Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session F1: Focus Session: Surface Chemistry and Catalysis III
Sponsoring Units: DCPChair: Feng Tao, University of Notre Dame
Room: 103/105
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F1.00001: Parahydrogen Induced Polarization Reactions on Supported Metal Nanoparticle Catalysts Clifford Bowers, Ronghui Zhou, Wei Cheng, Luke Neal, Helena Hagelin-Weaver ALTADENA type parahydrogen induced polarization (PHIP) signals were acquired using various oxide (e.g. Al$_{2}$O$_{3}$, TiO$_{2})$ supported Pt and Ir nanoparticle catalysts in the hydrogenation of small alkenes. The hydrogenation reactions were performed using a home-built mini-reactor installed on top of a 9.4 Tesla superconducting NMR magnet. Precise control of the gas mixture (i.e. alkene, para-H$_{2}$ and carrier gas) was achieved using mass flow controllers. Hyperpolarized adducts were delivered down the magnet bore from the reactor to the NMR probe for NMR detection. For certain substrates, long-lived hyperpolarized states were generated and detected. The PHIP signal enhancement and pairwise H$_{2}$ addition selectivity was measured as a function of the reactant partial pressures and reaction temperature. Activation energies and reaction kinetics were obtained for both pairwise and random addition. The reaction conditions and metal nanoparticle characteristics favoring pairwise selectivity were thus identified. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F1.00002: Nanosecond Dynamics in Pt Nanoparticles F.D. Vila, J.M. Moore, J.J. Rehr Understanding the physical and chemical behavior of supported catalysts is of fundamental and technological importance. However, due to the complex nature of their structure and dynamics at operando temperatures, their nanoscale behavior remains poorly understood. We have shown that DFT/MD calculations provide fundamental insight into the few ps dynamic structure of the nanoparticles, but such methods can be very computationally intensive.\footnote{F. Vila \textit{et al.}, Phys. Rev. B {\bf78}, 121404(R) (2008).}\footnote{F. Vila \textit{et al.}, J. Phys. Chem. C {\bf117}, 12446 (2013).} In order to examine relaxation dynamics in the ns regime here we present finite temperature MD simulations based on a modified Sutton-Chen (SC) model potential, supplemented with Lennard-Jones potentials for the interaction with the support. We find that bulk SC parameters tend to produce nanoparticles with less fluxional dynamics than those in ab initio simulations. To address this issue, we have determined modified SC parameters that capture the DFT dynamics. Nanosecond simulations reveal regimes controlled by internal particle melting and activation of surface mobility. The approach is illustrated for nano-catalysts of Pt/$\gamma$-alumina and compared with ab initio DFT/MD results. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F1.00003: First principles molecular dynamics of metal/water interfaces under bias potential Luana Pedroza, Pedro Brandimarte, Alexandre Rocha, Marivi Fernandez-Serra Understanding the interaction of the water-metal system at an atomic level is extremely important in electrocatalysts for fuel cells, photocatalysis among other systems. The question of the interface energetics involves a detailed study of the nature of the interactions between water-water and water-substrate. A first principles description of all components of the system is the most appropriate methodology in order to advance understanding of electrochemically processes. In this work we describe, using first principles molecular dynamics simulations, the dynamics of a combined surface(Au and Pd)/water system both in the presence and absence of an external bias potential applied to the electrodes, as one would come across in electrochemistry. This is accomplished using a combination of density functional theory (DFT) and non-equilibrium Green's functions methods (NEGF), thus accounting for the fact that one is dealing with an out-of-equilibrium open system, with and without van der Waals interactions. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F1.00004: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F1.00005: Manipulation of Single Molecular Hydrogen in a Size-tunable Nanogap Hui Wang, Haiyan He, Shaowei Li, Wilson Ho, Ruqian Wu The determination of weak bonds in physisorption systems remains as a major challenge for modern density functional approaches. In addition, it becomes important to use the tip of scanning tunneling microscope (STM) to manipulate chemical bonds. Here we study the adsorption geometries, translational and rotational motions, and vibrations of a single H2 molecule trapped in the gap of STM-tip and Au (110) reconstructed surface, using the density functional theory calculations. The tip-substrate separation is used as an adjustable parameters. We find that the stable adsorption geometry, H2 bondlength, H-H stretching frequency, and H2-Au bouncing frequency strongly depends on the tip-substrate distance. Computational results agree well with STM data, both indicate the strong role of STM tip on the behavior H2 motions. The new insights established through this work are useful for the understanding of puzzling observations, and should be applicable for the analysis of other physisorption systems. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F1.00006: A model of the ideal molecular surface Bryan Henson, Laura Smilowitz We utilize two manifestations of the phenomena of the quasiliquid phase on the surface of molecular crystals to formulate a universal thermodynamic theory describing the thickness of the layer as a function of the liquid phase activity. We use direct measurements of the liquid thickness as a function of temperature and measurements of the acceleration of thermal decomposition as a function of temperature approaching the melting point to illustrate the mechanism. We show that given the existence of a liquid phase below the melting point the ideal liquid activity is necessarily a fixed function of the free energies of sublimation and vaporization. We use this activity to create a reduced formula for the liquid thickness generally applicable to the molecular surface. We provide a prediction of the mechanism and kinetics of quasiliquid formation and show that the phase exists as a metastable kinetic steady state. We show that to first order the principle controlling feature of the system is the configurational entropy of the liquid/solid interface, rather than the specifics of the surface potential energy. This is analogous to other bulk colligative phenomena such as ideal gas and solution theories, and is thus an ideal, universal formulation of inherent, thermodynamically driven, surface disorder. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F1.00007: Origin of unexpected attractive adsorbate-adsorbate interactions between negatively charged ions on Mg (0001) surfaces Su-Ting Cheng, Mira Todorova, J{\"o}rg Neugebauer Electrostatic repulsion usually leads to an increase in work function and a decrease in binding energy when the coverage of electronegative elements adsorbed on a metal surface is increased. Using density-functional theory we investigate the adsorption of \{N,O,F,Cl\} on Mg$(0001)$ and find that only Cl complies with this expectations. All the considered $2^{\rm nd}$ row elements cause a decrease in work-function and an increase in binding energy with increasing coverage. We show that these counterintuitive phenomena can be understood in terms of an efficient embedding of the adsorbate atoms into the unusually large electronic surface spill-out of Mg$(0001)$. The described mechanism is based on purely electrostatic arguments and thus expected to be a generic feature on surfaces consisting of highly electropositive elements. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:24AM |
F1.00008: Multiscale Studies of Surface Chemistry of Catalysis: Au-Ag alloys Invited Speaker: Cynthia Friend Multiscale studies of gold-based materials spanning materials complexity and gas phase pressure demonstrate the predictive value of fundamental studies for selective oxidative transformations of organic oxygenates (alcohols and aldehydes) on gold-based materials. Model studies on single crystal surfaces under ultrahigh vacuum are used to understand surface structure and reaction mechanism on a molecular scale. The model studies use a combination of spectroscopy and imaging with scanning tunneling microscopy. The principles are derived from these used as a basis for predicting and understanding reactivity on complex, nonporous gold catalysts under steady-state conditions pressure. These nonporous materials are Au alloys with $\sim$3{\%} Ag. This work illustrates the predictive value of model studies and the potential for improving reaction selectivity in important catalytic reactions. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F1.00009: Fe adsorption on the hematite (0001) and magnetite (111) surface Adam Kiejna, Tomasz Pabisiak A detailed ab initio investigation of the structural, electronic and magnetic properties of Fe-atom adsorption on the hematite (0001) and magnetite (111) surfaces is presented. Spin-polarized density functional theory calculations are applied accounting for strong electron correlation effects by including a Hubbard-type on-site Coulomb repulsion (the DFT+U approach). For each oxide surface, the adsorption on two terminations has been studied: one terminated with Fe and the other with oxygen. The binding sites and coordination geometry of Fe adatoms are identified. Different adatom coverages were considered. The Fe atoms bind strongly to the Fe-oxide surfaces and induce large changes in their near surface geometry, and the electronic and magnetic properties. The binding of Fe is distinctly stronger at the O- than at the Fe-terminated surfaces of both oxides. The resulting adsorption energetics, structure and bonding are discussed based on the calculated local density of states and electron charge transfer. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F1.00010: Temperature programmed desorption of a binary gas mixture Nayeli Zuniga-Hansen, M. Mercedes Calbi Temperature programmed desorption (TPD) is an experimental technique that is widely used to determine the adsorption properties of a surface. Many existing theoretical studies have focused on the desorption of a single gas species, but the desorption of binary mixtures is a subject that has been relatively less explored. We perform computer simulations of the thermal desorption of binary gas mixtures using a kinetic Monte Carlo scheme. We start with a simple structure formed by a single line of adsorption sites and two species of adsorbates which bind to the surface with different energies. By varying the initial surface coverage, the particle-particle interactions and the concentration of the different adsorbates, we study the kinetics of desorption of the mixture and compare our results to available experimental data. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F1.00011: Compton Scattering from Bulk and Surface of Water Wenjie Wang, Ivan Kuzmenko, David Vaknin Elastic and Compton scattering at grazing angle X-ray incidence from water show distinct behaviors below and above the critical angle for total reflections suggesting surface restructuring of the water surface.~ Using X-ray synchrotron radiation in reflectivity mode, we collect the Thomson and Compton scattering signals with energy dispersive detector at various angles near the normal to surface as a function of the angle of incidence. Analysis of the ratio between the Thomson and Compton intensity above the critical angle (which mainly probes bulk water) is a constant as expected from incoherent scattering from single water molecule, whereas the signal from the surface shows strong angular dependence on the incident angle. Although we do not fully understand the phenomena, we attribute the observation to more organized water at the interface. [Preview Abstract] |
Session F2: Focus Session: Quantum Control of Molecular, Nano, and Plasmonic Materials III
Sponsoring Units: DCPChair: Maxim Sukharev, Arizona State University
Room: 102
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F2.00001: Ultrafast nanooptics: Using strong laser fields to control the motion of electrons in and around metallic nanotstructures Christoph Lienau Sharp metallic nanotapers irradiated with few-cycle laser pulses are emerging as a source of highly confined coherent electron wavepackets with attosecond duration and strong directivity. The possibility to steer, control or switch such electron wavepackets by light is expected to pave the way towards real-time probing of electron motion in solid state nanostructures. Such pulses can be generated by strong-field induced tunneling and acceleration of electrons in the near-field of sharp gold tapers within one half-cycle of the driving laser field. Here, we study for the first time the effect of the carrier envelope phase of few cycle laser pulses on the motion of electrons emitted from metallic nanostructures by strong-field tunneling [1]. We illuminate very sharp, single-crystalling gold tips with CEP-stable few-cycle near-infrared pulses at 1.5 $\mu$m and record angle-resolved kinetic energy spectra of the photoemitted electrons. Our experiments give first evidence for the effect of absolute phase of the laser pulses on the emission direction and kinetic energy distribution of the photoemitted electrons. \\[4pt] [1] B. Piglosiewicz\textit{ et al.}, \textit{Nature Photonics}, doi:10.1038/NPOTON.2013.288 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F2.00002: Resonant Scattering of Surface-Plasmon-Polariton Waves by a Dynamical Quantum Dot Danhong Huang, Michelle Easter, Godfrey Gumbs, Shawn-Yu Lin, Xiang Zhang, Alexei Maradudin The resonant scattering of a launched surface-plasmon-polariton wave by an embedded quantum dot above the dielectric/metal interface is explored in the strong-coupling regime. In contrast to non-resonant scattering by a localized dielectric surface defect, a strong resonant peak in the scattering-field spectrum is predicted and accompanied by the presence of two side valleys. The peak strength depends nonlinearly on the amplitude of surface-plasmon-polariton wave, reflecting the feedback dynamics from photoexcited electron-hole pairs inside the quantum dot. This unique behavior in the scattering-field peak strength is correlated with a resonant dip in the absorption spectrum of surface-plasmon-polariton wave due to interband photon-dressing effect. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F2.00003: Hybrid metal-dielectric nanocavity for ultrafast quantum dot optical field interaction Kevin Fischer, Thomas Babinec, Yousif Kelaita, Konstantinos Lagoudakis, Tomas Sarmiento, Armand Rundquist, Arka Majumdar, Jelena Vuckovic Efficient light-matter interfaces for solid-state quantum emitters offering high single-photon collection efficiency as well as strong light-matter interaction are an important ingredient to a variety of quantum technologies. In this talk we introduce and demonstrate a new light-matter interface based on a hybrid metal-dielectric nanopillar cavity coupled to a single InAs quantum dot (QD). Its essential design characteristics include low quality factor Q $\approx$ 25 resonance, ultrasmall mode volume V $\approx$ 0.04 $(\lambda /n)^3$, and record-high coherent coupling g/2$\pi$ $\approx$ 150-200GHz, exceeding those offered in other light-matter interfaces including in photonic crystal cavities coupled to single QDs. We have observed that the single QD emitters are both embedded in the nanometallic devices and well-coupled to the orthogonal nanocavity modes, that our devices significantly enhance the spontaneous emission rate of the QD transitions (Purcell factor Fp $\approx$ 8 relative to bulk), and that overall single photon flux from the QD is increased by nearly two orders of magnitude relative to bulk. We conclude with an outlook for applications of this nanocavity geometry in information processing. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F2.00004: Molecular controlled of quantum nano systems Invited Speaker: Yossi Paltiel A century ago quantum mechanics created a conceptual revolution whose fruits are now seen in almost any aspect of our day-to-day life. Lasers, transistors and other solid state and optical devices represent the core technology of current computers, memory devices and communication systems. However, all these examples do not exploit fully the quantum revolution as they do not take advantage of the coherent wave-like properties of the quantum wave function. Controlled coherent system and devices at ambient temperatures are challenging to realize. We are developing a novel nano tool box with control coupling between the quantum states and the environment. This tool box that combines nano particles with organic molecules enables the integration of quantum properties with classical existing devices at ambient temperatures. The nano particles generate the quantum states while the organic molecules control the coupling and therefore the energy, charge, spin, or quasi particle transfer between the layers. Coherent effects at ambient temperatures can be measured in the strong coupling regime. In the talk I will present our nano tool box and show studies of charge transfer, spin transfer and energy transfer in the hybrid layers as well as collective transfer phenomena. These enable the realization of room temperature operating quantum electro optical devices. For example I will present in details, our recent development of a new type of chiral molecules based magnetless universal memory exploiting selective spin transfer. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F2.00005: Three-Dimensional Plasmonic Nanoclusters Nicolas Large, Alexander Urban, Xiashuang Shen, Yumin Wang, Hong Wang, Mark Knight, Peter Nordlander, Hongyu Chen, Naomi Halas Recent developments in the control and manipulation of electromagnetic radiation allow for the emergence of new concepts, found only in artificially engineered nanoscale media. Assembling nanoparticles into well-defined structures is an important way to create and tailor the optical properties of materials. While displaying fascinating optical properties, nanostructures created by self-assembly or lithography have a major drawback; strong angular-dependent optical properties resulting from their two-dimensionality. Here, we present novel three-dimensional nanoclusters comprised of noble metal nanoparticles encapsulated in a polymer displaying interesting optical features in the visible (Fano resonances, optical isotropy,...). We investigate the nature of the optical properties and their dependence on cluster geometry. Such three-dimensional clusters show great promise as optical kernels for metafluids, imparting metamaterial optical properties into disordered media such as liquids, glasses, or plastics, free from the requirement of nanostructure orientation. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F2.00006: Long range emission enhancement and anisotropy in coupled quantum dots induced by aligned elongated and proximal gold nanoantenna Jaydeep Basu, Laxminarayan Tripathi, Praveena M, Pranay Valson Metal nanoparticles have been shown to considerably modify the optical properties of quantum emitters like quantum dots and molecules when they are in close proximity to each other. Understanding the microscopic nature of such interactions requires studying the optical properties in the near field. Here, we discuss experimental results on non-local long range emission intensity enhancement and anisotropy in quantum dot assemblies induced by isolated and partially aligned gold nanoantennas overlaid on the quantum dots. Sub-diffraction and near field, spatially resolved, photoluminescence spectroscopy of these hybrid films, clearly demonstrate that the effect is maximum when the longitudinal surface plasmon resonance of the nanoantenna is resonant with the emission maxima of the quantum dots. Numerical simulations qualitatively captures the near field behavior of the nanorods but fails to match the experimentally observed non-local effects. We have suggested how collective excitations of quantum dots in the close packed assemblies, mediated by the nanoantennas, could lead to such observed behavior. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F2.00007: Plasmon-enhanced energy transfer for improved upconversion of infrared radiation in doped-lanthanide nanocrystals Qi Sun, Haridas Mundoor, Josep Ribot, Vivek Singh, Ivan Smalyukh, Prashant Nagpal Upconversion of infrared radiation into visible light has been investigated for applications in biological imaging and photovoltaics. However, low conversion efficiency due to small absorption cross-section for infrared light (Yb$^{\mathrm{3+}})$, and slow rate of energy transfer (to Er$^{\mathrm{3+}}$ states) has prevented application of upconversion photoluminescence (UPL) for diffuse sunlight or imaging tissue samples. Here, we utilize resonant surface plasmon polaritons (SPP) waves to enhance UPL in doped-lanthanide nanocrystals. Our analysis indicates that SPP waves not only enhance the electromagnetic field, and hence weak Purcell effect, but also increases the rate of resonant energy transfer from Yb$^{\mathrm{3+}}$ to Er$^{\mathrm{3+}}$ ions by 6 fold. While we do observe strong metal mediated quenching (14 fold) of green fluorescence on flat metal surfaces, the nanostructured metal is resonant in the infrared, and hence enhances the nanocrystal UPL. This strong columbic effect on energy transfer can have important implications for other fluorescent and excitonic systems too. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:24AM |
F2.00008: Quantum and nonlinear optics at the single photon level with quantum dots in optical nanocavities Invited Speaker: Jelena Vuckovic By embedding a single InAs/GaAs quantum dot (QD) inside a nanocavity that strongly localizes optical field, it is possible to achieve a very strong light-matter interaction. The strength of this interaction is characterized by the coherent emitter-field coupling strength (g) which also sets the limit on the operational speed of such a system. While in systems consisting of a single neutral atom coupled to a cavity maximum $g/(2\pi) \sim$ 20 MHz has been demonstrated, InAs/GaAs QDs inside photonic crystal cavities have reached $g/(2\pi) \sim$ 40 GHz. Such a QD-cavity platform has also been employed in a series of quantum and nonlinear optics experiments at the single or few photon level which will be discussed in this talk, including: 1) photon blockade and photon induced tunneling (which can be employed to build high throughput sources of single or n-photons); 2) all optical switching at the single photon level and at the speed of 25GHz (which can be employed in all optical gates); 3) single quantum dot based optical modulators that operate at the sub-fJ control energies and potentially at $>$10GHz speeds; 4) single QD spin-photon interfaces that could be employed as nodes of a quantum repeater. However, considering that the speed of each of these elements is ultimately limited by g, which in turn scales as $\sim 1/\sqrt{V}$, where V is the optical mode volume, it is worthwhile building structures with V even smaller than those of photonic crystal cavities (which typically have V on the order of a cubic optical wavelength). With our recently demonstrated metal- GaAs nanocavity, V is squeezed by more than 10 times relative to photonic crystal cavities, and we demonstrate $g/(2\pi) > $ 100GHz with a single, embedded InAs/GaAs quantum dot. We are also working on extensions of this platform from two-level to multi-level quantum emitters strongly coupled to a cavity, as well as the extensions to emitters coupled to photonic molecules and cavity arrays, with applications in nonclassical light generation and quantum simulation. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F2.00009: Near-field Mediated Plexcitonic Coupling and Giant Rabi Splitting in Individual Metallic Dimers Andrea Schlather, Nicolas Large, Alexander Urban, Peter Nordlander, Naomi Halas We investigate hybrid metallic dimer -- J-aggregate nanostructures that show coherent coupling between the localized surface plasmon (LSP) of the metallic disks and the exciton of the J-aggregate molecular complex. This hybrid nanostructure, combining both bottom-up and top-down approaches, is designed to probe the limitations of coherent coupling detection. Indeed, this allows us to report, for the first time, an experimental investigation of a plexcitonic coupling mechanism at the single-particle regime. By varying the diameter of the nanodisks, the LSP energies of the dimers can be systematically modified and tuned across the exciton energy of the J-aggregate. This allows for the direct measurement of the plexcitonic coupling energy of the dimer -- J-aggregate hybrid nanostructures. In this work, using single particle dark-field scattering spectroscopy as well as Finite-Difference Time-Domain (FDTD) calculations, we report giant Rabi splitting energies up to 400 meV resulting in a plexcitonic-induced transparency window observed both experimentally and theoretically at the overlap energy of the individual excitations. Furthermore, through a rigorous study of the polarization dependence and of the tunable geometric parameter effects (gap, diameter), the plexcitonic coupling mechanism has been investigated in these hybrid nanostructures, leading to the determination of the crucial role played by the plasmonic hot spots in the formation of the hybrid plexcitonic modes. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F2.00010: Plasmon-assisted surface photochemistry and nanoassembly in silver nanoparticles Erich M. See, Seyyed Mohammad Hossein Abtahi, Xi Guo, Brenden A. Magill, Webster L. Santos, Richey M. Davis, Hans D. Robinson Bottom-up self-assembly of nanostructures into larger constructs remains a difficult proposition marred by low precision and low yield. Here we report on our effort to use optical activation to drive the assembly of particles onto silver nanospheres to form well-defined dumbells. The spheres were adsorbed onto a substrate and functionalized with a photocleavable o-nitrobenzyl-based ligand, which becomes positively charged upon photactivation. Illuminating the spheres with polarized light at either visible or ultraviolet wavelengths, plasmonic effects induce preferential photocleavage on opposite poles of the spheres, where negatively charged particles then can be adsorb. We will also discuss how this technique can be extended to enable the assembly of more complex nanostructures. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F2.00011: Lightning-rod-effect-directed photo assembly of gold nanorods and spheres in a colloidal suspension Seyyed Mohammad Hossein Abtahi, Xi Guo, Webster L. Santos, Hans D. Robinson, Richey M. Davis We describe a method for making colloidally stable gold nanorods that can be photo-functionalized at their ends---the plasmon hot spots---while dispersed in a fluid. Such particles could be used in supramolecular self-assembly and in developing chemical sensors. Gold nanorods---approximately 60 nm long and 20 nm in diameter---were functionalized with combination of mono-thiol PEG and a photophotocleavable {\em o}-nitrobenzyl ligand. The PEG serves to help stabilize the gold nanorods suspended in a mixture of water and alcohols so that the assembly can stably be done in suspension. The functionalized gold nanorods were then exposed to UV light that triggered photocleavage, resulting in the formation of positively charged amine groups. When these rods were mixed with negatively charged gold nanospheres, there was a red-shift in the wavelength of the longitudinal plasmon peak of more than 20-30 nm, indicating the preferential binding of gold nanospheres to the ends of the gold nanorods, which we attribute to the lightning rod effect. [Preview Abstract] |
Session F3: Undergraduate Research - Society of Physics Students IV
Sponsoring Units: SPSChair: Crystal Bailey, American Physical Society
Room: 107
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F3.00001: Design of Rectangular Coils for Control of Magnetic Fields Ryan Daniels, Changgong Zhou Over the last decade, cylindrical cross-section (CCS) coils have encompassed the majority of studies (i.e., ``Double-Helix'' coils): predominantly for use in particle accelerators (Goodzeit et al., Rochford et al., and Tominaka et al.). In this study, we investigate single and double-layered rectangular cross-section (RCS) coils of different inclination angles. RCS coils are a novel design, which does not require special machining of grooves on supporting structure for precise assembly of coils, and may lead to cost reduction. Numerical calculation of the field based on Biot-Savart's Law is conducted using Mathematica. Our goal is to generate a static and controllable time-varying magnetic field using a special configuration of four RCS coils, and impose the field on magnetic nanoparticles levitated by optical forces to study their behavior. The calculation provides guidance for optimizing the magnetic field in this application. Our current results indicate that the configuration produces highly uniform and controllable magnetic fields in the region where the nanoparticles are levitated. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F3.00002: Magnetic Phase Transitions in Intercalated Dichalcogenide Nanostructures Corbyn Mellinger, Corey Cooling, Kayla Boyle, Paul Shand, Tim Kidd, Laura Strauss Nanostructured Mn-intercalated TaS$_{\mathrm{2}}$ was prepared with a nominal Mn concentration of 23.5{\%}. Powder x-ray diffraction confirms incorporation on Mn in the Van der Waals gaps between TaS$_{\mathrm{2}}$ layers. AC susceptibility measurements in a DC bias field indicate the sample displays paramagnetic behavior down to its Curie-Weiss temperature of 75 K. An Arrott plot confirms the transition to the ferromagnetic state, with critical exponent $\beta $ larger than expected for the Heisenberg 3D model. Further analysis of the AC susceptibility indicates a transition to cluster-glass state around 40 K, indicated by Vogel-Fulcher analysis near the transition temperature. We believe the difference from expected critical exponent values to be due to the proximity of the ferromagnetic and cluster-glass transitions. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F3.00003: Field Directed Ordering in Magnetic Nanocrystal Structures Stuart Lawson, Joshua Wright, Robert Meulenberg Iron oxide nanocrystals (NCs) have been the focus of intense research owing to the observation of tunable magnetic properties which could lead to advances in many fields including magnetic storage devices and medicine. We have been targeting the use of iron oxide NCs as magnetoresistance (MR) based sensors using ordered NC arrays. In this work, we will present our efforts toward using external magnetic fields to induce intraparticle ordering in iron oxide NC drop cast films. We use x-ray diffraction to analyze effects of the external fields on the NC array structure, while using SQUID magnetometry to probe the effects of NC interactions on the magnetic properties of iron oxide NCs ranging from 5 - 20 nm in diameter. MR measurements suggest large changes in the MR ratio can be achieved using the directed ordering approach for NC arrays. Our work could provide new avenues towards the fabrication of new magnetic devices. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F3.00004: Temperature Dependence of the Spin Hall Effect in Perpendicularly Magnetized Magnetic Materials Shuoying Yang, Weifeng Zhang, Salvatore Mesoraca, Aakash Pushp, Timothy Phung, See-hun Yang, X.M. Cheng, Stuart S.P. Parkin The spin Hall effect (SHE) and spin torque generated from it have been of great interest recently due to their potential use in future spintronic memory and logic devices. A solid understanding of the detailed mechanisms behind SHE is key to effectively utilizing and enhancing this effect. In this work, we report the experimental study of switching perpendicularly magnetized magnetic layers using the spin torque from SHE. Multilayers with the repeated units consisting of normal metal (Pt or Ta)/ ferromagnet with perpendicular magnetic anisotropy (CoFeB, CoNiCo, or Co) were deposited on Si substrates by sputtering deposition. Magnetoresistance and Hall resistance of the samples were measured by the Quantum Design PPMS DynaCool system with the field up to 6 tesla at various temperatures ranging from 10 K to 300 K. The spin Hall angle, calculated by comparing the field dependence of Hall resistance measured with the currents of the same magnitude but opposite directions, depends linearly on temperature. The contributions of the skew-scattering and side-jump mechanisms to SHE have been quantitatively separated. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F3.00005: Distortions in 2p4d Partial Fluorescence yield for 4d elements Alexander Price, Frank de Groot, Trinanjan Datta X-ray absorption spectroscopy (XAS) is a standard tool to determine the electronic structure of molecules and materials. CTM4XAS and CTM4RIXS are semi-empirical programs to analyze transition metal L$-$ and M$-$ edge transitions by evaluating the effects of crystal field and charge transfer parameters on the atomic multiplets. We compute and compare the XAS and the fluorescence yield (FY) XAS, of the 3d and 4d transition metal ions. In the case of 2p edges of 3d elements Auger decay dominates and sets the time scale. The 2p3d X$-$ray emission spectra (XES) accounts for approximately 80\% of the radiative decay. The 2p3d partial FY is distorted and because it dominates the FY, the total FY is also distorted. For the 4d elements the 2p4d XES decay is approximately 10\% of 2p3d XES decay, implying that (the energy-constant) core-core XES and Auger channels dominate the decay. The computed 2p4d partial FY$-$XAS spectra are different from the 2p XAS. Although 2p4d partial FY is distorted, the total FY is not because it is dominated by 2p3d XES. We also find that the 2p3s and 2p4s XES channels contribute less than 1\% and can be neglected. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F3.00006: Structural and Electrical Properties of Thin Films of Electron-doped Mixed-Valent Rare Earth Manganites Zoey Warecki, Grace Yong, David Schaefer, Rajeswari Kolagani Research in thin films of mixed valent rare earth manganese oxides has largely been focused on hole-doped manganites that exhibit colossal magnetoresistance. Hole doped manganites are derived from trivalent rare earth manganese oxides, where the hole doping (introduction of Mn$^{\mathrm{4+}}$ ions to replace the Mn$^{\mathrm{3+}}$ ions) is the result of substitution of the trivalent rare earth site (such as La$^{\mathrm{3+}})$ by a divalent alkaline earth element (such as Ca$^{\mathrm{2+}})$. In contrast, electron doped manganites can be obtained by introducing Mn$^{\mathrm{3+}}$ ions to replace Mn$^{\mathrm{4+}}$ ions in an alkaline earth manganese oxide. We are currently investigating the properties of electron-doped manganites which are derived from CaMnO$_{\mathrm{3}}$. We use Pulsed Laser Deposition to grow these epitaxial thin films. One way to introduce electron carriers in the film is by creating an oxygen poor environment during the deposition, causing the film composition to be of the form Ca$^{\mathrm{2+}}$Mn$^{\mathrm{4+}}_{\mathrm{1-2x}}$Mn$^{\mathrm{3+}}_{\mathrm{2x}}$O$^{\mathrm{2\thinspace -}}_{\mathrm{3-x}}$. Another method is by substitution of the Ca$^{\mathrm{2+}}$ site by rare earth elements of valency 3$+$ or higher (such as Ce$^{\mathrm{4+}}$ or Ho$^{\mathrm{4+}})$ to introduce electron carriers. We will report our study of the structural, electrical, and magneto-transport properties of electron doped manganite thin films, focusing on the sensitivity of these properties to growth parameters. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F3.00007: Fluid Manipulation Utilizing Electrowetting Techniques Laura Kaiser, Laura Pyrak-Nolte The fraction of the pore space in rock occupied by a given fluid is called saturation. The relationship between saturation and capillary pressure for porous media is hysteretic between imbibition and drainage cycles. If the wetting phase saturation increases, the capillary pressure follows an imbibition curve, and, if the wetting phase saturation decreases, the capillary pressure follows the drainage curve. Due to this hysteresis, researchers have suggested that there is a third variable that should be considered called interfacial area per volume that removes the ambiguity in the capillary pressure - saturation relationship. Before the relationship can be explored in more detail, we first must be able to manipulate the saturation internally rather than externally. We used electrowetting techniques to manipulate the contact angle of a salt water drop. This technique affects the interfacial energy and, therefore, enables manipulation of the contact angles and saturation. Once mastered, the technique could be used to explore the effect of interfacial area per volume on micromodel systems. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F3.00008: Understanding the Cytoxicity of Permalloy Microdisks Aleksandra Karapetrova, Elena Rozhkova, Valentin Novosad, Philip Gach Nanomagnetic materials offer exciting opportunities when attempting remote control of biological processes. For example, ferromagnetic microdisks are able to induce apoptosis via magnetomechanical stimulus. The rotation of the disks occurs under an alternating magnetic field. A spin vortex state is formed in the microdisk that gives them the ability to not congregate in the absence of magnetic field but to be mechanically responsive in the presence of field. Iron-nickel Permalloy disks are fabricated using optical lithography and metal deposition. The disks can be made with a layer of gold on the top and bottom sides for the purpose of surface functionalization such that fluorescent dyes and biological compounds can be bound. Since the magnetic core of the disks consists of a transition metal alloy, there is a possibility of reactive oxidative species (ROS) forming in aqueous solution by a Fenton reaction. The chemical stability of disks not coated with a gold layer were studied. ROS formation was detected using fluorescent probe hydroxyphenyl fluorescein, X-ray fluorescence microscopy, and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). No significant levels of hydroxyl radicals were detected at neutral pH. However, X-Ray fluorescence and ICP-MS did detect leaching. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F3.00009: Influence of Crowding on Polymer Conformations in Polymer-Nanoparticle Mixtures: Monte Carlo Simulations Wei Kang Lim, Alan R. Denton Within the cytoplasm and nucleoplasm of eukaryotic cells, a complex mixture of macromolecules (biopolymers, such as proteins and RNA) and smaller molecules share a tightly restricted space. In this crowded environment, hard nanoparticles exclude volume to softer biopolymer coils, restricting protein and RNA conformations and folding pathways. At sufficiently high concentrations, nanoparticle crowding also can affect phase stability, inducing aggregation or separation into polymer-rich and polymer-poor phases. Through Monte Carlo simulations, we explore the impact of crowding on polymer conformations and phase behavior in a coarse-grained model of polymer-nanoparticle mixtures. Neglecting polymer self-interactions, we exploit the random-walk geometry of ideal coils to model the polymers as effective ellipsoids whose shapes fluctuate according to the probability distribution of the gyration tensor. Accounting for penetration of polymers by smaller nanoparticles, we calculate the crowding-induced shift in the polymer shape distribution. We compare our results with predictions of a free-volume theory and available experimental data. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F3.00010: Microwave Directional Coupler for Quantum Measurement Victoria Xu, Chris Macklin, Andrew Eddins, Irfan Siddiqi We present the design of a 20dB single-section directional coupler using two edge-coupled, conductor-backed coplanar waveguides (CPW). We begin with an electromagnetic analysis of the physical mechanisms that allow two waveguides to form a directional coupler. Based on the coplanar waveguide geometry used for the coupler, we experienced inherently limited directivity in the performance, and we discuss the mechanisms by which we optimize for directivity despite geometric limitations. After laying out the theory behind CPW directional couplers, an electromagnetic analysis of our simulated design is presented. Two iterations of designs were fabricated. The final directional coupler yields simulated and measured performance even beyond the level of our design goals. At the center frequency of 6 GHz, our coupler showed comparable performance to commercial directional couplers. The 20-dB directional coupler serves as a solid-state equivalent of a 99/1 beam splitter for microwave photons, and will further enable on-chip experiments in quantum measurement. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F3.00011: Non-linear Multidimensional Optimization for use in Wire Scanner Fitting Alyssa Henderson, Balsa Terzic, Alicia Hofler To ensure experiment efficiency and quality from the Continuous Electron Beam Accelerator at Jefferson Lab, beam energy, size, and position must be measured. Wire scanners are devices inserted into the beamline to produce measurements which are used to obtain beam properties. Extracting physical information from the wire scanner measurements begins by fitting Gaussian curves to the data. This study focuses on optimizing and automating this curve-fitting procedure. We use a hybrid approach combining the efficiency of Newton Conjugate Gradient (NCG) method with the global convergence of three nature-inspired (NI) optimization approaches: genetic algorithm, differential evolution, and particle-swarm. In this Python-implemented approach, augmenting the locally-convergent NCG with one of the globally-convergent methods ensures the quality, robustness, and automation of curve-fitting. After comparing the methods, we establish that given an initial data-derived guess, each finds a solution with the same chi-square- a measurement of the agreement of the fit to the data. NCG is the fastest method, so it is the first to attempt data-fitting. The curve-fitting procedure escalates to one of the globally-convergent NI methods only if NCG fails, thereby ensuring a successful fit. This method allows for the most optimal signal fit and can be easily applied to similar problems. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F3.00012: Simulating FinFET Self-Heating for Device Reliability James Ham, Lincoln Carr, Carole Graas The continual scaling of transistors has led to sharp gradients in temperature (from ballistic transport of carriers) that result in new difficulties modeling device reliability. Current device-level thermal simulations do not track phonon populations, which are necessary to understand damage from high temperatures in scaled devices. A model for simulating highly localized hot spots due to an optical phonon bottle-neck near the channel/drain interface of a device operating in a ballistic transport regime will be presented. Various expansions of the Boltzmann transport equation (spherical harmonic expansion and methods of moments) are compared to a hydrodynamic model for device thermal simulations. We will discuss the post-processing technique for arriving at phonon populations from technology computer aided design (TCAD) simulations. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F3.00013: Scanning capacitance microscopy using a relaxation oscillator Marie Pahlmeyer, Andrew Hankins, Sam Tuppan, Woo-Joong Kim We have performed scanning capacitance microscopy using a relaxation oscillator. Calibrations using precision capacitors indicate a sensitivity on the order of 0.05 pF, stabilizing in under 0.1s. Surface topography of metallic structures, such as machined grooves and coins, can be readily obtained either in the constant-height (non-contact) or tapping (contact) mode. Spatial resolution of sub-50 $\mu$ micron has been achieved. [Preview Abstract] |
Session F4: Focus Session: Frustrated Magnets and Spin-Orbit Coupling
Sponsoring Units: GMAGChair: Seunghun Lee, University of Virginia
Room: 112/110
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F4.00001: Reconstruction of Chiral Edge States in Magnetic Chern Insulators Ryo Ozawa, Masafumi Udagawa, Yutaka Akagi, Yukitoshi Motome Surface and interface properties of spin-charge coupled systems are one of the central issues not only in fundamental physics but also in application to spintronics. In particular, in magnetically-ordered insulators with topological nature, topologically protected surface states may emerge. On the other hand, the magnetic state near the surface suffer from a reconstruction due to the local symmetry breaking, which may alter the surface states. It is of great interest to clarify how such a reconstruction occurs in a microscopic way. For this purpose, we consider an example of such magnetically-ordered topological insulators, i.e., a spin scalar chiral ordered phase characterized by a nonzero Chern number, recently discovered in the classical Kondo lattice model on a triangular lattice[1]. We investigate this state numerically in finite-size systems with open edges by large scale simulation. As a result, we find that ferromagnetic spin correlations are induced near the edges. Surprisingly, at the same time, the chiral edge current is enhanced. We also clarify that the relation between penetration depth and bulk energy gap. [1] Y. Akagi and Y. Motme, J Phys. Soc. Jpn. {\bf{79}}, 083711 (2010). [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F4.00002: Ultrasound Velocity Measurements in the Orbital-Degenerate Frustrated Spinel MgV$_2$O$_4$ Tadataka Watanabe, Takashi Ishikawa, Shigeo Hara, A.T.M. Nazmul Islam, Elisa M. Wheeler, Bella Lake Magnesium vanadate spinel MgV$_2$O$_4$ is a geometrically frustrated magnet with $t_{2g}$-orbital degeneracy of V$^{3+}$ (3$d^2$), which undergoes a cubic-to-tetragonal structural transition at $T_s$ = 65 K and an antiferromagnetic (AF) transition at $T_N$ = 42 K. For MgV$_2$O$_4$, it is considered that the occurrence of $t_{2g}$-orbital order at $T_s$ causes the release of frustration by the AF ordering at $T_N$ lower than $T_s$. We performed ultrasound velocity measurements in high-purity single crystal of MgV$_2$O$_4$. Temperature dependence of the tetragonal shear modulus ($C_{11}-C_{12}$)/2 exhibits huge Curie-type softening in the cubic paramagnetic (PM) phase ($T>T_s$), which should be a precursor to the cubic-to-tetragonal lattice distortion at $T_s$. The trigonal shear modulus $C_{44}(T)$ exhibits softening with an upturn curvature in the cubic PM phase, indicating a coupling of the lattice to magnetic excitations. These softenings suggest the coexistence of the dynamical Jahn-Teller effect and the dynamical magnetic state in the cubic PM phase. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F4.00003: Spin-orbital liquids in non-Kramers magnets on Kagome lattice Robert Schaffer, Subhro Bhattacharjee, Yong Baek Kim Localized magnetic moments with crystal-field doublet or pseudo-spin 1/2 may arise in correlated insulators with an even number of electrons and strong spin-orbit coupling. Such a non-Kramers pseudo-spin 1/2 is the consequence of crystalline symmetries as opposed to the Kramers doublet arising from time-reversal invariance. We investigate possible spin-orbital liquids with fermionic spinons for such non-Kramers pseudo-spin 1/2 systems on the Kagome lattice. Using the projective symmetry group analysis, we find ten new phases that are not allowed in the corresponding Kramers systems. We compute the spin-spin dynamic structure factor that shows characteristic features of these non-Kramers spin-orbital liquids arising from their unusual coupling to neutrons, which is therefore relevant for neutron scattering experiments. We also point out possible anomalous broadening of Raman scattering intensity that may serve as a signature experimental feature for gapless non-Kramers spin-orbital liquids. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F4.00004: Study of the magnon spectrum in $FeV_{2}O_{4}$ using inelastic light scattering Y. Gim, S. Gleason, T. Byrum, G.J. MacDougall, H.D. Zhou, S.L. Cooper The interplay between orbital, spin and lattice dynamics create a rich environment for the study of novel properties and phases. Transition metal oxides with a spinel structure, $AB_{2}O_{4}$ are excellent systems in which to explore the interplay among these dynamics: By substituting on the $A$ and $B$ sites with various elements, various phenomena and ground states can be explored. $FeV_{2}O_{4}$ is a special spinel with two orbital-active $A-$ and $B-$ site cations. This material exhibits interesting magnetic and structural phenomena, such as multiferroic behavior and a strong dependence of its physical properties on external stimuli such as pressure and magnetic field. In this talk, we present an inelastic light (Raman) scattering study of the temperature- and magnetic field-dependence of the magnon spectrum of $FeV_{2}O_{4}$. We compare these results to of the magnon spectrum of $MnV_{2}O_{4}$ in order to examine the role of A-site substitution on the spin dynamics. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F4.00005: Co Doping Effect on the Crystal and Magnetic Phases in the Frustrated Spinel Mn$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$V$_{2}$O$_{4}$ Jie Ma, Tao Hong, Huibo Cao, Adam Aczel, Wei Tian, Zhiling Dun, Yiming Qiu, John Copley, H.D. Zhou, Masaaki Matsuda Co doping effect on the MnV$_{2}$O$_{4}$ spinel system has been investigated by the elastic and inelastic neutron scattering techniques. Our data present that a magnetic phase transition exists from collinear to noncollinear ferrimagnetic structure between the Mn$^{2+}$/Co$^{2+}$ and V$^{3+}$ moments and the Co doping decreases the V$^{3+}$ canting angle. The most remarkable finding is that with Co doping, the collinear to noncollinear transition, which coincides with the cubic to tetragonal structural transition related with the orbital ordering of the V$^{3+}$ ions in pure MnV$_{2}$O$_{4}$, occurs independently without the structural transition. Our results indicate that the Co doping changes the orbital nature of the V$^{3+}$ ions and enhances the magnetic coupling between the Mn$^{2+}$/Co$^{2+}$ and V$^{3+}$ moments. We discuss how the orbital and magnetic order are correlated in this system. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F4.00006: Magnetism in a new structural family of iridates James Analytis, Tess Smidt, Ross McDonald, Kim Modic, Itamar Kimchi, Ashvin Vishwanath, Radu Coldea, Alun Biffin, S.K. Choi, Julia Chan, Pilanda Watkins-Curry The physics of Mott insulators underlies diverse phenomena ranging from high temperature superconductivity to exotic magnetism. Although both the electron spin and the local orbitals play a key role in these phenomena, in most systems these are connected only indirectly --- via Pauli exclusion --- since the spin-orbit interaction is relatively weak. Iridium-based oxides (iridates) depart from this expectation, since the spin-orbit coupling dominates over other interactions, such that the Mott physics obtains a strong orbital character. In some cases this is thought to generate strongly spin-anisotropic exchange. Here we report a new family of iridates whose magnetic character shows that this material has highly spin-anisotropic interactions, a key ingredient of the exotic possibilities associated with these compounds. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F4.00007: Weak increase in ordering temperature with pressure in KCuF$_3$ Alexander Thaler, Andrew Christianson, Shi Yuan, Isaac Brodsky, Lance Cooper, Stephen Nagler, Gregory MacDougall The perovskite KCuF$_3$ has been extensively studied as a prototype for both orbital order and 1D Heisenberg antiferromagnetism. Despite decades of research, the nature of its orbital and spin order are still debated. Several interesting results have been shown recently, among them the discovery via Raman scattering of a glassy structural transition at $T_s=50$~K, well below the known Jahn-Teller transition at $T_{OO}=800$~K and argued to be a necessary precursor to the 3D Neel transition at $T_N=39$~K. Recent experiments have demonstrated that this transition can be suppressed to zero temperature with hydrostatic pressures as low as $P_c\sim7$~kbar. In order to directly probe the effect of pressure on the magnetic behavior at low-temperatures, we have followed the above Raman measurements with a neutron scattering study. Structural and magnetic properties of single-crystalline KCuF$_3$ were explored using elastic scattering of thermal neutrons under applied quasi-hydrostatic pressure. We will present data suggesting that the A-type antiferromagnetism observed at ambient pressure is slightly increased by our application of pressure well above 1~GPa, as well as showing a spin-reorientation with increasing pressure. We will discuss the results in the context of present literature. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F4.00008: Spin waves in the double orbital-order spinel, $FeV_2O_4$ G.J. MacDougall, A.A. Aczel, V.O. Garlea, G.E. Granroth, T. Hong, A.D. Christianson, S.E. Nagler, I. Brodsky, H.D. Zhou For the past several years, the spinel vanadates, $AV_2O_4$, have been central to the study of orbital degeneracy and the complex coupling of spin, charge and lattice degrees-of-freedom in frustrated antiferromagnets. One such material of recent interest is $FeV_2O_4$, which has orbital degeneracy at high temperatures on both cation sites of the spinel structure. Previous x-ray scattering and our own neutron powder diffraction results have identified three structural and two magnetic transitions in this compound, and the low-temperature non-collinear spin state has been associated with an emergent ferroelectric moment. In the past year, we have followed our initial results on powders with an elastic and inelastic neutron scattering study on large single crystals of $FeV_2O_4$. Our elastic data confirm the same structural and magnetic transitions inferred from powder measurements, with near identical transition temperatures. Our inelastic data reveal the presence of two low-energy spin-wave branches, qualitatively similar to what has been reported for the related compound, $MnV_2O_4$, but with an order of magnitude larger spin-gap. I will present these results, and discuss them in the context of $MnV_2O_4$ and the present state of the literature. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F4.00009: Magnetic excitations in spin-orbital liquid FeSc$_2$S$_4$ in zero and applied magnetic field probed by inelastic neutron scattering Alun Biffin, Radu Coldea, Christian R\"{u}egg, Oksana Zaharko, Jan Embs, Tatiana Guidi, Vladimir Tsurkan In systems where both spin and orbital frustration are present, an intriguing Spin Orbital Liquid (SOL) state is believed to occur where spin and orbital moments remain disordered down to the lowest measurable temperatures. The A-site spinel FeSc$_2$S$_4$ is believed to form such a SOL ground state, with its undistorted cubic structure and diamond lattice of Fe$^{2+}$ sites providing the ingredients for orbital and spin frustration, respectively. The system displays Curie-Weiss behaviour indicative of strong exchange between $S=2$, $L=2$ Fe$^{2+}$ ions, though it does not order down to the lowest measurable temperatures. Here I will present the results of inelastic, time-of-flight neutron scattering experiments that probe the full bandwidth of the magnetic excitations in a powder sample of FeSc$_2$S$_4$, and provide a consistent model of the observed dynamics in terms of spin-orbital excitations, in both zero-field and in-field measurements. I will discuss in particular how the application of a magnetic field elucidates the spin and orbital nature of these excitations, as the system shows behaviour drastically contrary to its spin-only analogue. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:24AM |
F4.00010: Quantum Spin States, Multiferroicity, Orbital Ordering, and Metal-Insulator Transition in New Layered-Perovskites Invited Speaker: Haidong Zhou The high chemical tunability of the layered-perovskites Ba$_3$BC$_2$O$_9$ (B = Co2+, Ni2+, Mn2+, and C = Nb5+, Ru5+, Ir5+) makes them idea systems to study various physical behaviors, such as quantum spin states, multiferroicity, orbital ordering, and metal-insulator transition, based on the geometrically frustrated lattices. In this talk we present several examples to discuss these intriguing properties: (i) Ba$_3$CoNb$_2$O$_9$, Ba$_3$NiNb$_2$O$_9$, and Ba$_3$MnNb$_2$O$_9$. For these samples, the only magnetic ions Co2+, Ni+, or Mn+ on the B sites form a triangular lattice in the ab plane, which makes them new triangular lattice antiferromagnets (TLAFs). The detailed magnetic and electric properties show that the samples not only exhibit successive spin state transitions under magnetic fields but also multiferroic behaviors [1]; (ii) Ba$_3$CoRu$_2$O$_9$. With Ru5+ ions occupy the face-shared bioctahedral C-sites, the system exhibits an orbital ordering for the Ru5+ orbitals which leads to complex magnetic and structural phase transitions [2]; (iii) Ba$_3$CoIr$_2$O$_9$. This system exhibits metal-insulator transition under high pressure, which is accompanied with complex magnetic behaviors. \\[4pt] [1] J. Hwang et al., Phys. Rev. Lett. 109, 257205 (2012).\\[0pt] [2] H. D. Zhou et al., Phys. Rev. B 85, 041201(R) (2012). [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F4.00011: Magnetic Field Imaging of the Spinel MnV$_{2}$O$_{4}$ Brian Wolin, Tyler Naibert, Taylor Byrum, Samuel Gleason, Haidong Zhou, S. Lance Cooper, Raffi Budakian The complex interplay of spin, orbital degeneracy, and lattice degrees of freedom result in many intriguing behaviors in condensed matter systems. Due to its simple lattice structure and extensive theoretical work, the spinel MnV$_{2}$O$_{4}$ is a prime candidate for archetypal study of these phenomena. We perform magnetic force microscopy imaging on single crystal samples of MnV$_{2}$O$_{4}$ at variable temperature and magnetic field. Our results show previously unobserved magnetic structure and behavior (including stripes and domain switching) as the phase diagram is explored. These represent the first direct imaging of the magnetic properties of a vanadium oxide spinel and inform the current debate over the low temperature magnetic phases of MnV$_{2}$O$_{4}$. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F4.00012: Magnon spectra and strong spin-lattice coupling in magnetically frustrated MnV$_2$O$_4$: Inelastic light scattering studies S.L. Gleason, T. Byrum, Y. Gim, S.L. Cooper, H.D. Zhou The spinel MnV$_2$O$_4$ exhibits a series of closely spaced magnetic and structural transitions at low temperatures, reflecting magnetic frustration and strong spin-lattice coupling. MnV$_2$O$_4$ has a canted ferrimagnetic ground state with an undetermined orbital configuration. Temperature dependent studies of magnetic and vibrational excitations in MnV$_2$O$_4$ are important for determining the role that spin-lattice coupling plays in the low temperature phase transitions of this material and setting constraints on the orbital ground state configuration. We report an inelastic light (Raman) scattering study of the temperature and magnetic field dependences of magnetic excitations in MnV$_2$O$_4$. We observe a pair of $\textbf{q}=0$ one-magnon modes at 74 cm$^{-1}$ and 81 cm$^{-1}$, which is in contrast with the single 80 cm$^{-1}$ $\textbf{q}=0$ magnon that has been reported for MnV$_2$O$_4$ from previous neutron scattering measurements and spin wave calculations. Additionally, we find that the two-magnon energy of MnV$_2$O$_4$ decreases with decreasing temperature below $T_N$, which we attribute to strong coupling between zone-boundary magnons and phonons. These results offer important clues to the orbital ground state and the nature of spin-lattice coupling in MnV$_2$O$_4$. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F4.00013: Temperature and magnetic field dependent Raman scattering study of magnetically frustrated Mn$_{3}$O$_{4}$ T. Byrum, S.L. Gleason, Y. Gim, A. Thaler, P. Abbamonte, G.J. MacDougall, S.L. Cooper The coupling between spin and lattice degrees of freedom is presumed to be responsible for many of the novel phenomena observed in the magnetically frustrated spinel Mn$_{3}$O$_{4}$. While the lattice excitations have previously been investigated by Kim $\textit{et al.}$,* the dependences of the spin excitations in Mn$_{3}$O$_{4}$ with magnetic field and temperature have not yet been reported. We perform inelastic light (Raman) scattering to study the spin excitations in Mn$_{3}$O$_{4}$ as functions of temperature and magnetic field. We observe both one- and two-magnon excitations below the magnetic transition temperature of Mn$_{3}$O$_{4}$. In this presentation, we will discuss the temperature and magnetic field dependent evolutions of these excitations. Interestingly, we conclude that some of the magnon excitations are likely associated with the frustrated B-site sublattice of the spinel (AB$_{2}$O$_{4}$) structure. These results set the stage for future studies of the coupling between spin and lattice degrees of freedom in Mn$_{3}$O$_{4}$ as functions of pressure, temperature, and magnetic field. *M. Kim, X. M. Chen, X.Wang, C. S. Nelson, R. Budakian, P. Abbamonte, and S. L. Cooper, Phys. Rev. B 84, 74424 (2011). [Preview Abstract] |
Session F6: Focus Session: Spin-Dependent Physics in Carbon-Based Materials II
Sponsoring Units: GMAG DMPChair: Tatiana Rappoport, Universidade Federal do Rio de Janeiro
Room: 108
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F6.00001: Electrical spin injection into graphene through hexagonal boron nitride tunnel barrier Takehiro Yamaguchi, Yoshihisa Inoue, Satoru Masubuchi, Sei Morikawa, Masahiro Onuki, Kenji Watanabe, Takashi Taniguchi, Rai Moriya, Tomoki Machida Two-dimensional crystals such as graphene, h-BN, and transition metal dichalcogenides are emergent material system and receiving much attention for spintronics applications. Particularly, these 2D crystals have significant advantages when they are used as a tunnel barrier. 1) These materials can be exfoliated with a monolayer thick resolution. 2) A single-crystalline flake can be fabricated. 3) A wide range of band gaps are available. However, up to now, spin polarized tunneling through these materials has not been fully explored experimentally. Here, we demonstrate spin polarized tunneling through one monolayer thick of hexagonal boron nitride (h-BN) layer and used it for electrical spin injection into graphene [1]. A NiFe/ML h-BN/bilayer graphene/h-BN structure is fabricated using a micromechanical cleavage and dry transfer technique. I-V curve across h-BN exhibits non-linear characteristics and suggests the successful fabrication of tunnel barrier. A spin signal is observed in non-local magnetoresistance measurement. Spin diffusion constant and spin relaxation time are obtained from the Hanle measurement.\\[4pt] [1] T. Yamaguchi, Y. Inoue, et al., Applied Physics Express 6, 073001 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F6.00002: Effect of contacts on spin lifetime measurements in Graphene Evan Sosenko, Vivek Aji Current spintronic devices favor Graphene's high carrier mobility, however spin precession measurements using the Hanle effect in nonlocal spin valve devices have yielded spin lifetimes between 100 ps and 1 ns. These are orders of magnitude smaller than what is observed in ESR measurements or expected theoretically. In this talk, I revisit the issue of contact induced losses, and establish the extent to which it accounts for this discrepancy. We use the standard approach of solving the Block equations augmented by boundary conditions characterizing the device. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F6.00003: Homoepitaxial Graphene Tunnel Barriers Adam Friedman, Olaf van 't Erve, Connie Li, Jeremy Robinson, Berend Jonker Tunnel barriers are key elements for spintronic devices. Such devices require mating dissimilar materials, raising issues of heteroepitaxy, interface stability, and electronic states that severely complicate fabrication and compromise performance. Graphene is the perfect tunnel barrier: It is an insulator out-of-plane, possesses a defect-free, linear habit, and is impervious to interdiffusion. Nonetheless, true tunneling between two stacked graphene layers is not possible except under extreme circumstances. However, two stacked graphene layers can be decoupled using chemical functionalization, which would allow tunneling between the two layers and the realization of an all graphene electronic tunneling device. Here, we demonstrate a homoepitaxial tunnel barrier device in which graphene serves as both the tunnel barrier and the high mobility transport channel. Beginning with bilayer graphene, we fluorinate the top layer to decouple it from the bottom layer, so that it serves as a single monolayer tunnel barrier for both charge and spin injection into the lower graphene transport channel. We demonstrate high spin injection efficiency and lateral transport of spin currents in non-local spin-valve structures and determine spin lifetimes with the non-local Hanle effect. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F6.00004: Tailoring Graphene Spintronics from first Principles Igor Zutic, Predrag Lazic, Guilherme Matos Sipahi, Nicolae Atodiresei, Roland Kawakami, Kirill Belaschenko, Branislav Nikolic Graphene/ferromagnet junctions provide large spin signals and important opportunities for spintronic devices [1,2]. However, for critical studies of such structures it is crucial to establish accurate predictive methods that would yield atomically-resolved information of interfacial properties and incorporate van der Walls interactions. We formulate a computationally-inexpensive model to study spin injection and proximity effects [3] and apply our finding to magneto-logic gates [2] using Ni(111) or Co(0001) as the ferromagnetic electrode. We show that spin polarization maps can be a versatile tool to tailor materials properties for graphene spintronics and explore their relation to computationally more demanding nonequilibrium transport codes [4]. [1] W. Han et al., Phys. Rev. Lett. 105, 167202 (2010); I. Neumann et al., Appl. Phys. Lett. 103,112401 (2013). [2] H. Dery et al., IEEE Trans. Electron Dev. 59, 259 (2012). [3] G. M. Siphai et al., J. Phys. Cond. Matter (in press); P. Lazic et al., preprint. [4] K. K. Saha, et al., Phys. Rev. B 85, 184426 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F6.00005: Cr2O3 Films for Magnetoelectric Gate Applications Sean Stuart, Edward Sachet, J.P. Maria, J.E. (Jack) Rowe, Marc C. Ulrich, Dan Dougherty The magnetoelectric properties of Cr2O3 have been extensively studied, including recent reports of a robust electrically switched magnetic surface state. We have identified Cr2O3 as a material whose magnetoelectric properties would enable voltage controlled switching of the exchange interaction with graphene, as in the Field Effect Transistor proposed by Semenov et al. (Appl. Phys. Lett. 91, 153105). We used pulsed laser deposition to grow thin Cr2O3 films directly on HOPG and sapphire. Atomic force microscopy for films grown on HOPG show closely packed Cr2O3 islands, with a smooth surface interrupted by grain boundaries. X-Ray Diffraction shows that the film has a (0001) texture for films grown at 650 deg. C, which is the ideal orientation for magnetoelectric gating. X-Ray photoelectron spectroscopy on incomplete films suggest strong chemical interactions between the graphite and Cr2O3. Films grown on sapphire have improved crystallinity and surface morphology, which allow for measurement of the surface magnetization by magnetic force microscopy after magneto-electric annealing. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F6.00006: Spin Transfer Torque in Graphene Chia-Ching Lin, Zhihong Chen Graphene is an idea channel material for spin transport due to its long spin diffusion length. To develop graphene based spin logic, it is important to demonstrate spin transfer torque in graphene. Here, we report the experimental measurement of spin transfer torque in graphene nonlocal spin valve devices. Assisted by a small external in-plane magnetic field, the magnetization reversal of the receiving magnet is induced by pure spin diffusion currents from the injector magnet. The magnetization switching is reversible between parallel and antiparallel configurations by controlling the polarity of the applied charged currents. Current induced heating and Oersted field from the nonlocal charge flow have also been excluded in this study. Next, we further enhance the spin angular momentum absorption at the interface of the receiving magnet and graphene channel by removing the tunneling barrier in the receiving magnet. The device with a tunneling barrier only at the injector magnet shows a comparable nonlocal spin valve signal but lower electrical noise. Moreover, in the same preset condition, the critical charge current density for spin torque in the single tunneling barrier device shows a substantial reduction if compared to the double tunneling barrier device. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:48AM |
F6.00007: Defect-Mediated Spin Relaxation and Dephasing in Graphene Invited Speaker: Joshua Folk This talk will describe a series of transport measurements that disentangle mechanisms of spin and orbital phase relaxation in graphene. The measurements are based on well-known quantum interference phenomena--weak localization and universal conductance fluctuations. We show that a careful analysis of the in-plane magnetic field and temperature dependences of these effects can separately quantify spin-orbit and magnetic scattering rates; this technique works especially well in graphene due to its single-atom thickness. Spin relaxation in exfoliated graphene on SiO$_2$ is found to be dominated by magnetic scattering (scattering off of magnetic defects), with a smaller contribution from spin-orbit interaction. A similar measurement performed in graphene on SiC suggests that both magnetic scattering and spin-orbit interaction are a factor of 10 stronger than in exfoliated graphene. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F6.00008: Graphene spin relaxation via resonant scattering off magnetic impurities Denis Kochan, Martin Gmitra, Jaroslav Fabian We will present phenomenological theory, based on first-principles calculations, of the exchange splitting and spin relaxation in graphene with hydrogen adatoms. The phenomenological modeling includes a symmetry based tight-binding model with the adatom interaction and local exchange couplings that are fitted to the first-principles electronic band structure data in the ferromagnetic ground state of hydrogenated graphene. We will show that resonant scattering and the exchange interaction with the paramagnetic impurities at the adatom site can explain the experimentally observed short spin relaxation times, providing a competitive mechanism to that based on spin-orbit coupling. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F6.00009: Inter-valley scattering and spin transport in graphene Sergio E. Ulloa, Mahmoud M. Asmar Electron scattering in graphene is characterized by a highly anisotropic behavior due to the helical nature of its charged carriers. This anisotropy has been experimentally verified in [1], as the ratio of transport to elastic times is found to take a constant value of ~2, consistent with the single valley Dirac equation description at low energies. It was also shown theoretically in [2] that the presence of spin orbit interactions (SOIs) transforms the intra-valley scattering process to be increasingly isotropic for stronger SOI. In this work we analyze the effects of inter-valley scattering on the electronic and spin transport of electrons in graphene. By considering the most relevant terms allowed by time reversal symmetry in the Dirac Hamiltonian, and using partial wave decomposition, we obtain full spin-dependent scattering amplitudes in the system. Here, we present the scattering in the absence and presence of SOIs, where we extract critical strengths of the inter-valley mixing terms that could lead to drastic changes in previous results [1,2]. We also obtain estimates of the critical parameter features of impurities for which the single valley description of graphene fails. [1] M. Monteverde, et al., PRL 104, 126801 (2010).[2] M. M. Asmar and S. E. Ulloa, arXiv:1311.1271 [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F6.00010: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F6.00011: Ferromagnetism on graphene multilayers by hydrogen adsorption Juan J. Palacios, Mohammed Moaied, Jose V. Alvarez, Maria J. Caturla A remarkable theoretical prediction for graphene is that, in theory, it can be permanently magnetized by the adsorption of H atoms. Unfortunately, this will only be possible if the adsorption is selectively realized in such a way that all H atoms occupy the same sublattice so that the contributions of the H-induced local magnetic moments add up due to the expected ferromagnetic coupling in this situation. Inspired by recent experiments, I will show that such selectivity can be naturally achieved on the graphite surface. Due to the sublattice broken symmetry on the surface, a spontaneous arrangement of the hydrogen atoms where all end up adsorbed on the same sublattice takes place at room temperature in a reasonable time scale. First-principles calculations combined with kinetic Monte Carlo simulations and model Heisenberg-like Hamiltonians derived from them give a complete account of the emergence of this novel ferromagnetism. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F6.00012: Magnetism of Adatom on Bilayer Graphene and its Control: A First-principles Perspective Tanusri Saha-Dasgupta, Dhani Nafday We present first-principles investigation of the electronic and magnetic properties of adatom on bilayer graphene within the framework of density functional theory. In particular, we study the influence of an applied gate-voltage which modifies the electronic states of the bilayer graphene as well as shifts the adatom energy states relative to that of the graphene energy states. Our study carried out for a choice of three different adatoms, Na, Cu and Fe, shows that the nature of adatom-graphene bonding evolves from ionic to covalent, in moving from alkali metal, Na to transition metal, Cu or Fe. This leads to the formation of magnetic moments in the latter cases (Cu, Fe) and its absence in the former (Na). Application of an external electric field to bilayer graphene, completely changes the scenario, switching on a magnetic moment for Na adatom, and switching off the magnetic moments for Cu, and Fe adatoms. Our results have important implications for fundamental studies of controlled adatom magnetism and spintronics application in nanotechnology. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F6.00013: Substrate Effects on Adsorbate-induced Magnetism in Graphene Pratibha Dev, Thomas Reinecke Using density functional theory, we show that substrates play an important role in the properties of layered systems such as graphene. In particular, we focus on the effects of a copper substrate on magnetic properties associated with functionalized graphene. Local magnetic moments are created in freestanding graphene by decorating it with an unequal number of fluorine (hydrogen) adatoms in the two sublattices. However, when the functionalized graphene is placed on copper, the local moments completely disappear. We attribute this to several interconnected effects -- doping by the substrate, increased distortion relative to the freestanding case and broadening of the defect states. We show that the interactions with the substrate and the formation of local magnetic moments can be modified by using multiple layers of graphene. This work also shows the importance of including the effects of the immediate environment in determining the properties of functionalized, layered structures such as graphene. [Preview Abstract] |
Session F7: Focus Session: Low-D Quantum Spins I
Sponsoring Units: GMAGChair: Chris Landee, Clark University
Room: 106
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F7.00001: Quantum magnetism in low dimensions and large magnetic fields Invited Speaker: Thierry Giamarchi The ability to control the properties of magnetic insulators by magnetic fields large enough to fully polarize the system has opened a host of possibilities. In addition to the intrinsic interest of such questions for magnetic systems, is has been shown that such systems could be efficiently used as quantum simulators to emulate problems pertaining to itinerant fermionic or bosonic systems. The magnetic field can then be viewed as similar to a gate voltage controlling the number of ``particles'' allowing an unprecedented level of control. In parallel with the experimental developments, progress on the theoretical front both on the numerical and the analytical side, have allowed a remarkable level of accuracy in obtaining the physical properties and in particular the correlation functions of these systems. A comparison between theoretical predictions without adjustable parameters or fudging with results from NMR, Neutrons or other probes such as ESR is thus now possible. This has allowed for example to test {\it quantitatively} the physics of Tomonaga-Luttinger liquids and also to tackle the effects of the interactions between spinons by comparing the physics of weak rung ladders with the one of strong rung ones. Comparison between the neutron results and theoretical calculations of the correlation functions has also been demonstrated as a way to reconstruct efficiently the Hamiltonian from the experimental data. I will review the recent results obtained in this domain with the different experimental compounds and will discuss the open questions and challenges. This concerns in particular the issues of finite temperatures, higher dimensional systems and effects of disorder. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F7.00002: The structure-magnetism relationship in a molecule-based magnetic system: magnetic order and quantum disorder in one and zero dimensions Tom Lancaster, Stephen Blundell, Johannes M\"{o}ller, Saman Ghannadzadeh, Paul Goddard, Peter Baker, Francis Pratt, Jamie Manson We have synthesized two distinct molecule-based magnets from the same starting components. These show different structural motifs which promote contrasting exchange pathways and consequently lead to markedly different magnetic ground states. Through examination of their structural and magnetic properties we show that [Cu(pyz)(H$_{2}$O)(gly)$_{2}$](ClO$_{4}$)$_{2}$ may be considered a quasi one-dimensional quantum Heisenberg antiferromagnet while the related compound [Cu(pyz)(gly)](ClO$_{4}$), which is formed from strongly antiferromagnetically interacting Cu$^{2+}$ dimers, remains disordered down to at least 0.03~K in zero field, but shows a field-temperature phase diagram reminiscent of that seen in materials showing a Bose-Einstein condensation of magnons. We emphasise the use of muon-spin relaxation as a probe of these materials, which has allowed us to determine magnetic ordering that is invisible to many conventional measurement techniques. This is especially useful in low-dimensional magnetism where strong thermal and quantum fluctuations often make transitions to states of long-range magnetic order difficult to observe. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F7.00003: Fractional spinon excitations in the quantum Heisenberg antiferromagnetic chain material CuSO$_4$.5D$_2$O Martin Mourigal, Mechthild Enderle, Axel Kl\"opperpieper, Jean-S\'ebastien Caux, Anne Stunault, Henrik R{\O}nnow One of the simplest quantum many-body systems is the spin-1/2 Heisenberg antiferromagnetic chain, a linear array of interacting magnetic moments. Its exact ground state is a macroscopic singlet entangling all spins in the chain. Its elementary excitations, called spinons, are fractional spin-1/2 quasiparticles created and detected in pairs by neutron scattering. Theoretical predictions show that two-spinon states exhaust only 71\% of the spectral weight and higher-order spinon states, yet to be experimentally located, are predicted to participate in the remaining. By accurate absolute normalization of our inelastic neutron scattering data on the spin-1/2 Heisenberg antiferromagnetic chain compound CuSO$_4$.5D$_2$O, we account for the full spectral weight to within 99(8)\% [1]. Our data thus establish and quantify the existence of higher-order spinon states. The observation that, within error bars, the experimental line shape resembles a rescaled two-spinon one with similar boundaries allows us to develop a simple picture for understanding multi-spinon excitations. \\[4pt] [1] Nature Physics {\bf 9}, 435--441 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F7.00004: Characterization of the Spin-1/2 Linear-Spin-Chain Ferromagnet CuAs$_{2}$O$_{4}$ Kevin Caslin, Reinhard Kremer, Fereidoon Razavi, Armin Schulz, Alfonso Munoz, Franz Pertlik, Jia Liu, Mike Whangbo, Joseph Law We are investigating Cu$^{2+}$ ($S=$1/2) linear-spin-chains systems exhibiting low-dimensional magnetism. Linear-spin-chains are formed when CuX$_{6}$ (X$=$O,Cl,Br,...) Jahn-Teller distorted octahedra link together via their trans-edges. Most often, these spin-chains support ferromagnetic (FM) nearest-neighbor (NN) and antiferromagnetic (AFM) next-nearest-neighbor (NNN) spin-exchange interactions, sometimes leading to an incommensurate spin-spiral structures with multiferroic behavior. There exists a magnetic phase diagram which can predict the intra-chain behavior using a ratio of spin-exchange constants, $\alpha =$Jnn/Jnnn. A quantum critical point exists on a boundary at $\alpha =$- 4, small spin exchange perturbations on a system with an $\alpha $ ratio in the vicinity of this point may induce a pronounced response of the system. In this study, we report on CuAs$_{2}$O$_{4}$ mineral name trippkeite, featuring CuO$_{2}$ ribbon chains. Trippkeite is an exceptional spin-chain system because it shows long-range FM ordering and has an $\alpha$ ratio close to -4. Measurements of magnetic susceptibility, heat capacity, Raman spectroscopy, and electron paramagnetic resonance were performed. DFT calculations and TMRG simulations were also carried out. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F7.00005: Dimerizations in spin-$S$ antiferromagnetic chains with three-spin interaction Zheng-Yuan Wang, Shunsuke C. Furuya, Masaaki Nakamura, Ryo Komakura We discuss spin-$S$ antiferromagnetic Heisenberg chains with three-spin interactions, next-nearest interactions, and bond alternation. First, we prove rigorouslly that there exist parameter regions of the exact dimerized ground state in this system. This is a generalization of the Majumdar-Ghosh model to arbitral $S$. Next, we discuss the ground state phase diagram of the models by introducing several effective field theories and universality classes of the transitions are described by the level-$2S$ $\mathrm{SU}(2)$ Wess-Zumino-Witten model and the Gaussian model. Finally, we determine the phase diagrams of $S=1$ and $S=3/2$ systems by using exact diagonalization and level spectroscopy method. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F7.00006: Spin-orbital entanglement or separation? Understanding elementary excitations in a spin-orbital chain Krzysztof Wohlfeld, Cheng-Chien Chen, Michel van Veenendaal, Thomas P. Devereaux Recent theories have suggested {\it separation} of elementary spin and orbital excitations in anisotropic spin-orbital chains with evidence coming from a number of experiments on various copper oxides [1]. However, it is well-known that elementary excitations in an idealized spin-orbital chain with isotropic SU(4) symmetric interactions contain {\it entangled} spin and orbital quantum numbers [2]. Using a combined analytical and numerical approach, we show that a common description of the excitations in these two limits is possible: the spin and orbital spectra can be described in terms of fractionalized `RVB-like' spinons and antispinons where each excitation carries both spin and orbital quantum numbers, thus showing spin-orbital entanglement. Spin-orbital separation occurs solely in the highly anisotropic limit, and such a description is allowed only due to a particular choice of the spin and orbital basis.\\[4pt] [1] Nature 485, 82 (2012); PRL 107, 147201 (2011); arxiv:1307.6180; arxiv:1310.8346.\\[0pt] [2] PRL 81, 3527 (1998). [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F7.00007: Thermal transport and spin-phonon coupling in the one-dimensional antiferromagnetic spin chain compound CuSb$_2$O$_6$ Narayan Prasai, Joshua Cohn, Alwyn Rebello, Michael Smith, John J. Neumeier We report thermal conductivity ($\kappa$) measurements on single crystals of the $S=1/2$ antiferromagnetic spin-chain compound CuSb$_2$O$_6$ over the temperature range $5{\rm K}\leq {\rm T}\leq 300 {\rm K}$. Similar measurements on the non-magnetic analog compound, ZnSb$_2$O$_6$, allow for a comparison of the lattice thermal conductivities. The role of spin-phonon coupling and twinning on the anisotropic thermal transport of CuSb$_2$O$_6$ will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F7.00008: Magnetic model of BaCuSi$_2$O$_6$ revisited: Bose-Einstein condensation of magnons on a non-frustrated spin lattice Alexander A. Tsirlin, Vladimir V. Mazurenko, Maria V. Valentyuk, Raivo Stern Bose-Einstein condensation (BEC) of magnons remains one of the most intricate collective phenomena observed in quantum magnets. In Han Purple the BEC physics is heavily influenced by structural peculiarities related to the low-temperature structural distortion taking place around 100 K. The crystal structure comprises structural and magnetic dimers forming bilayers, and the BEC transition is formally 2D. Frustrated couplings between the bilayers are believed to be responsible for this effect, because at low enough temperatures the bilayers become decoupled. We challenge this scenario using extensive density-functional (DFT) calculations. We will show that DFT can well reproduce the couplings of $J_A\simeq 50$~K and $J_A\simeq 60$~K in two nonequivalent bilayers. Our calculations also yield a new scenario of the interdimer exchange that takes place between the top site of one dimer and the bottom site of the neighboring dimer rather than top-to-top and bottom-to-bottom. This scenario is verified by INS data and by magnetostructural correlations for the superexchange. The new regime of the interdimer couplings implies that BaCuSi$_2$O$_6$ lacks any appreciable magnetic frustration, individual bilayers are not decoupled, and other explanations for the 2D BEC physics should be sought [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F7.00009: High-resolution thermal expansion measurements of BaCuSi$_4$O$_{10}$ and BaCuSi$_2$O$_6$ Sueli Masunaga, Alwyn Rebello, J.J. Neumeier BaCuSi$_4$O$_{10}$ and BaCuSi$_2$O$_6$ were used in many ancient Chinese artifacts as synthetic pigments, and recently named as Han Blue and Han Purple, respectively.\footnote{E. W. FitzHugh \emph{et al}., Stud. Conserv. \textbf{37}, 145 (1992).} Besides being important synthetic pigments of ancient and modern times, these compounds have attracted scientific and technological interest due to their luminescent properties.\footnote{S. M. Borisov \emph{et al}., Anal. Chim. Acta \textbf{787}, 219 (2003); G. Pozza \emph{et al}., J. Cult. Herit. \textbf{1}, 393 (2000); S. M. Borisov \emph{et al}., Anal. Chem. \textbf{85}, 9371 (2013).} Moreover, Han Purple is a spin-dimer compound with an interesting phase diagram and a potential solid state device for exploring quantum effects in magnetic field induced Bose-Einstein condensation.\footnote{M. Jaime \emph{et al}., Phys. Rev. Lett. \textbf{93}, 087203 (2004).} In this work, we study BaCuSi$_2$O$_6$ and BaCuSi$_4$O$_{10}$ single crystals grown by floating zone method and flux growth technique, respectively. The results of thermal expansion, specific heat, and magnetization measurements of these compounds will be presented in detail. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F7.00010: Thermal phase transitions to valence-bond-solid states in the two dimensional SU(N) Heisenberg models Takafumi Suzuki, Kenji Harada, Haruhiko Matsuo, Synge Todo, Naoki Kawashima The two-dimensional (2D) SU(N) Heisenberg model with n-fold singlet projectors, namely the $JQ_n$ model [1], is believed to be a good example to study the deconfined critical (DC) scenario[2], because this hosts a quantum phase transition between valence-bond-solid (VBS) and magnetic ordered states at $T=0$. The DC scenario tells us that the universality should be same and independent on the broken lattice-rotation symmetry: the same criticality is observed in both $JQ_{3}$ model on the honeycomb lattice ($Z_{3}$) and $JQ_{2}$ model on the square lattice ($Z_{4}$) [3]. However, the thermal phase transition to the VBS phase may be drastically affected by the breaking symmetry. In this study, we consider the SU(N) $JQ_{n}$ models on square and honeycomb lattices and study the thermal phase transition to the VBS phases. From the QMC calculations, we discuss the critical properties for (1) the lattice dependence, (2) N dependence, and (3) coupling ratio $Q_{n}/J$ dependence. [1] T. Senthil, et al., Science 303, 1490 (2004); M. Levin and T. Senthil, Phys. Rev. B 70, 220403(R) (2004). [2] A. W. Sandvik, Phys. Rev. Lett. 98, 227202 (2007). [3] K. Harada, et al., arXiv:1307.0501. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F7.00011: ABSTRACT MOVED TO Q4.00015 |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F7.00012: Giant magnetic anisotropy and quantum tunneling of the magnetization in Li$_2$(Li$_{1-x}$Fe$_x$)N Anton Jesche, R. William McCallum, Srinivasa Thimmaiah, Jenee L Jacobs, Valentin Taufour, Andreas Kreyssig, Robert S. Houk, Sergey L. Bud'ko, Paul C. Canfield The magnetic anisotropy of 3$d$ transition metals is usually considered to be weak, mainly due to the widely known paradigm of orbital quenching. However, a rare interplay of crystal electric field effects and spin-orbit coupling causes a large orbital contribution to the magnetic moment of iron in Li$_2$(Li$_{1-x}$Fe$_x$)N. This leads, not only to large magnetic moments of $\sim$\,5\,$\mu_{\rm B}$ per iron atom but, also, to an enormous magnetic anisotropy field that extrapolates to more than 200 Tesla. Magnetic hysteresis emerges for $T \leq 50$\,K and the coercivity fields of more than 11 Tesla exceed even the hardest 4$f$ electron based ferromagnets. Li$_2$(Li$_{1-x}$Fe$_x$)N not only has a clear and remarkable anisotropy, generally not associated with iron moments, but also shows time-dependence more consistent with molecular magnets. In particular for low iron concentrations $x \ll 1$ the spin-inversion is dominated by a macroscopic tunneling process rather than by thermal excitations. It is shown that the huge magnetic anisotropy makes Li$_2$(Li$_{1-x}$Fe$_x$)N (i) an ideal model system to study macroscopic quantum effects at elevated temperatures and (ii) a basis for novel magnetic functional materials. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F7.00013: Molecular $j_{\rm eff}$ states in ternery transition metal chalcogenides $AM_4X_8$ Heung-Sik Kim, Jino Im, Myung Joon Han, Hosub Jin Spin-orbit-coupling(SOC)-induced $j_{\rm eff}$ states, reported in several iridium oxide compounds, is the key ingredient in understanding the interesting cooperation between SOC and the electron correlations. From our density functional theory calculations we suggest that, a series of ternery transition metal chalcogenides $AM_4X_8$ ($A$ = Ga, $M$ = 4$d$ and 5$d$ transition metal atoms, $X$ = chalcogen atoms) host the $j_{\rm eff}$ states in a molecular form. Wide range of the bandwidth covered with the external or chemical pressure enable one to access a broad range of electron correlation strength in a single compound. Implications of our results in both the weak and strong coupling regime are discussed. Our finding provides an ideal playground in exploring the $j_{\rm eff}$ physics and the resulting emergent phenomena. [Preview Abstract] |
