Bulletin of the American Physical Society
APS March Meeting 2012
Volume 57, Number 1
Monday–Friday, February 27–March 2 2012; Boston, Massachusetts
Session Z1: General Atomic, Molecular and Optical Physics
Sponsoring Units: DAMOPRoom: 203
Friday, March 2, 2012 11:15AM - 11:27AM |
Z1.00001: The Timing of Sonoluminescence Thomas Brennan, Gustave Fralick We measured the timing of the sonoluminescence flash by scattering laser light from the bubble. We performed this measurement on 17.8 kHz, 13.28 kHz and 7920 Hz systems and found that the flash typically occurs 100 nanoseconds before the minimum radius, contrary to previous claims that the flash always occurs within a nanosecond of the minimum radius. These results are important because they imply that previous \emph{hot} models of sonoluminescence are wrong. We propose a new model: that the flash results from the discharge of an excited cold condensate, formed during the adiabatic expansion of the bubble. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z1.00002: Coherence Enhanced Transient Lasing Pankaj Jha, Anatoly Svidzinsky, Marlan Scully We study the effect of a coherent drive on transient lasing with inversion in three-level $\Lambda $ and $\Xi$ configurations ($c\leftrightarrow a\leftrightarrow b$). We show that the presence of a resonant coherent drive on the $a\leftrightarrow c$ optical transition can yield substantial enhancement of the output laser energy on a $a\rightarrow b$ XUV or X-ray transition. We demonstrate the crucial role of coherence $\varrho _{ac}$ for this laser power enhancement. Contrary to the forward direction, where forward gain can be enhanced for some choice of $\Omega_{c}$, coherent drive on the $ac$ transition always suppresses the backward gain. Thus, the use of a coherent drive at optical frequency could be a useful tool for increasing power of lasers in XUV and X-ray regions. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z1.00003: Variable Atomic Radius of Hydrogen Due to Vibrating Nucleus Stewart Brekke The H-atomic radius is variable because the H-nucleus is vibrating and the electric force field upon the electron is repeatedly changing due to the changing distance from the positive nucleus to the negatively charged electron. If the the distance from the nucleus to the electron is $d=r + Acos2\pi ft$ where $r=5.29x10^{-11}m$, the calculated Bohr radius, and $d=2.5x10^{-11}m$, the measured atomic radius of the H-atom, then the equation for the variable atomic radius of the H-atom is $5.29x10{-11}m + Acos2\pi ft= 2.5x10{-11}m$. If the RMS value for the average cosine is $0.707$, solving for A, the average amplitude of nuclear vibration, $A=3.95x10^{-11}m$. Therefore, the oscillating orbit of the electron in an H-atom has an average amplitude of $A=3.95x10^{-11}$. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z1.00004: Strong Field Control of Atomic and Molecular Dynamics: An Attosecond Resolved Study Niranjan Shivaram, Henry Timmers, Xiao-Min Tong, Arvinder Sandhu Strong laser fields are routinely used in attosecond pump-probe studies of atomic and molecular phenomena. However, even a moderately strong field ($\sim $ 10$^{12}$ W/cm$^{2})$ can significantly alter the electronic structure. By understanding and quantifying the effect of strong fields, one can obtain a high degree of control over the photo-absorption and photo-fragmentation processes. Here, we study the atomic and molecular response to the simultaneous presence of XUV attosecond pulse trains and strong IR fields. We describe an IR laser-dressed atom using Floquet picture. We observe quantum interference between XUV excitation paths to the Fourier components of a given Floquet state, which leads to oscillations in the ion-yield. By measuring the phase of the ion-yield oscillations, we extract the quantum phase difference between the Fourier components of that Floquet state. We obtain a quantitative understanding of how Floquet ionization channels change with intensity and what is the phase associated with each channel. We also extend our studies to molecular fragmentation processes. Our work represents real-time measurement and control of dynamics using strong-field modification of the atomic and molecular structure. This work was support by NSF grant PHY-0955274. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z1.00005: Steady State Ab Initio Laser Theory: Generalizations Alexander Cerjan, Yidong Chong, Li Ge, A. Douglas Stone We show that Steady-state Ab initio Laser Theory (SALT)\footnote{L. Ge, Y. D. Chong, and A. D. Stone, Phys. Rev. A \textbf{82}, 063824 (2010).} can be generalized to find the stationary multimode lasing properties of gain systems with $N$ levels, and to include carrier diffusion. The former result is achieved by mapping the $N$-level rate equations to an effective two-level inversion equation of the type typically quoted in the Maxwell-Bloch equations.\footnote{A. Cerjan, Y. D. Chong, L. Ge, and A. D. Stone, Opt. Express \textit{in press}, arXiv: 1111.2279v1 [physics.optics]} The latter result is found by rewriting the SALT algorithm to non-linearly solve two coupled non-linear equations in the steady state, with one equation determining the modal field intensities, given the inversion and the other equation determining the inversion given the field intensities. In both cases we find excellent agreement with more computationally demanding Finite-Difference Time-Domain (FDTD) simulations for the steady state. These results generalize the SALT algorithm to handle more realistic lasing systems, including semiconductor lasers. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z1.00006: Dynamical mean-field theory for transition metal dioxide molecules Nan Lin, Dominika Zgid, Chris Marianetti, David Reichman, Andrew Millis The utility of the dynamical mean-field approximation in quantum chemistry is investigated in the context of transition metal dioxide molecules including $TiO_2$ and $CrO_2$. The choice of correlated orbitals and correlations to treat dynamically is discussed. The dynamical mean field solutions are compared to state of the art quantum chemical calculations. The dynamical mean-field method is found to capture about 50\% of the total correlation energy, and to produce very good results for the d-level occupancies and magnetic moments. We also present the excitation spectrum in these molecules which is inaccessible in many wave-function based methods. Conceptual and technical difficulties will be outlined and discussed. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z1.00007: Disalignment of the Ne$^{\ast }$(2p$_{10}$ [J=1]) atoms induced by Helium atom collisions from 10K to 3000K Cristian Bahrim, Vaibhav Khadilkar Quantum close-coupling many-channel calculations using the new model potential for the interaction between Ne$^{\ast }$(2p$_{i}$ [J=1]) and He atoms proposed in [1] are performed in order to analyze the depolarization of Ne$^{\ast }$(2p$_{i}$ [J=1]) atoms in a gaseous mixture at thermal equilibrium. For temperatures above 77 K we successfully explain measurements of disalignment done with a laser-induced fluorescence spectroscopy method and destruction of alignment using a technique based on the Hanle effect for all four 2p$_{i}$ [J=1] states of the 2p$^{5}$3p configuration of neon [1, 2]. Our interpretation of the experimental data is based on the anisotropy between collisional channels which asymptotically converge toward the same 2p$_{i}$ [J=1] state [2]. Below 77 K our disalignment rate coefficients for the Ne$^{\ast }$(2p$_{10}$ [J=1]) atoms are much larger than the experimental data [3] after the radiation re-absorption is subtracted from the disalignment rates. The calculations indicate that for the 2p$_{10}$ state, at low collision energies, the nuclear rotation has a strong influence in the overall long-range interaction, while the experimental data suggests that below 16 meV, the intramultiplet transitions within the (2p$_{i}$ [J=1]) state of neon are completely negligible. The discrepancy between theory and experiment is carefully analyzed. \\[4pt] [1] Bahrim C and Khadilkar V 2009 \textit{Phys Rev A} \textbf{79} 042715. \\[0pt] [2] Khadilkar V and Bahrim C 2010 \textit{J Phys B }\textbf{43}235209. \\[0pt] [3] Matsukuma H, Shikama T, and Hasuo M 2011 \textit{J Phys B }\textbf{44 }075206. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z1.00008: Cold collisions of complex polyatomic molecules Zhiying Li, Eric Heller We introduce a method for classical trajectory calculations to simulate collisions between atoms and large rigid asymmetric-top molecules. Using this method, we investigate the formation of molecule-helium complexes in buffer-gas cooling experiments at the temperature of 6.5 K for molecules as large as naphthalene. Our calculations show that the mean lifetime of the quasi-bound collision complex is not long enough for the formation of stable clusters under the experimental conditions. Our results suggest that it may be possible to improve the efficiency of the production of cold molecules in buffer-gas cooling experiments by increasing the density of helium. In addition, we find that the shape of molecules is important for the collision dynamics where molecular vibrational motions are frozen. For some molecules, it is even more crucial than the number of accessible degrees of freedom. This indicates that by selecting molecules with suitable shape for buffer-gas cooling, one could cool molecules with a very large number of degrees of freedom. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z1.00009: Spin-boson models for periodic $N$-site systems Victor V. Albert The single/multi-mode spin-boson model provides a description for numerous two-level exciton-phonon and atom-cavity systems. Existing many-level extensions conserve symmetries but quickly become intractable due to the inclusion of multiple interacting modes. Other ad-hoc single-mode extensions contain arbitrary numbers of parameters and often ignore the symmetries of their respective systems. This work presents a simple model for the interaction of a periodic system of $N$ coupled sites with one or more non-interacting boson modes using a minimal number of parameters [1]. A group theoretic approach allows one to partially diagonalize the Hamiltonian, providing numerical advantages, physical insight, and a gateway to accurate approximations. The single-mode two-site system reduces to the single-mode spin-boson model, also known as the Rabi Hamiltonian. Two higher dimensional generalizations are reviewed in the exciton-phonon/atom-field interpretations and related to a new integrability criterion [2]. The model predicts that $2N$-level systems have parity symmetry and that the ground state of certain four-level atom-cavity systems will undergo parity change at large coupling. \\[4pt] [1] V. V. Albert, arXiv:1112.0849 \\[0pt] [2] D. Braak, PRL \textbf{107}, 100401 (2011) [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z1.00010: Thermal Effects on Quantum Sticking Yanting Zhang, Dennis Clougherty Many-body effects on the threshold law of quantum sticking of a particle coupled to an ohmic bosonic bath are examined for finite temperature surfaces. Generalizing a variational mean-field method\footnote{Y. Zhang and D.P. Clougherty, arXiv:1012.4405} previously applied to zero temperature surfaces, we obtain an explicit expression for the sticking probability of a particle with incident energy $E$. We find that there is a critical particle energy below which the probability of its sticking to the surface discontinuously drops to zero. We show that this singularity, whose origin is rooted by analogy to the localization transition in the spin-boson model, is experimentally accessible for ultracold particles. We provide detailed numerical results for this effect. [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z1.00011: Theory of Non-Markovian dynamics in resonance fluorescence spectra Abhishek Kumar, Sigurdur I. Erlingsson, Bill Coish Robust quantum coherence is an important prerequisite for any system that may be used to perform quantum information processing tasks. For systems that can be probed optically, the resonance fluorescence spectrum may provide indirect evidence of coherence times when supplemented with an appropriate model of the decay process. A common approach is to assume a Markovian system, resulting in exponential decay of correlation functions and Lorentzian features in the associated spectrum. For physical systems with strongly history-dependent (non-Markovian) dynamics, there is currently no satisfying systematic theoretical approach to establish the associated spectrum. We present a detailed theoretical method to obtain the resonance fluorescence spectrum for a general system undergoing non-Markovian dynamics. This procedure can be used to systematically account for features in resonance-fluorescence spectra due to genuine non-Markovian dynamics. Our approach is based on a Nakajima-Zwanzig generalised master equation for the dynamics of the reduced density matrix. We apply this theory to study the resonance fluorescence in the non-Markovian dynamics of a three level lambda system, relevant to recent experiments on heavy-hole spin dynamics in a quantum dot. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z1.00012: Quantum Revivals of the Morse Oscillator in Position Space and Momentum Space Alvason Zhenhua Li, William Harter Analytical solutions for the Morse oscillator are applied to investigate the quantum revivals both in position and momentum spaces. The properties of this anharmonic oscillator came across interesting space-time phenomena. These findings include simple Farey arithmetic revival structures. Such dynamic systems may have applications for quantum information technology and quantum computing. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z1.00013: Trapping of particles in the ray optics regime using DNG materials Joseph Shahbazian Optical tweezers use to confine and manipulate microscopic objects including living cells and bacteria, with high accuracy. The objective is calibrating the force on targets using DPS-DNG layered structure. Using this layered structure which acts as a tunable optical band-pass filter would assist calibration of the force on the target(s). Here shown that the proposed DNG-DPS structure would help to have highly focused calibrated tweezers without worrying about the polarization of optical wave. Calculation can describe well the experimental results. [Preview Abstract] |
Session Z2: Invited Session: Active Responses of Biological Materials to Mechanical Stress
Sponsoring Units: DCMP DBIOChair: Allen Ehrlicher, Harvard University and Chin-Lin Guo, California Institute of Technology
Room: 204AB
Friday, March 2, 2012 11:15AM - 11:51AM |
Z2.00001: Cellular Mechanics and Mechanotransduction Invited Speaker: Daniel Fletcher |
Friday, March 2, 2012 11:51AM - 12:27PM |
Z2.00002: Force Transmission in the Actin Cytoskeleton Invited Speaker: Margaret Gardel The ability of cells to sense and generate mechanical forces is essential to numerous aspects of their physiology, including adhesion, migration, division and differentiation. To a large degree, cellular tension is regulated by the transmission of myosin II-generated forces through the filamentous actin (F-actin) cytoskeleton. While transmission of myosin-generated stresses from the molecular to cellular length scale is well understood in the context of highly organized sarcomeres found in striated muscle, non-muscle and smooth muscle cells contain a wide variety of bundles and networks lacking sarcomeric organization. I will describe the \textit{in vitro} and \textit{in vivo} approaches we use to study force transmission in such disordered actomyosin assemblies. Our \textit{in vivo} results are showing that highly organized stress fibers contribute surprisingly little to the overall extent of cellular tension as compared to disordered actomyosin meshworks. Our in vitro results are demonstrating the mechanisms of symmetry breaking in disordered actomyosin bundles that facilitate the formation of contractile bundles with well-defined ``contractile elements.'' These results provide insight into the self-organization of actomyosin cytoskeleton in non-muscle cells that regulate and maintain cellular tension. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 1:03PM |
Z2.00003: Environmental properties set cell mechanics and morphology Invited Speaker: Paul Janmey Many cell types are sensitive to mechanical signals that are produced either by application of exogenous force to their surfaces, or by the resistance that their surroundings place on forces generated by the cells themselves. Cell morphology, motility, proliferation, and protein expression all change in response to substrate stiffness. Changing the elastic moduli of substrates alters the formation of focal adhesions, the assembly of actin filaments into bundles, and the stability of intermediate filaments. The range of stiffness over which different primary cell types respond can vary over a wide range and generally reflects the elastic modulus of the tissue from which these cells were isolated. Mechanosensing depends on the type of adhesion receptor by which the cell binds, and therefore on both the molecular composition of the extracellular matrix and the nature of its link to the cytoskeleton. Many cell types can alter their own stiffness to match that of the substrate to which they adhere. The maximal elastic modulus that cells such as fibroblasts can attain is similar to that of crosslinked actin networks at the concentrations in the cell cortex. The precise mechanisms of mechanosensing are not well defined, but they presumably require an elastic connection between cell and substrate, mediated by transmembrane proteins. The viscoelastic properties of different extracellular matrices and cytoskeletal elements strongly influence the response of cells to mechanical signals, and the unusual non-linear elasticity of many biopolymer gels, characterized by strain-stiffening, leads to novel mechanisms by which cells alter their stiffness by engagement of molecular motors that produce internal stresses. Cell cortical elasticity is dominated by cytoskeletal polymer networks and can be modulated by internal tension. Simultaneous control of substrate stiffness and adhesive patterns suggests that stiffness sensing occurs on a length scale much larger than single molecular linkages and that the time needed for mechanosensing is on the order of a few seconds. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:39PM |
Z2.00004: Molecular Mechanotransduction: how forces trigger cytoskeletal dynamics Invited Speaker: Allen Ehrlicher Mechanical stresses elicit cellular reactions mediated by chemical signals. Defective responses to forces underlie human medical disorders, such as cardiac failure and pulmonary injury. Despite detailed knowledge of the cytoskeleton's structure, the specific molecular switches that convert mechanical stimuli into chemical signals have remained elusive. Here we identify the actin-binding protein, filamin A (FLNa) as a central mechanotransduction element of the cytoskeleton by using Fluorescence Loss After photoConversion (FLAC), a novel high-speed alternative to FRAP. We reconstituted a minimal system consisting of actin filaments, FLNa and two FLNa-binding partners: the cytoplasmic tail of {\ss}-integrin, and FilGAP. Integrins form an essential mechanical linkage between extracellular and intracellular environments, with {\ss} integrin tails connecting to the actin cytoskeleton by binding directly to filamin. FilGAP is a FLNa-binding GTPase-activating protein specific for Rac, which in vivo regulates cell spreading and bleb formation. We demonstrate that both externally-imposed bulk shear and myosin II driven forces differentially regulate the binding of integrin and FilGAP to FLNa. Consistent with structural predictions, strain increases {\ss}-integrin binding to FLNa, whereas it causes FilGAP to dissociate from FLNa, providing a direct and specific molecular basis for cellular mechanotransduction. These results identify the first molecular mechanotransduction element within the actin cytoskeleton, revealing that mechanical strain of key proteins regulates the binding of signaling molecules. Moreover, GAP activity has been shown to switch cell movement from mesenchymal to amoeboid motility, suggesting that mechanical forces directly impact the invasiveness of cancer. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 2:15PM |
Z2.00005: The physics of 3D cell migration Invited Speaker: Denis Wirtz |
Session Z3: Invited Session: Frontiers of Non-Equilibrium Transport Theories
Sponsoring Units: DCMP DCOMPChair: Zhenyu Zhang, Oak Ridge National Laboratory
Room: 205AB
Friday, March 2, 2012 11:15AM - 11:51AM |
Z3.00001: DFT treatment of transport through Anderson junction: exact results and approximations Invited Speaker: Kieron Burke Since the pioneering break-junction experiments of Reed and Tour measuring the conductance of dithiolated benzene between gold leads, many researchers in physics and chemistry have been calculating conductance for such systems using density functional theory (DFT). Off resonance, the predicted current is often 10-100 times larger than that measured. This error is often ascribed to the application of ground-state DFT to a non-equilibrium problem. I will argue that, in fact, this is largely due to errors in the density functional approximations in popular use, rather than necessarily errors in the methodology. A stark illustration of this principle is the ability of DFT to reproduce the exact transmission through an Anderson junction at zero-temperature and weak bias, including the Kondo plateau, but only if the exact ground-state density functional is used. In fact, this case can be used to reverse-engineer the exact functional for this problem. Popular approximations can also be tested, including both smooth and discontinuous functionals of the density, as well as symmetry-broken approaches. \\[4pt] [1] Kondo effect given exactly by density functional theory, J. P. Bergfield, Z. Liu, K. Burke, and C. A. Stafford, arXiv:1106.3104; \\[0pt] [2] Broadening of the Derivative Discontinuity in Density Functional Theory, F. Evers, and P. Schmitteckert, arXiv:1106.3658; \\[0pt] [3] DFT-based transport calculations, Friedel's sum rule and the Kondo effect, P. Tr\"{o}ster, P. Schmitteckert, and F. Evers, arXiv:1106.3669; \\[0pt] [4] Towards a description of the Kondo effect using time-dependent density functional theory, G. Stefanucci, and S. Kurth, arXiv:1106.3728. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:27PM |
Z3.00002: Inelastic single-spin transport theory Invited Speaker: Stefano Sanvito Spin-flip inelastic spectroscopy is a powerful tool for investigating the magnetic excitations of nano-scale magnets deposited on a metallic surface. In this talk I will present a perturbative approach to the calculation of the inelastic spin-flip spectra of magnetic adatoms, small magnetic clusters and magnetic molecules. The theory is based on the non-equilibrium Green's function formalism combined with a model spin Hamiltonian, where the conduction electrons are exchanged coupled to a system of quantum spins. By expanding the self-energy describing the electron-spin interaction to the third order we are able to capture both inelastic spin-flip events and the signature of Kondo resonances. Furthermore, when our approach is combined with a Master equation describing spin-relaxation, effects related to the spin-pumping at the single spin level can be described. In the talk I will demonstrate that the method offers an extremely good quantitative agreement with published experimental data. Importantly the formalism is amenable to be implemented together with highly accurate electronic structure methods. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 1:03PM |
Z3.00003: Electron correlation effects on the diode properties and the local heating Invited Speaker: Yoshihiro Asai Single molecular bridge junctions and atomic wires provide one of the best test fields for non-equilibrium transport theories whose progress gives benefits over wide range of physics. Experimental progresses in inelastic tunneling spectroscopy (IETS) and break junction techniques have played very important roles to make this possible. Inelastic scatterings between electrons and phonons give ``local heating'' of the junctions. The effective temperature due to the local heating was discussed successfully in terms of a fully self-consistent theory treating energy dissipation processes as well as inelastic heat generation on equal footing [1]. Recently, we found two cases where electron correlation gives distinct changes. The first case was found in the local heating problem in the resonant systems, where phonon damping due to its coupling with electron-hole excitation is suppressed by the correlation. The suppression enhances heat release to electrodes leading to the effective temperature suppression [2]. Another example is the single molecular rectifier. First principle NEGF-GGA calculation fails to explain the large rectification ratio (RR) at high bias voltage. Separate GW calculation based on Keldysh Green's function gives clear enhancement of RR over the mean field NEGF results suggesting that RR could be enhanced by the electron correlation effect [3]. Thus latest non-equilibrium transport theories enable us to treat the important physical processes accompanying electric conduction allowing us to make more direct comparisons with experimental phenomena at nano-scale. \\[4pt] [1] Y. Asai, Phys. Rev. B78, 045434 (2008).\\[0pt] [2] Y. Asai, Phys. Rev. B84, 085436 (2011).\\[0pt] [3] Y. Asai, H. Nakamura, J. Hihath, C. Bruot, and N.J Tao, Phys. Rev. B 84, 115436 (2011). [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:39PM |
Z3.00004: Electron-ion correlations in electromigration: Coulomb's law and Landauer transport at the nanoscale Invited Speaker: Kirk Bevan Electromigration has gained increased prominence in recent years, as the rise of nanoelectronics has given way to higher current and power densities in computing interconnects and devices. In this context we address the fundamental materials question: what drives electromigration at the nanoscale? Our understanding of the forces that drive electromigration has remained at an uneasy juncture between the mesoscopic semi-classical and atomistic quantum mechanical regimes. At the nanoscale an atomistic understanding of materials is required. Through first-principles quantum transport calculations we show that a self-consistent Laundaer transport framework provides much needed atomistic insight into the fundamental Coulombic forces which drive both current flow and electromigration at the nanoscale. These insights provide a general timely overview of the importance of electromigration in modern nanoelectronic devices and materials, not only from an operational perspective but also from a novel materials design perspective. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 2:15PM |
Z3.00005: Probing DNA in nanopores via tunneling: from sequencing to ``quantum'' analogies Invited Speaker: Massimiliano Di Ventra Fast and low-cost DNA sequencing methods would revolutionize medicine: a person could have his/her full genome sequenced so that drugs could be tailored to his/her specific illnesses; doctors could know in advance patients' likelihood to develop a given ailment; cures to major diseases could be found faster [1]. However, this goal of ``personalized medicine'' is hampered today by the high cost and slow speed of DNA sequencing methods. In this talk, I will discuss the sequencing protocol we suggest which requires the measurement of the distributions of transverse currents during the translocation of single-stranded DNA into nanopores [2-5]. I will support our conclusions with a combination of molecular dynamics simulations coupled to quantum mechanical calculations of electrical current in experimentally realizable systems [2-5]. I will also discuss recent experiments that support these theoretical predictions. In addition, I will show how this relatively unexplored area of research at the interface between solids, liquids, and biomolecules at the nanometer length scale is a fertile ground to study quantum phenomena that have a classical counterpart, such as ionic quasi-particles, ionic ``quantized'' conductance [6,7] and Coulomb blockade [8]. Work supported in part by NIH. \\[4pt] [1] M. Zwolak, M. Di Ventra, Physical Approaches to DNA Sequencing and Detection, Rev. Mod. Phys. 80, 141 (2008).\\[0pt] [2] M. Zwolak and M. Di Ventra, Electronic signature of DNA nucleotides via transverse transport, Nano Lett. 5, 421 (2005).\\[0pt] [3] J. Lagerqvist, M. Zwolak, and M. Di Ventra, Fast DNA sequencing via transverse electronic transport, Nano Lett. 6, 779 (2006).\\[0pt] [4] J. Lagerqvist, M. Zwolak, and M. Di Ventra, Influence of the environment and probes on rapid DNA sequencing via transverse electronic transport, Biophys. J. 93, 2384 (2007).\\[0pt] [5] M. Krems, M. Zwolak, Y.V. Pershin, and M. Di Ventra, Effect of noise on DNA sequencing via transverse electronic transport, Biophys. J. 97, 1990, (2009).\\[0pt] [6] M. Zwolak, J. Lagerqvist, and M. Di Ventra, Ionic conductance quantization in nanopores, Phys. Rev.Lett. 103, 128102 (2009).\\[0pt] [7] M. Zwolak, J. Wilson, and M. Di Ventra, Dehydration and ionic conductance quantization in nanopores, J. Phys. Cond. Matt. 22 454126 (2011). \\[0pt] [8] M. Krems and M. Di Ventra, Ionic Coulomb blockade in nanopores arXiv:1103.2749. [Preview Abstract] |
Session Z4: Disorder and Pairing in Ultracold Systems
Sponsoring Units: DAMOPChair: Seth Rittenhouse, ITAMP, Harvard-Smithsonian Center for Astrophysics
Room: 205C
Friday, March 2, 2012 11:15AM - 11:27AM |
Z4.00001: Many Body Localization in Incommensurate Potentials Shankar Iyer, David Huse, Gil Refael A long-standing problem concerns the survival of Anderson localization in a many body system with interparticle interactions. In recent years, this problem has resurfaced due to work by Basko, Aleiner, and Altshuler, who have argued that highly excited states of an interacting, many body system can be localized in Fock space. Consequently, a dynamical quantum phase transition may separate such a many body localized phase from a delocalized, ergodic phase, and there is now numerical evidence for the existence of such a transition in disordered 1D systems. Meanwhile, 1D lattice models that lack genuine disorder, but which instead contain a periodic potential that is incommensurate with the lattice spacing, are known to have a localization transition even in the absence of interactions. Here, we numerically investigate whether this transition survives the introduction of interactions and, if so, how it is modified. These questions are increasingly experimentally relevant, because ultracold atom experiments sometimes use incommensurate potentials in place of true disorder to probe localization physics. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z4.00002: Mobile impurities in one-dimensional cold gases: subdiffusive, diffusive and ballistic regimes Adrian Kantian, Thierry Giamarchi Advances in cold gases physics are beginning to enable experiments involving the direct manipulation and observation of single- or few-atom mobile impurities [1] within a many-body quantum system, a topic of longstanding interest for condensed matter theory, where it is related to studies of e.g. conductivity and the X-ray edge problem. Further progress in this direction is expected from the latest generation of experiments offering single-site addressability in optical lattices [2,3]. In light of these developments we study the dynamics of mobile impurities in 1D quantum liquids using a DMRG technique. We address the recently proposed subdiffusive regime of impurity motion [4], a class of excitations beyond those described by the standard Tomonaga-Luttinger theory. We study the conditions for observing this regime and its' crossover to the ballistic regime. We furthermore examine the possibilities to observe the intermediate diffusive motion of impurities in these systems. \\[4pt] [1] J. Catani, G. Lamporesi, D. Naik et. al., arXiv:1106.0828\\[0pt] [2] W. S. Bakr, J. I. Gillen, A. Peng et. al., Nature 462, 74 (2009)\\[0pt] [3] J. F. Sherson, C. Weitenberg, M. Endres et. al., Nature, 467, 7311 (2010)\\[0pt] [4] M. B. Zvonarev, V. V. Cheianov, T. Giamarchi, PRL 99, 240404 (2007); PRL 103, 110401 (2009) [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z4.00003: Collective mode of an impurity and a Tonks-Girardeau gas Charles Mathy, Mikhail Zvonarev, Eugene Demler We investigate the quantum dynamics of an impurity immersed in a one-dimensional gas of strongly repulsive bosons, or equivalently fully-polarized fermions, interacting via a contact interaction. Using Bethe Ansatz we obtain essentially exact results at all timescales and for all couplings to the impurity. We find at strong coupling that if the impurity starts off with a momentum of the order of the Fermi momentum or higher a new type of collective mode is excited, corresponding to long lived oscillations of the impurity with respect to the background gas. We characterize this mode and discuss how it can be observed experimentally. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z4.00004: Dynamics of mobile impurities in one-dimensional quantum liquids Michael Schecter, Alex Kamenev, Dimitri Gangardt, Austen Lamacraft We consider the dynamics of mobile impurities immersed in one-dimensional (1d) quantum liquids. Such systems have been realized experimentally in the context of ultracold atomic gases in optical lattices.
We show that, on very general grounds, the dispersion relation of the impurity dressed by the liquid is a periodic function of momentum with period $2\pi \hbar n$, $n$ being the 1d density. An impurity subject to a small external force thus exhibits the phenomenon of Bloch oscillations about a fixed point in real space, in the absence of a periodic potential. To compare with experiments,we set out to address the consequences of both finite temperature and finite force on the Bloch oscillation sequence.
