ 2.1 s) maneuver through the gaps between model grass. We developed a simple energy minimization model, and found that body roll reduces the energy barriers that the animal must overcome during traversal. We hypothesized that the animal's ellipsoidal body shape facilitated traversal. To test our hypothesis, we modified body shape by adding either a rectangular or an oval plate onto its dorsal surface, and found that $P$ dropped by an order of magnitude and $t$ more than doubled. Upon removal of either plate, both $P$ and $t$ recovered. Locomotor kinematics and geometry effectively coupled to terrain properties enables negotiation of 3D, multi-component obstacles, and provides inspiration for small robots to navigate such terrain with minimal sensing and control. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W10.00013: ABSTRACT WITHDRAWN |
Session W11: Assembly and Function of Biomimetic and Bioinspired Materials
Sponsoring Units: DMP DBIOChair: Elaine Zhu, University of Notre Dame
Room: 203
Thursday, March 6, 2014 2:30PM - 2:42PM |
W11.00001: Impact of Hydrogel Structure and Composition on Autonomic Chemo-Mechanical Behavior Ryan Kramb, Philip Buskohl, Richard Vaia Autonomic materials harvest energy to change size, shape, or color in response to a set of environmental conditions. At the core of this biomimetic behavior is a material that transduces energy between forms (e.g. chemical to mechanical). The most widely studied of these materials are self-oscillating, Ru-containing PNIPAAm hydrogels driven by the Belousov$-$Zhabotinsky (BZ) reaction; and if correctly designed, BZ gels mimic biological process such as a quorum sensing or beating like a heart. However, establishing relationships between chemo-mechanical response and gel characteristics, such as crosslinking density, monomer composition, catalyst content, and gel stiffness, has remained elusive due to the limited material set and challenges in determining the appropriate balance of reaction kinetics and mechanical response necessary to establish self-sustaining oscillations. To address this challenge, we have broadened the suite of available monomers through a modular synthesis of the Ru-containing constituent; and thereby demonstrating facile tuning of the aforementioned characteristics of the BZ gel. From the resulting correlations between gel properties and chemo-mechanical response, we discuss the material design of composite autonomic gels for maximum shape change, directionality of oscillations, and synchronization of oscillations analogous to a natural pacemaker cell cluster. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W11.00002: Mechanical tuning of elastomers via peptide secondary structure Nandula Wanasekara, J. Casey Johnson, LaShanda T.J. Korley Nature utilizes an array of design tools for engineering materials with multiple functions and tunable mechanical properties. The precise control of hierarchical structure, self-assembly, and secondary structure is essential to achieve the desired properties in bio-inspired materials design. We have developed a series of peptidic-poyurea hybrids to determine the effects of peptide secondary structure and hydrogen bonding arrangement on morphology, thermal and mechanical properties. These materials were fabricated by incorporating peptide segments containing either poly($\beta $-benzyl-\textsc{l}-aspartate) or poly($\varepsilon $-carbobenzyloxy-\textsc{l}-lysine) into non-chain extended polyureas to form either $\beta $-sheets or $\alpha $-helix conformations based on peptide length. Infrared analysis proved the retention of peptide secondary structure when incorporated into peptidic-polyureas. The polymers containing $\beta $-sheet forming peptide blocks exhibited higher modulus and toughness due to intermolecular H-bonding. Additionally, higher peptide weight fractions lead to higher plateau moduli due to a transition of continuous domain morphology from a soft segment continuous to a fibrous and interconnected stiffer peptide domain. All the polymers exhibited microphase separated morphology with nanofibrous or ribbon-like structures. It is observed that fiber aspect ratio and percolation were influenced by the peptide secondary structure and the weight fraction. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W11.00003: Zwitterionic Hydrogel-Biopolymer Assembly towards Biomimetic Superlubricants Raymond Seekell, Elaine Zhu One superlubricant in nature is the synovial fluid (SF), comprising of a high molecular weight polysaccharide, hyaluronic acid (HA), and a globule protein, lubricin. In this bio-inspired materials research, we have explored hydrogel particles to mimic lubricin as a ``ball-bearing'' and control their interaction with the viscoelastic HA matrix. Biocompatible poly(N-[2-(Methacyloyloxy)ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide) (PMSA) hydrogel particles are synthesized to examine the electrostatic induced assembly of PMSA-HA supramolecular complexes in aqueous solutions. Fluorescence microscopy and rheology experiments have characterized the tunable network structure and viscoelastic properties of PMSA-HA aggregates by HA concentration and ionic conditions in aqueous solution. When being grafted to a solid surface, the PMSA-HA composite thin film exhibits superior low biofouling and friction performance, suggesting great promises as artificial superlubricants. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W11.00004: Metal-coordination: using more of nature's tricks to assemble new soft materials Niels Holten-Andersen Growing evidence supports a critical role of metal-coordination in soft biological material properties such as self-healing, underwater adhesion and autonomous wound plugging. Using bio-inspired metal-coordinating polymers, initial efforts to mimic these properties have shown promise. In addition, with polymer network mechanics dictated by coordinate crosslink dynamics material properties can be easily tuned from visco-elastic fluids to elastic solids. Given their exploitation in desirable material applications in nature, metal-coordinate crosslinking provides an opportunity to advance synthetic polymer materials design. Early lessons from this pursuit are presented. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W11.00005: Weak Polyelectrolyte-Clay Assemblies: Physical Mechanisms of Biological Response Svetlana Sukhishvili, Svetlana Pavlukhina, Iryna Zhuk We report on a highly efficient, non-leachable antibacterial coating, consisting of an ultrathin nanocomposite hydrogel capable of hosting, protecting and delivering antibiofilm agents in response to bacterial infection. Constructed using layer-by-layer (LbL) deposition of clay nanoplatelets and a weak polyelectrolyte and loaded with an antimicrobial agent (AmA), the coatings was highly resistant to colonization by \textit{Staphylococcus aureus}. The high antibiofilm activity of the coating results from a combination of highly localized, bacteria-triggered AmA release and hydrogel swelling, as well as retention of AmA by clay nanoplatelets. We discuss the dependence of rheological and swelling properties of weak polyelectrolyte-clay assemblies on film thickness, clay platelet orientation and environmental pH. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W11.00006: Bacillus spores as building blocks for stimuli-responsive materials and nanogenerators Ozgur Sahin, Xi Chen Materials that mechanically respond to external chemical stimuli have applications in a wide range of fields. Inspired by biological systems, stimuli-responsive materials that can oscillate, transport fluid, mimic homeostasis, and undergo complex changes in shape have been previously demonstrated. However, the effectiveness of synthetic stimuli-responsive materials in generating work is limited when compared to mechanical actuators. During studies of bacterial sporulation, we have found that the mechanical response of Bacillus spores to water gradients exhibits an energy density of more than 10 MJ/m3, which is two orders of magnitude higher than synthetic water-responsive materials. We also identified mutations that can approximately double the energy density of the spores, and found that spores can self-assemble into dense, submicron-thick monolayers on substrates such as silicon microcantilevers and elastomer sheets, creating self-assembled actuators that can remotely generate electrical power from an evaporating body of water. The energy conversion mechanism of Bacillus spores may facilitate synthetic stimuli-responsive materials with significantly higher energy densities. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W11.00007: Utilizing Chemo-mechanically Functionalized Oscillating Fins to ``Catch and Release'' Nanoparticles in Binary Flow Ya Liu, Olga Kuksenok, Amitabh Bhattacharya, Yongting Ma, Ximin He, Joanna Aizenberg, Anna Balazs In biomimetics, designing an effective ``catch and release'' device for the selective removal of target species from the surrounding solution is critical for developing autonomous sensors and sorters. Using computer simulations, we model an array of oscillating fins that are tethered on the floor of a microchannel and immersed in a mixture of binary fluid stream and binary nanoparticles. During the oscillation, the fins with the specific chemical wetting reach the upper fluid when they are upright and are entirely immersed within the lower stream when they are tilted. We introduce specific interaction between the fins and particulates in the solution and determine conditions where the oscillating fins can selectively ``catch'' target nanoparticles within the upper fluid stream and then release these particles into the lower stream. We isolate the effects of wetting contact angle between fins and fluid and the mode of fins' oscillations that lead to the efficient extraction of target species from the upper stream and their placement into the lower fluid. These studies provide fundamental insights into the system's complex dynamics and mechanism for detection, separation, and purification of multi-component mixtures. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W11.00008: Driven translocation of Polymer through a nanopore: effect of heterogeneous flexibility Ramesh Adhikari, Aniket Bhattacharya We have studied translocation of a model bead-spring polymer through a nanopore whose building blocks consist of alternate stiff and flexible segments and variable elastic bond potentials. For the case of uniform spring potential translocation of a symmetric periodic stiff-flexible chain of contour length N and segment length $m$ (mod(N,2m)=0), we find that the end-to-end distance and the mean first passage time (MFPT) have weak dependence on the length $m$. The characteristic periodic pattern of the waiting time distribution captures the stiff and flexible segments of the chain with stiff segments taking longer time to translocate. But when we vary both the elastic bond energy, and the bending energy, as well as the length of stiff/flexible segments, we discover novel patterns in the waiting time distribution which brings out structural information of the building blocks of the translocating chain. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W11.00009: The role of polyelectrolytes in the stabilization of calcium phosphate nanoparticles for the production of biomimetic materials Daniel Krogstad, Dongbo Wang, Sheng Lin-Gibson The exceptional mechanical properties of bone are a result of the hierarchical assembly of hydroxyapatite and the bone matrix, which is primarily composed of collagen. However, it has been shown that without highly acidic, non-collagenous proteins (NCP), which comprise only a few percent of the total organic material, collagen cannot be mineralized correctly. Although the exact roles of these NCP are unknown, it is believed that they are responsible for the stabilization and transportation of the apatite precursor, amorphous calcium phosphate (ACP). In this work, polyaspartic acid was used as a synthetic analog for NCP and the structure and kinetics of calcium phosphate nanoparticle formation were determined at various concentrations using cryo-TEM and scattering. From this investigation, it was determined that the size and stability of the ACP nanoparticles could be directly controlled by the relative ion and polymer concentrations. Interestingly, at high polymer concentrations, the particles remained suspended in solution even after they transformed from ACP to apatite indicating that the polymers have a strong ability to prevent particle aggregation. Through these results, control over the particle size and stability has been increased which will help in the design and development of biomimetic materials. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W11.00010: Lipid tail protrusions initiate spontaneous insertion of charged, amphiphilic nanoparticles into lipid bilayers Reid Van Lehn, Maria Ricci, Randy Carney, Kislon Voitchovsky, Francesco Stellacci, Alfredo Alexander-Katz Vesicle fusion is a primary mechanism used to mediate the uptake and trafficking of materials both into and between cells. The pathway of vesicle fusion involves the formation of a lipid stalk in which the hydrophobic core regions of two closely associated bilayers merge. The transition state for stalk formation requires the transient protrusion of hydrophobic lipid tails into solvent; favorable contact between these hydrophobic tails then drives stalk creation. In this work, we use unbiased atomistic molecular dynamics simulations to show that lipid tail protrusions can also induce the insertion of charged, amphiphilic nanoparticles (NPs) into lipid bilayers. As in the case of vesicle fusion, the rate-limiting step for NP-bilayer fusion is the stochastic protrusion of aliphatic lipid tails into solvent and into contact with hydrophobic material in the amphiphilic NP monolayer. We confirm our predictions with experiments on supported lipid bilayers. The strong agreement between simulation and experiments indicates that the pre-stalk transition associated with vesicle fusion may be a general mechanism for the insertion of amphiphilic nano-objects that could be prominent in biological systems given the widespread use of NPs in applications ranging from drug delivery to biosensing. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W11.00011: Hierarchical assembly of peptoid nanosheets catalyzed by an air-water interface Thomas Haxton, Babak Sanii, Gloria Olivier, Ranjan Mannige, Caroline Proulx, Andrew Cho, Ronald Zuckermann, Stephen Whitelam Peptoids are synthetic analogs of peptides created by moving sidechains from the alpha carbon to the nitrogen. Removing backbone hydrogen bonding and chirality allows for assembly of planar structures driven by sidechain interactions. For instance, when exposed to and subsequently compressed at an air-water interface, amphiphilic peptoids adsorb into monolayers, reversibly compress, and finally collapse into free-floating bilayer nanosheets. We use X-ray spectroscopy, coarse-grained modeling, and analytic theory to investigate the mechanisms for structure formation and catalytic activity at the air-water interface. We find that affinity for the air-water interface and neighboring polymers lowers the free energy barrier for nanosheet formation, creates substantial in-plane order in the monolayer phase, and open voids amenable to further adsorption. The resulting monolayer exhibits residue-scale in-plane order conserved and augmented by inter-leaf order during collapse into bilayers. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W11.00012: Ion Transport Characteristics of Individual Single-walled Carbon Nanotubes Mimic Those of Biological Ion Channels Hasti Amiri, Kenneth Shepard, Colin Nuckolls Transmembrane ionic channels play a crucial role in vital cellular activities by regulating the transport of ions and fluid across the cell membrane. Their structural complexity and flexibility as well as their many unique operational features, however, make their investigation extremely difficult. The simple, atomically smooth and well-defined structure of carbon nanotubes (CNTs) provides an excellent template for studying molecular transport at nanoscale. Additionally, CNTs have been suggested as analogues to biological pores since they share several common features such as nanometer size diameter, hydrophobic core and ultrafast water flow. Functionalizing the nanotube entrance can also mimic the selectivity filter of ion channels. In this work, we experimentally study ionic transport through individual single-walled CNTs connecting two fluid reservoirs as a function of pore properties and electrolyte type and concentration. We provide strong evidence that the electrostatic potentials arising from the ionized carboxyl groups at the pore entrance significantly influence the ion permeation in a manner consistent with a simple electrostatic mechanism. Lastly, the similarities of ionic transport mechanisms between individual single-walled CNTs and protein ion channels are discussed. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W11.00013: Diffusion and Controlled Localized Drug Release from an Injectable Solid Self-Assembling Peptide Hydrogel Jessie E.P. Sun, Brandon Stewart, Sigrid Langhans, Joel P. Stewart, Darrin J. Pochan We use an injectable solid peptide hydrogel (first assembled into a solid hydrogel, can shear-thin flow and immediately reheal on cessation of shear) as a drug delivery vehicle for sustained and active drug release. The triggered intramolecular peptide folding into a beta-hairpin leads to intermolecular assmebly of the peptides into the entangled and branched nanofibrillar hydrogel network responsible for its advantageous rheological properties. The hydrogel is used to encapsulate a highly effective chemotherapeutic, vincristine, with hydrophobic behavior. We show that we are able to constantly maintain drug release in low but still potent concentrations after the shear-thinning injection process. Similarly, the mechanical and morphoogical properties of the gels remains identical after injection. Characterization of the hydrogel construct is through tritiated vincristine release, TEM, confocal microscopy, and in vitro methods. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W11.00014: Soft hydrogel materials from elastomeric gluten-mimetic proteins Mehran Bagheri, Shane Scott, Fan Wan, Scott Dick, James Harden Elastomeric proteins are ubiquitous in both animal and plant tissues, where they are responsible for the elastic response and mechanical resilience of tissues. In addition to fundamental interest in the molecular origins of their elastic behaviour, this class of proteins has great potential for use in biomaterial applications. The structural and elastomeric properties of these proteins are thought to be controlled by a subtle balance between hydrophobic interactions and entropic effects, and in many cases their characteristic properties can be recapitulated by multi-block protein polymers formed from repeats of short, characteristic polypeptide motifs. We have developed biomimetic multi-block protein polymers based on variants of several elastomeric gluten consensus sequences. These proteins include constituents designed to maximize their solubility in aqueous solution and minimize the formation of extended secondary structure. Thus, they are examples of elastic intrinsically disordered proteins. In addition, the proteins have distributed tyrosine residues which allow for inter-molecular crosslinking to form hydrogel networks. In this talk, we present experimental and simulation studies of the molecular and materials properties of these proteins and their assemblies. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W11.00015: Controlled-surface-wettability-based fabrication of hydrogel substrates with matrix tethering density variations Md. Mahmudur Rahman, Donghee Lee, Divya Bhagirath, Xiangshan Zhao, Vimla Band, Sangjin Ryu It is widely accepted that cells behave differently responding to the stiffness of extracellular matrix (ECM). Such observations were made by culturing cells on hydrogel substrates of tunable stiffness. However, it was recently proposed that cells actually sense how strongly they are tethered to ECM, not the local stiffness of ECM. To investigate the hypothesis, we develop constant-stiffness hydrogel substrates with varying matrix tethering density (the number of anchoring sites between the gel and the ECM protein molecules). We fabricate polyacrylamide gel of static stiffness and conjugate ECM proteins to the gel using a cross-linker. When treating the gel with the cross-linker, we control positioning of cross-linker solutions with different concentrations using superhydrophobic barriers on glass, functionalize the gel by pressing it to the aligned cross-linker solutions, and conjugate an ECM protein of constant concentration to the gel. We expect that the gel will be functionalized to different degrees depending on the concentration distribution of the cross-linker and thus the gel will have variations of matrix tethering density even with constant ECM protein concentration. [Preview Abstract] |
Session W12: Invited Session: Active Matter and the Cytoskeleton
Sponsoring Units: DBIOChair: Daniel Chen, Brandeis University
Room: 205
Thursday, March 6, 2014 2:30PM - 3:06PM |
W12.00001: Active stresses and hydrodynamics of microtubule/motor-protein assemblies Invited Speaker: Meredith Betterton In biologically-inspired soft active materials, chemical energy (typically from ATP) is transduced to generate active stresses arising from reconiguration or forcing of the microstructure. This can lead to novel material organization, mechanical properties, and active flows. While much focus has been on active gels and motile suspensions, another important class are suspensions of microtubules (MTs) crosslinked by motile molecular motors. These are central actors in biological phenomena such as pronuclear transport and spindle formation. Here we develop a multi-scale theory for studying such systems. At the discrete level, we use Brownian dynamics of MTs with moving crosslinks to study microscopic organization and active stress development. We observe, surprisingly, that activity generated extensile stresses arise from both polarity sorting and crosslink relaxation. These simulations estimate polarity-dependent active stress coeffcients in a Doi-Onsager kinetic theory -- similar to those developed previously for motile suspensions -- that captures polarity sorting and induced hydrodynamic flows. In simulating recent experiments of active flows on immersed surfaces, the model exhibits turbulent-like dynamics, and the continous generation and annihilation of disclination defects associated with coherent flow structures. We can associate the system's coherent features with instabilities of aligned linear and nonlinear states. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W12.00002: Active Matter and the Spindle Invited Speaker: Daniel Needleman |
Thursday, March 6, 2014 3:42PM - 4:18PM |
W12.00003: Defect Dynamics in Active Nematics Invited Speaker: M. Cristina Marchetti In vitro suspensions of cytoskeletal filaments and motor proteins can form active fluids and gels with liquid crystalline order and self-sustained flows characterized by evolving topological defects. While in passive nematics opposite-sign defect attract and ultimately annihilate, in active liquid crystals defect pairs are continuously generated by activity. Using a continuum model of a planar active nematic in two dimensions, we have demonstrated that activity results in a turbulent-like state with a steady concentration of defect-antidefect pairs, as observed in recent experiments in suspensions of active microtubules-kinesin bundles. We have shown that these ``active defects'' behave as self-propelled particles with equilibrium interactions and a self-propulsion speed proportional to activity. This particle model quantitatively describes the dynamics of the four required defects in active nematics confined to the surface of vesicles that oscillate between tetrahedral and planar configurations at a tunable frequency. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W12.00004: Dynamics of active actin networks Invited Speaker: Simone Koehler Local mechanical and structural properties of a eukaryotic cell are determined by its cytoskeleton. To adapt to their environment, cells rely on constant self-organized rearrangement processes of their actin cytoskeleton. To shed light on the principles underlying these dynamic self-organization processes we investigate a minimal reconstituted active system consisting of actin filaments, crosslinking molecules and molecular motor filaments. Using quantitative fluorescence microscopy and image analysis, we show, that these minimal model systems exhibit a generic structure formation mechanism. The competition between force generation by molecular motors and the stabilization of the network by crosslinking proteins results in a highly dynamic reorganization process which is characterized by anomalous transport dynamics with a superdiffusive behavior also found in intracellular dynamics. \textit{In vitro}, these dynamics are governed by chemical and physical parameters that alter the balance of motor and crosslinking proteins, such as pH. These findings can be expected to have broad implications in our understanding of cytoskeletal regulation \textit{in vivo}. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:30PM |
W12.00005: Active mechanics and geometry of adherent cells and cell colonies Invited Speaker: Shiladitya Banerjee Measurements of traction stresses exerted by adherent cells or cell colonies on elastic substrates have yielded new insight on how the mechanical and geometrical properties of the substrate regulate cellular force distribution, mechanical energy, spreading, morphology or stress ber architecture. We have developed a generic mechanical model of adherent cells as an active contractile gel mechanically coupled to an elastic substrate and to neighboring cells in a tissue. The contractile gel model accurately predicts the distribution of cellular and traction stresses as observed in single cell experiments, and captures the dependence of cell shape, traction stresses and stress ber polarization on the substrate's mechanical and geometrical properties. The model further predicts that the total strain energy of an adherent cell is solely regulated by its spread area, in agreement with recent experiments on micropatterned substrates with controlled geometry. When used to describe the behavior of colonies of adherent epithelial cells, the model demonstrates the crucial role of the mechanical cross-talk between intercellular and extracellular adhesion in regulating traction force distribution. Strong intercellular mechanical coupling organizes traction forces to the colony periphery, whereas weaker intercellular coupling leads to the build up of traction stresses at intercellular junctions. Furthermore, in agreement with experiments on large cohesive keratinocyte colonies, the model predicts a linear scaling of traction forces with the colony size. This scaling suggests the emergence of an effective surface tension as a scale-free material property of the adherent tissue, originating from actomyosin contractility. [Preview Abstract] |
Session W13: Focus Session: Fe-Based Superconductors-Moments/Fluctuations/NMR
Sponsoring Units: DMPChair: Kenji Ishida, Kyoto University
Room: 207
Thursday, March 6, 2014 2:30PM - 2:42PM |
W13.00001: Itinerancy enhanced quantum fluctuation of magnetic moments in iron-based superconductors Yuting Tam, Dao-Xin Yao, Wei Ku The serious mismatch of large local magnetic moments and small ordered moments in iron-based superconductors is one of the unique and essential features of this new class of high-temperature superconductors. Here we demonstrate the active role of electron itinerancy in modulating strong anisotropic quantum spatial fluctuation and tuning the ordered anti-ferromagnetic moments. This is performed by first integrating out the itinerant degree of freedom of a degenerate spin-fermion model with Hund's coupling, followed by estimation of quantum fluctuation via spin-wave theory. Our results complement current emphasis on the temporal fluctuation in the literature, and highlight the essential interplay between itinerant and local degree of freedoms, paving the way to systematic studies of transport, superconductivity and other fluctuation dominant phenomena in iron-based superconductors. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W13.00002: Magnetic excitations in BaFe$_{2}$Se$_{3}$ Shan Wu, M. Mourigal, J.R. Nielson, M.B. Stone, T.M. McQueen, C. Broholm The ladder-like relative of the iron superconductors, BaFe2Se3, has recently been successfully synthesized and its thermo-magnetic and structural properties explored. Magnetic neutron diffraction and susceptibility data clearly reveal magnetic long-range order below $T_{N}=$256K. The proposed magnetic structure consists of antiferromagnetically aligned ferromagnetic blocks. To understand the origin of square plaquette formation, we carried out inelastic neutron scattering experiments on powder samples of BaFe$_{2}$Se$_{3}$. We identify spin-wave-like low energy excitations in the ordered state. The wave vector dependence of the low energy magnetic scattering is consistent with \textbf{\textit{k}}$=$(1/2,1/2,1/2) magnetic ordering. Comparison to a spin wave theory provides estimates for the dominant exchange interactions in BaFe$_{2}$Se$_{3}$. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W13.00003: Doping dependence of spin excitations and its correlations with high-temperature superconductivity in iron pnictides Meng Wang, Xingye Lu, Huiqian Luo, Xiaotian Zhang, Chenglin Zhang, Yu Song, Pengcheng Dai, Miaoyin Wang, Guotai Tan, E.A. Goremychkin, T.G. Perring, T.A. Maier, Zhiping Yin, Kristjan Haule, Gabriel Kotliar Since spin excitations may be responsible for electron pairing and superconductivity in iron pnictides, it is important to determine their electron/hole-doping evolution and connection with superconductivity. Here we use inelastic neutron scattering to show that while electron doping to the antiferromagnetic BaFe$_2$As$_2$ parent compound modifies the low-energy spin excitations and their correlation with superconductivity ($<50$ meV) without affecting the high-energy spin excitations ($>100$ meV), hole-doping suppresses the high-energy spin excitations and shifts the magnetic spectral weight to low-energies. In addition, our absolute spin susceptibility measurements for the optimally hole-doped iron pnictide reveal that the change in magnetic exchange energy below and above Tc can account for the superconducting condensation energy. These results suggest that high-Tc superconductivity in iron pnictides is associated with both the presence of high-energy spin excitations and a coupling between low-energy spin excitations and itinerant electrons. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W13.00004: Local moments in iron-based superconductors probed with x-ray emission spectroscopy Invited Speaker: Young-June Kim We report Fe K$\beta$ x-ray emission spectroscopy studies of local magnetic moments in various iron-based superconductors. X-ray emission spectroscopy (XES) is a fast, local probe that is bulk-sensitive and couples directly to the d-electron moment. Recently developed integrated absolute difference method of analyzing the XES signal [1] allows us to study even small fluctuating moments found in metallic systems such as iron-based superconductors. In our survey of various materials in their paramagnetic phases [2], we found local magnetic moments in all samples studied: PrFeAsO, Ba(Fe,Co)$_2$As$_2$, LiFeAs, Fe$_{1+x}$(Te,Se), and A$_2$Fe$_4$Se$_5$ (where A = K, Rb, and Cs). The moment size shows very little dependence on temperature or carrier concentration, but varies significantly across different families. Specifically, all iron pnictide samples have local moments of about 1.5-2 $\mu_B$/Fe, while FeTe and K$_2$Fe$_4$Se$_5$ families have much larger local moments of ?3 $\mu_B$/Fe and ?5 $\mu_B$/Fe, respectively. The extracted moment sizes agree well with energy and momentum integrated inelastic neutron scattering results. In addition, XES was used to study the spin-state transition in rare-earths doped CaFe$_2$As$_2$ [3]. When about 10-20\% of Ca is replaced with Pr or Nd ions, this material goes through so-called collapsed tetragonal transition at 70 K, below which the c-lattice constant shrinks by almost 10\% [4]. The XES data show that the local magnetic moment is quenched in this collapsed tetragonal phase. We also found that the moment size exhibits unusually large temperature dependence even in the high temperature regime, indicating that the crystal field splitting and the Hund's rule coupling are of similar strength in this compound [5]. Our experimental results illustrate the importance of multiorbital physics in describing magnetism of iron-based superconductors.\\[4pt] [1] G. Vanko et al., J. Phys. Chem. B 110, 11 647 (2006);\\[0pt] [2] H. Gretarsson et al., Phys. Rev. B 84, 100509(R) (2011);\\[0pt] [3] H. Gretarsson et al., Phys. Rev. Lett. 110, 047003 (2013);\\[0pt] [4] S. R. Saha et al., Phys. Rev. B 85, 024525 (2012);\\[0pt] [5] J. Chaloupka and G. Khaliullin, arXiv:1208.1197v1. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W13.00005: Electronic and magnetic properties of Ca(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}})_{2}$As$_{2}$ studied by $^{75}$As NMR Yuji Furukawa, Beas Roy, Shen Ran, Sergey L. Bud'ko, Paul C. Canfield Recently much attention has been paid to CaFe$_{2}$As$_{2}$ because the magnetic and electronic properties of the system can be controlled by changing the heat treatment conditions. CaFe$_{2}$As$_{2}$ annealed at 400 C for 24 hours undergoes a phase transition from a high-temperature tetragonal paramagnetic state to a low temperature orthorhombic antiferromagnetic state at T$_{\mathrm{N}}$ $\sim$ 160K. On the other hand, CaFe$_{2}$As$_{2}$ quenched from 960 C to room temperature shows a transition to a collapsed tetragonal non-magnetic phase below T$_{\mathrm{s}}$ $\sim$ 90 K. In order to investigate the difference in electronic and magnetic properties of the two different CaFe$_{2}$As$_{2}$ samples from a microscopic point of view, we have carried $^{75}$As-NMR spectra and spin-lattice relaxation measurements. We also performed $^{75}$As-NMR measurements on Co-doped CaFe$_{2}$As$_{2}$ superconductor. Based on our NMR data, we will discuss similarities and difference in magnetic fluctuations in the systems, and compare the NMR data with inelastic neutron scattering data. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W13.00006: NMR Evidence for Two Spin-Fluctuation Components and their Linear Scaling with Superconductivity in Fe Superconductors Weiqiang Yu, B. Normand, Long Ma, J. Dai, G.F. Ji, P. Fan, P.S. Wang, Y. Song, C.L. Zhang, Pengcheng Dai The relationship between spin fluctuations and superconductivity is one of the most fundamental questions in the study of iron-based superconductors. From a series of NMR measurements on different compounds and over wide ranges of temperature [1] and pressure [2], we have found strong evidence for two different components contributing to spin fluctuations in the normal state. One originates from Fermi-surface electrons and appears at low energies, while the other arises from the local magnetic moments and appears at higher energy scales [1]. In NaFe$_{1-x}$Co$_x$As, NMR studies demonstrate that both low-energy spin fluctuations and superconductivity are at first enhanced strongly under pressure, both scaling linearly, but decrease together above 2.2 GPa. A ``missing constant'' contribution to $T_c$ reveals the additional effect of the local spin fluctuations. This clear evidence for (i) separate itinerant and local spin fluctuations and (ii) the scaling between spin fluctuations and superconductivity provide essential input to support a magnetic origin of superconductivity based on a two-component model for spin fluctuations in Fe superconductors. \\[4pt] [1] L. Ma {\it et al.}, PRB {\bf 84}, 220505(R) (2011).\\[0pt] [2] G. F. Ji {\it et al.}, PRL {\bf 111}, 107004 (2013). [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W13.00007: High-energy spin excitations in heavily hole-doped superconductor KFe$_{2}$As$_{2}$ Kazumasa Horigane, Chul-Ho Lee, Kunihiro Kihou, Kay Fujita, Ryoichi Kajimoto, Sungdae Ji, Yasuhiro Inamura, Masatoshi Arai, Jun Akimitsu The understanding of overall spin dynamics over the wide hole doping region is a key to progress in the study of iron based superconductors. In contrast to experimental works in parent compounds, spin dynamics in over hole doping region is not yet provided. In this research, we have performed inelastic neutron scattering of heavily hole-doped KFe$_{2}$As$_{2}$ at 4SEASONS. We revealed a well-defined low-energy incommensurate spin fluctuation at (0.32, 0.32) and (0.68, 0.68). This incommensurability was consistent with the previous triple-axis neutron scattering study. A practically vertical dispersion was discovered up to 80meV and spin fluctuations clearly exist even in heavily hole doped KFe$_{2}$As$_{2}$. The energy dependence of dynamical magnetic susceptibility $\chi $(q,w) can be explained by a phenomenological function applicable to correlated spin system in Fermi liquid without magnetic long-range ordering. Therefore, magnetism in KFe$_{2}$As$_{2}$ likely originates from itinerant nature. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W13.00008: Spin excitations in Ba(Fe$_{0.957}$Cu$_{0.043})_{2}$As$_{2}$ single crystals near a putative antiferromagnetic quantum critical point Min Gyu Kim, P. Valdivia, S. Chi, S. Ran, G.S. Tucker, A.D. Christianson, E. Bourret-Courchesne, S.L. Bud'ko, P.C. Canfield, A. Kreyssig, R.J. McQueeney, A.I. Goldman, R.J. Birgeneau We report on inelastic neutron scattering measurements of the spin excitations in the non-superconducting Ba(Fe$_{0.957}$Cu$_{0.043})_{2}$As$_{2}$ compound that orders antiferromagnetically at $T_{\mathrm{N}}=$26(5) K in the near vicinity of a putative antiferromagnetic quantum critical point. These results are compared to those of previous studies on low Cu doped BaFe$_{2}$As$_{2}$ and high Co doped BaFe$_{2}$As$_{2}$ compounds. While Co doping induces superconductivity and Cu doping does not, we find that the Cu-doped and Co-doped spectra are qualitatively similar so additional factors must be considered to explain why the Cu-doped compound is not a superconductor. In addition, we show a possible E/T scaling in this compound, which has been observed in many heavy fermion and cuprate superconductors at/near putative quantum critical points. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W13.00009: NMR evidence for coexistence of cluster spin glass and superconductivity in Ba(Fe$_{1-\mathrm{x}}$Co$_{\mathrm{x}}$)$_2$As$_2$ Adam P. Dioguardi, John Crocker, Abigail C. Shockley, Ching H. Lin, Kent R. Shirer, David M. Nisson, Matthew M. Lawson, Nicholas apRoberts-Warren, Paul C. Canfield, Sergey L. Bud'ko, Sheng Ran, Nicholas J. Curro We present $^{75}$As nuclear magnetic resonance data from measurements of a series of Ba(Fe$_{1-\mathrm{x}}$Co$_{\mathrm{x}}$)$_2$As$_2$ crystals with $0 \leq \mathrm{x} \leq 0.075$. Spectral wipeout and the onset of stretched exponential spin-lattice relaxation as a function of decreasing temperature reveal the coexistence of frozen antiferromagnetic domains and superconductivity for $0.060 \leq \mathrm{x} \leq 0.071$. Although bulk probes reveal no long range antiferromagnetic order beyond $\mathrm{x} = 0.06$, we find that the local spin dynamics reveal no qualitative change across this transition. Domain sizes vary by more than an order of magnitude, reaching a maximum variation at $\mathrm{x} = 0.06$. This inhomogeneous glassy dynamics may be an intrinsic response to the competition between superconductivity and antiferromagnetism in this system. We also present field-dependent spin-lattice relaxation studies from 3.5 T to 30.4 T to further probe the glassy dynamics. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W13.00010: NMR study of the AF-SC-SC-AF phased transition in a pnictide superconductor LaFeAsO$_{\mathrm{1-x}}$H$_{\mathrm{x}}$ Naoki Fujiwara, Ryosuke Sakurai, Soushi Iimura, Satoru Matsuishi, Hideo Hosono, Youichi Yamakawa, Hiroshi Kontani We have performed $^{75}$As and $^{1}$H NMR measurements in LaFeAsO$_{\mathrm{1x}}$H$_{\mathrm{x}}$, an isomorphic compound of LaFeAsO$_{\mathrm{1x}}$F$_{\mathrm{x}}$. LaFeAsO$_{\mathrm{1x}}$H$_{\mathrm{x}}$ is an electron doped system, and O2- can be replaced with H$^{-}$ up to $x=$0.5. LaFeAsO$_{\mathrm{1x}}$H$_{\mathrm{x}}$ is known for having double superconducting (SC) domes on H doping. Recently, we discovered that a new antiferromagnetic (AF) phase follows the double SC domes on further H doping, forming a symmetric AF-SC-SC-AF phase alignment in the electronic phase diagram [1] Unlike the AF ordering in the lightly H-doped regime, the AF ordering in the highly H-doped regime is attributed to the nesting between electron pockets. In the conference, we will show the data of both NMR spectra and the relaxation rate 1/$T_{1}$ in the whole doping region. We will discuss the difference of electronic states between the lightly H-doped AF-SC phases and highly H-doped SC-AF phases.\\[4pt] [1] N. Fujiwara, et al., PRL \textbf{111} 097002 (2013)\\[0pt] [2] Y. Yamakawa, et al., PRB \textbf{88} 041106 (R) (2013) [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W13.00011: Neutron spin resonance study in Co-doped NaFeAs Chenglin Zhang, Egami Takeshi, Pengcheng Dai Since the discovery of iron superconductors, the (Ba,Sr,Ca)Fe2As2 (``122'') family especially electron doped side has been subjected to heavily study byneutron scattering. One of the pronounced features generally observed in bulk superconducting compositions is a broad resonance along antiferrromagnetic order wave vector. The resonance energy linearly scales with Tc. However, our neutron study shows that Co-doped NaFeAs system exhibits complexity, distinguishing itself from ``122'' system. We observed a sharp resonance in the electron-overdope regime, providing strong evidence for S$+$\textunderscore\ pairing symmetry in pnictide superconductors. In the underdoped regime, we find double resonances at commensurate wave vector, demonstrating the multi-orbital nature of pnictides. Our finding further suggests that the resonance energy and Tc may not be simply correlated in multiband superconductors such as iron pnictides. We will discuss in detail how resonances evolve with electron doping. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W13.00012: Compositional dependence of the superconducting resonance in underdoped Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ G.S. Tucker, D.K. Pratt, A. Thaler, N. Ni, K. Marty, A.D. Christianson, M.D. Lumsden, B.C. Sales, A.S. Sefat, S.L. Bud'ko, P.C. Canfield, A. Kreyssig, A.I. Goldman, R.J. McQueeney The low energy magnetic fluctuation spectra of Ba(Fe$_{1-x}$Co$_{x}$)$_2$As$_2$ single crystals in the range $0.040 \le x \le 0.055$ were studied in their antiferromagnetically ordered state, above and below their superconducting transition temperature ($T_c$), using inelastic neutron scattering. The normal state excitation spectra are best described by a model of overdamped correlated spin fluctuations characteristic of the paramagnetic phase. Upon entering the superconducting state the excitation spectra is modified by the superconducting resonance. The superconducting resonance energy and its relation to $T_c$, the dispersion of the resonance and its dimensionality, and the absolute spectral weight of the resonance will be discussed in terms of their composition dependence.\\[0pt] Work at the Ames Laboratory was supported by US DOE, Basic Energy Sciences, DMSE under Contract No. DE-AC02-07CH11358. Part of the research conducted at ORNL's High Flux Isotope Reactor was sponsored by the Scientific User Facilities Division, BES, US DOE. Some work at Oak Ridge was supported by US DOE, Basic Energy Sciences, DMSE. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W13.00013: Orthorhombic distortion, superconductivity and magnetic fluctuations in FeSe A.E. B{\"o}hmer, F. Hardy, P. Schweiss, T. Wolf, C. Meingast, T. Arai, T. Iye, T. Hattori, K. Ishida The recently observed scaling between magnetic and lattice fluctuations observed for Ba(Fe,Co)$_2$As$_2$ provides evidence that its tetragonal-to-orthorhombic structural transition is magnetically driven\footnote{Fernandes et al. PRL 111, 137001 (2013)}. Here, we study the interplay between structure, magnetism and superconductivity in FeSe, an iron-based superconductor which is particularly interesting because it orders magnetically only under high pressure, while a tetragonal-to-orthorhombic structural transition takes place at 90 K at ambient pressure. In contrast to the 122-systems, our high-resolution thermal-expansion data clearly demonstrate that orthorhombic distortion and superconductivity do not compete in FeSe\footnote{B{\"o}hmer et al. PRB 87, 180505 (2013)}, while the shear modulus softening is similar to the 122 systems. By studying magnetic fluctuations using nuclear magnetic resonance, we investigate whether FeSe is simply a case of extreme splitting of magnetic and structural phase transitions - and thus comparable to 122 systems - or whether its structural transition has a qualitatively different origin. [Preview Abstract] |
Session W14: Invited Session: Patterns in Polymers: Elasticity, Fluids, and Surfaces
Sponsoring Units: DPOLY GSNPChair: Kari Dalnoki-Veress, McMaster University
Room: 301-303
Thursday, March 6, 2014 2:30PM - 3:06PM |
W14.00001: Exotic nanoparticles with block copolymer design and solution construction with kinetic control Invited Speaker: Darrin Pochan Kinetic pathways and long temporal stabilities of different block copolymer micelles and nanoscale aggregates have been used to construct exotic nanoparticles in solution. Due to low chain exchange dynamics between block copolymeric micelles and solvent, global thermodynamic equilibrium is extremely difficult, if not impossible, to achieve in block copolymer assembly. However, by taking advantage of this slow kinetic behavior of polymeric micelles in solution, one can purposely produce multicompartment nanoparticles and mulitgeometry nanoparticles by forcing different block copolymers to reside in the same nanoscale structure through kinetic processing. While kinetically trapped in common nanostructures, local phase separation can occur producing compartments and surface patches uniquely displayed from the surface of the nanoparticle. This compartmentalization can be used within common micelle geometries to make complex spheres and cylinders or can be used to make new nanostructures such as multigeometry aggregates such hybrid cylinder-sphere aggregates, disk-cylinder nanoparticles, and hybrid inorganic-block copolymer nanoparticles. Additional results producing nanoparticles with blends of three or more different block copolymers and block copolymer-polypeptide hybrid particles will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W14.00002: Exploiting Elasticity with Thin Polymer Films Invited Speaker: Andrew Croll Soft matter is often dominated by long-ranging mechanical distortion and is thus intimately linked to elastic theory. The detailed understanding provided by theory has allowed remarkable technological achievements to be made with polymers and other soft systems. However, as technology pushes lengthscales downward many challenges have arisen and even basic problems such as measuring Young's modulus become difficult. To move forward, many polymer thin-film researchers have been attracted to the simple repetitive buckling pattern known as wrinkling because the instability provides a convenient tool to measure mechanical properties. As with all technology the wrinkle system does have physical limits on its applicability, several of which may not be obvious and may have implications for extreme measurement. Here we highlight some of our recent work examining the limits of this elastic pattern and the implications for thin polymer films. We first show how the morphology of ultra-thin wrinkled polystyrene and polystyrene-block-poly(2-vinylpyridine) films show signs of localization effects - a clear deviation from linear elasticity. We go on to show how roughness, in certain cases, can induce similar morphologies, even in the limits of vanishing applied stress. As random roughness influences a film's elastic behaviour it is natural to examine periodic roughness as means to control localization and create more complex morphologies. Colloidal polystyrene is an excellent test material as it can easily be assembled in highly ordered crystalline monolayers. Remarkably, this ``discrete'' polymer film shows the same wrinkled morphology as does a continuum film. We show how a completely different type of elasticity is necessary to explain the effect, that of a granular material. More disordered ``glassy'' colloidal monolayers provide a means to push our understanding of the granular elastic theory, and suggest an interesting, albeit highly speculative limit for extreme continuum behaviour. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 4:18PM |
W14.00003: Transition Pathways Connecting Stable and Metastable Ordered Phases Invited Speaker: An-Chang Shi Phase transitions are ubiquitous in nature. Understanding the kinetic pathways of phase transitions has been a challenging problem in physics and physical chemistry. From a thermodynamics point of view, the kinetics of phase transitions is dictated by the characteristics of the free energy landscape. In particular, the emergence of a stable phase from a metastable phase follows specific paths, the minimum energy paths, on the free energy landscape. I will describe the characteristics of the minimum energy paths and introduce an efficient method, the string method, to construct them. I will use membranes and block copolymers as examples to demonstrate the power of the method. In particular, I will show how precisely determined transition pathways provide understanding and surprises when we try to connect the different ordered phases. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W14.00004: Pattern formation and control in polymeric systems: From Minkowski measures to in situ AFM imaging Invited Speaker: Karin Jacobs Thin liquid polymer films are not only of great technical importance, they also exhibit a variety of dynamical instabilities [1]. Some of them may be desired, some rather not. To analyze and finally control pattern formation, modern thin film theories are as vital as techniques to characterize the morphologies and structures in and on the films. Examples for the latter are atomic force microscopy (AFM) as well as scattering techniques [2]. The talk will introduce into the practical applications of Minkowski measures to characterize patterns and explain what thin film properties (e.g. capillary number, solid/liquid boundary condition, glass transition temperature, chain mobility) can further be extracted including new technical possibilities by AFM and scattering techniques [3]. \\[4pt] [1] O. B\"{a}umchen, L. Marquant, R. Blossey, A. M\"{u}nch, B. Wagner and K. Jacobs, ``Influence of slip on the Rayleigh-Plateau rim instability in dewetting viscous films,'' (submitted). \\[0pt] [2] P. Gutfreund, O. B\"{a}umchen, R. Fetzer, D. van der Grinten, M. Maccarini, K. Jacobs, H. Zabel and M. Wolff, ``Solid surface structure affects liquid order at the polystyrene--self-assembled-monolayer interface,'' \textit{Phys. Rev. E }\textbf{87} (2013) 012306. \\[0pt] [3] O. B\"{a}umchen, R. Fetzer, M. Klos, M. Lessel, L. Marquant, H. H\"{a}hl, K. Jacobs, ``Slippage and nanorheology of thin liquid polymer films,'' \textit{J. Phys.:Condens. Matter }\textbf{24} (2012) 325102. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:30PM |
W14.00005: Nonequilibrium patterns in nanocomposite films and fluids Invited Speaker: Erik K. Hobbie Carbon and silicon impact a broad range of technologies from structural composites to microelectronics. Working at the nanoscale in the colloidal domain, we leverage soft matter to simplify processing and improve the performance of these materials. Our approach, which exploits concepts firmly rooted in polymer science, has potentially profound implications for a number of emerging technologies. Two specific examples will be discussed. In the first, thin films of purified single-wall carbon nanotubes are deposited on elastomer substrates for applications in flexible electronics. By adapting approaches developed to study the elasticity of thin polymer films, we infer the mechanics of the nanotube coatings from the pattern of wrinkles and folds that emerges under compression, and we relate this response to the electronic and optical properties of the films. In the second example, patterns of phase separation in drying nanocrystal-polymer mixtures are explored in the context of achieving homogeneous coatings. The results are interpreted using equilibrium theories of colloid-polymer mixtures coupled with lattice-Boltzmann simulations of drying complex fluids. I will conclude by discussing some examples of how ideas from both of these scenarios are being merged for new applications. [Preview Abstract] |
Session W15: Instabilities & Turbulence
Sponsoring Units: DFDChair: James Brasseur, Pennsylvania State University
Room: 304
Thursday, March 6, 2014 2:30PM - 2:42PM |
W15.00001: Linear stability analysis of two phase stratified one dimensional Poiseuille flow with surfactant at the interface, in a channel Ashutosh Singh, Subramaniam Pushpavanam Linear stability analysis of one dimensional Poiseuille flow of two superposed fluids with surfactant at the interface is considered in a channel . Three dimensional disturbance is considered for arriving at the Orr-Sommerfeld equations for both the fluids. The mathematical formulation yields a generalized eigen value problem Ax$=$cBx, which is solved by numerically by spectral collocation technique. Gaussian elimination is employed to recast the eigen value problem as A'x'$=$cB'x' in order to get rid of the zero rows in B as explained by Boomkamp. Dispersion curves are plotted for different Reynolds numbers in order to distinguish between interfacial and shear modes for both the cases ;with and without surfactant at the interface. The role of surfactant in stabilization is investigated and the results for pure interface case are compared with that of Yiantsios and Higgins. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W15.00002: Buoyancy Driven Mixing Induced by Volumetric Energy Deposition Adam J. Wachtor, Veronika Mocko, Farzaneh F. Jebrail, Malcolm J. Andrews, Robert A. Gore A two fluid, buoyancy driven mixing experiment, which transitions from a Rayleigh-Taylor stable configuration to a Rayleigh-Taylor unstable configuration via preferential heating obtained with microwaves, is presented. The experiment is initiated with a light, non-polar fluid at rest atop a heavier, more polar fluid. Microwave energy causes rotation of the polar molecules in the heavier fluid, and the density of the bottom fluid begins to drop due to thermal expansion. As heating of the bottom fluid continues, the system passes through the neutral stability point and buoyancy driven mixing ensues. Challenges for designing an experimental facility, and data collection limitations, for this investigation are discussed. Experimental and numerical predictions of the neutral stability point, and onset of buoyancy driven mixing, are compared, and differences with classical Rayleigh-Taylor driven turbulence are discussed. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W15.00003: Instability of displacement of Oldroyd-B fluid by air in a Hele-Shaw cell Prabir Daripa We study the displacement of an Oldroyd-B fluid in a Hele-Shaw cell when driven by air. In particular, we explicitly obtain an analytical expression for the growth rate of instability which depends on the relaxation and retardation (time) constants, denoted by $\lambda$, and $\lambda_1$ respectively, appearing in the Oldroyd-B constitutive relations. When these two constants are zero, we recover the classical Saffman-Taylor result for a Newtonian liquid displaced by air. Our results show that this displacement process is more unstable than the case when a Newtonian fluid is displaced by air. The analytical results are plotted and compared with numerical results on this unstable displacement process available in the literature. The agreement is found to be excellent. In particular, results show that the non-Newtonian case (i.e., Oldroyd-B) is more unstable than the Newtonian case. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W15.00004: The Transition in Structure of the Atmospheric Boundary Layer from Neutral with Surface Heating James Brasseur, Balaji Jayaraman The scales, strengths and detailed structure of atmospheric boundary layer (ABL) turbulence are strongly dependent on the relative contributions of buoyancy-driven vertical motions from surface heating and shear driven motions from geostrophic winds at the mesoscale, as characterized by the global stability state parameter --$z_{i}$/$L$. In the shear-dominant neutral limit, the ABL is characterized by streamwise-elongated coherent eddies of negative fluctuating horizontal velocity As surface heating is increased, buoyancy drives vertical fluctuations strongly correlated with shear-driven motions that eventually organize to generate streamwise rolls that couple upper with lower boundary layer. We use large-eddy simulation (LES) to study this transition between ``near neutral'' and ``moderately convective'' by quantifying correlations and integral scales as a function of -$z_{i}$/$L. $The interactions between outer and the surface layer eddies generate surprising turbulence dynamics that includes a special transitional stability state with unusually enhanced streamwise coherence. The transitional process includes a critical phenomenon with sudden dramatic change in ABL structure, and high sensitivity in horizontal fluctuations to surface heating at a low --$z_{i}$/$L.$ \textit{Supported by DOE}. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W15.00005: Logarithmic scaling in the longitudinal velocity variance explained by a spectral budget in a neutral and unstable atmosphere Tirtha Banerjee, Gabriel Katul, Scott Salesky, Marcelo Chamecki A logarithmic scaling for the streamwise turbulent intensity $\sigma_u^2/{{u_*}^2}=B_1-A_1\,\ln\left({z}/{\delta}\right)$ was reported across several high Reynolds number laboratory experiments as predicted from Townsend's attached eddy hypothesis, where $u_*$ is the friction velocity and $z$ is the height normalized by the boundary layer thickness $\delta$. A phenomenological explanation for the origin of this log-law in the intermediate region is provided here based on a solution to a spectral budget where the production and energy transfer terms are modeled. The solution to this spectral budget predicts $A_1=C_o/{\kappa^{2/3}}$ and $B_1=(3/2) A_1$, where $C_o$ and $\kappa$ are the Kolmogorov and von K\'arm\'an constants. The spectral budget approach is then extended to explore the scaling behavior of $\sigma_u/{{u_*}}$ in the unstably stratified atmosphere. It is demonstrated with support from recent datasets that $\sigma_u/{{u_*}}$ does not only depend on $\delta/L$ but also depends on the atmospheric stability parameter $\zeta=z/L$. Thus, the proposed spectral budget shows how Townsend's attached eddy hypothesis, the $k^{-1}$ spectral law in low wavenumbers and the similarity arguments for a stratified atmosphere are all interconnected. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W15.00006: Study of Turbulence Statistics in Large-Eddy Simulation of Ocean Current Turbine Environments Spencer Alexander, Peter Hamlington As ocean current turbines move from the design stage into production and installation, a better understanding of both oceanic turbulent flows and localized loading is needed by researchers and members of industry. The use of realistic ocean turbulence in Large-Eddy Simulations (LES) of ocean turbines is essential in obtaining realistic ocean turbine loading and characteristics. In this study, an ocean current turbine environment is simulated using the National Center for Atmospheric Research (NCAR) LES model. The inflow and boundary conditions are designed to represent conditions during an observational campaign at Admiralty Inlet in Puget Sound (Thomson, et al. 2012). Comparisons are first made between the LES simulation results and available measurements from the Admiralty Inlet, and further measures are then presented from the LES results, including vertical profiles of Reynolds stresses, anisotropy, turbulent loading, and two point correlations. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W15.00007: Local Dissipation Scales in Homogeneous Sheared Turbulence Ryan King, Peter Hamlington The effects of shear on local dissipation scales in turbulent flows have been examined using direct numerical simulations (DNS) of homogeneously sheared turbulence. In the classical theory of turbulence it is assumed that there is a universal equilibrium range and effects of a large-scale shear are lost at small spatial scales. Recent numerical and experimental studies have shown that large-scale anisotropy can remain significant at small scales. Furthermore, the local dissipation scale distributions have been found to depend on wall distance in turbulent pipe and channel flows and on measurement location in backward-facing step flows. The exact influence of mean shear and large-scale flow properties in these flows remains unclear due to the presence of confounding wall effects or flow separation. We use DNS of homogeneously sheared turbulence to determine the dependence of local dissipation scales on the shear and Reynolds number in the absence of these wall or flow separation effects. This study also tests the assumption that a universal equilibrium range exists and that small-scale behavior is independent of large-scale flow properties. Finally, comparisons are made with prior studies of local dissipation scales in both homogeneous, isotropic and wall-bounded shear flows. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W15.00008: Suppressing Turbulence and Enhancing the Liquid Suspension Flow in Pipeline with Electromagnetic Fields G.Q. Gu, R. Tao Flows through pipes are the most common and important transportation of fluids. To enhance the flow output along pipeline, it requires reducing the fluid viscosity and suppressing turbulence simultaneously and effectively. Unfortunately, no method is currently available to accomplish both goals simultaneously. Fore example, heating reduces the fluid viscosity, but makes turbulence worse. Here we show that the symmetry breaking physics provides an efficient solution for this issue. When a strong electromagnetic field is applied in the flow direction in a small section of pipeline, the field polarizes and aggregates the particles suspended inside the base liquid into short chains along the flow direction. Such aggregation breaks the symmetry and makes the fluid viscosity anisotropic. Along the flow direction, the viscosity is significantly reduced; in the directions perpendicular to the flow, the viscosity is substantially increased. The turbulence is thus suppressed as all rotating motions and vertexes are suppressed. Only the flow along the pipeline is enhanced and the outflow is improved. The method is extremely energy efficient since it only aggregates the particles and does not heat the suspensions. Recent field tests on pipeline fully support the theoretical prediction. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W15.00009: Modeling the Effects of Turbulence in Rotating Detonation Engines Colin Towery, Katherine Smith, Peter Hamlington, Marthinus Van Schoor Propulsion systems based on detonation waves, such as rotating and pulsed detonation engines, have the potential to substantially improve the efficiency and power density of gas turbine engines. Numerous technical challenges remain to be solved in such systems, however, including obtaining more efficient injection and mixing of air and fuels, more reliable detonation initiation, and better understanding of the flow in the ejection nozzle. These challenges can be addressed using numerical simulations. Such simulations are enormously challenging, however, since accurate descriptions of highly unsteady turbulent flow fields are required in the presence of combustion, shock waves, fluid-structure interactions, and other complex physical processes. In this study, we performed high-fidelity three dimensional simulations of a rotating detonation engine and examined turbulent flow effects on the operation, performance, and efficiency of the engine. Along with experimental data, these simulations were used to test the accuracy of commonly-used Reynolds averaged and subgrid-scale turbulence models when applied to detonation engines. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W15.00010: Studies in Quantum Plasma Turbulence Cassandra Oduola Turbulence is a phenomenon associated with chaotic and stochastic change in properties. At the quantum level, turbulence can be found in quantum fluids also known as super fluids; a friction free state of matter containing charged particles. Super fluidity has recently been observed at the core of neutron stars. These fluids containing also act as superconductors. Studies have found that the remaining protons in the star's core are also in a superfluid state and because they carry a charge also form a superconductor. This study employs the non-linear Schrodinger coupled with Poisson's equation for three dimensional quantum turbulence simulations. These simulations follow Fermi-Dirac statistics. Research has found evidence of soliton solutions to the non-linear Schrodinger. Solitons are self-reinforcing waves in nature that are also symmetric. Evidence of these solitons has been found in quantum turbulence. In order to verify the existence of solitons in this model, we aim to model solutions to the Non Linear Schrodinger in 1D and to obtain data to verify the these solitons. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W15.00011: Generalized Hasimoto Transform, Binormal Flow and Quantized Vortices Scott A. Strong, Lincoln D. Carr A quantized vortex is a topological object central to the study of quantum liquids. Current models of vortex dynamics are motivated by the nonlinear Schr\"odingier equation and porting techniques from classical vortices. Self induction of classical vorticity ideally localized to a space curve asserts that a curved vortex filament propagates at a speed proportional to its curvature, $|\textbf{v}|\propto\kappa$, in the binormal direction of the Frenet frame, $\hat{\textbf{b}}$. Interestingly, this autonomous dynamic can be mapped into the space of solutions to a cubic focusing nonlinear Schr\"odinger equation, $i\psi_{t}+\psi_{ss}+\frac{1}{2}|\psi|^{2}\psi=0$, where $\psi$ is a plane-wave defined by curvature and torsion of the vortex filament, $\psi=\kappa\exp[i\int ds\tau]$. Using these two results, one can define a vortex configuration, within superfluid helium or a Bose-Einstein condensate, and prescribe a binormal evolution. In general, however, binormal flow depends nonlinearly on local curvature and maps to a class of nonlinear integro-differential Schr\"odinger equations. In this talk we discuss how system size affects higher-order nonlinearity and filament geometry which is applicable to theoretical and numerical investigations of vortex dominated quantum hydrodynamics. [Preview Abstract] |
Session W16: General Statistical and Nonlinear Physics I
Room: 401
Thursday, March 6, 2014 2:30PM - 2:42PM |
