Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session Q1: Memory and Focusing in Catastrophic Deformations
Sponsoring Units: DCMPChair: Michael Brenner, Harvard University
Room: Spirit of Pittsburgh Ballroom A
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q1.00001: Tsunami Asymptotics Invited Speaker: Optical analogies, and some singularity theory, give new information about tsunamis. For most of their propagation, tsunamis are linear dispersive waves whose speed is limited by the depth of the ocean and which can be regarded as diffraction-decorated caustics in spacetime. For constant depth, uniform asymptotics gives a very accurate compact description of the tsunami profile generated by an arbitrary initial disturbance. Variations in depth act as lenses and can focus tsunamis onto cusped caustics, and this ``singularity on a singularity'' constitutes an unusual diffraction problem, whose solution indicates that focusing can amplify the tsunami energy by an order of magnitude. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:27PM |
Q1.00002: Memory encoding vibrations in a disconnecting air bubble Invited Speaker: The implosion that disconnects a submerged air bubble into several bubbles provides a simple example of energy focusing. The most efficient disconnection is an entirely symmetric one terminating in a finite-time singularity. At the final moment, the potential energy at the start of the disconnection is entirely condensed into the kinetic energy of a vanishingly small amount of liquid rushing inwards to disconnect the bubble. In reality, however, the initial shape always possesses slight imperfections. We show that a memory of the imperfection remains and controls the final fate of the focusing. Linear stability reveals that even an infinitesimal perturbation is remembered. A slight initial asymmetry excites vibrations in the cross-section shape of the bubble neck. The vibrations persist over time. Near the singularity, their amplitudes freeze, locking onto constant values, while their frequencies chirp, increasing more and more rapidly. The net effect is that the singularity remembers exactly half of the information about the initial imperfection, the half encoded by the vibration amplitudes. We check this scenario in an experiment by releasing an air bubble from a nozzle with an oblong cross-section. This excites an elongation-compression vibrational mode. We measure the vibration excited and find quantitative agreement with linear stability. When the initial distortion has a small, but finite, size, the saturation of the vibration amplitude causes the symmetric singularity to be pre-empted by an asymmetric contact between two distant points on the interface. Numerics reveal that the contact is typically smooth, corresponding to two inward-curving portions of the bubble surface colliding at finite speed. Both the contact speed and curvature vary non-monotonically with the initial distortion size, with abrupt jumps at specific values. This is because the vibration causes contact to occur at different values of the phase. A contact produced when the shape distortion is pronounced requires a smaller initial amplitude than a contact produced when the vibration is out of phase. (Joint work with Nathan C. Keim, Lipeng Lai, Laura E. Schmidt, Konstantin Turitsyn and Sidney R. Nagel.) [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 1:03PM |
Q1.00003: Pattern Transformation Triggered by Deformation Invited Speaker: Periodic elastomeric cellular solids are subjected to uniaxial compression and a novel uniform transformation of the structure is found above a critical value of applied load. The results of a numerical investigation reveal that the pattern switch is triggered by a reversible elastic instability. The mechanism has proved to be useful for controlled imprinting of complex patterns in phononic and photonic crystals. The material also provides an example of a simple, tunable and robust negative Poisson ratio foam. More recently, the inverse problem of an appropriate array of elastic particles has been shown to provide another example of an intriguing pattern switch. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:39PM |
Q1.00004: Resonant generation of internal waves on a model continental slope Invited Speaker: Away from shallow, well-mixed surface regions, the density of sea water increases with depth due to variation in salinity and temperature. This continuous density stratification supports \textit{internal} gravity waves, which are the counterpart within the fluid interior of \textit{surface} gravity waves. Internal gravity waves are import for many oceanic processes, such as sediment transportation and ocean mixing. We study internal wave generation in a laboratory model of oscillating tidal flow on a continental margin. Waves are found to be generated only in a near-critical region where the slope of the bottom topography matches that of internal waves. Fluid motion with a velocity an order of magnitude larger than that of the forcing occurs within a thin boundary layer above the bottom surface. The resonant wave is unstable because of strong shear; Kelvin-Helmholtz billows precede wave breaking. We construct a model to extrapolate our results to oceanic conditions. This work [1] provides a new explanation for the intense boundary flows on continental slopes. \\[4pt] [1] H. P. Zhang, B. King and Harry L. Swinney, Phys. Rev. Lett. 100, 244504 (2008). [Preview Abstract] |
Session Q2: New Insights into Hidden Order in URu2Si2
Sponsoring Units: DCMPChair: John Mydosh, Leiden University
Room: Spirit of Pittsburgh Ballroom BC
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q2.00001: Sleuthing Hidden Order in $URu_2Si_2$ Invited Speaker: In this talk, I will provide an overview of recent experimentally-driven advances in our understanding of the hidden order in $URu_2Si_2$; constraints and implications for future theoretical work will be discussed. State-of-the-art pressure, neutron and transport measurements [1] on this material have led to the confirmation of the phase diagram proposed on theoretical grounds for $URu_2Si_2$ several years ago [2], while recent neutron [3], ARPES and STM studies provide growing evidence for the formation of a density wave of unknown character [2]. I will also describe the challenge of linking the observed excitations to the underlying hidden order, and will discuss ongoing theoretical advances in this direction.\\[4pt] References:\\[0pt] [1] A. Villaume et al., arXiv:0805.0672.\\[0pt] [2] P. Chandra et al., Nature 417, 831 (2002); V. Tripathi et al., J. Phys. Cond. Mat. 17, 5285 (2005).\\[0pt] [3] C.R. Wiebe et al., Nature Physics 3, 96 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:27PM |
Q2.00002: ARPES Clues to the Hidden Order in URu$_{2}$Si$_{2}$ Invited Speaker: The three-dimensional electronic structure of UHV-cleaved URu$_{2}$Si$_{2}$ is investigated using photon-dependent angle-resolved photoemission (ARPES). Wide angle Fermi-surface (FS) maps as well as high-resolution spectroscopy focused on key high symmetry points reveal high U 5$f$ spectral weight at the hole-like regions of the $\Gamma $ and Z-points. The small hole-surface FS topologies have good size correspondence to dHvA FS orbit frequencies, but do not agree well with LDA band structure calculations. More favorable correspondence of the URu$_{2}$Si$_{2}$ ARPES is made to LDA+DMFT calculations as well as to detailed ARPES measurements of 5$f^{0}$ ThRu$_{2}$Si$_{2}$. Special attention was given to spatial-dependent characterization of the cleave surface in order to understand the possible cleave terminations and to avoid surface effects related to disorder or non-bulk coordinated U-termination. Theoretical surface slab calculations assist in identifying surface-termination related features at the X-point. In addition, we propose a model for the incommensurate nesting vectors, 0.6a* and 1.4a*, observed by inelastic neutron scattering\footnote{C. Wiebe \textit{et al}., Nat Phys. \textbf{3}, 96 (2007)} to be characteristic of the hidden order phase of URu$_{2}$Si$_{2}$. Finally, preliminary ARPES results for URu$_{2-x}$Re$_{x}$Si$_{2}$ give a clue as to the mechanism by which Re doping suppresses the hidden order phase in favor of ferromagnetism. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 1:03PM |
Q2.00003: Neutron Scattering Studies of Hidden Order in URu$_2$Si$_2$ Invited Speaker: The heavy fermion superconductor URu$_2$Si$_2$ has held the attention of physicists for the last two decades due to the presence of a mysterious hidden order phase below 17.5 K. Previous neutron scattering measurements indicate that the ordered moment is 0.03 {$\mu_{B}$}~, much too small to account for the large heat capacity anomaly at 17.5 K. We present recent neutron scattering experiments which unveil a new piece of this puzzle - the spin excitation spectrum above 17.5 K exhibits well-correlated, \emph{itinerant}-like spin excitations up to at least 10 meV emanating from incommensurate wavevectors. The gapping of these excitations corresponds to a large entropy release and explains the reduction in the electronic specific heat through the transition. We also present new neutron scattering data linking the spin excitations to Fermi surface instabilities, and discuss the remaining candidates for the identity of the hidden order phase. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:39PM |
Q2.00004: Competition of Hidden Order and Antiferromagnetism in URu$_2$Si$_2$ under Pressure Invited Speaker: URu$_2$Si$_2$ is a heavy fermion compound with an ordered phase below $T_0=17.5$~K at ambient pressure. The signature of the transition at $T_0$ in macroscopic quantities is very large and indicates a partial gap opening on the Fermi surface. However, the order parameter could not be identified yet and therefore, the phase is called hidden order (HO). Additionally, the compound becomes superconducting below $T_{sc}=1.4$~K. Here, we\footnote{Collaborators : A. Villaume, G. Knebel, F. Bourdarot, V. Taufour, S. Raymond, J. Flouquet} focus on the difference between HO and the pressure induced antiferromagnetic state (AF) in order to shed light on the HO itself. By specific heat and resistivity measurements under pressure\footnote{E. Hassinger {\em et al.}, Phys. Rev. B \textbf{77}, 115117 (2008)}, we were able to confirm the pressure-temperature phase diagram determined by neutron scattering\footnote{H. Amitsuka {\em et al.}, J. Magn. Magn. Mater. \textbf{310}, 214 (2007)}. For pressures higher than $P_c=0.5$~GPa the antiferromagnetic phase develops and superconductivity is suppressed at the same time. The transition line between HO and AF can be seen as a small anomaly in resistivity and specific heat data until 1.3~GPa, where it seems to join the transition line between the paramagnetic and the HO phase. The nesting-like signature at $T_0$ in resistivity surprisingly does not change qualitatively between low pressures at the transition to HO and high pressures at the transition to the AF. The differences in the low energy excitations between the HO and AF phases have been investigated by neutron scattering measurements at 0.67~GPa\footnote{A. Villaume {\em et al.}, Phys. Rev. B \textbf{78}, 012504 (2008)}, where three phases can be detected on cooling: paramagnetic, HO and AF phase. The inelastic response at the antiferromagnetic wavevector $Q_0=(1,0,0)$ and at the position of the second minimum in the dispersion relation $Q_1=(1.4,0,0)$ was measured in the three distinct phases. The sharp excitation at $Q_0$ with a gap of 1.8~meV exists only in the hidden order phase and disappears in the antiferromagnetic phase whereas the excitation at $Q_1$ persists in both phases. Therefore only the excitations at the commensurate wavevector $Q_0$ are characteristic of the HO phase. [Preview Abstract] |
Session Q3: Physics and Culture
Sponsoring Units: DMPChair: Charles Falco, University of Arizona
Room: 301/302
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q3.00001: Computerized Comparison and Analysis of Vincent van Gogh's Painting Brushstrokes Invited Speaker: With advanced digitization techniques, museums have routinely begun to assemble vast digital libraries of images of their collections. These images can be analyzed by computers to assist art historians for a number of tasks. In our work, we focus on three challenges: artist identification, dating of an art work, and finding distinguishing features among artists. Two complementary approaches were taken: (1) the analysis of the geometric statistics based on the extracted individual brushstroke, and (2) the modeling of overall brushstroke texture. These approaches aim at assisting art historians in comparing a painting or parts of a painting to a group of paintings based on multiple criteria. Statistical methods have been used to compare groups of paintings. Each painting image is divided into subimages. Individual brushstrokes are segmented automatically. Geometric features, including the curvature, the overall orientation, and the size, are computed for each brushstroke. We also compute the features representing the interactivity of the brushstrokes extracted. The statistics, including average and standard deviation, of those features are used to model certain aspects of the artist's brushstrokes. For capturing the local brushstroke texture, we first apply a wavelet transform to the image. A spatial model, the 2-D hidden Markov model, is used to model the texture features of each subimage. The methods have shown to be able to distinguish van Gogh paintings and non van Gogh paintings to a great extent. The techniques can provide clues for the dating of van Gogh paintings. A comparison of the van Gogh paintings, Monticelli's paintings, and paintings by contemporary artists provides insights on their similarities and differences. The analysis has provided numerical statistics for further studying these and other paintings. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:27PM |
Q3.00002: Prime-Time Science: The Beginning of a Beautiful Friendship? Invited Speaker: The portrayal of science has often posed a challenge to Hollywood. Though it has provided some of the compelling storylines, the many complexities of science (and scientists) have confounded even the most talented writer or director, pitting creative license against scientific accuracy again and again. Likewise, the scientific community has struggled to find a conduit through which it can communicate its story to the general public on a mass scale. To bridge this gap, the National Academy of Sciences has partnered with leading Hollywood professionals to launch The Science and Entertainment Exchange; its mission is to connect entertainment industry professionals with top scientists from across the country to foster creative collaborations. The timing has never been better: there is a plethora of network TV shows currently on the air with science-based themes and plot lines, and a renewed interest -- even with science fiction films and TV -- in coming up with more plausible futuristic scenarios. Working together, the two communities can create a ``win-win'' synergy between accurate science and engaging entertainment. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 1:03PM |
Q3.00003: Hollywood Science: Good for Hollywood, Bad for Science? Invited Speaker: Like it or not, most science depicted in feature films is in the form of science fiction. This isn't likely to change any time soon, if only because science fiction films are huge moneymakers for Hollywood. But beyond that, these films are a powerful cultural force. They reach millions as they depict scientific ideas from DNA and cloning to space science, whether correctly or incorrectly; reflect contemporary issues of science and society like climate change, nuclear power and biowarfare; inspire young people to become scientists; and provide defining images -- or stereotypes -- of scientists for the majority of people who've never met a real one. Certainly, most scientists feel that screen depictions of science and scientists are badly distorted. Many are, but not always. In this talk, based on my book \textit{Hollywood Science }[1], I'll show examples of good and bad screen treatments of science, scientists, and their impact on society. I'll also discuss efforts to improve how science is treated in film and ways to use even bad movie science to convey real science. \\[4pt] [1] Sidney Perkowitz, \textit{Hollywood Science: Movies, Science, and the End of the World} (Columbia University Press, New York, 2007). ISBN: 978-0231142809 [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:39PM |
Q3.00004: The Physics of the Blues Invited Speaker: In looking at the commonalities between music and science, one sees that the musician's palette is based on the principles of physics. The pitch of a musical note is determined by the frequency of the sound wave. The scales that musicians use to create and play music can be viewed as a set of rules. What makes music interesting is how musicians develop those rules and create ambiguity with them. I will discuss the evolution of western musical scales in this context. As a particular example, ``Blue'' notes are very harmonic notes that are missing from the equal temperament scale. The techniques of piano blues and jazz represent the melding of African and Western music into something totally new and exciting. Live keyboard demonstrations will be used. Beyond any redeeming entertainment value the talk will emphasize the serious connections between science and art in music. Nevertheless tips will be accepted. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 2:15PM |
Q3.00005: Joys and Pain in Making a Science Movie Invited Speaker: This talk will describe the joys and pains in making a multiple award winning science movie: ``When Things Get Small.'' We found that in order to reach a wide public of non experts, the best approach is to develop a collaboration between a scientist and a TV producer. In order to keep scientific accuracy and at the same time maintain public interest it is crucial to keep the message well defined and crisp and not dwell on too many details. The creative process, coming from these two different cultures, makes this a very rewarding experience. However, there are also ``pains'' associated with it which will be described. Movie available at http://ischuller.ucsd.edu/movies/movies.php or http://uctv.tv/getsmall/ movie was produced in collaboration with R. Wargo and UC-TV(http://www.ucsd.tv/getsmall/) and had the contribution from many other movie professionals. [Preview Abstract] |
Session Q4: Polymer Surface Instabilities
Sponsoring Units: DPOLY GSNPChair: Alfred Crosby, University of Massachusetts Amherst
Room: 306/307
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q4.00001: Elastic instabilities in rubber Invited Speaker: Materials that undergo large elastic deformations can exhibit novel instabilities. Several examples are described: development of an aneurysm on inflating a rubber tube; non-uniform stretching on inflating a spherical balloon; formation of internal cracks in rubber blocks at a critical level of triaxial tension or when supersaturated with a dissolved gas; surface wrinkling of a block at a critical amount of compression; debonding or fracture of constrained films on swelling, and formation of ``knots'' on twisting stretched cylindrical rods. These various deformations are analyzed in terms of a simple strain energy function, using Rivlin's theory of large elastic deformations, and the results are compared with experimental measurements of the onset of unstable states. Such comparisons provide new tests of Rivlin's theory and, at least in principle, critical tests of proposed strain energy functions for rubber. Moreover the onset of highly non-uniform deformations has serious implications for the fatigue life and fracture resistance of rubber components. \\[4pt] References: \\[0pt] R. S. Rivlin, Philos. Trans. Roy. Soc. Lond. Ser. A241 (1948) 379--397. \\[0pt] A. Mallock, Proc. Roy. Soc. Lond. 49 (1890--1891) 458--463. \\[0pt] M. A. Biot, ``Mechanics of Incremental Deformations'', Wiley, New York, 1965. \\[0pt] A. N. Gent and P. B. Lindley, Proc. Roy. Soc. Lond. A 249 (1958) 195--205. \\[0pt] A. N. Gent, W. J. Hung and M. F. Tse, Rubb. Chem. Technol. 74 (2001) 89--99. \\[0pt] A. N. Gent, Internatl. J. Non-Linear Mech. 40 (2005) 165--175. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:27PM |
Q4.00002: Creasing instability of solvent-swelled polymer films Invited Speaker: A thin layer of polymer bound to a rigid substrate develops compressive stresses when it is swelled by solvent, due to the constraint against lateral expansion imposed by the substrate. For sufficiently large stresses, the surface becomes unstable to a buckling mode in which tightly-folded ``creases'' form on the surface to relieve compressive stress. While this instability has been known in practice for more than a century, it remains poorly characterized and incompletely understood. I will describe experiments on model systems of surface-attached hydrogels to characterize the onset and growth mechanisms of creases, as well as methods that allow control of crease formation in both space and time. In addition to the implications that this instability has for any type of polymeric coating undergoing swelling, it also provides an opportunity to create surfaces with switchable topography and chemistry. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 1:03PM |
Q4.00003: The macroscopic delamination of thin films from elastic substrates Invited Speaker: The wrinkling and delamination of stiff thin films adhered to a polymer substrate have important applications in ``flexible electronics.'' The resulting periodic structures, when used for circuitry, have remarkable mechanical properties since stretching or twisting of the substrate is mostly accommodated through bending of the film, which minimizes fatigue or fracture. To date, applications in this context have used patterning of the substrate-film adhesion energy to produce a controlled array of delamination ``blisters.'' However, even in the absence of such patterning, blisters have a characteristic size. We use macroscopic experiments to study what sets the dimensions of these blisters in terms of the material properties, which we explain using a combination of scaling and analytical methods. This points to a novel method for determining the interfacial toughness. Finally, we suggest a number of design guidelines for the thin films used in flexible electronic applications. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:39PM |
Q4.00004: Elastic Instability and Pattern Formation in Confined Soft Elastomeric Films Invited Speaker: When a rigid flat object or a flexible plate is removed from a thin soft film, instability patterns appear at the interface in the form of bubbles or fingers. The wavelengths of these instabilities are independent of all material and geometrical properties of the system except the thickness of the film. These observations contrast the classical Saffman-Taylor type instability in which the instability pattern depends on the viscous and surface tension forces in addition to the thickness of the liquid film. In the case of elastic instability of the kind described here, the wavelength depends on the material properties of the films only when soft films of different elastic properties are separated from each other. In the later case, a co-operative instability mode develops, which is a non-linear function of the thicknesses and the elastic moduli of both the films. In contrast to the wavelengths of these instabilities, their amplitudes are strong functions of several material and geometric properties of the system. These problems can be analyzed using regular perturbation technique to obtain the excess deformations of the film over and above the base quantities. Furthermore, by estimating the excess energy of the system, it can be shown that instability develops when the films are critically confined. This point can be illustrated by pre-stretching the film or simply by adjusting the contact width between the film and the plate. The instabilities that develop at the interface are critical to understanding adhesion and friction of soft thin films as they act like nucleated interfacial cracks. We performed a simple experiment, in which a flat rigid glass prism is sheared off a soft elastomeric film. At a given tangential force, the prism starts to slide on the elastomeric film accompanied with the formation of bubbles at the interface due to elastic instability. These bubbles, the lateral dimensions of which are comparable to the thickness of the film, move across the interface with speeds 1000 times faster than the overall sliding speed of the prism. The process continues till the critical condition for fracture is reached. These studies may shed some light on the fast dynamics of shear crack propagation in other systems. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 2:15PM |
Q4.00005: Micro-Origami: Elastic Instability of Polymer Films Invited Speaker: Upon compression, thin (rigid) elastic membranes supported on a soft elastic solid spontaneously deviate from their flat geometry by forming regular wrinkles. How can we control the wavelength and the symmetry of the wrinkle patterns? What is the influence of defects? Why should we observe focalisation of deformation in folds for large strains? During my talk, I will review our approaches to tackle these questions. [Preview Abstract] |
Session Q5: Plasmonics in Future Electronics
Sponsoring Units: FIAPChair: Boris Luk`yanchuk, Agency for Science, Technology and Research
Room: 401/402
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q5.00001: A Technique for Nanoscale Plasmonic Imaging via Photoemission Invited Speaker: The scientific community is witnessing increased research activity on Surface Plasmon Polaritons (SPPs). The potential applications of SPPs and plasmonic structures based on their control and manipulation are truly multi-disciplinary, spanning high speed nano-scale interconnects, meta-materials, chemical and biological sensing, sub-wavelength optics and waveguides, near-field optical trapping, high-density data storage, and the enhancement of non-linear effects. Measurement of the localized optical field intensity is a critical component in validating physical models and characterizing plasmonic structures. The dominant technique employed for this task is the Scanning Near-Field Optical Microscope (SNOM) or Photon Scanning Tunneling Microscope (PSTM), whose contrast mechanism is based on measuring light scattered from the near-field with a probe. These techniques can provide high resolution images of the localized fields, but they are slow. Furthermore, tip-sample interactions can perturb the fields, yielding ambiguity between electric and magnetic fields and frustrating attempts at accurate optical characterization. One way to facilitate the advance of plasmonics is to develop new techniques for imaging and characterizing SPP behavior on the nanoscale. Recent efforts employing photoemission to reveal the localized fields have demonstrated that this technique can provide both high spatial ($\sim$10nm) and temporal (fs) resolution when combined with a Photoelectron Emission Microscope (PEEM)[1-3]. The PEEM does not require a probe so the fields can be imaged without perturbation. It also provides a parallel image of the full field, so acquisition times are fast. We are expanding the capabilities of the PEEM to exploit a novel contrast mechanism which will broaden the spectrum of plasmonic devices observable. We present our experimental efforts in this area, detail the underlying physics of the contrast mechanism and discuss how it can be controlled to enable unique spatial and temporal information on the propagation of SPPs within plasmonic structures. \\[4pt] [1] M. Cinchetti, A. Gloskovskii, S. A. Nepjiko, G. Schonhense, H. Rochholz and M. Kreiter, PRL 95*, *047601 (2005) \\[0pt] [2] Atsushi Kubo, Ken Onda, Hrvoje Petek, Zhijun Sun, Yun S. Jung, and Hong Koo Kim, Nano Letters, 2005, Vol. 5, No. 6, 1123-1127 \\[0pt] [3] M. Stockman, M. Kling, U. Kleineberg, F. Krausz, Nature photonics, VOL 1, Sept 2007, 539-544 [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:27PM |
Q5.00002: Ultrafast and Quantum Nanoplasmonics: SPASER and Control Invited Speaker: Nanoplasmonics is presently experiencing a period of unprecedented growth and has numerous applications. These include sensing and detection of minute amount of chemical and biological objects for biomedicine and defense [1], near-field scanning optical microscopy [2], immunological tests, labels for biomedical research, nanoantennas for efficient coupling of light to semiconductor devices, etc. Nanoplasmonics still greatly needs active elements to generate optical energy on the nanoscale and serve as amplifiers. We have proposed a quantum nanoplasmonic generator and amplifier of the local optical fields, SPASER [surface plasmon amplification by stimulated emission (of radiation)]. [3-5]. A SPASER is analogous to laser except that light (photons) is replaced by local optical fields (surface plasmons). This is responsible for the principal difference: laser cavity must support photonic modes and its size is on order or much greater than the optical wavelength, cf. [6]. In contrast, the surface plasmons in the spaser are purely electric oscillations whose localization size is nanometric. SPASER will transform nanoplasmonics the same way as the laser transformed optics. In particular nanoplasmonic processors working at THz operation rates will become possible. Another important area is the active control of nanoplasmonic phenomena. One approach to it is coherent control, where a shaped optical pulse dynamically, on the femtosecond scale controls the nanoscale distribution of local fields [7-12]. \newline \newline \textbf{References \newline }[1] J. N. Anker\textit{ et al.}, Nature Materials \textbf{7}, 442 (2008). \newline [2] L. Novotny, and B. Hecht, \textit{Principles of nano-optics }(Cambridge University Press, Cambridge, New York, 2006). \newline [3] D. J. Bergman, and M. I. Stockman, Phys. Rev. Lett. \textbf{90}, 027402 (2003). \newline [4] K. Li\textit{ et al.}, Phys. Rev. B \textbf{71}, 115409 (2005). \newline [5] M. I. Stockman, Nature Photonics \textbf{2}, 327 (2008). \newline [6] M. T. Hill\textit{ et al.}, Nature Photonics \textbf{1}, 589 (2007). \newline [7] M. I. Stockman, S. V. Faleev, and D. J. Bergman, Phys. Rev. Lett. \textbf{88}, 67402 (2002). \newline [8] M. I. Stockman, D. J. Bergman, and T. Kobayashi, Phys. Rev. B \textbf{69}, 054202 (2004). \newline [9] M. I. Stockman, and P. Hewageegana, Nano Lett. \textbf{5}, 2325 (2005). \newline [10] M. Durach\textit{ et al.}, Nano Lett. \textbf{7}, 3145 (2007). \newline [11] M. I. Stockman, and P. Hewageegana, Appl. Phys. A \textbf{89}, 247 (2007). \newline [12] X. Li, and M. I. Stockman, Phys. Rev. B \textbf{77}, 195109 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 1:03PM |
Q5.00003: Magnetic Light Emitters: Plasmon-enhanced Magnetic Dipole Transitions Invited Speaker: Over the past decade, advances in both negative index metamaterials and resonant optical antennas have challenged traditional assumptions about light-matter interactions. While metamaterials research has shown that metallic structures can be engineered to support strong optical frequency magnetic resonances, resonant optical antennas have been designed to amplify and re-direct the emission from electric dipole emitters. In this talk, we explore the intersection of these distinct fields and investigate how resonant optical effects may be used to challenge the electric dipole approximation. Specifically, we will show how Purcell effects may be used to enhance the natural optical frequency magnetic dipole transitions in Lanthanide ions. We will present experimental and numerical results that demonstrate enhanced magnetic dipole emission from trivalent Europium ions near metallic films and nanoparticle composites. We will explore how the varying symmetries of electric and magnetic dipoles can be used to characterize and optimize magnetic light emission. Finally, we will discuss the implications of enhancing and controlling higher-order optical transitions for optical spectroscopy and photonic devices. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:39PM |
Q5.00004: Plasmonic nano-circuitry Invited Speaker: Photonic components are superior to electronic ones in terms of operational bandwidth but suffer from the diffraction limit that constitutes a major problem on the way towards miniaturization and high density integration of optical circuits. The degree of light confinement in dielectric structures, including those based on the photonic band-gap effect, is fundamentally limited by the light wavelength in the dielectric used. The main approach to circumvent this problem is to take advantage of hybrid nature of surface plasmons (SPs) whose subwavelength confinement is achieved due to very short (nm-long) penetration of light in metals. After briefly reviewing various SP guiding configuration the results of our investigations of subwavelength photonic components utilizing SP modes propagating along channels cut into gold films are overviewed [Nature \textbf{440}, 508 (2006); Nano Lett. \textbf{7}, 880 (2007)], demonstrating first examples of \textit{ultracompact} plasmonic components that pave the way for a new class of integrated optical circuits [Physics Today, May 2008, pp.44-50]. Recent results on the SP guiding along gold wedges at telecom wavelengths are also presented [Opt. Express \textbf{16}, 5252 (2008)]. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 2:15PM |
Q5.00005: Field enhancement by Plasmonic Nanostructures Invited Speaker: Plasmonics is a fascinating research theme undergoing rapid evoluations in recent years that focus on the study of light interaction with metallic nanostructures involving collective electron oscillations (plasmon). An exciting goal of plasmonics is to devleop fully integrated electro-optical nano-circuits in which photonics and electronics merge at nanoscale dimensions. Field localization, i.e., strongly enhanced optical hot spots in nanoscale near nanostructures, is one of the key features in light interaction with plasmonic nanostructures. In this talk, I will discuss the nanoscale hotspots induced by light interaction with several different types of plasmonic nanostructures such as isolated/chained nanoparticles, tips, and nanowires. Besides the intensity field information, I will also present the energy flows information of the hotspots (Poynting vector) which illustrates the formation of field enhancement effect in a more intuitive manner. Many interesting energy flows such as saddle, centre and vortex flows are seen in the near-field of the plasmonic nanostructures. Lastly, I will briefly discuss the possible thermal influence of these hotspots on the performance and operation of future integrated plasmonic/electronics nano-circuits. [Preview Abstract] |
Session Q6: Physics Demonstrations and Strategies for Teaching and Public Outreach
Sponsoring Units: FEdChair: Ernie Malamud, University of Nevada, Reno
Room: 406
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q6.00001: So, You Want to be a Science Communicator? Invited Speaker: The late Carl Sagan opined that somehow we have managed to create a global civilization dependant on science and technology in which almost no one understands science and technology. This is an unacceptable recipe for disaster with social, political and financial implications for the future of scientific research. And so, like it or not, popular science communication, more than ever before, is an important and necessary part of the scientific enterprise. Public outreach programs, media interviews, and popular articles have become required parts of the scientist's professional repertoire. But, what does it take to be a good science communicator? What is needed to develop and deliver meaningful public outreach programs? How do you handle non-technical presentations? And, what help is available in developing the necessary skills for good popular science communication? This presentation will look at the essential components of effective science communication aimed at a broad public audience. The components of successful science communication in programs, presentations and articles will be discussed. Specific attention will be given to how university-museum partnerships can expand the reach and enhance the quality of public outreach programs. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:27PM |
Q6.00002: Public Outreach for the International Year of Astronomy Through Faculty and Science Center Partnerships Invited Speaker: The International Year of Astronomy 2009 provides an opportunity to jump-start public education and outreach programs and to engage the community in a fascinating field. In my talk I will discuss a diverse program of education and outreach designed and implemented as a collaborative effort between the Astronomy faculty at the University of Pittsburgh and the Carnegie Science Center. I will highlight some of the unique benefits of such a partnership and some of the unique events such a partnership enables. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 1:03PM |
Q6.00003: Public Education and Outreach Through Full-Dome Video Technology Invited Speaker: My long-term goal is to enhance public understanding of complex systems that can be best demonstrated through richly detailed computer graphic animation displayed with full-dome video technology. My current focus is on health science advances that focus on regenerative medicine, which helps the body heal itself. Such topics facilitate science learning and health literacy. My team develops multi-media presentations that bring the scientific and medical advances to the public through immersive high-definition video animation. Implicit in treating the topics of regenerative medicine will be the need to address stem cell biology. The topics are clarified and presented from a platform of facts and balanced ethical consideration. The production process includes communicating scientific information about the excitement and importance of stem cell research. Principles of function are emphasized over specific facts or terminology by focusing on a limited, but fundamental set of concepts. To achieve this, visually rich, biologically accurate 3D computer graphic environments are created to illustrate the cells, tissues and organs of interest. A suite of films are produced, and evaluated in pre- post-surveys assessing attitudes, knowledge and learning. Each film uses engaging interactive demonstrations to illustrate biological functions, the things that go wrong due to disease and disability, and the remedy provided by regenerative medicine. While the images are rich and detailed, the language is accessible and appropriate to the audience. The digital, high-definition video is also re-edited for presentation in other ``flat screen'' formats, increasing our distribution potential. Show content is also presented in an interactive web space (www.sepa.duq.edu) with complementing teacher resource guides and student workbooks and companion video games. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:39PM |
Q6.00004: A New Approach to A Science Magnet School - Classroom and Museum Integration Invited Speaker: The Pittsburgh Science {\&} Technology Academy is a place where any student with an interest in science, technology, engineering or math can develop skills for a career in life sciences, environmental sciences, computing, or engineering. The Academy isn't just a new school. It's a new way to think about school. The curriculum is tailored to students who have a passion for science, technology, engineering or math. The environment is one of extraordinary support for students, parents, and faculty. And the Academy exists to provide opportunities, every day, for students to Dream. Discover. Design. That is, Academy students set goals and generate ideas, research and discover answers, and design real solutions for the kinds of real-world problems that they'll face after graduation. The Academy prepares students for their future, whether they go on to higher education or immediate employment. This talk will explain the unique features of the Pittsburgh Science {\&} Technology Academy, lessons learned from its two-year design process, and the role that the Carnegie Museums have played and will continue to play as the school grows. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 2:15PM |
Q6.00005: Fractured Physics and the Great Color Caper: Large Scale Physics Outreach Invited Speaker: |
Session Q7: Physics of the Immune System
Sponsoring Units: DBPChair: Aaron Dinner, University of Chicago
Room: 407
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q7.00001: How T lymphocytes see antigen Invited Speaker: Complex organisms, like humans, have an adaptive immune system that enables us to do battle with diverse pathogens. This flexible system can also go awry, and many diseases are the direct consequence of the adaptive immune system failing to discriminate between markers of self and non-self. The orchestrators of adaptive immunity are a class of cells called T lymphocytes (T cells). T cells recognize minute numbers of molecular signatures of pathogens, and T cell recognition of these molecular markers of non-self is both specific and degenerate. The specific (yet, cross-reactive), diverse, and self-tolerant T cell repertoire is designed in the thymus. I will describe how an approach that brings together theoretical and computational studies (rooted in statistical physics) with experiments (carried out by key collaborators) has allowed us to shed light on the mechanistic principles underlying how T cells respond to pathogens in a digital fashion (``on'' or ``off''), and how this molecular machinery coupled with frustration (a la spin glasses) plays a key role in designing the special properties of the T cell repertoire during development in the thymus. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:27PM |
Q7.00002: Physical Theory of Vaccine Design for Influenza and Dengue Fever Invited Speaker: The immune system normally protects the human host against death by infection. I will introduce a physical theory of the evolutionary dynamics that occurs in the antibody-mediated and T cell-mediated immune responses. The theory will be used to provide a mechanism for original antigenic sin, wherein an initial exposure to antigen can degrade the response of the immune system upon subsequent exposure to related, but different, antigens. A new order parameter to characterize antigenic distance will be introduced from the theory. This order parameter predicts effectiveness of the influenza vaccine more reliably than do results from animal model studies currently used by world health authorities. I will discuss how this order parameter might be a valuable new tool for making vaccine-related public health policy decisions. Next, I will briefly discuss dengue fever. Infection with, or vaccination against, one of the four serotypes of dengue fever typically increases susceptibility to dengue hemorrhagic fever from one of the other three serotypes. I will present a physical theory of this immunodominance and use this theory to quantify the predicted mitigation of immunodominance in a novel formulation of the dengue vaccine. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 1:03PM |
Q7.00003: Fast, continuous recirculation of germinal center B cell populations enhances robustness of immune response towards varying pathogens Invited Speaker: Germinal centers (GCs) are dynamic microstructures that form in lymphatic tissues during immune responses. There, B cells undergo rapid proliferation and mutation of their B cell receptors (BCRs). Selection of B cells bearing BCRs that bind to the pathogen causing the immune response ultimately leads to BCRs that, when secreted as antibodies, form a new, effective, and pathogen specific antibody repertoire. However, the details of this evolutionary process are poorly understood, since currently available experimental techniques do not allow for direct observation of the prevailing mechanisms [Or-Guil et al., Imm.Rev. 2007]. Based on optimality considerations, we put forward the assumption that GCs are not isolated entities where evolutionary processes occur independently, but interconnected structures which allow for continuous exchange of B cells. We show that this architecture leads to a system whose response is much more robust towards different antigen variants than a set of independently working GCs could ever be. We test this hypothesis by generating our own experimental data (time course of 3-D volume distribution of GCs, analysis of high-throughput BCR sequences), and show that available data is consistent with the outlined hypothesis. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:39PM |
Q7.00004: Fish Immunology Invited Speaker: |
Wednesday, March 18, 2009 1:39PM - 2:15PM |
Q7.00005: Genetic Circuit Architectures Underlying Cell Fate Choices for Immunity Invited Speaker: Antigen stimulated B cells follow an unusual developmental trajectory that transiently passes through a germinal center state, which promotes receptor affinity maturation and immunoglobulin class switching, before terminally differentiating into antibody secreting plasma cells. It was found that graded expression of the transcription factor IRF-4 regulates cell fate, but the relationship between antigen receptor signaling, the network of interactions with IRF-4, and cell fate was not known. This talk describes models that link ligand-receptor avidity with cell fate. The models have been validated experimentally by directly varying the levels and kinetics of IRF-4 accumulation. Furthermore, signaling through the antigen receptor is demonstrated to control the expression of IRF-4 and in turn the frequency of B cells that undergo class switching before differentiating into plasma cells. These findings provide an explanation for experiments that measure B cell numbers in transgenic mice. The architecture of our regulatory circuit provides a general mechanism for quantitative variations in a signal to be translated into a binary cell-fate choice involving transient expression of one of the two developmental fates. In collaboration with Aryeh Warmflash, Ying Li, Roger Sciammas, and Harinder Singh, The University of Chicago. [Preview Abstract] |
Session Q8: Quantum Spin Dynamics and Relaxation in Molecular Magnets
Sponsoring Units: GMAGChair: Andrew Kent, New York University
Room: 414/415
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q8.00001: Coherent Manipulation and Decoherence of S=10 Single-Molecule Magnets Invited Speaker: A single crystal of high-spin single-molecule magnets (SMMs) is an attractive testbed for quantum science and technologies. High-spin SMMs are suitable for applications to dense quantum memory and computing devices. Because SMM clusters are identical and interact weakly, the ensemble properties of single crystals of SMMs reflect the properties of a single cluster. However coherent manipulation of high-spin SMM crystals has never been demonstrated due to strong spin decoherence. For spins in the solid state, an interaction with fluctuations of surrounding spin bath is a major source of spin decoherence. One approach to reduce spin bath fluctuations is to bring the spin bath into a well-known quantum state that exhibits little or no fluctuations. A prime example is the case of a fully polarized spin bath. In diamond, spin decoherence has been quenched using high-frequency pulsed electron paramagnetic resonance (EPR) [1]. We present coherent manipulation and decoherence of a single-crystal of S=10 Fe8 SMMs. Through polarizing a spin bath in Fe8 single-molecule magnets at 4.6 T and 1.3 K, we demonstrate that spin decoherence is significantly suppressed to extend the spin decoherence time ($T_{2})$ up to 700 ns [2]. Investigation of temperature dependence of spin relaxation times reveals the nature of spin decoherence. This work is collaboration with J. van Tol, C. C. Beedle, D. N. Hendrickson, L.-C. Brunel, and M. S. Sherwin.\\[4pt] [1] S. Takahashi, R. Hanson, J. van Tol, M. S. Sherwin, and D. D. Awschalom, \textit{Phys. Rev. Lett.} \textbf{101}, 047601 (2008).\\[0pt] [2] S. Takahashi, J. van Tol, C. C. Beedle, D. N. Hendrickson, L.-C. Brunel, and M. S. Sherwin, arXiv: 0810.1254. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:27PM |
Q8.00002: The Role of Antisymmetric Exchange on the Quantum Interference between States of Different Spin Length in a dimeric Molecular Nanomagnet. Invited Speaker: We report direct evidence of quantum oscillations of the \textit{total spin length} of a dimeric molecular nanomagnet through the observation of quantum interference associated with tunneling trajectories between states having different spin quantum numbers. As we outline, this is a consequence of the unique characteristics of a molecular Mn$_{12}$ wheel which behaves as a (weak) ferromagnetic exchange-coupled molecular dimer: each half of the molecule acts as a single-molecule magnet (SMM), while the weak coupling between the two halves gives rise to an additional internal spin degree of freedom within the molecule, namely that its total spin may fluctuate. This extra degree of freedom accounts for several magnetization tunneling resonances that cannot be explained within the usual giant spin approximation. More importantly, the observation of quantum interference provides unambiguous evidence for the quantum mechanical superposition involving entangled states of both halves of the wheel. Magnetization results obtained in two other versions of this compound, in which the ligands have been modified, show that slight variations of the relative distance between the Mn ions determine whether the molecule behaves as a rigid magnetic unit of spin $S$~=~7 or as two exchange-coupled halves of spin $S$~=~7/2. We analyze the effect of the Dzyaloshinskii-Moriya antisymmetric exchange interaction in a molecule with a centre of inversion symmetry and propose a formal model to account for the observed broken degeneracy that preserves the molecular inversion symmetry. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 1:03PM |
Q8.00003: Spin dynamics in the single molecule magnet Ni$_4$ under microwave irradiation Invited Speaker: Quantum mechanical effects such as quantum tunneling of magnetization (QTM) and quantum phase interference have been intensively studied in single molecule magnets (SMMs). These materials have also been suggested as candidates for qubits and are promising for molecular spintronics. Understanding decoherence and energy relaxation mechanisms in SMMs is then both of fundamental interest and important for the use of SMMs in applications. Interestingly, the single-spin relaxation rate due to direct process of a SMM embedded in an elastic medium can be derived without any unknown coupling constant [1]. Moreover, nontrivial relaxation mechanisms are expected from collective effects in SMM single crystals, such as phonon superradiance or phonon bottleneck. In order to investigate the spin relaxation between the two lowest lying spin-states of the $S=4$ single molecule magnet Ni$_4$, we have developed an integrated sensor that combines a microstrip resonator and micro-Hall effect magnetometer on a chip [2]. This sensor enables both real time studies of magnetization dynamics under pulse irradiation as well as simultaneous measurements of the absorbed power and magnetization changes under continuous microwave irradiation. The latter technique permits the study of small deviations from equilibrium under steady state conditions, i.e. small amplitude cw microwave irradiation. This has been used to determine the energy relaxation rate of a Ni$_4$ single crystal as a function of temperature at two frequencies, 10 and 27.8 GHz. A strong temperature dependence is observed below 1.5 K, which is not consistent with a direct spin-phonon relaxation process. The data instead suggest that the spin relaxation is dominated by a phonon bottleneck at low temperatures and occurs by an Orbach process involving excited spin-levels at higher temperatures [3]. Experimental results will be compared with detailed calculations of the relaxation rate using the density matrix equation with the relaxation terms in the universal form.\\ 1. E. M. Chudnovsky, D. A. Garanin and R. Schilling, Phys. Rev. B \textbf{72}, 094426 (2005)\\ 2. G. de Loubens \textit{et al.}, J. Appl. Phys. \textbf{101}, 09E104 (2007)\\ 3. G. de Loubens, D. A. Garanin, C. C. Beedle, D. N. Hendrickson and A. D. Kent, Europhys. Lett. \textbf{83}, 37006 (2008)\\ [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:39PM |
Q8.00004: Multiphoton Coherent Manipulation in Large Spin Qubits Invited Speaker: Manipulation of quantum information allows certain algorithms to be performed at unparalleled speeds. Photons are an ideal choice to manipulate qubits as they interact with quantum systems in predictable ways. They are a versatile tool for manipulating, reading/coupling qubits and for encoding/transferring quantum information over long distances. Spin-based qubits have well known behavior under photon driving and can be potentially operated up to room temperature. When diluted enough to avoid uncontrolled spin-spin interactions, a variety of spin qubits show long coherence times, $e.g.$ the nitrogen vacancies in pure diamonds (1,2), nitrogen atoms trapped in a C60 cage (3), Ho3+ and Cr5+ ions (4,5) and molecular magnets (6,7). We have used large spin Mn2+ ions (S=5/2) to realize a six level system that can be operated by means of single as well as multi-photon coherent Rabi oscillations (8). This spin system has a very small anisotropy whose effect can be tuned \textit{in-situ} to turn the system into a multi-level harmonic system. This offer new ways of manipulating, reading and resetting a spin qubit. Decoherence effects are strongly reduced by the quasi-isotropic electron interaction with the crystal field and with the 55Mn nuclear spins. \\[0pt] 1. R. Hanson \textit{et al.,} \textit{Science} \textbf{320}, 352 (2008). \\[0pt] 2. M.V. Gurudev Dutt \textit{et al.,} \textit{Science} \textbf{316}, 1312 (2007). \\[0pt] 3. G.W. Morley \textit{et al., Phys. Rev. Lett.} \textbf{98}, 220501 (2007). \\[0pt] 4. S. Bertaina \textit{et al.}, \textit{Nat. Nanotech.} \textbf{2}, 39 (2007). \\[0pt] 5. S. Nellutla \textit{et al.}, \textit{Phys. Rev. Lett.} \textbf{99}, 137601 (2007). \\[0pt] 6. A. Ardavan \textit{et al.,} \textit{Phys. Rev. Lett.} \textbf{98}, 057201 (2007). \\[0pt] 7. S. Bertaina \textit{et al.}\textbf{, }\textit{Nature} \textbf{453}, 203,(2008). \\[0pt] 8. S. Bertaina \textit{et al., submitted.} [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 2:15PM |
Q8.00005: Understanding electron and nuclear spin dynamics in Cr$^{5+}$ doped K$_{3}$NbO$_{8}$ Invited Speaker: Chromium(V) doped in the diamagnetic host potassium niobate, a simple spin $S=\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $, $I$ = 0 system, has been proposed as an alternative standard for field calibration and g-standard for high-field EPR [1]. This system constitutes a dilute two-level model relevant for use as a electron spin qubit [2] and as such coherent electron spin manipulation at X-band ($\sim $9.5 GHz) was observed over a wide range temperature. Rabi oscillations are observed for the first time in a spin system based on transition metal oxides up to room temperature. At 4 K, a Rabi frequency $\Omega _{R}$ of 20 MHz together with the phase coherence relaxation (spin-spin relaxation) time, $T_{2}$ of $\sim $10 $\mu $s results in the single qubit figure of merit $Q_{M}$ (=$\Omega _{R}T_{2}$/$\pi )$ as about 500, showing that a diluted ensemble of Cr(V)$ ($S = 1/2) doped K$_{3}$NbO$_{8}$ is a potential candidate for solid-state quantum information processing. Also, the field and temperature dependence of the $T_{1}$ (spin-lattice relaxation) and $T_{2}$ times was investigated [3] for a further understanding of the relaxation mechanisms governing the phase decoherence in this system. These studies show that the coupling of the electron spin with the neighboring $^{39}$K nuclei ($I$ = 3/2) is one of the prominent $T_{2}$ mechanisms. The hyperfine and quadrupole interactions with $^{39}$K nuclei was resolved by using the high-frequency (240 GHz) pulsed electron nuclear double resonance (ENDOR). \\[3pt] [1]. B. Cage, A. Weekley, L. -C. Brunel and N. S. Dalal, \textit{Anal. Chem.} \textbf{71}, 1951 \textbf{(}1999). \\[0pt] [2]. S. Nellutla, K.-Y. Choi, M. Pati, J. van Tol, I. Chiroescu and N. S. Dalal, \textit{Phys. Rev. Lett.} \textbf{99}, 137601 (2007). \\[0pt] [3]. S. Nellutla, G. W. Morley, M. Pati, N. S. Dalal and J. van Tol, \textit{Phys. Rev. B.} \textbf{78}, 054426 (2008). [Preview Abstract] |
Session Q9: Focus Session: Dynamics of Glassy Systems
Sponsoring Units: GSNPChair: David Grier, New York University
Room: 303
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q9.00001: Geometric interpretation of pre-vitrification in hard sphere glasses Invited Speaker: I will derive a geometric condition for the stability of hard sphere configurations, and I will show empirically that this condition is saturated in the glass phase. The marginal stability observed explains the presence of slow modes in the short term dynamics of the glass, and supports a microscopic interpretation for the rapid initial rise of the viscosity, or pre-vitrification, when the packing fraction increases. This observation also suggests a possible cause for the collective nature of the structural relaxation near the glass transition. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q9.00002: Aging of a Colloidal Suspension of Thermosensitive Particles Kyaw Win, Gregory McKenna, Tetsuharu Narita, Francois Lequeux, Srinivas Pullela, Zhengdong Cheng We have studied the aging behavior of a concentrated suspension of thermosensitive particles (PNIPAM).~ We found that the characteristic time of the dynamics of the system as a function of waiting time obeys the usual power law with exponent $\sim $ 1 both after shear melting the system and after a jump in temperature which is equivalent to a jump in concentration.~ We also report for the first time the observation of an extreme asymmetry of approach in the aging behavior with respect to down jump and up jump in temperature.~ While an asymmetry effect is already known in conventional glass formers such as polymers and small molecule liquids, it is both much more pronounced in the colloidal PNIPAM suspension and of a fundamentally different character. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q9.00003: Rejuvenation and memory in a 2D colloidal glass Jennifer M. Lynch, Zexin Zhang, Peter Yunker, Arjun G. Yodh, Eric R. Weeks We work with a 2D colloidal system that has a glass transition and use this system to experimentally observe memory and rejuvenation effects as the sample ages. In particular, we study a system of colloidal particles made of thermosensitive poly(N-isopropylacrylamide) (NIPA) polymer. The sample is confined in a narrow quasi-2D gap between parallel glass plates, which allows easy observation and rapid temperature response. Lowering temperature increases the size of the colloidal particles, which can induce the glassy state due to the crowding of the particles. When our colloidal sample is quenched into the glassy state, particle motion slows over time; this is aging. In molecular glasses, prior experiments studied how aging is modified when the temperature is changed while the sample ages, finding that aging at a first temperature $T_1$ and aging at a later time with second temperature $T_2$ are mostly independent. ``Memory'' relates to the case $T_1 < T_2$ and "rejuvenation" to the opposite case. Our colloidal system allows us to observe both of these effects. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q9.00004: A continuous time random walk description of the hopping dynamics in an aging polymer glass Mya Warren, Joerg Rottler Due to the non-equilibrium nature of the glassy state, structural relaxation becomes increasingly sluggish with the wait time $t_w$ since vitrification. As a result, dynamical correlation functions age, and often obey a simple rescaling with $t_w$: $C(t,t_w) = C_0(t) + C_{age}(t/t_w^{\mu})$. It has recently been shown that, to first order, scaling also applies to the distributions of local correlations and displacements (the van Hove function). In this study, we use molecular dynamics simulations to measure the statistics of the discontinuous hopping events that characterize structural relaxations during aging. This allows us to map the particle dynamics onto a continuous time random walk, which successfully reproduces the entire distribution of displacements. Our results bear a striking resemblance to the popular trap model of aging. We find that the hop time distribution takes the form of a power law which is independent of $t_w$, whereas the time to the first hop shifts to longer times with $t_w$. This two-timescale behavior explains not only the scaling of the distribution functions for times $t\sim t_w$, but also small deviations from perfect scaling that have been observed at longer times. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q9.00005: The Defect Diffusion Model and Isochoric Energy and Isobaric Enthalpy for Glass Formers Michael Shlesinger, John Bendler, John Fontanella, Mary Wintersgill The defect diffusion model produces stretched exponential relaxation, in supercooled liquids, through the sub-diffusive motion of defects. The aggregation of the defects produces a Vogel-Fulcher type law for the divergence of the time scale at a critical temperature. The model is employed to calculate the ratio of the apparent isochoric activation energy to the isobaric activation enthalpy, $E_{V}$*/$H*$ or $E_{V}$/$E_{P}$,. This ratio measures the relative sensitivity of kinetic processes to changes in volume and temperature respectively. This ratio equation is tested using dielectric relaxation data for poly(vinyl acetate), viscosity data for glycerol and ionic conductivity data for poly(propylene glycol) containing LiCF$_{3}$SO$_{3}$. Good agreement between theory and experiment is found using model parameters previously published. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q9.00006: Fluctuations in the relaxation of a strong glass Azita Parsaeian, Horacio E. Castillo, Katharina Vollmayr-Lee We present results of molecular dynamics simulations of amorphous silica, carried out by using the BKS inter-atomic potential. We quantify the evolution of fluctuations by studying the probability distributions of local observables such as individual particle displacements $\Delta x$ and local coarse grained intermediate scattering functions $C_r$. We test for universality by comparing the probability distributions with those in small molecule glasses and in polymer glasses. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q9.00007: A percolation model for dynamics in glass-forming materials Gregg Lois, Jerzy Blawzdziewicz, Corey O'Hern We characterize the glass to liquid transition as a percolation of mobile regions in configuration space. We find that many hallmarks of glassy dynamics, for example stretched-exponential response functions and a diverging structural relaxation time, result from critical properties of mean-field percolation. Specific predictions of the percolation model include the range of possible stretching exponents $1/3 \leq \beta \leq 1$ and the functional dependence of the alpha relaxation time and stretching exponent on temperature, density, wave number, and entropy. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q9.00008: Finite Temperature Simulations of Glassy Models with Patchwork Dynamics Creighton Thomas, Alan Middleton We present simulation results on aging effects in the late time dynamics of two glassy models: the Edwards-Anderson Ising spin glass and a disordered lattice dimer model. As these models have glassy dynamics, direct simulations take prohibitively long times. We use patchwork dynamics, in which we replace local Monte Carlo updates with efficient exact finite temperature equilibration of subsystems, or patches. We scale the simulation data and find a collapse to relate the dynamics for different patch sizes, with larger patches evolving the system more rapidly. We investigate the use of this technique to study rejuvenation and memory effects. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q9.00009: Connecting microscopic and phenomenological approaches to glassy dynamics Malcolm Kennett, Matthew Downton Kinetically constrained spin models are known to exhibit dynamical behavior mimicking that of glass forming systems. They are often understood as coarse-grained models of glass formers, in terms of some ``mobility'' field. The identity of this ``mobility'' field has remained elusive due to the lack of coarse-graining procedures to obtain these models from a more microscopic point of view. Here we exhibit a scheme to map the dynamics of a two-dimensional soft disc glass former obtained from Molecular Dynamics simulations onto a kinetically constrained spin model, providing an attempt at bridging these two approaches. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q9.00010: Glassy Dynamics in Systems of Ellipse-shaped Particles Carl Schreck, Mitch Mailman, Bulbul Chakraborty, Corey O'Hern Glass-forming materials possess a critical cooling rate r*; for thermal quench rates r $>$ r*, these systems form disordered solids; for r $<$ r*, they form (poly) crystalline materials. ~We investigate the influence of particle shape (or anisotropic interactions) on the critical cooling rate. In particular, we perform molecular dynamics (MD) simulations of ellipsoidal particles in 2D as a function of aspect ratio of the major to minor axes to optimize the local packing efficiency and the critical cooling rate to improve glass-forming ability. Also, previous mode-coupling theoretical studies have predicted that over a wide range of aspect ratios, the rotational and translational degrees of freedom undergo dynamical arrest at the same temperature. We will perform MD simulations as a function of the cooling rate, packing fraction, and aspect ratio to determine whether novel glass phases also exist in which the rotational and transitional degrees of freedom freeze at different temperatures. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q9.00011: Self-Assembly of Spherical Colloidal Particles at Low N Natalie Arkus, Vinothan Manoharan, Michael Brenner The number of rigid structures that a system of N particles can form grows exponentially with N. Stabilizing any one structure over all others is thus a challenging problem. We consider a system of N spherical colloidal particles that cannot deform or overlap, and which exhibit a short-range attractive force. We present a method, using graph theory and geometry, that solves for all possible rigid packings of N particles - the resultant set of packings is provably complete. We then present a mechanism that is capable of stabilizing any one structure over all others (in the zero temperature limit), and which is experimentally realizable - thereby, potentially allowing us to direct the self-assembly of a desired structure. We compare to preliminary experimental results. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q9.00012: Aging in the shear-transformation-zone theory of plastic deformation Joerg Rottler, Philipp Maass Aging phenomena in the plastic response of glassy solids are studied within the theory of shear transformation zones (STZs), which describes the kinetic rearrangement of localized defects in response to external stress. To account for the slow, non-equilibrium dynamics after a quench below the glass transition temperature, two possible models are considered. In the first model, transition rates between the internal states of STZs decrease with time, while in the second model aging occurs due to the relaxation of an effective temperature that determines the number density of STZs and other out-of-equilibium degrees of freedom. We show that for reasonable choices of parameters, both models capture qualitatively typical aging features seen in computer simulations and experiments on polymer and other soft glasses: (i) Compliance curves measured for different waiting times after the quench can be superimposed when the observation times are rescaled with relaxation times, and (ii) stress-strain curves show a stationary plateau stress independent of waiting time and a peak stress that increases logarithmically with both waiting time and the strain rate. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q9.00013: Stable glass transformation to supercooled liquid via surface-initiated growth front Mark Ediger, Stephen Swallen, Katherine Traynor, Robert McMahon, Thomas Mates Recently it has been established that vapor deposition onto substrates at 0.85 T$_{g}$ can produce high density, high stability, low enthalpy glasses. These glasses may be the most stable ever produced in a laboratory (using the glass formed by cooling the liquid as the reference state). Here we use SIMS to observe the transformation of isotopically layered stable glasses of trisnaphthylbenzene into a liquid during annealing above T$_{g}$. In contrast to the predictions of standard models, the observed transformation is spatially heterogeneous. The liquid grows into the stable glass with sharp growth fronts initiated at the free surface and at the interface with the substrate. For the free surface, the growth velocity is constant in time and has the same temperature dependence as self-diffusion in the equilibrium supercooled liquid. These stable glasses are packed so efficiently that defects such as surfaces and interfaces are required to initiate the transformation to the liquid even well above T$_{g}$. [Preview Abstract] |
Session Q10: Focus Session: Multiferroics III and Dielectrics
Sponsoring Units: FIAPChair: Delphine Lebuegle, University of Paris-Sud 11
Room: 304