Session F8: Focus Session: Magnetism Techniques: Temporal and Spatial Characterization
Sponsoring Units: GMAGChair: Avag Sahakyan, The State Engineering University of Armenia
Room: 104
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F8.00001: One-dimensional scattering of electrons and neutrons in nano heterostructures with magnetic inclusions Avag Sahakyan, Ruzan Movsesyan, Armen Kocharian The spin dependent scattering of electrons and neutrons in one dimension is investigated in systems, containing a nano size magnetic layers. Two thin systems are considered such as: a) magnetic layer with non magnetic surrounding and b) two magnetic layers divided by non magnetic layer or non magnetic surrounding. Magnetization of layers, in general, contains longitudinal and transverse components (parallel and perpendicular to the interface, respectively). It is shown that the Zeeman energy splitting of longitudinal component caused by magnetic field provides modulation of partial scattering amplitude for transmitted and reflected waves. On the other hand, the transverse field can have perceptible contribution into the backward and forward scattering phase. The latest provided an opportunity to manage continuously the scattering energy for resonance transmission. In particular, for electron system this modulation can be manifested in behavior both, conductance and reflectance. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F8.00002: Design considerations for a high sensitivity Barkhausen Noise sensor Neelam Prabhu Gaunkar, Orfeas Kypris, Cajetan Nlebedim, David Jiles Barkhausen emissions are produced due to sudden changes in magnetization when a continuously changing magnetic field is applied to a ferromagnetic material. The emissions described as Barkhausen noise can be observed as voltage signals using induction sensors. Effective capture of these emissions with high level of precision depends on several parameters influenced by the sensor design. For the magnetization unit, amongst others, the critical parameters include the magnetic field produced by the magnetizing coils, core geometry, sensor-to-specimen coupling, choice of core material, core length and operating frequency. Similarly, for the sensing unit the optimal pick-up coil material and number of winding turns need to be optimized. Enhancing these parameters will lead to improved sensitivity, reproducibility and reliability of the detected Barkhausen emissions. Using finite element analysis, this study shows design considerations for optimizing these parameters in order to achieve high accuracy in detection and analysis of Barkhausen signals especially as a tool for magnetic non-destructive evaluation. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F8.00003: Development of a new magnetic Barkhausen spectroscopy method for the non-destructive characterization of magnetic materials Orfeas Kypris, Ikenna Nlebedim, David Jiles Barkhausen emissions, which result from discontinuous, irreversible changes in magnetization, are related to the stress state, defect/inclusion sizes and microstructure of ferromagnetic materials. Time domain analysis of Barkhausen signals measured at the surface of a specimen can reveal the average magnitude of stress in the structure. Such analysis offers a powerful tool for magnetic nondestructive characterization of materials. However, determining the stress and other microstructural parameters as a function of depth still remains a challenging problem, which can be treated in the frequency domain. In this work, a model for stress-depth profiling of ferromagnets is developed. In the model, the frequency spectrum at the surface of a specimen is described in terms of two parameters; the average amplitude of Barkhausen emissions at their origin $V_{orig}$ and $\zeta$, which is proportional to the square root of magnetic permeability. A ferromagnetic structure is mathematically divided into homogeneous layers with each layer acting as a source of Barkhausen signal having a unique spectrum that is attenuated as it propagates to the surface. We show that $V_{orig}$ and $\zeta$ correlate with stress and we provide a framework for detecting stress variations as a function of depth. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F8.00004: Magneto-optic-Kerr-effect-based spin-orbit torque magnetometer Halise Celik, Xin Fan, Wenrui Wang, Jun Wu, Chaoying Ni, Kyung-Jin Lee, John Xiao, Virginia Lorenz Current-induced spin-orbit torques in heavy metal (HM)/ferromagnetic metal (FM) bilayers have attracted great attention for their potential in spintronic applications. It is essential to be able to measure the magnitude and direction of the spin-orbit torques. There have been several methods developed to measure spin-orbit torques based on second-order rectifying voltages, including spin-torque ferromagnetic resonance [1] and second-order harmonic voltage detection [2]. While these techniques have been widely used, they have their respective limits, e.g. requirement of an in-plane/out-of-plane magnetization configuration, small damping, etc. Here we present the development of a first-order spin-orbit torque magnetometer that is based on the magneto-optic Kerr effect (MOKE). The MOKE-based spin-orbit torque magnetometer is sensitive and versatile and can be used in both in-plane and out-of-plane magnetized samples. References: [1] L. Liu et al., Spin-Torque Ferromagnetic Resonance Induced by the Spin Hall Effect, Physical Review Letters 106, 036601 (2011). [2] J. Kim et al., Layer thickness dependence of the current-induced effective field vector in Ta\textbar CoFeB\textbar MgO, Nature Materials 12, 240-245 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F8.00005: Manipulating femtosecond magnetism through pressure: First-principles calculations Mingsu Si, Guoping Zhang Inspired by a recent pressure experiment in fcc Ni, we propose a simple method to use pressure to investigate the laser-induced femtosecond magnetism. Since the pressure effect on the electronic and magnetic properties can be well controlled experimentally, this leaves little room for ambiguity when compared with theory. Here we report our theoretical pressure results in fcc Ni: Pressure first suppresses the spin moment reduction and then completely diminishes it; further increase in pressure to 40 GPa induces a demagnetization-to-magnetization transition. To reveal its microscopic origin, we slide through the $L$-$U$ line in the Brillouin zone and find two essential transitions are responsible for this change, where the pressure lowers two valence bands, resulting in an off-resonant excitation and thus a smaller spin moment reduction. In the spin-richest $L$-$W$-$W^{'}$ plane, two spin contours are formed; as pressure increases, the contour size retrieves and its intensity is reduced to zero eventually, fully consistent with the spin-dipole factor prediction. These striking features are detectable in time- and spin-resolved photoemission experiments. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F8.00006: Magnetization process and topological plateau phase induced by circularly polarized laser Shintaro Takayoshi, Masahiro Sato, Takashi Oka One of the fundamental experiments to investigate magnetic properties of materials is a measurement of magnetization curve. Antiferromagnets with large exchange couplings, however, need high external field to achieve their saturated magnetization, and large equipment is required in experiments. We theoretically propose a new and dynamic way to realize magnetization processes of general quantum magnets without any static field. The way is to apply a circularly polarized laser to magnetic systems. We can show that the coupling between the laser and magnets is mapped to an effective static Zeeman term with a longitudinal magnetic field via a time-dependent unitary transformation or Floquet theory. It is hence expected that the magnetization curve of magnets can be realized by applying a suitable laser. We demonstrate dynamical magnetization processes by numerically solving Schr\"odinger equations for concrete quantum spin models under applied lasers. We also show that a laser-induced magnetization plateau state appears in a simple Ferro-Ferro-Antiferro spin chain model under a certain condition and it has a topological nature. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F8.00007: Circularly polarized high harmonic generation for element-selective probing of magnetic materials on a tabletop Patrick Grychtol, Emrah Turgut, Dmitriy Zusin, Dimitar Popmintchev, Tenio Popmintchev, Henry Kapteyn, Margaret Murnane, Ronny Knut, Hans Nembach, Justin Shaw, Ofer Kfir, Avner Fleischer, Oren Cohen Ultrafast short wavelength sources based on high harmonic upconversion of femtosecond lasers are unique in their ability to simultaneously probe the magnetically-sensitive M absorption edges of the 3d ferromagnets Fe, Co and Ni. This novel capability to capture the fastest spin dynamics in materials has uncovered a wealth of new fundamental understanding about spin scattering and transport on few-femtosecond timescales. However, to date these investigations have used linearly polarized higher harmonics, since it has not been possible to generate circularly polarized harmonics with sufficient flux for scientific applications. In this contribution, we present a simple setup that enables the efficient generation of circularly polarized harmonics, and demonstrates that they are bright enough for studies of magnetic materials. The fundamental and second harmonic of a Ti:sapphire laser are focused into a gas filled waveguide under good phase matching conditions, with opposite chirality circular polarizations. Thus, circularly-polarized harmonics are produced that are then used to perform magnetic circular dichroism studies in the extreme ultraviolet photon energy range. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F8.00008: Development of a microwave probe for the optical study of microwave-excited spin physics Yu-Sheng Ou, Yi-Hsin Chiu, Rohan Adur, Patrick Odenthal, Roland Kawakami, P. Chris Hammel, Ezekiel Johnston-Halperin We have developed an experimental probe that allows simultaneous broadband microwave excitation and optical excitation/detection at variable temperature and magnetic field. Specifically, we have designed a unique sample probe with a microwave stripline based sample mount that allows for direct optical access to the sample under study within a magneto- optical cryostat. This powerful combination enables optical studies of spintronic systems under microwave excitation using both CW (e.g. photo- and electro-luminescence) and time resolved (e.g. time resolved absorption/transmission and time resolved Kerr rotation, TRKR) techniques. To benchmark the capabilities of this probe we present data demonstrating simultaneous ferromagnetic resonance (FMR) and TRKR in a Fe/MgO/GaAs heterostructure. Such studies have potential applications in the study of FMR driven spin pumping and interaction of free carrier spins with native and engineered defects. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F8.00009: Optically Detected Scanned Probe Magnetic Resonance Imaging Christopher Wolfe, Vidya Bhallamudi, Hailong Wang, Chunhui Du, Sergei Manuilov, Rohan Adur, Fengyuan Yang, P. Chris Hammel Magnetic resonance is a powerful tool for studying magnetic properties and dynamics of spin systems. Scanned magnetic probes can induce spatially localized resonance due to the strong magnetic field and gradient near the magnetic tip.\footnote{K.C. Fong, M.R. Herman, P. Banerjee, D.V. Pelekhov, and P.C. Hammel, Phys. Rev. B 84, 220405(R) (2011).}$^,$\footnote{I. Lee, Y. Obukhov, G. Xiang, A. Hauser, F. Yang, P. Banerjee, D.V. Pelekhov, and P.C. Hammel, Nature 466, 845 (2010).} Nitrogen vacancy centers (NV) in diamond provide a sensitive means of measuring magnetic fields at the nanoscale. We report preliminary results towards using the high sensitivity of NV detection with a scanned magnetic probe to study local magnetic phenomena. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F8.00010: On-chip coplanar stripline for micron-scale ferromagnetic resonance and spin pumping devices Shane White, Andrew Berger, Rohan Adur, Sergei Manuilov, P. Chris Hammel Ferromagnetic resonance (FMR) spin pumping is a rapidly growing field. While previous measurements have focused on large-scale ($\sim$mm) devices, dimensions will need to be reduced to prove useful for applications. On-chip microwave structures offer a solution to this by providing localized microwave fields that can be used to resonate small device geometries while leaving them easily accessible for electrical connections, as opposed to the ``flip-chip'' or resonant cavity methods. Using a shorted coplanar strip (CPS) waveguide, we perform broadband (6-12 GHz) FMR measurements in a permalloy bar of dimensions 20$\mu$m x 1$\mu$m x 20nm--too small to be detected by measuring microwave reflections from a cavity. FMR is detected in the permalloy strip through changes in the anisotropic magnetoresistance [1]. This scheme allows for quantitative characterization of magnetization dynamics and microwave fields. These findings demonstrate that on-chip microwave structures will enable new, smaller device geometries and measurement possibilities for a variety of spin pumping systems.\\[4pt] [1] M. V. Costache, et. al, Appl. Phys. Lett. 89, 232115 (2006). [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F8.00011: Probe-localized modes in continuous YIG thin films Rohan Adur, Chunhui Du, Sergey A. Manuilov, Chi Zhang, Denis V. Pelekhov, Hailong Wang, Fengyuan Yang, P. Chris Hammel The measurement of damping in precessing ferromagnets is obscured by the excitation of spin waves of different wavelengths due to defects and inhomogeneities in the ferromagnetic material. In order to reduce this parasitic broadening the magnetic mode can be confined to small volumes (nm to $\mu$m) either by external fields or by patterning. While nanostructures have shown size-dependent effects such as suppression of inhomogeneity when the size of the nanostructure is sufficiently small [1], it has been vital to consider the effect of imperfections in lithography that can cause edge damage and hence extrinsic linewidth broadening. In contrast, the dipolar field from a micron-sized probe magnet can be used to localize a mode in a continuous thin film without lithographic modification to the film. This technique of localized mode ferromagnetic resonance force microscopy (FMRFM) has been demonstrated in permalloy [2] at liquid helium temperature. In the present study we demonstrate probe-localized modes in a YIG thin film (t=25nm) measured at room temperature. Using FMRFM we explore the spatial and size dependence of inhomogeneity and damping of a localized mode within a continuous film. [1] C Hahn et al, 58th MMM conference BC-09 (2013) [2] I Lee et al, Nature 466, 845 (2010) [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F8.00012: Scanned Spin-Precession Microscopy: Progress towards cantilever based approach Vidya Bhallamudi, Christopher Wolfe, Vivek Amin, Helena Reichlova, Andrew Berger, David Stroud, Jairo Sinova, P.Chris Hammel The principal spin microscopy tools for spintronic materials are primarily based on optical detection and are thereby limited to certain materials. There is a need for imaging tools that can address a wider range of materials. Towards this end we recently developed Scanned Spin-Precession Microscopy [1, 2], where we demonstrated the ability to extract local spin properties from a spatially-averaged signal. This is enabled by the modification of the precessional behavior of the spins in a small region by the strongly inhomogeous magnetic field from a micromagnetic probe. We will discuss this novel imaging tool and our recent efforts towards a cantilever-based approach for wider applicability, especially for electrical spin-based devices.\\[4pt] [1] V. P. Bhallamudi et.al., PRL 111, 117201 (2013).\\[0pt] [2] V. P. Bhallamudi et.al., JAP. 111, 013902 (2012) [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F8.00013: Soft x-ray coherent diffraction imaging on magnetic nanostructures Xiaowen Shi, James Lee, Shrawan Mishra, Daniel Parks, Tolek Tyliszczak, David Shapiro, Sujoy Roy, Steve Kevan Coherent soft X-rays diffraction imaging enable coherent magnetic resonance scattering at transition metal L-edge to be probed so that magnetic domains could be imaged with very high spatial resolution with phase contrast, reaching sub-10nm. One of the overwhelming advantages of using coherent X-rays is the ability to resolve phase contrast images with linearly polarized light with both phase and absorption contrast comparing to real-space imaging, which can only be studied with circularly polarized light with absorption contrast only. Here we report our first results on high-resolution of magnetic domains imaging of CoPd multilayer thin film with coherent soft X-ray ptychography method. We are aiming to resolve and understand magnetic domain wall structures with the highest obtainable resolution here at Advanced Light Source. In principle types of magnetic domain walls could be studied so that Neel or Bloch walls can be distinguished by imaging. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F8.00014: Deterministic propagation of nanomagnetic logic observed by time-resolved XMCD-PEEM Mark Nowakowski, Zhang Gu, Brian Lambson, Jeongmin Hong, Ralph Storz, Patrick Bennett, David Carlton, Weilun Chao, Scott Dhuey, Anthony Young, Andrew Doran, Matthew Marcus, Andreas Scholl, Jeffrey Bokor Nanomagnetic logic (NML) is a low-power logic architecture that relies on the dipolar coupling of closely spaced (30 nm) magnets (450x150 nm) to flow binary information down lithographically defined chains. A majority logic gate selects an output based on the magnetic orientation of three intersecting NML chains, permitting logic functions without requiring electrical currents like those used in Si-based transistors. The repeatable and reliable flow of magnetic signal propagation down a chain, a critical feature of this technology, has not been experimentally demonstrated, however computational models have predicted NML signal flow and have postulated a better performance from lithographically engineered magnets with configurational anisotropy. Using the PEEM-3 microscope at the Advanced Light Source, we perform an XMCD pump-probe measurement and observe signal propagation along a chain of 13 magnets with configurational anisotropy. We resolve successive individual magnets flipping on 100 ps time scales and complete signal propagation down the chain after 1-2 ns. This behavior is consistent with previous computational models. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F8.00015: Real Space Visualization of Mott Gap and Magnon Excitations Yao Wang, Chunjing Jia, Brian Moritz, Thomas Devereaux Real-space and time information plays a significant role in understanding inhomogeneous physical and chemical processes at the nano-scale. Experimentally, inelastic light scattering promises to become an important tool for characterizing the spatio-temporal properties of complex systems. To demonstrate the power of this technique, we perform a theoretical study of real-space charge and spin density response functions in the Hubbard model to track time-dependent Mott gap and magnon excitations. Carrier doping is found to affect the evolution of the charge and spin response with distinct timescales and real-space patterns appearing for n- or p-type materials. [Preview Abstract] |
Session F10: Focus Session: Evolutionary and Ecological Dynamics II
Sponsoring Units: DBIO GSNPChair: Pankaj Mehta, Boston University
Room: 201
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F10.00001: Environmental vs. demographic variability in stochastic lattice predator-prey models Invited Speaker: Uwe C. Tauber In contrast to the neutral population cycles of the deterministic mean-field Lotka-Volterra rate equations, including spatial structure and stochastic noise in models for predator-prey interactions yields complex spatio-temporal structures associated with long-lived erratic population oscillations. Environmental variability in the form of quenched spatial randomness in the predation rates results in more localized activity patches. Population fluctuations in rare favorable regions in turn cause a remarkable increase in the asymptotic densities of both predators and prey [1]. Very intriguing features are found when variable interaction rates are affixed to individual particles rather than lattice sites. Stochastic dynamics with demographic variability in conjunction with inheritable predation efficiencies generate non-trivial time evolution for the predation rate distributions, yet with overall essentially neutral optimization [2].\\[4pt] [1] U. Dobramysl and U.C.T., Phys. Rev. Lett. {\bf 101}, 258102 (2008);\\[0pt] [2] U. Dobramysl and U.C.T., Phys. Rev. Lett. {\bf 110}, 048105 (2013); J. Stat. Mech. P10001 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F10.00002: Stochastic recruitment leads to symmetry breaking in foraging populations Tommaso Biancalani, Louise Dyson, Alan McKane When an ant colony is faced with two identical equidistant food sources, the foraging ants are found to concentrate more on one source than the other. Analogous symmetry-breaking behaviours have been reported in various population systems, (such as queueing or stock market trading) suggesting the existence of a simple universal mechanism. Past studies have neglected the effect of demographic noise and required rather complicated models to qualitatively reproduce this behaviour. I will show how including the effects of demographic noise leads to a radically different conclusion. The symmetry-breaking arises solely due to the process of recruitment and ceases to occur for large population sizes. The latter fact provides a testable prediction for a real system. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F10.00003: Ising-like patterns of spatial synchrony in population biology Andrew Noble, Alan Hastings, Jon Machta Systems of coupled dynamical oscillators can undergo a phase transition between synchronous and asynchronous phases. In the case of coupled map lattices, the spontaneous symmetry breaking of a temporal-phase order parameter is known to exhibit Ising-like critical behavior. Here, we investigate a noisy coupled map motivated by the study of spatial synchrony in ecological populations far from the extinction threshold. Ising-like patterns of criticality, as well as spinodal decomposition and homogeneous nucleation, emerge from the nonlinear interactions of environmental fluctuations in habitat quality, local density-dependence in reproduction, and dispersal. In the mean-field limit, the correspondence to the Ising model is exact: the fixed points of our dynamical system are given by the equation of state for Weiss mean-field theory under an appropriate mapping of parameters. We have strong evidence that a quantitative correspondence persists, both near and far from the critical point, in the presence of fluctuations. Our results provide a formal connection between equilibrium statistical physics and population biology. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F10.00004: A Computational Approach to Competitive Range Expansions Markus F. Weber, Gabriele Poxleitner, Elke Hebisch, Erwin Frey, Madeleine Opitz Bacterial communities represent complex and dynamic ecological systems. Environmental conditions and microbial interactions determine whether a bacterial strain survives an expansion to new territory. In our work, we studied competitive range expansions in a model system of three \textit{Escherichia coli} strains. In this system, a colicin producing strain competed with a colicin resistant, and with a colicin sensitive strain for new territory. Genetic engineering allowed us to tune the strains' growth rates and to study their expansion in distinct ecological scenarios (with either cyclic or hierarchical dominance). The control over growth rates also enabled us to construct and to validate a predictive computational model of the bacterial dynamics. The model rested on an agent-based, coarse-grained description of the expansion process and we conducted independent experiments on the growth of single-strain colonies for its parametrization. Furthermore, the model considered the long-range nature of the toxin interaction between strains. The integration of experimental analysis with computational modeling made it possible to quantify how the level of biodiversity depends on the interplay between bacterial growth rates, the initial composition of the inoculum, and the toxin range. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F10.00005: The effects of psammophilous plants on sand dune dynamics Golan Bel, Yosef Ashkenazy Sand dune dynamics involve physical processes in many temporal and spatial scales. Many physical and mathematical models have been developed to explain the interesting patterns of sand dunes. While many works have focused on the formation and patterns of sand dunes, the observed bi-stability of fixed and active sand dunes under the same climatic conditions has received little attention. Many of the models considered different types of sand dune cover (affecting dune activity); however, despite their important role in dune dynamics, to our knowledge, psammophilous plants (special plants that flourish in moving sand environments) have never been incorporated into mathematical models of sand dunes. Here, we propose a non-linear physical model for the role of psammophilous plants in the dynamics of sand dunes. The model exhibits complex bifurcation diagrams and dynamics, which explain observed phenomena, and predicts new dune stabilization scenarios. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F10.00006: The fate of complex ecologies: How do species organize? An exact method Ahmed Roman, Michel Pleimling Complex ecology models present a bridge between far from equilibrium physics and biology of populations. The May-Leonard, Rock-Paper-Scissor and Lotka-Volterra models have been extensively studied in an attempt to understand the dynamics of finite but large populations. In this talk we present a new theoretical technique which predicts the dynamics of these models for any complex ecology with interactions similar to the aforementioned models. This method has applications to real-world systems as it presents a simple method to predict correlations among two or more species in a complex ecology. We apply this method to the models mentioned and show that exact agreement between predictions and Monte-Carlo simulation data is obtained. This method could be applied to a wide variety of problems from economics to biology and game theory. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F10.00007: Collapse of biodiversity in fractured metacommunities Charles Fisher, Pankaj Mehta The increasing threat to global biodiversity from climate change, habitat destruction, and other anthropogenic factors motivates the search for features that increase the resistance of ecological communities to destructive disturbances. Recently, Gibson et al (\emph{Science} 2013) observed that the damming of the Khlong Saeng river in Thailand caused a rapid collapse of biodiversity in the remaining tropical forests. Using a theoretical model that maps the distribution of coexisting species in an ecological community to a disordered system of Ising spins, we show that fracturing a metacommunity by inhibiting species dispersal leads to a collapse in biodiversity in the constituent local communities. The biodiversity collapse can be modeled as a diffusion on a rough energy landscape, and the resulting estimate for the rate of extinction highlights the role of species functional diversity in maintaining biodiversity following a disturbance. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F10.00008: Flow-driven Delocalization of Populations with Heterogeneous Growth Rates Thiparat Chotibut, David Nelson, Sauro Succi Growth in controlled laboratory environments such as a Petri dish can be used to study the spatial evolutionary dynamics of microorganisms. However, natural populations often grow up in heterogeneous environments with spatially varying growth rates, and can be subjected to fluid advection as well. Using lattice Boltzmann simulations, we study single species population dynamics subject to constant flows under heterogeneous growth conditions. We show that quenched random growth rates lead to localized growth niches even in the presence of a background fluid flow. Non-equilibrium steady states when the flow velocity is weak exhibit a mixture of localized high-density growth niches and a low-density background mass distribution influenced by extended states of the linearized growth operator. At sufficiently strong advection, however, the growth niches suddenly delocalize to form elongated parallel streaks of order the system size along the flow direction. We discuss the localized and delocalized growth eigenfunctions, as well as a phase transition characterized by a diverging correlation length in the flow direction. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F10.00009: Correlation between stability and resilience in multiple deteriorating environments Lei Dai, Kirill Korolev, Jeff Gore The recovery rate and the basin of attraction are two important properties that describe the local and global stability of dynamical systems. The idea that loss of stability (i.e. slower recovery) may indicate loss of resilience (i.e. shrinking size of basin of attraction), especially in the context of providing warning signals as a system is close to bifurcations, has been demonstrated before transitions in many systems, such as ecosystems, the climate, neurons and power grids. However, most empirical studies focus on the observation of warning signals with respect to a particular type of environmental change. Here we measure the stability-resilience relationship of laboratory microbial populations in different deteriorating environments (e.g. increasing death rate, nutrient limitation, etc.). We found that the loss of stability is correlated with loss of resilience before population collapsed in multiple scenarios of deterioration, but the warning signals increased with variable levels under different drivers. We mapped out the relationship between stability and resilience by tuning three drivers and also evaluated possible scenarios of environmental change where the positive correlation between the recovery rate and the basin of attraction may break down. Our results suggest the correlation between stability and resilience can be utilized to assess the fragility of dynamical systems under environmental changes; however, the stability-resilience relationship can be complex and will limit our assessment when multiple drivers are involved. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F10.00010: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F10.00011: Coarsening and biodiversity in cyclically competing species Ben Intoy, Michel Pleimling When four species compete stochastically in a cyclic way, the formation of two teams of mutually neutral partners is observed. We study through numerical simulations the extinction processes that can take place in this system both in the well mixed case as well as on different types of lattices [1]. The different routes to extinction are revealed by the probability distribution of the domination time, i.e. the time needed for one team to fully occupy the system. If swapping is allowed between neutral partners, then the probability distribution is dominated by very long-lived states where a few very large domains persist, each domain being occupied by a mix of individuals from species that form one of the teams. Many aspects of the possible extinction scenarios are lost when only considering averaged quantities as for example the mean domination time. We also discuss some results for a model where species, that compete in Rock-Paper-Scissor fashion, have mixed strategies rather than pure strategies. We compare the case with mixed strategy to the pure strategy case and look at similarities and differences.\\[0.2cm] [1] B. Intoy and M. Pleimling, J. Stat. Mech (2013) P08011. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F10.00012: Escaping an infestation of parasites by outrunning them: insights from a simple stochastic model Jiajia Dong, Brian Skinner, Nyles Breecher, Beate Schmittmann, Royce K.P. Zia Coexistence of multiple species abounds in ecological systems as a consequence of various interactions. Unlike predator-prey, the latter is not killed by the former in a parasite-host system. We study a simple lattice model, in which parasites wander randomly and die, giving birth only when they land on a square with the host. For a stationary host with certain boundary conditions, the stochastic process can be solved and the results match well to Monte Carlo simulations. In non-trivial stationary states, the characteristics of the ``parasite-cloud'' around the host are well understood. If the host moves with uniform velocity, solving the problem becomes much more challenging. Instead, we consider a stationary host with parasites performing \emph{biased} diffusion, for which our theoretical predictions (with no fitting parameters) also agree with simulation results. In the appropriate continuum limit, the two processes are identical but interesting differences emerge in our lattice model. The most notable phenomenon is that the stationary parasite population generally increases with the bias, reaching a maximum before vanishing at some critical value. These and other features will be illustrated by examples with realistic Verhulst factors, which model finite carrying capacities. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F10.00013: The Structure of Fitness Landscapes in Antibiotic-Resistant Bacteria Barrett Deris, Minsu Kim, Zhongge Zhang, Hiroyuki Okano, Rutger Hermsen, Jeff Gore, Terence Hwa To predict the emergence of antibiotic resistance, quantitative relations must be established between the fitness of drug-resistant organisms and the molecular mechanisms conferring resistance. We have investigated E. coli strains expressing resistance to translation-inhibiting antibiotics. We show that resistance expression and drug inhibition are linked in a positive feedback loop arising from an innate, global effect of drug-inhibited growth on gene expression. This feedback leads generically to plateau-shaped fitness landscapes and concomitantly, for strains expressing at least moderate degrees of drug resistance, gives rise to an abrupt drop in growth rates of cultures at threshold drug concentrations. A simple quantitative model of bacterial growth based on this innate feedback accurately predicts experimental observations without ad hoc parameter fitting. We describe how drug-inhibited growth rate and the threshold drug concentration (the minimum inhibitory concentration, or MIC) depend on the few biochemical parameters that characterize the molecular details of growth inhibition and drug resistance (e.g., the drug--target dissociation constant). And finally, we discuss how these parameters can shape fitness landscapes to determine evolutionary dynamics and evolvability. [Preview Abstract] |
Session F11: Focus Session: Active Soft Matter I - Transport, Biomimetics and Dynamic Response
Sponsoring Units: DPOLY GSNP DBIORoom: 203
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F11.00001: Polymer Prize Break |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F11.00002: Cytoskeletal organization by motor and polymerization forces Invited Speaker: Gijsje Koenderink Cells need to constantly change their change to perform vital functions, such as growth, division, and movement. Dysregulation of cell shape can have severe consequences such as cancer. Our goal is to resolve physical mechanisms that contribute to cell shape control. For this purpose, we study simplified experimental model systems reconstituted from purified cellular components. In this talk, I will give two examples of our recent work. The first example concerns active contractility of the actin cortex, which lies underneath the cell membrane and drives shape changes by means of myosin motors. Using in vitro models, we studied how myosin motors and actin filaments collectively self-organize into force-generating arrays. I will show that motors contract actin networks only above a sharp threshold in crosslink density. We discovered that right at this threshold, the motors rupture the network into clusters that exhibit a broad distribution of sizes, as expected in filamentous networks near a percolation threshold. The second example I will discuss concerns cell shape polarization directed by interactions between the actin and microtubule (MT) cytoskeletons. A prominent example is the guidance of MT growth along F-actin bundles towards specific targets, i.e. focal adhesions. It has been suggested that MT end-tracking proteins ($+$TIPs) that also bind F-actin are responsible for this process. We built an in vitro system involving a simplified actin-MT crosslinker molecule and could show that the interaction between MT ends and actin is sufficient to capture and re-direct MT growth along actin bundles. By keeping MT growth tightly coupled to F-actin, this mechanism allows linear arrays of actin bundles to act as templates for MT organization. Instead, when interacting with single actin filaments, MT ends become the dominant organizing factor, exerting forces that align, pull and even transport actin filaments in the direction of MT growth. We conclude that actin and MTs can influence each other's organization through coupling by $+$TIP proteins. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F11.00003: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F11.00004: Stress activated contractile wavefronts in the mechanically-excitable embryonic heart Kevin Chiou, Stephanie Majkut, Dennis Discher, Tom Lubensky, Andrea Liu The heart is a prime example of a robust, active system with behavior--the heart beat--that is extraordinarily well timed and coordinated. For more than half a century, electrical activity induced by ion release and diffusion has been argued to be the mechanism driving cardiac action. But recent work indicates that this phenomenon is also regulated by mechanical activity. In the embryonic avian heart tube, the speed of the contractile wavefront traversing the heart tube with each beat is measured to be a monotonic, linear function of tissue stiffness. Traditional electrical conduction models of excitation-contraction cannot explain this dependence; such a result indicates that the myocardium is mechanically excitable. Here, we extend this work by using experimental observations of stiffness-dependent behavior in isolated cardiomyocytes as an input to study contractile wavefronts in the tissue as a whole. We model the heart tube as an active, overdamped elastic network where the primary stress mediator is the extracellular matrix. Using this simple model, we explain experimental observations of the systolic wave and predict qualitatively new behavior. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F11.00005: Photo-induced Mass Transport through Polymer Networks Yuan Meng, Mitchell Anthamatten Among adaptable materials, photo-responsive polymers are especially attractive as they allow for spatiotemporal stimuli and response. We have recently developed a macromolecular network capable of photo-induced mass transport of covalently bound species. The system comprises of crosslinked chains that form an elastic network and photosensitive fluorescent arms that become mobile upon irradiation. We form loosely crosslinked polymer networks by Michael-Addition between multifunctional thiols and small molecule containing acrylate end-groups. The arms are connected to the network by allyl sulfide, that undergoes addition-fragmentation chain transfer (AFCT) in the presence of free radicals, releasing diffusible fluorophore. The networks are loaded with photoinitiator to allow for spatial modulation of the AFCT reactions. FRAP experiments within bulk elastomers are conducted to establish correlations between the fluorophore's diffusion coefficient and experimental variables such as network architecture, temperature and UV intensity. Photo-induced mass transport between two contacted films is demonstrated, and release of fluorophore into a solvent is investigated. Spatial and temporal control of mass transport could benefit drug release, printing, and sensing applications. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F11.00006: Characterization of Particulate Matter Transport across the Lung-Surfactant Barrier using Langmuir Monolayers Jeremy Eaton, Michael Dennin, Alex Levine, Steven George We investigate the transport of particulate matter acros the lung using a monolayer of bovine lung surfactant tagged with NBD in conjunction with alveolar lung cells below the air-water interface. The monolaye dynamically compressed and expanded to induce phase transitions as well as buckling and folding. Polystyrene spheres ranging from 20 to 500 nm in diameter were tagged with fluorescent molecules and deposited on the monolayer. We will present results of preliminary studies of the transport of beads from the air-water surface to the lung cells through the monolayer. Characterization of the transfer will focus on differential fluorescence microscopy to distinguish uncoated beads from beads from beads coated with surfactant monolayers. The presence or absence of surfactant associated with the beads provides insight into potential transfer mechanisms and will serve as an input into models of the bead transfer. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F11.00007: Activating membranes Ananyo Maitra, Pragya Srivastava, Sriram Ramaswamy, Madan Rao We formulate a hydrodynamic theory of a fluid membrane coupled to a bulk medium comprising treadmilling filaments endowed with active stresses and show that active membrane dynamics [Phys. Rev. Lett \textbf{84}, 3494 (2000)] and spontaneous shape oscillations emerge from this description. We also consider membrane instabilities and patterns induced by the presence of filaments with polar orientational correlations in the tangent plane of the membrane. The dynamical features we predict should be seen in a variety of cellular contexts involving the dynamics of the membrane-cytoskeleton composite and cytoskeletal extracts coupled to synthetic vesicles. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F11.00008: Protein-Polyelectrolyte Coacervation: Morphology Diagram, Binding Affinity, and Protein Separation David Hoagland, Xiaosong Du, Paul Dubin For aqueous mixtures of negatively charged polysaccharide, hyaluronic acid (HA), and globular protein, either bovine serum albumin (BSA) or beta-lactoglobulin (BLG), a pH-ionic strength (I) morphology diagram, with regions of homogeneous solution, soluble complex, coacervation, precipitation, and redissolution, was developed by pH titrations performed at fixed I. The systems are models for coacervation, or liquid-liquid phase separation, between flexible and compact solutes of opposite charge. Protein charge here is tuned by pH, and titration keeps the mixtures close to equilibrium. At high I, only homogeneous solution is observed, as true at high and low pH. Diagrams for the proteins differ because HA affinity for BSA is higher than for BLG, traced to BSA's greater charge patchiness and higher net charge; isothermal solution titration calorimetry finds a factor of two difference in binding energy. Dependences of transition pH on protein charge Z and solution I offer additional insights into interactions underlying morphology transitions. At optimal conditions, the affinity disparity is sufficient to achieve highly selective BSA coacervation in a 1:1 protein mixture, suggesting coacervation to separate similar proteins under mild, non-denaturing conditions. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F11.00009: Spontaneous motion and deformation of a droplet driven by chemical reaction Natsuhiko Yoshinaga Spontaneous motion has been attracting lots of attention in last decades in nonlinear and nonequilibrium physics partially for its potential application to biological problems such as cell motility. Recently several model experiments showing spontaneous motion have been proposed in order to elucidate underlying mechanism of the motion. The systems in these works consist of relatively simple ingredients, for instance oil droplets in water, but nevertheless the results show rich motion and deformation of the droplet. Importantly, the system breaks symmetry and chooses one direction of motion. In this work, we theoretically derive a set of nonlinear equations exhibiting a transition between stationary and motile states starting from advection-reaction-diffusion equation driven away from an equilibrium state due to chemical reactions. A particular focus is on how hydrodynamic flow destabilizes an isotropic distribution of a concentration of chemicals. We also discuss a shape of the droplet. Due to self-propulsive motion and flow around the droplet, a spherical shape becomes unstable and it elongates perpendicular to the direction of motion. This fact would imply that the self-propulsion driven by chemical reaction is characterized as a pusher in terms of a flow field. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F11.00010: Structural transitions in helical polymers Matthew Williams, Michael Bachmann Helical structures, as well as more complex tertiary structures, made up of helixes are relevant in biological systems. We perform generalized-ensemble Monte Carlo simulations to examine homopolymer models which include a torsional potential energy associated with each bond. With the inclusion of a torsional potential, helical structures emerge and can contort to form a variety of tertiary structural phases. We explore the two-dimensional space, parametrized by temperature and torsional energy scale, to map helical structures and to locate structural transitions. We see transitions occur between helical and non-helical secondary structures and also between various tertiary structures. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F11.00011: Peptidyl Materials Formed Through Click Chemistry Enhanced Coiled-Coil Interactions Kenneth Koehler Biologically derived materials offer a level of sophistication synthetically fabricated materials have only attempted to mimic. This level of complexity may be found in materials such as peptides. Implementing new theory and modeling, peptides with the propensity to form coiled-coil (CC) bundles were designed and synthesized. Through the use of this \textit{de novo} approach, modeling allowed prediction of the feasibility to include non-natural amino acids conducive to click chemistry into the peptide. Amino acids showcasing thiol or alkyne functionalities were considered owing to the ability of these moieties to participate in the thiol-ene and copper click reactions respectively. Once synthesized, the peptides decorated with these clickable motifs were placed in solution and allowed to self-assemble into CC's. CD spectroscopy and DLS experiments confirmed the formation and assembly of CC's. Click reactions were then incited to link the CC assemblies together and form a network with predictable dimensionality and pore size between CC bundles. To incite network formation, click reactions between CC side chain residues and suitably functionalized crosslinkers were implemented. The linking of coiled-coils and material formation were assessed using DLS and TEM. [Preview Abstract] |
Session F12: Biophysical Dynamics and Locomotion
Sponsoring Units: DBIO DFDChair: Arpita Upadhyaya, University of Maryland
Room: 205
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F12.00001: Dynamic Force Patterns of an Undulatory Microswimmer Rafael Schulman, Matilda Backholm, William Ryu, Kari Dalnoki-Veress {\it C. elegans} is a millimeter-sized nematode which has served as a model organism in biology for several decades, primarily due to its simple anatomy. Using an undulatory form of locomotion, this worm is capable of propelling itself through various media. Due to the small length scales involved, swimming in this regime is qualitatively different from macroscopic locomotion because the swimmers can be considered to have no inertia. In order to understand the microswimming that this worm exhibits, it is crucial to determine the viscous forces experienced during its motion. Using a micropipette deflection technique in conjunction with high speed imaging, we have directly measured the time-varying forces generated by {\it C. elegans} during swimming. Furthermore, by analyzing the body's kinematics over time and applying a model of locomotion, we can compute the theoretical force curves. We observe excellent agreement between the measured and calculated forces. The success of this simple model has important implications in the understanding of microswimming in general. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F12.00002: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F12.00003: Propulsion and locomotion in hexatic liquid crystal Thomas Powers, Madison Krieger, Saverio Spagnolie The long chainlike molecules in mucus can align and lead to liquid-crystalline order. The resulting anisotropy can affect swimming behavior of spermatozoa and bacteria. We study a simple model of swimming in an anisotropic fluid, that of an infinitely long two-dimensional sheet deforming via propagating transverse or longitudinal waves and immersed in a hexatic liquid crystal. The liquid crystal is categorized by the dimensionless Ericksen number Er, which compares viscous and elastic effects. We calculate how swimming speed depends on Er for small amplitude waves, and show that our perturbative approach breaks down at large Er for transverse waves but not longitudinal waves. We also calculate the fluid transported by the swimming motion. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F12.00004: Efficient swimming of a plunging elastic plate in a viscous fluid Peter Yeh, Alexander Alexeev We use three dimensional computer simulations to examine the combined hydrodynamics and structural response of a plunging elastic plate submerged in a viscous fluid with Reynolds number of 250. The plate is actuated at the root with a prescribed vertical sinusoidal displacement and a zero slope (clamped) boundary condition. We explore the steady state swimming velocity and the associated input power as a function of driving frequency, added mass, and aspect ratio. We find a universal bending pattern independent of geometry and added mass that maximizes the distance traveled per unit applied work. This bending pattern is associated with minimizing center of mass oscillations normal to the direction of travel. Subsequently, the flow around the sides of the swimmer, which does not aid in propulsion, is minimized, thereby reducing viscous losses. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F12.00005: Kinematic Matrix Analysis of Biological Swimmers and Artificial Nanomotors Amir Nourhani, Paul Lammert, Ali Borhan, Vincent Crespi In recent years, much attention has been attracted by autonomous movers (both natural, often biological, and synthetic) which exhibit a basic deterministic motion significantly perturbed by stochastic elements. Fokker-Planck equations are a traditional tool for investigating such phenomena, but can be cumbersome to apply, especially in complex situations of the sort now attracting attention. This is partly due to their giving complete probability distributions, which is a level of detail seldom needed, and potentially obscuring of the basic physics. We present a simple yet powerful new approach which can flexibly and easily handle a large variety of elementary deterministic and stochastic component processes to yield drift and diffusion characteristics with a minimum of fuss and effort. We use the clarity and power of the new methodology to discern several new universal emergent time scales in this class of physical systems. We also describe how these methods could now serve as a platform for further advances and insights. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F12.00006: Fluid flow enhances the effectiveness of toxin export by aquatic microorganisms: a first-passage perspective Nicholas Licata, Aaron Clark Aquatic microorganisms face a variety of challenges in the course of development. One central challenge is efficiently regulating the export of toxic molecules inside the developing embryo. The strategies employed should be robust with respect to the variable ocean environment and limit the chances that exported toxins are reabsorbed. In this talk we consider the first-passage problem for the uptake of exported toxins by a spherical embryo. A perturbative solution of the advection-diffusion equation reveals that a concentration boundary layer forms in the vicinity of the embryo, and that fluid flow enhances the effectiveness of toxin export. We highlight connections between the model results and recent experiments on the development of sea urchin embryos. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F12.00007: Evaluation of the mass transfer effect of the stalk contraction cycle of \textit{Vorticella} Jiazhong Zhou, David Admiraal, Sangjin Ryu \textit{Vorticella} is a protozoan with a contractile stalk that can contract pulling the cell body toward the substrate in less than 10 ms and return to the extended state in a few seconds. Although this stalk contraction is one of the fastest cellular motions, it is unknown why \textit{Vorticella} contracts. Because the flow field induced by \textit{Vorticella} shows different characteristics between contraction and relaxation, it has been suggested that \textit{Vorticella} augments mass transfer near the substrate based on its stalk contraction-relaxation. We investigate this hypothesis using computational fluid dynamics (CFD) simulations and particle image velocimetry (PIV) experiments. In both approaches, \textit{Vorticella} is modelled as a solid sphere that translates perpendicular to a solid surface in liquid based on the measured stalk length changes of \textit{Vorticella}. Based on the computationally and experimentally simulated flow, we evaluate the mass transfer capability of \textit{Vorticella}, for a possible application of the stalk contraction of \textit{Vorticella} as a biomimetic model system for microfluidic mixers. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F12.00008: Endothelial Interfaces -- Master Gatekeepers of the Cardiovascular System Sylvia Ann Junghans, Luka Pocivavsek, Noureddine Zebda, Konstantin Birukov, Mary Jo Waltman, Jaroslaw Majewski Endothelial cells, master gatekeepers of the cardiovascular system, line its inner boundary from the heart to distant capillaries constantly exposed to blood flow. Inter-endothelial signaling and the monolayer's adhesion to the underlying collagen rich basal lamina are key in physiology and disease. Using neutron scattering, we report the first-ever interfacial structure of endothelial monolayers under dynamic flow conditions mimicking the cardiovascular system. Endothelial adhesion strength (defined as the separation distance l between the basal cell membrane and solid boundary) is explained using developed interfacial potentials and intra-membrane segregation of specific adhesion proteins. Our method provides a powerful tool for the biophysical study of cellular layer adhesion strength in living tissues. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F12.00009: The Fast and Non-capillary Fluid Filling Mechanism in the Hummingbird's Tongue Alejandro Rico-Guevara, Tai-Hsi Fan, Margaret Rubega Hummingbirds gather nectar by inserting their beaks inside flowers and cycling their tongues at a frequency of up to 20 Hz. It is unclear how they achieve efficiency at this high licking rate. Ever since proposed in 1833, it has been believed that hummingbird tongues are a pair of tiny straws filled with nectar by capillary rise. Our discoveries are very different from this general consensus. The tongue does not draw up floral nectar via capillary action under experimental conditions that resemble natural ones. Theoretical models based on capillary rise were mistaken and unsuitable for estimating the fluid intake rate and to support foraging theories. We filmed (up to 1265 frames/s) the fluid uptake in 20 species of hummingbirds that belong to 7 out of the 9 main hummingbird clades. We found that the fluid filling within the portions of the tongue that remain outside the nectar is about five times faster than capillary filling. We present strong evidence to rule out the capillarity model. We introduce a new fluid-structure interaction and hydrodynamic model and compare the results with field experimental data to explain how hummingbirds actually extract fluid from flowers at the lick level. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F12.00010: Investigation of ciliary propulsion of \textit{Tetrahymena Pyriformis} in viscous solution Ilyong Jung, Eva Lyubich, James Valles Recent experiments by our group showed that the ciliated protist \textit{Paramecium Caudatum }swims with a constant propulsive force in solutions with viscosities 1 \textless $\eta $/ $\eta_{\mathrm{w}}$\textless 7 where $\eta_{\mathrm{w}}$ is the viscosity of water. Measurements of the geometry of its helical swimming trajectory combined with high speed video of the ciliary motion provided insight into this behavior. Using a phenomenological model we found that the body cilia beating frequency decreases while the beating angle remains roughly constant to produce the constant propulsive force dependence on viscosity. In this talk, we present studies of another ciliated protozoa, \textit{Tetrahymena Pyriformis} to determine whether the behavior of \textit{Paramecium} is general. Preliminary results indicate that \textit{Tetrahymena Pyriformis} also swims with a nearly constant propulsive force with increasing viscosity. Investigations similar to those performed on \textit{Paramecium} are underway and the latest results will be presented. This work was supported by NSF PHY0750360 and at the NHMFL by NSF DMR-0084173 [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F12.00011: The behavioral space of zebrafish locomotion and its neural network model Kiran Girdhar, Maria Benitez-Jones, Ha Pham Thi, Mark Nelson, Martin Gruebele, Yann Chemla How does one describe quantitatively the complex motion of vertebrates? To answer this question, we investigated a model system for vertebrate locomotion: zebrafish swimming. We performed a quantitative analysis of all stereotyped behavioral swimming patterns of zebrafish larvae: spontaneous swimming, escape response to stimulus, and prey tracking. Previous attempts to analyze zebrafish swimming motion quantitatively have imposed many arbitrary parameters. Here, we instead used a~parameter-independent method that produces an orthogonal set of ``eigen-shapes'' of fish backbones to describe swimming motion in a low-dimensional space. We show that a linear combination of only three such ``eigen-shapes'' is sufficient to describe 97{\%} of zebrafish shapes. Moreover, stereotyped swimming behaviors fall on two low-dimensional attractors embedded in this three dimensional behavioral space. We also show using a two-dimensional correlation analysis that ``scoots'' and ``R-turns,'' which were previously described as discrete behavioral states, in fact represent extrema in a continuum in this low-dimensional behavioral space. To understand the neural basis of~the~behavior, we have also developed a neural network model of spontaneous swimming of fish larvae. We present a set of neural parameters such as synaptic conductance, stimulus amplitude that produces swimming behavior and reconstructed the low-dimensional behavioral space obtained from experimental results. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F12.00012: Active microrheology of fluids inside developing zebrafish Mike Taormina, Raghuveer Parthasarathy Biological fluids are a source of diverse and interesting behavior for the soft matter physicist. Since their mechanical properties must be tuned to fulfill functional roles important to the development and health of living things, they often display complex behavior on length and time scales spanning many orders of magnitude. For microbes colonizing an animal host, for example, the mechanical properties of the host environment are of great importance, affecting mobility and hence the ability to establish a stable population. Indeed, some species possess the ability to affect the fluidity of their environment, both directly by chemically modifying it, and indirectly by influencing the host cells' secretion of mucus. Driving magnetically doped micron-scale probes which have been orally micro-gavaged into the intestinal bulb of a larval zebrafish allows the rheology of the mucosal layer within the fish to be measured over three decades of frequency, complementing ecological data on microbial colonization with physical information about the gut environment. Here, we describe the technique, provide the first measurement of mucosal viscosity in a developing animal, and explore the technique's applicability to other small-volume or spatially inhomogeneous fluid samples. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F12.00013: Alignment of active particles with hydrodynamic interactions and formation of a self-assembled pump Katrin Wolff, Marc Hennes, Holger Stark Hydrodynamically interacting active particles in an external harmonic potential are known to form a self-assembled pump at large enough Peclet numbers [1]. Here, we give a quantitative criterion for the formation of the pump for active Brownian particles depending on the rotational diffusion of particles, their swim speed and the strength of the harmonic trap. The emerging flow field caused by the swimmers corresponds to a regularized stokeslet and stabilises the pump. We find that the particle distribution settles into a non-equilibrium steady state with non-vanisihing flux. The particle orientations can be mapped onto an equilibrium system as they align along a common ``pump axis'' in analogy to dipoles in an electric field. We perform Brownian dynamics simulations with hydrodynamic interactions and compare the many-particle simulations with an analytically tractable mean field system. \\[4pt] [1] R. W. Nash et al., \emph{Phys. Rev. Lett.}~\textbf{104}, 258101 (2010) [Preview Abstract] |
Session F13: Focus Session: Fe Based Superconductors-Orbital Physics
Sponsoring Units: DMPChair: Adriana Moreo, University of Tennessee
Room: 207
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F13.00001: Orbital nematic order and its interplay with magnetism in iron based superconductors Zhentao Wang, Andriy Nevidomskyy The nematic order in the iron pnictide family of superconductors has received a lot of attention, with recent ARPES [1] and STM [2] experiments providing strong indication in favor of the orbital nature of the nematic phase. We study the spontaneous development of the orbital nematic order and its interplay with magnetism, using random phase approximation (RPA), mean field methods, and variational cluster approximation (VCA). We show that the orbital nematic order develops when inter-orbital Hubbard repulsion $U^\prime$ is strong enough, while the intra-orbital Hubbard $U$ and Hund's coupling $J$ tend to suppress nematicity. In addition to the pure orbital nematic phase and columnar antiferromagnetic phase, we find a broad region in the parameter space where the two orders coexist. We have studied the doping dependence of these phases and find that doping away from half-filling generally suppresses both orders, consistent with the experimental phase diagram of the pnictides. We also find that the doping effect on both orders is not particle-hole symmetric, also consistent with experiments.\\[4pt] [1] M. Yi {\it et al.}, PNAS \textbf{108}, 6878 (2011).\\[0pt] [2] T.-M. Chuang {\it et al.}, Science \textbf{327}, 181 (2010). [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F13.00002: Coexistence of orbital degeneracy lifting and superconductivity in iron-based superconductors Hu Miao, Pierre Richard, Shangfei Wu, Jun Ma, Tian Qian, Lingyi Xing, Xiancheng Wang, Changqing Jin, Hong Ding, Chungpin Chou, Limin Wang, Wei Ku, Ziqiang Wang In iron-based superconductors, local orbital fluctuations have been proposed to be directly responsible for the structural phase transition and closely related to the observed giant magnetic anisotropy and electronic nematicity. However, whether superconductivity can emerge from, or even coexist with orbital fluctuations, remains unclear. Here we report the angle-resolved photoemission spectroscopy observation of the lifting of symmetry-protected band degeneracy, and consequently the breakdown of local tetragonal symmetry in the SC state of Li(Fe$_{\mathrm{1-x}}$Co$_{\mathrm{x}}$)As. Supported by theoretical simulations, we analyse the doping and temperature dependences of this band-splitting and demonstrate an intimate connection between ferro-orbital correlations and superconductivity. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F13.00003: Effects of spin-orbit coupling and space group symmetry in multiorbital models for iron pnictides Rong Yu, Emilian Nica, Jian-Xin Zhu, Qimiao Si Motivated by recent experiments, we study the effects of spin-orbit coupling in multiorbital models for iron-based superconductors. We show that the spin-orbit coupling leads to a nontrivial hybridization among the three t2g bands in the Brilluion zone corresponding to the two-iron unit cell, as required by the space group symmetry of the system. We also consider the superconducting pairing in the presence of spin-orbit coupling, and in agreement with the space group symmetry. By calculating the dynamical spin susceptibility in the superconducting state, we find anisotropic spin resonance excitations in consequence of the breaking of spin rotational symmetry. We further discuss the connections between our results and recent ARPES and polarized inelastic neutron scattering measurements. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F13.00004: Linear Response Theory for Shear Modulus $C_{66} $ and Raman Quadrupole Susceptibility: Evidence for Orbital Fluctuations in Fe-Based Superconductors Hiroshi Kontani, Youichi Yamakawa, Seiichiro Onari Existence of strong nematic fluctuations in various Fe-based superconductors has been discussed as a central issue. To clarify the origin, we discuss both the softening of shear modulus $C_{66} $ and the enhancement of the charge quadrupole susceptibility observed by Raman spectroscopy $\chi _{x^{2}-y^{2}}^{\mbox{Raman}} $. Due to the Aslamazov-Larkin vertex correction (AL-VC), strong orbital nematic fluctuations are induced by spin fluctuations. The strong development of $1/C_{66} $ is given by the summation of the Pauli and Van-Vleck orbital susceptibilities due to AL-VC, whereas moderate enhancement of $\chi_{x^{2}-y^{2}}^{\mbox{Raman}} $ is induced by the Van-Vleck term only. Therefore, a consistent explanation for the difference behavior between two measurements is achieved based on the orbital fluctuation theory. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F13.00005: Orbital Resonance Mode in Superconducting Iron Pnictides Wei-Cheng Lee, Philip Phillips We show that the fluctuations associated with ferro orbital order in the $d_{xz}$ and $d_{yz}$ orbitals can develop a sharp resonance mode in the superconducting state with a nodeless gap on the Fermi surface. This orbital resonance mode appears below the particle-hole continuum and is analogous to the magnetic resonance mode found in various unconventional superconductors. If the pairing symmetry is s$_{\mathrm{\pm }}$ , a dynamical coupling between the orbital ordering and the d-wave subdominant pairing channels is present by symmetry. Therefore the nature of the resonance mode depends on the relative strengths of the fluctuations in these two channels, which could vary significantly for different families of the iron based superconductors. The application of our theory to a recent observation of a new $\delta $ -function-like peak in the B$_{\mathrm{1g}}$ Raman spectrum of Ba$_{\mathrm{0.6}}$K$_{\mathrm{0.4}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ is discussed. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F13.00006: Study of the multi-orbital Hubbard model at finite temperature Anamitra Mukherjee, Shuai Dong, Gonzalo Alvarez, Elbio Dagotto Research in pnictide superconductors have clearly established the need for the study of multi-orbital Hubbard models. With this motivation, here we apply a combination of the real-space Exact Diagonalization and Classical Monte Carlo (ED+MC) method, widely used in manganites, with the standard Hartree-Fock mean field (MF) theory to investigate the properties of multiorbital models as a function of temperature. In this approach the MF parameters are treated via a classical MC and the fermions moving in the MF background are solved by exact diagonalization. The temperature dependence of the dynamical spin susceptibility $S(\vec{q},\omega)$, orbital resolved single particle spectral function $A(\vec{k},\omega)$, optical conductivity, and real space charge/spin/orbital density maps are calculated at different dopings. These results are relevant in understanding the role of the multiple degrees of freedom in governing the magnetic and transport properties of the Fe based superconductor materials. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:48AM |
F13.00007: Selective Mottness as a key to iron superconductors: weak \textit{and} strong correlations Invited Speaker: Luca de Medici I will discuss the strength of electronic correlations in the normal phase of Fe-superconductors and trace a comparison with cuprates. The phase diagram of the high-Tc~cuprates is dominated by the Mott insulating phase of the parent compounds. Approaching it from large doping, a standard Fermi-liquid is seen to gradually turn into a bad non-Fermi liquid metal in which quasiparticles have heavily differentiated coherence depending on momentum, a process which culminates in the pseudogap regime, in which the antinodal region in momentum space acquires a gap before the material reaches a fully gapped Mott state. I will show that experiments for electron- and hole-doped BaFe2As2~support an analogous scenario. The doping evolution is dominated by the influence of a Mott insulator that would be realized for half-filled conduction bands, while the stoichiometric compound does not play a special role. Weakly and strongly correlated conduction electrons coexist in much of the phase diagram, a differentiation that increases with hole-doping. We identify the reason for this ``selective Mottness'' in a simple emergent mechanism, an ``orbital decoupling,'' triggered by the strong Hund's coupling. When this mechanism is active charge excitations in the different orbitals are decoupled and each orbital behaves as a single band Hubbard model, where the correlation degree almost only depends on how doped is each orbital from half-filling. This scenario reconciles contrasting evidences on the electronic correlation strength, implies a strong asymmetry between hole- and electron-doping and establishes a deep connection with the cuprates. L. de' Medici, G. Giovannetti and M. Capone, ArXiv:1212.3966 [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F13.00008: Spin excitation spectra of iron-based superconductors from the degenerate double-exchange model Zhidong Leong, Wei-Cheng Lee, Weicheng Lv, Philip Phillips Using a degenerate double-exchange model, we investigate the spin excitation spectra of iron pnictides. The model consists of local spin moments on each Fe site as well as itinerant electrons from the degenerate $d_{xz}$ and $d_{yz}$ orbitals. The local moments interact with each other through antiferromagnetic $J_1$-$J_2$ Heisenberg interactions, and they couple to the itinerant electrons through a ferromagnetic Hund's coupling. We employ the fermionic spinon representation for the local moments and perform a generalized RPA calculation on both spinons and itinerant electrons. We find that in the ($\pi$,0) magnetically-ordered state, the spin-wave excitation at ($\pi$,$\pi$) is pushed to a higher energy due to the presence of itinerant electrons, which is consistent with the previous study using Holstein-Primakoff transformation. In the non-ordered state, the particle-hole continuum keeps the collective spin excitation near ($\pi$,$\pi$) at a higher energy even without any $C_4$ symmetry breaking. The implications for the recent neutron scattering measurement at high temperature will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F13.00009: Orbital-dependent electronic correlations in iron chalcogenide superconductors M. Yi, Z.K. Liu, Y. Zhang, R. Yu, J.J. Lee, R.G. Moore, F.T. Schmitt, W. Li, S.C. Riggs, J.-H. Chu, B. Lv, J. Hu, T.J. Liu, M. Hashimoto, S.K. Mo, Z. Hussain, Z.Q. Mao, C.W. Chu, I.R. Fisher, Q. Si, Z.X. Shen, D.H. Lu The strength of electronic correlations is a fundamental question that has not been fully settled for the iron-based superconductors. There appears to be a systematic trend among the various families of FeSC, from relatively weak correlation in the iron phosphides, to moderate in iron arsenides, to relatively strong in iron chalcogenides. In this study using angle-resolved photoemission spectroscopy, we find a generic behavior in the three families of iron chalcogenides: KxFe2-ySe2, Fe(Te,Se), and FeSe thin films, in which there is an orbital-dependent correlation effect with the dxy orbital strongly renormalized. Moreover, with raised temperature, we find that the dxy orbital-dominated bands in all three materials lose spectral weight while the other orbitals remain metallic. Hence we find this crossover from a metallic superconducting state at low temperatures to an orbital-selective Mott phase at high temperatures to be universal for iron-chalcogenide superconductors. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F13.00010: Nematic state of the pnictides stabilized by the interplay of the lattice with the spin and orbital degrees of freedom Shuhua Liang, Adriana Moreo, Elbio Dagotto The nematic state of the iron-based superconductors is studied in the undoped limit of the three-orbital (xz, yz, xy) spin-fermion model [1] via the introduction of lattice degrees of freedom. Monte Carlo simulations show that in order to stabilize the experimentally observed lattice distortion and nematic order, and to reproduce photoemission experiments, both the spin-lattice and orbital-lattice couplings are needed. The interplay between their respective coupling strengths regulates the separation between the structural and Ne\'el transition temperatures. Experimental results for the temperature dependence of the resistivity anisotropy and the angle-resolved photoemission orbital spectral weight are reproduced by the present numerical simulations [2]. \\[4pt] [1] S. Liang, G. Alvarez, C. Sen, A. Moreo, and E. Dagotto, Phys. Rev. Lett. 109 047001 (2012)\\[0pt] [2] S. Liang, A. Moreo, and E. Dagotto, Phys. Rev. Lett. 111 047004 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F13.00011: Origin of Electronic Nematicity in the Iron Pnictide NaFe$_{1-x}$Co$_x$As Superconductor Verner Thorsmolle, Wei-Lu Zhang, Chenglin Zhang, Scott Carr, Pengcheng Dai, Girsh Blumberg Doped iron pnictides present a complex phase diagram with superconductivity in close proximity to antiferromagnetic and structural transitions (ST). In addition to these phases, an electronic nematic phase has been suggested to be associated with the tetragonal-to-orthorhombic transition at $T_S$. Electronic nematicity breaks $C_4$ rotational symmetry and is believed to be the driving force behind the ST. However, at present, the main interaction behind electronic nematicity and nematic fluctuations remain unexplained. Using electronic Raman spectroscopy we show nematic charge fluctuations in the $XY$ symmetry channel to follow a Curie-Weiss-like temperature dependence extending over a $\sim$200 K range above $T_S$ and in the entire phase diagram including the superconducting phase in NaFe$_{1-x}$Co$_{x}$As ($0 < x < 0.08$) single crystals. The nematicity is found to originate from orbital fluctuations, interconnected with local phonons, and are described in the frame of a classical Curie-Weiss law two-level system corresponding to the $d_{xz}$ and $d_{yz}$ Fe-orbitals. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F13.00012: Superconductivity in Ab initio Low-Energy Effective Model for Iron-Based Superconductor Takahiro Misawa, Masatoshi Imada To clarify microscopic mechanism of superconductivity in iron-based superconductors, we study the {\it ab initio} low-energy effective models for iron-based superconductor~[1], particularly for LaFeAsO by using multi-variable variational Monte Carlo (mVMC) method, which properly takes into account both spatial and dynamical quantum fluctuations. The calculated magnetic order was shown to correctly reproduce the experimental material dependences~[2,3]. By extending these normal state studies, we find that superconductivity emerges in the electron doped LaFeAsO in essential agreement with the experimental results. The pairing satisfies $s\pm$ symmetry. We discuss the role of antiferromagnetic correlations, Mott proximity, and charge and orbital fluctuations in stabilizing the superconductivity. The specific orbital ($d_{X^2-Y^2}$) is shown to play a role of orbital-selected doped Mott insulator in stabilizing the superconducting phase as well as the antiferromagnetic phase. We discuss similarity and dissimilarity to the cuprate superconductors. ~[1]~T. Miyake $et$ $al$., J. Phys. Soc. Jpn. {\bf 79}, 044705 (2010). ~[2]~T. Misawa $et$ $al$., J. Phys. Soc. Jpn. {\bf 80}, 023704 (2011). ~[3]~T. Misawa $et$ $al$., Phys. Rev. Lett. {\bf 108}, 177007 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F13.00013: Effect of electron-phonon interactions on orbital fluctuations in iron-based superconductors: a cDFPT study Yusuke Nomura, Kazuma Nakamura, Ryotaro Arita The pairing mechanism and symmetry of iron-based superconductors is still an open issue. So far, it has been shown that while spin-fluctuations mediate $s$-wave pairing with sign changes in the gap function ($s_{\pm}$-wave), orbital-fluctuations favor $s$-wave pairing without sign changes ($s_{++}$-wave). It has been recently proposed that electron-phonon (el-ph) interactions can enhance the orbital-fluctuations and thus contribute to the superconductivity. To examine the scenario quantitatively, it is highly important to derive, from first principles, an effective model including the phonon degrees of freedom. In this study, we develop an {\it ab initio} downfolding scheme for the el-ph coupled system, which we call constrained density-functional perturbation theory (cDFPT), and estimate the el-ph couplings and the phonon frequencies in the low-energy effective model for LaFeAsO [1]. We analyze the resulting model by the random phase approximation and show that, due to the small phonon-mediated effective exchange interaction, the $s_{\pm}$-wave instability is dominant. Therefore, we conclude that the el-ph interactions cannot be a driving force for the orbital-fluctuation-mediated $s_{++}$-wave state.\\[4pt] [1] Y. Nomura, K. Nakamura, R. Arita, arXiv:1305.2995 [Preview Abstract] |
Session F14: Invited Session: Delbrueck Prize Session: Chance and Necessity - Determinism in the Physics of Cancer
Sponsoring Units: DBIOChair: Daniel Cox, University of California, Davis
Room: 301-303
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F14.00001: Delbrueck Prize Talk: ``I Don't Believe a Word of It'': My Struggles With Max Delbruck's Ghost Invited Speaker: Robert Austin I never meet Max Delbruck, but I have worked with similarly great biological physicists. Because of this, I am familiar with the expression ``I don't believe a word of it'' that Max Delbruck was known to say, and other great biological physicists have been known to say. Lately I have been working on evolution dynamics and how one might accelerate evolution of de novo mutations by standing on the shoulders of great theoretical evolution biologists such as Sewall Wright and Conrad Waddington and following their insights. I have found Delbruck's pronouncement ``I don't believe a word of it'' ringing in my ears. I will try to convince the ghost of Delbruck and his present day acolytes that Darwin did not have the final say on evolution, that biological organisms are quite creative when it comes to evolution of new traits, and that failure to understand this complexity in evolution has had a major impact on our struggles with the physics of cancer. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F14.00002: The Atavistic Model of Cancer: Evidence, Objections, Therapeutic Value Invited Speaker: Charles Lineweaver As cancer progresses tumor cells dedifferentiate. In the atavistic model this dedifferentiation is interpreted as a reversion to phylogenetically earlier capabilities (Davies {\&} Lineweaver 2011). Since there is an identifiable order to the evolution of capabilities, the more recently evolved capabilities are more likely to be compromised first during cancer progression. A loss of capabilities based on the phylogenetic order of evolution suggests a therapeutic strategy for targeting cancer -- design challenges that can only be met by the recently evolved capabilities still intact in normal cells, but lost in cancer cells. Such a target-the-weakness therapeutic strategy contrasts with most current therapies that target the main strength of cancer: cell proliferation. Here, we describe several examples of this target-the-weakness strategy. Our most detailed example involves the immune system. As cancer progresses, the atavistic model suggests that cancer cells lose contact with the more recently evolved adaptive immune system of the host (the basis of vaccination). The absence of adaptive immunity in immunosuppressed tumor environments is an irreversible weakness of cancer that can be exploited by creating a challenge that only the presence of adaptive immunity can meet. Thus, we propose the post-vaccination inoculation of disease at dosages that the recently evolved (and vaccination-primed) adaptive immune system will be able to destroy in normal cells, but not in the immunosuppressed microenvironment of tumor cells. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:48AM |
F14.00003: The Role of the Microenvironment in the Origins of Cancer Invited Speaker: Thea Tlsty |
Tuesday, March 4, 2014 9:48AM - 10:24AM |
F14.00004: Are biomechanical changes necessary for tumor progression? Invited Speaker: Josef A. Kas Already the Roman Celsus recognized rigid tissue as characteristic for solid tumors. Conversely, changes towards a weaker cytoskeleton have been described as a feature of cancer cells since the early days of tumor biology. It remains unclear if a carcinoma's rigid signature stems from more inflexible cells or is caused by the stroma. Despite that the importance of cell biomechanics for tumor progression becomes more and more evident the chicken-and-egg problem to what extent cancer cells already change their mechanical properties within the solid tumor in order to transgress its boundary or mechanical changes are induced by the microenvironment when the cell has left the tumor has been discussed highly controversial. Comprehensive clinical biomechanical measurements only exist from tumor tissue without the possibility to identify individual cells or from individual cancer cells from pleural effusions. Since the biomechanical properties of cells in carcinomas remain unknown measurements on individual cells that directly stem out of primary tumor samples are required, which we have conducted. We found in cervix and mammary carcinomas a distinctive increase of softer cells as well as contractile cells. A soft and contractile cell is like a strong elastic rope. The cell can generate a strong tensile tension to pull its self along and is soft against compression to avoid jamming. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 11:00AM |
F14.00005: Cancer: The beat of an ancient drum? Invited Speaker: Paul Davies |
Session F15: Focus Session: Physics of Chromosomes
Sponsoring Units: DBIOChair: Dieter Heerman, University of Heidelberg
Room: 304
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F15.00001: Mechanics, Structure and Dynamics of Metaphase Chromosome Folding Invited Speaker: John F. Marko During cell division, eukaryote chromosomes are restructured from a relatively dispersed interphase form, into a relatively compact folded metaphase form. I will discuss experiments aimed at analyzing the folding scheme of metaphase chromosomes, where mechanical response and biochemical perturbation are used as tools for diagnosing structure. Experiments with nucleases reveal that the continuity of the metaphase chromosome depends on DNA, i.e., that the metaphase chromosome can be considered to be a ``chromatin gel.'' Experiments with topoisomerases indicate that chromatin entanglements play an appreciable role in determining chromosome mechanical properties, suggesting that they may play a structural role. We further show that perturbation of condensin complexes dramatically changes metaphase chromosome mechanics. Finally we report results of fluorescence visualization of distributions of condensin I and II along metaphase chromosomes. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F15.00002: Understanding DNA Condensation: From Simple Ions to Protamine-DNA Packaging in Sperm Jason DeRouchey DNA in nature exists primarily in a highly compacted state critical for most biological functions. DNA condensation, however, remains poorly understood at the molecular level. We are interested in understanding the fundamental interactions, molecular scale forces and elucidating mechanisms by which polycations interact with DNA in vitro and in vivo. We use osmotic stress coupled with x-ray scattering, to study packaging densities and compaction energies between DNA helices in the presence of various cations. In this talk, we will discuss from simple ions to complex proteins and how these cations modulate both the attractive and repulsive forces between DNA helices. Lastly, the biological implications of these forces will be discussed with regards to spermatogenesis where chromatin histones are replaced by arginine-rich protamines to densely compact DNA in sperm heads. Tight packaging from spermatogenesis is considered essential for both successful transport as well as to protect DNA from damage. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F15.00003: Folding of Nucleosome Arrays Steven Howell, Isabel Jimenez-Useche, Kurt Andresen, Chongli Yuan, Xiangyun Qiu Chromatin conformation and dynamics is central to gene functions including packaging, regulation, and repair. At the molecular level, the basic building block of chromatin is a nucleosome core particle (NCP) made of ~147 base pairs (bp) of dsDNA wrapped around an octamer of histone proteins. These NCPs are connected by short 10-90 bps of linker DNA as beads on a string. Key factors determining the packaging of NCP arrays to form chromatin include ionic condition, linker DNA length, and epigenetic modifications, especially of the histone tails. We have investigated how the conformations of model tetra-NCP arrays are modulated by these factors using small angle x-ray scattering (SAXS). Here we present recent studies of the effects of ion (KCl and MgCl2), linker length, and histone modification (tail deletions) on NCP arrays. Our SAXS measurement makes it possible to learn about both the global compaction of NCP arrays and local inter-NCP spatial correlations within the same array. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F15.00004: Entropy in DNA Double-Strand Break, Detection and Signaling Yang Zhang, Christina Schindler, Dieter Heermann In biology, the term entropy is often understood as a measure of disorder - a restrictive interpretation that can even be misleading. Recently it has become clearer and clearer that entropy, contrary to conventional wisdom, can help to order and guide biological processes in living cells. DNA double-strand breaks (DSBs) are among the most dangerous lesions and efficient damage detection and repair is essential for organism viability. However, what remains unknown is the precise mechanism of targeting the site of damage within billions of intact nucleotides and a crowded nuclear environment, a process which is often referred to as recruitment or signaling. Here we show that the change in entropy associated with inflicting a DSB facilitates the recruitment of damage sensor proteins. By means of computational modeling we found that higher mobility and local chromatin structure accelerate protein association at DSB ends. We compared the effect of different chromatin architectures on protein dynamics and concentrations in the vicinity of DSBs, and related these results to experiments on repair in heterochromatin. Our results demonstrate how entropy contributes to a more efficient damage detection. We identify entropy as the physical basis for DNA double-strand break signaling. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F15.00005: Nanofluidic laboratory-on-chip device for mapping of single molecule DNA extracted from single cells Sara Mahshid, Daniel Berard, Robert Sladek, Sabrina Leslie, Walter Reisner The aim of this project is to create a nanofluidic platform to provide comprehensive maps of single-cell genomes at 1 kbp resolution based on the direct analysis of single 1-10 Mbp extended DNA molecules extracted from individual cells on-chip. We have developed a nanodevice in which all biochemical processing of single cells (cell lysis, DNA purification and fragmentation) is performed in situ. The platform has the following three components: (1) a micro-cavity (50$\times$20 micron in dimension) for trapping and biochemical processing of single cells; (2) post arrays (1 micron depth) for untangling the released genomic contents and (3) parallel nanochannel arrays (100 nm) for extension of $\sim$ 1-10 Mbp DNA for high-throughput optical mapping. Moreover, we use ``Convex Lense-Induced Nanoconfinement'' (CLIC) technique for trapping of single cell and dragging DNA into nanochannels. The principle is that a convex lens is pushed down to deform a flexible coverslip lid above the aforesaid platform containing nano/micro patterns, creating a locally confined region that pins molecules in the embedded nano/micro features. CLIC is used to lower the device lid over a cell isolated in the microcavity with an adjustable gap for buffer exchange. The released DNA is untangled using 1 micron-deep post arrays and driven into nanochannel array where its genomic content is revealed. In particular, using CLIC we were able to successfully trap 20 micron lymphoblast cells inside microcavity and lyse the trapped cell to drive out DNA. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F15.00006: Active microrheology of a model of the nuclear micromechanical environment Henry Byrd, Maria Kilfoil In order to successfully complete the final stages of chromosome segregation, eukaryotic cells require the motor enzyme topoisomerase II, which can resolve topological constraints between entangled strands of duplex DNA. We created an in vitro model of a close approximation of the nuclear micromechanical environment in terms of DNA mass and entanglement density, and investigated the influence of this motor enzyme on the DNA mechanics. Topoisomerase II is a non-processive ATPase which we found significantly increases the motions of embedded microspheres in the DNA network. Because of this activity, we study the mechanical properties of our model system by active microrheology by optical trapping. We test the limits of fluctuation dissipation theorem (FDT) under this type of activity by comparing the active microrheology to passive measurements, where thermal motion alone drives the beads. We can relate any departure from FDT to the timescale of topoisomerase II activity in the DNA network. These experiments provide insight into the physical necessity of this motor enzyme in the cell. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 10:12AM |