W16.00001: Geometric criticality in random Ising models Erica Carlson, Shuo Liu, Karin Dahmen We have recently pioneered the use of geometric cluster techniques from disordered statistical mechanics to analyze scanning probe data by mapping two-component image data to random Ising models. The method is capable of extracting information from the data about disorder, interactions, and dimension. We have already successfully applied this new technique to uncover a unification of the fundamental physics governing the multiscale pattern formation observed in two disparate strongly correlated electronic materials (cuprate superconductors and vanadium dioxide). However, because the geometric clusters which are directly accessible experimentally via scanning probes do not directly encode thermodynamic critical behavior, little is known about the general theoretical structure of geometric clusters in random Ising models, and the critical exponents associated with the geometric clusters are unknown for many of the fixed points which are key to interpreting experimental data. We discuss our recent progress on uncovering the geometric critical behavior at several fixed points in random Ising models. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W16.00002: Flat-histogram Monte Carlo in the Classical Antiferromagnetic Ising Model G. Brown, P.A. Rikvold, D.M. Nicholson, Kh. Odbadrakh, J.-Q. Yin, M. Eisenbach, S. Miyashita Flat-histogram Monte Carlo methods, such as Wang-Landau and multicanonical sampling, are extremely useful in numerical studies of frustrated magnetic systems. Numerical tools such as windowing and discrete histograms introduce discontinuities along the continuous energy variable, which in turn introduce artifacts into the calculated density of states. We demonstrate these effects and introduce practical solutions, including ``guard regions'' with biased walks for windowing and analytic representations for histograms. The classical Ising antiferromagnet supplemented by a mean-field interaction is considered. In zero field, the allowed energies are discrete and the artifacts can be avoided in small systems by not binning. For large systems, or cases where non-zero fields are used to break the degeneracy between local energy minima, the energy becomes continuous and these artifacts must be taken into account. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W16.00003: The three-dimensional Edwards-Anderson spin glass in an external magnetic field David Yllanes Spin glasses are a longstanding model for the sluggish dynamics that appears at the glass transition. However, in order for spin glasses to be a faithful model for general glassy physics, we need to introduce an external magnetic field to eliminate their time-reversal symmetry. Unfortunately, little is known about the critical behavior of a spin glass in a field in three spatial dimensions. We have carried out a dynamical study combining equilibrium and non-equilibrium data. In particular, using the Janus computer, we have been able to simulate one thousand samples, each with half a million spins, along a time window spanning ten orders of magnitude for several magnetic fields and temperature protocols. Our main conclusion is that the system has a clearly identifiable dynamical transition, which we discuss in terms of different possibilities for the underlying physics (from a thermodynamical spin-glass transition to a mode-coupling crossover). In fact, we are able to make quantitative connections between the Edwards-Anderson spin glass and the physics of supercooled liquids. We also discuss ongoing work in equilibrium from parallel tempering simulations. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W16.00004: Direct evidence of reentrance in three-dimensional random-bond Ising models Helmut G. Katzgraber, Ruben S. Andrist We numerically investigate the reentrant behavior in the disorder--temperature phase diagram of the three-dimensional random-bond Ising model with a fraction $p$ of antiferromagnetic bonds using large scale Monte Carlo simulations. The two-point finite-size correlation function divided by the system size is ideally suited to pinpoint second-order phase transitions in disordered magnetic systems: Because the observable is dimensionless, data for different system sizes cross at the putative transition, up to corrections to scaling. Here we show that a direct measurement of the two-point finite-size correlation function divided by the system size down to very low temperatures shows two crossings at different temperatures for $p = 22.8$\%, therefore clearly signaling reentrant behavior in the phase diagram. This means that for a fraction $p = 22.8$\% of antiferromagnetic bonds the system undergoes two phase transitions with an ordered ferromagnetic phase existing only for intermediate temperatures. Furthermore, we attempt to probe the universality classes for both transitions via an innovative finite-size scaling analysis of the susceptibility. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W16.00005: Barkhausen noise and the random field Ising model Jian Xu, Daniel Silevitch, Thomas Rosenbaum, Karin Dahmen We measure Barkhausen noise in the rare-earth compound Nd2Fe14B prepared with large uniaxial anisotropy. A magnetic field applied transverse to the easy axis of magnetization introduces local random fields and tunes the pinning of domains. We compare the distribution of avalanche sizes and the spectral response with and without a transverse field to characterize the effects of disorder and to test predictions for critical exponents in the random field Ising model. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W16.00006: Average case complexity of low-dimensional Ising spin glasses Ilia Zintchenko, Matthew Hastings, Matthias Troyer Finding ground states of Ising spin glasses is a notoriously hard problem for which there is to date no known efficient general case algorithm [F.Barahona, Journal of Physics A, 15, 3241, 1982]. We present strong numerical evidence that the complexity of the average case low-dimensional spin glass is polynomial in the number of vertices. To this end we present an efficient exact solver based on local constraint satisfaction for finding ground states of this system with an average case complexity polynomial in system size, exponential in the degree of the graph and polynomial in $1/h$, where $h$ is the on-site field. Numerical studies are done in two and three dimensions, and using scaling arguments we conjecture that polynomial complexity holds. We present results for bi-modal and Gaussian distributed couplings and on-site fields and discuss boundary cases on which the complexity of the algorithm is exponential. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W16.00007: High $\textbf{q}$-State Clock Spin Glasses in Three Dimensions and the Lyapunov Exponents of Chaotic Phases and Chaotic Phase Boundaries Efe Ilker, A. Nihat Berker Spin-glass phases, phase transitions for q-state clock models and their q infinity limit XY model in d = 3 are studied by renormalization-group (RG) that is exact for the d=3 hierarchical lattice, approximate for the cubic lattice. In addition to the chaotic rescaling behavior of the spin-glass phase, each of the two types of spin-glass phase boundaries displays, under RG, its own distinctive chaotic behavior. These chaotic RG trajectories subdivide into two categories: strong-coupling chaos (in the spin-glass phase and, distinctly, on the spinglass-ferromagnetic boundary) and intermediate-coupling chaos (on the spinglass-paramagnetic boundary). We characterize each different phase and phase boundary exhibiting chaos by its distinct calculated Lyapunov exponent. We show that under RG, chaotic trajectories and fixed distributions are equivalent. The phase diagrams of arbitrary even q-state clock spin-glass models are calculated. These, for all non-infinite q, have a finite-temperature spin-glass phase. The spin-glass phases exhibit universal ordering behavior independent of q. The spin-glass phases and the spinglass-paramagnetic boundaries respectively have universal fixed distributions, chaotic trajectories, Lyapunov exponents.In the XY limit a T=0 spin-glass phase is indicated. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W16.00008: Applying tensor renormalization group methods to frustrated and glassy systems: advantages, limitations, and applications Zheng Zhu, Helmut G. Katzgraber We study the thermodynamic properties of the two-dimensional Edwards-Anderson Ising spin-glass model on a square lattice using the tensor renormalization group method based on a higher-order singular-value decomposition. Our estimates of the internal energy per spin agree very well with high-precision parallel tempering Monte Carlo studies, thus illustrating that the method can, in principle, be applied to frustrated magnetic systems. In particular, we discuss the necessary tuning of parameters for convergence, memory requirements, efficiency for different types of disorder, as well as advantages and limitations in comparison to conventional multicanonical and Monte Carlo methods. Extensions to higher space dimensions, as well as applications to spin glasses in a field are explored. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W16.00009: Partial solvability from dualities: Applications to Ising models in general dimensions and universal geometrical relations S. Vaezi, Z. Nussinov, G. Ortiz We illustrate that dualities or general series expansion parameter considerations lead to an extensive set of linear constraints that {\it partially solve} or, equivalently, {\it localize the computational complexity} associated with numerous systems. As an illustration, we examine both ferromagnetic and spin-glass type Ising models on hypercubic lattices in $D \ge 3$ dimensions and show that, by virtue of dualities alone, the partition functions of these systems can be determined by explicitly computing only $\sim 1/4$ of all coefficients of their high and low temperature series . For the self-dual two-dimensional Ising model, the fraction of requisite coefficients is further halved; all remaining series coefficients are determined by trivial linear combinations of this subset. These relations lead to a large set of non-trivial geometric equalities that hold in all dimensions. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W16.00010: Spinodal nucleation effects in heterogeneous systems with long range interactions James Silva, William Klein, Harvey Gould, Kang Liu The kinetics of phase transitions in heterogeneous systems remains an area that is not well understood due to experimental difficulties despite heterogeneous nucleation being an occurrence in real systems where impurities are a common reality. In this talk work is presented in developing an understanding of nucleation near the mean field spinodal in an Ising model modified to introduce heterogeneity to the system. The effect of heterogeneity on the critical droplet properties in this simple model is investigated. The question of using a mapping to a percolation transition is also investigated in this heterogeneous system with the goal of defining a critical droplet object allowing for a geometric interpretation of thermal fluctuations in this heterogeneous system. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W16.00011: Dynamical theory of spin noise and relaxation - prospects for real time NMR measurements Timothy Field The dynamics of a spin system is usually calculated using the density matrix. However, the usual formulation in terms of the density matrix predicts that the signal will decay to zero, and does not address the stochastic dynamics of individual spins. Spin fluctuations are to be viewed as an intrinsic quantum mechanical property of such systems immersed in random magnetic environments, and are observed as ``spin noise'' in the absence of any radio frequency (RF) excitation. Using stochastic calculus we develop a dynamical theory of spin noise and relaxation whose origins lie in the component spin fluctuations. This entails consideration of random pure states for individual protons, and how these pure states are correctly combined when the density matrix is formulated. Both the lattice and the spins are treated quantum mechanically. Such treatment incorporates both the processes of spin-spin and (finite temperature) spin-lattice relaxation. Our results reveal the intimate connections between spin noise and conventional spin relaxation, in terms of a modified spin density (MSD), distinct from the density matrix, which is necessary to describe non-ensemble averaged properties of spin systems. With the prospect of ultra-fast digitization, the role of spin noise in real time parameter extraction for (NMR) spin systems, and the advantage over standard techniques, is of essential importance, especially for systems containing a small number of spins. In this presentation we outline prospects for harnessing the recent dynamical theory in terms of spin noise measurement, with attention to real time properties. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W16.00012: Noise and noise reduction in coupled map lattice systems of different topologies with applications Behnam Kia, Sarvenaz Kia, John Lindner, Sudeshna Sinha, William Ditto A model will be presented to demonstrate how the effects of local noise can be controlled in a variety of topologies in coupled map lattices. Then we calculate the optimal value of coupling parameters between different nodes of the lattice to obtain the maximum amount of noise reduction. We argue that the dynamics of the coupled map lattice functions as an averaging filter to reduce noise. We study this effect in different types of networks, including globally coupled and small world networks. Different numerical simulations are presented, and it is observed that there is agreement between the theoretical predictions and numerical simulations. We compare the results of this approach with the ``majority wins'' approach where in order to obtain noise robustness, a series of similar systems operate at the same time and the result of the majority is selected as the final result. We will demonstrate that our approach gives a higher level of noise robustness compared to the ``majority wins'' technique. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W16.00013: Dissipative Processes with Infinite Memory Elvis Geneston, Mauro Bologna, Arkadii Krokhin, Paolo Grigolini We study the process of random growth of surfaces approximating it by fractional Brownian motion (FBM) with scaling
index $H$. The diffusion trajectories generated by the ballistic deposition ($H=1/3$) and Edward-Wilkinson ($H=1/4$) models are analyzed and the distribution of time intervals between two consecutive origin re-crossings are calculated numerically. This distribution follows the inverse power-law, $\psi(\tau) \propto 1/{\tau}^{\mu}$. For pure FBM $\mu = 2-H$ if $1/3 |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W16.00014: Topological supersymmetry breaking: the origin of 1/f noise Igor Ovchinnikov, Kang Wang The scientific community across disciplines is still puzzled by the mysterious phenomenon generically known as 1/f noise -- the long-range (temporal and spatial) correlations that always accompany dynamical behaviors that can be intuitively characterized as chaotic/complex. Here we discuss that within the recently proposed approximation-free cohomological (or Witten-type) Topological Field Theory of Dynamical Systems all (stochastic and deterministic) dynamical systems possess the so-called topological supersymmetry. In its turn, chaotic/complex dynamics is the result of the spontaneous breakdown of this supersymmetry and the emergence of the long-range correlations in the form of 1/f noise, butterfly effect (sensitivity to initial conditions), the power-law statistics for sandpile, neurodynamical and other avalanches, Kolmogorov power spectrum for turbulence etc. is an inevitable consequence of the Goldstone theorem. [Preview Abstract] |
Session W17: Jamming and the Glass Transition
Chair: Kai Zhang, Yale UniversityRoom: 402
Thursday, March 6, 2014 2:30PM - 2:42PM |
W17.00001: Evolution of force networks in dense particulate matter Lou Kondic, Miro Kramar, Arnaud Goullet, Konstantin Mischaikow We present novel methods used to describe temporal evolution of force networks in dense particulate matter. The methods, based on algebraic topology, allow to quantify the evolution of these networks in precise terms. Different measures that we have developed allow to distinguish between local and global changes of the networks and furthermore illustrate strong dependence of the evolution itself on the state of the system. We will focus in particular on discussing the crucial factors that determine the time scales on which the networks evolve. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W17.00002: Characterizing dense granular systems by percolation and statistical properties of force networks Lenka Kovalcinova, Arnaud Goullet, Lou Kondic We consider a two dimensional granular systems compressed isotropically within a square box, We study the force networks, including evoltion of their statistical and percolation properties. Using the information about the total forces between the particles, the number of contacts and forming clusters, we identify the phase transition in granular systems, as well as distinguish between the system that do and do not crystallize. We discuss the influence of various physical parameters inlcuding the speed of compression on jamming and percolation transitions, and on force statistics. For systems without cohesion, we find that the jamming and percolation transitions coincide in the quisistatic limit. In the last part of the talk, we will present preliminary results discussing the degree to which out finding extend to cohesive systems. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W17.00003: New order metric for 3D packings Zhusong Li, Corey S. O'Hern, Mark D. Shattuck Characterization of the structure of static granular packings is important in both theory and applications. For example, it is often useful to assign a value to the degree of structural order in a system. However, most current order metrics assume a particular symmetry for the ordered structure. For systems composed of monodisperse spheres, it is known that the face-centered cubic (FCC) structure is the most ordered. Thus, order metrics that quantify icosahedral order are often selected for studies of monodisperse spheres. However, what order metrics should be used for bi-disperse system with arbitrary size ratio and mixture fraction that do not possess strong icosahedral order? We propose using the Shannon entropy that counts the number of distinct local geometric structures (e.g Voronoi polyhedra) as a measure of generic order. Using this new order metric, we find that we can distinguish order in systems where other metrics fail. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W17.00004: Ioffe-Regel limits of marginally jammed solids Xipeng Wang, Ning Xu We measure both the transverse and longitudinal dynamical structure factors from the normal modes of vibration of marginally jammed solids, from which Ioffe-Regel limits for transverse and longitudinal modes are obtained. We find that the Ioffe-Regel limit for transverse modes lies at a lower frequency than the boson peak, in contrast to the previous observation that the Ioffe-Regel limit and boson peak coincide. At the unjamming transition, while the Ioffe-Regel limit for transverse modes approaches zero frequency, the Ioffe-Regel limit for longitudinal modes approaches a constant. We also find that the longitudinal dynamical structure factor consists of two components: the liquid-like Rayleigh part and solid-like Brillouin part. At fixed volume fraction, the Rayleigh contribution increases with decreasing the wavelength. We thus determine a crossover wavelength at which contributions of the Rayleigh and Brillouin parts to the dynamical structure factor are equal. This crossover wavelength increases with decreasing the volume fraction following a power-law scaling and diverges at the unjamming transition. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W17.00005: Nonlinear Vibrational Response in Frictional Sphere Packings Thibault Bertrand, Corey S. O'Hern, Mark D. Shattuck The response of frictional granular packings to vibrations can display complex spatiotemporal dynamics due to strong nonlinearities from contact breaking, Hertzian contact interactions, frictional sliding, and other sources that are inherent in granular media. However, most computational and theoretical studies of the vibrational response of packings of frictional spheres have only characterized the linear vibrational response using the dynamical matrix. Here, we directly measure the frequency content of the response of packings of frictional spheres to vibrations as a function of the amplitude and frequency of the perturbations. By doing this, we are able to capture the transition from linear to nonlinear response as a function of the driving and identify the largest source of the nonlinear response for systems with different friction coefficients and packing fraction. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W17.00006: Efficient determination of soft spots in amorphous solids using local structural information Ekin Cubuk, Samuel Schoenholz, Brad Malone, Andrea Liu, Efthimios Kaxiras Structural defects such as dislocations are also flow defects that control plastic flow in crystalline solids. In disordered solids, it is more challenging to identify such local regions that are susceptible to rearrangement. We propose an extremely fast method for identifying soft spots with high accuracy, which scales linearly with number of particles. We achieve this by training a supervised learning model with instances of local neighborhoods and their subsequent plastic flow behavior. By characterizing local neighborhoods with not just one structural quantity, such as bond orientational order, but a combination of multiple structural quantities, we are able to identify a population of regions that correlates just as strongly with rearrangements as do soft spots calculated from vibrational modes. This method does not require knowledge of the interparticle interactions and can readily be applied to experiments that measure the positions of constituent particles in a disordered packing. Furthermore, this also allows for the prediction of plastic behavior in systems like lithiated amorphous silicon, which is important for addressing the durability issues encountered in recent work on improving lithium-ion batteries. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W17.00007: Solitary Wave Interactions in the Hertzian System Paul Anzel, Chiara Daraio It is well known in nonlinear dynamics that when solitons or solitary waves collide, their interaction creates a phase change in the propagating waves. It is natural to expect a similar behavior in highly nonlinear (Nesterenko) solitary waves--waves of mechanical motion in a Hertzian system: a row of elastic spheres which have a non-linear contact force that grows as $F = kx^{3/2}$. However, while this phenomenon has been qualitatively observed in simulations, the size of the change has not been explored systematically and little experimental work has gone into confirming the phase changes. Here we present an experimental and numerical study of the phase shifts created by solitary wave interactions in both co-travelling and head-on collisions. We measure the influence of compressing the spheres, which has the effect of linearizing the system towards a Boussinesq-like equation of motion. Additionally, we measure the creation of secondary solitary waves from the interactions and compare their amplitudes to values previously found in the literature. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W17.00008: Experimental measure of sphere packing probability in a Quasi-2D channel Harry Charalambous, Mark D. Shattuck, Corey S. O'Hern We designed an experiment to test exactly solvable models for packings of frictionless disks in confined geometry. We place a fixed number of monodisperse spheres (grains) of diameter (D=3.2mm) in a quasi 2d rectangular column 1.5D wide by 1D thick by 100D tall. In this arrangement only two possible configurations are allowed for a pair of grains; either consecutive grains are on opposite sides of the column or they are on the same side. We used an electromagnetic shaker to create random states by tossing a range from 4 to 26 grains in the air. After each toss, a vibration (perturbation) was applied to remove frictional effects. We measure the probability of finding each possible state and compare with theory. We find that gravity affects the probability distribution and needs to be incorporated into a new theory. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W17.00009: Frictional families in 2D experimental disks under periodic gravitational compaction Aline Hubard, Mark Shattuck, Corey O'Hern We studied a bidisperse system with diameter ratio 1.2 consisting of four 1.26cm and three 1.57cm stainless steel cylinders confined between two glass plates separated 1.05 times their thickness with the cylinder axis perpendicular to gravity. The particles initially resting on a movable piston are thrown upward and allowed to come to rest. In general this frictional state is stabilized by both normal and tangential (frictional) forces. We then apply short (10ms) small amplitude bursts of 440Hz vibration, temporarily breaking tangential forces and then allow the system to re-stabilize. After N of these compaction steps the number of contacts will increase to an isostatic friction-less state and additional steps do not change the system. Many frictional states reach the same final friction-less state. We find that this evolution is determined by the projection of the gravity vector on the null space of the dynamical matrix of a normal spring network formed from the contacts of the frictional state. Thus each frictional contact network follow a one-dimensional path (or family) through phase space under gravitational compaction. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W17.00010: Conching Chocolate Gary L. Hunter, Paul Chaikin, Elena Blanco, Wilson Poon ``Conching'' is an intermediate step in the processing of chocolate where hydrophilic solid particles, such as sugar and milk proteins, are aggressively mixed into a fatty, fluid phase containing emulsifier, e.g. molten cocoa butter with lecithin. During conching, the system evolves from a fine powder to a coarser granulated material and ultimately into a thick cohesive paste. Our goal is to better understand the evolution of chocolate during conching and the transition from an effectively dry to a wet or immersed granular material. In particular, we focus on how mixing times change in response to variations in solid particle volume fractions and emulsifier concentration. As a function of volume fraction, mixing times are well-described by a conventional form that diverges at a finite volume fraction. Furthermore, mixing times can be collapsed onto a universal curve as a function of mixing speed and emulsifier concentration. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W17.00011: Static quasi-2D emulsion as a granular system Rui Wu, Carlos Orellana, Xia Hong, Kenneth Desmond, Eric Weeks We study the forces between emulsion droplets and the properties of force chains in a static oil-in-water emulsion system near jamming. The emulsion is confined between two parallel glass plates in order to construct a quasi-2D system. Quasi-2D emulsion systems are somewhat analogous to 2D granular disks, except for the absence of static friction between the droplets. We focus on samples at an area fraction $\phi$ that is higher than the jamming point, $\phi_{c}$, and test the robustness of the power law dependence of pressure and the contact numbers on $\phi-\phi_{c}$. Specifically, we vary the surface tension by adding surfactants in the water, and examine the power law relationship under such variations. We also compare our result to simulations as well as established experimental results of true granular systems. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W17.00012: Starch Suspensions with Different Fluids Melody Lim, Audrey Melville, Joshua Dijksman, Robert Behringer A suspension made of starch particles dispersed in water displays significant non-Newtonian behavior for high enough particulate concentration. This surprising behavior has recently inspired a series of experiments that have shed much light on the possible mechanism behind this phenomenon. In our studies we assess the role of the fluid phase in these suspensions. We find that using fluids other than water can significantly alter the behavior of starch suspensions. Through mechanical tests of various kinds, we assess the interaction between starch particles and different liquids, and how this interaction affects the non-Newtonian behavior of starch suspensions. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W17.00013: Marginal rigidity and history dependence in packings of attractive athermal emulsions Dylan Bargteil, Lea-Laetitia Pontani, Jasna Brujic The geometry and stress through particulate packings depends on the method of preparation and the interaction potential between the particles. Previously, we discovered that creaming frictionless, athermal emulsions with a short-range depletion attraction leads to an initial increase in the packing density above random close packing, followed by a monotonic decrease in density (Jorjadze et al, PNAS, 2011). This decrease is because the attractive force stabilizes loose voids, thus reducing the average coordination number, $<$z$>$, of the packing. In order to understand the mechanism of packing creation, we investigate whether the final density is influenced by the polydispersity or the initial volume fraction of droplets, as it is in frictional packings. Finally, we compress the attractive packings by centrifugation to probe the scaling laws of pressure versus density and $<$z$>$ and compare them with those found in repulsive packings (Jorjadze et al, PRL, 2013).\\[4pt] [1] I. Jorjadze, L. Pontani, K. A. Newhall, and J. Brujic, Proc. Natl. Acad. Sci. U.S.A. 108, 4286 (2011).\\[0pt] [2] I. Jorjadze, L.-L. Pontani and J. Brujic, Phys. Rev. Lett. 110, 048302 (2013). [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W17.00014: Jamming and free energy landscapes for three caged soft disks Xin Du, Eric Weeks We study jamming in a model of three soft Brownian disks confined in a circular corral. For large corrals, the disks can freely rearrange where one particle passes in between the other two, but for small corrals rearrangements become rare. We use a Monte Carlo simulation to study the dynamics of the three disks, and calculate the Helmholtz free energy from the distribution of configurations in the system. The free-energy landscape in a one-dimensional space contains two symmetric energy minima separated by an energy barrier. Rearrangement events correspond to the system crossing over the free-energy barrier. With low temperature and/or small corral size, the energy barrier becomes larger and the system approaches glass transition. The free energy barrier has both energy and entropy components. We compare our results to a model of hard disks, for which the free energy barrier for rearrangements is entirely entropic. In particular we find that we cannot simply model the soft disks as hard disks with a temperature-dependent effective size. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W17.00015: Elastic probes of length scales in jammed packings: from global response to point response Kamran Karimi, Craig Maloney We probe amorphous packings in different ways to determine whether or not characteristic length scales govern the elastic response and how these lengths depend on the area fraction of disks, $\phi$. First we drive the system globally using either: i) a homogeneously deforming periodic cell of length $L$, ii) a force field having a plane-wave structure with wavelength $L$, iii) a homogeneously deforming rigid wall of length $L$. Methods i) and ii) give elastic moduli values that converge rapidly to the infinite system size limit and have $\phi$-independent functional forms. Method iii), however shows a dramatic decrease in the shear modulus $\mu$ with increasing $L$. At low $L$, $\mu$ has a value that depends only weakly on $\phi$, whereas, as $L$ goes to infinity, $\mu$ must approach zero near jamming point $\phi_c$. We show that the $\mu$ vs $L$ curves at various $\phi$ can be collapsed into a master curve after scaling $L$ by a quantity $\xi$ that grows near $\phi_c$. Secondly, we study the point response. We show that the response, in Fourier space, crossovers to the Kelvin solution for small wave vectors. This cross-over exhibits a lenghtscale that grows with $\phi$ in a similar fashion to the lengthscale determined by the global shear with a rigid box. [Preview Abstract] |
Session W18: Colloidal Particles at Interfaces
Chair: Matthew Lohr, University of PennsylvaniaRoom: 403
Thursday, March 6, 2014 2:30PM - 2:42PM |
W18.00001: Solids under Stress: Lessons from Simple Problems of Elastic String Depinning Stefanos Papanikolaou When stress is applied on solid structures, deformation ultimately becomes permanent/plastic. Plasticity at the mesoscale proceeds through abrupt events, naturally resembling the jumps that a rubber band makes when driven through a landscape of pins. Elastic string depinning has been a very useful analogy for elucidating the statistical character of either crystalline or amorphous plasticity. Recent experiments and simulations, however, point out that the analogy is not complete, as naturally expected: In crystals, plasticity-mediating dislocation defects may ``jump'' through multiple slow relaxation channels beyond the fast gliding one. In amorphous solids, the ``pins'' for each particle have dynamics that may not be neglected since they correspond to neighboring particles. For each case, I will describe a generalized depinning model that aims to minimally include the additional physical mechanism and then compare to recent experiments and simulations. These simple depinning models suggest that plasticity of solids may cross-over to a state of stochastic relaxation oscillations through a mechanism that resembles a singular Hopf bifurcation. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W18.00002: Hysteresis of the Contact Angle around Spheres Adsorbed at Curved Fluid Interfaces Nesrin Senbil, Wei He, Benjamin Davidovitch, Anthony Dinsmore When a particle adsorbs to a fluid interface, the geometry of the contact between the interface and the particle determines the force acting on the particle. We find a significant hysteresis in the contact angle, and --surprisingly-- a strong dependence of the hysteresis on the shape of the interface. Hysteresis in the wetting of a fluid on a flat substrate is well known, whereby two contact angles are typically defined, corresponding to the advancing and receding cases. We find that the receding angle around the sphere changes with the shape of the interface. We use millimeter-sized glass spheres coated with PDMS and adsorbed at an air-water interface. High-resolution images are analyzed to obtain the contact geometry as the spheres are raised or lowered across the interface. We find advancing contact angles of approximately 107$^{\mathrm{o}}$ and receding angles that range between 90$^{\mathrm{o}}$ and 97$^{\mathrm{o}}$ depending on the interface shape. Our results are important for understanding interactions between particles at interfaces and may shed new light on the origin of contact-angle hysteresis. This work is funded by the NSF through CBET-0967620 and by the Gulf of Mexico Research Initiative through the C-MEDS consortium. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W18.00003: Squares of spheres: capillarity-induced ordering of spherical colloids on an interface with anisotropic curvature Jasper Van der Gucht, Dmitry Ershov Objects floating at a liquid interface, such as breakfast cereals floating in a bowl of milk or bubbles at the surface of a soft drink, clump together in space-saving hexagons to minimize the disruption of the liquid interface. Micrometer-sized colloidal particles embedded in a liquid interface normally do not disrupt the interface, so that such clustering does not occur. Here, we show that this is different when the interface has a curvature that is anisotropic. We find that in this case the condition of constant contact angle along the three-phase contact line can only be satisfied when the interface is deformed. We present experiments and numerical calculations that demonstrate how this leads to quadrupolar capillary interactions between the particles, giving rise to organization into regular square lattices. We demonstrate that the strength of the governing anisotropic interactions can be rescaled with the deviatoric curvature alone, irrespective of the exact shape of the liquid interface. Our results suggest that anisotropic interactions can easily be induced between isotropic colloids through tailoring of the interfacial curvature. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W18.00004: Dynamics of 2D Colloidal Crystals Under Microscopic Shear Colm Kelleher, Paul Chaikin Since the early 1980's, 2D colloidal crystals have been used as model systems for studying a variety of basic problems in condensed matter physics - for instance, the KTHNY theory of melting, and the ``Thomson problem'' of finding the ground states of crystals in curved space. However, many non-equilibrium phenomena, such as the response of these crystals to external forces, remain poorly understood. We study systems of 2D colloidal crystals which are formed when charged PMMA microspheres bind to a flat oil-water interface. Using optical tweezers, we apply forces to individual particles, or selected groups of particles, in the crystal lattice. These forces can be precisely controlled in time, space and intensity. We then use video microscopy to study defect formation and dynamics in the crystal. We are particularly interested in the issue of reversibility of dislocation dynamics - as shown recently [Irvine et al. \textit{PNAS} \textbf{2013} 110 (39)], simple dislocation-dislocation interactions tend to be reversible, while more complex, many-dislocation interactions tend to be irreversible. This talk will discuss the above topics in the context of an experiment where the crystal was sheared periodically between two parallel rows of optically trapped colloids. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W18.00005: Crowding and Ordering in the Assembly of Nanoparticles at Liquid Interfaces Konrad Schwenke, Lucio Isa, Emanuela Del Gado Experiments with self-assembly of nanoparticles at liquid interfaces suggest that cooperative and slow dynamical processes due to particle crowding at the interface govern the adsorption and properties of the final assembly [1]. We report a numerical approach to study non-equilibrium adsorption, which elucidates these experimental observations. The analysis of particle rearrangements shows that local ordering processes are directly related to adsorption events at high interface coverage. Interestingly, this feature and the mechanism coupling local ordering to adsorption do not seem to change qualitatively upon increasing particles size polydispersity, although the latter changes the interface microstructure and its final properties. Our results indicate how adsorption kinetics can be used for the fabrication of two-dimensional nano-composites with controlled microstructure. \newline \newline [1] L. Isa, E. Amstad, K. Schwenke, E. Del Gado, P. Ilg, M. Kroger and E. Reimhult, Soft Matter, 2011, 7, 7663-7675.\newline [2] K. Schwenke, L. Isa and E. Del Gado, Assembly of nanoparticles at liquid interfaces: Crowding and ordering, submitted [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W18.00006: Ordering in Conformal Crystals Vishal Soni, Leopoldo Gomez, William Irvine Condensed matter systems commonly undergo ordering processes that are frustrated by geometric constraints. Experiments on interfacial colloidal systems have resulted in several recent insights into the two dimensional ordering of crystalline lattices frustrated by Gaussian curvature. We study the ordering of flat colloidal Wigner crystals immersed in an axially symmetric potential. By relating the resulting inhomogenous structure to a lattice with Gaussian curvature, we investigate the role of topological defects in organizing the conformal crystal-like ground state. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W18.00007: Dynamic Regimes for Driven Colloidal Particles on a Periodic Substrate at Commensurate and Incommensurate Fillings Danielle McDermott, Jeffery Amelang, Cynthia Reichhardt, Charles Reichhardt We numerically examine colloidal particles driven over a muffin tin substrate. Previous studies of this model identified a variety of commensurate and incommensurate static phases in which topological defects can form domain walls, ordered stripes, superlattices, or disordered patchy regimes as a function of the filling fraction. Here we show that the addition of an external drive to these static phases can produce distinct dynamical responses. At incommensurate fillings the flow occurs in the form of localized pulses or solitons correlated with topological defect structures. Transitions between different modes of motion can occur as a function of increasing drive. We measure the average particle velocity for specific ranges of external drive and show that changes in the velocity response correlate with changes in the topological defect arrangements. We also demonstrate that in the different dynamic phases, the particles have distinct trajectories and velocity distributions. Dynamic transitions between ordered and disordered flows exhibit hysteresis, while in strongly disordered regimes there is no hysteresis and the velocity-force curves are smooth. When stripe patterns are present, transport can occur at an angle to the driving direction. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W18.00008: Orientation Waves as the Order Parameters for Crystals Rolfe Petschek Even rather simple particle-particle interactions can result in very complicated crystal structures. Simple Landau or density theories that use particle density as the primary order parameter generically suggest that crystal structures should generically be simple -- e.g. bcc, particularly if the interactions are short ranged and uncomplicated. Convincing evidence is presented that the order parameters for the complicated crystal that forms in the crystallization of hard tetrahedra is an orientation wave: dependent on the periodic ordering of a third and sixth rank traceless symmetric tensor rather than a density is presented. A simple Landau / density-like theory presentation of such orientation waves is presented. Such orientation waves are known to be order parameters for the blue phases which have crystalline symmetry, in which a second rank tensor varies periodically in space. It is argued that the complicated nature of the Landau theory of high rank tensor order parameters makes it plausible that they result in complicated crystals and quasicrsystals. This, in turn, suggests that complicated crystals and quasicrystals have high rank tensor order parameters. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W18.00009: Colloidal diffusion over a periodic energy landscape Xiao-guang Ma, Pik-Yin Lai, Penger Tong A two-layer colloidal system is developed for the study of colloidal diffusion over a two-dimensional periodic energy landscape. The energy landscape is made from the bottom layer of colloidal spheres forming a honey-comb crystalline pattern above a glass substrate. The corrugated surface of the bottom colloidal crystal provides a gravitational potential field for the diffusing particles in the top layer. The obtained population probability histogram $P(x,y)$ of the diffusing particles is used to fully characterize the energy landscape $U(x,y)$ via the Boltzmann distribution. The dynamical properties of the diffusing particle, such as its escape time $t_R$ and diffusion coefficient $D$ are simultaneously measured from the particle's trajectories. The long-time diffusion coefficients $D$ is found to be in good agreement with the theory for all colloidal samples studied. The experiment demonstrates the applications of this newly constructed colloidal energy landscape. *Work supported in part by the Research Grants Council of Hong Kong SAR. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W18.00010: Hydrodynamic damping of collective motion in a quasi-two-dimensional dense colloidal particle suspension Michael Ryan, Tim Still, Arjun Yodh, Kevin Aptowicz Dense colloidal suspensions confined to a monolayer are often used to explore physical phenomena such as the glass transition, crystallization, and frustration. Although hydrodynamic damping is known to play a significant role in the dynamics of these systems, it is difficult to quantify due to the collective nature of the particle motions. In this work, we employ digital video-microscopy to explore the phonon dynamics of an entropic 2D colloidal crystal. Friction coefficients along high symmetry directions in q-space are extracted and provide insight about the hydrodynamic forces at play. Preliminary results suggest the friction coefficient decreases with increasing phonon wavelength, but it does not appear to vanish.~ [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W18.00011: 2D Melting in General: Solid/hexatic/liquid Phase Transitions in Soft Spheres using Event-Chain Monte Carlo Sebastian C. Kapfer, Manon Michel, Werner Krauth The melting of two-dimensional solids has been the subject of continued research for more than fifty years, with the prevalent scenarios being the KTHNY theory of defect unbindung and a conventional first-order liquid/solid transition. For hard disks, a rather unexpected hybrid transition has recently been found with both a first-order transition and an intermediate hexatic phase [1], while magnetic colloid experiments support the KTHNY scenario [2]. To resolve this discrepancy, we here address the melting problem for soft interaction potentials, in particular the nature of the liquid/hexatic and hexatic/solid transitions, and the defects driving melting. Simulations were performed using a new rejection-free irreversible Monte Carlo algorithm generalizing event-chain Monte Carlo to arbitrary pair potentials. In addition to fast equilibration, this algorithm permits to deduce the pressure in the NVT ensemble without any additional computations [3]. References: [1] E. P. Bernard and W. Krauth, Phys. Rev. Lett. 107, 155704 (2011). [2] P. Keim et al. Phys. Rev. Lett. 92, 215504 (2004). [3] M. Michel, S. C. Kapfer and W. Krauth, preprint at arXiv:1309.7748. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W18.00012: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W18.00013: A field-directed colloidal phase transition Eric Furst, James Swan, Jonathan Bauer Suspensions of polarizable colloids are expected to form crystalline equilibrium phases when exposed to a steady, uniform field. However, when colloids become localized this field-induced phase transition arrests and the suspension persists indefinitely as a kinetically trapped, percolated structure. We show that by toggling the applied field on and off gels formed in MR fluids can be annealed. There is a stark boundary as a function of magnetic field strength and toggle frequency that distinguishes arrested states from phase separation. A key advantage of self-assembly in toggled fields is the relatively large range of field-strengths (effective temperatures) that lead to phase separation. Finally, we demonstrate that such directed self-assembly can be used to create colloidal crystals of uniform size. These results demonstrate how kinetic barriers to a colloidal phase transition are subverted through measured, periodic variation of driving forces. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W18.00014: The role of hydrodynamic forces in the confinement and assembly of magnetic dipoles M. Prikockis, A. Chen, R. Sooryakumar The confinement of interacting magnetic dipoles provides a means to probe the assembly of and many-body coupling within a mesoscopic system. Using a previously developed confinement method (Scientific Reports 3, 3124 (2013)), we investigate the role of hydrodynamic forces in one such mesoscopic system that supports a fluid borne suspension of microscopic beads that contain embedded superparmagnetic particles. Our confinement platform consists of a thin permalloy disk patterned on a silicon surface and a precessing magnetic field. By adjusting the orientation of the field, inter-particle dipolar and trap confinement forces are tuned - thereby enabling the plane-confined beads to repel or attract one another. At a specific field orientation, the dipolar interaction is weakened to provide a regime where the hydrodynamic forces, stemming from rotational motion of the beads, play a role in bead assembly. We investigate the dependence of dipole ordering on the hydrodynamic forces by varying the frequency of the field rotation in this special field configuration. This represents a unique system where the hydrodynamic forces of fluid borne magnets are tuned independently of the magnetic forces in a magnetic dipolar confinement scheme. [Preview Abstract] |
Session W19: Focus Session: Thin Films of Block Copolymers and Hybrid Materials III - Surface, Wetting, and Confinement Interactions
Sponsoring Units: DPOLYChair: Bradley D. Olsen, Massachusetts Institute of Technology
Room: 404
Thursday, March 6, 2014 2:30PM - 2:42PM |
W19.00001: Understanding the surface chemistry of amphiphilic copolymer thin films in aqueous environments Hilda Buss, Nathaniel Lynd, Ronald Zuckermann, Ed Kramer, Rachel Segalman Controlling the surface chemistry of polymer coatings which are stable in aqueous environment is a complex problem which depends heavily on the hydrophobicity of the polymer. Poly(styrene)-$b$-(ethyleneoxide-\textit{co}-allylgycidylether)-$b$-poly(styrene) [PS-$b$-(PEO-\textit{co}-AGE)-$b$-PS] triblock copolymers functionalized at the pendant allyl groups with fluorinated moieties are a promising class of polymers for applications in antifouling coatings. These polymers gain their water stability from the PS blocks and their antifouling character from the PEO block. Surface active fluorinated groups are used to direct the surface chemistry of the film during annealing. However, the surface can rearrange or become damaged upon immersion in water. Near edge X-ray absorption fine structure spectroscopy (NEXAFS) of films after soaking in water shows that the surface composition as characterized by the PS and PEO content in the first 6 nm of the film is directly related to the relative sizes of the PS and the P(EO-\textit{co}-AGE) blocks as well as the fluorine content. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W19.00002: Effects of Substrate Interactions on Out-of-Plane Order in Thin Films of Lamellar Copolymers Indranil Mitra, Nikhila Mahadevapuram, Alona Bozhchenko, Joseph Strzalka, Gila E. Stein Block copolymer (BCP) thin films are widely studied and applied for low cost, large area nanopatterning of semiconductor devices and has a very low tolerance for both in-plane or out of plane defects. Here we study, defects in lamellar diblock copolymers as a function of film thickness and the types of interactions at the substrate interface. Thin films of poly (styrene-b-methyl methacrylate) (PS-PMMA) with equilibrium periodicity 46nm were prepared and annealed on silicon substrates that were functionalized with a random copolymer P(s-r-MMA) brush. The resulting structures were evaluated with optical, scanning force and, scanning electron microscopy, along with grazing-incidence small-angle X-ray scattering (GISAXS). The in-plane correlation length (OCL) increased with brush grafting density, and increased with distance from the substrate interface. Out-of-plane order improved with brush grafting density, but thick films always contain a high density of misoriented domains. Based on these findings, we propose that (1) substrate pinning either induces or traps the mis-oriented domains, and (2) out-of-plane orientation defects are difficult to remove, from a thick film, because the energetic penalty for bending a ``tall'' domain is very low. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W19.00003: Evolution of directed and self-assembled structure in free standing and confined PS-b-PMMA thin films Lingshu Wan, Hyo Seon Suh, Xuanxuan Chen, Paulina Delgadillo, Zhang Jiang, Paul Nealey Directed self-assembly of block copolymer films is promising for sub-10-nm lithography. One strategy for obtaining perpendicularly oriented domains is to confine the film with a non-preferential wetting ``top coat''. Here we investigate the evolution of structure in films on non-preferential and chemically patterned substrates; use of PS-b-PMMA enables the direct comparison of free standing films and films that are confined with a ``top coat''. The films were characterized by grazing incidence small-angle X-ray scattering (GISAXS). For self-assembled films, the correlation length of the block copolymer structure was calculated using the Scherrer equation. Results indicate that the effects of the top coat on the self-assembling dynamics depend on the thickness of top coats, molecular weight of block copolymer, and annealing temperature. In the assembly on chemical patterns with density multiplication, the presence of the top coat resulted in more symmetric PS and PMMA domains, and comparable or possibly faster rates of assembly compared to free standing films. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W19.00004: Self-Assembly of Diblock Copolymers in Half-Ellipsoid-Shape Confinements So Jung Park, Myong-Hyun Kim, Dagam Lee, Jin Kon Kim, Jaeup Kim AB block copolymers can assemble into various nanoscale morphologies such as lamella, cylinder, sphere and gyroid depending on their composition and the interaction strength. In this work, we theoretically study various block copolymer morphologies in hemispherical and ellipsoidal shape confinements and compare the results with experiments. In the experiment, PS-PMMA block copolymers are physically confined by air and surface of nanobowl which interacts preferentially or randomly depending on the coating of the nanobowl. Our theoretical modeling uses self-consistent field theory (SCFT) which calculates the mean field density distribution of AB block copolymers in this confined geometry. The key parameters for the morphology determination are the size and shape of the container and the surface tension between components. For example, when the container wall is coated with PS polymers, onion-shape lamellar phase with PS at the bottom is observed rather than the parallel lamella r phase. It is also found that preferential air-polymer surface interaction promotes the alignment of domains. Our versatile method allows us to model ellipsoid-shaped confinements, and other interesting morphologies are found depending on the eccentricity of the ellipsoid. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W19.00005: Manipulating Nanoscale Morphologies in Block Copolymer Thin Films Using Gradient Approaches Ming Luo, Jonathan Seppala, Julie Albert, Ronald Lewis, III, Nikhila Mahadevapuram, Gila Stein, Thomas Epps, III Controlling the nanostructure of self-assembled block copolymer (BCP) thin films is critical for templating and membrane applications. The surface interactions and commensurability (film thickness) strongly influence the phase behavior of substrate supported BCP thin film. In this work, we employed a gradient approach to examine the effects of substrate surface chemistry and film thickness on the self-assembly of cylinder-forming poly(styrene-$b$-isoprene-$b$-styrene) (SIS) thin films. We identified an interesting phase transformation from parallel cylinders to hexagonally perforated lamellar (HPL) structures on chlorosilane modified substrates, and the through-film morphology was further characterized using ultraviolet ozone (UVO) etching, cross-sectional transmission electron microscopy (TEM) and grazing incidence small angle X-ray scattering (GISAXS) techniques. We demonstrated the use of film thickness and monolayer substrate surface chemistry gradients to manipulate the nanostructure of SIS thin films. These gradients represent a high-throughput screening tool that facilitates the examination of new materials and furthers the understanding of block copolymer thin film self-assembly. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W19.00006: Patterning square and rectangular arrays using shear-aligned block copolymer thin films So Youn Kim, Raleigh L. Davis, Richard A. Register, Jessica Gwyther, Adam Nunns, Ian Manners, Paul M. Chaikin Microphase separation of block copolymers in thin films can generate periodic structures: hexagonally packed arrays of dots from spherical or cylindrical phase block copolymers, or periodic stripes from cylindrical or lamellar phase block copolymers. Square or rectangular patterns, however, do not naturally form by spontaneous self-assembly of a simple diblock copolymer, and are a challenge to create. We present a simple way to create nano-square/rectangular arrays by building up a double-layer film of a cylinder-forming diblock, where each layer is sequentially deposited, shear-aligned independently, and cross-linked. Any block copolymer with at least one crosslinkable block can in principle be employed; in this study we use cylinder-forming polystyrene-b-poly(ferrocenylisopropylmethylsilane) and polystyrene-b-poly(hexylmethacrylate). The pitch of the array is tunable by varying polymer molecular weight. Oxygen reactive ion etching is used to reveal the grid structures, and these grids can in turn form nano-wells in the silicon substrate when the cylinder-forming block is very etch-resistant under the conditions used for silicon etching. Additionally, metal dots ordered in square arrays can be created using these grids as templates, via metal evaporation and lift-off. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W19.00007: Capillary instabilities of non-axisymmetric stripe arrays atop a viscous medium Zheng Zhang, Yifu Ding We study the simultaneous capillary breakup of parallel polymer stripes, which were non-axisymmetrically embedded in another immiscible polymer medium. Polystyrene (PS) and poly(methyl methacrylate) (PMMA) were used because of both their immiscibility and well-characterized properties. The influences of the stripe-to-matrix viscosity ratio, volume ratio and substrate energy on the morphological development were examined. Notably, phase-correlation in the breakup of neighboring stripes was observed. The transition between in-phase and out-of-phase breakup was controllable with the degree of substrate confinement. Under strong substrate confinement, the simultaneous breakup of the parallel polymer stripes transitioned from non-correlated to in-phase, irrespective of the viscosity ratio between PS and PMMA. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W19.00008: Capillary wrinkling in thin film polymer annuli David J. Farmer, James S. Sharp A capillary driven wrinkling instability was studied in thin film ($\sim$ 350nm thick) annuli of polystyrene (PS) suspended on the aqueous sub-phase of a Langmuir-Blodgett (LB) trough. Surfactant was added around the outside of the annuli and the surface pressure/surface tension difference, P, between the inside and outside of the annuli was varied via the motion of the PTFE barriers of the LB trough. Radially oriented wrinkles were formed on the surface of the annuli above a critical value of the surface pressure difference and the number of wrinkles formed, n, was found to increase with increasing P. In this talk we will present a combination of experiments and a theoretical model that attempts to explain the process of wrinkle formation in these samples. This model shows that it is possible to extract parameters such as the elastic modulus of ultrathin film polymer samples from simultaneous measurements of n and P for annuli with internal and external diameters (a and b respectively) with values in the range 10 mm \textless a,b \textless 25 mm. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W19.00009: Tension Amplification and Structural Rearrangement in Tethered Bottle-Brush Layers via Molecular Dynamics Simulation Gary Leuty, Mesfin Tsige, Michael Rubinstein, Gary S. Grest Bottle-brush polymers are a subgroup of comb polymers -- linear polymer backbones to which linear polymer side chains are grafted. What sets bottle-brushes apart is the density of side chains grafted to the backbone; in bottle-brushes, that density is high enough that neighboring side chains experience significant mutual steric repulsion. This repulsion restricts the conformations available to the backbones and forces them to stretch, generating considerable tension that is controlled primarily by the density of side chains and their length. When bottle-brush polymers are end-tethered to a solid substrate, however, intermolecular interactions can augment the generation of tension as the number of side chains per unit area increases, leading to crowding. Using coarse-grained molecular dynamics simulations of homopolymer bottle-brush layers tethered to a flat wall, we have examined the effects of varying the chain length of both the backbone and side chains and the number of chains per unit surface area. We then show that the side chains are compressed and reorient during the transition from isolated macromolecules to dense but unentangled layers. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W19.00010: A single liquid on a homogeneous substrate can lead to quantized contact angles and running droplets Mark Ilton, Pawel Stasiak, Mark Matsen, Kari Dalnoki-Veress We have observed for the first time a quantized spectrum of contact angles in the dewetting of a liquid from a homogeneous solid substrate. Using structured liquids of a lamellar diblock copolymer above the order-disorder transition temperature, we observe that predominantly disordered droplets coexist with different discrete thicknesses of wetting layer. At a fixed temperature, the measured contact angle of a droplet depends only on the number of monolayers in the wetting layer, resulting in a temperature dependent spectrum of contact angles. To describe the behavior of this system, a self-consistent field theory calculation was performed to calculate the effective interface potential of a lamellar diblock copolymer in its disordered state. The calculation shows excellent qualitative agreement with experiment. Further experiments were performed examining droplets which coexist with two different thicknesses of wetting layer. These droplets experience an unbalanced force which leads to ``running droplets''--droplets that move and whose dynamics can be understood from the effective interface potential. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W19.00011: Morphology Development and In-situ Crosslinking in Electrosprayed Thin Films Hanqiong Hu, Jonathan Singer, Chinedum Osuji Electrospray has been recently developed as a novel technique for continuously depositing ordered block copolymer thin films. The development of well phase-separated microstructures is achieved by balancing thermal equilibration, deposition rate and residual solvent content, which are all readily tuned by spray parameters. Here we describe the morphology development of a lamellae-forming PS-b-P4VP and explore the preservation of non-equilibrated vertical orientation through in-situ crosslinking in a cylinder-forming SBS deposited by electrospray. We conducted parametric studies of solvent composition, flow rate, substrate temperature, solution concentration and molecular weight on determining morphology. Special emphasis was given to equilibration kinetics tuned by the residual solvent content. Film morphology transitioned from alternating lamellae to hexagonally packed micelles in the dry spray limit. In the ``wet'' spray regime, the evaporation of solvent from deposited material led to perpendicular alignment of cylinders in the SBS system. In-situ crosslinking through the addition of thermal initiator enabled quasi-epitaxial growth of vertically oriented domain when the competition between crosslinking and ordering were well balanced. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W19.00012: Protonation-induced microphase separation in thin films of a polyelectrolyte-hydrophilic diblock copolymer Charlotte Stewart-Sloan, Bradley Olsen Materials with easily and controllably tuneable morphologies are of interest for many applications where the relevant properties depend upon the microstructure. Here, we present a novel double hydrophilic diblock copolymer whose solid state morphology is responsive to protonation. It contains one block which is neutral and hydrophilic at all values of pH, poly(oligoethylene glycol methyl ether methacrylate) (POEGMA), and one block which is neutral and hydrophobic above its pKa but positively charged and hydrophilic when protonated, poly(2-vinylpyridine) (P2 VP). This material is disordered when cast from acid-free solutions but displays increasing segregation between the two blocks with increasing protonation of the pyridine groups. The protonation-induced microphase separation is shown to be due to ionomer-like effects and not to the selective solubilzation of ions in one of the blocks. Order-disorder transitions occur between 1:0.28 and 1:0.55 pyridine group:acid content for thin films of a 50kg/mol POEGMA-30kg/mol P2VP diblock and between 1:0.8 and 1:0.9 pyridine group:acid content for thin films of a 43kg/mol POEGMA-13kg/mol P2VP diblock. The latter also displays an order-order transition between spheres and in-plane cylinders between 1:1 and 1:1.1 pyridine group:acid loading. These films can be annealed in aqueous as well as polar organic solvents, allowing for both traditional polymer processing and environmentally friendly water-based casting and annealing. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W19.00013: Factors Influencing Shear Alignment of Cylinder-Forming Block Copolymer Thin Films Raleigh Davis, Richard Register, Paul Chaikin Application of shear stress to block copolymers is known to preferentially orient the microdomains in the direction of applied shear. While this phenomenon has been well studied for bulk block copolymer systems, the use of shear stress to align microdomains in block copolymer thin films (typically one to several microdomain layers) is still an active area of research. Numerous experimental factors influence the ease with which orientation is achieved as well as the ultimate quality of alignment observed. The present work investigates several of these factors using a series of cylinder-forming poly(styrene)-poly(hexylmethacrylate) copolymers. Parameters studied include film thickness, block copolymer molecular weight and composition, substrate wetting conditions (controlled via grafted polymer brush layers of either polystyrene or polyhexylmethacrylate), and applied shear stress. Quality of alignment is assessed via atomic force microscopy and subsequent computation of an orientational order parameter and the density of defects in the microdomain lattice. The results are compared to a melting-recrystallization model, thus providing greater insight into the fundamental mechanisms and key parameters which control how microdomains order in response to shear. In general monolayers are observed to align more poorly than thicker films, though the influence of film thickness on orientation depends strongly on polymer composition. Alignment quality is ultimately limited by inherent fluctuations in the cylinder trajectories as well as the presence of isolated dislocations. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W19.00014: Direct Write Thermocapillary Dewetting of Polymer Thin Films by a Laser-Induced Thermal Gradient Jonathan Singer, Pao-Tai Lin, Steven Kooi, Jurgen Michel, Lionel Kimerling, Edwin Thomas Laser direct write (DW) is an attractive alternative to the slower vacuum chamber particle beam techniques as it can achieve up to cm/s patterning rates. The materials employed for both laser and charged particle DW, however, are often expensive, designer materials. By taking advantage of the interaction between optical and thermal effects, we have developed a positive-tone laser DW technique that can induce controlled dewetting conventional polymer systems (here polyvinylacetate, polystyrene, and polyvinylpyrrilidone). Via this combination of antireflection, dewetting, and thermal absorption, features <100 nm can be achieved through exploiting overlap-based pattern formation, but with a much greater degree of deliberate control than is usually achieved by bottom-up dewetting. This is accomplished with a continuous wave, 532 nm source and free space optics that have a relatively low numerical aperture (NA=0.4), thus representing a sub-diffraction limit patterning technique. Using experiments and simulations, we demonstrate the mechanism and efficacy of this technique and investigate the effects of material parameters such as molecular weight and glass transition temperature. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W19.00015: Patterning Thin Polymer Films by Photodirecting the Marangoni Effect Christopher Ellison, Chae Bin Kim, Dustin Janes, Joshua Katzenstein New methodologies for patterning micro- and nano- scale features in polymer thin films are desired because of their high technological relevance to a range of applications, including microelectronics fabrication. A new non-contact strategy for high-speed patterning of arbitrary shapes in polymer films that involves photochemically directing the Marangoni effect will be described. The Marangoni effect drives the formation of thin film topography by causing liquid flow in response to surface energy gradients. In this approach, a topographical pattern can be preprogrammed and stored in a smooth glassy film using light activated chemistry to pattern surface energy gradients. The topography can be later revealed by heating the film to the liquid state without use of a wet or dry etch step, unlike traditional photoresist methods. The use of grafting reactions from small molecule photosensitizers to change the surface energy locally in polymers that do not intrinsically undergo photochemical reactions will also be discussed. Judicious selection of the photosensitizing compound in an otherwise transparent polymer expands the use of this method to more readily available light sources. We believe this methodology will be potentially useful as a facile and ubiquitous patterning technique for many polymers. [Preview Abstract] |