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q10.00001: Low-energy Electron Microscopy (LEEM) Imaging and Diffraction of Ho$_{1-x}$Y$_{x}$MnO$_{3}$(0001) M.D. Ulrich, Relja Vasic, J.T. Sadowski, J.E. (Jack) Rowe, H.D. Zhou, C.R. Wiebe We have investigated the (0001) surfaces of several hexagonal perovskite alloys Ho$_{1-x}$Y$_{x}$MnO$_{3}$ (x=0-1), by low-energy electron microscopy (LEEM) using both mirror-mode imaging and diffraction (LEED) techniques. We find LEEM structured domains for our (0001) surfaces of single crystals grown by a traveling-solvent-floating zone technique which are likely due to work function variations on a scale of $\sim $100 nm to 5 $\mu $m. The domains are visible up from 500\r{ }C to $\sim $900\r{ }C, giving possible evidence for the expected 180\r{ } ferroelectric domain structure below and above the Curie temperature of $\sim $630 \r{ }C. The domain contrast in LEEM was lower upon cooling and we are exploring several mechanisms for this result. In addition, we observed LEED patterns from $\sim $24 to 1100\r{ }C, with electron energies in the range of 15eV to 50eV. Above 500\r{ }C, 1$\times $1 and 2$\times $2 patterns were obtained for all samples with the sharpest LEED contrast near T $\sim $1000\r{ }C. In some cases a 2$\times $4 surface reconstruction was also evident but with less consistent order. The 2-D lattice constant parameters in (0001) plane are consistent with previous literature values for pure YMnO$_{3}$ (see: Aberdam \textit{et al., Surface Science }\textbf{14, }pp.121-140 (1969)). [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q10.00002: Spin Correlation and Magnetically Induced Ferroelectricity in YMn$_2$O$_5$ J. Okamoto, D. J. Huang, W. B. Wu, S. L. Cheng, C. T. Chen, C. Y. Mou, K. S. Chao, S. W. Huang, S. Park, S-W. Cheong There is great interest in understanding the microscopic nature of the coupling between ferroelectricity and magnetic ordering in several multiferroic frustrated manganites $R$MnO$_3$ and $R$Mn$_2$O$_5$ ($R$= rare earth and Y). For $R$MnO$_3$, the multiferroicity can be understood in terms of the anti- symmetric spin interaction, whereas the underlying mechanism of multiferroicity in $R$Mn$_2$O$_5$ remains controversial, because of its structural complexity. We unraveled the temperature-dependent spin ordering of multiferroic YMn$_2$O$_5 $ by using resonant soft x-ray magnetic scattering at Mn $L_3$ edge. For temperatures below the onset temperature $T_C$ of ferroelectricity, the handedness of cycloidal spin spirals exists, but vanishes above $T_C$. The spin handedness perpendicular to the induced polarization reverses at the temperature where the polarization changes its sign. The temperature dependence of spin correlation along the propagating direction of spin spirals resembles the temperature behavior of polarization. These data imply that both symmetric and antisymmetric spin interactions involve in the magnetoelectric coupling in YMn$_2$O$_5$. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q10.00003: Magnetic field effects on charge and magnetic structures in a new multiferroic LuFe$_2$O$_4$ Jinsheng Wen, Guangyong Xu, Genda Gu, Steve Shapiro LuFe$_2$O$_4$ is a new multiferroic where the ferroelectric polarization originates from valence order of Fe$^{2+}$ and Fe$^{3+}$ ions instead of cation displacements. It evolves from two- to three-dimensional charge ordered state upon cooling, and the bulk polarization appears when the charges order three dimensionally. A ferrimagnetic order appears with further cooling. Bulk polarizations and the charge order are both enhanced when the magnetic order occurs, suggesting a strong coupling between the two orders in the system. We have performed neutron scattering measurements on both the charge and magnetic orders under external magnetic fields. We will discuss the field effects and their implications. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q10.00004: Reentrant ferroelectricity and the multiferroic phase diagram of Mn$_{1-x}$Fe$_{x}$WO$_{4}$ Rajit Chaudhury, Bernd Lorenz, Yaqi Wang, Yanyi Sun, Ching Chu Recently MnWO$_{4}$ has attracted attention because of its multiferroic properties. In MnWO$_{4}$ the Mn$^{2+}$ ions can be substituted by Fe$^{2+}$ since MnWO$_{4}$ and FeWO$_{4}$ are isomorphic. This opens the possibility to tune the magnetic orders by Fe-substitution for a better understanding of the microscopic interactions resulting in the multiferroic properties. We report the discovery of reentrant ferroelectricity in the phase diagram of multiferroic Mn$_{1-x}$Fe$_{x}$WO$_{4}$ single crystals. At zero magnetic field (H) the spin-spiral ferroelectric (FE) state is completely suppressed at Fe substitutions (x) exceeding 0.04. For x<0.04 a ferroelectric phase exists in a narrow temperature (T) range at zero magnetic field. This FE phase shows a reentrant behavior at lower T above a critical magnetic field H$_{c}$(x). The reentrant FE transition is explored by polarization, dielectric constant, and magnetization measurements. The complete multiferroic x-T-H phase diagram of Mn$_{1-x}$Fe$_{x}$WO$_{4}$ is derived. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q10.00005: X-ray resonant magnetic scattering study of multiferroic $R$MnO$_{3 }(R$~=~Dy, Ho, Er) compounds A.I. Goldman$^{1}$, S. Nandi$^{1}$, A. Kreyssig$^{1}$, L. Tan$^{1}$, J.W. Kim$^{1}$, J.Q. Yan$^{1}$, M.D. Vannette$^{1}$, J.C. Lang$^{3}$, D. Haskel$^{3}$, T.A. Lograsso$^{2}$, R.J. McQueeney$^{1}$ Element specific x-ray resonant magnetic scattering (XRMS) investigations were undertaken to determine the magnetic structure of multiferroic hexagonal RMnO$_{3}$ compounds. In the intermediate temperature phase (ITP) (8-68~K for the Dy$^{3+}$ and 4.5-40~K for Ho$^{3+})$ the moments are aligned and antiferromagnetically correlated in the \textbf{c }direction according to the same magnetic representation $\Gamma _{3}$. Below the ITP, the Dy$^{3+}$/Ho$^{3+}$ moments order differently and according to the magnetic representations $\Gamma _{2 }$/$\Gamma _{1}$. The temperature dependence of the observed intensity in the ITP can be modeled assuming the splitting of ground-state doublet/quasi-doublet crystal-field levels of Dy$^{3+}$/Ho$^{3+ }$by the exchange field of Mn$^{3+}$. No resonant signals could be found for Er$^{3+}$ from 2-80~K. Specific magnetic representations can be excluded for the magnetic order of Er$^{3+}$ moments but can not be uniquely determined within the sensitivity of XRMS. -- The support by U.S. DOE (DE-AC02-07CH11358 and DE-AC02-06 CH11357) is acknowledged. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q10.00006: Frequency dependent magneto-dielectric effect in bilayer manganite Pr(Sr$_{0.1}$Ca$_{0.9})_{2}$Mn$_{2}$O$_{7}$ Barnali Ghosh-Saha, D. Bhattacharya, S. Patnaik, A.K. Raychaudhuri, S. Arumugam We report novel frequency dependent magneto-dielectric effect and a strong dielectric anomaly near Neel temperature (T$_{N})$ in a single crystal of bilayer manganite Pr(Sr$_{0.1}$Ca$_{0.9})_{2}$Mn$_{2}$O$_{7}$ system. The magneto-dielectric effect measured in a field of 3T shows large frequency dependence and reaches a maximum ($\sim $25{\%}) near T$_{N}$ at a measurement frequency of 1 kHz. Change in frequency leads to a change in the sign of the effect. There is a sizeable dielectric relaxation process near T$_{N}$, which exhibits an activated behavior and strongly non-Debye nature at or below T$_{N}$ while becoming Debye like at higher temperature. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 1:03PM |
Q10.00007: Flexoelectricity in nanostructures Invited Speaker: Crystalline piezoelectric dielectrics electrically polarize upon application of uniform mechanical strain. Inhomogeneous strain, however, locally breaks inversion symmetry and can potentially polarize even non-piezoelectric (centrosymmetric) dielectrics. Flexoelectricty--the coupling of strain gradient to polarization-- is expected to show a strong size-dependency due to the scaling of stain gradients with structural feature size. In this study, using a combination of atomistic and theoretical approaches, we elucidate the ``effective'' size-dependent piezoelectric and elastic behavior of inhomogeneously strained non-piezoelectric and piezoelectric nanostructures. We argue, through computational examples, the tantalizing possibility of creating ``apparently piezoelectric'' nano-composites without piezoelectric constituents and the emergence of size-dependent ``giant piezoelectricity'' in paradigmatical nanostructures. Finally, we propose that flexoelectricity is an important and essential contributor to the intrinsic dead-layer effect in high permittivity ferroelectric based nanocapacitors. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q10.00008: Evidence for the Potential Barrier Height Reduction in Metal-Oxide-Metal Tunnel Junctions due to the Interface Dependent Metal-Induced-Gap-States Lee Hosik, Hyuntae Jung, Yongmin Kim, Kyooho Jung, Hyunsik Im, Yuri Pashkin, O. Astafiev, J. S. Tsai, Yoshiyuki Miyamoto We have performed transport measurements on Al and Nb based metal-oxide-metal tunnel junctions with an AlO$_{x}$ tunnel barrier and found a strong dependence of the effective potential barrier height on the oxide-metal interface properties. Our estimations of the barrier height based on a phenomenological Simmons' model are consistent with the values obtained from the first-principle calculations. The calculations clearly show that the barrier height is strongly affected by the formation of metal induced gap states originating from the hybridization between metallic bands and Al$_{2}$O$_{3}$ conduction band. These findings are important for nanoelectronic devices containing tunnel junctions with a thin insulating layer. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q10.00009: Role of defects in the interfacial conductivity at interfaces in Pt/ SrTiO3/Pt heterostructures Naga Phani Aetukuri, Guenole Jan, Mahesh Samant, Kevin Roche, Stuart Parkin Oxides exhibit a wide variety of diverse phenomena including ferromagnetism, anti-ferromagnetism, ferroelectricity, superconductivity and multiferroicity. Furthermore, they display complex phase diagrams which result from the sensitivity of their properties to subtle changes in structure, doping, temperature and pressure. We will present the effect of interfaces and interface defects on the electrical switching properties of metal-insulator-metal (MIM) structures where the insulator is strontium titanate (STO) grown by pulsed laser deposition. The role of growth temperature and pressure and the effect of intentionally created oxygen vacancies on MIM device characteristics will be discussed. An attempt will be made to rationalize the role of intrinsic mechanisms and interface effects on the variability seen in device properties. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q10.00010: High-quality SrTiO$_{3}$ films using a hybrid MBE approach Bharat Jalan, Roman Engel-Herbert, Nick Wright, Susanne Stemmer A novel hybrid molecular beam epitaxy (MBE) approach for atomic-layer controlled growth of high-quality, epitaxial, stoichiometric SrTiO$_{3}$ films is presented. A solid source is used for Sr, an rf oxygen plasma source for oxygen and a metal-organic source for Ti. High-resolution x-ray diffraction revealed high quality single crystalline films with rocking curve full width half maxima similar to those of the substrates (i.e., 0.0095\r{ } for a LSAT substrate). RHEED showed persistent layer-by-layer growth ($>$ 180 oscillations), which has previously been observed only in a very few other systems (Si and GaAs). Surface reconstructions were observed during growth and related to the growth modes. Depending on growth conditions, step flow growth was also observed. The film surface RMS roughness was less than 0.2 nm. Excellent film stoichiometry was confirmed by homoepitaxy (prefect overlap of film and substrate reflections). Films were very insulating, consistent with oxygen stoichiometry. Oxygen gettering Ti contacts and vacuum anneals were used to produce n-type, oxygen deficient films to study carrier mobilities. We demonstrate the relative influence of hydrogen and oxygen vacancies on the electrical conductivity of SrTiO$_{3}$. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q10.00011: Conductance bistability in metal oxide junctions Zhongkui Tan, Vijay Patel, Esteban Monge, Shih-Sheng Chang, Shawn Pottorf, James Lukens, Konstantin Likharev We are exploring conductance bistability (memory) effects in junctions based on metal oxides, in the context of their possible applications in hybrid CMOS/nanoelectronic (e.g., CMOL [1]) circuits. So far, we have investigated CuO$_{x}$, NbO$_{x}$ and TiO$_{x}$ formed by thermal and plasma oxidation, with or without rapid thermal post-annealing (at 200 to 800$^{\circ}$C for 30 to 300 seconds). Conductance switching effects have been observed for all these materials. Particularly high endurance (over 1000 switching cycles) has been obtained for TiO$_{x}$ junctions plasma oxidized in 15mTorr oxygen and then post-annealed at 700$^{\circ}$C. However, the ON/OFF conductance ratio for these junctions is only about 5, and the sample-to-sample reproducibility is much lower than required for integrated circuit applications. Our plans are to extend our studies to a-Si junctions with one Ag electrode, and multilayer TiO$_{x}$ junctions, with the main goal to improve device reproducibility. The work is supported in part by AFOSR. \\[3pt] [1] K. K. Likharev, ``Hybrid CMOS/Nanoelectronic Circuits,'' accepted for publication in \textit{J. Nanoelectronics and Optoelectronics}, Nov. 2008. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q10.00012: Electrical characterization of epitaxial MgO/SiC capacitor structures Agham Posadas, Fred Walker, Charles Ahn, Trevor Goodrich, Zhuhua Cai, Kate Ziemer Epitaxial heterostructures of MgO (111) have been grown on hydrogen-cleaned 6H-SiC (0001) substrates and characterized using capacitance vs. voltage (C-V) and current vs. voltage (I- V) measurements. Low frequency capacitance measurements of MgO/SiC under strong accumulation conditions as a function of MgO layer thickness show that the epitaxial MgO has a dielectric constant of 10. The C-V measurements show modulation of the SiC from accumulation to depletion, consistent with the wafer conductivity type. The density of interface states was determined from ac conductance vs. frequency measurements, indicating a density of 6 x 10$^{11}$ eV$^{-1}$ cm$^{-2}$ at an energy level close to the conduction band edge, similar to what has been reported for native SiO$_{2}$ capacitor structures. In order to determine the dielectric breakdown field of the epitaxial MgO, leakage current as a function of gate voltage was measured for the capacitor structures. The median gate voltage at which the MgO breaks down was found to correspond to an electric field of 12 MV/cm, similar to that of bulk, single crystal MgO. These results show that epitaxial MgO has potential for use as a gate dielectric in SiC MOSFET applications. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q10.00013: Observation and coupling of domains in a spin spiral multiferroic Dennis Meier, Michael Maringer, Thomas Lottermoser, Manfred Fiebig, Petra Becker, Ladislav Bohaty The intrinsically strong cross coupling between magnetism and ferroelectricity in spin spiral multiferroics suggests these systems as prime candidates for novel multifunctional devices. Comprehension and controlling of the correlated antiferromagnetic (AFM) and ferroelectric (FE) domain structures by external fields is an indispensible prerequisite for future device design. However, very few is know about the domain topology and switching of AFM spin spirals and the magnetically induced FE domains. Here we present the spatial distribution of AFM and FE domains in MnWO$_4$, revealed by optical second harmonic generation. Electric fields are used to uniquely control the magnetic domain structure, while applied magnetic fields influence the poling behavior of the FE domains. [Preview Abstract] |
Session Q11: Focus Session: Optical Properties of Nanostructures IV: Optical Antennas and Plasmonics
Sponsoring Units: DMPChair: P. James Schuck, Molecular Foundry, Lawrence Berkeley National Laboratory
Room: 305
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q11.00001: Dielectric Optical Antenna Emitters and Metamaterials Invited Speaker: Optical antennas are critical components in nanophotonics research due to their unparalleled ability to concentrate electromagnetic energy into nanoscale volumes. Researchers typically construct such antennas from wavelength-size metallic structures. However, recent research has begun to exploit the scattering resonances of high-permittivity particles to realize all-dielectric optical antennas, emitters, and metamaterials. In this talk, we experimentally and theoretically characterize the resonant modes of subwavelength rod-shaped dielectric particles and demonstrate their use in negative index metamaterials and novel infrared light emitters. At mid-infrared frequencies, Silicon Carbide (SiC) is an ideal system for studying the behavior of dielectric optical antennas. At frequencies below the TO phonon resonance, SiC behaves like a dielectric with very large refractive index. Using infrared spectroscopy and analytical Mie calculations we show that individual rod-shaped SiC particles exhibit a multitude of resonant modes. Detailed investigations of these SiC optical antennas reveal a wealth of new physics and applications. We discuss the distinct electromagnetic field profile for each mode, and demonstrate that two of the dielectric-type Mie resonances can be combined in a particle array to form a negative index metamaterial [1]. We further show that these particles can serve as ``broadcasting'' antennas. Using a custom-built thermal emission microscope we collect emissivity spectra from single SiC particles at elevated temperatures, highlighting their use as subwavelength resonant light emitters. Finally, we derive and verify a variety of general analytical results applicable to all cylindrical dielectric antennas and discuss extensions of the demonstrated concepts to different materials systems and frequency regimes. [1] J.A. Schuller, et al., Phys. Rev. Lett. 99, 107401 (2007) [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q11.00002: Quantum Confined Stark Effect for Exciton-Plasmons in Carbon Nanotubes Igor Bondarev, Justice McConnell We study theoretically the perpendicular electrostatic field effect (the quantum confined Stark effect) for excitons and interband plasmons in small-diameter ($<\sim $1nm) semiconducting carbon nanotubes (CNs). The exciton excitation energy and the plasmon energy both shift to the red due to the decrease in the CN band gap as the field increases. However, the exciton red shift is much less than the plasmon one due to the decrease in the absolute value of the (negative) exciton binding energy [1]. This brings the exciton in resonance with the interband surface plasmon. The exciton total energy may be tuned to the nearest interband plasmon resonance this way, to form the strongly coupled exciton-surface-plasmon excitation[2,3]. We propose this effect for the development of CN based tunable optoelectronic device applications in areas such as nanophotonics, nanoplasmonics, and cavity quantum electrodynamics.\\[0pt] REFERENCES: [1] I.V. Bondarev, K .Tatur and L.M. Woods, Phys. Rev. B, submitted. [2] I.V. Bondarev and H. Qasmi, Physica E 40, 2365 (2008). [3] I.V. Bondarev, K. Tatur and L.M. Woods, Optics Communications, to appear in Dec. 2008. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q11.00003: Resonant Plasmonic and Vibrational Coupling in a Tailored Nanoantenna for Infrared Detection Javier Aizpurua, Aitzol Garcia-Etxarri, Thomas W. Cornelius, Shafkat Karim, Frank Neubrech, Annemarie Pucci Gold nanowires are introduced as plasmonic infrared antennas for effective molecular spectroscopy. A novel resonant mechanism involving the interference of the broadband plasmon with the narrowband vibration from molecules is presented in the context of Surface-Enhanced Infrared Absorption (SEIRA). With the use of this concept, we demonstrate experimentally the enormous enhancement of the vibrational signals from less than one attomol of molecules on individual gold nanowires, tailored to act as plasmonic nanoantennas in the infrared. By detuning the resonance via a change in the antenna length, a Fano-type behavior of the spectral signal is observed, which is clearly supported by full electrodynamical calculations. This resonant mechanism can be a new paradigm for sensitive infrared identification of molecular groups. \\[0pt] F. Neubrech {\it et al}. Phys. Rev. Lett. {\bf 101}, 157403 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q11.00004: A novel near field transducer for efficient energy transfer William Challener, Amit Itagi, Chubing Peng, Nils Gokemeijer, Ganping Ju, Michael Seigler, Edward Gage Interest in the localized surface plasmon resonance (LSPR) of metallic nanoparticles has been piqued by single molecule detection in surface enhanced Raman spectroscopy, scanning optical microscopy with sub-20 nm resolution, particle capture using the optical tweezers effect, and proposed applications in nanolithography and data storage. We have designed a gold near field transducer (NFT) that combines the LSPR effect, the lightning rod effect, and the dual dipole effect. Optical energy that is focused onto the NFT is coupled into a metallic thin film within a spot that is an order of magnitude smaller than the free space wavelength with an efficiency of about 5{\%}. With approximately 40 mW of optical power from a laser diode at a wavelength of 830 nm, data has been recorded at a track width less than 50 nm onto a high coercivity magnetic medium by heating it to its Curie point of 650 K while the medium was rotating at 2700 RPM and the NFT was separated from the medium surface by 15 nm. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q11.00005: Near-field imaging and probe-assisted nano-mechanical control of plasmonic antennas Aitzol Garcia-Etxarri, Isabel Romero, F. Javier Garcia de Abajo, Rainer Hillenbrand, Javier Aizpurua Imaging plasmon-resonant gold nanodisks acting as optical nanoantennas by scattering-type near-field optical microscopy (s-SNOM), we identify weak and strong coupling regimes between the near-field probe and the plasmonic nanoantenna sample. By means of rigorous electrodynamical calculations based on a model system, we find that in the weak coupling regime, s-SNOM can be applied for direct mapping of plasmonic nanoantenna modes, while in the strong coupling regime, the near-field probe allows for high-precision opto-mechanical control of the antenna response. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q11.00006: Direct Near-field Imaging of UV Surface Plasmon of a Bowtie Optical Nano-antenna Liangcheng Zhou, Qiaoqiang Gan, Volkmar Dierolf, Filbert Bartoli Study of Ultraviolet Surface Plasmon Polariton (SPP) is of special interest because of UV light's wide applications. Near-field Scanning Microscopy (NSOM) has been proven to be one of the most effective ways of characterizing SPP modes thanks to its highly localized signal collection from surface with a sub-wavelength resolution. A resolution of 60nm is achieved on our NSOM which is capable of working with deep-UV (down to 244nm) light. By utilizing this NSOM working under collection mode, we directly imaged the UV SPP modes on various nanostructures on an Al/Al2O3 thin film, among which a bowtie antenna structure showed extraordinary quality of both confining UV light field to a sub-wavelength size and enhancing the optical intensity as well. Numerical simulation of said structure is also reported and discussed, revealing that a bowtie antenna is a promising candidate for many uses such as a novel NSOM tip, optical sensors and optical nano-trap etc. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q11.00007: Cavity resonances of metal-dielectric-metal nanoantennas Bhuwan Joshi, Qi-Huo Wei We numerically study the optical properties of metal-dielectric-metal nanoantennas. The nanoantennas consist of two metal nanocylinders stacked vertically with a dielectric disk spacer. The numerical analysis using finite difference time domain method (FDTD) shows that nanoantennas exhibit two types of resonances when the gap between the metal cylinders is below 5nm. One of the resonance corresponds to the antenna resonance, generates a peak in scattering spectra and the other corresponds to cavity resonance, produces multiple dips in the scattering spectra. The multiple dips are corresponding to the different cavity resonant modes; the resonant frequencies of these modes depend upon the gap size between the cylinders. It is found that as the gap size decreases, enormous electric field enhancement can be generated inside the cavity. For a particular gap size, electric field enhancement can be maximized by varying diameter of the dielectric disk and optimum condition is obtained when dielectric disk diameter is roughly half that of the metal cylinders. The cavity resonance can be explained as interference of gap surface plasmons between two metal cylinders. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q11.00008: Plasmonic hysteresis: temperature dependent resonance of vanadium-dioxide coated gold nanoparticle arrays Davon Ferrara, Joyeeta Nag, Eugene Donev, Jae Suh, Richard Haglund The optical properties of metal nanostructures are dominated by the free-electron, or plasmonic, response of the material. In the case of metal nanoparticles, this leads to a resonant extinction with wavelength determined by the particles' size, shape, material, and surrounding dielectric. Vanadium-dioxide has a hysteretic transition from a semiconductor to a metal about 68C accompanied by a change in its structural, electrical and optical properties. Using vanadium dioxide as a thermochromic dielectric switch, we map out the hysteresis of the plasmonic resonance of gold nanoparticle arrays coated with the metal-oxide as a function of temperature. To study this plasmonic dependence on temperature, a sample of 20nm thick Au nanoparticle arrays with various particle sizes and grating constants were coated with a 60nm thick vanadium dioxide film. We find that near the transition, the particle plasmon resonance can shift position over 250nm. Measurements of the line shape show the effects of strong correlation in the vicinity of the switching temperatures. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q11.00009: Surface plasmon lifetime in metal nanoshells A.S. Kirakosyan, T.V. Shahbazyan Lifetime of localized surface Plasmon in metal nanostructures plays important role in many aspects of plasmonics and its applications. In small nanometer-sized spherical particles, the dominant mechanism is size-dependent Landau damping that limits plasmon lifetime to t$<$R/v, where R is nanoparticle radius and v is the electron Fermi velocity. It has long been expected that for other nanostructures, the plasmon lifetime should be similarly determined by their characteristic size. We performed quantum-mechanical calculations of Landau damping in metal nanoshells with dielectric core, and found a significant difference of plasmon lifetime from the expected behavior. In particular, due to electron scattering on two metal surfaces, the damping rate exhibits pronounced quantum oscillations with changing shell thickness. Our calculations explain the results of recent measurements of plasmon lifetime in gold nanoshells. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q11.00010: Finite Size Effects on the Electromagnetic Field Enhancement from Low-dimensional Silver Nanoshell Dimer Arrays Youlin Song, Ke Zhao, Yu Jia, Xing Hu, Zhenyu Zhang Finite size effects on the optical properties of one-dimensional (1D) and 2D nanoshell dimer arrays are investigated using generalized Mie theory and coupled dipole approximation within the context of surface-enhanced Raman spectroscopy (SERS). It is shown that the huge enhancement in the electromagnetic (EM) field at the center of a given dimer oscillates with the length of the 1D array. For an array of fixed length, the EM enhancement also oscillates along the array, but with a different period. Both types of oscillations can be attributed to the interference of the dynamic dipole fields from different dimers in the array. When generalized to 2D arrays, EM enhancement higher than that of the 1D arrays can be gained with a constant magnitude, a salient feature advantageous to experimental realization of single-molecule SERS. [K. Zhao et al, J. Chem. Phys. \textbf{125}, 081102 (2005); Y. L. Song et al, accepted by J. Chem. Phys.] [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q11.00011: System-bath approach to electronic effect in Surface Enhanced Raman Scattering Semion Saikin, Roberto Olivares-Amaya, Cesar Rodriguez-Rosario, Michael Stopa, Alan Aspuru-Guzik Raman scattering from molecules is greatly enhanced in proximity of a metal nanoparticle or a rough metal surface. The strong interest in this effect is driven by applications to selective detection of toxic chemicals, warfare agents, etc. The scattering enhancement has two distinct contributions. The electromagnetic effect originates in the field concentration by surface plasmons excited in the metal. The second, electronic or chemical contribution, which is important for molecules in direct contact with the surface, is more controversial. It is controlled by the charge transfer between a molecule and a metal with nanoscale roughness. We develop an open quantum system approach to the formation of charge-transferred states and apply it to describe electronic effect in SERS using specific examples of organic molecules adsorbed on a surface of a silver nanoparticle. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q11.00012: Single molecule surface-enhanced Raman spectroscopy in nanogap structures Daniel Ward, Naomi Halas, Douglas Natelson Single molecule sensitivity in surface enhanced Raman scattering (SERS) is of significant interest to multiple fields of study but has been difficult to demonstrate conclusively. We have developed a planar nanogap structure with single molecule Raman sensitivity ( Nano Lett. 7, 2007; Nano Lett. 8, 2008). Nanogap devices offer a reliable way to probe SERS phenomena often thought to be the hallmarks of single molecule sensitivity: intensity fluctuations and spectral diffusion. We present a series of experiments on intensity fluctuation and spectral diffusion rates as a function of temperature to better understand the mechanism driving these phenomena. We also explore how the gap width affects overall Raman intensity. Additionally, time permitting, we present results on plasmonic light emission from nanogap devices when excited by hot electrons. The spectrum shows intensity peaks at energies well above the excitation energy revealing a wealth of interesting physics. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q11.00013: Plasmonic Nanolens Arrays for Enhanced Raman Spectroscopy E.V. Ponizovskaya, I. Naumov, Z. Li, Jing Tang, A.M. Bratkovsky Surface-enhanced Raman scattering (SERS) is the 4th-order process with regards to a local electric field, $\sim $E$^{4}$, and therefore, may be extraordinarily enhanced well in excess of 10-11 orders of magnitude. The ``chemical' enhancement factor of less clear origin may also reach few orders of magnitude. We are looking at engineering various nanoparticle arrays that may focus local field and may be fabricated in top-down manner or self-assembled. Nanocrystals of Au and Ag with different shapes, such as octahedra, cubes, stars etc and their 2-dimensional and 3-dimensional assembly have been studied for plasmonic applications. One promising way of reaching the enhancement $\ge 10^{12}$is to use arrays of plasmonic nanolenses with with binary or even ternary nanoparticles arrangements with certain patterns. The nanoparticles arrangements were modeled numerically using Finite Difference Time Domain method. and results are compared with the data collected by our team on some fabricated high-performance SERS substrates. [Preview Abstract] |
Session Q12: Structure, Dynamics, and Diffusion at Surfaces
Sponsoring Units: DMP DCMPChair: Kristen Fichthorn, Penn State University
Room: 308
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q12.00001: Water Adsorption on Wurtzite GaN Surfaces Xiao Shen, Philip B. Allen, Mark S. Hybertsen, James T. Muckerman A solid solution of wurtzite GaN/ZnO absorbs light in the visible and can photosplit water.\footnote{K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue, and K. Domen, Nature 440, 295 (2006)} The water is oxidized by the photo-holes at the surface of the semiconductor alloy. However, microscopic details of the oxidation process are unknown. We present a first-principles study of water adsorption on wurtzite GaN. We study the structures and energetics of water adsorption, calculate the energy barrier for water dissociation, analyze the water-water interactions, and study the electronic structure. The results are compared with water adsorption on ZnO surface. We also study the behavior of the holes near the water-semiconductor interface. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q12.00002: Does Water Adsorb Molecularly or Dissociatively on a Plutonium Surface? Asok Ray, Raymond Atta-Fynn DFT-GGA has been used to study adsorption of water in molecular and dissociative configurations on $\delta $-Pu (111) surface. In molecular state, water is physisorbed in an almost flat-lying orientation at a one-fold coordinated on-top site. The interaction of the water 1$b_{1}$ orbital and the Pu-6$d$ orbital provides the stability of water on the surface, implying that the Pu-5$f$ electrons remain chemically inert. The coadsorption cases of partially dissociated and fully dissociated products at the three-fold hollow sites yield chemisorption, coupled with rumpling of the surface layer and delocalization of the Pu-5$f$ electrons and formation of strong ionic bonds. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q12.00003: Reaction path for dissociative water adsorption on vicinal anatase (101) surface Bernard Delley, Alfonso Baldereschi, Michel Posternak Using density functional calculations and a periodical slab model, we investigate water adsorption on edges formed by intersection of two anatase TiO$_2$ (101) surfaces. We find that after adsorption of a water molecule on a low-coordinated Ti atom on the ridge, decomposition may happen over a moderatly high barrier. In this process, a proton gets abstracted by bonding to a low-coodinated ridge oxygen atom. The hydroxyl anion remains bonded to the acidic Ti site on the edge. The methods used to find the saddle points and to map out the reaction paths are briefly discussed. We also give a discussion of the reaction rates that may be expected based on these calculations. The presence of hydroxyl groups and protons is favoring nucleation of Ca phoshate bonding by allowing exchange of the adsorbed proton against Ca$^{2+}$ ions. Such processes are thought to be essential for the biocompatibility of titanium and its use in dental and orthopedic applications. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q12.00004: First Principles Study of Bulk and Surface Ordering Phenomena in Pt-X Binary Alloys in the Presence of Oxygen Wei Chen, Chris Wolverton, William Schneider Alloying in metal catalyst particles (such as platinum) may change surface structure due to segregation and ordering, which may in turn significantly impact catalytic activity. Using first principles density functional theory (DFT) calculations in conjunction with a cluster expansion (CE) technique, we have studied the ordering/phase-separation phenomena of bulk and surface Pt-X binary alloys, with a specific focus on Pt-Au. The surface DFT+CE calculations are performed both in the presence and absence of oxygen. For Pt-Au, the calculated results reveal a phase separating tendency in bulk Pt-Au and a small coherency energy between these two elements. The Pt-Au phase diagram calculated by combining DFT+CE with Monte Carlo simulation shows a slightly asymmetric miscibility gap is in good agreement with experimental results. In contrast to the bulk tendency, the surface does not show a pronounced phase-separation tendency, with several low-energy ``striped'' ordered structures (with a very small, negative formation energy). The presence of oxygen qualitatively changes the surface segregation tendency of Au, and our DFT+CE results show that the equilibrium structure of the Pt-Au (111) surface varies with oxygen coverage. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q12.00005: A first-principles study of the electropotential dependent shape stability of metal nanoparticles Nicephore Bonnet, Ismaila Dabo, Nicola Marzari Understanding the catalytic activity of transition metal nanoparticles is a central issue in the development of novel fuel cell materials. Observed trends are often interpreted in terms of the size dependent shape of nanoparticles, in particular the relative density of low coordination sites. However, no consensus exists regarding the direction or even the existence of such an effect. In this context, ab-initio methods can be useful to extract relevant parameters. Here, we calculate surface energies under realistic electrochemical conditions and use the Wulff construction to infer stable nanoparticle contours. The electropotential is adjusted through its conjugate variable, the charge, and density countercharge periodic-image corrections are applied. The surrounding solvent is treated as a combination of a continuum dielectric and a classical ionic distribution at equilibrium. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q12.00006: Lattice Dynamics of Cu$_2$O: Bulk and (110) Surface Klaus-Peter Bohnen, Rolf Heid, Aloysius Soon, Catherine Stampfl A number of theoretical studies have been carried out in the past to investigate the stability of various surface oxides for the O/Cu system however despite the fact that catalytic processes usually proceed at elevated temperatures stability at finite temperatures has never been studied for these systems. Modern ab-initio methods however allow for the determination of the lattice dynamics and the phononic contribution to the free energy. Using density functional perturbation theory we have studied the lattice dynamics of Cu$_2$O-bulk as well as Cu$_2$O(110). In calculating the free energy as function of lattice constant we obtained for the bulk a negative thermal expansion up to roughly 300 K in excellent agreement with experiments. This is due to anomalous mode Gr\"uneisen parameters for vibrational modes in the low energy regime. Due to the anomalous behavior of the mode Gr\"uneisen parameter the bulk system is highly unstable against variations of the lattice constant by more than 2\%. To investigate the stability of the O/Cu surfaces we have investigated the lattice dynamics of Cu$_2$O(110) as a prototype. Despite a large number of low lying modes no instability has been found. These calculations allow also for the O/Cu system for the first time for a realistic estimation of the surface-free energy which is important for the determination of surface thermodynamic properties. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q12.00007: Ab-initio Study of the Erlich-Schoewebel Barriers for fcc(100) Talat S. Rahman, Handan Yildirim We present the results of density functional theory based calculations for the activation energies for the diffusion of adatoms (Cu or Ag) on Cu(100) and Ag(100) surfaces with and without steps\textbf{.} For Ag adatom diffusion via hopping on Cu(100), we find the energy barrier to be 0.37 eV, which is less than that (0.60 eV) of Cu adatom diffusion on Ag(100). In the presence of a step edge, we find the Erlich-Schoewebel (ES) barriers (via hopping process) for both Ag and Cu atoms on Cu(100) to be 170 meV\textbf{.} For Ag and Cu adatoms on Ag(100), the corresponding barriers are 50 meV and 60 meV, respectively\textbf{.} The ES barrier (via exchange process) for Ag on Ag(100) is found to be 20 meV and for Cu on Cu(100) it is 60 meV\textbf{. }The barriers for diffusion along the step edges at the lower terraces are 0.36 eV and 0.20 eV for Cu on Ag(100) and Ag on Cu(100), respectively. We trace the differences in the diffusion barriers of the homo-and hetero-epitaxial systems to the differences in the corrugation of the potential energy surface, and discuss the implications for homo-and heteroepitaxial growth on these surfaces. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q12.00008: On the $\textrm{Si(111)}5\times2\textrm{-Au}$ surface, Si adatom diffusion is defect-mediated Ezra Bussmann, S. Bockenhauer, F.J. Himpsel, B.S. Swartzentruber The $\mathrm{Si(111)}5\times2\textrm{-Au}$ surface is a member of a family of metal-induced chain reconstructions of Si. Studies of these reconstructions have led to new understanding of the physics of one-dimensional electronic states. The $5\times2\textrm{-Au}$ surface is speckled with Si adatoms, which are intimately linked with the surface electronic properties. At temperatures $>423\textrm{ K}$, the adatoms diffuse along the chains between adjacent $5\times2$ cells. We have measured scanning tunneling microscopy movies of the diffusing adatoms. Distinctive diffusion statistics, e. g. correlations between displacements, imply that the displacements are triggered by an interaction with a defect. By a statistical characterization of the diffusion, we show that the adatoms move by a defect-mediated mechanism similar to the vacancy-mediated diffusion observed on some metal surfaces. We use a Monte Carlo simulation to model the diffusion process, accurately reproducing the unique diffusion statistics over the temperature range ($145-215^o$C) of our experiments. We have also determined the diffusion activation barrier=$1.24\pm0.08$ eV. Sandia National Labs is operated by Sandia Corp, a Lockheed-Martin Company, for the DOE under Contract No. DE-AC04-94AL85000. FJH and SB acknowledge NSF support under DMR-0705145. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q12.00009: The diffusion pathways of phosphorus atoms in the silicon (001) surface Jennifer Bennett, Oliver Warschkow, Nigel Marks, David McKenzie Effective use of surface chemical reactions to control the placement of dopants in semiconductors requires a detailed understanding of the reaction pathways involved. The most highly developed approach to accurate placement of phosphorus in silicon involves reacting phosphine gas with selected areas of the silicon (001) surface, incorporating the phosphorus atom into the surface and silicon overgrowth. In this computational study we investigate the least understood chemical reactions involved in this process, namely the phosphorus incorporation and associated diffusion reactions. We use density functional theory, combined with an efficient method for locating transition states, to identify the reaction pathways involved in three processes: (1) the diffusion of a phosphorus adatom along the silicon (001) surface, (2) the incorporation of a phosphorus atom into the surface, and (3) the migration of the incorporated phosphorus atom within the surface. The calculated pathways and corresponding reaction barriers provide insight into the conditions required for accurate incorporation and encapsulation of phosphorus atoms into the silicon surface. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q12.00010: Kinetic Monte Carlo studies of the behavior of CO on sulfur-covered Pd(100) surface Dominic Alfonso Investigations of the behavior of CO on the surface of Pd modified with sulfur were carried out using first-principles Kinetic Monte Carlo method. In particular, the influence of adsorbed sulfur on the adsorption, diffusion and desorption of CO on the Pd(100) surface was studied. A kinetic Monte Carlo code was developed which enables the simulation of hosts of competing elementary steps with lateral interaction between the adspecies taken into account. The barriers and energetics of the relevant elementary processes were determined by density-functional theory. The rates entering the simulation were derived using transition state theory. The adsorbates were assumed to interact via pairwise additive interactions. We demonstrate that adsorbed sulfur has an adverse effect on the behavior of CO on Pd(100). [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q12.00011: Charge-State Dependent Hydrogen Diffusion on Silicon (001) Oliver Warschkow The diffusion of hydrogen atoms is relevant to a number of chemical and technological processes of the silicon (001) surface. These include the dissociative adsorption of molecules, the growth of overlayers by chemical vapor deposition (CVD), and the directed atomic-scale functionalization of the surface by scanning tunnelling microscopy (STM) lithography. The basic inter- and intradimer shift reactions of hydrogen are well studied, and activation energies of respectively 1.7 eV and between 1.0 and 1.4 eV are commonly cited. In this presentation, I will pose and discuss two questions: (1) Are single energy barriers adequate to describe H-shift reactions on silicon, and (2) are STM measurements of H-diffusion truly representative for hydrogen desorption in the absence an STM tip? These questions warrant examination because hydrogen adatoms on Si(001) are known to adopt a variety of charge states depending on factors such as the doping level of the silicon substrate, the defect density on the surface, or the presence of an STM tip. High-level density functional calculations are reported to shed some clarity on these questions. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q12.00012: Unidirectional Linear Diffusion on an Isotropic Cu(111) Surface in a Periodic and Asymmetric Potential Dezheng Sun, Ki-Young Kwon, Kin L. Wong, Greg Pawin, Eric Chu, Zhihai Cheng, Dae-Ho Kim, Miaomiao Luo, Sampyo Hong, Talat S. Rahman, Michael Marsella, Ludwig Bartels We performed an STM study of the diffusion of 1,4-benzenedithiol, 9-thioanthracene, 9,10-dithioanthracene (DTA) and 2,3-dimethyl-9,10-dithioacethylanthracene (DMDTA) as well as napthaquinone, anthraquinone and pentacenetetrone on Cu(111). Inherently uniaxial motion of all species with two thiol groups and at least three aromatic rings are observed. Sequential placement of the substrate linkers prevents DTA and DMDTA from rotating or veering off course. Asymmetric methylation impacts DTA's diffusive behavior by about 100-fold decrease in surface mobility caused by a about 2-fold increase of the diffusion barrier, with the overall symmetry of DTA diffusion not affected: A forward/backward ratio of 1.009$\pm $0.01, were found i.e. less than 1{\%} deviation from unity. This is in stark contrast to the classical behavior but in perfect agreement with Tolman's ``Principle of Microscopic Reversibility?. Density functional theory (DFT) calculations are performed and results are discussed [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q12.00013: Surface Morphological Response of Stressed Elastic Solids under Electromigration Conditions Vivek Tomar, Rauf Gungor, Dimitrios Maroudas We present a theoretical analysis of the surface morphological response of electrically conducting, stressed elastic crystalline solids under the simultaneous action of an electric field that drives surface electromigration. The analysis is based on a fully nonlinear model of driven surface morphological evolution and combines linear stability theory with self-consistent dynamical numerical simulations. We report results of the surface morphological response of a uniaxially stressed solid as a function of electric field strength, surface crystallographic orientation, and temperature. We find that a properly directed and sufficiently strong electric field can stabilize the surface morphology of the stressed solid against crack-like surface instabilities, as well as surface rippling instabilities, and determine the required critical electric-field strength over a broad temperature range and for various surface crystallographic orientations. We also demonstrate the superior morphological response of $<$111$>$-oriented surfaces of face-centered cubic metals. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q12.00014: Theory of the lifetime of adsorbate vibrations on semiconductor surfaces Peter Kratzer, Sung Sakong On semiconductor surfaces, the vibrational lifetime of covalently bonded adsorbates is rather long (nanoseconds or more) since the band gap precludes electronic dissipation. Due to the quantum nature of vibrational states, such slow relaxation is expected, as the large quantum of the bond stretching must be converted into several smaller (phononic or vibronic) quanta by a high-order process. We use density functional theory calculations to map out the high-dimensional potential energy surface governing the anharmonic coupling of the stretching to the local bending and shift modes, while the coupling to the substrate phonons is treated perturbatively. Applying our method to the vibrational lifetime of CO on Si(100), we find that the CO stretching relaxes predominantly via an intermediate state consisting of four shift and/or bending quanta and one phonon. Good agreement with the measured lifetime of 2.3 ns at $T=100$~K is achieved. As a second application, we elucidate the role of intermediate vibrational states in the relaxation of the stretching vibration at Ge(100):H surfaces. For Ge surface dimers saturated by one H and one D atom, the lifetime of the Ge--H stretching vibration is up to five times shorter and less temperature-dependent than in Ge dimers homogeneously saturated by H. Our analysis shows that the symmetry breaking associated with the isotope mixture opens up additional relaxation channels. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q12.00015: Modeling H$_2$-Surface Interactions on Interstellar Dust Grains: A Classical Molecular Dynamics Study Vijay Veeraghattam, Steven Lewis, Phillip Stancil, Junko Takahashi A classical Molecular Dynamics (MD) method is employed to model hydrogen molecules interacting with the surface of amorphous ice and to calculate the sticking coefficient as a function of various system parameters. This study, combining molecular physics and surface science, is part of a larger program of research to provide theoretical input for models of dust-grain-mediated physico-chemical processes in the interstellar medium. Many dust-grain species are thought to be clad in amorphous ice, which motivates the choice of substrate for this study. Our method simulates the various dynamical processes associated with H$_2$-ice scattering events, including collision, sticking, diffusion, and ejection. Variables such as angle of incidence, molecular rotational state, substrate temperature ($T_D$), and H$_2$ temperature ($T_{\rm H_2}$) are monitored and allowed to vary. In this talk, we will present our results for the H$_2$-ice sticking coefficient as a function of $T_D$ and $T_{\rm H_2}$. [Preview Abstract] |
Session Q13: Focus Session: Extreme Conditions and High Pressure II: Phase Transitions
Sponsoring Units: DCOMP GSCCMChair: Amy Lazicki, Geophysical Laboratory
Room: 309
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q13.00001: Computation of the Iron and Iron-Nickel Phase Diagrams from Ambient to Earth's Inner Core Xuan Luo, R.E. Cohen We have performed first-principles computation for the magnetic iron and iron-nickel over a wide range of pressure (0-400GPa) and temperature (0-6000K), within density-functional theory (DFT) in the generalized-gradient approximation using the projector augmented wave (PAW) method with the ABINIT code. We computed the free energies of hcp, bcc and fcc phases for iron and nickel including the thermal excitation of electrons and quasiharmonic phonons computed using the supercell method. We find that at high temperatures and pressures random stacking of fcc and hcp (rhcp) is most stable since the difference in Gibbs free energy between fcc and hcp Fe is smaller than the thermal energy. We computed the free energy of stacking faults, the rhcp phase and the percentage of fcc and partial ordered sequences along the melting curve. For the first time, we used first-principles calculation to be able to produce the pure-Fe phase diagram including the magnetic contributions at low pressures. In the Fe-Ni system, we find that FeNi3 is an intermediate phase below 700K at 0 GPa, consistent with experiment, and below 50 GPa at 0K. Finally, we obtained the T-P-x phase diagram for FeNi from 0-400 GPa and 0-6000K. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q13.00002: Plasticity-Mediated Structural Transformation of bcc Ta: Bridging Laser Heated Diamond-Anvil Cell and Shock Melting Christine Wu, Per Soderlind, James Glosli, John Klepeis Determination of the melting curve of a metal under high pressures is essential for establishing its phase diagram, and has wide scientific implications, including our understanding of the Earth's interior. Currently, melting temperatures at high pressure are primarily measured by \textit{in situ} laser-heated diamond-anvil cell (DAC) or shock wave experiments. Often, but not always, these two methods yield significantly different results for metals with non close-packed structures, such as bcc metals. For instance, anomalously flat melting slopes were reported for numerous bcc metals by DAC. The flatness of the melting slope is in sharp contrast to the classical Lindemann behavior which shock-melting temperatures follow closely. In this presentation, we will report our finding of a plasticity-mediated structural transition of bcc Ta to a partially disordered glassy structure obtained from molecular dynamics (MD) simulations. This transition is fully consistent with reported DAC low melting, thus provide a highly probable resolution to the long-standing controversy in melting of metals under high pressures. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q13.00003: High Pressure-High Temperature Phase Diagram of Beryllium M.J. Lipp, B.J. Baer, H. Cynn, Z. Jenei, J.-H. Klepeis, W.J. Evans, H.-P. Liermann, Y. Meng, S.V. Sinogeikin, W. Yang, A. Lazicki, Y. Ohishi A detailed understanding of the phase diagram of beryllium and its alloys impacts fundamental science and technological applications. Despite a simple atomic structure, theoretical modeling of the phase diagram of beryllium has been extremely challenging and remains an area of active investigation [Kadas, ,PRB 07]. Extension of the experimental understanding of beryllium will serve to inform and advance theoretical efforts and technological applications. To address these needs, we have extended our previous work [Evans, PRB 05], and performed x-ray diffraction and melt studies beryllium and beryllium alloys at high pressure. We will describe our measurements of the crystal structure, lattice constants, and melt curve of high-pressure beryllium and beryllium alloys. We will discuss insights into this simple yet challenging system. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q13.00004: High Pressure-Temperature Studies of Vanadium Z. Jenei, B.J. Baer, H. Cynn, J.-H. Klepeis, M.J. Lipp, W.J. Evans, H.-P. Liermann, Y. Meng, S.V. Sinogeikin, W. Yang Vanadium, a seemingly simple metal, has captured the interest of high-pressure scientists following the discovery (Ding et al. PRL 2007) of a subtle pressure-induced phase transition from bcc to a rhombohedral phase. Recent first-principles electronic-structure studies (Lee et al. PRB 2007) are consistent with these experiments and extend beyond the range of the measurements, predicting a reentrant phase transition back to bcc at high pressure. Further experiments in the regime of these predictions can validate and advance the understanding of simple metals at high-pressures. We have made x-ray diffraction measurements of the crystal structure and lattice parameters of vanadium at high-pressure and temperature. Detailed comparisons will challenge/validate models and guide development of predictive codes. We will discuss our measurements including high temperature behavior, the EOS, and transitions of vanadium at high pressure. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q13.00005: Coupling of Atomistic and Meso-scale Phase-field Modeling of Rapid Solidification J. Belak, P.E.A. Turchi, M.R. Dorr, D.F. Richards, J.-L. Fattebert, M.E. Wickett, F.H. Streitz Recently, phase field models have been introduced to model the crystallography during polycrystal microstructure evolution [1,2]. Here, we assess these models with molecular dynamics and phase field simulations that overlap in time and space. Large parallel computers have enabled MD simulations of sufficient scale to observe the formation of realistic microstructure during pressure driven solidification [3]. We compare the two methods by calculating the phase field order parameter (quaternion) from the atomic coordinates and drive the evolution with the MD. Results will be presented for the solidification of tantalum. [1] R. Kobayashi and J.A. Warren, Physica A, \textbf{356}, 127-132 (2005). [2] T. Pusztai, G. Bortel and L. Granasy, Europhys. Lett, 71, 131-137 (2005). [3] F. H. Streitz, J. N. Glosli, and M. V. Patel, Phys. Rev. Lett. 96, 225701 (2006). [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q13.00006: New high-pressure phases of calcium and their finite-temperature phase boundaries Amanuel Teweldeberhan, Stanimir Bonev The phase diagram of Ca has been studied using first-principles density functional theory. The simple cubic structure hitherto believed to exist between 32 and 109 GPa is found to be mechanically and thermodynamically unstable. Instead we propose two new solid phases with orthorhombic {\it Cmcm} and {\it Pnma} structures and determine their finite-temperature phase boundaries. We also predict liquid transitions in molten Ca under compression, which together with the new solid phases provide a consistent description of the Ca phase diagram. The implications of our findings and extensions of the work to other alkali and alkaline-earth metals will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q13.00007: Physical and Chemical Transformations of Sodium Cyanide at High pressures Jing-Yin Chen, Choong-Shik Yoo Pressure-induced physical and chemical transformations of Sodium Cyanide (NaCN) have been studied up to 50 GPa in diamond-anvil cells, using micro-Raman spectroscopy and angle-resolved synchrotron x-ray diffraction. The present results suggest three phase transitions to occur in this pressure range: from NaCN-I (cubic) to NaCN-II (orthorhombic) at 2 GPa, to NaCN-III (monoclinic) at 8 GPa, and to NaCN-IV (tetragonal) at 15 GPa. At higher pressures, NaCN-IV undergoes irreversible chemical changes, which occurs over a large pressure range between 25 and 34 GPa. The new material exhibits a broad yet strong Raman band at around 1600 cm$^{-1}$, indicating the formation of C=N bonds in a similar configuration of carbon graphite. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q13.00008: First-order liquid-liquid phase transition in compressed nitrogen Brian Boates, Stanimir Bonev We present results of first-principles molecular dynamics simulations, which provide evidence for the existence of a first-order liquid-liquid phase transition in compressed nitrogen [1]. The transition is from a molecular to a polymeric liquid. It is characterized by a discontinuous loss of molecular stability followed, upon further compression, by gradual transformation until the local order of the liquid becomes similar to that of cg-N. We have computed the phase boundary of the liquid-liquid transition to be first-order between 2000 and 4000 K and determined that above 4000 K it becomes continuous. Comparison with measurements and suggestions for experimental confirmation of our predictions will be discussed as well. [1] B. Boates and S.A. Bonev, submitted. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q13.00009: High-temperature high-pressure properties of silica from Quantum Monte Carlo and Density Functional Perturbation Theory R.E. Cohen, K. Driver, Z. Wu, B. Militzer, P.L. Rios, M. Towler, R. Needs We have used diffusion quantum Monte Carlo (DMC) with the CASINO code with thermal free energies from phonons computed using density functional perturbation theory (DFPT) with the ABINIT code to obtain phase transition curves and thermal equations of state of silica phases under pressure. We obtain excellent agreement with experiments for the metastable phase transition from quartz to stishovite. The local density approximation (LDA) incorrectly gives stishovite as the ground state. The generalized gradient approximation (GGA) correctly gives quartz as the ground state, but does worse than LDA for the equations of state. DMC, variational quantum Monte Carlo (VMC), and DFT all give good results for the ferroelastic transition of stishovite to the CaCl$_2$ structure, and LDA or the WC exchange correlation potentials give good results within a given silica phase. The $\Delta V$ and $\Delta H$ from the CaCl$_2$ structure to $\alpha$-PbO$_2$ is small, giving uncertainly in the theoretical transition pressure. It is interesting that DFT has trouble with silica transitions, although the electronic structures of silica are insulating, simple closed-shell with ionic/covalent bonding. It seems like the errors in DFT are from not precisely giving the ion sizes. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q13.00010: Quantum Monte Carlo Equations of State of $\alpha$- and $\beta$-Magnesium Silicate Kevin P. Driver, John W. Wilkins The 410 km seismic discontinuity in Earth's mantle is ascribed to the $\alpha$ to $\beta$-(Mg,Fe)$_2$SiO$_4$ phase transformation. Considering Mg-endmembers, density functional theory (DFT) predictions within LDA and GGA disagree on the phase boundary by 50\% [1]. Quantum Monte Carlo (QMC) offers a route to avoid the approximation of the exchange-correlation potential in DFT and provide a benchmark for the phase boundary, elastic moduli, and thermodynamic properties. Zero point and thermal contributions are included by using DFT linear response within the quasiharmonic approximation. Preliminary results indicate the QMC phase relations and bulk moduli are in reasonable agreement with experiment. \\[4pt] [1] Y. Yu, Z. Wu., R. M. Wentzcovitch, Earth Planet. Sci. Lett. 273, 115 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q13.00011: High Pressure Studies on Group IV Transition Metals Based Metallic Glasses Andrew Stemshorn, Yogesh Vohra The compression behavior of Group IV transition metals based metallic glasses Ti37Zr29Cu15.5Ni14.5Be4 and Zr57Cu15.4Ni12.6Al10Nb5 are investigated at room temperature up to 74 GPa in a diamond anvil cell using in-situ energy and angular dispersive x-ray diffraction with a synchrotron radiation source. The x-ray diffraction studies did not reveal any pressure induced crystallization phenomenon in metallic glasses to a volume compression of 35 percent. In Zr-based metallic glass, a nanostructured tetragonal Zr2Ni phase was observed and also found to be stable to the highest pressure. The measured equation of state (Pressure-Volume curve) of Group IV transition metals based metallic glasses is compared to the known high phases of transition metals. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q13.00012: High pressure-High temperature phases of Carbon Dioxide Amartya Sengupta, Choong-Shik Yoo The phase diagram of CO$_{2}$ has not been understood adequately above 40 GPa and high temperatures, particularly regarding the stabilities and boundaries of various extended phases that include a-carbonia, Phase V, Phase VI, and to an extent Phase III. We have studied the phase diagram of CO$_{2}$ above 40 GPa and at high temperatures, using both ohmically and laser-heated diamond anvil cells. We found the co-existence of several extended phases over a large pressure region, which we attribute to the metastability of the extended phases and the extraordinarily large pressure gradients at these pressures. We determined the relative stability fields of the co-existing phases, which may offer the physico-chemical mechanism for the existence of carbonate minerals in deep Earth's mantle. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q13.00013: First-principles study of BC$_2$N Eunja Kim, Tao Pang, Wataru Utsumi, Vladimir Solozhenko, Yusheng Zhao First-principles calculations are performed and analyzed to identify different cubic phases of BC$_2$N synthesized experimentally. With a proper choice of the supercell, cutoff energy, and sampling $k$ points, the cubic phases are found to be stable theoretically. The bulk modulus from elastic stiffness constants for each of the phases is in excellent agreement with available experimental data. All the phases are defect free and do not possess any B--B or N--N bond. Two high-density phases with nearly degenerate energies are interpreted to represent two experimental systems of different x-ray patterns. The high-density phases are characterized by the existence of C--C bonds whereas the low-density phase is characterized by the absence of C--C bonds. From the calculated equation of state and the available experimental data, we show for the first time that the unique feature of each of the cubic BC$_2$N phases is a direct result of the corresponding local electronic structure and chemical bonding in the system. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q13.00014: Large volume change across OI --$>$ OII phase transition in transition-metal dioxides TiO$_{2}$, ZrO$_{2}$, and HfO$_{2}$ as determined by experiment and theory Yahya Al-Khatatbeh, Kanani K.M. Lee, Boris Kiefer The nature of bonding in transition-metal dioxides TiO$_{2}$, ZrO$_{2}$, and HfO$_{2}$ is of interest as they are potential superhard materials with many industrial applications. Using high-resolution synchrotron x-ray powder diffraction for TiO$_{2}$ and ZrO$_{2}$, and complementary \textit{ab-initio} computations of these dioxides, we have determined the equation of state of the orthorhombic I (OI) and orthorhombic II (OII) phases. Our measurements are in agreement with the computationally predicted phase sequence of these oxides. The measured volume change across OI --$>$ OII transition is 8.3{\%} for TiO$_{2}$ and 10{\%} for ZrO$_{2}$ in good agreement with our density-functional theory (DFT) calculations that predict a large volume change for all of these dioxides across the OI --$>$ OII phase transition. For TiO$_{2}$, this volume collapse is significantly higher than previously measured (2.6{\%}), but consistent with the volume decreases observed in both ZrO$_{2}$ and HfO$_{2}$ across this transition. Furthermore, the OII phase was observed to be the most stable phase of TiO$_{2}$ and ZrO$_{2}$ at high pressure (56 GPa) after heating to high temperatures (above $\sim $1800 K) and no post-OII phase was observed under these conditions. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q13.00015: Crystal Structure and Phase Transition of XeF$_{2}$ at High pressures Minseob Kim, Choong-Shik Yoo We have investigated the crystal structure and phase transition of solid XeF$_{2}$ up to 51 GPa in diamond anvil cells by using Raman and synchrotron x-ray diffraction. The x-ray data indicates the tetragonal-to-orthorhombic phase transition at 7 GPa, which accompanies a small distortion ($<$ 1{\%}) in the ab-plane of the tetragonal structure. The Rietveld refinement further indicates a rapid change of intermolecular F...F contact distance with increasing pressures and, thereby, a rotation of linear symmetric XeF$_{2}$ molecules along the c-axis and the observed distortion in the ab-plane. This symmetry lowering tetragonal-to-orthorhombic transition also induces the Davydov splitting of symmetric stretching $\nu _{1}$ and bending $\nu _{2}$ modes in the Raman spectrum. [Preview Abstract] |
Session Q14: Granular Fluctuations
Sponsoring Units: DFDChair: Wolfgang Losert, University of Maryland
Room: 315