F15.00007: Models of chromatin spatial organisation in the cell nucleus Invited Speaker: Mario Nicodemi In the cell nucleus chromosomes have a complex architecture serving vital functional purposes. Recent experiments have started unveiling the interaction map of DNA sites genome-wide, revealing different levels of organisation at different scales. The principles, though, which orchestrate such a complex 3D structure remain still mysterious. I will overview the scenario emerging from some classical polymer physics models of the general aspect of chromatin spatial organisation. The available experimental data, which can be rationalised in a single framework, support a picture where chromatin is a complex mixture of differently folded regions, self-organised across spatial scales according to basic physical mechanisms. I will also discuss applications to specific DNA loci, e.g. the HoxB locus, where models informed with biological details, and tested against targeted experiments, can help identifying the determinants of folding. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F15.00008: Is DNA a non-draining, swollen coil? Abhiram Muralidhar, Douglas Tree, Patrick Doyle, Kevin Dorfman Double-stranded DNA has long been used as a model polymer in a wide variety of experiments, particularly in single molecule studies. However, there is little consensus about whether molecules used commonly in experiments, such as $\lambda$-DNA (48.5 kbp, kilo base pairs) and T4-DNA (169 kbp), are long enough to exhibit universal, long-chain behavior. To resolve this point of contention, we use Pruned-Enriched Rosenbluth Method (PERM) simulations to calculate static and near-equilibrium dynamic properties of DNA ranging from a molecular weight of 100 bp to nearly 1 Mbp (mega base pairs). By evaluating metrics such as the end-to-end distance, and comparing these results with renormalization group theory predictions, we show that molecules such as $\lambda$-DNA and T4-DNA are far from the swollen coil limit. Our results indicate that DNA exhibits flexible swollen coil behavior when the contour length is approximately 1 Mbp. Moreover, computation of the Kirkwood diffusivity from equilibrium configurations reveals that DNA is partially draining to chain lengths as big as 1 Mbp. We attribute this slow transition to universal behavior to the semiflexible nature of DNA, that gives rise to weak intramolecular excluded volume and hydrodynamic interactions. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F15.00009: Escape of a knot from a DNA molecule in flow Benjamin Renner, Patrick Doyle Macroscale knots are an everyday occurrence when trying to unravel an unorganized flexible string (e.g. an iPhone cord taken out of your pocket). In nature, knots are found in proteins and viral capsid DNA, and the properties imbued by their topologies are thought to have biological significance. Unlike their macroscale counterparts, thermal fluctuations greatly influence the dynamics of polymer knots. Here, we use Brownian Dynamics simulations to study knot diffusion along a linear polymer chain. The model is parameterized to dsDNA, a model polymer used in previous simulation and experimental studies of knot dynamics. We have used this model to study the process of knot escape and transport along a dsDNA strand extended by an elongational flow. For a range of knot topologies and flow strengths, we show scalings that result in collapse of the data onto a master curve. We show a topologically mediated mode of transport coincides with observed differences in rates of knot transport, and we provide a simple mechanistic explanation for its effect. We anticipate these results will build on the growing body of fundamental studies of knotted polymers and inform future experimental study. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F15.00010: Depletion Zone Effects in Active Microrheology Studies of DNA Solutions Cole D. Chapman, Douglas E. Smith, Rae M. Robertson-Anderson In active microrheology studies, micron scale spheres are driven through complex fluids while forces imparted on the spheres are measured to determine valuable information about the fluids at the molecular level. However, when a microsphere is dragged through a polymer solution, polymers can amass along its leading surface while leaving an area devoid of polymers in its wake (depletion zone). Depletion zone effects can complicate the interpretation of the measured force, prohibiting standard continuum limit methods for analyzing microrheology data. Here, we examine the depletion zone created by dragging microspheres embedded within a network of DNA (a model polymer). Using dual-force optical tweezers, parallel microspheres are driven axially through a DNA solution, while measuring the force imparted on the individual microspheres. Thus, we are able to explore the effective `wake' created by the leading microsphere via the response of the lagging microsphere and its dependence on a variety of parameters, such as solution concentration, distance traveled, and driving rate. This technique is combined with single-molecule fluorescence microscopy, allowing for simultaneous visualization of the deformation of the individual DNA molecules surrounding the driven microspheres. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F15.00011: Dual-Colored DNA Comb Polymers for Single Molecule Rheology Danielle Mai, Amanda Marciel, Charles Schroeder We report the synthesis and characterization of branched biopolymers for single molecule rheology. In our work, we utilize a hybrid enzymatic-synthetic approach to graft ``short'' DNA branches to ``long'' DNA backbones, thereby producing macromolecular DNA comb polymers. The branches and backbones are synthesized via polymerase chain reaction with chemically modified deoxyribonucleotides (dNTPs): ``short'' branches consist of Cy5-labeled dNTPs and a terminal azide group, and ``long'' backbones contain dibenzylcyclooctyne-modified (DBCO) dNTPs. In this way, we utilize strain-promoted, copper-free cycloaddition ``click'' reactions for facile grafting of azide-terminated branches at DBCO sites along backbones. Copper-free click reactions are bio-orthogonal and nearly quantitative when carried out under mild conditions. Moreover, comb polymers can be labeled with an intercalating dye (e.g., YOYO) for dual-color fluorescence imaging. We characterized these materials using gel electrophoresis, HPLC, and optical microscopy, with atomic force microscopy in progress. Overall, DNA combs are suitable for single molecule dynamics, and in this way, our work holds the potential to improve our understanding of topologically complex polymer melts and solutions. [Preview Abstract] |
Session F16: Focus Session: Extreme Mechanics: Filaments and their Assemblies, Elasticity and Defects
Sponsoring Units: GSNP DPOLYChair: Christian Santangelo, University of Massachusetts-Amherst
Room: 401
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F16.00001: Twisted Ribbons: Theory, Experiment and Applications Julien Chopin, Benjamin Davidovitch, Flavio A. Silva, Romildo D. Toledo Filho, Arshad Kudrolli We investigate, experimentally and theoretically, the buckling and wrinkling instabilities of a pre-stretched ribbon upon twisting and propose strategies for the fabrication of structured yarns. Our experiment consists in a thin elastic sheet in the form of a ribbon which is initially stretched by a fixed load and then subjected to a twist by rotating the ends through a prescribed angle. We show that a wide variety of shapes and instabilities can be obtained by simply varying the applied twist and tension. The observed structures which include helicoids with and without longitudinal and transverse wrinkles, and spontaneous creases, can be organized in a phase diagram with the tension and twist angle as control parameters [J. Chopin and A. Kudrolli, PRL (2013)]. Using a far-from-threshold analysis and a slender body approximation, we provide a comprehensive understanding of the longitudinal and transverse instabilities and show that several regimes emerge depending on subtle combinations of loading and geometrical parameters. Further, we show that the wrinkling instabilities can be manipulated to fabricate structured yarns which may be used to encapsulate amorphous materials or serve as efficient reinforcements for cement-based composites. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F16.00002: Elastocapillarity and the curling of fibers Anupam Pandey, Suzie Protiere, Douglas Holmes Coalescence of paintbrush bristles removed from a bath of fluid is the result of competing elastic and surface energies. The lengthscale that emerges out of this energy balance is called the ``elastocapillary'' lengthscale. This phenomenon has been well studied both experimentally and theoretically at the desktop scale as well as microscale. But in many natural and synthetic systems, the fluid between the flexible fibers can swell the material and causes the fibers to curl. A natural example is human hair, which swells in humid conditions, dilating and becoming frizzy. In this presentation, we demonstrate experimental results on this coupled ``elastocapillary-elastoswelling'' system. Specifically, we identify two distinct regimes dominated by capillarity and swellability, and the transition between these two regimes is governed by the ``elastoswelling'' lengthscale. We also show that in the swelling dominated regime a small fluid droplet is being carried upward by the curling fibers that mimic a pipetting mechanism. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F16.00003: Handedness and self assembly of chiral rods Efi Efrati When handed building blocks, such as twisted fusilli, self-assemble the resulting assembled object is typically also handed (as are its physical response properties). This phenomenon plays a central role in fields raging from biological self-assembly to optimizing the design of optical meta-materials. Despite the importance of this problem, predicting the relation between the handedness of the constituents of an assembled object and its overall handedness has remained an elusive goal even for the simplest of cases. At the heart of this problem lies the difficulty of quantifying the handedness of even a single building block. In this talk I will show how a recent orientation-dependent interpretation of handedness as a relation between directions and rotations sidesteps most of the difficulties associated with the quantification of handedness and resolves an existing puzzle regarding the self-assembly of handed colloidal rods. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F16.00004: Spontaneous formation and evolution of kinks in elastic helical structures Shuangping Liu, Zhenwei Yao, Monica Olvera de la Cruz A variety of linear entities in many biological and chemical systems can spontaneously form helical structures to realize specific functions, notably the helical ribbons found in peptide amphiphiles whose closure can further lead to the formation of tubes. Of particular interest is the coexistence of helices with opposite chiralities connected by kinks in one structure that has been found inbacterial flagella, plant tendrils and peptide amphiphiles etc, in analogy to domain walls separating regions of spin up and spin down. The spatial distribution of chirality is completely controlled by these kinks. There is no topological constraint on the number of kinks in a helical system. The introduction and evolution of kinks are largely determined energetically. In this work, using the three-dimensional pre-strained elastomeric bi-strip model, we investigate the general principles underlying the emergence of regular helical shapes and the proliferation of kinks. Specifically, it is found that if the ends of the belt can freely rotate can have significant influence on the behavior of kinks, opening the possibility of using boundary conditions to control the chirality of these systems. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F16.00005: Stretchable nanoparticle helical ribbons through asymmetric cross-sectional geometry Alfred Crosby, Jonathan Pham, Jimmy Lawrence, Gregory Grason, Todd Emrick Helical objects are ubiquitous. From macroscopic plant tendrils to nanoscopic DNA, the geometry of a coiled helix is fundamentally interesting for its mechanical energy storage and tunable mechanical properties, like the spring stiffness. To create helices on micro- and nano- length scales, it is often necessary to have bilayer materials systems or chiral structures. However, we show in thin ribbons, where the thickness is on a similar order to the elastocapillary length, that having an asymmetric cross-sectional geometry can drive helical formation. We create long, nanoparticle-based ribbons using an evaporative assembly technique called flow coating, which produces non-rectangular cross-sections on the nanoscale. When released into water, interfacial tension balances with elasticity to form spring-like structures. These helical ribbons can be extended to high strains, show good shape recovery, and can display mechanical stiffness values ranging from 10-6 N/m at low strains to 10-2 N/m when highly stretched. In addition, the mechanical properties of these structures can be predictably tuned by controlling the ribbon dimensions or the material composition. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F16.00006: Theory of equilibria of elastic braids with applications to DNA supercoiling Gert van der Heijden, Eugene Starostin Motivated by supercoiling of DNA and other filamentous structures, we formulate a new theory for equilibria of 2-braids, i.e., structures formed by two elastic rods winding around each other in continuous contact and subject to a local interstrand interaction. Unlike in previous work no assumption is made on the shape of the contact curve. Rather, this shape is solved for. The theory is developed in terms of a moving frame of directors attached to one of the strands with one of the directors pointing to the position of the other strand. The constant-distance constraint is automatically satisfied by the introduction of what we call braid strains. The price we pay is that the potential energy involves arclength derivatives of these strains, thus giving rise to a second-order variational problem. The Euler-Lagrange equations for this problem give balance equations for the overall braid force and moment referred to the moving frame as well as differential equations that can be interpreted as effective constitutive relations encoding the effect that the second strand has on the first as the braid deforms under the action of end loads. Both open braid and closed braid solutions (links and knots) are computed and current applications to DNA supercoiling are discussed. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:48AM |
F16.00007: Geometrically frustrated filament assemblies: Unravelling the connection between bundle shape and inter-filament order Invited Speaker: Gregory Grason From steel cables and textile fibers to filamentous protein bundles in cells and tissues, densely-packed assemblies of filaments are vital structural elements of the worlds around us and inside of us. Despite the ubiquity and utility of dense-filament assemblies in such diverse materials (across 7 orders of magnitude in size!) surprisingly little is known about the fundamental rules that govern their structure. This talk will discuss recent progress in our understanding of the non-linear relationship between the geometry of a rope-like assembly and the structure and energetics of inter-filament packing. In particular, we focus on mathematical models of the geometric frustration between twist -- as in macroscopic cables or chiral biofilament bundles -- and the preference for isometric, or ``constant spacing,'' packing of filaments in the cross section. Any measure of twist makes it geometrically impossible to evenly space filaments in bundles, begging the question what is the optimal packing of a twisted bundle? We show that geometry of interfilament contact can be mapped formally onto a problem of packing on a 2D non-Euclidean surfaces, whose intrinsically-curved geometry points to the necessity of a complex spectrum defects in the ground-state packing. We confirm the existence of defects and their sensitivity to bundle twist and radius through simulations of energy-minimizing assemblies of cohesive filaments. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F16.00008: Defect-induced shape transitions in filament bundles Isaac Bruss, Gregory Grason From extracellular proteins to artificially fabricated materials, cohesive filament bundles are found across many systems. Employing continuum elasticity theory and numerical simulations, we study the interdependence between the organization of cohesive filaments arranged into a bundle, and their global structure, focusing on the effects of topological defects on equilibrium bundle shape. We analyze the structural stability of parallel filament bundles possessing 5- and 7-fold disclinations in their cross section, whose presence gives rise to inhomogeneous patterns of compressive and tensile stress. We argue that a generic coupling between filament tilt and inter-filament strains leads to a class of defect-induced shape instabilities, which are the filamentary analogue of defect-induced buckling transitions of 2D membranes, and can be understand as a consequence of the generic Helfrich-Hurault instability of layered materials under tension. We show that bundles containing 5-fold disclinations prefer twisted motifs, and 7-fold disclinations give rise to radial undulations. Furthermore, the pitch and wavelength of these deformations are conditional on the relative cost of filament bending and cohesive interactions. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F16.00009: Optimal packing of curved filaments Luis Cajamarca, Gregory Grason The interactions between straight filaments generically favor a uniform hexagonal arrangement, a packing motif that is frustrated when filaments are {\it curved} which forces a compromise between uniform spacing and uniform shape. Examples of curved biological filaments include bacterial flagella and filamentous components of the bacterial cytoskeleton. We address a simple question: what is the optimal ground state packing of $N$ curved filaments? We present a geometric and mechanical model that incorporates the helical shape of the filaments and adhesive interactions, described by hard tube short-range repulsion and larger range of inter-filament attraction. We discuss two generic geometric classes of helical filament packings: vertically-stacked ($N$-plies) and side-to-side (N-packs). While $N$-plies maintain constant spacing with neighbors at constant shape, the cylindrical structure of the enclosing packing space limits the number and coordination of helices of a given geometry, resulting in fewer adhesive contacts than the ``looser" $N$-pack class, where the lateral packing is unconstrained. We show that this geometric interplay gives rise to rich phase diagram of optimal packing, sensitively dependent to helical geometry, range of adhesion and filament number. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F16.00010: 3D Filament Network Segmentation with Multiple Active Contours Ting Xu, Dimitrios Vavylonis, Xiaolei Huang Fluorescence microscopy is frequently used to study two and three dimensional network structures formed by cytoskeletal polymer fibers such as actin filaments and microtubules. While these cytoskeletal structures are often dilute enough to allow imaging of individual filaments or bundles of them, quantitative analysis of these images is challenging. To facilitate quantitative, reproducible and objective analysis of the image data, we developed a semi-automated method to extract actin networks and retrieve their topology in 3D. Our method uses multiple Stretching Open Active Contours (SOACs) that are automatically initialized at image intensity ridges and then evolve along the centerlines of filaments in the network. SOACs can merge, stop at junctions, and reconfigure with others to allow smooth crossing at junctions of filaments. The proposed approach is generally applicable to images of curvilinear networks with low SNR. We demonstrate its potential by extracting the centerlines of synthetic meshwork images, actin networks in 2D TIRF Microscopy images, and 3D actin cable meshworks of live fission yeast cells imaged by spinning disk confocal microscopy. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F16.00011: Elasticity using Nambu-Goldstone modes of isometries Salem Al Mosleh, Christian Santangelo, Arthur Evans Thin shells have a natural separation in energetic scales between bending and stretching. Owing to the prohibitively high cost for stretching, the elastic energy is approximately invariant under isometric deformations, associated with symmetry there will be Nambu-Goldstone modes which can be described by an effective theory in the diffuse deformation limit. We apply this method to study small deformations of elastic shells, and to the evolution of growing shells under an imposed swelling pattern as well as the effect of imperfection in the swelling pattern on bending and stretching rigidities. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F16.00012: Optomechanical elastomeric engine Milos Knezevic, Mark Warner Efficiently converting solar energy to mechanical or electrical energy is one of the greatest contemporary challenges in science and technology. We present a conceptual design for an engine based on liquid crystal elastomers (LCEs) that extracts mechanical work from heat or light. Unusual properties of LCEs arise from a coupling between the liquid crystalline ordering of mesogenic molecules and the elasticity of the underlying polymer network. The external heat or light cause reversible contractions of monodomain LCEs along their nematic director, with recovery elongations on stimuli removal. The contraction-elongation cycle can be repeated many times, and can be exploited to construct a continuosly operating engine. The material parameters and the geometry of such an engine are explored, and it is shown that its efficiency can go up to 20\%. \\[4pt] [1] M. Kne\v{z}evi\'c and M. Warner, Phys. Rev. E 88, 040501(R) (2013)\\[0pt] [2] I. Z. Steinberg, A. Oplatka, and A. Katchalsky, Nature 210, 568 (1966) [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F16.00013: Defect interactions in blueprinted Liquid Crystal Polymer Networks Vianney Gimenez-Pinto, Andrew Konya, Robin Selinger, Fangfu Ye Using finite element simulation we investigate the shape transformation in liquid crystal elastomers imprinted with several defects of different topological charge arranged in a pattern. We investigate how the distance between defects affects the overall shape distortion of the material. These numerical studies represent an efficient method to predict shape distortions in elastomers imprinted with defects depending on the defect's topological charge, the number or disclinations in the director field, the spatial distribution of the defects cores, among other design aspects. Supported by NSF-DMR 1106014. [Preview Abstract] |
Session F17: Focus Session: Glass Formation and Crystallization in Anisotropic Particles
Sponsoring Units: GSNP DPOLYChair: Corey O'Hern, Yale University
Room: 402
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F17.00001: Odd-even Effects of Glass Transition Temperature with a Network-forming Ionic Glass Ke Yang, Madhusudan Tyagi, Jeffrey Moore, Yang Zhang Odd-even effects, the non-monotonic dependency of physical properties on odd/even structural units, are widely observed in homologous series of crystalline materials. However, such alternation is not expected for molecular amorphous materials. Herein, we report the synthesis of a class of network-forming ionic glasses (IG) using non-spherical multivalent ammonium cations and citrate anions. The glass transition temperatures of these amorphous solids show an alternating pattern with increasing backbone length. To understand the phenomenon's molecular origin, we performed incoherent elastic neutron scattering measurements of the nanosecond atomic dynamics. In addition, quasi-elastic neutron scattering measurements were performed to measure two very discrete relaxation process in this ionic glass. Our results suggest that the molecules' mobility, thus the glass transition temperature, correlates with their structural symmetry. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F17.00002: Phase behavior and crystal nucleation and growth in a system of short semi-flexible chains Bart Vorselaars, David Quigley A system of semi-flexible short chains is simulated to study its phase behavior and ability to crystallize, by using a combination of molecular dynamics and other techniques. For calculating the free energy of the liquid phase a new method is introduced. It is very simple to implement in practice and leads to accurate computation of the melting curve. Furthermore we determine the rate of nucleation and crystal growth in this system via a combination of path-sampling and brute-force simulation techniques. By comparing these quantities, we infer the initial microstructure of the solid phase. Due to the strong anisotropy in the crystal growth rate grains no thicker than a single chain are common, even at moderate supercoolings. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 9:00AM |
F17.00003: Effect of chain topology and angular interactions on the competition between crystallization and glass-formation Invited Speaker: Robert S. Hoy We investigate the role of chain topology and angular interactions on the competition between crystallization and glass formation by mapping out the phase diagram of model ``soft'' colloidal polymers as a function of temperature and bending stiffness $k_b$, spanning the range from fully flexible to rodlike chains. For small $k_b$, monomers occupy the sites of close-packed crystallites while chains retain random-walk-like order. For large $k_b$, for short chains, nematic chain ordering typical of lamellar precursors coexists with a high degree of close-packing, while for longer chains, close-packed chain-folded lamellae separated by amorphous regions are formed. At intermediate values of bending stiffness, the competition between random-walk-like and nematic chain ordering produces glass-formation, as indicated by both dynamical heterogeneity and the growth of icosahedral order near and below $T_g$. The kinetics of the ordering transition depend strongly on chain flexibility: in the flexible limit chains crystallize at lower temperatures and the disorder-order transition occurs very sharply, while as the rodlike limit is approached, the initial phase of ordering occurs at significantly higher temperatures but the crystallization rate is much slower. We also examine the crystallization behavior of short (unentangled) branched polymers as a function of $k_b$. Branching points significantly suppress crystallization, especially for stiffer chains, because their preferred bond angles are incompatible with close-packing. Finally, we discuss the degree to which colloidal polymers can serve as proxies for their microscopic counterparts. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F17.00004: Colloidal analogues of spin systems: Order and phase transitions in dense suspensions of magnetic ellipsoids Peter Schurtenberger, Ilya Martchenko, Jerome Crassous We have determined the phase diagram of magnetic colloidal ellipsoids as a function of both packing fraction $\phi$ and external magnetic field $B$. We use core-shell particles with a magnetic core where the magnetic moment of the core is sufficiently small to avoid additional dipole-dipole interactions, but high enough to induce preferential particle alignment with an external magnetic field. By using a combination of small-angle x-ray scattering, microscopy and magnetometry we have examined positional correlations of the charged ellipsoids (aspect ratio $p = 2.7$) and orientational order of their magnetic moments. We establish structural criteria for the different phase and arrest transitions and map distinct isotropic, nematic and crystalline phases over an extended range of $\phi - B$ coordinates. We demonstrate that upon crystallization of the ellipsoids, the bulk magnetic behavior of the suspensions switches from superparamagnetic to ferromagnetic. We extend the often-used atom-colloid analogy to spin systems and present a relationship between the structural topology of suspensions of magnetic colloids and their macroscopic magnetic response. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F17.00005: Structural signatures of dynamic heterogeneities in monolayers of colloidal ellipsoids Yilong Han, Zhongyu Zheng, Ran Ni, Feng Wang, Marjolein Dijkstra, Yuren Wang we discovered two relationships between dynamic heterogeneity and structure for both translational and rotational motion in monolayers of colloidal ellipsoids by video microscopy: (1) the onsets of translational and rotational dynamic heterogeneities coincide with the maximum density fluctuation and the maximum orientation fluctuation respectively at aspect ratio > 2.5; and (2) the dynamic slowest-moving clusters, the static glassy clusters and the low-entropy clusters are strongly correlated and their sizes diverge at the ideal glass transition point with the same power-law length scaling as a function of density, whereas the size of the fastest-moving clusters diverges at the mode-coupling critical point. These results show that the glass transition has a thermodynamic origin. In addition, we observed one-step and two-step glass transitions at different aspect ratios. All experimental results were confirmed by simulations. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F17.00006: Characterizing the local environment for self-assembly Wenbo Shen, Greg van Anders, Eric Harper, Matthew P. Spellings, Michael Engel, Sharon C. Glotzer Recent advances in synthesis techniques of nano- and micrometer sized colloids have produced a diversity of enthalpically and entropically patchy particles. Relating particle patchiness and structure is necessary for the successful self-assembly of these building blocks. We investigate the relationship between patchiness and local structure by quantifying variations in the relative alignment of particle pairs. Using these methods we compare the efficiency of different types of patchiness for inducing ordering and apply them to self-complementary and actively driven shapes. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F17.00007: Jamming transition in hierarchical networks Xiang Cheng, Stefan Boettcher Jamming transitions arise in disordered granular materials where the systems fall out of equilibrium due to an increase in the packing density. A kinetically constrained lattice gas model due to Biroli and Mezard (BM) has connected the jamming transition to an equilibrium phase transition.\footnote{F. Krzakala {\it et al.}, Phys. Rev. Lett. {\bf 101}, 165702 (2008).} In this description, before this equilibrium transition can be reached, any experiment or simulation would fall out of equilibrium at a Kauzmann transition. However, this analysis is based on a mean-field calculation which, for disordered systems, may have limited relevance in finite dimensions. We study the BM-model on a lattice-like network,\footnote{S. Boettcher and A. K. Hartmann, Phys. Rev. E {\bf 84}, 011108 (2011).} which mixes geometric and mean-field features, to reproduce such a phase transition. Computationally, we use the Wang-Landau algorithm which should be less affected by the jamming near the phase transition. The algorithm produces the density of states and, hence, the entropy directly, in addition to many critical properties, such as packing fraction, compressibility, etc. Also, lattice-like hierarchical networks conveniently allow exact or approximate renormalization group treatments, extending analytical results to the thermodynamic limit. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F17.00008: The Effect of Particle Shape on the Density of Acoustic Modes in Granular Materials Alex Mauney, Sara Berry, Theodore Brzinski, Karen Daniels Most granular simulations and experiments are conducted using circular particles. We report on the effect of particle shape on the density of vibrational modes in a real two dimensional granular material. We acoustically excite static packings composed of one of four shapes: circles, ellipses, pentagons, and concave stars. Using embedded piezoelectric sensors we measure the particle-scale vibrational response. We observe shape-dependence in the acoustic spectra and density of modes, particularly at low frequency. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F17.00009: Stabilizing Liquid Drops in Nonequilibrium Shapes by the Interfacial Jamming of Nanoparticles Mengmeng Cui, Todd Emrick, Thomas Russell Nanoparticles can assemble at the interface between two fluids into a 2-D, forming liquid-like arrays where the nanoparticles can diffuse laterally at the interface. By changing the shape of the liquid domain with an external field, the surface area increases and more nanoparticles adsorb to the interface. By releasing the field, the interfacial area decreases and the nanoparticles are jammed, arresting further change in the shape of the drop. The shapes of the liquid can be tailored and indefinitely remain trapped into shapes far different than spherical indefinitely. Limitations related to the inherent weak forces holding the nanoparticles at the interface are overcome by generating nanoparticle-surfactants in situ. The ability to generate and stabilize liquids with a prescribed shape poses unique opportunities for reactive liquid systems, packaging and delivery, and storage. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F17.00010: Tumbling Motion of Interacting U-Shaped Particles in a Uniform Shear Flow Near Jamming Theodore Marschall, Scott Franklin, Stephen Teitel We simulate a system of overdamped frictionless U-shaped particles (staples) in a uniformly sheared host fluid. An isolated staple in such a shear flow undergoes a tumbling motion due to its asymmetric shape, with average angular velocity proportional to the shear strain rate. We investigate how this tumbling motion is modified in a dense system of interacting staples as we approach the jamming transition. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F17.00011: Strain-rate and temperature-driven transition in the shear transformation zone Penghui Cao, Xi Lin, Harold S. Park We couple the recently developed self-learning metabasin escape algorithm, which enables efficient exploration of the potential energy surface (PES), with shear deformation to elucidate strain-rate and temperature effects on the shear transformation zone (STZ) characteristics in two-dimensional amorphous solids. In doing so, we report a transition in the STZ characteristics that can be obtained through either increasing the temperature or decreasing the strain rate. The transition separates regions having two distinct STZ characteristics. Specifically, at high temperatures and high strain rates, we show that the STZs have characteristics identical to those that emerge from purely strain-driven, athermal quasistatic atomistic calculations. At lower temperatures and experimentally relevant strain rates, we use the newly coupled PES + shear deformation method to show that the STZs have characteristics identical to those that emerge from a purely thermally activated state. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F17.00012: An experimental study of the phases of hard squares Lee Walsh, Narayanan Menon We study the phase diagram of hard squares in two dimensions using millimeter-sized square particles on a vibrated plate. The plate serves as a quasi-thermal noise source which generates translational and rotational diffusion of isolated particles. As area density increases, the spatial arrangement of the squares undergoes a transition from isotropic to phases with four- and six-fold ordering, and subsequently develops crystalline order. This succession of transitions in orientational and translational ordering is in qualitative agreement with recent simulations [C. Avenda\~no and F. Escobedo, Soft Matter 2012 (8) 4675]. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F17.00013: Tunneling percolation behavior and filling factors in metal-insulator nanocomposites Rupam Mukherjee, Zhi-Feng Huang, Boris Nadgorny We have studied conventional transport and tunneling in nanocomposite metal-insulator systems. Two types of percolation thresholds P$_{\mathrm{C}}^{1}$ and P$_{\mathrm{C}}^{2}$, associated with both conventional transport and quantum tunneling respectively are identified in the same nanocomposite systems and the functional dependence between the two thresholds is investigated. In addition, we have studied the relationship between filling factors and percolation thresholds, particularly the importance of geometric effects of nanoparticles of different sizes and shapes in metal-insulator composite systems. A non-monotonic dependence of filling factor as a function of filler volume fraction is established and possible implications of this non-trivial behavior is discussed. [Preview Abstract] |
Session F18: Emulsions and Foams
Sponsoring Units: GSNPChair: Stephan Koehler, Harvard University
Room: 403
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F18.00001: Elastic turbulent-like flow of disordered solid composed of polydisperse emulsion drops Peter Yunker, Shima Parsa, Stephan Koehler, David Weitz We experimentally study the low Reynolds number flow of polydisperse emulsion drops through a wide microfluidic channel. Water drops dispersed in oil flow through a microfluidic channel that is 50 microns in height and 1700 microns in width. The drop area fraction is $\sim0.50$, and drop size polydispersities range from 5\% to 40\%. Polydisperse drops are observed to form solid-like plugs in the middle of the channel. These solid plugs of polydisperse drops are squeezed by faster moving oil at the channel edges; in response, the polydisperse drops collectively deform, forming long-lived force chains that resist the faster moving oil. Conversely, monodisperse drops do not form solid-like plugs or long-lived force chains, but instead spread throughout the channel. Surprisingly, the speed fluctuations for flows of polydisperse drops are nonperiodic, and exhibit power-law-like spectral decays similar to those seen in elastic turbulence; for flows of monodisperse drops, the spectra are largely flat. Decreasing interfacial tension causes force chains in flows of polydisperse drops to decrease in size, as drops deform individually rather than collectively. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F18.00002: Experimental measurements of stress redistribution in flowing emulsions Eric Weeks, Kenneth Desmond We study how local rearrangements alter droplet stresses within flowing dense quasi-two-dimensional emulsions at area fractions $\phi > 0.87$. Using microscopy, we measure droplet positions while simultaneously using their deformed shape to measure droplet stresses. We find that rearrangements alter nearby stresses in a quadrupolar pattern: stresses on neighboring droplets tend to either decrease or increase depending on location. The stress redistribution is more anisotropic with increasing $\phi$. The spatial character of the stress redistribution influences where subsequent rearrangements occur. Our results provide direct quantitative support for rheological theories of dense amorphous materials that connect local rearrangements to changes in nearby stress. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F18.00003: Influence of interfacial area on the rheological behavior of heavy oil emulsions Enrique Soto, Patsy V. Ram\'Irez-Gonz\'alez, Roc\'Io G. de la Torre, Jos\'e M. Guadarrama-Cetina, Sergio H. Qui\~nones-Cisneros Experimental observations of the rheological behavior of heavy oil emulsions ARE presented. The emulsions were prepared from mixtures of the oil and brine in different rations and controlled mixing conditions. It was observed that the oil is the continuous phase and the brine the dispersed one. The drop size distribution and water fraction were measured from digital images obtained by a camera and a microscopy. The viscosity of the emulsions increases, when the drop size decreases and The interfacial area increases. The fluid exhibits a shear thinning and elastic rheological behavior below a critical drop size and concentration. The emulsions are stable for long periods of time. The increase in viscosity and non Newtonian behavior are strongly related to the interfacial area. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F18.00004: Towards an easy way to fabricate small clusters with microfluidic device Bingqing Shen, Mathilde Reyssat, Patrick Tabeling We present a novel approach of clusters elaboration by utilizing microfluidic devices with a T junction combined with a step emulsification generator. The droplets in colloidal size can be directly assembled into clusters in a reproducible manner within a shear flow environment. The shear stress is evidenced to influence the clusters morphology: at low shear, the clusters adopt equilibrium configurations that maximize the number of contact points, consistently with observations made in fluids at rest; at high shear, diverse non-equilibrium configurations are observed. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F18.00005: Ternary liquid mixtures control the multiplicity, shape and internal structure of emulsion droplets Martin F. Haase, Jasna Brujic It is important to control the shape, internal structure and stability of emulsion droplets for drug delivery, biochemical assays, and the design of materials with novel physical properties. Successful methods involve the mechanical manipulation of the flow of oil in water using complex microfluidic devices to make multiple emulsions with a sequential introduction of specific reactants. Instead, here we show how the thermodynamics of immiscible liquid mixtures tailor emulsions using a single dripping instability. For example, the initial composition and choice of surfactant govern the multiplicity of concentric alternating oil and water layers inside the droplets. Stabilizing ternary droplets using nanoparticles gives rise to a plethora of shapes whose geometry is defined by the deformability of the shell and the flow rate. Another option is to incorporate lipids to the multiple emulsion droplet, which form vesicles upon expulsion of the inner water droplets. Depending on the number of initial water droplets, these vesicles eventually form complex hollow topologies, which can be used as junctions or scaffolds for the self-assembly of colloidal particles in the future. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F18.00006: Theoretical Analysis for the Optical Shaping of Emulsion Droplets David Tapp, Jonathan Taylor, Alex Lubanksy, Colin Bain, Buddhapriya Chakrabarti Motivated by recent experimental observations, I discuss a theoretical framework to predict the three-dimensional shapes of optically deformed micron-sized emulsion droplets with ultra-low interfacial tension. The resulting shape and size of the droplet arises out of a balance between the interfacial tension and optical forces. Using an approximation of the laser field as a Gaussian beam, working within the Rayleigh-Gans regime and beyond, and assuming isotropic surface energy at the oil-water interface, the resulting shape equations are numerically solved to elucidate the three-dimensional droplet geometry. A plethora of shapes as a function of the number of optical tweezers, their laser powers and positions, surface tension, initial droplet size and geometry are obtained. Experimentally, two-dimensional emulsion droplet silhouettes have been imaged from above, but their full side-on view has not been observed and reported for current optical configurations. This experimental limitation points to ambiguity in differentiating between droplets having the same two-dimensional projection but with disparate three-dimensional shapes. The model I present elucidates and quantifies this difference for the first time. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F18.00007: Liquid domains of lipid monolayers on the surface of oil-in-water emulsions Lea-Laetitia Pontani, Dylan Bargteil, Martin Haase, Jasna Brujic Immiscible lipids spontaneously decompose into domains, both in cellular membranes and monolayers of amphiphilic films. Here we show that they also form on the surface of oil in water droplets, produced by a microfluidic device. In this case, curvature induced instabilities are balanced by surface tension to produce diverse surface morphologies, such as spots, stripes and hemispheres. Surprisingly, the ternary phase diagram shows that these structures are present even in binary mixtures and can be stable over weeks. We investigate the origin of domain stability by tuning the parameters of the forces that play a role in this process, such as the electrostatic repulsion between the domains, the surface tension of each phase or the size, i.e. the curvature of the droplets. Understanding those mechanisms will not only shed light on the physics of lipid domains in biological membranes but will also allow us to tune this stability to produce droplets with a given number of patches that can then be functionalized for self-assembly with controlled valency. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F18.00008: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F18.00009: Similarity between humans and foams in aging dynamics Byung Mook Weon, Peter S. Stewart Foams are cellular networks between two immiscible phases. Foams are initially unstable and finally evolve toward a state of lower energy through sequential coalescences of bubbles. In physics, foams are model systems for materials that minimize surface energy. We study coalescence dynamics of clean foams using numerical simulations with a network model. Initial clean foams consist of equally pressurized bubbles and a low fraction of liquid films without stabilizing agents. Aging of clean foams occurs with time as bubbles rapidly coalesce by film rupture and finally evolve toward a new quasi-equilibrium state. Here we find that foam aging is analogous to biological aging: the death rate of bubbles increases exponentially with time, which is similar to the Gompertz mortality law for biological populations. The coalescence evolution of foams is self-similar regardless of initial conditions. The population change of bubbles is well described by a Boltzmann sigmoidal function, indicating that the foam aging is a phase transition phenomenon. This result suggests that foams can be useful model systems for giving insights into biological aging. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F18.00010: Foam morphology, frustration and topological defects in a Negatively curved Hele-Shaw geometry Adil Mughal, Gerd Schroeder-Turk, Myfanwy Evans We present preliminary simulations of foams and single bubbles confined in a narrow gap between parallel surfaces. Unlike previous work, in which the bounding surfaces are flat (the so called Hele-Shaw geometry), we consider surfaces with non-vanishing Gaussian curvature. We demonstrate that the curvature of the bounding surfaces induce a geometric frustration in the preferred order of the foam. This frustration can be relieved by the introduction of topological defects (disclinations, dislocations and complex scar arrangements). We give a detailed analysis of these defects for foams confined in curved Hele-Shaw cells and compare our results with exotic honeycombs, built by bees on surfaces of varying Gaussian curvature. Our simulations, while encompassing surfaces of constant Gaussian curvature (such as the sphere and the cylinder), focus on surfaces with negative Gaussian curvature and in particular triply periodic minimal surfaces (such as the Schwarz P-surface and the Schoen's Gyroid surface). We use the results from a sphere-packing algorithm to generate a Voronoi partition that forms the basis of a Surface Evolver simulation, which yields a realistic foam morphology. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F18.00011: Testing for Hyperuniformity in Two Dimensional Foam Anthony Chieco, Douglas Durian, Salvatore Torquato It has been conjectured that all maximally random, strictly jammed, saturated systems are hyperuniform, i.e. the standard deviation of the number, N, of particles inside a region goes like the square root of the number, N\textunderscore b, of particles on the boundary. By contrast, for a normal system the standard deviation of N goes like the square root of N. We study a two dimensional dry foam, which is a heterogeneous media that is jammed and therefore should be hyperuniform. For foam, images are taken so that the grayscale values are high in the liquid phase and zero in the gas phase. To test for hyperuniformity, grayscale values are then considered as proxy for the number of particles in each pixel. We probe our system by randomly placing many windows throughout an image to find an average number, \textless N\textgreater , of particles for the whole sample and calculate the standard deviation of N for each set of windows. Our preliminary results show that the system appears closer to normal than hyperuniform but we are examining larger sample sizes in order to get a definitive conclusion. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F18.00012: Diagnosing the hyperuniformity of two-dimensional jammed packings of spheres Remi Dreyfus, Ye Xu, Salvatore Torquato, Arjun Yodh In the colloidal domain, ascertaining the degree to which disordered jammed structures are hyperuniform is gaining interest because the hyperuniformity property (vanishing of infinite-wavelength density fluctuations) seems to endow the jammed structure with novel physical properties. Indeed, it has recently been shown that hyperuniform disordered structures can be produced to exhibit a complete photonic bandgap. However, determining whether a 2D packing of spheres is hyperuniform or not is non-trivial, especially from experimental datasets where imperfection exists. In this talk, we will use numerical simulations and experimental investigations to show how we can diagnose whether a packing is hyperuniform or not. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F18.00013: Swelling/deswelling of Toroidal Hydrogels Ya-Wen Chang, Michael Dimitriyev, Samantha Marquez, Paul Goldbart, Alberto Fernandez-Nieves Swelling/deswelling of hydrogel spheres proceeds with the increase/decrease of particle radius that corresponds to the change in overall volume. When the hydrogel has a toroidal geometry, which is characterized by two principal radii --radius from the center of the donut hole to the center of the tube, and the tube radius, it is not obvious how swelling proceeds. We prepare thermo-sensitive poly(N-isopropylacrylamide) pNIPAM toroidal gel particles of different aspect ratios. At equilibrium deswelling, i.e., slow heating rate, we find that the aspect ratio remains constant for both fat and thin tori. This is explained by linear elasticity. On the other hand, when the heating rate is sufficiently high, the toroid buckles due to the presence of a water-impermeable skin layer that develops in the initial deswelling stages. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F18.00014: Deriving the microstructural parameters of sea foam from experimental measurements Wai Soen Chan, Hon Ping Lee, Kin Wah Yu We have studied the effective dielectric constant of sea foam by exploiting its spectral structure. We have considered sea foam as a two-phase composite containing air and sea water, at scale where the quasi-static limit is valid. McPhedran and co-workers derived tight bounds of the structural parameters of such composite when a set of measured data is given. However, determining the exact structural parameters have not been successful. We have performed an inverse algorithm, attempted to determine the structure of the foam given measured data of dielectric constant. We model the sea foam by a multilayered Hashin-Shtrikman structure consisting of air embedded in sea water with decreasing air volume fraction from the top to bottom. We first express the effective permittivity of the foam using spectral representation as proposed by Bergman and Milton. Then, by an optimization approach, we determine the spectral parameters, namely the zeros and poles. Next, we convert these spectral parameters into structural parameters by an algorithm proposed by Sun and Yu. Hence the structure of foam could be determined. The inverse problem of determining the sea foam structure is important in marine science. Sea surface wind speed and salinity could be determined from properties of sea foam. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F18.00015: Evidence of nanobubbles in alcohol-water mixtures via production of optically-induced breathing modes in nanofluids Luat T. Vuong, J.-Luis Dominguez-Juarez, Matthew Moocarme When light of sufficient intensity enters a liquid close to a meniscus, thermal and mechanical effects lead to the spontaneous formation of ``leaky-faucet'' breathing modes. The modes are also associated with Marangoni convection. We have recently studied these modes in highly disperse solutions containing 80-nm gold plasmonic spheres (0.01mg/mL fill factor) in alcohol-water mixtures. Our investigations are focused on characterizing and understanding the dynamically-coupled light, heat, and electrical currents that are produced via the ``osmotic stress'' of hydration and solvation. The materials of focus are plasmonically-absorbing gold and silver nanoparticles in \textit{alcohol-water mixtures }because it has been observed that the robust breathing modes occur in such nanofluids with extremely low-power light illumination (\textless 50 mW). In addition to new nonlinear dynamics associated with the anomalous physical properties of alcohol-water mixtures-- i.e., partial molar volume, adiabatic compressibility, heat capacity , ultrasonic speed, and light scattering-- we observe evidence of the formation of nanobubbles, which agree with recent hypotheses that alcohol organic-aqueous mixtures form local 100-nm inhomogeneities described as nanobubbles. [Preview Abstract] |
Session F19: Focus Session: Theory and Simulations of Macromolecules IV
Sponsoring Units: DPOLYChair: Anupriya Agarwal, Clemson University
Room: 404
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F19.00001: POLYMER PHYSICS PRIZE BREAK |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F19.00002: Twinkling Fractal Analysis of PolyVinyl Acetate (PVAc) Yutao Zhang, Richard P. Wool In amorphous polystyrene melts we have shown by Atomic Force Microscopy (Height and Phase) that dynamic rigid fractal clusters form in equilibrium with the fractal liquid and their relaxation behavior determines the kinetic nature of T$_{\mathrm{g}}$ [J. Non Cryst Solids 357(2): 311-319 2011]. The fractal clusters of size R $\sim$ 1-100 nm have relaxation times $\tau $ $\sim$ R$^{1.8}$ (solid-to-liquid) where the exponent is related to the Fractal dimension D$_{\mathrm{f}}$ and Fracton dimension d$_{\mathrm{f}}$ via Df/d$_{\mathrm{f}} =$1.8. Israeloff et al (2006) showed nanoscale spatio-temporal thermo fluctuations in PVAc using a non-contact Dielectric Force Microscopy technique; PVAC shows similar dynamic clustering using both phase and height tapping AFM modes. The dynamic clusters are clearly evident in the range 1-700 nm. The cluster relaxation behavior was explored in both height and phase modes and found to be different. The fractal clusters have a TFT vibrational density of states G(w) $\sim$ w$^{\mathrm{df-1}}$ with eigenvalues (frequencies) and eigenvectors (displacements) and these are expected to manifest differently in these AFM studies on PVAc thin films. We examine the cluster relaxation functions C(t)$\sim$ t$^{\mathrm{-4/3}}$ predicted by the TFT and look for the presence of highly mobile layers near surfaces and holes in nanothin films. These results are in accord with computer simulations of anharmonically interacting particles and the recent observation of ``Dancing Molecules'' in strained ceramic glass (Huang et al, Science Oct 2013), as predicted by the TFT. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F19.00003: Viscoelasticity of crosslinked epoxy networks under extreme conditions from molecular dynamics simulation Timothy Sirk, Mir Karim, Ketan Khare, Rajesh Khare, Jan Andzelm Understanding viscoelastic behavior at ballistic conditions is critical for the design of new polymeric-based protective materials in civilian and military applications. The relaxation mechanisms available to polymer networks at ballistic conditions (strain rate \textgreater 10$^{\mathrm{5}}$ s$^{\mathrm{-1}})$ include both segmental and chain relaxations, and thus cannot be understood as a simple superposition of the relaxations acting at the much lower strain rates typically considered in experiments. We present viscoelastic properties of polymer networks found from atomistic molecular dynamics simulation, where we consider an epoxy monomer, di-glycidyl ether of bisphenol A, reacted with binary amine mixtures, 4,4'-methylenebis(cyclohexylamine) and poly(oxypropylene) diamine. Our results show that (1) oscillatory strain simulations similar to experimental dynamic mechanical analysis are capable of predicting the complex modulus at high frequencies, (2) the maximum of the loss modulus can be expected to occur well-above the glass transition temperature predicted by simulated volumetric data, and (3) the molecular weight between crosslinks strongly influences the thermodynamic state required for ideal energy dissipation. These results from oscillatory strain will also be compared with other methods of evaluating linear viscoelasticity from molecular dynamics simulation. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F19.00004: Optimization of constant pH replica exchange molecular dynamics method Danial Sabri Dashti, Adrian Roitberg Improvement in sampling of configuration space enhances sampling in protonation space. Recently a constant pH replica exchange (PHREM) method has been developed by Itoh et al to improve the coupling between conformational and protonation sampling. We present a technique for estimating the exchange acceptance ratio (EAR) between two arbitrary replicas (i.e., with distinct pHs) in PHREM. Moreover, we designed a scoring function to optimize the position of each replica on pH ladder. Maximizing the scoring function results in equal EAR between all neighbor pairs, which increases the efficiency of PHREM. We have tested our method on erabutoxin and hen egg white lysozyme (HEWL). We found that the estimations of pKa values in the optimized set of replicas converge faster respect to equally spaced set. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F19.00005: Evaluating the Applicability of the Fokker-Planck Equation for Polymer Translocation James Polson, Taylor Dunn Computer simulation methods are used to study the dynamics of polymer translocation through a nanopore for a coarse-grained model. The variation of the translocation time distributions with nanopore friction strength is examined using Brownian dynamics simulations. The distributions are analyzed using the Fokker-Planck (FP) formalism together with free energy functions explicitly calculated using Monte Carlo simulations. When the pore friction is weak, translocation is rapid and the polymer is not conformationally relaxed. In this regime, the FP equation yields quantitatively poor predictions. By contrast, in the limit of sufficiently strong pore friction, the translocation is slow and the polymer maintains a state of conformational quasi-equilibrium. In this regime, the theoretical predictions for both driven and non-driven translocation are in good agreement with the simulation results. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F19.00006: Ab initio calculations of the atomic and electronic structure of crystalline PEO$_{3}$:LiCF$_{3}$SO$_{3}$ electrolytes Sha Xue, Yingdi Liu, Hongli Dang, Dale Teeters, Daniel Crunkleton, Sanwu Wang With the advent of high conductivity polymer batteries, a great deal of research interest has been generated in the study of PEO:LiCF$_{3}$SO$_{3}$ polymer electrolyte, because of its enhanced stability at the lithium/polymer interface. Experimental studies have concluded that both the PEO$_{3}$:LiCF$_{3}$SO$_{3}$ crystalline complex and the PEO$_{3}$:LiCF$_{3}$SO$_{3}$ amorphous phase are both present when PEO/Li ratio is greater than 3. However, most theoretical investigations to date are concerned about the short chain amorphous PEO:LiCF$_{3}$SO$_{3}$ system. We report first-principles-density-functional-theory calculations of crystalline PEO$_{3}$:LiCF$_{3}$SO$_{3}$. In particular, we provide the atomic-scale characteristics and electronic structures. The calculated results about the bonding configuration, electronic structures, and conductivity properties are in good agreement with the experimental measurements. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F19.00007: Cellulose microfibril formation within a coarse grained molecular dynamics Abdolmadjid Nili, Oleg Shklyaev, Vincent Crespi, Zhen Zhao, Linghao Zhong Cellulose in biomass is mostly in the form of crystalline microfibrils composed of 18 to 36 parallel chains of polymerized glucose monomers. A single chain is produced by cellular machinery (CesA) located on the preliminary cell wall membrane. Information about the nucleation stage can address important questions about intermediate region between cell wall and the fully formed crystalline microfibrils. Very little is known about the transition from isolated chains to protofibrils up to a full microfibril, in contrast to a large body of studies on both CesA and the final crystalline microfibril. In addition to major experimental challenges in studying this transient regime, the length and time scales of microfibril nucleation are inaccessible to atomistic molecular dynamics. We have developed a novel coarse grained model for cellulose microfibrils which accounts for anisotropic interchain interactions. The model allows us to study nucleation, kinetics, and growth of cellulose chains/protofibrils/microfibrils. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F19.00008: Novel and Efficient Methods for Calculating Pressure in Polymer Lattice Models Pengfei Zhang, Qiang Wang Pressure calculation in polymer lattice models is an important but nontrivial subject. The three existing methods -- thermodynamic integration, repulsive wall, and sedimentation equilibrium methods -- all have their limitations and cannot be used to \textit{accurately} calculate the pressure at \textit{all} polymer volume fractions $\varphi $. Here we propose two novel methods. In the first method, we combine Monte Carlo simulation in an expanded grand-canonical ensemble with the Wang-Landau -- Optimized Ensemble (WL-OE) simulation to calculate the pressure as a function of polymer volume fraction, which is very efficient at low to intermediate $\varphi $ and exhibits negligible finite-size effects. In the second method, we introduce a repulsive plane with bridging bonds, which is similar to the repulsive wall method but eliminates its confinement effects, and estimate the two-dimensional density of states (in terms of the number of bridging bonds and the contact number) using the 1/$t$ version of Wang-Landau algorithm. This works well at all $\varphi $, especially at \textit{high} $\varphi $ where all the methods involving chain insertion trial moves fail. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F19.00009: Statistical Behavior of Polymer Chains in Curved Space Jianfeng Li, An-Chang Shi, Hongdong Zhang, Feng Qiu, Yuliang Yang Recently, we have derived the modified diffusion equation of the propagator (the end-segment distribution function) in general curved space for both Gaussian and wormlike chains. Mathematically, a Gaussian-curvature term appears as an extra external field in the diffusion equation of Gaussian chains while there is an additional normal-curvature term for the case of wormlike chains. The basic statistical behavior of polymer chains in curved space can be also extracted by examining these newly derived diffusion equations revealing that Gaussian chains are aware of the intrinsic curvature of the space but are blind to the external curvature while wormlike chains can feel both. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F19.00010: Formation and structural properties of multi-block copolymer vesicles Rong Wang, Shiying Ma Due to the unique structure, vesicles have attracted considerable attention for their potential applications, such as gene and drug delivery, microcapsules, nanoreactors, cell membrane mimetic, synthetic organelles, \textit{etc}. By using dissipative particle dynamics, we studied the self-assembly of amphiphilic multi-block copolymer. The phase diagram was constructed by varying the interaction parameters and the composition of the block copolymers. The results show that the vesicles are stable in a large region which is different from the diblock copolymer or triblock copolymer. The structural properties of vesicles can be controlled by varying the interaction parameters and the length of the hydrophobic block. The relationship between the hydrophilic and hydrophobic block length \textit{vs} the aqueous cavity size and vesicle size are revealed. The copolymers with shorter hydrophobic blocks length or the higher hydrophilicity are more likely to form vesicles with larger aqueous cavity size and vesicle size as well as thinner wall thickness. However, the increase in hydrophobic-block length results to form vesicles with smaller aqueous cavity size and larger vesicle size. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F19.00011: Diamond-Forming Block Copolymers and Diamond-like Morphologies: a New Route towards efficient Block Copolymer Membranes? Igor Erukhimovich, Yury Kriksin Formation of ordered (microphase separated) block copolymer nanostructures is a promising route towards creating isoporous membranes suitable for technological applications. We propose a new route to achieve this target: to choose such block copolymer architectures, which would provide a practically isotropic permeability both in the bulk and in thin films. Basing both on the weak segregation theory extension into the thin films and the self-consistent field theory numerical procedure we present the results concerning the effects of the wall confinement both with neutral, selective and patterned walls on the structure and stability of the block copolymer ordered films. The diamond-like morphology is found to be the most promising one as to optimizing the permeability of thin films. A new effect of the diamond morphology stability enhancement in the presence of a properly designed lamellar-like wall pattern is discovered and the corresponding phase diagram demonstrating the effect of the pattern scale and film width on the diamond morphology stability is presented. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F19.00012: The influence of chain length polydispersity of of ABA triblock copolymers on bicontinuous network structures Zhong-Yuan Lu, Yue Li, Hu-Jun Qian, An-Chang Shi We study the polydispersity effect on microphase separation of ABA triblock copolymers using dissipative particle dynamics simulations, focusing on the formation of bicontinuous structures. The composition window for observing the bicontinuous network structures can be controled by designing polydispersity distributions of ABA triblock copolymers. We find that increasing polydispersity in both A and B blocks can significantly enhance the composition window for observing bicontinuous network structures. The network structures possess good continuity throughout the material, implying possible applications in photovoltaic devices. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F19.00013: Phase Behavior of Semiflexible Block Copolymer droplets in isotropic homopolymer matrix Ping Tang, Jie Gao, Jianfeng Li, Yuliang Yang We investigate the phase behavior of semiflexible-coil block copolymer droplets in the matrix of isotropic homopolymers by using an efficient pseudo spectral method to solve self-consistent field theory (SCFT) equations. The semiflexible blocks are described with worm-like chain model and the Maier-Saupe orientational interactions are included to deal with semiflexible chains. We will interested that the influence of microphase separation of semiflexible-coil block copolymers coupling with liquid crystalline behavior of rod blocks on the interface and droplets shapes. [Preview Abstract] |
Session F20: Focus Session: Organic Electronics and Photonics - Small Molecules
Sponsoring Units: DMP DPOLYChair: Rodrigo Noriega, University of California, Berkeley
Room: 405
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F20.00001: POLYMER PHYSICS PRIZE BREAK |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F20.00002: Effect of mechanical deformation on the electrical properties of organic single crystals Marcos Reyes-Martinez, Alfred Crosby, Alejandro Briseno Despite efforts in the flexible electronics field, relatively little research quantified the effects of mechanical strain on the electrical properties of organic single crystals (OSCs) and their device performance in deformed geometries. Single crystals of organic semiconductors are ideal systems for the elucidation of these effects without having to account for imperfections, grain boundaries and other defects. The aim of this presentation is to bring new understanding of the effects of mechanical strain in charge transport phenomena on OSCs. First, the existence of a piezoresistive effect in rubrene crystals is demonstrated and experimentally quantified by the application of in-plane strain along its [010] axis. A piezoresistive coefficient approximately 50 is determined. Second, the effect of local mechanical deformation on the conductive channel is investigated in rubrene single-crystal field-effect transistors. A wrinkling instability is used as a technique to apply local strains of different magnitudes to the conducting channel of field-effect transistors. All devices maintain excellent transistor behavior, and small, reversible changes in performance are observed during wrinkling. This work provides useful knowledge for the effective application of organic semiconductors in strain intensive applications such as pressure sensors, electronic skins and strained-channel organic transistors. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F20.00003: Near Surface Structure of Organic Semiconductor Tetracene Single Crystal Yusuke Wakabayashi, Hazuki Morisaki, Tsuyoshi Kimura, Kazumoto Miwa, Takashi Koretsune, Jun Takeya Electric conduction in organic crystals is highly anisotropic because of the anisotropic molecular orbitals. Crystal structure governs the transfer through the overlap integral among the highest occupied (or lowest unoccupied) molecular orbitals. In case of organic devices, the place where electrons conduct is the interface. Therefore, the surface structure of organic single crystals is relevant. Surface relaxation of the structure of rubrene single crystal was firstly observed by means of surface x-ray diffraction a few years ago [1]. This time we performed similar measurement on tetracene single crystal, whose molecular shape has large similarity with rubrene while the crystal structure is very different. Tetracene single crystal was grown by the physical vapor transport method, and the surface x-ray diffraction experiments were performed at BL-3A and 4C of the Photon Factory, KEK, Japan. Obtained electron density profile shows a large structural deformation at the surface layer of tetracene. \\[4pt] [1] Y.Wakabayashi, J.Takeya and T.Kimura, Phys. Rev. Lett. 104, 066103 (2010). [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F20.00004: Trap healing and ultra low-noise Hall effect at the surface of organic semiconductors Vitaly Podzorov Fundamental studies of intrinsic charge transport properties of organic semiconductors are often hindered by charge traps associated with static disorder present even in optimized single-crystal devices. Here, we report a novel method of surface functionalization using an inert non-conjugated polymer, perfluoropolyether (PFPE), deposited at the surface of organic molecular crystals, that results in accumulation of mobile holes and ``trap healing'' effect at the crystal/PFPE interface [1]. As a consequence, a remarkable ultra low-noise, trap-free conduction regime characterized by intrinsic mobility and transport anisotropy emerges in organic single crystals, and Hall effect measurements with unprecedented signal-to-noise ratio are demonstrated. This general method to convert trap-dominated organic semiconductors to intrinsic systems may enable the determination of intrinsic transport parameters with high accuracy and make Hall effect measurements in molecular crystals ubiquitous.\\[4pt] [1] B. Lee, Y. Chen, D. Fu, H. T. Yi, K. Czelen, H. Najafov, V. Podzorov, ``Trap healing and ultra low-noise Hall effect at the surface of organic semiconductors,'' Nature Mater. DOI 10.1038NMAT3781 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:48AM |
F20.00005: Effect of pressure on electronic charge coherence in organic semiconductor single crystals Invited Speaker: Jun Takeya Small molecular organic semiconductor crystals form interesting electronic systems of periodically arranged ``charge reservoirs'' whose mutual electronic coupling determines whether or not electronic states can be coherent over molecular distances. Recently, it turned out that band transport is realized in high-mobility organic semiconductor crystals though this situation is not common to all organic semiconductors. Series of Hall-effect measurements on different molecular crystal systems indicated that the extent of the charge coherence depends on molecular species at room temperature. In this presentation, we focus on the single-crystal molecular assembly of pentacene which does not exhibit full charge coherence at room temperature under atmospheric pressure. Hall coefficient, telling us the extent of the electronic coherence, is precisely measured for accumulated charge in pentacene single-crystal field-effect transistors at various temperatures with varied pressure. With the application of external pressure, the electronic coupling between pentacene molecules is continuously modified so that the extent of the intermolecular coherence grows with increasing pressure. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F20.00006: Controlling Conformations of Conjugated Polymers and Small Molecules: The Role of Nonbonded Interactions Kevin Kohlstedt, Nicholas Jackson, Brett Savoie, Lin Chen, Monica Olvera de la Cruz, George Schatz, Mark Ratner The chemical variety present in the organic electronics literature has motivated us to investigate potential nonbonding interactions often incorporated into conformational ``locking'' schemes. We have examined a variety of potential interactions, including oxygen-sulfur and nitrogen-sulfur, using accurate quantum-chemical wave function methods on a selection of high-performing conjugated polymers and small molecules. In addition, we evaluate a set of nonbonding interactions occurring between various heterocyclic and pendant atoms taken from a group of representative pi-conjugated molecules. From our survey, it is determined that while many nonbonding interactions possess weak binding capabilities, hydrogen bonding interactions, namely oxygen-hydrogen and nitrogen-hydrogen, are alone in inducing conformational control and enhanced planarity along a polymer or small molecule backbone at room temperature. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F20.00007: \textit{Ab Initio} Investigation of Conformal and Dipolar Effects on Subphthalocyanine Photovoltaic Properties Michael Waters, Guangsha Shi, Hossein Hashemi, Emmanouil Kioupakis, John Kieffer Boron subphthalocyanine chloride (B-SubPc-Cl) currently has the highest reported open circuit voltages of any organic photovoltaic donor coupled with C$_{60}$. In our density functional theory (DFT) investigations, we sought to understand the origins of this performance by substituting boron and chlorine with other trivalent and halogen elements, respectively. Substitution of the trivalent and halogen elements distorts the porphyrin ring and changes the molecular dipole moment. For the equilibrium conformation of each isolated molecule, time-dependent DFT was used to compute the optical absorption. Using DFT with added Van der Waals interactions, experimentally unknown crystal structures were predicted. The electronic and optical excitation energies of these crystal structures were calculated using the GW/Bethe-Salpeter equation method. We found that the optical absorption spectra are significantly affected by the strong exciton binding energies in these materials ($\sim$0.5 eV for B-SubPc-Cl). [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F20.00008: Tunable Molecular Orientation of Organic Semiconductors in Vapor-Deposited Amorphous Solids Diane Walters, Shakeel Dalal, Mark Ediger Amorphous solids made by physical vapor deposition (PVD) of organic molecules have found increasing use in organic LEDs and photovoltaics. PVD is favored because it allows precise control of layer thickness and high material purity, however the impact of deposition conditions on the structure of amorphous solids has been largely uninvestigated. We have previously shown that solid films prepared by PVD can have drastically higher densities, moduli and thermal stability than are obtainable by cooling the liquid. Using a high-throughput characterization technique, we show that PVD is also able to impart significant molecular orientation into amorphous solids. We present work on several common molecules used in organic semiconducting devices including AlQ$_{3}$, NPB, TPD, CBP, DSA-Ph, and BSB-Cz. The molecular orientation depends systematically on the substrate temperature during deposition. At low temperatures there is a strong tendency to lie parallel to the substrate, while at higher temperatures there is a tendency to stand vertically on end. It is anticipated, and in some limited cases has been previously shown, that this orientation can significantly affect charge mobility and light out-coupling efficiency in devices. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F20.00009: The nanoscale morphology of new types of solar cells based on solution-processed small-molecules Nuradhika Herath, Valeria Lauter, Jim Browning Organic electronics have become promising alternatives for the today's energy demand, owing to their low cost fabrication processes, ability to performance under low light, and flexibility. Solution processed small molecule (SM)- fullerene based solar cell devices have been subjected to number of studies recently with significant progress of power conversion efficiency (PCE). The bulk hetero junction (BHJ) consisting SM-fullerene blend is the most critical part of the solar cell device as nano-to-meso-scale morphology of BHJ plays a significant role in the device performances and properties. In this study we investigate the morphological structure of a device constructed from solution processed SM-molecule $p-$DTS(FBTTh$_{2}$)$_{2}$ with fullerene PC$_{70}$BM BHJ blend using neutron reflectometry (NR). Here we present the scattering length density changes of PC$_{70}$BM concentration along the film depth and the history dependence of the BHJ device by taking the measurements as-cast as thermally annealed (150 $^{\circ}$C). [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F20.00010: Photo-oxidation degradation mechanisms in P3HT for organic solar cells: Insights from first-principles simulations Kevin Leung, Na Sai, Judit Zador, Graeme Henkelman Photo-oxidation is one of the leading chemical degradation mechanisms in polymer solar cells. In this work, using hybrid density functional theory and periodic boundary condition, we investigate reaction pathways that may lead to the sulfur oxidation in poly(3-hexylthiophene)(P3HT) as a step toward breaking the macromolecule backbone. We calculate energy barriers for reactions of P3HT backbone with oxidizing radicals suggested by infrared spectroscopy (IR) and XPS studies. Our results strongly suggest that an attack of hydroxyl radical on sulfur as proposed in the literature is unlikely to be thermodynamically favored. On the other hand, a reaction between the alkylperoxyl radical and the polymer backbone may provide low barrier reaction pathways to photo-oxidation of conjugated polymers with side chains. Our work paves way for future studies using ab-initio calculations in a condensed phase setting to model complex chemical reactions relevant to photochemical stability of novel polymers.\\[4pt] 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-AC04-94AL850. [Preview Abstract] |
Session F21: Focus Session: Polymeric Fibers and Superstructures
Sponsoring Units: DPOLYChair: Hyun-Joong Chung, University of Alberta
Room: 406