Our main results are as follows: (i) There exists a finite window of parameters where Bloch oscillations exist $F_{\textrm{min}}(T) |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z4.00005: Gapless superfluid phase with spin-dependent disorder Mi Jiang, Ravindra Nanguneri, Nandini Trivedi, George Batrouni, Richard Scalettar Motivated by the recent experimental development on spin-dependent optical lattices and disordered lattices, we show that the presence of a spin-dependent random potential on a superconductor or a superfluid atomic gas leads to distinct transitions at which the energy gap and average order parameter vanish, generating an intermediate gapless superfluid phase. This behavior is in marked contrast to the case of spin-symmetric randomness. The calculations are performed for a two dimensional attractive Hubbard model within Bogoliubov-de Gennes mean field theory. We characterize the different phases by correlating the local order parameter and the density of states. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z4.00006: Superdiffusive nonequilibirum transport of an impurity in a Fermi sea Hyungwon Kim, David Huse We discuss a nonequilibrium transport of a single impurity atom immersed in a low-temperature Fermi sea with a short range interaction. We find that the impurity does a superdiffusive geometric random walk in which the characteristic momentum decay rate shows a quartic decrease in its momentum in three dimension. Then, we construct a master equation and its scaled form that governs the time evolution of the impurity. Next, we discuss two dimensional case in which the momentum decay rate decreases with the third power of its momentum. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z4.00007: Repulsive polarons in two-dimensional Fermi gases Vudtiwat Ngampruetikorn, Jesper Levinsen, Meera Parish We consider a single spin-down impurity atom interacting via an attractive, short-range potential with a spin-up Fermi sea in two dimensions (2D). Similarly to 3D, we show that the impurity can form a metastable state (the ``repulsive polaron'') with energy greater than that of the non-interacting impurity. Moreover, we find that the repulsive polaron can acquire a finite momentum for sufficiently weak attractive interactions. Even though the energy of the repulsive polaron can become sizeable, we argue that saturated ferromagnetism is unfavorable in 2D because of the polaron's finite lifetime and small quasiparticle weight. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z4.00008: Universal bound states of two atoms near a Feshbach resonance Shina Tan The Efimov effect was traditionally thought to exist for three or more particles only. It will be shown how to make universal shallow bound states of TWO atoms only, which will exhibit a universal energy spectrum reminiscent of the Efimov effect, by using potentials to constrain the spatial motion of atoms. Several related types of such two-body states will be described. These diatomic ``artificial molecules", if isolated from each other, will be free from three-body recombination, and can have long lifetimes in principle. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z4.00009: Feshbach Correlations and Closed Channel Amplitudes Nicolas Lopez, Shan-Wen Tsai, Eddy Timmermans The magnetically controlled Feshbach resonance is a prominent member of the cold atom toolkit. The ability to tune binary particle interactions in a quantum many body system has given access to collapsing BEC-physics in bosenovas, to BEC-BCS crossover physics, to the unitarity regime, and to quantum phase transitions. The resonance is accessed by tuning the energy of a quasi-bound spin-rearranged molecular state near the vaccuum of the interacting particles. Does the amplitude of the spin-rearranged or ``closed channel'' state play a significant role in the many body physics? We present a microscopic derivation of the Feshbach resonance interactions and obtain the parameters of the two-channel model in a optical lattice. We study two atoms interacting in a harmonic oscillator potential near a Feshbach resonance to derive the closed channel probabibilty and to uncover the validity-range of the two channel lattice model. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z4.00010: Superfluid pairing in a mixture of a spin-polarized Fermi gas and a dipolar condensate Ben Kain, Hong Ling We consider a mixture of a spin-polarized Fermi gas and a dipolar Bose-Einstein condensate in which s-wave scattering between fermions and the quasiparticles of the dipolar condensate can result in an effective attractive Fermi-Fermi interaction anisotropic in nature and tunable by the dipolar interaction. We show that such an interaction can significantly increase the prospect of realizing a superfluid with a gap parameter characterized with a coherent superposition of all odd partial waves. We formulate, in the spirit of the Hartree-Fock-Bogoliubov mean-field approach, a theory which allows us to estimate the critical temperature when the anisotropic Fock potential is taken into consideration and study how to prepare the mixture in order to optimize the critical temperature at which such a superfluid emerges before the system starts to phase separate. [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z4.00011: Superfluid transition temperature and its zero density limit extrapolation in a unitary atomic Fermi gas on a lattice Qijin Chen The superfluid transition temperature $T_c$ of a unitary Fermi gas has been of great interest. One way to study $T_c$ in a 3D continuum is to study fermions on a lattice at finite densities and then extrapolate to the zero density limit, as has been done in quantum Monte Carlo (QMC) simulation studies. For this extrapolation to work, it is essential to probe the densities in the asympotic regime. In this talk, we study fermions on a three-dimensional isotropic lattice with an attractive on-site interaction as a function of density $n$, from half filling down to $5.0\times 10^{-7}$ per unit cell, using a pairing fluctuation theory. As $n$ decreases towards $n=0$, $T_c/E_F$ increases to the leading order linearly in $n^{1/3}$, and reaches the zero density limit $T_c/E_F = 0.256$, consistent with that calculated directly in the continuum for a contact potential. Inclusion of the particle-hole channel reduces $T_c/E_F$ to 0.217, in agreement with experiment. However, except for very low $n$, $T_c/E_F$ exhibits significant higher order nonlinear dependence on $n^{1/3}$. The densities accessed by QMC studies are still not low enough to be in the asymptotic regime. References: Q.J. Chen, arXiv:1109.5327; arXiv:1109.2307; Q.J. Chen et al, PRL 81, 4708(1998); PRB 59, 7083(1999). [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z4.00012: Pair Condensation in a Finite Trapped Fermi Gas Christopher Gilbreth, Yoram Alhassid Cold atomic fermi gases are widely studied examples of strongly interacting quantum systems. Examples include $^{40}{\rm K}$, $^6{\rm Li}$ and neutron matter. In the unitary regime, where the scattering length is very large compared to the mean inter-particle distance, they are nonperturbative and exhibit universal behavior. Moreover, they can be created in the lab, providing an excellent testing ground for theory. In this talk I will describe quantum Monte Carlo calculations we have been performing to study the signatures of pairing and the superfluid phase transtion in finite-size systems. Using the Auxillary Field Monte Carlo (AFMC) method, we study the pairing gap, condensate fraction, pair wavefunction and density profile as a function of temperature. Defining the onset of condensation $T_{\rm cond}$ as the temperature when the condensate fraction crosses its (finite) noninteracting limit, we consider the question of whether pairing occurs prior to condensation in the unitary regime. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z4.00013: Cooling across the superfluid-normal interface of a unitary Fermi gas - an analogue of a dilution fridge? Arijeet Pal, David Huse Phase separation between paired superfluid and partially polarized normal phases has been observed by various experimental groups around the world using resonantly-interacting spin-imbalanced, hyperfine states of fermionic atoms. In this work we phenomenologically study the effect of the evaporation of atoms and explore the possibility of realizing a non-equilibrium steady state with chemical potential and temperature gradients in some of these experiments. [Preview Abstract] |
Friday, March 2, 2012 1:51PM - 2:03PM |
Z4.00014: Upper Branch Bosons at High Temperatures Weiran Li, Tin-Lun Ho We use a generalized Nozieres-Schmitt-Rink (NSR) approach, which excludes the molecule poles in the T-matrix, to study the ``upper branch'' Bose gases at high temperatures. We show that when we tune the scattering length from positive side across the resonance, the Bose system can remain stable even with attractive interactions at relatively high temperatures. The energy of this upper branch Bose gas has a maximum at negative scattering length, which indicates pair formations are enhanced by Bose statistics in a many body system, in contrast to the Fermionic case where the maximum occurs at positive scattering length.\footnote{V.B.Shenoy and Tin-Lun Ho, arXiv:1106.0960, to appear in PRL.} [Preview Abstract] |
Session Z5: Synthesis, Transport, and Devices Based on Artificially Structured Materials
Sponsoring Units: DCMPChair: Alan Bristow, West Virginia University
Room: 206A
Friday, March 2, 2012 11:15AM - 11:27AM |
Z5.00001: Ultrasmall Silver Nanopores Fabricated by Femtosecond Laser Pulses Jimin Zhao, F. Bian, Y.C. Tian, R. Wang, H.X. Yang, Hongxing Xu, Sheng Meng Ultrasmall nanopores in silver thin films with a diameter of about 2 nm have been fabricated using femtosecond laser ablation in liquid [1]. Ultrafast laser pulse ablation generates highly nonequilibrium excitated states, from which silver thin films emerge and progressively grow with the assistance of capping agent molecules. During this growth process, capping agent molecules are enclaves within the film, leaving individual ultrasmall pores in the thin film. Our first-principles calculations show that the pore size is critically determined by the dimension of the confined molecules. Furthermore, by using smaller capping agent molecules, we were able to fabricate smaller nanopores with 1.6nm diameter. Our approach advances the capability of optical methods in making nanoscale structures with potential applications in areas such as near-field aperture probes, imaging masks, magnetic plasmonic resonances, and biosensing with individual nanopores. \\[4pt] [1] F. Bian, Y. C. Tian, R. Wang, H. X. Yang, H. X. Xu, Sheng Meng, and \underline {Jimin Zhao}, \textit{Ultrasmall Silver Nanopores Fabricated by Femtosecond Laser Pulses}, Nano Lett. \textbf{11}, 3251--3257 (2011). [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z5.00002: Effect of Li+3 Ion Irradiation on Properties of Ta and Dy Doped Bi1.5Zn0.92Nb1.5O6.92 Pyrochlores Bilgehan Yumak, Mehmet Yumak, Ayhan Mergen, Anjum Qureshi Pyrochlore compounds with a general formula of A2B2X7 where A and B are B cations and X is anion exhibit a variety of interesting properties which allow for a broad range of applications, such as high-permittivity dielectrics, as cathode and electrolyte materials in solid electrolytes, host materials for the immobilization of fission products, catalysis, and thermal barrier coatings and fluorescence centers. Pyrochlore oxides can accommodate a wide range of solid solutions between BO2 and A2O3 compounds. Radiation effects in a wide range of pyrochlore compositions have been extensively investigated due to the potential application of pyrochlores. In this study, Ta and Dy-doped Bi1.5Zn0.92Nb1.5O6.92 (BZN) pyrochlore compounds were produced by mixed oxide technique. After determining the solubility limit of Ta and Dy in BZN by XRD, single phase Ta and Dy-doped BZN ceramics were irradiated with different fluences of Li3+ ion irradiation. The microstructures of sintered ceramics before and after irradiation were discovered by scanning electron microscopy. The effect of irradiation on dielectric properites at different frequencies and temperatures were also investigated. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z5.00003: Development of ZnO/Cu Nanolaminate Materials Seth King, Loralee Bilke, Joseph Krueger, Benjamin Oleson, Ian Smith, Benjamin Sturomski Promising materials to replace cost inhibitive indium-tin oxide as transparent conductive oxide layers are ZnO based alloys. Unlike In, Zn is cheap and abundant with a stable supply. Furthermore, ZnO is easily fabricated using standard industrial scale techniques. Therefore, the development of ZnO based materials may greatly advance modern electrical devices. The undoped bandgap of ZnO is 3.4 eV, and may be degenerately doped with donor species such as Al, B, or Ga. For pure ZnO, the electron mobility is ca. 200 cm$^{2}$/V, but decreases significantly with doping due to impurity scattering. Recent studies have suggested that bilayers of doped ZnO and metal may offer low enough resistivity for industrial application. Therefore, it is a logical extension to investigate the properties of ZnO/metal nanolaminate films fabricated from multiple, thin, alternating layers of doped ZnO and a metal. We will present the preliminary results of the development of ZnO based nanolaminate materials consisting of alternating layers of ZnO and Cu fabricated by reactive DC sputter deposition. Our preliminary results suggest that these materials may have applications in photovoltaic devices as well as infrared mirrors. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z5.00004: Nano-structured Au surfaces by off-normal gas cluster ion beam technique Buddhi Tilakaratne, Dharshana Wijesundera, Xuemei Wang, Wei-Kan Chu Surface nano pattern formation has generated great interest in semiconductor, optoelectronics and bio-medical industries. For the past three decade cluster ion beam technology has developed significantly. A gas cluster ion consists thousands of atoms and usually energy per atom in the cluster is about 10eV and cluster ion surface penetration is minimum. Therefore, cluster ion impact form shallow depth modifications. When a cluster ion incident on to the surface off-normal beyond a certain angle from the normal surface target atoms gains forward momentum and creates clear surface nano-ripples between angles 40 to 60 degrees from surface normal. Even though many experiments have demonstrated cluster ion ripple formation, a mechanism of these formations has limited cluster ion beam industrial applications. We will discuss experimental results of surface evolution during the cluster ion off-normal incident irradiation and azimuthal sample rotation dependence of surface patterns, and compare results with a theoretical model. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z5.00005: Diamondoids enables 10nm resolution on X-ray PEEM Hitoshi Ishiwata, Hendrik Ohldag, Andreas Scholl, Olav Hellwig, Peter Schreiner, Jeremy Dahl, Nick Melosh, Z.X. Shen Diamondoids are the smallest sp3 bonded carbon cage that can be found in different sizes and shapes, starting from single cage called adamantine that contain 10 carbons all hydrogen terminated on outside. While the electronic structure and material properties of diamondoids are based on bulk diamond, their nanostructure allows for tailoring their properties to a particular task. Diamondoids can be produced reliably and cost effective in different sizes and quantities. They exhibit a tremendous potential for real-world applications, e.g. as seed crystals in diamond growth, as robust mechanical coatings or as highly efficient electron cathodes. Here, we will focus on the latter, showing how diamondoid coating can be used to push the spatial resolution to the limit and significantly increasing the efficiency of an x-ray based cathode lens microscope (X-ray Photoemission Electron Microscope or XPEEM) by minimizing the effect of chromatic aberrations caused by the energy spread of the electrons emitted from the cathode. The unique feature of XPEEM microscopy is that it is capable of acquiring images with magnetic, chemical, structural as well as topographical contrast of surfaces and shallow interfaces. The capability to obtain such images with 10nm spatial resolution through simple, low-cost and readily available nanodiamond coating represents a major step forward towards real world application of these promising nano materials. (Presenting author: Z.X. Shen) [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z5.00006: ABSTRACT HAS BEEN MOVED TO B17.00008 |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z5.00007: Quantum-Dot Cellular Automata On A Hydrogenated Silicon Surface: An Exact Diagonalization Study Burkhard Ritter Quantum-dot cellular automata (QCA) is an alternative computing paradigm for molecular-scale quantum devices. We present results from the first detailed exact diagonalization study of QCA systems using a richly extended Hubbard model. A controlled Hilbert space truncation alleviates the scaling problem and provides access to moderate system sizes. We characterize the static signal transmission for short wires and identify suitable material and system parameters for dangling bonds on a hydrogen-terminated silicon surface, a recent experimental realization. We discuss the challenges for a realistic, working implementation. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z5.00008: The chemical analysis about post annealing effect of HfO2 on Si-passivated GaAs Sang Han Park, Hyo Jin Kim, Dae-kyong Kim, Mann-Ho Cho, Chung Yi Kim, Hyoungsub Kim In order to develop a high performance MOS device, 3-5 based semiconductors as a high carrier transport semiconductors have been seriously considered. Especially, GaAs with HfO2 as gate dielectric material attract as a candidate for future MOS FET device. Since, larger trap density at HfO2/GaAs interface than for HfO2/Si interface degrade device performance, Si interfacial layer was introduced to reduce interfacial trap. Moreover, Si reduces intrinsic defects at GaAs surface by reconstruct Ga or As homo bonds. In this study, we focused on changes in the chemical and structural characteristics of HfO2/Si/GaAs film as a function of post annealing temperature. The interfacial reactions induced by post annealing were investigated by XPS, REELS, and XAS. The results show that Si layer decrease the diffusion and oxide formation of Ga and As. Also, the post nitridation significantly improve the diffusion barrier by forming the Ga-N layer. XAS result also consists with the fact that the post nitridation suppress Ga diffusion. The band offsets between GaAs and high-k gate dielectric were aligned using XPS and REELS. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z5.00009: Terahertz Generation Based on Gunn Oscillations in Unipolar Nanodiodes Mubarak Ali, Aimin Song A pressing concern in the terahertz (THz) deployment of technology is the lack of efficient, compact, solid-state THz emitters. Recently, self-switching devices (SSDs) have been demonstrated to offer a 2D planar technology with ultra-low parasitic capacitance, which enabled detection of microwave radiation up to 2.5 THz.$^{ }$Furthermore, it is possible to integrate a large array of SSDs in parallel in order to reduce overall impedance and hence reduce thermal noise. Monte Carlo simulations showed that the SSD can also emit radiation based on Gunn oscillations. In this work, we modeled using Silvaco Atlas to provide evidence of dipole domain formation in the channel and systematically study the dependence of emission frequency and intensity as a function of channel length and width as well as interface-charge density. The results showed that the fundamental oscillation frequency can reach as high as 400 GHz, whereas higher harmonics go well beyond 1.2 THz. By constructing an array that contains different geometries of SSDs placed in parallel, we expect to achieve frequency tuning in wide or narrow bands, which may have useful implications to practical applications. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z5.00010: Tuning electric and magnetic responses with structure evolution in a metamaterial Xiang Xiong, Mu Wang, Ruwen Peng We demonstrated that in an assembly of double-layered metallic U-shaped resonators, magnetic and electric responses are realized respectively. The two different types of responses are realized at higher and lower frequency. When the U-shaped structure resonators evolve into H-shaped structure gradually, the electric and magnetic frequencies move forward to each other. In this case, when electric response and magnetic response are overlapped in frequency, negative refractive index can be realized. [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z5.00011: Configuration interaction in a tunable wavelength shifter Aleksej Mialitsin, Ilia Solov'yov, Stefan Schmult, Brian Fluegel, Angelo Mascarenhas It has recently been demonstrated how hybridized plasmon-phonon collective excitations in GaAs can be blue shifted by about 20 wavenumbers (0.6 THz) relative to the unperturbed longitudonal optical lattice vibration frequency as a function of excitation beam intensity [Fluegel et al., Nat. Phot. 1, 701 (2007)]. At beam intensities greater than 10 mW/cm$^{-2}$ the wavelengh shifted mode broadens and begins to exhibit a double-peak structure. We attribute this line-shape modification to configuration interaction of the named mode with the edge of the continuous background and discuss potential implications for coupled plasmon-phonon modes generated in semiconductor hetero-structures. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z5.00012: Secondary Electron Emission (SEE) Calculations Harold P. Hjalmarson, Ronald P. Kensek, Kenneth E. Kambour Secondary electron emission (SEE) from solids is a consequence of energy loss by charged particles such as electrons. One important energy loss mechanism to be considered involves plasmon emission by the charged particle. Plasmons create electron-hole pairs when they decay. Under certain conditions, a sufficiently energetic electron, a secondary electron, may be emitted from the solid surface. In this presentation, results from two different approaches to this process will be presented. A particle-based Monte Carlo method and a continuum method will be used for these calculations. The results from the two different methods will be compared to each other to understand the effects of the various approximations in the two methods. The methods will be illustrated by an application to titanium dioxide (rutile). --Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z5.00013: Dark Field Imaging of Plasmonic Resonator Arrays Atilla Aydinli, Sinan Balci, Ertugrul Karademir, Coskun Kocabas We present critical coupling of electromagnetic waves to plasmonic cavity arrays fabricated on Moir\'{e} surfaces. The critical coupling condition depends on the superperiod of Moir\'{e} surface, which also defines the coupling between the cavities. Complete transfer of the incident power can be achieved for traveling wave plasmonic resonators, which have relatively short superperiod. When the superperiod of the resonators increases, the coupled resonators become isolated standing wave resonators in which complete transfer of the incident power is not possible. Dark field plasmon microscopy imaging and polarization dependent spectroscopic reflection measurements reveal the critical coupling conditions of the cavities. We image the light scattered from SPPs in the plasmonic cavities excited by a tunable light source. Tuning the excitation wavelength, we measure the localization and dispersion of the plasmonic cavity mode. Dark field imaging has been achieved in the Kretschmann configuration using a supercontinuum white light laser equipped with an acoustooptic tunable filter. Polarization dependent spectroscopic reflection and dark field imaging measurements are correlated and found to be in agreement with FDTD simulations. [Preview Abstract] |
Friday, March 2, 2012 1:51PM - 2:03PM |
Z5.00014: Engineered Uniform Conduction Fronts in Memristive/Memcapacitive Systems Patrick Mickel, Conrad James We introduce here a novel ``memristor'' design enabling the uniform propagation of the conduction front within the device, improving performance as well as device-to-device consistency. Typically, resistive switching in memristors occurs due to the localized formation of conductive filaments. Electric fields are magnified at filament tips (due to decreased separation, E = V/d), amplifying growth rates for select filaments and producing a localized and highly non-uniform conduction front. However, we show that by incorporating specifically spaced layers with alternating ionic mobilities the electric field magnification can be counterbalanced, resulting in a uniform conduction front. The uniform conduction front lowers device-to-device variability, improves analog tuning and significantly amplifies the memcapacitive properties of the device. These multilayered engineered nanostructures have potential applications in multi-bit memory storage and neuromorphic computing architectures. [Preview Abstract] |
Session Z6: Focus Session: Carbon Nanotube Electronics, Properties, and Devices
Sponsoring Units: DMPChair: Gyula Eres, Oak Ridge National Laboratory and Eric Pop, University of Illinois at Urbana-Champaign
Room: 206B
Friday, March 2, 2012 11:15AM - 11:27AM |
Z6.00001: ABSTRACT WITHDRAWN |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z6.00002: Exploring adiabatic/non-adiabatic phase transitions in suspended metallic carbon nanotubes Shun-Wen Chang, Rohan Dhall, Zuwei Liu, Stephen Cronin We investigate the non-adiabatic Kohn anomaly in suspended pristine metallic single-walled carbon nanotubes by studying the dependence of the Raman $G$ band and $2D$ band frequency on Fermi energy. We find that by varying temperature, metallic nanotubes can switch between a regime in which the non-adiabatic Kohn anomaly is clearly observed, to a regime where the non-adiabatic Kohn anomaly is absent. Furthermore, we find that the non-adiabatic Kohn anomaly is always accompanied by a dramatic gate-induced modulation of the $G$ band Raman intensity. By establishing a quantitative correlation between the strength of the non-adiabatic Kohn anomaly and the modulation of Raman intensity, we determine that the underlying mechanism that leads to both these effects is the same. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z6.00003: Current-Induced Cleaning of Adsorbates from Suspended Semiconducting Carbon Nanotube Diodes Argyrios Malapanis, Everett Comfort, Ji Ung Lee Single-walled carbon nanotubes (SWNT) are prime candidates for future applications, including nanoelectronic and nanophotonic devices. Because of their large surface-to-volume ratio compared to that of a bulk seminconductor, however, SWNTs are very sensitive to their environment. Others have already established that their electronic properties can be dramatically changed by exposure to air, particularly oxygen or water. In this paper,\footnote{A. Malapanis, E. Comfort, and J. U. Lee, Appl. Phys. Lett., 98, 263108 (2011).} we show that the electronic and optical properties of $p-n$ diodes fabricated with suspended semiconducting SWNTs degrade over time with exposure to ambient conditions, mainly due to adsorption onto the tube's suspended part, which creates band-gap states. We provide the first correlation between adsorbate-generated electronic states and their impact on diode performance. Specifically, we show that the ideality factor, a fundamental parameter used to measure defect states in a $p-n$ diode, increases with the degree of adsorbate coverage. We also demonstrate a simple technique---current annealing---that can thermally reverse such degradation, returning device properties to their original characteristics. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z6.00004: Electronic performance of flexible single-wall carbon nanotube films: The role of electronic type John M. Harris, Steven D. Hudson, Jeffrey A. Fagan, Erik K. Hobbie Recent advances in the separation of single-wall carbon nanotubes (SWCNTs) by length and electronic type have made highly monodisperse SWCNT membranes a reality, opening up new realms of potential application in flexible electronics. By measuring the coupling between mechanical flexibility and electronic performance for thin transparent films of metallic and semiconducting SWCNTs assembled on elastic polymer substrates, we demonstrate a marked difference in the electronic manifestations of thin-film deformation for the two electronic SWCNT types. We relate these differences to mechanical and interfacial phenomena that stem from the distinct optical resonances characteristic of metallic or semiconducting nanotubes, and we evaluate the durability of each film type in response to repeated mechanical strain. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z6.00005: Origin of Capacitance Change in Semiconducting and Metallic Carbon Nanotubes Seong Chu Lim, Dinh Loc Duong, Young Woo Jo, Tae Hyung Kim, Soo Min Kim, Jung Hyun Yoon, Ha Ryong Hwang, Slava Rotkin, Young Hee Lee Different mechanisms are reported for the capacitance changes in metallic (m-) and semiconducting (s-) single-walled carbon nanotubes (SWCNTs) upon gas adsorption. The sensitivity, i.e., the change in capacitance, was high and reached approximately 2500{\%} in the aligned s-SWCNT network, whereas the sensitivity was found to be low in the m-SWCNT network. The charge transfer/Fermi level shift and quantum capacitance related to the localized electronic density of states near the Fermi level were key contributors to sensitivity changes upon gas adsorption, although the polarization effect also played a role in the capacitance changes, particularly in the aligned CNTs, under a strong electric field. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:51PM |
Z6.00006: Carbon based devices for molecular quantum spintronics Invited Speaker: Wolfgang Wernsdorfer This presentation will address a new field called molecular quantum spintronics, which combines the concepts of spintronics, molecular electronics and quantum computing [1]. Various research groups are currently developing low-temperature scanning tunnelling microscopes to manipulate spins in single molecules, while others are working on molecular devices (such as molecular spin-transistors, spin valves and filters, and carbon-nanotube-based devices [1]) to read and manipulate the spin state and perform basic quantum operations. The talk will discuss the read-out of the spin states of single-molecule magnets using carbon based devises. In particular, carbon nanotube devices [2] and graphene nano-constrictions [3] will be discussed.\\[4pt] [1] L. Bogani \& W. Wernsdorfer, Molecular spintronics using single-molecule magnets, Nature Mat. 7, 179 (2008).\\[0pt] [2] M. Urdampilleta, S. Klyatskaya, J.-P. Cleuziou, M. Ruben, W. Wernsdorfer. Supramolecular Spin Valves. Nature Mat. 10, 502 (2011).\\[0pt] [3] A. Candini, S. Klyatskaya, M. Ruben, W. Wernsdorfer, M. Affronte. Graphene Spintronic Devices with Molecular Nanomagnets. Nano Lett. 11, 2634, (2011). [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z6.00007: Poole-Frenkel emission by carbon nanotube defect sites Deng Pan, Brad Corso, Philip Collins Single walled carbon nanotubes (SWCNTs) have a conductance that is particularly sensitive to the presence of defects and disorder. Here, we fabricate field effect devices out of individual SWCNTs in order to investigate this effect. The bias- and gate-dependent conductance of SWCNT devices is measured over a temperature range of 77 -- 400 K. By performing these measurements on the same SWCNT before and after the incorporation of a point defect, we clearly discern the electronic consequences of the addition. Specifically, the initial recording of the pristine SWCNT determines the energy-dependent resistances of the SWCNT itself. After electrochemical point functionalization to introduce a defect site, the additional resistance and its energy-dependence is determined by properly accounting for the initial contributions. We find the defect scattering to be well fit by a Poole-Frenkel emission model, with the consequence that barrier widths and heights can be extracted for different defect types. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z6.00008: Scanning Probe Characterization of Electronic Scattering by Carbon Nanotube Defects Elliot Fuller, Steven Hunt, Brad Corso, Philip Collins Single-walled carbon nanotubes (SWCNTs) are quasi-ballistic, one-dimensional conductors with unique electronic properties. The controlled addition of a single covalent, sidewall defect interrupts this conduction pathway and produces a disproportionate change in the device properties. Here, we investigate the electronic effects in the immediate vicinity of a defect site using Kelvin force microscopy (KFM) and scanning gate spectroscopy (SGS). KFM images the local electrostatic surface potential along a SWCNT and thus can directly measure the potential drop at defects and other electronic surface features. Using KFM, we clearly observe the bias dependence of resistance at the site of defects and/or chemical attachments. The SGS technique provides complementary information by mapping their gate dependence. Combined, these two scanning probe techniques resolve the full energy dependence of scattering by a defect site and allow the determination of effective barrier widths and heights for different types. [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z6.00009: Fundamental Limits of Current Flow in One-dimensional Carbon Nanomaterials Albert Liao, Christopher Neumann, Eric Pop Carbon nanotubes (CNTs) and graphene nanoribbons (GNRs) carry very high current densities given their strong sp2 bonds and high carrier mobility. However, the fundamental limitations to their maximum current flow are still not well understood. We measured the maximum current flow of CNTs and GNRs in substrate-supported configurations in ambient air where devices break from Joule heating at $\sim $600 C [1,2], revealing information about their power dissipation. Interestingly, thermal coupling with the substrate increases with CNT diameter but decreases with GNR width, due to competing roles of thermal boundary resistance and heat spreading into the substrate. To further study quasi-metallic single-wall CNTs at very high fields, we performed measurements in vacuum. We found some devices show current saturation as expected [3], but in many the current continues to increase with a constant slope of $\sim $~1~$\mu $A/V at fields $>$10 V/$\mu $m. This is observed even in small-diameter ($\sim $1.2 nm) CNTs whose dimensions were verified by AFM and Raman spectroscopy. We suggest such behavior can be explained by a combination of better heat coupling with the substrate and higher subband conduction. [1] A. Liao \textit{et al}, PRB \textbf{82}, 205406 (2010). [2] A. Liao \textit{et al}, PRL \textbf{106}, 256801 (2011). [3] Z. Yao \textit{et al}, PRL \textbf{84}, 2941 (2000). [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z6.00010: Terahertz Detection in an Individual Single-Walled Carbon Nanotube Joel Chudow, Daniel Santavicca, Chris McKitterick, Luigi Frunzio, Daniel Prober, Philip Kim Carbon nanotubes (CNTs) serve as a test experimental system for verification of physical models of one-dimensional (1-D) conduction. We aim to excite terahertz standing wave resonances on a CNT, which are predicted to display Luttinger-liquid behavior due to the lack of screening in 1-D. We describe measurements of terahertz (THz) absorption in individual single-walled carbon nanotubes and distinguish between two response mechanisms: bolometric detection due to heating a CNT with a temperature-dependent resistance and the response due to non-thermal electrical contact nonlinearities. This is the first frequency-domain demonstration of THz detection in an individual CNT. The effect of the contact nonlinearity is not decreased at THz frequencies and allows for analysis of the parallel contact capacitance to an individual CNT. Both detection mechanisms are expected to give evidence of the Luttinger-liquid resonant behavior. This experimental technique provides a method to study high-frequency charge excitations in the nanotube as a probe of the strength of the electron-electron interactions in this 1-D system. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z6.00011: Individual Single-Walled Carbon Nanotube Device Fabrication for Terahertz Detection Chris McKitterick, Joel Chudow, Daniel Santavicca, Luigi Frunzio, Daniel Prober, Philip Kim We describe the fabrication process for the development of antenna-coupled single-walled carbon nanotube (CNT) terahertz (THz) detectors. This requires the development of a lithographically-defined gate electrode, as the device is fabricated on a THz-transparent insulating silicon substrate. Antenna development is geared towards accessing the largest possible bandwidth to facilitate the study of standing wave resonances in the CNT. These standing wave resonances are expected from the one-dimensional Tomonaga-Luttinger Liquid theory, which predicts collective charge oscillations, known as plasmons, which travel faster than the Fermi velocity. Using a Fourier transform interferometer, we excite the antenna-coupled CNT with a broadband blackbody source and probe the THz impedance of the CNT through Joule heating. [Preview Abstract] |
Friday, March 2, 2012 1:51PM - 2:03PM |
Z6.00012: In-situ TEM study of heat transfer between a carbon nanotube and contacting material Merijntje Bronsgeest, Norvik Voskanian, Hanna Nilsson, John Cumings Due to their interesting intrinsic properties and 1D nature, carbon nanotubes may be (part of) the solution to the heat management challenge in nanoelectronics in multiple ways: e.g. as thermal interface material and/or interconnects in existing Si-based electronics, or as a building block in carbon-based electronics. Of crucial importance for any of these applications is the (in)ability to get heat in and out of the tube: the thermal contact resistance. Thermal contact resistance for carbon nanotubes is not fully understood yet, and theory is under construction. Measuring it is a challenge of its own as it is intimately connected to the thermal conductivity of the connected materials which are not necessarily known that well either. We study the thermal properties of carbon nanotubes with Electron Thermal Microscopy [T. Brintlinger et al., Nano Lett. 8, 582 (2008)], which allows for thermal imaging with a resolution of 150 nm, and combine that with finite-element modeling. With this approach we have already demonstrated that also on a substrate the CNT thermal conductivity is high, and that different heat transfer mechanisms (e.g. phonon-phonon, electron-surface polariton interaction) can be important. Our goal is to study and quantify thermal contact resistance between a carbon nanotube and its surroundings and we will present our latest results. [Preview Abstract] |
Friday, March 2, 2012 2:03PM - 2:15PM |
Z6.00013: Remote Joule heating of crossed nanotube Norvik Voskanian, Kamal H. Baloch, Merijntje S. Bronsgeest, John Cumings The high thermal conductivity of carbon nanotubes makes them an excellent candidate for thermal management and thermal logic devices. We have studied the thermal characteristics of Joule-heated MWNTs in a crossed geometry, using an established thermal measurement technique that relies on the solid to liquid phase transition of indium islands [1]. Our experimental observations show the efficient transfer of heat from CNT to substrate but inefficient heat transfer past the crossing point. This supports the presence of a nonlocal joule heating phenomenon in which the hot electrons from the biased CNT directly transfer their energy to a nearby material. This talk will cover the developed thermal characterization technique, experimental results and simulations. \\[4pt] [1] T. Brintlinger, et al., Nano Lett. \textbf{8}, 582 (2008). [Preview Abstract] |