Session W20: Focus Session: Membranes and Confinement
Sponsoring Units: DPOLYChair: Pullickel Ajayan, Rice University
Room: 405
Thursday, March 6, 2014 2:30PM - 2:42PM |
W20.00001: Poly(styrene-b-dimethylsiloxane-b-styrene) Membranes in Pervaporation for In Situ Product Recovery during Fermentation Chaeyoung Shin, Zachary Baer, Ali Evren Ozcam, Douglas Clark, Nitash Balsara In situ product recovery was investigated in fermentation experiments to enable the development of a continuous fermentation process. Our pervaporation membranes are based on poly(styrene-b-dimethylsiloxane-b-styrene) (SDS) block copolymers. Polydimethylsiloxane (PDMS) is the best known organophilic pervaporation membrane material and was utilized as the transporting phase for selective permeation of organic molecules. The polystyrene (PS) block added structural integrity to the membrane due to the high modulus of PS. SDS membranes were found to have both the enhanced robustness as well as comparable pervaporation performance to that of cross-linked PDMS membranes. The permeabilities of water and organic components through SDS membranes were studied to elucidate the sorption and transport phenomena in this system. Furthermore, experiments combining fermentation with pervaporation were performed, and continuous fermentation by using pervaporation as the sole means of removing products was successfully demonstrated for the first time. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W20.00002: Microphase Separated Block Copolymers in Pervaporation Membranes for Biofuels Processing Douglas Greer, Chae-Young Shin, Evren Ozcam, Jeffrey Skerker, Thalita Basso, Dacia Leon, Stefan Bauer, Nitash Balsara The production of transportation biofuels requires numerous continuous separation processes. We designed block copolymer membranes for pervaporation as a means to achieve these separations. These block copolymers contain a glassy structure block for support and a rubbery transport block for sorption and diffusion. We create membranes with nanoscale conducting channels using the unique trait of block copolymers to assemble into ordered morphologies. We have previously used nanostructured membranes to separate ethanol/water binary mixtures [J. Membr. Sci. 373, 112 (2011)], [J. Membr. Sci. 401, 125 (2012)]. We report this type of membranes is effective in other, more complex separations important to biofuel production. These separations increase yield and decrease process time. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W20.00003: Controlling Solution Self-assembly and Non-Solvent Induced Microphase Separation of Triblock Terpolymers to Generate Nanofiltration Membranes with Chemically-Tailored Pore Walls Bryan Boudouris, Ryan Mulvenna, Jacob Weidman, William Phillip Block polymer-based templates have been utilized in a number of membrane applications; however, there has yet to be a demonstration of a nanoporous block polymer thin film that can achieve high flux and high selectivity simultaneously while also allowing for the facile tuning of the pore wall chemistry. Here, we demonstrate that by synthesizing and controlling the solution self-assembly of a triblock terpolymer, polyisoprene-$b$-polystyrene-$b$-poly($N$,$N$-dimethylacrylamide) (PI-PS-PDMA), and precisely inducing non-solvent induced phase separation during the self-assembly process allows for the creation of an asymmetric nanoporous membrane with PDMA-lined pore walls. This PDMA functionality is then converted to any number of side chain functionalities through simple chemistry in the solid state. In this way, we are able to show a highly selectivity membrane that can separate analytes of interest based both on size and chemical composition at a high solution flux. In fact, this high fidelity structure has a very narrow distribution of pore sizes (\textless 10{\%} variation in diameter) over large areas (\textgreater 500 cm$^{\mathrm{2}})$. This has allowed for the separation of particles with hydrodynamic radii as low as 0.8 nm, which is the smallest separation achieved using a block polymer-based membrane to date. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W20.00004: N/A Invited Speaker: Ramanan Krishnamoorti |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W20.00005: Mechanical Properties of Two-Dimensional Alkanethiol-Coated Gold Nanoparticle Membranes K. Michael Salerno, Dan S. Bolintineanu, J. Matthew D. Lane, Gary S. Grest Membranes formed from nanoparticle monolayers have been shown to have mechanical properties that may make them suitable for use in micro-scale devices. Metallic-core nanoparticles with short, organic ligands can form membranes with dimensions up to several micrometers, with large elastic moduli. Experimental tests of membranes with different cores and ligands indicate that ligand length as well as core-ligand and ligand-ligand interactions can influence membrane mechanical response. We use explicit-atom molecular dynamics simulations to examine the properties of membranes formed from a two-dimensional hexagonal array of alkanethiol-coated Au nanoparticles. Results are presented for nanoparticle core diameters from 4-6nm, ligand lengths of 10-18 units and carboxyl and methyl end groups, all of which influence the mechanical properties of the membranes. Knowledge of how microstructure and composition influence membrane properties could lead to efficient membrane manufacture with improved mechanical properties. 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 3:54PM - 4:06PM |
W20.00006: Nanoparticle Encapsulation in Diblock Copolymer/Homopolymer Blend Thin Film Mixtures Junnan Zhao, Xi Chen, Peter Green We investigated the organization of low concentrations of poly (2-vinylpyridine) (P2VP) grafted gold nanoparticles within a diblock copolymer polystyrene-b-poly (2-vinylpyridine) (PS-b-P2VP)/homopolymer polystyrene (PS) blend thin film. The PS-b-P2VP copolymers formed micelles, composed of inner cores of P2VP block and outer coronae of PS blocks, throughout the homopolymer PS. All nanoparticles were encapsulated within micelle cores and each micelle contained one or no nanoparticle, on average. When the host PS chains are much longer than corona chains, micelles tended to self-organize at the interfaces. Otherwise, they were dispersed throughout the PS host. In comparison to the neat PS-b-P2VP/PS blend, the nanoparticles/PS-b-P2VP/PS system had a higher density of smaller micelles, influenced largely by the number of nanoparticles in the system. The behavior of this system is understood in terms of the maximization of the nanoparticle/micelle core interactions and of the translational entropies of the micelles and the nanoparticles. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W20.00007: Bicontinuous Porous Carbon Films Templated with ABC Triblock Copolymers Kevin Cavicchi, Guodong Deng, Bryan Vogt Mesoporous carbons are useful for a range of applications such as separation and catalysis. A route to prepare porous materials is through cooperative self-assembly of a carbon precursor (e.g. phenolic resin) and a block copolymer, in which the precursor is selectively soluble, to drive mesophase formation. Typical soft templating uses AB or ABA block copolymers, which form classical morphologies, such as spheres, cylinders, and lamellae. Switching to an ABC type block copolymer provides greater flexibility in the design of the morphology potentially opening up larger processing windows for complex structures, such as bicontinuous morphologies. This presentation will discuss efforts to prepare bicontinuous porous carbon thin films using an ABC triblock copolymer of poly(ethylene oxide)-block-poly(ethyl acrylate)-block-polystyrene via spin-coating and a series of thermal annealing steps. It will be shown that direct thermal annealing can produce high porosity ($\sim$60\%) carbon fiber networks. In addition, adding a solvent annealing step prior to the thermal annealing steps is able to produce longer range order structures with a small window of an ordered bicontinuous morphology. These high porosity films with organized fibers are promising for energy and separation applications. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W20.00008: Crystallization and Phase Transitions in Polymer Nanolayered Systems under Confinement Invited Speaker: Eric Baer Forced assembly multiplication coextrusion has been reported as an advanced technique to study crystallization and phase transitions for polymers under confinement. This technique can readily produce continuous alternating layered structures composed of two or three polymers [1, 2]. Multilayer films fabricated by multiplication coextrusion consist of hundreds or thousands of layers with individual layer thickness varying from 10 nanometers to several micrometers. The flexibility of this novel multiplication coextrusion process, particularly at the nanoscale, enables the study of confinement effects on polymer crystallization and phase transitions. We have discovered that the hierarchical morphology of many polymers can be manipulated by confinement between rigid layers [2]. Spherulites are flattened and lamella single crystals are oriented as the confining scale is decreased towards the nano-level. Poly(ethylene oxide) (PEO), poly($\varepsilon $-caprolactone) (PCL), syndiotactic polypropylene (sPP) and poly(vinylidene fluoride) (PVDF) will be given as unique examples of this phenomena [2, 3]. Depending on the crystallization temperature, two major lamellae orientations ``in-plane'' or ``on-edge'' can be achieved, which dramatically affect the multilayer film characteristics, such as film barrier properties [2, 4].For some polymers such as high density polyethylene (HDPE), in-plane lamellae orientation is difficult to achieve. However, at the micro-scale, confined HDPE spherulites have tilted lamellae, which also improve gas barrier properties. Nanoscale multilayer films were also utilized to produce submicron size polymer droplets by thermal breakup of the layers [4]. Phase transitional behaviors during fractionated crystallization of these droplets will be described as a powerful tool for the study of both heterogeneous and homogeneous nucleation. \\[4pt] [1] M. Ponting, A. Hiltner and E. Baer, Macromolecular Symp, 294(2010), 19 -32.\\[0pt] [2] J. l M. Carr, D.S. Langhe, M. T. Ponting, A. Hiltner, and E. Baer, J. Mater. Res, 27(2012), 1326-1350.\\[0pt] [3] J. M. Carr, M. Mackey, L. Flandinb, A. Hiltner and E. Baer, Polymer, 54(2013), 1679--1690.\\[0pt] [4] D.S. Langhe, A. Hiltner and E. Baer, Polymer, 52(2011), 5879--5889. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W20.00009: Estimation of the Thickness of the Interface in Polyoctenamer-Single Walled Carbon Nanotube Composites by Thermogravimetric Analysis Alin Cristian Chipara, Robert Vajtai, Pulickel M. Ajayan, Dorina M. Chipara, Elamin Ibrahim, James Hinthorne, Mircea Chipara In polymer-based nanocomposites, macromolecular chains surround the nanoparticles interacting with them and thus defining a thin layer of material known as interface. The interface exhibits modified physical properties compared to the polymeric matrix; shifts of the glass, melting, and crystallization temperatures. A simple method for the estimation of the thickness of the interface in polymer based nanocomposites, by using thermogravimetric analysis is presented. The method is exemplified through experimental data on polyoctenamer-single walled carbon nanotube nanocomposites obtained by melt mixing. The thermal stability of the as obtained nanocomposites has been investigated by thermogravimetric analysis, using a Q50 TGA instrument from TA Instruments. The measurements have been performed in air and in nitrogen atmosphere at various heating rates (5, 10, 20, 30, and 40 $^{o}$C/min). Additional measurements by Raman, and Wide Angle X Ray are supporting thermal analysis data. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W20.00010: Programmable Crafting of Hierarchically Structured Block Copolymer/Nanoparticles (and Nanorods) via Flow Enabled Self-Assembly Zhiqun Lin, Bo Li, Wei Han Hierarchical assembly of diblock copolymer/nanocrystals (e.g., Au and CdSe nanoparticles and nanorods) was successfully crafted into parallel stripes by flow enabled self-assembly (FESA). They were precisely and programmably patterned at desired position on the substrate. Remarkably, a minimum spacing between two adjacent stripes was observed and a model was proposed to understand the relationship between the width of stripes and the minimum spacing. FESA of diblock copolymer/nanocrystals is a lithography-free method and facile to implement, offering opportunities for creating functional hierarchically structured materials and devices. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W20.00011: On the Glass Transition in Polystyrene-TiO2 Nanocomposites Jorge Alarcon, Dorina M. Chipara, Karen Lozano, Mircea Chipara, Alin Cristian Chipara, Robert Vajtai, Pulickel M. Ajayan Nanocomposites of atactic polystyrene (PS) filled with TiO2 nanoparticles of about 15 nm have been prepared. A dilute solution of PS in a good solvent (chlorophorm) has been prepared by stirring the components at room temperature for 24 h at 500 rotations per minute. The solution was then sonicated for 5 minutes by using a high power sonicator. TiO2 nanoparticles were added in the sonicating bath and the sonication continued for 1 hour in order to achieve an uniform dispersion on nanoparticles. Then, a non solvent (distilled water) has been suddenly added under sonication. The sonication continued for about 30 minutes. After 30 minutes, the polymer nanocomposite was isolated from the liquid mixture by filtration. The residual amount of solvent and water was removed by placing the nanocomposites into a vacuum oven at 100 C for 12 hours. The complete removel of water and solvent was confirmed by TGA. The as obtained samples were measured by Differential Scanning Calorimetry and the effect of TiO2 on the glass transition temperature was investigated. The effect of TiO2 on the glass transition of PS is discussed. [Preview Abstract] |
Session W21: Polymer Melts and Solutions
Sponsoring Units: DPOLYChair: Rob Hoy, University of South Florida
Room: 406
Thursday, March 6, 2014 2:30PM - 2:42PM |
W21.00001: Multi-level slip-link modeling Jay Schieber That the dynamics of concentrated, high-molecular-weight polymers are largely governed by entanglements is now widely accepted, and typically understood by the tube model. Although the original idea for slip-links was proposed at the same time as tubes, only recently have detailed, quantitative mathematical models arisen based on this picture. We argue here for the use of a slip-link model that has strong connections to atomistic, multichain levels of description, agrees with non-equilibrium thermodynamics, applies to any chain architecture and can be used in linear or non-linear rheology. We present a hierarchy of slip-link models that are connected to each other through successive coarse graining. One might choose a particular member of the hierarchy depending on the problem at hand, in order to minimize computational effort. In particular, the most detailed level of description has four parameters, three of which can be determined directly from atomistic simulations. The least-detailed member is suitable for predicting non-linear, non-uniform flow fields. We will show how using this hierarchy of slip-link models we can make predictions about the nonlinear rheology of monodisperse homopolymer melts, polydisperse melts, or blends of different architectures. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W21.00002: Synthesis of amphiphilic diblock copolymer for surface modification of Ethylene-Norbornene Copolymers Simon Levinsen, Winnie Edith Svendsen, Andy Horsewell, Kristoffer Almdal The aim of this work is to produce polymer modifiers in order to develop hydrophilic polymeric surfaces for use in microfluidics. The use of hydrophilic polymers in microfluidics will have many advantages e.g. preventing protein absorbance. Here we present an amphiphilic diblock copolymer consisting of a bulk material compatible block and a hydrophilic block. To utilize the possibility of incorporating diblock copolymers into ethylene-norbornene copolymers, we have in this work developed a model poly(ethylene-1-butene) polymer compatible with the commercial available ethylene-norbornene copolymer TOPAS. Through matching of the radius of gyration for the model polymer and TOPAS the miscibility was achieved. The poly(ethylene-1-butene) polymer was synthesized from a hydrogenated anionic polymerized polybutadiene polymer. As hydrophilic block poly(ethylene oxide) was subsequently added also with anionic polymerization. Recent miscibility results between the model polymer and TOPAS will be presented, as well ongoing efforts to study the hydrophilic surface. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W21.00003: Pulling a Polymeric Chain through Tiny Grommets Xiaorong Wang, Yongli Mi In this model, the N beads of mass m are connected in a chain by entropy springs. The friction of the chain is described by a viscous-force for energy dissipation. The interaction of the chain with the grommets is represented by a narrow Gaussian potential. The equations of motion for this model are Langevin type. It is shown that in this model the chain displays oscillated motions under a constant pulling condition. This stick-slip dynamics in a polymer system has not been discussed previously, but may really exist in some systems such as polymer nano-composites and polymer-entangled fluids. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W21.00004: Unravelling Popular Myths in the Rheology of Entangled Polymer Melts Richard P. Wool \underline {Myth No 1}: \textbf{Constraint release and Chain End Fluctuation} coupled with Reptation dominate stress relaxation of highly entangled chains. Fact: Experiments show that Percolation processes account for about 50{\%} of the relaxation, coupled with deGennes Reptation dynamics. In fact, the random coils are not relaxed (via Neutrons) when the stress (Birefringence) goes to zero, a critical prediction of the percolation mechanism, which is not in violation of the stress-optical law.. \underline {Myth No 2}: The \textbf{Packing Length Entanglement Model }for the critical entanglement molecular weight, M$_{\mathrm{e}} =$ 354 p$^{3}$, provides a fundamental description of entanglements at the molecular level. Fact: Experiments show that the Packing Length model is fundamentally incorrect in all its predictions of rheological properties via M$_{\mathrm{e}}$ $\sim$ [ M$_{\mathrm{o}}$/C$_{\mathrm{\infty }}$]$^{\mathrm{3}}$, especially at the nanoscale, as well as the bulk. This is due to an incidental relationship between the monomer molecular weight M$_{\mathrm{o}}$ and the characteristic ratio C$_{\mathrm{\infty }}$ for vinyl type polymers. The correct entanglement model is given by M$_{\mathrm{e}}$ $\sim$ C$\infty $M$_{\mathrm{o}}$. \underline {Myth No 3}: \textbf{The Glass Transition T}$_{\mathrm{\mathbf{g}}}$\textbf{ is dominated by Segmental Dynamics and Free Volume}: Fact: Experiments show that T$_{\mathrm{g}}$ is dominated by the cluster dynamics of anharmonically interacting fractal aggregates which range from 1-100 nm in size (ave $\sim$ 5 nm), as described by the TFT. In nanoconfined thin films, the segmental dynamics does not change much while huge decreases in T$_{\mathrm{g}}$ can be observed due to cluster size effects. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W21.00005: Shear and extensional rheology of model branched polymer melts (H shaped and grafted) Gengxin Liu, Konstantinos Ntetsikas, Kostas Misichronis, Namgoo kang, Jimmy Mays, Apostolos Avgeropoulos, Shi-Qing Wang While nonlinear rheology of entangled linear polymers has been fully explored in recent years, the effect of chain architecture remains the last frontier in polymer rheology. Here we study two H-shape and one grafted-polyisoprene (3 branches) using startup and step extension and shear. Long chain branches (LCB) impede yielding and prevent entangled network from full disentanglement. Thus, nonlinear rheological behavior of LCB polymers forms a sharp contrast to that of linear chains. We will demonstrate these striking differences. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W21.00006: Visualize space-dependence of viscosity Lingxiang Jiang, Boyce Tsang, Steve Granick The space-time dependence of viscosity plays a fundamental, crucial role in a number of natural and industrial processes, where the time dependence has been extensively studied by conventional methods, yet its spatial counterpart has not been directly determined. Here, we propose an imaging based method to measure the space-time dependent cooperative viscosity and confirm its validity in a biopolymer, F-actin solution. A space dependent master curve of cooperative viscosity is identified with an exponential growth at short distance (correlation length 8 times of mesh size) and a plateau at long distance (surprisingly large crossover distance 18 times of mesh size), therefore visualizing the discrete-to-continuum transition of viscosity in real space. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W21.00007: Synthesis and rheological behavior of atactic polypropylene molecular bottlebrushes Samuel Dalsin, Frank Bates, Marc Hillmyer Molecular bottlebrushes are branched polymer structures characterized by an extremely high density of polymeric side chains emanating from a central backbone. Due to unique conformational and rheological properties, molecular bottlebrushes have become attractive candidates for developing new photonic bandgap materials, nanotubes and nanowires, and rheological modifiers. In this study, bottlebrushes comprised of atactic polypropylene (aPP) side chains were synthesized via ring-opening metathesis polymerization of norbornenyl-terminated aPP macromonomers. A series of bottlebrush polymers with fixed side chain length and variable backbone length was prepared using Grubbs' third-generation catalyst, yielding products with low dispersity in less than five minutes reaction time. Small-amplitude oscillatory shear measurements were performed to examine linear viscoelastic properties. Master curves of all bottlebrush polymers exhibited relaxation spectra devoid of any entanglement plateau, despite high molecular weights (up to 892 kg/mol). Lack of entanglement was further confirmed by zero shear viscosity experiments, which displayed a nearly linear dependence on molecular weight. These rheological properties are compared directly with a linear aPP control sample. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W21.00008: Structure of a bottlebrush melt Jaroslaw Paturej, Sergei Sheiko, Sergey Panyukov, Michael Rubinstein A bottlebrush polymer is a branched macromolecule composed of a linear chain (backbone) with side chains densely tethered to it. High grafting density of side chains gives rise to various unique structural properties, such as highly extended conformations of their backbones and tunable character of their stiffness and rheological properties with degree of polymerization of the side chains. We conducted coarse-grained molecular dynamics simulations to determine how the number of Kuhn segments in a bottlebrush backbone $L$ and in the side chains $N$ affect size, stiffness, and structure of these molecules. We found that the size (root-mean-squared radius of gyration and end-to-end distance) and persistence length of bottlebrushes in a melt state scales as $N^{1/2}$. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W21.00009: Role of the Entanglements and Bond Scission in High Strain-Rate Fracture of Polymer Melts Yelena Sliozberg, Robert Hoy, Randy Mrozek, Joseph Lenhart, Jan Andzelm We present coarse-grained molecular dynamics simulations of the effects of solvent molecular weight on the toughness of entangled and non-entangled polymer gels. Our results demonstrate that higher molecular weight solvents enhance gel toughness, and that mechanical properties including strength and toughness can be related to bond scission. We find a remarkable two-step gel fracture mechanism: network chains undergo scission first (and well before fracture), followed by scission of solvent chains. Even after the network chains break, long highly entangled solvent chains provide fracture resistance by effectively increasing the number of chains that must be broken as a crack propagates [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W21.00010: Controlling solubility of pNIPAM in aqueous solutions using hydrophobic and photoresponsive molecular units Rahul Singh, Sanket Deshmukh, Subramanian Sankaranarayanan, Ganesh Balasubramanian The structural properties of pNIPAM (poly-N-isopropylacrylamide), which is a thermally sensitive polymer, are investigated by copolymerizing it with molecular units that are either (1) hydrophobic (polystyrene) or (2) photoresponsive (spiropyran-merocyanine pair). We employ molecular dynamics (MD) simulations to examine aqueous solutions of pNIPAM (modified with these molecules) across a temperature range below and above the LCST of pure pNIPAM to understand the fundamental physics underlying the coil-to-globule transition in pNIPAM and the contribution of the attached constituents on the LCST. The LCST can be tuned by copolymerizing pNIPAM with polystyrene (PS), a hydrophobic molecule. We prepare a number of copolymers with different chain lengths of the hydrophobic units (PS) and observe the lowering of the LCST of the modified pNIPAM by computing the radius of gyration and end-to-end distances across the temperature range. Also, the aqueous solubility of pNIPAM can be controlled by functionalizing it with a photoresponsive moiety as this new copolymer exhibits a shifted LCST phase transition. Thus, the temperature sensitive behavior of pNIPAM can be tuned by copolymerizing it with varying molecular lengths of hydrophobic block units or attaching reversibly switchable photoresponsive moieties. Our work demonstrates the controllability of pNIPAM solubility aqueous solutions and recommends strategies to design complex programmable polymers that have wide-ranging applications in several biomedical and industrial processes. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W21.00011: Effect of Chain Architecture on the Structural and Rheological Properties of Dilute Polymer Solutions: A Molecular Simulation Study Fardin Khabaz, Rajesh Khare Advances in chemistry have allowed synthesis of polymer chains of specified architecture. The effect of chain architecture on the structure and rheology of dilute polymer solutions is not fully understood. Furthermore, possible breakage of polymer chains at high shear rates is important for several industrial applications. In this work, these effects are investigated by performing molecular dynamics simulations. Structure and rheology of dilute polymer solutions containing four types of chains - linear, comb shaped, H-shaped and star - are compared. Simulations are used to determine the scaling of the intrinsic viscosity with molecular weight for chains of different architectures. These simulation results are also compared with theoretical predictions from literature. In addition, structural stability of the chains is studied by monitoring their local deformation at high shear rates. Possible sites for chain scission are identified from these high shear rate simulations for each type of chain architecture. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W21.00012: Direct observation of polymer dynamics in semi-dilute solutions Kai-Wen Hsiao, Christopher Brockman, Charles M. Schroeder In this work, we use single molecule techniques to study polymer dynamics in semi-dilute solutions. Here, we study the steady state extension and relaxation time dynamics of polymer molecules in semi-dilute DNA solutions in extensional flow. Polymer chain dynamics are complicated in semi-dilute solutions due to chain overlap, hydrodynamic interactions, and excluded volume interactions. We use single molecule fluorescence microscopy and a microfluidic-based hydrodynamic trap to directly observe the dynamics of polymers in non-dilute solutions. We report the scaling of polymer relaxation time as a function of polymer concentration, and we observe a crossover in chain behavior from the dilute to semi-dilute regime. Interestingly, we observe a pronounced center-of-mass drift of single polymer chains in directions orthogonal to flow in semi-dilute solutions, which is characterized as a function of concentration and flow rate. By using the automated hydrodynamic trap coupled with a piezoelectric stage, we are able to track the 3-D position of single polymer molecules and deduce the relationship between strain rate and polymer extension. Overall, our work reports on a key advance in the field of polymer dynamics via direct observation of dynamics in semi-dilute solutions in strong flows [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W21.00013: Locality of entangled polymer dynamics Chi Hang Boyce Tsang, Lingxiang Jiang, Steve Granick A combination of sparse and full fluorescence labeling of entangled actin solutions (filaments about 15 $\mu $m long at 1 mg/ml concentration) allowed us to probe both filament-scale polymer dynamics and effectively monomer dynamics. On the filament scale, the reptation tube idea of classical polymer physics works well. However, on a local scale comparable to mesh size, local tube width fluctuation becomes important. For the first time, the dependence of longitudinal diffusion on local tube width was quantified. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W21.00014: Shear-induced irreversible breakdown of shear thickening fluids Jonathan Seppala, Kirk Rice, Gale Holmes Amorphous fumed silica/polypropylene glycol (PPG) suspensions were subjected to multiple steady shear and oscillatory shears above the critical strain rate and critical strain amplitude. After each strain sweep, the steady shear viscosity and oscillatory shear moduli decreased over the entire measured range, and the on-set of shear thickening occurred at increasingly higher critical strain rates or strain amplitudes. Analysis of the oscillatory intra-cycle stress-strain (Lissajous-Bowditch) curves indicated a single-cycle shear thickening occurred at strain amplitudes below the traditionally defined critical strain and only during the first pass. The changes in the material properties appear to be irreversible and are attributed to breakdown of fumed silica-PPG agglomerates. Simultaneous rheology and small angle neutron scattering (RheoSANS) was also used to test this hypothesis. Finally intra-cycle and non-linear responses for fumed silica-PPG on parallel plate and cone and plate were analyzed via the MITlaos package. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W21.00015: Application of scaling model to investigate solvent quality and functionality in star polymers Durgesh Rai, Gregory Beaucage, Ramanth Ramachandran, Kedar Ratkanthwar, Nikos Hadjichristidis, Hong Kunlun, David Uhrig, Andy Tsou Symmetric star polymers serve as model systems to understand branching effects in long chain macromolecules. Generally, the solution properties of stars have been modeled based on ideal Gaussian statistics or using empirical approaches that incorporate fractal scaling neither of which provide satisfactory complete understanding of thermodynamic or structural details across different solvent quality and temperature ranges. A coupling of the unified scattering function with the RPA equation and Benoit's approach to model inter-arm and intra-arm interactions is proposed to analytically quantify thermodynamic effects along with topological variations using the proposed scaling model. Detailed topological quantification of star polymers systems have been able to describe both, good and theta solvent conditions along with effects of functionalities, as well as resolve deviations in chain conformations due to steric interactions between star arms. The scaling model quantifies the distinction between invariant topological features for star polymers and chain tortuosity, which changes with functionality as well as goodness of solvent and steric interactions. [Preview Abstract] |
Session W22: Focus Session: Dynamics of Polymers Under Nanoscale Confinement III
Sponsoring Units: DPOLYChair: Robert Riggleman, University of Pennsylvania
Room: 407
Thursday, March 6, 2014 2:30PM - 2:42PM |
W22.00001: Physical Aging within Hairy NanoParticle Assemblies H. Koerner, M. Bockstaller, A. Dang, C. Mahoney, K. Matyjaszewski, C.-M. Hui, R. Vaia Polymer grafted nanoparticles provide solutions to overcome dispersion challenges in conventional polymer-inorganic nanocomposites (NCs). While most research has focused on blends of these hairy nanoparticles (HNPs) into polymer matrices, recent work has demonstrated substantial promise for solvent- or matrix-free assemblies of HNPs (aHNPs). Significant progress has been made in understanding the relationship between the structure of the polymer corona at intermediate and high graft densities and the morphology, mechanical properties and melts dynamics of the assembly. However, very little is known about the behavior of aHNPs with low graft densities ($\sigma $\textless 0.05 nm$^{-2})$ of high molecular weight chains that are above entanglement (\textgreater 60kDa). Such aHNPs contain more than 30 vol{\%} inorganic, with maximum separation between particle surfaces less than 10 nanometers. For such materials, we discuss the physical aging characteristics from enthalpy relaxation experiments of these highly confined poly(styrene) and poly(methylmethacrylate) grafts. Physical aging is substantially suppressed in the low $\sigma $ ($\sigma $\textless 0.05) regime, as compared to conventional NCs at similar nanoparticle loadings. Furthermore, relaxation rate, distribution and fragility indicate that aHNPs with high $\sigma $ exhibit behavior deep within the glass similar to conventional NCs and their neat polymers, however deviate substantially from Arrhenius behavior as Tg-T approaches 0. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W22.00002: Glassy structural relaxation of star-shaped polymers Bradley Frieberg, Emmanouil Glynos, Georgios Sakellariou, Peter Green Time-dependent changes of thermodynamic properties due to structural relaxations, physical aging, occur in all glasses. In the case of linear polymers, the aging rate is independent of the degree of polymerization at a given aging temperature, relative to the average glass transition temperature. In contrast, we demonstrate that star-shaped macromolecules exhibit average structural relaxations that are dependent on both the number of arms, f, and the degree of polymerization of each arm, Narm. In particular, while increasing f, and/or decreasing Narm, the average segmental relaxation rate decreases, and can be up to a factor of two times lower for star-shaped molecules compared to their linear analogs. We reconcile these differences in terms of the free volume diffusion and its relation to the segmental motions in the glass state. We propose that this ideal class of polymeric materials, star-shaped molecules, can be used in order to tailor the physical properties on a molecular level, by simply changing the polymer architecture. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W22.00003: Glass transition of star-shaped thin polymer films Emmanouil Glynos, Bradley Frieberg, Georgios Sakellariou, Peter Green We show that the glass transition temperatures, $T_{\mathrm{g}}$, of thin polystyrene (PS) films, supported by oxidized silicon substrates (SiOx), depends on functionality, $f$, and the degree of polymerization of the arm, N$_{\mathrm{arm}}$, of the macromolecule. The $T_{\mathrm{g}}$s of star-shaped PS films with thickness H$=$30nm, $T_{\mathrm{g}}$(30nm) with $f$ as high as 64 were investigated. The $T_{\mathrm{g}}$(30nm) of linear PS thin films, is less than the average bulk $T_{\mathrm{g}}$, $T_{\mathrm{g}}$(bulk). For molecules of N$_{\mathrm{arm}}$ about100, the $T_{\mathrm{g}}$(30nm) of a macromolecule with $f=$3 was equal to that of a linear PS. However, the $T_{\mathrm{g}}$(30nm) increased with increasing $f$, reaching a maximum of at $f=$8 where $T_{\mathrm{g}}$(30nm) was higher than $T_{\mathrm{g}}$(bulk). For larger values of $f$, T$_{\mathrm{g}}$ (30nm) decreased monotonically with increasing $f$ and for $f=$64the$ T_{\mathrm{g}}$(30nm) became comparable to $T_{\mathrm{g}}$(bulk). The magnitude of this effect is weaker for much larger values of N$_{\mathrm{arm}}$. We rationalized these observations in term of two competing entropic: the increasing entropic attraction of the macromolecules to interfaces, with increasing $f$, and an increasing intermolecular entropic repulsion of these macromolecules with increasing f, and/or decreasing N$_{\mathrm{arm}}$. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W22.00004: Dynamics of Hyperbranched Polymers in the Bulk and under Confinement: Effect of Dendritic Generation Kiriaki Chrissopoulou, Krystalenia Androulaki, Spiros H. Anastasiadis, Daniele Prevosto, Massimiliano Labardi The structure and dynamics of three generations of a hyperbranched polyester polyol (Boltorn) and their nanocomposites with natural montmorillonite (Na$^{+}$-MMT) are investigated to offer a detailed picture of the behavior in bulk and under confinement. The structure was studied with X-ray diffraction (XRD) and differential scanning calorimetry (DSC), while the dynamics using dielectric spectroscopy (DS). XRD reveals that the polymer chains reside within the galleries of the Na$^{+}$-MMT producing an intercalated nanocomposite. The glass transition temperature, Tg, of the bulk polymers shows a dependence on the generation whereas the transition is completely suppressed when all chains are intercalated. The dynamics of the polymers and nanocomposites with $\sim$50wt\% polymer, where all chains are confined, were investigated for temperatures both below and above the polymer Tg. A sub-Tg process was found, showing similar features for the three polymers whereas the segmental relaxation was observed around Tg. For the nanocomposites, the dynamics that are observed show similarities and differences with the respective of the pure polymers depending on the specific process. Partially sponsored by EU (COST Action MP0902) and by the Greek GSRT (Research Funding Program: THALES (MIS 377278)) [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W22.00005: Confinement Effects on Molten Thin Cyclic Polystyrene Films Qiming He, Suresh Narayanan, David Wu, Mark Foster The surface fluctuations of melt film of 6k cyclic polystyrene (CPS) and its linear analog were measured using X-ray photon correlation spectroscopy (XPCS) for films of various thicknesses. The surface fluctuations of the 6k linear PS melt films 17 nm and thicker and the 6k cyclic melt films 28 nm and thicker can be described using a hydrodynamic continuum theory (HCT) that assumes the film is characterized only by the bulk viscosity. When a film of CPS is 24 nm or thinner, the behavior can no longer be captured using the HCT with bulk viscosity. The surface fluctuations behave as though the film has an effective viscosity higher than the bulk value. There is no evidence of an effective modulus in the very thin films of cyclic chains. The thickness at which confinement effects are seen for the 6k CPS chains is larger than that for the linear analogs. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W22.00006: Chain configurations, glass transition and polymer dynamics in polymer nanoparticles under 3D-confinement Aurora Nogales, Daniel E. Martinez-Tong, Michelina Soccio, Alejandro Sanz, Tiberio A. Ezquerra Polymer nanospheres with different size distributions of several polymer systems are prepared by a variety of methods, including miniemulsion and flash precipitation among others. The physical properties of the obtained nanoparticles have been studied. The calorimetric trace of these spheres shows an increase of the glass transition temperature that has been evaluated by means of an entropy model. This 3D-confinement, imposed by the nanospheres, leads to a limiting number of repeating polymer units in the sphere and thus to a reduction of the possible configuration states of the polymer chains, which is ultimately related to variations in the bulk value of the glass transition temperature. Our model is evaluated against our calorimetric measurements as well as with the data available in the literature. Good agreement between data and model is found for many cases, proving that confinement is related to reductions in entropy for these systems. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 4:18PM |
W22.00007: Structural Relaxations in Bulk and Thin Film Polymers: Role of Macromolecular Architecture Invited Speaker: Peter Green Structural relaxations that occur in polymers quenched below the glass transition temperature, T$_{\mathrm{g}}$, are responsible for time-dependent changes in physical properties that include optical, specific volume and the enthalpy. This phenomenon, physical aging, has been of interest in bulk polymers for decades and much is understood. The aging rate, $R$, is known to increase as the temperature at which the glass ages, $T',$ decreases below $T_{g}$, due to the increasing departure from structural equilibrium; it then decreases with further decreases in T. We show that the aging rates of star-shaped polymers are slower than their linear analogs of the same degree of polymerization, N. Whereas the temperature dependence of $R$ is independent of N for linear chains, it depends on the functionality, $f,$ and on the degree of polymerization per arm, N', of the star-shaped macromolecules. $R$ decreases with increasing $f, $for N' less than a threshold degree of polymerization, which increases with increasing $f$. The aging of very thin films (h $\sim$ 100 nm) is dependent on the distance, z, from an external interface. With the use positron annihilation spectroscopy (PALS) we determined the z-dependence of the T$_{\mathrm{g}}$ in thin films and showed that the aging rate is largely determined by the difference between the local glass transition temperature of the films and T'. Finally we show that the aging rates of linear and star-shaped macromolecules are consistent with experimental findings of glassy dynamics in both systems. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W22.00008: Effect of Confinement on Glass Transition Behavior in Polymeric Nanotubes and Nanorods: Comparison of DSC and Fluorescence Measurements Anthony Tan, John Torkelson The effects of nanoscale confinement on the glass transition temperature, Tg, and related behavior are studied in polystyrene nanotubes and nanorods made using anodized aluminum oxide templates. Tube thickness as small as 19 nm has been achieved by melt infiltration methods. Substantial Tg reductions are observed with both DSC and fluorescence measurements of nanotubes supported by the templates, with confinement effects being comparable in magnitude to those obtained via ellipsometry and fluorescence for supported polymer films. Free-standing nanotubes can also be characterized by DSC, yielding much larger Tg reductions than observed in supported nanotubes. Effects of confinement on fragility and physical aging in the supported polystyrene nanotubes and nanorods will also be discussed. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W22.00009: Glass Transitions in Polymer Nanocomposites Dong Meng, Sanat Kumar For polymers are under geometric confinement, it is generally believed that the glass transition temperature (T$_{\mathrm{g}})$ increases with favorable interfacial interactions. Experiments [1] and simulations [2] have reported that T$_{\mathrm{g}}$ increases almost proportionally to the attractive polymer-surface interactions. However, recent studies [3,4] have reported the contradictory finding that the T$_{\mathrm{g}}$ shift is rather modest and insensitive to the strength of interfacial attractions. In this study, we investigate the glass transition in polymer nanocomposites using molecular dynamics simulations. With attractive polymer-nanoparticle (NP) interactions, we find that T$_{\mathrm{g}}$ is increased by $\sim$ 3{\%} at moderate loadings and that the shift stays almost unchanged when the polymer-NP attractions are further increased by one order of magnitude. Both are in agreement with the recent experiments at comparable NP loadings [4]. We show that this is because the strongly adsorbed polymer segments do not participate in the glass transition. In other words, strong polymer-NP attractions create immobile polymer ``coatings'' around NPs that shield them from direct contact with the mobile polymers. \\[4pt] [1] Tate, R. S.; de Pablo, J. J.; Nealey, P. F. Journal of Chem. Phys. 2001, 115 (21), 9982-9990. \\[0pt] [2] Torres, J. A.; Nealey, P. F.; de Pablo, J. J. Phys. Rev. Lett. 2000, 85 (15), 3221$-$3224. \\[0pt] [3] Lu, H. Y.; Chen, W.; Russell, T. P. Macromolecules 2009, 42 (22), 9111$-$9117. \\[0pt] [4] Moll, J.; Kumar, S.K. Macromolecules 2012, 45 (32), 1131$-$1135. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W22.00010: Different Effects of Confinement on the Glass Transition Behavior of Supported Polymer Films and Model Polymer Nanocomposites Made with Carbon Based vs. Silica Based Substrates Lawrence Chen, John Torkelson While the effect of confinement on the glass transition temperature, Tg, of polymeric materials has been studied for two decades, only limited work has focused in a systematic way on the effect of different substrates, in the case of polymer films, or different nanofillers, in the case of nanocomposites. We employ both silica based and carbon based substrates on which films have been spin coated to study how the potential for pi-pi bonding interactions between polymer and substrate can modify the Tg-confinement effect in thin polystyrene (PS) films. Characterization is done in single-layer supported films by fluorescence spectroscopy and ellipsometry and in multilayer films by fluorescence. Model nanocomposite studies are also done by layering films supported on substrates, yielding a film sandwiched between substrates. Major differences in the Tg-confinement effect are observed in sufficiently thin PS films, with silica supported films possessing a free surface exhibiting major Tg reductions while the carbon supported analogs exhibit little Tg reduction. Bilayer film studies demonstrate an enhancement in Tg in a sufficiently thin PS layer in contact with a carbon substrate, which is not observed with a silica substrate. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W22.00011: Creating monodisperse polyacrylamide free-radically via thermal frontal polymerization in confined geometries Preeta Datta, Kirill Efimenko, Jan Genzer Bulk free radical polymerization reactions lead to highly polydisperse polymers (polydispersity index, PDI $\gg$ 1.5). In the past, researchers have shown that polymerization in porous microreactors can lower polydispersity (PDI $\sim$1.5-1.7) by promoting gelation. We employ free-radical thermal frontal polymerization reaction of acrylamide (AAm) in DMSO in highly confined reactors (height \textless 1mm) to produce high molecular weight ($\sim$300 kDa) PAAm of relatively low PDI ($\sim$1.2). In frontal polymerization systems, a localized reaction zone propagates in space along the direction of heat transfer, sustained by the interplay of heat diffusion and Arrhenius reaction kinetics. The directional heat transfer assists in maintaining the uniformity of the front temperature. While convection improves thermal transport, it causes inhomogeneity in the propagating front in horizontal reactors. In highly confined systems, convection is heavily suppressed, as manifested by the ``flattening'' of the reaction front and the absence of ``fingering''. Gelation lowers termination rate and increases the life time of the active reaction centers. Elimination of convection in confined geometries coupled with directional heat transfer and gelation results in polymers with high molecular weights and low PDIs. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W22.00012: The Effect of Nanoconfinemnt on Methyl Methacrylate Polymerization: Reactivity and Resulting Properties Haoyu Zhao, Ziniu Yu, Ronald Hedden, Sindee Simon The effect of nanoconfinement is well known to affect the properties of polymers. In this work, free radical polymerization of methyl methacrylate (MMA) is performed in hydrophilic or hydrophobic 13 nm diameter controlled pore glass (CPG). Changes in polymerization kinetics and the properties of the synthesized polymer are quantified. Reaction kinetics and glass transition temperatures are followed by differential scanning calorimetry (DSC). After polymerization, the changes in the molecular weights and tacticity are measured using gel permeation chromatography (GPC) and 1H nuclear magnetic resonance (1H NMR). Nanoconfinement is found to result in earlier onset of autoacceleration leading to the increase in both number-average and weight-average molecular weights, whereas the polydispersity index at full conversion decreases relative to the bulk value. Moreover, for both pore surfaces, the glass transition temperature increases compared with the bulk, but the increase in hydrophilic pores is more pronounced at 20 $^{\circ}$C. In addition to the changes in molecular weight and Tg, the tacticity changes from syndiotactic-rich triads for the bulk PMMA to a higher percentage of isotacticity under nanoconfinement. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W22.00013: Finite size effects on irreversible chain adsorption: a new probe of dynamics under nanoscale confinement Simone Napolitano, Caroline Housmans, Michele Sferrazza We investigated the role of finite size effects on the dynamics of thin polymer films, analyzing the thickness dependence of the kinetics of irreversible chain adsorption of polystyrene onto silicon oxide. We identified two growth regimes - linear at short times and logarithmic at long times - separated by a molecular weight independent crossover time, and by a crossover thickness scaling as predicted by the reflected random walk. Film thickness did not affect the dynamics at short time scales, while in the logarithmic growth regime we observed slower adsorption for melts confined in slabs thinner than 4-6 R$_{g}$'s. Given the correlation between the dynamics of the whole film and the structure of the adsorbed layer [1, 2], our findings suggest that the time necessary to equilibrate a polymer melt increases upon confinement. [1] Napolitano et al. Nature Comm., 2, 260 (2011) [2] Napolitano et al. EPJE 36, 61 (2013) [Preview Abstract] |
Session W23: Invited Session: Industrial Physics Forum: Panel Discussion: Industrial Innovation: An Intersection Among Industry, Academia and Government
Sponsoring Units: FIAPChair: David Seiler, National Institute of Standards and Technology
Room: 505-507
Thursday, March 6, 2014 2:30PM - 2:36PM |
W23.00001: Introduction and Overview of the Industrial Interactive Panel Session David Seiler A unique industrial panel covering the challenges and needs of various industries and how being innovative is important. The session involves two invited industry speakers (24 minutes each) who will set the stage for the interactive round table panel session. The Panel, led by moderator Mark Bernius (Morgan Advanced Materials), consists of the two invited speakers plus an additional four industry panelists. The first twenty-four minutes of the panel session has the four additional panelists introducing themselves and their work/company. These introductions could include what they or their company does, sharing one or two technical highlights, listing some challenges or needs for physicists, and what innovation breakthroughs are needed in their industries. The final hour of the session will be highly interactive with questions to the panel coming from the moderator, the audience, and the panelists themselves. Questions that might be addressed include: how physicists are or could be critical in advancing innovation; how can AIP/APS/FIAP help industry get the physics help they need to be innovative (knowledge, the right staff, etc.); what role can students and post docs play in advancing industry's mission; etc. We invite you to participate in this interactive session and ask our industry experts your own interesting and challenging questions. The invited speakers are George Thompson, Intel, and James Hollenhorst, Agilent Technologies. The panel members also include Jason Cleveland, Asylum Research; Robert Doering, Texas Instruments; William Gallagher, IBM T.J. Watson Research Center; and Martin Poitzsch, Schlumberger-Doll Research. [Preview Abstract] |
Thursday, March 6, 2014 2:36PM - 3:00PM |
W23.00002: Industry - Government Collaboration as an Engine of Innovation Invited Speaker: George Thompson The role of the government in encouraging innovation is a widely discussed topic in science policy today. This talk will review at a high level some of the different models for collaboration between industry and government, with an eye towards elucidating some of the collaboration characteristics that may be best correlated to successful innovation. Specific examples of programs based on goal setting, supporting the relevant national labs and universities, and direct public - private partnerships will be described in order to provide concrete examples. [Preview Abstract] |
Thursday, March 6, 2014 3:00PM - 3:24PM |
W23.00003: Perspectives on Industrial Innovation from Agilent, HP, and Bell Labs Invited Speaker: James Hollenhorst Innovation is the life blood of technology companies. I will give perspectives gleaned from a career in research and development at Bell Labs, HP Labs, and Agilent Labs, from the point of view of an individual contributor and a manager. Physicists bring a unique set of skills to the corporate environment, including a desire to understand the fundamentals, a solid foundation in physical principles, expertise in applied mathematics, and most importantly, an attitude: namely, that hard problems can be solved by breaking them into manageable pieces. In my experience, hiring managers in industry seldom explicitly search for physicists, but they want people with those skills. [Preview Abstract] |
Thursday, March 6, 2014 3:24PM - 3:30PM |
W23.00004: Connecting Physics with Engineering in Industrial R&D Invited Speaker: Robert Doering Profitable products are frequently enabled by innovations that prevent early commoditization. At its best, industrial physics research provides the key differentiators for such products. To fulfill this goal, it's necessary to establish effective working relationships between R\&D staff with both physics and engineering backgrounds. In the semiconductor industry, the ``middle ground'' is often materials science, electromagnetics, or a wide range of phenomena useful for creating sensors. In this brief presentation, we will mention a few examples of such collaborative development at Texas Instruments, including MEMS devices, ferroelectric memory, and silicon-IC-based radar. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:36PM |
W23.00005: Physics-Driven Innovation In the Oil and Gas Industry Invited Speaker: Martin Poitzsch In terms of sheer scale and financial investment and geographical footprint, nothing is bigger than the oil and gas industry. This ``mature industry'' employs a bewildering mix of technologies dating from the 19$^{\mathrm th}$ century to the 21$^{\mathrm th}$. Oil well construction represents one of the largest volume markets for steel tubulars, Portland cement, and high-quality sand. On the other hand, advanced 3D seismic data processing, shaped-charge perforating, and nuclear well logging have consistently driven forward the state of the art in their respective areas of applied science, as much or more so than defense or other industries. Moreover, a surprising number of physicists have made their careers in the oil industry. To succeed at introducing new technology requires understanding which problems most need to be solved. The most esoteric technology can take off in this industry if it honestly offers the best solution to a key problem that is costing millions of dollars in risk or inefficiency. When the right breakthrough solution emerges, the resources to implement it can be almost limitless. However, the prevailing culture is conservative and brutally cost-driven: any cheaper or simpler solution that performs as well will prevail, no matter how inelegant! [Preview Abstract] |
Thursday, March 6, 2014 3:36PM - 3:42PM |
W23.00006: A Physicist Role in Innovation within IBM Research Invited Speaker: William Gallagher The broad and deep insight a physicist brings to the goings on in a large technology company lead to many varied and exciting opportunities. Examples in my own career include contributions to important understanding of new breakthroughs (understanding the basic anisotropy of high temperature superconductivity), bringing vital physics understanding to ambitious engineering projects (basic switching and noise margins in digital Josephson junction technology), and initiating and growing large applied projects based on fundamental physics breakthroughs (magnetoresistive random access memory -- MRAM). Success at such undertakings within a large enterprise involves a number of factors. Always seeking out the best expert advice and the best collaborators in unfamiliar technical areas as new ideas develop is enormously helpful and not at all difficult within a large innovative organization. While being imaginative and optimistic, one must also remain brutally honest about the potential value of new endeavors, the hurdles ahead, and the likelihood of success. Always, however, there is no substitute hard work. I can attest that the results of efforts along these directions within a technology company can be very exciting and satisfying, and the process along the way a whole lot of fun. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:48PM |
W23.00007: Entrepreneurial Physics Invited Speaker: Jason Cleveland As a physicist that co-founded a startup Atomic Force Microscopy company and co-managed it for 15 years, I bring a small business perspective to the panel. Our scientific and management teams are heavily weighted by physicists and the generalist skills and attitudes that they bring to the table have been extremely valuable at all stages of our company's growth. For physicists interested in entrepreneurship, scientific instrumentation companies represents a unique niche where the barrier to entry is lower for them than in many other areas. Besides the science and technology behind our company, I can address other factors that may be of interest to budding entrepreneurs including funding models and intellectual property. [Preview Abstract] |
Thursday, March 6, 2014 3:48PM - 4:48PM |
W23.00008: Interactive Panel Discussion Invited Speaker: Mark Bernius Quo Vadis? Here is the opportunity to ask panel members your questions: seek a forecast of current trends, where are we going as a collection of physicists in a wide variety of employment settings? What is the likelihood of remaining cohesive as those schooled in the fundamentals of physics? How might we better foster collaboration, with the disparate agendas of academia, government and commerce? Come with your questions, and share in this unique opportunity to quiz the experts. [Preview Abstract] |
Session W24: Scattering and Diffraction
Sponsoring Units: GIMSChair: Yaohua Liu, Argonne National Laboratory
Room: 504
Thursday, March 6, 2014 2:30PM - 2:42PM |