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F21.00001: POLYMER PHYSICS PRIZE BREAK |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F21.00002: Edge electrospinning: a facile needle-less approach to realize scaled up production of quality nanofibers J.R. Bochinski, C. Curtis, M.P. Roman, L.I. Clarke, Q. Wang, N.M. Thoppey, R.E. Gorga Utilizing unconfined polymer fluids (e.g., from solution or melt), edge electrospinning [1] provides a straightforward approach for scaled up production of high quality nanofibers through the formation of many parallel jets. From simple geometries (using solution contained within a sharp-edged bowl [2,3] or on a flat plate [4]), jets form and spontaneously re-arrange on the fluid surface near the edge. Using appropriate control of the electric field induced feed rate, comparable per jet fabrication as traditional single-needle electrospinning can be realized, resulting in nanofibers with similar diameters, diameter distribution, and collected mat porosity. The presence of multiple jets proportionally enhances the production rate of the system, with minimal experimental complexity and without the possibility of clogging. Extending this needle-less approach to commercial polyethylene polymers, micron scale fibers can be melt electrospun using a similar apparatus. [1] N. M. Thoppey et al., \textit{Polymer} \textbf{51}, 4928 (2010). [2] N. M. Thoppey et al., \textit{Nanotechnology} \textbf{22}, 345301 (2011). [3] N. M. Thoppey et al., \textit{Macromolecules} \textbf{45}, 6527 (2012). [4] M. P. Roman et al., \textit{Macromolecules} \textbf{46}, 7352 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F21.00003: A new method for the alignment of electrospun nanofibers by oxygen plasma treatment Natsumi Kobayashi, Norihisa Miki, Koichi Hishida, Atsushi Hotta An effective way of controlling the alignment of electrospun nanofibers using oxygen plasma treatment was introduced. Poly (dimethylsiloxane) (PDMS) was selected as a base material for electrospinning and polyvinyl alcohol (PVA) was chosen as an electrospun-nanofiber material. It was found that most of PVA nanofibers were selectively deposited on the O2 plasma-treated area of PDMS, while only a few PVA nanofibers were randomly deposited on the untreated area of the PDMS film. Interestingly, a number of PVA nanofibers were neatly aligned along the border of the untreated area and the O2 plasma-treated area of PDMS. The surface structures and the morphology of the PDMS films with PVA nanofibers were analyzed by scanning electron microscopy, water contact angle measurements, and X-ray photon spectroscopy. By selecting the optimized ratio of treated and untreated area of PDMS film, it was found that more than 80{\%} of PVA nanofibers could be deposited parallel to the border of the treated and untreated area of PDMS. We used PVA as a reference material for the nanofiber alignment in this study, but similar deposition behavior was also observed for polyurethane (PU) fibers. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F21.00004: Molecular dynamics simulations of electron irradiated PVDF nanofibers Jiayuan Miao, Ram Bhatta, Christian Kisielowski, Dinesh Lolla, Darrell Reneker, Mesfin Tsige, Philip Taylor High-resolution, aberration corrected transmission electron microscopy was used to observe morphological changes and segmental motion of electrospun poly(vinylidene fluoride) nanofibers in an 80 kilovolt electron beam. Atomic and molecular scale high-resolution images of fibers were made with an aberration corrected electron microscope. Chemical and morphological changes, which include the breaking of the fiber, loss of fluorine atoms and cross-linking of chains, caused by the high-energy electron beam were observed. We present the results of molecular dynamics (MD) simulations of such atomic and molecular level observations. The calculational models include the influence of chain scission, chain recoiling, and torsional defects on the morphology of a nanofiber. The effects of the loss of fluorine atoms and the applied tension on the morphology of the fibers were also investigated. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F21.00005: Synchrotron X-ray Scattering Studies of Poly(lactide) Electrospun Fibers Containing Carbon Nanotubes Yazhe Zhu, Peggy Cebe Carbon nanotubes(CNTs) often serve as an effective nucleating agent that facilitates the crystallization of semicrystalline polymers. Here we study the influence of CNTs on thermal and structural properties of Poly-lactide (PLA), which is well-known as a biodegradable and biocompatible thermoplastic polymer. The effect of CNTs on the crystallization and melting behavior of electrospun fibers of poly (L-lactide) (PLLA, with 100{\%} L-isomer) and poly (D-lactide) (PDLA, containing 4{\%} D-isomer) was systemically studied by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), Fourier transform spectroscopy(FT-IR) and real time synchrotron wide-angle X-ray scattering (WAXS) . Multi-walled CNTs were co-electrospun with the poly(lactides) in weight ratios ranging from 0.1 to 4.0 wt{\%} MW-CNT. PLA/carbon nanotubes composite electrospun fibers were successfully produced by appropriate choice of processing conditions and solution concentration. The morphologies of neat and CNT-filled electrospun nanofibers were observed by scanning electron microscopy. WAXS and DSC results show that lower content of CNTs contributes to higher speed of crystallization. However the results also showed that at the highest concentration of CNTs the ultimate crystallinity was reduced. FTIR and X-ray results show that PLA fibers have different crystal forms at high and low crystallization temperature. DSC results also show that D-lactide has reduced crystallinity compared to L-lactide. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F21.00006: Nanofibers from Melt Blown Fiber-in-Fiber Polymer Blends Zaifei Wang, Feng Zuo, Dawud Tan, Soondeuk Jeung, Christopher Macosko, Frank Bates Nanofibers were generated by melt blowing three sets of polymer blends each comprised of pairs of immiscible components. Blends containing minority phases of poly(ethylene-\textit{co}-chlorotrifluoroethylene) (PECTFE) in poly(butylene terephthalate) (PBT), PECTFE in poly(styrene) (PS), and PBT in PS, were melt blown into long (\textgreater 100 microns) fibers with average diameters of several microns. Electron microscope revealed that melt blowing transformed the initial spherical dispersions into a nanofibers-in-fiber morphology. Macroscopic mats of nonwoven PBT and PECTFE nanofibers, with average diameters as small as 70 nm, were isolated by selectively removing the majority phase with a solvent. This method provides a potentially inexpensive, high throughput, one step route to scalable quantities of polymeric nanofibers. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F21.00007: Multi-scale modeling for the self-assembly of DNA-functionalized nanoparticle into supperlattice and Wulff polydedra Ting Li, Evelyn Auyeung, Chad Mirkin, Monica Olvera de la Cruz Since 1996, DNA hybridization has proven robust for programmable self-assembly of nanoparticles (NPs). Recently, we showed that through a ``slow cooling'' method, DNA functionalized nanospheres or so-called ``programmable atom equivalents'' can assemble into crystals with a specific and uniform habit. Here we perform molecular dynamics simulations on multi-scale models to study and predict the corresponding shapes. Firstly, we use a scale-accurate coarse-grained model with explicit DNA chains to estimate surface energy ratios for different surface orientations, and predict the corresponding Wulff polyhedra based on these values. Secondly, we use a colloidal model in which each DNA coated nanosphere is represented by a single bead to simulate the growth dynamics of the crystals. By this method, we confirm the shape for the body-centered-cubic system to be a (110)-enclosed rhombic dodecahedron. But the face-centered-cubic system does not show any uniform shape yet except triangular features with (111) and (100) facets due to crystallographic defects including twinning and stacking faults. These simulated crystal shapes agrees very well with experiments. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F21.00008: Giant Polyhedra based on Nano-atoms Stephen Cheng In order to create new functional materials for advanced technologies, both Precisely control over functionality and their hierarchical structures and orders are vital for obtaining the desired properties. Among all the giant molecules, giant polyhedra are a class of materials which are utilized via deliberately placing precisely functionalized polyhedral oligomeric silsesquioxane (POSS) and fullerene (C60) molecular nano-particles (MNPs) (so-called ``nano-atoms'') at the vertices of a polyhedron. These giant polyhedra capture the essential structural features of their small-molecule counterparts in many ways but possess much larger sizes, and therefore, they are recognized as size-amplified versions of those counterparts. One of the most illustrating examples is a series of novel giant tetrahedral which possessing precisely-defined amphiphilic MNPs with different geometric combinations. With both geometrical and chemical symmetry breakings, these tetraphedra perform as building blocks to construct When specific interactions are introduced, these polyhedral are functioned as building blocks to construct into hierarchical ordered structures. A range of ordered super-lattice structures of this class of materials have been investigated in the condensed bulk state. The study has also expanded to other types of giant polyhedra to identify the general role in their assembly processes. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F21.00009: Programmable Nanoparticle clusters via DNA linking Xu Ma, Mark J. Bowick, Alisha Lewis, Mathew M. Maye, Rastko Sknepnek Due to selective recognition, short complementary DNA strands have been widely used as linkers to direct the crystallization or the formation of larger assemblies of nanoparticles. We study the self-assembly of small clusters through DNA hybridization in a binary mixture of spherical nucleic acid gold nanoparticles (SNA-GNPs) with the larger SNA-GNPs in excess by performingthe molecular dynamics simulationson the graphical processing unit (GPU).The resultant structures are self-assembled clusters with a varying number of large SNA-GNPs clustersaround the small ones, and the structure of the clusters varies as the ratio of large to small hydrodynamic radii changes. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F21.00010: Fluorinated Polyhedral Oligomeric Silsesquioxane Based Giant Molecular Shape Amphiphiles: Hierarchical Self-Assembly with Unusual Chain Conformation Xue-Hui Dong The fluorous phase has thus been considered as the third phase that repels both oil and water due to its ultra-low surface energy. Incorporation of fluorinated component into hydrophilic/hydrophobic polymers is anticipated to bring novel self-assembly behaviors in the bulk, solution and thin film states, which are not only academically intriguing but also technological relevant. Among them, fluorous molecular clusters are of particular interest. A topologic isomer pair of giant molecular shape amphiphiles can be constructed by tethering molecular nanoparticle at different location of block polymers. In this study, a fluorinated polyhedral oligomeric silsesquioxane (FPOSS) was precisely fixed onto polystyrene\textit{block}poly(ethylene oxide) (PS-$b$-PEO) at chain end (FPOSS-PS-$b$-PEO), or junction point [PS-(FPOSS)-PEO]. The interplay between nanoparticle and block polymers results in hierarchical structures with three types of order. The incommensuration of cross-sectional area between FPOSS and block polymer stretches polymer chains, which found to enhance the immiscibility between PEO and PS block. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 11:00AM |
F21.00011: Large-scale electrohydrodynamic organic nanowire printing, lithography, and electronics Invited Speaker: Tae-Woo Lee Although the many merits of organic nanowires (NWs), a reliable process for controllable and large-scale assembly of highly-aligned NW parallel arrays based on ``individual control (IC)'' of NWs must be developed since inorganic NWs are mainly grown vertically on substrates and thus have been transferred to the target substrates by any of several non-individually controlled (non-IC) methods such as contact-printing technologies with unidirectional massive alignment, and the random dispersion method with disordered alignment. Controlled alignment and patterning of individual semiconducting NWs at a desired position in a large area is a major requirement for practical electronic device applications. Large-area, high-speed printing of highly-aligned individual NWs that allows control of the exact numbers of wires, and dimensions and their orientations, and its use in high-speed large-area nanolithography is a significant challenge for practical applications. Here we use a high-speed electrohydrodynamic organic nanowire printer to print large-area organic semiconducting nanowire arrays directly on device substrates in an accurately individually-controlled manner; this method also enables sophisticated large-area nanowire lithography for nano-electronics. We achieve an unprecedented high maximum field-effect mobility up to 9.7 cm$^{2}$$\cdot$V$^{-1}$$\cdot$s$^{-1}$ with extremely low contact resistance (\textless 5.53 $\Omega \cdot$ cm) even in nano-channel transistors based on single-stranded semiconducting NWs. We also demonstrate complementary inverter circuit arrays consist of well-aligned p-type and n-type organic semiconducting NWs. Extremely fast nanolithography using printed semiconducting nanowire arrays provide a very simple, reliable method of fabricating large-area and flexible nano-electronics. [Preview Abstract] |
Session F22: Focus Session: Biological and Bio-inspired Adhesive Polymers I
Sponsoring Units: DPOLY DBIO GSNPChair: Devin Kachan, University of California, Los Angeles
Room: 407
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F22.00001: POLYMER PHYSICS PRIZE BREAK |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F22.00002: Toughening elastomers with sacrificial bonds and watching them break Invited Speaker: Costantino Creton Most unfilled elastomers are relatively brittle, in particular when the average molecular weight between crosslinks is lower than the average molecular weight between entanglements. We created a new class of tough elastomers by introducing isotropically prestretched chains inside ordinary acrylic elastomers by successive swelling and polymerization steps. These new materials combine a high entanglement density with a densely crosslinked structure reaching elastic moduli of 4 MPa and fracture strength of 25 MPa. The highly prestretched chains are the minority in the material and can break in the bulk of the material before catastrophic failure occurs, increasing the toughness of the material by two orders of magnitude up to 5 kJ/m$^{\mathrm{2}}$. To investigate the details of the toughening mechanism we introduced specific sacrificial dioxetane bonds in the prestretched chains that emit light when they break. In uniaxial extension cyclic experiments, we checked that the light emission corresponded exactly and quantitatively to the energy dissipation in each cycle demonstrating that short chains break first and long chains later. We then watched crack propagation in notched samples and mapped spatially the location of bond breakage ahead of the crack tip before and during propagation. This new toughening mechanism for elastomers creates superentangled rubbers and is ideally suited to overcome the trade-off between toughness and stiffness of ordinary elastomers. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F22.00003: Bond-breaking in semiflexible networks and the peeling dynamics of a filament from a random array of pinning sites Christian Vaca, Alex J. Levine Recent rheological experiments on cross-linked microtubule networks suggest that the principal dissipative mechanism at low frequencies is cross linker breakage. In such networks, applied stress leads to both the breaking of old cross links and the formation of new ones, allowing the network to maintain its elastic modulus while dissipating energy. We present a model of the underlying microscopic processes in such networks: the force-induced unbinding of a semiflexible filament from an array of randomly distributed pinning sites. These pinning sites dissociate from the filament with a force-dependent rate, as prescribed by e.g., the Bell model. This problem is part of a larger class of nonequilibrium systems that includes the driven motion of three phase contact lines and flux lines in superconductors, in which an elastic object is pulled through a quenched random potential. Using transfer matrix methods and numerical simulations, we explore the distribution of forces on the various pinning sites, and calculate the statistical properties of the filament's peeling dynamics under a constant force applied at one point perpendicular to its length. The mean peeling rate depends on the filament's bending modulus, the elasticity of the pinning sites, and their spatial distribution. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F22.00004: Mussel Adhesion is Significantly Enhanced Due to the Shape and Mechanics of Its Holdfast Kenneth Desmond, Nicholas Zacchia, Herbert Waite, Megan Valentine Mussels permanently adhere to surfaces through a circular plaque that is attached to the animal body via a long thin thread; forming a mushroom-shaped geometry. A plaque just a few millimeters in diameter with a 250-micron diameter thread can withstand large pull forces of a several Newtons without debonding. While the strength of individual chemical bonds plays a role in determining the adhesive strength, the contact mechanics associated with the mushroom shape is also critically important. In fact, numerous other organisms also use mushroom-shaped holdfasts to create strong bonds, suggesting the mushroom geometry is particularly effective for adhesion. To better understand the role of contact mechanics on the adhesive strength of mussels, we study mussel detachment using a custom built load frame capable of pulling on samples along any orientation and measuring the resulting force, while simultaneously imaging the plaque deformation and the glass-plaque interface. We will show that the holdfast shape improves bond strength by an order of magnitude compared to other simple geometries and that force-induced yielding of the mussel plaque improves the bond strength by another two orders of magnitude. These results show that by optimizing for contact mechanics, adhesive strength can be finely tuned for a particular application without changing the interface chemistry. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F22.00005: Equilibrium phase behavior of labile cross inkers in semiflexible networks Devin Kachan, Alex Levine, Robijn Bruinsma The equilibrium phase behavior of cross linkers in a network of semiflexible filaments is complex. The binding of the cross linkers affects the transverse undulations of the filaments leading to a fluctuation-mediate attractive or Casimir interaction between them. If the cross linkers also provide constraint torques to enforce a preferred binding angle between filaments, the resulting networks can have complex spatial distributions of filaments and of cross linkers bound to those filaments. Simulations report both the formation lamellar network structures and the aggregation of cross-linkers in thermal equilibrium. In this talk, we explore the the Casimir interaction between cross linkers bound to a given filament. We report on the spatial correlations between cross linkers bound to a given filament due to their Casimir interactions, and compare these theoretical predictions to the results of Brownian dynamics based finite element simulations of the system. We conclude with a discussion of the implications of these results for the equilibrium structure of semiflexible filament networks with labile cross linkers. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F22.00006: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F22.00007: The Role of Salts in the Evolution of Modern Orb-Webs. Vasav Sahni, Toshikazu Miyoshi, Kelley Chen, Dharamdeep Jain, Sean J. Blamires, Todd A. Blackledge, Ali Dhinojwala The evolution of modern viscid silk webs from ancient cribellate silk webs is associated with a 95{\%} increase in diversity of orb-weaving spiders, and their dominance as predators of flying insects. Yet the transition's mechanistic basis is an evolutionary puzzle. Ancient cribellate silk is a dry adhesive that functions through van der Waals interactions. Viscid threads adhere more effectively than cribellate threads due to high extensibility of their axial silk fibers, and firm adhesion of the viscid glue droplets. The organic and inorganic salts present in viscid glue sequester atmospheric water that plasticizes the axial silk fibers and renders them extensible. Here, we provide direct molecular and macro-scale evidence to show that salts also generate adhesion by directly solvating the glycoproteins, regardless of water content, thus imparting viscoelasticity and enabling the glue droplets to establish good contact. This `dual role' of salts provides a crucial link to the evolutionary transition from cribellate silk to viscid silk. In addition, salts also provide a simple mechanism to adhere even at the extremes of relative humidity, a feat eluding most synthetic adhesives. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F22.00008: Synthetic Adhesive Attachment Discs based on Spider Pyriform Silk Architecture Dharamdeep Jain, Vasav Sahni, Ali Dhinojwala Among the variety of silks produced by spiders, pyriform silk is used in conjunction with the dragline silk to attach webs to different surfaces. Cob weaver spiders employ different architectural patterns to utilize the pyriform silk and form attachment joints with each pattern having a characteristic adhesive performance. The staple pin architecture is a one of the strongest attachment designs employed by spiders to attach their webs. Here we use a synthetic approach to create the a similar patterned architecture attachment discs on aluminum substrate using thermoplastic polyurethane. Measurable pull off forces are generated when the synthetic discs are peeled off a surface. This innovative adhesive strategy can be a source of design in various biomedical applications. [Preview Abstract] |
Session F23: Invited Session: Industrial Physics Forum: The Squid at 50: Impact and Future
Sponsoring Units: FIAPChair: John Clarke, University of California, Berkeley, Dale Van Harlingen, University of Illinois at Urbana-Champaign
Room: 505-507
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F23.00001: First SQUIDs Invited Speaker: Arnold Silver The Superconducting QUantum Interference Device (SQUID) is the most sensitive magnetic flux sensor and the most widely applied superconductor electronic device, whose applications range from magnetocardiography to picovoltmeters, from digital logic to quantum computing, and from non-destructive testing to Gravity Probe B, a spaceborne test of Einstein's theory of gravity. In this presentation, I describe the initial experiments and device modeling at the Ford Scientific Laboratory that produced the early versions of the SQUID during the 1960's. That history originated in an anomalous observation during microwave ENDOR experiments and led to the first report of macroscopic quantum interference in superconductors in 1964 [Phys. Rev. Letters 12 (1964)]. The SQUID is based on London's electrodynamic theory of multiply-connected superconductors [Superfluids Wiley, New York (1950)], the magnetic flux quantum (h/2e=2.07E-15 Wb), and Josephson's theory of weakly-connected superconductors [Phys. Lett. 1 (1962)]. Physically, it incorporates Josephson tunnel junctions in a low inductance, superconducting ring. Two distinct types of SQUIDs were demonstrated: first the ``dc SQUID'' and then the ``rf SQUID.'' The former has two Josephson junctions and produces a dc frequency response; the latter has only one junction and responds only at rf and microwave frequencies. The first phase, conducted by Lambe, Jaklevic, Mercereau, and Silver, used type I thin film superconductors and Josephson tunnel junctions. The second phase, conducted by Silver and Zimmerman, used bulk niobium structures with ``cat whisker'' junction technology [Phys.Rev. 157 (1967)]. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F23.00002: SQUIDs: Then and Now Invited Speaker: John Clarke In 1964, Jaklevic, Lambe, Silver and Mercereau demonstrated quantum interference in a superconducting ring containing two Josephson tunnel junctions. This observation marked the birth of the SQUID---Superconducting QUantum Interference Device. The following year saw the appearance of the SLUG (Superconducting Low-inductance Undulatory Galvanometer)---a blob of solder frozen around a length of niobium wire---that was used as a voltmeter with femtovolt resolution. Although extremely primitive by today's standards, the SLUG was used successfully in a number of ultrasensitive experiments. Today, the square washer dc SQUID, fabricated on a wafer-scale from thin films with an integrated input coil, finds a wide range of applications. One example is the use of a SQUID amplifier to read out ADMX---Axion Dark Matter eXperiment---at the University of Washington, Seattle. This experiment, which involves a cooled microwave cavity surrounded by a superconducting magnet, searches for the axion, a candidate for cold dark matter. In the presence of a magnetic field the axion is predicted to decay into a photon, which is detected by the SQUID. In another example, the combination of a SQUID with prepolarized proton spins enables one to perform magnetic resonance imaging (MRI) in magnetic fields of the order of 0.1 mT, four orders of magnitude lower than in conventional MRI systems. In vivo images of the human brain acquired at these ultralow fields are able distinguish brain tissue, blood, cerebrospinal fluid and scalp fat using a combination of inversion recovery and multiple echo sequences. Potential clinical applications are briefly discussed. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:48AM |
F23.00003: SQUID-amplified photon detection: from cosmology to material science Invited Speaker: Kent Irwin Superconducting photon detectors amplified by SQUIDs are playing an increasingly important role in science ranging from cosmology to materials characterization. The most widely used superconducting photon detector uses a superconducting transition-edge sensor (TES), which is a superconducting film biased in the narrow transition region between the normal and superconducting state. The film is voltage biased, and the current flowing through it is measured with a SQUID. An incident photon increases the resistance of the TES, which reduces the current through the SQUID. Large arrays of SQUID-coupled TES detectors are read out by cryogenic multiplexing of the SQUIDs with a time-division, frequency-division, or code-division multiplexing scheme. SQUID-coupled TES detectors are now widely deployed in ground- and balloon-borne observatories to measure the cosmic microwave background (CMB) radiation. By measuring the power and the polarization of the CMB, new constraints have been placed on cosmological parameters, as well as the absolute masses and number of neutrino species. Experiments are now being conducted to search for the signature of gravitational waves in the polarization of the cosmic microwave background, which would provide strong evidence of inflation at GUT energy scales. Remarkably, very similar sensor arrays to those developed for CMB measurements can also be used for spectroscopic measurements at synchrotron and free-electron laser x-ray light sources. SQUID-coupled TES sensors provide spectroscopic resolution previously only achieved with dispersive detectors based on gratings and crystal diffraction, but with the high efficiency of semiconductor x-ray detectors. I will describe experiments using SQUID-coupled TES arrays for x-ray emission and x-ray absorption spectroscopy of materials, and plans to develop much larger arrays for next-generation light sources. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:24AM |
F23.00004: SQUID use for Geophysics: finding billions of dollars Invited Speaker: Catherine Foley Soon after their discovery, Jim Zimmerman saw the potential of using Superconducting Quantum Interference Devices, SQUIDs, for the study of Geophysics and undertook experiments to understand the magnetic phenomena of the Earth. However his early experiments were not successful. Nevertheless up to the early 1980's, some research effort in the use of SQUIDs for geophysics continued and many ideas of how you could use SQUIDs evolved. Their use was not adopted by the mining industry at that time for a range of reasons. The discovery of high temperature superconductors started a reinvigoration in the interest to use SQUIDs for mineral exploration. Several groups around the world worked with mining companies to develop both liquid helium and nitrogen cooled systems. The realisation of the achievable sensitivity that contributed to successful mineral discoveries and delineation led to real financial returns for miners. By the mid 2000's, SQUID systems for geophysics were finally being offered for sale by several start-up companies. This talk will tell the story of SQUID use in geophysics. It will start with the early work of the SQUID pioneers including that of Jim Zimmerman and John Clarke and will also cover the development since the early 1990's up to today of a number of magnetometers and gradiometers that have been successfully commercialised and used to create significant impact in the global resources industry. The talk will also cover some of the critical technical challenges that had to be overcome to succeed. It will focus mostly on magnetically unshielded systems used in the field although some laboratory-based systems will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 11:00AM |
F23.00005: Magnetoencephalography: From first steps to clinical applications Invited Speaker: Risto Ilmoniemi Magnetoencephalography (MEG), the study of femtotesla-range magnetic fields produced by neuronal currents in the brain, originated in the 1960's. After Baule and McFee's (Am Heart J 66:95-6,1963) measurement of the cardiac magnetic field using induction-coil sensors, Cohen (Science 16:784-6, 1968) used a similar multi-turn coil to detect the brain's alpha rhythm. The introduction of the superconducting quantum interference device (SQUID) by Zimmerman et al. (J Appl Phys 41: 1572-80) improved the sensitivity of magnetic sensing by several orders of magnitude, making MEG practical. The SQUID enabled the unaveraged recording of spontaneous brain rhythms (D. Cohen, Science 175:664-6, 1972) as well as evoked magnetic fields (Brenner et al., Science 190:480-2, 1975; Teyler et al., Life Sci 17:683-91, 1975). Subsequently, a large number of evoked-field variants were demonstrated. The main benefit of MEG is its ability to locate electrical activity in the brain at high temporal resolution. For practical work, we need large arrays of highly sensitive SQUIDs; such arrays were first built in the 1990's (Knuutila et al., IEEE Trans Magn 29:3315-20, 1993). While the intrinsic spatial accuracy of locating sources with well-calibrated large sensor arrays is better than one millimeter, uncertainties in determining the location and geometry of the cortex with respect to the array may lead to source-location errors of 5--10 mm or more. These errors could be reduced to 1 mm if one could obtain the structural image of the brain with the same sensors that are used for MEG and if the conductivity geometry of the head would be accurately known. This may indeed be possible if MRI is recorded with SQUIDs (McDermott et al., PNAS 21:7857-61, 2004) concurrently with MEG (Zotev et al., J Magn Reson 194:115-20, 2008), especially if large arrays are developed (Vesanen et al., Magn Reson Med 69:1795-1804, 2013); the conductivity distribution of the head might be possible to determine with current-density imaging (Nieminen et al. Magn Reson Imaging, 2013). MEG has established itself as a standard tool in human neuroscience (Hamalainen et al., Rev Mod Phys 65:413-97, 1993). It is used increasingly in clinical applications such as in locating motor or language areas prior to brain surgery or in determining characteristics of epileptic activity of patients. [Preview Abstract] |
Session F24: Photovoltaics, including Cu, Sulfides, and Si Alloys
Sponsoring Units: GERAChair: Gergely Zimanyi, University of California, Davis
Room: 504
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F24.00001: Scaleable photovoltaic absorber materials within the Cu-Sb-S system Adam Welch, William Tumas, David Ginley, Colin Wolden, Andriy Zakutayev The Cu-Sb-S system contains four ternary compounds which may hold promise for scalable, non-toxic, and efficient solar photoconversion. Like similar compounds CuInSe$_2$, CIGS, and CZTS, the Cu-Sb-S compounds are predicted to offer high absorption coefficients, and electrically benign grain boundaries. Antimony, instead of indium or gallium, has the advantage of lower cost and greater availability, as well as theoretically predicted better photon absorption. It also has a potential advantage over CZTS, as the Cu-V-S compound avoids the deep traps associated with antisite defects. Here, we synthesize two compounds within the Cu-Sb-S system, Cu$_{12}$Sb$_4$S$_{14}$ (tetrahedrite) and CuSbS$_2$ (chalcostibite), by combinatorial RF magnetron co-sputtering from Cu$_2$S and Sb$_2$S$_3$ targets. Chalcostibite films were found to have good optical and electrical properties, with a steep absorption onset at 1.5eV, high absorption coefficient ($>10^{5}$cm$^{-1}$), good carrier concentration ($p=10^{17}$cm$^{-3}$) and mobility (0.2 cm$^2$/V-s). Chalcostibite growth conditions were therefore further optimized and it was found that an overflux of vapor phase Sb$_2$S$_3$ allowed strict control of stoichiometry for better device integration. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F24.00002: Theoretical study on the growth conditions for single-phase stability of kesterite-Cu$_{2}$ZnSnS$_{4}$ Pranab Sarker, Tyler J. Harrison, Mowafak M. Al-Jassim, Muhammad N. Huda Nowadays, kesterite-Cu$_{2}$ZnSnS$_{4}$ (CZTS) is being pursued as an efficient solar absorber materials for PV cells. By chemical potential landscape analysis of CZTS we will show that the formation of stoichiometric CZTS is practically impossible at thermodynamic equilibrium. This analysis verifies the experimental fact that non-stoichiometry is evident for high efficiency CZTS. In addition, the co-existence of ZnS is found to be highly probable if high efficiency growth condition (Zn rich, Cu-poor) is pursued. Moreover, it is found that Zn-richer growth condition is necessary to minimize the number of competitive secondary phases. Cu-poor condition should be chosen in such a way so that the occurrence of Cu$_{2}$S can be prevented irrespective of the value available chemical potential for S. In addition, defect calculation shows that the suitable Cu-poor condition can prevent anionic defects as well. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F24.00003: Growth of Cu$_{2}$ZnSnS$_{4}$(CZTS) by Pulsed Laser Deposition for Thin film Photovoltaic Absorber Material Abhishek Nandur, Bruce White CZTS (Cu$_{2}$ZnSnS$_{4}$) has become the subject of intense interest because it is an ideal candidate absorber material for thin-film solar cells with an optimal band gap (1.5 eV), high absorption coefficient (10$^{4}$ cm$^{-1}$) and abundant elemental components. Pulsed Laser Deposition (PLD) provides excellent control over film composition since thin films are deposited under high vacuum with excellent stoichiometry transfer from the target. CZTS thin films were deposited using PLD from a stoichiometrically close CZTS target (Cu$_{2.6}$Zn$_{1.1}$Sn$_{0.7}$S$_{3.44}$). The effects of laser energy fluence and substrate temperature and post-deposition sulfur annealing on the surface morphology, composition and optical absorption have been investigated. Optimal CZTS thin films exhibited a band gap of 1.54 eV with an absorption coefficient of 4x10$^{4}$cm$^{-1}$. A solar cell utilizing PLD grown CZTS with the structure SLG/Mo/CZTS/CdS/ZnO/ITO showed a conversion efficiency of 5.85{\%} with V$_{\mathrm{oc}} = $ 376 mV, J$_{\mathrm{sc}}=$ 38.9 mA/cm$^{2}$ and Fill Factor, FF $=$ 0.40. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F24.00004: Non-equilibrium phase map, optical and electrical properties of Cu-Zn-O alloys Archana Subramaniyan, John Perkins, Ryan O'Hayre, David Ginley, Stephan Lany, Andriy Zakutayev Cuprous oxide (Cu$_{2}$O) is a candidate p-type solar cell absorber material that has been spotlighted recently due to its low cost, earth abundant and non-toxic nature. The maximum reported efficiency of Cu$_{2}$O based solar cells is rather low (5. 38{\%}) and it can in part be attributed its forbidden direct band gap (2.1 eV) and higher absorption threshold (2.6 eV). Here, we alloy Cu$_{2}$O with ZnO via combinatorial RF magnetron sputtering as a function of temperature (T) and composition at fixed 20 mTorr Ar pressure to modify the electronic band structure and reduce its absorption threshold, which can potentially enhance the solar cell performance. A non-equilibrium Cu-Zn-O phase map was generated in the T range 100 -- 400 $^{\circ}$C and Zn composition 0 -- 37 at{\%}. Highly crystalline Cu$_{2}$O structured Cu-Zn-O alloys with Zn content of 0 to 17 at{\%} were synthesized in the T range 200 -- 270 $^{\circ}$C. With increasing Zn at{\%}, the preferential orientation in Cu-Zn-O alloy changes from (200) to (111) direction. At lower T (\textless 200 $^{\circ}$C), either amorphous or poor crystalline Cu$_{2}$O structured alloys were observed, whereas at higher T (\textgreater 270 $^{\circ}$ C) and higher Zn composition (\textgreater 25 at{\%}), CuO or ZnO second phases were observed. The absorption coefficient of all Cu-Zn-O alloys was higher than that of phase pure Cu$_{2}$O. The absorption threshold () was also reduced significantly, for example, at $=$ 2*10$^{4}$ cm$^{-1}$ the absorption threshold of Cu-Zn-O alloy with 10 at{\%} Zn reduced from 2.4 eV to 2.1 eV. The electrical conductivity of all Cu-Zn-O alloys was measured to be within 2 -- 5 mS/cm. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F24.00005: First principles simulations of Cu$_2$ZnSnS$_x$O$_{4-x}$ alloys Chaochao Dun, N.A.W. Holzwarth, Yuan Li, Wenxiao Huang, David Carroll Crystalline Cu$_2$ZnSnS$_4$ (CZTS) has been well studied for its photo-voltaic properties. This paper reports a systematic computational study of CZTS alloys with oxygen substituting for S in the form Cu$_2$ZnSnS$_x$O$_{4-x}$ in order to understand their stability and structural forms. The calculations find the heat formation of Cu$_2$ZnSnO$_{4}$ (CZTO) to be 4.7 eV lower than that of CZTS, a result which is consistent with the general observation that CZTS is very reactive when exposed to air. Interestingly, the results find that CZTS is stable with respect to its decomposition products; the calculated enthalpy for CZTS $\rightarrow$ Cu$_2$S + ZnS + SnS$_2$ is $\Delta H_{cal}$= +0.6 eV. However, for CZTO the corresponding decomposition is predicted to be exothermic; the calculated enthalpy for CZTO $\rightarrow$ Cu$_2$O + ZnO + SnO$_2$ is $\Delta H_{cal}$= -1.7 eV. The simulations of S/O alloys show that there are preferred structures for the O configurations. For example, for alloys with $x=2$, the energy difference between the lowest and highest energy O arrangements is 0.25 eV/formula unit. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F24.00006: An EXAFS Analysis of Cu$_2$SnS$_3$ for Extremely Thin Absorber Layer Leila Jewell, Andrew Short, Frank Bridges, Glenn Alers, John Norman, Sue A. Carter We present local structure studies of Cu$_2$S and Cu$_2$SnS$_3$ composite films prepared with CVD, using extended x-ray absorption fine structure (EXAFS) technique. The EXAFS technique has the ability to probe the local environment of specific atoms, and can also give very precise ratios of elements using their fluorescence peaks. Chemical vapor deposition (CVD) deposits highly conformal films and hence is an important tool for developing nanostructured solar cells with scalability. Cu$_2$SnS$_3$ is an earth-abundant absorber that is even more cost-effective when used in an extremely thin absorber solar cell. Composite films of Cu$_2$SnS$_3$ were made using CVD layers of Cu$_2$S and Tin (IV) Sulfide (SnS$_2$) with an anneal step. Cu$_2$SnS$_3$ also has the same structure as ZnS, which allows for the formation of the quaternary Cu$_2$ZnSnS$_4$ by depositing ZnS on top of the Cu$_2$S and SnS$_2$ layers determined for Cu$_2$SnS$_3$. Stoichiometric control was established by varying the deposition times of the binary compounds and was measured using energy-dispersive x-ray spectroscopy (EDX), x-ray diffraction (XRD), and EXAFS techniques. Optical absorption results are promising for forming a photovoltaic device with copper-based ternary and quaternary materials as the absorber. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F24.00007: Comparative nm-Resolution Electrical Potential and Resistance Mapping of Cu(In,Ga)Se$_{2}$, Cu$_{2}$ZnSnSe$_{4}$, and CdTe Thin Films Chunsheng Jiang, Ingrid Repins, Lorelle Mansfield, Miguel Contreras, Helio Moutinho, Kannan Ramanathan, Mowafak Al-Jassim We report on a comparative study of three leading thin-film PV materials of Cu(In,Ga)Se$_{2}$ (CIGS), Cu$_{2}$ZnSnSe$_{4}$ (CZTS), and CdTe, by mapping the local electrical potential and resistance using atomic force microscopy (AFM)-based electrical techniques of scanning Kelvin probe force microscopy (SKPFM) and scanning spreading resistance microscopy (SSRM). The SKPFM potential mapping shows consistent results among the three films. The energy bands around the grain boundaries (GBs) bent downward and the GBs are positively charged. However, whether the carriers around the GBs are depleted or inverted could not be determined solely by the potential contrast between the GB and grain surface because surface band bending decreases this contrast. The SSRM resistance mapping shows different results between the films. A higher conduction channel was imaged along the GBs of CIGS and CZTS, indicating an inversion of carriers around the GBs. However, no characteristic resistance was imaged on the GBs of CdTe. This difference of local resistance on the GBs suggests a depletion of carriers in CdTe, in contrast to CIGS and CZTS. These nm-electrical mapping proposes an active GB of CdTe for minority carrier recombination, but inactive GBs of CIGS and CZTS. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F24.00008: Structure prediction and electronic structure study of pristine and doped cuprous sulfide (Cu$_{2}$S) Prashant Khatri, Mowafak M. Al-Jassim, Muhammad N. Huda Cuprous sulfide (Cu$_{2}$S) is among the materials that have high potential of being used in solar cells, but it is highly unstable mainly due to the formation of Cu vacancies. Due to this instability of Cu$_{2}$S and mobile nature of Cu in Cu$_{2}$S, it is hard to study Cu$_{2}$S, and as a result not much is known about its structural details. A systematic theoretically understanding is necessary to utilize its potential fully in photovoltaic devices. The goal of this study is to predict the most probable structure for stoichiometric Cu$_{2}$S which is energetically favorable, and to find a mechanism to stabilize it against the formation of Cu vacancy. DFT, DFT$+$U and DFT-Hybrid functional theory has been used in predicting the structure and studying the properties. Many different structures have been considered while performing the calculations. Acanthite like Cu$_{2}$S structure has been found to be the most favorable structure energetically. We have also studied the structures with Cu-vacancy. A detail theoretical analysis of these aspects will be presented. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F24.00009: Structural Order and Thermodynamic Stability of Disordered Cu$_{2}$ZnSnS$_{4}$ Alloys Sin Cheng Siah, Rafael Jaramillo, Pete Erslev, Glenn Teeter, Tonio Buonassisi Crystalline kesterite Cu$_{2}$ZnSnS$_{4}$ (c-CZTS) thin films, of interest for photovoltaics, has a narrow window of thermodynamic stability and complex point defect chemistry. Hence, c-CZTS solar cells are thought to suffer from the effects of secondary phase segregation and further improvements in device efficiency may hinge on using kinetic stabilization to inhibit decomposition. By growing films at room temperature (T), we achieve a disordered (CuZnSn)S$_{4}$ alloy with an expanded solid solution window in the pseudo-ternary CuS--ZnS--SnS phase diagram that allows independent tuning of bandgap and carrier concentration. We use extended x-ray absorption fine structure to quantify short range order, and x-ray absorption near edge structure to quantify phase segregation of this new alloy. X-ray diffraction is used to elucidate the long range structural order. We study the structural evolution of the alloy as a function of annealing temperature and see continuous evolution towards c-CZTS phase that is nearly complete at 450$^{\circ}$C. Our results inform the fabrication of conventional c-CZTS solar cells by establishing the temperature range over which thin films transform from a kinetically stabilized, metastable phase to a thermodynamically stabilized, crystalline phase. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F24.00010: Defects in buffer layers: electrical and optical properties of point defects in CdS and ZnS Joel Varley, Vincenzo Lordi The rapid development of thin-film solar cells has largely focused on alternative absorber materials, while the choices for buffer layers remain somewhat limited. The most common buffer layer material is cadmium sulfide (CdS), which exhibits good electrical properties leads to a loss of solar photocurrent due to its band gap of 2.4 eV. Wider band gap alternatives with good electrical properties are desired, but the precise material characteristics dictating the electrical properties are not fully understood. Here we present a first principles study to benchmark the electrical and optical characteristics of intrinsic and common extrinsic point defects in CdS and ZnS, a larger band gap alternative buffer layer. We discuss the role of defects in carrier compensation and recombination events that strongly impact the buffer layer electrical properties in a thin-film solar cell and overall device performance. Correlation of defect properties with growth conditions is made in terms of film stoichiometry and presence of impurities. We also calculate the band alignments in a conventional Cu(In,Ga)Se$_2$ solar cell, showing why CdS performs well and why Zn(O,S) is a promising alternative buffer layer for high-efficiency devices. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F24.00011: Evidence of p- to n-type inversion at CIGS grain boundaries: A depth-dependent surface electron microscopy study Calvin Chan, Taisuke Ohta, Gary Kellogg, Lorelle Mansfield, Rommel Noufi Chalcopyrite Cu(In$_{1-x}$Ga$_x$)Se$_2$ (CIGS) is an interesting photovoltaic material because it holds the laboratory record for thin-film solar power conversion efficiency ($\eta > 20$\%) despite its disordered microcrystalline structure. However, commercialization of this technology has been limited by structural and chemical variations in CIGS films. Many microscopic and spectroscopic studies have shown built-in electric potentials ($\Phi_{\textrm{bi}}$) at CIGS grain boundaries. This may assist with electron-hole separation, but the reported magnitude and statistical distribution of $\Phi_{\textrm{bi}}$ remains inconsistent between studies. In this work, photoemission and low-energy electron microscopies (PEEM and LEEM) were used to reconcile these reported differences. Highly surface sensitive PEEM measurements showed $\Phi_{\textrm{bi}} \sim 0.5$ V, which was consistent with most other reports. However, more bulk sensitive LEEM measurements showed $\Phi_{\textrm{bi}} \sim 1.5$ V, which strongly suggests p- to n-type inversion at CIGS grain boundaries. This formation of pn junctions at CIGS grain boundaries is likely responsible for the high performance of CIGS photovoltaics. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F24.00012: Plasma and Thermal Assisted Selenization for Preparation of CuInGaSeS absorber film Zehra Cevher, Zhi Huang, Ren Yuhang Cu(In,Ga)Se semiconductor alloys have been the center of attention over the past decades to potentially replace high efficient silicon based~photovoltaic devices.~ In order to improve the conversion efficiency of CuInGaSe photovoltaic devices, the CuInGa precursor film along with a suitable selenization technique must be enhanced.~ We demonstrate the plasma assisted selenium cracking technique and thermal assisted selenium cracking by radio frequency plasma or increased temperature and the deposition of a selenium cap layer above CuInGa metallic precursors.~ The two stage plasma enhanced selenization process includes the modification of the ionization state of the Se species by radio frequency plasma and the deposition of a selenium cap layer above CuInGa precursors and thermal assited selenization includes cracking large selenium molecules without further modification during selenization.~ Improved homogeneity and crystallization is realized in both techniques~as opposed to conventional selenization procedure. The result is explained by the enhancement of reaction kinetics between the reduced Se phase and metallic precursor layers.~ We further demonstrate improvement in CIGS film morphology using sulfurization technique.~ [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F24.00013: Possible reasons for low open circuit voltage in pyrite (FeS$_2$) Predrag Lazic, Rickard Armiento, William Herbert, Rupak Chakraborty, Ruoshi Sun, Maria Chan, Katherine Hartman, Tonio Buonassisi, Bilge Yildiz, Gerbrand Ceder Pyrite (FeS$_2$), being a promising material for future solar technologies, has so far exhibited in experiments an open-circuit voltage (OCV) of around 0.2 V, which is much lower than the frequently quoted ``accepted'' value for the fundamental bandgap of 0.95 eV. Absorption experiments show large subgap absorption, commonly attributed to defects or structural disorder. However, computations using density functional theory with a semi-local functional predict that the bottom of the conduction band consists of a very low intensity sulfur p-band that may be easily overlooked in experiments. The intensity of absorption into the sulfur p-band is found to be of the same magnitude as contributions from defects and disorder. Our findings suggest the need to re-examine the value of the fundamental bandgap of pyrite presently in use in the literature. If the contribution from the p-band has so far been overlooked, the substantially lowered bandgap would partly explain the discrepancy with the OCV. Also, we show that more states appear on the surface within the low energy sulfur p-band, which suggests a mechanism of thermalization into those states that would further reducing the OCV. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F24.00014: Si3AlP: A New Promising Material for Solar Cell Absorber Jihui Yang, Yingteng Zhai, Hengrui Liu, Hongjun Xiang, Xingao Gong, Suhuai Wei First-principles calculations are performed to study the structural and optoelectronic properties of the newly synthesized nonisovalent and lattice-matched (Si2)0.6(AlP)0.4 alloy [T. Watkins et al., J. Am. Chem. Soc. 2011, 133, 16212.] The most stable structure of Si3AlP is a superlattice along the \textless 111\textgreater direction with separated AlP and Si layers, which has a similar optical absorption spectrum to silicon. The ordered C1c1-Si3AlP is found to be the most stable one among all the structures with --AlPSi3- motifs, in agreement with the experimental suggestions. We predict that C1c1-Si3AlP has good optical properties, i.e., it has a larger fundamental band gap and a smaller direct band gap than Si, thus it has much higher absorption in the visible light region, making it a promising candidate for improving the performance of the existing Si-based solar cells. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F24.00015: Si-Based Earth Abundant Clathrates for Solar Energy Conversion Yuping He, Giulia Galli We show that recently synthesized Si-based clathrates[1], composed entirely of Earth abundant elements are promising materials for solar energy conversion. Using ab initio calculations we found that the type I clathrate K$_{8}$Al$_{8}$Si$_{38}$ exhibits a quasi-direct band gap of $\simeq$ 1 eV, which may be tuned to span the IR and visible range by strain engineering. We also found that electron and hole states generated by photon absorption are spatially separated on different cages in the material, with low probability of charge recombination. Finally, we computed electron and hole mobilities and obtained values much superior to those of amorphous silicon and approximately six and ten time smaller than those of crystalline silicon.\\[4pt] [1] Y. He, F. Sui, S. Kauzlarich and G. Galli (submitted) [Preview Abstract] |
Session F25: Focus Session: Materials for Electrochemical Energy Storage: Beyond Li-ion
Sponsoring Units: DMP GERA DCOMPChair: Paul Kent, Oak Ridge National Laboratory
Room: 503
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F25.00001: Electrochemistry of dioxygen in lithium-air batteries Invited Speaker: Laurence Hardwick The non-aqueous lithium-oxygen battery is one of a host of emerging opportunities available for enhanced energy storage [1]. Unlike a conventional battery where the reagents are contained within the cell, the lithium-oxygen cell uses dioxygen from the atmosphere to electrochemically form the discharge product lithium peroxide. Degrees of reversible oxidation and formation of lithium peroxide has been demonstrated in a number of non-aqueous electrolyte classes, mostly notably in dimethysulfoxide based electrolytes [2], thus making the lithium-oxygen cell a potential energy storage device. This talk will present our groups recent results of the electrochemistry of dioxygen in non-aqueous electrolytes, of which particular electrolytes could have practical application within a lithium-oxygen cell. Discussion will touch upon how the electrochemistry can be related to electrode substrate and will be presented with in situ spectroscopic studies that identify intermediate and surface species during the oxygen reduction reaction. \\[4pt] [1] P.G. Bruce, S. Freunberger, L.J. Hardwick, J.-M. Tarascon, Nature Mater. (2012) 11 19\\[0pt] [2] Z. Peng, S.A. Freunberger, Y. Chen, P.G. Bruce, Science, (2012) 337 563 [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F25.00002: First Principles Investigation of Li/Fe-Oxide as a High Energy Material for Hybrid All-in-One Li-ion/Li-O$_{2}$ Batteries Alper Kinaci, Lynn Trahey, Michael M. Thackeray, Scott Kirklin, Christopher Wolverton, Maria K.Y. Chan We recently introduced a vision for high energy all-in-one electrode/electrocatalyst materials that can be used in hybrid Li-ion/Li-O$_{2}$ (Li-air) cells [1]. Recent experiments using Li$_{5}$FeO$_{4}$ demonstrated substantially smaller voltage polarizations and hence higher energy efficiency compared to standard Li-O$_{2}$ cells forming Li$_{2}$O$_{2}$ [2]. The mechanism by which the charge process activates the Li$_{5}$FeO$_{4}$, however, is not well understood. Here, we present first principles density functional theory (DFT) calculations to establish the thermodynamic conditions for the extraction of Li/Li$+$O from Li$_{5}$FeO$_{4}$. A step-by-step, history-dependent, removal process has been followed and the stability of the Li and Li$+$O deficient samples is investigated on the basis of the energies of the extraction reactions. Various stages of Li/Li$+$O removal are identified, and structural changes and electronic structure evolution, as well as computed XRD, XANES, and PDF characterizations are reported. \\[4pt] [1] M. M. Thackeray, M. K. Y. Chan, L. Trahey, S. Kirklin, and C. Wolverton, Journal of Physical Chemistry Letters, 4, 3607 (2013).\\[0pt] [2] L. Trahey, C. S. Johnson, J. T. Vaughey, S.-H. Kang, L. J. Hardwick, S. A. Freunberger, P. G. Bruce, M. M. Thackeray, Electrochemical and Solid-State Letters, 14, A64 (2011). [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F25.00003: First-principles study on structure stabilities of $\alpha$-S and Na-S battery systems Hiroyoshi Momida, Tamio Oguchi To understand microscopic mechanisms of charge and discharge reactions in Na-S batteries, there has been increasing needs to study fundamental atomic and electronic structures of elemental S as well as that of Na-S phases. The most stable form of S is known to be an orthorhombic $\alpha$-S crystal at ambient temperature and pressure, and $\alpha$-S consists of puckered S$_8$ rings which crystallize in space group $Fddd$. In this study, the crystal structure of $\alpha$-S is examined by using first-principles calculations with and without the van der Waals interaction corrections of Grimme's method, and results clearly show that the van der Waals interactions between the S$_8$ rings have crucial roles on cohesion of $\alpha$-S. We also study structure stabilities of Na$_2$S, NaS, NaS$_2$, and Na$_2$S$_5$ phases with reported crystal structures. Using calculated total energies of the crystal structure models, we estimate discharge voltages assuming discharge reactions from 2Na+$x$S$\rightarrow$Na$_2$S$_x$, and discharge reactions in Na/S battery systems are discussed by comparing with experimental results. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F25.00004: Indirect correlation between superionc sodiums in $\beta $-alumina -First principles molecular dynamic study- Kazuo Tsumuraya, Shoichi Yasuda The atoms are ionized in the ion conductors like the atoms in the ionic crystals, yet either cations or anions are mobile under the electric field unlike the ions in the ionic crystals. The elucidation of the conduction mechanism is essential for the development of the secondary batteries which operate at low temperature. Since the Na-Na correlation peak in superionic $\beta $-alumina has been located at a well separated from the peak position arising from the commensurate sites, the analyses of the origin of the correlation peak allows us to give the nature of the conduction mechanism in conductors. The first principles molecular dynamic study shows that split-interstitial sodiums on mid-oxygen produce the correlation. This is an indirect correlation between the superionic sodiums and is consistent with low Haven's ratios in the $\beta $-alumina. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F25.00005: Structure of Imidazolium-based Ionic Liquids Marie-Louise Saboungi, Miguel Gonz\'alez, Bachir Aoun, Oleg Borodin, Wesley Henderson, Shinji Kohara, Maiko Kofu, Osamu Yamamuro Room-temperature ionic liquids (RTILs) are receiving increased attention due to their unique properties, including nonvolatility and solvating capability, leading to a wide range of potential uses in catalysis, separation technology, photovoltaics, batteries and fuel cells. We have studied the structures of liquid and solid 1-ethyl, 1-butyl and 1-hexyl-3-methylimidazolium bromide with high-energy x-ray diffraction measurements and atomistic molecular dynamics numerical simulations. Excellent agreement between experiment and simulation is obtained, including the region of the low-Q peak that characterizes the nanoscale heterogeneity in these liquids. Significant changes in this heterogeneity are observed when water is added to the ionic liquid, depending on the length of the ethyl chain. With a longer (octyl) chain length, the degree of heterogeneity is enhanced, possibly reflecting the water nano-domains observed in simulations. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F25.00006: First principles simulations of structural phase transformations in the solid electrolyte LiBH$_4$ with chemical substitutions Noam Bernstein, Khang Hoang, Michelle Johannes The proposed hydrogen storage material LiBH$_4$ has been shown to have possible applications as a Li-ion battery solid electrolyte, due to its high Li-ion conductivity over 10$^{-3}$ S/cm$^{-1}$ [1], comparable to polymer gel electrolytes. The high conductivity is only observed above a phase transition temperature that is outside of the useful operating range, but doping the material with various substitutions for the Li or BH$_4$ units can bring the phase transition below room temperature. Both smaller and larger substituting species can stabilize the high T structure, indicating that it is not a simple volume effect. We show that variable-cell-shape molecular-dynamics simulations using density functional theory forces and stresses reproduce the structural phase transition. Using umbrella integration to compute the free energy differences between the two structures, we calculate the phase transition temperature and its dependence on substitutional I, Cl, and Na concentrations, and show that they are in very good agreement with experiment. We calculate the effect of K substitution, and predict that it will be even more effective at stabilizing the high T structure. Decomposing the free energy difference changes into enthalpy and entropy contributions shows that the mechanis [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F25.00007: Determination of Raman Spectrum of Li$_{28}$La$_{12}$Zr$_{8}$O$_{48}$ as a Function of Dopant Saikat Mukhopadhyay, Travis Thompson, Jeff Sakamoto, Michelle Johannes, Derek Stewart Li$_{28}$La$_{12}$Zr$_{8}$O$_{48}$ is a supervalent conductor with a low conductivity tetragonal phase and a high conductivity cubic phase, making it a strong candidate as a practical Li ion rechargeable battery solid electrolyte. The high conductivity phase can be stabilized via supervalent doping that drives Li$^{+}$ ions out of the lattice, creating vacancies that both relieve the necessity for Li sublattice ordering and provide easier pathways for ionic conduction. The conductivity strongly depends on both doping concentration and site preference. Ta$^{5+}$ has been suggested as an optimal dopant as it likely substitutes for Zr$^{4+}$, thereby leaving the Li sublattice undisturbed. However, it is difficult to accurately establish the actual, as compared to nominal, amount of Ta doped into the lattice which, in turn, determines the vacancy concentration and conductivity. In this talk, we will present the variation of Raman intensities of LLZO as a function of Ta concentration to determine the role of dopant and vacancies in deciding measured Raman intensities via first principles calculations based on Density Functional Theory. A direct comparison of calculated and measured Raman spectrum may provide a definitive measure of vacancy concentration. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F25.00008: First principles modeling of interfaces of lithium (thio) phosphate solid electrolytes and lithium metal anodes N.A.W. Holzwarth, N.D. Lepley, A.N.M. Al-Qawasmeh, C.M. Kates Computer modeling studies show that while lithium phosphate electrolytes form stable interfaces with lithium metal anodes, lithium thiophosphate electrolytes are typically structurally and chemically altered by the presence of lithium metal. On the other hand, experiments have shown\footnote{Z. Liu, W. Fu, {\em{et. al.}}, {\bf{\em{J. Am. Chem. Soc.}}} {\bf{135}} 975-978, (2013).} that an electrochemical cell of Li/Li$_3$PS$_4$/Li can be cycled many times. One possible explanation of the apparent experimental stability of the Li/Li$_3$PS$_4$/Li system is that a stabilizing buffer layer is formed at the interface during the first few electrochemical cycles. In order to computationally explore this possibility, we examined the influence of ``thin film'' buffer layers of Li$_2$S on the surface of the electrolyte. Using first principles techniques,\footnote{N. D. Lepley, N. A. W. Holzwarth, Y. A. Du, {\bf{\em{Phys. Rev. B}}} {\bf 88}, 104103 (2013).} stable electrolyte-buffer layer configurations were constructed and the resulting Li$_3$PS$_4$/Li$_2$S and Li$_2$S/Li interfaces were found to be structurally and chemically stable. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F25.00009: Mechanisms of dendrite formation in Lithium Ion Batteries Ning Sun, Dilip Gersappe The formation of dendrite on the anode of Lithium-Ion Batteries during charging process can compromise the safety of the battery. By using a Lattice Boltzmann Method we simulated the mechanisms of dendrite formation. We postulated a way to monitor the growth of dendrites by recording the rate of change of the surface area of anode. Our results showed that the onset of dendrite will happen after the Sand's time when the current density is larger than a critical value. We also show that the Sands time is affected by the local curvature, particularly at low current densities. We also find that the roughness of the anode influences dendrite formation only when current density is not very high (around the critical current density). Our results show that it is possible to~suppress the growth of dendrites by~applying pulses during the charging process if the frequency of the pulse is chosen properly. Our model is able to study the discharge process as well, and we find that during the cycling process, high aspect ratio regions formed during charging, might break off from anode during discharging, and the anode surface will get rougher and rougher during the cyclic process, thus possibly increasing the propensity to form dendrites. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F25.00010: Lithium Ion Solvation and Diffusion in Bulk Organic Battery Electrolytes from First Principles Molecular Dynamics Mitchell Ong, Vincenzo Lordi, Erik Draeger, John Pask Lithium-ion batteries are commonly used to power many consumer devices. One of the key properties that influence the performance of lithium-ion batteries is the ionic conductivity of the electrolyte. This is dependent on the speed at which Li ions diffuse across the cell and related to the solvation structure of the Li ions. The choice of the electrolyte can greatly impact both solvation and diffusivity of Li ions. In this work, we use first principles molecular dynamics to examine the solvation and diffusion of Li ions in several bulk organic electrolytes. We find that differences in the local environment throughout the liquid can lead to solvation of Li ions by either carbonyl or ether oxygen atoms. In addition, we examine the differences in solvation of associated and dissociated Li(PF$_{6})$ salts, showing that the bulky PF$_{6}$ group blocks complete solvation of Li$^{+}$ by solvent oxygen atoms. Finally, we calculate Li diffusion coefficients in each electrolyte, finding slightly larger diffusivities in a linear carbonate such as ethyl methyl carbonate (EMC) compared to a cyclic carbonate like ethylene carbonate (EC). Results from this work can be used to design new bulk electrolytes that will improve the performance of current Li-ion batteries. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F25.00011: First-principles estimates of free energy barriers for Mg desolvation and intercalation at electrolyte/electrode interfaces Liwen Wan, David Prendergast There is a growing interest in developing multivalent ion batteries that could, in principle, double or triple the energy density compared to the monovalent Li-ion batteries. However, the strong electrostatic interaction caused by the extra charge also makes it very challenging to find appropriate intercalation compounds that allow for relatively fast and reversible ion transport. An established working multivalent battery is comprised of Mg(AlCl2BuEt)2 salts in THF solution as the electrolyte, and Mg metal and Mo6S8 Chevrel phase as the anode and cathode, respectively. Currently, we lack a clear understanding of the mechanism for Mg desolvation and intercalation at the interface between the electrolyte and Chevrel phase surfaces, which is critical in designing new advanced battery systems with improved ion diffusion rate. Here, we present a theoretical investigation of the dynamics and kinetics of the Mg desolvation/intercalation process. The surface properties of Mo6S8 are studied for the first time using density functional theory (DFT) and its interaction with the electrolyte is simulated via an ab initio molecular dynamics (AIMD) approach. The free energy barrier for Mg diffusing through the interface is then calculated by performing a set of biased AIMD simulations. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F25.00012: Investigation of the Silicon Solid Electrolyte Interface in Lithium Ion Batteries using the Technique of Hard X-Ray Photoelectron Spectroscopy Benjamin Young, David Heskett, Mengyun Nie, Brett Lucht, Joseph Woicik Formation of a stable Solid Electrolyte Interface (SEI) between the anode and electrolyte material of a lithium ion battery (LIB) is essential to battery performance. Silicon anodes represent a theoretical tenfold increase in energy density over more thoroughly investigated carbonaceous anodes, but experience large volume changes during normal cycling which represents a challenge to stable SEI formation. Overcoming this challenge demands more thorough understanding of SEI formation which can be achieved through the technique of Hard X-ray Photoemission Spectroscopy (HAXPES). This work is focused on addition of ethylene carbonate (EC) and fluoroethylene carbonate (FEC) solvents to the base electrolyte LiPF$_{\mathrm{6}}$ material in coin cell LIBs using binder-free silicon nanoparticle anodes. The results of HAXPES experiments carried out at beamline X24-A of the National Synchrotron Light Source at Brookhaven National Laboratory are presented, revealing depth dependent composition information at various points of SEI development. [Preview Abstract] |
Session F26: Focus Session: Materials in Extremes: High Pressures and Temperatures
Sponsoring Units: GSCCM DCOMP DMPChair: Anatoly Belonoshko, KTH Royal Institute of Technology
Room: 502
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F26.00001: A multiphase equation of state for carbon addressing high pressures and temperatures Lorin Benedict, Kevin P. Driver, Sebastien Hamel, Burkhard Militzer, Tingting Qi, Alfredo A. Correa, Eric Schwegler We present a 5-phase equation of state (EOS) for elemental carbon. The phases considered are: diamond, BC8, simple-cubic, simple-hexagonal, and the liquid/plasma state. Free energy models for the various phases are constrained by Density Functional theory (DFT) and path integral quantum Monte Carlo calculations. The precise manner in which the ideal gas limit is reached is greatly constrained by both the highest temperature DFT data and the path integral data, forcing us to discard an ion-thermal model we had used previously in favor of a new one. Predictions are made for the principal Hugoniot and the room-temperature isotherm, and comparisons are made to recent experimental results. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F26.00002: Lattice dynamics and thermal equation of state of cubic CaSiO$_{3}$ perovskite Tao Sun, Renata Wentzcovitch CaSiO$_{3}$ perovskite (CaPv) is believed to be the third most abundant mineral in the Earth's lower mantle and is a major component of mid-ocean ridge basalt (MORB). A well constrained thermal equation of state for CaPv is key to several geophysical problems, e.g., lower mantle composition, density contrast between mantle and plates, nature of D'' region, etc. Its experimental and theoretical determination have been very challenging because the cubic structure that CaPv adopts at lower mantle conditions is unstable at low temperatures and some of its harmonic phonons have imaginary frequencies. We have used a recently developed hybrid method combining ab initio molecular dynamics with vibrational normal mode analysis to compute its free energy and thermal equation of state at lower mantle conditions. These results are essential to understand the fate of subducted MORB in the mantle. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F26.00003: QMC Benchmarks of Density Functionals for High-Pressure Hydrogen Applications Raymond Clay, Jeremy McMinis, Jeffrey McMahon, Carlo Pierleoni, David Ceperley, Miguel Morales It has recently been shown in high-pressure hydrogen that the predicted locations of the liquid-liquid phase transition and the solid insulator-to-metal transition are very sensitive to the choice of density functional employed. We use Quantum Monte Carlo to benchmark some of the most commonly used DFT functionals for dense hydrogen in these two regions of the phase diagram. We find that in both of these phases, van der Waals and hybrid functionals noticeably outperform LDA and PBE functionals, and recommend the use of the vdW-DF [M. Dion \textit{et al.}, Phys. Rev. Lett. \textbf{92}, 246401 (2004)] functional for structural relaxation and molecular dynamics. We look at the impact of the functional on enthalpies, bond lengths, and the location of the liquid-liquid phase transition. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F26.00004: Computation of the Principal Deuterium Hugoniot with quantum Monte Carlo Norm Tubman, David Ceperley We have performed extensive calculations of the principal deuterium Hugoniot using the Coupled Electron Ion Monte Carlo method (CEIMC). In this method we use Monte Carlo to simulate both the electronic and ionic degrees of freedom. We will discuss finite size effects, effects of zero point motion of the ions, as well as convergence issues with the simulation. We compare the predicted Hugoniot with previous simulations and experimental results. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F26.00005: Intermediate-spin ferrous iron in the Earth's lower mantle? Han Hsu, Renata Wentzcovitch Using density functional theory $+$ self-consistent Hubbard U (DFT$+$Usc) calculations, we investigate intermediate-spin (IS) ferrous iron (Fe$^{\mathrm{2+}})$ in major lower-mantle minerals, ferropericlase (Fp) and magnesium silicate (MgSiO$_{\mathrm{3}})$ perovskite (Pv). In both minerals, two distinct types of IS Fe$^{\mathrm{2+}}$ are found. In Fp, while both types of IS Fe$^{\mathrm{2+}}$ are configured t$_{\mathrm{2g}}^{\mathrm{5}}$ e$_{\mathrm{g}}^{\mathrm{1}}$, one has a d$_{\mathrm{z2}}$ electron$_{\mathrm{\thinspace }}$and the other has a d$_{\mathrm{x2-y2}}$ electron, referred to as the IS(z$^{\mathrm{2}})$ and IS(x$^{\mathrm{2}}-$y$^{\mathrm{2}})$ state, respectively.$_{\mathrm{\thinspace }}$The IS(z$^{\mathrm{2}})$ state has an exceptionally high QS ($\ge $ 5.5 mm/s); the IS(x$^{\mathrm{2}}-$y$^{\mathrm{2}})$ state has a quite low QS (\textless 0.5 mm/s). Also, the IS(z$^{\mathrm{2}})$ state has a stronger on-site Coulomb interaction and much higher energy. In Pv, while Fe$^{\mathrm{2+}}$ substitutes Mg in the dodecahedral site, it is effectively under a distorted octahedral crystal field, and the two IS states can be characterized by their filled e$_{\mathrm{g}}$-like orbitals as well. These two IS Fe$^{\mathrm{2+}}$, in contrast to those in Fp, are energetically competitive, and they both have a small QS (\textless 1.6 mm/s). Our calculations show that all IS Fe$^{\mathrm{2+}}$ in lower-mantle minerals are unfavorable, and their QSs are all inconsistent with experiments. Therefore, IS Fe$^{\mathrm{2+}}$ is highly unlikely in the Earth's lower mantle. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F26.00006: Predicted novel hydrogen hydrate structures under pressure from first principles Guangrui Qian, Andriy Lyakhov, Qiang Zhu, Artem Oganov, Xiao Dong Gas hydrates are systems of prime importance. In particular, hydrogen hydrates are potential materials of icy satellites and comets, and may be used for hydrogen storage. We explore the H2O-H2 system at pressures in the range 0 $\sim$ 100 GPa with ab initio variable-composition evolutionary simulations. According to our calculation and previous experiments, the H2O-H2 system undergoes a series of transformations with pressure, and adopts the known open-network clathrate structures (sII, C0), dense ``filled ice'' structures (C1, C2) and two novel hydrogen hydrate phases. One of these structures is based on the hexagonal ice framework and has the same H2O:H2 ratio (2:1) as the C0 phase at low pressures and similar enthalpy (we name this phase Ih-C0). The other newly predicted hydrate phase has a 1:2 H2O:H2 ratio and structure based on cubic ice. This phase (which we name C3) is predicted to be thermodynamically stable above 38 GPa when including van der Waals interactions and zero-point vibrational energy. This is the hydrogen-richest hydrate and this phase has the highest gravimetric densities (18 wt.{\%}) of extractable hydrogen among all known materials. We thank the DARPA (Grants No. W31P4Q1310005 and No. W31P4Q1210008), National Science Founda- tion (EAR-1114313, DMR-1231586), AFOSR (FA9550- 13-C-0037), DOE (DE-AC02-98CH10886), CRDF Global (UKE2-7034-KV-11) for financial support. We thank Purdue University Teragrid for providing computational resources and technical support for this work (Charge No.: TG-DMR110058). [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F26.00007: High Pressure Study of Electrical Resistivity of CeB6 to 136 GPa Neda Forouzani, Jinhyuk Lim, James Schilling, Gilberto Fabbris, Zachary Fisk Since the 1960's the dense Kondo compound cerium hexaboride (CeB6) has attracted a great deal of interest. To investigate whether this material might evolve into a topological insulator under sufficient pressure, we have carried out four-point electrical resistivity measurements on CeB6 over the temperature range 1.3 K to 295 K in a diamond anvil cell to 136 GPa. Although a transition into an insulating phase is not observed, the evolution of the initial dense Kondo state under such extreme pressures is of considerable interest. As reported in earlier studies to 13 GPa [1], the temperature of the resistivity maximum near 3 K initially increases with pressure. We observe that between 33 and 53 GPa the resistivity maximum disappears and by 83 GPa CeB6 appears to have settled into a Fermi liquid state. The marked changes observed under pressure suggest that a change in valence and/or a structural transition may have occurred. Synchrotron x-ray diffraction measurements are being carried out to investigate possible changes in crystal structure under extreme pressures.\\[4pt] [1] Kobayashi et al., Physica B 281\&282, 553 (2000) [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F26.00008: Compression effects on electrons for chemical bonding Anguang Hu, Fan Zhang How electrons move under compression as chemical bonds between atoms are broken and formed is central to a number of challenges on the performance of materials in extreme conditions. This is not only associated with the fundamental knowledge of material response to compressive loading but also would advance many aspects of material science towards future energy technologies. First-principles simulations of enthalpy minimization, in various target pressures on chemical transformation bonding pathways, reveal that high pressure can push electrons away from their denser regimes where the kinetic energy rises steeply on compression, causing a destabilization of intramolecular bonds. The high-pressure pushing of electrons from one regime to another thus leads to chemical bond destruction and formation with a cell volume collapse accompanied by a drop in stress components. Determination of such electron pathways following bonding conformations of molecular precursors would then result in a number of chemical transformations for novel materials, including high energy density materials. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F26.00009: Elasticity of Fe- and Al-bearing and --free MgSiO$_{3}$-perovskite Gaurav Shukla, Zhongqing Wu, Renata Wentzcovitch We present a thorough analysis of the elastic properties of iron- and aluminum-bearing and --free MgSiO3-perovskite. Results from different first principles methods are compared to experimental data available and results for aggregate elastic moduli and velocities are analyzed at lower mantle conditions. Velocity heterogeneities produced by temperature variations and variation of aluminum and iron content are carefully examined and contrasted. This analysis is essential for improving understanding of the constitution of Earth's lower mantle. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F26.00010: Evolutionary Structure Prediction of Stoichiometric Compounds Qiang Zhu, Artem Oganov In general, for a given ionic compound A$_{m}$B$_{n\thinspace }$at ambient pressure condition, its stoichiometry reflects the valence state ratio between per chemical specie (i.e., the charges for each anion and cation). However, compounds under high pressure exhibit significantly behavior, compared to those analogs at ambient condition. Here we developed a method to solve the crystal structure prediction problem based on the evolutionary algorithms, which can predict both the stable compounds and their crystal structures at arbitrary P,T-conditions, given just the set of chemical elements. By applying this method to a wide range of binary ionic systems (Na-Cl, Mg-O, Xe-O, Cs-F, etc), we discovered a lot of compounds with brand new stoichimetries which can become thermodynamically stable. Further electronic structure analysis on these novel compounds indicates that several factors can contribute to this extraordinary phenomenon: (1) polyatomic anions; (2) free electron localization; (3) emergence of new valence states; (4) metallization. In particular, part of the results have been confirmed by experiment, which warrants that this approach can play a crucial role in new materials design under extreme pressure conditions. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F26.00011: Investigation of structural and magnetic properties of LaCo5 under pressure Markus Daene, Jason R. Jeffreys, Jon R.I. Lee, Daniel Aberg, Patrick Huang, Nick P. Butch, Scott K. McCall, Lorin X. Benedict, Babak Sadigh We report a joint experimental and theoretical investigation of the crystal structure parameters and magnetic moments of LaCo5 under hydrostatic compression. Theoretical predictions were made using density-functional-based electronic structure methods; special attention was paid to the dependence of the results on the choice of exchange-correlation functional. We comment on the degree to which our predictions match those of our measurements and relate both to earlier studies of Koudela et al. [1]. [1] D. Koudela et al., Phys. Rev. B vol.77, 024411 (2008). [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F26.00012: Hydrostatic High-Pressure Studies to 25 GPA on the Model Superconducting Pnictide LaRu2P2 Jinhyuk Lim, Neda Forouzani, James Schilling, Roxanna Fotovat, Chong Zheng, Roald Hoffmann Prior to the discovery of the Fe-pnictides in 2008, the ruthenium phosphide LaRu2P2 possessed the highest value of the su- perconducting transition temperature, Tc$\approx$ 4 K, in the entire pnictide family. Recently, there has been renewed interest in this compound in an effort to better understand why the Fe-pnictides have much higher values of Tc [1]. In related phosphides superconductivity appears to only be present if the separation be- tween the phosphor ions dp-p in neigh- boring Ru2P2 planes is greater than the critical value 2.8 {\AA}, too great for a P-P covalent bond to be formed. For example, in superconducting LaRu2P2, the value of dp-p is 3.0 {\AA}. To test these ideas directly, we have carried out hydro- static high-pressure studies on single-crystalline LaRu2P2 in a diamond-anvil cell using He pressure medium to pres- sures as high as 25 GPa and temperatures as low as 1.5 K. We find that Tc initially increases under pressure, but suddenly disappears above 2.1 GPa. Since dp-p decreases under pressure, the sudden disappearance of superconductivity is likely due to the formation of a covalent P-P bond between adjacent Ru2P2 planes and a possible structural phase transition.\\[4pt] [1] Razzoli et al., Phys. Rev. Lett. 108, 257005 (2012) [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F26.00013: Hyperstoichiometric Oxygen in Fluorite-type U$_{3}$O$_{8}$ Formed at Extreme Conditions Fuxiang Zhang, Maik Lang, Rod Ewing U$_{3}$O$_{8}$ was obtained by annealing UO$_{3}$ in a reducing atmosphere at 200 $^{\circ}$C. Powder sample of $\beta $-U$_{3}$O$_{8}$ was pressurized at room temperature up to 37.5 GPa and XRD patterns clearly indicated that a phase transition occurred between 3-11 GPa. The high-pressure phase is a fluorite-like structure. The high-pressure phase was then laser heated to over 1700 K in the diamond anvil cell at high pressure conditions. No phase transition was found at high pressure/ temperature conditions, and the fluorite-like structure of U$_{3}$O$_{8}$ is even fully quenchable. The lattice parameter of the fluorite-like high-pressure phase is 5.425 {\AA} at ambient conditions, which is smaller than that of the stoichiometric UO$_{2}$. Previous experiments have shown that the stoichiometric uranium dioxide (UO$_{2})$ is not stable at high pressure conditions and starts to transform to a cotunnite structure at $\sim$ 30 GPa. When heating the sample at high pressure, the critical transition pressure is greatly reduced. However, the fluorite-like high-pressure phase of U$_{3}$O$_{8}$ is very stable at high pressure/high temperature conditions. The enhanced phase stability is believed to be related to the presence of extra oxygen (or U vacancies) in the structure. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F26.00014: Strong Pressure Dependence of Electrical Transport in V$_{2}$O$_{3}$ Thin Films Ilya Valmianski, Gabriel Ramirez, Siming Wang, Xavier Batlle, Ivan K. Schuller We present results of electrical transport measurements in V$_{2}$O$_{3}$ thin films under hydrostatic pressure from 100 KPa to 1.6 GPa. Uniaxial pressure and strain dependences of the metal-insulator transition temperature in V$_{2}$O$_{3}$ were extracted using a method previously established for high Tc superconductors [1]. Strain in the $z$ direction was calculated using V$_{2}$O$_{3}$ stiffness along the growth direction, while lateral strain was determined by the substrate properties. V$_{2}$O$_{3}$ thin films (100 nm) were grown epitaxially on three differently oriented single crystal Al$_{2}$O$_{3}$ substrates (a-plane, m-plane, and r-plane). Crystal phase purity and film quality were confirmed using high angle X-ray diffraction and X-ray reflectometry. All of the films showed a more than a four order of magnitude resistance change between the metallic and insulating states. The obtained pressure and strain dependences of the transition temperature may lead to novel device applications. \\[4pt] [1] S Bud'ko, J. Guimpel, O. Nakamura, M. Maple and I. K. Schuller, Phys. Rev. B, 1992, 46 1257 [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F26.00015: Comparative study of helimagnets MnSi and Cu$_2$OSeO$_3$ at high pressures Sergei Stishov, Vladimir Sidorov, Alla Petrova, Peter Berdonosov, Valery Dolgikh The heat capacity of helical magnets Cu$_2$OSeO$_3$ and MnSi has been investigated at high pressures by the ac-calorimetric technique. Despite the differing nature of their magnetic moments, Cu$_2$OSeO$_3$ and MnSi demonstrate a surprising similarity in behavior of their magnetic and thermodynamic properties at the phase transition. Two characteristic features of the heat capacity at the phase transitions of both substances (peak and shoulder) behave also in a similar way at high pressures if analyzed as a function of temperature. This probably implies that the longitudinal spin fluctuations typical of weak itinerant magnets like MnSi contribute little to the phase transition. The shoulders of the heat capacity curves shrink with decreasing temperature suggesting that they arise from classical fluctuations. In case of MnSi the sharp peak and shoulder at the heat capacity disappear simultaneously probably signifying the existence of a tricritical point and confirming the fluctuation nature of the first order phase transition in MnSi as well as in Cu$_2$OSeO$_3$. [Preview Abstract] |
Session F27: Electronic Structure Methods II
Sponsoring Units: DCOMPChair: Jia-An Yan, Towson University
Room: 501