Session Z7: Transport and Optical Properties of Non-Carbon Nanotubes and Nanowires
Sponsoring Units: DCMPChair: Xiuling Li, University of Illinois at Urbana-Champaign
Room: 207
Friday, March 2, 2012 11:15AM - 11:27AM |
Z7.00001: Exchange induced electron transport in heavily n-doped Si nanowires Joonyeon Chang, Tae-Eon Park, Byoung-Chul Min, Ilsoo Kim, Jae-Eun Yang, Moon-Ho Jo, Heon-Jin Choi Silicon nanowires (Si NWs) have been used as building blocks in the ``bottom-up'' approach to nanoelectronics because of their excellent electrical properties. Despite the potential of Si NWs, unfortunately, there is little known about the electrical properties of heavily doped Si NWs. We have synthesized heavily doped $n$-type Si NWs and measured the electrical resistance using four-probe method. As we decrease the temperature, the resistivity of Si NWs decreases initially, shows a resistivity minimum around 60 K, and thereafter increases logarithmically. Below the resistivity minimum temperature (T$_{min})$, we have observed a dip around zero-bias in the differential conductance, and a negative magnetoresistance (MR) which depends on the angle between the applied magnetic field and current flow. These results are associated with the impurity band conduction and electron scattering by the localized spins at phosphorus donor states. The analysis on the MR reveals that the localized spins are coupled antiferromagnetically at low temperature via the exchange interaction. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z7.00002: Low-field mobility in ultrathin silicon nanowire junctionless transistors Bart Soree, Wim Magnus, William Vandenberghe We theoretically investigate the phonon, surface roughness and ionized impurity limited low-field mobility of ultrathin silicon n-type nanowire junctionless transistors in the long channel approximation with wire radii ranging from $2$ to $5$ nm, as function of gate voltage. A few years ago, the junctionless nanowire transistor (JNT) or pinch-off nanowire was proposed by several research groups and was recently fabricated for the first time. The JNT is a uniformly doped nanowire with no junctions, i.e. source, channel and drain are doped with the same doping type. The main motivation for introducing this novel device concept are the absence of doping junctions which makes the fabrication easier, and the reduction of detrimental interactions occuring at the interface between the silicon body of the wire and the insulator (surface roughness). We investigate the case where due to quantum mechanical confinement the surface roughness scattering becomes again important and we report on the behavior of phonon, ionized impurity and surface roughness limited mobility as a function of radius and gate voltage. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z7.00003: Photovoltaic properties of axial in-situ doped SiGe heteronanowires Son T. Le, A.D. Mohite, D.E. Perea, H. Htoon, P. Jannaty, A. Zaslavsky, S.T. Picraux We report on vapor-liquid-solid (VLS) growth and photovoltaic properties of axial in-situ doped Ge-Si heteronanowire (hetero-NW) solar cells. Modern VLS growth has recently demonstrated the growth of Si-Ge axial NWs with abrupt heterojunctions [1], and simultaneous control of both material composition and doping profiles. This advance allows for the integration of Ge with Si in a hetero-NW structure for broad-spectrum and high absorption efficiency solar cells. Our preliminary optical measurements on $p$-Ge/$i$-Si/$n-$Si wires provide important ingredients for a hetero-NW photovoltaic device. We achieved good rectification with $\sim $ 10$^{3}$ ratio between forward and reverse bias currents at moderate voltage. Under laser illumination (532 nm), we measured a large open circuit voltage $V_{OC} \quad \sim $ 0.54 V and a high short-circuit current density $J_{SC} \quad \sim $ 4x10$^{3}$A/cm$^{2}$, comparable to state-of-the-art reported single NW results [2]. Additional optimization of separate Ge and Si \textit{pin} NW structures and their integration in a combined Ge/Si tandem hetero-NW solar cell will be reported. \\[4pt] [1] D. E. Perea \textit{et al.}, \textit{Nano Lett.} \textbf{11}, 3117 (2011). \\[0pt] [2] B. Tian \textit{et al.}, \textit{Chem. Soc. Rev.} \textbf{38}, 16 (2009). [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z7.00004: Strong Tuning of Spin Orbit Interaction in an InAs Nanowire by an Electrolyte Gate Dong Liang, Xuan P.A. Gao Manipulating electron spin in solid state devices has been the main target of spintronics. A key concept in many spintronic devices is to apply gate voltage or electric field ($E)$ to tune spin precession via the Rashba spin orbit interaction (SOI). Quasi-one dimensional (1D) indium arsenide (InAs) nanowires are promising platforms in this regard due to InAs's strong intrinsic SOI and more effective control of Rashba SOI in 1D system with uni-directional momentum. Here, we demonstrate efficient control of Rashba SOI where $E$ is created at the surface of InAs nanowire by solid electrolyte surrounding gate. Six-fold tuning of Rashba coefficient and 2.5 order of magnitude tuning of spin relaxation time are achieved within only 1 V of gate bias. Such a dramatic tuning of SOI paves a way towards quasi-1D nanowire spintronic devices with low power consumption. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z7.00005: InAs/InSb axially heterostructured nanowires for small junction area semiconductor diodes Alessandro Pitanti, Daniele Ercolani, Lucia Sorba, Stefano Roddaro, Fabio Beltram, Lucia Nasi, Giancarlo Salviati, Alessandro Tredicucci Semiconductor nanowires are low-dimensional systems which appear as ideal candidates to enable ultra-fast electronic technology. Even if the realization of complex [1], nanowire-based electronic devices such as transistors and diodes has been demonstrated, the ultra-small axial electronic capacitance, related to the semiconductor tiny cross-section, is rarely exploited, due to the difficulty in creating homo/hetero-junctions within the nanowire itself. We propose a solution to produce axial, majority-carrier diodes, by employing growth interruption to create InAs/InSb heterojunction along the nanowire axis [2]. Despite being both InAs and InSb n-type materials, their broken gap band alignment produces strong asymmetry in the I-V characteristic, similarly to standard Schottky-barrier diodes. The band line-up determines a strong nonlinear current response at positive source-drain bias: when an additional InP barrier layer is inserted in-between the heterojunction, the maximum nonlinearity is drawn towards zero bias and the leakage current reduced, making these devices promising candidates for high cut-off frequency rectifying detectors/photomixers. \\[4pt] [1] N. Wang et al., Mat. Sci. Eng. R 60, 1 (2008).\\[0pt] [2] A. Pitanti et al. Phys. Rev. X 1, 011006 (2011). [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z7.00006: Growth and electrical properties of vertically oriented organic single-crystalline nanostructures San Theingi, Tim Mirabito, Alejandro Briseno, Katherine Aidala We present our growth of vertically oriented organic single crystalline nanowires of copper tetra-tertbutylphthalocyanine (t-butyl-CuPc) on highly ordered pyrolytic graphite (HOPG). The nanowires are grown by using physical vapor transport (PVT) method, with directed crystallization on ordered substrates. Single crystal nanostructures are free of grain boundaries and this promotes charge transport through the material. The pi-pi interactions between adjacent molecules of the single crystal provide a direct path for charge transfer, leading to high carrier mobility. Vertically oriented single-crystal nanostructures have applications as high surface area solar cells. We are studying the electrical conductivity of these nanostructures using conductive atomic force microscopy (CAFM). The AFM tip makes an electrical contact to an individual wire, and current is measured as a function of applied voltage. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z7.00007: Rectification in an idealized model junction of strongly correlated electrons Florian Sabou, Natalie Bodington-Rosen, Brad Marston Junctions of oppositely doped Mott insulators offer the possibility of rectification at extremely high frequencies. To simulate an idealized junction we use a model of spinless electrons moving in one dimension, the t-V chain, and control the chemical potential on the two halves of the chain to create a p-n junction. For short chains the many-body Schrodinger equation can be integrated forward in time numerically exactly, and we find that when subjected to an oscillating electric field the system rectifies by transferring charge in a preferred direction. Dissipation can be included in a phenomenological way by rotating time slightly off the real axis. The time dependent density-matrix renormalization-group may be used to extend the simulation to longer chains with spinning electrons. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z7.00008: Temperature effects on the Raman lineshape of cupric oxide thin films and nanowires Shrividya Ravi, Alan Kaiser, Christopher Bumby The Raman spectrum can be used as a fingerprint for a given material but the details of the Raman spectrum (e.g. the peak lineshape) can also impart considerable information about various environmental effects. In this study, we will describe the effects of heating on the Raman lineshape using a temperature-dependent model. Homogeneous heating (i.e. spatially uniform temperature) leads to symmetrically broadened lineshapes while asymmetric lineshapes can be associated with electron-phonon coupling in or phonon confinement effects. Recently, several groups have found that asymmetric lineshapes from nanoparticles and nanowires require an additional term that describes the inhomogeneous heating - temperature gradients within the sample - due to a gaussian laser beam. In our samples, phonon confinement does not play a role at all as nanowire dimensions are well above 20 nm. Hence, we have invoked a purely temperature-dependent model. This model has considerable flexibility as it can explain both the symmetrical and asymmetrical broadening of the lineshape. We have used it to describe and compare a thin film of cupric oxide (which mostly exhibits homogeneous heating effects) and a vertical forest of cupric oxide nanowires (which exhibit effects of inhomogeneous heating). [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z7.00009: Nonlocal Response of the Plasmonic Nanowire Metamaterials Brian Wells, Anatoly Zayats, Viktor Podolskiy Nanowire metamaterials are a class of composite photonic media formed by an array of aligned plasmonic nanowires embedded in a dielectric matrix. Depending on exact composition, geometry, and excitation wavelength, nanowire structures are known to exhibit elliptical, hyperbolic, or epsilon-near-zero (ENZ) responses. In the ENZ regime optical response of the composite becomes strongly nonlocal. Excitation of an additional wave, caused by nonlocality, has been experimentally demonstrated in nanowire-based metamaterials. Here we present numerical and analytical studies of the nonlocal optical response of plasmonic nanowire metamaterials. Dispersion of photonic modes of plasmonic metamaterials has been studied in finite-element-method (FEM) simulations as a function of wavelength, geometry, and material parameters. Analytical description of nonlocal effective permittivity tensor has been developed. These analytical results are in agreement with FEM simulations and experimental data. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z7.00010: ABSTRACT WITHDRAWN |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z7.00011: Equilibration of Luttinger liquid and conductance of quantum wires K.A. Matveev, A.V. Andreev Luttinger liquid theory describes one-dimensional electron systems in terms of non-interacting bosonic excitations. In this approximation thermal excitations are decoupled from the current flowing through a quantum wire, and the conductance is quantized. We show that relaxation processes not captured by the Luttinger liquid theory lead to equilibration of the excitations with the current and give rise to a temperature-dependent correction to the conductance. In long wires, the magnitude of the correction is expressed in terms of the velocities of bosonic excitations. In shorter wires it is controlled by the relaxation rate. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z7.00012: Rectification in Y-junctions of Luttinger liquid wires Chenjie Wang, D.E. Feldman We investigate rectification of a low-frequency ac bias in Y-junctions of one-channel Luttinger liquid wires with repulsive electron interaction. Rectification emerges due to three scatterers in the wires. We find that it is possible to achieve a higher rectification current in a Y-junction than in a single wire with an asymmetric scatterer at the same interaction strength and voltage bias. The rectification effect is the strongest in the absence of the time-reversal symmetry. In that case, the maximal rectification current can be comparable with the total current $\sim e^2V/h$ even for low voltages, weak scatterers and modest interaction strength. In a certain range of low voltages, the rectification current can grow as the voltage decreases. This leads to a bump in the $I$-$V$ curve.\\[4pt] [1] Chenjie Wang and D. E. Feldman, Phys. Rev. B {\bf 83}, 045302 (2011). [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z7.00013: Phonon scattering in a strongly-driven 1D electron system Kathleen E. Hamilton, Leonid P. Pryadko, Alexey A. Kovalev We consider phonon relaxation kinetics of a non-equilibrium, one-dimensional electron system driven by a strong, high frequency electric field. For a single-band system, and assuming that the phonon scattering rate is small on the scale of the driving frequency $\omega$, we derive and solve the coupled equations for the Keldysh Green's functions and self energies. In the presence of the periodic driving field, the electrons' energy is replaced by the Floquet quasienergy; in the single-band case it just equals the energy averaged over the period (modulo $\hbar\omega$). The energies of the phonons emitted or absorbed by the system correspond to transitions between the Floquet energy bands, and are strongly dependent on both the amplitude and the frequency of the driving field. Of note is a system in the regime of dynamical localization, where the average electron kinetic energy vanishes. Here, the phonon energies must be in resonance with harmonics of the driving frequency, and the stationary electron distribution function reduces to a constant, with infinite effective temperature. [Preview Abstract] |
Friday, March 2, 2012 1:51PM - 2:03PM |
Z7.00014: Nonequilibrium transport through a spinful quantum dot with superconducting leads Jens Paaske, Brian Moeller Andersen, Karsten Flensberg, Verena Koerting We study the nonlinear cotunneling current through a spinful quantum dot contacted by two superconducting leads. Applying a general nonequilibrium Green function formalism to an effective Kondo model, we study the rich variation in the IV-characteristics with varying asymmetry in the tunnel coupling to source and drain electrodes. The current is found to be carried respectively by multiple Andreev reflections in the symmetric limit, and by spin-induced Yu-Shiba-Russinov bound states in the strongly asymmetric limit. The interplay betweeen these two mechanisms leads to qualitatively different IV-characteristics in the cross-over regime of intermediate symmetry, consistent with recent experimental observations of negative differential conductance and re-positioned conductance peaks in sub-gap cotunneling spectroscopy. [Preview Abstract] |
Session Z8: Focus Session: Frustrated Magnetism - Magnetic insulators
Sponsoring Units: DMP GMAGChair: Vesna Mitrovic, Brown University
Room: 208
Friday, March 2, 2012 11:15AM - 11:27AM |
Z8.00001: Nature of the low energy excitations in the spin liquid state of Cs$_2$CuCl$_4$ Altan Allawala, Vesna Mitrovic, Brad Marston, Georgios Koutroulakis, Radu Coldea We have performed detailed measurements as a function of temperature and applied magnetic field of the NMR rate in the spin liquid phase of the spin-1/2 frustrated antiferromagnet Cs$_2$CuCl$_4$. Comparison of the magnetization and relaxation rate to the spin-1/2 antiferromagnetic chain $\alpha$-CuNSal and to variational calculations using Gutzwiller-projected mean-field theory implies that the low energy excitations in Cs$_2$CuCl$_4$ are characterized by gapless fermionic excitations in the spin liquid phase at non-zero temperature and applied field. To investigate the ability of one dimensional versus two dimensional models to reproduce the low energy properties of Cs$_2$CuCl$_4$ \footnote{M.-A. Vachon {\it et al.}, New J. Phys. {\bf 13} 093029 (2011)} we compare the measured T$^{-1}$ NMR rate to a field theoretical description of a Luttinger liquid\footnote{H. K{\"u}hne {\it et al.}, Phys. Rev. B {\bf 83} 100407(R) (2011)}. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z8.00002: ESR studies of the quasi-2D frustrated Cs2CuBr4 S.A. Zvyagin, D. Kamenskyi, J. Wosnitza, M. Ikeda, T. Fujita, M. Hagiwara, O.A. Starykh, *R. Hu, H. Ryu, C. Petrovic We report low-temperature electron spin resonance (ESR) studies of single-crystalline samples of Cs$_2$CuBr$_4$, a spin-1/2 antiferromagnet with a triangular spin-lattice structure. A remarkable angular dependence of the resonance field, including the splitting of the ESR line for some orientations of the magnetic field, and the presence of a gap in the ESR excitation spectrum at temperatures above $T_N\sim$ 1.3~K have been revealed. Our observations suggest that uniform Dzyaloshinskii-Moriya interaction affects the low-energy excitation spectrum in this frustrated compound. The results are compared with that obtained recently for the isostructural material Cs$_2$CuCl$_4$ [Povarov et al., Phys. Rev. Lett. 107, 037204 (2011)]. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z8.00003: NMR Spectra in 2D Anisotropic Triangle Lattice Adam Frees, Vesna Mitrovic The spin 1/2 Heisenberg antiferromagnet on the 2D anisotropic triangle lattice represents an important example of the frustrated quantum magnets. The materials Cs$_2$CuCl$_4$ and Cs$_2$CuBr$_4$ are relevant model systems of such a frustrated magnet. In these compounds magnetic fields can induce numerous exotic quantum states. The states are characterized by their spin texture, which can be measured by NMR. In this talk, we discuss how to model different spin arrangements, and compute the resulting NMR spectra. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z8.00004: Distinct magnetic regimes through site-selective atom substitution in the frustrated quantum antiferromagnet Cs$_2$CuCl$_{4-x}$Br$_x$ Kateryna Foyevtsova, Harald O. Jeschke, P.T. Cong, B. Wolf, M. de Souza, N. Krueger, A.A. Haghighirad, F. Ritter, W. Assmus, M. Lang, Roser Valenti A good realization of a spatially anisotropic spin 1/2 triangular lattice is provided by the isostructural layered compounds Cs2CuCl4 and Cs2CuBr4. In this talk, we shall present electronic structure calculations and magnetic susceptibility measurements[1] for the mixed systems Cs$_2$CuCl$_{4-x}$Br$_x$. We shall discuss the existence of three distinct concentration regimes which are separated by critical concentrations x$_{c1}$ = 1 and x$_{c2}$ = 2. The role of frustration effects at the critical concentrations will be highlighted. \\[4pt] [1] Cong et al. Phys. Rev. B 83, 064425 (2011) [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z8.00005: Two-stage orbital order and dynamical spin frustration in KCuF$_3$ James C.T. Lee, Shi Yuan, Siddhartha Lal, Young-Il Joe, Yu Gan, Serban Smadici, Paul M. Goldbart, S. Lance Cooper, Peter Abbamonte, Ken Finkelstein, Yejun Feng, Andrivo Rusydi Results from our x-ray and Raman scattering studies on KCuF$_3$, a model orbital order system, strongly link a low-temperature orbital order transition to a previously unidentified structural phase transition at 50 K. Raman scattering shows softening of phonon modes linked to F---ions that ceases upon a splitting of a degenerate E$_g$ mode at 50 K. This, along with the emergence of diffuse scattering around an orbital order Bragg peak at low temperature, suggests an onset of GdFeO$_3$-like octahedral tilting, which serves to stabilize the Neel spin order at 39 K. To explain these effects, we have added to the Kugel-Khomskii model a term for direct orbital exchange driven by electron-electron interactions and ligand distortions. This term creates a near degeneracy, which dynamically frustrates the spin order at high temperature, that is lifted by orbital---lattice interactions at low temperature. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z8.00006: Evidence for pressure-tuned quantum structural fluctuations in KCuF$_{3}$ S. Yuan, M. Kim, J. Seeley, S. Lal, P. Abbamonte, S.L. Cooper Frustrated magnetic systems are currently of great interest because of the possibility that these materials exhibit novel ground states such as orbital and spin liquids. We provide evidence in the orbital-ordering material KCuF$_{3}$ for pressure-tuned quantum melting of a static structural phase to a phase that dynamically fluctuates even near T $\sim $ 0K.[1] Pressure-dependent Raman scattering measurements show that applied pressure above P* $\sim $ 7kbar reverses a low temperature structural distortion in KCuF$_{3}$, resulting in the development of a $\omega \quad \sim $ 0 fluctuational (quasielastic) response near T $\sim $ 0K. This pressure-induced fluctuational response is temperature independent and exhibits a characteristic fluctuation rate that is much larger than the temperature, $\Gamma \quad >>$ K$_{B}$T, consistent with quantum fluctuations of the CuF$_{6}$ octahedra. We show that a previous developed model of pseudospin-phonon coupling qualitatively describes both the temperature- and pressure-dependent evolution of the Raman spectra of KCuF$_{3}$. Work supported by the U.S. Department of Energy under Award No. DE-FG02-07ER46453 and by the National Science Foundation under Grant NSF DMR 08-56321. \\[4pt] [1] S. Yuan et al., arXiv:1107.1433 (2011). [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z8.00007: Bose metal phase in a simple honeycomb lattice model Christopher Varney, Kai Sun, Victor Galitski, Marcos Rigol The existence of quantum spin liquids was first conjectured by Pomeranchuk some 70 years ago, who argued that frustration in simple antiferromagnetic theories could result in a Fermi-liquid-like state for spinon excitations. Here we present evidence that a simple quantum spin model on a honeycomb lattice hosts the long sought for Bose metal with a clearly identifiable Bose surface. The complete phase diagram of the model is determined via exact diagonalization and is shown to include four distinct phases separated by three quantum phase transitions. The stability of the Bose metal phase in the presence of other interactions is also discussed. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z8.00008: Spin liquid ground state on the honeycomb Heisenberg spin 1/2 model with nearest and next nearest neighbor interaction Zhenyue Zhu, Steven White, David Huse We numerically investigate the S=1/2 Heisenberg model on the honeycomb lattice with nearest ($J_1$) and next-nearest neighbor ($J_2$) interactions with the density matrix renormalization group (DMRG). We are able to study open cylinders with widths up to 12 lattice spacings. For $J_2/J_1$ near $0.3$, we find a spin liquid phase, bordered by an antiferromagnetic phase for smaller $J_2$ and a valence bond crystal for larger $J_2$. For the spin liquid phase we find finite spin singlet and triplet gaps and short spin-spin and bond-bond correlation lengths. We also find that the energy splitting between the two different topological sectors decays exponentially with the system width, consistent with a gapped Z2 spin liquid. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z8.00009: Exact Spin Liquid Ground States of the Quantum Dimer Model on the Square and Honeycomb Lattices Hong Yao, Steve Kivelson We study a generalized quantum hard-core dimer model on the square and honeycomb lattices, allowing for first and second neighbor dimers. At generalized Rohksar-Kivelson points, the exact ground states can be constructed, and ground-state correlation functions can be equated to those of interacting 1+1 dimensional Grassmann fields. When the concentration of second neighbor dimers is small, the ground state correlations are shown to be short-ranged corresponding to a (gaped) spin liquid phase. On a 2-torus, the ground states exhibit fourfold topological degeneracy. On a finite cylinder we have found a dramatic even-odd effect depending on the circumference, and propose that this can be used as a numerical diagnostic of the phase, more generally. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z8.00010: Physical solutions of the Kitaev honeycomb model Stefano Chesi, Fabio L. Pedrocchi, Daniel Loss We have investigated the exact solution of the honeycomb model proposed by Kitaev and derived an explicit formula for the projector onto the physical subspace. The physical states are simply characterized by the parity of the total occupation of the fermionic eigenmodes. We consider a general lattice on a torus and show that the physical fermion parity depends in a nontrivial way on the vortex configuration and the choice of boundary conditions. In the vortex-free case with a constant gauge field we are able to obtain an analytical expression of the parity. For a general configuration of the gauge field the parity can be easily evaluated numerically, which allows the exact diagonalization of large spin models. We consider physically relevant quantities, as in particular the vortex energies, and show that their true value and associated states can be substantially different from the one calculated in the unprojected space, even in the thermodynamic limit. [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z8.00011: An FRG approach to the Heisenberg-Kitaev model Johannes Reuther, Ronny Thomale, Simon Trebst We apply the Functional Renormalization Group (FRG) method to frustrated spin-1/2 systems on two dimensional lattices such as the Heisenberg-Kitaev model. In order to be able to perform diagrammatic approximations, we use the pseudo fermion representation of spin operators. The FRG provides a systematic scheme for infinite order resummations in different interaction channels and hence allows to treat magnetic order and disorder effects on an equal footing. Calculating the magnetic susceptibility we identify different magnetically ordered and paramagnetic phases. In particular, the Heisenberg-Kitaev model exhibits magnetically ordered states well beyond the isotropic Heisenberg limit as well as an extended gapless spin-liquid phase around the highly anisotropic Kitaev limit. From the RG flow of the magnetic susceptibility we extract both, the Curie-Weiss scale and the critical ordering scale (for the magnetically ordered states). The Curie-Weiss scale changes sign, indicating a transition of the dominant exchange from antiferromagnetic to ferromagnetic, deep in the magnetically ordered regime. We discuss our results in light of recent experimental susceptibility measurements for Na$_2$IrO$_3$ and Li$_2$IrO$_3$. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z8.00012: DMRG study of the spin-1/2 J1-J2 Honeycomb Antiferromagnetic Heisenberg Lattice Shoushu Gong, Dongning Sheng, Matthew Fisher A possible quantum spin liquid state has been revealed in the Hubbard model on the honeycomb lattice by Quantum Monte Carlo study, which has stimulated a lot of recent interest in the quantum spin models on honeycomb lattice. In a recent paper (Phys. Rev. B 84, 024406 (2011)), the J1-J2-J3 Heisenberg model has been studied by exact diagonalization (ED), establishing a rich phase diagram with the Neel and plaquette valence-bond crystal (VBC) phases depending on couplings J2 and J3, while it remains unclear if there is a featureless spin liquid phase exists in such a model. Here, by implementing DMRG method with the full rotational SU(2) symmetry, we study the J1-J2 Heisenberg model on honeycomb lattice at larger sizes. We analyze the spin-spin and dimer-dimer correlation functions for different sizes and extrapolate the structure factors to the thermodynamic limit to determine the nature of the quantum state. Our results suggest that the intermediate phase (with J2/J1$\sim $0.2-0.35) may be a spin liquid phase with vanishing spin/dimer correlations at the large distance limit. The nature of such a phase will be explored based on comparison with variational wavefunctions. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z8.00013: Searching for the Topological Degeneracy in the Hubbard Model on a Honeycomb Lattice Bryan Clark Recent quantum Monte Carlo calculations by Meng, et. al [1] have produced strong numerical evidence for a topological Z2 spin liquid on the Hubbard model on the honeycomb lattice. One feature of these spin liquids is the presence of a ground state degeneracy that depends on the manifold on which the system lives. Using finite temperature QMC calculations, we identify what states can live in the low-lying spectra, constraining the options for topologically degenerate ground states. In this talk we discuss these bounds and the implications for the Z2 spin liquid. [1] Z. Y. Meng, T. C. Lang, S. Wessel, F. F. Assaad, and A. Muramatsu, Quantum spin-liquid emerging in two-dimensional correlated Dirac fermions," Nature 464, 847 (2010). [Preview Abstract] |
Friday, March 2, 2012 1:51PM - 2:03PM |
Z8.00014: Emergent critical phase in a 2D frustrated Heisenberg model Peter P. Orth, Premala Chandra, Piers Coleman, Joerg Schmalian It is well-known that a discrete Ising ($Z_2$) order parameter emerges in the frustrated square lattice $J_1$-$J_2$-Heisenberg model, which may be broken at finite temperature. We ask whether a different discrete symmetry $Z_q$ with $q>2$ may be found in other frustrated Heisenberg models, giving rise to a different finite temperature phase transition. Indeed, we identify an emergent $Z_6$ symmetry at low temperatures in a frustrated Heisenberg model on a 2D lattice that contains both the sites of the triangular and its dual honeycomb lattice. Our analysis combines a spin-wave expansion, susceptible to short-distance physics, with renormalization group arguments of the corresponding long-wavelength non-linear sigma model. Our results are even more appealing since the $Z_6$ clock model has a rich finite temperature phase diagram with two distinct Berezinskii-Kosterlitz-Thouless (BKT) phase transitions separated by a massless critical phase. We also discuss possible realizations of this phenomenon using cold-atoms in optical lattices. [Preview Abstract] |
Session Z9: Focus Session: Complex Bulk Oxide: General Properties
Sponsoring Units: DMP GMAGChair: Tsuyosh Kimura, Osaka University
Room: 209
Friday, March 2, 2012 11:15AM - 11:27AM |
Z9.00001: Local atomic structure of the layered compound SrFeO$_{2}$ Kazumasa Horigane, Anna Llobet, Keeseong Park, Despina Louca SrFeO$_{2}$ exhibits several unexpected structural and physical properties. Its antiferromagnetic transition temperature T$_{N}$=473K is unusually high for a two-dimensional layered structure. First-principle calculations on SrFeO$_{2}$ showed that the Fe 3d down-spin elections occupy the nondegenerate d$_{z2}$ level rather than the degenerate (d$_{xz}$, d$_{yz})$ levels. This is in good agreement with the absence of a Jahn-Teller instability and the existence of the three dimensional antiferromagnetic ordering because the out-of plane direct Fe-Fe exchange is comparable in strength to the in-plane Fe-O-Fe superexchange. Therefore, it is expected that there is no structure instability in SrFeO$_{2}$. Using the pair distribution function (PDF) analysis to characterize the local structure of SrFeO$_{2}$, we observed that the local symmetry is lower than the average P4/mmm crystal symmetry. In particular, the FeO$_{2}$ planes are buckled, with two unique buckling angles along the a-axis. The buckling angle of Fe-O-Fe is reduced from 180$^{o }$with increasing temperature, accompanied by a reduction of the Fe magnetic moment. Thus the local structure instability correlates with the magnetism where the distortions suppress orbital overlap. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z9.00002: Phonon anomalies and critical fluctuations associated with charge stripes in La$_{1.67}$Sr$_{0.33}$NiO$_4$ Svetlana Anisimova, Dan Parshall, Dmitry Reznik, Daniel Lamago, Douglas Abernathy, Karol Marty, Mark Lumsden, G. Gu, J.M. Tranquada We will report inelastic neutron scattering measurements of the spectrum of charge excitations in the stripe-ordered phase of La$_{2-x}$Sr$_{x}$NiO$_{4}$ ($x = \frac{1}{3} $). We identified clear signature of charge stripes at low energies in the nickelates. Our results imply that dynamic stripes are critical fluctuations associated with the stripe-ordering transition. We also observed a phonon anomaly correlated with dynamic stripes above the static-ordering transition, which occurs at 241~K. Our results elucidate the nature of dynamic charge stripes and their signature in the neutron spectra. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z9.00003: Rare orbital glass state in single crystalline Y$_{2}$Mo$_{2}$O$_{7}$ Harlyn Silverstein, Haidong Zhou, Alannah Hallas, Jason Gardner, Yiming Qiu, Georg Ehlers, Andrei Savici, Zahra Yamani, Michel Gingras, Bruce Gaulin, Katharina Fritsche, Kate Ross, Christopher Wiebe Perhaps one of the most curious cases of frustrated pyrochlores, Y$_{2}$Mo$_{2}$O$_{7 }$was first classified as a spin glass in 1986. Conventionally, spin glasses must exhibit some sort of chemical disorder although oxygen vacancies and Y-Mo site mixing is virtually absent in all studies to date. NMR and neutron PDF experiments show the presence of local disorder. While other studies have shown a lattice deformation occurring near T$_{g}$=22K, these distortions cannot be detected globally and may not be enough to explain the spin glass behavior. For 25 years, researchers have struggled to resolve spin glass theory with the data; the problem lies in that, until now, scientists have been unable to grow single crystal samples due to the oxidation of Mo$^{4+}$ to Mo$^{6+}$ at low temperatures. Here, we report the synthesis and characterization of the world's first single crystalline sample of Y$_{2}$Mo$_{2}$O$_{7}$. Unlike powder samples, single crystalline Y$_{2}$Mo$_{2}$O$_{7}$ heat capacity measurements show a T$^{2}$ dependence. Neutron scattering experiments show isotropic, broad, liquid-like collective modes and high-Q diffuse scattering characteristic of an orbital liquid to orbital glass transition at T$_{g}$. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z9.00004: Anisotropic magnetic properties of the KMo$_{4}$O$_{6}$ M. Andrade, M. L. Maffei, C. A. M. dos Santos, B. Ferreira, A. F. Sartori Electrical resistivity measurements in the tetragonal KMo$_{4}$O$_{6}$ single crystals show a metal-insulator transition (MIT) near 100K. Magnetization measurements as a function of temperature show no evidence of magnetic ordering at this MIT [1]. Single crystals of KMo$_{4}$O$_{6}$ were obtained by electrolysis of a melt with a molar ratio of K$_{2}$MoO$_{4}$:MoO$_{3}$ = 6:1. The process were carried out at 930\r{ }C with a current of 20-25mA for 52h in argon atmosphere. After that, electrodes were removed from the melt alloying the crystals to cool down to room temperature rapidly. Scanning Electron Microscopy (SEM) showed that the black single crystals were grown on the platinum cathode. Typical dimensions of the single crystals are 1x0.2x0.2mm$^{3}$. X-ray diffractometry confirmed that the single crystals have KMo$_{4}$O$_{6}$ tetragonal crystalline structure with space group P$\overline{4}$. Magnetization measurements were performed parallel and perpendicular to the c-axis from 2 to 300K. The results show anisotropic behavior between both directions. Furthermore, the temperature independence of the magnetization at high temperature and the upturn at low temperature are observed in agreement with previous results [1]. MxH curves measured at several temperatures show nonlinear behavior and a small magnetic ordering. The magnetic ordering seems to be related to the MIT near 100K. This material is based upon support by FAPESP (2009/14524-6 and 2009/54001-6) and CNPq/NSF (490182/2009-7). M. Andrade is CAPES fellow and C.A.M. dos Santos is CNPq fellow. \\[4pt] [1] K. V. Ramanujachary et al., J. Sol. State Chem.102 (1993) 69. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z9.00005: Cu3Nb2O8: A Multiferroic with Chiral Coupling to the Crystal Structure Roger Johnson, Sunil Nair, Laurent Chapon, Alessandro Bombardi, Carlo Vecchini, D. Prabhakaran, Andrew Boothroyd, Paolo Radaelli We will present recent bulk properties, neutron powder diffraction, and non-resonant x-ray diffraction measurements on a new multiferroic material, Cu$_3$Nb$_2$O$_8$. We demonstrate than an electric polarization evolves simultaneously with a generalised helicoidal magnetic ordering. Contrary to conventional models of multiferroicity, the electric polarization was found to lie perpendicular to the common rotation plane of the magnetic moments. We present a new model applicable to a specific class of crystals; those that support a macroscopic``ferroaxial'' vector and adopt a helicoidal magnetic structure. Our new interpretation, which may explain the multiferroic properties of a number of systems reported in recent literature, is based upon the well-understood inverse Dzyaloshinskii-Moriya interaction, but requires an altogether different interpretation. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z9.00006: THz and infrared excitation spectrum below the Jahn-Teller transition in Sr3Cr2O8 Zhe Wang, Michael Schmidt, Axel G\"{u}nther, Sebastian Schaile, Nikola Pascher, Franz Mayr, Yurii Goncharov, Hans-Albrecht Krug von Nidda, Alois Loidl, Diana Quintero-Castro, A.T.M.N. Islam, Bella Lake, Joachim Deisenhofer We report on optical excitations observed recently in Sr3Cr2O8 by THz and infrared spectroscopy. Low-energy excitations below 3 THz are detected by THz time domain spectroscopy. These excitations can be divided into two different classes according to the temperature-dependent properties. One is emergent right below the Jahn-Teller transition temperature, which is determined by specific heat measurement to occur at 285 K [1, 2]. The other appears only below 100 K, where the fluctuations are sufficiently suppressed, consistent with the temperature dependence of low-energy Raman modes [3]. Infrared transmission measurements reveal a broad crystal-field excitation, which can be associated with an electronic transition from E to T2 orbital states. \\[4pt] [1] Zhe Wang \textit{et al}., Phys. Rev. B \textbf{83}, 201102 (2011) \\[0pt] [2] D. L. Quintero-Castro \textit{et al}., Phys. Rev. B \textbf{81}, 014415 (2010) \\[0pt] [3] D. Wulferding \textit{et al}., Phys. Rev. B \textbf{84}, 064419 (2011) [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z9.00007: Magnetic and Orbital Excitations in Sr$_3$CuIrO$_6$ J.P. Hill, X. Liu, M. Dean, W. Yin, A. Tsvelik, J. Kim, D. Casa, M.H. Upton, H. Gretarsson, Y.-J. Kim, T. Qi, G. Cao, L. Hozoi, V. Katukuri, J. v.d. Brink We report resonant inelastic x-ray scattering studies of the one-dimensional Sr$_3$CuIrO$_6$ at the Ir L$_3$ edge, with $\Delta$E=40 meV. At high energies, we find peaks at 0.6 eV, 0.9 eV, 3 eV, 4 eV and 6 eV. These peaks are non-dispersive, well-defined excitations. On the basis of quantum chemistry calculations, we are able to identify the first two as excitations within the $t_{2g}$ manifold and the next two as between the $t_{2g}$ and $e_g$ manifolds, together with charge transfer excitations from the O $2p$ to the Ir 5$d$. The 6 eV feature is another $t_{2g}$ - $e_g$ excitation. From these we are able to determine both the spin-orbit and non-cubic crystal field splittings. We find that they are of comparable strength. In addition, magnetic excitations are observed, corresponding to excitations of the $j_{1/2}$ isospin. We find these are highly dispersive along the chain direction with a bandwidth of 20 meV and a gap of 30 meV. These results allow a complete modeling of the spin and orbital degrees of freedom in this model compound and we conclude an atomic, spin-orbit coupled description works well. