Bulletin of the American Physical Society
APS March Meeting 2012
Volume 57, Number 1
Monday–Friday, February 27–March 2 2012; Boston, Massachusetts
Session Y1: Measurement Science from Optics through Thermodynamics
Sponsoring Units: GIMSChair: Albert Migliori, Los Alamos National Laboratory
Room: 203
Friday, March 2, 2012 8:00AM - 8:12AM |
Y1.00001: Robustness of holographic particle tracking and characterization against defects in illumination Henrique W. Moyses, Bhaskar Jyoti Krishnatreya, David G. Grier Images obtained with holographic video microscopy can be interpreted with predictions of the Lorenz-Mie theory of light scattering to track individual colloidal particle's motions with nanometer resolution in three dimensions over ranges extending to hundreds of micrometers, to measure their radii with nanometer resolution, and to characterize their complex refractive indexes with part-per-thousand precision. In this work we numerically and experimentally investigate how defects in the illumination system, such as divergence and tilt of the illuminating laser beam, and spherical aberrations in the optical train affect the measured properties of the particles. We show that for the usual conditions where the experiments are performed divergence and tilt of the laser beam do not affect the measured parameters significantly, while spherical aberration can introduce significant errors. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y1.00002: Quantification of metallic nanoparticle morphology with tilt series imaging by transmission electron microscopy Aniruddha Dutta, Biao Yuan, Christopher J. Clukay, Christopher N. Grabill, Helge Heinrich, Aniket Bhattacharya, Stephen M. Kuebler We report on the quantitative analysis of electrolessly deposited Au and Ag nanoparticles (NPs) on SU8 polymer with the help of High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) in tilt series. Au NPs act as nucleating agents for the electroless deposition of silver. Au NPs were prepared by attachingAu$^{3+}$cations to amine functionalized SU8 polymeric surfaces and then reducing it with aqueous NaBH$_{4}$. The nanoscale morphology of the deposited NPs on the surface of polymer has been studied from the dark field TEM cross sectional images. Ag NPs were deposited on the cross-linked polymeric surface from a silver citrate solution reduced by hydroquinone. HAADF-STEM enables us to determine the distances between the NPs and their exact locations at and near the surface. The particle distribution, sizes and densities provide us with the data necessary to control the parameters for the development of the electroless deposition technique for emerging nanoscale technologies. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y1.00003: Novel Imaging and Nano Fabrication with a Focused Beam of Helium Ions John Notte A newly introduced commercial instrument can produce a focused beam of helium ions with a focused probe size of 0.35 nm, and an energy range from 5 to 35 keV. While using only small beam currents (0.1 to 10 pA), it provides a means of generating images with high lateral resolution and surface specific information. The imaging is based up the generation of detectable particles (such as secondary electrons) as the beam interacts with the sample. Although similar to the scanning electron microscope, this instrument offers several unique imaging advantages. In addition to imaging, the focused helium beam has also been used for fabrication at the nanometer scale. Recent result have shown that this beam can be very effective for sputtering away materials to produce fine patterns for applications in biosensors, graphene, and plasmonic devices. The helium beam has also been used for lithographic purposes, producing 7 nm features with no apparent proximity effects. In another application, when the beam interacts with adsorbed molecules, the molecules are fixed -- permitting the fabrication of three dimensional nano-structures. A review of the recent work, and future plans will be presented. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y1.00004: Neutral Atom Microscopy: A New Surface Imaging Probe Philip Witham, Erik Sanchez Recent advances have made microscopy using scanned neutral atom beams a practical reality. This technique is also called Atomic DeBroglie Microscopy, Neutral Beam Microscopy, and Scanning Helium Microscopy. Using thermal energy (under 70 meV) gas particles with neutral charge results in a probe beam that scatters from the first atomic layer of samples, with little chance of beam damage. The technique presented eliminates any need to focus the beam by using an aperture in close proximity to the sample, and has produced the first published images from gas scattering. Resolution has reached 0.6 $\mu $m and much higher resolution is possible$^{1}$. Now that NAM is a reality, a great deal of research can be done to show what it is uniquely useful for, and to explain the image contrast mechanisms. Molecular beam experiments show a wide range of surface properties that may be possible to image with such a microscope, some that are difficult to see otherwise. For example, thermal helium has a strong scattering interaction with surface hydrogens. Imaging un-coated surfaces with high electric fields is possible and imaging through high magnetic fields has been demonstrated. Recent image results and the basic instrument design will be presented. $^{1 }$A simple approach to neutral atom microscopy, Rev. Sci. Instrum. 82, 103705 (2011) [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y1.00005: Imaging the solar cell p-n junction and depletion region using secondary electron contrast Jennifer Heath, Chun-Sheng Jiang, Mowafak Al-Jassim We report on secondary electron (SE) images of cross-sectioned multicrystalline Si and GaAs/GaInP solar cell devices, focusing on quantifying the relationship between the apparent n$^{+}$-p contrast and characteristic electronic features of the device. These samples allow us to compare the SE signal from devices which have very different physical characteristics: differing materials, diffused junction versus abrupt junction, heterojunction versus homojunction. Despite these differences, we find that the SE image contrast for both types of sample, and as a function of reverse bias across the diode, closely agrees with PC1D simulations of the bulk electrostatic potential in the device, accurately yielding the depletion edge and width. A spatial derivative of the SE data shows a local maximum at the metallurgical junction. Such data are valuable, for example, in studying the conformity of a diffused junction to the textured surface topography. These data also extend our understanding of the origin of the SE contrast. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y1.00006: Resonant excitation of Rayleigh waves in a narrow fluid channel clad between two metal plates Nagaraj Nagaraj, Arkadii Krokhin, Jos\'e S\'anchez-Dehesa, Victor M. Garcia-Chocano We study extraordinary absorption of acoustic energy due to resonant excitation of Rayleigh waves in a narrow water channel clad between two unidentical metal plates with Brass plate on one side of the channel and Aluminium plate on the other. The extraordinary absorption is observed at discrete resonant frequencies. From the elastic properties of the metal plates we derive a dispersion equation for coupled Rayleigh waves. Two different types of resonances, corresponding to different polarizations of the coupled waves, are studied for different channel widths and are experimentally confirmed. We also present the experimental confirmation of coupling through measurements of change in transmission minima with channel aperture. Experimental, theoretical, and numerical results are in a good agreement. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y1.00007: Probing Heat Transfer in the Nanoscale Using Optomechanical Sensors. Brian Burg, Jonathan Tong, Poetro Sambegoro, Anastassios Mavrokefalos, Gang Chen The transition from heat conduction to radiation at extremely small gaps cannot be captured by current theories. Experimentally researchers are only slowly starting to learn how to approach this domain. To this end, the development of a measurement platform based on the picowatt sensitivity of optomechanical sensors will be presented. The bending of a custom designed bimorph cantilever accurately allows the absolute amount of transferred heat to be extracted and temperature to be determined based on the response from thermal inputs. The versatility of the platform permits thermal radiation and conduction measurements, as well as the characterization of material thermal conductivities and absorptivities in nearly identical configurations. Results of this measurement platform for fundamental heat transfer measurements will considerably improve the current understanding of nanoscale energy transport and conversion, as well as lead to advanced design guidelines for energy capture and conversion devices, in particular thermophotovoltaic cells, (solar) thermoelectric generators and waste heat recovery heat exchangers. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y1.00008: Heat Capacity Measurements by Simultaneous Relaxation and AC-Calorimetry H. Kashuri, K. Kashuri, G.S. Iannacchione A high-resolution method for measuring the heat capacity $C_p$ using simultaneously AC and Relaxation Calorimetry techniques has been developed. This technique is useful for both first and second-order phase transitions of liquids and complex fluids. The difference of the $C_p$'s measured by the Relaxation and AC calorimetry is a direct measurement of a phase transitions' latent heat. As a test, the $C_p$ of two cyanobiphenyl liquid crystals, 5CB and 8CB, were measured using a square wave modulation pulse train over a base temperature range from $300$ to $320$~K in which 5CB exhibits a first-order phase transition and 8CB exhibits a first and second-order phase transition. Fourier transform analysis allows for the direct $C_p$ measurement at the fundamental frequency of the square wave pulse train (as well as higher frequency orders) as function of temperature (i.e., AC-mode). The heating and cooling relaxations at the beginning and end of the square pulse heating allows for a relaxation analysis of $C_p$ by applying the dual slope-method that includes all enthalpic conversions. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y1.00009: The State of the Unit: A documentary film about the kilogram Amy Young The definition of the SI unit of mass is based on the international prototype of the kilogram, created in 1879 [1]. In the next years, metrologists will redefine the kilogram in relation to fundamental physical constants [2]. Intended for a general audience, the forthcoming documentary, \textit{The State of the Unit: The Kilogram}, presents the history of the kilogram, interviews with researchers at national metrology institutes in the U.S., France, and Germany, and everyday mass measurement activities at varying scales. Excerpts of the film will be shown, and followed by a discussion with the filmmaker about the project to date. This film is supported in part by the Materials Computation Center at the University of Illinois at Urbana-Champaign, the California Institute of the Arts, Valencia, California, and La F\'{e}mis, Paris, France.\\[4pt] [1] The Kilogram and Measurements of Mass and Force, Z. J. Jabbour and S. L. Yaniv. J. Res. Natl. Inst. Stand. Technol. 106, 25--46 (2001).\\[0pt] [2] Redefining the SI Base Units, Peter Mohr. National Institute of Standards and Technology website. November 1, 2011. http://www.nist.gov/pml/newsletter/siredef.cfm. Accessed November 3, 2011. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y1.00010: Wall friction measurement in the absence of mean shear Stefanus Stefanus, James Castiglione, Rory Cerbus, Walter Goldburg The dimensionless frictional force $f$ between a pipe wall and a flowing turbulent fluid is $f=\nu \overline {s} /U^2$, where $U$ is mean flow speed in the $x$-direction, $\nu$ is kinematic viscosity, and $\overline s=\frac{\partial u }{\partial y }$, where the $y$ axis is perpendicular to the flow direction. The derivative is evaluated at the wall, $y$ = 0. Described here a scheme for measuring $f$ in a turbulent fluid where $\overline{s}$ is close to zero. Hence the source of frictional dissipation is from $fluctuations$ in the shear about its mean, namely $\overline {s^2}$. This type of shear is encountered in turbulence in a closed container such as a food mixer. The scheme, which involves photon correlation spectroscopy, averages the shear rate over a laser spot size $w$ $\simeq$ 100 $\mu$m or smaller. The scheme yields the probability density function (PDF) of components of the shear rate tensor and the moments of of the PDF. The theory will be described briefly and measurements will be presented where $\overline{s} \simeq$ 0. In that limit $f$ is redefined to be $f =u' \overline{s_{ij}}/\nu$, where $\overline{s_{ij}}$ is the dominant component being measured, and $u'$ is the rms fluctuations of the velocity. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y1.00011: Mid-Infrared Photothermal Response in a Liquid Crystal Using a Quantum Cascade Laser Alket Mertiri, Mi Hong, Jerome Mertz, Hatice Altug, Shyamsunder Erramilli We report on a new technique to measure the mid-infrared photothermal response induced by a tunable Quantum Cascade Laser (QCL) in the neat liquid crystal 4-Octyl-4'-Cyanobiphenyl (8CB), without using any intercalated dye. The modulated pump QCL range spanned a weak combination absorption band centered at $1912 cm^{-1}$. The thermally induced modulation of a Ti:Sapphire probe laser operating at 800 nm was measured by lockin detection. Heterodyne measurement of the response in the solid, smectic, nematic(N) and isotropic(I) liquid crystal phases allows direct detection of a weak mid-infrared normal combination mode absorption using an inexpensive room temperature silicon photodetector. The sensitivity of the response exceeds that of a conventional FTIR spectrometer equipped with a liquid nitrogen cooled detector. At high pump power in the nematic phase close to the N-I phase transition, we observe an interesting peak splitting in the photothermal response. The advent of tunable lasers that can access still stronger modes suggests that the photothermal mid-infrared response has the potential to detect ultralow concentration of absorbers. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y1.00012: Optical detection of thermal noise modes in torsional microelectromechanical oscillators Vincent Vlaminck, Jeffrey R. Guest, Dario Antonio, Daniel Lopez, John E. Pearson, Axel Hoffmann We present the optical detection of the thermal noise spectrum for different torsional MEMS that will be implemented in a study of magnetomechanical coupling at the nanoscale. The interferometric measurement yields the differential dynamic displacement between two diffraction-limited spots on the surface to sub-pm precision, allowing us to identify the thermal modes in the low MHz frequency range. Flexion and torsional modes from thermal noise at room temperature can be distinguished by different amplitudes at different positions of the probe beam. The different mechanical eigenmodes are identified with the help of finite element simulations. This study of the thermal oscillation serves to identify the torsional mode frequencies that can be matched to low frequency magnetization dynamics of magnetic domain wall oscillators. At this point we have fabricated torsional oscillators with a resonance frequency of 6.53 MHz and a Q-factor of 1030, which are at the same time compatible with magnetic domain wall oscillators. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y1.00013: Advances in Surface Plasmon Resonance Imaging allowing for quantitative measurement of laterally heterogeneous samples Adam Raegen, Kyle Reiter, Anthony Clarke, Jacek Lipkowski, John Dutcher The Surface Plasmon Resonance (SPR) phenomenon is routinely exploited to qualitatively probe changes to materials on metallic surfaces for use in probes and sensors. Unfortunately, extracting truly quantitative information is usually limited to a select few cases -- uniform absorption/desorption of small biomolecules and films, in which a continuous ``slab'' model is a good approximation. We present advancements in the SPR technique that expand the number of cases for which the technique can provide meaningful results. Use of a custom, angle-scanning SPR imaging system, together with a refined data analysis method, allow for quantitative kinetic measurements of laterally heterogeneous systems. The degradation of cellulose microfibrils and bundles of microfibrils due to the action of cellulolytic enzymes will be presented as an excellent example of the capabilities of the SPR imaging system. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y1.00014: Use of Cavity Ring Down Spectroscopy to Characterize Organic Acids and Aerosols Emitted in Biomass Burning Solomon Bililign, Marc Fiddler, Sujeeta Singh One poorly understood, but significant class of volatile organic compounds (VOC) present in biomass burning is gas-phase organic acids and inorganic acids. These acids are extremely difficult to measure because of their adsorptive nature. Particulates and aerosols are also produced during biomass burning and impact the radiation budget of the Earth and, hence, impact global climate. Use cavity ring down spectroscopy (CRD) to measure absorption cross sections for OH overtone induced photochemistry in some organic acids (acetic acid and peracetic acid) will be presented and planed measurements of optical properties of aerosols composed of mixtures of different absorbing and non-absorbing species using CRD will be discussed. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y1.00015: Modeling large screening length effects in Electrostatic Force Microscopy P.L. Lang, A. Mottaghizadeh, L. Cui, A. Zimmers, H. Aubin, J. Li, D.N. Zheng Electrostatic Force Microscopy (EFM) and its variants such as Kelvin Probe Microscopy (KPM) are ordinarily used to image charged states or electrochemical surface potentials. However, EFM can also be used to measure the local capacitance between the tip and the substrate. For perfectly metallic substrates, this capacitance is purely geometrical, i.e. it is set by the tip shape and substrate geometry. In semi-metals with long screening length, the measured capacitance contains a ``quantum'' component, which is set by the electronic compressibility. Using finite element calculations, we demonstrate that this quantum capacitance component can be measured by EFM. We apply these calculations to the analysis of EFM data on magnetite nanoparticles presented by A. Mottaghizadeh during this meeting. [Preview Abstract] |
Session Y2: Invited Session: Magnetic Materials and Magnetism Research for Energy Applications
Sponsoring Units: GMAG FIAPChair: Dario Arena, Brookhaven National Laboratory
Room: 204AB
Friday, March 2, 2012 8:00AM - 8:36AM |
Y2.00001: Magnetic Materials in sustainable energy Invited Speaker: Oliver Gutfleisch A new energy paradigm, consisting of greater reliance on renewable energy sources and increased concern for energy efficiency in the total energy lifecycle, has accelerated research in energy-related technologies. Due to their ubiquity, magnetic materials play an important role in improving the efficiency and performance of devices in electric power generation, conversion and transportation. Magnetic materials are essential components of energy applications (i.e. motors, generators, transformers, actuators, etc.) and improvements in magnetic materials will have significant impact in this area, on par with many ``hot'' energy materials efforts. The talk focuses on the state-of-the-art hard and soft magnets and magnetocaloric materials with an emphasis on their optimization for energy applications. Specifically, the impact of hard magnets on electric motor and transportation technologies, of soft magnetic materials on electricity generation and conversion technologies, and of magnetocaloric materials for refrigeration technologies, will be discussed. The synthesis, characterization, and property evaluation of the materials, with an emphasis on structure-property relationships, will be examined in the context of their respective markets as well as their potential impact on energy efficiency. Finally, considering future bottle-necks in raw materials and in the supply chain, options for recycling of rare-earth metals will be analyzed.\footnote{O. Gutfleisch, J.P. Liu, M. Willard, E. Bruck, C. Chen, S.G. Shankar, Magnetic Materials and Devices for the 21st Century: Stronger, Lighter, and More Energy Efficient (review), Adv. Mat. 23 (2011) 821-842.} [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 9:12AM |
Y2.00002: Spintronics Device for Stand-by Power Free Nonvolatile CMOS VLSI Invited Speaker: Hideo Ohno Recent progress in perpendicular magnetic-easy axis magnetic tunnel junctions (MTJs), a spintronics device, offers a high potential building block for constructing not only stand-alone fast and nonvolatile RAMs in the 30 nm feature size and beyond but also nonvolatile CMOS VLSI employing logic-in-memory architecture [1]. The shift from in-plane to perpendicular is prompted by the need for a high crystalline anisotropy that is available in perpendicular materials for reducing the device size. In addition, current-induced switching is inherently more efficient with perpendicular easy axis. However, satisfying both high tunnel magnetoresistance (TMR) ratio over 100{\%} and low switching current was a challenge, because of the mismatch between MgO (100) - CoFe(B) bcc (100) structure needed to obtain high TMR and the crystal structure of perpendicular materials. It was shown that a strong perpendicular interface anisotropy exists at the MgO-CoFeB interface [2, 3], strong enough ($K_{i}$ = 1.3 mJ/m$^{2})$ to overcome demagnetization energy and make the easy axis perpendicular when the ferromagnetic electrode thickness is thin enough. First principle calculation by Nakamura \textit{et al.} showed that the perpendicular anisotropy is due to the oxygen-iron bond that reduces contribution of in-plane crystalline anisotropy [4]. By the use of this perpendicular easy axis, a 40 nm$\phi $ MgO-CoFeB MTJ with high TMR ($>$100 {\%}) and low switching current of 49 $\mu $A was realized [2]. It was also pointed out that activation volume for reversal plays an important role in determining the thermal stability of the MTJs [5]. I will discuss how the MTJs are incorporated in CMOS VLSIs to make them nonvolatile and stand-by power free. \\[4pt] [1] S. Ikeda, \textit{et al.} IEEE Trans. Electron Devices, \textbf{54}, 991, 2007. \\[0pt] [2] S. Ikeda, \textit{et al.} Nature Mat., \textbf{9}, 721, 2010. \\[0pt] [3] M. Endo, \textit{et al.} Appl. Phys. Lett., \textbf{96}, 212503, 2010. \\[0pt] [4] K. Nakamura \textit{et al}., Phys. Rev. B, 81, 220409(R), 2010 \\[0pt] [5] H. Sato, \textit{et al. }Appl. Phys. Lett. \textbf{99}, 042501, 2011. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:48AM |
Y2.00003: Optimized Magnetocaloric Materials Invited Speaker: Vitalij Pecharsky The discovery of the giant magnetocaloric effect in Gd$_{5}$Si$_{2}$Ge$_{2}$ and other R$_{5}$T$_{4}$ compounds (R = rare earth metal and T is a Group 14 element) generated a broad interest in the magnetocaloric effect and magnetostructural transitions. Reports on the giant magnetocaloric effect in other systems soon followed. These include MnFeP$_{x}$As$_{1-x}$ and related compounds, La(Fe$_{1-x}$Si$_{x})_{13}$ and their hydrides, Mn(As$_{x}$Sb$_{1-x})$, CoMnSi$_{x}$Ge$_{1-x}$ and related compounds, Ni$_{2}$MnGa and some closely related Heusler phases, and a few other systems. A common feature is the enhancement of the magnetic entropy effect by the overlapping contribution from the lattice, regardless whether it is a massive structural change like in R$_{5}$T$_{4}$ compounds, or only a phase volume change as in La(Fe$_{1-x}$Si$_{x})_{13}$. Both the magnetic and lattice entropies are, therefore, important and each contribution must be maximized in order to have the optimum magnetocaloric effect. Both of these entropy terms and the potential pathways towards a further enhancement of the giant magnetocaloric effect will be discussed. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:24AM |
Y2.00004: High-Performance Permanent Magnets for Energy-Efficient Devices Invited Speaker: George Hadjipanayis Permanent magnets (PMs) are indispensable for many commercial applications including the electric, electronic and automobile industries, communications, information technologies and automatic control engineering. In most of these applications, an increase in the magnetic energy density of the PM, usually presented via the maximum energy product (\textit{BH})$_{max}$, immediately increases the efficiency of the whole device and makes it smaller and lighter. Worldwide demand for high performance permanent magnets has increased dramatically in the past few years driven by hybrid and electric cars, wind turbines and other power generation systems. New energy challenges in the world require devices with higher energy efficiency and minimum environmental impact. The potential of 3d-4f compounds which revolutionized the PM science and technology is almost fully utilized, and the supply of 4f rare earth elements does not seem to be much longer assured. This talk will address the major principles guiding the development of PMs and overview state-of-the-art theoretical and experimental research. Recent progress in the development of nanocomposite PMs, consisting of a fine (at the scale of the magnetic exchange length) mixture of phases with high magnetization and large magnetic hardness will be discussed. Fabrication of such PMs is currently the most promising way to boost the (\textit{BH})$_{max}$, while simultaneously decreasing, at least partially, the reliance on the rare earth elements. Special attention will be paid to the impact which the next-generation high-(\textit{BH})$_{max}$ magnets is expected to have on existing and proposed energy-saving technologies. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 11:00AM |
Y2.00005: Soft Magnetic Materials for Improved Energy Performance Invited Speaker: Matthew Willard A main focus of sustainable energy research has been development of renewable energy technologies (e.g. from wind, solar, hydro, geothermal, etc.) to decrease our dependence on non-renewable energy resources (e.g. fossil fuels). By focusing on renewable energy sources now, we hope to provide enough energy resources for future generations. In parallel with this focus, it is essential to develop technologies that improve the efficiency of energy production, distribution, and consumption, to get the most from these renewable resources. Soft magnetic materials play a central role in power generation, conditioning, and conversion technologies and therefore promoting improvements in the efficiency of these materials is essential for our future energy needs. The losses generated by the magnetic core materials by hysteretic, acoustic, and/or eddy currents have a great impact on efficiency. A survey of soft magnetic materials for energy applications will be discussed with a focus on improvement in performance using novel soft magnetic materials designed for these power applications. A group of premiere soft magnetic materials -- nanocrystalline soft magnetic alloys -- will be highlighted for their potential in addressing energy efficiency. These materials are made up of nanocrystalline magnetic transition metal-rich grains embedded within an intergranular amorphous matrix, obtained by partial devitrification of melt-spun amorphous ribbons. The nanoscale grain size results in a desirable combination of large saturation induction, low coercivity, and moderate resistivity unobtainable in conventional soft magnetic alloys. The random distribution of these fine grains causes a reduction in the net magnetocrystalline anisotropy, contributing to the excellent magnetic properties. Recently developed (Fe,Co,Ni)$_{88}$Zr$_{7}$B$_{4}$Cu$_{1}$ alloys will be discussed with a focus on the microstructure/magnetic property relationship and their effects on the energy efficiency of these materials for AC applications. [Preview Abstract] |
Session Y3: Invited Session: Competing Phases and Quantum Criticality in Strongly Correlated Systems
Sponsoring Units: DCMPChair: Piers Coleman, Rutgers University
Room: 205AB
Friday, March 2, 2012 8:00AM - 8:36AM |
Y3.00001: Correlated Electron State in Ce$_{1-x}$Yb$_{x}$CoIn$_{5}$ Stabilized by Cooperative Valence Fluctuations Invited Speaker: Lei Shu Heavy fermion superconductivity has continuously attracted broad scientific attention. One of the important issues in this study is the relationship between quantum criticality, non-Fermi-liquid behavior (NFL), and unconventional superconductivity. It is generally thought that critical fluctuations associated with a magnetic quantum critical point (QCP) can provide a mechanism for NFL behavior and unconventional superconductivity in a narrow dome around the QCP. However, the precise nature of the relationship between these phenomena remains to be understood, particularly since many compounds have been reported where the NFL behavior persists over an extended region of the phase diagram in the absence of any identifiable QCP. Recently, intermediate valence phenomena has been found in the heavy fermion superconductor system Ce$_{1-x}$Yb$_{x}$CoIn$_{5}$. X-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements reveal that many of the characteristic features of the x = 0 correlated electron state are stable for 0 $\le $ x $\le $ 0.775, and that phase separation occurs for x $>$ 0.775. The stability of the correlated electron state is apparently due to cooperative behavior of the Ce and Yb ions, involving their unstable valences. Low temperature NFL behavior is observed which varies with x, even though there is no readily identifiable quantum critical point. The NFL state is tuned by valence fluctuations. The strongly intermediate-valence state of Yb in Ce$_{1-x}$Yb$_{x}$CoIn$_{5}$ has recently been verified by angle-resolved photoemission spectroscopy, extended x-ray absorption fine structure, and x-ray absorption near-edge structure measurements. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 9:12AM |
Y3.00002: Tuning Correlations in Low-Dimensional Electron Systems: Fermi liquid versus non-Fermi-liquid behavior in organic conductors Invited Speaker: Martin Dressel While the electronic properties of cuprates can be modified by electron or hole doping, organic conductors provide the opportunity to tune the strength of electronic correlations more directly. Varying the bandwidth by (physical or chemical) pressure, the $\kappa$-phase BEDT-TTF compounds cross over from a Fermi liquid to a Mott insulator by increasing effective correlations. We systematically investigate the electronic transport properties in organic conductors by dc resistivity and optical measurements in order to extract the temperature and frequency-dependent scattering rate $1/\tau = A(k_BT)^2 + B(\hbar\omega)^2$. We find corresponding temperature and frequency ranges in which the parabolic behaviors are observed. For the first time, we can quantitatively relate the two prefactors ($A/B=56$) and their enhancement as correlations increase upon approaching the Mott transition. Conceptually low-dimensional organic conductors are also good candidates for quantum criticality because often an ordered state is located next to a metallic state when the system is tuned by pressure. Interestingly both are found, order in the spin as well as in the charge sector. Fermi-liquid behavior observed in the metallic state seems to be limited to certain regions of the phase diagram with non-Fermi-liquid properties evolving as the ordered phase is approached. It is not clear whether these deviations from Fermi liquid behavior are actually a signature of quantum criticality.\\[4pt] [1] M. Dressel, {\it Quantum criticality in organic conductors? Fermi-liquid versus non-Fermi-liquid behavior}, J. Phys.: Condens. Matter {\bf 23}, 293201 (2011).\newline [2] S. Yasin, M. Dumm, B. Salameh, P. Batail, C. M{\'e}zi{\'e}re and M. Dressel, {\it Transport studies at the Mott transition of the two-dimensional organic metal} $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br$_x$Cl$_{1-x}$, Eur. Phys. J. B {\bf 79}, 383 (2011).\newline [3] M. Dumm, D. Faltermeier, N. Drichko and M. Dressel, {\it Bandwidth-controlled Mott transition in} $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Br$_x$Cl$_{1-x}$: {\it Optical studies of correlated carriers}, Phys. Rev. B {\bf 79}, 195106 (2009)\newpage [4] J. Merino, M. Dumm, N. Drichko, M. Dressel and R. H. McKenzie, {\it Quasiparticles at the verge of localization near the Mott metal-insulator transition in a two-dimensional material}, Phys. Rev. Lett. {\bf 100}, 086404 (2008) [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:48AM |
Y3.00003: Superconducting condensation energy of CeCu2Si2 and theoretical implications Invited Speaker: Stefan Kirchner Unconventional superconductivity occurs in a broad range of strongly correlated electron systems including the newly discovered iron pnictides and chalcogenides, various intermetallic rare earth metals, the cuprates and the organic superconductors. These systems are not only of varying effective dimensionality but their parent compounds out of which superconductivity emerges ranges from metals to bad metals and Mott insulators. The only unifying characteristic features seems that unconventional superconductivity occurs in close vicinity of zero-temperature instabilities which are most often magnetic in nature. Heavy fermion compounds represent prototype systems to address the interplay between quantum criticality and unconventional superconductivity [1]. In CeCu2Si2, the magnetic quantum phase transition and superconductivity occur at ambient pressure which allows for a detailed study of the energetics across the superconducting transition. Based on an in-depth study of the magnetic excitation spectrum of CeCu2Si2 in the normal and superconducting state we obtain a lower bound for the change in exchange energy [2]. The comparison with the superconducting condensation energy demonstrates that the built-up of magnetic correlations near the quantum critical point does drive superconductivity in CeCu2Si2. In addition, our comparison establishes a huge kinetic energy loss which we relate to the competition of Kondo screening and superconductivity as the opening of the gap weakens the Kondo effect [2,3]. We discuss the relation between kinetic energy loss and the nature of the underlying quantum critical point [1,3]. Our unexpected findings sheds further light on the emerging global phase diagram of heavy fermion compounds [4] and are believed to be relevant to other families of superconductivity which are also located in close proximity to magnetism.\\[4pt] [1] O. Stockert, S. Kirchner, F. Steglich, Q. Si, ``Superconductivity in Ce- and U-based 122 heavy-fermion compounds,'' to be published in JPSJ (invited review paper).\\[0pt] [2] O. Stockert, J. Arndt, E. Faulhaber, C. Geibel, H. S. Jeevan, S. Kirchner, M. Loewenhaupt, K. Schmalzl, W. Schmidt, Q. Si, F. Steglich, ``Magnetically driven superconductivity in CeCu2Si2,'' Nature Physics, 7, 119-124 (2011).\\[0pt] [3] S. Kirchner and Q. Si, to be published.\\[0pt] [4] Q. Si, ``Quantum Criticality and Global Phase Diagram of Magnetic Heavy Fermions,'' Phys. Status Solidi B247, 476 (2010). [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:24AM |
Y3.00004: Quantum critical scaling in beta-YbAlB4 and theoretical implications Invited Speaker: Andriy Nevidomskyy Emergent phenomena in quantum materials are subject of intense experimental and theoretical research at present. A wonderful example thereof are the sister phases of YbAlB$_4$ - a newly discovered heavy fermion material [1]. While one phase ($\alpha$-YbAlB$_4$) is a heavy Fermi liquid, its sibling $\beta$-YbAlB$_4$ is quantum critical, supporting an unconventional superconductivity with a tiny transition temperature of $\sim80$ mK. Latest experiments [2] uncover the quantum critical $T/B$-scaling in $\beta$-YbAlB$_4$ and prove that superconductivity emerges from a strange metal governed by an extremely fragile quantum criticality, which apparently occurs at zero field, without any external tuning. \newline Here, we will present a theoretical perspective on the quantum critical scaling in $\beta$-YbAlB$_4$ and will show that the critical exponents can be derived from the nodal structure of the hybridization matrix between Yb $f$-band and the conduction electrons. It follows that the free energy at low temperatures can be written in a scaling form $F\propto [(k_B T)^2 + (g\mu_B B)^2]^{3/4}$, which predicts the divergent Sommerfeld coefficient $\gamma$ and quasi-particle effective mass as $B\to 0$: $\gamma\sim m^*/m \propto B^{-1/2}$. This is indeed observed in the experiment [1,2], which places a tiny upper bound on the critical magnetic field $B_c<0.2$~mT. We will discuss theoritical implications of this fragile intrinsic quantum criticality in $\beta$-YbAlB$_4$ and discuss the possibility of a quantum critical phase, rather than a quantum critical point, in this material. \newline \newline [1] S. Nakatsuji \emph{et al.}, Nature Physics {\bf 4}, 603 (2008). \newline [2] Y. Matsumoto, S. Nakatsuji, K. Kuga, Y. Karaki, Y. Shimura, T. Sakakibara, A. H. Nevidomskyy, and P. Coleman, Science {\bf 331}, 316 (2011). [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 11:00AM |
Y3.00005: Chromium at High Pressure Invited Speaker: Rafael Jaramillo Chromium has long served as the archetype of spin density wave magnetism. Recently, Jaramillo and collaborators have shown that Cr also serves as an archetype of magnetic quantum criticality. Using a combination of x-ray diffraction and electrical transport measurements at high pressures and cryogenic temperatures in a diamond anvil cell, they have demonstrated that the N\'{e}el transition ($T_{N})$ can be continuously suppressed to zero, with no sign of a concurrent structural transition. The order parameter undergoes a broad regime of exponential suppression, consistent with the weak coupling paradigm, before deviating from a BCS-like ground state within a narrow but accessible quantum critical regime. The quantum criticality is characterized by mean field scaling of $T_{N}$ and non mean field scaling of the transport coefficients, which points to a fluctuation-induced reconstruction of the critical Fermi surface. A comparison between pressure and chemical doping as means to suppress $T_{N}$ sheds light on different routes to the quantum critical point and the relevance of Fermi surface nesting and disorder at this quantum phase transition. The work by Jaramillo \textit{et al.} is broadly relevant to the study of magnetic quantum criticality in a physically pure and theoretically tractable system that balances elements of weak and strong coupling. \\[4pt] [1] R. Jaramillo, Y. Feng, J. Wang {\&} T. F. Rosenbaum. Signatures of quantum criticality in pure Cr at high pressure. \textit{Proc. Natl. Acad. Sci. USA} \textbf{107}, 13631 (2010). \\[0pt] [2] R. Jaramillo, Y. Feng, J. C. Lang, Z. Islam, G. Srajer, P. B. Littlewood, D. B. McWhan {\&} T. F. Rosenbaum. Breakdown of the Bardeen-Cooper-Schrieffer ground state at a quantum phase transition. \textit{Nature} \textbf{459}, 405 (2009). [Preview Abstract] |
Session Y4: Cold Quantum Gases in Reduced Dimensions
Sponsoring Units: DAMOPRoom: 205C
Friday, March 2, 2012 8:00AM - 8:12AM |
Y4.00001: Tunneling-driven transitions in magnetization compressibility and density redistributions in a fermionic superfluid of cold atoms trapped in an array of one-dimensional tubes Kuei Sun, C.J. Bolech We study two-species fermion gases with attractive interactions in optical lattices that are made as an array of one-dimensional tube confinements. With the decrease in lattice depth, we find that the increase in tunneling between tubes leads to an incompressible-compressible transition in magnetization. The role of pair tunneling is considered, as well as the experimental implications. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y4.00002: Fermion Pairing in a One-Dimension Optical Lattice Ariel Sommer, Lawrence Cheuk, Mark Ku, Waseem Bakr, Martin Zwierlein Strongly correlated fermions in an array of two-dimensional planes coupled via tunneling serve as an important model system for high-temperature superconductors and layered organic conductors. We realize this model using ultracold fermionic $^6$Li atoms in a one-dimensional optical lattice near a Feshbach resonance. The depth of the lattice controls the interlayer coupling, and tunes the system between three and two dimensions. Pairing between fermions is studied using radio-frequency spectroscopy. The binding energy of fermion pairs is determined along the dimensional crossover and for different interaction strengths through the BEC-BCS crossover. Probes of superfluidity in the coupled layer system are also discussed. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y4.00003: ABSTRACT WITHDRAWN |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y4.00004: Entanglement-based perturbation theory for highly anisotropic Bose-Einstein condensates Alexandre Tacla, Carlton Caves We investigate the emergence of three-dimensional behavior in a reduced-dimension Bose-Einstein condensate trapped by a highly anisotropic potential. We handle the problem analytically by performing a perturbative Schmidt decomposition of the condensate wave function between the tightly confined direction(s) and the loosely confined direction(s). The perturbation theory is valid when the nonlinear scattering energy is small compared to the transverse energy scales. Our approach provides a straightforward way, first, to derive corrections to the transverse and longitudinal wave functions of the reduced-dimension approximation and, second, to calculate the amount of entanglement that arises between the transverse and longitudinal spatial directions. Numerical integration of the three-dimensional Gross-Pitaevskii equation for different cigar-shaped potentials and experimentally accessible parameters reveals good agreement with our analytical model even for relatively high nonlinearities. In particular, we show that even for such stronger nonlinearities the entanglement remains remarkably small, which allows the condensate to be well described by a product wave function that corresponds to a single Schmidt term. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y4.00005: Phase Diagram of the Bose Hubbard Model with Weak Links Kalani Hettiarachchilage, Valy Rousseau, Ka-Ming Tam, Juana Moreno, Mark Jarrell, Daniel Sheehy We study the ground state phase diagram of strongly interacting ultracold Bose gas in a one-dimensional optical lattice with a tunable weak link, by means of Quantum Monte Carlo simulation. This model contains an on-site repulsive interaction (U) and two different near-neighbor hopping terms, $J$ and $t$, for the weak link and the remainder of the chain, respectively. We show that by reducing the strength of $J$, a novel intermediate phase develops which is compressible and non-superfluid. This novel phase is identified as a Normal Bose Liquid (NBL) which does not appear in the phase diagram of the homogeneous bosonic Hubbard model. Further, we find a linear variation of the phase boundary of Normal Bose Liquid (NBL) to SuperFluid (SF) as a function of the strength of the weak link. These results may provide a new path to design advanced atomtronic devices in the future. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y4.00006: The effective mass of ultracold atoms in one-dimensional optical lattices Federico Duque Gomez, J.E. Sipe According to the effective mass theorem, in the presence of an external force the wavepacket associated with a crystal electron in one band accelerates as a particle with an effective mass. However, when the force is turned on suddenly, the expectation value of the acceleration initially behaves according to the electron's bare mass, and afterwards oscillates around the value given by the usual effective mass.\footnote{D. Pfirsch and E. Spenke, Z. Physik \textbf{137}, 309 (1954).} These oscillations are difficult to measure in typical solid state systems because they decay after a time of the order of femtoseconds.\footnote{Y. M. Zhu, et al., phys. stat. sol. (c) \textbf{5}, 240 (2008).} We consider this oscillatory behaviour with ultracold atoms in a one-dimensional optical lattice where the time scale of the oscillations and the coherence times are much longer. Our theoretical analysis is based on a perturbation scheme that decouples the bands to any order in the external force.\footnote{G. H. Wannier, Phys. Rev. \textbf{117}, 432 (1960).} We check the validity of this perturbative approach, comparing its results with those obtained from a full numerical calculation. Experimental investigations are underway.\footnote{A. Steinberg, private communication.} [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y4.00007: Supercurrent decay via quantum nucleation of phase slips in one-dimensional lattice bosons Ippei Danshita, Anatoli Polkovnikov We study transport properties of one-dimensional (1D) Bose gases in a periodic potential. In 1D, superflow at zero temperature can decay via quantum nucleation of phase slips even when the flow velocity is much smaller than the critical velocity predicted by mean-field theories. We use instanton techniques to find that the decay rate $G$ is algebraically increases with the flow momentum $p$ as $G/L \propto p^{2K - 2}$, where $L$ is the system size, $K$ the Luttinger parameter. We also discuss the relation between the nucleation rate and the quantum superfluid-insulator transition in order to present a physical interpretation of the scaling formula. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y4.00008: Compressibility and Entropy of One Dimensional Fermions in a combined Harmonic and Periodic Potential Andrew Snyder, Theja De Silva We solve the homogeneous Hubbard model for repulsively interacting fermions using thermodynamic Bethe anzatz method. Treating the harmonic potential in local density approximation, we calculate particle density, various compressibilities, double occupancy, and entropy as a function of temperature and interaction. These quantities show characteristic features that can be used to detect temperature, metal-insulator transition, and coexistence of metallic and insulating phases. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y4.00009: From GPE to KPZ: Finite temperature dynamical structure factor of the 1D Bose gas Manas Kulkarni, Austen Lamacraft Recent experiments on 1D Bose gases have raised interest in the investigation of dynamical properties at finite temperature such as the structure factor. For weak enough interaction and high enough temperature, we expect a classical description in terms of the Gross--Pitaevskii equation with thermally populated modes to be valid. Here, we present numerical results for the finite temperature dynamical structure factor and its universal anomalous scaling behavior, arising from resonant interactions between phonons. Our results are also relevant to sound damping in 1D classical fluids. Somewhat more surprisingly, there is a deep connection to systems in the Kardar--Parisi--Zhang universality class, describing growing fluctuating interfaces. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y4.00010: Photoinduced phase transition in one dimensional extended Hubbard model Hantao Lu, Shigetoshi Sota, Hiroaki Matsueda, Takami Tohyama We illustrate one interesting example of the photoinduced phase transitions due to a nonequilibrium process. The impact of laser pump on one dimensional half-filled extended Hubbard model in the spin-density-wave (SDW) phase is investigated by using time-dependent density-matrix renormalization group. With proper laser frequencies and strengths, we find that charge-density wave (CDW) can be observed during the pulse. Further, in some situations, for instance, near the boundary between SDW and CDW in the ground state, the CDW signature can be sustained even after the pulse turned off. The underlying physics and possible experimental realization are discussed. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y4.00011: Separation induced resonances in quasi-one-dimensional ultracold atomic gases Wenbo Fu, Zhenhua Yu, Xiaoling Cui We study the effective one-dimensional (1D) scattering of two distinguishable atoms confined individually by {\it separated} transverse harmonic traps. With equal trapping frequency for two s-wave interacting atoms, we find that by tuning the trap separations, the system can undergo {\it double} 1D scattering resonance, named as the separation induced resonance(SIR), when the ratio between the confinement length and s-wave scattering length is within $(0.791,1.46]$. Near SIR, the scattering property shows unique dependence on the resonance position. Right at SIR, the universal property of a many-body system is manifested by studying the interaction effect of a localized impurity immersed in a Fermi sea of light atoms. The proposed SIR can be realized in cold atom experiment. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y4.00012: Boson pairing and unusual criticality in a generalized XY model Yifei Shi Motivated by the physics of condensates of boson pairs, we study the generalized XY model in two dimension, which has a term proportional to cos(2 $\theta$) in addition to the normal XY Hamiltonian. This corresponds to having half vortices connected by solitons, as well as integer vortices. From both renormalization group analysis and Monte Carlo simulation using the worm algorithm, we find that the phase diagram includes Kosterlitz-Thouless transitions of half and integer vortices, together with an Ising transition. Remarkably, part of the Ising line is a direct transition from the quasi-long-ranged ordered state to the disordered state. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y4.00013: Pairing and pseudogap for ultracold fermions in two dimensions J. Tempere, S.N. Klimin, J.T. Devreese The T-matrix approach, straightforwardly applied to cold fermions in two dimensions, leads to a divergent fermion density for any finite temperature. We have shown that the Gaussian pair fluctuation theory, which is an improvement of the Nozi\`{e}res -- Schmitt-Rink approach, provides a convergent density in the paired fermion state. In our work, special attention is paid to the pseudogap state above the BKT transition temperature. In the pseudogap state, the modulus of the order parameter is finite, while phase coherence is absent. The pairing crossover temperature in 2D has been determined. Owing to the fluctuations, this pairing temperature is considerably lower than the mean-field critical temperature. With increasing coupling strength, the pairing temperature behaves non-monotonically reaching a maximum before decreasing to a finite value. For an imbalanced Fermi gas, the fluctuations lower the critical value of the imbalance at which the superfluid or non-coherent paired state is formed. This effect exists even at zero temperature, where only the quantum fluctuations survive. The obtained pairing temperatures and spectral functions are in fair agreement with recent experimental results on pairing of fermionic atoms in strongly anisotropic optical lattices. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y4.00014: Excitation spectrum of two-dimensional cold fermionic gases in the dilute limit C. Berthod, C. Kollath, T. Giamarchi, M. Feld, B. Fr{\"o}hlich, M. Koschorreck, E. Vogt, M. K{\"o}hl Two-dimensional gases of fermionic atoms have been recently realized, and cooled down to temperatures a few tenths of the Fermi temperature. Such ultracold atom systems are ideal tools to investigate the fundamental properties of Fermi ensembles subject to short-range interactions. One of the key questions is whether the interaction changes the ground state and excitation spectrum in a non-perturbative way, or whether the weak-coupling perturbation theory and Fermi-liquid idea remain valid in two dimensions. In contrast to condensed-matter systems, in atomic gases the perturbation theory must be carried out at finite temperature and far from the Fermi surface for a meaningful comparison with experiment. We have calculated the electronic self-energy of dilute two-dimensional Fermi gases at arbitrary temperature and momentum, using the ladder approximation. This scheme is expected to become exact (in a perturbative sense) in the low-density limit. For short-range attractive interaction, we study the evolution of the excitation spectrum as a function of temperature and interaction strength, and we compare our results with recent experiments. [Preview Abstract] |
Session Y5: Optical Properties and Excitations in Semiconductor Quantum Dots
Sponsoring Units: DCMPChair: Eric Stinaff, Ohio University
Room: 206A
Friday, March 2, 2012 8:00AM - 8:12AM |
Y5.00001: Elastic vs Inelastic Light Scattering from a Quantum Dot K. Konthasinghe, J. Walker, M. Peiris, C.K. Shih, Y. Yu, M. Li, J. He, L. Wang, H. Ni, Z. Niu, A. Muller We spectrally resolve the light scattered by a single InAs semiconductor quantum dot and analyze in detail the contribution from elastic and inelastic scattering processes. The measurements are well described by the theoretical expression given by Mollow. High resolution measurements reveal that the elastically scattered light is highly phase coherent with the laser. Thus a quantum dot elastically scattering a pulsed laser may serve as a triggered single photon source. In this regime, spectral diffusion and other broadening mechanisms are not a bottleneck for obtaining transform-limited photons. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y5.00002: Polarized luminescence characterization of charged quantum dot molecules Ramana Thota, Swati Ramanathan, Kushal C. Wijesundara, Eric Stinaff, Allan Bracker, Dan Gammon Polarization sensitive spectroscopy of self assembled quantum dots (QDs) has been shown to yield important information about spins associated with the charge carriers in various excitonic states. As pairs of quantum dots are brought together, and the formation of molecular states via tunneling becomes relevant, the interactions that determine the polarization state can be modified. In this talk we will present polarization dependent photoluminescence and photoluminescence excitation studies on vertically stacked InAs QDs grown by molecular beam epitaxy. We will discuss the characterization of the Stokes parameters of singly and doubly charged exciton states in these coupled QDs and compare with the results reported for single QDs. This study will help in identifying appropriate charge states for potential spin manipulation and entanglement measurements. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y5.00003: Exciton fine structure splitting in self-assembled semiconductor quantum dots: Intrinsic and extrinsic effects Ranber Singh, Gabriel Bester We investigate the excitonic fine structure splitting (FSS) in InGaAs/GaAs and GaAs/AlGaAs quantum dots (QDs) of different shapes and sizes using an atomistic pseudopotential approach [1,2]. We consider intrinsic effects originating from the atomistic symmetry of the structure. We highlight the effects of the growth direction and the repercussions it has on the point group symmetry and the FSS. We give predictions for the cases where the semiconductor alloy has a certain degree of atomic order, and for the case, where the QDs are influenced by charged point defects. These effects are contrasted to the extrinsic effect of applied stress.\\ \noindent [1] R. Singh and G. Bester, {\it Lower bound for the excitonic fine structure splitting in self-assembled quantum dots.} Phys. Rev. Lett. {\bf 104}, 196803 (2010).\\ \noindent [2] R. Singh and G. Bester, {\it Nanowire Quantum Dots as an Ideal Source of Entangled Photon Pairs.} Phys. Rev. Lett. {\bf 103}, 063601 (2009). [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y5.00004: Coulomb Enhanced Nonlinear Optical Properties of Strongly Confined Excitons in InAs/GaAs Quantum Dots Hanz Ramirez, Jefferson Florez, Angela Camacho Nonlinear optical response in parametric crystals has become widely used in entanglement production; however, low generation rate is an undesirable feature of this technique. Self-assembled quantum dots arrays (QDAs) may be a promising alternative with a larger nonlinear coefficients respect to higher dimensionality systems. Previously, some works have dealt with nonlinear susceptibilities of InAs/GaAs quantum dots, studying intraband transitions of either electrons or holes. The second order susceptibilities found there, are substantially bigger than those in bulk samples, although interband excitations and Coulomb effects were not considered. In this work we study the effects of strong confinement and Coulomb interactions on exciton states in fully 3D axially symmetric QDs. By using partial CI approach, we obtain eigenenergies and envelope eigenfunctions. Second order optical susceptibilities and their dependence on quantum dot size and aspect ratio are calculated. As a main result, we observe a Coulomb related enhancement in the second order optical susceptibilities of exciton transitions as compared to those in bulk, 2D, and even 0D purely intraband systems. This increased nonlinear response suggests interband excited QDAs as efficient entanglement sources. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y5.00005: Diamagnetic Exciton Properties in Quantum Dot Molecules Fino Puerto Nelson Ricardo, Ramirez Hanz Yecid, Camacho Angela S The magnetic properties of nanostructures like quantum dots and rings are the subject of intense research. In particular, magnetic control of coupled quantum dots (artificial molecules) has become subject of interest. The diamagnetic shift of confined excitons complexes has been used as a measured of the wave function spatial extent in semiconductor nanostructures. In weak magnetic field, the diamagnetic shift is expected to exhibit quadratic dependence. However, for exciton complexes the diamagnetic behavior is expected to exhibit more complicated features related to electron-hole asymmetry effects on Coulomb interactions. In this work we study the magnetic response of neutral and charged excitons in InAs/GaAs asymmetric artificial molecules By using a first order perturbation approach, and within the effective mass approximation, we calculate magnetic field dependent electronic structures of confined excitons and trions in vertically coupled quantum dots. These predicted regions, which show coexistence of crossing and anticrossing exciton states, because of allowing control of charge localization and polarization of emitted photons. . [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y5.00006: ABSTRACT WITHDRAWN |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y5.00007: Observed Shifts in Unoccupied States for Cu Doped CdSe Quantum Dots Observed via Synchrotron Techniques Joshua Joshua, Robert Meulenberg Recent work has been targeted on examining the optical properties of guest ions in quantum dot (QD) lattice; however, very few studies have attempted to understand the effect the dopant has on the host electronic structure. In this talk, we will present data that suggests copper doping of CdSe QDs leads to trapped states below the conduction band (CB) minimum of the host CdSe particle. We propose that one possible reason for this could be hybridization between copper and cadmium, lowering the energy for the cadmium 5s states below the CB minimum of bulk CdSe. X-ray absorption near edge structure spectroscopy measurements at the Cd $M_3$-edge for bulk, undoped, and doped QDs are compared and an unexpected lowering in the CB minimum is observed. We also present a first order theoretical model, for describing our results considering the effects caused by confinement, doping, and hybridization. Numerical approximations for atomic interactions suggest the hybridization parameter can lead to a lowering of the CB minimum by as much as 1.5 eV, as observed experimentally. Future work will include more in depth modelling of hybridization starting from tight binding calculations, developing a predictive model, applicable to more than existing data. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y5.00008: Elementary electronic excitations in quantum wires made up of vertically stacked quantum dots M.S. Kushwaha We investigate the elementary electronic excitations in quantum wires made up of vertically stacked (self-assembled) InAs/GaAs quantum dots. The length scales (of a few nm) involved in the experimental setups prompt us to consider an infinitely periodic system of two-dimensionally confined (InAs) quantum dot layers separated by GaAs spacers. The resultant quantum wire is characterized by the Bloch functions and the Hermite functions. We make use of the Bohm-Pines' RPA in order to derive a general nonlocal, dynamic dielectric function. The theoretical framework is then specified to work within a two-subband model that enables us to scrutinize the single-particle as well as collective responses of the system. We also size up the importance of studying the inverse dielectric function in relation with the quantum transport phenomena. It is remarkable to notice how the variation in the barrier- and well-widths can allow us to tailor the excitation spectrum in the desired energy range. Given the foreseen applications in the single-electron devices and in the quantum computation, it is quite tempting to explore the electronic, optical, and transport phenomena in such systems.\footnote{M.S. Kushwaha, J. Chem. Phys. {\bf 135}, 124704 (2011).} [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y5.00009: Anomalous Suppression of Valley Splittings in Lead Salt Nanocrystals Alexander Poddubny, Mikhail Nestoklon, Serguei Goupalov Atomistic $sp^3d^5s^*$ tight-binding theory of PbSe and PbS nanocrystals is developed. It is demonstrated, that the valley splittings of confined electrons and holes strongly and peculiarly depend on the geometry of a nanocrystal. When the nanocrystal lacks a microscopic center of inversion and has $T_d$ symmetry, the splittings are strongly suppressed as compared to the more symmetric nanocrystals with $O_h$ symmetry, having an inversion center. This effect is quite unusual because typically a higher symmetry of a physical system implies a higher degeneracy of its energy levels, while in our case the suppression of the splittings occurs in NCs having lower symmetry. Nevertheless, we were able to explain this puzzling behavior using mathematical apparatus of the group theory. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y5.00010: First-principles spectroscopic characterization of PbSe nanoparticles passivated with Fe complexes Keith Gilmore, Aaron Hammack, April Sawvel, Evelyn Rosen, D. Frank Ogletree, Jeffrey Urban, Delia Milliron, Brett Helms, Bruce Cohen, David Prendergast Given that defining characteristics of nanoparticles -- morphology, catalytic reactivity, optical and electronic properties -- are often dictated by their surfaces, it is informative to investigate how surface chemistry and structure change as different ligands are introduced to the surface. Starting with oleate-passivated PbSe nanoparticles, we remove the oleate ligands and replace them with an organometallic complex: cyclopentadienyl iron dicarbonyl. Measured and calculated x-ray photoemission core-level shifts indicate a charge transfer between surface Pb atoms and Fe atoms. We investigate the nature of this charge transfer in more detail through analysis of x-ray absorption spectra (XAS) at the Fe L-edge. Fe XAS are calculated from first-principles using a GW-based Bethe-Salpeter approach. The spectra reveal that the extent to which pi-backbonding is possible between the Fe and associated carbonyls varies with the charge density on the Fe atom. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y5.00011: Preservation of the optical properties of small Si quantum dots in the face of oxidation Huashan Li, Zhigang Wu, Mark Lusk Rapid oxidation occurs when small, unpassivated Si quantum dots are subjected to ambient conditions, and it may at first seem that complete oxidation of 1-3 nm QDs is inevitable, since their initial oxidation rate is expected to be greater than that for bulk Si for a more open surface structure, and the oxide layers on bulk Si tend to be thicker than the dot radii. We use computations based on density functional theory show that sufficiently small, appropriately terminated dots might actually be able to resist oxidation better than their larger brethren. This is because, on well-passivated bulk surfaces, oxygen attacks silicon through vulnerable sites and defects, and defects are much less likely to be present as dot size decreases. Although the more open surface structure of these small dots does leave certain keys sites more vulnerable to oxidation than their bulk counterparts, the oxygen atoms absorbed there are essentially immobile due to large hopping barriers. Furthermore, computations employing the many-body Green function perturbation theory show that the oxidation of these QDs has a relatively small impact on optical character. Therefore, the best defense against oxidation is to eliminate defects; a strategy that becomes increasingly reasonable as dot size decreases. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y5.00012: Confinement effects on the vibrational properties of colloidal quantum dots Peng Han, Gabriel Bester We present a first-principles study of the confinement effects on the vibrational properties of colloidal III-V and II-VI nanoclusters with thousand atoms and radii up to 16.2~\AA. We describe the connection between the vibrational properties including surface-optical and -acoustic modes, coherent acoustic modes and the structural changes induced by the surface. We highlight the qualitative difference between III-Vs and II-VIs. We can clearly ascribe most of the observations to the large relaxation of the clusters dominated by an inward relaxation of the surface penetrating deep inside the cluster in case of the III-Vs and a large distribution of bond length at the surface of II-VIs. These strong confinement effects tend to disappear for clusters with more than 1000 atoms, where a small red shift of the Raman peaks remains, due to a softening in response to undercoordination. The coherent acoustic phonons are identified and found to be in good agreement with results from the Lamb model and experiment. We explain why the simple model by Lamb gives an accurate description in case of the breathing modes while the vibrational properties of small NCs are poorly described by continuum models in general. [Preview Abstract] |
Session Y6: Carbon Nanotubes: Devices, Capacitors and Other Applications
Sponsoring Units: DMPChair: Mauricio Terrones, Pennsylvania State University
Room: 206B
Friday, March 2, 2012 8:00AM - 8:12AM |
Y6.00001: The Ultimate Electron Sources Using Millimeter Long Carbon Nanotubes N. Perea, B. Rebollo, J.A. Briones, A. Morelos, D. Hernandez, E. Munoz, F. Lopez-Urias, A.R. Botello, J.C. Charlier, V. Meunier, G.A. Hirata, B. Maruyama, M. Terrones, H. Terrones We are reporting the fabrication of a very efficient electron source using long and crystalline carbon nanotubes. These devices start to emit electrons at fields as low as 0.10 V/$\mu $m and reach threshold emission at 0.164 V/$\mu $m. In addition, these electron sources are very stable for long operation periods up to 200 hrs and can achieve peak current density of 2 Acm$^{-2}$ at only 0.28 V/$\mu $m. To demonstrate intense electron beam generation, these devices were used to produce white light by cathodoluminescence. Finally, to rational the measured properties in open carbon nanotubes of different lengths we used density functional theory. The modeling establishes a clear correlation between length and field enhancement factor. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y6.00002: One-dimensional nature in transport property of SWNT thin film electrochemical transistor Hidekazu Shimotani, Satoshi Tsuda, Hongtao Yuan, Yohei Yomogida, Rieko Moriya, Taishi Takenobu, Kazuhiro Yanagi, Yoshihiro Iwasa Recent success in isolating single-walled carbon nanotubes (SWNTs) of narrow chirality distribution enabled making pure metallic (m-) and semiconducting (s-) SWNT films. Such films are expected to reflect the nature of individual SWNTs, that is their one dimensional subband structure. Therefore, it is interesting to investigate electronic transport in m- and s-SWNT films by controlling their Fermi level ($E_{F})$. Chemical doping or FET is unsuitable for the purpose because of the lack of precise and reversible $E_{F}$ controllability, and the narrow controllable $E_{F}$ range, respectively. The problems are solved by our electric double layer transistor technique,$^{1}$ where the gate voltage ($V_{G})$ is applied through an electrolyte. The conductance and optical absorption spectra of the resistance of s- and m-SWNT films were measured at various $V_{G}$. The conductance of the s-SWNT film showed stepwise change against $V_{G}$. The absorbance spectra indicate the steps correspond to reaching of the $E_{F}$ to a vHs. Furthermore, even m-SWNT films showed steep increases of conductance, demonstrating that the conductance strongly depend on the subband filling. $^{1}$ H. Shimotani et al., Appl. Phys. Lett. 88, 073104 (2006). [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y6.00003: A general approach for high yield fabrication of CMOS compatible all semiconducting carbon nanotube field effect transistor Muhammad R. Islam, Kristy Kormondy, Eliot Silbar, Saiful Khondaker We report strategies of achieving both high assembly yield of carbon nanotubes at selected position of the circuit via dielectrophoresis (DEP) and field effect transistor (FET) yield using semiconducting enriched single walled carbon nanotube (s-SWNT) aqueous solution. When the DEP parameters were optimized for the assembly of individual s-SWNT, 97{\%} of the devices show FET behavior with a maximum mobility of 210 cm$^{2}$/Vs, on-off current ratio $\sim $ 10$^{6}$ and on conductance up to 3 $\mu $S, however with an assembly yield of only 33{\%}. As the DEP parameters were optimized so that 1-5 s-SWNTs are connected per electrode pair, the assembly yield was almost 90{\%} with $\sim $ 90{\%} of these assembled devices demonstrating FET behavior. Further optimization gives an assembly yield of 100{\%} with up to 10 SWNT/site, however with a reduced FET yield of 59{\%}. Improved FET performance including higher current on--off ratio and high switching speed were obtained by integrating a local Al$_{2}$O$_{3}$ gate to the device. Our 90{\%} FET with 90{\%} assembly yield is the highest reported so far for carbon nanotube devices. Our study provides a pathway which could become a general approach for the high yield fabrication of CMOS compatible carbon nanotube FETs. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y6.00004: Scanning Photocurrent Characterization of Absorption Resonances and Photocarrier Generation in Single-Walled Carbon Nanotubes Tristan DeBorde, Tal Sharf, Joshua W. Kevek, Ethan D. Minot We use a scanning photocurrent microscope (SPCM) in conjunction with a supercontinuum laser source to study resonant absorption in individual single-walled carbon nanotubes (SWNTs). Characterization by spectrally-resolved SPCM is much faster than using resonant Raman scattering. The technique also complements existing Rayleigh scattering techniques because measurements can be performed on SWNTs in a standard field-effect transistor geometry. The broad band light source (0.67 eV to 2.76 eV) is monochromated and then focused onto the SWNT. Resonant absorption is manifested as peaks in photo-induced conductivity as a function of excitation energy. The wide range of photon energies gives us access to the excitonic transitions E11 to E44 in typical semiconducting SWNTs. This allows us to access information about the diameter/chirality of the nanotube, as well as probing phenomena associated with photogenerated carriers in SWNTs at room temperature. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y6.00005: Local and broadband photovoltaic response of aligned carbon nanotube films Sebastien Nanot, Darius Morris, Lisa A. Hendricks, Junichiro Kono, Cary Pint, Robert H. Hauge, Francois Leonard Although individual semiconducting single-walled carbon nanotubes (SWCNT) have exhibited clear photovoltaic responses, it remains unclear whether macroscopic films of carbon nanotubes can also behave this way. While some groups have explained finite photovoltages as Schottky barrier effects, other groups have proposed photo-thermoelectric effects in suspended films. Here, we have studied highly aligned SWCNT films that work well as photovoltaics. SWCNTs grown by CVD were transferred onto a SiO2 substrate. There was a broad diameter distribution in the films to obtain a large wavelength range of interband absorption. The films were top-contacted with various metals. We made a systematic scanning photocurrent study of such samples at 660 and 1350 nm. A strong local photovoltage appeared at electrode-SWCNT interfaces. Detailed comparison with theoretical calculations of the dependence of photo-response on the nanotube orientation, metal electrode type, and temperature unambiguously revealed the photovoltaic nature of the observed photovoltage. We assign these effects to the doping of both metallic and semiconducting SWCNTs under the electrodes, in a similar fashion to graphene, its lineshape being determined by the diffusion of photoexcited carriers. Finally, to obtain a finite net signal under global illumination, we utilized different electrode combinations and studied their photoresponses from the visible up to mid-infrared and terahertz. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y6.00006: Separated Carbon Nanotube Macroelectronics for Active Matrix Organic Light-Emitting Diode Displays Yue Fu, Jialu Zhang, Chuan Wang, Pochiang Chen, Chongwu Zhou Active matrix organic light-emitting diode (AMOLED) display holds great potential for the next generation visual technologies due to its high light efficiency, flexibility, lightweight, and low-temperature processing. However, suitable thin-film transistors (TFTs) are required to realize the advantages of AMOLED. Pre-separated, semiconducting enriched carbon nanotubes are excellent candidates for this purpose because of their excellent mobility, high percentage of semiconducting nanotubes, and room-temperature processing compatibility. Here we report, for the first time, the demonstration of AMOLED displays driven by separated nanotube thin-film transistors (SN-TFTs) including key technology components such as large-scale high-yield fabrication of devices with superior performance, carbon nanotube film density optimization, bilayer gate dielectric for improved substrate adhesion to the deposited nanotube film, and the demonstration of monolithically integrated AMOLED display elements with 500 pixels driven by 1000 SN-TFTs. Our approach can serve as the critical foundation for future nanotube-based thin-film display electronics. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y6.00007: Semiconducting Enriched Carbon Nanotube Aligned Arrays of Tunable Density and Their Electrical Transport Properties Saiful I. Khondaker, Biddut K. Sarker, Shashak Shekhar Many proposed applications of semiconducting single walled carbon nanotubes (s-SWNT) require massively parallel array as they can average out inhomogeneity of individual tubes, provide larger on- currents and better transistor properties. Here, we report assembly of solution processed semiconducting enriched (99{\%}) SWNT in an array with varying linear density via ac-dielectrophoresis and investigate electronic transport properties of the fabricated devices. We show that (i) the quality of the alignment varies with frequency of the applied voltage and that (ii) by varying the frequency and concentration of the solution, we can control the linear density in the array from 1 to 25 s-SWNT/$\mu $m. We found that with increasing nanotube density the device mobility increases while the current on-off ratio decreases dramatically. For the dense array, the device current density was 16 $\mu $A/$\mu $m, on-conductance was 390 $\mu $S, and sheet resistance was 30 k$\Omega $/. These values are the best reported so far for any semiconducting nanotube array. Our study will have important implications in fabricating high quality devices for digital and analog electronics. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y6.00008: Fully Transparent Separated Carbon Nanotube Based Thin-film Transistors and their Application in Display Electronics Jialu Zhang, Chuan Wang, Chongwu Zhou Transparent electronics have attracted numerous research efforts in recent years due to its great potential to make significant commercial impact in a wide variety of areas such as transparent displays. High optical transparency as well as good electrical performance is required for this kind of applications. Pre-separated, semiconducting enriched carbon nanotubes are excellent candidates for this purpose due to their excellent mobility, high percentage of semiconducting nanotubes, and room-temperature processing compatibility. Here in this paper, we report fully transparent high-yield transistors based on separated carbon nanotube random network. High electrical performance is achieved by using large work function thin metal layer and indium-tin oxide (ITO) as contacts and all devices show excellent transparency ($\sim $82{\%}). Furthermore, OLED control circuit has been demonstrated with transparent separated nanotube thin-film transistors and large range output light intensity modulation has been observed. Our results suggest the promising future of separated carbon nanotube based transparent electronics and can serve as the critical foundation for the next generation transparent display applications. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y6.00009: Fabrication of Carbon nanotube TFTs for pressure-sensing device by printing method Hiroyuki Endoh, Fumiyuki Nihey, Hideaki Numata, Kazuki Ihara, Tsuyoshi Sekitani, Takao Someya Printing technology is very promising from many advantages, for example, low cost, flexible etc. We previously have developed a high-performance printed thin-film transistor(TFT) using single-walled carbon nanotube(CNT). A simple ink-jet printing system was used for drawing the device patterns. The maximum temperature was 200 degrees during nano-silver electrode fabrication. The temperature of CNT-channel patterning is under 50 degrees. The widths of the source and drain electrodes were about 1 mm and the channel length were about 150 $\mu$m. The thickness of the gate insulator was about 650 nm. The estimated mobilities using over 95{\%} purified semiconductive-CNT were $\sim $5.1 cm2/Vs for the TFTs whose on/off ratio were more than 5,000. This time we fabricated CNT-TFT arrays for pressure-sensing sheet devices, using the printed-process on a plastic film. CNT-TFT sheet has a dimension of 16 x 16 TFTs. The average mobility of the TFTs is 4.6 cm2/Vs. The pressure-sensing cell was prepared to combine Printed-TFT, a conductive rubber sheet and a film with a copper foil. Drain current changes in response to pressure applied to the current changes were observed up to 100nA from 10pA. These results were very promising for CNT-TFT applications to printable electronics. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y6.00010: Flexible Single-wall Carbon Nanotube Membrane Symmetric Aqueous Double Layer Electrochemical Capacitor Pralav Shetty, Jim Kapulson, Ramakrishnan Rajagopalan, Kofi Adu We present preliminary results on an aqueous symmetric double layer electrochemical capacitor (EDLC) constructed with flexible binder-free single wall carbon (SWCNTs) membrane as electrodes. The capacitors were cycled from 0 to 1V @ 10 A/g for 10,000 cycles with 99.9{\%} coulombic efficiency and 94{\%} energy efficiency, and 100{\%} depth of discharge. The power performance of the aqueous symmetric SWCNTs membrane capacitor is almost 100 --1000 times better than commercial non-aqueous EDLC capacitors. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y6.00011: Optimizing Efficiency in Conducting Polymer/Single-walled Carbon Nanotube Hybrids for Organic Photovoltaics Josh Holt, Fritz Prehn, Martin Heeney, Nikos Kopidakis, Garry Rumbles, Jeffrey Blackburn Several unique properties of single-walled carbon nanotubes (SWCNTs) have motivated their investigation as potential replacements for fullerene derivatives as the acceptor phase of organic photovoltaic (OPV) devices. Although replacement of the ubiquitous fullerene acceptors by SWCNTs in OPV devices has shown limited success thus far, better understanding of charge transfer between SWCNTs and conjugated polymers has promoted its viability. We provide experimental evidence that m-SWNTs limit the generation efficiency and lifetime of the charge-separated state in these composites. We also probe the photo-carrier generation and decay dynamics in poly(3-hexylthiophene) (P3HT) paired with a broad diameter range of SWCNTs. We witness electron transfer from the polymer to SWCNT and \textit{selective} hole transfer from the SWCNT to polymer by varying the nanotube HOMO via its diameter. We finally extend our investigation to additional semi-conducting polymers that have contributed to high OPV efficiencies, pBTTT and PCDTBT. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y6.00012: Ultra-high density aligned Carbon-nanotube with controled nano-morphology for supercapacitors Mehdi Ghaffari, Ran Zhao, Yang Liu, Yue Zhou, Jiping Cheng, Roberto Guzman de Villoria, B.L Wardle, Q.M. Zhang Recent advances in fabricating controlled-morphology vertically aligned carbon nanotubes (VA-CNTs) with ultrahigh volume fractioncreate unique opportunities for developing unconventional supercapacitors with ultra-high energy density, power density, and long charge/discharge cycle life.Continuous paths through inter-VA-CNT channels allow fast ion transport, and high electrical conduction of the aligned CNTs in the composite electrodes lead to fast discharge speed. We investigate the charge-discharge characteristics of VA-CNTs with $>$20 vol{\%} of CNT and ionic liquids as electrolytes. By employing both the electric and electromechanical spectroscopes, as well as nanostructured materials characterization, the ion transport and storage behaviors in porous electrodes are studied. The results suggest pathways for optimizing the electrode morphology in supercapacitorsusing ultra-high volume fraction VA-CNTs to further enhance performance. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y6.00013: Capacitance of highly ordered nanocapacitors arrays: Model and microscopy Andrea Cortes, Carlos Celedon, Pablo Ulloa, Patricio Haberle It is described briefly the process used to build an ordered porous array in an anodic aluminum oxide (AAO) template, filled with multiwall carbon nanotubes (MWCNTs). The MWCNTs were grown directly inside the template through chemical vapor deposition (CVD). The role of the CNTs is to provide narrow metal electrodes wich contact with a dielectric surface barrier, hence, forming a capacitor. This procedure allows the construction of an array of 10$^{10}$ parallel nano-spherical capacitors/cm$^{2}$. A central part of this contribution is the use of physical parameters obtained from processing high-resolution transmission electron microscopy (HRTEM) images, to predict the specific capacitance of the AAO arrays. Electrical parameters were obtained by solving Laplace's equation through finite element methods [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y6.00014: Gecko inspired carbon nanotube based thermal gap pads Sunny Sethi, Ali Dhinojwala Thermal management has become a critical factor in designing the next generation of microprocessors. The bottleneck in design of material for efficient heat transfer from electronic units to heat sinks is to enhance heat flow across interface between two dissimilar, rough surfaces. Carbon nanotubes (CNT) have been shown to be promising candidates for thermal transport. However, the heat transport across the interface continues to be a challenging hurdle. In the current work we designed free standing thermal pads based on gecko-inspired carbon nanotube adhesives. The pads were made of metallic carbon nanotubes and the structure was designed such that it would allow large area of intimate contact. We showed that these adhesive pads can be used as electrical and thermal interconnects. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y6.00015: Stiff and Multifunctional Carbon Nanotube Composites Yuntian Zhu, Xin Wang, Qingwen Li, Philip Bradford, Fuh-Gwo Yuan, Dennis Tucker, Wei Cai, Hsin Wang It has been a challenge for two decades to assemble the extremely strong carbon nanotubes (CNTs) into macroscopic CNT composites that break the strength ceiling of carbon fiber composites. Here we report the fast incorporation of long CNTs into polymer matrix using a novel approach, stretch-winding, to produce composites that are much stronger than any current engineering composite. The CNT composites reach a strength of 3.8 GPa, an excellent electrical conductivity and a high thermal conductivity. These superior properties are primarily derived from the long length, high volume fraction, good alignment and reduced waviness of the CNTs that are produced. The combination of high strength and excellent electrical and thermal conductivities makes CNT composites a promising enabler of new aerospace technologies and adventures. [Preview Abstract] |