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q14.00001: Cooling and aggregation in wet granulates Annette Zippelius, Stephan Ulrich, Timo Aspelmeier, Klaus Roeller, Axel Fingerle, Stephan Herminghaus Wet granular materials are characterized by a defined bond energy in their particle interaction such that breaking a bond implies an irreversible loss of a fixed amount of energy. Associated with the bond energy is a nonequilibrium transition, setting in as the granular temperature falls below the bond energy. The subsequent aggregation of particles into clusters is shown to be a self-similar growth process with a cluster size distribution that obeys scaling. In the early phase of aggregation the clusters are fractals with $D_f=2$, for later times we observe gelation. We use simple scaling arguments to derive the temperature decay in the early and late stages of cooling and verify our results with event-driven simulations. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q14.00002: Propagating Waves in a Monolayer of Self-Propelling Gas-Fluidized Rods Lynn J. Daniels, Douglas J. Durian We report on the existence of propagating compression waves in a quasi-two-dimensional monolayer of self-propelling rods fluidized by an upflow of air. This behavior is unique to rods; a comparable system of spheres exhibits no waves and displays `thermal' number fluctuations, proportional to N$^{1/2}$. The waves, however, give rise to anomalously large number fluctuations, having both magnitude and exponent greater than `thermal' fluctuations. This occurs as rarefaction zones relax after a compression front has traveled through a region. We characterize the waves by calculating a dynamic structure factor. The position of observed peaks, as a function of frequency $\omega$ and wavevector $k$, yield a linear dispersion relationship in the long-time, long-wavelength limit and a wavespeed $\omega/k$ = 20 cm/s. By contrast, spheres exhibit $1/\omega^{2}$ decay for all wavevectors in the hydrodynamic limit, consistent with the diffusive decay of density fluctuations. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q14.00003: Phase diagram of wet granular matter under vertical vibrations Kai Huang, Klaus Roeller, Stephan Herminghaus The phase diagram of vertically vibrated wet granular matter is investigated by both experiments and simulations. We find a critical point where the coexistence (C) regime of the fluid (F) and gas (G) phases terminates. The energy driven F-C transition is found to scale with the rupture energy of a liquid bridge if the corresponding vibration amplitude(A) is less than particle diameter(d). This is in good agreement with our simulations. Close to the F-G transition line, the variation of the size of the gas bubble with vibration amplitude shows a hysteretic behavior. Within the hysteresis loop, we observe temporary gas bubbles with strong fluctuations in size. The F-G boundary is shown to have an interfacial tension and non-trivial wetting behavior at container walls. Focusing on the solid (S)- F transition line, we find that the fluidization is a surface melting process. This is demonstrated by detecting the mobility of ruby tracers utilizing ruby fluorescence. This as well agrees with our simulation results. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q14.00004: Stress wave mitigation in granular media Chiara Daraio, F. Fernando, Mason Porter We study stress wave mitigation in one- and two-dimensional granular media employing evolutionary algorithms to investigate the optimal design of composite protectors using granular chains composed of beads of various sizes, masses, and stiffnesses. We define a fitness function using the maximum force transmitted from the protector to a ``wall'' that represents the body to be protected and accordingly optimize the topology (arrangement), size, and material of the chain. We obtain optimally randomized granular protectors characterized by high-energy equipartition and the transformation of incident waves into interacting solitary pulses. We provide a quantitative characterization of dissipative effects using the propagation of highly nonlinear solitary waves as a diagnostic tool and develop optimization schemes that allow one to compute the relevant exponents and prefactors of the dissipative terms in the equations of motion. We thus propose a quantitatively-accurate extension of the Hertzian model encompassing realistic material dissipative effects. Experiments and computations with steel, brass, and polytetrafluoroethylene reveal a common dissipation exponent (for a discrete Laplacian of the velocities) with a material-dependent prefactor. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q14.00005: Anisotropies in granular temperature in a dense sheared granular flow Chris Rycroft, Ashish Orpe, Arshad Kudrolli We investigate a three-dimensional, slow, gravity-driven, sheared granular flow, making use of both simulation (carried out using the Discrete-Element Method) and experiment (using glass beads, imaged via an index-matched fluid). We begin by performing a quantitative comparison between the two procedures, concentrating on the level of agreement at the microscopic scale. After establishing how well the simulation can reproduce the microscopic fluctuations in particle velocities seen in experiment, we proceed to carry out a tensorial analysis of granular temperature. Our results show different types of behavior near the boundary and in the bulk of a granular flow, due to differences in the particle packing structure, and highlight anisotropies that may have implications for granular continuum modeling. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q14.00006: Correlation Functions of a Homogeneously Driven Granular Fluid in Steady State Katharina Vollmayr-Lee, Timo Aspelmeier, Annette Zippelius We study a homogeneously driven granular fluid of hard spheres at intermediate volume fractions and focus on time-delayed correlation functions in the stationary state. The results of computer simulations using an event driven algorithm are compared to the predictions of generalized fluctuating hydrodynamics. The incoherent scattering function ($F_{\rm incoh}(q,t)$) follows time-superposition and is well approximated by a Gaussian $F_{\rm incoh}=\exp \left ( - \frac{q^2}{6} \langle \Delta r^2(t) \rangle \right )$. For sufficiently small wavenumber $q$ we observe sound waves in the intermediate scattering function $F(q,t)$ and in the longitudinal current correlation function $C_l(q,t)$. We determine their dispersion and damping. Temperature fluctuations are predicted to be either diffusive or nonhydrodynamic, depending on wavenumber and inelasticity as characterized by incomplete normal restitution. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q14.00007: Interparticle friction between gently contacting spheres Greg Farrell, Narayanan Menon In previous experimental work we have found that the packing fraction of gently-sedimented monodisperse spheres is affected by particle roughness as well as the viscosity and buoyancy provided by the surrounding fluid. In order to provide a macroscopic quantification of the microscopic effects of particle surface and of the fluid, we have developed a new technique to measure the coefficients of static and kinetic friction between two spheres in a fluid. We find that even in fluid environments, there are static and kinetic coefficients of friction characteristic of solid-on-solid contact. Surprisingly, even for a given pair of spheres, we measure a broad range of friction coefficients corresponding to contacts made at different locations on the surface. Thus, even for lubricated surfaces, surface heterogeneity is more apparent for small normal forces than at familiar force-scales. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q14.00008: Spatial Force Correlations in 3D Granular Flow . Nalini Easwar, Kelsey Hattam, Efrosyni Seitaridou, Alisa Stratulat, Narayanan Menon We measure the force delivered at four locations on the boundary of a 3D flow of mono-disperse glass spheres in a vertical, cylindrical chute. A variable opening at the bottom is used to change the flow velocity v$_{f}$ from 3 to 30cm/s. The force is measured at 80KHz, allowing us to resolve individual collisions. We measure two-point spatial correlations in the flow direction and normal to it. The equal-time correlation between forces that are higher than a threshold shows a weak but measurable spatial correlation. This correlation shows no spatial directionality or dependence on flow rate. The time correlations are synchronous between diametrically opposed locations, and shifted in time between locations along the flow. From the time-lag we determine that the correlations are carried up the flow at speeds $\sim $ 1000 v$_{f}$ . This speed increases as the flow approaches jamming. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q14.00009: Impact phenomena in fluidized granular matter Patrick Mayor, Hiroaki Katsuragi, Douglas Durian Projectiles dropped into granular media form a crater and come to rest in a particular way that has been actively investigated in numerous studies. These impact phenomena illustrate how particulate materials respond to externally applied forces. Several recent experiments have focused on the penetration of projectiles impacting granular materials at relatively low speeds, and measured the dynamics of the impact process, yielding force laws accounting for the observations. We present results showing how granular impacts are affected when the load on the grains is modified using a vertical gas flow. Balls or cylinders are dropped into a dry, noncohesive granular medium and we measure the penetration depth when gas is flown upward (thus unloading the contacts) or downward (loading the contacts). We observe that the frictional drag decreases linearly with the flow rate, and vanishes completely once the system is fluidized. Different projectile geometries allow us to separate the effect of normal and tangential frictional forces. We also consider the case of objects that are lowered quasistatically into the granular medium and measure the net vertical force exerted by the granular system on the objects at each immersion depth. We then discuss how this resistance force compares with the forces observed in actual impacts experiments. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q14.00010: A Statistical Approach to the Filtration of Rods Scott Franklin We investigate the efficacy of a square-grid mesh at filtering rods from solution. The volume fraction $\phi$ is kept low, reducing the chance of rods cooperatively jamming at the mesh. For round particles, filtering at low $\phi$ is trivially determined by the ratio of particle diameter to mesh size. Because rods have two length scales, filtering is non-trivial for meshes larger than the rod width but smaller than the length, a potentially very large range. We have measured experimentally the probability for a rod to be filtered as a function of mesh size, particle length, and aspect ratio. Results are compared with a theoretical extension of the Buffon-Laplace Needle problem that accounts for finite rod width and an isotropic distribution in the zenith angle. The solution is the probability that a sphero-cylinder in three dimensions makes contact with a 2D sieve-like mesh, a necessary but not sufficient condition for filtration. Comparison of experiment and theory is then suggestive of what conditions are both necessary and sufficient. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q14.00011: Granular Breathing Surajit Sen, Robert Simion, Adam Sokolow We study the dynamics of monodispersed and tapered granular alignments held within a fixed boundary and a moving boundary. The system is assumed to be driven at one end by imparting a constant or time dependent acceleration to the edge grain. Analytical and simulational studies show that such a driven system can eventually get ``over-compressed" and begin to dilate due to repulsive grain-grain interactions. Continuous driving results in the phenomenon of granular breathing. The talk shall discuss the dynamical processes associated with granular breathing for time-independent and time-dependent driving. The phenomenon of nonlinear resonance and related processes that arise in these systems will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q14.00012: Combustion of Micropowdered Biomass Ethan Geil, Robert Thorne Combustion of finely powdered biomass has the potential to replace heating oil, which accounts for a significant fraction of US oil consumption, in heating, cooling and local power generation applications. When ground to 30-150 micron powders and dispersed in air, wood and other biomass can undergo deflagrating combustion, as occurs with gaseous and dispersed liquid fuels. Combustion is very nearly complete, and in contrast to sugar/starch or cellulose-derived ethanol, nearly all of the available plant mass is converted to usable energy so the economics are much more promising. We are exploring the fundamental combustion science of biomass powders in this size range. In particular, we are examining how powder size, powder composition (including the fraction of volatile organics) and other parameters affect the combustion regime and the combustion products. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q14.00013: On the nonlocality of the fractional Schr\"{o}dinger equation Shiliyang Xu, Monwhea Jeng, Eli Hawkins, J.M. Schwarz A wide variety of stochastic processes are more general than the familiar Brownian motion, but presumably can still be described by modifying the diffusion equation using a fractional Laplacian operator. In analogy with fractional diffusion, the fractional Schr\"{o}dinger equation is the ordinary Schr\"{o}dinger equation with the fractional Laplacian operator replacing the ordinary one. Over the past eight years, a number of papers have claimed to solve the fractional Schr\"{o}dinger equation for systems ranging from the one-dimensional infinite square well to the Coulomb potential to one-dimensional scattering with a rectangular barrier. However, some of the claimed solutions ignore the fact that the fractional diffusion operator is inherently nonlocal, preventing the fractional Schr\"{o}dinger equation from being solved in the usual piecewise fashion. We focus on the one-dimensional infinite square well and show that the purported groundstate, which is based on a piecewise approach, is definitely not a solution of the fractional Schr\"{o}dinger equation for general fractional parameters $\alpha$. On a more positive note, we present a solution to the fractional Schr\"{o}dinger equation for the one-dimensional harmonic oscillator with $\alpha=1$. Potential physical applications will also be discussed. [Preview Abstract] |
Session Q15: General Topics in Statistical and Nonlinear Physics and Applications to Social Systems
Sponsoring Units: GSNPChair: Sid Redner, Boston University
Room: 316
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q15.00001: Phase transitions by means of information theory E.E. Vogel, G. Saravia, F. Bachmann, B. Fierro, J. Fischer Sequential files for observables show different compression capabilities depending on the temperature T at which the simulation is done. Compressed files reach maximum size at a T coinciding with the critical temperature TC; these TCs can be independently determined by well of established methods (Binder cumulants or time autocorrelation functions). This behavior can be explained by information theory: near the critical temperature values for the observable span a large universe and the sequence is chaotic; under TC the system is trapped within few states yielding monotonous sequences of values; over TC values tend to the corresponding thermal noise. Compression is maximal in presence of repeated information which is to be found away from TC. This new method to obtain TC is successfully applied to the Edwards-Anderson model where a ferromagnetic 2D Ising system is progressively changed by randomly introducing antiferromagnetic interactions in concentration x. Analysis is done for classical Monte Carlo simulations on square lattice of different sizes, increasing x, varying T. Possible extensions of this treatment are discussed. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q15.00002: An Efficient Numerical Approach for Nonlinear Fokker-Planck equations Dustin Otten, Prakash Vedula Fokker-Planck equations which are nonlinear with respect to their probability densities that occur in many nonequilibrium systems relevant to mean field interaction models, plasmas, classical fermions and bosons can be challenging to solve numerically. To address some underlying challenges in obtaining numerical solutions, we propose a quadrature based moment method for efficient and accurate determination of transient (and stationary) solutions of nonlinear Fokker-Planck equations. In this approach the distribution function is represented as a collection of Dirac delta functions with corresponding quadrature weights and locations, that are in turn determined from constraints based on evolution of generalized moments. Properties of the distribution function can be obtained by solution of transport equations for quadrature weights and locations. We will apply this computational approach to study a wide range of problems, including the Desai-Zwanzig Model (for nonlinear muscular contraction) and multivariate nonlinear Fokker-Planck equations describing classical fermions and bosons, and will also demonstrate good agreement with results obtained from Monte Carlo and other standard numerical methods. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q15.00003: Universal formulation of Casimir forces for static objects with arbitrary shapes and susceptibilities Sahand Rahi We have derived a general formula for the Casimir energy of any number of objects with arbitrary but linear electric permittivities and magnetic permeabilities. The formula is more general than existing ones since it applies to more than two objects and allows arbitrary shapes. It even allows for objects to be inside of one another and thus enables the study of the Casimir force between one object and a cavity. It requires as input the T-matrix of each individual object. The formula is applied to the geometry of two infinite cylinders, two cylinders opposite one or two metallic plates, and a dielectric sphere and dielectric plate. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q15.00004: Triggering and control of stick-slip friction Shmuel M. Rubinstein, Gil Cohen, Jay Fineberg Even regular stick slip frictional sliding always has some stochasticity associated to it. This stochasticity appears as uncertainty in the period between consecutive slip events. We show that once harmonic perturbations are introduced to the shear loading this picture changes significantly. Even relatively small perturbations can trigger the slip instability causing it to occur at a specific phase of the perturbation. This triggering either eliminates the stochastic element completely, or constrains it so that the stick-slip periods differ by discrete multiples of the period perturbation. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q15.00005: Topological order and topological memories at finite temperature Alioscia Hamma, Claudio Castelnovo, Claudio Chamon We discuss the notion of topological order and topological memories at finite temperature. We argue that topological order is given by a long range pattern of entanglement and relaxation times that scale with the size of the system. We obtain the behavior of topological entropy in 3D for the toric code and similar models, showing that half of it is completely washed out in the thermodynamic limit at any finite temperature. This corresponds to the fact that quantum memory is spoiled at any finite temperature and relaxation times are independent of the size of the system. We also study the possibility of obtaining long lived metastable states by confining defects by means of effective long ranged interactions. We discuss the implications for the existence of topological order and stable quantum memories at temperatures different from zero. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q15.00006: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q15.00007: Extended Universality in Potts Models on Square and Triangular Lattices Mikael Wood, Carlos Wexler It has been recently discovered [1] that some families of systems exhibit universal behavior both near and away from any critical point. Specifically, it has been shown that all thermodynamic observables of the p-state Potts model on a square lattice collapse to the values of the 2-D planar XY model above a certain ``extended universality'' temperature $T_{eu}$ [1]. We have extended these results to the Potts model on triangular and honeycomb lattices. We hypothesize that such sharp transition between discrete and continuous behavior of the observables is due to a Nyquist-Shannon type theorem for statistical mechanics. We present evidence for this interpretation and discuss its relevance to emergent systems. \\[4pt] [1] {\em Universality away from critical points in two-dimensional phase transitions}; C.M. Lapilli, P. Pfeifer, and C. Wexler; Phys. Rev. Lett. {\bf 96}, 140603 (2006). [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q15.00008: Enhanced Transition Matrix Methods David Yevick, Michael Reimer Recently we have adapted the transition matrix Monte-Carlo method to general communication systems [IEEE Photon.Technol. Lett. 1529 (2007), IEEE Communications Letters, 755 (2008)]. In these studies we compared and integrated different multicanonical and transition-matrix methods. We determined that the standard multicanonical method can be reformulated more simply and accurately by constructing the intermediate probability density function (density of states) after a small number of Markov transitions from the ratios of the elements of the transition matrix between adjacent states. Further, we considered an algorithmically more simple procedure in which transitions only occur from a given state to another state that has previously been less frequently sampled. Here we found that while substantial errors often result if the perturbation that links adjacent Markov states is small, numerical accuracy can be restored by restricting the Markov chain to within a single histogram bin for a certain number of transitions before allowing transitions to other bins. We finally summarize our application of these procedures to several problems in optical and wireless communication theory. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q15.00009: $S$-index: Measuring significant, not average, citation performance Manolis Antonoyiannakis We recently [1] introduced the ``citation density curve'' (or cumulative impact factor curve) that captures the full citation performance of a journal: its size, impact factor, the maximum number of citations per paper, the relative size of the different-cited portions of the journal, etc. The citation density curve displays a universal behavior across journals. We exploit this universality to extract a simple metric (the ``$S$-index'') to characterize the citation impact of ``significant'' papers in each journal. In doing so, we go beyond the journal impact factor, which only measures the impact of the average paper. The conventional wisdom of ranking journals according to their impact factors is thus challenged. Having shown the utility and robustness of the $S$-index in comparing and ranking journals of different sizes but within the same field, we explore the concept further, going beyond a single field, and beyond journals. Can we compare different scientific fields, departments, or universities? And how should one generalize the citation density curve and the $S$-index to address these questions? [1] M. Antonoyiannakis and S. Mitra, ``Is PRL too large to have an `impact'?'', Editorial, Physical Review Letters, December 2008. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q15.00010: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q15.00011: Statistical laws for career longevity Alexander Petersen, Woo-Sung Jung, Jae-Suk Yang, H. Eugene Stanley Career length distinguishes successful long tenures from unsuccessful short stints, and partially reflects the contributions of an employee to the goals of the employer. In some professions, there are well-defined metrics that quantify career longevity, prowess, and productivity, which together contribute to the overall success rating for an individual employee. In this talk, I motivate a stochastic model for career development that relies on two key ingredients, random progress within the career and random stopping times terminating the career. This model is exactly solvable, predicting the probability density function (pdf) of career longevity, characterized by two parameters, $\alpha$ and $x_{c}$. The parameter $\alpha$ quantifies the power-law scaling of the pdf, which is terminated by an exponential cutoff after a crossover value $x_{c}$, representing the mean career lifetime. We test the model with the large quantity of empirical data available for several professional sports leagues, American baseball, Korean baseball, American basketball, and English soccer, finding excellent agreement with the model's predictions. In all, the generality of the model suggests that there may be common stochastic forces that underly progress, success, and longevity in various professions. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q15.00012: Battles: Intelligent Army versus Insurgency Linda Shanahan, Surajit Sen A ``simple'' battle can be thought of as a conflict between two parties, each with finite reserves, and typically fought on one sideās territory. Modern battles are often strategic, based largely on the speed of information processing and decision making and are mission oriented rather than to annex new territory. Here, we analyze such battles using a simple model in which the ``blue'' army fights a {\bf strategic} battle against a ``red'' army that is well matched in combat power and in redās territory. We assume that the blue army attacks strategically while the red army attempts to neutralize the enemy when in close enough proximity, implemented here as ``on- site,'' with randomly varying force levels to potentially confuse and drive the blue's strategies. The temporal evolution of the model battles incorporate randomness in the deployment of the reds and hence possess attendant history dependence. We show that minimizing risk exposure and making strategic moves based on local intelligence are often the deciding factors that determine the outcome of battles among well matched adversaries. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q15.00013: Complete trails of social network evolution: The past 10 years of complex network research Deokjae Lee, Kwang-Il Goh, Byungnam Kahng, Doochul Kim During the last 10 years since the publication of pioneering papers on small-world and scale-free networks, more than 5,500 distinct researchers produced more than 4,000 research papers on complex networks, setting an unprecedented example in the history of science. Based on the dataset of published papers on complex networks during the years 1998--2007, here we study the complete evolution of the co-authorship network in network science. This dataset allows us to study the complete trail of social network evolution from the inception, in particular in the early transient stage, which has not been addressed empirically in previous studies. We find that distinct patterns in network topology emerge during the evolution: A fractal, tree-like giant cluster forms in the early stage through the cluster aggregation process, akin to the pattern near the percolation point, then followed by the entanglement process due to appearance of large-scale loops in later times. This evolution pattern is also observed in the co-authorship network on string theory. The giant cluster is found to be dynamic yet robust upon removal of obsolete inactive links, providing the core part underneath the further developed network. Finally, based on the empirical observations, we introduce a network evolution model, successfully reproducing the observed patterns. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q15.00014: Statistical mechanics of image processing by digital halftoning Jun-ichi Inoue, Wataru Norimatsu, Yohei Saika, Masato Okada We consider the problem of digital halftoning (DH). The DH is an image processing representing each grayscale in images in terms of black and white dots, and it is achieved by making use of the threshold dither mask, namely, each pixel is determined as {\it black} if the grayscale pixel is greater than or equal to the mask value and as {\it white} vice versa. To determine the mask for a given grayscale image, we assume that human-eyes might recognize the BW dots as the corresponding grayscale by linear filters. Then, the Hamiltonian is constructed as a distance between the original and recognized images which is written in terms of the mask. Finding the ground state of the Hamiltonian via deterministic annealing, we obtain the optimal mask and the BW dots simultaneously. From the spectrum analysis, we find that the BW dots are desirable from the view point of human-eyes modulation properties. We also show that the lower bound of the mean square error for the inverse process of the DH is minimized on the Nishimori line which is well-known in the research field of spin glasses. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q15.00015: Molecular Kinetic Analysis of a Finite-Time Carnot Heat Engine Yuki Izumida, Koji Okuda We show the first derivation of the efficiency at the maximum power for a finite-time Carnot heat engine of a weakly interacting gas which we can regard as a nearly ideal gas. Using this simple model, we check the celebrated Curzon-Ahlborn (CA) efficiency by performing the event-driven MD simulation as a numerical experiment for the first time[1,2]. This numerical experiment reveals that the CA efficiency is realized only in the limit of the small temperature difference $T_{\mathrm{c}}$ $\rightarrow$ $T_{\mathrm{h}}$ where $T_{\mathrm{h}}$ and $T_{\mathrm{c}}$ are the temperatures of the hot and cold heat reservoirs, respectively. Our molecular kinetic analysis can explain the numerical results theoretically. \newline[1] F. Curzon and B. Ahlborn, Am. J. Phys. \textbf{43}, 22 (1975). \newline[2] Y. Izumida and K. Okuda, Europhys. Lett. \textbf{83}, 60003 (2008). [Preview Abstract] |
Session Q16: Focus Session: Dipolar Gases and Ultra-Cold Molecules
Sponsoring Units: DAMOPChair: Dan Stamper-Kurn, University of California, Berkeley
Room: 317
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q16.00001: Novel quantum magnets: Dipolar quantum gases Invited Speaker: Experimental creation of chromium condensate with large magnetic dipole moment and ultracold polar molecules with large electric dipole moment has generated significant interest in dipolar quantum gases. The long-range and anisotropic nature of the dipolar interaction potential greatly enriches the properties of the system, many of which are not present in their non-dipolar counterparts. In this talk, I will present our recent studies on dipolar BEC as well as dipolar fermions, with the emphasis on the latter which is relatively less studied. From a semi-classical calculation based upon the phase space representation of the Fermi gas, I will show how the dipolar interaction deforms the Fermi surface and manifests itself in collective excitations and time-of-flight expansion dynamics of the cloud. Finally, I will describe a self-consistent Hartree-Fock-Bogoliubov theory that also takes dipolar induced fermionic superfluid pairing into account. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:27PM |
Q16.00002: Ultracold Heteronuclear Fermi-Fermi Molecules Invited Speaker: Spin mixtures of quantum-degenerate fermionic gases exhibit long lifetimes in the strongly-interacting regime near a Feshbach resonance. This has opened the door for numerous key experiments like the creation of Fermi-Fermi molecules, the realization of molecular BEC, the observation of a pairing gap and of superfluidity in a fermionic gas in the BEC-BCS cross-over region near a Feshbach resonance. We present the production of $^{6}$Li-$^{40}$K heteronuclear molecules based on our experimental platform for the production of a two-species mixture of quantum-degenerate Fermi gases. Our production scheme for quantum-degenerate fermionic $^{6}$Li and $^{40}$K and bosonic $^{87}$Rb gases is based on multiple species magneto-optical trapping [1] and sympathetic cooling of the fermions by rubidium. We demonstrated catalytic cooling of lithium by potassium, overcoming the small lithium rubidium cross section. We achieved to simultaneously enter quantum degeneracy for all three species [2] with lowest temperatures of 0.25 and 0.35 times the Fermi temperature for lithium and potassium at about 260 nK. The highest atom numbers achieved are 1.8x10$^{5}$ for lithium as well as potassium, and about 1x10$^{5}$ for rubidium. We studied two s-wave Feshbach resonances between lithium and potassium [3] at 155 G and 168 G. By magnetic field sweeps we created about 4$\cdot $10$^{4 6}$Li-$^{40}$K molecules at conversion efficiencies of up to 50 {\%} [4]. With a Stern-Gerlach purification technique we are able to image molecules and atoms spatially separated from each other. We discuss the lifetime of the molecule-atom mixture close to resonance. \\[4pt] References: \\[0pt] [1] M.Taglieber, A.-C.Voigt, F.Henkel, S.Fray, T.W.H\"{a}nsch, and K. Dieckmann, Phys. Rev. A 73, 011402(R) (2006)\\[0pt] [2] M. Taglieber, A.-C. Voigt, T. Aoki, T. W. H\"{a}nsch, and K. Dieckmann, Phys. Rev. Lett., 100, 010401, (2008)\\[0pt] [3] E.Wille, et al., PRL, 100,053201,(2008)\\[0pt] [4] A.-C. Voigt, M. Taglieber, L. Costa, T. Aoki, W. Wieser, T.W. Haensch, and K. Dieckmann, accepted for publication in Phys. Rev. Lett. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q16.00003: Quantum phases of a two-dimensional dipolar Fermi gas Georg Bruun, Edward Taylor We examine the superfluid and collapse instabilities of a quasi two-dimensional gas of dipolar fermions aligned by an orientable external field. It is shown that the interplay between the anisotropy of the dipole-dipole interaction, the geometry of the system, and the $p$-wave symmetry of the superfluid order parameter means that the effective interaction for pairing can be made very large without the system collapsing. This leads to a broad region in the phase diagram where the system forms a stable superfluid. Analyzing the superfluid transition at finite temperatures, we calculate the Berezinskii--Kosterlitz--Thouless temperature as a function of the dipole angle. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q16.00004: Fermi surface distortions in a neutral Fermi fluid with dipolar interactions Benjamin M. Fregoso, Kai Sun, Eduardo Fradkin, Benjamin Lev We show that the Fermi surface of a neutral fluid of fermions with aligned dipole moments by an external field is elongated along the direction of the aligning field. The distortion of the Fermi surface can be expressed as a linear combination of Legendre polynomials $P_l(\hat{k})$ with $l=even$, with $l=2$ being the leading term. The possible existence of a phase transition to a spontaneous biaxial phase is discussed. The zero- sound collective modes of the system are found to be strongly anisotropic. We discuss the possible use of light scattering experiments to detect spatial anisotropies in dipolar gases. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q16.00005: Devil's staircase and supersolidity in one-dimensional dipolar Bose gases S.L. Sondhi, F.J. Burnell, Meera M. Parish, N.R. Cooper The classical ground states of particles in a convex repulsive potential are known to have a phase portrait displaying a complete devil's staircase structure. We consider a single- component gas of dipolar bosons confined in a one-dimensional optical lattice, where the dipoles are aligned such that the long-ranged dipolar interactions are maximally repulsive. Introducing a kinetic term tunes the system away from the classical limit and results in a phase diagram with a Mott- Hubbard lobe for each rational filling fraction. Tuning the on- site interaction away from convexity yields alternative commensurate states with double occupancies which can form a staircase of their own, as well as one dimensional ``supersolids'' which simultaneously exhibit discrete broken symmetries and superfluidity. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q16.00006: Roton softening and supersolidity in Rb spinor condensates Robert Cherng, Eugene Demler Superfluids with a tendency towards periodic crystalline order have both a phonon and roton like spectrum of collective modes. The softening of the roton spectrum provides one route to a supersolid. We show that roton softening occurs in $^{87}$Rb spinor condensates once dipolar interactions and spin dynamics are taken into account. By including the effects of a quasi-two-dimensional geometry and rapid Larmor precession, we show a dynamical instability develops in the collective mode spectrum at finite wavevectors. We construct phase diagrams showing a variety of instabilities as a function of the direction of the magnetic field and strength of the quadratic Zeeman shift. Our results provide a possible explanation of current experiments in the Berkeley group Phys. Rev. Lett. 100:170403 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q16.00007: Hexatic, Wigner crystal and superfluid phases of dipolar bosons Kaushik Mitra, Carl Williams, Carlos Sa de Melo The finite temperature phase diagram of two-dimensional dipolar bosons versus dipolar interaction is discussed for different values of short range repulsions. We identify the stable phases as superfluid, dipolar Wigner crystal (DWC), dipolar hexatic liquid crystal (DHLC), and normal fluid. In particular, we show that the DWC exists at low temperatures for large dipolar interactions, but it melts into a DHLC at higher temperatures, where translational lattice order is destroyed, but orientational order is preserved. Upon further increase in temperature the DHLC phase melts into the normal fluid, where both orientational and translational lattice order are absent. We also find that the supersolid phase has always higher energy than the superfluid or Wigner crystal phases at low temperatures, but the supersolid is metastable, having an energy minimum that may be accessed through thermal quenching. Lastly, we calculate the static structure factor for each of the stable phases and show that each phase can be identified uniquely in an optical Bragg scattering experiment. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q16.00008: Dipole moments of ultra-cold polar molecules: a quantum Monte Carlo study Michal Bajdich, Shi Guo, Lubos Mitas, Peter J. Reynolds Recently, there has been a great deal of interest in the production of ultra-cold heteronuclear molecules having large electric dipole moments [1]. This is of interest both for fundamental reasons as well as for applications such as qubits for quantum computing [2]. In this work, we calculate the dipole moment of a potentially implementable two-atom alkaline-alkaline-earth molecule, LiSr. We use correlated wave-function methods including both the quantum chemical configuration interaction (CI) method, and a stochastic quantum Monte Carlo (QMC), to calculate the potential energy surface and dipole moment. We study the dipole moment with increasing accuracy of correlated wave-functions. We then variationally re-optimize the wave-functions, which then serve as the representation of the Fermion nodes in the fixed-node QMC. To treat the Sr atom we employ two types of effective core potentials (ECPs), large core ECPs have only $s$-states in the valence space, while the small core ECP's valence space includes also the highest $s$ and $p$ semi-core sub-shells. We find significant sensitivity of thedipole moment on both the size of the valence space and on the accuracy of the Fermion nodes. [1] B. Damski, {\it et. al} Phys. Rev. Lett. 90, 110401 (2003). [2] D. DeMille, Phys. Rev. Lett. 88, 067901 (2002). [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q16.00009: Using an optical lattice to preform KRb molecules and enhance the efficiency of ultracold polar molecule formation James Freericks, Maciej Maska, Romuald Lemanski, Thomas Hanna, Paul Julienne We will discuss recent computational work that employs both direct quantum Monte Carlo simulation and inhomogeneous dynamical mean-field theory to study the efficiency of preforming KRb pairs in an optical lattice. We will describe how to optimize the efficiency by adjusting the lattice depth and the interspecies interaction (via the Feshbach resonance) with parameters specific for fermionic $^{40}$K and bosonic $^{87}$Rb (since the ground-state dipolar molecule has already been formed from those atoms in free space). We work with a deep enough lattice that the K atoms are mobile, but the Rb atoms are localized, so the system is described by the spinless Falicov-Kimball model on a two-dimensional lattice. We also calculate the entropy and estimate the temperature that one can achieve by cooling the atoms and adiabatically turning on the lattice. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q16.00010: The bound states of ultracold KRb molecules Paul Julienne, Thomas Hanna Recently ultracold vibrational ground state $^{40}$K$^{87}$Rb polar molecules have been made using magnetoassociation of two cold atoms to a weakly bound Feshbach molecule, followed by a two-color optical STIRAP process to transfer molecules to the molecular ground state [1]. We have used accurate potential energy curves for the singlet and triplet states of the KRb molecule [2] with coupled channels calculations to calculate all of the bound states of the $^{40}$K$^{87}$Rb molecule as a function of magnetic field from the cold atom collision threshold to the {\$}v=0{\$} ground state. We have also developed approximate models for understanding the changing properties of the molecular bound states as binding energy increases. Some overall conclusions from these calculations will be presented. [1] K.-K. Ni, S. Ospelkaus, M. H. G. de Miranda, A. Peer, B. Neyenhuis, J. J. Zirbel, S. Kotochigova, P. S. Julienne, D. S. Jin, and J. Ye, Science, 2008, 322, 231--235. [2] A. Pashov, O. Docenko, M. Tamanis, R. Ferber, H. Kn\"{o}ckel, and E. Tiemann, Phys. Rev. A, 2007, 76, 022511. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q16.00011: Multichannel quantum defect theory model of Feshbach resonances Thomas Hanna, Eite Tiesinga, Paul Julienne Multichannel quantum defect theory (MQDT) has a large number of applications in atomic physics, including the properties of collisions near threshold. The key concept is that the short range physics can be accounted for very simply and then matched to the asymptotic long range interaction. We have developed a model of Feshbach resonances based on the ideas of MQDT. This model allows calculation of the magnetic fields at which resonances occur, as well as properties such as the resonance width and background scattering length. Apart from known atomic properties, only three input parameters are required: the singlet and triplet scattering lengths, and the coefficient of the long range van der Waals potential. Analytic reference functions defined by the potential [1] are used to calculate the long range properties, which are linked to the short range physics through a frame transformation. We apply our theory to $^{6}$Li-$^{40}$K scattering, and obtain good agreement with experimental data and full coupled channels calculations, but with far less computational effort. This makes MQDT a useful tool for investigating collisions of new combinations of species. [1] B. Gao \textit{et al.}, Phys. Rev. A \textbf{72}, 042719 (2005). [Preview Abstract] |
Session Q17: Superconducting Qubits: Coherence and New Implementations
Sponsoring Units: DPOLYChair: Matthias Steffen, IBM
Room: 318
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q17.00001: Experimental progress toward single phonon creation in a mechanical resonator Aaron O'Connell, M. Ansmann, R. C. Bialczak, M. Hofheinz, E. Lucero, M. Neeley, D. Sank, H. Wang, J. Wenner, J. M. Martinis, A. N. Cleland Coupling a high frequency ($\sim $6 GHz) mechanical resonator to a Josephson phase qubit may enable the creation and manipulation of single phonons. Previously, we have shown the creation of arbitrary photon states in a superconducting coplanar waveguide (CPW) resonator coupled to a phase qubit (Max Hofheinz et al., Nature 454, 310-314, 2008). That experiment illustrated the capability of the phase qubit to controllably create and measure quantum states in a capacitively coupled resonator. By replacing the CPW resonator with a film bulk acoustic resonator (FBAR) we can potentially transfer the quantum state of the phase qubit to a vibrational mode of the FBAR. This talk will focus on our experimental progress to date in realizing this aim. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q17.00002: Lower limit on the achievable temperature in resonator-based sideband cooling M. Grajcar, S. Ashhab, J.R. Johansson, F. Nori A resonator with eigenfrequency $\omega_r$ can be effectively used as a cooler for another linear oscillator with a much smaller frequency $\omega_m \ll \omega_r$. A huge cooling effect, which could be used to cool a mechanical oscillator below the energy of quantum fluctuations, has been predicted by several authors. However, here we show that there is a lower limit $T^*$ on the achievable temperature, given by $T^* = T_{m} \; \omega_m / \omega_r$, that was not considered in previous work and can be higher than the quantum limit in realistic experimental realizations. We also point out that the decay rate of the resonator, which previous studies stress should be small, must be larger than the decay rate of the cooled oscillator for effective cooling. M. Grajcar, S. Ashhab, J.R. Johansson, F. Nori, Lower limit on the achievable temperature in resonator-based sideband cooling, Phys. Rev. B 78, 035406 (2008). URL: http://link.aps.org/abstract/PRB/v78/e035406 [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q17.00003: Strong single-qubit lasing and cooling at the symmetry point? Carsten Hutter Recent theoretical and experimental work discussed the possibility to achieve single-qubit lasing and cooling in systems of driven superconducting qubits coupled to an oscillator. The considered system and Hamiltonian in Refs. [1,2] were such that a first-order coupling term vanishes at the symmetry point. While Ref. [2] operated close to the symmetry point for using both first and second order coupling terms, another interesting regime would be the one with the opposing conditions of lowest dephasing (at the symmetry point) and strongest coupling (far away from the symmetry point).\\ Here, I address the question whether it is possible by different design to achieve single-qubit lasing or cooling with both strongest coupling and lowest dephasing, without compromising between the two. Starting from a more general model Hamiltonian than Refs. [1,2], I find the optimal conditions for a strong first order coupling at the symmetry point, realized by a different Hamiltonian than the one used in Refs. [1,2]. I also suggest first designs, which could realize this alternative model Hamiltonian, and discuss their practical limitations.\\ {[1]} J. Hauss, A. Fedorov, C. Hutter, A. Shnirman, G. Sch{\"o}n, Phys. Rev. Lett. 100, 037003 (2008)\\ {[2]} M. Grajcar et al, Nature Physics 4, 612 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q17.00004: Two-level systems driven by large-amplitude fields F. Nori, S. Ashhab, J.R. Johansson, A.M. Zagoskin We analyze the dynamics of a two-level system subject to driving by large-amplitude external fields, focusing on the resonance properties in the case of driving around the region of avoided level crossing. In particular, we consider three main questions that characterize resonance dynamics: (1) the resonance condition, (2) the frequency of the resulting oscillations on resonance, and (3) the width of the resonance. We identify the regions of validity of different approximations. In a large region of the parameter space, we use a geometric picture in order to obtain both a simple understanding of the dynamics and quantitative results. The geometric approach is obtained by dividing the evolution into discrete time steps, with each time step described by either a phase shift on the basis states or a coherent mixing process corresponding to a Landau-Zener crossing. We compare the results of the geometric picture with those of a rotating wave approximation. We also comment briefly on the prospects of employing strong driving as a useful tool to manipulate two-level systems. \\ \\ S. Ashhab, J.R. Johansson, A.M. Zagoskin, F. Nori, Two-level systems driven by large-amplitude fields, Phys. Rev. A 75, 063414 (2007). S. Ashhab et al, unpublished. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q17.00005: Quantum coherence in a Josephson junction array circuit. Vladimir Manucharyan, Jens Koch, Leonid Glazman, Robert Schoelkopf, Steven Girvin, Michel Devoret We introduce a novel superconducting quantum electrical circuit where a small capacitance Josephson tunnel junction is shunted by an array of larger junctions to form a loop. The loop is capacitively coupled to a microwave transmission line resonator in order to perform a dispersive readout of the qubit state. The low-lying energy states of such circuit belong to the microwave band and tune with magnetic flux threading the loop. Our circuit differs significantly from the well-established charge, flux and phase qubit circuits. Namely, while staying highly anharmonic, the energy spectrum is neither sensitive to the offset charges nor it is exponentially sensitive to the junction parameters or flux bias. We demonstrate experimentally strong coupling to the readout resonator, map the spectrum over wide range of bias fluxes and frequencies and observe coherence times in excess of one microsecond. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q17.00006: Quantum information processing with a Josephson ring modulator Nicolas Bergeal, Flavius Schackert, Michael Metcalfe, R. Vijay, Vladimir Manucharyan, Luigi Frunzio, Robert Schoelkopf, Steven Girvin, Michel Devoret We have developed and operated a new type of phase preserving parametric amplifier, the Josephson Parametric Converter, which approaches the quantum limit. Our device consists of two microwave resonators coupled to each other through a Josephson Ring Modulator. This latter element resembles a DC-SQUID, but has four junctions, and four active current modes instead of two. A pump line is non-resonantly coupled to one of the modes of the ring while the signal and idler are serviced by two others and are tuned in the band of the resonators. The fourth mode, which is the dc superconducting circulating current in the ring, is biased with half a flux quantum. Our design ensures that the non-linearity presented by the Ring Modulator is pure and involves the minimal number of modes, thus placing the JPC very close to the ideal non-degenerate parametric amplifier. This is supported by recent results on the amplification and frequency conversion operations. Furthermore, measurements of the noise temperature with an auto-calibrated source based on a nanowire in the hot electron regime will be presented. In combination with correlation measurements of the noise at the signal and idler ports, these results show that the JPC can perform two-mode squeezing of quantum noise. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q17.00007: 1/f Flux Noise in SQUIDs and Josephson Junction Qubits David Cardamone, Clare Yu 1/f flux noise represents an important, universal source of noise and decoherence in Josephson junction devices, one which must be overcome if their promise of a scalable, reliable mesoscopic qubit is to be achieved. Recent experiments at millikelvin temperatures (S. Sendelbach et al., Phys. Rev. Lett. 100, 227006(2008)) have suggested that this noise may be due to magnetic impurities residing at the surface of the superconductor. We examine this possibility, considering various models for the spacing and interactions of the impurities, and comparing the results of our numerical Monte Carlo simulations with experiment. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q17.00008: Josephson junction array protected from local noises. Sergey Gladchenko, David Olaya, Eva Dupont-Ferrier, Benoit Doucot, Lev Ioffe, Michael Gershenson We have developed small arrays of Josephson junctions (JJs) that can be viewed as prototypes of superconducting qubits protected from local noises [1]. The array consists of twelve superconducting loops interrupted by four sub-micron JJs. The protected state is realized when each loop is threaded by half of the magnetic flux quantum. It has been observed that the array with the optimized amplitude of quantum fluctuations is protected against magnetic flux variations well beyond linear order, in agreement with theoretical predictions [2]. 1. S. Gladchenko et al., ``Superconducting Nanocircuits for Topologically Protected Qubits'', arXiv:cond-mat/0802.2295, to be published in \textit{Nature Physics.} 2. L.B. Ioffe and M.V. Feigelman, \textit{Phys.~Rev.}~B~\textbf{66}, 224503 (2002); B. Doucot \textit{et al}.,\textit{ Phys. Rev}. B \textbf{71}, 024505 (2005); B. Doucot and L.B.~Ioffe,\textit{ Phys. Rev}. B \textbf{76}, 214507 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q17.00009: Optimization and characterization of protected Josephson circuits Eva Dupont-Ferrier, Sergey Gladchenko, Lev Ioffe, Michael Gershenson Recently, it was proposed that small Josephson arrays can operate as superconducting qubits protected from local noises [1,2]. Here we present measurements of several optimized array designs. The read-out circuit for these arrays consists of an inductively-coupled DC SQUID, which helps to minimize perturbations of the system during measurement. We will discuss the current-phase characteristics of these arrays and their response to microwave radiation. Our results indicate that the scattering of Josephson junction parameters can be made small enough to implement the symmetry-protected superconducting qubits; our theoretical model [1] captures all essential features of real devices. 1. see e.g., B. Doucot and L.B.~Ioffe,\textit{ Phys. Rev}. B \textbf{76}, 214507 (2007) and references therein. 2. S. Gladchenko, D. Olaya, E. Dupont-Ferrier, B. Dou\c{c}ot, L.B.~Ioffe, and M.E. Gershenson, ``Superconducting Nanocircuits for Topologically Protected Qubits'', arXiv:cond-mat/0802.2295, to be published in \textit{Nature Physics.} [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q17.00010: Controllable scattering of photons in a one-dimensional resonator waveguide C.P. Sun, L. Zhou, Z.R. Gong, Y.X. Liu, F. Nori We analyze the coherent transport of a single photon, which propagates in a one-dimensional coupled-resonator waveguide and is scattered by a controllable two-level system located inside one of the resonators of this waveguide. Our approach, which uses discrete coordinates, unifies low and high energy effective theories for single-photon scattering. We show that the controllable two-level system can behave as a quantum switch for the coherent transport of a single photon. This study may inspire new electro-optical single-photon quantum devices. We also suggest an experimental setup based on superconducting transmission line resonators and qubits. \\[4pt] L. Zhou, Z.R. Gong, Y.X. Liu, C.P. Sun, F. Nori, Controllable scattering of photons in a 1D resonator waveguide, Phys. Rev. Lett. 101, 100501 (2008). URL: http://link.aps.org/abstract/PRL/v101/e100501 [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q17.00011: Quantum two-level systems in Josephson junctions as naturally formed qubits Alexander Zagoskin, Sahel Ashhab, Robert Johansson, Franco Nori The two-level systems (TLSs) naturally occurring in Josephson junctions constitute a major obstacle for the operation of superconducting phase qubits. Since these TLSs can possess remarkably long decoherence times, we show that such TLSs can themselves be used as qubits, allowing for a well controlled initialization, universal sets of quantum gates, and readout. Thus, a single current-biased Josephson junction (CBJJ) can be considered as a multiqubit register. It can be coupled to other CBJJs to allow the application of quantum gates to an arbitrary pair of qubits in the system. We also show that using the dynamics of a driven qubit, it could be possible to characterize the nature of the two-level systems and their coupling to the phase qubit. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q17.00012: Topological states and braiding statistics using quantum circuits Xiao-Feng Shi, Jianqiang You, Franco Nori Using superconducting quantum circuits, we propose an approach to construct a Kitaev lattice, i.e., an anisotropic spin model on a honeycomb lattice with three types of nearest-neighbor interactions. We study two particular cases to demonstrate topological states (i.e., the vortex and bond states) and show how the braiding statistics can be revealed. Our approach provides an experimentally realizable many-body system for demonstrating exotic properties of topological phases. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q17.00013: Topological Transition in a Non-Hermitian Quantum Walk: a new test for quantumness in driven artificial atoms and Josephson arrays Mark Rudner, Leonid Levitov We analyze a quantum walk on a bipartite one-dimensional lattice, in which the particle can decay whenever it visits one of the two sublattices. The corresponding non-Hermitian tight-binding problem with complex potential for the decaying sites exhibits two distinct phases, distinguished by a winding number defined in terms of the Bloch eigenstates in the Brillouin zone [1]. We find that the mean displacement of a particle initially localized on one of the non-decaying sites is quantized as an integer, changing from zero to one at the critical point. By mapping this problem onto a Jaynes-Cummings-type model with decay, we find that the topological transition is relevant for a variety of experimental settings, in particular for superconducting qubits coupled to high quality resonators [2]. The quantized behavior stands in contrast with the smooth dependence expected for a classical random walk, and can serve as a hallmark of coherent quantum dynamics in ladder-like multilevel systems. A real-space implementation of the quantum walk may help to verify quantum coherence in vortex transport in Josephson arrays [3]. [1] M. S. Rudner, L. S. Levitov, arXiv:0807.2048. [2] A. Wallraff et al., Nature 431, 162-167 (2004). [3] A. van Oudenaarden, S. J. K. Vardy, and J. E. Mooij, Phys. Rev. Lett. 77, 4257 (1996). [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q17.00014: Two Types of Loss Expected for Josephson Qubit Circuits Arthur Davidson The energy of a coulomb blockade capacitor, or of a current biased Josephson junction, is known to depend on the difference between a continuous charge term and a discrete one. For example the energy in the coulomb blockade is the square of (ne-k), where n is an integer, e is the electron charge, and k is a continuous charge value. This suggests that there may be two types of quantum losses in the Schroedinger dynamics of these systems. One type would reduce k to ne to arrive at the ground state energy and would be the quantum analog of external classical resistance. The other type of loss would couple energy bands at constant k, minimizing n, and corresponding to tunnel losses. Historically, two quantum loss terms have been proposed: one due to Mortin Kostin in 1972; and another from the present author in 1990. The Kostin type of loss affects the continuous charge term, while the Davidson type affects the discrete charge term. These properties are linked to the boundary conditions of these systems. The loss mechanisms are likely to be important for understanding coherence times in Josephson qubit circuits. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q17.00015: Entanglement of Two Josephson Vortex Qubits in Resonant Cavity Ramesh Dhungana, Isaac O'Bryant, Ju Kim We discuss the entanglement of two Josephson vortex qubits (JVQs) interacting via the magnetic induction effect. A JVQ may be fabricated by implanting two closely spaced microresistors in the insulating layer of a long Josephson junction (LJJ). These two microresistors generate a double-well potential which traps a Josephson vortex (i.e., fluxon). The macroscopic quantum tunneling (MQT) of the fluxon from one well to another gives rise to a two-state system. The magnetic induction effect in a stack of LJJs introduces an asymmetry in the double-well potential and leads to the interaction between the qubits. We compute the MQT of the fluxons between the minima of the symmetric and asymmetric double-well potentials by using the instanton and the valley-instanton approaches, respectively. We compute the concurrence to estimate the level of entanglement of two JVQs. Our result indicates that the entanglement between the two JVQs is significant. Also, we show that the concurrence of the JVQs, placed in a resonant cavity, is enhanced since the cavity acts as a mediator for the interactions between the JVQs. This suggests that the degree of entanglement may be controlled by varying either the resonant frequency or the strength of coupling between the LJJ and cavity. [Preview Abstract] |
Session Q18: Theory and Simulation I
Sponsoring Units: DPOLYChair: August Bosse, National Institute of Standards and Technology
Room: 319
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q18.00001: Design of Thermoplastic Elastomers with Self-Consistent Mean Field Theory: Radial (ABA)$_n$ and A(BA)$_n$ Miktoarm Architectures Nathaniel Lynd, Folusho Oyerokun, Donal O'Donoghue, Dale Handlin, Glenn Fredrickson Two thermoplastic elastomer designs were evaluated using self-consistent mean field theory. The phase diagram of a radial (A$_1$BA$_2$)$_n$ block copolymer was calculated at $ \chi N$ = 40 per arm as a function of composition ($\emph{f}_A$) and asymmetry between the A end-blocks ($\tau = N_{A1}/(N_{A1}+N_{A2})$). Significant deflection of the phase boundaries towards larger $\emph{f}_A$ occurred for asymmetric triblock copolymers ($\tau\approx 0.15$ and $ \tau\approx 0.90$) due to the interplay between bidispersity and chain-pullout of the A-blocks. The phase diagram of an A$_1$(BA$_2$)$_n$ miktoarm star triblock copolymer was also investigated as a function of $\emph{f}_A$ and $\tau$ at $\chi N$ = 40 per A$_1$BA$_2$ unit. Similar deflections in phase boundaries towards higher $\emph{f}_A$ resulted. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q18.00002: Unified mathematical model for linear viscoelastic predictions of linear monodisperse and polydisperse and branched polymers. Renat Khaliullin, Jay Schieber We present an application of a single-chain mean-field model for entangled linear blends and star-branched systems. Slip-links instead of tubes are employed. The entanglements on a chain are destroyed by two coupled relaxation processes: so-called sliding dynamics; and relaxation of the environment, so-called constraint dynamics. The constraint dynamics are implemented by destruction and creation of the entanglements in the middle of the chain in a way statistically self consistent with sliding dynamics. In contrast to previous tube models, Rouse dynamics is completely avoided. Nonetheless, the implementation of constraint dynamics in tube models is different for linear and branched chains; the slip-link model shows no need for modification of constraint dynamics. Moreover, our slip-link model requires a single fitting parameter $\tau_{\rm K}$ that depends on the temperature of the melt, but not on chain length. The parameter can be fixed from a single fit to linear viscoelastic data. In addition, for branched polymers the branch point movements are determined by the free energy, so that its position is allowed to fluctuation, and even slide through the slip-links. The resulting model exhibits primitive-path fluctuations and chain stretching, so could be applied to flow and generalized to more complicated branches or cross-linked networks without significant modifications. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q18.00003: Self-assembly of rod coil block copolymers under confinement Manas Shah, Venkat Ganesan The interplay of microphase separation and liquid crystalline ordering in rod-coil block copolymers leads to formation of complex morphologies distinct from that of conventional flexible block copolymer phases. In order to be used for organic electronic applications such as photovoltaic cells, rod-coil block copolymers must be patterned into thin films. The final morphology and the nature of orientation of rod units would now depend (in addition to the constituent interactions) on the interactions of the blocks with the confining surfaces. We combine the self-consistent field theory models of rod-coil block copolymers in a thin film framework to understand the effect of confinement on the morphology and the nature of orientation of rod-units. Also for nearly symmetric rod-coil copolymers, we analyze the parallel -- perpendicular lamellae transitions using a free energy framework. Also, we consider morphologies of such block copolymers (and blends) which can be utilized for higher device efficiency in photovoltaic cells. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q18.00004: Theoretically Informed Particle-Based Simulations of Polymers in Arbitrary Ensembles Darin Pike, Francois Detcheverry, Marcus Mueller, Juan de Pablo A new, particle-based formalism is proposed for simulation of polymeric materials, where the interaction energy is given by the standard functional employed in field-theoretic models. The main features of the proposed formalism reside in its ability to enable simulations at constant stress or constant pressure, thereby permitting accurate estimation of free energies and phase boundaries. The usefulness of the proposed approach is illustrated in a series of thermodynamic property calculations from Monte Carlo simulations in the nVT, nPT, semi-grandcanonical and Gibbs ensembles. In particular, we consider the phase separation of a binary homopolymer blend and a symmetric diblock copolymer. For the blend, we present results for the phase diagram and the critical point of the model. For symmetric copolymers, we study the distribution of local stress in lamellae and the location of the first-order transition from a disordered to a lamellar phase. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q18.00005: Fields Help Particles -- Fast Off-Lattice Monte Carlo Simulations of Soft Materials Yuhua Yin, Qiang Wang Conventional molecular simulations of multi-chain systems are hindered by ``hard'' excluded-volume interactions (e.g., the Lennard-Jones potential in off-lattice simulations and the self- and mutual-avoiding walks in lattice simulations). Although such interactions are necessary for obtaining realistic dynamics, they significantly slow down the chain relaxation towards equilibrium configurations and efficient sampling of the configurational space. The idea of fast off-lattice Monte Carlo (FOMC) simulations is to perform particle-based Monte Carlo simulations in continuum with a Hamiltonian commonly used in polymer field theories, where individual polymer segments are modeled as ``soft'' particles whose interaction energy is finite when they overlap. This leads to much faster chain relaxation and better sampling of the configurational space. Furthermore, using the same Hamiltonian in both polymer field theories and FOMC simulations enables quantitative comparisons between them without any parameter-fitting to unambiguously reveal the effects of fluctuations and correlations in the system. Here we demonstrate these great advantages of FOMC simulations using several model systems. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q18.00006: Particles vs. Fields -- Finite-Range Interactions in Polymer Field Theories Qiang Wang Recently, we proposed a particle-based, fast off-lattice Monte Carlo (FOMC) simulation that uses the same Hamiltonian as in polymer field theories, which has great advantages over conventional molecular simulations. However, the continuous Gaussian chain model and $\delta$-function interactions widely used in polymer field theories (such as the self-consistent field theory) cannot be directly used in FOMC simulations. We therefore extend the field theories to the discrete Gaussian chain model and finite-range interactions. Taking the microphase separation of diblock copolymers as an example, a finite interaction range increases the order-disorder transition from the well-known result of $\chi N \approx 10.5$, as well as the bulk lamellar period. More importantly, this work allows direct comparisons between the polymer field theories and FOMC simulations without any parameter-fitting to unambiguously and quantitatively reveal the effects of fluctuations and correlations in the system. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q18.00007: Mean Field Theory for Ionomer Melts Erica Saltzman, Sanat Kumar Single Chain Mean Field theory is applied to melts of charged polymers. Control parameters include temperature, chain length, and monomer density. Equilibrium variations of polymer conformational, translational, and rotational degrees of freedom and counterion translational degrees of freedom are studied; in particular we are interested in conformational and morphological transitions which occur in ionomers with changes in temperature and apparently dominate their macroscale behavior. The equilibrium theoretical results, which are compared to simulation findings of low temperature condensation of chains and counterions to form ordered sheets of charges, form the basis for stochastic theories which model the temporal evolution of these structures, with immediate relevance to measurable dynamic properties. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q18.00008: Equilibrium and Beyond Equilibrium Properties of Polyelectrolytes - Ewald-Like Approach for Fluctuating Hydrodynamic and Electrostatic Interactions Juan P. Hernandez-Ortiz, Michael D. Graham, Juan J. de Pablo A method is proposed for self consistent simulations of the equilibrium and beyond equlibrium structures and transport properties of polyelectrolytes in solution. The method incorporates solution of the Nernst-Planck diffusion equation for ions and counter-ions within the solvent, and simultaneous description of fluctuating hydrodynamic interactions by means of a Green's function formalism. The proposed approach generalizes our $O(N)$ general geometry Ewald-like method to simultaneous treatment of hydrodynamics and electrostatics. With this method, we examine the transport properties of polyelectrolytes solutions at rest and in various flow fields, and we make direct comparisons to results from explicit ion Brownian dynamics simulations and experimental observations. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q18.00009: Self Consistent Field Theory Study of the Effect of Grafting Density on the Height of a Weak Polyelectrolyte Brush Kevin Witte, You-Yeon Won The height of weakly basic polyelectrolyte brushes in the osmotic brush regime is studied as a function of the grafting density using a numerical self-consistent field (SCF) theory derived from the (semi-) grand canonical partition function. The theory is shown to properly account for the local nature of the charge equilibrium and to be able to capture the basic behaviors of polyelectrolyte brushes, including brush height variation with salt concentration and scaling with respect to degree of polymerization. However, we find, in agreement with recent experiments, that the scaling of brush height with grafting density is qualitatively different than that predicted by basic scaling arguments. This difference is attributed to the relative strength of electrostatic type interactions compared to finite segment size packing constraints. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q18.00010: Adsorption and depletion of polyelectrolytes in charged Dadong Yan, Xingkun Man, An-Chang Shi Self-consistent field theory is presented to study the adsorption of flexible polyelectrolytes (PE) onto uniformly oppositely charged cylinders. We focus on the curvature effect of adsorbing surface on the adsorption-depletion phase- transition-like behavior. In terms of the scaling expression of the critical quantities, i.e., the salt concentration, the charge fraction of PE chain and the area density of surface charge, at the adsorption-depletion transition point have been obtained. Moreover, we find a critical line for the dependence of the critical radius of cylinder on the salt concentration, which separates the adsorption and depletion states. The theoretical results are in good agreement with the Monte Carlo simulations and the experimental results. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q18.00011: Field-Based Modeling and Simulation of Interfacial Fluctuations in Block Copolymers August Bosse The Edwards-model-based, field-theoretic simulation framework of Fredrickson is the cutting edge methodology in coarse-grained, field-based simulation of fluctuating copolymer systems [V. Ganesan and G.H. Fredrickson, \textit{Europhys. Lett.} \textbf{55}, 814 (2001); G.H. Fredrickson, V. Ganesan, and F. Drolet, \textit{Macromolecules} \textbf{35}, 16 (2001)]. Coarse graining the standard Edwards model yields the classic phenomenological ``phase field'' model of Ohta and Kawasaki [T. Ohta and K. Kawasaki, \textit{Macromolecules} \textbf{19}, 2621 (1986)]. Further coarse graining, coupled with the assumption of weak segregation, yields the ubiquitous Leibler-Brazovskii-Fredrickson-Helfand model [G.H. Fredrickson and E. Helfand, \textit{J. Chem. Phys.} \textbf{87}, 697 (1987)]. Each of these field-based models is capable of capturing thermodynamic fluctuations; however, the applicability of each model depends on the quench depth, the molecular weight, and the composition of the constituent copolymers, among other variables. Here we examine fluctuation effects in, and limitations of field-based models in the context of measuring interfacial fluctuations in a two dimensional diblock copolymer melt. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q18.00012: Variational Coarse-Graining of Discretized Field Theories of Fluids Michael Villet, Glenn Fredrickson Statistical field theory models have proven to be valuable tools for studying the equilibrium behavior of polymeric fluids, but direct simulation of these field theories without use of the mean field approximation is computationally demanding. Computational resources can be extended to simulate larger systems by discretizing the field variables with a coarsely spaced lattice, but indelicate coarse graining risks truncation of important short-wavelength physics. We introduce a variational method for systematically coarse-graining discretized field theoretic models of fluids while minimizing this truncation error. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q18.00013: Simulation of Fluctuations in Diblock Copolymer Melts: Testing an Alternative to the Fredrickson-Helfand Theory Jian Qin, David Morse Simulations of a bead-spring model of disordered diblock copolymer melts have been conducted to test a renormalized one-loop (ROL) theory of composition fluctuations recently proposed by the authors. The simulations use hybrid Monte Carlo (MC) / Molecular Dynamics (MD), reptation and double-rebriding moves, combined with replica exchange, to relax chain conformations. The quantitative comparison of simulation results with theory relies on a procedure that uses perturbation theory to independently identify the self-consistent-field (or RPA) interaction parameter. For the modest chain lengths accessible to simulations ($N \leq 64$ here), results for the maximum $S(q^{*})$ of the structure factor are quite different from both RPA and Fredrickson-Helfand predictions, but agree very well with renormalized one-loop predictions. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q18.00014: Microphase Separation Induced by the Differential Monomer-Monomer Interactions in Diblock Copolymer/Homopolymer Blends Jiajia Zhou, An-Chang Shi Phase diagrams of blends composed of diblock copolymer (AB) and homopolymer (C) are obtained using the random phase approximation and self-consistent field theory. Emphasis is placed on the special case where all three monomer pairs, A/B, B/C and C/A, are miscible. Despite the miscibility of the binary pairs, a close-loop immiscible region exists in the AB/C blends when the pair interaction parameters are sufficiently different. Inside the close-loop, the system undergoes microphase separation, exhibiting different morphologies. This phenomenon is enhanced when the homopolymer interacts much strongly to one of the blocks of the diblock copolymer. The theoretical results are used to explain some recent experiments. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q18.00015: Study on the strength of intermonomer interactions for PS-b-PMMA using compressible RPA Hyungju Ahn, Du Yeol Ryu, Youngmin Kim, Kwang Hyun Song, Kyung Wook Kwon, Junhan Cho PS-$b$-PMMA copolymer is one of the most useful nanoscopic materials that can be used as passive electronic materials, and also as templates and scaffolds. It is then clear that the better knowledge on the strength of intermonomer interactions for the PS-$b$-PMMA is of great importance in fabricating nanomaterials from it. Using a compressible random-phase approximation (RPA) theory, we discuss mainly the second-order vertex function in the compressible Landau free energy. This vertex function is involved in the exchange energy between self and cross interactions along with the self interaction difference. Ordering transition temperatures are predicted and compared with experimental measurements using small-angle X-ray scattering (SAXS) and depolarized light scattering. A close relationship between barotropicity (ordering upon pressurization) and the energetic vertex term for the copolymer is argued. [Preview Abstract] |
