Bulletin of the American Physical Society
APS March Meeting 2012
Volume 57, Number 1
Monday–Friday, February 27–March 2 2012; Boston, Massachusetts
Session X1: Focus Session: Advances in Scanned Probe Microscopy IV: New Instrumentation & Techniques
Sponsoring Units: GIMSChair: Alex de Lozanne, University of Texas at Austin
Room: 203
Thursday, March 1, 2012 2:30PM - 2:42PM |
X1.00001: ezAFM: A low cost Atomic Force Microscope(AFM) Umit Celik, Kubra Celik, Husnu Aslan, Ihsan Kehribar, Munir Dede, H. Ozgur Ozer, Ahmet Oral A low cost AFM, ezAFM is developed for educational purposes as well as research. Optical beam deflection method is used to measure the deflection of cantilever. ezAFM scanner is built using voice coil motors (VCM) with $\sim $50x50x6 $\mu $m scan area. The microscope uses alignment free cantilevers, which minimizes setup times. FPGA based AFM feedback Control electronics is developed. FPGA technology allows us to drive all peripherals in parallel. ezAFM Controller is connected to PC by USB 2.0 interface as well as Wi-Fi. We have achieved $<$5nm lateral and $\sim $0.01nm vertical resolution. ezAFM can image single atomic steps in HOPG and mica. An optical microscope with $<$3 $\mu $m resolution is also integrated into the system. ezAFM supports different AFM operation modes such as dynamic mode, contact mode, lateral force microscopy. Advanced modes like magnetic force microscopy and electric force microscopy will be implemented later on. The new ezAFM system provides, short learning times for student labs, quick setup and easy to transport for portable applications with the best price/performance ratio. The cost of the system starts from {\$}15,000, with system performance comparable with the traditional AFM systems. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X1.00002: Compact Scanning Tunneling Microscope for Spin Polarization Measurements Seong Heon Kim, Leujen Chen, Alex de Lozanne We have built a low temperature scanning tunneling microscope for spin-polarized studies. An important aspect of our design philosophy is to keep everything small, starting with a one-inch STM body that fits in the UHV bore of a small superconducting solenoid that provides up to 8 Tesla parallel to the tip. This, in turn, makes the liquid helium and liquid nitrogen dewars smaller and leads to a compact UHV chamber. The largest flange in the system is 10 inches in outer diameter. The benefits of a smaller system include lower consumption of cryogens and a reduced footprint. The STM has been tested from 300K to 77K and has achieved atomic resolution. A test at 4.2K will be done soon. We have imaged cobalt clusters deposited in situ using a simple and compact design for an electron-beam evaporator. We have developed new electronics for z-approach and a novel magnetically-coupled manipulator with an actuated grabber for tip and sample exchange. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X1.00003: Development of Molecular Beam Epitaxy/Pulsed Laser Deposition/Low Temperature Spin-Polarized Scanning Tunneling Microscopy System Jeongihm Pak, Wenzhi Lin, Kangkang Wang, Andrew Foley, Tianjiao Chen, Yinghao Liu, Abhijit Chinchore, Daniel Bergman, Meng Shi, Arthur R. Smith Spin-polarized scanning tunneling microscopy and spectroscopy have shown tremendous abilities to obtain detailed spin information about surfaces down to the atomic scale. In order to take full advantage of this method for studying pristine, as-prepared sample surfaces, we couple an SP-STM system to a sophisticated ultra-high-vacuum growth facility. The hybrid molecular beam epitaxy/pulsed laser deposition/spin-polarized STM system is home-designed and constructed with many unique features. A wide variety of engineered spintronic materials can be grown in the 8 source growth chamber, or using the 9 source laser deposition system. Samples may be heated during growth to as high as 1300 K or cooled using LN2 to temperatures below 195 K, while being simultaneously probed using reflection high energy electron diffraction. The system is currently configured for nitride systems as well as transition metal or rare earth ultra-thin films. Prepared samples are transferred through a central distribution chamber to the LHe-cooled, spin-polarized STM operating inside an integral superconducting magnet (0-4.5 T). The system is outfitted with magnetic tip preparation. The magnetic field allows us to manipulate the magnetic structure of samples during SP-STM experiments. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X1.00004: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X1.00005: A Dual Tip STM for Imaging the Superconducting Phase Difference Anita Roychowdhury, M.A. Gubrud, D. Sullivan, Michael Dreyer, J. R. Anderson, C.J. Lobb, F.C. Wellstood We have built a dual tipped millikelvin STM with each tip capable of independently scanning a sample. We will use the STM to measure spatial variations of the gauge-invariant phase difference at the atomic scale. The two tips along with the superconducting sample constitute a SQUID. This configuration is designed to minimize fluctuations in the superconducting phase of one of the tips as it scans the sample, hence inhibiting supercurrent suppression. We are currently developing a technique to fabricate superconducting Niobium tips for use with this system. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X1.00006: mK-Scanning Probe Microscope(mK-SPM) operating in a Cryogen-Free Dilution Refrigerator at 20mK Munir Dede, Ozgur Karci, Chris Snelling, Ahmet Oral Dramatic increase in liquid helium price limits the usage of cryogenic equipment. Dry cryogen-free dilution refrigerators(DR) systems are promising platforms to run mK-Scanning Probe Microscopes(mK-SPM) systems with a number of operating modes: STM, AFM, MFM, EFM, SSRM, PFM, etc. We present the design of a mK-Scanning Probe Microscope (mK-SPM) operating in a cryogen-free DR. An Oxford Instrument cryogen-free DR(Triton DR200) with 200uW cooling power and 7mK base temperature is used for the experiments. A 1W Pulse Tube cryocooler is integrated into the DR. After wiring and attaching the microscope we achieved 20mK base temperature. Piezo driven Stick slip coarse approach mechanism is used to bring the sample in to close proximity of the sample. In these initial results we deliberately did not take any precautions to isolate the pumping lines, attached to the DR and the DR itself. The turbomolecular pump was attached directly to the top plate of the DR. We first tested our mK-SPM in Scanning Tunnelling Microscope (STM) mode as it is the most sensitive of the SPM techniques. An image, using a gold coated 6$\mu $m period calibration grating at 20mK, obtained under these rudimentary conditions. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X1.00007: Atom-Specific Interaction Quantification and Identification by 3D-SPM Todd C. Schwendemann, Mehmet Z. Baykara, Harry Monig, Milica Todorovic, Jan Gotzen, Ozhan Unverdi, Ruben Perez, Eric I. Altman, Udo D. Schwarz Entire scientific disciplines such as mechanics and chemistry are governed by the interactions between atoms and molecules. On surfaces, forces extending into the vacuum direct the behavior of phenomena such as thin film growth, nanotribology, and surface catalysis. To advance our knowledge of the fundamentals governing these topics, it is desirable to simultaneously map electron densities and quantify force interactions between the surface of interest and a probe with atomic resolution. Using the oxygen-reconstructed copper (100) surface as a model system, we demonstrate that much of this information can be derived from combining three-dimensional atomic force microscopy (3D-AFM) with simultaneous STM. The three-dimensional scanning probe microscopy (3D-SPM) variant resulting from this combination provides complementary information in the various interaction channels recorded. The surface oxide layer of copper (100) features defects and a distinct structure of the Cu and O sublattices that is ideally suited for such model investigations. The analysis of our experimental results allows for the identification of atomic species and defects on the sample surface through the comparison of simultaneously recorded force and current data. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X1.00008: Laser Scanning Microscopy for Quantitative Measurement of the Local Microwave-Photonic Properties of Advanced Materials and Devices Behnood Ghamsari, Jacob Tosado, Mahito Yamamoto, Jinglei Ping, Daniel Lenski, Michael Fuhrer, Steven Anlage We present laser scanning microscopy (LSM) as a non-contact and non-invasive instrumentation technique for quantitative measurement of the local microwave, optoelectronic, and optical properties of advanced materials including superconductors and graphene. Since an LSM setup may be configured in different modes of operation, we will focus on the photoresponse and reflectivity imaging modes. It will be discussed how an LSM, in the photoresponse measurement mode, may be used to image the distribution of rf/microwave currents at the surface of a superconducting device, such as a resonator. In addition, relevant techniques for distinguishing the kinetic and resistive parts of the superconductor's photoresponse as well as imaging its possible anisotropic microwave properties are addressed. With regard to the reflectivity mode, we will present how this method enables precise measurement of the topological features and optical properties of non-opaque samples. As an example, we will show how the thickness of few-layer graphene flakes may be measured by this method. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X1.00009: Torsional tapping atomic force microscopy for molecular resolution imaging of soft matter Jamie Hobbs, Nic Mullin Despite considerable advances in image resolution on challenging, soft systems, a method for obtaining molecular resolution on `real' samples with significant surface roughness has remained elusive. Here we will show that a relatively new technique, torsional tapping AFM (TTAFM), is capable of imaging with resolution down to 3.7 Angrstrom on the surface of `bulk' polymer films [1]. In TTAFM T-shaped cantilevers are driven into torsional oscillation. As the tip is offset from the rotation axis this provides a tapping motion. Due to the high frequency and Q of the oscillation and relatively small increase in spring constant, improved cantilever dynamics and force sensitivity are obtained. As the tip offset from the torsional axis is relatively small (typically 25 microns), the optical lever sensitivity is considerably improved compared to flexural oscillation. Combined these give a reduction in noise floor by a factor of 12 just by changing the cantilever geometry. The ensuing low noise allows the use of ultra-sharp `whisker' tips with minimal blunting. As the cantilevers remain soft in the flexural axis, the force when imaging with error is also reduced, further protecting the tip. We will show that this combination allows routine imaging of the molecular structure of semicrystalline polymer films, including chain folds, loose loops and tie-chains in polyethylene, and the helical conformation of polypropylene within the crystal, using a standard, commercial AFM. \\[4pt] [1] N Mullin, JK Hobbs, PRL 107, 197801 (2011) [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X1.00010: Reference system for scanning probe tip fingerprinting Robert Turansky, Joseph Bamidele, Yasuhiro Sugawara, Lev Kantorovitch, Ivan Stich Knowledge of the chemical structure of the tip asperity in Non-Contact Atomic Force Microscopy (NC-AFM) is crucial as controlled manipulation of atoms and/or molecules on surfaces can only be performed if this information is available. However, a simple and robust protocol for ensuring a specific tip termination has not yet been developed. We propose a procedure for chemical tip finger printing and an example of a reference system, the oxygen-terminated Cu(110) surface, that enables one to ensure a specific tip termination with Si, Cu, or O atoms. To follow this up and unambiguously determine tip types, we performed a theoretical DFT study of the line scans with the tip models in question and found that the tip characterization made based on experimental results (Cu/O-terminated tip imaging Cu/O atoms) is in fact incorrect and the opposite is true (Cu/O-terminated tip imaging O/Cu atoms). This protocol allows the tip asperity's chemical structure to be verified and established both before as well as at any stage of the manipulation experiment when numerous tip changes may take place. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X1.00011: Micromolding fabrication of ``T'' cross section SiC SPM probes ChiaYun Wu, Shihang Wang, Daniel Schmidt, Joel Therrien Micromolding techniques using pre-ceramic polymers have been demonstrated for the creating silicon carbide based AFM cantilevers. In an attempt to reach higher resonance frequencies without significantly increasing the spring constant of the cantilevers, a ``T'' cross section cantilever was fabricated. The resonant frequency and spring constant have been compared to the standard rectangular cross section cantilevers of both Sic and commercially available silicon cantilevers. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X1.00012: Single molecule and single atom sensors for atomic resolution imaging of chemically complex surfaces F. Stefan Tautz, Georgy Kichin, Christian Weiss, Christian Wagner, Ruslan Temirov To resolve single atoms has always been a major goals of surface science. Mapping forces with a dynamic AFM, it is possible to reconstruct atomic resolution of various surfaces and of large organic molecules[1]. At the same time scanning tunneling hydrogen microscopy (STHM) reaches atomic scale resolution and reveals intermolecular interactions with much less technical effort[2]. Besides H$_{2}$ and D$_{2}$, also individual Xe atoms, single CO and CH$_{4}$ molecules adsorbed at the tip apex of an STM function as microscopic force sensors that change the tunneling current in response to the forces acting from the surface. An STM equipped with any of these sensors is able to image the Pauli repulsion and thus resolve the inner structure of organic adsorbates. The more rigidly bounded CO yields the strongest, least distorted contrast. Thus, the sensor functionality can be tailored by tuning the interaction between sensor particle and STM tip. Hence, STHM belongs to a wider family of atomic-sensor microscopy techniques.\\{[}1] L. Gross et al., Science 325, 1110 (2009)\\{[}2] R. Temirov et al., New J. Phys. 10, 053012 (2008); C. Weiss et al., Phys. Rev. Lett. 105, 086103 (2010), C. Weiss et al., J. Am. Chem. Soc. 132, 11864 (2010); G. Kichin et al., J. Am. Chem. Soc. 133, 16847 (2011) [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X1.00013: Detection of cantilever thermal motion and feedback cooling using a quantum point contact Michele Montinaro, Sarah Hellm\"{u}ller, Klaus Ensslin, Martino Poggio Nanomechanical oscillators enable ultrasensitive detection of force, mass and displacement. In particular, recent measurements of oscillator displacement have achieved an imprecision below that at the standard quantum limit (SQL), using optical [1] or microwave techniques [2]. A quantum point contact (QPC) has been employed as a transducer of nanomechanical motion, thanks to the sensitive dependence of its conductance on electrostatic fields. Such an approach has been demonstrated in combination with an off-board micromechanical cantilever in a versatile design compatible with nanoscale oscillators and, in principle, with a variety of force-sensing applications, including magnetic resonance force microscopy [3]. The aim of the research we present here is to improve this technique and to approach the SQL by accessing a regime in which, due to the one-dimensional electron transport, quantum mechanical back-action effects emerge on the mechanical resonator. We demonstrate the use of different types of QPCs as sensitive detectors of the low-temperature thermal motion of an off-board cantilever and their ability to cool the cantilever oscillation mode through feedback.\\[4pt] [1] Phys. Rev. A 82, 061804 (2010)\newline [2] Nat. Nano. 4, 820 (2009)\newline [3] Nat. Phys. 4, 635 (2008) [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X1.00014: Design of a Self-Aligned, High Sensitivity Fiber Fabry-Perot Interferometer for Low Temperature Atomic Force Microscope/Magnetic Force Microscope Ozgur Karci, Munir Dede, Ahmet Oral We describe the design of a high sensitivity fiber Fabry-Perot interferometer for low temperature atomic force microscope/magnetic force microscope. This is a self-aligned system utilizing an alignment chip and eliminating all tedious alignment procedures. Our interferometer cavity is composed of a cleaved fiber, which is coated using dielectric to increase the reflectivity of laser from fiber-air interface, and a cantilever. 50 percent of the incident laser beam is reflected at the end of the fiber. The transmitted light propagates from the fiber end and hits the cantilever. Multiple reflections occur between cantilever and the fiber then the beams go into the fiber again. These two beams interfere and generate a photocurrent at the PD which is used for deflection measurement. We designed a special stick-slip coarse approach mechanism using piezoelectric tube scanner of the microscope. We have measured 8fm per square root Hz noise level at 300K, while the shot noise limit was 2fm per square root Hz. Our previous Michelson interferometer design had 20 fm per square root Hz noise level and gave better than 10nm MFM resolution on hard disk. Our goal is to further enhance the noise levels and achieve 6 nm resolution for LT-MFM with this new interferometer. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X1.00015: Next Generation SPM Control System with ppm precision Alessandro Pioda Novel scanning probe microscopy techniques, modes of operation, and advances in microscope hardware are pushing the boundary for signal resolution and flexibility in SPM measurements. Here we present a new state of the art SPM control system, which improves signal precision, resolution, bandwidth and noise performance by about one order of magnitude compared to current generation controllers. The controller incorporates the performance of expensive dedicated instruments in a compact modular multichannel package. In combination with the well proven and flexible Nanonis SPM control software, this next generation controller is the ideal platform for the most demanding microscopy, spectroscopy and transport measurements tasks, opening the door to a larger range of applications compared to current systems. Furthermore, the flexible and easily configurable user interface of the controller and the large number of measurement channels allows its operation as a high performance DC and AC source and measurement interface with ppm precision and multiple lock-in amplifiers, opening new perspectives for materials research. [Preview Abstract] |
Thursday, March 1, 2012 5:30PM - 5:42PM |
X1.00016: Conical spin-spiral ground state of a Mn double layer on W(110) driven by higher-order exchange interactions Yasuo Yoshida, Silke Schroeder, Paolo Ferriani, David Serrate, Kirsten von Bergmann, Andre Kubetzka, Stefan Heinze, Roland Wiesendanger The magnetic properties of transition-metal nanostructures are commonly explained based on the interplay of Heisenberg exchange, Dzyaloshinskii-Moriya (DM) interaction and magnetocrystalline anisotropy while higher order terms such as the biquadratic exchange and the four-spin interaction are typically neglected due to their small strength. Here, we demonstrate that higher-order terms can play a crucial role for the magnetic ground state and report as an example a transverse conical spin-spiral state in an ultra-thin film composed of two atomic layers of Mn on W(110). This spin structure is characterized by magnetic moments rotating on a cone that is perpendicular to the [001] propagation direction of the spin-spiral with a periodicity of 2.4 nm. The cones of nearest-neighbor Mn atoms point into opposite directions which results in nearly antiferromagnetic alignment. This intriguing spin structure has been resolved on the atomic-scale using spin-polarized scanning tunneling microscopy and confirmed to be the ground state by first-principles calculations based on DFT. Our calculations also reveal that the canting of the spins is induced by higher-order exchange interactions while the spiraling along the [001]-direction is due to frustrated Heisenberg exchange and DM interaction. [Preview Abstract] |
Session X2: Invited Session: History of Metrology and Today's Frontiers of Measurement
Sponsoring Units: FHPChair: Richard Davis, BIPM
Room: 204AB
Thursday, March 1, 2012 2:30PM - 3:06PM |
X2.00001: Dreams of a Final System: Origins of the Quest for an Absolute Standard Invited Speaker: Robert Crease The first attempts to find unchanging phenomena that could be used to evaluate the accuracy of standards and recreate them if lost predated the metric system. As early as the seventeenth century, members of the French Academy and British Royal Society sought to use the seconds pendulum and the Earth's meridian as tethers for length standards. These efforts ultimately failed. The vision of an absolute standard was revived in the 1870s, when C. S. Peirce was the first to experimentally tie a unit, the meter, to a natural standard, the wavelength of a spectral line, using a diffraction grating. This work inspired A. Michelson and E. Morley, in the 1880s, to apply the interferometer with which they were attempting to detect ether drift to this purpose. Michelson further pursued this work at the BIPM in 1892, which set the stage for the later redefinition, in 1960, of the meter in terms of the wavelength of a spectral line. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:42PM |
X2.00002: Beller Lectureship: From Artefacts to Atoms: The Origins and Early Years of the International Bureau of Weights and Measures (BIPM) Invited Speaker: Terry Quinn The BIPM was founded by the Metre Convention in 1875. Its main task was to maintain and disseminate the units of length and mass using the new International Prototypes of the Metre and Kilogram. My talk will be based on the opening chapters of my book ``From Artefacts to Atoms'' which recount the story of the Metre Convention and the creation of the BIPM at the Pavillon de Breteuil in S\`{e}vres on the outskirts of Paris, as the first international scientific institute. I shall include a brief outline of the sometimes acrimonious discussions at the Diplomatic Conference of the Metre, which opened on 1 March 1875 and concluded with the signing of the Convention on 20 May, of the construction of a new laboratory building, recruitment of staff, purchase of instruments and equipment and the beginning of scientific work. There was no precedent for any of this, success was due to the wisdom and foresight of those who drafted the Convention and to the founder Members of the International Committee for Weights and Measures overseeing the BIPM and to the high quality of the original scientific staff. However, success came at a price, the decision to define the Metre at 0 $^{\circ}$C, for example, led to much ill health in the early years among the staff from working in cold damp laboratories, an aspect of metrology that is easy to forget these days. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 4:18PM |
X2.00003: Measurement of the gravitational quantities g and G: How ideas for precision measurement experiments come about Invited Speaker: James Faller I will talk about g and G whose determinations go back to some of the earliest measurements in the history of metrology. Although today's measurement accuracy for g, the free-fall acceleration due to the Earth's gravity, has improved by nearly nine orders of magnitude, the measurement accuracy of G, the Newtonian Constant of Gravitation, has improved by only two orders of magnitude over its 300 year measurement history. I will discuss what has driven (and impeded) this progress, and how ideas for improvements in these measurements have helped advance the frontiers of measurement science. Finally, I will point out the interconnectedness of all precision measurement experiments. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:54PM |
X2.00004: The Odyssey of the Frequency Measurements of Visible Light at NBS/NIST Invited Speaker: Howard Layer The long term goal of defining the length standard based on a constant of nature rather than an artifact was pursued at the National Bureau of Standards/National Institute of Standards of Technology during the period of 1968 to 1983. With the invention of the laser it became possible to measure the frequency of lasers, stabilized on atoms or molecules, with the cesium atomic clock as the time standard. Using lasers, high speed MIM (metal insulator metal) diodes, and summing the radiation from three lasers in a He-Ne plasma the frequency of the iodine stabilized HeNe laser at 633 nm was measured by direct frequency counting to an accuracy of 1.6 parts in 10$^{10}$. The major consequences of this accomplishment were the adoption of a fixed value for the speed of light and the redefinition of the SI meter based on the speed of light. After a brief historical review, the milestones of the research will be outlined and the principle researches indicated. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:30PM |
X2.00005: Evolving Planck Constant Measurements into the SI Kilogram Standard Invited Speaker: Richard Steiner This is a very brief 100 year history of measuring 2$e$/$h$ (pre-and post-Josephson), with a little on $e^{2}$/$h$ (quantum Hall Effect, QHE), and then on to a direct measure of Planck constant $h$, where the watt balance technique combines four basic standards, i.e., physical constants of time, length, voltage, and resistance into a mass redefinition. There are parallels between old and new controversies. In the 1970's and 80's the controversy was in the changeover from standard cells to the Josephson effect as voltage reference. A slightly similar and briefer one concerned the ohm and QHE. Today's discussion is about changing definitions from an artifact mass standard to the Planck constant (or Avogadro constant) using the different methods as realization. The mass redefinition concerns are two orders of magnitude down from those of voltage, and the discrepancies between $h$ are probably more systemic rather than artifact related (or not) as compared to the Josephson effect testing. This shows how far electronic metrology has progressed but also that is it not completed research. The conclusion summarizes the latest efforts on the watt balances. [Preview Abstract] |
Session X3: Invited Session: Full Counting Statistics, Fluctuation Theorems, and Many-Body Entanglement
Sponsoring Units: DCMPChair: Andrew Jordan, University of Rochester
Room: 205AB
Thursday, March 1, 2012 2:30PM - 3:06PM |
X3.00001: Towards Measuring the Many-Body Entanglement from Fluctuations Invited Speaker: Karyn Le Hur The degree of entanglement in a many-body quantum system is often characterized using the bipartite entanglement entropy. We propose that bipartite fluctuations are also an effective tool for studying many-body physics [1] particularly its entanglement properties, in the same way that noise and full counting statistics have been used in mesoscopic transport and cold atoms. We apply some concepts underlying the field of full counting statistics to the study of the ground states of many-body Hamiltonians, with the boundary introduced by the bipartition playing the role of the scattering or interacting region. For systems that are equivalent to non-interacting fermions, we show that fluctuations and higher-order cumulants fully encode the information needed to determine the entanglement entropy [1-3]. In the context of quantum point contacts, measurement of the second charge cumulant showing a logarithmic dependence on time [2] then would constitute a strong indication of many-body entanglement [1]. Here, the measurability of the entanglement entropy, while suggestive, is particular to the nature of non-interacting particles [4,5]. \\[4pt] [1] H. Francis Song, S. Rachel, C. Flindt, I. Klich, N. Laflorencie and K. Le Hur, arXiv:1109.1001. 30 pages + 25 pages supplementary information.\\[0pt] [2] I. Klich and L. Levitov, Phys. Rev. Lett. 102, 100502 (2009).\\[0pt] [3] H. F. Song, C. Flindt, S. Rachel, I. Klich and K. Le Hur, Phys. Rev. B 83, 161408R (2011).\\[0pt] [4] B. Hsu, E. Grosfeld and E. Fradkin, Phys. Rev. B 80, 235412 (2009).\\[0pt] [5] H. Francis Song, Stephan Rachel and Karyn Le Hur, Phys. Rev. B 82, 012405 (2010). [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:42PM |
X3.00002: Charge sensing and real-time electron counting in quantum dots Invited Speaker: Thomas Ihn The charge state of a Coulomb blockaded quantum dot can be sensed at the single-electron level using charge sensors integrated on-chip [1]. Quantum point contacts or quantum dots have proven to work as reliable charge sensors. Coupling between the quantum dot's charge state and the sensor is provided by Coulomb interactions between electrons in the two systems. The technique is therefore independent of the material of quantum dot and sensor, and works for diverse systems such as GaAs [1] and graphene [2]. Different materials, such as InAs and GaAs have even been combined. Time-resolved charge sensing is of fundamental interest for studying the statistical properties of charge flow, like the full counting statistics. Single-shot charge and spin read-out schemes are also needed for the measurement of qubits. Some of our recent work has focused on the exploration and optimization of the read-out bandwidth limits. Towards this goal, we learned how to influence the dot-detector coupling [3], and how to employ high-frequency techniques in the range of a few hundred megahertz [4], or even up to a few gigahertz for improved performance. New experiments using the slower conventional sensing schemes have allowed us to explore non-equilibrium statistics and the fluctuation theorem. In our measurements, rare events can be detected, where electrons flow against the applied source-drain bias direction thereby consuming entropy in agreement with theoretical predictions. \\[4pt] [1] T. Ihn, S. Gustavsson, U. Gasser, R. Leturcq, I. Shorubalko, and K. Ensslin, Physica E: Low-dimensional Systems and Nanostructures {\bf 42}, 803-808 (2010).\\[0pt] [2] J. G\"uttinger, J. Seif, C. Stampfer, A. Capelli, K. Ensslin, and T. Ihn, Phys. Rev. B {\bf 83}, 165445 (2011).\\[0pt] [3] C. R\"ossler, B. K\"ung, S. Dr\"oscher, T. Choi, T. Ihn, K. Ensslin, and M. Beck, Appl. Phys. Lett. {\bf 97}, 152109 (2010).\\[0pt] [4] T. M\"uller, B. K\"ung, S. Hellm\"uller, P. Studerus, K. Ensslin, T. Ihn, M. Reinwald, and W. Wegscheider, Appl. Phys. Lett. {\bf 97}, 202104 (2010). [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 4:18PM |
X3.00003: High-order counting statistics and interactions Invited Speaker: Christian Flindt Full counting statistics concerns the stochastic transport of electrons in mesoscopic structures [1]. Recently it has been shown that the charge transport statistics for noninteracting electrons in a two-terminal system is always generalized binomial: it can be decomposed into independent single-particle events, and the zeros of the generating function are real and negative [2]. In this talk I show how the zeros of the generating function move into the complex plane due to interactions and demonstrate how the positions of the zeros can be detected using high-order factorial cumulants [3]. As an illustrative example I discuss electron transport through a Coulomb blockade quantum dot for which the interactions on the quantum dot are clearly visible in the high-order factorial cumulants. These findings are important for understanding the influence of interactions on counting statistics, and the characterization in terms of zeros of the generating function provides a simple interpretation of recent experiments, where high-order statistics have been measured [4]. \\[4pt] [1] Yu. V. Nazarov, ed., Quantum Noise in Mesoscopic Physics, NATO Science Series, Vol. 97 (Kluwer, Dordrecht, 2003) \newline [2] A. G. Abanov and D. A. Ivanov, Phys. Rev. Lett. 100, 086602 (2008), Phys. Rev. B 79, 205315 (2009) \newline [3] D. Kambly, C. Flindt, and M. B\"{u}ttiker, Phys. Rev. B 83, 075432 (2011) -- Editors' Suggestion \newline [4] C. Flindt, C. Fricke, F. Hohls, T. Novotn\'{y}, K. Netocn\'{y}, T. Brandes, and R. J. Haug, Proc. Natl. Acad. Sci. USA 106, 10116 (2009) [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:54PM |
X3.00004: Hidden correlations in the zero-point motion of electrons revealed by non-Gaussian quantum noise Invited Speaker: Bertrand Reulet We have performed the measurement of the correlation between the low frequency voltage fluctuations and the power fluctuations of the high frequency electromagnetic field generated by a tunnel junction at very low temperature. Our experiment provides the first observation of the correlation between the electron transport at low frequency and the photon field at high frequency, both when real photons are emitted (\textit{eV$>$hf}, with $V$ the dc voltage and $f$ the frequency) and when the electromagnetic field is solely due to vacuum fluctuations. In terms of electrons only, our observations indicate that the intrinsic current fluctuations in a tunnel junction are given by $S_{3}(0,f)=e^{2}I$, regardless of the frequency $f$. Despite its classical look, this result expresses that the high frequency current fluctuations, caused by the zero-point motion of electrons, are correlated with their low frequency counterpart, associated with ``real'' motion of electrons. In terms of photons only, we observe that the electromagnetic field exhibits a discontinuity at \textit{eV=hf}, not in the amplitude of the fluctuations but in their relative phases. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:30PM |
X3.00005: Noise and fluctuation statistics in mesoscopic heat transport Invited Speaker: Dmitri Averin Fluctuations play important role in thermodynamics of small systems. In the talk, I will discuss two recent results on fluctuations in mesosopic heat transport. One is the demonstration [1] that the fluctuation-dissipation theorem for thermal conductance of a mesocopic junction is not valid at non-zero frequencies $\omega $. Finite relaxation energy creates fluctuations of the energy flux in the junction even at vanishing temperature, T=0, when the conductance vanishes. This suggest that in contract to electrical conductance, there is no ``Kubo-Green formula'' for equilibrium thermal conductance at $\omega $ $\ne $0. Non-equilibrium heat transfer satisfies general ``fluctuation relations'' of non-equilibrium thermodynamics. Recently, we have established the conditions of applicability of these relations to single-electron tunneling (SET), and calculated explicitly the statistics of dissipated energy in driven SET transitions [2], which gives an example of general statistics of energy dissipation in reversible information processing. An interesting consequence of this statistics is the possibility of implementing the electronic version of Maxwell's demon in the SET structures [3]. \\[4pt] [1] D.V. Averin and J.P. Pekola, Phys. Rev. Lett. \textbf{104}, 220601 (2010). \\[0pt] [2] D.V. Averin and J.P. Pekola, arXiv:1105.041. \\[0pt] [3] D.V. Averin, M. Mottonen, and J.P. Pekola, arXiv:1108.5435. [Preview Abstract] |
Session X4: Focus Session: Quantum Quench Dynamics in Cold Atom Systems
Sponsoring Units: DAMOPChair: Anatoli Polkovnikov, Boston University
Room: 205C
Thursday, March 1, 2012 2:30PM - 3:06PM |
X4.00001: Integrability versus Thermalizability in Isolated Quantum Systems Invited Speaker: Maxim Olshanii The purpose of this presentation is to assess the status of our understanding of the transition from integrability to thermalizability in isolated quantum systems. In Classical Mechanics, the boundary stripe between the two is relatively sharp: its integrability edge is marked by the appearance of finite Lyapunov's exponents that further converge to a unique value when the ergodicity edge is reached. Classical ergodicity is a universal property: if a system is ergodic, then every observable attains its microcanonical value in the infinite time average over the trajectory. On the contrary, in Quantum Mechanics, Lyapunov's exponents are always zero. Furthermore, since quantum dynamics necessarily invokes coherent superpositions of eigenstates of different energy, projectors to the eigenstates become more relevant; those in turn never thermalize. All of the above indicates that in quantum many-body systems, (a) the integrability-thermalizability transition is smooth, and (b) the degree of thermalizability is not absolute like in classical mechanics, but it is relative to the class of observables of interest. In accordance with these observations, we propose a concrete measure of the degree of quantum thermalizability, consistent with the expected empirical manifestations of it. As a practical application of this measure, we devise a unified recipe for choosing an optimal set of conserved quantities to govern the after-relaxation values of observables, in both integrable quantum systems and in quantum systems in between integrable and thermalizable. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X4.00002: Chaos and statistical relaxation in quantum systems of interacting particles Lea Santos, Fausto Borgonovi, Felix Izrailev Recent experimental progresses in the studies of quantum systems of interacting particles with optical lattices have triggered the interest in basic problems of many-body physics. One of the issues that has been widely discussed in the literature is the onset of thermalization in an isolated quantum system caused by interparticle interactions. A prerequisite for thermalization is the statistical relaxation of the system to some kind of equilibrium and its viability has been associated with the onset of quantum chaos. We propose a method to study the transition to chaos in isolated quantum many-body systems, which is based on the concept of delocalization of eigenstates in the energy shell. We show that although the fluctuations of energy levels and delocalization measures in integrable and non-integrable systems differ, global properties of the eigenstates may be quite similar, provided the interaction between particles exceeds some critical value. In this case the quench dynamics can be described analytically, demonstrating the universal statistical relaxation of the systems irrespectively of whether they are chaotic or not. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X4.00003: Non-adiabatic ramps in quantum many-particle systems Masud Haque A change of system parameter can be neither truly instantaneous nor truly adiabatic in real life. For several quantum many-particle systems, I will consider non-equilibrium dynamics induced by finite-rate ramps. The ramp rate extrapolates between an instantaneous quench and an adiabatic sweep. I will characterize the deviation from adiabaticity through the excess energy or ``heating'' of the system. For cold-atom systems in a harmonic trapping potential, I will show that the non-adiabatic heating in finite-time ramps has universal features common to a wide range of systems. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X4.00004: Entropy of Isolated Quantum Systems after a Quench Marcos Rigol, Lea Santos, Anatoli Polkovnikov A diagonal entropy, which depends only on the diagonal elements of the system's density matrix in the energy representation, has been argued to be the proper definition of thermodynamic entropy in out-of-equilibrium quantum systems. We study this quantity after an interaction quench in lattice hardcore bosons and spinless fermions, and after a local chemical potential quench in a system of hard-core bosons in a superlattice potential. The former systems have a chaotic regime, where the diagonal entropy approaches the equilibrium microcanonical entropy, coinciding with the onset of thermalization. The latter system is integrable. We show that its diagonal entropy is additive and different from the entropy of a generalized Gibbs ensemble, which has been introduced to account for the effects of conserved quantities at integrability [1]. \\[4pt] [1] Lea F. Santos, Anatoli Polkovnikov, and Marcos Rigol, Phys. Rev. Lett. 107, 040601 (2011). [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X4.00005: Mode coupling induced dissipative and thermal effects at long times after a quantum quench Aditi Mitra, Thierry Giamarchi An interaction quench in a Luttinger liquid can drive it into an athermal steady state. We analyze the effects on such an out of equilibrium state of a mode coupling term due to a periodic potential. Employing a perturbative renormalization group approach we show that even when the periodic potential is an irrelevant perturbation in equilibrium, it has important consequences on the athermal steady state as it generates a temperature as well as a dissipation and hence a finite life-time for the bosonic modes. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X4.00006: Random phase approximation study of one-dimensional fermions after a quantum quench Jarrett Lancaster, Thierry Giamarchi, Aditi Mitra The effect of interactions on a system of fermions that are in a nonequilibrium steady state due to a quantum quench is studied employing the random phase approximation. As a result of the quench, the distribution function of the fermions is greatly broadened. This gives rise to an enhanced particle-hole spectrum and overdamped collective modes for attractive interactions between fermions. On the other hand, for repulsive interactions, an undamped mode above the particle-hole continuum survives. The sensitivity of the result to the nature of the nonequilibrium steady state is explored by also considering a quench that produces a current-carrying steady state. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X4.00007: Ballistic expansion of interacting fermions in one-dimensional optical lattices Fabian Heidrich-Meisner, Stephan Langer, Martin J.A. Schuetz, Ian McCulloch, Ulrich Schollwoeck In most quantum quenches, no net particle currents arise. Access to studying transport properties can be gained by letting a two-component Fermi gas that is originally confined by the presence of a trapping potential expand into an empty optical lattice. In recent experiments, this situation was addressed in 2D and 3D optical lattices [1]. We focus on the 1D case in which an exact numerical simulation of the time-evolution is possible by means of the DMRG method. Concretely, we study the expansion in the 1D Hubbard model with repulsive interactions, driven by quenching the trapping potential to zero, and we concentrate on the most direct experimental observable, namely density profiles [2]. In the strict 1D case, we identify conditions for which the expansion is ballistic, characterized by an increase of the cloud's radius that is linear in time. This behavior is found whenever initial densities are smaller or equal to one, both for the expansion from box and harmonic traps. We make quantitative predictions for the expansion velocity as a function of onsite repulsion and initial density that can be probed in experiments. \\[4pt] [1] Schneider et al., arXiv:1005.3545\\[0pt] [2] Langer et al., arXiv:1109.4364 [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X4.00008: Quenching across a quantum critical point: dependence of scaling laws on spatial periodicity Smitha Vishveshwara, Manisha Thakurathi, Wade DeGottardi, Diptiman Sen We study the quenching dynamics of a quantum many-body system in one dimension described by a Hamiltonian having spatial periodicity. Specifically, we consider a spin-1/2 $XX$ chain subject to a periodically varying magnetic field in the $\hat z$ direction or, equivalently, a tight-binding model of spinless fermions having a periodic local chemical potential. If the strength of the magnetic field (or chemical potential) is varied slowly in time at a rate $1/\tau$ so as to take the system across a quantum critical point, we find that the density of excitations thereby produced scales as a power of $1/\tau$.Remarkably, the power depends on the spatial periodicity of the field and deviates from the $1/\sqrt{\tau}$ scaling that is ubiquitous to a range of systems. This behavior is analyzed by mapping the slow quenching problem to a collection of fermionic two-level systems, labeled by the lattice momentum $k$, for which the effective Hamiltonians vary as a power of the time close to the quantum critical point. For a magnetic field described by multiple periodicities, the power depends on the smallest period for very large values of $\tau$. Finally, we find that if there are interactions between the fermions, the power varies continuously with the interaction strength. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X4.00009: Kibble-Zurek Scaling: Universality and scaling Anushya Chandran, Amir Erez, Shivaji L. Sondhi, Steven S. Gubser Near a critical point, the equilibrium relaxation time of a system diverges and any change of control/thermodynamic parameters leads to non-equilibrium behavior. The Kibble-Zurek (KZ) problem is to determine the dynamical evolution of the system parametrically close to its critical point when the change is parametrically slow. We formulate the KZ problem as a scaling limit and compute its universal content analytically (critical exponents+scaling functions) in a few classical and quantum models. We also use gauge-gravity duality to compute KZ response functions in more exotic critical theories. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X4.00010: Universal Quantum Dynamics of the Transverse-Field Ising Model Michael Kolodrubetz, Bryan Clark, David Huse The one-dimensional transverse field Ising model is a prototypical example of a quantum phase transition. While its equilibrium scaling has been known for more than half a century, we discuss the non-equilibrium quantum dynamics as the system is swept slowly through the critical point (a Kibble-Zurek ramp). Scaling is well understood for Kibble-Zurek ramps that end at the quantum critical point or deep in the ferromagnetic regime. We solve for the full finite-size scaling forms of excess heat and spin-spin correlation function for an arbitrary point along the ramp. We also confirm the postulated universality of the dynamic scaling forms by numerically simulating Mott insulating bosons in a tilted potential, an experimentally realizable model in the same universality class [Simon et. al., Nature 472, 372 (2011)]. Our numerics indicate that the time-scales necessary to see non-equilibrium scaling should already be within the reach of experiment. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X4.00011: Self-consistent theory of instabilities in the spin-1 Bose gas Austen Lamacraft, Ryan Barnett We discuss instabilities of a spin-1 Bose condensate using a Hartree-Fock-Bogoliubov approximation to account for the interactions between the unstable modes. There is a close analogy to the ``S-theory'' that describes parametric excitation of magnons in solid state systems. We particularly emphasize the pair-breaking effect of phase fluctuations in the parent condensate and their role in inhibiting the instability. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X4.00012: Decay of classical quasiperiodic state and emergence of prethermalization in quenched Fermi-Pasta-Ulam system Rafael Hipolito, Ippei Danshita, Vadim Oganesyan, Anatoli Polkovnikov We will discuss the relaxation of the Fermi Pasta Ulam system in the presence of quantum fluctuations. In order to make comparisons with the classical relaxation, we strongly excite a single normal mode, while the rest of the modes are initially in the quantum ground state. We confine ourselves to the quasiperiodic regime where the classical system never thermalizes. We show that the short time dynamics of the quantum problem are very different from classical evolution, with the quantum zero point energy playing a key role. The short time dynamics can be viewed as an enhancement of zero point energy, parametrically driven by the classical degrees of freedom. This introduces nontrivial off-diagonal correlations in the low momentum sector and dampens the classical oscillations eventually leading to both dephasing and decay, and we identify the time scales associated with these processes. Eventually the system reaches a nontrivial very long lived quasistationary regime where off-diagonal correlations disappear and the energy remains mostly localized in the low q sector while the high q sector relaxes to a uniform effective temperature. In this regime, correlations are very well described by a generalized Gibbs ensemble. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X4.00013: Fisher zeroes and non-analytic real time evolution for quenches in the transverse field Ising model Stefan Kehrein, Markus Heyl, Anatoli Polkovnikov We study quenches of the magnetic field in the transverse field Ising model. For quenches across the quantum critical point, the boundary partition function in the complex temperature-time-plane shows lines of Fisher zeroes that intersect the time axis, indicating non-analytic real time evolution in the thermodynamic limit (analogous to well-known thermodynamic phase transitions). We obtain exact analytical results for these dynamic transitions and show that the dynamic behavior cannot be obtained from a naive analytic continuation of the thermal equilibrium partition function: Real time evolution across this quantum critical point generates a new non-equilibrium energy scale. We argue that this behavior is expected to be generic for interaction quenches across quantum critical points in other models as well. [Preview Abstract] |
Session X5: Structure and Formation of Oxide Surfaces and Interfaces
Sponsoring Units: DCMPChair: Michael Pierce, Rochester Institute of Technology
Room: 206A
Thursday, March 1, 2012 2:30PM - 2:42PM |
X5.00001: Quasi-long range ordered hole-adatoms pairs on SrTiO$_{3}$(110)-(4$\times $1) surface Jiandong Guo, Fengmiao Li, Zhiming Wang, Sheng Meng, Zhiqiang Zhang The surface structure of transition metal oxides (TMOs) has been an important issue for chemistry and photocatalysis. We studied the surface of SrTiO$_{3}$, which is a wide-gap semiconductor and has been believed useful for photo-induced water splitting. Specifically we focused on the (110) surface that bears intrinsic instability of reconstruction due to the surface polarity. The monophased (4$\times $1)-reconstructed surface was obtained with the treatment of argon ion sputtering followed by annealing in ultra high vacuum. More interestingly, we observed a (4$\times $10) quasi-long range ordered hole-adatom structure. The atomic configuration was identified by both experimentally adsorbing additional Sr atoms and density functional calculations. The ordering of the hole-adatom pairs was robust since its formation effectively released the stress on (4$\times $1)-reconstructed SrTiO$_{3}$(110) surface. Such a surface with ordered defects served as a good template for the guided growth of noble metal nanoclusters with controlled size and density. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X5.00002: Growth modes of multiferroic BiFeO$_{3}$ (001) thin films on SrTiO$_{3}$ -- Real time X-ray synchrotron study Priya V. Chinta, Randall Headrick, Ashrafi Almamun, Sara Callori, Matthew Dawber Real time X-ray scattering measurements during heteroepitaxial film deposition provide details of initial nucleation and growth, thus giving insight into atomic-scale processes and growth mechanisms. In this work the growth of pulsed laser deposited (PLD), and RF-magnetron sputter deposited multiferroic BiFeO$_{3}$ (001) thin films on SrTiO$_{3}$ substrates are studied using \textit{in-situ} X-ray specular and diffuse scattering.~ Both the out-of-plane (height distribution) and lateral (surface mounds and correlations) information is obtained from these measurements. Specular X-ray intensity monitored at (00 $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} )$ shows unit-cell growth oscillations during the first few monolayers, while both diffuse scattering width and intensity oscillate out-of-phase with specular, characteristic of 2D layer-by-layer growth for PLD deposited films. The coarsening process is consistent with growth and coalescence of islands - identified as key fundamental processes in epitaxial growth. Beyond several monolayers oscillations decay and the diffuse intensity increases indicating rapid growth of mounds.~ However, the mounds themselves subsequently merge after a few more deposited layers, leaving arrays of mesas with some holes due to incomplete coalescence. This behavior was corroborated with ex-situ Atomic Force Microscopy (AFM) at each stage. Results for sputter deposited films showed unusually strong specular and diffuse oscillations; implications for interface structure and formation will be discussed. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X5.00003: Polarization Dependent Reconstructions of Ferroelectric Surfaces John Mark Martirez, Wissam Al-Saidi, Andrew Rappe We present an \emph{ab initio} density-functional study of the surface structures and the thermodynamic stability of ferroelectric tetragonal (001)-oriented slabs of BaTiO$_3$ and PbTiO$_3$, including both cases where the bulk polarization is directed towards (positive) and away (negative) from the surface. We analyzed systematically the total energies of surfaces with different reconstruction symmetries and stoichiometries. The thermodynamic stability diagrams show that the stable surface compositions for the positive and negative surfaces are different, which are in turn different from those of the paraelectric phase. We show that the prevalence of certain surface reconstructions can be understood in the light of the ability of ions to passivate ferroelectric surface charges. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X5.00004: STM study of the Mn-dopants on the surface of Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$(x=6{\%}, 16{\%}) Ward Plummer, Guorong Li, Qing Li, Minghu Pan, Biao Hu, Von Braun Nascimento, Jiandi Zhang, Rongying Jin The double-layered Sr$_{3}$Ru$_{2}$O$_{7 }$is a paramagnetic metal, but the substitution of Mn for Ru (Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7})$ results in a metal-to-insulator transition at T$_{MIT}$ and antiferromagnetic (AF) ordering at T$_{M}$ (the two transitions are closely coupled for x $<$ 6{\%}). STM measurements at 4.2 K and 100 K on the surface of Sr$_{3}$(Ru$_{1-x}$Mn$_{x})_{2}$O$_{7}$ ($x$ = 6{\%}, 16{\%}) reveal a ($\surd $2 $\times \surd $2)R45$^{o}$ unit cell, consistent with the orthorhombic bulk structure. The Mn dopant has been identified through bias-dependent STM topography and dI/dV mapping. The Mn dopant equally occupies two sites which are anti-phase - one sitting at the center and the other on the corner of the ($\surd $2 $\times \surd $2)R45$^{\circ}$ unit cell. We have directly imaged the chirality of MnO$_{6}$ rotation at the anti-phase sites. In contrast to the bulk measurements, the surface is always metallic for $x $= 16{\%} and insulating only for 4.2K measurements on the $x$ = 6{\%} sample. The surface apparently suppresses the Mn-induced insulating (AF) phase observed in the bulk. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X5.00005: Morphology and Magnetism of Atomically Thin Layers of Chromia -- An STM Investigation Xumin Chen, Donna Kenkel, Geoffrey Rojas, Xi He, Christian Binek, Axel Enders A low temperature scanning tunneling microscopy (LT-STM) study of ultrathin chromium oxide films on Cu(111) is presented. The (0001) surface of Cr$_{2}$O$_{3}$ (chromia) exhibits long-range antiferromagnetic ordering, and its usefulness for electric field control of exchange bias has been recently established [Xi He, Ch. Binek, et al., Natuer Materials 9, 579 -- 585 (2010)]. We deposited ultrathin chromium layers on Cu(111), followed by post-annealing in oxygen partial pressures to promote oxidation of the chromium films. We find that chromium grows on Cu(111) in the Volmer-Weber mode, resulting in small 3D islands. During annealing, the small islands coalesce to form large, flat terraces and the most stable oxide of chromium, Cr$_{2}$O$_{3}$, is formed at 630\r{ }C. Spin-polarized scanning tunneling microscopy has been performed to image the layer-wise antiferromagnetism in different structural layers of the chromia films. Thin layers of cobalt have been deposited on the chromia so that the magnetism of the Co couples to that of the chromia. Imaging the magnetism of the Co islands on the Cr$_{2}$O$_{3}$ terraces with spin-polarized STM helped us distinguishing between magnetic and electronic contrast in the complex dI/dV maps of the chromia surface. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X5.00006: Structures of Stoichiometric and Oxygen Deficient Sapphire Surfaces Hawoong Hong, Aaron Gray, T.-C. Chiang, Shawn Haydn, Michael Gramlich, Yiyao Chen, Paul Miceli Sapphire, $\alpha $-Al$_{2}$O$_{3}$ is one of widely used oxide substrates. Its (0001) surface is presumed quite stable. However, when it is heat-treated under vacuum, the surface shows the variety of different surface structures, mostly due to oxygen deficiency. The initial surface structure could be also modified by covering the substrate with another sapphire substrate during air annealing above 1500\r{ } C. During the course of the heat-treatment under UHV condition, the x-ray reflectivity and crystal truncation rods were measured in addition to observing reflection high energy electron diffraction patterns. Metal films such as Pb, Ag, and Pd were deposited onto these various sapphire surfaces. Contrary to common belief that metal films would not grow well on insulating oxide substrates, metal films could be formed on some of the sapphire surfaces. The influence of the sapphire surface structure to the morphology of the metal films will be discussed. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X5.00007: Crystallographic Orientation(s) of Vanadium Dioxide Nano-Grains on Various Single-Crystal Sapphire Substrates Felipe Rivera, Joyeeta Nag, Richard Haglund, Robert Davis, Richard Vanfleet Vanadium dioxide (VO$_{2}$) is a material of particular interest due to its reversible structural semiconductor to metal phase transition near room temperature (\~ 68 $^{\circ}$C) and its accompanied hysteresis. Electron Back-Scattered Diffraction (EBSD) was used to study the orientation of the crystalline VO$_{2}$ grains deposited on three cuts of sapphire (a-, c-, and r-cuts) by pulsed laser deposition. EBSD showed a predominant family of crystallographic relationships present in all cuts of sapphire wherein the rutile VO$_{2}$ \{001\} planes tend to lie parallel to the substrate's \{10-10\} and the rutile VO$_{2}$ \{100\} planes lie parallel to the substrate's \{1-210\} and \{0001\}. This family of relationships accounts for the majority of the VO$_{2}$ grains observed on all the studied sapphire substrates. However, due to the symmetry of the substrate, there were variations of these same relationships that prevent a single epitaxy from taking place in these cuts as the VO$_{2}$ grains did orient themselves with equivalent out-of plane directions in the substrate. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X5.00008: Growth of Vanadium Dioxide Thin Films Using Magnetron Sputtering Fangfang Song, B.E. White Jr. In this work, we present our experimental investigation of vanadium oxide thin film deposition. RF and DC magnetron sputtering are used for thin film deposition. Post deposition anneal are introduced to stabilize the vanadium dioxide phase. The impact of deposition conditions and anneal conditions on the structural and morphological properties of the thin films, as determined by x-ray diffraction and scanning electron microscopy, will be discussed. Results indicate that on the technologically relevant silicon dioxide surface, the transitional phase of vanadium dioxide can be stabilized by post deposition anneal at 550$^{\circ}$C and oxygen partial pressures between $3.6 \times 10^{-4}$ torr and $10^{-2}$ torr. The films obtained show a resistivity change of a factor of 200 at 350k. The shift in transition temperature is attributed to thin film stress. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X5.00009: Studies of high coverage oxidation of the Cu(100) surface using low energy positrons N.G. Fazleev, W.B. Maddox, A.H. Weiss The study of oxidation of single crystal metal surfaces is important in understanding the corrosive and catalytic processes associated with thin film metal oxides. The structures formed on oxidized transition metal surfaces vary from simple adlayers of chemisorbed oxygen to more complex structures which result from the diffusion of oxygen into subsurface regions. In this work we present the results of theoretical studies of positron surface and bulk states and annihilation probabilities of surface-trapped positrons with relevant core electrons at the oxidized Cu(100) surface under conditions of high oxygen coverage. Calculations are performed for various high coverage missing row structures ranging between 0.50 and 1.50 ML oxygen coverage. The results of calculations of positron binding energy, positron work function, and annihilation characteristics of surface trapped positrons with relevant core electrons as function of oxygen coverage are compared with experimental data obtained from studies of oxidation of the Cu(100) surface using positron annihilation induced Auger electron spectroscopy (PAES). [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X5.00010: \textit{In situ} TEM Studies of the Initial Oxidation stage of Cu and Cu Alloy Thin Films Judith Yang, Yihong Kang, Langli Luo, James Ciston, Eric Stach, Guangwen Zhou The fundamental understanding of oxidation at the nanoscale is important for the environmental stability of coating materials as well as processing of oxide nanostructures. Our previous studies show the epitaxial growth of Cu$_{2}$O islands during the initial stages of oxidation of Cu thin films, where surface diffusion and strain impact the oxide development and morphologies. The addition of secondary elements changes the oxidation mechanism. If the secondary element is non-oxidizing, such as Au, it will limit the Cu$_{2}$O island growth due to the depletion of Cu near the oxide islands. When the secondary element is oxidizing, for example Ni, the alloy will show more complex behaviour, where duplex oxide islands were observed. Nucleation density and growth rate of oxide islands are observed under various temperatures and oxygen partial pressures (pO$_{2})$ as a function of time by \textit{in situ }ultra high vacuum (UHV)-transmission electron microscopy (TEM). Our initial results of Cu-Ni(001) oxidation is that the oxide epitaxy and morphologies change as function of Ni concentration. For higher spatial resolution, we are examining the atomic scale oxidation by aberration-corrected ETEM with 1{\AA} resolution. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X5.00011: Quantum-Mechanical Investigations for the Oxidation Mechanism of Ti$_{3}$Al Wenhua Xue, Shi-Yu Liu, Shiyang Liu, Dejun Li, Tara Drwenski, Shuxia Yin, Sanwu wang First-principles density-functional theory and thermodynamics calculations are combined to establish a microscopic mechanism for the oxidation of the $\alpha _{2}$-Ti$_{3}$Al(0001) surface. The surface energies as functions of the chemical potentials, as well as structural relaxations and electronic densities of states, are determined. The surface phase diagram (SPD) of the $\alpha _{2}$-Ti$_{3}$Al(0001) systems with different defects and at various oxygen coverages is constructed. It is found that the Al antisite defect prefers to segregate on the $\alpha _{2}$-Ti$_{3}$Al(0001) surface and oxygen adsorption enhances the segregation with the formation of the surface with three Al antisites per unit surface cell (i.e., the top surface layer is full of Al atoms) at the initial stage of oxidation, accounting for the aluminum selective oxidation observed experimentally. After the initial stage of oxidation, the O/$\alpha _{2}$-Ti$_{3}$Al(0001) system manifests itself with a non-uniform double-phase SPD, suggesting the competition between oxidations of the Al and Ti elements in the oxidation process. This result explains the experimentally observed second regime of oxidation in which both metal elements are oxidized. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X5.00012: Site Specific Molecular Chemisorption of O$_{2}$ on TiO$_{2}$(110): A Scanning Tunneling Microscopy Study Zhitao Wang, Yingge Du, Zdenek Dohn\'alek, Igor Lyubinetsky The investigation of O$_{2}$ adsorption on TiO$_{2}$ is critical since it can help us to better understand the photooxidation mechanism of TiO$_{2}$. In our work, molecularly chemisorbed O$_{2}$ were directly imaged on reduced TiO$_{2}$(110) at 50 K with scanning tunneling microscopy (STM). Two O$_{2}$ adsorption channels, one at bridging oxygen vacancies (V$_{O})$ and another at five-fold coordinated Ti atoms (Ti$_{5c})$, have been identified. While O$_{2}$ at Ti$_{5c}$ appears as a single protrusion, the O$_{2}$ at V$_{O}$ manifests itself by a disappearance of the V$_{O}$ feature. It is found that STM tip can easily dissociate O$_{2}$ and the dissociation details strongly depend on the tunneling conditions and the type of the O$_{2}$ adsorption site. The chemisorbed O$_{2}$ at these two distinctive sites are the most likely precursors for the two previously established O$_{2}$ dissociation channels, observed at temperatures above 150 and 230 K at the V$_{O}$ and Ti$_{5c}$ sites, respectively [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X5.00013: Hard x-ray photoemission studied and band alignment in TiO$_{2}$/HfO$_{2}$/Ge heterojunctions Abdul Rumaiz, Joseph Woicik, Qi Xie, Peter Siddons, Conan Weiland, Christophe Detavernier Novel high K oxides such as hafnium oxide and zirconium oxide have replaced silicon dioxide as gate oxide. Although titanium oxide has a high dielectric constant, the poor conduction band offset between titanium oxide and Si/Ge makes it a poor choice for gate oxide. One way to address this issue is to have a thin intermediate layer with appropriate band alignment between titanium oxide and the semiconductor. Here we present hard x-ray photoelectron spectroscopy (HAXPES) study on the band alignment between atomic layer deposited (ALD) TiO$_{2}$/HfO$_{2}$/Ge heterojunctions. The exact position of the valence band maximum was determined by convoluting theoretical calculated density of states from first-principles calculations and comparing with experimental valence band data. We will also discuss the dependence of the band alignment on the thickness of the intermediate layer. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X5.00014: Synchrotron Photoemission Characterization of Process Dependent Oxidation Control in InGaAs/High-k Film Systems Conan Weiland, Joseph Woicik, Jimmy Price, Pat Lysaght, Jeff Huang To continue CMOS scaling, higher mobility substrates, such as III-V semiconductors, are being investigated. However, certain technological barriers must be overcome before high-mobility devices can be employed; notably, defects at the semiconductor/high-k interface need to be well controlled. Here, we investigate the role of an AlN interlayer between InGaAs and ALD ozone deposited ZrO2 using synchrotron source X-ray photoemission spectroscopy (XPS). XPS is able to quantify the relative levels of oxidation from the In, Ga and As signals. Also, by comparing the relative As 3d3/2: As 3d5/2 peak intensities, it is possible to quantify the level of As-As bond formation, which is common to coarse As-O reduction processing and consistent with mobility degradation. To elucidate the role of the AlN interlayer, XPS spectra of InGaAs/AlN/ZrO2, InGaAs/ZrO2 and InGaAs/AlN films will be presented and compared. We will demonstrate that an AlN interlayer is effective in reducing In, Ga and As interfacial oxides, as well as As-As bonding, yet is not sufficient to passivate the interface against further oxidation from ozone based ALD processing. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X5.00015: Engineering an epitaxial dielectric/metal/dielectric trilayer system Jisun Kim, Yang Meng, Z.Q. Qiu, Chih-Kang Shih Ag is a favorable material platform for plasmonic applications. Most plasmonic devices to date have been based on granular polycrystalline metal films, which suffer from scattering due to surface roughness. Recently, superior plasmonic properties have been demonstrated by using atomically smooth epitaxial Ag(111) films grown on Si(111), illustrating the advantages of an epitaxial approach. For further device applications of the above example, it is desirable to create epitaxially grown dielectric/metal/dielectric multi-layer systems. Here we report successful epitaxial growth of a MgO(001)/Ag(001)/MgO(001) trilayer system which can be used as a material platform for creating multi-layered (dielectric/metal)$_{N}$ heterostructures with any desired number, N. This type of new structure can be utilized for plasmonic devices with a wider wavelength range due to the transparency of MgO substrates in visible light. Moreover, it can open a door to new integrated devices with high quality atomically smooth films. [Preview Abstract] |
Session X6: Carbon Nanotube Optics II: Absorption and Raman Spectroscopy
Sponsoring Units: DMPChair: Steve Cronin, University of Southern California
Room: 206B
Thursday, March 1, 2012 2:30PM - 2:42PM |
X6.00001: Measurement of Absolute Absorption Cross-section of Individual Carbon Nanotubes on a Substrate Lihong Herman, Adam Tsen, Jiwoong Tsen The absorption cross-section is one of the central parameters that determine the efficiency of most optical and optoelectronics processes in single-walled carbon nanotubes (SWNT), including photoluminescence, photodetection, and photovoltaic energy conversion. While absorption measurements on SWNT ensembles in solution provide a reliable estimate, the absolute absorption cross-secions of individual carbon nanotubes on and off resonance have not been reported. Here, we measure the absorption cross-section of SWNTs on chip with spatial resolution using lock-in technique with a spatial modulation of a focused laser spot near SWNTs. We measure the absorption cross-section of SWNTs near resonance to be on the order of 10$^{-17}$ cm$^2$ per carbon atom, which is consistent with on-chip Rayleigh measurements as well as recent time-resolved photoluminescence measurements. Since our measurement is performed on SWNTs on chip, it can be directly applied to various optoelectronic devices made with SWNTs, thus allowing quantitative analysis of the fundamental performance limits of SWNT photovoltaic devices. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X6.00002: Measurement of Absorption Cross-section for Single-walled Carbon Nanotubes Xiaoping Hong, Kaihui Liu, Feng Wang Optical absorption is the most fundamental optical property. Quantitative knowledge of absorption cross-section provides valuable information on the material electronic structure, and is necessary for evaluating quantum efficiency of other important optical processes, such as photoluminescence and photocurrent generation. However, absorption measurement of individual single-walled carbon nanotubes(SWCNTs) is quite challenging. Here we used an efficient interferometric method to obtain the absorption cross-section of individual chirality-defined SWCNTs. We will discuss how the absorption cross-section varies in different carbon nanotubes. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X6.00003: Enhancement of photoluminescence from single-walled carbon nanotubes by photonic crystal microcavities R. Watahiki, T. Shimada, P. Zhao, S. Chiashi, S. Iwamoto, Y. Arakawa, S. Maruyama, Y.K. Kato Single-walled carbon nanotubes are bright nanoscale emitters, while photonic crystal microcavities offer the possibility for efficient optical coupling at the nanoscale because of their small mode volumes and high quality factors. Here we report on the enhancement of photoluminescence from single-walled carbon nanotubes by L3 cavities in hexagonal lattice photonic crystals. Free-standing photonic-crystal membranes are fabricated from silicon-on-insulator substrates, and micelle-encapsulated carbon nanotubes are dispersed on the devices. We observe sharp peaks with a typical spectral width of 0.4 nm which corresponds to a quality factor of $\sim$3000. As the peaks appear at wavelengths longer than those of Si photoluminescence, they are attributed to carbon nanotube emission coupled to the microcavity modes. We find that the photoluminescence intensity is enhanced by more than a factor of four compared to luminescence from an unpatterned area. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X6.00004: Can we look into carbon nanotubes by infrared light? Katalin Kamaras, Aron Pekker, Zsolt Szekrenyes, Ferenc Simon, Bea Botka, Rudi Hackl, Akos Botos, Andrei Khlobystov Individual molecules filled into carbon nanotubes exhibit Raman activity but very weak, if any, infrared absorption. We will present infrared (transmission and ATR), Raman and transmission electron microscopy data of various filled nanotubes (sorted by diameter and metallicity; encapsulating organometallic, aromatic and fullerene-based molecules) to illustrate this puzzling behavior. In the infrared spectra of double-walled carbon nanotubes, however, vibrational signatures of the inner and outer tubes are clearly discernible. A strong proof for this assignment is the shift of the inner-tube modes with $^{13}$C isotope content in samples where the inner tube is enriched with $^{13}$C. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X6.00005: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X6.00006: Intersubband Edge Singularity in Metallic Nanotubes Eugene Mishchenko, Oleg Starykh Tunneling density of states of both the massless and massive (gapped) particles in metallic carbon nanotubes is known to have anomalous energy dependence. This is the result of coupling to multiple low-energy bosonic excitation (plasmons). For both kinds of particles the ensuing effect is the suppression of the density of states by electron-electron interactions. We demonstrate that the optical absorption between gapless and gapped states is affected by the many-body effects in the opposite way. The absorption probability is enhanced compared with the non-interacting value and develops a power-law frequency dependence, with the -0.2 exponent for typical nanotubes. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X6.00007: Indication of Long Lived States in Decay Associated Spectra of Single-Walled Carbon Nanotubes Vasudev Lal, Roseanne Sension, Duncan Steel The direct bandgap nature of Single-Walled Carbon Nanotubes(SWCNTs) along with the quantized energy level structure due to reduced dimensionality makes them useful elements in chip-based photonic devices for sensing and communications. However, their fundamental linear and nonlinear optical properties remain poorly understood. Using various nonlinear optical spectroscopy techniques with micelle encapsulated SWCNTs, we have measured carrier dynamics at both the picosecond, and tens of nanosecond timescale. We measure a fast 20ps timescale decay that agrees well with the lifetime of the lowest excited state measured before for such samples but surprisingly we also obtain an optical double-resonance signal on the slow timescale (~10nsec). Such slow timescale signals due to artifacts such as thermal effects have been ruled out. Decay associated spectra shows striking differences between the spectral lineshapes arising from the fast and slow components of the nonlinear optical signal which might indicate the creation of a long-lived state by the pump pulse that changes the subsequent probe spectra. This indicates the possibility of the presence of either a trap state or possibly a more complex energy level structure for the SWCNT involving the presence of a metastable state. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X6.00008: Asymmetric resonance Raman excitation profiles and violation of the Condon approximation in single-wall carbon nanotubes Stephen Doorn, Juan Duque, Hagen Telg, Hang Chen, Anna Swan, Erik Haroz, Junichiro Kono, Xiaomin Tu, Ming Zheng DNA wrapping-based ion exchange chromatography and density gradient ultracentrifugation provide nanotube samples highly enriched in single chiralities. We present resonance Raman excitation profiles for the G-band of several single chirality semiconducting and metallic species. The expected incoming and outgoing resonance peaks are observed in the profiles, but contrary to long-held assumptions, the outgoing resonance is always significantly weaker than the ingoing resonance peak. This strong asymmetry in the profiles arises from a violation of the Condon approximation [1]. Results will be discussed in the context of theoretical models that suggest significant coordinate dependence in the transition dipole (non-Condon effects). The generality of the behavior across semiconducting and metallic types, nanotube family, phonon mode, and Eii will be demonstrated. \\[4pt] [1] J. Duque et. al., ACS Nano, 5, 5233 (2011). [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X6.00009: Raman Spectroscopy characterization of individual triple-walled carbon nanotubes Thomas Hirschmann, Paulo Araujo, Mildred Dresselhaus, Kornelius Nielsch The characterization of individual triple-walled carbon nanotubes (TWCNT) was studied in detail by using Raman spectroscopy resonant signals taken with various laser excitation energies. TWCNTs are in fact an assembly of three concentric weakly coupled single-walled carbon nanotubes (SWCNTs) or, equivalently, a double-walled carbon nanotube (DWCNT) concentric to an external SWCNT. An isolated TWCNT consists of an inner, middle and outer tube, each of which can be either metallic or semiconducting. All of the eight possible combinations provide a multitude of information about the electrical and optical properties. Among numerous applications, TWCNTs offer an ideal structure to study and understand how an interacting medium influences the properties of both, SWCNTs and DWCNTs structures. The measured spectra show exceptionally distinctive radial breathing modes, G-, G'-band and further modes of the three concentric tubes. All of the Raman three bands described above are very sensitive to changes in the structure of each tube. By following the spectral changes of these frequency modes, we can extract important information about, for example, the respective tube distances, inter-tube interaction in TWCNTs systems and its consequences on their related SWCNTs and DWCNTs counterparts. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X6.00010: Resonance Raman Spectroscopy of Separated Single-Wall Carbon Nanotube J.R. Simpson, J.A. Fagan, X. Tu, M. Zheng, A.R. Hight Walker, J.G. Duque, J. Crochet, S.K. Doorn The heterogeneity of single-wall carbon nanotubes (SWCNTs) produced by typical techniques complicates characterization and presents a barrier for technological applications. Improvements in separation and purification techniques enable detailed studies of specific nanotube properties by providing samples of unique chirality, length, metallicity, bundling, and interior filling. We report resonant Raman spectroscopy (RRS) measurements on these samples over a wide range of excitation wavelengths using a series of discrete and continuously tunable laser sources coupled to a triple-grating spectrometer. RRS of these homogeneous samples reveals unique spectral features and affords interpretation of intrinsic nanotube optical properties. Of particular interest are the G-band of chirally-pure armchair metallic SWCNTS and shifts of the radial breathing mode and excitation energy with water filling. Additionally, we will compare Raman results with other optical characterization techniques. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X6.00011: Optical properties of single-walled carbon nanotube aerogels Gordana Ostojic A network of connected single-walled carbon nanotubes (SWNT) is created by a novel DNA-protein complex directed assembly. Due to a point-like nature of connectors, the SWNT aerogel represents a network of self-suspended nanotubes with a record ultra-low density of less 0.75 mg/cm$^{3}$. The assembly method and low density enables a direct comparison of optical properties of nanotubes in solvent and air to surfactant solubilized nanotubes. Optical properties of SWNT gels are investigated using optical absorption, photoluminescence and Raman spectroscopy. Gelled nanotubes in water and in the low population regime behave similar to solubilized nanotubes. In contrast, photoluminescence of SWNT aerogels exhibit nonlinear effects and a phonon-induced broadening. In addition, aerogels show a previously unobserved photoluminescence peak at 1.3 eV that corresponds to a phonon-assisted recombination of photoexcited charges. Raman spectra of carbon nanotube aerogels display narrow peaks due to the phonon decoupling of suspended SWNTs in air and a redistribution of G phonon population due to nonlinear effects. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X6.00012: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X6.00013: Coupled Radial Breathing Oscillation in Double-Walled Carbon Nanotubes Kaihui Liu, Xiaoping Hong, Enge Wang, Feng Wang Double-walled carbon nanotubes (DWNTs) provide a model system for quantitative study of electronic and vibrational couplings in the nanoscale. Here we investigate coupled radial breathing mode (RBM) oscillations in structurally defined DWNTs by combining electron diffraction, Rayleigh scattering, and Raman scattering spectroscopy on the same individual nanotubes. We find that the two RBM oscillations in DWNTs are strongly coupled, with vibration energies significantly higher than those of constituent inner- and outer-wall nanotubes. In addition, the oscillation strength of these two coupled modes shows an interesting quantum interference behavior between the inner- and outer-wall electronic resonance channels. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X6.00014: Observation of Optical Phonon Emission Threshold in the Current-Voltage Characteristics of Suspended Carbon Nanotubes Moh Amer, Adam Bushmaker, Ikai Hsu, Steve Cronin Electrically-heated suspended, nearly defect-free, carbon nanotubes (CNTs) exhibiting negative differential conductance in the high bias regime experience a sudden drop in current (or ``kink''). The bias voltage at the kink ($V_{kink})$ is found to depend strongly on the applied gate voltage, substrate temperature, pressure, and gas environment. After subtracting the voltage drop across the contacts, however, the kink bias voltages converge around 0.2V, independent of gate voltage and gas environment. Due to the ballistic nature of these defect free carbon nanotubes, this bias voltage of 0.2V corresponds to the threshold energy of optical phonon emission. This phenomenon is corroborated by simultaneously monitoring the Raman spectra of these nanotubes as a function of bias voltage. At the kink bias voltage, the $G $band Raman modes experience a sudden downshift, further indicating threshold optical phonon emission. A Landauer model is used to fit these kinks in various gas environments where the kink is modeled as a sudden change in the optical phonon lifetime, which corresponds to a change in the non-equilibrium factor that describes the existence of hot phonons in the system. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X6.00015: The Electronic and Vibrational Properties of Carbon Nanorings Hang Chen, Anna Swan [n]-Cycloparaphenylenes ([n]-CPPs), carbon nanorings with n benzene units reminiscent of a single unit of a (n, n) armchair carbon nanotubes, show unusual optical behaviors: the absorption maximum is almost size independent at 340 nm, while the lower energy fluorescence exhibits multiple peaks with strong size dependence, opposite of quantum confinement behavior [1]. The energy levels for CPPs with various sizes are calculated using time-dependent density functional theory (TDDFT) and compared to the experimental results. Moreover, we calculate the Raman modes of various [n]-CPPs and propose that the multiple peaks observed in the fluorescence spectra could be due to the electron-phonon coupling effects. We also consider possible excitonic effects in these carbon nanorings, and discuss their similarities as well as differences to carbon nanotubes. [1] T. Iwamoto, Y. Watanabe, Y. Sakamoto, T. Suzuki, S. Yamago. \textit{J. Am. Chem. Soc.}, 133, 2011; [Preview Abstract] |
Session X7: Electronic, Structural and Chemical Properties of Non-Carbon Nanotubes and Nanowires
Sponsoring Units: DCMPChair: Paul McIntyre, Stanford University
Room: 207
Thursday, March 1, 2012 2:30PM - 2:42PM |