Session Y7: Glassy Metallic, Semiconductor, Oxide and Chalcogenide Systems
Sponsoring Units: DCMPChair: Punit Boolchand, University of Cincinnati
Room: 207
Friday, March 2, 2012 8:00AM - 8:12AM |
Y7.00001: Ion irradiation induced suppression of shear banding in amorphous ZrCuAl nanowires under simulated compression Qiran Xiao, Howard Sheng, Yunfeng Shi Metallic glasses (MG) are amorphous metallic solids which exhibit extraordinary mechanical properties including large elastic strain limit and high tensile strength; at the same time, MGs are generally brittle due to catastrophic failure of shear banding. Controversy remains, however, as to whether shear banding is size-dependent in MG nanopillar samples prepared using Focused Ion Beam (FIB) technique. Here we modeled Zr50Cu40Al10 MG nanopillars under irradiation via molecular dynamic simulations (MD) and found a transition in deformation mode from dominant shear-banding to homogeneous shear-flow. Our results call for careful consideration of irradiation damage as a result of FIB sample-preparation-technique. In addition, we also show the amorphization of icosahedral structures as a result of irradiation, which might be responsible for the homogeneous deformation mode of irradiated MG nanowires upon compression tests. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y7.00002: Two Level Systems in Amorphous Silicon Daniel Queen, Julie Karel, Xiao Liu, Gregory Hohensee, David Cahill, Frances Hellman The specific heat of e-beam evaporated a-Si thin films prepared at various growth temperatures was measured from 2-300K. Below 20K, films with low density have a specific heat in excess of the predicted Debye value while higher density films do not. The excess heat capacity is typical of a glass with a linear contribution that is characteristic of two-level systems (TLS) and a T$^{3}$ contribution that is in excess of the Debye specific heat calculated from the measured sound velocity. The excess specific heat is independent of the elastic properties of the materials as determined by shear modulus and sound velocity measurements but depends on film density. The density dependence suggests that the low energy excitations form in voids or low density regions and are not intrinsic to the amorphous silicon network. A correlation is found between the density of TLS and the excess T$^{3}$ specific heat suggesting that they have a common origin. Comparisons will be made between the specific heat and internal friction. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y7.00003: First principles calculations of the Urbach tail in the optical absorption of silica glass Babak Sadigh, Paul Erhart, Daniel Aberg, Eric Schwegler, Jeff Bude We present density-functional theory calculations of the optical absorption spectra of silica glass for temperatures up to 2400\,K. The calculated spectra exhibit exponential tails near the fundamental absorption edge that follow the Urbach rule, in good agreement with experiments. We also discuss the accuracy of our results by comparing to hybrid e xchange correlation functionals. By deriving a simple relationship between the exponential tails of the absorption co efficient and the electronic density-of-states, we establish a direct link between the photoemission and the absorpti on spectra near the absorption edge. This relationship is subsequently employed to determine the lower bound to the U rbach frequency regime. Most interestingly, in this frequency interval, the optical absorption is Poisson distributed with very large statistical fluctuations. Finally, We determine the upper bound to the Urbach frequency regime by id entifying the frequency at which transition to Poisson distribution takes place. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y7.00004: Substrate- and interface-mediated photocrystallization in a-Se films and multi-layers G.P. Lindberg, R.E. Tallman, B.A. Weinstein, S. Abbaszadeh, K.S. Karim, A. Reznik Photocrystallization in a-Se films and layered a-Se structures is studied by Raman scattering as a function of temperature for photon energies near or slightly below the band gap. The samples are $\sim $16.5 $\mu $m thick films of a-Se grown i) directly on glass, ii) on indium tin oxide (ITO) coated glass, iii) on glass that is spin coated with 800nm polymide, and iv) on a Capton sheet. A low As-concentration ($<$ 0.2 {\%}) is present in several of the a-Se films. We compare the results on these samples to prior findings on a-Se HARP targets, and on a polymer-encapsulated a-Se film [1]. We observe strong evidence that the interface between the a-Se film and the underlying substrate and/or multi-layers plays an important role in the onset time and growth rate of photocrystallized Se domains. In some samples a discontinuous increase in the onset time with increasing temperature occurs near the glass transition ($\sim $310K), and there is a surprising ``dead zone'' of no crystallization in this region. Other samples merely show a minimum in the onset time at similar temperatures, but no discontinuity and no region where crystallization is absent. Soft intermediate layers appear to increase stability against crystallization in an overlying a-Se film. The competing effects of substrate shear strain and thermal driving forces on the photocrystallization process are considered to account for these findings. \\[4pt] [1] R.E. Tallman et. al. J. Non-crystalline Sols. \textbf{354,} 4577-81 (2008) [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y7.00005: Lattice irreversibility and enhanced fragility under fatigue in amorphous solids Despina Louca, Peng Tong, Gongyao Wang, Peter Liaw, Yoshihiko Yokoyama, Anna Llobet, Yiming Qiu The enhanced brittleness observed in glassy ZrCuAl metallic alloys under mechanical cyclic fatigue loading is linked to a local atomic structure reorganization and suppression of atomic fluctuations. From the analysis of neutron scattering and the pair density function technique, an atomic cluster restructuring is observed that intensifies with increasing the compression cycles ex-situ. This is accompanied by an attenuation of the extended phonon-like lattice dynamics, beyond the region in the momentum space where the Boson peak appears, observed by inelastic neutron scattering. Together both effects most likely render the glass more fragile. These findings provide a direct link between the plasticity and the internal structure of metallic glasses under fatigue. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y7.00006: Probing the liquid behavior in La-based metallic glasses using NMR spectroscopy Magdalena Sandor, Wei Xu, Hai-Bo Ke, Xue-Kui Xi, Yue Wu, Wei-Hua Wang The nature of liquid structure and its temperature and/or pressure dependent behavior is currently an active area of scientific investigation. Temperature dependent $^{27}$Al nuclear magnetic resonance (NMR) experiments were carried out above the liquid temperature in La-based metallic glasses. The strong coexistence of two liquid states was observed in addition to nonlinear liquid behavior. NMR results also provide thermodynamic insight for the structural changes observed. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y7.00007: Chemical ordering in Cu-Zr and Cu-Hf liquids and glasses Nicholas Mauro, Adam Vogt, Matt Blodgett, James Bendert, Kenneth Kelton Evidence for chemical ordering in Cu-Zr and Cu-Hf metallic liquids and glasses is presented. High-energy X-ray diffraction data has been taking using the Beamline Electrostatic (BESL) technique and stationary scattering methods up to a high momentum transfer. From this data, we obtained the total pair-correlation functions. By examining the structural evolution in the equilibrium and undercooled liquid, and the glassy state, we find that ordering occurs, increasing the Cu-Zr (Cu-Hf) and Zr-Zr (Hf-Hf) correlations over a wide composition and temperature range. This ordering is evident by a judicious choice of scattering lengths and compositions. Growth is observed at two distinct r-space coordinates in the pair-correlation function, unique to these two metallic systems. These results will be discussed in the context of previous experiments which indicate that this ordering is chemical and topological in nature, likely due to the development of Cu-centered icosahedral clusters. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y7.00008: Molecular structure of Si$_x$Ge$_x$Te$_{100-2x}$ glasses K. Gunasekera, P. Boolchand, S. Mamedov Bulk glasses of the titled ternary have been synthesized in the 8\% $<$ x $<$ 16\% range by melt quenching. Glass transition temperatures, T$_g$(x), increase linearly from 140$^{\circ}$C at x = 8\% to 200$^{\circ}$C at x = 12\%, then decrease steadily to 160$^{\circ}$C thereafter. The non-reversing enthalpy of relaxation at T$_g$ shows a broad minimum near 9\% but a maximum near 12\%. Glasses appear to be fully polymerized at x $<$ 12\%, but segregate as x $>$ 12\%. The broad minimum near 9\%, most likely, represents the opening of an Intermediate Phase. Raman scattering, excited using 785 nm radiation of a glass at x = 10\%, shows two modes, a broad one near 160 cm$^{-1}$ and a narrow one near 127 cm$^{-1}$. A mode near 157 cm$^{-1}$ has been previously\footnote{M.Brodsky, Phys. Stat. Solidi(b) 52, 609 (1972)} identified with polymeric Te$_n$ chains of a-Te. Tentatively, we assign the 127 cm$^{-1}$ mode with face-sharing\footnote{M. Malyj et al. Phys. Rev. B 31, 3672 (1985)} GeTe$_4$, and a mode near 170 cm$^{-1}$ with edge-sharing SiTe$_4$ tetrahedra in these glasses. The nature of glass structure evolution with composition will be elucidated. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y7.00009: The sharpness of thermally reversing windows as a measure of glass network homogeneity S. Bhosle, K. Gunasekera, P. Boolchand, M. Micoulaut, C. Massobrio Reversibility windows(RWs) have been observed\footnote{Boolchand et al., Phil. Mag 85, 3823 (2005)} in Chalcogenides, modified- and unmodified-oxides, and solid electrolytes. These are identified with isostaticaly rigid networks formed in narrow compositional windows between flexible and stressed-rigid elastic phases. Until recently, we found RWs in oxides and solid electrolytes to be sharper than in chalcogenides. Recently we introduced\footnote{Bhosle et al., Sol. St. Comm. 151, 1851 (2011)} a Raman profiling method to track homogeneity of melts during synthesis, and found that the kinetics of homogenization of chalcogenide melts are slow. The enthalpy of relaxation at T$_g$ measured in binary Ge$_x$Se$_{100-x}$ glasses show the RW to be intrinsically square-well like with sharp edges in homogeneous samples ($\Delta$x $\sim$ 0), and becomes trapezoidal ($\Delta$x = 1.5\%), then triangular ($\Delta$x = 3\%) and eventually disappears as glass heterogeneity increases. The heterogeneity deduced from Raman profiling experiments provides the Ge-stoichiometry variation ($\Delta$x) across a batch composition, which can be used to predict the observed RWs. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y7.00010: Boson mode, Medium Range Structure and Intermediate Phase (IP) in (Na$_{2}$O)$_{x}$(B$_{2}$O$_{3})_{1-x}$ glasses K. Vignarooban, P. Boolchand, M. Micoulaut, M. Malki Raman scattering of titled glasses are examined using a T64000 Dispersive system. Scattering strengths of the Boson mode (40 cm$^{-1}$, 70 cm$^{-1})$ and the Boroxyl ring (BR) mode (808 cm$^{-1})$ are found to decrease with increasing x at the same rate in the 0 $<$ x $<$ 20{\%} soda range. Apparently, the 2D character of BRs embedded in a 3D network gives rise to the Boson mode.\footnote{M. Flores-Ruiz and G. Naumis, PRB, 2011. \textbf{83}: p. 184204} The triad of modes (705, 740, 770 cm$^{-1})$ near the 808 cm$^{-1}$ mode are found to display a maximum in scattering strength near x = 37{\%} (705 cm$^{-1})$, 33{\%} (740 cm$^{-1})$ and 25{\%} (770 cm$^{-1})$, suggesting that these are also ring modes of Na-tripentaborate (STPB), Na-diborate (SDB) and Na-triborate (STB) super-structures. Variations in Raman scattering strengths also suggest that STB percolate near x = 20{\%}, the \textit{stress} transition, while the STPB and SDTB percolate near x = 40{\%}, the \textit{rigidity} transition. These transitions were inferred from m-DSC experiments that show an intermediate phase in the 20{\%} $<$ x $<$ 40{\%} range in dry and homogeneous glasses. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y7.00011: Boson mode, Intermediate Phase, and glass molecular structure of heavy metal Oxides S. Chakraborty, P. Boolchand We have synthesized bulk glasses of (B$_{2}$O$_{3})_{5}$(TeO2)$_{95-x}$(V$_{2}$O$_{5})_{x}$ ternary and examined their thermal and optical properties as a function of composition ``x.'' The enthalpy of relaxation at T$_{g}$ shows a global minimum in the Vanadia concentration of 24{\%} $<$ x $<$ 26.5{\%}, which we identify\footnote{P. Boolchand et al. Phil. Mag. \textbf{85}, 3823 (2005)} as the intermediate phase (IP) with compositions at x $<$ 24{\%} to be in stressed-rigid and those at x $>$ 26.5{\%} in the flexible phase. Raman scattering reveals a rich lineshape including a Boson mode, whose scattering strength steadily decreases with increasing x, possibly due to bifurcation of weak (Te-O$_{axial})$ and strong (Te-O$_{equatorial})$ springs characteristic of TeO$_{2}$ building blocks.\footnote{S. Sakida, J. Phys.: Condens. Matter \textbf{12} (2000)} Vanadia alloying brings in isostatic building blocks, pyramidal V(O$_{1/2})_{3}$ and quasi-tetrahedral O= V(O$_{1/2})_{3 }$as suggested by present Raman scattering and recent $^{51}$V NMR data.\footnote{Ibid.} We describe the structure evolution of these glasses in terms of the Te-centered and V-centered local structures. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y7.00012: Identification of strong and weak interacting two-level systems in KBr:CN Alejandro Gaita-Arino, Moshe Schechter Tunneling two-level systems (TLSs) are believed to be the source of phenomena such as the universal low temperature properties in disordered and amorphous solids, and $1/f$ noise. The existence of these phenomena in a large variety of dissimilar physical systems testifies for the universal nature of the TLSs, which however, is not yet known. Following a recent suggestion that attributes the low temperature TLSs to inversion pairs [M. Schechter and P.C.E. Stamp, arXiv:0910.1283] we calculate explicitly the TLS-phonon coupling of inversion symmetric and asymmetric TLSs in a given disordered crystal. Our work (a) estimates parameters that support the theory in M. Schechter and P.C.E. Stamp, arXiv:0910.1283 in its general form, and (b) positively identifies the relevant TLSs in a given system. Consequences to the understanding of the microscopic structure of amorphous solids will be discussed. A. Gaita-Arino and M. Schechter, Phys.~Rev.~Lett.~{\bf 107}, 105504 (2011) [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y7.00013: ABSTRACT WITHDRAWN |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y7.00014: Coulomb Glass: a Mean Field Study Salvatore Mandra, Matteo Palassini We study the Coulomb glass model of disordered localized electrons with long-range Coulomb interaction, which describes systems such as disordered insulators, granular metals, amorphous semiconductors, or doped crystalline semiconductors. Long ago Efros and Shklovskii showed that the long-range repulsion induces a soft Coulomb gap in the single particle density of states at low temperatures. Recent works suggested that this gap is associated to a transition to a glass phase, similar to the Almeida-Thouless transition in spin glasses. In this work, we use a mean field approach to characterize several physical properties of the Coulomb glass. In particular, following a seminal work of Bray and Moore, we show that the Edward-Anderson parameter $q_{EA}$ and the spin glass susceptibility $\chi_{SG}$ are directly related to spectrum distribution of the Hessian matrix around free energy minima. Using this result, we show that no glass transition is associated to the gap formation. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y7.00015: Non-linear dielectric response of glass formers under oscillating temperature Takashi Odagaki, Masatoshi Kuroda, Hideaki Katoh, Yasuo Saruyama Exploiting a simple model we investigate linear and non-linear dielectric responses of glass formers under oscillating temperature. We demonstrate that three characteristic temperatures, Vogel-Fulcher, glass transition and cross-over temperatures, can be determined from the analysis of the dielectric response. We first show that the real part of the linear susceptibility at the static limit becomes a cusp below the cross-over temperature and its curvature changes at the glass transition temperature. We also analyze the non-linear susceptibility under oscillating temperature which contains information of the response of the system, in particular of the free energy landscape, against the temperature modulation. We find that the real part of the non-linear susceptibility shows anomalies at the characteristic temperatures similar to the linear susceptibility. [Preview Abstract] |
Session Y8: Focus Session: Frustrated Magnetism - Experiment
Sponsoring Units: DMP GMAGChair: Benjamin Ueland, Los Alamos National Lab
Room: 208
Friday, March 2, 2012 8:00AM - 8:12AM |
Y8.00001: Magnetic ordering in SrEr$_2$O$_4$ and SrHo$_2$O$_4$ O.A. Petrenko, T.J. Hayes, O. Young, G. Balakrishnan, L.C. Chapon, A. Wildes, P. Manuel, P.P. Deen Single crystal neutron diffraction reveals two distinct components to the magnetic ordering in geometrically frustrated compounds $\rm SrEr_2O_4$ and $\rm SrHo_2O_4$. One component, a long-range ordered ${\bf k}=0$ structure, is associated with the appearance of resolution limited Bragg peaks below the ordering temperature. Another component is a quasi 1D short-range structure which manifests itself by the presence of a strong diffuse scattering signal forming {\it planes} in reciprocal space. On cooling from higher temperatures down to 0.06~K, the partially ordered component develops gradually and does not undergo a pronounced phase transition. The magnetic moments in the long-range structure are pointing along the [001] axes in both compounds. In the short-range structure (which is incommensurate in $\rm SrEr_2O_4$ and ${\bf k} \approx 1/2$ in $\rm SrHo_2O_4$) the moments are predominantly pointing along the [001] and [010] axes in these two compounds respectively. The unusual coexistence of two magnetic structures is probed using XYZ-polarised neutron scattering techniques [1,2]. [1] T.J. Hayes {\it et al.}, to appear in Phys. Rev. B (2011). [2] O. Young {\it et al.}, in preparation (2011). [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y8.00002: Magnetic order in GdBiPt studied by x-ray resonant magnetic scattering A. Kreyssig, M.G. Kim, J.W. Kim, S.M. Sauerbrei, S.D. March, G.R. Tesdall, S.L. Bud'ko, P.C. Canfield, R.J. McQueeney, A.I. Goldman Rare earth ($R$) half-Heusler compounds, $R$BiPt, exhibit a wide spectrum of novel ground states.[1] We have employed x-ray resonant magnetic scattering to elucidate the microscopic details of the magnetic structure in GdBiPt below $T_{\textrm{N}}$ = 8.5\,K. Experiments at the Gd $L_{2}$ absorption edge show that the Gd moments order in an antiferromagnetic stacking along the cubic diagonal [1\,1\,1] direction satisfying the requirement for an antiferromagnetic topological insulator as proposed previously[2] where both time-reversal symmetry and lattice translational symmetry are broken, but their product is conserved. \\ The work at the Ames Laboratory was supported by US DOE, Office of Basic Energy Sciences, DMSE, contract DE-AC02-07CH11358. \\ \,[1] P. C. Canfield et al., J. Appl. Phys. \textbf{70}, 5800 (1991). \\ \,[2] R. S. K. Mong et al., Phys. Rev. B \textbf{81}, 245209 (2010). [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y8.00003: X-ray resonance Exchange scattering study of Field induced meta-magnetic phases in TbNi$_2$Ge$_2$ R. Das, Z. Islam, J.P.C. Ruff, P.C. Canfield Rare-earth inter-metallic compound, TbNi$_2$Ge$_2$ is an interesting material for its uniaxial anisotropy and the presence of a number of meta-magnetic phases (MP), which we have investigated using x-ray resonant exchange scattering techniques. Two distinct field induced MPs have been revealed at 5 K for fields below 3 T applied along the c axis. In zero field, the magnetic structure is characterized primarily by commensurate q$_1$=(0,0,0.75), q$_2$=(0.5,0.5,0) and q$_3$=(0.5,0.5,0.5) wave vectors, implying a complex sequence of FM and AFM planes. With increasing magnetic field, a weak (0,0,1) peak starts to evolve reaching a maximum in the first MP (1.2 T $<$ H $<$ 1.6 T) and sharply disappears on entering the second MP (H $>$ 1.6 T). In the second MP, q$_1$ becomes incommensurate, (0,0,0.766), q$_2$ gets strongly suppressed, and q$_3$ peak splits into combination harmonics of q$_1$ and q$_2$. The persistence of AF peaks imply that AF planes are quite robust to spin-flip transitions and MP phases are intricate in nature due to several competing interactions in this compound. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y8.00004: Inelastic neutron scattering of the itinerant magnets Cr2Te3 and tr-Cr5Te8 Adam Aczel, Garrett Granroth, Nirmal Ghimire, Michael McGuire, David Mandrus, Steve Nagler Itinerant magnets based on transition metal chalcogenide compounds are of current interest, in part due to their relationship to the parent compounds of Fe-based superconductors. Two particularly interesting systems in this family are the chromium tellurides Cr2Te3 and trigonal (tr) Cr5Te8. These materials crystallize in layered structures with alternating partially and fully-occupied planes of Cr atoms stacked along the c-axis. Magnetization measurements along different crystallographic directions show a net ferromagnetic response and large magnetic anisotropy. In addition, the saturation moments are smaller than predicted by an ionic model; consistent with itinerant behavior. Previous neutron diffraction results for Cr2Te3 revealed an ordered moment of $<$ 0.2 $\mu $B in the partially-occupied planes. We examined the magnetic excitations in these materials by powder neutron spectroscopy measurements using the SEQUOIA instrument at the SNS. We find similar moment sizes for the magnetic Cr atoms of both systems. However, despite their similar crystal structures, ordered moment sizes, and chemical compositions, their magnetic excitation spectra are strikingly different. We compare our data to the predictions of various models in an effort to determine the relevant exchange parameters, put constraints on their magnitudes, and understand the differences between the inelastic magnetic spectra. We find that exchange along the c-direction is critical to explain our data. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y8.00005: Magnetic structure and site occupancies in Fe containing $\mu$-phases AFe (A=Ta, Nb, Mo) Nirmal Ghimire, Ovidiu Garlea, Michael McGuire, David Mandrus Binary mu-phases are intermetallic compounds occurring in numerous systems combining heavy and light transition metals. They have compositions close to AM where A is a 4d or 5d element (Nb, Ta, Mo, W) and M is a 3d element (Fe, Co, Ni, Zn). The possibility of mixed site occupancies results in stoichiometries ranging from $A_{~7}M_{~6}$ to $A_{~6}M_{~7}$. Interestingly, only the Fe containing compounds have been found to show magnetic ordering. NbFe and TaFe are known to be antiferromagnetic, with Neel temperatures near 280 and 320 K, respectively. It has been reported that in these materials ferromagnetic kagome planes stacked antiferromagnetically along the rhombohedral (111) direction, but the exact site occupancies in these layers remain unclear. In our investigation we have found mu-phase of MoFe also to be antiferromagnetic, but with a unique magnetic structure and significantly lower transition temperature about 110 K. Here we discuss in detail the crystallographic and magnetic structures of TaFe, NbFe and MoFe mu-phases based on our recent neutron diffraction studies and physical property measurements on polycrystalline samples. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y8.00006: Magnetic Excitations in the Nearly Localized, Itinerant Magnet Gd, Studied by Neutron Spectroscopy G.E. Graroth, A.A. Aczel, J.A. Fernandez-Baca, S.E. Nagler Many of the current questions about magnetic superconductors are present when these complex materials are in the normal state. Therefore studies of simpler itinerant magnets may help provide understanding of these phenomena. We chose to study an Itinerant magnet near to the fully localized limit. The system of choice, Gd has a total moment size of $\sim 7.6 \mu_B$ of which $\sim 0.6\mu_B$ of that is itinerant. We used the SEQUOIA spectrometer, at the Spallation Neutron Source at Oak Ridge National Laboratory, to measure the magnetic excitations in a 12 gm $^{160}$Gd single crystal. The fine resolution Fermi chopper was spun at 360 Hz and phased for $E_i$ = 50 meV. The crystal was mounted with the $h0l$ plane horizontal and then rotated around the vertical axis in $1^{\circ}$ steps. This method, and the large out of plane detector coverage of SEQUOIA, provided continuous coverage of a large region of reciprocal space allowing us to map the magnetic excitations. This map provides a measured structure factor for comparison to spin wave models with and without itinerancy effects. There measurements also more clearly resolve the excitations along the $h00$ direction than in previous studies (J. W. Cable, R. M. Nicklow and N. Wakabayashi Phys. Rev. B {\bf 32}, 1710 (1985)). [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y8.00007: Magnetic and charge carriers properties of metamagnetic Fe$_3$Ga$_4$ Joshua Mendez, Yan Wu, Bradford Fulfer, Julia Chan, John DiTusa Single crystals of Fe$_3$Ga$_4$ were grown via an iodine vapor transport method. Previous investigations of arc-melted polycrystalline samples identify metallic conduction with a magnetic phase transition at 400 K and interesting temperature-dependent metamagnetic behavior. The single crystal samples allow a much fuller exploration of the magnetic properties and have yielded some interesting differences with the previous data. This includes a sharp reduction of the magnetization within the magnetically ordered phase associated with a sharp onset of the metamagnetic behavior in the field dependence near room temperature. A previously identified second phase transition occurs below 50 K where the metamagnetic behavior is replaced by a ferromagnetic magnetization with little hysteresis. We find substantial anisotropy in the magnetization which is particularly apparent between 50 and 300 K. Charge transport experiments are underway to explore the magnetoresistance and Hall effect of this magnet. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y8.00008: Structural and magnetic characterization of the complete delafossite solid solution (CuAlO$_2$)$_{1-x}$(CuCrO$_2$)$_x$ Phillip Barton, Ram Seshadri, Andrea Kn\"{o}ller, Matthew Rosseinsky We have prepared the complete delafossite solid solution between diamagnetic CuAlO$_2$ and the $t_{2g}^3$ frustrated antiferromagnet CuCrO$_2$. The crystal structure and magnetism were studied with powder x-ray diffraction and magnetometry. The unit cell parameters follow the V\'{e}gard law and $\mu_{\rm{eff}}$ is equal to the Cr$^{3+}$ spin-only $S = 3/2$ value. $\Theta_{\rm{CW}}$ is negative and its magnitude increases with Cr substitution. For dilute Cr compositions, $J_{\rm{BB}}$ was estimated by mean-field theory to be 3.0\,meV. Despite the sizable $\Theta_{\rm{CW}}$, long-range antiferromagnetic order does not develop until $x$ is almost 1, and is preceeded by glassy behavior. For all samples, the 5\,K isothermal magnetization is sub-Brillouin and does not saturate in fields up to 5\,T. A scaled inverse susceptibility plot reveals that significant short-range antiferromagnetic interactions occur in CuCrO$_2$ above its N\'eel temperature. Additionally, the Al-substituted samples exhibit uncompensated short-range behavior and $x = 0.75$ shows glassy characteristics. It is suggested that reduction in magnetic frustration due to the presence of non-magnetic Al does not have as dominant an effect on magnetism as do chemical disorder and dilution of magnetic exchange. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y8.00009: Quasi-Spin Glass in a Geometrically Frustrated Magnet Arthur Ramirez, Andrew LaForge, Benny Chan, Gavin Lawes A spin glass state is observed in the B-spinel ZnCr$_{2(1-x)}$Ga$_{2x}$O$_{4}$ for x $<$ 0.05 via low-temperature magnetization and specific heat. The spin glass phenomenology is conventional. The degrees of freedom (quasi-spins) that undergo freezing are unconventional, however, both in structure as well as their mutual interactions. In particular, below x = 0.05, the freezing temperature is independent of quasi-spin density, yielding a strong violation of mean field theory. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y8.00010: Thermodynamic tuning in the dilute Ising magnet LiHo$_{0.045}$Y$_{0.955}$F$_4$ D.M. Silevitch, M.A. Schmidt, G. Aeppli, T.F. Rosenbaum The LiHo$_x$Y$_{1-x}$F$_4$ family of dipole-coupled Ising magnets has been found to exhibit a rich variety of magnetic phases, ranging from ferromagnet to spin glass to decoupled spin clusters. We examine the behavior for x=4.5\% at dilution refrigerator temperatures as a function of thermal coupling to a heat reservoir. We show that by cooling the sample in different thermodynamic limits, we can select qualitatively different low temperature magnetic states with different quantum characters. Furthermore, we demonstrate that an external transverse magnetic field, which acts to tune the rate of quantum tunneling, can be used to switch the system between the two states. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y8.00011: High Pressure $^{31}$P-NMR Study of the 2D Frustrated Square-Lattice Compound BaCdVO(PO$_{4})_{2}$ at Low Temperatures B. Roy, Y. Komaki, R. Nath, D.C. Johnston, Y. Furukawa BaCdVO(PO$_{4})_{2}$ is a $S $= 1/2 frustrated square-lattice (FSL) compound with a nearest neighbor exchange coupling $J_{1}\sim $ - 3.36 K and a next-nearest neighbor exchange coupling $J_{2}\sim $ 3.53 K bearing $J_{2}$/$J_{1}\sim $ 1.05. Based on the $J_{2}$/$J_{1}$ ratio, the system is known to be located close to the disordered ground state (known as `nematic state') regime of the phase diagram. We have carried out $^{31}$P-NMR measurements under high pressure ($\sim $ 1 GPa) and at low temperatures using dilution refrigerator to investigate the pressure effects on the magnetic properties of the system. Under ambient pressure at $H $= 2.67 T, we observed a sharp peak in $^{31}$P spin lattice relaxation rate (1/$T_{1})$ at $T_{N}\sim $ 1.05 K, which corresponds to the antiferromagnetic ordering temperature. On the other hand, under a pressure of 1 GPa, the peak in 1/$T_{1}$ is suppressed and 1/$T_{1}$ shows a sudden decrease below $\sim $ 0.75 K. This indicates that $T_{N}$ decreases with the application of pressure. We will report the temperature dependence of the NMR spectra and of the 1/$T_{1}$ under different magnetic fields and pressures. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y8.00012: Triplet state and in-gap scattering from the spin-$^1$/$_2$ quantum antiferromagnet Ba$_2$YMoO$_6$ J.P. Carlo, J.P. Clancy, T. Aharen, Z. Yamani, J.P.C. Ruff, J.J. Wagman, G.J. Van Gastel, H.M.L. Noad, G.E. Granroth, J.E. Greedan, H.A. Dabkowska, B.D. Gaulin The double perovskite material Ba$_2$YMoO$_6$ is known to exhibit an absence of long-range and short-range magnetic order down to at least 2K, indicating a high degree of geometric frustration of its tetrahedrally-coordinated spin-$^1$/$_2$ Mo$^{5+}$ moments, and NMR results have implied the existence of a spin-singlet ground state. Though geometric frustration in both 3D and quasi-2D systems has been of intense interest in recent years, comparatively little attention has been given to FCC systems, which may exhibit geometric frustration as the FCC lattice can be viewed as a network of edge-sharing tetrahedra. We have conducted inelastic neutron scattering measurements using triple-axis and time-of-flight instruments, revealing a band of scattering at 28 meV which disappears above $\sim$125K; we identify this scattering band as the triplet excitation out of a singlet ground state. We also identify a weaker population of in-gap states which are reminiscent of spin-polaron states induced by weak disorder. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y8.00013: Magnetothermal transport in monoclinic Nd$_{2}$Ti$_{2}$O$_{7}$ Hui Xing, Hongwang Zhang, Hanjie Guo, Chunmu Feng, Guanghan Cao, Zhuan Xu, Hao Zeng The monoclinic Nd$_{2}$Ti$_{2}$O$_{7}$ is an interesting material due to the existence of a novel field-induced slow spin relaxation (SSR) in the paramagnetic state. The SSR is attributed to the cooperative relaxation mode formed through the spin-spin interactions between partially polarized spins. Here we report the magnetothermal transport measurement on Nd$_{2}$Ti$_{2}$O$_{7}$ single crystals. The zero field thermal conductivity is dominated by the phonon contribution. The magnetothermal conductivity reveals significant field dependence. The field and temperature dependence is discussed in connection with the field-induced slow spin relaxation. . [Preview Abstract] |
Session Y9: Focus Session: Complex Bulk Oxide: Orbital Physics
Sponsoring Units: DMP GMAGChair: David Vanderbilt, Rutgers University
Room: 209
Friday, March 2, 2012 8:00AM - 8:12AM |
Y9.00001: Temperature dependence of Photoinduced dynamics in the orbital-ordered state of $A$V$_{10}$O$_{15}$ ($A$ = Ba, Sr) Ayaka Nogami, Yuya Onishi, Kou Takubo, Takurou Katsufuji In $A$V$_{10}$O$_{15}$ ($A$ = Ba, Sr), the V ions with mixed-valence states, V$^{2+}$/V$^{3+}$ ($3d^{3}$/$3d^{2}$), are located on the triangular lattice. BaV$_{10}$O$_{15}$ exhibits a structural phase transition with V trimerization caused by the orbital ordering of V ions at $T_{c}$ = 123 K, whereas SrV$_{10}$O$_{15}$ does not exhibit such a phase transition. We performed a femtosecond pump-probe reflection spectroscopy on BaV$_{10}$O$_{15}$ and SrV$_{10}$O$_{15}$ to clarify their photoinduced dynamics. For $A$ = Ba, a photoinduced melting of V trimerization, i.e. a photoinduced phase transition, was observed at 10 K ($< T_{c}$). At $T$ = 200 K ($>> T_{c}$), the photoinduced reflectivity change ($\Delta R/R$) for $A$ = Ba shows an oscillation with the period of several tens picoseconds, similarly to the behavior for $A$ = Sr at 10 K. This oscillation can be explained by assuming that the photoinduced state at the sample surface propagates into the inside of the sample. At $T$ = 135 K, immediately above $T_{c}$, we found that $\mid\Delta R/R\mid$ for BaV$_{10}$O$_{15}$ increases with time, suggesting that the area of the photoinduced state on the sample surface increases with time. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y9.00002: Chemical pressure effects on structural, magnetic, and transport properties of Mn$_{1-x}$Co$_{x}$V$_{2}$O$_{4}$ Andhika Kiswandhi, James Brooks, Jun Lu, Jeffrey Whalen, Theo Siegrist, Haidong Zhou The low-temperature x-ray diffraction, susceptibility, specific heat, and resistivity of the single crystal Mn$_{1-x}$Co$_x$V$_2$O$_{4}$ have been investigated. With increasing Co-doping, the chemical pressure related to the decreasing V-V distance drives the system towards the itinerant electron limit, accompanied with the increase of the ferrimagnetic transition temperature and the suppression of the structural distortion. These effects are compared to the effects from the application of physical pressure, and show that the V-V distance is the critical parameter controlling the properties of AV$_2$O$_4$. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y9.00003: Thermal conductivity of spinel MnV$_{2}$O$_{4}$ with doping Takuro Katsufuji, T. Omura, T. Ishikawa, Y. Ishitsuka Spinel MnV$_{2}$O$_{4}$ exhibits a structural phase transition and ferrimagnetic ordering simultaneously at 57 K. The crystal symmetry in the low-temperature phase obtained by the x-ray diffraction of a single crystal indicates an antiferro-orbital ordering of V $t_{2g}$ states [1]. It was also found that Al doping into the V site suppresses the orbital-ordering temperature ($T_{\rm oo}$) but barely affects the ferrimagnetic-ordering temperature ($T_{\rm N}$); thus two transition temperatures are separated in the Al-doped samples. We measured the thermal conductivity of Mn(V$_{1-x}$Al$_{x}$)$_{2}$O$_{4}$, and found that thermal conductivity sharply increases (thermal resistivity decreases) below $T_{\rm oo}$. It was also found that, with applied magnetic field, thermal resistivity increases above $T_{\rm oo}$ but decreases below $T_{\rm oo}$. These results indicate that thermal conductivity is dominated by the fluctuation of orbital ordering. [1] T. Suzuki {\em et al.}, Phys. Rev. Lett. 98, 127203 (2007). [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y9.00004: Magnetic properties driven by local structure in quasi-1D Ising chain system cobaltate system Bongjae Kim, Beom Hyun Kim, Kyoo Kim, Hong Chul Choi, Sang-Yeon Park, Y.-H Jeong, B. I. Min Using {\it ab-initio} band structure method and the microscopic model calculation, the origins of the large orbital magnetic moment and unique magnetic anisotropy in the quasi-1D magnetic cobaltate, $\alpha$-CoV$_{2}$O$_{6}$, is investigated. Unique crystal electric field effect in $\alpha$-CoV$_{2}$O$_{6}$ is combined with the strong spin-orbit coupling, results in intriguing magnetic properties of the system. Based on the estimated strengths of the intra- and the inter-chain exchange interaction, experimentally found 1/3 magnetization plateau in the $MH$ curve can be attributed to spin-flop mechanism. Origin of the reduced magnetic entropy behavior is found to be the strong uniaxial magnetic anisotropy in the quasi-1D Ising chain system. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y9.00005: A Comparative Study of Magnetic and Structural Transitions Focusing on Dielectric and RF Measurements Laurel Winter, James Brooks, Haidong Zhou High frequency inductive and dielectric measurements were used to study the magnetic and structural transitions of a number of compounds, including a series of spinel vanadates of the form Mn$_{1-x}$Co$_x$V$_2$O$_4$. We then compared the results to those found using other measurement techniques, such as resistivity and specific heat. The high frequency inductive measurements were conducted using a tunnel diode oscillator (TDO), and proved to be an effective and simple way to observe the magnetic transitions. From capacitance and dissipation measurements we were able to observe both magnetic and structural transitions, but only in the more insulating samples. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y9.00006: MicroRaman study of orbitonphonon coupling in YbVO3 Benoit Roberge, Serge Jandl, Thomas T.M Palstra, A.A. Nugroho Owing to their strong electron correlation, transition-metal oxides with perovskyte related structures display a variety of interesting properties such as Mott transition, high-T$_{c}$ superconductivity and colossal magnetoresistance. YbVO$_{3}$ belong to this family and exhibits magnetic and orbital orderings at low temperatures. In this communication, we present a study of its first order and multiphonon Raman active excitations as a function of temperature. Impact of various orbital, magnetic and structural transitions is analyzed and possibilities of orbiton-phonon coupling in the observed phonon combinations around 1400 cm$^{-1}$ are discussed. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:48AM |
Y9.00007: Controlling Magnetism in Multiferroic BiFeO$_3$ Invited Speaker: Valery Kiryukhin BiFeO$_3$ (BFO) is a room-temperature multiferroic combining large electric polarization with an antiferromagnetic structure with a superimposed long-wavelength (62 nm) cycloidal modulation. Large single crystals of BFO have become available recently, opening up new opportunities for experiments. In this talk, we discuss various ways of controlling magnetism in BFO single crystals using coupling between the ferroelectric and magnetic order parameters. Electric field can rotate the electric polarization and Fe spins simultaneously, and a chiral magnetic monodomain state can be obtained. Populations of the 3 equivalent cycloidal magnetic domains can be controlled by an electric field through piezoelectric coupling. Alternatively, they can be controlled via the inverse effect by applying uniaxial pressure. Very small ($\sim$50 bar) pressures producing tiny elastic strain ($\sim 10^{-5}$) are needed to move the magnetic domain walls. Using polarized small-angle neutron scattering, we show for the first time that the spins in the cycloid are tilted, producing local weak ferromagnetism (0.06 $\mu_B$ rms value), confirming a long-standing theoretical prediction. This shows that intrinsic macroscopic ferromagnetism could be expected in strained BFO, in which the cycloid is suppressed. Combined with the ability to control the magnetic domains by an electric field or tiny deformation, this observation accentuates the potential of BFO for room-temperarture applications involving magnetoelectric effects. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y9.00008: Pressure-driven changes in electronic structure of BiCoO$_3$ Sudipta Kanungo, Tanusri Saha-Dasgupta Using first-principles DFT based calculations, carried out on the recently measured crystal structure data [Oka et.al. J.Am.Chem.Soc.132, 9438 (2010)], we study the changes in the electronic structure of BiCoO$_3$ between the ambient-pressure and the high-pressure conditions. Our study shows that the application of high pressure drives the high-spin-to-low-spin transition at the Co site. The obtained results for the ambient pressure phase shows C-type AFM alignment of Co high spins, while the electronic structure at the high-pressure phase shows the presence of a finite energy gap at E$_f$ in contrast with previously reported metallic or semimetallic character with low-spin state of Co. This semiconducting behavior in the nonmagnetic BiCoO$_3$ with LS state of Co is found to be driven by the presence of the GdFeO$_3$ type of orthorhombic distortion which arises due to finite mixing of Bi lone-pair states with O-p states, as opposed to previously predicted cubic or tetragonal symmetry of the high-pressure. The ambient pressure phase shows an order of magnitude larger energy gap arising due to the AFM alignment of Co in the high-spin state than the energy gap at the high pressure phase, explains the observed 3-order of magnitude jump in resistivity. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y9.00009: Long Range Ordering in Oxygen Doped SrCoO$_{x}$ F.J. Rueckert, F.Z. He, H. Mohottala, J.I. Budnick, W.A. Hines, B. Dabrowski, B.O. Wells Recent investigations have shown magnetic phase separation in polycrystalline samples of SrCoO$_{x} (2.5 \le x \le 3.0)$.\footnote{Xie et al. Appl. Phys. Lett 99, 052503 (2011)} As the samples are oxidized electrochemically, distinct ferromagnetic phases are formed at SrCoO$_{2.75}$ (T$_{C}$ = 165 K), SrCoO$_{2.88}$ (T$_{C}$ = 220 K), and SrCoO$_{3}$ (T$_{C}$ = 280 K). In the polycrystalline bulk samples, two magnetic phases are seen at oxygen concentrations between 2.875 and 3.0, always occurring in only a single structural phase. The distinct phases are also evident in epitaxial 100 nm films, but the mixed magnetic phase at intermediate concentrations is suppressed. The occurrence of separate magnetic phases at concentrations of 2.75 [3-1/4], 2.88 [3-1/8], and 3.0 imply an electronic ordering commensurate with the lattice. Using resonant x-ray diffraction in thin film samples, we have discovered the existence of a superlattice peak at (1/4, 1/4, 1/4), revealing a previously hidden order with a periodicity four times the basic perovskite cell in all directions. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y9.00010: Structural and magnetic properties of Nd$_{1-x}$Ca$_{x}$BaCo$_{2}$O$_{5.5}$ Omar Chmaissem, Stanislaw Kolesnik, Bogdan Dabrowski, Sevda Avci, Maxim Avdeev, Jason Hodges R$_{1-x}$A$_{x}$BaCo$_{2}$O$_{5.5}$ (R = rare earth, A = alkaline metal) is a relatively new class of complex oxide materials that exhibit a wide range of magnetic attributes in addition to metal/insulator switching properties, structural transitions and superstructure order parameters. In many ways, this family exhibits behaviors similar to those of the famous colossal magnetoresistive manganites; however, more complex properties have also been identified owing to the fact that the oxidation state of the cobalt ions often behave in unpredictable ways depending on the chemical composition of the investigated material and the corresponding Co local environment. Thus, Co$^{3+}$ and Co$^{4+}$ ions with high, intermediate and low spin states may be produced offering an additional degree of freedom to be accounted for when designing new materials with tunable magnetic properties. In this talk, I will discuss the effects of calcium substitution at the Nd sites and the various structural and magnetic models as determined by neutron powder diffraction and complementary magnetic measurements. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y9.00011: Peculiarities of spin and charge degrees of freedom in strongly correlated layered cobaltates Frank Lechermann, Lewin Boehnke, Antoine Georges, Oleg E. Peil, Christoph Piefke The layered cobaltate systems stand for the rare case of metallic quasi-2D triangular lattices at doping $x$. While $x$=0 corresponds to a half-filled scenario, $x$=1 marks the band-insulating limit. Surprisingly, the phase diagram displays especially for $x>$0.5 a rich competition between different spin-orderings as well as intriguing charge-ordering processes. Though already at high doping, strong electronic correlations are mainly responsible for the complex physics [1-4]. By means of combinations of band-structure techniques with many-body approaches it is shown that various cobaltate features may be tackled successfully. For instance the in-plane crossover from antiferromagnetic spin correlations towards the onset of ferromagnetism as well as charge ordering tendencies favoring an effective kagome lattice close to $x$=0.67 in agreement with experiment. The charge order also substantially effects the spectral and transport properties, giving rise to a specific low-energy scale susceptible to nonlocal correlations. \\[4pt] [1] F. Lechermann, Phys. Rev. Lett. 102, 046403 (2009).\\[0pt] [2] C. Piefke, L. Boehnke, A. Georges and F. Lechermann, Phys. Rev. B 82, 165118 (2010).\\[0pt] [3] L. Boehnke and F. Lechermann, arXiv:1012.5943.\\[0pt] [4] Oleg E. Peil, A. Georges and F. Lechermann, arXiv:1107.437 [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y9.00012: Stability of Multiferroic Transition in FeVO$_{4}$ against Transition Metal Doping Akila Kumarasiri, Ambesh Dixit, Gavin Lawes FeVO$_{4}$ is a recently discovered multiferroic material which undergoes successive antiferromagnetic phase transitions at T$_{N1} \quad \sim $ 22 K and T$_{N2} \quad \sim $ 15 K, with ferroelectricity developing at the T$_{N2}$ transition. FeVO$_{4}$ is a type II multiferroic where the ferroelectricity is magnetically driven. We have studied the effect of transition metal doping on these two phase transitions in order to explore how the multiferroic order is affected by introducing perturbations into the lattice. We synthesized polycrystalline M$_{x}$Fe$_{1-x}$VO$_{4}$ samples (M = Zn, Mn, Cr) using a standard solid state reaction method, and we used magnetic, dielectric, and heat capacity measurements to track the transition temperatures. Both magnetic and heat capacity measurements show clear peaks at the two transitions, enabling us to map how the transitions are suppressed as the doping fraction is changed. On doping with non magnetic Zn, we find only a minimal suppression of both transition temperatures, indicating the magnetic interactions producing the multiferroic order are surprisingly robust against non-magnetic perturbations. We will also present preliminary results of the effects of magnetic dopants, specifically Mn and Cr. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y9.00013: $^{51}$V single-crystal NMR Study on multiferroic FeVO$_4$ Jinshan Zhang, Long Ma, Weiqiang Yu, Zhangzhen He The multiferroicity in FeVO$_4$ is so far not well understood because its spin-orbit coupling is probably very weak. In this talk, we report our $^{51}$V single crystal NMR study on FeVO$_4$ under zero field and a finite field. The double magnetic transitions with $T_{SDW}\approx$ 19 K and $T_{helical}\approx$ 13 K are clearly shown by NMR spectra and the spin-lattice relaxation rate $1/T_1$. Two noneqivalent $^{51}$V sites are identified with different hyperfine couplings. Just below $T_{helical}$, a large RF enhancement is seen, which indicates rich magnetic domain walls formed in the helical state and not in the SDW state. Based on our results, we discuss the coupling between the magnetism and the ferro-electricity in FeVO$_4$. [Preview Abstract] |
Session Y10: Invited Session: Modeling & Simulation of the Impact of Space Radiation on Electronic Systems (Avionic and Astronautic)
Sponsoring Units: DCOMPChair: William Atkinson, Boeing Company
Room: 210A
Friday, March 2, 2012 8:00AM - 8:36AM |
Y10.00001: Mitigation of Space Radiation Effects Invited Speaker: William Atwell During low earth orbit and deep space missions, humans and spacecraft systems are exposed to high energy particles emanating from basically three sources: geomagnetically-trapped protons and electrons (Van Allen Belts), extremely high energy galactic cosmic radiation (GCR), and solar proton events (SPEs). The particles can have deleterious effects if not properly shielded. For humans, there can be a multitude of harmful effects depending on the degree of exposure. For spacecraft systems, especially electronics, the effects can range from single event upsets (SEUs) to catastrophic effects such as latchup and burnout. In addition, some materials, radio-sensitive experiments, and scientific payloads are subject to harmful effects. To date, other methods have been proposed such as electrostatic and electromagnetic shielding, but these approaches have not proven feasible due to cost, weight, and safety issues. The only method that has merit and has been effective is bulk or parasitic shielding. In this paper, we discuss in detail the sources of the space radiation environment, spacecraft, human, and onboard systems modeling methodologies, transport of these particles through shielding materials, and the calculation of the dose effects. In addition, a review of the space missions to date and a discussion of the space radiation mitigation challenges for lunar and deep space missions such as lunar outposts and human missions to Mars are presented. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 9:12AM |
Y10.00002: The Space Radiation Environment in Energetic Particles at the Earth Invited Speaker: Gary Zank Understanding the radiation environment in energetic particles at the Earth is critical to the stability, integrity, and longevity of satellite subsystems. The radiation environment comprises particles trapped in the Earth's radiation belts and magnetosphere, those generated by solar energetic particle events (SEPs), and galactic and anomalous cosmic rays. Of these different populations, the most highly variable, and consequently difficult to anticipate, is the SEP population. This is also the population that can often cause the most damaging effects. SEP events can be either impulsive or gradual (sometimes a mixture of the two) with the gradual events being larger, much longer lasting, and often with higher particle energies. Diffusive shock acceleration at a coronal mass ejection driven shock wave is generally invoked to explain gradual SEP events. The detailed [plasma] physics of the acceleration mechanism remains to be elucidated. We are fortunate in that very detailed observations of particle acceleration at shock waves, particularly in the guise of Space Weather, are providing considerable experimental insight into the basic physics of particle acceleration at a shock wave. Detailed interplanetary observations are not easily interpreted in terms of simple steady-state models of particle acceleration at shock waves. Three fundamental aspects make the interplanetary problem much more complicated than the typical astrophysical problem: the time dependence of the acceleration and the solar wind background; the geometry of the shock; and the long mean free path for particle transport away from the shock wave. An interplanetary shock is not steady, as it decelerates and expands into an expanding, temporal solar wind. Furthermore, the shock geometry varies from quasi-parallel to quasi-perpendicular along a shock front, and multiple shocks can be present simultaneously in the solar wind. Consequently, the shock itself introduces a multiplicity of time scales, ranging from shock propagation time scales to particle acceleration time scales at parallel and perpendicular shocks, and many of these time scales feed into other time scales (such as determining maximum particle energy scalings, escape time scales, etc.). We will discuss the basic physics of particle acceleration via scalings, their relationship to particle acceleration models, observations and geometry in both an astrophysical and space physics context. This will include discussing the physics of perpendicular and parallel shocks, upstream turbulence, particle spectra, and particle injection and the seed population. After acceleration of particles at an interplanetary shock, the transport of energetic particles is non-diffusive because of their large mean free path in the quiet solar wind. We will address the coupled acceleration and transport of heavy ions, Fe/O ratios, the variability among individual events, and seed particle populations. We will discuss theoretical models and address recent modeling efforts. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:48AM |
Y10.00003: Monte Carlo Simulation of Radiation Effects in Microelectronics Invited Speaker: Robert A. Weller Microelectronic devices are susceptible to disruption by ionizing radiation, and with the scaling of devices to ever-smaller dimensions they are increasingly vulnerable to single event effects. Single event effects are transient errors in active devices, usually although not exclusively in digital logic, that are caused by the interaction of ionizing particles with the materials from which the devices are made. This presentation describes a Monte Carlo approach for predicting the rate of single event effects from knowledge of radiation environments and device structure. The approach combines detailed physical modeling of discrete radiation events, semiconductor device simulation to estimate charge transport and collection, and circuit simulation to determine the macroscopic electrical effects of collected charge. Details of the Monte Carlo simulation will be presented, and a mathematical analysis that establishes its relationship to earlier single event rate prediction methods will be discussed. Recent experimental and computational results on the rate of single event effects in highly scaled devices will be reviewed. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:24AM |
Y10.00004: Data-driven Three-Dimensional (3D) Global Magnetohydrodynamic (MHD) Model with Radiation to Study the Solar Atmospheric Dynamics Invited Speaker: Shi Wu In this presentation, we describe a self-consistent, three-dimensional, global compressible, and resistive magnetohydrodynamic (MHD) model together with time-dependent boundary conditions based on the projected method of characteristics at the source surface (photosphere) to accommodate the observations. The additional physics included in this model are differential rotation, meridional flow, effective diffusion, and cyclonic turbulence effects in which the complex magnetic field structure can be generated through the nonlinear interactions between the plasma flows and magnetic field. To illustrate the capability of this model, we selected GONG's global transverse velocity measurements of synoptic chart CR2009 near the photosphere and SOLIS full-resolution LOS magnetic field maps of synoptic chart CR2009 on the photosphere as the inputs to drive the model to simulate the equilibrium state and compute the energy transport across the photosphere. To show the advantage of using both measured magnetic field and transverse velocity data, we have investigated two cases: (1) with the inputs of the LOS magnetic field and transverse velocity measurements, and (2) with the input of only the LOS magnetic field. For these two cases, the simulation results presented here are a three-dimensional coronal magnetic field configuration, density distribution on the photosphere and 1.5 solar radii, and the solar wind in the corona. The deduced physical characteristics are the total current helicity and the synthetic emission. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 11:00AM |
Y10.00005: Multi-Scale Modeling of the Plasma Flow and Magnetic Fields in the Entire Heliosphere Invited Speaker: Nikolai Pogorelov Numerical model of the solar wind (SW) interaction with the local interstellar medium (LISM), developed in UAHuntsville and implemented in the Multi-Scale Fluid-Kinetic Simulation Suite, treats ions with MHD equations while the transport of neutral atoms is performed kinetically by solving the Boltzmann equation. Pickup ions are treated as a separate fluid or kinetically. The evolution of SW turbulence is addressed on the differential equation level. Time-dependent, based on observational data, modeling of the SW in the entire heliosphere is critical for the space weather modeling and interpretation of the spacecraft data. We choose LISM properties using remote observations and appropriate numerical modeling which allows us to constraint them by matching the ribbon of the energetic neutral atom flux detected in different energy bands by the Interstellar Boundary Explorer. We used the Ulysses data to model the SW-LISM interaction during the period of the mission and matched rather well the timing of the termination shock crossing by Voyager 1 and Voyager 2. The SW boundary conditions include those provided by the interplanetary scintillation measurements and obtained by numerical modeling of the inner heliosphere from the Sun's surface to the Earth orbit. Numerical results are extracted as time series along real spacecraft trajectories and compared with in situ measurements. [Preview Abstract] |
Session Y11: Focus Session: Graphene Devices - Fabrication
Sponsoring Units: DMPChair: Hyun-Jong Chung, Samsung Advanced Institute of Technology
Room: 210B
Friday, March 2, 2012 8:00AM - 8:12AM |
Y11.00001: Lattice-Nanotomy for Large-Scale Production of Transferrable and Dispersible Graphene-Nanostructures of Controlled Shape and Size Balabalaji Padavala, Nihar Mohanty, David Moore, Zhiping Xu, Ashvin Nagaraja, Alfredo A. Rodriguez, Vikas Berry In this talk, we will present a novel graphite-lattice-nanotomy (nanoscale-cutting) process for high throughput production of monodispersed graphene nanostructures (GNs) with controlled shape (square, rectangle, ribbons and triangle), dimensions (sizes at 5 nm resolution with a range of 5--600 nm) and chemical-construct. We demonstrate that this versatile process enables the realization of unprecedented graphene-nanostructures, which exhibit the evolution of semiconductor-characteristics and electrical transport mechanism. Further, we will present in detail the size and shape-dependent electrical and optical properties of these GNs via various microscopic and spectroscopic techniques. This nanotomy process can provide access to virtually-infinite and unprecedented GNs for development of fundamental optical/electrical/structural correlations and novel applications. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y11.00002: Direct Writing of Graphene-based Nanoelectronics via Atomic Force Microscopy Michael Haydell, Elena Cimpoiasu, Woo Kyung Lee, Rory Stine, William P. King, Paul Sheehan We use direct writing with an atomic force microscope (AFM) to fabricate simple, graphene-based electronic components to explore their electronic properties and the feasibility of manufacturing electronic devices via AFM. The process being studied, thermochemical nanolithography (TCNL), involves flowing current through an AFM cantilever to provide thermal energy to a chemically modified graphene (CMG) film, either graphene oxide or graphene fluoride. The heat reduces the insulating CMG film back into conductive graphene. Thus, these nanoribbons can be used to fabricate nano-scale electronic components such as resistors, capacitors, and transistors. The technique, as compared to other attempts to produce graphene-based devices, is simple, does not involve solvents or other complicated fabrication steps, and allows for the exact placement of the devices on the substrate. The electronic properties of the devices produced using the two materials, measured using current-voltage characteristics at various temperatures down to 2 K and in variable magnetic fields up to 9 T, will be discussed. This work was partially supported by the US Naval Academy Research Office and the Nanoscience Institute at NRL. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y11.00003: A study on tapered graphene nanoribbons with controlled angle: Fabrication and conductivity studies Sreeprasad Theruvakkattil Sreenivasan, Balabalaji Padavala, Phong Nguyen, Vikas Berry Graphene, the newest member in the nanocarbon family, is a perfect single atom thick 2D sheet made up of carbon with exceptional electrical and mechanical properties. It is well-known that the band-gap of graphene nanoribbons (GNRs) can be controlled via their width. Here we demonstrate that GNRs with tapered morphology have semiconducting-to-metallic continuum along its length, and thus exhibit unique electrical properties. The device is fabricated from a single layer graphene grown on a Cu foil \textit{via} the standard CVD process. Here, the graphene is transferred on a silica substrate and electron beam lithography etching is performed to produce a tapered graphene GNR device, followed by with Pd-Au electrode-deposition. We demonstrate the unique carrier transport properties, electrical rectification and carrier modulation in these novel devices. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y11.00004: Toward graphene based electronics from homogeneous top-gated graphene devices Jinseong Heo, Hyunjong Chung, Heejun Yang, David Seo, Hyun Jae Song, Kyoung-Eun Byun, Seongjun Park We report high quality top-gated graphene field effect devices integrated on 150mm substrate, which involves unprecedented homogeneous chemical vapor deposition growth of monolayer graphene on Ni/Cu. Statistics of maximum resistance, dirac voltage and also mobility over a thousand devices are shown for the first time, in which 5\% of measurable devices are in 3,000$\sim$5,000 cm2/V$\cdot$s of mobility. Furthermore, logic gates such as NOT and AND based on top-gated graphene devices were demonstrated. Our results imply a route toward graphene based electronics with complementary metal-oxide-semiconductor (CMOS) compatible process. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y11.00005: Gate-Tunable Superconductor-Insulator Transition in Bilayer-Graphene Josephson Junctions Dongchan Jeong, Gil-Ho Lee, Yong-Joo Doh, Hu-Jong Lee Bilayer graphene shows opening of electric-field-induced band gap, the size of which is proportional to the intensity of the electric field. We report electronic transport measurements on superconducting proximity effect in planar dual-gated bilayer-graphene Josephson junction with Pb$_{0.93}$In$_{0.07}$ (PbIn) electrodes ($\Delta_{PbIn}$ $\sim$ 1.1meV, $T_c$ = 7.0 K). The junction resistance along the charge-neutral point (CNP) increases as we modulate top- and back-gate voltages away from the zero-gap CNP. The resistive state near the CNP shows a variable-range-hopping-type insulating behavior in $R$-$T$ curve with lowering temperature crossing the superconducting transition of PbIn electrodes. However, a highly doped regime shows metallic $R$-$T$ behavior and junction becomes superconducting below $T_c$. Moreover, magnetic-field-induced Fraunhofer supercurrent modulation, microwave-induced Shapiro steps, and multiple Andreev reflection (MAR) are observed, which indicate the formation of genuine Josephson coupling across the planar junctions below $T_c$ with sufficiently transparent superconductor$-$bilayer graphene interface. The separatrix of the superconductor-insulator transition corresponds to the square junction conductance of $G_{sq}\sim$ 6$-$8$e^{2}/h$. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y11.00006: Characterization of graphene quantum dot hybrid structures Ting-Fung Chung, Jiuning Hu, Luis A. Jauregui, Liangliang Chen, Qing Zhao, Xiulin Ruan, Yong P. Chen We report electrical transport, photo-electric response and Raman spectroscopy measurements in macroscopic samples of graphene decorated with inorganic quantum dots (CdSe QDs). QDs are deposited on chemical vapor deposition (CVD) graphene by spin-coating. Raman measurements of graphene decorated with QDs on Si wafer show very similar spectra with clear G and 2D peaks that reveal no degradation of graphene during the QDs deposition process. Furthermore, two types of device architectures (QDs-graphene and graphene-QDs-graphene) are fabricated with graphene as a transparent electrode and QD as a light absorbent for electrical photoresponse characterization. Upon application of either a broadband light source or a 532-nm monochromatic laser source, graphene-QDs-graphene devices demonstrate photoconducting response, but not in the case of QDs-graphene devices. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y11.00007: Fabrication of nanometer-scale suspended graphene transistors Vahid Tayari, Joshua O. Island, Serap Yi\u{g}en, James Porter, A.R. Champagne We present a method to fabricate suspended ultra-short graphene transistors. We define narrow bowtie gold junctions on exfoliated graphene, and then use an oxygen plasma to etch away the graphene except under the gold junctions. The next step is to wet etch the SiO$_{2}$ under the junctions to suspend the devices. Finally, we use a feedback-control electromigration procedure to break the gold junctions and expose sections of graphene which are 100 to 300 nm wide and as short as $\approx$ 10 nm. Using electron transport, we show that these suspended graphene nanocrystals form ballistic two-dimensional Dirac electron gas systems. We study them as a function of temperature and charge carrier density. These ultra-short NEMS transistors offer the prospect of exploring the coupling between flexural vibrons and charge carriers in graphene. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y11.00008: Implementation of embedded-gate graphene field effect transistors on flexible substrates Jongho Lee, Li Tao, Milo Holt, Deji Akinwande In this work we present embedded-gate graphene field effect transistors (GFETs) on flexible polyimide films for high frequency RF and sensor applications. Graphene transistors with gate lengths from 1um to 4um and width of 8um have been realized by e-beam lithography. Chemical vapor deposited monolayer graphene with negligible defects is transferred to flexible polyimide substrates for device fabrication. The electrostatic measurement reveals that fabricated device shows a mobility of over 1000cm$^{2}$/Vs at room temperature, the highest reported among graphene FETs implemented on flexible films. Surface roughness and impurity doping are of crucial importance in fabricating flexible graphene devices since its charge transport is limited by mobility degradation due to surface roughness and doping related with the chemical transfer process. The stack of atomic layer deposited Al$_{2}$O$_{3}$ as gate dielectric and gate pads underneath provides smooth surface for graphene to sit on as confirmed by atomic force microscopy. This results in a field effect mobility of an embedded gate GFET twice that of a top gated GFET on polyimide substrates. The minimum conduction point is close to zero, indicting the impurity induced doping for the device is negligible. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y11.00009: Molecular Layer-seeded Ultra-thin Top-gate Dielectrics for High Transconductance Graphene Transistors Vinod Sangwan, Deep Jariwala, Hunter Karmel, Justice Alaboson, Lincoln Lauhon, Tobin Marks, Mark Hersam The potential of graphene in integrated analog and digital circuits can only be fully realized through incorporation of ultra-thin gate dielectrics to enable large-scale small-channel graphene field-effect transistors (GFETs). Atomic-layer deposition (ALD) is a viable technique to fabricate gate-dielectrics, however, it requires a seeding layer on otherwise inert graphene. Here, we demonstrate a single molecule thick perylene-3,4,9,10-tetracarboxylic dianhydride overlayer as an effective seeding layer to grow high-$\kappa $ Al$_{2}$O$_{3}$ on mechanically exfoliated graphene for high-performance GFETs. Using an ultra-thin ($<$ 1nm) seeding layer, in contrast to polymer films (5-10 nm), we demonstrate fabrication of the thinnest ALD-grown gate-dielectric (4 nm) reported to date in top-gated GFETs. This yields high performance GFETs with the intrinsic transconductance parameter approaching 2.4 mS and the field-effect mobility $\sim $3000 cm$^{2}$/Vs. We also demonstrate generalization of this molecular layer seeded-ALD growth method to higher- $\kappa $ gate dielectrics, yielding further enhanced GFET transconductance for possible application to radio-frequency circuits. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y11.00010: Conductance through two-terminal graphene junctions with wetting metal contacts Salvador Barraza-Lopez, Markus Kindermann, Mei-Yin Chou Metallic contacts become a relevant factor for the behavior of nanoscale graphene devices. A thin layer of a wetting metal --a metal that forms covalent bonds to graphene-- is customarily placed in between graphene and bulk leads. The most common choices for this wetting metal are Titanium, Chromium, and Palladium. We will present the equilibrium conductance through crystalline (defect- and impurity-free) two-terminal graphene junctions attached to normal, spin-unpolarized Titanium metal leads. In addition, we discuss the equilibrium potential profile across the junctions, and the presence of Fabry-Perot oscillations. The conductance shows pronounced noise, and the Fano factor near the Dirac point is seen to fluctuate, as in experiment [1]. The distribution of transmission eigenvalues is bimodal, indicating a disordered-metal-like charge transport through nanoscale two-terminal graphene junctions with wetting metals.\\[4pt] [1] L. DiCarlo, J. R. Williams, Y. Zhang, D. T. McClure, and C. M. Marcus. Phys. Rev. Lett. 100, 156801 (2008).\\[0pt] [2] S. Barraza-Lopez, M. Kindermann, and M.-Y. Chou. (Manuscript in preparation.) [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y11.00011: Fabrication of a tunable quantum point contact in bilayer graphene Patrick Maher, Carlos Forsythe, Takashi Taniguchi, Kenji Watanabe, Philip Kim The realization of a quantum point contact (QPC) in graphene is of interest for both physical and technological reasons. Fabricating a tunable QPC thus far, however, has been technologically challenging due to the inability to electrostatically deplete graphene. Recent advances have allowed for the creation of high mobility dual-gated bilayer samples sandwiched in hexagonal boron nitride. These samples display a robust, tunable band gap, which opens the door to electrostatically defining the conductance channel. In this presentation, we report the fabrication and characterization of point contact structures in high mobility dual-gated bilayer graphene samples employing hBN as both the top and bottom dielectric layer. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y11.00012: Surperconducting Graphene Nanodevice in Ballistic Regime Joel I-Jan Wang, Kenji Watanabe, Takashi Taniguchi, Pablo Jarillo-Herrero Superconductivity carried by Dirac fermion can be realized through induced superconductivity in graphene. Observation of novel phenomena anticipated by theories requires superconducting graphene devices with low disorder so that the transport is ballistic. In this talk we present the fabrication and characterization of superconducting graphene nanodevices that are built on hexagonal Boron Nitride. The ultra flatness and lack of dangling bond in the boron nitride substrate reduces the disorder in graphene, opening the door to the study of ballistic Dirac fermion in superconducting regime. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y11.00013: Scanning Tunneling Potentiometry Mapping of Electron Transport in Graphene on SiC Kendal Clark, Shengyong Qin, Xiaoguang Zhang, Guowei He, Gong Gu, Randall Feenstra, An-Ping Li Single layers of graphene formed on SiC look to be a promising system for the realization of graphene electronics. To utilize the full potential of graphene on SiC a complete understanding of the physical and electronic properties of this system is needed. Scanning Tunneling Microscope (STM) images along with scanning tunneling spectroscopy is used to characterize the sample surface. STM images clearly show the distinction between 1 monolayer (ML) and 2ML regions and this transition is further confirmed by point spectroscopy and spectroscopic mapping across the boundary. Defects, grain boundaries, step edges and other potential scattering centers are thought to play a major role in the electronic properties, especially in transport, along the graphene sheets. Using a low temperature four-probe scanning tunneling microscope, potentiometry measurements are performed on the epitaxial graphene grown on 4H-SiC. Potentiometry maps spanning the transition from 1ML to 2ML graphene show a contrast change indicating a potential change at this interface. Preliminary results of the transport along this potentially revolutionary new electronic system will be presented. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y11.00014: Atomic Force Microscopy-Based Local Tunable Oxidation of Graphene Satoru Masubuchi, Miho Arai, Tomoki Machida We have fabricated graphene/graphene oxide/graphene (G/GO/G) junctions by local anodic oxidation lithography using atomic force microscopy (AFM). The conductance of the G/GO/G junction decreased with the bias voltage applied to the AFM cantilever $V_{tip}$. For G/GO/G junctions fabricated with large and small $\vert V_{tip}\vert $. GO was semi-insulating and semiconducting, respectively. AFM-based LAO lithography can be used to locally oxidize graphene with various oxidation levels and achieve tunability from semiconducting to semi-insulating GO [S. Masubuchi \textit{et al.}, Nano Lett. \textbf{11}, 4542 (2011).] [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y11.00015: High mobility Single Layer Epitaxial Graphene on 4H-SiC (000-1) Yike Hu, Zelei Guo, Ming Ruan, John Hankinson, James Palmer, Baiqian Zhang, Rui Dong, Jan Kunc, Claire Berger, Walt deHeer Multi-layer Epitaxial Graphene on 4H-SiC (000-1) has demonstrated very high mobility up to$\sim $27,000 cm$^{2}$/Vs [1]. Recently single layer graphene grown by the Confinement Control Growth method [2] exhibits mobility up to $\sim $ 25,000cm$^{2}$/V$\cdot $s at 4K and 13,000 cm$^{2}$/V$\cdot $s at 300K with p=3 x 10$^{12}$ cm$^{-2}$ The relation between Raman G peak features (FWHM and position) and carrier density of Epitaxial Graphene on carbon face is revealed. Quantum Hall Effect [3] is observed both for p and n type carriers on top gated sample. This indicates that top gated single layer graphene can be produced on the Carbon face with high quality and high carrier mobility. \\[4pt] [1] Science \textbf{312}, 1191 (2006) \\[0pt] [2] PNAS \textbf{108 }(41) 16900 (2011) \\[0pt] [3] APL \textbf{95}, 223108 (2009) [Preview Abstract] |