Session Q19: The Physics of Polymer Nanocomposites: Rheology and Mechanical Properties
Sponsoring Units: DPOLYChair: Francis Starr, Wesleyan University
Room: 320
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q19.00001: Influence of Nanoparticles on the Amplitude of Molecular Motions and the Fragility a Model Glass-Forming Polymer Melt. Jack Douglas, Francis Starr We investigate the impact of the addition of nanoparticles on both the fast and slow dynamics of a coarse-grained polymer fluid by molecular dynamics. The fast dynamics is characterized by the Debye-Waller factor (the average mean square particle displacement at a characteristic time in the caged particle motion regime) and the slow structural relaxation is characterized by the coherent intermediate scattering function. Our study explores how both the polymer-particle and nanoparticle volume fraction change the amplitude of the high frequency molecular motions (relative to the pure melt reference condition) and the strength of the temperature dependence of the structural relaxation time (defining the fragility of glass formation, as well as the glass transition temperature). Substantial variations of the Debye-Waller factor are observed and we test the effectiveness of the Buchenau relation linking the Debye-Waller factor to the long time structural relaxation time. We also consider how the presence of nanoparticles in the polymer melt influence the fragility of glass formation, where a range of criteria are utilized to define fragility. Appreciable changes of fragility are observed, these changes being dependent on the nanoparticle concentration and particle-polymer interaction. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q19.00002: Macroscopic dynamics of polystyrene grafted silica nanoparticles in a homopolymer matrix Joseph Moll, Pinar Akcora, Sanat Kumar, Ralph Colby Silica nanoparticles grafted with polymers, dispersed in a homopolymer matrix, and annealed over time adopt a broad range of dispersion states which depend on grafting density, annealing time, weight percent silica, and the molecular weights of the polymers.~ We tuned these variables to give desired dispersion states, from uniformly dispersed particles to agglomerated clusters.~ Rheology was used to critically determine how the dispersion state affects the mechanical reinforcement of the composite.~ We have run both steady shear and small amplitude oscillatory shear experiments on nanocomposites comprising a range of dispersion states. By mapping the observed reinforcement on a morphology diagram, we observe the location of a maximum in reinforcement. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q19.00003: Structure, ion transport and rheology of nanoparticle organic hybrids Haibo Qi, Lynden Archer We report a new class of liquid nanoparticle organic hybrid materials (NOHMs), produced by tethering an organic oligomer corona to the surface of inorganic nanoparticles, and investigate their use as electrolytes. This talk focuses on the structure factor and transport properties of these materials. Specifically, because the suspending solvent is covalently tethered to the NOHMs cores, the structure factor is predicted to vanish in the limit of small q. This behavior arises fundamentally from the presence of a new entropic attraction force produced by the tethered solvent, which constrains separation of the nanoparticle cores. Additionally, we show that NOHMs based on lithium conducting corona provide high ionic conductivities and lithium transfer numbers when doped with lithium salts. The enhanced conductivity is investigated in detail by studying how the core particles affect the melting transition, rheology, and activation energy for ion transport in the corona. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q19.00004: Effect of filler surface properties on stress relaxation behavior of carbon nanofiber/polyurethane nanocomposites I. Sedat Gunes, Guillermo Jimenez, Sadhan Jana The effect of carbon nanofiber (CNF) surface properties on tensile stress relaxation behavior of CNF/polyurethane (PU) nanocomposites was analyzed. PU was synthesized from methylene diisocyanate, polypropylene glycol (PPG diol), and butanediol. CNF, oxidized CNF (ox-CNF), and PPG diol grafted CNF (ol-CNF) were selected as fillers. ol-CNF was obtained by grafting PPG diol onto ox-CNF by reacting it with the carboxyl groups present on ox-CNF surface. The atomic ratios of oxygen to carbon present on the filler surfaces were 0.13 and 0.18 on ox-CNF and on ol-CNF as compared to 0.015 on CNF, mostly due to the presence oxygen containing polar groups on the surfaces of the former. The composites were prepared by in-situ polymerization and melt mixing in a chaotic mixer. The stress relaxation behavior of composites was determined at room temperature after inducing a tensile strain of 100{\%}. The presence of fillers augmented the rate of stress relaxation in composites which was highest in the presence of CNF. The results suggested that relatively weak polymer-filler interactions in composites of CNF promoted higher stress relaxation. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q19.00005: Synthesis of metal-molecule-metal structures for single-molecule transport and spectroscopy measurements Alex Neuhausen, David Goldhaber-Gordon, Chris Chidsey Robust, repeatable metal-molecule contacts are an elusive yet important hurdle in the development of molecular electronic devices. This project explores the chemical synthesis of metal-molecule-metal structures for single-molecule spectroscopy and transport measurements. Conjugated thiol-azide molecules are self-assembled on gold nanoparticles, which are then linked with dialkyne bridge molecules using Sharpless ``click'' reactions. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q19.00006: Impact of Nanofillers on the Durability of Polymeric Coatings and Composites Li-Piin Sung, Stephanie Watson, Aaron Forster, Sheng Lin-Gibson Metal oxide nanoparticles have been incorporated into polymer systems to improve durability performance properties, for example Ultra Violet (UV) degradation and scratch resistance. In this paper, we present recent research results on (1) the effect of particle dispersion and photorectivity of TiO$_{2}$ on the UV degradation of polymeric coatings exposed to high intensity UV radiations at two different humidity conditions; (2) the impact of nano-SiO$_{2}$ concentration on surface mechanical properties (surface morphology and scratch behavior) of polymeric coatings and composites. The physical and chemical degradation of the coatings were monitored in periodic intervals using a combination of laser confocal scanning confocal microscopy (LSCM) and attenuated total reflectance{\-}Fourier transform infrared spectroscopy. An instrumented nanoindentation and LSCM are utilized to measure surface modulus, perform scratch testing, and map scratch damage patterns. A strong impact on the durability performance in both studies was observed in the presence of nanofillers. Particularly in the scratch resistance study, the addition of nanofillers reduces surface roughness and increase scratch resistance of the nanofiller-polymer composites. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q19.00007: The ``Music" of Silica-Poly(methyl methacrylate) Core-Shell Spheres: Eigenvibrations and Mechanical Properties at the Nanoscale Tim Still, Rebecca Sainidou, Goetz Hellmann, George Fytas We report on the measurement of elastic vibrational modes (eigenvibrations) in silica--poly(methyl meth\-acrylate) ($\mathrm{SiO}_2$--PMMA) core-shell spheres and corresponding spherical hollow capsules (PMMA) with different particle size (dia\-meter: 232~nm--405~nm) and shell thickness (25~nm--112~nm) using Brillouin light scattering, supported by numerical calculations.~[T.\ Still et al., Nano Lett.\ \textbf{8}, 3194 (2008)] These localized modes allow to access the mechanical moduli of core and shell material. We observe reduced mechanical strength of the porous silica core and for the core-shell spheres a striking increase of the moduli in both the $\mathrm{SiO}_2$ core and the PMMA shell. The peculiar behavior of the vibrational modes in the hollow capsules is attributed to antagonistic dependence on overall size and layer thickness. The present investigation of the acoustical properties of the individual core-shell particles can lead to the use of such nanoscale engineered particles in more eloborate systems to control hypersonic phonons. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q19.00008: A Microscopic Model for the Reinforcement and the Non Linear Behaviour of Filled Elastomers and Thermoplastic Elastomers (Payne and Mullins Effects) Didier Long, Samy Merabia, Paul Sotta We present a model regarding reinforcement properties of nano- structured polymers. Then, we show how it can be solved numerically by Dissipative Particles Dynamics. The model is based on the presence of glassy layers around the fillers. Strong reinforcement is obtained when these layers overlap. Key is the life-times distribution of these glassy bridges. The latter depend on polymer-filler interaction, the thermo- mechanical history, on the temperature, on the distance between fillers, and on the local stress in the material. Under applied strain, we show how the dynamics of yield and rebirth of glassy bridges account for the non-linear Payne and Mullins effects, which are a large drop of the elastic modulus at intermediate deformations, and a progressive recovery of the initial modulus when the samples are subsequently put at rest, respectively. These mechanisms account also for dissipative properties of filled elastomers. Our model opens the way for predicting mechanical behavior of nano-filled elastomers according to the filler structures and dispersion, polymer- filler interactions and temperature, in order to prepare systems with taylored properties. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q19.00009: Molecular Simulation of Highly Crosslinked Epoxy Resin and POSS-Epoxy Nanocomposites Po-Han Lin, Rajesh Khare Generation of atomistic model structures of crosslinked epoxy at realistic density is a challenging task. In this work, we present an efficient approach for generating such model structures of highly crosslinked matrices. The approach utilizes simulated annealing optimization technique for carrying out one-step polymerization of the reaction mixture in the simulation box. The structures so generated are relaxed using a combination of molecular mechanics and molecular dynamics (MD) simulations. The developed technique is computationally efficient and has been used for creating atomistic model structures of both crosslinked epoxy and a nanocomposite formed by the incorporation of the polyhedral oligomeric silsesquioxane (POSS) molecules in the crosslinked epoxy matrix. MD simulations are used to determine the volume-temperature behavior of these structures. The density and the glass transition temperature of the simulated structures are compared with the literature experimental data. Furthermore, the molecular packing behavior of the POSS-epoxy nanocomposite is compared with the molecular packing in the crosslinked epoxy matrix. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q19.00010: Reducing Strain in Electrophoretically Deposited Nanocrystal Films by Post-Deposition Incorporation of Polymers Theodore Kramer, Steffen Jockusch, Michael Steigerwald, Nicholas Turro, Irving Herman We have made dense nanoparticle-polymer films and investigated their mechanical properties using nano-indentation and other methods. Electrophoretically deposited (EPD) films of cadmium selenide nanocrystals were infiltrated with network-forming monomers and subsequently exposed to UV radiation in the presence of photoinitiators to facilitate polymerization of the monomer. This hybrid material exhibits the desirable photoluminecent properties of CdSe nanocrystals but does not fracture, as do thick electrphoretically grown nanoparticle films. This may be the result of effectively reducing strain in the films via void filling. The mechanical properties of these films differ from those of EPD films without the introduction of polymer, as seen by nanoindentation studies. These films offer the benefit of high particle density, as well as large film thickness ($>$ 2 micron), and may have useful applications in the area of flexible photovoltaic devices. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q19.00011: Mechanics of nanoscale composite films from stress-electrical measurements: A nanoscale foam Chieu Nguyen, Vivek Maheshwari, Ravi Saraf Nanometer thin ($>$ 100nm) composite films consisting of polymers and organic-inorganic materials such as nanoparticles, quantum dots, nanotubes and dyes are widely researched for applications in designing a bio-mimetic cell membrane, solar cells, electronic and optical sensors, ion separation membranes and coatings. Being nanoscale in dimensions the mechanical properties of the film is critically governed by its morphology at nanoscale and the mutual interaction between the constituents of the film. The assembly process and the components of the film are detrimental in defining its morphology. A vast array of film morphologies is possible due to the multitude of combinations in processing and the components available to make the film. The study of mechanical properties of the film is hence important due their application in multitude of fields and correlating it to the nanoscale morphology and properties of its constituents. Here we present the stress-electrical measurements on a nanoscale ($\sim $100nm) nanocomposite film prepared using the well known spin assisted ionic self-assembly process. The film is a stack of nanoparticle layers, spaced by dielectric layer. Each dielectric layer consists of a stack of alternating anionic and cationic polyelectrolyte layers. The separation between the nanoparticle layers can be controlled with nanometer scale precession by modulating the number of polyelectrolyte layers in each dielectric layer. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q19.00012: Local dielectric permittivity profiles of sapphire/polypropylene interfaces Liping Yu, V. Ranjan, M. Buongiorno Nardelli, J. Bernholc Recently, the need for high-power-density capacitors has stimulated research to develop composite dielectric materials with high-k nanoparticles embedded in a polymer matrix. In these materials, surfaces and interfaces may play an important role in determining the overall dielectric properties. We present first-principles investigations of the dielectric permittivity profiles across slabs and interfaces of sapphire($\alpha$-Al$_2$O$_3$)/isotactic-polypropylene(iPP). Our results indicate that the permittivity profile at interface strongly depends on the nanoscale averaging procedure. We propose an averaging model that ensures near-locality of the dielectric function. We find that: (i) the dielectric permittivity approaches the corresponding bulk value just a few atomic layers away from the interface or surface; (ii) the dielectric constant is enhanced at the surfaces of the isolated $\alpha$-Al$_2$O$_3$ slabs, while no enhancement is observed at the iPP slab surfaces; and (iii) the dielectric transition at the $\alpha$Al$_2$O$_3$/iPP is mainly confined in the $\alpha$Al$_2$O$_3$ side. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q19.00013: Flow induced orientation behavior of concentrated dispersions of multi-wall carbon nanotube suspensions under shear flow: Effect of aspect ratio and concentration Saswati Pujari, Wesley Burghardt, Sameer Rahatekar, Jeffrey Gilman, Krzysztof Koziol, Alan Windle We report studies of average orientation state of concentrated dispersions of multi-walled carbon nanotube (MWNTs) in steady shear flow. Uncured epoxy was used as a viscous, Newtonian suspending medium, and samples were prepared from 'aligned' MWNTs using methods previously reported (Rahatekar et al., J Rheol 40:599, 2006). Flow induced structural measurements were made in the vorticity (1-3) plane of simple shear flow using in- situ wide angle x-ray scattering techniques in a rotating disc shear cell. Azimuthally-dependent diffraction from the internal layered structure of the MWNTs was used to characterize aligment. Steady state anisotropy of MWNT dispersions decrease with increasing the length of the MWNTs. Surprisingly, the anisotropy is seen to increase with increasing concentration. For one of the samples, more detailed orientation dynamics are studied in steady shear and transient shear flow both in the 1-2 (flow gradient) and 1-3 (vorticity) planes of shear flow, and through comparison of wide-angle and small-angle scattering signatures of flow-induced nanotube alignment. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q19.00014: Polymorphism in electrospun poly(vinylidene fluoride)/nanoclay composite nanofibers Lei Yu, Peggy Cebe We investigated the morphology and polymorphism behavior of electrospun (ES) composite nanofibers of poly(vinylidene fluoride) (PVDF) with two nanoclays: Lucentite$^{TM}$ STN and SWN. Lucentite$^{TM}$ STN and SWN synthetic nanoclays are based on hectrite structure, but only STN contains an organic modifier between the hectrite layers. The PVDF was dissolved, and nanoclay was dispersed, in N,N-dimethylformamide/acetone and then electrospun into nanofibers with diameters ranging from 100$\sim $1000 nm. The nanoclay content ranged from 0.2{\%} to 10{\%}. The addition of STN can greatly decrease the number of beads and makes the diameter of the ES nanofibers more uniform due to an increase of solution conductivity. From wide angle X-ray scattering and Fourier transform infrared spectroscopy, we found both STN and SWN can induce more beta phase PVDF crystals and TTT conformers, while reducing the alpha phase crystal content in ES PVDF/nanoclay composite nanofibers. STN can completely eliminate the alpha phase crystals, even at low STN content. The ionic organic modifier makes STN much more effective than SWN in promoting beta phase PVDF crystals. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q19.00015: Polymer/inorganic nanocomposites with prescribed morphologies E. Manias, M.J. Heidecker, J. Zhang, G. Polizos Despite the proliferation of polymer/inorganic nanocomposites in academic research and the commercialization of tens of products based on such materials, their true potential still remains largely untapped. One of the major hurdles in this endeavor is to capitalize on the novel properties afforded by a true--'nano'morphology, {\it i.e.}, beyond simple nanoparticulate dispersions and towards prescribed filler/phase arrangements and tailored filler--polymer interfaces. We comparatively present nanocomposites with prescribed {\it nano}morphologies, which can be made in large, industrial-scale, quantities (e.g. composites with spatially arranged fillers: such as shear--aligned fillers in blown PE films and filler-induced compatibilization of PC/PET blends). We discuss the fundamental mechanisms of achieving the prescribed {\it nano}morphologies and the related novel functionalities. In particular, we emphasize on extraordinary properties achieved by simultaneous control of the composite morphology and of the polymer--filler interface, such as an impressive toughening effectin PC/PET nanocomposites, and PE nanocomposites with a predetermined tensile strength by tailoring the polymer--filer interfacial adhesion. [Preview Abstract] |
Session Q20: Thin Films and Adhesion I
Sponsoring Units: DPOLYChair: Chris Stafford, National Institute of Standards and Technology
Room: 321
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q20.00001: Capillary wrinkling of a floating sheet under differential surface tension Jiangshui Huang, Wim H. de Jeu, Narayanan Menon, Thomas P. Russell We have previously studied the radial wrinkling of a thin polymer film floating on the surface of water under the capillary force exerted by a drop of water placed on its surface. Here, the same surface tension both sets the radial stress in the unperturbed film as well as the source of the perturbation that leads to the wrinkling instability. We now report the effect on the wrinkling instability of a differential surface tension by using fluids with different surface tensions for the liquid the film is floating on and the drop put on the film. We use both surfactants and a variety of pure liquids to control the surface tension of water. When the base radial stress of the the films floating was decreased, the length of the wrinkles increased, but the number of wrinkles decreased. \\[3pt] [1] Full reference here. Science 317, 650(2007) [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q20.00002: Effect of Long-chain Branching on Surface Dynamics of Polymer Films Mark D. Foster, Shih-fan Wang, Jae Sik Lee, Sewoo Yang, Zhang Jiang, Suresh Narayanan, David Wu Thermally stimulated fluctuations of the surface of films of branched polystyrene chains have been studied using x-ray photon correlation spectroscopy (XPCS), a recently-developed technique that has already been applied to study the surfaces of melts of linear polystyrene chains. Surface relaxations of films of branched chains are faster than are those of films of linear analogs. However, the Tg's of the branched molecules are also lower. The variation in surface relaxation time as a function of scattering vector can be described well by a continuum hydrodynamic theory of thermally stimulated capillary waves with a nonslip boundary condition. However, the film viscosities inferred from fits of the theory to the data differ markedly from viscosities from bulk measurements. Acknowledgements: NSF support (CBET 0730692) [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q20.00003: Entanglement swelling in polymer glasses Joshua D. McGraw, Kari Dalnoki-Veress A polymer system in which the chains are much longer than the entanglement molecular weight, $M_{+} \gg M_{e}$, is well entangled. When a thin, glassy polymer film is uniaxially strained, deformations which are almost visible to the naked eye called crazes may be formed. Measuring volume fractions of deformed to undeformed regions provides a method by which entanglement densities of similar systems can be compared\footnote{ACM Yang, EJ Kramer, CC Kuo, SL Phoenix, \emph{Macromolecules} \textbf{19} 2020 (1986)}. We present results of deformation experiments, probed using atomic force microscopy, in which well entangled polystyrene networks have been diluted with various weight fractions of polystyrene with molecular weight in the vicinity of $M_{e}$. Upon dilution the system assumes an effective reduction in the entanglement density. The effective entanglement density is predicted by a conceptually simple model with $M_e$ as the only free parameter. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q20.00004: Microstructure Evolution during Solvent Evaporation from Thin Film Polymer Mixtures Nigel Clarke, Mireille Souche, Gavin Buxton We present simulations of the phase separation dynamics in a thin film polymer blend solution subject to solvent evaporation [1]. If the upper and lower surfaces are neutral with respect to the different components, we find that as the solvent diffuses through the film, and evaporates from the surface, phase separation becomes energetically favourable progressively throughout the film. This produces an ordering front which propagates through the film and leaves an ordered lateral morphology in its wake. In order to understand microstructure evolution if the surface interactions are strong enough that the film initially separates into a two layers, we have perfomed a linear analysis of the Marangoni instability of a deformable interface between two fluid layers of finite depths, submitted to a gradient of solvent concentration induced by the evaporation [2]. Qualitative comparison with experimental observations of spin-coating processes of solution of two immiscible polymers are then performed, yielding satisfactory agreement.\\[0pt] [1] G. A. Buxton and N.Clarke, Europhysics Letters, 78, 56006, 2007.\\[0pt] [2] M. Souche and N. Clarke, European Physical Journal E, in press. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q20.00005: Dimension-Dependent Mechanical Properties of Pure and Antiplasticized Polymer Nanostructures Sean Delcambre, Robert Riggleman, Juan de Pablo, Paul Nealey Dense arrays of poly(methyl methacrylate) (PMMA) grating nanostructures 80 nm to 120 nm in pitch were fabricated by electron-beam and extreme ultraviolet interferometric lithography. During development and rinse drying, the nanostructures are subjected to capillary forces that are defined by the rinse fluid properties and spacing between adjacent structures. The applied capillary forces and structure aspect ratios were varied experimentally to induce structure collapse. By coupling nanostructure collapse data with continuum cantilever beam bending models, mechanical properties such as the elastic modulus and yield stress are determined. The elastic moduli of PMMA structures at this scale are observed to decrease with structure linewidth. This behavior is counteracted by the addition of a low molecular weight diluent, tris(2-chloropropyl) phosphate (TCPP). At concentrations up to 5 wt{\%}, TCPP acts as an antiplasticizing agent, decreasing the glass transition temperature while simultaneously increasing the elastic modulus. For a given applied capillary force, nanostructures containing 5 wt{\%} TCPP are observed to remain stable at aspect ratios up to 20{\%} higher than the pure material. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q20.00006: The stiffening of ultrathin polymer films in the rubbery regime -- the relative contributions of bending, membrane stress and surface tension Paul O'Connell, Gregory McKenna A novel nano-bubble inflation technique has been developed which allows the determination of the absolute creep compliance of ultrathin polymer films as thin as 9 nm. Previous results have shown that the degree of reduction in Tg with film thickness is not universal, with PVAc showing no change in Tg down to 23nm while PS shows a significant reduction at thicknesses below approximately 80nm. Interestingly the rubbery plateau region for both materials shows a similar stiffening as the thickness is reduced. At low inflation pressures the film is dominated by the bending stiffness of the film while at higher pressure the film is under membrane conditions and the response is dominated by the biaxial stiffness of the film. In addition the film is subject to surface tension effects. Compliance data have been measured as a function of applied pressure and have been analyzed to determine the relative contribution to the response from these three modes. The results suggest that at sufficiently large deformations the bending contribution is small while the surface tension contribution varies depending on loading conditions, film thickness etc. However neither contribution is sufficient to account for the observed stiffening in the rubbery regime [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q20.00007: Molecular confinement and residual stress in ultrathin polymer films Arnold Yang, Gunter Reiter, Yi-Hsin Chang, Yi Chien The residual stress operative in thin films of a polymer (polystyrene) prepared by spin coating was determined from local elastic stress release induced by pinhole nucleation during dewetting instability. The measured stress was orders of magnitude greater than the capillary force and attributed to chain recoiling of the confined macromolecules. The entropy-driven stress was found to be small for thicker films but increase dramatically as film thickness became less than the unperturbed molecular dimensions. The chain conformations in these films can only be described by the Langevin, rather than Gaussian, statistic and the draw ratio was determined to be around 5, comparable to that in craze fibrils, for film thickness of 4 nm. The effects of spin speed, aging-induced relaxation, and molecular packing were investigated. The molecular processes during spin coating were proposed. In addition, conjugated polymers when squeezed into the molecular thicknesses were found to emit light with much enhanced efficiencies due to the large molecular deformation. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q20.00008: Semifluorinated Polymers Confined at the Solid-Air Interface Umesh Shrestha, Stephen Clarson, Dvora Perahia Effective responsive layers should exhibit stability while retaining a dynamic mode that will allow reaction of the interface to external stimuli. Semifluorinated polymers have a potential for forming energy controlled responsive interfaces. Because of the high segregation between the fluorinated and protonated segments, well defined structures are induced at relatively short chains, retaining the capability to rearrange on short time scales. Fluorinated segments affect the interfacial energies as well as enhance thermal stability and controls the refractive index and dielectric properties. The present study investigates the interfacial response of poly trifluoro propylmethyl siloxane-polystyrene diblock copolymer (PTFPMS-PS) at volume fractions varying from 0.003 to 0.5 of fluorinated block, at the interface of oxidized silicon wafers. In all volume fractions we found that the air interface is fluorine rich and the solid surface in proton rich. Layering is detected across the films for all volume fractions. Upon annealing the layering is retained, however the interfacial compositions changes. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q20.00009: Crosslink Density Variations with Process Conditions in Plasma Polymerized Photonic Films Somesh Peri, Mark Foster, Jesse Enlow, Hao Jiang, Timothy Bunning, Bulent Akgun, Sushil Satija, Charles Majkrzak The structures of plasma polymerized homopolymer octafluorocyclobutane (PP-OFCB) films made under different processing conditions were studied using x-ray reflectivity (XR) and neutron reflectivity (NR). The processing parameters varied were monomer feed location, plasma power, and pressure. Each dry film had a surface layer of thickness $\sim $20{\AA} and a thin layer of $\sim $10{\AA} thickness at the substrate in which the crosslink density was lower than in the bulk polymer film. The region of lower cross-link density at the film-air interface reflects the extent of a reaction zone that moves with the deposition and is responsible for dictating the width of interfaces that are formed when a layer of different precursor is deposited atop the first layer. Results from bilayer films support this view. Such a reaction zone is also seen for benzene and iron-containing plasma polymerized films. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q20.00010: Spatially Organized Polymer Films Prepared by Oblique Angle Polymerization Niranjan Malvadkar, Melik Demirel We developed a novel, bottom-up method to prepare nanostructured poly($p$-xylylene) (PPX) films called oblique angle polymerization (OAP). In OAP, the monomer vapor flux is directed at an angle ($<$ 10$^{o})$ to the substrate. The morphology of the film is influenced by the combination of nucleation, surface diffusion and geometrical self-shadowing. The final nanostructure consists of 40 $\times $ 10$^{6}$ /mm$^{2}$ obliquely aligned, quasi-periodic PPX nanowires on the substrate[1]. The nanostructure can be controlled by tuning the physical deposition parameters and/or the monomer chemistry. Functional materials prepared by depositing conformal metal layer on these nanostructured PPX films have opened new avenues of application in the areas of biodetection[2] and catalysis[3]. [1] Cetinkaya, M., Malvadkar, N., Demirel, M. J. Poly. Sci. B, 46, 640 (2008). [2] Kao, P., Malvadkar N., Wang, H., Allara, D., Demirel, M. Adv. Mat., 20, 3562 (2008). [3] Malvadkar, N., Park, S., MacDonald, M., Wang, H., Demirel, M. J. Power Sources, 182, 323 (2008). {\copyright} 2008 Elsevier [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q20.00011: Rubber Friction -A Molecular Picture Anish Kurian, Kumar Nanjundiah, Ali Dhinojwala Understanding the relationship between adhesion, friction and the interfacial structure has been of significant interest for many years. Most tribological experiments involve measuring friction and adhesion values to develop a molecular model to explain the macroscopic results. Here, we have used surface sensitive infrared-visible sum frequency generation spectroscopy (SFG) to study the interface between elastomer and solid surfaces. SFG is a second order nonlinear optical technique that provides information on the chemical identity; orientation and concentration of the molecules at the interface. We have designed a friction cell to probe the changes in the interfacial structure during sliding using a femto-second laser spectroscopy. These results will be presented. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q20.00012: In-situ Adhesion Measurements Utilizing Layer-by-layer Functionalized Surfaces Christopher M. Stafford, Adam J. Nolte, Jun Young Chung, Marlon L. Walker The adhesion between poly(dimethylsiloxane) (PDMS) hemispheres coated with layer-by-layer (LbL) assemblies of polyelectrolytes and rigid, planar substrates was investigated using Johnson, Kendall, and Roberts (JKR) contact mechanics. Measurements were performed against amine-functionalized glass slides both in air and in aqueous solutions of controlled pH. Despite the increased density of negatively charged carboxylate groups, LbL-functionalized PDMS exhibited lower adhesion due to the combined effects of increased surface roughness and the high Young's modulus of the coating. Measurements of coated PDMS in aqueous solutions revealed tunable adhesion behavior dominated by pH-mediated changes in the mechanical properties of the coating. Smoothing the surface of the LbL coatings by aqueous salt annealing led to a significant increase in adhesion. Our results suggest that LbL assembly can be an effective means of surface functionalization for in-situ adhesion measurements, but understanding and predicting the adhesion behavior requires comprehensive knowledge of the chemical, mechanical, and topological properties of the coating and how such properties change in response to the ambient environment. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q20.00013: Tethered Lubricant Films Based On Cross-linked Polydimethylsiloxane Lucas Landherr, Claude Cohen, Lynden Archer We report on the interfacial friction and wear properties of surface-tethered cross-linked polymer thin films. We show that thin, two-tiered films produced by covalently tethering polydimethylsiloxane (PDMS) networks to self-assembled monolayers manifest the lowest friction coefficient (mu = 0.0039) recorded for a dry lubricant film. Using a combination of lateral force microscopy, equilibrium swelling, and adhesion measurements we determine the physical processes responsible for these low friction coefficients. We also investigate the effect of free (unattached) and pendent polymer chains dispersed in thin PDMS network films on transient mechanical properties, interfacial friction, and wear characteristics. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q20.00014: Effect of Adsorbed Films on Nanoscale Mechanical Contacts Shengfeng Cheng, Binquan Luan, Mark Robbins For surfaces exposed to ambient air, the presence of adsorbed molecules cannot generally be avoided. Molecular simulations are presented which show that the compliance of these adsorbed films can have a profound effect on the mechanical behavior of contacts. An adsorbed film of short chain molecules is equilibrated on a flat, elastic substrate. The film is then contacted by a non-adhesive spherical tip. The atomic scale structure of the tip is varied from amorphous to crystalline, since this has a substantial effect on contacts with clean substrates. Including adsorbed molecules reduces sensitivity to tip geometry, but introduces new effects. One is that the contact region is broadened dramatically, with measured contact radii increased from predictions of continuum theory by a constant shift. The variation of tip displacement and substrate deformation with normal load show a crossover between two regimes. At small loads, the effective elastic modulus is set by the soft adsorbed film, while at large loads the modulus is that of the substrate. Variations in friction with tip geometry are much smaller than for bare substrates and the friction rises linearly with load in almost all cases. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q20.00015: The Promotion of Au Adhesion on Polymer Surfaces Using Polyhedral Oligomeric Silsequioxane. Chris Hughes, Brian Augustine, Alan Mo, Jonathan Wyrick, Bruno Caputo, Ethan Rosenthal The adhesion of Au on polymer surfaces is weak because of the inert nature of Au and the non-polarity of the hydrocarbon surface of the polymer. We seek to fabricate microfluidic devices in which vacuum deposited gold thin films will be used in electrical contacts and optical reflectors. Various fabrication steps involve the use of solvents which easily wash away the gold. To overcome this, we have explored the use of a thin layer of polyhedral oligomeric silsequioxane-methacrylate (POSS-MA) which is a nanocomposite having both polymer and inorganic silica glass characteristics. The POSS-MA is spun cast onto the surface of PMMA creating a film which is on the order of 100 nm thick. Au dots that are 1 mm in diameter were deposited onto both the virgin PMMA surface and the POSS-MA coated surface and the samples were covered with acetone, a known solvent for PMMA. Optical microscope video images of the dots revealed their delamination from the surface and image analysis was used to determine the time that it took for the dots to be undercut -- typically in the range of seconds to minutes. An obvious increase in the time required to undercut the Au was observed for the POSS-MA treated surface. A model explaining the improved adhesion will be discussed as will future plans for device fabrication. [Preview Abstract] |
Session Q21: Focus Session: Dopants and Defects in Semiconductors III
Sponsoring Units: DMPChair: Michael Stavola, Lehigh University
Room: 323
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q21.00001: High-Resolution Spectroscopy with a Free-Electron Laser: Vibrational Lifetimes of Hydrogen-related Defects in Silicon Invited Speaker: Gunter Luepke, Department of Applied Science, The College of William and Mary, Williamsburg, VA 23187 Vibrational lifetimes of hydrogen- and deuterium-related bending and stretching modes in crystalline silicon are measured by high-resolution infrared absorption spectroscopy and pump-probe transient bleaching technique using the Jefferson Lab. Free-Electron Laser. We find that the vibrational lifetimes of the bending modes follow a universal frequency-gap law, i.e., the decay time increases exponentially with increasing decay order, with values ranging from 1 ps for a one-phonon process to 265 ps for a four-phonon process. The temperature dependence of the lifetime shows that the bending mode decays by lowest-order multi-phonon process. In contrast, the lifetimes of the stretching modes are found to be extremely dependent on the defect structure, ranging from 2 to 295 ps. Against conventional wisdom, we find that lifetimes of Si-D stretch modes typically are longer than for the corresponding Si-H modes. Our results provide new insights into vibrational decay and the giant isotope effect of hydrogen in semiconductor systems. The potential implications of the results on the physics of electronic device degradation are discussed. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q21.00002: First-Principles Calculation of Carrier Lifetimes in Semiconductors Vincenzo Lordi, Paul Erhart, Daniel Aberg We have developed first-principles methods based on density functional theory to calculate carrier lifetimes in semiconductors related to trapping on deep-level defects. Lifetimes are determined based on Schottky-Read-Hall theory, using recombination rates calculated from first principles for several possible, competing mechanisms: radiative recombination, phonon-assisted (nonradiative) recombination, and Auger recombination. The recombination rates are calculated within a fully first-principles framework with no empirical parameters. We have recently applied these methods to study the role of native and impurity defects in reducing carrier lifetimes in bulk single-crystal aluminum antimonide (AlSb) and cadmium telluride (CdTe), two promising materials for high-resolution room-temperature gamma radiation detection. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q21.00003: Fully \textit{ab initio} supercell corrections for charged defects Christoph Freysoldt, J\"{o}rg Neugebauer, Chris G. Van de Walle Charged point defects govern the carrier densities in semiconductors and are crucial for the performance of electronic devices. However, quantifying the thermodynamical, chemical, and electrical properties of such defects is a challenge to both experiment and theory. In \textit{ab-initio} calculations, the defect is usually modeled in a periodic supercell with a few dozen to a few hundred atoms. Unfortunately, this introduces artificial electrostatic interactions between charged defects. A number of correction schemes such as Makov-Payne corrections, potential alignment, scaling laws, or Coulomb truncation, are available in the literature, but they often fail to remove the supercell dependence completely. The assumptions behind these schemes are sometimes unclear and all schemes lack a stringent theoretical foundation. From a formal analysis within linear-response theory, we propose a new and simple scheme that combines the strengths of Makov-Payne corrections and potential alignment. Our scheme requires no empirical parameters or fitting procedures. Its reliability is demonstrated even in extreme cases. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q21.00004: Optimal Silicon for Photovoltaic Applications Georgy Samsonidze, Marvin L. Cohen, Steven G. Louie A small overlap of the silicon optical absorption spectrum with the solar emission spectrum limits the efficiency of silicon-based solar cells. We conduct a theoretical search for substitutionally doped silicon with the aim to maximize the spectral overlap. Different dopant species at various concentrations compatible with the existing silicon technology are examined in the virtual crystal approximation using the empirical pseudopotential method. The optimal doping configurations found are further investigated with a first-principles many-electron Green's function approach. The optical absorption spectrum of the doped silicon is calculated by solving the Bethe-Salpeter equation which includes excitonic effects. This work was supported by National Science Foundation Grant No. DMR07-05941, and by the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. Computational resources have been provided by NSF through TeraGrid resources at Indiana University and TACC. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q21.00005: Microprobe X-ray Absorption Spectroscopy of Chalcogen Doped Silicon Bonna Newman, Joe Sullivan, Mark Winkler, Meng-Ju Sher, Matthew Marcus, Matthew Smith, Silvija Gradecak, Eric Mazur, Tonio Buonassisi Doping Si with chalcogen atoms (S, Se, and Te) in excess of the solubility limit has been shown to result in optical absorption below the bandgap. This material, known as ``black silicon", is promising for infrared photon detectors and possibly photovoltaic devices. We report on the relationship between the chemical state of the dopant atoms and infrared absorption properties. A high concentration of 10$^{20}$ dopant atoms/cm$^{3}$ in the near-surface layer allows for extended X-ray absorption fine structure (EXAFS) investigations and determination of chemical state. We combine these results with absorption measurements and Auger spectroscopy to understand the correlations between optical and structural properties of chalcogen doped Si. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q21.00006: Hydrogen in multi-crystalline Si used for the fabrication of solar cells Chao Peng, Michael Stavola, W. Beall Fowler, Lode Carnel The multicrystalline Si materials that are used by industry to fabricate solar cells often contain a high concentration of carbon impurities. Furthermore, hydrogen is also commonly introduced during processing to improve solar-cell performance.[1,2] At present, the H- and C-related defect reactions that occur and what their effect might be remain poorly understood. We have performed a series of experiments in which IR spectroscopy has been used to study a family of defect complexes that are formed when H is trapped by substitutional carbon impurities in multi-crystalline Si. The structures, concentrations, and thermal stabilities of these defects have been investigated. 1. F. Duerinckx and J. Szlufcik, Sol. Energy Mater. Sol. Cells \textbf{72}, 231 (2002). 2. H. Dekkers, Dissertation, Catholic Univ. of Leuven, 2008. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q21.00007: Effect of copassivation of Cl and Cu on CdTe grain boundaries Lixin Zhang, Juarez L.F. Da Silva, Jingbo Li, Yanfa Yan, T.A. Gessert, Su-Huai Wei Grain boundaries (GBs) and dislocations are generally viewed as detrimental to device applications, because they usually contain a high density of deep defect levels that act as recombination centers for charge carriers. Surprisingly, two leading polycrystalline thin-film solar cells based on CuInSe2 (CIS) and CdTe have produced very high efficiencies of 20\% and 16.5\%, respectively, despite that these materials contain significant amounts of GBs. Using a first-principles method, we investigate the structural and electronic properties of GBs in polycrystalline CdTe and the effects of copassivation of elements with far distinct electronegativities. Of the two types of GBs studied in this work, we find that the Cd core is less harmful to the carrier transport, but is difficult to passivate with impurities such as Cl and Cu, whereas the Te core creates a high defect density below the conduction band minimum, but all these levels can be removed by copassivation of Cl and Cu. Our analysis indicates that for most polycrystalline systems copassivation or multipassivation is required to passivate the GBs. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q21.00008: Influence of Crystalline Defects in GaN-InGaN Solar Cells Balakrishnam Jampana, Nikolai Faleev, Ian Ferguson, Robert Opila, Christiana Honsberg Crystalline defects originating from lattice-mismatch in epitaxial materials appear to be the dominant factor reducing high efficiency solar cell performance. In this paper we present an explanation of the observed structural and optical characteristics originating in lattice-mismatched III-nitride epitaxial materials. This model is based on creation, diffusion, accumulation and structural transformation of point defects to extended crystalline defects. In this work InGaN photovoltaic structures are grown by MOCVD on GaN templates with thicknesses in the 50 to 400nm range. The types and spatial distribution of crystalline defects are determined from XRD rocking curves and reciprocal space maps. The crystalline quality is observed to deteriorate with increasing thickness and growth rate. Wide band gap InGaN based solar cells require 150 to 400nm of active layer thicknesses and crystalline defects are observed in this thickness range degrading the solar cell performance. A physical model correlating the response of the solar cell to the type and spatial distribution of the defects will be presented. The work will aid improve the crystalline quality of InGaN for application as high efficiency solar cells. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q21.00009: Scanning Probe Spectroscopy of Individual Dopants in Silicon Morewell Gasseller, Matty Caymax, Roger Loo, Sven Rogge, Stuart Tessmer A key goal of semiconductor nanoelectronics is to develop devices based on manipulating the charge and spin of individual dopant atoms. Elucidating the quantum structure of these minute systems is a difficult technical challenge. Here we present capacitance-based scanned-probe measurements that both spatially-resolve individual subsurface boron dopants in silicon and detect spectroscopically single holes entering the B+ state of these atoms. We observe that, on average, acceptors with a closer nearest neighbor exhibit stronger binding. This finding is consistent with the interpretation of resonant tunneling measurements performed on a similar sample. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q21.00010: Vacancy-related defects and the E'$_{\delta }$ center in amorphous silicon dioxide Blair Tuttle, Sokrates Pantelides The microscopic identification of vacancy-related defects in silicon dioxide has been a major challenge. Particularly in amorphous silica, the role of vacancy clusters is still controversial. Experimental data have led to suggestions that the E'$_{\delta }$ center is a four-vacancy cluster instead of a single vacancy. Here we report density functional calculations of single vacancies and clusters of four vacancies in realistic models of amorphous silica. Results for single vacancies compare well to previous theory. A key result for four-vacancy clusters is that relaxations localize the unpaired electron preferentially on one Si atom, resulting in a strongly anisotropic electron-paramagnetic-resonance signal. Electrons at single vacancies have a more benign anisotropy which is more compatible with the observed isotropic signal. This work was supported by the Air Force Office of Scientific Research under a MURI grant (FA9550-05-1-0306) and by the US Navy. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q21.00011: Enhanced defect generation in gate oxides of p-channel MOS transistors in a moisture ambient Aritra Dasgupta, S.A. Francis, D.M. Fleetwood Transistors and ICs built in Sandia's 4/3 $\mu $m technology were exposed to moisture, irradiated, and annealed. The moisture exposures were performed using highly accelerated stress test (HAST) at 130$^{\circ}$C and 85\% relative humidity. Irradiation of n-channel transistors exposed to HAST followed by a long-term anneal resulted in some increase in interface-trap and oxide-trapped charge buildup. We observed enhanced post-irradiation defect generation of oxide trapped charge, interface traps and border traps in the gate oxides of p-channel MOS transistors that were exposed to humidity. This is characterized by enhanced voltage shifts due to oxide trapped charge and interface traps observed in the p-channel transistors. Low frequency noise measurements also showed enhanced low frequency noise power in the moisture exposed p-channel transistors. Our results indicate that there are enhanced precursor hole trap defects or oxygen vacancies present in the gate oxide of p-channel transistors as a result of presence of moisture or hydrogenous ambient. The smaller voltage shifts in the n-channel transistors may be related to the presence of phosphorus atoms in the gate oxides. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q21.00012: Carbon clusters as possible defects at the SiC-SiO$_{2}$ interface Yingdi Liu, Hongli Dang, Yang Liu, Ying Li, Matthew Chisholm, Trinity Biggerstaff, Gerd Duscher, Sanwu Wang High state densities in the band gap of the SiC-SiO$_{2}$ interface significantly reduce the channel mobilities in SiC-based high-temperature/high-power microelectronics. Investigations of the nature of the interface defects are thus of great importance. While several possible defects including very small carbon clusters with up to four carbon atoms have been identified by first-principles theory, larger carbon clusters as possible defects have attracted less attention. Here, we report first-principles quantum-mechanical calculations for two larger carbon clusters, the C$_{10}$ ring and the C$_{20}$ fullerence, at the SiC-SiO$_{2}$ interface. We find that both carbon clusters introduce significant states in the band gap. The states extend over the entire band gap with higher densities in the upper half of the gap, thus accounting for some of the interface trap densities observed experimentally. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q21.00013: First-principles study of local p(2$\times$2) structures on Si(100) surface Min-Kook Kim, Hyoung Joon Choi We study structural defects inducing local p(2$\times$2) structures in c(4$\times$2)-reconstructed Si(100) surface, using an \textit{ab-initio} pseudopotential density functional method. The local density approximation to the density functional theory is used and electronic wavefunctions are expanded with pseudo-atomic orbitals. The atomic structures of defects are optimized by minimizing the total energy. Our calculations show that the defects increase the total energy of the system but they are energetically stable with energy barrier. STM images for occupied and unoccupied states are simulated to investigate the surface electronic structures. Effects of electron doping and external electric field on the defects are also studied. This work was supported by the KRF (KRF-2007-314-C00075) and by the KOSEF Grant No. R01-2007-000- 20922-0. Computational resources have been provided by KISTI Supercomputing Center (KSC-2008-S02-0004). [Preview Abstract] |
Session Q22: Focus Session: Spins in Quantum Dots and Mn in Arsenides
Sponsoring Units: GMAG DMP FIAPChair: Xinyu Liu, University of Notre Dame
Room: 324
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q22.00001: Intershell Exchange and Sequential Electrically Injected Spin Populations of InAs Quantum-Dot Shell States Invited Speaker: Quantum dots (QDs) are attractive for a variety of spintronic applications. Their electronic structure exhibits the $s,p,d,f$ shells characteristic of atoms. We report electrical injection of spin-polarized electrons from Fe contacts into the individual shells of highly uniform self-assembled InAs QDs, and we determine the s-p and p-d inter-shell exchange energies. The electron population and polarization of each shell are controlled by the spin bias current. The circular polarization of the electroluminescence (EL) spectra exhibits maxima red-shifted with respect to the EL intensity peaks, in contrast with simple models of shell occupation. Using exact diagonalization techniques, calculations of spectra from multi-exciton complexes show that this is due to inter-shell exchange [1]. We determine exchange energies for the $s-p $shells $\sim $ 6-7 meV, and for the $p-d$ shells $\sim $ 13-14 meV. These results are significant to our understanding of QD behavior, and provide a mechanism for electrical control of spins in QDs.\\[4pt] [1] M. Korkusinski and P. Hawrylak, Phys. Rev. Lett. 101, 027205. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q22.00002: Magnetically induced spin relaxation in InAs QDs M. Yasar, I. Khan, T. Ali, A. Petrou, G. Kioseoglou, C. Li, A. Hanbicki, B. Jonker, M. Korkusinski The circular polarization $P$ of light emitted by Fe/InAs QDs spin LEDs has been studied as function of magnetic field $B $and temperature $T$. $P $shows a pronounced decrease around $B_{o }$= 5 T, in the form of a resonance with a full width of $\approx $ 0.75 T with the following characteristics: (i) The resonance strength is quite sensitive to the bias voltage $V$. At low $V $ the resonance is strong, but, as $V$ is increased, it becomes progressively weaker. (ii) The resonance is pronounced at $T$ = 5 K but loses strength with increasing temperature, and disappears above 60 K. The decrease in $P$ around $B_{o }$ is attributed to a spin relaxation mechanism that is induced by magnetic field. The sensitivity of the resonance to $V$ suggests that the origin of the spin relaxation mechanism is connected to the spin-orbit interaction. By changing $B$ we tune the energies of different electron states and thereby change the rate of spin relaxation in the system. We compare experimental results with calculations, in which many-body energies and wave functions are obtained using the effective-mass configuration-interaction approach; the spin-orbit interaction, is treated perturbatively. Work at SUNY was supported by ONR and NSF. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q22.00003: Mn doping of InAs quantum dots studied by X-STM. Paul Koenraad, Murat Bozkurt, Jens Garleff, Vicky Grant, Richard Campion, Tom Foxon, Euclydes Marega, Greg Solomon We report on the X-STM analysis of Mn doped quantum dots. The X-STM technique allows for the atomic scale analysis of single Mn acceptors and their incorporation in III/V nanostructured materials. We will show the detrimental effect of segregation which complicates the doping process of InAs quantum dots in GaAs. Several routes to attain doping of quantum dots are addressed. Only in structures with extremely high doping concentrations we could show the incorporation of a few or a single Mn atom(s) in the InAs dots. We will present low temperature (5 K) spectroscopic measurements that allow studying the interaction of the Mn acceptor state with the quantum dot. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q22.00004: Thermo-spin effects in quantum dots connected to ferromagnetic leads Yonatan Dubi, Massimiliano Di Ventra We study a system composed of a quantum dot in contact with ferromagnetic leads held at different temperatures, which we suggest can be used as a source of spin-voltage. Spin analogs to the thermopower and thermoelectric figure of merit are defined and studied as a function of junction parameters. It is shown that in contrast to bulk ferromagnets, the spin thermopower coeffcient in a junction can be as large as the Seebeck coefficient, resulting in a large spin figure of merit. In addition, it is demonstrated that the junction can be tuned to supply only spin current but no charge current. We also discuss experimental systems where our predictions can be verified. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q22.00005: Tunnel magnetoresistance in mesoscale (Ga,Mn)As magnetic tunnel junctions. Partha Mitra, Mark J. Wilson, Meng Zhu, Peter Schiffer, Nitin Samarth, Kiran V. Thadani, Dan C. Ralph We recently demonstrated exchange-biased magnetic tunnel junctions (MTJs) built from the ferromagnetic semiconductor (Ga,Mn)As [Phys. Rev. B. {\bf 78}, 195307 (2008)]. Here, we report measurements of the tunnel magnetoresistance (TMR) in mesoscale (Ga,Mn)As MTJ devices with areas that range from $\sim 1 - 100 \mu \rm{m}^2$, mapping out the TMR as a function of the magnetic field vector and the sample temperature. The vector field measurements provide insights into the interplay between TMR and the magnetic anisotropies characteristic of (Ga,Mn)As. In contrast to our earlier studies large area devices, we find that the TMR in these mescoscale devices increases exponentially with decreasing temperature, with a form exp (-T/T*). At temperatures lower than T*, the conductance-voltage characteristics show a $\sqrt{V}$ dependence, suggesting the role of Coulomb interactions in the spin- dependent tunneling process in these small area MTJs. Work supported by the ONR MURI program. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q22.00006: Spectra broadening in Point-Contact Andreev Reflection Measurement on GaMnAs Shang-Fan Lee, T.W. Chiang, Y.H. Chiu, S.Y. Huang, J.J. Liang, H. Jaffres, J.M. George, A. Lemaitre Point-Contact Andreev Reflection (PCAR) technique has been considered as a reliable method for determining electron spin polarization of novel metallic ferromangets. For dilute magnetic semiconductors, this technique is less applicable due to the resistive nature of the material. We investigate PCAR spectra of Ga$_{0.94}$Mn$_{0.06}$As using Pb tips. The observed spectrum exhibits behaviors described in the Modified Blonder-Tinkham-Klapwijk (MBTK) model but with a significant spectrum broadening. Modified BTK theory is commonly applied to analyze PCAR spectra with electron polarization, superconducting energy gap, and interface transparency as parameters. We present an analysis based on the introduction of spreading resistance and the inelastic scattering at the interface. In as-grown and annealed samples, we extract spin polarization of 76{\%}/74{\%} from our analysis, both smaller than the value obtained from approach of effective temperature$^{3}$, 90{\%}/82{\%}. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q22.00007: Ultrafast Photoinduced Coherent Spin Dynamics in Ferromagnetic Ga$_{1-x}$Mn$_x$As/GaAs Structure Jingbo Qi, Ying Xu, Andrew Steigerwald, Norman Tolk, Xinyu Liu, Jacek Furdyna, Ilias Perakis Ultrafast pump-probe magneto-optical spectroscopy is used to study
coherent spin dynamics in the ferromagnetic semiconductor
Ga$_{1-x}$Mn$_x$As systems. Above GaAs bandgap $E_g$, the
temporal Kerr signal is found to be strongly dependent on pump
photon polarization. This polarization dependence is attributed
to spins of electrons photoexcited to the conduction band, and
disappears for $E_{ph} |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q22.00008: Temperature Dependence of Anomalous Hall Effect in Metallic (Ga,Mn)As films Xinyu Liu, Zhiguo Ge, Shaoping Shen, Margaret Dobrowolska, Jacek Furdyna We present a systematic study of the temperature dependence of anomalous Hall effect (AHE) in metallic (Ga,Mn)As films. The Hall effect in (Ga,Mn)As is described as $\rho_{xy}$ =$R_{0}B$+$c\rho_{xx}^{n}M_{z}$, where $R_{0}$ ($1/ep$) is the ordinary Hall coefficient, $\rho_{xy}$ and $\rho_{xx}$ are the transverse and longitudinal resistivities, and $n$ is a scaling parameter. In this work we have developed a self-consistent method to determine $R_{0}$, $c$ and $n$ simultaneously. We use this method to analyze the Hall and resistivity data measured up to B = 6 T at various temperatures. We find that for the metallic samples, a distinct evolution of the AHE occurs as temperature increases, as evidenced by the temperature or magnetization dependence of the parameter c. We propose that the correlation between the AHE and the resistivity should be reconsidered by using a two- component model in order to separate contributions due to different scattering mechanisms. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q22.00009: Antiferromagnetic interlayer exchange coupling in Ga$_{1-x}$Mn$_x$As/GaAs diluted ferromagnetic semiconductor multilayers Jae-Ho Chung, S.J. Chung, Sanghoon Lee, B.J. Kirby, J.A. Borchers, Y.J. Cho, X. Liu, J.K. Furdyna We use neutron reflectometry to investigate the interlayer exchange coupling between Ga$_{0.97}$Mn$_{0.03}$As ferromagnetic semiconductor layers separated by non-magnetic Be-doped GaAs spacers. Polarized neutron reflectivity measured below the Curie temperature of Ga$_{0.97}$Mn$_{0.03}$As reveals a characteristic splitting at the wave vector corresponding to twice the multilayer period, indicating that the coupling between the ferromagnetic layers are antiferromagnetic (AFM). When the applied field is increased to above the saturation field, this AFM coupling is suppressed. This behavior is not observed when the spacers are undoped, suggesting that the observed AFM coupling is mediated by charge carriers introduced via Be doping. The behavior of magnetization of the multilayers measured by DC magnetometry is consistent with the neutron reflectometry results. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q22.00010: Engineering the interlayer exchange coupling in hybrid ferromagnetic metal/semiconductor heterostructures Mark J. Wilson, Meng Zhu, Peter Schiffer, Nitin Samarth, Roberto C. Myers, David D. Awschalom, Michael E. Flatte The systematic engineering of exchange coupling in ferromagnetic semiconductor heterostructures is important for developing proof-of-concept spin transfer semiconductor devices. We recently demonstrated interlayer exchange coupling between a ferromagnetic semiconductor (Ga$_{1-x}$Mn$_x$As) and a ferromagnetic metal (MnAs) [APL {\bf 91}, 192503 (2007)]. Here, we report a comprehensive magnetometry study of the underlying exchange coupling in this hybrid system. We vary key parameters such as the thicknesses of both the ferromagnetic layers and the composition of the Ga$_{1-x}$Mn$_x$As layer, and explain our observations using an ``exchange spring'' model. We also demonstrate the propagation of the exchange coupling through a non-magnetic spacer layer (p-doped GaAs) and examine the variation of this coupling as a function of the spacer layer thickness and doping. Work supported by the ONR MURI program and by NSF. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q22.00011: A novel random fixed point in diluted magnetic semiconductors Donald Priour Jr, Sankar Das Sarma We examine the critical behavior of strongly disordered Heisenberg models at the Curie Temperature $T_{c}$, where the disorder is manifest as missing magnetic ions (i.e. as in diluted magnetic semiconductors) with the aid of large-scale Monte Carlo. We calculate the magnetic susceptibility critical exponent, obtaining a value ($\gamma = 1.1 \pm 0.05$) lower than that of the corresponding pure model. We obtain the same reduced value for $\gamma$ even for weak disorder. We find that both the amplitude and exponent of the first correction to leading order scaling are invariant with respect to the strength of the disorder, suggesting that critical behavior is controlled by a single random fixed point. Nevertheless, we show that for reduced temperature $t = (T - T_{c})/T_{c}$ regimes accessible in experiment, one would actually measure an effective exponent $\gamma_{\textrm{eff}}$ markedly higher than that of either the random fixed point or the pure Heisenberg model. We find self-averaging parameters to be non-monotonic in system size, initially increasing with the number of magnetic ions $N$, and ultimately decreasing beyond $N \sim 10^{3}$ as per the Harris Criterion. We acknowledge support from US-ONR and NRI-NSF. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q22.00012: Solubility control in dilute magnetic semiconductors by using the co-doping method Kazunori Sato, Hitoshi Fujii, Lars Bergqvist, Peter H. Dederichs, Hiroshi Katayama-Yoshida To overcome low solubility limit of magnetic impurities in dilute magnetic semiconductors (DMS) and realize room temperature ferromagnetism, we propose a co-doping method to increase solubility of magnetic impurities in DMS [1]. We calculate electronic structure of (Ga, Mn)As, (Ga, Mn)N, (Ga, Cr)N and (Zn, Cr)Te with interstitial impurities, such as Li, Na and Cu, from first-principles by using the Korringa-Kohn- Rostoker coherent potential approximation (KKR-CPA) method. From the total energy results, it is shown that the mixing energy of magnetic impurity becomes negative and the solubility of magnetic impurities is strongly enhanced under the existence of interstitials [1]. In general, the co-dopants compensate hole carriers, thus the system becomes paramagnetic. However, owing to the large diffusivity of these interstitial impurities, we can anneal out the co-dopants after the crystal growth to recover the ferromagnetism. As an example, kinetic Monte Carlo simulations for the diffusion of interstitial co-dopants in DMS will be shown. [1] K. Sato et al., Jpn. J. Appl. Phys. 46 L1120 (2007) [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q22.00013: Time Resolved Spectroscopy of InMnAs Using Differential Transmission Technique in Mid-Infrared Region M. Bhowmick, K. Nontapot, G.A. Khodaparast, B.W. Wessels The emergence of III-Mn-V magnetic semiconductors, such as GaMnAs, InMnAs, and InMnSb has led to a number of exciting results relevant to the new field of spintronics. In contrast to earlier MBE work, InMnAs structures grown by MOVPE at the Northwestern University are room temperature ferromagnetic semiconductors with a {\it T$_C$} of 330 K. The origins of ferromagnetism and the interactions between itinerant carriers and localized spins in these structures are open and interesting questions. The samples are grown on GaAs substrates with the Mn content ranging from 1-4{\%}. The carrier and spin life time in these structures were probed using a differential transmission technique by tuning the pump-probe radiations from 3-3.6 microns. The relaxation times are in order of 2-4 ps similar to the observations in the MBE grown structures. The temperature dependence of the carrier and spin lifetimes will be presented and related to recent models for spin recombination. [Preview Abstract] |