X7.00001: Electronic properties of fluorine passivated silicon nanowires: density functional calculations Keenan Zhuo, Mei-Yin Chou Arrays of silicon nanowire(SiNWs) have recently gained attention as a promising new photovoltaic technology. Previous studies show that halogen passivated SiNWs have good chemical stability and also form a critical pathway towards organic group functionalization. Yet, surface halogens are known to alter the SiNW electronic structure, most notably by reducing the band gap. We explore the fundamental physics behind this effect through first principles calculations on hydrogen and fluorine passivated (110) SiNWs. Electronic structure analysis reveals that the highly electronegative fluorine passivation modifies the quantum confinement potential and induces changes in the energy level ordering. Furthermore, we show how a modified cylindrical potential well model can demonstrate a link between this quantum confinement modification and the shift in energy levels responsible for the band gap reduction. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X7.00002: Molecular Hydrogen and Oxygen Interactions with Armchair Si Nanotubes Haoliang Chen, Asok Ray First principles calculations based on hybrid density functional theory have been used to study the electronic structure properties of armchair silicon nanotubes from (3, 3) to (12, 12). Full geometry and spin optimizations have been performed without any symmetry constraints with an all electron 3-21G* basis set and the B3LYP functional. The largest silicon nanotube studied has a cohesive energy of 3.47eV/atom. Molecular hydrogen and oxygen adsorptions on a (6, 6) tube have been studied by optimizing the distances of the admolecules from both inside and outside the tube. The molecule is originally placed perpendicular or parallel to the tube axis. The on-top external site is the most preferred site for the hydrogen molecule with adsorption energy of 3.71eV and an optimized distance of 3.31 {\AA} when it is perpendicular to the tube axis. For oxygen, the molecule dissociates into two atoms with adsorption energy of 7.45eV, the optimized distances being 1.65/1.68{\AA}. The buckling of the nanotubes increased significantly indicating structural deformation and an increase of sp$^{3}$ structure. The band gap increases from 0.98eV of bare nanotube to 1.26eV after adsorption of hydrogen molecule. For oxygen molecule, the band gap slightly increases from 0.98eV to 1.01eV. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X7.00003: Modeling Earle-Stage Kinking during VLS Ge Nanowire Growth Seunghwa Ryu, Yanying Li, Ann F. Marshall, Wei Cai, Paul C. McIntyre The catalyzed growth of Ge nanowires from gold nanoparticles via the vapor-liquid-solid (VLS) mechanism has been the subject of intense research worldwide, due to their potential applications in nanotechnology. Understanding the fundamental mechanisms underlying kinking during Ge nanowire growth, especially at the early-stage, is helpful for better control of Ge nanowire growth for technological applications. We report an investigation of wire morphology before and during Ge nanowire kinking in early stage growth under typical nucleation conditions. The Ge nanowires grew either along the vertical [111] direction or kinked away onto inclined $<$111$>$ axes early on during their growth. We found that most kinked Ge nanowire deposited under these conditions kinked at similar height, and had similar sidewall facet structure in the kinked region. High-resolution transmission and scanning electron microscopy investigations also showed that the typical kinking-structure was bounded by (111) and other relatively stable Ge surface facets. We construct 3D phase field model of the nanowire based on the transmission and scanning electron microscopy and compare the evolution of the droplet and nanowire with experimental observations. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X7.00004: Detection of bio-molecules of different polarities by Boron Nitride Nanotoube Saikat Mukhopadhyay, Ralph Scheicher, Ravindra Pandey, Shashi Karna The effect of molecular polarity on the interaction between a boron nitride nanotube (BNNT) and amino acids is investigated with density functional theory. Three representative amino acids, namely, tryptophane (Trp), a nonpolar aromatic amino acid, and asparatic acid (Asp) and argenine (Arg), both polar amino acids are considered for their interactions with BNNT. The polar molecules, Asp and Arg, exhibit relatively stronger binding with the tubular surface of BNNT. The binding between the polar amino acid molecules and BNNT is accompanied by a charge transfer, suggesting that stabilization of the bioconjugated complex is mainly governed by electrostatic interactions. The results show modulation of the BNNT band gap by Trp. Interestingly, no change in band gap of BNNT is seen for the polar molecules Asp and Arg. The predicted higher sensitivity of BNNTs compared to carbon nanotubes (CNTs) toward amino acid polarity suggests BNNTs to be a better substrate for protein immobilization than CNTs. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X7.00005: Quantum Confinement and Surface Relaxation Effects in Rutile TiO$_{2}$ Nanowires Abraham Hmiel, Yongqiang Xue Recent developments in synthesis of TiO$_{2}$ nanowires with diameters on the atomic scale have opened up new grounds for studying structure-property relationships in the regime where quantum confinement effects are important. In the sub-nanometer range, the properties of nanowires can be sensitive to atomic-level control of surface morphology, functionalization, and nanowire stoichiometry during the growth and fabrication processes, thereby opening the way for new applications. First-principles density functional theory calculations have been applied to investigate the size- and shape-dependent properties of [001]-oriented rutile TiO$_{2}$ nanowires of rectangular cross section. We find that the pronounced oscillation in the formation energy and band structure of the nanowires as a function of the number of TiO$_{2}$ trilayers is largely connected to the presence or absence of a mirror Ti-O plane along each confinement direction. We demonstrate that the relative stability and the indirect or direct character of the band structure of the rutile TiO$_{2}$ nanowires arise from the interplay between surface relaxation and quantum confinement effects that depend on the even-odd parity of the number of TiO$_{2}$ trilayers. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X7.00006: Simulation of Nanowires on Metal Vicinal Surfaces: Effect of Growth Parameters and Energetic Barriers Ajmi B.H. Hamouda, Sonia Blel, T.L. Einstein Growing one-dimensional metal structures is an important task in the investigation of the electronic and magnetic properties of new devices. We used kinetic Monte-Carlo (kMC) method to simulate the formation of nanowires of several metallic and non-metallic adatoms on Cu and Pt vicinal surfaces. We found that mono-atomic chains form on step-edges due to energetic barriers (the so-called Ehrlich-shwoebel and exchange barriers) on step-edge. Creation of perfect wires is found to depend on growth parameters and binding energies. We measure the filling ratio of nanowires for different chemical species in a wide range of temperature and flux. Perfect wires were obtained at lower deposition rate for all tested adatoms, however we notice different temperature ranges. Our results were compared with experimental ones [\textit{Gambardella }\textit{et al.}, Surf. Sci.449, 93-103 (2000), PRB \textbf{61}, 2254-2262, (2000)]. We review the role of impurities in nanostructuring of surfaces [Hamouda \textit{et al.}, Phys. Rev. B \textbf{83}, 035423, (2011)] and discuss the effect of their energetic barriers on the obtained quality of nanowires. Our work provides experimentalists with optimum growth parameters for the creation of a uniform distribution of wires on surfaces. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X7.00007: Escape of an axial dislocation from a thin rod Zhaoxuan Wu, Yong-Wei Zhang, Mark Jhon, David Srolovitz Whiskers, nanowires and nanorods have been supposed to grow by preferential attachment to the atomic step formed from a screw dislocation intersecting the surface. This is expected to leave behind an axial screw dislocation, as has been observed in ionic nanowires such as NaCl, PbS and PbSe. However, we are not aware of any studies that have directly observed axial dislocations in pure FCC metal nanowires. To explain this, we speculate that thermal vibrations will be enough to kick out dislocations due to their high mobility. We consider two models of how a dislocation might escape from a nanowire. The first model is that the dislocation vibrates inside the nanowire. The second is that the nanowire itself vibrates, causing deformations of the nanowire that push the dislocation out. Analysis of these models imply that dislocations in thin nanowires are remarkably thermally stable. We test this prediction with molecular dynamics calculations on Cu nanowires, and find that the preparation of these systems is critical to the dislocation stability. Preparing the sample by simply raising the MD temperature will cause the dislocation to run out. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X7.00008: Ensemble Measurements of Temperature Dependence in High-Q GaN Nanowire Mechanical Resonators Joshua Montague, Kris Bertness, Norman Sanford, Victor Bright, Charles Rogers We report on measurements of c-axis oriented, single crystal, gallium nitride nanowire (GaN NW) mechanical resonators. Our measurements use a capacitively-coupled, microwave homodyne reflectometer that allows for simultaneous detection of large ensembles of the as-grown, GaN NW resonators. The NWs - grown via molecular beam epitaxy - behave as singly clamped beams, have lengths near 15 microns, and radii near 100 nm. We observe, respectively, typical resonance frequencies and mechanical quality factors (defined as the ratio of resonance frequency to full width at half maximum power), Q, near 1 MHz and above 10,000, near room temperature [1]. At lower temperatures, we observe increases in resonance frequencies consistent with temperature-dependent elastic moduli. Measured Q factors demonstrate a minimum near 150 K and typically increase an order of magnitude - to above 100,000 - below 100 K.\\[4pt] [1] S.W. Hoch et al., Appl. Phys. Lett. 99(5), 053101 (2011). [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X7.00009: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X7.00010: Low-frequency noise in gallium nitride nanowire mechanical resonators Jason Gray, Kris Bertness, Norman Sanford, Charles Rogers We report on the low-frequency 1/f (flicker) parameter noise displayed by the resonance frequency and resistance of doubly clamped c-axis gallium nitride nanowire (NW) mechanical resonators. The resonators are electrostatically driven and their mechanical response is electronically detected via NW piezoresistance. With an applied dc voltage bias, an NW driven near its mechanical resonance generates a dc and Lorentzian rf current that both display 1/f noise. The rf current noise is proportional to the square of the derivative of the Lorentzian lineshape with a magnitude highly dependent on NW dc bias voltage conditions, consistent with noise in the NW's resistance leading to temperature noise from local Joule heating, which in turn generates resonance frequency noise. An example device with a 27.8 MHz resonance frequency and 220 k$\Omega$ resistance experiences an approximate resonance frequency shift of -5.8 Hz/nW. In terms of NW resistance change, this corresponds with shifts of 0.1 Hz/$\Omega$ and 2.6 Hz/$\Omega$ at 1 V bias and 4 V bias, respectively, with an average resistance fluctuation of 1 k$\Omega$ in a 1-second bandwidth. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X7.00011: Unusual Germanium Nanowire Crystal Structures Formed During Low Temperature Catalytic Growth Andrew Gamalski, Jerry Tersoff, Caterina Ducati, Renu Sharma, Stephan Hofmann Implementing bottom-up grown semiconductor nanowires (NW) in technological applications will require a detailed understanding of the electronic/physical properties of the NWs, which are closely related to their crystal structure. We present environmental transmission electron microscopy video data of Au catalyzed Ge NW growth under digermane exposure at 240 -- 320 \r{ }C. The catalyst particles are initially liquid after gas exposure [1], and Ge NWs temporarily grow by step flow from the liquid AuGe alloy [2] before the liquid catalyst crystallizes into a metastable solid phase. Nucleating solid Ge at low temperatures requires a considerable supersaturation in the liquid catalyst, which drives the formation of a NW with a non-equilibrium crystal structure [3]. We explore how nanoscale systems with high supersaturations lead to the formation of metastable phases whose properties can be dramatically different from those of the common bulk phases. [1] A. D. Gamalski \textit{et al.}, Nano Lett., 10, 2972 (2010) [2] A. D. Gamalski \textit{et al.}, J. Phys. Chem. C, 115, 4413 (2011) [3] A. D. Gamalski \textit{et al.}, in preparation [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X7.00012: Strong Spin-Orbit Interaction and Helical Hole States in Ge/Si Nanowires Christoph Kloeffel, Mircea Trif, Daniel Loss We study theoretically the low-energy hole states of Ge/Si core/shell nanowires. The low-energy valence band is quasidegenerate, formed by two doublets of different orbital angular momenta, and can be controlled via the relative shell thickness and via external fields. We find that direct (dipolar) coupling to a moderate electric field leads to an unusually large spin-orbit interaction of Rashba type on the order of meV which gives rise to pronounced helical states enabling electrical spin control. The system allows for quantum dots and spin qubits with energy levels that can vary from nearly zero to several meV, depending on the relative shell thickness [1].\\[4pt] [1] C. Kloeffel, M. Trif, and D. Loss, Phys. Rev. B {\bf 84}, 195314 (2011). [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X7.00013: Spontaneous Formation of A Nanotube From A Square Ag Nanowire: An Atomistic View Mine Konuk Onat, Sondan Durukanoglu We have performed molecular static calculations to investigate the recently observed phenomenon of the spontaneous formation of a nanotube from a regular, square Ag nanowire[1]. In the simulations, atoms are allowed to interact via the model potential obtained from the modified embedded atom method. Our simulations predict that this particular type of structural phase transformation is controlled by the nature of applied strain, length of the wire and initial cross-sectional shape. For such a perfect structural transformation, the $<$100$>$ axially oriented fcc nanowire needs (1) to be formed by stacking A and B layers of an fcc crystal, both possessing the geometry of two interpenetrating one-lattice-parameter-wide squares, containing four atoms each, (2) to have an optimum length of eight layers, and (3) to be exposed to a combination of low and high stress along the length direction. The results further offer insights into atomistic nature of this specific structural transformation into a nanotube with the smallest possible cross-section. [1] M.J. Lagos et al., Nature Nanotech. \textbf{4}, 149 (2009). [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X7.00014: Crossed Andreev Reflection in Quantum Wires with Strong Spin-Orbit Interaction Koji Sato, Daniel Loss, Yaroslav Tserkovnyak We theoretically study tunneling of Cooper pairs from an s-wave superconductor into two semiconductor quantum wires with strong spin-orbit interaction under magnetic field, which approximate helical Luttinger liquids. The entanglement of the two electrons within a Cooper pair can be detected by the electric current cross correlations in the wires. By controlling the relative orientation of the wires, either lithographically or mechanically, on the substrate, the ensuing current correlations can be tuned, as dictated by the initial spin entanglement. This proposal of a spin-to-charge readout of quantum correlations is alternative to a recently proposed utilization of the quantum spin Hall insulator. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X7.00015: A Microscopic View of Repeated Polytypism in Self-organization of Hierarchical Nanostructures Mu Wang, Ming LIu, Guo-Bin Ma, Zhao-Wu Wang, Ru-Wen Peng We demonstrate that the stacking-fault-induced repeated polytypism of wurtzite (WZ) and zinc-blend (ZB) phases may lead to a hierarchical nanostructure of ZnO possessing a hexagonal central trunk decorated with thin blades. Each blade keeps a fixed angle to the central trunk, resembling propellers with seemingly six-fold symmetry. The blades epitaxially nucleate on the ZB stripe assisted by the defect-induced reentrant corners at the interface of ZB-WZ phases. Our experiments reveal a unique approach to assemble hierarchical nanostructures, and the mechanism could be general to many materials. [Preview Abstract] |
Session X8: Focus Session: Frustrated Magnetism - Theory
Sponsoring Units: DMP GMAGChair: Mike Zhitomirsky, CEA
Room: 208
Thursday, March 1, 2012 2:30PM - 2:42PM |
X8.00001: Spin wave theory study of neutron intensity, magnetic field, and anisotropy of Type IIA FCC antiferromagnet Trinanjan Datta, Dao-Xin Yao We study the spin dynamics in a 3D quantum antiferromagnet on a face-centered cubic (FCC) lattice. The effects of magnetic field, single-ion anisotropy, and biquadratic interactions are investigated using linear spin wave theory with Dyson-Maleev transformation for spins in a canted basis. We calculate the expected finite frequency neutron scattering intensity and give qualitative criteria for typical FCC materials MnO and CoO. The magnetization reduction due to quantum zero point fluctuations is also analyzed. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X8.00002: Lifetime of gapped excitations in antiferromagnets Sasha Chernyshev, Michael Zhitomirsky We show that local modulations of magnetic couplings may have a profound effect on the temperature-dependence of the relaxation rate of gapped excitations in a class of antiferromagnets in which gapless modes are also present. Considering a prototypical 2D XY antiferromagnet with random disorder we find that the disorder-induced relaxation rate of the gapped mode should greatly exceed the effect of conventional magnon-magnon scattering, which becomes negligible at low temperatures. Our results compare favorably with the available experimental data. Generalizations to the other systems are discussed. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X8.00003: Effect of Interlayer Coupling on Field-Induced Magnon Decays in Square-Lattice Antiferromagnet Wesley Fuhrman, Martin Mourigal, Michael Zhitomirsky, Sasha Chernyshev We study the high-field magnon dynamics in the quasi-2D tetragonal Heisenberg antiferromagnet. Within spin-wave theory, we show that non-zero interlayer coupling mitigates singular corrections to the excitation spectrum occurring above the threshold field that would otherwise require a self-consistent approach beyond the Born approximation. Increasing field yields widening two-magnon sidebands with significant shifting of the spectral weight away from the quasi-particle peak. We examine the dynamic structure factor with interlayer coupling corresponding to realistic materials. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X8.00004: Detecting non-magnetic excitations in quantum magnets Zhihao Hao Many unconventional quantum phases host special non-magnetic excitations such as photons and visons. We discuss two possible ways to detect these excitations experimentally. Firstly, spin-lattice coupling mixes the excitations with phonons. The phonon spectral function acquires new features that can be detected by neutron or X-ray scattering. Secondly, valence-bond fluctuations translate into charge density fluctuations on non-bipartite lattices. Such charge fluctuations can be characterized by conventional spectroscopies such as Terahertz spectroscopy. Observation of exotic singlet excitations would provide positive identification of unconventional quantum phases in frustrated antiferromagnets. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X8.00005: Finite temperature phase diagram of quantum compass model Fei Lin, Vito Scarola The quantum compass model has been proposed as a simplified model to study Mott insulators with orbital degeneracy and, more recently, as a candidate model to study protected non-local qubits. In this talk we will show that in addition to well known bond-directional ordering, there is additional ordering associated with long range string operators. We will discuss quantum Monte Carlo results that explore the thermodynamics of non-local order in the quantum compass model. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X8.00006: Order by geometrical disorder in a 2D quantum antiferromagnet Anuradha Jagannathan, Benoit Doucot, Attila Szallas, Stefan Wessel We consider the effects of random fluctuations in the local geometry on the ground state properties of a two-dimensional quantum antiferromagnet. We analyse the behavior of spins described by the Heisenberg model as a function of what we call ``phason flip disorder,'' following a terminology used for aperiodic systems. The calculations were carried out both within linear spin wave theory and using quantum Monte Carlo simulations. An ``order by disorder'' phenomenon is observed in this model, wherein antiferromagnetism is found to be enhanced by phason disorder. The value of the staggered order parameter increases with the number of defects, accompanied by an increase in the ground state energy of the system. We furthermore find a long-ranged attractive Casimir-like force between two domain walls of defects separated by a finite distance. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X8.00007: From Popov-Fedotov case to universal fermionization Boris Svistunov, Nikolay Prokof'ev We show that Popov-Fedotov trick of mapping spin-1/2 lattice systems on two-component fermions with imaginary chemical potential readily generalizes to bosons with a fixed (but not limited) maximal site occupation number, as well as to fermionic Hamiltonians with various constraints on the site Fock states. In a general case, the mapping---fermionization---is on multi-component fermions with many-body non-Hermitian interactions. Additionally, the fermionization approach allows one to convert large many-body couplings into single-particle energies, rendering the diagrammatic series free of large expansion parameters; the latter is essential for the efficiency and convergence of the diagrammatic Monte Carlo method. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X8.00008: Universality of modulation length exponents Saurish Chakrabarty, Alexander Seidel, Zohar Nussinov We study systems (classical or quantum) with general pairwise interactions. Our prime interest is in frustrated spin systems. First, we focus on systems with a crossover temperature $T^*$ across which the correlation function changes from exhibiting commensurate to incommensurate modulations. We report on a {\em new exponent}, $\nu_L$, characterizing the universal nature of this crossover. Near the crossover, the characteristic wave-vector $k$ on the incommensurate side differs from that on the commensurate side, $q$ by $|k-q|\propto|T-T^*|^{\nu_L}$. We find, in general, that $\nu_L=1/2$, or in some special cases, other rational numbers. We discuss applications to the axial next nearest neighbor Ising model, Fermi systems (with application to the metal to band insulator transition) and Bose systems. Second, we obtain a universal form of the high temperature correlation function in general systems. From this, we show the existence of a diverging correlation length in the presence of long range interactions. Such a correlation length tends to the screening length in the presence of screening. We also find a way of obtaining the pairwise interaction potentials in the high temperature phase from the correlation functions. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X8.00009: Restricted Spin Set Lattice Models: A Route to Topological Order R. Zachary Lamberty, Stefanos Papanikolaou, C.L. Henley A typical lattice gauge model configuration consists of elements of a finite symmetry group $G$ placed on directed edges of a two-dimensional lattice. We consider generalized models\footnote{C. L. Henley, J. Phys. Condens. Matter. 23, 164212 (2011).} which are defined by instead only allowing elements from a subset $S \subset G$ consisting of certain classes of group elements. The subset restriction can be regarded as a new (but discrete) tunable model parameter, providing a novel pathway to topologically ordered phases. Taking a small allowed set $S$, we can realize well understood critical models (e.g. the square lattice ice model or dimer covering); in contrast, for large enough $S$ the configuration ensemble realizes a form of topological order. Using a sequence of sets $S_1 \subset S_2 \subset \ldots$, we can ``interpolate'' from one kind of state to the other. This is confirmed by Monte Carlo simulations, measuring two characteristic properties: (1) the distribution of separations between two (possibly deconfined) topological defects, and (2) the relative probabilities of different sectors (sub-ensembles with inequivalent products of the group elements around the periodic boundary conditions). We also discuss how to construct quantum-mechanical extensions of these models. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X8.00010: Stability of the quantum Lifshitz model in 2+1 Dimensions Benjamin Hsu, Eduardo Fradkin Magnetic and electric perturbations to the quantum Lifshitz model in 2+1 dimensions are examined in this paper. The quantum Lifshitz model is an effective field theory for quantum critical systems that include generalized 2D quantum dimer models in bipartite lattices. Magnetic perturbations break the dimer conservation law. Electric excitations have been studied extensively both in the classical 3D model and in the quantum 2D model, but magnetic vortex excitations have been ignored. While they are forbidden in classical 3D statistical mechanics, they are allowed in the quantum version. To study the interplay of both excitations, we perform a perturbative renormalization group study to one loop order. This is done by generalizing the operator product expansion to anisotropic models. The relation with recent classical Monte Carlo simulations will be discussed. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X8.00011: Supersymmetry in strongly correlated fermion models Dimitrios Galanakis, Stefanos Papanikolaou, Chris Henley We investigate the Fendley and Schoutens~\footnote{ P. Fendley and K. Schoutens, Phys. Rev. Lett. 90, 120402 (2003).} model of hard core fermions on a lattice which have hopping elements $t$, and potential terms $V$ which include a second-neighbor repulsion with some multi-particle terms. At the special point $t=V$, the Hamiltonian is $H = \{Q^\dagger(r), Q\}$ with $Q = \sum_r q(r)= \sum_r c(r)P(r)$, where $c(r)$ is an annihilation operator and $P(r)$ enforces the hard core. That means the system acquires an exact non-relativistic supersymmetry, and for a range of fillings has a large number of zero-energy ground states~$^1$. To better understand the nature of the zero-energy states and excitations, we perform exact diagonalizations on finite clusters for the square and triangular lattice, different fillings and center of mass momenta. In momentum sectors with unique zero energy ground states we find a menagerie of symmetry breaking patterns in the density-density correlation functions and we investigate them further by evaluating the entanglement spectrum. In momentum sectors with degenerate ground states we search for topological ground states using using the numerical Berry matrix method~\footnote{ E. Kapit, P. Ginsparg, and E. Mueller, arXiv:1109.4561}. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X8.00012: First-principles study on electronic properties of the pyrochlore oxide Cd2Os2O7 Hiroshi Shinaoka, Takashi Miyake, Shoji Ishibashi The pyrochlore oxide Cd2Os2O7 is one of 5d pyrochlore oxides in which the interplay of geometrical frustration, electron correlation, and spin-orbit coupling is expected. This compound exhibits a metal-insulator transition at 227 K, below which the emergence of a magnetic order has been suggested by experiments. Despite extensive studies for over thirty years, however, the nature of the low-temperature phase remains to be clarified. We depict the ground-state phase diagram based on the LSDA+$U$ method (LSDA denotes local spin density approximation). We find that the all-in/all-out non-collinear magnetic order is stable in a wide range of $U$. We show that the easy-axis anisotropy arising from the spin-orbit coupling plays a significant role in stabilizing the all-in/all-out magnetic order. A pseudo gap extending up to high energy is found to appear near a continuous metal-insulator transition between an antiferromagnetic metallic phase and an antiferromagnetic insulating phase. Based on the computed results, we discuss possible origins of peculiar low-temperature properties observed in experiments. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X8.00013: Exchange and Magnetic Anisotropic Interactions of Magnetic Ions in Antiferromagnetic Materials Alexander Bazhan Investigations of exchange and magnetic anisotropic interactions, based on materials crystallographic and magnetic symmetry, introducing quadratic forms of thermodynamic potentials, invarianted with respect to operations of magnetic symmetry groups and presented in irreducible representations of interacting ions magnetic moments, are in discussions in connection with considerations of symmetric, Anderson exchange interactions, based on Hubbard Hamiltonians, that indicates dependencies of symmetric exchange on electrons transferrings between magnetic, nonmagnetic ions, electrons kinetic energy, Coulomb interactions and, determined by crystal fields, energy levels. As example, according to symmetry for some rhombohedral structures, spin Hamiltonians of symmetric, Anderson and antisymmetric, Dzyaloshinskii-Moria exchange, H$_{ex}=\sum _{ij}$J$_{ij}$(\textbf{S}$_{i}$\textbf{S}$_{j})-\sum _{ij}$D$_{ij,z}$(\textbf{S}$_{ix}$\textbf{S}$_{jy}-$\textbf{S}$_{iy}$\textbf{S}$_{jx})$, with D$_{ij,z}$ in abs values. Magnetic field dependencies of separate, three components of samples magnetic moments of vector v.s.magnetometer, indicating magnetic moments orientations, present direct information about interactions of magnetic ions, especially with high spin orbit interactions. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X8.00014: ABSTRACT WITHDRAWN |
Session X9: Focus Session: Complex Bulk Oxides: Magnetic Phase Transitions
Sponsoring Units: DMP GMAGChair: Mohit Randeria, Ohio State University
Room: 209
Thursday, March 1, 2012 2:30PM - 2:42PM |
X9.00001: Temperature and pressure induced Cu-Fe intermetallic charge transfer in LaCu$_{3}$Fe$_{4}$O$_{12}$ Youwen Long, T. Kawakami, Y. Shimakawa An A-site ordered double perovskite LaCu$_{3}$Fe$_{4}$O$_{12}$ was prepared at high pressure and high temperature. At ambient condition, the charge combination was proved to be LaCu$^{3+}_{3}$Fe$^{3+}_{4}$O$_{12}$. When the temperature was increased to 393 K, however, an intermetallic charge transfer was found to occur between the A-site Cu and the B-site Fe ions, giving rising to the change of charge combination from the low-temperature LaCu$^{3+}_{3}$Fe$^{3+}_{4}$O$_{12}$ to the high-temperature LaCu$^{2+}_{3}$Fe$^{3.75+}_{4}$O$_{12}$.[1,2] Similarly, at room temperature, if high pressure was applied, the Cu-Fe intermetallic charge transfer could also be induced. This charge transfer led to a first-order isostructural phase transition with sharp reduction in unit cell, negative thermal expansion and unusual softening (decreased bulk modulus). Moreover, the material experienced antiferromagnetism-to-paramagnetism and insulator-to-metal transitions accompanying with the charge-transfer transition. \\[4pt] [1] Y. W. Long \textit{et al}, Nature, 458, 60-63 (2009). \\[0pt] [2] Y. W. Long and Y. Shimakawa, New J. Phys. 12, 063029 (2010). [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X9.00002: Spin-phonon coupling effect in $A$MnO$_3$ ($A$=Ca, Sr, Ba) and La$M$O$_3$ ($M$=Cr, Fe, Cr/Fe) from DFT+$U$ and hybrid functional methods Jiawang Hong, Alessandro Stroppa, Jorge Iniguez, Silvia Picozzi, David Vanderbilt Spin-phonon coupling effects, as reflected in phonon frequency shifts between ferromagnetic (FM) and G-type antiferromagnetic (AFM) configurations in cubic CaMnO$_3$, SrMnO$_3$, BaMnO$_3$, LaCrO$_3$, LaFeO$_3$ and La$_2$(CrFe)O$_6$, are investigated using density-functional methods. The calculations are carried out using the DFT+$U$ method with a $U$ that has been extracted by comparing with hybrid-functional (HSE) calculations. The phonon frequency shifts $\Delta \omega = \omega_{\rm AFM} - \omega_{\rm FM}$ obtained in this way agree well with those computed directly from the more accurate HSE approach, but are obtained with much less computational effort. We find that in the $A$MnO$_3$ materials class with ($A$=Ca,Sr,Ba), the $\Gamma$ ($R$) phonon frequency shift $\Delta \omega$ decreases (increases) as the $A^{2+}$ size increases. In La$M$O$_3$ ($M$=Cr, Fe, Cr/Fe), the phonon frequencies at $\Gamma$ decrease as spin order changes from AFM to FM for LaCrO$_3$ and LaFeO$_3$, but they increase for double perovskite La$_2$(CrFe)O$_6$. We discuss the prospects for bulk and superlattice forms of these materials to be useful as multiferroics. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X9.00003: Nd magnetic order in NdFe$_x$Ga$_{1-x}$O$_3$ Fernando Bartolome, Maria Parra-Borderias, Jose Alberto Rodriguez-Velamazan, Juan Bartolome, Burzuri Enrique, Fernando Luis, Marco Evangelisti The Nd magnetic order in NdFe$_x$Ga$_{1-x}$O$_3$ has been studied as a function of Fe content along the whole $0 |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X9.00004: Magnetic phase transitions in single crystalline Mn$_{0.68}$Ni$_{0.32}$TiO$_3$ Songxue Chi, Huibo Cao, Feng Ye, Jaime A. Fernandez-Baca, Haidong Zhou The magnetoelectric MnTiO$_3$ has the ilmenite structure and order antiferromagnetically with neighboring Mn$^{2+}$ spins antiparallel to each other both within the ab-plane and along the c-axis. We have observed a magnetic field induced electric polarization in the 32$\%$Ni-doped MnTiO$_3$. To understand the origin of this magnetoelectric effect, we have carried out neutron diffraction study on single crystalline Mn$_{0.68}$Ni$_{0.32}$TiO$_3$. The Mn$^{2+}$ spins order below 27 K and arrange in the same antiparallel manner as the parent compound, but with different spin direction. On cooling the magnetic phase goes through a second transition at 15 K, below which the spins lock into a new direction. We have also determined the spin structure under a magnetic field applied along the c-axis. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X9.00005: Electron energy loss spectroscopy study of Sr$_{2-x}$Gd$_x$TiMnO$_6$ Nevenko Biskup, Inmaculada Alvarez-Serrano, Maria Luisa Veiga, Mar Garcia-Hernandez, Maria Luisa Lopez, Maria Varela The newly synthesized double perovskite family Sr$_{2-x}$Gd$_x$TiMnO$_6$ (0$<$x$<$1) is magnetically frustrated system that orders magnetically at T=40K. In spite of the probable double exchange interaction around x=0.5, no metallic state is established and the magnetoresistance is weak in the whole family. The most interesting feature in this material is the giant electroresistance that persists even at room temperature. We have studied the microscopic composition of these polycrystals by the electron energy loss spectroscopy. We find that, is spite of some precipitations of Mn and Ti rich regions that exist in 0.25$\leq$x$\leq$0.75, the manganese and titanium ions are generally well intermixed in both interior of the grains and on the grain boundaries. We discuss these results in the frame of highly non-linear electrical conductivity found in these materials. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X9.00006: Density functional modeling for a perovskite SrTi$_{1-x}$Co$_{x}$O$_3$ system: Beyond GGA+U functional Juan Manuel Florez, Shyue Ping Ong, Gerbrand Ceder, Gerald F. Dionne, Patricio Vargas, Caroline A. Ross Understanding the exact mechanisms of spin-stabilization according to the oxygen stoichiometry and substituting metal-transition ions is essential to finding new perovskites-based technologies to augment silicon-based devices with room-temperature spintronic materials. We perform ab-initio modeling for the SrTi$_{1-x}$Co$_{x}$O$_3$ system with x = 0.0, 0.25, 1.0, and by using a HSE06 hybrid functional. Electronic structure for x=(0.0,1.0) predicts 3.1 eV band gap and a metallic behavior, respectively, predicting lattice parameters in agreement with experiments. Different positions for the cobalt ions are taken into account for x=0.25, and predicted ground states and Co energy-correlations suggest the structurally stabilized Co-site positions as fingerprints of whether possible intrinsic magnetic ordering or other phenomena is giving rise to the macroscopic magnetism. Passing from Stoichiometric SrTi$_{0.75}$Co$_{0.25}$O$_3$ to 1-O-vacancy SrTi$_{0.75}$Co$_{0.25}$O$_{3-\delta}$ produces a lattice parameter expansion due to the valence state distribution of the substituting B (ABO$_3$) ions, which is turn lead us to a band gap expansion. The stabilized valence spin states of the magnetic ions are discussed in comparison with the ligand-field theory. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X9.00007: Role of cooperative structural distortions in the metal--insulator transitions of perovskite ferrates Antonio Cammarata, James Rondinelli Transition-metal oxides within the perovskite crystal family exhibit strong electron--electron correlation effects that coexist with complex structural distortions, leading to metal-insulator (MI) transitions. Using first-principles density functional calculations, we investigate the effects of cooperative octahedral rotations and dilations/contractions on the charge-ordering MI-transition in CaFeO$_3$. By calculating the evolution in the lattice phonons, which describe the different octahedral distortions present in the low-symmetry monoclinic phase of CaFeO$_3$ with increasing electron correlation, we show that the MI-transition results from a complex interplay between these modes and correlation effects. We combine this study with group theoretical tools to disentangle the electron--lattice interactions by computing the evolution in the low-energy electronic band structure with the lattice phonons, demonstrating the MI-transition in CaFeO$_3$ proceeds through a symmetry-lowering transition driven by a cooperative three-dimensional octahedral dilation/contraction pattern. Finally, we suggest a possible route by which to control the charge ordering by fine-tuning the electron--lattice coupling. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X9.00008: Far infrared spectroscopy of magnons and phonons in TbFeO$_{3}$ single crystal T.N. Stanislavchuk, E.C. Standard, A.A. Sirenko, G.L. Carr, A.A. Mukhin, M.V. Mostovoy, N. Lee, S-W. Cheong Far-infrared spectra of TbFeO$_{3}$ single crystals have been studied in the temperature range between 1.6 K and 300 K using transmission in high magnetic field and rotating analyzer ellipsometry. The symmetry of the IR optical phonons and their oscillator strengths were determined. Polarization and frequencies of two AFM resonances at around 18 and 23 cm$^{-1}$ were analyzed around the spin reorientation (SR) transition at $\sim $8K and magnetic fields up to 9 T. Intensity of the AFM resonances exhibit an unusual oscillator dependence on both temperature and magnetic field. The observed effects are analyzed taking into account main magnetic interactions in the system including exchange of the Fe$^{3+}$ spins with Tb$^{3+}$ paramagnetic moments as well as the geometry of the measurements. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:42PM |
X9.00009: Optical properties of iron oxides Invited Speaker: Janice Musfeldt Magnetoelectric coupling in materials like multiferroics, dilute magnetic semiconductors, and topological insulators has attracted a great deal of attention, although most work has been done in the static limit. Optical spectroscopy offers a way to investigate the dynamics of charge-spin coupling, an area where there has been much less effort. Using these techniques, we discovered that charge fluctuation in LuFe$_2$O$_4$, the prototypical charge ordered multiferroic, has an onset well below the charge ordering transition, supporting the ``order by fluctuation'' mechanism for the development of charge order superstructure. Bragg splitting and large magneto-optical contrast suggest a low temperature monoclinic distortion that can be driven by both temperature and magnetic field. At the same time, dramatic splitting of the LuO$_2$ layer phonon mode is attributed to charge-rich/poor proximity effects, and its temperature dependence reveals the antipolar nature of the W layer pattern. Using optical techniques, we also discovered that $\alpha$-Fe$_2$O$_3$, a chemically-similar parent compound and one of the world's oldest and most iconic antiferromagnetic materials, appears more red in applied magnetic field than in zero field conditions. This effect is driven by a field-induced reorientation of magnetic order. The oscillator strength lost in the color band is partially transferred to the magnon side band, a process that also reveals a new exciton pattern induced by the modified exchange coupling. Analysis of the exciton pattern exposes $C2/c$ monoclinic symmetry in the high field phase of hematite. Taken together, these findings advance our understanding of iron-based materials under extreme conditions. \\[4pt] Collaborators include: X. S. Xu, P. Chen, Q. -C. Sun, T. V. Brinzari (Tennessee); S. McGill (NHMFL); J. De Groot, M. Angst, R. P. Hermann (Julich); A. D. Christianson, B. C. Sales, D. Mandrus (ORNL); A. P. Litvinchuk (Houston); J. -W. Kim (Ames); Z. Islam (Argonne); N. Lee, S. -W. Cheong (Rutgers). [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X9.00010: Theory of Spectroscopy and Transport in Half-metallic Double Perovskites Julia Janczak, Oinam Nganba Meetei, Mohit Randeria, Nandini Trivedi Half-metallic double perovskites hold great promise in spintronics applications, hence we are motivated to understand the spectroscopy and charge transport in these materials. We present theoretical calculations of the temperature and disorder dependence of the spin-resolved density of states of the conduction electrons, the optical conductivity $\sigma(\omega)$, and the anomalous Hall conductivity for Sr$_2$FeMoO$_6$ (SFMO), a half-metal with 100\% spin-down polarized charge carriers at $T=0$. We build on the recent progress [1] in modeling magnetic properties of SFMO by using an exact diagonalization plus Monte Carlo scheme. We obtain $\sigma(\omega)$ as a function of disorder as well as temperature using the Kubo formula for linear response in the exact eigenstate basis. In agreement with experiment, we find a secondary peak in $\sigma(\omega)$ at $\omega\sim0.5\,eV$, and attribute it to a spin-up density of states at the chemical potential induced by disorder and/or thermal fluctuations. We propose that the size of the secondary peak can be used to determine the polarization of conduction electrons at the chemical potential, facilitating experimental measurements. \newline [1] O. Erten et al, arXiv:1107.0983; Phys. Rev. Lett. (to appear) [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X9.00011: High T$_c$ Ferrimagnetism, Multiband Mott Transition and Spin-Orbit Coupling in Double Perovskites Onur Erten, O. Nganba Meetei, Anamitra Mukherjee, Mohit Randeria, Nandini Trivedi, Patrick M. Woodward The ferrimagnetic insulator Sr$_2$CrOsO$_6$ (SCOO), which has the highest $T_c = 725$K among all double perovskites, raises several questions. Why is this material an insulator? What sets the scale for the high T$_c$? Why is there a net moment given that both Cr and Os have d$^3$ configurations? What is the role of spin-orbit coupling in Os? Finally, why does SCOO show a highly unusual, non-monotonic magnetization $M(T)$ as a function of temperature? We address all of these questions. First, we describe the charge sector using slave-rotor mean field theory and obtain an analytic Mott criterion $\sqrt{U_{Cr}U_{Os}} > 2.5W$ relating the Hubbard $U$'s to the bandwidth $W$. We argue that SCCO is a multiband Mott insulator. Next, we argue that the orbital moment on Os is quenched in SCOO and spin-orbit coupling does not play a major role in this material. Finally, we show that the effective spin Hamiltonian for SCOO has both Cr-Os and Os-Os superexchange interactions that are frustrated. This leads to a canted ground state with a net moment at $T=0$ and a nonmonotonic magnetization $M(T)$. Our results are in excellent agreement with available data and we make predictions to test our theory. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X9.00012: Magnetically driven metal-insulator transition in NaOsO$_3$ Stuart Calder, Vasile Garlea, Desmond McMorrow, Mark Lumsden, Mathew Stone, Jonathan Lang, Jong-Woo Kim, John Schlueter, Youguo Shi, Ying Sun, Yoshira Tsujimoto, Kazunaria Yamaura, Andrew Christianson The metal-insulator transition (MIT) is one of the most dramatic manifestations of electron correlations in materials, enjoying interest both for its fundamental nature and technological application. Various mechanisms producing MITs have been extensively considered over the years associated with the names of their originators, including most especially Mott (electron localization through Coulomb repulsion) and Anderson (localization through disorder). An alternative route due to Slater dating back to 1951, in which long-range magnetic order in a three dimensional system drives the MIT, has received relatively little attention, particularly from an experimental viewpoint. Using neutron and X-ray scattering we have shown that the MIT in NaOsO$_3$ is coincident with the onset of long-range commensurate three dimensional magnetic order at 410 K. NaOsO$_3$ thus encompasses all of the expected features of the long predicted Slater transition. Our results are the first definitive experimental example of a Slater MIT and we discuss them in the light of recent reports of a Mott spin-orbit insulating state in other $5d$ oxides. [Preview Abstract] |
Session X10: Invited Session: Thermal Properties and Electron-Phonon Coupling from First Principles
Sponsoring Units: DCOMPChair: Anderson Janotti, University of California, Santa Barbara
Room: 210A
Thursday, March 1, 2012 2:30PM - 3:06PM |
X10.00001: Competition between electron-Â?phonon coupling and spin fluctuations in superconductors Invited Speaker: Silke Biermann |
Thursday, March 1, 2012 3:06PM - 3:42PM |
X10.00002: Phonon Raman scattering from first principles Invited Speaker: Claudia Ambrosch-Draxl Light propagating through a crystal can be inelastically scattered by lattice vibrations. Thereby the exciting light beam experiences a frequency shift which is detected in the Raman experiment. This {\it Raman shift} is caused by the modulation of the material's polarizability upon time-dependent ionic displacements. Experimental Raman data thus contain a wealth of information about phonons, electronic structure, as well as their interplay. For instance, temperature-dependent lineshapes and overtone spectra can be related to the extent of anharmonic effects, and strong electron-phonon interaction is expressed in terms of large scattering intensities. The assignment of frequencies to specific vibrations typically relies on comparison with reference systems by considering atomic masses and the symmetry of the crystal. For an in-depth understanding of the measured features and an unambiguous assignment of modes, {\it ab initio} theory can provide valuable insight. In the Raman spectra, the peak positions correspond to the phonon frequencies and thus can be solely determined from frozen-phonon or linear-response calculations. The scattering intensities, however, involve the change of the frequency-dependent complex dielectric tensor with atomic displacements along the phonon eigenvector. All these quantities can be obtained from density-functional theory (DFT) in combination with time-dependent DFT or many-body perturbation theory. In this talk, I will review how to compute Raman spectra from first principles. Selected examples will demonstrate the effects of isotope substitution, anharmonicity, temperature, and excitation energy, as well as the role of symmetry. They reveal that Raman scattering is a beautiful technique as it reflects basic principles of quantum mechanics as much as complex excitations of complex matter. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 4:18PM |
X10.00003: Pushing the limits of first-principles electron-phonon calculations: from photoemission kinks to band gaps Invited Speaker: Feliciano Giustino The electron-phonon interaction is key to some of the most intriguing and technologically important phenomena in condensed matter physics, ranging from superconductivity to charge density waves, electrical resistivity, and thermoelectricity. Starting from the late nineties first-principles calculations of electron-phonon interactions in metals have become increasingly popular, mainly in connection with the study of conventional superconductors and with the interpretation of angle-resolved photoemission experiments. In contrast, progress on first-principles calculations of electron-phonon interactions in insulators has been comparatively slower. This delay is arguably due to the conventional wisdom that the signatures of electron-phonon interactions in semiconductor band structures are so small that they fall within the error bar of the most accurate electronic structure calculations. In order to fill this gap we developed, within the context of state-of-the-art density-functional techniques, a theory proposed by Allen and Heine for calculating the temperature dependence of band gaps in semiconductors [P. B. Allen, V. Heine, J. Phys. C: Solid State Phys. 69, 2305 (1976)]. This methodology allows us to calculate both the temperature dependence of the quasiparticle energies and the renormalization due to zero-point quantum fluctuations. In order to demonstrate this technique an application to the intriguing case of diamond will be discussed [F. Giustino, S. G. Louie, M. L. Cohen, Phys. Rev. Lett. 105, 265501 (2010)]. In this case the calculated temperature dependence of the direct band gap agrees well with spectroscopic ellipsometry data, and the renormalization due to the electron-phonon interaction is found to be spectacularly large ($>$0.6 eV). This unexpected finding might be only the tip of the iceberg in a research area which remains largely unexplored and which, from a first glimpse, appears rich of surprises. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:54PM |
X10.00004: Phonon-assisted Auger recombination and optical absorption in semiconductors Invited Speaker: Emmanouil Kioupakis The coupling of charge carriers to lattice vibrations is an important process in materials that enables higher-order electronic transitions. We employed first-principles methods based on density functional theory to study various phonon-assisted electronic processes in semiconductors, such as Auger recombination and optical absorption. Auger recombination is a three-particle non-radiative recombination process that affects optoelectronic devices at high carrier densities. Phonon-assisted Auger recombination, in particular, is expected to be important in wide-band-gap materials. We describe the computational formalism to study phonon-assisted Auger recombination in semiconductors. We show that these indirect Auger processes are strong in the group-III nitrides and affect the high-power performance of visible light-emitting diodes. Moreover, the electron-phonon interaction facilitates the absorption of light by free carriers in doped semiconductors and transparent conducting oxides, which limits the output power of optoelectronic devices. We describe how first-principles techniques can be used to calculate the phonon-assisted free-carrier absorption coefficient in semiconductors and discuss our results for the group-III nitrides. In addition, the electron-phonon coupling enables indirect interband optical transitions in indirect-gap materials such as silicon. These processes are instrumental for the absorption of visible light and the operation of silicon solar cells. We present our first-principles formalism and calculated results for the phonon-assisted absorption of visible light in silicon. Our calculated results are in very good agreement with experiment. Our work highlights the significance of first-principles methods in understanding key microscopic quantum phenomena in technologically important materials and devices. Work done in collaboration with C. G. Van de Walle, P. Rinke, K. Delaney, A. Schleife, F. Bechstedt, D. Steiauf, H. Peelaers, J. Noffsinger, S. G. Louie, and M. L. Cohen. Support was provided by CEEM, SSLEC, NERSC, and Teragrid. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:30PM |
X10.00005: Large scale atomistic approaches to thermal transport and phonon scattering in nanostructured materials Invited Speaker: Ivana Savic Decreasing the thermal conductivity of bulk materials by nanostructuring and dimensionality reduction, or by introducing some amount of disorder represents a promising strategy in the search for efficient thermoelectric materials [1]. For example, considerable improvements of the thermoelectric efficiency in nanowires with surface roughness [2], superlattices [3] and nanocomposites [4] have been attributed to a significantly reduced thermal conductivity. In order to accurately describe thermal transport processes in complex nanostructured materials and directly compare with experiments, the development of theoretical and computational approaches that can account for both anharmonic and disorder effects in large samples is highly desirable. We will first summarize the strengths and weaknesses of the standard atomistic approaches to thermal transport (molecular dynamics [5], Boltzmann transport equation [6] and Green's function approach [7]) . We will then focus on the methods based on the solution of the Boltzmann transport equation, that are computationally too demanding, at present, to treat large scale systems and thus to investigate realistic materials. We will present a Monte Carlo method [8] to solve the Boltzmann transport equation in the relaxation time approximation [9], that enables computation of the thermal conductivity of ordered and disordered systems with a number of atoms up to an order of magnitude larger than feasible with straightforward integration. We will present a comparison between exact and Monte Carlo Boltzmann transport results for small SiGe nanostructures and then use the Monte Carlo method to analyze the thermal properties of realistic SiGe nanostructured materials. This work is done in collaboration with Davide Donadio, Francois Gygi, and Giulia Galli from UC Davis.\\[4pt] [1] See e.g. A. J. Minnich, M. S. Dresselhaus, Z. F. Ren, and G. Chen, Energy Environ. Sci. 2, 466 (2009).\\[0pt] [2] A. I. Hochbaum et al, Nature 451, 163 (2008).\\[0pt] [3] R. Venkatasubramanian, E. Siivola, T. Colpitts, and B. O'Quinn, Nature 413, 597 (2001).\\[0pt] [4] B. Poudel et al, Science 320, 634 (2008).\\[0pt] [5] See e.g. Y. He, D. Donadio, and G. Galli, Nano Lett. 11, 3608 (2011).\\[0pt] [6] See e.g. A. Ward and D. A. Broido, Phys. Rev. B 81, 085205 (2010).\\[0pt] [7] See e.g. I. Savic, N. Mingo, and D. A. Stewart, Phys. Rev. Lett. 101, 165502 (2008).\\[0pt] [8] I. Savic, D.Donadio, F.Gygi, and G.Galli (in preparation).\\[0pt] [9] See e.g. J. E. Turney, E. S. Landry, A. J. H. McGaughey, and C. H. Amon, Phys. Rev. B, 79, 064301 (2009). [Preview Abstract] |
Session X11: Focus Session: Graphene Structure, Stacking, Interactions: Twisting, Stretching, Folding, Wrinkling
Sponsoring Units: DMPChair: Jeanie Lau, University of California, Riverside
Room: 210B
Thursday, March 1, 2012 2:30PM - 2:42PM |
X11.00001: Graphene Origami Melina Blees, Arthur Barnard, Samantha Roberts, Peijie Ong, Aliaksandr Zaretski, Si Ping Wang, Paul McEuen Graphene, which features unparalleled in-plane strength and low out-of-plane bending energy, is an ideal material with which to tackle the challenge of building three-dimensional structures and moving parts at the nanoscale. Here we demonstrate laser-induced folding and scrolling of large-area monolayer graphene in solution. Monolayer graphene is typically well-adhered to its substrate, but we have achieved control of the adhesion using a combination of an aluminum sacrificial layer and surfactants. Once the graphene can move, local heating with an infrared laser and the interfacial tension of laser-nucleated bubbles allow us to lift, fold, and scroll the graphene. We have also formed a regular array of polymer dots on the graphene surface which can be easily imaged in three dimensions, allowing us to optically track the shape of the graphene as it moves. And finally, we establish graphene's viability as a strong but flexible sheet hinge by building and manipulating structures of rigid metallic panels connected by strips of graphene. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X11.00002: Twisting Graphene into Carbon Nanotubes Oleg O. Kit, Tuomas Tallinen, L. Mahadevan, Jussi Timonen, Pekka Koskinen Carbon nanotubes are usually described as being rolled up from graphene sheets; this process, however, have never been realized experimentally. We showed that graphene can indeed be transformed into nanotube by twisting [1]. Further, we showed that tube formation can be well-explained within classical theory of elasticity---in fact the very mechanism of tube formation can be observed by twisting a strap from one's backpack (try now!). Furthermore, we showed that nanotube chirality may not only be predicted, but can also be controlled externally. The quantum molecular dynamic simulations at T=300K were achieved thanks to the revised periodic boundary conditions (RPBC) approach [2-3]. The structures similar to simulated have been recently observed experimentally [4]. This novel rote for nanotube formation opens new opportunities in nanomaterial manipulation not restricted to carbon alone. In the presentation, I will describe tube formation, as well as outline the easy and efficient technique for distorted nanostructures simulation, the RPBC approach. \\[4pt] [1] O. O. Kit et al. arXiv:1108.0048\\[0pt] [2] P. Koskinen \& O. O. Kit PRL 105, 106401 (2010)\\[0pt] [3] O. O. Kit, L. Pastewka, P. Koskinen PRB 84, 155431 (2011)\\[0pt] [4] A. Chuvilin et al. Nature Materials 10, 687 (2011) [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X11.00003: Wrinkling instability in graphene supported on nanoparticle-patterned SiO$_{2}$ William Cullen, Mahito Yamamoto, Olivier Pierre-Louis, Theodore Einstein, Michael Fuhrer Atomically-thin graphene is arguably the thinnest possible mechanical membrane: graphene's effective thickness (the thickness of an isotropic continuum slab which would have the same elastic and bending stiffness) is significantly less than 1 {\AA}, indicating that graphene can distort out-of-plane to conform to sub-nanometer features. Here we study the elastic response of graphene supported on a SiO$_{2}$ substrate covered with SiO$_{2}$ nanoparticles. At a low density of nanoparticles, graphene is largely pinned to the substrate due to adhesive interaction. However, with increasing nanoparticle density, graphene's elasticity dominates adhesion and strain is relieved by the formation of wrinkles which connect peaks introduced by the supporting nanoparticles. At a critical density, the wrinkles percolate, resulting in a wrinkle network. We develop a simple elastic model allowing for adhesion which accurately predicts the critical spacing between nanoparticles for wrinkle formation. This work has been supported by the University of Maryland NSF-MRSEC under Grant No. DMR 05-20471 with supplemental funding from NRI, and NSF-DMR 08-04976. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X11.00004: Molecular Dynamics Study of Ripples in Graphene and Bilayer Graphene Arunima Singh, Richard G. Hennig Transmission electron microscopy experiments have shown that suspended graphene is not perfectly flat, but displays ripples such that the surface normal of graphene varies by several degrees [1,2]. For multi-layered graphene, the ripples are suppressed with increasing numbers of layers. Recent experiments demonstrated that ripples in suspended graphene can also be controlled by mechanical and thermally induced strain [3]. Knowledge of and control over the ripples in graphene is desirable for fabricating and designing of strain-based devices. We show using molecular dynamics simulation that thermally induced ripples in suspended single and multi-layer graphene at room temperature result in deviations of the local surface normal by $\pm$ 7 $^{\circ}$ and $\pm$ 4 $^{\circ}$ for single and bilayer graphene, respectively. These angular deviations are in excellent agreement with transmission electron microscopy results [2] and confirm that these ripples can be dynamic and thermally induced. We also study how these angles change as a function of applied tensile and shear strain. [1] Meyer J. C., Geim A. K., et al. Solid State Communications, 143, 101 (2007). [2] Meyer J.C., Geim A.K., et al. Nature, 446, 60 (2007). [3] Bao W., Miao F., et al. Nature Nanotechnology, 4 (9), 562 (2009). [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X11.00005: Ideal strength of graphene Sungjong Woo, Young-Woo Son We have investigated the ideal strength of graphene by calculating the response under strain based on density functional theory. The strength of materials with usual contaminations and impurities is typically weaker than the ideal one. However, we have considered realistic factors and calculated several electromechanical properties of perturbed graphene showing that under certain conditions graphene can endure more strain than the pristine one. The interpretation of our result will be presented in the context of competitions among different energy scales under mechanical strains that eventually leads to the structure failure. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X11.00006: Determination of Young's Modulus of Graphene by Raman Spectroscopy Jae-Ung Lee, Duhee Yoon, Hyeonsik Cheong The mechanical properties of graphene are interesting research subjects because its Young's modulus and strength are extremely high. Values of $\sim $1 TPa for the Young's modulus have been reported [Lee et al. Science, \textbf{321}, 385 (2008), Koenig et al. Nat. Nanotech. \textbf{6}, 543 (2011)]. We made a graphene sample on a SiO$_{2}$/Si substrate with closed-bottom holes by mechanical exfoliation. A pressure difference across the graphene membrane was applied by putting the sample in a vacuum chamber. This pressure difference makes the graphene membrane bulge upward like a balloon. By measuring the shifts of the Raman G and 2D bands, we estimated the amount of strain on the graphene membrane. By comparing the strain estimated from the Raman measurements with numerical simulations based on the finite element method, we obtained the Young's modulus of graphene. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X11.00007: Role of the effective tensile strain in the electromechanical response of helical graphene nanoribbons with open and closed edges Traian Dumitrica, Dong-Bo Zhang There is a growing need to understand the electronic properties of non-ideal graphene nanoribbons. Using objective molecular dynamics [1] and a density-functional based tight-binding model, we investigate the effects of torsion on the electromechanical properties of graphene nanoribbons with armchair edges. We propose to characterize with an effective tensile strain scalar [2] the torsional mechanical response, including a reverse Poynting effect, and the fundamental band gap modulations. The demonstrated utility of this concept in both the mechanical and electrical domains provides a perspective for understanding electromechanical response in a unified way, and for designing NEMS devices with graphene components. [1] T. Dumitric? and R.D. James, J. Mech. Phys. Sol. 55, 2206-2236 (2007). [2] D.-B. Zhang and T. Dumitric?, Small 7, 1023 (2011) . [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X11.00008: Failure mechanisms of graphene under tension Chris Marianetti Recent experiments established pure graphene as the strongest material known to mankind, further invigorating the question of how graphene fails. Using density functional theory, we reveal the mechanisms of mechanical failure of pure graphene under a generic state of tension at zero temperature. One failure mechanism is a novel soft-mode phonon instability of the $K_1$-mode, whereby the graphene sheet undergoes a phase transition and is driven towards isolated hexagonal rings resulting in a reduction of strength. The other is the usual elastic instability corresponding to a maximum in the stress-strain curve. Our results indicate that finite wave vector soft modes can be the key factor in limiting the strength of monolayer materials. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X11.00009: Characterization of strain in CVD graphene on copper substrates Rui He, Liuyan Zhao, Nicholas Petrone, Michael Roth, James Hone, Philip Kim, Abhay Pasupathy, Aron Pinczuk Strain plays an important role in controlling graphene's properties and induces significant changes in the electronic band structure. Strain and morphology of CVD (chemical vapor deposition) graphene layers grown on Cu substrates are studied by Raman spectroscopy and scanning tunneling microscopy (STM). We find that CVD graphene on Cu surfaces are subject to strain which depends on the orientation of the underlying Cu surfaces. The strain is compressive on Cu (111) surface and estimated to be on the order of 0.5 percent by molecular dynamics (MD) simulations. For graphene grown on Cu (100) surface the strain is highly nonuniform and includes both compressive and tensile components. MD simulations of graphene on Cu (100) show highly nonuniform strain patterns including linear superstructures, consistent with the patterns seen in STM. For graphene grown on Cu foil the strain is partially released after graphene is removed from Cu surfaces and transferred onto oxidized Si substrate. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X11.00010: Local nanoscale frictional variations of graphene investigated with lateral force microscopy Patrick Hunley, Tyler Flynn, Tom Dodson, Abhishek Sundararajan, Douglas Strachan Lateral force microscopy is used to investigate the local nanoscale frictional variations on single- and multi-layered graphene films. Employing novel calibration methods, quantitative frictional measurements are taken for a range of normal loads. The coverage of specific high-friction regions with a single layer of graphene shows a significant reduction in the frictional characteristics. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X11.00011: Adsorption-induced Pore Expansion and Contraction in Activated Carbon Matthew Connolly, Carlos Wexler Adsorbent materials such as activated carbon and Metal-Organic Frameworks (MOFs) have received significant attention as a potential storage material for hydrogen and natural gas.1 Typically the adsorbent material is assumed to consist of rigid slit- or cylindrical-shaped pores. Recent work has revealed the importance of the mechanical response of the adsorbent in the presence of an adsorbate. Here, we first demonstrate the flexibility of pore walls in activated carbon and the effect this has on the pore structure of the bulk samples. The interaction is modeled as a competition between Van der Waals interactions between neighboring walls and a resistance to bending due to the rigidity of graphene. Minimal energy configurations were calculated analytically for a simplified potential and numerically for a more realistic potential. The pore structures are discussed in the context of pore measurements on activated carbon samples. Following recent work by Cole and Neimark, large pressures due to an adsorbed film are predicted in the narrow pores of activated carbon. The coverage-dependent nature of adsorbed-film pressure, indicating a pressure-variant pore structure, is discussed in terms of adsorption isotherms. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X11.00012: Hierarchical graphene materials: from monolayers to papers and nanocomposites Zhiping Xu Macroscopic graphene materials such as papers and nanocomposites consist of chemically modified graphene sheets and intersheet crosslinks of various types, which hold great promises in high-performance, multifunctional and light-weighted applications. In this talk, we will present a multiscale approach (density functional theory, molecular dynamics and continuum mechanics) to understand simultaneously atomistic mechanisms, microscale structures and microscopic performance of these hierarchical graphene materials, and provide general principles for materials design that are supported by experimental evidence. Hierarchical structures of graphene-based materials will be also discussed comparably with biological materials that are widely studied recently, such as bones, nacre and collagen fibrils, which possess similar materials hierarchies and inspire many novel concepts in new materials development. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X11.00013: Layer Effects in Graphene NEMS Resonators Britton Baugher, Tchefor Ndukum, Kevin Fischer, Pablo Jarillo-Herrero In this talk we will present new data on mono, bi, and tri-layer graphene nanoelectromechanical (G-NEMS) resonators.~ G-NEMS resonators utilize graphene's high mobility, high Young's modulus, Dirac-like dispersion relation, and other unique qualities to produce unusual physical phenomena at the intersection of electronics and mechanics on the quantum scale.~ We extend the work done on these devices with a study into the effects of layer number on temperature dependence, quality factor, tunability, and frequency amongst others. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X11.00014: Ultrastrong Adhesion of Graphene Membranes Steven P. Koenig, Narasimha G. Boddeti, Martin L. Dunn, J. Scott Bunch As mechanical structures enter the nanoscale regime, the influence of van der Waals forces increases. Graphene is attractive for nanomechanical systems because its Young's modulus and strength are both intrinsically high, but the mechanical behavior of graphene is also strongly influenced by the van der Waals force. For example, this force clamps graphene samples to substrates, and also holds together the individual graphene sheets in multilayer samples. Here we use a pressurized blister test to directly measure the adhesion energy of graphene sheets with a silicon oxide substrate. We find an adhesion energy of 0.45 +/- 0.02 J m-2 for monolayer graphene and 0.31 +/- 0.03 J m-2 for samples containing two to five graphene sheets. These values are larger than the adhesion energies measured in typical micromechanical structures and are comparable to solid--liquid adhesion energies. We attribute this to the extreme flexibility of graphene, which allows it to conform to the topography of even the smoothest substrates, thus making its interaction with the substrate more liquid-like than solid-like. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X11.00015: Graphene coatings: An efficient protection from oxidation Mehmet Topsakal, Hasan Sahin, Salim Ciraci We demonstrate that graphene coating can provide an efficient protection from oxidation by posing a high energy barrier to the path of oxygen atom, which could have penetrated from the top of graphene to the reactive surface underneath. Graphene bilayer, which blocks the diffusion of oxygen with a relatively higher energy barrier provides even better protection from oxidation. While an oxygen molecule is weakly bound to bare graphene surface and hence becomes rather inactive, it can easily dissociates into two oxygen atoms adsorbed to low coordinated carbon atoms at the edges of a vacancy. For these oxygen atoms the oxidation barrier is reduced and hence the protection from oxidation provided by graphene coatings is weakened. Our predictions obtained from the state of the art first-principles calculations of electronic structure, phonon density of states and reaction path will unravel how a graphene can be used as a corrosion resistant coating and guide further studies aiming at developing more efficient nanocoating materials. [Preview Abstract] |
Session X12: Focus Session: Graphene: Growth, Mechanical Exfoliation, and Properties - Morphology and Interactions
Sponsoring Units: DMPChair: Luigi Colombo, Texas Instruments
Room: 210C
Thursday, March 1, 2012 2:30PM - 3:06PM |
X12.00001: Atomic Scale Properties of Chemically Doped Graphene Invited Speaker: Abhay Pasupathy In monolayer graphene, substitutional doping during growth can be used to alter its electronic properties. We used scanning tunneling microscopy, Raman spectroscopy, x-ray spectroscopy, and first principles calculations to characterize individual nitrogen dopants in monolayer graphene grown on a copper substrate. Individual nitrogen atoms were incorporated as graphitic dopants, and a fraction of the extra electron on each nitrogen atom was delocalized into the graphene lattice. The electronic structure of nitrogen-doped graphene was strongly modified only within a few lattice spacings of the site of the nitrogen dopant. These findings show that chemical doping is a promising route to achieving high-quality graphene films with a large carrier concentration. Ref: L. Zhao et al, \textit{Science} \textbf{333},999 (2011). [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X12.00002: Quantum Hall Effect in Graphene Bilayers Grown on Copper by Chemical Vapor Deposition Babak Fallahazad, Yufeng Hao, Kayoung Lee, Seyoung Kim, Rodney Ruoff, Emanuel Tutuc We report an investigation of quantum Hall effect in graphene bilayers grown on Cu substrates by chemical vapor deposition. Raman spectroscopy of the as grown graphene bilayers reveals a position dependent full width half maximum of the 2D peak, ranging from 22 to 55 cm$^{-1}$, suggesting the bilayer is a mixture of Bernal stacked and decoupled graphene monolayers. Using scanning Raman spectroscopy we identify areas with either wide (45-55 cm$^{-1})$, as well as narrow (22-26 cm$^{-1})$ 2D peaks, in order to fabricate back-gated Hall bars on such grains. Magnetotransport measurements in bilayer regions characterized by a wide 2D peak reveal quantum Hall states (QHS) at filling factors $\nu = \pm$ 4, 8, 12 consistent with a Bernal stacked bilayer, which develop at magnetic fields higher than 15 T. In contrast, magnetotransport measurements in bilayer regions defined by a narrow 2D peak shows QHSs down to 3 T, and with a sequence consisting of the superposition of the QHSs of two independent monolayers. We compare the QHS energy gaps extracted from activation measurements with the theoretical Landau level (LL) separation, and estimate the LL broadening. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X12.00003: Layer Number and Stacking Order Imaging of Few-layer Graphenes by Transmission Electron Microscopy Jinglei Ping, Michael Fuhrer A method using transmission electron microscopy (TEM) selected area electron diffraction (SAED) patterns and dark field (DF) images is developed to identify graphene layer number and stacking order by comparing intensity ratios of SAED spots with theory. Graphene samples are synthesized by ambient pressure chemical vapor depostion and then etched by hydrogen in high temperature to produce samples with crystalline stacking but varying layer number on the nanometer scale. Combined DF images from first- and second-order diffraction spots are used to produce images with layer-number and stacking-order contrast with few-nanometer resolution. This method is proved to be accurate enough for quantative stacking-order-identification of graphenes up to at least four layers. This work was partially supported by Science of Precision Multifunctional Nanostructures for Elecrical Energy Storage, an Energy Frontier Research Center funded by the U.S. DOE, Office of Science, Office of Basic Energy Sciences under Award Number DESC0001160. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X12.00004: Metal surface melting effect on the formation of graphene wrinkles Tereza Paronyan, Elena Pigos, Avetik Harutyunyan The synthesis of high quality large area graphene is a significant challenge that needs to be overcome for practical applications of this material. Therefore, understanding the growth mechanism of graphene is crucial in order to explain the origin of defects on it. Different thickness of Copper and Nickel films deposited on silica substrates, and foils with various degrees of purity were used in this study. We investigated the changes in the surface morphology of the thin films and foils with temperature at 860-1100$^{\circ}$C with and without growth of graphene by Chemical Vapor Deposition method. Detailed investigation by Raman spectroscopy, SEM and AFM analysis revealed that thermal treatment of the metal substrates at high temperatures $\sim $1000$^{\circ}$C causes formation of dendritic like structures on the surface. We attributed these structures to the non-equilibirium solidification of the melted surface of the metal during the cooling, which are also present in the case of graphene growth. We concluded that the reconstruction of the metal surface morphology in the case of graphene growth, due to the surface melting, significantly affects on the final topography of the graphene wrinkles and, thereby on the quality of graphene. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 4:18PM |
X12.00005: Growth and Characterization of Graphene-Boron Nitride Heterostructures Invited Speaker: Peter Sutter Graphene has been used to explore the fascinating properties of two-dimensional sp$^{2}$ carbon, and shows great promise for applications. Heterostructures of graphene (G) and hexagonal boron nitride (h-BN) have the potential for extended functionality, e.g., providing high carrier mobilities in graphene devices supported on h-BN and giving rise to emergent electronic behavior near in-plane G/h-BN junctions. While significant progress has been made recently in separate graphene and boron nitride growth on transition metals, the controlled synthesis of high-quality G/h-BN heterostructures poses new challenges. We discuss the fundamental growth mechanisms underlying the synthesis of G/h-BN heterostructures, studied by a combination of in-situ surface microscopy methods. Real-time low-energy electron microscopy (LEEM) provides a mesoscale view of the nucleation and growth of h-BN in the presence of graphene, and vice-versa. LEEM imaging together with diffraction and angle resolved photoemission spectroscopy (micro-ARPES) gives insight into the interaction between graphene and h-BN. Scanning tunneling microscopy has been used to probe intermixing and the atomic-scale structure of interfacial boundaries. Combining real-time and atomic-resolution imaging, we identify successful approaches for achieving atomically sharp G/h-BN junctions. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X12.00006: Physical and electrical properties of graphene folds grown by Chemical Vapor Deposition Wenjuan Zhu, Tony Low, Yu Zhu, Ageeth Bol, Hugen Yan, Xuesong Li, Yu-ming Lin, Yanqing Wu, Fengnian Xia, Vasili Perebeinos, Phaedon Avouris We found that there is a large density of wrinkles on CVD graphene transferred to a SiO$_{2}$/Si substrate. At these graphene wrinkles, the SEM signal intensity is lower, the AFM height is higher, and the Raman G-band intensity is stronger as compared to the surrounding single-layer graphene, due to extra layers of graphene at the wrinkles. TEM images confirmed that wide wrinkles are folds instead of ridges. The channel resistance near the Dirac point along graphene folds is significantly lower than the resistance without folds. However, as the gate field or the carrier density is increased, the difference between graphene channel along the fold and without the fold is reduced, due to carrier screening. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X12.00007: Superior Mobility in Chemical Vapor Deposition Synthesized Graphene by Grain Size Engineering Nicholas Petrone, Cory Dean, Inanc Meric, Arend van der Zande, Pinshane Huang, Lei Wang, David Muller, Kenneth Shepard, James Hone Chemical vapor deposition (CVD) offers a promising method to produce large-area films of graphene, crucial for commercial realization of graphene-based applications. However, electron transport in CVD grown graphene has continued to fall short of the performance demonstrated by graphene derived from mechanical exfoliation. Lattice defects and grain boundaries developed during growth, structural defects and chemical contamination introduced during transfer, and charged scatterers present in sub-optimal dielectric substrates have all been identified as sources of disorder in CVD grown graphene devices. We grow CVD graphene and fabricate field-effect transistors, attempting to minimize potential sources of disorder. We reduce density of grain boundaries in CVD graphene by controlling domain sizes up to 250 microns. By transferring CVD graphene onto h-BN utilizing a dry-transfer method, we minimize trapped charges at the interface between graphene and in the underlying substrate. We report field-effect mobilities up to 110,000 cm2V-1s-1 and oscillations in magnetotransport measurements below 1 T, confirming the high quality and low disorder in our CVD graphene devices. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X12.00008: Transfer-Free, Wafer-Scale, and Patterned Synthesis of Graphene on Dielectric Substrates Hossein Sojoudi, Samuel Graham We report a method for the direct, wafer-scale synthesis of graphene on dielectric substrates using a solid carbon source. Graphene films were synthesized through the thermal decomposition of poly(methyl methacrylate) on copper coated quartz and Si/SiO2 substrates in a low pressure H2/Ar environment. The Cu film partially evaporated during growth, leaving a graphene layer directly on the dielectric substrate. Polyacrylamide was used for synthesis of N-doped graphene due to its nitrogen content. A similar method with process optimization was utilized to grow graphene without any external carbon source. Trace amounts of carbon in metal films result in direct formation of graphene on insulators through a segregation process. A wet etch step allowed complete removal of the metal film, leading to continuous graphene coverage of the surface. This technique utilized for patterned synthesis of graphene and can be used in Si-wafer compatible device fabrication. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X12.00009: Transport studies of dual-gated ABA- and ABC-stacked trilayer graphene K. Zou, F. Zhang, C. Clapp, A.H. MacDonald, J. Zhu We present electrical transport studies of dual-gated ABA- and ABC-stacked trilayer graphene field effect transistors. Employing high-quality thin HfO$_{2}$ layers as the top and back gate dielectrics, we independently tune the carrier density and control the band structure of trilayer graphene via the application of a perpendicular electric field E$_{perp}$. The large gating efficiency of the two gates (5.53 x 10$^{12} $/cm$^{2}$ per Volt) and their high breakdown voltage ($>$ 6 V) enable us to reach exceedingly large carrier densities and E$_{perp}$ values, which results in wide tuning of the conductivity of the trilayer devices. Results on ABA-stacked trilayer graphene confirm its semi-metallic nature and reveal evidence of the band structure changes induced by E$_{perp}$. The resistance at the charge neutrality point of ABC-stacked trilayer graphene increases by many orders of magnitude with increasing E$_{perp}$ due to the gradual opening of a band gap. Our results suggest a saturation of the gap size at perpendicular displacement fields greater than 3.5 V/nm, in agreement with theoretical calculations. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X12.00010: Graphene growth by CVD using liquid precursors Jessica Campos Delgado, Andr\'es Botello-M\'endez, Benoit Hackens, Jean-Christophe Charlier, Thomas Pardoen, Jean-Pierre Raskin Chemical vapor deposition (CVD) represents an attractive route to synthesize large-area graphene. Its catalytic growth using metals has been studied in recent years, the most popular being nickel and copper. We have successfully grown graphene on copper by CVD at ambient pressure using alcohols as the carbon feedstock. The produced materials are analyzed by SEM, TEM, Raman spectroscopy and transferred onto Si/SiO$_{2}$ substrates using standard methods. Raman fingerprint of monolayer graphene is present in our samples. This technique represents a safer and more versatile alternative to the production of graphene compared to the synthesis of graphene using methane at low pressure. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X12.00011: Vibrational properties in graphene-on-metal systems: A scanning tunneling microscopy study and density functional theory calculations Haigang Zhang, Yuyang Zhang, Wende Xiao, Shixuan Du, Hongjun Gao Combing inelastic electron tunneling microscopy (IETS) measured by scanning tunneling microscopy and quantum mechanical calculations based on density functional theory, we investigate and compare the vibrational properties of graphene on various transition metal substrates. The observed d2I/dV2 spectra and mappings indicate there is a mode dependant spatial localization for graphene on Ru(0001) surface In contrast, vibrations of graphene on Pt(111) surface and Ni(111) surface is homogeneous. Out-of-plane vibration modes of graphene are tuned by the different interactions on these three substrates. Vibrational density of states of graphene and graphene/Ni(111) system are calculated and make comparison to the IETS experiments. We also calculate the inelastic and elastic tunneling coefficients in Graphene/Ni(111) system to understand the missing peak in IETS experiments. Our results point to the importance of interfacial bonding on phonon properties and, consequently, electronic and thermal transport properties of graphene based devices. [Preview Abstract] |
Session X13: Focus Session: Magnetic Nanostructures-Hard Magnetic Materials and Magnetocaloric Materials
Sponsoring Units: DMP GMAGChair: Sam Jiang, Argonne National Laboratory
Room: 211
Thursday, March 1, 2012 2:30PM - 2:42PM |