Session Y12: Graphene: Electronic Structure and Interactions - STM and Nanoribbons
Sponsoring Units: DCMPChair: Brian LeRoy, University of Arizona
Room: 210C
Friday, March 2, 2012 8:00AM - 8:12AM |
Y12.00001: Mapping Dirac Quasiparticles near a Single Coulomb Impurity on Graphene Yang Wang, Victor Brar, Andrey Shytov, Qiong Wu, Willian Regan, Hsin-zon Tsai, Alex Zettl, Leonid Levitov, Michael Crommie We have locally mapped the response of charge carriers to a single Coulomb potential placed on a gated graphene device. Scanning tunneling microscopy and spectroscopy were used to fabricate a tunable charge impurity and to measure how Dirac fermions screen it. By mapping spatial variation in the electronic structure of graphene we have directly probed the strength of screened electronic interactions, obtaining a value of epsilon = 3 for the intrinsic graphene dielectric constant. This small value suggests that microscopic electron-electron interactions contribute significantly to intrinsic graphene properties. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y12.00002: Graphene Tunneling Heterostructures Benjamin Hunt, Javier Sanchez-Yamagishi, Pablo Jarillo-Herrero, R. C. Ashoori We have fabricated tunneling heterostructures comprising graphene on boron nitride (BN) substrates and tunnel barriers constructed of exfoliated BN or MoS$_{2}$. We present measurements of the low-temperature tunneling spectrum as a function of the tunneling energy and the carrier density in the graphene, with the latter controlled by a back-gate voltage. We observe a series of tunneling resonances, reminiscent of those seen in STM and planar tunneling experiments on graphene, whose energies disperse with the back-gate voltage. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y12.00003: Scanning Tunneling Spectroscopy of Graphene on Hexagonal Boron Nitride Matthew Yankowitz, Jiamin Xue, Daniel Cormode, Javier Sanchez-Yamagishi, Pablo Jarillo-Herrero, K. Watanabe, T. Taniguchi, Philippe Jacquod, Brian LeRoy Recent work has found hexagonal boron nitride (hBN) to be a good substrate for graphene devices due to its ability to screen charged impurities in the underlying substrate and increase graphene mobility. We investigated graphene on hBN heterostructures using scanning tunneling microscopy and spectroscopy. Because hBN has the same bond structure as graphene with a slightly longer lattice constant, a rotationally dependent Moir\'{e} pattern is formed in graphene on hBN heterostructures. The Moir\'{e} pattern creates a weak periodic potential for the charge carriers in graphene. We performed an experimental and theoretical investigation of its effect on the local density of states. We observed a Moir\'{e} wavelength dependent modification of the local density of states in good agreement with theory predictions. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y12.00004: Local spontaneous time-reversal symmetry breaking and interacting Dirac fermions in strained CVD-grown graphene on copper R.T.-P. Wu, M.L. Teague, H. Chu, D.A. Boyd, N.-C. Yeh, M.W. Bockrath Atomically resolved imaging and spectroscopy of CVD-grown graphene on Cu are studied using scanning tunneling microscopy and spectroscopy. Under strain-induced giant pseudo-magnetic fields (\textit{B$_{S}$}), quantized Landau levels are manifested by peaks of density of states (DOS) at quantized energies. While global time-reversal symmetry is preserved, local spontaneous time-reversal symmetry breaking (TRSB) for the two inequivalent lattice sites due to opposite \textit{B$_{S}$} directions is evidenced by the presence or absence of the zero-mode tunneling conductance peak, confirming theoretical predictions for gauge fields in graphene causing local TRSB while preserving the chiral symmetry. Additionally, the finding of both integer and fractional quantum Hall states due to strain-induced \textit{B$_{S}$} may be attributed to significant short-range Coulomb interactions of Dirac fermions in graphene mediated by the underlying Cu substrate, which yields an onsite Coulomb interaction \textit{U} $\sim$ 3.2 eV larger than the nearest-neighbor hopping energy \textit{t} $\sim$ 2.8 eV. Finally, effects on the DOS of graphene due to pseudo-magnetic fields are compared with those due to applied fields. This work was supported by NSF. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y12.00005: Real-Space Magnitude and Spatial Extent of the Surface Charge Density of Graphene P. Xu, Y. Yang, S.D. Barber, M.L. Ackerman, J.K. Schoelz, Salvador Barraza-Lopez, L. Bellaiche, P.M. Thibado, Igor A. Kornev The discovery of graphene, a unique two-dimensional electron system with extraordinary physical properties, has ignited tremendous research activity in both science and technology. One such extraordinary property is its enormous current-carrying capacity of 1$\mu $A per atomic row. Fundamentally, this suggests that graphene possesses an unusually large electronic density of states (DOS). Surprisingly, a detailed atomic-scale investigation of the DOS has yet to be completed. Here we present, for the first time, variable-current scanning tunneling microscopy (STM) images, which reveal an unusual three-dimensional picture of graphene's orbitals. Furthermore, density functional theory was used to simulate the variable-current STM images. From this we found that the orbitals expand to fill the holes in the honeycomb structure, making atomic-resolution STM more difficult at lower currents. Also, we discovered that the wavefunctions expand into the vacuum an unusually large amount. Identical studies were performed on graphite, revealing that the DOS of graphene is 300\% larger. Other significant differences found between graphite and graphene will be discussed. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y12.00006: Direct Imaging of Intrinsic Molecular Orbitals Using Graphene-based Moir\'e Pattern Haitao Zhou, Geng Li, Jinhai Mao, Yeliang Wang, Shixuan Du, Hongjun Gao Direct imaging of the intrinsic electronic structure at high resolution is of both fundamental and technological importance for investigating molecular interaction and mechanisms. Metallic or semiconducting materials are commonly used as substrates for molecular adsorption. Generally, the strong interactions between molecules and these substrates significantly change the intrinsic electronic and geometric structures of the adsorbed molecules. In order to overcome this problem, much effort has been made by passivating substrates with various buffer layers, for instance, thin organic films, NaCl, and oxides. We demonstrate the graphene grown epitaxially on Ru(0001) can be used as a buffer layer to study the intrinsic electronic properties of adsorbed molecules. The intrinsic molecular orbitals of C60, pentacene and perylene-3,4,9,10-tetracarboxylic dianhydride molecules were imaged by scanning tunneling microscope (STM). High resolution STM images of the molecules reveal that the graphene layer decouples the individual molecules electronically from the metallic substrate, which is also verified by density functional theory calculations. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y12.00007: Tip Induced doping effects in the local tunnel spectra of graphene Anjan K. Gupta, Shyam K. Choudhary We report on tip induced carrier density changes in local tunnel spectra of single layer graphene (SLG) with backgate using room-temperature scanning tunneling microscopy (STM) and spectroscopy. The SLG samples, prepared by exfoliation method and verified by Raman spectra, show atomically resolved honeycomb lattice. Local tunnel spectra show two minima with the two moving in opposite directions along the bias axis. One minimum shows nearly a square-root dependence, and the other shows a linear dependence on the gate voltage. We understand these features as arising from the STM tip induced and bias voltage dependent change in carrier density in SLG. Other than the tip induced doping we also see the effect of charge inhomogeneity on the local tunnel spectra of SLG. The charge inhomogeneity is also seen in bilayer graphene but no new features due to tip induced doping are observed in the local spectra. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y12.00008: Electronic Structures of Graphene on Ru(0001): Scanning Tunneling Spectroscopy Study Won-Jun Jang, Jeung-Hum Jeon, Jong Keon Yoon, Se-jong Kahng Graphene has inspired remarkable advances in nanotechnology due to its unusual electronic band structures represented by massless Dirac cones. Graphene can be epitaxially grown on metal surfaces by chemical vapor deposition method. Due to the lattice mismatch, epitaxial graphene grown on Ru(0001) shows hexagonal Moir\'e patterns with subatomic height variations. We studied local electronic structures of the epitaxial graphene using low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS). Different spectra were observed at top, bridge, and hollow regions of the Moir\'e patterns. Observed STS data will be explained with structural models. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y12.00009: Fourier transform-STM: signatures of impurity scattering in graphene ribbons Anders Bergvall, Tomas Lofwander We report results of a theoretical investigation of the effects of impurity scattering on the Fourier transformed local density of states (FT-LDOS) in graphene ribbons. We derive analytic expressions, within the Dirac approximation, for the Green's functions for armchair ribbons. Utilizing these, we show that the FT-LDOS contains distinct features that can be understood in terms of intra- and intersubband scattering processes. The ribbon band structure can then be reconstructed from the FT-LDOS. This makes the FT-LDOS a valuable spectroscopic tool. These predictions can be directly confirmed by Fourier transform scanning tunneling microscopy. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y12.00010: Tunable gap graphene micro-ribbons for terahertz plasmonics Danhong Huang, Godfrey Gumbs, Oleksiy Roslyak Maxwell's equations are solved for an array of graphene micro-ribbons located at the interface between a vacuum half-space and a half-space of a dielectric substrate. Our calculations are include mode-mixing in the optical-response function. A closed-form analytic expression is obtained for the nonlocal optical-response function of a graphene layer with an induced energy gap which is then employed in our calculations beyond the long-wavelength approximation. Both the reflectivity and transmissivity spectral functions are calculated. Specifically, we obtain their dependences on the period of the array, the ribbon width, chemical potential of doped graphene, energy gap between the valence and conduction bands, substrate refractive index, and incident angle of a plane-wave electromagnetic field. Additionally, a qualitative comparison is made between our calculated results in this paper and the recent experimental data given by Ju, {\em et al.\/}, [Nature Nanotechnology, {\bf 6}, 630 (2011)]. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y12.00011: Electron-phonon coupling effects in the (8-5-5) line defect of graphene nanoribbon Heng Luo, Yongwoo Shin, Xi Lin A ``metallic'' line defect in graphene nanoribbon consisting of alternating units of octagon and a pair of pentagons (8-5-5) is modeled using the adapted Su-Schrieffer-Heeger model Hamiltonian to include explicit electron-phonon coupling effects. Our results indicate that the 8-5-5 line defect has a finite optical gap of 0.4 eV with a broken charge conjugation symmetry, the bottom conduction band state mimic the nanoribbon edge case and the top valence state identical to the polyacetylene case. Upon photoexcitation, a small self-localized polaron state is found along one zigzag side of the line defect and a soliton-antisoliton pair is found on the other side. When the line defect is sandwiched between two graphene nanoribbons, the finite optical gap and photo-induced self-localized states persist as long as the nanoribbon width is smaller than 2 nm. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y12.00012: One Dimensional Massless Dirac Fermions in Functionalized Graphene Nanoribbons R. Tu\u{g}rul Senger, Ozan Ari Low-energy excitations of graphene are massless Dirac fermions due to presence of linear bands crossing at the Fermi level. Zigzag-edged nanoribbons of graphene (ZGNR), however, being semiconducting, do not possess this property. Using ab initio density-functional theory calculations we find that it is possible to close the band gap of ZGNRs through edge-functionalization with Na atoms. Moreover, the resulting band structure displays tilted-v-shaped linear bands crossing at the Fermi level, corresponding to one-dimensional massless Dirac fermions. We discuss mechanism of formation of such a band structure and its consequences in terms of electronic and transport properties. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y12.00013: Band Structure Measurements of Bottom-up Fabricated Graphene Nanoribbons Christopher Bronner, Felix Leyssner, Stephan Meyer, Manuel Utecht, Tillmann Klamroth, Peter Saalfrank, Petra Tegeder Along with the growing interest in graphene, other low-dimensional carbon nanostructures are currently in the focus of research since these materials offer a wide variety of properties interesting e.g. for nanotechnology application. Among these carbon systems, quasi-one-dimensional graphene nanoribbons (GNR) introduce a possibility to tune the electronic structure - for example, GNRs exhibit a band gap which is inversely proportional to their width and can thus be adjusted over a wide range. While many theoretical studies have been published on the band structure of GNRs, experiments are usually limited by the quality of the GNRs' fabrication, e.g. using lithography or unzipping of carbon nanotubes. In order to avoid defects and irregular edges that are inevitable in these methods, lately a surface-assisted bottom-up synthesis has been demonstrated which yields quasi-perfect GNR structures. [1] In the present study we employ complementary surface-sensitive spectroscopies to investigate occupied and unoccupied bands and the band gap in an armchair GNR which has been synthesized on the Au(111) surface. DFT calculations were performed to obtain a thorough understanding of the nature of the observed states.\\[4pt] [1] J. Cai \emph{et al.}, Nature (London) {\bf466}, 470-473 (2010) [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y12.00014: Synthesis and characterization of nitrogen-doped graphitic nanoribbons Josue Ortiz, M. Luisa Garcia, Xiaoting Jia, Rafael Martinez, Miguel A. Pelagio, David Swanson, A. Laura Elias, Humberto Gutierrez, Fernando Rodriguez, Emilio Munoz, Mildred Dresselhaus, Humberto Terrones, Mauricio Terrones Nitrogen doping of carbon nanostructures such as nanotubes and graphene is a practical approach for tailoring their electronic and chemical properties. However, the doping of graphene nanoribbons still remains to be a challenge. Here we discuss a novel synthetic route to N-doped graphitic nanoribbons using chemical vapor deposition. The morphology of the new nanomaterial resembles the observed for the undoped graphitic nanoribbons, with particular differences specially at the ribbons' edges. We performed scanning and transmission electron microscopy as well as Raman and X-ray photoelectron spectroscopies in order to confirm the nitrogen presence within the nanoribbons. In addition, the electrical response for individual nanoribbons was obtained. We observed that N-doped nanoribbons exhibit a clear semiconductor-like behavior depending on the amount of nitrogen embedded in the hexagonal carbon network (undoped nanoribbons always showed a metallic response). These doped nanostructures could find applications in the fabrication of electronic devices. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y12.00015: Graphyne- and Graphdiyne-based Nanoribbons: Density Functional Theory Calculations of Electronic Structures Lida Pan, Lizhi Zhang, Boqun Song, Shixuan Du, Hongjun Gao Graphdiyne, a carbon allotrope, which has the same symmetry as graphene and has butadiyne linkages between its nearest-neighbor hexagonal rings, has recently synthesized and fabricated on copper, showing experimentally the semiconductor property with conductivity of 2.516 $\times$ 10$^{-4}$ S m$^{-1}$, which is comparable to silicon. We investigate the configurations and electronic properties of graphyne and graphdiyne nanoribbons with armchair and zigzag edges by using first principles calculations. Our results show that all the nanoribbons are semiconductors with suitable band gaps similar to silicon. And their band gaps decrease as widths of nanoribbons increase. We also find that the band gap is at the $\Gamma$ point for all graphdiyne ribbons and it is at the X point for all graphyne ribbons. Of particular interest, the band gap of zigzag graphyne nanoribbons show a unique ``step effect'' as the width increases. This property is good for tuning of the energy band gap, as in a certain range of the ribbon width, the energy gap remains constant and in reality the edge cannot be as neat as that in a theoretic model. Graphyne and graphdiyne with tunable gaps are promising candidates for future carbon-based electronic devices. [Preview Abstract] |
Session Y13: Focus Session: Low-Dimensional and Molecular Magnetism - Nanomagnetism on Surfaces - Magnetic Adatoms and Clusters
Sponsoring Units: DMP GMAGChair: Juan Bartolome, University of Zaragoza - Spain
Room: 211
Friday, March 2, 2012 8:00AM - 8:12AM |
Y13.00001: Magnetism of single-vacancy defects in graphene and boron-nitride nanoflakes Silvia Fernandez-Sabido, Carlos Ramos, Eduardo Cifuentes-Quintal, Romeo de Coss In this work we have used the hexagonal zigzag graphene and boron-nitride nanoflakes as a simple systems for studying the new class of magnetic materials obtained by structural vacancies in nonmagnetic s-p nanostructures. We have shown that for these systems, it is possible to predict the total spin moment from a electron counting analysis. Employing DFT calculations based on the LCAO approximation and the Fixed Spin Moment method, we have determinate the ground state spin multiplicity and the spin magnetic distribution for these structures. We have found that the ground state multiplicity of graphene nanoflakes is triplet, corresponding to a spin magnetic moment of $M=2\mu_{\beta}$. Analyzing the spin orbital distribution we have determinate that the spin-polarized for the graphene nanoflakes is equally distributed in the $sp^{2}$ and $p_{z}$ orbitals. For the boron-nitride nanoflakes we have obtained a quartet state ($M=3\mu_{\beta}$) in the case of a boron vacancy, and a doublet state ($M=1\mu_{\beta}$) for a nitrogen vacancy. We have found that for the boron-vacancy the spin-polarized is mainly localized on the $sp^{2}$ orbitals of nitrogen atoms. In contrast, for the nitrogen-vacancy the spin-polarized is concentrated at the $p_{z}$ orbitals of boron atoms. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y13.00002: Dynamical magnetic excitations in adatoms and dimers on metallic surfaces Samir Lounis, Antonio T. Costa, Roberto B. Muniz, Doug L. Mills There is hardly any method which has shaped nanoscience and nanotechnology more profoundly than the scanning tunneling microscope. Such a tool is used nowadays to probe spin-excitations in nano-objects[1,2,3,4]. A key quantity describing these excitations is the transverse dynamical magnetic susceptibility that we calculate using the Korringa-Kohn-Rostoker Green function method within the framework of time-dependent density functional theory[5]. The behavior of adatoms and dimers will be discussed and comparison to experimental works will be provided when available. \\[4pt] [1] C. F. Hirjibehedin {\it et al.}, Science 317, 1199 (2007)\newline [2] T. Balashov {\it et al.}, Phys. Rev. Lett. 102, 257203 (2009)\newline [3] A. A. Khajetoorians {\it et al.}, Phys. Rev. Lett. 106, 037205 (2011)\newline [4] B. Chilian {\it et al.}, Arxiv:1108.2443\newline [5] S. Lounis {\it et al.}, Phys, Rev, Lett. 105, 187205 (2010); Phys. Rev. B 83, 035109 (2011) [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y13.00003: Real Space Observation of Inelastic Kondo Effect and Interorbital Spin-Coupling in Molecule-Metal Contacts Aitor Mugarza, Cornelius Krull, Roberto Robles, Nicolas Lorente, Richard Korytar, Sebastian Stepanow, Gustavo Ceballos, Pietro Gambardella We present a comparative scanning tunneling spectroscopy study of four different types of MPc complexes (M = Fe, Co, Ni, Cu) adsorbed on the Ag(100) surface. Their magnetic properties are studied via the Kondo interaction with the substrate. Whereas the spectra of FePc and CoPc near the Fermi level is featureless, CuPc and NiPc show a Kondo resonance arising from the interaction of a ligand spin with conduction electrons. The spin at the organic macrocycle is induced by charge transfer from the Ag substrate. In CuPc, the coexistence of ion and ligand spin gives rise to interorbital coupling and spin excitations. The latter are observed via inelastic tunneling, where the Kondo interaction appears coupled to spin and vibrational excitations. By using the tip as a mobile electron we find that each type of excitation occupy mutually exclusive regions within the molecule, and result in different spin relaxation dynamics, reflecting the need of an atomic control of the molecule-metal interface to obtain reproducible transport properties. Finally, we study the influence of intermolecular interactions on the electronic and magnetic properties by creating artificial clusters in a controlled manner by manipulation of individual molecules. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y13.00004: Emerging magnetic stability in atomically assembled spin arrays Sebastian Loth, Susanne Baumann, Christopher P. Lutz, D.M. Eigler, Andreas J. Heinrich Magnetic stability is usually created by the interaction of a large ensemble of atomically small magnetic moments that are themselves unstable. We make use of the Scanning Tunneling Microscope's ability to move individual atoms and construct arrays of interacting spins. Owed to their smallness, the magnetic states of these spin arrays are quantized and we probe their energies by inelastic electron tunneling spectroscopy [1]. To gain access to the equally important dynamical properties we employ an all-electronic pump-probe measurement scheme with which we follow the evolution between the spin states at nanosecond speed [2]. The combination of energetic and dynamical information allows identification of the relevant spin interaction and spin relaxation mechanisms at the atomic level. We design arrangements of atoms that suppress quantum tunneling of magnetization and drastically stabilize different spin configurations. Tracing the emergence of magnetic stability in the progression from individual atoms to arrays of spins points to new avenues for spintronic applications at atomic dimensions. \\[4pt] [1] A. J. Heinrich, J. A. Gupta, C. P. Lutz, D. M. Eigler, Science 306 466 (2004).\\[0pt] [2] S. Loth, M. Etzkorn, C. P. Lutz, D. M. Eigler, A. J. Heinrich, Science 329 1628 (2010). [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y13.00005: Magnetocrystalline anisotropy of 3d transition metal atoms on graphen Jun Hu, Ruqian Wu Graphene has attracted most attention in the field of condensed matter physics, chemistry and material science since the first day when it was produced in lab. It's an ideal material for two-dimensional electron gas (2DEG). Most intriguingly, the electronic and magnetic properties can be easily engineered by decorating with external elemental atoms ranging from nonmetal to transition metal. Especially, graphene is attractive for spintronics due to its long spin life time and high mobility. So far, ultrathin Co films have been deposited on graphene which exhibit perpendicular magnetic anisotropy. In this work, first-principles calculations are performed to systematically study the magnetic properties of graphene decorated by 3d transition metal atoms with several covering patterns. We find that Fe/graphene always exhibits in-plane anisotropy regardless the coverage, while Mn/graphene and Co/graphene tend to have perpendicular easy axis for most range of coverage. The spin moments of Ni atoms are largely quenched for Ni/graphene. Moreover, the Mn atoms on graphene prefer ferromagnetic exchange coupling which are totally different from the pure counterpart without graphene support. The strong hybridization between 3d orbitals of transition metal atoms and pi states of graphene are responsible for the modification of magnetic properties. \textbf{Acknowledgement.} This work was supported by DOE Grant DE-FG02-05ER46237. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y13.00006: Exploring Kondo Phenomena in Physisorbed Nitric Oxide Ryan Requist, Alexander Smogunov, Pierpaolo Baruselli, Michele Fabrizio, Erio Tosatti The NO molecule is a S=1/2 system that can be physisorbed on gold surfaces. As there are presently no data indicating whether or not physisorbed NO retains its spin and displays an observable Kondo effect, we investigate that question by means of our ab-initio based DFT+NRG approach [1]. DFT calculations for NO/Au(111) confirm that at low coverage the on-top adsorption site is the most stable, with the NO molecule forming an angle of approximately 60 degrees with the surface normal. Spin-polarized DFT calculations reveal that the molecule retains one unpaired electron in an antibonding $\pi$ orbital that hybridizes, albeit moderately, with the surface. Based on these ingredients, we discuss the possibility of observing a zero-bias Kondo anomaly in scanning tunneling spectroscopy above NO/Au(111). The influence of gold surface states and rovibronic motion of the molecule are also investigated. [1] P. Lucignano et al., Nature Mat. 8, 563 (2009). [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y13.00007: Magnetic Interaction between Surface Engineered Rare-earth Atomic Spins Barbara Jones, Chiung-Yuan Lin We report an ab initio study of rare-earth adatoms (Gd) on a surface, where it has been demonstrated previously that the STM can build and manipulate spin-coupled transition-metal atoms on such a surface one atom at a time, and have their spin excitation measured to be antiferromagnetic. The present work is the first attempt of studying rare-earth spin-coupled adatoms, the geometry effect of spin coupling, and the underlying mechanism of ferromagnetic coupling. The exchange coupling between Gd atoms on the surface is calculated to be antiferromagnetic in one geometry and ferromagnetic in another, by considering their collinear spins and using the PBE+U exchange correlation. We also find the Gd dimer in these two geometries is similar to nearest-neighbor and next-nearest-neighbor Gd atom pairs in GdN bulk. We analyze how much direct exchange, superexchange, and RKKY interactions contribute to the exchange coupling for the ferromagnetic arrangement by additional first-principles calculations of alternative model systems. Our calculations also show that the Gd spin of these structures is 7/2, the same as that of a GdN bulk. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y13.00008: Atom-by-atom engineering and atomic magnetometry of tailored nanomagnets with SP-STM Alexander A. Khajetoorians, Jens Wiebe, Bruno Chilian, Samir Lounis, Stefan Bl\"ugel, Roland Wiesendanger Nanomagnets, namely arrays of a few exchange coupled atomic magnetic moments, possess a rich variety of magnetic properties and are explored as constituents of nano-spintronics technologies. They have been realized as magnetic clusters or molecular nanomagnets. Individual nanomagnets, built from magnetic atoms adsorbed onto a nonmagnetic surface (adatoms) coupled by Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange, exhibit a high level of versatility resulting from distance-dependent interactions. Here, we combine spin-resolved scanning tunneling microscopy (SP-STS), atom manipulation and simulations to tailor nanomagnets ranging from linear chains to complex two-dimensional arrays and perform magnetometry in an atom-by-atom fashion. Distinct ground states of each chain, depending on even or odd numbers of constituent atoms, and magnetic frustration within the arrays have been observed directly. Our work demonstrates real space access to the magnetic states of tailored nanostructures providing an approach to tackling open fundamental questions in magnetism. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y13.00009: First-principles study of magnetic exchange interactions in scanning probe microscopy Stefan Heinze, Cesar Lazo, Paolo Ferriani In the last years, spin-polarized scanning tunneling microscopy (SP-STM) has been established as a technique to resolve complex magnetic structures down to the atomic scale. More recently, it has even become possible to detect the exchange interactions between tip and sample by magnetic exchange force microscopy (MExFM). However, the interpretation of such measurements is non-trivial, especially on the atomic scale. Here, we use density functional theory in order to study the effect of exchange interactions in SP-STM and MExFM measurements. First, we demonstrate the occurrence of a spin-valve effect for single Co and Cr atoms on Fe islands on W(110) contacted by an SP-STM tip as a result of the spin-dependent orbital symmetry of the states in the vicinity of the Fermi energy [1]. We find that the exchange interaction between tip and adsorbed atoms affects the magnetoresistance in the tunneling regime. Second, we explain the quantitative measurement of the exchange interaction across a vacuum gap using MExFM applied to an Fe monolayer on W(001) [2]. We show how the chemical tip composition influences the magnitude and distance dependence of the exchange forces.\\[4pt] [1] M. Ziegler et al., New J. Phys. 13, 085011 (2011).\\[0pt] [2] R. Schmidt et al., Phys. Rev. Lett. 106, 257202 (2011). [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y13.00010: Evidence of Kondo effect in organic radical nanoassemblies Mohammad Rashidi, Stefan Mullegger, Michael Fattinger, Reinhold Koch The outstanding spatial resolution of low temperature (LT) scanning tunneling microscopy (STM) and spectroscopy (STS) enables to probe the frontier orbital electronic structure of single magnetic molecules and clusters adsorbed on substrates. Here, we study self-aligned nanostructures of (spin-1/2) hydrocarbon radicals on a metal surface by means of LT-STM and STS. Pronounced involvement of surface state electrons is observed in the frontier molecular orbital (MO) resonances. An empty hybrid state closely above the substrate Fermi level exhibits the characteristic properties of surface Kondo effect reported for similar systems in the literature. By identifying three electronic states as hybrids of molecular orbitals and surface state electrons (two of them directly related to the Kondo effect), we are able to present a modified picture of the surface Kondo effect. It is based on a valence-bond model, where the bonding state represents Kondo's virtual bound state and the antibonding state is the so called 'Kondo resonance' reported in STM studies of the surface Kondo effect. Furthermore, double occupation of the originally singly unoccupied MO by tunneling electrons leads to the third state well above the Fermi level due to Coulomb repulsion as described by the Anderson model. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y13.00011: Spin inelastic electron transport through magnetic nanostructures Aaron Hurley, Nadjib Baadji, Stefano Sanvito Recent experimental advances in scanning tunneling microscopy make the measurement of the conductance spectra of isolated and magnetically coupled atoms on nonmagnetic substrates possible. Notably, these spectra are characterized by a competition between the Kondo effect and spin-flip inelastic electron tunneling. In particular they include Kondo resonances and a logarithmic enhancement of the conductance at voltages corresponding to magnetic excitations, two features that cannot be captured by second order perturbation theory in the electron-spin coupling. We have now derived a third order analytic expression for the electron-spin self-energy, which can be readily used in combination with the non-equilibrium Green's function scheme for electron transport at finite bias. We demonstrate that our method is capable of semi-quantitative description of the competition between Kondo resonances and spin-flip inelastic electron tunneling at a computational cost significantly lower than that of other approaches. The examples of Co and Fe on CuN are discussed in detail. We also explain the theoretical origin of the conductance assymetry that is present for both spin and non-spin polarized STM tips in the experimentally determined spectra of these atoms. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y13.00012: Real-space imaging of inelastic Kondo effect in a single O$_{2}$ molecule Ying Jiang, Wei Ji, Qing Huan, Arthur Yu, Shaowei Li, Wilson Ho Inelastic Kondo effect of the single O$_{2}$ molecule physisorbed on Ag(110) surface is investigated in real space using a low-temperature scanning tunneling microscope (STM) at 10 K. The O$_{2}$ molecule carries an unpaired spin, as supported by density functional theory, showing a Kondo resonance at the Fermi energy. The coupling between the vibrations and the unpaired electron in the O$_{2}$ molecule results in the inelastic Kondo effect, which is manifested as striking side peaks at finite biases in the dI/dV spectra, in clear contrast to the normal vibrational inelastic tunneling spectroscopy (IETS). Spectroscopic imaging shows that two vibrational modes are coupled to the Kondo resonance with different strengths, which arises from the symmetry match between the Kondo state and the vibrational modes. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y13.00013: Scanning Tunneling Microscopy and Spectroscopy of Fe[H$_2$B(Pz)$_2$]$_2$(bipy) Spin-Crossover Complex on Au(111) Alex Pronschinske, Geoff Lewis, Yifeng Chen, Robert Bruce, Wei You, Marco Buongiorno-Nardelli, David Shultz, Daniel Dougherty Spin crossover compounds have externally-tunable magnetic moments that will be useful for spintronic applications if they result in correspondingly tunable electronic properties. In order to assess this electronic tunability, we have performed variable temperature scanning tunneling microscopy and spectroscopy on a known Fe(II) spin crossover compound, Fe[H$_2$B(Pz)$_2$]$_2$(bipy), thermally-evaporated onto a single crystal Au(111) surface. We report on the surface assembly of this molecule in few layer films including imaging with submolecular resolution. In addition, we use STS to probe the local density of electronic states across a temperature range spanning the low spin to high spin transition and compare with first principles electronic structure calculations. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y13.00014: Frustration in the Magnetic Molecules W$_{72}$V$_{30}$ and Pr$_{13}$ Probed by NMR and Tunnel-diode Resonator Steven Yeninas, M. Luban, R. Prozorov, Y. Furukawa, C. Schr\"{o}der, J. Schnack The magnetic molecules Pr$_{13}$ and W$_{72}$V$_{30}$ have been studied by DC and AC magnetic susceptibilities and NMR. These molecules exhibit geometrical frustration resulting from antiferromagnetic intramolecular coupling of nearest neighbor paramagnetic ions. In Pr$_{13}$, 12 Pr$^{III}$ ions (S=1) interact along edge sites of an icosahedron centered about an additional Pr$^{III}$ ion. For W$_{72}$V$_{30}$, 30 V$^{IV}$ ions (S=1/2) interact along corner sharing sites of a near perfect icosidodecahedron, a spherical representation of a 2-D Kagome Lattice. Characterizing the DC magnetic response requires a distribution of exchange constants for both molecules at low temperatures. Results are compared to similar frustrated systems which feature a variety of magnetic phenomena - metamagnetic phase transitions, magnetic hysteresis without anisotropy, spin glass behavior, and superparamagnetism. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y13.00015: Unification of ultrafast, laser-driven and slow, phonon-driven spin-flip scenarios in the three-magnetic-center cluster Ni$_3$Na$_2$ Hongping Xiang, Georgios Lefkidis, Wolfgang H\"{u}bner Recently the use of the three-magnetic-center Ni$_3$Na$_2$ on a fictitious, inert surface as a prototypical system for ultrafast, laser-driven spin manipulation and logic functionalization was reported [1]. Here we extend this investigation of spin dynamics on the same system by including vibronic motion and recalculating all electronic states for each structural distortion. The first immediate finding is that the system exhibits an unexpectedly rich set of magnetic phases (geometry sets with different orientations of the highly localized spin-density). Exploiting those phases allows us (a) to establish the combination of spin dynamics and phonons as an unprecedentedly accurate sensor of the bond length between metallic centers, and (b) to present for the first time a unified picture of ultrafast (fs), laser-driven and slow (ps), phonon-driven spin dynamics in molecular magnets.\\[4pt] [1] W. H\"{u}bner, S. Kersten, G. Lefkidis, Phys. Rev. B {\bf 79}, 184431 (2009). [Preview Abstract] |
Session Y14: Focus Session: Spins in Carbon - Organic Spintronics
Sponsoring Units: DMP GMAGChair: Roland Kawakami, University of California, Riverside
Room: 212
Friday, March 2, 2012 8:00AM - 8:36AM |
Y14.00001: A new twist on organic spintronics: Controlling transport in organic sandwich devices using fringe fields from ferromagnetic films Invited Speaker: Markus Wohlgenannt Organic spintronics studies the physics of spin-injection and magnetic-field dependent transport phenomena in organic semiconductors, possibly leading to devices with added functionality. So far, studies have focused on spin-valve architectures as well as entirely non-magnetic devices that nevertheless show large room-temperature magnetoresistance through the so-called organic magnetoresistive effect. We demonstrate a new method of controlling the electrical conductivity of an organic film at room temperature, using the spatially-varying magnetic fringe fields of a magnetically-unsaturated ferromagnet. A large variation from hopping site to hopping site of the nuclear hyperfine field is known to dramatically affect electronic transport in organics, whose resistances are very sensitive to small applied magnetic fields, so the ferromagnet's fringe fields might act as a substitute either for the applied magnetic field or for the inhomogeneous hyperfine field. The size of the effect, magnetic field dependence, and hysteretic properties rule out a model where the fringe fields from the ferromagnet provide a local magnetic field that changes the electronic transport properties through the hyperfine field, and show our effects originate from electrical transport through the inhomogeneous fringe fields coming from the ferromagnet. Surprisingly these inhomogeneous fringe fields vary over length scales roughly two orders of magnitude larger than the hopping length in the organic materials, challenging the fundamental models of magnetoresistance in organic layers which require the correlation length of the inhomogeneous field to correspond roughly to the hopping length. Our devices, which do not rely on spin injection, tunneling anisotropic magnetoresistance or spin-valve behavior, may provide a simple approach to integrating magnetic metals and organics for hybrid spintronic devices. These devices may find application as high-voltage readouts of the magnetic state of low-impedance ferromagnetic films. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y14.00002: Magnetic Field Effects: Triplet-Charge Annihilation versus Triplet-Triplet Annihilation in Organic Semiconductors Bin Hu, Ming Shao, Liang Yan, Mingxing Li Triplet-charge reaction and triplet-triplet annihilation are two important processes in generating magnetic field effects in organic semiconductors. This presentation reports experimental studies on triplet-charge annihilation (TCA) versus triplet-triplet annihilation (TTA) in organic semiconductors. Specifically, we separately adjust the triplet-charge and triplet-triplet interactions towards the generation of TCA and TTA by changing triplet density, charge confinement, and charge/exciton ratio based on organic light-emitting diodes. We then use magnetic field effects of electroluminescence (MFE$_{EL})$ to study the TCA and TTA through spin interactions. We observe that the electroluminescence can clearly show negative response to applied magnetic field when triplets and charges are simultaneously confined within close proximity. On contrast, the electroluminescence only exhibits positive MFE$_{EL}$ when triplets are confined within close proximity. Therefore, it can be concluded that the TCA is a major process to annihilate triplets through Coulomb interaction in organic semiconductors. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y14.00003: Electron Spin Relaxation: The Role of Spin-Orbit Coupling in Organic Semiconductors M. Willis, L. Nuccio, L. Schulz, W. Gillin, T. Kreouzis, F. Pratt, J. Lord, M. Heeney, S. Fratini, C. Bernhard, A. Drew Rapid development of organic materials has lead to their availability in commercial products. Until now, the spin degree of freedom has not generally been used in organic materials. As well as engineering difficulties, there are fundamental questions with respect to the electron spin relaxation (eSR) mechanisms in organic molecules. Muons used as a microscopic spin probe, localized to a single molecule, can access information needed to identify the relevant model for eSR. In this presentation I will introduce the ALC-MuSR technique describing how eSR can be extracted and the expected effects. I will show how the technique has been applied to small organic molecules such as the group III Quinolate series and functionalized molecules with a pentacene-like backbone. Lastly I will present the Z-number and temperature dependence in these organic molecules and show strong evidence for a spin-orbit based eSR mechanism. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y14.00004: Using quantum spin-phase to elucidate weakly coupled pair hopping rates in organic semiconductors William Baker, Dane Mccamey, Tom Keevers, John Lupton, Christoph Boehme We have carried out electrically detected spin echo experiments on recombining electron-hole pairs (polaron pairs)\footnote{McCamey, D.~R. \textit{et al}., \textit{Nat Mater} \textbf{7}, 728 (2008).} in MEH-PPV organic light emitting diodes at different temperatures and device currents. We find long spin-phase relaxation times [$T_{2}$ = 324(18) ns] at room temperature and less than twice this value [$T_{2}$ = 611(39) ns] at $T$ = 10K. Next to this very weak temperature dependency, we also observe nearly no dependency of $T_{2}$ on the free carrier density in the material. We attribute this coherence decay behavior to charge carrier hopping transitions between localization sites which are exposed to differing hyperfine fields. Although the nuclear spin relaxation times in our material are much longer than the time scale of our experiments, the stochastic movement through this random environment leads to a time dependent fluctuating field causing an irreversible phase loss. We have simulated the echo experiments with this hopping process incorporated, and find good agreement with experimental data. The results of our study contrast free polaron transport processes which show an Arrhenius-type activation in time-of-flight experiments. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y14.00005: Spin-flip induced magnetoresistance in positionally disordered organic solids Nicholas Harmon, Michael Flatt\'e A theory for magnetoresistance in positionally disordered organic materials is presented and solved using percolation theory. The model describes the effects of spin dynamics on hopping transport by considering changes in the effective density of hopping sites, a key quantity determining the properties of percolative transport. Faster spin-flip transitions open up `spin-blocked' pathways to become viable conduction channels and hence produce magnetoresistance. Features of this percolative magnetoresistance can be found analytically in several regimes, and agree with previous measurements, including the sensitive dependence of the magnetic-field-dependence of the magnetoresistance on the ratio of the carrier hopping time to the hyperfine-induced carrier spin precession time. Studies of magnetoresistance in known systems with controllable positional disorder would provide an additional stringent test of this theory. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y14.00006: Magnetoresistance in organic spin-valve devices with low work-function metal H.-Jae Jang, Kurt P. Pernstich, David J. Gundlach, Oana D. Jurchescu, Curt A. Richter Organic thin-films have recently generated interest for studies related to spin-based phenomena. Long spin transport through organic semiconductors has been predicted due to their weak spin-orbit and small hyperfine interactions. This property gives them great potential for spintronic applications. While magnetoresistance (MR) has been reported by multiple research groups, controversy still remains over whether the basic mechanism is the transport of spin-polarized carriers through the organic semiconducting thin films or transport via unintended tunneling paths through the devices. We investigate the MR of organic thin-film spin valve devices fabricated from tris[8-hydroxyquinoline]aluminium (Alq3), high Tc transition ferromagnetic metals, and low work-function metals. The devices are fabricated without the exposure to the air to minimize the oxidation. We observe the MR as large as 3 {\%} at 4.5K in devices containing Alq3 as thick as 150 nm. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y14.00007: Origin of magnetoresistance in organic spin-injection structures Zhi-Gang Yu Spin injection into organic materials is usually inferred from the device resistance difference between antiparallel and parallel magnetic configurations (magnetoresistance, or MR) in an organic spin valve (OSV). The common features of the observed MR in OSVs include: 1) the MR is pronounced only at a low bias, where the device I-V characteristic is essentially linear; 2) the MR quickly decreases with bias and temperature while the decrease in device resistance is insignificant; 3) the MR is usually negative for OSVs with thick organic films, particularly when the two ferromagnets are similar in electronic structure. Despite superficial resemblance between OSVs and inorganic spin-injection structures, the MR in the former cannot be explained by theories developed for the latter. Here we show that the resistance of an OSV is controlled by the carrier density deep inside the organic and the MR is due to the difference in the carrier density for the two magnetic configurations. The sign of MR is determined by the electron spin polarization at a finite energy above the Fermi level in the electrodes. This picture explains common features of MR and suggests new strategies for probing and manipulating spin in organic spintronic structures. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y14.00008: Magnetoresistance in an all-organic-based spin valve Bin Li, Chi-Yueh Kao, Jung-Woo Yoo, Yu Lu, Mengqi Zhou, Vladimir Prigodin, Arthur Epstein We demonstrate spin injection and detection in an all-organic-based magnetic tunnel junction using two layers of organic-based magnet V[TCNE]$_x$ (x$\sim$2, TCNE: tetracyanoethylene) as the magnetic contacts and organic semiconductor rubrene (C$_{42}$H$_{28}$) as the spacer. For the V[TCNE]$_x$ film growth, we exploited two different growth techniques, chemical vapor deposition and molecular layer deposition, which result in different coercivities of V[TCNE]$_x$ films. The spin valve devices show negative magnetoresistance (MR), the sign of which does not change with temperature and bias. To explain the unusual negative MR, we propose a simple phenomenological bias-enhanced selective tunneling (BEST) model based on the different spin polarizations of the molecular energy levels of V[TCNE]$_x$. Our results show the significance of bias induced energy level shift in organic spintronic devices due to relatively narrow spin polarized bandwidths. This work was supported in part by AFOSR Grant No. FA9550-06-1-0175, DOE Grant Nos DE-FG02-01ER45931, DE-FG02-86ER45271, NSF Grant No. DMR-0805220, the Center for Emergent Materials (an NSF-MRSEC; Award Number DMR-0820414) at The Ohio State University and the Institute for Materials Research at The Ohio State University. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y14.00009: Manganese Phthalocyanines on manganese surfaces: An ab-initio approach to the tunneling magnetoresistance Maria Soriano, Juan Jose Palacios We have performed computational studies of spin polarized transport based on Density Functional Theory (DFT) using a non-equilibrium Green's functions formalism implemented in our ANT code [1] to compute tunneling magnetoresistance (TMR) on different manganese based systems. Specifically we present results for STM tips on a clean manganese surface, which is well known to show an antiferromagnetic coupling between layers when grown on an iron surface. In this system we have studied the dependence of the TMR on the tip-surface distance, the position of the tip, and on the bias voltage. Also we present a comparison between the Landauer formalism and the Tersoff-Hamman approach, usually used in this context. Finally we analyse the interaction between Manganese Phthalocyanine (MnPc) molecules on the manganese surface, focusing on the TMR signature and its dependence on the different adsorption sites.\\[4pt] [1] J.J. Palacios et. al. Ab-initio Quantum Transport (ANT). alacant.dfa.ua.es [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y14.00010: Spin injection in cobalt and copper phthalocyanines David Ciudad, Marco Gobbi, Dario A. Arena, Jagadeesh S. Moodera An organic light emitting diode (OLED) is a heterojunction comprising two different organic semiconductors (OS): one for hole and another one for electron transport. Electrons and holes recombine at the interface between them. Depending on their total spin the recombination may (singlet state) or may not (triplet state) produce visible light. The efficiency of OLEDs could be doubled by injecting a spin-polarized current (Spin-OLED).\footnote{I. Bergenti et al. Organic Electronics, 2004, 5, 3} One obstacle to attain a spin-OLED is the lack of studies of spin diffusion lengths ($\lambda_{s}$) and injection efficiencies on hole-transport OS. We investigate spin injection in phthalocyanines (Pcs). The Pcs are ideal candidates to reach the above goal because: 1) of their high hole mobility; 2) Cu-Pc in particular is currently used in OLEDs; 3) their high thermal and chemical stability; 4) these molecules contain a single metal atom making them a unique system to study its effect on the spin injection. To determine $\lambda_{s}$ we investigate multilayer structures having transition metal/ Cu-Pc or Co-Pc interfaces including magnetic tunnel junctions with OS as barriers. Further characterization of these interfaces has been performed by soft X-Ray measurements at the NSLS-BNL facility. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y14.00011: The role of thin MgO(100) epilayer for polarized charge injection into top-emitting OLED Tae Hee Kim, Nyun Jong Lee, Yu Jeong Bae, Hyunduck Cho, Changhee Lee, Eisuke Ito A new top-emitting OLED (TOLED) structure, which is formed on an Si(100) substrate and an epitaxial MgO(100)/Fe(100)/MgO(100) bottom electrode, was investigated. Our TOLED design included a semi-transparent cathode Al, a stack of conventional organic electroluminescent layers ($\alpha $-NPD/Alq$_{3}$/LiF) and a thin Cu-Phthalocyanine (CuPc) film to enhance the hole injection into the luminescent layers. At room temperature (RT), magnetoluminescence of $\sim $5 {\%} was observed in low magnetic field up to 1 Tesla , which is obviously larger than that of the OLEDs with epitaxial and polycrystalline Fe anodes without MgO(100) covering layer. Our results indicate that the magnetic field effect on the electroluminescence could be strongly related to the magnetic properties of bottom electrode, more precisely the interfacial properties between CuPc layer and the anode. Therefore, we focused on understanding interface electronic states and energy alignment by using x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy. Our results showed that the use of appropriate oxide layers could represent a new interface engineering technique for improving reliability and functionality in organic semiconductor devices. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y14.00012: Spin Polarized Scanning Tunneling Microscopy of Alq3 on Cr(001) Daniel Dougherty, Zhengang Wang, Alex Pronschinske The field of organic spitronics has been strongly motivated in recent years by the observation of giant magnetoresistive effects in tris-(8-hydroxyquinoline) aluminum (Alq3) films and nanostructures. It is crucial to understand the spin- dependent electronic structure at metal-Alq3 interfaces. We have carried out spin polarized scanning tunneling microscopy to measure the local density of electronic states for submonolayer films of Alq3 grown on the layered anitferromagnetic Cr(001) surface. We report an energy-dependent tunneling conductance asymmetry for single molecules adsorbed on differently magnetized (001) terraces and discuss its connection with metal-molecule hybridization and magnetoresistive effects in Alq3 spintronic devices. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y14.00013: Ferroelectric control of the spin polarization in an organic spin valve Dali Sun, Xiaoshan Xu, Lu Jiang, Ho Nyung Lee, Hangwen Guo, Paul C. Snijders, T. Zac Ward, Zheng Gai, X.-G Zhang, Jian Shen Recently engineering the spin propagation in organic spin valves has shown increasingly interesting properties. In this work we demonstrate novel ferroelectric control of the spin polarization in an organic spin valve. By inserting a thin ferroelectric buffer layer between a bottom La$_{0.67}$Sr$_{0.33}$MnO$_{3 }$(LSMO) electrode and the organic Alq$_{3}$ layer, a controlled spin polarization through the ferroelectric interface is achieved. The spin valve exhibits both positive and negative magnetoresistance depending on the applied bias. We conclude that this results from the energy level shift by the ferroelectric dipoles between Alq$_{3}$ and LSMO (Research sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, managed by UT-Battelle, LLC, for the U. S. Department of Energy). [Preview Abstract] |
Session Y15: Focus Session: Spins in Metals: Spin Torque Theory, Spin Dependent Transport
Sponsoring Units: DMP FIAP GMAGChair: Markus Eisenbach, Oak Ridge National Laboratory
Room: 213
Friday, March 2, 2012 8:00AM - 8:12AM |
Y15.00001: Newtonian reciprocality between spinmotive forces (SMF) and spin-torque-transfer (STT) Stewart Barnes An SMF and the STT effect are reflected in definitions $$ \vec p = m\vec v + e\vec A_s \ \ \ \ \ {\rm and } \ \ \ \ \ H = H_0+e\Phi_s + eA_{0s}, $$ valid beyond the adiabatic approximation, where the momentum $m\vec v$ is mechanical, while $\vec p$ is that conjugate to the position $\vec r$, $H$ is the full Hamiltonian while $H_0$ is that for uniform magnet. Here $(\vec A_s, A_{0s})$ is the four vector which reflects the {\it linear\/} momentum of the magnetic system. The spin magnetic and electric fields $$ B_i = - \frac{im^2}{\hbar}\epsilon_{ijk}[v_j,v_k]\ \ \ \ {\rm and} \ \ \ \ E_i = - \frac{im^2}{\hbar} [v_i,v_0]; \ \ \ \ i = \{x,y,z\} $$ where $i\partial_t = mv_0 + e A_0$, and involve commutators. The Landau-Lfttshitz equations are an {\it emergent}. They correspond to $$ [S_z, (H_0+ e\Phi_s + eA_{0s})] = 0 $$ as required to separate the {\it slow\/} magnetic and {\it fast\/} electronic degrees of freedom. E.g. $ L = (enA \dot z - \dot q)\frac{\hbar}{e} \phi + g \mu_B B nAx - q{\mathcal E} $ is the Lagrangian for simple domain wall connected to a battery emf ${\mathcal E}$. SMF-SST reciprocality reflects Newtonian third law and {\it not\/} an Onsager relationship between transport coefficients. Experiment for spin-valves and MTJs will be reviewed. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y15.00002: Theoretical investigation of the dynamics of a magnetic bilayer in the presence of spin transfer torque Lydia Novozhilova, Sergei Urazhdin Magentoelectronic nanodevices, such as magnetic memory cells and spin torque nano-oscillators, generally utilize at least two magnetic layers, the ``reference'' and the ``free'' layer. While most previous studies focused only on the behaviors of the ``free'' layer, the dynamics of the ``reference'' layer can also be important due to the coupling between the layers. We utilize Landau-Lifshitz-Slonczewski equations in the macrospin approximation to analyze the dynamics of a magnetic bilayer driven by a dc current. We show that even in the absence of magnetic or RKKY interactions, coupling between the layers is efficiently mediated by the spin transfer torque. We use an axially symmetric approximation to derive an analytical stability condition for the bilayer. The stability is determined by the physical characteristics of the bilayer and two control parameters - the dc current and the external time-independent magnetic field. Additionally, we use an averaging technique to find synchronized precessional motions of the two nanomagnets. Finally, we show that for some conditions, the dynamics of the bilayer becomes chaotic. Such chaotic dynamics is prohibited for a single macrospin due to the limitations imposed by the dimensionality of the system. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y15.00003: Interplay between conduction electron spin current and magnon spin current Shulei Zhang, Shufeng Zhang Spin current is a flow of angular momentum that can be carried by conduction electrons as well as magnons. In conducting ferromagnets, both electron spin currents and magnon currents are present. By using a semiclassical description for conduction electrons and magnons, we evaluate the distribution functions of the non-equilibrium electrons and magnons in the presence of electron-magnon interactions. In some interesting limiting cases, we have derived novel diffusion equations for both electrons and magnons. We then apply our formalisms to study the spin currents for various bilayer and trilayer systems. In particular, we predict how an electron spin current in a metallic layer can be converted into a pure magnon current in a ferromagnetic insulator layer at interface. We also discuss possible experimental realizations for non-equilibrium magnon currents. \newline [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 9:12AM |
Y15.00004: A Unified Quantum Theory of Spin Transfer Torque and Electron Transport in Magnetic Nanostructures Invited Speaker: Yong Wang Dynamical control of nanoscale magnetization by spin-polarized current has attracted intensive studies due to its vast potential applications in spintronics. The magnetization dynamics driven by spin transfer torque is usually described by the semiclassical theory, where the quantum nature of the magnet is neglected. Here, we present a unified theory which describes the magnetization and the spin-polarized electrons at the full quantum level. The quantum state of the magnetization is given by the coherent state in angular momentum space, and is driven by the continuous scatterings with the electrons in the stochastic nature. Based on this picture, the Monte Carlo simulation is applied to get the mean trajectory and fluctuations of the magnetization, and also the electric current and the current shot noise. We also analyze the different sources which contribute to the magnetization noise and current noise, and show that the quantum noise due to the scattering can be comparable to the thermal noise at low temperature. An analytic theory is also formulated in terms of the density matrix technique. We obtain the master equation of the density matrix of the magnet driven by spin transfer torque, and find that it becomes a Fock-Planck equation in the coherent representation. Solutions from this approach is highly consistent with the results from the Monte Carlo method. We forsee that our theory will play an important role when the quantum nature of the nanomaget emerges in the spintronics devices. For references, see [1] Yong Wang and L.J. Sham, Quantum Dynamics of a Nanomagnet driven by Spin-Polarized Current, arxiv : 1106.2359. [2] Yong Wang and L.J. Sham, A Quantum Theory of Spin Transfer Torque, to be submitted. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y15.00005: Spin-population inversion in magnetic point contacts under non-equilibrium conditions Torsten Pietsch, Stefan Egle, Elke Scheer The creation of a novel type of spin-based electronics is one of the most intensively researched topics in current solid-state physics. The unifying characteristics in this advancing field is that the spin degree of freedom of the electron rather than its charge is exploited to achieve a specific device functionality. Recently, theoretical predictions suggest that spin-inversion in metallic point contacts under strong non-equilibrium conditions may enable the design of novel types of radiation sources. These radiation sources are highly tunable and of giant intensity compared to cutting-edge semiconductor devices, due to the much larger electron density in metals. Moreover, the accessible frequency range covers both, microwave (GHz) and THz radiation. Especially the later one is of great interest, since up to date there is no miniaturized, high intensity THz source available. Therefore, the experimental demonstration of this lasing effect in metallic systems is an important breakthrough in solid state physics. Presently the concept of spin-flip lasing in magnetic point contacts rests on theoretical predictions and first proof of principle studies. Herein we present detailed investigations on the magneto-transport properties of magnetic herterostructures and -point contacts. In particular, we study the complex interplay between magnetization, current density and the influence of high frequency (GHz and THz) fields on the magneto-transport properties of magnetic point contacts. The results illustrate that a successful spin-population inversion can be detected via transport spectroscopy. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y15.00006: Two-channel resonant tunneling in the presence of spin-orbit coupling T.V. Shahbazyan, L.S. Petrosyan We study resonant tunneling through a pair of localized states independently coupled to two-dimensional electron gas (2DEG) in the leads. The shape of tunneling conductance is determined by the coherent coupling of localized states via electron continuum in the leads. We show that spin-orbit coupling (Rashba and Dresselhaus) in 2DEG leads to a splitting of the narrow (subtunneling) peak by the amount proportional to spin-orbit coupling constant. The conductance lineshape is highly sensitive to the magnitude and orientation of in-plane magnetic field. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y15.00007: Non-equilibrium coherent potential approximation for electron transport Alan Kalitsov, Mairbek Chshiev, Julian Velev Treatment of disorder in extended systems presents essential difficulties because the disorder breaks the periodicity of the system. The coherent potential approximation (CPA) solves this problem by replacing the disordered medium with a periodic effective medium [1]. However, calculating the electron current within CPA requires summing scattering diagrams to infinite order, the so called vertex corrections. In this work we reformulate CPA for non-equilibrium electron transport (NE-CPA). This approach, based on the non-equilibrium Green function formalism, provides an efficient and precise way to solve the transport problems in the presence of disorder. We demonstrate that the NE-CPA current is equivalent to the CPA current with vertex corrections to infinite order. [1] B. Velicky, Phys. Rev. 184, 614 (1969). [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y15.00008: Semiclassical dynamics of Bloch electrons to second order in electromagnetic fields Yang Gao, Shengyuan Yang, Qian Niu Berry curvature appears in the semi-classical theory of Bloch electrons already to first order in electromagnetic fields, resulting in profound modification of the carrier velocity and phase space density of states. Here we derive the equations of motion for the physical position and crystal momentum to second order in the fields. The dynamics still has a Hamiltonian structure, albeit with noncanonical Poisson brackets between the physical variables. We are able to expand both the carrier energy and the Poisson brackets to second order in the fields with terms of clear physical meaning. To demonstrate the utility of our theory, we obtain with much ease the electromagnetic response and orbital magnetic susceptibility. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y15.00009: Zero bias STS Kondo anomalies of Co impurities on Cu surfaces: do ab initio calculations work? Pier Paolo Baruselli, Alexander Smogunov, Michele Fabrizio, Ryan Requist, Erio Tosatti Transition metal atoms such as Co on Cu (111), (100), and (110) surfaces produce STS I-V spectra showing different zero bias Kondo anomalies [1] but these differences have been neither quantitatively predicted nor fully explained theoretically. We apply to this problem the DFT+NRG scheme of Lucignano et al [2], where one solves by NRG an Anderson model built from ab initio phase shifts provided by DFT. For Co/Cu(100) and Co/Cu(110) our calculations describe correctly the experimental trend of Kondo temperatures, and fairly the lineshapes too. By contrast, they fail to describe Co/Cu(111) where in particular the anti-lorentzian lineshape found in experiment remains unexplained. This failure underscores the role of surface states, probably relevant for Co/Cu(111) [3] but not correctly described by our thin slab calculations. Future efforts to quantitatively include Kondo screening by surface states are therefore called for. 1. N. Knorr et al PRL 88, 096804 (2002); M. Ternes et al 2009 J. Phys.: Cond. Matt. 21, 053001 (2009); A. Gumbsch et al PRB81, 165420 (2010). 2. P. Lucignano et al Nature Mat. 8, 563 (2009); P.P. Baruselli et al, Physica E, doi:10.1016/j.physe.2011.05.005. 3. C. Lin et al. PRB 71, 035417 (2005). [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y15.00010: ABSTRACT WITHDRAWN |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y15.00011: Linear magnetoresistance as a Berry phase effect Hua Chen, Di Xiao, Zhenyu Zhang, Qian Niu Conventional theory of charge carrier dynamics in metals and semiconductors predicts a quadratic field dependence in the magnetoresistance (MR) in the weak field limit. A linear dependence is usually explained as a quantum effect known as weak localization. However, in systems with time reversal symmetry breaking, a linear dependence can be observed even above room temperature, where quantum coherence is absent. Here we show that linear MR can arise from the Berry curvature modified semiclassical dynamics of the charge carriers, and evaluate its magnitude in several model systems and make preliminary comparisons with experimental results. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y15.00012: Theory of conducting electrons in textured antiferromagnets Ran Cheng, Qian Niu The dynamics of conducting electrons in antiferromagnetic texture is shown to exhibit an SU(2) geometric phase in the iso-spin that characterizes a locked spin-sublattice degree of freedom. The iso-spin rotation is purely geometrical and can be expressed in terms of the flux of a novel Berry curvature jointing the real and momentum space, a crucial quantity that has been overlooked in conventional transport theory. We provide an example of one dimensional domain wall, where the iso-spin rotation is topologically quantized. Scattering by skyrmion centers in two dimensions are also discussed, where a Mott like scattering of the iso-spins is predicted. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y15.00013: Exchange field induced large magnetoresistance in the correlated insulator phase of ultrathin Beryllium films Tijiang Liu, Yiming Xiong, Shane Stadler, Joseph Prestigiacomo, Philip Adams We present a detailed study of low-temperature magnetotransport properties of ultrathin, amorphous Be films in the EuS/Be bilayers. Significant magnetoresistance (MR) of pure insulating Be films can only be observed in fairly high magnetic field ( $> \quad \sim $5 tesla ), but by depositing insulating ferromagnetic EuS on top of Be film, one can obtain the same value of MR at low magnetic field ($\sim $ 0.2 tesla). We argue that this shift of MR from high field to low field may be caused by the exchange field in the Be component of Be/EuS bilayers. [Preview Abstract] |
Session Y16: Strongly Correlated Electrons - Experiment and Theory
Sponsoring Units: DCMPChair: Ryan Baumbach, Los Alamos National Laboratory
Room: 251
Friday, March 2, 2012 8:00AM - 8:12AM |
Y16.00001: Electronic Excitations in CuO from a Many-Body Point of View Claudia R\"{o}dl, Francesco Sottile, Lucia Reining The insulating transition-metal oxide CuO is considered a key to understanding the electronic structure of high-temperature superconducting cuprates, since it features similar bonding geometries. The photoemission spectra of this oxide, which is usually termed to be strongly correlated, have not been explained satisfactorily by first-principles calculations up to now. Special difficulties arise from the close entanglement of the structural and electronic degrees of freedom in this compound, which is due to the unoccupied Cu~$3d$ orbitals. In contrast to the local-density approximation of density-functional theory (DFT), which predicts CuO to be a metal, we obtain finite band gaps by means of hybrid functionals containing screened exchange and the DFT+$U$ method. Starting from these qualitatively correct band structures, we perform many-body calculations in the $GW$ approximation. The various approaches to the one-particle excitation spectra are compared to experimental results. We discuss whether or not the peaks occurring in the photoemission data are quasiparticle excitations or satellite structures, respectively. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y16.00002: Pressure Effects on the Vanadium Oxides V$_{6}$O$_{11}$, V$_{7}$O$_{13}$, and V$_{8}$O$_{15}$ Stella Kim, Estelle Colombier, Ni Ni, Sergey Bud'ko, Paul Canfield Members of the V$_{N}$O$_{N-1}$ Magneli Series (3~$<$~N~$<$~9) exhibit metal to insulator transitions (MIT) as well as antiferromagnetic (AFM) transitions at ambient pressure, with the exception of V$_{7}$O$_{13}$ which remains metallic to lowest measured temperatures. In this talk we present pressure and temperature dependent measurements of electrical resistivity for V$_{6}$O$_{11}$, V$_{7}$O$_{13}$ and V$_{8}$O$_{15}$ samples. For V$_{6}$O11 and V$_{8}$O$_{15}$ MIT can be suppressed by 4 and 1.3~GPa respectively. For V$_{7}$O$_{13}$ and V$_{8}$O$_{15}$ T$_{SDW}$ can be suppressed by 3.5~GPa. Composite phase diagrams will be presented and low temperature data, focusing on the possibility of quantum criticality will be discussed. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y16.00003: Magnetic properties of Fe$_{1-y}$Co$_y$Si near insulator-metal transition Yan Wu, Bradford Fulfer, Julia Chan, David Young, John DiTusa FeSi is a nonmagnetic narrow gap insulator with interesting temperature-dependent magnetic and optical properties. When doped with Mn, holes are introduced to the FeSi system along with S=1 magnetic moments. As a result, a non-Fermi liquid metallic state is observed due to the underscreening of the S=1 moments. Here we investigate the effects of Co doping of FeSi (Fe$_{1-y}$Co$_y$Si, 0$\leq$y$\leq$0.03) to explore the insulator-to-metal transition and to compare with Mn-doped effects. We find a systematic increase of the Curie constant and Weiss temperature of the impurity-induced low temperature susceptibility with y. The Curie constant and saturated magnetization indicates that there is an underlying competition between screening of the magnetic moments at low y and ferromagnetic ordering at higher Co-concentrations. Transport measurements are underway to explore the associated charge carrier properties in this semiconducting system on the verge of ferromagnetism. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y16.00004: Signatures of Correlation Effects and Thermopower in FeSi Jan Tomczak, Kristjan Haule, Gabriel Kotliar Correlated semiconductors have been studied intensively over the years, because they exhibit an unusual metalization process which is poorly understood. At low temperatures FeSi behaves as an ordinary semiconductor, while at high temperatures the system is a bad metal with a Curie like susceptibility. Analogies with heavy fermion Kondo insulators and mixed valence compounds, and anomalous electron phonon coupling have been invoked to account for this behavior, but lacking quantitative methodologies applied to this problem, a consensus remained elusive to date. Here, we use realistic many-body methods to elucidate the metalization of FeSi. Our methodology accounts for all substantial anomalies observed in FeSi : lack of conservation of spectral weight in optics, Curie susceptibility and an anomalous thermoelectric power. Having quantitatively validated our approach, we propose a new scenario. In correlated insulators such as FeSi the metalization is induced by the emergence of non-quasiparticle incoherent states in the gap. This coherence-incoherence crossover is accompanied by a massive reorganization of the spin excitations. Besides the fundamental interest of our theory, our work is relevant to the design of thermoelectric materials based on correlated insulators. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y16.00005: FeGa3 -- a strongly correlated insulator? Monika Gamza, Meigan Aronson FeGa3 is one of few Fe-based nonmagnetic semiconductors, in which a small gap is produced by the hybridization of Fe 3d states with p states of a group 13 or 14 element. The role of strong electron-electron correlation effects in formation of the gap is unclear. In case of FeSi and FeSb2, a metal-insulator transition was observed at temperatures low relative to the gap energy, which is a hallmark of a Kondo nature of the gap. For FeGa3, a combination of photoemission data and DFT-based electronic structure calculations indicated a rather strong on-site effective Coulomb repulsion within the Fe 3d shell Ueff$\sim $3 eV and a sizable Ueff/W $\sim $ 0.6 (W -- band width) [1]. Interestingly, recent ARPES measurements revealed a Fe 3d derived state located around 0.4 eV away from the top of the valence band [1]. Thermodynamic and transport measurements do not give any sign of a metal-insulation transition up to 1000 K [2]. To get insight into the importance of e-e correlation effects in formation of the gap in FeGa3, we drive the system towards a metallic state by doping. The results of resistivity, specific heat and magnetization measurements on doped single crystals of FeGa3 grow by Ga-flux technique will be discussed. \\[4pt] [1] Arita M et al., Phys. Rev. B 83, 245116 (2011) \\[0pt] [2] Hadano Y et al., J. Phys. Soc. Jpn. 78, 013702 (2009) [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y16.00006: Hybridization wave as the cause of the metal-insulator transition in rare earth nickelates Hyowon Park, Chris A. Marianetti, Andrew J. Millis The metal-insulator transition driven by varying rare earth ($Re$) ion in $ReNiO_3$ has been a longstanding challenge to materials theory. Experimental evidence suggesting charge order is seemingly incompatible with the strong Mott-Hubbard correlations characteristic of transition metals. We present density functional, Hartree-Fock and Dynamical Mean field calculations showing that the origin of the insulating phase is a hybridization wave, in which a two sublattice ordering of the oxygen breathing mode produces two $Ni$ sites with almost identical $Ni$ $d$-charge densities but very different magnetic moments and other properties. The high temperature crystal structure associated with smaller $Re$ ions such as $Lu$ is shown to be more susceptible to the distortion than the high temperature structure associated with larger $Re$ ions such as $La$. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y16.00007: Non-thermal dynamics of the spin and charge order in striped nickelates Wei-Sheng Lee In the striped nickelate La$_{2-x}$Sr$_{x}$NiO$_{4}$, spin order coexists with charge order, whose periodicity is half of spin order. So far, most of the studies on the stripe phase were performed in the thermal equilibrium state by varying temperatures; the dynamics of stripe phase in the time domain when the system is driven-out-of-equilibrium has not yet been studied. Using the x-ray free electron laser (XFEL) at LCLS, we performed time-resolved optical-pump and resonant soft x-ray diffraction probe experiments to study the dynamics of the spin and charge order. Comparisons of charge and spin order dynamics will be discussed. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y16.00008: The temperature evolution of NiO$_{6}$ octahedral tilts as a probe to study charge stripes in Ln$_{1:67}$Sr$_{0:33}$NiO$_{4}$ (Ln=La/Nd) Milinda Abeykoon, Emil Bozin, Genda Gu, John Hill, John Tranquada, Simon Billinge Nd$_{1:67}$Sr$_{0:33}$NiO$_{4}$ and La$_{1:67}$Sr$_{0:33}$NiO$_{4}$ exhibit long range ordered (LRO) charge stripes below Tco$\sim $240K. When the charge stripes are static and long-range ordered, they have been seen as superlattice Bragg peaks in single crystal neutron and x-ray diffraction. We used neutron atomic pair distribution function (NPDF) technique to investigate the possibility of existence of localized charges above Tco, where, the LRO of the stripes disappears. Rigid body type NiO$_{6}$ octahedral tilts were used as a probe to study the local and average structural response to charge order. The amplitude of rotation of the octahedral units was used to investigate the observed large apical oxygen thermal ellipsoids (anisotropic thermal displacement parameters) in the a-b crystal plane. We measured enhanced tilt amplitudes in the local structure, compared to the average. We will discuss the temperature dependence of the tilt amplitudes and of the correlation length of the local tilt order and explore the potential relationship to local stripe order. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y16.00009: Time-Dependent Recovery of Charge and Spin Order in Striped Nickelate Y.F. Kung, C.-C. Chen, A.F. Kemper, W.-S. Lee, B. Moritz, A.P. Sorini, Z.-X. Shen, T.P. Devereaux Using time-dependent Ginzburg-Landau theory, we study the melting and recovery of charge and spin order in the striped nickelate La$_{1.75}$Sr$_{0.25}$NiO$_4$ in response to an ultrashort pump pulse that destroys the order. Treating the behavior of both the amplitudes and phases of the order parameters, we examine their effects on the recovery time scales of the charge and spin order. We compare the temporal dynamics of our model to experimental observations at the Linac Coherent Light Source (LCLS). [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y16.00010: ARPES studies on metal-insulator-transition in NiS$_{2-x}$Se$_{x}$ Garam Han, Y.K. Kim, W.S. Kyung, Chul Kim, Y.Y. Koh, K.D. Lee, C. Kim Understanding Metal insulator transition (MIT) is one of the most challenging issues in condensed matter physics. NiS$_{2-x}$Se$_{x}$ (NSS) is a well known system for band width controlled MIT studies while most of High-Tc superconductors (HTSCs) are described within band filling MIT picture. Cubic pyrite NiS$_{2}$ is known as a charge-transfer (CT) insulator and easily forms a solid solution with NiSe$_{2}$, which is a good metal even though it is isostrucural and isoelectronic to NiS$_{2}$. MIT is induced by Se alloying and is observed at a low temperature for x=0.5. The important merit is that there is no structure transition which often accompanies MIT. In spite of the importance of the system, even the experimental band dispersion is not known so far along with many controversies. For this reason, we performed angle resolved photoemission spectroscopy on high quality single crystals and successfully obtained Fermi surface maps of x=0.5, x=0.7 and x=0.8 systems (the metallic side). By doping dependent systematic studies on NSS and comparison with LDA calculation, we try to explain the relationship between band width and the MIT. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y16.00011: Quantum criticality at a Mott-Hubbard Metal-Insulator Transition Arnab Banerjee, Daniel Silevitch, Alexander Palmer, Yejun Feng, Thomas Rosenbaum The Mott insulator nickel disulfide undergoes an insulator to metal transition that can be driven by either hydrostatic pressure or doping-induced chemical pressure. Previously,NiS2~doped with selenium to just below the quantum critical point showed anomalous critical exponents as a function of pressure. Doping, however, introduces positional disorder and charge transfer effects from the selenium orbitals, potentially altering the critical behavior. We report here on a set of transport experiments on pure~NiS2, where the pressure and temperature scales for the critical regime require the use of a diamond anvil cell integrated with a helium dilution refrigerator, allowing us to compare the critical behavior of the pure and doped materials. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y16.00012: Redefining the metal/charge-transfer insulator paradigm in transition metal oxides Hung Dang, Xinyuan Ai, Chris Marianetti, Andrew Millis The universality of the phase diagram in the variables of interaction strength and d-occupancy, shown for late transition metal oxides in Ref.[1], is examined for two series of early transition metal oxides: (SrVO$_3$, SrCrO$_3$, SrMnO$_3$) and (LaTiO$_3$, LaVO$_3$, LaCrO$_3$) using density functional theory (DFT), DFT+U and DFT+dynamical mean field theory methods. The interaction required to drive the metal-insulator transition is found to depend sensitively on the d-occupancy $N_d$, and beyond a threshold value of the d-occupancy an insulating state cannot be achieved for any practical value of the interaction. The critical $N_d$ values are determined and compared to ab initio and experimental estimates where available. Additionally, the minimal model for the transition is determined and the crucial role played by the Hunds coupling is demonstrated. \\[4pt] [1] Xin Wang, M. J. Han, Luca de' Medici, C. A. Marianetti, and Andrew J. Millis (2011). arXiv:1110.2782 [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y16.00013: Implementation of a dynamical cluster approximation within an augmented plane-wave framework Hunpyo Lee, Kateryna Foyevtsova, Johannes Ferber, Markus Aichhorn, Harald O. Jeschke, Roser Valenti Even though the local density approximation in combination with dynamical mean field theory (LDA+DMFT) accounts for various and interesting physics such as Mott insulator and enhancement of effective mass for real compounds, the pseudo-gap and spin density wave behaviors are missed due to the absence of spatial correlations. In order to consider the short-range spatial correlations beyond the LDA+DMFT approach for real compounds, we introduce a combination of LDA with a dynamical cluster approximation (LDA+DCA) in the framework of the full-potential linear augmented plane-wave basis and show results for SrVO3. We discuss how to implement the LDA+DCA approach in the WIEN2k density functional theory code and analyze the momentum spectral properties on SrVO3 as a result. We compare our LDA+DCA results with LDA+DMFT as well as with angle-resolved photoemission spectra (ARPES). We find that LDA+DCA results compare better with ARPES than the LDA+DMFT results due to inclusion of spatial correlations. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y16.00014: Pressure induced insulator to metal transition and orbital dynamics in Sr2VO4 Sukanta Karmakar The unusual ground states of transition metal oxides with layered perovskite structure (K$_{2}$NiF$_{4}$ type) are driven by a complex interplay between charge, orbital, spin and lattice degrees of freedom. The S=1/2 and 3d$^{1}$ system Sr$_{2}$VO$_{4}$, a Mott insulator, is of particular interest because of its vicinity to the insulator-metal boundary. The strong two-dimensionality along with a weak Jahn-Teller distortion (slightly elongated VO$_{6}$ octahedra along c-axis) causes splitting of t$_{2g}$ orbitals (\textit{xy} orbital and doubly degenerate \textit{yz} and \textit{zx} orbitals), pressure-tuning of which is expected to reveal interesting physical properties. Our recent high pressure Raman spectroscopic investigations at low temperature describe the orbital dynamical changes in this system. The anomalous pressure dependence of the strong magnetic excitations in Raman spectra confirms the unconventional spin-orbital composite nature (magnetic octupolar type order) of the d\textit{xy}/\textit{zy} levels [PRL \textbf{103}, 067205, (2009)]. The system transforms from high temperature orbital liquid like state to the low temperature orbital ordered state via a short range orbital order precursor at 150K. With increasing pressure the orbital ordering transition temperature decreases, predicting a QCP at $\sim $8 GPa. At this pressure the system undergoes an insulator to metal transition (as observed from mid IR reflectance measurements as well as sharp rise in Raman intensity of phonon modes due to occurrence of intraband transitions). [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y16.00015: ab-initio calculations of electronic structure and magnetism of O2MF6 (M=Sb, Pt) : Coulomb correlation and spin-orbit interaction effects in 2p and 5d electrons Minjae Kim, B. I. Min We have investigated electronic structures and magnetic properties of O$_{2}$$M$F$_{6}$ ($M$=Sb, Pt), which are composed of two building blocks of strongly correlated electrons: O$_{2}^{+}$ dioxygenyls and $M$F$_{6}^{-}$ octahedra, by employing the first-principles electronic structure band method. For O$_{2}$SbF$_{6}$, as a reference system of O$_{2}$PtF$_{6}$, we have shown that the Coulomb correlation of O(2$p$) electrons drives the Mott insulating state. For O$_{2}$PtF$_{6}$, we have demonstrated that the Mott insulating state is induced by the combined effects of the Coulomb correlation of O(2$p$) and Pt(5$d$) electrons and the spin-orbit (SO) interaction of Pt(5$d$) states. The role of the SO interaction in forming the Mott insulating state of O$_{2}$PtF$_{6}$ is similar to the case of Sr$_{2}$IrO$_{4}$ that is a prototype of a SO induced Mott system with J$_{eff}=1/2$. [Preview Abstract] |