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z9.00008: Probing Spin-Orbit Coupling in Iridium 5d Compounds with X-ray Absorption Spectroscopy J.P. Clancy, N. Chen, W.F. Chen, C.Y. Kim, K.W. Plumb, Y.J. Kim Iridium-based transition metal oxides have begun to attract considerable interest following recent proposals of exotic electronic and magnetic ground states (topological insulators, topological semi-metals, spin liquids, and spin-orbital Mott insulators) driven by large spin-orbit coupling effects. We have performed x-ray absorption spectroscopy (XAS) measurements on a series of iridium-based compounds (such as Ir, IrCl3, IrO2, Na2IrO3, Sr2IrO4, and Y2Ir2O7) in order to investigate the magnitude of the spin-orbit interactions in these systems. By determining the branching ratio of the XAS white-line intensities at the Ir L2 and L3 absorption edges, we obtain a direct measure of the expectation value for the spin-orbit operator $\langle$L $\cdot$ S$\rangle$. These measurements reveal remarkably strong spin-orbit interactions in almost all Ir-based compounds, with branching ratios up to several times larger than the statistical value. The branching ratio is found to be largely independent of electronic properties (metal or insulator), ionization state (Ir$^{3+}$ [5d$^{6}$] or Ir$^{4+}$ [5d$^{5}$]), and chemical composition (oxide or halide), although it does appear to be correlated with local Ir site symmetry and Ir-Ir bond distances. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z9.00009: Investigation of magnetic phase transitions in B-site disordered PbBxB'1-xO3 (B = Fe, Co and B' = Nb, Ta) Shravani Chillal, Elena Popova, Erik Eulf, Severian Gvasaliya, Tatiana Shaplygina, Sergey Lushnikov, Andrey Zheludev Materials such as PbFe$_{0.5}$Nb$_{0.5}$O$_{3}$ (PFN-0.5)/ PbFe$_{0.5}$Ta$_{0.5}$O$_{3}$ (PFT-0.5) and PbCo$_{0.33}$Nb$_{0.67}$O$_{3}$ (PCN-0.33) are relaxor ferroelectrics. PbFe$_{0.5}$Nb$_{0.5}$O$_{3}$ (PFN-0.5) also shows anti-ferromagnetic order below $\sim$ 143 K. Though multiferroicity is an important property of PFN-0.5, its uniqueness stems from coexisting anti-ferromagnetic and spinglass phases below $\sim$ 12 K. Presently, it is the only known such case in a Heisenberg 3D spin system. We report a first systematic study of the H-T phase diagram of PFN-0.5 and discuss the results in the context of existing theories. In addition, we study the magnetic properties of PFT-0.5 and PCN-0.33. While PFN-0.5 and PFT-0.5 demonstrate similar behavior, PCN-0.33 does not show any anomalies that could signify magnetic ordering. We propose that the observed dramatic differences may result from partial B-site ordering. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z9.00010: Direct neutron scattering-based observation of spin excitons in LaCoO$_{3}$ S. El-Khatib, D. Phelan, J.A. Borchers, J. Barker, P. Butler, H. Zheng, J.F. Mitchell, C. Leighton We report temperature-dependent small angle neutron scattering (SANS) results on undoped LaCoO$_{3}$ single crystals. The data reveal two main scattering components; a high $q$ signal increasing with $T$ above 50 K, and a low $q$ component that turns on sharply below 60 K. The former is shown to be due to inelastic scattering quantitatively consistent with the excitations associated with the spin-state transition. Of greater interest, the low $q$ scattering is shown to be of Guinier form, revealing scattering from a dilute assembly of magnetic objects of size approximately 15-20 nm. The abrupt onset of this scattering at 60 K, i.e. very close to the point at which prior work provided evidence of formation of interacting magnetic excitons, suggests that this represents the first direct scattering-based observation of magnetic excitons forming around O defects in LaCoO$_{3}$. [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z9.00011: High-pressure studies on Ba-doped cobalt perovskites by neutron diffraction Huibo Cao, Vasile Garlea, Fangwei Wang, Antonio dos Santos, Zhaohua Cheng Cobalt perovskite possess rich structural, magnetic and electrical properties depending on the subtle balance of the interactions among the spin, charge, and orbital degrees of freedom. Divalent hole-doped cobalt perovskites LaA$^{2+}$CoO$_{3}$ exhibit structural phase transitions, metal-insulator transitions, and multi-magnetic phase transitions. High-pressure measurement is believed to mimic the size effects of the doped ions. We performed neutron diffraction experiments on selected Ba-doped LaCoO$_{3}$ under pressures up to 6.3 GPa at SNAP at Spallation Neutron Source of ORNL. This work focuses on the high-pressure effects of the selected Ba-doped samples and the change of the phase diagram with pressure. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z9.00012: Electric field tuning of the metal-magnetic transition in ionic liquid gated Ca$_3$Ru$_2$O$_7$ flakes Conor Puls, Xinxin Cai, Jin Peng, Zhiqiang Mao, Ying Liu The recent application of ionic liquids in electric field-effect devices has provided for unprecedentedly high surface charge accumulations ($\approx$ 10$^{14}$ cm$^{-2}$) due to the formation of an electronic double layer at their interface with various materials. This technique provides a tool to explore metal-insulator transitions and superconductivity in various materials including transition metal oxides. We extend this technique to the study of the physics of strongly correlated electrons in complex oxides such as the Ruddlesden-Popper (R-P) series Ca$_ {n+1}$Ru$_n$O$_{3n+1}$, where $n$ = 1, 2, $\ldots$ $\infty$. Ca$_ 3$Ru$_2$O$_7$, the $n$ = 2 member of the R-P series, features antiferromagnetic ordering below 56 K and a first-order metal-insulator transition along with a structural transition at 48 K. We developed a technique for fabricating Hall bar devices on Ca$_ 3$Ru$_2$O$_7$ flakes prepared by mechanical exfoliation from bulk crystals, and performed low-temperature measurements using the ionic liquid DEME-TFSI in a top-gate configuration. We will discuss electrical transport results and the metal-magnetic transition with gate voltage tuning in these devices. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z9.00013: p-Electron Magnetism in anion doped BaTiO$_{3-x}$X$_x$ (X=C,N,B) Christoph Gruber, Pedro Osvaldo Bedolla Velazquez, Josef Redinger, Peter Mohn, Martijn Marsman We present VASP calculations using the HSE functional for carbon, nitrogen, and boron doped BaTiO$_{3-x}$X$_x$ (X=C,N,B). We calculate a 40-atom supercell and replace one oxygen atom by C,N, or B. For all three substituents we find a magnetically ordered groundstate which is insulating for C and N and halfmetallic for B. The changes in the electronic structure between the undoped and the doped case are dominated by the strong crystal field effects together with the large band splitting for the impurity p-bands. Using an MO picture we give an explanation for the pronounced changes in the electronic structure between the insulating non-magnetic state and the as well insulating magnetic state for doped BaTiO$_3$. p-element doped perovskites could provide a new class of materials for various applications ranging from spin-electronics to magneto-optics. [Preview Abstract] |
Friday, March 2, 2012 1:51PM - 2:03PM |
Z9.00014: Spin-dependent Optical Properties of Multiferroic EuO and GdN Chun-Gang Duan We present a first-principles study on the linear and non-linear optical properties of multiferroic EuO and GdN. In particular, we have decomposed the linear and non-linear optical spectra into spin-dependant terms, i.e., we could obtain optical spectra from individual spin channel. With the help of such method, a regular optical spectrum could present information of spin-dependent band structures, which is of great importance to the study of ferromagnetic insulators. [Preview Abstract] |
Session Z11: Focus Session: Graphene Devices - Hybrid Structures
Sponsoring Units: DMPChair: Saverio Russo, University of Exeter
Room: 210B
Friday, March 2, 2012 11:15AM - 11:27AM |
Z11.00001: Nano-patterning of fluorinated graphene by electron beam Saverio Russo, Freddie Withers, Thomas Bointon, Marc Dubois, Monica Craciun The development of transparent electronics is reliant on achieving high conductivity materials with a gate tuneable carrier mobility and low contact resistance at the interface with metals. Graphene --a layer of carbon atoms in a honeycomb lattice- offers just such a possibility. Functionalizing graphene with fluorine induces the opening of a band gap in the otherwise semimetallic graphene. Here we demonstrate that fluorinated graphene --a wide gap semiconductor with sp3 electron orbital hybridization- can be selectively reduced to graphene by electron-beam irradiation. We employ this functionality to pattern conductive nanostructures in a sheet of fluorinated graphene, realizing transparent graphene-based electronic devices such as nanoribbons without the need for etching of graphene. Electrical transport experiments over a wide range of temperatures (from 300K to 4K) of the ribbons show a transport gap whose size is inversely proportional to the width of the patterned ribbons. In this gap, electrons are localized, and charge transport is dominated by variable range hopping. Charging effects constitute a significant portion of the activation energy, and we find that the activation energy scales well with the width of the ribbons [Nano Lett. 11, 3912 (2011)]. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z11.00002: Room-temperature gating of molecular junctions using few-layer graphene nanogap electrodes Amelia Barreiro, Ferry Prins, Justus Ruitenberg, Johannes Seldenthuis, Nuria Aliaga-Alcalde, Lieven Vandersypen, Herre van der Zant We report on a new method based on feedback controlled electroburning to controllably form nanogaps in few-layer graphene [1]. The gaps have separations on the order of 1-2 nm as estimated from a Simmons model for tunneling. Furthermore, molecules are deposited in the nanogaps. These molecular junctions display gateable IV-characteristics at room temperature. Gateable transport through molecules contacted between the electrodes demonstrates the potential of room-temperature operation of our molecular devices. Combined with the observed stability in time, our study shows that few-layer graphene nanogaps are an interesting alternative to metal electrodes. [1] Ferry Prins, Amelia Barreiro, Justus Ruitenberg, Johannes Seldenthuis, N\'{u}ria Aliaga-Alcalde, Lieven Vandersypen, Herre van der Zant, Nanoletters 11 (2011) 4607 - 4611 [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z11.00003: Graphene Nanogap for Gate Tunable Quantum Coherent Single Molecule Electronics Tomas Lofwander, Anders Bergvall, Kristian Berland, Per Hyldgaard, Sergey Kubatkin We present atomistic calculations$^1$ of quantum coherent electron transport through fulleropyrrolidine terminated molecules bridging a graphene nanogap. We predict that three difficult problems in molecular electronics with single molecules may be solved by utilizing graphene contacts: (1) a back gate modulating the Fermi level in the graphene leads facilitate control of the device conductance in a transistor effect with high on/off current ratio; (2) the size mismatch between leads and molecule is avoided, in contrast to the traditional metal contacts; (3) as a consequence, distinct features in charge flow patterns throughout the device are directly detectable by scanning techniques. We show that moderate graphene edge disorder is unimportant for the transistor function.\\ $^1~$ A. Bergvall, K. Berland, P. Hyldgaard, S. Kubatkin, and T. Lofwander, Phys. Rev. B. {\bf 84}, 155451 (2011). [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z11.00004: The Role of Surfactant Adsorbates on Hysteresis and Carrier Mobility in Graphene Transistors Chih-Jen Shih, Geraldine Paulus, Qing Hua Wang, Zhong Jin, Moon-Ho Ham, Daniel Blankschtein, Michael Strano Understanding the role of polar and ionic adsorbates on the transport characteristics of graphene transistors is important for the development of graphene-based sensor devices and printable electronics using graphene solutions. We have investigated the effects of commonly used surfactants for graphene dispersion in aqueous solution on transport characteristics of graphene transistors. The adsorbates are found to transfer electrons to graphene, scatter carrier transport, and induce more electron-hole puddles when the graphene is on an SiO$_{2}$ substrate. We relate the change in transport characteristics to specific properties of a series of anionic, cationic, and neutral surfactants using a modification of a self-consistent transport theory. To understand the effects of surfactant adsorbates trapped on either side of the graphene, suspended devices were fabricated. Strong hysteresis is observed when both surfaces were exposed to the surfactants, attributable to their function as charge traps. This work is the first to demonstrate the control of hysteresis, allowing us to eliminate it for sensor and device applications or enhance it for non-volatile memory applications. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z11.00005: Model results for graphene electrodes in molecular junctions Carlo Motta, Mario Italo Trioni, Gian Paolo Brivio, Kizhakeyil Lukose Sebastian, Daniel S\'anchez Portal Graphene is a material with a high potential for nanoelectronic applications as transparent conductive electrode. In view of a graphene-based electronics, a possible way to bridge graphene electrodes is by molecular linkers either organic molecules or graphene-derived systems such as graphene nanoribbons (GNR). In the present work, we investigated the electronic and conductive properties of such junctions for two different devices. First we modeled a photochromic switch based on diarylethene molecules, which can be activated/deactivated by light. We found a large on/off current ratio, which can be improved by modifying the functional groups of the molecule. We then investigated the properties of graphene/GNR/graphene junctions, showing that their conductive properties can be tailored by changing the length and width of the GNR. The calculations have been carried out by using the non-equilibrium Green's function method combined with density functional theory. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z11.00006: ABSTRACT WITHDRAWN |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z11.00007: Graphene Chemical sensors on Flexible Substrates Benjamin Mailly, Allen Hsu, Francesco Pappalardo, Dawn Nida, Elio Guidetti, Luigi Occhipinti, Salvatore Coffa, Jing Kong, Tomas Palacios Thanks to its all-surface 2D structure combined with a very high carrier mobility, Graphene is a very promising candidate for high sensitivity and low noise chemical sensing. Indeed, graphene devices can perform electrical detection for chemical sensing in a wide variety of applications, including pH monitoring in electrolytes and glucose measurements in blood. Furthermore, the fabrication of low cost and flexible sensors can be made possible by enabling the integration of graphene on plastic substrates. Our group has developed the first solution-gated graphene field effect transistor (SGFET) on a flexible substrate, PolyEthylene Naphthalate (PEN). For this purpose, graphene grown by chemical vapor deposition is transferred on the PEN substrate, where the metal contacts had already been evaporated. The characterization of our devices in a phosphate buffer solution displays good transconductance around 0.9 mS.mm$^{-1}$. pH measurements have been performed using these devices and a sensitivity as high as 22 mV/pH have been demonstrated. In addition, long term pH monitoring was demonstrated in these devices. Our on-going work focuses on studying the influence of the substrate as well as the presence of residues on the graphene surface in the pH sensing mechanism. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z11.00008: Graphene field-effect biosensor arrays B. Wang, K.L. Liddell, C.D. Keating, J. Zhu We report on the study of chemical and biological sensing using graphene field-effect transistor (GFET) arrays. Large-scale single layer graphene sheets are synthesized by low-pressure chemical vapor deposition on copper. We fabricate GFET arrays capable of operating in solutions by passivating the graphene channel with a thin oxide layer. This oxide layer also serves as the electrolyte gate dielectrics and the sensing surface. The GFET arrays exhibit an average field-effect mobility of $\sim $ 5000 cm$^{2}$/Vs and small hysteresis in gate sweeps. We demonstrate the sensing operation of the GFET via measuring the pH value of phosphate buffer solutions. Gate sweeps indicate an approximately linear shift of the Dirac point with increasing pH, with an average slope of +46mV/pH. The viability of using GFETs to detect the specific binding of biomolecules will also be discussed. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z11.00009: A model of large volumetric capacitance in graphene supercapacitors based on ion clustering Brian Skinner, Michael Fogler, Boris Shklovskii Electric double layer supercapacitors are promising devices for high-power energy storage based on the reversible absorption of ions into porous, conducting electrodes. Graphene is a particularly good candidate for the electrode material in supercapacitors due to its high conductivity and large surface area. In this paper we consider supercapacitor electrodes made from a stack of graphene sheets with randomly-inserted ``spacer" molecules. We show that the large volumetric capacitances $C > 100$ F/cm$^3$ observed experimentally can be understood as a result of collective intercalation of ions into the graphene stack and the accompanying nonlinear screening by graphene electrons that renormalizes the charge of the ion clusters. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z11.00010: Morphology-controlled graphene aerogel for energy storage Son Truong Nguyen, Hoa Tien Nguyen, Hai Minh Duong The development of new anode/cathode materials with highly conductive, non-corrosive, high specific surface area and high porosity for energy storage devices is highly desirable. Graphene aerogels has been focused emergently recently due to novel properties of the graphene. However, the aerogel-based application performance strongly depends on the morphology and structure of the graphene aerogels. The graphene aerogels with low-density have thinner struts, a different distribution of particle sizes, and less internal connectivity. This, in turn, changes the way the material can transport electric charge. As a result, the highest surface area graphene aerogels end up having the worst electrical conductivity, and the most conductive graphene aerogels have lowest surface areas. So the best designs of the developed graphene aerogel nanostructures in terms of pore size, porosity, density and mechanical properties for energy storage devices are essential. In this work, we develop a new fabrication method of graphene aerogels with well-controlled morphology and high electrical conductivity from graphene oxide through the supercritical drying process. The morphology and electrical conductivity of the graphene aerogels are controlled by the precursor contents and the synthesis conditions. The experimental results are very useful for experimentalists deciding the best graphene aerogel nanostructures for their needs. [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z11.00011: Electrostatic control of many-body interactions in graphene: Observation of the effects of doping on the saddle-point exciton Kin Fai Mak, Keliang He, Nick Petrone, Jim Hone, Jie Shan, Tony Heinz Significant excitonic effects were recently identified in the optical response of graphene through the asymmetric resonance feature at 4.62 eV in the optical conductivity. The peak, which is red-shifted by nearly 600 meV from the predicted band-to-band transition energy,\footnote{Mak et al. Phys. Rev. Lett. 106, 046401, (2011).} can be considered as a saddle-point exciton. Here we report a systematic study of this excitonic feature as a function of the doping density, for densities extending up to the metallic regime\footnote{Efetov \& Kim, Phys. Rev. Lett. 105, 256805, (2010).} ($\sim$ $10^{14} cm^{-2}$). With increasing density of either electrons or holes, the excitonic resonance is found to shift to the red and to become more symmetric in form. These experimental features agree very well with a recent ab-initio GW-Bethe-Salpeter calculation\footnote{Felipe H. da Jornada, J. D., Steven G. Louie. Private communications. (2011).} and can be understood as a consequence of enhanced ``metallic'' screening of the graphene dielectric function.\footnote{Hwang, E. H. \& Das Sarma, S. Phys. Rev. B 75, 205418} In addition, analysis of the width of the excitonic peak provides information on the quasiparticle lifetime. Mechanisms for the inferred rapid quasiparticle decoherence will be discussed. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z11.00012: Chemically Tunable Transport Phenomena of Functionalized Graphene Nicolas Leconte, Aur\'elien Lherbier, Francois Varchon, Jean-Christophe Charlier, Juan Jose Palacios, David Soriano, Pablo Ordejon, Stephan Roche We present an ab initio multiscale study and quantum transport simulations using the Kubo formalism [1] of chemically modified graphene based materials, whose properties are tuned by changing the density and nature of grafted molecular units. Depending on the nature of the introduced molecular bonding different conduction mechanism are obtained, including transition from weak to strong Anderson localization [2,3], as well as spin-dependent phenomena [4] and magnetoresistive fingerprints [5]. \\[4pt] References: [1] H. Ishii, F. Triozon, N. Kobayashi, K. Hirose, and S. Roche, C. R. Physique 10, 283 (2009) [2] N. Leconte, J. Moser, P. Ordejon, H. Tao, A. Lherbier, A. Bachtold, F. Alsina, C.M. Sotomayor Torres, J.-C. Charlier, and S. Roche, ACS Nano 4, 7, 4033-4038 (2010) [3] N. Leconte, A. Lherbier, F. Varchon, P. Ordejon, S. Roche, and J.-C. Charlier (accepted in PRB) [4] N. Leconte, D. Soriano, S. Roche, P. Ordejon, J.-C. Charlier, and J.J. Palacios, ACS Nano 5, 5, 3987-3992 (2011) [5] D. Soriano, N. Leconte, P. Ordejon, J.-C. Charlier, J.J. Palacios, and S. Roche, Phys. Rev. Lett. 107, 016602 (2011) [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z11.00013: First-principles study of hybrid Graphene/MnO$_2$ bilayers Miguel Pruneda Oxide nanosheets are an important and promising component for creating new materials. Their capacitance properties are particularly appealing for electric batteries. Manganese oxide nanosheets are abundant, environmental friendly, have low cost, and high electrochemical activity. However MnO$_2$ poor electrical conductivity and chemical stability limits its applicability as electrode material. Hybrid graphene/oxide nanostructures have been proposed to overcome these difficulties. Here, density functional theory calculations are performed to better understand the electronic properties of heterobilayers made from graphene and MnO$_x$ monolayers. The charge transfer between graphene and MnO$_2$ monolayers are analyzed and related to the presence of oxygen vacancies in different concentrations, which are known to induce atomic reconstructions and phase transformations of the oxide. Magnetic properties of the heterobilayers will also be discussed. [Preview Abstract] |
Friday, March 2, 2012 1:51PM - 2:03PM |
Z11.00014: Boron Nitride Nanoribbons: Synthesis and Future Directions Ashley Gibb, Kris Erikson, Alex Sinitskii, Michael Rousseas, Nasim Alem, James Tour, Alex Zettl Boron Nitride Nanoribbons (BNNR) have been theorized to have many interesting electrical and magnetic properties and edge states, but these characteristics have not been experimentally verified due to challenges in synthesis and purification. We have produced BNNRs by longitudinally splitting~boron nitride nanotubes (BNNT) using potassium vapor as an intercalant. Due to the strong interactions between boron nitride sheets, separation of nanoribbons from their parent tubes is challenging. We have used various solvent systems to assist with separation of the ribbons with the goal of probing their properties. [Preview Abstract] |
Friday, March 2, 2012 2:03PM - 2:15PM |
Z11.00015: Electron-Hole Polarization Dynamics in Graphene Oxide Annemarie L. Exarhos, Michael E. Turk, Zhengtang Luo, A.T.C. Johnson, James M. Kikkawa Graphene oxide (GO) has been shown to emit broadband visible and near infrared photoluminescence (PL). Here we use polarization sensitive optical spectroscopy to study spectral diffusion and temporal dynamics of electron-hole polarization in this material. Steady state polarization memory measurements show strong polarization memory close to the excitation energy, which weakens gradually in moving toward lower emission energies. To understand the dynamics underlying this behavior, we also perform time-resolved PL studies using an optical Kerr gate with sub-picosecond temporal resolution. Polarization memories show ultrafast dynamics within the PL lifetime in solid GO preparations where the incident light lies fully in the plane of the GO flakes. Using additional knowledge gained from optical anisotropy measurements, we discuss the relevance of our polarization memory data to the origins of PL in these systems. [Preview Abstract] |
Session Z12: Graphene: Electronic Structure and Interactions - Spectroscopy
Sponsoring Units: DCMPChair: Tony Heinz, Columbia University
Room: 210C
Friday, March 2, 2012 11:15AM - 11:27AM |
Z12.00001: Phonon self-energy corrections to non-zero wavevector phonon modes in single-layer graphene Paulo Araujo, Daniela Mafra, Kentaro Sato, Richiiro Saito, Jing Kong, Mildred Dresselhaus Phonon self-energy corrections have mostly been studied theoretically and experimentally for phonon modes with zone-center ($q $= 0) wave-vectors. Here, gate-modulated Raman scattering is used to study phonons of a single layer of graphene (1LG) in the frequency range from 2350 to 2750 cm$^{-1}$, which shows the G* and the G'-band features originating from a double-resonant Raman process with \textit{q $\ne $} 0. The observed phonon renormalization effects are different from what is observed for the zone-center $q $= 0 case. To explain our experimental findings, we explored the phonon self-energy for the phonons with non-zero wave-vectors (\textit{q $\ne $ } 0) in 1LG in which the frequencies and decay widths are expected to behave oppositely to the behavior observed in the corresponding zone-center $q $= 0 processes. Within this framework, we resolve the identification of the phonon modes contributing to the G* Raman feature at 2450 cm$^{-1}$ to include the iTO+LA combination modes with \textit{q $\ne $} 0 and the 2iTO overtone modes with $q $= 0, showing both to be associated with wave-vectors near the high symmetry point \textbf{K }in the Brillouin zone. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z12.00002: Impact of Graphene-Metal Interfaces on the Raman and Transport Properties of Graphene Devices Allen Hsu, Mario Hofmann, Wenjing Fang, Ki Kang Kimg, Jing Kong, Tomas Palacios Graphene is an amazing nano-material with many exciting properties and applications. However, due to its low dimensionality, the performance of this material is mainly limited by interfaces and surface properties. One of these interfaces, important for graphene field effect transistors and catalysts supported on graphene membranes, is that between the graphene and a metal layer. In this study, we experimentally examine the impact of various metals on graphene through Raman and Transmission Electron Microscopy. We find that strong graphene-metal interactions have significant impacts on the phonon structure in graphene. Furthermore, we observe changes in our Raman spectra relating to the crystallographic orientation between a metal and graphene. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z12.00003: Raman spectroscopy of sputtered metal-graphene and metal-oxide-graphene interfaces Ching-Tzu Chen, Marcin Gajek, Marcus Freitag, Marcelo Kuroda, Vasili Perebeinos, Simone Raoux In this talk, we report our recent development in sputtering deposition of magnetic and non-magnetic metal and metal-oxide thin films on graphene for applications in spintronics and nanoeleoctronics. TEM and SEM images demonstrate homogeneous coverage, uniform thickness, and good crystallinity of the sputtered films. Raman spectroscopy shows that the structure of the underlying graphene is well preserved, and the spectral weight of the defect D mode is comparable to that of the e-beam evaporated samples. Most significantly, we report the first observation of graphene-enhanced surface excitations of crystalline materials. Specifically, we discover two pronounced dispersive Raman modes at the interface of graphene and the nickel-oxide and cobalt-oxide films which we attribute to the strong light absorption and high-order resonant scattering process in the graphene layer. We will present the frequency-dependent, polarization-dependent Raman data of these two modes and discuss their microscopic origin. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z12.00004: Effects of sliding on Raman spectrum of bilayer graphene Seon-Myeong Choi, Seung-Hoon Jhi, Young-Woo Son Electronic properties of graphene are changed by various external effects such as strain, doping, and defects, etc. Such effects can be measured by Raman spectroscopy. For example, doping and defects increase the intensities of D-peak of single layer graphene, while the external strains generate splitting of G- and 2D-peaks respectively. In bilayer graphene, sliding motions between two layers also can change its electric and optical properties. Based on the first principles calculation methods, the sliding between two layers of bilayer graphene is shown to change the electronic and phonon dispersions, thereby altering its Raman spectrum significantly. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z12.00005: Electronic Structure of Suspended Bilayer Graphene Po-Chun Yeh, Kevin Knox, Wencan Jin, Jerry Dadap, Philip Kim, Richard M. Osgood, Alexei Barinov Bilayer graphene is an important medium for achieving band-gap tuning for graphene applications in digital electronics, such as in graphene nanoribbon FETs. ARPES is an important tool for measuring the electronic structure of crystals in the vicinity of the Dirac point. While accurate measurements have been made on the bilayer graphene band-structure on SiC, similar effort on free-standing graphene has not yet been reported. In this paper, we describe measurements of the band-structure of a $\sim $ 650 $\mu $m$^{2}$ sample of exfoliated bilayer graphene suspended over 5-$\mu $m-diameter wells formed in SiO$_{2}$ on a Si substrate. The measurements, performed at the Spectromicroscopy Beamline at ELETTRA with photon energy of 27eV, show that this material is essentially undoped and we compare its band structure with that expected from tight-binding calculations. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z12.00006: Electrical transport between single-crystal domains in graphene: bigger is not always better Adam Tsen, Lola Brown, Mark Levendorf, Fereshte Ghahari, Pinshane Huang, Carlos Ruiz-Vargas, David Muller, Philip Kim, Jiwoong Park Single-layer graphene can now be produced on the centimeter or even meter scale using chemical vapor deposition. These large-scale graphene films are polycrystalline, consisting of many separate single-crystal domains, as was recently identified using transmission electron microscopy. Understanding the electrical transport across these domains is relevant not only for device applications, but has also been the focus of many fundamental studies. Here, we first examine the structure of graphenes produced under different growth conditions using dark-field transmission electron microscopy (DF-TEM). We find three classes of grain boundaries--continuous, amorphous, and overlapped. Next, we study the electrical properties of graphene devices consisting of individual grain boundaries that have been first imaged by DF-TEM. We find that the grain boundaries exhibit an additional gate-dependent resistance that is keenly sensitive to growth conditions. Surprisingly, this resistance is an order of magnitude greater for growths with larger grain size due to more poorly-connected domains. Our results show that domain size is not the single most important parameter determining electrical performance of large-scale graphene films--the quality of inter-domain connections is just as crucial. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z12.00007: Trapping Image State Electrons on Graphene Layers and Islands Jerry Dadap, Daniel Niesner, Thomas Fauster, Nader Zaki, Kevin Knox, Po-chi Yeh, Rohan Bhandari, Richard M. Osgood, Marin Petrovic, Marko Kralj The understanding of graphene-metal interfaces is of utmost importance in graphene transport phenomena. To probe this interface we use time- and angle-resolved two-photon photoemission to map the bound, unoccupied electronic structure of the weakly coupled graphene/Ir(111) system. The energy, dispersion, and lifetime of the lowest three image-potential states are measured. In addition, the weak interaction between Ir and the smooth, epitaxial graphene permits observation of resonant transitions from an unquenched Shockley-type surface state of the Ir substrate to graphene/Ir image-potential states. The image-potential-state lifetimes are comparable to those of mid-gap clean metal surfaces. Evidence of localization of the excited image-state electrons on single-atom-layer graphene islands is provided by coverage-dependent measurements. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z12.00008: Image potential states of Graphene/Ru(0001) interface Shengmin Zhang, Xuefeng Cui, Leiming Wang, Hrvoje Petek Graphene, the parent of all graphitic forms, has become one of the most exciting topics in condensed matter physics. It is now well understood that the low-energy electronic properties of graphene are described by two-dimensional Dirac equation for massless particles, but the unoccupied bands are hardly unexplored in experimental research. In this talk we will present new results on the unoccupied electronic structure, namely the image potential states (IPS), measured by two-photon photoemission spectroscopy (2PPS). The single layer graphene is prepared by thermal decomposition of ethylene on a Ru(0001) surface. Through low energy electron diffraction (LEED) and 2PPS, we verify formation of high-quality, single layer thick graphene samples.Based on the graphene/Ru(0001) system, we first measured the surface structure and IPS on Ru(0001) by angle-resolved 2PP. From the angle-resolved spectra, we obtain the effective mass $m_{eff} = 1.1m_{e}$, which is close to the expected value for a free electron state of $m_{eff} = 1m_{e}$.With the same method, we measured the IPS on the graphene/Ru(0001) surface. However, we observe complex structure consisting of two nearly degenerate states that reflect the mutual interactions between graphene and Ru(0001) substrate. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z12.00009: Energy loss spectroscopy of epitaxial and free-standing multilayer graphene Oleksiy Roslyak, Godfrey Gumbs, Danhong Huang We present a formalism and numerical results for the energy loss of a charged particle scattered at an arbitrary angle from epitaxially grown multilayer graphene (MLG). It is compared with that of free-standing graphene layers. Specifically, we investigated the effect of the substrate induced energy gap on one of the layers. The gap yields collective plasma oscillations whose characteristics are qualitatively and quantitatively different from those produced by Dirac fermions in gapless graphene. The range of wave numbers for undamped self-sustaining plasmons is increased as the gap is increased, thereby substantially increasing and red-shifting the MLG stopping power for some range of charged particle velocity. We also applied our formalism to interpret several distinct features of experimentally obtained electron energy loss spectroscopy (EELS) data. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z12.00010: Interface Science in Graphene Materials: An Electronic Structure View of Soft X-Ray Spectroscopy Liang Zhang, Junfa Zhu, Wei-Cheng Wang, Chinglin Chang, Per-Anders Glans, Jinghua Guo The ability to control the morphology of nanostructured carbon-based materials is of crucial importance for the applications of photovoltaic, energy storage such as Li-ion batteries, etc. The properties of matter at nanoscale dimensions are dramatically different from the bulk. The differences arise through quantum confinement, altered thermodynamics or changed chemical reactivity. In general, electronic structure ultimately determines the properties of matter, thus understanding of electronic structure is crucial for tailoring the properties of nanoscale systems. The graphene/Cu and SiO$_{2}$ composites have been studied using XAS, XES and RIXS. New electronic states in the conduction band are observed, which are ascribed to the monovacancy defect state and interfacial interaction. The polarized XAS spectra demonstrate that the graphene/Cu exhibits high alignment and weak corrugation. Significant intensity modulation of resonant XES spectral shape upon different excitation energies near the C K-edge, indicates that graphene preserves an intrinsic symmetry and the interaction between graphene and Cu has unique influence on the electronic structure of graphene. The broad RIXS features and subtle shifts are observed in the RIXS spectra of graphene/Cu, which can be attributed to the strong electron-phonon scattering, charge transfer from the Cu sites [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z12.00011: Electron-Phonon Coupling and Large Intrinsic Bandgap in Highly-Screened Graphene David Siegel, Choongyu Hwang, Alexei Fedorov, Alessandra Lanzara The electron-electron and electron-phonon interactions are two of the most fundamental interactions in many-body physics, leading to Mott insulating behavior and superconductivity. In graphene, the pointlike Fermi surface and linear band dispersions give these interactions distinctive characteristics with respect to normal metals. Here we present data on the electron-phonon interaction in highly-screened graphene, which we have analyzed by angle-resolved photoemission spectroscopy (ARPES). In contrast with every previous ARPES study, the magnitude of our extracted electron-phonon coupling constants generally agree with theoretical predictions. We also demonstrate the presence of a 400 meV bandgap at the Dirac point in epitaxial graphene on a copper substrate. These results shed light on the nature of the electron-phonon interaction and the tunability of the Dirac quasiparticles in graphene. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z12.00012: Photoemission on highly-ordered arrays of structured epitaxial graphene Jeremy Hicks, J. Palmer, H. Tinkey, K. Shepperd, A. Tejeda, A. Taleb-Ibrahimi, P. Le F\`{e}vre, C. Berger, W.A. de Heer, E.H. Conrad We use the recently-demonstrated method of growing pre-patterned epitaxial graphene directly from structured silicon carbide (SiC) to produce dense, highly-ordered arrays of graphene nanoribbons suitable for macroscopic characterization methods. Specifically, we use angle-resolved photoemission spectroscopy (ARPES) to observe the ensemble-averaged band structure of nanoribbons with nominal widths of 10nm and 30nm. A Dirac-like cone is visible down to 10nm and, although the graphene is a single layer, it is undoped, in contrast to single layer graphene nanoribbons on either the Si- or C-terminated faces of SiC. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z12.00013: Spectroscopic Ellipsometry, Auger and STM Characterization of Epitaxial Graphene grown on 6H-SiC (0001) Florence Nelson, Alain C. Diebold, Andreas Sandin, Dan Dougherty, Dave Aspnes, Jack Rowe Graphene grown by the thermal decomposition of SiC has become of interest to the semiconductor industry due to its unique, high-mobility electronic structure. The growth is of a more scalable nature when compared to exfoliated flakes produced from the ``scotch tape'' method. The resulting film rests on a ``buffer layer'' separating the graphene from the underlying substrate, which is thought to consist of a mixture of sp$^{2}$ and non-sp$^{2}$ bonding due to the sp$^{3}$ bonding of the SiC substrate. The mobilities of the graphene layer have previously been shown to differ from that of the interface layer. We investigate the difference in the optical response of the two layers using Spectroscopic Ellipsometry and find a red-shift of the $\sim $4.5 eV absorbance found in graphene due to the exciton-domianted transition at the M point of the Brilloun Zone. The structural characterization of the films are performed through Auger and STM on substrates which were cleaned by CMP and chemical etching methods prior to the epitaxial growth in UHV. [Preview Abstract] |