Session Q23: Quantum Hall Effect: Tunneling, High-Frequency
Sponsoring Units: FIAP DCMPChair: Ramesh Mani, Georgia State University
Room: 325
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q23.00001: Landau Level Spectrum in In$_{0.53}$Ga$_{0.47}$As/InP Heterostructures Chi Zhang, Yanhua Dai, Kristjan Stone, Rui-Rui Du We report on magnetotransport results from a high-quality Rashba two-dimensional electron gas (2DEG) formed in undoped In$_{0.53}$Ga$_{0.47}$As/InP heterostructures, which were MOCVD grown on (100) InP substrate. The 2DEG has a density of n$_{e}\sim $ 1.1 x 10$^{11}$/cm$^{2}$ and a mobility of $\mu \sim $ 2 x 10$^{5}$ cm$^{2}$/Vs at T = 300 mK, and with illumination from a light-emitting diode, the n$_{e}$ can be tuned to 1.5 x 10$^{11}$/cm$^{2}$. A systematic pattern in R$_{xx}$ and R$_{xy}$, corresponding to the opening and closing of the integer quantum Hall gaps, was observed as a function of n$_{e}$, as well as a function of tilt angle in tilted field experiments. We are interested in the possible observation of resonant spin Hall conductance in this system. Experimental data and a brief discussion will be presented. The work at Rice was supported by NSF DMR-0706634. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q23.00002: The quenching of scattering-enhanced tunneling in the Quantum Hall Regime Kasey Russell, Federico Capasso, Venkatesh Narayanamurti, Hong Lu, Joshua Zide, Arthur Gossard Using capacitance-voltage spectroscopy, we are able to probe the magnetic field dependence of the quasi-bound state lifetime of a quantum well. Our measurements are done on a special InGaAs/InAlAs heterostructure that is designed to promote scattering-enhanced tunneling out of the quantum well. The bound state lifetime shows large oscillations as the magnetic field is varied, which result from the quenching of electron scattering as the Fermi level crosses a Landau Level. The results are interpreted in terms of the edge states and cyclotron orbitals of the Integer Quantum Hall Effect. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q23.00003: Microwave modes of a two dimensional electron systems in the presence of a perpendicular magnetic anisotropy provided by a macroscopic ferromagnet Brenden Magill, L. W. Engel, M. P. Lilly, J. A. Simmons, J. L. Reno We report on a resonance in the microwave absorption spectrum of a high mobility two dimensional electron system (2DES) in a spatially varying magnetic field from a ferromagnet placed near the sample. The ferromagnet, made of Dy, cylinder or plates, with a hole through it. Microwave transmission between contacts capacitively coupled to the 2DES near the hole or cylinder shows a resonant absorption peak f$_{pk}$ for holes and cylinders with varying radii, r$_{m}$, from 0.5 mm to .125 mm. The resonance absorption is present for a uniform external field B$_{o }$less than 1 T and only for specific magnetization of the ferromagnet with respect to B$_{0}$ with f$_{pk}$ decreasing as either B$_{o}$ or r$_{m}$ increase. We will interpret the data in terms of plasma excitations similar to edge magnetoplasmons [1] confined along the magnetic field inhomogeneity by the large magnetic field gradients there. [1] See, for example, V. A. Volkov and S. A. Mikhailov, Sov. Phys.-JETP \textbf{67}, 1639(1988). [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q23.00004: Itinerant electron-driven chiral magnetic ordering and spontaneous quantum Hall effect in triangular lattice models Ivar Martin, C.D. Batista We study the Kondo Lattice and the Hubbard models on a triangular lattice. We find that at the mean field level, these rotationally invariant models naturally support a non-coplanar chiral magnetic ordering. It appears as a weak-coupling instability at the band filling factor 3/4 due to the perfect nesting of the itinerant electron Fermi surface. This ordering is a triangular-lattice counterpart of the collinear Neel ordering that occurs on the half-filled square lattice. While the long-range magnetic ordering is destroyed by thermal fluctuations, the chirality can persist up to a finite temperature, causing a spontaneous quantum Hall effect in the absence of any externally applied magnetic field. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q23.00005: Comparative study of radiation-induced transport in Wegscheider's and Umansky's GaAs/AlGaAs material R.G. Mani, W. Wegscheider, V. Umansky Transport studies of GaAs/AlGaAs specimens have shown radiation-induced, periodic-in-the-inverse-magnetic-field, magnetoresistance oscillations that saturate into novel radiation-induced zero- resistance states (RIZRS) at the deepest oscillatory minima.[1] The origin of these RIZRS remains a topic for further experimental investigation, as does the dependence of these phenomena on the impurity configuration and the material quality. On the latter point, it remains to be understood if similar material prepared in different laboratories yield a similar response under the same conditions. In addressing this issue, we examine here the radiation-induced transport in GaAs/AlGaAs material prepared by W. Wegscheider and co-workers. In a previous study, Simovic et al.[2] have reported the observation of B-periodic radiation- induced oscillations and the strong suppression of the inverse- B periodic oscillations in Wegscheider's GaAs/AlGaAs material. Here, we compare our experimental results to their study and also to our own previous results obtained on specimens prepared by V. Umansky and co-workers. 1) R. G. Mani, Appl. Phys. Lett., 91, 132103 (2007). 2) B. Simovic et al., Phys. Rev. B 71, 233303 (2005). [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q23.00006: Transport measurements and simulations of GaAs/AlGaAs ``anti-Hall-bar within a Hall bar'' devices Annika Kriisa, Ramesh G. Mani Hall effect measurements are often carried out in the Hall geometry, which is a thin rectangular plate with current and Hall voltage contacts at the external boundary. The motivation of this study is to further understand the impact on Hall effect when a hole is inserted inside Hall geometry. One way on conducting this investigation is to superimpose an ``anti-Hall bar'' inside the standard Hall bar, where the anti Hall bar is actually the hole inside the Hall device with contacts on the inside boundary of this hole. This configuration is thought to generate an ordinary Hall effect within the interior boundary such that the interior Hall voltage divided by the interior injected current equals the Hall resistance. One believes that it might also be possible to simultaneously realize multiple independent Hall effects by injecting multiple currents into the multiply connected device [1]. We have studied Hall effect in the doubly connected ``anti-Hall bar within a Hall bar'' geometry fabricated out of the GaAs/AlGaAs semiconductor system. Also the simulations of the distribution of the Hall current and potential profile within the specimen are conducted. To attain understanding of how the Hall effect arises in this geometry, the simulation plots are compared to the experimental results. [1] R. G. Mani and K. von Klitzing, App. Phys. Lett. 1993, 64, 1262-1264. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q23.00007: Effect of strain on nematic phases of two-dimensional hole gases Sunanda Koduvayur, Leonid Rokhinson, Michael Manfra We study the effect of uniaxial strain on high Landau levels(LL), $N\geq2$ (N is the LL index), in two dimensional hole gases(2DHG). The presence of anisotropic magnetotransport at certain half-integer filling factors in these systems has been understood as a signature of stripe or nematic phases. Recent studies on 2DHG in a perpendicular field have shown anisotropic transport at filling factors $\nu=7/2$ and $11/2$ accompanied by an isotropic $9/2$ state. These results differ from those of 2D electrons where anisotropy is only observed for LLs with $N\geq3$. While this difference has been attributed to stronger spin-orbit interactions in holes, the origin and conditions necessary for the stabilization of these states are still open questions. We study samples fabricated in the Van der Pauw geometry from C-doped GaAs/AlGaAs 2DHG grown on (001) substrate. We apply uniaxial strain along $[110]$ and study the transport properties in a perpendicular field at $10$mK. We introduce nematic states earlier on for $2\leq N\leq7$ with large enough strain. Furthermore, we demonstrate reversal in direction of anisotropy at filling factors $\nu=7/2$ and $5/2$ with strain modulation. We also see a difference in strain response of the resistances along $[110]$ and $[1\overline{1}0]$. We try to understand the observed effects using an electrostatic model which incorporates the anisotropy of the elastic moduli of GaAs. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q23.00008: Dependence of Effective Mass on Spin and Valley Degrees of Freedom Mansour Shayegan, Tayfun Gokmen, Medini Padmanabhan We measure the effective mass ($m^*$) of interacting two- dimensional electrons confined to an AlAs quantum well at a fixed density while we change the conduction-band valley occupation and the spin polarization via the application of strain and magnetic field, respectively [1]. Compared to its band value, $m^*$ is enhanced when the electrons are valley or spin unpolarized, and the largest enhancement is observed for the case where both spin and valley are unpolarized. Consistent with the study of M. Padmanabhan et al., in the fully spin- and valley-polarized regime, the measured $m^*$ is suppressed compared to the band value. Incidentally, in the fully spin- and valley-polarized regime, the electron system exhibits an insulating behavior. [1] T. Gokmen \textit{et al.}, Phys. Rev. Lett. {\bf 101}, 146405 (2008). [2] M. Padmanabhan, \textit{et al.}, Phys. Rev. Lett. {\bf 101}, 026402 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q23.00009: Anomalous Effective Mass of Two-dimensional Holes in a Strong Parallel Magnetic Field YenTing Chiu, Medini Padmanabhan, Javad Shabani, Mansour Shayegan, Roland Winkler We report effective hole mass (m*) measurements through analyzing the temperature dependence of the Shubnikov-de Haas oscillations in dilute (density $\sim $ 5x10$^{10}$cm$^{-2})$ two-dimensional (2D) hole systems confined to a 20nm-wide, (311)A GaAs quantum well. In this system the 2D holes occupy two spin-subbands whose m* we measure to be $\sim $ 0.2 (in units of free electron mass), in good agreement with the theoretical band calculations. We then apply a sufficiently strong ($>$10T) parallel magnetic field to fully depopulate one of the spin subbands, and measure m* for the populated subband. We find that this latter m* is close in magnitude to the m* we measure in the absence of the parallel field. This is a surprising observation as it is in stark disagreement with the results of our band calculations which take into account the spin-orbit interaction and the holes' finite layer thickness, and predict a large enhancement of m* in a strong parallel magnetic field. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q23.00010: Quantum Hall Effect and Field Dependent Valley Splitting on High Mobility Silicon-(111) Surfaces Tomasz M. Kott, Robert N. McFarland, Luyan Sun, Bruce E. Kane, Kevin Eng We have developed a method for fabricating field effect transistors, using vacuum as the dielectric, in order to study electron transport on a clean, flat, chemically prepared hydrogen-terminated surface. Resulting devices display high mobilities (110,000 cm$^2$/V s at 70 mK), enabling us to probe field dependent transport dynamics of this six-fold valley degenerate surface. I will present evidence that a low oxygen environment during sample preparation is necessary to achieve high mobilities. To support the correlations between surface chemistry and electronic properties, I will show AFM images of the surface for various preparation techniques. Finally, I will describe high field magneto-transport measurements (up to 12 T) that indicate field-dependent valley splitting. In particular, we find easily resolvable filling factors of, amongst others, 3, 5, and 7; an indication that the six-fold degeneracy is possibly broken by many-body effects. I will also show preliminary data with hints of the FQHE at $\nu = \frac{4}{3}$ and $\frac{8}{5}$. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q23.00011: Interaction Effects in Conductivity of a Two-Valley Electron System in High-Mobility Si Inversion Layers Nikolai N. Klimov, Dmitry A. Knyazev, Oleg E. Omel'yanovskii, Vladimir M. Pudalov, Harry Kojima, Michael E. Gershenson We have measured the conductivity of high-mobility (001)~Si metal-oxide-semiconductor field-effect transistors over wide ranges of electron densities $n = (1.8-15)\times 10^{11}$\,cm$^{-2}$, temperatures $T = 30$\,mK$-4.2$\,K, and in-plane magnetic fields $B_\parallel= 0-5$\,T [1]. The experimental data have been analyzed using the theory of interaction effects [2] in the conductivity $\sigma$ of disordered two-dimensional (2D) systems. The parameters essential for comparison with the theory, such as the intervalley scattering time and valley splitting, have been measured or evaluated in independent experiments [1,3]. The observed behavior of $\sigma$, including its quasi-linear increase with decreasing $T$ down to $\sim 0.4$\,K and its downturn at lower temperatures, is in agreement with the theory. The values of the Fermi-liquid parameter obtained from the comparison agree with the corresponding values extracted from the analysis of Shubnikov--de Haas oscillations based on the theory of magneto-oscillations in interacting 2D systems [4]. [1] N.\,N.\,Klimov {\em et. al.}, PRB {\bf 78}, 195308 (2008). [2] G.\,Zala {\em et. al.}, PRB {\bf 64}, 214204 (2001); {\bf 65}, 020201(R) (2001). [3] A.\,Yu.\,Kuntsevich {\em et. al.}, PRB {\bf 75}, 195330 (2007). [4] Y.\,Adamov {\em et. al.}, PRB {\bf 73}, 045426 (2006). [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q23.00012: 2DEG effect on vibration of piezoelectric plates Alexey Suslov Resonances of a GaAs wafer with a GaAs/AlGaAs heterostructure grown on one of its sides were studied in the temperature range 0.05-10K in magnetic fields of up to 18 T. To the best of our knowledge, this is the first use of the Resonant Ultrasound Spectroscopy in a dilution refrigerator. Observed quantum oscillations of the resonance frequencies and linewidths were caused by the Quantum Hall Effect in the 2DEG. The wafer with the 2D gas can be conceived as a film with field dependent conductivity deposited on a piezoelectric plate. Being dielectric, the film does not affect properties of GaAs and, thus, the resonance frequencies are defined only by the elastic, piezoelectric, and dielectric constants of GaAs. Being metallic, the 2D sheet effectively screens the parallel electric field, so the ultrasound wave velocities and resonance frequencies decrease with the increase of the sheet conductivity. Oscillations of the resonance linewidth reflect the influence of the 2D system on the ultrasound attenuation. A metallic film as well as a dielectric one does not affect this attenuation but at some finite nonzero value of the conductivity the linewidth approaches a maximum value. The observed phenomena can be described by the relaxation type equations. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q23.00013: Quantum Hall transition on a triangular lattice; network model and analytical renormalization-group treatment Vagharsh Mkhitaryan, Mikhail Raikh Common approach to the theoretical study of the quantum Hall transition is the Chalker-Coddington network model on the square lattice. We introduce a new version of the network model formulated on the triangular lattice, where the scattering at the sites is described by $3\times 3$ matrix. Extending renormalization-group description of the classical site percolation to the quantum case, we derive a closed equation for the distribution function of conductance. Solving this equation numerically, we get for the critical exponent of the correlation radius $\nu \approx 2.3\div2.76$ in good agreement with established value $\nu=2.33$. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q23.00014: Half integer features in the quantum Hall Effect: experiment and theory Tobias Kramer, E.J. Heller, R.E. Parrott, C.-T. Liang, C.F. Huang, K. Y. Chen, L.-H. Lin, J.-Y. Wu, S.-D. Lin We discuss experimental data and a new model of the integer quantum Hall effect (IQHE), which explains an intriguing substructure within Landau levels observed at higher currents. The experiments show inflection points in the Hall resistivity around filling factors 5/2 and 7/2. The experiments require to revisit the foundations of the IQHE and to establish an injection model which incorporates the correct boundary conditions imposed by a real Hall device and the Lorentz force. We have to follow the electrons to their source: one corner of the Hall bar and its steep electric field gradients, rather than focusing on the middle of the Hall device. We find the entire Hall resistivity curve is calculable as a function of magnetic field, temperature, and current. In contrast to previous theories of the IQHE, disorder plays no fundamental role in our theory. Contrary to the standard picture of Landau levels in disorder system, we predict and observe gaps right in the middle of certain Landau levels. The Hall plateaus and half integer inflections are shown to result from the LDOS appropriate to the magnetic field and the strong electric field at the injection corner. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q23.00015: Operation of a Single Electron Transistor Placed on Stacked Integer Quantum Hall Layers as a Magnetometer Hailing Cheng, Yu Jin, Rachel Goldman, Cagliyan Kurdak A single electron transistor (SET) placed on an integer quantum Hall liquid (IQHL) can detect small time varying magnetic fields in the presence of a large constant magnetic field. To enhance the sensitivity, we placed a SET made out of Al/AlOx/Al tunnel junctions on top of a GaAs/AlGaAs heterostructure with 25 identical quantum well structures. By monitoring the conductance of the SET following a small change in magnetic field, we studied the equilibration processes in IQHLs. The equilibration times associated with small changes in magnetic field are found to be strongly dependent on the magnetic field and became unmeasurably long (many days) as we got closer to the center of the quantum Hall plateaus. We have also characterized the SET magnetometer by an ac technique where we applied 170 nT ac magnetic field and measured the response of the SET using a double lock-in technique. At T=20 mK, the SET magnetometer worked at filling fractions up to n=12 and was most sensitive at the filling fraction n=6. [Preview Abstract] |
Session Q24: Focus Session: Optical Response of Nanotubes
Sponsoring Units: DMPChair: Achim Hartschuh, University of Munich
Room: 326
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q24.00001: Optical near-field investigations of individual single-walled carbon nanotubes Invited Speaker: Optical excitation of semiconducting nanotubes creates excitons that determine nearly all light-based applications. Near-field photoluminescence (PL) and Raman imaging with a spatial resolution better than 15 nm was used to probe the spectroscopic properties of excitonic states along single nanotubes on substrates [1,2]. The PL intensity was found to decrease towards the nanotube ends on a length scale of few 10 nm probably caused by exciton transport to localized end states followed by efficient non-radiative recombination. DNA-wrapping of nanotubes results in pronounced emission energy variations on a length scale of few 10 nm indicating the potential of the material for nanoscale sensing applications [3]. Inter-nanotube energy transfer was studied for different pairs of semiconducting nanotubes forming bundles and crossings [4]. Efficient transfer is found to be limited to a few nanometres because of competing fast non-radiative relaxation and can be explained in terms of electromagnetic near-field coupling. We also report on our recent experimental results on time-resolved near-field PL measurements, electrically gated nanotubes and the PL of nanotubes on metal surfaces. \\[4pt] References: \\[0pt] [1] A. Hartschuh, Angew. Chem. Int. Ed. 47, 8178 (2008). \\[0pt] [2] I. O. Maciel et. al, Nature Mat. 7, 878 (2008). \\[0pt] [3] H. Qian et. al, Nano Lett. 8, 2706 (2008). \\[0pt] [4] H. Qian et. al, Nano Lett. 8, 1363 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q24.00002: Electroluminescence Spectral Shape in Carbon Nanotube Field Effect Transistors under High Bias Conditions Megumi Kinoshita, Vasili Perebeinos, Mathias Steiner, Phaedon Avouris In carbon nanotube field effect transistors, electroluminescence excited by intra-nanotube impact excitation at high source-drain bias reveals strongly broadened electronic transitions (FWHM $\sim$150 to $\sim$300 meV for the lowest energy peak observed) in the E$_{11}$ to E$_{22}$ energy range. Through the bias and polarization dependence of the spectra, we investigate the production mechanism of these states and consider possible causes for their bias-dependent broad lineshapes, including exciton-exciton annihilation, and high electron and phonon temperatures. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q24.00003: Electroluminescence from Carbon Nanotube Network Transistors Elyse Adam, Carla Aguirre, Matthieu Paillet, Benoit Cardin St-Antoine, Francois Meunier, Patrick Desjardins, David Menard, Richard Martel A spectroscopic study of the electroluminescence properties of individual carbon nanotube (CNFET) and carbon nanotube network (NNFET) field effect transistors has been performed. As expected, the measurements on metallic and semiconducting CNFET showed that only semiconducting carbon nanotubes produce electroluminescent signals. The narrow emission line widths of CNFET ($\sim $80 meV) compared to that of NNFETs ($\sim $180 meV) indicates that the light emission from carbon nanotube networks involves many carbon nanotubes. Moreover, electroluminescence spectra from NNFETs made from three different sources of carbon nanotubes (laser ablation, CoMoCAT and DWNT) have shown major differences which, based on comparisons with their corresponding absorption spectra, indicate that only larger diameter carbon nanotubes contribute to light emission. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q24.00004: Confocal imaging and excitation spectra of photoluminescence from carbon nanotubes suspended over trenches of various widths S. Moritsubo, T. Murai, T. Shimada, Y. Murakami, S. Maruyama, Y. K. Kato Carbon nanotubes (CNTs) have novel optical properties, such as strongly bound excitons and sensitivity to surrounding environments, because of their unique structure. In particular, it is well known that CNTs lying on substrates show photoluminescence (PL) quenching caused by very effective substrate-induced nonradiative decay of photoexcited excitons. In this study, we formed trenches with various widths on SiO$_{2}$/Si substrates in order to prepare suspended CNTs. Using ethanol as carbon source, chemical vapor deposition was used for the synthesis of CNTs. PL images were collected by a home-built laser-scanning confocal microscope system utilizing a fast steering mirror. In addition, PL excitation spectra were taken using a wavelength tunable Ti:sapphire laser. We analyzed these PL images and excitation spectra in order to clarify the interaction between excitons and the substrate. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q24.00005: Near infrared photoresponse study of large area multi-walled carbon nanotube film with different electrode spacing Biddut Sarker, M. Arif, Paul Stokes, Alamgir Kabir, Saiful I. Khondaker Photoconductivity of carbon nanotube have generated considerable debate in terms of whether the photoresponse is (i) due to photon induced charge carrier (excitonic), (ii) due to heating of the CNT network (bolometric), or (iii) caused by photodesorption of oxygen molecules at the surface of the CNT. In addition, the role of the metal electrode -- CNT contact's effect on the photoresponse has also been debated. In this talk, we will present near -infrared photoresponse study of multi-walled carbon nanotube (MWCNT) film with different electrode spacings. We found that there is a large enhancement of photocurrent upon laser illumination and the photocurrent strongly depends on the position of the laser spot with maximum response occurring at the metal -- film interface. We also show that the photoresponse is rather slow ($\sim $1s) and increases with increasing electrode spacing. We will discuss the origin of the position dependent photocurrent and slow time response. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q24.00006: Transparency and conductivity of carbon nanotube networks Jan Obrzut The conductivity of films made of single wall carbon nanotubes longer than 200 nm closely follows the percolation theory for two-dimensional (2D) networks . The scaling universal exponents describing the ``percolation'' transition from an insulating to conducting state with increasing concentration are consistent with 2D percolation model. A sheet of tubes about 820 nm long becomes conducting at an amazingly low concentration of about 18x10$^{-9}$ g/cm$^{2}$. In comparison, batches of short nanotubes or mixed-length batches form more 3D networks that conduct noticeably worse. Furthermore, the conductivity percolation threshold (x$_{c}$) varies with the aspect ratio Length (L) as, x$_{c} \sim$1/L, a result that is also in accordance with the percolation theory. We also show that contrary to current predictions, these sheets do not have optical properties similar to thin metallic films. Our results indicate that the correlation between the optical properties and the electrical conductivity of these sheets is again better predicted by the general percolation theory. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q24.00007: Carrier renormalization effects on the optical response of doped semiconducting single-walled-carbon nanotubes Sheng Ju, Cheol-Hwan Park, Steven Louie It is known that many-electron effects dramatically change the optical properties of single-walled carbon nanotubes (SWCNTs). Recently, researchers have succeeded in tuning the Fermi energy of an individual SWCNT by applying a gate voltage or by introducing adsorbate dopants. Therefore, the optical response of doped SWCNTs is not only interesting from a pure scientific point of view but also important for the application of these systems. We present here first-principles calculations, based on the GW-Bethe Salpeter equation (GW-BSE) approach, of the quasiparticle (single-particle excitation) spectrum and the optical (electron-hole excitation) spectrum of doped SWCNTs. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q24.00008: Optical properties of doped semiconducting single-walled carbon nanotubes. Catalin D. Spataru, Francois Leonard We studied how the optical response of semiconducting single-walled carbon nanotubes changes upon doping. We performed ab initio calculations of the optical absorption spectrum of the p-doped (10,0) SWCNT, employing a many-electron Green's function approach that determines both the quasiparticle and electron-hole excitations from first principles. We found that the absorption spectrum changes dramatically upon doping, due to both quasiparticle and excitonic effects. In the independent quasiparticle picture, the electron-hole continua are strongly red-shifted with respect to the undoped case due to the metallic character acquired by the electronic screening upon doping. However, the main optical features in the absorption spectrum, including both quasiparticle and electron-hole interaction effects, are only slightly shifted, but qualitatively very different, with respect to the undoped case. Small doping levels (where the Fermi level lies below the valence band maximum by an energy much smaller than the binding energy of excitons in the undoped SWCNT) are sufficient to bleach band-gap absorption. In addition, while resonant excitons associated with the second electron-hole continuum can still exist in the doped SWCNT, their binding energy is much reduced, to a level typical of metallic SWCNTs of similar diameter. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q24.00009: Origins of optical absorption components of metallic and semiconducting single-wall carbon nanotubes in ultra-violet region Kazuhiro Yanagi, Takeshi Saito, Yasumitsu Miyata, Hiromichi Kataura There are large absorption components in the optical absorption spectra of single-wall carbon nanotubes (SWCNTs) in the ultraviolet (UV) region ($\sim $5 eV). Clarification of the origins of the UV absorption is important, since the tails of the UV components influence the transparency of nanotubes and impede their uses for transparent conducting films. However, the origins have not been correctly understood yet. Such UV absorption components are assumed to be caused by $\pi $-plasmons, however, recently contributions from $\pi -\pi $* transition at the M point were also suggested. To understand the origins of UV component, here we clarified how the electronic structure (metallic or semiconducting) and the diameters of SWCNTs influence the UV optical absorption features. We clearly identified two components in UV region, and revealed dependence of the components on their diameters. Remarkably, dependence of the peak-energies of one component on diameters could not be explained by plasmon model, implying the presence of different origins than plasmons in the UV absorption components. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q24.00010: Environmental Change in UV Absorption by Single-Walled Carbon Nanotubes Yoichi Murakami, Shigeo Maruyama We investigated the UV absorption of single-walled carbon nanotubes (SWNTs) in the 4 - 6 eV range, which has been customarily referred to as the ``pi-plasmon'' as a whole. The optical absorption spectra of aligned SWNTs were measured in different dielectric environments. The experimental results unambiguously show that, for the two different UV absorption components existing in the 4 - 6 eV range, only the feature at 5.0 - 5.3 eV exhibits remarkable spectral changes, while the other feature at $\sim $ 4.5 eV remains unchanged. We attribute the former (5.0 - 5.3 eV) to a dipolar surface plasmon in the radial direction of SWNTs. On the other hand, the experimental result raises a fundamental question as to whether it is appropriate to classify the UV feature at $\sim $ 4.5 eV as a pi-plasmon. We will discuss its relation with the UV absorption at $\sim $ 4.5 eV in graphite/graphene that has long been recognized as the interband transition at M point of the Brillouin Zone. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q24.00011: Electron-electron interaction effects on the photophysics of metallic single-walled carbon nanotubes Sumit Mazumdar, Demetra Psiachos, Zhendong Wang, Roberto Badilla Within a molecular Hamiltonian appropriate for correlated $\pi$-electron systems, we show that optical excitations polarized parallel to the nanotube axes in the so-called metallic single-walled carbon nanotubes are to excitons. Our calculated absolute exciton energies in twelve different metallic single-walled carbon nanotubes, with diameters in the range 0.8 - 1.4 nm, are in nearly quantitative agreement with experimental results. We have also calculated the absorption spectrum for the (21,21) single-walled carbon nanotube in the E$_{22}$ region. Our calculated spectrum gives an excellent fit to the experimental absorption spectrum. In all cases our calculated exciton binding energies are only slightly smaller than those of semiconducting nanotubes with comparable diameters. As in the semiconducting nanotubes we predict in the metallic nanotubes a two-photon exciton above the lowest longitudinally polarized exciton that can be detected by ultrafast pump-probe spectroscopy. We also predict a subgap absorption polarized perpendicular to the nanotube axes below the lowest longitudinal exciton, blueshifted from the exact midgap by electron-electron interactions. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q24.00012: Fano Resonance in Single-Walled Cabron Nanotube Devices Gang Liu, Yong Zhang, Chunning Lau We have observed Fano resonance in a short carbon nanotube device. The device's transport spectroscopy exhibits inverse Coulomb blockade structures superimposed on Fabry Perot resonance patterns, indicating the quantum interference between a well-coupled channel and a poorly-coupled channel. Our results have implication on the detection of charges' phase and phase coherence in an electronic interferometer. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q24.00013: Optical Absorption Spectra of Charge Doped Single-Walled Carbon Nanotubes from First-principles Calculations Guangfu Luo, Jing Lu, Wai-Ning Mei, Lu Wang, Lin Lai, Jing Zhou, Rui Qin, Hong Li, Zhengxiang Gao The optical absorption spectrum of single-walled carbon nanotubes (SWCNTs) under charge doping is often interpreted within the graphene zone-folding and rigid-band model. Based on the periodic boundary model together with a uniform background countercharge, our density functional theory calculations, however, show that the spectrum response deviates from the expectations of such model. Specifically, the SWCNT band structures can differ qualitatively from the zone-folding ones, and with the increasing doping level, the absorption peaks will blench in a non-sequential energy order. The on-tube bands in SWCNTs sometimes change obviously even under low charge doping level, and accordingly cause spectra peaks to shift, split, and merge. At the end of this paper, we discuss briefly the applicability of the present results at the GW-BSE theory level and in other SWCNT-like systems. [Preview Abstract] |
Session Q25: Focus Session: Graphene IX: Structure and Strain
Sponsoring Units: DMPChair: Antonio Castro Neto, Boston University
Room: 327
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q25.00001: Interaction of magneto-excitons with phonons in graphene and graphite Jun Yan, Trevor David Rhone, Sarah Goler, Melinda Han, Vittorio Pellegrini, Philip Kim, Aron Pinczuk We study the Landau levels of graphite and single layer graphene by measurements of the magneto-phonon resonance effect in which there is coupling between the inter-Landau level magneto-exciton with the long wavelength optical phonon (G band). In graphite the G band displays a rich line shape evolution as the magnetic field is finely tuned between 5 and 7 Tesla. These observations indicate that the G band is resonantly coupled to the magneto-excitions at these fields. In the interpretation we postulate that the anticrossing of the phonon band with the inter-Landau level transitions results in a mode-splitting at around 6.2 T. The evolution of the energy and spectral weight of the two coupled modes indicates that the phonon is a probe of the unique structure of Landau levels in graphene-related materials. In an as-prepared single-layer graphene, much smaller changes are observed for fields reaching as high as 12 Tesla. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q25.00002: Klein Backscattering and Fabry-Perot Interference in Graphene Heterojunctions Andrei Shytov, Mark Rudner, Leonid Levitov Fabry-Perot (FP) interference in a lateral p-n-p structure in graphene is proposed as a vehicle to probe Klein scattering phenomenon [Phys. Rev. Lett. 101, 156804 (2008)]. In ballistic regime, interference between waves scattered from p-n interfaces leads to oscillations in conductance as a function of electron density. Perfect transmission at zero incidence angle (Klein effect) implies a sign change of the backreflection amplitude. This change contributes a phase~$\pi$ to interference and shifts FP fringes by half a period. Alternatively, the $\pi$ phase can be understood as Berry's phase accrued by electron bouncing between p-n boundaries. This effect is revealed in the evolution of fringes when a relatively weak, non-quantizing magnetic field is applied. The behavior of the interference fringes recently observed by Young and Kim (arXiv:0808.0855) is consistent with this picture. The observed crossover to Shubnikov-de Haas oscillations can be also understood from quantization of perioidic orbits bouncing between the two p-n interfaces. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 12:15PM |
Q25.00003: Graphene electronics via strain engineering Invited Speaker: Recently, graphene has been confirmed as the strongest material ever measured, being able to sustain reversible deformations in excess of 20\%. These mechanical measurements arise at a time where graphene draws considerable attention on account of its unusual and rich electronic properties. Besides the great crystalline quality, high mobility and resilience to high current densities, they include a strong field effect, absence of backscattering and a minimum metallic conductivity. While many such properties might prove instrumental if graphene is to be used in future technological applications in the ever pressing demand for miniaturization in electronics, the latter is actually a strong deterrent: it hinders the pinching off of the charge flow and the creation of quantum point contacts. In addition, graphene has a gapless spectrum with linearly dispersing, Dirac-like, excitations. Although a gap can be induced by means of quantum confinement in the form of nanoribbons and quantum dots, these ``paper-cutting'' techniques are prone to edge roughness, which has detrimental effects on the electronic properties. We explore an alternative route for tailoring the electronic structure of graphene, based on a strain engineering. We will discuss how local and global strain profiles can be suitably tailored to impact the bandstructure of graphene and control its transport characteristics. Electron confinement, electron beam collimation, energy filtering, surface modes and bulk spectral gaps are some examples of what might be achieved. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q25.00004: Fabrication and transport measurement of suspended graphene devices. Wenzhong Bao, Feng Miao, Gang Liu, Chunning Lau We developed a lithography-free technique to fabricate suspended graphene devices. Graphene sheets are exfoliated over pre-defined trenches over the substrates, and electrodes are deposited via shadow mask evaporation. This technique eliminates resist residues which may affect electrical properties of graphene. We will discuss results from electrical transport measurement at different temperatures and magnetic fields. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q25.00005: Temperature dependent resistivity of suspended graphene Eros Mariani, Felix von Oppen In this talk we discuss the temperature dependence of the resistivity for suspended single layer graphene, due to electron-phonon scattering. All the temperature regimes are studied, as well as the contributions due to the different acoustic phonon branches in graphene. We show how tension in the membrane suppresses the otherwise dominant contribution due to flexural phonons [1], leaving a linear temperature scaling compatible with recent experiments. The eventual crossover to quadratic temperature dependence at very high temperatures could be used as an experimental tool to investigate the otherwise unknown strength of the tension. Finally, we discuss the transition to the quasi-nondegenerate regime for electrons in graphene. This is relevant for current experiments on the temperature dependent resistivity in most temperature regimes, and can shed light on the unexpected density dependence of the linear-T resistivity. \\[3pt] [1] Eros Mariani and Felix von Oppen, Phys. Rev. Lett. {\bf 100}, 076801 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q25.00006: Neutron Scattering Studies of Graphene Alice Acatrinei, Zhijun Lin, Luke Daemen We synthesized graphene by thermal exfoliation at 500 deg C. The material is nanosize, as confirmed by TEM/SEM, with flake transverse dimensions 50-100 nm. We present the first detailed measurement of the vibrational Spectrum of both graphene and graphite using INS, along with a neutron Scattering study of hydrogen adsorbed on graphene. Our measurements were collected at 10K using the Filter Difference Spectrometer (FDS) at Lujan Neutron Scattering Center, Los Alamos National Laboratory. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q25.00007: Effect of film corrugation on the optical phonon lifetime in graphene Peter Eklund, Awnish Gupta We present results of a microRaman study of n-layer graphene films supported on $\sim $atomically flat mica, Si/SiO$_{2}$ (or varying roughness) and suspended above a trench. Using the Raman G-band line width \textit{$\Gamma $}$_{G }$, we find that the optical phonon lifetime \textit{$\tau \sim $1/$\Gamma $} decreases linearly with increasing rms substrate roughness $\delta $, and independent of the chemical composition of the substrate. In agreement with this general observation, we find that \textit{$\Gamma $}$_{G}$ for unsupported graphene is significantly higher (i.e., \textit{the q=0} optical phonon lifetime is significantly lower) than observed when the film is supported on mica. Correlating \textit{$\Gamma $}$_{G}$ with values obtained from supported films, we infer an inherent rms roughness \textit{$\delta \quad \sim $ 2 nm} for unsupported graphene, in reasonable agreement with recent STM reports that first suggested that graphene might prefer to spontaneously convert to a corrugated system. Our observations may then relate to the effect of the local bending of the sp$^{2}$ sheet on the electron-phonon interaction. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q25.00008: Edge stress induced warping of graphene sheets and nanoribbons Vivek Shenoy We show that edge stresses introduce intrinsic ripples in free-standing graphene sheets even in the absence of any thermal effects. Compressive edge stresses along zigzag and armchair edges of the sheet cause out-of-plane warping to attain several degenerate mode shapes. Based on elastic plate theory, we identify scaling laws for the amplitude and penetration depth of edge ripples as a function of wavelength [1]. We also demonstrate that edge stresses can lead to twisting and scrolling of nanoribbons as seen in experiments. Our results underscore the importance of accounting for edge stresses in thermal theories and electronic structure calculations for free-standing graphene sheets. [1] V. B. Shenoy, C. D. Reddy, A. Ramasubramaniam and Y. W. Zhang, \textit{Phys. Rev. Lett} (in press) [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q25.00009: Infrared spectroscopy of electronic bands in bilayer graphene Alexey Kuzmenko, Erik van Heumen, Dirk van der Marel, Philippe Lerch, Peter Blake, Konstantin Novoselov, Andre Geim We present infrared spectra (0.1-1 eV) of electrostatically gated bilayer graphene as a function of doping and compare them with tight binding calculations. All major spectral features corresponding to the expected interband transitions are identified in the spectra: a strong peak due to transitions between parallel split-off bands and two onset-like features due to transitions between valence and conduction bands. A significant electron-hole asymmetry is observed. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q25.00010: Sharp Landau Levels in Scanning Tunneling Spectroscopy of Epitaxial Graphene on SiC(000 -1) David Miller, Kevin Kubista, Gregory Rutter, Ming Ruan, Walt de Heer, Phillip First, Joseph Stroscio Monolayer graphene has unique electronic properties stemming from a low-energy band structure that is linear, with chiral Dirac quasiparticles. In a magnetic field, the Landau level (LL) energies for graphene $E_n$ vary proportional to $\sqrt{nB}$, where $n$ is the LL index. Conversely, Bernal-stacked bilayer graphene and graphite have parabolic dispersion at low energies, resulting in $E_n \propto B$. In this talk we measure the LL spectrum of the top graphene layer directly via scanning tunneling spectroscopy (STS) at a 4.3 K. We show that for $\approx 10$-layer epitaxial graphene grown on SiC(000 -1), the spectrum exhibits very sharp peaks (including a strong n=0 peak) spaced as $E_n \propto \sqrt{nB}$. This spectrum indicates that the rotational stacking in multilayer epitaxial graphene effectively decouples the layers, producing single-layer graphene behavior. Work supported in part by NSF, NRI-INDEX, and the W. M. Keck Foundation. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q25.00011: Electronic structures of single- and multi-layer epitaxial graphene on SiC (0001) Seungchul Kim, Jisoon Ihm, Young-Woo Son The electronic structures of single- and multi-layered epitaxial graphene on silicon carbide (0001) surface are studied theoretically. To calculate energy bands of the systems, we construct the simple Hamiltonian with tight-binding approximations. We confirm that the present simple model do give identical electronic structure to the previous ab-initio study on the single layer case [1]. We extend the model up to four epitaxial graphene layers to explain various interesting experimental findings. The roles of the coupling between graphenes and the buffer layer, and their large scale reconstructions to the electronic structures are also investigated. [1] S. Kim, J. Ihm, H. J. Choi, Y.-W. Son, Phys. Rev. Lett. 100, 176802 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q25.00012: G-band Phonon Symmetry Breaking of Graphene Monolayers Lain-Jong Li, Yang Zhao, Xiaochen Dong, Peng Chen Aromatic molecules can effectively exfoliate graphite into graphene monolayers. And the resulting graphene monolayers sandwiched by the aromatic molecules exhibit pronounced Raman G-band splitting, similar to that observed in rolled-up graphene sheet (single-walled carbon nanotubes). Raman measurements and the theoretical calculation based on force-constant model demonstrate that aromatic molecules are able to induce G-band splitting via breaking the symmetry of two in-plane longitudinal and transverse optical phonons at Gamma-point. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q25.00013: Edge states in a honeycomb lattice: effects of anisotropic hopping and mixed edges. Zi-Xiang Hu, Hari Dahal, Nikolai Sinitsyn, Kun Yang, Alexander Balatsky We study the effects of anisotropic hopping and mixed edges on the edge states of graphene. The discussion of the edge states in graphene so far is focused on either zigzag or armchair edge with isotropic hopping. In this case the zigzag (arm chair) edge has enhanced (suppressed) local density of states at E=0 near the edge. In practice electrons in graphene can have anisotropic hopping. The lattice can have mixed (zigzag and arm chair) edges. Hence we study the effects of the anisotropic hopping and mixed edges on the edge states. We show that the mixed edges smear the enhanced local density of states at E=0 of the zigzag edge and, the anisotropic hoping enhanced the LDOS at E=0 in the armchair edge. Edge states in graphene can be studied using scanning tunneling microscopy (STM) experiments. We suggest that care must be taken while interpreting the STM data because the distinction between the zigzag and arm chair edge will be affected by the anisotropic electron hopping and the mixed edges. [Preview Abstract] |
Session Q26: Focus Session: Graphene X: Theory
Sponsoring Units: DMPChair: Tobias Stauber, University of Minho, Portugal
Room: 328
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q26.00001: Electron-electron interactions in graphene bilayers Fan Zhang, Hongki Min, Marco Polini, Allan MacDonald Electrons in condensed matter normally form Fermi-liquid states in which e-e interactions play an inessential role. A well known exception is the case of 1D electron systems in which the Fermi-surface consists of two points and divergences associated with low-energy particle-hole excitations abound when e-e interactions are described perturbatively. Corresponding divergences normally occur in systems with higher space dimensions when Fermi lines or surfaces satisfy idealized nesting conditions. Here we discuss the role of e-e interactions in 2D graphene bilayers which behave in many ways as if they were 1D because they have point-like Fermi surfaces which satisfy the nesting condition and have two-layer chirality and because their particle-hole energies have a quadratic dispersion which compensates for the difference between 1D and 2D phase spaces. We conclude, on the basis of a perturbative RG calculation, that interaction in neutral graphene bilayers drive the system into a spontaneously broken symmetry state with layer-pseudospin ferromagnetism. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q26.00002: Ferromagnetism in Graphene Stacks Dagim Tilahun, Allan MacDonald Because the density of states at the Fermi level of neutral graphene layers is proportional to external magnetic field, the ground state in strong fields is expected to have a broken symmetry - most likely ferromagnetism. In systems with stacked graphene layers this tendency competes with inter-layer hopping which favors paramagnetic ground states or perhaps other types of broken symmetries. We present a criterion for the stablility of the ferromagnetic state and discuss its application to single-layer graphene, to weakly coupled epitaxial graphene layers on SiC or other substrates, and to bulk graphite. We use the Slonczewksi-Weiss-McClure model to explain why the dominant inter-layer hopping process in Bernal (AB) stacked graphite does not compete with ferromagnetism. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q26.00003: Theory of inter-edge superexchange in zigzag edge magnetism Jeil Jung, Tami Pereg-Barnea, Allan MacDonald A graphene nanoribbon with zigzag edges has a gapped magnetic ground state with an antiferromagnetic inter-edge superexchange interaction. We present a theory based on the asymptotic properties of the Dirac-model ribbon wavefunction which predicts $W^{-2}$ and $W^{-1}$ ribbon-width dependences for the superexchange interaction strength and the gap respectively. Unlike in conventional superexchange we find that both the kinetic and exchange energy contributions favor the antiferromagnetic inter-edge coupling with a dominant role of exchange several times larger in magnitude than the kinetic energy contribution. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q26.00004: Numerical study on electron-electron interaction and ferromagnetic fluctuation in graphene Tianxing Ma, Feiming Hu, Zhongbing Huang, Hai-Qing Lin Within the Hubbard model on a honeycomb lattice, we investigate the effect of electron-electron interactions and ferromagnetic fluctuations in graphene numerically. We find that the system in the filling region $<$$n$$>$=1.60-1.90 shows a short-ranged ferromagnetic correlation, and the on-site Coulomb interaction tends to strengthen ferromagnetic fluctuation slightly. Furthermore, the ferromagnetic fluctuation is strengthened markedly as the next-nearest neighbor hoping energy increases, which indicate that the next-nearest neighbor hoping term plays an important role in graphene since it breaks the particle-hole symmetry. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q26.00005: Gate-induced interlayer asymmetry in ABA-stacked trilayer graphene Edward McCann, Mikito Koshino We model the electronic band structure and conductivity of ABA- stacked trilayer graphene in the presence of external gates, self-consistently calculating the electric potential of the three layers. We show that a gate field perpendicular to the layers breaks mirror reflection symmetry with respect to the central layer, leading to hybridization of the linear and parabolic low-energy bands. For large gate fields, we derive an effective two-component Hamiltonian describing chiral electrons in two low-energy bands that exhibit an anti-crossing with a small hybridization gap. The magnitude of the gap is largely independent of the gate field, but the momentum at the anti- crossing and the typical band velocity both increase with it. Using the self-consistent Born approximation, we find that the density of states and the minimal conductivity in the presence of disorder generally increase as the gate field increases, in sharp contrast with bilayer graphene. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q26.00006: Aharonov-Bohm-like scattering, localization, and novel electronic states in hydrogenated graphene Andrey Shytov, Dmitry Abanin, Leonid Levitov Metallic nature of transport in graphene, which is fairly robust with respect to varying amounts of disorder, changes in an unexpected way when vacancies are introduced in this material. At low energies, near the Dirac point, electron scattering on vacancies mimics scattering on Aharonov-Bohm solenoids carrying unit flux. This type of scattering results in a very narrow band of states at the Dirac point with properties resembling those of zeroth Landau level, which is positioned in the middle of a (pseudo)gap created by vacancies and resembling the cyclotron gap around zeroth Landau level. The fictitious magnetic field describing vacancies has opposite signs for the valleys K and K'. As a result of this, an externally applied magnetic field has opposite effects in the two valleys, suppressing (reinforcing) the gap in the K (K') valley. We show that this picture is in agreement with the behavior observed in a recent study [1] of electronic properties of graphene, which can be transformed from metallic state to insulating state by hydrogenation. [1] D. C. Elias, R. R. Nair, T. M. G. Mohiuddin, S. V. Morozov, P. Blake, M. P. Halsall, A. C. Ferrari, D. W. Boukhvalov, M. I. Katsnelson, A. K. Geim, K. S. Novoselov, arXiv:0810.4706 [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q26.00007: Rashba spin-orbit interactions in zigzag graphene nano-ribbons Mahdi Zarea, Nancy Sandler The crystalline structure of graphene can be described in terms of a pseudo-spin degree of freedom and spinor wavefunctions. This characteristic has important physical consequences not observed in normal semiconductors. For zigzag ribbons, for instance, this translates into the existence of localized chiral edge states, with momentum coupled to pseudo-spin. In the presence of Rashba spin-orbit interactions (RSOI), this special feature makes the material a good candidate to produce localized spin polarized currents. To address this issue we investigated the role of the RSOI on the band-structure and wavefunctions of an infinite graphene plane and a zigzag nano-ribbon. We present analytic and numerical results showing that the spin profile along the edge is state-dependent. We compare these results with the profiles obtained in the presence of the intrinsic spin-orbit interaction [1]. We show that the RSOI can create average localized spin polarized currents along the edges of zigzag ribbons with appropriate applied voltages. [1] Zarea, M., Busser, C. and Sandler, N. PRL 101, 196804 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q26.00008: Plasma Instabilities in Graphene Ben Yu-Kuang Hu, Antti-Pekka Jauho We discuss the possibility of the occurrence of plasma instabilities under non-equilibrium conditions in graphene. Specifically, we investigate the stability of the electronic collective modes in graphene with two counter-streaming distributions of carriers by studying the frequency-dependent dielectric function $\epsilon({\bf q},\omega)$ of the system. We find that the linear electronic dispersion of graphene results in instabilities that are qualitatively different from the standard two-stream instabilities for classical plasmas and parabolic-band systems. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q26.00009: Is suspended graphene an insulator? Joaquin Drut, Timo Lahde Graphene at low energies resembles massless quantum electrodynamics in a strongly coupled regime, away from the usual perturbative region where the fine structure constant is $\alpha \simeq 1/137$. Indeed, a single sheet of graphite in vacuum presents $\alpha \sim 1$. At such strong couplings the U(4) chiral symmetry of graphene can spontaneously break, inducing a gap in the quasiparticle spectrum. The question of whether chiral symmetry is broken represents a computational challenge that lies outside the domain of analytic techniques. In this talk, we will present the results of the first Monte Carlo simulation of the low-energy effective theory of graphene in vacuum (see abstract by T.~A.~L\"ahde). We have computed the chiral condensate, which is the order parameter for the insulating charge density wave state, as a function of $\alpha$, and found a chiral phase transition that is compatible with suspended graphene being in the gapped phase. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q26.00010: Many-body effects in neutral graphene bilayers Csaba Toke, Vladimir I. Falko A graphene bilayer is studied within the Hartree-Fock approximation in the tight-binding model. The exchange self-energy is studied systematically in an momentum expansion. Up to first order in the coupling constant (the effective fine structure constant) and to first order of the nonperpendicular hopping parameter we find that, for zero magnetic field, the exchange interaction with the valence band contributes with a logarithmically divergent correction to the Fermi velocity, the perpendicular inter-layer hopping, and the trigonal warping. The effective mass renormalization in the two-band effective Hamitonian is studied. For a strong perpendicular magnetic field the exciton dispersions are calculated. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q26.00011: Berry phase and the role of trigonal warping in graphene systems Ivan Stanic, Karyn Le Hur In general Dirac fermions exhibit a Berry phase of $\pi$ whereas non-relativistic free-like particles have a Berry phase of $2\pi $. The two cases have been reported, both theoretically and experimentally, in single-layer and bi-layer graphene respectively. On the other hand, if one considers, for example, bi-layer graphene in more detail, its band structure shows both the Dirac-type, coming from the trigonal warping, and the parabolic, non-relativistic aspect in different energy regimes. This gives an unprecedented opportunity to investigate the \textbf{crossover between non- relativistic and Dirac fermions in a Berry phase formulation}. Here, we propose a scattering type-experiment (reflecting the Berry phase) to demonstrate this crossover. We present our theoretical results on scattering cross-sections taking into account the trigonal warping term, which confirm the jump in the Berry phase from $\pi$ to $2\pi$ as the energy of the incoming electrons is increased. This jump in the Berry phase can be understood from a general theorem relating the Berry phase and the band structure of a material. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q26.00012: Trigonal Band Structure and Time-Reversal Invariance in Graphene Roland Winkler, Ulrich Zuelicke We present a symmetry analysis of the trigonal band structure in graphene. While the energy spectrum near the Fermi edge equals the spectrum of massless Dirac fermions, the transformational properties of the underlying basis functions are qualitatively different. Using group theory we develop an invariant expansion of the Hamiltonian for the electron states near the $\mathbf{K}$ points of the graphene Brillouin zone. We find that the $k$-linear dispersion near the band edge arises as an unusual consequence of time-reversal invariance. We suggest to divide the electronic properties of graphene into two categories, those that depend and those that do not depend on the transformational properties of the Bloch functions at $\mathbf{K}$. See arXiv:0807.4204. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q26.00013: Two dimensional massless Dirac fermions with Coulomb interaction and random gauge potential Sen Zhou, Oskar Vafek We present a numerical study of the two dimensional massless Dirac fermions of monolayer graphene with long-range Coulomb interaction and random gauge potential. The Coulomb interaction renormalizes logarithmically the electron velocity at low energies, leading to a decrease in the density of states. While the density of states is enhanced by the random gauge potential, and has a power-law dependence in low energies, $\rho(E)\sim E^{-1+2/z}$ with $z=1+\sqrt{3}\Delta/\pi$, where $\Delta$ measures the disorder strength. The combined effect of interaction and disorder gives rise to a line of fixed points where both the interaction and disorder are finite, and the low-energy density of states is exactly linear. Results are consistent with previous renormalization group argument. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q26.00014: Gap opening due to topological defects in graphene Ricardo Nunes, Joice Ara\'ujo, Helio Chacham Stone-Wales defects (SWD = two adjacent pentagon-heptagon pairs) are common low-energy defects in carbon nanotubes. Previously, Crespi et al.[{\bf PRB, 53, 1996}] have proposed a purely-carbon covalent metal sheet called ``pentaheptite,'' consisting entirely of SWDs, with a relatively low formation energy of 0.32 eV/atom, with respect to graphene. In this work, we consider three different families of periodic carbon sheets containing topological defects (TDs = pentagons and heptagons). The families differ by the density of TDs in a seed structure. In each family, we generate periodic structures in which isolated pentagons and heptagons are surrounded only by hexagons. By means of ab initio calculations, we propose that, depending on the density and distribution of TDs, these carbon sheets may behave as a semiconductor, a metal or a semimetal. In the range of TD concentrations we examine, the sheets are stable in a planar form, but, allowing for the corrugation generated by the curvature fields associated with the isolated TDs, leads to lower formation energies and to either a reduction of the density of states or to gap opening at the Fermi level. Formation energies can be very small: in particular, we obtain a semiconducting structure with a formation energy of only 92 meV/atom with respect to graphene. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q26.00015: Transport of massless Dirac fermions in graphene layers in presence of electromagnetic potential barriers Sankalpa Ghosh, Manish Sharma We study the transport of massless Dirac fermions in Graphene layers through electromagnetic potential barriers. The barriers consist of periodically arranged delta function-like magnetic fields superposed with periodic electrostatic potentials. We show that such an arrangement provides a wide range of control on the electron transport through Graphene, and the associated problems can be mapped on certain classes of optical problems. We discuss the related band structure and its effect on transport over a range of magnetic field strengths and barrier widths. We also discuss the typical experimental set up where related properties can be verified. [Preview Abstract] |
Session Q27: Focus Session: X-ray and Neutron Instruments and Sciences II
Sponsoring Units: GIMSChair: Dennis Mills, Argonne National Laboratory
Room: 329