Session Y17: Nanostructures and Interfaces: Electrons, Phonons, and Plasmons
Sponsoring Units: DCMPChair: Evan Glaser, Naval Research Laboratory
Room: 252A
Friday, March 2, 2012 8:00AM - 8:12AM |
Y17.00001: Transport and Kondo correlations in magnetic break junction devices Gavin D. Scott, Douglas Natelson A single molecule transistor device fabricated with a break junction technique is utilized as a tunable model system for probing transport properties of the highly correlated Kondo state. The emergence of this collective phenomenon occurs due to an antiferromagnetic interaction between conduction electrons and a local magnet moment represented by an unpaired spin on the molecule. Low energy non-equilibrium conductance measured in the Kondo regime has been shown to obey a particular scaling relationship with respect to different perturbations. Devices are now fabricated with ferromagnetic and strongly paramagnetic electrodes, and the applicability of such scaling behavior is investigated in the presence of magnetic interactions and anomalous transport characteristics. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y17.00002: Phonon transmission across Si/Ge interface from first-principles by the Green's function method Zhiting Tian, Keivan Esfarjani, Gang Chen Modeling phonon transmission as a function of phonon frequency and incidence angle is vital for multiscale modeling of heat transport in nanostructured materials. In this study, we calculate the phonon transmission in three-dimensions via Green's function method. It will be applied to silicon/germanium interface for which the force constants will be calculated from either the Stillinger-Weber semi-empirical potential, or from first-principles density functional methods. Both the perfect interface and the rough interface will be investigated. The transmission as a function of interface roughness will give us more guidance for surface engineering. Results between first-principles and the SW potential will be compared to see how reliable the predictions from SW potential are. The contribution of optical modes is illustrated by comparing the results will the prediction of the acoustic mismatch model (AMM) which is also harmonic and the long-wavelength limit of the general theory. It will now be possible to integrate the information on momentum and frequency-dependent transmission and the bulk mean free paths, both calculated from first-principles DFT, to accurately model heat transport in complex nanostructured materials. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y17.00003: Ultrafast Characterization of Nanostructures in GaAs Stephanie Gilbert, Jason Kawasaki, Andrew Steigerwald, Justin Gregory, Chris Palmstr{\O}m, Norman Tolk We combine ultrafast pump-probe and optoacoustic spectroscopy with magneto-optical Kerr rotation measurements to characterize embedded, self-assembled magnetically active nanostructures in a GaAs host matrix. We observe variations in the pump-probe and optoacoustic signals depending on the composition and growth characteristics of the embedded layers. Further, we observe (a) distinct behaviors in the femtosecond response of the composite structures when the probe photon energy is tuned near the GaAs band edge and (b) strong modulation of the optoacoustic signal inside the embedded layer. These results indicate an effective change in the transient femtosecond response of the composite structure, likely originating in strain effects due to the presence of nanoparticles within the host lattice. We additionally probe ultrafast magneto-optical interactions through time-resolved Kerr measurements. Finally, we present a potential method for high-resolution depth-dependent magnetic characterization by combining the Kerr rotation and optoacoustic experimental techniques. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y17.00004: Position-dependent Diffusion Coefficient as Localization Criterion in Non-Conservative Random Media Ben Payne, Alexey Yamilov Characterization of different regimes of electromagnetic wave transport in random media is an important area of research with ramifications in condensed matter physics. For passive systems, transport can be either ballistic, diffusive, or localized. Wave corrections to otherwise classical transport in a finite random media result in position-dependent diffusion coefficient $D(z)$. If media is active or dissipative (contains optical gain or absorption), then $D(z)$ can be used to distinguish a multitude of distinct wave transport regimes. Using a numerical model, we validate the existence of the position-dependent diffusion, including for media with absorption. We have previously developed a phase space enumerating all regimes exhibiting distinct transport behavior. Here we present recent results from our numerical simulations which demonstrate the ability to use $D(z)$ to distinguish between these regimes. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y17.00005: Probing phonon surface scattering in nanostructures Richard Robinson, Jared Hertzberg, Obafemi Otelaja In insulating materials, heat is transmitted by atomic vibrations (``phonons''). In nanostructured materials such as nanowires and nanosheets, the characteristic length scale of a material can be less than the mean free path of a phonon. The phonon transport is then drastically altered and becomes dominated by scattering from surfaces. We demonstrate a method to assess the scattering rate and transmission factor of phonons traversing a silicon nanosheet. Generation and detection of phonons is accomplished by a superconducting tunnel junction attached to the silicon nanostructure and operated at a temperature of 0.3K. Decay of excited states in the superconductor is employed as a tunable narrow-band source of phonons [1,2]. This tunable source enables investigation of the phonon mean free path as a function of phonon frequency and surface roughness, for frequencies from $\sim$100 GHz to $\sim$500 GHz in nanosheets 100 to 200 nm thick. This work is supported by DOE (DE-SC0001086). \\[4pt] [1] H. Kinder. Phys. Rev. Lett. 28, 1564 (1972)\\[0pt] [2] J. B. Hertzberg et al, Rev. Sci. Inst. 82, 104905 (2011). [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y17.00006: Self-assembled, vertically aligned, epitaxial nanoscale $p-n$ heterojunctions for thin film based photovoltaic applications Daniela F. Bogorin, Tolga Aytug, Parans M. Paranthaman, Andrew A Lupini, Adam Rondinone, Kyle Winters Using \textit{rf-} sputtering technique we have exploited phase-separated self-assembly and developed epitaxial, nanostructured composite films composed of phase separated, and vertically oriented $p-n$ interfacial nanocolumns of Cu$_{2}$O (p type; 2eV bandgap) and TiO$_{2 }$(n type; 3.2 eV bandgap). The characteristic band gaps of these phases allow extension of the solar capture from ultraviolet to a visible wavelenght. The composite films were grown on perovskite substrates and exhibit single crystalline epitaxy in both phases. We have investigated crystalline structure, interfacial quality and optical properties of the nanopillar arrays using XRD, TEM, SEM, AFM, and optical absorption techniques. Here, we show nearly complete atomic order at Cu$_{2}$O-TiO$_{2}$ interface (i.e., $p-n $junction) and an absorption profile that captures a wide range of solar spectrum extending from ultraviolet to visible wavelengths. Compared to layered thin film architectures, the use of such vertically aligned nanostructures in solar cells can promote cost-effective fabrication of high efficiency PV devices by providing low defect concentrations, improved absorption and light trapping capabilities, and increased minority carrier diffusion lengths. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y17.00007: Ab initio calculation of the electron-phonon coupling for transport Nathalie Vast, Jelena Sjakste, Paola Gava, Iurii Timrov, Valery Tyurerev We have developed an approach which enables us to compute matrix elements of the electron-phonon coupling within the density functional perturbation theory for the electronic interaction with short-wavelength phonons.\footnote{J. Sjakste, N. Vast, V. Tyuterev, Phys. Rev. Lett. 99, 236405 (2007).} Combining this {\it ab initio} approach to the Boltzmann transport equation, we have obtained the thermoelectric coefficients of silicon.\footnote{Z. Wang, S. Wang, S. Obukhov, N. Vast, J. Sjakste, V. Tyuterev, and N. Mingo, Phys. Rev. B 83, 205208 (2011).} The lifetime of the 2p$_0$ shallow impurity state in doped-silicon turns out to be shorter than expected.\footnote{V. Tyuterev, J. Sjakste, N. Vast, Phys. Rev. B 81, 245212 (2010)} The lifetime of the exciton in germanium under pressure\footnote{V.G. Tyuterev and S.V. Obukhov N. Vast and J. Sjakste,Phys. Rev. B 84, 035201} is found to be well described. Effect of the material nanostructuring on the electron-phonon coupling constants will be shown for small semiconducting superlattices. Finally, the calculation of deformation potentials for intravalley scattering will be discussed, and results shown for silicon and for bismuth, which is the prototype material for thermoelectricity. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y17.00008: ABSTRACT WITHDRAWN |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y17.00009: Effect of Interlayer Interaction on the Structural, Electronic, and Thermal Properties of Layered MS$_2$ (M=W, Mo) Structures Seoung-Hun Kang, Sora Park, Young-Kyun Kwon Using density functional theory (DFT) supplemented with van der Waals interaction, we investigate the effect of interlayer interaction on the structural, electroinic, and thermal properties of transition-metal disulfides MS$_2$, such as MoS$_2$ and WS$_2$. We calculate the relative stability of various layer-layer stacking configurations determined by considering relative positions and orientations between neighboring layers. We find that MS$_2$ layers may slide over each other with a small sliding barrier. We explore the effect of layer stacking on the electronic structure, and find an intriguing coupling effect. We also calculate their thermal properties including thermal expansion behavior especially along the direction normal to the plane. Such thermal expansion behavior is considered for our study of Li-intercalation into layered MS$_2$, which may become a fundamental understanding for future development of Li-ion battery. We evaluate thoroughly the diffusion paths and barriers in between layers and compare them with those on the surface. Interestingly we find that the diffusion barrier between layers is ${\sim}100$~meV smaller than that on a single layer, implying layered MS$_2$ may be a good candidate for Li-ion battery electrodes. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y17.00010: Surface plasmon modes management by Thompson plasmonics Nai Jing Deng, Cheung Wai Chau, Sai Kit Yung, Kin Wah Yu We have studied the dispersion and propagation of the surface plasmons in a structure consisting of a metal slab and a dielectric slab, the latter of which contains randomly distributed small metal particles. In our model, the metal material is characterized by the Drude model and the pudding structure is studied with Maxwell-Garnett effective medium theory. This construction of material can bring a new hybridized band in the dispersion relation where light has a relatively small group velocity. The geometric profile of volume fraction of metal balls in pudding structure can effectively change the behaviour of the plasmon propagation. For example, by adding a parabolic confinement, it is shown by the Hamiltonian optics that the light propagation is trapped, i.e., the light experiences an oscillation in a small space. Experimentally, the confinement condition can be achieved with various means, thus it may be useful in development of new mechanism of solar cell. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y17.00011: Tunable Propagation and Localization of Hybrid Surface Plasmon Polaritons in Chirped Metal-Dielectric Waveguides Cheung Wai Chau, Sai Kit Yung, Kin Wah Yu We have studied the propagation of coupled surface plasmon polariton (SPP) waves in a metal-dielectric-metal (e.g. Au/MgF2/Au) waveguide by the transfer matrix method. Due to the evanescent coupling of the SPP waves at the two interfaces in the dielectric layer, three hybridized surface plasmon polariton (HSPP) branches are achieved with a nearly flat branch at intermediate frequencies. The flat HSPP branch is tunable by varying the thickness and/or the permittivity of the dielectric layer. Moreover, by imposing a gradual variation of the permittivity (or thickness) of the dielectric layer along the propagation direction, it is possible to alter the local dispersion relation so that a localization of SPP waves can be realized. Under a parabolic confinement, Hamiltonian optics is used for simulating the propagation and localization of HSSP waves. We demonstrate that HSSP are localized at different locations at different frequencies, which is useful for achieving trapped SPP rainbow. The results of this research can have fruitful applications in optical computing, etc. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y17.00012: Energy concentration in plasmonic nanostructures: Green function formalism Sai Kit Yung, Cheung Wai Chau, Kin Wah Yu We have developed the Green function formalism (GFF), which can be used to study the field distribution and electrostatic resonance of different nanostructures. In the GFF, a surface integral equation is formulated for the scalar potential for an arbitrary number of nanostructures of various shapes. This formalism has the advantage of avoiding matching the complicated boundary conditions on the surfaces of the nanostructure. In particular, we have studied the cases of two approaching metal cylinders and non-touching metal crescent under a uniform applied electric field. It is shown that there is an energy concentration within the air narrow gap and the metal narrow gap in the cases of approaching cylinders and non-touching crescent respectively. The numerical GFF results are compared with the analytic results by conformal transformation. The results are useful in designing plasmonic light-havesting devices. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y17.00013: Circular Dichroism and Spin Polarization of Rashba Split Surface States on a Bi/Ag Surface Alloy Guang Bian, Longxiang Zhang, Yang Liu, T. Miller, T.-C. Chiang The Bi/Ag surface alloy possesses a huge Rashba splitting in its surface bands due to the prominent corrugation in the surface reconstruction and the large atomic spin-orbit coupling of the Bi atom. This system is an intriguing candidate to realize the 2D p$_{x}$+ip$_{y}$ superconductor and further, Majorana states. In this work, we study the electronic structure of the Bi/Ag surface alloy prepared by depositing Bi onto ultrathin Ag films followed by annealing. The electronic structure of the system is measured using circular angle resolved photoemission spectroscopy (CARPES). The results reveal two interesting phenomena: the hybridization of spin polarized surface states with Ag bulk quantum well states and the umklapp scattering by the perturbed surface potential. In addition, our CARPES spectra show clearly a unique dichroism pattern which is closely related to the spin texture of this 2D strongly spin-orbit coupled electron system. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y17.00014: Atomic structure, energetics, and dynamicsof topological solitons inindium chains on Si(111) surfaces Hui Zhang, Jin-Ho Choi, Jun-Hyung Cho, Changgan Zeng, Zhenyu Zhang, Jianguo Hou Besides the presence of exotic ground states, potentially more intriguing are the elementary excitations of the One-dimensional charge density waves (1D-CDWs), including the nonlinear topological excitation or soliton. Solitons may possess spin-charge inversion properties, and act as the effective carriers that account for the high conductivity in conducting polymers. Howevercomprehensive quantitative study of topological solitary excitations at the atomic level remains a challenge. In this talk, I will present our recent work on the quantitative haracterization of solitons in In chains grown on Si(111) surfaces at atomic scale. The precise atomic structure of the topological soliton in In/Si(111) is determined based on scanning tunneling microscopy and first-principles calculations.Variable emperature measurements of the soliton population allow us to determine the soliton formation energy to be $\sim $60 meV, smaller than one-half of the band gap of $\sim $200 meV. Once created, these solitons have very low mobility; the sluggish nature is attributed to the exceptionally low attempt frequency for soliton migration. We furtherdemonstrate local electric ?eld enhanced soliton dynamics, and the feasibility of aggregating solitonsinto soliton polymers. [Preview Abstract] |
Session Y18: Focus Session: Interfaces in Complex Oxides - Functional Interfaces
Sponsoring Units: DMPChair: J.D. Burton, University of Nebraska
Room: 252B
Friday, March 2, 2012 8:00AM - 8:36AM |
Y18.00001: Tunable ferroelectricity and magnetoelectricity at the interfaces of superlattices of antiferromagnets Invited Speaker: Christos Panagopoulos In heterostructures composed of transition metal oxides, the disruption introduced even by an ideal interface can drastically upset the delicate balance of the competing interactions among electronic spins, charges and orbitals, leading to a range of exotic phenomena. These include interfacial conduction, magnetism, superconductivity but also improved ferreoelectric properties. Possible incorporation of magnetoelectrics in these structures would further enable electrical control of magnetization as well as magnetic sensing of the polarization state. Naturally occurring magnetoelectrics are however, rare and generally with weak coupling. In this work we report a strategy and evidence for the emergence and control of ferroelectricity and strong magnetoelectric coupling in superlattices \textit{consisting of solely non-ferroelectric antiferromagnetic layers}. By selecting appropriate yet abundant starting materials one may now design a plethora of low dimensional heterostructures with the desired relative arrangement of the oxide layers, of potential utility to miniature valves with electric and magnetic field tunable functions, promising to set new standards in future electronics. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y18.00002: Interfacial Properties of Ultra-thin YBCO/LCMO Superlattices Benjamin Gray, Jian Liu, E.J. Moon, M. Kareev, D.J. Meyers, I.C. Tung, M.J. Bedzyk, M. Veenendaal, J.W. Freeland, J. Chakhalian The rational design of complex oxide heterostructures enables the investigation of novel materials with antagonistic order parameters. Our previous work using resonant x-ray spectroscopies has provided insight into the role of orbital reconstruction and covalent bonding at the interface of such heterostructures. In this talk, we will further address the intriguing interfacial electronic and magnetic properties and possible coupling between layers in superlattices composed of alternating superconductive YBa$_{2}$Cu$_{3}$O$_{7}$ and ferromagnetic La$_{2/3}$Ca$_{1/3}$MnO$_{3}$ layers upon approaching the ultra-thin limit, where a superconductor to insulator transition with increasing thickness N is observed in [YBCO (1 u.c)/LCMO (N u.c.)] superlattices. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y18.00003: Scanning tunneling microscope investigation of the interfacial electronic properties of YBCO/LCMO J.C. Wang, B.C. Huang, Y.P. Chiu, Y.C. Huang, Y.C. Chen, V.T. Tra, J.Y. Lin, J.C. Yang, Y.H. Chu Direct measurements of the interfacial electronic structures in YBCO/LCMO hetero-structures have been performed using cross-sectional scanning tunneling microscopy (STM) and spectroscopy (STS). Both scanning tunneling spectroscopy and analysis of the local electronic states across the interface of YBCO/LCMO reveal the evolution of the energy-band structures. Closely examining the recognition of the electronic structure by the unique combination of STM and STS reveals the direct information on the local interaction between superconductivity and magnetism in this work. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y18.00004: Quasiparticle dynamics in YBCO and YBCO/LSMO Using Femtosecond Optical Pulses J. Lee, D. Talbayev, J. Xiong, J. Zhu, Q. Jia, A.J. Taylor, R.P. Prasankumar The properties of various complex oxide systems, such as multiferroics, high-T$_{c}$ superconductors and colossal magnetoresistance manganites, have been extensively studied for the past $\sim $25 years. In particular, the interplay between superconductivity (SC) and ferromagnetism (FM) is interesting from both academic and applied viewpoints. we have temporally resolved quasiparticle dynamics in multilayered films composed of the high-temperature superconductor YBCO and the ferromagnetic manganite LaSrMnO$_{3}$ (LSMO) by performing temperature-dependent UOS experiments. In YBCO alone, we observed two distinct decay relaxation channels that have previously been related to the pseudogap and superconducting gaps and can be explained with the phenomenological Rothwarf-Taylor (RT) model. However, the fast sub-picosecond relaxation related to the pseudogap was not observed in our YBCO/LSMO heterostructures, possibly due to the influence of FM order These first UOS experiments on SC/FM heterostructures demonstrate the ability of UOS to quantify the influence of ferromagnetism on superconductivity through time domain measurements. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y18.00005: Ba and Sr templates on Ge(001) for epitaxial oxide growth Miri Choi, Agham Posadas, Chih-Kang Shih, Alexander A. Demkov In the epitaxial growth of oxides on semiconductors, Ba and Sr templates are used as transition layers for perovskite deposition. Furthermore, the interface between complex oxides and semiconductors are relatively unexplored. The ability to control the interface between them can lead to new phenomena and new combinations of oxide-semiconductor heterostructures. In this work, we compare the differences between the Sr template with Ba templates on Ge(001) at various temperatures. The sub-monolayer coverage of Ba and Sr shows different morphology and nucleation behavior depending on the deposition temperature. For example, at higher deposition temperature, the electron diffraction pattern of Sr on Ge shows a 9x1 phase which was not observed at lower deposition temperature. We perform an atomic-scale study of Ba and Sr templates grown on Ge(001) by molecular beam epitaxy as a function of deposition temperature and coverage, using a combination of reflection high energy electron diffraction and \textit{in situ} scanning tunneling microscopy (STM). An understanding of the behavior of Ba and Sr template formation on germanium can potentially be used as a means of better controlling the subsequent oxide growth. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y18.00006: Structure of SrTiO$_3$ films on Si C. Stephen Hellberg, Kristopher Andersen, Hao Li, Philip Ryan, Joseph Woicik The epitaxial deposition of oxides on silicon opens the possibility of incorporating their diverse properties into silicon-device technology. Deposition of SrTiO$_3$ on silicon was first reported over a decade ago, but growing the coherent, lattice-matched films that are critical for many applications has been difficult for thicknesses beyond 5 unit cells. Using a combination of density functional calculations and x-ray diffraction measurements, we determine the atomic structure of coherent SrTiO$_3$ films on silicon, finding that the Sr concentration at the interface varies with the film thickness. The structures with the lowest computed energies best match the x-ray diffraction. During growth, Sr diffuses from the interface to the surface of the film; the increasing difficulty of Sr diffusion with film thickness may cause the disorder seen in thicker films. The identification of this unique thickness-dependent interfacial structure opens the possibility of modifying the interface to improve the thickness and quality of metal oxide films on silicon. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y18.00007: Low kV Atomic Resolution and First Principles Study of the Structure and Bonding at SrTiO$_{3}$/GaAs Hetero-interfaces Qiao Qiao, Robert Klie, Serdar Ogut Ultrathin transition-meal oxide films on polar substrates have attracted increasing attention in recent years, due to the emergence of novel interfacial phases, not seen in the bulk of either material. In this study, we have combined aberration-corrected atomic-resolution Z-contrast imaging, electron energy loss spectroscopy (EELS) with first-principles density functional theory calculations to examined the atomic and electronic structures of epitaxially grown, ultrathin SrTiO$_{3}$ (100) films on GaAs (001). We find that the interface is atomically abrupt and no surface reconstruction of the GaAs (001) surface is observed. Using atomic-column resolved EELS, we show that Ti diffuses into the first few monolayers of GaAs and we will present evidence for the formation of As-oxides at the interface depending on the thin film growth conditions. First-principles DFT calculations will be used to analyze the formation energies of Ti-related impurity defects in the bulk and surface regions of GaAs, as well as the stability of any surface reconstruction at the SrTiO$_{3}$/GaAs interface. These findings are used to explain transport behavior of the SrTiO$_{3}$ films as a function of deposition conditions. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y18.00008: Ultimate photovoltage in perovskite oxide heterostructures with critical film thickness Kui-juan Jin, Cong Wang One order larger photovoltage is obtained with critical thicknesses of La$_{0.9}$Sr$_{0.1}$MnO$_{3}$ films in both kinds of heterostructures of La$_{0.9}$Sr$_{0.1}$MnO$_{3}$/SrTiO$_{3}$ (0.8 wt {\%} Nb-doped) and La$_{0.9}$Sr$_{0.1}$MnO$_{3}$/Si fabricated at various oxygen pressures. Our self-consistent calculation reveals that the critical thickness of the La$_{0.9}$Sr$_{0.1}$MnO$_{3}$ film with the ultimate value of photovoltage is just the thickness of the depletion layer of La$_{0.9}$Sr$_{0.1}$MnO$_{3}$ in the $p-n$ heterostructures of La$_{0.9}$Sr$_{0.1}$MnO$_{3}$/SrTiO$_{3}$ (0.8 wt {\%} Nb-doped) and La$_{0.9}$Sr$_{0.1}$MnO$_{3}$/Si, respectively. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y18.00009: Topological insulators from complex orbital order in transition-metal oxides heterostructures Gregory Fiete, Andreas Ruegg Topological band insulators which are dynamically generated by electron-electron interactions have been the- oretically proposed in two and three dimensional lattice models. We present evidence that the two-dimensional version can be stabilized in digital (111) heterostructures of transition-metal oxides as a result of purely local interactions. The topological phases are accompanied by spontaneous ordering of complex orbitals and we discuss their stability with respect to the Hund's rule coupling, Jahn-Teller interaction and inversion symmetry breaking terms. As main competitors we identify spin-nematic and magnetic phases. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y18.00010: The electronic structure of epitaxially strained iridate thin films and superlattices from first principles Johannes Voss, Craig J. Fennie Within the Ruddlesden-Popper iridates Sr$_{n+1}$Ir$_n$O$_{3n+1}$, strong spin-orbit interactions lead to the formation of a half-filled, narrow $J_\textrm{eff}=1/2$ band and filled $J_\textrm{eff}=3/2$ bands. This places the iridates in the vicinity of a Mott transition, which is sensitive to perturbations in crystal structure, despite relatively weak on-site Coulomb interactions [1]. For example, Sr$_2$IrO$_4$ ($n=1$) is an antiferromagnetic Mott insulator that displays an almost rigid coupling between spin canting and IrO$_6$ octahedron rotations [2], while epitaxially stabilized SrIrO$_3$ ($n=\infty$) is a correlated metal. In this talk, we will discuss from first-principles within the LDA+SO+$U$ approach the possibility to engineer the electronic structure of SrIrO$_3$ and CaIrO$_3$ thin films using epitaxial strain and by creating superlattices of the form ($A$IrO$_3$)$_m$($A'B$O$_3$)$_{m'}$ with $A$, $A'$ = Ca, Sr. [1] S.J. Moon, H. Jin, K.W. Kim, W.S. Choi, Y.S. Lee, J. Yu, G. Cao, A. Sumi, H. Funakubo, C. Bernhard, and T.W. Noh, PRL 101, 226402 (2008). [2] B.J. Kim, H. Jin, S.J. Moon, J.-Y. Kim, B.-G. Park, C.S. Leem, J. Yu, T.W. Noh, C. Kim, S.-J. Oh, J.-H. Park, V. Durairaj, G. Cao, and E. Rotenberg, PRL 101, 076402 (2008). [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y18.00011: The Structure of Y$_2$O$_3$:ZrO$_2$ / SrTiO$_3$ Layered Heterostructures Matthew Dyer, George Darling, John Claridge, Matthew Rosseinsky Since the discovery of greatly increased conductivity in layered heterostructures of yttria stabilized zirconia (YSZ) and strontium titanate (STO) there has been considerable study and discussion regarding the origin of the high level of conductivity. The detailed atomic structure of layered YSZ-STO heterostructures is likely to be a crucial source of understanding, and as yet remains undetermined, particularly in the region of the buried interfaces between YSZ and STO. We present the results of density functional theory calculations on YSZ-STO heterostructures layered in the [001] direction, with different structures and compositions. We find that structures built using the terminations of the fluorite crystal structure of YSZ do not produce heterostructures which follow conventional solid state chemistry. These heterostructures contain Ti and Zr ions with unlikely O coordination geometries, reduced Ti$^{3+}$ and Zr$^{3+}$ ions and O-O bonding. In contrast, heterostructures built using a reconstructed YSZ termination, based on a rock-salt ordered ZrO layer, give more stable and more conventional atomic structures and are calculated to be electrically insulating. We describe these structures in detail, and the reasons behind the necessary reconstruction of YSZ at the interface. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y18.00012: Structural Rearrangement of Niobium Oxides from Lamellar Phases to Discrete Nanosheets and Nanoscrolls Probed by DFT Calculations Jhashanath Adhikari, Luis J. Smith Inorganic niobates ACa$_{2}$Nb$_{3}$O$_{10}$ (A= H and K) with layered structures are good photocatalytic materials due to their high surface areas accommodating a larger number of active sites and ease of processing through soft chemical techniques like exfoliation and restacking. Alkali metal phases can be ion-exchanged to the acid phase, which in turn can be easily exfoliated to individual nanosheets. The nanosheets can change their form to nanoscrolls with a curled geometry instead of a flat surface. During these morphological transformations, the local structure at the Nb-atom, H-atom and the interface may undergo rearrangement which is responsible for the alteration of properties of the materials. This presentation highlights the preliminary results on these structural modifications (interface variation, stacking of layers, lattice contraction and space group settings) and the possible positions of the proton. Our calculations show that the protons in the acid form are non-bridging and bonded to the same layer oxygen atoms unlike the K-atoms in its parent compound. The Electric field gradient (EFG) is a parameter very sensitive to the electron density around a quadrupolar nucleus $^{2}$H and $^{93}$Nb that can be detected using NMR. Changes in its magnitude/sign can be correlated to the change in the local environment (bond lengths and angles) around the sites of interest. EFG values from DFT calculations based on the proposed structural models will be used for the characterization of surface O-H bond lengths, H-bonding and Nb-O bond lengths and can be used to interpret NMR studies. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y18.00013: Study of interface interactions in ZnO/Mesoporous silica nanocomposite R.C. Reddy A, Sowri Babu K, Sujatha Ch, V.G. Reddy K The Photoluminescence (PL) properties of ZnO/Mesoporous silica (MPS) nanocomposite annealed under different temperatures were studied. A broad PL band at 395 nm has been observed in all samples and analysis was made by using Gaussian fitting. As the temperature increased, emission bands were blue shifted and the relative intensity ratio of the oxygen vacancies at ZnO-SiO$_{2}$ interface to the oxygen vacancies in inner ZnO crystallites was increased. The emission peaks at 363 nm and 384 nm are attributed to the near band edge emission (NBE) and to the phonon replica emission. At 550C the exciton confinement effect disappears due to the large amount of surface effects. The influence of porosity of host media on Si-O-Zn cross linking bonds was also investigated. ZnO nanoparticles were loaded into nanocrystalline silica (NCS) and silica gel (SG). The surface area increases monotonously from NCS to MPS through SG. Si-O-Zn cross linking bonds were almost absent in the sample prepared with NCS. It exhibits NBE emission at 360 nm which was found to be absent in other samples prepared with SG and MPS. It was also found that the emission intensity of the samples decreases with aging. This is due to diffusion of oxygen atoms from the atmosphere to interior of the sample. [Preview Abstract] |
Session Y19: Invited Session: Novel Mechanisms of Multiferrocity
Sponsoring Units: DCMPChair: Paolo Radaelli, Oxford University
Room: 253AB
Friday, March 2, 2012 8:00AM - 8:36AM |
Y19.00001: Multiferroicity due to Charge Ordering Invited Speaker: Jeroen van den Brink In this contribution I discuss multiferroicity that is driven by different forms of charge ordering, presenting first the generic mechanisms by which charge ordering can induce ferroelectricity in magnetic systems. In type-I multiferroics [1], ferroelectricity and magnetism have different origins and occur at different temperatures. There is a number of specific classes of materials for which this is relevant. Discussed will be in some detail (i) perovskite manganites of the type (PrCa)MnO3 [2,3], (ii) the complex and interesting situation in magnetite Fe3O4, (iii) strongly ferroelectric frustrated LuFe2O4 and (iv) an example of a quasi-one-dimensional organic system [4]. In type-II multiferroics [1], ferroelectricity is completely due to magnetism, but with charge ordering playing an important role [5], such as (v) multiferroic Ca3CoMnO6, (vi) possible ferroelectricity in rare earth perovskite nickelates of the type RNiO3 [6,7], (vii) multiferroic properties of manganites of the type RMn2O5 [8], (viii) perovskite manganites with magnetic E-type ordering. \\[4pt] [1] J. van den Brink and D. Khomskii, J. Phys.: Condens. Matter 20, 434217 (2008).\\[0pt] [2] D.V. Efremov, J. van den Brink and D.I. Khomskii, Nature Materials 3, 853 (2004).\\[0pt] [3] G. Giovannetti, S. Kumar, J. van den Brink, S. Picozzi, Phys. Rev. Lett. 103, 037601 (2009).\\[0pt] [4] G. Giovannetti, S. Kumar, A. Stroppa, J. van den Brink and S. Picozzi, Phys. Rev. Lett. 103, 266401 (2009). \\[0pt] [5] J. Betouras G. Giovannetti and J. van den Brink, Phys. Rev. Lett. 98, 257602 (2007).\\[0pt] [6] G. Giovannetti, S. Kumar, D. Khomskii, S. Picozzi and J. van den Brink, Phys. Rev. Lett. 103, 156401 (2009).\\[0pt] [7] S. Kumar, G. Giovannetti, J. van den Brink and S. Picozzi, Phys. Rev. B 82, 134429 (2010).\\[0pt] [8] G. Giovannetti and J. van den Brink, Phys. Rev. Lett. 100, 227603 (2008). [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 9:12AM |
Y19.00002: Soft X-ray Observation of electronic contribution to ferroelectric polarization Invited Speaker: Stuart Wilkins Multiferroic materials open up new interesting possibilities for devices by enabling the switching of an electric state by magnetic field or {\it vice-versa}. In addition to this functionality, multiferroics are intriguing from a fundamental physics perspective, raising interesting questions concerning coupling of the electric and magnetic order parameters. To date, most coupling mechanisms are understood to occur due to distortions in the crystal lattice. Here we present experimental evidence that in the multiferroics RMn$_2$O$_5$ (where R is a rare earth) there exists a new, purely \emph{electronic} contribution to the ferroelectric polarization, which can exist in the absence of any lattice distortions. This contribution arises due to spin-dependent hybridization of O $2p$ and Mn $3d$ states and was observed through soft x-ray resonant scattering, which has proved to be a very useful tool in the study of the magnetic structure of multiferroics[1]. Through resonant x-ray scattering at the oxygen K-edge, we find that such spin dependent hybridization occurs in both TbMn$_2$O$_5$[2] and YMn$_2$O$_5$[3]. Remarkably, in YMn$_2$O$_5$ we find that the temperature dependence of the integrated intensity of the signal at the oxygen K-edge closely follows the macroscopic electric polarization [3], and hence is proportional to the ferroelectric order parameter. This is in contrast with the temperature dependence observed at the Mn L$_3$ edge, which reflects the Mn magnetic order parameter. Work performed at BNL was supported by the US Department of Energy, Division of Materials Science, under contract No. DE-AC02-98CH10886. \\[4pt] [1] S.B. Wilkins et al., Phys. Rev. Lett. 103, 207602 (2009)\\[0pt] [2] T.A.W. Beale, S. B. Wilkins et al., Phys. Rev. Lett. 105, 087203 (2010)\\[0pt] [3] S. Partzsch, S. B. Wilkins et al., Phys. Rev. Lett. 107, 057201 (2011) [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:48AM |
Y19.00003: Dynamic magnetoelectric interaction in multi-orbital Mott insulators Invited Speaker: Maxim Mostovoy The control of spin textures in magnetic insulators with an applied voltage can be of great importance for dissipationless spintronics. The coupling between spin and charge degrees of freedom in Mott insulators originates from the fluctuations of electron occupancy of strongly correlated orbitals of transition metal ions. Several microscopic mechanisms resulting from the virtual hopping of electrons from a magnetic site to its neighbor, e.g. the ``inverse Dzyaloshinskii-Moriya'' and the Heisenberg exchange striction mechanisms, are responsible for the electric polarization induced by non-centrosymmetric magnetic orders, the excitation of spin waves with the oscillating electric field of a light wave, and other effects recently observed in multiferroic materials. I will discuss a new dynamic magnetoelectric interaction, which describes the electric polarization induced by time-dependent spin textures [1]. Simple symmetry arguments as well as the explicit derivation from an extended Hubbard model of multi-orbital Mott insulators are used to obtain the form of this interaction, which is the electric analogon of the coupling between the scalar spin chirality and magnetic field. It is closely related to the so-called spinmotive force exerted by spins on electrons in magnetic conductors. This interaction makes possible to displace spin textures in ferromagnetic insulators by applying a voltage. It couples the external electric field to the center-of-mass coordinates of topological spin textures in ferromagnetic thin films, such as Skyrmions and magnetic vortices. The effect of this coupling is dramatically amplified in the resonant absorption of circularly polarized light by spin vortices in nanodiscs. \\[4pt] [1] M. Mostovoy, K. Nomura and N. Nagaosa, Phys. Rev. Lett. 106, 047204 (2011). [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:24AM |
Y19.00004: Multiferroic vortices: arrested Kosterlitz-Thouless order Invited Speaker: Sang-Wook Cheong The fascinating concept of topological defects permeates ubiquitously our understanding of the early-stage universe, hurricanes, quantum matters such as superfluids and superconductors, and also technological materials such as liquid crystals and magnets. Large-scale spatial configurations of these topological defects have been investigated only in a limited degree. Exceptions include the cases of supercurrent vortices or liquid crystals, but they tend to exhibit either trivial or rather-irregular configurations. Hexagonal REMnO$_{3}$ (RE= rare earths) with RE=Ho-Lu, Y, and Sc, is an improper ferroelectric where the size mismatch between RE and Mn induces a trimerization-type structural phase transition, and this structural transition leads to three structural domains, each of which can support two directions of ferroelectric polarization. We reported that domains in h-REMnO$_{3}$ meet in cloverleaf arrangements that cycle through all six domain configurations, Occurring in pairs, the cloverleafs can be viewed as vortices and antivortices, in which the cycle of domain configurations is reversed. Vortices and antivortices are topological defects: even in a strong electric field they won't annihilate. These ferroelectric vortices/antivortices are found to be associated with intriguing magnetism. The seemingly-irregular configurations of a zoo of multiferroic vortices and antivortices in h-REMnO$_{3}$ can be neatly analyzed in terms of graph theory and this graph theoretical analysis reflects the nature of self-organized criticality in complexity phenomena as well as the condensation and eventual annihilation processes of topological vortex-antivortex pairs. Furthermore, these numerous multiferroic vortices/antivortices can be understood as an arrested Kosterlitz-Thouless phase. \\[4pt] [1] Insulating Interlocked Ferroelectric and Structural Antiphase Domain Walls in Multiferroic YMnO$_{3}$, T. Choi, - - -, S-W. Cheong, Nature Materials 9, 253 (2010). \\[0pt] [2] Self-organization, condensation, and annihilation of topological vortices and antivortices in a multiferroic, S. C. Chae, - - - S.-W. Cheong, PNAS 107,~ 21366 (2010). \\[0pt] [3] Direct observation of the proliferation of ferroelectric dislocation loops and vortex-antivortex pairs, S. C. Chae, - - -, S.-W. Cheong, Phys. Rev. Lett., submitted. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 11:00AM |
Y19.00005: Interplay between trimerization, ferroelectric, and magnetic order in the hexagonal manganites Invited Speaker: Manfred Fiebig The hexagonal manganites h-RMnO$_3$ with R = Sc, Y, In, Dy -- Lu are a model system for multiferroics with pronounced magnetoelectric effects in which the magnetic and ferroelectric order emerge independently (so-called split-order-parameter or type-I multiferroics). In spite of many years of intense investigations the system never seizes to surprise us with novel, unexpected manifestations of its five-fold long-range order (antiferrodistortive, ferroelectric, antiferromagnetic Mn order, and rare-earth order on the 2a and 4b sites). Here I will discuss several such examples: (i) ``Incompatible'' magnetic order of the Mn and rare-earth sublattices according to different symmetry representations in combination with triggered ordering at the 4b site. (ii) The {it absence} of ferroelectric order in h-InMnO$_3$ down to low temperatures in spite of its apparent similarities to, in particular, h-YMnO$_3$. (iii) Observation of a direct and rigid coupling of the ferroelectric to the antiferrodistortive order in annealing experiments at 1300 K. In summary, all these experiments allow us to present a comprehensive model for the microscopic origin of the ferroelectric and the multiple magnetic (re-) ordering in this important group of compounds. [Preview Abstract] |
Session Y20: Invited Session: New Anisotropy-Driven Phenomena in Colloidal Suspensions
Sponsoring Units: DCMP GSNPChair: David Weitz, Harvard University
Room: 253C
Friday, March 2, 2012 8:00AM - 8:36AM |
Y20.00001: Entropically patchy particles: Shape-driven self assembly of hard colloids Invited Speaker: Sharon Glotzer Although the structural diversity of colloidal fluids and crystals has grown substantially in recent years, it still aspires to that of atomic and molecular systems. Ionic colloidal crystals and binary nanoparticle superlattices, by exploiting electrostatic interactions in mixtures of particles of opposite charge, have substantially broadened the diversity of structures beyond those obtainable in traditional hard sphere systems, but rely on energetic interactions as well as entropy for their stability. Likewise, ``traditional'' patchy particles with sticky interactions exploit explicit attractive interactions for assembly. Here we explore the role of shape and entropy in phase transitions of hard particle fluids, in the absence of all other interactions. Using computer simulations, we show that particle shape alone can suffice to produce a rich diversity of colloidal crystals, quasicrystals, glasses and mesophases through thermodynamic self-assembly whose complexity rivals that of atomic analogues. We compare self-assembled phases of hard polyhedra with their dense packings obtained from small unit cell compressions, and show the packings tend to be less structurally complex than the assemblies. Based on our findings, we present design rules for anisotropic hard, facetted colloids as ``entropically patchy particles'' for self assembly. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 9:12AM |
Y20.00002: Lock-and-Key Colloids Invited Speaker: David Pine We have developed a new kind of colloidal particle that is spherical but with one or more spherical dimples in the particle surface. These dimples serve as docking points for colloidal spheres whose radius matches the radius of dimples. The attractive docking force is provided by the depletion interaction and can be controlled by changing the depletant concentration or, in some cases, the temperature. The docking is completely reversible and mimics the classic lock-and-key interaction often used to describe protein binding. The lock-and-key binding is size specific and can be used to assemble a number of interesting colloidal superstructures, including flexible jointed chains, dumbbells, trimers, tetramers, and other assemblies. A new synthetic method for making the dimpled particles can be generalized to make a number of other new structured colloidal particles, with different functionalities analogous to block copolymers. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:48AM |
Y20.00003: Glass transitions in quasi-two-dimensional suspensions of colloidal ellipsoids Invited Speaker: Yilong Han Colloidal glasses constitute of anisotropic particles were mainly studied by simulations in three dimensions with incomplete phase diagrams. Here we report the experiment about glass transitions in a colloidal suspension of micro-ellipsoids at the single-particle level. Video microscopy revealed a two-step glass transition corresponding to inter-domain freezing and inner-domain freezing respectively. The glass transition in the rotational degree of freedom was at a lower density than that in the translational degree of freedom. Between the two transitions, ellipsoids formed an ``orientational glass'' in the area fraction range 0.72 $<$\textit{ $\phi $} $<$ 0.79 for aspect ratio $p$ = 6 ellipsoids and 0.60 $<$\textit{ $\phi $} $<$ 0.72 for $p$ = 9. Such orientational glass is expected to be replaced by the rotator phase at small aspect ratios and the nematic phase at large aspect ratios. The observed decoupling between diffusion and relaxation for both of translational and rotational motions reflects the dynamic heterogeneity. Approaching the respective glass transitions, the rotational and translational fastest-moving particles in the supercooled liquid moved cooperatively and formed clusters with power-law size distributions. The mean cluster sizes diverge in power law as approaching the glass transitions. The translational and rotational fastest- and slowest-moving ellipsoids are all spatially anticorrelated: most translational fast-moving ellipsoids and rotational slow-moving ellipsoids formed at different areas within pseudonematic domains, while most rotational fast-moving ellipsoids and translational slow-moving ellipsoids formed at different areas around the domain boundaries. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:24AM |
Y20.00004: Suppression of the coffee-ring effect by shape-dependent capillary interactions Invited Speaker: Peter Yunker |
Friday, March 2, 2012 10:24AM - 11:00AM |
Y20.00005: Glassy dislocation dynamics in colloidal dimer crystals Invited Speaker: Sharon Gerbode Dislocation mobility is central to both the mechanical response and the relaxation mechanisms of crystalline materials. Recent experiments have explored the role of novel particle anisotropies in affecting the rules of defect motion in crystals. ``Peanut-shaped'' colloidal dimer particles consisting of two connected spherical lobes form densely packed crystals in 2D. In these ``degenerate crystals,'' the particle lobes occupy triangular lattice sites while the particle axes are randomly oriented among the three crystalline directions. One consequence of the random orientations of the dimers is that dislocation glide is severely limited by certain particle arrangements in the degenerate crystals. Using optical tweezers to manipulate single lobe-sized spherical intruder particles, we locally deform the crystal, creating defects. During subsequent relaxation, the dislocations formed during the deformation leave the crystal grain, either via annihilation with other dislocations or by moving to a grain boundary. Interestingly, in large crystalline grains this dislocation relaxation occurs through a two-stage process reminiscent of slow relaxations in glassy systems, suggesting the novel concept that glassy phenomena may be introduced to certain kinds of colloidal crystals via simple anisotropic constituents. [Preview Abstract] |
Session Y21: Bi-based Cuprates -- Experiment
Sponsoring Units: DCMPChair: Kui Jin, University of Maryland
Room: 254A
Friday, March 2, 2012 8:00AM - 8:12AM |
Y21.00001: Phase Determination for Intra-unit-cell Fourier Transform STM -- Picometer Registration of Zn Impurity States in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ Ines Firmo, Mohammad Hamidian, Kazuhiro Fujita, Sourin Mukhopadhyay, Joseph Orenstein, Hiroshi Eisaki, Shin-ichi Uchida, Michael Lawler, Eun-Ah Kim, J.C. Davis By studying the real and imaginary components of the Bragg peaks in Fourier transforms of electronic structure images, distinct types of intra-unit cell (IUC) symmetry breaking can be studied using SI-STM [Lawler \textit{et al.}, Nature \textbf{466} 347 (2010)]. However, establishing the precise r-space symmetry point of each unit cell is crucial in defining the phase for such analysis. Exemplary of this challenge is the high-T$_{c}$ superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ for which the Bi atoms in the surface BiO layer are observable, while it is the invisible Cu atoms that define the relevant CuO$_{2}$ unit-cell symmetry point. We demonstrate, by imaging with pm precision the electronic impurity states at individual Zn atoms substituted at Cu sites, that the phase established using the Bi lattice produces a $\sim $2{\%}(2$\pi)$ error relative to the Cu lattice. In this case, IUC C$_{4v}$ symmetry breaking in the CuO$_{2}$ plane can be determined reliably using the phase assignment from the BiO layer. Moreover, impurity atom substitution at the relevant symmetry site can be of general utility in phase determination for Bragg-peak Fourier analysis of IUC symmetry. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y21.00002: Imaging broken symmetry in the electronic structure of Bi-doped cuprates by scanning tunneling microscopy Yang He, Tess Williams, Anjan Soumyanarayanan, Michael Yee, Takeshi Kondo, Hiroshi Ikuta, Eric Hudson, Jennifer Hoffman The relationship between the mysterious pseudogap phase in the cuprate superconductors and the myriad broken symmetries observed by various experimental techniques is a source of much controversy. We use low-temperature scanning tunneling microscopy and spectroscopy to image electronic nematic order in the cuprate superconductor Bi2-xPbxSr2CuO6+$\delta $. We determine the robustness of the nematic order parameter to experimental conditions. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y21.00003: Investigating structural distortions in Bi based superconductors by scanning tunneling microscopy Michael M. Yee, Ilija Zeljkovic, Anjan Soumyanarayanan, Elizabeth Main, T. Williams, Takeshi Kondo, T. Takeuchi, Hiroshi Ikuta, G.D. Gu, E.W. Hudson, Jennifer E. Hoffman We use scanning tunneling microscopy to image symmetry-breaking structural distortions in the bismuth-based high-Tc superconductors, Bi$_{2}$Sr$_{2}$Ca$_{n-1}$Cu$_{n}$O$_{2n+4+x}$ (BSCCO). To elucidate the structure, we have implemented a new algorithm that quantifies atomic distortions with picometer resolution. Using this algorithm, we have observed an in-plane inversion-symmetry-breaking orthorhombic structural distortion in the BiO layer. We have also quantified the lattice distortions resulting from the use of Pb substitution to suppress the supermodulation in BSCCO. Each of these structural distortions break point group symmetries of the CuO$_{2}$ plaquette, which may be generally relevant to understanding the broken symmetries of the mysterious pseudogap state in high-Tc cuprates. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y21.00004: Exploring the Nature of the Electronic Scatters in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ Eduardo Calleja, Jixia Dai, Genda Gu, Kyle McElroy The superconducting cuprates have been one of the most studied classes of materials over the last 20 years. While much progress has been made in understanding the strange electronic properties which govern their rich phase diagram, the interplay between coulomb interactions (which are thought to play a key role in their unconventional superconductivity) and the corresponding quasi particle scattering caused by coulomb interactions, are still the subject of debate among the community. We attempt to explore this issue by using spectroscopic imaging scanning tunneling spectroscopy (SI-STS) at variable temperatures which allows us to probe the Local Density of States (LDOS). The LDOS can be fitted with the theorized Dynes' formula allowing for the extraction of the quasi particle scattering rates. Furthermore, we seek to understand the scattering process by simulating the patterns produced by quasi particle interference (QPI) by either a pair potential impurity or a non-magnetic impurity. Extraction of these rates at various dopings and temperatures in the phase diagram will be presented. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y21.00005: Imaging the impact of single dopant atoms on the electronic order and disorder of Bi$_{2+y}$Sr$_{2-y}$CaCu$_{2}$O$_{8+x}$ Ilija Zeljkovic, Genda Gu, Jennifer Hoffman High-T$_{c}$ cuprate superconductors display startling nanoscale disorder in essential properties such as critical temperature, pseudogap energy, and even band structure. The underlying cause of this disorder has remained mysterious; theoretical explanations have ranged from chemical inhomogeneity to spontaneous electronic phase separation. We extend the energy range of scanning tunneling spectroscopy, allowing the first-ever direct mapping of both types of interstitial oxygen dopants in Bi$_{2+y}$Sr$_{2-y}$CaCu$_{2}$O$_{8+x}$, as well as vacancies at the apical oxygen site. We show that a subset of these dopants are indeed the direct cause of the nanoscale disorder. We further explain how the spatial variations in electronic orders, such as the pseudogap and the charge order, are governed by the disorder in the dopant concentrations, particularly vacancies in the apical oxygen site. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y21.00006: Imaging Locally Oriented Charge Modulations in a Cuprate Superconductor Jennifer Hoffman, Elizabeth Main, Benjamin Phillabaum, Hiroshi Ikuta, Eric Hudson, Karin Dahmen, Erica Carlson We use scanning tunneling microscopy to image the local orientation of the static charge modulations in Bi$_{2-y}$Pb$_y$Sr$_{2-z}$LazCuO$_{6+x}$, for samples spanning a wide range of doping. For each sample, we compute the size distribution of locally $x$-oriented and locally $y$-oriented clusters. We analyze the size distributions within a random field Ising model to obtain the fractal dimension and other critical exponents. We discuss the utility of scaling collapse to extract the critical doping $x_c$ of the smectic charge order in Bi$_{2-y}$Pb$_y$Sr$_{2-z}$LazCuO$_{6+x}$. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y21.00007: Structure of intra-unit cell C$_{4v}$ symmetry breaking domains in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ from SI-STM Mohammad Hamidian, Ines Firmo, Kazuhiro Fujita, Hiroshi Eisaki, Shin-ichi Uchida, Michael Lawler, Eun-Ah Kim, J.C. Davis Mounting evidence from a number of experimental probes supports the idea that the electronic structure of the cuprate pseudogap phase breaks rotational symmetry. Furthermore, ARPES, neutron scattering and spectroscopic imaging scanning tunneling microscopy (SI-STM) data all point to an intra-unit cell origin. We present new findings for near optimally doped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$(BSCCO) which elucidate the domains associated with intra-unit cell C$_{4v}$ symmetry breaking in the electronic structure. The analysis method will be motivated by the preceding talk, `Phase Determination of Intra-Unit Cell Fourier Transform STM -- Picometer Registration of Zn Impurity States in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$' given by I. A. Firmo. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y21.00008: Optical conductivity of exfoliated Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ nanocrystals Luke Sandilands, Viktoriya Baydina, Alexander Su, Anjan Reijnders, Tor Pedersen, Ferenc Borondics, Genda Gu, Shimpei Ono, Yoichi Ando, Kenneth Burch We report on infrared spectromicroscopy of mechanically exfoliated under-doped and optimally-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ thin crystals on SiO$_2$/Si substrates. The infrared reflectance and transmission was measured for samples of various thicknesses and the the optical conductivity extracted in the frequency range 0.15 eV to 1 eV. Trends in the optical conductivity with thickness are discussed. In particular, we observe that the conductivity of thicker ($\sim$ 100 nm) samples is comparable to bulk while that of thinner ($\sim$ 20 nm) samples is markedly suppressed. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y21.00009: Evidence of two-dimensional quantum critical behavior in the superfluid density of deeply underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ Jie Yong, Michael Hinton, Andy Mccray, M. Naamneh, Amit Kanigel, Mohit Randeria, Thomas Lemberger Evidence of two-dimensional (2-D) quantum critical fluctuations is observed in the superfluid density n$_{s}$(T) $\propto \quad \lambda ^{-2}$(T) of deeply underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x }$(Bi-2212). Quantum critical behavior is indicated by the evolution of the T-dependence of n$_{s}$(T)/n$_{s}$(0), which loses any evidence for thermal critical behavior and becomes quasi-linear when underdoping drops the transition temperature T$_{c}$ below roughly 48K. Two-dimensionality is indicated by the linear scaling of transition temperature T$_{c}$ with n$_{s}$(0). The 2-D behavior contrasts with that of the less anisotropic YBa$_{2}$Cu$_{3}$O$_{7-\delta }$, which sustains 3D quantum critical fluctuations. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y21.00010: Coherent THz emission from isosceles-triangular mesas of Bi2Sr2CaCu2O8 Richard Klemm, Erica LaBerge, Candy Reid, Dustin Morley, Kavel Delfanazari, Kazuo Kadowaki Using the standard emission patterns predicted for thin, isosceles triangular patch antennae, combined with a spatially uniform ac Josephson current source, we have performed two-parameter least-squares fits to our recently obtained unpublished experimental emission data from isosceles-triangularly-shaped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ mesas. Our results support the notion that such irregular patch antennae shapes may be useful in constructing high-powered, tunable continuous-wave coherent sources of light in the sub-terahertz to terahertz regime. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y21.00011: Tunable sub-terahertz emission from intrinsic Josephson junctions in Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ enhanced by the internal cavity resonance Manabu Tsujimoto, Takashi Yamamoto, Kaveh Delfanazari, Ryo Nakayama, Takeo Kitamura, Masashi Sawamura, Takanari Kashiwagi, Hidetoshi Minami, Masashi Tachiki, Richard Klemm, Kazuo Kadowaki Intense, continuous and coherent terahertz electromagnetic wave emission from the intrinsic Josephson junction system in cuprate high-$T_{c}$ Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ with remarkably higher intensity [L. Ozyuzer \textit{et al.}, Science \textbf{318}, 1291 (2007).] than previously generated from single or arrayed Josephson junctions has been understood by the enhancement of the output intensity by the internal cavity resonance. However, we have recently observed emission, which seems to have low enhancement due perhaps to the relatively low-$Q$ factor, in a wide frequency range, covering almost all frequencies continuously as long as the ac Josephson effect is satisfied. This broadly tunable behavior is very different from that with the tunability found in the inner current-voltage branch region [M. Tsujimoto \textit{et al.}, to be published in 2011.] and enables us to design useful and tunable sub-terahertz source devises. In the presentation, we will also discuss the possible high-power devices using the high-$Q $electromagnetic cavity. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y21.00012: Effect of magnetic field on the coherent THz emission from mesas of single crystal Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ Takeo Kitamura, Takanari Kashiwagi, Manabu Tsujimoto, Kaveh Delfanazari, Ryo Nakayama, Masashi Sawamura, Takashi Yamamoto, Hidehiro Asai, Hidetoshi Minami, Masashi Tachiki, Kazuo Kadowaki Coherent and continuous electromagnetic (EM) waves radiation phenomena with a mesa structure of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ single crystal have been investigated precisely in magnetic field up to only 200 Oe where the emission intensity decreases sharply expectedly for the field $H$ parallel to the $c$-axis. The emission could not be observed above 20 Oe for $H$//$c$-axis whereas it persisted up to 160 Oe for $H$//\textit{ab} plane [1]. These results indicate that both pancake vortices as well as Josephson vortices suppress the THz emission very strongly. On the other hand, the Josephson plasma resonance phenomena have been observed in both $H$//\textit{ab} and $H$//$c$ even in very high fields ($\sim $Tesla). The emission processes are considered to be the reverse processes of the absorption. It is interesting to pose a question what happens in high fields in the EM waves emission. We show interesting experimental results of THz emission in high magnetic fields including low field region and will argue the mechanism of emission in high magnetic fields. \\[4pt] [1] K. Yamaki \textit{et al}., physica C \textbf{470} (2010) S804-805. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y21.00013: Quasiparticle dynamics in overdoped Bi$_{1.4}$Pb$_{0.7}$Sr$_{1.9}$CaCu$_{2}$O$_{8+\delta}$: Coexistence of superconducting gap and pseudogap below $T_{c}$ Saritha K. Nair, Xingquan Zou, Jian-Xin Zhu, Christos Panagopoulos, Shigeyuki Ishida, Shin-ichi Uchida, Elbert E.M. Chia Photoexcited quasiparticle relaxation dynamics in overdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ ($T_{c}$=65~K, hole doping $p$=0.22) single crystal is investigated as a function of temperature. We provide evidence of a $\sim$22~meV pseudogap ($T^{\ast}$$\approx$100~K) at this doping level. Moreover, this pseudogap vanishes at $T^{\ast}$. Our data support the scenario where both the superconducting gap and pseudogap coexist in the superconducting state. Our results also suggest an increased scattering rate between electrons and spin fluctuations as the sample enters the pseudogap phase. Phys. Rev. B \textbf{82, 212503 }(2010) [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y21.00014: THz wave radiation from the triangular mesas in layered high-$T_{c}$ superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ intrinsic Josephson junctions Kaveh Delfanazari, Manabu Tsujimoto, Takanari Kashiwagi, Hidehiro Asai, Takashi Yamamoto, Masashi Sawamura, Takeo Kitamura, Ryo Nakayama, Kazuya Ishida, Hidetoshi Minami, Toshiaki Hattori, Kazuo Kadowaki Recently, it has been reported that the mesa structures fabricated from single crystalline high-$T$c superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$ intrinsic Josephson junctions generate THz waves [1, 2]. However, most of radiations in the previous studies were observed either below the critical current where the junctions switched to the resistive state or around the retrapping region. We present here three types of emission for isosceles triangular mesas in which THz waves were also additionally detected above the critical current where the external current is increasing in the resistive branch structures.Furthermore, intense, coherent, monochromatic, continuous and tunable THz waves were observed in various triangular mesas fabricated by the focus ion beam (FIB) technique [3]. Moreover, elucidating the oscillating modes inside the mesas by precisely studying the IV characteristics, FT-IR spectrum and angular dependence of the emission intensity will be discussed in detail at meeting. \\[4pt] [1] L. Ozyuzer \textit{et al. }Science \textbf{318,} 1291 (2007). \\[0pt] [2] K. Kadowaki \textit{et al. }Physica C \textbf{468} 634 (2008). \\[0pt] [3] K. Delfanazari \textit{et al. }J. Phys. Conf. Ser. Accepted (2011). [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y21.00015: Particle-hole asymmetric QPI in the pseudogap phase of underdoped Bi-2212 Chung Koo Kim, Jhinhwan Lee, Kazuhiro Fujita, Hiroshi Eisaki, Shinichi Uchida, J.C. Seamus Davis, Jinho Lee Quasi-Particle Interference (QPI) measured by STM revealed many interesting phenomena in Correlated Electron Systems. Particle-hole (p-h) symmetric QPI observed in the superconducting Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ disappears around the reduced zone boundary [1], while p-h asymmetric QPI's on non-superconducting Sr$_{3}$Ru$_{2}$O$_{7}$ [2] and Ca(Fe$_{x}$Co$_{1-x}$)$_{2}$As$_{2}$ [3] identified dominant band structures, suggesting electronic nature of the nematic phases. In light of these discoveries, here we report the observation of p-h asymmetric QPI of underdoped Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ in the pseudogap phase continuously dispersing through E$_{F}$. The Fermi velocity measured from this dispersion is $\sim$0.12$\times$10$^{6}$ m/s. We will discuss the possible origin and implications by comparing its symmetry and dispersion using theoretical model and currently available experimental data from other probes.\\[4pt] [1] Y. Kohsaka et al., Nature 454, 1072 (2008)\\[0pt] [2] Jinho Lee et al., Nature Physics 5, 800 (2009)\\[0pt] [3] Chuang et al., Science 327, 181 (2010) [Preview Abstract] |
Session Y22: Focus Session: Fe-based Superconductors - Crystal Growth and New Materials
Sponsoring Units: DMP DCOMPChair: Cedomir Petrovic, Brookhaven National Laboratory
Room: 254B
Friday, March 2, 2012 8:00AM - 8:12AM |
Y22.00001: Single crystal growth and physical properties of SrCu$_2$As$_2$, SrCu$_2$Sb$_2$ and BaCu$_2$Sb$_2$ V.K. Anand, P.K. Perera, A. Pandey, R.J. Goetsch, A. Kreyssig, D.C. Johnston We present the physical properties of self-flux-grown single crystals of SrCu$_2$As$_2$, SrCu$_2$Sb$_2$, SrCu$_2$(As$_{0.84}$Sb$_{0.16}$)$_2$ and BaCu$_2$Sb$_2$ investigated by magnetic susceptibility $\chi $, specific heat $C_{\rm p}$ and electrical resistivity $\rho$ vs. temperature $T$ measurements. Contrasting structures occur for SrCu$_2$As$_2$ (ThCr$_2$Si$_2$-type), SrCu$_2$Sb$_2$ (CaBe$_2$Ge$_2$-type) and BaCu$_2$Sb$_2$ (a distorted intergrowth of ThCr$_2$Si$_2$-type and CaBe$_2$Ge$_2$-type unit cells). The $\chi(T) $ data for all these compounds exhibit weakly anisotropic diamagnetic behaviors. For $1.8 \leq T \leq 300$~K, the $\rho(T)$ data show metallic character and are well-described by the Bloch-Gr\"{u}neisen model, and the $C_{\rm p}(T)$ data are well-fitted by metallic $\gamma T$ plus Debye lattice contributions. From the low-$T$ $C_{\rm p}(T)$ data, we infer Sommerfeld coefficients $\gamma$ = 2.2--3.9 mJ/mol\,K$^2$ and Debye temperatures $\Theta_{\rm D}$ = 204--246 K\@. The electronic properties indicate that these compounds are $sp$ metals containing Cu in a nonmagnetic $3d^{10}$ electronic configuration.\footnote{D. J. Singh, Phys. Rev. B {\bf 79}, 153102 (2009).} [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y22.00002: Synthesis of Large Single Crystals Within the Zr - Fe - Si System Greg Smith, Jacob E. Grose, J.W. Simonson, M.C. Aronson The synthesis of large single crystals of intermetallic silicides from a metal flux is challenging, owing to the poor solubility of silicon in many of the traditional fluxes at low temperatures. Accordingly, single crystal syntheses of several compounds within the Zr - Fe - Si system from a gallium flux were investigated. Zr$_4$Fe$_4$Si$_7$, a member of the well-known V-phase family of compounds, forms as silver lustrous rods. Successful growths yielded crystals with dimensions up to 12.1 mm by 0.14 mm by 0.14 mm, large enough to perform the first transport measurements of this compound. We grew for the first time single crystals of the hexagonal Laves phase ZrFe$_{1.5}$Si$_{0.5}$, which has previously only been reported in the polycrystalline form. Two previously unreported phases, including the silver lustrous rods Zr$_{2-x}$Fe$_4$Si$_{12-y}$, have been discovered as single crystals within this system. The composition of all phases was verified with powder and single crystal x-ray diffraction. Our group is currently investigating the growth of similar compounds using the methods developed for this system. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 9:00AM |