X13.00001: Thermal Fluctuation and Finite- Temperature Performance of Hard-Soft Composite Magnets Alexander Belemuk, S.T. Chui The demagnetization behavior of exchange-coupled hard/soft magnets was studied by finite temperature Monte Carlo simulation. Hard phase cube inclusions (Nd$_2$Fe$_{14}$B, SmCo$_5$ and Sm$_2$Fe$_{17}$N$_3$) into a soft matrix (FeCo) and hard/soft multilayer structure were studied. The easy axis of the hard and soft phase and the initial magnetization are parallel to the applied field. We found significant thermal fluctuations and lowering of the remnant magnetization with increasing soft magnet content than is anticipated from zero-temperature models, especially at higher temperatures. This greatly diminishes the expected performances of composites. For cube inclusions there is a boundary mismatch of the magnetization on the hard/soft interface. We investigated this mismatch as function of the soft phase content and temperature. The spin wave spectrum due to the mismatched dipolar interaction will be discussed. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X13.00002: Control of Size and Magnetic Properties of L1$_{0}$ FePt islands by DC sputtering Xiaocao Hu, Ozan Akdogan, Wanfeng Li, George Hadjipanayis FePt particles with the tetragonal L1$_{0}$ structure are attractive for high density recording media. In this study, we have fabricated well-ordered and separated L1$_{0}$ FePt islands on MgO (100) substrates by DC sputtering at temperatures varying from room temperature to 700 $^{\circ}$C. The dependence of particle size, degree of ordering and magnetic properties on sputtering time (5-20s), power (5-20W) and substrate temperature (300-1000K) were investigated. Electron diffraction patterns from TEM showed that the islands had the L1$_{0}$ tetragonal structure. TEM data revealed that a higher substrate temperature significantly increased the size of the islands from 2 to 20 nm whereas a higher sputtering power and longer sputtering times did not change the island size much but made the islands more connected with each other. Larger size islands and inter-connected islands showed a higher degree of ordering with an ordering parameter of 0.8 achieved at 600 $^{\circ}$C. The magnetic properties are currently being measured and the results will be reported. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X13.00003: Nano-granular FePt thin films for thermally-assisted magnetic recording Tiffany Santos, Oleksandr Mosendz, Simone Pisana, James Reiner, Gregory Parker, Barry Stipe, Dieter Weller In order to extend the data storage density in hard disk drives beyond 1 Tb/in$^2$, nano-size grains of a high crystalline anisotropy ($K_u$) material are required to obtain these high densities and maintain thermal stability. A promising approach to recording using high-$K_u$ materials is thermally-assisted magnetic recording (TAR), in which the media is locally heated above the Curie temperature while a magnetic field is applied in order to write a bit. FePt with L1$_0$ crystalline order is a potential candidate for TAR media. We deposit FePt nano-granular films with carbon as the segregant material, by co-sputtering on glass substrates at elevated temperature. Underlayer materials are selected for heat-sinking and to attain high out-of-plane L1$_0$ order. Characterization of the media by x-ray diffraction, magnetometry and transmission electron microscopy show that we can achieve properties that are promising for TAR media, such as an average grain size $<$ 7.5nm, size distribution as low as 16\%, coercivity as high as 5 tesla and $K_u >$ 4.5 x 10$^7$ erg/cm$^3$. Recording densities exceeding 600 Gb/in$^2$ have been demonstrated for our FePt granular films using a static tester. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X13.00004: Confined stripe structure in periodically grooved NdCo Films with perpendicular magnetic anisotropy Jose I. Martin, Aurelio Hierro-Rodriguez, Rosalia Cid, Gabriel Rodriguez-Rodriguez, Maria Velez, Luis M. Alvarez-Prado, Jose M. Alameda Magnetic multilayers are broad research field with many interesting phenomena depending on interlayer coupling. Also, since the development of nanolithography techniques, magnetic nanowires and dots have been intensively investigated [1]. Recently, as a combination of these two fields, the concept of magnetic lateral superlattice has emerged: continuous magnetic films with a lateral modulation of their magnetic properties at submicrometric length scale [2]. In this work, we have fabricated amorphous Nd-Co films with perpendicular magnetic anisotropy and a periodic thickness modulation by e-beam lithography and ion milling. Lateral periods range from 2 $\mu$m - 500 nm and groove depths from 10 to 30 nm. MFM and Kerr magnetometry have been used for characterization. Lateral patterning modifies the interplay between magnetostatic energy, perpendicular and in plane anisotropy and exchange interaction resulting in confined magnetic stripe structures. The different regimes that appear depending on the size of the periodic thickness modulation relative to the magnetic stripe period will be discussed. [1] J.I Martin et al, JMMM, 256 (2003) 449 [2] S. P. Li et al, PRL 88 (2002) 087202; N. Martin et al, PRB 83 (2010) 174423 [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X13.00005: Intrinsic Magnetic Properties of fct FePt Nanocubes and Rods by Cluster Beam Deposition Ozan Akdogan, Wanfeng Li, George Hadjipanayis, Ralph Skomski, David Sellmyer In this work, single crystal fct FePt nanocubes have been successfully produced by a cluster beam deposition technique without the need of post annealing. Particles have been deposited by DC magnetron sputtering using high Ar pressures (0.5 to 2 Torr) on both single crystal Si substrates and Au grids for the measurement of magnetic and structural properties, respectively. The nanocubes have a uniform size distribution with an average size of 6.5 nm. At 1 Torr, the particles have the fct structure with an order parameter of 0.5 and a RT coercivity of 2 kOe with high switching fields seen in the hysteresis loop. Particle size was controlled by changing the pressure and power and also by ex-situ annealing. In addition to these nanocubes, micron size rods (which consist of 20 nm nanoparticles) with the fct structure have been observed near the cluster gun. These particles show a room temperature coercivity of 8 kOe with an order parameter of 0.85. Intrinsic magnetic properties (Curie temperature, H$_{A}$, M$_{S}$ and magnetic viscosity) of the nanocubes and the nanoparticles (separated from the rods) have been extensively studied and the results will be reported. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X13.00006: Confinement Effect on the Phase Transformation of FePt from A1 to L1 Andrew Gallagher, Levent Colak, Ozan Akdogan, George Hadjipanayis The major challenge for the application of chemically synthesized FePt nanoparticles (NPs) in magnetic storage media is the sintering problem encountered during the required high temperature annealing to obtain the high anisotropy $L$1$_{0}$ phase. In this work, we have used two methods to avoid sintering: coating the NPs with a protective layer of silica (SiO$_{2})$ and using porous aluminum oxide (Al$_{2}$O$_{3})$ as a template to hold the NPs. The NPs were synthesized via the synthesis method of Sun \textit{et al}.$^{[1]}$ The NPs were added to the Al$_{2}$O$_{3}$ by in-situ suctioning of the reaction solution into the porous Al$_{2}$O$_{3}$ template. Monodispersed FePt NPs with a size of 5.8 and 15 nm were coated with SiO$_{2}$ shells using a water-in-oil microemulsion method. High room temperature coercivities were only obtained after annealing the samples at 900\r{ }C for long times (24-48 h) under forming gas flow as compared to the usual 600-700\r{ }C. Values of 4.7 and 7.8 kOe were observed in SiO$_{2}$ and Al$_{2}$O$_{3}$ samples, respectively after annealing for 24 h at 900\r{ }C. This behavior suggests that the restricted geometry of the samples suppresses the phase transformation drastically. \\[4pt] [1] S. Sun, C. B. Murray, D. Weller, L. Folks, A. Moser\textit{ Science }\textbf{2000}, $287$, 1989. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 4:18PM |
X13.00007: Towards high strength nanocomposite magnets --- Approaches from the bottom Invited Speaker: J. Ping Liu Exchange-coupled nanocomposite magnets are regarded as the next generation of permanent magnetic materials, based on the theoretical predictions. However, many fundamental questions and technical challenges remain in understanding the inter-phase exchange interactions and in processing bulk nanocomposite magnets with enhanced energy products. We will review recent advancements in both the fundamental research and the materials processing technologies. New findings about the effects of soft-phase properties and interface conditions on the hard/soft phase exchange interactions will be presented. Particularly, the development of the bottom-up approaches in materials processing will be discussed. Novel methodology for nanoparticle synthesis including the salt-matrix annealing, surfactant-assisted ball milling and severe plastic deformation will be described. Unconventional compaction techniques including warm compaction and dynamic compaction are recommended because they can be used to retain desired nanoscale morphology for effective exchange coupling in bulk nanocomposite magnets. A perspective on fabrication of anisotropic nanocomposite magnets will be also given. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X13.00008: NdCo$_{5}$ Nanoflakes and Nanoparticles Produced by Surfactant-Assisted High Energy Ball Milling Nilay Gunduz Akdogan, Wanfeng Li, George Hadjipanayis The study of size and surface effects in rare earth transition metal nanoparticles is scientifically very important. In this work our studies were focused on NdCo$_{5}$ which is interesting because of its complex magnetic ordering behavior at different temperatures. Anisotropic NdCo$_{5}$ nanoparticles have been produced by surfactant-assisted high-energy ball milling (HEBM) of nanocrystalline precursor alloys. A two-stage ball milling method has been employed to produce the NdCo$_{5}$ nanoflakes and nanoparticles. NdCo$_{5}$ flakes have a thickness below 150 nm and an aspect ratio as high as 10$^{2}$ - 10$^{3}$; the nanoparticles have an average size of 7 nm. Both the nanoparticles and nano-flakes showed high coercivities at low temperatures, with values at 50 K of 3 kOe and 3.7 kOe, respectively. The high values of coercivity observed in a planar anisotropy phase can be attributed to the large surface anisotropy of nanoparticles that leads to an effective uniaxial-type of behavior. The nanoparticles also showed spin reorientation temperatures which are lower when compared to the bulk values. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X13.00009: Magnetic Studies on Nd$_{2}$Fe$_{14-x}$Mn$_{x}$B Nanoflakes and Nanoparticles Produced by Surfactant-Assisted High Energy Ball Milling George Hadjipanayis, Nilay Gunduz Akdogan, Wanfeng Li High temperature magnetic ordering studies on rare earth transition-metal nanoparticles and nanoflakes present a great challenge due to the very high reactivity of these materials. It is well known that Mn substitution for Fe in Nd$_{2}$Fe$_{14}$B compound decreases the Curie temperature to a temperature range that allows for reliable measurements to be made. In this work, we have studied the magnetic properties of Mn substituted Nd$_{2}$Fe$_{14}$B particles in the temperature range of 50-400 K. Nd$_{2}$Fe$_{14-x}$Mn$_{x}$B nanoparticles and nanoflakes have been produced by surfactant-assisted high-energy ball milling (SA-HEBM). Different size nanoparticles have been obtained by varying the milling conditions. Anisotropic Nd$_{2}$Fe$_{14-x}$Mn$_{x}$B nanoparticles have been found with a size from 13 to 25 nm. Both the nanoparticles and nano-flakes showed high coercivities at low temperatures, with values at 50 K of 2.4 kOe and 5.5 kOe, respectively. The Curie temperature was determined from the temperature dependence of magnetization. We have observed a different magnetic ordering behavior in the nanoparticles with Curie temperatures that are higher when compared to the bulk values. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X13.00010: Giant Coercive Fields of 2.5 Tesla in Nanostructured Mn$_{x}$Ga Films Steven Bennett, Thomas Nummy, Thomas Cardinal, Welville Nowak, Don Heiman There is a growing interest in designing new magnetic materials that are free of rare-earth elements. The magnetism of the Heusler ferrimagnet Mn$_{x}$Ga [1] was found to be enhanced when fabricated with nanoscale structural disorder. Films of Mn$_{x}$Ga (x=2 to 3) with thicknesses of 20 to 40 nm were grown using molecular beam epitaxy at 100\r{ }C then annealed at 400\r{ }C. Disordered films were grown on lattice mismatched Si (001) substrates, then compared to epitaxially grown films on desorbed GaAs (001) substrates. While the epitaxial films have small hysteresis in the magnetization with coercive fields in the range $\mu _{o}$H$_{C}$ = 10$^{-2}$ - 10$^{-1}$ T, the disordered films exhibited surprisingly wide hysteresis with record high coercive fields as large as $\mu _{o}$H$_{C}$ = 2.5 T. These magnitudes are comparable to those of rare-earth-based magnets. This hysteresis was also present in the anomalous Hall effect. The enhanced coercive field in the disordered material arises from a combination of the exceptionally large magnetocrystalline anisotropy and nanoscale structural disorder. These results point out a new opportunity for developing rare-earth-free magnetic materials. Discovery of this unusually high coercive field is outlined and its sources discussed. [1] J. Winterlik, et al., Phys. Rev. B \textbf{77}, 054406 (2008). [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X13.00011: Tunable exchange length in laminate exchange coupled composite media B.J. Kirby, Hao-Cheng Hou, Chih-Huang Lai Exchange coupled composites - with a hard layer (HL) to anchor against thermal instabilities, and a soft layer (SL) to assist magnetization reversal - have been proposed for advanced recording applications. The reversal assist relies on a non-coherent rotation between the HL and SL, - the exchange-spring (ES) - in which the interfacial domain wall traverses the hard/soft interface, and promotes switching. Typically, the soft region is a single layer, with the emergent composite properties mainly determined by the choice of soft material. We are pursuing a more sophisticated approach, using a hard CoPtCr-SiO2 layer adjacent to a multilayer of the same material, [CoPtCr-SiO2/Pt]N, softened by lamination with Pt layers. Simulations predict that when the SL thickness exceeds a critical exchange length, a significant portion should decouple from the HL, becoming a domain wall nucleation site. Thus, ES behavior should be tunable via the Pt thickness. To test this, we have used polarized neutron reflectometry to measure the field-dependent magnetic depth profiles - and directly characterize ES formation - for a series of samples with varying Pt laminate thickness. The experimentally determined relationship between laminate thickness and ES formation will be discussed. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X13.00012: Impact of Size Reduction on the Magnetocaloric Effect in Single- and Multi-Phase Manganites N.S. Bingham, P. Lampen, A. Puri, S. Chandra, M.H. Phan, H. Srikanth, C.L. Zhang, S.W. Cheong, T.H. Hoang, H.D. Chinh Mixed-valent manganites of the form R$_{1-x}$M$_{x}$MnO$_{3}$ (R=La, Pr, Nd, Sm and M=Sr, Ca, Ba, Pb) are of interest as low-cost materials for potential application in the area of active magnetic refrigeration (AMR). An important parameter to optimize for AMR is the refrigerant capacity (RC), which depends on both the magnitude and breadth of the magnetic entropy change peak. Reducing the dimensions of a system to the nanoscale has the potential to enhance the RC by broadening a transition, but can also lead to a drop in entropy change. In this study, we contrast the impact of size reduction on the magnetic and magnetocaloric properties of single-phase La$_{0.4}$Ca$_{0.6}$MnO$_{3}$ (LCMO) and phase-separated La$_{0.35}$Pr$_{0.275}$Ca$_{0.375}$MnO$_{3}$ (LPCMO). Nanoparticles of LCMO and LPCMO were prepared by a sol-gel method; single crystals were grown in an optical floating zone furnace. XRD, SEM, and TEM were used to characterize the samples and DC magnetometry measurements were performed using a Quantum Design VSM. We find that size reduction negatively impacts both magnetization and the magnetocaloric properties in LCMO, while enhancing RC and entropy change simultaneously in LPCMO. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X13.00013: Enhancement of magnetic refrigerant capacity in nanocrystalline LaMnO$_{3}$ Anis Biswas, Sayan Chandra, M.H. Phan, H. Srikanth Manganites are considered as potential magnetic refrigerants owing to their large magnetocaloric effect (MCE). However, the effective temperature span of large MCE ($\delta $T) is quite small in their bulk form resulting in small refrigerant capacity (RC). We have studied the magnetocaloric property of LaMnO$_{3 }$in its polycrystalline bulk and nanocrystaline form with particle size $\sim $50nm. MCE is quantified as the change in magnetic entropy (-$\Delta $S$_{M})$, which is calculated from the isothermal magnetization curves using Maxwell relation. The samples exhibit large -$\Delta $S$_{M}$ associated with their paramagnetic to ferromagnetic transition. Relative to the bulk sample, $\delta $T increases significantly in the nanocrystalline form giving rise to more than 63{\%} enhancement in RC. The calculated values of -$\Delta $S$_{M}$(RC) for bulk and nanocrystalline samples are $\sim $2.6J/KgK(173J/Kg) and 2.4J/KgK(282 J/Kg) respectively for magnetic field change of 50 kOe. From present study, it can be inferred that reduction of particle size to the nanoscale may be an effective way to increase $\delta $T and hence improve RC of a material. [Preview Abstract] |
Session X14: Focus Session: Spins in Semiconductors - Spin-Orbit Interaction and Relaxation in Si and Ge
Sponsoring Units: GMAG DMP FIAPChair: Nicholas Harmon, University of Iowa
Room: 212
Thursday, March 1, 2012 2:30PM - 2:42PM |
X14.00001: Field-induced negative differential spin lifetime in silicon Jing Li, Lan Qing, Hanan Dery, Ian Appelbaum Using experimental measurements of spin transport in undoped silicon, we show that the electric field-induced thermal asymmetry between the electron and lattice systems substantially impacts the identity of the dominant spin relaxation mechanism. In contrast to the Elliott-Yafet theory where intraband phonon absorption leads to spin relaxation, here we induce phonon \emph{emission} during which electrons are scattered between conduction band valleys that reside on different crystal axes. This leads to anomalous behavior, where reduction of the transit time between spin-injector and spin-detector with larger electric field is accompanied by a counterintuitive reduction in spin polarization and an apparent \emph{negative} spin lifetime.\\[4pt] Work at UMD is supported by the Office of Naval Research and the National Science Foundation. We acknowledge the support of the Center for Nanophysics and Advanced Materials and Maryland NanoCenter and its FabLab. Work at UR is supported by AFOSR and NSF (No. FA9550-09-1-0493 and No. DMR 1124601). [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X14.00002: Decrease of electron spin lifetime in external electric field due to intervalley phonon scattering in silicon Lan Qing, Hanan Dery, Jing Li, Ian Appelbaum We derive a simple approximate expression of the spin lifetime of drifting electrons in silicon. This expression agrees well with elaborate Monte Carlo simulations of the charge transport and spin relaxation of conduction electrons heated by the electric field. Already at low temperatures, the drifting electrons become hot enough to undergo $f$-processes (scattering between valleys of different crystal axes following emission of a shortwave phonon). Such a process involves a direct coupling of valence and conduction bands and dominates the spin relaxation. A sharp decrease of spin lifetime can then be expected in intermediate electric fields in between $\sim$100~V/cm and $\sim$1~kV/cm. When electrons are transported between a spin injector and a spin-resolved detector, the decrease of both transit time and spin lifetime results in a non-monotonic behavior of the detected spin polarization with the electric field. The theory shows excellent agreement with empirical results. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X14.00003: Spin-Polarized Luminescence Across the Indirect Band-Gap of Strained Ge, Si and Their Alloys Dhara Trivedi, Pengke Li, Hanan Dery We study optical orientation and circularly polarized photoluminescence in germanium, silicon and their alloys. We focus on phonon-assisted optical transitions across the indirect band-gap under conditions of biaxial strain (either compressive or tensile). The signature of strain on the band structure and phonon dispersion is observed in the luminescence spectra where spin properties are better resolved from the change in intensity ratio between left and right circularly polarized emission. The spectra is simulated using the combined results of a spin-dependent empirical pseudopotential method, adiabatic bond charge model, electric-dipole approximation, and rigid-ion model. An additional strain tensor has been introduced in calculating the strain effect. We have used group theory extensively to account for all possible transitions and to provide concise selection rules. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:42PM |
X14.00004: Spin-orbit interaction and spin relaxation of conduction band electrons in Si and Ge Invited Speaker: Pengke Li Group IV element semiconductors, silicon and germanium, are promising material candidates in spintronics due to their intrinsic long electron spin lifetimes. To describe the spin properties of conduction electrons in these indirect band-gap multivalley semiconductors, the method of invariants is employed to build $8 \times 8$ (for Si) and $10 \times 10$ (for Ge) spin-dependent Hamiltonians that capture the symmetries of the zone edge states (X-point of Si [1] and L-point of Ge) and their spin dependent parameters. Concise expressions of the energy bands, and more importantly, of the spin mixed states are derived and verified by numerical results of an empirical pseudopotential method. These analytical state expressions are powerful tools to study the behavior of electron spins similar to the way that the Kane model in being used in direct band-gap semiconductors. Mechanisms of spin relaxation by electron-phonon scattering are studied based on this model. We reveal fundamental differences between spin and momentum relaxation mechanisms. In the case of silicon, intravalley spin flipping is governed by scattering with transverse acoustic (TA) phonons via the interband deformation potential that couples the upper and lower conduction bands (this deformation potential would also break the degeneracy of the conduction band at the X point if off-diagonal stress is applied). The intervalley g-process spin flipping couples the lowest conduction bands of different irreducible representations in opposite valleys via acoustic phonons; the intervalley f-process spin flipping, which is the dominant contribution of spin relaxation in a wide temperature range, couples the conduction and valence band components of the states by scattering with $\Sigma_1$ and $\Sigma_3$ phonons. In the case of germanium, intervalley spin-flip scattering, which is also the main contribution of spin relaxation, couples the lowest and upper conduction bands of different valleys by X-phonons. The intravalley spin flip scattering, which is about two orders of magnitude smaller, couples the conduction and valence bands mostly via TA phonons.\\[4pt] [1] Pengke Li and Hanan Dery, Phys. Rev. Lett. 107, 107203 (2011) [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X14.00005: Group theory analysis of phonon-induced spin relaxation in silicon Yang Song, Hanan Dery Selection rules and leading order matrix element expressions are derived for all important spin flip processes in the multivalley conduction band of bulk silicon. All results are generalized to arbitrary spin orientation directions. Intervalley $f$-process scattering induced by all phonon modes are analyzed using double group irreducible representation matrices of the $\Delta$ axis and independent integrals are identified for transitions between states of either spin. Intervalley $g$-process and intravalley spin flips are analyzed using the $X$ point single group and detailed selection rules are derived using a four-band basis. Together with electronic states obtained by a spin-dependent k$\cdot$p expansion, wavevector dependent spin-flip matrix elements are derived for all phonon modes in intravalley and for the leading order phonon mode in $g$-process scattering. Higher order matrix elements are qualitatively studied. Comparison with deformation potential theory in momentum scattering is made. Integrations for spin relaxation rate are carried out. Symmetry breaking mechanisms such as stress and electric field are discussed and quantified. We benchmark all of our analysis with numerical results of strain-dependent empirical pseudopotential and adiabatic-bond-charge models. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X14.00006: Influence of spin polarization on resistivity of a two-dimensional electron gas in Si MOSFET at metallic densities I. Shlimak, A. Butenko, D.I. Golosov, K.-J. Friedland, S.V. Kravchenko Positive magnetoresistance (PMR) of a silicon MOSFET in parallel magnetic fields $B$ has been measured at high electron densities $n \gg n_{\rm c}$ where $n_{\rm c}$ is the critical density of the metal-insulator transition (MIT). It turns out that the normalized PMR curves, $R(B)/R(0)$, merge together when the field is scaled according to $B/B_{\rm c}(n)$ where $B_{\rm c}$ is the field in which electrons become fully spin polarized. The values of $B_{\rm c}$ have been calculated from the simple equality between the Zeeman splitting energy and the Fermi energy taking into account the experimentally measured dependence of the spin susceptibility on the electron density. This extends the range of validity of the scaling all the way to a deeply metallic regime far away from MIT. The subseqent analysis of PMR for low $n\stackrel{>}{\sim} n_{\rm c}$ demonstrated that the merging of the initial parts of curves can bee achieved only with taking into account the temperature dependence of $B_{\rm c}$. It is shown that the shape of the PMR curves at strong magnetic fields is affected by a crossover from a purely two-dimensional (2D) electron transport to a regime where out-of-plane carrier motion becomes important (quasi-three-dimensional regime). [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X14.00007: Spin phase coherence of donor nuclear spins in silicon: the influence of electrical readout Dane McCamey, Johan van Tol, Gavin Morley, Christoph Boehme Storing information in spin underpins the operation of a wide range of emerging technologies. However, the ability to interact with, and thus control electron spin implies a reasonable coupling to the environment, and a correspondingly limited spin coherence time. This problem can be overcome by using nuclear spins for long term information storage, and significant experimental progress in this direction has been seen recently. Readout of stored information can be achieved in a variety of ways, with electrical approaches offering substantial benefit with regard to integration of spintronic and classical electronic applications. Here, we discuss electrical readout of coherent nuclear spin states of donor nuclei in silicon. By utilizing nuclear Hahn echo sequences, we are able to demonstrate that nuclear spin phase coherence can exceed 3 ms with electrical readout. We find that the spin phase coherence is in this case limited by the spin lifetime of the donor electron which mediates our readout scheme, and discuss approaches to ameliorate this effect. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X14.00008: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X14.00009: Optical Orientation and Spin Relaxation of electrons and holes in Strained Germanium Quantum Wells Fabio Pezzoli, Federico Bottegoni, Franco Ciccacci, Stefano Cecchi, Emanuele Grilli, Mario Guzzi, Giovanni Isella, Dhara Trivedi, Pengke Li, Yang Song, Hanan Dery We demonstrate optical orientation in strained Ge/SiGe quantum wells and study their spin properties. The energy proximity between the center of the Brillouin zone to its edge allows us to achieve high spin-polarization efficiency and to resolve the spin dynamics of holes and electrons. The circular polarization degree of the direct-gap photoluminescence is 37\% and 86\% for transitions with heavy and light holes states, respectively. Considering the ultrafast transition of electrons to L valleys, the extracted spin lifetime of holes at the top of the valence band is found to be 0.5 ps. This lifetime is governed by transitions between heavy and light hole states. The indirect-gap photoluminescence via the no-phonon line and its LA phonon replica allows us to study spin properties of electrons at the bottom of the conduction band. Taking into account the recombination lifetime of electrons (radiative and non-radiative channels), we find that their spin lifetime exceeds 5 ns below 150 K. Theoretical analysis of the electrons spin relaxation indicates that phonon-induced intervalley scattering by the X point phonon modes dictates the spin lifetime. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X14.00010: Infrared Kerr measurements in ferromagnetic silicon carbide Alok Mukherjee, Chase Ellis, Nada Tesarova, John Cerne, Yu Liu, Shunchong Wang, Gang Wang We measure the infrared (100-1000 meV) polar Kerr angle in ferromagnetic silicon carbide (SiC). The Kerr angle is sensitive to the Hall conductivity $\sigma _{xy}$ and measures the difference of optical responses for left and right circularly polarized light, which makes it a sensitive spectral probe for small changes in the symmetry of the system due to magnetic order. Both neutron-irradiated and Al-doped samples are studied in the 10-300K temperature range. This study provides new insights into the mechanisms by which non-magnetic impurities and defects can produce magnetic order. Strong frequency dependence and hysteresis are observed in the Kerr measurements. Work supported by NSF-DMR1006078. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X14.00011: Low-temperature scattering-Scanning Near-field Optical Microscopy of Strongly Correlated Materials Molly May, Joanna Atkin, Markus Raschke Strongly correlated electron materials display diverse complex phenomena such as metal-insulator transitions and ferroelectric and ferromagnetic ordering, with characteristic lengths on the nanometer scale. In order to directly access and study the associated nano-phase behavior and domains for a wide range of materials, we have developed a low temperature tip-enhanced scattering-type scanning near-field optical microscope (s-SNOM). A microscopy flow cryostat reservoir is coupled to a shear-force atomic force microscope, with illumination of electrochemically etched Au tips provided by an on-axis high numerical aperture parabolic mirror. We will discuss the use of this system for the study and imaging of ferroic ordering in multiferroic and ferroelectric materials through the symmetry selectivity provided by tip-enhanced second harmonic generation (SHG) and nano-Raman crystallography via the tensor based selection rules. [Preview Abstract] |
Session X15: Focus Session: Spin and Dynamics in Metal, Resonance Phenomena II: FMR in Magnetic Nanostructures
Sponsoring Units: DMP FIAP GMAGChair: Xin Fan, University of Delaware
Room: 213
Thursday, March 1, 2012 2:30PM - 2:42PM |
X15.00001: Two dimensional spectroscopic imaging of individual ferromagnetic nanostripes Robert McMichael, Han-Jong Chia, Lyuba Belova We report on high resolution imaging of the center and edge modes of individual Permalloy nanostripes using ferromagnetic resonance force microscopy (FMRFM). Fabrication of future spintronic devices requires an understanding of how edge damage affects a device's magnetic properties, and the highly localized edge modes of a nanostripe provide a direct method to investigate edge properties. While previous studies have measured spectra of the edge modes in large arrays of stripes, in this work we use FMRFM to image and probe the center and edge modes in individual NiFe stripes, 400 and 700 nm in width and 20 nm in thickness. Our spectroscopic measurements reveal different resonance fields for opposite edges of a stripe, which also exhibit different structural profiles. 2-D spatial imaging of the bulk and edge mode resonances demonstrates the inhomogeneity of the edge modes along their lengths with a resolution of 300 nm. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X15.00002: Size dependent nonlinear effects of magnetization dynamics in Permalloy disks Feng Guo, Han-Jong Chia, Lyuba Belova, Robert McMichael We use ferromagnetic resonance force microscopy (FMRFM) to probe nonlinear magnetization dynamics in individual Ni$_{80}$Fe$_{20}$ disks with diameters ranging from 100 nm to 500 nm. The nonlinear behavior of magnetic nanostructures has important implications for rapid switching of memory devices and for frequency stability of spin torque oscillators. In the studied size range, micromagnetic modeling predicts a transition from complex power dependent behavior in the larger disks to simple behavior in the smallest disks where only a few modes are observed. At low power levels in the measurements, precession produces a force-detected signal that is linear in power and that displays a Lorentzian line shape. At higher powers, we observe nonlinear effects, including asymmetric line shapes and foldover where the resonance shifts with power. In addition, complex, jagged line shapes appear at high powers. We report that the onset power level of the nonlinear regime was found to be size dependent. In smaller disks, higher microwave power is required to drive nonlinear precession, and we see fewer complex peaks. Furthermore, the direction of foldover also depends upon the disk size. We have modeled our results using micromagnetic simulations and they display good correspondence with our experimental data. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X15.00003: Electrically Detected Broadband Ferromagnetic Resonance in Individually Defined Permalloy Nanowires Zheng Duan, Carl Boone, Ilya Krivorotov, Nathalie Reckers, Sven Stienen, Juergen Lindner We report measurements of electrically detected broadband ferromagnetic resonance (FMR) in lithographically defined Permalloy nanowires. For these measurements, the Permalloy nanowire is placed in close proximity to the short of a gold coplanar strip waveguide. The microwave power applied to the waveguide drives the magnetization precession in the wire and the four-point resistance of the wire is measured as a function of DC external magnetic field. The time-averaged resistance of the wire depends on the amplitude of the magnetization precession via anisotropic magnetoresistance (AMR), and thus peaks and dips with resistance dependence on the bias magnetic field arise from resonant excitation of spin wave modes in the wire. Using this electrically detected FMR technique, we measure the frequency and linewidth of the quasi-uniform mode of magnetization precession as well as bulk modes and edge modes that exist for the magnetic field applied in the plane of the sample perpendicular to the wire. We will present measurements of the resonance frequency and linewidth of various modes for several values of the wire width and compare our results to theoretical predictions of the field dependence of the mode frequency. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:42PM |
X15.00004: Spin torque phenomena originating from the spin Hall effect: resonance, magnetic switching, and magnetic dynamics Invited Speaker: Luqiao Liu The spin Hall effect (SHE) generates a transverse spin current with the passage of a current through a non-ferromagnetic metal (NM) film. Quite different results have been obtained for the magnitude of this effect. Here I will discuss a new technique where, by applying an RF current to a NM/FM thin film bilayer, the spin current onto the FM layer can induce a spin torque ferromagnetic resonance (ST-FMR). This enables the determination of the SHE strength with precision and without the need to assume the values of unmeasured parameters. The large magnitude of the SHE that we have established in several types of NM is sufficient to reversibly switch the magnetic orientation of a FM layer and I will discuss two different implementations of this. In the first the FM layer has a perpendicular-to-plane magnetic moment at equilibrium and the SHE injected spins are orthogonal to the moment. The spin torque can overcome the anisotropy (coercive) field restoring torque, with the polarity of the current-induced switching being determined by the sign of a small external field applied along the current direction. In the second approach, the FM moment lies in plane and the spins injected by the SHE exert a negative damping just as in conventional ST switching of a spin valve or magnetic tunnel junction (MTJ). We have fabricated three-terminal devices that incorporate a MTJ and a SHE layer to induce in-plane reversal switching. The simple architecture of this three terminal device and the high efficiency of the SHE induced switching made it a promising technique for future memory and non-volatile logic applications. We have also used this three terminal device to demonstrate DC induced dynamics in the magnetic layer due to the SHE. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X15.00005: Magnetostatics and magnetodynamics in single crystal Ni on MgO Michael Pechan, Brian Kaster, Michael Sinko, Dan Dahlberg, C.A. Ross, Gye Hyun Kim, Carl V. Thompson We present an investigation of the magnetic properties of 120 nm thick (100) and (110) oriented single crystal Ni films grown on MgO via evaporative deposition. Xray diffraction analysis was used to confirm the single crystal nature and crystallographic orientations of the films. Magnetization measurements reveal anisotropy and magnetic moment consistent with bulk Ni values. Ferromagnetic resonance measurements have been made as a function of in-plane angle and temperature at 36 GHz. Resonance field maps confirm the anisotropy expected for high quality single crystal films. Both the anisotropy and damping are presented at temperatures ranging from 50 K to room temperature. Additional FMR results at 10 GHz are also presented from 4 K to room temperature. These results are discussed in the context of the temperature-dependent magnetocrystalline anisotropy of Ni and magnetoelastic anisotropy resulting from thermal mismatch between the Ni and MgO. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X15.00006: Localized Edge Modes in Permalloy Square Antidot Arrays J.B. Ketterson, J. Sklenar, V.S. Bhat, L. DeLong, O. Heinonen We have carried comprehensive experiments on and simulations of the ferromagnetic resonance spectrum of various thin films of permalloy patterned with a periodic array of holes on a square lattice. These so-called antidot lattices show a rich multi-line spectrum covering a wide range of frequencies and magnetic fields; they also exhibit striking angular dependences. One mode that is predicted in our simulations is highly localized at the edges of the holes. Our simulations further show that for a fixed field or excitation frequency the angular dependence of this mode strongly depends on the shape of the holes in the antidot array. It has been suggested\footnote{S. Neusser, B. Botters and D. Grundler, Phys. Rev. B, 78, 054406 (2008).} that, in comparison to other more extended modes, this localized mode would be difficult to find experimentally due to: i) the effects of varying shape and roughness present in an actual array, and ii) the fact that the mode is concentrated within a small fraction of the unit cell of the sample. Our experiments, performed with a broadband meanderline-based ferromagnetic resonance (FMR) spectrometer,\footnote{C. C. Tsai, J. Choi, S. Cho, B. K. Sarma, C. Thompson, O. Chernyashevskyy, I. Nevirkovets, and J. B. Ketterson, Rev. of Sci. Instr. \textbf{80}, 023904 (2009).} show a very weak resonance (requiring extensive signal averaging) that is in rough agreement with the simulations, which we propose as a candidate for this elusive edge mode. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X15.00007: Topological Localization of FMR Modes by Antidot Arrays Patterned into Permalloy Thin Films Vinayak Bhat, Justin Woods, Lance De Long, Joseph Sklenar, John Ketterson, Olle Heinonen We have observed novel broad-band FMR spectra for 25-nm-thick Permalloy films patterned with square arrays of diamond antidots with axes D$_{x }$= 1430 nm and D$_{y}$ = 860 nm. The y-axis lattice spacing was held constant at d$_{y}$ = 2000 nm, and variable x-axis lattice spacings d$_{x }$= 1730, 2000, 2267 and 2730 nm. The applied DC magnetic field H (in-plane at angle $\theta $ with respect to x-axis) spanned the hysteretic regime $\vert $H$\vert \quad \le $ 150 Oe, to $\vert $H$\vert $ = 3 kOe in the saturated regime, corresponding to mode frequencies f $\approx $ 250 MHz to 14 GHz. In spite of hysteretic evolution of domain walls in the low-field regime, highly reproducible absorption peaks appear at f $<$ 3 GHz. Static and dynamic micromagnetic simulations agree with DC magnetization and FMR dispersion curves, and show domain pinning by the antidot edges is responsible for the reproducible spectra in the hysteretic regime. For H = 1 kOe along the x-axis, we observe two localized modes: one (f = 9 GHz) in a narrow gap between the accute vertices, and another (f = 10.25 GHz) between the oblique vertices, of adjacent diamonds. For $\theta $ = 45$^{\circ}$, one mode (f = 8.7 GHz) extends along the (-1,1) direction with strong angular variation of f, and a standing mode (f = 9.87 GHz) is localized between nearly parallel edges of adjacent antidots. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X15.00008: Ferromagnetic Resonance Studies of Individual Ferromagnetic Nanowires Rohan Adur, Inhee Lee, Stefan Philippi, Thomas Muhl, Denis V. Pelekhov, P. Chris Hammel We investigate magnetization dynamics in ferromagnetic nanowires using two techniques: Ferromagnetic Resonance Force Microscopy (FMRFM) and Vector Network Analyzer Ferromagnetic Resonance (VNA-FMR). Using FMRFM we report our attempts at scanned probe FMR imaging of individual ferromagnetic nanowires. By placing ferromagnetic nanowires on permalloy we can use the capability of localized mode imaging (I. Lee et. al, Nature 2010) to measure local fields both in and around the ferromagnetic nanowire. In addition, we use VNA-FMR to study dipolar interactions in densely packed nanowire arrays. The dipolar fields between neighboring nanowires introduce an anisotropy field that can dominate over the shape anisotropy expected for isolated nanowires, and this effect is observed in the angular dependence of VNA-FMR spectra. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X15.00009: Damping Dependence in Microwave Assisted Magnetization Reversal Yunpeng Chen, Xin Fan, Qi Lu, John Xiao Microwave assisted magnetization reversal (MAMR) is one possible technique to mitigate the writability problem in ultrahigh density magnetic recording. In the presence of microwaves, the magnetization reversal of a magnetic recording material could be triggered at significantly reduced switching field. The damping constant is one of the critical parameters in magnetization switching for the magnetization precession spiraling down to the direction along the external field. We demonstrate microwave assisted magnetization reversal (MAMR) in a CoFeB film and the damping dependence in MAMR through the measurement of ferromagnetic resonance (FMR). Spin-pumping in non-ferromagnetic/ferromagnetic films provides a large range variation of Gilbert damping constants in magnetic samples when changing the thickness of non-ferromagnetic layers without changing the ferromagnetic film. An evident dependence of switching fields on the damping constant is observed in the presence of microwaves. The trend of the experiment data is well reproduced by a numerical simulation based on the Landau-Lifshitz-Gilbert equation. The result indicates that the large damping decreases the efficiency of microwaves in reducing the magnetization switching field. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X15.00010: Theory of the ac spin valve effect: a new method to measure spin relaxation time Denis Kochan, Martin Gmitra, Jaroslav Fabian Parallel (P) and antiparallel (AP) configurations of FNF junctions have, in a dc regime, different resistivities ($R_{AP}>R_{P}$), giving rise to the giant magnetoresistance (GMR) effect, which can be explained within the spin injection drift-diffusion model. We extend the model to include ac phenomena and predict new spin dynamical phenomenon; the resonant amplification and depletion of spin accumulation in the P and AP configurations, respectively. As the major new effect, the spin valve magnetoimpedance of the FNF junction oscillates with the driving ac frequency, which leads to negative GMR effect ($|Z_{AP}|<|Z_{P}|$). We show that from the spin-valve oscillation periods, measured all electrically in the GHz regime, the spin relaxation times could be extracted without any magnetic field and sample size changes (contrary to other techniques). For thin tunnel junctions the ac signal becomes pure Lorentzian, also enabling one to obtain the spin relaxation time of the N region from the signal width. This work, was published in Physical Review Letters,10, 176604 (2011). [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X15.00011: Vortex Resonance in Coupled Ferromagnetic Disks Shikha Jain, Helmut Schultheiss, John Pearson, Frank Fradin, Samuel Bader, Valentyn Novosad Advances in nanolithography and thin film growth techniques offer the unique opportunity to prepare a variety of laterally confined nanostructured magnets. Of particular interest are lithographically patterned micron and submicron disk-shaped particle arrays. The magnetic ground state in confined geometries consists of a curling spin configuration, known as a vortex state. Studies of vortex dynamics have mainly focused on circular or elliptical dots at remanence. In this work, we investigate the dynamic response of vortex gyration in interacting systems where two circular dots are statically exchange coupled. The induced coupling due to the interacting area forces the disks to have antiparallel chirality of magnetization. Apart from different vortex polarity combinations, various frequency modes were observed as a function of external magnetic field and contact length. Moreover, due to the induced configurational anisotropy in the system, vortex resonance in the two disks was found to be strongly dependent on the orientation of the static magnetic field. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X15.00012: Study of the coupling of vortices in magnetic nanodisks Alberto Guimaraes, Gabriel Fior, Flavio Garcia There is a marked interest nowadays in the dynamic behavior of magnetic nanodisks that present magnetic vortices [1]. The tailoring of vortex features, including their gyrotropic frequency, critical vortex core switching velocity and strength of the coupling between nanodisks, is very desirable for future applications, such as vortex magnetic memories (VRAMs) and spin transfer nanooscillators (STNOs) [2]. An original way to tune some of the static vortex properties (specially the vortex core diameter) is simply to introduce a uniaxial perpendicular magnetic anisotropy, as has been recently shown [3]. Here we have studied the coupling between vortices as a function of the magnetic properties of the disks and their separation, using micromagnetic simulations. We analyzed the motion of a vortex core caused by the motion of the second one, excited by static or rotating magnetic fields. A splitting of the gyrotropic frequency is also observed. Exciting one of the disks, it is possible to switch the core polarity of the other. These results open new possibilities for applications of magnetic vortices.\\[4pt] [1] Ruotolo et al. Nat Nano, 4(8):528-532, (2009);\\[0pt] [2] Jung et al. Sci. Rep. 1 ,59;DOI:10.1038/ srep00059 (2011);\\[0pt] [3] Garcia et al. APL. 97, 022501 (2010). [Preview Abstract] |
Session X16: Manganites: Perovskite and Bilayer
Sponsoring Units: DCMPChair: Mark Golden, University of Amsterdam
Room: 251
Thursday, March 1, 2012 2:30PM - 2:42PM |
X16.00001: Persistence of Jahn-Teller Distortion up to the Insulator to Metal Transition in LaMnO$_{3}$ Maria Baldini, Viktor Struzhkin, Alex Goncharov, Paolo Postorino, Wendy Mao High pressure, low temperature Raman measurements performed on LaMnO$_{3}$ up to 34~GPa provide the first evidence for the persistence of the Jahn-Teller distortion over the entire stability range of the insulating phase. This result resolves the ongoing debate about the nature of the pressure driveninsulator to metal transition (IMT), demonstrating that LaMnO$_{3}$ is not a classical Mott insulator. The formation of domains of distorted and regular octahedra, observed from 3 to 34~GPa, suggests that LaMnO$_{3}$ becomes metallic when the fraction of undistorted octahedra domains increases beyond a critical threshold. In this scenario, it is interesting to consider whether or not the CMR effect may be induced in LaMnO$_{3}$ by applying pressure. Preliminary results obtained performing high pressure resistivity measurements in a magnetic field will be reported. \\[4pt] [1] I. Loa, et al., \textit{Phys. Rev. Lett.} 87, 125501 (2001). \\[0pt] [2] A.Y. Ramos et al., \textit{Phys. Rev. B} 75, 052103(2007). \\[0pt] [3] A.Y. Ramos et al., \textit{J. Phys. Conf. Ser}. 190, 012096 (2009). \\[0pt] [4] A. Yamasaki et al., \textit{Phys. Rev. Lett.} 96, 166401 (2006). \\[0pt] [5]J. D. Fuhr et al., \textit{Phys. Rev. Lett.} 100, 216402 (2008). \\[0pt] [6] M. Baldini et al.\textit{, Phys. Rev. Letter} 106, 066402 (2011). [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X16.00002: Influence of Gd-doping in La0.7Ca0.3MnO3 on its structural and Magneto-Electrical Properties Sevgi Polat Altintas, Abderrezak Amira, Cabir Terzioglu We present a study of the structural and electrical properties of lanthanum-based manganite, La0.7Ca0.3MnO3 with x=0.0 and 0.1. The samples synthesized by the conventional solid state reaction method. The samples are characterized by X-ray diffraction, scanning electron microscope and energy dispersive X-ray spectrometer. The electrical and magneto-transport properties of bulk samples have been investigated in the temperature range 5-300 K and a magnetic field up to 7 T. Although the replacement of La ion by Gd results a decrease in metal-insulator transition temperature TMI, the magnetoresistance and resistivity are found to be increased. The electrical resistivity in the entire temperature range fit well with the phenomenological percolation model, which is based upon an approach that the system consists of the phase separated ferromagnetic metallic and paramagnetic insulating regions. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X16.00003: Size control of Magnetism, Charge and Orbital Order in Half-Doped Manganite, La$_{0.5}$Ca$_{0.5}$MnO$_3$ Tanusri Saha-Dasgupta, Hena Das, G. Sangiovanni, A. Valli, K. Held Motivated by recent experimental results, we study the effect of size reduction on half-doped manganite, La$_{0.5}$Ca$_{0.5}$MnO$_3$, using the combination of density functional theory (DFT) and dynamical mean field theory (DMFT). We find that upon size reduction, the charge-ordered antiferromagnetic phase, observed in bulk, to be destabilized, giving rise to the stability of a ferromagnetic metallic state. Our theoretical results, carried out on defect-free nanocluster in isolation, establish the structural changes that follow upon size reduction to be responsible for this. Our study further points out the effect of size reduction to be distinctively different from application of hydrostatic pressure. Interestingly, our DFT+DMFT study, additionally, reports the correlation-driven stability of charge-orbitally ordered state in bulk La$_{0.5}$Ca$_{0.5}$MnO$_3$, even in absence of long range magnetic order. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X16.00004: Measuring Coexisting Phases in La$_{0.35}$Pr$_{0.275}$Ca$_{0.375}$MnO$_3$ Mark H. Burkhardt, M.A. Hossain, S. Sarkar, H.A. D\"{u}rr, J. St\"{o}hr, Y.-D. Chuang, A.G. Cruz Gonzalez, A. Doran, A. Scholl, A.T. Young, Y.J. Choi, S.-W. Cheong Manganite compounds in the La$_{0.625-y}$Pr$_y$Ca$_{0.375}$MnO$_3$ series are known for exhibiting phase separation over a large temperature range. We combined the x-ray photoemission electron microscopy (PEEM) and resonant elastic soft x-ray scattering (RSXS) techniques to study the interplay between the low-temperature ferromagnetic and intermediate temperature charge-ordered/antiferromagnetic phases, respectively, in La$_{0.35}$Pr$_{0.275}$Ca$_{0.375}$MnO$_3$. We found that the system is driven by glassy polarons, which are present above the curie temperature $T_C$ in many ferromagnetic metallic manganites. They stunt the growth of the ferromagnetism on cooling: we clearly observe the onset of small, strained ferromagnetic domains almost $30\,\mathrm{K}$ above the temperature where ferromagnetism fully sets in, and the ferromagnetism has a very unconventional temperature dependence even below $T_C$. This relationship could explain the need for such high magnetic fields to induce colossal magnetoresistance. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X16.00005: Oxygen Vacancy Induced Metal Insulator Transition in Epitaxial Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ Thin Films Jingdi Zhang, Kebin Fan, Ryuhei Kinjo, Weiming Xu, Iwao Kawayama, Masayoshi Tonouchi, Xin Zhang, Richard Averitt We report the metal-insulator transition in epitaxial Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ thin film by introducing oxygen vacancies, which assist the nucleation of ferromagnetic metallic domains in a antiferromagnetic insulating matrix. The hysteresis of the resistivity indicates the transition is first-order, and covers a broad temperature range from 80K to 220K. Such novel transport properties of the x=0.3 doped manganite may result from strong spin-lattice coupling which stabilizes the system to a metallic metastable state at low temperature. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X16.00006: Search for Multi-Ferroic Manganites with Elongated Mn-O bonds Bogdan Dabrowski, James Mais, Stanislaw Kolesnik Development of multi-ferroic materials, where magnetism and ferroelectricity are strongly coupled near room temperature, is of fundamental technological and theoretical importance. Typically, both phenomena tend to be mutually exclusive because ferroelectricity is usually present for d$^{0}$ and magnetism for non-d$^{0}$ transition metals. By analogy to perovskite Ba$^{2+}$Ti$^{4+}$O$_{3}$ (d$^{0})$ for which [Ti-O] bonds are highly elongated beyond their equilibrium lengths resulting in Ti distortion out of the center of the TiO$_{6}$ octahedral unit, resulting in T$_{F}\sim $400 K ferroelectricity, we have projected that similar effect should be observed for the non-d$^{0}$ insulating and antiferromagnetic (T$_{N}\sim $240 K) perovskites of Mn$^{4+}$ (d$^{3})$. I will describe our search for such compounds guided by our ``tolerance factor design rules'' in the (Sr,Ba)Mn O$_{3}$ system for which strong multi-ferroic behavior was achieved near room temperature. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X16.00007: Formation of mesoscopic metallic filaments in manganite thin films imaged by microwave impedance microscopy Worasom Kundhikanjana, Keji Lai, Yongliang Yang, Yue Ma, Michael Kelly, Zhi-Xun Shen, Masao Nakamura, Zhigao Sheng, Masashi Kawasaki, Yoshi Tokura We study the ferromagnetic metallic domains from the charge-order insulating background at mesoscopic length scale in a Pr0.55Ca0.75Sr0.25MnO3 thin film using a variable temperature microwave impedance microscope (MIM). The metallic state in this compound can be easily induced at a moderate magnetic field as low as 2 T observed by both the transport and MIM. The temperature dependent transport under 1.2 T shows a large hysteresis loop. MIM allows us to observe the formation and melting of metallic domains at different temperatures during the cooling and warming processes. At higher temperatures, the metallic domains first emerge in small isolated filaments along certain crystal axes of the LSAT(110) substrate, suggesting that the local strain plays an important role. Surprisingly, small insulating islands remain in the metallic ground state and persist up to very high magnetic fields, indicating strong pining sites. Lastly, the sizes of the insulating islands at the ground state increase when the film is field cooled at lower speeds, suggesting s glassy order in this compound. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X16.00008: Imaging of nano domain fluctuations in manganites Mirian Garcia Fernandez, Stuart Wilkins, Hong Zheng, John Mitchell We present a soft x-ray resonant diffraction study of the electronic spatial nano-scale domains present in manganites. We explain the details of our new set up that allow us to perform this diffraction contrast microscopy in the nano regime. We will present preliminary results on the nanoscale domains in the bilayer manganite LaSr$_{2}$Mn$_{2}$O$_{7}$. The structural reflection (002) and the A-type antiferromagnetic reflection (001) have been investigated with soft x-rays in the vicinity of the manganese $\mathit{L}$$_{3}$ edge. A resolution of $\sim$ 150-200 nm has been achieved by implementing a zone plate focusing optic in our diffraction set up. These two reflections have been mapped in the same region of the sample. This region has dimensions of 20 x 20 microns. We will present measurements comparing the magnetic nano scale domains in LaSr$_{2}$Mn$_{2}$O$_{7}$ probed directly by measuring the (001) antiferromagnetic reflection, with crystallographic inhomogeneities that are observed in the mapping of the (002) crystallographic reflection. Finally, future directions using this setup will be discussed for the study of strongly correlated systems. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X16.00009: Fragile Magnetic Ground State in Half-Doped Manganite LaSr$_2$Mn$_2$O$_7$: Orbital Instability J.-S. Lee, C.-C. Kao, C.S. Nelson, S.B. Kim, Y.J. Choi, S.-W. Cheong, S. Smadici, P. Abbamonte, H. Jang, K.-T. Ko, J.-H. Park Recently, a careful doping control study, however, reported that the La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ material has an exotic phase diagram very near the half doping with extremely narrow antiferromagnetic phase boundaries at x$\simeq$ 0.5 $\pm$ 0.005 -- the $CE$-type within the boundaries but the $A$-type outside.\footnote{Q. Li $et~al.$, Phys. Rev. Lett. 98, 167201 (2007).} To understand a complexity on this material's phase diagram, we investigated the orbital and antiferromagnetic ordering behaviors of the half-doped bilayer manganite La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ (x $\simeq$ 0.5) by using Mn $L_{2,3}$-edge resonant soft x-ray scattering. We confirmed the predicted $CE$-type antiferromagnetic order for the true half-doped (x = 0.5) case. Moreover, we found that such a narrow phase boundary is due to the close competition of the two antiferromagnetic ordering phases via $3d$ Mn $e_g$ orbital instability. Our study reveals the spin and orbital orders of electrons in the sample as well as information about their ground states.\footnote{J.-S. Lee $et~al.$, Phys. Rev. Lett. 107, 037206 (2011).} [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X16.00010: Bilayer manganites reveal polarons in the midst of a metallic breakdown F. Massee, S. de Jong, Y. Huang, W. K. Siu, I. Santoso, A. Mans, A. T. Boothroyd, D. Prabhakaran, R. Follath, A. Varykhalov, L. Patthey, M. Shi, J.B. Goedkoop, M.S. Golden Just what tips the balance between the wealth of competing phases and textures in spin, charge and orbital degrees of freedom in the CMR manganites such as La$_{2-2x}$Sr$_{1+2x}$Mn$_{2}$O$_{7}$ (LSMO)? Combining ARPES and STM/S measurements on bilayered LSMO (0.30$\le $x$<$0.5), we arrive at a compelling explanation for the seemingly contradictory data on the electronic structure of bilayered LSMO that has appeared in the literature. We show that the true signature of bilayered (N=2) LSMO is that of a gapped non-metal on the verge of a metallic breakdown. All the former confusion stems from the intrinsic presence of stacking faults leading to either more gapped single layer (N=1) LSMO or sharply peaked, N$>$2 slabs displaying true metallic double-exchange. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X16.00011: The intrinsic electronic structure of bilayer manganites from Angle Resolved Photoemission Sanne de Jong, R. Kukreja, M.A. Hossain, M.S. Golden, E. van Heumen, F. Massee, Y. Huang, A.T. Boothroyd, P. Pabhakaran, A. Walter, A. Bostwick, E. Rotenberg, H.A. Durr The Colossal MagnetoResistant (CMR) manganites are one of the most studied condensed matter physics systems since decades. Yet, the mechanism behind the CMR effect and their electronic structure are still under hot debate. Recent angle resolved photoemission (ARPES) studies on the bilayer manganite La$_{(2-2x)}$Sr$_{(1+2x)}$Mn$_3$O$_7$, LSMO327, reported contradictory results [1]. Here we present an ARPES study unveiling the intrinsic $k$-- and temperature dependent electronic structure of LSMO327, while carefully steering away from the recently reported sample inhomogeneities [2] that have caused all the confusion. \\[4pt] [1] N. Mannella, Nature (2005); S. Sun Nature Phys. (2007); S. de Jong, PRB (2007)\\[0pt] [2] F. Massee, Nature Phys(2011) [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X16.00012: Unconventional response of acoustic phonons to the onset of charge order in a bilayer manganite S. Rosenkranz, F. Weber, J.P. Castellan, R. Osborn, H. Zheng, J.F. Mitchell, S. Chi, J.W. Lynn, D. Reznik The acoustic phonons in the 50\% doped bilayer manganite LaSr$_2$Mn$_2$O$_7$ exhibiting CE type charge order were investigated using inelastic neutron scattering. At the onset of charge ordering, we observe an abrupt increase of the energies and a decrease of the linewidts of the transverse mode along (1,1,0), which crosses the CE ordering wavevector. This effect is however not localized to the CE ordering wavevector, but is observed over an extended range of momentum transfers,for which the phonon energy is lower than 15 meV. These observations indicate a reduced electron-phonon coupling due to a partial removal of the Fermi surface and provide direct evidence for a link between electron-phonon coupling and charge order in manganites. However, the observed response of the phonons is not consistent with a standard CDW mechanism, clearly showing that the transition is unconventional. \\ \\ Work supported by US DOE BES-DMS DE-AC02-06CH11357. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X16.00013: Magnetoelastic Spin Flip in La$_{1.4}$Sr$_{1.6}$Mn$_2$O$_7$ K.-T. Ko, H. Jang, J.-H. Park, B.-G. Park, J.-Y. Kim, Sung Baek Kim, S-W. Cheong The magnetoelastic coupling in a bilayer manganite was investigated by using x-ray absorption spectroscopy (XAS) and resonant soft x-ray scattering (RSXS) at Mn $L_{2,3}$-edge. Huge occupation reversal of $e_g$ level from $d_{3z^2-r^2}$ to $d_{x^2-y^2}$ was observed at the temperature and magnetic field induced phase transition in La$_{1.4}$Sr$_{1.6}$Mn$_2$O$_7$. The CI model calculation indicated that the direction of magnetocrystalline anisotropy is affected by the configuration of $e_g$ level, and the sharp spin flip transition was expected. The field dependent RSXS measurements demonstrated a strong magnetoelastic coupling in La$_{1.4}$Sr$_{1.6}$Mn$_2$O$_7$, where the AFM spin axis was changed from out-of-plane to in-plane as a result of the field induced change of $e_g$ orbital occupation. Finally, we discuss the spin-orbital-lattice coupling in bilayer manganites. [Preview Abstract] |
Session X17: Focus Session: Nanostructures and Metamaterials, Growth, Structure, and Characterization -- Improved Materials and Applications II
Sponsoring Units: DMPChair: Luca Dal Negro, Boston University
Room: 252A
Thursday, March 1, 2012 2:30PM - 3:06PM |
X17.00001: Low-loss multilayered metamaterial exhibiting a negative index of refraction at visible wavelengths Invited Speaker: Carlos Garcia-Meca Over the last decade, metamaterials have attracted a great interest thanks to their potential to expand the range of electromagnetic properties found in natural materials. In particular, the possibility of achieving negative refractive index media (NIM) enables us to implement superlenses and optical storing devices. Since the first experimental demonstration at microwave frequencies, much effort has been put in extending negative refraction to the visible spectrum, where we can take full advantage of NIM properties. For instance, the superior imaging ability of NIM would be essential for visible microscopy. The desired features for NIM are low loss and isotropy. This last property includes polarization independence and negative-index behavior in all spatial directions. None of these features have been attained in previous experiments. Thus, the current challenge is to improve such aspects in order to make NIM suitable for practical applications. In this work, we experimentally demonstrate a low-loss multilayer metamaterial exhibiting a double-negative index in the visible spectrum, while presenting polarization independence at normal incidence. This has been achieved by exploiting the properties of a second-order magnetic resonance of the so-called fishnet structure, in contrast to previous works that used first-order magnetic resonances, both related to gap surface plasmon polariton (SPP) modes. The low-loss nature of the employed magnetic resonance, together with the effect of the interacting adjacent layers, results in a figure of merit as high as 3.34. A wide spectral range of negative index is achieved, covering the wavelength region between 620 and 806 nm with only two different designs. The fabricated metamaterials are the first experimental multilayer NIM in the visible spectrum, which entails an important step towards homogeneous NIM in this range. Finally, we found that the SPP modes determining the permeability resonance display weak angular dispersion. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X17.00002: Photonic-Plasmonic Coupling and Near Field Engineering in Nanoparticle Necklaces Alyssa Pasquale, Bj\"orn Reinhard, Luca Dal Negro Particle clusters with different degrees of rotational symmetry consisting of circular loops of gold nanoparticles, dubbed nanoplasmonic necklaces, are proposed as a novel, reproducible platform for elastic and inelastic optical sensors with polarization insensitive behavior. Engineering of the necklaces allows for full control of the plasmonic hot-spot locations and near-field strength by coupling photonic resonances to the circular resonator structure. The polarization insensitivity of necklaces guarantees that the plasmonic hot-spots remain excited within the necklaces irrespective of the incident polarization of the excitation field, which is a significant advantage compared to hot-spots in dimer configurations. Near-fields can be further enhanced using radiative coupling in concentric necklaces having integer multiple diameters. Engineering design rules are determined for hot-spot formation, polarization insensitivity, and intensity distribution in necklaces using 3-dimensional Finite-Difference Time-Domain simulations. Plasmonic necklaces of different rotational axes were fabricated using electron-beam lithography and electron-beam deposition of gold films. Surface enhanced Raman scattering measurements were used to experimentally validate our near-field calculations. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X17.00003: Probing Thermomechanics at the Nanoscale: Impulsively Excited Pseudosurface Acoustic Waves in Hypersonic Phononic Crystals D. Nardi, Q. Li, K. Hoogeboom-Pot, M. Murnane, H. Kapteyn, M. Siemens, M. Travagliati, F. Parmigiani, G. Ferrini, F. Banfi Hypersonic-frequency surface acoustic waves can be generated by ultrafast laser excitation of nanoscale patterned surfaces and are of great interest because of their high sensitivity to the mechanical properties of the material in which they propagate. By modeling nanoscale thermomechanics from first principles, we can calculate a composite system's initial heat-driven response and follow its evolution in time. A spectral decomposition of the response on the calculated eigenmodes of the system allows evaluation of impulsively excited pseudosurface acoustic wave frequencies and lifetimes, expanding our understanding of surface waves scattering in mesoscale metamaterials, while providing crucial information about non-destructive photoacoustic characterization and imaging of nanostructures for nanoelectronics, nanomedicine and photovoltaic applications. The model is successfully benchmarked against time-resolved optical diffraction measurements performed on 1D and 2D surface phononic crystals, probed using extreme ultraviolet and near-infrared light. Reference: D. Nardi et al., Nano Lett. 11, 4126 (2011). [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X17.00004: Plasmonic nanotips for spectroscopy with nanometer-scale resolution Johannes Feist, Semion K. Saikin, M.T. Homer Reid, Al\'an Aspuru-Guzik, Mikhail D. Lukin We theoretically investigate the use of metallic nanotips, i.e. nanowires with a sharp tip, as tools for spectroscopic applications such as surface-enhanced Raman scattering (SERS). Nanotips can provide strong coupling between guided plasmon modes and single emitters such as atoms or molecules. At the same time, the spatial localization of the plasmon response can potentially provide nanometer-scale spatial resolution in a scanning-tip setup. In particular, we will focus on the possibilities of transporting the electromagnetic field to the target through the surface plasmon mode on the wire, and on coupling the emitted radiation into the same mode. We compare the performance of such an approach with more conventional SERS setups, where localized surface plasmons are used to enhance the local field of an incoming laser beam and the emitted free-space radiation. Finally, we will discuss whether single-molecule sensitivity can be reached. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X17.00005: Structural and Optical Properties of Metals in Photonic Semiconductor Devices Meng-Mu Shih Metals can play important roles in semiconductor photonics if properly incorporated and designed. The mode-coupling coefficient of the periodic waveguide in the device is the key parameter for analyzing optoelectronic performance. This work constructs a modified model to show how the metal gratings on the semiconductor surfaces and how the metal compositions inside the semiconductors can affect the coupling coefficients. Metal gratings with various materials and nano-structures can affect optical interactions at corrugated metal-semiconductor interfaces. Optical effects such as wavelength and wave polarization can affect the optical properties of metals and semiconductors. In addition, metal compositions in semiconductor compounds can change the optical properties of semiconductors. Semiconductors with different optical and structural properties can generate specific wavelengths. Consequently, the above factors related to optical properties and interactions of metals can affect the coupling coefficients. Computational results with physical interpretations provide insights into photonic devices for more applications. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X17.00006: Cavity Resonances in Plasmonic Patch Nanoantennas Ayan Chakrabarty, Feng Wang, Fred Minkowski, Qi-Huo Wei Plasmonic nanoantennas allow for confining and detecting photons at very small length scales. This work presents our recent experimental and theoretical studies of two dimensional periodic arrays of elliptical metal nano-patches on a silver film with a dielectric gap layer. Simulation and theoretical results shows that various cavity modes can be excited with tilted or normal incident light, and that the azimuthal symmetry breaking makes the nanoantennas polarization sensitive due to different resonant frequencies of the even and odd cavity modes. Particularly, it is shown that the cavity modes can be well described by a product of Mathieu functions, providing good agreements with both simulations and experiments. The effects of coupling between the cavity modes and the propagating plasmons will be discussed. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X17.00007: Efficient calculation for waveguides and for photonic crystal band structures using improved finite elements in 2D C.R. Boucher, C.I. AhHeng, L.R. Ram-Mohan We employ scalar, fifth-order Hermite interpolation polynomials to solve Maxwell's equations in two dimensions in the finite element method. We analyze homogeneous conducting waveguides, inhomogeneous waveguides, and photonic crystals. The Hermite interpolation functions provide greater accuracy than vector finite elements of equal polynomial order, while bypassing the issue of spurious modes observed when using Lagrange polynomials. The scalar Hermite elements offer a level of flexibility which is not seen with vector finite elements, as in multiphysics problems such as coupled Schr\"odinger-Maxwell problems. The use of Hermite elements is an attractive alternative to plane-wave methods for modeling photonic crystals, leading to sparse matrices due to local connectivity of the finite elements, greater flexibility in modeling, and lower computational costs. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X17.00008: Diffusion of a Plasmon-Exciton Polaron Charles Cherqui, David Dunlap, Andrei Piryatinski We consider the motion of an exciton constrained to a quasi-one-dimensional geometry in the vicinity of a metal interface. For weak coupling, the metal causes damping of the center of mass motion, leading to a decrease in the exciton diffusion constant. This can be modeled as non-contact dielectric friction between an oscillating dipole and a substrate, where the frictional force is related to the response of the metal through the fluctuation dissipation theorem [1]. When the exciton frequency is in the neighborhood of the plasma resonance, the interaction can no longer be described by linear response theory, for the exciton and plasmon form a quasiparticle, an exciton-plasmon polaron. We calculate the transmission and reflection coefficients for the exciton-plasmon polaron in the neighborhood of a metal interface, as well as the diffusion rate and radiative lifetime versus coupling strength.\\[4pt] [1] Seppe Kuehn, John A. Marohn, and Roger F. Loring, 110(30) J. Phys. Chem. B, (2006) 1425 [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X17.00009: Coherent interband transport in binary zigzag optical waveguide arrays Kin Chung Au Yeung, Nai Jing Deng, Cheung Wai Chau, Sai Kit Yung, Kin Wah Yu We have studied the optical oscillation and Zener tunneling of light waves in binary zigzag optical waveguide arrays in which the evanescent coupling in the array is included up to the second order. By tuning the ratio of the first order and second order coupling strengths, there is a miniband-minigap structure in the dispersion diagram. Moreover, by adding a gradient in the propagation constant transverse to propagation, Bloch-Zener oscillation (BZO) and Zener tunneling between two bands can be realized. The occurrence of BZO and Zener tunneling is simulated by the field-evolution analysis using an input Gaussian beam. Through a visual band picture, the simulation results confirm the band structure of the waveguide arrays. A rate equation is proposed to understand the coherent transport behaviors between the two minibands across the gap. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X17.00010: Tunable rainbow trapped in a self-similar liquid crystal waveguide Qing Hu, Si-Hui Wang, Di-Hu Xu, Yu Zhou, Ru-Wen Peng, Mu Wang We have investigated the optical propagation through a self-similar dielectric waveguide, where a hollow core is surrounded by a coaxial Thue-Morse multilayer. It is found that due to the self-similar furcation feature in the photonic band structure, the transmission multibands are achieved. And different frequency ranges of the transmission modes can be selectively guided and spatially confined along the waveguide. Consequently, a rainbow can be trapped in the waveguide. Then by infiltrating liquid crystal into the cladding layers, the transmission modes and rainbow trapping can be tuned by altering the temperature. And transverse electric (TE) and transverse magnetic (TM) polarizations present different propagating features. The attenuation and energy density distributions of different modes in the waveguide are also discussed. The finding can be applied to designing miniaturized compact photonic devices, such as a spectroscopy on a chip, color-sorters on a chip, and photon sorters for spectral imaging. Reference: Qing Hu, Jin-Zhu Zhao, Ru-Wen Peng, Feng Gao, Rui-Li Zhang, and Mu Wang, Appl. Phys. Lett. (2010) 96, 161101; and Qing Hu, Ru-Wen Peng, Si-Hui Wang, and Mu Wang, manuscript prepared(2011). [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X17.00011: A New Strategy for Designing Broadband Epsilon-Near-Zero Metamaterials Hon Ping Lee, Ka Shing Hui, Lei Sun, Kin Wah Yu We have developed a new strategy for designing metamaterials in multi-layered film with permittivity being closed to zero over a broad frequency range, which is as known as broadband epsilon-near-zero (ENZ) materials. Milton representation, Bergman-Milton representation and electromagnetic representation of the effective permittivity ($\epsilon_{eff}$) are used, and the strategy consist of the following 3 parts: choosing the operation frequency range, properly placing the poles and zeros into the range, and solving the inverse problem by equating different representations of $\epsilon_{eff}$. Demonstration of the strategy is carried out by zeroth and first order design with several examples. The distribution of electric field inside the designed materials is investigated to reveal the physical principles of the broadband ENZ phenomenon. The study would be further extended to other geometries (e.g. multi-shell cylinder) through conformal transformation. The results obtained are useful for designing ENZ metamaterials. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X17.00012: Effective permittivity of ensemble-averaged waves in dense random plasmonic composites Satvik Wani, Ashok Sangani, Radhakrishna Sureshkumar Random composites of metallic nanospheres (Ag, Au, Cu, etc.) in transparent media are highly opaque due to absorption resulting from plasmon resonance. A new technique for calculating the effective properties of such dense composites is described. The underlying physical motif is the separation of the space surrounding any inclusion into two regions, one immediately surrounding the particle with the properties of the matrix (the size of this region depends on the static structure factor) and an effective medium. Self consistent closure relations are found for the conditionally averaged fields by solving the vector Helmholtz equations for a layered sphere in an infinite matrix by utilizing a multipole expansion technique. For finitely large $\phi $, the effective permittivity is given by $\varepsilon _{eff} /\varepsilon _m =1+3\beta \phi +\raise0.7ex\hbox{$3$} \!\mathord{\left/ {\vphantom {3 4}}\right.\kern-\nulldelimiterspace}\!\lower0.7ex\hbox{$4$}(\beta +4)\beta ^2\phi ^2+O(\phi ^3)$ where $\varepsilon _m $ is the permittivity of the medium and $\beta $ is the particle polarizability per unit volume. For denser systems, the particle and effective medium fields interfere to give rise to a Fano-like line shape for $Im\,(\varepsilon _{eff} )$. The resonance conditions result in a $\phi $ dependent red-shift of the resonance peak. Effects of polydispersity and multiple particle species on $\varepsilon _{eff} $ will be discussed. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X17.00013: Microfabrication of highly absorbent THz metafilms tuned for specific frequencies Dragoslav Grbovic, Fabio Alves, Brian Kearney, Gamani Karunasiri THz imaging has gained increased attention in recent years. The main motivation is the property of THz radiation to penetrate through most non-metalic materials and fabrics, making it attractive for medical and security applications. In addition, the radiation is non-ionizing and therefore does not present a risk for human health. However, THz imaging requires external illumination, often by quantum cascade lasers (QCLs). Metafilm absorbers with nearly 100{\%} absorption, at frequencies matched to appropriate QCLs, have been designed and fabricated using a periodic array of aluminum (Al) squares and an Al ground plane, separated by a thin SiO$_{2}$ dielectric film. All metafilms are below 2 $\mu $m and are suitable for integration with microbolometers or bimaterial sensors for THz imaging. The THz spectral characteristics of structures were probed using FTIR spectrometer. Films with different dielectric layer thicknesses exhibited resonant absorption at close to 100{\%} at respective frequencies. The measured THz reflection, from thin film of both broad-band and resonant metamaterial structures, exhibit excellent agreement with their respective finite element models. [Preview Abstract] |
Session X18: Growth and Properties of Nanoparticles and Nanowires
Sponsoring Units: DMP DCMPChair: Matthew McCluskey, Washington State University
Room: 252B
Thursday, March 1, 2012 2:30PM - 2:42PM |