Friday, March 2, 2012 1:51PM - 2:03PM |
Z12.00014: Vibrational spectroscopy of molecules adsorbed on graphene Yaqing Bie, Jason Horng, Feng Wang Graphene, being a monolayer membrane, is extremely sensitive to the environment. Understanding how it interacts with adsorbed molecules and polymers is fundamental for improving graphene electronic devices. Previous studies show that electrons in graphene can couple efficiently to phonon vibrations of the substrate, which can become a limiting factor for graphene mobility. Here we investigate the interactions between vibrations of adsorbed molecules with graphene using vibrational spectrsocopy. We performed the vibrational spectroscopy using a broadband tunable infrared laser source and a high precision spectrometer. We will discuss how the vibration frequencies of the adsorbed molecules get modified through their interactions with graphene. [Preview Abstract] |
Friday, March 2, 2012 2:03PM - 2:15PM |
Z12.00015: Magneto-phonon resonance of shear modes of bilayer graphene Kostyantyn Kechedzhi, Mark Olivier Goerbig, Jean-Noel Fuchs, Vladimir Fal'ko We describe the frequency renormalization of the recently observed shear phonon mode of bilayer graphene due to electron-phonon coupling. In presence of a relatively strong magnetic field resonances with electronic inter-Landau level transitions are possible. When the resonance condition is staisfied a fine structure of the Raman line corresponding to the shear mode arises which is linear in electron-phonon coupling constant. This effect can be used to measure the strength of the electron-phonon coupling for the shear mode. [Preview Abstract] |
Session Z13: Focus Session: Magnetic Nanostructures-Nanoparticles and Granular Systems
Sponsoring Units: DMP GMAGChair: Renat Sabirianov, University of Nebraska at Omaha
Room: 211
Friday, March 2, 2012 11:15AM - 11:27AM |
Z13.00001: Unexpected magnetization in highly pure metal oxide nanoparticles C.B. Hanna, Gordon Alanko, Boone Beausoleil, Jordan Chess, Aaron Thurber, Alex Punnoose We report the synthesis and characterization of a large set of highly pure metal oxide (CeO$_{2}$, SnO$_{2}$ and ZnO) nanoparticles of ultra-small size (2-10 nm). While the metal oxide systems in this study are non-magnetic as bulk materials, our prepared nanoparticles possess an unexpected small room-temperature ferromagnetic magnetization on the order of 0.001 emu/g. This magnetization is shown to not be a result of magnetic impurities, and is discussed in terms of modification of the electronic structure and crystal lattice. These nanoparticles were thoroughly characterized in their size and phase by x-ray diffraction, morphology by transmission electron microscopy, chemical state and elemental purity by x-ray photoelectron spectroscopy, electronic bandgap by UV-vis absorption spectroscopy, and magnetic properties by vibrating sample magnetometry and electron paramagnetic resonance. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z13.00002: Magnetic Transitions in Granular FeRh B. Kaeswurm, F. Jim\'enez-Villacorta, R. Barua, D. Heiman, L.H. Lewis The relationship between the crystallographic lattice and magnetism in materials undergoing first order thermodynamic phase transitions, where structural and magnetic phase transitions occur simultaneously, are not fully understood. Nanostructuring of such materials offers a route to tailor these transitions through alteration of free energy terms dependent upon the surface area and volume of finite systems. Previous studies in melt spun ribbons of FeRh nanoprecipitates in a Cu matrix have shown a reduced phase transition compared to bulk [1]. In this study nanostructured FeRh films were obtained by RF sputter deposition in nonmagnetic matrices of Cu, Si and alumina. After vacuum annealing, the structure and magnetism of the samples were studied. Preliminary results highlight the relationships between chemistry, nanostructuring and magnetic response in the FeRh system. \\[4pt] [1] Evidence for Highly Suppressed Magnetostructural Transition Temperature in Nanostructured FeRh, R. Barua; F. Jimenez-Villacorta; H. Jiang; J.E. Shield; D. Heiman; L.H. Lewis, IEEE MMM Conference 2011, Scottsdale, Arizona, US, Abstract No CE-05 [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z13.00003: Investigation of Local Structures and Magnetism in Mn-doped Y$_{2}$O$_{3}$ Nanocrystals T.S. Wu, S.L. Chang, Y.L. Soo Nanocrystals of Mn-doped Y$_{2}$O$_{3}$ were prepared by thermal decomposition method and alternately annealed in oxygen and forming gas to vary the oxygen deficiency. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HR-TEM), x-ray absorption fine structures (XAFS), and superconducting quantum interference device (SQUID) techniques were applied before and after each annealing to monitor structural and magnetic variations of the sample. The XRD data show that these annealing treatments do not appreciably change the average particle size of the sample. An amorphous-to-crystalline long-range-order structural change was observed for the first annealing applied to the as-made samples. The short-range-order structure exhibited by XAFS reveals that O vacancies surrounding magnetic impurity atoms were appreciably increased by forming-gas-annealing and decreased by oxygen-annealing in the samples. The increase and decrease of O vacancies are accompanied by enhanced and reduced saturation magnetization as demonstrated by SQUID, respectively. Our experimental results demonstrate clear correlation between magnetism and O vacancies around magnetic ions and therefore strongly support the bound magnetic polaron model in these nanocrystal DMO samples. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z13.00004: Preparation and Characterization of Mg Substituted NiCuZn Nano Ferrites for Multilayer Chip Inductors R.C. Reddy A, Sujatha Ch, Venugopal Reddy K, Sowri Babu K, H.Rao K The present paper examines the effect of magnesium substitution on structural and magnetic properties of NiCuZn nano ferrites synthesized by sol - gel method. Formation of single phase spinel structure was confirmed both from XRD and FTIR. The initial permeability shows decreasing trend with increasing Mg concentration due to reduced magnetization, grain size and increased magneto - crystalline anisotropy constant. At the same time, the cut off frequency increases with increasing Mg content. This is attributed to domain wall pinning arising due to the presence of non magnetic magnesium ions. Also the permeability is observed to be constant up to 10MHz frequency range showing compositional stability and quality of the material. The magnetic loss factor shows very low values at higher frequencies. It is concluded that even though both zinc and magnesium are non magnetic ions, substitution of one cation by another prone to influence the magnetic properties due to their change in dimension and cation distribution among the two available sites of a spinel system. These samples have advantages of low sintering temperature find applications in multilayer chip inductors due to their high and constant permeability even at higher frequencies. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z13.00005: Ferromagnetic Resonance on Micro- and Nanoferrites in Millimeter Waves Konstantin Korolev, John McCloy, Mohammed Afsar Complex magnetic permeability and dielectric permittivity of micro- and nano-sized powdered barium ferrite (BaFe$_{12}$O$_{19})$ and strontium ferrite (SrFe$_{12}$O$_{19})$ have been studied in a broadband millimeter wave frequency range for the first time. Transmittance measurements have been performed using a free space quasi-optical millimeter wave spectrometer, equipped with a set of high power backward wave oscillators. Backward wave oscillators have been used as sources of tunable coherent radiation at each individual $Q$-, $V$- and $W$- frequency bands. Real and imaginary parts of dielectric permittivity for both types of micro- and nanoferrites have been calculated using analysis of recorded high precision transmittance spectra. Frequency dependences of the magnetic permeability have been obtained from Schl\"{o}mann's equation for partially magnetized ferrites. Tunable millimeter wave absorber, based on micro- and nano-sized powdered ferrite materials is presented. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z13.00006: Polarized Magnetic Induced Broadening of Plasmon-photonics in Fe$_{3}$O$_{4}$- Silicone Elastomer Composite Films Danhao Ma, Derek Caplinger, Dustin Hess, Kofi Adu, Richard Bell We report systematic studies of polarization dependence of magneto-optical response of Fe$_{3}$O$_{4}$-silicone elastomer composite. The Fe$_{3}$O$_{4}$ particles were aligned in the elastomer matrix with static magnetic field. The optical response of two composites containing 5wt{\%} and 15wt{\%} of 20nm-30nm diameter Fe$_{3}$O$_{4}$ particle aligned in- and out-of-plane were measured with an absorption spectrometer. We observed a systematic redshift in the optical response of the out-of-plane samples with increasing static magnetic field. Furthermore, the observed redshift increases with increasing weight percent of Fe$_{3}$O$_{4}$ in the composite; obtaining a maximum shift of $\sim $ 174 nm at 600 Gauss in the 15wt{\%} Fe$_{3}$O$_{4}$-elastomer composite films. The observed redshift in the optical response of the out-of-plane composite is attributed to the effect of magnetic field strength and the metal particle/cluster size in the elastomer. However, there were no observable shifts in the in-plane samples, suggesting that the orientation (polarization) of the magnetic dipole and the induced electric dipole play a crucial role in the optical response. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z13.00007: Precipitation of coherent FeRh nanoparticles with highly suppressed magnetostructural transition temperatures in rapidly solidified (FeRh)$_{5}$Cu$_{95}$ alloys Radhika Barua, Xiujuan Jiang, Jeff Shield, Don Heiman, Laura Lewis Magnetostructural phase transitions have the capability of delivering large functional effects in response to small excursions in magnetic field, temperature and strain; this potential might be amplified in nanostructured systems by virtue of large surface:volume ratios. Nanoprecipitates ($\sim$10nm) of FeRh, a well-known magnetostructural material, were studied with structural and magnetic probes in a rapidly solidified phase-separated system of (FeRh)$_{5}$Cu$_{95}$. Magnetization studies indicate a dramatic reduction in the magnetostructural phase transition temperature (T$_{t}$) of the nanoscaled FeRh phase relative to the bulk value ($\Delta$T=T$_{t,Bulk}$ - T$_{t,Nano}$ = 220 K). Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) reveals a coherent orientational relationship between the FeRh (a$_{FeRh}$ = 3.09 {\AA})and Cu (a$_{Cu}$ = 3.78 {\AA}) phases. At the matrix/precipitate interface a constrained misfit strain of $\epsilon$ = 0.18 is observed. The reduction of the magnetostructural phase transition temperature and evolution of the magnetic properties with system annealing is analyzed in the context of the strain between the FeRh nanoparticles and the Cu matrix. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z13.00008: Biomimetic Control of Magnetite Shape and Morphology using Polyaminoacids Cem Levent Altan, Seyda Bucak, Nico Sommerdijk Inspired by nature, this work explores the use of randomly sequenced poly(aminoacids)s to control the size, morphology and magnetic properties of magnetite via synthetic methods in a controlled manner as in the case of magnetotactic bacteria. Aqueous partial oxidation and chemical precipitation methods are employed for the synthesis of 7 - 50 nm iron oxide nanoparticles at room temperature. X -- ray diffraction (XRD) and Transmission Electron Microscopy (TEM) revealed formation of iron oxide nanoparticles both in the presence and absence of poly(amino acids). In the presence of random poly(amino acid)s with different compositions consisting of E, K and A amino acids the mean particle size for the chemical precipitation method is decreased regardless of amino acid composition. For partial oxidation method, mean particle size is also decreased and nanoparticle strings are observed while synthesized in the presence of poly(aspartic acid). Magnetic properties of particles obtained via different routes are also investigated. This provides a bio-inspired route for control over size, morphology and magnetic properties of magnetite nanoparticles. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z13.00009: High Resolution Far Infrared Study of Antiferromagnetic Resonance Transitions in ${\alpha-}Fe_{2}O_{3}$ (hematite) Shin Grace Chou, David F. Plusquellic, Paul E. Stutzman, Shuangzhen Wang, Edward J. Garboczi, William F. Egelhoff In this study, we report high resolution optical measurements of the temperature dependence of the antiferromagnetic (AFM) transition in ${\alpha-}Fe_{2}O_{3}$ (hematite) between $(0.5$ and $10)$ cm$^{-1}$. The absorption peak position, over a large temperature range, is found to be in agreement with a modified spin-wave model at both the high and low temperature phases, where the temperature is above and below the Morin transition temperature, respectively. The high spectral resolution optical measurements as demonstrated in this study allow unprecedented zero-field spectral analysis of the zone center AFM magnon in a previously challenging spectral region, giving insights into the role of temperature and strain on the exchange and anisotropy interactions in the system. The results also suggest that the frequency-resolved measurement platform could be extended for room-temperature non-destructive examination and imaging applications for antiferromagnetic materials and devices. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z13.00010: Mapping Nanomagnetic Fields Using a Radical Pair Reaction Hohjai Lee, Nan Yang, Adam Cohen We visualized the magnetic field around ferromagnetic nanostructures using a combination of a standard epifluorescence microscope and a fluorescence chemical indicator of magnetic field (H. Lee et al., Nano Lett. DOI: 10.1021/nl202950h). The indicator was a chain-linked electron donor-acceptor molecule, phenanthrene-(CH$_{2})_{12}$-O-(CH$_{2})_{2}$-dimethylaniline, that forms spin-correlated radical pairs upon photoexcitation. The magnetic field altered the coherence spin dynamics, yielding an 80{\%} increase in exciplex fluorescence in a 0.1 T magnetic field. The magnetic field distributions were quantified to precision of 1.8 $\times $ 10$^{-4}$ T by image analysis and agreed with finite-element nonmagnetic simulations. [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z13.00011: Electrostatic Force Microscopy of Fe$_3$O$_4$ nanoparticles A. Mottaghizadeh, P.L. Lang, L. Cui, J. Lesueur, A. Zimmers, H. Aubin, J. Li, D.N. Zheng, V. Rebuttini, N. Pinna The electronic compressibility is a fundamental property that characterizes the electronic properties of materials submitted to an external electric field. In metals (insulators), the electronic compressibility is large (small) and leads to a small (large) screening length. Variations of the screening length can be observed through measurements of the ``quantum'' capacitance between one material and a metallic counter-electrode. Using an Electrostatic Force Microscope (EFM), we measured maps of the local capacitance of 8 nm magnetite nanoparticles synthesized following the ``benzyl alcohol route'' deposited on a metallic substrate. Magnetite, an inverse spinel structure of composition Fe$_3$O$_4$, is a material with strongly correlated electronic properties and presents a metal-insulator transition at 120 K, the so-called Verwey transition. We present EFM measurements of these nanoparticles as a function of tip-sample distance and temperature. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z13.00012: Nano-scaled magnetic domains in CMR-manganites Yoshihiko Togawa, Tsukasa Koyama, Ken Harada, Shigeo Mori La$_{1-x}$Sr$_x$MnO$_3$ (LSMO) is one of interesting materials with strongly-correlated electrons, wherein a complex variety of ground states are generated depending on the Sr doping concentration $x$. In this work, we have microscopically investigated changes of the magnetic states by applying magnetic fields in single crystals of LSMO by using Lorentz transmission electron microscopy. In the specimen with $x$ = 0.175, the magnetic stripe domains appear at regular intervals of about 200 nm as a magnetic ground state in zero magnetic field at 110 K. Importantly, we have clarified that magnetic domains as large as 100 nm are generated in the magnetic stripe domains in vertical magnetic fields and take a form of the magnetic vortex with tilted magnetic components. To the best of our knowledge, these magnetic domains are new kinds of magnetic ground states (spin textures) in manganites. In the presentation, we will explain detailed responses of magnetic vortices to various experimental parameters of external magnetic fields and discuss the nucleation and growth mechanism of magnetic vortices in the magnetic stripe domains and the expected functionality of magnetic vortices in manganites. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z13.00013: Anomalous thermal hysteresis in the high-field magnetic moments of magnetic nanoparticles embedded in multi-walled carbon nanotubes Guo-meng Zhao, Jun Wang, Yang Ren, Pieder Beeli We report high-temperature (300-1120 K) magnetic properties of Fe and Fe$_{3}$O$_{4}$ nanoparticles embedded in multi-walled carbon nanotubes. We unambiguously show that the magnetic moments of Fe and Fe$_{3}$O$_{4}$ nanoparticles are seemingly enhanced by a factor of about 3 compared with what they would be expected to have for free (unembedded) magnetic nanoparticles. What is more intriguing is that the enhanced moments were completely lost when the sample was heated up to 1120 K and the lost moments at 1120 K were completely recovered through several thermal cycles below 1020 K. The anomalous thermal hysteresis of the high-field magnetic moments is unlikely to be explained by existing physical models except for the high-field paramagnetic Meissner effect due to the existence of ultrahigh temperature superconductivity in the multi-walled carbon nanotubes. [Preview Abstract] |
Friday, March 2, 2012 1:51PM - 2:03PM |
Z13.00014: Carbon Nanostraws with Novel Magnetic Properties for Microwave Devices and Biomedical Applications K. Stojak, S. Chandra, S. Pal, H. Khurshid, M.H. Phan, H. Srikanth Carbon nanotubes (CNTs) have stirred interest in many areas of current research because of their unique electrical properties and potential use in the biomedical field. Here, we report on the synthesis, structural, and magnetic characterization of ``nanostraws,'' which consist of nanoparticle-filled CNTs made by a template-assisted chemical vapor deposition method. In this study, the nanoparticle fillers are magnetite, cobalt ferrite, and nickel ferrite. These high-aspect ratio magnetic nanostructures have a tunable anisotropy in addition to enhanced magnetic interactions amongst the CNT-encapsulated magnetic nanoparticles. Enhanced magnetic interactions include higher saturation magnetization and higher blocking temperature. These properties are desirable for microwave devices and biosensing applications. [Preview Abstract] |
Session Z16: Disordered and other Strongly Correlated Systems
Sponsoring Units: DCMPChair: Adrian Del Maestro, University of Vermont
Room: 251
Friday, March 2, 2012 11:15AM - 11:27AM |
Z16.00001: Composition-tuned smeared phase transitions David Nozadze, Fawaz Hrahsheh, Christopher Svoboda, Thomas Vojta Phase transitions in random systems are smeared if individual spatial regions can order independently of the bulk system. We study such smeared phase transitions (both classical and quantum) in substitutional alloys A$_{1-x}$B$_x$ that can be tuned from an ordered phase at composition $x=0$ to a disordered phase at $x=1$. We show that the ordered phase develops a pronounced tail that extends over all compositions $x<1$. Using optimal fluctuation theory, we derive the composition dependence of the order parameter and other quantities in the tail of the smeared phase transition. We also investigate the influence of spatial disorder correlations on smeared phase transitions. We compare our results to computer simulations of a toy model, and we discuss experiments. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z16.00002: Understanding zero-bias anomalies in disordered strongly-correlated electron systems: An atomic-limit perspective Rachel Wortis, Lister Mulindwa Many interesting phenomena arise in strongly-correlated electron systems which are disordered, either intrinsically or due to doping. In trying to understand these phenomena, the single-particle density of states provides a useful bridge between theory and experiment. Progress has recently been made in understanding the origins of the zero-bias anomaly that appears in these systems, and how this zero-bias anomaly differs from that studied by Altshuler and Aronov in weakly-correlated systems . Because both interactions and disorder reduce the importance of kinetic energy, the atomic limit provides a useful perspective. The case of long-range 1/r interactions in the atomic limit was addressed by Efros and Shklovskii, who showed the density of states is suppressed to zero at the Fermi level. However, the argument they used does not address screened interactions or the effect of double occupancy. This talk presents classical Monte Carlo results for the density of states in the atomic limit of the extended Anderson-Hubbard model. The origin of the zero-bias anomaly in this system is explained, and the results are compared both with those obtained when hopping is allowed and with those of Efros and Shklovskii. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z16.00003: Novel critical point in the random quantum Ashkin-Teller model Fawaz Hrahsheh, Thomas Vojta The first order phase transition of the quantum Ashkin-Teller model has been intensely studied over many decades. In this work, we study the effect of disorder on this quantum phase transition using a strong-disorder renormalization group approach. Specifically, we develop an implementation of the strong-disorder renormalization group that works for both weak and strong four-spin couplings. For large four-spin coupling, we find a novel type of infinite-randomness fixed point. We investigate the critical properties of this fixed point, and we discuss broader implications for the fate of the first-order quantum phase transitions in disordered systems. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z16.00004: Spectral function of two-dimensional disordered Hubbard model Oinam Nganba Meetei, Nandini Trivedi We show that moderate disorder introduces extended states in the Mott gap which upon further increase of disorder strength become localized states [1]. We propose that the inverse of the Lorentzian broadening of the spectral function A({\bf k}, $\omega$=0) as a function of {\bf k} can be used as an order parameter for describing the both the transition from a Mott insulator to an unusual metallic state and the transition from the metal to a localized insulator of spin singlets. We further track the evolution of A({\bf k},$\omega$) as a function of disorder and interaction strength. We also obtain the screening length of an external Coulomb potential from the density-density correlation function and find that the screening length is shortest at intermediate disorder in the metallic region. Our calculations are performed within an exact eigenstate formalism that treats the disorder exactly. The single particle Green's function is calculated within self-consistent mean field theory. In real space the bubble diagrams for this on-site interaction are an exact representation of density-density correlation function. \\[4pt] [1] D. Heidarian and N. Trivedi, Phys. Rev. Lett. {\bf 93}, 126401 (2004) [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z16.00005: Variational Monte Carlo Study of Anderson Localization in the Hubbard Model William Atkinson, Avid Farhoodfar, Robert J. Gooding We have studied the effects of interactions on persistent currents in half-filled and quarter-filled Hubbard models with weak and intermediate strength disorder. Calculations are performed using a variational Gutzwiller ansatz that describes short range correlations near the Mott transition. A persistent current is induced with an Aharonov-Bohm flux, and the Anderson localization length is extracted from the scaling of the current with system size. We find that, at half filling, the localization length grows monotonically with interaction strength, even though the current itself is suppressed by strong correlations. This supports earlier dynamical mean field theory predictions that the elastic scattering rate is reduced near the Mott transition. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z16.00006: Spin fluctuations near a spin-density-wave instability in periodic Anderson model studied by two-particle approach in dynamical mean field theory Wenhu Xu, Cedric Weber, Gabriel Kotliar We study the magnetic properties of periodic Anderson model when the system approaches to the vicinity of a spin-density-wave(SDW) instability from paramagnetic phase. Static and dynamical $Q$-dependent susceptibility are calculated using a two-particle approach in dynamical mean field theory. The SDW instability at a critical value of hybridyztion $V_c$ is identified by the divergence of static susceptibility at low temperature and at a wavevector $Q_c$ which connects the ``hot zones'' of the conduction band. Away from $V_c$, spin fluctuations at $Q_c$ is suppressed at low energy and at low temperature in the heavy Fermi liquid regime, while near $V_c$, spin fluctuations at $Q_c$ are significantly enhanced as temperature decreases. This indicates that the SDW instability is due to the competition between RKKY interaction and Kondo coupling in the crossover regime. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z16.00007: Large Disorder Renormalization Group Study of Singularities in the Insulating Phase of the Anderson Model of Localization Sonika Johri, R.N. Bhatt A recent study\footnote{S. Johri, R.N. Bhatt, arXiv:1106.1131v2 } of Anderson's 1958 model of localization\footnote{P. W. Anderson, Physical Review \textbf{109}, 1492 (1958).} reveals singular behavior of electronic eigenstates, as displayed by the density of states and inverse participation ratio, as a function of energy. This behavior occurs inside the insulating phase and separates typical Anderson localized from rare configuration, resonant Lifshitz states. Here, we use the large disorder renormalization group (LDRG) approach to study this problem. In particular, we study, using the LDRG approach, how the singular behavior evolves as a function of system size, starting from a toy model with two-sites which can be solved analytically. We assess the accuracy of the LDRG approach in obtaining the singular behavior in the thermodynamic limit for different disorder strengths, by comparing with results obtained by exact numerical diagonalization. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z16.00008: Long-range spatial correlations in one-dimensional Anderson models Greg Petersen, Nancy Sandler The study of metal-insulator transitions (MIT) in one-dimensional (1d) Anderson disordered systems remains an active topic of research. Analytic and numerical results have confirmed the scaling prediction on the absence of MIT for short-range correlated disorder potentials. Solutions for long-range correlated potential models (i.e. the dimer model and those with power-law spectral densities) have shown MITs in 1d. However, long-range correlations remain poorly understood. In order to gain some insight, we study a 1d Anderson model with disorder potential correlations described by a power-law model with $\langle \epsilon_r \epsilon_{0} \rangle = 1/(1 + r/a)^\alpha$. Here $\epsilon_i$, $r$, $a$, and $\alpha$ are the on-site energy, position, lattice constant, and strength of the correlation respectively. We obtained results with various methods (wave packet diffusion, participatio ratio, transfer matrix and Green's function) that support the absence of a MIT in these models in. We further show that an analysis of the beta function provides evidence for the validity of the same one-parameter scaling law valid for short-range correlated potentials. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z16.00009: Dispersive Impurities in one-dimensional Fermi Gases: From one to two Channel Kondo Polarons Karen Hallberg, Julian Rincon, Daniel Garcia, Matthias Vojta We consider the problem of a dispersive magnetic impurity interacting antiferromagnetically with a one dimensional fermionic gas. By combining general considerations and extensive numerical simulations we show that the problem displays a quantum phase transition between two-channel and one-channel Kondo behaviour upon increasing the Kondo coupling and construct a phase diagramme. We also discuss possible experimental realisations. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z16.00010: ABSTRACT WITHDRAWN |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z16.00011: Charge fractionalization on quantum Hall edges Marianne Rypestoel, Mats Horsdal, Hans Hansson, Jon Magne Leinaas Interactions between edges of quantum Hall bars give rise to Luttinger Liquid behavior with a nontrivial interaction parameter g. This leads to fractionalization of localized charges that propagate along the edges. We focus on fractionalization in systems with variable g and the separation of a charge into a sharply defined front pulse and a broader tail. The possibility of detecting the front pulse through noise measurement is discussed and illustrated by numerical simulations of a simplified Hall bar model. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z16.00012: Study of spontaneous anomalous Hall effect in 2-D electron fluid by bosonization Wathid Assawasunthonnet, Eduardo Fradkin We explore spontaneous time-reversal symmetry breaking in two-dimensional electron fluids using the method of higher dimensional bosonization. We focus on a fluid phase in which time-reversal symmetry and chiral symmetry are broken, but the space inversion and the combination of chiral and time-reversal symmetries are intact. This phase exhibits non-quantized anomalous Hall effect in the absence of external magnetic fields which corresponds to the Berry curvature on the Fermi surface. Furthermore we investigate the Berry phase connection and its representation in terms of bosonized fields. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z16.00013: Non-chiral Bosonization of Fermions in One Dimension Girish Setlur An alternative to the conventional approach to bosonization in one dimension that invokes the Dirac equation in 1+1 dimension with chiral ``right-movers'' and ``left-movers'' is proposed that works directly with the bounded parabolic energy bands relevant to Condensed Matter problems. This technique allows us to use a basis different from the plane wave basis that makes this non-chiral approach ideally suited to study Luttinger liquids that have boundary or impurities that break translational symmetry. We provide a simple solution to the electron Green function for the problem of Luttinger liquid (LL) with a boundary and also for a LL with a single impurity. The present method is significantly easier than the g-ology based standard bosonization and other methods that require a combination of RG along with bosonization/refermionization techniques. Our results are broadly consistent with these more established approaches. [Preview Abstract] |
Friday, March 2, 2012 1:51PM - 2:03PM |
Z16.00014: A Partially-ordered-set Based Approach to the Dirac Equation in 3+1 space-time Keith Earle, Kevin Knuth Recent work by Knuth and co-workers has shown how insights into Einstein's Theory of Special Relativity may be obtained by careful reasoning about consistent quantification of a poset. The Feynman Chessboard problem in 1+1 spacetime can be treated from this perspective, for example. Alternative methods of solution based on techniques borrowed from statistical mechanics have also been developed over the years to solve the Feynman Chessboard model in 1+1 spacetime. One particularly intriguing solution is based on a master-equation approach developed by McKeon and Ord for 1+1 spacetime. We will show how this model may be extended to 3+1 spacetime using techniques developed by Bialynicki-Birula, thus providing an alternative derivation of the Dirac equation. An external electromagnetic field can be accommodated very naturally in the formalism from which a pleasing pictorial representation of electromagnetic interactions in the lattice picture emerges. [Preview Abstract] |
Session Z17: 2D Crystals: Beyond Graphene
Sponsoring Units: DCMPChair: Sefeattin Tongay, University of Florida
Room: 252A
Friday, March 2, 2012 11:15AM - 11:27AM |