Y22.00003: Exploration of Superconductivity in Layered Structures Invited Speaker: Athena Sefat Superconductivity at high temperatures is of great interest in terms of both basic science and applications. Although there are many findings on the chemistry and physics of superconductors, there are a lot of questions that remain unanswered. We will review experimental routes to possibly finding and identifying new superconducting materials. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y22.00004: High T$_{c}$ electron doped Ca$_{10}$(Pt$_{3}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$ and Ca$_{10}$(Pt$_{4}$As$_{8})$(Fe$_{2}$As$_{2})_{5 }$superconductors Ni Ni, Jared M. Allred, Benny C. Chan, Robert J. Cava In this talk, we will present the crystal structures and physical properties of two new iron arsenide superconductors, Ca$_{10}$(Pt$_{3}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$ (the ``10-3-8 phase") which crystallizes in the triclinic structure and Ca$_{10}$(Pt$_{4}$As$_{8})$(Fe$_{2}$As$_{2})_{5}$ (the ``10-4-8 phase") which crystallizes in the tetragonal structure. They are very similar compounds for which the most important differences lie in the structural and electronic characteristics of the intermediary platinum arsenide layers. Electron doping through partial substitution of Pt for Fe in the FeAs layers leads to $T_{c}$ of 11 K in the 10-3-8 phase and 26 K in the 10-4-8 phase. The anisotropic H$_{c2}$ measurement indicates the multiband superconductivity in these compounds. The often-cited empirical rule in the arsenide superconductor literature relating $T_{c }$to As-Fe-As bond angles does not explain the observed differences in $T_{c}$ of the two phases; rather, comparison suggests the presence of stronger FeAs interlayer coupling in the 10-4-8 phase due to the two-channel interlayer interactions and the metallic nature of its intermediary Pt$_{4}$As$_{8}$ layer. The interlayer coupling is thus revealed as important in enhancing $T_{c}$ in the iron pnictide superconductors. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y22.00005: Influence of Cr-doping on the magnetic structure of the FeAs strip compounds CaFe4As3 : a single crystal neutron diffraction study Pascal Manuel, Laurent Chapon, Illiya Todorov, Duck Y. Chung, Bachir Ouladdiaf, Mercouri G. Kanatzidis CaFe$_4$As$_3$ offers the unique opportunity to modify the topology of the Fe$_2$As$_2$ layers, key in the understanding of superconductivity in the new iron pnictide superconductors, from infinite layers to strips of finite width linked a rectangular cross pattern. Bulk measurement on CaFe$_4$As$_3$ indicate a magnetic ordering at ~90K with a second transition seen at about 26K. Neutron diffraction allowed to ascribe the high temperature transition to a Spin Density Wave(SDW) with a propagation vector k=(0,$\delta$,0) eventually locking to $\delta$=3/8 at the lower transition. As expected a profound effect on the magnetic properties of CaFe$_4$As$_3$ can be obtained by chemically doping or applying pressure. We report here on the consequence of Cr-doping on the magnetic structure of CaFe$_4$As$_3$ derived from single crystal neutron diffraction. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y22.00006: Anisotropic $H_{c2}$ curves determined up to 92 T and the signature of two-band superconductivity in the novel superconductor Ca$_{10}$(Pt$_{4}$As$_{8})$((Fe$_{1-x}$Pt$_{x})_{2}$As$_{2})_{5}$ Eundeok Mun, Vivien Zapf, Oscar Ayala, Ross McDonald, Neil Harrison, Ni Ni, Jared Allred, Robert Cava The upper critical fields, $H_{c2}(T)$, of single crystals of the novel superconductor Ca$_{10}$(Pt$_{4}$As$_{8})$((Fe$_{1-x}$Pt$_{x})_{2}$As$_{2})_{5}$ with $x$=0.02 were determined over a wide range of temperatures down to $T$ = 1.42 K and magnetic fields up to $H $= 92 T. The measurements of anisotropic $H_{c2}(T)$ curves are performed in pulsed magnetic fields using radio-frequency contactless penetration depth measurements for magnetic field applied both parallel and perpendicular to the ab-plane. Whereas a clear upward curvature in $H_{c2}^{c}(T)$ along H$\vert \vert $c is observed with decreasing temperature, the $H_{c2}^{ab}(T)$ along H$\vert \vert $ab shows a flattening at low temperatures. The rapid increase of the $H_{c2}^{c}(T)$ suggests that the superconductivity can be described by two dominating bands. The anisotropy parameter, $H_{c2}^{ab}(T)$ /$ H_{c2}^{c}(T)$ , is $\sim $7 close to $T_{c}$ and decreases considerably to $\sim $1 with decreasing temperature, showing rather weak anisotropy at low temperatures. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y22.00007: Direct measurement of the absolute value of the magnetic penetration depth in two-dimensional pnictide superconductor Ca$_{10}$(Pt$_{3}$As$_{8})$[(Fe $_{1-x}$ Pt$_{x})_{2}$As$_{2}$]$_{5}$ Jeehoon Kim, Filip Ronning, Evgeny Nazaretski, Ni Ni, J.M. Allred, R.J. Cava, J.D. Thompson, R. Movshovich We have measured the absolute value of the magnetic penetration depth $\lambda $ in a single crystal of the Ca$_{10}$(Pt$_{3}$As$_{8})$[(Fe $_{1-x}$ Pt$_{x})_{2}$As$_{2}$]$_{5}$ (``10-3-8'') superconductor using low temperature magnetic force microscopy (MFM). We directly probed local values of $\lambda $ in the ``10-3-8'' sample using Meissner response measurements and compared Meissner curves to those obtained in a Nb reference sample. The Meissner response measured at different locations on the sample shows similar behavior indicating homogeneity of the superconducting state, in accord with tunnel-diode resonator measurements. We also discuss larger values of $\lambda $ in 10-3-8 relative to $\lambda $ values measured in other pnictide systems. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y22.00008: Local Displacements, Magnetoelastic Coupling, and Bonding in Spin-Ladder Iron Selenides James Neilson, Joseph Caron, David Miller, Anna Llobet, Tyrel McQueen The spin-ladder ``1-2-3'' compounds BaFe$_2$Se$_3$ and KFe$_2$Se$_3$, built of double-chains of edge-sharing [FeSe$_4$] tetrahedra, are localized-moment antiferromagnetic semiconductors. Total neutron scattering of BaFe$_2$Se$_3$ reveals local iron displacements coupled to long-range ordered antiferromagnetism comprised of ferromagnetic Fe$_4$ plaquettes. The magnitude of the iron displacements follow the antiferromagnetic order parameter: a manifestation of magnetoelastic coupling. These local displacements are essential for properly understanding the electronic structure of these systems, as local structural modulations necessarily perturb the ground state wavefunctions. Furthermore, while iron displacements from magnetoelastic coupling in Fe$X_4$-based materials are hypothesized to be important in the emergence of superconductivity, the spin-ladders remain insulating down to 1.8 K, even upon hole doping by substitution of K for Ba. As with the copper oxide superconductors two decades ago, our results highlight the importance of reduced dimensionality spin-ladder compounds in the study of the coupling of spin, charge, and atom positions in superconducting materials. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y22.00009: Spin glass and semiconducting behavior of the flux grown BaFe$_{2-\delta }$Se$_{3}$ crystals Bayrammurad Saparov, Stuart Calder, Balazs Sipos, Huibo Cao, Songxue Chi, David Singh, Andrew Christianson, Mark Lumsden, Athena Sefat In this talk, physical properties and crystal and electronic structures of BaFe$_{2-\delta }$Se$_{3}$ crystals, synthesized using tellurium flux, will be discussed. This phase is an iron-deficient derivative of the ThCr$_{2}$Si$_{2}$-type and its structure is made of double chains formed from edge-sharing FeSe$_{4}$ tetrahedra. The semiconducting BaFe$_{2-\delta }$Se$_{3}$ with \textit{$\delta $} $\approx $ 0.2 does not order magnetically, however, there is evidence for short-range magnetic correlations of spin glass type below 50 K in magnetization, heat capacity and neutron diffraction results. The semiconducting behavior of BaFe$_{2-\delta }$Se$_{3}$ is in line with the detrimental influence of iron deficiency to the superconductivity in $A_{x}$Fe$_{1.8}$Se$_{2}$ ($A$ = alkali metal) superconductors. The electronic structure calculations suggest that this compound can be considered as a low-dimensional (1D) ladder structure with a weak interchain coupling. Based on the survey of available data on BaFe$_{2}$Se$_{3}$ so far, lower concentrations of iron vacancies may lead to a long range antiferromagnetic order, whereas higher concentrations of iron vacancies may suppress long range order and then lead to a spin glass behavior. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y22.00010: Ba$_{1-x}$K$_x$Mn$_2$As$_2$: An Antiferromagnetic Local Moment Metal Abhishek Pandey, R.S. Dhaka, J. Lamsal, Y. Lee, V.K. Anand, A. Kreyssig, R.J. McQueeney, A. Goldman, B.N. Harmon, A. Kaminski, D. C. Johnston, T.W. Heitmann The syntheses of K-doped single crystalline $\rm{Ba_{0.984}K_{0.016}Mn_2As_2}$ and polycrystalline $\rm{Ba_{0.95}K_{0.05}Mn_2As_2}$ with the tetragonal ${\rm ThCr_2Si_2}$ structure are reported. Electrical resistivity, heat capacity, magnetic susceptibility, angle-resolved photoemission spectroscopy and neutron diffraction measurements and spin-polarized electronic structure calculations consistently establish that these hole-doped Ba$_{1-x}$K$_x$Mn$_2$As$_2$ samples are antiferromagnetic local-moment metals, in contrast to the parent BaMn$_2$As$_2$ [1-3] which is an antiferromagnetic local-moment semiconductor. This new class of materials bridges the gap between the iron pnictide and cuprate high $T_{\rm c}$ materials. Investigations of the phase diagram of the Ba$_{1-x}$K$_x$Mn$_2$As$_2$ system and other similar systems are underway.\\[4pt] [1] Y. Singh et al., Phys. Rev. B \textbf{79}, 094519 (2009).\\[0pt] [2] Y. Singh et al., Phys. Rev. B \textbf{80}, 100403 (2009).\\[0pt] [3] D. C. Johnston et al., Phys. Rev. B \textbf{84}, 094445 (2011). [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y22.00011: Role of ruthenium in iron-based superconductors and related materials Michael McGuire, Athena Sefat, Brain Sales Ruthenium and iron share the same valence electron count, and form many isostructural compounds. However, the larger covalent radius and extent of the d-electrons of ruthenium lead to interesting and sometimes unexpected behavior when iron is partially or fully substituted by ruthenium. For example, ``doping'' layered iron compounds with ruthenium has been shown to produce superconductivity in some cases but not others, and ruthenium analogs of certain layered iron compounds do not form under similar conditions. We have investigated full and partial ruthenium substitution in several iron-based materials, including the superconducting 1111 and 122 families, and studied the effects on formation, crystal structures, and physical properties. Our new experimental findings and results from available literature will be used to discuss the unusual role that ruthenium plays in iron-based superconductors and related materials. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y22.00012: Neutron Diffraction Studies on Ba$_{0.95}$K$_{0.05}$Mn$_{2}$As$_{2}$ J. Lamsal, T.W. Heitmann, A. Pandey, V.K. Anand, Y. Lee, R.S. Dhaka, A. Kaminski, B.N. Harmon, D.C. Johnston, R.J. McQueeney, A. Kreyssig, A.I. Goldman There has been a great deal of interest in compounds, such as BaMn$_{2}$As$_{2}$, which are closely related to the iron pnictide superconductors. Although undoped BaMn$_{2}$As$_{2}$ is an antiferromagnetic (AF) insulator[1, 2], recent studies of lightly doped Ba$_{1-x}$K$_{x}$Mn$_{2}$As$_{2}$ have shown a striking change to metallic behavior for $x \quad >$ 0.01 and, therefore, may offer a bridge between the high $T_{c}$ cuprates and the iron-pnictide superconductors. We will present neutron diffraction measurements on polycrystalline Ba$_{0.95}$K$_{0.05}$Mn$_{2}$As$_{2}$ performed on the powder diffractometer at the Missouri Research Reactor. Our measurements reveal that the antiferromagnetism in the doped compound remains nearly the same as that found for undoped BaMn$_{2}$As$_{2}$, characterized by a G-type collinear AF spin structure below $T_{N }\sim $ 607(2) K with an ordered moment of 4.21(6) $\mu _{B}$/Mn ion at 14 K. Research at Ames Lab was supported by the USDOE, Basic Energy Sciences under Contract No. DE-AC02-07CH11358. \\[4pt] [1] Y. Singh et al., Phys. Rev. B \textbf{80}, 100403 (2009). \\[0pt] [2] D. C. Johnston et al., Phys. Rev. B \textbf{84}, 094445 (2011). [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y22.00013: Growth of parent and electron doped NaFeAs Yu Song It has been found hole doping on Fe sites in BaFe$_{2}$As$_{2}$ does not induce superconductivity with Cr and Mn as dopants, but doping on Ba sites with K induces superconductivity as high as 38K. We have investigated hole doping with Titanium to be compared with other hole doping compounds. Single crystals of Titanium doped BaFe$_{2}$As$_{2}$ were grown by flux method. Transport and susceptibility measurements were done showing doping Titanium suppresses the Neel temperature but no superconductivity was found up to 4{\%} doping. Susceptibility measurements also showed spin glass behavior. Phase diagrams of temperature vs doping concentration have been constructed from transport and Susceptibility measurements. [Preview Abstract] |
Session Y23: Superconductor-Insulator Transitions
Sponsoring Units: DCMPChair: Michael Osofsky, NRL
Room: 255
Friday, March 2, 2012 8:00AM - 8:12AM |
Y23.00001: Theory of Giant Magnetoresistance Peak in InO Superconducting Films Anirban Gangopadhyay, Joe Mitchell, Victor Galitski, Markus Mueller It has been suggested that the giant magnetoresistance peak seen on the insulating side of a superconductor-insulator transition in In$_2$O$_{3-x}$ films is a signature of remanant localized pairs which can participate in variable-range-hopping transport. A theory of pair-transport must necessarily take into account the bosonic nature of the pairs, as opposed to the fermionic nature of single-electron carriers. We show that this opposite statistical nature of the carriers alone can lead to opposite variations of the localization length with magnetic field and thus be a possible candidate for explaining the giant magnetoresistance peak. We further provide an explanation of the pairing mechanism in In$_2$O$_{3-x}$ films. We argue that the electron pairing mechanism here is non-BCS-type and is related instead to the local effect of electron pairs occupying Oxygen vacancies (with charge +2e). The neutral state of these defects may be more stable than the singly-charged defect, as supported by an ab initio analysis. This simple phenomenon leads naturally to a negative-U Hubbard model with strong disorder. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y23.00002: Broadband microwave study of 2D superconductor-insulator quantum phase transition Wei Liu, Lidong Pan, Minsoo Kim, Sambandamurthy Ganapathy, Peter Armitage Using our broadband microwave spectrometer, we investigate the complex AC conductance of disordered InO$_x$ films as a function of magnetic field through the 2D superconductor-insulator quantum phase transition. We have studied the behaviors of the frequency dependent complex response function of a particular InO$_x$ sample near the critical point in the limit of $\hbar \omega < K _{B} T$ and $\hbar \omega > K _{B} T$ and compare our results to theoretical models. We discuss the possibility for a novel insulating state on the insulating side of the transition through the frequency dependent conductance. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y23.00003: Superconductor-insulator transition in amorphous indium oxide films: Role of in-plane magnetic field Minsoo Kim, Tailung Wu, Ganapathy Sambandamurthy We present experimental results from transport studies on amorphous indium oxide films that are driven through a superconductor-insulator transition by applying a pair-breaking magnetic field. The direction of the magnetic field is varied continuously from being perpendicular to the film plane to parallel to the film plane and we identify characteristically different transport regimes at different magnetic field values when the film is rotated in a magnetic field. We also study the evolution of these transport regimes as a function of disorder in these films. A distinctly clear magnetic field value, in the insulating phase, at which the sample resistance is independent of the angle and very weakly dependent on temperature and disorder is observed. Implications for our current understanding of the 2D superconductor - insulator transition will be presented. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y23.00004: Evolution of magnetoresistance peak on disordered systems Yeonbae Lee Measurements of the electric and magnetic properties of amorphous indium oxide films are reported. The carrier densities of films have been tuned by employing electrostatic gating using electric double layer transistor (EDLT) configurations employing the ionic liquid DEME{\_}TFSI. We have observed the emergence of the magnetoresistance peak as films are tuned from the insulating to the superconducting regimes, and its disappearance as the system is doped further with additional carriers. The result also demonstrates the reversible tuning of the superconductor-insulator transition with low gate voltages [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y23.00005: Superconductor-Insulator transition in long Mo-Ge nanowires Hyunjeong Kim, Andrey Rogachev We have studied transport properties of two series of long (1-25$\mu $m) and very narrow (9-20nm) homogenous amorphous Mo-Ge wires fabricated by advanced electron beam lithography. We observed that the wires undergo a superconducting-insulator transition that is controlled by the wire cross sectional area, i.e. by local physics. Reduction of mean-field critical temperature can be explained by the fermionic mechanism. We also observed an unusual zero-bias anomaly in the insulating state that has signatures of both Coulomb blockade and perturbative electron-electron interaction correction. In addition, some of our long superconducting wires appear to be localized superconductor, namely in these wires one-electron localization lengths is several times shorter than the length of a wires. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y23.00006: The extent of the Cooper pair insulator phase in amorphous Pb$_{0.9}$Bi$_{0.1}$ nanohoneycomb films S.M. Hollen, J.C. Joy, A.H. Berg, G.E. Fernandes, J. Shainline, J.M. Xu, J.M. Valles, Jr. Amorphous Bi nanohoneycomb (NHC) thin films, which contain a nanometer-scale array of holes and regular thickness undulations, exhibit an insulating phase made up of localized Cooper pairs (CPs) near their thickness-driven insulator to superconductor transition (IST). This Cooper pair insulator (CPI) phase includes a giant magnetoresistance peak, also observed in InO$_{x}$ and TiN. We have now produced NHC films of a new material, Pb$_{0.9}$Bi$_{0.1}$, that show a qualitatively similar CPI phase. We will show the evolution of this CPI phase from deep in the insulating state to the IST using transport measurements. Throughout this regime, we will track the appearance, growth, and range of the magnetoresistance oscillations (which indicate CP transport) and giant magnetoresistance peak. Considering these observations, we will discuss the likely extent of CP transport in these insulators. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y23.00007: Superconductor-to-Insulator Transition in BaPb$_{1-x}$Bi$_x$O$_3$ Paula Giraldo-Gallo, Han-Oh Lee, Theodore Geballe, Ian Fisher BaBiO$_3$ is a charge density wave (CDW) insulator. Hole doping with Pb, i.e. BaPb$_{1-x}$Bi$_x$O$_3$, eventually suppresses the CDW, leading to superconductivity, with a maximum critical temperature Tc of 10 to 13K for $x\approx 0.25$. The origin of the CDW state, its doping dependence, the nature of the pairing mechanism and the effects introduced by disorder remain open questions. Here we investigate the normal state properties in the vicinity of the metal-semiconductor transition. Single crystalline samples of the solid solution BaPb$_{1-x}$Bi$_x$O$_3$, with $0 |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y23.00008: Electrostatic doping of high T$_{c}$ superconductors Joseph Kinney, Javier Garcia-Barriocanal, Xiang Leng, Alexey Kobrinskii, Boyi Yang, Stephen Snyder, Allen Goldman The application of field effect transistor concepts to electrostatically doped strongly correlated electron systems has been the focus of intense research during the last years [C. H. Ahn et al., Rev. Mod. Phys. 78, 1185 (2006)]. In this talk we will show our recent results on Electronic Double Layer Transistor (EDLT) techniques applied to high T$_{c}$ cuprates. The EDLT configuration, which employs ionic liquids as gate dielectrics, has succeeded in achieving unprecedented charge transfers, of the order of 10$^{15}$ carriers/cm$^{2}$. This large accumulation and depletion of carriers allows us to explore the phase diagram of YBa$_{2}$Cu$_{3}$O$_{7-x}$ and La$_{2}$CuO$_{4+\delta}$. We will focus on the physics of the superconductor to insulator transition [X. Leng et al., Phys. Rev. Lett. 107, 027001 (2011)] and discuss the magneto-transport properties of the underdoped and overdoped regions of the phase diagram [X. Leng, et al., arXiv:1108.0083v1]. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y23.00009: Dynamic charge order and superconductivity in lightly doped La$_{2-x}$Sr$_x$CuO$_4$ thin films Xiaoyan Shi, D. Popovic, C. Panagopoulos, A. Bollinger, G. Logvenov, I. Bozovic Recent experiments have demonstrated the existence of a charge glass state at very low temperatures deep within the spin glass phase ($T\ll T_{SG}$) in lightly doped, insulating La$_{2-x}$Sr$_{x}$CuO$_4$ (LSCO) with $x = 0.03$. Here we use magnetotransport measurements in a series of high quality MBE-grown LSCO films ($x=0.03$, 0.05, 0.055, and 0.06) to investigate the doping dependence of this dynamically inhomogenous, charge ordered state as the superconductor-insulator transition (SIT) is approached. We show that the charge glassiness is suppressed with doping. Nevertheless, close enough to the SIT on the insulating side, it coexists and competes with superconducting fluctuations (SCFs), leading to a suppression of SCFs at low $T$. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y23.00010: Two component Coulomb glass in disordered superconducting films Joe Mitchell, Anirban Gangopadhyay, Victor Galitski, Markus Mueller We propose a new two component Coulomb glass model which includes strong disorder, Coulomb interaction, and on-site electron pairing to investigate the effects of localized pairing in disordered films on the insulating side of a superconductor-insulator transition. In particular, we show how the density of states (DOS) changes with increasing on-site coupling between electrons, from an Efros-Shklovskii linear DOS for the electrons at weak coupling, to a strongly modified, non-monotonic DOS with nonuniversal Coulomb gap for electrons and on-site pairs at moderate coupling, and finally to an Efros-Shklovskii linear DOS for pairs at strong coupling. We discuss the effects of a spatially random coupling. We use a Miller Abrahams resistor network mapping to numerically calculate resistance for samples of this model, given temperature and localization length. With certain parameter choices, we can obtain a peak in resistance with respect to magnetic field, reminiscent of magnetoresistance peaks reported experimentally. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y23.00011: ABSTRACT WITHDRAWN |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y23.00012: Superconductor-insulator transition in Josephson junction arrays screened by a superconducting ground plane Joshua Paramanandam, Michael Gershenson We have studied quantum phase transitions in unconventional Josephson arrays with a large number of interacting nearest-neighbour islands (typically, 10). The range of inter-island interactions was controlled by the presence/absence of a superconducting ground plane placed in close proximity to the array. We have found that the superconductor-to-insulator transition occurs in the arrays with both short- and long-range interactions at $E_{J}/E_{C}\sim1$ . Here $E_{J}$ is the effective Josephson energy per island, $E_{C}$ is its charging energy. However, the critical resistance for the arrays with short-range interactions greatly exceeds the quantum resistance, in contrast to the case of arrays with long-range interactions. These experiments clearly show that the ratio $E_{J}/E_{C}$ is the only relevant (for the SIT) parameter, while the critical resistance can vary a great deal depending on the interaction range. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y23.00013: Strong quantum interference in strongly disordered bosonic insulators Sergey Syzranov, Andreas Moor, Konstantin Efetov We study the model of variable-range hopping of bosons in an array of sites with short-range interactions and a large characteristic coordination number, which describes conduction in a broad class of insulators, such as strongly disordered superconductive films, systems close to the Anderson transition, Josephson networks, etc. Large coordination number leads to strong quantum interference phenomena yet allows for their analytical study. We develop a functional renormalization group scheme that repeatedly eliminates high-energy sites properly renormalizing the tunneling between the low-energy ones. Using this approach we determine the temperature and magnetic field dependence of the hopping conductivity and find a large positive magnetoresistance. With increasing magnetic field the behaviour of the conductivity crossovers from the Mott's law to an activational behaviour with the activation gap proportional to the magnetic field. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y23.00014: Numerical identification of three distinct phases in a $(2+1)D$ array of quantum dissipative Josephson junctions Einar Stiansen, Iver Bakken Sperstad, Asle Sudb{\O} We have performed large-scale Monte-Carlo simulations on a model describing a $(2+1)D$ array of quantum dissipative Josephson junctions. With the superconducting phases as our fundamental degrees of freedom we are able to identify three distinct phases as function of Josephson coupling and dissipation strength. Apart from the fully superconducting state, where fluctuations in both space and time are at bay, and the normal phase, characterized by wild fluctuations, we find an additional phase featuring spatial phase coherence coinciding with temporal disorder. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y23.00015: Metallic state in La-doped YBa$_2$Cu$_3$O$_y$ thin films with n-type charge carriers S.W. Zeng, X. Wang, W.M. Lv, Z. Huang, M. Motapothula, Z.Q. Liu, Y. L. Zhao, A. Annadi, S. Dhar, T. Venkatesan, - Ariando Through substitution of La for Ba and reduction of oxygen, we successfully doped n- and p-type charge carriers into La-doped YBa$_2$Cu$_3$O$_y$ thin films synthetized by pulsed laser deposition technique. The n-type samples demonstrated metallic behaviors and in-plane resistivity exhibited a quadratic temperature dependence within the metallic regime, and then evolved into a ln T-dependence insulating-like state. Furthermore, the doping evolution of temperature with minimum resistivity (T$_{min}$) and electron-electron rate were investigated and showed asymmetry between p- and n-side. The present results suggest the potential of obtaining n-type superconductivity in La-doped YBa$_2$Cu$_3$O$_y$ and investigating n-p asymmetry (symmetry) in cuprates with the same crystallographic structure. [Preview Abstract] |
Session Y24: Quantum Many-Particle Systems: DMFT & DMRG
Sponsoring Units: DCOMPChair: R. Torsten Clay, Mississippi State Unviersity
Room: 256
Friday, March 2, 2012 8:00AM - 8:12AM |
Y24.00001: A New Lanczos-Based Low Rank Algorithm for Inhomogeneous Dynamical Mean-Field Theory Pierre Carrier, Yousef Saad, James K. Freericks Inhomogeneous DMFT is used to approximately solve models of ultracold atoms in optical lattices. The intensive part of the IDMFT algorithm is the solution of the Dyson equation for the local Green's function, which involves the computation of the diagonal of the inverse of a sparse matrix. Our new algorithm for finding the diagonal of the inverse of a large sparse matrix is based on domain decomposition into interior and interface points. Since the number of interface points is much less than the interior points, it is a low-rank matrix. Using this matrix allows for a much smaller number of Lanczos steps to obtain the exact solution of the diagonal of the inverse and hence reduces the need for as many re-orthogonalization steps in Lanczos. We show that the problem of finding the diagonal of the inverse is transformed into a naturally parallel GMRES solver (based on the domain decomposition) solved at each of the Lanczos iterations. We successfully implemented a coarray fortran (CAF) program code of this new algorithm for the 2D Fermionic-Bosonic Falicov-Kimball Hamiltonian (mixture of light and heavy atoms). Results of parallel performance and advantages of using a CAF implementation are discussed, in terms of a 3D implementation which is planned for the Hubbard model. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y24.00002: Continuum Numerical Renormalization Group Nandan Pakhira, Hullikel Krishnamurthy, James Freericks The numerical renormalization group (NRG) has emerged as one of the most powerful techniques for calculating emergent renormalized low-temperature properties of strongly correlated systems. When applied as an impurity solver within the dynamical mean-field theory (DMFT), it allows one to directly find spectral functions and how they evolve with temperature. In spite of this success, the NRG method has a number of well-known shortcomings. It fails to properly produce the Fermi liquid state down to T=0 in DMFT and hence it does not properly calculate transport, and it produces only semiquantitative features of the spectral functions. We believe, this is mainly due to the fact that the spectral representation involves a discrete set of delta function peaks at logarithmically discretized frequency intervals which are broadened to the continuum. The broadening parameters are chosen in an {\it ad-hoc} basis. Here we formulate this problem as a discrete degree of freedom embedded in a continuum, which involves coupling the original semi-infinite NRG chain to another semi-infinite chain, arising from the neglected continuum degrees of freedom. This residual coupling can be solved perturbatively and provides an {\it ab initio} approach to constructing spectral functions from the NRG. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y24.00003: Nonequilibrium dynamical mean-field study of correlated electron systems driven by a mono-cycle pulse Naoto Tsuji, Takashi Oka, Hideo Aoki, Philipp Werner A few-cycle pulse can be generated and has been widely used in recent ultrafast optical experiments, but its potential application to correlated electron systems has remained unexplored in contrast to many-cycle pulses. What is characteristic of the few-cycle pulse is to induce a shift of electron's momentum dynamically. To reveal its effect on the electronic properties of the system, we study the single-band Hubbard model driven by half-cycle and mono-cycle pulses using the nonequilibrium dynamical mean-field theory. As an impurity solver, we employ the continuous-time quantum Monte Carlo method and the iterative perturbation theory. We show that when the momentum shift is nearly $\pi$ (half of the Brillouin zone) the shifted population relaxes to a negative-temperature state, where the electron-electron interaction is effectively switched from repulsive to attractive. The shift is found to deviate from the dynamical phase $\phi=\int F(t)dt$ due to electron correlation effects, which suggests that one can generate the repulsion-to-attraction transition by a mono-cycle pulse with $\phi=0$. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y24.00004: PIMC study of spin-polarized 1D trapped fermions with strong attractive contact interaction ChangMo Yang Spin imbalance in a trapped one-dimensional gas of ${}^6$Li atoms ($F=1/2$) is studied with continuous-space path-integral Monte Carlo simulation. This follows closely the experiment of Liao et al. [1], which aims to confirm the existence of the FFLO pairing predicted from the Bethe ansatz [2,3] and DMRG [4,5]. Algorithmic improvements [6] to the configuration-space sampling efficiency of a previous work [7] is made in order to explore the conditions where the attractive contact interaction between unlike-spin atoms can be stronger than in the previously accessible. Signatures of FFLO pairing is looked for in the pair momentum distribution.\\ \\ $\left[1\right]$ Y.-A. Liao et al., Nature \textbf{467}, 567 (2010).\\ $\left[2\right]$ X.-W. Guan et al., Phys. Rev. B \textbf{76}, 085120 (2007).\\ $\left[3\right]$ E. Zhao et al., Phys. Rev. Lett. \textbf{103}, 140404 (2009).\\ $\left[4\right]$ M. Rizzi et al., Phys. Rev. B \textbf{77}, 245105 (2008).\\ $\left[5\right]$ F. Heidrich-Meisner et al., Phys. Rev. A \textbf{81}, 023629 (2010).\\ $\left[6\right]$ M. Boninsegni et al. Phys. Rev. E, \textbf{74}, 036701, (2006).\\ $\left[7\right]$ M. Casula et al. Phys. Rev. A, \textbf{78}, 033607, (2008). [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y24.00005: Stability of Topological Quantum Phases at Zero Temperature Spyridon Michalakis, Justyna Pytel We prove stability of the spectral gap for gapped, frustration-free Hamiltonians under general, quasi-local perturbations. We present a necessary and sufficient condition for stability, which we call {\it Local Topological Quantum Order} and show that this condition implies an area law for the entanglement entropy of the groundstate subspace. This result extends previous work by Bravyi \textit{et al}, on the stability of topological quantum order for the groundstate subspace of Hamiltonians composed of commuting projections with a common zero-energy subspace. Moreover, our result implies that zero-temperature topological order is robust against quasi-local perturbations, for all topologically ordered subspaces that correspond to the groundstate space of a gapped, frustration-free Hamiltonian. Finally, even in the absence of topological order, we show that symmetry-protected sectors are also stable against perturbations respecting the same symmetries. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y24.00006: Recurrence relations and time evolution in the relativistic electron gas at long wavelengths Erica Silva Some years ago M. Howard Lee developed the recurrence relations method to solve the Heisenberg motion equation in an exact way. The relaxation and memory functions, as other linear-response quantities, e.g., the density-density response function and the dynamic structure factor, were obtained for the two- and three-dimensional non-relativistic electronic systems at long wavelengths. In this work we study the time- and frequency-dependent behavior of the relativistic electron gas. As some applications, one can cite graphene, in two dimensions, and dwarf stars, in the three-dimensional case, since both systems have a relativistic electron gas in their compositions. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y24.00007: Combining Tensor Networks with Monte Carlo: Applications to the MERA Andy Ferris, Guifre Vidal Our recent understanding of the entanglement properties of ground states of many-body quantum systems has led to the development of a variety of variational wavefunctions based on tensor networks. The so-called \emph{bond dimension} of the tensor network, $\chi$, sets both the limit to the amount of entanglement allowed in the ansatz as well as the required computational power to contract the tensor network. Because the cost scales very strongly with $\chi$ in higher dimensions, these approaches are currently challenging in 2D and currently unusable in 3D. We present our efforts in combining Monte Carlo techniques with tensor networks to ease the computational bottleneck. Classical Monte Carlo sampling can be used to estimate the contracted value of the network, allowing one to sample expectation values and vary parameters to optimize ground states. In particular, we show a perfect sampling scheme can be efficient for tensor networks which are also unitary quantum circuits. We apply this to the Multi-scale Entanglement Renormalization Ansatz (MERA) in 1D, formally reducing the cost from $O(\chi^9)$ to $O(\chi^5)$ per sample, and demonstrate that we can optimize wavefunctions. We expect the advantages from Monte Carlo sampling will be stronger in 2D and 3D systems. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y24.00008: Recent Progress In Exactly Solvable Discrete Models for Topological Phases in Two Dimensions Yong-Shi Wu, Yuting Hu, Spencer D. Stirling The study of two-dimensional topological phases in condensed matter systems is a frontier in the field of condensed matter theory as well as topological quantum computation. Discrete or lattice models, which are exactly solvble have been proposed by Kitaev and by Levin and Wen, respectively, some years ago. Here we present a summary of recent progress in studying these models and their generalizations. The topics to be covered include 1) Duality between the Kitaev and Levin-Wen models in certain special cases; 2) General procedure for computing ground state degeneracy when the models are put on a topologically non-trivial surface; 3) More detailed study of the properties (exchange and exclusion statistics etc) of topological excitations (e.g. fluxons); 4) General framework for studying constraints of topological invariance on a wide class of discrete models on more general fluctuating graphs; 5) Generalization of these models to general graphs that incorporates more general degrees of freedom. Our approach, though closely related to topological field theory and tensor category theory, could be understood by physicists. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y24.00009: Time Evolution of Density Matrices Using BBGKY Hierarchy Ali Akbari, Javad Hashemi, Risto Nieminen, Robert Van Leeuwen, Angel Rubio Our work starts with the BBGKY hierarchy equations which can be straightforwardly derived from the time-dependent Schr\"odinger equation for each $n$-body reduced density matrices ($n$-RDM). The equations of the BBGKY hierarchy in each level, couple an $n$-RDM to the $(n+1)$-RDM. In order to make this set of equations tractable we need to truncate the hierarchy. While people usually truncate the hierarchy at the first level, one can also truncate it at the level of second equation by approximating $3$-RDM in terms of $2$-RDM and $1$-RDM. Regardless of approximations that we choose, the total energy and momentum will be conserved if we solve the first and second equation together. However, we will show that most of the existing approximations are unstable and even diverging in time and ponder the reasons behind it. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y24.00010: Optimizing Matrix- and Tensor-Product Algorithms for Momentum-Space Hamiltonians using Quantum Entropy Reinhard Noack, \"Ors Legeza, Jen\"o S\'olyom Momentum-space formulations of local models such as the Hubbard model are hard to treat using matrix- and tensor-product-based algorithms because they contain contain non-local interactions. Quantum entropy-based measures such as the single-site and block entropies and the mutual information can be used to map the entanglement structure in order to gain physical information and to optimize algorithms. In this contribution, we will discuss the optimization of density-matrix-renormalization-group and tree-tensor-network algorithms and their application to the two-dimensional Hubbard model. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y24.00011: Topological liquid nucleation induced by vortex-vortex interactions in Kitaev's honeycomb model Ville Lahtinen, Andreas Ludwig, Jiannis Pachos, Simon Trebst We provide a microscopic understanding of the nucleation of topological quantum liquids for interacting non-Abelian anyons by making an explicit connection between the microscopics of the pairwise interactions - typically showing oscillations in sign, but decaying exponentially with distance - and the nature of the collective many-anyon state. We investigate this issue in the context of Kitaev's honeycomb lattice model, where non-Abelian vortex excitations can be arranged on superlattices. Depending on microscopic parameters such as the vortex-spacing, we observe the nucleation of several distinct Abelian topological phases. By reformulating the collective behavior of the interacting vortex superlattice in terms of an effective lattice model of tunneling Majorana fermion zero modes, we show that the pairwise interactions fully determine the phase diagram of the nucleated phases. We find that due to the oscillations longer-range interactions beyond nearest neighbor can influence the nature of the collective state and thus need to be included for a comprehensive microscopic picture. Correspondind results should hold for vortices forming an Abrikosov lattice in a p-wave superconductor or quasiholes forming a Wigner crystal in non-Abelian quantum Hall states. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y24.00012: Optimizing the Hartree-Fock orbitals by the DMRG Mingpu Qin, Honggang Luo, Tao Xiang We have proposed a density matrix renormalization group (DMRG) scheme to optimize the one-electron basis states of molecules. It improves significantly the accuracy and efficiency of the DMRG in the study of quantum chemistry or other many-fermion system with nonlocal interactions. For a water molecule, we find that the ground state energy obtained by the DMRG with only 61 optimized orbitals already reaches the accuracy of best quantum Monte Carlo calculation with 92 orbitals. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y24.00013: Determination of Boundary Scattering, Intermagnon Scattering, and the Haldane Gap in Heisenberg Chains Hiroshi Ueda, Koichi Kusakabe Low-lying magnon dispersion in a $S=1$ Heisenberg antiferromagnetic (AF) chain with boundary $S/2$ spins coupling antiferromagnetically ($J_{\rm end} > 0$) is analyzed by use of the non-Abelian DMRG method. The Haldane gap $\Delta$, the magnon velocity $v$, the inter-magnon scattering length $a$, and the scattering length $a_{\rm b}$ of the boundary coupling are evaluated. The length $a_{\rm b}$, which represents the contribution of boundary effects, depends on $J_{\rm end}$ drastically, while $\Delta$, $v$, and $a$ are constant irrespective of $J_{\rm end}$. Our method estimates the gap of the $S=2$ AF chain as $\Delta = 0.0891623(9)$ using a chain length up to 2048, which is longer than the correlation length. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y24.00014: Fractional exclusion statistics: the paradigm to describe interacting particle systems Anghel Dragos-Victor The thermodynamics and statistical mechanics calculations for systems of interacting particles represent in general a difficult task. Even in relatively simple cases, like systems described in the Fermi liquid theory or in the Hartree or Hartree-Fock approximations, the dependence of the quasiparticle energies on the population of all the quasiparticle energy levels makes it impossible to apply the standard formalism. This is because either the sum of quasiparticle energies is different from the total energy of the system or the typical Bose and Fermi populations do not maximize the partition function. The solution to this problem is provided by the application of the fractional exclusion statistics. In this presentation I will compare the standard treatment of systems of interacting particles, given in terms of the Bose or Fermi populations of the quasiparticle energy levels, with a method based on the fractional exclusion statistics. This method is the only paradigm for describing rigorously the interacting particle systems in terms of quasiparticles. [Preview Abstract] |
Session Y25: Focus Session: Multiscale Modeling
Sponsoring Units: DCOMP DMPChair: Turab Lookman, Los Alamos National Laboratory
Room: 257A
Friday, March 2, 2012 8:00AM - 8:36AM |
Y25.00001: The heterogeneous multiscale method: A ten-year review Invited Speaker: Weinan E The heterogeneous multiscale method (HMM) was proposed 10 years ago, as a unified framework for designing multiscale algorithms in different applications. It is a top-down strategy in the sense that it relies on a preconceived form of macroscale model. Missing data in the macroscale model are estimated on the fly using a reliable microscale model. In this talk, we will take a critical look at HMM. We will discuss applications to various problems, including dynamic fracture, complex fluids, transition pathways in complex systems and stochastic simulation algorithms. We will also examine areas where improvement are needed in order to make HMM more successful. ~ [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 9:12AM |
Y25.00002: Progress Towards Atomistic Simulations that Reach Anthropological Timescale and Beyond Invited Speaker: Ju Li Atomistic and first-principles modeling, which describe the world as assembly of atoms and electrons, provide the most fundamental answer to problems of materials. However, they also suffer the most severe timescale limitations. For instance, in molecular dynamics (MD) simulations, in order to resolve atomic vibrations, the integration time step is limited to hundredth of a picosecond, and therefore the simulation duration is limited to sub-microsecond due to computational cost. Although a nanosecond simulation is often enough (surprisingly) for many physical and chemical properties, it is usually insufficient for predicting microstructural evolution and thermo-mechanical properties of materials. In this invited talk I will discuss recent attempts at overcoming the timescale challenges of atomic-resolution simulations: (a) strain-boost hyperdynamics [Phys. Rev. B 82 (2010) 184114] for simulating primarily displacive events and associated issues of activation entropy and the Meyer-Neldel compensation rule, (b) diffusive molecular dynamics (DMD) [Phys. Rev. B 84 (2011) 054103] for microstructural evolution driven by repetitive diffusion events and coupled displacive-diffusive processes, and (c) a Markovian network statistical mechanical treatment of the energy-landscape basin connectivity and a formula for the viscosity of supercooled liquid and glass [PLoS ONE 6 (2011) e17909]. Challenges and future directions are discussed. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y25.00003: Adaptation of multiscale thermal modeling methods to general crystalline solids Brent Kraczek, Peter Chung From the nano- to micro-scale, phonons act as the primary mechanism for energy transport and storage in most non-metalic, crystalline materials. While phonon properties are determined by interatomic interactions on the scale of the atomistic unit cell, phenomena and microstructures of interest are often at length and time scales that are too large for atomistic simulations. The phonon Boltzmann transport equation (pBTE) enables continuum-scale calculations that capture both atomistic- and continuum-scale thermal behavior, though so far pBTE-based methods have been limited primarily to silicon [1]. We have developed a pBTE-based method for general crystaline materials with up to hundreds of atoms per unit cell. We demonstrate its applicability in silica (9-atom unit cell) and RDX (168-atom unit cell), though presently limited to one dimension in k-space. We discuss the main challenges when moving to larger unit cells, including the automated untangling of phonon dispersion data, discretization of phonon modes and the treatment of high energy modes, including the omission and/or lumping of modes and the use of reservoir modes. \\[4pt] [1] Chunjian Ni and J. Murthy, in \textit{11th Intersociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems, ITHERM}, 2008, 1097 [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y25.00004: Extended Lagrangian Born-Oppenheimer Molecular Dynamics: quantum mecahnical molecular dynamics for extended time and length scales Anders Niklasson, Marc Cawkwell Born-Oppenheimer molecular dynamics (BOMD) based on density functional theory offers a very accurate quantum mechanical approach to atomistic simulations that is more reliable and general compared to classical MD. Unfortunately, BOMD simulations are often limited by a high computational cost or by problems such as unbalanced phase space trajectories, numerical instabilities and a systematic long-term energy drift. These problems become particularly severe in combination with reduced complexity or linear scaling algorithms that are necessary for the study of large systems. We have recently taken some steps toward a new generation of first principles MD, which combines some of the best features of regular BOMD and Car-Parrinello MD, while avoiding their most serious shortcomings. The new dynamics is given in terms of an extended Lagrangian (XL), where auxiliary extended electronic degrees of freedom are added to the nuclear part. Our framework enables accurate geometric integration of both the nuclear and electronic degrees of freedom that provide a time-reversible and energy conserving dynamics on the ground state BO potential energy surface that is stable also under approximate SCF convergence. XL-BOMD provides a surprisingly simple and general framework for atomistic simulations [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y25.00005: Multi scale modeling of atomic layer deposition Mahdi Shirazi, Simon D. Elliott Atomic layer deposition (ALD) is one of the techniques now used to grow conformal nanometer thin films with the quality required for electronic devices. ALD is a type of chemical vapor deposition that depends on self-limiting surface chemistry. Many aspects of chemical reactions and their effect on the stoichiometry of the film remain unclear. We have used therefore density functional theory (DFT) to explore those reactions. The growth of HfO$_{2}$ from H$_{2}$O and Hf(N(CH$_{3}$)$_{2}$)$_{4}$ was regarded as a sample ALD system. The process of densification was explained accurately. A new mechanism of multiple proton diffusion was propounded and DFT energetics showed it to be the most favourable path way. We also found that reaction rates are strongly coupled with the coordination number of Hf atoms at the surface. Then this complex chemistry was implemented in kinetic Monte-Carlo (KMC). The KMC calculation showed the effect of intermediate reactions on the growth rate. Furthermore the morphology of the film and the growth rate under different reaction conditions were compared with experimental data. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y25.00006: A unified approach to preserve structure and thermodynamics in a coarse-grained model of aqueous mixtures Pritam Ganguly, Debashish Mukherji, Christoph Junghans, Nico van der Vegt Biological organizations depend on a sensitive balance of noncovalent interactions, in particular also those involving interactions of small molecules, including inorganic salts and urea, with biomolecules in aqueous solution. Computer simulations of these types of systems require simple-yet-specific models in order to cover all relevant time and length scales. We present a method to systematically coarse-grain liquid mixtures using Kirkwood-Buff theory of solution combined with an iterative Boltzmann inversion technique that infers single-site interaction potentials for the solution components from the pair correlation functions. Our method preserves both the solution structure at pair level and variations of solution components' chemical potentials with compositions within a unified coarse-graining framework. To test the robustness of our approach, we simulated urea-water and benzene-water systems over a wide-range of concentrations. We also observe the coarse-grained potentials to be reasonably transferable with varying concentrations. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y25.00007: Novel mesoscopic approach for modeling Carbon Nanotube System Igor Ostanin, Roberto Ballarini, Traian Dumitrica, David Potyondy We present a distinct spherical element concept for simulating morphologies and mechanical properties of carbon nanotube systems. The important interactions present at the microscopic level are encapsulated into two types of contact models that act simultaneously. Each individual nanotube is coarse-grained into a chain of spherical elements interacting by parallel contact bonds, representing the microscopic covalent bonding. An anisotropic model with aligning moments acts at the contact between elements located in different tubes to represent the van der Waals long-ranged interactions. The accuracy, computational efficiency, and capabilities of the created mesoscopic model are discussed along with illustrative examples, including self-folding of individual nanotubes, mechanical testing of nanotube ropes, self-assembly of a high-porosity nanotube paper, and mechanical testing of a low-porosity nanotube paper. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y25.00008: Structure formation of toluene around C60: Application of the Adaptive Resolution Scheme Christoph Junghans, Sebastian Fritsch, Kurt Kremer In the adaptive resolutions scheme (AdResS) a local, typically all-atom cavity is coupled to a surrounding medium of coarse grained, simplified molecules. This methodology cannot only be used to reduce the CPU time demand of atomistic simulations but also to systematically investigate the relative influence of different interactions on structure formation. For this, we vary the thickness of the all atom layer of toluene around a C60, analyze the first toluene layers and compare the result to a full resolution simulation. With this system, we also introduce the implementation of AdResS for molecular simulations into GroMaCS. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y25.00009: Characterization of phase behavior in assemblies of colloidal nanoparticles Ray Sehgal, Daniel Beltran-Villegas, Michael Bevan, Dimitrios Maroudas, David Ford We report results of a systematic investigation of the phase behavior of clusters of colloidal nanoparticles, which interact via a hard core and depletion attraction potential, as a function of system size and inter-particle interaction strength. To describe the various phases that may be present in such nanoparticle assemblies, we carry out a set of windowed Monte Carlo umbrella sampling (MC-US) simulations to generate free energy landscapes (FELs). For the MC-US generation of the FELs, we use the diffusion mapping method to identify the system's underlying dimensionality and define the dynamically relevant coarse variables. The resulting set of FELs samples broad ranges of the system size and interaction potential strength. These computed FELs describe the phase behavior of the nanoparticle assemblies and allow us to analyze the effects of interaction strength and system size as these assemblies approach the bulk thermodynamic limit. In very small clusters, only a single stable liquid-like phase exists. However, as the number of nanoparticles in the cluster increases, a second crystalline phase emerges in coexistence with the liquid-like phase. The corresponding critical cluster size marks the onset of nucleation of crystalline assemblies of colloidal nanoparticles. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y25.00010: New algorithms for field-theoretic block copolymer simulations: Progress on using adaptive-mesh refinement and sparse matrix solvers in SCFT calculations Scott Sides, Ben Jamroz, Robert Crockett, Alexander Pletzer Self-consistent field theory (SCFT) for dense polymer melts has been highly successful in describing complex morphologies in block copolymers. Field-theoretic simulations such as these are able to access large length and time scales that are difficult or impossible for particle-based simulations such as molecular dynamics. The modified diffusion equations that arise as a consequence of the coarse-graining procedure in the SCF theory can be efficiently solved with a pseudo-spectral (PS) method that uses fast-Fourier transforms on uniform Cartesian grids. However, PS methods can be difficult to apply in many block copolymer SCFT simulations (eg. confinement, interface adsorption) in which small spatial regions might require finer resolution than most of the simulation grid. Progress on using new solver algorithms to address these problems will be presented. The Tech-X Chompst project aims at marrying the best of adaptive mesh refinement with linear matrix solver algorithms. The Tech-X code PolySwift++ is an SCFT simulation platform that leverages ongoing development in coupling Chombo, a package for solving PDEs via block-structured AMR calculations and embedded boundaries, with PETSc, a toolkit that includes a large assortment of sparse linear solvers. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y25.00011: Concurrent multiscale modeling of 3D granular systems Holger Meier, Steven Meier, Fushen Liu, Peter Gordon, Tuan Tran Large-scale granular mechanics simulations are often based on continuum approaches such as the finite element method (FEM). However, these approaches require continuum descriptions of the constitutive relationship between stresses and strains. As a result, grain-scale dynamics are not explicitly considered. Therefore, modeling of large-scale history dependent phenomena due to grain-scale rearrangement and strain localization remains a long-standing challenge. For small-scale studies, discrete element method (DEM) simulations model grain-scale interactions and thus capture history dependent phenomena. However, the application of this approach to large-scale systems is computationally expensive and impractical. We demonstrate a scalable multiscale approach where large-scale granular systems are discretized with the classical FEM simulation, while the necessary constitutive relation is calculated concurrently from DEM simulations of representative volume elements of grains subject to the loading and deformation prescribed by each finite element. [Preview Abstract] |
Session Y26: General Theory / Computational Physics I
Sponsoring Units: DCOMPChair: Amy Bug, Swarthmore College
Room: 257B
Friday, March 2, 2012 8:00AM - 8:12AM |
Y26.00001: Computation of Collision-Induced Absorption by dense Hydrogen-Helium gas mixtures up to Thousands of Kelvin, for Astrophysical Applications Martin Abel, Lothar Frommhold, Xiaoping Li, Katharine L.C. Hunt The interaction-induced absorption by collisional pairs of H$_{2}$ molecules is an important opacity source in the atmospheres of the outer planets and cool stars. The emission spectra of cool white dwarf stars differ significantly in the infrared from the expected blackbody spectra of their cores, which is largely due to absorption by collisional H$_{2}$--H$_{2}$, H$_{2}$--He, and H$_{2}$--H complexes in the stellar atmospheres. Using quantum-chemical methods we compute the atmospheric absorption from hundreds to thousands of kelvin, as required, for example, in astrophysical analyses of objects, including cool white dwarf stars, brown dwarf stars, M dwarfs, cool main sequence stars, solar and extra-solar planets, and the formation of so-called first stars [1]. Comparisons of our calculations with laboratory measurements, which exist only at room temperature and below, show close agreement. \\[4pt] [1] Martin Abel, Lothar Frommhold, Xiaoping Li, and Katharine L. C. Hunt, ``Collision-Induced Absorption by H$_{2}$ pairs: From Hundreds to Thousands of Kelvin,'' J. Phys. A, 2011, 115 (25), pp 6805-6812 [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y26.00002: Microcanonical entropy inflection points and their relationship to cooperative behavior Michael Bachmann We discuss a method for the systematic classification of the analogs of phase transitions in finite systems. This completely general analysis, which is applicable to any physical system, is based on the microcanonical entropy and its energetic derivative, the inverse caloric temperature. Inflection points of this quantity signal cooperative activity and thus serve as distinct indicators of transitions. The microcanonical entropy as the logarithm of the density of states is the fundamental quantity of statistical mechanics and can directly be obtained by means of contemporary generalized-ensemble computer simulation methodologies. Nowadays, the statistics achieved by employing these methods is sufficiently high such that the accuracy of the data allows for a very precise microcanonical analysis of ``phase'' transitions, even in mesoscopic systems, where finite-size and surface effects are significant. This can hardly be achieved by the far more prevalent conventional canonical approach. We demonstrate the power of this method in exemplified applications to long-standing problems of polymer and protein nucleation transitions. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y26.00003: Improving Event Identification in the HF Calorimeter of CMS Christopher Frye, Jeremiah Mans The Forward Hadron Calorimeter (HF) of the Compact Muon Solenoid (CMS) at the Large Hadron Collider (LHC) lies in a region not covered by an inner tracking system, and we can rely only on the shapes of showers that hit the HF to determine whether or not they are due to electromagnetic particles. We review the current method of distinguishing shower types in the HF, and we bring attention to a drawback that will become present as the luminosity of the LHC increases and creates a need for tighter shower-shape cuts. We provide a method to correct this drawback, and we analyze the effectiveness of various tight cuts at isolating signal from background. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y26.00004: The Effect of White Nonstationary and Colored Nonstationary Noise on Signal Detection Mauricio Flores, Matthew Benacquista, Alexander Stroeer We analyze the effect of non-stationary noise on the detection of signals on unevenly sampled data. Initial frequency estimation is obtained from a Lomb-Scargle periodogram; which is followed by a global multi-start optimization, as working on a dense local Nelder-Mead iterator for parameter estimates. It has been found that a varying white noise level has no effect on the required relative signal-to-noise ratio for detection in the proposed algorithm, though affecting the absolute amplitude strength of the signal recording. Further analysis has been done on realistic colored noise. Different whitening routines have been incorporated to the proposed algorithm. Detection efficiency is compared for these different routines. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y26.00005: Deteriorated Quality of Phase Distorted Gaussian Beams Sergiy Mokhov, Leonid Glebov, Boris Zeldovich The divergence of coherent optical beam is often characterized by the parameter $M^{2}$. For one transverse axis, this beam quality parameter is proportional to the product of the minimal observed beam size divided by the wavelength and multiplied by the divergence angle. Furthermore, $M^{2}$ is normalized such that its minimum value equals unity, which is achievable only by a Gaussian beam. An arbitrary phase distortion of a Gaussian beam increases $M^{2}$. Self-phase modulation is a common distortion in which the phase difference across the beam aperture is proportional to the beam intensity profile. For self-phase modulation, the deterioration of the beam quality parameter will depend on only one parameter, the phase at the center of the beam. We have found this dependence analytically. A general phase distortion profile can be represented by higher-order radial phase modes. We have also found the analytical dependence of $M^{2}$ in this generalized case. In addition, we derived expressions for beam quality deterioration of super-Gaussian beams due to phase distortions. If the waists of Gaussian and super-Gaussian beams are defined by a residual power criterion, which means both beams outside the same radius have the same amount of residual power, then a super-Gaussian beam will better tolerate phase distortions. It is important that all of our results cannot be efficiently reproduced by the traditional approach, based on the polynomial representation of aberrations, due to the poor convergence of power series for Gaussian profiles. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y26.00006: Exponential tails near the band edges of a one-dimensional disordered exciton system in the Coherent Potential Approximation Abdelkrim Boukahil, Nouredine Zettili, David Huber We report the results of studies of the tails near the band edges of a one-dimensional Frenkel exciton system in the Coherent Potential Approximation (CPA). A Gaussian distribution of the transition frequencies with rms width $\sigma $ (0.1 $\le \sigma \le $ 2.0) is used. We found that the tails obey two different exponential power laws depending on the value of $\sigma $. In the weak disorder limit 0.1 $\le \sigma <$ 0.5, the tails of the absorption line shape and the density of states behave like $exp(-k|E|^{3/2} / \sigma^2)$, and in the strong disorder limit,\textit{0.5 $< \sigma \le $ 2.0}, the tails behave like $exp(-|E|^2 / \sigma^2)$. In the weak disorder limit, our CPA results are in excellent agreement with previous investigations. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y26.00007: Elastic impurity scattering and relaxation in electronic lattice under finite electric-field Jong Han, Woo-Ram Lee, Kwon Park From the birth of solid state physics, understanding electron transport in solids has been one of the central questions. Recently, rigorous quantum mechanical treatments of solid nonequilibrium have been sought by many groups. Here, we discuss electron transport in tight-binding lattice with disordered potential scatterers when driven by a finite electric field. Based on Floquet formalism with non-perturbative treatment of the electric field, we investigate how the spectral properties evolve when the system is non-dissipative, governed by a \textit{closed} Hamiltonian. As the disorder becomes stronger, the spectra evolve from $\delta$-peaks representing Bloch oscillations to a continuous spectral distribution which is distinctly different from the non-interacting limit. We investigate the fate of electric current in the steady-state nonequilibrium. Finally, we discuss an implementation of energy dissipation channels and the way that the conventional Drude picture is recovered within the Floquet method as a function of Bloch oscillation frequency and electron relaxation-time. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y26.00008: Universal resistance quantum in multichannel transport and dissipative field theory Prasenjit Dutt, Thomas Schmidt, Karyn Le Hur The Landauer formula for coherent DC transport lies at the heart of nanoelectronics and embodies a startling prediction: the quantization of the conductance in one-dimensional metallic wires for ballistic transport, in steps of $R_q^{-1}=e^2/h$ for each channel. Scattering proccesses undergone by the electrons cause a deviation from this result. The resistance then depends on the transparency of the channel and assumes a nonuniversal value. Recently, the unit of resistance $R_q$ has been shown to be a universal feature for AC transport through a single-channel quantum RC circuit with a large cavity. This result can be understood by mapping the system to the one-channel Kondo model and the emergent low-energy Fermi-liquid theory. In a different context $R_q$ arises in a certain nonequilibrium setting for the multichannel quantum RC circuit. In this work, we study AC transport in the many-channel quantum RC circuit. Under certain well-defined conditions the charge relaxation resistance remains universal and equals $R_q$. We study the emergence of this universal resistance in the multi-channel limit by using the mapping with a dissipative particle on a ring and making an analogy with the Kondo model. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y26.00009: Holon-Doublon Dynamics in Hubbard Ladders Luis Dias da Silva, Gonzalo Alvarez, Elbio Dagotto Studies of the dynamics of holon-doublon pairs in Mott insulators have unveiled interesting and contrasting results with their band-insulator counterparts [1,2]. In the 1D Hubbard model, a paradigm for a Mott insulator, numerical evidence suggests that the mechanism for exciton decay into magnetic excitations is inefficient [1]. However, transition metal oxides are usually grown in layered superlattices and a real-time study of the holon-doublon propagation on ladders and other layered structures is therefore needed. In this talk, we present results for the real-time dynamics of holon-doublon pairs propagating in a \textit{two-leg} Hubbard ladder, a more realistic model for several SCMs. We use the time-dependent density matrix renormalization group (tDMRG) algorithm with a time-step-targetting Krylov method. We find that the ladder geometry changes the dynamics of the holon-doublon pair. A ``transfer" of the excitation between the ladder legs is seen, depending on the ratio between the couplings in the two leg directions. Furthermore, the time decay of the total double occupation is modified in the ladder as compared to the 1D case. \\[4pt] [1] K.A. Al-Hassanieh et al., PRL {\bf 100} 166403 (2008).\\[0pt] [2] L. Dias da Silva et al., PRB {\bf 81} 125113 (2010). [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y26.00010: First principles study of the spin-orbit coupling effect on the Tl-Pb superconducting alloy Omar De la Pe\~na-Seaman, Rolf Heid, Klaus-Peter Bohnen We have studied the influence of spin-orbit coupling (SOC) on the phonon dispersion, the electron-phonon (e-ph) coupling and on the superconducting properties for the Pb-Tl alloy in the stable fcc-phase doping regime. This system have been studied within the framework of density functional perturbation theory, using a mixed-basis pseudopotential method and the virtual crystal approximation (VCA) for modeling the alloy. The Eliashberg spectral function ($\alpha^{2}F(\omega)$) and the electron-phonon coupling parameter ($\lambda$) have been calculated with and without SOC. The observed effects of SOC in the full phonon dispersion and $\alpha^{2}F(\omega)$ consist in a softening of the phonon frequencies and an increase of the e-ph coupling matrix elements, which become weaker on the Tl-rich side. SOC enhances $\lambda$ by as much as 48\% in some cases and improves its overall behavior as a function of the concentration for the alloy, leading to a very nice agreement with experimental data from tunneling measurements. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y26.00011: Critical point of phase transitions of fractional order at Superconductors Vladimir Udodov This paper describes the behavior of thermodynamic values superconducting transition at temperature, tending to absolute zero. Using the thermodynamic arguments we demonstrate that superconductors can undergo third, fourth-, fifth- and higher (including fractional) order phase transitions (PT's) within the meaning of Baxter [1] (or Ehrenfest) as the temperature tending to zero. It is proved that the order of PT can be any real (fractional) number at some interval (from 2 to 8). It is established relation between critical exponent of specific heat and critical magnetic field for superconductor as the temperature tending to zero. It is proved that Ehrenfest classification of phase transitions does not work with a zero critical temperature. Note that the results are valid only in the case of thermodynamic equilibrium, making it difficult to reach experimentally at very low temperatures.\\[4pt] [1] Baxter R.J. Exactly Solved Models In Statistical Mechanics. -- London, New York, Sydney, Tokyo, Toronto. -- Academic press. -- 1982. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y26.00012: Numerical simulation of dynamical response in superconducting quantum detectors Yukihiro Ota, Keita Kobayashi, Masahiko Machida, Tomio Koyama, Franco Nori We numerically study the dynamical response of superconducting quantum detectors such as a single-photon detector, using the time-dependent Ginzburg-Landau equation coupled with heat diffusion and Maxwell equations. Our simulation shows a dynamical transition to a resistive state via an incident particle with energy higher than $T_{\rm c}$. We find that such a transition is associated with the generation of vortex-antivortex pairs and the occurrence of a normal-state region within the superconductor. We also discuss the applicability of this approach to a superconducting single-photon detector. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y26.00013: Quantitative examination of out-of-phase mixed holographic gratings Kay-Michael Voit, Hauke Bruening, Mirco Imlau Modern holographic applications require advanced photosensitive materials that particularly obey alterations of the complex permittivity with pronounced amplitudes of both real and imaginary parts on the sub-ps-time scale. Promising candidates such as amorphous and crystalline materials remarkably show a mutual phase-shift between phase and absorption gratings that complicates the analysis of the underlying wave-coupling mechanisms. Hence, theoretical descriptions that are simply based on Kogelnik's coupled-wave theory can not be applied, i.e., a formal approach to systematically derive the full parameter space of the gratings from diffraction efficiency measurements is missing in literature. We revised the analysis of the wave-coupling theory omitting former approximations or applying them later. As a result we derived a formal description for mixed gratings allowing for a full description of beam-coupling experiments. Both the modulations of the refractive index and the absorption coefficient as well as the phase shift between these gratings can be determined through measurements of the angular dependent diffraction efficiency around the positive and the negative Bragg angle. Our approach and results are demonstrated along a mixed grating with a most common parameter set. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y26.00014: Extended phase diagram of ASEP with two types of particles Ayse Yesil, M. Cemal Yalabik First introduced by Evans et al. (PR E, 74 208, (1995)), the Asymmetric Simple Exclusion Process (ASEP) has become an important model in the field. The model is simple in structure, however it offers an oppurtunity to study some of the basic characteristics of nonequilibrium systems. We use mean field, Monte Carlo, and RG methods to present a detailed phase diagram of the ASEP with two types of particles moving in opposite directions. The nature of the spontaneous symmetry broken phases will also be discussed. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y26.00015: Non-Equilibrium Phase Transitions in the Baxter Model Basak Renklioglu, M. Cemal Yalabik The Baxter model in equilibrium is an exactly solved model for which the universality of the static critical phenomena does not hold [1]. The free energy has a branch-point singularity at a phase transition. The exponent of this singularity can range continuously from one to infinity. That is to say, this model has non-universal equilibrium critical exponents [2]. In our study, the non-equilibrium critical dynamics of the Baxter model which is in contact with two thermal baths is analyzed. Monte Carlo methods are applied to the system in which one of the bath is fixed at infinite temperature. The Baxter model is formulated as two interlacing spin-$1/2$ Ising models on a square lattice, interacting through a four spin coupling. The dynamics of the system is taken to be driven by ``spin exchanges'' of the neighbor spins in each lattice. We focus on the phase transitions of the system under this spin exchange dynamics. Preliminary results on the universality class properties of the non equilibrium phase transitions is presented. \\[4pt] [1] R.J. Baxter, Phys. Rev. Lett., {\bf 26}, 14 (1971).\\[0pt] [2] R.J. Baxter, J. Stat. Phys., {\bf 8}, 25 (1973). [Preview Abstract] |
Session Y27: Invited Session: Topological Phases in Magnets
Sponsoring Units: DCMPChair: Achim Rosch, University of Cologne
Room: 258AB
Friday, March 2, 2012 8:00AM - 8:36AM |
Y27.00001: Formation and dynamics of Skyrmions in B20-type chiral magnets Invited Speaker: Naoya Kanazawa The topological stable spin texture called a Skyrmion, in which the directions of the spins wrap a sphere, has been attracting much attention as an arena for unconventional magneto-transport effects. Small angle neutron scattering (SANS) studies identify the formation of two-dimensional triangular Skyrmion lattice in B20-type transition-metal monosilicides, such as MnSi\footnote{S. M\"{u}hlbauer {\it et al}., Science {\bf 323}, 915 (2009).} and Fe$_{1-x}$Co$_x$Si.\footnote{W. M\"{u}nzer {\it et al}., Phys. Rev. B {\bf 81}, 041203 (2010).} In addition, novel transport properties due to its topological spin arrangement, namely the topological Hall effect\footnote{A. Neubauer {\it et al}., Phys. Rev. Lett. {\bf 102}, 186602 (2009).} and the current-induced rotation of the Skyrmion lattice\footnote{F. Joneitz {\it et al}., Science {\bf 330}, 1648 (2010).} are observed in MnSi. We have provided crucial evidence of the existence of Skyrmions in B20-type compounds by direct real-space observation using Lorentz transmission electron microscopy (TEM)\footnote{X. Z. Yu {\it et al}., Nautre {\bf 465}, 901 (2010).}$^,$\footnote{X. Z. Yu {\it et al}., Nautre Materials {\bf 10}, 106 (2011).} and also found a large topological Hall effect in much wider temperature region in MnGe than other B20-type magnets.\footnote{N. Kanazawa {\it et al}., Phys. Rev. Lett. {\bf 106}, 156603 (2011).} TEM observation reveals the detailed information on nucleation and fusion processes and topological defects besides the perfect hexagonal arrangement of Skyrmions. Furthermore, we have found that the Skyrmion lattice state is quite stabilized in a thin-plate formed sample with its thickness smaller than the skyrmion lattice constant. The orders of magnitude larger topological Hall effect in MnGe indicates the high-density Skyrmion crystal formation and distinguishable large responses in some novel electromagnetic phenomena. In addition, we have fabricated B20-type thin films where Skyrmions are more stabilized and applied electric current is more easily controlled than in bulk samples. This work was done in collaboration with X. Z. Yu, Y. Onose, J. H. Park, J. H. Han, N. Nagaosa, K. Kimoto, W. Z. Zhang, S. Ishiwata, Y. Matsui, T. Arima, D. Okuyama, K. Ohoyama, S. Wakimoto, K. Kakurai, A. Tsukazaki, M. Ichikawa, Y. Li, Y. Shiomi, K. Shibata, D. Inosov, J. H. Kim, J. White, N. Egetenmeyer, J. Gavilano, B. Keimer, and Y. Tokura. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 9:12AM |
Y27.00002: Magnetic Blue Phase in the Chiral Itinerant Magnet MnSi Invited Speaker: Dmitry Reznik In MnSi with the cubic crystal structure B20, the lack of inversion symmetry induces a chiral Dzyaloshinsky-Moriya interaction between magnetic moments. It locally favors helical magnetic order. In addition to a long-range helical order with the well-defined propagation vector, other locally helical phases such as skyrmions and partial order appear in its phase diagram. These enigmatic phases are suspected to be behind many unusual properties of MnSi. I will report results of model calculations, which show that such phases should arise naturally as a result of the basic Dyaloshinsky-Moriya Hamiltonian. Specifically, in the absence of an applied magnetic field, the free energy in MnSi should be minimized by the magnetic analogue of blue phases, which have previously been observed in chiral liquid crystals. This result is consistent with our recent neutron scattering measurements as well as those of others. The properties of this blue phase explain a number of previously reported puzzling features of MnSi such as partial magnetic order and a two-component specific-heat as well as, possibly, non-Fermi-liquid resistivity at high pressure. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:48AM |
Y27.00003: Quasi-two-dimensional Skyrmion lattices and other exotic structures in confined chiral nematic liquid crystals Invited Speaker: Slobodan Zumer Skyrmions are localized particle-like topological entities in a number of continuous fields that play important roles in various condensed matter systems, including two-dimensional electron gases exhibiting the quantum Hall effect, Bose--Einstein condensates, and chiral ferromagnets [1, 2]. Here we are using Landau - de Gennnes theoretical approach and numerical techniques to show that in a highly chiral nematic liquid crystal confined to a thin film between two parallel surfaces imposing normal alignment nematic director can exhibit thermodynamically stable states characterized by quasi-two-dimensional Skyrmion lattices of disclinations [3]. By confining a chiral nematic, that in bulk forms blue phases characterized by cubic lattices of nematic defect lines, to a layer with thickness comparable to the lattice constant of the blue phase [3-5], various quasi 2D defect lattices can be stable. Depending on the anchoring direction of the nematic director on confining surfaces, temperature, and layer thickness, beside skyrmion structures, lattices of double helical half-integer dislinations running along the layer [4], and lattices of ring defects [5] are the most interesting. A chiral nematic liquid crystal film can thus serve as a model system, allowing direct investigation of numerous defect lattices by a variety of optical techniques at conditions that are less demanding than used for other condensed matter Skyrmion systems. \\[4pt] [1] U.K. R\"{o}{\ss}ler, A. N. Bogdanov, {\&} C. Pfleiderer, Spontaneous skyrmion ground states in magnetic metals. Nature 442, 797 (2006). \\[0pt] [2] A. Hamann, D. Lamago, Th. Wolf, H. v. L\"{o}hneysen, and D. Reznik, Magnetic Blue Phase in the Chiral Itinerant Magnet MnSi, Phys. Rev. Lett. 107, 037207 (2011). \\[0pt] [3] J. Fukuda and S. Zumer, Quasi-two-dimensional Skyrmion lattices in a chiral nematic liquid crystal, Nature Communications 2, 246 (2011). \\[0pt] [4] J. Fukuda and S. Zumer, Novel defect structures in a strongly confined liquid-crystalline blue phase. Phys. Rev. Lett. 104, 017801 (2010). \\[0pt] [5] J. Fukuda and S. Zumer, Ring Defects in a Strongly Confined Chiral Liquid Crystal, Phys. Rev. Lett. 106, 097801 (2011). [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:24AM |
Y27.00004: Unusual Hall effect Anomaly in MnSi under pressure Invited Speaker: Minhyea Lee Recent works in B20 type transition metal compounds have revealed a new topological object in spin systems -- the skyrmion, a particle-like object in which spins point all directions to wrap around the sphere. While neutron scattering and scanning probe experiments\footnote{S. M\"{u}hlbauer {\it et al.,} Science {\bf 323,} 915 (2009).}$^,$\footnote{X. Z. Yu, {\it et al.}, Nature, {\bf 465,} 901 (2010).} confirmed the existence of individual skyrmions and skyrmion lattices in a particular part of the phase diagram, the interaction between skyrmions and electronic degrees of freedom remains to be unveiled. In this talk, we report the observation of a highly unusual Hall current in the helical magnet MnSi under pressure.\footnote{M.Lee {\it et al.,} Phys. Rev. Lett {\bf 102,} 186601 (2009).} In addition to the normal Hall effect and the anomalous part that arises from spontaneous magnetization, the Hall conductivity displays a distinctive stepwise field profile quite unlike any other Hall response observed in solids. This additional contribution was observed in a much larger range of temperature and applied field than the so-called $A$-phase,where the skyrmion lattice was observed in ambient pressure. It suggests that fluctuating, {\it i.e.} non-static, skyrmions might be present over a broad range of the phase diagram under pressure when the magnetic ordering becomes weakened. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 11:00AM |
Y27.00005: Cosmic Strings Meet Multerroics: Understanding topological defects in spontaneous symmetry breaking phase transitions Invited Speaker: Sinead Griffin Jumping from the expanse of galactic scales to land in the laboratory might seem a gargantuan task. Common to both, however, is the the concept of symmetry breaking and in particular the formation of topological defects. Here I discuss the formation of topological defects in multiferroic YMnO3 whose ferroelectric behavior enables the direct imaging of these defects. I also show how this material can be used to study the Kibble-Zurek model of topological defect formation in the early universe and give quantitative insights on the number of domains formed during the spontaneous symmetry breaking phase transition. [Preview Abstract] |
Session Y29: Focus Session: Quantum Optics with Superconducting Circuits: Nonlinearity and Itinerant Microwave Photons
Sponsoring Units: GQIChair: Andreas Wallraff, ETH Zurich
Room: 259A
Friday, March 2, 2012 8:00AM - 8:12AM |