W24.00001: A Novel X-ray Diffractometer for the Florida Split Coil 25 Tesla Magnet Shengyu Wang, Alexey Kovalev, Alexey Suslov, Theo Siegrist At National High Magnetic Field Laboratory (NHMFL), we are developing a unique X-ray diffractometer for the 25 Tesla Florida Split Coil Magnet for scattering experiments under extremely high static magnetic fields. The X-ray source is a sealed tube (copper or molybdenum anode), connected to the magnet by an evacuated beam tunnel. The detectors are either an image plate or a silicon drift detector, with the data acquisition system based on LabVIEW. Our preliminary experimental results showed that the performance of the detector electronics and the X-ray generator is reliable in the fringe magnetic fields produced at the highest field of 25 T. Using this diffractometer, we will make measurements on standard samples, such as LaB$_{\mathrm{6}}$, Al$_{\mathrm{2}}$O$_{\mathrm{3}}$ and Si, to calibrate the diffraction system. Magnetic samples, such as single crystal HoMnO$_{\mathrm{3}}$ and stainless steel 301 alloys will be measured subsequently. The addition of X-ray diffraction to the unique split coil magnet will significantly expand the NHMFL experimental capabilities. Therefore, external users will be able to probe spin -- lattice interactions at static magnetic fields up to 25T. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W24.00002: Ion-Irradiation Induced Vacancy and Interstitial Clusters in Fe Investigated by X-Ray Diffuse Scattering and Molecular Dynamics Simulations Bennett Larson, Jon Tischler, Yury Osetskiy, Roger Stoller, Yanfei Gao, Yanwen Zhang The size and nature of vacancy and interstitial clusters in 15 MeV Ni-ion irradiated Fe have been investigated using x-ray diffuse scattering combined with scattering cross-sections based on continuum elasticity and molecular dynamics (MD) simulations. X-ray diffuse scattering measurements performed at the Advanced Photon Source within the so-called asymptotic regime near the (002) reflection of \textless 001\textgreater oriented single crystal Fe have been analyzed using diffuse scattering cross-sections based on continuum elasticity and MD simulated lattice displacements around \textless 111\textgreater and \textless 100\textgreater surface-normal interstitial and vacancy loops. To assess the sensitivity of diffuse scattering measurements to loose vacancy clusters or voids, molecular dynamics based cross-sections were calculated for 3D vacancy structures as well as for planar vacancy loops. The diffuse scattering measurements for ambient temperature Ni-ion irradiations of Fe corresponding to 1 displacement per atom (DPA) will be presented, and the results of the analysis of the diffuse scattering measurements in terms of vacancy and interstitial cluster type, surface-normal orientation, and size distributions will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W24.00003: Correlated x-ray scattering, from nanoparticle solutions to proteins Derek Mendez, Thomas Joseph Lane, Jongmin Sung, Herschel Watkins, Daniel Ratner, Sebastian Doniach Recent developments in x-ray source technology have shed new light on the Kam correlated x-ray scattering (CXS) theory, first proposed in 1977 [1]. The goal of CXS is to obtain high resolution structural information for the individual particle in a solution of disoriented, identical particles. This is achieved by exposing the solution to bright, short (shorter than the particle diffusion time) pulses of x-rays and then calculating angular intensity correlations in the plane of an area detector. The resulting correlations contain more information than standard small and wide angle x-ray scattering (SAXS and WAXS) measurements. This information can be used to place constraints on low-resolution particle models. We have demonstrated the feasibility of CXS at atomic length scales for solutions of nanoparticles (in review), where recovery of the signal involved non-linear filtering of the intensities. This eliminated dominant systematic noises in the data, which was recorded at the Stanford Synchrotron Radiation Lightsource (SSRL). We are refining analysis techniques which will be applied to CXS measurement of biomolecules at the SPring-8 angstrom compact free electron laser (SACLA) facility. \\[4pt] [1] Kam, Zvi 1977 Macromolecules 10, 927-934. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W24.00004: Fourier-transform inelastic x-ray scattering from time- and momentum- dependent phonon-phonon correlations Mariano Trigo, David Reis In a solid, the elementary excitations of the crystalline lattice (phonons) determine the macroscopic properties such as thermal transport and structural stability. The spectrum of these elementary excitations is normally obtained from inelastic neutron and x-ray scattering near equilibrium conditions, which is a Fourier transform of the spatial and temporal correlations of the system. Recent advances in Free Electron Laser sources provide sufficient flux and time-resolution to explore the dynamics of solids at the fundamental time- and length-scales of the atomic motions. In this talk I will show that by probing phonon correlations by femtosecond diffuse scattering in photoexcited germanium, we were able to obtain the phonon dispersion with extreme frequency and momentum resolution without analyzing the energy of the outgoing photon. I will show that time-dependent coherences are generated when an ultrafast laser pulse slightly quenches the phonon frequencies, generating pairs of correlated phonons at equal and opposite momenta. Using this approach we obtain an extremely high-resolution probe of the excited-state phonon dispersion over large sections of momentum space by a simple Fourier transform. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W24.00005: Magnetostriction in Ni Nanowires: Coherent X-ray Diffraction and Density Functional Analysis Erandi Wijerathna, Jong Woo Kim, San Wen Chen, Ross Harder, Sohini Manna, Boris Kiefer, Eric Fullerton, Edwin Fohtung, Jose De la Venta, Oleg Shpyrko Three-dimensional magnetostriction is mapped in Ni nanowires with the aid of Bragg coherent X-ray diffraction (BCXDI). By inverting the measured BCXDI patterns using iterative phase retrieval algorithms giant magnetostrictive strain are observed due to a differential anisotropy of the lattice displacements along the [111] and (001) directions. Density functional calculations performed is consistent with the experimental observation. Our finding paves the way for the fabrication and development of novel magnetostrictive sensor elements. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W24.00006: Acoustic Pulses in Iron Observed by Femtosecond X-ray Diffraction Tom Henighan, Stefano Bonetti, Patrick Granitzka, Diling Zhu, Stuart Parkin, Mariano Trigo, David Reis, Herman Durr Interest in improving the performance of memory storage devices has fueled recent discoveries in novel mechanisms for manipulating magnetic spins on ultrafast timescales, including magnetoacoustics. Direct measurement of ionic motion could allow one to observe the coupling between the magnetic spins and lattice dynamics in a crystal. In this talk, I will discuss recent results on time-resolved acoustics observed by time-resolved diffuse X-ray scattering in a 25 nm thick alpha-iron crystal of high quality. Acoustic pulses are generated using a femtosecond optical laser which provides an impulsive strain in the crystal. The ensuing phonon dynamics are resolved by scattering of femtosecond X-ray pulses provided by the Linac Coherent Light Source. In particular, we observe terahertz oscillations in the Fourier components of the acoustic pulses imaged by the detector. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W24.00007: Near-Equilibrium Structural Dynamics on the 20ps Time Scale Michael Kozina, Te Hu, Apurva Mehta, David Reis, Aaron Lindenberg We observed near-equilibrium optically-induced structural dynamics at the Stanford Synchrotron Radiation Lightsource (SSRL). Hard x-ray diffraction experiments were performed in a typically unavailable regime: low optical pump fluence (20-250 $\mu$J/cm$^{2})$ and short x ray pulses (15-60ps). We studied several different thin films (BiFeO$_{3}$ (BFO), Pb(Zr)TiO$_{3}$ (PZT), and Bi), pumping above bandgap using 343nm (BFO, PZT) or 1030nm (Bi) laser light at a 1.28MHz repetition rate in a special low alpha (short pulse) mode of the synchrotron. The corresponding small diffraction pattern changes (including fractional changes in lattice constant on order 10$^{-6}$ were made much more apparent because of the ability of the experimental apparatus to make use of the high repetition rate of the x rays; it is precisely this rapid data acquisition process that allows such small changes to be measured. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W24.00008: Measurement of the background in Auger-Photoemission Spectra (APECS) associated with multi-electron and inelastic valence band photoemission processes Prasad Joglekar, Karthik Shastry, Steven Hulbert, Alex Weiss Auger Photoelectron Coincidence Spectroscopy (APECS), in which the Auger spectra is measured in coincidence with the core level photoelectron, is capable of pulling difficult to observe low energy Auger peaks out of a large background due mostly to inelastically scattered valence band photoelectrons. However the APECS method alone cannot eliminate the background due to valence band VB photoemission processes in which the initial photon energy is shared by 2 or more electrons and one of the electrons is in the energy range of the core level photoemission peak. Here we describe an experimental method for estimating the contributions from these background processes in the case of an Ag N23VV Auger spectra obtained in coincidence with the 4p photoemission peak. A beam of 180eV photons was incident on a Ag sample and a series of coincidence measurements were made with one cylindrical mirror analyzer (CMA) set at a fixed energies between the core and the valence band and the other CMA scanned over a range corresponding to electrons leaving the surface between 0eV and the 70eV. The spectra obtained were then used to obtain an estimate of the background in the APECS spectra due to multi-electron and inelastic VB photoemission processes. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W24.00009: A new TOF-SANS instrument at the Helmholtz-Zentrum Berlin Karsten Vogtt, Miriam Siebenbuerger, Daniel Clemens, Christian Rabe, Peter Lindner, Margarita Russina, Ferenc Mezei, Matthias Ballauff The V16/VSANS is a new small angle neutron scattering (SANS) instrument at the Helmholtz-Zentrum Berlin in Germany. It employs the time-of-flight (TOF) technique, i.e. the sample is irradiated with a broad band of neutron wavelengths rather than operating under monochromatic conditions. Thus a broader and dynamic range in momentum transfer $q$ can be accessed. Four choppers allow tailoring the wavelength band to individual requirements in terms of resolution in $q$ and neutron flux. Long pulse lengths lead to a broad wavelength band and a concomitant broad range in $q$ as well as high neutron flux, while short pulse lengths have the opposite effect. The TOF-mode permits free selection of time intervals from a sample file and thus allows tracing the chronological development of a sample run. Moreover the wavelength range of the experiment can be freely narrowed down for the data reduction process, providing a tool for further data optimization after the finish of the experiment. Special software and hardware is required to deal with the large volumes of data generated and to perform data correction and normalization. The talk addresses the instrumental setup as well as data processing procedures and discusses the challenges and opportunities of the method. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W24.00010: Novel Multidimensional Cross-Correlation Data Comparison Techniques for Spectroscopic Discernment in a Volumetrically Sensitive, Moderating Type Neutron Spectrometer Cory Hoshor, Stephan Young, Brent Rogers, James Currie, Thomas Oakes, Paul Scott, William Miller, Anthony Caruso A novel application of the Pearson Cross-Correlation to neutron spectral discernment in a moderating type neutron spectrometer is introduced. This cross-correlation analysis will be applied to spectral response data collected through both MCNP simulation and empirical measurement by the volumetrically sensitive spectrometer for comparison in 1, 2, and 3 spatial dimensions. The spectroscopic analysis methods discussed will be demonstrated to discern various common spectral and monoenergetic neutron sources. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W24.00011: Neutron Scattering Dependence on Proximate Human Tissue from Fast Neutrons Brent Rogers, Cory Hoshor, Paul Scott, Joseph Crow, Noah Kramer, Anthony Caruso Neutrons incident on a human may undergo scattering, altering their energy. It is necessary to acknowledge that a proximate human moderator may non-trivially alter the neutron flux and source spectrum for a given neutron detection/spectroscopic device. Using primarily the Monte Carlo N-Particle (MCNP) transport code, the neutron-human moderator dependence will be discussed with respect to the rigorous quantification of human tissue composition and proximity/geometry on neutron moderation. MCNP simulated results will then be discussed in view of empirical results. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W24.00012: Digital holographic microscopy for imaging and characterization of micron-sized particles. Nava Subedi, Matthew Berg We use the digital holographic microcopy (DHM) technique for particle imaging and characterization. In this work, the interference pattern produced by superposition of unscattered reference light and the scattered light is recorded by a digital camera. This pattern constitutes a hologram from which an image of the particle is computationally reconstructed. This technique has the potential to provide \textit{in situ }particle information up to the sub-micron resolution level and helps in the development of instrumentation capable to characterize respirable-sized (1-10 $\mu $m) aerosol particles. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W24.00013: Operando characterization of nanocatalysts via spectroscopy, scattering and imaging techniques in the same micro-reactor Yuanyuan Li, Anatoly Frenkel, Philipp Baumann, Ryan Tappero, Dmitri Zakharov, Eric Stach, Annika Elsen, Ulrich Jung, Ralph Nuzzo The increasing demand to rationally design new catalysts for energy generation/conversion calls for improvements in research methodology which enables multi-technique investigations of working catalysts in reaction conditions. Using the operando approach is necessary to establish structure activity/selectivity relationship. However, this approach is hindered by many challenges, e.g., the incompatibility of different characterization methods with respect to the sample concentration and environment, and, hence, the need to use multiple in situ reactor designs. We report on the development and tests of the single, portable reactor compatible with most useful techniques for operando studies of nanocatalysts: X-ray absorption, transmission electron microscopy, infrared and Raman spectroscopies. The test system was Pt/SiO2 nanocatalyst and the reaction was the ethylene hydrogenation. The reactor was a closed cell with SiN windows enabled catalytic reactions under atmospheric pressure. Both XAFS and TEM experiments were conducted in identical conditions, while monitoring the product formation using mass spectrometry. Comparison of TEM and XAFS results provided new information on the structure-activity relationship of these catalysts. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W24.00014: Tin particle size measurements in high explosively driven shockwave experiments using Mie scattering method Shabnam Monfared, William Buttler, Martin Schauer, Brandon LaLone, Cora Pack, Gerald Stevens, Joseph Stone Los Alamos National Laboratory is actively engaged in the study of material failure physics to support the hydrodynamic models development, where an important failure mechanism of explosively shocked metals causes mass ejection from the backside of a shocked surface with surface perturbations. Ejecta models are in development for this situation. Our past work has clearly shown that the total ejected mass and mass-velocity distribution sensitively link to the wavelength and amplitude of these perturbations. While we have had success developing ejecta mass and mass-velocity models, we need to better understand the size and size-velocity distributions of the ejected mass. To support size measurements we have developed a dynamic Mie scattering diagnostic based on a CW laser that permits measurement of the forward attenuation cross-section combined with a dynamic mass-density and mass-velocity distribution, as well as a measurement of the forward scattering cross-section at 12 angles (5- 32.5 degrees) in increments of 2.5 degrees. We compare size distribution followed from Beers law with attenuation cross-section and mass measurement to the dynamic size distribution determined from scattering cross-section alone. We report results from our first quality experiments. [Preview Abstract] |
Session W25: Focus Session: Organic Electronics and Photonics - Thermoelectric Properties of Polymers
Sponsoring Units: DMP DPOLYChair: Bryan Boudouris, Purdue University
Room: 503
Thursday, March 6, 2014 2:30PM - 3:06PM |
W25.00001: The Prospects of Organic Semiconductors for Thermoelectrics Invited Speaker: Michael Chabinyc Organic semiconductors have moved from a laboratory curiosity to commercial use in displays with organic light emitting diodes. In comparison to inorganic semiconductors, a remaining challenge for organic materials is the rational control of their electrical conductivity by doping. Due to the low lattice thermal conductivity of organic materials and their high electrical conductivities, organic semiconductors represent a promising class of solution processable thermoelectrics. The state of organic thermoelectrics and work from our lab on electrical doping of both p- and n- type semiconducting polymers will be discussed. The phase behavior of blends of semiconducting polymers and molecular dopants plays a critical role in their ultimate performance. Recent work on charge transfer doping and self-doping of polymers and molecular materials will be presented. Data mining from the literature along with results on recently developed materials systems suggests directions for optimization of organic thermoelectrics. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W25.00002: Investigation of the Wiedemann-Franz law in the conducting polymer PEDOT Annie Weathers, Li Shi, Zia Ullah Khan, Olga Bubnova, Xavier Crispin The conducting polymer PEDOT:PSS (Poly-3,4-ethylenedioxythiophene poly-styrenesulfonate) has been shown to have promising thermoelectric properties for a polymer system, with a reported ZT on the order of 0.3 at room temperature. Previous measurements of the thermoelectric properties has suggested a violation of the Wiedemann-Franz law, with a reported total thermal conductivity less than the estimated electronic contribution. The validity of the Wiedemann-Franz law in these systems has remained an open question, as the charge transport mechanism can be different than in inorganic materials. However, no measurements have been done to measure directly all three thermoelectric properties in the same direction. We present the in-plane thermoelectric properties of suspended PEDOT samples of varying electrical conductivities and discuss the implications of the results on the validity of the Wiedemann-Franz law for conducting polymer systems.~ [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W25.00003: Optimization of thermoelectric performance in semiconducting polymers for understanding charge transport and flexible thermoelectric applications Anne Glaudell, Michael Chabinyc Organic electronic materials have been widely considered for a variety of energy conversion applications, from photovoltaics to LEDs. Only very recently have organic materials been considered for thermoelectric applications - converting between temperature gradients and electrical potential. The intrinsic disorder in semiconducting polymers leads to an inherently low thermal conductivity, a key parameter in thermoelectric performance. The ability to solution deposit on flexible substrates opens up niche applications including personal cooling and conformal devices. Here work is presented on the electrical conductivity and thermopower of thin film semiconducting polymers, including P3HT and PBTTT-C$_{14}$. Thermoelectric properties are explored over a wide range of conductivities, from nearly insulating to beyond 100 S/cm, enabled by employing different doping mechanisms, including molecular charge-transfer doping with F4TCNQ and vapor doping with a fluoroalkyl trichlorosilane (FTS). Temperature-dependent measurements suggest competing charge transport mechanisms, likely due to the mixed ordered/disordered character of these polymers. These results show promise for organic materials for thermoelectric applications, and recent results on thin film devices will also be presented. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W25.00004: Power factor enhancement in solution-processed organic n-type thermoelectric materials through side chain design Boris Russ, Maxwell J. Robb, Fulvio G. Brunetti, Levi Miller, Shrayesh Patel, Victor Ho, Jeffrey J. Urban, Michael L. Chabinyc, Craig J. Hawker, Rachel A. Segalman Building efficient organic thermoelectric architectures requires complementary p-type (hole transporting) and n-type (electron transporting) components. While several high performance hole-transporting polymers have been developed, the design of n-type organics has proven challenging, and thermoelectric studies of organic n-type systems are scarce. We investigate the properties of a series of charged perylene diimide (PDI) derivatives. Charged side chains in these materials enable both water solubility and self-doping. We show that changing the length of the alkyl spacer between the charged end groups and the PDI core dramatically improves thin film thermoelectric properties. The top derivatives in our study demonstrated the highest power factor reported for n-type solution-processed films. By complementing thermoelectric characterization of these variants with insight on the electronic and structural property changes from optical spectroscopy, EPR, and GIWAXS experiments, our findings shape a promising molecular design strategy for future enhancements in thermoelectric performance. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W25.00005: Thermoelectric Properties of Conjugated Polyelectrolytes Cynthia Chen, Cheng-Kang Mai, Michael Chabinyc, Jeffrey Urban, Guillermo Bazan, Rachel Segalman Conjugated polymers are emerging as promising thermoelectric materials due to their solution processability, low thermal conductivity, and tunability of electrical properties via chemical modification. For the first time, conjugated polyelectrolytes, which are conjugated polymers with charged side chains, are being explored for thermoelectric applications. Charged side chains may be able to dope directly conjugated polymers by stabilizing the radical cations on the $\pi $-conjugated backbone. In this work, we investigate the thermoelectric properties of a novel narrow band gap conjugated polyelectrolyte with anionic side chains, poly[2,6-(4,4-bis-potassiumbutanylsulfonate-4H-cyclopenta-[2,1-b;3,4-b']-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (CPE-K). We show that doping CPE-K with hydrochloric acid can raise electrical conductivity without significantly changing Seebeck coefficient, resulting in an overall increase in power factor and an indication of how molecular design can be used to increase thermoelectric efficiency. Our results also shed some light on the role of charged side chains and the mechanism of doping in conjugated polyelectrolytes, which is different from that of doping in inorganic materials. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W25.00006: Synthesis and Solid State Charge Transport in Radical Polymers Lizbeth Rostro, Aditya Baradwaj, Bryan Boudouris Conducting polymers have been studied extensively for their applicability in a wide range of electronic devices. Previously, $\pi $-conjugated polymers have dominated the research focus due to the high degree of electronic delocalization associated with their molecular structure; however many challenges continue to prevent their viability in consumer applications. Here, we report on an emerging class of transparent non-conjugated conducting polymers, radical polymers, which circumvent many of the challenges faced by $\pi $-conjugated polymers. Specifically, a model radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), was synthesized in a controlled manner using the RAFT polymerization mechanism, which produced polymers with readily-tunable molecular weights and narrow molecular weight distributions. Additionally, the solid state charge transport ($i.e.,$ conductivity) was characterized in radical polymers. Furthermore, we demonstrate that the chemistries of the radical polymer functionalities can be tuned readily, and this tuning leads to critical changes in the charge transport ability of these types of macromolecules in the solid state; this tunability allows the materials to be used in high-performing photovoltaic and thermoelectric devices. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W25.00007: Anisotropic Thermal Conduction in Polymers and its Molecular Origins David Nieto Simavilla, David Venerus, Jay Schieber Anisotropy in thermal conductivity has a significant impact on both processing and final properties of materials. Simple molecular arguments suggest that Fourier?s law must be generalized to allow for anisotropic thermal conductivity. We present two complementary experimental methods to obtain quantitative measurements of the thermal diffusivity (conductivity) tensor. We report anisotropic thermal diffusivity and stress in molten, cross-linked and solid polymers under several types of flows. Our results support the validity of a linear relationship between stress and anisotropy in thermal conductivity. When the proportionality constant, the stress-thermal coefficient, is made dimensionless by the plateau modulus of the polymer melt, a universal value of approximately 0.03 is observed for all chemistries. Such a universality is surprising, since phonon transport mechanisms are sensitive to chemical structure. For instance, the analogous stress-optic coefficient depends strongly on chemistry, and can even change sign. Connecting these measurements with current theories for thermal transport in amorphous materials, such as Minimum Thermal Conductivity (MTC) model, is crucial to understand the molecular origins of anisotropic thermal conduction in polymers. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W25.00008: Thermal Conductivities of Crystalline Organic Semiconductors Invited Speaker: Joseph Brill As applications for organic semiconductors grow, it is becoming increasingly important to know their thermal conductivities, k. For example, for sub-micron electronic devices, values of k\textgreater k$_{0}$ $\sim$ 5 mW/cm/K are needed, while values k\textless k$_{0}$ are required for desired thermoelectric applications. Whereas it is not surprising that semiconducting polymers typically have room temperature thermal conductivities below k$_{0}$, many molecular organic crystals also have values of k below this value. We have started measurements of both the in-plane and interplane thermal diffusivities of layered crystalline organic semiconductors using frequency\footnote{H. Zhang and J.W. Brill, J. Appl. Phys. \textbf{114}, 043508 (2013).} and position dependent\footnote{I. Hatta \textit{et al}, Jpn. Jnl Appl. Phys. \textbf{25}, L493 (1986).} ac-calorimetry; the thermal conductivities are then determined from the specific heats measured with differential scanning calorimetry. For rubrene, which has k\textless k$_{0}$, the interplane thermal conductivity is several times smaller than the in-plane value, although its temperature dependence indicates that the phonon mean-free path is at least a few layers.\footnote{H. Zhang and J.W. Brill} On the other hand, the in-plane thermal conductivity of TIPS-pentacene,\footnote{J.E. Anthony, Chem. Rev. \textbf{106}, 5028 (2006).} is several times greater than k$_{0}$, similar to that of the quasi-one dimensional organic metal TTF-TCNQ.\footnote{M.B. Salamon \textit{et al};, Phys. Rev. B \textbf{11}, 619 (1975).} Remarkably, its interlayer thermal conductivity is several times larger than its in-plane value,\footnote{H. Zhang and J.W. Brill} perhaps due to interactions between the large (triisopropylsilylethynyl) side groups on the pentacene backbone. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W25.00009: Measurements of In-Plane Thermal Diffusivities of Layered Organic Semiconductors by ac-Calorimetry Hao Zhang, Yulong Yao, Joseph Brill We are using the position-dependent ac-calorimetric technique of Hatta \textit{et al}\footnote{I. Hatta \textit{et al}, Jpn. Jnl Appl. Phys. \textbf{25}, L493 (1986).} to measure the in-plane thermal conductivity of layered organic semiconductors, such as 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pn).\footnote{J.E. Anthony, Chem. Rev. \textbf{106}, 5028 (2006).} Chopped light is used to heat the sample, part of which is screened from the light, with the thermometer placed on the back of the sample in the screened region. In the ``infinite crystal length'' limit, the logarithm of the oscillating temperature as well as its phase shift decrease linearly with distance of the thermometer from the edge of the screen, with a slope inversely related to the thermal diffusivity. Materials like TIPS-pn have surprisingly large values of thermal diffusivity, D \textgreater 1 mm$^{2}$/s, making finite size effects important, since crystal lengths are typically \textless 1 cm. We will discuss our technique and results in detail, including the effects of finite crystal size on the measurements of phase and magnitude of the oscillating temperature. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W25.00010: Polymer Thermoelectric Generators: Device Considerations Shannon Yee Recent control of the transport properties in polymers has encouraged the development of polymer thermoelectric (TE) devices. Polymer TEs are thought to be less expensive and more scalable than their inorganic counterparts. The cost of the raw material is less and polymer TEs can leverage the large areal manufacturing technique established by the plastics industry. Additionally, while the overall ZT of polymer TEs appears attractive, individual polymer properties have a very different scale than their inorganic counterparts (i.e., the thermal conductivity and electrical conductivity are approximately one and two orders of magnitude smaller, respectively). Furthermore, the majority of TE measurements on polymers have been limited to thin-films where traditional TE materials are measured in bulk. So why should it be expected that polymer TE devices resemble traditional TE devices? Given the uniqueness of polymers, different device architectures are proposed that can leverage the unique strengths of polymer films. It will be shown that by logically considering device requirements, new polymer TE devices have non-linear features that are more attractive than linear inorganic TE devices. This leads to very different device optimizations that favor polymer TEs. [Preview Abstract] |
Session W26: Focus Session: Explicitly Correlated Methods and Quantum Few-Body Systems
Sponsoring Units: DCOMPChair: Sergiy Bubin, University of Rochester
Room: 502
Thursday, March 6, 2014 2:30PM - 3:06PM |
W26.00001: Applications of the Stochastic Variational Method Invited Speaker: Kalman Varga The variational method complemented with the use of explicitly correlated Gaussian basis functions is one of the most powerful approaches currently used for calculating the properties of few-body systems. Despite its conceptual simplicity, the method offers great flexibility, high accuracy, and can be used to study diverse quantum systems, ranging from small atoms and molecules to light nuclei, hadrons, quantum dots, and Efimov systems. One of the biggest computational issue is the optimal choice of basis parameters. The stochastic variational method is a random trial and error approach which proved to be very efficient in minimizing the variational energy. The basic computational foundations are discussed, recent advances in the applications of the stochastic variational method in physics and chemistry are reviewed, and the strengths and weaknesses of the explicitly correlated Gaussians approach are compared with other few-body techniques. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W26.00002: Perspectives and Current the Development of Non-Born-Oppenheimer Atomic and Molecular Quantum Mechanical Variational Calculations using Explicitly Correlated Gaussian Basis Functions Keeper L. Sharkey The development of highly accurate theoretical quantum mechanics models for atomic and molecular calculations is crucial for the verification of the results of high-resolution experimental spectroscopy. High accuracy in the calculations can be achieved by not assuming the Born-Oppenheimer approximation (non-BO) and by using the variational principle. The non-relativistic Hamiltonian describing the internal state of the considered system used in the approach is obtained by separating out the center-of-mass motion from the laboratory frame Hamiltonian. The wave functions used in the calculations are expanded in terms of explicitly correlated Gaussian (ECG) functions. The optimization of the Gaussian non-linear parameters is aided by the analytical energy gradient determined with respect to these parameters. Examples of some very accurate calculations of small atoms and diatomic molecules will be presented. The presentation will also include a discussion of the extension of the approach to perform non-BO calculations of bound states of small triatomic molecules (e.g. H$_3^+$). [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W26.00003: Low-energy D-wave positronium-hydrogen scattering Denton Woods, P. Van Reeth, S.J. Ward We are investigating the four-body Coulomb process of positronium-hydrogen (Ps-H) scattering below the Ps(n=2) excitation threshold using the Kohn variational method and variants. Our Ps-H $^1D$-wave phase shifts compare reasonably well with the close-coupling results [1,2], but our $^3D$-wave phase shifts are appreciably lower. In an attempt to improve the accuracy of these, we are employing a sectors-based approach [3] and the modification of the short-range Hylleraas terms with an exponential in the $r_{12}$ coordinate. We are investigating the use of the Born approximation for higher partial waves. We plan also to present our latest S-wave and P-wave results using the Kohn variational method [4]. \\[4pt] [1] H.R.J. Walters \emph{et al}, Nucl. Instrum. Methods B \textbf{221}, 149-159 (2004).\\[0pt] [2] J. Blackwood \emph{et al}, Phys. Rev. A \textbf{65}, 032517 (2002).\\[0pt] [3] Zong-Chao Yan and Y.K. Ho, Phys. Rev. A \textbf{59}, 2697 (1999).\\[0pt] [4] Denton Woods, S. J. Ward and P. Van Reeth, http://meetings.aps.org/link/BAPS.2013.DAMOP.Q1.122 (and references within). [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 4:06PM |
W26.00004: Asymptotic Expansions, 1/Z Expansions, and the Critical Nuclear Charge Invited Speaker: Gordon Drake The quantum mechanical three-body problem defies analytic solution, and so computationally intensive approximation methods involving, for example, variational calculations with large correlated basis sets must be used. This talk will review recent work to explore the outer fringes of the quantum mechanical three-body problem for heliumlike atoms. Asymptotic expansions provide a surprisingly simple and accurate account of highly excited Rydberg states with high angular momentum. $1/Z$ expansions, where $Z$ is the nuclear charge, provide results for an entire isoelectronic sequence within a single calculation. Its radius of convergence is thought to be related to the critical nuclear charge $Z_c$ for a state to be bound. For $Z < Z_c$, there may still be quasibound states (shape resonances) imbedded in the scattering continuum. Relationships amongst all three topics will be discussed, and new results presented for both asymptotic expansions and the critical nuclear charge. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W26.00005: Stability of positron--atom complexes in ground and excited states Sergiy Bubin, Oleg Prezhdo Using a variational method with an explicitly correlated Gaussian basis set we have studied the stability of weakly bound positron--atom complexes in the ground and lowest excited states with higher spin multiplicity. Our calculations provide rigorous theoretical confirmation that a positron can be attached to the lowest quartet state of Li and triplet state of Be. The result is particularly notable for the positron--Be complex, as the excited triplet state lies below the autoionization threshold. The simultaneous existence of the ground and meta-stable excited states of positronic Li and Be opens up new possibilities for the experimental detection of positron--atom complexes. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W26.00006: Challenges in calculating molecular systems with Coulomb interactions Nikita Kirnosov, Keeper Sharkey, Ludwik Adamowicz The highly accurate quantum mechanical calculations are not only crucial for high-resolution experimental data verification, but may also serve as a guide in the field of exotic systems exploration. Including all non-relativistic effects in a single-step variational approach and rigorously separating out the center of mass motion allows us to build a reliable model for calculating bound states of molecular systems with Coulomb interactions. In these calculations the wave function of the system is expanded in terms of explicitly correlated Gaussian (ECG) basis functions. Examples of calculations of energies and other properties of some molecular systems will be presented. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W26.00007: Developments for a Relativistic Four-Component Many-1/2-Fermion Theory Benjamin Simmen, Edit M\'atyus, Markus Reiher Explicitly correlated configuration interaction methods have proven to be highly successful in the study of non-relativistic many-electron systems. They are also suited for pre-Born--Oppenheimer calculations where nuclei and electrons are treated on equal footing. Relativistic quantum chemistry is based on the no-pair approximation and provides a four-component Hamiltonian capturing the essential aspects of special relativity for molecular systems. Two fundamental issues arise when aiming at four-component pre-Born--Oppenheimer calculations. The concept of a center of mass cannot be exploited for the Dirac--Coulomb Hamiltonian: It is not possible to separate the overall motion of the system through a linear transformation of the one-particle Cartesian coordinates [1]. Second, a finite number of basis functions leads to an artificial decrease of the bound state energies since the Dirac--Coulomb Hamiltonian is not bounded from below [2]. Kinetic balance solves this for Slater determinants, but its explicitly correlated variant is considerably more involved.\\[4pt] [1] B. Simmen, E. M\'atyus, M. Reiher; Mol. Phys. 111; 2086 (2013)\\[0pt] [2] B. Simmen, M. Reiher; In: Many-Electron Approaches in Physics, Chemistry and Mathematics; Eds.: V. Blum, L. Delle Site; Springer (in press); (2014) [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W26.00008: Correlation-bound anion states Vamsee Voora, Kenneth Jordan In a correlation-bound anion, the excess electron is bound to the molecule in a diffuse non-valence orbital and electron correlation is crucial for the electron binding. Examples of such anions include Xe$_{\mathrm{n}}^{\mathrm{-}}$ clusters and certain (H$_{\mathrm{2}}$O)$_{\mathrm{n}}^{\mathrm{-}}$ clusters. Using many-body methods we have characterized correlation-bound anion states of C$_{\mathrm{60}}$, C$_{\mathrm{6}}$F$_{\mathrm{6}}$ and several large acenes. The correlation-bound anion states of these species are related to the image potential states of graphene. Modeling correlation-bound anion states presents challenges for \textit{ab initio }approaches. Hartree-Fock based approaches such as MP2 and CCSD fail to describe these states. The key to treating these species theoretically is to employ a method that allows the singly occupied orbital to relax in the presence of the long-range correlation effects. A model potential approach accounting for image effects for describing the binding of the excess electron will be presented for C$_{\mathrm{60}}$. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W26.00009: Particle Number Conserving Approach to the Collective States in a Small Fermi-System Jennifer Glick, Vladimir Zelevinsky The standard Bardeen-Cooper-Schrieffer (BCS) description of pairing theory, random phase approximation (RPA) and Hartree-Fock-Bogoliubov (HFB) methods, routinely used in macroscopic many-body physics when the dimension of the Hamiltonian matrix is prohibitively large, include features which are not well suited to describe mesoscopic systems such as nuclei or cold atoms in traps. Two important disadvantages are the non-conservation of exact particle number through the introduction of quasiparticles, and the absence of a non-trivial paired solution in the discrete spectrum with weak pairing. We develop the pairing theory based on the exact particle number conservation, whose first applications to the ground state physics presented in [A. Volya and V. Zelevinsky, in {\sl 50 Years of Nuclear BCS}, World Scientific, 2012] demonstrated that such an approach avoids well known deficiencies of the standard treatment, especially in the region of weak pairing. Now, we use the method for low-lying collective excitations which in many cases are even more sensitive to conservation laws. We show that the RPA version based on solving the operator equations of motion is reduced to the set of recurrence relations for neighboring systems which precisely conserve the exact particle number. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W26.00010: Nearly-exact calculation of chromium dimer binding with auxiliary-field quantum Monte Carlo Wirawan Purwanto, Shiwei Zhang, Henry Krakauer The binding of the strongly correlated Cr$_2$ molecule has long resisted accurate theoretical description, and Cr$_2$ has become a landmark test for many-body computational methods. We first performed exact auxiliary-field quantum Monte Carlo (AFQMC) calculations using a moderately-sized basis set. In parallel, phaseless AFQMC\footnote{% Zhang and Krakauer, \textit{Phys. Rev. Lett.} \textbf{90}, 136401 (2003)} calculations were carried out using the same and larger basis sets to remove the finite-basis errors from the exact AFQMC calculations. Results on Cr$_2$ ground-state properties, including binding energy, equilibrium distance, and vibrational frequency, are in excellent agreement with experiment. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W26.00011: Imaginary-time nonuniform mesh method for solving the multidimensional Schr\"odinger equation Alberto Hernando de Castro, Jiri Vanicek An imaginary-time nonuniform mesh method for diagonalizing multidimensional quantum Hamiltonians is proposed and used to find the first 50 eigenstates and energies of up to $D=5$ strongly interacting spinless quantum Lennard-Jones particles trapped in a one-dimensional harmonic potential. We show that the use of tailored grids allows exploiting the symmetries of the system---in our case the $D!$ degeneracy derived from all possible permutations of distinguishable particles---reducing drastically the computational effort needed to diagonalize the Hamiltonian. This leads to a favorable scaling with dimensionality, requiring for the 5-dimensional system four orders of magnitude fewer grid points than the equivalent regular grid. Solutions to both bosonic and fermionic counterparts of this strongly interacting system are constructed, the bosonic case clustering as a Tonks-Girardeau crystal exhibiting the phenomenon of fermionization. The numerically exact excited states are used to describe the melting of this crystal at finite temperature. [Preview Abstract] |
Session W27: Quantum Many-Body Systems II
Sponsoring Units: DCOMPChair: Aldo Romero, West Virginia University
Room: 501
Thursday, March 6, 2014 2:30PM - 2:42PM |
W27.00001: Generalized Cumulant Expansion for the One Electron Green's Function J.J. Kas, J.J. Rehr, L. Reining The cumulant expansion has proved extremely useful in describing many-body excitations. For example, the approach rectifies the failure of the GW approximation to account for multiple satellites in x-ray photoemission spectra.\footnote{Matteo Guzzo, Giovanna Lani, Francesco Sottile, Pina Romaniello, Matteo Gatti, Joshua J. Kas, John J. Rehr, Mathieu G. Silly, Fausto Sirotti, and Lucia Reining, Phys. Rev. Lett. 107, 166401 (2011)} However, current implementations are inadequate since they ignore diagrams that lead to partial occupations and satellite features in the spectral function both above and below the Fermi surface. Here, we correct these limitations using a cumulant expansion of the retarded one-electron Green's function. At 2nd order in the effective boson couplings, the cumulant is proportional to the GW self-energy. Thus the cumulant method extends the GW Green's function without additional computational expense, and can therefore be used for complex systems. We test the approach on the homogeneous electron gas, and present results for a range of parameters that are physically relevant for condensed matter systems. The resulting spectral function is used for calculations of occupation numbers, quasiparticle properties, and correlation energies. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W27.00002: Solid-state calculations using the second-order M{\o}ller-Plesset perturbation theory combined with the transcorrelated method Masayuki Ochi, Shinji Tsuneyuki Recently, wave-function theory has been actively applied to solid-state calculations, where the Hartree-Fock (HF) method is used as a starting point. Transcorrelated (TC) method [1-5] is expected to be an attractive alternative to the HF method, in which the total wave function is assumed to be the Jastrow-Slater-type wave function, and the many-body Hamiltonian is similarity-transformed by the Jastrow factor. Then the electron correlation effects are taken into account through the similarity-transformed Hamiltonian. However, the band gaps calculated using the TC method have about 1 or 2 eV errors for some semiconductors. For improving accuracy, we apply the second-order M{\o}ller-Plesset (MP2) perturbation theory to the similarity-transformed Hamiltonian, and will show that the band structures of solids are corrected well with the same level of computational cost as that for conventional MP2 applied to the HF method.\\[4pt] [1] S. F. Boys and N. C. Handy, Proc. R. Soc. London Ser. A 309, 209 (1969).\\[0pt] [2] S. Ten-no, Chem. Phys. Lett. 330, 169 (2000).\\[0pt] [3] N. Umezawa and S. Tsuneyuki, J. Chem. Phys. 119, 10015 (2003).\\[0pt] [4] R. Sakuma and S. Tsuneyuki, J. Phys. Soc. Jpn. 75, 103705 (2006).\\[0pt] [5] M. Ochi, K. Sodeyama, R. Sakuma, and S. Tsuneyuki, J. Chem. Phys. 136, 094108 (2012). [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W27.00003: Statistical mechanics of Coulomb gases as quantum theory on Riemann surfaces Tobias Gulden, Michael Janas, Alex Kamenev Statistical mechanics of 1D Coulomb gases may be mapped onto (in general) non-Hermitian quantum mechanics. We use this example to develop non-Hermitian instanton calculus. Treating momentum and coordinate as independent complex variables, constant energy manifolds are given by Riemann surfaces of genus $g\geq1$. The actions along principal cycles on these surfaces obey an ODE in the moduli space of the Riemann surface known as the Picard-Fuchs equation. Solving the Picard-Fuchs equation yields semiclassical spectra as well as instanton effects such as width of Bloch bands (the latter determines energy barrier for charge transport). Both are shown to be in perfect agreement with numerical simulations. Applications include transport through biological ion channels as well as nanofluidics, e.g water filled nanotubes. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W27.00004: Off-Center Thawed Gaussian Multi-Dimensional Approximation for Semiclassical Propagation Lucas Kocia, Eric Heller The Off-Center Thawed Gaussian Approximation's (OCTGA) performance in multi-dimensional coupled systems is shown in comparison to Herman-Kluk (HK), the current workhorse of semiclassical propagation in the field. As with the Heller-Huber method and Van Voorhis \emph{et~al.}'s nearly-real method of trajectories, OCTGA requires only a single trajectory and associated stability matrix at every timestep to compute Gaussian wave packet overlaps under any Hamiltonian. This is in sharp contrast to HK which suffers from the necessity of having to propagate thousands or more computationally expensive stability matrices at every timestep. Unlike similar methods, the OCTGA relies upon a single \emph{real} guiding trajectory, which in general does not start at the center of the initial wave packet. This guiding ``off-center" trajectory is used to expand the local potential, controlling the propagating ``thawed" Gaussian wavepacket such that it is led to optimal overlap with a final state. Its simple and efficient performance in any number of dimensions heralds an exciting addition to the semiclassical tools available for quantum propagation. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W27.00005: Behavior of the $GW$ approximation of Many-Body Pertubation Theory upon electron addition or removal Fabien Bruneval, Miguel Marques Within Many-Body Perturbation Theory (MBPT), the position of highest occupied molecular orbital (HOMO) is a quasiparticle energy. It should be hence stable upon an electron removal. The situation is slightly more complicated within Density-Functional Theory, for which the exchange-correlation potential may experience discontinuities. However, once this technicality has been considered, the HOMO energy should also be stable [1]. In other words, within an exact theory, the LUMO of a positive ion should be equal to the HOMO of the neutral molecule. It is remarkable that most approximations within DFT and MBPT fail with this sanity check. Here we demonstrate for isolated atoms and molecules that the $GW$ approximation, though not perfect, presents the weakest deviation from the ideal behavior among all the approximation studied [2]. The results have been obtained with a newly developed $GW$ code based on the Gaussian basis, which does not employ any further technical approximation besides the basis set [3]. We show that the convergence is unexpectedly slow, in constrast with earlier reports. \\[4pt] [1] A.J. Cohen, P. Mori-Sanchez, and W.T. Yang, Science 321, 792 (2008).\\[0pt] [2] F. Bruneval, JCP 136, 194107 (2012).\\[0pt] [3] F. Bruneval and M.A.L. Marques, JCTC 9, 324 (2013). [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W27.00006: Single-site density matrix embedding theory: from one to infinite dimensions Zhengqian Cheng, Chris Marianetti The recently developed density matrix embedding theory (DMET) has proven to be reliable for ground state properties in the Hubbard model in one and two dimensions. Here we focus on the single-site DMET, which has potential as a highly efficient method to treat actinides and oxides. We apply DMET in infinite dimensions where it can be compared to the exact solution via the dynamical mean-field theory. The results for the single band model, in addition to the two-band model with exchange and a crystal-field will be presented. Different magnetic solutions will also be presented in 1D, 2D, and infinite dimensions. We show that single-site DMET can be very reliable if one allows for magnetic solutions. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W27.00007: Many-Body Density Matrix Theory C.J. Tymczak, Kostyantyn Borysenko We propose a novel method for obtaining an accurate correlated ground state wave function for chemical systems beyond the Hartree-Fock level of theory. This method leverages existing linear scaling methods to accurately and easily obtain the correlated wave functions. We report on the theoretical development of this methodology, which we refer to as Many Body Density Matrix Theory. This theory has many significant advantages over existing methods. One, its computational cost is equivalent to Hartree-Fock or Density Functional theory. Two it is a variational upper bound to the exact many-body ground state energy. Three, like Hartree-Fock, it has no self-interaction. Four, it is size extensive. And five, formally is scales with the complexity of the correlations that in many cases scales linearly. We show the development of this theory and give several relevant examples. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W27.00008: Energy density matrix decomposition of interacting quantum systems Jaron Krogel, Jeongnim Kim, Fernando Reboredo We develop energy density matrices that parallel the one-body reduced density matrix for many-body quantum systems. Just as the density matrix gives access to the number density and orbital occupations, the energy density matrix yields the energy density and orbital energy levels. The eigenvectors of the matrix provide a natural orbital partitioning of the energy density while the eigenvalues comprise a single particle like energy spectrum obeying a total energy sum rule. In systems where a single particle picture is valid (e.g. for mean-field or weak interactions), the spectrum gives the expected results. We demonstrate that the QMCPACK implementation of the energy density matrix approach is correct for the cases of the non-interacting electron gas and the spherical harmonic oscillator. We further explore the meaning of the computed spectrum in the case of the fully interacting electron gas. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W27.00009: Precise estimate of correlation length exponents from simple real-space renormalization group analysis Aleksander Kubica, Beni Yoshida We invent a novel real-space renormalization group (RG) scheme which accurately estimates correlation length exponents $\nu$ near criticality of quantum Ising and clock models in higher dimensions. The method, based on a recent proposal by Miyazaki et al., Phys. Rev. E 83, 051103 (2011), is remarkably simple (often analytical), grouping only a few spins into a block spin so that renormalized Hamiltonian has a closed form. A previous difficulty of spatial anisotropy and unwanted terms arising in higher-dimensional RG schemes is avoided by incorporating rotational invariance and internal $Z_q$ symmetries of the Hamiltonian. By applying this scheme to (2+1)-dim Ising model on a triangular lattice, we obtained $\nu=0.6300$ which is within statistical error of the current best Monte-Carlo result and $\phi^4$ theory estimation with seven-loop corrections. We also apply the scheme to higher-dimensional clock (Potts) models for which ordinary Monte-Carlo methods are not efficient due to suppression of quantum fluctuation in first-order phase transition. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W27.00010: Nearest neighbor interaction in the Path Integral Renormalization Group method Wasanthi De Silva, R. Torsten Clay The Path Integral Renormalization Group (PIRG) method is an efficient numerical algorithm for studying ground state properties of strongly correlated electron systems. The many-body ground state wave function is approximated by an optimized linear combination of Slater determinants which satisfies the variational principle. A major advantage of PIRG is that is does not suffer the Fermion sign problem of quantum Monte Carlo. Results are exact in the noninteracting limit and can be enhanced using space and spin symmetries. Many observables can be calculated using Wick's theorem. PIRG has been used predominantly for the Hubbard model with a single on-site Coulomb interaction $U$. We describe an extension of PIRG to the extended Hubbard model (EHM) including $U$ and a nearest-neighbor interaction $V$. The EHM is particularly important in models of charge-transfer solids (organic superconductors) and at $\frac{1}{4}$-filling drives a charge-ordered state. The presence of lattice frustration also makes studying these systems difficult. We test the method with comparisons to small clusters and long one dimensional chains, and show preliminary results for a coupled-chain model for the (TMTTF)$_2$X materials. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W27.00011: Correlation effects in metallic cohesion Roger Haydock The electronic contribution to the cohesive energy of a correlated metal is the sum of the transition energies for adding successive electrons at successive Fermi levels until the system reaches its final electron density. This can be computed as the integral of energy over the projected density of transitions for adding single electrons to localized orbitals. In the case of independent electrons, this reduces to the usual integral over the projected density of states. As an example, cohesive energies for some simple transition metal structures are calculated using the recursion method$^{\ast }$ with a Hubbard repulsion between electrons. * Phys. Rev. B \underline {61}, 7953-64 [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W27.00012: Local susceptibility and Kondo scaling Andreas Weichselbaum, Markus Hanl The Kondo scale $T_K$ for quantum impurity systems is typically assumed to guarantee universal scaling of physical quantities. In practice, however, not every definition of $T_K$ necessarily supports this notion away from the strict scaling limit for finite bandwidth $D$. Various theoretical definitions of $T_K$ are analyzed based on the inverse magnetic impurity susceptibility at zero temperature. While conventional definitions in that respect quickly fail to ensure universal Kondo scaling for all $D$, an altered definition of $T_K^{\mathrm{sc}}$ is presented which allows universal scaling of dynamical or thermal quantities for a given fixed Hamiltonian. If the scaling is performed with respect to an external parameter which directly enters the Hamiltonian, such as magnetic field, the corresponding $T_K^{\mathrm{sc,B}}$ for universal scaling may differ, yet becomes equivalent to $T_K^{\mathrm{sc}}$ in the scaling limit. The only requirement for universal scaling in the full Kondo parameter regime with a residual error of less than $1\%$ is a well-defined isolated Kondo feature with $T_K\leq 0.01\,D$. By varying $D$ over a wide range relative to the bare energies of the impurity, this allows a smooth transition from the Anderson to the Kondo model. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W27.00013: Study of Electron Distribution and Magnetism at the Relaxed SrTiO$_3$/LaAlO$_3$ Interface Soham Ghosh, Efstratios Manousakis The presence of a two-dimensional electron gas (2DEG) at the interface between two insulators SrTiO$_3$ and LaAlO$_3$ makes it an interesting topic of condensed matter research. It exhibits a variety of properties such as high mobility, magnetism and superconductivity. Bandstructure calculations have linked the presence of the electon gas to polar catastrophe and oxygen vacancy, but the value of the carrier density and its distribution is a matter of debate. In the present work, we use Density Functional Theory to study the electron density distribution and the effect of ionic relaxations on the properties of the 2DEG. In order to understand the nature of magnetism, we construct localized Wannier functions from Bloch states given by DFT and use them to calculate hopping matrix elements and exchange integrals, which act as parameters in a model to understand electron-electron correlation at the interface. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W27.00014: Electronic and optical properties of $LaVO3$ and $LaVO3$/$SrTiO3$ interface using Ab Initio techniques Suvadip Das, Efstratios Manousakis We have investigated the electronic structure and optical anisotropy of the strongly correlated system $LaVO_3$ using DFT+U and Many body perturbation theory (MBPT) techniques implemented by the Vienna Ab Initio Simulation Package (VASP) code. LDA+U predicts $LaVO_3$ to be an antiferromagnetic insulator with C-type spin and G-type orbital ordering in the monoclinic phase. We will discuss the nature of the transitions leading to the in-plane and out of plane anisotropy in the optical conductivity of $LaVO_3$ using GGA+U. The GW self-energy correction have been incorporated by solving the quasiparticle energies self consistently and the two particle-hole excitonic effects have been included by further solving the Bethe Salpeter equations (BSE). The electronic structure and nature of the low energy optical peaks and their dependence on temperature in the $LaVO_3$/$SrTiO_3$ interface will be presented. The prospect of using $LaVO3$/$SrTiO3$ as a photovoltaic cell with enhanced photo current by the generation of multiple elctron-hole pairs will also be discussed. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W27.00015: The Extended Haldane Phase in Bilinear-Biquadratic Spin-1 chains Colin West, Artur Garcia-Saez, Tzu-Chieh Wei We study the gap of the Haldane phase in the biliniear-biquadratic model as parameterized by $\beta$, the coefficient of the biquadratic term. We then investigate the effect of additional local perturbations on the phase diagram, as first studied at $\beta = 0$ by Gu \& Wen [PRB 80, 155131 (2009)], and later by Pollmann \& Turner [PRB 86, 125441, (2012)]. In particular, we explore the extended Haldane phase under such perturbations, using the guidance of the perturbation-free gap, Pollmann-Turner topological order parameters, and other physical quantities. [Preview Abstract] |
Session W32: Invited Session: Quantum Mechanics Applied to Biophysical Problems
Sponsoring Units: DCOMP DBIOChair: Susan Rempe, Sandia National Laboratories
Room: 708-712
Thursday, March 6, 2014 2:30PM - 3:06PM |
W32.00001: Principles Governing Metal Ion Selectivity in Ion Channel Proteins Invited Speaker: Carmay Lim Our research interests are to \begin{enumerate} \item (i) unravel the principles governing biological processes and use them to identify novel drug targets and guide drug design, and \item (ii) develop new methods for studying macromolecular interactions. \end{enumerate} This talk will provide an overview of our work in these two areas and an example of how our studies have helped to unravel the principles underlying the conversion of Ca$^{2+}$-selective to Na$^{+}$-selective channels. Ion selectivity of four-domain voltage-gated Ca$^{2+}$(Ca$_{\mathrm{v}})$ and sodium (Na$_{\mathrm{v}})$ channels, which is controlled by the selectivity filter (SF, the narrowest region of an open pore), is crucial for electrical signaling. Over billions of years of evolution, mutation of the Glu from domain II/III in the \textbf{EEEE}/\textbf{DEEA} SF of Ca$^{2+}$-selective Ca$_{\mathrm{v}}$ channels to Lys made these channels Na$^{+}$-selective. This talk will delineate the physical principles why Lys is sufficient for Na$^{+}$/Ca$^{2+}$selectivity and why the \textbf{DEKA} SF is more Na$^{+}$-selective than the \textbf{DKEA} one. \\[4pt] References:\\[0pt] [1] Competition among metal ions for protein binding sites: Determinants of metal ion selectivity in proteins. Todor Dudev {\&} Carmay Lim, \textit{Chemical Reviews} (\textbf{2013}) http://dx.doi.org/10.1021/cr4004665\\[0pt] [2] Effect of Metal Hydration on the Selectivity of Mg$^{2+}$ vs. Ca$^{2+}$ in Magnesium Ion Channels. Todor Dudev {\&} Carmay Lim \textit{J. Am. Chem. Soc.}\textbf{ (2013) }\underline {135}: 17200-17208. \\[0pt] [3] Competition among Ca$^{\mathrm{2+}}$, Mg$^{2+}$, and Na$^{+}$ for ion channel selectivity filters: Determinants of metal ion selectivity. Todor Dudev {\&} Carmay Lim,$ J$. \textit{Phys. Chem. B} (\textbf{2012)} \underline {116}: 10703--10714.\\[0pt] [4] Why voltage-gated Ca$^{2+}$ and bacterial Na$^{+}$ channels with the same EEEE motif in their selectivity filters confer opposite metal selectivity. Todor Dudev {\&} Carmay Lim,\textit{ Phys. Chem. Chem. Phys. }(\textbf{2012)}\underline { 14}: 12451--12456. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W32.00002: Quantum Mechanical Studies of Asparaginase Reaction Invited Speaker: Maria J. Ramos |
Thursday, March 6, 2014 3:42PM - 4:18PM |
W32.00003: Free Energy Wells and Barriers to Ion Transport Across Membranes Invited Speaker: Susan Rempe The flow of ions across cellular membranes is essential to many biological processes. Ion transport is also important in synthetic materials used as battery electrolytes. Transport often involves specific ions and fast conduction. To achieve those properties, ion conduction pathways must solvate specific ions by just the ``right amount.'' The right amount of solvation avoids ion traps due to deep free energy wells, and avoids ion block due to high free energy barriers. Ion channel proteins in cellular membranes demonstrate this subtle balance in solvation of specific ions. Using ab initio molecular simulations, we have interrogated the link between binding site structure and ion solvation free energies in biological ion binding sites. Our results emphasize the surprisingly important role of the environment that surrounds ion-binding sites for fast transport of specific ions. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W32.00004: An Unusual Co(I)--H Interaction: Structural and Mechanistic Ramifications for Methyltransferases Invited Speaker: Manoj Kumar The cob(II)alamin cob(I)alamin (Co$^{2+}$/Co$^{1+})$ reduction is a common chemical event in a broad family of cytoplasmic methyltransferases and ATP:corrinoid adenosyltransferases, respectively. Despite its broad and general chemical appeal, the Co$^{2+}$/Co$^{1+}$ reduction continues to remain one of the least understood aspects of corrinoid chemistry. This is due in part to the inaccessible redox chemistry of Co$^{2+}$/Co$^{1+}$ couple under cellular conditions i.e., the reduction potential of cob(II)alamin (-500 mV vs SHE) is more negative than that of the common physiological reductants (-280 mV to -440 mV vs SHE) present in the cellular environments. To gain better understanding about the Co$^{2+}$/Co$^{1+}$ reduction, we have utilized the density functional theory and quantum mechanics/molecular mechanics (QM/MM) computational methods. The calculations indicate that cob(I)alamin, a ubiquitous B$_{12}$ intermediate, is not square planar as has been commonly accepted, but a square pyramidal species due to the unusual hydrogen bonding interaction between the Co$^{1+}$ ion and its axial ligands (\textit{Angew. Chem. Int. Ed.} \textbf{2011}, \textit{50}, 8702-8705; \textit{Inorg. Chem. }\textbf{2012}, \textit{51}, 5533-5538). The Co$^{1+}$--H interaction exerts an anodic shift of 100 mV vs SHE upon the reduction potential of the Co$^{2+}$/Co$^{1+}$ couple, which explains why this redox process is observed inside transferases. Building upon these new insights, an alternate mechanism for the enzyme-bound Co$^{2+}$/Co$^{1+}$ redox process is suggested that is mediated by the square pyramidal cob(I)alamin rather than its commonly accepted square planar analogue. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:30PM |
W32.00005: From Computational Photobiology to the Design of Vibrationally Coherent Molecular Devices and Motors Invited Speaker: Massimo Olivucci In the past multi-configurational quantum chemical computations coupled with molecular mechanics force fields have been employed to investigate spectroscopic, thermal and photochemical properties of visual pigments. [1,2] Here we show how the same computational technology can nowadays be used to design, characterize and ultimately, prepare light-driven molecular switches which mimics the photophysics of the visual pigment bovine rhodopsin (Rh) [2,3]. When embedded in the protein cavity the chromophore of Rh undergoes an ultrafast and coherent photoisomerization. In order to design a synthetic chromophore displaying similar properties in common solvents, we recently focused on indanylidene-pyrroline (NAIP) systems. [3,4] We found that these systems display light-induced ground state coherent vibrational motion similar to the one detected in Rh. Semi-classical trajectories provide a mechanistic description of the structural changes associated to the observed coherent motion which is shown to be ultimately due to periodic changes in the $\pi $-conjugation. \\[4pt] [1] I. Schapiro, M. N. Ryazantsev, L. M. Frutos, N. Ferr\'{e}, R. Lindh {\&} M. Olivucci; \textit{J Am Chem Soc }\textbf{2011}\textit{, 133}, 3354. \\[0pt] [2] S. Gozem, I. Schapiro, N. Ferr\'{e}, M. Olivucci; \textit{Science} \textbf{2012}, \textit{337}, 1225.\\[0pt] [3] A. Melloni, R. Rossi Paccani, D. Donati, V. Zanirato, A. Sinicropi, M. L. Parisi, E. Martin, M. Ryazantsev, W. J. Ding, L. M. Frutos, R. Basosi, S. Fusi, L. Latterini, N. Ferr\'{e}, M. Olivucci; \textit{J Am Chem Soc} \textbf{2010},\textit{ 132}, 9310. \\[0pt] [4] J. L\'{e}onard, I. Schapiro, J. Briand, S. Fusi, R. Rossi Paccani, M. Olivucci, S. Haacke \textit{Chem Eur J }\textbf{2012}, \textit{18}, 15296. [Preview Abstract] |
Session W33: Cold Quantum Gases
Sponsoring Units: DAMOPChair: Luming Duan, University of Michigan
Room: 706
Thursday, March 6, 2014 2:30PM - 2:42PM |