X18.00001: Electronic structures and adsorption configurations of gold nanoclusters on cerium oxide defect surfaces Lu Wang, Wai-Ning Mei, Neil Lawrence, Joseph R. Brewer, James Wells-Kingsbury, Marcella Ihrig, Gonghua Wang, Chin Li Cheung, Yun-Liang Soo Fluorite-structured cerium oxide (or ceria, CeO$_{2-x}$, 0 $\le x\le $ 0.5) has been shown to be an important material in catalysis, yet few study has investigated the effect of non-dopant introduced oxygen vacancy defect (OVD). In addition, we found experimentally that when doped with Au nanoclusters, the catalytic ability of ceria enhanced. In this work, we modeled and optimized an (111) fluorite-structured slab model of defective ceria with a chemical formula corresponding to CeO$_{1.5}$. The optimized surface structure of this model was found to contain both surface and sub-surface OVDs, similar to those observed in our HRTEM data for low pressure activated nanoceria. Significantly, the model captures comparable reduction in the average Ce-O bond distance and also atomic coordination numbers observed in our EXAFS data. To explore the roles of Au nanoclusters, we adsorbed flat clusters of 3, 4, 9, 10, and 19 Au atoms on ceria slabs, optimized their configurations, and computed the corresponding electronic structures applying first-principle approach. Consequently, we present the density of states results to elucidate the experimentally observed optical property change and $s-d$ hybridization. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X18.00002: The Kirkendall Effect in Copper Nanocrystals Mark Stoykovich, Katherine Rice The study of copper nanocrystals, unlike nanocrystals of other noble metals such as gold and silver, has been limited due to challenges in the synthesis of monodisperse copper nanoparticles and their reactivity at ambient conditions. Copper is a material of broad interest, however, because of its unique optical and catalytic properties, as well as its non-toxic nature and relative abundance. Copper nanocrystals in the process of being oxidized are subject to the Kirkendall effect, which describes independent diffusion rates in a binary system (Cu and oxygen in this case) and that causes the formation of voids at the core of spherical nanocrystals. Previous studies have attributed the Kirkendall effect in Cu/Cu2O nanocrystals to interactions between the organic passivation layer and the solvent. Here we will present our results on the kinetics of oxidation of Cu nanocrystals in solvent-less conditions as a function of temperature and show that void formation occurs at a relatively narrow range of temperatures. In-situ UV-vis spectroscopy, x-ray diffraction, and electron microscopy have been used to monitor the oxidation process in Cu nanocrystals and a model has been developed to describe hollow particle formation in the Cu/Cu2O system. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X18.00003: Systematic Study of Supported Ni, Pd, Pt Metal Nanocrystals for Catalytic Energy Conversion Vicky Doan-Nguyen, Matteo Cargnello, Thomas R. Gordon, Kevin Bakhmutsky, Paolo Fornasiero, Raymond Gorte, Christopher B. Murray Ni, Pd, and Pt nanocrystals ranging from 4-12 nm have been synthesized with great control in size and shape using a high-temperature chemical synthetic method for gas-phase catalytic testing. The tunability in size allows for controlled catalytic study of model reactions such as CO oxidation, CO hydrogenation and methane oxidation. The monodispersity of the particles allow for a systematic correlation between size and catalytic activity. There is consistent size dependence of CO oxidation for each of the metal systems on CeO2 support as indicated by the lower temperatures needed for full conversion. Our calculated activation energy using the Arrhenius equation for each size and material correlated with surface-area-to-volume ratio. Consistent with the light-off studies, the smallest particles were the most catalytically active under differential conditions. The same trend was observed for CO hydrogenation amongst each system. This trend was inverted for the oxidation of methane. This current study aims to elucidate the size dependence of catalytic activity in model systems with supported uniform nanocrystals. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X18.00004: Ab Initio Investigation of the Structures of Fe-Doped Carbon Clusters Christella Lovato, Clifton Brownrigg, Ajit Hira We continue our interest in the theoretical study of carbon clusters to examine the effects of the doping of small carbon clusters (C$_{n}$, n = 2 - 15) with iron atoms. This work applies the hybrid ab initio methods of quantum chemistry to derive the different Fe$_{m}$C$_{n}$ (m = 1-3) geometries. Of particular interest are linear, fan, and cyclic geometries. Electronic energies, rotational constants, dipole moments, and vibrational frequencies for these geometries are calculated. Exploration of the singlet, triplet, quintet, and septet potential energy surfaces is performed. The type of bonding in terms of competition between sp$^{2}$ and sp$^{3}$ hybridization is examined, with a view to addressing the possibility of the stabilization of the doped carbon nano-particles in a diamond type structure. The potential for the existence of new pathways to the fabrication of nanotubes is explored. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X18.00005: Electrical Detection of Mechanical Resonance of ZnO Nanowhiskers Deepika Saini, Doyl Dickel, Ramakrishna Podila, Herbert Behlow, Malcolm Skove, Apparao Rao Here, we present the fundamental mechanism for the observation of electrically actuated resonances in semi-conducting ZnO nanowhiskers (NWs). Previous studies have claimed that various mechanisms including charge induction lead to a mechanical resonance in NWs. Many of such studies employ an electron beam to visualize the resonance of NWs. However, we find that the use of an electron beam changes the electrical character of the NWs making it difficult to understand fundamental actuation mechanism. In this article, we developed a novel, fully electrical harmonic detection of resonance (HDR) method that enables us to probe mechanical resonances of NWs even in the absence of an electron beam. In contrast to the traditional optical detection scheme, the HDR method allows us to successfully decouple the effects of the electron probe beam from the actual driving force. Interestingly, we find that the observed mechanical resonance of ZnO NWs is dominated by their interactions with the electron probe beam. Importantly, ZnO NWs exhibit strong (weak) mechanical resonance only in presence (absence) of the electron probe beam suggesting that the observed behavior originates from dynamically induced (static) charges. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X18.00006: Direct growth of vertically aligned long ZnO nanowires on FTO substrates and their application for dye sensitized solar cells Liyou Lu, Jiajun Chen, Wenyong Wang In this research we report a direct growth of vertically aligned ZnO nanowires on fluorine-doped-tin-oxide (FTO) coated substrates for dye sensitized solar cell (DSSC) applications. ZnO nanowires with length of more than 30 $\mu $m were synthesized using a fine-tuned chemical vapor deposition method at temperatures as low as 550 $^{\circ}$C. The nanowires grew along the [0001] direction and exhibited a need-like shape with a wurtzite single crystal structure. Compared to the ZnO nanowires fabricated by solution based methods, the nanowires synthesized in this study have much longer length, thus could increase the amount of dye loading and improve DSSC performance. Furthermore, since the nanowires were directly synthesized on FTO substrates, DSSCs could be fabricated using the as-grown nanowires without the usual wire transfer processing that caused nanowire breaking and created additional transport barriers and recombination possibilities for photo-generated carriers. DSSCs fabricated in this study showed attractive performance with short-circuit current density of 5.1 mA/cm$^{2}$ and power conversion efficiency of 1.66{\%}. Dependence of the DSSC performance on nanowire length and annealing processing was also examined in this research. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X18.00007: Competing gas phase reactions during vapor transport deposition of ZnO nanowires Eric Driscoll, Kevin Range, Marian Tzolov We present results illuminating some of the major chemical processes in the vapor deposition of ZnO nanowires. The analysis of our deposition experiments indicates that carbon dioxide is a major oxidizing agent rather than carbon monoxide as previously thought. Additionally, we present evidence that carbon monoxide will etch zinc oxide at high temperatures. Zinc oxide nanowires have been prepared by using chemical vapor deposition on silicon (100) substrates with a 10-15nm layer of gold as a catalyst. Zinc oxide and graphite powders were heated to approximately 1000$^{\circ}$C in a tube furnace in a flow of argon. We have delivered oxygen gas specifically in the growth zone to facilitate the formation of high aspect ratio nanowire growth. Thermodynamics calculations were used to justify the growth and etching processes. Imaging of samples was performed with scanning electron microscopy. Chemical composition was determined by energy dispersive x-ray spectroscopy. Photoluminescence spectroscopy was used to characterize the emission properties of the zinc oxide samples. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X18.00008: Study on Pure Phase Formation of Lead Oxide Nanowires by Oxidation of Lead Nanowires Kuo Hai, Xiaogang Wang Lead-oxide nanowires were synthesized by oxidizing lead metal nanowires. The phase structures, sizes and morphologies of the nanowires were investigated by atomic force microscopy and x-ray diffraction, and the band gap of the nanowires was determined by UV-Vis-NIR reflectance diffusion spectrums. The thermodynamic environment for the pure phase formation has been studied. The first-principle computation has been done to help understand the phase formation. Our results reveal that the pure phase formation strongly relies on both the process temperature and the oxygen flow/oxygen partial pressure, and the pure phase ?-PbO nanowires can be obtained only in a narrow, low temperature range under a low oxygen flow. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X18.00009: Formation of metallic gold chain on patterned hydrogen terminated Si(001)-2$\times $1 surface: Density functional study Bikash Gupta, Purusottam Jena Metal adsorption on silicon surface for the formation of linear metallic chain is one of the important research areas for the advancement of nanotechnology. Due to the presence of dangling bonds all over the surface of bare Si(001), metals when deposited, generally do not tend to form stable wire structures. However, patterned hydrogen terminated Si-surface may be a good choice for the formation of atomic chain structures of metals. Since patterned hydrogen terminated Si(001):2x1 surface is very stable, we consider patterning it by removing desired hydrogen atoms and adsorbing gold atoms. We have examined the structure, energetic and electrical properties of such gold adsorbed surface by varying gold coverage. We have found that linear gold chain structures may be formed by controlling gold coverage. Some of these gold chain structures are metallic in nature. We hope that our results will motivate synthesis of gold chains on patterned hydrogen terminated Si(001): 2$\times $1 surface. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X18.00010: Catalytic role of Au nanoparticle in GaAs nanowire growth Peter Kratzer, Sung Sakong, Volker Pankoke The energetics of Ga, As and GaAs species on the Au(111) surface (employed as a model for Au nanoparticles) is investigated by means of density-functional calculations. Apart from formation of the compound Au$_7$Ga$_2$, Ga is found to form a surface alloy with Au, with comparable $\Delta H \sim 0.5$~eV for both processes. Dissociative adsorption of As$_2$ is found to be exothermic by more than 2~eV on both clean Au(111) and AuGa surface alloys. The As-Ga species formed by reaction of As with the surface alloy is sufficiently stable to cover the surface of an Au particle {\it in vacuo} in contact with a GaAs substrate. Concerning the Au-catalysed growth of GaAs nanowires, we conclude that impingement of As$_2$ or As$_4$ molecules on the Au particle suffices as supply of arsenic to the growth zone. We identify a regime of temperatures and As$_2$ partial pressures suitable for Au-catalysed nanowire growth in molecular beam epitaxy. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X18.00011: Engineering Enhanced Optical Properties of Near-IR Upconverting Nanoparticles Daniel Gargas, Alexis Ostrowski, Emory Chan, Delia Milliron, Bruce Cohen, P. James Schuck Due to their unique properties in converting low energy light into higher energy electronic transitions, upconverting nanoparticles (UCNPs) have garnered considerable interest in bio-imaging, photovoltaic, and opto-electronic applications. In particular, lanthanide-doped UCNPs have demonstrated a host of functionalities due to their nanoscale dimensions, wide range in transition-metal doped compounds, and high photostability in both aqueous and ambient environments.\footnote{P.J. Schuck, et al \textit{Proc. Nat. Acad. Sci.} \textbf{106}, 10917, 2009} In addition, their mixed electric and magnetic dipole transitions make them ideal materials for study of plasmon-enhanced properties with metal nanostructures in which tunable surface properties can mediate energy transfer processes. Here we report on the luminescence properties of Er$^{3+}$, Yb$^{3+}$-doped NaYF$_{4}$ UCNPs with diameters ranging from 5 -- 50 nm in both core and core-shell architectures. Optical characterization of the luminescence lifetime and spectral emission from both UCNP films and single particles reveal a strong dependence on particle size and surface functionalization. Furthermore, by utilizing the large shift (anti-stokes) in absorption energy versus transition energy, we investigate the interaction of energy transfer across metal-semiconductor nano-interfaces whereby the intrinsic luminescence lifetimes are probed for Purcell enhancement and emission rate modification. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X18.00012: Tuning the parameters to establish Quenching and enhancement regimes in hybrid Gold nanoparticles and CdSe Quantum dots monolayer Laxminarayan Tripathi, Praveena M., J.K. Basu The multi-component nanomaterials combine the individual properties and give rise to emergent phenomenon. Optical excitations in such hybrid nonmaterial's ( for example Exciton in semiconductor quantum dots and Plasmon in Metal nanomaterials) undergo strong \ weak electromagnetic coupling. Such exciton-plasmon interactions allow design of absorption and emission properties, control of nanoscale energy-transfer processes, and creation of new excitations in the strong coupling regime.This Exciton plasmon interaction in hybrid nanomaterial can lead to both enhancement in the emission as well as quenching. In this work we prepared close-packed hybrid monolayer of thiol capped CdSe and gold nanoparticles. They exhibit both the Quenching and enhancements the in PL emission.The systematic variance of PL from such hybrid nanomaterials monolayer is studied by tuning the Number ratio of Gold per Quantum dots, the surface density of QDs and the spectral overlap of emission spectrum of QD and absorption spectrum of Gold nanoparticles. Role of Localized surface Plasmon which not only leads to quenching but strong enhancements as well, is explored. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X18.00013: Enhanced Free Exciton and Direct Band-Edge Emissions at Room Temperature in Ultrathin Zn0 Films Grown on Si Nanopillars by Atomic Layer Deposition Pei-Yuan Chu, Yuan-Ming Chang, Jiann Shieh, Jenh-Yih Juang Room-temperature ultraviolet (UV) luminescence was investigated for the atomic layer deposited ZnO films, which were grown on silicon nanowires (Si-NWs) fabricated by self-masking dry etching in hydrogen-containing plasma. For films deposited at $200^\circ$ C, an intensive UV emission corresponding to free-exciton recombination (~3.31eV) was observed with a nearly complete suppression of the defect-associated broad visible range emission peak. On the other hand, for ZnO films grown at $25^\circ$ C, albeit the appearance of the abovementioned defect-associated broad visible emission, the UV emission peak was observed to shift by ~60meV to near the direct band edge (3.37 eV) recombination emission. The high resolution transmission electron microscopy (HRTEM) examinations showed that, indeed, the microstructure of the obtained ZnO films for the former case was of continuous crystalline nature, while that for the latter displayed a microstructure consisting of ZnO nanocrystals with a mean diameter of $4$nm embedded in a largely amorphous matrix. The blue shift of the UV emission peak in the latter films, thus, might have been due to the effects of quantum confinement on the free-exciton recombination. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X18.00014: Growth of Molybdenum Oxide Nano-Micro Structures by Thermal Annealing Process Seyad Ahmad Mahdavi Ardakani, Sajjad Tollabimazraehno, Maryam Raftari, Abas Azarian, Seyed Mohammad Mahdavi, Azam Irajizad The needle and planar molybdenum oxide structures on molybdenum foil were grown by thermal annealing process. The effects of different parameters such as oxygen flow rate, presence of oxygen, annealing temperatures and annealing time on structures, grain size and aspect ratio of nano/micro structures were studied. It is found that the density of structures is only function of oxygen flow rate and annealing temperature. The maximum density of MoO3 nanorods were observed at annealing temperature 530$^{\circ}$C in air. The mechanism of the crystals growth was found for synthesized nanorods. [Preview Abstract] |
Session X19: Invited Session: Two Dimensional Electron Systems at Oxide Interfaces
Sponsoring Units: DMP DCMPChair: Maitri Warusawithana, Florida State University
Room: 253AB
Thursday, March 1, 2012 2:30PM - 3:06PM |
X19.00001: Electronic structure of the interfacial LaAlO$_3$/SrTiO$_3$ 2D electron gas Invited Speaker: Jean-Marc Triscone The interface between LaAlO$_3$ and SrTiO$_3$, two good band insulators, was found in 2004 to be conducting with a high mobility [1] and, in some doping range, superconducting with a maximum critical temperature of about 200~mK [2,3]. I will describe recent experiments aiming at determining the origin of the electron gas. I will then discuss the transport properties of high mobility samples that display Shubnikov de Haas (SdH) oscillations [4]. In such high mobility samples, electric field tuning of the carrier density allows the electronic structure to be followed through analysis of the evolution of the SdH oscillations. \\[4pt] [1] A. Ohtomo, H. Y. Hwang, Nature 427, 423 (2004).\\[0pt] [2] N. Reyren, S. Thiel, A. D. Caviglia, L. Fitting Kourkoutis, G. Hammerl, C. Richter, C. W. Schneider, T. Kopp, A.-S. Ruetschi, D. Jaccard, M. Gabay, D. A. Muller, J.-M. Triscone and J. Mannhart, Science 317, 1196 (2007).\\[0pt] [3] A. Caviglia, S. Gariglio, N. Reyren, D. Jaccard, T. Schneider, M. Gabay, S. Thiel, G. Hammerl, J. Mannhart, and J.-M. Triscone, Nature 456, 624 (2008).\\[0pt] [4] A.D. Caviglia, S. Gariglio, C. Cancellieri, B. Sac\'ep\'e, A. F\^ete, N. Reyren, M. Gabay, A.F. Morpurgo, J.-M. Triscone, Physical Review Letters 105, 236802 (2010). [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:42PM |
X19.00002: Fundamental Properties of the 2-D Electron Liquid Generated by LaAlO$_{3}$-SrTiO$_{3}$ Interfaces Invited Speaker: Jochen Mannhart Extraordinary electron systems can be generated at well-defined interfaces between complex oxides. Much more so than the 2-D electron gases formed at interfaces between conventional semiconductors, the electron systems at oxide interfaces may be shaped by the character of the underlying ionic lattice and be characterized by substantial correlations. Focusing on the electron liquid produced by n-type LaAlO$_{3}$-SrTiO$_{3}$ interfaces, I will present our studies of such structures and discuss in particular our results pertaining to the magnetism, superconductivity, and negative electronic compressibility of these systems. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 4:18PM |
X19.00003: Gate control of the mobility, carrier density and superconductivity at the LaAlO$_3$/SrTiO$_3$ interface Invited Speaker: Christopher Bell The conductivity at the LaAlO$_3$/SrTiO$_3$ interface [1], and in particular its control with a back-gate electrode in the superconducting regime [2], offers a powerful route to explore the 2D superconductor-insulator transition. An essential requirement is to fully understand how the gate voltage changes the various characteristic properties of the system, such as the mobility, carrier density, critical temperature, critical fields and superfluid density. One important point, for example, is that with back-gating the Hall mobility variation is significantly larger than the change in sheet carrier density [3]. These results indicate that the relative disorder strength increases across the superconductor-insulator transition, and that disorder is the primary control parameter associated with back-gating. I will discuss how this 2D superconductor-insulator transition can be understood, in analogy to thickness variations in other more conventional systems, and from studies of symmetrically confined 2D superconductivity in STO [4,5]. The importance of the strong dielectric nonlinearity in STO at low temperatures will be stressed, leading to the result that the gating phase diagram is a nonlinear function of the starting free carrier density. Our collaborative efforts using real space imaging of the superfluid density by scanning SQUID microscopy, offering an extremely powerful complementary tool to transport studies, will also be discussed, as well as the relationship between the superconductivity and the ferromagnetism in this fascinating system [6]. \\[4pt] [1] A. Ohtomo and H. Y. Hwang, Nature {\bf 427}, 423 (2004).\\[0pt] [2] A. D. Caviglia $et$ $al.$, Nature {\bf 456}, 624 (2008).\\[0pt] [3] C. Bell $et$ $al.$, Phys. Rev. Lett. {\bf 103}, 226802 (2009).\\[0pt] [4] Y. Kozuka $et$ $al.$, Nature {\bf 462}, 487 (2009).\\[0pt] [5] M. Kim $et$ $al.$, arxiv/1106.5193 (2011). \\[0pt] [6] J. Bert $et$ $al.$, Nature Phys. {\bf 7}, 767 (2011). [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:54PM |
X19.00004: Electronic and structural correlations at 2DEG oxide heterointerfaces Invited Speaker: M.S. Rzchowski The formation of a two-dimensional electron gas (2DEG) at complex oxide interfaces is directly influenced by the rich electronic and structural characteristics of the bulk oxides, as well as new phenomena arising at the interface. We investigated how local correlations control oxide 2DEGs by inserting a single atomic layer of a rare-earth oxide (RO) [(R is lanthanum (La), praseodymium (Pr), neodymium (Nd), samarium (Sm), or yttrium (Y)] into an epitaxial SrTiO$_3$ matrix using pulsed-laser deposition with atomic layer control [1]. We find that structures with La, Pr, and Nd ions result in conducting 2DEGs at the inserted layer, whereas the structures with Sm or Y ions are insulating. Our local spectroscopic results, and our theoretical results, indicate that the interfacial conductivity is dependent on electronic and structural correlations that decay spatially into the SrTiO$_3$ matrix and are determined by the local rare-earth ion. We also find that at least the structural correlations play a role in the properties of the 2DEG at LaAlO$_3$/SrTiO$_3$ interfaces. We used different lattice constant single-crystal substrates to produce LaAlO$_3$/SrTiO$_3$ interfaces with controlled levels of biaxial epitaxial strain [2], finding that tensile-strained SrTiO$_3$ destroys the conducting 2DEG, while compressively strained SrTiO$_3$ retains the 2DEG, but with reduced carrier concentration. In addition, the critical LaAlO$_3$ overlayer thickness for 2DEG formation increases with SrTiO$_3$ compressive strain. Our first-principles calculations suggest that a strain-induced electric polarization in the SrTiO$_3$ layer, stabilized by the LaAlO$_3$ overlayer, is responsible for this behavior. This work was done in collaboration with H.W. Jang, C.W. Bark, D.A. Felker, T. Hernandez, Y. Wang, M.K. Niranjan, C.T. Nelson, Y. Zhang, D. Su, C.M. Folkman, S.H. Baek, S. Lee, K. Janicka, H. Zhou, Y. Zhu, X.Q. Pan, D.D. Fong, E.Y. Tsymbal, C. B. Eom. This work was supported by the National Science Foundation through grant DMR-0906443.\\[4pt] [1] H.W. Wang et al., Science {\bf 331}, 886 (2011). \newline [2] C.W. Bark et al., P. Natl Acad Sci USA {\bf 108}, 4720 (2011). [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:30PM |
X19.00005: Electronic structure and two-dimensional electron gas at the surface of SrTiO$_{3}$ Invited Speaker: Andres Felipe Santander-Syro Similar to silicon that is the basis of conventional electronics, strontium titanate (SrTiO$_{3})$ is the bedrock of the emerging field of oxide electronics. SrTiO$_{3}$ is the preferred template to create exotic two-dimensional (2D) phases of electron matter at oxide interfaces, exhibiting metal-insulator transitions, superconductivity, large magnetoresistance, or coexistence of superconductivity and ferromagnetism. However, the physical nature of the electronic structure underlying these 2D electron gases (2DEGs) remains elusive, although its determination is crucial to understand their remarkable properties. In this talk, we present our angle-resolved photoemission spectroscopy (ARPES) results showing that there is a highly metallic universal 2DEG at the vacuum-cleaved surface of SrTiO$_{3}$, independent of bulk carrier densities over more than seven decades, including the undoped insulating material [A. F. Santander-Syro \textit{et al}. Nature \textbf{469}, 189-193 (2011)]. Our data unveil a remarkable electronic structure consisting on multiple subbands of heavy and light electrons. We find that the 2DEG is confined within a region of \textit{$\sim $}5 unit cells and has a sheet carrier density of \textit{$\sim $}0$.$33 electrons per $a^{2}$ ($a $ is the cubic lattice parameter). The similarity of this 2DEG with those reported in SrTiO$_{3}$-based heterostructures and field-effect transistors suggests that different forms of electron confinement at the surface of SrTiO$_{3}$ lead to essentially the same 2DEG. Our discovery provides thus a model system for the study of the electronic structure of 2DEGs in SrTiO$_{3}$-based devices and a novel route to generate 2DEGs at surfaces of other functional oxides. [Preview Abstract] |
Session X20: Invited Session: Physics of Color Reflective Displays
Sponsoring Units: FIAPChair: Gary Gibson, Hewlett Packard - Palo Alto
Room: 253C
Thursday, March 1, 2012 2:30PM - 3:06PM |
X20.00001: Bio-Inspired Adaptive Coloration -- Knowledge Gained by Comparison of Nature and Man-Made Technologies Invited Speaker: Jason Heikenfeld Adaptive coloration, achieved through the use of pigments and reflective surfaces, is used by biological organisms to resemble natural surfaces and/or vividly communicate information. The key to this approach is that light incident on the organism is manipulated to perform the adaptive coloration (i.e., no light is created in the process). Only recently have man-made display technologies (E-paper) attempted to achieve similar adaptive reflective properties. For biological organisms, as well as display technologies, the following features must be controlled simultaneously while minimizing optical losses: pattern, texture, multiple colors, diffuseness, reflectance, and polarization. Many e-Paper technologies have attempted to duplicate optical effects that are utilized in nature. However, to date, they fail in comparison to the optical performance of biological systems. Thus, engineers working on adaptive reflective surfaces may benefit by examining equivalent biological systems in greater detail than previously achieved. On the other hand, intense research and development into adaptive reflective surfaces has given us a mature understanding of the optics of man-made surfaces, and the advanced measurement standards required for scientific involvement. Although this framework currently exists, it is underutilized for the analysis of biological coloration. To advance the field of adaptive coloration, the gap between biology and engineering must be bridged by developing a consistent framework of scientific metrics important to the performance of all platforms of adaptive reflective surfaces. In this presentation, the optics of adaptive coloration are presented in detail. Biological and technological methods are compared based on the construction, physics, and optical performance of each type of adaptive coloration. These comparisons are discussed at the system (organism), device (organ), and pixel/materials (cellular) levels. The main outcomes of this investigation are: display engineers gain insight from techniques perfected in nature; biologists benefit from an understanding of the types of characterization and metrics that could be extracted from biological organisms; and all scientists gain a clearer picture of the long-term prospects for adaptive reflective technologies. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:42PM |
X20.00002: Hewlett-Packard's Approaches to Full Color Reflective Displays Invited Speaker: Gary Gibson Reflective displays are desirable in applications requiring low power or daylight readability. However, commercial reflective displays are currently either monochrome or capable of only dim color gamuts. Low cost, high-quality color technology would be rapidly adopted in existing reflective display markets and would enable new solutions in areas such as retail pricing and outdoor digital signage. Technical breakthroughs are required to enable bright color gamuts at reasonable cost. Pixel architectures that rely on pure reflection from a single layer of side-by-side primary-color sub-pixels use only a fraction of the display area to reflect incident light of a given color and are, therefore, unacceptably dark. Reflective devices employing stacked color primaries offer the possibility of a somewhat brighter color gamut but can be more complex to manufacture. In this talk, we describe HP's successes in addressing these fundamental challenges and creating both high performance stacked-primary reflective color displays as well as inexpensive single layer prototypes that provide good color. Our stacked displays utilize a combination of careful light management techniques, proprietary high-contrast electro-optic shutters, and highly transparent active-matrix TFT arrays based on transparent metal oxides. They also offer the possibility of relatively low cost manufacturing through roll-to-roll processing on plastic webs. To create even lower cost color displays with acceptable brightness, we have developed means for utilizing photoluminescence to make more efficient use of ambient light in a single layer device. Existing reflective displays create a desired color by reflecting a portion of the incident spectrum while absorbing undesired wavelengths. We have developed methods for converting the otherwise-wasted absorbed light to desired wavelengths via tailored photoluminescent composites. Here we describe a single active layer prototype display that utilizes these materials along with an innovative optical out-coupling scheme. Further benefits of our approach include means for highly power-efficient back-lighting under low ambient light conditions and the possibility of video rate operation. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 4:18PM |
X20.00003: Opalux Photonic Ink: Full-color, bistable, reflective displays Invited Speaker: Andre Arsenault |
Thursday, March 1, 2012 4:18PM - 4:54PM |
X20.00004: Cholesteric Liquid Crystal Based Reflex Color Reflective Displays Invited Speaker: Asad Khan Bistable color cholesteric liquid crystal displays are unique LCDs that exhibit high reflectivity, good contrast, extremely low power operation, and are amenable to versatile roll-to-roll manufacturing. The display technology, now branded as Reflex{\texttrademark} has been in commercialized products since 1996. It has been the subject of extensive research and development globally by a variety of parties in both academic and industrial settings. Today, the display technology is in volume production for applications such as dedicated eWriters (Boogie Board{\texttrademark}), full color electronic skins (eSkin), and displays for smart cards. The flexibility comes from polymerization induced phase separation using unique materials unparalleled in any other display technology. The blend of monomers, polymers, cross linkers, and other components along with nematic liquid crystals and chiral dopants is created and processed in such ways so as to enable highly efficient manufactrable displays using ultra thin plastic substrates -- often as thin as 50$\mu $m. Other significant aspects include full color by stacking or spatial separation, night vision capability, ultra high resolution, as well as active matrix capabilities. Of particular note is the stacking approach of Reflex based displays to show full color. This approach for reflective color displays is unique to this technology. Owing to high transparency in wavelength bands outside the selective reflection band, three primarily color layers can be stacked on top of each other and reflect without interfering with other layers. This highly surprising architecture enables the highest reflectivity of any other reflective electronic color display technology. The optics, architecture, electro-topics, and process techniques will be discussed. This presentation will focus on the physics of the core technology and color, it's evolution from rigid glass based displays to flexible displays, development of products from the paradigm shifting concepts to consumer products and related markets. This is a development that spans a wide space of highly technical development and fundamental science to products and commercialization to enable the entry of the technology into consumer markets. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:30PM |
X20.00005: Mirasol displays: the revolution in display technology Invited Speaker: Brian Gally |
Session X21: Fluctuation Phenomena (noise, nonequilibrium effects, localization effects)
Sponsoring Units: DCMPChair: Carmen Almasan
Room: 254A
Thursday, March 1, 2012 2:30PM - 2:42PM |
X21.00001: Fluctuoscopy of Disordered Two-Dimensional Superconductors Andreas Glatz, Andrey Varlamov, Valerii Vinokur In this talk I will present our results for the fluctuation conductivity (FC) in disordered two-dimensional superconductors placed in a perpendicular magnetic field. In our works [1,2] we finally derived the complete solution in the temperature-magnetic field phase diagram. The obtained expressions allow both to perform straightforward (numerical) calculation of the FC surface $\delta\sigma(T,H)$ and to get all 27 asymptotic expressions in the seven qualitatively different domains of the phase diagram. This surface becomes in particular non-trivial at low temperatures, where it is trough-shaped and close to the quantum phase transition non-monotonic, in agreement with experimental findings. I will show our main results and demonstrate how these can be used as a high precision tool (fluctuoscope) to determine the critical temperature, critical magnetic field, and dephasing time from experimental data in superconducting films. \\[4pt] [1] A. Glatz, A. A. Varlamov, and V. M. Vinokur, EuroPhys. Lett. {\bf 94}, 47005 (2011).\\[0pt] [2] A. Glatz, A. A. Varlamov, and V. M. Vinokur, Phys. Rev. B {\bf 84}, 104510 (2011). [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X21.00002: Majorana modes in a superconducting wire with quasiperiodic and Wade DeGottardi, Manisha Thakurathi, Diptiman Sen, Smitha Vishveshwara We present a systematic study of the role quasiperiodic and disordered potentials play in the topology of 1D $p$-wave superconducting systems based on a lattice model which we analyze using a transfer matrix approach. We employ a similarity transformation to demonstrate that the existence of Majorana modes is intimately connected to the band structure of the corresponding normal state system (i.e one which, though otherwise identical, lacks superconducting order). We illustrate this correspondence using the case of an electron moving in a quasiperiodic potential. The Hamiltonian of this system is the so-called ``almost Mathieu operator'' whose bulk spectrum (for a certain choice of parameters) is described by a fractal known as Hofstadter's butterfly. Specifically, we prove that states belonging to this spectrum host end Majoranas for arbitrarily weak superconductivity and that increasing the magnitude of the superconducting gap causes these topologically non-trivial regions of the phase diagram to expand and fill in the butterfly. We show that similar considerations give us excellent theoretical control over the topological phase diagram of systems with disordered potentials. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X21.00003: A Preformed Pair Approach to Diamagnetism in the Cuprates Vivek Mishra, Dan Wulin, K. Levin Enhanced diamagnetism extending beyond the critical regime is associated with the pseudogap. It has been one of the key experiments supporting the precursor superconductivity scenario for this normal state gap. In this talk we demonstrate how non-condensed pairs (rather than normal state vortices) yield a large low field diamagnetic signal. Our work is based on a 3d BCS-BEC crossover scenario where the diamagnetic susceptibility calculation is well controlled and built on a sum-rule compatible correlation function approach. We demonstrate reasonable semi-quantitative agreement with experiment, with no adjusted parameters. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X21.00004: Transport theory of superconductors in paramagnetic limit Maxim Khodas, Alex Levchenko, Gianluigi Catelani We report on the study of the quantum phase transition from metastable normal to superconductive state in thin films driven by in-plane magnetic field. In this system resistivity exhibits hysteresis at low temperature when the strong Zeeman field is gradually turned off and on. Quantum fluctuations smear the transition form the metastable normal state to paramagnetically limited superconductivity. The typical energy scale for fluctuations is $\bar{\Omega}=\sqrt{E_z^2 - \Delta^2}$, where $E_z$ is the Zeeman energy and $\Delta$ is zero temperature gap. At the onset of the transition $\bar{\Omega}\rightarrow 0$ quantum fluctuations cause non-analytic corrections to the conductivity. The most singular corrections are presented. The result is strongly sensitive to the strength of the spin-orbit scattering. We argue that our theory qualitatively agrees with experimental findings provided the spin-orbit scattering is taken into account. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X21.00005: Proliferation of mesoscopic effects in transport of superconductors Mengling Zhang, Alex Levchenko Universality of conductance fluctuations is the hallmark of mesoscopic physics. This phenomenon emerges from the quantum coherence of electron trajectories and is sensitive to changes in external magnetic field or gate voltage. There exists compelling physical evidence, ranging from the experiments in sub-micron scale superconducting rings to granular films driven across superconductor-insulator transition, that the role of mesoscopic fluctuations proliferate in the presence of superconducting correlations. We thus study theoretical the fate of universality of mesoscopic fluctuations in superconductors focusing on the kinetic characteristics such as conductance, thermopower, NMR relaxation rate, spin susceptibility etc. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X21.00006: Superconducting fluctuation regime in the cuprates revealed by torque magnetometry Guichuan Yu The extent of the superconducting fluctuation regime in the normal state of the cuprate superconductors has remained unclear. For the single-CuO$_{2}$-layer compounds La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO) and Bi$_{2}$(Sr,La)$_{2}$CuO$_{6+\delta }$ (Bi2201), one class of experiments indicates characteristic temperatures as high as 2-3 times $T_{c}$ at optimal doping, whereas a second class reveals superconducting fluctuations in a relatively narrow temperature range above $T_{c}$. Here we report a systematic torque magnetometry study of the superconducting fluctuation regime in three single-layer compounds, LSCO, Bi2201 and HgBa$_{2}$CuO$_{4+\delta }$. We find in all three cases that the regime of fluctuating diamagnetism is narrow and closely tracks the doping dependence of $T_{c}$, consistent with the second class of experiments [1]. The seemly controversial results can be understood if short-range phase correlations develop only in the vicinity of $T_{c}$, whereas local pair formation appears at a relatively high temperature that is universal among all single-layer cuprates. \\[4pt] [1] G. Yu, D.-D. Xia, N. Bari\v{s}ic, R.-H. He, N Kaneko, Yangmu Li, Yuan Li, T. Sasagawa, A. Shekhter, X. Zhao and M. Greven, unpublished [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X21.00007: The potential for persistent current measurements in mesoscopic rings of underdoped cuprates Thomas Lippman, Ilya Sochnikov, Kathryn Moler Dimensionality, disorder, order parameter symmetry, competing phases, quantum critical points, thermal phase fluctuations, and quantum phase fluctuations all play a role in the physics of the underdoped cuprates. Measurements of the persistent current in mesoscopic rings near phase transitions can provide valuable information about underlying coherent states. We evaluate the prospects for such measurements in the underdoped cuprates, considering both likely signatures of various theoretical scenarios and experimental feasibility in terms of fabrication and signal levels. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X21.00008: Cantilever torque magnetometry study of multiply connected BSCCO arrays near Tc Grigoriy Polshyn, Raffi Budakian The goal of this work is to study the superconducting coherence length in the fluctuation regime in cuprate superconductors. In this work we present cantilever torque magnetometry measurements of micron-size BSCCO flakes patterned with arrays of nanometer scale rings or holes. Using ultrasensitive dynamic torque magnetometry, oscillations in magnetization are observed near Tc as a function of the applied magnetic flux threading the array. Special effort was made to detect the oscillations in magnetization at temperatures above Tc, where the Nernst effect and magnetization measurements suggest the possibility of pairing. To constrain the magnitude of the coherence length in the fluctuation regime, we will present the dependence of the amplitude of the h/2e period oscillations as a function of temperature and hole size. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X21.00009: Properties of ultrathin Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ films grown by pulsed laser deposition Michael Hinton, Jie Yong, Stanley Steers, Adam Ahmed, John Draskovic, Tom Lemberger Thermal and quantum fluctuations in superconductors are expected to grow as film thickness decreases. Such behavior has been observed in YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ (YBCO) films as thickness is reduced to two unit cells. In particular, a Kosterlitz-Thouless like drop in superfluid density appears and shows that 2D fluctuations are correlated through the YBCO film thickness. Since Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+x}$ (Bi-2212) plays such a prominent role in important ARPES and Scanning Probe measurements of the superconducting gap, it is important to see how this compound behaves in reduced dimensions. To that end, we are working to grow films of Bi-2212 as thin as possible, by pulsed laser deposition in on-axis and off-axis geometries. One goal is to look for the KT transition in the superfluid density, to see whether 2D fluctuations are correlated layer-to-layer even in this highly anisotropic compound, and complement the cuprate picture dominated by the wealth of YBCO data. This requires films to be quite homogeneous. To effectively improve homogeneity, we focus the ac magnetic field of our two-coil measurement system by masking the film of interest with a thick superconducting film with a hole etched through it. Thus, the film area probed is greatly reduced and transition widths are narrowed. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X21.00010: Ghost critical field and weak localization phenomena in superconducting Tantalum Nitride films Nicholas Breznay, Aharon Kapitulnik We study the appearance of superconducting fluctuations and weak localization effects in disordered conducting thin films using magnetotransport measurements. At temperatures above Tc, we observe a positive magnetoresistance that is 4 orders of magnitude larger than the predicted classical effect. Well above Tc this behavior is consistent with the magnetic field dependence of localization quantum corrections to the conductivity in the presence of strong spin-orbit scattering. Close to Tc the observed magnetoresistance is well described by theories that describe both localization and superconducting fluctuations effects. This analysis allows for careful study of the so-called ghost critical field and inelastic scattering rates close to Tc. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X21.00011: Coexistence of superconducting gap and pseudogap above and below $T_c$ in cuprates Jeffery Tallon, James Storey, John Loram We develop a new method to calculate the full $T$-dependent superconducting (SC) gap from thermodynamic measurements and apply to the cuprates. We find that the SC gap persists high above $T_c$ due to strong SC fluctuations and coexists there with the pseudogap. This allows the (disputed) phase diagram to be mapped more accurately than previously. We apply the results below $T_c$ to understand the doping and temperature evolution of critical currents in practical conductors. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X21.00012: On-chip detection of photon-assisted shot noise and photocurrent of quantum point contact Y. Jompol, P. Roulleau, F. Portier, P. Roche, D.C. Glattli, I. Farrer, D.A. Ritchie We present the first experimental realization of on-chip shot noise detection in capacitively-coupled quantum point contacts (QPCs). The detection is based on the photon-assisted effects. A dc voltage biased QPC: the emitter generates broad frequency current shot noise, inducing voltage fluctuations on the second QPC: the detector. This yields photon induced electron-hole pairs whose partitioning between left and right contact causes a current noise called photon-assisted shot noise (PASN). Alternatively, the electron-hole pairs may also generate a photon-assisted current for energy-dependent transmission. Both the photocurrent and PASN are proportional to the product D(1-D), where D is the transmission of a QPC. Our sample is realized using a set of quantum point contacts positioned onto two separate mesas of a two-dimensional electron gas. The QPC emitter is DC biased and emits high-frequency shot noise that is converted into voltage fluctuations at the QPC detector via an interdigital capacitor, $C$. Two additional QPCs next to the emitter and the detector serve as tunable quantum resistors and are used to control impedance of the circuit. We show that by varying both the emitter and detector transmissions, D$^{E}$, D$^{D}$, the measured current at the QPC detector is proportional to D$^{D}$(1-D$^{D})$ D$^{E}$(1-D$^{E})$ for a given V$_{ds}$, which is in good agreement with the theoretical prediction. This new way of detection should find fundamental applications in electronic quantum physics. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X21.00013: Direct access to quantum non-Gaussian noise through cross-correlation measurements Iurii Chernii, Eugene Sukhorukov Detection of quantum non-Gaussian fluctuations is often difficult since they are dominated by the classical Gaussian noise, and requires the use of high frequency amplifiers. In our work we investigate the possibility to employ the cross-correlation technique in order to overcome these difficulties. We propose to measure the cross-correlator of outputs of a pair of two-level detectors, coupled to the source of fluctuations via an electric circuit. In the weak coupling regime, the noise induces rare stochastic transitions in the detectors, that allows one to perform the long time measurement. The transition rates can be derived from the evolution of the density matrix, calculated to the fourth order in level mixing of the two-level detectors. We express the cross-correlator in terms of these rates, and demonstrate that there is a range of parameters, where the main contribution to the cross-correlator is proportional to the intensity of the quantum non-Gaussian noise. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X21.00014: Interferometric and Noise Signatures of Majorana Fermion Edge States in Transport Experiments Wolfgang Belzig, Gregory Struebi, Mahn-Soo Choi, Christoph Bruder Domain walls between superconducting and magnetic regions placed on top of a topological insulator support transport channels for Majorana fermions. We propose to study noise correlations in a Hanbury Brown-Twiss type interferometer and find three signatures of the Majorana nature of the channels. First, the average charge current in the outgoing leads vanishes. Furthermore, we predict an anomalously large shot noise in the output ports for a vanishing average current signal. Adding a quantum point contact to the setup, we find a surprising absence of partition noise which can be traced back to the Majorana nature of the carriers. [Preview Abstract] |
Session X22: Focus Session: Fe-based Superconductors - Exchange Coupling Theory
Sponsoring Units: DMP DCOMPChair: Thomas Maier, Oak Ridge National Laboratory
Room: 254B
Thursday, March 1, 2012 2:30PM - 3:06PM |
X22.00001: Strong Versus Weak Coupling Pairing in Iron-Based Superconductors Invited Speaker: B. Andrei Bernevig We use the functional renormalization group as well as strong-coupling methods to analyze the phase diagram of several of the iron-based superconductors. As in the previous studies by F. Wang, D.H. Lee et. al., we observe a nodeless sign-changing order parameter to be favored over a sizable part of the parameter space, but the physics quickly develops peculiarities depending on the doping, shape, size and orbital content of the Fermi surfaces in the different superconducting compounds. Using several new one-body models available in the literature (due to Kuroki, Graser and Raghu), we analyze the orbital content of the superconducting gap, which should be observable in spin-polarized ARPES experiments. We find that the effective theory of the Iron-based superconductors is a $J_1-J_2$ model in orbital space with $J_2> |J_1|$ and antiferromagnetic, and analyze the behavior of the physical properties such as superconducting gap for systems ranging from electron overdoped $(K, Cs)Fe_{2?x}Se_x$ to hole-doped $ K_{x}Ba_{1-x}Fe_2As_2$. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X22.00002: Hund's metal physics in iron-based superconductors Gabriel Kotliar, Zhiping Yin, Kristjan Haule The role of Hubbard U and Hund's J in a material depends on the energy scale of the crystal field splitting. In transition metal oxides, the crystal field splitting is usually considerably larger than Hund's J thus Hubbard U plays the dominating role. However, the crystal field splitting in iron-based superconductors is substantially smaller and the physics in this family is governed primarily by Hund's rule. In this talk, we will show that the combination of density functional theory and dynamic mean field theory properly incorporates the Hund's physics as well as realistic band structure thereby is well suited to capture and predict a wide range of physical properties and their trends in iron pnictides and chalcogenides, such as optical conductivity, x-ray spectroscopy, Fermi surface, magnetic ordering and moments, spin excitations, effective masses and so on. We will demonstrate two important mechanisms operating in this family, namely, Hund's blocking and Kinetic frustration. The importance of electronic correlation caused by the Hund's physics and its relation to various experimental observations will also be discussed. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X22.00003: Consistent theory of magnetism and superconductivity in iron pnictides Michail Katsnelson, Vladimir Antropov, Liqin Ke, Mark van Schilfgaarde We show that iron pnictide systems possess unusual quantum spin fluctuations which strongly affect their magnetic properties and may be relevant in the mechanism of superconductivity. These fluctuations represent highly non-linear aharmonic excitations and have both transversal and longitudinal components which may contribute differently to the observed properties. From the point of view of magnetism, these fluctuations are responsible for the stability of the observed magnetic ground states, and thus determine the spin-wave spectra. The anharmonic character of the excitations under consideration provides a strong coupling with electron degrees of freedom which may be relevant for the appearance of high-temperature superconductivity. We discuss a mechanism of superconductivity related to spin fluctuation which uses the same pool of fluctuations to explain the Cooper pairing. Theory predicts some correlations between superconductivity temperature and magnetic characteristics and they seem to be in agreement with the available experimental data for pnictides and selenides. We discuss also other materials where one can expect a similar behavior. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X22.00004: Giant spin zero-point fluctuations in iron superconductors Vladimir Antropov, Pawel Buczek, Frank Essenberger, Arthur Ernst, Leonid Sandratskii, E.K.U. Gross We analyze the strength of quantum spin fluctuations in novel iron superconductors. By using linear response calculations and the density functional approach, we show that many of these materials can be classified as highly responsive magnetic materials with very strong non-linear quantum spin fluctuations. Furthermore, by using the newly developed theory of magnetic instabilities in systems with strong spin zero-point motion, we show that the inclusion of fluctuations dramatically improves an agreement with the experiment for systems such as CaFe$_{2}$As$_{2}$, LiFeAs and FeSe, while for FeTe, the results are less affected by fluctuations. We demonstrate how spin fluctuations influence the criteria of local magnetic moment and long range magnetic order existence. In addition to iron superconductors, several other groups of materials with possible giant quantum spin fluctuations are identified. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X22.00005: Spectral and magnetic properties of the iron-based superconductors: The role of electronic correlation Alessandro Toschi, P. Hansmann, R. Arita, S. Sakai, G. Sangiovanni, K. Held Electronic correlation plays a subtle role in Fe-based superconductors. In fact, due to the presence of several moderately correlated bands close to the Fermi level, one observes the formation of localized magnetic moments driven by the Hund's exchange interactions, which takes place, however, in a mainly metallic background (``Hund's metal'' [1]). This physical scenario provides the key to understand [2,3] the discrepancies observed between experimental estimates of the magnetic moments in the magnetically ordered phase and those obtained via standard LSDA calculations. The magnitude of the discrepancy observed in different compounds would be hence related to the efficacy of the metallic screening, which is decreasing when going from the 1111 (e.g., LaFeAsO) to the 122 class, and eventually to the 11 materials (like FeTe). Also important to be considered for the interpretation of the ARPES experiments and of the symmetry of the superconducting pairing within the Hund's exchange scenario is the interplay between the electronic correlations and the details of the band-structure of the specific compound considered.\\ \\ \noindent [1] K. Haule and G. Kotliar, NJP {\bf 11} 025021 (2009). [2] P. Hansmann, {et al.}, PRL {\bf 104}, 197002 (2010). [3] A. Toschi, {et al.} in preparation [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X22.00006: {\it Ab initio} Evidence of Strong Correlation Associated with Mott Proximity in Iron-Based Superconductor Takahiro Misawa, Kazuma Nakamura, Masatoshi Imada Recently discovered iron-based superconductors have attracted much interest because of their high superconducting critical temperatures ($T_{c}$). Although it is believed that electron correlations play key roles in the unconventional high-$T_{c}$ superconductivity, their roles are not fully understood yet. To clarify electron correlation effects from a microscopic point of view, we study the {\it ab initio} low-energy effective models for iron-based superconductors by using multi-variable variational Monte Carlo (mVMC) method. From the {\it ab initio} calculations, we show that the iron-based superconductors found around $d^6$ configuration (namely, five Fe 3$d$ orbitals filled by 6 electrons on average) are under the umbrella of an unexpectedly large-scale dome of correlated-electron matter centered at the Mott insulator at $d^5$ (namely, half filling). This proximity of the large-dome of strong electron correlations yields a variety with bad insulating (or incoherent metallic) states, quantum criticality of antiferromagnetism, orbital fluctuations and differentiations arising from interplay between the Hund's rule coupling and Mott physics. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X22.00007: Two-orbital quantum spin model of magnetism in the iron pnictides Chen Liu, Dao-Xin Yao, Anders Sandvik We study a two-orbital spin model to describe $(\pi,0)$ stripe antiferromagnetism in the iron pnictides. The ``double-spin'' model has an on-site Hunds's coupling and inter-site interactions extending to second neighbors on the square lattice. Using a variational method based on a cluster decomposition, we optimize wave functions with up to $8$ cluster sites (up to $2^{16}$ variational parameters). We focus on the anomalously small ordered moments in the stripe state of the pnictides. To account for it, and large variations among different compounds, we show that the second-neighbor cross-orbital exchange constant should be ferromagnetic, which leads to ``partially hidden'' stripe order. In a different parameter region, we confirm a canted state previously found in spin-wave theory. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X22.00008: Spin waves of 2D $J_1$-$J_2$ model and single hole dynamics via nearest neighbor hole hopping Satyaki Kar We study the two dimensional $J_1$-$J_2$ model using the spin wave approximation and find the spin wave modes at different values of parameter $\lambda$=$J_2/J_1$. Competing antiferromagnetic ($\pi,\pi$) and ($\pi,0$) (or, ($0,\pi$)) orders preferred by the $J_1$ and $J_2$ terms respectively creates frustration in such a system and the spin wave excitations about different vacuum states are observed for different values of $\lambda$. We investigate the stability of different zero spin deviation states as $\lambda$ is varied from 0 to 1.5. We discuss the band folding of the super-lattice structure that appears with the emergence of the $J_2$ term and study the different spin wave modes that the system inherits. Dynamics of a foreign hole put in such a system is studied using a $t$-$J_1$-$J_2$ model and the hole spectra obtained by solving the Dyson's equation self-consistently within the non-crossing approximation are compared with the ARPES spectra from the newly discovered FeAs superconducting compounds. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X22.00009: An unified minimum effective model of magnetism in iron-based superconductors Jiangping Hu, Bao Xu, Wuming Liu, Ningning Hao, Yupeng Wang Since 2008, many new families of iron-based high temperature (high-Tc) superconductors have been discovered. Unlike all parent compounds of cuprates that share a common antiferromagnetically (AF) ordered ground state, those of iron-based superconductors exhibit many different AF ordered ground states, including collinear-AF (CAF) state in ferropnictides, bicollinear-AF (BCAF) state in 11-ferrochalcogenide FeTe, and block-AF vacancy (BAFv) order state in 122-ferrochalcogenide K0.8Fe1.6Se2. While the universal presence of antiferromagnetism suggests that superconductivity is strongly interrelated with magnetism, the diversity of the AF ordered states obscures their interplay. Here we show that all magnetic phases can be unified within an effective magnetic model. This model captures three incommensurate magnetic phases as well, two of which have been observed experimentally. The model characterizes the nature of phase transitions between the different magnetic phases and explains a variety of magnetic properties, such as spin-wave spectra and electronic nematism. Most importantly, by unifying the understanding of magnetism, we cast new insight on the key ingredients of magnetic interactions which are critical to the occurrence of superconductivity. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X22.00010: Spin dynamics of the J1-J2 Model with Biquadratic Spin Interactions for the Paramagnetic Phase of the Iron Pnictides Zhentao Wang, Rong Yu, Pallab Goswami, Andriy Nevidomskyy, Qimiao Si, Elihu Abrahams The parent compounds of the iron pnictides are bad metals, and are hence interpreted as being located at the boundary of localization and itinerancy. As such, the effective exchange interactions will naturally include more than two spin components. Here we study the $J_1-J_2$ antiferromagnetic Heisenberg model with a biquadratic spin-spin coupling $-\kappa (\vec{S}_i \cdot \vec{S}_j)^2$ to explore the spin excitations in the paramagnetic phase of the iron pnictides which has a $(\pi,0)$ collinear antiferromagnetic ground state. By using the modified spin wave theory and Schwinger Boson mean field theory, we determine the spin dynamics at finite temperatures. We show that a moderate biquadratic coupling $\kappa$ can induce sizable anisotropy in the spin excitation spectrum. The calculated dynamical structure factor $S(\vec{q},\omega)$ shows anisotropic elliptic features near $(\pi,0)$, which expand with increasing frequency ($\omega$), in a way that agrees with recent experiment on BaFe$_2$As$_2$ above its Neel transition temperature (L.~W.~Harriger {\it et.al}, PRB {\bf 84}, 054544, 2011). [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X22.00011: Dimensional crossover in the quasi-two-dimensional Ising-O(3) model Naoki Kawashima, Yoshitomo Kamiya, Cristian Batista We present results of our Monte Carlo simulation of the Ising-O(3) model on the two-dimensional (2D) and quasi-2D lattices. This model is an effective classical model for the stacked square-lattice $J_1$-$J_2$ Heisenberg model where the nearest neighbor ($J_1$) and next-nearest neighbor ($J_2$) couplings are frustrated and we assume that $J_2$ is dominant. We find an Ising ordered phase where the O(3) spins remain disordered in a moderate quasi-2D region. There is a single first order transition for a sufficiently large 3D coupling in agreement with a renormalization group treatment. The subtle region where the single transition splits into two transitions is also discussed and compared against recent measurements of two very close transitions in BaFe$_2$As$_2$. Our results can provide a qualitative explanation on the experiments on ferropnictides, namely observed sequence and orders of the structural and magnetic transitions, in terms of the ratio between the inter-layer and intra-layer coupling. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X22.00012: Are Spinwaves Glue for Cooper Pairs in Iron-Pnictide High-T$_{\rm c}$ Superconductors? Jose Rodriguez We study the 2-orbital t-J model over the square lattice via Schwinger-boson-slave-fermion mean-field theory and by exact numerical diagonalization of two holes over a 4$\times$4 grid. The two orbitals in question are the degenerate $d\pm = 3d_{(x\pm iy)z}$ ones, which maximize the Hund's Rule coupling. The mean-field theory predicts the existence of a quantum critical point (QCP) that separates a commensurate spin-density-wave (cSDW) metal at strong Hund's Rule coupling from a hidden half metal at weak Hund's Rule coupling. Holes in the hidden half metal hop through a $\nwarrow_{d+}\searrow_{d-}$ spin background without much hopping across orbitals. Mean-field theory further predicts a critical spin-wave spectrum that shows hidden low-energy excitations at zero momentum, and that shows observable low-energy excitations at cSDW momenta. We find that the virtual exchange of such spin-waves by mobile holes can be attractive, and that it can result in the formation of hole pairs. We seek to corroborate this by exact diagonalization of two holes in the 2-orbital t-J model. [Preview Abstract] |
Session X23: Magnetic Field Effects: Experimental II
Sponsoring Units: GMAGChair: Milind Kunchur, University of South Carolina
Room: 255
Thursday, March 1, 2012 2:30PM - 2:42PM |
X23.00001: Investigating the field evolution of four-fold anisotropy in the basal plane of TmNi$_2$B$_2$C P. Das, C. Rastovski, K. Schlesinger, M.R. Eskildsen, J.M. Densmore, S.L. Bud'ko, P.C. Canfield The superconductor TmNi$_2$B$_2$C possesses a significant 4-fold in-plane anisotropy originating from the Fermi surface and possibly also from the superconducting pairing. However, unlike other members of the borocarbide superconductors, the anisotropy appears to decrease with increasing field, attributed to strong Pauli paramagnetic effects (PPE) and a vortex core expansion close to $H_{c2}$. We have investigated the field evolution of the four-fold anisotropy by small-angle neutron scattering (SANS) of the vortex lattice (VL), measuring several higher order Bragg peaks which allow a real space reconstruction of the VL field modulation. The measurements are possible due to the PPE which lead to a large field modulation (e.g. $\sim$~65 \% of the applied field at $0.2$~T). Our results present the first {\em direct} demonstration of the decreasing anisotropy and furthermore, allows this to be measured quantitatively. This provides an explanation for the reentrant square VL phase observed in TmNi$_2$B$_2$C. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X23.00002: Vortex Lattice Anisotropy in Sr$_2$RuO$_4$ with $H \parallel a$ M.R. Eskildsen, C. Rastovski, C.D. Dewhurst, W.J. Gannon, D.C. Peets, H. Takatsu, Y. Maeno We have studied the vortex lattice (VL) in superconducting Sr$_2$RuO$_4$ using spin-polarized small-angle neutron scattering (SP-SANS) and with magnetic fields ($H$) close to, but not perfectly aligned with, the crystalline basal plane. In this configuration the VL possesses a large transverse field component, due to the large anisotropy of Sr$_2$RuO$_4$, which greatly increases the spin-flip SANS scattered intensity. Bragg reflections indicative of a highly distorted hexagonal VL were observed, and an analysis of the magnitude of the scattering vector yielded an anisotropy of $50 - 60$, roughly two times the $H_{c2}$ anisotropy. We discuss implications of this result in relation to theoretical predictions of Pauli paramagnetic effects in Sr$_2$RuO$_4$ with $H \perp c$ [K. Machida and M. Ichioka, Phys. Rev. B {\bf 77}, 184515 (2008)]. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X23.00003: Influence of intrinsic superconducting parameters on the vortex dynamics and critical currents of different superconductor materials N. Haberkorn, Jeehoon Kim, I. Usov, B. Maiorov, M. Weigand, L. Civale Vortex physics became a major field in condensed matter and statistical physics since the discovery of the oxide high temperature superconductors (HTS). Although the rich HTS vortex phenomenology arises from the large influence of thermal fluctuations, there is no hard boundary between HTS and conventional low temperature superconductors. The discovery of the iron-based superconductors provided a chance to study vortex matter in a new family of materials with broad ranges of superconductor transition temperature and crystalline anisotropy, where the small coherence length in some of them results in large fluctuation effects similar to those found in the oxide HTS. In this presentation, we will discuss the vortex dynamics of iron arsenide and oxide HTS by performing magnetization measurements of the critical current density ($J_{c})$ and flux creep rate ($S)$. We will analyze the different creep regimes and crossovers that appear in the different samples and their relations to intrinsic superconducting parameters. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X23.00004: Electrostatic manipulation of magnetic flux quanta at the nanoscale Arnaud Crassous, Rozenn Bernard, Stephane Fusil, Karim Bouzehouane, Javier Briatico, Manuel Bibes, Agnes Barthelemy, Javier Villegas The electrostatic tuning of physical properties in materials offers significant potential in a large variety of systems. For example, the application of an electric field allows depressing or enhancing superconductivity in certain oxides. Using heterostructures that combine a large-polarization ferroelectric (BiFeO3) and a high-temperature superconductor (YBa2Cu3O7-x), we demonstrate here the nanoscale modulation of the superconducting condensate via ferroelectric field effects [1]. The ability to design the ferroelectric domain structure at will enables us to create nanoscale ``patterns'' of normal regions within the superconductor, in a reversible and modifiable way. This produces an energy landscape for magnetic flux quanta and, in turn, couples the local polarization in the ferroelectric to the local magnetic induction in the superconductor. This new form of magnetoelectric coupling allows the electrostatic manipulation of magnetic flux quanta. \\[4pt] [1] A. Crassous et al., Phys. Rev. Lett. in press (2011) [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X23.00005: Strong periodic flux pinning in oxygen-ion-irradiated high-T$_{C}$ superconductors Javier Villegas, I. Swiecicki, J. Briatico, R. Bernard, A. Crassous, T. Wolf, N. Bergeal, J. Lesueur, C. Ulysse, G. Faini, X. Hallet, L. Piraux We used oxygen ion irradiation to transfer into high-T$_{C}$ superconducting thin films the nanoscale pattern of different types of masks (alumina [1] and [2] PMMA templates with ordered arrays of holes). This causes a nanoscale spatial modulation of superconductivity, and strongly affects the magneto-transport in the mixed-state. By tuning the irradiation dose and the array parameters, it is possible to engineer vortex energy landscapes sufficiently strong to govern flux dynamics. This is evidenced by a periodic series of strong magneto-resistance oscillations, the well-known fingerprint of periodic flux pinning. Interestingly, this irradiation technique allows tuning the \textit{geometry} and the \textit{strength} of the pinning potential wells at the nanoscale. This allows the observation of unusually strong matching effects at relatively high fields (up to several kOe). We show that the amplitude of the magneto-resistance oscillations is intimately connected with vortex channeling effects. [1] J.E. Villegas \textit{et al.} Nanotechnology\textbf{ 22}~075302~(2011). [2] I. Swiecicki \textit{et al.} submitted [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X23.00006: Single vortex manipulation in superconducting NdFeAsO$_{1-x}$F$_x$ Magdalena Huefner, Jeehoon Kim, Matt Tillman, Paul Canfield, Jennifer Hoffman Vortex pinning challenges have severely hampered attempts to incorporate cuprate high $T_c$ superconductors into technology. Understanding and improving the pinning of quantized magnetic vortices in high $T_c$ superconductors remains an important challenge. We use a homebuilt low temperature magnetic force microscope to image and manipulate individual vortices in single crystal NdFeAsO$_{1-x}$F$_x$ ($T_c$=50K). By exerting a large force on a single vortex we can deliberately depin it from its original position and permanently move it to a different, predetermined position. We can also drag the top of a single vortex for a short distance without permanently depinning its full length. By dragging individual vortices along different directions, we observe a 4-fold anisotropy of the dragging distance, with the easy-drag direction along the Fe-Fe axis. Our results shed light on the questions of anisotropy and pinning mechanisms in iron pnictides. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X23.00007: Anisotropy and Vortex Pinning of Heavy Ion irradiated SmFeAsO$_{0.8}$F$_{0.15}$ and BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ Crystals Wai-Kwong Kwok, Lei Fang, Carlos Chaparro, Ying Jia, Ulrich Welp, Alexei Koshelev, Shaofei Xu, George Crabtree, Janusz Karpinski We report specific heat and magnetization measurements on SmFeAsO$_{0.8}$F$_{0.15}$ and BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ single crystals irradiated with high energy heavy ions of 1.4GeV Pb to dose matching fields up to 4 Tesla. We find a nearly one half reduction in the superconducting anisotropy and doubling of the irreversibility field in SmFeAsO$_{0.8}$F$_{0.15 }$after irradiation and virtually no change in the zero-field superconducting transition temperature. In both SmFeAsO$_{0.8}$F$_{0.15}$ and BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ crystals, we find a substantial increase in the critical current determined from SQUID and micro-Hall probe magnetization measurements. Pinning force analysis on proton and heavy ion irradiated pristine overdoped BaFe$_{2}$(As$_{1-x}$P$_{x})_{2}$ crystals indicates presence of induced $\Delta $T$_{c}$-type pinning defects in these samples. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X23.00008: High Dose Heavy-Ion Irradiation Effects on the Multiband Superconductor (Ba$_{0.6}$K$_{0.4})$Fe$_{2}$As$_{2}$ Lei Fang, Carlos Chaparro, Goutam Sheet, Ying Jia, Shao-Fei Zhu, He-Fei Hu, Jian-Min Zuo, Hai-Hu Wen, Ulrich Welp, Alexei Koshelev, George Crabtree, Wai-Kwong Kwok Optimal doped crystals of (Ba$_{0.6}$K$_{0.4})$Fe$_{2}$As$_{2 }$were irradiated with 1.4 GeV Pb ions to dose-matching fields ranging from 4 Tesla to 21 Tesla. Plan-view transmission electron microscopy shows creation of defects with diameters of 2 $\sim $ 5 nm. Post-irradiation characterization shows that the superconducting anisotropy is reduced to near unity, probably due to the increase in intra-band scattering. In addition, the critical current density $J_{C}$ determined from magnetization measurements shows systematic enhancement up to $\sim $5 MA/cm$^{2}$ at T=5K. We show that the decay of the critical current with magnetic field can be greatly mitigated with dense defects with approximately 20nm spacing produced by a dose matching field irradiation of 21T. Remarkably, the superconducting transition temperature remain unchanged for all matching field irradiation, suggesting that inter-band scattering due to non-magnetic impurity does not play a dominant role in pair-breaking. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X23.00009: Angular dependence of magneto-transport properties in Ba$_{1-x}$K$_{x}$(FeAs)$_{2}$ and Ba (Fe$_{1-x}$Co$_{x})_{2}$As$_{2 }$single crystals Goutam Sheet, Ulrich Welp, Ying Jia, Wai-Kwong Kwok, E.C. Blomberg, M.A. Tanatar, N. Ni, S.L. Bud'ko, P.C. Canfield, R. Prozorov, H.H. Wen We find unexpectedly sharp minima in the angular dependence of the flux-flow resistance of a series of Ba$_{1-x}$K$_{x}$(FeAs)$_{2}$ and Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2 }$crystals when the magnetic field is applied parallel to the FeAs-planes. These minima are too sharp to be accounted for by effective mass anisotropy. Furthermore, since the c-axis coherence length is substantially larger than the FeAs layer spacing, intrinsic pinning mechanisms are ruled out. However, high-resolution cross-sectional SEM reveal the presence of plate-like inclusions of 50 - 100 nm thickness, which in a natural way can account for the enhanced pinning for H $\vert \vert $ ab. On crystals that have been irradiated with heavy ions along the c-axis we observe additional resistance minima for H $\vert \vert $ c, indicating the correlated nature of the irradiation induced defects. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X23.00010: Combination of strong natural and artificial pinning centers in Co-doped BaFe$_{2}$As$_{2}$ films Boris Maiorov, T. Katase, M. Weigand, I. Usov, N. Haberkorn, H. Hiramatsu, H. Hosono, L. Civale Studying the angular dependence of the critical current density (J$_{c})$ as a function of temperature in superconductors with complex pinning landscapes is very important from technical and fundamental points of view. The low anisotropy found in the BaFe$_{2}$As$_{2 }$(Ba122) family together with strong naturally grown pinning make Ba122 films very attractive. We present results on Ba122 superconducting films with naturally grown correlated defects with the addition of different amounts of random defects produced by consecutive irradiations with 3MeV protons. We analyze the changes of J$_{c}$, H$_{irr}$ and H$_{c2}$ as a function of field, angle and temperature. Irradiations produce a small decrease in critical temperature (T$_{c})$ of 0.5 K per 1x10$^{16}$ cm$^{-2 }$dose. After the irradiation, the pinning near the c-axis remains almost unchanged (except for the effects from the decrease of T$_{c})$. On the other hand, an increase of J$_{c}$ is observed for other field orientations indicating and stronger random pinning contribution, particularly at high fields. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X23.00011: Highly Mobile In-plane Vortex Arrangement in SmFeAs(O,F) Philip Moll, Luis Balicas, Janusz Karpinski, Nikolai D. Zhigadlo, Bertram Batlogg We observed a strong enhancement of flux flow dissipation for current flowing along the inter-planar direction in single crystals of the iron pnictide high-T$_c$ superconductor SmFeAs(O,F) (T$_c \sim$ 50K) in high magnetic fields precisely aligned with the FeAs planes. The dissipation reaches significant fractions of the resistance in the normal state at all temperatures and fields, far below H$_{c2}||ab$, estimated to be well above 100T at low temperatures. Even slightest field misalignments from the FeAs planes ($<$0.1deg) restore the dissipation free state characterized by very high critical current densities ($\sim10^6 A/cm^2$) at low temperatures. We attribute this feature to vortices arranging themselves between the FeAs layers, accompanied by a reduced effectiveness of pinning. The qualitative features are reminiscent of the well-known lock-in effect in the cuprates, yet there are clear differences evident: The angular range of enhanced dissipation is reduced upon cooling in SmFeAs(O,F), whereas in the cuprates it significantly broadens as H$_{c1}$ increases at lower temperatures. Furthermore, the lock-in effect is most pronounced in strongly anisotropic materials, while SmFeAs(O,F) is moderately anisotropic ($\gamma \sim$ 6-8), becoming more isotropic at low temperature. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X23.00012: Meissner holes in iron-based superconductors Tsuyoshi Tamegai, Shyam Mohan, Yuji Tsuchiya, Yasuyuki Nakajima Magnetic flux penetrates into a superconductor in the form of quantized vortices. This process is usually described by the Bean model, and the flux front forms a regular pattern reflecting the shape of the sample. However, a novel form of flux penetration accompanying wiggling fronts between vortices and antivortices has been observed in YBa$_{2}$Cu$_{3}$O$_{7-\delta }$ upon remagnetization [1]. Such a phenomenon is ascribed to the presence of special arrangements of vortices at the front accompanying flux free regions and excess current around it. The flux free region is called as `Mesissner hole'. We have performed extensive magneto-optical imagings of iron-based superconductor single crystals and found similar anomalous features for the first time in superconductors other than 123-type cuprates [2]. Implications of this finding will be discussed with possible origins of the anomalous vortex arrangements. [1] V. K. Vlasko-Vlasov \textit{et al}., Phys. Rev. B \textbf{56}, 5622 (1997). [2] S. Mohan, Y. Tsuchiya, Y. Nakajima, and T. Tamegai, Phys. Rev. B \textbf{84}, 18050X (2011). [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X23.00013: Thermo-magnetic stability of NbN films with controlled nano-granularity Vitaliy Yurchenko, Konstantin Ilin, Pavlo Mikheenko, M. Siegel, Yuri Galperin, Tom Henning Johansen The critical state in superconductors (SC) is metastable and can be destructed either by flux creep or by abrupt massive flux avalanches. The avalanches are associated with thermo-magnetic instability (TMI) of superconductors, which appears when the heat generated by the moving vortices is greater than the heat released into an environment. In that sense NbN films are known to be unstable, i.e. even the smallest increase of the external field may trigger a massive flux avalanche. Most theories developed to formulate the criteria of TMI operate in terms of intrinsic parameters of SC, such as heat capacity, critical current etc., and disregard the origins and the nature of the triggering mechanisms of the avalanches. We will present the most recent results of magneto-optical visualizations of flux dynamics in a series of NbN films with nano-scale disorder (ND) introduced in a well controlled fashion. We will demonstrate that not only do ND stipulate increase of the critical current but also promote correlated motion of large vortex bundles - micro jumps, which in turn trigger the macro avalanches. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X23.00014: Quasi-1D intermittent flux dynamics in superconducting films Tom H. Johansen, Atle J. Qviller, Vitaliy V. Yurchenko, Jorn I. Vestgarden, Peter B. Mozhaev, Jorn B. Hansen, Yuri M. Galperin The stability of pinned vortex systems is constantly challenged in superconductors. In this work, magneto-optical imaging was used to reveal a new type of intermittent flux behavior in films of YBa$_{2}$Cu$_{3}$O$_{x}$. Films were grown on tilted NdGaO$_{3}$ substrates, where the terrace structure creates a high density of planar defects. The flux penetration along the terrace steps consists of numerous 1-dimensional avalanches, some starting at the film edge, some fully internal. In spite the vivid dynamics the flux front advances in accordance with the critical state model. Analysing more than 10000 avalanche events, we find a power-law size distribution and finite-size-scaling with the depth of the flux front as crossover length. The intermittent behaviour shows no threshold value in the applied field. These new characteristics largely contrast those of the thermo-magnetic avalanches observed in many superconducting films, and suggest that a different mechanism is responsible for the 1-dimensional avalanches. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X23.00015: Investigation of the magnetic field angle dependence of resistance, irreversibility field, upper critical field and critical current density in DC sputtered Bi-2223 thin film Murat Erdem, Mustafa Akdogan, Gurcan Yildirim, Sevgi Polat Altintas, Ahmet Varilci, Cabir Terzioglu We measured resistivity and transport critical current density as a function of DC magnetic field and the angle between the surface of the film and the magnetic field on ex-situ annealed, c-axis oriented Bi-2223 thin films fabricated by DC sputtering method. Irreversibility field and upper critical field were determined from the resistivity vs. the applied magnetic field graph. It is observed that the superconducting properties of the films strongly depend on the direction and strength of the field. Penetration depths and coherence lengths were also determined from the irreversibility field and upper critical field versus temperature graphs, respectively. The anisotropic J$_{c}$ behavior of the film is found to be intrinsic. We provided a theoretical analysis of the obtained results in the framework of intrinsic pinning theory of superconductors. Microstructural properties of the produced films were investigated by XRD and SEM measurements. XRD patterns indicate that the films are c-axis oriented based on the prominent (00l) peaks. SEM images show needle-like grain structures dominate the surface morphology of the films. [Preview Abstract] |
Session X24: Electronic Structure: Quantum Monte Carlo
Sponsoring Units: DCOMPChair: Jonathan Dubois, Lawrence Livermore National Laboratory
Room: 256
Thursday, March 1, 2012 2:30PM - 2:42PM |