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q27.00001: 3D Mapping of Strain and Dislocation Gradients near Surfaces and Interfaces via Polychromatic Microdiffraction Rozaliya Barabash, Gene Ice The results of 3D polychromatic X-ray microbeam analysis (PXM) of strain and dislocation gradients are presented. Two examples are considered: (1) FIB machined nano-size trenches in thin GaN/InGaN multylayers; (2) natural nanosize Mo pillars in the NiAl matrix of the eutectic composite alloy. Position sensitive $d$-spacings were obtained from Laue patterns. The PXM results show that FIB induces structural changes and \textit{lattice rotations} in the InGaN/GaN layer not only in the immediate trench region but in the surrounding area as well. For embedded nanosize Mo fibers, the measured elastic strain is consistent with the predicted thermal mismatch strain between the NiAl and Mo phases. However, when the NiAl matrix is etched back to expose Mo micro-pillars, the $d$-spacing increases to that of unconstrained Mo, indicating release of the compressive residual strain in the Mo fibers. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q27.00002: Strain Fields in Zeolite Microcrystals by Coherent X-ray Diffraction Hyunjung Kim, Wonsuk Cha, Sanghoon Song, Nak Cheon Jeong, Kyung Byung Yoon, Ross Harder, Ian K. Robinson We measured coherent X-ray diffraction (CXD) on zeolite microcrystals in order to get information on internal density distribution and to map deformation field of stress or strain during the fabrication process. The experiments were performed at the beamline 34-ID-C in Advanced Photon Source and employed monochromatic radiation with x-ray energy of 9 keV. The sample size was about $\sim$2{$\mu$}m. The diffraction patterns were obtained at (200) Bragg condition with unfocused beam. We inverted the diffraction patterns to obtain three dimensional images of the shapes and internal strain fields of zeolite microcrystals using phase retrieval algorithms of error reduction and hybrid input-output method. The internal density and strain distribution as a function of temperature will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q27.00003: Silicon x-ray monochromator surfaces by independent oxidation and etching steps Albert Macrander, Kimberley MacArthur, Josef Maj, Jun Qian, Dan Linnen, Ruben Khachatryan, Michael Wieczorek, Ray Conley, Alan Genis X-ray monochromators should ideally possess a surface that does not distort a diffracted beam. Beam distortions have been observed at the APS for rough surfaces. Mechanical polishing leaves sub-surface damage. The standard method to remove this damage is to wet etch Si crystals in a mixture of nitric acid and hydrofluoric acid. During the etch an oxide is produced and removed in the same acid bath. X-ray diffraction from a bulk reflection that is largely unaffected by strain can be obtained by this method. However, the smoothness is degraded to produce an orange-peel morphology. For the present study we carried out the oxidation and etching steps independently. By first growing an oxide layer in a furnace and subsequently etching away the the oxide layer, we find that sub-surface damage can be removed and the surface quality can be improved over that found with only wet etching. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q27.00004: Observation of annealing of grains in high purity aluminum using High Energy X-ray Diffraction Microscopy C.M. Hefferan, S.F. Li, U. Lienert, R.M. Suter High energy x-ray diffraction microscopy (HEDM) is capable of measuring volumes of polycrystal microstructure on a granular basis, producing spatial maps of crystallographic orientation covering ensembles of grains with micron resolution. A non-destructive experimental probe capable of observing the response of polycrystals to thermo-mechanical stimulus, HEDM establishes constraints on analytic materials models. HEDM uses high-brilliance, line focused synchrotron x-rays to image diffracted beams emanating from individual grains in a succession of planar sections. Area detector images of diffraction patterns are collected as the sample rotates normal to the beam plane. A forward modeling computer simulation adjusts local crystallographic orientations and compares simulated scattering to experimental images in order to optimize the match with observations. Three dimensional digital representations are generated from large numbers of reconstructed sections. Growth measurements on high purity polycrystalline aluminum have been conducted at the 1-1D beamline at the Advance Photon Source at Argonne National Laboratory and reconstructions have been obtained using the Pittsburgh Supercomputing Center. Both defect annealing and grain boundary motions have been observed and will be described. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q27.00005: Strain Analysis in 2D and 3D X-ray Microscopy J.Z. Tischler, B.C. Larson, Wenjun Liu, Lyle Levine Spatially resolved strain distributions on the submicron length scale are important for materials problems such as deformation and phase separation, and in heterogeneous systems in general. High-energy 3DXRD techniques have been developed by RISO with a few micron 3D resolution for lightly deformed materials. However, submicron 3D spatial distributions of local strain in heavily deformed materials are often required, such as in for multiscale materials modeling. We have developed a scanning-monochromatic x-ray microbeam technique on sector-34 of the Advanced Photon Source to measure the Q-distribution from submicron volume elements in lightly and heavily deformed materials and in single, polycrystalline, or composite materials. By sorting the intensity from every pixel in an area detector during nested energy and differentialāaperture depth scans, Q-distributions are obtained for every spatially resolved volume element. We will present measurement examples and discuss the range of applications. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q27.00006: Towards mapping of defected grains using high energy x-ray diffraction microscopy Shiu Fai Li, C.M. Hefferan, U. Lienert, R.M. Suter High energy x-ray diffraction microscopy (HEDM), the use of focused high energy synchrotron x-ray radiation diffraction imaging, is becoming a promising technique for orientation mapping of polycrystalline material. The nondestructive nature of HEDM makes real-time observation of inter- and intra-grain dynamics possible. Collected HEDM data in the form of diffracted images on a high resolution 2D detector is analyzed by a Monte Carlo fitting algorithm using a forward modeling method, which simulates a set of detector images based from a specified orientation field. Since no specific assumptions are made regarding grain shapes and topologies, internal mosaic structures may be captured. The combination of nondestructive nature and intra-grain resolution makes it an ideal candidate for in situ studies of grain damage due to strain/stress treatment. A proof of concept has been demonstrated in observations of intra-grain orientation mosaics from both orientation maps of polycrystalline aluminum obtained from experiments conducted at beam line 1-ID of the Advanced Photon Source and forward modeling simulations. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q27.00007: A Holographic Iterative Algorithm for X-ray Microscopy Diling Zhu, Benny Wu, Ramon Rick, Joachim St\"{o}hr, Andreas Scherz In recent years X-ray Fourier transform holography has gained recognition as a high resolution microscopy technique. The phase information is encoded in the hologram which renders this lensless approach as a true imaging technique by applying a simple inverse Fourier transform. In previous experiments the resolution was limited by imperfect knowledge of the reference and therefore was determined by the size of the reference. We report an alternative technique based on direct calculation of the encoded phase by recording multiple holograms. This phase information provides additional constraints to uniquely deconvolve the reference and the object using iterative phase retrieval algorithms. In numerical simulations we observe rapid convergence of this new reference-guided phase retrieval method which also shows high immunity against noise. We present two different experimental implementations and their results to demonstrate the feasibility of the concept. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q27.00008: New Method for Inverting X-ray Holographs Yuhao Wang, Jianming Bai, Trevor A. Tyson, Peter Siddons, Gianluigi De Geronimo The matrix solving method is a new class of methods to be applied to inverting an x-ray holograph for obtaining real space structures. The method is shown to provide better resolution and more flexibility than Fourier transform methods. Simulations suggest that non-direct scheme non-indirect scheme x-ray fluoresce holograph, measured with both fixed light source and fixed detector can be inverted with the matrix solving method. Applying pre-determined non negative restrictions can improve the spatial resolution and approach the wavelength of the measuring x-rays. Experimental details and methods for measuring x-ray florescence holography with the matrix solving inversion is discussed. This work is supported by NSF DMR grant MRI-0722730. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q27.00009: Growth and characterization of high k ZrO$_{2}$ on Ge (100) Abdul Rumaiz, Gabriella Carini, Peter Siddons, Joseph Woicik, Pavel Rehak The higher mobility of carriers combined with a low effective mass in Ge as compared to Si has generated a lot of interest in Ge based devices. This is particularly so in X-ray radiation detectors where Si based detectors become transparent at higher energy. The challenge in realizing a Ge based detector is having a robust barrier oxide since the native Ge oxide is hygroscopic. We have grown high k ZrO$_{2}$ on Ge (100) using direct metal sputtering followed by UV oxidation [1]. High energy X-ray photoelectron spectroscopy (XPS) was performed to study the oxidation state of ZrO2 as well the interface with Ge. A simple structure with Ge/GeO/ZrO$_{2}$(25 nm)/Al (200 nm) was created. A significant hysteresis was observed in the capacitance-voltage measurement which is indicative of some interface states [2]. The effect of the intermediate layer between ZrO$_{2}$ and Ge on the interface states will be addressed. Valence band measurement done using high energy XPS will be discussed. [1] C. O. Chui, S. Ramanathan, B. B. Triplett, P. C. McIntrye and K. C. Sarawat, IEEE Electron Dev. Lett. 28, 473 (2002) [2] H. Kim, C.O. Chui, K. C. Sarawat and P. C. McIntrye, Apl. Phys. Lett. 83, 2647 (2003) [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q27.00010: High-field pulsed magnet instruments for x-ray studies of materials at the Advanced Photon Source Zahirul Islam, J.P.C. Ruff, Y. Matsuda, Z. Qu, H. Nojiri, B.D. Gaulin, S. Yoshii, Z. Mao, J.C. Lang High-field pulsed magnets are not the solution to x-ray studies of all problems requiring high magnetic fields, but, they are the only solution to many. We present a very high-field pulsed magnet system for x-ray studies of materials at the Advanced Photon Source (APS). The high-field instruments for x-ray studies are unique in the United States. Currently, 30 Tesla split-coil and long-pulse solenoid magnets are in use for scattering and spectroscopic experiments, respectively. The coils are made of CuAg wires. Pulsed fields (1-10 ms in duration) are generated using a configurable bipolar capacitor bank (40 kJ). For scattering studies split coils are mounted on the cold finger of a closed-cycle He cryostat capable of a repetition rate of $\sim$10-20 minutes for peak fields in the range of 20-30 Tesla. Time-resolved scattering data are typically collected using a fast APD detector. Initial scattering studies of a geometrically frustrated magnet will be presented. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q27.00011: Longitudinal and Transverse Components in the X-ray Resonant Magnetic Reflectivity Experiment J.-S. Lee, E. Vescovo, C.-C. Kao, J.-M. Beaujour, A.D. Kent, H. Jang, J.-Y. Kim, J.-H. Park X-ray Resonant Magnetic Reflectivity (XRMR) is a powerful tool: It allows to simultaneously probe the structural and the magnetic properties of complex multilayer structures. It is often advantageous to utilize circular polarized synchrotron radiation to obtain magnetic information. Unfortunately, in XRMR measurement using circular polarization, the transverse and longitudinal components are intrinsically mixed, making a proper vector-analysis of the magnetization usually impossible. In this work, we strive to overcome this restriction. In particular we demonstrate how to effectively separate the transverse and longitudinal components in the scattering experiment using circular polarized light. This is accomplished by taking advantage of x-ray interference effects which fully suppress the longitudinal component at all angles where the magnetic asymmetry ratio is null. At these angles only the purely transverse component is therefore left in the data. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q27.00012: Monte-Carlo Resampling Analysis of Neutron and X-ray Reflection Data F. Heinrich, P. Shekhar, P. Kienzle, M. Loesche In most cases, reflectivity data analysis relies on the use of a structural model with reasonable physical constraints. Commonly, parameter confidence intervals are estimated, and the choice of the adequate model solely relies on the of the experimenter. We present the implementation of a Monte-Carlo resampling technique for reflectometry data analysis [F. Heinrich et al. A new lipid anchor for sparsely-tethered bilayer lipid membranes. Langmuir, submitted]. It is based on the statistical evaluation of a large number of fits with resampling. This technique provides access to precise confidence intervals for model parameters and parameter correlation matrices. It allows one to identify an over-parameterization of the model and provides a tool for a controlled model extension with additional parameters. This becomes especially important in the prospect of the new neutron sources where data with higher information content will be available. We also present an emerging application of the Monte-Carlo resampling technique that allows the determination of unknown elements free of any structural assumptions. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q27.00013: Scatterless Hybrid Metal-Single Crystal Slit for Small Angle X-ray Scattering and High Resolution X-ray Diffraction. Youli Li, Roy Beck, Tuo Huang, Myung Chul Choi, Morito Divinagracia A simple hybrid design has been developed to produce effectively scatterless aperture slits for small angle x-ray scattering (SAXS) and high resolution x-ray diffraction. The hybrid slit consists of a single crystal (Si, Ge) edge bonded to a tapered high density metal base. The beam-defining single crystal tip is oriented at a large tilt angle with respect to the beam and far from any Bragg peak position, and hence should produce no slit scattering commonly associated with conventional metal slits. The scatterless performance of the new slit design was confirmed by experiments conducted with laboratory x-ray sources as well as third generation synchrotron radiation. The new scatterless slits have been successfully used for SAXS application, where it led to a greatly simplified Single Aperture SAXS design with dramatically increased intensity (3-fold observed) as well as improved low angle resolution compared to a conventional three-pinhole set up.* Y. Li, R. Beck, T. Huang, M.C. Choi, M. Divinagracia, J. Appl. Cryst. (2008) 41, 1134-1139 [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q27.00014: Resonant soft x-ray scattering from Cu valence modulations in oxygen ordered YBCO David Hawthorn, K.M. Shen, J. Geck, D.C. Peets, H. Wadati, Ruixing Liang, D.A. Bonn, W.N. Hardy, G.A. Sawatzky, J. Okamoto, D.J. Huang, H.-J. Lin, Jonathan Denlinger Recently resonant elastic soft x-ray scattering (RSXS) has emerged as a powerful new tool to study electronic ordering in materials like cuprates and manganites. The power of this technique is to combine x-ray scattering, which is sensitive to spatial order, with x-ray spectroscopy, which is sensitive to the valence, spin and orbital symmetry of specific atoms. This combination allows one to probe very directly and considerable detail a variety of exotic spin, charge, orbital or structural ordering phenomena. I will discuss the application of this technique to an important test case, oxygen ordering in YBCO. In this system we are able to accurately calculate the energy dependence of the scattering intensity, providing a basis for understanding the spectroscopy of more complex systems. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q27.00015: Phase Separation of Water/Glycerol Binary Mixtures Next to Lipid Monolayers -- An X-ray and Neutron Reflectivity Study Luka Pocivavsek, Brian Leahy, Mati Meron, Binhua Lin, Jarek Majewski, Ka Yee Lee We recently developed a general model for studying instabilities like wrinkling and folding in interfacial membranes on fluid substrates. The dominant length scales describing the instability are set by the elastic response of the membrane (primarily bending) and the ``stiffness'' of the substrate. These length scales, like the wrinkle wavelength and fold amplitude, are independent of the particular interfacial molecular interactions for micron thick membranes where typical system energies like the membrane bending stiffness are thousands of times larger than intermolecular potentials. However, as the membranes become thinner and thinner and eventually approach molecular membranes only a couple of nanometers thin, the chemical interactions between the membrane and the fluid substrate strongly influence the wrinkling and folding length scales. We present data for two such systems (a lipid monolayer and a gold nanoparticle layer) on different hydrogen bonding fluids and discuss possible mechanisms and modifications of our wrinkle-to-fold scaling laws to account for this new degree of freedom. [Preview Abstract] |
Session Q28: Focus Session: Hydrogen Storage/Nanomaterials for Energy
Sponsoring Units: DMPChair: Jason Graetz, Brookhaven National Laboratory
Room: 330
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q28.00001: Carbon Dioxide Capture in Microporous Metal-Organic Frameworks Invited Speaker: Metal-organic frameworks represent a new class of materials exhibiting high internal surface areas, tunable pore dimensions, and tailorable surface functionality. Research in our laboratory has focused on the development of metal-organic frameworks with surfaces bearing open metal coordination sites for high-enthalpy hydrogen adsorption. Recently, we have initiated efforts to utilize such materials for the selective capture of CO$_{2}$ from flue gas. Here, open metal coordination sites can deliver a high CO$_{2}$ loading capacity at low pressures. However, additional criteria, such as water stability and the selective binding of CO$_{2}$ over N$_{2}$, must also be taken into consideration. Towards that end, we have targeted air- and water-stable frameworks bearing surfaces coated with amine groups. For example, the use of 1,3,5-benzenetristriazolate (BTTri$^{3-})$ as a bridging ligand has led to sodalite-type frameworks such as HCu[(Cu$_{4}$Cl)$_{3}$(BTTri)$_{8}$], possessing open Cu$^{2+}$ coordination sites and exhibiting good chemical and thermal stability. Attachment of ethylenediamine to the Cu$^{2+}$ sites within this structure generates a material that selectively binds small amounts of CO$_{2}$ over N$_{2}$. Details of the characterization of this and related materials will be presented. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q28.00002: Hydrogen storage in a metal-organic-framework structure from a nonempirical van der Waals density functional approach Lingzhu Kong, Valentino C. Cooper, Nour Nijem, Yves J. Chabal, KunHao Li, Jing Li, David C. Langreth Hydrogen adsorption in the metal-organic-framework structure Zn$_2$(BDC)$_2$(TED) (BDC=benzenedicarboxylate; TED=triethylenediamine) is studied using a van der Waals-density-functional approach.\footnote{M. Dion et al., PRL {\bf 92}, 246401 (2004); T. Thonhauser et al., PRB {\bf 76}, 125112 (2007).} Two types of adsorbtion sites are located in the structure. The binding energies and the number of such sites are in good agreement with the values obtained from the experimental isotherms and isosteric heat of adsorption.\footnote{J.Y. Lee et al., Adv. Func. Mater. {\bf 17}, 1255 (2007).} The stretching mode frequency of the adsorbed H$_2$ is calculated for various H--H bond orientations at the two positions. The frequency changes by approximately $-30$ cm$^{-1}$ for the strongest binding direction at each of the two points, which is consistent with the measured infrared absorption band measured at 4120 cm$^{-1}$ at room temperature and high pressures (300-800 psi). [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q28.00003: Enhanced H$_{2}$ adsorption in metal-organic frameworks with open metal sites: Binding mechanism and strong dependence on metal ions Wei Zhou, Hui Wu, Taner Yildirim Metal-organic frameworks (MOFs) with open metal sites exhibit much stronger H$_{2}$ binding strength than classical MOFs, due to the direct interaction between H$_{2}$ and the coordinately unsaturated metal ions. [1] Here we will present a systematic study of the H$_{2}$ adsorption on a series of isostructural MOFs, M$_{2}$(dhtp) with open metals M = Mg, Mn, Co, Ni, Zn. The experimental, initial isosteric heats of adsorption for H$_{2}$ ($Q_{st})$ of these MOFs range from 8.5 to 12.9 KJ/mol, with increasing $Q_{st}$ in the following order: Zn, Mn, Mg, Co, and Ni.[2] The H$_{2}$ binding energies derived from first-principles calculation follow the same trend as the experimental observation on $Q_{st}$, confirming the electrostatic Coulomb attraction between the H$_{2}$ and the open metals being the major interaction. We also found a strong correlation between the metal ion radius, the M-H$_{2}$ distance and the H$_{2}$ binding strength, which provides a viable, empirical method to predict the relative H$_{2}$ binding strength of different open metals. [1] J. Phys. Chem. C, 112, 8132 (2008). [2] J. Am. Chem. Soc., 130, 15268 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q28.00004: Noncovalent hydrogen bonding in metal-organic structures Norm M. Tubman, Jonathan L. Dubois, Randolph Q. Hood, Sebastien Hamel, Eric R. Schwegler Transition metal sites in metal-organic frameworks and in doped carbon structures are actively being studied for their binding properties of molecular hydrogen. We present a study of prototypical metal-organic structures that can be used to bind molecular hydrogen non-covalently. Due to the well known limitations of current density functional theory based descriptions of non-covalent hydrogen bonding we have focused our efforts on a consistent many-body approach based on the fixed-node diffusion Monte Carlo method. Accurate studies of binding energies and the effects of multiple hydrogens in these structures are presented. Prepared by LLNL under Contract DE-AC52-07NA27344 [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q28.00005: Pd-assisted hydrogen spillover on graphene and carbonanotubes Sa Li, Puru Jena Addition of a small amount of Pd precursors on carbon nanotubes has recently been found to substantially improve the hydrogen uptake. In spite of several attempts, a fundamental understanding of how the catalyst works has remained unattainable. Using first principles methods we have investigated hydrogen spillover on Pd-doped graphene and (8,8) carbon nanotube. Through molecular dynamics (MD) simulations, we found that each Pd can bind to three pairs of hydrogen molecules on graphene and only one pair of hydrogen molecule on (8,8) nanotube at 300K. This difference is attributed to the effect of curvature. The hydrogen molecules were found to dissociate and bind to carbon surface once the Pd atom is saturated with hydrogen. These results provide important new insight to understand hydrogen spillover on carbon based materials. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q28.00006: Calcium-Decorated Carbon Nanotubes for Hydrogen Storage Hoonkyung Lee, Jisoon Ihm, Marvin L. Cohen, Steven G. Louie Using the first-principles pseudopotential density-functional method, we carry out a systematic search for high-capacity hydrogen storage media based on individually dispersed calcium atoms on carbon nanotubes (CNTs). We find that Ca clustering is suppressed on boron-doped and defective carbon nanotubes and that up to six H$_{2}$ molecules can bind to a Ca atom with a binding energy of $\sim $0.2 eV/H$_{2}$. We show that Ca-decorated CNTs with a concentration of $\sim $6 at. {\%} B doping can reach the gravimetric capacity of $\sim $5 wt {\%} hydrogen storage. We also will discuss the binding mechanism of the H$_{2}$ molecules. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q28.00007: Theoretical study of hydrogen storage in Ca-coated fullerenes Qiang Sun, Qian Wang, Yoshi Kawazoe, Puru Jena First principles calculations based on gradient corrected density functional theory and molecular dynamics simulations of Ca decorated fullerene yield some novel results: (1) C60 fullerene decorated with 32 Ca atoms on each of its 20 hexagonal and 12 pentagonal faces is extremely stable. Unlike transition metal atoms that tend to cluster on a fullerene surface, Ca atoms remain isolated even at high temperatures. (2) C60Ca32 can absorb up to 62 H2 molecules in two layers. The first 30 H2 molecules dissociate and bind atomically on the 60 triangular faces of the fullerene with an average binding energy of 0.45 eV/H, while the remaining 32 H2 molecules bind on the second layer quasi-molecularly with an average binding energy of 0.11 eV/H2. These binding energies are ideal for Ca coated C60 to operate as a hydrogen storage material at near ambient temperatures with fast kinetics. (3) The gravimetric density of this hydrogen storage material can reach 5.8 wt {\%}. Simple model calculations show that this density is the limiting value for higher fullerenes. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q28.00008: Rotor in a Cage: Infrared Spectroscopy of an Endohedral Hydrogen-Fullerene Complex Toomas R{\~o}{\~o}m, Min Ge, D. H{\"u}vonen, U. Nagel, S. Mamone, A. Danquigny, F. Cuda, M. C. Grossel, M. Carravetta, M. H. Levitt, Y. Murata, K. Komatsu We report the observation of quantized translational and rotational motion of molecular hydrogen inside the cages of C$_{60}$. Narrow infrared absorption lines at the temperature of 6\,K correspond to vibrational excitations in combination with translational and rotational excitations and show well-resolved splittings due to the coupling between translational and rotational modes of the endohedral H$_2$ molecule. A theoretical model shows that H$_2$ inside C$_{60}$ is a three-dimensional quantum rotor moving in a nearly spherical potential. The theory provides both the frequencies and the intensities of the observed infrared transitions. Good agreement with the experimental results is obtained by fitting a small number of empirical parameters to describe the confining potential, as well as the {\it ortho\/} to {\it para\/} ratio at 6\,K and at elevated temperatures [S. Mamone, et al., arXiv:0807.1589v2]. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q28.00009: Hydrogen storage in charge compensated organic molecular crystals Mina Yoon, Matthias Scheffler We propose charge compensated organic molecular crystals as a promising class of materials for hydrogen storage. Using quantum mechanical first-principles calculations based on numerical atom-centered orbitals as all-electron basis functions [1] we study the basic structural properties of molecular crystals consisting of parallel sheets of cations and anions (such as DMPH and TCNQ) stacked alternatingly. The long range dispersion interactions between the cations and anions, which are important for the stability of the crystals, were studied and compared using various DFT xc functionals, semi- empirical approach [2], and M\o ller-Plesset perturbation theory. The molecular configuration causes accumulation of electrons at acceptors and depletion at donors, which results in finite dipolar fields. Our study indicates that these fields make it possible to use charge compensated organic molecular crystals for hydrogen storage. \\[3pt] [1] V. Blum {\it et al}., FHI ab initio molecular simulations (FHI-aims) project.\\[0pt] [2] A. Tkatchenko and M. Scheffler, to be published. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q28.00010: Mg-doped GaN nanostructures: Energetics, magnetism and H2 adsorption Qian Wang, Qiang Sun, Puru Jena It has been shown that p-type GaN can greatly improve the performance of GaN-based devices. Mg is a suitable candidate dopant for p-type GaN. Since the ionic radius of Mg is comparable with that of Ga, Mg doping can be expected to eliminate self-compensation effects. Thus, synthesis of Mg-doped p-type GaN for fabrication of optoelectronic devices has been hotly pursued. Using density functional theory and generalized gradient approximation for exchange and correlation potential we show that Mg doped GaN nanocage and nanotube can be magnetic with Mg contributed spins distributed over the neighboring N sites. Mg atoms show no tendency for clustering and due to the positive charge residing on them; they can trap hydrogen in molecular form via the charge polarization mechanism. The binding energies of hydrogen lie in the range of 0.1$\sim $0.2 eV/H2 which are ideal for storage applications under ambient thermodynamic conditions. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 2:03PM |
Q28.00011: The USDOE Hydrogen Program: Status and Performance Gaps of On-board Hydrogen Storage Technologies Grace Ordaz, Monterey Gardiner, Carole Read, Ned Stetson The USDOE Hydrogen Program's mission is to reduce oil use and carbon emissions in the US transportation sector and to enable clean, reliable energy for stationary and portable power generation. The requirements for vehicular hydrogen storage continue to be one of the most technically challenging barriers to the widespread commercialization of hydrogen fueled vehicles. The DOE applied hydrogen storage activity focuses primarily on the research and development of low-pressure, materials-based technologies to allow for a North American market driving range of more than 300 miles (500 km) while meeting packaging, cost, safety, and performance requirements to be competitive with current vehicles. This presentation summarizes the status, recent accomplishments and current performance gaps of hydrogen storage technologies primarily for transportation applications. Materials projects are focused in three main areas: metal hydrides, chemical hydrogen storage materials, and hydrogen sorbents. A new effort is the Hydrogen Storage Engineering Center of Excellence which will provide a coordinated approach to the engineering research and development of on-board storage and refueling systems. The presentation will especially highlight topics emphasized in the session theme. [Preview Abstract] |
Session Q29: Focus Session: Ordering in Complex Oxides
Sponsoring Units: DMP GMAGChair: Jaime Fernandez-Baca, Oak Ridge National Laboratory
Room: 333
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q29.00001: Orbitals, reduced dimensionality and spin gaps in correlated oxides Invited Speaker: Due to directional character of orbitals very often orbital ordering leads to an effective dimensionality reduction. Especially strong are these effects in systems with triply-degenerate t$_{2g}$ electrons. One of the striking consequences thereof is the possibility to form spin-gap states instead of long-range magnetic ~ ordering. Such spin gap states may be formed on dimers, on trimers, on bigger clusters and even on chains. In the talk I will review this question, discuss different situations and different mechanisms of spin gap formation, and illustrate these general ideas on several examples (perovskite KCuF3; spinels MgTi2O4 and CuIr2S4 [1]; pyroxene NaTiSi2O6 [2]; layered systems La4Ru2O10 with square [3], LiVO2 with triangular [4], Li2RuO3 with honeycomb lattices [5], pyrochlore Tl2Ru2O7 [6]) \\[4pt] [1] D.I.Khomskii and T.Mizokawa, Phys.Rev.Lett. \textbf{94}, 156402 (2005); \\[0pt] [2] S.V.Streltsov, O.A.Popova and D.I.Khomskii, Phys. Rev. Lett. 96, 249701 (2006) \\[0pt] [3] Hua Wu et al., Phys.Rev.Lett. 96, 256402 (2006) \\[0pt] [4] H. Pen et al., Phys. Rev. Lett. 78, 1323 (1997) \\[0pt] [5] G.Jackeli and D.I.Khomskii, Phys. Rev. Lett. 100, 147203 (2008) \\[0pt] [6] Seongsu Lee et al., Nature Mater. 5, 471 (2006) [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q29.00002: Phase transitions and magnetostructural coupling in ZnCr$_2$O$_4$ from first principles Carl-Johan Eklund, Craig J. Fennie, Karin M. Rabe In the spinel structure oxide ZnCr$_2$O$_4$, a phase transition is observed from the high-temperature cubic phase to a low-temperature low-symmetry phase, reported as tetragonal$^1$ or orthorhombic$.^2~$ Building on a previous first-principles analysis of the zone-center phonons and spin-phonon coupling$,^3~$ we construct a first-principles effective Hamiltonian to investigate this transition. The local modes included are the Cr displacements, distortions of the Zn-centered tetrahedra, and the homogeneous strain. The magnetostructural coupling of these degrees of freedom to the spins of the Cr$^{3+}$ ions is included in the effective Hamiltonian parameterization and first-principles determination using a symmetry analysis. The role of the magnetostructural coupling in the phase transition will be analyzed and discussed. \\ 1. S. H. Lee et al., J. Phys. Cond. Matt. 19, 145259 (2007) \\ 2. V. N. Glazkov et al., http://arxiv.org/abs/0807.0546 \\ 3. C. J. Fennie and K. M. Rabe, Phys. Rev. Lett. 96, 205505 (2006) [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q29.00003: Charge-ordering in Magnetite studied by Magnetic Compton scattering B. Barbiellini, H. Kobayashi, M. Itou, S. Todo, P.E. Mijnarends, A. Bansil We present the [100]-[110] anisotropy of the magnetic Compton profile (MCP) in magnetite (Fe$_3$O$_4$). Good agreement is found between theory (within the local spin density approximation) and experiment with respect to the anisotropy in the metallic phase at 300K. However, the experimental curve for the insulating phase at 12K presents important changes at the low-momentum peak near 1 a.u. indicating that the bonding electrons are significantly affected by the Verwey transition. We will discuss whether these MCP anisotropy changes can be explained in terms of a charge-ordering mitigated by covalent effects [1]. Our study illustrates the high sensitivity of the Magnetic Compton scattering technique for extracting information on the electrons involved in the Verwey transition. Work supported in part by U.S. DOE. \\ \mbox{[1] M. Coey, Nature {\bf 430}, 155 (2004)} [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q29.00004: First-principles determined charge and orbital interactions in Fe$_3$O$_4$ Fei Zhou, Gerbrand Ceder In this work we have attempted to describe the charge and orbital degrees of freedom in fo with a classical effective energy model. Electronic and lattice effects are both included through first-principles calculated energies from which the model is parametrized. The calculated charge and orbital interactions in fo are found to be physically meaningful. The energy landscape is complex in terms of frustrated charge and orbital interactions as well as their competition. Additionally, although our predicted ground state structure has smaller periodicity than experimentally observed, it reveals the possibility that not only charge and orbital ordering, but the Jahn-Teller lattice distortions may also decide the structure. Therefore this work may help better understand the problem of the low-T magnetite structure. Beyond magnetite, our approach can be easily adapted to explore other transition metal oxides where charge and/or orbital order exist. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q29.00005: Magnetic control of crystal chirality and gigantic magneto-chiral effect in CuB$_{2}$O$_{4}$ Mitsuru Saito, Kenta Ishikawa, Kouji Taniguchi, Taka-hisa Arima The possibility of a magnetic-field control of the chirality of matter has been debated since 19th century, because of its importance in the problem of homochirality. However, the difference of symmetry between magnetic fields and chirality implies that it would be impossible to induce chirality by a magnetic field alone. Here, we report the successful induction of crystal chirality in a noncentrosymmetric canted antiferromagnet, CuB$_{2}$O$_{4}$, by the application of a low-intensity static magnetic field[1]. The chirality is reversed by a 90-degree rotation of the direction of the magnetic field. This is the first successful magnetic control of chirality to the best of our knowledge. The induction of chirality by a magnetic field gives rise to a gigantic enhancement of magneto-chiral dichroism in this material. The magnitude of the effect is larger by three orders than the previous reports. The extraordinary enhancement allows us to design new magneto-optical devices. [1] M. Saito \textit{et al.,} Phys. Rev. Lett. \textbf{101 }117402 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q29.00006: Proposed Orbital Ordering in MnV$_2$O$_4$ from First-principles Calculations Tanusri Saha-Dasgupta, Soumyajit Sarkar, Tulika Maitra, Roser Valenti Based on density functional calculations, we propose a possible orbital ordering in MnV$_2$O$_4$ which consists of orbital chains running along crystallographic $a$ and $b$ directions with orbitals rotated alternatively by about 45$^{\circ}$ within each chain. We show that the consideration of correlation effects as implemented in the local spin density approximation (LSDA)+U approach is crucial for a correct description of the space group symmetry signifying a strong influence of the correlation-driven orbital ordering on the structural transitions in this system. Inclusion of spin-orbit effects does not seem to influence the orbital ordering pattern. We further find that the proposed orbital arrangement favors a non-collinear magnetic ordering of V spins, as observed experimentally. Exchange couplings among V spins are also calculated and discussed. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q29.00007: Magnetic relaxation in TM$_{3}$V$_{2}$O$_{8}$ (TM = Cu, Ni, Co and Mn) staircase Kagome compounds Ambesh Dixit, C. Sudakar, N. Rogado, E. Morosan, R.J. Cava, A.P. Ramirez, Gavin Lawes The transition metal vanadate oxides having a staircase Kagome lattice structure exhibit rich magnetic phase diagrams, which arise from the complex geometry of these materials. Among these compounds, Ni$_{3}$V$_{2}$O$_{8}$ is particularly widely studied, as it develops simultaneous ferroelectric and incommensurate magnetic ordering at a single phase transition. In order to investigate the low frequency spin dynamics in these layered materials, we have probed the ac magnetic properties in these systems. We find that the Cu, Co, and Mn systems display strong magnetic relaxation in a spin ordered phase, with activation energies on the order of hundreds of Kelvin. We discuss these results in the context of spin-glass behaviour and domain wall motion. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q29.00008: Effect of Zn doping on the phase transition temperatures of Ni$_{3}$V$_{2}$O$_{8}$ Akila Kumarasiri, Parashu Kharel, Ambesh Dixit, Gavin Lawes There is a considerable interest in understanding the nature of magnetic phase transition in geometrically frustrated materials. Ni$_{3}$V$_{2}$O$_{8}$ is one such system, with spin-1 Ni$^{2+}$ ions forming a layered buckled Kagome structure. We have studied the effects of doping spin-0 Zn ions on the magnetic phase transitions of powder Ni$_{3}$V$_{2}$O$_{8}$ using dielectric and heat capacity measurements. (Ni$_{1-x}$Zn$_{x})_{3}$V$_{2}$O$_{8}$ powder samples were synthesized starting with a mixture of Ni, V and Zn metal organic solutions mixed at appropriate atomic ratio. XRD and Raman studies show that (Ni$_{1-x}$Zn$_{x})_{3}$V$_{2}$O$_{8}$ powder samples annealed at 1000$^{\circ}$C crystallize in Ni$_{3}$V$_{2}$O$_{8}$ structure without forming any secondary phases. We have observed from heat capacity measurements that the phase transitions T$_{H}$, T$_{L,}$ and T$_{C}$ at 9.2K, 6.4K, and 3.9K expected for Ni$_{3}$V$_{2}$O$_{8}$ are present in our (Ni$_{1-x}$Zn$_{x})_{3}$V$_{2}$O$_{8}$ samples up to a Zn concentration of 20{\%}. The transition at 2.4 K was not clearly observed. All three transitions shift toward lower temperatures with an increase in Zn concentration. We will present the experimental results on the strong suppression of both T$_{H}$ and T$_{L}$ due to dilution of Ni$_{3}$V$_{2}$O$_{8}$ with non-magnetic Zn. Furthermore, we will present a quantitative comparison of this suppression with the 2D Ising and Heisenberg models. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q29.00009: Electrical control of direction of orbital stripes in charge-orbital ordered state of single-layered manganite La$_{1/2}$Sr$_{3/2}$MnO$_{4}$ Shota Konno, Kouji Taniguchi, Hajime Sagayama, Taka-Hisa Arima Electrical control of the localized electron such as an electric-field induced metal-insulator transition in charge-orbital ordered (COO) state of perovskite-related manganese oxides has been intensively studied, since the large electroresistance effect in Pr$_{0.7}$Ca$_{0.3}$MnO$_{3}$ was reported[1]. Moreover, the formation of orbital stripes in the COO state gives rise to in-plane anisotropies in the electrical, magnetic, and optical properties. However, there are few reports on controlling anisotropic properties in the COO state. We report an electric-field effect on the in-plane anisotropy in the COO state of a layered manganite La$_{1/2}$Sr$_{3/2}$MnO$_{4}$. After applying an electric field, a 90-degree rotation of COO states has been observed by using a polarizing microscope. A drastic change of the volumes of two COO domains was confirmed by means of synchrotron X-ray diffraction. [1]A. Asamitsu \textit{et al}., Nature \textbf{388}, 50(1997). [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q29.00010: Intersite charge transfer in the $A$-site-ordered LaCu$_{3}$Fe$_{4}$O$_{12}$ perovskite Youwen Long, Naoaki Hayashi, Takashi Saito, Masaki Azuma, Shigetoshi Muranaka, Yuichi Shimakawa A novel transition metal oxide LaCu$_{3}$Fe$_{4}$O$_{12}$ was prepared at 10 GPa and 1400 K. It crystallizes an $A$-site-ordered perovskite structure, which has a general chemical formula A$^{\prime}$A$_{3}$B$_{4}$O$_{12}$, with space group \textit{Im}-3. Bond valence sum calculations and M\"{o}ssbauer spectra confirm the change of charge combination at 393 K from a high-temperature La$^{3+}$Cu$^{2+}_{3}$Fe$^{3.75+}_{4}$O$_{12}$ with unusually high oxidation Fe$^{3.75+}$ ions at the $B$ site to a low-temperature La$^{3+}$Cu$^{3+}_{3}$Fe$^{3+}_{4}$O$_{12}$ with exceptional Cu$^{3+}$ ions at the $A$ site. The results strongly suggest an unusual intermetallic charge transfer between the $A$-site Cu and $B$-site Fe ions. The simultaneous valence change (3Cu$^{2+}$-3$e^{-}\to $3Cu$^{3+}$ and 4Fe$^{3.75+}$+3$e^{-}\to $4Fe$^{3+})$ caused by the $A-B$-site charge transfer leads to a first-order and reversible isostructural phase transition accompanied by an anomalous volume contraction as large as 1.0{\%}. Meanwhile, a paramagnetic metal to antiferromagnetic insulator transition is also induced by the $A-B$-site charge transfer. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q29.00011: $A$-site Magnetism in Perovskites CaCu$_{3}B_{4}$O$_{12}$ ($B$ = Ge, Ti, Sn). Takashi Saito, Hiroshi Shiraki, Yuichi Shimakawa, Masaichiro Mizumaki $A$-site-ordered perovskites CaCu$_{3}$Ge$_{4}$O$_{12}$ and CaCu$_{3}$Sn$_{4}$O$_{12}$, both isostructural to antiferromagnetic CaCu$_{3}$Ti$_{4}$O$_{12}$, were found to be ferromagnets, which are very rare in cuprates. All of these materials may be called ``$A$-site magnets'', since they contain magnetic species only at the $A$-site of the perovskite \textit{AB}O$_{3}$ structure. The ferromagnetism of CaCu$_{3}B_{4}$O$_{12}$ ($B$ = Ge, Sn) is attributed to the ferromagnetic direct exchange interaction, whereas antiferromagnetic superexchange interaction, due to the Cu(3$d)$-O(2$p)$-Ti-(3$d)$ orbital hybridization, is dominant in antiferromagnetic CaCu$_{3}$Ti$_{4}$O$_{12}$. The $A$-site magnetism is controlled by the electronic structure of the non-magnetic $B$ site. Solid solutions CaCu$_{3}$(Ge,Ti)$_{4}$O$_{12}$ and CaCu$_{3}$(Ti,Sn)$_{4}$O$_{12}$ display phase boundary between ferromagnetic and antiferromagnetic phases. [1] H. Shiraki, T. Saito, Y. Shimakawa et al., \textit{Phys. Rev. B}, \textbf{76}, (2007) 140403. [2] Y. Shimakawa, H. Shiraki and T. Saito, \textit{J. Phys. Soc. Jpn.}, \textbf{77}, (2008) 113702. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q29.00012: Structure and properties of high-oxygen-pressure annealed Sr$_{1-x}$La$_{x}$Co$_{0.5}$Fe$_{0.5}$O$_{3-d}$ (0$\le $x$\le $0.5) S. Remsen, K. Swierczek, B. Dabrowski, L. Suescun, S. Kolesnik Synthesis, oxygen content, structural, magnetic, and resistive properties will be discussed for the Sr$_{1-x}$La$_{x}$Co$_{0.5}$Fe$_{0.5}$O$_{3-d}$ perovskites. The x=0 sample shows oxygen-vacancy ordered Sr$_{8}$Co$_{4}$Fe$_{4}$O$_{23}$ tetragonal I4/mmm structure. With an increase of the La content the materials became oxygen stoichiometric and a lowering of the crystal symmetry is observed from cubic Pm3m (x=0.1 and 0.2) to tetragonal I4/mcm (x=0.3 and 0.4), and finally to monoclinic I12/c1 (x=0.5). All samples show ferromagnetic ordering with the maximum Curie temperature near 290 K at x=0.2. Conductivity is enhanced and small negative magneto-resistance is observed below T$_{C}$. Transport measurements up to 1100$^{o}$C show high conductivity that is affected by the varying oxygen content. Work at NIU was supported by the NSF (DMR-0706610) and at ANL by the U.S. DOE under contract No. DE-AC02-06CH11357. [Preview Abstract] |
Session Q30: Focus Session: Oxide Superlattices
Sponsoring Units: DMP GMAGChair: Maria Varela, Oak Ridge National Laboratory
Room: 334
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q30.00001: Colossal ionic conductivity at ZrO$_{2}$:Y$_{2}$O$_{3}$ /SrTiO$_{3}$ interfaces Jacobo Santamaria, J. Garcia Barriocanal, A. Rivera Calzada, Z. Sefrioui, C. Leon, E. Iborra, M. Varela, S.J. Pennycook We describe the strong enhancement of the conductivity occurring at the interfaces of superlattices made by alternating 10 nm strontium titanate and 1 nm yttria stabilized zirconia $^{[1]}$layers. Conductivity is found to be as high as 0.014 S/cm at 357 K, with a substantial decrease of the activation energy for the dc ionic conductivity from 1.1 eV down to 0.64 eV. EELS analysis is consistent with a large number of interfacial oxygen vacancies and high disorder in the interface oxygen plane between YSZ and STO layers. Our results demonstrate that the design of suitable heterogeneous interfaces in epitaxial heterostructures might have important implications in the search of artificial nanostructures with high ionic conductivity. Work at UCM and UPM supported by MCINN MAT2008 6517and Division of Materials Sciences and Engineering of the US Department of Energy. [1] J. Garcia-Barriocanal, et al. Science \textbf{\textit{321}}, 676 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q30.00002: Origin of Colossal Ionic Conductivity in YSZ-STO Superlattices Timothy Pennycook, Matthew Beck, Kalman Varga, Maria Varela, Stephen Pennycook, Sokrates Pantelides An eight order of magnitude increase in the ionic conductivity of yttria-stabilized zirconia (YSZ) has recently been demonstrated in YSZ/strontium titanate (STO) epitaxial heterostructures. YSZ is the preferred electrolyte for solid oxide fuel cells (SOFC), in which the ionic conductivity is the major factor limiting the energy conversion efficiency. A colossal increase in the ionic conductivity, therefore, goes a long way towards increasing SOFC practicality by increasing efficiency overall and reducing the operating temperature necessary for efficient operation. We report density functional calculations that explain this colossal ionic conductivity as the result of a large 7\% expansive in-plane strain of the YSZ. Molecular dynamics simulations of strained zirconia yield an activation energy for ionic conduction in agreement with experiment. Additionally, simulated annealing under these strain conditions reveals a new lowest energy structure for which EELS simulations using the Z+1 approximation for the core hole are consistent with electron energy loss spectra from the thin, coherently strained, YSZ layers of the heterostructures. This work is supported by NSF grant DMR-0513048 and DOE Office of Basic Energy Sciences, Division of Materials Science and Engineering. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q30.00003: Non-uniform magnetization in LaAlO$_{3}$/SrTiO$_{3}$ superlattices. M.R. Fitzsimmons, M. Zhernenkov, N. Hengartner, A. Sharoni, Ivan K. Schuller, J. Garcia-Barriocanal, F.Y. Bruno, J. Santamaria Recently, Brinkman et al., [Nature \textbf{6}, 493 (2007)] reported magnetism induced at the interface between LaAlO$_{3}$ (LAO) and SrTiO$_{3 }$(STO)$_{ }$inferred from transport measurements. They found the magnetization to be greatly enhanced at low temperatures (i.e., liquid He temperature) and by application of high (10+ T) fields. We report polarized neutron reflectometry measurements of the magnetization depth profile of two LAO/STO superlattices with the same number of bilayer repeats. For low temperatures and a field of 11 T, the intensities of the superlattice Bragg reflections for both samples exhibited a dependence upon neutron beam polarization. The spin dependence was much weaker at small field (and low temperature) and disappeared altogether at 11 T and 300 K. These observations demonstrate that the magnetization depth profile has the period of the LAO/STO superlattice. The neutron spin dependence was more pronounced for the sample with a thin LAO layer compared to one with a thick LAO layer, suggesting that the magnetism may be interfacial in origin. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:27PM |
Q30.00004: How to make a cuprate Fermi surface out of a nickelate heterostructure Invited Speaker: Chaloupka and Khaliullin had the idea that it might be possible to make Ni$^{3+}$-based high-temperature superconductors by sandwiching NiO$_{2}$ layers between insulating layers through heterostructuring.$^{2}$ Provided that spin-, charge-, and orbital ordering can be avoided, the confinement should make it possible to empty the Ni $3z^{2}-1$ band, thus leaving the conduction electron in the Ni $x^{2}-y^{2}$ band. Fabrication of such heterostructures are now being pursued in many laboratories. We$^{3}$ have attempted to give theoretical guidance by performing calculations for numerous heterostructures using the local density-approximation in combination with static (LDA+U) and dynamical (LDA+DMFT) mean-field theory. We show how confinement together with electronic correlations can lead to a single-sheet Fermi surface with a shape like that of the cuprate superconductors with the highest transition temperatures$;$ the Ni $3z^{2}-1$ Wannier orbital now plays the role of the axial, Cu $4s$-like orbital in the cuprates.$^{4}$ Since also strong antiferromagnetic fluctuations are present, the low-energy electronic and spin excitations should resemble those of high-temperature cuprate superconductors. Chemical modification of the insulating layers should make it possible to avoid spin-, charge-, and orbital ordering.\newline $^{2}$J. Chaloupka and G. Khaliullin, Phys. Rev. Lett. \textbf{100,} 016404 (2008).\newline $^{3}$P. Hansmann, Xiaoping Yang, A. Tosci, G. Khaliullin, O.K. Andersen, and K. Held, arXiv: 0807.0407.\newline $^{4}$E. Pavarini, I. Dasgupta, T. Saha-Dasgupta, O. Jepsen, and O. K. Andersen, Phys. Rev. Lett. \textbf{87}, 047003 (2001). [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q30.00005: Interfaces in $\rm La_{2}NiO_{4} - La_{2}CuO_{4}$ superlattices S. Smadici, J. C. T. Lee, S. Wang, P. Abbamonte, G. Logvenov, A. Gozar, I. Bozovic Ni substitution on Cu sites in underdoped $\rm La_{2-x}Sr_{x} CuO_{4}$ quickly restores Neel order. This was attributed to strong interaction between the Ni and doped holes. An open question was whether the additional Ni empty orbital or the different spin on Ni sites was at the origin of this strong interaction. We have addressed this problem with resonant soft x-ray scattering on a $\rm La_{2}NiO_{4} - La_{2}CuO_{4}$ heterostructure. $\rm La_{2}NiO_{4}$ and $\rm La_{2}CuO_{4}$ have close lattice structures and electronic configurations. However, the x-ray scattering contrast between superlattice layers is greatly enhanced at soft x-ray resonant energies. Based on our measurements at the O K, La M, Cu L and Ni L edges a model of the charge, orbital and spin structures in these superlattices will be presented with a special emphasis on the interface region. This work was supported by Grants. DE-FG02- 06ER46285, DE-AC02-98CH10886, MA-509-MACA, DE-FG02-07ER46453 and DE-FG02-07ER46471. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q30.00006: Layer-by-layer growth by pulsed laser deposition in the unit-cell limit. M. Kareev, S. Prosandeev, J. Liu, P. Ryan, J.W. Freeland, J. Chakhalian Unlike conventional growth of complex oxide heterostructures, the ultimate unit cell limit imposes strict constrains for a multitude of parameters critical to layer-by-layer growth. Here we report on detailed analysis of far-from-equilibrium growth by interrupted pulsed laser deposition with application to RENiO$_{3}$/LaAlO$_{3 }$superlattices grown on a diverse set of substrates SrTiO$_{3}$, NdGaO$_{3}$, LSAT and LaAlO$_{3}$. A combination of \textit{in-situ} high-pressure RHEED and AFM along with extensive data obtained from synchrotron based XRD and resonant XAS allows us critically assess the meaning of RHEED intensity oscillation and the effect of a polar/non-polar interface on the heteroepitaxial growth. The role of defects formed during the initial stages of growth is also addressed. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q30.00007: Perfect dc conductance of a finite width Mott insulator sandwiched between metallic leads at zero temperature: a quantum emergent phenomenon in strongly correlated multilayers Hand Zenia, Jim Freericks, Hulikal Krishnamurthy, Thomas Pruschke Self-consistent inhomogeneous DMFT calculations as well as analytical investigations of the electronic structure of a multilayered device are presented. The device consists of two semi-infinite leads of a ballistic metal that sandwich an interacting barrier. The interactions in the barrier are described by the Hubbard model with the whole system particle-hole symmetric. We find that for a finite barrier no matter how strong the interaction, the system becomes a Fermi liquid with a perfect metallic conductivity at low enough temperature. We argue that at zero temperature and frequency the Luttinger theorem holds and that the system has a well defined Fermi surface. The perfect conducting state may be extremely fragile to finite temperature, finite driving electric fields, finite driving frequencies, or disorder, so it will often be difficult to see experimentally. We will discuss possible experimental realizations of the phenomena [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:39PM |
Q30.00008: Strongly Interacting Electrons at the Oxide Interfaces. Invited Speaker: Utilizing the recent advances in complex oxide synthesis, one can now combine materials with antagonistic order parameters to create new compounds in the form of heterostructures often with properties not attainable in the bulk$^{1}$. Broken symmetries, strain, and modified local environment at the interface provide a unique route to manipulate the subtle energy balance in correlated materials with promise to create novel material phases and quantum states. Here we report on how the interface can be used to alter electronic, magnetic and orbital structure of multilayers composed of late transition metal oxides with specific examples from cuprates, manganites and nickelates. We will discuss the underlying challenges in growth of ultra-thin layers of complex oxides and illustrate the ways synchrotron based resonant x-ray spectroscopies and resonant x-ray diffraction can be used to probe bulk vs. interface properties to gain unique insight into the underlying physics. J. Chakhalian et al, Science, v. 314, 1114, (2007). [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q30.00009: X-ray standing wave photoemission from multilayer nanostructures C. Papp, B. Balke, C. Sakai, S. Ueda, H. Yoshikawa, Y. Yamashita, S. L. He, K. Kobayashi, G. Conti, D. Buergler, C. Schneider, C. S. Fadley, S. Doering, U. Berges, C. Westphal We have used soft and hard x-ray standing wave excitation of photoelectrons to study buried layers and interfaces in multilayer nanostructures. The samples were grown on synthetic multilayer mirrors, and the x-ray incidence was tuned to 1$^{st}$ order Bragg reflection. Scanning angle, photon energy, or distance along a wedge profile in the sample permits scanning the resultant standing wave field through nm-scale structures and analyzing the depth distribution of their chemical, electronic, magnetic, and structural properties. Using harder x-ray excitation permits via the higher kinetic energy of the electrons studying those properties at greater depths. The systems discussed will be two related to magnetic tunnel junctions (magnesium oxide/iron and STO/LSMO), and one related to integrated circuit production (titanium nitride on silicon). [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q30.00010: Competing anisotropies and complex magnetism in SrRuO$_{3}$/SrMnO$_{3}$ superlattices Omar Chmaissem, Y. Choi, Y.C. Tseng, D. Haskel, D.E. Brown, S. Kolesnik, D. Danaher Using element-specific x-ray resonance techniques, we have investigated the interfacial magnetic coupling in SrRuO$_{3}$/SrMnO$_{3}$ superlattices. A strong out-of-plane SRO anisotropy coupled with AFM Ru-Mn interactions result in a canted Mn structure with a significant induced net Mn moment that reduces to zero under a strong magnetic field. At T $>$ T$_{C SRO}$, the SRO anisotropy is removed and the planar Mn AFM structure cants to produce a net Mn moment along the field direction. Below T$_{C}$, the net development of in-plane Mn moment is suppressed by partially frustrated exchange interactions at the AFM-SMO/FM-SRO interfaces and competing Mn-Ru anisotropies. Hysteretic magnetization curves show a two-step magnetization reversal and enhanced coercivity. X-ray measurements confirm that the low-field magnetization reversal coincides with ``free'' Ru moments inside the SRO layers and that the high-field magnetization reversal involves the interfacial magnetization in the SMO layers and provide strong evidence for the presence of pinned SRO moments at the SRO/SMO interface. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q30.00011: Electrically controlled magnetization in tricolor superlattices Jaekwang Lee, Na Sai, Alexander A. Demkov With recent breakthroughs in fabricating high-quality oxide films, ultra thin ferroelectric (FE) films have attracted significant attention. Many FE-based electronic devices proposed to date have a capacitor configuration, where a FE layer is inserted between two identical metal electrodes. We consider theoretically so-called tricolor structures or asymmetric capacitors with one electrode being ferromagnetic and other normal metal. An interesting aspect of a tricolor structure is breaking of the inversion symmetry which is expected to generate new properties. Of particular interest is the control of the magnetization in a ferromagnetic layer without using an external magnetic field. The effect may find applications in low-power and high-density integration in future spintronics devices. To investigate the polarization-dependent magnetization change in the iron layer we construct the tricolor superlattices comprised of Fe/BaTiO$_{3}$/Pt, Fe/PbTiO$_{3}$/Pt and perform first principles calculations at the LSDA+U level. We find the electrode magnetization sensitive to the polarization direction in the FE layer, which suggests a multiferroic character of the structure. The effect is much stronger than in the analogous symmetric structures. [Preview Abstract] |
Session Q31: Focus Session: Spinels, Pyrochlores and General
Sponsoring Units: GMAGChair: Gavin Lawes, Wayne State University
Room: 335
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q31.00001: Magnetic and Transport Properties of Tb and Y Molybdate Pyrochlores R.P. Guertin, E.-S. Choi, C. Wiebe, H. Zhou The transport, magnetic, and magnetotransport properties of single crystal Tb$_{2}$Mo$_{2}$O$_{7}$ and Y$_{2}$Mo$_{2}$O$_{7}$ are reported. The Mo cation carries a small magnetic moment ($<$1 \'{E}$_{B})$, but neither system shows long-range magnetic order due geometrical frustration. Short-range ($\sim $5 A) correlations [1] cause Mo magnetization irreversibility below T$_{irr}\sim $24 K - a spin glass-like anomaly. The pressure dependence of T$_{irr}$ for Tb$_{2}$Mo$_{2}$O$_{7}$ is strongly negative (-0.24 K/kbar), consistent with similar pressure studies in (Tb$_{1-x}$La$_{x})_{2}$Mo$_{2}$O$_{7}$ [2]. High dc field (33 T) isothermal magnetization shows Tb$_{2}$Mo$_{2}$O$_{7 }$M(H) is not fully saturated due to crystal field splitting and nearly linear Y$_{2}$Mo$_{2}$O$_{7}$ M(H) attains only $\sim $0.4 \'{E}$_{B}$/f.u, A metal-insulator transition (MIT) in Tb$_{2}$Mo$_{2}$O$_{7}$ occurs at T$\sim $50 K, with resistivity \'{E}{\oe}(T) rising $>$10X by T=10 K. The temperature dependence of the Hall resistivity, thermopower, and magnetoresistance are consistent with the MIT. The buildup of the magnetic clusters and the MIT occur at substantially the same temperature, and is discussed in terms of the electronic localization of the charge carriers.\\[0pt] [1] B. D. Gaulin et al, Phys. Rev. Lett. 69, 3244 (1992).\\[0pt] [2] A. Apetrei et al, Phys. Rev. Lett. 97, 206401 (2006). [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q31.00002: Mott transition between a spin-liquid insulator and a metal in three dimensions Daniel Podolsky, Arun Paramekanti, Yong Baek Kim, T. Senthil We study a bandwidth controlled Mott metal-insulator transition (MIT) between a Fermi liquid metal and a quantum spin-liquid insulator at half-filling in three dimensions (3D). Using a slave rotor approach, and incorporating gauge field fluctuations, we find a continuous MIT and discuss the finite temperature crossovers around this critical point. We show that the specific heat $C \sim T \ln \ln (1/T)$ at the MIT and argue that the electrical transport on the metallic side near the transition should exhibit a `conductivity minimum' as a function of temperature. A possible candidate to test these predictions is the 3D spin liquid insulator Na$_4$Ir$_3$O$_8$ which exhibits a pressure-tuned transition into a metallic phase. We also present the electron spectral function of Na$_4$Ir$_3$O$_8$ at the transition. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q31.00003: Transport and ordering properties of geometrically frustrated metallic pyrochlore magnets. Ka-Ming Tam, Michel Gingras Motivated by recent experiments on the Pr$_{2}$Ir$_{2}$O$_{7}$ metallic pyrochlore material, we study the ordering of the localized 4f Pr moments coupled to the itinerant 5d electrons from Ir. A Monte Carlo method is used to study the condition for spin-ice ordering, and the accompanying change of transport property due to spin ordering. We find that the effects from hybridization between the Pr moments and the Ir electrons should be included to describe the metallic spin liquid phase at low temperatures. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q31.00004: Lattice vibration modes in CdCr2O4 J.-H. Kim, S.-H. Lee, M. Matsuda, H. Ueda, Y. Ueda, J.-H. Chung, S. Tsutsui, A. Baron In geometric frustration magnets, spin-lattice coupling can play an important role in lifting the magnetic frustration. In order to understand the mechanism, we have performed inelastic synchrotron x-ray measurements on a single crystal of a frustrated magnet CdCr2O4 that undergoes such a three-dimensional spin-Peierls phase transition at T{\_}N = 7.8 K. Our data taken above and below T{\_}N could be well explained by the rigid ion model, which led to a full identification of the lattice vibration modes in CdCr2O4. A phonon anomaly that might be associated with the transition, however, was not observed. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q31.00005: Crystal and magnetic structures of the three-dimensional spin-Peierls state of ZnCr2O4 Sungdae Ji, S.-H. Lee, C. Broholm, W. Ratcliff II, S.-W. Cheong, P. Zschack The geometrically frustrated spinel ZnCr$_2$O$_4$ undergoes a three-dimensional spin-Peierls phase transition at T$_N$ = 12.5 K from a cubic paramagnetic state to a tetragonal Neel state. The exact nature of the lattice distortion, and the one-to-one-correspondence between the distortion and the magnetic ground state has been a long standing issue. To unveil the mystery, we have performed synchrotron X-ray diffraction measurements on a single crystal of this compound and neutron diffraction measurements on a powder sample. Our detailed analysis of the single crystal X-ray data shows that below T$_N$ the symmetry of the crystal structure is lowered from the cubic $Fd\bar3m$ to the tetragonal $I\bar4m2$ due to formation of a complex pattern of Cr-Cr clustering. The relation between the distorted crystal structure and the magnetic structure will also be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q31.00006: Magnetic Correlations in YBaCo$_{4}$O$_{7}$: A Frustrated Magnet with Novel Trigonal Bipyramidal Chains John Mitchell, Pascal Manuel, Laurent Chapon, Paolo Radaelli, Hong Zheng Novel structural motifs generate opportunities to explore unique geometrically frustrated groundstates and their properties. One such new material,YBaCo$_{4}$O$_{7}$ (Y114), is closely related to the pyrochlore lattice, differing only in the stacking sequence of triangular layers. This alternative stacking sequence leads to chains of corner sharing trigonal bipyramids with magnetic ion vertices, a novel motif among geometrically frustrated lattices. We have studied Y114 using single crystal neutron diffraction above its ordering temperature. Strong magnetic diffuse scattering can be understood using Monte-Carlo simulations, and a simple nearest-neighbor model explains the magnetic structure. Along the c-axis, long-range correlations arising from the corner-sharing bipyramids create a quasi one-dimensional order at finite temperature. In contrast, the spin-spin correlation function decays rapidly in the ab-plane, following a unique short-range configuration that enforces an$ L$=0 for the bipyramid. Approaches to suppressing long-range order in the system will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q31.00007: Specific heat of gadolinium garnets Jeffrey Quilliam, Shuchao Meng, Linton Corruccini, Oleg Petrenko, Michel Gingras, Jan Kycia Specific heat measurements on two different geometrically frustrated, Heisenberg, garnet lattices will be presented.Ā The specific heat of an isotopically pure, single crystal sample of Gd$_3$Ga$_5$O$_{12}$, or GGG, is found to be consistent with previous measurements of the specific heat of GGG \footnote{Schiffer et al. Phys. Rev. Lett. 74, 2379 (1995).} and shows no signs of a standard ordering anomaly despite sharp Bragg peaks that were seen in neutron diffraction experiments.Ā A first measurement of the specific heat of polycrystalline Gd$_3$Li$_2$Te$_3$O$_{12}$, in contrast, shows a sharp first-order phase transition at 240 mK. Ā We will discuss possible explanations for such diverse behavior in very similar systems. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q31.00008: Long-range antiferromagnetic interactions in ZnFe$_{2}$O$_{4}$ and CdFe$_{2}$O$_{4}$ Ching Cheng, Ching-Sheng Liu, Hsueh Fang Yeh For the first time, the Fe-Fe interactions in the well known geometrically frustrated antiferromagets of Zinc and Cadmium ferrite are determined quantitatively by considering structures of different collinear magnetic distributions using first-principles methods. Both the generalized gradient approximation (GGA) and GGA plus the onsite Coulomb interaction (GGA+U) are considered for the exchange-correlation energy functional. The interactions up to third neighbors are determined to be all antiferromagnetic regardless of which approximation scheme (GGA or GGA+U) is used. Surprisingly, the third neighbor interaction is much stronger than the second-neighbor one. The magnetic distributions have prominent effects on energies as well as density of states, including band gaps, for both normal and inverse spinel structures of these two materials. The possible magnetic distributions coincident with neutron scattering results are also investigated. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:27PM |
Q31.00009: Frustrated and Quantum Antiferromagnetism on the Diamond Sublattice of A-site Magnetic Spinels Invited Speaker: Spinel crystals, with the chemical formula AB$_2$X$_4$, in which only the A atom is magnetic, realize antiferromagnetism on a diamond sublattice. We first discuss examples, such as CoAl$_2$O$_4$ and MnSc$_2$S$_4$, which exhibit ``bond frustration'' due to the competing effects of first and second neighbor interactions. This is well modeled by a classical Heisenberg Hamiltonian, which leads to a remarkable ground state degeneracy of coplanar spirals, in which the wavevector of the spiral can lie anywhere on a ``spiral surface'' in momentum space. We describe how thermal fluctuations lead to a broad spin liquid regime, with unique properties, and magnetic ordering at low temperatures. We next discuss the intriguing case of FeSc$_2$S$_4$, in which orbital degeneracy leads to a persistant ``spin orbital liquid'' down to the lowest temperatures. We argue that this material is in the vicinity of an unusual quantum critical point driven by a competition between exchange and spin-orbit interactions. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q31.00010: Phase competition and large residual entropy in the pyrochlore double-exchange system Yukitoshi Motome, Nobuo Furukawa Strong interplay between spin and charge degrees of freedom gives rise to fascinating phenomena, in particular when the system undergoes severe geometrical frustration, such as anomalous magneto-transport and heavy-mass behavior. We investigate this intriguing problem in the double-exchange model on the frustrated pyrochlore lattice by employing an unbiased Mote Carlo simulation. We find that as increasing the antiferromagnetic superexchange between localized moments, the ferromagnetic metallic state stabilized by the double-exchange mechanism becomes unstable, and is taken over by a paramagnetic metal in which spin correlations are strongly suppressed by the frustration. In the critical region, the system exhibits a peculiar electronic and magnetic state; the competing interactions cancel with each other, resulting in an almost temperature independent state. This indicates a large entropy remaining at low temperatures, which potentially leads to heavy mass behavior or some nontrivial symmetry breaking at a lower temperature. We also discuss the relevance to experiments in itinerant pyrochlore materials. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q31.00011: Magnetic Bose condensation vs. magnon localization in a model magnet with site dilution Tommaso Roscilde, Stephan Haas, Rong Yu We report on the theoretical field-temperature phase diagram of anisotropic coupled S=1 chains with site dilution, modeling the magnetic behavior of doped NiCl$_2$-\emph{tetrakis} thiourea (DTN). In absence of doping, this compound clearly displays field-induced Bose-Einstein condensation of magnons [V. Zapf \emph{et al.}, Phys. Rev. Lett. {\bf 98}, 047205 (2007)], as revealed by the mean-field scaling of the field-induced ordering temperature, $T_c \sim |H-H_c|^{\phi}$ with $\phi=2/3$. The critical field $H_c$ corresponds to a $T=0$ quantum phase transition (QPT) between a spin gap phase and a gapless ordered phase. Here we show that site dilution opens a novel gapless spin-liquid phase close to the ordering transition, corresponding to a \emph{Bose glass} phase of localized magnons. Disorder leads to a radical change in the universality class of the QPT (which turns into a \emph{quantum percolation} transition), and in the critical temperature scaling, which exhibits a novel universal exponent $\phi\approx 1.2$. A crossover to mean-field scaling of $T_c$ at finite temperature is observed, and explained via a scenario of \emph{thermal percolation} of magnons. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q31.00012: Effect of chemical substitution on quantum spin ladders Tao Hong, Andrey Zheludev, Hirotaka Manaka, Yiming Qiu Unlike in any other quantum spin ladder system, the magnitude of the spin gap in IPA-CuCl$_{3}$ [1] can be tuned by chemical doping. We describe inelastic neutron scattering measurements on powder samples of IPA-Cu(Br$_{x}$Cl$_{1-x})_{3}$ for a different values of x. Br-doping directly affects the key interactions involving Cu-Cl-Cl-Cu superexchange pathway and induces bond randomness [2]. In the pure system the spin gap is readily visible. At 3{\%} and 7{\%} Br-doping, the gap models are weakened and presumably broadened, while additional scattering emerges at low energies. The gap excitations totally disappear by x=10{\%}. [1] T. Masuda, A. Zheludev, H. Manaka, L.-P. Regnault, J.-H. Chung, Y. Qiu, Phys. Rev. Lett. 96, 047210 (2006). [2] H. Manaka, I. Yamada, H. Mitamura and T. Goto, Phys. Rev. B \textbf{66}, 064402 (2002); H. Manaka \textit{et al.}, Phys. Rev. Lett. \textbf{101}, 077204 (2008). [Preview Abstract] |
Session Q32: Magnetic Domains and Domain Walls
Sponsoring Units: GMAGChair: Kristen Buchanan, Colorado State University
Room: 336