Z17.00001: Electronic and Transport Properties of Few-Layer MoS$_{2}$ Crystals Doron Naveh, Ashwin Ramasubramaniam, Elias Towe We investigate the electronic properties of few-layer MoS$_{2}$ flakes prepared by mechanical exfoliation. Field-effect transistors from MoS$_{2}$ flakes were fabricated and their properties were systematically characterized as a function of sample thickness. Scanning probe measurements are employed to characterize the interface between MoS$_{2}$ flakes and metal contacts. Transport properties of these devices and their correlation to electronic structure calculations are discussed. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z17.00002: Optical pump-probe studies of carrier dynamics in few-layer MoS$_2$ Rui Wang, Brian Ruzicka, Nardeep Kumar, Matthew Bellus, Hsin-Ying Chiu, Hui Zhao Molybdenum disulfide is a transition metal dichalcogenide with an indirect band gap of 1.29 eV. Its layered structure allows fabrication of atomically-thin films where the quantum confinement can significantly modify the electronic and optical properties. We demonstrate that a femtosecond pump-probe technique can be used to study charge carrier dynamics in few-layer MoS$_2$ samples fabricated on Si/SiO$_2$ substrates by mechanical exfoliation. Carriers are injected by a 780-nm pump pulse via phonon assisted indirect absorption in bilayers or through edge states. Their dynamics are probed by measuring differential reflection of a probe pulse tuned to the excitonic transition near 670 nm. We found that the magnitude, the sign, and the decay time of the signal change dramatically as the probe wavelength is tuned within the excitonic resonance. With a fixed probe wavelength, the differential reflection signal is proportional to the carrier density. The decay time is independent of the carrier density. Besides providing quantitative information on the carrier dynamics in this promising two-dimensional material, our experiment may stimulate further optical studies of carrier dynamics in this material system. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z17.00003: Integrated circuits and logic operations with high room temperature voltage gain based on single-layer MoS$_{2}$ Andras Kis, Branimir Radisavljevic, Michael Whitwick Two-dimensional materials such as single-layer MoS2 represent the ultimate limit of miniaturization in the vertical dimension, are interesting as building blocks of low-power nanoelectronic devices and are suitable for integration due to their planar geometry. Because they are less than 1 nm thin, 2D materials in transistors could also lead to reduced short channel effects and result in fabrication of smaller and more power efficient transistors. Here, we report on the first integrated circuit based on a two-dimensional semiconductor MoS2. Our integrated circuits are capable of operating as inverters, converting logical ``1'' into logical ``0'', with room-temperature voltage gain higher than 4.5, making them suitable for incorporation into digital circuits. We also show that electrical circuits composed of single-layer MoS2 transistors are capable of performing the NOR logic operation, the basis from which all logical operations and full digital functionality can be deduced. We have also fabricated suspended single-layer MoS2 membranes and have performed mechanical measurements using an atomic force microscope. Our results show that single-layer MoS2 has a Young's modulus higher than steel and can withstand deformation up to 11{\%}, making it suitable for integration with flexible electronic devices. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z17.00004: ABSTRACT WITHDRAWN |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z17.00005: ABSTRACT WITHDRAWN |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z17.00006: Transient Photoluminescence in MoS$_{2}$ layered crystals Tung-Wu Hsieh, Chih-Wei Lai We report sub-10-ps transient exciton photoluminescence (PL) in mechanically exfoliated few- and mono-layered crystals of MoS$_{2}$. We characterize layered crystals with thickness of $\sim $1$\mu $m, 100 nm, 10nm, and down to few-layers on SiO2/Si and mica substrates using luminescence and Raman spectroscopy spectroscopy. A frequency shift of $\sim $2 cm$^{-1}$ is observed on sub-10-nm-thick samples for the in-plane $E_{2g}^1 $ and the out-of-plane $A_{1g} $ Raman modes. The relative intensities of Stokes and Anti-Stokes Raman components are used to determine the lattice temperature under a laser excitation with a spot diameter of 1 $\mu $m and an average power 0.5 to 20mW. PL spectra are measured for lattice temperatures from $\sim $70K to 500K. We observe two groups of luminescence emissions with comparable peak intensities centered at 1.85eV (VIS) and 1.35eV for samples of a thickness 1 $\mu $m down to 10 nm under a cw laser excitation at a wavelength of 532nm (2.33eV). The VIS luminescence emissions are enhanced under a 2-ps pulsed laser excitation at a wavelength of 633nm (1.96eV). The rise and decay times of the luminescence are found to be less than 5 ps. Our results suggest that excitonic effects play a role in enhancing the luminescence quantum efficiency. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z17.00007: Quasiparticle self-consistent GW calculations of monolayer, bilayer and bulk MoS$_{2}$ Tawinan Cheiwchanchamnangij, Walter Lambrecht, Jie Shan Photoluminescence and absorption spectra of monolayer MoS$_{2}$ indicate a direct gap behavior while bulk MoS$_{2}$ is known to have an indirect gap. The details of these spectra and the band structure are not yet fully understood. Here, the quasiparticle self-consistent GW method is used to study the electronic structure of monolayer, bilayer, and bulk MoS$_{2}$. Band structures, effective masses, and dielectric functions are extracted from our calculations. In contrast to another recent GW calculation, we find the monolayer to have a direct gap of 2.84 eV at K, which is large compared to the photoluminescence energies. The exciton binding energy for this transition is estimated within an effective mass approximation using our calculated effective masses and dielectric constants and amounts to about 0.90 eV, leading to good agreement with the experimental data for the lowest direct transition. We find a second conduction band local minimum along $\Gamma $-K about 0.44 eV higher but do not find it to give rise to a bound state exciton. When spin-orbit coupling is included, we find a spin-splitting of the levels along $\Gamma $-K in the monolayer related to the absence of an inversion center. In the bilayer, we find an indirect gap from $\Gamma $-K and a splitting of the valence band at K, mainly due to the interlayer interaction but also increased by spin-orbit coupling. The splitting between the lowest two absorption features A and B is consistent with the slightly larger splitting calculated in the bilayer than the monolayer. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z17.00008: High Performance Two Dimensional MoS$_{2}$ MOSFET with ALD Al$_{2}$O$_{3}$ Gate Stacks Han Liu, Kun Xu, Peide Ye We study the growth mechanism of atomic layer deposition (ALD) of Al$_{2}$O$_{3}$ on layered MoS$_{2}$. We demonstrate the feasibility of direct growth of Al$_{2}$O$_{3}$ on this 2D material by trimethylaluminum (TMA) and water as precursors. Atomic force microscopy study shows that the quality of the Al$_{2}$O$_{3}$ film is degraded at elevated temperatures, originated from impeded surface absorption of precursors. We also apply density functional theory (DFT) study of the reaction which is in good agreement with our experimental observations. In addition, we fabricate dual gate MoS$_{2}$ metal-oxide-semiconductor field effect transistors (MOSFET). From the transport study we find out the lowering the growth temperature will result in a huge negative threshold voltage shift, which can be improved by either forming gas anneal after Al$_{2}$O$_{3}$ deposition or insertion of an Al seeding layer which would facilitate higher growth temperature with better film quality. Further details will be provided in the presentation. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z17.00009: Novel valley and spin physics in monolayer MoS$_{2}$ Di Xiao, G.B. Liu, Wenguang Zhu, Xiaodong Xu, Wang Yao We show that the valley Hall effect and valley-dependent optical selection rule can be realized in monolayer MoS$_{2}$, which is a direct bandgap semiconductor with non-central valleys. In addition, spin-orbit coupling in this materials is large, which gives rise to novel spin physics tied to the valley degree of freedom. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z17.00010: Enhanced thermal conductivity and isotope effect in single-layer hexagonal boron nitride David Broido, Lucas Lindsay We have calculated the lattice thermal conductivity, $k$, of both naturally occurring and isotopically enriched single layers of hexagonal boron nitride (h-BN) as well as bulk h-BN using an exact numerical solution of the Boltzmann transport equation for phonons [1]. Good agreement is obtained with measured bulk h-BN data [2], and the stronger phonon-phonon scattering identified in these systems explains why their $k$ values are significantly lower than those in graphene and graphite. A reduction in such scattering in the single layer arising mainly from a symmetry-based selection rule leads to a substantial increase in$ k$, with calculated room temperature values of more than 600 W/m-K. Additional enhancement is obtained from isotopic enrichment, which exhibits a strong peak as a function of temperature, with magnitude growing rapidly with crystallite size. [1] L. Lindsay and D. A. Broido, Phys. Rev. B 84, 155421 (2011). [2] E. K. Sichel, R. E. Miller, M. S. Abrahams, and C. J. Buiocchi, Phys. Rev. B 13, 4607 (1976). [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z17.00011: Two-dimensional Dirac fermions and quantum transport in (Sr/Ca)MnBi$_{2}$ Kefeng Wang, D. Graf, Limin Wang, Hechang Lei, S.W. Tozer, Cedomir Petrovic We report two dimensional Dirac fermions and quantum transport behavior in single crystals of SrMnBi$_{2}$ and CaMnBi$_{2}$. The non-zero Berry's phase, small cyclotron resonant mass and first-principle band structure suggest the existence of the Dirac fermions in the Bi square nets. Angular dependent magnetoresistance and quantum oscillations suggest dominant two-dimensional (2D) Fermi surfaces. The in-plane transverse magnetoresistance exhibits a crossover at a critical field $B^{\ast }$ from semiclassical weak-field $B^{2}$ dependence to the high-field unsaturated linear magnetoresistance ($\sim $120{\%} in 9 T at 2 K) due to the quantum limit of the Dirac fermions. The temperature dependence of $B^{\ast }$ satisfies quadratic behavior, which is attributed to the splitting of linear energy dispersion in high field. Our results demonstrate the universal existence of two dimensional Dirac fermions in different materials with Bi square nets. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z17.00012: Field-induced polarization of Dirac valleys in bismuth Kamran Behnia, Zengwei Zhu, Aurelie Callaudin, Benoit Fauque, Woun Kang The principal challenge in the field of ``valleytronics'' is to lift the valley degeneracy of electrons in a controlled way. In graphene, a number of methods to generate a valley-polarized flow of electrons have been proposed, which are yet to be experimentally realized. In bulk semi-metallic bismuth, the Fermi surface includes three cigar-shaped electron valleys lying almost perpendicular to the high-symmetry axis known as the trigonal axis. The in-plane mass anisotropy of each valley exceeds 200 as a consequence of Dirac dispersion, which drastically reduces the effective mass along two out of the three orientations. We present a study of angle-dependent magnetoresistance in bismuth which shows that a flow of Dirac electrons along the trigonal axis is extremely sensitive to the orientation of in-plane magnetic field. The effect is visible even at room temperature. Thus, a rotatable magnetic field can be used as a valley valve to tune the contribution of each valley to the total conductivity. At high temperature and low magnetic field, the three valleys are interchangeable and the three-fold symmetry of the underlying lattice is respected. As the temperature is decreased or the magnetic field increased, this symmetry is spontaneously lost. This loss may be an experimental manifestation of the recently proposed valley-nematic Fermi liquid state. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z17.00013: STM measurements of Charge Density Waves in S-doped NbSe$_{2}$ Ulrich Wurstbauer, Ethan Rosenthal, Erick F. Andrade, Carlos Arguello, Subbaiah P. Chockalingam, Shuang Jia, Robert J. Cava, Abhay N. Pasupathy Recent scanning tunneling microscopy (STM) measurements have shown that crystal defects have a profound influence on the nature of the charge density wave (CDW) transition in the transition-metal dichalcogenides (TMD). In order to examine this effect systematically, we employ low temperature STM to visualize the CDW transition in intentionally doped TMD crystals. We use a newly designed homebuilt ultra-low-loss, variable temperature STM to perform measurements on crystal samples of NbS$_{x}$Se$_{2-x}$. We describe the effect of the sulphur atoms on both the local and global charge order in this material. [Preview Abstract] |
Friday, March 2, 2012 1:51PM - 2:03PM |
Z17.00014: Imaging Charge Density Wave Nucleation in NbSe$_{2}$ Ethan Rosenthal, Carlos Arguello, Subbaiah Chockalingam, Woo Chang Chung, Liuyan Zhao, Christopher Gutierrez, Joon Ho Kang, Abhay Pasupathy, Shuang Jia, Robert Cava Understanding the effects of spatial inhomogeneity in complex materials is necessary to achieve a fundamental understanding of their quantum states. NbSe$_{2}$ serves as a clean and relatively simple system for understanding the emergence of one such state -- the charge density wave (CDW) phase. Using variable temperature scanning tunneling microscopy (STM), we visualize the nucleation of CDWs about crystal defects at temperatures well above T$_{CDW}$. The CDW correlation length increases with decreasing temperature, until global order is reached below T$_{CDW}$. We also employ scanning tunneling spectroscopy in order to visualize the energy-dependent, spatial phase of the CDW state. With both topographic and spectroscopic data, we will provide a clear picture of the CDW transition and insight into the microscopic mechanisms at work. [Preview Abstract] |
Friday, March 2, 2012 2:03PM - 2:15PM |
Z17.00015: Nature of Electronic States in Ultrathin MoS$_{2}$ Field Effect Transistor Subhamoy Ghatak, Atindra nath Pal, Arindam Ghosh Molybdenum disulphide (MoS$_{2})$ is a layered transition metal dichalcogenide with a Mo layer sandwiched between two S layers (S-Mo-S), which forms its basic unit. Each basic unit is attached to other units only with weak Van der Waals force. This enables to make an atomically thin single layer of MoS$_{2}$ with a bandgap 1.9 eV. The presence of bandgap has made it an interesting material in thin film transistors. It has been reported [1] recently that very high on/off ratio ($\sim $10$^{8})$ can be obtained in single layer MoS$_{2}$ transistor due to the presence of this bandgap. Though the on/off ration is very high, mobility in these transistors are considerably low. Here we have investigated the origin of such low mobility. From our temperature dependent study we find that atomically thin MoS$_{2}$ layer becomes highly disordered in the presence of the substrate and electron got localised in the traps created by the charge impurities at substrate-MoS$_{2}$ interface. We propose that high mobility can be obtained in these transistors by removing the charge impurity background. \\[4pt] [1] Radisavljevic, B. \textit{et al}. Nature Nanotechnology \textbf{2011}, 6, 147--150. \\[0pt] [2] Ghatak, S. \textit{et al}. ACS Nano \textbf{2011}, 5, 7707. [Preview Abstract] |
Session Z18: Enhanced Optical Properties Using Plasmonics, Metamaterials, and Nanoparticles
Sponsoring Units: DCMPChair: Xiaoqin Li, University of Texas, Austin
Room: 252B
Friday, March 2, 2012 11:15AM - 11:27AM |
Z18.00001: Nonresonant Broadband Funneling of Light via Ultrasubwavelength Channels Ganapathi Subramania, Stavroula Foteinopoulou, Igal Brener Efficient control of light-matter interaction, which is key to many photonics applications such as detectors, sensors and novel light sources, can be achieved by enhancing and funneling light efficiently through deep subwavelength channels. Thus far, this has been accomplished by exciting the structural surface plasmon resonances of perforated nanostructured metal films, a phenomenon known as extraordinary optical transmission. The resonant nature of the phenomenon makes it inherently narrowband. Here, we introduce a new paradigm structure consisting of double-grooved metallic structure that possesses all the capabilities of extraordinary optical transmission platforms, yet operates nonresonantly and across broadband (Phys. Rev. Lett. 107, 163902(2011)). As a result, our proposed platform demonstrates efficient ultrabroadband funneling of optical power confined to an area as small as $\sim $ ($\lambda $/500)$^{2}$, where optical fields are enhanced, thus exhibiting functional possibilities beyond resonant platforms. We explain the nonresonant mechanism underlying this phenomenon with a simple quasistatic picture that shows excellent agreement with our numerical simulations. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z18.00002: Measurement of Near-Field Thermal Radiation through a Nanometer-Sized Gap Anastassios Mavrokefalos, Poetro Sambegoro, Gang Chen Radiation heat transfer in nanostructures can differ significantly from that in macrostructures due to wave effects. Theory has predicted that thermal radiation heat transfer between two surfaces separated by tens of nanometers can exceed that of Planck's blackbody radiation law by several orders of magnitude. Our AFM-inspired heat flux sensor, comprising of a sphere attached to the tip of a bimetallic cantilever, can measure the radiation exchange across nanometer-scale gaps between a sphere and a flat surface. The objective of this work is to experimentally study thermal radiative transfer at very small separation gaps. In previous experiment, our group has successfully measured near-field radiative heat transfer through gap as small as 30 nm. In this work, we extend this technique to decrease the gap down to a few nanometers and show that existing fluctuating electrodynamics theory cannot predict experimental results in the extreme limit of small separation between two surfaces. Our experiments raise interesting question on the convergence of radiation heat transfer mechanism and interfacial heat conduction mechanism. Theoretical approaches bridging these two regimes will be discussed. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z18.00003: Making metals transparency for white light by surface plasmons Ru-Wen Peng, Xian-Rong Huang, Ren-Hao Fan, Jia Li, Qing Hu, Mu Wang We demonstrate both experimentally and theoretically that metallic gratings consisting of narrow slits become transparent for extremely broad bandwidths under oblique incidence. This phenomenon can be explained by a concrete picture in which the incident wave drives free electrons on the conducting surfaces and part of the slit walls to form surface plasmons (SPs). The SPs then propagate on the slit walls but are abruptly discontinued by the bottom edges to form oscillating charges that emit the transmitted wave. This picture explicitly demonstrates the conversion between light and SPs and indicates clear guidelines for enhancing SP excitation and propagation. Making structured metals transparent may lead to a variety of applications. References: Xian-Rong Huang, Ru-Wen Peng, and Ren-Hao Fan, Phys. Rev. Lett. (2010)105, 243901; and Ren-Hao Fan, Ru-Wen Peng, Xian-Rong Huang, Jia Li, Qing Hu, and Mu Wang, manuscript prepared(2011). [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z18.00004: Fabrication of Broad Band Mid-Infrared Absorber based on Periodic Dielectric-Thin Metal Film Multilayer Structures Shy-Hauh Guo, Andrei B. Sushkov, Timothy Corrigan, Dong Park, H. Dennis Drew, Paul Kolb, Warren Herman, Raymond Phaneuf We present results from measurements on periodic multilayer structure of alternating dielectric and thin metal layers to achieve a broadband absorber of mid-infrared radiation. We examine the effect on performance of a back-reflective metallic bottom layer, surface roughness at interfaces, the metal conductance, the thickness of dielectric layers, and a patterned anti-reflective layer. We determine optimum structure parameters for absorption of a 500 K-black body spectrum, and find that the numerical results agree well with the measured absorption spectra. We also investigate the possibility of fabricating a patterned anti-reflective layer to further increase the absorption. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z18.00005: Simultaneous Bulk and Surface Plasmon Resonance and Radiative Polaritons excited in RuO$_2$ films grown on glass and on TiO$_2$ (001) L. Wang, C. Clavero, K. Yang, E. Radue, G. Scarel, I. Novikova, R. Lukaszew Conducting oxides, such as RuO$_2$, have a much lower carrier concentration as compared to metals, leading to a lower plasma frequency of $3.3eV$ which lies in the infrared (IR) region. This unique feature of conducting oxides allows for simultaneous observation of surface and bulk polariton modes in the IR range. Here we have investigated bulk and surface plasmons as well as radiative polaritons in RuO$_2$ thin films. The RuO$_2$ thin films investigated were grown using DC magnetron sputtering on glass and on TiO$_2$ (001). We have used X-ray Diffraction and Reflection High-Energy Electron Diffraction to characterize the microstructure of these samples. Four-point probe and ellipsometry were used to investigate the electrical conductivity and the optical properties. The optical measurements were carried out using HeNe red laser ($632nm$) and IR laser ($1520nm$) radiations to illuminate RuO$_2$ thin films. We will show that bulk plasmons can be excited in RuO$_2$ thin films in the visible red region, while simultaneous bulk plasmons as well as surface plasmons excitation are observed in the IR region. We also studied the substrate influence on the radiative polaritons in the middle IR region ($20$-$2.2um$) by measuring films grown on glass and on TiO$_2$ (001). [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z18.00006: Simulations of enhanced absorption in composite embedded, insulated metal nanopatterns for ultrathin film photovoltaics Xinyu Liu, Fan Ye, Michael J. Burns, Michael J. Naughton In recent work [1], a concept of employing embedded metallic nanopatterns (EMN) in ultrathin film solar cells was discussed. Elsewhere in this conference, Fan {\it et al.} advance this with a scheme for embedded insulated metallic nanopatterns (EIMN) that is designed to avoid deleterious carrier recombination as would result from bare metal inclusions in a PV film. However, a practical route to fabricating EIMNs of desired shapes for eventual scale production is nontrivial. Here, we introduce two notions toward that goal, nano-stamping and spin-coating, of compact arrays of metallic core/insulating shell nanoparticles (MNP). We show by simulations that optical absorption of an EIMN composed of arrays of core-shell MNPs having SiO2 coatings is essentially the same as that of an EMN composed of solid metals without insulation, with absorption concentrated in the surrounding PV medium. These concepts may provide practical routes for scalability of EIMN-based ultrathin film plasmonic solar cells.\\[4pt] [1] F. Ye, M. J. Burns, M. J. Naughton, Proc. SPIE {\bf 8111}, 811103 (2011), and this conference. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z18.00007: Embedded insulated metallic nanopatterns for enhanced optical absorption and photovoltaics Fan Ye, Michael J. Burns, Michael J. Naughton Recently, we have shown embedded metallic nanopatterns (EMN) in ultrathin PV films to be candidates for high efficiency thin-film solar cells, owing to prominent metamaterial/plasmonic-enhanced light trapping, as compared to unpatterned, surface- or bottom-patterned [1]. We also showed that hot electron effects emerge in ultrathin a-Si-based solar cells [2]. The EMN in the semiconductor layer, however, can also serve as a source of recombination for photogenerated electrons and holes, leading to decreased current. Here, we propose the idea of an embedded insulated metallic nanopattern (EIMN) to efficiently avoid the recombination effect while maintaining high light absorption in an ultrathin film format in which hot electron physics can contribute. Simulations show that an EIMN with a ~10 nm layer of dielectric insulation provides essentially the same absorption as its EMN counterpart. Measurements on several EMN structures will be presented. This EIMN architecture may provide a practical route to high efficiency, hot electron solar cell technology using ultrathin films.$\\$[1]F. Ye, M.J. Burns, M.J. Naughton, Proc. SPIE {\bf8111}, 811103 (2011).$\\$[2]K. Kempa, M.J. Naughton, Z.F. Ren, A. Herczynski, T. Kirkpatrick, J. Rybczynski, Y. Gao, Appl. Phys. Lett. {\bf95}, 233121(2009) [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z18.00008: An effective medium formulation to estimate the plasmonic dispersion of a randomly distributed gallium nanoparticle ensemble Yang Yang, John M. Callahan, Tong-Ho Kim, April S. Brown, Maria Losurdo, Gianni Bruno, Henry O. Everitt Quantum confinement causes the dielectric function of nanometer-sized metal particles to depart from metallic bulk dispersion in a manner correlating with the size, shape, and spacing of the nanostructures. An improved effective medium approximation is formulated to reconcile angle-dependent ultraviolet/visible spectroscopy and spectroscopic ellipsometry measurements of the collective optical dispersion of randomly distributed hemispherical gallium nanoparticles deposited on a sapphire surface using ultra high vacuum molecular beam epitaxy. Atomic force microscopy and scanning electron microscopy analyze the size distribution of the nanoparticle ensembles to estimate their volume fraction. The optical constants are then estimated using a modified Maxwell-Garnett effective medium approximation that treats the ambient vacuum as a host and the bare sapphire substrate as a semi-infinite layer. The refined dielectric function improves estimates of the collective plasmonic response of the nanoparticle ensemble. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z18.00009: Plasmon responses and optical chirality of helical nanoparticle assemblies Zhiyuan Fan, Alexander Govorov Helical gold nanoparticle assemblies exhibit strong circular dichorism (CD) in the plasmonic band. This CD effect comes from dipolar Coulomb and electromagnetic interactions between spherical gold nanoparticles. A typical CD spectrum of chiral plasmonic assembly includes positive and negative bands. The shape of CD spectra is sensitive to geometrical parameters of the assembly. In this study, we show that the CD signal is stable against structural defects, which makes experimental realizations of strong CD effect feasible. To date, several recent experimental papers reported CD effects in helical plasmonic systems. In addition, we found that the sign of CD signal can flip as a function of the inter-particle distance and, for very long helices, the long-range electrodynamic interactions become essential. These results are important for designing nanocomposite materials with strong optical chirality in the visible wavelength range. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z18.00010: Determining 3D Relative Orientation between Plasmonic Nanosparticles Farbod Shafiei, Chihhui Wu, Patrick Putzke, Yanwen Wu, Akshay Singh, Gennady Shvets, Xiaoqin Li Polarization resolved far-field scattering measurements were used to determine 3D relative orientation between plasmonic nanosparticles placed few nanometers apart. The metallic nanostructure was assembled using the atomic force microscope (AFM) nanomanipulation method. When a gold nanosphere (150 nm in diameter) was placed within a few nanometers to the end of a gold nanorod (20 nm in diameter and 180 nm in length), near field coupling between them introduced new features in the scattering spectra. Specifically, a Fano resonance emerged, due to the interference between a dark mode, corresponding to the quadrupole charge distribution on the rod, and the bright, dipole mode of the sphere. As linear polarizer in the path of the incident and emission light was rotated, the scattering spectra evolve systematically, enabling us to determine the 3D relative orientation between these two plasmonic nanoparticles. This orientation information cannot be accurately and dynamically obtained using other scanning probe techniques, especially when one nanoparticle is partially hidden under the other nanoparticles. [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z18.00011: Plasmonic Circular Dichroism Effect in Nanomaterials Hui Zhang, Zhiyuan Fan, Alex Govorov By employing a numerical solution of Maxwell's equations beyond the dipole limit, we studied the circular dichroism (CD) signal of a chiral molecule in the presence of a gold nanopartical (NP) dimer. The CD signals come from two parts: The first one is Coulomb interaction within the molecule-dimer complex giving rise to the plasmon peak in the CD spectrum, while the other one is the plasmonic enhancement of the absorption process in a chiral molecule. Typically, the CD signals of chiral molecules are very weak in the visible range, but are strong in the UV range. In the presence of gold dimer, however, we found a strong CD signal emerges in the visible range of photon energies, where the plasmon effect makes the main contribution to CD signal at the plasmon frequency. Furthermore, we propose that, by using the plasmon-induced CD signals, one can design optical sensors to study chirality of biomolecules. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z18.00012: Metamaterial Enhanced Terahertz Spectroscopy of Biomolecules Logan Chieffo, George Keiser, Andrew Strikwerda, Kebin Fan, Shyamsunder Erramilli, Xin Zhang, Richard Averitt As the field of metamaterials experiences exponential growth, an increasing number of studies have focused on understanding near-field coupling between arrays of metamaterials and their local environment. Examples include a second array of metamaterials, phonon bands in semiconductors, and even small molecules. In this work, we demonstrate metamaterial coupling to a thin film of biomolecules. Protein films are deposited onto arrays of gold split ring resonators (SRRs) on thin silicon nitride substrates. As the LC resonance of the SRR is tuned across the low frequency terahertz (THz) modes of the biomolecule, hybridization results in mode splitting. The thin substrates (400nm) and the large electric field enhancement intrinsic to the SRRs provide ultrasensitive detection of the THz modes in the protein, allowing for the THz response to be measured from thin films which would otherwise not be observable. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z18.00013: Poynting vector in magnetic materials: from thermomagnetic effects to metamaterials Andrei Sergeev, Michael Reizer, Vladimir Mitin In a finite sample, besides the bulk currents given by the Kubo formula, additional charge and energy are transferred by surface magnetization currents. We show that for the energy current, the corresponding surface correction to the Kubo's current is expressed in terms of the magnetization component of the Poynting vector, MxE. Magnetization currents, like persistent currents, are dissipationless and do not transfer entropy. Only in this way, one can obtain the Nernst and Ettingshausen coefficients that satisfy to the Onsager relation. In the microscopic transport theory, for maintaining gauge invariance in a magnetic field, the heat current operator should include the magnetic term. Both magnetization-related effects - the dissipationless nature and strong surface contribution to the energy transfer - have been also overlooked in the recent works on the Poynting vector in metamaterials. The paradoxes in this area are resolved, if one accurately considers a balance of electromagnetic energy transferred by bulk and surface magnetization currents. [Preview Abstract] |
Friday, March 2, 2012 1:51PM - 2:03PM |
Z18.00014: Polarization Conversion Using the Cavity Resonances of Plasmonic Patch Nanoantennas Feng Wang, Ayan Chakrabarty, Fred Minkowski, Qi-Huo Wei The control of light polarization is essential to all optical experiments and photonic devices. Here we propose and demonstrate a novel method to realize the polarization conversion by using an array of non-chiral plasmonic patch nano-antennas. The patch nano-antennas are composed of a 2D array of elliptical Ag nano-disks and a base Ag substrate spaced by an ultra-thin dielectric layer. The Ag nano-disks and the base metal substrate would form an array of plasmonic nanocavities with ultrasmall mode volumes. We will show the excitation of the fundamental mode of the plasmonic cavity resonance could introduce a 90$^{\circ}$ phase delay to the reflected light from the patch antenna. This phase delay can be utilized to realize the polarization conversion between linearly polarized light and elliptically or circularly polarized light. [Preview Abstract] |
Friday, March 2, 2012 2:03PM - 2:15PM |
Z18.00015: Plasmonic nanowire transmision lines Yun Peng, Kris Kempa Metallic nanowires could be used as nano-optical transmission lines. The important factors characterizing such lines are the subwavelength operation, long propagation length, and low penetration of the propagating modes into the environment outside wires. In this work we study these factors in silver nanowires operating as surface plasmon polariton (SPP) waveguides, by employing the finite difference time domain (FDTD) and the finite difference frequency domain (FDFD) simulations. In addition to the dispersion relation of the SPP mode, we investigate the inter-wire crosstalk, an important feature of the nano-optical circuits. We compare our results with the available experimental results.~ [Preview Abstract] |
Session Z19: Invited Session: Non-Abelian States in the 1st Excited Landau Level: Experimental Status and Theoretical Outlook
Sponsoring Units: DCMPChair: Aron Pinczuk, Columbia University
Room: 253AB
Friday, March 2, 2012 11:15AM - 11:51AM |
Z19.00001: Interferometric evidence for non-Abelian quasiparticles at filling factor 5/2 Invited Speaker: Robert Willett The 5/2 fractional quantum Hall state charge e/4 excitations are proposed to follow non-Abelian statistics [1]. In edge state interference these purported non-Abelian quasiparticles should display period e/4 Aharonov-Bohm oscillations if the interfering quasiparticle encircles an even number of localized e/4 charges, but suppression of oscillations if an odd number is encircled [2-3]. To test this, we have performed swept area interference measurements at 5/2 [4-5]. We observe an alternating pattern of e/4 and e/2 period oscillations in resistance for a large change in the interferometer area, with the area sweep changing the enclosed localized e/4 quasiparticle number. This observed aperiodic alternation is consistent with proposed non-Abelian e/4 properties: the e/4 oscillations occur for encircling an even number of localized quasiparticles over their aperiodic spatial distribution, and the lower amplitude e/2 oscillations are observed when encircling an odd number as the e/4 oscillations are suppressed, allowing observation of the persistent smaller Abelian e/2 oscillations. Importantly, adding localized quasiparticles to the encircled area by changing magnetic field can change the parity of the enclosed quasiparticle number and should induce interchange of the expressed e/4 and e/2 periods: such interchange is observed in these measurements. In further experiments with the goal of understanding specific e/4 edge propagation properties, a series of interferometers of different sizes have been tested. The range of device dimensions has allowed measurement of the e/4 quasiparticle propagation attenuation length, demonstrating that small interferometric pathlengths are necessary to observe the interference oscillations. The stability in phase and amplitude of the e/4 oscillations has been tested with respect to sample dimensions, time, and temperature using this set of interferometers, and these results will be discussed. \\[4pt] [1] Moore, G. and Read, N., Nucl. Phys. B360, 362 (1991). \\[0pt] [2] Stern, A. and Halperin, B. I., Phys. Rev. Lett. 96, 016802-016805 (2006). \\[0pt] [3] Bonderson, P., Kitaev, A. and Shtengel, K., Phys. Rev. Lett. 96, 016803-016806 (2006). \\[0pt] [4] Willett, R.L., Pfeiffer, L.N., West, K.W., PNAS 106: 8853-8858 (2009). \\[0pt] [5] Willett, R.L., Pfeiffer, L.N., West, K.W., PRB 82: 205301 (2010). [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:27PM |
Z19.00002: Observation of Braiding Statistics in the Fractional Quantum Hall Effect Invited Speaker: Woowon Kang It has been postulated that the quasiparticles excitations of fractional quantum Hall (FQH) states are Abelian anyons with fractional statistical angles. More interestingly, non-Abelian anyons have been predicted for certain FQH states such as that found at filling factors $nu = 5/2$ and 12/5. To date experimental detection of anyons and their braiding statistics in quantum interference experiments has remained controversial. In this talk I will present results from the study of Abelian and non-Abelian braiding statistics of anyons in the fractional quantum Hall (FQH) systems through Fabry-Perot interferometry. In the $\nu = 7/3$ FQHE state we confirm the anyonic braiding statistics by detecting the postulated statistical phase angle of $2\pi/3$. This result is consistent with a change of the anyon number by one. In the interference study of the $\nu = 5/2$ FQH state we observe phase slips $5\pi/4, \pi,$ and $\pi/4$. These observed statistical phase slips agree with a theoretical model of braiding of Majorana modes of the $\nu = 5/2$ non-Abelian state that is strongly coupled to each other and to the edge modes of interferometer in presence of Coulomb interaction. Our results provide compelling support for the existence of non-Abelian anyons. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 1:03PM |
Z19.00003: Quantum Hall interferometry in abelian and non-abelian states Invited Speaker: Ady Stern This talk deals with the theory of quantum Hall interferometers. I will first consider an idealized model of such interferometers and review how they may potentially identify anyonic quasi-particles, both abelian and non-abelian. I will then describe lessons learned from experiments regarding this idealized model, and how it should be amended to take into account effects of coupling between the bulk and the edge. In particular, I will describe how electron-electron interaction affects the outcome of interferometry experiments and distinguishes between ``Aharonov-Bohm'' and ``Coulomb-dominated'' regimes. These two regimes are characterized by different magnetic field periodicities and different directions of the equi-phase lines in the plane of magnetic field and voltage applied to a side gate. I will discuss how the contributions of the Aharonov-Bohm effect, the Coulomb interaction and the fractional statistics (abelian and non-abelian) may be distinguished from one another in the two regimes. Finally, I will address issues unique to non-abelian states, particularly the effect of bulk-edge tunnel coupling on the interference, and comment on experimental observations of interference in the $\nu=5/2$ state. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:39PM |
Z19.00004: Plasma Analogy and non-Abelian Braiding Statistics in Ising-type Quantum Hall States Invited Speaker: Parsa Bonderson Quantum Hall systems are effectively two-dimensional and thus allow for quasiparticle excitations with exotic exchange statistics based on the braid group. These may be ``anyons'' with fractional statistics that lie somewhere between that of bosons and fermions, or, even more exotically, ``non-Abelian anyons'' with quasiparticle exchange represented by matrices. The quasiparticle statistics of candidate quantum Hall states (universality classes) is usually conjectured, but in some instances can be deduced from their representative trial wavefunctions. I will explain how this can be done for wavefunctions with a ``plasma analogy'' similar to Laughlin's. Then I will construct a plasma analogy for the non-Abelian Ising-type states, e.g. the Moore-Read Pfaffian state. This provides the first complete proof of the non-Abelian braiding statistics of quasiparticles in these states, which are likely candidates to explain the observed Hall conductivity plateaus in the second Landau level, most notably the one at filling fraction $\nu=5/2$. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 2:15PM |
Z19.00005: Insights into the Nature of the Exotic Fractional Quantum Hall States from Ultra-low Temperature Transport Invited Speaker: Gabor Csathy It is believed that the $\nu=5/2$ and 12/5 fractional quantum Hall states (FQHS) in the second Landau level of a two-dimensional electron gas support excitations with non-Abelian braiding statistics. Two outstanding questions concerning the nature of the both the odd and even denominator FQHS of the second Landau level as probed by transport measurements at temperatures as low as 5mK will be addressed. We report the discovery of a new odd denominator FQHS state at $\nu=2+6/13$. The energy gaps of this and other states at $\nu=2+1/3$, 2+2/3, and 2+2/5 reveal a markedly different dependence on the effective magnetic field as compared to that of the corresponding lowest Landau level states at 6/13, 1/3, 2/3, and 2/5. If, in addition, we assume a Landau level-independent effective mass, we find that the 7/3 and 8/3 states are consistent, whereas the 2+2/5 and the 2+6/13 states show a strong deviation from the predictions of the model of free composite fermions. For the even denominator states at $\nu=5/2$ and 7/2 we extended measurements to the new regime of very low densities and to samples grown in two MBE chambers: one at Princeton and one at Purdue. Comparisons found in the literature of the experimentally measured intrinsic gaps at $\nu=5/2$ with numerical results are confusing: three methods find a large difference whereas a fourth method finds a good agreement. Our data suggests that the former three methods have deficiencies and therefore cannot be used. Using the fourth and a new method we introduce we find an excellent agreement of the experimental and numerical intrinsic gaps at $\nu=5/2$. These findings lend a strong support to the Pfaffian description of the $\nu=5/2$ fractional state. Work done in collaboration with A. Kumar, N. Samkharadze, N. Deng, J. Watson, G. Gardner, M. Manfra, L. Pfeiffer, and K. West. G.A.C. has been supported by the NSF DMR-0907172 and DOE DE-SC0006671 grants. [Preview Abstract] |
Session Z20: Invited Session: Optical Processes in Nitrides and Other Wide-Band-Gap Semiconductors
Sponsoring Units: DCMPChair: Joel Ager, Lawrence Berkeley National Laboratory
Room: 253C
Friday, March 2, 2012 11:15AM - 11:51AM |
Z20.00001: Electronic Properties of ZnO: Reconciling Multiple Techniques Invited Speaker: Steven M. Durbin Significant progress has been made recently in our understanding of the electronic properties of ZnO and their physical origins, and yet we are far from being in possession of a complete picture despite decades of effort. Given the ultraviolet bandgap of this semiconductor (essentially the same as GaN), its easy synthesis using a wide variety of crystal growth techniques, the ready availability of high-quality single crystals, and intrinsic highly efficient luminescence, it is a material of great interest for a wide variety of device applications. Still, reliable p-type doping remains a considerable obstacle to realizing junction devices, and luminescence features often attributed to acceptor related transitions in fact have alternative physical origins. The role of impurities is reasonably well-understood, but a detailed understanding of dominant defects is somewhat elusive, although several techniques based directly and indirectly on Schottky contacts have provided some illumination on the topic. In this talk, I will summarize recent results in the field, and outline some of the key issues to which definitive answers are desirable if ZnO is going to be commercially competitive with GaN. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:27PM |
Z20.00002: Loss mechanisms in nitrides Invited Speaker: Chris Van de Walle Indium gallium nitride alloys are successfully being used for light emitting diodes (LEDs) and laser diodes (LDs) in the green to ultraviolet part of the optical spectrum. These devices are the key enablers to Solid-State Lighting, which promises to significantly cut electricity consumption. Applications are still limited, however, by the declining efficiency of LEDs at high currents (``droop'') and by absorption losses of undetermined origin in LDs. Several mechanisms have been suggested as the cause of this efficiency loss, such as Auger recombination and free carrier absorption. Experimentally it is very difficult to discriminate between different nonradiative processes. We have therefore addressed the loss mechanisms based on state-of-the-art first-principles computational theory. We use ab initio wave functions and bands that are accurate throughout the entire Brillouin zone (as opposed to k.p band structures). For Auger recombination we find that both electron-electron-hole and hole-hole-electron processes contribute, and that indirect processes assisted by alloy scattering and by electron-phonon coupling dominate. The magnitude of the resulting Auger coefficient indicates that Auger recombination is indeed responsible for the efficiency reduction at high carrier densities. Strategies for overcoming this limitation will be discussed. For free-carrier absorption, the relevant optical processes are again indirect. We determine the optical absorption coefficient and the corresponding photon mean free path as a function of carrier concentration. The computed values indicate that the effect is weak in LEDs but constitutes an important loss mechanism in LDs. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 1:03PM |