X24.00001: The Homogeneous Electron Gas: Beyond Fixed Nodes James Shepherd, George Booth, Andreas Gruneis, Ali Alavi The ground state energy of the homogeneous electron gas (HEG) still presents a significant challenge to Quantum Chemical methods, in spite of being a model Hamiltonian that has been studied for many decades and is often regarded as the archetypal system in solid state physics and in Fermi liquid theory. To date the only truly successful methods to yield accurate ground state energies at a range of densities have been quantum Monte Carlo techniques, in particular Diffusion Monte Carlo (DMC). Attempts to go beyond the fixed-node approximation have been met with some success, however elimination of this error all-together has not been achieved. Full Configuration Interaction (FCI) would provide an exact solution to this problem in the limit of an infinite basis set, which can be approached in a systematically improvable way. However, this is prohibitively expensive, scaling exponentially in the electron number and the size of the underlying one-electron basis with very large pre-factors. We present the application of a new method, FCI Quantum Monte Carlo, which stochastically samples the exact wavefunction producing FCI accuracy at a greatly improved computational cost, to the high-density HEG. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X24.00002: Simulation of the Warm-Dense Homogeneous Electron Gas Ethan Brown, David Ceperley, Jonathan DuBois Warm-dense matter (WDM), where both the Coulomb coupling parameter ($\Gamma \equiv q^{2}/(r_{s} k_{B} T)$) and the electron degeneracy parameter ($\Theta \equiv k_{B} T/\epsilon_{F}$) are approximately unity, exists in systems as disparate as planetary interiors and along the pathway to inertial confinement fusion. Attempts to characterize this regime through the use of Density Functional Theory (DFT) require an accurate equation of state. Here we present results for a first-principles simulation of the homogeneous electron gas (HEG) in the warm-dense regime through Restricted Path Integral Monte Carlo (RPIMC). These results could be used as a benchmark for improved functionals, as well as input for orbital-free DFT formulations. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X24.00003: Improved Full Configuration Interaction Monte Carlo for the Hubbard Model Hitesh Changlani, Adam Holmes, Frank Petruzielo, Garnet Chan, C.L. Henley, C.J. Umrigar We consider the recently proposed full configuration interaction quantum Monte Carlo (FCI-QMC) method and its ``initiator'' extension, both of which promise to ameliorate the sign problem by utilizing the cancellation of positive and negative walkers in the Hilbert space of Slater determinants. While the method has been primarily used for quantum chemistry by A.Alavi and his co-workers [1,2], its application to lattice models in solid state physics has not been tested. We propose an improvement in the form of choosing a basis to make the wavefunction more localized in Fock space, which potentially also reduces the sign problem. We perform calculations on the 4x4 and 8x8 Hubbard models in real and momentum space and in a basis motivated by the reduced density matrix of a 2x2 real space patch obtained from the exact diagonalization of a larger system in which it is embedded. We discuss our results for a range of fillings and U/t and compare them with previous Auxiliary Field QMC and Fixed Node Green's Function Monte Carlo calculations. \\[4pt] [1] George Booth, Alex Thom, Ali Alavi, J Chem Phys, 131, 050106,(2009)\\[0pt] [2] D Cleland, GH Booth, Ali Alavi, J Chem Phys 132, 041103, (2010) [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X24.00004: Semi-stochastic full configuration interaction quantum Monte Carlo Adam Holmes, Frank Petruzielo, Mihir Khadilkar, Hitesh Changlani, M.P. Nightingale, C.J. Umrigar In the recently proposed full configuration interaction quantum Monte Carlo (FCIQMC) [1,2], the ground state is projected out stochastically, using a population of walkers each of which represents a basis state in the Hilbert space spanned by Slater determinants. The infamous fermion sign problem manifests itself in the fact that walkers of either sign can be spawned on a given determinant. We propose an improvement on this method in the form of a hybrid stochastic/deterministic technique, which we expect will improve the efficiency of the algorithm by ameliorating the sign problem. We test the method on atoms and molecules, e.g., carbon, carbon dimer, N2 molecule, and stretched N2. \\[4pt] [1] Fermion Monte Carlo without fixed nodes: a Game of Life, death and annihilation in Slater Determinant space. George Booth, Alex Thom, Ali Alavi. J Chem Phys 131, 050106, (2009).\\[0pt] [2] Survival of the fittest: Accelerating convergence in full configuration-interaction quantum Monte Carlo. Deidre Cleland, George Booth, and Ali Alavi. J Chem Phys 132, 041103 (2010). [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X24.00005: Excited state calculations in solids by auxiliary-field quantum Monte Carlo Fengjie Ma, Shiwei Zhang, Henry Krakauer We have studied electronic excitations in solid systems using the phaseless auxiliary-field quantum Monte Carlo (AFQMC) method.\footnote{S.~Zhang and H.~Krakauer, Phys.~Rev.~Lett.~{\bf 90}, 136401 (2003); W.~Purwanto, S.~Zhang, and H.~Krakauer, J. Chem. Phys. {\bf 130}, 094107 (2009).} Trial wave functions for excited states are simply constructed from the corresponding density functional theory (DFT) ground state orbitals by promoting electrons to conduction bands. The post-processing finite size (FS) correction method\footnote{F.~Ma, S.~Zhang, and H.~Krakauer, Phys.~Rev.~B {\bf 84}, 155130 (2011); H.~Kwee, S.~Zhang, and H.~Krakauer, Phys. Rev. Lett. {\bf 100}, 126404 (2008).} is applied to remove the many-body FS effects. By fitting the calculated excitation energies at various crystal momentum values, a many-body electronic band structure is obtained. Our results for prototypical semiconductors such as silicon are compared to those from the GW approximation\footnote{M.~Rohlfing, P.~Kr\"uger, and J.~Pollmann, Phys.~Rev B.~{\bf 48}, 17791 (1993).} and diffusion Monte Carlo calculations.\footnote{A.~J.~Williamson, R.~Q.~Hood, R.~J.~Needs, and G.~Rajagopal, Phys.~Rev.~B {\bf 57}, 12140 (1998).} [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X24.00006: Frozen core method in auxiliary-field quantum Monte Carlo Wirawan Purwanto, Shiwei Zhang, Henry Krakauer We present the implementation of the frozen-core approach in the phaseless auxiliary-field quantum Monte Carlo method (AFQMC). Since AFQMC random walks take place in a many-electron Hilbert space spanned by a chosen one-particle basis, this approach can be achieved without introducing additional approximations, such as pseudopotentials. In parallel to many-body quantum chemistry methods, tightly-bound inner electrons occupy frozen canonical orbitals, which are determined from a lower level of theory, e.g. Hartree-Fock or CASSCF. This provides significant computational savings over fully correlated all-electron treatments, while retaining excellent transferability and accuracy. Results for several systems will be presented. This includes the notoriously difficult Cr$_2$ molecule, where comparisons can be made with near-exact results in small basis sets, as well as an initial implementation in periodic systems. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X24.00007: Tests on novel pseudo-potentials generated from diffusion Monte Carlo data. Fernando Reboredo, Randolph Hood, Michal Bajdich Since Dmitri Mendeleev developed a table in 1869 to illustrate recurring ("periodic") trends of the elements, it has been understood that most chemical and physical properties can be described by taking into account the outer most electrons of the atoms. These valence electrons are mainly responsible for the chemical bond. In many ab-initio approaches only valence electrons are taken into account and a pseudopotential is used to mimic the response of the core electrons. Typically an all-electron calculation is used to generate a pseudopotential that is used either within density functional theory or quantum chemistry approaches. In this talk we explain and demonstrate a new method to generate pseudopotentials directly from all-electron many-body diffusion Monte Carlo (DMC) calculations and discuss the results of of the transferability of these pseudopotentials. The advantages of incorporating the exchange and correlation directly from DMC into the pseudopotential are also discussed. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X24.00008: A diffusion Monte Carlo study of sign problems from non-local pseudopotentials Norm Tubman, Miguel Morales-Silva, Jonathan Dubois, Randolph Hood Difficulties can arise in simulating various Hamiltonian operators efficiently in diffusion Monte Carlo (DMC) such as those associated with non-local pseudopotentials which require the introduction of an approximate form. The locality approximation and T-moves are two widely used techniques in fixed-node diffusion Monte Carlo (FN-DMC) that provide a tractable approach for treating non-local pseudopotentials, however their use introduces an uncontrolled approximation. Exact treatment of the non-local pseudopotentials in FN-DMC introduces a sign problem with the associated Green's function matrix elements which take on both positive and negative values. Here we present an analysis of the nature of the sign problem that non-local operators introduce into the Green's function. We then consider the feasibility of running DMC simulations in which the non-local pseudopotentials are treated exactly and demonstrate the algorithm on a few molecular systems. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X24.00009: Variational and diffusion quantum Monte Carlo methods for spin-orbit interactions in heavy element systems Lubos Mitas, Rene Derian, Shi Guo In most electronic structure quantum Monte Carlo calculations spins of individual electrons have fixed values which are determined by the spin and spatial symmetries of the desired eigenstate. However, for heavy atoms the spin-orbit interaction becomes important and its influence on electronic structure becomes comparable to the electron exchange, correlations or other many-body effects. In such cases the simplest antisymmetric wave function based on one-particle states is a determinant of spinors, while the simplest pairing wave function is a pfaffian. We will present calculations using variational and diffusion Monte Carlo for heavy atoms systems with spin-orbit operators. We eliminate the atomic cores by effective core potentials (pseudopotentials) which are formulated so as to include also effective spin-orbit operators. We test both discrete spin sampling as well as continuous spin representation in the variational and diffusion Monte Carlo methods. Corresponding generalizations of the fixed-node/fixed-phase methods will be discussed as well. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X24.00010: Progress in exact treatment of fermions at finite temperature Jonathan L. DuBois, Berni J. Alder We will discuss some key features of the structure of permutation space for interacting Fermi systems. Exploiting these features, we will then demonstrate improved efficiency in the exact path integral Monte Carlo treatment of liquid $^3$He by using importance sampling to deemphasize the contribution of long permutation cycles to the partition function. Finally, a route to a polynomial scaling algorithm for homogeneous Fermi systems will be presented. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X24.00011: An energy density estimator for quantum Monte Carlo calculations Jaron Krogel, Jeongnim Kim, David Ceperley We establish a physically meaningful representation of a quantum energy density for use in quantum Monte Carlo calculations. The energy density operator, defined in terms of Hamiltonian components and density operators, returns the correct Hamiltonian when integrated over a volume containing a cluster of particles. This property is demonstrated for a Helium-Neon ``gas,'' showing that atomic energies obtained from the energy density correspond to eigenvalues of isolated systems. The formation energies of defects or interfaces are typically calculated as total energy differences. Using a model of delta doped Silicon (where dopant atoms form a thin plane) we show how interfacial energies can be calculated more efficiently with the energy density, since the region of interest is small. We also demonstrate how the energy density correctly transitions to the bulk limit away from the interface where the correct energy is obtainable from a separate total energy calculation. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X24.00012: Metal-Insulator Transition in Low Density Atomic Hydrogen Jeremy McMinis, Jeongnim Kim, David Ceperley At low density BCC hydrogen undergoes a metal-insulator transition. We compute the zero temperature equation of state for the paramagnetic and anti-ferromagnetic phases using diffusion Quantum Monte Carlo. We predict the phase transition density, investigate the shape of the anti-ferromagnetic curve, and compare to previous results [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X24.00013: Quantum Monte Carlo of ThO2 Shuming Hu, Lubos Mitas Thorium dioxide solid is a unique optical and heat-resistant actinide material with large gap and cohesion. It is a diamagnet, unlike a number of other similar actinide oxides. We investigate the electronic structure of ThO2 using Density Functional Theory (DFT) and quantum Monte Carlo (QMC) methods. We adopt Stuttgart RLC and RSC effective core potentials (pseudopotentials) for the Th atom. In the DFT calculations, some of the properties are verified in all-electron calculations using the FLAPW techniques. Using the fixed-node diffusion Monte Carlo we calculate the ground state and several excited states from which we estimate the cohesion and the band gap. Simulation cells of several sizes are used to estimate/reduce the finite size effects. We compare the QMC results with recent DFT calculations with several types of functionals which include hybrids such as PBE0 and HSE. Insights from QMC calculations give us understanding of the correlations beyond the DFT approaches and pave the way for accurate electronic structure calculations of other actinide materials. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X24.00014: Quantum Monte Carlo for the Spectroscopy of Core Excited States Dmitry Zubarev X-ray absorption spectroscopy is a powerful experimental tool that is capable of delivering valuable information about very delicate aspects of electronic structure and reveals details of the local chemical environment in many systems of fundamental and applied importance. The rigorous interpretation of core-level spectra requires very accurate quantum chemical simulations. The trade-off between feasibility of treatment of large systems and consistency in description of electron correlation tremendously hinders the generation of accurate theoretical results for many experimental studies. We show that the fixed-node diffusion Monte Carlo (FN-DMC) approach can be used straightforwardly for the accurate simulation of core-level spectra. Basic methodological aspects are addressed, including the strategy for the construction of adequate trial wave functions. Examples of FN-DMC calculations of core-level spectra of water and pyrrole are presented. The possibility of the simulation of X-ray absorption spectra of solvent-solute systems is discussed. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X24.00015: Density Functional Monte Carlo: an efficient way to calculate the ground state density profile of nanostructures Fons Brosens, Katrijn Putteneers We present a method in which the Hohenberg-Kohn theorems are implemented directly by simulating the density profile using Bernouilli walkers and conserving the total number of particles during the Monte Carlo process. This leads to a much faster algorithm than, e.g., by solving the Kohn-Sham equations. The method is explained in detail and results are shown for a nanoshell which contains several millions of conduction electrons. [Preview Abstract] |
Session X25: Focus Session: Modeling of Rare Events: Methods and Applications II
Sponsoring Units: DCOMPChair: Amit Samanta, Program in Applied and Computational Mathematics, Princeton University
Room: 257A
Thursday, March 1, 2012 2:30PM - 3:06PM |
X25.00001: Accelerated Molecular Dynamics of Rare Events with the Bond-Boost Method Invited Speaker: Kristen Fichthorn A continuing challenge in materials simulation is to conduct long time and large length simulations of structural evolution, while accurately retaining atomic detail. For many materials, dynamical evolution occurs through a series of ``rare events,'' in which the system spends a long-time period in one free-energy minimum before escaping and moving on to another. To address the rare-event problem for materials evolution, we developed the bond-boost method, which is a variant of hyperdynamics. We will introduce the bond-boost method and demonstrate several applications of it to thin-film growth and surface kinetics. A significant problem that plagues rare-event simulations is the ``small barrier problem'' and we will discuss how this problem can be addressed within the bond-boost method. We will also discuss our recent efforts to combine this method with kinetic Monte Carlo simulations. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:42PM |
X25.00002: Generalizations of the string method Invited Speaker: Eric Vanden-Eijnden Generalizations of the string method will be presented that permit to identify the saddle points by which the process escapes the basin of attraction of a minimum of the potential due to thermal noise, or to map out the volume of this basin. These methods will be illustrated via examples such as thermal reorganization of Lennard-Jones clusters and packings of soft-spheres. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X25.00003: String method for the computation of minimum energy paths and saddle points Weiqing Ren Many problems arising from applied sciences can be abstractly formulated as a system that navigates over a complex energy landscape of high or infinite dimensions. The system is confined in metastable states for long times before making important transitions from one metastable state to another. For gradient systems driven by small noise, the transitions follow the minimum energy path, i.e. the heteroclinic orbit connecting the metastable states. I will talk about the zero-temperature string method for computing the minimum energy paths and the saddle points. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X25.00004: Rare events in viscoelastic escape dynamics and subdiffusive transport Igor Goychuk Anomalously slow relaxation, escape and transport processes become increasingly important in many research domains. This talk is focused on a profound question: Does anomalously slow subdiffusive dynamics imply rare events with a divergent mean time separating such events in multistable potentials? The answer depends on the underlying physical mechanism. It is shown that the subdiffusive dynamics originated due to the medium's viscoelasticity and described by a power law memory kernel within the non-Markovian generalized Langevin equation (GLE) approach does not imply such anomalously rare events [1]. Based on a Markovian multidimensional embedding of the GLE dynamics it is shown that the kinetics of subdiffusive Kramers escape is asymptotically stretched exponential for intermediate barriers. It is characterized by a finite mean escape time, and all the higher moments are also finite. Moreover, it approaches normal exponential kinetics for very high barriers which coexists with anomalously slow subdiffusion and transport in washboard potentials [1]. Rare escape events for such subdiffusive dynamics are not presenting the transport limiting step. \\[4pt] [1] I. Goychuk, Phys. Rev. E 80, 046125 (2009); Adv. Chem. Phys. 150 (2012, in press). [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X25.00005: Non-axisymmetric Deformations of Monovalent Metallic Nanowires: A Stochastic Field Theory Approach Lan Gong, Daniel Stein A stochastic Ginzburg-Landau field theory is used to describe the noise-induced transition of monovalent metallic nanowires. Here the transition consists of the nanowire changing from one locally stable radius to another (and thereby changing its conductance as well). The lifetime of a specific wire configuration is obtained from the Kramers rate formula in the limit of of weak noise. We found a ``phase transition'' in the saddle state under general conditions, and have constructed a phase diagram of the activated transition. To study the transition process further, we employed a numerical approach called the string method in order to obtain in detail the complete transition path, crucial to solving the dynamics of wire deformation and transition under noise. We also discuss several interesting applications to lifetime calculations for real nanowires. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X25.00006: Study of Evaporation Rate of Water in Hydrophobic Confinement using Forward Flux Sampling Sumit Sharma, Pablo G. Debenedetti Drying of hydrophobic cavities is of interest in understanding biological self assembly, protein stability and opening and closing of ion channels. Liquid-to-vapor transition of water in confinement is associated with large kinetic barriers which preclude its study using conventional simulation techniques. Using forward flux sampling to study the kinetics of the transition between two hydrophobic surfaces, we show that a) the free energy barriers to evaporation scale linearly with the distance between the two surfaces, d; b) the evaporation rates increase as the lateral size of the surfaces, L increases, and c) the transition state to evaporation for sufficiently large L is a cylindrical vapor cavity connecting the two hydrophobic surfaces. Finally, we decouple the effects of confinement geometry and surface chemistry on the evaporation rates. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X25.00007: Reaching extended length-scales with accelerated dynamics Bradley Hubartt, Yunsic Shim, Jacques Amar While temperature-accelerated dynamics (TAD) has been quite successful in extending the time-scales for non-equilibrium simulations of small systems, the computational time increases rapidly with system size. One possible solution to this problem, which we refer to as parTAD$^1$ is to use spatial decomposition combined with our previously developed semi-rigorous synchronous sublattice algorithm$^2$. However, while such an approach leads to significantly better scaling as a function of system-size, it also artificially limits the size of activated events and is not completely rigorous. Here we discuss progress we have made in developing an alternative approach in which localized saddle-point searches are combined with parallel GPU-based molecular dynamics in order to improve the scaling behavior. By using this method, along with the use of an adaptive method to determine the optimal high-temperature$^3$, we have been able to significantly increase the range of time- and length-scales over which accelerated dynamics simulations may be carried out. [1] Y. Shim et al, Phys. Rev. B {\bf 76}, 205439 (2007); ibid, Phys. Rev. Lett. {\bf 101}, 116101 (2008). [2] Y. Shim and J.G. Amar, Phys. Rev. B {\bf 71}, 125432 (2005). [3] Y. Shim and J.G. Amar, J. Chem. Phys. 134, 054127 (2011). [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X25.00008: A computational atomistic study of the relaxation of ion-bombarded \emph{c}-Si on experimental time-scales: an application of the kinetic Activation Relaxation Technique Laurent Karim B\'{e}land, Normand Mousseau The kinetic activation relaxation technique (kinetic ART) method, an off-lattice, self-learning kinetic Monte Carlo (KMC) algorithm with on-the-fly event search,\footnote{L. K. B\'{e}land, P. Brommer, F. El-Mellouhi, J.-F. Joly and N. Mousseau, Phys. Rev. E \textbf{84}, 046704 (2011).} is used to study the relaxation of \emph{c}-Si after Si$^-$ bombardment at 3 keV. We describe the evolution of the damaged areas at room-temperature and above for periods of the order of seconds, treating long-range elastic deformations exactly. We assess the stability of the nanoscale structures formed by the damage cascade and the mechanisms that govern post-implantation annealing. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X25.00009: Dislocation Cross-slip Mechanisms in Aluminum Yang Xiang, Congming Jin, Gang Lu We have systematically studied dislocation cross-slip in Al at zero temperature by atomistic simulations, focusing on the dependence of the transition paths and energy barriers on dislocation length and position. We find that for a short dislocation segment, the cross-slip follows the uniform Fleischer (FL) mechanism. For a longer dislocation segment, we have identified two different cross-slip mechanisms depending on the initial and final positions of the dislocation. If the initial and final positions are symmetric relative to the intersection of the primary and cross-slip planes, the dislocation cross-slips via the Friedel-Escaig (FE) mechanism. However, when the initial and final positions are asymmetric, the dislocation cross-slips via a combination of the FL and FE mechanisms. The leading partial folds over to the cross-slip plane first, forming a stair-rod dislocation at the intersection with which the trailing partial then merges via the FL mechanism. Afterwards, constrictions appear asymmetrically and move away from each other to complete the cross-slip via the FE mechanism. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X25.00010: The Effects of Hydrogen on the Potential-Energy Surface of Amorphous Silicon Jean-Francois Joly, Normand Mousseau Hydrogenated amorphous silicon (a-Si:H) is an important semiconducting material used in many applications from solar cells to transistors. In 2010, Houssem et al. [1], using the open-ended saddle-point search method, ART nouveau, studied the characteristics of the potential energy landscape of a-Si as a function of relaxation. Here, we extend this study and follow the impact of hydrogen doping on the same a-Si models as a function of doping level. Hydrogen atoms are first attached to dangling bonds, then are positioned to relieve strained bonds of fivefold coordinated silicon atoms. Once these sites are saturated, further doping is achieved with a Monte-Carlo bond switching method that preserves coordination and reduces stress [2]. Bonded interactions are described with a modified Stillinger-Weber potential and non-bonded Si-H and H-H interactions with an adapted Slater-Buckingham potential. Large series of ART nouveau searches are initiated on each model, resulting in an extended catalogue of events that characterize the evolution of potential energy surface as a function of H-doping. \\[4pt] [1] Houssem et al., Phys Rev. Lett., 105, 045503 (2010)\\[0pt] [2] Mousseau et al., Phys Rev. B, 41, 3702 (1990) [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X25.00011: Order-order Nucleation in Copolymers: A String Method Approach Xiuyuan Cheng, Weinan E, Pingwen Zhang, Anchang Shi, Chu Wang The mechanism of nucleation in order-order phase transitions of block copolymers is an interesting problem presenting both theoretical and numerical challenges. In this talk we will introduce our recent work of applying the string method to the order-order nucleation in diblock copolymer. We use the self-consistent field model, and search for the saddle point of the free energy functional by solving a variational problem. We will also talk about a study of the epitaxial relation between ordered phases (work by Wang Chu et al.), which verifies one of the assumptions taken by the numerical method beforehand. [Preview Abstract] |
Session X26: Focus Session: Non-Adiabatic Dynamics in Irradiated Materials
Sponsoring Units: DCOMPChair: Alfredo Caro, Los Alamos National Laboratory
Room: 257B
Thursday, March 1, 2012 2:30PM - 3:06PM |
X26.00001: First-principles calculation of electronic stopping power in metals and insulators via time-dependent DFT simulations Invited Speaker: Jorge Kohanoff Projectiles interacting with solid or liquid targets are subject to two main inelastic collision channels: electronic and nuclear. The end result is a slowing down -- or stopping -- of the projectile due to energy deposition onto electronic excitations or motion of the target nuclei. At high projectile velocities, cross sections for nuclear stopping are exceedingly small, thus leaving electronic stopping as the only relevant channel. At low velocities nuclear stopping becomes predominant. In metals, it coexists with electronic stopping, but in insulators the existence of an energy gap for electronic excitations translates into a velocity threshold for electronic stopping. In this presentation I will introduce a recently developed computational methodology designed to study electronic stopping at the first-principles level, by means of time-dependent density-functional (TDDFT) simulations. I will then discuss the results for a variety of systems, starting from the first application to the ionic crystal LiF, moving to metallic systems like Al and Au, and then to the insulating water ice. In all cases I will compare to available experimental data to emphasize the astonishing accuracy of TDDFT results. I will also discuss how these simulations provide further insight into open questions such as the difference in stopping between H and He projectiles, and I will focus on the interplay between electronic excitations and non-adiabatic forces on the nuclei. Finally, I will touch on computational methods to simulate the combined dynamics of nuclei and electrons. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:42PM |
X26.00002: Nonadiabatic Molecular Dynamics with Trajectories Invited Speaker: Ivano Tavernelli In the mixed quantum-classical description of molecular systems, only the quantum character of the electronic degrees of freedom is considered while the nuclear motion is treated at a classical level. In the adiabatic case, this picture corresponds to the Born-Oppenheimer limit where the nuclei move as point charges on the potential energy surface (PES) associated with a given electronic state. Despite the success of this approximation, many physical and chemical processes do not fall in the regime where nuclei and electrons can be considered decoupled. In particular, most photoreactions pass through regions of the PES in which electron-nuclear quantum interference effects are sizeable and often crucial for a correct description of the phenomena. Recently, we have developed a trajectory-based nonadiabatic molecular dynamics scheme that describes the nuclear wavepacket as an ensemble of particles following classical trajectories on PESs derived from time-dependent density functional theory (TDDFT) [1]. The method is based on Tully's fewest switches trajectories surface hopping (TSH) where the nonadiabatic coupling elements between the different potential energy surfaces are computed \textit{on-the-fly} as functionals of the ground state electron density or, equivalently, of the corresponding Kohn-Sham orbitals [2]. Here, we present the theoretical fundamentals of our approach together with an extension that allows for the direct coupling of the dynamics to an external electromagnetic field [3] as well as to the external potential generated by the environment (solvent effects) [4]. The method is applied to the study of the photodissociation dynamics of simple molecules in gas phase and to the description of the fast excited state dynamics of molecules in solution (in particular Ruthenium (II) tris(bipyridine) in water). \\[4pt] [1] E. Tapavicza, I. Tavernelli, U. Rothlisberger, \textit{Phys. Rev. Lett.,} \textbf{98}, (2007) 023001. \\[0pt] [2] Tavernelli I.; Tapavicza E.; Rothlisberger U., \textit{J. Chem. Phys}., \textbf{130}, (2009) 124107; Tavernelli I., Curchod B.F.E., Rothlisberger U., \textit{J. Chem. Phys}., \textbf{131}, (2009) 196101; Tavernelli I., Curchod B.F.E., Laktionov A., Rothlisberger U., \textit{J. Chem. Phys.}, \textbf{133}, (2010) 194104. \\[0pt] [3] Tavernelli I., Curchod B.F.E., Rothlisberger U., \textit{Phys}. \textit{Rev. A}, \textbf{81}, (2010) 052508. \\[0pt] [4] Tavernelli I., Curchod B.F.E., Rothlisberger U., \textit{Phys. Chem.,} accepted 2011. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X26.00003: Advances on Time-dependent DFT Simulations of Electronic Stopping Alfredo Correa, Andre Schleife, Yosuke Kanai, Jorge Kohanoff, Alfredo J. Caro Radiation damage of reactor materials is a topic of interest and continuous research in the nuclear industry. A single nuclear decay event (e.g. alpha ) produces a cascade of collisions involving the displacement of thousands of atoms in a crystalline material. While atomistic-scale simulations would be the ideal tool to understand these processes, the fact that they currently work on the assumption that electrons respond adiabatically to the atomic motion does not provide valid answers, for example to the stopping power problem. An alternative approach to attacking this problem is a method which explicitly takes into account electron non-adiabatic dynamics. We will present results obtained by time-depending DFT on the electronic stopping power of channeling protons in prototypical metals and insulators obtained by recent implementations of non-adiabatic electron dynamics methods. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X26.00004: Electronic stopping of slow H and He atoms in gold from first principles M. Ahsan Zeb, Jorge Kohanoff, Daniel Sanchez-Portal, Andres Arnau, J.I. Juaristi, Emilio Artacho In spite of a long history, the quantitative understanding of non-adiabatic processes in condensed matter and our ability to perform predictive theoretical simulations of processes coupling many adiabatic energy surfaces is very much behind what accomplished for adiabatic situations, for which first-principles calculations provide predictions of varied properties within a few percent accuracy. We will present here high-accuracy results for the electronic stopping power of H and He moving through gold, using time-evolving density-functional theory, thereby conveying usual first-principles accuracies to strongly coupled, continuum non-adiabatic processes in condensed matter. The two key unexplained features of what observed experimentally have been reproduced and understood: ($i$) The non-linear behavior of stopping power versus velocity is a gradual crossover as excitations tail into the $d$-electron spectrum; and ($ii$) the higher stopping for He than for H at low velocities is explained by the substantial involvement of the $d$ electrons in the screening of the projectile even at the lowest velocities where the energy loss is generated by $s$-like electron-hole pair formation only. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X26.00005: Golden Rule of Radiation Hardness: a Study of Strain Effect on Controlled Radiation Damage Qing Peng, Wei Ji, Suvranu De Stain is widely presented in microstructures. Strain effect to radiation hardness is critical in understanding and engineering nano-materials. Here we studied the strain effect on the controlled radiation damage in monolayer hexagonal boron nitride (h-BN) through {\em ab initio} density functional theory calculations. We observed a general behavior of reduction in the radiation hardness by the strain, for both B-vacancy and N-vacancy configurations, in both compressive and tensive strain states, at the directions of zigzag, armchair and bi-axial. We proposed a golden rule of the radiation hardness states that any effort adding energy to the system will reduce the radiation hardness. Such golden rule of radiation hardness could be widely applied to material design and engineering for those devices working in irradiation-enrich environments, for example, electronic and optoelectronic devices in outer space. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X26.00006: First-principles simulation of laser irradiation of graphene and graphane Kalman Varga, Sergiy Bubin In the framework of real-time real-space time-dependent density functional theory complemented with classical molecular dynamics for ions, we have studied the behavior of graphene and graphane fragments irradiated with strong laser pulses. In particular, we have investigated how the response of graphene and graphane changes when laser pulses of different frequency (near IR, visible, and UV) are shot. Damage thresholds have been established and compared with existing experimental data. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X26.00007: Electron-Phonon Coupling in a CdSe Nanowire Christopher Barrett, Lin-Wang Wang It is important to calculate the coupling between phonons and electrons in realistic nanostructures, e.g. to understand carrier cooling and dynamics in a nanowire. In this talk, we will present results of phonon spectrum calculations using a customized valence force field (VFF) method. This customized VFF method is developed to be fittable to the results of any ab-initio calculations, with density functional theory (DFT) results being used in this work. By fitting many different DFT calculations on different motifs and their perturbations, we have obtained in the custom VFF a very efficient method that closely reproduces DFT phonons for CdSe nanowires with (10-10) surfaces having Cd-Se dimerization. We have also combined the results of these phonon spectrum calculations with electronic structure calculations to obtain the electron-phonon coupling. We will present this result and and show how the electron-phonon coupling affects the carrier dynamics in the nanowire. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X26.00008: Deviational formulations for efficient simulation of multiscale phonon transport Jean-Philippe Peraud, Nicolas Hadjiconstantinou We show that by simulating only the deviation from equilibrium, considerable computational savings can be realized in Monte Carlo solutions of the Boltzmann equation describing phonon transport at small scales. The computational savings manifest themselves in the form of significantly smaller statistical uncertainty compared to standard Monte Carlo solution methods in the limit of small deviation from equilibrium (e.g. small temperature differences). Additional computational savings are realized in multiscale problems where the degree of deviation from equilibrium varies considerably over the simulation domain. By developing rigorous evolution equations for the deviation from equilibrium from the governing kinetic equation, the resulting algorithm seamlessly bridges the near- and far-equilibrium regions without introducing any approximation. We also show that considering an energy-based Boltzmann equation lends itself naturally to algorithms that conserve energy exactly, thus improving the simulation fidelity. Application of the proposed methods to practical problems of current interest, such as a transient thermoreflectance experiment used to extract information about the mean free path of heat carriers in various materials, will be presented and discussed. [Preview Abstract] |
Session X27: Invited Session: Physics for Everyone: Innovative Materials for Energy
Sponsoring Units: DMP DCMPChair: David Cahill, University of Illinois at Urbana-Champain
Room: 258AB
Thursday, March 1, 2012 2:30PM - 3:06PM |
X27.00001: Materials research needs for energy Invited Speaker: Arun Majumdar |
Thursday, March 1, 2012 3:06PM - 3:42PM |
X27.00002: Water Splitting with Materials and Sunlight Invited Speaker: Tom Jarvi Sun Catalytix has been developing technology for energy storage and the generation of renewable fuels in an ARPA-E sponsored program. The program has focused on the development and deployment of low-cost earth-abundant catalytic materials coupled to light-absorbing semiconductor systems to capture and convert solar energy into chemical species. This talk will focus on recent work done to couple cobalt-based water-oxidation and nickel-based hydrogen evolution catalysts with silicon-based solar cells. The results demonstrate direct wireless coupling of solar collection with catalytic materials and operate in relatively benign conditions at reasonable conversion efficiency. These results suggest development pathways for solar hydrogen generation using catalyzed particulate solar absorbing materials. Such pathways will be discussed as they may offer relevant means to generate solar-derived hydrogen as a cost-effective fuel for the future. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 4:18PM |
X27.00003: Advanced materials manufacturing for solar energy Invited Speaker: Frank van Mierlo The US has a robust technical roadmap to get to a \$1/W total installed cost with several potential winners in the race. We dominate in the new technology arena and there is a good chance that tomorrow's winning technology will be from the current crop of contenders. One potential breakthrough is Direct Wafer$^{\rm TM}$ a new manufacturing technique to make silicon wafers at a fraction of the traditional cost. Current wafer manufacturing is a multi-step, energy- and capital-intensive process that wastes half of the valuable silicon feedstock. 1366's Direct Wafer technology forms a standard, 156mm multi-crystalline wafer directly from molten silicon in a semi-continuous, efficient, high-throughput process that eliminates silicon waste. Direct Wafer$^{\rm TM}$ cuts the amount of consumables by a factor of four and requires only half the capital per GigaWatt production capacity thus enabling solar to compete successfully with coal generated electricity. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:54PM |
X27.00004: Genetically Engineered Materials for Biofuels Production Invited Speaker: Michael Raab Agrivida, Inc., is an agricultural biotechnology company developing industrial crop feedstocks for the fuel and chemical industries. Agrivida's crops have improved processing traits that enable efficient, low cost conversion of the crops' cellulosic components into fermentable sugars. Currently, pretreatment and enzymatic conversion of the major cell wall components, cellulose and hemicellulose, into fermentable sugars is the most expensive processing step that prevents widespread adoption of biomass in biofuels processes. To lower production costs we are consolidating pretreatment and enzyme production within the crop. In this strategy, transgenic plants express engineered cell wall degrading enzymes in an inactive form, which can be reactivated after harvest. We have engineered protein elements that disrupt enzyme activity during normal plant growth. Upon exposure to specific processing conditions, the engineered enzymes are converted into their active forms. This mechanism significantly lowers pretreatment costs and enzyme loadings ($>$75{\%} reduction) below those currently available to the industry. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:30PM |
X27.00005: Innovative oxide materials for electrochemical energy conversion Invited Speaker: Eric D. Wachsman Research in functional materials has progressed from those materials exhibiting structural to electronic functionality. The study of ion conducting ceramics ushers in a new era of ``chemically functional materials.'' This chemical functionality arises out of the defect equilibria of these materials, and results in the ability to transport chemical species and actively participate in chemical reactions at their surface. Moreover, this chemical functionality provides a promise for the future whereby the harnessing of our natural hydrocarbon energy resources can shift from inefficient and polluting combustion - mechanical methods to direct electrochemical conversion. The unique properties of these materials and their applications will be described. The focus will be on the application of ion conducting ceramics to energy conversion and storage, chemical sensors, chemical separation and conversion, and life support systems. Results presented will include development of record high power density (3 kW/kg) solid oxide fuel cells, NO$_{x}$/CO species selective solid-state sensors, high yield membrane reactors, and regenerative life support systems that reduce CO$_{2}$ to O$_{2}$ and solid C. [Preview Abstract] |
Session X29: Focus Session: Semiconductor Qubits - Managing or Eliminating Nuclei
Sponsoring Units: GQIChair: Thaddeus Ladd, HRL
Room: 259A
Thursday, March 1, 2012 2:30PM - 3:06PM |
X29.00001: Ultrafast optical coherent control of individual electron and hole spins in a semiconductor quantum dot Invited Speaker: Kristiaan De Greve We report on the complete optical coherent control of individual electron and hole spin qubits in InAs quantum dots. With a magnetic field in Voigt geometry, broadband, detuned optical pulses couple the spin-split ground states, resulting in Rabi flopping. In combination with the Larmor precession around the external magnetic field, this allows an arbitrary single-qubit operation to be realized in less than 20 picoseconds [1,2]. Slow fluctuations in the spin's environment lead to shot-to-shot variations in the Larmor precession frequency. In a time-ensemble measurement, these would prevent a measurement of the true decoherence of the qubit, and instead give rise to ensemble dephasing. This effect was overcome by implementing a spin echo measurement scheme for both electron and hole spins, where an optical $\pi $-pulse refocuses the spin coherence and filters out the slow variations in Larmor precession frequency. We measured coherence times up to 3 microseconds [2,3]. Finally, our optical pulse manipulation scheme allows us to probe the hyperfine interaction between the single spin and the nuclei in the quantum dot. Interesting non-Markovian dynamics could be observed in the free-induction decay of a single electron spin, whereas the complete absence of such effects illustrates the reduction of the hyperfine interaction for hole spin qubits. We measured and modeled these effects, and explain the non-Markovian electron spin dynamics as involving a feedback effect resulting from both the strong Overhauser shift of the electron spin and spin dependent nuclear relaxation [2,4]. \\[4pt] [1] D. Press, T. D. Ladd, B. Zhang and Y. Yamamoto, Nature \textbf{456}, 218 (2008)\\[0pt] [2] K. De Greve, P. McMahon, D. Press \textit{et al.}, Nat. Phys. \textbf{7}, 872 (2011)\\[0pt] [3] D. Press, K. De Greve, P. McMahon \textit{et al.}, Nat. Phot. \textbf{4}, 367 (2010)\\[0pt] [4] T. D. Ladd, D. Press, K. De Greve \textit{et al.}, Phys. Rev. Lett. 105, 107401 (2010) [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X29.00002: Ultrafast optical control of interacting hole spins in coupled quantum dots Samuel Carter, Alex Greilich, Danny Kim, Allan Bracker, Daniel Gammon Recently, hole spins in quantum dots (QDs) have shown great promise as quantum bits due to a reduced hyperfine interaction with nuclear spins, the primary source of decoherence for electron spins. We have developed a system of two vertically stacked InAs QDs that can be charged with a number of holes. In this way, an isolated hole in one QD or two interacting holes in separate dots can be studied. We demonstrate ultrafast optical control of both systems and find a number of differences compared to electron spins. Complete control of the single hole qubit is obtained through optical initialization and single qubit rotations. These control measurements give a hole spin T$_{2}^{\ast }$ of 20ns, an order of magnitude longer than electrons in similar QDs. Spin echo experiments extend the coherence time but are complicated by oscillations in the echo amplitude. For the case of two hole spins, we can observe and tune the coherent exchange interaction that acts as a two qubit gate. We also initialize and perform gates on an entangled spin state, taking a significant step toward a scalable platform for quantum information processing. [1] ``Optical control of one and two hole spins in interacting quantum dots,'' A. Greilich, S. G. Carter, D. Kim, A. S. Bracker and D. Gammon. \textit{Nature Photon.}\textbf{5}, 702 (2011). [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X29.00003: Electrical control of single hole spins in InSb nanowire quantum dots Vlad Pribiag, Stevan Nadj-Perge, Johan van den Berg, Sergey Frolov, Sebastien Plissard, Erik Bakkers, Leo Kouwenhoven The spin-orbit interaction provides an efficient handle for all-electric control of individual spins in quantum dots. Recently, III-V semiconductor nanowires, which have a strong spin-orbit coupling, have emerged as a promising platform for spin-based qubits. Previous work has been focused on the electrical control of electron spins in InAs nanowires. In contrast, spin-dependent quantum transport with holes has so far remained largely unexplored. Here, we demonstrate gate tuning from the few-electron double dot to the few-hole double-dot regimes in InSb nanowires and observe Pauli spin-blockade for both electrons and holes. We use electric-dipole spin resonance (EDSR) to determine the effective g-factors of the two types of carriers. EDSR control over the hole spins is promising for driving coherent rotations of hole-spin qubits. These hole qubits are expected to be less sensitive to hyperfine-mediated decoherence effects than electron-spin qubits as a result of the p-wave symmetry of the hole wavefunction. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X29.00004: Theory of heavy-hole spin-echo decay Xiaoya Judy Wang, William Coish Heavy-hole spin states have emerged as a robust new candidate for realizing a qubit. Nevertheless, the coupling of the hole spin to nuclei in the surrounding medium likely limits hole-spin coherence and has, until very recently, been overlooked. We describe the real-time spin decoherence of a heavy-hole in a semiconductor quantum dot, subject to spin echo pulses. We obtain an analytical expression which we compute numerically for an experimentally realistic number ($\sim 10^4$) of nuclear spins. Including the (previously neglected) nuclear Zeeman term in the Hamiltonian, we observe novel effects uniquely characterizing the decoherence mechanisms under study. In particular, we find a nontrivial dependence of the decay on the applied magnetic field, as well as novel predictions for motional narrowing and envelope modulation, which could significantly extend the hole-spin memory time in near-future experiments. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X29.00005: High Quality Two-Dimensional Electron Gases (2DEGs) in Isotopically-Enriched Strained Silicon Jiun-Yun Li, Chiao-Ti Huang, Leonid Rokhinson, James Ohlhausen, Malcolm Carroll, James Sturm Silicon quantum dots (QDs) formed in a Si/SiGe two-dimensional electron gas (2DEG) are a promising candidate for quantum computation. To capture a single electron in a QD, the dot must be very small, which requires a short distance from the surface to Si 2DEG layer for fine gating. Here we demonstrate a high quality modulation-doped Si 2DEG grown by chemical vapor deposition (CVD), with a distance of 65 nm from the surface to 2DEG layer. The electron mobility at 0.3K of 504,000 cm$^{2}$/V-s (density 4.3 x 10$^{11}$ cm$^{-2})$ is the highest yet reported by CVD for ungated Si 2DEGs. Further, a Si 2DEG layer consists of isotopically-enriched $^{28}$Si to minimize spin decoherence due to $^{29}$Si. SIMS results show that in the Si 2DEG layer, $^{28}$Si is enriched from natural abundance of 92.2{\%} to 99.8{\%} with $^{29}$Si reduced from 4.7{\%} to an upper limit of $\sim $ 0.24{\%} and $^{30}$Si reduced from 3.1{\%} to $\sim $ 63ppm. Finally, effective Schottky gating requires a sharp turn-off slope in phosphorus from the doped layer to the surface for low electric fields near the surface. We have achieved ultra-sharp turn-off slope of $\sim $16 nm/dec, and demonstrate Schottky gating to fully deplete the 2DEG with extremely low leakage current. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X29.00006: Implanted bismuth donors in 28-Si: Process development and electron spin resonance measurements C.D. Weis, C.C. Lo, V. Lang, R.E. George, A.M. Tyryshkin, J. Bokor, S.A. Lyon, J.J.L. Morton, T. Schenkel Spins of donor atoms in silicon are excellent qubit candidates. Isotope engineered substrates provide a nuclear spin free host environment, resulting in long spin coherence times [1,2]. The capability of swapping quantum information between electron and nuclear spins can enable quantum communication and gate operation via the electron spin and quantum memory via the nuclear spin [2]. Spin properties of donor qubit candidates in silicon have been studied mostly for phosphorous and antimony [1-3]. Bismuth donors in silicon exhibit a zero field splitting of 7.4 GHz and have attracted attention as potential nuclear spin memory and spin qubit candidates [4,5] that could be coupled to superconducting resonators [4,6]. We report on progress in the formation of bismuth doped 28-Si epi layers by ion implantation, electrical dopant activation and their study via pulsed electron spin resonance measurements showing narrow linewidths and good coherence times. \\[4pt] [1] A. M. Tyryshkin, et al. arXiv: 1105.3772 [2] J. J. L. Morton, et al. Nature (2008) [3] T. Schenkel, et al APL 2006; F. R. Bradbury, et al. PRL (2006) [4] R. E. George, et al. PRL (2010) [5] G. W. Morley, et al. Nat Mat (2010) [6] M. Hatridge, et al. PRB (2011), R. Vijay, et al. APL (2010) This work was supported by NSA (100000080295) and DOE (DE-AC02-05CH11231). [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X29.00007: Isotopic enrichment and growth of material for quantum coherent devices Kevin Dwyer, Joshua Pomeroy We demonstrate isotopic enrichment and growth of highly pure materials in support of quantum coherent devices. Efforts to produce devices capable of quantum computation rely on long coherence times of the electron or nuclear spin being used. Impurities with nuclear spin are a major cause of decoherence in such systems, and their elimination is essential towards longer T$_{2}$ realization. The produced material must be isotopically enriched as well as chemically pure and defect free, and we present an alternative method for achieving these goals. Unenriched material is ionized and filtered using a mass selecting magnet and then epitaxially deposited. As an initial check on enrichment, $^{22}$Ne is implanted into Si demonstrating an isotopic selectivity over 1800:1 which extrapolates to a $^{28}$Si enrichment better than 99.994{\%}. In progression towards Si deposition from a silane precursor, methane is used as an analog to grow enriched $^{13}$C on semiconductor grade silicon. Analysis of this material by SIMS and ESR as a check on estimated levels of isotopic and chemical purity is presented. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X29.00008: Realization of a double quantum dot in an isotopically purified $^{28}$Si 2DES Andreas Wild, Juergen Sailer, Gerhard Abstreiter, J.W. Ager, E.E. Haller, Stefan Ludwig, Johannes Kierig, Dominique Bougeard The Si/SiGe material system shows great promise for the realization of electron spin qubits due to the weak hyperfine interaction in natural silicon [2]. The electron spin coherence time is expected to further increase for spins embedded in a nuclear spin-refined $^{28}$Si host crystal. In this contribution, we report on the realization and characterization of a 2DES in a MBE grown hybrid $^{28}$Si/SiGe heterostructure with a record mobility of $5.5\cdot10^4 cm^2/Vs$ at an electron density of $3\cdot10^{11}/cm^2$ in which the electron-nuclear spin overlap is greatly suppressed [1]. Based on this heterostructure, we present the first double quantum dot device in isotopically purified silicon. Our device can be operated down to the few electron regime and by using an additional global topgate above the quantum dot gates, the overall charge noise performance can be optimized significantly. This recent progress is fundamental for further experiments towards e.g. measurements of spin relaxation times in $^{28}$Si. \\[4pt] [1] J. Sailer et al., Phys. status solidi RRL 3, 61 (2009)\\[0pt] [2] A. Wild et al., New J. Phys. 12, 113019 (2010) [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X29.00009: Deterministic preparation of Dicke states of donor nuclear spins in silicon Yu Luo, Hongyi Yu, Wang Yao We present a scheme to deterministically prepare various symmetric and asymmetric Dicke states for donor nuclear spins in silicon. The state preparation is realized by cooperative pumping of nuclear spins by coupled donor electrons, and the required controls are in situ to the prototype Kane's architecture for quantum computation. This scheme only requires a sub-gigahertz donor exchange coupling which can be achieved without atomically precise donor placement, hence it could be a practical way to prepare multipartite entanglement of spins in silicon with current technology. All desired Dicke states appear as the steady state under various pumping scenarios and therefore the preparation is robust and does not require accurate temporal controls. Numerical simulations with realistic parameters show that Dicke states of 10 - 20 qubits can be prepared with high fidelity in the presence of decoherence and undesired dynamics. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X29.00010: Optical Patterning of Nuclear Polarization in Gallium Arsenide Jonathan King, Yunpu Li, Carlos Meriles, Jeffrey Reimer Large enhancements of nuclear spin polarization can quench weak electron-nuclear fluctuations, mitigate electron spin decoherence, and provide control of electron spins in devices for quantum information processing. Such enhancements might include spatially patterned regions of polarized nuclei so as to prepare a spatially-dependent effective Zeeman field acting on electron spins as well as coherently manipulate the spin state of drifting electrons. By exploiting two competing mechanisms for optical nuclear polarization in semi-insulating GaAs, we use high field stray-field NMR imaging to demonstrate all-optical creation of three-dimensional patterns of positive and negative nuclear polarization without the need for ferromagnets or lithographic patterning techniques. These patterns may be controlled on the micron length scale. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X29.00011: Integrating MREV and XYXY Pulse Sequences to Decouple Dipolar Interactions in ESR Experiments Alexei Tyryshkin, Stephen Lyon Dynamical decoupling (DD) techniques employ a series of strong refocusing pulses to combat decoherence in quantum systems. A great number of the new improved DD sequences have been designed recently aiming to decouple from a specific kind of decohering noise, arising in the direction of an externally applied quantizing field. MREV-type pulse sequences have long been used in NMR community to decouple spins from another source of decoherence - the dipolar interactions between like spins. Here, we report on our experience while using MREV sequences in ESR experiments on donors in silicon. We find MREV sequences to be very sensitive to even small instrumental errors in the applied pulses. The errors accumulate upon repeating the MREV pulse sequence, destroying the coherenence instead of protecting it. A possible solution to this pulse error problem is found by integrating an MREV pulse sequence with a self-correcting XYXY sequence. The new MREV-XYXY sequence appears to satisfy all the requirements of a ``good'' DD sequence: (1) providing a protection to an arbitrary coherent state with good fidelity (at least 95\% after more than 17,000 pulses), (2) decoupling from dipolar interactions of like spins, and (3) cancelling the phase noise arising from fluctuations in the magnetic field. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X29.00012: Master equation approach to the central spin decoherence problem Edwin Barnes, Lukasz Cywinski, Sankar Das Sarma The electron-nuclear hyperfine interaction is the leading source of decoherence for electron spin qubits in III-V semiconductor quantum dots. For sufficiently low external B-field, the dynamics is purely hyperfine-induced. Generalized master equations embody an attractive approach to this problem due to strong analytical control, but so far they have only been applied in the case of high magnetic fields where a standard perturbative treatment is reliable. In the low field regime where pure hyperfine effects are measurable, this standard treatment breaks down. We show how to overcome this problem by first arguing that the detailed shape of the electron wavefunction is irrelevant for the electron spin decoherence at low B-fields. We then employ a powerful technique involving so-called correlated projection operators to vastly improve the convergence of the perturbative master equation approach by taking advantage of the symmetries that arise when the electron wavefunction is coarse-grained. This brings the low B-field regime into the scope of the master equation description, paving the way for the development of a well-controlled theory of pure hyperfine decoherence that is relevant for current experiments. [E. Barnes, L. Cywinski, S. Das Sarma, Phys. Rev. B 84, 155315 (2011)] [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X29.00013: Two-electron-spin dephasing due to hyperfine interaction in a GaAs double dot Jo-Tzu Hung, Lukasz Cywinski, Xuedong Hu, Sankar Das Sarma We study hyperfine interaction induced pure dephasing of two electron spin states in a GaAs double quantum dot. We construct the effective pure dephasing Hamiltonian for the two electron spins, and apply the ring-diagram theory [1] to calculate the decoherence function of the double dot two-spin system. With a finite exchange coupling, singlet and triplet states are the electron spin eigenstates, and we focus on the dephasing between the singlet state S and the unpolarized triplet state T$_{0}$. We find that the effective Overhauser fields for these two states are suppressed because of their state symmetries, leading to weaker effective coupling between the nuclear spins. On the other hand, the weaker Overhauser field also allows more nuclear spins to flip-flop with each other. We show that these competing effects lead to interesting two-spin decoherence dynamics. We calculate the coherence decay as functions of the external field, the exchange splitting, and the quantum dot size, and compare our results with those for a single spin and for two uncorrelated spins. [1] L. Cywinski, W. Witzel, and S. Das Sarma, Phys. Rev. B 79, 245314 (2009). [Preview Abstract] |
Session X30: Focus Session: Quantum Information for Quantum Foundations - Foundational Experiments and Experimental Proposals
Sponsoring Units: GQIChair: Christopher Fuchs, Perimeter Institute for Theoretical Physics
Room: 259B
Thursday, March 1, 2012 2:30PM - 2:42PM |
X30.00001: Experimental violation of the Leggett-Garg inequality under decoherence Jin-Shi Xu, Chuan-Feng Li, Xu-Bo Zou, Guang-Can Guo Despite the great success of quantum mechanics, questions regarding its application still exist and the boundary between quantum and classical mechanics remains unclear. Based on the philosophical assumptions of macrorealism and noninvasive measurability, Leggett and Garg devised a series of inequalities (LG inequalities) involving a single system with a set of measurements at different times. Introduced as the Bell inequalities in time, the violation of LG inequalities excludes the hidden-variable description based on the above two assumptions. Here, we experimentally investigate the single photon LG inequalities in a dephasing environment simulated by birefringent media. By implementing an optical Controlled-Not gate on a single photon, the LG inequalities are shown to be maximally violated in a coherent evolution process. This disproves its classical realistic description with the two assumptions of the LG inequalities. With the increase of birefringent media, the violation of LG inequalities becomes weaker and is shown to be not violated anymore at some time. The ability to violate the LG inequalities can be used to set the boundary of the classical realistic description. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X30.00002: A condition for macroscopic realism beyond the Leggett-Garg inequalities Johannes Kofler In 1985, Leggett and Garg have put forward the concept of macroscopic realism (macrorealism), stating that the properties of macroscopic objects exist independent of and are not influenced by measurement. In analogy to Bell's theorem, they derived a necessary condition in terms of inequalities, which are now known as the Leggett-Garg inequalities. In this talk, a mathematical condition is introduced which is not only necessary but also sufficient for macrorealism. More importantly, the structure of this condition intuitively encompasses the physical meaning of macrorealism and allows for its experimental test in situations where the paradigm of Leggett-Garg inequalities might not be applicable. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X30.00003: A quantum delayed-choice {\em gedanken} experiment Radu Ionicioiu, Daniel Terno {\em Gedanken} experiments are important conceptual tools in the quest to reconcile our classical intuition with quantum mechanics and nowadays are routinely performed in the laboratory. An important open question is the quantum behaviour of the controlling devices in such experiments. We propose a framework to analyse quantum-controlled experiments and illustrate the implications by discussing a quantum version of Wheeler's delayed-choice experiment. The introduction of a quantum-controlled device (i.e., quantum beamsplitter) has several consequences. First, it implies that we can measure complementary phenomena with a single experimental setup, thus pointing to a redefinition of complementarity principle. Second, a quantum control allows us to prove there are no consistent hidden-variable theories in which ``particle'' and ``wave'' are realistic properties. Finally, it shows that a photon can have a morphing behaviour between ``particle" and ``wave''; this further supports the conclusion that ``particle" and ``wave'' are not realistic properties but merely reflect how we ``look'' at the photon. The framework developed here can be extended to other experiments, particularly to Bell-inequality tests. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X30.00004: The Pauli Exclusion Principle for electrons -- a high sensitivity test in Gran Sasso underground laboratory Johann Marton One of the fundamental rules of nature and a pillar in the foundation of quantum theory and thus of modern physics is represented by the Pauli Exclusion Principle. We know that this principle is extremely well fulfilled due to many observations like the order of the elements and the stability of matter. Numerous experiments were performed to search for tiny violation of this rule in various systems. The experiment VIP at the Gran Sasso underground laboratory is searching for possible small violations of the Pauli Exclusion Principle for electrons leading to an ``anomalous'' X-ray transition in copper atoms. VIP is aiming at a test oft he Pauli Exclusion Principle for electrons with unprecedented accuracy, down to the level of 10-29 - 10-30, thus improving the previous limit by 3-4 orders of magnitude. The experimental method, the setup, results obtained so far and future plans to further increase the precision by 2 orders of magnitude will be presented. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X30.00005: Quantum metamaterials as a tool for investigating the quantum-classical transition Sergey Saveliev, Alexandre Zagoskin, Mark Everitt, Richard Wilson Quantum metamaterials are optical media comprised of individually controllable unit elements (e.g., qubits), which maintain quantum coherence for periods sufficient for an electromagnetic wave to pass through the system. They represent macroscopic, spatially extended quantum scatterers, which can be put in a superposition of states with different properties (e.g., different refractive indexes) and can thus provide new ways of testing different scenarios of quantum-classical transition. We consider an inverse of the classic double-slit experiment, where a classical electromagnetic wave is scattered by a quantum metamaterial in a superposition of states, and discuss the possibilities of its experimental realization. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X30.00006: Progress towards a loophole-free test of nonlocality Kevin McCusker, Bradley Christensen, Paul Kwiat, Joseph Altepeter We report on our progress towards a loophole-free test of nonlocality using spontaneous parametric down-conversion (SPDC). While the timing loophole can be easily closed in such a system by moving the detectors far apart [1], closing the detector loophole is significantly more difficult. In the standard Bell entangled states with the maximal violation of the CHSH inequality [2], an overall efficiency of 83\% is required. This limit can be lowered to 67\% by using non-maximally entangled states (although sensitivity to noise is greatly increased) [3]. We are carefully engineering our source to achieve maximal heralding efficiency, by optimizing both the spatial and spectral filtering, while keeping noise low using high-extinction-ratio polarizing beamsplitters. Combined with high-efficiency detectors, either optimized visible-light photon counters [4] or transition-edge sensors [5], closure of the detection loophole is within reach. \\[4pt] [1] G. Weihs et al., Phys. Rev. Lett. 81, 5039 (1998).\newline [2] J. F. Clauser et al., Phys. Rev. Lett. 23, 880 (1969).\newline [3] P.H. Eberhard, Phys. Rev. A 47, R747 (1993).\newline [4] S. Takeuchi et al., Appl. Phys. Lett. 74, 1063 (1999).\newline [5] A. E. Lita, A. J. Miller, and S. Nam, Opt. Exp. 16, 3032 (2008). [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X30.00007: Loophole-free Quantum Steering Sven Ramelow, Bernhard Wittmann, Fabian Steinlechner, Nathan K. Langford, Nicolas Brunner, Howard Wiseman, Rupert Ursin, Anton Zeilinger Experiments testing quantum mechanics have provided increasing evidence against local realistic theories. However, a conclusive test that simultaneously closes all major loopholes (the locality, freedom-of-choice, and detection loopholes) remains an open challenge. An important class of local realistic theories can be tested with the concept of ``steering.'' Schr\"{o}dinger introduced this term for entanglement seemingly allowing to remotely steer the state of a distant system [1]. Einstein called this ``spooky action at a distance.'' Steering was recently formalized by deriving steering inequalities allowing experimental tests. Here, we present the first loophole-free steering experiment [2]. We use entangled photons shared between two distant laboratories and close all loopholes by a large separation, ultra-fast switching and quantum random number generation, and high, overall detection efficiency. Beside its foundational importance loop-hole-free steering is relevant for is relevant for device-independent certification of quantum entanglement. \\[4pt] [1] E. Schr\"{o}dinger, Proc. Camb. , Phil. Soc. 31, 553 (1935) \\[0pt] [2] B. Wittmann, S. Ramelow, F. Steinlechner, N. K. Langford, N. Brunner, H. Wiseman, R. Ursin, A. Zeilinger, arXiv:1111.0760, (2011) [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X30.00008: Conclusive quantum steering with superconducting transition edge sensors Marcelo P. de Almeida, Devin H. Smith, Geo Gillett, Cyril Branciard, Alessandro Fedrizzi, Till J. Weinhold, Adriana Lita, Brice Calkins, Thomas Gertis, Sae Woo Nam, Andrew G. White Quantum steering allows two parties to verify shared entanglement even if one measurement device is untrusted. A conclusive demonstration of steering through the violation of a steering inequality is of considerable fundamental interest and opens up applications in quantum communication. To date all experimental tests with single photon states have relied on post-selection, allowing untrusted devices to cheat by hiding unfavorable events in losses. Here we close this ``detection loophole'' by combining a highly efficient source of entangled photon pairs with superconducting transition edge sensors. We achieve an unprecedented $\sim $62{\%} conditional detection efficiency of entangled photons and violate a steering inequality with the minimal number of measurement settings by 48 standard deviations. Our results provide a clear path to practical applications of steering and to a photonic loophole-free Bell test. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X30.00009: Experimental Violation of Heisenberg's Precision Limit by Weak Measurements Ardavan Darabi, Lee A. Rozema, Dylan H. Mahler, Alex Hayat, Yasaman Soudagar, Aephraim M. Steinberg Along with the uncertainty principle, Heisenberg postulated another set of relations, which set a lower limit on the disturbance caused by a measurement [1]. These relations were shown by Ozawa to be inaccurate [2], shedding doubt on widely accepted bounds on the information left in a system after a measurement, and offering new insights into the foundations of quantum physics and quantum information. A theoretical scheme for testing Ozawa's precision-disturbance relations was proposed [3]. In this proposal the hurdle of destructive measurements is addressed by the weak value approach [4]. This scheme is based on a 3-qubit quantum circuit that requires two CNOT gates of variable strength with a common control qubit. Here, we present an experimental realization of Heisenberg's precision limit violation based on weak value measurements. We implement a technique inspired by the one-way quantum computing using entanglement as the substrate for quantum gates. In this way, we demonstrate a violation of Heisenberg's relation for measurement disturbance, confirming the revised bound due to Ozawa. \\[4pt] [1] Z. Phys. 43 172(1927); [2] Ann. Phys. NY 311 350(2004); [3] New J. Phys. 12 093011(2010); [4] Phys. Rev. Lett. 60 1351(1988) [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X30.00010: The rise of long-distance entanglement within a linear chain of ions Thomas Fogarty, Endre Kajari, Bruno G. Taketani, Alexander Wolf, Thomas Busch, Giovanna Morigi One stumbling block which limits our observation of quantum effects in the macroscopic world is decoherence. For this reason the study of decoherence and dissipation in open quantum systems has attracted a lot of attention. It has been shown that the generation of long distance entanglement is possible between oscillators via a harmonic crystal (Wolf et al, EPL, 95(2011) 60008). The aim of this current work is to propose an experimentally feasible setup to test the possibility of the creation of long distance entanglement. For this purpose we consider an ion chain in a linear Paul trap with two embedded impurities, whose transverse modes resemble the two degrees of freedom that we aim to entangle via the rest of the chain. With the aid of appropriately designed laser fields, the dynamics described in (Wolf et al, EPL, 95(2011) 60008) is reproduced. The resulting entanglement between the transverse modes of the impurities is analysed by means of the logarithmic negativity. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X30.00011: Popper's Thought Experiment Reinvestigated Chris Richardson, Jonathan Dowling Karl Popper posed an interesting thought experiment in 1934. With it, he meant to question the completeness of quantum mechanics. He claimed that the notion of quantum entanglement leads to absurd scenarios that cannot be true in real life and that an implementation of his thought experiment would not give the results that QM predicts. Unfortunately for Popper, it has taken until recently to perform experiments that test his claims. The results of the experiments do not refute QM as Popper predicted, but neither do they confirm what Popper claimed QM predicted. Kim and Shih implemented Popper's thought experiment in the lab. The results of the experiment are not clear and have instigated many interpretations of the results. The results show some correlation between entangled photons, but not in the way that Popper thought, nor in the way a simple application of QM might predict. A ghost-imaging experiment by Strekalov, et al. sheds light on the physics behind Popper's thought experiment, but does not try to directly test it. I will build the physics of Popper's thought experiment from the ground up and show how the results of both of these experiments agree with each other and the theory of QM, but disprove Popper. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X30.00012: Positive Noise Cross Correlation in a Copper Pair Splitter. Anindya Das, Yuval Ronen, Moty Heiblum, Hadas Shtrikman, Diana Mahalu Entanglement is in heart of the Einstein-Podolsky-Rosen (EPR) paradox, in which non-locality is a fundamental property. Up to date spin entanglement of electrons had not been demonstrated. Here, we provide direct evidence of such entanglement by measuring: non-local positive current correlation and positive cross correlation among current fluctuations, both of separated electrons born by a Cooper-pair-beam-splitter. The realization of the splitter is provided by injecting current from an Al superconductor contact into two, single channel, pure InAs nanowires - each intercepted by a Coulomb blockaded quantum dot (QD). The QDs impedes strongly the flow of Cooper pairs allowing easy single electron transport. The passage of electron in one wire enables the simultaneous passage of the other in the neighboring wire. The splitting efficiency of the Cooper pairs (relative to Cooper pairs actual current) was found to be $\sim $ 40{\%}. The positive cross-correlations in the currents and their fluctuations (shot noise) are fully consistent with entangled electrons produced by the beam splitter. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X30.00013: Scalable fiber integrated source for higher-dimensional path-entangled photons Robert Polster, Christoph Schaeff, Radek Lapkiewicz, Robert Fickler, Sven Ramelow, Anton Zeilinger Higher dimensional Hilbert spaces are expected to show intriguing higher order effects. Examples are higher dimensional perfect correlations or the unsolved problem of finding a complete set of MUBs for dimension 6. Higher dimensional systems also have advantages for QKD protocols. Our approach is to build an easy to use platform to access these dimensions. We realized an in-fiber, high brightness and high fidelity source for path-entangled quNits in the telecom band. It is purely integrated in fiber and only standard off-shelf components are used. This results in high stability and scalability in terms of complexity with increasing dimension. In order to manipulate and transform the produced entangled states we implemented multiports in integrated optical technology, enabling us to perform any unitary transformation depending on its internal settings[1]. Results up to dimension 4 (ququarts) will be presented in the talk. [1] M. Reck, A. Zeilinger, H. J. Bernstein and P. Bertani, PRL 73, 1 (1994) [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X30.00014: New Near-Deterministic Teleportation Protocol with Linear Optics Mladen Pavicic We present a new near-deterministic method of separating all four photon Bell states by means of concatenated Mach-Zehnder interferometers. Realistic proposals for implementations of teleportation, superdense coding, and cryptographic ping-pong protocols will be presented. Discrimination of the Bell states is made possible by two two linear chains of concatenated Mach-Zehnder interferometers each fed with photons emerging from two opposite sides of a beam splitter. This amounts to detecting two Bell states $|\Phi^\pm\rangle$ while keeping the third one $|\Psi^+\rangle$ conditionally at bay---thus going around Vaidman-L{\"u}tkenhaus 50\%-limit. Realistic implementation with an efficiency of 90\%\ is feasible with today's technology. Channel capacity of 1.98 can be ideally achieved for superdense coding with 5 concatenated Mach-Zehnder interferometers but already with only two easily implementable ones we obtain 1.74 capacity. The setup is based on a revised and corrected method given in M.\ Pavi{\v c}i{\'c}, {\it Phys.\ Rev\ Lett.\/} {\bf 107}, 080403 (2011). [Preview Abstract] |
Session X31: Focus Session: Topological Insulators: Synthesis and Characterization - Scanning Tunneling Spectroscopy
Sponsoring Units: DMPChair: Ali Yazdani, Princeton University
Room: 260
Thursday, March 1, 2012 2:30PM - 2:42PM |
X31.00001: Scanning tunneling spectroscopic studies of Dirac fermions and impurity resonances in the surface-state of a strong topological insulator Bi$_{2}$Se$_{3}$ H. Chu, M.L. Teague, C.-C. Hsu, N.-C. Yeh, L. He, K.-L. Wang, F.-X. Xiu Scanning tunneling spectroscopic studies of MBE-grown Bi$_{2}$Se$_{3}$ epitaxial films on Si (111) revealed surface-state (SS) characteristics of Dirac fermions and signatures of strong impurity resonances. The impurity resonances in this three-dimensional strong topological insulator (3D-STI) occurred near the Dirac energy (E$_{D})$ and diverged as the Fermi level (E$_{F})$ approached E$_{D}$. They were also highly localized within a region of radius $\sim $ 0.2 nm, beyond which the SS spectra of the 3D-STI recovered quickly, suggesting robust topological protection against non-magnetic impurities. Similar spectral characteristics and separations between E$_{F}$ and E$_{D}$ were also observed in the MBE-grown Bi$_{2}$Se$_{3}$ films on CdS. For sufficiently thin samples, opening of an energy gap due to wave-function overlap between the surface and interface layers was observed. The Rashba-like spin-orbit splitting further gave rise to spin-preserving quasiparticle interferences. Finally, the effect of different impurities (e.g. Cr and Mn) on the SS spectra of Bi$_{2}$Se$_{3}$ as a function of magnetic field will be reported. This work was supported by FENA and DARPA. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X31.00002: Scanning tunneling spectroscopy studies of Bi2Te2Se Ilya Drozdov, Haim Baidenkopf, Jungpil Seo, Shuang Jia, Robert Cava, Ali Yazdani Topological insulators are a class of semiconductors characterized by the presence of current-carrying helical surface states lying within the bulk gap. The surface states of these materials possess massless Dirac-like dispersion. Helical spin texture of the surface states leads to suppression of backscattering in these materials. Results of scanning tunneling spectroscopy study of Bi2Te2Se (BTS) topological insulator will be presented.~~Similar to previously studied Bi2Te3 and Bi2Se3 the new material shows a relatively large band gap and a simple surface band structure. High bulk resistivity and high surface electron mobility make it a compound of interest for potential applications. Differential conductance mapping with scanning tunneling microscope is used to visualize surface states of this novel highest-bulk-resistivity topological insulator. These experiments enable us to assess the variation of local density of states in this compound under different growth conditions and to correlate the findings with transport properties [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X31.00003: Scanning tunneling spectroscopy of stripe induced spatial modulations in the electronic structure of Bi2Te3+d Yoshinori Okada, Wenwen Zhou, Daniel Walkup, Chetan Dhital, Ying Ran, Ziqing Wang, Stephen Wilson, Vidya Madhavan To take full advantage of the unique properties of topological insulators, a huge amount of effort has been spent in exploring the tunability of their charge carrier density or magnetism by chemical doping, creating an ever increasing need for probing and understanding the real-space electronic response. In this study, we investigate the effects of a 1D periodic modulation (stripes) on the electronic structure of Bi2Te3+d. Using a combination of spectroscopic mapping, Fourier transform spectroscopy and Landau level measurements we map out the energy-momentum dispersion of the surface state over a large energy range and show that the stripes have a non-trivial effect on the bulk and surface state band structure. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X31.00004: Scanning tunneling microscopy study of ultrathin topological insulator Bi$_2$Te$_3$ nanoribbons Carolina Parra, Desheng Kong, Jason C. Randel, Alex W. Contryman, Francis Niestemski, Ming Rue D. Thian, Yi Cui, Hari C. Manoharan Currently there is an increasing interest in the study of topological insulators (TI) nanostructures as a result of their large surface-to-volume ratio, which allows the manifestation of surface conduction states without masking by bulk carriers. We performed low-temperature scanning tunneling microscopy (STM) measurements of the surface of TI Bi$_2$Te$_3$ nanostructures grown on HOPG. Although surface states of TIs are inherently robust against almost any surface modifications, these materials are prone to various surface chemical reactions which are taken into account when preparing samples for devices and STM study. Our STM measurements reveal the presence of ultrathin nanostructures (nanoribbons and nanoplates) which show an important growth anisotropy, with lateral dimension growing much faster than the vertical thickness dimension. Nanoribbons group mainly in bunches, with an aspect ratio of 1:300 and thickness down to 6 nm (6 quintuple layers). Nanoplates with hexagonal morphology of lower than 20 quintuple layers thick were also found, suggesting both one- and two-dimensional preferential growth. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X31.00005: The field dependence of quantized Landau levels on Bi$_{2}$Te$_{3+d}$ via scanning tunneling spectroscopy Daniel Walkup, Yoshinori Okada, Wenwen Zhou, Chetan Dhital, Stephen Wilson, Vidya Madhavan Measurements of Landau level (LL) spectra by scanning tunneling spectroscopy can provide important information on quasi-particle lifetime, effective g-factor as well as the dispersion of surface state bands. We have studied the effect of magnetic fields on spatially inhomogeneous samples of the topological insulator Bi$_{2}$Te$_{3+d}$. Using spatial maps of Landau levels as a function of magnetic field, we show that the surface state electrons near the Dirac point are surprisingly sensitive to perturbations. The magnetic field dependence also allows us to obtain an upper limit to the effective g-factor for the surface state electrons. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X31.00006: Landau quantization and the thickness limit of topological insulator thin films of Sb$_{2}$Te$_{3}$ Xucun Ma, Yepeing Jiang, Yilin Wang, Mu Chen, Canli Song, Zhi Li, Lili Wang, Ke He, Xi Chen, Qi-Kun Xue We report the experimental observation of Landau quantization of molecular beam epitaxy grown Sb$_{2}$Te$_{3}$ thin films by a low-temperature scanning tunneling microscope. Different from all the reported systems, the Landau quantization in Sb$_{2}$Te$_{3}$ topological insulator is not sensitive to the intrinsic substitutional defects in the films. As a result, a nearly perfect linear energy dispersion of surface states as 2D massless Dirac fermion system is observed. We demonstrate that 4 quintuple layers are the thickness limit for Sb$_{2}$Te$_{3}$ thin film being a 3D topological insulator. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X31.00007: Multiband quasiparticle interference in the topological insulator Cu$_x$Bi$_2$Te$_3$ Erik van Heumen, S. Johnston, E. Rienks, A. Varykhalov, F. Massee, N. de Jong, Y. Huang, J. Kaas, J.B. Goedkoop, M.S. Golden One of the main interests in topological materials is their purported robustness against disorder. In the Bi$_{2}$X$_{3}$ family (X=Se,Te) cubic spin-orbit coupling terms play an important role through their effect on the dispersion of the surface states. The cubic terms are also responsible for the restoration of Friedel like oscillations around impurity sites, which may be important in surface transport processes. We have therefore investigated how impurity scattering affects the surface states using a combination of Fourier-transform scanning tunneling spectroscopy (FT-STS) and calculations of the charge density oscillations expected from the cubic spin-orbit coupling terms [1]. FT-STS allows us to map out the energy-momentum relation of the important scattering wave-vectors, which can be compared to scattering vectors predicted from a self-consistent single impurity scattering calculation. To fully explain the features observed in our experiments we need to take the condu ction band into account. Namely, for energies where the surface states and the conduction band overlap, the dominant scattering process turns out to be interband impurity scattering. \\[4pt] [1] E. van Heumen et al., arXiv: 1110.4406 [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X31.00008: LT-STM study of the topological insulator Bi$_{2}$Se$_{3}$ with superconducting and/or magnetic over layers Rami Dana, Anita Roychowdhury, Irek Miotkowski, Yong P. Chen, Michael Dreyer Superconducting and/or magnetic over layers structures on topological insulator (TI) surfaces were suggested as a potential candidate to create Majorana bound states and 1D Majorana fermions (spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $, particle = antiparticle). The latter can serve as a platform for topological quantum computation since two Majorana bound states equal one fermion bound state and two degenerate states (full/empty) forming one qubit. In this work we study the TI Bi$_{2}$se$_{3}$ using a low temperature STM. The data shows the signature energy gap $\Delta \quad \sim $ 0.3 eV, metallic surface states and typical defects. Following the experimental challenges to create and detect Majorana bound or chiral edge states, we present our study of Nb and/or Fe films on the Bi$_{2}$se$_{3}$ surface. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X31.00009: Visualizing the inhomogeneous response of Dirac surface states to bulk disorder in topological insulators Haim Beidenkopf, Pedram Roushan, Jungpil Seo, Lindsay Gorman, Ilya Drozdov, Yew San Hor, Robert J. Cava, Ali Yazdani The Dirac dispersion and helical spin texture of surface states in topological insulators render them resilient to backscattering. This remarkable property, assured by time reversal symmetry, should give rise to enhanced surface conductivity as the Dirac states anti-localize in presence of disorder. We have used scanning tunneling microscopy (STM) and spectroscopy to study the response of the surface states to both magnetic and non-magnetic dopants [HB et al. Nat. Phys. doi:10.1038/nphys2108]. We find that helicity provides protection from scattering, irrespective of the magnetic nature of the individual scaterrers and even of ferromagnetic correlations among them. However, the charged defects in the bulk induce pronounced fluctuations in energy, momentum and helicity of the surface states. Agreement with a theoretical model, derived for the response of Dirac states to charged disorder in graphene, further implies that such fluctuations limit the attainable surface mobility. Although we show that the potential energy landscape induced by the bulk defects does not localize the Dirac surface states, our results suggest that reducing charged defects content is essential for tuning the chemical potential to the Dirac energy and enhancing mobility of the novel surface states. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X31.00010: Probing surface state conductance of topological insulator Bi$_{2}$Se$_{3}$ with scanning tunneling potentiometry Chockalingam Subbaiah, Frances Ross, Matthew Brahlek, Seongshik Oh, Abhay Pasupathy Topological insulators such as Bi$_{2}$Se$_{3}$ have unique surface states. How do electrons actually flow on the surface of a real Bi$_{2}$Se$_{3}$ sample? We study this question using scanning tunneling potentiometry. In this measurement, a lateral current flows in the sample while the local potential is mapped on the surface using a scanning tunneling microscope. This technique can be used to identify with atomic resolution the potential drops in the current-carrying pathways at the surface, and is ideally suited to measure the properties of quasi-2D materials such as graphene. Our topological insulator samples are MBE grown films of Bi$_{2}$Se$_{3}$ on a sapphire substrate. We will describe both the surface morphology and its effect on the current carrying pathways in the material. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X31.00011: Probing high energy, unoccupied states on the surface of pristine and Fe doped Bi$_{2}$Te$_{3}$ by scanning tunneling spectroscopy Wenwen Zhou, Yoshinori Okada, Daniel Walkup, Chetan Dhital, Hsin Lin, Arun Bansil, Stephen Wilson, Vidya Madhavan We probe the surface state of pristine and Fe-doped topological Insulator Bi$_{2}$Te$_{3}$ by using Fourier Transform Scanning Tunneling Spectroscopy (FT-STS). FT-STS allows us to probe the surface state dispersion far above the Fermi energy, a regime inaccessible to angle resolved photoemission spectroscopy. We report the observation of novel multi scattering channels that emerge at high energies along the $\Gamma $M and $\Gamma $K directions. The possible origins of these channels including spin-orbit scattering will be discussed. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X31.00012: STS studies of the surface of Bi2Se3 Megan Romanowich, Mal-Soon Lee, S.D. Mahanti, Stuart Tessmer, Duck Young Chung, Jung-Hwan Song, Mercouri Kanatzidis We apply scanning tunneling spectroscopy to characterize the surface of the topological insulator Bi2Se3. Spectroscopy reveals that the minimum in the local density of states (LDOS) does not actually vanish in the region where Dirac cone states exist. We demonstrate with density functional theory calculations that this can be understood in terms of an asymmetric addition to the LDOS associated with a contribution from the bulk valence band that overlaps in energy with the Dirac point. We will discuss the origin of the fluctuations in the LDOS seen in the experiment near 0.2 eV above the Dirac point, which are associated with tunneling into the lowest conduction band states. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X31.00013: STM imaging of impurity resonances on Bi2Se3 Zhanybek Alpichshev, Rudro Biswas, Alexander Balatsky, James Analytis, Jiun-Haw Chu, Ian Fisher, Aharon Kapitulnik In this paper we present detailed study of the density of states near defects in Bi2Se3. In particular, we present data on the commonly found triangular defects in this system. While we do not find any measurable quasiparticle scattering interference effects, we do find localized resonances, which can be well fitted by theory [1] once the potential is taken to be extended to properly account for the observed defects. The data together with the fits confirm that while the local density of states around the Dirac point of the electronic spectrum at the surface is significantly disrupted near the impurity by the creation of low-energy resonance state, the Dirac point is not locally destroyed. We discuss our results in terms of the expected protected surface state of topological insulators. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X31.00014: Probing Dirac Fermions in Bi-based Semimetals by Cryomagnetic Scanning Tunneling and Point Contact Spectroscopy J.Y.T. Wei, I. Fridman, Y.T. Yen, Hechang Lei, Kefeng Wang, C. Petrovic The topological semimetal Bi$_2$Se$_3$ is distinguished by the presence of two-dimensional Dirac fermions with strong spin-orbit coupling. The linear dispersion of Dirac fermions in Bi$_2$Se$_3$ was recently observed by scanning tunneling spectroscopy measurements of the Landau level spacing versus magnetic field. In this work we extend the field-dependent spectroscopy study of Dirac fermions to other Bi-based semimetals, for both topological and non-topological cases, using cryomagnetic scanning tunneling and point contact probes on single crystals down to 300 mK and up to 9 T. The spectral dependences on field strength and field direction are examined, in an effort to elucidate the role of spin-orbit coupling in each case. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X31.00015: Thickness and Wave-Vector Dependence in the Inter-Surface Coupling of Topological Surface States of Sb(111) Films Guanggeng Yao, Feng Pan, Ziyu Luo, Wentao Xu, Jiatao Sun, Andrew Thye Shen Wee, Yuanping Feng, Xue-Sen Wang Sb is a semimetal but possesses topological surface states (SSs) [1]. Taking advantage of quantum confinement effect for bulk states and topological protection for SSs, an Sb thin film could be a topological insulator. We explore this possibility using Fourier-transform scanning tunneling spectroscopy (FT-STS) and \textit{ab initio} calculations for Sb(111) films of thickness $\le $ 30 bilayers (BL). Quasiparticle interference (QPI) patterns of SSs and calculated band structures exhibit dramatic dependence on film thicknesses, reflecting variation of inter-surface coupling of SSs with film thickness and wave vector \textbf{k}. One Kramers-pair of SSs forming a Dirac point at \textbf{k} = 0 exist on each surface for 6-BL or thicker films. The inter-surface coupling of SSs not far away from \textbf{k} = 0 is significant in the QPI patterns at $\sim $ 10 BL. Such coupling is due to a relative large penetration depth of these SSs results in unpolarized states with large wave function amplitude in the interior of the film. [1] D. Hsieh et al., Science 323, 919 (2009); K. K. Gomes et al., e-print arXiv:0909.0921 (2009); J. Seo et al., Nature 466, 343 (2010). [Preview Abstract] |