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F27.00001: Determination of the one-particle Green's function: freedom and constraints Pina Romaniello, Giovanna Lani, Lucia Reining In this work we explore an approach for the calculation of the one-particle Green's function that is an alternative to standard methods based on approximations to the self-energy, namely, the solution of Schwinger's functional integro-differential equations [1]. These equations relate the one-particle Green's function to its functional derivative with respect to an external source. Here we start from an approximate version of these equations, where the Hartree potential is linearized with respect to the source [2]. We show that this set of equations has, in principle, multiple solutions. However, only one can be identified as the physical solution. We provide an expression for the formally exact family of solutions with the help of an auxiliary quantity q. The latter is defined by a number of exact constraints. Our findings suggest that once q is known, the physical solution is uniquely fixed by the limit of vanishing Coulomb interaction. [1] L. P. Kadanoff and G. Baym, Quantum Statistical Mechanics (W.A. Benjamin Inc., New York, 1964) [2] G. Lani, P. Romaniello and L. Reining, New Journal of Physics 14, 013056 (2012); in preparation [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F27.00002: The exact solution of the many-body problem in one-point: insights in approximate Green's function approaches Arjan Berger, Pina Romaniello, Lucia Reining In this work we obtain the exact one-body Green's function in one point by solving the Kadanoff-Baym equation. The result is a family of solutions. We show that only one of these solutions is a physical solution. We compare the exact physical solution to the exact solution of an approximate Kadanoff-Baym equation that was obtained recently [1] as well as to standard approximations such as GW. We show that the iterative solution of the GW equations is not always equal to the exact GW result. \\[4pt] [1] G. Lani, P. Romaniello and L. Reining, New J. Phys. 14, 013056 (2012) [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F27.00003: Acceleration of screened-exchange density-functional calculations with approximate differential overlap Jonathan Moussa, Peter Schultz We implement the Heyd-Scuseria-Ernzerhof (HSE) screened-exchange density functional in the \textsc{SeqQuest} electronic structure code. HSE calculations are accelerated by approximating differential overlap in the Fock exchange based on an atomic-orbital partitioning scheme. All one-center and two-center exchange integrals are calculated. A subset of three-center exchange integrals are calculated for one-center Fock exchange matrix elements and for exchange mediated by one-center density matrix elements. Four-center exchange integrals are not calculated. We test the validity of this approximation by examining the number and magnitude of these different classes of exchange integrals. Basis set and pseudopotential errors in HSE calculations are benchmarked on atoms. Differential overlap approximation errors are benchmarked on small molecules. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F27.00004: Implementation of DFT$+$DMFT in local-orbital pseudopotential code Hyungju Oh, Choong-ki Lee, Hyoung Joon Choi Density functional theory (DFT) has been remarkably successful at describing ground-state properties of many solids from first principles. This is also the state-of-the-art method for band structure calculations, with the additional assumption that Kohn-Sham eigenvalues can be interpreted as single-particle excitations. However, DFT has limitations for strongly correlated materials. Dynamical mean-field theory (DMFT) is one of various approaches that have been developed for overcoming the shortcomings of DFT. DMFT goes beyond DFT by allowing the interaction potential of the correlated orbitals to be energy (frequency) dependent. This frequency dependent potential, or self-energy, is computed for the correlated orbitals using many-body techniques within an accurate impurity solver. We have implemented DMFT to the SIESTA code based on pseudo-atomic orbital basis set. For an impurity solver, we use exact diagonalization. We calculate electronic states of LaFeAsO using our DFT$+$DMFT code and confirm the band-narrowing, corresponding to an enhancement of the effective masses of quasiparticles. This work was supported by the NRF of Korea (Grant No. 2011-0018306). Computational resources have been provided by KISTI Supercomputing Center (Project No. KSC-2013-C3-008). [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F27.00005: {\it Ab initio} Sternheimer-GW method for quasiparticle calculations Henry Lambert, Feliciano Giustino The GW method has emerged as the standard computational tool for investigating electronic excitations in bulk and nanoscale systems. Recently significant efforts have been devoted to extending the range of applicability of the GW method. With this aim, Ref.~[1] introduced the Sternheimer-GW method, reformulating the standard GW approach so that no unoccupied electronic states are required in the calculations. Here we present the implementation of the Sternheimer-GW method using planewaves and norm-conserving pseudopotentials [2]. In our method we calculate the complete position- and energy-dependent GW self-energy operator, and as a by-product we obtain the entire $G_{0}W_{0}$ quasiparticle spectral function. We have validated our method by calculating the quasiparticle band structures of standard semiconductors and insulators (Si, SiC, diamond, LiCl) and by comparing the results with previous GW calculations. This method is currently being used for investigating the electronic structure of novel materials of reduced dimensionality. \\[4pt] [1] F.\ Giustino, M.\ L.\ Cohen, and S.\ G.\ Louie, Phys.\ Rev.\ B {\bf 81}, 115105 (2010).\\[0pt] [2] H.\ Lambert and F.\ Giustino, Phys.\ Rev.\ B.\ {\bf 88}, 075117 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F27.00006: Efficient calculation of random-phase approximation correlation energies using Lanczos chains and an optimal basis set Dario Rocca A new \emph{ab initio} approach is introduced to compute the correlation energy within the adiabatic connection fluctuation dissipation theorem in the random phase approximation. First, an optimally small basis set to represent the response functions is obtained by diagonalizing an approximate dielectric matrix containing the kinetic energy contribution only [1]. Then, the Lanczos algorithm is used to compute the full dynamical dielectric matrix and the correlation energy [1,2]. The convergence issues with respect to the number of empty states or the dimension of the basis set are avoided and the dynamical effects are easily kept into account. To demonstrate the accuracy and efficiency of this approach the binding curves for three different configurations of the benzene dimer are computed: T-shaped, sandwich, and slipped parallel.\\[4pt] [1] D. Rocca, J. Chem. Phys. (2014), to appear in the special issue Advances in DFT Methodology. \\[0pt] [2] T.A. Pham, H.-V. Nguyen, D. Rocca, G. Galli, Phys. Rev. B 87, 155148 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F27.00007: Double-counting corrections to the LDA+DMFT method in the exact density limit Andrei Valentin Plamada, Peter Staar, Anton Kozhevnikov, Bart Ydens, Thomas C. Schulthess The LDA+U method is commonly used for ab-initio studies of strongly correlated electron materials, and it has been successful in predicting spectral properties of prototypical systems such as NiO when used in conjunction with Dynamical Mean Field Theory (DMFT). Presently the method still includes an empirical term to correct doubly counted correlations. Assuming the double-counting correction is a constant $\mu_{DC}$ multiplied by the identity operator in the correlated subspace and that the electron density is well approximated with the Local Density Approximation (LDA) to Density Functional Theory, we devise a method to determine $\mu_{DC}$ directly from LDA and DMFT calculations. The method has been validated for prototypical transition metal oxides and shows promising results that agree with commonly used values for the double counting correction in the respective systems. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F27.00008: An experimentally realized 1-D correlated system for which DFT, DFT$+U$, and DFT$+$DMFT fall short Nader Zaki, Hyowon Park, Richard Osgood, Andrew Millis, Chris Marianetti Density functional theory (DFT) has been immensely successful in its ability to predict physical properties of condensed matter systems, and it is generally qualitatively correct when predicting structural properties. Here, however, we show that DFT qualitatively fails to predict the dimerized structural phase for a monatomic Co wire system that is self-assembled on a vicinal, i.e. stepped, Cu(111) substrate [1]. To elucidate the nature of this failure, we compute the energetics of a Co chain on a Cu surface, step, notch, and embedded in bulk, which demonstrates that increasing coordination and hybridization extinguishes the dimerization. We attribute the failure of DFT for Co on the Cu step to excessive hybridization, which both weakens the ferromagnetic correlations that drive the dimerization and increases the bonding that opposes dimerization. Additionally, we show that accounting for local interactions via DFT$+U$ or DFT$+$DMFT also fails at predicting the correct structural phase for the step-substrate supported wire, though the Co wire does dimerize in DFT$+$DMFT for the isolated vacuum case. [1] N. Zaki, et al, Phys. Rev. B \textbf{87}, 161406 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F27.00009: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F27.00010: Development of noncollinear-spin DFT$+$U method with spin-orbit interaction Eunjung Ko, Hyungjun Lee, Hyungju Oh, Se Young Park, Hyoung Joon Choi We developed a DFT$+$U$+$SOI method by incorporating spin-orbit interaction (SOI) into a noncollinear-spin generalization of the density functional theory (DFT) plus Coulomb interaction among $d$ electrons, parameterized by U and J. The Coulomb interaction, which is based on the rotationally invariant form, is generalized for noncollinear-spin configuration, and the fully localized limit is adopted for the double-counting term. The spin-orbit interaction is treated in the $l$-dependent fully separable nonlocal form using additional Kleinman-Bylander projectors generated by relativistic calculations of atoms. We implemented our DFT$+$U$+$SOI method into the SIESTA code and performed test calculations for the 4$d$ or 5$d$ transition metal oxides, the all-in-all-out noncollinear magnetic insulator Cd$_{\mathrm{2}}$Os$_{\mathrm{2}}$O$_{\mathrm{7}}$, the canted antiferromagnetic order insulator Sr$_{\mathrm{2}}$IrO$_{\mathrm{4}}$, and the paramagnetic insulator Ca$_{\mathrm{2}}$RuO$_{\mathrm{4}}$. This work was supported by NRF of Korea (Grant No. 2011-0018306) and KISTI supercomputing center (Project No. KSC-2012-C3-046). [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F27.00011: Multi-orbital time-dependent spin-density functional theory for strongly correlation systems: Application to Ce and YTiO$_{3}$ Volodymyr Turkowski, Syed Islamuddin Shah, Talat S. Rahman We present a methodology for examining the spectral properties and nonequilibrium response of strongly-correlated electron systems within multi-orbital time-dependent spin-density functional theory. The key element of the theory -- exchange-correlation (XC) kernel - is derived from dynamical mean-field theory (DMFT) expressions for two-particle susceptibilities and the electron self-energy for the effective Hubbard model. We demonstrate that the appropriate description of strongly-correlated materials requires a non-adiabatic (time non-local) XC kernel, though the spatial locality in general is not necessary. We apply the formalism to study the spectral properties of cerium and YTiO$_{3}$, and establish that the method is capable of describing both metallic and insulating systems. In addition, we present results of the nonequilibrium response of YTiO$_{3}$ under an applied short laser pulse. In particular, we analyze the role of inter-orbital interactions in the relaxation dynamics of the system. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F27.00012: Developments in Coupled Cluster Theory for the Homogenous Electron Gas James J. Shepherd, Tom M. Henderson, Andreas Gr\"uneis, Gustavo E. Scuseria In a series of recent communications the correlation energy for the ground-state homogeneous electron gas has been precisely determined by full configuration interaction quantum Monte Carlo. The power of this new approach is that energies going beyond fixed-node approxmation and at finite basis set sizes are now available. This has opened up the possibility of benchmarking and further developing quantum chemical methods which involve finite basis sets for periodic systems, in particular coupled cluster theory. We will discuss: A) extensivity and divergences in approximate correlation energies, B) diagrammatic channels in the gas, and C) screening and range-separation in modern coupled cluster theory. This talk will draw on material from: 1) Phys. Rev. B 85, 081103 (2012); 2) Phys. Rev. B 86, 035111 (2012); 3) Phys. Rev. Lett., 110, 226401 (2013); 4) arXiv: 1310.6425; 5) arXiv:1310.6806. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F27.00013: Spin-Orbit Effects in the Quasiparticle Bandstructure of Noble Metals Jamal Mustafa, Steven Louie Applications of the $GW$ approximation to the electron self-energy have proven quite successful for calculating the quasiparticle properties of materials. We find that for the noble metals, in line with previous work in such calculations, the semicore states need to be taken into account. We show that, with these semicore states, a large cutoff must be used to describe the screening and, in turn, a large number of empty states must be included. Taking all of this into account, and carefully checking convergence, shows $G_{0}W_{0}$ can describe experimental results from angle-resolved photoemission spectroscopy quite well when the effects of spin-orbit coupling is also included. We compare our results to recent self-consistent $GW$ calculations on gold. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F27.00014: Effect of spin fluctuations on quasiparticles in simple metals Johannes Lischner, Timur Bazhirov, Allan MacDonald, Marvin Cohen, Steven Louie We present a first-principles theory for quasiparticle excitations in condensed matter systems that includes their interaction with spin fluctuations. We apply this theory to sodium and lithium. Despite several previous studies, the importance of spin fluctuations in these materials and, in particular, their effect on the occupied band width remains unclear. We show that the coupling to spin fluctuations does not significantly change the occupied band width, but gives an important contribution to the quasiparticle lifetime. To obtain quantitative agreement with experiment for the occupied band width, we find that it is necessary to include vertex corrections beyond the random-phase approximation in the screening by charge fluctuations. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F27.00015: The computational foundations of time dependent density functional theory James Whitfield The mathematical foundations of TDDFT are established through the formal existence of a fictitious non-interacting system (known as the Kohn-Sham system), which can reproduce the one-electron reduced probability density of the actual system. We build upon these works and show that on the interior of the domain of existence, the Kohn-Sham system can be efficiently obtained given the time-dependent density. Since a quantum computer can efficiently produce such time-dependent densities, we present a polynomial time quantum algorithm to generate the time-dependent Kohn-Sham potential with controllable error bounds. Further, we find that systems do not immediately become non-representable but rather become ill-representable as one approaches this boundary. A representability parameter is defined in our work which quantifies the distance to the boundary of representability and the computational difficulty of finding the Kohn-Sham system. [Preview Abstract] |
Session F28: Focus Session: Superconducting Qubits: Coherence & Noise
Sponsoring Units: GQIChair: David Schuster, University of Chicago
Room: 601
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F28.00001: Towards a Spin-Ensemble Quantum Memory for Superconducting Qubits Invited Speaker: P. Bertet A multi-mode quantum memory able to store coherently large numbers of qubit states is a desirable resource for quantum information. We report progress towards this direction, using an ensemble of electronic spins (NV centers in diamond) coupled to a superconducting transmon qubit via a tunable resonator. We demonstrate the reversible coherent storage and retrieval of a single microwave photon from the qubit into the spin ensemble [1]. In this experiment the storage time was however limited by inhomogeneous broadening of the ensemble of spins. We propose a realistic protocol [2] that should extend the ensemble storage time by several orders of magnitude, based on spin-echo like pulse sequences; first experimental results will be presented [3].\\[4pt] [1] Y. Kubo et al., PRL \textbf{107}, 220501 (2011).\\[0pt] [2] B. Julsgaard, C. Grezes, P. Bertet, and K. Moelmer, \textbf{PRL} 110, 205503 (2013).\\[0pt] [3] C. Grezes et al., submitted (2014). [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F28.00002: Quantum memory operations in a flux qubit - spin ensemble hybrid system S. Saito, X. Zhu, R. Amsuss, Y. Matsuzaki, K. Kakuyanagi, T. Shimo-Oka, N. Mizuochi, K. Nemoto, W. J. Munro, K. Semba Superconducting quantum bits (qubits) are one of the most promising candidates for a future large-scale quantum processor. However for larger scale realizations the currently reported coherence times of these macroscopic objects (superconducting qubits) has not yet reached those of microscopic systems (electron spins, nuclear spins, etc). In this context, a superconductor-spin ensemble hybrid system has attracted considerable attention. The spin ensemble could operate as a quantum memory for superconducting qubits. We have experimentally demonstrated quantum memory operations in a superconductor-diamond hybrid system [1]. An excited state and a superposition state prepared in the flux qubit can be transferred to, stored in and retrieved from the NV spin ensemble in diamond. From these experiments, we have found the coherence time of the spin ensemble is limited by the inhomogeneous broadening of the electron spin (4.4 MHz) and by the hyperfine coupling to nitrogen nuclear spins (2.3 MHz). In the future, spin echo techniques could eliminate these effects and elongate the coherence time. Our results are a significant first step in utilizing the spin ensemble as long-lived quantum memory for superconducting flux qubits. [1] S. Saito, et al., Phys. Rev. Lett. 111, 107008 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F28.00003: Improved coherence times for transmon qubits in two-dimensional resonators Simon Gustavsson, Archana Kamal, Theodore Gudmundsen, Jonilyn Yoder, Paul Welander, Xiaoyue Jin, Fei Yan, David Hover, Andrew Kerman, Adam Sears, Terry Orlando, William Oliver We have designed, fabricated and characterized the coherence of transmon qubits coupled to planar microwave resonators. By using high-quality, epitaxially grown aluminum, we see a significant increase in coherence times compared to samples fabricated with evaporated metal. We also study how the coherence time scales with qubit dimensions, and for the device with largest spacing between the fingers of the interdigitated capacitance we report an energy-relaxation time (T1) of 34 us. The Lincoln Laboratory portion of this work was sponsored by the Assistant Secretary of Defense for Research {\&} Engineering under Air Force Contract number FA8721-05-C-0002.~ Opinions, interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the United States Government. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F28.00004: Geometrical description of nonreciprocity in coupled two-mode systems Jose Aumentado, Leonardo Ranzani Traditional microwave and optical devices that break reciprocal symmetry are based on the Faraday effect in anisotropic materials such as ferrites. These devices contain permanent magnets and are therefore not compatible with superconducting quantum circuits. Various nonreciprocal devices that do not employ dc magnetic fields to break reciprocal systems have been discussed in the literature, but it is not obvious if and how these different systems might be connected conceptually. In this talk we explore the concept of nonreciprocity in coupled two-mode systems using a geometric mapping to the Poincar\'{e} sphere. In this picture the evolution of the system is described by a rotation sequence of the state vector, where the axis of rotation is determined by the matrix of the coupled-mode system and a different order for the rotations corresponds to a different direction of propagation of the signal. The requirements for reciprocity are then expressed in terms of geometric properties of the rotation axis of the system. We provide a few examples (the microwave circulator, parametric up/down converter, and traveling wave frequency converter) to demonstrate how this general geometric picture can provide insight into specific physical systems. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F28.00005: Trapping a single vortex in a superconducting microwave resonator Ibrahim Nsanzineza, B.L.T. Plourde Trapped vortices in a superconducting microwave resonant circuit can have a significant influence on the loss and resonance frequency. By varying the linewidths of our resonators in different configurations and weakly coupling them to the measurement circuitry, we are able to resolve the shift of the resonance caused by the addition of individual vortices that become trapped following a field-cooling process. In addition, by probing harmonics with different driving forces on the vortices, we are able to study interactions between the trapped vortices and non-equilibrium quasiparticles in the superconducting film. We will discuss prospects for upcoming microwave experiments based on the trapping of a single vortex. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F28.00006: Quasiparticle Trapping and Dynamics in Superconducting Nanobridges E.M. Levenson-Falk, F. Kos, R. Vijay, L. Glazman, I. Siddiqi Quasiparticle excitations can cause loss and noise in superconducting circuits. Recent experiments [1-4] have probed the bulk density of nonequilibrium quasiparticles and their tunneling rates in aluminum superconducting qubits and resonators at low temperature. We perform dispersive measurements of quasiparticle trapping in phase-biased aluminum nanobridge Josephson junctions incorporated into a superconducting resonator. The trapped quasiparticles populate Andreev states formed in the biased nanobridge. We use our technique to not only infer the quasiparticle density, but also to probe the quasiparticles' energy distribution and trapping statistics, to perform spectroscopy on the trap states, and to measure trapping dynamics. We find that the quasiparticle energy distribution is non-thermal below 75 mK, with non-Poissonian trapping statistics. The evolution of the trapping time with the phase bias is consistent with electron-phonon relaxation as the dominant mechanism for quasiparticle trapping in Andreev states.\\[4pt] [1] Manucharayan et al., Science 326, 113 (2009)\\[0pt] [2] Paik et al., Phys. Rev. Lett. 107, 240501 (2011)\\[0pt] [3] Barends et al., Appl. Phys. Lett. 99, 113507 (2011)\\[0pt] [4] Rist\'e et al., Nature Communications 4, 1913 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F28.00007: Parity switching and decoherence by quasiparticles in single-junction transmons Gianluigi Catelani Transmons are at present among the most coherent superconducting qubits, reaching quality factors of order $10^6$ both in 3D and 2D architectures. These high quality factors enable detailed investigations of decoherence mechanisms. An intrinsic decoherence process originates from the coupling between the qubit degree of freedom and the quasiparticles that tunnel across Josephson junctions. In a transmon, tunneling of a single quasiparticle is associated with a change in parity. I will discuss the theory of the parity switching rate in single-junction transmons, compare it with recent measurements, and consider the role of parity switching in limiting the coherence time. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F28.00008: Progress towards a metastable RF squid (MRFS) qubit Archana Kamal, Andrew Kerman, Simon Gustavsson, Xiaoyue Jin, Fei Yan, Ted Gudmundsen, David Hover, Adam Sears, Jonilyn Yoder, Terry Orlando, William Oliver The MRFS qubit [1] consists of an RF squid with a very high loop inductance, and whose two lowest quantum states are very well-defined, equal and opposite persistent supercurrents. These states can be strongly decoupled from each other, such that spontaneous electromagnetic decay processes of the excited state are extremely slow. Also, the large loop inductance suppresses the magnetic flux sensitivity of the design. We have realized these large inductances with NbN nanowires whose kinetic inductance is around 0.5 ?H. We will discuss experimental progress in measuring MRFS qubits fabricated using these inductors, and expected improvements in coherence. Future directions include studying the dynamics of quantum phase slips through these nanowires. [1] A. J. Kerman, PRL 104, 027002(2010). This research was funded in part by the Office of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA); and by the Asst Secretary of Defense for Research \& Engineering under Air Force Contract number FA8721-05-C-0002. All statements of fact, opinion or conclusions contained herein are those of the authors and should not be construed as representing the official views or policies of IARPA, ODNI or the US government [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F28.00009: Decoherence of superconducting flux qubits in coplanar waveguide resonators Adrian Lupascu, Jean-Luc Orgiazzi, David Layden, Ryan Marchildon, Mustafa Bal, Chunqing Deng, Florian Ong We present detailed measurements of decoherence of persistent current qubits coupled to coplanar waveguide resonators. We find energy relaxation times reaching up to 10 $\mu$s. Dephasing is characterized in detail for different flux biasing points, corresponding to coupling of flux noise with different strength, using Ramsey, spin-echo, and multiple pulse dynamical decoupling. The coherence decay changes in a continuous manner from Gaussian to exponential as the strength of the coupling to flux noise is reduced. This indicates the presence of a source of noise with a flat spectrum around the flux insensitive point of the qubit, a result which is also confirmed by extracting the spectral density of the noise based on different sets of measurements with decoupling sequences. This noise source limits dephasing times at the flux insensitive point to about 1-2 $\mu$s. In qubits with a smaller Josephson to charging energy ratio, we observe decoherence induced by quasiparticles. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F28.00010: First-principles simulation of magnetic defects on the substrate of noisy superconducting qubits Nicole Adelstein, Jonathan DuBois, Vincenzo Lordi Superconducting qubits represent one of the more promising routes to realization of a scalable quantum computer. Current performance as measured by the lifetime of quantum states in these systems is, however, largely limited by an as yet unidentified source of low frequency flux noise. Recent experimental and theory efforts suggest that the noise in flux qubits arises from hopping of unpaired spins on the silica or sapphire substrate. In addition, noise could be due to defects with low energy magnetic excited states, though neither noise source is known at the atomic level. We have performed a comprehensive study of the magnetic defects on the surface of SiO$_{\mathrm{2}}$ and investigated barriers to magnetic fluctuations using first-principles density functional theory. Within this framework, we show how defects, such as oxygen vacancies, and adsorbents, such as water, on the substrate represent possible sources of magnetic flux fluctuations. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F28.00011: 1/f flux noise and field-dependent spin susceptibility Pradeep Kumar, Taylor Klaus, Antonio Puglielli, Steven Sendelbach, Robert McDermott Low-frequency 1/f magnetic flux noise is a dominant source of dephasing in superconducting Qubits. It is believed that the noise originates in a high density of surface magnetic defects, but the microscopic noise mechanism is not understood. Here, we describe investigations of the field-dependent complex susceptibility of the surface magnetic system. We have fabricated and characterized asymmetric dc SQUIDs that allow injection of a low-frequency excitation current directly into the SQUID loop to allow measurement of the SQUID inductance, which contains a contribution from the surface spin system. We observe a strong dependence of the SQUID inductance on applied dc field, which we attribute to field-dependent surface spin susceptibility. The data constrains possible models for 1/f flux noise from surface spin states. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F28.00012: Temperature dependent spin-diffusion as a mechanism of intrinsic flux noise in in SQUIDs Rogerio de Sousa, S.-F. Chen, Stephanie Laforest, Trevor Lanting, Mohammad Amin The intrinsic flux noise observed in superconducting quantum interference devices (SQUIDs) is thought to be due to the fluctuation of electron spin impurities, but the frequency and temperature dependence observed in experiments do not agree with the usual 1/f models. We present theoretical calculations of flux noise in rf-SQUID flux qubits that shows how these observations can be interpreted in terms of a spin-diffusion constant that increases with temperature. A comparison of our theory to measurements of flux noise in the 20-80 mK temperature range allows the extraction of the spin-diffusion constant and its temperature dependence, suggesting that the spin system is close to a phase transition. See our paper at http://arxiv.org/abs/1306.1512. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F28.00013: Geometric inductance effects in the spectrum of split transmon qubits R.T. Brierley, J. Blumoff, K. Chou, R.J. Schoelkopf, S.M. Girvin The low-energy spectra of transmon superconducting qubits in a cavity can be accurately calculated using the black-box quantization approach [1]. This method involves finding the normal modes of the circuit with a linearized Josephson junction and using these as the basis in which to express the non-linear terms. A split transmon qubit consists of two Josephson junctions in a SQUID loop. This configuration allows the Josephson energy to be tuned by applying external flux. Ideally, the system otherwise behaves as a conventional transmon with a single effective Josephson junction [2]. However, the finite geometric inductance of the SQUID loop causes deviations from the simplest ideal description of a split transmon. This alters both the linearized and non-linear behaviour of the Josephson junctions in the superconducting circuit. We study how these changes can be incorporated into the black-box quantization approach and their effects on the low-energy spectrum of the split transmon. \\[4pt] [1] S. E. Nigg et al, Phys. Rev. Lett., 108, 240502 (2012)\\[0pt] [2] J. Koch et al, Phys. Rev. A, 76, 042319 (2007) [Preview Abstract] |
Session F29: Magnetotransport in Graphene
Sponsoring Units: DCMPChair: Kathleen McCreary, U.S. Naval Research Laboratory
Room: 603
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F29.00001: Theoretical study of disorder induced magnetoresistance in graphene Shaffique Adam, Jinglei Ping, Indra Yudhistira, Navneeth Ramakrishnan, Sungjae Cho, Michael S. Fuhrer In this work we predict theoretically that carrier density inhomogeneity provides a new mechanism for classical magnetoresistance. For concreteness, we study the case of graphene where density inhomogeneity and carrier scattering is dominated by charged impurities, although the mechanism itself is quite general and applies to other systems in which there are large spatial fluctuations of the carrier density. Calculations using an effective medium approximation show that low-field magnetoresistance becomes a universal function of the ratio between the average carrier density and the fluctuations of the carrier density, and scales as a power-law when this ratio is large. Our finding is in excellent agreement with recent experimental results. This work is supported by the Singapore National Research Foundation NRF-NRFF2012-01. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F29.00002: Examining the Quantum Nature of Edge Magnetism in Graphene Nanoribbons Michael Golor, Cornelie Koop, Thomas C. Lang, Manuel J. Schmidt, Stefan Wessel Based on low-energy theories for interaction effects along graphene edges, which preserve the full quantum nature, we study edge magnetism in various types of graphene nanoribbons. We find that the relevant physics is well captured by an effective Heisenberg model with extended ferromagnetic interactions along and antiferromagnetic interactions across the ribbon edges. The basic principles of edge magnetism are then studied in short and narrow armchair ribbons, for which we predict magnetic response and STS signatures that could be probed in future experiments. For the case of macroscopically large chiral ribbons, we find the spin-spin-correlation length to grow exponentially with the ribbon width, demonstrating the importance of quantum fluctuations in these systems. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F29.00003: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F29.00004: Magnetoresistance induced by inhomogeneity in graphene Jinglei Ping, Indra Yudhistira, Navneeth Ramakrishnan, Sungjae Cho, Shaffique Adam, Michael Fuhrer We study the magnetoresistance of graphene samples with varying disorder as a function of carrier density. We observe a quadratic low-field classical magnetoresistance which is largest at low carrier density reflecting the inhomogeneous nature of transport in the electron-hole puddle dominated minimum conductivity region, as observed previously[\textit{Phys. Rev. B} \textbf{77}, 084102(R) (2008)]. However we observe the magnetoresistance persists to carrier densities well outside the electron-hole puddle region, where single band transport is expected. We find that the magnetoresistance for all samples follows a universal form which depends only on the ratio of the carrier density $n$ to the characteristic electron-hole puddle density $n*$. The results are in excellent quantitative agreement with a recent theory based on an effective medium approximation for disordered graphene. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F29.00005: Magnetotransport properties of graphene devices contacted by resist-free stencil lithography Ather Mahmood, Cheol-Soo Yang, Won Jin Choi, Jean-Fran\c{c}ois Dayen, Jeong-O Lee, Bernard Doudin We demonstrate large-scale fabrication of high-quality contaminant-free graphene devices, a prerequisite for chemical functionalization applications. We investigate CVD graphene transferred from Cu substrates to Si/SiO$_{2}$. Patterning of graphene and metal evaporation are performed through a multi-step mechanical stencils methodology. Microlithography through stencil masks is well known, but patterning graphene while keeping its outstanding electrical properties remains challenging. Magnetotransport measurements at low temperature show the existence of Shubnikov-de Hass oscillations and Quantum Hall plateaus. Weak (anti-) localization signatures of monolayer graphene validate the excellent intrinsic properties of our samples. Finally, we show this technique is extended to complex geometries and smaller device feature sizes. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F29.00006: Effect of spin-orbit interaction on the conductance fluctuation in disordered graphene Duk-Hyun Choe, K.J. Chang Recent findings of topological insulators have demonstrated the importance of spin-orbit interaction in low dimensional systems. In particular, the spin-orbit coupling gives rise to the formation of topological surface states that are protected by time-reversal symmetry. The universal conductance fluctuation (UCF) in spin-orbit coupled systems, however, has received comparatively little attention. It has been known that the universality characterized by the value of UCF only depends on the dimensionality and symmetry ($\beta = $ 1,2,4 according to the random matrix theory) of the system. Here, we investigate the effect of spin-orbit interaction on the UCF behavior in disordered graphene by considering Kane-Mele (KM) and Rashba type interactions. Following the random matrix theory, both KM and Rashba Hamiltonians belong to the circular symplectic ensemble ($\beta =$ 4), because in both cases time-reversal symmetry is maintained while spin-rotational symmetry is broken. Interestingly, conductance fluctuation in the KM Hamiltonian exhibits the same UCF value as that for the circular unitary ensemble ($\beta =$ 2). We reveal the origin of such inconsistency and furthermore find that there exist new types of universality class, different from the conventional ones. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F29.00007: Electronic transport in graphene sheets in a random magnetic field Caio Lewenkopf, Rhonald Burgos, Jesus Warnes, Leandro Lima We present a theoretical study of the effect of ripples and strain fields in the transport properties of diffusive deposited graphene flakes. Defects in the crystalline structure, adsorbed atomic impurities and charge inhomogeneities at the substrate are believed to be the dominant disorder sources for the electronic transport in graphene at low temperatures. We show that intrinsic ripples also effect the conductivity, in particular, its quantum corrections. To this end, we analyze recent experimental results on the conductivity of rippled monolayer graphene sheets subjected to a strong magnetic field parallel to the graphene-substrate interface, $B_\parallel$ [M. B. Lundeberg and J. A. Folk, Phys. Rev. Lett. 105, 146804 (2010)]. In this setting, $B_\parallel$ gives rise to a random magnetic field normal to graphene sheet, that depends on the local curvature of the smooth disordered ripples. The analysis of the weak localization corrections of the magnetoconductance allows to establish the dependence of electronic dephasing rate on the magnitude of the random magnetic field. We compare the results for $B_\parallel$ with the conductivity and weak localization corrections due to the pseudo-magnetic fields originated by intrinsic ripples and strain fields. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F29.00008: Density-matrix renormalization group studies on a magnetic impurity in graphene Tomonori Shirakawa, Seiji Yunoki Motivated by recent experiments on the emergence of magnetism in graphene induced by lattice defects or by magnetic adatoms, we have theoretically studied the ground state properties of single magnetic impurity in grapheme. First, we have developed a new numerical technique within the density-matrix renormalization group (DMRG) scheme for magnetic impurity model in general to study site-dependent quantities including local density of states even away from magnetic impurity site, Friedel density oscillations, and spin-spin correlation functions between the magnetic impurity and the surrounding conduction electrons. This new technique is applied to three different models: (i) a magnetic adatom on grapheme, (ii) a substitutional magnetic impurity in grapheme, and (iii) a model on defect in the graphene. Our systematic study of these models reveals that, in the presence of particle-hole symmetry, the ground state of model (i) exhibits the formation of local moments without Kondo-screening, whereas the others behave very similarly to the Kondo singlet states. We also discuss the real-space decay of spin-spin correlations between magnetic impurity and surrounding conduction electrons in these models. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F29.00009: Magneto-optics of general pseudospin-$s$ two-dimensional Dirac-Weyl fermions John Malcolm, Elisabeth Nicol The popularity of graphene--a pseudospin-$\frac{1}{2}$ two-dimensional Dirac-Weyl material--has prompted the search for related materials and the characterization of their properties. The magneto-optical conductivity is calculated for systems that obey the general pseudospin-$s$ two-dimensional Dirac-Weyl Hamiltonian, with particular focus on $s=\left\{\frac{1}{2},1,\frac{3}{2},2\right\}$. This follows previous work on the optical response of these systems in zero field [1]. In the presence of a magnetic field, Landau levels condense out of the $2s+1$ energy bands [2]. As the chemical potential in a system is shifted, patterns arise in the appearance and disappearance of certain peaks within the optical spectra. These patterns are markedly different for each case considered, creating unique signatures for potential experimental observations. The general structure of each spectrum and how they compare is discussed. \\ \\ \noindent [1] B. D\'{o}ra, J. Kailasvuori, and R. Moessner, Phys. Rev. B {\bf 84}, 195422 (2011). \\ \noindent [2] X. Lan, N. Goldman, A. Bermudez, W. Lu, and P. \"{O}hberg, Phys. Rev. B {\bf 84}, 165115 (2011). [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F29.00010: Study of Magneto-transport in Niobium Nitride (NbN) - Graphene Josephson weak links Piranavan Kumaravadivel, Xu Du Proximity induced superconductivity in graphene - superconductor (SC) hybrid devices has revived interest in the study of electronic transport of Andreev bound states in the Quantum Hall (QH) regime. This is mainly due to the ability to fabricate ballistic superconducting weak links with type II SC where the interplay of proximity effect and QH effect can be studied at low magnetic fields ($\sim$ 0.5T) and extended up to the upper critical field (Hc$_{2})$ [1]. In our work we use sputtered NbN which has an upper critical field of 16T. Below the SC transition temperature of NbN (approx. 12K) we observe Andreev reflection and super-current which are indicative of transparent superconductor-normal interface. We study magneto- transport measurements at different fields applied perpendicular to the graphene channel. Our results suggest that the diamagnetic current and Abrikosov vortices which are formed in the NbN leads influence the SC-proximity effect in graphene. \\[4pt] [1] Mizuno, N. et al. Ballistic-like supercurrent in suspended graphene Josephson weak links. Nat. Commun. 4:2716 doi: 10.1038/ncomms3716 (2013) [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F29.00011: Ballistic transport in CVD graphene V.E. Calado, S.E. Zhu, S. Goswami, Q. Xu, K. Watenabe, T. Taniguchi, G.C.A.M. Janssen, L.M.K. Vandersypen Chemical vapor deposition (CVD) synthesis of graphene is a scalable and controllable method for the production of single layer CVD graphene (CVDg). Up to now its electronic and structural quality is considered to be inferior to exfoliated graphene, and in particular no ballistic phenomena have been observed in CVDg. Here we synthesize and measure CVDg that shows ballistic transport on a micron length-scale at 4 K. With a dry transfer method we transferred 100 $\mu$m size single crystals of CVDg onto hexagonal boron nitride. Using non-local measurements we show that electrons can be ballistically directed by a magnetic field (transverse magnetic focussing) over length scales of about 1 micron. These findings suggest that CVD graphene is suitable for electron optics experiments. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F29.00012: Pseudogap opening and localization in disordered graphene: frustration effects at the Fermi energy due to the underlying triangular symmetry Eduardo Barrios-Vargas, Gerardo G. Naumis An intuitive explanation of the increase in localization observed near the Dirac point in doped graphene is presented. To do this, we renormalize the tight binding Hamiltonian in such a way that the honeycomb lattice maps into a triangular one [1]. Then, we investigate the frustration effects that emerge in this Hamiltonian. In this doped triangular lattice, the eigenstates have a bonding and antibonding contribution near the Dirac point, and thus there is a kind of Lifshitz tail [2]. The increase in frustration is related to an increase in localization, since the number of frustrated bonds decreases with disorder, while the frustration contribution raises. Then we show that states have a multifractal nature, with a fractal spectrum that approaches freezing as disorder increases [2]. We compute exacty the first spectral moments of the DOS using statistical averages and counting paths. Finally, the number of states at the Dirac point is obtained using a configurational counting [3]. \\[4pt] [1] Barrios-Vargas, Naumis, J. Phys.: Condens. Matter 23, 375501 (2011)\\[0pt] [2] Barrios-Vargas, Naumis, J. Phys.: Condens. Matter 24 (2012)\\[0pt] [3] Barrios-Vargas, Naumis, Solid State Communications 162, 23-27 (2013) [Preview Abstract] |
Session F30: Focus Session: Graphene Devices: Fabrication, Characterization and Modeling: Graphene Quantum Dots
Sponsoring Units: DMPChair: Vincent Meunier, Rensselaer Polytechnic University
Room: 605
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F30.00001: Graphene quantum dots: localized states, edges and bilayer systems Invited Speaker: Klaus Ensslin Graphene quantum dots show Coulomb blockade, excited states and their orbital and spin properties have been investigated in high magnetic fields. Most quantum dots fabricated to date are fabricated with electron beam lithography and dry etching which generally leads to uncontrolled and probably rough edges. We demonstrate that devices with reduced bulk disorder fabricated on BN substrates display similar localized states as those fabricated on the more standard SiO$_2$ substrates. For a highly symmetric quantum dot with short tunnel barriers the experimentally detected transport features can be explained by three localized states, 1 in the dot and 2 in the constrictions. A way to overcome edge roughness and the localized states related to this are bilayer devices where a band gap can be induced by suitable top and back gate voltages. By placing bilayer graphene between two BN layers high electronic quality can be achieved as documented by the observation of broken symmetry states in the quantum Hall regime. We discuss how this method can be exploited to achieve smoother and better tunable graphene quantum devices. This work was done in collaboration with D. Bischoff, P. Simonet, A. Varlet, Y. Tian, and T. Ihn. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F30.00002: Multiple quantum dot behavior in short and wide graphene devices, with disorder Joseph Lambert, Steven Carabello, Roberto Ramos Quantum dot (QD) behavior in graphene has been investigated previously in several different types of systems. These systems range from QDs etched in graphene, to impurity induced QDs in graphene nanoribbons. Here, we report on QD behavior in a new system where the graphene channel between two superconducting leads is short (a few hundred nanometers long) and wide (5-10 microns across). Measurements of conductance as a function of gate voltage and bias voltage at temperatures between 20 mK and 10 K revealed long-range tapestry patterns extending across a wide range of voltages, from -60 to $+$20 volts. Applying filtering techniques reveals Coulomb diamond features of varying sizes, suggestive of multiple QDs contributing to the conductance. The minimum conductance values for our devices range from G$_{min}\approx $ 40 e$^{2}$/h to 100 e$^{2}$/h, which are several orders magnitude larger than in typical QD systems. For several samples, measurements of conductance versus gate voltage show a broad and relatively flat minimum conductance region $\Delta $V$_{g}\approx $ 10V to 20V wide, with a center that is shifted in gate voltage to V$_{g}\approx $ -10V to -20V. This indicates impurity doping and the formation of electron/hole puddles on the graphene surface. The Coulomb diamonds uncovered by filtering is consistent with the presence of several low-barrier QDs in parallel. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F30.00003: Wigner localization in a graphene quantum dot with a mass gap Karina Andrea Guerrero Becerra, Massimo Rontani The role of electron-electron interactions in graphene is an open issue that impacts on the operation of quantum dots (QDs) and other graphene-based devices. Whereas electrons in bulk graphene allegedly behave as noninteracting particles except for subtle effects, there is strong evidence that electrons in carbon-based nanostructures--nanotubes--form Wigner molecules [Nat. Phys. 9, 576 (2013)]. Besides, a significant effort is presently devoted to minimize the role of disorder in next-generation graphene QDs. Here we show theoretically that Dirac electrons in a clean, circular graphene QD with a mass gap induced by the breaking of sublattice symmetry form a Wigner molecule for realistic values of device parameters. The evidence is the combined analysis of many-body energies, one-body densities, and pair correlation functions obtained through the exact diagonalization of the interacting Dirac-Weyl Hamiltonian. This method, which uses two different sublattice envelopes and includes both inequivalent Dirac cones, allows us to take all many-body correlations into account. The experimental signature of Wigner localization is the suppression of the fourfold periodicity of the filling sequence and the quenching of excitation energies, accessible through Coulomb blockade spectroscopy. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F30.00004: Narrow graphene nanoribbons with atomically precise armchair edges: Solution synthesis and characterization Alexander Sinitskii Although graphene is a semimetal, a substantial electronic band gap could be found in narrow graphene nanoribbons (GNRs) with atomically precise armchair edges and widths less than 2 nm. Different top-down approaches typically yield ribbons with widths \textgreater 10 nm and have a limited control over the edge structure in GNRs. Much narrower GNRs with atomically precise edges could be synthesized by a surface-assisted bottom-up approach. This method provides small amounts of GNRs of exceptional quality, but it cannot be used to produce large quantities of GNRs for bulk applications. Therefore, a complimentary chemical approach for bulk quantities of high-quality GNRs is in order. This talk will be focused on a recently developed bottom-up approach for gram quantities of narrow GNRs that are less than 2 nm wide and have atomically precise armchair edges. STM studies of these GNRs show that their structural quality is comparable to that of surface-synthesized GNRs. These nanoribbons have a bandgap of about 1.3 eV, which makes them promising for applications in field-effect transistors with high on-off ratios, as well as bulk applications, including coatings, composites and photovoltaic devices. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F30.00005: Single channel ballistic transport in epitaxial graphene nanoribbons Claire Berger, Ming Ruan, Jens Baringhaus, Frederik Edler, James Palmer, Zelei Guo, John Hankinson, Christoph Tegenkamp, Walt A. de Heer We present transport results on high quality epitaxial graphene nanoribbons about 40 nm in width, with annealed edges, grown on sidewall SiC. The nanoribbons are produced directly in their final shape with no post-graphene growth patterning. We show that the nanoribbons are neither semiconductors, nor have a transport gap, but are single channel room temperature ballistic conductors. The graphene ribbons behave as electronic waveguides or quantum dots. The low-temperature transport properties of top-gated ribbons indicates that transport is dominated by two components of the ground state transverse waveguide mode, one that is ballistic and temperature independent, and a second thermally activated component that appears to be ballistic at room temperature and insulating at cryogenic temperatures. These properties appear to be related to the lowest energy quantum states in the charge neutral ribbons.\\[4pt] [1] J. Baringhaus et al. ArXiv: 1301.5354. Nature (in print).\\[0pt] [2] M. Sprinkle et al. Nature Nanotech 5, 727 (2010).\\[0pt] [3] Y. Hu et al. J. Phys. D: Appl. Phys 45 154010 (2012).\\[0pt] [4] M. Ruan, MRS Bulletin 37, 1146 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F30.00006: Transport properties of nanoconstrictions in armchair graphene ribbons Igor Romanovsky, Constantine Yannouleas, Uzi Landman The transport properties of nanoconstrictions and quantum-point contacts formed in atomically precise segmented armchair graphene nanoribbons ( SaGRs) are investigated using a tight-binding non-equilibrium Green's function (TB-NEGF) approach and relativistic quantum-field theory modeling.\footnote{% I. Romanovsky, C. Yannouleas, and U. Landman, Phys. Rev. B {\bf 87}, 165431 (2013)} The TB behavior is accounted for by a one-dimensional Dirac-transfer-matrix (DTM) model using variable-mass (scalar-field) barriers assigned to the junctions between the nanoribbon segments. It is shown that the topology of the junctions (sharp versus smooth) and the ratio of length over width of the constriction are the principal factors influencing the height of the mass barriers, and thus they control the extent of trapping and confinement by the constriction of graphene's relativistic carriers, even in the case of all-metallic SaGRs. A rich variety of transport patterns ensues, ranging from ballistic quantized conductance to resonant tunneling associated with Coulomb blockade. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F30.00007: Atomic-scale measurements of graphene nanoribbon edge properties Patrick Han, Katsuya Iwaya, Susumu Shiraki, Naoki Asao, Taro Hitosugi, Paul Weiss Graphene edges are predicted to be a type of defects that can be utilized to tailor both the electronic and the magnetic properties of graphene structures. However, to date, there is little experimental result on how graphene size and structure affect these edge properties. For this purpose, we fabricate defect-free graphene nanoribbons (GNRs) by self-assembly of organic precursor molecules on a Cu(111) single-crystal surface in ultrahigh vacuum. We use low-temperature scanning tunneling microscopy to image and measure the electronic properties of these ribbons, comparing GNR edges and centers. We discuss the results of our fabrication process and of our local spectroscopic measurements of individual GNRs. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F30.00008: Electronic transport in graphene ribbons with a Gausssian deformation Ramon Carrillo, Daiara Faria, Andrea Latg\'{e}, Francisco Mireles, Nancy Sandler The coupling of geometrical and electronic properties is a promising avenue to engineer conduction properties in graphene. Confinement added to strain allows for interplay of different transport mechanisms with potential device applications. In particular, strain-predicted to produce localized states similar to those in an external magnetic field--can be tailored for desired transport properties. To investigate specific strain signatures on transport in confined geometries, we focus on graphene nanoribbons with different edge terminations and circularly symmetric deformations. In particular, we study nanoribbons with an inhomogeneous, out of plane Gaussian bump deformation, connected to reservoirs, with and without external magnetic field. We use the tight-binding approximation with the deformation described by elasticity theory. Using the recursive Green function algorithm, we calculate the local density of states and obtain the Landauer conductance. An enhancement of the density of states in the deformed region, similar to the one appearing with constant fields in confined regions is observed. We show how these confined states give rise to peculiar features in the emerging Landau levels and discuss their effect on the overall conductance. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F30.00009: Bandgap Engineering of Bottom-up Synthesized graphene nanoribbon junctions Zahra Pedramrazi, Yen-Chia Chen, Chen Chen, Danny Haberer, Ting Cao, Dimas Oteyza, Felix Fischer, Steven Louie, Michael Crommie Bandgap engineering is a key concept in electronic device fabrication, through which various types of semiconductor heterostructures have been realized. However, as the size of electronic building blocks is approaching the physical limits of well-established top-down methods, the need for alternative strategies towards electronic devices becomes apparent. Considering the recent progress in bottom-up synthesis of graphene nanoribbons (GNRs), components with single-atom thickness and sub-2 nm width may be realized based on GNRs. The electronic properties of GNRs are crucially depending on their width and edge geometry, and it has been predicted that intra-ribbon bandgap engineering may be achieved by varying width or doping at desired positions. Here, we demonstrate the successful realization of bottom-up narrow-wide GNR junctions, consisting of covalent bonding of armchair segments having either 7 or 13 carbon dimer lines across the width (i.e. the n$=$7 and n$=$13 segments are ``welded together'' at the atomic scale). We study the resultant~7-13~junctions with scanning tunneling microscopy (STM) and spectroscopy (STS), and identify distinct electronic structures in different GNR segments. We have further performed first-principles calculations to support our experimental results. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F30.00010: Monomer Doping of Self-Assembled Graphene Nanoribbons for Band Gap Alignment Christopher Bronner, Stephan Stremlau, Marie Gille, Felix Brau{\ss}e, Anton Haase, Stefan Hecht, Petra Tegeder In order to exploit the technologically interesting electronic properties of graphene, several concepts have been discussed which would lead to the opening of a band gap. One approach is spatial confinement of the charge carriers in quasi-one-dimensional graphene nanoribbons (GNRs). The band gap of a GNR scales inversely with its width and particularly nanometer-scale widths are desirable for application e.g. in transistor devices. Since the electronic properties of GNRs depend critically on their structure, precise synthesis is necessary but challenging for conventional methods such as lithography. In contrast, self-assembly from molecular precursors is an intriguing approach which has been employed to fabricate defect-free GNRs with well-defined widths and edge structures. Only this high level of structural precision allows introduction of dopant atoms at specific doping sites and concentrations in the graphene lattice. Nitrogen doping has been known to shift the band structure of GNRs with respect to the Fermi level which is interesting for GNRs in contact with electrodes and other device materials. Using surface-sensitive electron spectroscopies we demonstrate a continuous down-shift of the band structure with increased nitrogen doping of the monomers. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F30.00011: Polarization Dependent Optical Responses of Graphene Nanoribbons Ting Cao, Sangkook Choi, Steven Louie The optical response of an anisotropic system depends on light's polarization direction. In this study, we perform first-principle calculations on polarization dependent optical absorption spectra of graphene nanoribbons at the RPA and GW-BSE level. We observe significant polarization dependent features. We demonstrate the many-body origins of these features. We also discuss the polarization dependent optical responses of other carbon nanostructures, and connect our work to experimental measurements. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F30.00012: A structural and electronic comparison of armchair and zigzag epitaxial graphene sidewall nanoribbons Meredith Nevius, F. Wang, I. Palacio, A. Celis, A. Tejeda, A. Taleb-Ibrahimi, W. de Heer, C. Berger, E. Conrad Graphene grown on sidewalls of trenches etched in SiC shows particular promise as a candidate for post-Si CMOS electronics because of its ballistic transport, exceptional mobilities, low intrinsic doping, and the opening of a large band gap. [1,2] However, before definitive progress can be made toward epitaxial graphene-based transistors, we must fully understand the nuances of graphene ribbon growth on different SiC facets. We have now confirmed that sidewall ribbons grown in graphene's two primary crystallographic directions (``armchair'' and ``zigzag'') differ greatly in both structure and electronic band-structure. We present data from both geometries obtained using low-energy electron microscopy (LEEM), low-energy electron diffraction (LEED), angle-resolved photoemission spectroscopy (ARPES), photoemission electron microscopy (PEEM), micro-ARPES and dark-field micro-ARPES. We demonstrate that while graphene grows on stable facets of trenches oriented for armchair edge growth, trenches oriented for zigzag edge growth prefer narrow ribbons of graphene on the (0001) surface near the trench edge. The structure of these zigzag edge graphene ribbons is complex and paramount to understanding their transport. [1] J. Baringhaus et al. arXiv:1301.5354 Nature to be published [2] J. Hicks et al. Nature Physics (2012). This work was supported by the NSF under grants DMR-1005880 and DMR-0820382, the W. M. Keck Foundation and the Partner University Fund from the Embassy of France. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F30.00013: Interference between Fano resonances for impurities in graphene nanoribbon C.H. Chiu, C.S. Chu The presence of impurities in a gapless armchair graphene nanoribbon (AGNR) is expected to exhibit Fano resonances in the conductance $G$ of the AGNR. For instance, in the low energy range, when the only propagating subband is gapless, an impurity with an on-site potential $V$\textgreater 0 will give rise to the formation of the Fano resonance just above the first hole-like subband. This Fano characteristics, however, is masked in the total $G$ by the contribution from the first hole-like subband. In this work, we show that two neighboring impurities of the same type could recover the peak-dip Fano characteristics by shifting the peak-part of the Fano structure away from the first hole-like subband. Factors that affect, separately, the peak- and the dip-part of the Fano profile will be elucidated and discussed. Furthermore, the interference of the Fano resonances from two neighboring impurities, and the sensitivity to their respective locations in the sublattice, will be studied in detail. For comparison, we also consider the case of gapped AGNRs. [Preview Abstract] |
Session F31: Focus Session: Van der Waals Interactions in Complex Materials: Bridging Theory and Experiment II
Sponsoring Units: DMPChair: Robert DiStasio, Princeton University
Room: 607
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F31.00001: Polymorphism and Elastic Response of Molecular Materials from First Principles: How Hard Can it Be? Anthony Reilly, Alexandre Tkatchenko Molecular materials are of great fundamental and applied importance in science and industry, with numerous applications in pharmaceuticals, electronics, sensing, and catalysis. A key challenge for theory has been the prediction of their stability, polymorphism and response to perturbations. While pairwise models of van der Waals (vdW) interactions have improved the ability of density functional theory (DFT) to model these systems, substantial quantitative and even qualitative failures remain. In this contribution we show how a many-body description of vdW interactions can dramatically improve the accuracy of DFT for molecular materials, yielding quantitative description of stabilities and polymorphism for these challenging systems. Moreover, the role of many-body vdW interactions goes beyond stabilities to response properties. In particular, we have studied the elastic properties of a series of molecular crystals, finding that many-body vdW interactions can account for up to 30\% of the elastic response, leading to quantitative and qualitative changes in elastic behavior. We will illustrate these crucial effects with the challenging case of the polymorphs of aspirin, leading to a better understanding of the conflicting experimental and theoretical studies of this system. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F31.00002: Structure and properties of fullerene molecular crystals with linear-scaling van der Waals density functional theory Arash Mostofi, Lampros Andrinopoulos, Nicholas Hine Fullerene molecular crystals are of technological promise for their use in heterojunction photovoltaic cells. An improved theoretical understanding of their structure and properties would be a step towards the rational design of new devices. Simulations based on density-functional theory (DFT) are invaluable for developing such insight, but standard semi-local functionals do not capture the important inter-molecular van der Waals (vdW) interactions in fullerene crystals. Furthermore the computational cost associated with the large unit cells needed are at the limit or beyond the capabilities of traditional DFT methods. In this work we overcome these limitations by using our implementation of a number of vdW-DFs in the ONETEP linear-scaling DFT code to study the structural properties of C$_{60}$ molecular crystals. Powder neutron diffraction shows that the low-temperature Pa-3 phase is orientationally ordered with individual C$_{60}$ units rotated around the [111] direction. We fully explore the energy landscape associated with the rotation angle and find two stable structures that are energetically very close, one of which corresponds to the experimentally observed structure. We further consider the effect of orientational disorder in very large supercells of thousands of atoms. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F31.00003: Van der Waals Interactions in Pyridine and Pyridine-like Molecular Crystals: An \textit{ab initio} Molecular Dynamics Study Hsin-Yu Ko, Robert A. DiStasio Jr., Biswajit Santra, Roberto Car Pyridine has recently been investigated as a potentially effective material for use in artificial light harvesting.\footnote{ EB Cole, PS Lakkaraju, DM Rampulla, AJ Morris, E Abelev, AB Bocarsly J. Am. Chem. Soc., \textbf{132}, 11539 (2010).} In this work, we propose the use of \textit{ab initio} molecular dynamics (AIMD) to gain valuable physical insight into the artificial photosynthetic processes occurring in condensed-phase pyridine, the study of which has been limited to semi-empirical force fields to date.\footnote{AT Anghel, GM Day, SL Price CrystEngComm., \textbf{4}, 348 (2002).} For this purpose, we introduce an accurate and efficient AIMD method, based on density functional theory (DFT) and a self-consistent pairwise description of van der Waals (vdW) interactions, for use in finite temperature and pressure (NPT) simulations on pyridine and several pyridine-like molecular crystals (PLMCs). Utilizing this approach, we demonstrate that vdW forces play a crucial role in the theoretical prediction of the structure and density of pyridine and PLMCs, and therefore must be accounted for in studies of these potential alternative energy materials. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F31.00004: Lowering the desorption temperature of Mg(BH$_4$)$_2$ through doping D. Harrison, T. Thonhauser Magnesium borohydride Mg(BH$_4$)$_2$ is a very promising hydrogen storage material due to its high gravimetric (14.9 mass\%) and volumetric density. However, it is limited for practical storage applications by its high hydrogen desorption temperature of 270$^\circ$C. Arguments have been made for both high thermodynamic stability and slow kinetics to be responsible for this high desorption temperature. In our study we show that doping of Mg(BH$_4$)$_2$ can address the thermodynamic stability issue and predictably lower its desorption enthalpy. We use ab initio calculations at the DFT level (utilizing vdW-DF) and calculate the change in desorption enthalpy from ground state energy and phonon contributions for several possible hydrogen release reactions. Note that van der Waals interactions are crucial to correctly describe the ground state of this complex hydride. We find that, depending on the reaction, the undoped phase has a desorption enthalpy of 50--75 kJ/mol H$_2$ and doping can lower this number by approximately 5~kJ/mol per 10\% doping at 300~K, making the desired range of 40~kJ/mol easily accessible. We argue that this lowering of desorption enthalpy will correspond to a lowering of the desorption temperature. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F31.00005: Ab initio study of hydrogen-halo rotation in ammonia borane Evan Welchman, T. Thonhauser The van der Waals crystal ammonia borane NH$_{3}$BH$_{3}$ is a promising hydrogen-storage material due to its large gravimetric storage density. In an isolated molecule, the H atoms reside in halos about either end of a central B--N backbone with \emph{three-fold} rotational symmetry. However, in the solid phase at ambient temperature and pressure, experiments reveal a tetragonal unit cell with a \emph{four-fold} rotational symmetry about the same axis, creating a geometric incompatibility. Using ab initio calculations at the DFT level (with vdW-DF to capture crucial van der Waals interactions), we elucidate this incompatibility by simulating the behavior of the hydrogen-halos and their substituent H atoms. We use Car-Parrinello molecular dynamics at several different temperatures to simulate behavior in the solid and NEB calculations to find barriers to rotation in solid and gas phase. We find that at room temperature the halos can rotate several degrees per fs and that the four-fold symmetry thus results from a time average. We further show that in the solid phase the complex network of dihydrogen bonds affects the torsional barrier for the halos, and thus their rate of rotation in the solid phase, contributing to the experimentally observed positional uncertainty. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F31.00006: \textit{Atoms in Solids} Perspective on Polarizabilities and van der Waals Coefficients in Semiconductors Guo-Xu Zhang, Anthony M. Reilly, Alexandre Tkatchenko, Matthias Scheffler The calculation of response properties of solids including their polarizabilities and van der Waals (vdW) coefficients usually requires the knowledge of the full electronic bandstructure. For non-covalently bound solids, such as noble-gas and ionic crystals, atoms-in-solids model can be successfully utilized to define their polarizabilities. Here we critically assess the atoms-in-solids model for covalently-bound solids, ranging from wide-gap ($\sim$10 eV) to narrow-gap ($\sim$1 eV) semiconductors. We model their response by assigning a single quantum harmonic oscillator to every atom, where the parameters of the oscillators are defined as functionals of the electron density, following the Tkatchenko-Scheffler method [1]. The response function is then calculated by solving self-consistent screening equations of classical electrodynamics, without any explicit information about the electronic bandstructure [2]. The calculated polarizabilities and vdW coefficients for 23 semiconductors are compared with TDDFT and experimental benchmark data, revealing an overall agreement within 10\%. We demonstrate the crucial role of vdW interactions in the cohesive properties of the 23 semiconductors.\\[4pt] [1] Tkatchenko and Scheffler, PRL (2009);\\[0pt] [2] Tkatchenko, DiStasio, Car, Scheffler, PRL (2012). [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F31.00007: van der Waals interactions in MoS$_2$ and MoO$_3$ Hartwin Peelaers, Chris G. Van de Walle Molybdenum disulfide (MoS$_2$) is a layered material that attracted a lot of attention recently for use in electronic devices, such as field-effect transistors, as it has high electron mobilities and high on/off ratios. MoO$_3$ is a layered n-type semiconductor that shows good properties for energy applications. The layers in both materials are weakly bound by van der Waals interactions. A good theoretical description of these interactions is thus required. In this talk I will discuss different approaches to include van der Waals interactions in density-functional theory (DFT), focusing on MoS$_2$ and MoO$_3$. In particular, a combination of hybrid functionals, which correct for the DFT band gap problem, and explicit inclusion of van der Waals interactions, to correct for the long range interactions, shows a lot of promise. The validity of this approach will be demonstrated by comparing the structural parameters of MoS$_2$ under hydrostatic pressure with experimental data. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F31.00008: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F31.00009: The Structure, Density, and Local Environment Distribution in \emph{Ab Initio} Liquid Water Biswajit Santra, Robert A. DiStasio, Jr., Xifan Wu, Roberto Car We have performed extensive \emph{ab initio} molecular dynamics (AIMD) simulations of liquid water at ambient conditions in the canonical (NVT) and isothermal-isobaric (NPT) ensembles to understand the individual and collective importance of exact exchange, van der Waals interactions, and nuclear quantum effects on the structural properties of liquid water. AIMD simulations which include these effects result in oxygen-oxygen radial distribution functions which are in excellent agreement with experiments and a liquid water structure having an equilibrium density within 1\% of the experimental value of 1 g/cm$^3$. A detailed analysis of the distribution of local structure in ambient liquid water has revealed that the inherent potential energy surface is bimodal with respect to high- and low-density molecular environments, consistent with the existence of polymorphism in the amorphous phases of water. With these findings in mind, the methodology presented herein overcomes the well-known limitations of semi-local density functional theory (GGA-DFT) providing a detailed and accurate microscopic description of ambient liquid water. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F31.00010: Nuclear Zero Point Effects as a Function of Density in Ice-like Structures and Liquid Water from vdW-DF \textit{Ab Initio} Calculations Bet\"{u}l Pamuk, Philip B. Allen, Jose M. Soler, Marivi Fern\'andez-Serra The contributions of nuclear zero point vibrations to the structures of liquid water and ice are not negligible. Recently, we have explained the source of an anomalous isotope shift in hexagonal ice, representing itself as an increase in the lattice volume when H is replaced by D, by calculating free energy within the quasiharmonic approximation, with \textit{ab initio} density functional theory [1]. In this work, we extend our studies to analyze the zero point effect in other ice-like structures under different densities: clathrate hydrates, LDL and HDL-like amorphous ices with different densities, and a highly dense ice phase, ice VIII. We show that there is a transition from anomalous isotope effect to normal isotope effect as the density increases. We also analyze nuclear zero point effects in liquid water using different vdW-DFs and make connections to this anomalous-normal isotope effect transition in ice. [1] B. Pamuk \textit{et. al}, Phys. Rev. Lett. \textbf{108}, 193003 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F31.00011: Local order of liquid water at the electrochemical interface Marivi Fernandez Serra, Luana Pedroza Understanding the aqueous electrochemical interface in an atomic level is of fundamental importance in many areas, such as catalysis and materials science. In this work we analyze in detail the structural, dynamic and energetic properties of liquid-water interacting with (111) Pd and Au surfaces at ambient temperature, using first principles molecular dynamics, with and without van der Waals interactions. We show that, contrary to what was found when studying ice-like water layers, van der Waals interactions play a critical role in modeling the aqueous/electrode interface. We show the differences in the ordering of water at the interface for Pd and Au, and we explain the change in work functions of these two metals in aqueous solution. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F31.00012: 5-Methylation of Cytosine in CG:CG Base-Pair Steps: A Physicochemical Mechanism for the Epigenetic Control of DNA Nanomechanics Tahir Yusufaly, Wilma Olson, Yun Li Van der Waals density functional theory is integrated with analysis of a non-redundant set of protein-DNA crystal structures from the Nucleic Acid Database to study the stacking energetics of CG:CG base-pair steps, specifically the role of cytosine 5-methylation. Principal component analysis of the steps reveals the dominant collective motions to correspond to a tensile ``opening'' mode and two shear ``sliding'' and ``tearing'' modes in the orthogonal plane. The stacking interactions of the methyl groups are observed to globally inhibit CG:CG step overtwisting while simultaneously softening the modes locally via potential energy modulations that create metastable states. The results have implications for the epigenetic control of DNA mechanics. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F31.00013: Validation Challenge of Density-Functional Theory for Peptides: Example of Ac-Phe-Ala$_5$-LysH$^+$ Volker Blum, Mariana Rossi, Sucismita Chutia, Matthias Scheffler We assess the performance of a group of exchange-correlation functionals for predicting the secondary structure of peptide chains, up to a new many-body dispersion corrected hybrid density functional, coined PBE0+MBD*. For the purpose of validation, we first compare to published, high-level CCSD(T) benchmark conformational energy hierarchies for 73 conformers of small three-residue peptides, establishing that the van der Waals corrected PBE0 functional yields an average error of only $\approx$ 20 meV ($\approx$ 0.5 kcal/mol). This compares to $\approx$ 40-50~meV for non-dispersion corrected PBE0 and 50-100~meV for different empirical force fields. For longer peptide chains that form secondary structure, CCSD(T) level benchmark data are currently unaffordable. We thus turn to the experimentally well studied Ac-Phe-Ala$_5$-LysH$^+$ peptide, for which four closely competing conformers were experimentally established. For comparison, an exhaustive conformational space exploration yields at least eleven competing low energy minima. We show that (i) the many-body dispersion correction, (ii) the hybrid functional nature of PBE0+MBD*, and (iii) zero-point corrections are needed to reveal the four experimentally observed structures as the minima that would be populated at low temperature. [Preview Abstract] |
Session F32: Invited Session: Quantum Communication and Cryptography
Sponsoring Units: GQI DAMOPChair: Jane Nordholt, Los Alamos National Laboratory
Room: 708-712
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F32.00001: Network-Centric Quantum Communications Invited Speaker: Richard Hughes Single-photon quantum communications (QC) offers ``future-proof'' cryptographic security rooted in the laws of physics. Today's quantum-secured communications cannot be compromised by unanticipated future technological advances. But to date, QC has only existed in point-to-point instantiations that have limited ability to address the cyber security challenges of our increasingly networked world. In my talk I will describe a fundamentally new paradigm of network-centric quantum communications (NQC) that leverages the network to bring scalable, QC-based security to user groups that may have no direct user-to-user QC connectivity. With QC links only between each of N users and a trusted network node, NQC brings quantum security to N$^{\mathrm{2}}$ user pairs, and to multi-user groups. I will describe a novel integrated photonics quantum smartcard (``QKarD'') and its operation in a multi-node NQC test bed. The QKarDs are used to implement the quantum cryptographic protocols of quantum identification, quantum key distribution and quantum secret splitting. I will explain how these cryptographic primitives are used to provide key management for encryption, authentication, and non-repudiation for user-to-user communications. My talk will conclude with a description of a recent demonstration that QC can meet both the security and quality-of-service (latency) requirements for electric grid control commands and data. These requirements cannot be met simultaneously with present-day cryptography. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F32.00002: Memory-assisted measurement-device-independent quantum key distribution Invited Speaker: Norbert Lutkenhaus |
Tuesday, March 4, 2014 9:12AM - 9:48AM |
F32.00003: Spectrally Multiplexed Solid-State Memories for Quantum Repeaters Invited Speaker: Neil Sinclair Quantum communication is currently limited to channel lengths on the order of 100 km. The possibility to overcome this limit hinges on using a quantum repeater that in turn relies on the heralded distribution of entangled photons across subsections of the whole communication channel, on the storage of entanglement (by quantum memories) at the end-nodes of each subsection, and on the swapping of entanglement established in neighbouring sections to the end-points of the total channel. Workable distribution rates can be obtained if multiplexing is used to overcome the low success probabilities of multi-photon operations required in such a repeater. To this end, quantum memory research has focused on photons arriving at different times at the memory (i.e. temporal multiplexing) and recall on demand via a variable storage time. However, quantum repeaters multiplexed with respect to other degrees-of-freedom, such as frequency (spectral multiplexing), can be utilized with memories having fixed storage times, supplemented with on-demand shifting in the degree-of-freedom of importance. In this talk we describe how to build a quantum repeater using qubits encoded into different spectral modes, and present experimental results showing readout on demand from a spectrally multiplexed quantum memory based on atomic frequency combs in a Ti:Tm:LiNbO$_3$ waveguide cooled to 3 K. Our measured fidelity of 0.95 $\pm$ 0.03 significantly violates the maximum fidelity of 0.67 achievable using a classical memory, confirming the validity of the spectral multiplexing approach. We anticipate that this will accelerate the development of quantum repeaters, linear optics quantum computing, and advanced quantum optics experiments. \\[4pt] In collaboration with E. Saglamyurek, H. Mallahzadeh, J.A. Slater, M.P. Hedges, D. Oblak, C. Simon and W. Tittel, Institute of Science and Technology, and Department of Physics \& Astronomy, University of Calgary; and M. George, R. Ricken and W. Sohler, Department of Physics -- Applied Physics, University of Paderborn, Germany. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:24AM |
F32.00004: Probabilistic protocols in quantum information science: Use and abuse Invited Speaker: Carlton Caves Protocols in quantum information science often succeed with less than unit probability, but nonetheless perform useful tasks because success occurs often enough to make tolerable the overhead from having to perform the protocol several times. Any probabilistic protocol must be analyzed from the perspective of the resources required to make the protocol succeed. I present results from analyses of two probabilistic protocols: (i) nondeterministic (or immaculate) linear amplification, in which an input coherent state is amplified some of the time to a larger-amplitude coherent state, and (ii) probabilistic quantum metrology, in which one attempts to improve estimation of a parameter (or parameters) by post-selecting on a particular outcome. The analysis indicates that there is little to be gained from probabilistic protocols in these two situations. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 11:00AM |
F32.00005: Toward Noiseless Amplification and Frequency Conversion Invited Speaker: C.J. McKinstrie In this talk, I will review recent progress toward noiseless amplification and frequency conversion by four-wave mixing (FWM) processes in optical fibers. In these processes, one or two strong pump waves drive weak signal and idler waves (or photon wavepackets). Depending on the relative frequencies of the waves, FWM can amplify the signal (without frequency conversion) or frequency convert it (with or without amplification). These functions enable a variety of applications. Amplification with a noise figure of 1 dB (close to the quantum limit of 0 dB) has been demonstrated. So also has the frequency conversion of single photons. I will review these results in the contexts of conventional communication systems and quantum information science. A theme of current research is the encoding of information in different temporal eigenfunctions of arbitrary single-photon wavepackets. FWM driven by pulsed pumps provides the means to detect and manipulate information encoded in these eigenfunctions. [Preview Abstract] |
Session F34: Focus Session: Thermoelectrics - Novel Approaches
Sponsoring Units: GERA DMPChair: Alex Zevalkink, Jet Propulsion Laboratory
Room: 704
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F34.00001: Profiling the Local Seebeck Coefficient of InAs-GaAs Quantum Dots Using Scanning Thermoelectric Microscopy Yen-Hsiang Lin, Jenna Walrath, Simon Huang, Rachel Goldman Thermoelectric (TE) devices offer a method of recovering waste heat through solid state conversion of heat to electricity. However, the typical efficiencies of TE devices are 5-10{\%} which constitutes a barrier to wide spread use. There have recently been a number of reports of an increase in the bulk thermopower due to nanostructuring. In addition to our recent report of enhanced thermopower for GaAs embedded with indium nanocrystals [1], a theoretical study by Mahan and Sofo suggested that the best thermoelectric materials have a delta function density of states [2]. Quantum dots fit ideally into such a picture. To date, the influence of nanostructuring on the electronic LDOS and thermopower has been studied using spatially averaged measurements; a nanoscale investigation of the effects of nanostructures on thermopower has yet to be presented. To investigate the link between dimensionality and TE properties, we are examining structures ranging from QDs to bulk-like layers, comparing SThEM measurements of the local Seebeck coefficient, S, with STS measurements of the local density of states (LDOS). STM, STS, and SThEM performed on InAs quantum dots (QDs) grown on GaAs. SThEM reveals enhanced S-values near the QD edge; STS reveals band-bending at the QD/GaAs interface, suggesting that the S enhancement is due to interfacial charge accumulation. \\[4pt] [1] M. V. Warren, et. al, J. Appl. Phys. 114, 043704 (2013).\\[0pt] [2] Mahan and Sofo. Proc. Natl. Acad. Sci. USA 93:7436(1996). [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F34.00002: Experimental determination of the valence band of Bi$_2$Se$_3$ Yi-Bin Gao, Bin He, Ioannis Androulakis, Joseph P. Heremans P-type Bi$_2$Se$_3$ is predicted theoretically to have good thermoelectric properties[1], because its valence band has a high calculated density of states (DOS). In this presentation, p-type Bi$_2$Se$_3$ samples are prepared both as single crystals and as polycrystals. Shubnikov - de Haas (SdH) measurements are carried out in a rotating stage on single crystals to obtain the Fermi surface cross-sections and the cyclotron effective masses. Thermoelectric transport measurements are done on polycrystals, and used to construct Pisarenko plots of Seebeck coefficient versus hole concentration. The Fermi surface cross-section measurements confirm the theoretically predicted [1] shape of the Fermi surface.Both cyclotron masses and Pisarenko plotsare in good agreement and show that p-type Bi$_2$Se$_3$ has a hole effective mass smaller than the theoretically predicted value. The reason for the discrepancy is not yet understood at this time. Reference: [1] Phys. Rev. X 1, 021005 (2011) [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F34.00003: Correlation between defect transition levels and thermoelectric operational temperature of doped CrSi$_{2}$ Abhishek Singh, Tribhuwan Pandey The performance of a thermoelectric material is quantified by figure of merit ZT. The challenge in achieving high ZT value requires simultaneously high thermopower, high electrical conductivity and low thermal conductivity at optimal carrier concentration. So far doping is the most versatile approach used for modifying thermoelectric properties. Previous studies have shown that doping can significantly improve the thermoelectric performance, however the tuning the operating temperature of a thermoelectric device is a main issue. Using first principles density functional theory, we report for CrSi$_{2}$, a linear relationship between thermodynamic charge state transition levels of defects and temperature at which thermopower peaks. We show for doped CrSi$_{2}$ that the peak of thermopower occurs at the temperature T$_{m}$, which corresponds to the position of defect transition level. Therefore, by modifying the defect transition level, a thermoelectric material with a given operational temperature can be designed. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F34.00004: First-principles study of thermoelectric properties of pyrite Yi Xia, Fei Zhou, Vidvuds Ozolins Due to its natural abundance, moderate band gap and good light absorption properties, pyrite (FeS$_2$) is being considered for use in nanocrystalline solar cells. High-quality n-type samples show high electron mobility, but their adoption in solar cells is hampered by low open circuit voltages. Here, using density-functional theory (DFT), we study charge and thermal transport properties of FeS$_2$. Using the Debye-Callaway model, we obtain lattice thermal conductivity in good agreement with experimental data, suggesting that significant reduction of lattice thermal conductivity would be needed for thermoelectric applications. In addition, we find that holes in p-type pyrite form localized small polaron states, which naturally explains low hole mobilities observed experimentally. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F34.00005: Efficient simulation of quasi-ballistic heat transport in nanostructured materials Giuseppe Romano, Jeffrey Grossman Modeling nanoscale heat transport is challenging because of the presence of phonon size effects, which cannot be captured by Fourier's law. Furthermore, accurate phonon transport calculations require the knowledge of phonon dispersion curves and scattering times, which are unknown for most promising thermoelectric materials. We introduce a model based on the Boltzmann Transport Equation that computes heat transport in nanostructured materials by only using the bulk thermal accumulation function, which is a material property that can be directly obtained by experiments. Furthermore, our model is computationally convenient compared with other frequency-dependent approaches. We apply this method to nanoporous Silicon and find good agreement with experiments. The presented method could be useful in the design of high-efficiency thermoelectric materials. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F34.00006: Experimental and Theoretical Studies of Thermoelectric Properties of Manganese (IV) Oxide Particles as a Function of Electrical Resistance Morgan Hedden, Nicholas Francis, Jason Haraldsen, Costel Constantin Thermoelectric (TE) materials show great promise for converting waste heat energy into electricity. TE systems have many unique advantages such as silent operation, time reliability, and dimensional scalability. Recently, researchers have found that MnO$_{\mathrm{2\thinspace }}$nanoparticles show a giant Seebeck coefficient of S $=$ 20 mV/K, which is 100 times higher than that of bismuth telluride-one of the best TE materials. However, no figure-of-merit measurements (ZT) have been reported so far. In this project, we present preliminary results of ZT, Seebeck coefficient, thermal and electrical conductivities as a function of particle electrical resistance in the range of 10-80 $\Omega $ for particle sizes in the range of 5 nm -- 150 $\mu $m. ZT values ranged between 0.12-0.18. The samples with the smallest particle size show the greatest promise for further increasing the ZT. For comparison to experiment, we also present density functional theory results for conductance and other transport properties. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F34.00007: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F34.00008: Thermoelectric properties of electron-doped SrTiO3 thin films Elias Ferreiro-Vila, Alexandros Sarantopoulos, Victor Leboran, Cong-Tinh Bui, Francisco Rivadulla Two dimensional conductors are expected to show an improved thermoelectric performance due the positive effect of quantum confinement on the thermoelectric power, and the decrease of thermal conductivity by interface boundary scattering. The recent report of a large increase of the thermoelectric power in quantum wells of Nb-doped SrTiO3 (STO) seems to be in agreement with this hypothesis. However, extrinsic effects like the existence of oxygen vacancies that propagate away from the interface cannot be ruled out, and the results are far from clear. Here we will show the thermoelectric properties (electrical conductivity, Seebeck coefficient, and Hall effect), of epitaxial thin-films of (La,Nb)-doped STO. The films have been deposited by PLD on different substrates (STO, LAO...) to study the effect of tensile/compressive stress on the thermoelectric properties of the system. The oxygen pressure during the deposition was carefully controlled to tune the amount of oxygen vacancies and to compare with the cation doping. We have performed a systematic study of the transport properties as a function of thickness and doping, which along with the effect of stress, allows to understand the effect of charge density and dimensionality in an oxide system with promising thermoelectric properties. [1] H. Ohta et al. Nat. Mat. \textbf{6}, (2007) 129. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F34.00009: Large transverse thermoelectric effects in single crystals of the quasi-one-dimensional metal Li$_{0.9}$Mo$_6$O$_{17}$ Saeed Moshfeghyeganeh, Joshua Cohn, Carlos A.M. dos Santos, John J. Neumeier We present measurements of transverse thermoelectric (TE) effects in the temperature range 300-500 K for single crystals of the quasi-one-dimensional (q1D) metal Li$_{0.9}$Mo$_6$O$_{17}$ (lithium purple bronze). Prior work demonstrates a highly anisotropic Seebeck coefficient (S), with metallic $n$-type behavior along the q1D chains (crystallographic \emph{b} axis), $p$-type semiconductor behavior in the perpendicular, inter-chain direction (\emph{c} axis), and a difference $\Delta S\simeq 200 \mu$V/K near $T=450$~K. Significant transverse TE voltages, induced by applied temperature differences, and Peltier cooling, induced by applied currents, in specimens with body axes misaligned with the \emph{b} and \emph{c} axes will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F34.00010: Photon-assisted thermoelectric properties of noncollinear spin-valves Xiaobin Chen, Dongping Liu, Wenhui Duan, Hong Guo We report theoretical analysis of thermal-spin and thermoelectric properties of noncollinear spin-valves driven by a high frequency AC voltage bias. The spin-valve consists of two ferromagnetic contacts sandwiching a single-level or multi-level quantum dot (QD). A general formulation for the time-averaged thermal-spin and thermoelectric properties of spin-valves is derived within the nonequilibrium Green's function theory, which provides a starting point for further numerical calculations of these properties. Numerical results of a spin-valve having a spin-degenerate single-level QD are given as an example. The AC bias induces various photon-assisted transmission peaks which can greatly enhance the Seebeck coefficients and the figures of merit, and offer a new possibility to tune both the spin-dependent and normal thermoelectric properties of the spin-valve. Details of these properties and how they depend on the non-collinearity of the spin-valve, magnetic polarization, temperature, AC bias, and other control parameters are reported. A particularly interesting result is the opposite dependency of the thermoelectric properties on the magnetic polarization and non-collinearity for contacts with or without spin accumulation. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F34.00011: Extreme Seebeck anisotropy in the quasi-one-dimensional metal, Li$_{0.9}$Mo$_6$O$_{17}$ Joshua Cohn, Saeed Moshfeghyeganeh, Carlos A.M. dos Santos, John J. Neumeier We present resistivity and thermopower measurements in the range $300~K\leq T\leq 500$~K on single crystals of the quasi-one-dimensional (q1D) metal, Li$_{0.9}$Mo$_6$O$_{17}$ (LiPB) transverse to the q1D metallic chains. Direct electron transfer between the chains of this material is sufficiently weak that inter-chain transport above 400 K is predominated by thermal activation of valence-band states ($\sim 0.14$~eV below $E_F$), yielding a large, {\emph p}-type inter-chain Seebeck coefficient that coexists with {\emph n}-type metallic behavior confined along the q1D chains. A significant Seebeck anisotropy, $\Delta S\simeq 200\ \mu$V/K, along mutually perpendicular directions gives LiPB potential as a transverse thermoelectric. This anisotropy along with a relatively low inter-chain thermal conductivity $(\kappa\simeq 2$W/mK) results in a substantial transverse Peltier effect. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F34.00012: Photo-Seebeck effect of ZnO single crystals and thin films Ryuji Okazaki, Ayaka Horikawa, Masaru Fujita, Hiroki Taniguchi, Ichiro Terasaki, Hiromichi Ohta We have investigated the thermoelectric properties of ZnO single crystals and thin films under illumination. In both samples, the electrical conductivity and the Seebeck coefficient are varied under ultraviolet light ($h\nu$ = 3.4 eV) while a negligible change is observed under visible green light ($h\nu$ = 2.4 eV), indicating a carrier excitation across the band gap of ZnO ($E_g\sim$ 3.3 eV) by the ultraviolet illumination. This phenomenon thus can be ascribed to a photo doping effect into thermoelectric materials [1]. The carrier concentration doped by illumination is estimated to be about 10$^{19}$ cm$^{-3}$, which is close to the optimal value for conventional thermoelectrics, suggesting a possible optical control of the thermoelectric efficiency. We also investigate the sample thickness dependence of the photo-Seebeck effect in ZnO thin films, whose thickness is comparable to the absorption length of ultraviolet light. These results are compared with the bulk sample results in terms of a parallel-circuit model consisting of photo-excited metallic and unexcited insulating layers. \\[4pt] [1] R. Okazaki {\it et al}., J. Phys. Soc. Jpn. {\bf 81}, 114722 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F34.00013: Infrared power generation in an insulated environment Yosyp Schwab, Harkirat Mann, Brian Lang, Jarrett Lancaster, Ronald Parise, Anita Vincent-Johnson, Giovanna Scarel Alternative energy sources are an increasingly popular field of research, in particular energy harvesting through solar radiation, focusing on infrared (IR) radiation. Exploitation of readily available thermal energy is particularly interesting due to possible widespread applications. This work examines the behavior of thermoelectric devices exposed to infrared radiation in a controlled environment. Thermoelectric power generators work according to the Seebeck effect, where the temperature difference $\Delta $V induced between the two junctions is linearly proportional to the voltage difference $\Delta $T across the two contacts. Our experimental results show [1] that heat and radiation do not activate the same mechanisms in the thermoelectric power generator. Analysis and simulation further support the distinction between the IR and heat power generation. In particular, $\Delta $T and $\Delta $V have a linear and exponential behavior versus time with heat and IR, respectively. Our work is of significant importance for designing IR sensors, detectors, and radiation harvesting devices. \\[4pt] [1] Y. Schwab, H. S. Mann, B. N. Lang, J. L. Lancaster, R. J. Parise, A. J. Vincent-Johnson, and G. Scarel, ``Infrared power generation in an insulated compartment,'' Complexity, in press (2013). [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F34.00014: Modulation of thermopower in heater integrated field-effect molecular devices Youngsang Kim, Wonho Jeong, Kyeongtae Kim, Woochul Lee, Pramod Reddy Study of thermopower in molecular junctions is of great importance for understanding charge transport mechanisms as well as for making efficient energy conversion devices. In order to achieve a large thermoelectric efficiency (figure of merit, \textit{ZT}), it is crucial to simultaneously increase the electrical conductance and Seebeck coefficient of junctions. The electrical conductance of molecular junctions is directly proportional to the transmission ($T(E))$ at the Fermi level ($E_{F})$, while the Seebeck coefficient is proportional to the energy derivative of $T(E)$ at $E_{F}$. In this study, we successfully fabricated electromigrated break junction with integrated heater devices to establish temperature differentials across molecular junctions and study the possibility of tuning the electronic structure to simultaneously increase $T(E)$ and its energy derivative at $E_{F}$. Further, using this platform, we studied the thermopower and the I-V characteristics of molecular junctions while modulating the electronic structure using a gate electrode. Our results unambiguously show that the thermopower and the electrical conductance of molecular junctions can be simultaneously enhanced by tuning the electronic structure. These results could pave the way for both understanding energy conversion at the molecular scale and the development of novel thermoelectric devices. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F34.00015: Manipulating thermal conductance across 3D/1D interface by impedance matching Jingjie Zhang, Carlos Polanco, Avik Ghosh Self-assembled monolayers (SAMs) are of special interest to nano-electronic and thermal devices, because we can tune their properties by changing the bonding strength that links the SAMs to a thin film layer. We explain how this bonding strength influence heat across this 3D-1D interfaces based on a frequency dependent broadening matrix that acts as a generalization of acoustic impedance. We demonstrate both how to build an equivalent ``impedance'' broadening matrix that captures the dimensionality mismatch at the 3D-1D transition and the ``matching'' effect of the end group on an equivalent 1D-1D interface. We calculate thermal boundary conductance (TBC) at metal/polymer interfaces with different terminal groups and polymers. The calculations are done with non-equilibrium Green's function formalism coupled with ab-initio parameters for the chemical group functionalized systems. Our results confirm that in the low frequency spectrum, the stronger the bonding the larger the TBC. Nevertheless, when we consider the whole phonon spectrum, there is a sweet spot in the bonding strength that maximizes TBC. [Preview Abstract] |