Y29.00001: Microwave Photon Counter Based on Josephson Junctions Guilhem Ribeill, David Hover, Umeshkumar Patel, Yung-Fu Chen, Robert McDermott We describe progress in the development of a microwave photon counter based on current biased Josephson junctions; absorption of a single photon causes the junction to switch to the voltage state, producing a large and easily measured classical signal. We have combined multiple junctions with a broad-band, on-chip microwave beam splitter to realize a multiplexed microwave photon detector. We discuss application of the Josephson microwave counter to the study of full counting statistics of the microwave emission from various mesoscopic conductors, and we describe alternative biasing schemes to enable operation in more traditional photon counting modes. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y29.00002: Quantum efficiency of a single microwave photon detector based on a current-biased Josephson junction Amrit Poudel, Robert McDermott, Maxim Vavilov We analyze the quantum efficiency of a single microwave photon detector based on a current-biased Josephson junction. We consider the Jaynes-Cummings Hamiltonian to describe coupling between the photon field and the junction. We then take into account coupling of the junction and the resonator to the environment. Numerically solving the equation of motion of the density matrix of the resonator-junction system, we compute the quantum efficiency of the photon detector as a function of detection time, bias current and the junction's decay time. For current-biased Josephson junctions, the efficiency to detect a single photon with frequency in the microwave regime is around 50\%. Our results also indicate that a highly efficient single microwave photon detector is feasible for a moderate improvement in the junction's decay time. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y29.00003: Backaction of Microwave Photon Detection by a Strongly Coupled Josephson Junction Emily Pritchett, Luke Govia, Seth Merkel, Frank Wilhelm We analyze the functionality of on-chip Josephson junctions as single microwave photon detectors, as has been demonstrated recently in Chen, {\it et al.}, arXiv:1011.4329. The Josephson junction device, which we refer to as a Josephson Photomultiplier (JPM), acts as a nearly perfect binary detectors of microwave photons by undergoing an observable switching event when there are one or more photons in an incident cavity. We analyze the backaction of this switching event on the state of incident light, including the energy dissipation and dephasing affecting an imperfect JPM. This analysis improves the efficiency and fidelity with which a JPM reconstructs the state of light in an incident transmission line `cavity', which are commonly used to store and transfer quantum states in implementations of circuit-QED. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y29.00004: Cooperative effects for Qubits in a Transmission Line: Theory K. Lalumi\`ere, A. Blais, B.C. Sanders, A.F. Van Loo, A. Fedorov, A. Wallraff Strong extinction of the transmitted power in a 1D transmission line coupled to an artificial atom has recently been achieved [1]. In contrast to the 3D case, large extinctions are made possible by the strong light-matter coupling occurring because of reduced dimensionality. Motivated by this, here we consider the situation where multiple artificial atoms (ie transmon qubits) are coupled to the 1D line. Following the work of Lehmberg for the 3D case [2], we obtain a master equation describing the dynamics of an arbitrary number of qubits coupled to the line. This master equation reveals interaction between the qubits mediated by the line. Using the input-output formalism, the model is compared to experimental results for multiple qubits coupled to the 1D line. [1] O. Astafiev et al., Science 327, 840 (2010) [2] R. H. Lehmberg. Phys. Rev. A 2, 883 (1970). [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y29.00005: Cooperative Effects for Qubits in a Transmission Line: Experiment A.F. van Loo, A. Fedorov, K. Lalumi\`{e}re, B.C. Sanders, A. Blais, A. Wallraff The interaction probability between freely propagating photons and atoms or atom-like systems is greatly enhanced in one dimension. Thus a system of many atoms coupled to a one-dimensional continuum of electromagnetic modes is expected to reveal many interesting phenomena - the photons emitted by an atom can be absorbed by other atoms and coherently interact with propagating modes of the continuum. We implement three superconducting qubits coupled strongly to an open transmission line to investigate such light-matter interactions in one dimension. We characterize our system by scattering radiation off the qubits and measuring the transmitted and reflected field. For low driving powers, a single qubit reflects nearly all incident radiation resonant with its transition frequency [1]. When two qubits are tuned such that their emitted radiation has a wavelength close to twice the distance between the qubits we observe interference effects in the reflection and transmission spectra. At high driving powers resonance fluorescence is measured for single qubits and multiple qubits in resonance. These results present first steps towards investigating cooperative effects for multiple qubits in open one-dimensional space.\\[4pt] [1] O.~Astafiev {\em et~al.}, Science, {\bf327}, 840 (2010) [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y29.00006: Hong-Ou-Mandel Interference in Circuit QED Experiments Matthew Woolley, Christian Lang, Christopher Eichler, Andreas Wallraff, Alexandre Blais The Hong-Ou-Mandel (HOM) effect is a quantum interference effect whereby two indistinguishable photons incident at either side of a balanced beam splitter will be detected together at one output port or the other, but never with one photon at each output port. Such experiments have long been performed in the optical domain, but recent developments have raised the possibility of performing such experiments in the microwave domain, using linear amplifiers and quadrature amplitude detectors instead of photon counting [Bozyigit \emph{et al.}, Nat. Phys. \textbf{7}, 154-158 (2010)]. Here we determine the signature of HOM interference in a system consisting of two independent circuit QED systems out-coupled into an on-chip microwave beam splitter. We have calculated the beam splitter output intensity auto- and cross-correlations for both trains of pulsed Lorentzian photons, and continuously-driven sources based on photon blockade. The HOM interference is manifest as antibunching in the output intensity cross-correlation. Controllable distinguishability may be introduced via a time delay in the pulsed case, or via a frequency offset in the continuously-driven case. The frequency offset leads to a quantum beat effect. Preliminary experimental results will be discussed. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y29.00007: Generating distributable and unconditional entanglement on-chip at microwave frequencies W.F. Kindel, H.S. Ku, S. Glancy, G.C. Hilton, K.D. Irwin, E. Knill, L.R. Vale, K.W. Lehnert Entanglement is a critical requirement for quantum teleportation protocols. In a strategy to generate entanglement between two separate microwave lines, we integrate two Josephson Parametric Amplifiers (JPAs) and a quadrature hybrid onto a single chip making the entangler circuit. When two squeezed states created by the JPAs are combined on the hybrid (microwave beam splitter), the two output modes are entangled. We observe entanglement using a quantum efficient two channel quadrature measurement device. In our initial tests, the degree of entanglement has been limited by undesirable coupling among the elements of our entangler circuit. We present our investigation of the undesired coupling along with design strategies to reduce it. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y29.00008: Quantifying distributable and unconditional entanglement at microwave frequencies H.S. Ku, W.F. Kindel, S.C. Glancy, E. Knill, L.R. Vale, G.C. Hilton, K.D. Irwin, K.W. Lehnert Unconditional and distributable entanglement can be created by combining a squeezed state and a vacuum state at a beam splitter. We create a single integrated circuit designed to pursue this strategy at microwave frequencies. The squeezed state is created with a Josephson Parametric Amplifier and then combined with a vacuum state in a hybrid (microwave beam splitter) producing entanglement of the output modes. In this talk, we will present the measurement and quantification of entanglement between separate microwave transmission lines. We quantify the entanglement and estimate the fidelity when applying this entangled state as a quantum teleportation channel. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y29.00009: Microwave photonics and Josephson junction arrays Juan J. Mazo, David Zueco, Enrique Solano, Juan J. Garcia-Ripoll We present an architecture for a photonic crystal in the microwave regime based on superconducting transmission lines interrupted by Josephson junctions. A study of the scattering properties of a single junction in the line shows that the junction behaves as a perfect mirror when the photon frequency matches the Josephson plasma frequency. We generalize our calculations to periodic arrangements of junctions, demonstrating that they can be used for tunable band engineering, forming what we call a quantum circuit crystal. As a relevant application, we discuss the creation of stationary entanglement between two superconducting qubits interacting through a disordered media. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y29.00010: Modeling of a Flying Microwave Qubit Kyle Keane, Alexander N. Korotkov We investigate the transfer efficiency of sending a flying microwave qubit through a transmission line from one resonator to another resonator. Our model is based on the current technology of coupling superconducting phase qubits to microwave resonators and transmission lines. Analytical as well as numerical results are presented. Procedural imperfections have been modeled, including weak detuning, imperfect timing, and deviations from the ideal time dependence of the coupling modulation. The effects of multiple reflections within the transmission line and energy dissipation in the system are also considered. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y29.00011: A Four-wave Mixing Toolbox For Photon State Manipulation in Superconducting Resonators A.V. Sharypov, Xiuhao Deng, Lin Tian We present a circuit scheme to generate quantum operations on superconducting resonators by engineering effective interaction Hamiltonians. We show that both the linear Bogoliubov transformations, including the beam-splitter operation, the squeezing operation, and the phase shifter, and the nonlinear interactions such as the cross-Kerr interaction can be realized with one single circuit. Our circuit is composed of two superconducting qubits coupled with each other to form a quantum four-level system. Each qubit interacts directly with one superconducting resonator. We exploit the four-wave mixing (FWM) approach and use the circuit as a toolbox to generate the above-mentioned quantum operations by controlling circuit parameters with external sources. Using numerical simulations to study the error rates, we show that the transformations can be realized with high fidelity. Arbitrary quantum operations on the microwave photons can be realized by combining these effective interactions. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y29.00012: Dephasing and Kerr type interaction effects in circuit quantum electrodynamics Eran Ginossar, Steven Girvin There has been recently a significant advance in obtaining high quality factor resonators in superconducting circuit architectures. The reduction of the resonator line width motivates us to consider subtle Kerr type interaction effects in small clusters of cavities and transmon type qubits. The Kerr interaction leads to entanglement of cavities, which in the transient regime is manifested in collapse-revival dynamics. For longer time scales, the interaction of the system with its environment becomes important and we discuss how the entangled states are modified. The signal of this steady-state Kerr interaction is a multi-photon port-to-port scattering process which can be observed in homodyne measurements or in a spectral analysis (correlations). We discuss the relevance of these effects to the challenge of building quantum memories. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y29.00013: Dynamic quantum Kerr effect in circuit quantum electrodynamics Yi Yin, Haohua Wang, Matteo Mariantoni, Rami Barends, Radoslaw C. Bialczak, Yu Chen, Mike Lenander, Julian Kelly, Erik Lucero, Anthony Megrant, Peter O'Malley, Daniel Sank, Jim Wenner, Ted White, Andrew Cleland, John Martinis In the dispersive regime of circuit quantum electrodynamics (QED), where the qubit and resonator frequencies differ slightly, photons in the resonator exhibit induced frequency and phase shifts. The qubit-state dependent phase shift is usually measured by monitoring the resonator transmission spectrum at fixed qubit-resonator detuning. In this static scheme, the phase shift can only be monitored in the far-detuned, linear dispersion regime, in order to avoid measurement-induced demolition of the quantum state. By using a dynamic procedure to adiabatically drive the qubit frequency, here we are able to explore the dispersive interaction over a much broader range, and we further monitor the interaction using resonator Wigner tomography. Exotic non-linear effects on different photon states, e.g., Fock states, coherent states and Schrodinger cat states, are thereby directly revealed. Correspondingly, we demonstrate a quantum Kerr effect in the dynamic framework in circuit QED. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y29.00014: Shaping an Itinerant Quantum Field into a Multimode Squeezed Vacuum by Dissipation Juan Jose Garcia-Ripoll, Diego Porras This work shows how to create tunable continuous sources of single and multimode squeezed light by controlling single emitters coupled to propagating modes of the EM field. Our work builds on recent experiments that implement the main tools of cavity Quantum Electrodynamics (QED) using superconducting qubits coupled to microwave transmission lines, as well as quantum dots coupled to microcavity photons, or plasmons. The main results of this letter, presented in sequential order are: A multicolor driving of an artificial atom modifies its coupling to the EM field, inducing sidebands. Combining the sidebands with an auxiliary bath, a single qubit may cool a quantum field in a single mode cavity to a squeezed vacuum. If instead of a cavity, the driven qubit is placed in a waveguide, the high energy modes play the role of a dissipative bath and the result is tunable multimode squeezing of the propagating quantum field. Through the manuscript we will also discuss implementations, measurement schemes and further outlook. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y29.00015: Effects of nonlinearity in the emission spectrum of the driven nonlinear oscillator Michael Marthaler, Stephan Andre, Lingzhen Guo, Vittorio Peano, Gerd Sch\"on Motivated by many experiments on nonlinear driven systems realized using superconducting circuits we investigate the properties of a coherently driven nonlinear resonator. By using Josephson junctions in superconducting circuits, strong nonlinearities can be engineered, which lead to a relatively low number of photons in the resonator and the appearance of pronounced nonlinear effects. Based on a master equation approach, which takes into account the quantum nature of noise, we determine the emission spectrum and observe for typical circuit QED parameters, in addition to the primary side-peaks, second-order peaks not predicted by a linearized theory. [Preview Abstract] |
Session Y30: Quantum Computing, Quantum Algorithms, and Quantum Simulation
Sponsoring Units: GQIChair: Liang Jiang, Caltech
Room: 259B
Friday, March 2, 2012 8:00AM - 8:12AM |
Y30.00001: Phase Slips in Topological Superconductor Wire Devices Samuel Goldberg, Doron Bergman, David Pekker, Gil Refael We make a detailed study of phase slips in topological superconducting wires and devices based on topological wires. We begin by investigating a device composed of a topological superconducting wire connected to a non-topological wire (T-S). In the T-segment only slips of the phase by multiples of 4$\pi $ are allowed, while in the S-segment slips by 2$\pi $ are also allowed. We show that near the interface, 2$\pi $ phase slips are also allowed and we comment on the consequences of such phase slips for the Aharonov-Casher effect. We also consider an implementation of a q-bit consisting of a T-S-T device, where the quantum information is stored in the parity of the two topological segments via the four Majorana modes. We show that the central S-segment of this type of device can support 2$\pi $ phase-slips which result in the decoherence of the q-bit. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y30.00002: ABSTRACT WITHDRAWN |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y30.00003: String localization and delocalization in the disordered toric code Pejman Jouzdani, Eduardo R. Mucciolo Topological quantum memories based on the toric code model have the ability to protect quantum information by self correcting a large class of errors. However, excitations such as a string of spin flips, when allowed to perform a quantum walk, can change the logical state encoded in the system every time they wind around the torus. It has been proposed that by adding randomness to the local spin exchange couplings, one can localize these string excitations and avoid logical errors. In our work, we investigate this proposal numerically through the use of an efficient time-dependent numerical quantum evolution method. We determine the dependence of the winding time on the torus size and on the amount of randomness. We study the effect of dephasing in the quantum evolution of the string excitations and show that a transition to delocalization can occur. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y30.00004: Implementing quantum gates through scattering between an electron and a magnetic impurity in a graphene nanoribbon Guillermo Cordourier-Maruri, Romeo de Coss, Sougato Bose We study the feasibility of implementing quantum logic gates and generate entanglement when a ballistic electron is scattered by a magnetic impurity fixed in a graphene nanoribbon. Because electrons in graphene behaves like massless Dirac Fermions, we use the Dirac equation to describe the system. In our model we consider the ballistic electron spin as a relativistic flying qubit and the impurity spin as a static qubit. The interaction between spins is described by a Heisenberg - like operator. The interaction by electron scattering shows the advantage of a low control in the interaction, and the operation success can be measured by means the electron transmition probability. We show that is possible to implement quantum logic gates of the type SWAP, and partial SWAP with a physicaly feasible coupling strenght between spins. We also present the condition to generate states of maximum entanglement. The possible use of the graphene pseudospin as an additional degree of freedom is discussed. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y30.00005: Long-lived qubit constructed from three identical atoms Rui Han, Hui Khoon Ng, Niels L\"orch, Jun Suzuki, Berge Englert In this talk, I will present a scheme for constructing logical qubits from clusters of three identical atoms that are long-lived against decoherence from fluctuating magnetic fields, a limiting source of noise in many experiments. Each qubit is stored in a rotationally invariant subsystem of the total angular momentum states of the three atoms, and can persist with high fidelity for time-scales on the order of hours. This is to be compared with a fraction of a millisecond for an unprotected atomic qubit. I will first present the scheme of rotationally invariant subsystems in atomic systems and show that the information stored in the system is robust against decoherence. Then I will move on to discuss a proposal for an experiment to demonstrate the feasibility of the scheme. In our proposal, the state preparation is done with the help of Rydberg blockade for three atoms, where the atoms are localized in space and addressed by a sequence of laser pulses simultaneously. By carefully selecting the atomic levels addressed and tuning the parameters of the applied lasers, an arbitrary logical qubit state can be prepared. Lastly, the fidelity of state preparation will be discussed. Ref: R. Han, N. L\"{o}rch, J. Suzuki and B. G. Englert, Phys. Rev. A 84, 012322 (2011) [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y30.00006: Deterministic and Cascadable Conditional Phase Gate for Photonic Qubits Christopher Chudzicki, Isaac Chuang, Jeffrey Shapiro Cross-phase modulation (XPM) at the single-photon level, if strong enough, would enable a simple conditional $\pi$-phase gate for photonic qubits. Together with easily realized single-qubit rotations for such qubits, this would provide a universal gate set for quantum computation. However, previous analyses of photonic conditional $\phi$-phase gates that treat XPM in a causal, multimode, quantum field setting suggest that a large ($\sim$$\pi$\,rad) nonlinear phase shift is always accompanied by fidelity-degrading noise. We present a conditional phase gate that, for sufficiently small nonzero $\phi$, has high fidelity. Moreover, our gate is cascadable, in that it preserves the structure of the principal modes used to encode qubit information, and can therefore be cascaded to realize a high-fidelity conditional $\pi$-phase gate. The key components of our gate are: (1) an atomic $\vee$-system to create XPM; (2) a principal-mode restorer that compensates the evolution a principal-mode incurs when the $\vee$-system is driven by a single photon; and (3) a principal-mode projector that exploits the quantum Zeno effect to preclude the accumulation of fidelity-degrading departures from the principal-mode Hilbert space when both control and target photons illuminate the gate. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y30.00007: Dynamic Phases and Robust Quantum Gates Yasushi Kondo, Tsubasa Ichikawa, Masamitsu Bando, Mikio Nakahara We are interested in composite pulses widely employed in Nuclear Magnetic Resonance (NMR) and geometric phase gates (GQGs) with vanishing dynamic phases in Quantum Information Processing (QIP). A composite pulse in NMR is constructed with poor quality pulses but becomes more reliable than its components. We found: a composite pulse robust against pulse length error in NMR is always a GQG [1]. We then extended this observation to two-qubit operations. Let us consider the interaction $e^{-i \theta \sigma_z \otimes \sigma_z}$ and assume that there is a systematic error in $\theta$. When we construct a ``composite pulse'' robust against this error, we obtain a two-qubit GQG [2]. We clarified that geometric phase gates are really useful in QIT. \\[4pt] [1]Y.\ Kondo \& M.\ Bando, {\it J. Phys. Soc. Jpn.} {\bf 80}, 054002.\\[0pt] [2] T.\ Ichikawa, M.\ Bando, Y.\ Kondo \& M.\ Nakahara, submitted to {\it philosophical transaction} A. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y30.00008: Non-Adiabatic Holonomic Quantum Gates in an atomic system Vahid Azimi Mousolou, Carlo M. Canali, Erik Sjoqvist Quantum computation is essentially the implementation of a universal set of quantum gate operations on a set of qubits, which is reliable in the presence of noise. We propose a scheme to perform robust gates in an atomic four-level system using the idea of non-adiabatic holonomic quantum computation proposed in [1]. The gates are realized by applying sequences of short laser pulses that drive transitions between the four energy levels in such a way that the dynamical phases vanish. \\[4pt] [1] E. Sjoqvist, D.M. Tong, B. Hessmo, M. Johansson, K. Singh, arXiv:1107.5127v2 [quant-ph] [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y30.00009: Single-qubit gates by graph scattering Michael S. Underwood, Benjamin A. Blumer, David L. Feder Continuous-time quantum walkers with tightly peaked momenta can simulate quantum computations by scattering off finite graphs. We enumerate all single-qubit gates that can be enacted by scattering off a single graph on up to $n=9$ vertices at certain momentum values, and provide numerical evidence that the number of such gates grows exponentially with $n$. The single-qubit rotations are about axes distributed roughly uniformly on the Bloch sphere, and rotations by both rational and irrational multiples of $\pi$ are found. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y30.00010: Recent Results in Photonic Quantum Computations, Simulations and Quantum Networks Philip Walther The applications of photonic entanglement manifold and reach from quantum communication [1] to quantum metrology [2] and optical quantum computing [3]. The advantage of the photon's mobility makes optical quantum computing unprecedented in speed, including feed-forward operations with high fidelity [4]. During the last few years the degree of control over photonic multi-particle entanglement has improved substantially and allows for not only overcoming the random nature of spontaneous emission sources [5], but also for the quantum simulation of other quantum systems. Here, I will also present the simulation of four spin-1/2 particles interacting via any Heisenberg-type Hamiltonian [6]. Moreover, recent experimental and theoretical progress, using the concepts of measurement-based quantum computation, indicates that photons are best suited for quantum networks. I will also present present results for the realization for such a client-server environment, where quantum information is communicated and computed using the same physical system [7]. References: [1] PRL 103, 020503 (2009); [2] Nature 429, 158 (2004); [3] Nature 434, 169 (2005); [4] Nature 445, 65 (2007); [5] Nature Photon 4, 553 (2010); [6] Nature Physics 7, 399 (2011); [7] in press. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y30.00011: Physics of an isolated electron puddle revealed via dephasing in thermal equilibrium Emil Weisz, Hyungkook Choi, Oktay G\"okta\c{s}, Moty Heiblum, Yuval Gefen, Vladimir Umansky, Diana Mahalu Low dimensional electron systems serve as a good setup for studying interactions among quantum systems. In our study, we examined a system comprised of an electron puddle, confined in a quantum dot, coupled to an electronic Mach-Zehnder interferometer via Coulomb interactions. Surprisingly, even when the electron puddle was in thermal equilibrium and nearly isolated, it induced full and robust dephasing in the nearby interferometer when the average puddle's occupation was N+1/2. We attribute this unexpected behavior to a unique manifestation of the Friedel Sum Rule, which connects the occupation of a system with its scattering phase. Furthermore, this phenomena allowed accessing various properties of the isolated electron puddle, such as its average occupation, in thermal equilibrium and under bias, and decoherence rate of the confined electrons. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y30.00012: Quantum trajectories for systems probed by fields in multimode Fock and Schrodinger cat states Joshua Combes, Ben Q. Baragiola, Agata M. Branczyk, Robert L. Cook, John E. Gough, Matthew R. James, Hendra I. Nurdin Using Gardiner and Collet's input-output theory we derive system and output field master equations for an arbitrary quantum system probed by a field in a non-classical state of light. Specifically the field states we study are arbitrary combinations (superpositions and / or mixtures) of continuous-mode Fock states or continuous mode-coherent states. We also unravel the master equations for the system state to get the conditional evolution (the stochastic master equation) for homodyne and photon counting measurements. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y30.00013: Optimal Experimental Detection and Characterization of SU(2) Decoherence Lee A. Rozema, Ardavan Darabi, Dylan H. Mahler, Aephraim M. Steinberg, Robin Blume-Kohout Quantum metrology is the art of measuring tiny forces by preparing quantum states, then measuring how much they get displaced by the force. The resulting precision often exceeds classical limits. The best state depends on what we want to detect. N00N states [1] are optimal for U(1) phase shifts. Measuring arbitrary dynamical shifts requires quantum process tomography (QPT)[3], in which the process is applied to a complete set of input states. In this work, we experimentally study the important intermediate case of SU(2) decoherence (SD), fluctuating SU(2) rotations, by probing it with biphotons [2]. We show that N00N states are optimal for detecting SD. Then, we turn to QPT, and examine how accurately SU(2)-covariant sets of states can identify SD. The set of spin-coherent states (generated by SU(2)-displacements of a ``highest-weight'' state) are sufficient for QPT [4], but exponentially insensitive to some parameters. We show that N00N states (though optimal at detecting SD) generate even less effective input sets, with zero sensitivity to some parameters. Finally, we show evidence that optimal input sets are 2-designs, which can be generated from a fiducial state and SU(2) rotations.\\[4pt] [1] PRL 85,2733(2000) [2] Nature 457,67(2009) [3] PRL 91,120402(2003) [4] Science 322,563(2008) [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y30.00014: Complete characterization of linear amplifiers including the quantum limits for nongaussian noise Shashank Pandey, Zhang Jiang, Joshua Combes, Carlton Caves We characterize the quantum limitations on the entire probability distribution of added noise in a phase-preserving linear amplifier. Previously the quantum limits on amplifiers have been given entirely in terms of second moments, i.e., noise power or noise temperature [1]. As Josephson parametric amplifiers approach fundamental quantum limits on noise temperature [2,3,4], it becomes important to investigate the limits on higher moments of the amplifier noise. We prove that all phase-preserving linear amplifiers with arbitrary noise are formally equivalent to a parametric amplifier: $\rho_{out}= {\rm tr}[S(r) \,\rho_{in}\otimes \sigma\, S^{\dag}(r)]$, where the gain is $g^{2}= \cosh^{2}(r)$, $S$ is a two-mode squeeze operator, and $\sigma$ is a physical state of an ancillary mode whose quantum noise determines the noise properties of the amplifier. We discuss generalization of these limits to the nondeterministic linear amplifiers proposed by Ralph and Lund [5]. [1] C. M. Caves, Phys. Rev. D {\bf 26}, 1817 (1982). [2] A. A. Clerk et al., Rev. Mod. Phys. {\bf 82}, 1155-1208 (2010). [3] N. Bergeal et al., Nature 465, 64--68 (2010). [4] D. Kinion and John Clarke Appl. Phys. Lett. 98, 202503 (2011). [5] T. C. Ralph and A. P. Lund, in QCMC Vol.~1110 of AIP Conf. Proc. (2009). [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y30.00015: Playing the Aharon-Vaidman quantum game with a Young type photonic qutrit Piotr Kolenderski, Urbasi Sinha, Youning Li, Tong Zhao, Matthew Volpini, Adan Cabello, Raymond Lafalmme, Thomas Jennewein The Aharon-Vaidman (AV) game exemplifies the advantage of using simple quantum systems to outperform classical strategies. We present an experimental test of this quantum advantage by using a three-state quantum system (qutrit) encoded in a spatial mode of a single photon passing through a system of three slits. The preparation of a particular state is controlled as the photon propagates through the slits by varying the number of open slits and their respective phases. The measurements are achieved by placing detectors in the specific positions in the near and far-field after the slits. This set of tools allowed us to perform tomographic reconstructions of generalized qutrit states, and implement the quantum version of the AV game with compelling evidence of the quantum advantage. [Preview Abstract] |
Session Y31: Focus Session: Topological Insulators: Synthesis & Characterization - Optical & THz Spectroscopy
Sponsoring Units: DMPChair: Kenneth Burch, University of Toronto
Room: 260
Friday, March 2, 2012 8:00AM - 8:12AM |
Y31.00001: Topological surface state dispersion measured using THz magneto-ellipsometry Jason N. Hancock, J.L.M. van Mechelen, Alexey Kuzmenko, Dirk van der Marel, Christophe Brune, Elena Novik, Georgy Astakhov, Hartmut Buhmann, Laurens Molenkamp We present a magneto-optical study of the three-dimensional topological insulator, strained HgTe. Using polarization-sensitive time-domain THz spectroscopy in a magnetic field, reliable information on the Drude weight and cyclotron resonance frequency severely constrain the details of surface state dispersion within 1meV of the Fermi level. Details of the technique and its prospect for future observation of axion electrodynamics using THz spectroscopy will also be discussed. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y31.00002: Terahertz Kerr Measurements of the Surface States on n- and p-type Bi$_2$Se$_3$ Topological Insulators G.S. Jenkins, A.B. Sushkov, D.C. Schmadel, M.-H. Kim, K.M. Bhamidipati, P. Syers, J. Paglione, H.D. Drew, N.P. Butch, J.G. Analytis, I.R. Fisher We report terahertz magneto-optical characterization of a single surface state on bulk crystals of n-type Bi$_2$Se$_3$ and p-type Bi$_2$Se$_3$ doped with Mg as well as Sm, breaking time reversal symmetry and opening a gap at the Dirac point. A gate is used to apply an electric field which creates and modulates a small depletion layer. The modulated Fermi energy of the surface state produces differential optical signals with no contribution arising from bulk carriers. The real and imaginary parts of the differential Kerr angle yield the transport scattering rate, spectral weight, and mass of the surface state carriers. Comparison with ARPES and other transport measurements will be discussed. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y31.00003: Nonlinear optical responses to circularly polarized lights of the surface state of a topological insulator Tetsuro Misawa, Takehito Yokoyama, Shuichi Murakami Recent photoelectron spectroscopy experiments have revealed the presence of the Dirac cone on the surface of the topological insulator and its spin-splitting due to the spin-orbit interaction. In general, on spin-orbit coupled systems, electric fields induce spin polarizations as linear and nonlinear responses. Here we investigate the inverse Faraday effect on the surface of the topological insulator. The inverse Faraday effect is a non-linear optical effect where a circularly polarized light induces a dc spin polarization. We employ the Keldysh Green's function method to calculate the induced spin polarization and discuss its frequency dependence. In particular, in the low frequency limit, our analytical result gives the spin polarization proportional to the frequency and the square of the lifetime. As for the finite frequency regime, we employ numerical methods to discuss the resonance due to interband transitions. We also discuss the photogalvanic effect, where an illumination of a circular polarized light generates the dc charge current. Lastly, we evaluate those quantities with realistic parameters.\\[4pt] [1] T. Misawa, T. Yokoyama, S. Murakami, Phys. Rev. B84, 165407 (2011). [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y31.00004: Optical properties of highly resistive Bi$_2$Te$_2$Se Anjan Reijnders, S. Y. F. Zhao, Louise Niemeyer, Raphael Bouskila, H. J. Cho, Seyedyara Mohajerani, Luke J. Sandilands, Robert J. Cava, Kenneth S. Burch Topological insulators are characterized by strong spin-orbit coupling, producing metallic surface states while the bulk remains insulating. Experimental observations of surface states in transport and optical measurements have been obstructed by bulk contributions due to sample defects, placing the Fermi level in the bulk energy gap. Bi$_2$Te$_2$Se (BTS), a three-dimensional topological insulator, has recently been shown to exhibit a bulk resistivity larger than any other topological insulator measured to date. However, quantum oscillations are still observed, and attributed to the surface states. In this talk I will present the optical properties of BTS, obtained by broadband FTIR spectroscopy combined with ellipsometry, over a range of 20 - 45,000 $cm^{-1}$, as well as the Raman spectrum of BTS. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y31.00005: Quasiparticle dynamics of topological insulator Bi2Se3 studied by time and angle-resolved photoemission spectroscopy Yihua Wang, David Hsieh, Edbert Sie, Dillon Gardner, Hadar Steinberg, Young Lee, Pablo Jarillo-Herrero, Nuh Gedik Topological insulator is a new state of matter that hosts spin helical surface states that may be important for future spintronic applications. Even though the conical bandstructure of the helical Dirac fermions is well established, elastic and inelastic scattering rates of their excitations are less well known because conventional transport or optical techniques can not easily separate surface contribution from bulk effects. Here we use time and angle-resolved photoemission spectroscopy to study the surface electron time-domain dynamics of a prototypical topological insulator Bi$_{2}$Se$_{3}$. We observe non-Fermi liquid behavior of quasiparticles both above and below the Fermi level. Our result suggests that surface-bulk scattering can play an important role in the transport properties of topological insulator. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y31.00006: Ultrafast Bulk and Surface Dynamics of Bi2Se3 Measured by Time-Resolved ARPES Jonathan Sobota, Shuolong Yang, James Analytis, Yulin Chen, Ian Fisher, Patrick Kirchmann, Zhi-Xun Shen We investigated the nonequilibrium carrier dynamics of the topological insulator Bi$_2$Se$_3$ using femtosecond time- and angle- resolved photoemission spectroscopy. Optical excitation leads to a metastable population of bulk carriers due to the presence of the bandgap. We discuss the coupling of these carriers to the Dirac surface state, which results in a long-lived nonequiilibrium surface carrier distribution. We will comment on the implications for topological insulator device applications. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y31.00007: Optical and magneto-optical spectra of Bi$_{1-x}$Sb$_x$ with x = 0.015 M.S. Wolf, S.V. Dordevic, N. Stojilovic, S.S. Vujatovic, M.V. Nikolic, P.M. Nikolic, L.C. Tung Bismuth and its alloys with antimony have attracted attention in recent years due to possible realization of topological insulating state. In this study we have used infrared and magneto-optical spectroscopies to probe the electrodynamic response of bismuth doped with 1.5 $\%$ of antimony. The spectra will be presented for temperatures down to 5 K, and in magnetic fields as high as 18 Tesla. The results reveal strong magneto-optical activity, especially around the plasma minimum in reflectance. These findings will be compared and contrasted with magneto-optical results on pure bismuth. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y31.00008: Magneto-reflectance of Bi$_2$Se$_3$ in 18 Tesla fields S.V. Dordevic, M.S. Wolf, N. Stojilovic, H. Lei, C. Petrovic, L.C. Tung Magneto-optical measurements can provide valuable information in studies of topological insulators. We will report magneto-reflectance spectra of Bi$_2$Se$_3$ in magnetic fields up to 18 Tesla. Magnetic-field induced changes in reflectance are most pronounced around the plasma minimum and around the 60 cm$^{-1}$ mode. Such large magneto-optical activity is unusual for a phonon, and might indicate coupling to collective modes of magnetic origin. Model fits will provide deeper insight into evolution of optical functions with magnetic field. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y31.00009: Photo-induced Chiral Edge Current in the 3D Topological Insulator Bi$_{2}$Se$_{3}$ Ricky Roy, Mao Li, Grant Aivazian, Wang Yao, David Cobden, Chuanwei Zhang, Xiaodong Xu We perform scanning photocurrent (PC) microscopy measurements on the 3D topological insulator Bi$_{2}$Se$_{3}$, and observe spatially separated PC peaks of opposite sign, localized along the sample edges. The physics origin of this experimentally observed two-way PC is attributed to the chiral nature of the TI surface states. We show that the coupling between the optical field and the topological surface states yields a spin population imbalance along the edges of the surface, leading to the first order chiral edge charge current observed in the experiment. Further experimental observations of the weak polarization and strong temperature dependence of the PC agree with the theoretical predications. The PC fades away at a low temperature indicating a mean-free path of $\sim $ 3 $\mu $m for topologically protected surface spins. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y31.00010: Terahertz studies and colossal Kerr rotation in the topological insulator Bi$_2$Se$_3$ Rolando Vald\'es Aguilar, A.V. Stier, L. Wu, L.S. Bilbro, W. Liu, N.P. Armitage, N. Bansal, M. Brahlek, S. Oh, D.K. George, J. Cerne, A.G. Markelz We report a study of high quality MBE grown Bi$_2$Se$_3$ topological insulator thin films using time domain terahertz spectroscopy (TDTS). We explicitly demonstrate the 2D character of the response by studying films of different thicknesses. In addition, we take advantage of a unique feature of TDTS that allows to measure the time structure of the THz pulses. In this way we measure the Faraday and Kerr rotation angles in a single experiment. We find an unprecedentedly large value of the Kerr rotation that is due to the cyclotron resonance of the 2D Dirac fermions \footnote{R. Vald\'es Aguilar, \textit{et al}. Arxiv:1105.0237.}. We will also show results on the effect of exposure of the thin films to atmospheric conditions for prolonged periods of time. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y31.00011: Size effect in Bi2Se3 Topological Insulator micro and nanoparticles Fangze Liu, Eugen Panaitescu, Swastik Kar 3D Topological insulators are electronic materials that have a semiconducting bulk interior but a metal-like (zero-gap) surface. We present the investigations of optical transitions in bulk and nanoscale structures of the Topological Insulator Bi$_{2}$Se$_{3}$, using optical absorption spectroscopy over a range of energies between 1.1 eV and 6.5 eV. Bi$_{2}$Se$_{3}$ micro and nanoparticles were obtained by a sequence of ultrasonication steps developed by our group for this purpose. Investigations were performed on particles of different size scales ranging from 10 microns to sub-25 nanometers. While the bulk samples demonstrated expected absorption properties, the smallest (sub-25 nm) particles showed the appearance of a number of new high-energy absorption edges. The fabrication, detailed characterization, and possible reasons for the appearance of the high-energy excitation states will be discussed within the framework of its electronic band-structure description [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y31.00012: Temperature-Dependence in Optical Properties of Topological Insulator Bi$_{1.5}$Sb$_{0.5}$Te$_{1.8}$Se$_{1.2}$ Chi Sin Tang, Xing Quan Zou, Bin Xia, Mingyi Liao, Lan Wang, Elbert E.M. Chia Topological Insulators are materials with insulating bulk states and conducting states on their edge or surface. Using Terahertz Time-Domain Spectroscopy (THz-TDS), we studied the temperature-dependence of the optical properties of Bi$_{1.5}$Sb$_{0.5}$Te$_{1.8}$Se$_{1.2}$ single-crystals from 5~K to 300~K in the terahertz regime ($0.4$ THz to $3.0$ THz). We observed a spectral weight shift from low frequencies to frequencies above 1.0 THz in the real conductivity at temperatures below $\sim$100 K. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y31.00013: Efficient THz emission from a topological insulator surface Li-Guo Zhu, Brian Kubera, Kin Fai Mak, Jie Shan Bi$_{2}$Se$_3$ is a 3D topological insulator (TI) recently confirmed by the ARPES.\footnote{Hsieh et al. Nature 460, 1101 (2009).} Direct optical probe of its metallic surface states is, however, hindered by the remnant Drude response of the bulk material. Second-order nonlinear optical techniques with their surface specificity provide unique opportunities for studying surface electronic transitions in TIs such as Bi$_{2}$Se$_3$ with bulk inversion symmetry.\footnote{Hsieh et al. Phys. Rev. Lett. 106, 057401 (2011).} Here we demonstrate efficient THz emission from the surface of Bi$_{2}$Se$_3$ under the excitation of a femtosecond optical pulse. The emission arises from optical rectification of the optical pulse at the TI surface and the transient current within the surface depletion region. By spectrally resolving the emission under different pump and emission polarizations, we separate the different contributions. Effects arising from just a few atomic layers of the sample surface due to resonance enhancement of the quasi-real optical transitions between the surface electronic states will be discussed. [Preview Abstract] |
Session Y32: Topological Insulators: General Theory
Sponsoring Units: DCMPChair: Bodgan Bernevig, Princeton University
Room: 261
Friday, March 2, 2012 8:00AM - 8:12AM |
Y32.00001: Z$_Q$ topological invariants for identification of short rangeentangled states Isao Maruyama, Sho Tanaya, Yasuhiro Hatsugai Since the Berry phase is quantized as $Z_2$ value, i.e., 0 or $\pi$, due to the time-reversal, or lattice-inversion symmetry in any dimension, the quantized Berry phase[1] is useful for characterization of a topological or quantum order in various models including strongly correlated electron systems[2] and spin systems[3]. Recently, we have proved $Z_Q$ quantization of Berry phases for the general lattice symmetry, where $Z_{Q}$ ($Q=d+1$) Berry phases are defined for $d$-dimensional lattices: Polyacetylene, Kagome and Pyrochlore lattice respectively for $d=1,2$ and $3$.[4]. We have also characterized the dimer-plaquette transition of the orthogonal dimer model in two dimension[5]. [1]{Y.Hatsugai, J. Phys. Soc. Jpn, {\bf 75}, 123601 (2006).} [2]{IM, Y. Hatsugai , J. Phys. Soc. Jpn, {\bf 76}, 113601 (2007).} [3]{IM, T.Hirano, Y.Hatsugai, Phys. Rev. B. {\bf 79}, 115107 (2009)}[4]{Y.Hatsugai, IM, Euro. Phys. Lett., {\bf 95}, 20003 (2011) }[5] I.Maruyama, S.Tanaya, M.Arikawa, Y.Hatsugai, J. Phys.:Conf. Ser., {\bf 320}, 01219 (2011) [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y32.00002: Trace index and spectral flow in the entanglement spectrum of topological insulators Aris Alexandradinata, Taylor Hughes, Andrei Bernevig We investigate the entanglement spectra of topological insulators, which manifest edge states on a lattice with spatial boundaries. In the physical energy spectrum, a subset of the edge states that intersect the Fermi level translates to discontinuities in the trace of the single-particle entanglement spectrum, which we call a ``trace index.'' We find that any free-fermion topological insulator that exhibits spectral flow has a nonvanishing trace index, which provides us with a new description of topological invariants. In addition, we identify the signatures of spectral flow in the single-particle and many-body entanglement spectrum; in the process we present new methods to extract topological invariants and establish a connection between entanglement and quantum Hall physics. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y32.00003: Axions strings and Goldstone-Wilczek currents in quantum spin systems and topological insulators Akihiro Tanaka Axion, originally a high-energy physics entity, has in recent years come into focus in the condensed matter community due to its incarnation in the physics of topological insulators. Much of the discussion so far has concentrated on the physics at the interface/junction of these insulators, which can be viewed as axion domain walls. Here we look into a slightly different situation: axion strings, which are vortex configurations of the axion fields. We discuss how these objects can arise in quantum spin systems and topological insulators,along with their electrodynamic and elastic implications. We also point to some novel Goldstone-Wilczek fermionic currents which can be relevant to the systems considered. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y32.00004: Casimir Interaction Between Topological Insulator Thin Films and Graphene Sheets Wang-Kong Tse, A.H. MacDonald The Casimir effect is a peculiar manifestation of quantum vacuum fluctuations of the electromagnetic field, resulting in an attractive force between closely-spaced conductors. Because of the advent of new materials like graphene and topological insulators, it is of fundamental and potentially also of practical interest to investigate the Casimir effect between conductors described by two-dimensional Dirac models. In this work we study the Casimir effect between topological insulator thin films and graphene planes in the presence of a time-reversal breaking perturbation, most practically an external magnetic field, that gives rise to a half-quantized quantum Hall effect. We evaluate the Casimir energies and forces from the reflection properties of the double layer system. We demonstrate the essential role that is normally played by the Dirac system's dissipative longitudinal conductivity which is neglected in topological field theory descriptions of the Casimir effect. We also show that repulsive Casimir forces are possible in the presence of a magnetic field. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y32.00005: Exact duality between symmetry protected topological order and intrinsic topological order Zhengcheng Gu, Michael Levin The discovery of topological insulator(TI) motivates the intensive study of symmetry protected topological(SPT) order. Different from the intrinsic topological order, SPT order is only distinguishable from a trivial disorder phase when certain symmetry is preserved. Indeed, SPT order has a long history in 1D, it has been shown the well known Haldane phase of S=1 Heisenberg chain belongs to this class. However, in higher dimensions, most of the previous studies focus on free electron systems with a time reversal symmetry. Until very recently, it was realized that SPT order also exists in interacting boson/spin systems in higher dimensions. In this talk, I will show in 2D boson/spin systems, there exists an interesting duality map between the intrinsic topological order and the SPT order. The duality map implies the SPT orders are stable and distinguishable against arbitrary perturbations if the symmetry is preserved. I will focus on a simplest exact solvable model with an Ising symmetry and discuss the nature of its symmetry protected low-lying edge excitations. The duality map can of course be generalized into arbitrary symmetry group $G$ in any dimension. In principle, the duality map is also applicable for interacting fermion/electron systems. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y32.00006: Asymmetric spatial structure of zero modes for birefringent Dirac fermions Malcolm P. Kennett, Bitan Roy, Peter M. Smith We study the zero energy modes that arise in an unusual vortex configuration involving both the kinetic energy and an appropriate mass term in a model which exhibits birefringent Dirac fermions as its low energy excitations. We find the surprising feature that the ratio of the length scales associated with states centered on vortex and anti-vortex topological defects can be arbitrarily varied but that fractionalization of quantum numbers such as charge is unaffected. We discuss this situation from a symmetry point of view and present numerical results for a specific lattice model realization of this scenario. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y32.00007: Location of Singularities in tight-binding wave function of loop-ordered states with Chern numbers Yan He, Chandra Varma As a pedagogic exercise, we consider time-reversal violating tight-binding wave-functions for Haldane states in the Graphene model and in a Copper-Oxide model. We locate the singularities in the wave-functions and derive the movement in their location as well as the change in the Berry phase with the choice of Gauge. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y32.00008: Effects of spin-orbit interaction on a triangular lattice potential patterned two-dimensional electron gas Chon-Saar Chu, Wie-Lin Su, RongSing Chang We have studied theoretically the electronic states of a two-dimensional electron gas (2DEG) in the presence of a triangular lattice of muffin-tin potentials. In particular, spin-orbit interaction due to the in-plane potential gradient is included. At the $K $and $K$' points in the Brilluoin zone, the Dirac cones open up a gap and their respective Berry curvatures are of the same sign which, in turn, depends on the electron spin. This contrasting feature, from that of graphene, is related to the fact that inversion symmetry is preserved in our case. The system thus exhibits Z$_{2}$ physics. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y32.00009: Smooth gauge for topological insulators Alexey Soluyanov, David Vanderbilt We develop a technique for constructing Bloch functions for ${Z}_2$ quantum spin-Hall insulators that are smooth functions of ${\bf k}$ on the whole Brillouin-zone torus. As the initial step, the occupied subspace of the insulator is decomposed into a direct sum of two ``Chern bands,'' i.e., topologically non-trivial subspaces with opposite Chern numbers. This decomposition remains robust independently of underlying symmetries or specific model features. Starting with the Chern bands obtained in this way, we construct a topologically non-trivial unitary transformation that rotates the occupied subspace into a direct sum of topologically trivial subspaces. The possibility of using such a transformation is validated, and the entire procedure is illustrated, by applying it to the Kane-Mele model. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y32.00010: Stable nontrivial $Z_2$ topology in ultrathin Bi (111) films: a first-principles study Zheng Liu, Chao-Xing Liu, Yong-Shi Wu, Wen-Hui Duan, Feng Liu, Jian Wu Recently, there have been intense efforts in searching for new topological insulator (TI) materials. Based on first-principles calculations, we find that all the ultrathin Bi (111) films are characterized by a nontrivial $Z_2$ number independent of the film thickness, without the odd-even oscillation of topological triviality as commonly perceived. The stable nontrivial $Z_2$ topology is retained by the concurrent band gap inversions at multiple time-reversal-invariant k-points and associated with the intermediate inter-bilayer coupling of the multi-bilayer Bi film. Our calculations further indicate that the presence of metallic surface states in thick Bi(111) films can be effectively removed by surface adsorption. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y32.00011: Breakdown of Landau-Ginzburg-Wilson Scheme by Spontaneous Orbital Currents in Zero-gap Semiconductors Moyuru Kurita, Youhei Yamaji, Masatoshi Imada Critical phenomena of phase transitions are classified into a small number of universality classes. For understanding such symmetry-breaking transitions, a Landau-Ginzburg- Wilson (LGW) scheme greatly helps us. Here, we study the critical phenomena of topological Mott insulator (TMI)[1] where spontaneous orbital currents induced by electron correlation bring the system to topological insulator by formally following the spirit of LGW. Then an unconventional criticality surprisingly emerges around the quantum critical point of TMI: It is governed not only by the symmetry as in the LGW framework but also by topology through electron dispersions and spatial dimension. We demonstrate it by studying an extended Hubbard model on several lattices such as honeycomb and pyrochlore lattice[2] within the Hartree-Fock mean-field level. Both fully numerical calculations and analytical calculations using effective band dispersions are performed. \\ $[1]$ S. Raghu {\it et al}., Phys. Rev. Lett. {\bf 100} 156401 (2008) \\ $[2]$ M. Kurita {\it et al}., J. Phys. Soc. Jpn {\bf 80} 044708 (2011) [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y32.00012: Transition in topological phase of Ge-Sb-Te and Ge-Bi-Te heterostructures Jeongwoo Kim, Jinwoong Kim, Ki-Seok Kim, Seung-Hoon Jhi We studied the transition of topological phase of ternary chalcogenide compounds using first-principles methods. Our calculations show that they undergo a transition between band insulator and topological insulator phases upon the variation of atomic composition and layer thickness. In order to understand the transition, we developed a model Hamiltonian of 1D Dirac fermionic superlattice. We introduced to the Hamiltonian an effective interaction between Dirac fermions that is mediated by the electrons from the band insulating layers. We obtained the phase diagram of the transition as a function of materials parameters including layer thickness, band gap, and Dirac fermionic mass gap. We also discussed the implication of our ressults to the conducting properties of chalcogenide compounds [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y32.00013: Nontrivial topological effects on the surface of strong and weak topological insulators and superconductors Ching-Kai Chiu, Michael Stone We investigate states on the surface of strong or weak topological insulators and superconductors that have been gapped by a magnetic material or by a charge density wave. The surface of a strong 3D topological insulator gapped by a magnetic material is well known to possess a half quantum Hall effect. Furthermore, a recent paper (arXiv:1110.3420) showed that the surface of a weak 3D topological insulator gapped by charge density wave has a half quantum spin Hall effect. We generalize these results to all classes of topological insulator and superconductors. We find that if, in d-1 dimension and that symmetry class, there is a $Z$ or $Z_2$ topological invariant, then the resulting surface state may have a nontrivial topological phase. [Preview Abstract] |
Session Y40: Focus Session: Cytoskeleton and Biomechanics - Role of Motors (Including DBIO Best Thesis Award Lecture)
Sponsoring Units: DBIO DPOLYChair: Arpita Upadhyaya, University of Maryland
Room: 156A
Friday, March 2, 2012 8:00AM - 8:12AM |
Y40.00001: Forces driving three-dimensional tissue patterning during morphogenesis Heng Lu, Adam Sokolow, U. Serdar Tulu, Daniel Kiehart, Glenn Edwards Dorsal closure is an essential stage of \textit{Drosophila} embryogenesis and is a model system for \textit{in-vivo} investigations of cell sheet morphogenesis. During closure a system of four biological processes work collectively to close a gap in the epithelium, which initially is filled with a transient tissue. The geometry of the dorsal opening is similar to that of two intersecting circular arcs being pulled apart at a nearly constant rate. Substantial progress in understanding the dynamics has been made in the past by largely viewing closure as a two-dimensional process. However, tissue and cell dynamics are not confined to the embryo surface. We have been investigating the three-dimensional kinematics of dorsal closure by imaging the actomyosin purse strings at the periphery of the dorsal opening and by imaging the apical belts of DE-cadherin in each cell within the opening. We have analyzed the results with the methods of analytic geometry. In addition, in the past we have determined the relative magnitudes of the forces that drive dorsal closure. We have been using magnetic tweezers, time-resolved \textit{in-vivo} microscopy, and biophysical modeling to measure the net force and to determine the magnitude of each force. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y40.00002: Cell Autonomous Shape Changes in Germband Retraction Holley Lynch, Elliott Kim, Robert Gish, M. Shane Hutson Germband retraction involves the cohesive movement and regulated cellular mechanics of two tissues on the surface of fruit fly embryos, the germband and the amnioserosa. The germband initially forms a `U' shape, curling from the ventral surface, around the posterior of the embryo, and onto the dorsal surface; the amnioserosa lies between the arms of this `U'. Retraction straightens the germband and leaves it only on the ventral side. During retraction, the germband becomes clearly segmented with deep furrows between segments, and its cells elongate towards the amnioserosa, along what becomes the dorsal-ventral axis. To determine the importance of these changes for the overall movement of the tissues, we observed embryos that did not complete germband retraction due to targeted laser ablation of half the amnioserosa. Without the chemical and mechanical influence of the amnioserosa, germband furrows still formed and germband cells still elongated; however, this elongation was misaligned compared to unablated embryos. Thus, furrow formation and cell elongation in the germband are autonomous, but insufficient to drive proper tissue motion. These results suggest that part of the necessary role of the amnioserosa is proper orientation of germband cell elongation. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y40.00003: Wavefronts and mechanical signaling in early Drosophila embryos Timon Idema, Julien Dubuis, Lisa Manning, Philip Nelson, Andrea Liu Mitosis in the early syncytial Drosophila embryo has a high degree of spatial and temporal correlations, visible as mitotic wavefronts that travel across the embryo. This mitosis wavefront is preceded by another wavefront which corresponds to chromosome condensation. The two wavefronts are separated by a time interval that is independent of cell cycle and propagate at the same speed for a given embryo in a given cycle. We study the wavefronts in the context of excitable medium theory, using two different models, one with biochemical signaling and one with mechanical signaling. We find that the dependence of wavefront speed on cell cycle number is most naturally explained via a mechanical signaling, and that the entire process suggests a scenario in which biochemical and mechanical signaling are coupled. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y40.00004: Passive cellular microrheology in developing fruit fly embryos Sarah Crews, Xiaoyan Ma, Stacey Lawrence, M. Shane Hutson The development of fruit fly (\textit{Drosophila)} embryos involves spatial and temporal regulation of cellular mechanical properties. These properties can be probed \textit{in vivo }using laser hole drilling experiments; however, this technique only infers relative forces. Conversion to absolute forces requires measurement of cellular viscoelastic properties. Here, we use passive microrheology of fluorescently labeled cell membranes to measure the viscoelastic properties of amnioserosa cells. These dynamic epithelial cells play an important mechanical role during two developmental stages: germ band retraction and dorsal closure. Passive microrheology in this system is confounded by active contractions in the cytoskeleton. Thus, the fruit fly embryos are transiently anesthetized with CO$_{2}$, halting active cellular movements, leaving only passive Brownian motion. The power spectra of these fluctuations are well fit by a Lorentzian -- as expected for Brownian motion -- and allow us to extract cellular viscoelastic parameters at different developmental stages. These measured parameters inform previous hole-drilling experiments and provide inputs for quantitative computational models of fruit fly embryonic development. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y40.00005: Remodeling of cellular cytoskeleton drives tissue level morphogenesis Madhav Mani, Thomas Lecuit, Boris Shraiman Mechanical stresses are central to morphogenesis, both as a cause that generates geometric and topological change, and as regulatory signals that couple cells. Live imaging of fluorescently tagged tissues gives us insight into the cellular processes underlying tissue dynamics during morphogenesis. Amongst these is the remodeling of the cytoskeleton and cellular adhesion. Here, following observations from \textit{drosophila} germ band extension and ventral furrow formation, we a) investigate the mechanical state of the tissue b) perform a quantitative analysis and verification of the cell and tissue level stresses and c) determine how conserved cellular processes are regulated to generate tissue level stresses that drive morphogenesis. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y40.00006: Investigation of autonomous cell dynamics using holographic laser microsurgery Aroshan Jayasinghe, M. Shane Hutson Laser-microsurgery has emerged as a powerful technique for evaluating \textit{in vivo} tissue mechanics. We extend this technique by using a spatial light modulator (SLM) to diffract a single UV laser pulse to simultaneously ablate multiple points in living tissue. Using this method, we can quickly and cleanly isolate a single cell by destroying all its nearest neighbors. The post-ablation dynamics of such an isolated cell are then largely dependent on autonomous intracellular forces. Here, we use this technique to investigate cell shape pulsations in amnioserosa cells in \textit{Drosophila} embryos during dorsal closure -- specifically to address the degree to which these pulsations are cell autonomous or driven by the contractions of neighboring cells. When cells are isolated at different points in the pulsation cycle, we find that the post-isolation dynamics are strongly dependent on the pre-isolation pulsation phase: cells in a contractile phase collapse immediately, but cells in an expansionary phase continue to expand -- at least for 20-60 s before collapsing. These results are in conflict with previous pulsation models that place expanding cells under large extensional strain, and instead suggest that even the expansion phase has a significant cell autonomous component. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:48AM |
Y40.00007: DBIO Best Thesis Award: Mechanics, Dynamics, and Organization of the Bacterial Cytoskeleton and Cell Wall Invited Speaker: Siyuan Wang Bacteria come in a variety of shapes. While the peptidoglycan (PG) cell wall serves as an exoskeleton that defines the static cell shape, the internal bacterial cytoskeleton mediates cell shape by recruiting PG synthesis machinery and thus defining the pattern of cell-wall synthesis. While much is known about the chemistry and biology of the cytoskeleton and cell wall, much of their biophysics, including essential aspects of the functionality, dynamics, and organization, remain unknown. This dissertation aims to elucidate the detailed biophysical mechanisms of cytoskeleton guided wall synthesis. First, I find that the bacterial cytoskeleton MreB contributes nearly as much to the rigidity of an \textit{Escherichia coli} cell as the cell wall. This conclusion implies that the cytoskeletal polymer MreB applies meaningful force to the cell wall, an idea favored by theoretical modeling of wall growth, and suggests an evolutionary origin of cytoskeleton-governed cell rigidity. Second, I observe that MreB rotates around the long axis of \textit{E. coli}, and the motion depends on wall synthesis. This is the first discovery of a cell-wall assembly driven molecular motor in bacteria. Third, I prove that both cell-wall synthesis and the PG network have chiral ordering, which is established by the spatial pattern of MreB. This work links the molecular structure of the cytoskeleton and of the cell wall with organismal-scale behavior. Finally, I develop a mathematical model of cytoskeleton-cell membrane interactions, which explains the preferential orientation of different cytoskeleton components in bacteria. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y40.00008: Coupling of Active Motion and Advection Shapes Intracellular Cargo Transport Philipp Khuc Trong, Jochen Guck, Raymond Goldstein Three different mechanisms can contribute to intracellular cargo transport: (1) passive diffusion, (2) active motor-driven transport along cytoskeletal filament networks and (3) passive advection by fluid flows. Active and advective transport are coupled because cytoplasmic flows can arise through entrainment of the fluid that surrounds actively moving cargo on cytoskeletal networks. Here, we report a reaction-advection-diffusion model for transport of a passive mass-conserved scalar that can cycle between a bound state, where advection represents active transport on a cytoskeletal network, and an unbound state, where the advecting fluid flow field is driven by forces from the cytoskeletal network. Cargo transport and localization patterns are explored for different cytoskeletal network topologies and for varying reaction kinetics. We find that for sufficiently low diffusion, localization of cargo to a target area is optimized either by low reaction kinetics and decoupling of bound and unbound state, or by a mostly disordered cytoskeletal network with only weak directional bias. The principles exemplified by this model likely have implications for our understanding and interpretation of transport patterns and cytoskeletal network structures. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y40.00009: Motion in partially and fully cross-linked F-actin networks Eliza Morris, Allen Ehrlicher, David Weitz Single molecule experiments have measured stall forces and procession rates of molecular motors on isolated cytoskeletal fibers in Newtonian fluids. But in the cell, these motors are transporting cargo through a highly complex cytoskeletal network. To compare these single molecule results to the forces exerted by motors within the cell, an evaluation of the response of the cytoskeletal network is needed. Using magnetic tweezers and fluorescence confocal microscopy we observe and quantify the relationship between bead motion and filament response in F-actin networks both partially and fully cross-linked with filamin We find that when the transition from full to partial cross-linking is brought about by a decrease in cross-linker concentration there is a simultaneous decline in the elasticity of the network, but the response of the bead remains qualitatively similar. However, when the cross-linking is reduced through a shortening of the F-actin filaments the bead response is completely altered. The characteristics of the altered bead response will be discussed here. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y40.00010: Measurements of the constituent contributions to the physical properties of fibroblast populated collagen microtissues with magnetic micro-tissue stretchers Ruogang Zhao, Alan Liu, Thomas Boudou, Christopher Chen, Daniel Reich The mechanical properties of fibroblast populated collagen matrix (FPCM) provide important physical cues to regulate physiological and pathological processes of encapsulated cells. The mechanical strength of FPCM is arises from both of its constituents: the collagen matrix and the fibroblasts. Existing methods to separate the contribution of individual constituents by treating cm-scale tissue samples with decellularization drugs for prolonged periods have been shown to adversely affect the properties of the collagen matrix. To minimize such matrix damage, we have developed a magnetic microtissue stretching system that allows us to grow arrays of sub-mm scale microtissues that can be rapidly decellularized. This consists of arrays of paired micro-cantilevers that support the 3D FPCM and can be driven by incorporated magnetic material via externally applied magnetic fields. By measuring the tensile force applied to the FPCM and the tissue strain, we found the stiffness of the matured FPCM is 28.1 +- 1.8 kPa and that of the decellularized collagen matrix is 23.1 +- 3.1 kPa. These measurements of the stiffness of the intact collagen matrix in a remodeled FPCM can provide important clues on the mechanical environment that regulates the biological function of encapsulated cells. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y40.00011: Crowding of molecular motors determines microtubule depolymerization Louis Reese, Anna Melbinger, Erwin Frey Assembly and disassembly dynamics of microtubules (MTs) is tightly controlled by MT associated proteins. Here, it is investigated how plus-end-directed depolymerases of the kinesin-8 family regulate MT depolymerization dynamics. We reproduce experimental findings within the framework of a totally asymmetric simple exclusion process with Langmuir kinetics (TASEP/LK). Thereby, crowding is identified as the key regulatory mechanism of depolymerization dynamics. The analysis gives two qualitatively distinct phases. For motor densities above a particular threshold, a macroscopic traffic jam emerges at the plus-end and the MT dynamics become independent of the motor concentration. Below this threshold, microscopic traffic jams at the tip arise which cancel out the effect of the depolymerization kinetics such that the depolymerization speed is determined by the motor density. Because this density varies over the MT length, length-dependent regulation is possible. The critical length at which MTs start to depolymerize in a length-dependent way is discussed. Reference: Louis Reese, Anna Melbinger and Erwin Frey. Biophys. J. 101, 2190 (2011) [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y40.00012: Physics of the actin cortex in shape oscillations of dividing cells Guillaume Salbreux During cytokinesis, a contractile actomyosin cortex is present at the poles of the dividing cell. For a large enough tension of the polar cortex, a physical symmetry-breaking instability of the cell shape can occur where one pole transfers its volume to the other. Such a shape instability can indeed be observed in control and treated dividing cultured cells where it results in cell shape oscillations and in some cases leads to cytokinesis failure. The cell oscillation properties can be accurately described with a theoretical model based on a competition between cortex turnover and contraction dynamics. Interestingly, our results indicate that a sufficiently large cell elasticity is needed to ensure successful cytokinesis. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y40.00013: Geometrical Reorganisation of the Cytoskeleton and Changes in Cellular Stiffness Following Stretch Harikrishnan Parameswaran, Bela Suki Cells in the lung and the vasculature are under a highly dynamic mechanical environment where they are constantly exposed to stretch. Cells adapt to these fluctuations in stretch by remodeling their cytoskeleton. However, the influence of these geometrical changes on cell stiffness is not well understood. We developed a computational model to simulate the geometrical reorganization of the actin by non-muscle myosin-II under conditions of monotonous cyclic stretch, where amplitude and frequency is constant from cycle to cycle and variable stretch, where the amplitude is varied from cycle-to-cycle and the frequency is inversely proportional to amplitude. With the monotonous cyclic-stretch, the network exhibited significant hysteresis in geometry, it reorganized itself into a more stable configuration and the internal prestress decreased after each cycle. In contrast, the more realistic variability in stretch amplitude prevented these stable configurations from forming and preserved the prestress. This behavior was dependent on the variability in stretch amplitude and the timing of the large amplitude stretches. We conclude that prestress is a consequence of cytoskeletal reorganization which exhibits structural hysteresis and is dependent on the nature of the stretch pattern. [Preview Abstract] |
Session Y41: Focus Session: Physics of Cancer III -- Imaging
Sponsoring Units: DBIOChair: Robert Austin, Princeton University
Room: 156B
Friday, March 2, 2012 8:00AM - 8:36AM |
Y41.00001: Cancer Imaging Invited Speaker: Chenghang Zong |
Friday, March 2, 2012 8:36AM - 9:12AM |
Y41.00002: Optimality in the Development of Intestinal Crypts Invited Speaker: Alexander van Oudenaarden Intestinal crypts in mammals are comprised of long-lived stem cells and shorter-lived progenies, maintained under tight proportions during adult life. Here we ask what are the design principles that govern the dynamics of these proportions during crypt morphogenesis. We use optimal control theory to show that a stem cell proliferation strategy known as a `bang-bang' control minimizes the time to obtain a mature crypt. This strategy consists of a surge of symmetric stem cell divisions, establishing the entire stem cell pool first, followed by a sharp transition to strictly asymmetric stem cell divisions, producing non-stem cells with a delay. We validate these predictions using lineage tracing and single molecule fluorescent in-situ hybridization of intestinal crypts in newborn mice and find that small crypts are entirely composed of Lgr5 stem cells, which become a minority as crypts further grow. Our approach can be used to uncover similar design principles in other developmental systems. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y41.00003: Molecular Imaging System for Monitoring Tumor Angiogenesis Esra Aytac, Mehmet Burcin Unlu In cancer, non-invasive imaging techniques that monitor molecular processes~associated with the tumor angiogenesis could have a central role in the evaluation of novel antiangiogenic and proangiogenic therapies as well as early detection of the disease.~Matrix metalloproteinases (MMP) can serve as specific biological targets for imaging of angiogenesis since expression of MMPs is required for angiogenesis and has been found to be upregulated in every type of human cancer and correlates with stage, invasive, metastatic properties and poor prognosis. However, for most cancers it is still unknown when, where and how MMPs are involved in the tumor angiogenesis [1].~Development of high-resolution, high sensitivity imaging techniques in parallel with the ~tumor models could prove invaluable for assessing the physical location and the time frame of MMP enzymatic acitivity. The goal of this study is to understand where, when and how MMPs are involved in the tumor angiogenesis.~ We will accomplish this goal by following two objectives: to develop a high sensitivity, high resolution molecular imaging system, to develop a virtual tumor simulator that can predict the physical location and the time frame of the MMP activity. In order to achieve our objectives, we will first develop a PAM system and develop a mathematical tumor model in which the quantitative data obtained from the PAM can be integrated.~~So, this work will develop a virtual tumor simulator and a molecular imaging system for monitoring tumor angiogenesis.~1.Kessenbrock, K., V. Plaks, and Z. Werb,~MMP:regulators of the tumor microenvironment.~Cell, 2010.~\textbf{141}(1) [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y41.00004: Characterizing Spatial Organization of Cell Surface Receptors in Human Breast Cancer with STORM Evan Lyall, Matthew R. Chapman, Lydia L. Sohn Regulation and control of complex biological functions are dependent upon spatial organization of biological structures at many different length scales. For instance Eph receptors and their ephrin ligands bind when opposing cells come into contact during development, resulting in spatial organizational changes on the nanometer scale that lead to changes on the macro scale, in a process known as organ morphogenesis. One technique able to probe this important spatial organization at both the nanometer and micrometer length scales, including at cell-cell junctions, is stochastic optical reconstruction microscopy (STORM). STORM is a technique that localizes individual fluorophores based on the centroids of their point spread functions and then reconstructs a composite image to produce super resolved structure. We have applied STORM to study spatial organization of the cell surface of human breast cancer cells, specifically the organization of tyrosine kinase receptors and chemokine receptors. A better characterization of spatial organization of breast cancer cell surface proteins is necessary to fully understand the tumorigenisis pathways in the most common malignancy in United States women. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y41.00005: In Vivo Fluorescence Resonance Energy Transfer Imaging for Targeted Anti-Cancer Drug Delivery Kinetics Kevin Webb, Vaibhav Gaind, Hsiaorho Tsai, Brian Bentz, Venkatesh Chelvam, Philip Low We describe an approach for the evaluation of targeted anti-cancer drug delivery {\em in vivo}. The method emulates the drug release and activation process through acceptor release from a targeted donor-acceptor pair that exhibits fluorescence resonance energy transfer (FRET). In this case, folate targeting of the cancer cells is used - 40~\% of all human cancers, including ovarian, lung, breast, kidney, brain and colon cancer, over-express folate receptors. We demonstrate the reconstruction of the spatially-dependent FRET parameters in a mouse model and in tissue phantoms. The FRET parameterization is incorporated into a source for a diffusion equation model for photon transport in tissue, in a variant of optical diffusion tomography (ODT) called FRET-ODT. In addition to the spatially-dependent tissue parameters in the diffusion model (absorption and diffusion coefficients), the FRET parameters (donor-acceptor distance and yield) are imaged as a function of position. Modulated light measurements are made with various laser excitation positions and a gated camera. More generally, our method provides a new vehicle for studying disease at the molecular level by imaging FRET parameters in deep tissue, and allows the nanometer FRET ruler to be utilized in deep tissue. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y41.00006: Exploring X-Ray Phase-Contrast Imaging using Photon Counting Detectors for Early Breast Cancer Detection Mini Das X-ray attenuation-contrast (AC) imaging in the form of digital mammography (DM) is the current gold standard of screening for deep-set cancers like breast cancer. DM creates images based on the attenuation differences between normal and malignant breast tissue, but the extremely low attenuation contrast poses severe challenges for early cancer detection. In order to overcome these limitations posed by AC imaging, there is a growing interest in exploring phase changes in x-rays as they propagate through the tissue. Theoretical estimations show that the x-ray phase difference between normal and malignant breast tissue is three orders of magnitude higher than the corresponding absorption contrast. While x-ray attenuation is determined by the atomic number of the elements forming the tissue, the phase change is determined by the density or refractive index. Due to high energy of x-rays, absolute measurement of phase change is challenging. We will present our efforts to understand x-ray phase contrast in biological tissue using a photon counting detector (TIMEPIX), which is capable of energy and time resolved measurements with very high spatial resolution (about 50 microns). We are exploring novel methods, which will also be clinically feasible to extract phase information using a combination of PCDs and phase retrieval techniques. Phase changes and contrast details of various breast cancer types will also be investigated using the energy resolved measurements obtained using PCDs. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y41.00007: Uncovering cancer cell behavioral phenotype in 3-D \textit{in vitro} metastatic landscapes Liyu Liu, Bo Sun, Guillaume Duclos, Yoonseok Kam, Robert Gatenby, Howard Stone, Robert Austin One well-known fact is that cancer cell genetics determines cell metastatic potentials. However, from a physics point of view, genetics as cell properties cannot directly act on metastasis. An agent is needed to unscramble the genetics first before generating dynamics for metastasis. Exactly this agent is cell behavioral phenotype, which is rarely studied due to the difficulties of real-time cell tracking in \textit{in vivo} tissue. Here we have successfully constructed a micro \textit{in vitro} environment with collagen based Extracellular Matrix (ECM) structures for cell 3-D metastasis. With stable nutrition (glucose) gradient inside, breast cancer cell MDA-MB-231 is able to invade inside the collagen from the nutrition poor site towards the nutrition rich site. Continuous confocal microscopy captures images of the cells every 12 hours and tracks their positions in 3-D space. The micro fluorescent beads pre-mixed inside the ECM demonstrate that invasive cells have altered the structures through mechanics. With the observation and the analysis of cell collective behaviors, we argue that game theory may exist between the pioneering cells and their followers in the metastatic cell group. The cell collaboration may explain the high efficiency of metastasis. [Preview Abstract] |