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q32.00001: Effect of Wall Width on Spin Torque in Ferromagnetic Domain Walls E.A. Golovatski, M.E. Flatt\'e The amount of spin torque exerted on a domain wall in a ferromagnetic semiconductor depends on the amount of spin flip that occurs during the transport process. Starting with a model Hamiltonian[1], we calculate the total amount of spin torque exerted on a $\pi$ wall and a 2$\pi$ wall for ballistic transport across the domain wall, and calculate the dependence of the torque on the width of the domain wall. In very thin 2$\pi$ walls, transport occurs with almost no spin flip. As the wall width increases, spins precess more inside the domain wall, increasing the spin torque. In a $\pi$ wall, where most spins will flip during transport through a thick wall, we find that the spin torque increases monotonically with wall width. In contrast, spins in a thick 2$\pi$ wall will continue to precess back towards their original configuration, and there will be much less net spin flip. Thus there is very little spin torque in both very thin and very thick 2$\pi$ walls, but significant spin torque is possible in a range of intermediate widths. This non-trivial dependence on the width of the domain wall leads to an optimal wall width for achieving a maximum amount of spin torque. For a 2$\pi$ wall with an exchange-induced spin splitting of 100 meV, and an effective carrier mass equal to the electron mass, we calculate this optimal width to be $\sim$ 5nm. This work was supported by an ONR MURI. [1] G. Vignale and M.E. Flatt\'e, PRL 89, 098302 (2002). [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q32.00002: Transport across a pinned domain wall across a GaMnAs constriction: from AMR to spin-dependent tunneling. Sung Un Cho, Hyung Kook Choi, Fabio C.S. DaSilva, Teresa Osminer, David P. Pappas, Yun Daniel Park We report on the different magnetotransport mechanism across a pinned domain wall in a GaMnAs nanowire dependent on constriction size. Nanometer-sized constrictions are realized in LT-MBE epifilm GaMnAs by standard e-beam lithography and wet-etch chemistries, as well as a ``break-junction method'' to further decrease constriction size. Four-point probe DC \textit{IV} measurements- with applied fields at varying angles to wire axis- are utilized to study the transport mechanism- as well as magnetic properties. As constriction size approaches epifilm thickness, nonlinear \textit{IV }response is observed with a differing field dependence on temperature. As constrictions become smaller, we observe a tunneling AMR-like behavior. This effect is more evident after series of high current pulses are applied to decrease the constriction width. ``Break-junction'' method results in higher constriction resistances and increases in resulting MR values. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q32.00003: Mechanisms for low-field, and multiple domain wall injection into magnetic nanowires. Sarah Reiff, Andrew Kunz The motion of a domain wall within a magnetic nanowire is important for the development of future recording, sensing and logic devices.~ The speed of a field driven domain wall is quickest when the applied magnetic field is below the so-called Walker Field which depends on the size and material properties of the wire.~ However, the field needed to inject a domain wall into a wire is much greater than the Walker Field which leads to slow wall motion because of the nucleation of vortices and anti-vortices, or fast motion with complicated domain wall structures. We present Landau-Lifshitz simulation results showing a significant decrease in the field needed to inject a domain wall into the wire for a variety of injection designs including: pads, rings, straight ends, and tapered ends.~ We also find that by applying a transverse field the required driving field to inject the wall decreases, and that the domain wall motion in the wire is faster.~ The magnetization of the pad and ring injection designs can be easily manipulated so that multiple walls with a known magnetization structure are injected allowing for faster, more reliable domain wall motion. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q32.00004: The influences of transverse magnetic anisotropy on field-induced domain wall propagation in magnetic nanowires Jie Lu, Peng Yan, Xiangrong Wang Domain wall (DW) propagation in magnetic nanowires is an important subject in nanomagnetism because of its fundamental interest and potential applications in spintronic devices. It is well known that a head-to-head (or tail-to-tail) domain wall in a nanowire will propagate along the wire under an axial magnetic field. In this talk, we shall show that a new velocity-field formula can fit well with numerical results obtained from the open-source micromagnetic simulation package OOMMF. The fitting parameters have clear physical meanings that relate to the transverse magnetic anisotropy. How the transverse magnetic anisotropy, which can be modified by both transverse magnetic field and the aspect ratio of wire cross section, affects the DW structure and hence the DW propagation velocity will be discussed systematically. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q32.00005: Near-field interaction between domain walls in adjacent permalloy nanowires Liam O'Brien, D. Petit, H. T. Zeng, D. Read, E. R. Lewis, R. P. Cowburn Proposed data storage schemes based on ferromagnetic nanowires rely on the controlled propagation of domain walls (DWs) along nanoscale shift registers [Allwood \textit{et al. }Science 309, 1688 (2005)]. To make technologically relevant devices, these nanowires must be fabricated to within a wire width of one another. However, the effect of magnetostatic interactions between DWs on their propagation through closely spaced nanowires has not been well studied. Using MOKE magnetometry we have experimentally observed the interaction between two DWs of opposite charge travelling in adjacent permalloy nanowires (8nm thick, 100 nm wide), with inter-wire separation between 125 and 13nm. For the smallest separations, depinning fields (H$_{D})$ as high as 93 Oe were measured. Considering the energy landscape experienced by the two DWs under the approximation they are isolated and rigid and accounting for finite temperature we can completely reproduce the experimental dependence of H$_{D}$ on the inter-wire spacing. Our results suggest that the interaction causes little perturbation to the DW shape. Pinning resulting from localised stray fields is of interest for studying the fundamental properties of DWs as it occurs without modification of the DW or nanowire shape. Our results suggest propagation could be compromised by DW-DW interactions unless careful DW control is used. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q32.00006: Control of Domain Wall Structure and Pinning In Spin-Valve Nanowires J. Sampaio, L. Thevenard, E. Lewis, L. O'Brien, H.T. Zeng, D. Petit, D. Read, R.P. Cowburn Domain walls (\textbf{DWs}) in magnetic nanowires are the basis for several proposed data storage devices [D Allwood et al. Science 309, 1688 (2005), SS Parkin, US Patent 6,834,005 (2004)]. Most schemes use artificial defects (\textbf{ADs}) to modify the potential landscape seen by the DW, and thereby control its propagation. This potential modification depends on the DW structure. Integrating the nanowire in a Spin-Valve (\textbf{SV}) stack allows the electrical probing of the magnetization as well as electronic integration in future devices. However, using SV systems introduces strong stray fields from the reference layer, especially on the ADs. These can significantly alter the internal structure and propagation of DWs. The study of their influence has been hindered so far by the difficulty of creating DWs of known internal structure and to propagate them at low fields. Here we demonstrate low field (20Oe) propagation of DWs and their pinning by ADs in L-shaped SV nanowires with dimensions for which only transverse DWs are stable (200nm width, free layer 8nm Ni$_{19}$Fe$_{81}$, pinned layer 2nm CoFe).This was verified with micromagnetic simulations. Moreover we show DW depinning at protrusions along the wire with fields lower than that required to nucleation (80/140Oe). These results contribute to furthering the electrical integration of DW based data storage devices. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q32.00007: Detection of EMF induced by domain wall motion Geoffrey Beach, Shengyuan Yang, Carl Knutson, Di Xiao, Qian Niu, Maxim Tsoi, James Erskine It is now well established that an electric current can drive magnetic domain wall (DW) motion via coupling between conduction electrons and local magnetic moments. The reverse of this effect, i.e., an emf induced by a DW moving through a stationary electron gas, has also been predicted [1]. DW-induced emf has been explored in more detail in recent theoretical work [2,3], but has yet to be observed. In this talk, we describe the experimental detection of an emf induced by a field-driven DW in a Permalloy nanowire [4]. This DW-driven emf is discussed in terms of a generalized two-dimensional theoretical framework [4] capable of treating vortex DWs. Supported by NSF DMR-0404252, NSF DMR-0606485, DOE DE-FG03-02ER45958, and the Welch Foundation. [1] L. Berger, Phys. Rev. B 33, 1572 (1986). [2] S. E. Barnes et al., arXiv:cond-mat/0410021 (2004); Appl. Phys. Lett. 89, 122507 (2006) [3] R. A. Duine, Phys. Rev. B 77, 014409 (2008). [4] S. Yang, et al., submitted (2008). [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q32.00008: Domain Properties of a Single Magnetic Nanorod Investigated by Cantilever Magnetometry SangGap Lee, Eric Moore, Steven A. Hickman, John A. Marohn Single Ni nanorods having 50 to 100 nm diameter were integrated as the tip of ultra-sensitive cantilevers, having a force sensitivity of 8 aN/Hz$^{1/2}$ at 4.2 K, designed for use in scanned-probe magnetic resonace force microscopy. We measured cantilever frequency, dissipation, and frequency fluctuations as a function of magnetic field, applied along both the easy axis and the hard axis of the nanorods while the cantilevers were self-oscillated. The nanorods exhibit bulk magnetization. Hard-axis magnetometry experiments show the nanorods have B$_{switch} \quad \sim $ 300 mT, in which the magnetization switches from being orthogonal to being parallel to the applied field, and the three observables each show multiple sharp peaks. We find the cantilever frequency shift is well described by modeling the tip as a single-domain of uniformly-magnetized spins interacting with the applied field (the Stoner-Wohlfarth model). [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q32.00009: Magnetic Domain Structures in an In-plane Array of Cobalt Filaments with Periodic Structures Mu Wang, Wei Han, Xiang Xiong With a unique electrochemical deposition method we fabricated in-plane arrays of straight cobalt filaments with periodic corrugations over a silicon substrate without using templates. The periodic corrugations on the filaments are induced by spontaneous oscillation in electrodeposition, and the periodicity can be tuned from a few tens of nanometers to a few hundreds of nanometers by controlling the electric current in experiments. Magnetic force microscopy indicates that each corrugated structure on the filament may correspond to a local single magnetic domain. When the inter-filament separation is large, the magnetic domains are regularly aligned along the filament. The domains become random when the filaments are closely packed. We suggest that our results could be helpful in understanding the evolution of magnetic domain patterns on microscopic scale and may have potential application in spintronics. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q32.00010: Direct evidence of imprinted vortices in exchange-biased patterned bilayer nanomagnets J.J. Kavich, G. Salazar-Alvarez, J. Sort, A. Potenza, A. Mugarza, S. Stepanow, J. Nogues, P. Gambardella We investigate the magnetic domain structure in lithographically patterned nanomagnet arrays using element-sensitive circularly polarized XPEEM imaging. ZFC Py/IrMn (FM/AFM) bilayer nanodot (0.5 um -- 4um dia.) arrays imaged across the Fe and Ni L edges clearly demonstrate spontaneous formation of vortex states. Magnetic contrast at the Mn L edge resonance indicates that the vortex state is transferred into the underlying AFM layer. The exchange-bias, measured through magneto-optical hysteresis measurements, verifies the AFM nature of the underlying IrMn, suggesting that the imprinted vortex state is confined to the interface by local interactions of the uncompensated interfacial Mn spins with the FM Py layer. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q32.00011: Confinement distance of the closure structure around a single hole in a 2D magnetic thin film M. Velez, G. Rodriguez-Rodriguez, H. Rubio, A. Perez-Junquera, J.I. Martin, J.M. Alameda, J.V. Anguita One common feature in many magnetic nanostructures, such as nanorings or patterned thin films [1], is the existence of non magnetic holes within the magnetic material. However, up to now, the simple problem of a a single non magnetic hole in a 2D magnetic film has received little attention, even though it is qualitatively different from the blade domains that appear around holes in 3D magnetic material. In this work [2] this basic problem has been analyzed in detail by magnetic force microscopy, micromagnetic simulations and an analytical model. The closure magnetization configuration can be described by two -1/2 half vortices located at the hole edge along the easy anisotropy axis, and confined within a distance L that is determined by the minimization of magnetostatic and anisotropy energies constrained by the magnetic charge conservation within the system. [1] A. Perez-Junquera et al, J. Appl. Phys. 99 (2006) 033902 [2] G. Rodriguez-Rodriguez et al, Phys. Rev. B (2008) in press. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q32.00012: A shape phase diagram for a ferromagnetic liquid drop Shubho Banerjee, Travis Rasor, Mike Widom A ferromagnetic liquid phase has been predicted by mean field theory and computer simulations but conclusive experimental evidence is lacking. Liquids such as ferrofluids, that are suspensions of ferromagnetic particles in solvents, magnetize only in the presence of an applied magnetic field and thus are paramagnets, not ferromagnets. A ferromagnetic liquid, if it existed, would spontaneously magnetize even in the absence of a magnetic field. A droplet of such a liquid will likely not be a sphere due to the magnetostatic energies involved. These energies will induce a magnetization texture in the drop analogous to domain formation in solids. We examine possible shapes for the droplet by optimizing its shape and magnetization textures with respect to its overall energy. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q32.00013: GGA+$U$ calculation of the magnetic ground state of GdB$_{4}$ Leonard Kleinman, Muhammad Huda We have studied eight collinear and non-collinear magnetic orientations of GdB$_{4}$ using the GGA + $U$ method, without and with spin-orbit coupling, for values of $U - J$ between 0 and 6. For $U $-- $J$ = 6, the value which had been found to yield the correct Gd lattice constants, we obtain GdB$_{4}$ lattice constants within 0.26{\%} of experiment. We find the magnetization lies in-plane but is collinear, in disagreement with the most recent experimental determination. [Preview Abstract] |
Session Q33: Superconductivity: Response to Electromagnetic Fields and Proximity Effect
Sponsoring Units: DCMPChair: Art Hebard, University of Florida
Room: 403
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q33.00001: Magnetic field dependence of the infrared transmittance of superconducting NbTiN D.B. Tanner, J. Hwang, X. Xi, H. Zhang, G.L. Carr Superconductivity may be destroyed by raising the temperature of the superconductor above the transition temperature or by increasing an applied magnetic field above the upper critical field. We have studied the behavior of key microscopic properties, the superconducting energy gap and the superfluid density, through far-infrared magnetospectroscopy measurements on thin-film Nb$_{0.5}$Ti$_{0.5}$N. The measurements were performed at the National Synchrotron Light Source, Brookhaven National Laboratory. As temperature is increased, the gap and the superfluid density are reduced, both reaching zero at $T_{c}.$ The behavior with field is different. Over much of the range between 0 and 10 T, the gap is almost unchanged, while the superfluid density is reduced, roughly following the area not in vortices in the film. Only near the highest field does the superconducting gap become reduced. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q33.00002: Strong correlation effects and optical conductivity in electron doped cuprates Tanmoy Das, R.S. Markiewicz, A. Bansil We demonstrate that most features ascribed to strong correlation effects in various spectroscopies such as angle-resolved photoemission spectroscopy (ARPES) and optical spectra of the cuprates are captured by a calculation of the self-energy incorporating effects of spin and charge fluctuations[1]. The self-energy is calculated over the full doping range of electron-doped cuprates from half-filling to the overdoped system. The spectral function reveals four subbands, two widely split incoherent bands representing the remnant of the split Hubbard bands, and two additional coherent, spin- and charge-dressed in-gap bands split by a spin-density wave, which collapses at the AFM quantum critical point (QCP) in the overdoped regime. The transition between the in-gap states leads to pseudogap features in the mid-infrared region of the optical spectra, where the incoherent features persist to high doping even above the QCP, producing a remnant Mott gap. Notably, our results are also in good accord with variational cluster and quantum Monte Carlo calculations. Work supported in part by the USDOE. \\[3pt] [1] Tanmoy Das, R. S. Markiewicz, and A. Bansil, arXiv:0807.4257. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q33.00003: A broadband microwave study of the superconducting~fluctuations in 2D InOx thin films Wei Liu, Minsoo Kim, Tailung Wu, Sambandamurthy Ganapathy, Peter Armitage We apply a broadband microwave `Corbino' spectrometer covering the~range from 10MHz to 20GHz to the study of 2D disordered~superconducting InOx thin films.~ Explicit frequency dependency of~the superfluid stiffness and conductivity are obtained down to 270mK.~The AC measurements are sensitive to different time scales of the~superconducting fluctuations.~ A number of fluctuation regimes are~investigated (gaussian fluctuations, vortex proliferation) as we cool~the sample into the low-temperature Kosterlitz-Thouless-Berezinskii- like phase.~We discuss our results in terms of prevailing scenarios~for fluctuation superconductivity and make connection to other~experimental results. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q33.00004: Time-resolved terahertz photoconductivity of insulating cuprates Amir Farahani, Jesse Petersen, Ruixing Liang, J. Steven Dodge We use a visible pump, terahertz probe technique to study the photoconductivity of the undoped cuprates. We use ultrafast optical pulses ($E_{\mathrm{pump}} = 3.1$~eV) to create photocarriers in high quality single crystals of Sr$_2$CuO$_2$Cl$_2$ and YBa$_2$Cu$_3$O$_6$, and time-domain terahertz spectroscopy to probe the resulting photoconductivity. We observe a rapid onset of photoconductivity followed by a non-exponential relaxation on a picosecond timescale. This dynamics is independent of photocarrier concentration over the range of 0.2 to 1.7 percent excitations per copper atom. Assuming a quantum efficiency of unity, we infer a mobility of $\sim$0.2 cm$^2$/Vs, significantly lower than the Hall mobility in chemically doped systems \footnote{Y. Ando {\em et al.} PRL {\bf 87} 017001 (2001)}. As the fluence is increased, there is a weak decrease in the photoconductivity amplitude. We also measured the frequency dependence of the photoconductivity in the terahertz range, and observe an increase in photoconductivity with frequency up to 600 GHz, suggesting polaronic effects. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q33.00005: Magnetic field-induced modification of superfluid density and interplane spectral weight in YBa$_2$Cu$_3$O$_y$ Andrew LaForge, Willie Padilla, Kenneth Burch, Zhiqiang Li, Alexander Schafgans, Kouji Segawa, Yoichi Ando, Dimitri Basov We report on the interlayer infrared response of YBa$_2$Cu$_3$O$_y$ in an applied magnetic field. This study explores both the underdoped ($y$ = 6.67 and 6.75) and optimally doped ($y$ = 6.95) regions of the phase diagram, and includes data for fields applied both parallel to the $c$ axis and to the CuO$_2$ planes in this anisotropic superconductor. A sum rule analysis reveals that magnetic fields $H \parallel c$ eliminate the high-frequency contribution to the superfluid density, returning the system to a more BCS-like energy scale [1]. For fields $H \parallel$ CuO$_2$, however, the high-energy component scales with the superfluid density, and the anomalous scheme of condenstate formation is maintained, at least in underdoped $y$=6.67 and 6.75 samples. This behavior is discussed in relation to the change of electronic kinetic energy and the suppression of interplane phase coherence. [1] A. D. LaForge \emph{et al.}, Phys. Rev. Lett. \textbf{101}, 097008 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q33.00006: A Terahertz Conductivity Study of Pseudogap Phase in Underdoped LSCO. Lucas Bilbro, Ivan Bozovic, N. Peter Armitage Using Terahertz Time Domain Spectroscopy (TTDS), we investigate the pseudogap phase of the high-temperature cuprate superconductors. We measure the frequency and temperature dependence of the complex conductivity for a number of underdoped thin films of La$_{2-x}$Sr$_{x}$CuO$_{4}$ at frequencies well below the superconducting gap. A number of issues are investigated, including evidence for a non-zero finite frequency superfluid stiffness in regions outside the superconducting phase boundary. We discuss the extent of the fluctuation regime and the impact of these fluctuations on the physics of the pseudogap. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q33.00007: Scaling of the superfluid density in severely underdoped \boldmath YBa$_2$Cu$_3$O$_{6+y}$\unboldmath C.C. Homes, W.A. Huttema, P.J. Turner, D.M. Broun, Ruixing Liang, W.N. Hardy, D.A. Bonn Microwave impedance techniques have been used to determine the critical temperature ($T_c$), the in-plane superfluid density ($\rho_{s0}$), and the dc conductivity ($\sigma_{dc}$) just above $T_c$ in a highly-underdoped sample of YBa$_2$Cu$_3$O$_{6+y}$ for $y\simeq 0.333$. In this state the sample may be annealed to yield different levels of chain oxygen order and electronic doping in the copper-oxygen planes, resulting in a range of $T_c \simeq 2 - 17$~K.\footnote{D. M. Broun {\it et al.}, Phys. Rev. Lett. {\bf 99}, 237003 (2007).} The linear relation between $\rho_{s0}$ and $T_c$ is not observed, instead $\rho_{s0}\propto T_c^2$. However, the results do follow the more general scaling relation $\rho_{s0}/8 \simeq 4.4\,\sigma_{dc}T_c$,\footnote{C. C. Homes {\it et al.}, Phys. Rev. B {\bf 72}, 134517 (2005).} extending the validity of this relation for the in-plane data by an order of magnitude. In addition, these new results now provide a region of overlap between the scaling observed in the copper-oxygen planes, and perpendicular to the planes along the poorly-conducting {\it c} axis. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q33.00008: Real-time photoinduced quasi-particle relaxation of superconductors Jianmin Tao, Jian-Xin Zhu Ultrafast optical phenomena are of fundamental importance in the investigation of electronic dynamics of metals and superconductors [1]. By considering a model Hamiltonian with electron-boson coupling of a superconductor exposed to a time-dependent laser field, we calculate the current density, which can be expressed in terms of the quasi-particle density matrices. The time evolution of these density matrices is derived within a mean-field approximation using the equation-of-motion approach and is numerically investigated with Runge-Kutta method. we discuss the consequence of the $d$-wave pairing symmetry in the quasi-particle relaxation process. \\[3pt] [1] R. D. Averitt and A. J. Taylor, J. Phys: Condensed Matter 14, R1357 (2002). [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q33.00009: Raman scattering from the CaC6 superconductor in the presence of disorder Aleksej Mialitsin, Jun Sung Kim, Reinhard Kremer, Girsh Blumberg Polarized Raman scattering has been performed on CaC$_6$ single crystal superconductor. We identify two of the three Raman active E$_g$ phonon modes at 440 and 1508\,cm$^{-1}$ expected for the $R\overline{3}m$ space group of CaC$_6$. These first order scattering modes appear along with the D and G bands around 1300\,cm$^{-1}$ and 1600\,cm$^{-1}$ that are similar in origin to the corresponding bands in plain graphite. The intensities of the D and G bands in CaC$_6$ correlate with degree of disorder. The D band arises from the double resonant Raman scattering process; its frequency shifts as a function of excitation energy with $\sim$\,35\,cm$^{-1}$/eV. The double resonant Raman scattering probes phonon excitations with finite wave vector $\emph{\textbf{q}}$. We compare experimental results to from-first-principles calculations. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q33.00010: Possibility of p-wave triplet pairing in Nb/Ni bilayers Wenjian Lu, Kookrin Char, Y.K. Bang, P. SanGiorgio, M.R. Beasley We have experimentally investigated the density of states (DOS) in Nb/Ni (S/F) bilayers and found the anomalous double peak structure. In order to analyze the measured DOS data, we propose a theory model in which $p$-wave triplet pairing correlations are induced by an inhomogeneous magnetization in the ferromagnet. The induced triplet component penetrates into the ferromagnet over a long length (much larger than a characteristic length scale $\xi_F=\sqrt{D/h}$, where $D$ is the diffusion coefficient and $h$ exchange energy). We numerically calculate the DOS of Nb/Ni bilayers based on Eilenberger equation with various parameters and discuss the unusual sub-gap structure in the DOS. We find a good qualitative and quantitative agreement between the model calculations and our measurements and therefore suggest the possibility of $p$-wave triplet correlations in Nb/Ni bilayers. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q33.00011: 1D Chain of Interacting Majorana Bound States at the Edge of a Topological Insulator Vasudha Shivamoggi, Joel Moore We study a realization of a 1d chain of Majorana bound states that consists of alternating ferromagnetic and superconducting regions at the edge of a quantum spin hall insulator. Each boundary between a ferromagnetic and superconducting region supports a Majorana bound state, and the pair-wise interaction energies have previously been calculated in the weakly interacting limit. By adjusting the phases of the order parameters in these regions, it is possible to create a Majorana bound state localized at each interface. In the limit of well separated Majorana fermions, the system can be mapped to the transverse field Ising model. To reach the random critical Majorana chain studied by Bonesteel and Yang, the phases of the ferromagnetic and superconducting order parameters must be drawn from essentially the same random distribution. We examine factors in an experimental system that will move the system away from the critical point, such as Coulomb interactions and breaking of the duality between the ferromagnetic and superconducting regions. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q33.00012: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q33.00013: Superconducting Proximity Effect in Thin Semiconducting Films Michael Vissers, Kevin Inderhees, Tim McArdle, Stephanie Law, Paul Goldbart, Laura Greene, Jim Eckstein We report results using a novel 3 terminal device to study the influence of the superconducting proximity effect on the sheet resistance of the N-layer, Rs, as well as the junction conductance across the N-S boundary, Gc. When the N-layer is a degenerate semiconductor the changes in these quantities are large. In samples with large Gc, we find that the N-layer sheet resistance below the SC transition increases as T decreases. We can also extract the actual J-E relationship by modeling the spatial current profile beneath the injector electrode. We interpret these effects as being due to a phase stiff proximity effect in the N layer moving the N-S electrical boundary into the semiconductor as the temperature is lowered, thereby removing volume available to normal transport, and causing the measured Rs to increase. This work was supported by the DOE BES at the F. Seitz Materials Research Laboratory at the University of Illinois, Urbana. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q33.00014: I-V studies of proximity-induced superconductivity in long nanowires Haidong Liu, Zuxin Ye, K.D.D. Rathnayaka, Wenhao Wu Recently we have observed an anomalous long-range superconducting proximity effect in superconducting Zn and Sn nanowires in contact with bulk film electrodes of Au, Sn, or Pb. With electrodes having a higher transition temperature, nanowires up to 60 micron in length are observed to superconduct at the transition temperature of the electrodes. With Au electrodes, superconductivity in the nanowires is largely suppressed. These samples are fabricated by electroplating nanowires into porous membranes where the electrode-nanowire contacts were made via a self-contacting technique. In this talk, measurements of the I-V characteristics at various temperatures and magnetic fields will be discussed to further understand the observed proximity effect. The most striking feature observed in the dc I-V characteristics is a reproducible step-like feature, identical to that observed in superconducting whiskers and micro-bridges. Such multiple steps are attributed to the successive establishment of localized phase slip centers (PSCs) along a nanowire since some sections of a nanowire have smaller critical currents. With our data, we estimate that the length of a typical PSC is 5-10 microns. These results suggest that the observed proximity effect occurs along the entire length of a nanowire, rather than only at the nanowire-electrode interfaces [Preview Abstract] |
Session Q34: Focus Session: Superconductivity: Magnetic Properties
Sponsoring Units: DMP GMAGChair: Carmen Almasan, Kent State University
Room: 404
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q34.00001: Influence of Magnetic Domain Structure on Abrikosov Vortex Dynamics in Superconductor-Ferromagnet Hybrids Invited Speaker: We will review the experimental and theoretical aspects of transport properties and vortex static and dynamic characteristics in magnetically coupled superconductor-ferromagnet hybrid structures. Magnetotransport characteristics and scanning tunneling microscopy (STM) images of vortex structures reveal rich superconducting phase diagram in these systems. Focusing on particular combination of a Permalloy ferromagnet with a well ordered rotatable periodic stripe-like magnetic domain structure with alternating out-of-plane component of magnetization, and a small coherence length superconductor, we find directed nucleation of superconductivity above domain wall boundaries. We show that near the superconductor-normal state phase boundary the superconductivity is localized in narrow mesoscopic channels. Changing the in-plane direction of magnetic stripe domains it is possible to re-configure the direction of the superconducting nano-channels and controllably rotate the direction of the in-plane anisotropy axis in the superconductor. Deeper into the superconducting state we observe strongly anisotropic vortex pinning effect due to the presence of the ordered magnetic domain structure. We show that the hybrid exhibits commensurability features that are related to the matching periodicities of the Abrikosov vortex lattice and the magnetic stripe domains. Using STM vortex imaging we show that the periodic magnetic induction in the superconductor creating a series of (anti)pinning channels for externally added magnetic flux quanta forcing confinement of the Abrikosov vortices and formation of quasi-1D vortex arrays. We will also discuss potential for electronic applications of ferromagnet-superconductor hybrid systems. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q34.00002: The influence of magnetic domain landscape on the flux pinning in ferromagnetic/superconducting bilayers Marta Z. Cieplak, Z. Adamus, M. Konczykowski, L. Y. Zhu, C. L. Chien A line of miniature Hall sensors has been used to study the influence of the disorder in the magnetic domain landscape on flux pinning in the ferromagnetic/superconducting (F/S) bilayers. The bilayers consist of Nb as the S layer and Co/Pt multilayer with perpendicular magnetic anisotropy as the F layer, separated by a Si buffer layer to avoid the proximity effect. By changing of the Pt layer thickness, the magnetic domain landscape with different degree of disorder, ranging from uniformly distributed narrow domains (quasi-ordered landscape) to highly disordered landscape with domains of different sizes, can be predefined in the F layer. The flux behavior is then measured in the superconducting state using the Hall sensors. It is found that the quasi-ordered landscape with domains width comparable to the magnetic penetration depth produces large enhancement of the vortex pinning and smooth flux penetration. The more disordered magnetic domain patterns cause less pinning and create large edge barrier for vortex entry followed by strongly inhomogeneous flux penetration. The possible origins of this behavior will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q34.00003: Soft magnetic lithography and giant magnetoresistance in superconducting/ferromagnetic hybrids. V. Vlasko-Vlasov, U. Welp, A. Imre, D. Rosenmann, J. Pearson, W. Kwok We report on direct visualization confirmed by the transport measurements of strong interactions between superconducting vortices and ferromagnetic domains in bilayers of type-II SC lead films and FM permalloy films with perpendicular magnetic anisotropy. Domains in permalloy formed a submicron stripe lattice that could be easily aligned in the film plane. We show that domain walls yield a robust magnetic pinning potential providing preferential vortex motion along the stripe domains. The effect is observed in a wide temperature range and results in a noticeable anisotropy of critical currents. The anisotropy increases near Tc when the core pinning becomes inefficient and the anisotropy direction is changed by reorienting the stripe domains. Such a tunable magnetic lithography is a convenient way of varying transport properties of superconductors and developing new cryotronic devices such as microscale superconducting switches and modulators. In our samples we found an unusually high magnetoresitance of 10$^{6 }${\%} in the fields of $\sim $10 Oe for the currents perpendicular to the domain walls. It can be referred to the granular structure of the lead films assisting the formation of easy flux flow channels along the stripe domains. --- The work was supported by the U.S. DOE Office of Science under Contract No. DEAC02-06CH11357. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q34.00004: Experimental approach to search magnetic pinning in YBCO films grown by chemical deposition method Carlos Monton, Anna Palau, Jone Zabaleta, Narcis Mestres, Teresa Puig, Xavier Obradors In the last 10 years we have developed experience in generating YBa$_{2}$Cu$_{3}$O$_{7}$ (YBCO) films and coated conductors by chemical solution deposition (CSD) capable to carry current densities in the range of 3 to 4 MA/cm$^{2}$ (at 77K and H=0). To improve these performances specific defects were grown by chemical nanostructurated routes. Interfacial pinning was obtained by the growth of nanostructurated templates generated by strain induced or assisted self-assembled processes [1]. On the other hand isotropic defect pinning contribution was increased by adding nanocomposites with second phase within the YBCO matrix. These samples were grown by modified solution precursors [2] reaching the maximum value of the critical current density, Jc (1T, 77K) = 2.2MA/cm$^{2}$, reported so far [2]. However recent theoretical works suggest that vortex pinning can be improved even more by using hybrids superconductor/ferromagnetic (Sc/FM) materials. In this work we explore the interaction of FM random distributed CSD La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ nanoislands with the YBCO film and their effect on vortex pinning. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q34.00005: Interplay between ferromagnetism and superconductivity at interfaces of La$_{0.7}$Ca$_{0.3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-\delta}$/ La$_{0.7}$Ca$_{0.3}$MnO$_3$ trilayers Felio Perez, Eval Bacca, Maria E. Gomez, Hongtao Shi, David Lederman We report studies of La$_{0.7}$Ca$_{0.3}$MnO$_3$/YBa$_2$Cu$_3 $O$_{7-\delta}$/ La$_{0.7}$Ca$_{0.3}$MnO$_3$ trilayers onto (001) SrTiO$_3$ substrates by high-pressure dc sputtering. We have grown heterostructures with a constant thickness of the ferromagnetic layers of 287 unit cells (110~nm) and ranging the thickness of the superconducting-middle layer between 5 (6~nm) and 15 unit cells (17~nm). The transport measurements show a strong suppression of the superconducting properties when the thickness of superconducting layer is reduced below 10 unit cells. However, the magnetic response out of plane shows the presence of the superconductor until 5 unit cells. The difference between the electrical characterization and the onset of the diamagnetic transition might to be related of presence of the spontaneous vortex phase in this temperature interval. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q34.00006: Probing the magnetic states in a ferromagnet using a superconductor Leyi Zhu, Tingyong Chen, Chia-Ling Chien In a superconductor (S)/ferromagnet (F) bilayer, the superconducting properties of the S layer are sensitive to the domain pattern in the adjacent F layer.[1-2] We exploit this effect to investigate Ni films, which instead of retaining in-plane anisotropy as usual, unexpectedly acquire perpendicular anisotropy when the thickness is above a critical value. Using Ni/Nb bilayers, the perpendicular magnetization component can be sensitively probed by the measurements of the superconducting transition of Nb in a magnetic field, which alters the domain pattern in Ni. Above the critical Ni thickness, an in-plane magnetic field can manipulate the stripe domains in Ni between parallel stripes and random labyrinth states resulting in as much as 90 mK in the transition temperature of the Nb layer. This clearly demonstrates that superconductor is a sensitive probe of the magnetic domain state of a ferromagnet. In turn, the results also show that along the parallel stripe domains, superconductivity is less detrimental. [1]. A. Yu. Rusanov, M. Hesselberth, J. Aarts, and A. I. Buzdin, Phys. Rev. Lett. 93, 057002 (2004). [2]. L. Y. Zhu, T. Y. Chen, and C. L. Chien, Phys. Rev. Lett. 101, 017004 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q34.00007: Evidence for Induced Magnetization in Superconductor/Ferromagnet Bilayers Itay Asulin, Ofer Yuli, Gad Koren, Oded Millo The inverse proximity effect in superconductor/ferromagnet heterostructures has been the focus of recent theoretical studies. The different approaches share the basic conclusion that a sizable magnetic moment should penetrate into the superconductor side. The sign of this moment, its spatial behavior and the actual mechanism of this effect are still controversial. Very few experimental works provided evidence for the existence of such a magnetic moment inside the superconductor. However, the effects of such an induced magnetization on the density of states of the superconductor was not observed so far and needs further theoretical and experimental clarification. We have performed scanning tunneling spectroscopy of (001)YBa$_{2}$Cu$_{3}$O$_{7-\delta }$/SrRuO$_{3}$ and (110)YBa$_{2}$Cu$_{3}$O$_{7-\delta }$/SrRuO$_{3}$ bilayers, where the SrRuO$_{3}$ is an itinerant ferromagnet, and found an anomalous splitting of both the gap and zero bias conductance peak features that may provide evidence for the existence of such an induced magnetic moment inside the superconductor. The relevant length and energy scales of the effect will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q34.00008: Interaction between magnetism and superconductivity in La$_{0.7}$Ca$_{0.3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-\delta}$ multilayers T. Hu, H. Xiao, C. C. Almasan, C. Visani, Z. Sefrioui, J. Santamaria Angular and field dependent magnetoresistance measurements were
performed on
La$_{0.7}$Ca$_{0.3}$MnO$_3$/YBa$_2$Cu$_3$O$_{7-\delta}$
(LCMO/YBCO) heterostructures below and above the superconducting
transition temperature $T_{c} \approx$ 28 K of the YBCO layer, in
order to address the origin of the long range proximity effect
found in these heterostructures. The proximity-induced
conductance in the LCMO layer at $T |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q34.00009: Directional control of the inverse superconducting spin-switch Cristina Visani, Norbert M. Nemes, Mirko Rocci, Christian Miller, Javier Garcia-Barriocanal, Diego Arias, Zouhair Sefrioui, Carlos Leon, Jacobo Santamaria, Mar Garc\'Ia-Hern\'andez, Suzanne G. E. te Velthuis , Axel Hoffmann, Mike R. Fitzsimmons We report results on the study of spin dependent transport in La$_{0.7}$Ca$_{0.3}$MnO$_{3}$/YBa$_{2}$Cu$_{3}$O$_{7-\delta }$/La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ trilayers, focusing on the effects induced by magnetic anisotropy of the LCMO. The study of magnetic anisotropy through the combination of transport measurements, magnetometry, x-ray magnetic circular dichroism and polarized neutron reflectometry, allowed us to unravel the origin of the inverse superconducting spin switch. Applying the magnetic field along the easy axis we obtained a plateau-like response reflecting a well defined antiparallel alignment of the LCMO layers over a wide magnetic field range yielding large magnetoresistance that we ascribe unambiguously to the spin dependent transport. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q34.00010: Oxide thin film based inverse superconducting spin switches. Norbert M. Nemes, C. Visani, C. Miller, M. Rocci, F. Bruno, J. Garcia-Barriocanal, Z. Sefrioui, C. Leon, J. Santamaria, M. Iglesias, F. Mompean, M. Garcia-Hernandez, A. Hoffmann, S. G. E. te Velthuis Thin film F/S/F trilayers made of YBa2Cu3O7 (S, YBCO) and La0.7Ca0.3MnO3 (F, LCMO) behave as inverse superconducting spin switches (SSS) as the critical temperature of the superconductor depends on the relative orientation of the magnetization of the F layers in a way that the resistivity is increased in the antiparallel configuration. This is caused by enhanced pair-breaking due to the spin dependent transport of quasiparticles transmitted from the ferromagnetic electrodes into the superconductor. Similar inverse SSS is obtained from exchange biased LCMO/YBCO/Cobalt trilayers with a broad AP field range. Spin diffusion across the superconductor, proximity effect at the F/S interface, stray fields due to domain walls of the ferromagnet and the magnetic anisotropy all play a role. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q34.00011: Direct and Inverse Spin Switch Effect in Superconducting Spin Valves Jian Zhu, Carl Boone, Xiao Cheng, Ilya Krivorotov We report the observation of direct and inverse spin switch effects in ferromagnet/ superconductor/ ferromagnet/ antiferromagnet (FM/SC/FM/AF) spin valves with FM = Ni$_{81}$Fe$_{19}$, AF = Ir$_{25}$Mn$_{75}$ and SC=Nb. In these spin valve structures, the magnetization of the free layer can be switched between parallel (P) and antiparallel (AP) orientations with respect to the orientation of the fixed layer by a small in-plane magnetic field. Near the superconducting transition temperature, the P state has a higher resistance than the AP state. This is the direct spin switch effect expected from the proximity effect in superconducting spin valves. However, when the fixed ferromagnetic layer is brought into a multi-domain state in zero magnetic field, resistance in this state becomes significantly higher than that of the P state and rapidly decreases to the P state value with increasing magnetic field. We explain this inverse spin switch effect by the formation of N\'eel - quasi-N\'eel domain wall pairs in the ferromagnetic bilayer. The N\'eel - quasi-N\'eel domain wall pairs induce stray magnetic fields with a significant component perpendicular to the plane of the sample. This field penetrates the SC and gives rise to vortex flow resistance. Our work shows that the inverse spin switch effect is magnetostatic in origin. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q34.00012: Even- to Odd-frequency Pair Conversion by Magnetic Interfaces in SC/N Junctions Matthias Eschrig, Jacob Linder, Takehito Yokoyama, Asle Sudb{\O} We study the proximity-induced superconducting correlations in a ballistic or diffusive normal metal (N) connected to a superconductor (SC) when the interface between them is spin-active. One of the hallmarks of the proximity effect in a non-magnetic bilayer is a minigap in the density of states of the normal metal. It scales with the Thouless energy of the normal metal and with the transmission probability of the interface. For a spin-active interface, the transmission properties of spin-$\uparrow$ and spin-$\downarrow$ electrons into N are different, giving rise to spin-dependent phase shifts at the interface. This leads to a rather surprizing result. Remarkably, for any interface spin polarization there is a critical interface resistance, above which the conventional singlet proximity component vanishes at the chemical potential, while an odd-frequency triplet component remains finite. At the same time, the minigap is replaced by a low-energy band with enhanced density of states. We propose a way to unambiguously observe the odd-frequency component. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q34.00013: Proximity effect in Exchange-spring/Superconductor hybrid system Jiyeong Gu, Jesse Burgess In ferromagnet/superconductor hybrid systems, it has been well known that the superconducting property is modified when the local magnetic environment changes in the adjacent ferromagnetic layers. In exchange-coupled hard/soft ferromagnet systems, the mutual coupling of the soft and hard magnetic layers creates a spiral magnetic domain structure. We utilized the exchange-coupled hard/soft ferromagnet system, such as NiFe/SmCo and NiFe/SmFe, to vary the magnetic environment near the superconducting layer. The switching behavior of both NiFe/SmCo and NiFe/SmFe systems were studied. In NiFe/SmFe systems, the reversible switching of the soft-layer was sharper, and occurred over a much smaller applied magnetic field range, than for NiFe/SmCo. The magnetic field required to achieve soft-layer saturation was also an order of magnitude smaller, making it more responsive to weaker applied fields. The superconducting property of the hybrid system was investigated as functions of the temperature and magnetic field. [Preview Abstract] |
Session Q35: Focus Session: Iron Pnictides and Other Novel Superconductors X: Spectroscopy
Sponsoring Units: DCMPChair: Michelle Johannes, Naval Research Laboratory
Room: 405
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q35.00001: NMR Studies of Iron-Oxypnictide Superconductor LaFeAs(O$_{1-x}$F$_x$) Invited Speaker: We present experimental results of $^{75}$As and $^{139}$La nuclear magnetic resonance (NMR) in the layered oxypnictide system LaFeAs(O$_{1-x}$F$_x$) ($x$ = 0.0, 0.04, 0.07, 0.11 and 0.14) where superconductivity occurs in $x$ greater than 0.04 [1,2]. In the undoped LaFeAsO, $1/T_1$ of $^{139}$La exhibits a distinct peak at $T_N \sim$ 142 K below which the La-NMR spectra become broadened due to the internal magnetic field attributed to an antiferromagnetic (AFM) ordering[1]. In the $x$=0.04 sample, $1/T_1T$ of $^{75}$As exhibits a Curie-Weiss temperature dependence down to 30 K, suggesting the development of AFM spin fluctuations with decreasing temperature. The AFM fluctuations are significantly suppressed with F-doping, and pseudogap behavior is observed in $1/T_1T$ in the $x$=0.11 sample with a gap value of $\Delta_{PG} \sim$ 175 K[1]. The spin dynamics vary markedly with F-doping, which is ascribed to the change of the Fermi-surface structure by the electron doping. As for the superconducting properties for $x$=0.04, 0.07 and 0.11, $1/T_1$ of $^{75}$As in all compounds does not exhibit a coherence peak just below $T_c$ and follows a $T^3$ dependence at low temperatures. These results seemingly suggest that unconventional superconductivity with zero gap along lines, whereas the lack of the residual density of states at the low temperatures is incompatible with the presence of the line-nodes. We discuss similarity and difference between LaFeAs (O$_{1-x}$F$_x$) and cuprates, and also discuss the relationship between spin dynamics and superconductivity on the basis of F-doping dependence of $T_c$ and $1/T_1$[2]. \\[4pt] [1] Y. Nakai, K. Ishida, Y. Kamihara, M. Hirano, and H. Hosono; J. Phys. Soc. Jpn. 77, 073701 (2008). \\[0pt] [2] Y. Nakai, S. Kitagawa, K. Ishida, Y. Kamihara, M. Hirano, and H. Hosono; cond/mat 0810.3569. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q35.00002: Electron itinerancy and Strong Itinerant Spin Fluctuations in the Normal State of CeFeAsO$_{0.89}$F$_{0.11}$ Iron-Oxypnictides F. Bondino, E. Magnano, M. Malvestuto, F. Parmigiani, M.A. McGuire, A.S. Sefat, B.C. Sales, R. Jin, D. Mandrus, E.W. Plummer, D.J. Singh, N. Mannella The recent discovery of high-temperature superconductivity in iron-oxypnictides and related materials has generated enormous excitement in the community. The electronic structure of the normal state of CeFeAsO$_{0.89}$F$_{0.11}$ has been measured with photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS). The Fe XAS and PES spectra do not display satellite features commonly found in the Cu spectra of cuprates HTCS, indicative of the absence of strong electron correlation and localization effects in the electronic structure. In sharp contrast to the cuprates HTSC, the Fe XAS and PES spectra exhibit spectral signatures which are typical of delocalized, itinerant electrons. The Fe 3$s$ spectra show exchange multiplets due to the coupling of the final Fe 3$s$ core hole state with the conduction band states, indicative of the presence of fluctuating spin moments on the Fe sites. These findings indicate that the FeSC must be considered a new class of materials, quite unlike the cuprate HTSC or conventional BCS superconductors [F. Bondino et al. http://arxiv.org/abs/0807.3781 (arXiv:0807.3781)]. [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q35.00003: Core Level and Valence Band Studies of the Novel Iron Pnictide Superconductors Daniel Garcia, Chris Jozwiak, Choonkyu Hwang, Alexei Fedorov, Stephen Hanrahan, Steven Wilson, Costel Rotundu, Byron Freelon, Robert Birgeneau, Edith Bourret-Courchesne, Alessandra Lanzara Towards understanding the physics of the superconducting iron pnictides, critical information can be gained through exploring the electronic structure of these novel materials. We have used photoemission spectroscopy to study the LaFeAsO$_{1-x}$F$_{x}$ and the PrFeAsO$_{1-x}$F$_{x}$ superconductor. The evolution of valence band density of states, hybridization energy, Fermi surface topology and many body interaction are presented as a function of doping, photon energy and temperature. We explore the significance of these results to the question of electron correlation and spin physics in these Fe-based superconductors. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q35.00004: Core-level study of high-temperature superconductivity iron arsenide (Ba$_{1-x}$K$_{x})$Fe$_{2}$As$_{2}$. Yi Li, Haizhong Guo, Jiandi Zhang, Darwin Urbina, H. Ding, Genfu Chen, N.L. Wang We have used high-resolution x-ray photoemission spectroscopy (XPS) techniques to investigate the core-level x-ray photoemission spectra for high-temperature superconductivitor iron arsenide (Ba$_{1-x}$K$_{x})$Fe$_{2}$As$_{2}$ ($T_{C}$ = 32 K) and its parent compound BaFe$_{2}$As$_{2}$. One important issue for understanding the nature of the superconductivity in the compound is the determination of the electron-electron correlation in the system which in principle should be reflected by the appearance of core-level satellites. We have measured the temperature-dependence of Fe-$2p$ and $3s$ core-level spectrum in both parent and doped superconductor compounds and found that the core-level electronic structure is quite different from that observed in cuprates. The origin and nature of the core spectra in these iron-based materials will be discussed. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q35.00005: $^{75}$As NMR study of spin-spin relaxation and impurity effects in (Ba,K)Fe$_2$As$_2$ and CaFe$_2$As$_2$ S. Mukhopadhyay, S. Oh, A.M. Mounce, M. Lee, W.P. Halperin, A.P. Reyes, P. Kuhns, P.C. Canfield, N. Ni, S. Bud'ko We report here NMR measurements on Ba$_{0.55}$K$_{0.45}$Fe$_2$As$_2$ single crystals ($T_c \sim 30 $ K) grown from Sn flux. The variations of the NMR line widths and shifts with temperature (40 -- 150 K) and with magnetic field (6.4 -- 14 T) are indicative of the presence of local magnetic impurities in these crystals, in contrast with crystals of CaFe$_2$As$_2$. Both the shift and line width are linearly dependent on the bulk magnetization and are sufficiently large that the impurity magnetic moments must be coupled through the hyperfine interaction to the As nuclei. However, this coupling is somewhat weaker than for impurities in YBCO and $^{17}$O NMR~[1]. Measurements of the spin-spin relaxation from the Hahn echo decay envelope and CPMG sequences indicate slow magnetic fluctuations, $\approx 350$ Hz, whose origin will be discussed.\\[3pt] [1] J.~Bobroff. Ph. D. Thesis, Universite De Paris-Sud (1997) [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q35.00006: NMR study of the FeAs parent compounds, AFe$_2$As$_2$ (A=Ba,Ca) Eric Bauer, Seung-Ho Baek, Nicholas Curro, Filip Ronning, Joe Thompson We present $^{75}$As NMR results of the FeAs 122 parent compounds, AFe$_2$As$_2$ (A=Ba,Ca) single crystals. For BaFe$_2$As$_2$, we find that Sn impurities in the single crystal dramatically alter the low energy spin fluctuations and suppress the ordering temperature from 138 K to 85 K, and that the temperature dependence of the $^{75}$As NMR spectra and spin lattice relaxation rates reveal a second order phase transition to a state of incommensurate magnetic order. On the other hand, CaFe$_2$As$_2$ shows a commensurate first order magnetic transition which is coupled to the structural transition. By comparing the two compounds, we show that the static and dynamic properties of the FeAs systems is extremely sensitive to the microscopic out-of-plane structure in microscopic level. Our results may shed light on the superconductivity observed under pressure. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q35.00007: NMR relaxation rate in the FeAs superconductors Meera Parish, Jiangping Hu, B. Andrei Bernevig We consider how different symmetries of the superconducting order parameter will affect the NMR spin relaxation rate in the newly discovered iron-based superconductors. We particularly focus on a nodeless order parameter of unconventional extended s-wave symmetry, which changes sign between the electron and hole Fermi surfaces. Using a two-band model, we show that the extended s-wave order parameter is consistent with the results of recent NMR measurements, which exhibit a characteristic $T^3$ dependence of the NMR spin relaxation rate, only if the inter-band contribution dominates the response. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q35.00008: Infrared properties of Sr(Fe,Ni)2As2 superconducting single crystals. Kevin Kirshenbaum , A.B. Sushkov, S.R. Saha, N.P. Butch, J. Paglione, H.D. Drew We report on temperature dependence of the bulk single crystal reflectivity and transmission of thin crystals of iron pnictides Sr(Fe,Ni)$_2$As$_2$ in the broad frequency range from far infrared to UV. We will discuss our data in comparison to results of other experiments and theory. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q35.00009: Nonequilibrium quasiparticle dynamics in single crystals of Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ Darius Torchinsky, G.F. Chen, J.L. Luo, N.L. Wang, Nuh Gedik We report on measurements of the quasiparticle dynamics in single-crystals of Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ in the superconducting state via ultrafast pump-probe spectroscopy. Quasiparticles are injected into the samples by ultrashort laser pulses. Once injected, they cause a reflectivity change of the sample at the laser frequency, allowing time-resolved optical measurements of their density. We describe the temperature and excitation density dependence of the quasiparticle recombination rate and discuss the implications of these measurements on the nature of the superconducting gap and the electron-phonon coupling in these materials. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q35.00010: Charge dynamics of the spin-density-wave state in BaFe$_2$As$_2$ F. Pfuner, L. Degiorgi, J.G. Analytis, J.-H. Chu, I.R. Fisher We report on a thorough optical investigation of BaFe$_2$As$_2$ over a broad spectral range and as a function of temperature, focusing our attention on its spin- density-wave (SDW) phase transition at $T_{SDW}=135$ K. While BaFe$_2$As$_2$ remains metallic at all temperatures, we observe a depletion in the far infrared energy interval of the optical conductivity below $T_{SDW}$, ascribed to the formation of a pseudogap-like feature in the excitation spectrum. This is accompanied by the narrowing of the Drude term consistent with the $dc$ transport results and suggestive of suppression of scattering channels in the SDW state. About 30\% of the spectral weight in the far-infrared energy interval of the optical conductivity is affected by the SDW phase transition. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q35.00011: Temperature Dependent Local Structure of LaO$_{1-x}$F$_{x}$FeAs T.A. Tyson, T. Wu, J. Woicik, B. Ravel, A. Ignatov, C. Zhang, Z. Qin, T. Zhou, S.-W. Cheong The local structure of the parent and doped LaO$_{1-x}$F$_{x}$FeAs compounds were studied by x-ray absorption spectroscopy. The Fe-As correlations are well modeled by an Einstein model with no low temperature anomalies. While the Einstein temperatures are identical for the doped (11{\%}) and undoped samples, the doped sample is found to have a lower level of static disorder in the Fe-As distribution. For the Fe-Fe correlation, doping enhances the effective Einstein temperature. Comparisons with the temperature dependent structure of the simpler FeSe$_{0.88}$ systems will be made. This work is supported by DOE Grant DE-FG02-07ER46402. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q35.00012: Exploring Local Density of States in FeSe-based Superconductors D. Zhang, C.S. Ting Starting from two Fe ions per unit cell and two degenerate orbitals per Fe ion, we construct an effective four-band model for the FeSe-based superconductors, which Fermi surfaces are consistent with those from the ARPES experiments and LDA calculations. The hole pockets at $(0,0)$ and the electron pockets at $(\pi,\pi)$ are controlled by hopping between the same orbitals on the nearest and second neighboring sites while the intervals between the inner and outer Fermi surfaces around $(0,0)$ and $(\pi,\pi)$ are determined by hopping between different orbitals on the same sublattice. On the basis of the mean field theory for superconductivity, we also investigate the local density of states for different pairing symmetries and compare them with the recent STM experiments. [Preview Abstract] |
Session Q36: Carbon Nanotubes and Other Nanostructured Materials: Sensing, Transport, and Optical Properties
Sponsoring Units: DCMPChair: Zhigang Jiang, Georgia Tech
Room: 408
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q36.00001: Effect of Localized Covalently Attached Oxygen Functionalization on the Transport Properties of Metallic Carbon Nanotubes Md. Ashraf, Nicolas Bruque, Rajeev Pandey, Philip Collins, Roger Lake We perform of a comprehensive study of the effect of covalent oxygen attachment on the transmission of metallic carbon nanotubes (CNTs). Oxygen attachment on the CNT surface favors an ether type bond. Two oxygen atoms attached on the CNT surface within the same carbon ring on parallel bonds are energetically the most stable attachment configuration. In an armchair CNT, oxygen attachment favors the C-C bonds orthogonal to the CNT axis. Correlated addition propagates axially along parallel orthogonal bonds. In a zigzag CNT, oxygen attachment prefers the slanted bond, and correlated addition propagates spirally along parallel slanted bonds. Closely spaced oxygen attachment on the armchair and zigzag CNT surfaces causes a dip in transmission symmetrically away from the Fermi level. A clustered group of oxygen atoms covalently attached to a single-walled metallic zigzag CNT can result in a one order of magnitude drop in transmission that is asymmetric with respect to the Fermi energy resulting in a qualitative resemblance to conductance versus gate voltage curves observed experimentally [Science 315, 77 (2007)]. Calculations use density functional theory combined with non-equilibrium Green functions. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q36.00002: Molecular Modifications of the Electronic Characteristics of Carbon Nanotube Field-Effect Transistors Kan-Sheng Chen, P. Xiong, S.A. McGill We report a systematic examination of the effects of an organic self-assembled monolayer at the electrode/nanotube interface on the key electrical properties, including on/off ratio, subtheshold swing, and threshold voltage, of single-walled carbon nanotube field-effect transistors (SWNT-FETs). For the study, we utilize a unique device structure made of three adjacent Au electrodes, defined by electron-beam lithography, on doped-Si/SiO$_{2}$ substrates. A SAM of thiol molecules was formed selectively on one of side electrodes by dip-pen nanolithography. A single SWNT was then directed to assemble across the three electrodes, forming two FETs of essentially identical structure with the difference of the presence of the SAM at one of the electrodes of one FET. The device enables a direct unambiguous comparison of the electrical characteristics of the two SWNT-FETs with and without molecular modification of a Au electrode. We will present results of modification by molecules of different end-groups. Specifically, when one electrode was covered with 16-Mercaptohexandecanoic acid (MHA), a polar molecule, we observed a significant increase of the on/off ratio, decrease of the subthreshold swing, and shift of the threshold voltage for the SWNT-FET. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q36.00003: Electrochemically mediated charge transfer effects in Single Walled Carbon Nanotubes (SWCNTS) Buddhika Abeyweera, Sharvil Desai, Gamini Sumanasekera Electrochemically mediated charge transfer has been studied by its effect on the surface conductivity of diamond. Here we show that the effect is not restricted to diamond, but may occur in other material systems as well, for example, semiconducting single-walled carbon nanotubes (s-SWNTs). For s-SWNTs the electron energies of the redox couple involving oxygen lie within or below the band gap. Nanotubes are known to be inherently n-type in vacuum and p-type under ambient conditions. Systematically sweeping the Fermi energy by controlled removal of oxygen through the redox couple and exposing to moist oxygen and ammonia with controlled pH, the charge transfer effect of the SWNTs was studied by \textit{in situ} monitoring of the Thermoelectric power (S) and Resistance (R). The changes in both R and S correlate very well with the relative position of the Fermi level with respect to the equilibrium chemical potential of the electrons. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q36.00004: Does Moisture Influence the Chemical Detection of Gas Molecules Adsorbed on Single-Wall Carbon Nanotubes? Ming Yu, W.Q. Tian, C.S. Jayanthi, S.Y. Wu In this work, the role of water in the detection of hydrazine (N$_{2}$H$_{4})$ by a single-wall carbon nanotube (SWCNT) is investigated using first principles electronic structure calculations (DFT/GGA--USPP)[1]. This calculation is undertaken to interpret the experimental resistivity measurements for N$_{2}$H$_{4}$ adsorbed on SWCNT that reveal an $n$-type behavior [2]. Our preliminary theoretical studies of the adsorption of N$_{2}$H$_{4}$ on SWCNT revealed physisorption for N$_{2}$H$_{4}$ and an unaltered band structure for the SWCNT [3]. This prompted us to look into the role of water on the bonding of N$_{2}$H$_{4}$ to the SWCNT. We found that, by introducing a monolayer of water film on the (8,0) SWCNT, the adsorption of N$_{2}$H$_{4}$ can introduce occupied states near the Fermi level, exhibiting an $n$-type behavior. However, the introduction of just few water molecules was not sufficient to influence the electronic structure of N$_{2}$H$_{4}$/SWCNT. Presently, we are studying the influence of water films on the chemical detection of a variety of other gas molecules (N$_{2}$, NH$_{3, }$\textit{etc}.) by SWCNTs, and the results from such studies will also be reported. [1]. G. Kresse \textit{et al}. Phys. Rev. B \textbf{54}, 11169 (1996). [2]. S. Desai, \textit{et al}. (APS, March 2008). [3]. M. Yu, \textit{et al}. (APS, March 2008). [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q36.00005: ``Nanocavity'' Biochemical Sensor T. Kirkpatrick, J.I. Oh, P. Dhakal, D. Cai, H.Z. Zhao, A. Cimeno, L. Ren, K. Kempa, Z.F. Ren, T.C. Chiles, M.J. Naughton A biochemical sensor, based on a coaxial ``nanocavity,'' is described. The sensor is capable of detecting small changes in complex impedance, resulting from the presence of target entities in, and near, the device. Its nanoscale dimensions can be tuned for size-specificity, and its constituent components functionalized, for biochemical specificity. The measured capacitance of the bare sensor is in good agreement with calculations for a parallel array of 10$^{6}$/mm$^{2}$ vertically orientated coaxial capacitors. Here, changes in the complex impedance of the sensor are reported in the presence of various organic solvents. Molecules entering the coax annuli result in a significant change in impedance as a function of time, frequency and concentration. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q36.00006: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q36.00007: Optimizing the signal-to-noise ratio for biosensing with carbon nanotube transistors Iddo Heller, Jaan Mannik, Serge G. Lemay, Cees Dekker The signal-to-noise ratio (SNR) for real-time biosensing with liquid-gated carbon nanotube transistors is crucial for exploring the limits of their sensitivity, but has not been studied thus far. We show that, surprisingly, the maximum SNR is obtained when the device is operated in the sub-threshold regime. In the ON-state, additional contributions to the noise lead to a reduction of the SNR by up to a factor of 5. For devices with passivated contact regions, the SNR in ON-state is even further reduced than for bare devices. Interestingly, a conventional back gate provides a handle to improve the SNR in ON-state by increasing the conductivity of the contact regions. The results presented here demonstrate that biosensing experiments can best be performed in the sub-threshold regime for optimal SNR. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q36.00008: Scanning Tunneling Microscopy of DNA-Carbon Nanotube Hybrids Dzmitry Yarotski, Svetlana Kilina, Alec Talin, Alexander Balatsky, Sergei Tretiak, Antoinette Taylor Production of carbon nanotube-based (CNT) devices holds a great promise for bringing the size of electronic circuits down to molecular scales. Recently, yet another step has been made towards achieving this goal by developing a new method for metal-semiconductor CNT separation, which relies on wrapping the CNT with ssDNA molecule[1]. Though it was shown that the outcome of the separation process strongly depends on the DNA sequence, further investigations have to be conducted to determine detailed structure of the hybrids and their electronic properties. Here, we use STM to characterize structural and electronic properties of the CNT-DNA hybrids and compare experimental results to theoretical calculations. STM images reveal 3.3 nm DNA coiling period, which agrees very well with the theoretical predictions. Additional width modulations with characteristic lengths of 1.9 and 2.6 nm are observed along the molecule itself. Although scanning tunneling microscopy confirms the presence of DNA in the hybrid and visualizes its structure, further experimental work is required to reveal the dependence of electronic properties of hybrids on their internal structure. [1] M. Zheng et al., Science 302, 1545 (2004). [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q36.00009: Electron Transport in Quasi-1D, DNA-Templated Nanoparticle Arrays M. S. Fairbanks, G. J. Kearns, B. C. Scannell, A. Loftus, R. P. Taylor, J. E. Hutchison Devices based on self-assembled metal nanoparticle arrays are good model systems for investigating the physics of the nanoscale regime, where size quantization effects and the Coulomb charging energy can dominate transport even at high temperatures. We apply a novel, highly parallel fabrication technique [1] that creates quasi-1D (200 nm x $\sim $20 nm) arrays of Au nanoparticles (r = 1.8 nm) bonded to DNA between predefined electrodes. These devices are found to exhibit Coulomb blockade over a wide range of operating temperatures (0.24 K to $>$80 K). We present an analysis of our results in comparison to recent theoretical predictions for 1- and 2D tunnel junction arrays and highlight some effects that arise from our devices' particular geometry. [1] M. G. Warner, J. E. Hutchison. Nature Materials 2, 272 (2003).; G. J. Kearns, et al. to be published. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q36.00010: Determining the optical modes of solid and core-shell nanowires using relativistic electrons Jerome Hyun, Mark Levendorf, Martin Blood-Forsythe, Jiwoong Park, David Muller Nanowires serve as building-blocks for miniaturized optoelectronic devices. Determining the dispersion properties of the nanowires is necessary for device-engineering, but can be experimentally difficult with conventional optical techniques because of fundamental diffraction limitations. Fast electrons, on the other hand, can be focused to nanometer or sub-nanometer probes, providing spatial resolutions far superior to existing optical techniques. The time-varying Fourier components of the electron's evanescent electric field can extend beyond the far ultra-violet regime, providing a near-field, broad-band light source. Using scanning transmission electron microscopy and electron energy loss spectroscopy, we report on the relativistic calculations and measurements of the optical eigenmodes of single Ge nanowires. We also present calculations of a dielectric core/metallic shell system, where couplings between the surface plasmonic modes and the cavity modes occur. The work demonstrates a powerful optical characterization solution for nanowire systems. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q36.00011: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q36.00012: Transport properties of transition metal impurities on gold nanowires Renato B. Pontes, Edison Z. da Silva, Adalberto Fazzio, Ant\^{o}nio J. R. da Silva Performing first principles density functional theory (DFT) we calculated the electronic and transport properties of a Au thin nanowire with transition metal atoms (Mn, Fe, Ni or Co) bridging the two sides of the Au nanowire. We will show that these systems have strong spin dependent transport properties and that the local symmetry can dramatically change them, leading to a significant spin polarized conductance. This spin dependent transport is also associated with the transition metal in the nanowire, in particular with the d-level positioning. Using Co, for example [1], when the symmetry permits the mixing between the wire s-orbitals with the transition metal d-states, there are interference effects that resemble Fano-like resonances with an anisotropy of 0.07 at the Fermi level. On the other hand, if this symmetry decouples such states, we simply have a sum of independent transmission channels and the calculated anisotropy was 0.23. The anisotropies for the other transition metals, as well as calculated transmittances for two Co impurities will also be presented \textbf{[1] }R. B. Pontes, E. Z. da Silva, A. Fazzio and Ant\^{o}nio J. R. da Silva, J. Am. Chem. Soc. 130 (30), 9897-903, 2008 [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q36.00013: Quantum simulation of four-probe measurement of carbon nanotube Asako Terasawa, Keiji Tobimatsu, Tomofumi Tada, Satoshi Watanabe, Keiji Tobimatsu The four-probe method is widely used to measure the intrinsic resistance of various materials without the effects of sample-probe contacts. Recently, there have been many attempts to apply this method to nanoscale objects. Also, anomalous behaviors of nanoscale four-probe measurements were reported such as the negative four-probe resistance of single-walled carbon nanotube. To investigate quantum effects on the four-probe measurements in nanoscale, we examined the four-probe resistance of (5,5)-carbon nanotube with a vacancy or without a vacancy theoretically on the basis of density functional tight-binding method and Green's function method. We found that the calculated four-probe resistance is sensitive to the position of the vacancy relative to the probes even when the sample-probe connections are weak. Such a behavior is unlikely to be seen in the two-probe resistance, and suggests that the four-probe resistance of nanoscale systems depend on the sample-probe geometry in a complicated manner. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q36.00014: Coherent Acoustic Spectroscopy of Nanorod Arrays Masashi Yamaguchi, Jianxun Liu, Dexian Ye, Toh-Ming Lu Coherent acoustic transport through vertically grown nanorod array on substrate and coherent acoustic vibration of nanorod arrays are experimentally studied by using femtosecond laser based acoustic spectroscopy in GHz-THz frequency range. In nanorod materials, acoustic phonon dispersion and life time are altered by the spatial confinement, and many of nanoscale materials have mechanical resonance in GHz-THz frequency range. Si and Ni nanorod arrays are grown vertical on top of a thin Al transducer layer deposited on substrate. Longitudinal acoustic phonons are excited coherently in the transducer layer. We have observed the transport of the coherent acoustic pulses through the nanorod arrays. The center frequency of the acoustic pulse was comparable to the diameter of the nanorods. Also, we have observed the transfer of the acoustic energy to the nanorod vibration mode while the coherent acoustic pulse propagates through. Mean-Free-Path and time of flight of the coherent acoustic phonons is determined by comparing the results of the samples with different thicknesses. [Preview Abstract] |