W33.00001: Dynamics of Fermionic Impurity in One Dimension Huijie Guan, Natan Andrei We study the dynamics of a fermionic impurity propagating in a one dimensional infinite line. The system is described by the Gaudin-Yang Model and is exactly solvable by the Nested Bethe Ansatz. Starting from a generic initial state, we obtain the time evolution of the wavefunction by the Yudson Approach in which we expand the initial state with the Nested Bethe Ansatz solutions. One situation that we are interested in is where, initially, the impurity is embedded in host fermions with a lattice configuration and one remove the periodic potential at time zero. We calculate the density profile and correlation functions at a later time. Another situation is to shoot an impurity into a cloud of fermions and calculate the probability for it to pass through. While the repulsive case has been studied already\footnote{C. J. Mathy and M. B. Zvonarev and E. Demler, Nature Physics \textbf{8}, 881(2012)}, we extend it to the attractive case and study the role of bound states in the evolution. We are also interested in boson impurity problem, where not only impurity interacts with host particles, all host particles interact with each other. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W33.00002: Momentum relaxation of a mobile impurity in a one-dimensional quantum gas Oleksandr Gamayun, Evgeni Burovski, Vadim Cheianov, Oleg Lychkovskiy We investigate the time evolution of the momentum of an impurity atom injected into a degenerate Tonks-Girardeau gas. We establish that given an initial momentum $p_0$ the impurity relaxes to a steady state with a non-vanishing momentum $p_{\infty}$. The nature of the steady state is found to be drastically different for integrable and non-integrable impurity models, which is due to multiple coherent scattering processes leading to a resonant interaction between the impurity and the host in the integrable case. The dependence of $p_{\infty}$ on $p_0$ remains non-trivial even in the limit of vanishing interaction between the impurity and host particles. In this limit $p_{\infty}(p_0)$ is found explicitly and the case of the external force applied to the impurity is analyzed as well. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W33.00003: Transport induced dynamical flat-band phases in optical kagome lattices Chih-Chun Chien, Gia-Wei Chern, Massimiliano Di Ventra We consider quantum transport of ultracold fermions in an optical kagome lattice with a barrier keeping part of the lattice initially empty. The kagome lattice has two dispersive bands at low energy and one flat band at higher energy. When the barrier is removed, mobile atoms in the dispersive bands flow to the empty region. With atoms excited and removed by photons in the initially empty region, mobile atoms are depleted and a flat-band insulating phase emerges. Since the flat band of the kagome lattice is a high-energy one compared to the dispersive bands, this dynamically generated flat-band insulator is a population-inversion phase with no pumping required for maintaining it after its formation. In a similar setup a dynamical stripe phase emerges in the flat band when two-component fermions with weakly repulsive onsite interactions evolve in a static kagome lattice or even in the absence of interactions when the optical lattice is modulated. By considering nearest-neighbor repulsion, the system supports topologically non-trivial phases and their dynamics can be monitored at the mean-field level. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W33.00004: Energy dissipation in drag dynamics of one-dimensional Fermi systems Jun'ichi Ozaki, Masaki Tezuka, Norio Kawakami We study the drag dynamics of a few fermions in a cloud of another fermion species in one-dimensional continuous systems, from interest in characteristic many-body effects in cold atom systems whose parameters change gradually in real time. We adopt the Fermi--Hubbard model and the time-dependent density matrix renormalization group method to calculate the energy cost needed to drag a trapped fermion cluster in a cloud of another type of fermions with contact interaction. We plot the energy cost per unit time as a function of the cloud density, and observe two peaks of the energy loss. This result provides, for example, the guide to reduce energy cost when one moves fermions in another type of fermion pool: move them independently or as a cluster. We explain the origin of the two peaks by using a schematic model which describes the detail of the excitation process. The peak in the small density region comes from the quasiparticle modes, while the other peak, in the large density region, corresponds to the collective mode of the whole cluster. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W33.00005: An Interaction quench of strongly correlated heavy-light fermion mixtures khadijeh Najafi, Jim Freericks We use nonequilibrium dynamical mean-field theory to study the strongly correlated heavy-light fermion mixtures after making quench of its interaction parameter. We consider mixture of spinless heavy-light fermion at nonzero temperature and perform the sudden quench of the interaction parameter between the homogeneous metallic and insulating phase. Furthermore we present out result for the case of slow ramps and discuss about the possible optimized ramp for these system. We also discuss how close the system is to a thermal state after the quench of the interaction. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W33.00006: Boson mediated collapse and revival of the Fermi sea in a Bose-Fermi mixture Deepak Iyer, Sebastian Will, Marcos Rigol The collapse and revival dynamics of quantum fields is one of the most pristine forms of nonequilibrium quantum dynamics. It has so far only been observed in the dynamical evolution of bosonic systems, such as coherent light or matter wave fields. We report on the first experimental observation of the collapse and revival of the Fermi sea in a Bose-Fermi mixture. The dynamics is generated by quenching the mixture to a deep 3D optical lattice and letting it evolve. To describe the observations, we develop an analytical model of the dynamics after the quench based on a spin-polarized Fermi sea that interacts with a coherent Bose-Einstein condensate. A remarkable outcome of the exact analytical solution is the robustness of the collapse and revival dynamics to the presence of an underlying confining potential in the initial state and/or during the time evolution, which suggests that such experiments can be used to accurately characterize interactions between bosons and fermions. Furthermore, the analytical solution makes apparent that the fermonic dynamics are independent of whether one starts with a bosonic coherent state or a collapsed Fock state with random occupation numbers. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W33.00007: Log divergence in finite-size quantum Riemann metric Tiago Grangeiro Souza Barbosa Lima, Michael Kolodrubetz, Anatoli Polkovnikov We study the metric tensor, an object that describes distances between quantum states within a ground state manifold. Traditionally, it has been studied for changes in external parameters (e.g., magnetic field) at fixed system size. Here, we instead treat the system size as a tunable parameter and analyze the distance between wave functions at different system sizes. To emulate the effect of a change in the size of the system, we calculate the metric with respect to the position of a movable delta function potential, starting with the simplest case of free fermions. We find that the metric tensor diverges logarithmically with system size, similar to the entanglement entropy in a CFT. We also calculate the same metric tensor for the transverse field Ising model via perturbation theory, and comment on the relationship of our results to the spacetime metric in general relativity. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W33.00008: Quench Dynamics of the Anisotropic Heisenberg Model Wenshuo Liu, Natan Andrei We develop an analytic approach for the study of the quench dynamics of the anisotropic Heisenberg model (XXZ model) on the infinite line. We present the exact time-dependent wavefunctions after a quench in an integral form for any initial state and for any anisotropy ? by means of a generalized Yudson contour representation. We calculate the evolution of several observables from two particular initial states: starting with a local N\`eel state we calculate the time evolution of the antiferromagnetic order parameter-staggered magnetization; starting with a state with consecutive flipped spins we calculate the propagation of magnons and bound state excitations, and the induced spin currents. We also show how the ``string'' solution of Bethe Ansatz equations emerge naturally from the contour approach. We confront our results with experiments and numerical methods where possible. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W33.00009: Heating and decoherence from continuous measurement of the local density of lattice bosons Yariv Yanay, Erich Mueller, Mukund Vengalattore We explore the dynamics of a Bose Hubbard system when a weak local probe continuously measures the occupation of all sites. We find that this poissonian measurement process drives the system towards a thermodynamic distribution with high entropy. The final distribution does not depend on the interaction strength, but the time until steady-state does. Using a master equation for quantum observables, we calculate the heating rate and decoherence time for the system, and follow the time evolution of the two-point and four-point correlation functions in real and momentum space. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W33.00010: Quantized Superfluid Vortex Rings in the Unitary Fermi Gas Michael Forbes, Aurel Bulgac, Michelle Kelley, Kenneth Roche, Gabriel Wlazowski In a recent article, Yefsah \textit{et al.} [Nature \textbf{499}, 426 (2013)] report the observation of an unusual excitation in an elongated harmonically trapped unitary Fermi gas. After phase imprinting a domain wall, they observe oscillations almost an order of magnitude slower than predicted by any theory of domain walls which they interpret as a ``heavy soliton'' of inertial mass some 200 times larger than the free fermion mass or 50 times larger than expected for a domain wall. We present compelling evidence that this ``soliton'' is instead a quantized vortex ring by showing that the main aspects of the experiment can be naturally explained within the framework of time-dependent superfluid DFT. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W33.00011: Transverse Demagnetization Dynamics of a Unitary Fermi Gas Edward Taylor, Alma Bardon, Scott Beattie, Christopher Luciuk, William Cairncross, Daniel Fine, Nathan Cheng, Graham Edge, Shizhong Zhang, Stefan Trotzky, Joseph Thywissen Understanding the quantum dynamics of strongly interacting fermions is a challenge raised by diverse forms of matter, including high-temperature superconductors, neutron stars, and quark-gluon plasmas. An appealing benchmark is offered by cold atomic gases in the unitary limit of strong interactions, where the system is both scale-invariant and known to obey universal thermodynamics in equilibrium. Here we study the dynamics of a transversely magnetized unitary Fermi gas in an inhomogeneous magnetic field. We find that demagnetization is caused by diffusive spin transport with a diffusion constant that saturates at low temperatures to the conjectured quantum-mechanical lower bound $\hbar/m$, where $m$ is the particle mass. The development of pair correlations is observed by measuring Tan's contact parameter. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W33.00012: Numerical generation of solitons, vortex rings, and vortices in an ultracold Fermi gas Peter Scherpelz, Karmela Padavic, Adam Rancon, Andreas Glatz, Igor Aranson, K. Levin Using the complex time-dependent Ginzburg Landau (TDGL) equation [1], we study quenches associated with phase imprinting, temperature sweeps and other density disturbances in three and two dimensional trapped Fermi gases. We consider variations in the TDGL equation due to the BCS-BEC crossover. While solitons are generally seen after the quenches, they are often accompanied by vortices and occasionally by vortex rings. Our work is partly motivated by the experimental observation of solitons in ultracold Fermi gases [2] which display both unusually slow oscillations and remarkable stability in a three-dimensional atomic gas. We discuss the stability and nature of the decay of these 3 types of collective superfluid inhomogeneities and their dependence on fluctuations, trap effects, and the trap aspect ratio. [1] A. Glatz, H. Roberts, I. Aranson, and K. Levin, PRB 84 180501 (2011). [2] T. Yefsah et al., Nature 499 426 (2013). [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W33.00013: Interacting Dark-Resonances for Sub-Natural Spectral Response: From Atoms to Meta-Atoms Pankaj Jha, Michael Mrejen, Jeongmin Kim, Chihhui Wu, Yuan Wang, Xiaobo Yin, Xiang Zhang Coherent interaction between dark-resonances have been extensively studied in atomic molecular and optical (AMO) physics to alter the interaction between atoms and electromagnetic fields. Here we theoretically investigate a classical analogue of interacting dark-resonance type physics in a plasmonic meta-molecule consisting of a radiative(bright) atom coupled to cascaded subradiant (dark) atoms. We theoretically demonstrate crude-damping limited absorptive response of the plasmonic molecule which also exhibits efficient excitation transfer within the elements. We provide numerical results in support of our analysis and develop an analytical description of the response of the meta-molecule in the limit of weak cascaded dark atoms coupling. The proposed scheme may be useful, in principle, for enhanced non-linearity, energy transport via coupled dark-resonances in plasmonics. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W33.00014: Dynamics of Vector Solitons in Bose Einstein Condensates Majed O.D. Alotaibi, Lincoln D. Carr We analyze the dynamics of two-component vector solitons, namely bright-in-dark solitons, via variational approximations in Bose-Einstein condensates. We calculate the binding energy and the oscillation modes between the two components analytically for special cases. The variational approximation is based on hyperbolic secant (hyperbolic tangent) for the bright (dark) component, which leads to a system of ordinary differential equations for the evolution of the ansatz parameters. Analytical calculations are performed for same width components in the vector soliton, and numerical calculations extend the results to arbitrary widths. The system is described by a vector nonlinear Shr\"odinger equation appropriate to the mean field theory of Bose-Einstein condensates [Preview Abstract] |
Session W34: Topological Systems and Quantum Dynamics
Sponsoring Units: DAMOPChair: Alexey Gorshkov, Joint Quantum Institute
Room: 704
Thursday, March 6, 2014 2:30PM - 2:42PM |
W34.00001: Spin supercurrents and torquing with Majorana fermions Kirill Shtengel, Alexey Kovalev, Amrit De We show that resonant coupling and entanglement between a mechanical resonator and majorana bound states can be achieved via spin supercurrents in a 1D quantum wire with strong spin-orbit interactions in the proximity of s-wave superconductor. The bound states induced by vibrating and stationary magnets can hybridize thus resulting in spin-current induced {\$}4$\backslash $pi{\$}-periodic torque, as a function of the relative field angle, acting on the resonator. We propose a realization based on spin transistor like architecture in which a heterostructure nanowire consists of semiconductors with large and small g-factors in order to form the topological and non-topological regions. We also study the feasibility of detecting and manipulating majorana bound states with the use of magnetic resonance force microscopy techniques. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W34.00002: Floquet generation of Majorana edge modes and topological invariants Diptiman Sen, Manisha Thakurathi, Aavishkar Patel, Amit Dutta, Krishnendu Sengupta We show that periodic driving of one of the parameters in the Hamiltonian of a system can produce Majorana modes at its edges. The systems studied include a $p$-wave superconducting wire and the Kitaev model on the honeycomb lattice. For the wire, we show that periodic $\delta$-function kicks of the on-site potential can produce a number of Majorana modes at the two ends; these modes can appear or disappear as the driving frequency is varied. The end modes correspond to eigenvalues of the Floquet operator equal to $\pm 1$. Using Floquet theory for the bulk, we derive a topological invariant which correctly predicts the number of these modes as a function of the frequency and the Floquet eigenvalue. We also discuss the generation of end modes by periodic kicking of the hopping and superconducting terms. For the Kitaev model, we derive the phase diagram where Majorana edge modes appear on zigzag and armchair edges. We then show that if one of the couplings is given periodic $\delta$-function kicks, modes can appear on some edges even when the corresponding equilibrium Hamiltonian has no modes on those edges. The Floquet theory of the bulk can again be used to predict the frequencies at which edge modes appear or disappear for different values of the momentum of the modes. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W34.00003: Quantum Quenches in Topological Systems Graham Kells, Diptiman Sen, J.K. Slingerland, Smitha Vishveshwara We study the non-equilibrium dynamics of quenching through a quantum critical point in topological systems, focusing on one of their characteristic features, namely, ground state degeneracies, and associated topological sectors. We present the notion of ``topological blocking,'' experienced by the dynamics due to the mismatch in degeneracies between two phases. We demonstrate the interplay between quenching and topology in two extensively studied systems, the transverse Ising chain and the Kitaev honeycomb model. Casting these systems in the language of fermionic spinless p-wave paired superconductors enables us to cleanly address degeneracies, subtle issues of fermion occupation and parity, and mismatches between topological sectors. We show that several features of the quench, which are related to Kibble-Zurek physics, are sensitive to the topological sector being probed. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W34.00004: Hidden-Symmetry-Protected Topological Semimetals on a Square Lattice Jing-Min Hou We study a two-dimensional fermionic square lattice, which supports the existence of two-dimensional Weyl semimetal, quantum anomalous Hall effect, and $2\pi$-flux topological semimetal in different parameter ranges. We show that the band degenerate points of the two-dimensional Weyl semimetal and $2\pi$-flux topological semimetal are protected by two distinct novel hidden symmetries, which both corresponds to antiunitary composite operations. When these hidden symmetries are broken, a gap opens between the conduction and valence bands, turning the system into a insulator. With appropriate parameters, a quantum anomalous Hall effect emerges. The degenerate point at the boundary between the quantum anomalous Hall insulator and trivial band insulator is also protected by the hidden symmetry. [PRL 111, 130403(2013)] [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W34.00005: Topological superfluids with finite-momentum pairing and Majorana fermions Chunlei Qu, Zhen Zheng, Ming Gong, Yong Xu, Li Mao, Xubo Zou, Guangcan Guo, Chuanwei Zhang Majorana fermions (MFs), quantum particles that are their own antiparticles, are not only of fundamental importance in elementary particle physics and dark matter, but also building blocks for fault-tolerant quantum computation. Recently MFs have been intensively studied in solid state and cold atomic systems. These studies are generally based on superconducting pairing with zero total momentum. On the other hand, finite total momentum Cooper pairings, known as Fulde-Ferrell (FF) Larkin-Ovchinnikov (LO) states, were widely studied in many branches of physics. However, whether FF and LO superconductors can support MFs has not been explored. Here we show that MFs can exist in certain types of gapped FF states, yielding a new quantum matter: topological FF superfluids/superconductors. We demonstrate the existence of such topological FF superfluids and the associated MFs using spin-orbit-coupled degenerate Fermi gases and derive their parameter regions. The implementation of topological FF superconductors in semiconductor/superconductor heterostructures is also discussed. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W34.00006: Singlet Mott State Simulating the Bosonic Laughlin Wave Function Biao Lian, Shou-Cheng Zhang We study properties of a class of spin singlet Mott states for arbitrary spin $S$ bosons on a lattice, with particle number per cite $n=S/l+1$, where $l$ is a positive integer. We show that such a singlet Mott state can be mapped to a bosonic Laughlin wave function on the sphere with a finite number of particles at filling $\nu=1/2l$. Bosonic spinons, particle and hole excitations in the Mott state are discussed, among which the hole excitation can be mapped to the quasi-hole of the bosonic Laughlin wave function. We show that this singlet Mott state can be realized in a cold atom system on optical lattice, and can be identified using Bragg spectroscopy and Stern-Gerlach techniques. This class of singlet Mott states may be generalized to simulate bosonic Laughlin states with filling $\nu=q/2l$. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W34.00007: Realizing topological states with polyatomic symmetric top molecules Michael Wall, Kenji Maeda, Lincoln Carr We show how to use polyatomic symmetric top molecules, such as methyl fluoride, in an optical lattice to produce states with non-trivial topology via a self-consistent analog of the proximity effect in the internal state space of the molecule. The key ingredient is pairwise transitions between internal states of a molecule which are generated by the dipole-dipole interaction and made resonant by a combination of static and AC electric field dressing. These pairwise transitions endow the effective many-body Hamiltonian with a U(1)$\times$Z$_2$ symmetry leading to topologically nontrivial states. We will present results of matrix product state simulations demonstrating non-trivial topology, and also provide mappings of the many-body description to models of quantum spins with un-conserved magnetization as well as to systems with Majorana fermion excitations. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W34.00008: Topological Bose-Mott Insulators in a One-Dimensional Optical Superlattice Shi-Liang Zhu, Z.D. Wang, Y.H. Chan, L.M. Duan In this talk, I will introduce topological Bose-Mott insulators we found in a one-dimensional optical superlattice. We study topological properties of the Bose-Hubbard model with repulsive interactions in a one-dimensional optical superlattice, and find that the Mott insulator states of the single-component (two-component) Bose-Hubbard model under fractional fillings are topological insulators characterized by a nonzero charge (or spin) Chern number with nontrivial edge states. For ultracold atomic experiments, we show that the topological Chern number can be detected through measuring the density profiles of the bosonic atoms in a harmonic trap. Ref.: S.L. Zhu, Z.D. Wang, Y.H. Chan, and L.M. Duan, Phys. Rev. Lett. 110, 075303 (2013). [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W34.00009: Detection of topological excitations in atom circuits via phase reference C. Lanier, N. Murray, M. Edwards, C.W. Clark Atom circuits (such as ring Bose--Einstein condensates [BECs]) can now be implemented in ultracold--atom systems confined in a horizontal plane with a red--detuned light sheet plus an essentially arbitrary two--dimensional potential in the plane. Atom--circuit operation may be effected by subsequent interaction with the system (such as stirring a ring BEC with a blue--detuned laser). These interactions will create topological excitations such as solitons and ring-- and line--vortices which may be critical to circuit operation. It is therefore interesting to study methods by which such topological excitations can be detected and to identify the various signatures whereby the different excitations can be distinguished. We have investigated methods for doing this in multiply connected BECs in which part of the condensate participates in the atom circuit while another part is left alone so that its phase profile is undisturbed. By releasing the confinement the presence of these topological excitations may be detected via the resulting interference pattern. Using the time--depdendent Gross--Pitaevskii equation, we demonstrate ways in which this may be done for BECs confined in ring--ring and disk--plus--ring traps. We show how to detect vortices, solitons, and phonons. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W34.00010: Detangling Flat Bands via Fano Structures Joshua Bodyfelt, Sergej Flach, Daniel Leykam, Anton Desyatnikov, Peter Matthies Translationally invariant lattices with flat bands (FB) in the band structure possess irreducible compact localized flat band states (FBS). The number of unit cells involved in one irreducible FBS defines the FB class of the model. For class 1, we transform and detangle the FBS and dispersive states into a Fano lattice. Inverting the scheme, we end up with a continuum of FB models for any FB class. In the case of an on-site disorder potential, the symmetric part lifts the FB degeneracy, keeping compact localization of FBS. The antisymmetric part yields Fano-induced Cauchy tails for the potential felt by the dispersive states. As a result, weak disorder enforces different energy dependent localization length scales, and highly nontrivial mode profiles at the FB energy. Scattering by perturbed FBS can then be understood as Fano resonance. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W34.00011: Interacting gauge theories in ultracold gases Manuel Valiente, Matthew J. Edmonds, Luis Santos, Gedeminas Juzeliunas, Patrik Ohberg We consider ultracold atoms coupled to a near-resonant laser field, and show how weak interacting -- yet not dynamical -- gauge fields can be induced in the system. The resulting microscopic effective theory corresponds to a physical realization of one-dimensional anyons, while its semiclassical (or mean-field) approximation supports chiral solitons and persistent currents on a ring.\\[4pt] [1] M.J. Edmonds et al., Phys. Rev. Lett. 110, 085301 (2013) [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W34.00012: Three `species' of Schrodinger cat states in an infinite-range spin model Bo Zhao, Cotty Kerridge, David Huse We explore a transverse-field Ising model that exhibits both spontaneous symmetry-breaking and eigenstate thermalization. Within its ferromagnetic phase, the exact eigenstates of the Hamiltonian of any large but finite-sized system are all Schrodinger cat states: coherent linear superpositions of states with `up' and `down' spontaneous magnetization. This model exhibits two dynamical phase transitions {\it within} the ferromagnetic phase between regimes where the motion of the order parameter between `up' and `down' is via quantum tunneling or not, and is always overdamped or not. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W34.00013: The Hilbert-glass transitions: new universality of temperature-tuned many-body dynamical quantum criticality David Pekker, Gil Refael, Ehud Altman, Eugene Demler, Vadim Oganesyan We study a new class of unconventional critical phenomena that is characterized by singularities only in dynamical quantities and has no thermodynamic signatures. Describing this purely dynamical quantum criticality is technically challenging as understanding the finite-temperature dynamics necessarily requires averaging over a large number of matrix elements between many-body eigenstates. Here we develop a real-space renormalization group method for excited state (RSRG-X) that allows us to overcome this challenge in a large class of models. We characterize a specific example: the 1D disordered transverse field Ising model with generic interactions. While thermodynamic phase transitions are generally forbidden in this model, using RSRG-X we find a finite-temperature dynamical transition between two localized phases. The transition is characterized by non-analyticities in the low frequency heat conductivity and in the long-time (dynamic) spin correlation function. The latter is a consequence of an up-down spin symmetry that results in the appearance of an Edwards-Anderson-like order parameter in one of the localized phases. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W34.00014: Probing Real-Space and Time-Resolved Correlation Functions with Many-Body Ramsey Interferometry Michael Knap, Adrian Kantian, Thierry Giamarchi, Immanuel Bloch, Mikhail D. Lukin, Eugene Demler We propose to use Ramsey interferometry and single-site addressability, available in synthetic matter such as cold atoms, polar molecules, or trapped ions, to measure real-space and time resolved spin correlation functions. These correlation functions directly probe the excitations of the system, which makes it possible to characterize the underlying many-body states. Moreover, they contain valuable information about phase transitions where they exhibit scale invariance. We explicitly consider examples of the two-dimensional, antiferromagnetic Heisenberg model and the one-dimensional, long-range transverse field Ising model to illustrate the technique. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W34.00015: Quasiclassical molecular dynamics for the dilute Fermi gas at unitarity Kevin Dusling, Thomas Schafaer We study the dilute Fermi gas at unitarity using molecular dynamics with an effective quantum potential constructed to reproduce the quantum two-body density matrix at unitarity. Results for the equation of state, the pair correlation function and the shear viscosity are presented. These quantities are well understood in the dilute, high temperature, limit. Using molecular dynamics we determine higher order corrections in the diluteness parameter $n\lambda^3$, where $n$ is the density and $\lambda$ is the thermal de Broglie wave length. In the case of the contact density, which parameterizes the short distance behavior of the correlation function, we find that the results of molecular dynamics interpolates between the truncated second and third order virial expansion, and are in excellent agreement with existing T-matrix calculations. For the shear viscosity we reproduce the expected scaling behavior at high temperature, $\eta\sim 1/\lambda^3$, and we determine the leading density dependent correction to this result. [Preview Abstract] |
Session W35: Focus Session: Quantum Computing Architectures and Algorithms: Characterization, Verification, & Validation
Sponsoring Units: GQIChair: Robin Blume-Kohout, Sandia National Laboratories
Room: 702
Thursday, March 6, 2014 2:30PM - 2:42PM |
W35.00001: Weak value amplification considered harmful Christopher Ferrie, Joshua Combes We show using statistically rigorous arguments that the technique of weak value amplification does not perform better than standard statistical techniques for the tasks of parameter estimation and signal detection. We show that using all data and considering the joint distribution of all measurement outcomes yields the optimal estimator. Moreover, we show estimation using the maximum likelihood technique with weak values as small as possible produces better performance for quantum metrology. In doing so, we identify the optimal experimental arrangement to be the one which reveals the maximal eigenvalue of the square of system observables. We also show these conclusions do not change in the presence of technical noise. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W35.00002: The information inequality in postselection on parameter estimation problems Saki Tanaka, Naoki Yamamoto The weak measurement proposed by Aharonov and his co-workers allows the meter to generate an infinitely large measurement result. The essence of this measurement is the operation called postselection; more precisely, we read the measurement result displayed by the probe only when we get a particular postselected state of the system, after the system-probe interaction. When the postelected state is nearly orthogonal to the preselected state, this weak measurement significantly amplifies the amount of displacement of the probe, which is regarded as a signal amplification. However, does the large displacement really mean signal amplification? We show that this is not true in the parameter estimation context. In general, the estimation error of a parameter is lower bounded by the inverse of the SLD-Fisher information, known as the Cram\'er-Rao inequality. We compare the SLD-Fisher information of the states with and without postselection. Taking into account the success rate of the postselection, we then derive an inequality showing that the postselection never decrease the estimation error. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W35.00003: Quantum Limits on Probabilistic Amplifiers Shashank Pandey, Zhang Jiang, Joshua Combes, Carlton Caves An ideal phase-preserving linear amplifier is a deterministic device that adds to an input signal the minimal amount of noise consistent with the constraints imposed by quantum mechanics. A noiseless linear amplifier takes an input coherent state to an amplified coherent state, but only works part of the time. Such a device is actually better than noiseless, since the output has less noise than the amplified noise of the input coherent state; we refer to such devices as immaculate. We bound the working probabilities of probabilistic and approximate immaculate amplifiers and construct theoretical models that achieve some of these bounds. Our chief conclusions are the following: (i) the working probability of any phase-insensitive immaculate amplifier is very small in the phase-plane region where the device works with high fidelity;(ii) phase-sensitive immaculate amplifiers that work only on coherent states sparsely distributed on a phase-plane circle centered at the origin can have a reasonably high working probability. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W35.00004: Optimal quantum-enhanced interferometry using a laser power source Matthias Lang, Carlton Caves We consider an interferometer powered by laser light (a coherent state) into one input port and ask the following question: what is the best state to inject into the second input port, given a constraint on the mean number of photons this state can carry, in order to optimize the interferometer's phase sensitivity? This question is the practical question for high-sensitivity interferometry. We answer the question by considering the quantum Cramer-Rao bound for such a setup. The answer is squeezed vacuum, if there are no photon losses in the interferometer. For a lossy interferometer, the squeezed vacuum is the best choice for the practical case where the laser power is much bigger than the power put into the squeezing. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W35.00005: Optimal phase estimation using photon number counting in the presence of dephasing noise Kaushik Seshadreesan, Bhaskar RoyBardhan, Hwang Lee, Jonathan Dowling We study interferometric phase estimation in the presence of dephasing noise. We show that photon number counting and photon number parity measurement achieve the quantum Cramer-Rao bound of the optimal cosine state. Furthermore, we show that, when operated using a Bayesian update protocol, photon counting saturates the bound independently of the actual value of the unknown phase, thereby allowing for globally optimal phase estimation. We also show that both photon counting and parity measurement achieve the quantum Cramer-Rao bound of all path-symmetric probe states (a class which includes the optimal cosine state) in the presence of dephasing noise. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W35.00006: Quantum process tomography of near-unitary maps Amir Kalev, Charles Baldwin, Ivan Deutsch We study the problem of quantum process tomography given the prior information that the implemented map is near to a unitary map on a $d$-dimensional Hilbert space. In particular, we show that a perfect unitary map is completely characterized by a minimum of $d^2 + d$ measurement outcomes. This contrasts with the $d^4$ measurement outcomes required in general. To achieve this lower bound, one must probe the system with a particular set of $d$ states in a particular order. This order exploits unitarity but does not assume any other structure of the map. We further consider the more general case of noisy quantum maps, with a low level of noise. Our study indicates that transforming to the interaction picture, where the noiseless map is represented by a diagonal operator, can provide a useful tool to identify the noise structure. This, in turn, can lead to a substantial reduction in the numerical resources needed to estimate the noisy map. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W35.00007: Quantum process estimation via compressed sensing with convex optimization Charles Baldwin, Amir Kalev, Ivan Deutsch Quantum process tomography is the standard method for diagnosing an unknown process. However, it is extremely resource intensive requiring $O(d^4)$ measurement outcomes for a $d$ dimensional Hilbert space. In previous work, researchers have applied compressed sensing techniques allowing us to make use of previous knowledge of the system in order to reduce the resources required. We study different procedures for reconstructing a process matrix from compressed sensing in the form of convex-optimization. We show that different estimation applied to the same data are sensitive to different types of noise. The estimators could, therefore, be used as indicators of particular error models. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W35.00008: Compressed Sensing for Reconstructing Sparse Quantum States Kenneth Rudinger, Robert Joynt Compressed sensing techniques have been successfully applied to quantum state tomography, enabling the efficient determination of states that are nearly pure, i.e, of low rank. We show how compressed sensing may be used even when the states to be reconstructed are full rank. Instead, the necessary requirement is that the states be sparse in some known basis (e.g. the Pauli basis). Physical systems at high temperatures in thermal equilibrium are important examples of such states. Using this method, we are able to demonstrate that, like for classical signals, compressed sensing for quantum states exhibits the Donoho-Tanner phase transition. This method will be useful for the determination of the Hamiltonians of artificially constructed quantum systems whose purpose is to simulate condensed-matter models, as it requires many fewer measurements than demanded by standard tomographic procedures. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W35.00009: High-Confidence Quantum Gate Tomography Blake Johnson, Marcus da Silva, Colm Ryan, Shelby Kimmel, Brian Donovan, Thomas Ohki Debugging and verification of high-fidelity quantum gates requires the development of new tools and protocols to unwrap the performance of the gate from the rest of the sequence. Randomized benchmarking tomography\footnote{Kimmel et al. arXiv:1306.2348 [quant-ph] (2013)} allows one to extract full information of the unital portion of the gate with high confidence. We report experimental confirmation of the technique's applicability to quantum gate tomography. We show that the method is robust to common experimental imperfections such as imperfect single-shot readout and state preparation. We also demonstrate the ability to characterize non-Clifford gates. To assist in the experimental implementation we introduce two techniques. ``Atomic Cliffords'' use phase ramping and frame tracking to allow single-pulse implementation of the full group of single-qubit Clifford gates. Domain specific pulse sequencers allow rapid implementation of the many thousands of sequences needed. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W35.00010: Robust Characterization of Quantum Processes Invited Speaker: Shelby Kimmel Accurate characterization of the errors that occur in quantum systems will help to improve the performance of quantum computers. However, many characterization procedures suffer from systematic errors because they assume state preparation, measurement, and other controlling gates are error free. In this talk I will describe a method that can provide estimates of almost all parameters of a quantum map, yet is robust to many types of errors.\\[4pt] In collaboration with Marcus P. da Silva, Colm A. Ryan, Blake R. Johnson, and Thomas Ohki, Raytheon BBN Technologies. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W35.00011: Tomography via Correlation of Noisy Measurement Records Colm Ryan, Blake Johnson, Jay Gambetta, Jerry Chow, Marcus Silva, Oliver Dial, Thomas Ohki We present methods and results of shot-by-shot correlation of noisy measurements to extract entangled state and process tomography in a superconducting qubit architecture \footnote{Ryan et al. arXiv:1310.6448 (2013)}. We show that averaging continuous values, rather than counting discrete thresholded values, is a valid tomographic strategy and is in fact the better choice in the low signal-to-noise regime. We show that the effort to measure $N$-body correlations from individual measurements scales exponentially with $N$, but with sufficient signal-to-noise the approach remains viable for few-body correlations. We provide a new protocol to optimally account for the transient behavior of pulsed measurements. Despite single-shot measurement fidelity that is less than perfect, we demonstrate appropriate processing to extract and verify entangled states and processes. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W35.00012: Simpler, faster, better: robust randomized benchmarking tests for non-unitality and non-Markovianity in quantum devices Joel Wallman, Steve Flammia, Marie Barnhill, Joseph Emerson Characterizing and suppressing noise in quantum systems is the major obstacle to developing a universal quantum computer. It is impossible to completely characterize the noise acting on $n$ qubits efficiently, since a complete characterization would require an exponential number of parameters. However, we can efficiently obtain parameters of interest, such as the average gate fidelity (which gives the average error rate introduced by using a noisy gate instead of an ideal gate) using protocols such as randomized benchmarking, which will be reliable under certain strong assumptions (such as time-independent and gate-independent noise). In this talk, we will present recent results that allow randomized benchmarking to be used in a wider array of scenarios (in particular, to characterize time-dependent noise), with simpler operations (e.g., using only single-qubit operators, which makes the assumption of gate-independent noise more reasonable) and to obtain additional information about the noisy processes (e.g., whether they are unital and/or Markovian). These results allow a more efficient and complete characterization of noisy quantum systems. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W35.00013: Determining individual gate error models from the output of quantum error detection circuits Austin Fowler, John Martinis There are many different ways of determining an error model for a quantum gate, including process tomography and randomized benchmarking. These techniques focus on individual gates. A frequently discussed concern is whether any given error model accurately reflects how errors will compose and propagate when multiple gates are applied in a practical circuit. We approach the error model problem in a new manner, starting with the experimental output of a complete quantum error detection circuit, and determine error models for all gates from this single source of data. We argue that these error models are the most appropriate for predicting the error suppression performance of larger quantum circuits. [Preview Abstract] |
Session W36: Invited Session: Panel Discussion: Increasing the Participation of Women in Physics--The Conferences for Undergraduate Women in Physics (CUWiP)
Sponsoring Units: CSWPChair: Theodore Hodapp, American Physical Society
Room: 703
Thursday, March 6, 2014 2:30PM - 3:06PM |
W36.00001: Overview and History of CUWiP Invited Speaker: Kevin Pitts The APS Conferences for Undergraduate Women in Physics (CUWiP) are a continuation of a grass-roots collaborative effort of physicists from around the country that has provided conferences for undergraduate women annually since 2006. The goals of the CUWiP effort are to~increase retention and improve career outcomes of undergraduate women in physics by (i) communicating the breadth of education and career paths open to physics majors; (ii) disseminating information and advice on applying for summer research, graduate school and professional employment; and (iii) providing opportunities to share experiences, advice and ideas with women at different stages of their education or career paths. Since the first conference in 2006, the number of CUWiP locations has grown from one in the first two years to six in 2012 and 2013. The number of participants has shown more than commensurate growth---increasing from 29 students to approximately 650 in the first seven years. Close to 900 women attended the 2013 conferences and almost 300 applicants had to be turned away. The 2014 Conferences again saw record number of applicants and attendees, with 1300 students participated in conferences at eight different sites. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W36.00002: Initial findings from CUWiP Invited Speaker: Eric Brewe The 2014 APS Conferences for Undergraduate Women in Physics gathered approximately a thousand participants at six sites across the nation. As part of the overall project evaluation, we collected pre and post workshop surveys from conference participants. The surveys were designed to investigate how features of the CUWiP conferences impacted participants' physics-related attitudes (e.g. physics identity) and knowledge (e.g. opportunities, career options) as well as to gauge how the conferences promoted the development of networks among participants. In this talk we present initial findings from the surveys and discuss how these surveys might influence future iterations of the CUWiP. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 4:18PM |
W36.00003: Hosting a CUWiP -- the fun and the challenges Invited Speaker: Mette Gaarde Louisiana State University is hosting the South Eastern Conference for Undergraduate Women in Physics (CUWiP) in January 2014, as one out of eight regional CUWiPs. As a first-time organizer of a CUWiP I will share our local experiences with organizing and hosting the conference: the timeline for the work that is involved in hosting, what we learned, and what were our biggest successes and challenges. Since the different regional conferences have different ``flavors'' and somewhat different issues, I will also briefly give an overview of the more general lessons learned with highlights from some of the different conferences. Finally, I will try to convey the incredibly empowering experience, for both organizers and participants, of being in a room with more than 100 female physicists!~ [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W36.00004: Panel Discussion on the Conferences for Undergraduate Women in Physics Invited Speaker: Nicole Johnson |
Thursday, March 6, 2014 4:54PM - 5:30PM |
W36.00005: Panel Discussion on the Conferences for Undergraduate Women in Physics Invited Speaker: Hsiu-Chuan Hsu |
Thursday, March 6, 2014 5:30PM - 6:06PM |
W36.00006: Panel Discussion on the Conferences for Undergraduate Women in Physics Invited Speaker: Shannon Glavin |
Thursday, March 6, 2014 6:06PM - 6:42PM |
W36.00007: Panel Discussion on the Conferences for Undergraduate Women in Physics Invited Speaker: Noelle Stiles |
Thursday, March 6, 2014 6:42PM - 7:18PM |
W36.00008: Panel Discussion on the Conferences for Undergraduate Women in Physics Invited Speaker: Laurel Winter |
Session W37: Focus Session: Carbons Nanotubes: Towards More Complex Circuits
Sponsoring Units: DMPChair: Xia Hong, University of Nebraska-Lincoln
Room: 705/707
Thursday, March 6, 2014 2:30PM - 3:06PM |
W37.00001: Carbon Nanotube Computer: Transforming Scientific Discoveries into Working Systems Invited Speaker: Subhasish Mitra The miniaturization of electronic devices has been the principal driving force behind the semiconductor industry, and has brought about major improvements in computational power and energy efficiency. Although advances with silicon-based electronics continue to be made, alternative technologies are being explored. Digital circuits based on transistors fabricated from carbon nanotubes (CNTs) have the potential to outperform silicon by improving the energy-- delay product, a metric of energy efficiency, by more than an order of magnitude. Hence, CNTs are an exciting complement to existing semiconductor technologies. However, carbon nanotubes (CNTs) are subject to substantial inherent imperfections that pose major obstacles to the design of robust and very large-scale CNFET digital systems: \begin{itemize} \item It is nearly impossible to guarantee perfect alignment and positioning of all CNTs. This limitation introduces stray conducting paths, resulting in incorrect circuit functionality. \item CNTs can be metallic or semiconducting depending on chirality. Metallic CNTs cause shorts resulting in excessive leakage and incorrect circuit functionality. \end{itemize} A combination of design and processing technique overcomes these challenges by creating robust CNFET digital circuits that are immune to these inherent imperfections. This imperfection-immune design paradigm enables the first experimental demonstration of the carbon nanotube computer, and, more generally, arbitrary digital systems that can be built using CNFETs. The CNT computer is capable of performing multitasking: as a demonstration, we perform counting and integer-sorting simultaneously. In addition, we emulate 20 different instructions from the commercial MIPS instruction set to demonstrate the generality of our CNT computer. This is the most complex carbon-based electronic system yet demonstrated. It is a considerable advance because CNTs are prominent among a variety of emerging technologies that are being considered for the next generation of highly energy-efficient electronic systems. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W37.00002: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W37.00003: Carbon Nanotube Network Anti-fuses Paulo Araujo, Albert Liao, Joaquin Rodriguez-Nieva, Eduardo Barros, Hyun Jung, Ji Hao, Yung Jung, Mildred Dresselhaus Copper interconnects are known to fail from electromigration at $\sim$10$^{6}$~A/cm$^{2}$. However, to continue aggressive scaling in integrated circuits (ICs), new materials that can carry much higher current densities will be required. For this reason, Carbon nanotubes (CNTs), which can carry up to 10$^{9}$ A/cm$^{2}$, are a promising replacement. We discover that after Joule breakdown, CNTs can be healed by applying a healing voltage $V_{H}$. Such a technology could be useful to repair failed interconnects, creating anti-fuses for field programmable gate array (FPGA) or memory technology. We fabricate a CNT network using a dip-coating method and then we sweep a voltage across the device until the CNT network undergoes Joule breakdown, creating a physical gap (of $\sim$ 10-40~nm) within the network. Making hysteretic $I-V$ sweeps, we observe a sudden increase in current at a voltage $\sim$50 -- 80~{\%} of the breakdown voltage $V_{BD}$. We can reliably break and heal the device multiple times. We also observe the Raman coalescence induced mode (CIM), which is characteristic of \textit{sp} hybridized carbon chains, after the breaking and healing process. According to our analysis, we conclude that the formation of carbon chains is key to promote the electrical restoration of the broken CNT network. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W37.00004: Fabrication and electrical properties of single wall carbon nanotube channel and graphene electrode based transistors; Toward all carbon electronics Sang Wook Lee, Miri Seo, Junhong Na, Yong Hyeon Kim, Byeong-Joo Lee, Jin-Ju Kim, Hoyeol Yun, Hakseong Kim, Ho-Ang Yoon, Keun Soo Kim, Goo-Hwan Jeong, Gyu Tae Kim A transistor structure composed of an individual single-walled carbon nanotube (SWNT) channel with a graphene electrode was demonstrated. The integrated arrays of transistor devices were prepared by transferring patterned graphene electrode array on top of the pre-deposited SWNTs which were aligned along one direction. Aligned arrays of SWNTs were synthesized by thermal chemical vapor deposition (CVD) method on quartz substrate. The micro scale contact electrodes and following circuit structures were defined by photo lithography on the large area graphene produced by CVD. Both of the single and multi layer graphene were used for the electrode materials. In this presentation, the device fabrication procedure, the contact properties, and the transistor performances of the device structures were discussed. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W37.00005: Capacitance of nanostructures Junqiang Lu, Jonathan Gonzalez, Carlos Sierra, Yang Li Modeling capacitance of nanostructures in nanoscale circuits presents particular challenge because of contribution from electrodes, which can usually be neglected in modeling conductance, or even capacitance of macrodevices. We present a model of capacitance of a nano-gap configuration and applied to calculate capacitance of a carbon nanotube nano-gap and effective capacitance of a buckyball inside the nano-gap. The capacitance of the carbon nanotube nano-gap increases with length of electrodes, which shows the important contribution from the electrodes in dynamic transport properties of nanoscale circuits. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W37.00006: Self-aligned T-gate Nanotube Radio Frequency Transistors and Circuits Application Yuchi Che, Yu Cao, Chongwu Zhou We applied self-aligned T-gate design to aligned carbon nanotube array transistors and achieved an extrinsic current-gain cut-off frequency (ft) of 25 GHz, which is the best on-chip performance for nanotube RF transistors reported to date. Meanwhile, an intrinsic current-gain cut-off frequency up to 102 GHz is obtained, comparable to the best value reported for nanotube RF transistors. Armed with the excellent extrinsic RF performance, we performed both single-tone and two-tone measurements for aligned nanotube transistors at a frequency up to 8 GHz. Furthermore, we utilized T-gate aligned nanotube transistors to construct mixing and frequency doubling analog circuits operated in gigahertz frequency regime. Our results confirm the great potential of nanotube-based circuit application and indicate that nanotube transistors are promising building blocks in high-frequency electronics.. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W37.00007: Towards parallel, CMOS-compatible fabrication of carbon nanotube single electron transistors Muhammad Islam, Daeha Joung, Saiful Khondaker We demonstrate an approach for the parallel fabrication of single electron transistor (SET) using single-walled carbon nanotube (SWNT). The approach is based on the integration of individual SWNT via dielectrophoresis (DEP) and deposition of metal top contact. We fabricate SWNT devices with a channel length of 100 nm and study their electron transport properties. We observe a connection between the SET performance and room temperature resistance ($R_{T})$ of the devices. Majority (90{\%}) of the devices with 100 K$\Omega $ \textless $R_{T}$ \textless 1 M$\Omega $, show periodic, well defined Coulomb diamonds with a charging energy around 15 meV, corresponding to transport through a single quantum dot (QD), defined by the top contact. For high $R_{T}$ (\textgreater 1M$\Omega )$, devices show multiple QD behaviors, while QD was not formed for low $R_{T}$ (\textless 100 K$\Omega )$ devices. This easy, simple and CMOS-compatible fabrication process will provide a much desired insight towards the wide spread application and commercialization of SWNT SET devices. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W37.00008: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W37.00009: Improving the conductivity of carbon nanotube wires through resonant momentum exchange Robert Bell, Mike Payne, Arash Mostofi Carbon nanotubes (CNTs) have remarkable properties that make them excellent candidates for nano-electronic devices. Retaining these properties in CNT networks scalable for manufacture is a significant challenge. Experiment shows that conductivities of CNT networks are at least an order of magnitude lower than the theoretical maximum based on single CNT performance. In a CNT network, typically no single tube spans the device. As a result, electrons must travel between CNTs in order to contribute to the conductivity. Optimizing the conductivity of CNT networks, therefore, requires a detailed understanding of inter-tube electron transport. To this end, we present theoretical investigations of inter-tube conductivity of CNTs. We find, in agreement with previous studies, that conductivity between CNTs of different chirality is strongly suppressed as a consequence of the requirement for momentum conservation. We show that this problem can be overcome by providing a weak perturbation to the system, resulting in increases in inter-tube conductivity by over one order of magnitude. We will discuss practical realizations of the required perturbation and its experimental relevance for enhancing the conductivity of CNT networks. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W37.00010: High Frequency Generation from Horizontally Aligned Carbon Nanotube Field-effect Transistors Da Song, Vince Ballarotto, John Cumings Horizontally aligned carbon nanotubes grown on quartz substrates are used to fabricate top-gated field-effect transistors. Second, third and even higher order harmonic products are observed when high frequency signals are applied on gate side and detected from drain side. Measurements on control devices with identical geometry but without carbon nanotubes indicate all the harmonic generations are due to the carbon nanotubes in the channel. The second harmonic generation can be explained by following a traditional transistor model. The third and even higher harmonics observed are attributed to the Schottky barrier between the metal contacts and the carbon nanotubes. Devices with different metals as source and drain are fabricated to evaluate the effects of Schottky barrier. Taking the harmonic generation as the combination of a field-effect transistor and a Schottky diode, high frequency measurements and corresponding DC characterization data are combined to quantify the contribution of the non-linear elements on the measured output signal. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W37.00011: Comparative study of gel-based separated arcdischarge, HiPCO, and CoMoCAT carbon nanotubes for macroelectronic applications Hui Gui, Jialu Zhang, Bilu Liu, Jia Liu, Chongwu Zhou Due to their excellent electrical properties and compatibility with room-temperature deposition/printing processing, single-walled semiconducting carbon nanotubes (SWNTs) hold great potential for macroelectronic applications. However, the relative advantages and disadvantages of various SWNTs for macroelectronics remains an open issue, despite the great significance. Here we report a systematic study of three kinds of mainstream SWNTs (arc-discharge, HiPCO, CoMoCAT) separated using gel-based column chromatography for thin-film transistor applications, and high performance transistors---which satisfy the requirements for transistors used in active matrix organic light-emitting diode displays---have been achieved. We observe a trade-off between transistor mobility and on/off ratio depending on the SWNT diameter. While arc-discharge SWNTs with larger diameters lead to high device mobility, HiPCO and CoMoCAT SWNTs with smaller diameters can provide high on/off ratios (\textgreater 10$^{\mathrm{6}})$. Furthermore, we compare gel-based separated SWNTs with SWNTs separated by the density gradient ultracentrifuge (DGU) method, and find that gel-separated SWNTs can offer purity and thin-film transistor performance as good as DGU-separated SWNTs. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W37.00012: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W37.00013: In-plane Thermal and Electrical Transport Through Single-walled Carbon Nanotube Thin Films A.J. Ferguson, A.D. Avery, K.S. Mistry, B.L. Zink, M.L. Olsen, P.A. Parilla, J.L. Blackburn Recent advances in both chemical processing and fabrication techniques have enabled the development of a variety of new nanostructured materials for energy conversion technologies. Single-walled carbon nanotube (SWNT) networks may enable a number of cost-effective energy technologies, including transparent conductors for photovoltaics and thermoelectric composites. For such applications, a fundamental understanding of the physics governing their thermal and electrical properties is needed. Transport in SWNT networks is highly anisotropic; therefore the ability to measure the in-plane transport, both thermal and electrical, for these systems is extremely important. In this talk, we discuss the dispersion of highly enriched semiconducting SWNTs in organic solvents and deposition techniques optimized to enable measurements of in-plane transport of uniform thin films. We present results from in-plane thermal and electrical measurements as well as optical properties of SWNT:polymer thin films. Finally, we discuss the application of these results to developing nanocomposite films optimized for thermoelectric applications. [Preview Abstract] |
Session W38: Invited Session: 20th Century Chinese Physicists and Physics
Sponsoring Units: FHP FIPChair: Danian Hu, City College of New York, CUNY
Room: 709/711
Thursday, March 6, 2014 2:30PM - 3:06PM |
W38.00001: Chien-Shiung Wu: An Icon of Physicist and Woman Scientist in China Invited Speaker: Yuelin Zhu Chien-Shiung Wu, the first female president of APS, is a well-known figure in China, a figure who serves as an inspiration for youths, especially young women, to study science and particularly physics. In this presentation, a historical perspective will be used to show how such an icon was formed. Born in 1912, the year of the Republic Revolution, Wu was in the first generation of physicists in China and her college mentor was a student of Marie Curie. When Wu came to the U.S. for graduate studies in the 1930s, it was a ``golden age'' for nuclear physics, and the invention of the cyclotron by E. O. Lawrence put UC Berkeley at the frontier. Wu was trained there, with Lawrence as her advisor, and later became an expert in Beta-decay. In 1956, Wu conceived and initiated the experiment of Cobalt-60, which, together with other two experiments, eventually proved the asymmetry of parity in weak-interactions, a hypothesis proposed by T. D. Lee and C. N. Yang. The importance of the experiment gained Wu an enormous reputation which spread even to China, when this was a period of hostility in Sino-American relations, and near total isolation due to the Cold-War. Wu was the daughter of a revolutionary, and an activist in college in patriotic student movements, and she combined this background with her scientific career as the way of ``Saving China with Science,'' a common belief reflecting the Zeitgeist of her time. Although she spent most of her life in the U.S., Wu never wavered in her love for or loyalty to her motherland. Her patriotism, as well as her scientific achievement, made Wu a legend in China, being called ``the Chinese Madam Curie.'' Even during the Cultural Revolution, a novel supposedly taking Wu as the original model was very popular in underground circles, widely spread by hand-written-copies. From 1979-1988, the CUSPEA program enrolled hundreds of China's best graduate students into physics departments in American universities. Although Wu herself was not the initiator of it, many participates of the program were inspired by her. From this perspective, Wu's story may also help to understand the cultural characteristics of the Chinese born American physicists which have been a phenomenon in the American physics community since the 1940s till today. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W38.00002: 2014 Beller Lectureship: Chinese Physicists Educated in the Great Britain during the First Half of the 20th Century Invited Speaker: Xiaodong Yin More than 30 Chinese students went to Great Britain to study physics during the first half of the 20th century. They were concentrated in London University (13), Cambridge University (9), Edinburgh University (5) and Manchester University (3) and so on. All these students returned to China after finishing their study and most of them later became excellent physicists. They contributed to the development of physics in China. Based on newly discovered primary materials concerning these Chinese physicists, I examine their study in UK and subsequent accomplishments after their return to China. I will then analyze these British-trained Chinese physicists and compare them with those studying in Japan and America. I would argue that Chinese physicists educated in Britain have high degree of specialization as a whole and formed unique style. They made certain unique contributions to Chinese physics development. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 4:18PM |
W38.00003: Mao and physics research in China in the 1950s-1960s: the H-bomb project and the Straton model Invited Speaker: Tian Yu Cao |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W38.00004: Some problems in the competition of high-temperature superconductivity research during the late 1980s Invited Speaker: Bing Liu After A. M\"{u}ller and J.G. Bednorz found that Oxide Ba-La-Cu-O could have Tc for 30K in 1986, a special competition in High Temperature superconductors research began in the world, especially among American, Japanese and Chinese scientists in late 1980's. By investigating that competition in history, some interesting problems were found. There are strategy used by scientists in different country which differ from normal science period; Question about the peer review and competition in that special period; ``Matthew's Effect'' in that competition; some question about the disclosure of the secret information and competition; and, finally what methodology was used by Chinese scientists. All that problems are not only historically, but also have some sociological and philosophical meaning. Based on historical re-investigation, all those problems were discussed in the paper. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:30PM |
W38.00005: A Brief History of the Institute of Theoretical Physics in the Chinese Academy of Sciences since 1978 Invited Speaker: Liu Jinyan The Institute of Theoretical Physics (ITP), Chinese academy of Sciences (CAS), founded in June 1978, is a specialized institute studying major issues in the fundamental research of theoretical physics. ITP has played an important role in the development of theoretical physics in China, especially in organizing and undertaking major national projects, expanding international exchanges and cooperation, and nurturing advanced researchers. My presentation will examine the reasons why ITP was founded in 1978 and why Peng Huanwu and Zhou Guangzhao, two prominent Chinese theorists, were chosen as the first and second directors of ITP. Moreover, I will summarize ITP's scientific activities and achievements in the past 35 years. Last but not least, I will compare ITP with university physics departments and explore its unique characters (both strength and weakness). [Preview Abstract] |
Session W39: Invited Session: Emergent Symmetries in Magnetic and Ferroelectric Systems
Sponsoring Units: DCMP DMPRoom: Mile High Ballroom 2A-3A
Thursday, March 6, 2014 2:30PM - 3:06PM |
W39.00001: Critical Properties of the Kitaev-Heisenberg Model Invited Speaker: Natalia B. Perkins |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W39.00002: Spin dynamics in layered honeycomb iridates: implications for Kitaev physics Invited Speaker: Sungkyun Choi We explore the spin dynamics in the frustrated honeycomb magnets Na$_{2}$IrO$_{3}$ [1] and Li$_{2}$IrO$_{3}$, candidates to display novel magnetic states stabilized by the strong spin-orbit coupling at the 5d Ir ions. Theory [2] predicts composite spin-orbital J$=$1/2 moments at the Ir ions coupled by strongly-anisotropic and bond-directional exchanges, the so-called Kitaev honeycomb model, which has in its phase diagram novel magnetically-ordered ordered phases and a quantum spin liquid with exotic excitations. To search for such physics the experimental technique of choice is inelastic neutron scattering to probe the spin dynamics, however this is technically very challenging due to the large absorption cross-section of neutrons by the Ir nuclei. Using an optimised setup to minimise neutron absorption we have been successful in observing strongly dispersive spin-wave excitations of the Ir moments in both compounds and results are compared with predictions for a Kitaev-Heisenberg model as well as a Heisenberg model with further neighbour couplings. \\[4pt] [1] S. K. Choi, R. Coldea, A. N. Kolmogorov, T. Lancaster, I. I. Mazin, S. J. Blundell, P. G. Radaelli, Yogesh Singh, P. Gegenwart, K. R. Choi, S.-W. Cheong, P. J. Baker, C. Stock, and J. Taylor, Phys. Rev. Lett. 108, 127204 (2012). \\[0pt] [2] J. Chaloupka, G. Jackeli, and G. Khaliullin, Phys. Rev. Lett. 105, 027204 (2010). [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 4:18PM |