Session X32: Focus Session: Dielectric, Ferroelectric, and Piezoelectric Oxides - Elastic and Optical Properties
Sponsoring Units: DMP DCOMPChair: Marty Gregg, Queen's University Belfast
Room: 261
Thursday, March 1, 2012 2:30PM - 3:06PM |
X32.00001: Flexoelectricity in Nanoscale Ferroelectrics Invited Speaker: Gustau Catalan All ferroelectrics are piezoelectric and thus have an intrinsic coupling between polarization and strain. There exists an additional electromechanical coupling, however, between polarization and strain gradients. Strain gradients are intrinsically vectorial fields and, therefore, they can in principle be used to modify both the orientation and the sign of the polarization, thanks to the coupling known as flexoelectricity. Flexoelectricity is possible even in paraelectric materials, but is generally stronger in ferroelectrics on account of their high permittivity (the flexoelectric coefficient is proportional to the dielectric constant). Moreover, strain gradients can be large at the nanoscale due to the smallness of the relaxation length and, accordingly, strong flexoelectric effects can be expected in nanoscale ferroelectrics. In this talk we will present two recent results that highlight the above features. In the first part, I will show how polarization tilting can be achieved in a nominally tetragonal ferroelectric (PbTiO$_{3})$ thanks to the internal flexoelectric fields generated in nano-twinned epitaxial thin films. Flexoelectricity thus offers a purely physical means of achieving rotated polarizations, which are thought to be useful for enhanced piezoelectricity. In the second part, we will show how the large strain gradients generated by pushing the sharp tip of an atomic force microscope against the surface of a thin ferroelectric film can be used to actively switch its polarity by 180$^{\circ}$. This enables a new concept for ``multiferroic'' memory operation in which the memory bits are written mechanically and read electrically. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X32.00002: Giant Flexoelectric Effect in Ferroelectric Epitaxial Thin Films Tae Won Noh, Daesu Lee, A. Yoon, M. Kim, J.-G. Yoon, J.-S. Chung, J.F. Scott The flexoelectric effect describes an electric field that is generated by a strain gradient. Owing to its universal nature, flexoelectricity has inspired broad scientific interest and has application potential, particularly in flexible systems. In solids, however, there has been little investigation into flexoelectricity, due to its minuscule magnitude by limited elastic deformation. In this presentation, we will develop a general framework for realizing and modulating the giant flexoelectric effect in epitaxial oxide thin films, emphasizing the key role of flexoelectricity in solids.\footnote{D. Lee \textit{et al.}, Phys. Rev. Lett. \textbf{107}, 057602 (2011).} In epitaxial oxide thin films, a lattice mismatch between the film and the substrate can result in strain relaxation within tens of nanometers of the film/substrate interface, inducing a large strain gradient. We observed the nanoscale strain gradients in ferroelectric HoMnO$_{3}$ epitaxial thin films, which were 6 or 7 orders of magnitude larger than typical values reported for bulk oxides. Giant flexoelectric effect by the nanoscale strain gradient provides a means of tuning the physical properties of ferroelectric epitaxial thin films, such as domain configurations and hysteresis curves.\footnote{D. Lee \textit{et al.}, Phys. Rev. B \textbf{81}, 012101 (2010).} [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X32.00003: Thermal Stresses in Ferroelectric Thin Films and Their Role on the Dielectric, Pyroelectric, and Electrocaloric Properties Jialan Zhang, S. Pamir Alpay In-plane strains develop in thin films due to thermal stresses that arise from differences between the thermal expansion coefficients of the film and the substrate upon cooling from a growth temperature ($T_{G})$ to room temperature (RT). In ferroelectric (FE) films, there is a coupling between strain and polarization through electrostriction. Therefore, thermal strains may have profound effects on the dielectric, pyroelectric, and electrocaloric (EC) responses of FE films. We provide here a quantitative thermodynamic model to investigate the role of in-plane thermal strains on these properties. We show that there is a substantial degradation in the dielectric response and tunability of SrTiO$_{3}$ films on IC-friendly substrates such as Si and $c$-sapphire due to tensile thermal in-plane strains. Our analysis on (001)-textured polycrystalline Ba$_{x}$Sr$_{1-x}$O$_{3}$ (BST) films with different compositions indicates that for BST 60/40 and BST 70/30 films, the pyroelectric response does not display a significant dependence on$ T_{G}$ if the FE is in a paraelectric state. Furthermore, we show that for BaTiO$_{3}$ films on Si, the thermal stresses are sufficient to shift the zero-field Curie temperature to RT, resulting in a strong enhancement of the EC properties as compared to the bulk material. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X32.00004: Polarization in Perovskite Manganites induced by Shear Stress K. Miyano, N. Ogawa, Y. Ida, R. Tamaki, K. Shimizu, Y. Nomura, R. Arita, Y. Ogimoto We found static polarization in perovskite manganite films when they are under shear stress. The phenomenon is omnipresent in films deformed in (at least) monoclinic fashion due to the substrate-imposed strain, whereas it is absent in bulk crystals even though they are distorted in a similar manner in thermal equilibrium. The substrate stress of low symmetry is clearly the driving force for the appearance of the polarization. Optical second-harmonic generation (SHG) confirms the loss of inversion symmetry in strained films and pyroelectricity was detected in insulating films confirming the presence of the static polarization. DFT calculations show that the stable atomic positions in the experimentally observed structure is polar with the shift of the center of gravity of anions relative to that of cations as much as $10^{-2} {\AA}$. The calculated polar structure is consistent with the symmetry obtained from the SHG polarimetry. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X32.00005: Discovery of strain glass transition in non-metallic ferroelastic Pei Zhang, Dezhen Xue, Xiaobing Ren Strain glass, a glassy state of lattice strain, has been identified in alloys with shuffle being the principle order parameter and strain being the secondary order parameter. However, it is well known that many non-metallic ferroelastic systems possess long range order with tilt being the first order parameter. But the existence of the glassy state of such strain caused by tilt remains unclear. In the present study, we report that the strain glass indeed exists in the non-metallic ferroelastic material, a Sr and Nb co-doped LaAlO$_{3}$ system, with randomly frozen tilt strain local order. With increasing defect concentration x in La$_{1-x}$Sr$_{x}$Al$_{0.95}$ Nb$_{0.05}$O$_{3}$, the martensitic transition is gradually suppressed and finally strain glass transition occurs. The glassy transition is characterized by a typical frequency dispersion of modulus, a broken of ergodicity for static strain, as well as the formation of nano-domains with R local structure. Due to the strong local barrier caused by the randomly distributed point defects, the ideal freezing temperature $T_{0}$ of strain glass in this system increases with defect concentration, which can be well understood by a modified Landau free energy landscape. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X32.00006: Enhanced Piezoelectricity in PbTiO$_3$/BaTiO$_3$ Superlattices T. Yusufaly, B. Ziegler, V.R. Cooper, S.J. Callori, J. Sinsheimer, K.M. Rabe, P. Chandra, M. Dawber First-principles calculations by Cooper and Rabe predict an enhancement of the piezoelectric coefficient ($d_{33}$) in PbTiO$_3$/BaTiO$_3$ (PTO/BTO) superlattices for intermediate values of PTO concentration. PTO/BTO superlattices have been fabricated using an off-axis RF magnetron sputtering technique, enabling x-ray diffraction, electrical measurements and atomic force microscopy on this system. The experimental results agree with the calculated polarization, tetragonality and enhanced piezoelectricity as a function of PTO concentration. Additional first-principles calculations indicate that the enhancement in $d_{33}$ is more pronounced in shorter-period superlattices. By applying a Landau-Devonshire model to this system, we find that the enhanced piezoelectricity is due to the combination of a bulk effect associated with the presence of finite electric fields in each layer and interfacial effects. Implications of our results for future experiments are discussed. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X32.00007: Interpolation schemes for high-throughput prediction of new piezoelectric alloys Rickard Armiento, Boris Kozinsky, Marco Fornari, Gerbrand Ceder Systematic discovery of materials with optimized properties based on first principles methodologies requires well-defined descriptors, in addition to the automation infrastructure for calculations and data analysis. We have designed a set of computationally affordable descriptors for enhanced piezoelectric performances and analyzed the chemical space for oxides with the perovskite structure. Our results include phase stability for the most promising compositions and ad hoc interpolation schemes that have been exploited to identify 49 alloys with favorable properties. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X32.00008: Piezoelectric characteristics of PZT thin films on polymer substrate Min-Gyu Kang, Younh-Ho Do, Seung-Min Oh, Rheza Rahayu, Yiyein Kim, Chong-Yun Kang, Sahn Nahm, Seok-Jin Yoon The goal of piezoelectric energy harvesting is to improve the power efficiency of devices. One of the approaches for the improvement of power efficiency is to apply the large strain on the piezoelectric materials and then many scientists approached using thin films or nano-structured piezoelectric materials to obtain flexibility. However, the conventional thin film processes available for the fabrication of piezoelectric materials as PbZr$_{0.52}$Ti$_{0.48}$O$_{3}$ (PZT) are not compatible with flexible electronics because they require high processing temperatures ($>$700$^{o}$C) to obtain piezoelectricity. Excimer laser annealing (ELA) is attractive heat process for the low-temperature crystallization, because of its material selectivity and short heating time. In this study, the amorphous PZT thin films were deposited on polymer substrate by rf-sputtering. To crystallize the amorphous films, the ELA was carried out with various conditions as function of the applied laser energy density, the number of pulse, and the repetition rate. To evaluate the piezoelectric characteristics, piezoelectric force microscopy (PFM) and electrometer are used. As a result, we obtained the crystallized PZT thin film on flexible substrate and obtained flexible piezoelectric energy harvester. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X32.00009: Characterizing the Angular Frequency of Radiative Polaritons using Infrared Spectroscopy Anita Vincent-Johnson, Giovanna Scarel Polaritons are important for understanding the optical properties of oxide films and possibly also for energy conversion. The two known types of polaritons: surface phonon polaritons (SPP) and radiative polaritons (RP), form when infrared (IR) photons enter a crystal lattice material and couple with the phonons present. While SPP's are largely studied for their heat transfer properties, RP's are typically not studied; therefore, much is still not understood about RP's. It is known, however, that RP's have a complex angular frequency, which includes a real part, Re($\omega$), and an imaginary part, Im($\omega$). Investigations done by our Group suggests that Im($\omega$) indicates the frequency of the radiated field. What is unknown is the relationship between Re($\omega$) and Im($\omega$). Therefore, we experimentally compare three different oxides deposited on silicon and aluminum by atomic layer deposition (ALD). This allows us to characterize proportionality between Re($\omega$) and Im($\omega$) with respect to oxide film thickness. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X32.00010: Dielectric and optical properties of SrTiO$_3$ films deposited from metallo-organic solution M. Spies, L.S. Abdallah, S. Zollner, C.V. Weiss, J. Zhang, S.P. Alpay, M.W. Cole SrTiO$_3$ thin films on Si were grown by metallo-organic solution deposition. Spectroscopic ellipsometry was used to determine the ellipsometric angles $\psi$ and $\Delta$ in the 0.6 to 6.6 eV spectral range at three angles of incidence. From the region below 3.5 eV (where the films are transparent), we are able to determine the refractive index and the film thickness, which ranges from 140 to 340 nm for different films. The refractive index is similar for all of our films, but 25\% lower than that of bulk SrTiO$_3$. By contrast, the low-frequency dielectric constant of similar films grown on metalized Si substrates is about the same as bulk SrTiO$_3$. Using a B-spline parametrization, we are able to determine the dielectric function of our films from the ellipsometric angles. We find an onset of absorption (band gap) of about 3.7 eV and similar interband electronic transitions of our films as for bulk SrTiO$_3$. For our films, the interband peaks are broadened due to poly-crystalline disorder and have a lower amplitude. The reason for the discrepancy between the low-frequency (vibrational) and the high-frequency (electronic) dielectric constant is unclear. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X32.00011: Nonlinear photonic crystals of BaTiO$_{3}$ and their electro-optic properties Bruce Wessels, Jianheng Li, Zhifu Liu Future optical systems will require electro-optic (EO) modulators with bandwidths of 100 GHz and low drive voltage. To achieve this, non-linear photonic crystals using epitaxial BaTiO$_{3}$ have been proposed. In our work two-dimensional photonic crystal (PhC) structures with a hexagonal array were fabricated from an epitaxial BaTiO$_{3}$ thin film using focused ion beam milling. The PhC waveguides were based on Si$_{3}$N$_{4}$/BTO/MgO multilayer epitaxial thin film structure. Simulation shows that sufficient refractive index contrast is achieved at 1550 nm to form a bandgap in the PhC structure by milling through the Si$_{3}$N$_{4}$ and BTO layers. The measured transmission spectrum of the PhC waveguide exhibited a stop-band centered at 1550 nm with well-defined band edges. Photonic crystal electro-optic modulators with a bandwidth of greater than 50 GHz have now been realized. The question of what dielectric properties limit the ultimate bandwidth of the PhC will be addressed. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X32.00012: Electro-optic contribution of optically generated small bound polarons in nominally undoped, thermally reduced LiNbO$_3$ Mirco Imlau, Hauke Bruening Recently we have shown that a spatial modulation of optically generated densities of small bound polarons can be applied for hologram recording in LiNbO$_3$ [1]. This new type of recording mechanism is of particular interest for the field of nonlinear and ultrafast photonics because of small bound polaron generation on the fs-scale. The grating recording via the photochromic response of small bound polarons was successfully applied to explain gratings recorded with a grating vector aligned orthogonal to the polar c-axis. In this contribution we study the relation of optically generated small bound polarons with pronounced index changes, that were found with values up to 10$^{-4}$ and a grating vector parallel to the c-axis. The Pockels effect that must be driven by an internal electric space-charge field is taken into account. In contrast to the classical photorefractive effect, where slow and long-ranging transport mechanisms must be considered, we discuss the build-up of the space-charge field on the sub-ps-time scale in the model of optical absorption of small polarons, i.e., the optically generated polaron hopping to next-neighboring lattice sites.\\[4pt] [1] M. Imlau et al., Optics Express 19, 15322 (2011) [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X32.00013: Dispersive properties of small polaron-based hologram recording in nominally undoped, thermally reduced LiNbO$_3$ Hauke Bruening, Mirco Imlau We recently discovered a new type of hologram recording in nominally undoped, thermally reduced LiNbO$_3$ by means of a single intense ns-laser pulse ($\lambda = 532\,$nm) yielding short-lived volume phase-gratings with unique features [1]: a diffraction efficiency of more than 20\% in the NIR spectral range ($\lambda = 785\,$nm), a stretched-exponential relaxation behavior of the grating efficiency with a lifetime in the ms-range at room temperature and a pronounced dependence on the orientation of the grating vector with respect to the polar c-axis. This type of hologram recording could be successfully modeled by taking into account an optically generated spatial modulation of small electron bipolarons, small bound and free electron polarons. In this contribution we face the unique dispersive properties of this type of hologram recording and particularly present our results for probing light in the blue-green spectral range ($\lambda = 488\,$ nm). We show that the further contribution of small bound hole polarons must be taken into account. Furthermore, we conclude that a considerable diffraction efficiency at the telecommunication wavelength ($\lambda = 1550\,$ nm) can be expected.\\[4pt] [1] M. Imlau et al., Optics Express 19, 15322 (2011) [Preview Abstract] |
Session X33: Focus Session: X-ray, Gamma Ray, and Electron Diffraction
Sponsoring Units: GIMSChair: Zahir Islam, Argonne National Laboratory
Room: 106
Thursday, March 1, 2012 2:30PM - 2:42PM |
X33.00001: First-principles study of $\gamma$-ray detectors: Cs-based compounds Hosub Jin, Jino Im, Arthur Freeman, Bruce Wessels, Mercouri Kanatzidis In an effort to find good candidate materials for $\gamma$-ray detectors, Cs-based compounds containing heavy elements, such as in Cs$_2$Hg$_6$S$_7$, were investigated. We performed $ab$-$initio$ density functional theory calculations using the full-potential linearized augmented plane wave method\footnote {Wimmer, Krakauer, Weinert, Freeman, Phys. Rev. B, {\bf 24}, 864 (1981)}. The screened-exchange local density approximation (sX-LDA) scheme was employed to correct the underestimation of the band gap in the LDA method. As a result, the band gap of Cs$_2$Hg$_6$S$_7$ is predicted to be 1.23 eV by sX-LDA, corrected from 0.51 eV by LDA. Therefore, Cs$_2$Hg$_6$S$_7$ seems to be useful as a $\gamma$-ray detecting material in terms of the high density and the band gap. Not only the pristine bulk but also several defect configurations were calculated, which affects the transport properties. Defect formation energy determinations allow us to predict which defect configuration is most likely in Cs$_2$Hg$_6$S$_7$. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X33.00002: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X33.00003: Resolving Interface Interactions in Layered Structures in 3 Dimensions Rozaliya Barabash, Jon Tischler, John Bidai, Wenjun Liu 3D micro-Laue diffraction was used to probe interface interactions in layered structures. Indented Cr/NiAl composite with alternating submicron size Cr and NiAl lamellae was chosen as a model material. Differential aperture microscopy revealed a twin orientation relationship at the interface between the Cr and NiAl lamellae in the as grown state. The indentation-induced alternation of compressive/tensile residual strains in the neighboring Cr and NiAl lamellae was observed. Line broadening analysis found a two orders of magnitude increase of dislocation density in the near-surface zone in the center of the indent. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X33.00004: X-ray Studies of Quantum Fluctuations in SrTiO3 across its Quantum Paraelectric Phase Transition Shih-Chang Weng, Ruqing Xu, Ayman Said, Hawoong Hong, Tai-Chang Chiang Strontium titanate (SrTiO3), with a simple cubic perovskite structure at room temperature, displays a number of interesting phase transitions as a function of temperature and pressure. It exhibits an antiferrodistortive transition at Tc = ~105 K, resulting in a cubic-to-tetragonal structural distortion. This transition has been studied in detail by M. Holt and H. Hong using thermal diffuse scattering and inelastic x-ray scattering methods. Another phase transition takes place at 37 K, below which the system assumes a quantum paraelectric phase. It is generally believed that the classical free energy of the system favors a ferroelectric phase, but this transition never fully develops because of quantum fluctuation. Despite a large number of prior investigations, this quantum phenomenon is still not well understood in terms of the underlying lattice dynamics. Using inelastic x-ray scattering, we have obtained new information about this classic system, including mode softening as a function of temperature. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X33.00005: X-ray nanotomography study of insulator-coated tips with sub-micron conducting apex for the combination of scanning probe microscopy and synchrotron radiation Volker Rose, TeYu Chien, John Freeland, Daniel Rosenmann, Robert Winarski Hard X-ray nanotomography provides an important three-dimensional view of insulator-coated ``smart tips'' that can be utilized for modern emerging scanning probe techniques. Tips, entirely coated by an insulating SiO2 film except at the very tip apex, are fabricated by means of electron beam physical vapor deposition, focused ion beam milling and ion beam-stimulated oxide growth. Although x-ray tomography studies confirm the structural integrity of the oxide film, transport measurements suggest the presence of defect-induced states in the SiO2 film [1]. The development of insulator-coated tips can facilitate nanoscale analysis with electronic, chemical, and magnetic contrast by synchrotron-based scanning probe microscopy. \\[4pt] [1] Rose at al., Appl. Phys. Lett. 99, 173102 (2011). [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X33.00006: Grazing-incidence coherent x-ray imaging in true reflection geometry Tao Sun, Zhang Jiang, Joseph Strzalka, Jin Wang The development of the 3$^{rd}$ and 4$^{th}$ generation synchrotrons has stimulated extensive research activities in x-ray imaging techniques. Among all, coherent diffractive imaging (CDI) shows great promise, as its resolution is only limited by the wavelength of the source. Most of the CDI work reported thus far used transmission geometry, which however is not suitable for samples on opaque substrates or in which only the surfaces are the regions of interest. Even though two groups have performed CDI experiments (using laser or x-ray) in reflection geometry and succeeded in reconstructing the planar image of the surface, the theoretical underpinnings and analysis approaches of their techniques are essentially identical to transmission CDI. Most importantly, they couldn't obtain the structural information along sample thickness direction. Here, we introduce a reflection CDI technique that works at grazing-incidence geometry. By visualizing Au nanostructures fabricated on Si substrate, we demonstrate that this innovative imaging technique is capable of obtaining both 2D and 3D information of surfaces or buried structures in the samples. In the meanwhile, we will also explain the grazing-incidence-scattering based-algorithm developed for 3D phase retrieval. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X33.00007: Soft x-ray absorption spectroscopy measurement methods with using x-ray scattering techniques H. Jang, J.-S. Lee, W.-S. Noh, K.-T. Ko, K.-B. Lee, B.-G. Park, J.-Y. Kim, S.H. Chun, K.H. Kim, J.-H. Park Methodology via x-ray absorption spectroscopy (XAS) has been actively employed for exploring the microscopic aspects of materials. In particular, such method within soft x-ray energy range is very useful for investigating strongly correlated systems, such as high T$_C$ superconductor, and multiferroic, including heterostructures. While XAS approach on such materials has been used, however we sometimes confront a few of experimental difficulties; electron motion distortion under external fields, charging effect, and saturation effect. In this presentation, we introduce an alternative approach for overcoming the difficulties in conventional XAS measurement, which uses soft x-ray scattering techniques, i.e., reflection and diffraction. Due to photon-in and photon-out nature, probing depth becomes longer and possible to reduce several problems in conventional total electron yield method. The results of demonstrations on simple monoxide CoO and multiferroic Y-type hexaferrites will be given. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X33.00008: Structure uncovering from fluctuation x-ray scattering of randomly oriented nanoparticles Gang Chen, Miguel Modestino, Billy Poon, Andre Schirotzek, Stefano Marchesini, Rachel Segalman, Alexander Hexemer, Peter Zwart We have carried out a fluctuation x-ray scattering experiment on platinum coated gold nanoparticles randomly oriented on a substrate. A complete algorithm for determining the electron density of an individual particle from diffraction patterns of many particles, randomly oriented about a single axis is demonstrated. This algorithm operates on angular correlations among the measured intensity distributions and recovers the angular correlation functions of a single particle from measured diffraction patterns. Taking advantage of the cylindrical symmetry of the nanoparticles, we proposed a cylindrical model to reconstruct the structure of the nanoparticle by fitting both the experimental ring angular auto-correlation and the small angle scattering data. The physical meaning of the resulted structure is discussed in terms of their statistical distributions of the shape and electron density profile. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X33.00009: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X33.00010: Hard X-ray resonance in sapphire crystal cavities using back diffraction Y.-H Wu, Y.-W Tsai, Y.-Y Chang, C.-H Chu, David G. Mikolas, C.-C Fu, S.-L Chang The Fabry-Perot type resonators using back diffraction from sapphire crystals for hard X-ray was investigated. On the basis of its less absorption and hexagonal structure, the resonator in sapphire crystals underwent a pure 2-beam diffraction which could enhance the resonance interference and improve finesse compared with the one in silicon crystals. The resonators were manufactured from sapphire crystals using microelectronic lithography process with thickness of a few tens $\mu $m. With synchrotron radiation of energy resolution $\Delta $E=0.82 meV at 14.315 keV, X-ray back diffraction from two monolithic sapphire crystal plates shows resonance fringes clearly resulting from coherent interaction inside the energy gap of the (0 0 30) reflection. These experimental results of sapphire cavities imply the potential application for X-ray optics. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X33.00011: X-ray wave guiding using three-beam Bragg-surface diffraction Shih-Lin Chang, Yan-Zong Zheng A diffraction-type of X-ray wave guide, in contrast to refraction-type, is proposed using three-beam diffraction geometry to generate a surface diffracted beam propagating along the direction of the wave guide. The three-beam Bragg-Surface diffraction involves a symmetric Bragg reflection and a surface diffraction. The former is used to guide a wide-angle incident beam into a silicon crystal. The simultaneously occurring surface diffraction then guides the diffracted beam propagating along the direction of the wave guide that is parallel to the crystal surface. A wave guide with a shallow ditch is then manufactured along the direction of the surface diffraction using the conventional lithographic technique. As a whole the wave guide consists of a three-layer structure of tantalum/photon resist (PMMA)/tantalum, on the Silicon substrate. The surface diffracted X-rays can then be confined in and guided along the layer of photon resist. Details of the design of the wave guide and synchrotron diffraction experiments will be reported. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X33.00012: Combined surface plasmon resonance and X-ray absorption spectroscopy Miguel Angel Garcia, Aida Serrano, Oscar Rodriguez de la Fuente, German R. Castro We present a system for the excitation and measurement of surface plasmons in metallic films based on the Kretschmann-Raether configuration that can be installed in a synchrotron beamline. The device was mounted an tested in a hard X-ray Absorption beamline, BM25 Spline at ESRF. Whit this device it is possible to carry on experiments combining surface plasmon and X-ray absorption spectroscopies. The surface plasmons can be use to monitor in situ changes induced by the X-rays in the metallic films or the dielectric overlayer. Similarly, the changes in the electronic configuration of the material when surface plasmons are excited can be measured by X-ray absorption spectroscopy. The resolution of the system allows to observe changes in the signals of the order of 10$^{-3}$ to 10$^{-5}$ depending on the particular experiment and used configuration. The system is available for experiments at the beamline. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X33.00013: Probing of Strain Mediated Hybrid Multiferroic Devices Edwin Fohtung, J. KIm, M. Marsh, Na Lei, S. Chen, S. Sinha, D. Ravelosona, Eric Fullerton, Oleg Shpyrko Smart materials for sensor technology, (non) volatile device memories for information technology, and ultrasound generators in medical imaging have one thing in common, their active elements consist of ferroelectrics (FE) driven by voltages or ferromagnetics (FM) driven by magnetization. In the quest to design high functionality devices to meet today's consumer technological demands, high focus has been given to multiferroic [1]. However, the coexistence of magnetic order and ferroelectric polarization combined in a single-phase material has proven to be rear as most of these materials tend to have low magnetic ordering temperatures and are often antiferromagnets, in which the magnetoelectric (ME) coupling effect is intrinsically small. We utilize an alternative approach to design multiferroic-hybrid devices based on FE-FM composites where the ME coupling emerges from strain-mediated interaction between individual phases [2]. We develop a nonlinear thermodynamic theory for strain-mediated direct ME effect and Bragg Ptychographic Coherent Diffraction Imaging (BCDI) serves as the unique tool of choice for sub-nanometer resolution nondestructive probing of the order parameters in the devices [1] N. Spaldin and M. Fiebig, Science 309, 391 (2005). [2] E. Fohtung et al., submitted (2012) [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X33.00014: First-Principles Modeling for Low-Energy Electron Diffraction Spectra John McClain, Jiebing Sun, James Hannon, Karsten Pohl, Jian-Ming Tang We present a computational method to incorporate density-functional theory (DFT) into the calculation of the reflectivity in low-energy electron diffraction (LEED). Rapid and accurate analysis of diffraction spectra will facilitate the development of low-energy electron microscopy. The dynamical analysis of the electron reflectivity is traditionally carried out using multiple scattering theory with spherically symmetric (muffin-tin) potentials. However, for directionally bonded materials, such as semiconductors, the actual crystal potentials in the interstitial region can be significant, particularly for very low energy electrons. DFT with nonlocal pseudopotentials yields the low-energy electronic structure more accurately. In typical DFT calculations for surfaces, a finite slab is set up in a large unit cell with periodic boundary conditions. By matching the plane waves representing the LEED beams to the Kohn-Sham wave functions at the boundaries of the supercell, we determine the diffraction intensities. To demonstrate that our matching approach is not limited by the finite size of the supercell, we first consider trial models with exact solutions. We then use this approach to analyze the electron diffraction from graphene and compare with features in the band structure. [Preview Abstract] |
Session X34: Focus Session: Nano VI: Junctions and Transport
Sponsoring Units: DCPChair: Shiv Khanna, Virginia Commonwealth University and Gabor Somorjai, UC Berkeley
Room: 107A
Thursday, March 1, 2012 2:30PM - 2:42PM |
X34.00001: Electronic transport through a light-driven azobenzene molecule switch: A revisit by density functional theory study Yan Wang, Hai-Ping Cheng Azobenzene, a molecule that changes conformation between \emph{trans} and \emph{cis} configurations, is a candidate light-driven molecule switch. Recent experiments showed that the ``on'' state with larger measured conductance is associated with the \emph{cis} isomer, which is in contrast with our previous theoretical prediction. Here we reconsider the issue of the molecule-electrode and electrode-electrode coupling by performing a first-principles study of the electronic structures and transport properties of Au-azobenzene-Au molecule junctions. Specifically, we investigate the dependence of the conductance and the current-voltage characteristics in two types of Au electrode, 2-D Au(111) surface and 1-D Au STM tip. We find that, not only the \emph{trans} to \emph{cis} transformation of the molecule, but also the electrode-electrode coupling plays a critical role in determining the conductance near the Fermi level. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X34.00002: Reliable anchoring groups for single-molecule junctions M. Teresa Gonz\'alez, Edmund Leary, Charalambos Evangeli, Carlos Arroyo, Gabino Rubio-Bollinger, Nicol\'as Agra\"It In the field of molecular electronics, thiols have been extensively used as the most common anchoring groups to bind molecules to gold electrodes. However, other anchoring groups as amines can provide interesting advantages. Recently, C$\-{60}$ has been also proposed as a possible very efficient binding group. In this talk, I will present our studies on molecular junctions formed by thiol-, amine-, and C$\-{60}$-terminated molecules. We use a STM (scanning tunneling microscope) break-junction technique to create and characterized single-molecule junctions both in ambient and liquid environment. We compare thiols and amines on the alkane family and an oligo(phenylene ethynylene). Our study of the molecular-junction stretching length allows us to conclude that thiols affect atomic rearrangement at the electrodes significantly more than amines. Using C$\-{60}$-terminated molecules, we have recently introduced a new technique for controllably wiring one molecule at a time. We first get STM images to located isolated molecules on a gold substrate, which are then specifically targeted and contacted using a STM gold tip. This technique offers a significant improvement over other techniques, as it guaranties that one and only one molecule is contacted at a time between the electrodes. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X34.00003: Novel quantum interference effects in transport through molecular radicals Justin Bergfield, Gemma Solomon, Charles Stafford, Mark Ratner In molecules with an unpaired electron (radicals), we predict a correlation-induced `Mott-node' in the transmission spectrum arising from destructive interference between transport contributions from different charge states of the molecule. This class of quantum interference effect has no single-particle analog and cannot be described by effective single-particle theories. Large errors in the thermoelectric properties and nonlinear current-voltage response of molecular radical junctions are introduced when the complementary wave and particle aspects of the electron are not properly treated. A method to accurately calculate the low-energy transport through a radical-based junction using an Anderson model is given. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X34.00004: Electron Transport through Porphyrin in Nanoscale Junctions Swatilekha Saha, Guoguang Qian, Kim M. Lewis As electronic devices become exceedingly small, incorporation of molecules as circuit elements is an attractive option due to their small size and the new functions that molecules can bring to the existing microelectronics. To realize such devices we have fabricated nanogaps of size 2-3 nm on gold wires and positioned porphyrin molecules in the gap. I-V characteristics with and without molecules in the junction is performed and signatures of molecular transport has been identified. We measure inelastic electron tunneling spectra (IETS) from molecular junctions at 4K in ultra-high vacuum to study its vibrational modes. These measurements will be compared to Raman spectra. We discuss how IETS can provide valuable insight to the metal-molecule coupling and the role ligated atoms can play in electron transport properties. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X34.00005: Modeling Surface and Stress-Anisotropy Effects on Transformations in Lead Sulfide Nanocrystals Under Pressure Clive Bealing, Richard Hennig The semiconductor PbS, which displays a small band gap and large excitonic Bohr radius, presents an ideal candidate material for such devices as infrared photon detectors and nanocrystal solar cells.\footnote{J. Choi, et al., Nano Lett., {\bf9}, 3749 (2009)} This has motivated a number of studies into the effect of pressure on bulk PbS and PbS nanocrystals (NCs), which organize into highly periodic superlattices with interesting mechanical properties.\footnote{P.~Podsiadlo, \emph{et al.}, Nano Lett.,{\bf11}, 579 (2010)} The ambient-pressure NaCl-type structure of PbS undergoes a transformation to an orthorhombic structure close to 2.5 GPa, which itself transforms to the CsCl-type structure at 21.5 GPa. We have identified competitive minimum energy paths between the different modifications of PbS using density-functional calculations, and have calculated the associated enthalpy barriers over a range of pressures. In empirical molecular dynamics simulations of the PbS NC transformation under pressure, the effect on the transformation of anisotropic stresses, applied perpendicular to the $\{100\}$- and $\{111\}$-type facets of the NC, has been investigated. The effect of the NC surface on the stability of metastable modifications in PbS NCs is also considered. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X34.00006: Compositional Distribution and Electronic Structure of Ternary Compound Semiconductor Nanocrystals Sumeet Pandey, Dimitrios Maroudas We present a first-principles-based theoretical study of compositional distribution and the resulting electronic structure of ternary quantum dots (TQDs) of compound semiconductor nanocrystals. The analysis is based on first-principles Density Functional Theory (DFT) calculations of atomic and electronic structure and on Monte Carlo (MC) simulations of compositional distribution according to DFT-parameterized valence force field models. We report results for ZnSe$_{1-x}$S$_{x}$ (type-I), ZnSe$_{1-x}$Te$_{x}$ (type-II), and In$_{x}$Ga$_{1-x}$As (Reverse type-I) TQDs with nm-scale diffusion lengths and large surface-to-volume ratios. The equilibrium compositional distribution is predicted as a function of overall composition (x) and TQD diameter and its impact on the electron density distribution, electronic density of states, and band gap of the TQDs is analyzed. We find that thermodynamically stable atomic distributions allow for optimal band-gap tenability and wave function confinement in TQDs. Our findings explain the possibility for compositional redistribution that may cause, over time, favorable or adverse changes of the TQD electronic properties with far reaching implications for the synthesis and applications of such nanostructures in devices. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X34.00007: The Role of morphology and interface in photoluminescent properties in CdSe/CdS Heterostructure Nanocrystals Arnaud Demortiere, Chunxing She, Donovan N. Leonard, Soma Chattopadhyay, Matthew Pelton, Elena Shevchenko Semiconductor core/shell colloidal nanocrystals (NCs) are promising materials for many applications such as luminescent solar concentrators and lasing media, due to their high PL quantum yield (PLQY) and their charge separation effect. The role of interface~in PLQY and in band alignment type is studied in this nanoscale heterojunction. Rod-shaped CdSe/CdS NCs have been synthesized by colloidal chemical approach via epitaxial process, which gives us a fine-tuning of both the spherical core size and rod-like shell length. The modification of the relative morphology changes the effective core/shell band alignment, which impacts the electron and hole delocalization into the nanorods. The evolution of the PLQY and PL lifetimes has been studied as a function of the relative core/shell sizes. High PLQY of up to 80{\%} and PL lifetime of 36 ns have been obtained for a shell-excitation wavelength of 450 nm with a large quasi-Stokes shift ($\sim $100nm). Radiative decay rates have been correlated with the rod volume and the band alignment type has been identified as being core size dependent. Finally, UltraSTEM and EXAFS analyses have been used to characterize the crystalline configuration at the core/shell interface and the lattice strain. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X34.00008: Electrostatic gating and single-molecule Raman spectroscopy Yajing Li, Joseph Herzog, Douglas Natelson SERS(surface enhanced raman spectroscopy) is a useful tool for single molecule spectroscopic investigations. We fabricated nanoscale Au bowtie structures to function as SERS substrates. Following electromigration, these metal nanostructures possess nanometer-scale interelectrode gaps that support highly localized surface plasmon resonances, resulting in SERS electromagnetic enhancements sufficient for single-molecule studies. These structures have also proven suitable for single-molecule electronic transport experiments, in which the underlying substrate can function as a gate electrode to shift molecular electronic levels relative to the metal source and drain. We will present preliminary results of the effect of gate modulation on the SERS and electrical properties of molecules in such junctions. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X34.00009: Thermoelectricity and transmission eigenchannels in buckyball junctions Michael Stefferson, Jarred Hudson, Joshua Barr, Justin Bergfield, Charles Stafford Transmission through nanoscale junctions consisting of a single Buckminsterfullerene molecule between two Pt or Au electrodes is investigated in the Coulomb blockade regime using the nonequilibrium Green's function approach. The Green's function of the buckyball is calculated in the isolated-resonance approximation, including the degenerate HOMO and LUMO orbitals. Electron-electron interactions were included in a constant-interaction model derived from $\pi$-electron effective field theory. For junctions with Pt electrodes, we find two transmission channels (despite the 5-fold degenerate HOMO and 3-fold degenerate LUMO resonances) and a positive thermopower. For Au electrodes, the thermopower is strongly affected by quantum interference, and we find just one transmission channel. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X34.00010: Study of Raman Stark Effect in Self-Aligned Nanojunctions Joseph Herzog, Daniel Ward, Douglas Natelson Plasmonically-active nanojunctions have been used to study the electrical and optical properties of single molecules by using surface-enhanced Raman spectroscopy (SERS). A new, ``self-aligned'' fabrication technique has been developed to mass-produce more robust nanojunctions. Similar to nanogaps made by electromigration, these self-aligned nanojunctions have been shown to exhibit strong SERS signal. In addition to having the capabilities of fabricating SERS substrates on a massive scale, the self-aligned technique also produces devices with a longer shelf-life than those fabricated by electromigration. Preliminary studies of the electomigrated devices have demonstrated Raman Stark shifts under DC bias. This work aims to study the Stark effect more in-depth and with the self-aligned nanogaps geometry by integrating the self-aligned structures into electrical circuits. Initial findings and current progress of these simultaneous optical and electrical measurements of a single molecule's vibrational modes will be discussed. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X34.00011: Hopping And Trapping of F4TCNQ on h-BN Nanomesh Huanyao Cun, Silvan Roth, Ari Seitsonen, Haifeng Ma, Jurg Osterwalder, Thomas Greber A single layer of hexagonal boron nitride (h-BN) on Rh(111) (nanomesh) [1] is an excellent template for trapping and self-assembly of molecules. This hexagonal structure has a periodicity of 3.22 nm and an appearance with strongly bound h-BN regions called ``pores'' of about 2 nm in diameter surrounded by h-BN regions called ``wires.'' The trapping mechanism traces back to a corrugated electrostatic potential at the surface [2]. We have investigated the adsorption behavior of an electron acceptor molecule, tetrafluoro-tetracyano-quinodimethane (F4TCNQ) on nanomesh by combining XPS, UPS, STM and DFT calculation. The work function increase upon F4TCNQ adsorption indicates electron transfer to the molecules, and is in good agreement with the DFT result. At room temperature, F4TCNQ adsorbs on the ``wires'' and in the ``pores'' because of a high mobility. STM measurements allow us to detect the hopping rate of molecules. Upon cooling, molecules are trapped inside of pores, which gives insight into the behavior of a negatively charged molecule, compared to neutral species, in the trapping potential of h-BN nanomesh. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X34.00012: Quantum-Enhanced Thermoelectric Effects in Polycyclic Molecular Junctions Joshua Barr, Charles Stafford We calculate the thermoelectric response of a polycyclic molecular junction including electron-electron interactions. To do this, the molecular Green's function is determined via a Lanczos-based technique and $\pi$-electron effective field theory is used to model the degrees of freedom most relevant to transport. In these junctions we find that the presence of multiple rings leads to higher order quantum interference features giving rise to dramatic enhancements of molecular thermoelectric effects, consistent with previous predictions based on Hueckel theory, which neglected electron correlations. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X34.00013: Decoherence Assisted Single Electron Trapping at Room Temperature Ahmed Elhalawany, Michael Leuenberger In this work, we theoretically investigate electron transport in heterostructure semiconductor nanowire (NW). We develop a new mechanism to trap an electron in a quantum dot (QD) by means of decoherence. There are six QDs in the NW. Bias voltage (Vb) is applied across the NW and gate voltage (Vg) is applied to the auxiliary QD to control single charge tunneling. The single electron dynamics along the NW is calculated by means of the generalized master equation based on the tight binding model taking into account electron LO phonon interaction (ELOPI) and thermal broadening inside the QDs. It is shown that the decoherence, which is in the pico-second (ps) regime, speeds up the trapping of the electron in the central QD with probability of 70{\%} in less than 2 ps. Our results can be used for the implementation of high temperature single photon source (SPS) or single electron transistor (SET). We acknowledge support from NSF (Grant No. ECCS-0725514), DARPA/MTO (Grant No. HR0011-08-1-0059), NSF (Grant No. ECCS-0901784), AFOSR (Grant No. FA9550-09-1-0450), and NSF (Grant No. ECCS-1128597). [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X34.00014: First-Principles Studies on Photoinduced Charge Transfer in Functionalized Carbon Nanotubes Iek-Heng Chu, Dmitri Kilin, Hai-Ping Cheng We have studied the binding energy, electronic structure, optical excitation, and relaxation of dinitromethane molecules (CH$_{2}$N$_{2}$O$_{4})$ on semiconducting carbon nanotubes (CNTs) of chiral index (n, 0) (n=7,10,16,19). The electronic structures calculated from density functional theory (DFT) show that the dinitromethane introduces a localized state inside the band gap of CNT systems of n=10,16 and19, which indicates that the state can trap an electron when the CNT is photoexcited. The dynamics of intra-band relaxations in such systems has been investigated using reduced density matrix formalism combined with DFT. For pristine CNTs, we have found that the calculated charge relaxation time constants agree well with the experimental time scales. Upon adsorption, these constants are modified and there is not a clear trend for the direction and magnitude of the change. However, our calculations predict that electron relaxation in the conduction band is faster than hole relaxation in the valence band, for CNTs with and without molecular adsorbates. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X34.00015: Understanding Crosstalk Between Parallel Molecular Wires Matthew Reuter, Tamar Seideman, Mark Ratner Cooperative effects between molecular wires affect conduction through the wires, and studies have yet to clarify the conditions under which these effects enhance (diminish) conduction. Using a simple but general model, we attribute this crosstalk to the duality of energetic splitting and phase interference between the wires' conduction channels. In most cases, crosstalk increases (decreases) conductance when the Fermi level is far from (close to) an isolated wire's resonance. Finally, we discuss strategies for controlling crosstalk between parallel molecular wires. \\[4pt] [1] M. G. Reuter et al. J. Phys. Chem. Lett. 2, 1667-1671 (2011).\\[0pt] [2] M. G. Reuter et al. Nano Lett. 11, 4693-4696 (2011). [Preview Abstract] |
Session X39: Focus Session: Materials and Functional Structures for Biological Interfaces - Nanoscale Materials
Sponsoring Units: DMP DBIO DPOLYChair: Marcella Chiari, ICRM, Italy
Room: 109B
Thursday, March 1, 2012 2:30PM - 3:06PM |
X39.00001: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X39.00002: Designing super-selectivity in multivalent nano-particle binding Francisco Martinez-Veracoechea, Daan Frenkel A key challenge in nano-science is to design ligand-coated nano-particles that can bind selectively to surfaces that display the cognate receptors above a threshold (surface) concentration. Nano-particles that bind monovalently to a target surface do not discriminate sharply between surfaces with high and low receptor coverage. In contrast, ``multivalent'' nano-particles that can bind to a larger number of ligands simultaneously, display regimes of ``super-selectivity'' where the fraction of bound particles varies sharply with the receptor concentration. We present numerical simulations that show that multivalent nano-particles can be designed such that they approach the ``on-off'' binding behavior ideal for receptor-concentration selective targeting. We propose a simple analytical model that accounts for the super-selective behavior of multi-valent nano-particles. We propose a simple rule of thumb to predict the conditions under which super-selectivity can be achieved. We validate our model predictions against the Monte Carlo simulations. Finally, we investigate the role of multi-component ligand-receptor interactions in the enhancement of targeting selectivity. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X39.00003: Nano-Engineered Cubic Zirconia for Orthopaedic Implant Applications F. Namavar, A. Rubinstein, R. Sabirianov, G. Thiele, J. Sharp, U. Pokharel, R. Namavar, K. Garvin Osseointegration failure of the prosthesis prevents long-term stability, which contributes to pain, implant loosening, and infection that usually necessitates revision surgery. Cell attachment and spreading in vitro is generally mediated by adhesive proteins such as fibronectin and vitronectin. We designed and produced pure cubic zirconia (ZrO2) ceramic coatings by ion beam assisted deposition (IBAD) with nanostructures comparable to the size of proteins. Our ceramic coatings exhibit high hardness and a zero contact angle with serum. In contrast to Hydroxyapatite (HA), nano-engineered zirconia films possess excellent adhesion to all orthopaedic materials. Adhesion and proliferation experiments were performed with a bona fide mesenchymal stromal cells cell line (OMA-AD). Our experimental results indicated that nano-engineered cubic zirconia is superior in supporting growth, adhesion, and proliferation. We performed a comparative analysis of adsorption energies of the FN fragment using quantum mechanical calculations and Monte Carlo simulation on both types of surfaces: smooth and nanostructured. We have found that the initial FN fragment adsorbs significantly stronger on the nanostructured surface than on the smooth surface. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X39.00004: Computational characterization of DNA/peptide/nanotube self assembly for bioenergy applications Vanessa Ortiz, Ruriko Araki, Galen Collier Multi-enzyme pathways have become a subject of increasing interest for their role in the engineering of biomimetic systems for applications including biosensors, bioelectronics, and bioenergy. The efficiencies found in natural metabolic pathways partially arise from biomolecular self-assembly of the component enzymes in an effort to avoid transport limitations. The ultimate goal of this effort is to design and build biofuel cells with efficiencies similar to those of native systems by introducing biomimetic structures that immobilize multiple enzymes in specific orientations on a bioelectrode. To achieve site-specific immobilization, the specificity of DNA-binding domains is exploited with an approach that allows any redox enzyme to be modified to site-specifically bind to double stranded (ds) DNA while retaining activity. Because of its many desirable properties, the bioelectrode of choice is single-wall carbon nanotubes (SWNTs), but little is known about dsDNA/SWNT assembly and how this might affect the activity of the DNA-binding domains. Here we evaluate the feasibility of the proposed assembly by performing atomistic molecular dynamics simulations to look at the stability and conformations adopted by dsDNA when bound to a SWNT. We also evaluate the effects of the presence of a SWNT on the stability of the complex formed by a DNA-binding domain and DNA. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 4:18PM |
X39.00005: Engineering upconverting nanophosphors as biosensors and biotherapeutic agents Invited Speaker: Shuang Fang Lim Contrast agents play an important role in the study of biological tissues and whole organisms, since they enable visualization of functional structures. Fluorescent contrast agents also enable specific targeting in therapeutic approaches. Developing optimized contrast agents is central to optimizing the performance of both imaging and therapy. Upconversion nanophosphors (UCNPs) are a class of nanoparticles which enable efficient 2-photon fluorescence. Their intrinsic properties of low toxicity, low excitation intensity, narrow fluorescence line width, multiplexing capability, zero fluorescent background, and absence of bleaching and blinking make them strong candidates as a contrast agent with wide applicability. I am working to develop these nanomaterials with biological significance as contrast agents in biolabels , in biosensors and as therapeutic agents in photodynamic therapy. This will be combined with the enhancement of brightness of the UCNPs through coupling to resonant gold nanofeatures. The study is necessary in order to bring about a break through in upconversion luminescence enhancement, particularly at ever decreasing nanoparticle sizes necessary for biological applications. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X39.00006: Optically Trappable Single Wall Carbon Nanotubes Samantha Roberts, Arthur Barnard, Joshua Kevek, Alexander Ruyack, Ive Silvestre, Rodrigo Lacerda, Paul McEuen We are developing a single walled carbon nanotube (NT) force probe which would utilize an optical trap to manipulate NTs fitted with micron scale dielectric handles. With its nanometer diameter and micron length a NT harnessed in this way may eventually allow us to probe a cell's membrane and interior. In pursuit of this goal we have developed a method to create parallel arrays of aligned NT cantilevers. In this process we transfer highly aligned NTs to a substrate composed of alternating regions of Si separated by SiO$_{2}$. Patterning the NTs and etching away the oxide leaves behind ridges supporting arrays of NT cantilevers 0.7nm-2nm in diameter and up to 700nm in length with densities of over one cantilever per micron. We will discuss our work modifying this technique to pattern and release micron scale NT cantilever probes into solution. We have designed and optically tested lithographically patterned SiO$_{2}$ and SU-8 handles, shaped such that they can be manipulated with an optical trap in predictable orientations. We will focus on our current efforts in attaching these optically trappable dielectric handles to individual nanotubes so they can be implemented to make direct force measurements. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X39.00007: Self-assembled peptide nanowires on single-layer graphene and MoS$_{2}$ with biomolecular doping effect Yuhei Hayamizu, Christopher R. So, Mehmet Sarikaya Developing elegant hybrid systems of biological molecules on atomic single layers (ASLs), such as graphene, is a key in creating novel bio-nanoelectronic devices, where versatile biological functions are integrated with electronics of ASLs. Biomolecules self-assembling into ordered structures on ASLs offer a novel bottom-up approach, where organized supramolecular architectures spatially govern the ASL electronics. Despite the potential in bridging nano- and bio-worlds at the molecular scale, no work has yet realized a way to control electronic and optical properties of ASLs by the biomolecular structures. Here, we demonstrate that engineered dodecapeptides self-assemble into supramolecular networks of peptide nanowires on single-layer graphene and MoS$_{2}$. Peptide nanowires introduce electric charge into these ASLs \textit{via} biomolecular doping. Abrupt boundaries of nanowires create electronic junctions in graphene, which manifest themselves within the single-layer as a self-assembled electronic network. Furthermore, the designed peptides modify both conductivity and photoluminescence of single-layer MoS$_{2}$. Controlling nano-electronics through engineered peptides potentially opens up new avenues in self-assembly of nanodevices for future bioelectronics. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X39.00008: {\it {Ab initio}} Calculations of Electronic Fingerprints of DNA bases on Graphene Towfiq Ahmed, John J. Rehr, Svetlana Kilina, Tanmoy Das, Jason T. Haraldsen, Alexander V. Balatsky We have carried out first principles DFT calculations of the electronic local density of states (LDOS) of DNA nucleotide bases (A,C,G,T) adsorbed on graphene using LDA with ultra-soft pseudo-potentials. We have also calculated the longitudinal transmission currents $T(E)$ through graphene nano-pores as an individual DNA base passes through it, using a non-equilibrium Green's function (NEGF) formalism. We observe several dominant base-dependent features in the LDOS and $T(E)$ in an energy range within a few eV of the Fermi level. These features can serve as electronic fingerprints for the identification of individual bases from $dI/dV$ measurements in scanning tunneling spectroscopy (STS) and nano-pore experiments. Thus these electronic signatures can provide an alternative approach to DNA sequencing. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X39.00009: Controlled Chemical Patterns with ThermoChemical NanoLithography (TCNL) Keith Carroll, Anthony Giordano, Debin Wang, Vamsi Kodali, W.P. King, S.R. Marder, E. Riedo, J.E. Curtis Many research areas, both fundamental and applied, rely upon the ability to organize non-trivial assemblies of molecules on surfaces. In this work, we introduce a significant extension of ThermoChemical NanoLithography (TCNL), a high throughput chemical patterning technique that uses temperature-driven chemical reactions localized near the tip of a thermal cantilever. By combining a chemical kinetics based model with experiments, we have developed a protocol for varying the concentration of surface bound molecules. The result is an unprecedented ability to fabricate extremely complex patterns comprised of varying chemical concentrations, as demonstrated by sinusoidal patterns of amine groups with varying pitches ($\sim $5-15 $\mu $m) and the replication of Leonardo da Vinci's \textit{Mona Lisa} with dimensions of $\sim $30 x 40 $\mu$m$^2$. Programmed control of the chemical reaction rate should have widespread applications for a technique which has already been shown to nanopattern various substrates including graphene nanowires, piezoelectric crystals, and optoelectronic materials. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X39.00010: Interface-Limited Spherulitic Growth of Hydroxyapatite/Chondroitin Sulfate Composite Enamel-like Films Guobin Ma, Yifei Xu, Xiyan Wang, Mu Wang Understanding and mimicking the growth of hard tissues such as tooth enamel may lead to innovative approaches toward engineering novel functional materials and providing new therapeutics. Up to now, in vitro growth of enamel-like materials is still a great challenge, and the microscopic formation mechanisms are far from well understood. Here we report synthesis of large-scale hydroxyapatite (HAP) and chondroitin sulfate (ChS) composite films by an efficient solution-air interface growth method. The products have the characteristic hierarchical prism structures of enamel and the mechanical properties comparable to dentin. We demonstrate that the films are assembled by spherulites nucleated at the solution surface. The growth of the spherulites is limited by the interfaces between them as well as between the solution and air, leading to the ordered prism structure. The results are beneficial for a clearer understanding of the fundamentals of tooth enamel formation. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X39.00011: Biomimetic Calcium Phosphate Crystallization: Synchrotron X-ray Studies Ahmet Uysal, Benjamin Stripe, Pulak Dutta, Binhua Lin, Mati Meron The nucleation and growth of calcium phosphate by organic templates attract great attention due to its relevance to bone biomineralization. In spite of the vast studies in the field, the role of the organic templates in the process is still not well understood. One reason for this drawback is the lack of experimental tools to probe the organic template structure during the process. We studied the nucleation and growth of calcium phosphate under floating Langmuir monolayers, at the air/water interface, using two complementary X-ray scattering methods. We show that Grazing Incidence X-ray Diffraction (GID) and Grazing Incidence X-ray off-Specular Scattering (GIXOS) can reveal the organic-inorganic interface properties \textit{in situ}. By using GID and GIXOS together, we can simultaneously determine the lateral interface structure and the electron density profile normal to the interface. Combined with \textit{ex situ} methods, these techniques can improve our understanding of the role of the organic template during biomineralization. [Preview Abstract] |
Session X40: Focus Session: Single Molecule Biological Physics - Nucleic Acids
Sponsoring Units: DBIO DPOLY DCOMPChair: Lori Goldner and Jennifer Ross, University of Massachusetts
Room: 156A
Thursday, March 1, 2012 2:30PM - 3:06PM |
X40.00001: From force-fields to photons: MD simulations of dye-labeled nucleic acids and Monte Carlo modeling of FRET Invited Speaker: Lori Goldner Fluorescence resonance energy transfer (FRET) is a powerful technique for understanding the structural fluctuations and transformations of RNA, DNA and proteins. Molecular dynamics (MD) simulations provide a window into the nature of these fluctuations on a different, faster, time scale. We use Monte Carlo methods to model and compare FRET data from dye-labeled RNA with what might be predicted from the MD simulation. With a few notable exceptions, the contribution of fluorophore and linker dynamics to these FRET measurements has not been investigated. We include the dynamics of the ground state dyes and linkers in our study of a 16mer double-stranded RNA. Water is included explicitly in the simulation. Cyanine dyes are attached at either the 3' or 5' ends with a 3 carbon linker, and differences in labeling schemes are discussed.\\[4pt] Work done in collaboration with Peker Milas, Benjamin D. Gamari, and Louis Parrot. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X40.00002: Quantifying screening ion excesses surrounding stretched, charged polymers Jonathan Landy, Dustin McIntosh, Omar Saleh We present the results of a combined theoretical/experimental study in which we have applied thermodynamic identities to infer -- from single molecule force-extension curves taken at different salt concentrations -- how the number of screening ions associated with a charged polymer changes as a function of its end to end extension. This number, which can change only through non-linear screening mechanisms, turns out to depend non-trivially on both the concentration of salt and the inherent rigidity of the polymer. In the case of a flexible polymer, such as ssDNA, our data indicates that the excess can change substantially between the fully extended and globule states. The effect is reduced for semi-flexible polymers, such as dsDNA, at physiologically-relevant salt concentrations, but it can again become substantial at lower salt concentrations. Based on these findings, we argue that small ion entropic effects should often contribute substantially to free energy differences between the competing conformational states of charged polymers -- both \textit{in vivo} and in certain polymer-based materials systems. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X40.00003: Euler buckling and nonlinear kinking of double-stranded DNA Alexander Fields, Kevin Axelrod, Adam Cohen Bare double-stranded DNA is a stiff biopolymer with a persistence length of roughly 53 nm under physiological conditions. Cells and viruses employ extensive protein machinery to overcome this stiffness and bend, twist, and loop DNA to accomplish tasks such as packaging, recombination, gene regulation, and repair. The mechanical properties of DNA are of fundamental importance to the mechanism and thermodynamics of these processes, but physiologically relevant curvature has been difficult to access experimentally. We designed and synthesized a DNA hairpin construct in which base-pairing interactions generated a compressive force on a short segment of duplex DNA, inducing Euler buckling followed by bending to thermally inaccessible radii of curvature. The efficiency of F\"{o}rster resonance energy transfer (FRET) between two fluorophores covalently linked to the hairpin indicated the degree of buckling. Bulk and single-molecule measurements yielded distinctly different force-compression curves for intact DNA and for strands with single nicks, base pair mismatches, and damage sites. These results suggest that changes in local mechanical properties may play a significant role in the recognition of these features by DNA-binding proteins. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X40.00004: Measuring fluctuations in shear stretched DNAs using site specific labeling Allen Price, Thomas Graham, Joseph Loparo, Joel Eaves We report a new technique for measuring the internal dynamics of surface tethered DNAs in shear flow. Previous studies have used end labeling or intercolating dyes which label the entire length of the DNA. Neither prior method can resolve the internal longitudinal fluctuations of the DNA. Our technique accomplishes this by site specific labeling of five sites in lambda phage DNA using EcoRI labeled with fluorescent quantum dots. We used our technique to determine the two point cross correlation functions of the longitudinal and transverse fluctuations of the DNA under shear flow. Our technique allows us to test current models of the non-equilibrium fluctuations of DNA in shear flow in a way previously inaccessible. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X40.00005: Electric-field Assisted Deposition of the DNA on Polymer Surface JunHwan Ryu, Ke Zhu, Julia Budassi, Jonathan Sokolov Recently, the interaction of DNA with surfaces has been widely studied for its range of applications, including mapping, sequencing and analyzing DNAs. In this study, the Lambda DNA molecules were aligned in 6:50(0.1M NaOH:0.02M MES) buffer solution with different electric fields and deposited onto polymethylmetacrylate (PMMA) surfaces by dipping and retracting PMMA coated silicon wafers into the solution. Electric field was set up with platinum wire and gold plated Si wafer. The DNA strands were dyed with YoYo-1 and observed using a fluorescence microscope. The efficiency of deposition was optimized with respect to DNA concentration, DNA length and electric field. The results indicate that the density and possibly the lengths of the DNA deposited on surface can be controlled by this method. Enhancement of adsorption density of greater than twenty-fold were found using electric field strengths of 10v/cm. This study is supported by NSF-DMR-MRSEC program. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X40.00006: Using Microcontact Printing as a Novel Method for Patterned Dyeing of Surface-adsorbed DNA Emily Shea, Julia Budassi, Ke Zhu, Jonathan Sokolov We use microcontact printing (MCP)$^{1}$ to stain individual DNA molecules adsorbed and combed onto a polymer-coated silicon surface. Polydimethylsiloxane (PDMS) stamps with micron-sized features have been used to selectively stain lambda DNA molecules with SyBr Gold dye. DNA was deposited out of dilute solution onto polymethylmethacrylate (PMMA) layers, 70nm thick, spun-coated on Si wafers, producing linearly stretched and aligned molecules. The stamps were soaked in dye solutions for one minute, followed by wiping of excess solution with a swap. The stamp was pressed onto the surface, varying the pressure and time of application (typically 5-10 minutes) to control the staining. The DNA molecules were imaged with a fluorescence microscope equipped with a cooled CCD camera. Single molecules of DNA were successfully dyed and imaged with stamps having a grating pattern either parallel to or perpendicular to the DNA orientation. Supported by NSF-DMR MRSEC program. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X40.00007: Stretching DNA Molecules on a Polymer Surface Jonathan Rosenberg, Ke Zhu, Julia Budassi, Jonathan Sokolov DNA's stretched form is one of great importance to the study of its structural characteristics and sequence. In our experiment, we studied the effects of stretching on lambda DNA, deposited onto Polydimethyl siloxane (PDMS, silicone) using the evaporating drop method. The DNA was dyed with SyBr Gold dye, or YOYO dye, which does not drastically affect the stretching properties of the DNA molecules while being deposited. Different DNA concentrations were used to optimize the density of the DNA on the surface. Once deposited, the DNA was imaged using a confocal microscope, for further measurements and to image stretching, in situ. To stretch the DNA molecules after deposition onto PDMS, the PDMS sample was placed onto a modified linear stage, pinched at the ends. The DNA length was measured throughout stretching. The result shows we successfully stretched DNA strands by 68{\%} without breakage of the strands and without the strands coming off of the PDMS surface. This study is supported by NSF-DMR-MRSEC program. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X40.00008: Polarization and Angle Dependence of Fluorescence from Aligned DNA Ashish Sridhar, Suri Bandler, Ke Zhu, Yingzhan Gu, Julia Budassi, Jonathan Sokolov DNA molecules can be deposited and aligned on various surfaces and imaged by confocal microscopy when labeled with fluorescent dyes. SyBr Gold dye, is known to possess a high angle and polarization dependence. We measured the emission intensity for various incident angles as a function of incident polarization angle. Samples were created by means of dipping PMMA-coated silicon wafers into dyed DNA solutions with DC electric field setup or drop evaporation. The blue laser as the imaging light source was mounted on an optical rail with a polarizer with rotatable half wave plate to change the incident polarization relative to the DNA molecular orientation. When applied to samples dyed using SyBr Gold, a clear change in the intensity of imaged DNA strands was observed though a range of input polarization angle. We have shown that it is possible to optimize the conditions in which aligned DNA is imaged using confocal microscopy by varying the polarization and angle of incidence of laser light on the sample. This study is supported by NSF-DMR-MRSEC program. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X40.00009: Calorimetric and Low-Frequency Dielectric Studies of Mesoscopic Ordering in Solutions of Engineered DNA Hairpin Fragments K. Kashuri, H. Kashuri, G.S. Iannacchione Calorimetry (both AC and MDSC) from $20$ to $100$~$^o$C, as well as low-frequency ($0.1$ to $100$~kHz) isothermal dielectric measurements have been performed on solutions of DNA fragments as a function of concentration. Custom hairpin DNA fragments were obtained with $13$-base unit length and samples made in solution at various concentration. Results show a reproducible heat capacity $C_p$ signature on heating and cooling scans. This thermal behavior of a diluted oligonucleotide chain is very different from that seen for mesoscopic ordering of liquid crystals. The AC Cp peak vanishes and new features are revealed as the temperature scan rate is lowered to $0.017$~K~min$^{-1}$. The observed real, $\epsilon'$, and imaginary, $\epsilon''$, permittivity of the suspended DNA show features indicating low-frequency dynamics that in turn suggests large-scale ordering or agglomeration of the DNA hairpin loops. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X40.00010: Modeling of DNA zipper reaction rates Preston Landon, Casey Sanchez, Alexander Mo, Ratnesh Lal DNA zippers are a thermodynamically driven system consisting of three DNA oligonucleotides. Two of the strands are designed to create a small helix the third is designed to invade and separated the helix. A zipper system consisting of a normal strand (N), a weak strand (W), and an opening strand (O). N is made up of normal DNA bases, while W is engineered with inosine bases substituted for guanine. Inosine forms one less hydrogen bond with cytosine than guanine. By varying the number and order of inosine, W is engineered to provide less than natural bonding affinities to N in forming the [N:W] helix. When O is introduced (a natural complement of N), it competitively displaces W from [N:W] and forms [N:O]. DNA zippers have been used to create new DNA devices such as springs and tweezers and to create functionalized DNA origami structures. Currently, The basic principles and interactions of DNA zippers are not well understood. Here we will report the results on an investigation of several different DNA zipper constructs designed to aid in the creation of a mathematical prediction of the reaction rate for DNA zippers. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X40.00011: Moving Beyond Watson-Crick Models of Coarse Grained DNA Kevin Dorfman, Margaret Linak, Richard Tourdot DNA structure possesses several levels of complexity, ranging from the sequence of bases (primary structure) to base pairing (secondary structure) to its three-dimensional shape (tertiary structure) and can produce a wide variety of conformations in addition to canonical double stranded DNA. By including non-Watson-Crick interactions in a coarse-grained model, we developed a system that not only can capture the traditional B-form double helix, but also can adopt a wide variety of other DNA conformations. In our experimentally parameterized, coarse-grained DNA model we are able to reproduce the microscopic features of double-stranded DNA without the need for explicit constraints and capture experimental melting curves for a number of short DNA hairpins. We demonstrate the utility of the model by simulating more complex tertiary structures such as the folding of the thrombin aptamer, which includes G-quartets, and strand invasion during triplex formation. Our results highlight the importance of non-canonical interactions in DNA coarse- grained models. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X40.00012: Direct comparison of the theory of molecular solvation with molecular dynamics and experiment Tyler Luchko, In Suk Joung, George Giambasu, Darrin York, David A. Case The reference interaction site model (RISM) provides complete equilibrium sampling of bulk solvent and solvent around a solute of arbitrary shape and size at a fraction of the computational cost of explicit solvent molecular dynamics (MD). Though based on first principles, approximations must be made to achieve numerical solutions. In this study, we first compare RISM to MD and experimental results for bulk solutions of aqueous monovalent ions using the Joung-Cheatham parameters with SPC/E and TIP3P water subject to several approximate closures. Then, using the same parameters and approximations, we evaluate the distribution of water and ions around a 24 base pair strand of DNA, once again, comparing to MD results and experimental observables. In both cases RISM gives the correct qualitative behavior and, often, the correct quantitative behavior. However, this strongly depends on the closure relation used, with higher order, HNC-like closures usually giving better results. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X40.00013: Information-based measure of chirality for biomolecules Karol Baca Lopez, B. Roy Frieden, Roberto Lopez, Miguel Mayorga, Enrique Henandez Lemus Homochirality is a common property of biomolecules such as DNA, RNA and proteins. In particular, {\scshape d}-ribose and {\scshape d}-deoxyribose enantiomers are found within living cells, while their mirror images, the {\scshape l}-enantiomers, are not known to occur naturally even though the configurations are highly stable. On the other hand, proteins are formed by {\scshape l}-amino acids, not by their mirror images. Why? In this work, we propose the use of Fisher Information (FI) $I$ as a measure of chirality or dissimilarity between enantiomers. We performed Hartree-Fock (HF) and Density Functional Theory (DFT) calculations to obtain the electronic wave function $\Psi (x,y,z)$ and corresponding density function $\rho(x,y,z)$ for each of the natural and synthetic forms of oligoribonucleotides and alanine amino acid. The four wave functions $\Psi (x,y,z)$ are used to compute the FI evaluated from two different view points: a coherent viewpoint, which includes the phase part of each $\Psi (x,y,z)$, and an incoherent or classical viewpoint, which ignores the phase. Our goal is to describe the extent to which the information content in chiral molecules ({\scshape d}- and {\scshape l}-) plays a role in selecting one or the other isomer in nature. [Preview Abstract] |
Session X41: Focus Session: Physics of Proteins II: Folding and Structure
Sponsoring Units: DBIO DPOLY DCOMPChair: Ruhong Zhou, IBM T.J. Watson Research Center
Room: 156B
Thursday, March 1, 2012 2:30PM - 3:06PM |