Session Q37: Focus Session: Structure and Dynamics of Interfacial Water II
Sponsoring Units: DCPChair: Jin Zhao, University of Pittsburgh
Room: 409
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q37.00001: Surveying the potential landscapes controlling the accommodation of excess electrons by water networks Invited Speaker: We present recent results of a new experimental approach where we use ir-ir pump-probe methods to measure the transition states and relative energies of isomers associated with the negatively charged water clusters. First, the vibrational spectra of various isomers are systematically disentangled using hole-burning Ar predissociation spectroscopy in a triple-stage time-of-flight mass spectrometer. We then monitor the spectra of fragment ions that are created by photoevaporation of Ar atoms through the various vibrational levels identified in the spectroscopic step. The major conclusion is the weak binding isomers are readily transformed into more strongly bound forms, while the reverse process is very inefficient. We describe progress on using this strategy to identify the large amplitude motions available in the finite systems using trace isotopes. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:27PM |
Q37.00002: X-ray Studies of Hydrogen Bonding of Water; the Liquid Phase and on Surfaces Invited Speaker: Many of the unique properties of water have been interpreted as due either to a continuum of distorted mainly near-tetrahedral hydrogen bond or a mixture of differently coordinated distinct species. Here we report high-resolution measurements of small-angle x-ray scattering and x-ray emission spectroscopy from liquid water at ambient conditions. A model based on 10-20 {\AA} diameter size density fluctuations with energetically strongly tetrahedral bonded water in a soup of disordered entropic structures with large hydrogen bond distortion can explain the spectroscopy and scattering data. I will also address fundamental questions regarding geometric structure, electronic structure, nature of surface chemical and hydrogen bonding and reactivity of water on surfaces. The connection between studies performed at both UHV and ambient conditions will be emphasized. Several examples of different water adsorption system will be illustrated such as Pt(111), Ru(001), Cu(110), Cu(111), TiO$_{2}$, Fe$_{2}$O$_{3}$ and MgO. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 1:03PM |
Q37.00003: Femtosecond Sum-Frequency Generation Studies of the Structure and Dynamics of Interfacial Water Invited Speaker: Water interfaces play a central role in a wide variety of disciplines including electrochemistry, (photo-) catalysis and biophysics. Knowledge of the details of water interfacial structure is thus essential both for a fundamental understanding of this ubiquitous liquid and for a basic understanding of the many systems in which aqueous interfaces play a key role. Although considerable progress has been made in understanding of bulk water, substantially less progress has been made at the interface. Here we report a series of surface specific studies of various water interfaces using surface-specific vibrational spectroscopies, both in equilibrium and on ultrafast (femtosecond) time scales. Our approach allows us to selectively investigate the one monolayer of water molecules at the different water interfaces. Water is characterized through its O-H stretch vibration. We find that interfacial hydrogen bonding depends strongly in the type of interface. Remarkably, for the water-air interface, interfacial hydrogen bonding is very similar to that occurring in bulk. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q37.00004: Ultrafast response of water near hydrophobic and hydrophilic surfaces Chang-Ki Min, Juan Guan, Sung Chul Bae, David Cahill, Steve Granick Ultrafast response of water near hydrophobic and hydrophilic surfaces is investigated by femtosecond pump-probe ellipsometry. Pump and probe pulses are from a dual Ti:sapphire laser with stable difference repetition rate. Every difference repetition rate, time delay is swept the whole pulse-to-pulse interval without an optical delay stage. Pump pulses induce heating and acoustic vibration to a Pd surface. The Pd surface is modified by thiol chemistry. Thiols with --OH and -CH3 end groups generate uniform hydrophilic and hydrophobic surfaces, and these mixtures modulate hydrophobicity two-dimensionally. Probe pulses with circular polarization impinge at the Brewster's angle and are analyzed by a polarizer. The transient ellipticity shows a refractive index change of water by thermal conductance and novel insight into the peculiar qualities of interfacial water. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q37.00005: Dynamics of Water Confined in Partially Hydrophobic Nanosized Cylindrical Sieves Antonio Faraone, Yang Zhang, Kao-Hsiang Liu, Chung-Yuan Mou, Sow-Hsin Chen Using three high resolution quasielastic neutron scattering spectrometers we have investigated the single particle dynamics of water confined in a hydrophobically modified MCM-41-S sample. This latter is a silica matrix containing cylindrical sieves with diameter $<$ 20 {\AA} arranged in a hexagonal geometry. In the hydrophobically modified sample some of the sylanol groups in the pores' wall have been substituted with methanol groups resulting in a partially hydrophobic confining surface, which could be used as a model system. We have been able to analyze the data in the temperature range from 300 K to 210 K using a single consistent model, the Relaxing Cage Model (RCM) for the dynamics of supercooled water. Because of the heterogenous environment experienced by the water molecules in the pores, the relaxational dynamics show a broad distribution of relaxation times. However, the Fickian diffusive behaviour is retained. The obtained results help clarify the role that the chemical interaction between the water molecules and the walls of the confining host plays in determining the characteristics of the water dynamics, as compared to purely geometric constraints such as the size and shape of the pores. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q37.00006: Reconstructing the dynamic hydration around hydrophobic molecular solutes using inelastic x-ray scattering R. Coridan, N. Schmidt, G.H. Lai, P. Abbamonte, G.C.L. Wong, M. Marucho, N. Baker We combine inelastic x-ray scattering (IXS) data and liquid-state theory to image the dynamical hydration structure of water solvating molecular, hydrophobic solutes. Using `linear response imaging', we computationally reconstruct the {\AA}-scale spatial and fs-scale temporal evolution of density fluctuations in water using IXS. The imaginary part of density propagator $\chi $(q,$\omega )$ is directly extracted from the IXS data, and the real part recovered using Kramers-Kronig relations. The resultant complex-valued $\chi $(q,$\omega )$ is the Fourier transform of the density-density response function $\chi $(r,t) which measures the dynamical density fluctuations of water due to a point-like instantaneous pulse. We have shown that this propagator can be used to reconstruct the dynamical hydration around prototypical charge distributions. To extend this technique to more realistic solutes, Ornstein-Zernike integral equations from liquid state theory are used to study hydration around small excluded volumes. We will present results for simple geometries and discuss the implications of combining exclusion and charge. [Preview Abstract] |
Session Q38: Focus Session: Nanomaterials for Energy Applications II
Sponsoring Units: DCPChair: Geoff Hutchison, University of Pittsburgh
Room: 410
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q38.00001: Nanostructured Multimetallic Catalysts in Fuel Cells Invited Speaker: There are two major driving forces for the global interests in research and development of fuel cells: the reality that fossil fuels are running out and the increasing environmental concern over pollution from using fossil fuels. Fuel cells utilizing hydrogen as fuels represent an important form of energy because hydrogen is a highly-efficient fuel and it is environmentally clean. Fuel cells such as proton exchange membrane fuel cell and direct methanol fuel cell are attractive because of their high conversion efficiency, low pollution, lightweight, and high power density. However, one of the important challenges for fuel cell commercialization is the preparation of active, robust and low-cost catalyst, which is key component in fuel cells counting for $\sim $30{\%} of the cost in manufacturing fuel cells. The durability of the catalysts can also be compromised by sintering and dissolution, especially at high electrode potentials or under load-cycling. We have been developing nanotechnological approaches and investigating nanostructured materials to address some of the fundamental issues in terms of catalyst activity, stability and cost. This presentation discusses recent findings of our investigations of the synthesis and processing for nanostructured catalysts with controlled size, composition, and surface properties by highlighting a few examples of bimetallic/trimetallic nanoparticles and supported catalysts. The results from the characterization of the nanoparticles and catalysts using an array of techniques and computational modeling will be discussed. The synergistic properties of the nanostructured materials in fuel cell reactions, including electrocatalytic methanol oxidation reaction and oxygen reduction reaction, will also be discussed, along with current challenges and opportunities. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:27PM |
Q38.00002: Reticular chemistry for clean energy Invited Speaker: Linking molecular building blocks by strong bonds to make networks (Reticular Chemistry) has yielded a number of new classes of materials such as metal-organic frameworks, zeolitic imidazolate frameworks and covalent organic frameworks. These are new classes of porous materials in which inorganic 'joints' are linked by organic 'struts' to give extended structures with surface areas greater than 5000 m2/g. Their ultra-high surface area is useful in storing hydrogen and natural gas, and for capturing carbon dioxide. Recently we have shown that MOFs can be quite effective as air purification and capture of harmful gases. This presentation will highlight the milestones and future prospects of this new field [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 1:03PM |
Q38.00003: Chemical tools for creating energy-relevant nanomaterials Invited Speaker: An important pre-requisite for using nanoscale materials in energy-related applications is the ability to make them on-demand and to rigid and pre-determined standards. For example, creating nanoscale solids with controllable composition, crystal structure, size, morphology, and surface chemistry is necessary for optimizing and fine-tuning their properties, as well as spatially organizing them and interfacing them with other components in a device. This talk will summarize our efforts to controllably and rationally synthesize shape-controlled nanocrystals of complex multi-element metallic and semiconducting materials. Collectively, these results provide reliable and predictable guidelines for designing and synthesizing complex nanomaterials of solids that are typically viewed as challenging to make. The focus will be on applying these ideas to energy-relevant nanomaterials, including nanostructured superconductors with high critical fields, metal hydrides for hydrogen storage applications, nanoparticle catalysts relevant to fuel cells, and metal-based compounds for thermoelectric, battery, and photovoltaic applications. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q38.00004: Structural and electrochemical properties of V$_{2}$O$_{5}$ and Ag$_{x}$V$_{2}$O$_{5}$ nanowries prepared by template assisted method M.B. Sahana, C. Sudakar, R. Naik, V.M. Naik Vanadium pentaoxide and silver vanadium oxides are promising cathode materials for lithium ion battery as it allows easy intercalation/deintercalation of Li+ due its open layered structure. During Li+ intercalation energy is stored as chemical potential and during deintercalation the energy is released as electricity. Because of the large surface area nanostructured materials have enhanced energy storage capacity. We have prepared V$_{2}$O$_{5}$ and Ag$_{x}$V$_{2}$O$_{5}$ (x= 0.1, 0.5) nanowires by template assisted method using radiation track etched hydrophilic PC membrane. The nanowires were grown on ITO coated glass substrates for optical analysis and on stainless steel substrate for XRD, SEM, Raman and electrochemical measurements. The effects of Ag doping on the electrochemical properties of V$_{2}$O$_{5}$ nanowires were investigated using a three electrode cell with nanowries as working electrode and Li as counter and reference electrode and lithium perchlorate in propylene carbonate as the electrolyte. The electrochemical characteristics of V$_{2}$O$_{5}$ and Ag$_{x}$V$_{2}$O$_{5}$ nanowries such as lithium intercalation capacity, cyclic stability and diffusion coefficient will be presented. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q38.00005: Stable Room Temperature Hydrogen Storage in Titainum-Doped Silica. Jason Simmons, Taner Yildirim, Ahmad Hamaed, David Antonelli The optimum conditions for viable room temperature hydrogen storage require materials that possess isoteric heats of adsorption in between that of standard physisorbers and chemisorbers, typically in the $\sim $20-30~kJ/mol regime. It has been theoretically predicted that transition metal atoms incorporated onto high surface area materials could enable significant room temperature storage; herein we demonstrate a possible experimental proof of these predictions. Titanium(III) complexes are grafted onto porous silica hosts, then activated to generate sites for dissociative adsorption of hydrogen gas. Using a combination of sorption measurements and inelastic neutron scattering, we show that the activated titanium provides strong hydrogen binding sites at room temperature and that adsorbed hydrogen is stable for long periods of time at ambient conditions. Further, the hydrogen can be desorbed under mild processing conditions. Neutron vibrational spectra agree well with theoretically predicted vibrational modes of the Ti---H complex. These results represent an important step towards reversible room temperature hydrogen storage. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q38.00006: Universal Behavior of Core-Shell Preferences in Transition-Metal Nanoparticles Lin-Lin Wang, Duane D. Johnson Transition-metal, core-shell nanoparticles are becoming ubiquitous from (electro-) catalysis to biomedical applications, due to control of size, performance, biocompatibility, and cost. We investigate 66 binary core-shell nanoparticle systems (groups 8 to 11 in the periodic table) using density functional theory (DFT) and systematically explore their segregation energies to determine core-shell preferences. We find that core-shell preferences are described by two simple factors: (1) cohesive energy (related to vapor pressure) and (2) atomic size (quantified by Wigner-Seitz radius). Core-shell preferences determined from DFT segregation energies agree with all available observations, and predict others, which can be used for design purposes. We then provide a universal description of core-shell preference via tight-binding band-energy differences that (i) quantitatively reproduces the DFT segregation energies and (ii) confirms the electronic origins for core-shell behavior. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q38.00007: Catalytic Gold Nanoparticles on an Iron Oxide Surface: A Scanning Tunneling Microscopy/Spectroscopy Study Kwang Taeg Rim, Daejin Eom, Li Liu, Elena Stolyarova, Joan Raitano, Siu-Wei Chan, Maria Flytzani-Stepanopoulous, George Flynn We present a Scanning Tunneling Microscopy/Spectroscopy study of a model catalyst system consisting of supported gold nanoparticles on a reduced Fe$_{3}$O$_{4}$(111) surface in ultrahigh vacuum. Gold forms two electrically distinct types of nanoparticles on an iron oxide surface upon annealing a multilayer Au/Fe$_{3}$O$_{4}$(111). STS measurements show that large nanoparticles ($\sim $8nm) are metallic while single gold adatoms are bonded to the oxygen sites on the Fe$_{3}$O$_{4}$(111) surface. Site-specific adsorption at oxygen surface atoms and the size sensitive nature of the electronic structure (Coulomb blockade) suggest that Au adatoms are positively charged. When this Au/Fe$_{3}$O$_{4}$(111) catalyst system is dosed with CO at 260K, there is evidence for CO adsorption at gold adatom sites. These observations are consistent with the proposal that nonmetallic, positively charged, ``invisible'' Au particles are the catalytically active species for the water-gas-shift reaction on Au/metal oxide surfaces. http://clippercontrols.com/info/dielectric\_constants.html [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q38.00008: Intercalation dynamics in rechargeable batteries Liam Stanton, Martin Bazant We consider the ion intercalation of rechargeable battery electrode particles during charging (or discharging). We have developed a general phase-field model which incorporates entropic, enthalpic and elastic effects within the particle as well as the nonlinear chemical reactions at the particle- electrolyte interface. It is shown through linear stability analysis and numerical simulations that particle size and elastic effects will decrease or even eliminate both the spinodal region and the miscibility gap in the phase diagram. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q38.00009: Nanowire-based solar cell fabricated by nanosphere lithography Oki Gunawan, Supratik Guha Nanowire (NW) structures have been predicted to provide performance enhancement for solar cells due to improved light absorption [1] and (for radial $p-n$ junction geometry) improved carrier collection [2]. We report the development of NW-based solar cells fabricated using nanosphere lithography. This method provides a simple, scalable, low cost and high throughput technique to define large scale NW structures. The fabricated NW solar cells (0.25 $\mu $m diameter and 1.3 $\mu $m tall) on a $p$-Si (100) substrate show $\sim $30 {\%} higher short-circuit current and $\sim $4 {\%} higher open circuit voltage compared to the control cells (without any NWs) with baseline efficiency of 6.2 {\%}. The reflectance and quantum efficiency spectra reveal some advantages and shortcomings of the NW-based solar cell. This work marks some progress in the development of a scalable nanowire-based solar cell and highlights some key issues such as conformal-junction formation, surface passivation, and contact formation. [1] L. Hu and G. Chen, Nano Lett. 7, 3249 (2007). [2] B. M. Kayes \textit{et. al. }, J. Appl. Phys. 97, 114302 (2005). [Preview Abstract] |
Session Q39: Physical Mechanisms of Membrane Remodeling
Sponsoring Units: DBPChair: Markus Deserno, Carnegie Mellon
Room: 411
Wednesday, March 18, 2009 11:15AM - 11:51AM |
Q39.00001: How some proteins tubulate membranes Invited Speaker: Endocytosis, exocytosis, membrane transport between intracellular compartments, virus or toxin entry or exit out of the cell, all imply to deform membrane. Membrane deformation mechanisms of cell membranes by proteins are currently actively studied. Giant vesicles (GUV) are interesting model membrane systems because they are composed of a very limited number of components compared to cellular membranes. The deformations induced by the interaction with a specific protein or any other additional components to the system, can then be directly monitored and the deformation mechanism eventually understood. In this talk, we will focus on different tubular structures induced by proteins. We will show that the B-subunits of Shiga toxin or Cholera Toxin, binding to their lipid receptors, Gb3 or GM1 respectively, incorporated in GUV membrane, induce negative membrane curvature and form tubular invaginations, in absence of any other cellular machinery. Tubular structures can also be obtained when molecular motors walking along microtubules exert a pulling force on the membrane of GUV. The helicoidal assembly of dynamin, a protein involved \textit{in vivo} in membrane fission can also produce tubular structures. This assembly has been reconstituted around membrane nanotubes of controlled diameter; we will show that the initial tube diameter strongly influences dynamin polymerisation. In each case, a physical framework for understanding deformation mechanism will be presented [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q39.00002: Membrane Disruption Effects of antimicrobial Peptide Protegrin-1 Hao Wang, James Kindt Molecular dynamics simulations have been performed to better understand membrane disruption induced by antimicrobial peptide Protegrin-1 (PG-1). Two distinct setups were adopted for atomistic simulations for DMPC/PG-1 systems. One started from bilayered ribbons either with or without the PG-1 peptides embedded and another one started from random lipid mixtures in the presence of the PG-1 peptides. Line tensions deduced from the ribbon simulations were generally lower with the PG-1 peptides embedded in ribbon edge, which supports edge-active role of the peptides. The random mixtures self-assembled into various structures. Extended simulations are being carried out to investigate the relation between concentration of the PG-1 peptides and the resultant structures. Furthermore, coarse-grained models have been used to simulate larger DMPC bilayers with the PG-1 peptides embedded. The PG-1 peptides were found to self-assemble into clusters. However, pore formation was not observed within our simulation period up to 3 microseconds. (DMPC: 1,2-Dimyristoyl-sn-Glycero-3-Phosphocholine) [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q39.00003: Activation Dependent Organization of T Cell Membranes Martin Forstner, Bjoern Lillemeier, Mark Davis, Jay Groves We investigate the role of lipid anchor motifs in the micro-organization of T-cell plasma membranes. To that end we generated a combinatorial library of protein constructs by fusing different lipid-modification sites of lipid anchored proteins with one of two fluorescent proteins (EGFP and Cherry). Two of these constructs that encode either for myristoylation, palmitoylation, geranylation or glycosylphosphatidylinositol (GPI) elaboration were co-expressed and dual color fluorescence cross-correlation spectroscopy (FCCS) was used to exploit co{\-}movement as a signature of co-localization. We find that in living T cells most anchors only co-localize with themselves, while different anchors move independently from each other. This suggests that a variety of domains with different chemical compositions exist in the plasma membrane and that the lipid anchor structure plays a key role in domain- specific recruitment of proteins. Furthermore, the degree of aggregation is found to depend on the activation state of the T cells. Cholesterol depletion and actin-drug experiments indicate that both are involved in the dynamic organization of the T cell plasma membrane. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q39.00004: Blebs in Model Lipid Membranes M. Laradji, C.W. Harvey, E.J. Spangler, P.B. Sunil Kumar It is now widely recognized that biomembranes exhibit complex lateral heterogeneities. Among these are blebs, which are localized balloon-like membrane protrusions observed during cell apoptosis, necrosis, and cytokinesis. Despite the poorly understood mechanism of bleb formation and their physiological role, they recently received a renewed attention. In order to investigate the physical mechanism leading to bleb formation, we developed a model based on an implicit-solvent lipid membrane model with soft interactions recently proposed by us [J. Chem. Phys. {\bf 128}, 035102 (2008)]. The model also incorporates an explicit fluctuating polymer meshwork simulating a cytoskeleton, which is anchored to the membrane. Using systematic large-scale simulations of membranes with varying values of the lipid density, cytoskeleton grafting-sites density and cytoskeleton tension, we found that localized blebs are formed on the membrane exoplasmic side in the presence of mismatch between tensions of the bare membrane and cytoskeleton. The blebs are pinned by the cytoskeleton anchors, reminiscent to those observed in apoptotic cells. The distance between neighboring anchors determines the neck of a bleb. The remaining membrane surrounding the blebs stiffens to accommodate the tensed cytoskeleton. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 1:03PM |
Q39.00005: Membrane Fusion Proteins as Nanomachines Invited Speaker: Membrane fusion is key to fertilization, virus infection, and neurotransmission. Specific proteins work like nanomachines to stitch together fluid, yet highly ordered lipid bilayers. The energy gained from large exothermic conformational changes of these proteins is utilized to fuse lipid bilayers that do not fuse spontaneously. Structural studies using x-ray crystallography and NMR spectroscopy have yielded detailed information about architecture and inner workings of these molecular machines. The question now is: how is mechanical energy gained from such protein transformations harnessed to transform membrane topology? To answer this question, we have determined that a boomerang-shaped structure of the influenza fusion peptide is critical to generate a high-energy binding intermediate in the target membrane and to return the ``boomerang'' to its place of release near the viral membrane for completion of the fusion cycle. In presynaptic exocytosis, receptor and acceptor SNAREs are zippered to form a helical bundle that is arrested shortly before the membrane. Ca binding to interlocked synaptotagmin releases the fusion block. Structural NMR and single molecule fluorescence data are combined to arrive at and further refine this picture. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q39.00006: Miscibility Critical Points in Plasma Membranes Benjamin Machta, Sarah Veatch, Stefanos Papanikolaou, James P. Sethna Lipid bilayers surround all cells and are home to a host of proteins and lipids that mediate interactions between the cell and its environment. Recent experimental work has shown that simple membranes composed of three lipid components show complex phase behavior at temperatures in the physiological range. For example, two liquid phases and a gel or solid phase are seen, and a second order phase transition with Ising critical behavior can be reached at a boundary of the liquid-liquid coexistence region [1]. Surprisingly, membrane vesicles isolated from living cells can be tuned with a single parameter (temperature) to criticality [1]. This suggests that cell membranes in vivo sit near miscibility critical points, and may help explain some of the paradoxes associated with putative lipid rafts proposed in other experiments. Here we report on work mapping phase diagrams for the simple membranes utilizing NMR and microscopy data. In addition, we use canonical models of phase transitions to understand the qualitative features of the membranes. Finally we explore ideas from information theory and self organized criticality to understand how and why real cells maintain a membrane near criticality. [1] Honerkamp-Smith, Veatch, and Keller, Biochim Biophys Acta. 2008 (in press) [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q39.00007: Calcium-induced gel domains in bilayer -- more elusive than thought Dennis Discher, David Christian, Wouter Ellenbroek, Andrea Liu As a highly bioactive divalent cation, calcium can in principle crossbridge anionic groups and induce domain formation and rigidification, but past reports with lipid systems appear conflicted. We mix, as a robust model system, anionic and neutral polymer amphiphiles within vesicle and cylinder micelle morphologies and add calcium. Based on micro- measurements, calcium forms definitive crossbridges between the anionic amphiphiles, rigidifying the charged membranes across a fluid-gel transition and also leading to lateral phase separation without disrupting the assemblies. A systematic phase diagram shows that long-lived domains occur in a narrow region near the polyanion's p$K$'s. The phase behavior appears well described by a relatively simple model in which -- among electrostatic and entropic contributions -- counterion entropy outcompetes attractive crossbridging to drive remixing of the highly charged polyacid at high pH. Initial observations extend from polymers to a polyanionic lipid involved in cell signaling [phosphatidylinositol (4,5)-bisphosphate], highlighting both the generality and limits of calcium effects. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q39.00008: Nonequilibrium instabilities of membranes with multiple-state active proteins Hsuan-Yi Chen, Alexander Mikhailov We present a theoretical model for the dynamics of membranes containing active proteins that have several conformational states. The proteinss are active because their conformational transition rates depend on the strength of external energy source that drives the system out of equilibrium. We show that there exist several types of nonequilibrium phase transitions for a membrane with proteins that have typical transition rates and in-plane diffusion constant. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q39.00009: Physical Foundations of PTEN/Phosphoinositide Interaction Arne Gericke, Zhiping Jiang, Roberta E. Redfern, Edgar E. Kooijman, Alonzo H. Ross Phosphoinositides act as signaling molecules by recruiting critical effectors to specific subcellular membranes to regulate cell proliferation, apoptosis and cytoskeletal reorganization, which requires a tight regulation of phosphoinositide generation and turnover as well as a high degree of compartmentalization. PTEN is a phosphatase specific for the 3 position of the phosophoinositide ring that is deleted or mutated in many different disease states. PTEN association with membranes requires the interaction of its C2 domain with phosphatidylserine and the interaction of its N-terminal end with phosphatidylinositol-4,5-bisphophate (PI(4,5)P$_{2})$. We have investigated PTEN/PI(4,5)P$_{2}$ interaction and found that Lys13 is crucial for the observed binding. We also found that the presence of cholesterol enhances PTEN binding to mixed PI(4,5)P$_{2}$/POPC vesicles. Fluorescence microscopy experiments utilizing GUVs yielded results consistent with enhanced phosphoinositide domain formation in the presence of cholesterol. These experiments were accompanied by zeta potential measurements and solid state MAS $^{31}$P-NMR experiments aimed at investigating the ionization behavior of phosphoinositides. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q39.00010: Multiscale modeling of phospholipid bilayers: from explicit-solvent all-atom to solvent-free coarse-grained simulations Zun-Jing Wang, Markus Deserno To advance the efficiency of phospholipid bilayer simulations and permit the treatment of challenging phenomena on mesoscopic length- and time-scales, several solvent-free Coarse-Grained (CG) phospholipid bilayer models have been presented in the past ten years. Most of them were derived in a top-down scheme, aiming to qualitatively reproduce phase diagrams, bending and stretching rigidity, area per lipid, and the thickness of a generic bilayer in experiments. Here, we derive a bottom-up CG model of an implicit-solvent lipid bilayer by matching its structural and mechanical properties with that of a membrane in all-atom resolution. Besides preserving chemical specificity and quantitative accuracy, we expect to gain a more fundamental understanding of the relationship between the elastic, mechanical, and diffusive properties of implicit solvent bilayers and their underlying CG interaction potentials, specifically bonded and non-bonded forces, as well as the effective interactions replacing the solvent. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q39.00011: Effects of cholesterol and PIP2 on membrane domain formation Paul Janmey, Fitzroy Byfield, David Christian, Ilya Levental Lipid head group size, acyl chain saturation, the relative amounts of cholesterol, phospholipids and sphingolipids, and electrostatic effects due to highly charged anionic lipids such as phosphatidylinositol bisphosphate (PIP2) all contribute to the force balance that determines the conditions at which domains form as well as their size, shape and stability. Giant plasma membrane vesicles derived from intact cells reveal lipid phase separation in a system with appropriate biological complexity. Formation of liquid ordered domains large enough to visualize by light microscopy form under physiologically realistic conditions in cell-derived vesicles, and their dependence on cholesterol content and temperature are consistent with studies of purified lipids. Compared to the effects of cholesterol, PIP2 has a smaller but still significant effect on liquid ordered / liquid disordered domain formation, but compared to other lipids, PIP2 is much more strongly segregated in the liquid disordered domains, away from those enriched in cholesterol. These results suggest physical mechanisms by which the cell can rapidly alter local PIP2 concentration to trigger cellular signals. [Preview Abstract] |
Session Q40: Neural Computation
Sponsoring Units: DBPChair: John Beggs, Indiana University
Room: 412
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q40.00001: Deriving functional structure of neuronal networks from spike train data Sarah Feldt, Vaughn Hetrick, Joshua Berke, Michal Zochowski We present a novel algorithm for the detection of functional clusters in neural data. In contrast to many clustering techniques which convert functional interactions to topological distances to determine groupings, our algorithm directly utilizes the dynamics of the neurons to obtain functional groupings. No prior knowledge of the number of groups is needed, as the algorithm determines statistically significant clusters through a comparison to surrogate data sets. Additionally, we introduce a new synchronization measure and use this measure in the algorithm to observe known groupings in simulated data. We then apply our algorithm to experimental data obtained from the hippocampus of a freely moving mouse and show that it detects known changes in neural states associated with exploration and slow wave sleep. Finally, we show that the new synchronization measure can detect changes which are consistent with known neurophysiological processes involved in memory consolidation. [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q40.00002: Bursting dynamics of in vitro neural networks and their stimulation driven learning. Joon Ho Choi, June Hoan Kim, Kyoung J. Lee Recent studies have indicated that recurring neural ``bursts'' may play an essential role in neural information processing and memory. One key element of this hypothesis involves the translation of temporal patterns of stimuli into spatiotemporally distributed information. One ideal system to investigate this issue is cultured network of neurons grown on multi-electrode array (MEA). Based on such in vitro systems, we have investigated the changes incurred by extrinsic stimuli in the spontaneously recurring bursting activities. We have employed, in particular, two-channel paired, delayed, tetanic stimuli to evoke different patterns of bursting activities. Our preliminary data suggests that the neural network can exhibit some learning behavior. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q40.00003: Burst switching between incoherence and synchrony Nathan Crosby, Joseph Tranquillo Studies of coupled oscillators often use diffusive connections to ensure that the coupled quantity is conserved. Signals between neurons, however, are not diffusive and may propagate unattenuated throughout a network. We compare diffusive and synapse-like coupling of Hindmarsh-Rose (HMR) oscillators through numerical simulations. HMR parameters are tuned to either oscillate continuously or alternate bursts of oscillations and periods of quiescence. In diffusive coupling, two HMR units synchronize bursts and individual oscillations within a burst at nearly the same coupling strength. Synapse-like coupling, however, shows a new behavior, called burst switching, between incoherence and synchrony. For example, a bursting unit can entrain an oscillating unit of a different frequency during the burst but then force the oscillator into quiescence. Burst switching in various network topologies synchronizes inhomogeneous units for the duration of the burst, followed by a period of network quiescence and a return to incoherence. The summed activity resembles the progression of an epileptic seizure including the ``spike and wave'' at the transition from synchrony to quiescence. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q40.00004: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q40.00005: Trajectories through similarity space produced by local neocortical circuits John Beggs, Wei Chen, Jon Hobbs, Aonan Tang The dynamics found in local cortical networks strongly impact the types of computations they can perform. Major classes of cortical network models assume that spatio-temporal activity evolves with either ultra-stable, chaotic or neutral dynamics. While experimental evidence has demonstrated that repeatable activity states can exist in cortical networks, it is still unclear what the spatio-temporal dynamics near these states are. To accurately address this question, the trajectories of similar, but not identical, inputs must be quantified. We use 60 channel microelectrode arrays to measure spatio-temporal trajectories through similarity space at 4 ms resolution in organotypic cortical cultures and acute cortical slices. Here we show that while attractive, chaotic and neutral trajectories can exist in these networks, the average trajectory has a Lyapunov exponent near zero (0.01 $\pm $ 0.2, mean $\pm $ s.d.), indicating that neutral dynamics prevail. [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q40.00006: Synaptic weight distribution under spike-timing dependent plasticity Chun-Chung Chen, David Jasnow We consider a network of integrate-and-fire neurons with random connections driven by noise triggered firings. The synaptic weights between the neurons are allowed to evolve under spike-timing dependent plasticity rules with additive potentiation and multiplicative depression. Under realistic physiological parameters, the network was equilibrated with simulations for a physical time of days. For lower potentiation-to-depression ratio $w^{\star}$, the synaptic weights forms a unimodal distribution which decays for large weights following a power law with a strong negative exponent. The decay exponent increases with $w^{\star}$, and runaway synaptic weights were observed as the exponent approaches $-1$. In the stationary state under the plasticity, for low $w^{\star}$, triggering the firing of a single neuron in a quiet network typically leads to a bursting event that lasts for seconds in a small network of 32 neurons. For high $w^{\star}$, the induced activities can persist in the network indefinitely. A mean-field theory combined with a master equation describing the distribution of synaptic weights predicts a power-law regime under the small jump assumption of synaptic weight changes. The exponents of predicted power law depends on the deviation of the mean synaptic weight from the $w^{\star}$ parameter and is to be determined self-consistently. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q40.00007: Hebbian based learning with winner-take-all for spiking neural networks Ankur Gupta, Lyle Long Learning methods for spiking neural networks are not as well developed as the traditional neural networks that widely use back-propagation training. We propose and implement a Hebbian based learning method with winner-take-all competition for spiking neural networks. This approach is spike time dependent which makes it naturally well suited for a network of spiking neurons. Homeostasis with Hebbian learning is implemented which ensures stability and quicker learning. Homeostasis implies that the net sum of incoming weights associated with a neuron remains the same. Winner-take-all is also implemented for competitive learning between output neurons. We implemented this learning rule on a biologically based vision processing system that we are developing, and use layers of leaky integrate and fire neurons. The network when presented with 4 bars (or Gabor filters) of different orientation learns to recognize the bar orientations (or Gabor filters). After training, each output neuron learns to recognize a bar at specific orientation and responds by firing more vigorously to that bar and less vigorously to others. These neurons are found to have bell shaped tuning curves and are similar to the simple cells experimentally observed by Hubel and Wiesel in the striate cortex of cat and monkey. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q40.00008: Neural Decision Boundaries Predict Maximum Entropy Parameters Jeff Fitzgerald, Tatyana Sharpee Previous studies have shown that the response properties of neural networks can be well described by a pairwise maximum entropy model (PMEM). The coupling constants in this model can be calculated from experimental data, but it is unknown how they would need to change to optimally encode different distributions of stimuli. To determine the optimal coupling constants for a given stimulus distribution, we extended the model of neural decision boundaries to networks of neurons. This model of neural responses assumes stimuli that elicit a spike are separated from those that do not by a decision boundary. We demonstrate that the coupling constants of the PMEM which maximize information can be found from smoothness conditions on the decision boundaries. We considered exponentially distributed stimuli that mimic the large deviations found in signals in the natural environment and found that the optimal coupling constants between pairs of neurons are indeed non-zero, in agreement with experimental data. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q40.00009: Relationship between higher-order correlations in stimulus and information in the receptive fields of visual neurons Ryan Rowekamp, Tatyana Sharpee Neurons encode incoming signals in a series of spikes in the voltage trace across their cell membranes. This encoding is known to change in response to stimulus mean, variance, and power spectrum. Natural signals are known to have strong higher-than-second order correlations that cannot be described by a Gaussian distribution. To examine whether these higher-order statistics can also cause neurons to adapt their codes, we modeled the neural spike probability as an arbitrary nonlinear function with respect to two stimulus dimensions. The relevant stimulus dimensions were found as those that accounted for the largest mutual information between stimuli and spikes. We found that the contribution of the second dimension on the spike probability was stronger for natural, rather than Gaussian noise, stimuli and increased with the kurtosis of the stimulus distribution. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q40.00010: Robust Motion Processing in the Visual Cortex Audrey Sederberg, Julia Liu, Matthias Kaschube Direction selectivity is an important model system for studying cortical processing. The role of inhibition in models of direction selectivity in the visual cortex is not well understood. We probe the selectivity of an integrate-and-fire neuron with a noisy background on top of a deterministic input current determined by a temporal-lag model for selectivity, including first only excitatory inputs and later both excitatory and inhibitory input. In this model, postsynaptic potentials are fully synchronous for the preferred direction and maximally dispersed in time for the null direction. Further, any inhibitory inputs lag excitatory inputs, as Priebe and Ferster have observed (2005). At any level of input strength, the selectivity is weak when only excitatory inputs are considered. The inclusion of inhibition significantly strengthens selectivity, and this selectivity is preserved over a wide range of background noise levels and for short stimulus durations. We conclude that inhibition likely plays an essential role in the mechanism underlying direction selectivity. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q40.00011: Rhythmogenic Neuronal Networks and k-Core Percolation David Schwab, Robijn Bruinsma, Alex Levine The \textit{preB\"{o}tzinger Complex} (pBC) is a small ($\sim $10$^{2})$ network of identical excitatory neurons that collectively generate a temporally stable pattern of firing bursts interspersed by quiescent periods. The voltage output of this system is essential to the control of the mammalian breathing rhythm under certain physiological conditions. The network is also remarkable in that a small set of coupled identical neurons can generate a collective behavior that is not inherent in any one of them: individual neurons do not exhibit rhythmic bursting. We develop a simple model of interacting excitatory neurons that demonstrates this behavior as one of its dynamical regimes, and show that while some of its dynamical transitions can be understood in terms of mean field theory, others cannot. The non-mean-field behavior can be understood in terms of purely topological properties of random networks. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q40.00012: ABSTRACT WITHDRAWN |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q40.00013: Determining information flow in networks containing one hundred neocortical neurons Aonan Tang, Jon Hobbs, Wladek Dabrowski, Pawel Hottowy, Alexander Sher, Alan Litke, John Beggs How does information flow through networks of neurons? The type of network topology revealed could have important consequences for network efficiency and robustness to damage. Several tools, including transfer entropy, Granger causality, and directed information can be applied to this question. Yet indirect connections, connections with various delays, and feedback loops can complicate the task of uncovering the information flow structure. We have applied the above methods in simple validation studies, demonstrating that many of these issues can in principle be overcome. We will present preliminary results from neocortical networks of neurons recorded with a 512 electrode array. [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q40.00014: Integration of neuroblasts into a two-dimensional small world neuronal network Casey Schneider-Mizell, Michal Zochowski, Leonard Sander Neurogenesis in the adult brain has been suggested to be important for learning and functional robustness to the neuronal death. New neurons integrate themselves into existing neuronal networks by moving into a target destination, extending axonal and dendritic processes, and inducing synaptogenesis to connect to active neurons. We hypothesize that increased plasticity of the network to novel stimuli can arise from activity-dependent cell and process motility rules. In complement to a similar in vitro model, we investigate a computational model of a two-dimensional small world network of integrate and fire neurons. After steady-state activity is reached in the extant network, we introduce new neurons which move, stop, and connect themselves through rules governed by position and firing rate. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q40.00015: Phase dependent and independent responses in auditory cortex Didier Depireux, Barak Shechter Responses of auditory neurons are often characterized by their spectro-temporal receptive field (STRF). This linear measure has been shown to capture the overall trend of the response, but by its nature, it does not reflect any nonlinear processing. We have recently shown that neurons in primary auditory cortex (AI) of the awake ferret respond with non-trivial nonlinearities (not solely the result of rectifying or saturating nonlinearities). We developed new techniques to reveal additional phase independent (DC) and dependent (quadratic) tuning in the tuning of single neurons. One of the assumptions in the STRF model is that the mean firing rate (averaged over any single period of the stimulus) does not depend on the spectro-temporal modulations, but rather on the overall level of the stimulus. The phase-independent tuning to the spectro-temporal envelope is analogous to complex visual neural responses, in which responses to an auditory grating stimulus do not depend on its spatial phase. We show the existence of neurons tuned in 1) a phase-independent manner, 2) a linear manner and 3) a quadratic manner to the time-frequency content of the spectral envelope of sounds. [Preview Abstract] |
Session Q41: Strongly Correlated Electron Theory
Sponsoring Units: DCMP DMPChair: Andrey Chubukov, University of Wisconsin
Room: 413
Wednesday, March 18, 2009 11:15AM - 11:27AM |
Q41.00001: Quantum dots: a new tool for studying quantum phase transitions (QPT) N. Roch, S. Florens, V. Bouchiat, W. Wernsdorfer, F. Balestro QPT were studied in many different systems: spin chains, strongly correlated materials, high Tc superconductors, etc. but all the properties (magnetism, superconductivity ...) of these materials can be difficult to control. On the other hand, thanks to microelectronic technologies, it is now possible to obtain taylor-made quantum dots in which all the interactions can be tuned finely. It was then proposed by several theoretic papers [1] to use them as model systems for probing QPT. In this experimental work, we observed a screening/non screening QPT transition in a single-molecule transistor. We will present a full study as a function of magnetic field, bias voltage and temperature [2].\\[3pt] [1] M.Vojta, Philosophical Magazine,86:13,1807 - 1846 (2006)\\[0pt] [2] N.Roch et al. , Nature 453, 633-637 (2008) [Preview Abstract] |
Wednesday, March 18, 2009 11:27AM - 11:39AM |
Q41.00002: Signs of quantum Griffiths effects in the weak itinerant ferromagnet Ni-V Almut Schroeder, Sara Ubaid-Kassis ?Magnetization and ac-susceptibility data of $Ni_{1- x}V_{x}$ alloys are presented in the vicinity of the critical concentration, $x_c \approx 11\%$, where the ferromagnetic transition temperature is expected to vanish. For $x > x_c$ power laws with unusual exponents are observed, which change with further dilution. [Preview Abstract] |
Wednesday, March 18, 2009 11:39AM - 11:51AM |
Q41.00003: Effects of retardation on a system of polarized fermions Ling Yang, Filippos Klironomos, Shan-Wen Tsai When fermion-fermion interactions are frequency dependent, retardation effects may play an important role in determining the phase diagram and critical energy scales of the system. These effects are particularly significant when there is competition between two or more instabilities of the Fermi liquid state. In order to elucidate these effects, we study a simple model of spinless fermions on a square lattice. We employ a functional renormalization-group method to obtain the flows of vertices and correlation functions of this system. The phase diagram, critical scales and sub-dominant types of order can be obtained this way for various types of interactions. We discuss an analytical understanding for simple cases, and possible ways to observe these effects in fermion-boson cold-atom mixtures on optical lattices. [Preview Abstract] |
Wednesday, March 18, 2009 11:51AM - 12:03PM |
Q41.00004: Phase diagram, correlation gap, and critical properties of the Coulomb glass Matteo Palassini, Martin Goethe We investigate the lattice Coulomb glass model in three dimensions via extensive Monte Carlo simulations. 1. No evidence for an equilibrium glass phase is found down to very low temperatures, contrary to mean-field predictions, although the correlation length increases rapidly near $T=0$. 2. The single-particle density of states near the Coulomb gap satisfies the scaling law $g(e,T)=T^\lambda f(e/T)$ with $\lambda\approx 2.2$. 3. A charge-ordered phase exists at low disorder. The phase transition from the fluid to the charge ordered phase is consistent with the Random Field Ising universality class, which shows that the interaction is effectively screened at moderate temperature. Results from nonequilibrium simulations will also be briefly discussed. Reference: M.Goethe and M.Palassini, arXiv:0810.1047 [Preview Abstract] |
Wednesday, March 18, 2009 12:03PM - 12:15PM |
Q41.00005: Fingerprints of intrinsic phase separation in magnetically-doped 2DEG Hanna Terletska, Vladimir Dobrosavljevic We theoretically study the properties of a recently observed [1] inhomogeneous phase preceding the metal-insulator transition in a magnetically-doped two-dimensional electron gas (2DEG). We show that, due to competition between (ferromagnetic) double-exchange and (anti-ferromagnetic) super-exchange, at very low carrier density such a system is unstable toward intrinsic phase separation (PS). Here, ferromagnetic carrier-rich (metallic) ``droplets'' emerge within a magnetically disordered carrier-poor (insulating) matrix. Our calculations indicate that this regime should display very unusual transport, featuring colossal magneto-resistance with exceptionally weak density dependence - in striking agreement with experiments [1] on CdMnTe quantum wells. Such exotic transport properties - we argue - should be considered as ``fingerprints'' for intrinsic phase separation, a behavior very different from situations where phase coexistence is driven by disorder due to extrinsic impurities or defects. [1] J. JaroszyƱski \textit{et al}., Phys. Rev. B \textbf{76}, 045322 (2007). [Preview Abstract] |
Wednesday, March 18, 2009 12:15PM - 12:27PM |
Q41.00006: The Exponential Downfall of the Weak-Coupling SDW State in Chromium Yejun Feng, R. Jaramillo, T.F. Rosenbaum, J.C. Lang, Z. Islam, G. Srajer, P.B. Littlewood Elemental chromium is the archetypical model system for itinerant antiferromagnetism. The incommensurate spin density wave ground state, originating from the nested Fermi surface, is readily observable with direct scattering probes. Through a combination of low temperature cryogenic, diamond anvil cell, and synchrotron x-ray diffraction techniques, we measure directly the spin and charge order in the pure metal as it is driven towards its quantum critical point under pressure. We observe that both the spin and charge order are suppressed exponentially with pressure, well beyond the region where disorder cuts off such a simple evolution, and they maintain a harmonic scaling relationship over decades in scattering intensity. The observed exponential behavior of the order parameter follows a weak-coupling BCS theory for the ground state, even in the presence of strong pairing correlations that survive to surprisingly high temperatures and energies, as observed by inelastic scattering, transport, and thermal expansion measurements. This duality points to the fundamental issue of how mean-field behavior can describe so successfully important aspects of strongly coupled electron systems. [Preview Abstract] |
Wednesday, March 18, 2009 12:27PM - 12:39PM |
Q41.00007: Nonanalytic spin susceptibility of interacting fermions away and near a ferromagnetic quantum phase transition Dmitrii Maslov, Andrey Chubukov We study nonanalytic paramagnetic response of an interacting Fermi system both away and in the vicinity of a ferromagnetic quantum phase transition (QCP). Previous studies found that the spin susceptibility $\chi$ scales linearly with either the temperature $T$ or magnetic field $H$ in the weak-coupling regime and that the interaction in the Cooper channel affects this scaling via logarithmic renormalization of prefactors of the $T$, $|H|$ term. We show that Cooper renormalization becomes effective only at very low energies, which get even smaller near a QCP. We derive the thermodynamic potential as a function of magnetization and show that it contains, in addition to regular terms, a non-analytic $|M|^3$ term, which becomes $M^4/T$ at finite $T$. We consider the vicinity of a ferromagnetic QCP by generalizing the Eliashberg treatment of the spin-fermion model to finite magnetic field, and show that the $|M|^3$ term crosses over to a non-Fermi-liquid form $|M|^{7/2}$ near a QCP. The prefactor of the $|M|^{7/2}$ term is negative, which indicates that the system undergoes a first-order rather than a continuous transition to ferromagnetism. We compare two scenarios of the breakdown of a continuous QCP: a first-order instability and a spiral phase and show that in a model with a long-range interaction in the spin channel first-order transition occurs before the spiral instability. [Preview Abstract] |
Wednesday, March 18, 2009 12:39PM - 12:51PM |
Q41.00008: Fermionic propagators for 2D systems with singular interactions Tigran Sedrakyan, Andrey Chubukov We analyze the form of the fermionic propagator for 2D fermions interacting with massless overdamped bosons. Examples include a nematic and Ising ferromagnetic quantum-critical points, and fermions at a half-filled Landau level. Fermi liquid behavior in these systems is broken at criticality by a singular self-energy, but the Fermi surface remains well defined. These are strong- coupling problems with no expansion parameter other than the number of fermionic species, $N$. The two known limits, $N >>1$ and $N=0$ show qualitatively different behavior of the fermionic propagator $G(\epsilon_k, \omega)$. In the first limit, $G(\epsilon_k, \omega)$ has a pole at some $\epsilon_k$, in the other it is analytic. We analyze the crossover between the two limits. We show that the pole survives for all $N$, with residue $Z = O(1)$, however at small $N$ it only exists in a range $O(N^2)$. At $N=0$, the range collapses and the behavior of $G (\epsilon_k, \omega)$ becomes analytic. [Preview Abstract] |
Wednesday, March 18, 2009 12:51PM - 1:03PM |
Q41.00009: Non-Fermi-liquid behavior of quantum magnetooscillations near a quantum critical point Chungwei Wang, Dmitrii Maslov We study many-body effects in quantum magnetooscillations of a 2D strongly correlated system near ferromagnetic and antiferromagnetic quantum critical points (QCP). The amplitude of magnetooscillations is determined by the electron self-energy $\Sigma(\pi T, \mathbf{k};T)$ averaged over the Fermi surface, at the Matsubara frequency $\omega=\pi T$ for $T>\omega_c$, where $\omega_c$ is the cyclotron frequency. The major contribution of the bosonic propagator to the electron self-energy comes from static spin fluctuations. In the spin-fermion model, the self-energy behaves as $\Sigma \propto \sqrt{T}$ in the ferromagnetic system and as $\Sigma \propto T\ln{T}$ in the antiferromagnetic system. This shows the non-Fermi-liquid temperature dependence of the self-energy near the QCP and the oscillation amplitude $A\propto \exp{\left[-2\pi (\pi T + \Sigma)/\omega_c\right]}$ can be very different from the Lifshitz-Kosevich form. The momentum dependence of the self-energy contribution to the oscillation amplitude is also discussed. [Preview Abstract] |
Wednesday, March 18, 2009 1:03PM - 1:15PM |
Q41.00010: Mechanism of multifractal spectrum termination at the Anderson metal-insulator transition Matthew Foster, Shinsei Ryu, Andreas Ludwig We revisit the problem of wavefunction statistics at the Anderson metal-insulator transition (MIT) of non-interacting electrons in $d > 2$ spatial dimensions. At the transition, the complex spatial structure of the critical wavefunctions is reflected in the non-linear behavior of the multifractal spectrum of generalized inverse participation ratios (IPRs). For sufficiently large moments of the wavefunction intensity, the spectrum obtained from a \textit{typical} wavefunction associated to a particular disorder realization differs markedly from that obtained from the \textit{disorder-averaged} IPRs---the phenomenon known as the termination of the multifractal spectrum. We provide a derivation for the termination of the typical multifractal spectrum, by fusing the non-linear sigma model framework, conventionally used to access the MIT in $d = 2 + \epsilon$ dimensions, with a functional renormalization group (FRG) methodology. The FRG was previously used to demonstrate the termination of the multifractal spectrum in a very special model of 2D Dirac fermions, subject to a particular type of quenched disorder. [D.\ Carpentier and P.\ Le Doussal, Phys.Rev.\ E \textbf{63}, 026110 (2001)]. Our result shows that the FRG framework can be generalized to the much broader context of the delocalization transition of ordinary electrons in higher dimensions. [Preview Abstract] |
Wednesday, March 18, 2009 1:15PM - 1:27PM |
Q41.00011: Effects of Strong Correlations on the Disorder-Induced Zero Bias Anomaly William Atkinson, Yun Song, Sinan Bulut, Rachel Wortis In conventional metals and semiconductors, density of states anomalies result from the interplay between disorder and interactions. Motivated by a number of experiments that find zero bias anomalies (ZBA) in transition metal oxides, we have performed calculations to determine the effect of strong correlations on the ZBA in disordered interacting systems. We use a self-consistent mean-field theory that incorporates strong correlations and treats spatial fluctuations of the disorder potential exactly. We discuss both the Anderson-Hubbard model and the extended Anderson-Hubbard model. We find that, even for a zero-range interaction, nonlocal self-energy corrections lead to the formation of an Altshuler-Aronov-like ZBA. In the extended Anderson-Hubbard model, Efros-Shklovskii-like physics dominates at large disorder. [Preview Abstract] |
Wednesday, March 18, 2009 1:27PM - 1:39PM |
Q41.00012: Detrended Fluctuation Analysis for Dynamics in an Electron Glass Stephen Arnason As a result of the correlations between electrons, electron glasses show enhanced fluctuations in coductance with a $1/f^{\alpha}$ frequency dependence. We are interested in looking at the time dependence of the fluctuation spectra as the system relaxes towards equilibrium after a discontinuous change in chemical potential. Our measurements are taken on FET structures where the conductance channel is fabricated from amorphous Indium Oxide with stoichiometry close to the superconductor to insulator transition. Changing the potential on the gate electrode allows us to change the chemical potential and we measure the resistance of the conductance channel as a function of time. In addition to the fluctuations there is a slow, logarithmic relaxation of the channel conductance. Because of this slow relaxation it is hard to characterize our signal as stationary, calling into question the application of Fourier transform based analysis techniques. One approach to coping with this problem is the subtraction of the slowly varying background before the calculation of the Fourier transforms, so called detrended fluctuation analysis. This paper presents results on simulations of this technique as applied to computer generated signals with characteristics similar to our actual data. The frequency dependence of the fluctuation spectra is imperfectly preserved but can be similar to the actual fluctuation spectra within certain bounds of analysis parameters. [Preview Abstract] |
Wednesday, March 18, 2009 1:39PM - 1:51PM |
Q41.00013: Transport in a Dissipative Luttinger Liquid Zoran Ristivojevic, Thomas Nattermann We study theoretically the transport through a single impurity in a one-channel Luttinger liquid coupled to a dissipative Ohmic bath. For nonzero dissipation, the single impurity is always a relevant perturbation which suppresses transport strongly. At zero temperature, the current voltage relation of the link is $I\sim \exp(-E_0/eV)$, where $E_0\sim\eta/\kappa$ and $\kappa$ denotes the compressibility and $\eta$ the dissipation strength. At nonzero temperature $T$, the linear conductance is proportional to $\exp(-\sqrt{{\mathcal{C}}E_0/k_B T})$. The decay of Friedel oscillation saturates for at distances larger than $L_{\eta}\sim 1/\eta$ from the impurity.\newline [1] Z. Ristivojevic and T. Nattermann, Phys. Rev. Lett. \textbf{101}, 016405 (2008). [Preview Abstract] |
Wednesday, March 18, 2009 1:51PM - 2:03PM |
Q41.00014: Mapping the Driven Interacting Resonant Level Model to an Equilibrium Problem Eduardo Novais, M.R. Plesser, Harold U. Baranger We map the driven Interacting Resonant Level Model (IRLM) to an equivalent statistical mechanical problem. Correlation functions in the nonequilibrium model are given, to all orders in perturbation theory, by thermal averages in the statistical system. This enables us to apply the traditional theoretical techniques for thermal problems, such as the renormalization group and diagrammatic expansions, to a far from equilibrium problem. Using these tools, we study the current as a function of bias, as well as of the interactions in the leads and the resonant level. As a simple example of a strongly interacting system far from equilibrium, the IRLM has played an important role in motivating and evaluating recent theoretical advances. We compare our new strategy to other recent proposals for studying far from equilibrium interacting systems. [Preview Abstract] |
Wednesday, March 18, 2009 2:03PM - 2:15PM |
Q41.00015: Numerical study of relaxation dynamics in photoexcited states of one-dimensional Mott insulators Hiroaki Matsueda, Takami Tohyama, Sadamichi Maekawa We examine relaxation dynamics of one-dimensional Mott insulators after photoirradiation. This study is motivated by ultrafast metal-insulator switching seen in Sr2CuO3, halogen-bridged Ni compounds, and organic materials. In order to examine energy dissipation due to relaxation processes, we take account of strongly correlated electrons as well as environmental degrees of freedom by introducing the Hubbard--Holstein model. Then, we perform density matrix renormalization group calculations. We find quite large number of phonons excited just after irradiation even for very small electron-phonon coupling. We will discuss the nature of the phonon relaxation characteristic of strongly correlated systems, and how the relaxation is associated with other internal degrees of freedom of correlated electrons. [Preview Abstract] |
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