Z20.00003: Carrier localisation mechanisms and efficiency droop in nitride quantum wells Invited Speaker: Colin Humphreys A variety of experimental evidence indicates that the carriers in InGaN quantum wells (QWs) in InGaN/GaN QW structures are localised at room temperature, for example the S-shape temperature dependence of the peak photoluminescence (PL) energy with increasing excitation power density, a key fingerprint of carrier localisation. This localisation is believed to be responsible for the high efficiency of light emission from InGaN QWs since it prevents the carriers from moving to non-radiative recombination centers such as dislocations. In-rich clusters in the InGaN QWs were widely believed to be responsible for the localisation of the carriers. However, careful electron microscopy (EM) and atom probe tomography (APT) have shown that such clusters do not exist in InGaN QWs, at least for In contents less than 25{\%}, and hence such clusters cannot be responsible for the localisation of the carriers. So what mechanisms are localising the carriers? APT and EM have shown that the InGaN in the QWs is a random alloy, with the In atoms distributed at random on the Ga sites. They have also shown that the InGaN QWs have monolayer and bilayer thickness fluctuations. Quantum mechanical calculations show that the holes in the InGaN QWs are localised on a scale of 1-2nm by the random indium fluctuations, and the electrons are localised on a scale of about 5nm by the QW thickness fluctuations. This localisation prevents the carrier from diffusing to dislocations and hence results in a high efficiency of light emission at room temperature. At high carrier densities the localised states saturate with carriers and the additional non-localised carriers can then diffuse to defects and recombine non-radiatively. It is suggested that this is a significant contributory factor to the efficiency droop observed in InGaN/GaN QW structures at higher current densities. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:39PM |
Z20.00004: Photoluminescence as a tool for characterizing point defects in semiconductors Invited Speaker: Michael Reshchikov Photoluminescence is one of the most powerful tools used to study optically-active point defects in semiconductors, especially in wide-bandgap materials. Gallium nitride (GaN) and zinc oxide (ZnO) have attracted considerable attention in the last two decades due to their prospects in optoelectronics applications, including blue and ultraviolet light-emitting devices. However, in spite of many years of extensive studies and a great number of publications on photoluminescence from GaN and ZnO, only a few defect-related luminescence bands are reliably identified. Among them are the Zn-related blue band in GaN, Cu-related green band and Li-related orange band in ZnO. Numerous suggestions for the identification of other luminescence bands, such as the yellow band in GaN, or green and yellow bands in ZnO, do not stand up under scrutiny. In these conditions, it is important to classify the defect-related luminescence bands and find their unique characteristics. In this presentation, we will review the origin of the major luminescence bands in GaN and ZnO. Through simulations of the temperature and excitation intensity dependences of photoluminescence and by employing phenomenological models we are able to obtain important characteristics of point defects such as carrier capture cross-sections for defects, concentrations of defects, and their charge states. These models are also used to find the absolute internal quantum efficiency of photoluminescence and obtain information about nonradiative defects. Results from photoluminescence measurements will be compared with results of the first-principle calculations, as well as with the experimental data obtained by other techniques such as positron annihilation spectroscopy, deep-level transient spectroscopy, and secondary ion mass spectrometry. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 2:15PM |
Z20.00005: Numerical Simulation of III-Nitrides Materials and Light Emitting Devices Invited Speaker: Enrico Bellotti The versatility of III-Nitrides semiconductors has led to their use in an increasing number of technologically important applications. Their ability to operate over a wide spectral range from the infrared to the deep ultraviolet has propelled this material system into the field light emitters and detectors. Furthermore, their desirable high field transport properties make III-Nitrides an ideal platform for power semiconductor devices. Along with the experimental activity to fabricate and characterize optoelectronic and electronics devices, a number of significant theoretical efforts are underway to understand the novel properties of this material system. This presentation will discuss the unique characteristics that the III-Nitrides material presents from the point of view of the carrier transport and optical properties. The quantum mechanical processes that are responsible for breakdown at high fields will also be discussed. In particular it will be show that these quantum mechanical effects have to be taken in to account to reproduce a number of experimental results. Furthermore, an analysis of the non-radiative recombination processes that are relevant in analyzing the quantum efficiency data of III-Nitride based light emitters will be presented. Finally the model used and the results obtained for the direct and assisted (phonons and electrons) Auger recombination rates and their impact on the calculated quantum efficiency of III-Nitrides based LEDs will be presented. [Preview Abstract] |
Session Z22: Focus Session: Fe-based Superconductors - Orbital Order and Chalcogenides
Sponsoring Units: DMP DCOMPChair: Tom Devereaux, Stanford University
Room: 254B
Friday, March 2, 2012 11:15AM - 11:51AM |
Z22.00001: Orbital Order and Orthorhombic Anisotropy in Iron Pnictides Invited Speaker: Cheng-Chien Chen Orthorhombic anisotropy has been reported in iron-pnictide superconductors by a broad range of experiments, including neutron scattering, transport measurements, and a variety of spectroscopies. We explore the idea that these observed anisotropies of broken tetragonal symmetry stem from an ordering of the partially-filled iron d-orbitals. In particular, we will consider a model Hamiltonian that couples the spin and orbital variables, and show that this spin-orbital model captures several observed behaviors in the iron-pnictide materials. We will conclude the talk by discussing x-ray absorption linear dichroism and other recent experiments supportive of theories highlighting the orbital degrees of freedom. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z22.00002: Orbital ordering in Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ revealed by X-ray absorption Spectroscopy Y.K. Kim, W.S. Jung, G.R. Han, C. Kim, K.-Y. Choi, A.P. Singh, J.Y. Kim, J. Miyawaki, Y. Takata, A. Chainani Recently, anomalous in-plane anisotropy was observed by various experiment in iron pnictide systems. To explain the anomalous in-plane anisotropic behavior observed in iron pnictide system, orbital ordering was suggested as an origin of it. Among the various possible ordering, Ferroorbital ordering was proposed which occurs unequal occupation number of d$_{yz}$ and d$_{zx}$ orbital. it was theoretically predicted that such orbital ordering could be observed by performing X-ray Linear Dichroism experiment. To figure out, we performed the experiment on the most studied iron pnictide system, Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$. We obtained linear dichroism signal which indicates different occupation number for different orbital. And we observed temperature and doping dependence of the dichroism signal. Our results support the existence of ferro-orbital ordering. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z22.00003: Non-Fermi Liquid due to Orbital Fluctuations in Iron Pnictide Superconductors Wei-Cheng Lee, Philip Phillips We propose that the quantum fluctuations associated with quasi-1D $d_{xz}$ and $d_{yz}$ bands could result in a non-Fermi liquid behaviour in iron-pnictide superconductors. Using a five orbital tight binding model with generalized Hubbard on-site interactions, we find that within a one-loop treatment, a branch of overdamped collective modes develops at low frequency in channels associated with quasi-1D $d_{xz}$ and $d_{yz}$ bands. When the critical point for the $C_4$ symmetry broken phase (structural phase transition) is approached, the overdamped collective modes soften, and acquire increased spectral weight, leading to a non-Fermi liquid behavior at the Fermi surface. We argue that this non-Fermi liquid behavior is responsible for the recently observed zero-bias enhancement in the tunneling signal in quantum point contact spectroscopy. A key experimental test of this proposal is the absence of the non-Fermi liquid behaviour in the hole-doped materials. Our result suggests that quantum criticality plays an important role in understanding the normal state properties of iron-pnictide superconductors. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z22.00004: Spin and orbital characters of excitations in iron arsenide superconductors revealed by simulated resonant inelastic x-ray scattering Takami Tohyama, Eiji Kaneshita, Kenji Tsutsui We theoretically examine the orbital excitations coupled to the spin degree of freedom in the parent state of the iron-arsenide superconductor, based on the calculation in a five-band itinerant model [1]. The calculated Fe L$_3$-edge resonant inelastic x-ray scattering (RIXS) spectra disclose the presence of spin-flip excitations involving several specific orbitals. Magnon excitations predominantly composed of a single orbital component can be seen in experiments, although its spectral weight is smaller than spin-flipped interorbital high-energy excitations. The detailed polarization and momentum dependence is also discussed with predictions for the experiments. [1] E. Kaneshita, K. Tsutsui, and T. Tohyama, Phys. Rev. B {\bf 84}, 020511(R) (2011) [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z22.00005: Orbital fluctuation mediated superconductivity and structure transition in iron-based superconductors Hiroshi Kontani The main features in Fe-based superconductors are summarized as (i) orthorhombic transition accompanied by remarkable softening of the shear modulus $C_{66}$, (ii) high-$T_{\rm c}$ superconductivity close to the orthorhombic phase, and (iii) stripe-type magnetic order induced by orthorhombicity. To understand them, we analyze the multiorbital Hubbard-Holstein model with Fe-ion optical phonons. In the random-phase-approximation (RPA), a small electron-phonon coupling constant ($\lambda\sim0.2$) is enough to produce large orbital (=charge quadrupole) fluctuations. The most divergent susceptibility is the $O_{xz}$-antiferro-quadrupole (AFQ) susceptibility, which causes the $s$-wave superconductivity without sign reversal ($s_{++}$-wave state). \footnote{H. Kontani and S. Onari, Phys. Rev. Lett. {\bf 104}, 157001 (2010).} The $_{s++}$-wave state is robust against impurities, \footnote{S. Onari and H. Kontani, Phys. Rev. Lett. {\bf 103}, 177001 (2009).} consistently with experimental observations. At the same time, divergent development of $O_{x^2-y^2}$-ferro-quadrupole (FQ) susceptibility is brought by the ``two-orbiton process'' with respect to the AFQ fluctuations. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z22.00006: A Monte Carlo simulation study of phase transitions in spin-orbital models for iron pnictides Ryan Applegate, Rajiv Singh, Cheng-Chien Chen, Thomas Devereaux The common phase diagrams of superconducting iron pnictides show interesting material specificities in the structural and magnetic phase transitions. In some cases the two transitions are separate and second order, while in others they appear to happen concomitantly as a single first order transition. We explore these differences using Monte Carlo simulations of a two-dimensional Hamiltonian with coupled Heisenberg-spin and Ising-orbital degrees of freedom. In this spin-orbital model, the finite-temperature orbital-ordering transition results in a tetragonal-to-orthorhombic symmetry reduction and is associated with the structural transition in the iron-pnictide materials. With a zero or very small spin space anisotropy, the magnetic transition separates from the orbital one in temperature, and the orbital transition is found to be in the Ising universality class. With increasing anisotropy, the two transitions rapidly merge together and tend to become weakly first order. We also study the case of a single-ion anisotropy and propose that the preferred spin-orientation along the antiferromagnetic direction in these materials is driven by orbital order. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z22.00007: Local Quantum Criticality of an Iron-Pnictide Tetrahedron Tze Tzen Ong, Piers Coleman Motivated by the close correlation between transition temperature ($T_c$) and the tetrahedral bond angle of the As-Fe-As layer observed in the iron-based superconductors, we study the interplay between spin and orbital physics of an isolated iron-arsenide tetrahedron embedded in a metallic environment. Whereas the spin Kondo effect is suppressed to low temperatures by Hund's coupling, the orbital degrees of freedom are expected to quantum mechanically quench at high temperatures, giving rise to an overscreened, non-Fermi liquid ground-state. Translated into a dense environment, this critical state may play an important role in the superconductivity of these materials. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z22.00008: Hint of a condensate in K$_{\mathbf{0.8}}$Fe$_{\mathbf{2-y}}$Se$_{\mathbf{2}}$ C.C. Homes, J.S. Wen, Z.J. Xu, G.D. Gu The optical properties of the iron-chalcogenide superconductor K$_{0.8}$Fe$_{2-y}$Se$_2$ with a critical temperature $T_c = 31$~K have been measured over a wide frequency range in the {\em a-b} planes above and below $T_c$. The conductivity is incoherent at room temperature, but becomes coherent (Drude-like) with $\omega_{p,D}\simeq 430 \pm 20$~cm$^{-1}$ and $1/\tau_D \simeq 70\pm 5$~cm$^{-1}$ at $T\simeq T_c$; however, $\omega_{p,D}$ is an order of magnitude smaller than what is observed in other iron-based superconductors. The highly anisotropic nature of these materials suggests that the transport is best described by a sheet resistance $R_\Box = \rho_{dc}/d \simeq 64$~k$\Omega$ (per sheet), well above the threshold for the superconductor-insulator transition at $R_\Box = h/4e^2 \simeq 6.9$~k$\Omega$. Below $T_c$, $\omega_{p,S} \simeq 220\pm 20$~cm$^{-1}$ resulting in a superfluid density $\rho_{s0} \equiv \omega_{p,S}^2 \simeq 48 \times 10^3$~cm$^{-2}$, placing this material on the scaling line $\rho_{s0}/8 \simeq 4.4\, \sigma_{dc}\, T_c$ observed for the cuprates, but in a region associated with Josephson coupling, suggesting this material is inhomogeneous and constitutes a Josephson phase.\footnote{C. C. Homes {\em et al.}, arXiv:1110.5529} [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z22.00009: Optical spectroscopy of K$_{x}$Fe$_{2-y}$Se$_{2-z}$S$_{z}$ superconductors Catalin Martin, K. Miller, Hechang Lei, C. Petrovic, D. B. Tanner We measured the temperature dependent optical reflectivity $R(\omega$) and extracted the complex optical conductivity $\sigma(\omega)$ of the K$_{x}$Fe$_{2-y}$Se$_{2-z}$S$_{z}$ superconductors. At room temperature, both $R(\omega$) and $\sigma_{1}(\omega)$ have semiconducting behavior. Upon cooling, at temperatures depending on the S-concentration, the low frequency reflectivity increases in absolute value and shows a sharp upturn, consistent with metallic behavior. The phonon spectrum of $\sigma_{1}(\omega)$, which is very different from similar Fe-based superconductors and also doping dependent will be explained in terms of the changes in occupancy of the Fe-sites with doping. An anomalous feature in optical reflectivity is observed in the superconducting state and its possible origin and association with the condensation of free carriers will be discussed. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z22.00010: Electronic Structure of K$_{0.8}$Fe$_2$Se$_2$ High Temperature Superconductor Shizhong Yang, Rui Guo, Ebrahim Khosravi, Guang-Lin Zhao, Diola Bagayoko Since the synthesis of the first ones in 2008, iron-based high temperature superconductors have been the subject of many studies. This great interest is partly due to their higher, upper magnetic field, smaller Fermi surface around the $\Gamma $ point, and a larger coherence length. This work is focused on A$_{x}$Fe$_{2}$Se$_{2}$ structural superconductor (FeSe, 11 hierarchy; A=K, Cs) as recently observed. ARPES data show novel, electronic structure and a hole-free Fermi surface which is different from previously observed Fermi surface images. \textit{Ab initio} density functional theory GW method was used to simulate the electronic structure of the novel superconductor A$_{x}$Fe$_{2}$Se$_{2}$. We compare this electronic structure with those of other Fe-based superconductors. Possible explanations for the hole-free Fermi surface were discussed. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z22.00011: ARPES studies of the AFe2Se2 (A=K, Rb, Cs) iron-based superconductors Ming Yi, D.H. Lu, Z.K. Liu, S. Riggs, J.-H. Chu, B. Lv, S.-K. Mo, M. Hashimoto, R.G. Moore, Z. Hussain, I.R. Fisher, C.W. Chu, Z.-X. Shen The AFe2Se2 (A=K, Rb, Cs) family is one of the newest iron-based superconductors that has attracted considerable attention in the pnictide community due to its many differences with the other iron pnictide compounds, including a very large magnetic moment indicative of strong electron correlation, insulating behavior in non-superconducting compounds, and even in superconducting compounds resistivity shows a bad metallic behavior that crosses over into insulating behavior at higher temperatures. Despite such marked differences with the other pnictides, Tc in AFe2Se2 can be as high as 30K. Such interesting properties suggest that the AFe2Se2 superconductors may be close to a Mott insulating state, and many theoretical efforts have followed in this regard. Here we present detailed angle-resolved photoemission studies of AFe2Se2 to address the issue of whether these materials are indeed close to a Mott insulating state. [Preview Abstract] |
Friday, March 2, 2012 1:51PM - 2:03PM |
Z22.00012: One-Fe versus Two-Fe Brillouin Zone of Fe-Based Superconductors: Creation of the Electron Pockets via Translational Symmetry Breaking Chia-Hui Lin, Tom Berlijn, Limin Wang, Chi-Cheng Lee, Wei-Guo Yin, Wei Ku We investigate the physical effects of translational symmetry breaking in Fe-based high-temperature superconductors due to alternating anion positions [1]. In the representative parent compounds, including the newly discovered Fe-vacancy-ordered $\mathrm{K_{0.8}Fe_{1.6}Se_2}$, an unusual change of orbital character is found across the one-Fe Brillouin zone upon unfolding the first-principles band structure and Fermi surfaces [2], suggesting that covering a larger one-Fe Brillouin zone is necessary in experiments. Most significantly, the electron pockets (critical to the magnetism and superconductivity) are found only created with the broken symmetry, advocating strongly its full inclusion in future studies, particularly on the debated nodal structures of the superconducting order parameter. [1] C.-H. Lin et al, arXiv:1107.1485 (2011). [2] Wei Ku et al, Phys. Rev. Lett. {\bf 104}, 216401 (2010). [Preview Abstract] |
Friday, March 2, 2012 2:03PM - 2:15PM |
Z22.00013: Metal-to-Insulator Transition in Multi-Orbital Models for A$_x$Fe$_y$Se$_2$ Rong Yu, Qimiao Si The degree of electron correlations remains a central issue in the iron-based superconductors. Compared to other compounds, the newly discovered A$_x$Fe$_y$Se$_2$ family is unique in some aspects: the Fermi surface consists of only electron pockets, while T$_c$ is as high as 30 K ; the superconducting compound is close to an antiferromagnetically insulating phase with a large magnetic moment. These features suggest that the A$_x$Fe$_y$Se$_2$ system contains stronger electron correlations than pnictides.To investigate the correlation effects in A$_x$Fe$_y$Se$_2$, we study the metal-to-insulator transition in multi-orbital models for this system using slave-spin mean-field method. We show that when electron correlations are tuned, the system undergoes a metal-to-Mott-insulator transition at commensurate electron filling. We also find that the Mott insulator is close to an orbital-selective Mott phase in the phase diagram. [Preview Abstract] |
Session Z23: Superconductivity: Magnetic Field & Vortex Related (Experimental)
Sponsoring Units: DCMPChair: Michael Bleiweiss, Naval Academy Preparatory School
Room: 255
Friday, March 2, 2012 11:15AM - 11:27AM |
Z23.00001: Flux avalanches triggered by microwave depinning of superconducting vortices Ahmad Awad, Farkhad Aliev, Weldeslassie Ataklti, Alejandro Silhanek, Victor Moshchalkov, Yuri Galperin, Valeri Vinokur We observe abrupt changes in broadband microwave permeability of thin Pb superconducting films as functions of the microwave frequency and intensity, as well as of external magnetic field. These changes are attributed to vortex avalanches generated by microwave induced depinning of vortices close to the sample edges. We map the experimental results on the widely used theoretical model assuming reversible response of the vortex motion to a. c. drive. It is shown that our measurements provide an efficient method of extracting the main parameter of the model- depinning frequencies for different pinning centers. The observed dependences of the extracted depinning frequencies on the microwave power, magnetic field and temperature support the idea that the flux avalanches are generated by microwave induced thermomagnetic instabilities. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z23.00002: Free flux flow: a probe into the field dependence of vortex core size in clean single crystals A.A. Gapud, O. Gafarov, S. Moraes, J.R. Thompson, D.K. Christen, A.P. Reyes The free-flux-flow (FFF) phase has been attained successfully in a number of clean, weak-pinning, low-anisotropy, low-$T_{c}$, single-crystal samples as a unique probe into type II superconductivity that is independent of composition. The ``clean'' quality of the samples have been confirmed by reversible magnetization, high residual resistivity ratio, and low critical current densities $J_{c}$ with a re-entrant ``peak'' effect in $J_{c}(H)$ just below the critical field $H_{c2}$. The necessity of high current densities presented technical challenges that had been successfully addressed, and FFF is confirmed by a field-dependent ohmic state that is also well below the normal state. In these studies, the FFF resistivity \textit{$\rho $}$_{f}(H)$ has been measured in order to observe the field-dependent core size of the quantized magnetic flux vortices as modeled recently by Kogan and Zelezhina (KZ) who predicted a specific deviation from Bardeen-Stephen flux flow, dependent on normalized temperature and scattering parameter $\lambda $. The compounds studied are: V$_{3}$Si, LuNi$_{2}$B$_{2}$C, and NbSe$_{2}$, and results have shown consistency with the KZ model. Other applications of this method could also be used to probe normal-state properties, especially for the new iron arsenides, as will be discussed. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z23.00003: Vortex exclusion transition Milind Kunchur, Manlai Liang, Alexander Gurevich Ordinarily, Abrikosov vortices penetrate a type-II superconductor in a magnetic field larger than the lower critical field. However if the sample dimensions/s transverse to the applied magnetic field become smaller than some limit (found by Likharev to be $d<4.4 \xi$ for a film of thickness $d$ where $\xi$ is the coherence length), the formation of a parallel vortex in a film is excluded. We measured the transport response of a Molybdenum-Germanium superconducting film in parallel magnetic field and observed a sharp change in the V-I characteristics at a temperature which corresponds to the condition that $d \approx 4.4 \xi(T)$. We present the evolution of the observed transport behavior as a function of temperature and magnetic field. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z23.00004: Vortex motion in superconducting single-crystal microstructures of NbSe$_2$ Shaun Mills, Neal Staley, Conor Puls, Chenyi Shen, Linjun Li, Zhuan Xu, Ying Liu Superconducting microstructures prepared by advanced nanofabrication methods can be used to address long-standing, fundamental questions concerning vortex motion, including vortex tunneling and the Aharonov-Casher effect of vortices. The observation of these phenomena requires devices with minimal disorder and the fewest dissipative normal electrons. We have developed a process to fabricate superconducting microstructures from single-crystal ultrathin flakes of the layered Type II superconductor NbSe$_2$. Our process utilizes a multi-step electron beam lithography technique, whereby a NbSe$_2$ flake is cut into a desired microstructure with appropriate electrical leads. Despite the small device dimensions, which feature line widths less than 40 nanometers, our devices are superconducting. We are currently working on superconducting microstructures of NbSe$_2$ that involve integration of aluminum leads, aiming at the control and measurement of vortices in these novel structures. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z23.00005: Evidence of Vortex Jamming in Abrikosov Vortex Flux Flow Regime Goran Karapetrov, V. Yefremenko, G. Mihajlovic, J.E. Pearson, M. Iavarone, V. Novosad, S.D. Bader We report on dynamics of non-local Abrikosov vortex flow in mesoscopic superconducting Nb channels. Magnetic field dependence of the non-local voltage induced by the flux flow shows that vortices form ordered vortex chains. Voltage asymmetry (rectification) with respect to the direction of vortex flow is evidence that vortex jamming strongly moderates vortex dynamics in mesoscopic geometries. The findings can be applied to superconducting devices exploiting vortex dynamics and vortex manipulation, including superconducting wires with engineered pinning centers. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z23.00006: Vortex Pinning Enhancement in Patterned MoGe Thin Films Coated with Permalloy Michael Latimer, Zhili Xiao, Ralu Divan, Il Woong Jung, Wai-Kwong Kwok Resistivity measurements on permalloy (Py) coated MoGe thin films containing periodic hole arrays were carried out to study the effects of magnetic material filling the hole array. Thin films of MoGe were patterned via focused-ion-beam (FIB) milling to create pinning sites for the vortex lattice. A Py layer was deposited onto the hole array using magnetron sputtering. We investigate periodic hole arrays coated with magnetic material to determine the change in transport properties with varying magnetic fields. Samples with and without a Py coating were tested to determine the effect of magnetic material filling the hole array on the vortex lattice. Significant pinning enhancement was found in the Py coated samples. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z23.00007: Vortex Confinement in Planar S/F Hybrid Structures S. Moore, V. Novosad, V.G. Yefremenko, G. Karapetrov, M. Iavarone We have investigated the effect of periodic stray fields of the ferromagnet on the vortex dynamics in superconductor/ferromagnet (S/F) systems. Magnetization measurements were performed using SQUID magnetometry for Permalloy/Niobium (Py/Nb) samples of varying Py domain widths and Nb thicknesses. The hysteresis loops show an increase of the critical current for some values of magnetic domain width and superconductor thickness in some portion of the H-T phase diagram. However, below a threshold temperature sudden jumps in magnetization are observed during a slow sweep of the external magnetic field, which indicate the occurrence of vortex avalanches. These avalanches have been confirmed by scanning probe microscopy and they can cause a collapse of the critical state below a threshold temperature. Static and dynamics of these systems will be discussed. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z23.00008: Spatial resolution of MFM measurements of penetration depth Eric Spanton, Lan Luan, John Kirtley, Kathryn Moler The penetration depth and its temperature dependence are key ways to characterize superconductors. Measurements of the local Meissner response of a superconductor can determine the local penetration depth. To quantify the spatial resolution of such measurements, we seek to characterize the point spread function of magnetic force microscope (MFM) measurements of the penetration depth both numerically and experimentally. Modeling various geometries of MFM tips (pyramid, dipole, and long thin cylinder) in the presence of various geometries of spatial variation in the penetration depth (point variation, columnar defects, and planar defects or twin boundaries) shows the importance of the MFM tip geometry to achieving both excellent spatial resolution and quantitatively interpretable results. We compare these models to experimental data on pnictides and cuprates to set upper limits on the sub-micron-scale variation of the penetration depth. These results demonstrate both the feasibility and the technical challenges of submicron penetration depth mapping. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z23.00009: Direct observation of jamming of superconducting vortices in a funnel structure Vitalii Vlasko-Vlasov, Tim Benseman, Ulrich Welp, Wai Kwok Arrest of the vortex dynamics at elevated temperatures is an imperative for using benefits of high Tc in cuprate superconductors. Here we explore experimentally a possibility of using funnel geometries for attenuation of the vortex motion. A single funnel structure is sculptured into a twin free single crystal of YBCO using laser lithography and ion milling. The magnetic field penetration patterns are studied using magneto-optical imaging technique at different temperatures below Tc. In ramping up fields we observe an increase of the vortex density in the throat of the funnel structure showing the jamming of vortices at the entry into narrow vortex channel. The effect is discussed using results of recent numerical simulations. This work was supported by UChicago Argonne, LLC, under contract No. DE-AC02-06CH11357. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z23.00010: Interplay between Superconducting Vortices in the Dynamic Regime and Magnetism in Borocarbides Marcus Weigand, Christian Batista, Lev Bulaevskii, Shizeng Lin, Francisco Baca, Leonardo Civale, Sergey Bud'ko, Paul Canfield, Boris Maiorov In superconductors with coexisting magnetic order an interaction is expected between vortices moving above a certain speed and the intrinsic magnetic moments. It has been predicted that in this dynamic regime vortices emit energy through the radiation of spin waves, thus slowing down and reducing the dissipation associated with their movement.$^{1}$ While of potential interest for applications, this effect has not yet been proven experimentally. In order to elucidate the phenomenon, we have carried out electrical transport measurements on ErNi$_2$B$_2$C single crystals, covering a broad range of temperatures, applied magnetic fields and field orientations. We observe a distinct change in the shape of current-voltage curves measured above and below the N\'{e}el temperature ($T_N$), which implies that the features seen below $T_N$ are related to the material's antiferromagnetism. We complement these results with measurements of the irreversible magnetization, which also show significant anomalies around $T_N$. Angular critical current measurements have also been performed to investigate the influence of the material's intrinsic magnetic moments on its current-carrying capabilities. \\[4pt] $^{1}$A.~Shekhter, L.~N.~Bulaevskii, and C.~D.~Batista, Phys. Rev. Lett. 106, 037001 (2011). [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z23.00011: Vortex pinning effects in the Corbino geometry Yaniv Rosen, Stefan Gu\'enon, Ivan Schuller We probed a dynamic system of superconducting vortices with an artificial pinning landscape in the Corbino geometry. Current was applied from the center of the disc and propagated radially outward to produce a circular force with strength proportional to 1/r on the vortices. For small injection currents the vortex lattice is rigid, however large currents can cause shearing of the lattice. In order to investigate the temperature, current, and pinning lattice dependencies in different samples, we have defined Nb discs with a diameter of 60 $\mu $m on a circularly symmetric lattice of magnetic dots. In particular we present data that show steps instead of minima in the magnetoresistance curves at the position of the matching field indicating an unexpected influence of the pinning array on the motion of the vortex lattice. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z23.00012: Zero average and net flows of vortices in hybrid nanostructures with asymmetric pinning potentials Jose L. Vicent, David Perez de Lara, Alicia Gomez, Fernando Galvez, Miguel A. Garcia, Elvira M. Gonzalez We have fabricated hybrid nanostructures with superconducting film on top of an array of magnetic nanotriangles. In these structures, non-zero DC and AC voltages (V$_{DC}$, V$_{AC})$ are generated by alternating currents injected in the hybrid device. The V$_{DC }$and V$_{AC}$ behaviors give us an overall picture of the vortex dynamics and the rectification effects in these superconducting devices. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z23.00013: Vortex configurations and geometrical shapes of superconducting MoGe networks revealed by scanning SQUID microscope Takekazu Ishida, Ho Thanh Huy, Tsutomu Yotsuya, Masahiko Hayashi We prepared square networks of MoGe films by a standard photolithographic technique to observe vortex penetration into the network by means of a scanning SQUID microscope under different conditions of applied magnetic fields and temperatures. We found that vortex distribution in network evolves with applied magnetic field. At half matching field, vortices showed a checkerboard pattern, being in good agreement with theoretical predictions. We also investigated how vortices occupy network holes at different temperatures. Vortices tend to align in a ``diagonal'' direction at high temperatures while vortices repel each other to become isolated vortices at lower temperatures. Our results are consistent with theoretical calculation for nanoscopic superconducting network using the nonlinear Ginzburg-Landau equation. We also demonstrate that our data processing method appreciably improved a spatial resolution of the SQUID microscope. [Preview Abstract] |
Friday, March 2, 2012 1:51PM - 2:03PM |
Z23.00014: Threshold Critical Current Density to Trigger Flux Avalanches in Superconducting Thin Films W.A. Ortiz, M. Motta, F. Colauto, R. Zadorosny, T.H. Johansen, R. Dinner, M. Blamire, G.W. Ataklti, V.V. Moshchalkov, A.V. Silhanek Under certain conditions of temperature and magnetic field, sudden flux bursts (avalanches) develop into superconducting films, as a consequence of thermomagnetic instabilities, which occur when heat dispersion is slower than magnetic diffusion. Based on a systematic study of the magnetic response (including magneto-optical imaging) of two Nb films - one plain and the other decorated with a square array of square antidots - we have found the existence of a threshold critical current density above which vortex avalanches are triggered. The experimental results reveal that this threshold value is nearly constant within the whole range of temperatures and magnetic fields investigated. The fact that an avalanche is triggered once the critical current reaches the threshold is in close correspondence with the behavior of granular material in sandpiles, which slides down whenever the slope exceeds the threshold repose angle. Our results are in perfect agreement with the predictions of a model for thermomagnetic instabilities in superconducting films, published previously by Yurchenko and coworkers [PRB 76, 092504 (2007)]. [Preview Abstract] |
Friday, March 2, 2012 2:03PM - 2:15PM |
Z23.00015: Vortex-state electrodynamics in superconducting thin films studied by infrared spectroscopy Xiaoxiang Xi, D.B. Tanner, G.L. Carr, J.-H. Park, D. Graf In a type-II superconductor, a magnetic field above the lower critical field creates vortices and dramatically changes the superconductor's electrodynamic response. Such changes have been observed in thin film samples by our infrared magneto-spectroscopy experiments with field normal to the film surface. The complex optical conductivity was extracted, and was compared to existing models for the effective electrodynamic response of the vortex state. We found a good agreement between our optical data and a calculation using Maxwell Garnett theory, which treats the mixed state as having normal-metal disks (representing the vortex cores) surrounded by superconductor. Our data also show the effect of magnetic-field-induced pair breaking on the superconducting fraction outside of the vortices. [Preview Abstract] |
Session Z26: General Theory / Computational Physics II
Sponsoring Units: DCOMPChair: Timothy Germann, Los Alamos National Laboratory
Room: 257B
Friday, March 2, 2012 11:15AM - 11:27AM |