W39.00003: Emergent Criticality and Ricci Flow in a 2D Frustrated Heisenberg Model Invited Speaker: Peter P. Orth In most systems that exhibit order at low temperatures, the order occurs in the elementary degrees of freedom such as spin or charge. Prominent examples are magnetic or superconducting states of matter. In contrast, emergent order describes the phenomenon where composite objects exhibit longer range correlations. Such emergent order has been suspected to occur in a range of correlated materials. One specific example are spin systems with competing interactions, where long-range discrete order in the relative orientation of spins may occur. Interestingly, this order parameter may induce other phase transitions as is the case for the nematic transition in the iron pnictides. In this talk, we introduce and discuss a system with emergent $Z_6$ symmetry, a two-dimensional frustrated Heisenberg antiferromagnet on the windmill lattice consisting of interpenetrating honeycomb and triangular lattices. The multiple spin stiffnesses can be captured in terms of a four-dimensional metric tensor, and the renormalization group flow of the stiffnesses is described by the Ricci flow of the metric tensor. The key result is a decoupling of an emergent collective degree of freedom given by the relative phase of spins on different sublattices. In particular, our results reveal a sequence of two Berezinskii-Kosterlitz-Thouless phase transitions that bracket a critical phase. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W39.00004: Z$_{2}$ x Z$_{3}$ Symmetry of Multferroic Vortices Invited Speaker: Sang-Wook Cheong Hexagonal REMnO$_{3}$ (RE$=$ rare earths) with RE$=$Ho-Lu, Y, and Sc, is an improper ferroelectric where the size mismatch between RE and Mn induces a trimerization-type structural phase transition, and this structural transition leads to three structural domains, each of which can support two directions of ferroelectric polarization. We reported that domains in h-REMnO$_{3}$ meet in cloverleaf arrangements that cycle through all six domain configurations, Occurring in pairs, the cloverleafs can be viewed as vortices and antivortices, in which the cycle of domain configurations is reversed. Vortices and antivortices are topological defects: even in a strong electric field they won't annihilate. These ferroelectric vortices/antivortices are found to be associated with intriguing collective magnetism at domain walls, reflecting the multiferroic nature of vortices. We have found that an intriguing, but seemingly irregular network of a zoo of multiferroic vortices and antivortices in h-REMnO$_{3}$ can be neatly analyzed in terms of graph theory, and this graph theoretical analysis reveals the emergence of Z$_{2}$ $\times$ Z$_{3}$ symmetry in the vortices/antivortices network. In addition, poling or self-poling due to a surface charge boundary condition induces global topological condensation of the network through breaking of the Z$_{2}$ part of the Z$_{2}$ $\times$ Z$_{3}$ symmetry. The opposite process of restoring the Z$_{2}$ symmetry can be considered as topological evaporation. It turns out that these Z$_{2}$xZ$_{3}$ vortices are, in fact, three-dimensional vortex loops, which result from the emergent continuous U(1) symmetry near the critical temperature. This spontaneous trapping of topological defects in the process of undergoing a continuous phase transition is important to understand numerous novel phenomena such as the early stage of universe after big bang. The so-called Kibble-Zurek mechanism was proposed for the trapping process of topological defects right after big bang. It appears that the Kibble-Zurek mechanism is also responsible for the network formation of multiferroic vortices, and thus, hexagonal REMnO$_{3}$ is a test bed for the birth of this cosmos. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:30PM |
W39.00005: Hidden Monopolar Order in Magnetoelectrics Invited Speaker: Nicola Spaldin I will discuss a recently proposed [Monopole-based formalism for the diagonal magnetoelectric response, N. A. Spaldin, M. Fechner, E. Bousquet, A. Balatsky and L. Nordstrom, Phys. Rev. B 88, 094429 (2013)] form of hidden order -- the magnetoelectric monopole -- and its relationship to a material's magnetoelectric response. Using density functional calculations for the Li transition metal phosphate series, LiMPO$_4$, with M = Mn, Fe, Co and Ni, I will show that materials with the same overall antiferromagnetic ordering can have distinct ferromonopolar or antiferromonopolar orderings, that lead to different, and in principle measureable, magnetoelectric responses. The current status and open questions in both the theoretical formalism and experimental verification will be outlined. [Preview Abstract] |
Session W40: Invited Session: Electronic Correlations in Unconventional Superconductors
Sponsoring Units: DCMPChair: Dung-Hai Lee, University of California, Berkeley
Room: Mile High Ballroom 2B-3B
Thursday, March 6, 2014 2:30PM - 3:06PM |
W40.00001: Angular fluctuations of a multi-component order describe the pseudogap regime of the cuprate superconductors Invited Speaker: Subir Sachdev The hole-doped cuprate high temperature superconductors enter the pseudogap regime as their superconducting critical temperature, $T_c$, falls with decreasing hole density. Experiments have probed this regime for over two decades, but we argue that decisive new information has emerged from recent X-ray scattering experiments. The experiments observe incommensurate charge density wave fluctuations whose strength rises gradually over a wide temperature range above $T_c$, but then decreases as the temperature is lowered below $T_c$. We propose a theory in which the superconducting and charge-density wave orders exhibit angular fluctuations in a 6-dimensional space. The theory provides a natural quantitative fit to the X-ray data, and is consistent with other observed characteristics of the pseudogap. Results will also be presented on the microscopic origins of these order parameters. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W40.00002: Quantum critical point underlying the pseudogap state in underdoped cuprate superconductors Invited Speaker: Catherine Pepin Cuprate superconductors rank among the most complex materials that are known in the universe. Faced with this complexity, scientists have adopted two types of approaches. In a bottom up approach, one considers that strong correlations occur at a high energy scale of roughly 1 eV upon very strong Coulomb interactions. In the top down approach one considers that one universal singularity at very low temperatures is responsible for complexity of the phase diagram. In this talk we will argue that the strong quantum fluctuations experienced at the proximity to a anti-ferromagnetic Quantum Critical Point (QCP) is responsible for a cascade of phase transitions in the charge and superconducting channels. We will discuss in this context the emergence of the pseudo-gap and charge order modulations. Symmetries and relations to experimental observations will be addressed. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 4:18PM |
W40.00003: A universal order underlying the pseudogap regime of the underdoped high $T_{\rm c}$ cuprates Invited Speaker: Neil Harrison A major achievement in condensed matter physics in the last quarter century has been a step towards the understanding of the unconventional d-wave superconducting state in the copper-oxide materials. Surprisingly, the normal state out of which the superconducting state emerges remains a mystery at low charge carrier densities, i.e., in the underdoped regime. This regime is of particular interest because it is characterised by an unusual momentum dependent energy pseudogap in the excitation spectrum that has defied explanation and is key to a full understanding of the unconventional d-wave superconducting state. I will present new quantum oscillation experimental results within the pseudogap regime of the high $T_{\rm c}$ superconductors YBa$_2$Cu$_3$O$_{6+x}$ and YBa$_2$Cu$_4$O$_8$ which now extend up to the optimally-doped regime. These data reveal the evolution of the Fermi surface approaching the putative quantum critical point under the superconducting dome. A comprehensive angle-resolved study of the Fermi surface enables us to unambiguously identify a specific form of order that accounts for the observed quantum oscillations as well as other spectroscopic, transport and thermodynamic probes within the pseudogap regime. The author would like to thank B. Ramshaw, S. Sebastian, F. Balakirev, C. Mielke, M. Altarawneh, P. Goddard, S. Sabok, B. Babrowski, D. Bonn, W. Hardy, R. Liang and G. Lonzarich. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W40.00004: Electron Correlations and Superconductivity in Iron Pnictides and Chalcogenides Invited Speaker: Qimiao Si In the iron pnictides, the bad metal behaviour in the normal state suggests the importance of electron correlations, which is further underscored by the existence of a Mott insulator state in the overall phase diagram of the iron chalcogenides. This has motivated a strong-coupling approach based on a proximity to the Mott transition. In this talk, I will briefly summarize earlier theoretical studies within this approach, which led to the proposal for a quantum critical point in the isoelectronically tuned iron pnictides [1]; this has since been verified in the P-doped iron arsenides. I will in addition show how the approach provides a natural understanding of a major issue in the field, namely the superconducting $T_c$ of the iron chalcogenides is comparably high as in the iron pnictides in spite of their qualitatively distinct Fermi surfaces [2]. I will also consider the multi-orbital aspects of the electron correlations more generally, including a proposed orbital-selective Mott phase [3] in the normal state and implications for gap anisotropy and spin resonances [4] in the superconducting state. Finally, I will discuss how these results expand on the notion [5] that the iron-based superconductivity primarily originates from strong electron correlations, as well as some implications for the general phenomenon of unconventional superconductivity at the border of magnetism. \\[4pt] [1] J. Dai, Q. Si, J.-X. Zhu, \& E. Abrahams, PNAS {\bf 106}, 4118 (2009). \\[0pt] [2] R. Yu, P. Goswami, Q. Si, P. Nikolic, \& J.-X. Zhu, Nat. Commun. {\bf 4}, 2783 (2013). \\[0pt] [3] R. Yu and Q. Si, Phys. Rev. Lett. {\bf 110}, 146402 (2013); M. Yi et al., Phys. Rev. Lett. {\bf 110}, 067003 (2013). \\[0pt] [4] R. Yu, J.-X. Zhu, \& Q. Si, arxiv:1306.4184; C. Zhang et al., Phys. Rev. Lett. {\bf 111}, 207002 (2013). \\[0pt] [5] Q. Si and E. Abrahams, Phys. Rev. Lett. {\bf 101}, 076401 (2008). [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:30PM |
W40.00005: Emergence of superconductivity, valence bond order and Mott insulators in Pd[(dmit)2] based organic salts Invited Speaker: Hae-Young Kee The EtMe$_3$P and EtMe$_3$Sb nearly triangular organic salts are distinguished from most other Pd[(dmit)$_2$] based salts, as they display valence bond and no long range order, respectively. Under pressure, a superconducting phase is revealed in EtMe$_3$P near the boundary of valence bond order. We use slave-rotor theory with an enlarged unit cell to study competition between uniform and broken translational symmetry states, offering a theoretical framework capturing the superconducting, valence bond order, spin liquid, and metallic phases on an isotropic triangular lattice. Our finite temperature phase diagram manifests a remarkable resemblance to the phase diagram of the EtMe$_3$P salt, where the re-entrant transitions of the type insulator-metal-insulator can be explained by an entropy difference between metal and the U(1) spin liquid. We find that the superconducting pairing symmetry is $d \pm i d$, and predict different temperature dependences of the specific heat between the spin liquid and metal. [Preview Abstract] |
Session W41: Topological Order
Sponsoring Units: DCMPChair: Tanmoy Das, Los Alamos National Laboratory
Room: Mile High Ballroom 3C
Thursday, March 6, 2014 2:30PM - 2:42PM |
W41.00001: The uses of Instantons for classifying Topological Phases Juven Wang, Xiao-Gang Wen A strategy of using instantons, zero modes and the index theorem for classifying topological phases is developed in this work. We argue that this approach is very powerful and can be applied to topological phases with or without a global symmetry in any (higher) dimensional spacetime. (this URL for a work summary: www.mit.edu/~juven/) [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W41.00002: Surface Theory of Topological Insulators Pak On Chan, Taylor Hughes, Shinsei Ryu, Eduardo Fradkin We discuss a hydrodynamic effective field theory description of 3+1 dimensional topological insulators. The effective field theory is a BF type topological field theory augmented with an axion (theta) term, which is obtained from the functional bosonization technique that introduces one form as well as two form gauge fields, the latter of which describes the conserved U(1) charge. We construct various kinds of non-local operators describing topological excitations in the bulk and study their algebraic properties thereof. Furthermore, we derive a hydrodynamic effective field theory for the gapless surface of 3+1 dimensional topological insulators. Such theory, which is essentially a BF-CS theory with a non-local Maxwell term, reproduces all the physical properties appeared in the bulk and is hence compatible to the bulk theory. Current-current correlation function is calculated and its transformation under modular transform is also discussed. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W41.00003: Topological invariants and the ground state wavefunction of topological insulators Zhong Wang, Shou-Cheng Zhang We will talk about precise topological invariants defined in terms of the ground state wavefuntion. The Hall coefficients in even spatial dimensions and the magnetoelectric theta terms in odd spatial dimensions are expressed in terms of the ground state wavefunctions under generalized twisted-boundary conditions. This formulation is valid in the presence of arbitrary interaction and disorder, in particular, it is applicable to both integer and fractional topological insulators. ( arXiv:1308.4900) [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W41.00004: Topological order in correlated topological insulators Joseph Maciejko, Andreas R\"uegg, Victor Chua, Gregory A. Fiete Motivated by recent experiments on correlated transition-metal oxides, an important outstanding issue in the field of topological insulators is to understand the effect of electron-electron interactions beyond the relatively well-understood perturbative limit. Using the $Z_2$ slave-spin theory, we theoretically predict that interaction effects in a spinful Chern insulator (CI) can give rise to a novel strongly correlated topological state of matter, the CI*, which is distinct from both the weakly correlated CI and the recently studied fractional CI. In the CI* the Hall conductivity and the quasiparticle charge are integer, but the quasiparticle statistics are fractional (semionic). In a time-reversal invariant 3D topological insulator strong interactions can give rise to a novel strongly correlated topological state of matter, the TI*, that is distinct from both the weakly correlated TI and other recently proposed fractionalized phases such as the topological Mott insulator and the fractional TI. In the TI* the weak-field magnetoelectric response is quantized as in a weakly correlated TI, but the state is a symmetry-enriched topological phase, with eight degenerate ground states on the 3-torus and emergent particle and string-like excitations with nontrivial mutual statistics. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W41.00005: The Role of Space Group Symmetries in Many Body Interacting Systems Rahul Roy The constraints placed by space group symmetries in two and three dimensions on the ground state degeneracies of a many body interacting system in the thermodynamic limit are studied. It is shown that the number of such degenerate ground states have a lower bound which depends on the details of the space group. These results may be seen as a generalization of some free fermionic band touching theorems to interacting systems and also provide guidance for the search for topological phases in interacting systems. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W41.00006: Topological order in lattice models of strongly interacting electrons Stefanos Kourtis, Titus Neupert, Claudio Chamon, Christopher Mudry Fractional Chern insulators are a class of strongly interacting topological states of electronic matter. So far, the paradigm of fractional Chern insulators was that they appear when interacting electrons with frozen spin degree of freedom populate relatively flat topological bands, with the interaction strength being smaller than the gap to other bands. In this talk, it will be shown that this limit is adiabatically connected to the opposite one, in which the interaction strength goes to infinity, thus exceeding the gap to other bands. Electrons then become extended hard-core particles, the notion of bands becomes meaningless and the connection to Landau-level physics of the fractional quantum-Hall effect is much less obvious. We also find fractional Chern-insulator states to be extremely robust in this hardcore limit, reaching up to, or possibly beyond, the noninteracting topological phase transition. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W41.00007: Topological non-symmorphic crystalline insulators Chaoxing Liu, Ruixing Zhang, Brian Vanleeuwen In this talk, we will describe a new class of Z2 topological insulator protected by non-symmorphic crystalline symmetry, dubbed a ``topological non-symmorphic crystalline insulator.'' We construct a concrete tight-binding model with the non-symmorphic space group pmg and confirm the topological nature of this model by directly calculating topological surface states. In analog to ``Kramers' pairs'' due to time reversal symmetry, we introduce the ``doublet pairs'' originating from non-symmorphic symmetry to define the corresponding Z2 topological invariant for this phase. Based on the projective representation theory, we extend our discussion to other non-symmorphic space groups that allows to host topological non-symmorphic crystalline insulators. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W41.00008: Topology of Symmetry Protected Gapless Modes in Insulators and Superconductors Masatoshi Sato There has been much recent interest in topological insulators and superconductors. Whereas the recent developments are based on topological classifications using the general symmetries of time-reversal and charge conjugation, systems often have other symmetries specific to their structures such as point group symmetries . Interestingly, additional symmetries can give rise to a nontrivial topology of the bulk wave functions and gapless states on the boundaries. Although these specific symmetries are microscopically sensitive to a small disturbance, recent studies of topological crystalline insulators have shown that if the symmetries are preserved on average, then the existence of gapless boundary states is rather robust. Therefore, it is expected that the symmetry-protected topological phase can provide an alternative platform of topological materials. Here we argue symmetry protected gapless modes in topological insulators and superconductors. We consider topological objects described by non-interacting Bloch and Bogoliubov de Gennes Hamiltonians that support an additional spatial symmetry, besides any of ten classes of symmetries defined by time-reversal and charge conjugation. Varoius symmetry protected gapless modes will be discussed in a uniform manner. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W41.00009: (3 + 1)-dimensional BF theory from a tight-binding model of interacting spinless fermions Mauro Cirio, Giandomenico Palumbo, Jiannis K. Pachos Currently, there is much interest in discovering analytically tractable (3 + 1)-dimensional models that describe interacting fermions with emerging topological properties. Towards that end we present a three-dimensional tight-binding model of spinless interacting fermions that reproduces, in the low energy limit, the (3 + 1)-dimensional Abelian BF topological theory. We first consider the non- interacting case. By employing a mechanism equivalent to the Haldane's Chern insulator we can turn the model into a (3 + 1)-dimensional chiral topological insulator. We then isolate energetically one of the two Fermi points of the lattice model. In the presence of fermionic interactions we can map the system to a generalised version of the (3 + 1)-dimensional Thirring model with low energy behaviour that is faithfully described by the BF theory. This approach directly establishes the presence of (2 + 1)-dimensional BF theory at the boundary of the lattice and it provides an observable for the topological order of the model through fermionic density measurements. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W41.00010: Hopf insulators and their topologically protected surface states Sheng-Tao Wang, Dong-Ling Deng, Chao Shen, Lu-Ming Duan Three-dimensional (3D) topological insulators in general need to be protected by certain kinds of symmetries other than the presumed $U(1)$ charge conservation. A peculiar exception is the Hopf insulators which are 3D topological insulators characterized by an integer Hopf index. To demonstrate the existence and physical relevance of the Hopf insulators, we construct a class of tight-binding model Hamiltonians which realize all kinds of Hopf insulators with arbitrary integer Hopf index. These Hopf insulator phases have topologically protected surface states and we numerically demonstrate the robustness of these topologically protected states under general random perturbations without any symmetry other than the $U(1)$ charge conservation that is implicit in all kinds of topological insulators. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W41.00011: 3D topological states - layer construction, surface topological order and surface symmetry Chao-Ming Jian, Xiao-Liang Qi 3D topological states can be constructed by stacking layers of 2D topological states and introducing coupling between them. The coupling between layers can effectively drive condensation of anyons in the stacked 2D systems. In the talk, we shall discuss a general layer construction of 3D topological states using the anyon condensation technique for Abelian topologically ordered states in each layer. For the finite size 3D system constructed this way, the emergent surface topological order (including that at the top/bottom and side surfaces) can also be described using the same technique. Extra symmetries can be cooperated into this construction to obtain 3D SPT phases with the symmetries realized in an anomalous way on the gapped surface states. We propose a general criterion to distinguish the symmetry operations that can be realized in a purely two-dimensional topological state from those that can only be realized anomalously on the surface of a higher dimensional state. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W41.00012: Combined topological and Landau order from strong correlations in Chern bands Maria Daghofer, Stefanos Kourtis In recent years, topologically nontrivial and nearly dispersionless bands have attracted attention as hosts for states analogous to fractional quantum-Hall states, but without a magnetic field. Indeed, such fractional Chern insulators were found and connections to fractional quantum-Hall states in Landau levels were established. We discuss here aspects where fractional Chern insulators differ from Landau levels. In particular, we present a class of states where both topological order and symmetry breaking arise spontaneously: the states show both fractional Hall conductivity and charge order. This coexistence of topological and conventional Landau order relies on the geometric frustration of the underlying lattice and consequently goes qualitatively beyond physics found in continuous Landau levels with their weak lattice. [1] S. Kourtis, J. W. F. Venderbos and M. Daghofer, PRB {\bf 86}, 235118 (2012);S. Kourtis and M. Daghofer, arXiv:1305.6948. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W41.00013: Symmetry-protected topological phases and orbifolds: Generalized Laughlin's argument Olabode Sule, Xiao Chen, Shinsei Ryu We consider non-chiral symmetry-protected topological phases of matter in two spatial dimensions protected by a discrete symmetry such as $Z_K$ or $Z_K \times Z_K $ symmetry. We argue that modular invariance/noninvariance of the partition function of the one-dimensional edge theory can be used to diagnose whether, by adding a suitable potential, the edge theory can be gapped or not without breaking the symmetry. By taking bosonic phases described by Chern-Simons K-matrix theories and fermionic phases relevant to topological superconductors as an example, we demonstrate explicitly that when the modular invariance is achieved, we can construct an interaction potential that is consistent with the symmetry and can completely gap out the edge. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W41.00014: Interactions and Bosonization for Topological Insulators David Schmeltzer The time reversal symmetry $ T^2=-1$ imposes restriction on the eigenvectors when transported in the Brillouin Zone, resulting in momentum dependent vector potentials which is sensitive to obstructions. The study of electron-electron interactions is done using the Hubbard Stratonovici field which are treated similarly to external electromagnetic fields. The integration of the fermion field is done using the gauge fields in momentum space, which obey special gauge imposed by the eigenvectors. The effect of the interaction is similar to the magnetoelectric response, due to the Hubbard Stratoonovici field we find that the magnetoelectric response is controlled by a fractional topological angle. Using the time reversal time reversal symmetry $ T^2=-1$ we construct the Bosonization for one and two dimensions and use the formulation to study interactions. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W41.00015: ABSTRACT WITHDRAWN |
Session W42: Focus Session: Magnetic Topological Insulators
Sponsoring Units: DMPChair: Don Heiman, Northeastern University
Room: Mile High Ballroom 4A
Thursday, March 6, 2014 2:30PM - 3:06PM |
W42.00001: Magnetism in Magnetically Doped Topological Insulators Invited Speaker: Ke He A topological insulator (TI) has topologically non-trivial electronic property induced by spin-orbit coupling (SOC) and protected by time reversal symmetry (TRS). Breaking the TRS of a three-dimensional (3D) TI with ferromagnetism can gap the Dirac surface states and induce novel quantum phenomena. Magnetic doping is a convenient approach to introduce magnetism into a TI. A crucial issue is if long-range ferromagnetic order could be built in magnetically doped TIs in the insulating regime. Combining angle-resolved photoemission spectroscopy, scanning tunneling microscopy, transport measurement, and first principles calculation, we have systematically studied the surface band structure, magnetism and transport properties of molecular beam epitaxy-grown Cr-doped 3D TIs: Bi$_{2}$Se$_{3}$, Bi$_{2}$Te$_{3}$, and Sb$_{2}$Te$_{3}$. We have found that Cr-doped Bi$_{2}$Te$_{3}$, Sb$_{2}$Te$_{3\, }$ and their alloys show long-range ferromagnetic order robust against variation in charge carriers. The ferromagnetism is likely mediated by the strong van Vleck susceptibility of the host materials due to the SOC-induced inverted band structure. Cr-doped Bi$_{2}$Se$_{3}$, on the other hand, could not show long-range ferromagnetic order, but exhibit gap-opening at the Dirac surface states. The absence of long-range ferromagnetic order and the observed gapped surface states are partly due to the superparamagnetic multimers formed by Cr dopants, and partly due to significant reduction of the SOC of Bi$_{2}$Se$_{3}$ by Cr substitution for Bi, which turns off the van Vleck mechanism. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W42.00002: Quantum Anomalous Hall Effect with Higher Plateaus Jing Wang, Biao Lian, Haijun Zhang, Yong Xu, Shou-Cheng Zhang The quantum anomalous Hall (QAH) effect in magnetic topological insulators is driven by the combination of spontaneous magnetic moments and spin-orbit coupling. Its recent experimental discovery raises the question if higher plateaus can also be realized. Here, we present a general theory for a QAH effect with higher Chern numbers and show by first-principles calculations that a thin film magnetic topological insulator of Cr-doped Bi2(Se,Te)3 is a candidate for the C$=$ 2 QAH insulator. Remarkably, whereas a higher magnetic field leads to lower Hall conductance plateaus in the integer quantum Hall effect, a higher magnetic moment leads to higher Hall conductance plateaus in the QAH effect. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W42.00003: Measurement of optical rotation in the magnetic topological insulator (Bi,Sb)2Te3 Shreyas Patankar, Daniel Golubchik, Joseph Orenstein, Eli Fox, David Goldhaber-Gordon, Xiao Feng, Ke He, Yayu Wang, Qi-kun Xue Topological insulators with surface states that break time reversal symmetry have been predicted to have exotic topological quantum properties. One way of realizing these is through topological insulators that also have magnetic ordering. Recently, measurements of quantum anomalous hall effect were reported in thin films of chromium doped (Bi,Sb)2Te3, which gave evidence for the presence of spontaneous magnetic order in this topological insulator. We report measurements of magneto-optic Kerr effect in this material, which provides an alternative quantification of magnetization. Kerr rotation was measured as a function of applied magnetic field and of temperature. Preliminary data suggests a transition to a ferromagnetic state at 12K and a coercive field of 30mT. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W42.00004: Magneto-optical Kerr effect in Cr-doped (Bi,Sb)$_{2}$Te$_{3}$ Thin Films Yu Pan, Bing Yao, Anthony Richardella, Abhinav Kandala, Robert Fraleigh, Joon Sue Lee, Nitin Samarth, Andrew Yeats, David D. Awschalom When a three-dimensional (3D) topological insulator (TI) is interfaced with magnetism, the breaking of time reversal symmetry results in new phenomena such as the recently observed quantum anomalous Hall effect [C.-Z. Zhang \textit{et al., Science} \textbf{340}, 167 (2013)]. Thus motivated, we use the polar-mode magneto-optical Kerr effect (MOKE) to probe the temperature- and field-dependent magnetization in molecular beam epitaxy grown Cr-doped thin films of the 3D TI (Bi,Sb)$_{2}$Te$_{3}$. Square MOKE hysteresis loops observed at low temperatures indicate robust ferromagnetism with a perpendicular magnetic anisotropy and Curie temperature that varies from $\sim$ 5 K to $\sim$ 150 K, depending on sample details. A key question is the nature of the ferromagnetism: is it a carrier-mediated mechanism, Van Vleck mechanism or due to extrinsic clusters? We address this issue by varying the magnetic ion concentration and carrier density via sample composition as well as by varying the chemical potential by back gating. Finally, we use spatially-resolved MOKE to image the magnetization in these samples. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W42.00005: Proximity effect induced magnetoresistance hysteresis loops in a topological insulator/YIG heterostructure Mohammad Montazeri, Murong Lang, Mehmet C. Onbasli, Xufeng Kou, Liang He, Caroline A. Ross, Kang L. Wang We experimentally demonstrate the proximity effect induced hysteretic magnetoresistance in an 8 quintuple layers of Bi2Se3 films grown on Gallium Gadolinium Garnet (GGG) (111) substrates with a 50 nm Yttrium Iron Garnet (YIG) buffer layer by molecular beam epitaxy. With in-plane and out-of-plane magnetic field, square wave shaped and butterfly shaped resistance hysteresis loops can be observed up to 25 K, respectively. The relationship between the hysteretic MR curves and the magnetic switching of the YIG will be discussed in the context of a proximity effect between the YIG and the TI. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W42.00006: Correlation of Lattice-Symmetry, Electronic Anisotropy and Transport in Topological Insulators Thin Films and Heterostructures Ferhat Katmis, Valla Fatemi, Peng Wei, Hadar Steinberg, John Freeland, Pablo Jarillo-Herrero, Jagadeesh Moodera To explore the intrinsic features of topological insulators (TIs) thin films and thus to correlate structure with the exotic electronic properties as well as interaction with other material systems careful structural studies are needed. Molecular beam epitaxy ideally allows us to engineer the required system for observing the intrinsic properties of TI thin films and heterostructures, thereby accessing the optimum Dirac surface states. In well-defined films and heterostructures, we elucidated the role of imperfections e.g. vacancies, line defects, twinning etc., on the symmetry of the material that leads to internal atomic ordering by the decoration of the defects. Charge transport is seen to relate with film growth induced strain and relaxation, as well as exhibit strong directional dependence on the defect geometry. In TI with ferromagnetic insulator (FI), the observation of symmetry breaking strongly depends on the interface coupling between FI and TI, where the exchange interaction occurs defined by the hybridization at interface. Synchrotron based GIXRD, XAFS, and XAS/XMCD helped reveal strain, hybridization, and magnetic interaction between FI and TI thin films show strong structural and magnetic interactions. MIT MRSEC through NSF Grant No. DMR-0819762. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W42.00007: Heterostructure of a Topological Insulator and a Ferromagnetic Insulator bi-layer Valeria Lauter, Ferhat Katmis, Badih Assaf, Jagadeesh Moodera The short-range nature of magnetic proximity coupling with a ferromagnetic insulator (FI) induces ferromagnetic interactions in the TI surface state with the symmetry breaking right at the interface. Here we investigate Bi$_{2}$Se$_{3}$/EuS heterostructures and the mechanism to induce ferromagnetic order at the surface of Bi$_{2}$Se$_{3}$ thin films by using the FI EuS. SQUID measurements demonstrated excessive magnetic moment for the EuS film alone, thus indicating that EuS might induce a magnetization in Bi$_{2}$Se$_{3}$. Using PNR we reveal that EuS induces a significant magnetic moment in the Bi$_{2}$Se$_{3}$ films. Thus, it creates broken time-reversal symmetry and should appear as magnetic signatures in electrical transport. The ferromagnetism of EuS and the coupling between EuS and Bi$_{2}$Se$_{3}$ have to be strong to signify surface magnetization effect. These findings may be used to investigate interesting emergent phenomena, because the local time-reversal symmetry breaking is essential for inducing a quantized topological magnetoelectric response. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W42.00008: Gate-Tunable Quantum Corrections in Topological Insulator/ Insulating Ferromagnet Heterostructures Joon Sue Lee, Anthony Richardella, Robbie Fraleigh, Chao-xing Liu, Nitin Samarth Heterostructures that interface a topological insulator (TI) and an insulating ferromagnet (IFM) are of current interest for potential spintronics applications, as well as for fundamental explorations of quantum phenomena resulting from broken time reversal symmetry (TRS). Since angle resolved photoemission spectroscopy cannot directly probe the modified topological surface state at the buried interface between a TI and an IFM, we use the quantum corrections to the magneto-conductance (MC) as a possible probe of broken TRS. We report systematic studies of the quantum corrections by varying temperature and chemical potential in (Bi,Sb)$_{2}$Te$_{2}$Se/ Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As/ InP (111)A heterostructures grown by molecular beam epitaxy. We select the Ga$_{\mathrm{1-x}}$Mn$_{\mathrm{x}}$As composition to yield a conductivity of orders of magnitude lower than the TI, with a ferromagnetic Curie temperature of 16 $\sim$ 40 K. At fixed chemical potential, we observe a crossover from negative MC (weak anti localization) to positive MC (weak localization) as the temperature is lowered. A similar crossover is observed when the chemical potential is electrically tuned using a top gate. The results are interpreted in terms of the opening of a gap at the Dirac point. Funded by ONR and DARPA. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W42.00009: Magnetotransport in Gate Tunable Insulating Ferromagnet -Topological Insulator Heterostructure Devices Abhinav Kandala, Anthony Richardella, Robert Fraleigh, Nitin Samarth Magnetic perturbations to topological insulator (TI) surface states are predicted to result in a host of exotic effects that are of great fundamental and applied interest. Complementary to magnetic doping, interfacing TI's with an insulating ferromagnets (IF) enables transport studies of magnetism solely at the surface state. In a previous study [A. Kandala et al., Appl. Phys. Lett. 103, 202409 (2013)], we used IF-TI heterostructure devices wherein the chemical potential in the TI was fixed in the bulk conduction band. By using Sb doping and growth on SrTiO$_3$ substrates, we have now fabricated IF-TI devices where the surface states are accessed by back-gate tuning of the chemical potential. Our unique device geometry enables a direct comparison of magneto-conductance in bare (control) and magnetically capped TI channels as the chemical potential is swept through the Dirac point. Analysis of the low-temperature magneto-conductance provides insights into the influence of the magnetic overlayer on quantum corrections to the diffusive transport. Supported by DARPA and ONR. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W42.00010: Characterization of Chemical Trends in Magnetically Doped, Electrically Gated Topological Insulator Thin Films Anthony Richardella, Abhinav Kandala, Joon Sue Lee, Robbie Fraleigh, Nitin Samarth, Minhao Liu, Nai Phuan Ong, Jing Tao Interfacing topological insulators (TIs) with magnetism breaks time reversal symmetry and opens a gap in the surface states at the Dirac point. This results in novel phenomena, such as the recently reported quantized conductance at zero applied external magnetic field due to the quantum anomalous Hall effect (QAHE) in Cr doped (Bi$_{\mathrm{x}}$Sb$_{\mathrm{1-x}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ [C-Z. Chang, et al., Science 340, 167 (2013)]. We have studied magnetically doped (Bi$_{\mathrm{x}}$Sb$_{\mathrm{1-x}})_{\mathrm{2}}$Te$_{\mathrm{3}}$ thin films grown by MBE on SrTiO3(111) (STO) substrates using Cr, Fe and Mn as magnetic dopants and as a function of the Bi and Sb composition. These films are carefully characterized by XRD, AFM, SQUID magnetometry and TEM. The chemical composition is determined using SIMS, RBS and XRF. Low temperature transport shows a large gate-tunable Hall effect in Cr doped samples and systematically varying longitudinal magneto-conductance as the Fermi energy is tuned through the Dirac point. The origin of ferromagnetism and its dependence on the chemical potential, chemical composition and sample thickness is discussed. Funded by DARPA and ARO-MURI. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W42.00011: The anomalous Hall effect in ultrathin films of gate-tuned Cr-doped (Bi,Sb)2Te3 Minhao Liu, Anthony Richardella, Abhinav Kandala, Nitin Samarth, N. P. Ong The surface states of topological insulator are predicted to display exotic properties in a magnetization field \textbf{M}. Quantized anomalous Hall effect was recently observed in Cr-doped (Bi,Sb)2Te3 ultrathin films, in which the surfaces states hybridize with each other. We investigate the situation when surface states are not coupled. SrTiO3 (111) crystals are chosen as the substrate due to its large electric polarization at low temperatures. Large anomalous Hall effect is observed for 8QL-thick films. The anomalous Hall conductance shows a maximum slightly lower than half conductance quanta (0.46e2/h) at 300mK when the Fermi level is tuned close to the neutral point by electric gating. It decreases monotonically as the hole carrier density becomes higher by negative gate voltage. On the electron doping side, the anomalous Hall conductance decreases and then saturates at a value of 0.4e2/h when the gate voltage is higher than 5V. The electron density is pinned at about 3e12/cm2 in the whole positive gate voltage range, only slightly increased (10{\%}) by a gate voltage of 150V. These results shed light on the robustness of the quantized Hall response of magnetically gapped Dirac surface state. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W42.00012: Engineering thin films of magnetically doped topological insulators for quantum anomalous Hall effect Xiao Feng, Yang Feng, Yunbo Ou, Kang Li, Jinsong Zhang, Minghua Guo, Zuocheng Zhang, Xintong Li, Liguo Zhang, Chang Liu, Zhenqi Hao, Yayu Wang, Shuaihua Ji, Xi Chen, Lili Wang, Ke He, Xucun Ma, Qikun Xue Quantum anomalous Hall effect (QAHE) is a kind of quantum Hall effect that can occur in zero external magnetic field. Recently, QAHE has been experimentally observed in thin films of Cr-doped (Bi,Sb)$_{2}$Te$_{3}$ topological insulator grown on SrTiO$_{3}$ (111) substrate by molecular beam epitaxy. The QAHE in Cr-doped (Bi,Sb)$_{2}$Te$_{3}$ films is found to be easily destroyed by slight variations in sample chemical composition, film thickness, substrate condition and capping layer. We have systematically investigated the influence of these parameters on the magnetism and anomalous Hall effect of Cr-doped (Bi,Sb)$_{2}$Te$_{3}$ films. The crucial factors preventing QAHE are discussed based on the results. This work is helpful for a detailed understanding of QAHE and for routine preparation of QAHE samples for further studies. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W42.00013: High-Temperature Quantum Anomalous-Hall Effect in a $n$-$p$ Codoped Topological Insulator Shifei Qi, Zhenhua Qiao, Hua Chen, Xiaohong Xu, Zhenyu Zhang Quantized anomalous-Hall effect (QAHE) has been theoretically predicted for over twenty years. Recently, it has been experimentally observed in magnetic topological insulators (TI). However, the extremely small band gap severely limits its potential application in novel nanoelectronics. In the present work, we use density functional theory calculations to establish a new materials design approach, compensated $n-p$ codoping, to predict a long-range ferromagnetic and insulating topological insulator with a relatively larger intrinsic band gap. By analyzing the band gap evolution as a function of the sample thickness and further calculating the corresponding Berry curvature, we show that the surface of the n-p codoped topological insulator supports a quantum anomalous Hall state at much higher temperatures than previously observed or predicted systems. [Preview Abstract] |
Session W43: Topological Insulators: New Materials Predictions
Sponsoring Units: DCMPChair: Nuh Gedik, Massachusetts Institute of Technology
Room: Mile High Ballroom 4B
Thursday, March 6, 2014 2:30PM - 2:42PM |
W43.00001: A DFT study of supported 2D-Sn (Stannanane) films Ana Suarez Negreira, Max Fischetti Theoretical studies indicated that Sn monolayer is a 2D topological insulator with robust properties against small perturbations, thus resulting in large tolerance against variations induced by a manufacturing process. The downside of many topological properties is that they manifest themselves only at very low temperatures. However, thin film of Sn have a significantly higher phase transition temperature, up to 120 $^{\circ}$C, creating new opportunities of using this material in nano-electronics applications. Since stannanane has never been synthesized before, its existence and mechanical stability are open questions. Using density functional theory (DFT), we study the growth of stannanane on various substrates (i.e., InSb(100) and CdTe(100)). The impact of the substrate on the electronic properties of this topological insulator is studied through Bader charge and density of states (DOS) analyses. Finally, ab initio thermodynamics methodology is used to study the stability of different Sn surface terminations as a function of temperature and pressure. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W43.00002: Giant topological insulator gap and Rashba splitting in honeycomb Pb Haiyan He, Jun Hu, Ruqian Wu It was predicted that graphene can be a topological insulator (TI) due to its special Dirac states and spin-orbit coupling (SOC). However, the SOC gap of pure graphene is too small for experimental observation. It was found that heavier group IV elements, such as Si and Ge, can also produce the TI state in the tow-dimensional honeycomb lattice, with their SOC gaps a few orders larger than that of graphene. In the present work, we find that the honeycomb Pb is also a TI, with a SOC gap as large as 250 meV. We demonstrate the feasibility of making a honeycomb Pb monolayer on the Al$_2$O$_3$(0001) substrate. Moreover, Pb/Al$_2$O$_3$(0001) has a giant Rashba splitting of 270 meV, useful for spintronics and topotronics applications. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W43.00003: Prediction of a Two-Dimensional Organic Topological Insulator Zhengfei Wang, Ninghai Su, Feng Liu Topological insulators (TI) are a class of materials exhibiting unique quantum transport properties with potential applications in spintronics and quantum computing. To date, all of the experimentally confirmed TIs are inorganic materials. Recent theories predicted the possible existence of organic TIs (OTI) in two-dimensional (2D) organometallic frameworks. However, those theoretically proposed structures do not naturally exist and remain to be made in experiments. Here, we identify a recently experimentally made 2D organometallic framework, consisting of $\pi $-conjugated nickel-bis-dithiolene with a chemical formula Ni$_{\mathrm{3}}$C$_{\mathrm{12}}$S$_{\mathrm{12}}$, which exhibits nontrivial topological states in both a Dirac band and a flat band, therefore confirming the existence of OTI. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W43.00004: Topological phase transition driven by electron-phonon interaction Kush Saha, Ion Garate We study the effect of electron-phonon interactions in the band topology of Dirac insulators, both at zero and finite temperature. Elaborating on recent theoretical work [1], we determine how and when phonons can drive a trivial insulator into a topological insulating phase. As an application, we evaluate the temperature-dependence of the critical thickness for the topological transition in CdTe/HgTe quantum wells. \\ \\ Ref[1]: Ion Garate, PRL 110, 046402 (2013). [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W43.00005: Topological effects in an electric-field-driven hexagonal lattice Woo-Ram Lee, Kwon Park In this work, using the Floquet theory, we investigate the topological effects in a hexagonal lattice under the influence of in-plane electric field. It is found that the Bloch oscillation of an electron in a hexagonal lattice causes topologically nontrivial energy shift in Wannier-Stark energy ladders, depending on the strength and relative angle of the electric field. Importantly, the energy shift is connected to the Berry curvature effect, which induces Hall current. In the presence of spin-orbit coupling, the competition between the electric field and the spin-orbit coupling is also studied. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W43.00006: Weak Topological Insulators in PbTe/SnTe superlattice Gang Yang, Junwei Liu, Liang Fu, Wenhui Duan, Chaoxing Liu It is desirable to realize topological phases in artificial structures by engineering electronic band structures. In this paper, we investigate (PbTe)$_m$(SnTe)$_{2n-m}$ superlattices along the [001] direction and find a robust weak topological insulator phase for a large variety of layer numbers $m$ and $2n-m$. We confirm this topologically non-trivial phase by calculating $Z_2$ topological invariants and topological surface states based on the first-principles calculations. We show that the folding of Brillouin zone due to the superlattice structure plays an essential role in inducing topologically non-trivial phases in this system. This mechanism can be generalized to other systems in which band inversion occurs at multiple momenta, and gives us a brand-new way to engineer topological materials in artificial structures. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W43.00007: Numerical Study of a Bosonic Topological Insulator in three dimensions Scott Geraedts, Olexei Motrunich We construct a model which realizes a (3+1)-dimensional symmetry-protected topological phase of bosons with $U(1)$ charge conservation and time reversal symmetry, envisioned by A. Vishwanath and T. Senthil [PRX 4 011016]. Our model works by introducing an additional $O(3)$ degree of freedom, and binding its hedgehogs to a species of charged bosons; the continuous symmetry is thus enlarged to $SO(3)\times U(1)$. We study the model using Monte Carlo and determine its bulk phase diagram; the phase where the bound states of hedgehogs and charges condense is the topological phase. We also study surface phase diagram on a (2+1)-dimensional boundary between the topological and trivial insulators. The theory for the surface is the same as for a (2+1)D hedgehog-suppressed non-linear sigma model, which confirms the proposed so-called NCCP$^1$ field theory. We apply a Zeeman field to the surface, which breaks time reversal on the surface only, and observe a surface Hall conductivity which is half of a quantized value allowed for bosons in strictly (2+1)D, thus establishing topological nature of the (3+1)D bulk phase. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W43.00008: Wannier Center Sheets in Topological Insulators Maryam Taherinejad, Kevin Garrity, David Vanderbilt The electronic ground state in a periodic crystalline insulator can be described by hybrid Wannier functions $\vert W_{nl_z}(k_x,k_y)\rangle$ which are maximally localized in one direction and Bloch-like in the other two. In 3D insulators the Wannier charge centers (WCCs), defined as $\bar{z}_n(k_x,k_y)=\langle W_{n0}(k_x,k_y) \vert \hat{z} \vert W_{n0}(k_x,k_y)\rangle$, are functions of momentum in two dimensions and can be plotted as sheets over the 2D Brillouin zone. We show that the symmetry group of the WCCs $\bar{z}_n(k_x,k_y)$ includes all the symmetries of surface energy bands $\epsilon_n(k_x,k_y)$. More importantly, the WCCs contain the same kind of topological information as is carried in the surface energy bands, with the crucial advantage that the topological properties of the bulk can be deduced from bulk properties alone. The distinct topological behavior of these WCC sheets in trivial, Chern, weak, strong, and crystalline topological insulators are demonstrated using different tight-binding models. The WCC sheets calculated from first-principles calculations in $Z_2$-even Sb$_2$Se$_3$, weak $Z_2$-odd KHgSb, and strong $Z_2$-odd Bi$_2$Se$_3$ confirm the results from the tight-binding models. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W43.00009: Localization and topology protected quantum coherence at the edge of `hot' matter Yasaman Bahri, Ronen Vosk, Ehud Altman, Ashvin Vishwanath Topological phases are often characterized by special edge states confined near the boundaries by an energy gap in the bulk. On raising temperature, these edge states are lost in a clean system due to mobile thermal excitations. Recently, however, it has been established that disorder can localize an isolated many-body system, potentially allowing for a sharply defined topological phase even in a highly excited state. Here we show this to be the case for the topological phase of a one-dimensional magnet with quenched disorder which features spin one-half excitations at the edges. The time evolution of a simple, highly excited initial state is used to reveal quantum coherent edge spins. In particular, we demonstrate, using theoretical arguments and numerical simulation, the coherent revival of an edge spin over a time scale that grows exponentially larger with system size. This is in sharp contrast to the general expectation that quantum bits strongly coupled to a `hot' many body system will rapidly lose coherence. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W43.00010: Floquet Topological Insulators in Uranium Compounds Shu-Ting Pi, Sergey Savrasov A major issue regarding the Uranium based nuclear fuels is to conduct the heat from the core area to its outer area. Unfortunately, those materials are notorious for their extremely low thermal conductivity due to the phonon-dominated-heat-transport properties in insulating states. Although metallic Uranium compounds are helpful in increasing the thermal conductivity, their low melting point still make those efforts in vain. In this report, we will figure out potential Uranium based Floquet topological insulators where the insulating bulk states accompanied with metallic surface states is achieved by applying periodic electrical fields which makes the coexistence of both benefits possible. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W43.00011: Broadband spectroscopic characterization of topological crystalline insulator Pb$_{0.77}$Sn$_{0.23}$Se Anjan Reijnders, Jason Hamilton, Quinn D. Gibson, Robert J. Cava, Kenneth S. Burch Topological crystalline insulators (TCI) are novel materials in which mirror symmetry protects the presence of spin polarized surface states. In this talk I will present the temperature dependent optical properties of Pb$_{0.77}$Sn$_{0.23}$Se, a compound with a temperature dependent trivial to nontrivial topological phase transition. Reflectance and ellipsometry data between 6 meV - 5.95 eV will be discussed in conjunction with optical conductivity and the frequency dependent scattering rate, revealing hints of the topological phase transition. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W43.00012: Properties of 2D Chiral Tensor Network States Barry Bradlyn, Jerome Dubail, Nicholas Read States that can be represented as a sum over local auxiliary degress of freedom are known as tensor network states (TNSs) [1]. In a recent paper [2], Dubail and Read gave a construction for free fermion TNSs in the chiral $p+ip$ and $\nu=1$ Chern insulator topological phases in two dimensions, and gave a generalization to Laughlin-like states. However, on general principles these free fermion states must be ground states of gapless local Hamiltonians. In this talk, we address the issue of what topological properties persist in these gapless states. We show analytically that the DC Hall conductivity for the $\nu=1$ Chern insulator TNS is quantized, although the conductivity tensor at finite frequency suffers from non-analytic corrections. Additionally, we investigate the issue of the energy gap for the interacting $\nu=1/2$ Laughlin-like TNS through Monte Carlo simulations.\\ \\ {\bf References} \\ $[1]$ F. Verstraete and J.I. Cirac, cond-mat/0407066 (2004).\\ $[2]$ J. Dubail and N. Read, arXiv:1307.7726 (2013). [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W43.00013: Localized electron basis sets and the electronic properties of novel materials Pablo Rivero, Victor Manuel Garcia-Suarez, Jaime Ferrer, Kyungwha Park, Salvador Barraza-Lopez Density functional theory algorithms based on localized electron basis sets permit calculation of material properties with modest computational cost, provided quantitative benchmarks against known properties available. Within the SIESTA computational package, we present a pragmatic and quantitative method to optimize basis sets and pseudopotentials of ordinary and novel materials. The method gives us a solid foundation to explore the electronic properties of new materials such as the strong topological insulator Bi$_{2}$Se$_{3}$. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W43.00014: Solitons, charge fractionization, and the emergence of topological insulators in graphene rings Constantine Yannouleas, Igor Romanovsky, Uzi Landman The doubly-connected polygonal geometry of planar graphene rings is found to bring forth topological configurations for accessing nontrivial relativistic quantum field (RQF) theory models that carry beyond the constant-mass Dirac-fermion theory. These include generation of sign-alternating masses, solitonic excitations, and charge fractionization. The work integrates a RQF Lagrangian formulation with numerical tight-binding Aharonov-Bohm electronic spectra and the generalized position-dependent-mass Dirac equation. In contrast to armchair graphene rings (aGRGs) with pure metallic arms,\footnote{% I. Romanovsky, C. Yannouleas, and U. Landman, Phys. Rev. B {\bf 87}, 165431 (2013)} certain classes of aGRGs with semiconducting arms, as well as with mixed metallic-semiconducting ones, are shown to exhibit properties of one-dimensional nontrivial topological insulators. This further reveals an alternative direction for realizing a graphene-based nontrivial topological insulator through the manipulation of the honeycomb lattice geometry, without a spin-orbit contribution. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W43.00015: Topological magnetic crystalline insulators and co-representation theory Ruixing Zhang, Chaoxing Liu We introduce a new type of topological insulator protected by magnetic group symmetry, which is a combined symmetry of point group symmetry and time reversal symmetry. Based on the Herring rule of the co-representation theory of magnetic group, we systematically show that systems with certain magnetic group symmetries can have Kramers'-like degeneracies and admit a Z2 classification. We establish a tight-binding model describing a layered magnetic structure with combined C4 rotation and time reversal symmetry. We show that this model can support non-trivial topological phases by calculating its gapless surface states and defining its Z2 topological invariant. [Preview Abstract] |
Session W44: Graphene and Carbon Nanotubes: Functionalization and Sensing Applications
Sponsoring Units: DCMPChair: Enrique Cobas, Naval Research Laboratory
Room: Mile High Ballroom 4C
Thursday, March 6, 2014 2:30PM - 2:42PM |
W44.00001: Fabrication and Investigation of resonance frequency sensitive Graphene/ZnO based gas sensor in room temperature Mehdi Namazi, Seyyed Mohsen Jebreiil Khadem, Yaser Abdi, Sara Darbari, Fatemeh Ostovari Graphene/ZnO hybrid was used to fabricate a highly selective and sensitive gas sensor. ZnO nanowires in the structure have three important roles: reduction of grapheme oxide to obtain graphene, acting as sensing element and mechanical actuation using their piezoelectric properties. A selected set of chemicals vapors was tested on the fabricated sensor. We have found that chemical vapors change the resonance frequency of the graphene/ZnO in addition to the electrical resistivity of the structure. Variation of the mechanical and electrical characteristics of the graphene/ZnO due to gas exposure make the graphene/ZnO based sensors highly selective and reliable device for gas sensing with distinctive signatures for specific gases. We have introduced an alternative frequency modulation gas detection method here in which the gas absorption on the graphene/ZnO can affects resonance frequency of the ZnO nanowires. Such alternative method can be utilized for detection of absorbed gases which do not change the resistivity of the sensing element significantly. Also the sensitivity of the graphene/ZnO based gas sensor was investigated under mechanical actuation. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W44.00002: Gase sensing using on terahertz emissions from graphene-coated InP surfaces Tonouchi Masayoshi, Iwao Kawayama, Yuki Sano, Khandoker Salek, Hironaru Murakami, Mika Tabata, Minjie Wang, Junichiro Kono, Robert Vajtai, Pulickel Ajayan Electrical and optical properties of graphene are known to be affected by the adsorption of gas molecules, which can be used for developing a highly sensitive gas sensor. In this study, we demonstrate a new approach for environmental gas sensing using terahertz emission from graphene-coated semiconductor wafers. Specifically, we show that the waveforms of terahertz radiation from graphene-coated InP sensitively change with the type of the atmospheric gas and the laser illumination time. The change of the terahertz waveforms in different environmental gases can be explained by modification of the surface depletion-layer potential of InP due to the surface dipole induced by the adsorbed gas molecules. Moreover, additional UV light illumination enhances the change of terahertz waveforms in oxygen, apparently due to photo-oxidation of graphene. We have developed a theoretical model that can explain these experimental observations in a semi-quantitative manner. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W44.00003: Detection of the first order phase transition in water with carbon nanotube layer Vladimir Samuilov, Nikolay Poklonski We have developed a new generation of the icing conditions sensors. These sensors are based on the detection of a molecular thin layer of absorbed water molecules, transforming into ice by detection of nonmonotonic variation of the resistance of the carbon nanotube sensor. Carbon nanotube layers could be utilized as an inexpensive and effective sensors of humidity and icing conditions, suitable for applications in aviation and different industries. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W44.00004: Transport of nanoparticles in porous media with Dissipative Particle Dynamics (DPD) simulation Minh Vo, Dimitrios Papavassiliou Nanoparticles can serve as nanosensor devices in oil recovery processes, because of their ability to propagate in porous media. We employ DPD simulations to explore the factors affecting the retention and mobility of carbon nanotubes (CNT) in porous media. Compared to molecular dynamics simulations, longer time and length scales can be obtained with DPD, while the hydrodynamic properties of system are also maintained. Besides, complex flow structures can be handled by DPD in a simple manner (using frozen DPD beads for solid surface). In our calculations, packed-sphere geometry is utilized to create porous media. After equilibrium, CNTs are released into the flow. The interaction between the CNTs and the solid surface is considered by applying both shifted force Lenard-Jones and Morse potential in the DPD model. Different sizes of CNTs are investigated, in order to study the effect of the aspect ratio on the hydrodynamic forces as well as the rotation of CNTs while moving with the flow. In addition, the mobility of CNTs is discussed by computing their trajectory in the flow and comparing the cylindrical particles to spherical. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W44.00005: Using graphene to track the conductivity of C$_{60}$ Claudia Ojeda-Aristizabal, Seita Onishi, Haider Rasool, Celeste Carruth, Alex Zettl C$_{60}$ exhibits superconductivity when intercalated with alkali metals [1,2]. This intercalation originates charge doping as well as a modification of the lattice constant, giving rise to an increased density of states at the Fermi energy of C$_{60}$. Here we study the change of the electronic transport in C$_{60}$ by charge doping alone. We deposit C$_{60}$ on a graphene device that has two metallic electrodes and a back gate. By measuring the conductance of graphene, we track any changes in the conductance of C$_{60}$, connected in parallel to graphene. We will show preliminary transport data that demonstrates charge transfer into C$_{60}$. \\[4pt] [1] A. F. Hebard, M.J. Rosseinsky, R. C. Haddon, D. W. Murphy, S. H. Glarum, T.T. M. Palstra, A. P. Ramirez and A. R. Kortan. Nature 350, 600 (1991).\\[0pt] [2] R. M. Fleming, A.P. Ramirez, M.J. Rosseinsky, D. W. Murphy, R.C. Haddon, S. M. Zahurak and A. V. Makhija. Nature 352, 787 (1991). [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W44.00006: First-principles study of multi-control doping using azobenzene adsorbed on graphene Jonathan Trinastic, Hai-Ping Cheng The high carrier mobility in graphene promises its utility in electronics applications, however more research is necessary to find optimal doping methods. Azobenzene (AB) is a widely used organic molecule for switchable optoelectronic devices that can be synthesized with a wide variety of ligands and deposited on graphene. Using first-principles calculations, we investigate the doping of graphene by physisorbed azobenzene molecules with various electron-donating and --accepting ligands. We confirm previous experimental results (Peimyoo et al 2011, \textit{ACS Nano},6(10),8878) that demonstrate greater p-doping of graphene for the \textit{trans }compared to \textit{cis }configuration when using a SO$_{\mathrm{3}}$ electron-accepting ligand, and we find different levels of p-doping when using other ligands. We extend these findings by examining the doping effects of an applied electric field and applied strain to the graphene, which leads to changes in doping for both the \textit{trans }and \textit{cis} isomers. These findings demonstrate a new type of multi-switch device combining light, electric field, and strain to change carrier concentration in graphene. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W44.00007: Graphene- and graphene oxide- based multisensor arrays for selective gas analysis Alexey Lipatov, Alexey Varezhnikov, Victor Sysoev, Andrei Kolmakov, Alexander Sinitskii Arrays of nearly identical graphene devices on Si/SiO$_{2}$ exhibit a substantial device-to-device variation, even in case of a high-quality chemical vapor deposition (CVD) or mechanically exfoliated graphene. We propose that such device-to-device variation could provide a platform for highly selective multisensor electronic olfactory systems. We fabricated a multielectode array of CVD graphene devices on a Si/SiO$_{2}$ substrate, and demonstrated that the diversity of these devices is sufficient to reliably discriminate different short-chain alcohols: methanol, ethanol and isopropanol. The diversity of graphene devices on Si/SiO$_{2}$ could possibly be used to construct multisensor systems trained to recognize other analytes as well. Similar multisensory arrays based on graphene oxide (GO) devices are also capable of discriminating these short-chain alcohols. We will discuss the possibility of chemical modification of GO for further increase the selectivity of GO multisensory arrays. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W44.00008: Gas sensing of a saturated tin/defective graphene device Sherif Tawfik, X. Cui, Damien Carter, S. Ringer, C. Stampfl The sensitivity and selectivity of defective graphene to gases is enhanced by implanting single metal adatoms into vacancy sites. Knowledge of the behavior of these devices under the incremental adsorption of gas molecules until saturation is essential for determining the sensitivity of the device in realistic situations as well as for evaluating the applicability of the device in molecular capture and storage. We present a DFT study of incremental gas adsorption of CO$_{2}$, NO$_{2}$, SO$_{2}$ and H$_{2}$S gases on tin adatom-double vacancy graphene system, in the presence and absence of O$_{2}$. Within the NEGF formalism, we analyze the sensitivity and selectivity of the saturated device to the gas species, showing distinctive transport features for each of the gas species. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W44.00009: Electronic transport properties of graphene with 5 \textit{d} metal adatoms Yilin Wang, Shudong Xiao, Wenzhong Bao, Janice Reutt-Robey, Michael Fuhrer Electronic transport properties of graphene are highly sensitive to adsorbates on its surface. Adsorbates can affect graphene through doping and scattering, and 5$d$ metal adsorbates have been predicted to induce strong spin-orbit coupling and open a bandgap [1]. Here we study the\textit{ in-situ} transport properties of single-layer graphene doped with 5$d$ heavy metal atoms. Iridium was deposited on graphene at a substrate temperature of 7 K under ultra-high vacuum condition. Gate-dependent conductivity measurements show that the mobility and minimum conductivity of graphene decrease with increasing Iridium concentration. The results are in agreement with the self-consistent theory of graphene with random charged impurities [2], and do not indicate any significant bandgap in graphene with Iridium adatoms. We will also discuss temperature-dependent measurements, and co-adsorption of Iridium and krypton.\\[4pt] [1] Phys. Rev. Lett. \textbf{109}, 266801 (2012)\\[0pt] [2] \textit{PNAS} \textbf{104}, 18392 (2007) [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W44.00010: Rigid band shifts, charge pinning, and charge transport through graphene junctions with wetting metal contacts Tobias Bothwell, Salvador Barraza-Lopez It is a common perception that graphene band shifts cannot be determined directly when attached to chemisorbed (``wetting'') metals due to the hybridization of graphene bands around the Dirac point. Graphene has deeper energy (sigma) bands which don't hybridize with the metal's bands, providing a definite measure of actual shifts. Looking at hybridization in a controlled way (by varying the metal/graphene separation by hand) one realizes the shifts can actually be considered rigid, i.e., $\sigma -$ and $p-$ bands shift by about the same energy $\Delta_{\mathrm{E}}$. In a related context, charge depinning is the modification of graphene's electron density at a metal/graphene interface with a (back) gate. Depinning happens at metal/graphene interfaces with physisorbed (non-wetting) metals. Oxidation or contamination at the interface can lead to charge depinning as well. Using first-principles calculations, we establish a link between charge depinning at a wetting metal/graphene interface and the quality of such an interface. For this purpose, metal/graphene/insulator structures are studied under transverse bias. We also report transmission coefficients through nanoscale two-terminal graphene/metal junctions. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W44.00011: Controlling the electronic structure of graphene using surface-adsorbate interactions Piotr Matyba, Adra Carr, Cong Chen, Margaret M. Murnane, Henry C. Kapteyn, David L. Miller, Mark W. Keller, Guowen Peng, Manos Mavrikakis, Stefan Mathias Strong coupling with the substrate, i.e. C-Ni hybridization, causes that the $\pi $ state maximum in graphene on Ni(111) is shifted to below the Fermi level, resulting in a band gap of 2.8 eV.[1, 2] The intercalation of noble metals to underneath graphene can reduce this band gap by decoupling graphene from the substrate.[3] Here we use angle-resolved x-ray photoemission spectroscopy and DFT calculations to explore the influence of Na adsorbate on the electronic structure of graphene on Ni(111).[1] We show that the carefully controlled electronic surface-adsorbate interactions reduce the band gap to 1.3 eV with no need of intercalation, since the strong graphene-substrate coupling is counterbalanced by the coupling to the adsorbate. Subsequent intercalation drastically changes the electronic structure, reducing the band gap to \textless 180 meV, and decoupling graphene from the substrate. Our results demonstrate full band gap closing using an adsorbate for the first time, and suggest that surface-adsorbate interactions make it possible to control the band gap, either statically or dynamically, which is potentially useful for novel electronics. [1] P. Matyba et al., in submission (2013), [2] A. Gruneis et al., Physical Review B 77 (2008), [3] A. Varykhalov et al., Physical Review B 82 (2010). [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W44.00012: Improving the electrical characteristics of graphene field effect transistors by hexamethyldisilazane interaction Sk. Chowdhury, Somayyeh Rahimi, Sushant Sonde, Li Tao, Sanjay Banerjee, Deji Akinwande We report the improvement of the electrical characteristics of graphene field effect transistors (FET) by hexamethyldisilazane (HMDS) passivation. Sample is left in liquid HMDS after complete back gated FET fabrication. Both electron and hole field effect mobilities are improved by 1.5X - 2X, accompanied by effective residual carrier concentration reduction. Dirac voltage also moves closer to zero. Various techniques for HMDS application are investigated. Time evolution of mobility data shows that mobility improvement saturates after a few hours of HMDS dosing. Temperature-dependent transport measurements show small mobility variation between 77K and room temperature (295K) before HMDS application. But mobility at 77K is almost 2 times higher than mobility at 295K after HMDS application. The best CVD devices achieve a mobility of $\sim$ 20,000 cm2/V-s at 77K. Performance improvement is observed for FETs made with exfoliated graphene and for FETs made on hydrophobic substrate- an HMDS-graphene-HMDS sandwich structure. Raman spectroscopic analysis shows that G peak width is increased, G peak position is down shifted and intensity ratios between 2D and G peak is increased after HMDS application. AFM data shows increased RMS roughness after HMDS application. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W44.00013: The range of two-body adsorbate-adsorbate interactions on the surface of graphene Dmitry Solenov, Chad Junkermeier, Thomas L. Reinecke, Kirill A. Velizhanin Tunable functionalization of ``all-surface'' materials is a promising area of fundamental and applied research. Two-body interactions between adsorbed atoms or molecules on surfaces, such as graphene, are crucial to a variety of applications, ranging from transport to photovoltaics and DNA manipulations. We present our recent results for adsorbate-adsorbate interactions between mono-valent and between bi-valent adsorbates on graphene. These interactions are dominated by direct Coulomb coupling and exchange of itinerant electrons. A model Hamiltonian that encompasses different types of adsorbates is constructed and parameterized from ab initio density functional theory. The range of interactions is found to depend strongly on the local adsorbate-substrate bonding mechanism and on the chemical potential. [Preview Abstract] |