Session F35: Focus Session: Quantum Computing Architectures and Algorithms: Quantum Control
Sponsoring Units: GQIRoom: 702
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F35.00001: A Fault-Tolerant Scheme of Holonomic Quantum Computation on Stabilizer Codes with Robustness to Low-weight Thermal Noise Yicong Zheng, Todd Brun We show an equivalence relation between fault-tolerant circuits for a stabilizer code and fault-tolerant adiabatic processes for holonomic quantum computation (HQC), in the case where quantum information is encoded in the degenerated ground space of the system Hamiltonian. By this equivalence, we can systematically construct a fault-tolerant HQC scheme, which can geometrically implement a universal set of encoded quantum gates by adiabatically deforming the system Hamiltonian. During this process, quantum information is protected from thermal excitation by an energy gap that does not change with the problem size. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F35.00002: Martin-Siggia-Rose approach to quantum error correction in the presence of time-dependent noise Rafael Hipolito, Paul Goldbart We consider the basic task of obtaining a target unitary operation (quantum gate) via external control fields coupled to a quantum system, while compensating for time-dependent noise. We address this problem by means of a formulation rooted in the MSR approach to noisy, classical, nonequilibrium systems. We express the noisy control problem as a path integral, and integrate out the noise to arrive at an effective noise-free description. To illustrate the approach, we consider a single spin-$s$ degree of freedom (with $s$ arbitrary) in the presence of Gaussian time-dependent noise, though our approach can be generalized to more complicated systems and noise distributions. Success is characterized via a ``fidelity,'' measuring the overlap between the ideal noise-free evolution and the noisy one. To make connection with MSR, we reformulate the fidelity in terms of a Schwinger-Keldysh path integral, with an added twist: ``forward'' and ``backward'' branches of the contour are inequivalent with respect to noise. We explore the effective description, and show how to evaluate the path integral to arbitrary order in noise strength. Our approach naturally treats the problem for arbitrary $s$ under a unified protocol, valid from the qbit limit ($s=1/2$) to the classical limit ($s \to \infty$). [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F35.00003: A recursive construction of noiseless subsystem for qudits Utkan G\"ung\"ord\"u, Chi-Kwong Li, Mikio Nakahara, Yiu-Tung Poon, Nung-Sing Sze The noiseless subsystem is a method of using the inherent permutation symmetry of the noise to protect a subsystem against errors. Its construction becomes a formidable task with the growing number of qudits. In this work, we describe a recursive way of constructing noiseless subsystem for qudits, that is robust against collective noise of the form $W^{\otimes n}$, where $n$ is the number of qudits and $W$ is the Kraus operator acting on a single site. This kind of error appears when the wavelength of an environmental disturbance is much larger than the size of the quantum system, which makes it natural to assume all the qubits in the register suffer from the same error operator. The presented recursive scheme is a direct generalization of the recursive scheme described in \emph{Phys. Rev. A, {\bf 84}, (2011) 044301} for qubits. We show that the quantum error correction rate, i.e., the ratio of correctable qudits and the number of transmitted qudits, approaches $1/d$ as $n$ goes to infinity in this recursive scheme. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F35.00004: Gap protection and dynamical decoupling for reliable multi-qubit gates Invited Speaker: Wayne Witzel We propose a scheme for producing multi-qubit gates by adiabatically shuttling an electron between donors in silicon to produce operations that are diagonal in the computational basis. Exploiting the commutation of these diagonal operations, we can use single-qubit refocusing gates to cancel the sensitivity to low-frequency noise and details of the shuttling. This strategy of cancelling unwanted portions of an adiabatic process to build up robust multi-qubit operations could be applied to other systems. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F35.00005: Robustness of composite pulse sequences to time- dependent noise Chingiz Kabytayev, Todd J. Green, Kaveh Khodjasteh, Lorenza Viola, Michael J. Biercuk, Kenneth R. Brown Quantum control protocols can minimize the effect of noise sources that reduce the quality of quantum operations. Originally developed for NMR, composite pulse sequences correct for unknown static control errors \footnote{True J. Merrill and Kenneth R. Brown. arXiv:1203.6392v1. In press Adv. Chem. Phys. (2013)}. We study these compensating pulses in the general case of time-varying Gaussian control noise using a filter-function approach \footnote{T. J. Green et al. New J. Phys. 15 095004 (2013)} and detailed numerics. Three different noise models were considered in this work: amplitude noise, detuning noise and simultaneous presence of both noises. Pulse sequences are shown to be robust to noise up to frequencies as high as $\sim$10\% of the Rabi frequency. Robustness of pulses designed for amplitude noise is explained using a geometric picture that naturally follows from filter function. We also discuss future directions including new pulses correcting for noise of certain frequency. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F35.00006: Optimal arbitrarily accurate composite pulse sequences Guang Hao Low, Theodore Yoder Implementing a single qubit unitary is often hampered by imperfect control. Systematic amplitude errors $\epsilon$, caused by incorrect duration or strength of a pulse, are an especially common problem. But a sequence of imperfect pulses can provide a better implementation of a desired operation, as compared to a single primitive pulse. We find optimal pulse sequences consisting of $L$ primitive $\pi$ or $2\pi$ rotations that suppress such errors to arbitrary order $\mathcal{O}(\epsilon^{n})$ on arbitrary initial states. Optimality is demonstrated by proving an $L=\mathcal{O}(n)$ lower bound and saturating it with $L=2n$ solutions. Closed-form solutions for arbitrary rotation angles are given for $n=1,2,3,4$. Perturbative solutions for any $n$ are proven for small angles, while arbitrary angle solutions are obtained by analytic continuation up to $n=12$. The derivation proceeds by a novel algebraic and non-recursive approach, in which finding amplitude error correcting sequences can be reduced to solving polynomial equations. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F35.00007: Dynamically Corrected Gates for Qubits with Always-on Ising Interactions: Error model and fault-tolerance Amrit De, Leonid Pryadko We prescribe a method to implement a universal set of dynamically-corrected quantum gates on any qubit network that forms a sparse bipartite graph using sequences of decoupling pulses. The qubit networks have Ising interactions that are always turned on and our method works to selectively decouple the interactions even when they differ. We study the error operators associated with the constructed gates for small qubit clusters and give bounds on high-order errors. We find that the present gate set can be used to achieve fault-tolerance with a concatenated code by choosing a suitable qubit network. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F35.00008: Qubits with always-on couplings and gates based on decoupling pulse sequences: fault tolerance with quantum LDPC codes Kathleen Hamilton, Alexey Kovalev, Amrit De, Leonid Pryadko Universal gate sets based on decoupling pulse sequences can be efficiently constructed to a given order of the Magnus series by working with small qubit clusters. However, the most likely systematic errors of such gates typically involve few qubits, with a possibility of run-away large error cluster formation when scaled to large systems. We analyze the existence of a fault-tolerant decoding threshold when such gate sets are used with a quantum low-density parity check (LDPC) code. In particular, we show that the existence of such a threshold when the code is used for quantum memory can be related to the existence of a finite percolation transition between random clusters on a graph associated with the code. The results also apply to other systems where gates are constructed perturbatively, e.g., by tuning qubits in and out of resonance. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F35.00009: Precision Quantum Control with Trapped $^{171}$Yb$^{+}$ Ions Alexander Soare, David Hayes, James McLoughlin, Xinglong Zhen, Michael Lee, M.C. Jarrat, Harrison Ball, Todd Green, Michael Biercuk We present our recent work in developing and characterizing novel methods for quantum error suppression using trapped~$^{171}$Yb$^{+}$~ions as a model experimental platform. A flexible, robust microwave system allows us to access the 12.6 GHz, hyperfine qubit manifold in trapped~$^{171}$Yb$^{+}$. The ultra low phase noise characteristics of our source allow the realization of free-evolution coherence times in excess of three seconds, and operational fidelities F\textgreater 99.99{\%}, characterized by randomized benchmarking. Starting from this baseline, we leverage high-bandwidth vector modulation capabilities to experimentally validate our recent theoretical work developing Walsh-modulated control operations for error-resilient single-qubit control in the presence of synthesized noise. ~This theory is based on a generalized filter-transfer-function formalism useful for predicting the fidelity of arbitrary operations in the presence of general Gaussian noise. We provide the first experimental validation of this formalism, showing good agreement between experimental measurements and theoretical predictions with no free parameters. ~These demonstrations support the notion of physical-layer error evasion as an efficient means to realize high-fidelity quantum control across a wide range of quantum technologies. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F35.00010: Improving Quantum Gate Performance using Optimal Control with Feedback Yuchen Peng, Frank Gaitan We present a procedure for improving the performance of a quantum gate based on optimal control theory with feedback. Starting with a quantum gate $U_{0} $ produced by a known control field ${\rm {\bf F}}_{0} (t)$ that provides a good approximation to a target gate $U_{t} $, we show how optimal control theory with feedback can be used to determine a modified control ${\rm {\bf F}}(t)={\rm {\bf F}}_{0} (t)+\Delta {\rm {\bf F}}(t)$ which yields a quantum gate $U$ that better approximates the target gate $U_{t} $. We illustrate the procedure by applying it to the gates in a universal set of quantum gates produced using non-adiabatic rapid passage [1]. We first examine the performance improvements produced with ideal controls, and then examine the robustness of these improvements in the presence of control field imperfections such as finite bandwidth, finite precision control parameters, and phase jitter. We find that this procedure reduces the gate error probability $P_{e} $ by 1-4 orders of magnitude even in the presence of control imperfections ($P_{e} \sim 10^{-4}$ improved to $10^{-8} < P_{e} < 10^{-5})$. \\[4pt] [1] R. Li and F. Gaitan, \textit{J. Mod. Opt.}~\textbf{58}, 1922 (2011). [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F35.00011: Near-optimal measurement-based feedback control for a single qubit Ashkan Balouchi, Kurt Jacobs Feedback control of quantum systems via continuous measurements involves complex nonlinear dynamics. As a result, even for a single qubit the optimal measurement for feedback control is known only in very special cases. We show here that for a broad class of noise processes, a series of compelling arguments can be applied to greatly simplify the problem of steady-state preparation of the ground-state, while loosing little in the way of optimality. Using numerical optimization to solve this simplified control problem, we obtain for the first time a non-trivial feedback protocol valid for all feedback strengths in the regime of good control. The protocol can be described relatively simply, and contains a discontinuity as a function of feedback strength. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F35.00012: Quantum logic gates by Walsh modulation Harrison Ball, David Hayes, Michael J. Biercuk We study a new class of error suppressing protocols for nontrivial quantum logic gates robust against band-limited stochastic noise to high order. Our underlying mathematical framework is to generate an amplitude modulated control field via synthesis of Walsh functions (an orthonormal set of basis functions well-known in signal processing) resulting in a composite pulse sequence parameterized in the amplitudes of the Walsh spectral components. In this work we show how one Walsh amplitude may be constrained to generate a target Bloch rotation while the remainder may be fine-tuned to optimize the decoupling power of the sequence. We use the filter function formalism to quantify the decoupling power and to derive a decoupling condition which enables us to prescribe an optimization procedure, searching over Walsh spectral weights. With these insights we characterize the robustness of a generalized family of rotary spin echo sequences against both dephasing noise and relaxation noise coaxial with control. We further derive a family of nontrivial, bounded, amplitude modulated gates decoupled to first order against dephasing noise, and describe a method to discover similar families of higher order protocols intrinsically compatible with control hardware and digital control circuitry. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F35.00013: Robust quantum channel for optical coherent-state qubits under environment noise Shin-Tza Wu, Ming-Jay Yang We study the non-Markovian dynamics of optical qubits encoded via coherent states with opposite phases which are exposed to environment noises. Making use of a coherent-state path integral formulation, we are able to study non-perturbatively the dynamics of the coherent-state qubits under strong environment coupling. We apply this formulation to examine the time evolution of a noisy quantum channel formed by two coherent-state qubits that are subject to uncorrelated local environment noises. In particular, we examine the time evolution of entanglement and maximal teleportation fidelity of the noisy quantum channel and show that at strong coupling, due to large feedbacks from the environment noise, it is possible to maintain a robust quantum channel in the long-time limit if appropriate error-correcting code is applied. [Preview Abstract] |
Session F36: Focus Session: Semiconductor Qubits: Impurities & Quantum Devices
Sponsoring Units: GQIChair: Sven Rogge, University of New South Wales
Room: 703
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F36.00001: Single-spin quantum coherence beyond 10 seconds in an isotopically engineered silicon nanostructure Andrea Morello, Juha Muhonen, Juan Pablo Dehollain, Arne Laucht, Fay Hudson, Kohei Itoh, David Jamieson, Jeffrey McCallum, Andrew Dzurak The single-shot readout and coherent control of both the electron and the nuclear spin of a single P atom in silicon has been recently demonstrated, using ion-implanted donors in MOS nanostructures. It is known from bulk experiments that P donors in isotopically purified $^{28}$Si exhibit record coherences, but it is also suspected that the proximity to a Si/SiO$_2$ interface will deteriorate the coherence time. Here we present the first experiment on single electron and nuclear spin qubits in an isotopically engineered $^{28}$Si nanostructure. We measured exceptionally sharp electron spin resonance lines ($< 2$ kHz wide), and we obtained single-qubit control fidelities in excess of $99 \%$. We performed noise spectroscopy experiments to extract the power spectral density of the decoherence sources acting on the electron and the nucleus. Contrary to widespread belief, our data show that the ultimate limit for single-spin coherence in our nanostructure is not set by charge noise and interface effects, but simply by broadband thermal radiation coupled to the qubit through a high-bandwidth transmission line. Using dynamical decoupling, we measured coherence times up to $T_{2e} = 0.5$ s for the electron, and $T_{2n} = 18$ s for the $^{31}$P nucleus. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F36.00002: Nuclear spin coherence of neutral $^{31}$P donors in isotopically enriched $^{28}$Si E.S. Petersen, A.M. Tyryshkin, S.A. Lyon, S. Tojo, K.M. Itoh, M.L.W. Thewalt, H. Riemann, N.V. Abrosimov, P. Becker, H.-J. Pohl In natural silicon the nuclear spin coherence of neutral $^{31}$P donors is limited to about 1 second by flip-flopping $^{29}$Si nuclear spins. Here we eliminate this process by using isotopically enriched $^{28}$Si with 50 ppm of $^{29}$Si. This allows us to examine other processes which may decohere the $^{31}$P nuclear spins. We use X-band pulsed ENDOR at 1.7 K to examine isotopically enriched Si crystals with donor concentrations from 10$^{14}$ to 4x10$^{15}$ P/cm$^{3}$ and find a dependence of $^{31}$P nuclear spin coherence time on donor concentration. The measured nuclear spin echo decays are fit by a stretched exponential function, exp(-(t/T$_{2})^{\mathrm{n}})$, with n ranging from 0.7 to 1. This differs from n of about 2 commonly seen for spectral diffusion due to indirect spin flip-flops. The measured T$_{2}$ times decrease significantly when the donor concentration increases, changing from 8 s at 10$^{14}$ to 0.2 s at 4x10$^{15}$ P/cm$^{3}$. From the observed donor concentration dependence at higher densities, we conclude that direct electron spin flip-flops are responsible for $^{31}$P donor nuclear spin decoherence. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F36.00003: Spin Measurements of an Electron Bound to a Single Phosphorous Donor in Silicon D.R. Luhman, K. Nguyen, L.A. Tracy, S.M. Carr, J. Borchardt, N.C. Bishop, G.A. Ten Eyck, T. Pluym, J. Wendt, M.S. Carroll, M.P. Lilly The spin of an electron bound to a single donor implanted in silicon is potentially useful for quantum information processing. We report on our efforts to measure and manipulate the spin of an electron bound to a single P donor in silicon. A low number of P donors are implanted using a self-aligned process into a silicon substrate in close proximity to a single-electron-transistor (SET) defined by lithographically patterned polysilicon gates. The SET is used to sense the occupancy of the electron on the donor and for spin read-out. An adjacent transmission line allows the application of microwave pulses to rotate the spin of the electron. We will present data from various experiments designed to exploit these capabilities. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE Office of Basic Energy Sciences user facility. The work was supported by Sandia National Laboratories Directed Research and Development Program. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the U. S. Department of Energy under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F36.00004: Bottom-up superconducting and Josephson junction devices and qubits inside a Group-IV semiconductor Invited Speaker: Yun-Pil Shim The Nb/AlOx/Nb (or Al/AlOx/Al) Josephson junction (JJ) has become ubiquitous for superconducting (SC) applications such as magnetometers, voltage standards, logic, and qubits. But heterogeneous devices such as these can pose problems, especially for low-power or quantum applications, where losses in or at the interfaces of the various materials can limit device quality dramatically. Possible solutions include better materials, weak-link junctions, symmetry protection, or 3D cavity qubits. Here we consider another alternative: atomically-precise, hole-doped SC silicon (or germanium) JJ devices and qubits made entirely out of the same crystal [1]. Like the Si spin qubit, our super-semi JJ devices exist inside the ``vacuum'' of ultra-pure silicon, far away from any dirty interfaces. We predict the possibility of SC wires, JJs, and qubits, calculate their critical parameters, and find that most known SC qubits should be realizable. This approach could enable better devices, hybrid superconducting-spin qubit systems, and exotic SC circuits, as well as a new physical testbed for superconductivity. \\[4pt] [1] Yun-Pil Shim and Charles Tahan, arXiv:1309.0015. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F36.00005: Electron shuttling in phosphorus donor qubit systems N. Tobias Jacobson, John King Gamble, Erik Nielsen, Richard P. Muller, Wayne M. Witzel, Ines Montano, Malcolm S. Carroll Phosphorus donors in silicon are a promising qubit architecture, due in large part to their long nuclear coherence times and the recent development of atomically precise fabrication methods. Here, we investigate issues related to implementing qubits with phosphorus donors in silicon, employing an effective mass theory that non-phenomenologically takes into account inter-valley coupling. We estimate the significant sources of decoherence and control errors in this system to compute the fidelity of primitive gates and gate timescales. We include the effects of valley repopulation during the process of shuttling an electron between a donor and nearby interface or between neighboring donors, evaluating the control requirements for ensuring adiabaticity with respect to the valley sector. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F36.00006: Transport Measurements on Si Nanostructures with Counted Sb Donors Meenakshi Singh, Edward Bielejec, Elias Garratt, Gregory Ten Eyck, Nathaniel Bishop, Joel Wendt, Dwight Luhman, Malcolm Carroll, Michael Lilly Donor based spin qubits are a promising platform for quantum computing. Single qubits using timed implant of donors have been demonstrated.$^{\mathrm{1}}$ Extending this to multiple qubits requires precise control over the placement and number of donors. Such control can be achieved by using a combination of low-energy heavy-ion implants (to reduce depth straggle), electron-beam lithography (to define position), focused ion beam (to localize implants to one lithographic site) and counting the number of implants with a single ion detector.$^{\mathrm{2}}$ We report transport measurements on MOS quantum dots implanted with 5, 10 and 20 Sb donors using the approach described above. A donor charge transition is identified by a charge offset in the transport characteristics. Correlation between the number of donors and the charge offsets is studied. These results are necessary first steps towards fabricating donor nanostructures for two qubit interactions. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. DOE Office of Basic Energy Sciences user facility. The work was supported by Sandia National Laboratories Directed Research and Development Program. Sandia National Laboratories is a multi-program laboratory operated by Sandia Corporation, a Lockheed-Martin Company, for the U. S. Department of Energy under Contract No. DE-AC04-94AL85000. $^{\mathrm{1}}$J. J. Pla et al., Nature \textbf{496}, 334 (2013) $^{\mathrm{2}}$J. A. Seamons et al., APL \textbf{93}, 043124 (2008). [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F36.00007: Phonon induced spin relaxation times of single donors and donor clusters in silicon Yuling Hsueh, Holger Buch, Lloyd Hollenberg, Michelle Simmons, Gerhard Klimeck, Rajib Rahman The phonon induced relaxation times (T1) of electron spins bound to single phosphorous (P) donors and P donor clusters in silicon is computed using the atomistic tight-binding method. The electron-phonon Hamiltonian is directly computed from the strain dependent tight-binding Hamiltonian, and the relaxation time is computed from Fermi's Golden Rule using the donor states and the electron-phonon Hamiltonian. The self-consistent Hartree method is used to compute the multi-electron wavefunctions in donor clusters. The method takes into account the full band structure of silicon including the spin-orbit interaction, and captures both valley repopulation and single valley g-factor shifts in a unified framework. The single donor relaxation rate varies proportionally to B$^{5}$, and is of the order of seconds at B$=$2T, both in good agreement with experimental single donor data (A. Morello et. al., Nature 467, 687 (2010)). T1 calculations in donor clusters show variations for different electron numbers and donor numbers and locations. The computed T1 in a 4P:5e donor cluster match well with a scanning tunneling microscope patterned P donor cluster (H. Buch et. al., Nature Communications 4, 2017 (2013)). [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F36.00008: Electronic Structure of donor pairs in Si Belita Koiller, Andre Saraiva, Maria Jose Calderon, Fernando Gonzalez-Zalba, Dominik Heiss, Andrew J. Ferguson We develop an effective mass theory for a pair of substitutional group-V donors in Si. An empirical central cell correction for a single donor, applied to energies and wavefunctions, leads to an accurate description of the $D_2$ ``molecular'' spectrum. No configuration averages simulating an ensemble of pairs are taken: our formalism applies to the single pair regime, including the A1, T and E single donor states in the hydrogenic S-like manifold and a Configuration Interaction approach to account for electron-electron correlations. We also obtain the inter donor distance dependence of experimentally accessible quantities: first ionization $(D_2^0 \to D_2^+)$, second ionization $(D_2^+ \to D_2^{++})$ energies, charging energy and singlet-triplet splitting. All results are consistent with recently performed experiments on As doped Si, suggesting that our approach is reliable down to distances $\sim$ 2 nm, and possibly smaller. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F36.00009: Quantum information processing using acceptors in silicon and phonon entanglement Susan Clark, Charles Reinke, Hayden McGuinness, Ihab El-Kady Quantum computing with large numbers of qubits remains challenging due to the decoherence and complexity that arise as more qubits are added to a system. Here I propose a new platform for semiconductor quantum computing which may be robust to common sources of decoherence and may not be difficult to fabricate repeatedly. This system consists of a hole bound to an acceptor in silicon which has been implanted in the center of a mechanical cavity (similar to a photonic crystal cavity) and connected to other cavities by a system of waveguides. I will outline a basic entangling gate and calculations showing the promise of this platform as the ideal qubit. 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] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F36.00010: A donor molecule in silicon M. Fernando Gonzalez Zalba, Dominik Heiss, Andrew J. Ferguson, Andre Saraiva, Maria J. Calderon, Belita Koiller In 1954 Kohn and Luttinger introduced the description for a single donor in silicon as a hydrogen atom analogue in a semiconductor environment. Generalizing the concept, a donor pair may behave as a hydrogen molecule. However, a detailed understanding of the electronic structure of these molecular systems is a challenge to be overcome. Here we present an experimental demonstration of the energy spectrum of a strongly interacting donor pair in the channel of a silicon nanotransistor. We show the first evidence of a simultaneous enhancement of the binding and charging energies with respect to the single donor spectrum as well as a measurable exchange coupling. The measured data can be accurately matched by an effective mass theory incorporating the Bloch states multiplicity in Si, a central cell donor corrected potential and a full configuration interaction. These results suggest a novel physical mechanism to increase the operation temperature of conventional single-atom transistors and improve their robustness against interfacial electric fields. Furthermore, the data describes the Kane basic quantum processing element in the range of molecular hybridization. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F36.00011: Qubits Based on Shallow Donor Spins in Ge Phononic Crystals Vadim Smelyanskiy, Vasyl Hafiychuck, Mark Dykman, Andre Petukhov We propose qubits based on shallow donor electron spins in germanium. Spin-orbit interaction for donor spins in germanium is in many orders of magnitude stronger than in silicon. In a uniform bulk material it leads to very short spin lifetimes. However the lifetime increases dramatically when the donor is placed into a quasi-2D phononic crystal (PHC) and the energy of the Zeeman splitting is tuned to lie within a phonon bandgap. In this situation single phonon processes are suppressed by energy conservations. The remaining two-phonon decay channel is very slow. The Zeeman splitting within the gap can be fine tuned to induce a strong coupling between the spins of remote donors via exchange of virtual phonons. The analysis immediately extends to the interaction between nuclear spins. We also show that the long-range longitudinal interaction (z-z) between localized electron spins in PHC is similar to that mediated by Lamb waves in elastic plates. We explore various shapes of PHC cells in order to maximize the coherent effects of the spin-spin coupling while keeping the decay rate minimal. We find that phononic crystals with unit cell sizes ~ 100-150 nm are viable candidates for quantum computing applications. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F36.00012: Using bound exciton transitions to optically resolve neutral donor hyperfine states of various donor species in Silicon-28 Jeff Salvail, Phillip Dluhy, Kamyar Saeedi, Michael Szech, Helge Riemann, Nikolai Abromisov, Peter Becker, Hans-Joachim Pohl, Michael Thewalt Phosphorus in silicon is established as a promising resource for use in quantum information processing tasks. The neutral donor hyperfine states have been shown to have record long coherence times, high fidelity gates via RF pulses, and projective readout via optical bound exciton transitions. As Shannon's theory of information tells us, we can process more information in an alphabet of more symbols, so there is motivation to look at donors with higher nuclear spin than the $I=1/2$ of $^{31}$P, which provide access to Hilbert spaces of dimension greater than two. In this talk I will describe optical studies of the donors $^{75}$As ($I=3/2$), $^{121}$Sb ($I=5/2$), and $^{209}$Bi ($I=9/2$) in $^{28}$Si. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F36.00013: Experimental quantum simulation using 1D LaAlO$_3$/SrTiO$_3$ nanostructures Megan Kirkendall, Patrick Irvin, Jeremy Levy, Sangwoo Ryu, Chang-Beom Eom Quantum simulation of important Hamiltonians could lead to new insights into quantum matter, for example, high-temperature superconductors. The 2DEG at the LaAlO$_3$/SrTiO$_3$ interface\footnote{A. Ohtomo and H.Y. Hwang, Nature \textbf{427}, 423 (2004)} exhibits a wide variety of phenomena including a tunable metal-insulator transition, magnetism, strong spin-orbit coupling, and superconductivity. These properties can be controlled at extreme nanoscale dimensions using a conductive-AFM writing technique\footnote{C. Cen \textit{et al.}, Nat. Mater. \textbf{7}, 298 (2008)}. Here we describe experiments in which 1D lattice structures are created at the LaAlO$_3$/SrTiO$_3$ interface and investigated using low temperature magnetotransport. These devices will allow us to modify the effective interactions between Cooper pairs and quasiparticles in the superconducting lattice, and represent an early demonstration of the potential of this solid-state quantum simulation platform. [Preview Abstract] |
Session F37: Focus Session: Functionalization and Atomic Engineering of Graphene
Sponsoring Units: DMPChair: Alexander Tzalenchuk, National Physics Laboratory, United Kingdom
Room: 705/707
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F37.00001: Atomic Scale Visualization of Dopant-Induced Unconventional Kondo Effect in Boron-doped graphene Minghu Pan, Qing Li, Liangbo Liang, Ruitao Lv, Wenzhi Lin, Eduardo Costa Gir\~ao, Andr\'es R. Botello-M\'endez, Ana Laura El\'Ias, Rodolfo Cruz-Silva, Jean Christophe Charlier, Mauricio Terrones, Vincent Meunier We describe the synthesis of large-area, highly-crystalline monolayer Boron-doped graphene (BG) sheets via atmospheric-pressure chemical vapor deposition, yielding unique and diverse B-doping site composed of substitutional Boron atoms and carbon vacancies. Scanning tunneling microscopy and spectroscopy (STM and STS) of BG reveal the presence of localized states in both the conduction and valence bands induced by Boron pz orbitals, confirmed by ab initio calculations. Furthermore, we demonstrate for the first time that atomic-resolved spin-polarization in a graphene sublattice via spectroscopic imaging of zero-energy states, induced by Boron incorporation. BG acts as a Kondo system with magnetic dopants embedded in C lattices, fully described by using the non-equilibrium Green's function method within the slave-boson mean-field approximation. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F37.00002: STM Observation of Molecular Adsorption on Graphene and Nitrogen Doped Graphene Seiji Obata, Koichiro Saiki Carbon alloy catalyst (CAC) shows catalytic activity to oxygen reduction reaction (ORR) and it is expected as a substitution of Pt in fuel cells due to its catalytic property. At present CAC are synthesized by burning organic compounds which contain nitrogen atoms such as phthalocyanine. The catalytic activity of CAC is lower than Pt. Since catalytic sites and oxygen reduction process is still unknown, elucidation of catalytic sites of CAC helps to synthesize high performance CAC. STM is a useful tool to investigate adsorption and reaction at atomic level. However, disordered structure of CAC makes it difficult to use STM for catalytic site observation. To overcome this difficulty, we synthesized nitrogen doped graphene (NG) and and pristine graphene (PG) on Pt (111) and used it as model catalyst to study the catalytic property of CAC. Oxygen adsorption is the first step of oxygen reduction reaction. Therefore we investigated the oxygen adsorption to NG and PG by STM. Oxygen adsorbed at domain boundary (DB) of NG?According to XPS measurement nitrogen atoms exist at edge site preferably. These results indicate that nitrogen atom enhances oxygen adsorption activity. In addition, actual reaction process occurs in H$_2$O. Thus we also investigated H$_2$O adsorption on NG. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F37.00003: Scanning Tunneling Microscopy and Scanning Tunneling Spectroscopy Studies of Chromium Clusters Deposited on Moir\'e Patterns on HOPG Xin Zhang, Hong Luo Moir\'e patterns (MP) formed by twisted graphene layers, present great potential for use as periodic substrates to facilitate the growth of nanostructures to obtain useful electronic and/or magnetic properties. The growth of Chromium (Cr) deposited on MPs on the surface of highly ordered pyrolitic graphite (HOPG) and its effects on the electronic structure in the MPs were studied by scanning tunneling microscopy/spectroscopy (STM/STS). Without Cr, two van Hove singularities (VHSs) were observed by STS on the MPs. With low coverage of Cr, atoms deposited on graphite Moir\'e form small clusters randomly distributed over the surface. With the presence of Cr clusters, the energy difference between the two VHS peaks enlarged while its linear dependence on the twisting angle remains. Compare to the situation before deposition, the graphite's Fermi velocity increased while the interlayer interaction decreased. The electronic structure modification caused by a Cr cluster as a function of distance from the cluster was studied with extremely low coverage. The effective distance can reach about 10 lattice cells of the Moir\'e pattern. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F37.00004: Local Probe Measurement of Atomically-Engineered Graphene Nanostructures Invited Speaker: Mike Crommie Graphene's unique electronic properties give rise to novel defect behavior at impurities and at edges. This can be seen in graphene's response to charged impurities, where graphene's ultra-relativistic nature leads to impurity states that are unlike those found in any other material. We have explored such impurity states across different impurity-charge regimes by building artificial charge centers (i.e., ``artificial nuclei'') atom-by-atom at the surface of graphene devices and probing them via scanning tunneling microscopy. New results on this topic, including the observation of ``atomic collapse'', will be discussed. The properties of graphene's edges become increasingly important when graphene is cut into nanoscale structures having sharp boundaries. While such structures are difficult to fabricate via traditional ``top-down'' lithography, new ``bottom-up'' synthesis techniques utilizing molecular self-assembly show great promise for creating flexible, atomically-engineered networks. We have recently made progress at fabricating new graphene nanostructures in this way from chemically engineered precursor molecules. New measurements on these systems will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F37.00005: First-Principles Studies of Ordered Potassium Monolayers on Graphite and Graphene Rodrigo B. Capaz, Josu\'e X. de Carvalho, Jorge L.B. Faria We investigate the structural and electronic properties of ordered monolayers of potassium adosorbates on graphite and graphene using first-principles methods based on density-functional theory, pseudopotentials and periodic supercells. Several ordered structures are investigated and their total energies are mapped onto an effective Ising-like hamiltonian. Monte-Carlo simulations using this hamiltonian are performed in order to construct the phase diagram for this system, which is then compared to experimental results on graphite surfaces. In agreement with experiments, we find that structures with potassium concentrations larger than 1/3 ($\sqrt{3}\times \sqrt{3}$) are unstable with respect to metallic potassium segregation at the surface. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F37.00006: The stability, energetics, and magnetic states of Co adsorption on graphene Yudistira Virgus, Wirawan Purwanto, Henry Krakauer, Shiwei Zhang The adsorption of transition metal adatoms on graphene has attracted significant research interest due to their possible use to induce magnetism on graphene for spintronic applications. Single Co atoms on graphene have been extensively studied both theoretically and experimentally. In our previous work, we used auxiliary-field quantum Monte Carlo (AFQMC) and a size-correction embedding scheme to calculate the binding energy of Co/graphene for the six-fold hollow site.\footnote{Y. Virgus, W. Purwanto, H. Krakauer, and S. Zhang, Phys. Rev. B, \textbf{86}, 241406(R) (2012).} Recent experimental results show that single Co atoms can be adsorbed on graphene at both the hollow and the top sites.\footnote{T. Eelbo, M. Wasniowska, M. Gyamfi, S. Forti, U. Starke, and R. Wiesendanger, Phys. Rev. B, \textbf{87}, 205443 (2013).} We use AFQMC to investigate Co/graphene for the three high-symmetry adsorption sites; six-fold hollow site, two-fold bridge site, and top site. Highly accurate binding energy curves for the three sites are obtained. The stabilities of the different magnetic states and adsorption sites will be examined and discussed in relation to the experimental observations. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F37.00007: Surface fluorination on graphene field effect transistors Xu Zhang, Han Wang, Yi Song, Allen Hsu, Jing Kong, Mildred Dresselhaus, Tomas Palacios Graphene, a zero-gap semiconductor with massless charge carriers, has attracted tremendous interest because of its outstanding electronic properties. One very special property of graphene is the fact that graphene is an all-surface material, so every atom has access to the surface, which has a direct impact on its electronic and chemical performance. Therefore, surface functionalizations provide very effective methods to engineer its electronic properties and make it even more suitable for electronic device applications. Here, we demonstrate that controlled exposure of graphene devices to XeF2 is an effective way to open a bandgap in graphene. High on/off ratio fluorograhene-based field effect transistors are fabricated and analyzed. Raman characterization is also carried out to investigate their structural changes as a result of fluorination treatment. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F37.00008: Infrared Spectroscopy of Functionalized Graphene Sheets from First Principle Calculations Cui Zhang, Daniel Dabbs, Ilhan Aksay, Roberto Car, Annabella Selloni Detailed characterization of the structure of functionalized graphene sheets (FGSs) is an important and challenging task which could help to improve the performance of FGS materials for technological applications. We present here first principles calculations for the infrared (IR) spectra of different FGS models aimed at identifying the IR signatures of different functional groups and defect sites on FGSs. We found that vacancies and edges have significant effects on the IR frequencies of the functional groups on FGSs. In particular, hydroxyl groups close to vacancies have higher stretching and lower bending frequencies in comparison to hydroxyls in defect free regions of FGSs. More interestingly, the OH vibrations of carboxyl groups at edges exhibit unique features in the high frequency IR bands, which originate from the interactions with neighboring groups and the relative orientation of the carboxyl with respect to the FGS plane. Our results are supported by experimental IR measurements on FGS powders. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F37.00009: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F37.00010: Electric Field Tunable Spin-Flip Scattering in Dilute Fluorinated Bilayer Graphene Adam Stabile, Jing Li, Jun Zhu In earlier work, we showed that a dilute coverage of fluorine adatoms covalently bonded to single-layer graphene leads to intriguing and striking phenomena including metal-insulator transition, very large negative magneto-resistance and enhanced spin-flip scattering. By fluorinating only the top layer of a bilayer graphene sheet, this work investigates the possibility of tuning the spin-flip scattering rate $in$ $situ$ via a perpendicular electric field $D$. Dual HfO$_2$ gated field effect transistors of dilute fluorinated bilayer graphene (DFBG) (F:C ~ 0.03 \%) are used, in which we independently control $D$ and the carrier density $n$. The $n$-dependence of the conductance exhibits signatures of midgap state scattering. The midgap states also lead to increased conduction in the band gap of biased DFBG. Magneto-resistance measurements and weak localization analyses over a wide range of $n$, temperatures, and $D$-fields indicate the presence of spin-flip scattering, similar to what is observed in dilute fluorinated single-layer graphene. Most strikingly, the spin-flip rate can be tuned by over a factor of 2 via controlling the direction and magnitude of the $D$-field. These results demonstrate the potential of DFBG in spintronic applications. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F37.00011: Coupled Chemisorption and Physisorption of Oxygen on Single Layer Graphene Devices Hua Wen, Adrian Swartz, Dante O'Hara, Patrick Odenthal, Jen-Ru Chen, Roland Kawakami We investigate adsorption of molecular oxygen on single layer graphene devices and demonstrate that chemisorption of molecular oxygen at low temperatures is strongly coupled to the physisorption process. Through low temperature adsorption and variable-temperature desorption studies, we establish the ability to use electrical measurements to separately identify the physisorption and chemisorption of oxygen on graphene: chemisorption is identified by a change in Dirac point voltage, while physisorption is identified through its increase of the mobility. By utilizing the electrostatic gate controlled chemisorption, we demonstrate that the chemisorption at low temperatures is driven by a two-step process in which free oxygen molecules are first captured onto graphene by physisorption, and then the oxygen undergoes a physisorption-to-chemisorption conversion. Our study provides a better understanding of the effect of gas adsorbates on graphene and could be useful in future applications of graphene-based gas sensors. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F37.00012: Lithium-Intercalated Few Layer Graphene: Approaching the Limits of Transparency and Conductivity in Graphene-based Materials Wenzhong Bao, Jiayu Wan, Xiaogang Han, Xinghan Cai, Hongli Zhu, Dohun Kim, Yunlu Xu, Jeremy Munday, H. Dennis Drew, Michael Fuhrer, Liangbing Hu We measure simultaneous \textit{in situ} optical transmittance spectra and electrical transport properties of few-layer graphene (FLG) nanostructures upon electrochemical lithiation/delithiation. Reversible Li-intercalation stages and a two-phase boundary are observed optically. Due to the unusual electronic structure of FLG, upon intercalation we observe a simultaneous increase of both optical transmittance and DC conductivity, strikingly different from other materials. Transmission as high as 91.7{\%} for sheet resistance of 3.0 $\Omega $/square is achieved for 19 layer LiC$_{6}$, corresponding to a figure of merit (FOM) $\sigma_{dc}$/$\sigma_{opt\, \, }=$ 1400, five times higher than any previously demonstrated for a continuous transparent electrode. The unconventional modification of FLG optoelectronic properties is explained by the suppression of interband optical transitions and a small intraband Drude conductivity near the interband edge. Our techniques can enable investigation of other aspects of intercalation in nanostructures, for example intercalation dynamics and solid-electrolyte interface formation. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F37.00013: Achieving robust n-type nitrogen-doped graphene via a binary-doping approach Hyo Seok Kim, Han Seul Kim, Seong Sik Kim, Yong-Hoon Kim Among various dopant candidates, nitrogen (N) atoms are considered as the most effective dopants to improve the diverse properties of graphene. Unfortunately, recent experimental and theoretical studies have revealed that different N-doped graphene (NGR) conformations can result in both p- and n-type characters depending on the bonding nature of N atoms (substitutional, pyridinic, pyrrolic, and nitrilic). To overcome this obstacle in achieving reliable graphene doping, we have carried out density functional theory calculations and explored the feasibility of converting p-type NGRs into n-type by introducing additional dopant candidates atoms (B, C, O, F, Al, Si, P, S, and Cl). Evaluating the relative formation energies of various binary-doped NGRs and the change in their electronic structure, we conclude that B and P atoms are promising candidates to achieve robust n-type NGRs. The origin of such p- to n-type change is analyzed based on the crystal orbital Hamiltonian population analysis. Implications of our findings in the context of electronic and energy device applications will be also discussed. This work was supported by the Basic Science Research Grant (No. 2012R1A1A2044793), Global Frontier Program (No. 2013-073298), and Nano-Material Technology Development Program (2012M3A7B4049888) of the National Research Foundation funded by the Ministry of Education, Science and Technology of Korea. [Preview Abstract] |
Session F38: Invited Session: Keyhole to the World: Public Access to Satellite Data for Environmental, Security, and Social Ends
Sponsoring Units: FPSChair: Micah Lowenthal, National Academy of Sciences
Room: 709/711
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F38.00001: 40 years of Landsat images: What we learned about science and politics Invited Speaker: Jeff Dozier The first Landsat (then called ERTS -- Earth Resources Technology Satellite) launched in 1972. Landsat 8 launched in February 2013. The 40$+$ years of images have yielded a remarkable history of changes in Earth's land surface, and the program has accomplished significant technological achievements. However, the sustained long-term record owes more to luck than careful program planning, and especially benefitted from the remarkable 27-year life of Landsat 5. Recommendations for the future center mainly on making the program a real Program with a commitment to sustaining it, as well as some ideas to reduce cost and improve effectiveness. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F38.00002: Bringing the Crowd to Environmental Investigation and Monitoring Invited Speaker: John Amos In the nearly 70 years since scientists at the White Sands Missile Range first sent cameras into space on captured German V-2 rockets, humans have launched thousands of satellites into Earth orbit. While some look out at the heavens, many are Earth observation instruments pointing back at us and collecting images and measurements of our planet's dynamic physical and biological systems. Civilian access to, and use of, satellite imagery increased dramatically with the early 1970s advent of the Landsat satellite program, accelerating in recent years with the commercialization of high resolution ``spy satellite'' imagery and the advent of ubiquitous, easy-to-use web interfaces such as Google Earth. SkyTruth applies satellite remote sensing to illuminate environmental issues and incidents. New data sources expected to come on line in 2014 present opportunities for independent, third-party investigation and near-real-time response to environmental and humanitarian crises. New approaches to distributing, processing, sharing and analyzing the expected torrent of image data must be developed to take full advantage of this potential. Our goal is to build a global community of ``skytruthers'' regularly engaged in collaborative image analysis and mapping, and conducting routine oversight of environmental change in the places they care about. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:48AM |
F38.00003: Nuclear Verification from Space? Satellite Imagery in Support of Non-Proliferation and Arms Control Invited Speaker: Irmgard Niemeyer In the last decades, the international community has negotiated a number of multilateral agreements on nuclear non-proliferation and arms control, including also provisions for the verification of compliance. Among the different verification measures, earth observation (EO) by scientific or commercial satellite imaging sensors has been considered as an important source of information. If the area of interest is not accessible, remote sensing sensors offer one of the few opportunities to gather almost real-time data over the area. The study reviews the technical progress in the field of satellite imaging sensors and explores the recent advances in satellite imagery processing and geoinformation technologies as to the extraction of significant observables and signatures of possible non-compliance to non-proliferation and arms control. Moreover, it discusses how satellite data and geoinformation technologies could be used complementary for confirming information gathered from other systems or sources. The study also aims at addressing legal and political aspects and the cost benefits of using satellite imagery in the nuclear verification procedure. The study concludes that satellite imagery and geoinformation technologies are expected to support the efficient management of nuclear non-proliferation and arms control issues and to improve the effective performance of the Treaty. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:24AM |
F38.00004: Earth Science Serving Society: Using NASA Earth-observing Satellites for Policy, Management, and Capacity Building Invited Speaker: Lawrence Friedl |
Tuesday, March 4, 2014 10:24AM - 11:00AM |
F38.00005: Panel Discussion |
Session F39: Invited Session: Cold Atomic Gases with Synthetic Spin-Orbit Coupling
Sponsoring Units: DCMP DAMOPRoom: Mile High Ballroom 2A-3A
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F39.00001: Spin-orbit coupled bosons in optical lattices Invited Speaker: William Cole The interplay between strong correlations and spin-orbit coupling has recently become a significant theme in condensed matter physics. In light of this, we review the proposed realization of a spin-orbit coupled Hubbard model using cold atoms in a synthetic gauge field and an optical lattice potential. Focusing our attention on two component bosons, a variety of theoretical techniques are used to identify broken symmetry states, such as magnetically textured superfluids and Mott insulators. We discuss the spin Hall effect and anomalous Hall effect (in the presence of spontaneous time reversal symmetry breaking). Spin-orbit coupling also leads to interesting plaquette current patterns, and we describe the possibility for experimental confirmation of these using a quench of the lattice potential. Reference: Phys. Rev. Lett. 109, 085302 (2012) [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F39.00002: The spin Hall effect in a quantum gas Invited Speaker: Ian Spielman |
Tuesday, March 4, 2014 9:12AM - 9:48AM |
F39.00003: Many-body physics of spin-orbit-coupled quantum gases Invited Speaker: Victor Galitski Spin-orbit-coupled systems provides a unique area in which a fascinating variety of novel and fundamental phenomena occur. Recent theoretical and experimental works have demonstrated that dressed states of ultra-cold atoms coupled to light acquire effective spin-orbit (SO) interactions. In this talk, I will review recent progress in theoretical understanding of these synthetic spin-orbit coupled quantum systems. First, I will discuss possible single-particle Hamiltonians for dressed states that arise in various laser schemes, including Rashba and Dresselhaus-type Hamiltonians, three-dimensional isotropic SO (Weyl) interaction, and su(3) SO coupling. Then, I will discuss many-body quantum physics that arise in bosonic systems. Possible ground states of SO-coupled Bose-Einstein condensates will be discussed. It will be shown that when put on a lattice, SO-coupled, interacting bosons give rise to Mott insulators with exotic spin orders, such as a skyrmion lattice phase and various stripe orders. Finally, I will discuss non-equilibrium phenomena in SO-coupled systems and show how the interplay between spin-orbit coupling and interactions results in interesting quantum dynamical systems, which feature a rich variety of time-dependent behaviors and dynamical transitions. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:24AM |
F39.00004: Spin-Orbit Coupled Quantum Gases: New Physics and Challenges Invited Speaker: Hui Zhai In this talk I will review recent progresses in studying spin-orbit coupling in ultracold quantum gases. I will discuss several examples of new states or phenomena when spin-orbit coupling is introduced to ultracold atomic gases. i) the single particle ground state degeneracy will lead to condensate with stripe order for bosons and interesting finite temperature phase diagram; ii) the new feature in two-body physics strongly modifies the scenario of fermion pairing; On the other hand, I will also discuss great challenges in this direction that is the heating problem. I will present several ways to overcome the difficult, for instance, by utilizing highly magnetic lanthanide atoms. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 11:00AM |
F39.00005: Spin-Orbit Coupled Fermi Gases and Solitons in Fermionic Superfluids Invited Speaker: Lawrence Cheuk The coupling of the spin of electrons to their motional state lies at the heart of topological phases of matter. We have created and detected spin-orbit coupling in an atomic Fermi gas via spin-injection spectroscopy, which characterizes the energy-momentum dispersion and spin composition of the quantum states. For energies within the spin-orbit gap, the system acts as a spin diode. To fully inhibit transport, we open an additional spin gap with radio-frequency coupling, thereby creating a spin-orbit coupled lattice whose spinful band structure we probe. In the presence of s-wave interactions, spin-orbit coupled fermion systems should display induced p-wave pairing and consequently topological superfluidity. Such systems can be described by a relativistic Dirac theory with a mass term that can be made to vary spatially. Topologically protected edge states are expected to occur whenever the mass term changes sign. A system that similarly supports edges states is the strongly interacting atomic Fermi gas near a Feshbach resonance. Topological excitations, such as vortices - line defects - or solitons - planar defects - have been described theoretically for decades in many different physical contexts. In superconductivity and superfluidity they represent a defect in the order parameter and give rise to localized bound states. We have created and directly observed solitons in a fermionic superfluid. These are found to be stable for many seconds, allowing us to track their oscillatory motion in the trapped superfluid. Their trapping period increases dramatically as the interactions are tuned from the BEC to the BCS regime. At the Feshbach resonance, their period is an order of magnitude larger than expected from mean-field Bogoliubov-de Gennes theory, signaling strong effects of quantum fluctuations and possible filling of Andreev bound states. Our work paves the way towards the experimental study and control of fermionic edge states in ultracold gases. [Preview Abstract] |
Session F40: Invited Session: New Magnetoresistance in Metal/Magnetic Insulator Heterostructures
Sponsoring Units: GMAGChair: Sufeng Zhang, Arizona University
Room: Mile High Ballroom 2B-3B
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F40.00001: Theory of spin Hall magnetoresistance (SMR) and related phenomena Invited Speaker: Gerrit Bauer A new anisotropic magnetoresistance effect has recently been found for a Pt film on top of the insulating ferrimagnet Yttrium-Iron-Garnet (YIG) [1-6]. We interpret this effect by the simultaneous action of spin Hall and inverse spin Hall effects as a non-equilibrium proximity phenomenon dubbed spin Hall magnetoresistance (SMR). This mechanism does not require the equilibrium proximity magnetization in Pt, which was assumed in [5]. We computed the SMR in F\textbar N and F\textbar N\textbar F layered systems, where F is a magnetic insulator, treating the normal metal N by spin-diffusion theory with quantum mechanical boundary conditions at the interfaces in terms of the spin-mixing conductance [7]. Our results explain the experimentally observed spin Hall magnetoresistance in F\textbar N bilayers. An analysis of the Hall effect when magnetization is normal to the plane allowed the experimental observation of the imaginary part of the mixing conductance [4]. For F\textbar N\textbar F spin valves we predict enhanced SMR amplitudes when magnetizations are collinear. In this talk I review the state of the art and discuss recent extensions of the SMR theory. \\[4pt] [1] H. Nakayama et al., Phys. Rev. Lett. 110, 206601 (2013).\\[0pt] [2] C. Hahn, Phys. Rev. B 87, 174417 (2013).\\[0pt] [3] M. Althammer et al., Phys. Rev. B 87, 224401 (2013).\\[0pt] [4] N. Vlietstra, et al., Appl. Phys. Lett. 103, 032401 (2013).\\[0pt] [5] Y. M. Lu et al., Phys. Rev. B 87, 220409 (2013).\\[0pt] [6] M. Isasa et al., arXiv:1307.1267\\[0pt] [7] Y. Chen et al., Phys. Rev. B 87, 144411 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F40.00002: Spin Hall magnetoresistance in ferromagnetic insulator/normal metal hybrids Invited Speaker: Matthias Althammer Pure spin currents, i.e. the net flow of spin angular momentum without an accompanying charge current, represent a new paradigm for spin transport and spintronics. We have experimentally studied a new type of magnetoresistance effect, which arises from the interaction of charge and spin current flows in ferromagnetic insulator/normal metal hybrid structures. In more detail, we measured the resistance of yttrium iron garnet(YIG)/Pt, YIG/nonferromagnet/Pt, nickel ferrite/Pt, and magnetite/Pt hybrid structures as a function of the magnitude and the orientation of an external magnetic field. The resistance changes observed can be quantitatively traced back to the combined action of spin Hall and inverse spin Hall effect in the Pt metal layer, and are thus termed spin Hall magnetoresistance (SMR) $[1,2]$. We show that the SMR is qualitatively different from the conventional anisotropic magnetoresistance effect arising in magnetic metals. From the magnetoresistance measurements in YIG/Au/Pt and YIG/Cu/Pt structures and from x-ray magnetic circular dichroism measurements on YIG/Pt heterostructures we exclude a static proximity magnetization in Pt as the origin of the magnetoresistance, in contrast to the mechanism proposed by Huang et al.~$[3]$. Furthermore, the SMR enables us to quantify the spin Hall angle as a function of temperature in our Pt layers. In addition, we analyze the anomalous Hall type contribution of the SMR to quantify the imaginary part of the spin mixing conductance. Financial support by the DFG via SPP 1538 (project no. GO 944/4) and the Nanoinitiative Munich (NIM) is gratefully acknowledged.\\[4pt] [1] Nakayama et al., PRL, \textbf{110}, 206601 (2013)\\[0pt] [2] Althammer et al., PRB, \textbf{87}, 224401 (2013)\\[0pt] [3] Huang et al., PRL, \textbf{109}, 107204 (2012) [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:48AM |
F40.00003: Hybrid magnetoresistance in the proximity of a ferromagnet Invited Speaker: Chia-Ling Chien A new type of magnetoresistance (MR) effect has recently been observed in nominally nonmagnetic metal (Pt) thin films in contact with either a ferromagnetic (FM) insulator such as yttrium iron garnet (YIG),\footnote{H. Nakayama \textit{et al., }Phys. Rev. Lett. \textbf{110}, 206601(2013).} or a FM metal,\footnote{Y. M. Lu \textit{et al,,} Phys. Rev. B \textbf{87}, 220409(R) (2013).} such as permalloy (Py). The resistivities with in-plane magnetic fields parallel ($\rho _{\mathrm{\parallel }})$ and transverse ($\rho_{\mathrm{T}})$ to a current and a perpendicular field ($\rho_{\mathrm{\bot }})$ at room temperature show the behavior of $\rho_{\mathrm{\bot }}\approx \rho _{\mathrm{\parallel }}$\textgreater $\rho_{\mathrm{T}}$, distinctively different from all other known MR effects, including the well-known anisotropic MR in FMs of $\rho_{\mathrm{\parallel }}$\textgreater $\rho _{\mathrm{T}}\approx \rho_{\mathrm{\bot }}$. The key question is whether the new MR is the proposed spin Hall MR (SMR) based on spin current conversion in Pt, or due to magnetic proximity effects (MPE), for which Pt is highly susceptible when in contact with a FM. Recent experiments show that the characteristics of $\rho_{\mathrm{\bot }}\approx \rho _{\mathrm{\parallel }}$\textgreater $\rho_{\mathrm{T}}$, for which the SMR theory accounts, do not hold at low temperatures nor at different magnetic fields. Furthermore, the new MR persists even after altering the Pt/YIG interface thereby blocking the spin current.\footnote{B. F. Miao \textit{et al.,} Phys. Rev. Lett. \textbf{111}, 066602 (2013).} The feature of new MR can also be reproduced when Pt is in contact with a non-magnetic insulator doped with a few percent of Fe impurities. These results show that the new MR is probably due to both spin current and MPE. Through tuning the YIG surface and the insertion of other layers between Pt and YIG, we are able to separate the two contributions of spin current and MPE of the new hybrid MR. This work, in collaboration with S. Y. Huang, D. Qu (JHU) B. F. Miao (JHU and Nanjing University), Y. M. Lu and J. W. Cai (Institute of Physics, Chinese Academy of Sciences), has been supported in part by NSF DMR1262253. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:24AM |
F40.00004: Distinguishing spin Hall MR from anisotropic MR by temperature dependence Invited Speaker: Jing Shi In bilayers consisting of a strong spin-orbit coupling metal such as Pt or Pd and a magnetic insulator such as yttrium iron garnet, magnetoresistance is found in the magnetic field range where the magnetization of YIG reverses. This magnetoresistance resembles the conventional anisotropic magnetoresistance (AMR) in ferromagnetic conductors, which arises from the difference in resistance between the current parallel and perpendicular to the magnetization. At room temperature, however, no difference is observed when the magnetization is rotated between these two orientations; therefore, this phenomenon is clearly not the conventional AMR. An alternative explanation is based on the magnetization-dependent spin current reflection effect, called the spin Hall magnetoresistance (SMR). In our bilayer systems, we find that a finite AMR appears and increases monotonically as the temperature is decreased. In the meantime, SMR increases first, reaches a peak at an intermediate temperature, and then decreases at low temperatures. We will show that this characteristic temperature dependence is consistent with the SMR model. The SMR peak occurs when the spin diffusion length is approximately equal to the metallic layer thickness. These two magnetoresistance effects coexist but can be distinguished from each other from their distinct temperature dependences. [Preview Abstract] |
Session F41: Focus Session: BiFeO3
Sponsoring Units: DMP DCOMPChair: Neil Mathur, University of Cambridge
Room: Mile High Ballroom 3C
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F41.00001: First-principles-derived spin models for the electric control of magnetism in BiFeO$_{3}$ Jun Hee Lee, Randy Fishman While various spin models excellently describe long-range spin spiral states in complex materials, they do not capture atomistic behavior with respect to external perturbations such as electric field or strain. On the other hand, while first-principles approaches capture the atomistic behavior, they cannot practically deal with large systems with long-range spin ordering. In this talk, we demonstrate how spin models and first principles can be synergetically combined to understand the response of complex spin systems with respect to electric field or strain. We present first-principles-derived spin models that show excellent agreement with experimental spin-wave excitations driven by electric field or strain in BiFeO$_{3}$. With the atomistic model, we will discuss how to effectively control magnetism in BiFeO$_{3}$ with the combination of electric field and strain. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F41.00002: Magnetoelectric Control of Exchange Coupling in Monodomain BiFeO$_{3}$ Heterostructures Julian Irwin, W. Saenrang, B. Davidson, S. Ryu, S.-B. Baek, C.B. Eom, M.S. Rzchowski, J. Freeland The electric field control of magnetization via the exchange bias coupling of a ferromagnetic and antiferromagnetic orderings has exciting applications in spintronic devices such as magnetic tunnel junctions. We investigate the exchange coupling between the monodomain multiferroic BiFeO$_{3}$(BFO) thin film [1] and a ferromagnetic Co layer. Recently, X-ray magnetic circular dichromism (XMCD) has been used to observe a $\sim$20$^{\circ}$ rotation in the magnetization of the Co when the electric polarization of the BFO is reversed [2]. Due to the formation of an antiferromagnetic surface ``dead layer'' at high temperatures, observed using X-ray linear magnetic dichromism, this rotation is only seen at temperatures below $\sim$150K. Here we investigate the exchange coupling using anisotropic magnetoresistance (AMR) measurements that detect changes in the magnetization of the Co layer. Out approach using AMR can be applied more generally to study exchange coupling in multiferroic systems.\\[4pt] [1] S.H. Baek et al., ``Ferroelastic Switching for Nanoscale Nonvolatile Magnetoelectric Devices'' Nature Materials, 9, 309 (2010).\\[0pt] [2] W. Saenrang et al, ``Magnetoelectric Control of Exchange Coupling in Monodomain BiFeO$_{3}$ Heterostructures,'' in preparation [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F41.00003: Large room temperature ferroelectric polarization in thin films of solid solution of bismuth ferrite and lead titanate Rajesh Katoch, Rajeev Gupta, Ashish Garg BiFeO$_{3}$ and PbTiO$_{3}$ form a solid solution i.e. (1-x)BiFeO$_{3}$-xPbTiO$_{3}$ showing a morphotropic phase boundary (MPB) at x $=$ 0.30 with high Curie temperature (T$_{\mathrm{c}}$ $\sim$ 630 $^{\circ}$C).Here we present the results of our investigations on the structure and properties of thin films grown by pulsed laser deposition and chemical solution deposition method on Pt/Si substrates and show that the use of PbTiO$_{3}$ buffer layer leads to improvement in the room temperature (RT) ferroelectric response. X ray diffraction and Raman spectroscopy reveal structure to be tetragonal (\textit{P4mm}) at x $=$ 0.35, rhombohedral (\textit{R3c}) at x $=$ 0.25 and two phase (\textit{R3c}$+$\textit{P4mm)} at x $=$ 0.30 exhibiting giant tetragonality (c/a$=$1.17) in bulk. However, PLD grown films remain tetragonal (\textit{P4mm}) for all compositions with P$_{\mathrm{r}}=$40 microcoulombs/cm$^{2}$ while solution grown films showed the structure to be monoclinic (\textit{Cm})at x$=$0.25 and a coexistence of \textit{Cm} and \textit{P4mm} phases for x$=$0.30 and x$=$0.35 with c/a$=$1.02 resulting in large polarization with P$_{\mathrm{r}}=$80microcoulombs/cm$^{2}$ and E$_{\mathrm{c}}=$130 kV/cm. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F41.00004: Magnetic coupling in Epitaxial BiFeO$_{3}$-La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ Heterostructures Integrated on Si(100) Srinivasa Rao Singamaneni, J.T. Prater, Fan Wu, C.T. Shelton, J.-P. Maria, J. Narayan We present and discuss the magnetic characteristic of BiFeO$_{3}$ (BFO)/La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) heterostructure, integrated on Si (100) using pulsed laser deposition (PLD) via the domain matching epitaxy (DME) paradigm. The magnetic behavior of this heterostructure, in which a d$^{5}$ system (Fe$^{3+})$ manifested in FE-AFM BFO is epitaxially conjoined at the interface to a multivalent transition metal ion such as Mn$^{3+}$/Mn$^{4+}$ in LSMO exhibits interesting magneto electric coupling phenomenon. The temperature- and magnetic field-dependent magnetization measurements reveal an unexpected enhancement in magnetization and improved magnetic hysteresis squareness originating from the BFO/LSMO interface. We observe a stronger temperature dependence of exchange coupling when the polarity of field cooling is negative as compared to positive field cooling. We believe such an enhancement in magnetization and magnetic coupling is likely directly related to an electronic orbital reconstruction at the interface and complex interplay between orbital and spin degrees of freedom.\\[4pt] [1] S. S. Rao \textit{et al}, Nano Letters, http://dx.doi.org/10.1021/nl4023435. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F41.00005: Electro-photo double modulation on the resistive switching behavior and switchable photoelectric effect in BiFeO films Kuijuan Jin, Le Wang We present an electro-photo double modulation on the resistive switching behavior, combining the electro-resistance effect and the photo-resistance effect. The pulse voltages can lead to nonvolatile resistance variations in the Au/BiFeO3/La0.7Sr0.3Mno3 structure, and the laser illumination can also modulate the high and low resistance states. Consequently, four stable resistance states are achieved. Furthermore, we report a switchable photoelectric effect, in which a photocurrent can be created under illumination of the ultraviolet laser, and the direction of the photocurrent depends on the ferroelectric polarization. The present results should have potential applications to develop multi-state memory devices based on perovskite oxides. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F41.00006: X-Ray Imaging and Multiferroic Coupling of Cycloidal Magnetic Domains in Ferroelectric Monodomain BiFeO$_3$ P.G. Radaelli, R.D. Johnson, A. Bombardi, Y.-S. Oh, S.-W. Cheong, L.C. Chapon BiFeO$_3$ is perhaps the most studied material among the multiferroics. It is both magnetic and strongly ferroelectric at room temperature, making it potentially suitable for applications. Understanding the interplay between ferroelectric and magnetic domains is, however, essential to control device functionality. In BiFeO$_3$, the Dzyaloshinsky- Moriya interaction promotes the formation of cycloidal magnetic domains with magnetic polarity \emph{co-aligned} with the electrical polarization. We have imaged these magnetic domains at the surface of a ferroelectric monodomain BiFeO$_3$ single crystal by hard x-ray magnetic scattering. Domains up to several hundred microns in size have been observed, corresponding to cycloidal modulations along the wave vector ${\bf k}=(\delta, \delta, 0)$ and symmetry equivalent directions. The rotation direction of the magnetization in all magnetic domains, determined by diffraction of circularly polarized light, was found to be unique and in agreement with first-principle calculations. Imaging of the surface shows that the largest adjacent domains display a 120$^{\circ}$ vortex structure. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F41.00007: Modulating the ratio of tetragonal/rhombohedral phases in strained BiFeO$_3$ films by varying the oxygen pressure during deposition Margo Staruch, Heungsoo Kim Room-temperature multiferroic BiFeO$_3$ (BFO) has been the subject of recent research interest due to its potential applications in random access memory and other spintronic devices. Compressive strain in the BFO lattice results in a symmetry change from a rhombohedral to a monoclinically-distorted tetragonal structure, with intermediate strains lying near a morphotropic phase boundary. This has been demonstrated to lead to enhanced piezoelectric and dielectric properties. However, the effect of growth conditions, such as substrate temperature and oxygen partial pressure during deposition, on the multiferroic properties of strained BFO films has yet to be systematically studied. In this work, BiFeO$_3$ thin films were grown on (001) LaAlO$_3$ single crystal substrates by pulsed laser deposition at different oxygen partial pressures. By examining the structure and microstructure of the resulting films, the ratio of the tetragonal-like and rhombohedral phases was found to vary with oxygen deposition pressure. The effects of this modulation on the magnetic and ferroelectric properties will be presented. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F41.00008: Symmetry of Highly-Strained BiFeO$_3$ Films in the Ultrathin Regime Yongsoo Yang, Nancy Senabulya, Roy Clarke, Christian M. Schlep\"utz, Christianne Beekman, Wolter Siemons, Hans M. Christen At room temperature, highly-strained BiFeO$_3$ (BFO) films grown on LaAlO$_3$ substrates exhibit a monoclinic structure with a giant c/a ratio ($\sim$1.3) when the films are thicker than 4 nm. Their structural symmetry can be controlled by adjusting the temperature [Appl. Phys. Express {\bf 4}, 095801 (2011), Adv. Mater. {\bf 25}, 5561 (2013)], with a high-temperature tetragonal phase being observed. We report that a structural phase transition can also be achieved by controlling the film thickness: synchrotron x-ray diffraction data shows that the Bragg peak splitting associated with the monoclinic phase disappears as the film thickness decreases below 3 nm, indicating a tetragonal symmetry, but still maintaining the giant c/a ratio. Unlike a similar transition reported for moderately strained BFO grown on SrTiO$_3$ [APL Mater. {\bf 1}, 052102 (2013)], the half-order Bragg peaks indicate that this transition does not involve a significant change in the octahedral tilt pattern of the film. This suggests that the structural evolution of highly-strained BFO films should be understood in terms of the unique (non-octahedral) oxygen coordination of the Fe ion in this highly-strained BFO, not the corner-connectivity of the oxygen octahedra between the film and the substrate. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F41.00009: Neutron Scattering Study on Low Energy Phonons in BiFeO$_3$ Guangyong Xu, Zhijun Xu, Jinsheng Wen, Peter Gehring, Stephen Shapiro, Masaaki Matsuda, Toshimitsu Ito, Robert Birgeneau, Barry Winn, Genda Gu We have performed neutron scattering studies on low energy phonon modes in the multiferroic BiFeO$_3$. We show measurements near (100), (110), (200) Bragg peaks on the TA, LA and lowest energy TO modes in a broad temperature range from 300 K to 700 K. The intensities, dispersion, and life times (inversed energy width) of these phonon modes are plotted vs. temperature, and anomalies related to the AFM order (Neel temperature of 640 K) are discussed. We also will also discuss additional low energy modes observed that may be related to the ``electro-magnon'' excitations in this material. This work is supported by the Office of Basic Energy Sciences, DOE. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:24AM |
F41.00010: Electric-field control of magnetic order above room temperature Invited Speaker: Manuel Bibes Controlling magnetism by electric fields is a key issue for the future development of low-power spintronics. Progress has been made in the electrical control of magnetic anisotropy, domain structure, spin polarization or critical temperatures. However, the ability to turn on and off robust ferromagnetism at room temperature and above has remained elusive. Here we will present a new approach for the electrical control of magnetic and spintronic properties based on the combination of ferroelectric materials with magnetic transition-metal alloys. We demonstrate a giant, low-voltage control of magnetism, just above room temperature. The data are interpreted in the light of first-principles in terms of both strain and field-effect. Our results correspond to a magnetoelectric coupling larger than previous reports by at least one order of magnitude and open new perspectives for the use of ferroelectrics in spintronics.\\[4pt] Work done in collaboration with Ryan Cherifi, Viktoria Ivanovskaya, Lee Phillips, Unite Mixte de Physique CNRS/Thales; Alberto Zobelli, Laboratoire de Physique des Solides; Ingrid Infante, Ecole Centrale Paris; Eric Jacquet, Stephane Fusil, Unite Mixte de Physique CNRS/Thales; Patrick Briddon, University of Newcastle; Ahmet Unal, Helmholtz Zentrum Berlin; Alexandra Mougin, Laboratoire de Physique des Solides; Sergio Valencia, Helmholtz Zentrum Berlin; Florian Kronast, Laboratoire de Physique des Solides; Brahim Dkhil, Ecole Centrale Paris; and Vincent Garcia, Agnes Barthelemy, Unite Mixte de Physique CNRS/Thales. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F41.00011: The interplay of Dzyaloshinskii-Moriya interaction and single-ion anisotropy in multiferroic BiFeO$_{3}$ Jaehong Jeong, P. Bourges, S. Petit, S. Furukawa, M.D. Le, S.-A. Kim, S. Lee, S.-W. Cheong, Je-Geun Park Multiferroic compounds are promising materials for new spintronic devices utilising the coupling between magnetism and ferroelectricity. Among them, BiFeO$_{3}$ is the only example that has both magnetic and ferroelectric transitions above room temperature. It also has the cycloid spin structure with an extremely long period. In order to understand the microscopic magnetic interactions, several inelastic neutron scattering (INS) experiments were carried out using co-aligned single crystals. We could, for the first time, measure the magnon dispersion over the full Brillouin zone and determine the interaction parameters in a Hamiltonian with two Heisenberg interactions between the nearest and the next nearest neighbors. For the further study on the detailed magnetic excitations at low energy, we performed two INS experiments using the triple-axis spectrometer 4F2 at LLB. We also calculated the magnon dispersion using the Hamiltonian that includes Dzyaloshinskii-Moriya (DM) interaction and single-ion anisotropy (SIA), which are associated with the distortion of Fe$^{3+}$ ion in the FeO$_{6}$ octahedra, allowing us to understand the unusual low-energy excitations in BiFeO$_{3}$ by examining the interplay of the DM interaction and SIA. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F41.00012: Unified model for spin order induced polarization in multiferroics Hongjun Xiang The microscopic origins of ferroelectricity in different multiferroic systems were theoretically investigated. We proposed a unified model [1,2] which includes purely electronic and ion-displacement contribution simultaneously to describe spin-order induced ferroelectricity. An efficient method [3] was developed to compute the model parameters from first-principles. On the basis of the unified model and density functional calculations, we explained the ferroelectricity induced by the proper-screw spin spiral [2], discovered a novel magnetoelectric coupling mechanism in which the magnitude of the polarization is governed by the exchange striction with the direction by the spin chirality [4], proposed that the ferroelectricity in the chiral-lattice magnet Cu2OSeO3 is due to the unusual single-spin site term [5], unraveled that the magnetoelectric effect observed in BiFeO3 originates from the exchange striction [2].\\[4pt] [1] H. J. Xiang \textit{et al.}, Phys. Rev. Lett. 107, 157202 (2011).\\[0pt] [2] H. J. Xiang \textit{et al.}, Phys. Rev. B 88, 054404 (2013).\\[0pt] [3] H. J. Xiang \textit{et al.}, Phys. Rev. B 84, 224429 (2011).\\[0pt] [4] X. Z. Lu, M.-H. Whangbo, S. Dong, X. G. Gong, and H. J. Xiang, Phys. Rev. Lett. 108, 187204 (2012).\\[0pt] [5] J. H. Yang, Z. L. Li, X. Z. Lu, M.-H. Whangbo, S.-H. Wei, X. G. Gong, and H. J. Xiang, Phys. Rev. Lett. 109, 107203 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F41.00013: Atomic-Scale Electronic Spectra across BiFeO$_{3}$/La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ Complex Oxide Heterointerfaces Ya-Ping Chiu, Bo-Chao Huang, Pu Yu, Ramamoorthy Ramesh, Ying-Hao Chu Atomic-scale evolution of electronic structures across BiFeO$_{3}$/La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ complex oxide heterointerfaces has been revealed using cross-sectional scanning tunneling microscopy and spectroscopy. Analysis of scanning tunneling spectroscopy results exploits the interfacial valence mismatch to influence the electrostatic configurations across the BiFeO$_{3}$/La$_{0.7}$Sr$_{0.3}$MnO$_{0.3}$ heterointerfaces. Spatially unit-cell-by-unit-cell resolved electronic states at the atomic level reveal how the control of material interfaces at the atomic level to determine the ferroelectric polarization in BiFeO$_{3}$. [Preview Abstract] |
Session F42: Focus Session: Scanning Tunneling from Topological Insulators
Sponsoring Units: DMPChair: Arun Bansil, Northeastern University
Room: Mile High Ballroom 4A
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F42.00001: Mapping the unconventional orbital texture in topological crystalline insulators Ilija Zeljkovic, Yoshinori Okada, Cheng-Yi Huang, R. Sankar, Daniel Walkup, Wenwen Zhou, Maksym Serbyn, Fangcheng Chou, Wei-Feng Tsai, Hsin Lin, Arun Bansil, Liang Fu, M. Zahid Hasan, Vidya Madhavan The newly discovered topological crystalline insulators (TCIs) harbor a complex band structure involving multiple Dirac cones. These materials are potentially highly tunable by external electric field, temperature or strain and could find future applications in field-effect transistors, photodetectors, and nano-mechanical systems. Theoretically, it has been predicted that different Dirac cones, offset in energy and momentum-space, might harbor vastly different orbital character, a unique property which if experimentally realized, would present an ideal platform for accomplishing new spintronic devices. In this study, we unveil the orbital texture in a prototypical TCI Pb$_{1-x}$Sn$_x$Se by using Fourier-transform (FT) scanning tunneling spectroscopy (STS) to measure the interference patterns produced by the scattering of surface state electrons. We discover that the intensity and energy dependences of FTs show distinct characteristics, which can directly be attributed to orbital effects. Our experiments also reveal the complex band topology involving two Lifshitz transitions. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F42.00002: Photoemission studies of topological crystalline insulator Pb$_{1-x}$Sn$_{x}$Se Ivo Pletikosic, Genda Gu, Tonica Valla Topological crystalline insulators is a class of narrow-gap semiconductors with spin-orbit induced gap inversion and surface states whose topological protection arises from the crystal symmetry.