Session Y42: Physics of Bacteria and Viruses
Sponsoring Units: DBIOChair: Aparna Baskaran, Brandeis University
Room: 156C
Friday, March 2, 2012 8:00AM - 8:12AM |
Y42.00001: Membrane-mediated interaction between retroviral capsids Rui Zhang, Toan Nguyen A retrovirus is an RNA virus that is replicated through a unique strategy of reverse transcription. Unlike regular enveloped viruses which are assembled inside the host cells, the assembly of retroviral capsids happens right on the cell membrane. During the assembly process, the partially formed capsids deform the membrane, giving rise to an elastic energy. When two such partial capsids approach each other, this elastic energy changes. Or in other words, the two partial capsids interact with each other via the membrane. This membrane mediated interaction between partial capsids plays an important role in the kinetics of the assembly process. In this work, this membrane mediated interaction is calculated both analytically and numerically. It is worth noting that the diferential equation determining the membrane shape in general nonlinear and cannot be solved analytically,except in the linear region of small deformations. And it is exactly the nonlinear regime that is important for the assembly kinetics of retroviruses as it provides a large energy barrier. The theory developed here is applicable to more generic cases of membrane mediated interactions between two membrane-embedded proteins. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y42.00002: Discrete fracture patterns of virus shells reveal mechanical building blocks Irena L. Ivanovska, Roberto Miranda, Jos\'e L. Carrascosa, Gijs J.L. Wuite, Christoph F. Schmidt Viral shells are self-assembled protein nanocontainers with remarkable material properties. They combine simplicity of construction with toughness and complex functionality. To date we know little about how virus structure determines assembly pathways and shell mechanics. We have used atomic force microscopy to study structural failure of the shells of the bacteriophage $\Phi $29. We observed rigidity patterns following the symmetry of the capsid proteins and under prolonged force exertion, we see fractures along well-defined lines of the 2D crystal lattice. We found the mechanically most stable building block of the shells was a trimer. Our approach of ``reverse engineering'' the virus shells thus made it possible to identify stable structural intermediates. Such stable intermediates point to a hierarchy of interactions among equal building blocks correlated with distinct next-neighbor interactions. The results also demonstrate that concepts from macroscopic materials science, such as fracture, can be usefully employed in molecular engineering. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y42.00003: Pili-driven surface motility of Myxococcus xanthus Maxsim Gibiansky, Wei Hu, Kun Zhao, Hongwei Pan, Wenyuan Shi, Karin Dahmen, Gerard Wong Myxococcus xanthus is a common, rod-shaped soil-dwelling bacterium with complex motility characteristics. In groups, M. xanthus bacteria can move via social ``S'' motility, in which the Type IV Pili (TFP) attach to secreted exopolysaccharides (EPS). We examine this motility mechanism using high-framerate video acquisition, taking data on individual bacteria at 400 frames per second; using particle tracking algorithms, we algorithmically reconstruct the bacterial trajectories. The motion of a single bacterium as it is pulled by its TFP through the EPS layer on the surface is not smooth, but instead displays distinct plateaus and slips, with a wide range of plateau and slip lengths. The distribution of slips exhibits power law scaling, consistent with a crackling noise model; crackling noise has previously been used to model nonbiological systems such as earthquake dynamics and Barkhausen noise. We show quantitative agreement between mean field friction models and observed bacterial dynamics. We demonstrate that the crackling noise behavior of M. xanthus is strongly dependent on the presence of EPS, but is unaffected by the chemotactic behavior of the bacterium; we also demonstrate velocity coupling between pairs of bacteria in the early stages of social motility. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y42.00004: Pili-taxis: Clustering of \textit{Neisseria gonorrhoeae} bacteria Johannes Taktikos, Vasily Zaburdaev, Nicolas Biais, Holger Stark, David A. Weitz The first step of colonization of \textit{Neisseria gonorrhoeae} bacteria, the etiological agent of gonorrhea, is the attachment to human epithelial cells. The attachment of \textit{N. gonorrhoeae} bacteria to surfaces or other cells is primarily mediated by filamentous appendages, called type IV pili (Tfp). Cycles of elongation and retraction of Tfp are responsible for a common bacterial motility called twitching motility which allows the bacteria to crawl over surfaces. Experimentally, \textit{N. gonorrhoeae} cells initially dispersed over a surface agglomerate into round microcolonies within hours. It is so far not known whether this clustering is driven entirely by the Tfp dynamics or if chemotactic interactions are needed. Thus, we investigate whether the agglomeration may stem solely from the pili-mediated attraction between cells. By developing a statistical model for pili-taxis, we try to explain the experimental measurements of the time evolution of the mean cluster size, number of clusters, and area fraction covered by the cells. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y42.00005: The Psl economy in early P. aeruginosa biofilm development Kun Zhao, Boo Shan Tseng, Fan Jin, Max Gibiansky, Joe Harrison, Matthew Parsek, Gerard Wong Psl from P. aeruginosa (PAO1) is a mannose- and galactose-rich exopolysaccharide (EPS). It has been shown that Psl plays an important role in bacterial surface adhesion. Here, we examine role of Psl in controlling motility and microcolony formation during early biofilm development, by translating video microscopy movies into searchable databases of bacterial trajectories. We use a massively-parallel cell tracking algorithm to extract the full motility history of every cell in a large community. We find that at early stages of growth, P. aeruginosa motility is guided by Psl and self-organize in a manner analogous to a capitalist economic system, resulting in a power law bacterial distribution where a small number of bacteria are extremely ``rich'' in communally produced Psl. By comparing overproducers and underproducers of Psl, we find that local Psl levels determine post-division cell fates: High local Psl levels drive the formation of sessile microcolonies that grow exponentially. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y42.00006: Roles of Pel and Psl in very early biofilm development B.J. Cooley, Travis Thatcher, Guillaume L'Her, Erin Reed, Jamie Stuart, April Kissinger, Vernita Gordon Biofilms are dynamic, multicellular communities of unicellular organisms. Biofilms cause many chronic infections; an important case is the opportunistic human pathogen \textit{Pseudomonas aeruginosa}. Bacteria in biofilms produce an extracellular matrix that binds bacteria to each other and to a surface. The two primary extracellular matrix components produced by \textit{P. aeruginosa} are the polysaccharides Pel and Psl. Here we examine the roles of Pel and Psl in the very early stages of biofilm development, just after initial surface attachment. We use high-throughput automated tracking and analysis to compare wild-type bacteria with mutants incapable of producing Pel, Psl, or both. We examine motion on a surface as well as inter-bacterial interactions. These results quantify the unique roles played by Pel and Psl and show an unexpected relationship between Pel expression and adhesion to a surface. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y42.00007: Quorum activation at a distance: spatiotemporal patterns of gene regulation from diffusion of an autoinducer signal Gabriel Dilanji, Jessica Langebrake, Patrick Deleenheer, Stephen J. Hagen Bacteria in colonies coordinate gene regulation through the exchange of diffusible signal molecules known as autoinducers (AI). This ``quorum signaling'' often occurs in physically heterogeneous and spatially extended environments such as biofilms. Under these conditions the space and time scales for diffusion of the signal limit the range and timing of effective gene regulation. We expect that spatial and temporal patterns of gene expression will reflect physical environmental constraints as well as nonlinear transcriptional activation and feedback within the gene regulatory system. We have combined experiments and modeling to investigate how these spatiotemporal patterns develop. We embed engineered plasmid/GFP quorum sensor strains or wild type strains in a long narrow agar lane, and then introduce AI signal at one terminus of the lane. Diffusion of the AI initiates reporter expression along the length of the lane, extending to macroscopic distances of mm-cm. Resulting patterns are captured quantitatively by a mathematical model that incorporates logistic growth of the population, diffusion of AI, and nonlinear transcriptional activation. Our results show that a diffusing quorum signal can coordinate gene expression over distances of order 1cm on time scales of order 10 hrs. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y42.00008: Biofilm Formation in Microscopic Double Emulsion Droplets Connie Chang, David Weitz In natural, medical, and industrial settings, there exist surface-associated communities of bacteria known as biofilms. These highly structured films are composed of bacterial cells embedded within self-produced extracellular matrix, usually composed of exopolysaccharides, proteins, and nucleic acids; this matrix serves to protect the bacterial community from antibiotics and environmental stressors. Here, we form biofilms encapsulated within monodisperse, microscopically-sized double emulsion droplets using microfluidics. The bacteria self-organize at the inner liquid-liquid droplet interfaces, multiply, and differentiate into extracellular matrix-producing cells, forming manifold three-dimensional shell-within-a-shell structures of biofilms, templated upon the inner core of spherical liquid droplets. By using microfluidics to encapsulate bacterial cells, we have the ability to view individual cells multiplying in microscopically-sized droplets, which allows for high-throughput analysis in studying the genetic program leading to biofilm development, or cell signaling that induces differentiation. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y42.00009: Atomic Force Microscope Investigations of Bacterial Biofilms Treated with Gas Discharge Plasmas Kurt Vandervoort, Anna Zelaya, Graciela Brelles-Marino We present investigations of bacterial biofilms before and after treatment with gas discharge plasmas. Gas discharge plasmas represent a way to inactivate bacteria under conditions where conventional disinfection methods are often ineffective. These conditions involve biofilm communities, where bacteria grow embedded in an exopolysaccharide matrix, and cooperative interactions between cells make organisms less susceptible to standard inactivation methods. In this study, biofilms formed by the opportunistic bacterium \textit{Pseudomonas aeruginosa} were imaged before and after plasma treatment using an atomic force microscope (AFM). Through AFM images and micromechanical measurements we observed bacterial morphological damage and reduced AFM tip-sample surface adhesion following plasma treatment. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y42.00010: High speed dynamic characterization of an \textit{E. coli} population using advanced optical methods (DDM and DFM) Rongjing Zhang, Laurence Wilson The motility of microbes/bacteria in a complex environment, especially the average motility of the whole group of microorganisms, is directly related to behavior such as virulence, biofilm formation, etc. It is challenging to use traditional tracking methods to quantify the average motility of a large population. It is even more challenging when the environment is constantly changing. Two optical methods were developed to solve the problem: differential dynamic microscopy (DDM) and dark field flickering microscopy (DFM). The key features of bacteria motility were quantified automatically: average swimming speed, motile fraction, diffusion coefficient, cell body rotation speed and flagellar bundle rotation speed. This method is able to measure $\sim $10$^{4}$ cells simultaneously. With the help of a high speed camera, the timescale of the dynamic measurement can be in a wide range from 10$^{-4}$ s to 10$^{5}$ s. Using this tool, temperature effects on \textit{E. coli} motility were studied. Potential biomedically-relevant applications will also be discussed. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y42.00011: Bacterial Swimming at Air/Water and Oil/Water Interfaces Michael Morse, Athena Huang, Guanglai Li, Jay Tang The microbes inhabiting the planet over billions of years have adapted to diverse physical environments of water, soil, and interfaces between water and either solid or air. Following recent studies on bacterial swimming and accumulation near solid surfaces, we turn our attention to the behavior of Caulobacter crescentus, a singly flagellated bacterium, at water/air and water/oil interfaces. The latter is motivated by relevance to microbial degradation of crude oil in light of the recent oil spill in the Gulf of Mexico. Our ongoing study suggests that Caulobacter swarmer cells tend to get physically trapped at both water/air and water/oil interfaces, accumulating at the surface to a greater degree than boundary confinement properties like that of solid surfaces would predict. At the water/air interface, swimmers move in tight circles at half the speed of swimmers in the bulk fluid. At the water/oil interface, swimming circles are even tighter with further reduced swimming speed. We report experimental data and present preliminary analysis of the findings based on low Reynolds number hydrodynamics, the known surface tension, and surface viscosity at the interface. The analysis will help determine properties of the bacterium such as their surface charge and hydrophobicity. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y42.00012: Bacterial thermotaxis by modulation of the swimming speed Mahmut Demir, Hanna Salman It has been long established that random walkers such as bacteria can migrate in inhomogeneous environments, even without actively responding to changes they sense around them, by modulating their swimming speed and/or run time. We will show that \textit{E.coli} bacteria migrate in shallow temperature gradients due to their speed dependence on temperature even without the presence of sensing receptors. Interestingly however, we find that the direction of their migration in the gradient depends on the serine concentration in the medium. This results from the fact that the bacterial swimming speed exhibits a two-state function of serine concentration and the difference between the two states increases with temperature. Our results show that the swimming speed of the bacteria increases monotonically with temperature when serine is present at high concentration, while it decreases for temperatures above 30\r{ }C at low serine concentration. This observed difference in the speed dependence on temperature is found to be due to a change in the intracellular pH of the bacteria when serine is added to their surroundings, which occurs only in the presence of the serine receptor Tsr. We will discuss some details of the mechanism underlying this effect and its consequences for the bacterial behavior. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y42.00013: Role of cell bending and slime navigation in swarms of \textit{M. xanthus} Cameron Harvey, Dale Kaiser, Mark Alber Many bacteria use motility described as swarming to colonize surfaces that allows them to optimize their access to nutrients. The swarming of the bacterium \textit{M. xanthus} on surfaces is a remarkable interplay between motility mechanisms, cell flexibility, cell-cell adhesive interactions and directional reversals.~ The properties of individual cells from different mutant strains and density regimes will be demonstrated in this talk. Then, a computational model based on subcellular elements for cell representation and implemented on graphical processing units (GPUs) will be presented. High-quality high magnification movies of bacterial motility together with biologically justified computational simulations will be used for investigation of collective motion and order in swarming populations of bacteria.~ Collective motion will be shown to include the dynamical formation of cell clusters as well as streams of cells moving over networks of cell-generated slime tracks. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y42.00014: Effect of Antimicrobial Agents on MinD Protein Oscillations in \textit{E. coli} Bacterial Cells Corey Kelly, Maximiliano Giuliani, John Dutcher The pole-to-pole oscillation of MinD proteins in~\textit{E. coli} cells determines the location of the division septum, and is integral to healthy cell division. It has been shown previously that the MinD oscillation period is approximately 40 s for healthy cells [1] but is strongly dependant on environmental factors such as temperature, which may place stress on the cell [2,3]. We use a strain of \textit{E. coli} in which the MinD proteins are tagged with green fluorescent protein (GFP), allowing fluorescence visualization of the MinD oscillation. We use high-resolution total internal reflection fluorescence (TIRF) microscopy and a custom, temperature controlled flow cell to observe the effect of exposure to antimicrobial agents on the MinD oscillation period and, more generally, to analyze the time variation of the spatial distribution of the MinD proteins within the cells. These measurements provide insight into the mechanism of antimicrobial action. [1] Raskin, D.M.; de Boer, P. (1999) Proc. Natl. Acad. Sci. 96: 4971-4976. [2] Touhami, A.; Jericho, M; Rutenberg, A. (2006) J. Bacteriol. 188: 7661-7667. [3] Downing, B.; Rutenberg, A.; Touhami, A.; Jericho, M. (2009) PLoS ONE 4: e7285. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y42.00015: Residence and transit times of MinD in \textit{E. coli} bacterial cells Maximiliano Giuliani, Corey Kelly, John Dutcher A key step in the life of a bacterial cell is its division into two daughters cells of equal size. This process is carefully controlled and regulated so that an equal partitioning of the main cell components is obtained, which is critical for the viability of the daughter cells. In \textit{E. coli} this regulation is accomplished in part by the Min protein system, that determines the localization of the division machinery. Of particular interest is the MinD protein that exhibits an oscillation between the poles in the rod shaped bacteria. The oscillation relies on a ATP mediated dimerization of the MinD protein that allows its insertion into the inner membrane at one of the poles of the cell, followed by an interaction with the MinE protein, which releases the MinD from the membrane, allowing it to travel to the other pole of the cell where the cycle is repeated. We have studied the spatio-temporal characteristics of the MinD oscillation from which we extract the average times for the two main processes that determine the oscillation period: the residence time in the membrane and the transit time to travel the length of the cell. Additionally, we explore how these two timescales are affected by stresses on the bacterial cells due to unfavorable physiological conditions. [Preview Abstract] |
Session Y43: Invited Session: Physical Organizing Principles for Amyloid Matter: Prediction, Structure, Function
Sponsoring Units: DBIOChair: Daniel L. Cox, University of California, Davis
Room: 157AB
Friday, March 2, 2012 8:00AM - 8:36AM |
Y43.00001: Prions, From Structure to Epigenetics and Neuronal Functions Invited Speaker: Susan Lindquist Prions are a unique type of protein that can misfold and convert other proteins to the same shape. The well-characterized yeast prion [PSI+] is formed from an inactive amyloid fiber conformation of the translation-termination factor, Sup35. This altered conformation is passed from mother cells to daughters, acting as a template to perpetuate the prion state and providing a mechanism of protein-based inheritance. We employed a variety of methods to determine the structure of Sup35 amyloid fibrils. First, using fluorescent tags and cross-linking we identified specific segments of the protein monomer that form intermolecular contacts in a ``Head-to-Head,'' ``Tail-to-Tail'' fashion while a central region forms intramolecular contacts. Then, using peptide arrays we mapped the region responsible for the prion transmission barrier between two different yeast species. We have also used optical tweezers to reveal that the non-covalent intermolecular contacts between monomers are unusually strong, and maintain fibril integrity even under forces that partially unfold individual monomers and extend fibril length. Based on the handful of known yeast prion proteins we predicted sequences that could be responsible for prion-like amyloid folding. Our screen identified 19 new candidate prions, whose protein-folding properties and diverse cellular functions we have characterized using a combination of genetic and biochemical techniques. Prion-driven phenotypic diversity increases under stress, and can be amplified by the dynamic maturation of prion-initiating states. These qualities allow prions to act as ``bet-hedging'' devices that facilitate the adaptation of yeast to stressful environments, and might speed the evolution of new traits. Together with Kandel and Si, we have also found that a regulatory protein that plays an important role in synaptic plasticity behaves as a prion in yeast. Cytoplasmic polyAdenylation element binding protein, CPEB, maintains synapses by promoting the local translation of mRNAs. We postulate that the self-perpetuating folding of the prion domain acts as a molecular memory. Thus yeast prions have provided evidence for the surprising possibility that amyloid protein folds can serve as the basis for memory and inheritance. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 9:12AM |
Y43.00002: Exploring the Amylome: Predicting Amyloidogenic proteins & mapping their roles, functional or not Invited Speaker: Roland Riek |
Friday, March 2, 2012 9:12AM - 9:48AM |
Y43.00003: Toxic $\beta $-Amyloid (A$\beta )$ Alzheimer's Ion Channels: From Structure to Function and Design Invited Speaker: Ruth Nussinov Full-length amyloid beta peptides (A$\beta _{1-40/42})$ form neuritic amyloid plaques in Alzheimer's disease (AD) patients and are implicated in AD pathology. Recent biophysical and cell biological studies suggest a direct mechanism of amyloid beta toxicity -- ion channel mediated loss of calcium homeostasis. Truncated amyloid beta fragments (A$\beta _{11-42}$ and A$\beta _{17-42})$, commonly termed as non-amyloidogenic are also found in amyloid plaques of Alzheimer's disease (AD) and in the preamyloid lesions of Down's syndrome (DS), a model system for early onset AD study. Very little is known about the structure and activity of these smaller peptides although they could be key AD and DS pathological agents. Using complementary techniques of explicit solvent molecular dynamics (MD) simulations, atomic force microscopy (AFM), channel conductance measurements, cell calcium uptake assays, neurite degeneration and cell death assays, we have shown that non-amyloidogenic A$\beta _{9-42}$ and A$\beta _{17-42}$ peptides form ion channels with loosely attached subunits and elicit single channel conductances. The subunits appear mobile suggesting insertion of small oligomers, followed by dynamic channel assembly and dissociation. These channels allow calcium uptake in APP-deficient cells and cause neurite degeneration in human cortical neurons. Channel conductance, calcium uptake and neurite degeneration are selectively inhibited by zinc, a blocker of amyloid ion channel activity. Thus truncated A$\beta $ fragments could account for undefined roles played by full length A$\beta $s and provide a novel mechanism of AD and DS pathology. The emerging picture from our large-scale simulations is that toxic ion channels formed by $\beta $-sheets are highly polymorphic, and spontaneously break into loosely interacting dynamic units (though still maintaining ion channel structures as imaged with AFM), that associate and dissociate leading to toxic ion flux. This sharply contrasts intact conventional gated ion channels that consist of tightly interacting $\alpha $-helices that robustly prevent ion leakage, rather than hydrogen-bonded $\beta $-strands. Moreover, in comparison with $\beta $-rich antimicrobial peptide (AMP) such as a protegrin-1 (PG-1), both A$\beta $ and PG-1 are cytotoxic, and capable of forming fibrils and dynamic channels which consist of subunits with similar dimensions. These combined properties support a functional relationship between amyloidogenic peptides and $\beta $-sheet-rich cytolytic AMPs, suggesting that PG-1 is amyloidogenic and amyloids may have an antimicrobial function. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:24AM |
Y43.00004: Predicting Amyloid Structure Without Bioinformatic Support: Threading the Left Handed Beta Helix Invited Speaker: Alexander Kluber |
Friday, March 2, 2012 10:24AM - 11:00AM |
Y43.00005: Functional Hydrogel Materials Inspired by Amyloid Invited Speaker: Joel Schneider Protein assembly resulting in the formation of amyloid fibrils, assemblies rich in cross beta-sheet structure, is normally thought of as a deleterious event associated with disease. However, amyloid formation is also involved in a diverse array of normal biological functions such as cell adhesion, melanin synthesis, insect defense mechanism and modulation of water surface tension by fungi and bacteria. These findings indicate that Nature has evolved to take advantage of large, proteinaceous fibrillar assemblies to elicit function. We are designing functional materials, namely hydrogels, from peptides that self-assembled into fibrillar networks, rich in cross beta-sheet structure. These gels can be used for the direct encapsulation and delivery of small molecule-, protein- and cell-based therapeutics. Loaded gels exhibit shear-thinning/self-healing mechanical properties enabling their delivery via syringe. In addition to their use for delivery, we have found that some of these gels display antibacterial activity. Although cytocompatible towards mammalian cells, the hydrogels can kill a broad spectrum of bacteria on contact. [Preview Abstract] |
Session Y44: Focus Session: Directed Assembly of Hybrid Materials - Particle Organization and Arrays
Sponsoring Units: DPOLYChair: Dvora Perahia, Clemson University
Room: 157C
Friday, March 2, 2012 8:00AM - 8:36AM |
Y44.00001: Simulations of nanoparticle ionic and organic hybrid materials Invited Speaker: Athanassios Z. Panagiotopoulos We have used molecular dynamics simulations over microsecond time scales to study the structure and dynamics of coarse-grained models for nanoparticle-based ionic and organic hybrid materials [1]. The systems of interest consist of particles with charged surface groups and linear or three-arm counterions, which also act as the solvent. A comparable uncharged model of nanoparticles with tethered chains is also studied. The pair correlation functions display a rich structure resulting from the packing of cores and chains, as well as electrostatic effects. Even though electrostatic interactions between oppositely charged ions at contact are much greater than the thermal energy, we find that chain dynamics at intermediate time scales are dominated by chain hopping between core particles. We have also used molecular dynamics simulations and density functional theory to investigate the structure of solvent-free oligomer-grafted nanoparticles [2,3]. At low temperatures and moderate to high oligomer lengths, the qualitative features of the core particle pair probability, structure factor, and the oligomer brush configuration obtained from the simulations can be explained by a density-functional theory that incorporates the configurational entropy of the space-filling oligomers. In particular, the structure factor at small wave numbers attains a value much smaller than the corresponding hard-sphere suspension, the first peak of the pair distribution function is enhanced due to entropic attractions among the particles, and the oligomer brush expands with decreasing particle volume fraction to fill the interstitial space. At higher temperatures, the simulations reveal effects that differ from the theory and are likely caused by steric repulsions of the expanded corona chains. \\[4pt] [1] B. Hong, A. Chremos, and A. Z. Panagiotopoulos; Faraday Disc., DOI: 10.1039/c1fd00076d (2012).\\[0pt] [2] A. Chremos, A. Z. Panagiotopoulos, H.-Y. Yu, and D. L. Koch; J. Chem. Phys., 135:114901, 12pp (2011).\\[0pt] [3] A. Chremos and A. Z. Panagiotopoulos; Phys. Rev. Lett., 107:105503, 5pp (2011). [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y44.00002: Reverse-engineering the anisotropic assembly of grafted nano-particels Behnaz Bozorgui, Sanat Kumar We use computer simulations to study the self assembly and directed phase separation of nanoparticles grafted with polymeric chains in an implicit solvent. The formation of anisotropic clusters from building blocks that are symmetric (both in shape and interactions) has been the subject of recent studies. This anisotropy has roots in many body physics, which manifests itself as a directional assembly. Here we apply both full and coarse-grained simulations to get a better insight of the physics behind the assembly of such systems. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y44.00003: Crystalline assembly of hard polyhedra via directional entropic forces Pablo F. Damasceno, Michael Engel, Sharon C. Glotzer Entropic forces are effective forces that result from a system's statistical tendency to increase its entropy. Hard rods and disks spontaneously align and can assemble into layers and columns if those structures increase the configurational space available to the particles. Hard spheres, cubes and even tetrahedra order for the same reason. Here we extend those findings by showing that hard polyhedra can self-assemble into a variety of complex phases, most of them never before reported in systems of single-component hard particles. The role of shape and directional entropic forces in stabilizing these structures will be discussed. Our results suggest new possibilities for self-assembling complex target structures from colloidal building blocks. \\[4pt] [1] Damasceno, PF; Engel, M; Glotzer, SC. arXiv:1109.1323v1 [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y44.00004: Facile Fabrication of 3-D Nanoparticle Arrays in Thin Films toward Photonic Materials Joseph Kao, Seong-Jun Jeong, Vivian Peng-Wei Chuang, Peter Jin Bai, Zhen Zhang, Cheng Sun, Ting Xu 3-D hierarchical assemblies of nanoparticles in thin films enable one to exploit their collective properties to generate functional electronic, magnetic, and photonic materials. Among many templates, block copolymer-based supramolecules is one of the most promising candidates since they combines solution processiblity and high precision in nanoparticle spatial distribution. Herein, we report a facile way of fabricating 3-D nanoparticle arrays in thin films of diblock copolymer-based supramolecules exhibiting bandgap in the visible regime. The entropy-dominant assembly drives the formation of 3-D lattice of nanoparticle arrays with precise inter-array spacings in thin films. The coupling effect between the precisely positioned nanoparticle arrays with adjustable lattice spacings in the hierarchically-structured nanocomposites allows potentially interesting optical properties. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y44.00005: Creating Opal Templated Continuous Conducting Polymer Films with Ultralow Percolation Thresholds Using Thermally Stable Nanoparticles D.J. Kang, T. Kwon, M.P. KIM, B.J. Kim, H. Jung, J. Bang We propose a novel and robust strategy for creating continuous conducting polymer films with ultralow percolation thresholds using polymer-coated gold nanoparticles (Au NPs) as surfactant. Continuous poly(triphenylamine) (PTPA) films of high internal phase polymeric emulsions were fabricated using an assembly of crosslinked polystyrene (PS) colloidal particles as template. Polymer-coated Au NPs localize at the PS/PTPA interface and function as surfactant to efficiently produce a continuous conducting PTPA polymer film with very low percolation thresholds. The volume fraction threshold for percolation of the PTPA phase with insulating PS colloids was found to be 0.20. In contrast, with the addition of an extremely low volume fraction of surfactant Au NPs, the volume fraction threshold for percolation of the PTPA phase was dramatically reduced to 0.05. The SEM and TEM measurements clearly demonstrated the formation of a continuous PTPA phase within the polyhedral phase of PS colloids. To elucidate the influence of the nanoparticle surfactant on the blend films, the morphology and conductivity of the blends at different PS colloid/PTPA volume ratios were carefully characterized as a function of the Au NP concentration. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y44.00006: Magnetic fields in long-range alignment of functional hybrid soft materials Pawel Majewski, Manesh Gopinadhan, Candice Pelligra, Shanju Zhang, Lisa Pfefferle, Chinedum Osuji We present a magnetic field-based method to impose long range order in self-assembled soft materials including polymer-nanowire composites, block copolymers and surfactant mesophases. We discuss the broad utility of this approach, indicating its advantages and limitations. Our method yields highly anisotropic materials with quality of alignment in many cases comparable to that of single crystals as assessed by X-ray scattering techniques. We take advantage of the high fidelity of alignment to systematically explore and characterize the anisotropic properties of these materials. We present a perspective for improving electron and hole transport, as well as exciton utilization in magnetically doped ZnO nanowire-polythiophene composites for photovoltaic applications by global alignment of the nanowires. For block copolymers, we focus on enhancing Li-ion transport in membranes with self-assembled cylindrical and lamellar morphology by alignment of the Li-conducting PEO domains. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y44.00007: $^{2}$H NMR study of the Assembly of Gold Nanoparticles Dispersed in Nematic Liquid Crystal Linda Reven, J Milette, R. Bruce Lennox, Alejandro Rey, Ezequiel Soule Gold nanoparticles (AuNP) with mesogenic ligands, tailored to be highly miscible in the liquid phase of 4-n-pentyl-4'-cyanobiphenyl (5CB) liquid crystal (LC), form reversible, micron-scale cellular networks upon cooling to the nematic phase. The network topology and LC director field orientation are controlled by the cooling rate, film thickness, ligand shell composition and AuNP concentration. Isotopically labeled samples allow selective probing of the orientational order of the NP ligands and host LC by $^{2}$H NMR. The $^{2}$H NMR spectra of AuNPs dispersed in nematic 5CB display isotropic and doublet peaks assigned to disordered and field aligned ligands respectively. The intensity of the ligand doublet signal increases relative to the isotropic peak with decreasing temperature and NP concentration. Conversely an isotropic peak along with the expected doublet is observed for the host 5CB in the nematic phase. The intensity of the host 5CB isotropic peak, which persists far into the nematic phase and decreases with NP concentration, shows the coexistence of isotropic and nematic phases. The NP ligand and host LC orientational orders are related the formation of the cellular network and theoretical phase diagrams of NP-LC dispersions. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y44.00008: Organization of Gold Nanorods in Cylinder-Forming Block Copolymer Films Guoquian Jian, Robert Riggleman, Russell Composto The addition of gold nanorods (AuNRs) to copolymer films can impart unique optical and electrical properties. To take full advantage of this system, the AuNRs must be dispersed in a self-organizing copolymer that directs the orientation of the anisotropic particle. In the present work, AuNRs with aspect ratio 3.6 (8 nm x 29 nm) are grafted with poly(2-vinyl pyridine) (P2VP) brushes and dispersed in a cylindrical forming diblock copolymer of polystyrene-b-P2VP (180K-b-77K, 29.6 wt{\%} P2VP). Films are spun cast and solvent annealed in chloroform to produce a perpendicular cylindrical morphology at the surface. Using TEM and UV-ozone etching combined with AFM, the AuNRs are well dispersed and co-locate (top down view) with the P2VP cylinders, $\sim $50nm diameter. However, the AuNRs mainly lie parallel to the surface indicating that they likely locate at the junction created at the intersection between P2VP cylinders and P2VP brush layer adjacent to the silicon oxide surface. Self-consistent field calculations of the Au:PS-b-P2VP morphology as well as the effect of adding P2VP homopolymer to the nanocomposite will be discussed. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y44.00009: Nanopatterned poly(ethylene glycol) brushes: A route for highly tunable assembly of Au nanoparticles M. Serdar Onses, Paul F. Nealey Assembly of metallic nanoparticles (NPs) on surfaces offers many interesting opportunities for scientific studies as macroscopic properties of the assemblies depend on organization of the particles at the nanometer length scale. On the other hand, new/improved functionalities of the assemblies are promising in the potential technological applications such as chemical sensing and metamaterials. Chemical patterning of surfaces for directed assembly of ex-situ synthesized NPs is advantageous due to controllable substrate-particle interaction and applicability to a range of different size and type of particles. Here we present assembly of Au NPs (15-50 nm) on lithographically patterned poly(ethylene glycol) (PEG) brushes. The density of NPs on the patterns can be controlled with the molecular weight of PEG brush and the size of particles. The ability to pattern PEG brushes with high chemical contrast and resolution (sub 50 nm) with the interesting particle brush interaction provides routes for highly controllable assembly of NPs. For example, number of particles per spot can be precisely controlled with high yields (single 95{\%} and dimer 70{\%} for 30 nm Au NP) and NPs of two different sizes (heterostructures) can be assembled on patterned spots. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y44.00010: Distribution of POSS Nanoparticles in Symmetric Diblock Copolymer Thin Films Umesh Shrestha, Dilru Ratnaweera, Stephen Clarson, Dvora Perahia The distribution of Polyhedral Oligomeric Silsesquioxanes (POSS) nanoparticles) in thin films of symmetric diblock copolymer polystyrene-b-poly dimethyl siloxane (PS-$b$-PDMS) was studied by neutron reflectometry. Blending of nanoparticles (NPs) with polymers provide a pathway to tune properties of the polymer such as conductivity, mechanical strength, optical activity as well as dewetting, where the properties depend on the distribution of the NPs. Controlling the distribution of NPs in polymers thin films however, remains a challenge where entropic contributions drive segregation of the NPs to the interface. Different approaches were taken including modifying the interfacial interactions of the NP. The current study investigates effects of the relative size of the NP with respect to that of the polymer layers following distribution of POSS cages which consist of a silicon cage in a symmetric PS-PDMS. The temperature was varied from room temperature to 150$^{o}$C, investigating the distribution above and below the glass transition temperature of the styrene blocks. The distribution of the NPs as well as structural changes for two molecular weights of the diblock 10kg/mol and 50kg/mol within the films will be discussed. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y44.00011: Long-range Ordering of Symmetric Block-copolymers by Chaining of Superparamagnetic Nanoparticles in External Magnetic Fields V. Raman, A. Bose, B.D. Olsen, T.A. Hatton Chaining of superparamagnetic nanoparticles is numerically investigated as a new method to align the block copolymer without additional lithographic processing. The method relies on the chaining of superparamagnetic nanoparticles, sequestered preferentially in one of the blocks, in the direction of in-plane external magnetic fields to achieve long-range order in the block copolymer. Effects of nanoparticle size, concentration, and magnetization strength are explored using Hybrid-Particle-Field technique (Sides et al, 2006). The 2D simulations reveal that, for the same nanoparticle loading, the nanoparticle sizes commensurate with the domain sizes yield defect-free alignment. While small sizes lead to jamming and kinetic trapping of defects, larger sizes lead to swelling of domains and break the symmetry of the lamellar phase. A window of optimal nanoparticle concentrations exists over which orientational order is achieved. For low concentrations, only local alignment is observed, while high concentrations lead to order-order phase transition from lamellae to cylindrical phase. Scaling calculations corroborate the effect of high magnetization strengths in lowering the equilibrium defect density for such nematic-isotropic phase transitions observed in monolayers. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y44.00012: Highly Ordered Superstructures of Single-Walled Carbon Nanotubes in Polymeric Systems Sung-Min Choi, Hyoung-Sik Jang, Changwoo Doe, Tae-Hwan Kim Fabrication of highly ordered arrays of single-walled carbon nanotubes (SWNTs) has been of great interest for a wide range of potential applications. Block copolymers exhibit rich phase behavior and have been extensively used as excellent templates for highly ordered nanostructure materials with various architectures. Therefore, utilization of the rich phase behavior of block copolymers may provide a general and inexpensive way for fabricating a large variety of self-assembled and highly ordered arrays of SWNTs without going through complicated preparative procedures. Here, we investigated the cooperative self-assembling behavior of functionalized isolated SWNTs in Pluronic block copolymer systems using small angle neutron and x-ray scattering techniques, which show highly ordered superstructures of SWNTs with different symmetries. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y44.00013: Block copolymer modified epoxies: Role of localized network damage Carmelo Declet-Perez, Erica Redline, Lorraine Francis, Frank Bates Adding block copolymers to epoxy resins has proven to be an effective approach to toughening these materials while retaining commercially relevant properties such as high modulus and glass transition temperature. When properly designed, block copolymers self-assemble into spherical micelles that disperse in the monomer resin. These structures survive curing into a dense network. At this point, a complete description of the toughening mechanism for block copolymer modified epoxies is still lacking. Here we present new experimental evidence that challenges the current understanding in this area. We compared the toughening effect of spherical micelle forming block copolymers with rubbery and glassy cores. Consistent with previous reports rubbery cores produced significant toughening, but surprisingly the glassy core micelles also imparted some toughness. These results suggest that the block copolymer/epoxy interface plays a significant role during deformation. We propose that the micelles compromise the integrity of the glassy matrix by damaging or plasticizing the crosslinked network in contact with the corona blocks, thereby facilitating shear yielding. [Preview Abstract] |
Session Y45: Focus Session: Soft Matter Physics of Drops, Bubbles, Foams and Emulsions - Bubbles, Films, Foams
Sponsoring Units: DPOLYChair: Robert Cohen, Massachusetts Institute of Technology
Room: 159
Friday, March 2, 2012 8:00AM - 8:12AM |
Y45.00001: Mechanical properties of surface modified microbubbles by Atomic Force Microscopy (AFM) Jonathan Mckendry, Colin Grant, Stephen Evans Atomic force microscopy has been used to investigate the mechanical properties of phospholipid coated microbubbles and to quantify their stiffness. The mechanical properties were investigated using tipless AFM cantilevers to compress microbubbles attached to a gold surface in aqueous conditions. The phospholipid microbubbles were produced by microfluidic flow focusing and were found to have a stiffness of 25 mN/m. The attachment of a streptavidin coating increased the microbubble stiffness by a factor of 30 to around 750 mN/m. Further, the effect of temperature on the mechanical and time dependent properties of bubbles has been studied, the results of which have demonstrated that increasing temperature leads to a decrease in microbubble stiffness and an increase creep-displacement. The standard linear model was used to extract to the visco-elastic parameters at different temperatures, this allowed the first determination of the activation energy for creep for a microbubble. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y45.00002: Cylindrical bubbles and blobs from a Class II Hydrophobin Paul Russo, Michael Pham, Brad Blalock \textit{Cerato ulmin} is a class II hydrophobin. In aqueous suspensions, it easily forms cylindrical air bubbles and cylindrical oil blobs. The conditions for formation of these unusual structures will be discussed, along with scattering and microscopic investigations of their remarkable stability. Possible applications in diverse fields including polymer synthesis and oil spill remediation will be considered. Acknowledgment is made to Dr. Wayne C. Richards of the Canadian Forest Service for the gift of \textit{Cerato ulmin}. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y45.00003: The collapse and the folding of a particle rafts under compression Chin-Chang Kuo, Michael Dennin Compressing a single-layer of particles or bubbles that are confined to the air-water interface results in a range of interesting collapse dynamics. We report on the collapse modes of two systems: (1) a single layer of gas bubbles at the surface and (2) a single layer of polypropylene beads. Under compression, both systems exhibit a critical areal density beyond which there is a transition to a multi-layered structure. Generally speaking, the transformation is characterized by localized submergence into the subphase of bubbles or beads. For both systems, we observe single bubbles/beads being pushed underneath surrounding particles. However, for sufficiently small beads, we observe a folding mode, which corresponds to the long-ranged one dimensional wrinkling of the monolayer surface. In this talk, we will report on the transition between single particle submergence and folding, as well as general characterization of the collapse as a function of compression speed and initial structure of the particle raft. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 9:12AM |
Y45.00004: Falling drops skating on a film of air Invited Speaker: Shmuel Rubinstein When a raindrop hits a window, the surface immediately becomes wet as the water spreads. Indeed, this common observation of a drop impacting a surface is ubiquitous in our everyday experience. I will show that the impact of a drop on a surface is a much richer, more complex phenomenon than our simple experience may suggests: To completely wet the surface the drop must first expel all the air beneath it; however, this does not happened instantaneously. Instead, a very thin film of air, only a few tens of nanometers thick, remains trapped between the falling drop and the surface as the fluid spreads. The thin film of air serves to lubricate the drop enabling the fluid to skate laterally outward at strikingly high velocities. Simultaneously, the wetting fluid spreads inward at a much slower velocity, trapping a bubble of air within the drop. However, these events occur at diminutive length scales and fleeting time scales; therefore, to visualize them we develop new imaging modalities that are sensitive to the behavior right at the surface and that have time resolution superior to even the very fastest cameras. These imaging techniques reveal that the ultimate wetting of the surface occurs through a completely new mechanism, the breakup of the thin film of air through a spinodal like dewetting process that breaks the cylindrical symmetry of the impact and drives an anomalously rapid spreading of a wetting front. These results are in accord with recent theoretical predictions and challenge the prevailing paradigm in which contact between the liquid and solid occurs immediately, and spreading is dominated by the dynamics of a single contact line. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y45.00005: Compression of multiwall microbubbles Natalia Lebedeva, Sam Moore, Andrey Dobrynin, Michael Rubinstein, Sergei Sheiko Optical monitoring of structural transformations and transport processes is prohibited if the objects to be studied are bulky and/or non-transparent. This paper is focused on the development of a microbbuble platform for acoustic imaging of heterogeneous media under harsh environmental conditions including high pressure ($<$500 atm), temperature ($<$100\r{ }C), and salinity ($<$10 wt{\%}). We have studied the compression behavior of gas-filled microbubbles composed of multiple layers of surfactants and stabilizers. Upon hydrostatic compression, these bubbles undergo significant (up to 100$\times )$ changes in volume, which are completely reversible. Under repeated compression/expansion cycles, the pressure-volume P(V) characteristic of these microbubbles deviate from ideal-gas-law predictions. A theoretical model was developed to explain the observed deviations through contributions of shell elasticity and gas effusion. In addition, some of the microbubbles undergo peculiar buckling/smoothing transitions exhibiting intermittent formation of facetted structures, which suggest a solid-like nature of the pressurized shell. Preliminary studies illustrate that these pressure-resistant microbubbles maintain their mechanical stability and acoustic response at pressures greater than 1000 psi. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y45.00006: The life cycle of individual boiling bubbles: Insights from beyond optical imaging Scott Parker, Sung Chul Bae, David Cahill, Steve Granick With a high-speed camera, we have investigated the dynamics of individual vapor bubbles boiling on a laser-heated surface.~ Their sizes and shapes as they grow and depart from a surface are correlated with simultaneous thermal imaging measurements of the boiling surface using thermoreflectance-based microscopy to measure temperatures of individual stochastic events.~ Analysis of both the thermal profiles and the bubble shapes suggests the presence of an evaporating liquid microlayer under the developing bubble.~ Tuning surface and heating properties, we control the shapes of bubbles, ranging from regular periodic growth and departure to stochastic bubbles which exhibit rapid cavitation-like expansion and collapse.~ Unlike typical cavitation bubbles which collapse and form jets pointed towards the surface, jets from bubbles observed during boiling were observed to be directed away from the surface.~ By tuning the wettability of the substrate, we will report on how wettability affects the strength and direction of these jets. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y45.00007: Nonclassical Thermomigration of an Air Bubble Dominik Michler, Rudolf Sprik, Peter Schall, Daniel Bonn We study air bubbles confined in capillaries with a temperature gradient. Classically, air bubbles move in a temperature gradient due to decreased surface tension at higher temperatures, creating a net surface traction towards the cold pole, pushing the bubble towards the hot pole for mass conservation. Here we report non-classical thermo-migration of confined air bubbles: in the presence of surfactant the bubbles can go the other way. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y45.00008: Two-Dimensional Microfluidics: Stable Island Emulsions in Freely Suspended Smectic Liquid Crystal Films Cheol Park, Zoom Nguyen, Chaney Cranfill, Sarah Radzihovsky, Joe Maclennan, Matt Glaser, Noel Clark Islands (circular regions of greater thickness) in smectic films are easily created and manipulated, but are generally unstable, tending to grow or shrink over time. We have recently created stable emulsions of smectic islands by ``work hardening'' of the smectic film using shear and extensional flow to form a dense, mechanically stable network of edge dislocations. In this talk, we discuss this novel type of two-dimensional colloidal system, in particular the island size distribution, network of edge dislocations and topological defects that form stable two-dimensional emulsions. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y45.00009: Spherical and Non-Spherical Double Emulsions with Multiple Components Laura Adams, Thomas Kodger, Jacy Bird, Shin-Hyun Kim, Louise Jawerth, Vinothan Manoharan, David Weitz Monodispersed double emulsions, drops inside of drops, with multiple and tunable components are generated using microfluidics. A fluid dynamic model based on fast camera images of the single step emulsification technique is being developed to determine the critical separation between channels in the injection capillary; this model addresses the maximium number of distinct drops that can be controllably loaded inside a double emulsion for a given diameter capillary. New stable, non-spherical emulsions with two and three different components, Janus and Cerberus emulsions, are also reported. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y45.00010: Stability of thin liquid films: Influence of interfacial viscoelasticity Gerald Fuller, Liat Rosenfeld Lipid layer spreading and liquid film dewetting are important variables influencing numerous processes, including the stability of the tear film. The viscoelasticity of insoluble monolayers may govern thin liquid film dewetting phenomena. The purpose of this work is to gain insight into the effects of surface viscoelasticity elasticity by insoluble monolayers on dewetting of thin films of water with a particular attention paid to materials, such as meibum, that stabilize the tear film. For this purpose an experiment has been devised wherein monolayers of known surface pressure and surface rheology are introduced atop thin, liquid films that would normally spontaneously dewet .The results reveal that monolayers of viscoelastic surfactants are able to stabilize thin films against spontaneous dewetting. As the surface pressure and surface rheology of these layers is increased, their effectiveness is enhanced. Meibum is particularly effective in stabilizing thin films. These results suggest that the role of the meibum is to offer the tear film enhanced stability and not only to suppress evaporation. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y45.00011: Light-controlled air-water interfaces and thin liquid films using photo-surfactants Eloise Chevallier, Christophe Tribet, Francois Lequeux, Cecile Monteux We study the interfacial behavior of photosurfactants containing an azobenzene moiety in the apolar tail --which switch from cis to trans conformation depending on the wavelength of light. We present here results on the effet of stimulation on the interfacial dynamics of such photosurfactants upon illumination. First, without light stimulus, the trans isomers is found to desorb more slowly than the cis, this leads to a fast enrichment of the interface with trans. Under light, adsorbed trans convert to their cis form which quickly desorb resulting in an important decrease of the surface excess and an increase of the surface tension. Besides, we stimulate thin-liquid films stabilized by such surfactants. Several types of hydrodynamical instabilities in the thin-liquid films are generated. We show that these instabilities are due to a strong rise of concentration in-situ but also a light-induced variation of DLVO interactions that usually stabilize the films. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y45.00012: Supercritical Carbon Dioxide Assisted Processing of Silica/PMMA Nanocomposite Foams Deniz Rende, Linda S. Schadler, Rahmi Ozisik Polymer nanocomposite foams receive considerable attention in both scientific and industrial communities. These structures are defined as closed or open cells (pores) surrounded by bulk material and are widely observed in nature in the form of bone structure, sponge, corals and natural cork. Inspired by these materials, polymer nanocomposite foams are widely used in advanced applications, such as bone scaffolds, food packaging and transportation materials due to their lightweight and enhanced mechanical, thermal, and electrical properties compared to bulk polymer foams. The presence of the nanosized fillers facilitates heterogeneous bubble nucleation as a result, the number of bubbles increases while the average bubble size decreases. Therefore, the foam morphology can be controlled by the size, concentration, and surface chemistry of the nanofiller. In the current study, we used supercritical carbon dioxide as a foaming agent for silica/poly(methyl methacrylate), PMMA, foams. The silica nanoparticles were chemically modified by fluoroalkane chains to make them CO$_{2}$-philic. The surface coverage was controlled via tethering density, and the effect of silica surface coverage and concentration on foam morphology was investigated through scanning electron microscopy and image processing. Results indicated that nanofiller concentration and filler surface chemistry (CO$_{2}$-philicity) had tremendous effect on foam morphology but surface coverage did not have any effect. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y45.00013: Drainage dynamics of aqueous foams generated by sparging and turbulent mixing Matthew J. Kennedy, Michael W. Conroy, Ramagopal Ananth, James W. Fleming We investigate the effect of bubble size on the drainage dynamics of aqueous fire-suppression foams using laboratory-scale foam generators and theoretical modeling. We generate foams over a wide range of bubble sizes using two foam generation methods--sparging using fritted sheets of steel, and turbulent mixing using high-pressure T-junctions. The sparged foams comprise bubbles of mean diameter 0.5 mm or larger and begin draining immediately whereas the turbulently mixed foams comprise bubbles of mean diameter 0.15 mm or smaller and begin draining after induction times of 5-15 minutes. We study two proprietary fire-suppression foam solutions: a non-fluorinated surfactant solution containing viscous additives intended for use as a wet foam, $i.e.$ liquid fraction $>$ 0.1, and a sodium dodecyl sulfate surfactant solution intended for use as a dry foam, $i.e.$ liquid fraction $<$ 0.005. The change in liquid retention time due to change in mean bubble size differs between these two solutions. We compare our experimental results with theoretical models to examine the reasons for the difference in liquid retention time. [Preview Abstract] |
Session Y47: Soft Matter Physics of Biological Systems
Sponsoring Units: DPOLYChair: Maria Santore, University of Massachusetts
Room: 160C
Friday, March 2, 2012 8:00AM - 8:12AM |
Y47.00001: Antiviral activity of squalamine: Role of electrostatic membrane binding Bernard Beckerman, Wei Qu, Abhijit Mishra, Michael Zasloff, Gerard Wong, Erik Luijten Recent work\footnote{M. Zasloff \emph{et al.}, Proc. Nat. Acad. Sci. (USA) \textbf{108}, 15978 (2011).} has demonstrated that squalamine, a molecule found in the liver of sharks, exhibits broad-spectrum antiviral properties. It has been proposed that this activity results from the charge-density matching of squalamine and phospholipid membranes, causing squalamine to bind to membranes and displace proteins such as Rac1 that are crucial for the viral replication cycle. Here we investigate this hypothesis by numerical simulation of a coarse-grained model for the competition between Rac1 and squalamine in binding affinity to a flat lipid bilayer. We perform free-energy calculations to test the ability of squalamine to condense stacked bilayer systems and thereby displace bulkier Rac1 molecules. We directly compare our findings to small-angle x-ray scattering results for the same setup. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y47.00002: Simulating ligand receptor binding at a membrane interface with graphics processing accelerated coarse-grained molecular dynamics Sharon M. Loverde, David N. LeBard, Zhengyu Ma, Michael L. Klein, Dennis E. Discher Motivated by a deeper understanding of the immunological synapse, we develop a molecular-based model to understand receptor-polymer/ligand binding at a membrane interface. ~ We examine the case of weak ligand binding in the limit of confined polymer chains as a function of chain length, binding constant, and system size. ~We utilize a coarse-grained (CG) model of poly(ethylene oxide) and dimyristoylphosphatidylcholine (DMPC) previously developed by the Klein group and mimic weak binding with a sticky potential. ~This work employs graphics processing units (GPU) to accelerate the CG-MD simulations, where each simulation is run with multiple random-walker replicas to enhance sampling and facilitate statistical convergence of physical observables. ~Our results demonstrate that such an aggressive combination of GPU acceleration with CG modeling can yield accurate and precise data on polymer-DMPC binding, and, more importantly, hints at the mechanism behind empirical data of polymer binding to a T-cell receptor protein. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y47.00003: Stabilization of concentration fluctuations in mixed membranes by hybrid lipids Benoit Palmieri, Samuel Safran Finite-size domains have been observed at the surface of cells. These lipids ``rafts'' are stable nanodomains enriched in saturated lipids and cholesterol. While line tension favors macrodomains, one explanation for raft stabilization suggests that the membrane composition is tuned close to a spinodal temperature. From this point of view, rafts are long-lived concentration fluctuations in the mixed phase. We propose a ternary mixture model for the cell membrane that includes hybrid lipids which have one saturated and one unsaturated hydrocarbon chain. Finite amount of hybrid lipids reduces the packing incompatibility at the saturated/unsaturated lipid interface and stabilizes the concentration fluctuations. Hybrid-Hybrid interactions are included in the model and further increase the life-time of the rafts and decrease their length-scales. Moreover, the hybrid has extra orientational degrees of freedom that may lead to modulated phases. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y47.00004: Cooperativity in cholesterol-induced demixing of saturated and unsaturated lipids James Kindt The ternary DPPC/DOPC/cholesterol system exhibits a phase separation between liquid-ordered and liquid-disordered domains that may capture some features of demixing believed to occur in biomembranes. Using semi-grand canonical ensemble Monte Carlo approaches on a model with atomistic detail, we have investigated the degree of non-ideality of mixing of DPPC and DOPC as influenced by cholesterol. While the signature of phase separation is observed, more surprising is that in the region of composition space characterized by low DPPC and low cholesterol content, the mixing is approximately ideal. Through simple models we have shown that this behavior is inconsistent with a simple nearest-neighbor description of the interactions between these membrane components. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y47.00005: Steric Repulsion and Compressibility of Protein Resistant Brushes Maria Santore End-grafted polymer brushes, such as polyethylene glycol, have become commonplace as biocompatibilizers for medical devices and diagnostic surfaces. Key to their non-fouling character is the brush's steric repulsion towards biomolecules and cells. By functionalizing the substrate with small (order 10 nm) bioadhesive features around which the brush is placed, we gain insight into the repulsion between biomolecules and cells with the brush itself. While approximately exponential compression profiles are to be expected, some features of protein interactions with these brushes are unexpected, especially for small proteins whose dimensions are within a factor of 2 or 3 of the brush persistence length. The scaling of the compressive force, for example inferred from series of studies that vary the amounts and spacings of the adhesive elements, is weakly dependent on protein size, while one might expect a proportionality between this force and the effective protein footprint. These results are consistent with entropically inexpensive chain reconfigurations around the smaller proteins and the penetration of these proteins at least partially into the brush. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y47.00006: Topological defects induced by the retina's curvature improve vision Johnatan Aljadeff, Tatyana Sharpee The theory of disclinations and dislocations on curved surfaces predicts the length and density of grain boundary scars on a sphere. These predictions were successfully tested with colloids on droplets for systems satisfying $5\le R/a\le20$ ($R$- sphere radius, $a$- lattice constant). The foveal cone mosaic is another realization of this problem, for which $R/a\sim10^{4}$. New theories are needed to extend current predictions for scar length and density to this regime. We present a method of introducing the effect of irregularities that changes the prediction in the relevant regime. We do so by deriving a noise induced disclination density which truncates the scars: the cone density is mapped to an effective displacement $h_{eff}$ from the sphere; then the deviation from the constant curvature is computed to first order in $h_{eff}$; and finally the effective curvature is compared to a threshold above which noise induced disclinations appear. We compare stimuli projected on mosaics and on jittered lattices and show that the curvature induced correlations in the mosaics reduce aliasing by a factor of up to 50. This reduction increases with spatial frequencies, meaning that anti-aliasing is maximal in the visual acuity limit. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y47.00007: Exploring the low temperature thermodynamics of lattice proteins and polymers with chain lengths $> 1000$ Thomas Wuest Coarse-grained (lattice-) models have a long tradition in aiding to decipher the physical or biological complexity of polymers and proteins. Despite their simplicity however, numerical simulations of such models are often computationally very demanding and the quest for efficient algorithms is as old as the models themselves. I present a computational method based on Wang-Landau sampling in combination with suitable trial move sets which is particularly effective to study models such as the hydrophobic-polar (HP) lattice model of protein folding or its counterpart in polymer physics, the interactive self-avoiding walk (ISAW) at low temperatures. The approach provides a versatile and powerful mean for both the ground state search and the determination of the entire energy density of states (DOS) yielding reliable estimates of thermodynamic quantities for chain lengths $> 4000$ (ISAW) even in the very dense collapsed phase. The appearance of multiple low temperature pseudo-transitions for ISAWs will be elucidated. Further methodological improvements will be discussed. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y47.00008: Long-range mechanical force in epithelial tubule self assembly Chin-Lin Guo In vivo, epithelial cells can respond to extracellular matrix (ECM) molecules, type I collagen (COL), and switch their morphology from a lobular lumen (100-200 micron) to a tubular lumen (1mm-1cm). However, the mechanism is unclear. Using a temporal control of cell-ECM interaction, we find that epithelial cells, in response to a fine-tuned percentage of COL in ECM, develop various linear patterns. Remarkably, these patterns allow cells to self-assemble into a tubule of length $\sim $ 1cm and diameter $\sim $ 400 micron in the liquid phase. In contrast with conventional thought, the linear patterns arise through bi-directional transmission of traction force, but not through diffusible biochemical factors secreted by cells. In turn, the transmission of force evokes a long-range ($\sim $ 600 micron) intercellular mechanical interaction. A feedback effect is encountered when the mechanical interaction modifies cell positioning and COL alignment. Micro-patterning experiments further reveal that such a feedback is a novel cell-number-dependent, rich-get-richer process, which allows cells to integrate mechanical interactions into long-range ($>$ 1mm) linear coordination. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y47.00009: Bottom-up study of flaw tolerance properties of protein networks Zhao Qin, Markus Buehler We study the material properties of an intermediate filament proten network by computational modeling using a bottom-up approach. We start with an atomic model of each filament's and obtain the mechanical behavior of them. We then use these parameters in setting up a mesoscale model of the network material at scales of micrometers. Using this multi-scale method, we report a detailed analysis of the associated deformation and failure mechanisms of this hierarchical material. Our modeling reveals that a structure transition that occurs at the proteins' secondary structure level is crucial for the networks' flaw tolerance property, which implies that the material retains its mechanical function despite the existence of large defects. We also examine the effect of crosslink strength on the failure properties. We discover that relatively weaker crosslinks lead to a more flaw tolerant network that is 23{\%} stronger. This unexpected behavior is caused by that the crosslink strength functions as a switch to alter the failure mechanism. Weak crosslinks are able to efficiently diffuse the stress around the crack tip, making the crack more difficult to propagate. We compare our results to that of elastic and softening materials and find that the effect of crosslink strength is much smaller in those systems. These findings imply that the mechanical properties of both the filaments and interfaces among filaments are critical for bioinspired material designs, challenging the conventional paradigm in engineering design. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y47.00010: From Nano to Micro: Importance of Structure and Architecture in Spider Silk Adhesives Vasav Sahni, Ali Dhinojwala Spiders have developed outstanding adhesives over millions of years of evolution for prey capture and locomotion. We show that the structure and architecture of these adhesives play an important role in the adhesion. The adhesive produced by orb-weaving spiders to capture prey (viscid glue) is laid on a pair of silk fibers as micron-size glue drops composed of salts and glycoproteins. By stretching single drops, we show that viscid glue behaves like a viscoelastic solid and that elasticity is critical in enhancing adhesion caused by specific adhesive ligands by over 100 times. Comparing viscid glue with gumfoot glue, the glue produced by cob-weavers, the evolutionary descendants of orb-weavers, showed that, in spite of being produced in homologous aggregate glands, gumfoot glue behaves like a viscoelastic liquid. Moreover, gumfoot glue is humidity-resistant and viscid glue is humidity-sensitive. We use a synthetic strategy to spin beads-on-a-string (BOAS) architecture to mimic the adhesive properties of spider silk. Using these mimic threads, we show that the BOAS structure adheres more than a cylindrical structure during contact (collision of prey) and during separation (escape attempt of prey). These results inspire design of novel tunable adhesives. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y47.00011: Light scattering studies of human crystallin proteins and loss of transparency in cataracts Benjamin Mohr, Murugappan Muthukumar The human lens derives its transparency and refractive index from the interactions between crystallin proteins ($\alpha$-, $\beta$-, $\gamma$-crystallin). When the ordering of these crystallins is perturbed, insoluble macromolecular aggregates of crystalline proteins can occur resulting in cataracts. Using dynamic light scattering (DLS) and fast protein liquid chromatography (FPLC), we have conducted a detailed study of the formation of these aggregates. Our DLS results on $\gamma$-crystallin solutions exhibit the occurrence of slow and fast modes demonstrating the spontaneous formation of aggregates (hydrodynamic radius, Rh $\sim$ 200 nm) in equilibrium with monomeric proteins (Rh $\sim$ 3 nm). On the other hand, DLS results on $\alpha$-crystallin solutions clearly demonstrate that $\alpha$-crystallin molecules exist as a single population (Rh $\sim$ 18 nm). Our results on mixtures of $\alpha$- and $\gamma$-crystallin solutions show that the $\alpha$-crystallin tends to demolish the clumps of $\gamma$-crystallin. Our exploration of environmental effecs (temperature, pH, salt concentration) has revealed the macromolecular mechanism of dissolution of crystallin aggregates, providing a strategy for cataract prevention and insight into protein-protein interactions. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y47.00012: Quantifying the abnormal hemodynamics of sickle cell anemia Huan Lei, George Karniadakis Sickle red blood cells (SS-RBC) exhibit heterogeneous morphologies and abnormal hemodynamics in deoxygenated states. A multi-scale model for SS-RBC is developed based on the Dissipative Particle Dynamics (DPD) method. Different cell morphologies (sickle, granular, elongated shapes) typically observed in deoxygenated states are constructed and quantified by the Asphericity and Elliptical shape factors. The hemodynamics of SS-RBC suspensions is studied in both shear and pipe flow systems. The flow resistance obtained from both systems exhibits a larger value than the healthy blood flow due to the abnormal cell properties. Moreover, SS-RBCs exhibit abnormal adhesive interactions with both the vessel endothelium cells and the leukocytes. The effect of the abnormal adhesive interactions on the hemodynamics of sickle blood is investigated using the current model. It is found that both the SS-RBC - endothelium and the SS-RBC - leukocytes interactions, can potentially trigger the vicious ``sickling and entrapment'' cycles, resulting in vaso-occlusion phenomena widely observed in micro-circulation experiments. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y47.00013: The energetics of tightly bent DNA: a composite elastica model including local melting Arthur Evans, Alex Levine Melting transitions are well-known to be affected by the application of mechanical stress. Motivated by the experiments of Zocchi and collaborators (Qu and Zocchi 2011, EPL \textbf{94} 18003), we explore the effect of the application of mechanical stress on DNA melting in a particular composite of a stiff double stranded piece of DNA (dsDNA), shorter than its own persistence length, whose ends are linked by a flexible single stranded piece of DNA (ssDNA). The flexible ssDNA acts as a Gaussian polymer coil bending the stiff dsDNA through an elastic force that is controllable by the length of the ssDNA chain. In this talk we present theoretical predictions for two experimentally accessible features: the degree of local dsDNA melting and the local elastic energy of the dsDNA/ssDNA construct both as a function of the length of the attached ssDNA. We also address the effect of introducing a nick (broken covalent bond) in the dsDNA backbone on these results and discuss the implications of such data on the relative importance of backbone elasticity versus base stacking and base pairing interactions in determining the elasticity of dsDNA. This work also addresses open questions in the nonlinear elasticity of DNA in tightly bent curves. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y47.00014: Dispersion-relation phase spectroscopy of neuron transport Ru Wang, Zhuo Wang, Larry Millet, Martha Gillette, Joseph Robert Leigh, Nahil Sobh, Alex Levine, Gabreil Popescu Molecular motors move materials along prescribed biopolymer tracks. This sort of active transport is required to rapidly move products over large distances within the cell, where passive diffusion is too slow. We examine intracellular traffic patterns using a new application of spatial light interference microscopy (SLIM) and measure the dispersion relation, i.e. decay rate vs. spatial mode, associated with mass transport in live cells. This approach applies equally well to both discrete and continuous mass distributions without the need for particle tracking. From the quadratic experimental curve specific to diffusion, we extracted the diffusion coefficient as the only fitting parameter. The linear portion of the dispersion relation reveals the deterministic component of the intracellular transport. Our data show a universal behavior where the intracellular transport is diffusive at small scales and deterministic at large scales. We further applied this method to studying transport in neurons and are able to use SLIM to map the changes in index of refraction across the neuron and its extended processes. We found that in dendrites and axons, the transport is mostly active, i.e., diffusion is subdominant. [Preview Abstract] |