X41.00001: Simulations of the folding/unfolding of biomolecules under solvent, and pressure perturbations Invited Speaker: Angel Garcia Proteins exhibit marginal stability, determined by the balance of many competing effects. This stability can be perturbed by changes in temperature, pH, pressure, and other solvent conditions. Osmolytes are small organic compounds that modulate the conformational equilibrium, folded (F) and unfolded (U), of proteins as cosolvents. Protecting osmolytes such as trimethylamine N-oxide (TMAO), glycerol, and sugars that push the equilibrium toward F play a crucial role in maintaining the function of intracellular proteins in extreme environmental conditions. Urea is a denaturing osmolyte that shifts the equilibrium toward U. We will describe calculations of the reversible folding/unfolding equilibrium, under various solution conditions that include urea, high pressure, and different charge states of the Trp-cage miniprotein. The folding/unfolding equilibrium is studied using all-atom Replica exchange MD simulations. For urea, the simulations capture the experimentally observed linear dependence of unfolding free energy on urea concentration. We find that the denaturation is driven by favorable direct interaction of urea with the protein through both electrostatic and van der Waals forces and quantify their contribution. Though the magnitude of direct electrostatic interaction of urea is larger than van der Waals, the difference between unfolded and folded ensembles is dominated by the van der Waals interaction. We also find that hydrogen bonding of urea to the peptide backbone does not play a dominant role in denaturation. The unfolded ensemble sampled depends on urea concentration, with greater urea concentration favoring conformations with greater solvent exposure. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X41.00002: Accurate prediction of explicit solvent atom distribution in HIV-1 protease and F-ATP synthase by statistical theory of liquids Daniel Sindhikara, Norio Yoshida, Fumio Hirata We have created a simple algorithm for automatically predicting the explicit solvent atom distribution of biomolecules. The explicit distribution is coerced from the 3D continuous distribution resulting from a 3D-RISM calculation. This procedure predicts optimal location of solvent molecules and ions given a rigid biomolecular structure. We show examples of predicting water molecules near KNI-275 bound form of HIV-1 protease and predicting both sodium ions and water molecules near the rotor ring of F-ATP synthase. Our results give excellent agreement with experimental structure with an average prediction error of 0.45-0.65 angstroms. Further, unlike experimental methods, this method does not suffer from the partial occupancy limit. Our method can be performed directly on 3D-RISM output within minutes. It is useful not only as a location predictor but also as a convenient method for generating initial structures for MD calculations. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X41.00003: Analysis and Interpretation of Single Molecule Protein Unfolding Kinetics Herbert Lannon, Jasna Brujic The kinetics of protein unfolding under a stretching force has been extensively studied by atomic force microscopy (AFM) over the past decade [1]. Experimental artifacts at the single molecule level introduce uncertainties in the data analysis that have led to several competing physical models for the unfolding process. For example, the unfolding dynamics of the protein ubiquitin under constant force has been described by probability distributions as diverse as exponential [2,3], a sum of exponentials, log-normal [4], and more recently a function describing static disorder in the Arrhenius model [5]. A new method for data analysis is presented that utilizes maximum likelihood estimation (MLE) combined with other traditional statistical tests to unambiguously rank the consistency of these and other models with the experimental data. These techniques applied to the ubiquitin unfolding data shows that the probability of unfolding is best fit with a stretched exponential distribution, with important implications on the complexity of the mechanism of protein unfolding. \\[4pt] [1] Carrion-Vazquez, et. al. Springer Series in Biophys. 2006 \\[0pt] [2] Fernandez et. al. Science 2004 \\[0pt] [3] Brujic et. al. Nat. Phys 2006 \\[0pt] [4] Garcia-Manyes et. al. Biophys. J. 2007 \\[0pt] [5] Kuo et. al. PNAS 2010 [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X41.00004: Investigating protein structure and folding with coherent two-dimensional infrared spectroscopy Carlos Baiz, Chunte Peng, Michael Reppert, Kevin Jones, Andrei Tokmakoff We present a new technique to quantitatively determine the secondary structure composition of proteins in solution based on ultrafast two-dimensional infrared (2DIR) spectroscopy. The percentage of residues in alpha-helix, beta-sheet, and unstructured conformations is extracted from a principal component analysis of the measured amide-I 2DIR spectra. We benchmark the method against a library of commercially-available proteins by comparing the predicted structure compositions with the x-ray crystal structures. The new technique offers sub-picosecond time resolution, and can be used to study systems that are difficult to study with conventional methods such as gels, intrinsically disordered peptides, fibers, and aggregates. We use the technique to investigate the structural changes and timescales associated with folding and denaturing of small proteins via equilibrium and transient temperature-jump 2DIR spectroscopy. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 4:18PM |
X41.00005: Protein Folding and Functional Dynamics Invited Speaker: Feng Gai Proteins are intrinsically flexible and dynamic objects. Thus, protein dynamics are intimately coupled to their function. In many cases, however, it is difficult to directly probe protein conformational dynamics occurring on either very fast or very slow timescales with high spatial resolution. In this talk, several examples will be discussed to show how ensemble and single-molecule spectroscopic methods can be used to follow protein folding events taking place on the nanosecond timescale and slow conformational dynamics associated with function. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X41.00006: Folding a protein with equal probability of being helix or hairpin Chung-Yu Mou, Chun-Yu Lin, Nan-Yow Chen We explore the possibility for the native state of a protein being inherently a multi-conformation state in an ab initio coarse-grained model. Based on the Wang-Landau algorithm, the complete free energy landscape for the designed sequence 2D4X: INYWLAHAKAGYIVHWTA is constructed. It is shown that 2DX4 posesses two nearly degenerate native states: one has a helix structure, while the other has a hairpin structure and their energy difference is less than 2\% of that of local minimums. Furthermore, the hydrogen-bond and dipole-dipole interactions are found to be two major competing mechanims in transforming one conformation into the other. Our results indicate that degenerate native states are stablized by subtle balance between different interactions in proteins; furthermore, degeneracy only happens for small proteins of sizes being around 18 amino acides or 40-50 amino acides. These results provide important clues to the study of native structures of proteins. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X41.00007: Generic transition hierarchies of lattice HP protein adsorption: A Wang-Landau study Ying Wai Li, D.P. Landau, T. W\"{u}st We have applied Wang-Landau sampling with appropriate trial moves\footnote{T. W\"{u}st and D. P. Landau, Phy. Rev. Lett. \textbf{102}, 178101 (2009).} to investigate the thermodynamics and structural properties of the HP lattice protein model\footnote{K. A. Dill, Biochemistry \textbf{24}, 1501 (1985).} interacting with an attractive substrate. The conformational ``phase transitions'' of several benchmark HP sequences have been identified by a comprehensive canonical analysis of the specific heat and structural observables, e. g. radius of gyration and thermal derivatives of number of surface contacts. Three major ``transitions'': adsorption, hydrophobic core formation, and ``flattening'' of adsorbed structures, are observed. Depending on the surface attractive strength relative to the intra-protein attraction among the H monomers, these processes take place in a different order upon cooling. We identify a small number of generic categories that are sufficient to classify the folding hierarchies for different HP chains consisting of assorted sequences and chain lengths, regardless of the monomer type that the surface attracts. We thus believe that this classification scheme is generally applicable to lattice protein adsorption problems. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X41.00008: Elucidating Structure and Catalytic Cycles of Anti- or Ferro-magnetic Iron Enzymes from Spin Density Functional Theory Jorge H. Rodriguez Nature uses metal-containing enzymes to catalyze important biochemical reactions. Some enzymes, such as methane monooxygenase hydroxylase (MMOH), contain (anti)ferromagnetic binuclear iron centers that interact with dioxygen and/or other substrates to facilitate biochemical functions. We have studied the electronic and magnetic structures of several enzyme binuclear iron centers and predicted their spectroscopic properties. We have used spin density functional theory (SDFT) to predict $^{57}$Fe M\"ossbauer and other spectral parameters of MMOH and structurally related iron-containing enzymes. Upon dioxygen binding, the diiron center of MMOH undergoes a ferromagnetic to antiferromagnetic transition which may play an important role in its catalytic activity. In addition, based on our ability to predict spectroscopic data, we have been able to predict the structure of a key reaction intermediate in the MMOH catalytic cycle for which there is no X-ray structure. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X41.00009: AWSEM-MD: Coarse-grained Protein Structure Prediction Using Physical Potentials and Bioinformatically Based Local Structure Biasing Aram Davtyan, Weihua Zheng, Nick Schafer, Cecilia Clementi, Peter Wolynes, Garegin Papoian The Associative memory, Water mediated, Structure and Energy Model (AWSEM) is a coarse-grained protein model. When combined with a sequence alignment method, AWSEM can be used to perform de novo protein structure prediction. Herein we present structure prediction results for a particular choice of sequence alignment method based on short residue sequences called fragments. We demonstrate the model's structure prediction capabilities for three variants on a standard sequence alignment protocol, all of which assume that the structure of the target sequence is not known. We show that the inclusion of structures from homologous sequences in the fragment memory search improves structure prediction only marginally. However, when the fragment search is restricted to only homologous sequences, AWSEM can perform high-resolution structure prediction. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X41.00010: Combining first principles and replica exchange for the structure of two large peptides: Ac-Ala$_{19}$-LysH$^+$ vs. Ac-LysH$^+$-Ala$_{19}$ Franziska Schubert, Mariana Rossi, Carsten Baldauf, Volker Blum, Matthias Scheffler Predicting the structure of peptides requires a high accuracy for ``weak'' interactions. We here focus on the predominant structure types of two alanine-based peptides \emph{in vacuo} from first principles and in comparison to experimental IR spectroscopy$^1$: Ac-Ala$_{19}$-LysH$^+$, which is expected to be $\alpha$-helical [1,2], and Ac-LysH$^+$-Ala$_{19}$, where globular monomers, helical dimers, and helices with non-standard protonation sites are expected [2]. Despite supposedly very different conformers, Ac-LysH$^+$-Ala$_{19}$ and Ac-Ala$_{19}$-LysH$^+$ yield very similar experimental IR spectra in the $\approx$1000-2000 cm$^{-1}$ wavenumber range. We utilize a two-stage structure search approach: we begin by a force-field based replica exchange molecular dynamics (REMD) scan followed by further REMD scans based on density functional theory with the van der Waals corrected [3] PBE functional. We suggest plausible candidates for all likely structure prototypes. Helix-turn-helix motifs emerge as the most likely candidates and explain a subtle peak shift in experiment. [1] M. Rossi \textit{et al.}, JPCL \textbf{1}, 3465 (2010); [2] M. Jarrold, PCCP \textbf{9}, 1659 (2007); [3] A. Tkatchenko, M. Scheffler, PRL \textbf{102}, 073005 (2009). [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X41.00011: Enhanced Wang Landau Sampling of Adsorbed Protein Conformations Mithun Radhakrishna, Sumit Sharma, Sanat K. Kumar Using computer simulations to model the folding of proteins into their native states is computationally expensive due to the extraordinarily low degeneracy of the ground state. In this paper, we develop an efficient way to sample these folded conformations using Wang Landau sampling coupled with the configurational bias method (which uses an unphysical ``temperature'' which is between the collapse and folding transition of the protein). This method speeds-up the folding process by roughly an order of magnitude over existing algorithms. We apply this method to study the adsorption of HP protein fragments on a hydrophobic surface, a model which is a close analog of one presented recently by Shea and coworkers. We are able to readily capture the fact that these fragments, which are unstructured in the bulk, acquire secondary structure upon adsorption onto a strong hydrophobic surface. Apparently, the presence of a hydrophobic surface allows these random coil fragments to fold by providing hydrophobic contacts that were lost in protein fragmentation. [Preview Abstract] |
Session X42: Focus Session: The Physics of Genome Folding II: Chromosomes and Nucleosomes
Sponsoring Units: DBIO DPOLYChair: Erez Lieberman Aiden, Harvard Society of Fellows
Room: 156C
Thursday, March 1, 2012 2:30PM - 3:06PM |
X42.00001: Tensegrity Invited Speaker: Donald Ingber |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X42.00002: Polymer-nematic textures in spherical confinement: a coarse-grained model of DNA packing Gregory Grason, Homin Shin Inspired to understand the complex spectrum of space-filling organizations the dsDNA genome within the capsid of bacterial viruses, we study a minimal, coarse-grained model of single chains densely-packed into a finite spherical volume. We build the three basic elements of the model--i) the absence of chain ends ii) the tendency of parallel-strand alignment and iii) a preference of uniform areal density of chain segments--into a polymer nematic theory for confined chains. Given the geometric constraints of the problem, we show that axially symmetric packings fall into one of three topologies: the coaxial spool; the simple solenoid; and the twisted-solenoid. Among these, only the twisted-solenoid fills the volume without the presence of line-like disclinations, or voids, and are therefore generically preferred in the incompressible limit. An analysis of the thermodynamic behavior of this simple model reveals a rich behavior, a generic sequence of phases from the empty state for small container sizes, to the coaxial spool configuration at intermediate sizes, ultimately giving way, via a second-order, symmetry-breaking transition, to the twisted-solenoid structure above a critical sphere size, which we estimate to be within the range of bacteriophage capsid dimensions. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X42.00003: Conformational Fluctuations of Chromosomal DNA in \textit{E. coli} Clarissa Freeman, Jens-Christian Meiners We measured the conformational fluctuations of the bacterial chromosome in \textit{E. Coli in }vivo using fluorescence correlation spectroscopy (FCS). The chromosomal DNA was randomly decorated with a cell-permeable intercalating dye. Conformational fluctuations of the DNA move the fluorophores stochastically into the diffraction-limited excitation volume of a focused laser beam. The time correlation function of the fluorescence intensity reflects the underlying dynamics of the DNA on length scales down to $\sim $200 nm. A comparison between live cells and dead yet structurally intact cells shows identical fluctuation spectra for short time scales, yet substantial differences for frequencies below 100 Hz. Live cells show much stronger fluctuations in this regime. This observation points to the crucial importance of active molecular motor action, as opposed to passive thermal noise, in driving larger conformational fluctuations in the chromosomal DNA, in particular on length scales exceeding $\sim $500 nm. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X42.00004: Spatial organization and dynamics of interphase yeast chromosomes Baris Avsaroglu, Susannah Gordon-Messer, Miriam Fritsche, Jungoh Ham, Dieter W. Heermann, James E. Haber, Jane Kondev Understanding how the genome is spatially organized is an important problem in cell biology, due to its key roles in gene expression and DNA recombination. Here we report on a combined experimental and theoretical study of the organization and dynamics of yeast chromosome III which has a functional role in the yeast life cycle, in particular, it is responsible for mating type switching. By imaging two fluorescent markers, one at the spindle pole body (SPB) and the other proximal to the HML locus that is involved in DNA recombination during mating type switching, we measured the cell to cell distribution of distances and the mean square displacement between the markers as a function of time. We compared our experimental results with a random-walk polymer model that takes into account tethering and confinement of chromosomes in the nucleus, and found that the model recapitulates the observed spatial and temporal organization of chromosome III in yeast in quantitative detail. The polymer model makes specific predictions for mating-type switching in yeast, and suggests new experiments to test them. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X42.00005: Reverse-engineering Chromatin Folding via The Gaussian Polymer Model Manjul Apratim, Swagatam Mukhopadhyay, Anirvan Sengupta Recent technological advancements in techniques exploring chromatin conformation, such as 3C and its derivatives, provide us with information about contact frequency between chromatin segments. There is an urgent need for systematic methods of reconstructing folding patterns of the chromatin from such data. Dekker et al have previously summarized this experimental data in the form of a so-called `spatially averaged' conformation around which fluctuations occur. This would be accurate for regions where the chromatin is mostly frozen around one structure, but rather misleading for more dynamic euchromatin. To address the ill-posed problem of reconstruction of probability distribution from pairwise contact data, we propose a minimum relative entropy approach, which reduces to finding an interacting polymer model that reproduces the contact strengths, and allows for both very dynamic as well as frozen structures depending upon the indicated degree of interaction. While comparing contact frequencies computed from this model to observed data, we introduce the minimal number of interactions required as determined by criteria controlling model complexity. This technique allows us to reproduce known interactions for several biologically important regions like the beta-globin locus. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X42.00006: Electrohydrodynamics of DNA in confinement Jonathan K. Whitmer, Juan P. Hernandez-Ortiz, Juan J. de Pablo New methods of DNA sequencing aim to exploit the direct reading of individual DNA molecules. Such methods require one be able to elongate DNA molecules so that individual base-pairs may be accessed. In turn, this requires a detailed understanding of the mechanical and thermodynamic behavior of DNA, so that external manipulation and confinement successfully stretch the molecule. We aim to study the interplay between electrostatic and hydrodynamic interactions on the conformations of coarse-grained DNA through use of computer simulations with the general geometry Ewald-like method (GGEM), both in bulk and under geometric confinement. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X42.00007: Susceptibilities to DNA Structural Transitions within Eukaryotic Genomes Dina Zhabinskaya, Craig Benham, Sally Madden We analyze the competitive transitions to alternate secondary DNA structures in a negatively supercoiled DNA molecule of kilobase length and specified base sequence. We use statistical mechanics to calculate the competition among all regions within the sequence that are susceptible to transitions to alternate structures. We use an approximate numerical method since the calculation of an exact partition function is numerically cumbersome for DNA molecules of lengths longer than hundreds of base pairs. This method yields accurate results in reasonable computational times. We implement algorithms that calculate the competition between transitions to denatured states and to Z-form DNA. We analyze these transitions near the transcription start sites (TSS) of a set of eukaryotic genes. We find an enhancement of Z-forming regions upstream of the TSS and a depletion of denatured regions around the start sites. We confirm that these finding are statistically significant by comparing our results to a set of randomized genes with preserved base composition at each position relative to the gene start sites. When we study the correlation of these transitions in orthologous mouse and human genes we find a clear evolutionary conservation of both types of transitions around the TSS. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X42.00008: Mechanics of DNA Sticky End Joints Ehsan Ban, Catalin Picu Cohesive DNA sticky ends along with synthesis of stable branched DNA molecules have enabled self assembly of versatile new DNA structures including DNA crystals. Sticky end joints are formed by pairing of complimentary unpaired bases at the end of two DNA molecules. In this work mechanics of the DNA sticky end joints is investigated by using molecular dynamics simulations. Effects of base sequence, joint length, and salt concentration on the mechanical behavior of the joints is studied. The results have implications in understanding the mechanics of DNA crystals and other structures containing sticky end joints. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 5:06PM |
X42.00009: Statistical physics of nucleosome positioning and chromatin structure Invited Speaker: Alexandre Morozov Genomic DNA is packaged into chromatin in eukaryotic cells. The fundamental building block of chromatin is the nucleosome, a 147 bp-long DNA molecule wrapped around the surface of a histone octamer. Arrays of nucleosomes are positioned along DNA according to their sequence preferences and folded into higher-order chromatin fibers whose structure is poorly understood. We have developed a framework for predicting sequence-specific histone-DNA interactions and the effective two-body potential responsible for ordering nucleosomes into regular higher-order structures. Our approach is based on the analogy between nucleosomal arrays and a one-dimensional fluid of finite-size particles with nearest-neighbor interactions. We derive simple rules which allow us to predict nucleosome occupancy solely from the dinucleotide content of the underlying DNA sequences.Dinucleotide content determines the degree of stiffness of the DNA polymer and thus defines its ability to bend into the nucleosomal superhelix. As expected, the nucleosome positioning rules are universal for chromatin assembled in vitro on genomic DNA from baker's yeast and from the nematode worm C.elegans, where nucleosome placement follows intrinsic sequence preferences and steric exclusion. However, the positioning rules inferred from in vivo C.elegans chromatin are affected by global nucleosome depletion from chromosome arms relative to central domains, likely caused by the attachment of the chromosome arms to the nuclear membrane. Furthermore, intrinsic nucleosome positioning rules are overwritten in transcribed regions, indicating that chromatin organization is actively managed by the transcriptional and splicing machinery. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X42.00010: Interrogating Nucleosome Positioning Through Coarse-Grain Molecular Simulation Gordon S. Freeman, Daniel M. Hinckley, Vanessa Ortiz, Juan J. de Pablo Nucleosome positioning plays a crucial role in biology. As the fundamental unit in chromosome structure, the nucleosome core particle (NCP) binds to approximately 147 DNA base pairs. The location of bound NCPs in the genome, therefore, affects gene expression. The specific positioning of NCPs has been experimentally probed and competing viewpoints have been presented in the literature. Models for nucleosome positioning based on sequence-dependent flexibility (a genomic ``code" for nucleosome positioning) have been demonstrated to explain available experimental data. However, so do statistical models with no built-in sequence preference; the driving force for NCP positioning therefore remains an open question. We use a coarse-grain model for the NCP in combination with advanced sampling techniques to probe the sequence preference of NCPs. We present a method for determining the relative affinity of two DNA sequences for the NCP and use this method to compare high- and low-affinity sequences. We discuss several coarse-grain protein models with varying level of detail to examine the impact of model resolution on the agreement of our results with experimental evidence. We also investigate the dynamics of the NCP-DNA complex and their dependence on system characteristics. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X42.00011: Dynamics of Histone Tails within Chromatin Morgan Bernier, Justin North, Michael Page, Christopher Jaroniec, Christopher Hammel, Michael Poirier Genetic information in humans is encoded within DNA molecules that is wrapped around histone octamer proteins and compacted into a highly conserved structural polymer, chromatin. The physical and material properties of chromatin appear to influence gene expression by altering the accessibility of proteins to the DNA. The tails of the histones are flexible domains that are thought to play a role in regulating DNA accessibility and compaction; however the molecular mechanisms for these phenomena are not understood. I will present CW-EPR studies on site directed spin labeled nucleosomes that probe the structure and dynamics of these histone tails within nucleosomes. [Preview Abstract] |
Session X43: Invited Session: Bacterial Swimming and Chemotaxis
Sponsoring Units: DBIO DFDChair: Jay X. Tang, Brown University
Room: 157AB
Thursday, March 1, 2012 2:30PM - 3:06PM |
X43.00001: Bacterial Swimming and Accumulation at the Fluid Boundaries Invited Speaker: Jay Tang Micro-organisms often reside and thrive at the fluid boundaries. The tendency of accumulation is particularly strong for flagellated bacteria such as \textit{Escherichia coli}, \textit{Vibro alginolyticus}, and \textit{Caulobacter crescentus}. We measured the distribution of a forward swimming strain of \textit{Caulobacter crescentus} near a solid surface using a three-dimensional tracking technique based on darkfield microscopy and found that the swimming bacteria accumulate heavily within micrometers from the surface, even though individual swimmers are not trapped long enough to display circular trajectories. We attributed this accumulation to frequent collisions of the swimming cells with the surface, causing them to align parallel to the surface as they continually move forward. The extent of accumulation at the steady state is accounted for by balancing alignment caused by these collisions with the rotational Brownian motion of the micrometer-sized bacteria. We performed simulations based on this model, which reproduces the measured results. Additional simulations demonstrate the dependence of accumulation on swimming speed and cell size, showing that longer and faster cells accumulate more near a surface than shorter and slower ones do. Our ongoing experimental effort also includes observation of similar phenomena at the interfaces of either water-oil or water-air, noting even stronger trapping of the swimming bacteria than near a solid surface. These studies reveal a rich range of fluid physics for further analysis. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:42PM |
X43.00002: Bacterial Motility Patterns and Their Chemotaxis Behaviors Invited Speaker: Xiaolun Wu |
Thursday, March 1, 2012 3:42PM - 4:18PM |
X43.00003: Periplasmal Physics: The Rotational Dynamics of Spirochetal Flagella Invited Speaker: Greg Huber Spirochetes are distinguished by the location of their flagella, which reside within the periplasm: the tiny space between the bacterial cell wall and the outer membrane. In {\sl Borrelia burgdorferi\/} (the causative agent of Lyme Disease), rotation of the flagella leads to cellular undulations that drive swimming. Exactly how these shape changes arise due to the forces and torques acting between the flagella and the cell body is unknown. By applying low-Reynolds number hydrodynamic theory to the motion of an elastic flagellum rotating in the periplasm, we show that the flagella are most likely separated from the bacterial cell wall by a lubricating layer of fluid. We obtain analytical solutions for the force and torque on the rotating flagellum through lubrication analysis, as well as through scaling analysis, and find results are in close agreement numerical simulations. (Joint work with J. Yang and C.W. Wolgemuth.) [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:54PM |
X43.00004: Microfluidics for bacterial chemotaxis Invited Speaker: Mingming Wu The emerging microfluidic technology opens up new opportunities for bacterial chemotaxis studies. In this talk, I will present our efforts in correlating molecular level events with cellular phenotypes in bacterial chemotaxis using microfabricated device. I will present results of bacterial chemotaxis in both single and dual chemical gradients. In single gradient experiments, we demonstrated that bacteria sense the chemical concentration at a logrithmic scale, similar to sensory system in higher organism. In dual gradient experiments, we showed that the number ratio of the two different types of receptor plays a critical role in bacteria's chemotactic decision making process. Experimental results based on single cell analysis will be presented. This work is supported by the National Science Foundation and the Cornell Nanobiotechnology Center. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:30PM |
X43.00005: Directional swimming in bacteria: active and passive gradient responses Invited Speaker: Roman Stocker The ability to swim directionally is paramount for bacteria, in their quest for nutrients and favorable microhabitats. This ability depends on both active and passive responses to gradients. Here we bring an example from each case, based on novel microfluidic experiments that quantify the swimming behavior of bacteria. First, we describe their active response to oxygen gradients - or aerotaxis - and show the unexpected consequences of competing oxygen gradients with nutrient gradients. Then, we present the first observations of directional swimming by bacteria in response to fluid velocity gradients - or rheotaxis. Combining experiments with mathematical modeling we demonstrate that, unlike in larger organisms such as fish, rheotaxis in bacteria is passive, resulting from a previously undetected torque that originates from the chirality of the bacterial flagellum. [Preview Abstract] |
Session X44: Focus Session: Directed Assembly of Hybrid Materials - Nanoparticles in Micelles
Sponsoring Units: DPOLYChair: Lynden Archer, Cornell University
Room: 157C
Thursday, March 1, 2012 2:30PM - 2:42PM |
X44.00001: Multigeometry nanoparticles and higher-order assemblies from block copolymer blends via kinetic control and chemical modification Jiahua Zhu, Shiyi Zhang, Karen Wooley, Darrin Pochan Multigeometry micellar structures, due to segregation of unlike hydrophobic domains trapped within the same micelle core, have been produced via self-assembly of block copolymer mixtures in tetrahydrofuran/water solution. The mixture is composed of two block copolymers with distinctive hydrophobic blocks but the same poly(acrylic acid) (PAA) hydrophilic block. By taking advantage of the complexation in the hydrophilic corona between the acid side chains of the PAA block and added organoamine molecules, unlike hydrophobic blocks can be trapped in the same micelle core. Locally, the unlike hydrophobic blocks can segregate into compartments and even express different interfacial curvatures, or geometries, within the same nanoparticle. Through controlled kinetic pathways, block copolymer design and mixing ratios, both micelle compartment size and shape can be controlled to generate sphere-sphere, sphere-cylinder, cylinder-cylinder, cylinder-bilayer, and bilayer-bilayer blended multigeometry nanoparticles. Furthermore, higher order assembly behavior of the micelles has been investigated by taking advantage of chemical modification on the hydrophilic PAA shells. New mixtures using functionalized-PAA-containing block copolymers produce nanoparticles with a compartmentalized surface. These patchy surfaces can be used as templates for asymmetric hybrid nanoparticles, but also as building blocks for hierarchical assembly of the nanoparticles to produce one-dimensional arrays or three dimensional networks. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X44.00002: Polymer nano-particle hybrid micelles: Encapsulation of POSS into semi-fluorinated polymer micelles Dilru Ratnaweera, Dvora Perahia, Scott Iacono, Joseph Mabry, Dennis Smith Self-assembly of block copolymers in selective solvents was used to form a nanoparticle (NP)/polymer hybrid micelles. These micelles can be used as a cargo vehicle for other substances such as drug delivery, and as building blocks for polymer-nanocomposites with controlled NP distribution. Association of NPs into specific blocks of the copolymer depends on the compatibility between the NPs and the block as well as their preference to the solvent that micellization takes place. The current work introduces a small angle neutron scattering study of association of Polyhedral Oligomeric Silsesquioxane (POSS) NPs into micelles of a highly segregating random copolymer, Biphenyl Perfluorocyclobutane (BPh-PFCB), in toluene, which is a good solvent for BPh. Incompatibility between the blocks drives copolymer into micelles with PFCB in the core and BPh in swollen corona. Modification of NPs with polymer chains drives POSS cages into the micelle core and prevents the micelle dissociation at higher temperatures. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X44.00003: Efficient Encapsulation of Gold Nanorods into Block Copolymer Micelles Dae Hwan Kim, You-Yeon Won Gold nanorods (GNRs) have the potential to be used as an imaging and/or hyperthermia agent for cancer theragnosis. Clinical applications of as-synthesized GNRs (i.e., CTAB-coated CNRs) are currently limited by their cytotoxicity and insufficient colloidal stability. With an aim to address these problems, we developed a self-assembly processing technique for encapsulation of GNRs in block copolymer (BCP) micelles. This technique uses simple steps of solvent exchange processes designed based on the known principles of block copolymer self-assembly. It will be demonstrated that BCP-encapsulated GNRs are stable against aggregation under physiological conditions and non-toxic to cells. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:42PM |
X44.00004: Kinetic pathways to organized polymer/nanoparticle assemblies Invited Speaker: Ryan Hayward Processes that allow for controlled access to kinetically trapped non-equilibrium states have the potential to significantly expand the range of structures and properties that may be achieved by self-assembly. We will describe several recent examples from our group wherein new types of polymer/nanoparticle assemblies are enabled by designed processing pathways. In the first case, we study the formation of amphiphilic polymer micelles through hydrodynamic instabilities of solvent/water interfaces induced during emulsion processing. We show that this route allows for efficient co-encapsulation of multiple types of hydrophobic nanoparticles within the micelle cores. Second, we consider the influence of nanoparticles on spinodal decomposition of a polymer blend and find that the inclusion of aggregating particles provides a route to kinetically stabilize co-continuous structures through particle gelation in one of the polymer phases. Finally, we show how the structures of hybrid nanoparticle/conjugated polymer nanowires can be tuned using solution-state crystallization. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X44.00005: A coarse-grained molecular dynamics approach to shear-directed assembly of nanoparticle arrays in amphiphilic block copolymer solutions Bryan Rolfe, Jaehun Chun, Yong Joo Experiments by Pozzo and Walker (2007) demonstrated shear ordering of binary nanoparticle block copolymer micelle crystals and its potential in the development of new nanostructured materials. Specifically, they have shown that nanoparticles occupying interstitial sites within the micelle crystal affect a low-shear, long-range order and that the shear type and rate are important to the crystal structure. However, the connection between macroscopic variables and resulting crystal structure is yet to be understood. We present results which elucidate the underlying mechanisms governing shear-directed assembly of these binary nanocrystals. Our approach employs a coarse-grained molecular dynamics model (CGMD) that includes the hydrodynamics of the solvent and preserves dynamic effects by making no assumptions regarding the micelle structure. In fact, micelle self-assembly of the amphiphilic block copolymers occurs in-situ with the nanoparticles along with the shear ordering of the binary crystal. Tunability of the crystal structure is determined by varying shear rates/types, nanoparticle size/concentration, and block copolymer chemistry as simulation parameters. The simulation predicted results of shear-directed assembly of nanoparticles will be compared to the experimental results. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X44.00006: Nanoparticle Polymer Suspensions: Structure and Dynamics Samanvaya Srivastava, Lynden Archer We present structure and rheology measurements for model nanoparticle suspensions, which comprise of silica nanoparticles, densely grafted with small polyethylene glycol (PEG) chains and suspended in PEG oligomers. Structure characterization, using electron microscopy and X-ray scattering, reveals well-dispersed nanoparticles leading to stable suspensions across a range of particle volume fractions. At the same time, X-ray photon correlation spectroscopy studies reveal arrested dynamics and extremely small nanoparticle diffusivities in the high volume fraction suspensions, consistent with the expectations for a soft glass. The liquid-soft glassy solid transition is found to occur at strikingly low core volume fractions and the glassy suspensions are found to exhibit a range of unique features including strong shear thinning, presence of a zero shear Newtonian plateau, strain accelerated relaxation and prominent stress overshoots in flow startup. Comparisons of our experimental findings with the SGR model are provided and on this basis, we propose our suspensions as model systems for understanding the jamming transition and other properties of soft glasses. Further, we elucidate the form of particle interactions and compare them with models for spherical polymer brushes. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X44.00007: Pristine graphene dispersions and solution-cast composites Sriya Das, Fahmida Irin, Ahmed Wajid, Abel Cortinas, Micah Green Graphene holds potential as strong, conductive fillers in polymer nanocomposites; however, difficulties in dispersion quality and interfacial strength between filler and matrix have been a persistent problem for graphene-based nanocomposites, particularly for pristine, unfunctionalized graphene. We utilize a triphenylene based molecule (C10) to stabilize pristine graphene in water with a high graphene/stabilizer ratio. C10 molecules pi-pi stack with the graphene surface and prevent reaggregation. This dispersion can be reversibly destabilized based on pH and is stable against heat and lyophilization. Solution cast poly (vinyl alcohol) (PVA) composites prepared from these dispersions have enhanced mechanical and electrical properties (percolation threshold: 0.26 vol {\%} graphene). Also, for the first time, pristine graphene/PVA dispersions are electrospun to form graphene/polymer composite nanofibers. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X44.00008: Simulation of viscoelastic suspensions using regularized singularities Ronald Phillips, Ryota Aoki Particles interacting through viscoelastic fluids exhibit behavior that differs qualitatively from corresponding systems with Newtonian suspending fluids. Depending on the type of fluid involved, the polymer contribution to the stress can induce clustering or particle chaining, and impede efforts to form a homogeneous suspension. We are using regularized singularities in conjunction with the finite volume method to calculate velocity fields, stress fields, and particle displacements in viscoelastic suspensions. These singularities consist of Stokelets and stresslets, or regions of enhanced body forces and stresses, and provide a simple model of suspensions of weakly deformable, non-neutrally buoyant particles. Simulations of hundreds of two-dimensional particles in multimode viscoelastic fluids show behavior similar to what is seen experimentally, and provide insight into the physical causes of particle aggregation in complex fluids. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X44.00009: Polymer mediated solution self-assembly of nanorods Miguel A. Modestino, Rachel A. Segalman Control over the self-assembly of nanorods in polymer composites enables the design of hybrid materials in which the anisotropic properties of the nanocrystals can be harnessed efficiently. Here, we demonstrate that a delicate balance between entropic and enthalpic interactions controls the self-assembly behavior of nanorods in solutions and can lead to the formation of ordered nanorod arrays. Small angle X-ray scattering techniques were used to elucidate the phase behavior of CdSe nanorods in polymer solutions and to identify the concentration space that allows for the formation of nanorod superlattices. Furthermore, this work demonstrates that enthalpic interactions have strong effects on the nanorod self-assembly, and that the presence of reversible thermal transitions can allow for the growth of large nanorod arrays. The solution self-assembly behavior discussed in this study allows for the fabrication of solution processable composite thin films with vertically aligned nanorods over large areas. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X44.00010: Quantifying the Thermodynamic Interactions in Carborane Nanoparticle Solutions Mary Mutz, Eric Eastwood, Mark Dadmun The dissolution of nanoparticles, particularly those containing boron, is an important area of interest for polymer nanocomposite formation and material development (1-2). In this work, the solubility of four boron cage molecules are quantified in toluene, THF, and methyl ethyl ketone with static light scattering, refractometry, UV-Vis spectroscopy, and physical observations. UV-Vis spectroscopy provides a method to determine the concentration and solubility limits of the solutions tested. Using light scattering, the second virial coefficient, A2, was determined and used to calculate $\chi$, the solute-solvent interaction parameter. The Hildebrand solubility parameter, $\delta$, was then extracted from this data using the Hildebrand-Scatchard solution theory (3-4). A list of potential good solvents based on the extracted $\delta$ value is provided for each nanoparticle. Of the systems tested, 1,3-di-o-carboranylpropane was shown to be a thermodynamically stable in toluene, with a $\chi$ less than 0.5, a solubility limit of 2.47 mg/mL, and all solutions remaining clear with no visible particle settling. [Preview Abstract] |
Session X45: Focus Session: Soft Matter Physics of Drops, Bubbles, Foams, and Emulsions - Droplet spreading, colliding, wetting
Sponsoring Units: DPOLYChair: Esther Amstad, Harvard University
Room: 159
Thursday, March 1, 2012 2:30PM - 3:06PM |
X45.00001: Ultrafast interference of splashing dynamics: where is the air? Invited Speaker: Michelle Driscoll A drop impacting a solid surface with sufficient velocity will splash and emit many small droplets. While liquid and substrate properties obviously are important for determining the splashing threshold, it has also been shown that the surrounding gas is a crucial control parameter [1]. The mechanism underlying how the surrounding gas affects splashing remains unknown. One suggestion [2,3] has been that upon impact the liquid spreads outwards over a thin layer of gas that has been trapped beneath it during impact. In a sufficiently viscous liquid, splashing occurs at the edge of the drop several tenths of a millisecond after impact [4]. This large separation between impact and splashing, in both time and space, creates an ideal system in which to test whether the initial air pocket remains to influence splashing dynamics. We develop high-speed interference imaging to measure the air beneath all regions of a spreading viscous drop [5]. Although an initial air bubble is created on impact, no significant air layer persists up to the time a splash is created. This suggests that splashing in our accessible range of viscosities is not caused by the presence of a layer of air beneath the liquid, but rather it is initiated at the edge of the drop as it encroaches into the surrounding gas.\\[4pt] [1] L. Xu, W. W. Zhang, and S. R. Nagel, Phys. Rev. Lett. \textbf{94}, 184505 (2005).\\[0pt] [2] S. Mandre, M. Mani, and M. P. Brenner, Phys. Rev. Lett. \textbf{102}, 134502 (2009).\\[0pt] [3] L. Duchemin and C. Josserand, Phys. Fluids \textbf{23}, 091701 (2011).\\[0pt] [4] M. M. Driscoll, C. S. Stevens, and S. R. Nagel, Phys. Rev. E \textbf{82}, 036302 (2010).\\[0pt] [5] M. M. Driscoll and S. R. Nagel, Phys. Rev. Lett. \textbf{107}, 154502 (2011). [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X45.00002: The drop impact dynamics of complex fluids on textured surfaces Kyoo-Chul Park, Vivek Sharma, Robert Cohen, Gareth McKinley The deposition of aqueous drops on non-wetting surfaces is an important canonical problem for many applications, including suppressing splash or rebound of sprayed herbicides on intrinsically hydrophobic plant leaves. The addition of a small amount of high molecular weight polymer has been demonstrated to suppress drop rebound on smooth hydrophobic surfaces. The high extensional viscosity of polymer solutions and the increased viscous dissipation near the receding contact line are cited as two distinct anti-rebound mechanisms. Using drop impact experiments on both micro- and nano-textured surfaces with controlled wetting characteristics we examine the roles of viscosity, elasticity and inertia on expansion, retraction, and rebound of well-characterized viscoelastic fluids. By adopting a stick-slip flow model on textured surfaces with various topographic length scales and solid area fractions, we rationalize the dynamics leading to complete rebound following drop impact on nanotextured surfaces even for highly viscoelastic fluids. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X45.00003: Influence of nanoscale surface roughness on mechanism of dropwise water condensation Konrad Rykaczewski Adversely to most potential applications of superhydrophobic coatings, only a few natural and artificial surfaces retain their superhydrophobic characteristics during water condensation. This work addresses the key question of why condensation on such surfaces leads to self-propelled dropwise condensation but causes wetting of other surfaces with water contact angles above 150 degrees. The effects of gradually varying nanoscale roughness of a hydrophobic surface on the mechanism of drop growth and coalescence are observed using electron and light microscopy. It is demonstrated that increasing the nanoscale surface roughness confines the base diameter of the nucleating droplets causing them to grow by increasing their contact angle. The increase in the nanoscale surface roughness also decreases triple line pinning during coalescence, thus allowing formation of nearly spherical drops after merging of two high contact angle drops. The role of the nanoscale roughness in the diameter confinement effect is explained through thermodynamic calculations. Lastly, confined base diameter growth model is derived and compared with experimental results. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X45.00004: Effect of Nano-Scale Roughness on Particle Wetting and on Particle-Mediated Emulsion Stability Adriana San Miguel, Sven Behrens Colloidal particles can strongly adsorb to liquid interfaces and stabilize emulsions against droplet coalescence, the effectiveness of which depends crucially on the particle wettability. From the study of macroscopic solids, surface wetting is known to be influenced strongly by nano-scale roughness (as seen $e.g.$ in the ``Lotus effect'' or in anti-fog coatings); similarly, strong effects of particle roughness on particle-stabilized emulsions should be expected. Here we report the first experimental study of particle wetting and particle-mediated emulsion stability in which particle roughness could be varied continuously without varying the surface chemistry. We demonstrate an enabling method for preparing particles and macroscopic substrates with tunable nano-roughness and correlate the extent of roughness quantitatively with surface wetting (measured \textit{via} the three-phase contact angle) and with emulsion stability (quantifiable \textit{via} the maximum capillary pressure). Our results confirm a dramatic influence of roughness on wetting, emulsion stability, and even the type of emulsion formed (o/w \textit{vs.} w/o) upon mixing oil with an aqueous particle dispersion. Whether particle roughness benefits emulsion stability or not is seen to depend on both the size and shape of the surface features. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X45.00005: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X45.00006: Life Underneath a Leidenfrost Drop Aaron Sharpe, Justin Burton, Roeland van der Veen, Andres Franco, Sidney Nagel Liquid drops deposited on a hot surface undergo a dramatic transition from boiling to levitating, a phenomena known as the Leidenfrost effect. The drops float on an insulating layer of evaporated vapor, which forms a pocket of high pressure underneath the drop and distorts the liquid-vapor interface. Experiments [1] have examined the lifetime and maximum size of such levitated drops. However, the interface beneath the drop has not been visualized or characterized. Using high-speed laser-light interference, we measure the geometry and fluctuations of the liquid-vapor interface. The interference fringes produced between the bottom surface of the liquid and the hot substrate provide information about the curvature of the vapor pocket beneath the drop as well as the azimuthal undulations along the rim that resides closest to the surface. We measure the speed, wavelength, and amplitude of the fluctuations as a function of the temperature of the substrate, as well as compare our results to recent theoretical predictions concerning the size of the vapor pocket for large drops [2].\\[4pt] [1] A. Biance, C. Clanet, D. Qu\'{e}r\'{e}, Phys. Fluids \textbf{15}, 1632 (2003).\\[0pt] [2] J. H. Snoeijer, P. Brunet, J. Eggers, Phys. Rev. E \textbf{79}, 036307 (2009). [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X45.00007: Voltage Bursting Drops in Solids Qiming Wang, Zhigang Suo, Xuanhe Zhao Droplets in air or liquids under electrical voltages appear in diverse processes from thunderstorm cloud formation, ink-jet printing, electrospinning nanofibers to electrospray ionization. In these processes, the electrostatic energy competes with surface energy of the drops and causes sharp tips to form on the ends of the drops. Here, we report a physically distinct scenario for droplets in solid matrices under voltages. We show that water drops in elastic polymers can form sharp tips and surprisingly burst into long tubes under applied voltages. The new phenomenon is governed by the elasticity and fracture of the solids, instead of the drops' surface energy as in previous cases. A new scaling is derived for the critical electrical field of the voltage-induced instability of drops in solids. The observations and analyses have significant practical impacts, as they illustrate the mechanism of a major failure mode, defect-induced breakdown, of dielectric polymers, which are widely used as insulating cables and polymer capacitors and transducers. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X45.00008: Spreading dynamics of a water droplet on a soluble polymer: glass transition effects Emilie Verneuil, Julien Dupas, Marco Ramaioli, Laurent Forny, Laurence Talini, Francois Lequeux We study the wetting dynamics of a droplet of solvent spreading on a soluble polymer coating. The complexity arises from the transfers of solvent and of soluble material through 3 processes: liquid evaporation and recondensation on the substrate, diffusion of the liquid in the substrate from the droplet, and substrate dissolution within the droplet. Indeed, when completely dry, the substrate, although soluble, is initially poorly wetting. Hydration enhances the wettability of the coating and the contact angle is found to decrease at higher humidity or at lower spreading velocity. In this paper, we explore experimentally the situation where hydration itself induces a sharp change in the diffusion coefficient of water in the polymer: this is what happens when the polymer undergoes a glass transition in water content. Diffusion coefficient changes by orders of magnitude upon glass transition, and we show that it results in a sharp effect on the course of the spreading as the hydration will be affected by the change in diffusion in the coating. We validate the previously derived model describing the various spreading regimes observed, and we expand it to account for the glass transition effects. It also successfully describes the results we obtain with other solvents and substrates. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X45.00009: Water droplet spreading on a soluble polymer: what happens close to the contact line? Laurence Talini, Julien Dupas, Emilie Verneuil, Francois Lequeux, Laurent Forny, Marco Ramaioli We have studied the spreading of a water droplet on a water soluble substrate. Numerous coupled transfer processes are involved in such a situation, leading to complex wetting dynamics. In particular, previous studies have shown the major role of water evaporation from the droplet associated with water uptake by the substrate. However, the processes at stake close to the contact line, where the substrate properties set the wetting angle, have not been understood. We present an experimental study of the phenomena occurring within distances ranging from 10 to 1000 $\mu$m from the contact line of a water droplet spreading on a food polymer layer. We have evidenced a wrinkling pattern inside the droplet close to the contact line, and suggest it results from the swelling of the constrained polymer layer before it dissolves. In addition, using an optical method based on the analysis of Newton's rings, we have measured the hydration profile of the substrate ahead the contact line. We show that the profiles can be understood as a result of the evaporation/water uptake process through air combined with direct water diffusion in the substrate from the liquid wedge. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X45.00010: Contact angle dependence of the resonant properties of sessile drops James Sharp A simple optical deflection technique was used to monitor the vibrations of microlitre sessile drops of glycerol/water mixtures with glycerol compositions ranging from 0$\%$ to $75\%$. A photodiode was used to detect time dependent variations in the intensity of laser light reflected from the droplets. The intensity variations were Fourier transformed to obtain information about the resonant properties of the drops (frequency and width of the resonance). These experiments were performed on a range of different substrates where the contact angle formed by the droplets varied between $38\pm 2^{\circ}$ and $160\pm 4^{\circ}$. The measured resonant frequency values were found to be in agreement with a recently developed theory of vibrations which considers standing wave states along the profile length of the droplet. The widths of the resonances were also compared with theories which predict the influence of substrate effects, surface contamination effects and bulk viscous effects on the damping of capillary waves at the free surface of the droplets. These experiments indicate that the dominant source of damping in sessile liquid droplet is due to bulk viscous effects but that for small contact angles damping due to the droplet/substrate interaction becomes more important. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X45.00011: Interfacial Effects on Droplet Dynamics in Poiseuille Flow Steven Hudson, Jonathan Schwalbe, Kendra Erk, Frederick Phelan Jr., Petia Vlahovska Many properties of emulsions arise from interfacial rheology. Here we advance theoretical understanding and experimental observation of the dynamics of isolated drops suspended in Poiseuille flow. Stokes flow is assumed in the bulk phases, and a jump in hydrodynamic stress at the interface is balanced by Marangoni forces (linearized with respect to local deviations of interfacial surfactant concentration) and surface viscous forces according to the Boussinesq--Scriven constitutive law. Interfacial diffusion is also included. Our analysis predicts slip, cross-stream migration and droplet-circulation velocities. These results and the corresponding interfacial parameters are separable, enabling a new droplet-based interfacial rheology method to determine interfacial viscosities and Marangoni elasticity. We illustrate such measurements by particle tracking velocimetry of surfactant-stabilized droplets in the Poiseuille flow of a microfluidic device. Small molecule and block copolymer surfactants are examined. This droplet-based interfacial rheology method is attractive since it mimics the natural geometry and length scale of practical emulsions and suspensions. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X45.00012: Impact of liquid droplets on granular media Giles Delon, Denis Terwagne, Stephane Dorbolo, Nicolas Vandewalle, Herve Caps The crater formation due to the impact of a water droplet onto a granular bed has been experimentally investigated. Three parameters have been tuned: the impact velocity, the size of the droplet and the size of the grains. The shape of the crater depends on the Weber number at the moment the droplet starts to impact the bed. From the dynamical point of view, the spreading and the receding of the liquid during the impact have been carefully analyzed using image analysis of high speed video recordings. The different observed regimes are characterized by the balance between the impregnation time of the granular bed by the water contained in the droplet and the capillary time responsible for the receding of the drop. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X45.00013: Droplet Rearrangement in a Sheared Dense Emulsion S.K. Dutta, D.L. Blair The constituent particles of disordered colloidal dispersions compressed above the threshold for jamming flow with highly heterogeneous dynamics. Though this leads to the rich viscoelastic behavior that makes these materials so widespread, a clear description at the microscopic level has yet to emerge. We investigated the non-affine motion in a dense oil-in-water emulsion using a confocal rheometer, which can image individual droplets while applying a precisely controlled shear to the system. From these images, we identify the very fast rearrangements that accompany flow and determine the spatial and temporal distributions of the events as a function of the droplet volume fraction. In addition, it is possible to characterize the regions of the emulsion which are most susceptible to rearrangement. [Preview Abstract] |
Thursday, March 1, 2012 5:30PM - 5:42PM |
X45.00014: Wetting Colloidal Particles at a Curved Interfaces Colm Kelleher, Paul Chaikin At scales much smaller than the capillary length, surface tension plays a predominant role in the interactions that occur at an oil-water interface. When a spherical colloidal particle is adsorbed onto such an interface, two surface-tension-related effects occur: the adsorbed particle reduces the area of the interface, and the interface deforms in order to satisfy the requirement of constant contact angle at the three-phase contact line. If the interface is not flat or spherical, these effects depend on the position of the particle on the interface. In other words, the particle experiences an effective potential induced by the shape of the interface. [A. Wurger, PRE, 74, 041402 (2006).] We present results from an experiment in which a capillary bridge creates an interface of varying Gaussian curvature, onto which a colloidal particle is introduced. The shape of this interface is obtained by using confocal microscopy. We demonstrate that a shape-induced effective potential exists for this system, which attracts the wetting particle to the most curved regions. By tracking the motion of the particle in 3D, we are able to calculate the effective spring constant of this potential. We then compare our result to numerical and analytical predictions based on the geometry of the droplet. [Preview Abstract] |
Session X46: Invited Session: Deformation and Fracture of Soft Materials
Sponsoring Units: DPOLYChair: Kenneth Shull, Northwestern University
Room: 160AB
Thursday, March 1, 2012 2:30PM - 3:06PM |
X46.00001: Responsive Gel-Gel Phase Transitions in Artificially Engineered Protein Hydrogels Invited Speaker: B.D. Olsen Artificially engineered protein hydrogels provide an attractive platform for biomedical materials due to their similarity to components of the native extracellular matrix. Engineering responsive transitions between shear-thinning and tough gel phases in these materials could potentially enable gels that are both shear-thinning and tough to be produced as novel injectable biomaterials. To engineer a gel with such transitions, a triblock copolymer with thermoresponsive polymer endblocks and an artificially engineered protein gel midblock is designed. Temperature is used to trigger a transition from a single network protein hydrogel phase to a double network phase with both protein and block copolymer networks present at different length scales. The thermodynamics of network formation and resulting structural changes are established using small-angle scattering, birefringence, and dynamic scanning calorimetry. The formation of the second network is shown to produce a large, nonlinear increase in the elastic modulus as well as enhancements in creep compliance and toughness. Although the gels show yielding behavior in both the single and double network regimes, a qualitative change in the deformation mechanism is observed due to the structural changes. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:42PM |
X46.00002: Direct probes of molecular interactions at buried interfaces Invited Speaker: Ali Dhinojwala The molecular interactions at buried interfaces play an important role in adhesion, friction, and wetting. In my talk, I will discuss the use of interface-sensitive sum frequency generation (SFG) spectroscopy to study acid-base interactions at solid-liquid and solid-solid interfaces. The shift of the sapphire hydroxyl peak in contact with several polar and non-polar liquids and polymers can be used to determine the interaction energy. The magnitude of the interaction energies cannot be predicted based on measuring water contact angles. Molecular rearrangements at the sapphire interface, to maximize the interaction of the acid-base groups, play a dominant role and these effects are not accounted for in the current theoretical models. The importance of these interactions in controlling segregation of polymer blends and friction and adhesion of soft substrates will be discussed. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 4:18PM |
X46.00003: Adhesion of hydrogels under water by hydrogen bonding: from molecular interactions to macroscopic adhesion Invited Speaker: Costantino Creton Hydrogels are an essential part of living organisms and are widely used in biotechnologies, health care and food science. Although swelling properties, cell adhesion on gel surfaces and gel elasticity have attracted much interest, macroscopic adhesion of hydrogels on solid surfaces in aqueous environment is much less well understood. We studied systematically and in aqueous environment, the reversible adhesion by hydrogen bonding of macroscopic model hydrogels of polydimethylacrylamide (PDMA) or of polyacrylamide (PAAm) on solid surfaces functionalized with polyacrylic acid (PAA) polymer brushes. The hydrogels were synthesized by free radical polymerization and the brushes were prepared by grafting polytertbutyl acrylate chains and converting them by pyrolisis into polyacrylic acid. A new adhesion tester based on the flat punch geometry was designed and used to control the contact area, contact time, contact pressure and debonding velocity of the gels from the surface while the samples were fully immersed in water. The adhesion tests were performed at different pH and temperatures and the modulus of the gel and grafting density and molecular weight of the brushes was varied. Macroscopic adhesion results were compared with phase diagrams in dilute solution to detect molecular interactions. While the PDMA/PAA pair behaved very similarly in solution and in macroscopic adhesion tests, the PAAm/PAA pair showed an unexpectedly high adhesion level relatively to its complexation ability in dilute solution. Surprisingly, time dependent experiments showed that the kinetics of H-bond formation and breakup at interfaces was very slow resulting in adhesion energies which were very dependent on contact time up to one hour of contact. At the molecular level, neutron reflectivity showed that the equilibrium brush conformation when in contact with the gels was more extended at pH2 (H-bonds activated) than at pH9 (H-bonds deactivated) and that a certain applied pressure was necessary to bring the brush and the gel in contact. These combined results suggest that the mechanisms of formation and breakup of interactions at surfaces are more complex and cannot be understood from bulk interactions in solution alone. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:54PM |
X46.00004: Self-healing polymers and mechanochemistry Invited Speaker: Stephen Craig The forces that accompany macroscopic deformation and material failure are many orders of magnitude larger than the forces between the individual atoms of a molecule. The magnitude of macroscopic forces, in combination with the fact that they are directional, creates an opportunity to direct new, stress-responsive chemistry on demand in polymer materials. This talk will present studies of reactions under large, directional forces, and their applications in new classes of stress-responsive polymers, including a class of self-healing polymers in which mechanical activation of chemical reactions leads to improved structure and properties under conditions that are typically destructive to both. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:30PM |
X46.00005: Soft active materials---when mechanics meets chemistry Invited Speaker: Zhigang Suo Soft materials, such as elastomers and gels, can mimic a salient feature of life: deformation in response to diverse stimuli. For example, an electric field can cause an elastomer to stretch several times its length. As another example, a change in pH can cause a gel to swell many times its volume. The deformation is large and reversible. These soft active materials are being developed for soft robots, adaptive optics, self-regulated fluidics, and programmable haptic surfaces. This talk describes recent work in my group on the mechanics of soft active materials. We formulate theories to answer commonly asked questions. How do mechanics, chemistry, and electrostatics work together to generate large deformation? What is the maximal energy that can be converted by a material? We also develop experimental methods to characterize nonlinear time-dependent mechanical behavior. Also described in this talk are hydrogels of exceptional toughness and stretchability. [Preview Abstract] |
Session X47: Focus Session: Polymers for Energy Storage and Conversion - Nanostructures and Phase Separated Morphologies
Sponsoring Units: DPOLYChair: Bryan Boudouris, Purdue University
Room: 160C
Thursday, March 1, 2012 2:30PM - 2:42PM |
X47.00001: Nanostructured Organometallic Polymers for Enzymatic Bioenergy Moon Jeong Park, Jungphil Lee The development of efficient enzymatic biofuel cell is a subject of considerable studies in past decades for potential applications such as biomedical devices and microchip systems. One of the key challenges in advancing the technology lies in the power densities of the system. Limitations have been arisen from the buried redox active sites within enzyme structure and poor interplay between redox reactions. In present study, a glucose oxidase is employed as a model enzyme and ferrocene-containing organometallic block copolymers are chosen for the electron mediators. Wiring of glucose oxidase into electrode surface was successfully achieved by cross-linked networks of organometallic polymers and remarkably, catalytic current densities of the fabricated electrodes have proven be a sensitive function of the morphologies of electron mediators. Different nanoscale morphologies, i.e., bicontinous structure, nanowires, and nanoparticles, have been derived and the use of bicontinous morphology confirms 2-50 times improved catalytic current response than the values obtained from other morphologies. The bio-sensing ability of the fabricated electrode with structural optimization was also exploited and good sensitivity is obtained at the physiological concentration of glucose in blood. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X47.00002: Comparison of polymer-fullerene heterojunction morphology to bimolecular recombination kinetics Dean DeLongchamp, Deanna Rodovsky, Jeff Peet, Tracey Clarke, Attila Mozer, Lee Richter, Andrew Herzing, Joseph Kline One of the most important physical processes limiting the practical power conversion efficiency of bulk heterojunction (BHJ) organic photovoltaic devices is the bimolecular recombination of holes and electrons. Reduced recombination would permit the use of thicker BHJ layers, enabling greater light absorption without a penalty in device current. A few polymer light absorbers, when combined in a BHJ with a fullerene electron acceptor, exhibit recombination that is slower than Langevin-type, but the origins of this behaviour are not understood. This talk will describe our effort to determine whether slower-than-Langevin recombination can be attributed to features of the nanoscale morphology or crystalline microstructure within the BHJ film. In comparing BHJ films made from two silole-based monomers, one with Langevin recombination and one with slower-than-Langevin, we find many aspects of the BHJ material structure such as order, orientation, and nanoscale domain size and shape, to be surprisingly similar. We will compare the two materials and emphasize opportunities in data analysis and new measurements to determine whether a morphological basis underlies different recombination kinetics. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X47.00003: Confinement effects on P3HT-PCBM Morphology for Bulk Hetero-Junction Polymer Solar Cells Abul Huq, Dharmaraj Raghavan, Jolanta Marszalek, David Bucknall, Alamgir Karim Controlling morphology of the bulk hetero-junction in solar cell development is the key aspect for higher efficiency. We controlled the morphologies of phenyl-C61-butyric acid methyl ester (PCBM) and poly(3-hexylthiophene) (P3HT) blend thin films by tunable surface energy polydimethylsiloxane (PDMS) elastomer confinement. The advantages of replacing air-polymer interface with PDMS-polymer interface are its flexibility, easy detachability, facile tunability of surface energy by UVO exposure. We hypothesize that the confined annealing will suppress PCBM crystallinity, control crystallinity of P3HT, direct the vertical segregation of PCHT-PCBM blend, and thus influence the composition distribution of P3HT and PCBM at the interfaces. The annealed films were characterized by dark field optical microscopy, GISAXS, GIWAXS, AFM, and SANS. PCBM crystallization was indeed suppressed in the films during confinement annealing. On the other hand the phase separated interpenetrating network and favorable crystallinity of the P3HT evolved. The optimum surface energy of the confining PDMS yields the best structural features of the film. The morphology developed under different PDMS surfaces is an important step towards improvement of efficiency of OPV's. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X47.00004: The Role of P3HT Crystallization in the Morphological Development in P3HT/PCBM Thin Films Using Neutron Scattering Mark Dadmun, Wen Yin Small angle neutron scattering (SANS) provides an important method to characterize the morphology of PCBM/P3HT organic photovoltaics (OPVs), which is essential to improving the efficiency of plastic solar cell devices. Our recent SANS results indicate that fullerene derivatives and conjugated polymers employed in OPVs are significantly miscible, up to $\sim $20{\%}. In this work, the morphology of PCBM/P3HT composite \textit{thin films} is investigated via SANS and analyzed to document their morphology and miscibility. These results indicate that both 20 and 50 vol{\%} PCBM as-cast films exhibit relatively small low-Q scattering, suggesting an overall homogeneous mixture. After annealing at 150\r{ }C for 30 minutes, P3HT undergoes further crystallization in both mixtures. However, the low-Q scattering of the 20 vol{\%} PCBM sample remains low, indicating the film remains homogenous. On the other hand, 50{\%} PCBM sample undergoes phase separation between amorphous PCBM and P3HT. These results therefore exemplify the importance of P3HT crystallization in the structure development of OPV active layers, showing that the crystallization of P3HT is not sufficient to induce phase separation. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X47.00005: Understanding the Behavior of Poly(3-hexylthiophene) at Liquid/Vacuum Interfaces Yeneneh Yimer, Mesfin Tsige Among semiconducting polymers used in opto-electronic devices, poly(3-hexylthiophene) (P3HT) is one of the better candidates because of its good electrical properties and ease of processing. The performance of these devices strongly depends on the structural, morphological, dynamic and interfacial properties of P3HT. Using molecular dynamics simulation and utilizing two different models - all-atom and united-atom - we have studied the dynamic and structural properties of free-standing P3HT thin films at liquid/vacuum interfaces. To quantify these properties, the temperature and chain-length dependence of surface roughness, interfacial width, surface tension, torsional defects, and several other surface properties have been investigated. The results we obtained from all-atom and-united atom models are in reasonable agreement. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X47.00006: Understanding Schroeder's Paradox Adam Weber, Ahmet Kusoglu Schroeder's paradox is a well known, but not fully understood, phenomenon that exists in many polymers and gels. Essentially, the uptake of solvent in the polymer depends on the interaction with the boundary phase. Nafion, a polymer of interest for many electrochemical energy applications, is a classic example where the water uptake almost doubles by placement in liquid water versus saturated water vapor. In this talk, we examine the origin of this paradox through examination of Nafion morphology and water-uptake time constants using experiments in various solvents, vacuum, and small-angle X-ray scattering techniques. The results show that the interface controls the water uptake (even in bulk membranes) and that the interfacial morphology depends on the interactions of the different polymer moieties with the external environment including its density and dielectric constant. In addition, interactions with solid phases will be discussed which show similar impact on water uptake depending on whether they are hydrophilic or hydrophobic. Understanding the morphological changes and their associated impact on membrane properties is critical for optimizing polymers for use in energy applications. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 4:18PM |
X47.00007: Ions in block copolymers: Effects on thermodynamics, structural changes and electric field induced alignment. Invited Speaker: Thomas Thurn-Albrecht Block copolymers with added ions which selectively dissolve in one block are of interest as nanostructured polymeric ion conductors. In the microphase separated state such a system offers the possibility to simultaneously optimize different properties which would normally exclude each other. One block, being in the solid state, can give mechanical strength while the other block, typically in the liquid state, could be designed to achieve good ion transport. Oriented structures are especially interesting. We present two sets of experiments dealing with fundamental ion induced effects in block copolymers. It is generally observed that the addition of salt to a block copolymer leads to a strong increase of the order-disorder transition temperature and an increased domain spacing, i.e. conformational changes of the polymers. Based on a detailed analysis of small angle scattering data of two different copolymers (PS-b-PEO, PS-b-PVP) close to the order-disorder transition three contributions to the structural changes can be distinguished: an increased incompatibility between the different monomers, the additional volume of the added salt, and chain stretching due to coordination between polymer and salt. At the phase transition, i.e. at constant incompatibility $\chi$N, for low concentrations the increase in domain size is quantitatively explained by the volume of the added salt, at higher concentrations in addition chain stretching sets in. Structural and thermodynamic effects are considerably stronger in PEO than in P2VP. In a second experiment we study the effects of electric field induced interfacial polarization caused by the added salt. Impedance spectroscopy combined with orientation experiments at high fields enable a quantitative analysis of ionic polarization and a direct demonstration of its aligning effect on the interfaces. Field induced orientation effects are much stronger if the ionic charges come into play in comparison to much weaker dielectric effects. We present a physical model accounting for the differences. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X47.00008: Does filler surface chemistry impact filler dispersion, polymer dynamics and conductivity in nanofilled solid polymer electrolytes? Lalitha Ganapatibhotla, Janna Maranas We study the impact of nanofiller surface chemistry on filler dispersion, polymer dynamics and ionic conductivity in acidic $\alpha $-Al$_{2}$O$_{3}$ filled PEO+LiClO$_{4}$ solid polymer electrolytes (SPEs).SPEs are the key to light-weight and high energy density rechargeable Li ion batteries but suffer from low room temperature ionic conductivity. Addition of ceramic nanofillers improves conductivity of SPEs and their surface chemistry influences extent of conductivity enhancement. The ionic conductivity of acidic $\alpha $-Al$_{2}$O$_{3}$ filled SPE is enhanced for salt concentrations at and below eutectic, while neutral $\gamma $-Al$_{2}$O$_{3}$ filler enhances conductivity only at eutectic composition. Li ion motion is coupled to segmental mobility of polymer and we study how this is affected by addition of $\alpha $-Al$_{2}$O$_{3}$ using quasi-elastic neutron scattering. Aggregation extent of nanoparticles in SPE matrix, a less explored factor in filled SPEs, can affect segmental mobility of polymer. This can vary with surface chemistry of particles and we quantify this using small angle neutron scattering. All measurements are performed as a function of Li concentration, nanoparticle loading and temperature. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X47.00009: Regular Arrays of Germanium Nanoparticles Assisted by Thermoset Polymer Composites for High Capacity Lithium Ion Battery Gyuha Jo, Moon Jeong Park In recent years Li-batteries have attracted significant interests for a variety of applications such as portable electronics and electric vehicle (EV) batteries due to their high energy densities. Key challenges in advancing the technology lie in specific energy density, the long term cycle properties, and durability at elevated temperature. In present study, we were motivated to prepare high capacity Li-battery by creating regular arrays of germanium nanoparticles (GeNPs, 1600 mAh/g) to replace commercial graphite anode (370 mAh/g). Thermoset polymers were employed to prepare GeNPs/polymer composites with tunable NP loadings and spacings, followed by carbonization process to prepare GeNPs/carbon composite anode material. Due to the large volume change of GeNPs with charge/discharge cycles, the regular arrays of GeNPs are turned out to be a crucial parameter in obtaining enhanced cyclability. The GeNPs/carbon anode materials were cycle tested in a half cell configuration using Lithium foil as a counter electrode and lithium salt doped PS-PEO block copolymers as electrolytes. High capacity and rate capability were achieved, which demonstrate the role of nano-sized and regularly-arrayed anode active materials in obtaining the improved battery performance. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X47.00010: Disorder -- Order Transitions in Humidified Block Copolymer Electrolytes Studied by \textit{In Situ }SAXS Keith Beers, Andrew Jackson, Nitash Balsara The relationship between water uptake, proton conductivity and morphology in the dry and hydrated state for a series of poly(sulfonated styrene-\textit{block}-ethylene) was investigated. Specifically, the disorder-to-order transition (DOT) and hydrated morphology was characterized by \textit{in situ} humidity controlled small angle X-ray scattering (SAXS). The enhanced resolution afforded by SAXS allows for better characterization of the DOT than previous studies which have relied upon neutron scattering. The transition to an ordered state is found to display a coexistence of ordered and disordered states over a broad range of relative humidity values. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X47.00011: Effects of lithium salts on the lamellar phase of diblock copolymers Issei Nakamura, Zhen-Gang Wang We study the effects of lithium salts on the lamellar phase of AB diblock copolymers by means of the self-consistent field theory. We consider a model in which the A and B blocks have different dielectric constants and account for the tight binding of Li$^+$ to one of the blocks, the preferential solvation energy of anions in the higher-dielectric polymer domain, the translational entropy of anions, and change in the $\chi$ parameter due to the binding of Li$^+$. We study the effect of the strong polymer-Li$^+$ binding on the distribution of the salt ions. In particular, we show that local charge separation near the interface of the higher- and lower- dielectric polymers largely arises from the effect of the Born energy. We also examine the relationship between two definitions of the effective $\chi$ parameter, one based on the lamellar spacing and one based on the structure factor in the disordered phase, and show that these two definitions generally do not coincide. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X47.00012: Structure of Block Copolymer Hydrogel Formed by Complex Coacervate Process Soohyung Choi, Julia Ortony, Daniel Krogstad, Jason Spruell, Nathaniel Lynd, Songi Han, Edward Kramer Complex coacervation occurs when oppositely charged polyelectrolytes associate in solution, forming dense micron-sized droplets. Hydrogels with coacervate block domains were formed by mixing two ABA and A'BA' triblock copolymer solutions in water where the A and A' blocks are oppositely charged. Small-angle neutron scattering (SANS) was used to investigate the structure of hydrogels formed by ABA triblock copolymers (A block: poly(allyl glycidyl ether) functionalized with guanidinium (A) or sulfonate (A'), B block: poly(ethylene oxide)). By using an appropriate fitting model, structural information such as coacervate core block radius and water volume fraction w can be extracted from SANS data. The results reveal that w in the coacervate core block was significantly higher than in conventional triblock copolymer hydrogels where microphase separation is driven by the hydrophobicity of the core-forming blocks. [Preview Abstract] |
Session X48: Focus Session: Statistical Physics of Active Systems Away From Detailed Balance: Cytoskeleton, Flagella, and All That
Sponsoring Units: DPOLY DBIOChair: Michael Hagan, Brandeis University
Room: 161
Thursday, March 1, 2012 2:30PM - 2:42PM |