We present a photoemission study of the electronic structure of a rock-salt alloy $Pb_{1-x}Sn_{x}Se$ that forms for $0 |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F42.00003: Spin-filtered Edge States with an Electrically Tunable Gap in a Two-Dimensional Topological Crystallin Insulator Junwei Liu, Timothy H. Hsieh, Peng Wei, Wenhui Duan, Jagadeesh Moodera, Liang Fu Three-dimensional topological crystalline insulators (TCIs) were recently predicted and observed in the SnTe class of IV-VI semiconductors, which host metallic surface states protected by crystal symmetries. In this work, we study thin films of these materials and expose their potential device applications. We demonstrate that thin films of SnTe and Pb$_{1-x}$Sn$_{x}$Se(Te) grown along the (001) direction are topologically nontrivial in a wide range of film thickness and carry conducting spin-filtered edge states that are protected by the (001) mirror symmetry via a topological invariant. Application of an electric field perpendicular to the film will break the mirror symmetry and generate a band gap in these edge states. This functionality motivates us to propose a novel topological transistor device, in which charge and spin transport are maximally entangled and simultaneously controlled by an electric field. The high on/off operation speed and coupling of spin and charge in such a device may lead to electronic and spintronic applications for TCIs. \\[4pt] [1] J. Liu, \emph{et al.}, arXiv:1310.1044 (2013), (accepted by Nature materials) [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F42.00004: Scanning Tunneling Spectroscopy Investigations of Surface States in Three Dimensional Topological Insulators and Topological Crystalline Insulators Invited Speaker: Yoshinori Okada Three dimensional topological insulators (TIs) are a new class of material possessing topologically protected spin-polarized Dirac fermions on their surface. This new material has gathered much attention because of its great potential for realizing novel phenomena that are important for both fundamentals and applications. 3D topological insulators have been extensively probed by surface sensitive tools such as ARPES and spectroscopic imaging scanning tunneling microscopy (STM). In this talk, we will especially focus on STM measurements of Pb$_{1-x}$Sn$_{x}$Se. This material belongs to a recently discovered new category of topological insulators called topological crystalline insulators (TCIs). In TCIs, topology and crystal symmetry intertwine to create surface states with a unique set of characteristics different from conventional 3D TIs. We have discovered broken mirror symmetry driven states that coexist with massless Dirac electrons in different regions of momentum space. Our findings experimentally demonstrate the unique tunability of surface Dirac electrons which is promising for the future realization of novel electronic states within TCIs.\\[4pt] [1] Y. Okada \textit{et al}, Science \textbf{341,} 1496 (2013).\\[0pt] [2] Y. Okada \textit{et al}, Nature Commun. \textbf{3},1158 (2012).\\[0pt] [3] Y. Okada \textit{et al}, Phys. Rev. Lett. \textbf{109}, 166407 (2012).\\[0pt] [4] Y. Okada \textit{et al}, Phys. Rev. Lett. \textbf{106}, 206805 (2011). [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F42.00005: Experimental characterization and simulation of quasi-particle-interference in the Bi-bilayer topological insulator A. Eich, M. Michiardi, G. Bihlmayer, A.A. Khajetoorians, J. Wiebe, J.-L. Mi, B.B. Iversen, Ph. Hofmann, R. Wiesendanger Topological insulators are a new class of materials with a gapless surface state where spin and momentum are locked. A Bi-bilayer is predicted to be a 2D-topological insulator and well suited for scanning probe techniques that can be utilized to probe the topological edge states. Unfortunately there are only a few substrates that allow the growth of Bismuth in the rhombohedral structure, which is essential for the formation of the bilayer. Here we present a combined experimental and theoretical study of the quasi-particle interference (QPI) in the Bi-bilayer grown on the 3D-topological insulator Bi$_2$Se$_3$. Fourier-transform-scanning-tunneling-spectroscopy reveals additional features in QPI in comparison to a bare Bi$_2$Se$_3$ surface, indicating the development of new surface states below and above the Fermi energy. Via a comparison of measured QPI-patterns and simulated QPI-patterns based on DFT calculations, the bands participating in electron scattering are identified. DFT calculations further reveal a large influence of the bilayer-substrate-distance on the resulting band structure. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F42.00006: Real-space imaging of Dirac-Landau orbits in Bi2Se3 Yingshuang Fu, Minoru Kawamura, Kyushiro Igarashi, Hidenori Takagi, Tetsuo Hanaguri, Takao Sasagawa Dirac wave function has two-component spinors, which is associated with pseudo-spins in graphene and real spins in the surface state of topological insulators. To date, its direct observation is still elusive. Here we demonstrate it manifests itself in the Landau orbits drifting in a Coulomb potential. We perform spectroscopic imaging scanning tunneling microscopy on the topological surface state of Bi2Se3 to reveal the energy and spatial structures of Landau orbits. Our observations are qualitatively different from those reported in a conventional massive electron system but are well reproduced by a model based on a two-component Dirac Hamiltonian. Our model further predicts energy-dependent nontrivial spin textures in a Coulomb potential, providing a unique way to manipulate spins in the topological surface state. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F42.00007: Identifying Antisite and Vacancy Defects in n-doped Bi$_{2}$Se$_{3}$ Topological Insulators from Scanning Tunneling Microscopy and First Principles Calculations Jeong Heum Jeon, Joon-Suh Park, Howon Kim, Won Jun Jang, Jinhee Han, Hyungjun Lee, Hyung-Joon Choi, Se-Jong Kahng Intrinsic defects are the major sources of n-type doping character in Bi$_{2}$Se$_{3}$ topological insulators, but their structural nature remains unsettled; Theoretical calculations predicted that Se$_{\mathrm{Bi}}$ antisite was the most preferred under Se-rich, i.e. molecular beam epitaxy conditions, but there has been no report on its experimental observation. Here, we present our energy-dependent atomic resolution scanning tunneling microscopy (STM) images for intrinsic defects obtained from Bi$_{2}$Se$_{3}$ thin films grown under Se-rich conditions. We observed two types of defects, and identified them as Se$_{\mathrm{Bi}}$ antisite and Bi vacancy located at Bi layer right below surface Se layer, by comparing experimental STM images with the simulated ones obtained from first principles calculations. Our study shows that, in agreement with previous predictions, not Se-vacancy at surface but Se$_{\mathrm{Bi}}$ antisite is the origin of n-type doping in our Bi$_{2}$Se$_{3}$. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F42.00008: Visualizing a p-n junction of two-dimensional electronic gases on a polar semiconductor BiTeI Yuhki Kohsaka, Manabu Kanou, Tetsuo Hanaguri, Hidenori Takagi, Takao Sasagawa We report atomically-resolved spectroscopic imaging studies of bipolar two-dimensional gases and their junction on the surface of a polar semiconductor BiTeI with a scanning tunneling microscope. Topographic images of pristine and substituted samples reveal that this material shows domain structures composed of opposite stacking orders, Te-Bi-I and I-Bi-Te. We find that electrons are accumulated on Te-face and holes on I-face by elaborating electronic standing waves on the surfaces of each domain, and show atomic resolution imaging of a p-n junction on the domain boundary. Given that no chemical modifications such as surface contamination and additional defects are observed, the origin of the bipolar two-dimensional carriers as well as the formation of the domain structures are ascribed to spontaneous electric polarization in the bulk. Our results indicate that, besides chemical doping and electrostatic gating, spontaneous electric polarization can induce bipolar carriers, and demonstrate a platform to study spin-split two-dimensional p-n junction and edge states at atomic resolution. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F42.00009: One-dimensional Topological Edge States of Bismuth Bilayers Ilya Drozdov, Aris Alexandradinata, Sangjun Jeon, Stevan Nadj-Perge, Huiwen Ji, Robert Cava, B. Andrei Bernevig, Ali Yazdani The hallmark of a time-reversal symmetry protected topologically insulating state of matter in two-dimensions (2D) is the existence of chiral edge modes propagating along the perimeter of the sample. Bilayers of bismuth (Bi), an elemental system theoretically predicted to be a Quantum Spin Hall (QSH) insulator$^{\mathrm{1}}$, has been studied with Scanning Tunneling Microscopy (STM) and the electronic structure of its bulk and edge modes has been experimentally investigated. Spectroscopic mapping with STM reveals the presence of the state bound to the edges of the Bi-bilayer. By visualizing quantum interference of the edge state quasi-particles in confined geometries we characterize their dispersion and demonstrate that their properties are consistent with the absence of backscattering. Hybridization of the edge modes to the underlying substrate will be discussed. [1] Shuichi Murakami, Phys. Rev. Lett. 97, 236805 (2006). The work at Princeton and the Princeton Nanoscale Microscopy Laboratory was supported by ARO MURI program W911NF-12-1-0461, DARPA-SPWAR Meso program N6601-11-1-4110, NSF-DMR1104612, and NSF-MRSEC programs through the Princeton Center for Complex Materials (DMR-0819860) [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F42.00010: STM Studies of ternary topological insulators GeBi$_2$Te$_4$ and SnBi$_2$Te$_4$ Katsuya Iwaya, Tetsuo Hanaguri, Yuhki Kohsaka, Yingshuang Fu, Linda Ye, Joe Checkelsky, Yoshio Kaneko, Yoshinori Tokura We investigate topological surface states (TSSs) of three-dimensional topological insulators, GeBi$_2$Te$_4$ and SnBi$_2$Te$_4$, using low-temperature STM. Quasi-particle interference (QPI) patterns are clearly observed, as expected from the absence of back-scattering. The energy dispersions of the QPI are in good agreement with recent results from ARPES. We also find the energy of minimal local DOS, associated with the Dirac energy, spatially fluctuates due to n- and p-type atomic defects inherently existent in these intermixed systems. These results provide not only atomic-scale characterization of the defects but also a direct evidence for robust TSS against highly-disordered charged defects. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F42.00011: Imaging edge currents in HgTe quantum wells in the quantum spin Hall (QSH) regime Katja C. Nowack, Eric M. Spanton, Maria R. Calvo, Matthias Baenninger, Markus Koenig, Eli Fox, Andrew J. Bestwick, John R. Kirtley, Beena Kalisky, Mathias Muehlbauer, Chrsitian Ames, Philip Leubner, Christoph Bruene, Hartmut Buhmann, Laurens W. Molenkamp, David Goldhaber-Gordon, Kathryn A. Moler Conducting edge modes at the sample boundaries are a key feature of the QSH state, which was predicted and experimentally demonstrated in HgTe quantum wells. The existence of the edge modes has been inferred from conductance measurements on sufficiently small devices. Here we use a scanning SQUID to image current in devices made from HgTe quantum wells. First, I will show images of current in large Hallbars directly confirming the existence of the edge modes. Next, I will discuss progress on detecting persistent currents (PCs) flowing along the edges of anti-dots in HgTe quantum wells. The magnitude of the PC, which is periodic in flux, will depend on backscattering in the edge. Our scanning SQUID can probe currents of less than a nA, and allows us to characterize many anti-dots, one anti-dot at a time, on the same sample. The dependence of the PC on length and temperature, as well as variations between nominally identical anti-dots may provide insight into the scattering mechanisms that limit the ballistic nature of the QSH edge modes. [Preview Abstract] |
Session F43: Topological Superconductivity: Theory
Sponsoring Units: DCMPChair: Sasha Balatsky, Los Alamos National Laboratory
Room: Mile High Ballroom 4B
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F43.00001: Detecting Perfect Transmission in Josephson Junctions on the Surface of Three Dimensional Topological Insulators Jens H. Bardarson, Roni Ilan, H.-S. Sim, Joel E. Moore We consider Josephson junctions on surfaces of three dimensional topological insulator nanowires. We find that in the presence of a parallel magnetic field, short junctions on nanowires show signatures of a perfectly transmitted mode capable of supporting Majorana fermions. Such signatures appear in the current-phase relation in the presence or absence of the fermion parity anomaly, and are most striking when considering the critical current as a function of flux $\Phi$, which exhibits a peak around $\Phi = h/2e$. The peak sharpens in the presence of disorder at low but finite chemical potentials, and can be easily disentangled from weak-antilocalization effects. The peak also survives at small but finite temperatures, and represents a realistic and robust hallmark for perfect transmission and the emergence of Majorana physics inside the wire. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F43.00002: Proximity-induced unconventional superconductivity in topological insulators Annica Black-Schaffer, Alexander Balatsky We study proximity-induced superconducting pairing in a three-dimensional topological insulator - superconductor hybrid structure for superconductors with different pairing symmetries. The Dirac surface state in the topological insulator gives rise to a coupling between spin-singlet and spin-triplet pairing amplitudes as well as pairing that is odd in frequency for $p$-wave superconductors. We also find that all superconductors induce pairing that is odd in both frequency and orbital (band) index, with a complete reciprocity between pairing in orbital index and frequency. We show that the different induced pairing amplitudes significantly modify the density of states in the TI surface layer. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F43.00003: Dirac Fermion induced Parity Mixing in Superconducting Topological Insulators Takeshi Mizushima, Ai Yamakage, Masatoshi Sato, Yukio Tanaka We self-consistently study surface states of the recently discovered superconducting topological insulator Cu$_{\mathrm{x}}$Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$. We demonstrate that if a topologically trivial bulk s-wave paring symmetry is realized, parity mixing of pair potential near the surface is anomalously enhanced by surface Dirac fermions, opening an additional surface gap larger than the bulk one. Contrary to classical s-wave superconductors, the resulting surface density of state hosts an extra coherent peak at the induced gap besides a conventional peak at the bulk gap. In contrast, no such a surface parity mixing is induced by Dirac fermions for topological odd-parity superconductors. Our calculation suggests that a simple U-shaped spectrum of scanning tunneling microscope is not originated from s-wave superconductivity of Cu$_{\mathrm{x}}$Bi$_{\mathrm{2}}$Se$_{\mathrm{3}}$. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F43.00004: Anomalous Topological Pumps and Fractional Josephson Effects Fan Zhang, C.L. Kane We discover novel topological pumps in the Josephson effects for superconductors. The phase difference, which is odd under the chiral symmetry defined by the product of time-reversal and particle-hole symmetries, acts as an anomalous adiabatic parameter. These pumping cycles are different from those in the ``periodic table,'' and are characterized by $Z \times Z$ or $Z_2 \times Z_2$ strong invariants. We determine the general classifications in class AIII, and those in class DIII with a single anomalous parameter. For the $Z_2 \times Z_2$ topological pump in class DIII, one $Z_2$ invariant describes the coincidence of fermion parity and spin pumps whereas the other one reflects the non-Abelian statistics of Majorana Kramers pairs, leading to three distinct fractional Josephson effects. For the $Z \times Z$ topological pump in class DIII, Weyl or/and Dirac fermions appear in the Andreev spectrum. [arXiv:1310.5281] [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F43.00005: 4pi periodic Josephson current through a Quantum Spin-Hall edge Jan Dahlhaus, Carlo Beenakker, Dmitry Pikulin, Timo Hyart, Henning Schomerus The helical edge state of a quantum spin-Hall insulator can carry a supercurrent in equilibrium between two superconducting electrodes (separation L, coherence length ?). We calculate the maximum (critical) current Ic that can flow without dissipation along a single edge, going beyond the short-junction restriction L?? of earlier work, and find a dependence on the fermion parity of the ground state when L becomes larger than ?. Fermion-parity conservation doubles the critical current in the low-temperature, long-junction limit, while for a short junction Ic is the same with or without parity constraints. This provides a phase-insensitive, dc signature of the 4?-periodic Josephson effect. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F43.00006: Ac Josephson Effect in Topological Josephson Junctions Julia Meyer, Driss Badiane, Manuel Houzet, Leonid Glazman Topological superconductors admit zero-energy Majorana bound states at their boundaries. In Josephson junctions between two topological superconductors, the presence of these states gives rise to an Andreev bound state whose energy varies $4\pi$-periodically in the superconducting phase difference. An applied voltage bias leads to a dynamically varying phase according to the Josephson relation. Furthermore, it leads to dynamics of the occupation of the bound state via its non-adiabatic coupling to the continuum. While the Josephson relation suggests a fractional Josephson effect due to the $4\pi$-periodicity of the bound state, its observability relies on the conservation of the occupation of the bound state on the experimentally probed time scale. We study the lifetime of the bound state and identify the time scales it has to be compared to. In particular, we are interested in signatures of the fractional Josephson effect in the Shapiro steps and in current noise measurements. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F43.00007: Dynamics of Axion Vortices Akihiro Tanaka, Toru Kikuchi, Muneto Nitta Vortes lines of axion fields - axion strings - can arise in a variety of topological states of matter, e.g. in Weyl semimetals, topological superconductors, and dislocations in topological insulators. We derive the forces exterted on such axion vortices in analogy to the forces which a vortex in motion within a superfluid experiences. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F43.00008: Topological properties of possible singlet chiral superconducting states for URu$_2$Si$_2$ Pallab Goswami, Luis Balicas We show that the current thermodynamic measurements in the superconducting phase of URu$_2$Si$_2$ are compatible with two distinct singlet chiral paired states $k_z(k_x \pm i k_y)$ and $(k_x \pm i k_y)^2$. Despite possessing similar low temperature thermodynamic properties, these two pairings are topologically distinguished by their respective orbital angular momentum projections along the c-axis, $m= \pm 1$ and $m= \pm 2$. The point nodes of these states act as the charge-m monopoles and anti-monopoles of the Berry's gauge flux, which are separated in the momentum space along the c-axis, and the Berry's flux through the ab plane equals m. These topologically nontrivial point nodes, give rise to m copies of protected spin degenerate, chirally dispersing surface states on the ca and the cb planes, which carry surface current, and their energies vanish at the Fermi arcs. The Berry's flux through the ab plane gives rise to anomalous spin and thermal Hall conductivities, and various magnetoelectric effects. However, the clear determination of the bulk invariant can only be achieved by probing the pairing symmetry via a corner Josephson junction measurement, and Fourier resolved surface sensitive measurements of the Fermi arcs. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F43.00009: Robust Transport Signatures of Topological Superconductivity in Topological Insulator Nanowires Fernando de Juan, Roni Ilan, Jens H. Bardarson Finding a clear signature of topological superconductivity in transport experiments remains, to this date, an outstanding challenge. In this work, we propose to exploit the unique properties of nanowires made from three-dimensional topological insulators to generate a normal-superconductor junction in the single-mode regime, where an exactly quantized $2e^2/h$ zero-bias conductance can be observed over a wide range of realistic system parameters. Magnetic fields allow to reach the single mode regime in the normal part and to tune into and out of the topological regime in the superconducting region independently. The measurement proposed is insensitive to disorder, and the quantization of conductance survives at finite temperatures. Our proposal may be understood as an experimentally feasible variant of a Majorana interferometer. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F43.00010: Quantum anomalous Hall effect and novel topological superconductivity realized in thin-film SnTe Chen Fang, Matthew Gilbert, B. Andrei Bernevig The newly discovered topological crystalline insulator SnTe has surface states described by four Dirac cones protected by mirror symmetries. The properties of these novel topological surface states make realizing new topological phases possible. Here we propose that by using magnetic dopants and utilizing piezoelectric deposition, an anomalous Hall state can be obtained with Chern number tunable between $\pm4$ in thin-film SnTe. In another proposal, we propose that by proximity induced superconductivity in the thin film, a new kind of topological superconductivity can be obtained which hosts two Majorana states at a single superconducting vortex of unit flux, protected from hybridization (gapping) by magnetic group symmetries. The results are extended to other topological crystalline insulators. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F43.00011: Isotropic 3D f-wave topological Cooper pairing Wang Yang, Yi Li, Congjun Wu We generalize the 3D isotropic p-wave spin triplet Cooper pairing state of the $^3$He-B type into even high orbital partial-wave channels with large-spin fermions. In the spin-3/2 case, the $f$-orbital partial wave channel can support a spin-septet pairing yielding a fully gapped rotationally invariant pairing structure. Its topological properties are analyzed through the calculation of the gapless surface spectra. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F43.00012: Systematic Construction of tight-binding Hamiltonians for Topological Insulators and Superconductors Dong-Ling Deng, Sheng-Tao Wang, Lu-Ming Duan A remarkable discovery in recent years is that there exist various kinds of topological insulators and superconductors characterized by a periodic table according to the system symmetry and dimensionality. To physically realize these peculiar phases and study their properties, a critical step is to construct experimentally relevant Hamiltonians which support these topological phases. We propose a general and systematic method based on the quaternion algebra to construct the tight binding Hamiltonians for all the three-dimensional topological phases in the periodic table characterized by arbitrary integer topological invariants, which include the spin-singlet and the spin-triplet topological superconductors, the Hopf and the chiral topological insulators as particular examples. For each class, we calculate the corresponding topological invariants through both geometric analysis and numerical simulation. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F43.00013: Superconducting Proximity Effect in Topological Metal Kyungmin Lee, Abolhassan Vaezi, Mark H. Fischer, Eun-Ah Kim Much interest in the superconducting proximity effect in 3D topological insulators (TI) has been driven by the potential to induce exotic pairing states at the interface surface. However most candidate materials for 3D TI's are in fact bulk metals, due to the presence of bulk conduction states at the Fermi level. Nevertheless, such systems can have well-defined surface states exhibiting robust spin-momentum locking when the doping level is low enough. For such topological metals (TM), superconducting proximity effect can be qualitatively different from that in TI's. By studying a model topological metal-superconductor (TM-SC) heterostructure within Bogoliubov-de Gennes formalism, we show that the pairing amplitude is not confined to the interface as it is in topological insulator-superconductor (TI-SC) heterostructure and rather it reaches the naked surface. Furthermore, we predict vortex bound state spectra to contain a Majorana zero mode localized at the naked surface, separated from the bulk vortex bound state spectra by a finite gap in such a TM-SC heterostructure. Such naked-surface-bound Majorana modes are amenable to experimental observation and manipulation and hence present experimental advantage of TM-SC structure over TI-SC structure. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F43.00014: Symmetry breaking in topological insulators and high temperature superconductors Gayanath Fernando, Kalum Palandage, Kun Fang, Armen Kocharian We study symmetry breaking in three dimensional topological insulators due to various magnetic and nonmagnetic 3d transition metal dopants such as Cr and V. In addition, variational cluster approximation based Hubbard-Rashba systems are studied in order to identify effects of correlations on various geometries. Spin-Hall effect in small clusters, such as ladders with various boundary conditions, is also addressed. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F43.00015: Signatures of topological superconductivity in quantum spin Hall/superconductor junctions Shu-Ping Lee, Karen Michaeli, Jason Alicea, Amir Yacoby Interfacing s-wave superconductors with quantum spin Hall systems provides a highly favorable route to topological superconductivity and Majorana zero-modes. Indeed, once a proximity effect is successfully induced, topological superconductivity emerges very naturally -- tuning of the chemical potential in the quantum spin Hall system is unnecessary, and moreover disorder effects are greatly suppressed since time-reversal symmetry breaking is not required. The ability to implement such systems raises fundamental questions; for instance, how can one definitively expose the topological superconducting phase experimentally? We provide a possible answer by studying long Josephson junctions in quantum spin Hall systems. In particular, we predict fingerprints of topological superconductivity related to the ``fractional Josephson effect'' that, remarkably, survive even in the presence of parity relaxation processes. [Preview Abstract] |
Session F44: Focus Session: Defects in Semiconductors: Lighting Materials
Sponsoring Units: DMP FIAPChair: Anderson Janotti, University of California, Santa Barbara
Room: Mile High Ballroom 4C
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F44.00001: Theoretical stability of Eu dopant in diamond Wenhao Hu, Michael E. Flatt\'e Due to their extremely long spin coherence times, rare earth ions are promising candidates for high resolution magnetic sensing. In addition, recent progress on high resolution magnetometry based on the diamond NV center suggests that the combination of diamond and rare earth ions might have perform well. In this article, we simulated europium complexes in diamond using density functional theory in the LSDA+U approximation. We used a 64-atom supercell for the diamond host, inserted Eu and removed 1-4 carbon atoms; atomic positions were allowed to relax with a force precision of 1.0 mRy/a.u. The formation energies of possible configurations of charged Eu in diamond were investigated. The first order Markov-Payne correction was used to remove the effects of the supercell size and fictitious charge background. The formation energy for substitutional Eu is very large, originating from the limited relaxation space for the nearest neighbor carbons, however the formation energy is much lower with 1-3 surrounding carbon vacancies. We find the most stable configuration is the +1 charged Eu with 1 neighboring vacancy. The work was supported by an AFOSR MURI. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F44.00002: Bistable lattice position of a single magnetic dopant in a semiconductor Jeffrey M. Moore, Victoria R. Kortan, C\"uneyt \c{S}ahin, Juanita Bocquel, Paul M. Koenraad, Michael E. Flatt\'e Electronic control of the lattice position of individual dopants has been demonstrated recently, including displacement of a single Si dopant in the surface layer of GaAs by a scanning tunneling microscope (STM)[1]. Fe dopants in GaAs have internal spin degrees of freedom associated with their core d states which can also be manipulated using a STM[2]. A reversible and hysteretic change in the topography measured near a single Fe dopant is observed when a negative bias voltage is applied. To determine if a lattice displacement is responsible, we have performed first-principles calculations to evaluate the formation energy of a single Fe atom embedded in GaAs as a function of displacement from the substitutional site. Our calculations support the existence of a second stable configuration, characterized by a displacement accompanied by a change in atomic configuration symmetry about the Fe from four-fold to six-fold symmetry. These results expand the range of demonstrated local configurational changes induced electronically for dopants, and thus may be of use for sensitive control of spin-spin interactions between dopants.\\[4pt] [1] J. K. Garleff et al., Physical Review B 84, 075459 (2011).\\[4pt] [2] J. Bocquel et al., Physical Review B 87, 075421 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F44.00003: The Influence of Donor-Acceptor Pairs on Excitation Efficiency in GaN:Eu Brandon Mitchell, Jonathan Poplawsky, Dong-Gun Lee, Yasufumi Fujiwara, Volkmar Dierolf The nature of Eu incorporation and resulting luminescence efficiency, in GaN, has been extensively investigated. By performing a comparative study on Eu:GaN samples grown under a variety of controlled conditions, and using a variety of experimental techniques, the configuration of the majority site has been concluded to contain a nitrogen vacancy (V$_{\mathrm{N}})$. The nitrogen vacancy can appear in two symmetries, which has a profound impact on the luminescence and magnetic properties of the sample. The structure of the minority site has also been identified, and we further propose, and give evidence to the idea that the excitation efficiency, of both sites, is the result of a donor acceptor pair in the vicinity of the Eu. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F44.00004: Electronic and Optical Properties of Luminescent Centers in Halides and Oxides Invited Speaker: Mao-Hua Du Luminescent materials, such as phosphors and scintillators, are widely used for fluorescent lighting, laser, medical imaging, nuclear material detection, etc. . The luminescence is usually activated by impurities (or activators), which act as luminescence centers. The activators are typically multi-valent ions that insert multiple electronic states in the band gap of the host material. In this talk, first-principles calculations of electronic structure and optical transitions are shown for a wide range of activators, including rare-earth ions (e.g., Ce$^{\mathrm{3+}}$, Eu$^{\mathrm{2+}})$, ns$^{\mathrm{2}}$ ions (the ions that have outer electronic configurations of ns$^{\mathrm{2}}$, such as Tl$^{\mathrm{+}}$, Pb$^{\mathrm{2+}}$, Bi$^{\mathrm{3+}})$, and transition-metal ions (e.g., Mn$^{\mathrm{4+}})$, in a large number of halides and oxides. The results reveal how the activator-ligand hybridization affects the emission energy and the luminescence mechanism. New phosphors and scintillators are proposed based on the chemical trends emerging from the calculations of a large number of materials. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F44.00005: Stability of oxygen dopants in group-III nitride alloys Ji-Sang Park, K.J. Chang Group-III nitride materials have attracted much attention for their potential applications in light-emitting devices such as light-emitting and laser diodes. Low resistivity p-type layers are demanding, however, the p-type doping efficiency is still low due to high Mg acceptor level and compensating donor defects such as interstitial hydrogen and nitrogen vacancy. Several donor-acceptor co-doping methods have been suggested to enhance the p-type doping efficiency in group-III nitrides; however, there is a lack of study on the stability and electronic properties of donor dopants in nitride alloys. In this study, we investigate site preference of oxygen dopants in group-III nitride alloys including ternary AlGaN and quaternary AlInGaN alloys through first-principles density functional calculations. We adjust the composition ratio of Al and In to make the band gap of AlInGaN same to that of AlGaN. In AlGaN, we find that the oxygen dopants tend to bond with Al atoms due to the high bond energy between Al and O. The same tendency is found in AlInGaN, whereas the dopants also become stable as they are bonded to In atoms due to small strain. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F44.00006: Phonon-Assisted Auger Recombination in Medium and Wide Band-Gap Materials from First Principles Daniel Steiauf, Emmanouil Kioupakis, Chris G. Van de Walle GaN and GaAs and their alloys are technologically important materials for solid-state optoelectronic devices such as LEDs and lasers. The internal quantum efficiency of these devices, defined as the fraction of electron-hole pairs converted to photons, is limited by nonradiative loss mechanisms. Auger recombination is such a mechanism which limits the efficiency at high carrier densities. The energy and momentum of an electron-hole pair is transferred to a third carrier instead of creating a photon. We present state-of-the-art results of first-principles calculations of the Auger recombination rate coefficients both for the simple direct purely Coulombic process and the indirect phonon-assisted process. We find the absolute values of these recombination rates as well as their relative importance. In GaAs, when energy and momentum of the recombining pair are transferred to an Auger electron, the phonon-assisted process is several orders of magnitude stronger than the direct process, while for recombinations that create an Auger hole, the direct and phonon-assisted processes contribute almost equally. For lower band gaps, the electron processes become equally strong, and also the direct process becomes comparable in magnitude. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F44.00007: Efficiency Droop in Nanostructured III-N LEDs: Multiscale Numerical Analysis and Design Optimization Rezaul Nishat, Vinay Chimalgi, Krishna Yalavarthi, Shaikh Ahmed Recently, optical emitters using InGaN nanostructures have attracted much attention for applications in lasers, solid-state lighting, near-field photolithography, free-space quantum cryptography, consumer displays, as well as diagnostic medicine and imaging. Nanostructures can accommodate a broader range of lattice mismatch thereby allowing full-solar-spectrum emission characteristic, and provide larger active surface area and higher temperature stability. Nevertheless, performance of these III-N LEDs is determined by an intricate interplay of complex, nonlinear, highly stochastic and dynamically-coupled structural fields, charge, and thermal transport processes at different length and time scales. In this work, we study the effects of these coupled processes on the electronic and optical emission properties in nanostructured III-N LEDs. The multiscale computational framework employs the atomistic valence force-field molecular mechanics, the 10-band \textit{sp}$^{3}s^{\ast }$\textit{-SO} tight-binding models, and a coupling to a TCAD toolkit to determine the terminal properties of the device. Finally, a series of numerical experiments are performed (by varying different nanoscale parameters such as size, geometry, crystal cut, composition, surface and contacts, and electrostatics) that mainly aim to improve the efficiency \textit{droop} and reliability of these LEDs. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F44.00008: Blue luminescence and the Zn acceptor in GaN: test case for the hybrid functional approach Denis Demchenko, Michael Reshchikov We present a comparison of exchange tuned hybrid density functional calculations with experimental data obtained for the Zn acceptor in GaN. Since this acceptor is one of the few reliably identified defects in GaN, we use Zn-doped GaN as a test case for the widely used HSE06 hybrid functional method of calculations of defect properties in semiconductors. Here, we present the experimental results of luminescence measurements in Zn-doped GaN from which we obtain Zn acceptor defect levels. They are compared with theoretically calculated defect thermodynamic and optical transition levels, as well as the zero phonon line associated with this acceptor. We also analyze the dependence of the results on the exchange tuning procedure used in HSE06 hybrid functional. Excellent agreement with experiment is obtained when the amount of exact exchange in HSE06 is tuned to reproduce the GaN experimental band gap. This favorable comparison with the experimental results for a well-established defect suggests that the exchange tuned HSE06 hybrid functional yields accurate defect properties in GaN and therefore has significant predictive power. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F44.00009: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F44.00010: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F44.00011: Multivalency of group-V elements in SnO$_2$ Haowei Peng, Stephan Lany Multivalence is an intrinsic property of elements being capable to change their valence state which usually companies with environmental perturbation such as lattice distortion. It commonly shows in transition metal compounds, but also in some some main-group elements, especially heavy group-IV and -V elements. Group-V elements were proposed as $n$-type dopants in SnO$_2$, and compared with the commercial FTO (F-doped SnO$_2$), the cation-site incorporation can facilitate various growth techniques. However, substituting for Sn$^{4+}$ ions, the group-V elements can possess either the desired $5+$ oxidation state that generates electron charge carriers, or a compensating $3+$ oxidation state. Hence, specific attention to this multivalence characteristics is indispensable. To this end, we accurately determine the defect transition energy level $\epsilon(1-/1+)$ with respect to the conduction band minimum, by combining the state-of-art quasi-particle GW and hybrid functional calculations. Group-V elements including P, As, Sb and Bi are considered, which have strong site-preference on Sn instead of on O sites. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F44.00012: Excited States of the divacancy in SiC Michel Bockstedte, Thomas Garratt, Viktor Ivady, Adam Gali The divacancy in SiC - a technologically mature material that fulfills the necessary requirements\footnote{J.~R.~Weber \emph{et al.}, PNAS \textbf{107}, 8513 (2010).} for hosting defect based quantum computing - is a good candidate for implementing a solid state quantum bit. Its ground state is isovalent to the NV center in diamond as demonstrated by density functional theory (DFT).\footnote{A.~Gali, phys. status solidi (b) \textbf{248}, 1337 (2011); J.~P. Gross \emph{et al.} \textbf{77}, 3041 (1996).} Furthermore, coherent manipulation of divacancy spins in SiC has been demonstrated.\footnote{F.~Koehl \emph{et al.}, Nature\textbf{479}, 84 (2011).} The similarities to NV might indicate that the same inter system crossing (ICS) from the high to the low spin state is responsible for its spin-dependent fluorescent signal. By DFT and a DFT-based multi-reference hamiltonian we analyze the excited state spectrum of the defects. In contrast to the current picture of the spin dynamics of the NV center, we predict that a static Jahn-Teller effect in the first excited triplet states governs an ICS both with the excited and ground state of the divacancy. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F44.00013: Cs-based gamma-radiation detector material Cs$_2$Hg$_6$S$_7$: First-principles study of extrinsic doping Jino Im, Shichao Wang, John A. Peters, Zhifu Liu, Bruce W. Wessels, Mercouri G. Kanatzidis, Arthur J. Freeman Semiconductor X-ray/$\gamma$-ray radiation detectors have broad applications, yet finding superior detector materials that work at room temperature is a challenge because of its contradictory requirements. In a previous study, the ternary compound Cs$_2$Hg$_6$S$_7$ was proposed as a possible candidate because of its high density, optimal band gap and high $\mu\tau$ values. However, the low resistivity originating from p-type carriers is a detrimental factor that limits its performance. As a strategy to increase the resistivity, we investigated compensation by extrinsic doping. Using first-principles density functional theory calculations we focused on finding a proper dopant which gives a shallow donor level that leads to a compensation of hole carriers. We tested a number of extrinsic dopants and, as a result, we found that indium is a promising dopant for the strategy to increase the resistivity of Cs$_2$Hg$_6$S$_7$. [Preview Abstract] |
Session F45: Fractional Quantum Hall Effect: First Landau Level
Sponsoring Units: FIAPChair: Javad Shabani, University of California, Santa Barbara
Room: Mile High Ballroom 4D
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F45.00001: Phase Diagrams for the $\nu$ = 1/2 Fractional Quantum Hall Effect in Electron Systems Confined to Symmetric, Wide GaAs Quantum Wells L.N. Pfeiffer, J. Shabani, Y. Liu, M. Shayegan, K.W. West, K.W. Baldwin We report an experimental investigation of fractional quantum Hall effect (FQHE) at the even-denominator Landau level filling factor $\nu$ = 1/2 in high quality wide GaAs quantum wells. The quasi-two-dimensional electron systems we study are confined to GaAs quantum wells with widths, W, ranging from 41 to 96 nm and have variable densities in the range of $4 \times 10^{10}$ to $4 \times 10^{11} cm^{-2}$. We present several experimental phase diagrams for the stability of the $\nu$ = 1/2 FQHE in these quantum wells. We find that the densities at which the $\nu$ = 1/2 FQHE is stable are larger for narrower quantum wells. Moreover, even a slight charge distribution asymmetry destabilizes the $\nu$ = 1/2 FQHE and turns the electron system into a compressible state. We also present a plot of the subband separation ($\Delta_{SAS}$), which characterizes the interlayer tunneling, vs density for various W. Finally, we summarize the experimental data in a diagram that takes into account the relative strengths of the inter-layer and intra-layer Coulomb interactions and $\Delta_{SAS}$. We compare this experimental phase diagram of normalized inter-layer distance vs tunneling to recent theoretical calculations which have been used to conclude a two-component origin for the $\nu$ = 1/2 FQHE. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F45.00002: Fractional Quantum Hall Effect at $\nu = 1/2$ in Hole Systems Confined to GaAs Wide Quantum Wells Sukret Hasdemir, Yang Liu, Aurelius Graninger, Mansour Shayegan, Loren Pfeiffer, Ken West, Kirk Baldwin, Roland Winkler We observe fractional quantum Hall effect (FQHE) at the even-denominator Landau level filling factor $\nu = 1/2$ in two-dimensional hole systems confined to GaAs quantum wells of width 30 to 50 nm and having bilayer-like charge distributions. The $\nu = 1/2$ FQHE is stable when the charge distribution is symmetric and only in a range of intermediate densities, qualitatively similar to what is seen in two-dimensional electron systems confined to approximately twice wider GaAs quantum wells. Despite the complexity of the hole Landau level structure, originating from the coexistence and mixing of the heavy- and light-hole states, we find the hole $\nu = 1/2$ FQHE to be consistent with a two-component, Halperin-Laughlin ($\Psi_{331}$) state. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F45.00003: Even-denominator Fractional Quantum Hall Effect at a Landau Level Crossing Yang Liu, Sukret Hasdemir, Dobromir Kamburov, Aurelius Graninger, Mansour Shayegan, Loren Pfeiffer, Ken West, Kirk Baldwin, Roland Winkler The fractional quantum Hall (FQH) effect, observed in two-dimensional charged particles at high magnetic fields, occurs when the filling factor $\nu$ of the quantized Landau levels is a fraction which, with very few exceptions, has an odd denominator. Here we describe unexpected phenomena in two-dimensional hole systems confined to GaAs quantum wells. We observe an unusual crossing of the two lowest-energy Landal levels. The crossing leads to a weakening or disappearance of the commonly seen odd-denominator FQH states in the filling range $1/3 < \nu < 2/3$. But, surprisingly, a new FQH state at the even-denominator filling $\nu= 1/2$ comes to exist at the crossing. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F45.00004: Evolution of the $\nu=1/2$ Fractional Quantum Hall State in Tilted Magnetic Fields Hao Deng, Sukret Hasdemir, Yang Liu, Mansour Shayegan, Loren Pfeiffer, Ken West, Kirk Baldwin We report magneto-transport measurements of two-dimensional electron systems confined to 65-nm-wide GaAs quantum wells with density 1.4 10$^{11}$ cm$^{-2}$. We observe a remarkable evolution of the magnetoresistance around filling factor $\nu$=1/2 as we increase the tilting angle. The weak $\nu$=1/2 fractional quantum Hall (FQH) state at fully perpendicular field gets stronger as the sample is tilted, but abruptly disappears at higher tilting angles as an insulating phase moves from low fillings to higher ones near $\nu$=1/2. This insulating phase likely signals a bilayer, pinned Wigner crystal. At higher tilting angles, we observe a disappearance of the $\nu$=1 quantum Hall state and the appearance of even-numerator FQH states around $\nu$=1, which are also consistent with the interpretation that the system becomes bilayer. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F45.00005: State Counting for Excited Bands of the Fractional Quantum Hall Effect: Exclusion Rules for Bound Excitons Ajit Coimbatore Balram, Arkadiusz W\'ojs, Jainendra Jain Exact diagonalization studies have revealed that the energy spectrum of interacting electrons in the lowest Landau level splits, non-perturbatively, into bands. The theory of nearly free composite fermions (CFs) has been shown to be valid for the lowest band, and thus to capture the low temperature physics, but it over-predicts the number of states for the excited bands. We explain the state counting of higher bands in terms of composite fermions with an infinitely strong short range interaction between a CF particle and a CF hole. This interaction, the form of which we derive from the microscopic CF theory, eliminates configurations containing certain tightly bound CF excitons. With this modification, the CF theory reproduces, for all well-defined excited bands, an exact counting for $\nu>1/3$, and an almost exact counting for $\nu\leq 1/3$. The resulting insight clarifies that the corrections to the nearly free CF theory are not thermodynamically significant at sufficiently low temperatures, thus providing a microscopic explanation for why it has proved successful for the analysis of the various properties of the CF Fermi sea. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F45.00006: Integer and Fractional Quantum Hall Effect of Two-Component Bosons Yinghai Wu, Jainendra Jain We investigate integer and fractional quantum Hall states for two-component bosons in the lowest Landau level at filling factors $\nu=2/3$, 4/5, 4/3, and 2, using the generic label ``spin'' for the two components. We study ground states, excitations, edge states and entanglement spectrum for systems with up to 16 bosons, and construct explicit trial wave functions to clarify the underlying physics. For $\nu=4/3$ a ``non-Abelian spin-singlet'' state has been proposed to occur for a 2-body contact interaction; we find that it is more likely that the actual state here is a spin-singlet state of reverse-flux-attached composite fermions at filling $\nu^*=4$. The incompressible state at $\nu=2$ provides an example of bosonic integer topological states; it can be understood as the spin-singlet state of reverse-flux-attached composite fermions at $\nu^*=2$. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F45.00007: Momentum-resolved probing of the $\nu=2/3$ quantum Hall edge Hendrik Meier, Yuval Gefen, Leonid Glazman We evaluate the $I$-$V$ characteristic for momentum-resolved tunneling between the $\nu=2/3$ fractional quantum Hall state and a $\nu=1$ state in another layer of a two-dimensional electron gas (2DEG). In a version of a double-layer geometry, the momentum of tunneling electrons may be boosted by an auxiliary magnetic field parallel to the two planes of 2DEGs. The threshold behavior of the $I$-$V$ characteristic and its dependence on the boosting magnetic field yields information about the spectral function of the $\nu=2/3$ edge. It may bring insights into the nature of the various (counter)propagating modes inside the $\nu=2/3$ edge that have been discussed in the last twenty years. Effects due to in-plane disorder as well as of intralayer and interlayer Coulomb interaction are taken into account in our model. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F45.00008: Origin of composite particle ``mass'' in the fractional quantum Hall effect F.D.M. Haldane Composite particles in a partially-filled 2D Landau level are formed by ``flux attachment'' of q empty orbitals to p particles to form either a ``composite boson'' or a ``composite fermion''. The geometry of ``flux attachment'' (the shape of the q-orbital correlation hole that contains the p particles) is the principal degree of freedom of the composite particle, and can adjust to the local environment. An additional independent degree of freedom is the electric polarization of the composite particle by deformation of its inversion-symmetric charge profile to produce an electric dipole moment. In a magnetic field, the momentum is the magnetic flux density B times the electric dipole, rotated through 90 degrees. The energy increase of the composite boson as a function of its electric dipole moment is thus also its dispersion as a function of momentum. This then gives the quadratic dispersion that defines the analog of inertial ``mass'' of the composite particle. This gives both the stiffness constant of the Ginzburg-Landau term in the FQHE composite boson picture, and the dispersion of composite fermions in the $\nu$ = 1/2 composite Fermi liquid state. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F45.00009: Composite Fermion Spin Polarization Energy with Finite Layer Thickness Mansour Shayegan, Yang Liu, Sukret Hasdemir, Loren Pfeiffer, Ken West, Kirk Baldwin We study the spin polarization transitions of fractional quantum Hall (FQH) states in the filling range $1 < \nu < 2$ in symmetric quantum wells (QWs), as a function of density. Our results reveal a strong well-width dependence of the critical density $n_C$ and ratio between the Zeeman energy ($E_Z$) normalized to the Coulomb energy ($e^2/4\pi\epsilon l_B$), above which a certain FQH state becomes spin polarized. For example, the $\nu=7/5$ FQH state becomes spin polarized at about 3 times higher density or 1.7 times larger $E_Z$ in the 31-nm-wide QW than in the 65-nm-wide QW. This well-width dependence of the spin polarization stems from by the finite electron layer thickness in these QWs and the resulting softening of the Coulomb interaction. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F45.00010: Anisotropic Fermi Contour of (001) GaAs Electrons in Parallel Magnetic Fields M.A. Mueed, Dobromir Kamburov, Mansour Shayegan, L.N. Pfeiffer, K.W. West, K.W. Baldwin, J.J.D. Lee, Roland Winkler We demonstrate a severe Fermi contour anisotropy induced by the application of a parallel magnetic field to high-mobility electrons confined to a 30-nm-wide (001) GaAs quantum well. We study commensurability oscillations, namely geometrical resonances of the electron orbits with a unidirectional, surface-strain-induced, periodic potential modulation, to directly probe the size of the Fermi contours along and perpendicular to the parallel field. Their areas are obtained from the Shubnikov-de Haas oscillations. Our experimental data agree semi-quantitatively with the results of parameter-free calculations of the Fermi contours but there are significant discrepancies. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F45.00011: Fermi Contour Anisotropy of GaAs Electron-Flux Composite Fermions in Parallel Magnetic Fields Dobromir Kamburov, M.A. Mueed, Mansour Shayegan, Loren Pfeiffer, Kenneth West, Kirk Baldwin, J.J.D. Lee, Roland Winkler In high-quality two-dimensional electrons confined to GaAs quantum wells, near Landau level filling factors $\nu =$1/2 and 1/4, we observe signatures of the commensurability of the electron-flux composite fermion cyclotron orbits with a unidirectional periodic density modulation. Focusing on the data near $\nu =$1/2, we directly and quantitatively probe the shape of the composite fermions' cyclotron orbit, and therefore their Fermi contour, as a function of magnetic field (B$_{\mathrm{\vert \vert }})$ applied parallel to the sample plane. The composite fermion Fermi contour becomes severely distorted with increasing B$_{\mathrm{\vert \vert }}$ and appears to be elliptical, in sharp contrast to the electron Fermi contour which splits as the system becomes bilayer-like at high B$_{\mathrm{\vert \vert }}$. We present a simple, qualitative model to interpret our findings. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F45.00012: Spin-Polarization of $\nu =$ 3/2 Composite Fermions Insun Jo, Dobromir Kamburov, M.A. Mueed, Yang Liu, Mansour Shayegan, Loren Pfeiffer, Ken West, Kirk Baldwin, Jerry Lee We report the observation of ballistic transport commensurability minima in the magnetoresistance of $\nu =$ 3/2 composite fermions (CFs) in high-quality two-dimensional electron systems confined to wide GaAs quantum wells and subjected to a unidirectional periodic potential modulation. The positions of the minima are consisted with the magnetic commensurability condition implying the commensurability features originate from a periodic magnetic field. Their distance away from $\nu =$ 3/2 yields the size and shape of the CF Fermi contour. At a fixed electron density of n $\approx $ 1.8 x 10$^{11}$ cm$^{-2}$, as the quantum well width increases from 30 to 60 nm, the CFs become fully spin-polarized. The application of an additional parallel magnetic field (B$_{\mathrm{\vert \vert }})$ leads to a significant distortion of the CF Fermi contour. The distortion is much more severe compared to the $\nu =$ 1/2 CF case at comparable B$_{\mathrm{\vert \vert }}$. Furthermore, the applied B$_{\mathrm{\vert \vert }}$ spin-polarizes the $\nu =$ 3/2 CFs as evinced from the size of the CF Fermi contour. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F45.00013: Observation of Incompressibility of a New Type of Fractional Quantum Hall State in the Lowest Landau Level Nodar Samkharadze, Ian Arnold, Loren Pfeiffer, Ken West, Gabor Csathy We report on an ultra-low temperature study of a quantum Hall system in the 1/3\textless $\nu $\textless 2/5 region. Due to the residual interaction of composite fermions, this region is proposed to support a new type of fractional quantum Hall states at filling factors $\nu =$4/11 and 3/8. These states are expected to belong to a different universality class from those described by the weakly interacting composite fermion model. Despite the previous observations of magnetoresistance features at $\nu =$4/11,5/13 and 3/8 [Pan et al, Phys. Rev. Lett. 90, 016801], the hallmark property of activated behavior of the fractional quantum Hall states has not yet been observed for any of these states. In our study we have achieved an electronic temperature lower by a factor 5 in comparison to the previous work, revealing for the first time, an activation gap at $\nu =$4/11 and an incipient incompressibility at $\nu =$5/13. However, despite the considerable progress in identifying the later two fractional quantum Hall ground states, at $\nu =$3/8 in our sample we observe compressible state. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F45.00014: Weakly Pinned Wigner Solid-FQHE Liquid Phase Transition in the 2-Dimensional Hole System under Ultrahigh Magnetic Fields Chi Zhang, Rui-Rui Du, Junren Shi, Xincheng Xie, Michael J. Manfra, Loren N. Pfeiffer, Ken W. West, Ju-Hyun Park In the two dimensional systems, Wigner crystals (WC) solid and fractional quantum Hall effect (FQHE) liquid phase compete under low temperature and high magnetic fields. Here, we exhibit unusual experimental results in the new developed C-doped two-dimensional hole samples. Our derivative conductivity measurements elucidate the existence of reentrant insulating phase (RIP) around the Landau level filling factor $v=$1/5 in the 2D hole system. Moreover, we report the unexpected feature in the quantum phase transition between the Wigner Solid and FQHE liquid state in the 2D hole system under ultrahigh magnetic fields. Consequently, a systematic phase diagram is obtained based on our analysis. To our surprise, the excited electric field plays an equivalent role as the temperature in our specimen. From the duality of the electric field and temperature, a characteristic length of 450 nm is derived in our Analysis, which is the mean free path of the carriers. Based on the relation between the pinning gap and electric field, we obtained a characteristic domain size of the Wigner crystal. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F45.00015: Emergence of frustrated antiferromagnet in the lowest Landau level Jun Won Rhim, Alexander C. Archer, Jainendra K. Jain, Kwon Park We investigate the spin structure of the triangular composite fermion crystals (CFCs) in the lowest Landau level (LLL). In contrast to the usual Hund's rule, our Monte-Carlo (MC) calculation finds the spin exchange energy to be antiferromagnetic in certain parameter regimes in the vicinity of $\nu=1/5$. For further physical intuition, we develop an effective two-body potential between composite fermions in the crystal phase, which provides a reasonable account of the MC results. We discuss the experimental feasibility of this physics. [Preview Abstract] |
Session F46: Quantum Criticality: Theory and Experiment
Sponsoring Units: DCMPChair: Andriy Nevidomskyy, Rice University
Room: Mile High Ballroom 4E
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F46.00001: Finite-temperature spin dynamics near the quantum critical point of transverse field Ising chain with a small longitudinal field M\'arton Kormos, Jianda Wu, Qimiao Si When the transverse-field Ising chain at its quantum critical point is subjected to a small longitudinal field, the perturbed conformal field theory led to a field theory with an exotic E8 symmetry [1]. Recent neutron scattering experiments have provided evidence for the lightest two particles in this E8 model in the quasi-1D Ising ferromagnet CoNb$_2$O$_6$ [2]. While the zero temperature dynamic of the model is well known, its finite-temperature counterpart has not yet been systematically studied. We study the low-frequency dynamical spin structure factor at finite temperatures using the form-factor method. We show that the dominant contribution to the spin dynamics comes from the channel between two lightest particles, and demonstrate how the spin dynamics differ from a diffusion form. Using these results, we determine the temperature dependence of the NMR relaxation rate. We suggest that, for CoNb$2$O$6$, measurements of the NMR relaxation rate provide a means to further test the applicability of the E8 model. \\[4pt] [1] A. B. Zamolodchikov, Int. J. Mod. Phys. A4, 4235(1989).\\[0pt] [2] R. Coldea, D. A. Tennant, E. M. Wheeler, E. Wawrzynska, D. Prabhakaran, M. Telling, K. Habicht, P. Smeibidl, K. Kiefer2, Science, 327, 177 (2010). [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F46.00002: Isoelectronically tuned magnetic and Ising quantum phase transitions in iron-based superconductors Jianda Wu, Qimiao Si The bad-metal behavior of the iron arsenides motivated a proximity-to-Mott picture, which led to the theoretical proposal for a quantum critical point (QCP) under iso-electronic phosphorous for arsenic doping in the parent iron arsenides [1]. Here, P doping increases the in-plane electronic kinetic energy and thus the coherent electronic spectral weight, thereby weakening the magnetic order and the associated Ising-nematic spin order. Extensive experimental measurements in the P-doped CeFeAsO and BaFe$_2$As$_2$ [2,3] have provided strong evidence for such a QCP. Here, we explore these phases and their transitions by carrying out a large-N study of an effective low-energy Ginzburg-Landau model for these systems. We determine the parameter range over which second order magnetic and Ising quantum phase transitions arise. [1] J. Dai, Q. Si, J-X Zhu, and E. Abrahams, PNAS, 106, 4118 (2009) [2] C. de la Cruz, et al., Phys Rev Lett, 104, 017204 (2010) [3] S. Kasahara, et al., Phys Rev B, 81, 184519 (2010); K. Hashimoto et al, Science 336, 1554 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F46.00003: Metal-SDW phase transition in $3- \epsilon$ dimensions Shouvik Sur, Sung-Sik Lee The quantum phase transition associated with a spin density wave (SDW) instability in two dimensional metals is relevant to various strongly correlated electron systems including high-$T_c$ superconductors. However, the critical point associated with the phase transition has remained largely inaccessible due to the absence of a small parameter. In this work we use the recently developed dimensional regularization scheme, where the co-dimension of a Fermi surface is extended to general values while its dimension is held fixed, to perturbatively access the quantum critical point in $3-\epsilon$ space dimensions. We derive the beta functions and compute the critical exponents to the one loop order. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F46.00004: Optical conductivity of a two-dimensional metal at the onset of spin-density-wave order Dmitrii Maslov, Andrey Chubukov, Vladimir Yudson We consider the optical conductivity of a clean two-dimensional metal at $T=0$ near a spin-density-wave instability. Critical fluctuations destroy fermionic coherence at ``hot spots'' of the Fermi surface but a large part of the Fermi surface is neither ``hot'' or ``cold'' but rather ``lukewarm,'' in a sense that quasiparticles there are strongly renormalized compared to the non-interacting case. We discuss the self-energy of lukewarm fermions and their contribution to the optical conductivity, $\sigma(\Omega)$, due to scattering off composite bosons made of two critical magnetic fluctuations. Recent study [S.A. Hartnoll et al., Phys. Rev. B {\bf 84}, 125115 (2011)] found that composite scattering leads to a singular fermionic self-energy of lukewarm fermions at the quantum critical point. We show that, at the lowest frequencies, the most singular, $\ln^3\Omega/\Omega^{1/3}$ contribution to the conductivity is canceled between the self-energy, vertex-correction, and Aslamazov-Larkin diagrams. However, the cancellation does not extent beyond logarithmic accuracy, and the remaining conductivity still diverges as $1/\Omega^{1/3}$. At larger $\Omega$, $\sigma (\Omega)$ scales in a marginal FL way, as $1/\Omega$. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F46.00005: Anomalous Scaling of Magnetic Penetration Depth from Quantum Critical Fluctuations Jian-Huang She, Michael Lawler, Eun-Ah Kim Recently, systematic penetration depth (PD) measurements carried out over several families of unconventional superconductors suggest they are near quantum critical points (QCP). In particular, the temperature dependence of the PD shows anomalous power law scaling. We argue, because the momentum carried by critical fluctuations needs to connect nodal points, this anomalous behavior is not due to AFM ordering. So instead, we focus on instabilities of the d-wave superconducting state associated with developing additional Q=0 order that can alter the scaling behavior of the PD. This additional ordering can be in either the charge channel, the pairing channel or both. We find that fluctuations in the pairing channel leads to scaling exponents smaller than one, while fluctuations in the charge channel leads to scaling exponents larger than one. Based on these results, we argue that the temperature scaling of PD in CeCoIn5 is caused by close a proximity to a QCP associated predominantly with Fermi surface distortions such as a nematic QCP. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F46.00006: Unconventional Superconductivity near a Kondo Destroyed Quantum Critical Point Jedediah Pixley, Lili Deng, Kevin Ingersent, Qimiao Si Heavy fermion metals serve as prototypical correlated materials to study antiferromagnetic quantum critical points (QCPs). Theoretical studies have identified a class of unconventional quantum critical points, in which Kondo destruction accompanies the onset of magnetic order. Whether or not such a QCP may promote superconductivity is an open question. Experimentally, there is strong evidence, e.g. from the heavy-fermion material CeRhIn$_{5}$, that such a QCP underlies unconventional superconductivity. With this in mind, we study the superconducting pairing susceptibility in the periodic Anderson model, within a cluster extended dynamical mean field theory (C-EDMFT). We find that the Kondo energy scale is continuously suppressed at the magnetic QCP. In addition, we find the pairing susceptibility to be strongly enhanced when the QCP is approached, both from the paramagnetic Kondo screened side and from the Kondo-destroyed magnetically ordered side. Our results point to a new form of unconventional superconductivity associated with both the magnetic fluctuations and a proximity to electronic localization. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F46.00007: Pairing correlations near a Kondo-destruction quantum critical point Lili Deng, Kevin Ingersent, Jedediah Pixley, Qimiao Si Motivated by the unconventional superconductivity observed in heavy-fermion metals, we investigate the pairing susceptibility near a continuous quantum phase transition of the Kondo-destruction type. We first solve two-impurity Bose-Fermi Anderson models with Ising and Heisenberg forms of the inter-impurity exchange interaction using continuous-time Monte-Carlo and numerical renormalization-group methods [1]. For each model, we determine its phase diagram and show a Kondo-destruction quantum critical point separating Kondo-screened and local-moment phases. For antiferromagnetic inter-impurity exchange interactions, singlet pairing is found to be enhanced in the vicinity of the quantum critical point. We then proceed to study the Anderson lattice model based on a cluster extended dynamical mean-field theory (C-EDMFT). We show how the results of the two-impurity models connect to those near the Kondo-destruction quantum critical point of the lattice case, and discuss the implications of our results for superconductivity in quantum-critical heavy fermions.\\[4pt] [1] J. H. Pixley, L. Deng, K. Ingersent, Q. Si, arXiv:1308.0839. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F46.00008: Deconfined Quantum Criticality and Conformal Phase Transition Flavio Nogueira, Asle Sudbo We introduce a new perspective on deconfined quantum criticality within a field-theoretic framework. We show that in the allegedly weak first-order transition regime from a N\'eel to a valence-bond solid in $SU(N)$ antiferromagnets, a so-called conformal phase transition leads to a genuine deconfined quantum critical point. In such a transition, the gap vanishes as the critical point is approached, while the spin stiffness at zero temperature has a universal jump at the critical point. We discuss the logarithmic corrections to scaling observed numerically and interpret them in terms of the conformal phase transition. The behavior of the N\'eel and valence-bond solid susceptibilities are discussed at zero and finite temperatures. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F46.00009: The dynamics of quantum criticality: Quantum Monte Carlo and holography William Witczak-Krempa, Erik Sorensen, Subir Sachdev Understanding the real time dynamics of systems near quantum critical points at finite temperature constitutes an important yet challenging problem. We present quantum Monte Carlo results for 2 separate realizations of the superfluid-insulator transition of bosons on a lattice: their low-frequency conductivities are found to have the same universal dependence on imaginary frequency and temperature. We use the structure of the real time dynamics of conformal field theories described by the holographic gauge/gravity duality to make progress on the difficult problem of analytically continuing the Monte Carlo data to real time. Our method yields quantitative and experimentally testable results on the frequency-dependent conductivity near the quantum critical point. Connections to other observables and universality classes are discussed, as well as new holographic extensions. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F46.00010: Criticality and quenched disorder: rare regions vs. Harris criterion Thomas Vojta, Jose Hoyos We employ scaling arguments and optimal fluctuation theory to establish a general relation between quantum Griffiths singularities and the Harris criterion for quantum phase transitions in disordered systems. If a clean critical point violates the Harris criterion, it is destabilized by weak disorder. At the same time, the Griffiths dynamical exponent $z'$ diverges upon approaching the transition, suggesting unconventional critical behavior. In contrast, if the Harris criterion is fulfilled, power-law Griffiths singularities can coexist with clean critical behavior but $z'$ saturates at a finite value. We present applications of our theory to a variety of systems including quantum spin chains, classical reaction-diffusion systems and metallic magnets; and we discuss modifications for transitions above the upper critical dimension. Based on these results we propose a unified classification of phase transitions in disordered systems. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F46.00011: Divergence of the Thermopower Without a Quantum Phase Transition Kridsanaphong Limtragool, Philip Phillips It is generally believed that divergent thermopowers require an underlying quantum phase transition. We show here in two exactly solvable models that this is not the case. We study the quantum XY and Kitaev models and show that the thermopower diverges in a parameter space that has nothing to do with the phase transition. The divergence is tied to a zero of the Onsager coefficient $L_{11}$. The zero of $L_{11}$ is linked to a sign change in the effective charge and a vanishing of the band velocity. At such points, there is no thermodynamic signature only a divergent thermopower. Implications for interpreting divergent thermopowers as phase transitions will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F46.00012: Nernst and angle-dependent magneto thermopower measurements on Sr3Ru2O7 Chenyi Shen, Hui Xing, Xiaojun Yang, Qian Tao, Mingliang Tian, Zhiqiang Mao, Zhuan Xu, Ying Liu The behavior of the metamagnetic transition in Sr3Ru2O7, the double-layer member of the Ruddlesden-Popper homologous series the Srn$+$1RunO3n$+$1, depends strongly on the direction of the applied magnetic field. With the field applied along the c axis, the end point of the first order metamagnetic transition is located at a very low temperature, pushing the material to be close to a quantum critical point. Exotic phenomena such as an electronic nematic phase was proposed. However, important questions on this phase remains to be unresolved. Recent measurements on the field dependent specific heat appears to suggest a state with excessive entropy away from the quantum critical point. We present our Nernst effect and angle-dependent magneto thermopower measurements on high-quality single crystals of Sr3Ru2O7. The temperature gradient was applied along the a- (or b-) axis and 45-degree from it and the in-plane magnetic field was rotating in the ab plane. Interesting behavior was found in these measurements away from the quantum critical regime. The implications of our data on various issues on Sr3Ru2O7 will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F46.00013: Infrared spectroscopy of magnetic field-induced quantum criticality in Sr$_3$Ru$_2$O$_7$ Jesse S. Hall, Urmas Nagel, Toomas R{\~o}{\~o}m, J.F. Mercure, Robin S. Perry, Andrew P. Mackenzie, Thomas Timusk Infrared spectroscopy performed on the model quantum critical system Sr$_3$Ru$_2$O$_7$ offers an opportunity to study the frequency dependence of the magnetic field-tuned quantum critical behaviour. The temperature dependence of the resistivity is well characterized in magnetic fields, as is the Fermi surface, so the relevant optical properties can be determined and compared to the theory for non-Fermi liquids. We report infrared reflectance measurements from 1- 12 meV at temperatures below 10 K and magnetic fields up to 17 T. A sharp field-dependent feature appears in the reflectance at 3.8 meV, combined with a broad suppression of the reflectance across the measurement region. The optical scattering rate will be compared to other measurements and the implications for the understanding of quantum criticality will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F46.00014: Optical Detection of the Electron Nematic Phase in Sr$_3$Ru$_2$O$_7$ Colin Heikes, D. MacNeill, S. Ghosh, R. Perry, J.F. Mercure, E.A. Kim, A. Mackenzie, D.C. Ralph We report the implementation of a fiber-based optical microscope, capable of operating at temperatures below 100 mK and in magnetic fields in excess of 9 Tesla, with sub-micron spatial resolution. This microscope is integrated into the bore of a dilution refrigerator with an optical fiber coupling light to an external optical table. Bench-top optical elements allow for polarization analysis of the reflected light from a surface and thus the detection of magnetic or other polarization-sensitive properties of mater at low temperature and high fields. We are studying the proposed electron nematic phase of the n=2 Ruddlesden-Popper material Sr$_3$Ru$_2$O$_7$, which exhibits a low-temperature phase transition in the form of an in-plane conduction anisotropy. We report recent results from concurrent transport measurements and polarization analysis as well as polarization microscopy with sample temperatures below 150 mK and applied magnetic fields from 0 T to 9 T. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F46.00015: Field Tuned Quantum Criticality In YFe2Al10 Liusuo Wu, Monika Gamza, Keeseong Park, Moosung Kim, Manuel Brando, Markus Garst, Meigan Aronson Most studies of quantum criticality have been carried out in $f$-electron based heavy fermions, and the observation and description of the quantum critical behaviors in systems where magnetism comes from d-electrons have been very limited. YFe$_{2}$Al$_{10}$ is a rare d-electron compound that displays pronounced non-Fermi liquid behaviors, including divergencies in the magnetic susceptibility ($\chi $ $\sim$ T$^{-\gamma}$, $\gamma =$1.4) and magnetic specific heat (C$_{\mathrm{M}}$/T $\sim$ -log T). We propose a carried out a scaling analysis of $\chi $(B,T) and C(B,T)/T that indicates YFe2Al10 is located very close to a B$=$0 QCP. We propose a singular free energy and a scaling function that consistently explains the critical exponents as well as the QC-Fermi liquid crossover in terms of a scaling variable T/B$^{0.6}$ Unusually, we find that the spatial dimension d is equal to the dynamical exponent z, and considering the two-dimensional anisotropy of the magnetic susceptibility, we infer that d$=$z$=$2. Hyperscaling is established by the internal consistency of our analysis, and the decidedly non-mean field exponents argue that QC fluctuations are protected since YFe2Al10 is likely a system that is below its upper critical dimension. These experimental observations suggest that YFe$_{2}$Al$_{10}$ is a unique 3d-electron based system that is quantum critical without the need for fine tuning. [Preview Abstract] |
Session F47: Metal-Insulator and Other Electronic Phase Transitions: Experiment I
Sponsoring Units: DCMPChair: David Hsieh, California Institute of Technology
Room: Mile High Ballroom 4F
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F47.00001: Local modification of spin orbit coupling in Sr2IrO4 Kyle McElroy, Jixia Dai, Eduardo Calleja, Gang Cao Sr$_{2}$IrO$_{4}$ has a novel Mott insulating ground state that is a result of strong spin orbit coupling (SOC) splitting the t$_{\mathrm{2g}}$ states leaving a small bandwidth Jeff$=$1/2 valence band that can then be localized by the small 5d Coulomb repulsion. In order to investigate the effects that the strong SOC has on the novel ground state we have doped them with Rh$^{4+}$ atoms, which lower the SOC, which substitute for the the Ir$^{4+}$ ions. In bulk it has been shown that with only a small Rh concentration changes the insulating state to a metallic one and the low temperature magnetic state weakens. We have found several interesting results in these doped materials and will discuss them and what they tell us about the ground state of Sr$_{2}$IrO$_{4}$. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F47.00002: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F47.00003: A time- and wavelength-resolved optical pump-probe reflectivity study of the Metal-to-Insulator Transition in Sr$_{2}$IrO$_{4}$ Tejas Deshpande, Darius Torchinsky, Liuyan Zhao, Xiaoyue Ni, Tongfei Qi, Gang Cao, David Hsieh The iridates have been predicted to exhibit many exotic quantum phases due to a unique interplay of strong electron-electron correlations and spin-orbit coupling. The perovskite iridate Sr$_{2}$IrO$_{4}$ in particular has recently attracted a lot of attention owing to the possibility of high-temperature superconductivity upon doping and an unconventional phase transition between a metallic and spin-orbital entangled Mott Insulator ground state. Optical pump-probe reflectivity experiments using 1.5 eV pump and 1.5 eV probe light have demonstrated that the thermally induced metal-to-insulator transition in Sr$_{2}$IrO$_{4}$ exhibits both Mott- and Slater-type behavior [Phys. Rev. B 86, 035128 (2012)]. We extend these studies by performing optical pump-probe reflectivity experiments on Sr$_{2}$IrO$_{4}$ single crystals over a wide range of probe wavelengths in order to investigate the low energy electronic relaxation dynamics near the insulating gap. We will discuss the implications of our results on the nature of the metal-to-insulator transition. This work is supported by Army Research Office Grant Nos. W911NF-13-0059 and (ARO-DURIP) W911NF-13-1-0293. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F47.00004: Strongly-correlated 2D electron state by surface deposition in Sr$_2$IrO$_4$ Ilya Belopolski Strong electron correlations in crystals often play a more important role in lower dimensions. Here we use in situ potassium surface deposition to create a 2D strongly-correlated electronic state at the surface of Sr$_2$IrO$_4$. This compound is unusual because bandwidth, Coulomb repulsion and spin-orbit coupling are at comparable energy scales. In related compounds, it has been predicted that these competing interactions can give rise to exotic states such as a Weyl semimetal or an oxide topological insulator. Here, we use angle-resolved photoemission spectroscopy (ARPES) to study how the electron states in Sr$_2$IrO$_4$ change as a result of potassium deposition. We observe the formation of new electron states, which may be interpreted as a Rashba band splitting near the sample surface. Such a spin-textured surface electron state is unusual in a strongly-correlated compound. This result may allow us to realize novel strongly-correlated electron states by taking advantage of large spin-orbit coupling. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F47.00005: STM studies on the lightly doped Mott Insulator Sr$_{2-x}$Eu$_{x}$IrO$_{4}$ Carlos J. Arguello, Ethan P. Rosenthal, Qingbiao Zhao, Bum Joon Kim, Abhay N. Pasupathy Sr$_{2}$IrO$_{4}$ is a 5d$^{5}$ transition metal oxide that displays a novel J$_{eff}=$1/2 Mott insulator behavior. This has been attributed to a large spin-orbit coupling combined with narrow Hubbard bands. Several interesting effects of chemical doping on this system have been proposed, being of special interest the possibility of an insulating to metal transition and of induced superconductivity for large doping concentrations. However, a systematic study of this type requires an understanding of the effect of chemical dopants at the atomic scale. In this talk, we will present Scanning Tunneling Microscopy (STM) and Scanning Tunneling Spectroscopy (STS) measurements on lightly doped Sr$_{2-x}$Eu$_{x}$IrO$_{4}$. By obtaining atomic resolution images we have estimated the Eu doping concentration to be close to 0.5{\%}. This dilute doping allows us to isolate the effect of an individual doping atom or impurity on the electronic properties of the system. Furthermore, high spatial resolution STS measurements enable us to study the effective spatial range of the effect of a dopant on the local density of states. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F47.00006: Hole doping induced metal-insulator transition in Sr$_{1-x}$K$_x$IrO$_4$ Qing'an Li, Qingbiao Zhao, B.J. Kim, J.F. Mitchell We report a metal-insulator transition against temperature in hole doped Sr2IrO4. The temperature dependence of \textit{ab}-plane resistivity of the doped Sr2IrO4 shows a peak at 6.5K. The magnetization against temperature shows a magnetic transition temperature about 200 K that is significantly reduced compared with its pristine material (240K). Hall effect measurements confirm that the conduction carrier is hole. A small magnetoresistance $\sim$ 3.5{\%} with significant anisotropy with respect to magnetic field orientation is observed, indicating the importance of spin-orbital coupling on conduction mechanism of the materials. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F47.00007: Raman studies of electronic excitations in Sr$_{2}$Ir$_{1-x}$Rh$_{x}$O$_{4}$ Jhih-An Yang, Dmitry Reznik, Tongfei Qi, Gang Cao A novel Mott insulator Sr$_{2}$IrO$_{4}$ driven by strong spin-orbital interaction (SOI) has recently attracted a lot of attention. Small onsite Coulomb repulsion can open a gap in the SOC-induced J=1/2 states due to the narrow band. Interesting electronic phases in Sr$_{2}$Ir$_{1-x}$Rh$_{x}$O$_{4}$ were discovered by substituting Ir$^{4+}$ with Rh$^{4+}$, which can effectively tune the strength of spin-orbit interaction. We report results of a Raman scattering investigation of Sr$_{2}$Ir$_{1-x}$Rh$_{x}$O$_{4}$ from x=0 to x=0.7. The evolution of the phonon spectrum with Rh doping and temperature as well as the resonant profiles of phonons will be discussed. In addition, we also observed broad luminescence-like signal whose origin is not well understood. Latest results on high energy electronic excitations and luminescence will be presented. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F47.00008: Metal-insulator transition in doped iridates Sr$_3$Ir$_2$O$_7$ Wenwen Zhou, Yoshinori Okada, Chetan Dhital, Tom Hogan, Ilija Zeljkovic, Daniel Walkup, Hsin Lin, Tay-Rong Chang, Arun Bansil, Ziqiang Wang, Stephen Wilson, Vidya Madhavan Bilayer perovskite iridate Sr$_3$Ir$_2$O$_7$ (Ir327) is a spin-orbit Mott insulator with a small charge gap, and as such provides an ideal playground for exploring carrier-induced metal-insulator transition (MIT). In particular, site-dependent introduction of carriers is proposed to lead to vastly different effects on this transition. To probe this, we use scanning tunneling spectroscopy (STS) to spatially map out the local density of states of Ir327 doped via two distinct routes: Ru-doped Ir327 (Sr$_3$(Ir$_{1-x}$Ru$_x$)$_2$O$_7$) with in-plane, and La-doped Ir327 ((Sr$_{1-x}$La$_x$)$_3$Ir$_2$O$_7$) with out-of-plane carriers. We find that in-plane Ru doping leads to MIT at x $\sim$ 35 $\%$, while out-of-plane La doping shows homogeneous metallic phase, even with dilute La concentration of a few percent, indicating a different MIT mechanism arising from the site-dependent doping process. Our STS data, combined with transport and neutron scattering results, offer potential routes to obtaining metallic states as well as novel phases from the parent insulating states in iridates. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F47.00009: Surface Structure and Property Coupling of Sr$_{3}$(Ru$_{\mathrm{1-x}}$Mn$_{\mathrm{x}})_{2}$O$_{7}$ Chen Chen, Corentin Durand, An-Ping Li, Jiandi Zhang, Rongying Jin, Ward Plummer The double-layered Ruthenate Sr$_{3}$Ru$_{2}$O$_{7}$ exhibits interesting behavior under the influence of pressure, while partial substitution of Mn for Ru generates a dramatic response in the physical properties. Even more striking is the structure-property relationship observed at the surface. Combining LEED $I-V, $with STM/STS and high-resolution electron energy loss spectroscopy (HREELS), we document a very unique surface phase diagram. The octahedra are tilted at the surface (not in the bulk) for low Mn doping, and the surface stabilizes and enhances the octahedra rotation, present in the bulk for Mn doping less that $\sim$ 20{\%}. The structure-property relationship at the surface is consistent with calculations of a tilt/rotation phase diagram (PRB 64, 020509 (2001)). Tilt distortion at the surface favors an insulating AFM ordered phase and when tilt is removed by doping of Mn the surface becomes conducting. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F47.00010: Metal-insulator transition in epitaxial perovskite SrIrO$_3$ thin films via strain J.H. Gruenewald, J. Terzic, J. Nichols, G. Cao, S.S.A. Seo Iridates have drawn considerable interest due to their exotic phases arising from the interplay of the strong spin-orbit interaction and the electronic correlation. Here we will discuss our experimental investigations of the electronic properties of epitaxially strained SrIrO$_3$ thin-films. The orthorhombic perovskite crystal phase of SrIrO$_3$ is synthesized as a thin film ($\sim$ 20 nm) on various substrates of (LaAlO$_3$)$_{0.3}$-(Sr$_2$AlTaO$_6$)$_{0.7}$, SrTiO$_3$, GdScO$_3$, and MgO using pulsed laser deposition. We have observed that when the in-plane lattice parameters are tuned from tensile to compressive strain, the electronic behavior of the strained SrIrO$_3$ thin-films changes from metallic to insulating. All samples have sheet resistance below 13 k$\Omega$/$\Box$, and the insulating samples were fit using the Mott variable-range-hopping equation at low temperatures ($<$ 15 K), which is believed to be the conducing mechanism of Anderson localization at finite temperature. The strain-dependent metal-insulator transition in epitaxial perovskite SrIrO$_3$ thin-films offers an important insight into the electronic structure of these strongly correlated, spin-orbit-coupled materials. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F47.00011: Electronic, magnetic, and structural properties of ferromagnetic insulator K$_{2}$Cr$_{8}$O$_{16}$ Sooran Kim, Kyoo Kim, B.I. Min The hollandite-type material, K$_{2}$Cr$_{8}$O$_{16}$, exhibits a couple of phase transitions upon cooling: the first magnetic transition at T = 180K and the second metal-insulator transition at T = 95K. Namely, for 95K $<$ T $<$ 180K, K$_{2}$Cr$_{8}$O$_{16}$ is a ferromagnetic metal, while, for T $<$ 95K, it is a ferromagnetic insulator. Moreover, the metal-insulator transition is accompanied by the structural transition from tetragonal to monoclinic structure. In order to explore the underlying mechanisms of these phase transitions, we have investigated systematically electronic, magnetic, and structural properties of K$_{2}$Cr$_{8}$O$_{16}$ based on the first principles DFT (density-functional theory) band structure calculations taking into account the on-site Coulomb correlation interaction ${\it U}$. The role of the electron-electron correlation on the magnetic, metal-insulator, and the structural transitions will be discussed by comparing the band structures of DFT and DFT+${\it U}$. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F47.00012: Impact of Interface Roughness on the Metallic Transport of Strongly Correlated 2D Holes in GaAs Quantum Wells Nicholas Goble, John Watson, Michael Manfra, Xuan Gao Understanding the non-monotonic behavior in the temperature dependent resistance, $R(T)$, of strongly correlated two-dimensional (2D) carriers in clean semiconductors has been a central issue in the studies of 2D metallic states and metal-insulator transitions. We have studied the transport of high mobility 2D holes in 20nm wide GaAs quantum wells with varying interface roughness by changing the Al fraction $x$ in the Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As barrier. Prior to this work, no comprehensive study of the non-monotonic resistance peak against controlled barrier characteristics has been conducted. We show that the shape of the electronic contribution to $R(T)$ is qualitatively unchanged throughout all of our measurements, regardless of the percentage of Al in the barrier. It is observed that increasing $x$ or short range interface roughness suppresses both the strength and characteristic temperature scale of the 2D metallicity, pointing to the distinct role of short range versus long range disorder in the 2D metallic transport in this 2D hole system with interaction parameter $r_{s}$ $\sim$ 20. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F47.00013: Nonactivated transport driven by Coulomb interactions with tunable shapes Jian Huang, Loren Pfeiffer, Ken West In high quality updoped GaAs field-effect-transistors, the two-dimensional (2D) charge carrier concentrations can be tuned to very low values similar to the density of electrons on helium surfaces. An important interaction effect, screening of the Coulomb interaction by the gate, rises as a result of the large charge spacing comparable to the distance between the channel and the gate. Based on the temperature -dependence of the resistivity results from measuring four different GaAs heterojunction-insulated-gate field-effect-transistor (HIGFET) samples, the power law characteristics are found for 2D hole densities $\leq 2\times10^{9}$ cm$^{-2}$ with an exponent universally depending on a single dimensionless parameter [1], the ratio between the mean carrier separation and the distance to the metallic gate that screens the Coulomb interaction. Thus, the interaction-driven electronic properties are not only sensitive to the relative strength, but also the shape of the interaction potential. \newline [1] Jian Huang, L. N. Pfeiffer, K. W. West, to appear on Phys. Rev. Lett. (2013) [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F47.00014: Exploring Fe$_{1-y}$Co$_{y}$Si near the insulator-to-metal transition Yan Wu, Brad Fulfer, Julia Chan, David Young, John DiTusa FeSi is a nonmagnetic narrow gap insulator with interesting temperature-dependent magnetic and optical properties. Charge carriers, either holes or electrons, accompanied by a more localized magnetic moments, can be introduced by doping FeSi with Mn or Co. It has been reported that Mn doping of FeSi near the insulator-to-metal transition (IMT) exhibits an intriguing field sensitive non-Fermi-Liquid behavior due to an undercompensated Kondo effect where the spin-1/2 carriers underscreen the $S=$1 impurity moments. To compare with the case of Mn substitution (hole doping), we investigate the effect of Co substitution (Fe$_{1-y}$Co$_{y}$Si, 0$\le y \le $ 0.1)(electron doping)results. Our magnetic property measurements indicate an interesting evolution of the impurity magnetic moments with $y$. Our transport studies indicate a temperature and field dependence that does not conform to the standard disordered Fermi-liquid form for small $y$. Standard semiconducting behavior is restored either by applying a magnetic field or increasing $y$. A more detailed analysis is underway to compare with disordered Fermi liquid theory as well as to the previously reported behavior of Fe$_{1-x}$Mn$_{x}$Si. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F47.00015: Strain-induced metal-insulator transitions in $d^1$ perovskites within DFT+DMFT Krzysztof Dymkowski, Claude Ederer We present results of combined density functional theory plus dynamical mean-field theory (DFT+DMFT) calculations, assessing the effect of epitaxial strain on the electronic properties of the Mott insulator LaTiO$_3$ and the correlated metal SrVO$_3$. In particular, we take into account the effect of strain on the collective tilts and rotations of the oxygen octahedra in the orthorhombically distorted $Pbnm$ perovskite structure of LaTiO$_3$. We find that LaTiO$_3$ undergoes an insulator-to-metal transition under a compressive strain of about $-2$\,\%, consistent with recent experimental observations [1]. We show that this transition is driven mainly by strain-induced changes in the crystal-field splitting between the Ti $t_{2g}$ orbitals, which in turn are related to changes in the octahedral tilt distortion. We compare this with the case of SrVO$_3$, without octahedral tilts, where we find a metal-to-insulator transition under tensile epitaxial strain. Similar to LaTiO$_3$, this metal-insulator transition is linked to the strain-induced change in the crystal-field splitting within the $t_{2g}$ orbitals. [1] Wong et al., Phys. Rev. B 81, 161101 (2010) [Preview Abstract] |
Session F48: Focus Session: Spin-Dependent Phenomena in Semiconductors: Spins in 2D Systems and II-VI Quantum Dots
Sponsoring Units: GMAG DMP FIAPChair: Aubrey Hanbicki, Naval Research Laboratory
Room: Mile High Ballroom 1A
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F48.00001: Spin and Valley Noise in Two-dimensional Transition Metal Dichalcogenides Wang-Kong Tse, D.L. Smith, N.A. Sinitsyn We develop a theory for the spin dynamics and optical spin noise spectroscopy in two-dimensional transition metal dichalcogenides. Different from spin noise in conventional semiconductors, we find that the Faraday rotation fluctuations consist of intrinsic noise not only from spins but also the valley degrees of freedom in the presence of inter-valley scattering processes. When inter-valley scattering is fast compared to spin flip scattering, we find that the spin relaxation time is renormalized by spin-orbit coupling and the valley relaxation time. When spin flip scattering is, on the other hand, fast compared to inter-valley scattering, spin relaxation and valley relaxation processes decouple. The Faraday rotation noise power spectrum displays distinctive signatures in both cases. We propose optical spin noise spectroscopy as a useful nonperturbative technique for probing the spin and valley relaxation processes in transition metal dichalcogenides. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F48.00002: Suppression of Coulomb exchange energy in quasi-2D spin-3/2 hole systems R. Winkler, T. Kernreiter, M. Governale, U. Z\"ulicke We have calculated the exchange-energy contribution to the total energy of quasi-2D spin-3/2 hole systems in typical semiconductors [1]. The magnitude of the exchange energy turns out to be suppressed from the value expected for analogous spin-1/2 conduction electron systems whenever the mixing between heavy-hole and light-hole components is strong. Our results are obtained using a general formalism for calculating the exchange energy of many-particle systems where single-particle states are multicomponent spinors. We have applied this approach to obtain analytical results for spin-3/2 hole systems in limiting cases.\\{} [1] Kernreiter et al., Phys.\ Rev.\ B \textbf{88}, 125309 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F48.00003: Layered magnetic dichalcogenide in the nanoscale thickness regime W.J. Hardy, H. Ji, A. Marcinkova, E. Morosan, D. Natelson We report results of transport measurements on FexTaS2, with x approximately 0.1. This layered dichalcogenide material is a ferromagnet with a Curie temperature of about 80 K and very large magnetocrystalline anisotropy. Our study includes anisotropic magnetoresistance (AMR) and anomalous Hall effect (AHE) measurements on single crystals of nanoscale thickness, produced by tape exfoliation. We find an out-of-plane magnetoresistance effect considerably larger than was previously reported on bulk samples with x $=$ 0.25, as well as a less pronounced change with temperature of the Hall resistance hysteresis loops. These marked differences may help further elucidate the material's transport mechanisms and the nature of the ferromagnetic state. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F48.00004: Mn-doped monolayer MoS$_2$: An atomically thin dilute magnetic semiconductor Doron Naveh, Ashwin Ramasubramaniam We investigate the electronic and magnetic properties of Mn-doped monolayer MoS$_{2}$ using a combination of first-principles density functional theory (DFT) calculations and Monte Carlo simulations. Mn dopants that are substitutionally inserted at Mo sites are shown to couple ferromagnetically via a double-exchange mechanism. This interaction is relatively short ranged, making percolation a key factor in controlling long-range magnetic order. The DFT results are parameterized using an empirical model to facilitate Monte Carlo studies of concentration- and temperature-dependent ordering in these systems, through which we obtain Curie temperatures in excess of room temperature for Mn doping in the range of 10--15{\%}. Our studies demonstrate the potential for engineering a new class of atomically thin dilute magnetic semiconductors based on Mn-doped MoS$_{2}$ monolayers. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F48.00005: Current induced spin orbit torque in 2D ferromagnetic and anti-ferromagnetic system Huawei Gao, Tomas Jungwirth, Jairo Sinova Due to space inversion asymmetry in 2D ferromagnetic or anti-ferromagnetic system with spin orbit coupling, unpolarized electric current can induce non equilibrium spin polarization from carriers and act as torques on the magnetic order. We'll report analytical calculation of this effect in a simple 2D ferromagnetic model and numerical results for the 2D anti-ferromagnetic model using Kubo linear response formula. In ferromagnetic case, there is a disorder independent out-of-plane spin polarization component. This out-of-plane component has different signs for the sub-lattices in the anti-ferromagnetic case, which will exert a torque on the in-plane Neel order. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F48.00006: Probing the coupling between holes and nuclear spins in a 2D GaAs Hole System using a Landau level spin diode Alex Hamilton, Oleh Klochan, I. Farrer, D.A. Ritchie Hole spins have recently attracted significant interest due to their potential applications for quantum computing applications due to reduced coupling to the nuclear spin bath, which is the main source of decoherence in GaAs electron spin qubit systems. Here we report a study of the interaction between nuclear and hole spins in a two-dimensional hole system in the Quantum Hall regime, using the Landau level diode technique to make separate electrical contact to two edge states with different spin polarizations. Experiments on similar electron systems show hysteretic I-V traces and electrically detected nuclear magnetic resonance revealing coupling of electron and nuclear spins. For 2D hole systems however, although non-linear I-V characteristics are observed, we were unable to detect coupling between hole and nuclear spins over a wide range of hole densities, consistent with a greatly reduced hyperfine interaction. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F48.00007: Direct observation of an out-of-plane spin polarization caused by an in-plane magnetic field in a GaAs 2D hole system LaReine Yeoh, Ashwin Srinivasan, Oleh Klochan, Adam Micolich, Roland Winkler, Michelle Simmons, David Ritchie, Michael Pepper, Alexander Hamilton Recent interest in spin-orbit coupling has led to studies of quantum confined, hole based semiconductor devices, which naturally possess strong spin-orbit interaction due to the intrinsic spin-3/2 nature of holes. In general both crystal anisotropies and quantum confinement will affect the spin properties of holes. In high symmetry crystals such anisotropies can be ignored, however in low symmetry crystals this complex interplay between the crystal and the confining potential gives rise to intriguing spin behavior, which has no counterpart in spin-1/2 electron-based systems. Here I will present the first direct observations of an unusual effect where a magnetic field applied in the plane of the 2D hole system generates a spin polarization perpendicular to the 2D plane. This out-of-plane spin polarisation is detected in transport measurements of a symmetrically doped, GaAs 2D hole quantum well in tilted magnetic fields. We are able to extract the sign of this off-diagonal component of the Land\'{e} g-factor and show that it is consistent with theory. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F48.00008: Electric field control of cubic-Rashba spin orbit interaction in two-dimentional hole gas confined in Ge/SiGe quantum well Rai Moriya, Yusuke Hoshi, Kentarou Sawano, Yasuhiro Shiraki, Noritaka Usami, Satoru Masubuchi, Tomoki Machida Recently, control of hole spins in the semiconductor heterostructure have received considerable attention. Due to the strong spin orbit interaction (SOI) of the holes, efficient electrical control of hole spins can be demonstrated. However due to the difficulty in fabrication of high quality samples, the studies of the SOI in the holes are still limited. The high mobility two-dimentional hole gass (2DHG) can be obtained in Ge/SiGe quntum well structure due to its small hole effective mass. Moreover since Ge is inversion symmetric crystal, only the Rashba-type SOI due to the structural inversion asymmetry is allowed. Thus 2DHG in Ge is a good platform to study the SOI of the holes and to demonstrate its electric field control. We fabricated gated Hall bar device from Ge/SiGe quantum well and studied low magnetic field transport of the 2DHG. At low temperature, a weak anti-localization is observed. From the comparison with analytical model, we attribute this is due to the cubic-Rashba spin orbit interaction. The electric field applied with gate voltage significantly alter the weak anti-localization peak, thus enable us to control SOI with electric field. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F48.00009: Magneto-optical studies of (Zn,Mn)Se/ZnTe quantum dots B. Barman, Y. Tsai, T. Scrace, I. Zutic, B.D. McCombe, A. Petrou, W.C. Chou, M.H. Tsou, C.S. Yang, I.R. Sellers, R. Oszwaldowski, A.G. Petukhov We have recorded the circular polarization $P$ of photoluminescence from (Zn,Mn)Se/ZnTe quantum dots (QDs) as function of magnetic field $B$. The polarization at a fixed temperature increases monotonically with $B$ and saturates for B \textgreater 3 tesla at $P_{sat}$. The value of $P_{sat}$ depends strongly on the laser photon energy. When we excite above (below) the ZnMnSe gap with photons of energy of 3.81 eV (2.54 eV), we measure $P_{sat}=$55$\% (P_{sat}=$20$\% )$. We interpret these results as due to the difference in the Zeeman band splitting between the magnetic (Zn,Mn)Se matrix and the non-magnetic ZnTe QDs. For 3.81 eV excitation, electron-hole pairs are generated mainly in the (Zn,Mn)Se matrix. The majority of the holes relax to the $+$3/2 state before capture by the ZnTe QDs. With 2.54 eV excitation, all electron-hole pairs are excited in the QDs where the Zeeman splitting is negligible. Thus, $P_{sat}$ is determined in this case by the relatively small Zeeman splitting of ZnMnSe conduction band. We relate these findings to our previous results for magnetic type-II QDs, where $P_{sat}$ does not depend on the exciting photon energy. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F48.00010: Magneto-optical properties of core/shell quantum dots doped with radial position controlled magnetic impurities Gary Sanders, Chris Stanton We present a theory for the electronic and magneto-optical properties of spherical quantum dots consisting of an inner core surrounded by an outer shell. This core/shell quantum dot is doped by magnetic Mn impurities all of which are implanted at a preselected radius on a spherical surface withinthe dot. The spherical symmetry of the dot is broken by the application of an external magnetic field. The electronic states in the presence of a magnetic field are treated in an effective mass model which includes the s-d and p-d exchange interaction with localized Mn d electrons. The strain in the quantum dot due to lattice mismatch between core and shell regions is assumed to be pseudomorphic and the effect of this strain field on the electronic states is also included. The optical properties of the quantum dot are computed using the effective mass electronic states and Fermi's golden rule. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F48.00011: Magnetic polarons in type-II (Zn,Mn)Se/ZnTe quantum dots J.R. Murphy, B. Barman, Y. Tsai, T. Scrace, J.M. Pientka, I. Zutic, B.D. McCombe, A. Petrou, A.N. Cartwright, W.C. Chou, M.H. Tsou, C.S. Yang, I.R. Sellers, R. Oszwaldowski, A.G. Petukhov We have studied magnetic polaron formation dynamics in (Zn,Mn)Se/ZnTe quantum dots$^{\mathrm{2}}$ (QDs) using time-resolved photoluminescence (TRPL) spectroscopy. The emitted light was spectrally and temporally analyzed; the emission spectra were recorded as function of time delay ($\Delta t)$ from the exciting laser pulse. The recombination time at $T=$10 K in our samples is 2.3 ns. The peak energy of the emission red shifts with increasing $\Delta t $due to the lowering of the hole-Mn spin complex (magnetic polaron) energy. From this shift we determined the magnetic polaron formation energy ($E_{MP})$ at $T=$10 K to be 20 meV, which is half the value observed in the ZnSe/(Zn,Mn)Te system studied previously.$^{\mathrm{3}} \quad E_{MP\thinspace }$decreases with increasing temperature, in contrast to the behavior of the ZnSe/(Zn,Mn)Te system$^{\mathrm{3}}$ in which $E_{MP}$ is temperature independent. These results are discussed in terms of a theoretical model. [2] L. Lee, et al., J. Cryst. Growth \textbf{378}, 222 (2013). [3] I. R. Sellers, et al., Phys. Rev. B \textbf{82}, 195320 (2010). [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F48.00012: Time Resolved Kerr Rotation Studies on Sub-monolayer Type-II ZnTe/ZnSe Quantum Dots Vasilios Deligiannakis, Siddharth Dhomkar, Haojie Ji, Bidisha Roy, Daniela Pagliero, Igor L. Kuskovsky, Maria C. Tamargo, Carlos A. Meriles Semiconductor quantum dot (QD) systems have been proposed as possible candidates to store and transport quantum information. Systems with a type-II band alignment are of particular interest due to the spatial separation of electrons and holes. Yet, there is very little work that has been reported on the spin dynamics of type-II QDs. Here we report time resolved Kerr rotation (TRKR) measurements on sub-monolayer type-II ZnTe/ZnSe QDs. The TRKR results for three samples indicate that there is an increase spin lifetime with higher QD density. The spin relaxation rates increased with decreasing temperature. This behavior has been reported for undoped II-VI materials. In the low carrier density region, electron-hole exchange interaction is dominant at low temperatures. However, this enhanced relaxation rate with decreasing temperature is suppressed in samples with the highest quantum dot density, suggesting that the presence of type-II nanoislands modify the spin relaxation behavior in these materials. [Preview Abstract] |