Session W45: Graphene: Functional Interfaces
Sponsoring Units: DCMPChair: Yufeng Hao, University of Texas at Austin
Room: Mile High Ballroom 4D
Thursday, March 6, 2014 2:30PM - 2:42PM |
W45.00001: Giant Current-Perpendicular-to-Plane Magnetoresistance in Multilayer-Graphene Grown on Nickel Srikrishna Bodepudi, Abhay Singh, Sandipan Pramanik Magnetoresistance (MR), the change in electrical resistance as a function of an external magnetic field, is a key effect in magnetic field sensors. Ferromagnet-nonmagnet multilayers which often exhibit giant magnetoresistance or tunnel magnetoresistance effects are traditionally used to realize magnetic field sensors. MR in graphitic systems has drawn significant attention in recent years due to the unique crystal structures of these materials. In this work we explore another class of layered structure in which multilayer graphene (MLG) is as-grown on nickel substrate by chemical vapor deposition (CVD). We observed a large negative current-perpendicular-to-plane (CPP) MR (\textgreater 10$^{4}$ {\%}) in this system when the magnetic field is normal to the plane. The observed effect can be qualitatively explained within the framework of interlayer MR. Graphene layers in CVD-grown MLG are generally weakly coupled, which can be viewed as a stack of two dimensional Dirac electron system. A large negative interlayer MR can be expected in CPP geometry when the charge transport occurs between the zero mode Landau levels of weakly coupled Dirac electron system. This effect is stronger when the magnetic field is normal to the plane. We also showed that the defect free graphene is essential in addition to the weakly coupled graphene layers to observe the large negative MR. Due to large MR value and its persistence at room temperature, this effect is expected to have commercial implications and encourage further research on MLG physics. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W45.00002: Fabrication and transport studies of graphene-superconductor heterostructures Jiuning Hu, Tailung Wu, Jifa Tian, Yong Chen Recently, graphene based stacked heterostructures, e.g., graphene and boron nitride (BN) multi-layers, have attracted much attention as a system to study novel interaction-driven physics (e.g., excitonic condensation) and perform interesting measurements (eg. Coulomb drag and tunneling). The realm of graphene-superconductor heterostructures remains less unexplored, while such a system offers various interesting prospects (effects of superconductor vortices lattices on over-layering graphene and quantum Hall states, where novel phenomena such as anionic excitations have been predicted). We have used polyvinyl alcohol (PVA) based carrier films and a micro-manipulator to transfer mechanically exfoliated flakes and fabricated graphene/BN/NbSe$_2$ structures to study the transport properties of graphene in close proximity to electrically isolated superconducting NbSe$_2$ films. The NbSe$_2$ film shows the superconducting transition temperature of $\sim$7 K and upper critical field of $\sim$3.5 T after device fabrication. We will present results from magneto-transport in graphene and graphene-NbSe$_2$ Coulomb drag and tunneling measurements. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W45.00003: Two dimensional epitaxial graphene - SiC/SiO$_x$ field effect transistors Jan Kunc, Yike Hu, James Palmer, Zelei Guo, Claire Berger, Walter de Heer We have produced and measured two dimensional (2D) field effect transistors composed of graphene source and drain and a 2D SiC/SiO$_x$ channel supplied with a top gate. The devices have been measured in a wide range of gate voltages and temperatures. Careful attention was focused on the SiC/SiO$_x$ channel formation and graphitization conditions. The channel was characterized by XPS, LEED, atomic and electrostatic force microscopy and Raman spectroscopy. On to off current ratios up to 10$^6$ have been achieved and sub-threshold swings up to 200 mV/decade have been attained with on-state currents in the sub-miliamp range. The channel formation as well as graphene/SiC junction including charge transfer in the graphene are modeled solving the coupled Poisson equation and Schr\"{o}dinger equation in the effective mass approximation. The standard models of Metal Induced Gap States (MIGS) and Charge Neutrality Level concepts successfully reproduce the experimental data. The combined contributions of the space charge limited current in the channel and back-to-back Schottky diodes at the channel junctions are discussed. The thermionic and tunneling nature of the barriers is analyzed in these quasi two dimensional devices. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W45.00004: Measuring Schottky barrier height at graphene/SiC junction D. Tomer, L. Hudy, S. Rajput, L. Li When graphene is interfaced with a semiconductor, a Schottky junction forms with rectifying properties. In this work, we measured the Schottky barrier heights of graphene/SiC Schottky diodes using current-voltage (I-V) measurement. Chemical vapor deposited graphene was transferred onto semiconductor surfaces of opposite polarization: the hydrogen-terminated Si- and C-faces of $\alpha $-SiC, which was confirmed by Raman spectroscopy and scanning tunneling microscopy. The Schottky barrier height is found to be sensitive to the polarization of the substrate and surface preparation. On the Si-face, a barrier of 0.47 eV is found. These results will be compared with earlier work as well as our \textit{in situ} scanning tunneling spectroscopy results [1]. [1] Rajput et al., Nature Comm. (DOI: 10.1038/ncomms3752). [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W45.00005: Quasiparticle carrier dynamics in graphene from first principles Cheol-Hwan Park, Nicola Bonini, Thibault Sohier, Georgy Samsonidze, Boris Kozinsky, Matteo Calandra, Francesco Mauri, Nicola Marzari It is important to understand how a charge carrier in real materials interacts with other charge carriers or with the lattice vibration; these two effects, electron-electron interactions and electron-phonon interactions, respectively, largely determine the quasiparticle and transport properties of a material. In this presentation, we will show that some aspects of quasiparticle dynamics in graphene can be described by first-principles calculations considering these two effects. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W45.00006: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W45.00007: Electron-electron interaction effects in monolayer graphene Edwin Barnes Electron-electron interactions are expected to play an important role in graphene due to the absence of screening near the charge neutrality point, potentially leading to strong deviations from the Fermi liquid description. While such deviations have yet to be observed, there is experimental evidence of significant Dirac cone squeezing, a phenomenon which is consistent with renormalization of the Fermi velocity due to interaction effects. We show that while a first-order renormalization group analysis gives qualitative agreement with experimental observations of graphene both on substrates and in vacuum, a second-order analysis reveals an interacting critical point in suspended graphene, signifying either a quantum phase transition or a breakdown of the renormalization group approach. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W45.00008: Growth of graphene on sapphire by molecular beam epitaxy Sheng Wang, Lara Fernandes dos Santos, Ulrich Wurstbauer, Lei Wang, Loren N. Pfeiffer, James Hone, Jorge M. Garcia, Aron Pinczuk Graphene growth by direct deposition of carbon atoms on dielectric substrates in a MBE environment has potential for large area fabrication of graphene layers. We explore the optimal graphene growth on sapphire c-plane surface with gradients of carbon flux and substrate temperature. Single- and bi-layer nanocrystalline graphene with sharp Raman bands are achieved at temperature around 1200 C. Atomic force microscopy (AFM) images uncover the presence of etch pits which suggest a carbon removal mechanism known as ``carbo-thermal reduction''. The average spacing between etch pits (of about 100 nm) defines an upper limit of nanocrystal size. Tuning the easily controlled incident carbon flux and the markedly temperature dependent carbo-thermal reduction of sapphire should enable the growth of high quality graphene layers on large area sapphire substrates. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W45.00009: Piezoelectric surface acoustical phonon limited mobility of electrons in graphene on a GaAs substrate Samvel Badalyan, Shuhui Zhang, Wen Xu, Francois Peeters We study the mobility of Dirac fermions in monolayer graphene on a GaAs substrate, limited by the combined action of the extrinsic potential of piezoelectric surface acoustical phonons of GaAs (PA) and of the intrinsic deformation potential of acoustical phonons in graphene (DA). In the high temperature ($T$) regime the momentum relaxation rate exhibits the same linear dependence on $T$ but different dependences on the carrier density $n$, corresponding to the mobility $\mu\propto 1/\sqrt{n}$ and $1/n$, respectively for the PA and DA scattering mechanisms. In the low $T$ Bloch-Grueneisen regime, the mobility shows the same square-root density dependence, $\mu\propto \sqrt{n}$, but different temperature dependences, $\mu\propto T^{-3}$ and $ T^{-4}$, respectively for PA and DA phonon scattering. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W45.00010: Magneto-optical properties of graphene on polar substrates Benedikt Scharf, Vasili Perebeinos, Jaroslav Fabian, Igor \v{Z}uti\'c We theoretically study the effect of polar substrates on the magneto-optical conductivity of doped monolayer graphene, where we particularly focus on the role played by surface polar phonons (SPPs). Our calculations suggest that polaronic shifts of the intra- and interband absorption peaks and the loss of spectral weight at these peaks due to electron-SPP scattering can be significantly larger for polar substrates than in graphene on nonpolar substrates, where only intrinsic graphene optical phonons with much higher energies contribute. These effects can be strongly temperature dependent, most noticeably in polar substrates with small SPP energies such as HfO$_2$. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W45.00011: Characteristic of graphene field effect transistor with ferroelectric gate dielectric Somyeong Shin, Hyewon Du, Taekwang Kim, Jong-Hyuk Yoon, Eun-Kyu Lee, Seungmin Cho, Sunae Seo The increase of charge carrier concentration along with the mobility is essential to improve the conductance graphene. The replacement of high dielectric constant (high-k) materials provides this but in graphene charge carrier density could be limited by quantum capacitance of graphene with high-k gate dielectric. Y2O3 was one of materials intensively studied. Ferroelectric materials could also provide the other functionality using nonvolatile characteristic of remanent polarization as well as high charge carrier density. Several researches were reported for nonvolatile memory device combined with graphene and ferroelectric. everal previous experimental data seems to show the entangled hysteresis due to ferroelectric polarization and uncontrolled external impurity external charge in the electrical property and significant influences of interface states in ferroelectric and graphene interface.. In this study, we attempt to comprehend complicated hysteresis and the influence of charge carrier concentration by quantum capacitance and interface states qualitatively. We fabricated graphene ferroelectric field-effect transistor (GFFET) with PtZr$_{\mathrm{x}}$Ti$_{\mathrm{1-x}}$O$_{3}$(PZT) as gate dielectric and studied the effect on the transport property of electron or hoe conduction by interface states and ferroelectric polarization using gate voltage dependent capacitance and current --voltage experimental data at different temperature. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W45.00012: Ferroelectric Superlattices as a route to clean Graphene-ferroelectric Interfaces Mohammed Yusuf, Matthew Dawber, Xu Du A good interface between ferroelectric surfaces and graphene sheets can enable a new generation of multifunctional devices in which the ferroelectric material is used to control the properties of graphene. Ferroelectric superlattices, in particular PbTiO$_{3}$/SrTiO$_{3}$ (PTO/STO), provide us with a unique opportunity for studying the graphene-ferroelectric interface. The ferroelectric-paraelectric transition temperature of the superlattices is tunable by varying the PTO volume fraction. Using devices with different PTO volume fractions and different ferroelectric strength, we have successfully demonstrated ferroelectric hysteresis, charge-trapping associated anti-hysteresis, and cross-over from anti-hysteresis to hysteresis over a wide temperature range from 300K down to 4K. These results allow us to establish a deeper understanding of the graphene-ferroelectric interface. Contrary to the common understandings that the charge trapping centers and anti-hysteresis originate mainly from contaminants and adsorbates trapped between graphene and ferroelectric substrates during fabrication, we found that significant contribution of charge trapping may come from defects in the ferroelectric substrate itself, and we explore approaches to eliminating these. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W45.00013: Characterization of plasmon propagation in graphene on PZT substrates via infrared nano-imaging M.D. Goldflam, Guangxin Ni, Zhe Fei, A.S. McLeod, Barbaros Ozyilmaz, Antonio Castro Neto, Michael Fogler, D.N. Basov Using scattering-type scanning near-field optical microscopy, we have characterized graphene plasmons on a graphene-lead zirconate titanate (PZT) back-gated structure. By applying modest back-gate voltages of $\pm$1 V across the PZT, we have been able to induce variations in the graphene plasmon wavelength of more than $\sim$200 nm. The change in plasmon wavelength we observe corresponds to a shift in carrier concentration in the graphene by more than one order of magnitude. Additionally, we describe the plasmonic losses originating from the presence of PZT in such a device. Our results also suggest that persistent tuning of the graphene plasmon may be achieved by utilizing the ferroelectric nature of PZT. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W45.00014: Effect of Remote Surface Optical Phonon Scattering in Graphene Gated by Single Crystal Ferroelectric Oxide Thin Films Zhiyong Xiao, Anil Rajapitamahuni, Stefan Schoeche, Jason Hoffman, Charles Ahn, Mathias Schubert, Xia Hong We have studied the effect of remote surface optical (RSO) phonon on the carrier mobility in graphene gated by a ferroelectric Ba$_{0.6}$Sr$_{0.4}$TiO$_{3}$ (BSTO) substrate. Single crystal 100-400nm BSTO films are grown epitaxially on Nb doped SrTiO$_{3}$ substrates. Graphene flakes are mechanically exfoliated onto BSTO and single and bi-layer flakes are fabricated into field effect devices via e-beam lithography. All samples exhibit resistivity hysteresis induced by ferroelectric switching at low temperature, which can be used for nonvolatile memory operations. Single layer graphene exhibits high mobility with $\mu_{Hall}$ $\sim$ 10,000 cm$^{2}$/Vs at carrier density of 3.5x10$^{12}$ cm$^{-2}$ at 10K. Above 80K, We observe a sharp rise in resistivity as a function of temperature $\rho $(T), which is attributed to the RSO phonon scattering form the BSTO gate. We have extracted the dominant RSO phonon mode from $\rho $(T) and compared it with results extracted from independent spectroscopic ellipsometry measurements. We will also discuss the temperature dependence of resistivity in bi-layer graphene gated by BSTO. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W45.00015: Electrical quality improvement of thin Y$_{2}$O$_{3}$ topgates in graphene FETs by high-pressure O$_{2}$ post-deposition annealing Kosuke Nagashio, Kaoru Kanayama, Tomonori Nishimura, Akira Toriumi Although extensive research effort has focused on equivalent oxide thickness scaling by the deposition of ultrathin high-$k$ dielectrics on graphene, these dielectrics still suffer from leakage currents under high electric fields. This leakage is a critical concern for the increase in the on-current. Here, we demonstrate a considerable suppression of the gate leakage current by using Y$_{2}$O$_{3}$ film annealed in high-pressure O$_{2}$ at 100 atm (HP-PDA) in top-gated graphene FETs. Consequently, the quantum capacitance measurement for the monolayer graphene reveals the highest Fermi energy modulation ($E_{\mathrm{F}} =$ $\sim $0.52 eV, i.e., the carrier density of $\sim $2 $\times$ 10$^{13}$ cm$^{-2})$ in the solid-state topgate insulators reported so far. HP-PDA of Y$_{2}$O$_{3}$ enables to realize the robust and reproducible top-gated graphene FETs. [Preview Abstract] |
Session W46: Interacting Electron Systems: Theory
Sponsoring Units: DCMPChair: Sergey Pershoguba, University of Maryland
Room: Mile High Ballroom 4E
Thursday, March 6, 2014 2:30PM - 2:42PM |
W46.00001: Universal Conductance of Quantum Multiwire Junctions with Entanglement Renormalization Ya-Lin Lo, Yun-Da Hsieh, Chang-Yu Hou, Pochung Chen, Ying-Jer Kao We study the universal conductance of quantum multiwire junctions via muti-scale entanglement renormalization ansatz (MERA). MERA, in its scale invariant from, provides an efficient way to extract scaling operators and scaling dimensions for both bulk and boundary conformal field theories. By utilizing the key relationship between the conductance tensor and ground-state correlation function, the universal conductance can be evaluated within the framework of boundary MERA. In particular, we study the Kane and Fisher fixed point of two interacting wires with an impurity. We demonstrate how to construct boundary MERA to estimate the current-current correlation function and scaling dimensions. We show that the universal behavior of the junction can be clearly identified within MERA. This show the grand potential of using boundary MERA to classify the fixed points of the general multiwire junctions. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W46.00002: ABSTRACT MOVED TO Q29.00011 |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W46.00003: Coupling localized spins with free fermions - A model for magnetic interfaces Rubem Mondaini, Thereza Paiva, Richard Scalettar The study of transport and magnetism in surfaces is a topic of intense research and with potential applications to several materials as manganites and $Cu/CuO$ interfaces. We study a model in which an insulating magnetic material described by a collection of localized spins couples to a metallic region. For this we introduce a stacking of antiferromagnetic spin planes on top of free-fermion planes. The interaction of the spins with the free fermions is tuned and several fermionic and spin observables are calculated in a vast region of temperatures. To obtain it, the phase space of spin configurations is spanned by a usual Metropolis algorithm and allows us to have exact values for fermionic quantities, both magnetic and transport ones, at each of the visited configurations. We observed that the increase of this interaction not only helps in localizing the fermions in the adjacent plane but turns the magnetic order in the localized spin plane more robust by increasing its critical temperature when Ising spins are considered. On the situation this coupling is large, the more distant fermionic planes start to lose connection with the rest of the system and the information regarding magnetic ordering is not propagated in the free region. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W46.00004: Phase transitions in the two-dimensional electron-hole gas Roman Kezerashvili, Oleg Berman, Klaus Ziegler For a gas of spatially separated electrons and holes with tunable Coulomb interaction and variable density a first order phase transition between a Bardeen-Cooper-Schrieffer (BCS) phase and an insulating Mott phase is predicted. The phase diagram is obtained in the framework of a BCS-like mean-field approach and a Landau expansion in terms of the pairing order parameter. This phase diagram indicates several phases and phase transitions, including an electron-hole plasma at low density and weak interaction, an intermediate BCS phase with Cooper pairs and an electron-hole plasma at high density and weak interaction. The insulating Mott phase appears for the strong interaction and low temperatures. The possibilities to realize these phases in realistic systems such as coupled quantum wells and graphene double layers are discussed. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W46.00005: Dynamical frustration versus kinetic enhancement with excitons in strongly correlated bilayers Louk Rademaker Recently the condensation of electron-hole pairs in semiconductor bilayers has been achieved. This has opened up the pursuit of exciton condensation in other layered materials. Here I will present recent theoretical work on exciton physics in complex oxide heterostructures. The poorly understood high temperature superconducting cuprates are ideal candidates for bilayer exciton condensation. Therefore we study the dynamics and the phase diagram of bilayer excitons in a Mott insulating p/n heterostructure, which shows rich exciton-spin interaction phenomena. I will discuss the dynamical frustration experienced by an exciton moving through an antiferromagnetic background. In sharp contrast, I will show how in the exciton superfluid phase the magnetic excitations 'borrow' kinetic energy from the excitons. References: L. Rademaker, K. Wu, H. Hilgenkamp and J. Zaanen, EPL 97, 27004 (2012); L. Rademaker, K. Wu and J. Zaanen, New Journ. of Phys. 14, 083040 (2012); L. Rademaker, J. van den Brink, H. Hilgenkamp and J. Zaanen, Phys. Rev. B 88, 121101(R) (2013). [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W46.00006: Entanglement Spectroscopy using Quantum Monte Carlo Chia-Min Chung, Lars Bonnes, Pochung Chen, Andreas L\"auchli We present a numerical scheme to reconstruct a subset of the entanglement spectrum of quantum many body systems using quantum Monte Carlo. The approach builds on the replica trick to evaluate particle number resolved traces of the first $n$ of powers of a reduced density matrix. From this information we reconstruct $n$ entanglement spectrum levels using a polynomial root solver. We illustrate the power and limitations of the method by an application to the extended Bose-Hubbard model in one dimension where we are able to resolve the quasi-degeneracy of the entanglement spectrum in the Haldane-Insulator phase. In general the method is able to reconstruct the largest few eigenvalues in each symmetry sector and typically performs better when the eigenvalues are not too different. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W46.00007: Dynamically Generated Gaps in Holographic Models of Interacting Fermions Garrett Vanacore, Philip Phillips Previous investigations have shown that a dynamical gap forms in the holographic theory dual to fermions interacting via a dipole (Pauli) coupling in an AdS$_{d+1}$-Reissner-N\"ordstrom background. We examine the analogous problem in an AdS$_{d+1}$-Schwarzschild geometry with a probe gauge field, finding that the gap persists. Our results suggest that bulk field interactions -- rather than emergent symmetries of the boundary theory -- are the origin of the gap. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W46.00008: RG Analysis on a Neck-Narrowing Lifshitz Transition in the Presence of Weak Short-Range Interactions in Two Dimensions Sedigh Ghamari, Sung-Sik Lee, Catherine Kallin We present a pertrbative renormalization group (RG) analysis for a neck-narrowing Lifshitz transition in the presence of weak short-range interactions in two dimensions. The model we examine is described by the dispersion $\varepsilon(\mathbf{k}) = k_x^2-k_y^2$, which would be at the critical point of the neck-narrowing transition at zero chemical potential, $\mu=0$. At the critical point ($\mu=0$), we find that one-loop quantum corrections to the interaction vertex are non-analytic. This makes capturing the evolution of the low-energy effective theory, as the energy cutoff is progressively lowered, in terms of $\beta$-functions for local operators impossible. Thus we conjecture that any consistent RG description at the critical point of this neck-narrowing transition will involve non-local operators. Slightly away from the critical point ($\mu > 0$), where the Fermi surface has a narrow neck, we find that the quantum corrections are analytic only over a finite momentum range, which shrinks to zero as the chemical potential approaches zero. More importantly, within this analytic range, where a local RG description is possible, we show that the narrower the width of the neck, the larger the couplings of irrelevant interactions become, leading to the breakdown of perturbative RG. [Preview Abstract] |
Session W47: Superfluid He4 and Other Quantum Liquids
Sponsoring Units: DCMPChair: Shaun Fisher, Lancaster University
Room: Mile High Ballroom 4F
Thursday, March 6, 2014 2:30PM - 2:42PM |
W47.00001: Pressure driven flows of superfluid helium-4 through a single nanopipe Angel Velasco, Crystal Yang, Zuzanna Siwy, Peter Taborek We have measured flow rates of helium-4 through single etched nanopores of 72 nm and sub-20 nm diameter in PET and mica respectively with a mass spectrometer. Flow rates were measured as a function of pressure at constant temperature and at saturated vapor pressures along the coexistence curve between 0.5 K and 3.0 K. Due to the constraint of the mass spectrometer the low pressure side was maintained at P$=$0 creating an intrinsic superfluid/vapor interface which forms inside the pipe or at its exit. We observed flow velocities in the range of 2-4 m/s in the low temperature region which is consistent with Feynman's critical velocity. Near the lambda point our temperature dependent critical velocity did not agree with the thermal vortex nucleation theory. The superfluid transition temperature was measured to be suppressed by 2-3 mK in the 72 nm nanopore. We have also measured flow rates in the normal state and found rates in exact agreement with conventional viscous theory with zero slip length. The results were also consistent with previous nanofluidic studies. Supported by NSF DMR-0907495. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W47.00002: The dynamic pair distribution function of superfluid $^{4}$He Souleymane Diallo, Wojciech Dmowski, Kostantin Lokshin, Georg Ehlers, Takeshi Egami We present precision neutron scattering measurements of the spatial and time correlations between atoms in liquid $^{4}$He using dynamic pair distribution methods. As the Bose-Einstein condensate (BEC) sets in below the superfluid transition temperature $T_{\lambda}$, we observe clear changes in the local environment of the atoms. These local changes are investigated beyond the first coordination shell. We also test our observations against recent classical and ab-initio molecular dynamics simulations in which the local configurational excitations in the atomic connectivity network was found to be the elementary excitation in some liquid metals. [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W47.00003: Towards the in-situ detection of a single He2* excimer in superfluid helium Faustin Carter, Scott Hertel, Michael Rooks, Daniel Prober, Daniel McKinsey Incident radiation can excite superfluid helium into a diatomic He2* excimer, which decays through the emission of a 15 eV photon. Such excimers have been used as tracers to measure the superfluid's quantum turbulence, thanks partly to the long half-life of the He2* triplet state (13 seconds). However, the efficient detection of these excimers remains a challenge. We present a detector capable of in-situ detection of the He2* excimers either directly (the excimer collides with the detector), or by collecting the 15 eV photon emission upon decay. This detector is based on a tungsten superconducting transition edge sensor and is designed to operate near 100 mK in a dilution refrigerator. We will discuss operating characteristics and present preliminary data with an aim towards the detection of a single excimer. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W47.00004: Flow visualization in superfluid helium-4 using a thin line of He$_{2}$ excimer tracers Alex Marakov, Jian Gao, Wei Guo, Steven Van Sciver, Gary Ihas, Daniel McKinsey, William Vinen Cryogenic flow visualization techniques have been proven in recent years to be a very powerful experimental method to study turbulence in superfluid helium-4 (He II). In order to extract quantitative information of the flow field, we developed a new technique based on the generation of a thin line of He$_{2}$ excimer tracers via femtosecond-laser field ionization. These tracers move solely with the normal-fluid component in He II and can be imaged using a laser-induce fluorescence technique. Studying the drift and distortion of the tracer line in a turbulent flow shall allow us to measure the instantaneous flow velocity field and hence determine the structure functions and the energy spectrum of the turbulence. We discuss the preliminary results obtained that for the first time visually reveal the existence of a laminar-to-turbulent transition in the normal fluid in thermal counterflow. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W47.00005: Localized Bose-Einstein Condensation in Liquid $^4$He in Disorder Henry R. Glyde, Jacques Bossy, Jacques Ollivier, Helmut Schober Liquid $^4$He in porous media is an example of Bosons confined to nanoscales and in disorder. In porous media, the superfluid phase is suppressed to low temperature. The smaller the pore size, the further the transition temperature to the superfluid phase, $T_c$, is suppressed below the bulk value, $T_\lambda$. In 28 A pore diameter FSM, for example, $T_c$ = 0.8 K compared with $T_\lambda$ = 2.17 K at saturated vapor pressure[1]. We present measurements of the phonon-roton (P-R) modes and of Bose-Einstein condensation (BEC) of liquid $^4$He in porous media. These measurements show that the ``critical" temperature of BEC in porous media, $T_{BEC}$, lies close to $T_\lambda$ largely independent of the porous media investigated. There is a temperature range, $T_c$ $< T <$ $T_{BEC}$, in which there is BEC but no superflow. This is interpreted as a ``localized" BEC (LBEC) region. The model of the LBEC liquid is isolated islands or blobs of BEC separated by otherwise normal liquid. The phase of each island is independent so that there is no phase coherence across the sample as needed for superflow. The P-R data and LBEC phase will be discussed.\\[4pt] [1] J. Taniguchi, Y. Aoki and M. Suzuki, Phys. Rev. B{\bf 82}, 104509 (2010). [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W47.00006: Localized Bose-Einstein Condensation in films of Liquid $^4$He in Disorder Jacques Bossy, Jacques Ollivier, Helmut Schober, Henry R. Glyde When porous media is only partially filled, the helium is deposited as films on the porous media walls. The initial helium is tightly bound on the media walls, denoted ``dead layers". In subsequent fillings there is a transition to a superfluid phase below a critical temperature $T_c$ and $T_c$ increases with increasing filling. We present measurements of phonon-roton modes in liquid $^4$He films on 25 A diameter gelsil at fillings greater than 70\%. P-R modes are observed at low temperature and up to a maximum temperature denoted $T_{PR}$ which also increases with filling. Above $T_{PR}$ well-defined P-R modes are not observed. Since well-defined P-R modes exist where there is Bose-Einstein condensation (BEC), $T_{PR}$ is associated with $T_{BEC}$, the critical temperature for BEC in films. $T_{PR}$ lies above $T_c$ observed by Yamamoto et al. (Phys. Rev. Lett. 93, 075302 (2004)) in the same gelsil. There is a temperature range $T_c$ $ < T <$ $T_{BEC}$ where there is BEC but no superflow. This is identified as a ``localized" BEC region in which the BEC exists in islands or blobs separated by normal liquid. The P-R mode data and LBEC region will be discussed. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W47.00007: Third Sound Measurements of Superfluid $^4$He Films on Multiwall Carbon Nanotubes Below 1 K Emin Menachekanian, Vito Iaia, Andrew Li, Bob Chen, Gary Williams Third sound is studied for superfluid films of $^4$He adsorbed on multiwall carbon nanotubes of average diameter 12 Angstroms packed into an annular resonator. The third sound is generated with mechanical oscillation of the cell, and detected with carbon bolometers. A filling curve at temperatures near 250 mK shows oscillations in the third sound velocity, with maxima at the completion of the third and fourth atomic layers. The ``dead'' layer appears to be close to two atomic layers, about one layer thinner than previously found for flat graphite surfaces. We attribute this weaker binding to the effect of the cylindrical geometry on the van der Waals potential, the repulsive surface tension forces from the high curvature, and the lower density of the tubes compared to graphite. At the completion of the third layer there is a sudden reduction of the superfluid onset temperature, and then a recovery back to the Kosterlitz-Thouless linear dependence, forming re-entrant superfluidity. In a small region around 2.5 layers there is very anomalous behavior in the low-temperature variation of the third sound velocity, which is found to increase linearly with temperature. This could be related to changes in the gas-liquid coexistence at this intermediate fill. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W47.00008: Non-universal Casimir Effect in Saturated Superfluid $^4$He Films at T$_\lambda$ John Abraham, Gary Williams, Konstantin Penanen Measurements of Casimir effects in $^4$He films in the vicinity of the bulk superfluid transition temperature $T_\lambda$ have been carried out, where changes in the film thickness and the superfluid density are both monitored as a function of temperature. A new Casimir film-thickening effect is observed precisely at $T_\lambda$ when the temperature is swept extremely slowly. We believe this arises from the viscous suppression of any second sound modes in the superfluid film, while thermally excited second sound still propagates in the bulk superfluid to within microkelvins of $T_\lambda$, giving rise to a free energy difference between the bulk and film. At $T_\lambda$ this difference drops abruptly to zero, leading to a step increase in the film thickness that we have observed. The magnitude of the step increases rapidly with the equilibrium film thickness, in agreement with a calculation of the Casimir energy balance. From the amplitude of the increase we can extract the first measurement of the second-sound free energy at $T_\lambda$, found to be about 2.6 ergs/cc. This is at least roughly consistent with a Debye-type calculation of the free energy. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W47.00009: Dimensional crossover in quantum fluids Adrian Del Maestro, Bohdan Kulchytskyy, Guillaume Gervais In one spatial dimension it is not possible to break a continuous symmetry due to strong fluctuations, even at zero temperature. Although fermionic examples of quasi-one-dimensional systems abound, including carbon nanotubes and quantum wires, it is much more difficult to reach this limit in high density bosonic fluids due to a short coherence length on the atomic scale. Recent advances in nanofabrication techniques now allow for the confinement of helium-4 inside pores with nanometer radius, but unequivocal experimental evidence of one-dimensional behavior is still lacking. We have performed large scale quantum Monte Carlo simulations near the superfluid transition and have investigated how the signatures of dimensional crossover are reflected in the thermodynamic properties of nano-confined helium-4. The results hint at the emergence of quantum hydrodynamics inside the pore at length scales that can be probed in the laboratory. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W47.00010: Observation of quantum decay of homogeneous, isotropic (grid) turbulence Gary Ihas, Lydia Munday, Jihee Yang, Kyle Thompson, Wei Guo, Roman Chapurin, Shaun Fisher, Peter McClintock, W.F. Vinen In classical grid turbulence fluid is forced through a stationary grid. In the quantum case a grid moves through an initially stationary superfluid driven by a linear motor. We have developed a motor using superconducting drive coils and bearings, moving a grid at constant speed (0 and 15 cm/s). Stalp \textit{et al}\footnote{S. R. Stalp, L. Skrbek, and R. J. Donnelly, \textit{Phys. Rev. Lett}. \textbf{\textit{82}}, \textit{4831} (\textit{1999}).} report the decay of vortex-line density $L$ in the grid's wake measured by 2$^{\mathrm{nd}}$ sound attenuation. $L$ decayed at large times as $t^{-3/2}$, interpreted as a quasi-classical Richardson cascade of energy-containing eddies size limited by channel width, associated with a Kolmogorov energy spectrum. It is assumed eddies produced on a scale of the grid mesh grow through the classical fluids mechanism.\footnote{P.A. Davidson, \textit{Turbulence}, Oxford Univ. Press., UK (2004).} We can now test a semi-quantitative theory with different mesh grids or channel sizes, relating to the possible existence of inverse turbulent cascades. Our 2$^{\mathrm{nd}}$ sound system is conventional, but with a novel phase and amplitude feedback loop making stringent constant temperature unnecessary. Both $t^{-3/2}$ and non-$ t^{-3/2}$ decays have been observed with 2 mesh sizes. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W47.00011: Fluid Flow and Depinning of a Metastable Vortex Rena Zieve, Emily Hemingway, Ingrid Neumann We observe a vortex line pinned around a straight wire in superfluid helium, in the absence of external rotation. Unperturbed, the vortex line remains pinned indefinitely, but we can partially detach the vortex by heating the fluid in brief stints. Here we show that a key property of the heating cycle is the maximum rate of change of the temperature, which suggests that the fluid velocity generated by a temperature gradient plays an important role in the depinning. Our measurements of how depinning depends on both maximum temperature and velocity show a crossover from a velocity-dependent depinning temperature at slow heating rates to a velocity-independent temperature near 1.2 K for faster heating. We discuss how vortex loops generated by the thermal fluid flow may be responsible for the depinning. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W47.00012: Incompressible Quantum Glass State of Bosons in Two Dimensions Wenan Guo, Yancheng Wang, Anders Sandvik We study the quantum glass state intervening between the conventional superfluid and Mott-insulator states of lattice bosons with random potentials at average filling $\rho=1$. Its properties at temperature $T=0$ are controlled by rare large regions of superfluid surrounded by Mott insulator. These regions make the state gapless although it is insulating. Contrary to the commonly accepted theory of this state in two dimensions, we show here that a vanishing gap does not necessarily imply nonzero compressibility. Using quantum Monte Carlo simulations of the Bose-Hubbard model and a percolation theory, we show that the compressibility $\kappa$ follows the form $\kappa \sim {\rm exp}(-b/T^\alpha)$ with $\alpha <1$. In addition, the dynamic exponent of the superfluid-quantum glass transition is found to be smaller than 2. The system is, thus, incompressible at $T=0$ and should be classified as a Mott glass. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W47.00013: Dynamics of the Direct Vortex Cascade in 2D Quantum Turbulence Gary Williams, Andrew Forrester The growth and decay of the direct vortex cascade in 2D quantum turbulence is studied for different rates of injection of vortex pairs of a given separation (the stirring scale), with the system in contact with a heat bath at low temperature. At low injection rates the vortices have no effect on the superfluidity, and the distribution of pair separations spreads out until reaching the steady-state distribution of the direct cascade, where pairs annihilate at the same rate they are injected. This cascade has a k$^-3$ energy spectrum, the same spectrum as the direct enstrophy cascade in 2D classical turbulence. On switching off the injection, the pair distribution first decays starting from the initial stirring scale, with the total vortex density decreasing linearly in time. As pairs at smaller scales decay, the vortex density then falls off as a power law, the same power law found in recent exact solutions of quenched 2D superfluids. At high injection rates the large density of vortices drives the superfluid density to zero at long length scales, and the growth and decay of the cascade is found to be much slower for this case. [Preview Abstract] |
Session W48: Invited Session: Exotic Phase Transitions in 5d Compounds
Sponsoring Units: DCMP GMAGChair: Sang-Wook Cheong, Rutgers University
Room: Mile High Ballroom 1A-1B
Thursday, March 6, 2014 2:30PM - 3:06PM |
W48.00001: Exotic magnetism of Jeff$=$1/2 iso-spins in complex Ir oxides Invited Speaker: Hide Takagi In 5d Iridium oxides, a large spin-orbit coupling of $\sim$ 0.5 eV, inherent to heavy 5d elements, is not small as compared with other relevant electronic parameters including Coulomb U, transfer t and crystal field splitting D, which gives rise to a variety of exotic magnetic ground states. In the layered perovskite Sr$_{2}$IrO$_{4}$, spin-orbital Mott state with Jeff$=$1/2 is realized due to the novel interplay of those energy scales [1, 2]. Despite the strong entanglement of spin and orbital degrees of freedom, surprisingly isotropic, two-dimensional Heisenberg character of $J_{\mathrm{eff}}=$1/2 iso-spins was observed in Sr$_{2}$IrO$_{4}$ with 180 deg Ir-O-Ir bonds, by the recent resonant magnetic x-ray diffuse scattering and the magnetic susceptibility measurements [3]. Complex Na-Ir oxides with honeycomb and more recently identified \textit{hyper-honeycomb} lattices, where 90 deg Ir-O-Ir bonds are realized, are candidates for Kitaev spin liquid. Such exotic magnetism was recently shown to be tailored using super-lattice structure [4] In this talk, we review these unique magnetic phases in Ir oxides. \\[4pt] [1] B. J. Kim et al., \textit{Phys. Rev. Lett. }\textbf{101}, 076402 (2008).\\[0pt] [2] B. J. Kim, H. Ohsumi, T. Komesu, S. Sakai, T. Morita, H. Takagi, and T. Arima, \textit{Science} 323, 1329 (2009). \\[0pt] [3] S. Fujiyama\textit{ et al.} , \textit{Phys. Rev. Lett.} \textbf{108}, 247212 (2012).\\[0pt] [4] J. Matsuno \textit{et al.}, submitted. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:42PM |
W48.00002: Slater and Mott Insulating States in Os- and Ir-Based Transitional Metal Oxides Invited Speaker: A.D. Christianson The discovery of a novel J$_{\mathrm{eff}}=$1/2 electronic configuration and spin-orbit assisted insulating state in Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$ has stimulated a fresh look at metal-insulator transitions where relativistic effects participate on an even footing with other energy scales such as crystal field splitting and electron-electron correlations. There are several view points on the origin of the insulating state in Sr$_{2}$IrO$_{4}$, but the most prominent is that spin-orbit coupling modifies the electronic configuration such that a Mott insulting state emerges despite the relatively modest electron-electron correlations within the 5$d$ orbitals. An alternative viewpoint is that magnetic effects enable the opening of the electronic gap giving rise to the insulating state or a Slater metal-insulator transition. Here we describe realizations of both Mott and Slater insulators in the context of Os- and Ir-based 5$d$ transition metal oxides. NaOsO$_{3}$, exhibits a continuous phase transition at 410 K where antiferromagnetism appears in conjunction with the onset of insulating behavior. A combination of neutron diffraction and magnetic resonant x-ray scattering enables the conclusion that G-type magnetic order occurs at the metal-insulator transition providing microscopic evidence that NaOsO$_{3}$ is the first three dimensional realization of a Slater insulator. On the other hand we have probed the robustness of the J$_{\mathrm{eff}}=$1/2 Mott insulating state though studies of Sr$_{2}$Ir$_{\mathrm{1-x}}$T$_{\mathrm{x}}$O$_{4}$ (T$=$Mn, Ru). For both Mn and Ru doping we find that despite qualitative changes in the magnetic order the J$_{\mathrm{eff}}=$1/2 electronic configuration remains robust. In particular, for Ru-doping the signatures of the J$_{\mathrm{eff}}=$1/2 state are observed for all concentrations where magnetic order is present. Finally, we have investigated Ca$_{4}$IrO$_{6}$ which appears to exhibit a nearly ideal J$_{\mathrm{eff}}=$1/2 state which is unperturbed by deviations from cubic crystal field level splitting. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 4:18PM |
W48.00003: Novel phase transitions in iridium dichalcogenides Invited Speaker: Yoon Seok Oh 5d transition metal oxides has attracted lots of attention because of exotic electronic phase resulted from entanglement of strong spin-orbit coupling and electron correlation in 5d orbital. In this manner, 5d transition metal chalcogenides is another intriguing 5d compound to have a rich variety of strongly correlated electronic states. In fact, recent studies of IrTe$_{2}$ reported chemical-doping/intercalation (Pd, Pt, Cu, and Rh) induced superconductivity and the unconventional structural modulations below $\sim$260 K. The simple empirical features of IrTe$_{2}$ resemble the conventional charge density waves (CDW) in the 3d/4d layered chalcogenides (e.g. 1T-TaS$_{2}$, and 1T-TiSe$_{2}$, etc.). But, recent corroborative experimental results indicate that instability of covalency of Ir ions induces the structural phase transition associated with soliton lattice of Te-Te covalent bonding. So far, there exist controversy to identify the exotic phase transition of IrTe$_{2}$. In this talk, we introduce recent investigations and discuss the phase transition in IrTe$_{2}$. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:54PM |
W48.00004: Excitations, order, and criticality in quantum pyrochlores Invited Speaker: Lucile Savary I will present recent work on quantum criticality in the (conducting) pyrochlore iridates in the context of the wide range of exotic phenomena that occur on the pyrochlore lattice, such as Coulombic quantum spin liquids and quantum order-by-disorder. The physics of the highly-unusual super-universal quantum critical point between a non-Fermi liquid and a Weyl semimetal (with Ising-like order) uncovered in a model relevant to the pyrochlore iridates will be discussed in detail. There, the fluctuations in the parent non-Fermi liquid phase compete with the fluctuations due to the coupling to the Ising order parameter. Remarkably, the fluctuations of both origins are of the same order of magnitude and the resulting quantum critical regime belongs to a unique, very large, universality class, genuinely different from those obtained by considering the effects of a single phenomenon. Moreover, the perturbative analysis is controlled, yielding better faith in the theory. The scaling laws and some unusual coefficients of many physical quantities will be discussed, and a scheme to observe the quantum critical point in experiment provided. Further experimental connections, in particular to Pr$_2$Ir$_2$O$_7$, where Pr spins are important, will be made. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:30PM |
W48.00005: Quantum fluctuations in spin-ice-like Pr$_{2}$Zr$_{2}$O$_{7}$ Invited Speaker: Kenta Kimura Spin ice is a classical frustrated magnet in which ferromagnetic dipolar interactions stabilize a frozen disordered state with Pauling residual entropy and emergent magnetic monopolar quasi-particles [1]. A new class of spin ice has been recently proposed for Pr and Yb pyrochlores, in which quantum-mechanical exchange interactions predominately provides the ferromagnetic coupling between neighboring spins. The resultant strong quantum fluctuations were found to generate exotic quantum magnetism. For example, it has been demonstrated that the Pr-based metallic pyrochlore Pr$_{2}$Ir$_{2}$O$_{7}$ shows a novel chiral spin liquid phase, which may be explained by quantum melting of spin ice. In the talk, we report our recent experimental results on magnetic properties of the insulating analog Pr$_{2}$Zr$_{2}$O$_{7}$ [2]. Pinch-point features in quasi-elastic diffuse neutron scattering reflects adherence to a divergence free local constraint for disordered spins on long time scales. In sharp contrast to conventional ice, however, more than 90{\%} of the neutron scattering is inelastic and devoid of pinch points furnishing evidence for magnetic monopolar quantum fluctuations. \\[4pt] [1] S. T. Bramwell and M. J. P. Gingras, Science 294, 5546 (2001).\\[0pt] [2] K. Kimura \textit{et al}., Nat. Commun. 4, 1934-1-6 (2013). [Preview Abstract] |
Session W49: Focus Session: Titanate Interfaces, Layered Materials
Sponsoring Units: DMPChair: Emilio Artacho, CIC nanoGUNE, Spain
Room: Mile High Ballroom 1C
Thursday, March 6, 2014 2:30PM - 2:42PM |
W49.00001: Induced ferromagnetism and antiferromagnetism in perovskite quantum wells Clayton Jackson, Jack Zhang, Susanne Stemmer We report on induced magnetism in thin SrTiO$_{3}$ quantum wells embedded in ferrimagnetic GdTiO$_{3}$ and antiferromagnetic SmTiO$_{3}$, respectively. The SrTiO$_{3}$ quantum wells contain a high density of mobile electrons (7x10$^{14}$ cm$^{-2})$. We show that the longitudinal and transverse magnetoresistance in the structures with GdTiO$_{3}$ are consistent with anisotropic magnetoresistance, and thus indicative of induced ferromagnetism in the SrTiO$_{3}$. Measurements of the sheet and Hall resistances as a function of temperature in the structures with SmTiO$_{3}$ are consistent with two-dimensional itinerant antiferromagnetism induced in the SrTiO$_{3}$ layer as a result of the confinement of an extreme charge density coupled with proximity affects from the neighboring SmTiO$_{3}$. The studies show that the properties of thin SrTiO$_{3}$ quantum wells can be tuned to obtain magnetic states that do not exist in the bulk material. [Preview Abstract] |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W49.00002: Itinerant electron ferromagnetism at GdTiO$_{3}$/SrTiO$_{3}$ heterostructure Ming Xie, Allan MacDonald Interfaces between perovskite oxides have known to support conducting two dimensional electron gases (2DEGs) even when the parent materials are insulators. When one of the insulating parent materials is magnetic, the magnetism is inherited by the 2DEG. We will discuss the itinerant-electron magnetic 2DEGs which occur at GdTiO$_{3}$/SrTiO$_{3}$ interfaces in which the 2DEG resides on the otherwise non-magnetic SrTiO$_{3}$ side of the interface. Experimental studies [1,2] have shown magnetoresistance effects that are characteristic of itinerant electron ferromagnets, but the origin of the magnetic coupling is still not resolved. In this study, we propose that the ferromagnetic state of the 2DEG originates from exchange coupling between 2DEG electrons and electrons localized on GdTiO$_{3}$ Ti sites near the interface. We develop a tight binding model that accounts for this coupling and its relation to structural distortion of GdTiO$_{3}$ near the interface, and address its influence on 2DEG properties. \\[4pt] [1] P. Moetakef, J. R. Williams, D. G. Ouellette, A. P. Kajdos, D. Goldhaber-Gordon, S. J. Allen, and S. Stemmer, Phys. Rev. X 2, 021014 (2012). \\[0pt] [2] C. A. Jackson and S. Stemmer, arXiv:1311.0337 [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W49.00003: Evolution of the electronic structure of SrTiO$_3$/GdTiO$_3$ heterostructures with layer thickness Lars Bjaalie, Anderson Janotti, Chris G. Van de Walle A two-dimensional electron gas (2DEG), with density of 3e14cm$^{-2}$ (0.5 electrons per interface unit cell), has been observed at the SrTiO$_3$/GdTiO$_3$ interface, with potential applications in electronic devices [P. Moetakef, T.A. Cain, D.G. Ouellette, J.Y. Zhang, D.O. Klenov, A. Janotti, C.G. Van de Walle, S. Rajan, S.J. Allen, and S. Stemmer, Appl. Phys. Lett. 99, 232116 (2011)]. Yet, basic properties of the 2DEG is still poorly understood, in particular the variation of the electrical conductivity with the SrTiO$_3$ layer thickness. We performed density functional calculations with a hybrid functional to study the electronic structure of SrTiO$_3$/GdTiO$_3$ superlattices. We address the insulator to metal transition as a function of layer thickness, analyzing the effects of quantum confinement, charge ordering, and lattice distortions. Work supported by NSF and ARO. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W49.00004: Metal-insulator transitions in GdTiO$_3$/SrTiO$_3$ superlattices Se Young Park, Andrew Millis The density functional plus U (DFT+U) method is used to obtain electronic structures and metal-insulator phase diagrams of metal-insulator transition of (001) (GdTiO$_3$)$_m$/(SrTiO$_3$)$_n$ superlattices. In metallic phases, the mobile electrons are found in the SrTiO$_3$ layers, with near-interface electrons occupying $xy$-derived bands, while away from the interface the majority of electrons reside in $xz/yz$ bands. As the thickness $n$ of the SrTiO$_3$ layers decreases a metal-insulator transition occurs. Two insulating phases are found. At $n=1$ the hybridization of two TiO$_2$ layers across the SrO layer leads to a dimerized insulating state as previously proposed\footnote{Ru Chen, SungBin Lee, and Leon Balents, Phys. Rev. B {\bf 87}, 161119(R) (2013).} with relatively small U$_c \sim 2.5$ eV. For $n>1$ we find that insulating phases occur together with checkerboard charge and orbital ordering and variation of Ti-O bonds at a larger U$_c\sim3.5$eV. Inconsistencies with experiment suggest that many-body correlations are important. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W49.00005: Polarized Infrared Response of Subband Transitions in High Density 2DEG in GdTiO$_{3}$/SrTiO$_{3}$ Interfaces Bill Flaherty, Daniel Ouellette, Pouya Moetakef, Clayton Jackson, Susanne Stemmer, S. James Allen The 2-D electron gas at the interface between GdTiO$_{3}$ and SrTiO$_{3}$ layers has an electron density comparable to 3.4 x 10$^{14}$ cm$^{-2}$ per interface with potential applications for tunable plasmonic devices. Experiments are currently underway to measure the infrared response of this electron gas, with infrared electric fields perpendicular to the interface as well as parallel. The former may provide insight into the electric subband states. Using angle-resolved Fourier transform infrared spectroscopy with s- and p-polarized beams, we can compare the in- and out-of-plane response of the 2DEG. Normalizing it against the response of the bare substrate will allow us to extract the 2DEG contribution. These results will be compared to those predicted by Park and Millis, Phys. Rev. B87, 205145 (2013). Results to date display in-plane but little out-of-plane response. We will look at various GTO/STO interfaces, such as single interfaces and superlattices of alternating layers. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W49.00006: Electronic and structural reconstruction in titanate heterostructures from first principles Andrew T. Mulder, Craig J. Fennie Recent advances in transition metal oxide heterostructures have opened new routes to create materials with novel functionalities and properties. One direction has been to combine a Mott insulating perovskite with an electronic d$^1$ configuration, such as LaTiO$_3$, with a band insulating d$^0$ perovskite, such as SrTiO$_3$. An exciting recent development is the demonstration of interfacial conductivity in GdTiO$_3$/SrTiO$_3$ heterostructures that display a complex structural motif of octahedral rotations and ferromagnetic properties similar to bulk GdTiO$_3$. In this talk we present our first principles investigation of the interplay of structural, electronic, magnetic, and orbital degrees of freedom for a wide range of d$^1$/d$^0$ titanate heterostructures. We find evidence for both rotation driven ferroelectricity and a symmetry breaking electronic reconstruction with a concomitant structural distortion at the interface. We argue that these materials represent an ideal platform to realize novel functionalities such as the electric field control of electronic and magnetic properties. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W49.00007: Stoichiometry-Control of Electronic Transport at Complex Oxide Interface Peng Xu, Bharat Jalan Employing the hybrid molecular beam epitaxy approach to grow NdTiO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ heterostructures - a polar/nonpolar system sharing many similarities with LaAlO$_{\mathrm{3}}$/SrTiO$_{\mathrm{3}}$ with an added functionality of NdTiO$_{\mathrm{3}}$ being an antiferromagnetic Mott insulator- we will present a detailed film growth and transport study as a function of cation stoichiometry in NdTiO$_{\mathrm{3}}$. Irrespective of the cation stoichiometry (measured by high resolution x-ray diffraction and x-ray photoelectron spectroscopy), films grew in an atomic layer-by-layer fashion as evidenced by the reflection high-energy electron diffraction intensity oscillations, and films showed a temperature dependent metal-to-insulator (M-I) type behavior. Remarkably, T$_{\mathrm{MI}}$ was found to increase irrespective of whether films were Nd- or Ti-rich. Furthermore, hall measurement of a 3.5 nm NdTiO$_{\mathrm{3}}$ film grown on 3 nm SrTiO$_{\mathrm{3}}$ layer on LSAT substrate revealed n type carrier density, 3 x 10$^{\mathrm{14}}$ cm$^{\mathrm{-2}}$ for stoichiometric samples, which would be consistent with the interface conduction due to an interfacial polar discontinuity effect. Using detailed temperature dependent magneto-transport measurements, we will present a comprehensive study of correlation between film stoichiometry, interface conduction, and transport mechanisms. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W49.00008: Growth and Transport Studies of LaTiO$_{3}$ / KTaO$_{3}$ Heterostructures K. Zou, F.J. Walker, C.H. Ahn Perovskite oxide heterostructures provide a rich platform for exploring emergent electronic properties, such as 2D electron gases (2DEGs) at interfaces. In this talk, we present results on the growth of LaTiO$_{3}$ / KTaO$_{3}$ heterostructures by molecular beam epitaxy and subsequent measurements of transport properties. Although both oxide materials are insulating in the bulk, metallic conduction is observed from T = 2 - 300 K. We achieve a room temperature carrier mobility of $\sim$ 25 cm$^{2}$ /Vs at a carrier density of $\sim$ 10$^{14}$ /cm$^{2}$. By comparison, 2DEGs in LaTiO$_{3}$ / SrTiO$_{3}$ and LaAlO$_{3}$ / SrTiO$_{3}$ have lower carrier mobility, but the same carrier density. We attribute some of the increase in mobility to the smaller band effective mass of the Ta 4d electrons compared to the Ti 3d electrons. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W49.00009: Two-dimensional electron gas in tricolor oxide interfaces Yanwei Cao, Michael Kareev, Xiaoran Liu, Srimanta Middey, Derek Meyers, Jak Tchakhalian Understanding and manipulating spin of electrons in nanometer scale is the main challenge of current spintronics, recent emergent two-dimensional electron gas in oxide interface provides a good platform to investigate the spin behavior by covering an insulating magnetic oxide layer. In this work, take titanates as an example, ultra-thin tricolor (tri-compound) titanate superlattices ([LaTiO3/SrTiO3/YTiO3]) were grown in a layer-by-layer way by pulsed laser deposition. High sample quality and their electronic structures were characterized by the combination of in-situ photoelectron and ex-situ structure and surface morphology probes. Temperature-dependent sheet resistance indicates the presence of metallic interfaces in both [LaTiO3 /SrTiO3 ] and all the tricolor structures, whereas a [YTiO3 /SrTiO3] bi-layer shows insulating behavior. The tricolor titanate superlattices provide an opportunity to induce tunable spin-polarization into the two-dimensional electron gas (2DEG) with Mott carriers. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W49.00010: Controllable strain fields in multimonolayer 2D-layered TiO2 (110) crystals studied by STM Zhisheng Li, Denis Potapenko, Richard Osgood Strain of crystal lattice can change the electronic property of materials, such as oxides and semiconductors, significantly. However, experimental studies of lattice effects in oxides are limited especially in atomic scale, due to the difficulty of generating strain field experimentally. In this work, we generate a strain field in multiple monolayer sample of at TiO2 (110) by very low energy bombardment of single crystal TiO2 samples with argon ions at 1000$^{\mathrm{o}}$C. The interstitial argon diffuses so as to form nanometer scale regions of local exfoliated TiO2 layers. These layers retain their unstressed surface reconstruction although the top-most surface layers have a convex morphology. We use STM studies along with a continuum model to show the strain field. Our studies also show that the strained surface layers are free of oxygen vacancies and that the adsorption energy of hydrogen is altered by the local strain field. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W49.00011: The effect of hetero-structure material MoS$_{2}$-TiO$_{2}$(110) on CO and NO adsorption: insights from \textit{ab-initio} calculations Takat Rawal, Duy Le, Talat Rahman Using first-principles simulation based on the density functional theory, we show the effect of substrate on the adsorption of small gas molecules (CO and NO) on molybdenum disulfide (MoS$_{2})$ by investigating the adsorption on bare MoS$_{2}$ and on MoS$_{2}$-TiO$_{2}$(110) systems. First, our results show that MoS$_{2}$ binds to the rutile TiO$_{2}$ surface by forming bonds between unsaturated edge sulfur atoms of MoS$_{2}$ with both bridge oxygen atoms and five-fold titanium atoms of TiO$_{2}$. Second, results from structural optimizations show that CO prefers to adsorb on ($\bar{1}$010) edge (S-edge) of bare MoS$_{2}$ but on (10$\bar{1}$0) edge (Mo-edge) of MoS$_{2}$ when coupled to the TiO$_{2}$(110) surface. Third, results from Bader analysis indicate a very small difference in charge transfer (i.e. 0.01e) to CO molecule from these two systems. We also present detailed analysis of the electronic density of states and the charge density of adsorbate-substrate systems to explain the effect of substrates on the adsorption of CO. We compare and contrast the adsorption characteristics of NO with those of CO on these surfaces. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W49.00012: First Principles Study of Monolayer MoS$_{2}$ with Defects and Vacancy Yingye Gan, Huijuan Zhao Unlike graphene and silicene, monolayer molybdenum disulfide (MoS$_{2}$) is a direct band gap transition metal with interesting electrical, mechanical, and optical properties. As a potential material in NEMS application, it is necessary to systematically study the defect effects to the material properties of MoS$_{2}$ under strain tuning. We will perform ab initio density functional theory based calculations to study the mechanical and electronic property variation of MoS$_{2}$ with different vacancy types and vacancy densities, such as Young's modulus, Poisson's ratio, fracture strength and band gap. The failure mechanism under various strain conditions will be investigated through the phonon dispersion curves. We expect to elucidate the relation between material properties of MoS$_{2}$ with strain tuning and defect tuning. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W49.00013: Direct imaging of band profile in single layer MoS2 on graphite: metallic edge states and the lateral Schottky barrier Chendong Zhang, Chang-Lung Hsu, Yong-Huang Chang, Lain-Jong Li, Chih-Kang Shih Recently, single layer (SL) Transition metal dichalcogenides MX$_{2}$ has attracted intense interests as the band structures change from indirect to direct gap. In addition, the valley degeneracy is also lifted in SL MX$_{2}$. These properties have important implications in nanoelectronics and optoelectronics. The SL MX$_{2}$ islands often come with a triangular form with straight edges and it has been shown theoretically these are zig-zag edge with metallic states. Here we use scanning tunneling microscopy/spectroscopy (STM/S) to map out the electronic structure of single layer MoS$_{2}$ grown on HOPG (highly oriented pyrolytic graphite) using CVD. In the region away from the edge, the MoS$_{2}$ band profile shows a homogeneous band gap of about 1.95 $\pm$ 0.1 eV, consistent with the optical studies before. Moreover, the Fermi level locates at 0.15 $\pm$ 0.05 eV below the conduction band minimum (CBM), confirming its n-type nature. The band profile is bend upward by about 0.5 eV within 5 nm from the edge. At the edge, the metallic nature is observed from finite conductivity in the gap region. This study shows that the bulk SL MoS$_{2}$ and its metallic edge formed a lateral Schottky barrier with a narrow depletion region of 5 nm and the Fermi level is pinned at 0.65 eV below the CBM. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W49.00014: Imaginary geometric phases of quantum trajectories in high-order terahertz sideband generation Fan Yang, Ren-Bao Liu Quantum evolution of particles under strong fields can be described by a small number of quantum trajectories that satisfy the stationary phase condition in the Dirac-Feynmann path integral. The quantum trajectories are the key concept to understand the high-order terahertz siedeband generation (HSG) in semiconductors [1]. Due to the nontrivial ``vacuum'' states of band materials, the quantum trajectories of optically excited electron-hole pairs in semiconductors can accumulate geometric phases under the driving of an elliptically polarized THz field [2]. We find that the geometric phase of the stationary trajectory is generally complex with both real and imaginary parts. In monolayer MoS2, the imaginary parts of the geometric phase leads to a changing of the polarization ellipticity of the sideband. We further show that the imaginary part originates from the quantum interference of many trajectories with different phases. Thus the observation of the polarization ellipticity of the sideband shall be a good indication of the quantum nature of the stationary trajectory.\\[4pt] [1] B. Zaks, R. B. Liu, and M. S. Sherwin, Nature 483, 580 (2012).\\[0pt] [2] F. Yang and R.-B. Liu, arXiv:1211.3021. [Preview Abstract] |