Session Y48: Focus Session: Statistical Physics of Active Systems Away From Detailed Balance: Swimmers and All That
Sponsoring Units: DPOLY DBIOChair: Daniel Needleman, Harvard University
Room: 161
Friday, March 2, 2012 8:00AM - 8:12AM |
Y48.00001: Flow-controlled densification of E. Coli through a constriction Gast\'on Mi\~no, Ernesto Altshuler, Carlos P\'{e}rez-Penichet, Lenin del R\'{i}o, Anke Lindner, Annie Rousselet, Eric Clem\'{e}nt Bacterial suspensions are examples of ``active matter.'' Each bacteria can be regarded as a self-propelled particle that interacts hydrodynamically with its environment, including the surrounding ``passive'' fluid, the boundaries and other bacteria. In this presentation, we show a new phenomenon concerning E. Coli suspensions flowing through a funnel-like constrictions in micro-fluidic channels. The dynamics of bacterial suspensions flowing in confined spaces is relevant to understand their behavior in scenarios such as porous materials, soil, microbiology, water purification, and biomedical research. The applied flow induces a counter-intuitive symmetry breaking in the bulk bacteria concentration, which is found to increase past the funnel. The concentration enhancement persists over large distances and its amplitude increases linearly with the flow rate and disappears at large flow values. We show that the effect is reversible when the flow direction is reversed. We explain the observed effects on the interactions between the active bacteria and the channel boundaries. This experiment opens the possibility to control the concentration bacterial suspensions in microfluidic channels by simply tuning the flow of liquid. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y48.00002: Bidirectional sorting of flocking particles in the presence of asymmetric barriers Jeffrey Drocco, Charles Reichhardt, Cynthia Reichhardt We numerically demonstrate bidirectional sorting of flocking particles interacting with an array of funnel-shaped barriers. The particles choose a swimming direction by averaging the headings of their neighbors according to the Vicsek model, and experience additional steric interactions as well as repulsion from the fixed barriers. We show that particles preferentially localize to one side of the barrier array over time, and that the direction of this rectification can be reversed by adjusting the particle-particle exclusion radius or the noise term in the equations of motion. These results provide a conceptual basis for isolation and sorting of single- and multi-cellular organisms which move collectively according to flocking-type interaction rules. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y48.00003: Microfluidic one-way streets for algae Jorn Dunkel, Vasily Kantsler, Marco Polin, Raymond E. Goldstein Controlling locomotion and transport of microorganisms is a key challenge in the development of future biotechnological applications. Here, we demonstrate the use of optimized microfluidic ratchets to rectify the mean swimming direction in suspensions of the unicellular green alga Chlamydomonas reinhardtii, which is a promising candidate for the photosynthetic production of hydrogen. To assess the potential of microfluidic barriers for the manipulation of algal swimming, we studied first the scattering of individual C. reinhardtii from solid boundaries. High-speed imaging reveals the surprising result that these quasi-spherical ``puller''-type microswimmers primarily interact with surfaces via direct flagellar contact, whereas hydrodynamic effects play a subordinate role. A minimal theoretical model, based on run-and-turn motion and the experimentally measured surface-scattering law, predicts the existence of optimal wedge-shaped ratchets that maximize rectification of initially uniform suspensions. We confirm this prediction in experimental measurements with different geometries. Since the mechano-elastic properties of eukaryotic flagella are conserved across many genera, we expect that our results and methods are applicable to a broad class of biflagellate microorganisms. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y48.00004: Concentrating bacterial cells using a ratchet system: a lattice Monte Carlo simulation study Yuguo Tao, Gary Slater Rectification of motile E. coli bacteria has been observed in the presence of funnel-like channels. We present a lattice Monte Carlo model which takes into account both the size and the mechanical and thermodynamic properties of autonomous bacterial cells. The motion of the cells is composed of alternating run and tumble periods. We show that the rectification effect of the funnels is strongly dependent upon the effective random walk step length of the run/tumble cycle as well as the size of the funnel's aperture. Our results agree with experimental observations, and also confirm some conclusions from a previous simulation model of point-like bacteria. We also explore series of funnels as a means to pump and concentrate cells. We observe deviations from theoretical predictions when the size of the cells is comparable to that of the aperture of the funnel. The current model can be extended to study cells with different shapes, e.g. cigar-shape bacteria. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y48.00005: Spontaneous Segregation of Self-Propelled Particles with Different Motilities Samuel McCandlish, Aparna Baskaran, Michael Hagan We study mixtures of self-propelled and passive rod-like particles in two dimensions using Brownian dynamics simulations. The simulations demonstrate that the two species spontaneously segregate to generate a rich array of dynamical domain structures whose properties depend on the propulsion velocity, density, and composition. In addition to presenting phase diagrams as a function of the system parameters, we investigate the mechanisms driving segregation. We show that the difference in collision frequencies between self-propelled and passive rods provides a driving force for segregation, which is amplified by the tendency of the self-propelled rods to swarm or cluster. Finally, both self-propelled and passive rods exhibit giant number fluctuations for sufficient propulsion velocities. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y48.00006: Active Brownian Particles with Active Fluctuations Pawel Romanczuk, Robert Grossmann, Lutz Schimansky-Geier We study the effect of different types of noise on the dynamics of self-propelled particles with variable speed (active Brownian particles). We distinguish between passive and active fluctuations. Passive fluctuations are considered independent of the direction of particle's motion (e.g., thermal fluctuations). In contrast, active ones are assumed to be intrinsically connected with the propulsion mechanism of the active particle and, as a result, correlated with its time-dependent orientation. We calculate the stationary speed and velocity probability density functions of non-interacting active Brownian particles in the presence of both fluctuation types and discuss the generic signature of active fluctuations [1]. Furthermore, we discuss a model of active Brownian particles interacting via a velocity-alignment force [2]. We show, based on the results of a corresponding mean-field theory, how the type of fluctuations has a strong impact on the onset and stability of collective motion. \\[4pt] [1] P. Romanczuk and L. Schimansky-Geier, {\em Phys Rev Lett}, {\bf 106}, 230601 (2011) \\[0pt] [2] P. Romanczuk and L. Schimansky-Geier, {\em Ecol Compl}, in press, doi:10.1016/j.ecocom.2011.07.008 (2011) [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y48.00007: Run and tumble, run and reverse, or run reverse and flick - who wins the chemotaxis race? Vasily Zaburdaev, Sergey Denisov, David Weitz Run and tumble of {\em E.coli} bacteria is a well understood example of the stochastic cell motion that is alternated in the presence of signaling chemicals. By regulating the tumbling frequency bacteria are able to navigate toward the food sources. Another bacteria that use twitching to move on a surface, {\em M. xanthus}, utilize a different strategy - at the end of the run they completely reverse the direction of motion and continue moving in the opposite direction. The frequency of reversals was shown to be connected to the chemotactic response of the cell. Recently yet another pattern was discovered in marine bacteria {\em V. alginolyticus} which alternate sharp reversals with flicks -- making a turn to an angle with a broad distribution and centered around 90 degrees. In this work we are presenting a theoretical framework that describes all above motion patterns. As a highlight of the developed approach we find the exact analytical expressions for the mean squared displacement of moving cells for arbitrary distribution of run times. That allows us to quantitatively compare the performance of bacteria exploring the environment with and without signaling chemicals and, therefore, to find the winner of the chemotactic race. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y48.00008: Kinetic theory for systems of self-propelled particles Yen-Liang Chou, Thomas Ihle Models of self-driven particles similar to the Vicsek model [Phys. Rev. Lett. 75 (1995) 1226] are studied by means of kinetic theory. In these non-equilibrium models, particles try to align their travel directions with the average direction of their neighbors. At strong alignment a global flocking state forms. The alignment is defined by a stochastic rule, not by a Hamiltonian. The corresponding interactions are non-additive and are typically of genuine multi-body nature. The theory [1] is based on a Master equation in 3N-dimensional phase space, which is made tractable by means of the molecular chaos approximation. The phase diagram for the transition to collective motion is calculated and compared to direct numerical simulations. A stability analysis of a homogeneous ordered state is performed, which reveals a long wave length instability for some of the considered models. The mean-field calculations of one of the models show a tricritical point where the flocking transition changes its character from continuous to discontinuous. \\[4pt] [1] T. Ihle, Phys. Rev. E 83 (2011) 030901 [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y48.00009: Diffusional Variation in Autonomously Motile Catalytic Janus Particles R. Lloyd Carroll, Shengrong Ye, Zachary Jones Recently, we and others have studied the motion of Janus particles that undergo catalytically-induced motion in an appropriate chemical environment. Published studies indicate particles move with reduced rotational diffusion, resulting in enhanced effective diffusion coefficient -- motile particles move longer distances between each random rotation, resulting in much greater overall motion. We undertook to study the effective diffusion coefficient of janus particles with various structural designs (symmetric and asymmetric anisotropy, angular dislocation of catalytic regimes) and under environmental stresses (external fields, imposed gradients). We find that structural features have a strong effect on the characteristics of observed motion and effective diffusion, and that environmental stresses can change particle dynamics, suppressing rotational diffusion and enhancing correlated motion. These results are consistent, with some caveats, as particles grow smaller, and the effects of Brownian motion become more pronounced. Deeper understanding of the statistical behavior of the systems results in particles with practical applications in transport on a microfluidic scale. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:24AM |
Y48.00010: Emergent Collective Behavior of Microscopic Swimmers Invited Speaker: Guillaume Lambert The individual components of a biological system often express simple behaviors that lead to the spontaneous emergence of order and high levels of complexity; the whole is greater than the sum of its parts. Emergent complexity may arise spontaneously or, more interestingly, in response to external forces present in the surrounding environment. Here, we study the emergence of the emergent collective behavior of swimming {\em Escherichia coli} bacteria inside microfabricated environments. We first demonstrate how the swimming dynamics of single bacterium cells inside jagged, funnel-shaped geometries leads to the emergence of complex migratory patterns. In particular, by creating ratchet-like barriers that redirect the motion of single E. coli cells, we demonstrate that while a single bacterium is unable to ``escape'' an array of ratchets, a {\em population} of cells are able to collectively navigate against these motion-rectifying barriers. We then investigate the collective behavior of cells at increasing densities and witness the emergence of multicellularity in bacteria. As the local density of cell increases to reach physiologically relevant concentrations, individual swimming cells respond to the cell-cell interactions and collectively assemble into biofilms. We describe the physiological development of such biofilms inside microfluidics devices and, using in situ measurements, study the physical properties of various biofilms from their motion and deformation. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y48.00011: Active crystallization of Artificial Microswimmers Jeremie Palacci, Stefano Sacanna, Joyce Laine, Asher Preska Steinberg, David Pine, Paul Chaikin A novel type of light activated microswimmers is used to drive the system far from equilibrium. For sufficient concentrations they spontaneously assemble in crystalline clusters of particles. These clusters are mobile, with complex dynamics --explosion, self healing...-- and can reversibly melt down if the activity is shut down. The origin of the attraction between the active particles as well as the crystallization mechanism and the complex dynamics will be presented and rationalized quantitatively. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y48.00012: Collective motility of cells on deformable substrates Arvind Gopinath, Michael Hagan, Bulbul Chakraborty, Aparna Baskaran We consider a simple model for motile cells on deformable substrates. The cells interact through contact interactions and medium-mediated elastic interactions arising due to the adhesion of the cells to the substrate. Using Brownian dynamics simulations and systematic coarse graining of the microscopic model, we characterize the collective velocity field of such a collection of cells by calculating the velocity autocorrelation function and identify the role of elastic interactions in the emergent motility of cell sheets. Our findings include suppression of diffusivity due to the attractive part of the elastic interaction and a growing length scale in the velocity correlations. [Preview Abstract] |
Session Y49: Focus Session: Organic Electronics and Photonics - Small Molecules and General Advances
Sponsoring Units: DMP DPOLYChair: Richard Lunt, Michigan State University
Room: 162A
Friday, March 2, 2012 8:00AM - 8:12AM |
Y49.00001: Charge transport calculations of organic semiconductors by the time-dependent wave-packet diffusion method Hiroyuki Ishii, Nobuhiko Kobayashi, Kenji Hirose Organic materials form crystals by relatively weak Van der Waals attraction between molecules, and thus differ fundamentally from covalently bonded semiconductors. Carriers in the organic semiconductors induce the drastic lattice deformation, which is called as polaron state. The polaron effect on the transport is a serious problem. Exactly what conduction mechanism applies to organic semiconductors has not been established. Therefore, we have investigated the transport properties using the Time-Dependent Wave-Packet Diffusion (TD-WPD) method [1]. To consider the polaron effect on the transport, in the methodology, we combine the wave-packet dynamics based on the quantum mechanics theory with the molecular dynamics. As the results, we can describe the electron motion modified by (electron-phonon mediated) time-dependent structural change. We investigate the transport property from an atomistic viewpoint and evaluate the mobility of organic semiconductors. We clarify the temperature dependence of mobility from the thermal activated behavior to the power law behavior. I will talk about these results in my presentation. [1] H. Ishii, N. Kobayashi, K. Hirose, Phys. Rev. B, 82 085435 (2010). [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y49.00002: Investigation of Interfaces between Sub-Phthalocyanine and C60 using First-Principles Calculations Hossein Hashemi, Xiao Ma, John Kieffer, Steven E. Morris, Max Shtein, Shaohui Zheng, Eitan Geva, Barry Dunietz The structure and electronic properties of a boron subphthalocyanine (SubPc) adsorbed on buckminsterfullerene (C60) and of C60 on SubPc surfaces, mimicking reverse orders of deposition, have been studied using density-functional theory (DFT) including long-van der Waals. Total-energy calculations are used to elucidate the initial adsorption of SubPc on C60 low index surfaces and also C60 on SubPc surfaces. The energetics of crystalline substrates with different surface terminations were mapped out using a single molecule of the partnering species. Accordingly, the interfacial structure and properties are different depending on whether the substrate is SubPc or C60, due to the incongruency between lattices and the disorder that develops in the contact layers of C60 and SubPc, respectively. The dependence of the charge transfer energies on the interface morphology is studied using range separated hybrid functionals. The stabilization of charge transfer states to below the absorbing state, needed to optimize the fill factor, also depends on the order of layer deposition. These results are discussed in the context of experiments performed on organic solar cells, showing trade-offs in the short circuit current and open circuit voltage with varying deposition order of the organic layers. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y49.00003: First principles study of the electrical and optical properties of SubPc single crystal Xiao Ma, Hossein Hashemi, John Kieffer We studied the electrical and optical properties of the single crystal of boron subphthalocyanine chloride (SubPc), a popular donor material used in organic photovoltaic (OPV) devices within the framework of density-functional theory (DFT) with added van der Waals long range interactions to deepen our understanding of its performance in light absorption and charge transport. We calculated the frequency-dependent dielectric response, refractive index, extinction coefficient, and intrinsic charge mobility. The complex dielectric constant was computed using first-order perturbation theory, using the electronic wave functions and eigenvalues obtained from supercell DFT calculations. This was done for wave propagation in the (001), (010), (100) directions of optimized SubPc crystal with DFT calculations, revealing significant anisotropy in both electrical and optical properties. Comparison with experimental results allows us to draw conclusions regarding the structural organization of SubPc molecules deposited on a variety of substrates, as well as the conditions for thin film growth and property optimization. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y49.00004: Density-functional Theory and Beyond for Donor-Acceptor Complexes: The Example of TTF/TCNQ Viktor Atalla, Mina Yoon, Matthias Scheffler We study the performance of density-functional theory (DFT) with various exchange-correlation (XC) functionals in describing electronic and structural properties of the prototypical donor/acceptor complex TTF/TCNQ. We find that the binding energetics and the amount of electron transfer between TTF and TCNQ depends strongly on the functional. In particular, all semilocal functionals give rise to significant, aritificial electron transfer due to a wrong ordering of Kohn-Sham (KS) levels. We consider the HSE [1] ``family'' of XC functionals using the fraction of exact exchange ($\alpha$) as adjustable parameter. The optimum XC functional is then identified as that for which the $G_0W_0$ quasiparticle correction to the energy gap of the KS LUMO of the acceptor and the HOMO of the donor is minimized. We obtain $\alpha \sim 0.8$ which gives an electronic level alignment that is consistent with experiment and free from spurious asymptotic charge transfer. We conclude that the proposed scheme improves the KS spectrum, and that the investigated TTF-TCNQ dimer exhibits intra-molecular electron-density rearrangement rather than electron transfer. \\[4pt] [1] A.V. Krukau, et al., J. Chem. Phys. {\bf 125}, 224106 (2006) [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y49.00005: Understanding the high device efficiency of a class of solution-processed small-molecule solar cells Andriy Zhugayevych, Olena Postupna, Sergei Tretiak, Guillermo Bazan We perform a first principles study of light absorption, exciton and charge carrier transport for two recently synthesized molecular crystals which in bulk heterojunction solar cells with PC70BM acceptor show up to 6.7\% power conversion efficiency. Our results distinguish the following factors important for this high efficiency. The large conjugation length facilitates strong light absorption with low-energy absorption edge. The crystalline ordering of properly oriented molecules leads to exciton delocalization over the typical size of the crystallites. The tightly packed pi-stacks allow for fast disorder-resistant hole transport along these stacks. Nevertheless the microscopic characteristics of the considered crystals are typical and comparable with other materials used in photovoltaics. Therefore we conclude that the main factors of the high device efficiency should be searched at the mesoscale including interfaces and grain boundaries. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y49.00006: Relating Molecular-Scale Structure to Spectroscopy in Pentacene-Perfluoropentacene Donor-Acceptor Assemblies from First-Principles Sahar Sharifzadeh, Leeor Kronik, Jeffrey Neaton Using van der Waals-corrected density functional theory and many-body perturbation theory, we compute the spectroscopic properties of the archetypal organic semiconductors pentacene (PEN), perfluoropentacene (PFP), and their composite donor-acceptor blends. Band structures, bulk crystal densities of states, and low-energy optical excitations are computed for the isolated bulk crystals and their composite assemblies. For the individual crystals, transport and optical gaps are in good agreement with experiment, and the nature and orientation of the excitonic wavefunctions is found to be sensitive to the degree of co-facial packing. For the PEN-PFP systems, different molecular arrangements and compositions are considered in an effort to connect to thin film measurements. The relationship between packing in these structures, the transport gap, and the nature and binding energies of low-lying excitons are explored. We acknowledge DOE-BES, NSF, and US-Israel BSF for support, and NERSC for computational resources. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y49.00007: Quantitative structural analysis of organic thin film deposition: a real time synchrotron X-ray scattering study Ishviene Cour, Christian Schlepuetz, Yongsoo Yang, Songtao Wo, Ron Pindak, Randall Headrick Direct writing gives us the ability to deposit films from solution with controlled thickness, grain structure and orientation. We have investigated TIPS-Pentacene films deposited from toluene solution at various speeds via a combination of real time synchrotron x-ray scattering and polarized-light video microscopy. Through video microscopy we observe a well-defined crystallization front that becomes less defined as the writing speed is increased. In synchrotron x-ray scattering we observe that the ordering process is an order of magnitude slower than what is seen under the optical microscope. This discrepancy in the apparent crystallization rate raises questions such as, which part of the film actually rotates the polarized light and becomes visible under crossed polarizers? Observation with varying speeds and substrate temperatures suggest that the crystallization first occurs near the top surface of the drying film, while subsurface regions remain in a disordered state for up to several seconds. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y49.00008: Diindenoperylene as donor and acceptor for organic photovoltaic cells Andreas Wilke, Ben Br\"oker, Johannes Frisch, Patrick Amsalem, Jens Niederhausen, Antje Vollmer, Norbert Koch In organic photovoltaic cells (OPVCs) typically two organic materials with electron acceptor and donor character are sandwiched between anode and cathode, forming heterojunctions where charge separation occurs. To improve the efficiency of charge separation, understanding the mechanisms of the energy level alignment at these heterojunctions is crucial. We report on ultraviolet photoelectron spectroscopy (UPS) measurements on three different organic-diindenoperylene (DIP) heterojunctions formed on PEDT:PSS electrodes. The measurement reveal that the energy level alignment of C$_{60}$ on DIP/PEDT:PSS corresponds to a type II heterojunction, with DIP acting as donor. The offset between the highest occupied molecular orbital (HOMO) of DIP and the lowest unoccupied molecular orbital (LUMO) of the acceptor C$_{60}$, an estimate for the maximum achievable open circuit voltage, is 1.35 eV. In contrast, the energy level alignment of DIP on sexithiophene (6T) and poly(3-hexylthiophene) (P3HT) is of type II as well, but DIP acting as acceptor. The offset between the HOMO of the donors 6T and P3HT and the LUMO of DIP is found to be 1.75 eV and 1.6 eV, respectively. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y49.00009: Gradual thickness change of CuPc on MoOx on Oxygen Plasma Treated ITO Chenggong Wang, Irfan Irfan, Yongli Gao The thickness dependence of copper phthalocyanine (CuPc) interlayer on molybdenum trioxide (MoO$_3$) and conducting indium tin oxide (ITO) has been investigated with ultraviolet photoemission spectroscopy (UPS). We also investigated the air exposure effect on the CuPc/MoO$_3$/ITO interlayers. It was found that the MoO$_3$ interlayer substantially increased the substrate work function (WF). With the deposition of CuPc the WF decreased and saturated at the thickness of 80 {\AA}. We also found that 3x10$^6$ Langmuir (L) air exposure decreased both the MoO$_3$ WF and the interface dipole between CuPc/MoO$_3$ interface. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y49.00010: Voltage dependence of the electric double layer structure at an ionic liquid/Au interface Yusuke Wakabayashi, Ryosuke Yamamoto, Hazuki Morisaki, Osami Sakata, Hidekazu Shimotani, Hongtao Yuan, Yoshihiro Iwasa, Tsuyoshi Kimura Ionic liquids (ILs) have been studied extensively because of their unique characteristics. One of the utilization of them is applying a strong electric field to solids through the electric double layer. Using this field, one can reduce the gate voltage for an organic field effect transistors (FETs) to operate [1]. In order to clarify the microscopic structure of such IL-gated organic FETs, we have performed synchrotron x-ray scattering experiments at BL-13XU of the SPring-8, Japan. While the electric double layer structures at IL-solid interfaces have been studied by x-ray reflectometry [2], the electric double layer is stabilized by the natural polarity of the surface of the solid. In order to observe the electric field effect, we measured an electric double layer formed at the interface between an IL and a Au(111) single crystal under external electric field. The reflectivity profile was found to depend on the applied electric field, which reflects the formation of the electric double layer. \\[4pt] [1] T.~Uemura {\it et al.}, Appl. Phys. Lett., {\bf 95}, 103301 (2009).\\[0pt] [2] M.~Mezger {\it et al.}, Science {\bf 322}, 424 (2008). [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y49.00011: A critical assessment of thiolate self-assembled monolayers (SAMs) on platinum. Yenny Cardona Quintero, Hong Zhu, Rampi Ramprasad Thiolate SAMs have been successfully anchored on metal surfaces, but a critical assessment of the impact on the structural and electronic properties of the metal surfaces has remained elusive. CH$_{3}$S and CF$_{3}$S were selected as model systems in this work, because of their simple structures which can provide insights about how the composition and electronegativity of SAMs affect the properties of metal-SAM systems. Density functional theory calculations have been used in this work to study the adsorption of CH$_{3}$S and CF$_{3}$S molecules on the Pt (111) surface at different coverage (1/3, 1/4, 1/6, 1/9 and 1/12) and adsorption sites (fcc and hcp). The geometry, adsorption energy and the work function of the Pt-SAM systems have been determined. Several interesting observations could be made: (1) the optimized SAM is tilted with respect to the Pt surface and the tilted angle decreases with the molecular coverage on the Pt surface; (2) the adsorption energy of both systems are almost always lower at the fcc site compared to the hcp one and shows a coverage-dependence; (3) the work function of Pt-SAM also shows a dependence on coverage and hence controlling the molecular coverage is probably an effective technique to tune the work function. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y49.00012: Nonlinear light propagation in photopolymers: from self-trapped beams to 3-D optical lattices Kalaichelvi Saravanamuttu While liquid crystals, surfactants and colloidal crystal systems assemble into ordered phases to attain free energy minima, strikingly complex patterns can also emerge when condensed matter systems are perturbed away from equilibria. This talk will be an overview of research in our group into the dynamics of light beams that propagate while simultaneously initiating free-radical polymerisation in photopolymers. The consequent nonlinear and reciprocal interactions between the optical field and self-induced refractive index changes in the medium elicit a rich assortment of three-dimensional spatial patterns. These include self-trapping bright and dark beams, beam filamentation due to modulation instability, diffraction rings due to self-phase modulation and the formation of 2-D and 3-D bright and dark optical lattices. The potential of these optical phenomena to spontaneously inscribe complex 3-D polymer architectures that are inaccessible through conventional lithographic techniques and that would have advanced optical applications such as nonlinear photonic crystals will also be described. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y49.00013: Enhancing sensing of nitroaromatic vapors by thiophene-based polymer films G. Nagarjuna, Abhishek Kumar, Akshay Kokil, Kedar Jadhav, Serkan Yurt, Jayant Kumar, D. Venkataraman Sensing of nitroaromatic-based explosives is important for homeland security and for the detection of landmines in war zones. These compounds are detected using fluorescence quenching of poly(phenyleneethynylene) (PPE). When compared with PPE, polythiophenes have several features that make them excellent candidates for sensing. However, due to strong $\pi -\pi $ aggregation in polythiophenes, the permeation of the analyte in the thin film is poor. Therefore polythiophenes have poor quenching efficiencies. We have addressed this problem by tuning the side chains on these polymers to disrupt the polymer aggregation, thereby enhancing the analyte diffusion into the polymer thin film. We have now developed a materials platform for next generation sensory materials based on polythiophenes. This talk will discuss our approach and the studies that showed enhanced sensing of nitroaromatics in polythiophene thin films. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y49.00014: Giant Electrocaloric Effect in Ferroelectric Polymers with Great Impact on Energy and Environment Xinyu Li, Xiaoshi Qian, Haiming Gu, Shengguo Lu, Qiming Zhang Refrigeration and air conditioning overall consume around 20{\%} of the energy budget in developed countries which necessitates a search for new approaches to increase the energy efficiency of these cooling technologies. Cooling technologies based on the electrocaloric effect (ECE) hold great potential and promise in realizing these goals. The electrocaloric effect (ECE) refers to the change in temperature and/or entropy of a dielectric material by an applied voltage. Recently, a class of P(VDF-TrFE) based ferroelectric polymers have been discovered that provide a giant electrocaloric effect with an adiabatic temperature change of $\Delta $T $\sim $ 20 K and an isothermal entropy change $\Delta $S $>$ 90 J/kgK at room temperature. This talk will review the earlier works in the ECE, as well as present the basic materials considerations and experimental results of the ECE in both normal and relaxor ferroelectric polymers. It will be shown he relaxor ferroelectric polymer displays a nearly flat ECE response over a broad temperature range, which is very attractive for practical cooling device applications Furthermore, we will present our recent investigation, exploiting the giant ECE in these polymers for cooling devices with compact size, high cooling power and efficiency. [Preview Abstract] |
Session Y50: Focus Session: Micro and Nano Fluidics III: Microtransport and Thermophysical Properties
Sponsoring Units: DPOLY DFDChair: Ralph Sperling, Harvard University
Room: 162B
Friday, March 2, 2012 8:00AM - 8:12AM |
Y50.00001: Cavitation in confined water: ultra-fast bubble dynamics Olivier Vincent, Philippe Marmottant In the hydraulic vessels of trees, water can be found at negative pressure. This metastable state, corresponding to mechanical tension, is achieved by evaporation through a porous medium. It can be relaxed by cavitation, i.e. the sudden nucleation of vapor bubbles. Harmful for the tree due to the subsequent emboli of sap vessels, cavitation is on the contrary used by ferns to eject spores very swiftly. We will focus here on the dynamics of the cavitation bubble, which is of primary importance to explain the previously cited natural phenomena. We use the recently developed method of artificial tress, using transparent hydrogels as the porous medium. Our experiments, on water confined in micrometric hydrogel cavities, show an extremely fast dynamics: bubbles are nucleated at the microsecond timescale. For cavities larger than 100 microns, the bubble ``rings'' with damped oscillations at MHz frequencies, whereas for smaller cavities the oscillations become overdamped. This rich dynamics can be accounted for by a model we developed, leading to a modified Rayleigh-Plesset equation. Interestingly, this model predicts the impossibility to nucleate bubbles above a critical confinement that depends on liquid negative pressure and corresponds to approximately 100 nm for 20 MPa tensions. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y50.00002: Multiphase flow within 3D porous media Sujit Datta, David Weitz Multiphase flow through porous media is important for a diverse range of processes including aquifer remediation, CO2 sequestration, and oil recovery. Despite its enormous importance, exactly how flow proceeds within a porous medium is unknown; the opacity of the medium typically precludes direct imaging of the flow. Here, we present an experimental technique to directly visualize multiphase flow within porous media. Using this approach, we show how heterogeneity strongly affects flow behavior during the drainage of porous media. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y50.00003: Modeling capillary filling of micropores with nanoparticle-filled binary fluid Yongting Ma, Amitabh Bhattacharya, Olga Kuksenok, Dennis Perchak, Anna C. Balazs We examine the behavior of binary fluids containing nanoparticles that are driven by capillary forces to fill well-defined pores of microchannels. To carry out these studies, we use a hybrid computational approach that combines the lattice Boltzmann model for binary fluids and a Brownian dynamics model for the nanoparticles. The hybrid model allows us to capture the interactions between the binary fluids and the nanoparticles, as well as model the interactions among the fluid, the nanoparticles and the pore walls. We show that the nanoparticles dynamically alter both the interfacial tension between the two fluids and the contact angle on the pore walls; this, in turn, strongly affects the dynamics of the capillary filling. We demonstrate that by tailoring the properties of the nanoparticles, such as their affinity to the fluid components and their interaction with the pore walls, one can effectively control both the filling velocities and the deposition of nanoparticles on the pore walls. Our findings provide fundamental insights into the dynamics of this complex system, as well as potential guidelines for technological processes involving capillary filling with nanoparticles in microchannels with differing geometries. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y50.00004: Label-free Screening of Multiple Cell-surface Antigens Using a Single Pore Karthik Balakrishnan, Matthew Chapman, Anand Kesavaraju, Lydia Sohn Microfluidic pores have emerged as versatile tools for performing highly sensitive measurements. Pore functionalization can result in slower particle transit rates, thereby providing insight into the properties of particles that travel through a pore. While enhancing utility, functionalizing with only one species limits the broader applicability of pores for biosensing by restricting the insight gained in a single run. We have developed a method of using variable cross-section pores to create unique electronic signatures for reliable detection and automated data analysis. By defining a single pore into sections using common lithography techniques, we can detect when a cell passes through a given pore segment using resistive-pulse sensing$_{.}$ This offers such advantages as 1) the ability to functionalize each portion of a pore with a different antibody that corresponds to different cell surface receptors, enabling label-free multianalyte detection in a single run; and 2) a unique electronic signature that allows for both an accelerated real-time analysis and an additional level of precision to testing. This is particularly critical for clinical diagnostics where accuracy and reliability of results are crucial for healthcare professionals upon which to act. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y50.00005: Tracking rotation and translation simultaneously in confined liquids Subhalakshmi Kumar, SungChul Bae, Steve Granick At the same spatially-resolved spots when fluid is confined to molecularly-thin spacings between atomically-smooth mica crystals, we track simultaneously, using fluorescence correlation spectroscopy and time correlated single photon counting, the translational and rotational diffusion of small dyes suspended in octamethylcyclotetrasiloxane (OMCTS). The spatially-resolved quantification of both dynamical quantities gives insight, as it does in bulk glasses, into the origins of dynamical heterogeneity in confined fluids. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y50.00006: Neutron Scattering Applications for Characterizing Phase Behavior and Dynamics of Confined Fluids in Nanoporous Materials Yuri Melnichenko Fluid-solid interactions in natural and engineered porous solids underlie variety of technological processes, including sequestration of anthropogenic greenhouse gases, hydrogen storage, membrane separation, and catalysis. The size, distribution and interconnectivity of pores, the chemical and physical properties of the solid and fluid phases collectively dictate how fluid molecules migrate into and through the micro- and mesoporous media, adsorb and ultimately react with the solid surfaces. Due to the high penetration power and relatively short wavelength of neutrons, small-angle neutron scattering (SANS) as well as quasi elastic neutron scattering (QENS) techniques are ideally suited for \textit{in situ} studies of the structure and phase behavior of confined fluids under pressure as well as for evaluating structure of pores in engineered and natural porous systems. It has been demonstrated recently that SANS and USANS can also be used for evaluating the volume of closed pores as a function of pore sizes in the range from micrometer to sub-nanometer pores. In this talk I will overview some recent developments in the SANS and QENS methodology and give several examples of how it can be used for in-situ studies of the adsorption and dynamics of carbon dioxide and methane in porous fractal silica and carbon aerogels as well as characterizing the abnormal densification of hydrogen in activated carbons at ambient temperatures. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y50.00007: A mathematical model for the transport of a solute through a porous-walled tube Ian Griffiths, Rebecca Shipley Predicting the distribution of solutes or particles in flows within porous-walled tubes is essential to inform the design of cross-flow filtration devices. Here we use Taylor-dispersion theory to derive a radially averaged model for solute transport in a tube with porous walls, where the wall Darcy permeability may vary both spatially and in time. Crucially, this model includes solute advection via both radial and axial flow components, as well as diffusion, and the advection, diffusion and uptake coefficients in the averaged equation are explicitly derived. The model is used to explore the specific example of a hollow-fibre membrane bioreactor for tissue engineering applications - here membrane fouling and cell population expansion mean that the effective membrane permeability is intrinsically coupled to both fluid flow and nutrient transport. We conclude by presenting design considerations that promote spatially uniform cell population growth. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y50.00008: Biased and flow driven Brownian motion in periodic channels S. Martens, A. Straube, G. Schmid, L. Schimansky-Geier, P. H\"anggi In this talk we will present an expansion of the common Fick-Jacobs approximation to hydrodynamically as well as by external forces driven Brownian transport in two-dimensional channels exhibiting smoothly varying periodic cross-section. We employ an asymptotic analysis to the components of the flow field and to stationary probability density for finding the particles within the channel in a geometric parameter. We demonstrate that the problem of biased Brownian dynamics in a confined $2$D geometry can be replaced by Brownian motion in an effective periodic one-dimensional potential $\Psi(x)$ which takes the external bias, the change of the local channel width, and the flow velocity component in longitudinal direction into account. In addition, we study the influence of the external force magnitude, respectively, the pressure drop of the fluid on the particle transport quantities like the averaged velocity and the effective diffusion coefficient. The critical ratio between the external force and pressure drop where the average velocity equals zero is identified and the dependence of the latter on the channel geometry is derived. Analytic findings are confirmed by numerical simulations of the particle dynamics in a reflection symmetric sinusoidal channel. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y50.00009: ABSTRACT WITHDRAWN |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y50.00010: Crossover from the Hydrodynamic to the Kinetic Regime in Confined Nanoflows Charles Lissandrello, Victor Yakhot, Kamil L. Ekinci We present an experimental study of a confined nanoflow. The nanoflow is generated in a simple fluid by a sphere oscillating in the proximity of a flat solid wall. Varying the oscillation frequency, the confining length scale and the fluid mean free path over a broad range provides a detailed map of the flow. We use this experimental map to construct a scaling form, which seamlessly describes the nanoflow in both the hydrodynamic and the kinetic regimes. Furthermore, our scaling form unifies previous theories based on the slip boundary condition and the effective viscosity. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y50.00011: Viscosity of ultrathin water films confined between oxide surfaces -- ab initio and classical molecular dynamics simulations Peter J. Feibelman, Gary S. Grest, Neil Haria, Christian D. Lorenz We compare estimates based on ab initio (DFT/PBE) and on classical molecular dynamics simulations of the viscosity of 2, 3 and 5-layer water films confined between hydrophilic kaolinite surfaces. Results were obtained by constraining the confining surfaces to move in +x and -x directions at equal speeds of 1-200 m/sec and loads up to 1 GPa. In neither simulation approach did the calculated viscosity of the confined water exceed that of bulk water by more than an order of magnitude. Thus neither supports the idea that nano-confinement dramatically enhances water's viscosity. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y50.00012: Submicron flows of polymer solutions Hugues Bodiguel, Amandine Cuenca We study flow properties of high molecular weight polymer solutions below the micron scale. Fluorescence photobleaching is used as a non-invasive technique to evaluate the velocity of pressure-driven flows in channels from 100 to 4000 nm height. We observe a striking reduction of the effective viscosity of polyacrylamide solutiuons in the semi-dilute regime. This effect increases with molecular weight and concentration. Using a Rabinovitch-like approach, we correlate the data at different thicknesses to obtain the wall slip velocity and the flow curve at sub-microscale. Those properties are also evaluated using particle imaging velocimetry close to similar surfaces and standard rheometry. Comparing the measurements in bulk and in confined geometries, we conclude that the observed viscosity reduction can not be solely explained by slippage. We discuss the possible reasons of this effect that are size-dependant filtration and shear-thinning enhancement due to the confinement. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y50.00013: Nanomechanics and dynamics of confined water and other liquids Peter Hoffmann, Shah Khan From oil recovery to molecular biology, nanoconfined water plays an important role in many areas of research. However, the mechanics and dynamics of nanoconfined water are not well understood. Over the last ten years, a number of groups have measured the mechanics of confined water using atomic force microscopy (AFM) or surface force apparatus (SFA) - often with contradictory results. At Wayne State University, we have developed high resolution AFMs for ultra-small amplitude, linear measurements of the mechanics and dynamics of confined liquids. We have shown that water shows a distinct slow-down in dynamics under confinement (PRB 2004), co-discovered a dynamic ``solidification'' in a model liquid (Langmuir 2006), and showed that normal and shear stiffness are closely related in confined liquids (Rev. Sci. Instr. 2008). Recently, we found dynamic solidification also in water layers (PRL 2010), a finding that explains the contradictory findings in earlier measurements and points to surprisingly complex behavior in this seemingly simple system. Here we will review these findings, as well as present new findings that show the profound effects of ion concentration on these dynamical effects, as well as measurements of colloidal systems, which illustrate that some findings at the molecular scale can be understood from purely geometric considerations and are not dependent on molecular-scale interactions. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y50.00014: Investigation of Nanoscale Structure Using Spin-Echo Small-Angle Neutron Scattering (SESANS) Xin Li, Roger Pynn, Adam Washington, Wei-Ren Chen, Kunlun Hong, Gregory Smith, Yun Liu Spin-Echo Small-Angle Neutron Scattering (SESANS) is a new technique for probing structural correlations in real space over distances ranging from $\sim $20 nm to several microns. The measured SESANS correlation function is a projection of the normal Patterson correlation function on to a particular spatial direction. A framework to theoretically calculate this correlation function is laid out, followed by a general discussion of the features of the SESANS correlation function for colloidal systems with different interaction potentials. Our calculations for a system of monodisperse spherical particles, show that SESANS is much more sensitive to the intercolloid potential than conventional Small Angle Neutron Scattering. We have used SESANS to study the correlations between 300-nm-diameter surfactant-stablized poly(methyl methacrylate) (PMMA) spheres suspended in a good solvent, with and without an added polymeric depletant. Below a PMMA volume fraction of $\sim $30{\%} we find good agreement between the experimental data and theoretical prediction based on the Percus-Yevick approximation. With a small amount of polymer added to the suspension (less than 0.2{\%} by weight of 110 kD polymer), the short-range correlations between PMMA spheres are enhanced because of the presence of polymer depletant. The magnitude of the change is roughly as expected on the basis of calculations of a mixture of spherical particles of different sizes. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y50.00015: Non local rheology and near wall fluctuations in microgel jammed suspensions Patrick Tabeling, Choongyeop Lee, Fabrice Monti, Michel Cloitre We study flows of concentrated suspensions of soft nanoparticles in microchannels, over smooth hydrophilic and hydrophobic surfaces, using nano-PTV and $\mu $PIV techniques. With hydrophobic walls, the flow curves are in good agreeement with bulk rheology. With hydrophilic walls, substantial deviations from bulk rheology are observed. In the meantime, large velocity oscillations close to the wall are detected. We couple these observations by introducing a local rheology based on an energy barrier. As a whole, our work confirms the existence of non local rheological behavior in glassy systems. [Preview Abstract] |
Session Y52: Focus Session: Spin Glasses: Advances, Algorithms, and Applications
Sponsoring Units: GSNPChair: Daniel Stein, New York University
Room: 153C
Friday, March 2, 2012 8:00AM - 8:12AM |
Y52.00001: How the Edwards-Anderson Model reaches its Mean-Field Limit; Simulations in d=3,...,7 Stefan Boettcher, Stefan Falkner Extensive computations of ground state energies of the Edwards-Anderson spin glass on bond-diluted, hypercubic lattices are conducted in dimensions $d=3,\ldots,7$. Results are presented for bond-densities exactly at the percolation threshold, $p=p_{c}$, and deep within the glassy regime, $p>p_{c}$, where finding ground-states becomes a hard combinatorial problem. The ``stiffness'' exponent $y$ that controls the formation of domain wall excitations at low temperatures is determined in all dimensions. Finite-size corrections of the form $1/N^{\omega}$ are shown to be consistent throughout with the prediction $\omega=1-y/d$. At $p=p_{c}$, an extrapolation for $d\to\infty$ appears to match our mean-field results for these corrections. In the glassy phase, $\omega$ does not approach the value of $2/3$ for large $d$ predicted from simulations of the Sherrington-Kirkpatrick spin glass. However, the value of $\omega$ reached at the upper critical dimension \emph{does} match certain mean-field spin glass models on sparse random networks of regular degree called Bethe lattices.\\[4pt] [1] S. Boettcher and S. Falkner, arXiv:1110.6242;\hfil\break [2] S. Boettcher and E. Marchetti, PRB77, 100405 (2008);\hfil\break [3] S. Boettcher, PRL95, 197205 (2005). [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y52.00002: Ensemble Inequivalence in Spin Glasses Zsolt Bertalan, Hidetoshi Nishimori, Kazutaka Takahashi We report on the ensemble inequivalence in many-body spin-glass models with Ising and integer spins. In the Ising case, for many-body interactions the transition between the ferromagnetic and paramagnetic phases is of first order, and the microcanonical and canonical ensembles give different results. The spin-glass transition is of first order for certain values of the crystal field strength in the integer-spin model and is dependent whether it was derived in the microcanonical or the canonical ensemble. We also discuss the ensemble inequivalence of random energy models, corresponding to the limit infinitely many-body interactions. This is the first systematic treatment of spin glasses with long-range interactions in the microcanonical ensemble using the replica approach, which shows how the two ensembles give different results. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y52.00003: Algorithms and long-range order in the two-dimensional +/-J spin glass A. Alan Middleton, Creighton K. Thomas, David A. Huse Numerical methods and results of their application to the two-dimensional Ising spin glasses will be described. For a random mix of ferromagnetic and antiferromagnetic bonds of equal strength, long range correlations at zero-temperature are derived from scaling relations between computed exponents and are confirmed in numerical simulations. This long range order is stabilized by large entropy differences, as large domain walls often have zero energy cost. The order resembles that in higher-dimensional models at finite temperature. A publically distributed implementation of the algorithms has been developed for computing partition functions and exactly sampling configurations according to their Boltzmann weight for the general spin-glass and related two-dimensional models. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y52.00004: Replica theory of partition-function zeros in spin-glass systems Kazutaka Takahashi, Tomoyuki Obuchi We study the phase transitions in spin-glass systems by analysing the partition-function zeros (Lee-Yang zeros) with respect to the complex temperature/field. For several models as the random energy and spherical models with many-body interactions, we extend the replica method and the procedure of the replica symmetry breaking ansatz to be applicable in the complex-parameter case. We derive the phase diagrams in the complex plane and calculate the density of zeros in each phase. We find that there is a replica symmetric phase having a large density near the imaginary axis away from the origin. In the spin-glass phase, the density is finite only when the chaos effect is present. This result indicates that the density of zeros is more closely connected to the chaos effect than the replica symmetry breaking. We also investigate the relevance of our result to the finite-dimensional systems by studying the renormalization group flow in the complex plane. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y52.00005: Aging behavior in disordered and frustrated spin systems Hyunhang Park, Michel Pleimling Using Monte Carlo simulations we investigate aging in three-dimensional Ising spin glasses as well as in two-dimensional Ising models with disorder quenched to low temperatures. The time-dependent dynamical correlation length $L(t)$ is determined numerically and the scaling behavior of various two-time quantities as a function of $L(t)/L(s)$ is discussed. For disordered Ising models deviations of $L(t)$ from the algebraic growth law show up. The generalized scaling forms as a function of $L(t)/L(s)$ reveal a generic simple aging scenario for Ising spin glasses as well as for disordered Ising ferromagnets. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y52.00006: Critical behavior of the 1D L{\'e}vy lattice spin-glass: from mean-field threshold to the effective lower critical dimension Luca Leuzzi, Giorgio Parisi, Federico Ricci-Tersenghi, Juan-Jos\'e Ruiz-Lorenzo By means of Monte Carlo numerical simulations we analyze the critical behavior of a one dimensional spin-glass model with diluted interactions decaying, in probability, as an inverse power of the distance: the L{\'e}vy lattice spin-glass. Varying the power $\rho$, corresponds to change the effective dimension from mean-field-like (small power $\rho<4/3$) to finite dimensional-like short-range models ($<4/3\rho<2$) and, eventually, to 1D short-range models ($\rho>2$), where no phase transition occurs. The bond diluteness drastically reduces the computational time and large sizes can be approached. The one dimensionality allows for studying long systems, e.g., long correlation lenghts in the critical region. The spin-glass critical behavior can, therefore, be studied in and out of the range of validity of the mean-field approximation. After reviewing the main results in the L{\'e}vy lattice model about the spin-glass transition and the nature of the spin-glass phase for different values of the effective dimension, we will present new results on the critical behavior at $\rho=2$, corresponding to the lower critical dimension, and compare them with old and recent renormalization group approaches in this limit. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:48AM |
Y52.00007: Spin glasses: Still frustrating after all these years? Invited Speaker: Helmut G. Katzgraber Spin glasses are archetypal model systems to study the effects of frustration and disorder. Despite ongoing research spanning several decades, there remain many fundamental open questions, such as the existence of a spin-glass state in a field or the low-temperature structure of phase space for short-range systems. Novel applications across disciplines, as well as progress in algorithms and the advent of fast and cost-effective computers, have recently revived interest in the study of spin glasses. First, an overview of spin glasses will be given, followed by recent novel applications to fields as diverse as structural glasses and quantum computing. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y52.00008: The de Almeida-Thouless line of the four-dimensional Ising spin glass Victor Martin-Mayor We present the results of a large scale numerical simulation of the four dimensional Edwards-Anderson model in an external field. Using the Janus computer, as well as standard CPU clusters, we simulate lattices of size up to L=16 at several values of the external field. Our analysis method departs from the standard one. In fact, it has been previously noticed that the spin-glass susceptibility (i.e. the spin-glass propagator at zero external momentum) behaves anomalously. Instead, one should focus on the propagator at small but non-vanishing wave-vector. Starting from this observation, we obtain a simple and powerful finite-size scaling method. Clear evidence for a de Almeida-Thouless line is found. We compute critical exponents, widely differing from the zero field case, with an accuracy of five percent. The shape of the de Almeida-Thouless line in the (T,h) plane follows the Fisher-Sompolinsky scaling. Discrepancies with previous work are explained in terms of very strong scaling corrections. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y52.00009: Reentrance and ultrametricity in three-dimensional Ising spin glasses Helmut G. Katzgraber, Creighton K. Thomas, Alexander K. Hartmann We study the three-dimensional Edwards-Anderson Ising spin glass with bimodal disorder with a fraction of 22.8\% antiferromagnetic bonds. Parallel tempering Monte Carlo simulations down to very low temperatures show that for this fraction of antiferromagnetic bonds the phase diagram of the system is reentrant, in agreement with previous results. Furthemore, using a clustering analysis, we analyze the ultrametric properties of phase space for this model. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y52.00010: Replica exchange simulations of the three-dimensional Ising spin glass: static and dynamic properties Burcu Yucesoy, Jonathan Machta, Helmut G. Katzgraber We present the results of a large-scale numerical study of the equilibrium three-dimensional Ising spin glass with Gaussian disorder. Using replica exchange (parallel tempering) Monte Carlo, we measure various static, as well as dynamical quantities, such as the autocorrelation times and round-trip times for the replica exchange Monte Carlo method. The correlation between static and dynamic observables for 5000 disorder realizations ($N \le 10^3$ spins) down to very low temperatures ($T \approx 0.2T_c$) is examined. Our results show that autocorrelation times are directly correlated with the roughness of the free energy landscape. We also discuss the size dependence of several static quantities. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y52.00011: Monte Carlo Simulations of Random Frustrated Systems on Graphics Processing Units Sheng Feng, Ye Fang, Sean Hall, Ariane Papke, Cade Thomasson, Ka-Ming Tam, Juana Moreno, Mark Jarrell We study the implementation of the classical Monte Carlo simulation for random frustrated models using the multithreaded computing environment provided by the the Compute Unified Device Architecture (CUDA) on modern Graphics Processing Units (GPU) with hundreds of cores and high memory bandwidth. The key for optimizing the performance of the GPU computing is in the proper handling of the data structure. Utilizing the multi-spin coding, we obtain an efficient GPU implementation of the parallel tempering Monte Carlo simulation for the Edwards-Anderson spin glass model. In the typical simulations, we find over two thousand times of speed-up over the single threaded CPU implementation. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y52.00012: Monte Carlo simulations of the ${\rm LiHo_xY_{1-x}F_4}$ diluted dipolar magnet Juan Carlos Andresen, Moshe Schechter, Helmut G. Katzgraber Recent intriguing experimental results on ${\rm LiHo_xY_{1-x}F_4}$, a diluted dipolar magnet, along with new analytical insights, suggest that neither a mean-field treatment nor simulations using simplified versions of the underlying Hamiltonian adequately describe these materials. Not only does this imply that novel disordering mechanism might be present, it requires a detailed numerical analysis that incorporates all terms in the Hamiltonian. We present large-scale Monte Carlo simulations of the diluted dipolar magnet with competing interactions on a ${\rm LiHo}$ lattice with the inclusion of a random field term. For low concentrations of ${\rm Ho}$ atoms we reproduce the peculiar linear dependence of the transition temperature as a function of the random-field strength found in recent experimental results by Silevich {\em et al}.~[Nature {\bf 448}, 567 (2007)]. We then find a zero-temperature phase transition between the ferromagnetic and quasi-spin-glass phases, suggesting that it is the underlying spin-glass phase that dictates the above linear dependence of $T_c$ on the random field. For large concentrations we recover the quadratic dependence of the critical temperature as a function of the random field strength. [Preview Abstract] |
Friday, March 2, 2012 10:48AM - 11:00AM |
Y52.00013: Novel disordering mechanisms in dipolar spin glasses and ferromagnets Moshe Schechter, Juan Carlos Andersen, Helmut Katzgraber At and below the critical dimension the disordering of an ordered phase by a random field occurs via a collective effect of large domains at infinitesimal random field [Imry \& Ma, Phys.~Rev.~Lett.~{\bf 35}, 1399 (1975)]. At larger space dimensions the disordering requires a large random field, of the order of the interaction energy. In a random field, the lower critical dimension is 2 for Ising ferromagnets, whereas it is infinity for spin glasses. We have generalized the Imry-Ma argument for ferromagnets with competing interactions and an underlying spin-glass phase, and for dilute dipolar spin glasses. For dilute dipolar spin glasses we have found [EPL~{\bf 88}, 66002 (2009)] that the broad distribution of random fields dictates more efficient disordering of the glass phase, and domain sizes which depend explicitly on the concentration, i.e., do not obey simple scaling. Here we show that as a result of a competing spin-glass phase, the disordering of the ferromagnet occurs at a finite random field, which is yet much smaller than the interactions. Our results are verified numerically, explain the recently-observed peculiar linear dependence of $T_c$ on the random field strength [Nature~{\bf 448}, 567 (2007)], and predict a zero-temperature random-field driven transition between a ferromagnetic and a quasi spin glass phase. [Preview Abstract] |
Session Y53: Nonlinear Dynamics of Coupled Systems
Sponsoring Units: GSNPChair: Sid Redner, Boston University
Room: 153B
Friday, March 2, 2012 8:00AM - 8:12AM |
Y53.00001: Frequency Enhancement in Coupled Noisy Excitable Elements Wei-Yin Chiang, Pik-Yin Lai, C.K. Chan The oscillatory dynamics of coupled noisy excitable FitzHugh-Nagumo elements is investigated as a function of the coupling strength~$g$. For two such coupled elements, it~is found that their frequencies are enhanced by the coupling and will synchronize at a frequency~higher than the uncoupled frequencies of each element. As~$g$ increases, there is an unexpected peak~in the mean of frequency distribution~before reaching synchronization at the optimal coupling strength.~This phenomenon~can be understood with a simplified analytic model~based on the excitation~across a potential barrier whose height is controlled by $g$~as well as the formation of temporary coherent cluster. [Preview Abstract] |
Friday, March 2, 2012 8:12AM - 8:24AM |
Y53.00002: Ramification of stream networks Hansjoerg Seybold, Olivier Devauchelle, Alexander Petroff, Robert Yi, Daniel Rothman The geometric complexity of channel networks arises from their successive ramifications --- the splitting of a single tip into two branches. Here we show that streams incised by groundwater seepage split at a characteristic angle of $\alpha=2\pi/5=72^\circ$. Our theory represents streams as a collection of paths growing and bifurcating in a diffusing field, which can be described by Loewner dynamics. Analysis of thousands of bifurcations in a $\sim$100~km$^2$ stream network reveals that the mean branching angle is $72.5^\circ\pm 1.5^\circ$ (95\% C.I.) and that the five fold symmetry induced by the branching of the tips is observed on all scales in the network. This consistency between theory and observation suggests that the network geometry is determined by the external flow field rather than flow within the streams themselves, contrary to assumptions made by models that relate geometry to internal dissipation. [Preview Abstract] |
Friday, March 2, 2012 8:24AM - 8:36AM |
Y53.00003: Dynamical phyllotaxis: transition modes and Sine-Gorden-like solitons Yuanxi Wang, Cristiano Nisoli, Vincent Crespi Repulsive particles constrained to a cylindrical surface generate a rich set of static spiral patterns known as phyllotaxis. The ground states and quasi-static transitions of phyllotaxis has been well understood based on the conventional hypothesis that particles always form a cylindrical lattice. We investigate inhomogeneous transition modes which break this helical symmetry to connect monojugate and multijugate patterns. Furthermore, traveling Sine-Gordon-like solitons in dynamical phyllotaxis are observed in numerical simulations and explained using a modified Frenkel-Kontorova model. We show that kinks propagate spirally along selected parastichies and carry an intrinsic dipole. Applications in different areas of physics are discussed. [Preview Abstract] |
Friday, March 2, 2012 8:36AM - 8:48AM |
Y53.00004: Cascade of bifurcation type trajectories as a general type of attractors in fractional dynamical systems Mark Edelman Based on the results of computer simulations and analytical investigation of fractional maps, we present our latest results related to the existence and stability of the new type of attractors in the fractional dynamical systems: cascade of bifurcation type trajectories (CBTT). We show that in fractional Standard Map (FSM) this type of attractors appears in the area of parameters, where in the corresponding integer system (regular Standard Map) series of period doubling bifurcations and corresponding splitting of islands of stability leads to the disappearance of stability and transition to chaos. [Preview Abstract] |
Friday, March 2, 2012 8:48AM - 9:00AM |
Y53.00005: Scale-free avalanches in disordered systems of localized charges with long-range Coulomb interaction Matteo Palassini, Martin Goethe We study theoretically and numerically the charge avalanches created by a perturbation in disordered systems of localized charges with unscreened Coulomb interaction (the so-called electron glass model), in two and three dimensions. Starting from a low-lying local energy minimum, we perturb the system by inserting an extra charge or an extra dipole, and let it relax via one-particle hops until a new minimum is reached. We find that the size distribution of the avalanches created in this process displays generically a power-law tail with an exponent close to the mean-field value 3/2 both in 2D and 3D, without requiring any parameter tuning. We provide a qualitative explanation of these results in terms of the density of states of elementary charge and dipole excitations and the associated Coulomb gap, which shows that the power-law tail arises from arbitrarily long hops, without requiring to assume the existence of a glass phase. Finally, we discuss the experimental relevance of these results and compare our picture to similar scale-free avalanches observed in mean field spin glasses, in which they are are associated to a marginal glass phase. [Preview Abstract] |
Friday, March 2, 2012 9:00AM - 9:12AM |
Y53.00006: Cooperation-induced temporal complexity in networks of pulse-coupled units Elvis Geneston, Paolo Grigolini We study a network of stochastic pulse-coupled units generating bursts with the same size distribution as the neuronal avalanches in mature cultured neurons, recently revealed by the experimental observation. We prove that in addition to this form of complexity this model yields a form of phase transition generating also temporal complexity. This means that the distance from two consecutive bursts fits the prescription of a Mittag-Leffler (ML) function renewal theory. There exists a critical value of the cooperation parameter at which this description applies to the whole time regime. By increasing the cooperation parameter the ML theory breaks down and the sequence of bursts tend to become periodic with the same intensity. We make the conjecture that the analysis of this model may shed light into the theoretical foundation of neuronal burst leaders and that the recently discovered principle of complexity management may be conveniently applied to the neuro-physiological processes that are properly described by this model. [Preview Abstract] |
Friday, March 2, 2012 9:12AM - 9:24AM |
Y53.00007: Time dependence of reprecipitation rates in heterogeneous media Daniel Reeves, Daniel Rothman The analysis of spatial and temporal variations of the chemical and isotopic compositions of minerals in sedimentary systems provides a powerful tool for calculating dissolution and reprecipitation rates, and has previously been applied to find time-dependent rates in marine sediments. Dissolution and precipitation processes tend to shift the composition of the pore fluids toward that of the solid phase, and vice-versa. Current theory treats both the fluid and solid phases as well-mixed reservoirs, relying on mean-field theory that is inconsistent with the physical structure of the solid, as dissolution and precipitation occur only on the reactive surface of the solid. We present a model that accounts for the heterogeneity of the solid phase by adding and removing material only at the reactive surface. We therefore model the location of the surface with a 1-D random walk, in which the buried bulk of the solid phase can only be modified through repeated dissolution events. We approximate this physical scenario with a three-reservoir kinetic model and more detailed numerical simulations. We develop an understanding of two power-law scaling regimes, the second of which demonstrates 1/time aging in the rate constant, similar to those observed in marine sediment studies. [Preview Abstract] |
Friday, March 2, 2012 9:24AM - 9:36AM |
Y53.00008: Properties of compacton-anticompacton collisions Bogdan Mihaila, Andres Cardenas, Fred Cooper, Andres Saxena We study the properties of compacton-anticompacton collision processes. We compare and contrast results for the case of compacton-anticompacton solutions of the K(l,p) Rosenau-Hyman (RH) equation for l=p=2, with compacton-anticompacton solutions of the L(l,p) Cooper-Shepard-Sodano (CSS) equation for p=1 and l=3. This study is performed using a Pad\'e discretization of the RH and CSS equations. We find a significant difference in the behavior of compacton-anticompacton scattering. For the CSS equation, the scattering can be interpreted as ``annihilation'' as the wake left behind dissolves over time. In the RH equation, the numerical evidence is that multiple shocks form after the collision, which eventually lead to ``blowup'' of the resulting wave form. [Preview Abstract] |
Friday, March 2, 2012 9:36AM - 9:48AM |
Y53.00009: Cardiac arrhythmias and degradation into chaotic behavior prevention using feedback control Ilija Uzelac, Veniamin Sidorov, Marc Holcomb, John Wikswo, Richard Gray During normal heart rhythm, cardiac cells behave as a set of oscillators with a distribution of phases but with the same frequency. The heart as a dynamical system in a phase space representation can be modeled as a set of oscillators that have closed overlapping orbits with the same period. These orbits are not stable and in the case of disruption of the cardiac rhythm, such as due to premature beats, the system will have a tendency to leave its periodic unstable orbits. If these orbits become attracted to phase singularities, their disruption may lead to chaotic behavior, which appears as a life-threating ventricular fibrillation. By using closed-loop feedback in the form of an adjustable defibrillation shock, any drift from orbits corresponding to the normal rhythm can be corrected by forcing the system to maintain its orbits. The delay through the feedback network coincides with the period of normal heart beats. To implement this approach we developed a 1 kW arbitrary waveform voltage-to-current converter with a 1 kHz bandwidth driven by a photodiode system that records an optical electrocardiogram and provides a feedback signal in real time. Our goal is to determine whether our novel method to defibrillate the heart will require much lower energies than are currently utilized in single shock defibrillators. [Preview Abstract] |
Friday, March 2, 2012 9:48AM - 10:00AM |
Y53.00010: Experiments on oscillator ensemble with global nonlinear coupling Michael Rosenblum, Amirkhan Temirbayev, Zeinulla Zhanabaev, Stanislav Tarasov, Vladimir Ponomarenko We experimentally analyze collective dynamics of a population of 20 electronic Wien-bridge limit-cycle oscillators with a linear or nonlinear phase-shifting unit in the global feedback loop. With linear unit we observe, with increase of the coupling strength, a standard Kuramoto-like transition to a fully synchronous state; the threshold of the transition depends on the phase shift. In case of nonlinear global coupling we first observe a transition to a state when approximately half of the population forms a synchronous cluster. With further increase of the coupling strength we observe destruction of this cluster and formation of a self-organized quasiperiodic state, predicted in [M. Rosenblum and A. Pikovsky, PRL, 98, 064101 (2007)]. In this state, frequencies of all oscillators are smaller than the frequency of the mean field, so that the oscillators are not locked to the mean field they create and their dynamics is quasiperiodic. The transition is characterized by a non-monotonic dependence of the order parameter on the coupling strength. We demonstrate a good correspondence between theory and experiment. [Preview Abstract] |
Friday, March 2, 2012 10:00AM - 10:12AM |
Y53.00011: Dynamics of Confident Voting Daniel Volovik, Sidney Redner In the classical voter model, a voter has no intrinsic confidence in its current opinion. We introduce the confident voter model in which each voter can be in one of two opinions, and can additionally have two levels of commitment to an opinion --- confident and vacillating. Upon interacting with an agent of a different opinion, a confident voter becomes less committed, or vacillating, but does not immediately change opinion. However, a vacillating agent changes opinion upon interacting with an agent of a different opinion. In the mean-field limit, a population of size $N$ is quickly driven to a mixed state before consensus is eventually achieved in a time of order $\ln{N}$. In two dimensions, the distribution of consensus times is characterized by two distinct times --- one that scales linearly with $N$ and another that scales as $N^{3/2}$. The longer time arises from configurations that fall into long-lived stripe states, which are caused by an effective surface tension between domains of different opinion states, before consensus is finally reached. [Preview Abstract] |
Friday, March 2, 2012 10:12AM - 10:24AM |
Y53.00012: Nonuniversal Effects in Mixing Correlation-Growth Processes with Randomness Alice Kolakowska In mixed-growth dynamical process $P = Y \vee X$ there are two dynamical processes: $Y$ (in one universality class) and $X$ (in another universality class). They \textit{alternate} with each other: ``\textit{exclusively either} $Y$ (is active with probability $q$) \textit{or} $X$ (is active with probability $p$),'' $p+q=1$. When $P$ models surface growth via deposition/desorption/adsorption with $X$ building universal correlations and $Y$ representing randomness (e. g., thermal effects), in order to correctly construct a continuum growth equation for $P$ a distinction must be made \textit{within a single universality class} of $X$ between processes that do and do not create voids in the bulk of deposited material. Then, model-dependent prefactors in universal scaling of the surface roughness can be linked with the bulk morphology and determined from the bulk structures. This connection is essential to finding correct dynamical scaling and to interpretation of scaling laws for mixed-growth dynamical processes. [Preview Abstract] |
Friday, March 2, 2012 10:24AM - 10:36AM |
Y53.00013: Non-equilibrium modulated phases in a system with local energy input Linjun Li, Michel Pleimling The equilibrium phase diagram of the two-dimensional Ising model in contact with a single heat bath is well understood. We here study the properties of the two-dimensional Ising model with conserved dynamics where the two halves of the system are in contact with different heat baths. Using Monte Carlo simulations, we identify three different phases for this non-equilibrium system, as a function of the aspect ratio of the lattice and of the temperatures. The first phase is characterized by the complete disorder of the particles, while the second phase is characterized by the complete order of the particles. The third phase is the most interesting one as it displays stripes with widths that depend on the system parameters. The full phase diagram of our non-equilibrium system is determined through the study of the structure factor. [Preview Abstract] |
Friday, March 2, 2012 10:36AM - 10:48AM |
Y53.00014: Nonequilibrium relaxation and critical aging for driven Ising lattice gases George Daquila, Uwe C. Tauber We employ Monte Carlo simulations to study the non-equilibrium relaxation of driven Ising lattice gases in two dimensions. Whereas the temporal scaling of the density auto-correlation function in the non-equilibrium steady state does not allow a precise measurement of the critical exponents, these can be accurately determined from the aging scaling of the two-time auto-correlations and the order parameter evolution following a quench to the critical point. We obtain excellent agreement with renormalization group predictions based on the standard Langevin representation of driven Ising lattice gases, valid to all orders in the dimensional expansion. [Preview Abstract] |
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