X48.00001: The Role of Dynein in Microtubule Mechanics Tony Ladd, Gaurav Misra, Jun Wu, Robert Russell, Tanmay Lele, Richard Dickinson Experiments in Lele's group have shown that microtubules severed by laser ablation do not straighten, as would be expected from the large bending moments along their lengths. Instead, segments near newly created minus ends typically increased in curvature following severing, while segments near new microtubule plus ends depolymerize before any observable change in shape. However, in dynein-inhibited cells, segments near the cut straightened rapidly following severing. These observations suggest that microtubules are subject to significant tangential forces, and that lateral motion of the microtubule is primarily opposed by frictional rather than elastic forces. To interpret the experimental results, we have developed a numerical model for intracellular microtubule mechanics, accounting for dynein-generated forces on the microtubules. We have supplemented the Kirchoff model for an elastic filament with the stochastic growth and collapse of microtubules, and by a model for dynein generated forces. I will present simulations of the dynamics of individual microtubules that show how motor forces result in the localization of short-wavelength buckles near the cell periphery. Our results suggest that microtubule shapes in vivo reflect a dynamic force balance, where bending moments are opposed by dynein-motor forces that include a large effective friction from the stochastic binding and unbinding of the motors. Simulations of the motion of the centrosome are consistent with a mechanism for centrosome centering driven by pulling forces exerted by dynein motors. I will explain how tension on the centrosome can be reconciled with buckled filaments near the cell periphery. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X48.00002: Processivity and collectivity of molecular motors pulling membrane tubes Francisco Fontenele Araujo, Cornelis Storm In every cell, directed transport involves proteins that convert chemical energy into mechanical work. Molecular motors responsible for this vital task are mostly too weak to carry biological cargo by themselves and some cannot even take more than a single step before unbinding from their cytoskeletal track. By acting collectively, however, they can muster the required forces. In this talk, we discuss interactions among motors and their collective effects on the extraction of membrane nanotubes. Via a force balance coupled to binding kinetics, we sketch the phase diagram of tube formation. Three regimes are identified: (1) tip clustering, in the sense that the driving force is concentrated at the tip of the tube, (2) viscous extraction, in which motors axially drag membrane, and (3) hybrid extraction, such that tip clustering and axial drag are equally important. Comparison with experiments indicates that synthetic membranes mostly fall into regime (1), while biological membranes tend to fall into regime (2). Our model suggests a unifying picture of tube extraction by both processive and nonprocessive motors. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X48.00003: Carbon nanotubes as mechanical probes of equilibrium and non-equilibrium cytoskeletal networks Nikta Fakhri, Matteo Pasquali, Frederick C. MacKintosh, Christoph F. Schmidt Networks of filamentous proteins underlie the mechanics of cells. The activity of motor proteins typically creates strong fluctuations that drive the system out of equilibrium. Understanding the behavior of such networks requires probes that ideally span the characteristic length-scales, from nanometers to micrometers. Single-walled carbon nanotubes (SWNTs) are nanometer-diameter filaments with micrometer length and tunable bending stiffness. On a Brownian energy scale they have persistence lengths of about 20-100 micrometers and show significant thermal fluctuations on the cellular scale of a few microns. Diffusive motion and local bending dynamics of SWNTs embedded in an active polymeric network reflect forces and fluctuations of the embedding medium. We study the motion of individual SWNTs in equilibrium and non-equilibrium networks by near infrared fluorescence microscopy. We show that SWNTs reptate in the network. We will discuss the possibility of using SWNTs as multi-scale probes relating their local dynamic behavior to the viscoelastic properties of the surrounding network. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X48.00004: ABSTRACT HAS BEEN MOVED TO C1.00231 |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X48.00005: The metaphase and anaphase dynamics is dominated by the physical and mechanical properties of both microtubules and chromatin Luca Grisa, Maria Kilfoil One of the most interesting problems in biophysics involves the physical separation of chromosomes and the mechanical properties of both microtubules (MT's) and chromatin. This process involves the polymers MT's and chromatin, each of which has unique physical properties that have been determined extensively in vitro. Of further interest for physicists is the out-of-equilibrium nature of this process involving several force generators from motor proteins and MT depolymerization. We follow the dynamics of spindle pole bodies and centromeres of yeast cells during mitosis in three-dimensions at high spatial resolution. Using this novel approach, we are able to observe spindle oscillations during metaphase, and the three-dimensional dynamics of spindle elongation and chromosome separation during anaphase. With these data, we can separate the dynamics caused by MT depolymerization from those caused by the motors. This allows us to determine the depolymerization rate of the kinetochore MT's in vivo. Furthermore, we determine the temporal profile of the chromatin extension during anaphase we combine with the known force-extension curve of chromatin in vitro, to infer the expected force-velocity curve of the collective motors in vivo, which has never been measured in vivo or in vitro. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X48.00006: Effective temperature and spontaneous collective motion of active matter Shenshen Wang, Peter Wolynes Spontaneous directed motion, a hallmark of cell biology, is unusual in classical statistical physics. Here we study, using both numerical and analytical methods, organized motion in models of the cytoskeleton in which constituents are driven by energy-consuming motors. Although systems driven by small-step motors are described by an effective temperature and are thus quiescent, at higher order in step size, both homogeneous and inhomogeneous, flowing and oscillating behavior emerges. Motors that respond with a negative susceptibility to imposed forces lead to an apparent negative temperature system in which beautiful structures form resembling the asters seen in cell division. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X48.00007: An active matter analysis of intracellular Active Transport Bo Wang, Kejia Chen, Sung Chul Bae, Steve Granick Tens of thousands of fluorescence-based trajectories at nm resolution have been analyzed, regarding active transport along microtubules in living cells. The following picture emerges. Directed motion to pre-determined locations is certainly an attractive idea, but cannot be pre-programmed as to do so would sacrifice adaptability. The polarity of microtubules is inadequate to identify these directions in cells, and no other mechanism is currently known. We conclude that molecular motors carry cargo through disordered intracellular microtubule networks in a statistical way, with loud cellular ``noise'' both in directionality and speed. Programmed random walks describe how local 1D active transport traverses crowded cellular space efficiently, rapidly, minimizing the energy waste that would result from redundant activity. The mechanism of statistical regulation is not yet understood, however. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X48.00008: ABSTRACT WITHDRAWN |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X48.00009: Constant torque in flagellar bacterial motors optimizes space exploration Carlos A. Condat, Mario E. Di Salvo Experiments indicate that the torque provided by the bacterial rotary motor is approximately constant over a large range of angular speeds. Constant torque implies that the power spent in active motion is proportional to the instantaneous bacterial speed, if the relation between angular speed and swimming speed is linear. Here we show that a constant torque maximizes the volume of the region explored by a bacterium in a resource-depleted medium. Given that nutrients in the ocean are often concentrated in separate, ephemeral patches, we propose that the observed constancy of the torque may be a trait evolved to maximize bacterial survival in the ocean. We also discuss the dependence of the explored volume with the particular features of the bacterial propulsion mechanism. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:54PM |
X48.00010: Interplay between motor contractility and mechanical stability of active biopolymer networks Invited Speaker: Chase Broedersz The mechanical properties of cells are regulated in part by internal stresses generated actively by molecular motors in the cytoskeletal filamentous actin network. On a larger scale, collective motor activity allows the cell to contract the surrounding extracellular matrix, consisting also of biopolymer networks. Experiments show that such active contractility dramatically affects the networks' elasticity, both in reconstituted intracellular F-actin networks with myosin motors as well as in extracellular gels with contractile cells. We provide insight into this remarkable behavior with a model for the mechanics of contractile disordered networks consisting of simple straight fibers with linear bending and stretching elasticity. We find that these networks exhibit a low-connectivity rigidity threshold governed by fiber-bending elasticity and a high-connectivity threshold that controls a crossover between bending and stretching dominated network elasticity. Owing to their low connectivity, typical biopolymer networks fall below this upper threshold and their mechanical stability thus relies on the fibers' bending rigidity. The macroscopic elasticity of such networks is governed by soft fiber bending deformations. However, we find that motor-generated contractile forces can ``pull out'' these soft bending modes, thereby inducing a crossover to a mechanically more stable regime governed by stiff fiber stretching modes. Using scaling arguments and mean field theory, we show that this transition---induced by motor contractility---can be understood from the stress-dependence of the mechanical stability thresholds. These results suggest a physical principle by which active contractility can control biopolymer network mechanics, even when the fiber constituents are linear elastic elements. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X48.00011: Propulsion of microorganisms by a helical flagellum Chih-Hung Chen, Bruce Rodenborn, Harry Swinney, Bin Liu, Hepeng Zhang Many bacteria (e.g. \textit{E. coli} and \textit{Salmonella}) swim by rotating rigid helical flagella, which are typically several $\mu$m long and 0.4 $\mu$m in diameter. We investigate this propulsion in laboratory measurements on macroscopic rotating helices (typical diameter, 12 mm) in a fluid with viscosity $10^{5}$ times that of water; thus the Reynolds number in the experiments is much less than unity, just as for bacteria. We measure the propulsive force and torque generated by a rotating flagellum, and the drag force on a translating flagellum; thus we can determine all elements of the propulsion matrices along the axial direction. We also compute force, torque and drag using the regularized Stokeslets method of Cortez et al. (2005). Our experimental and numerical results are in excellent agreement. However, these results differ significantly from the predictions of resistive force theories developed by Gray and Hancock (1953) and Lighthill (1975). The difference between our measurements and resistive force theory is especially large for helices with small pitch/diameter ratios, which is the regime of many bacteria. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X48.00012: Energy Consumption of Actively Beating Flagella Daniel Chen, Daniela Nicastro, Zvonimir Dogic Motile cilia and flagella are important for propelling cells or driving fluid over tissues. The microtubule-based core in these organelles, the axoneme, has a nearly universal ``9+2'' arrangement of 9 outer doublet microtubules assembled around two singlet microtubules in the center. Thousands of molecular motor proteins are attached to the doublets and walk on neighboring outer doublets. The motors convert the chemical energy of ATP hydrolysis into sliding motion between adjacent doublet microtubules, resulting in precisely regulated oscillatory beating. Using demembranated sea urchin sperm flagella as an experimental platform, we simultaneously monitor the axoneme's consumption of ATP and its beating dynamics while key parameters, such as solution viscosity and ATP concentration, are varied. Insights into motor cooperativity during beating and energetic consequences of hydrodynamic interactions will be presented. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X48.00013: A particle based computational model for eukaryotic flagella Raghunath Chelakkot, Michael Hagan The structure of the eukaryotic flagella is very complex and the exact mechanisms responsible for flagellar beating are not clearly understood. Here we present a minimal model to study flagellar beating in two dimensions, which demonstrates that regular beating with a well defined characteristic frequency can arise spontaneously in the absence of external control. In this model, the flagella is represented by two stiff filaments clamped on a surface, on which model ``molecular motors'' take directed steps on one of the filaments and thereby apply a local force. The fluid medium is simulated using Multiparticle Collision dynamics (MPC), which is a particle based method for hydrodynamic simulations. Within a certain range of motor concentrations, large amplitude periodic oscillations with a well defined frequency are observed; other qualitatively different beating patterns arise outside of this range. We present a phase diagram that characterizes the beating behaviour as a function of relevant parameters such as filament length, motor density on the filament and motor velocity. [Preview Abstract] |
Thursday, March 1, 2012 5:30PM - 5:42PM |
X48.00014: Force generation in a regrowing eukaryotic flagellum Marco Polin, Bastien Bruneau, Thomas Johnson, Raymond Goldstein Flagella are whip-like organelles with a complex internal structure, the axoneme, highly conserved across eukaryotic species. The highly regulated activity of motor proteins arranged along the axoneme moves the flagellum in the surrounding fluid, generating forces that can be used for swimming or fluid propulsion. Although our understanding of the general mechanism behind flagellar motion is well established, the details of its implementation in a real axoneme is still poorly understood. Here we explore the inner working of the eukaryotic flagellum using a uniflagellated mutant of the unicellular green alga {\it Chlamydomonas reinhardtii} to investigate in detail the force and power generated by a moving flagellum during axonemal regrowth after deflagellation. These experiments will contribute to our understanding of the inner working of the eukaryotic flagellum. [Preview Abstract] |
Session X49: Focus Session: Organic Electronics and Photonics - Polymer Dielectrics and Charge Transport
Sponsoring Units: DMP DPOLYChair: Peter Green, University of Michigan
Room: 162A
Thursday, March 1, 2012 2:30PM - 2:42PM |
X49.00001: Inhomogeneous deformation and instability in soft dielectric transducers Tiefeng Li, Shaoxing Qu, Christoph Keplinger, Zhigang Suo, Wei Yang Dielectric elastomer (DE) is assembled by sandwich an elastomeric membrane with compliant electrodes on both sides. They are capable of converting mechanical into electrical energy (generator)or electrical into mechanical energy (actuator). The large actuation strain of DE has inspired intense development of dielectric elastomers as applications as actuators and generators. DE transducers are lightweight, compliant, rust-free, and can convert higher energy than those of conventional transducers. DE transducers often undergo inhomogeneous deformation and instability during operation. Inhomogeneous deformation can cause the DE membranes in have inhomogeneous fields distribution and fail locally. Instability during actuation highly affects the performance and safety of the DE transducer. We present an analyitial model of a dielectric elastomer transducers undergoing inhomogeneous deformation and snap-through instability during operation. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X49.00002: Dielectric Properties of Carbon, Silicon and Germanium Based Polymers: A First Principles Study Chenchen Wang, Ghanshyam Pilania, Chunsheng Liu, Rampi Ramprasad The field of high energy density capacitors would benefit from the discovery of new insulating polymers with high dielectric constant, low loss, large band gap and high breakdown strength. The current standards for capacitive energy storage applications are polypropylene and polyethylene which have large band gap and high breakdown strength, but a small dielectric constant. As an initial step aimed at the discovery of new polymers with better dielectric properties, we consider a class of chemically-modified polymers based on polyethylene. These polymers are composed of --XY$_{2}$-- building blocks, with X = C, Si or Ge, and Y = H, F or Cl. We use density functional perturbation theory and exchange-correlation functionals that include van der Waals and/or nonlocal exchange interactions to accurately predict the structure, dielectric constant (electronic and ionic) and band gap of this class of polymers. The computed properties have been correlated to the underlying electronic structure and phonon modes, and tradeoffs between the band gap and dielectric constant are established. The time-consuming dielectric computations have been optimized using a new ``single-chain'' method to allow for future extensive explorations of the polymer chemical space via automated high-throughput computations. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X49.00003: Paraelectric Crystalline Polymers for High Energy Density and Low Loss Dielectrics Lei Zhu, Run Su, Jung-Kai Tseng, Mao-Sheng Lu It is known that the high temperature phase above the Curie temperature (Tc) in poly(vinylidene fluoride-\textit{co}-trifluoroethylene) [P(VDF-TrFE)] is a typical paraelectric phase. Frequency-dependent electric displacement-electric field (D-E) loop tests are used to study the dielectric/ferroelectric properties of this paraelectric phase. It is observed that normal ferroelectric hysteresis loops are observed at a poling frequency of 1 Hz, while narrow paraelectric loops are observed at a high poling frequency of 1000 Hz. The poling mechanism is revealed by an in-situ electric field-dependent Fourier transform infrared study. From this study, we consider that paraelectric crystalline polymers are good candidates for high energy density and low loss dielectrics. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X49.00004: Influence of Aqueous Electrolytes on Electrical Insulating Properties of Polyethylene Adam Tornheim, Tom Devine Polyethylene is commonly used as electrical insulation in high voltage (3-35 kV), underground electrical distribution cables. During service conditions the insulation ``ages'' and may fail. One method of ageing is a consequence of long-time exposure of the polyethylene to humidity and groundwater. Chemical analyses by other researchers indicated iron was frequently detected in degraded areas of aged cable. In the current research we examine the effect of a ferric chloride electrolyte on the electrical insulating character of polyethylene. In earlier research we discovered that in the presence of high DC voltages (approximately 3kV-6kV) ferric chloride electrolytes markedly enhance electron injection into and subsequent electron transport through polyethylene. The present research shows that ferric chloride complexes in solution are likely responsible for electron injection. The effect of exposure to ferric chloride solution was permanent, causing an increase in current density when the polyethylene was subsequently exposed to other electrolytes. The effect of FeCl$_{3}$ exposure was observed in additive free polyethylene as well as commercially processed polyethylene. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X49.00005: Theoretically guided design of efficient polymer dielectrics Philip Taylor, Elshad Allahyarov We have used theory and molecular dynamics simulation as an aid to the development of polymeric materials with favorable properties for energy storage with low dielectric losses. We build on the principle that the stored energy in a capacitor is a sum of the intrinsic energy of the electric field and the energy of distortion of the molecular bonds within the dielectric. We have attempted to maximize the energy of bond distortion by increasing the polarization without introducing large losses. In this initial study we simulate the behavior of a system consisting of parallel chains of the antiferroelectric $\alpha$ phase of polyvinylidene fluoride that has been modified to increase the separation between chains through crosslinks that prevent chain rotation. By varying the crosslink density, we identify the optimum length of unlinked chain such that the polar entities that rotate when subject to a strong electric field will be neither so long that they rotate collectively with little stored energy, nor so short that the large distortion of bond angles necessary for dipole rotation reduces the polarizability. We have adopted a combined-atom model in order to make feasible the study of systems comprising up to 100 chains, each consisting of up to 500 monomers. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X49.00006: Interface conductivity contribution in Co-Phthalocyanines capacitive type devices Carlos Monton, Ilya Valmiansky, Ivan Schuller Metal Phthalocyanines are flat organic semiconductors which exhibit interesting magnetotransport properties. Recent experimental studies in a particular system together with theoretical calculations have shown that the temperature and thickness dependence of the ohmic conductivity can be universally described by two independent contributions: the organic film and the electrode-film interface [1]. In order to explore the implications of this model we performed transport measurements in sandwich devices with different bottom electrodes materials. These devices are grown in-situ by Organic Molecular Beam Epitaxy to assure ultra clean electrode-film interfaces. A combination of structural and transport studies are used to investigate the reason for the drastic change in ohmic conductance at metallo-organic film thickness around 100nm. \\[4pt] [1] C. N. Colesniuc, R. R. Biswas, S. A. Hevia, A. V. Balatsky, and I. K. Schuller, Phys. Rev. B 83, 085414 (2011). [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X49.00007: Nanoscale Correlation of Heterogeneous Morphology and Electrical Transport in Nanostructured Organic and Hybrid Solar Cells Jiebing Sun, Sean Wagner, Xiaoyu Liu, Phillip Duxbury, Pengpeng Zhang The knowledge of correlation between morphology and electrical properties is essential both to understand fundamental physics and to facilitate device optimization. We report a systematic study of the organic blend of PCBM particles and P3HT fibers formed via the thermal annealing method. We use conductive atomic force microscopy (c-AFM) to simultaneously map the surface morphology and collect the electrical current. Cross-section AFM is attempted to get the internal 3D morphology so as to establish its correlation with the electrical properties. The obtained heterogeneity in the current map with a resolution up to 20 nm is attributed to the formation of cross-linked three dimensional fiber network, which is further supported by a three dimensional device model incorporating the geometry of nanowire and the c-AFM tip. Results on the hybrid cell of ZnO nanowires infiltrated with P3HT will also be discussed. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X49.00008: Gigahertz Probing of Poly(3-hexylthiophene) With A Kilohertz Detection Scheme Jeff Worne, Corey Slavonic, Kevin Kelly, Douglas Natelson Organic semiconducting polymers have been well studied at DC and low-frequencies, giving important information about charge transport and metal-polymer interaction. However, comparatively little is known about the operation of these polymers at radio frequencies (~1 GHz). RF frequencies can be a useful tool to investigate high-frequency mobility and charge carrier dynamics relevant for the possibility of using these polymers in RF applications. We present a novel technique that probes poly(3-hexylthiophene) (P3HT) response in the RF regime but allows for detection in the kHz regime. We show transport data of P3HT and discuss a theoretical framework for inferring behavior at GHz frequencies. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X49.00009: Thickness dependence of hole mobility in regioregular poly(3-hexylthiophene) thin films Bingyuan Huang, Emmanouil Glynos, Bradley Frieberg, Peter Green Methods of time of flight (TOF) and charge extraction by linearly increasing voltage (CELIV) are used to investigate the hole mobility in regioregular poly(3-hexylthiophene) (RR-P3HT) thin films. The film thickness was varied by changing the RR-P3HT solution concentration and spin-coating speed. The hole mobility is found to monotonically increase from 10$^{-4}$ cm$^{2}$/Vs to 10$^{-3}$ cm$^{2}$/Vs along with film thickness from 100 nm to 700 nm and saturate at 10$^{-3 }$cm$^{2}$/Vs beyond 700 nm. X-ray diffraction and ellipsometry data showed that the film morphology changes with the thickness. The structural change supports the weak dependence of energetic and positional disorder on thickness analyzed by the Gaussian Disorder Model (GDM). [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X49.00010: Contribution of Increased Extraction Efficiency to Increased Photo-Luminescence in Strained Polymer Films Po-Jui Chen, Arnold Chang-Mou Yang, Jui-Hung Hsu, Jonathon D. White Potential applications of Luminescent Conjugated Polymers in thin film diodes, solar cells and flat panel displays have been limited by low efficiency. Craze formation in MEH-PPV/polystyrene thin film leads to a factor of 2 or 3 increase in collected photo-luminescence (PL) due to a combination of factors such as MEH-PPV chain conformation and increased extraction efficiency of PL. In order to determine the contribution of the latter effect, we used Monte Carlo based Ray Tracing to analyze the trajectory of photons generated in the thin film under different strain conditions. Our results indicate that increased PL extraction due to the existence of crazes contributes $\sim$15\% of the observed increase in PL, the majority of this being due to light emitted near the craze edges. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X49.00011: Modeling charge and energy transports in $\pi $-conjugated systems Yongwoo Shin, Xi Lin A generic 3D model Hamiltonian is developed to simulate the charge and energy transports in $\pi $-conjugated composite materials. The intrachain interactions are described by our recently developed adapted Su-Schrieffer-Heeger Hamiltonian and the interchain $\pi \pi $ interactions are modeled using distance-dependent hopping integrals. Excellent agreements in their binding energetics and geometries with post-Hartree-Fock ab initio methods and experiments are found in the cases of a benzene dimer, graphene bi-layer, and poly-($p$-phenylene vinylene) (PPV) crystal. The computed photoinduced charge separated states and associated adsorption spectra of the PPV-C$_{60}$ and PPV-C$_{60}$-graphene agree perfectly with experimental measurements. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X49.00012: NMR, magnetic susceptibility, and electrical conductivity investigation of doped poly-3-methylthiophene W.G. Clark, G. Gaidos, K.J. Singh, R. Menon, K.P. Ramesh, A.P. Reyes, P. Kuhns, J.D. Thompson We report $^{1}$H and $^{19}$F NMR spin-lattice relaxation rate (1/$T_{1})$ measurements over a wide range of temperature (3 K$ < T < $300 K) and magnetic field (0.9 T$ < B \quad < $23.4 T for $^{1}$H and 9.0 T for $^{19}$F) in the organic conductor poly-3-methylthiophene (P3MT) doped with hexafluorophosphate (PF$_{6})$. Also included are measurements of the electrical conductivity ($\sigma )$ at $B$ = 0 and 77 K $< \quad T \quad <$ 300 K and the magnetic susceptibility ($\chi )$ at $B$ = 0.1 T and 2 K $< \quad T \quad <$ 350 K. The doping level has been varied to tune the conductivity value at 300 K in the fully doped sample to $\sigma \quad \sim $ 120 S/cm and in the dedoped one to $\sigma \quad \sim $ 5 S/cm. This range enables investigation of the roles of carrier density and electron-electron interactions in the mechanisms for 1/$T_{1}$. A correlation between $\chi $ and the relaxation mechanisms is observed in these samples. The results are analyzed using the modified Korringa relation. Also, the proton and fluorine spin relaxation data give insight into the role of both inter-chain and intra-chain conduction mechanisms. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X49.00013: Improving Order and Mobility in MEH-PPV Films by Reducing Polydispersity Jonathon White The effect of polydispersity on morphology and charge transport in drop cast films of poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) was investigated using grazing incidence X-ray diffraction and time-of-flight respectively. Morphologically, reducing polydispersity by removing short chain segments promoted the capability of crystallization. This resulted in higher hole mobility and non-dispersive transport down to lower temperatures for the lower polydispersity sample. The slope for the Poole-Frenkel relationship at 298 K was increased, and its change with temperature decreased, indicating reduced spatial inhomogeneity. Analysis using Bassler's Gaussian disorder model (GDM), found that the value for energy disorder ($\sigma \sim $53meV for both films) and infinite temperature zero field mobility ($\mu $o$\sim $3 x10$^{-6}$ cm$^{2}$/Vs) were similar for both films. However, a good fit for hopping site separation and spatial disorder was only possible for the lower polydispersity device, suggesting that the lower polydispersity films have less mesoscopic inhomogeneity. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X49.00014: Charge Transport in Amorphous Polythiophene-Fullerene Blends Kiarash Vakhshouri, Derek Kozub, Chenchen Wang, Alberto Salleo, Enrique Gomez Energy-filtered transmission electron microscopy studies revealed that amorphous mixed phases are ubiquitous within mesostructured polythiophene/fullerene mixtures. Nevertheless, the role of mixing within nanophases on charge transport of organic semiconductor mixtures is not fully understood. We have examined the electron mobility in amorphous blends of poly(3-hexylthiophene) and phenyl-C$_{61}$-butyric acid methyl ester. Our studies reveal that the miscibility of the components strongly affects electron transport within amorphous blends. Immiscibility promotes efficient electron transport by promoting percolating pathways within organic semiconductor mixtures. As a consequence, partial miscibility may be important for efficient charge transport in polythiophene/fullerene mixtures and organic solar cell performance. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X49.00015: Modeling energy transport in $\pi $-conjugated dendrimers containing triple bonds Pak Kau Lim, Yongwoo Shin, Xi Lin An accurate, transferrable, and computational efficient adapted Su-Schrieffer-Heeger model Hamiltonian is developed to describe triple bonds in linear and fractal-dimensional $\pi $-conjugated systems. Chemical accuracy in the computed optical gaps is found for the cases of poly-(thiophene-ethynylene) and ploy-phenylacetylene of arbitrary lengths, with all errors less than 3{\%} as compared to existing UV-visible adsorption spectra. The computed exciton migration processes in the phenylacetylene dendrimers indicates that such conjugated Bethe tree structures are efficient energy transduction funnels. [Preview Abstract] |
Session X50: Focus Session: Micro and Nano Fluidics II: Structured or Active Surfaces and Electrotransport
Sponsoring Units: DPOLY DFDChair: Anna Balazs, University of Pittsburgh
Room: 162B
Thursday, March 1, 2012 2:30PM - 2:42PM |
X50.00001: Dynamics of self-oscillating cilia designed from active polymer gels Pratyush Dayal, Amitabh Bhattacharya, Olga Kuksenok, Anna C. Balazs Using theory and simulations, we design active synthetic surfaces which are capable of replicating functionalities of biological cilia. In order to design such exquisite biomimetic systems we harness unique properties of polymer gels that undergo photosensitive Belousov-Zhabotinsky (BZ) reaction. Powered by internalized BZ reaction these polymer gels swell and de-swell autonomously by chemo-mechanical transduction and therefore are ideal materials for designing our system. In order to simulate the dynamics of the BZ cilia in surrounding fluid we have developed a nonlinear hybrid 3D model which captures elasto-dynamics of polymer gel and diffusive exchange of BZ reagents between the gel and the fluid. Here we show that the geometrical arrangement of cilia and the distribution of BZ activator in the fluid determine the dynamic response of the cilia. We further show that using light as an external stimulus we can sequentially modulate height of individual cilium and thereby create the ``piano effect''. Finally, we demonstrate that synchronized oscillations in the cilia result from the distribution of BZ-activator in the surrounding fluid. Our findings can be used to design active surfaces which can be remotely tuned depending upon the magnitude of external stimuli. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X50.00002: Using actuated synthetic cilia to enhance microscale heat transport Zachary G. Mills, Alexander Alexeev We used three dimensional computer simulations to examine heat transport in a microchannel that encompasses a periodic array of actuated synthetic cilia. The channel was filled with a viscous fluid and its walls were maintained at different temperatures. Elastic synthetic cilia were attached to the bottom channel wall and were actuated by a periodic external force applied horizontally to their free ends. To model this multi-component system, we employed a thermal lattice Boltzmann model coupled with the lattice spring model. We probed how the beating cilia affect the heat transfer between channel walls, and how the thermal transport coefficient changes depending on the oscillating frequency and the relative distance between actuated filaments. Our findings could be useful for developing new methods for temperature control in microscale devices. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X50.00003: Transport of Micro-particles by Active Cilia Arrays Amitabh Bhattacharya, Gavin Buxton, O. Berk Usta, Anna. C. Balazs Biological organisms are known to use hair-like filaments called cilia to manipulate and transport particles. The coordinated motion of cilia is known to be effective at propelling surrounding fluid. In this work, we show that adhesive interaction between the actuated cilia and particulates can be crucial towards controlling particle transport. We model transport of a microscopic particle via a regular array of beating elastic cilium, whose tips experience an adhesive interaction with the particle's surface. At optimal adhesion strength, the average particle velocity is maximized. Using simulations spanning a range of cilia stiffness, particle radius, and cilia-particle adhesion strength, we explore the parameter space over which the particle can be ``released,'' ``propelled'' or ``trapped'' by the cilia. We use a low-order model to predict parameters for which the cilia are able to attach themselves to the particle. We also study the effect of varying the particle size and stiffness on its transport properties. This is the first study that shows how both stiffness and adhesion strength are crucial for manipulation of particles by active cilia arrays. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X50.00004: Fluid flows around nanoelectromechanical resonators O. Svitelskiy, V. Sauer, N. Liu, D. Vick, K.M. Cheng, M.R. Freeman, W.K. Hiebert To explore properties of fluids on a nanosize scale, we fabricated by a standard top down technique a series of nanoelectromechanical resonators (cantilevers and bridges) with widths w and thicknesses t from 100 to 500 nm; lengths l from 0.5 to 12 micron; and resonant frequencies f from 10 to 400 MHz. For the sake of purity of the experiment, the undercut in the widest (w=500 nm) devices was eliminated using the focused ion beam. To model the fluidic environment the devices were placed in the atmosphere of compressed gases (He, N$_{2}$, CO$_{2}$, Ar, H$_{2}$) at pressures from vacuum up to 20 MPa, and in liquid CO$_{2}$; their properties were studied by the real time stroboscopic optical interferometry. Thus, we fully explored the Newtonian and non-Newtonian flow damping models. Observing free molecular flow extending above atmospheric pressure, we find the fluid relaxation time model to be the best approximation throughout, but not beyond, the non-Newtonian regime, and both, vibrating spheres model and the model based on Knudsen number, to be valid in the viscous limit. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X50.00005: Dynamic Similarity Principle for Nanoscale Resonant Devices in Gaseous Environments Caryn Bullard, Michael Roukes, John Sader, Jianchang Li, Paul Mulvaney The mechanical performance of cantilevers on the nanoscale operating in atmosphere is dominated by gas damping. However, theoretical modeling of gas-solid interactions on the nanoscale is non-trivial due to the non-continuum nature of the gas flow. In addition, these gas-structure interactions can significantly affect the sensitivity of these devices. Instead of using numerical simulations to determine the gas flow and consequently, gas damping, of a nanoscale device, we used a general dynamic similarity principle to determine the gas damping of a nanoscale device by measuring the gas damping of a scaled up prototype device. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X50.00006: Computational modeling of traveling wave electrophoresis Robert Correll, Jarrod Schiffbauer, Lloyd Carroll Traveling wave electrophoresis (TWE) is a microfluidic separation technique in which electrodes flanking a microchannel apply a traveling potential wave along the channel. Charged particles, including small molecules, proteins, and nanoparticles are differentially transported along the channel at a rate dependent on their mobility. TWE is ideally suited for application in lab-on-a-chip and field deployed sensor systems. In order to fully exploit this technology, a series of computational models have been developed, including 1-dimensional and 2-dimensional models. These models allow for testable predictions of single-particle motion, and the effects of factors such as Ph and concentration upon separation efficiency. Efforts to include diffusive components within the model, and to consider the motion of bands, rather than single particles will be discussed. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X50.00007: Activated wetting dynamics in the presence of mesoscopic surface disorder Kristina Davitt, Michael Pettersen, Etienne Rolley Although disorder is commonly used to explain contact angle hysteresis, it is often neglected when considering wetting dynamics. When viscous forces are negligible, contact-line velocity is modelled by the Molecular Kinetic Theory [1], which predicts an activated motion driven by molecular jumps on preferential adsorption sites. We believe that in the presence of mesoscopic disorder, this model can be reinterpreted and that the activation length is no longer molecular-sized but is related to depinning events on the surface. This hypothesis is supported by a study of the wetting of cesium by liquid hydrogen in which it was shown that the activation length is of the order of the expected roughness [2]. However, no systematic study between the activation area and the length scale of the disorder has previously been made. We study wetting dynamics on metal films evaporated under different conditions, allowing us to obtain films with lateral grain sizes ranging from 10 to 200 nm. We find that the activation area deduced from wetting experiments is coherent with these sizes; however, its precise relation to the scale of disorder is not clear.\newline [1] T.D. Blake and J.M. Haynes, J. Colloid Interface Sci. 30, 421 (1969)\newline [2] E. Rolley and C. Guthmann, PRL 98, 166105 (2007) [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X50.00008: Experimental test of Schrodinger's first-passage-time theory using colloids in micro-channels Sungcheol Kim, Xinsheng Ling We report an experimental study of the first-passage-time problem of driven diffusion in micro-channels. Fluorescent microspheres of 190nm diameter are confined in channels of 1.0 micron in width and 1.0 micron depth and driven by an applied longitudinal electric field. The images are acquired by a fluorescent microscope. The time dependence of the particle positions is tracked using particle tracking algorithms. The first passage times at different electric field values are extracted from the real-time data and compared with the exact solution given by Schrodinger for the 1D biased diffusion equation with one absorbing boundary condition. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X50.00009: Liquid-impregnated surfaces: overcoming the limitations of superhydrophobic surfaces for robust non-wetting and anti-icing surface J. David Smith, Rajeev Dhiman, Ernesto Reza-Garduno, Gareth McKinley, Robert Cohen, Kripa Varanasi In this work we address fundamental limitations of superhydrophobic surfaces for non-wetting and anti-icing applications by impregnating them with a hydrophobic liquid. The impregnating liquid serves as a barrier to the penetration of impinging water droplets and forces preferential condensation and frost formation on texture tops. We predict the thermodynamically stable wetting states based on a free energy analysis, and model the behavior of rolling droplets on liquid-impregnating surfaces. We conducted droplet impact and roll-off experiments to assess the robustness of liquid-impregnateding micro- and nano-scale textured surfaces and found that their ability to shed droplets was improved dramatically. Furthermore, environmental scanning electron microscope experiments demonstrated that frost formation as well as condensation occurs preferentially on these surfaces thereby limiting ice contact to texture tops only. Ice adhesion strength was quantified using a custom-built adhesion testing apparatus to demonstrate greatly enhanced anti-icing performance of the liquid-impregnating surfaces compared to superhydrophobic surfaces. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X50.00010: Using Superhydrophobic Surfaces and Optical Caustics to Detect Nanoparticle Aggregation Antonio Garcia, James Lindsay, Eric Gilmore A 3-D envelope of refracted light known as an optical caustic, can be formed by shaping an aqueous drop on a superhydrophobic surface which is used to generate a signal that is very sensitive to changes in particle size. When the sample being detected is suspended in the drop, slow evaporation induces movement that segregates smaller from larger particles, enhancing the speed of detection via induced aggregation. While the unique properties of optical caustics have been used in engineering science to evaluate stress distributions and contact between material components, they have not been widely used in diagnostics or biological analyses. This paper demonstrates how this method can track aggregation of gold nanoparticles for rapid detection of molecular disease markers using immunoassays. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X50.00011: Droplet condensation and growth on nanotextured surfaces impregnated with an immiscible liquid Sushant Anand, Adam Paxson, Jonathan Smith, Rajeev Dhiman, Kripa Varanasi For effective dropwise condensation, a surface that sheds droplets easily is desirable due to the enhancement in accompanying heat transfer. Incorporating nano-textures on the surface can enhance the droplet shedding or spreading. We demonstrate that droplet shedding can be further influenced by impregnating the nano-textured surface with a liquid which is immiscible with respect to the droplet. In this study, the dynamics of dropwise condensation on such immiscible liquid impregnated nano-textured surfaces have been investigated in pure quiescent water vapor conditions. Condensation experiments were conducted using an Environmental Scanning Electron Microscope by controlling the chamber water vapor pressure and substrate temperature. We show preferential sites for condensation and different modes under which droplets grow, depending upon the surface chemistry, surface texture, and the impregnating liquid properties. Concurrently, we show an evolution of apparent contact angles during the condensation process on the impregnated surfaces. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X50.00012: Molecular diffusion and tensorial slip at surfaces with periodic and random nanoscale textures Nikolai Priezjev The influence of periodic and random surface textures on the flow structure and effective slip length in Newtonian fluids is investigated by molecular dynamics (MD) simulations. This study is motivated by the possibility to generate transverse flows in microfluidics devices to enhance mixing and separation processes. We consider a situation where the typical pattern size is smaller than the channel height and the local boundary conditions at wetting and nonwetting regions are characterized by finite slip lengths. In case of anisotropic textures, the interfacial diffusion coefficient of fluid molecules near heterogeneous surfaces correlates well with the effective slip length as a function of the shear flow direction with respect to the texture orientation. In addition, it was found that the angular dependence of the effective slip length obtained from MD simulations is in good agreement with hydrodynamic predictions provided that the pattern size is larger than several molecular diameters. These findings lend support for the microscopic justification of recently introduced tensor formulation of the effective slip boundary conditions in the case of noninertial flows of Newtonian fluids over smooth surfaces with nanoscale anisotropic textures. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X50.00013: Wetting as a basis for a highly selective colorimetric indicator for organic liquids Ian Burgess, Kevin Raymond, Natalie Koay, Anna Shneidman, Mathias Kolle, Marko Loncar, Joanna Aizenberg We present a colorimetric indicator for organic liquids that couples distinct macroscopic color patterns to minute differences in liquids' intrinsic wettability to a surface. We find that when a liquid percolates through the pores of large-area, defect-free silica inverse-opal films, a highly consistent re-entrant geometry leads to sharply defined threshold wettability for liquid infiltration, occurring at intrinsic contact angles near 20\r{ }. The structure also acts as a 3D photonic crystal, producing bright iridescent color that disappears when infiltrated with liquid, coupling the highly selective wetting observed to an easy-to-visualize colorimetric response. Combining a percolation model and FDTD optical simulations, we estimate the selectivity of the colorimetruic response. In addition, we present a technique to generate precisely controlled spatial patterns of surface chemistry throughout the porous network. This lets us tailor the wettability threshold to specific liquids across a continuous range. Using these techniques, we demonstrate the applicability of this indicator to colorimetrically distinguish: i) ethanol-water mixtures varying by only 2.5{\%} in concentration; ii) hexane, heptane, octane, nonane, and decane; and iii) samples of gasoline (regular unleaded) and diesel. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X50.00014: Drag Measurements in Laminar Flows over Superhydrophobic Porous Membranes Ozgur Ozsun, Victor Yakhot, Kamil L. Ekinci An anomalous hydrodynamic response has recently been observed in oscillating flows on mesh-like porous superhydrophobic membranes.\footnote{S. Rajauria, O. Ozsun, J. Lawall, V. Yakhot, and K. L. Ekinci, Phys. Rev. Lett. 107, 174501 (2011)} This effect was attributed to a stable Knudsen layer of gas at the solid-liquid interface. In this study, we investigate laminar channel flow over these porous superhydrophobic membranes. We have fabricated surfaces with solid area fraction $\phi_{s}$, which can maintain intimate contact with both air and water reservoirs on either side. Typical structures have linear dimensions of 1.5~mm $\times$ 15~mm $\times$ 1~$\mu$m and pore area of 10~$\mu$m $\times$ 10~$\mu$m. The surfaces are enclosed with precisely machined plastic microchannels, where pressure driven flow of DI water is generated. Pressure drop across the microchannels is measured as a function of flow rate. Slip lengths are inferred from the Poiseuille relation as a function of $\phi_{s}$ and compared to that of similar standard superhydrophobic surfaces, which lack intimate contact with an air reservoir. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X50.00015: Thermo-super-hydrophobic effect Jerzy M. Floryan Super-hydrophobic effect involves capture of gas bubbles in pores of solid wall. These bubbles separate moving liquid from the solid surface resulting in a substantial reduction of shear drag experienced by the liquid. The super-hydrophobic effect requires presence of two phases and thus drag reduction can be accomplished only for liquids. Thermo-super-hydrophobic effect takes advantage of the localized heating to create separation bubbles and thus can work with single phase flow systems. Analysis of a simple model problem shows that this effect is very strong in the case of small Re flows such as those found in micro-channels and can reduce pressure drop down to 50{\%} of the reference value if the heating pattern as well as the heating intensity are suitable chosen. The thermo-super-hydrophobic effect becomes marginal when Re increases above a certain critical value. [Preview Abstract] |
Session X52: Focus Session: Extreme Mechanics - Fracture, Friction, and Frequencies
Sponsoring Units: GSNP DFDChair: Karin Dahmen, University of Illinois
Room: 153C
Thursday, March 1, 2012 2:30PM - 2:42PM |
X52.00001: Geometry of Tearing: crack propagation in brittle sheets Benoit Roman, Jose Bico, Enrique Cerda, Eugenio Hamm, Francisco Melo, Victor Romero We experiment the fracture of thin object everyday when trying to open a packaging. From a physics point of vew, the propagation of cracks in thin brittle elastic sheets appears to be remarkably reproducible, with very regular crack path. We will present some examples where the crack path can be predicted using classical arguments in fracture and geometrical tools: this is another example where geometry plays a central role in the mechanics of thin sheets. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X52.00002: Spiral and croissant crack in drying thin films Joel Marthelot, Benoit Roman, Jose Bico, Etienne Barthel, Jeremie Teisseire, Davy Dalmas, Francisco Melo Drying mud or crazing in ceramics glaze leads to familiar hierarchical cracks network where a new crack connects perpendicularly to older ones. We report unusual spirals and croissants crack patterns in methylsiloxane drying thin films moderately adhering on a substrate. Such cracks are also observed in a very different situation when magnetron sputtering multilayers are under external tension. The amplitude and wavelength of the pattern are robusts and are orders of magnitude larger than the thickness of the layer. The propagation of the spiral and croissant cracks occurs in a narrow range of adhesion energy between the film and the substrate and strain in the film. We will show how the propagation is driven by a cooperation between fracture and adhesion. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X52.00003: Rupture of a highly stretchable acrylic dielectric elastomer George Pharr, Jeong-Yun Sun, Zhigang Suo Dielectric elastomers have found widespread application as energy harvesters, actuators, and sensors. In practice these elastomers are subject to large tensile stretches, which potentially can lead to mechanical fracture. In this study, we have examined fracture properties of the commercial acrylic elastomer VHB 4905. We have found that inserting a pre-cut into the material drastically reduces the stretch at rupture from $\lambda _{rup}$ = 9.43$\pm $1.05 for pristine samples down to only $\lambda _{rup}$ = 3.63$\pm $0.45 for the samples with a pre-cut. Furthermore, using ``pure-shear'' test specimens with a pre-crack, we have measured the fracture energy and stretch at rupture as a function of the sample geometry. The stretch at rupture was found to decrease with sample height, which agrees with an analytical prediction. Additionally, we have measured the fracture energy as a function of stretch-rate. The apparent fracture energy was found to increase with stretch-rate from $\Gamma \approx $1500 J/m$^{2}$ to $\Gamma \approx $5000 J/m$^{2}$ for the investigated rates of deformation. This phenomenon is due to viscoelastic properties of VHB 4905, which result in an apparent stiffening for sufficiently large stretch-rates. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X52.00004: How does adhesion impact the formation of telephone cord buckles? Etienne Barthel, Jean-Yvon Faou, Sergey Grachev, Guillaume Parry Compressively stressed thin films with low adhesion frequently buckle into telephone cords. Although telephone cord buckles have been studied for decades, no complete understanding of their origin and propagation has so far been presented. Here, using Finite Element Analysis, we have coupled non-linear plate deformation with a cohesive zone model to simulate the kinematics of a propagating telephone cord buckle. On the experimental side, we have developped model thin films with a precise adjustment of both adhesion and residual stresses. From the comparison of the simulations with some experimental observations, we propose a generic mechanism for the formation of telephone cord buckles. Proper inclusion of the dependence of interfacial toughness upon mode mixity proved to be central to the success of the approach so that this clarification of the mechanism of telephone cord formation promises better understanding of interfacial toughness through the analysis of buckle morphology. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X52.00005: Nonlinear modal interactions in a microcantilever Hidde Westra, Herre van der Zant, Warner Venstra We study the nonlinear interactions between vibrational modes in a microcantilever. The flexural-flexural, torsional-torsional and torsional-flexural modal interactions are investigated theoretically and experimentally. In a cantilever, the nonlinearity arising from geometrical and inertial effects couples the different modes. The motion of one mode influences the resonance frequency of the other modes. We show that depending on the amplitude of one mode, both frequency stiffening and weakening of the other mode occurs. The modal interactions in clamped-clamped beam resonators is recently studied, and several applications have been proposed. Microcantilevers are frequently used in instrumentation, and the modal interactions presented here enable such schemes, including Q-factor tuning and self-detection. [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X52.00006: Effects of Roughness and Inertia on Precursors to Frictional Sliding Mark O. Robbins, K. Michael Salerno Experiments show that when a PMMA block on a surface is normally loaded and driven by an external shear force, contact at the interface is modified in discrete precursor slips prior to steady state sliding.[1] Our simulations use an atomistic model of a rough two-dimensional block in contact with a flat surface to investigate the evolution of stress and displacement along the contact between surfaces. The talk will show how local and global stress conditions govern the initiation of interfacial cracks as well as the spatial extension of the cracked region. Inertia also plays an important role in determining the number and size of slips before sliding and influences the distribution of stresses at the interface. Finally, the geometry of surface asperities also influences the interfacial evolution and the total friction force. The relationship between the interfacial stress state and rupture velocity will also be discussed. [1] S.M. Rubinstein, G. Cohen and J. Fineberg, PRL 98, 226103 (2007) [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X52.00007: Scratch test as a fracture process: from soft to hard materials Ange-Therese Akono, Pedro Miguel Reis, Nicholas Xavier Randall, Franz-Josef Ulm The scratch test consists of driving a probe, at a certain depth, through a material and is most likely the oldest technique for the mechanical characterization of materials. Although it is widely used in strength testing, the presence of residual chips during the process suggests that a fracture mechanism is at play. We investigate the link between the material fracture properties, the probe geometry and the resulting forces using a combination of precision experiments and Linear Elastic Fracture Mechanics analysis. An analytical model is developed that is applicable both at the macro and micro scale, and that can take into account different probe geometries. Rationalizing the mechanics involved allows us to introduce a novel experimental technique to accurately determine the fracture toughness from scratch tests. Application of this technique to mechanical testing on metals, polymers and ceramics yields values for the fracture toughness that are in excellent agreement with conventional methods such as the three-point bending test, albeit in a way that is less destructive and more scalable. As such, our method to determine materials fracture properties represents an important new development in the field of mechanical micro-characterization. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X52.00008: Sliding on a Nanotube: Interplay of Friction, Deformations and Defects Hsiang-Chih Chiu, Suenne Kim, Erio Tosatti, Christian Klinke, Elisa Riedo Carbon nanotubes (CNT) have applications as composite material reinforcements and components in nanodevices due to their exceptional physical properties. However, CNTs have structural defects that can change their mechanical properties. For applications, CNTs have to be in contact with other surfaces, thus it is important to understand how defects change their frictional properties. Here, we show that defects can impact the frictional properties of supported Arc Discharge (AD) and Chemical Vapor Deposition (CVD) grown CNTs by sliding an AFM tip along (longitudinal) and across (transverse) the CNT axis. Larger friction coefficient is found during transverse sliding due to a lateral CNT deformation (called hindered rolling) that causes extra friction dissipation which is absent during longitudinal sliding.[1] A friction anisotropy, defined as the ratio of shear strength measured during both sliding directions, can be as high as 13.7 for AD CNTs but less than 6 for CVD CNTs. Extra defects in CVD CNTs couple both sliding motions, resulting in more energy dissipation and higher longitudinal friction. A simple analytical model is developed to explain the observed experimental behavior. Our finding provides a better understanding of tribological properties of individual CNT at the nanoscale. [1] M. Lucas et al., Nature Mater. 8, 876 (2009) [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X52.00009: Graphene Morphology on Nano-Patterned Electronic Substrates Guangxu Li, Cihan Yilmaz, Xiaohong An, Sivasubramanian Somu, Swastik Kar, Ahmed Busnaina, Kai-tak Wan In order to get high quality of graphene for the application in electronic devices, good transfer of graphene prepared by mechanical exfoliation or chemical vapor deposition is always required and substrate with flat surface is preferred to avoid the crack and destruction of the thin sheets. Here, we studied the graphene morphology on nano-patterned electronic substrates by transferring graphene grown from chemical vapor deposition onto the gold nano pillar patterns on silicon substrate. The adhesion between the graphene and the gold surface makes the flexible thin membrane conform to the substrate geometry and form a series of blisters. By measuring the blister radius and height, the adhesion energy of graphene and gold substrate can be deduced. In the meantime, the morphology of graphene on the pillar patterns was found to strongly related to the adhesion energy, the height and separation of pillars. By changing these parameters, the blisters may decrease size or expand to coalesce. The critical separation between pillars and the critical height of pillars were predicted to avoid the coalescence of the blisters when the adhesion energy was fixed. The results obtained here can be useful to increase the performance and the durability of the graphene based device. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X52.00010: Graphene Blister Adhesion Mechanics Narasimha Bodetti, Steven Koenig, Jianliang Xiao, Scott Bunch, Martin Dunn We describe graphene blister configurations to study the elasticity of mono- and multi-layer graphene as well as the adhesion of the blister to an SiO2 substrate. We create blisters by depositing graphene on a chip containing etched cavities of a prescribed volume. The chip is placed in a high-pressure chamber where the cavities are charged to a prescribed pressure. When the chip is removed from the chamber the pressure difference across the membrane causes it to bulge, while the number of gas molecules in the chamber remains constant. As the pressure is increased the membrane continues to bulge and at a critical pressure can delaminate (in a stable or unstable manner) permitting extraction of the adhesion energy from a combination of theory and measurements of the deformed blister configuration. We describe these experiments and develop a thermodynamic model of the system that identifies interesting nonlinear effects as the membranes deform including instabilities, delamination, and adhesion hysteresis, depending on the configurational parameters. We use the theory and experiments together to determine for the first time the adhesion energy between graphene and SiO2, as well as explore the interesting mechanics that occur. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X52.00011: Novel method for simulation of structural post buckling Rachmadian Wulandana, Sachin Velankar A new FEM-based method for simulating the onset of buckling instabilities and the post-buckling evolution is developed. The method consists of creating a random spatial perturbation of the elastic modulus and applying a step-by-step loading to approach the critical state and beyond. Prior to buckling, the non-uniform modulus triggers micro bending and lateral deformation. As the compressive load progresses, the micro displacement grows non-linearly causing the system to be biased toward the mode that minimizes energy. The system buckles in that mode and the post-buckling deformation can be examined. The technique has been applied to several buckling cases. The results show quantitative agreement with theory and experiments. For problems with continuously-distributed buckling modes and critical values that are close from one to another, the method is able to automatically select the correct critical configuration. Unlike other perturbation methods that are inspired by either Eigen vectors or experimental data, the current method does not need a priori knowledge of the expected buckling mode. This is especially useful in complex problems (e.g. wrinkling of stretched films) for which linear eigenvalue analysis cannot predict the critical conditions. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X52.00012: Buckling morphologies of crystalline shells with frozen defects Ee Hou Yong The crumpling of spherical crystalline lattices where the topological defects are frozen is studied. The geometry of the crumpled membrane is found to depend on the set of topological defects and more exotic defect sets can result in crumpled shapes resembling that of the Platonic and Archimedean solids. The phase diagram of the crumpled spheres can be categorized by two dimensionless numbers $h/R$ (aspect ratio) and $R/a$ (lattice ratio), where $h$ is the thickness of the shell, $R$ is the radius of the initial sphere and $a$ is the average bond length of the triangulation. The shapes of the crumpled membrane can be understood using rotationally invariant quantities formed from spherical harmonics coefficients and a Landau free energy can be written, involving quadratic and cubic rotational invariants. Shells with different topological defects have qualitatively different hysteresis behaviors and the transitions appear to be first order in general. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X52.00013: Do tidal stresses trigger large earthquakes early? Braden Brinkman, Michael LeBlanc, Jonathan Uhl, Yehuda Ben-Zion, Karin Dahmen The effect of tidal or other periodic stresses on the timing of large earthquakes is a hotly debated topic in geophysics and rock-friction or granular physics communities. I discuss a simple probabilistic model which captures the main qualitative features of several rock-friction or granular experiments and may resolve some outstanding discrepancies between different experimental results. With sufficiently accurate measurements, quantitative predictions for real experiments are possible, including the number of measured events needed to detect correlations between periodic stresses and large slip events for given amplitudes and frequencies. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X52.00014: Tattoo-Like Strain Gauges Based on Silicon Nano-Membranes Nanshu Lu This talk reports the in vivo measurement of tissue deformation through adhesive-free, conformable lamination of a tattoo-like elastic strain gauge consisted of piezoresistive silicon nano-membranes strategically integrated with tissue-like elastomeric substrates. The mechanical deformation in soft tissues cannot yet be directly quantified due to the lack of enabling tools. While stiff strain gauges for structural health monitoring have long existed, biological tissues are soft, curvilinear and highly deformable in contrast to civil or aerospace structures. An ultra-thin, ultra-soft, tattoo-like strain gauge that can conform to the convoluted surface of human body and stay attached during locomotion will be able to directly quantify tissue deformation without affecting the mechanical behavior of the tissue. While single crystalline silicon is known to have the highest gauge factor and best elastic response, it is intrinsically stiff and brittle. To achieve strain gauges with high compliance, high stretchability and reasonable sensitivity, single crystalline silicon nano-membranes will be transfer-printed onto polymeric support through carefully engineered stamps. The thickness and length of the Si strip will be chosen according to theoretical and numerical mechanics analysis which takes into account for the tradeoff between stretchability and sensitivity. [Preview Abstract] |
Session X53: Focus Session: Common Features of Soft Materials: Polymers, Colloids and Granular Media II
Sponsoring Units: GSNPChair: Joerg Rottler, University of British Columbia
Room: 153B
Thursday, March 1, 2012 2:30PM - 2:42PM |
X53.00001: Shear thickening and jamming in suspensions of different particle shapes Eric Brown, Hanjun Zhang, Nicole Forman, Douglas Betts, Joseph DeSimone, Benjamin Maynor, Heinrich Jaeger We investigated the role of particle shape on shear thickening and jamming in densely packed suspensions. Various particle shapes were fabricated including rods of different aspect ratios and non-convex hooked rods. A rheometer was used to measure shear stress vs. shear rate for a wide range of packing fractions for each shape. Each suspensions exhibits qualitatively similar Discontinuous Shear Thickening, in which the logarithmic slope of the stress vs. shear rate has the same scaling for each convex shape and diverges at a critical packing fraction $\phi_c$. The value of $\phi_c$ varies with particle shape, and coincides with the onset of a yield stress, a.k.a. the jamming transition. This suggests the jamming transition controls shear thickening, and the only effect of particle shape on steady state bulk rheology of convex particles is a shift of $\phi_c$. Intriguingly, viscosity curves for non-convex particles do not collapse on the same set as convex particles, showing strong shear thickening over a wider range of packing fraction. Qualitative shape dependence was only found in steady state rheology when the system was confined to small gaps where large aspect ratio particle are forced to order. [Preview Abstract] |
Thursday, March 1, 2012 2:42PM - 2:54PM |
X53.00002: Fluctuations and Avalanche Statistics in Sheared Systems of Elliptical Particles Somayeh Farhadi, Robert P. Behringer We have performed a series of 2D Couette shear experiment on a system of granular particles near jamming. In these experiments, the particles, which are photoelastic ellipses with aspect ratio 2, are confined between an inner rotating wheel and a fixed outer ring. The inner wheel rotation rate was varied between 0.01 and 0.1 rpm. Using photoelastic properties of the particles and two synchronized cameras, we are able to track the position, orientation and contact forces of each particle through the entire experiment. We determined the local mean, fluctuations, and correlations for various system properties, such as density (Voronoi volume), pressure, orientational order parameter and shear rate. A striking finding of this study is the fact that near jamming, the system can exist in an effectively shear jammed metastable state for very long times before relaxing to an unjammed state. We have also studied the dynamics of avalanches by analyzing the time series of global stress. This study gives us insight to the nature of failure in sheared systems of asymmetrical particles. [Preview Abstract] |
Thursday, March 1, 2012 2:54PM - 3:06PM |
X53.00003: Mapping From Soft to Hard-Core Disks Near the Athermal Shear Driven Jamming Transition Peter Olsson, Stephen Teitel We examine the rheology of soft-core, bidisperse, frictionless disks in two dimensions at zero temperature with overdamped dynamics. For shear driven flow at a uniform strain rate $\dot\gamma$, we find a simple expression for an effective hard-core packing fraction, $\phi_{\mathrm{eff}}$, such that the pressure equivalent to the shear viscosity, $p/\dot\gamma$, for different shear rates, packing fractions, and different contact interactions all collapse onto a common curve when plotted as a function of $\phi_{\mathrm{eff}}$. This function is a characteristic of the hard-core limit as it describes the system in the limit of vanishing particle overlaps. This mapping recovers all the critical behavior found in earlier scaling analyses. We use this mapping to derive a duality relation that gives the exponent of the non-linear Herschel-Bulkley rheology \emph{above} jamming in terms of the exponent of the diverging viscosity \emph{below} jamming. [Preview Abstract] |
Thursday, March 1, 2012 3:06PM - 3:18PM |
X53.00004: Hot spots in an athermal system Jerome Crassous, Axelle Amon, Van Bau Nguyen, Ary Bruand, Eric Clement We study experimentally the dynamical heterogeneities occuring at slow shear, in a model amorphous glassy material i.e. a 3D granular packing. The deformation field is resolved spatially using a Diffusive Wave Spectroscopy technique. The heterogeneities show up as localized regions of strong deformations spanning a mesoscopic size of about $10$ grains and called the 'hot spots'. The spatial clustering of hot-spots is linked to the subsequent emergence of shear bands. Quantitively, their apparition is associated with the macroscopic plastic deformation and their rate of occurence gives a physical meaning to the concept of ``fluidity,'' recently used to describe the local and non-local rheology of soft glassy materials. [Preview Abstract] |
Thursday, March 1, 2012 3:18PM - 3:30PM |
X53.00005: Effect of Inertial Mass on Velocity Correlations of Shear Driven Soft-Core Disks Approaching the Athermal Jamming Transition Daniel V{\aa}gberg, Peter Olsson, Stephen Teitel It was found numerically that overdamped, frictionless, soft-core disks undergoing uniform shear driven flow, show differences in behavior depending on how the viscous dissipation is introduced into the numerical simulation. When dissipation is with respect to a sheared external reservoir (the so-called ``mean-field'' approximation), velocity correlations are found to determine a finite length scale $\xi$ that diverges as the jamming transition is approached[1]. However, when dissipation is modeled by inter-particle inelastic collisions, the velocity correlations show no characteristic length other than the length of the system[2]. To study the relation between these two models of dissipation, we remove the overdamped constraint and consider particles with finite inertial mass $m$, and study how velocity correlations behave as the overdamped limit $m\to 0$ is approached. \newline [1] P. Olsson and S. Teitel, Phys.\ Rev.\ Lett.\ {\bf 99}, 178001 (2007). \newline [2] B. P. Tighe et al.\ Phys.\ Rev.\ Lett.\ {\bf 105}, 088303 (2010). [Preview Abstract] |
Thursday, March 1, 2012 3:30PM - 3:42PM |
X53.00006: Stretching dense colloidal suspensions: from flow to fracture Michael Smith, Rut Besseling, Andrew Schofield, James Sharp, Mike Cates, Volfango Bertola Concentrated suspensions of particles are commonly used in the pharmaceutical, cosmetic and food industries. Manufacture of these products often involves flow geometries that are substantially different from those studied by conventional shear rheology. Using a capillary break-up extensional rheometer we stretch fluids of different volume fraction at strain rates just below, at and above the critical rate required to induce jamming. We show that the jamming of a stretched colloidal column is closely related to that observed during shear rheology. However, fascinating additional effects due to the geometry are also observed. High speed photography of the filament shows evidence of dilatancy and granulation, leading finally to fracture at a critical strain rate. We also investigate an intriguing aspect of thin fluid filaments of the colloidal suspension, when stretched below the critical strain rate required to produce jamming. These filaments are observed to thin to a critical diameter before rupturing and displaying visco-elastic recoil. Finally, using fluorescent particles we visualise the flow fields inside these filaments to understand the dynamics. [Preview Abstract] |
Thursday, March 1, 2012 3:42PM - 3:54PM |
X53.00007: Fluctuations for Hopper Flow with Circular and Elliptical Particles Junyao Tang Recent studies have shown the importance of particle-scale fluctuations in granular flow, e.g. ``stick-slip'' and jamming. In this talk, we consider 2-D hopper flow, where we investigate how the mean and fluctuations of stress, velocity and density fields depend on hopper geometry (e.g. opening size and wall angle) and material properties (e.g. particle shape and initial filling height of materials). A particularly interesting observation is that the mean stress is a decreasing function of the filling height, even though the flux is nearly constant over the same range of material heights. We also find strong negative correlations between stress and velocity fluctuations. Also of interest is the effect of the particle orientation in the flow of elliptical particles. Related MD/DEM studies (supported by IFPRI and NSF) by Shattuck, Kondic and McCarthy et al. have found that good agreement between models and experiments for these flows. [Preview Abstract] |
Thursday, March 1, 2012 3:54PM - 4:06PM |
X53.00008: Rheology and Jamming in Soft Colloidal System Anindita Basu, Tim Still, Paulo Arratia, Kerstin Nordstrom, Jerry Gollub, Douglas Durian, Arjun Yodh Recent simulations have proposed that the jamming transition can be understood in the framework of critical phenomena, and thus can be described by various asymptotic scaling laws. We use thermosensitive colloidal suspensions and a commercially available rheometer to study the shear response of soft colloidal glass across the jamming transition. We carry out steady-state (viscometry) and time-dependent (oscillatory) rheology experiments in the vicinity of the jamming transition. Both viscometry and oscillatory stress data exhibit asymptotic scaling and presence of critical exponents reminiscent of second-order phase transition, as reported in recent simulations. Critical scaling of frequency indicates the presence of a diverging time-scale associated with the jamming transition. We attempt to understand these critical exponents based on the microscopic interactions of the colloidal systems. [Preview Abstract] |
Thursday, March 1, 2012 4:06PM - 4:18PM |
X53.00009: Local interactions and global rheology in disordered media Erik Woldhuis, Brian Tighe, Kerstin Nordstrom, Doug Durian, Jerry Gollub, Martin van Hecke We generalize our scaling model for the rheology of soft, frictionless repulsive spheres to include general local viscous and elastic interactions and come to a prediction of the effect on the global behavior of these local interactions. As our scaling model combines elastic and geometric ingredients and a power balance that depends on the local viscous law, we predict that the global rheology of disordered media depends on the details of the local interactions in a universal but non-trivial manner. We compare our predictions for the effect of different {\em elastic} interactions to recent experimental results on the interactions and rheology of NIPA particles. [Preview Abstract] |
Thursday, March 1, 2012 4:18PM - 4:30PM |
X53.00010: Jammed by shear: a new perspective of the jamming transition in frictional granular materials Dapeng Bi, Bulbul Chakraborty In the jamming diagram (\emph{Nature} \textbf{396} 21 (1998)) for athermal systems jamming is induced only through compression, and jammed states exist above a packing fraction $\phi_J$. Recent experiments in frictional disks clearly show shear induced jamming (D. Bi et al, \emph{Nature} doi:10.1038/nature10667 (2011)). A minimum shear stress, $\tau_0$, is needed to create robust, shear-jammed (SJ) states with a strong force network percolating along both the compressive and dilational directions. This percolation transition is controlled by the fraction of force-bearing grains, a parameter not previously discussed in the context of jamming. The minimum anisotropy of SJ states vanishes as $\phi\to\phi_J$ from below in a manner reminiscent of an order-disorder transition. To shed light on the origin of shear jamming we have constructed a lattice gas model of force-bearing grains with nematic interactions between the force tiles (objects that reflect force balance on each grain). We will show that there are clear signatures of nematic ordering in SJ states, and present analysis of the nature of the shear-induced jamming transitions as a function of $\phi$. We will also present numerical simulation results of sheared frictional particles. [Preview Abstract] |
Thursday, March 1, 2012 4:30PM - 4:42PM |
X53.00011: Deformation of inherent structures to detect short- and long-range correlations in supercooled liquids Emanuela Del Gado, Majid Mosayebi, Patrick Ilg, Hans Christian Oettinger We use deformation of inherent structures as a tool for detecting structural changes and the onset of cooperativity in supercooled liquids. The non-affine displacement (NAD) field resulting from the applied deformation shows characteristic differences between the high temperature liquid and supercooled state, that are typically observed in dynamic quantities and correlated to normal mode structure. The average magnitude of the NAD is very sensitive to temperature changes in the supercooled regime and is found to be strongly correlated with the inherent structure energy. We can rationalize such changes in terms of a crossover from a viscous liquid to a regime dominated by elastic effects. In addition, the NAD field is characterized by a correlation length that increases upon lowering the temperature towards the supercooled regime. By analysing different measures of correlations in the direction of the NAD field, we discuss their analogies with observations in the cooperative dynamics. [Preview Abstract] |
Thursday, March 1, 2012 4:42PM - 4:54PM |
X53.00012: Experiments on ordering transitions in mechanically stable structures of granular rods Vikrant Yadav, Jean-Yonnel Chastaing, Arshad Kudrolli We investigated the evolution of granular rods from mechanically stable disordered to crystalline states in response to vibrations. We obtained positions and orientations of the rods in three dimensions using micro-focus X-ray Computed Tomography. Above a critical aspect ratio, we find that rods align vertically in layers with hexagonal order within a layer, independent of the shape of the container and details of the form of vibration. We also quantitatively study the evolution of local and global ordering using density pair correlation function $g(r)$ and orientational order parameter $q_{6}$ as a function of aspect ratio. As the system compacts, local structures emerge and grow, their size and orientation being dependent on volume fraction. Although the initial nucleation of order occurs along the boundaries, we show that the geometry of boundaries have little overall effect on the observed ordering transition. Finally we show that configuration entropy arguments do not play a significant role in the observed ordering, and the system evolves towards increasing stability under small perturbations. [Preview Abstract] |
Thursday, March 1, 2012 4:54PM - 5:06PM |
X53.00013: Elasticity of floppy amorphous systems Gustavo D\"uring, Edan Lerner, Matthieu Wyart Simple amorphous solids made of repulsive particles display curious properties when they are barely mechanically stable, in particular near the unjamming transition where pressure vanishes. Here we focus on another class of materials, including granular flows, covalent glasses or gels of semi-flexible polymers. In such materials the coordination associated with the dominant interaction is too weak to guarantee mechanical stability. This fact implies the presence of floppy modes, collective motions of particles that have no or very little restoring force, and that strongly affect the properties of these materials. We use analytical methods to derive the response of these systems, their length scale and frequency dependence, and test these numerically. If time permits our results will be compared with numerical observations in simplified suspension flows. [Preview Abstract] |
Thursday, March 1, 2012 5:06PM - 5:18PM |
X53.00014: Granular Matter, Foams, and Beyond: Applications of the Granocentric Model Katherine Newhall We present a local stochastic model that predicts the statistical fluctuations in jammed packings of monodisperse and polydisperse spheres revealed by confocal microscopy. Moreover, we find that this model can account for the properties of looser and denser random packings that result from depletion attraction between the particles or compression by an applied load, respectively. Finally, we extend the model to space-filling packings of cells in tissues and biliquid foams by testing analytic predictions for the dependence of the number of neighbors of a given cell on its volume. Interestingly, the model distinguishes between scenarios in which size or positional disorder in the packing dominate, in good agreement with experimental data. This versatile model can be put into the statistical mechanics framework proposed by Edwards in order to compute the entropy and compactivity of each packing. [Preview Abstract] |
Thursday, March 1, 2012 5:18PM - 5:30PM |
X53.00015: Force landscape for particulate systems Lou Kondic, Miroslav Kramar, Arnaud Goullet, Konstantin Mischaikow We discuss the properties of force landscape for isotropically compressed particulate systems characterized by a wide range of packing fractions. The computational methods used are based on persistence diagrams which allow for clear identification of mathematical properties of force landscapes and help their physical interpretation. We find that using this technique which previously has not been applied to particulate matter, a significant new information can be extracted, going much beyond separation into `strong' and `weak' force networks. One result of this analysis is clear indication that for small packing fractions, polydispersity is a crucial parameter that defines the landscape, while for large packing fractions, friction, if present, becomes dominant. Preliminary results for dynamical features of force networks obtained from time-dependent analysis of force landscapes will be presented as well. [Preview Abstract] |
Thursday, March 1, 2012 5:30PM - 5:42PM |
X53.00016: Rearrangements in flow of foam Vijayakumar Chikkadi, Erik Woldhuis, Peter Schall, Martin van Hecke The nonaffine deformation of amorphous solids has attracted considerable attention. Recent simulations and experiments have shown that the non-affine particle rearrangements under an applied stress are localised, and they generate a long-ranged quadrupolar strain field. Here we have investigated the particle rearrangements in flows of soft, viscous spheres near the jamming point. The characteristics of the particle rearrangements are extremely different as we move away from the jamming point. [Preview Abstract] |
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