Session F49: Focus Session: Electricity and Magnetism: Manganites
Sponsoring Units: DMPRoom: Mile High Ballroom 1C
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F49.00001: Quantum Femtosecond Magnetism: Phase Transition in Step with Light in a Strongly Correlated Manganese Oxide Invited Speaker: Jigang Wang Research of non-equilibrium phase transitions of strongly correlated electrons is built around addressing an outstanding challenge: how to achieve ultrafast manipulation of competing magnetic/electronic phases and reveal thermodynamically hidden orders at highly non-thermal, femtosecond timescales? Recently we reveal a new paradigm called {\em quantum femtosecond magnetism}--photoinduced femtosecond magnetic phase transitions driven by quantum spin flip fluctuations correlated with laser-excited inter-atomic coherent bonding [1]. We demonstrate an antiferromagnetic (AFM) to ferromagnetic (FM) switching during about 100 fs laser pulses in a colossal magneto-resistive manganese oxide. Our results show a huge photoinduced femtosecond spin generation, measured by magnetic circular dichroism, with photo-excitation threshold behavior absent in the picosecond dynamics. This reveals an initial quantum coherent regime of magnetism, while the optical polarization/coherence still interacts with the spins to initiate local FM correlations that compete with the surrounding AFM matrix. Our results thus provide a framework that explores quantum non-equilibrium kinetics to drive phase transitions between exotic ground states in strongly correlated elecrons, and raise fundamental questions regarding some accepted rules, such as free energy and adiabatic potential surface. This work is in collaboration with Tianqi Li, Aaron Patz, Leonidas Mouchliadis, Jiaqiang Yan, Thomas A. Lograsso, Ilias E. Perakis.\\[4pt] [1] T. Li, {\em et al.}, Nature, 496, 69 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F49.00002: Quantum Femtosecond Magnetism in Strongly Correlated Electrons induced by Femtosecond Far-Infrared Pulses Tianqi Li, Aaron Patz, Jiaqiang Yan, Ilias Perakis, Jigang Wang There is growing evidence that femtosecond laser-induced transient polarization can be used to manipulate magnetic and electronic orders during a laser pulse. Recently we reveal a new paradigm called \textit{quantum femtosecond magnetism}---photoinduced femtosecond magnetic phase transitions driven by quantum spin flip fluctuations correlated with laser-excited inter-atomic bonding coherence. It provides the opportunity to study the non-equilibrium quantum dynamics of phase competitions in strongly correlated materials. In addition, the scheme of photo modulation of the magnetic/electronic properties of materials also provides potential candidates for industrial application. In this talk, we show our results of using femtosecond far-infrared to tune the ground state of a strongly correlated manganese oxide. A transient photo-induced coherence is introduced far below the band gap without electronic heating and inter-band transition. Such photo-induced coherence affects the spin correlations and the resonant phonon vibrational modes, which thus leads to femtosecond spin and charge dynamics. Such non-equilibrium quantum control of the magnetic/electronic order goes beyond the scope of the conventional thermal dynamics and provides new insights into correlation mechanisms in the materials. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F49.00003: Charge confinement in manganite thin film on stepped substrate Hoyoung Jang, B. Kim, C. Bell, Y. Hikita, X.M. Chen, P. Abbamonte, H.Y. Hwang, J.-S. Lee Technologies of fabricating the oxide films enable not only reproducing bulk properties even in the film form, but also generating new functionalities via the intrinsic interface effect in heterostructures. Beyond such aspects, nowadays, controlling a step terrace of single crystalline substrate that has been regarded as another playground of thin film research. We demonstrated resonant soft x-ray scattering (RSXS) experiment of thin La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) film grown on TiO$_{2}$ terminated SrTiO$_{3}$ (001) substrates which have the topographical step terrace. In this talk, we will present the site selective (i.e., step-edge sensitive) the RSXS results on the LSMO, showing the enriched Mn$^{3+}$ state distribution along step-edge. We propose that this distribution is associated with the anisotropic conductivity in the plane of the LSMO film. The details will be touched in presentation. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F49.00004: Chemically-induced Jahn-Teller ordering on manganite surfaces Zheng Gai, Wenzhi Lin, J.D. Burton, Evgeny Y. Tsymbal, K. Fuchigami, Jian Shen, P.C. Snijders, T.Z. Ward, Stephen Jesse, Sergei V. Kalinin, A.P. Baddorf Physical and electrochemical phenomena at the surfaces of transition metal oxides and their coupling to local functionality remains one of the enigmas of condensed matter physics. Understanding the emergent physical phenomena at surfaces requires the capability to probe the local composition, map order parameter fields, and establish their coupling to electronic properties. Here we demonstrate that measuring the sub 30 pm displacements of atoms from high-symmetry positions in the atomically resolved scanning tunneling microscopy (STM) allows the physical order parameter fields to be visualized in real space on the single atom level. Here, this local crystallographic analysis is applied to the in-situ grown manganite surfaces. In particular, using direct bond-angle mapping we report direct observation of structural domains on manganite surfaces, and trace their origin to surface-chemistry-induced stabilization of ordered Jahn-Teller displacements. Density functional calculations provide insight into the intriguing interplay between the various degrees of freedom now resolved on the atomic level. Research was supported by MSED and CNMS, which are sponsored at Oak Ridge National Laboratory by the Office of Basic Energy Sciences, U.S. Department of Energy. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F49.00005: Phase separation in complex oxides: RTiO3 Bo Shi, C. Schussler-Langeheine, J.B. Goedkoop, M.S. Golden, M. Buchholz, C. Trabant, C.F. Chang, A. Ricci, C. Gutt, M. Sprung, H.A. Durr, A. Robert, M. Sikorski, S. Song Complex oxides display an unparalleled richness of physical phenomena arising from the coupling of their charge, spin and orbital degrees of freedom, with cuprate high Tc superconductors and colossal magnetoresistive (CMR) manganites as flagship materials systems. For the CMR systems, phase separation is believed to play a crucial role in creating the hypersensitivity to external stimuli such as external field. In this contribution I will report our experiments on perovskite titanate systems, which are a t2g materials analogy to the CMR systems with which they share much underlying physics. In particular, I will deal with calcium-doped rare earth titanium oxides, which exhibit charge and orbital ordering during a temperature-driven metal-insulator transition (T-driven MIT). These systems are hypersensitive to the tuning of the hole-doping level, whereby the electrical transport then differs by several orders of magnitude, as occurs with external field in the CMR manganites. In this talk, I will present recently recorded data aimed at the investigation of the phase separation dynamics during T-driven MIT in titanates at LCLS. This is the first time that the single crystal coherent x-ray diffraction patterns have been recorded at 120Hz in the time domain. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F49.00006: Electrostatic phase control of half-doped manganites Takafumi Hatano, Yasushi Ogimoto, Zhigao Sheng, Naoki Ogawa, Masao Nakamura, Masaki Nakano, Masashi Kawasaki, Yoshihiro Iwasa, Kenjiro Miyano, Yoshinori Tokura In perovskite manganites as a strongly correlated electron system, the cross correlation among charge-spin-orbital degrees of freedom provides various electronic phases which have been controlled by external stimuli such as a magnetic field and light. Especially, the electric-field induced switching of electronic phases is of critical importance for the application toward future electronics. In this presentation, we demonstrate the gate control of the phase transition of manganites in the field effect transistor. From the variety of manganites, we chose the half-doped system of Pr$_{0.5}$Sr$_{0.5}$MnO$_3$, which is on the nearly vertical phase boundary between a ferromagnetic metallic phase and an anti-ferromagnetic insulating phase. By adopting the electric double layer transistor, we realized the phase switching accompanying with the huge resistance change by the slight modulation of the gate voltage, which may lead to beyond CMOS devices. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F49.00007: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F49.00008: Ordered phase separation in low dimensional manganite thin films B. Kim, C. Bell, Y. Hikita, H.Y. Hwang Two central challenges in ultra-thin oxide films are to understand the fundamental properties of low dimensional materials, and to create novel electronic ground states. In this context, manganites are of interest due to their complex phase diagrams which depend on the electronic band width. Here, we study in detail ultra-thin La$_{\mathrm{0.7}}$Sr$_{\mathrm{0.3}}$MnO$_{\mathrm{3}}$ films on TiO$_{\mathrm{2}}$ terminated SrTiO$_{\mathrm{3}}$ (001) substrates around the dead layer thickness. We find a strong anisotropy in the electronic transport properties depending on the current flow direction with respect to the step and terrace direction just above the dead layer thickness. Furthermore, the magnetoresistance showed significant differences when the bias current was parallel and perpendicular to the steps. This suggests the presence of an emergent insulating phase at the step edges and ordered phase separation in these low dimensional complex oxide films. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F49.00009: Screening of Strain Fields in Manganites Gian Guzman-Verri, Richard Brierley, Peter Littlewood It is well known that elastic couplings mediate long-range interactions between local degrees of freedom in colossal magnetoresistance manganites. Though the effects of elastic strain couplings on phase transitions have been extensively studied in the past [1], several important questions remain such as whether strain can induce inhomogeneous ordered states as those observed in manganites. In this talk, we address this question phenomenologically and propose that the observed scaling of the metal-to-insulator transition temperature on ionic radii in perovskite manganites [2] is the result of rotations of MnO$_6$ octahedra that screen the strain fields. \\[4pt] [1] D. J. Bergman and B. I. Halperin, Phys. Rev. B 13 2145 (1976). \\[0pt] [2] L. M. Rodriguez-Martinez and J. P. Attfield, Phys. Rev. B 54 R15622 (1996). [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F49.00010: Hybrid-Improper Ferroelectricity in a Cation Ordered Ruddlesden-Popper Manganite Antonio Cammarata, James Rondinelli There is strong interest in uncovering new routes to design multiferroic compounds, which combine ordered magnetic states with electric polarizations, since they have great potential in novel multifunctional devices. Using first principles calculations based on density functional theory, we show the ground state of the the cation ordered (La,Sr)MnO$_4$ Ruddlesden-Popper oxide is polar. The electric polarization arises from an anharmonic coupling mechanism, which can be designed at the atomic scale. We find that in addition to cooperative polar cation displacements, two non-polar distortions of the oxygen lattice, \emph{i.e.}, an octahedral rotation mode and Jahn-Teller bond distortions, are present in the polar phase. The latter originates from the \emph{electronic} susceptibility of the Mn $e_g$ orbitals to polarize, while the former is due to the La and Sr cation size mismatch. We find that the Jahn-Teller mode and the octahedral rotation mode are coupled through an anharmonic interaction and cooperatively stabilize the polar structure and induce a net electric polarization. Our survey of multiple transition paths reveal that this material is classified as a \emph{hybrid-improper} ferroelectric, and the Mn $d^4$ configuration makes it a potential multiferroic oxide. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F49.00011: Electronic structural origin of spin-phonon coupling in multiferroic CaMnO$_3$ Hongwei Wang, Hong Jiang, Lixin He, Xifan Wu Spin-phonon coupling is a functionality discovered recently in multiferroics and defined by the shift of polar phonon frequency as a function of varying magnetic ordering. In order to elucidate the electronic structural origin of this effect,in this work, we developed a novel computational method based on Extended Kugel-Khomskii (EKK) model. The maximally localized Wannier functions (MLWFs) are generated from density functional theory (DFT) band structure calculations and used as the basis to calculate the hopping integrals in the EKK model. In addition,the screened Coulomb interactions between MLWFs are computed by the random phase approximation. This method not only reproduces accurately the direct first-principles results but gives us a microscopic explanation. It is found that the large spin-phonon coupling in CaMnO$_3$ originates from in a large distortion of MLWFs generated by the slater phonon mode,which drastically affects the anitiferromagnetic hopping integral in the EKK model. On the other hand, phonon instabilities such as oxygen octahedral rotation will only result in a rigid rotation of MLWFs and the effect of spin-phonon coupling is much weaker. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F49.00012: Self-doping and emergent conductance at Mott interfaces via internal charge transfer Hanghui Chen, Andrew Millis, Chris Marianetti We use $ab$ $initio$ calculations to show that internal charge transfer can induce spatially separated electron-hole pairs at interfaces between two Mott insulators. DFT+ $U$ studies of multilayer systems consisting of Sr$_2$VO$_4$ and Sr$_2$MnO$_4$ (both Mott insulators) reveal that conductance emerges at their interface with electrons residing dominantly on the Mn $d_{x^2-y^2}$ orbital and holes on the V $d_{xy}$ orbital. With the transferred electron coupled to the core spin on Mn sites, ferromagnetism is significantly favored in the Sr$_2$MnO$_4$ layer, although this material is antiferromagnetic in bulk. Our work establishes internal charge transfer as a powerful method of tailoring correlation effects and that superlattices composed of Ruddlesden-Popper type oxides provide new possibilities for materials design, complementary to perovskite oxide heterostructures. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F49.00013: Controlling spontaneous symmetry breaking and topological defect duality in multiferroic InMnO3 from ab initio Sinead Griffin, Nicola Spaldin The rare earth hexagonal manganites are of considerable current interest because of the unusual nature of their ferroelectric phase transition that results in the formation of topological defects exhibiting universal scaling laws[1,2].~ Here we use first-principles density functional calculations and symmetry analysis to show that the spontaneous symmetry breaking in the related material InMnO3 can be described using dual Mexican-hat-like potentials that in turn explain the nature of the two recently reported low-symmetry structures. Our analysis also allows us to identify the third, previously unobserved structure that is allowed by symmetry to emerge from the high-symmetry prototype, giving a unified picture of the phase transitions and ground states in the multiferroic hexagonal manganites. [1] S.C. Chae et al., Phys. Rev. Lett. 108, 167603 (2012) [2] S.M. Griffin et al., Phys. Rev. X, 2, 041022 (2012) [Preview Abstract] |
Session F50: Superconductivity: Josephson and Proximity Effects
Sponsoring Units: DCMPChair: Jian-Xin Zhu, Los Alamos National Laboratory
Room: Mile High Ballroom 1D
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F50.00001: Gate-tuned Fraunhofer-type Conductance Modulation in Graphene-based Andreev Interferometers Minsoo Kim, Dongchan Jeong, Gil-Ho Lee, Yun-Sok Shin, Hyun-Woo Lee, Hu-Jong Lee The interplay between superconductivity and the Dirac-fermionic nature of electronic states of graphene leads to unique phase-coherent transport, when graphene is in proximity contact with superconducting electrodes. In this study, we report gate-tuned phase-coherent nonlocal magnetoconductance oscillations in Andreev interferometers consisting of a superconducting Al loop in contact with two ends of a T-shaped mono-layer graphene bar. The conductance oscillations arise from the flux change through the superconducting Al loop, with a gate-dependent Fraunhofer-type modulation of the envelope, which is independent of the sample-specific impurity configuration in the graphene sheet. We confirm that the modulation of envelope is caused by the gate-dependent nonlocal pair coherence along with the change of flux threading the phase-coherent region of graphene between the Al electrodes. The finite-bias effect on the conductance oscillations is also examined in terms of the Onsager-B\"uttiker relation and the BTK-type Andreev reflection probability. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F50.00002: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F50.00003: Effect of a spin-active interface on proximity-induced superconductivity in topological insulators Christopher Triola, Enrico Rossi, Alexander Balatsky We examine the effect of a spin-active interface on the symmetry of proximity-induced superconducting pairing amplitudes in topological insulators. We employ diagramatic techniques to investigate the leading order contribution to the pairing amplitude considering 3 different kinds of spin-active interfaces: 1) those for which the interface leads to the wavefunctions of transmitted electrons picking up spin-dependent phases in addition to flipping the spin of transmitted electrons, 2) those with only spin-dependent phases and no spin-flipping, and 3) those with only spin-flipping and no spin-dependent phases. We find that in cases (1) and (2) a considerable odd-frequency spinful-triplet pairing is induced in the TI while for case (3) no spin triplet pairing is induced to leading order. We compare our results to those for a normal metal and ferromagnetic materials finding that the nontrivial spin structure of the TI leads to qualitatively different behavior. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F50.00004: Josephson Junctions based Suspended Bi2Se3 nanoribbons Yanmeng Shi, Zhiyong Wang, Jing Shi, Chun Ning Lau As an important member of topological insulator family, Bi2Se3 has Dirac surface states and a 300meV bulk energy gap. Hybrid Bi2Se3/superconductor junctions have the promise of realizing Majorana fermions, and have attracted much interest recently. In our work, we fabricate suspended Bi2Se3 nanoribbon devices with superconducting Al electrodes, and study their transport transport properties. We will present our latest transport data at the meeting. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F50.00005: Observation of short ballistic Josephson effect in vertical graphene junctions Gil-Ho Lee, Hu-Jong Lee The current-phase relation (CPR) of vertical graphene Josephson junctions (vGJJs) was measured using phase-sensitive dc-SQUID interferometry. A vGJJ, realized by vertically sandwiching a monolayer graphene between two Al electrodes, had an atomically short channel with transparent contacts for the highly coherent junction nature. The measured CPR was almost perfectly skewed, which rigorously confirmed the short ballisticity of the vGJJs. The short ballistic character of a Josephson junction has been predicted since 1970's but has never been realized in scalable hybrid systems. The CPR also provided energy spectrum of Andreev levels formed inside the junction, which offered a promising prospect for scalable quantum information devices such as Andreev-level qubits. This vertical-junction scheme is also readily applicable to the other cleavable materials such as three-dimensional topological insulators or transition metal dichalcogenides, opening a new pathway for uncovering exotic coherence phenomena arising in an atomic scale. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F50.00006: Majorana Fermion Signatures in Flux Quantum Tunneling Pedro Lopes, Vasudha Shivamoggi, Amir Caldeira We propose Majorana fermions signatures in quantum tunneling experiments on SQUIDs composed of topological superconductors. Majorana fermions in a single Josephon junction have well-studied signatures which rely on single electron transfers. We investigate the effect of Majorana fermions on the flux, rather than charge, degree of freedom of a superconducting loop. Tuning of the applied magnetic flux through the loop can cause the system to tunnel between states with different enclosed flux quanta. We study how this tunneling picture may be modified in the presence of Majorana fermions. We use a 1+1D spinless p-wave model which hosts Majorana fermions in the topological phase to introduce and argue in favor of a phenomenological model, and demonstrate that novel phase slipping and quantum tunneling physics arise as a consequence. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F50.00007: Current filamentation in large Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ mesas observed by luminescent and scanning laser thermal microscopy Timothy Benseman, Yang Hao, Alex Koshelev, Vitalii Vlasko-Vlasov, Ulrich Welp, Wai-Kwong Kwok, Boris Gross, Matthias Lange, Dieter Koelle, Reinhold Kleiner, Kazuo Kadowaki Self-heating is a critical issue in stacked intrinsic Josephson junction devices designed for terahertz emission. Some theoretical models, as well as experimental evidence, suggest that self-heating may indeed be helpful for maximizing THz power output. Here we study the self-heating of a Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ mesa terahertz source via two techniques. We show that low-temperature scanning-laser microscopy measurements - a sensitive, but indirect probe of device temperature - agree well with direct temperature measurements obtained via a thermoluminescent imaging technique. Due to the semiconductor-like c-axis resistivity of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$, we find that at low temperatures device self-heating is highly non-uniform, displaying hysteretic nucleation of narrow hotspots with elevated current density. Also, the hotspot radius grows with increasing device temperature. These behaviors are consistent with theoretical predictions for a current filament forming in a material whose resistance falls with increasing temperature. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F50.00008: Spectral investigation of hot-spot and cavity resonance effects on the terahertz radiation emitted from high-$T_{\mathrm{c}}$ superconducting Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ single crystal mesa structures Kazuo Kadowaki, Chiharu Watanabe, Hidetoshi Minami, Takashi Yamamoto, Takanari Kashiwagi, Richard Klemm Terahertz (THz) electromagnetic radiation emitted from high-$T_{\mathrm{c}}$ superconducting Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ mesa structures in the case of single mesa and series-connected mesas is investigated by the FTIR spectroscopic technique while observing its temperature distribution simultaneously by a SiC photoluminescence technique. Changing the bias level, sudden jumps of the hot-spot position were clearly observed. Although the radiation intensity changes drastically associated with the jump of the hot spot position, the frequency is unaffected as long as the voltage per junction is kept constant. Since the frequency of the intense radiation satisfies the cavity resonance condition, we confirmed that the cavity resonance is of primarily importance for the synchronization of whole intrinsic Josephson junctions in the mesa for high power radiation. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F50.00009: Supercurrents in InSb nanowire Josephson junctions Jun Chen, Peng Yu, S\'ebastien Plissard, Diana Car, Vincent Mourik, Kun Zuo, David van Woerkom, Daniel Szombati, Leo Kouwenhoven, Erik Bakkers, Sergey Frolov Majorana fermions have been predicted in one-dimensional semiconductor nanowires with strong spin-orbit interactions coupled to superconductors. Effects such as odd number Shapiro steps disappearing and critical currents oscillating in magnetic field have been proposed as signatures of Majorana fermions in Josephson junctions. Here we investigate supercurrents in NbTiN-InSb nanowire-NbTiN Josephson junctions as a function of back gate and magnetic field. When an external magnetic field was applied along the nanowire, we observe gate-tunable oscillations in the critical current. To clarify the origin of this oscillating critical current, we are studying the spectra of Shapiro steps, which may give us a better understanding of such Josephson junctions and guide the search for additional signatures of Majorana fermions. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F50.00010: Anomalous oscillations of the Josephson supercurrent in InSb nanowires Attila Geresdi, D\'{a}niel B. Szombati, Ludo J. Cornelissen, Diana Car, S\'{e}bastien R. Plissard, Erik P.A.M. Bakkers, Leo P. Kouwenhoven Semiconductor nanowires proximity coupled to superconducting leads provide an ideal experimental platform to investigate the Josephson effect in tunable ballistic channels in the presence of strong spin-orbit coupling and large Land\'{e} g-factor. The interplay of an external magnetic field perpendicular to the intrinsic spin-orbit field may lead to an anomalous supercurrent which is a proposed signature of the coupling between two Majorana modes through the channel. Here we present our experimental studies of the Josephson supercurrent in InSb nanowires. Ohmic contacts to bulk superconductor NbTiN leads enable us to trace supercurrents up to $B=3\,$T magnetic field. The gate control over the channel allows us to investigate the amplitude of the critical current from the tunneling regime to a few transparent modes, where nonsinusoidal current-phase relationship (CPR) is expected, verified by the presence of fractional Shapiro steps under microwave irradiation. The evolution of the critical current with the external magnetic field is shown to exhibit non-monotonic behavior depending on the gate configuration, consistently with the theory of Josephson junctions hosting Majorana modes. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F50.00011: Nonlinear Superconducting Metamaterials in Free-Space at mm-wave Frequencies Steven Anlage, Daimeng Zhang, Melissa Trepanier, Oleg Mukhanov, K. Delfanazari, V. Savinov, N. Zheludev Superconducting metamaterials show the promise of low loss, compact size and extreme tunability and nonlinearity, allowing for new applications. Most demonstrations of these metamaterials have been conducted in waveguide geometries, either in co-planar form or three-dimensional single-conductor structures. Here we demonstrate for the first time a widely tunable superconducting metamaterial operating under the free-space illumination of a quasi-optical beam in the 100 GHz regime. The meta-atoms are Radio Frequency Superconducting QUantum Interference Devices (RF SQUIDs) that form compact self-resonant objects endowed with the nonlinearity of the Josephson effect. The metamaterial is tuned with dc magnetic flux, temperature and mm-wave power, and holds promise for a new generation of mm-wave agile devices. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F50.00012: Multi-photon Transitions in a near-millimeter-sized superconducting device Roberto Ramos, Steven Carabello, Joseph Lambert, Daniel Cunnane, Wenqing Dai, Ke Chen, Qi Li, Xiaoxing Xi The washboard potential well of a current-biased Josephson junction is a natural testbed for studying the quantum dynamics of trapped particles. At sufficiently low temperatures, the dynamics of the device is similar to that of a quantum ``phase particle'' that can access quantized states within the well. When photons are strongly coupled to such a quantum system such as by weak microwave irradiation, multi-photon transitions can be observed between two energy levels. This occurs when the quantum energy of the radiation, multiplied by an integer, matches the spacing between levels. These quantum-behaving devices are typically tens of microns across and are current-biased so that the well is shallow enough to accommodate few energy levels. In contrast, we have observed single- and multi-photon transitions in junctions with areas 600 times bigger than conventional junctions previously shown to display multi-photon transitions. These relatively large devices are MgB2 junctions, measuring up to 0.3 mm across. The data fits consistently with theoretical models of junctions behaving in the quantum limit. On the other hand, the data and the use of large capacitance junctions suggests observation of such transitions even in a deep well with many quantum levels. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F50.00013: A quantum accurate waveform synthesizer as a voltage reference for an electronic primary thermometer Alessio Pollarolo, Samuel Benz, Horst Rogalla, Paul Dresselhaus We are using a quantum voltage noise source (QVNS) for use as an intrinsically accurate voltage reference for a new type of electronic temperature standard. In Johnson Noise Thermometry (JNT) the noise of a resistor is used to measure temperature or Boltzmann's constant $k$, because the Nyquist equation \textless $V^{\mathrm{2}}$\textgreater $=$4\textit{kTR}$\Delta f$ shows that the power spectral density \textless $V^{\mathrm{2}}$\textgreater is proportional to $k$, temperature $T$, resistance $R$ and measurement bandwidth $\Delta f$. The QVNS is a digital to analog converter used to synthesize a voltage waveform that resembles pseudo-random noise comparable in amplitude to the resistor noise. The signal generated is a frequency comb of harmonics tones that are equally spaced in frequency, all having identical amplitudes but random phases. The QVNS is an array superconducting Josephson junctions that are biased with a pulsed waveform clocked at 10 GHz. The accuracy of the voltage waveform derives from the identical voltage pulses produced by each junction that are perfectly quantized because their time-integrals are always equal to flux quantum $h$/2$e$. The time-dependent output voltage waveform is determined by the number of pulses and their density in time. The measurement electronics exploits cross-correlation techniques to reduce the uncorrelated measurement noise so as to reveal the resistor noise, both of which are on the order of 2 nV/$\surd $Hz. With this technique we have measured $k$ with an uncertainty of about one part in 10$^{\mathrm{5}}$, which we hope to improve by another order of magnitude with further research. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F50.00014: Proximity induced superconductivity in Au(111) surface state with strong Rashba spin-orbit coupling Peng Wei, Andrew Potter, Ferhat Katmis, Patrick Lee, Jagadeesh Moodera The surface state with Rashba type spin-orbit coupling (SOC) and induced $s$-wave superconductivity (SC) has been predicted as an excellent platform for topological SC.$^{\mathrm{1,2}}$ Large SOC energy splitting is essential in protecting the topological SC from disorders. Metallic surface states, i.e. Au(111), has been known to possess SOC splitting as large as 50 meV - a candidate superior to many semiconductor materials. We present our experimental demonstrations on the proximity induced SC in Au(111), for the first time, by tunneling studies. We have successfully achieved epitaxial growth of gold/SC bilayers with clean interface. Structural analyses show excellent sample quality with minimal surface roughness. Planar tunnel junctions are made, where clear Shockley surface state of Au(111) was observed at about 450 meV below the Fermi energy. SC tunneling spectroscopy reveals unconventional two gap features on Au(111) that is attributed to the induced surface SC. Further results about ongoing tunneling studies in lithographically patterned Au(111) nanowires will also be discussed. [1] A. C. Potter {\&} P. A. Lee. \textit{Phys Rev B} \textbf{83}, 094525 (2011); [2] A. C. Potter {\&} P. A. Lee. \textit{Phys Rev B} \textbf{85}, 094516 (2012) [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F50.00015: Supercurrent Spectroscopy of Andreev States \c{C}a\u{g}lar Girit, Landry Bretheau, Cristian Urbina, Daniel Esteve, Hugues Pothier We measure the excitation spectrum of a superconducting atomic contact. In addition to the usual continuum above the superconducting gap, the single particle excitation spectrum contains discrete, spin-degenerate Andreev levels inside the gap. Quasiparticle excitations are induced by a broadband on-chip microwave source and detected by measuring changes in the supercurrent flowing through the atomic contact. Since microwave photons excite quasiparticles in pairs, two types of transitions are observed: Andreev transitions, which consists of putting two quasiparticles in an Andreev level, and transitions to odd states with a single quasiparticle in an Andreev level and the other one in the continuum. [Preview Abstract] |
Session F51: Focus Session: Beyond Graphene: Synthesis, Defects, Structure, and Properties IV
Sponsoring Units: DMPChair: Junhao Lin, Vanderbilt University
Room: Mile High Ballroom 1E
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F51.00001: Disorder in 2D Materials Invited Speaker: Manish Chhowalla Heterogeneity and aperiodicity in materials is typically viewed as undesirable but recent developments have shown that disorder in materials can lead to interesting and unexpected effects and that disorder and defect engineering are fundamental pathways for tailoring material properties. Towards this end, we utilize chemically exfoliated two-dimensional materials as model systems to study disorder. Chemical exfoliation leads to highly modified materials that are structurally and chemically heterogeneous, unlike the structurally pristine material obtained by mechanical exfoliation or chemical vapor deposition. In this talk, I will describe how several different structural phases with disparate properties in transition metal dichalcogenide (TMD) nanosheets such as MoS$_{\mathrm{2}}$ and WS$_{\mathrm{2}}$ are possible and how their concentrations can be controlled. I will also demonstrate metal-semiconductor transition in 2D material by phase transformation and how the metallic phase of 2D TMDs can be used to improve their catalytic activity for making hydrogen. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 8:48AM |
F51.00002: First-principles study on native defects and dopant impurities in single-layer MoS2 Ji-Young Noh, Minkyu Park, Hanchul Kim, Yong-Sung Kim We have carried out first-principles calculations on the atomic and electronic properties of a single layer MoS2 with various native defects and substitutional dopants (V, Nb, Ta, N, P, As, Sb for n-type and Mn, Tc, Re, F, Cl, Br for p-type). For charged defects, various supercell sizes are considered to investigate the finite-size supercell effects, and we apply the electrostatic energy correction and level alignment to obtain the formation energies and transition levels of the isolated defects. We find that the S-vacancy and S-interstitial on top of a S atom have low formation energies among the native defects. The S-interstitial is found to be a neutral defect, while the S-vacancy is a deep acceptor. We discuss possible origins of the natural n-type doping in exfoliated single-layer MoS2 based on the substitutional dopant impurities. [Preview Abstract] |
Tuesday, March 4, 2014 8:48AM - 9:00AM |
F51.00003: Intrinsic Magnetism of Grain Boundaries in Two-dimensional Metal Dichalcogenides Zhuhua Zhang, Xiaolong Zou, Vincent H. Crespi, Boris I. Yakobson In two-dimensional (2D) atomic crystals, ubiquitous grain boundaries (GBs) have been shown to cause considerable degradation in material properties. Using first-principles calculations, we show that dislocations and GBs in 2D metal dichalcogenides MX$_{2}$ (M$=$Mo,W; X$=$S,Se) exceptionally exhibit substantial magnetism, in sharp contrast to other 2D materials. All dislocations are shown to have a high magnetic moment of 1.0 Bohr magneton, mainly contributed by the Mo 4d orbitals. GB composed of pentagon-heptagon pairs shows ferromagnetic spin ordering and undergoes transitions from semiconductor to half-metal and to metal as tilt angle increases; when the tilt angle is over 47$^{\circ}$, GB prefers square-octagon pairs and turns to antiferromagnetic semiconductor. A novel mechanism based on interplay between dislocation-induced localized states and local unbalanced stoichiometry of GB is revealed for elucidating the magnetic behavior. Our findings suggest that purposeful engineering of topological GBs can upgrade 2D MX$_{2}$ into promising magnetic semiconductors for spintronic applications. [Preview Abstract] |
Tuesday, March 4, 2014 9:00AM - 9:12AM |
F51.00004: Monolayers of MoS2 and WS2 as oxidation protective nanocoating materials Huseyin Sener Sen, Engin Durgun, Hasan Sahin, Francois Peeters First-principles simulation techniques are employed to analyse the interaction of oxygen with MoS$_2$ and WS$_2$ monolayers. Our calculations show that while oxygen atoms are strongly bound on top of sulphur atoms, oxygen molecule only weakly interact with the system. The penetration of oxygen atom and molecule through MoS$_2$ monolayer require a very high energy barrier indicating that MoS$_2$ can serve as protective layer from oxidation. Not only ideal structures but also possible defect formations are considered and penetration/diffusion barriers of oxygen are calculated for each case. The study is extended for WS$_2$ as well, and obtained results are compared. Our predictions indicate that ideal and/or defected MoS$_2$ and WS$_2$ monolayers can improve the oxidation and corrosion-resistance of the covered surface and can be used as an efficient nanocoating material. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F51.00005: Stability and Electronic properties of Ultra-thin Metallic nanowires on MoS$_{2}$ monolayer Ashok Kumar, Xiaoliang Zhong, Sanjeev K. Gupta, P.K. Ahluwalia, Shashi P. Karna, Ravindra Pandey MoS$_{2}$ has emerged as a promising 2D nanomaterial for several technological applications. It has recently been shown that the highly capacitive Au nanoparticles raised the effective gate voltage for the MoS$_{2}$ based device by an order of magnitude (Nano Lett. 13, 4434-41, 2013). In this talk, we examine stability and electronic properties of commensurable ultra-thin noble-metal nanowires (Cu, Ag, Au, Pt) on MoS$_{2}$ monolayer. Results based on density functional theory will be presented to determine the preferred configuration for nanowires on the monolayer together with the enhancement in the conductivity of the composite system considered. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F51.00006: Understanding the intrinsic water wettability of graphite, graphene, and 2D materials Andrew Kozbial, Zhiting Li, Jianing Sun, Xiao Gong, Feng Zhou, Yongjin Wang, Haochen Xu, Haitao Liu, Lei Li Adsorption of airborne contaminants onto high energy surfaces can mask the intrinsic material properties and cause wettable surfaces to appear hydrophobic. We report the effect of airborne hydrocarbon contamination on the water wettability of graphite and its 2D counterpart, graphene. The WCA of HOPG was 64.4 $\pm$ 2.9$^{\circ}$ when measured within 10 seconds after exfoliation in air and increased to $\sim$90$^{\circ}$ after 15 minutes. Ellipsometry measurement showed growth of an adsorptive layer on exfoliated HOPG and ATR-FTIR data indicated that the layer is airborne hydrocarbon. Analogous experimental evidence on graphene indicated that a mildly hydrophilic and clean graphene surface with WCA of 44$^{\circ}$ (monolayer graphene on Copper) and 59.6$^{\circ}$ (2-3 layer graphene on Nickel) adsorbed airborne hydrocarbons resulting in a hydrophobic surface with WCA of 80$^{\circ}$. This indicates that graphite and graphene are intrinsically mildly hydrophilic and that surface adsorbed airborne hydrocarbon is the source of hydrophobicity. The results are extended to evaluating other 2D materials - MoS$_{2}$, WS$_{2}$, BN - to further elucidate the effect of hydrocarbon contamination. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F51.00007: Dirac fermions in monolayer TiB$_{2}$ Lizhi Zhang, Shixuan Du, Hongjun Gao, Feng Liu Monolayer TiB$_{2}$ sheet ($m$-TiB$_{2}$), a two-dimensional metal-diboride, is investigated by first-principles calculations. We demonstrate that $m$-TiB$_{2}$ maintains isotropic Dirac cones near the Fermi level, having a Fermi velocity about one-half of the Fermi velocity of graphene. Different form graphene, these Dirac cones are located between K and $\Gamma $ point in the Brillouin zone, and have primarily the transition metal \textit{d-orbit} characters. Further analysis illustrates that the $d$-band Dirac cones arise from the hybridization of B $p $and Ti $d$ orbitals. Calculations of adsorption of the m-TiB$_{2}$ on hexagonal BN ($h$-BN) substrate reveal a negligible influence of the $h$-BN substrate to the electronic properties of $m$-TiB$_{2}$. Our findings extend the Dirac-band materials to metal-diborides. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F51.00008: Effect of Defects on Doping, Magnetism, and Reactivity in Hexagonal Boron Nitride / Graphene Layered Systems Alejandro Suarez, Thomas L. Reinecke Boron nitride is a promising substrate for graphene devices due to its ultra-flat and insulating characteristics. However, the interactions between defects within a hexagonal boron nitride (h-BN) substrate and a graphene layer are not yet well understood. Using ab-initio methods, we calculate the ground state energies of h-BN/ graphene bilayer systems with a number of defects including vacancies, substitutions, and interstitials. Lattice distortion, charging, and magnetism due to defects are reported and compared with literature on boron nitride bilayers. We also model the adsorption of hydrogen and fluorine atop the various defect configurations. Differences in adsorption energy, bonding geometry, and density of states of such adsorbates help elucidate which defects may be desirable for controlling graphene reactivity. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F51.00009: Diffusion kinetics and capacity of defected and doped graphene in Li-ion battery Rahul Hardikar, Deya Das, Seungchul Kim, Sang Soo Han, Kwang-Ryeol Lee, Abhishek Singh Graphene, with high surface area, electrical conductivity, robust mechanical integrity, has been intensively studied as an anode material for Li ion batteries (LIBs). Better kinetics and reversible storage of Li are desirable characteristics in a LIB. Using first principles calculations, we study the diffusion of Li \textbf{through} and \textbf{across} the basal plane of defected and doped graphene. The di-vacancy graphene gives the lowest energy barrier of $1.34$ eV for Li to diffuse through the layer, while mono-vacancy B doped gives an energy barrier of $0.31$ eV for diffusion of Li across the basal plane, indicating across diffusion as the possible mechanism. It is also seen that the capacity of Li storage in doped and un-doped mono-vacancy graphene is several orders greater than its pristine counterparts. Through our study, we show that the defected and doped graphene structures emerge as promising anode materials for application in LIBs. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F51.00010: Electronic properties of functionalized MoS$_{2}$ Jyoti Katoch, Simranjeet Singh, Duy Le, Daniel Chenet, Arend van der Zande, James Hone, Talat Rahman, Laurene Tetard, Masahiro Ishigami We have measured the impact of ad-atoms on MoS$_{2}$ using photoluminescence (PL) and Raman spectroscopy. We find that ad-atoms induce a new peak in the PL spectra, indicating that excitons are bound at the ad-atom sites. Our results will be discussed in light of recent density functional theory calculations. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F51.00011: Origin and design of indirect-to-direct band gap transition in group-VIB transition metal dichalcogenide films and heterostructures Lijun Zhang, Liping Yu, Jun-Wei Luo, Alex Zunger Group VIB transition metal dichalcogenides (TMDs) often have indirect band gaps in bulk forms but become direct at monolayer thin films. This is an effect often associated with quantum confinement as demonstrated in semiconductor nanostructures. Using first-principle calculations with van der Waals interaction included, (i) we study the indirect-to-direct transition in films of a single TMD material as a function of its thickness for a series of TMDs (MX${_2}$, M = Cr, Mo, W; X = S, Se, Te). By systematic analysis of the obtained critical transition thickness, effective masses and energy evolution of the band edge states, we rule out the mechanism of (kinetic energy controlled) quantum confinement, in favor of an (potential energy controlled) inter-layer coupling. (ii) We explore the electronic structure of different layer stacking in the van der Waals heterostructures consisting of a few TMDs. We found in such multiple layered systems that certain stacking sequences result in a direct band gap, and thus accompanied by a remarkably different optical response. In some heterostructures, the behavior of charge separation, i.e. electron and hole in different layers, is observed. The results of our work provide new insight on engineering optoelectronic properties of TMDs. [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F51.00012: Annealing and adsorption effects on MoS2 electronic properties Alaric Bergeron, Alexandre Favron, Richard Martel, Richard Leonelli, Sebastien Francoeur Monolayers of MoS$_2$ and other transition metal dichalcogenides open up a number of promising opportunities in the fields of spintronics and flexible electronics. However, the properties of these 2D semiconductors are often altered by interactions with their environment that modify significantly their electrical behavior and their optical response. In this work, we investigated the influence of 1) laser annealing, 2) adsorbed gas molecules and 3) substrate characteristics on electronic states and properties of MoS$_2$ monolayers through low-temperature spatially resolved photoluminescence. We find that laser annealing suppresses low-temperature bound exciton and trion photoluminescence, as well as decreases the relative intensity of the B-exciton photoluminescence. The use of a polymer substrate enables a permanent five-fold enhancement of the room-temperature photoluminescence of the monolayer by laser annealing. This suggests that adsorbed molecules have a significant effect on doping levels. Further studies through annealing in different atmospheres allowed us to link this effect to specific adsorbates. Finally, MoS$_2$ monolayers were encapsulated in polymer to maximize the photoluminescence enhancement by completely shielding the surface from surrounding gas molecules. [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F51.00013: Synthesis and Photoresponse of Few Layer Liquid Phase Exfoliated Molybdenum Disulphide (MoS$_2$) Flakes Sujoy Ghosh, Baleeswaraiah Muchharla, Andrew Winchester, Simin Feng, Ana Laura Elias, Nestor Perea Lopez, Swastik Kar, Mauricio Terrones, Saikat Talapatra We report on the temperature dependent photo response of~thin films of MoS$_{2}$ consisting of few layered flakes obtained by liquid phase exfoliation of bulk MoS$_{2}$ powder. We found that under a constant laser power (wavelength $=$ 658 nm) the photocurrent (I$_{\mathrm{ph}}$) increases with increasing temperature and reaches a maximum value of I$_{\mathrm{ph(max)}}$ at T$=$T$_{\mathrm{m}}$ within the studied temperature range (330K \textless T \textless 25). Thereafter, I$_{\mathrm{ph}}$, decreases with further increase in temperature and also becomes temperature independent at low temperatures. Further, it was found that in such films I$_{\mathrm{ph}}$ $\sim$ (laser intensity)$^{\gamma}$ with 0.5 \textless $\gamma $ \textless 1.0. These findings will be presented and discussed under various available models related to photoconductivity in semiconductors. [Preview Abstract] |
Session F52: Focus Session: New and Improved Superconducting Materials
Sponsoring Units: DMPChair: Eric Palm, National High Magnetic Field Laboratory
Room: Mile High Ballroom 1F
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F52.00001: Optimization and doping of 112 Fe pnictide single crystals Akshat Puri, Jennifer Misuraca, Jedediah K. Morris, Meigan Aronson The recent discovery of Ca$_{1-x}$La$_{x}$FeAs$_{2}$, which when doped with Sb has a T$_{c}$ of 43K, has led to an increased interest in Fe pnictides in the 112 structure [1]. We have grown plate-like single crystals of LaFe$_{0.6}$Sb$_{2}$ from a self flux. These form in a tetragonal 112 structure with many Fe vacancies, as measured by single crystal x-ray diffraction. The crystal growths were optimized in two ways. Arc melting elemental Fe granules before use resulted in larger ($\sim$ 1 cm$^{2}$) crystals, and including a rapid cool-down during the growth avoided the formation of a parasitic phase, thus increasing the yield. Doping Ni into the structure resulted in a change in the lattice constants from a = 4.4026 {\AA}, c = 10.0341 {\AA} for undoped LaFe$_{0.6}$Sb$_{2}$ to a = 4.4343 {\AA}, c = 9.8911 {\AA} for LaNiSb$_{2}$. Energy dispersive x-ray spectroscopy showed that Ni replaces Fe and also occupies the vacancies, and at 89\% Ni doping, there are no vacancies in the structure. Due to the many vacancies in undoped LaFe$_{0.6}$Sb$_{2}$, the Sb residing near the vacant sites is strongly anharmonic in character; the electronic structure changes with doping and this is seen in the parameter becoming harmonic. [1] Kudo et al. arXiv:1311.1269 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F52.00002: Search for Superconductivity in Extraterrestrial Materials Ivan K. Schuller, S. Gu\'enon, J.G. Ramirez, Ali C. Basaran, M. Thiemens, S. Taylor Extraterrestrial and in particular presolar materials have formed under very extreme and unconventional growth conditions. They are highly heterogeneous and they consist of an unmatched variety of chemical compounds. In order to test this materials for superconductivity, we use the very sensitive and highly selective technique of Magnetic Field Modulated Microwave Spectroscopy. The sample is placed in a microwave cavity and the microwave reflectivity is monitored in the presence of a small AC magnetic field. Among others, we have investigated micrometeorites that were extracted from the water well of the Amundsen-Scott South Pole Station and materials from the Allende and Murchinson meteorite. First results will be presented and the challenges of this research project will be discussed. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F52.00003: Search for Superconductivity in Cu-Chlorides and Ferromagnetism in Partially Oxidized CuCl Thomas Saerbeck, Juan Pereiro, James Wampler, Jacob Stanley, James Wingert, Oleg G. Shpyrko, Ivan K. Schuller The phase diagram of copper-halides shows a rich diversity not only in crystalline structure, but also in magnetic and electronic properties. In particular, the chemically unstable CuCl has been proposed several times as a candidate for high-temperature superconductivity. We present a search for superconductivity in systems of CuCl/Si and CuCl$_{2}$/Si, which leads to the observation of ferromagnetism with a Tc of 18 K in bulk CuCl samples [1]. The hitherto unreported magnetism is found to emerge in pure CuCl upon prolonged exposure to humid air. Magnetic field modulated microwave spectroscopy in addition to SQUID magnetometry and x-ray diffraction are used to identify phase transitions and compare them to the antiferromagnetic transitions in other Copper-chloride structures. \\[4pt] [1] T. Saerbeck, J. Pereiro, J. Wampler, J. Stanley, J. Wingert, O. G. Shpyrko, and Ivan K. Schuller, Journal of Magnetism and Magnetic Materials \textbf{346}, 161 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F52.00004: Superconductivity in BiS$_2$-based compounds Invited Speaker: Duygu Yazici Polycrystalline samples of $Ln$O$_{0.5}$F$_{0.5}$BiS$_2$ ($Ln$ = La, Ce, Pr, Nd, Yb) were synthesized by solid-state reaction. These compounds form in a tetragonal structure with space group $P4/nmm$ conforming to the CeOBiS$_2$ crystal structure. Electrical resistivity, magnetic susceptibility and specific heat measurements were performed on all of the samples. All of the compounds exhibit superconductivity in the range 1.9 K - 5.4 K, and the YbO$_{0.5}$F$_{0.5}$BiS$_2$ sample was also found to exhibit magnetic order (probably antiferromagnetic order) at $\sim$2.7 K that appears to coexist with superconductivity below 5.4 K [1]. Electron-doping appears to induce superconductivity in the BiS$_2$-based superconductors as partial substitution of F for O is necessary to observe superconductivity. This was further demonstrated in a study where trivalent La$^{+3}$ was partially substituted with tetravalent Th$^{+4}$, Hf$^{+4}$, Zr$^{+4}$, and Ti$^{+4}$, all of which induced superconductivity [2]. We also observed that substitution of divalent Sr$^{+2}$ for La$^{+3}$ (hole doping) does not induce superconductivity [2]. Electrical resistivity measurements were also performed under applied pressure on $Ln$O$_{0.5}$F$_{0.5}$BiS$_2$ ($Ln$ = La, Ce, Pr, Nd) up to $\sim$3 GPa and down to 1 K. These studies revealed a universal behavior where the systems are tuned away from semi-conducting behavior towards metallic behavior. The superconducting states were stabilized by applied pressure, so that $T_c$ ~increased in all of the rare earth members listed. At a critical pressure $P_c$, $T_c$ increases rapidly from a low $T_c$ phase to a distinct high $T_c$ phase, after which additional pressure no longer suppressed the semiconducting behavior in the normal state [3,4]. In addition, the metallization of NdO$_{0.5}$F$_{0.5}$BiS$_2$ also occurs at $P_c$.\\[4pt] This work was performed in collaboration with M. B. Maple, K. Huang, B. D. White and C. T. Wolowiec. \\[4pt] [1] Yazici et al, Philos. Mag. 93, 673, (2012).\\[0pt] [2] Yazici et al, Phys. Rev. \textbf{B} 87, 174512, (2013).\\[0pt] [3] Wolowiec et al, Phys. Rev. \textbf{B} 88, 064503, (2013).\\[0pt] [4] Wolowiec et al, Journal of Physics: Condensed Matter 25, 422201, (2013). [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F52.00005: High Pressure Synthesis and properties of (Ca,Pr)Fe$_{2}$As$_{2}$ Tyler Drye, Valentin Taufour, Udhara Kaluarachchi, Sheng Ran, Paul Canfield, Johnpierre Paglione Despite impressively high superconducting transition temperatures approaching 50 K, superconductivity in rare earth-doped CaFe$_{2}$As$_{2}$ appears to only involve a small volume fraction as determined by shielding fraction. In addition, the amount of Pr that can be doped into the system via ambient pressure flux synthesis is limited to \textless 15{\%}, due to a width of formation limitation. We report a study using high-pressure flux growth to substitute higher levels of Pr approaching 40{\%} concentration. The superconducting properties of the resultant crystals are presented, including chemical composition, resistivity, and magnetization measurements. The final result is a complete phase diagram for the Pr-doped CaFe$_{2}$As$_{2}$ system. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F52.00006: Synthesis and characterization of single crystal of iso-valent doped K1-xNaxFe2As2 Yu Li, Chenglin Zhang, Pengcheng Dai KFe2As2 is a very special member of iron based superconductors and has attracted a lot of attention. With peculiar topology of Fermi surfaces and incommensurate spin fluctuation due to nesting between hole pockets and the electron-like band above Fermi level, different electron pairing symmetries were proposed. However, due to the limitation of single crystal size, there are not so many experiments (especially neutron scattering) on KFe2As2 systems. To solve this problem, we grow iso-valent doped K1-xNaxFe2As2. Big crystals are grown and they are much easier to handle than KFe2As2. By magnetic susceptibility and ICP measurements, we find a small doping dependent of Tc from 3.3K to 2.8K, confirming the chemical pressure effect of Sodium doping. Our neutron scattering data on nominal K0.5Na0.5Fe2As2 also shows identical incommensurate fluctuation discovered in KFe2As2, suggesting similar magnetic behavior between both of them. We will present some inelastic neutron scattering results on this system. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F52.00007: Correlation of superparamagnetism and self-assembled defects with non-bulk superconductivity up to 49 K in (Ca,Pr)Fe$_{2}$As$_{2}$ single crystal Bing Lv, F.Y. Wei, L.Z. Deng, Y.Y. Xue, C.W. Chu We have found the unusual simultaneous occurrence of superparamagnetism and superconductivity single crystals of (Ca$_{1-x}$Pr$_{x}$)Fe$_{2}$As$_{2}$ with an x-independent Tc and a close correlation of the superconducting volume fraction with the magnetic cluster density and self-assembled As-defect density. The finding demonstrates a close relationship of superconductivity with superparamagnetism associated with the self-assemble defects. In addition, we have detected extremely large magnetic anisotropy, doping level independent Tc, the existence of mesoscopic-2D structures and Josephson-Junction Array couplings in the system. All these observations provide the physical basis of interfaces for the proposed interface-mechanism, and the best evidence for interface-enhanced superconductivity in a naturally occurring (vs artificially synthesized) material system to date. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F52.00008: LaFe$_{0.6}$Sb$_{2}$: Strongly to weakly correlated system with Ni doping J.C. Misuraca, J.W. Simonson, J.J. Kistner-Morris, A. Puri, T. Orvis, L.H. Greene, M.C. Aronson Since the discovery of superconducting Ca$_{\mathrm{1-x}}$La$_{\mathrm{x}}$FeAs$_{2}$ with a T$_{\mathrm{c}}$ of 34 K [1], there has been an increasing interest in growing 112 iron pnictides in the search for high T$_{\mathrm{c}}$ superconductivity. We have grown large single crystals of LaFe$_{0.6}$Sb$_{2}$, which form in a tetragonal 112 structure with a significant amount of Fe vacancies, confirmed via single crystal x-ray diffraction. We present a doping study utilizing Ni which replaces both the Fe and vacancies while transforming the material from strongly to weakly correlated, as determined by low temperature heat capacity measurements. The Sommerfeld coefficient $\gamma $ of the undoped crystal is 50 mJ/mol Fe K$^{2}$, indicating a large mass enhancement, while LaNiSb$_{2}$ is 5 mJ/mol Ni K$^{2}$ with no vacancies and up to 18{\%} interstitial Ni according to energy-dispersive x-ray spectroscopy. When doping LaFeSb$_{2}$ with Ni, $\gamma $ remains constant when normalized per transition metal, possibly indicating a constant density of states. A divergence appears in C/T vs. T$^{2}$ once the vacancies are filled, at 89{\%} Ni, and the divergence remains until the LaNiSb$_{2}$ sample, which is a weakly correlated 1 K superconductor. [1] Katayama, et al. arXiv:1311.1303v1 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F52.00009: Interplay between magnetic impurity and superconductivity in annealed Fe1.05Te0.75Se0.25 Wenzhi Lin, Panchapakesan Ganesh, Anthony Gianfrancesco, Tom Berlijn, Thomas Maier, Sergei Kalilin, Brian Sales, Minghu Pan By annealing Fe1.05Te0.75Se0.25 in Te vapor, we are able to recover the moment of the magnetic impurity in the bulk chalcogenide superconductor, and enhance the superconductivity in the material. Scanning tunneling microscopy/ spectroscopy studies across a local magnetic impurity reveal the modification of electronic structure around the impurity on the surface of Fe1.05Te0.75Se0.25 sample after being annealed in the Te-vapor. The superconducting gap feature, normally seen on a pristine area, is suppressed around the magnetic impurity. In addition, density-functional theory calculations are carried out to identify the atomic structure, chemical composition and magnetic moment of impurity. \\ Research was supported (WL, BCS, SVK) by Materials Sciences and Engineering Division, Basic Energy Sciences, the U.S. Department of Energy. This research was conducted (WL, MP) at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. Fellowship support (AG) from the UT/ORNL Bredesen Center for Interdisciplinary Research and Graduate Education. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F52.00010: Vortex Matter Studies in Iron Arsenide Sr$_{2}$O$_{3}$VO$_{3}$FeAs Oscar Ayala-Valenzuela, Man-Jin Eom, Jong-Mok Ok, Jun-Sung Kim, Han-Woong Yeom, Jeehoon Kim In a high temperature superconductor (HTSC), at finite temperatures, vortices jump from one pinning center to another in response to the driving force of the current. In several cases this flux creep in Fe-based is larger than in cuprates HTSC. Thermal fluctuations in HTSC also produce melting of the vortex lattice and the appearance of vortex liquid phases, characterized by $J_{c}=$0, near the critical temperature ($T_{c})$. In general, Fe-based superconductors also exhibit large vortex fluctuation effects, in spite of their lower T$_{c}$. Liquid phases are observed in many of these compounds; their extension and characteristics are topics of extensive current research. We have explored vortex fluctuation effects in a single-crystal of Sr$_{2}$VO$_{3}$FeAs by measuring magnetization and its time decay in a SQUID magnetometer. Despite the lower T$_{c}$ and small anisotropy, we found creep rates even higher than in other HTSC. We also observed wider liquid phases that covers most of the mixed state region in the H-T phase diagram. These unusually strong fluctuations are a consequence of the very large penetration depth $\lambda $, which results in Ginzburg numbers ($G_{i})$ higher than in cuprates. In the present study we use classical theories developed for cuprates, and compare them with other Fe-based superconductors. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F52.00011: Electron-doping-induced insulator-to-superconductor transition in a BiS$_{2}$-based superconductor Sr$_{1-x}$La$_{x}$FBiS$_{2}$ Hideaki Sakai, Daichi Kotajima, Kosuke Saito, Hiroki Wadati, Yuki Wakisaka, Masaichiro Mizumaki, Kiyofumi Nitta, Yoshinori Tokura, Shintaro Ishiwata Recently, materials with BiS$_{2}$ layers have attracted much attention as a new family of layered superconductors. Superconductivity was first reported in Bi$_{4}$S$_{4}$O$_{3}$, followed by $R$O$_{1-x}$F$_{x}$BiS$_{2}$, Sr$_{0.5}$La$_{0.5}$FBiS$_{2}$, and Bi$_{3}$O$_{2}$S$_{3}$. So far, however, comprehensive studies about the dependence on carrier concentration have been still lacking. In this study, we have systematically synthesized polycrystalline Sr$_{1-x}$La$_{x}$FBiS$_{2}$ ($0\!\le\! x\!\le\! 0.6$) to reveal the electronic phase diagram associated with the superconductivity in the BiS$_{2}$ layer. Since the density of states of the Sr, La and F orbitals is negligibly small near the Fermi level, this series of compounds would allow the rigid-band carrier doping and provide an ideal arena to study the detailed concentration dependence. The obtained phase diagram is characterized by an insulator-superconductor transition with a steep phase boundary at $x\!\sim\!0.4$. This is markedly different from that for $R$O$_{1-x}$F$_{x}$BiS$_{2}$, indicating a strong impact of the blocking layer on the superconductivity. Unusual increase in $T_{\rm c}$ has been also revealed as the carrier concentration decreases toward the critical point [1]. [1] H. Sakai {\it et al.} JPSJ (accepted) [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F52.00012: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F52.00013: Copper Substituted Iron Telluride: A Phase Diagram Patrick Valdivia Investigations of superconductivity in the FeCh family (Ch$=$S,Se,Te) have produced rich physics and notable materials challenges despite the ostensible simplicity of thE system. We have studied the effects of copper substitution in iron-telluride. We map out basic physical parameters of this phase diagram and invesitgate structure-property relationships through a variety of transport and diffraction measurements. [Preview Abstract] |
Session F53: Focus Session: Growth of Metal Clusters and Islands on Surfaces
Sponsoring Units: DMPChair: Bene Poelsema, University of Twente
Room: Mile High Ballroom 2C
Tuesday, March 4, 2014 8:00AM - 8:12AM |
F53.00001: Inhomogeneous strain of single-crystalline polyhedral gold nanocrystals revealed by coherent x-ray diffraction imaging Jong Woo Kim, Edwin Fohtung, Sohini Manna, Sebastian Dietze, Andrew Ulvestad, Ross Harder, Eric Fullerton, Oleg Shpyrko Coherent x-ray diffractive imaging was used to measure strain in gold nanocrystals grown by a single-step thermal chemical vapor deposition (CVD). Gold nanocrystals with well-defined facets such as triangular thin plates and octahedra were investigated. The inhomogeneous strain distributions were observed in both nanocrystals. This strain likely results from defects on the substrate in triangular plate nanocrystal. The resulting strain on the spherical surface of octahedral nanocrystal shows good congruence with theoretical prediction, but shows a discrepancy near the bottom surface on a silicon substrate. This inhomogeneous stain fields may be attributed to not only the dissimilar interface energies during growth, but also different thermal expansions between nanocrystals and the substrate after cooling down. [Preview Abstract] |
Tuesday, March 4, 2014 8:12AM - 8:24AM |
F53.00002: Electromigration-driven assembly of single-layer epitaxial islands on substrates: An approach to nanopatterning Dwaipayan Dasgupta, Dimitrios Maroudas We study an approach to surface nanopatterning based on the current-driven assembly of single-layer epitaxial islands on crystalline substrates. We develop a fully nonlinear model for the driven morphological evolution of single-layer epitaxial islands on crystalline elastic substrates with diffusional mass transport limited to the island edge. We validate our model by comparisons of simulation results for individual islands with experimental data for Ag island morphology and migration speed. We report oscillatory dynamics for islands on $\left\langle {110} \right\rangle $-oriented substrate surfaces and explore the dependence of the stable time-periodic state on the angle between the applied electric field and fast edge diffusion directions. Toward current-driven nanopatterning, we study the evolution of different-size island pairs driven to coalescence and its dependence on three key geometrical parameters: the sizes of the two islands of the pair and their center-to-center line misalignment with respect to the electric-field direction. We discover various patterns ranging from equal- and different-size stable steady island-pair configurations to many-island patterns that can be tailored by controlling the initial-pair geometrical parameters as well as the duration of application of the electric field. [Preview Abstract] |
Tuesday, March 4, 2014 8:24AM - 8:36AM |
F53.00003: Role of strain in the stability of hetero-epitaxial island on nanopillars Maxime Ignacio, Yukio Saito, Peter Smereka, Olivier Pierre-Louis Optoelectronics and microelectronics call for new techniques aiming at producing even smaller crystalline components of higher quality. Hetero-epitaxial growth on nanopatterned substrates such as nanopillar forests, is a promising direction to reduce mismatch strain and to obtain higher quality crystals. Indeed, 3D islands are grown on top of the pillars in a configuration which is similar to that of superhydrophobic liquid drops. However, as opposed to the case of liquids, elastic strain plays a major role in hetero-epitaxy. Using Kinetic Monte Carlo Simulations including elastic effects, we have studied in details the stability of a solid hetero-epitaxial island at the top of a nanopillar. We show that mismatch strain strain induces novel states for the island, including spontaneous symmetry-breaking and partial impalement of the islands in the nanopillars. Our results also suggest possible instabilities for solid-state catalytic particles governing nanowire growth.\\[4pt] [1] M. Ignacio, Y. Saito, P. Smereka, O. Pierre-Louis, preprint (2013).\\[0pt] [2] M. Ignacio, O. Pierre-Louis, Phys Rev. B 86 23410 (2012).\\[0pt] [3] K. Takano, Y. Saito, O. Pierre-Louis, Phys Rev B 82 075410 (2010). [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F53.00004: Size-Ladder in Ripening by Cluster Diffusion Starting from Single Atoms Invited Speaker: Harald Brune We present a novel approach to create metal islands on close-packed single crystal metal surfaces with well defined sizes in the range of a few atoms. For elements with large cohesive energies, we observe that small clusters such as dimers and trimers diffuse as a whole at much lower temperatures than needed for their dissociation. Since the diffusion barriers increase with increasing island size we observe a stepwise increase of the mean island size from 1 to 2.5, to 4.5, to 7.0. The fact to able to produce large number densities of islands with these sizes enables to investigate the evolution of the chemical and physical properties with size in an atom-by-atom way. We demonstrate for the case of Co/Pt(111) how the experimental transition temperatures between the respective size plateaus can be used to infer monomer, dimer, and trimer migration barriers. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:24AM |
F53.00005: Controlled Growth of Silver Nanoclusters: A Molecular Dynamic Study Mine Konuk, Sondan Durukanoglu We have investigated the growth processes of various Ag nanoclusters with different shape and morphology. In order to understand the shape evaluation of nanocluster at the atomistic level, the energy barriers and reaction rates of different pathways are determined using nudged elastic band method and molecular dynamic simulations based on the potentials extracted from embedded atom method. Growth processes are controlled using varying initial nucleation conditions: deposition angle and rate, temperature, cluster size and shape. Our results show that the reaction conditions control the formation of atoms into clusters and determine the shape of nanocrystals. We also discuss our simulation results with the experimental studies based on the shape-controlled synthesis. [Preview Abstract] |
Tuesday, March 4, 2014 9:24AM - 9:36AM |
F53.00006: Interaction of Nanometer-Sized Gold Nanocrystals with Rutile (110) Surface Steps Revealed at Atomic Resolution Wenpei Gao, Ann Se Choi, Jian-Min Zuo We show that progress in atomic resolution Z-contrast imaging now enables a detailed understanding of nanocrystal (NC) interactions with surface steps. The interaction is studied based on the shape, orientation, strain and interfacial energy of Au NCs supported on surface steps of TiO$_{2}$ (110) surfaces with a small miscut angle. Au NCs with the approximate Au(111)$_{[-110]}\parallel $TiO$_{2}$ (110)$_{[001]}$ epitaxial relationship observed as it is on flat surfaces are selected for study. The presence of surface steps induces a small rotation in the NC in an amount less than the surface miscut angle. From the recorded images, we measured the atomic displacement around the surface steps. It shows that there is significant strain near the surface step inside Au NC. Experimental measurements of NC geometry on low miscut surfaces, however, reveal an approximate similarity in NC shapes. From this, an analysis of the average NC shape is performed using the modified Wulff-Kaishew theorem. Compared to NCs on flat surfaces, the measurements show a large height/width ratio, lowered interfacial energy and increased triple line energy for NCs observed on vicinal rutile (TiO$_{2})$ surfaces. [Preview Abstract] |
Tuesday, March 4, 2014 9:36AM - 9:48AM |
F53.00007: Structure and stability of stepped Au(111)/TiO$_{2}$(110) interface Bora Lee, Dallas R. Trinkle Au nanoparticles supported on TiO$_{2}$ surfaces has been widely studied due to its interesting catalytic properties. However, the Au/TiO$_{2}$ interface possesses a complex structure, making property determination difficult. In this study, Au layers on TiO$_{2}$ support associated with complex step structure have been investigated using energy density method (EDM) based on density functional theory. EDM provides the energy for each of atoms. This allows analysis of structure stability from the changes in atomic energy and work of adhesion is evaluated without spurious error, which leads accurate energy of complex interface structure of Au/TiO$_{2}$ with step configuration. We examine the changes with a stepped TiO$_{2}$ and Au surfaces; in particular, steps on surfaces of (110) TiO$_{2}$, (1x1) reduced surface and (1x2) added-row reconstructed surface, and both (001) step microfacet and with (111) step microfacet of Au (111) surfaces are considered. EDM results indicate that the step structure energy is localized, showing a large energy variation within one unit cell. The relaxed geometry of stepped interface Au/TiO$_{2}$ is consistent with the experimentally observed result with transmission electron microscopy. The detailed analysis including the charge density and electronic structures will be presented. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:00AM |
F53.00008: Composition dependent reduction of size-selected CoPt bimetallic clusters on Al2O3 thin film Bing Yang, Eric Tyo, Soenke Seifert, Ghassan Khadra, Juliette Tuaillon, Veronique Dupuis, Stefan Vajda Atomic ratio in CoPt bimetallic nanoparticles has a great impact on tailoring the oxidation state and catalytic performance of supported CoPt catalysts. Here, we produced size-selected CoPt bimetallic clusters with atomic precision in both size and composition, soft-landed on alumina thin films. Upon landing, an immediate oxidation of Co is observed and aging in air leads to further oxidation of both Co and Pt as characterized by XPS. In-situ grazing incidence X-ray Absorption Spectroscopy and Small Angle Scattering was performed to monitor the oxidation state, and the size and shape of the catalyst under reducing conditions, respectively. A strong composition dependent behavior is observed in the reduction of the two metallic components. Co reduction in CoPt cluster occurs at 65oC, while the reduction of other clusters (Co3Pt, Co, CoPt3) shifts to higher temperature range (105$^{\circ}$C-225$^{\circ}$C). Pt in all Pt containing clusters (Pt, CoPt3, CoPt and Co3Pt) compositions was reduced already at 25$^{\circ}$C. Our results open up the possibility to tune the physical/chemical properties of nanoscale matter by precise control of their atomic ratio. [Preview Abstract] |
Tuesday, March 4, 2014 10:00AM - 10:12AM |
F53.00009: The Investigation of Epitaxy and Morphology of Au on MgO (001), (110), and (111) Timothy Pulliam, Siddharth Gopal, Michael Pierce, Vladimir Komanicky, Hoydoo You, Andi Barbour, Chenhui Zhu Au nano-crystals serve a central role in catalysis and surface chemistry, with the catalytic properties of the crystals highly dependent on physical characteristics. Characteristics such as surface area to volume ratio, crystal symmetry, and surface energy define the catalytic properties. We present our analysis of the morphology of deposited Au on substrates and how they vary with macroscopic parameters. Au was evaporated onto single crystals of each of the MgO (001), (110), and (111) principal facets to study the epitaxy, morphology, and overall crystalline nature of the nano-particles on the substrates. The depositions were performed in vacuum at 700$^{\circ}$C using an e-beam evaporator. The samples were then analysed using x-ray diffraction (XRD) and atomic force microscopy (AFM) techniques revealing epitaxy, and morphology respectively. The samples were then annealed at progressively higher temperatures and the measurements repeated. Au nano-crystals deposited on TiO$_2$ (110) were also concurrently studied. [Preview Abstract] |
Tuesday, March 4, 2014 10:12AM - 10:24AM |
F53.00010: Microscopic insights into the pathways of mass transport in oxygen-induced reversible morphologic transformation of faceted rhenium surfaces Hao Wang, Wenhua Chen, Robert Bartynski The shape (morphology) of supported metal nanoparticles often varies under reaction conditions, which in turn can induce changes in their catalytic activity. Faceted metal surfaces, free of any support materials, can be used as model catalysts or templates for synthesizing new catalysts due to their well-defined facet structures and controlled facet sizes on the nanometer scale. Here we present reversible morphology changes on a faceted Re($11\bar{{2}}1)$ single crystal surface under ultra-high vacuum (UHV) conditions, which are controlled by tuning adsorbed oxygen coverage, using low energy electron diffraction (LEED) and scanning tunneling microscopy (STM). We find microscopic structural connections between the various morphologies on the faceted Re($11\bar{{2}}1)$ surface, which provide a natural explanation for the mass transport pathways in the structural evolution. Our findings motivate a more detailed future exploration of oxygen-induced morphology transitions on catalytically active metal single crystal surfaces, which is of importance for development of new catalysts operating under oxygen rich conditions. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 10:36AM |
F53.00011: Direct observation of frozen gallium gas on wurtzite gallium nitride (000\underline {1}) using low-temperature scanning tunneling microscopy Khan Alam, Andrew Foley, Wenzhi Lin, Joseph Corbett, YingQiao Ma, Jeongihm Pak, Arthur Smith Gallium nitride layers are ordinarily grown under gallium-rich growth conditions by molecular beam epitaxy (MBE) to obtain the highest material quality. In 1997, Smith \textit{et al.} reported the family of reconstructions existing on the growth surface at room temperature, the highest-order being the c(6x12).[1] Additional gallium deposition does not lead to new reconstructions. Instead, excess gallium atoms are presumed to exist in a 2-dimensional gas state. Using a custom-built MBE/low-temperature (4.2 K) STM system, we have imaged this gallium gas for the first time by freezing out the motion. The frozen-out gallium atoms are visualized as asymmetric `L-shaped' features, with left-handed and right-handed L's scattered randomly across the surface. Interestingly, on any given atomic terrace we observe a 4x greater probability of left-handed versus right-handed L's (or vice versa), which inverts across bilayer-height steps. The cause of this asymmetry is explored by zooming in with atomic resolution, revealing two inequivalent adsorption sites. [1] A. R. Smith \textit{et al.} Phys. Rev. Lett., \textbf{79}, 3934 (1997). [Preview Abstract] |
Tuesday, March 4, 2014 10:36AM - 10:48AM |
F53.00012: Heteroepitaxial growth and surface structure of L1$_{0}$-MnGa(111) ultra-thin films on GaN(0001) Andrada-Oana Mandru, Reyes Garcia Diaz, Kangkang Wang, Kevin Cooper, Muhammad Haider, David Ingram, Noboru Takeuchi, Arthur R. Smith Ferromagnetic MnGa(111) deposited on semiconducting GaN(0001) is a promising system due to the interest in developing nitride spintronic systems and the observed ideal lattice matching and sharp growth interface. [1] The experiments are carried out in a molecular beam epitaxy (MBE) system interconnected to an ultra-high vacuum (UHV) analysis chamber containing room-temperature scanning tunneling microscopy (STM). Ultra-thin MnGa films (23 nm) are grown heteroepitaxially on GaN(0001) substrates, while maintaining the Mn:Ga flux ratio at about 1.09. After growth, the sample is transferred in-situ to the analysis chamber for STM and Auger electron spectroscopy (AES) studies. STM imaging reveals the presence of smooth terraces and angular step edges and also the existence of both 1$\times$2 and 2$\times$2 surface structures. Additional Rutherford backscattering spectroscopy (RBS) measurements help clarify the important relationship between surface and bulk. Theoretical work has also been carried out and the resulting structural models and simulated STM images for both surface structures are compared to the STM images. [1] E. Lu, D. C. Ingram, A. R. Smith, J. W. Knepper and F. Y. Yang, Phys. Rev. Lett. 97, 146101 (2006) [Preview Abstract] |
Tuesday, March 4, 2014 10:48AM - 11:00AM |
F53.00013: Structural, electronic, and magnetic properties of the Mn3N2(001) surfaces J. Guerrero-Sanchez, Kangkang Wang, Noboru Takeuchi, Arthur R. Smith Structural, electronic, and magnetic properties of theMn3N2(001) surfaces have been investigated experimentally in recent years. The molecular beam epitaxy technique has been used for the sample preparation. Scanning tunneling microscopy measurements show two different surface terminations: a Mn3N2(001)-(1x1)structure with MnN-layer termination and a Mn3N2-c(4x2) structure with the formation of Mn tetramers [1]. Spin-polarized STM studies have revealed a spin anisotropy in antiferromagnetic Mn3N2(001) nano-pyramids. To explain these results it has been proposed an induced anisotropy as a result of the atomic strain generated by the absence of N atoms that drives the structure to a non-collinear magnetic configuration or by the formation of molecule-like Mn tetramers on the surface [2]. In this work we perform first principles total energy calculations to investigate the Mn3N2(001)-(1x1) and Mn3N2(001)-(2x2) surfaces. We have determined the structural, electronic and magnetic properties of bulk Mn3N2 and compare results with those reported previously. The most favorable configuration for both surfaces has been obtained by calculating the surface formation energy. We have also studied the electronic and magnetic properties of the most stable surface structures. References [1] Rong Yang.; et al. Appl. Phys. Lett. 88, 173101 (2006). [2] Kangkang Wang and Arthur R. Smith, Nano Lett. 12, 5443 (2012). [Preview Abstract] |
Session F55: Invited Session: Electronic Properties of Twisted Van der Waals heterostructures
Sponsoring Units: DCMPChair: Pablo Jarillo-Herrero, Massachusetts Institute of Technology
Room: Four Seasons Ballroom 1
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F55.00001: Quantum Spin Hall Effects and Interactions in Twisted Bilayer Graphene Invited Speaker: Javier D. Sanchez-Yamagishi Twisted bilayer graphene is the ultimate limit of a bilayer 2DEG, where two graphene layers are stacked directly on top of each other with an interlayer distance of only 0.34nm. This system owes its rich electronic structure to an interlayer tunnel coupling which can be continuously tuned by twisting the two layers. At large twist angles, the system behaves as two decoupled monolayer graphene sheets, where inter-layer and intra-layer Coulomb interactions compete to form new ground states. We investigate the possibility of realizing a quantum spin Hall state in twisted bilayer graphene when it is doped to form an electron-hole bilayer at moderate magnetic fields. In this regime, counter-propagating edge modes exist on different layers and the occupation of each mode can be independently controlled. We discuss the electronic properties of this twisted bilayer graphene quantum spin Hall state and the role of electron-electron interactions in its realization. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F55.00002: Stacking textures and singularities in bilayer graphene Invited Speaker: Eugene Mele Multilayer graphenes feature special functionalities that microscopically arise from the atomic registry when graphene sheets are stacked. These depend on relative lateral translations, rotations and layer symmetry breaking that can occur spontaneously or be induced. This talk will focus on bilayer graphenes (BLG) in which the stacking arrangement varies in space. We examine domain walls where the local stacking order switches from local AB to BA registry, and study the electronic modes at the boundary by analyzing their valley-projected four band continuum models augmented by numerical calculations on a lattice. We then consider the more general family of two dimensional strain-minimizing BLG stacking textures, finding that they are twisted textures of the interlayer displacement field. We study the interactions and composition rules for these elementary textures which permit a unified treatment of stacking point defects, domain walls and twisted graphenes. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:48AM |
F55.00003: Electronic properties of moire superlattice bands in layered two dimensional materials Invited Speaker: Jeil Jung When atomically thin two-dimensional materials are layered they often form incommensurate non-crystalline structures that exhibit long period moir\'e patterns when examined by scanning probes. In this talk, I will present a theoretical method which can be used to derive an effective Hamiltonian for these twisted van der Waals heterostructures using input from ab initio calculations performed on short-period crystalline structures. I will argue that the effective Hamiltonian can quantitatively describe the electronic properties of these layered systems for arbitrary twist angle and lattice constants [1-2]. Applying this method to the important cases of graphene on graphene and graphene on hexagonal-boron nitride, I will present a series of experimentally observable quantities that can be extracted from their electronic structure, including their density of states and local density of states as a function of twist angle, and compare with available experiments. \\[4pt] [1] Moire bands in twisted double-layer graphene, R. Bistritzer and A. H. MacDonald, PNAS 108 (30), 12233 (2011).\\[0pt] [2] Ab initio theory of moire bands in layered two-dimensional materials, J. Jung, A. Raoux, Z. H. Qiao and A. H. MacDonald, (submitted). [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:24AM |
F55.00004: Coexisting massive and massless Dirac fermions in symmetry-broken bilayer graphene Invited Speaker: Aaron Bostwick Charge carriers in bilayer graphene are widely believed to be massive Dirac fermions that have a bandgap tunable by a transverse electric field. However, a full transport gap, despite its importance for device applications, has not been clearly observed in gated bilayer graphene, a long-standing puzzle. Moreover, the low-energy electronic structure of bilayer graphene is widely held to be unstable towards symmetry breaking either by structural distortions, such as twist, strain, or electronic interactions that can lead to various ground states. Which effect dominates the physics at low energies is hotly debated. We find by direct band-structure measurements and by calculations that a native imperfection of bilayer graphene, a distribution of twists whose size is as small as $\sim$ 0.1$^{\circ}$, is sufficient to generate a completely new electronic spectrum consisting of massive and massless Dirac fermions. The massless spectrum is robust against strong electric fields, and has a unusual topology in momentum space consisting of closed arcs having an exotic chiral pseudospin texture, which can be tuned by varying the charge density. The discovery of this unusual Dirac spectrum may be widely relevant to charge transport in bilayer graphene. [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 11:00AM |
F55.00005: Imaging and Spectroscopy of Graphene Heterostructures Invited Speaker: Brian LeRoy Graphene on hexagonal boron nitride (hBN) is an example of a van der Waals heterostructure where the electronic properties of the composite material can be different from either individual material. The lattice mismatch and twist angle between graphene and hBN produces a moir\'{e} pattern in STM topographic images. For all angles, we have observed that the surface roughness of the graphene is reduced by at least an order of magnitude as compared to graphene on silicon oxide devices. Near the charge neutrality point, graphene breaks up into a series of electron and hole puddles due to potential fluctuations. Using scanning tunneling spectroscopy, we have shown that at large twist angles the potential fluctuations are reduced by an order of magnitude by the presence of the hBN [1]. Using heterostructures with graphite gates underneath the hBN [2], we have observed even further reduction in the potential fluctuations. At small twist angles, the hBN substrate produces a weak periodic potential which can have a wavelength of up to 14 nm. This periodic potential creates a new set of superlattice Dirac points at the wavevector of the potential. As the relative rotation angle between the graphene and hBN changes, the energy of this superlattice Dirac point changes. These new superlattice Dirac points have a reduced and anisotropic Fermi velocity. Using gate voltage dependent scanning tunneling spectroscopy, we have observed the effect of the new Dirac points on the local density of states in graphene [3]. Our latest results on other graphene heterostructures will also be discussed.\\[4pt] [1] J. Xue et al., Nature Materials 10, 282 (2011).\newline [2] B. Hunt et al., Science 340, 1427 (2013).\newline [3] M. Yankowitz et al., Nature Physics 8, 382 (2012). [Preview Abstract] |
Session F56: Invited Session: Polymer Physics Prize Symposium
Sponsoring Units: DPOLYChair: Tom Witten, University of Chicago
Room: Four Seasons Ballroom 4
Tuesday, March 4, 2014 8:00AM - 8:36AM |
F56.00001: Polymer Prize: The Many Varied Phenomena of Equilibrium Self-assembly/polymerization Invited Speaker: Karl Freed The self-assembly of molecules to form large clusters under equilibrium conditions is a ubiquitous phenomenon that impacts numerous systems of interest in physics, chemistry, and biology. While not a true phase transition, equilibrium self-assembly bears similarities to phase transitions but requires quite separate treatment. The simplest theory of self-assembly is provided by Flory-Huggins theory in which structureless monomers assemble into clusters of i-monomers, i$=$2,\textellipsis ,$\infty $. Despite its simplicity, the theory explains many experimental findings for a rich variety of different self-assembling systems, including the polymerization of actin, the influence of thermal activation, chemical initiation, hierarchical self-assembly, ring formation, soft interactions, crowding, and adsorption onto surfaces on the thermodynamics of self-assembling systems, the interplay between self-assembly and phase separation, the nature of cooperativity, and more. Extensions of the simplest theory are being developed for strongly interacting systems and for describing the evolution of non-equilibrium systems. [Preview Abstract] |
Tuesday, March 4, 2014 8:36AM - 9:12AM |
F56.00002: Polymer dynamics in turbulent flow Invited Speaker: Murugappan Muthukumar Presence of dilute amounts of high-molecular weight polymers in liquids undergoing turbulent wall-bounded shear flows leads to significant drag reduction. There are two major proposed mechanisms of drag reduction in the literature. One is based on enhanced viscosity due to chain extension; the other is based on the assumption that elastic energy stored in polymer conformations is comparable to the kinetic energy in some eddies. Using the Navier-Stokes equation for the fluid and the Kirkwood-Riseman-Zimm equation for polymer chains, we have addressed the coupling between the near-wall turbulence dynamics and polymer dynamics. Our theoretical results show that the torque associated with polymer conformations contributes more significantly than the chain stretching and that the characteristic dimensions of polymer coils are much smaller than eddy sizes required for possible exchange of energy. We thus emphasize an additional mechanism to the existing two schools of thought in the search of an understanding of drag reduction. [Preview Abstract] |
Tuesday, March 4, 2014 9:12AM - 9:48AM |
F56.00003: Path-Integration Computation of the Transport Properties of Polymers Nanoparticles and Complex Biological Structures Invited Speaker: Jack Douglas One of the things that puzzled me when I was a PhD student working under Karl Freed was the curious unity between the theoretical descriptions of excluded volume interactions in polymers, the hydrodynamic properties of polymers in solution, and the critical properties of fluid mixtures, gases and diverse other materials (magnets, superfluids,etc.) when these problems were formally expressed in terms of Wiener path integration and the interactions treated through a combination of epsilon expansion and renormalization group (RG) theory. It seemed that only the interaction labels changed from one problem to the other. What do these problems have in common? Essential clues to these interrelations became apparent when Karl Freed, myself and Shi-Qing Wang together began to study polymers interacting with hyper-surfaces of continuously variable dimension where the Feynman perturbation expansions could be performed through infinite order so that we could really understand what the RG theory was doing. It is evidently simply a particular method for resuming perturbation theory, and former ambiguities no longer existed. An integral equation extension of this type of exact calculation to ``surfaces'' of arbitrary fixed shape finally revealed the central mathematical object that links these diverse physical models- the capacity of polymer chains, whose value vanishes at the critical dimension of 4 and whose magnitude is linked to the friction coefficient of polymer chains, the virial coefficient of polymers and the 4-point function of the phi-4 field theory,\textellipsis Once this central object was recognized, it then became possible solve diverse problems in material science through the calculation of capacity, and related ``virials'' properties, through Monte Carlo sampling of random walk paths. The essential ideas of this computational method are discussed and some applications given to non-trivial problems: nanotubes treated as either rigid rods or ensembles worm-like chains having finite cross-section, DNA, nanoparticles with grafted chain layers and knotted polymers. The path-integration method, which grew up from research in Karl Freed's group, is evidently a powerful tool for computing basic transport properties of complex-shaped objects and should find increasing application in polymer science, nanotechnological applications and biology. [Preview Abstract] |
Tuesday, March 4, 2014 9:48AM - 10:24AM |
F56.00004: Temperature Dependence of Structural Relaxation: From ``Super-fragile'' Polymers to ``Super-strong'' Behavior of Water Invited Speaker: Alexei Sokolov The microscopic mechanism of the steep temperature dependence of structural relaxation upon approaching Tg still remains a puzzle in the field of dynamics of polymers and soft materials in general. The steepness of the temperature behavior and its deviation from classical Arrhenius law is usually characterized by the fragility index m. This contribution presents an overview of several models proposed to connect molecular parameters to the fragility. We emphasize the Generalized Entropy Theory [1] and its prediction on the role of chain packing in fragility of polymers. Based on this theory and many experimental studies we unravel the role of chain structure, intermolecular interactions and molecular weight in polymer fragility [2,3], providing a qualitative explanations of why many polymers exhibit extremely fragile behavior. Next we show that similar qualitative ideas about frustration in packing might be applicable to other glass forming systems. In the last part we discuss the recent discovery of ``super-strong'' behavior of deeply supercooled water and the role of quantum effects in this anomalously low fragility.\\[4pt] [1] Stukalin, E. B.; Douglas, J. F.; Freed, K. F. \textbf{J. Chem. Phys. 131}, 114905 (2009).\\[0pt] [2] K. Kunal, et al., \textbf{Macromolecules 41}, 7232 (2008).\\[0pt] [3] A. Agapov, et al., \textbf{Macromolecules 45}, 8430 (2012). [Preview Abstract] |
Tuesday, March 4, 2014 10:24AM - 11:00AM |
F56.00005: Interactive Phase Separation and Crystallization: from Dynamically Symmetric to Dynamically Asymmetric Blend Systems Invited Speaker: Charles C. Han Crystallization and phase separation are two intriguing phase transitions in nature and have been intensively studied in the past decades. Recently, the mechanism of simultaneous or interactive transitions of crystallization and phase separation of binary blend has became a popular research topic due to its importance to both fundamental understandings as well as technological applications. In this presentation, interactive phase separation and crystallization will be discussed. Situations where the two components are dynamically similar (symmetric) and dis-similar (un-symmetric) will be compared. Some interesting pattern formation, step-wise growth mechanism, and structure/morphology formation mediated under the competition between thermodynamic perturbation and asymmetric viscoelasticity will be presented. [Preview Abstract] |
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