Session W50: Focus Session: Mesoscopic Materials and Devices III
Sponsoring Units: DMPChair: Ivan Schuller, University of California, San Diego
Room: Mile High Ballroom 1D
Thursday, March 6, 2014 2:30PM - 3:06PM |
W50.00001: Low Frequency Noise in Mesoscopic Magnetic Dots Invited Speaker: E. Dan Dahlberg Measurements of random telegraph noise (RTN) in individual mesoscopic sized NiFe alloy dots will be presented; the dots dimensions are as small as 200nm x 200nm x 10nm. The temperature and magnetic field dependence of the RTN are explained by the energy landscape in the dots; the energy landscape RTN was independently measured [Appl. Phys. Lett. 103, 042409 (2013)]. The research was motivated by questions raised in understanding magnetic noise in magnetic tunnel junctions and giant magnetoresistance devices [Appl. Phys. Lett. 95, 062512 (2009) and Phys. Rev. B 88, 014409 (2013)]. This work was supported primarily by ONR Grant N00014-11-1-0850 and the MRSEC Program of the NSF under Grant No. DMR-0819885. Additional support for work done using the University of Minnesota Nanofabrication Center and Characterization Facility was provided by the NSF NNIN network. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W50.00002: Tuning the gate efficiency in epitaxially grown InGaAs-InAs heterostructures C.J. Palmstr{\O}m, J. Shabani Fabrication of gate-defined devices on epitaxially grown heterostructures containing InAs layers is highly desirable as it offers the possibility of tuning the confinement potential, carrier density and spin orbit coupling. However, reliable gating has proven difficult in these materials due to gate leakage and hysteretic behavior. In addition, charge traps and Fermi level pinning could screen the applied electric field and significantly reduce the gate efficiency. In this work, we have studied the effect of surface gating on epitaxially grown In$_{0.75}$Ga$_{0.25}$As-InAs-In$_{0.75}$Ga$_{0.25}$As quantum wells. We find that the application of the gate bias barely changes the carrier density and the efficiency of the gate to be poor. However when a positive voltage is applied to the gate during cool down, the gate efficiency is improved. Furthermore, the change in density as a function of gate bias becomes linear and the slope matches closely to the simple capacitance model. We have also fabricated a quantum point contact using a split gate design on a similar structure and achieved full depletion under the gates. The conductance plot as a function of side gate voltages shows quantized plateaus reminiscent of ballistic one-dimensional transport. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W50.00003: Quantum transport in a single quantum wire fabricated on epitaxially grown InGaAs-InAs heterostructures J. Shabani, Y. Kim, R.M. Lutchyn, C. Nayak, C.J. Palmstr{\O}m One-dimensional semiconducting quantum wires with strong spin orbit interaction represent a unique platform for realization of exotic topological states of matter such as Majorana fermions and novel spintronic devices. Self-assembled nanowires have shown great promise in providing a testbed for one-dimensional experiments. However, controlled assembly of nanowires for scaling and building complex architectures will be challenging. Molecular beam epitaxy (MBE) growth of large area two-dimensional systems combined with semiconductor processing could provide a venue to overcome these issues. In this work, we have studied quantum transport in a single quantum wire fabricated on MBE grown InGaAs-InAs two dimensional electron systems. The magneto-conductance measurements show a clear weak anti localization (WAL) peak and conductance fluctuation at low magnetic field. Further we show that the spin orbit interaction in this quantum wire can be controlled by changing the confinement potential using an external top gate. Quantitative analysis of measured WAL peaks using one-dimensional theoretical model shows an excellent agreement between theory and experiment. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W50.00004: Proximity effect in a Nb-InAs-Nb nanowire junction Jonathan Baugh, Kaveh Gharavi, Greg Holloway, Chris Haapamaki, Ray R. LaPierre Proximity effect superconductivity in semiconductor-superconductor hybrid devices contains rich physics and could be key to the realization of topological quantum information processing. We have performed a series of low temperature electronic transport measurements on an InAs nanowire contacted with Niobium leads. The channel length ($\sim4$ times the nanowire diameter) is shorter than the electronic phase coherence length, but longer than the elastic mean free path, leading to behaviour that can be modelled by a superconductor-normal-superconductor junction in the diffusive transport regime. A supercurrent is observed below a critical current $I_c$ of up to $\sim$50 nA. The critical current varies with local gate voltages and correlates with the normal state conductance, producing modulation of $I_c$ related to universal conductance fluctuations. An applied magnetic field produces a Gaussian decay of $I_c$, consistent with known theory. Analysis of multiple Andreev reflection corrections to conductance indicates a contact transparency $\approx$0.6. The full results help to shed light on the nature of proximity effect superconductivity in a quasi-one-dimensional semiconductor in the quasi-diffusive regime. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W50.00005: Thouless dephasing and amplitude modulation of Aharonov-Bohm oscillations in mesoscopic InGaAs/InAlAs interferometers J. J. Heremans, S.L. Ren, Yao Zhang, C.K. Gaspe, S. Vijeyaragunathan, T.D. Mishima, M.B. Santos Aharonov-Bohm oscillations in the low-temperature magnetoresistance of mesoscopic interferometric rings are investigated for their dependence on bias current and temperature, and to explore origins of the observed amplitude modulation in magnetic field. Single-ring interferometers of radius 650 nm and lithographic arm width 300 nm were fabricated on a high-mobility high-density InGaAs/InAlAs heterostructure. The rings show interference oscillations over a wide range of magnetic fields, with amplitudes subject to modulation with applied magnetic field. The quantum phase coherence length is extracted by analysis of the fundamental and higher Fourier components of the oscillations, and by comparative study of the amplitude. The variation of the amplitude with bias current and temperature shows the existence of a critical excitation energy consistent with the Thouless energy for quantum phase smearing. Autocorrelation and Fourier analysis are used to determine the quasi-period of the amplitude modulation, which is found to be consistent with an origin in the magnetic flux threading the finite width of the interferometer arms, changing the mesoscopic realization of the system. Supported by DOE DE-FG02-08ER46532 (VT) and NSF DMR-0520550 (UoO). [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W50.00006: Decoherence mechanisms of Aharonov-Bohm excitons in type-II quantum dots Bidisha Roy, Haojie Ji, Siddharth Dhomkar, Lev Murokh, Jonathan Ludwig, Dmitry Smirnov, Maria Tamargo, Igor Kuskovsky The Aharonov-Bohm (AB) effect is one of the most important verifications of phase coherence of quantum particles. It has been extensively used to study quantum coherence in mesoscopic systems by transport measurements, where contacts play a significant role. The AB effects can also be observed in magneto-photoluminescence (PL) of polarized excitons in quantum rings and type-II quantum dots (QD), which is a contactless technique. The AB effect reveals itself as oscillation(s) in both energy and intensity of the emission. The magnitude of these oscillations directly relates to the quantum coherence of the AB excitons. To study decoherence mechanisms for such AB excitons, and the AB effect in general, we performed temperature dependent magneto-PL on several samples consisting of stacked type-II ZnTe/ZnSe QDs. The PL as a function of the magnetic field exhibits a strong peak, whose magnitude decreases with increasing temperature, due to loss of coherence. The effect persisted up to 30-35 K depending on the sample. This observed decrease in the AB peak is modeled via one-dimensional electron-phonon and electron-electron scattering of ballistic electrons, assuming strong hole confinement, for temperatures above 3K. The physical meaning of the fitting parameters is discussed. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W50.00007: Generating electron-hole superfluidity in experimentally realizable graphene and GaAs heterostructures David Neilson, Andrea Perali, Andrew Croxall, Alexander Hamilton Exciton bound states in solids between electrons and holes have been predicted to form a superfluid at high temperatures. We present results of determination of the experimental parameter ranges needed for generating electron-hole superfluidity in three different heterostructures: double bilayer graphene, GaAs double quantum wells, and hybrid hole-bilayer graphene -- GaAs electron-quantum well structures. We find that in the double bilayer graphene [1] and GaAs quantum well systems, the sample parameters necessary to generate equilibrium superfluidity of the electron-hole pairs are close to values already achieved in experiments. Our results indicate that the superfluid transition temperatures should be at or above liquid helium in both cases. For the hybrid bilayer graphene -- GaAs quantum well structure, we obtain chiral superfluid states with phase coherence across the graphene--GaAs interface. Our results are based on a mean field approach with self-consistent screening of the pair Coulomb interaction. This approach has been successfully tested in a quantitative way [2] against recent Diffusion Quantum Monte Carlo results in a related system. \\[4pt] [1] A. Perali, \textit{et al.}, Phys. Rev. Lett. \textbf{110}, 146803 (2013)\\[0pt] [2] D. Neilson, \textit{et al., }arXiv:1308.0280 [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W50.00008: Spontaneously broken time reversal symmetry in strongly interacting two dimensional electron systems in Si and Ge Saquib Shamim, S. Mahapatra, G. Scappucci, W.M. Klesse, M.Y. Simmons, Arindam Ghosh Time reversal invariance is a fundamental and robust symmetry of nonmagnetic quantum systems whose violation often results in nontrivial and exotic phenomena ranging from delocalization of electrons, quantum Hall liquid, the quantum anomalous Hall effect in topological insulators or chiral superconductivity predicted in graphene. An external magnetic field or magnetic impurities is employed in experiments to break the time reversal symmetry. Here we show that strong Coulomb interactions can lift the time reversal symmetry in two dimensional systems formed by atomically confined doping of phosphorus (P) atoms inside bulk crystalline silicon and germanium at zero magnetic field. Weak localization corrections to the conductivity and the universal conductance fluctuations were both found to decrease with decreasing doping in the Si:P and Ge:P $\delta $-layers, suggesting delocalization driven by Coulomb interactions. In-plane magnetotransport measurements indicate the presence of local spin fluctuations at low doping, resulting in spontaneous lifting of the time reversal symmetry. Our experiments suggest the existence of a new delocalized many-body state in two dimensions when strongly interacting electrons are confined to narrow half-filled impurity bands. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W50.00009: Tailoring topological super\-conductivity using super\-cu\-rrents Panagiotis Kotetes, Andreas Heimes, Daniel Mendler, Alexander Shnirman, Gerd Sch\"{o}n Recent experiments have provided the first promising indications of Majorana fermions (MFs) in heterostructures consisting of semiconducting wires and superconductors in the presence of a Zeeman field. By performing a complete classification of engineered topological superconductors (TSCs) [1] we predict that MFs are accessible in quasi-1d Rashba semiconductors with proximity induced superconductivity, even in the absence of magnetism. The only requirement is the presence of a Josephson current, with a suitable direction of flow. Here, we demonstrate how MFs emerge in our proposed setup when multi-wire or multi-channel semiconductors are involved. The crucial effect of the supercurrent flow is to convert the inter-wire/channel spin-orbit coupling into an effective Zeeman term. Finally, we further extend the particular scheme and discuss how the control of supercurrents can be also used to engineer and manipulate TSC in antiferromagnetically doped conventional superconductors.\\[4pt] [1] P. Kotetes, 2013 \textit{New J. Phys.} \textbf{15} 105027 (Focus issue on MFs). [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W50.00010: Origin of the Au/Ge(001) metallic state Ren\'{e} Heimbuch, Mikhail Kuzmin, Nick de Jong, Mark Golden, Harold J.W. Zandvliet Electronic transport in one-dimensional systems is a highly investigated topic, as electronic devices continue to shrink in size further and further. To understand the exotic behavior of electrons in structures of atomic length scales is crucial for future technological advances in electronics. We studied the spatial variation of the metallic state of the Au-induced nanowires on Ge(001). Spatial maps of the differential conductivity of the metallic state, which has its energy minimum at 0.1-0.15 eV below the Fermi level, are recorded with a low-temperature scanning tunneling microscope. The metallic state is not located on the ridges of the nanowires, but in the troughs between the nanowires. Electronic end effects were investigated and spatial profiling of the density of states, as a function of temperature reveal great inside into Tomonaga-Luttinger liquid in 1D electron systems. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W50.00011: Quantum Transport in LaAlO$_3$/SrTiO$_3$ Nanowire Cavities Guanglei Cheng, Michelle Tomczyk, Shicheng Lu, Mengchen Huang, Josh Veazey, Patrick Irvin, Sangwoo Ryu, Chang-Beom Eom, Jeremy Levy Hybrid superconductor-nanowire devices have attracted extensive interest for quantum computation based on electron spins, superconducting quantum bits and Majorana fermions. Such devices, which regulate the flow of single Cooper pairs and electron quasiparticles, are conventionally created by aligning normal nanowires in intimate contact with superconductors. New opportunities for creating such devices exist using a new class of complex-oxide interfaces. In particular, the interface of two insulating oxides, LaAlO$_3$ and SrTiO$_3$, exhibits a rich set of gate-tunable phases including intrinsic superconductivity, metal-insulator transition, and spin-orbit interaction. Here we investigate a superconducting nanowire cavity created by reversible ``write'' and ``erase'' processes using a conductive atomic force microscope (c-AFM) tip.\footnote{C. Cen \textit{et al.} Nat. Mater. \textbf{7}, 298 (2008)} Low-temperature magnetotransport experiments show that electrons can be subject to Coulomb blockade, Cooper pair tunneling, Andreev reflection and Fabry-Perot interference in a single device. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W50.00012: Black Phosphorus Field-effect Transistors Likai Li, Yijun Yu, Guojun Ye, Qingqin Ge, Xuedong Ou, Hua Wu, Donglai Feng, Xianhui Chen, Yuanbo Zhang Black phosphorus is a layered allotropy of phosphorus that closely resembles graphite. But unlike graphene monolayer, black phosphorus is a semiconductor with a predicted band gap of $\sim$2 eV, which reduces to $\sim$0.3 eV in the bulk crystal. We investigate the electric property of black phosphors thin flakes with thickness down to a few nanometers. High conductance modulations up to 10$^{6}$ and field effect mobility up to 1000 cm$^{2}$/Vs at room temperature are achieved in a Metal-Insulator-Silicon (MIS) field effect transistor structure. We further uncover the mechanism that limits the mobility in black phosphorus thin flakes through temperature-dependent electronic transport measurements. Our results provide the first basic understanding of the electronic properties of black phosphorus thin flakes, and will greatly facilitate further exploration of its future applications. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W50.00013: The Role of Catalytic Substrate Morphology on the Shape and Domain Size of Two-Dimensional Boron Nitride Sheets Mark Griep, Roland Tay, Travis Tumlin, Edwin Teo, Govind Mallick, Shashi Karna Two-dimensional (2D) nanomaterials, including graphene and boron nitride (BN), has been of intense interest in recent years due to their exceptional electronic, thermal, and mechanical properties. Tailoring these novel properties to their maximum potential requires precise control of the atomic layer growth process. In recent years, catalytic growth of 2-D nanomaterials using chemical vapor deposition (CVD) process has emerged as an attractive approach due to their low-cost, scalalibility, and ability totransfer the grown materials on various substrates. In this approach, The the morphology and purity of the catalytic surface plays a critical role on the shape, size, and growth kintectics of the 2D nanomaterial. In this work, we present the results of our systematic studies of the role of catalytic surface morphology on the shape and domain size of CVD grown hexagonal boron nitride (hBN) films. The present work clearly demonstrates that that the presence of surface roghness in the form of ridges leads to a preferential growth of small-domain triangular BN sheets. A 10 to 100-fold reduction in the surfcae roughness leads to increased domain BN triangles, eventually transitioning to large-domain hexagonal shaped hBN sheets. [Preview Abstract] |
Session W51: Focus Session: Beyond Graphene Devices: Function, Fabrication, and Characterization VII
Sponsoring Units: DMPChair: David Cobden, University of Washington
Room: Mile High Ballroom 1E
Thursday, March 6, 2014 2:30PM - 3:06PM |
W51.00001: Spin-valley physics and field effect on transition metal dichalcogenides Invited Speaker: Yoshihiro Iwasa Transition metal dichalcogenide (TMD) is attracting growing interest as two dimensional (2D) crystals beyond graphene. Of particular importance is an optoelectronic functionality based on valleytronics, which is strongly coupled with spintronics through the spin-orbit interactions, making TMD a quite unique system. Field effect transistor (FET) plays crucial roles, not only because of its ambipolar operations [1] and electric field induced superconductivity [2], but also because of its inherent broken inversion symmetry causing electric field induced Zeeman splitting [3]. In this presentation, we review our latest achievements on spin-valley physics and FET functionalities in TMD materials. We demonstrated for the first time the spin/valley polarization using spin- and angle resolved-photoemission spectroscopy. This became possible by choosing noncentrosymmetric bulk crystals. Photoluminescence circular dichroism proved that the noncentrosymmetric stacking enhances the valley polarization in bilayer, indicating that the noncentrosymmetric MoS$_{2}$ crystals are useful materials for the future valleytronics. As for the field effect, we performed systematic investigations of ambipolar FETs in MoX$_{2}$ (X $=$ S, Se, and Te), and found new field induced superconductivity in MoSe$_{2}$. 2D nature of electric field induced superconductivity was unambiguously demonstrated by the anisotropic $H_{\mathrm{c2}}$. We also demonstrated electroluminescence using the field effect geometry. \\[4pt] [1] Y. J. Zhang et al., \textit{Nano Lett.} 12, 1136 (2012), \textit{ibid.} 13, 3023 (2013).\\[0pt] [2] J. T. Ye et al., \textit{Science} 338, 1193 (2013).\\[0pt] [3] H. T. Yuan et al., \textit{Nat. Phys.} 9, 563 (2013). [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W51.00002: Electric control of spin in monolayer WSe2 field effect transistors Lei Zhang, Kui Gong, Dongping Liu, Lei Liu, Yu Zhu, Yonghong Zhao, Hong Guo We report a first principles theoretical investigation of quantum transport in monolayer WSe2 field effect transistor (FET). Due to a strong spin-orbit interaction (SOI) and the atomic structure of the two-dimensional (2D) lattice, monolayer WSe2 has an interesting electronic structure that exhibits Zeeman-like up-down spin texture near the K and K' points of the Brillouin zone. In a FET, the gate electric field induces an extra, externally tunable SOI that re-orients the spins into a Rashba-like texture thereby realizing electric control of the spin. Quantum transport is modulated by the spin texture, namely by if the spin orientation of the carrier after the gated channel region, matches or miss-matches that of the FET drain electrode. The carrier current in the FET is labelled both the spin index and the valley index, realizing spintronics and valleytronics in the same device. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W51.00003: Carrier distribution and negative compressibility in graphene-MoS$_{2}$ heterostructures Stefano Larentis, John R. Tolsma, Babak Fallahazad, David C. Dillen, Kyoung Kim, Allan H. MacDonald, Emanuel Tutuc We report the investigation of electrical properties and magnetotransport in monolayer graphene - multilayered MoS$_{2}$ heterostructures. The devices are fabricated by dry transfer of graphene layers onto exfoliated MoS$_{2}$. The conductivity dependence on the back-gate bias shows the ambipolar behavior characteristic of graphene, along with a marked saturation of the conductivity on the electron branch. Magnetotransport measurements reveal that the conductivity saturation is the result of electrons populating the lower mobility MoS$_{2}$ layer at a positive, threshold back-gate bias. Experimental data from heterostructures with different thicknesses allow the extraction of the band offset between the MoS$_{2}$ conduction band and the graphene charge neutrality point. Surprisingly, the carrier density in graphene reveals a marked decrease as a function of gate bias near the MoS$_{2}$ population threshold, an observation which implies that electrons in MoS$_{2}$ have negative compressibility at low carrier density. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W51.00004: Negative Compressibility and Charge Partitioning Between Graphene and MoS$_{2}$ Two-Dimensional Electron Gases John Tolsma, Stefano Larentis, Emanuel Tutuc, Allan MacDonald Electron-electron interactions often have opposite influences on thermodynamic properties of electrons in graphene compared to conventional two-dimensional electron gases (2DEGs), for example by lowering charge and spin-susceptibilities in the graphene case and enhancing them in the ordinary 2DEG case [1]. In ordinary 2DEGs the charge susceptibility diverges at a finite carrier density, below which the compressibility becomes negative. We theoretically explore the influence of this qualitative difference on how charge is partitioned between a MoS$_{2}$ and a graphene sheet 2DEG when they act as a compound capacitor electrode. Our theory is based on a random phase approximation for charge fluctuations in the 2DEGS and the coupling constant formulation for the ground state energy. We find that in the ideal case the MoS$_{2}$ 2DEG carrier density jumps immediately to a finite value when it is initially populated and discuss how this effect is moderated by disorder. \\[4pt] [1] Yafis Barlas, T. Pereg-Barnea, Marco Polini, Reza Asgari, and A.H. MacDonald, \textit{PRL} \textbf{98, }236601 (2007). [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W51.00005: Optoelectronics with electrically tunable PN diodes in monolayer WSe2 Hugh Churchill, Britton Baugher, Yafang Yang, Pablo Jarillo-Herrero We describe the transport and optoelectronic behavior of ambipolar monolayer WSe$_2$ devices in which two local gates are used to define a PN junction exclusively within the sheet of WSe$_2$. With these electrically tunable PN junctions, we demonstrate both PN and NP diodes with ideality factors better than 2. Under excitation with light, the diodes show photodetection responsivity of 210 mA/W and photovoltaic power generation with a peak external quantum efficiency of 0.2\%, promising numbers for a nearly transparent monolayer sheet in a lateral device geometry. Finally, we demonstrate a light-emitting diode based on monolayer WSe$_2$. These devices provide a fundamental building block for ultra-thin, flexible, and nearly transparent optoelectronic and electronic applications based on ambipolar dichalcogenide materials. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W51.00006: Ambipolar light emitting transistors on transition-metal dichalcogenides Yijin Zhang, Ryuji Suzuki, Yoshihiro Iwasa Transition-metal dichalcogenides (TMDs) are known to show ambipolar transistor operation, in which both electron and hole can transport through TMD channel materials [1]. As widely investigated in organic transistors, ambipolar transistor has additional functionality of the efficient light emitting source by simultaneously introducing electron and hole in the channel, forming a bias tunable p-n junction [2]. Recently, tunable yet stable p-n junction has been realized in MoS2 using a device structure of electric double layer transistor (EDLT), taking advantage of liquid gate dielectric [3]. We fabricated EDLT devices with tungsten diselenide (WSe2), molybdenum diselenide (MoSe2), and molybdenum disulfide (MoS2) as channel materials, and observed electroluminescence (EL) from both monolayers and multilayers. The peak energy suggests that EL occurs at K point in the momentum space even in multilayer samples, in contrast with band modulation from monolayer to multilayers [4]. Such a light emitting device will be a fundamental device in opto-valleytronics application. [1] Y. J. Zhang et al. Nano Lett. 12, 1136 (2012) [2] J. Zaumseil et al. Nat. Mater. 5, 69 (2006) [3] Y. J. Zhang et al. Nano Lett. 13, 3023 (2013) [4] A. Splendiani, et al. Nano Lett. 10, 1271 (2010) [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W51.00007: RKKY interaction in MoS$_2$ Diego Mastrogiuseppe, Nancy Sandler, Sergio Ulloa MoS$_2$ belongs to a family of layered compounds --the transition metal dichalcogenides-- that are attracting increasing attention in the solid state community due to their very rich phase diagram. In particular, the semiconducting ones in their 2D form, are of particular interest in the search for a new generation of devices in nanoelectronics and nanophotonics. The hexagonal lattice allows one to describe the low-energy physics with a massive Dirac equation around the $K$ and $K'$ points. Moreover, the presence of a large intrinsic spin-orbit interaction due to the presence of transition metal atoms, leads to a valley-dependent splitting of the states of an otherwise spin-degenerate valence spectrum. We study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two magnetic impurities in the direct band gap semiconducting single-layer MoS$_2$, focusing in the p-doped case. Going beyond a recent study [1], we include the effects of the spin-degenerate valence bands at the center of the Brillouin zone, relevant for energies close to the valence band maximum. The easy experimental tunability of the carrier concentration by electrical or chemical means, makes possible the study of the carrier-mediated spin-spin interaction at different fillings.\\[4pt] [1] PRB 87, 125401. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W51.00008: Transport Properties and Devices of Molybdenum Disulfide Fenglin Wang, Petr Stepanov, Jeanie Lau Molybdenum Disulfide (MoS2) is a very promising material especially the monolayer MoS2 with a direct bandgap; however, the low mobility is the major obstacle currently. We have combined multiple methods to improve the mobility, also investigate into the possible mechanism of the mobility bottleneck. With the help of additional gates, we are able to achieve ambipolar transport in MoS2 devices.We will present the latest experimental results. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W51.00009: Single Layer Transition Metal Dichalcogenides Transistors Yuhang Jiang, Jinhai Mao, Eva Andrei Single layer Transition Metal Dichalcogenides (TMDs), such as MoS$_{\mathrm{2}}$, WS$_{\mathrm{2}}$ and TaS$_{\mathrm{2}}$, are atomically thin two-dimensional materials with unique electronic properties different from their bulk counterparts. The lack of inversion symmetry, high mass of the components and the 2D geometry lead to strong spin-orbit coupling and to a metal insulator transition (MIT). Recent progressing ultrathin sample preparation and nanodevice fabrication has opened new opportunities to explore the transport properties of these layers for potential applications in nanoelectronics. In particular the ability to gate these samples across the MIT, carries the promise of sharp switching characteristics that defeat the thermodynamically imposed limiton the sub-threshold slope in standard field effect transistors. We will report on the electronic properties of field effect transistors fabricated with monolayer in the TMD family under conditions of extreme doping achieved by ionic liquid gating. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W51.00010: Effects of the dielectric environment on the electron transport properties of single-layer MoS$_{2}$ Simone Bertolazzi, Adrien Allain, Dominik Lembke, Andras Kis Two-dimensional materials, such as graphene, boron nitride and transition metal dichalcogenides, offer a wide range of electronic, optical and mechanical properties that can be advantageous for several applications in nanotechnology. Among these materials, single-layer molybdenum disulfide (MoS$_{2})$ shows great potential for scaling field-effect transistor devices, due to an optimal electrostatic control of the 2D semiconducting sheet, large energy bandgap and minimal leakage currents. However, to fully exploit the potential of this atomically thin semiconductor, additional experimental efforts need to be undertaken to boost the device performance and access the theoretical intrinsic electron mobilities. To pursue this objective, it is mandatory to reduce the density of charged impurities, both in the semiconducting sheet and in its surrounding environment, and to limit carrier scattering induced by polar optical phonons in the dielectric surface. Here we present the results of our recent experimental investigation of the electron transport properties of single-layer MoS$_{2}$ mechanically exfoliated/transferred onto different substrates, with varying surface chemistry, surface roughness and dielectric permittivity. We will show temperature-dependent four-terminal measurements of the electrical conductivity of single-layer MoS$_{2}$ in contact with various insulating materials, including 2D sheets of hexagonal boron nitride, organic polymers and metal oxides. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W51.00011: Time-Dependent Structural Phase Transitions of Two-dimensional Intercalated Layered Oxides Kristie Koski, Philip Zucker, Bryan Reed We demonstrate time-dependent phase transitions in metal-intercalated 2D layered MoO$_{3}$. Copper metal atoms are chemically intercalated into ultrathin 2D nanocrystalline MoO$_{3}$ using a novel method we developed to intercalate high densities of zero-valent atomic species. In-situ transmission electron microscopy (TEM), operating on a timescale of seconds, and Dynamic TEM, operating on nanosecond time scales show that unique, time-dependent phase transitions can be driven in these two-dimensional layered oxide nanoribbons. Very different structures arise on different time scales, indicating a competition between kinetics and thermodynamics in determining the resulting structure. Control experiments in pure MoO$_{3}$ show no such transitions, thus it appears that the copper intercalant is an essential part of the process. Measurements of the nanosecond-scale transformation are consistent with a local reordering of material within the original unit cell, while the slower transition is characterized by an incommensurate superlattice possibly associated with a charge density wave. This work opens new ground for accessing novel phases of matter in two-dimensional layered nanomaterials. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W51.00012: Characterization of atomically thin layers of 1T-TaS$_{2}$ Adina Luican-Mayer, Jeffrey R. Guest, Saw Wai Hla 1T-TaS$_{2}$ is a transition metal dichalcogenide that shows a wealth of correlated phenomena: it is metallic at higher temperatures, it has four temperature-dependent charge density wave phases with distinct structures [1]; at low temperatures it shows Mott insulator behavior and it becomes superconducting under pressure [2,3]. Due to the weak van der Waals bonding between its layers we show that it is possible, by mechanical exfoliation, to obtain atomically thin 1T-TaS$_{2}$ crystals. In this talk we address the question of how the transition from bulk to few layers affects the different phases of this material. Specifically, we discuss resistivity measurements for flakes of 1T-TaS$_{2}$ exfoliated onto the surface of Si/SiO$_{2}$ complemented by temperature-dependent Raman spectroscopy characterization. \\[4pt] [1] Thomson, R. E. et al. Phys. Rev. B 49,16899-16916 (1994).\\[0pt] [2] Fazekas, P. and Tosatti, E. Phil. Mag. B 39, 229-244 (1979).\\[0pt] [3] Sipos, B. et al. Nature Materials 7,960-965 (2008). [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W51.00013: Tunable CDW and Superconductivity Phase Transitions in 1T-TaS$_2$ Thin Films through Gate-controlled Intercalation Yijun Yu, Fangyuan Yang, Yajun Yan, YoungJai Choi, Sejoong Kim, Young-Woo Son, Sang-Wook Cheong, Xianhui Chen, Yuanbo Zhang 1T-TaS$_2$ has a rich set of complex phases as a result of competition among multiple electronic orders in this layered material. The delicate balance among the various phases makes 1T-TaS$_2$ very sensitive to external modulations. Using PEO/LiClO$_4$ solid electrolyte as a medium between a gate electrode and the sample, we have successfully intercalated Li ions into 1T-TaS$_2$ thin flakes in a continuous and reversible way. This allows us to probe the interplay between CDW phases, Mott phase, and superconducting phase as the concentration of Lithium is varied. Apart from 1T-TaS$_2$, our method of controlled intercalation is a promising new technique which could be applied to other layered materials. [Preview Abstract] |
Session W52: Superconductivity: General Theory
Sponsoring Units: DCMPChair: Ronny Thomale, University of Wurzburg
Room: Mile High Ballroom 1F
Thursday, March 6, 2014 2:30PM - 2:42PM |
W52.00001: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 2:42PM - 2:54PM |
W52.00002: Nodeless versus Nodal Scenarios of Possible Triplet Superconductivity in the Quasi-One-Dimensional Layered Conductor Li$_{0.9}$Mo$_6$O$_{17}$ Otar Sepper, Andrei Lebed We solve a theoretical problem of the upper critical magnetic field, parallel to a conducting axis of a layered, quasi-one-dimensional (Q1D) superconductor. In particular, we consider two cases: a triplet superconducting order parameter with and without zeros on the Q1D Fermi surface. We demonstrate [1,2] that the orbital destructive effects can destroy the superconducting state with parallel magnetic fields much higher than the so-called Clogston-Chandrasekhar paramagnetic limit, and that the nodeless order parameter is in a better quantitative agreement with the recent experimental data [3]. Our results indicate strong evidence in favor of triplet superconducting pairing in the layered Q1D superconductor Li$_{0.9}$Mo$_6$O$_{17}$. This work was supported by the NSF under Grant DMR-1104512.\\[4pt] [1] O. Sepper and A. G. Lebed, Phys. Rev. B 88, 094520 (2013.)\\[0pt] [2] A. G. Lebed and O. Sepper, Phys. Rev. B (Rapid Communications) 87, 100511(R) (2013).\\[0pt] [3] J. F. Mercure et al., Phys. Rev. Lett. 108, 187003 (2012). [Preview Abstract] |
Thursday, March 6, 2014 2:54PM - 3:06PM |
W52.00003: Superconductivity and Gutzwiller correlations in a two band Hubbard-Fr\"{o}hlich model Tao Qin, Michele Fabrizio, S. Shahab Naghavi, Erio Tosatti We studied the two-band Hubbard-Fr\"{o}hlich model with the hopping parameters from the first-principle calculation for La-Phenanthrene. The mean-field approximation shows that two-band superconductivity with opposite signs is possible even with the presence of a large Hubbard $U$. Using Gutzwiller approximation, we show that the Hubbard $U$ can help the superconductivity in this system. When $U$ is too large, it will go into the insulator state. Roughly, we determined the critical $U$ for this superconductor-insulator (SI) transition. We further shows that before the SI transition, the antiferromagnetic order will come into being. However, there is still a large range of $U$ for the superconductivity to win. We also discussed the possibility to apply our model to other polycyclic aromatic hydrocarbons. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W52.00004: A $\frac{1}{4}$-filled band model for the organic superconductor $\kappa$-(ET)$_2$X R. Torsten Clay, S. Mazumdar The minimal model usually assumed for the conducting layers of the organic charge transfer solid superconductor $\kappa$-(ET)$_2$X is a $\frac{1}{2}$-filled anisotropic triangular lattice Hubbard model, where a dimer of molecules is replaced with a single effective site. Within this frustrated $\frac{1}{2}$-filled model a metal to antiferromagnetic (AFM) phase transition is found, but calculations beyond the mean field level do not find evidence for superconductivity. Recent dielectric constant measurements suggest that a coupling is present between AFM order and ferroelectric charge ordering. Such a charge order would lead to unequal charge densities on the ET molecules within each dimer and can not be described within the effective model. We present the results of correlated calculations on the full $\frac{1}{4}$-hole-filled $\kappa$-(ET)$_2$X lattice using the Path Integral Renormalization Group (PIRG) method. We show that AFM order does occur within the $\frac{1}{4}$-filled model and investigate the possibility that charge order can cooperatively enhance the AFM state. We further present results for superconducting pair-pair correlations within this model. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W52.00005: Chiral p-wave superconductivity in 2D lattices of magnetic atoms on a superconductor Jian Li, Bogdan Bernevig We investigate chiral p-wave superconductivity in 2D lattices of magnetic atoms on an s-wave superconductor. We identify criteria of obtaining topologically nontrivial phases in such systems. In particular, we prove that a non-commuting helix pattern along the x and y directions is a necessary condition. When such a condition is satisfied, the system displays a rich phase diagram that generically allows for an arbitrary Chern number. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W52.00006: First-order chiral to non-chiral transition in the angular dependence of the upper critical induction of the Scharnberg-Klemm $p$-wave pair state Richard Klemm, Jingchuan Zhang, Christopher Lorscher, Qiang Gu We calculate the temperature $T$ and angular $(\theta,\phi)$ dependence of the upper critical induction $B_{c2}(\theta,\phi,T)$ for parallel-spin superconductors with an axially symmetric $p$-wave pairing interaction pinned to the lattice and a dominant ellipsoidal Fermi surface (FS). When both parallel-spin states are allowed, the chiral Scharnberg-Klemm state $B_{c2}(\theta,\phi,T)$ exceeds that of the chiral Anderson-Brinkman-Morel state for all FS anisotropies, and exhibits a kink at $\theta=\theta^{*}(T,\phi)$, indicative of a first-order transition from its chiral, nodal-direction behavior to its non-chiral, antinodal-direction behavior. Potential applicability to Sr$_2$RuO$_4$, UCoGe, and topological superconductors is discussed. [Preview Abstract] |
Thursday, March 6, 2014 3:42PM - 3:54PM |
W52.00007: Theory of the magnetic ground states of TMTTF$_2$X Andrew Ward, R. Torsten Clay, Sumit Mazumdar As a function of pressure the ground state of the TMTTF$_2$X salts is either one of two different antiferromagnetic (AFM) phases, spin-Peierls (SP), or superconducting. Pressure is usually thought to increase the dimensionality of the TMTTF stacks. The occurrence of the SP phase is then counterintuitive, as it enters at higher pressure than the first AFM state. Here we examine a model for the pressure dependent phase of TMTTF$_2$X salts, the extended Hubbard model on a two dimensional lattice with inter- and intra-site electron-phonon coupling. Previous calculations have suggested that two distinct SP phases with different charge and bond distortions occur in this model. We argue that two distinct SP phases are not supported by experiment, and are a result of unsuitable parameter choices and finite-size effects within calculations. We present the results of further numerical calculations and investigate the effect of magnetic frustration on the AFM and SP phases. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W52.00008: Enhanced anti-ferromagnetic exchange between magnetic impurities in a superconducting host Jay Sau, Norman Yao, Leonid Glazman, Eugene Demler, Mikhail Lukin It is generally believed that superconductivity only weakly affects the indirect exchange between magnetic impurities. If the distance $r$ between impurities is smaller than than the superconducting coherence length $(r<\xi)$, this exchange is thought to be dominated by RKKY interactions, identical to the those in a normal metallic host. This perception is based on a perturbative treatment of the exchange interaction. Here, we discuss a non-perturbative analysis and demonstrate that the presence of Yu-Shiba-Rusinov bound states induces a strong $1/r^2$ anti-ferromagnetic interaction that can dominate over conventional RKKY even at distances significantly smaller than the coherence length $(r\ll\xi)$. Experimental signatures, implications and applications are discussed. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W52.00009: Unconventional Superconductivity in TiSe$_2$: a renormalization group approach R. Ganesh, Dmitry Efremov, Jeroen van den Brink, G. Baskaran TiSe$_2$ is a quasi-two dimensional material which hosts CDW and superconducting orders. Motivated by recent studies of single-layer transition metal dichalcogenides, we study the effect of electronic correlations in single-layer TiSe$_2$. This is a hexagonal system with an elegant band structure -- one hole-like Fermi pocket and three electron-like pockets at the edge centres of the Brillouin zone. We use Renormalization Group (RG) analysis to examine low energy interactions in this configuration. RG flow is governed by three fixed points corresponding to different long range orders: (i) The most exciting fixed point corresponds to `chiral superconductivity' -- the three electron-like pockets undergo pairing but each pair of Fermi pockets differs in phase by 2$\pi$/3. The phase increases as we move clockwise or anticlockwise. The state breaks time reversal symmetry and has interesting properties. (ii) Another fixed point corresponds to s$_{+-}$ superconductivity, in which the order parameter on the central pocket has its sign reversed. (iii) The third fixed point corresponds to CDW order. We discuss situations in which RG flow may be cutoff, possibly also giving SDW order. We suggest that TiSe$_2$ may host one of two possible superconducting orders, both unconventional. [Preview Abstract] |
Thursday, March 6, 2014 4:18PM - 4:30PM |
W52.00010: Spin-orbit scattering in inhomogeneous superconductors Charles Agosta, Daniel Ellowitz, Steven Butler, Evan Palmer Spin-orbit scattering is important in all superconductors (SC) to understand the critical magnetic field. In most SC, the application of a magnetic field will eventually destroy the SC mostly due to the formation of vortices. At higher fields, when the Zeeman Energy is closer to the SC energy gap, singlet Cooper pairs are separated due to Pauli paramagnetism. One would expect that when the magnetic energy equals the SC energy gap, all SC would be destroyed, and that limit is called the Chandrasakhar-Clogston Pauli paramagnetic limit. In the case that the orbital effects can be suppressed, Pauli paramagnetism can become the dominant cause of the destruction of SC, and an inhomogeneous SC can result. An inhomogeneous SC has an order parameter with nonzero pair momentum that oscillates periodically as a function of distance, unlike traditional SC where the order parameter is uniform. Furthermore, an inhomogeneous SC can form an exotic SC state above the paramagnetic limit. We have studied a class of anisotropic organic SC that have an exotic inhomogeneous SC state. Spin-orbit scattering has a strong affect on Pauli paramagnetism and must be taken into account carefully when developing model theories. We will discuss how we extract spin-orbit scattering amplitudes from our data. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W52.00011: Effects of columnar defects on fluctuation phenomena in superconductors Alexey Galda, Alexei Koshelev, Andrey Varlamov We investigate thermodynamic properties of superconducting materials with columnar defects in presence of external magnetic field. The defects, which can be produced by ion irradiation, have a dramatic effect on vortex pinning and critical parameters of the superconductor. We account for superconducting fluctuations in the system and calculate their effect on thermodynamic properties (e.g. specific heat, etc.) The results are valid for high-T$_{\mathrm{c}}$ superconductors. We compare our results with the recent experiments. [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W52.00012: ABSTRACT WITHDRAWN |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W52.00013: The Casimir effect across a superconducting transition Zachary Raines, Andrew Allocca, Victor Galitski We show that Casimir effect can be used as a means to probe electronic correlations. In particular, we consider an interacting electron system which undergoes a superconducting transition and calculate the Casimir force it exerts on a normal metal. We found an interesting non-analytic behavior of the Casimir force as a function of temperature across the superconducting transition. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W52.00014: Hall conductivity in the normal and topological superconducting phases of the Rashba system Suk Bum Chung, Rahul Roy The study of the intrinsic Hall conductivity in superconductors is attracting considerable theoretical and experimental efforts in recent years. In this paper we focus on the effect of Cooper pairing on a metallic system with non-zero intrinsic Hall conductivity - the Rashba metal with under the perpendicular Zeeman field. We find that there is a qualitatively larger change in the intrinsic Hall conductivity when there is interband pairing, with the change in magnitude linear in the pairing gap. Since the topologically non-trivial phase is unlikely to allow for significant interband pairing, our work leads to an interesting prediction : that the observation or lack thereof of the linear dependence of the intrinsic Hall conductivity on the pairing gap could be used to determine if the Rashba superconductor is topologically trivial or not. Our results are consistent with other investigations of interband pairing in the context of the proposed chiral $p$-wave superconducting state of Sr$_2$RuO$_4$. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W52.00015: Gravity at a Kelvin: the generalized rigidity of superconducting quantum nematics Kai Wu, Aron Beekman, Vladimir Cvetkovic, Jan Zaanen There is experimental evidence for the existence of zero temperature nematic quantum liquids. Resting on field theoretic dualities we demonstrate that its rigidity theory is closely related to linearized gravity. The difference is in the loss of Lorentz invariance and we show that the gravitons merge into a richer world of collective excitations such as rotational Goldstone modes deconfining at the crystal-nematic phase transition, yet to be detected experimentally. [Preview Abstract] |
Session W53: Focus Session: Electron, Ion, and Exciton Transport in Nanostructures III
Sponsoring Units: DMPChair: Yoshihiro Asai, National Institute of Advanced Industrial Science and Technology
Room: Mile High Ballroom 2C
Thursday, March 6, 2014 2:30PM - 3:06PM |
W53.00001: Modeling the Operation of Resistive Switching Memory Devices Invited Speaker: Blanka Magyari-Kope Resistance change based nonvolatile memory devices are currently considered as leading candidates for future memory modules. To assess the scalability, retention and endurance properties of these devices, however, a detailed understanding of the underlying resistive switching mechanism is imperative. Filamentary arrangements of oxygen vacancies in transition metal oxides under applied electric field were investigated theoretically and recently detected experimentally. Generally, the process of forming in these systems may include a mechanism by which oxygen vacancies can cluster into filaments and/or the diffusion of oxygen atoms away from the oxide region to form a thin interfacial reduced oxide. During electroforming, oxygen vacancies and/or ions drift due to the applied bias, trap electrons or holes and facilitate the formation of vacancy ordered domains. We review the implications on the electronic structure and energetics of conductive filament channels formation corresponding to the ``ON'' state and discuss the interplay between the ionic and electronic transport mechanisms. We show that charge trapping effects play a significant role in the switching process under applied electrical field affecting the atomistic pathways of the filament rupturing/dissolution process from the ``ON'' into the ``OFF'' state. Furthermore, in order to improve on the device characteristics, favorable effects and ``ON''-``OFF'' transition process control can be achieved with preferential impurity doping. [Preview Abstract] |
Thursday, March 6, 2014 3:06PM - 3:18PM |
W53.00002: Metal oxide resistive switching: evolution of the density of states across the metal insulator transition Alireza Mottaghizadeh, Qian Yu, Alexandre Zimmers, Herve Aubin Memristive devices have attracted considerable attention since the recognition that two-terminal resistive switching elements represents an example of a memristive element. In oxide materials such as SrTiO$_{3}$ (STO), oxygen vacancies are doping sites that can be displaced by an electric field. This allows for electric-field manipulation of doping as exploited in memristive devices. In this work, we present the study of metal-semiconductor-metal junctions formed on STO, where we demonstrate that the junction characteristics can be fine-tuned through electric field migration of oxygen vacancies at very low temperature (T $\sim$ 260 mK). At very low dopant concentration, the junction displays characteristic signatures of discrete dopants levels. As the dopant concentration increases, the semiconductor band gap fills in but a soft Coulomb gap remains, at even higher doping, a transition to a metallic state occurs where the density of states at the Fermi level becomes finite and Altschuler-Aharonov correction to the density of states is observed. This work demonstrates that electric field induced migration of dopants can be used to tackle open questions on the physics of correlated electron systems. This work was supported by the French ANR grants 10-BLAN-0409-01 and 09-BLAN-0388-01. [Preview Abstract] |
Thursday, March 6, 2014 3:18PM - 3:30PM |
W53.00003: \textit{In Situ} TEM of Conductive Bridge Formation in Nanoscale Resistive Memory Devices William A. Hubbard, Edward R. White, Jared Lodico, B.C. Regan We observe formation and dissolution of conductive filaments in nanoscale conductive bridge memory (CBRAM) devices \textit{in situ} by scanning and conventional TEM. Horizontally separated CBRAM devices are fabricated on electron-transparent membranes, and the solid electrolyte layer is deposited between the inactive and active metal layers via atomic layer deposition. An additional ALD film caps the entire device. This geometry allows for unambiguous determination of active filaments and precludes filament formation by surface migration of the active metal, ensuring that conductive paths form within the solid electrolyte. In this study the inactive metal, active metal, and solid electrolyte are platinum, copper or silver, and alumina, respectively. Devices exhibit repeatable switching between high and low resistance states and the impact of filament number, size, and geometry on device switching parameters is discussed. [Preview Abstract] |
Thursday, March 6, 2014 3:30PM - 3:42PM |
W53.00004: Memristor Physics Driven by Joule Heating Harold Hjalmarson, Michael McLain, Denis Mamaluy, Xujiao Gao Switching in bipolar memristive devices involves the growth of conductive filaments following the application of a voltage pulse that causes heating. This Joule heating by the electric field is a large contributor to the migration of atoms and vacancies. In this talk, the results of continuum calculations will be used to describe the switching of tantalum oxide devices. The continuum calculations include the effects of Joule heating, chemical species migration, ionizing radiation and chemical reactions. These calculations will be focused on the temporal evolution of a conductive filament in a simple structure. 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 3:42PM - 3:54PM |
W53.00005: \emph{Ab initio} Calculations for Hydrogen-Doped HfO$_{2-x}$ RRAM Dan Duncan, Blanka Magyari-Kope, Yoshio Nishi Hydrogen impurities are shown to have significant effects on the mechanism of electronic conduction in HfO$_2$-based resistance change memory (RRAM) devices, and to affect the ionic transport during the forming, set, and reset processes. Using density functional theory and employing the LDA+\emph{U} formalism, the diffusion of oxygen ions in hydrogen-doped HfO$_{2-x}$ was examined and its implications on the electronic structure are determined. Results indicate that hydrogen can have multiple substantial effects on device operation, and has a strong potential to improve device switching and uniformity. These hydrogen-doped devices make promising candidates for low-voltage and forming-free memory schemes, as well as for electronic synapses in neuromorphic systems. [Preview Abstract] |
Thursday, March 6, 2014 3:54PM - 4:06PM |
W53.00006: Theoretical Investigation of the Hafnia-Hafnium Interface in RRAM Devices Andrew O'Hara, Gennadi Bersuker, Alexander Demkov Oxide based resistive-switching memory devices (RRAM) utilizing hafnia (HfO$_{\mathrm{2}})$ as the dielectric serve as an attractive option for embedded non-volatile memory systems. Successful operation requires a degree of oxygen deficiency caused by application of a forming voltage. A recent approach to help facilitate this has been the use of an oxygen gettering layer overlaying hafnia. Using density functional theory (DFT) in the local density approximation (LDA), we construct and study a hafnia-hafnium interface to understand the reducing and gettering properties. With this interface, we compare two routes to the creation of substoichiometric hafnia: formation of oxygen vacancies that leave hafnium unoxidized and migration of oxygen to hafnium to form an extended Frenkel pair (FP). Our work shows that the presence of the interface lowers the vacancy formation energy by 1.1 eV from the bulk value of 7.5 eV. Using the nudged-elastic band method, we show that not only is the formation energy lower for an extended FP, but that the barrier to formation of the shortest such FP is only 1.3 eV implying the favorability of such defects. Finally, we study the diffusion of oxygen in bulk hafnium to learn how the defect would behave after disassociation of the FP. [Preview Abstract] |
Thursday, March 6, 2014 4:06PM - 4:18PM |
W53.00007: Physical Description and Experimental Characterization of the Resistive Switching Filament Andrew Lohn, Patrick Mickel, Conrad James, Matthew Marinella We derive an analytical, steady state solution for resistive switching from the heat equation. Fitting our equation to a single hysteresis loop (the most fundamental experiment in the field), provides experimental determination of the filament radius, conductivity, temperature and the thermal conductivity of the surroundings. These parameters are determined continuously and show excellent agreement with the detailed experimental work to date. This approach enables every researcher with a current-voltage sourcemeter to experimentally characterize their filament. The analytical nature of our equation also elucidates the relation between materials, design parameters, and performance metrics. We generalize the empirical relation for uniting resistive switches and we show that our steady state solution is valid over all relevant timescales by fitting to a hysteresis loop taken within 10 nanoseconds. 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 4:18PM - 4:30PM |
W53.00008: Multidimensional information storage in configurational changes of resistive switching filaments Patrick Mickel, Andrew Lohn, Conrad James, Matthew Marinella We present a new methodology which enables direct control of the geometry and radial composition of nanoscale filamentary resistive switches, and demonstrate the ability of this technique to store multidimensional information in a single device. Using bi-polar, power limited switching (as opposed to the common voltage or current sourcing), we demonstrate individual control over both the radius and the conductivity of the nanoscale conducting filaments which control resistive switching elements. Using this control, we show that degenerate resistances states may be composed of alternate radius/conductivity pairs which require distinct power thresholds to thermally activate resistive switching (thereby constituting 2D storage: R and P, or radius and conductivity). Finally, by implementing a series of alternate polarity power pulses, we show that the radial composition profile within the nanoscale filament may be precisely tuned leading to designed trajectories through P-R space and a third dimension of information storage. Using this technique we estimate that a single resistive switch may realistically supplant as many as 10 digital devices. [Preview Abstract] |
Thursday, March 6, 2014 4:30PM - 4:42PM |
W53.00009: Transient doping in atomic chains -- a case study in time-resolved STM Paul Snijders, Stefan Polei, Steve Erwin, Franz Himpsel, Karl-Heinz Meiwes-Broer, Ingo Barke Doping one-dimensional (1D) systems is notoriously difficult due to the structural disorder created by the dopants. The Si(553)-Au surface features an array of step edges with 1D chains of dangling bonds. These chains have a 1x3 ordered ground state [1]. Using a scanning tunneling microscope we inject electrons from the tip into these step-edge chains and we observe that the periodicity of the atomic chains changes from the 1x3 ordered ground state to a 1x2 ordered excited state with increasing tunneling current. The threshold current for this transition is reduced at lower temperatures. In conjunction with first principles density-functional calculations we conclude that the 1x2 phase is created by transient doping of the atom chains [2]. Random telegraph fluctuations between two levels of the tunneling current provide direct access to the dynamics of the phase transition, revealing a monostable state, and lifetimes in the millisecond range. Our method provides a possible avenue to map out a doping-dependent phase diagram in cases where conventional impurity doping is problematic. \\[4pt] [1] S.C. Erwin, F.J. Himpsel, Nat. Comm. 1:58 (2010).\\[0pt] [2] S. Polei et al., PRL 111, 156801 (2013). [Preview Abstract] |
Thursday, March 6, 2014 4:42PM - 4:54PM |
W53.00010: Local Gating of Carbon Nanotube inside TEM Li-Ying Chen, Yen-Song Chen, Chia-Seng Chang We report a new method of fabricating ultra-clean and hysteresis-free multi-wall carbon nanotube field-effect transistor (CNFET) inside an ultra-high vacuum transmission electron microscopy (TEM) equipped with a movable Au tip as a local gate. Local gating of CNFET is demonstrated concurrently with atomic-scale imaging. The development of the ambipolar characteristic of CNFET, the Vds effect on CNFET as well as the localized characteristics of CNFET have been investigated. [Preview Abstract] |
Thursday, March 6, 2014 4:54PM - 5:06PM |
W53.00011: Equilibrium charge fluctuations of a charge detector and its effect on a nearby quantum dot David Ruiz-Tijerina, Edson Vernek, Sergio Ulloa We study the Kondo state of a spin-1/2 quantum dot (QD), in close proximity to a quantum point contact (QPC) charge detector near the conductance regime of the 0.7 anomaly. The electrostatic coupling between the QD and QPC introduces a remote gate on the QD level, which varies with the QPC gate voltage. Furthermore, models for the 0.7 anomaly [Y. Meir et al., PRL 89,196802(2002)] suggest that the QPC lodges a Kondo-screened level with charge-correlated hybridization, which may be also affected by capacitive coupling to the QD, giving rise to a competition between the two Kondo ground states. We model the QD-QPC system as two capacitively-coupled Kondo impurities, and explore the zero-bias transport of both the QD and the QPC for different local gate voltages and coupling strengths, using the numerical renormalization group and variational methods. We find that the capacitive coupling produces a remote gating effect, non-monotonic in the gate voltages, which reduces the gate voltage window for Kondo screening in either impurity, and which can also drive a quantum phase transition out of the Kondo regime. Our study is carried out for intermediate coupling strengths, and as such is highly relevant to experiments; particularly, to recent studies of decoherence effects on QDs. [Preview Abstract] |
Thursday, March 6, 2014 5:06PM - 5:18PM |
W53.00012: Magnetosymmetries of nonlinear transport in dissipative conductors Salil Bedkihal, Dvira Segal We demonstrate with numerically exact simulations that nonlinear transport coefficients obey certain magnetic field symmetries. Our model includes a two terminal Aharonov-Bohm interferometer with a quantum dot located at each of its arms. One quantum dot is interacting electrostatically with a reservoir, a fermionic environment made of a quantum dot coupled to one or more leads. We study the dynamics and steady state properties of this many-body out of equilibrium setup, by using a numerically exact influence functional path integral technique (Phys. Rev.B 82, 205323 (2010)). We show that, in agreement with phenomenological treatments of dephasing and mean field approaches, even (odd) conductance terms obey odd (even) symmetry with threading magnetic flux, as long as system acquires spatial inversion symmetry. When spatial asymmetry is introduced, magnetic field symmetries are broken, but more general symmetries with respect to left-right interchange are obeyed. Finally we also numerically demonstrate that double quantum dot Aharonov-Bohm interferometer coupled electrostatically to a fermionic environment can act as a charge current rectifier when two conditions are met simultaneously (I)broken time reversal and (II) many body effects. [Preview Abstract] |
Thursday, March 6, 2014 5:18PM - 5:30PM |
W53.00013: Theoretical study of carbon-nanotube-based molecular sensors Yan Li, Miroslav Hodak, Jerry Bernholc Carbon Nanotubes (CNTs) are highly promising for chemical and biological sensing applications, owing to their high chemical and mechanical stabilities, high surface areas as well as unique electronic properties. We report results of theoretical studies of detection abilities of several small analyte molecules, such as ammonia and nitrogen dioxide. We use density functional theory (DFT) and Keldysh non-equilibrium Green's function (NEGF) formalism to investigate differences in transmission coefficients and current due to interactions between the CNT and analyte molecules. For nitrogen dioxide, which chemisorbs on the CNT, we show that its attachment produces significant differences in both transmission and Current-Voltage (I-V) curve. For ammonia, we find that it can be either physisorbed or chemisorbed on the CNT depending on its position relative to the metalic leads. The chemisorbed case shows detectable differences in transmission and I-V curve. We also investigate sensing mechanisms of CNTs functionalized with receptor molecules for specific analyte molecules. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700