Z26.00001: Corrections to the geometrical interpretation of bosonization Manfred Steiner, Brad Marston Bosonization is a powerful approach for understanding certain strongly-correlated fermion systems, especially in one spatial dimension but also in higher dimensions\footnote{A.Houghton, H.-J. Kwon and J. B. Marston, Adv. in Phys. { \bf 49}, 141 (2000).}. The method may be interpreted geometrically in terms of deformations of the Fermi surface, and the quantum operator that effects the deformations may be expressed in terms of a bilinear combination of fermion creation and annihilation operators. Alternatively the deformation operator has an approximate representation in terms of coherent states of bosonic fields\footnote{A. H. Castro Neto and E. Fradkin, Phys. Rev. B {\bf 49}, 10877 (1994).}. Calculation of the inner product of deformed Fermi surfaces within the two representations reveals corrections to the bosonic picture both in one and higher spatial dimensions. We discuss the implications of the corrections for efforts to improve the usefulness of multidimensional bosonization. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z26.00002: Emergent Anyons in Exactly Solvable Discrete Models for Topological Phases in Two Dimensions Yuting Hu, Spencer Stirling, Yong-Shi Wu Anyons can emerge as collective excitations in models of topological phases. Exactly solvable discrete models that describe two-dimensional topological phases were proposed by Kitaev, and Levin and Wen respectively. I will present the explicit form of the operators that create and move fluxons (anyonic quasiparticles living at the plaquettes) in the Levin-Wen models. The exchange and exclusion statistics of these fluxons are studied. In particular, I will discuss the topological properties of Fibonacci anyons emerging in a particular Levin-Wen model. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z26.00003: Uniform Approximation from Symbol Calculus on a Spherical Phase Space Liang Yu We use symbol correspondence and quantum normal form theory to develop a more general method for finding uniform asymptotic approximations. We then apply this method to derive a uniform approximation of the $6j$-symbol in terms of the rotation matrices. The derivation is based on the Stratonovich-Weyl symbol correspondence between matrix operators and functions on a spherical phase space. The resulting approximation depends on a canonical, or area preserving, map between two pairs of intersecting level sets on the spherical phase space. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z26.00004: Searching for B-modes with QUIET: Latest results from the maximum-likelihood pipeline Ingunn Kathrine Wehus The polarisation of the cosmic microwave background (CMB) gives us unique information about the existence of primordial gravitational waves and the energy scale of inflation. QUIET is a ground-based CMB polarisation experiment, taking advantage of the world's most sensitive microwave radiometers, to search for B-modes in the CMB polarisation. The pilot phase data taking was finished in December 2010, consisting of around 250 000 diode hours of Q-band data at 43 GHz and about ten times more W-band data at 94 GHz. The data are analyzed by two independent pipelines, the maximum-likelihood (ML) pipeline and the pseudo-$C_\ell$ (PCl) pipeline. I will present the ML pipeline and our latest results. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z26.00005: The Fourth Dimension and Transaction Line Peter Schick The fourth dimension is basically the depth between two objects or the depth between an object. It is the z compared to the length, width, height. When one looks at a few objects the depth in between them is the fourth dimension. The transaction line is a relationship between two objects that are falling. This line allows the relationship to form so that they fall at the same time, regardless how much they weigh. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z26.00006: Reconsidering Dark Matter Sol Aisenberg There is a difference between (a) distances of remote standard candles, SN Type Ia, and (b) distances based upon their red shifts. It was believed that these galaxies had accelerated and used Dark Energy. There are 2 assumptions not supported by observations. The first is that the red shifts for remote galaxies are due to the Doppler Effect associated with receding velocity. Hubble only observed red shifts as a function of distances of known stars, and never measured receding velocities. He suggested the Doppler Effect as a cause, but expressed doubt about the suggestion. There are other causes for a red shift - gravity red shift of light from the sun, and loss of photon energy by gravity interaction of photons with dust and gas in interstellar space. The second assumption is that Hubble's linear relationship between the observed red shift and the distance will be valid at very large distances. Increasing red shift corresponds to a decrease of photon energy towards zero, and cannot be used for very remote stars - where the photon energy approaches zero and the red shift dependence becomes nonlinear and asymptotic to a constant value. This predicts the difference between the galaxy distances and the distances determined from their observed red shifts. The recent Nobel Prize (to Schmidt, Reis, and Perlmutter) needs reexamination. Two basic assumptions that are the foundation of their work may not be accurate. Details are in my earlier essays in ``The Misunderstood Universe'', {\copyright} 2009. . [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z26.00007: The Exact Solution of The Pioneer Anomaly According to The General Theory of Relativity and The Hubble's Law Azzam Almosallami Radio metric data from Pioneer 10/11 indicate an apparent anomalous, constant, acceleration acting on the spacecraft with a magnitude $\sim 8 \times 10^{-10}$ m/s$^2$, directed towards the Sun[1,2]. Turyshev [7] examined the constancy and direction of the Pioneer anomaly, and concluded that the data a temporally decaying anomalous acceleration $-2 \times 10^{-11} \frac{m}{s^2 yr}$ with an over 10{\%} improvement in the residuals compared to a constant acceleration model. Anderson, who is retired from NASA's Jet Propulsion Laboratory (JPL), is that study's first author. He finds, so ``it's either new physics or old physics we haven't discovered yet.'' New physics could be a variation on Newton's laws, whereas an example of as-yet-to-be- discovered old physics would be a cloud of dark matter trapped around the sun[12]. In this paper I introduce the exact solution for the Pioneer anomaly depending on the general theory of relativity and the Hubble's law. According to my solution, there are two terms of decelerations that controls the Pioneer anomaly. The first is produced by moving the Pioneer spacecraft through the gravitational field of the Sun, which causes the velocity of the spacecraft to be decreased according to the Schwarzschild Geometry of freely infalling particle. This deceleration is responsible for varying behaviour of the Pioneer anomaly in Turyshev [7], depending on $1/r^{2.5}$ the distance from the sun. The second term is produced by the attractive force of the dark matter which is constant and equals to the Hubble's constant multiplied by the speed of light in vacuum. [Preview Abstract] |
Session Z27: Invited Session: Interaction Driven Broken Symmetry States in Bilayer Graphene
Sponsoring Units: DCMPChair: Allan MacDonald, University of Texas at Austin
Room: 258AB
Friday, March 2, 2012 11:15AM - 11:51AM |
Z27.00001: Theoretical Approach to Many-body Instabilities in Bilayer Graphene Invited Speaker: Oskar Vafek I will review current theoretical approach to electron-electron interaction driven many-body instabilities in bilayer graphene at the neutrality point. The role of competing interactions and the dependence of different ordering tendencies on the range of the interaction will be examined. In particular, I will argue that within the renormalization group approach, for longer range interaction giving predominantly forward scattering, the leading ordering tendency is towards a gapless electronic nematic state. For shorter range interactions giving additional back scattering comparable to the forward scattering, the leading ordering tendency is towards a gapped Neel antiferromagnet. These results will be discussed in the context of recent experiments reporting signatures of broken symmetry states in suspended bilayer graphene. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:27PM |
Z27.00002: Interaction-Driven Spectrum Reconstruction in Bilayer Graphene Invited Speaker: Kostya Novoselov The nematic phase transition in various two-dimensional electronic systems is a fascinating subject of an ongoing investigation. Driven by electron-electron interactions it represents a new class of strongly correlated electronic ground states. Thus it is extremely important and interesting to expand the list of materials where such transitions are observed to those with particularly unusual electronic dispersion. Here we show indications that bilayer graphene -- a truly two-dimensional material with complex chiral electronic spectrum -- undergoes such transition. This is especially surprising as no interaction effects have been observed so far in either mono- or bilayer graphene without a help of magnetic field. Gaining access to low-energy physics in bilayer graphene devices (by suspending our samples and achieving quasiparticle mobilities larger than 10$^{6}$ cm$^{2}$/V$\cdot $s) allowed us to observe strong spectrum reconstructions and electron topological transitions which we attribute being due to such nematic transition. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 1:03PM |
Z27.00003: Spontaneous Quantum Hall Liquids Invited Speaker: Fan Zhang Driven by electron-electron interactions, bilayer graphene and its thicker cousins, chirally (ABC) stacked multilayers, exhibit a variety of distinct broken symmetry states in which each spin-valley flavor spontaneously transfers charge between layers, because of their flat touching bands and large pseudospin chiralities. These gapped states are accompanied by large momentum space Berry curvatures and different types of topological orders. These competing ground states are distinguished by their flavor Hall conductivities, orbital magnetizations, edge state properties, and response to external fields. These spontaneous quantum Hall (SQH) states at zero field smoothly evolve into quantum Hall ferromagnet states at finite field. Various phase transitions occur by tuning carrier densities, temperature, and external fields. Recently, SQH states have started to be observed and explored in transport and Hall experiments on suspended devices with dual gates. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:39PM |
Z27.00004: Interaction-Driven Insulating States in Bilayer Graphene Invited Speaker: Chun Ning Lau Bilayer graphene (BLG) at the charge neutrality point (CNP) is unstable to electronic interactions, and expected to host a ground state with spontaneously broken symmetries. Here I will present our transport spectroscopy measurements on singly- and dual-gated suspended BLG devices, which have field effect mobility values up to 250,000 and 100,000 cm$^2$/Vs, respectively. We observe an insulating state at CNP with a gap $\sim $2 meV, which can be closed by elevated temperature, finite doping or a perpendicular electric field of either polarity. For magnetic field B$>$1T, the gap increases linearly with B. Our work contributes towards understanding the rich interaction-driven physics in BLG. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 2:15PM |
Z27.00005: Electronic structure of multilayer graphene Invited Speaker: Edward McCann The single-particle low-energy Hamiltonian of bilayer graphene describes chiral quasiparticles with a dominantly parabolic dispersion exhibiting Berry phase 2$\pi$. This chiral Hamiltonian produces a doubly-degenerate zero-energy Landau level incorporating two different orbital states with the same energy. Taking into account spin and valley degeneracies, the zero-energy Landau level in a bilayer is eightfold degenerate, as compared to the fourfold degeneracy of other bilayer states and the fourfold degeneracy of all levels in a monolayer. Such levels can be split by interlayer asymmetry, due to the presence of an external gate or doping, or by interaction effects. This talk will describe the electronic behavior of multilayer graphene, focusing on three, four and five layers. The goal will be to identify features that are distinct from those observed in monolayers and bilayers, and to highlight effects - such as level splitting and crossing - that can be explained either within the single-particle picture or that require an understanding of electronic interactions. For example, the low-energy Hamiltonian of ABA-stacked multilayer graphene may be partially diagonalized into an approximate block-diagonal form, with each diagonal block contributing parabolic bands except for an additional block describing Dirac-like bands with a linear dispersion in a multilayer with an odd number of layers. By taking into account the symmetry of the crystal structure, it is possible to fully include the band parameters and to analyze their effect on the block-diagonal Hamiltonian. Next-nearest-layer couplings are shown to be particularly important in determining the low-energy spectrum and the phase diagram of the quantum Hall conductivity by causing energy shifts, level anti-crossings, and valley splitting of the low-lying Landau levels. \\[4pt] This work was done in collaboration with Mikito Koshino of the Department of Physics, Tohoku University, Sendai 980-8578, Japan. [Preview Abstract] |
Session Z29: Focus Session: Superconducting Qubits: Noise and Anomalous Temperature
Sponsoring Units: GQIChair: Will Oliver, MIT/ Lincoln Laboratory
Room: 259A
Friday, March 2, 2012 11:15AM - 11:27AM |
Z29.00001: Measurement-induced qubit state mixing from upconverted low frequency noise D.H. Slichter, R. Vijay, S.M. Weber, I. Siddiqi We observe readout-induced qubit state mixing in a flux-tunable transmon qubit coupled to a planar resonator. Our results indicate that dephasing noise at the qubit-cavity detuning frequency $\Delta$ is upconverted by photons in the readout cavity, causing spurious qubit state transitions in agreement with theory [1]. Using a superconducting parametric amplifier to perform continuous high-fidelity qubit measurement, we characterize the transition rate dependence on cavity photon population and the intensity of added low frequency noise injected from a broadband fast flux excitation line. From the remnant excitation rate in the absence of added noise, we extract a noise spectral density at frequencies $\sim$ 1 GHz. We also measure the noise spectral density from 0.02-0.5 Hz and 1-20 MHz using Ramsey fringes and Rabi oscillations, respectively. Postulating that flux noise is the dominant source of dephasing in our qubit, we fit the measured noise to a $1/f^\alpha$ power law, finding a slope $\alpha=0.6$ and amplitude $(1.4\mu\Phi_0)^2$/Hz at 1 Hz. These values are in agreement with other measurements of low-frequency flux noise. Our results suggest that 1/f flux noise persists to GHz frequencies. \\[4pt] [1] Boissonneault et al., PRA 79, 013819 (2009). [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z29.00002: 1/f Noise of Josephson Tunnel Junction Embedded Microwave Resonators at Single Photon Energies and Millikelvin Temperatures I. Siddiqi, Kater Murch, Steven Weber, Eli Levenson-Falk, R. Vijay We present measurements of the 1/f frequency noise of superconducting aluminum lumped and distributed element resonators in the low power, low temperature operating regime characteristic of superconducting qubits. A comparison was made between these devices and similar co-fabricated linear resonators to infer the level of critical current noise associated with the tunnel junctions. At 25 mK and in the single photon regime, the observed frequency fluctuations of junction embedded and linear resonators were comparable. Attributing all the observed noise to critical current fluctuations, we report an upper bound for 1/f critical current fluctuations in 0.5-2 $\mu$A junctions of 3 x $10^{-8}(1/\sqrt{Hz})$ at 1 Hz. We note that for some samples we observed the activation of a single fluctuator above 50 mK which increased the level of noise significantly. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z29.00003: Flux Noise in SQUIDs Due to Hyperfine Interactions Jiansheng Wu, Clare Yu Although there have been significant advances in superconducting qubits, they continue to be plagued by noise and decoherence. Low frequency $1/f$ flux noise in superconducting quantum interference devices (SQUIDs) is one of the dominant sources of noise in superconducting flux and phase qubits. Recent experiments implicate spins on the surface of metals as the source of flux noise in SQUIDs, and indicate that these spins are able to relax without conserving total magnetization. We present a model of $1/f$ flux noise in which electron spins on the surface of metals can relax via hyperfine interactions. Our results indicate that flux noise would be significantly reduced in superconducting materials where the most abundant isotopes do not have nuclear moments such as zinc and lead. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z29.00004: Pure dephasing in flux qubits due to flux noise with spectral density scaling as $1/ f^\alpha$ Steven Anton, C. Muller, J.S. Birenbaum, S.R. O'Kelley, A.D. Fefferman, D.S. Golubev, G.C. Hilton, H.-M. Cho, K.D. Irwin, F.C. Wellstood, Gerd Schon, A. Shnirman, John Clarke Magnetic flux noise is a major source of pure dephasing in superconducting flux qubits. This noise, common to SQUIDs, is believed to arise from localized electrons whose spins reverse randomly. We present representative measurements on dc SQUIDs over a range of temperatures showing, in general, $S_\Phi(f)= A^2/(f/(1~$Hz$))^\alpha$. In our measurements, $A$ is of the order of 1~$\mu\Phi_0$Hz$^{-1/2}$ and $0.6<\alpha<1$. Motivated by these results, for arbitrary values of $\alpha$ we calculate pure dephasing times for both Ramsey and echo pulse sequences assuming linear coupling between the energy level splitting and the flux through the qubit. We find that the dephasing time $\tau_\phi$ decreases dramatically as $\alpha$ is reduced. In addition, the frequency bandwidth to which the qubit is sensitive---defined by the infrared and ultra-violet cutoff frequencies---can significantly affect $\tau_\phi$ in a manner depending on the type of sequence and value of $\alpha$. For each sequence, $\tau_\phi$ becomes independent of the ultra-violet cutoff frequency when its value exceeds $1/\tau_\phi$. Finally, we present calculated dephasing times corresponding to our measured spectra. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z29.00005: Very-low-frequency spectroscopy of both flux and tunnel-coupling noise in a flux qubit Jonas Bylander, Fei Yan, Simon Gustavsson, Fumiki Yoshihara, David G. Cory, Yasunobu Nakamura, William D. Oliver We inferred the very-low-frequency noise (0.01 to 100 Hz) of a superconducting flux qubit by repeatedly subjecting it to free induction during a fixed length of time and sampling the binary read-out signal. The excited-state probability varies as the qubit-transition frequency fluctuates due to noise, and we control the sensitivities to noise in the energy and tunnel coupling terms of the Hamiltonian by tuning the static flux (energy) bias. At low temperature, interestingly, both types of low-frequency noise follow the same 1/f-type power laws observed at much higher frequencies. We will further present the temperature dependence of both noises from 10 to 200 mK. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z29.00006: $T_{1\rho}$ experiment as a noise spectrum analyzer Fei Yan, Simon Gustavsson, Jonas Bylander, Fumiki Yoshihara, Yasunobu Nakamura, David Cory, William Oliver We performed a $T_{1\rho}$ (spin-locking) experiment on a superconducting flux qubit, enabling us to resolve the environmental noise in the intermediate-frequency range. By driving the qubit along its state polarization, in the rotating frame, it is effectively spin-locked: the decohering effect of low-frequency noise is thereby dramatically reduced compared to Rabi oscillations. We measured the $T_{1\rho}$ relaxation rate in the rotating frame, under different driving amplitudes and flux biases. Relating this driven relaxation rate to the noise at the corresponding Rabi frequency, we extracted the noise power spectral densities of the energy-bias (flux) and tunnel-coupling terms of the qubit's Hamiltonian at frequencies ranging from 0.5 to 100 MHz. In the flux-noise spectrum, we observed features due to non-Gaussian noise, which can be modeled by a strong random-telegraph fluctuator, supporting observations in the decoherence of a spin-echo. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z29.00007: Characterization of critical-current noise in Josephson junctions and its implications for qubit dephasing Christopher D. Nugroho, Vladimir Orlyanchik, Dale J. Van Harlingen Critical-current noise in Josephson junctions may ultimately limit the coherence of superconducting qubits. Presently qubit coherence times are limited by energy relaxation or other dephasing mechanisms, but recent qubit advances may put the coherence times in the regime where critical-current noise play an important role. We report on the measurement of $I_{c}$-noise in Josephson junctions and compare them to fluctuations in the normal state resistance when superconductivity is suppressed in a magnetic field. We measure the noise scaling with the junction area, normal state resistance, and temperature. We will then discuss the implication of this noise to qubit decoherence. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z29.00008: Pure dephasing from quasiparticle tunneling in superconducting qubits Gianluigi Catelani Quasiparticles tunneling across Jospehson junctions provide an intrinsic decoherence mechanism in superconducting qubits. The quasiparticle current spectral density $S_{\mathrm{qp}}(\omega)$ determines both the relaxation rate and the pure dephasing rate. The latter is in general proportional to the the spectral density evaluated at zero frequency. In the case of quasiparticle, however, $S_{\mathrm{qp}}(\omega)$ diverges logarithmically as frequency goes to zero, potentially leading to very fast dephasing. Here we show how to regularize this divergence in a self-consistent way. This enable us to estimate the dephasing rate due to quasiparticle tunneling and to study its magnetic flux dependence for various qubit designs. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z29.00009: Anomalous Temperatures of Superconducting Qubits K. Geerlings, S. Shankar, Z. Leghtas, M. Mirrahimi, L. Frunzio, R.J. Schoelkopf, M.H. Devoret We present qubit temperature measurements on several superconducting transmon qubits coupled to compact resonators. By addressing multiple transitions of the artificial atom and cavity system, we measured the temperature as a function of qubit and cavity frequency and cavity Q. For high cavity Q and large detuning, qubit temperatures were found to be greater than 120mK, well in excess of the dilution refrigerator base temperature of 15mK. This unanticipated effect can be explained by the decoupling of the qubit to the cold load damping the cavity. We will present our attempts to produce lower qubit temperatures with additional filtering and dynamical cooling experiments. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z29.00010: Minimizing environmental decoherence for a superconducting phase qubit - transmon architecture Rami Barends, J. Wenner, M. Lenander, Y. Chen, J. Kelly, J. Bochmann, B. Chiaro, E. Lucero, P. O'Malley, M. Mariantoni, A. Megrant, C. Neill, D. Sank, P. Roushan, A. Vainsencher, H. Wang, T. C. White, Y. Yin, A. N. Cleland, John M. Martinis, J. J. A. Baselmans The coherence of superconducting quantum systems is currently a major obstacle towards high gate fidelity and long-lived memory. We found that quasiparticle generation from stray infrared light is a significant source of energy relaxation. We show that resonator quality factors and phase qubit energy relaxation times are limited by a quasiparticle density of approximately 200 $\mu$m$^{-3}$, induced by 4 K blackbody radiation from the environment. We demonstrate how this influence can be fully removed by isolating the devices from the radiative environment using multistage shielding. In addition, we analyze the decoherence due to circuitry in our new phase qubit-transmon architecture. This architecture - consisting of transmon qubits, resonators, and phase qubits using a frequency domain multiplexed readout - is less affected by Purcell decay. At present, fabrication of samples is ongoing. [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z29.00011: Designs towards improved coherence times in superconducting qubits Antonio Corcoles, Jerry Chow, Jay Gambetta, Chad Rigetti, Jim Rozen, George Keefe, Mary Beth Rothwell, Stefano Poletto, Mark Ketchen, Matthias Steffen Coherence times for superconducting qubits in a planar geometry have increased drastically over the past 10 years with improvements exceeding a factor of 1000. However, recently these appeared to have reached a plateau around 1-2 microseconds, the limits of which were not well understood. Here, we present experimental data showing that one limit is due to infra-red radiation, confirming observations from other groups. We observe increased coherence times after appropriate IR shielding. Further improvements are shown to be possible by increasing the feature size of the interdigitated shunting capacitor, strongly indicating that surface losses at the metal/substrate interface are limiting qubit coherence times. In our experiments we kept the ratio of line width to gap size constant, but increased the overall feature size. We will discuss this and other similar design approaches towards better coherence in superconducting qubits. [Preview Abstract] |
Session Z30: Focus Session: Quantum Information for Quantum Foundations - Quantumness versus Classicality
Sponsoring Units: GQIChair: Philip Goyal, University of Albany
Room: 259B
Friday, March 2, 2012 11:15AM - 11:27AM |
Z30.00001: Nonequilibrium quantum correlations and Leggett-Garg inequalities J.C. Castillo, F.J. Rodriguez, L. Quiroga Theoretical guides to test 'macroscopic realism' in condensed matter systems under quantum control are highly desirable. We report the evaluation of Leggett-Garg inequalities (LGI) in an out-of-equilibrium set up consisting in two interacting qubits coupled to independent baths at different temperatures as can occur for two dipolar coupled spins or superconducting qubits in diverse solid-state environments. We find that LGI violations persist for a longer time in a thermal nonequilibrium scenario as compared with similar results at thermodynamic equilibrium. We contrast these findings with the behavior of non-locality-dominated quantum correlation measurements, such as concurrence, between the two qubits under similar temperature gradients. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z30.00002: Quantum Correlations in Large-Dimensional States of High Symmetry Eric Chitambar Multiparty quantum systems can possess non-classical correlations more general than those characterized by entanglement. In this talk, I will discuss various proposed measures of quantum correlations and investigate how these measures behave for the so-called Werner and isotropic families of states. In particular, I will provide closed expressions for the quantum discord (QD) and the relative entropy of quantumness (REQ) in these states for arbitrary dimensions. The QD and REQ will be shown to equal one another, as well as other well-known measures of quantum correlations. For all Werner states, the classical correlations are seen to vanish in high dimensions while the amount of quantum correlations becomes independent of whether or not the the state is entangled. For isotropic states, nearly the opposite effect is observed with both the quantum and classical correlations growing without bound as the dimension increases and only as the system becomes more entangled. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z30.00003: Nonnegative subtheories of qubits: stabilizer states and more Stephen Bartlett, Joel Wallman Negativity in a quasi-probability representation is typically interpreted as an indication of nonclassical behavior. However, this does not preclude bases that are nonnegative from having interesting applications---the single-qubit stabilizer states have nonnegative Wigner functions and yet play a fundamental role in many quantum information tasks. We determine what other sets of quantum states and measurements of a qubit can be nonnegative in a quasi-probability distribution, and identify nontrivial groups of unitary transformations that permute such states. These sets of states and measurements are analogous to the single-qubit stabilizer states. We show that no quasi-probability representation of a qubit can be nonnegative for more than 2 bases in any plane of the Bloch sphere. Furthermore, there is a unique set of 4 bases that can be nonnegative in an arbitrary quasi-probability representation of a qubit. We provide an exhaustive list of the sets of single-qubit bases that are nonnegative in some quasi-probability distribution and are also closed under a group of unitary transformations, revealing two families of such sets of 3 bases with quasi-probability distributions defined on a space of 8 ontic states. We extend several of these results to higher dimensions. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z30.00004: Symmetric States on the Octonionic Bloch Ball Matthew Graydon Finite-dimensional homogeneous self-dual cones arise as natural candidates for convex sets of states and effects in a variety of approaches towards understanding the foundations of quantum theory in terms of information-theoretic concepts. The positive cone of the ten-dimensional Jordan-algebraic spin factor is one particular instantiation of such a convex set in generalized frameworks for quantum theory. We consider a projection of the regular 9-simplex onto the octonionic projective line to form a highly symmetric structure of ten octonionic quantum states on the surface of the octonionic Bloch ball. A uniform subnormalization of these ten symmetric states yields a symmetric informationally complete octonionic quantum measurement. We discuss a Quantum Bayesian reformulation of octonionic quantum formalism for the description of two-dimensional physical systems. We also describe a canonical embedding of the octonionic Bloch ball into an ambient space for states in usual complex quantum theory. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z30.00005: Magically, the negativity of the discrete Wigner function is useful Victor Veitch, Christopher Ferrie, Joseph Emerson It is possible to represent $d$-dimensional quantum states as probability distributions over a phase space of $d^{2}$ points. However, to encompass the full quantum formalism we must allow negative representations. The well known magic state model of quantum computation gives a recipe for universal quantum computation using perfect Clifford operations and repeat preparations of a noisy ancilla state. It is an open problem to determine which ancilla states enable universal quantum computation in this model. In this talk we will show that for systems of odd dimension a necessary condition for a state to enable universal quantum computation is that it have negative representation in a particular quasi-probability representation. This representation is a natural discrete analogue to the Wigner function. This condition implies the existence of a large class of bound states for magic state distillation: states which cannot be prepared using Clifford operations but which are not useful for quantum computation. This settles in the negative the conjecture that all states not representable as a convex combination of stabilizer states enable universal quantum computation. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z30.00006: Anderson localization modeled by means of numerical solutions of the Schr\"{o}dinger equation Nagendra Dhakal, Sergio Tafur, Michael Leunberger We developed codes for simulating the Schr\"{o}dinger equation based on the finite-difference time-domain (FDTD) method. We model the 2 dimensional free electron gas system using perfectly matched layers for the open surrounding space. We study the effect of localized impurities on the time evolution of the electron wave function, thereby observing dephasing introduced by the impurities. Our numerical simulations show the decoherence due to the impurities at moderate impurity densities and Anderson localization at high impurity densities. Our results are important for the implementation of quantum computing, quantum communication, and spintronics. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z30.00007: Quench dynamics in the Anderson impurity model Deepak Iyer, Natan Andrei We study the non-equilibrium behavior of an interacting quantum dot following a quench, where it is suddenly attached to a lead. The system is modeled by a single level Anderson impurity model with infinite on-site repulsion attached via tunneling to non-interacting leads. We use the open system Bethe Ansatz solution of the Anderson model and develop a formal framework to implement Yudson's contour integral formalism in the presence of a Fermi sea. This framework allows the calculation of the full time evolution of the multi-particle wave function and various observables of the system. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z30.00008: Consistent quantum prediction and decoherence in quantum cosmology David Craig A complete ``consistent histories'' framework for certain symmetry-reduced models of quantum gravity is given, within which probabilities may be consistently extracted from quantum amplitudes. The decoherence functional for both a standard ``Wheeler-DeWitt'' quantization and a loop quantization of a flat Friedmann-Robertson-Walker cosmological model is constructed, from which consistent quantum predictions may be made in mathematically precise models of quantum cosmologies. Consistent (decoherent) families of histories are exhibited, with an emphasis on the crucial role played by the decoherence of histories in arriving at self-consistent quantum predictions for these closed quantum systems. By way of example, the problem of resolution of the classical ``big bang'' singularity is compared and contrasted in these two models. Special attention is given to consistent quantum predictions in these theories which are \emph{certain i.e.\ }predictions for which the problem of interpretation of probabilities for a closed quantum system is not present. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z30.00009: Wavefunction Collapse via a Nonlocal Relativistic Variational Principle Alan Harrison We propose a relativistically covariant variational principle (VP) capable of describing wavefunction collapse. This produces a nonlinear, nonlocal, time-reversal-invariant theory; the hidden variable is the phase of the wavefunction. The VP is $\delta (A_1 + A_2) = 0$, in which $A_1$ and $A_2$ are positive definite integrals over all spacetime of functions of $\psi(t,\vec x)$. $A_1$ is quadratic in deviations of the wavefunction from compliance with the standard quantum mechanical (SQM) wave equation. $A_2$ takes a minimum value when the wavefunction is a state of minimal uncertainty, penalizing certain kinds of superpositions and thus driving collapse. A multiplier sets the relative size of the terms so that (1) $A_1$ dominates in isolated microscopic systems, so they evolve according to the SQM wave equation; and (2) macroscopic superpositions cause $A_2$ to dominate, driving the system to collapse. Since any macroscopic measurement apparatus is entangled with the system being measured, process (2) explains the empirical observation that measurement collapses the wavefunction. We show that $A_2$ enforces the Born rule, under suitable assumptions and approximations. As an example, the theory predicts the results of the two-slit experiment, including the delayed-choice variant. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z30.00010: New proofs of the Kochen-Specker theorem for a system of three qubits Mordecai Waegell, P.K. Aravind In 1995 Kernaghan and Peres gave a transparent state-independent ``parity proof'' of the Kochen-Specker theorem by using a system of three qubits. They did this by using the observables of the 3-qubit system to construct a set of 40 rays in a real 8-dimensional space that formed 25 bases, and then picked out a subset of the bases that gave a parity proof. They showed that there are 320 different (but unitarily equivalent) versions of their proof in this 40-ray set. We extend their result in a number of ways. Firstly, we show that this 40-ray set contains five new types of parity proofs in addition to the one found by Kernaghan and Peres, and that the total number of versions of all six types of proofs under the symmetries of the system is 2$^{11}$ = 2048. Secondly, we point out the existence of a large number of state-independent KS proofs in the 3-qubit system that are structurally different from the Kernaghan-Peres proof, and we explore their features. The geometry of mutually unbiased bases (MUBs) in the 3-qubit system, which played a crucial role in the discovery of these new proofs, will be discussed. [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z30.00011: New Algorithms for Generating Arbitrary Kochen-Specker Sets Norman D. Megill, Mladen Pavicic The Kochen-Specker KS) sets (constructive proofs of quantum contextuality) have recently obtained a special significance as building blocks of quantum information protocols since quantum contextuality was revealed as property complementary to nonlocality and entanglement. [A. Cabello, {\it Phys.\ Rev.\ Lett.\/} {\bf 104}, 220401 (2010).] Thus, generating arbitrary KS sets becomes as needed as generating Bell states and this has been enabled by recent findings of a vast amount ($>10^{20}$) of new KS sets---we call them a ``KS sea.'' [N.D.\ Megill, K.\ Fresl, M.\ Waegell, P.K.\ Aravind, and M. Pavi{\v c}i{\'c}, {\it Phys.\ Lett.\ A}, {\bf 375} 3419 (2011); M.\ Waegell and P.K.\ Aravind, {it J. Phys.\ A} [to appear] (2011).] Here we present our newest algorithms and computer programs which enable us to obtain any desired KS set from the KS sea in a very short time without ever making a complete data base of KS sets---which would be an impossible task anyhow. This was made possible with the help of our representation of the KS sea as well as individual KS sets by means of MMP hypergraphs. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z30.00012: How Different Can Quantum States with the Given Fidelity Be? Viktor Dodonov, Matheus Horovits We address the following question: how big can the relative energy difference between two states of a harmonic oscillator (field mode) with the fixed value of fidelity $F$ be? Exact analytical bounds are found for several popular families of quantum states: coherent, squeezed, arbitrary (mixed) Gaussian, binomial and negative binomial. Numerical bounds are calculated for various superpositions of coherent states (``Schr\"odinger cat states'') and their generalizations. The restrictions on the minimal admissible fidelity levels for quite arbitrary (unknown) states belonging to selected families appear rather strong. For example, one can find two squeezed states with $F=0.9$ but with the relative mean energy difference exceeding 100\%. To guarantee the relative energy difference less than 10\% for arbitrary coherent states, the fidelity must exceed the level $0.995$. For many other sets of states (e.g., squeezed and negative binomial) the restrictions can be much stronger. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z30.00013: Physical meaning of the de Broglie waves Jingzheng Qin, Simon Berkovich Commonly regarded as an abstract entity, de Broglie waves represent a real thing making the wave-particle duality an operative factor with a wealth of consequences. Waves as probabilities in Schr\"{o}dinger's quantum mechanics are due to interactive holography that builds up on top of the suggested cellular automaton model of the physical Universe [1]. This model implementing a rule of mutual synchronization produces a set of helicoidal solitons that corresponds exactly to the whole spectrum of elementary particles of matter, no more and no less. The particle-like behavior of the solitons exhibits periodic synchro activities that constitute a wave. The wavelength is inversely proportional to the momentum of these elementary particles, and they distinctly show characteristic qualities of the micro world. The condition of translation-rotation congruity foresees a very small decrease of photons speed with their frequency and polarization, while smoothness of this condition for neutrinos accounts for their already observed faster-than-light propagation. Helicoidal solitons create identical impacts from rotation resulting in the same value of spin for different particles irrespective of variations in mass and speed that are related to translational motion. For any given orientation, spin is either ``up'' or ``down'' since helicoidal solitons are flipping this way with every half-turn. Deep inelastic scattering performs sampling of the wave structure of the helicoidal solitons deceptively portraying the outcomes as isolated quarks. [1] S. Berkovich, ``A comprehensive explanation of quantum mechanics,'' http://www.cs.gwu.edu/research/technical-report/170 [Preview Abstract] |
Friday, March 2, 2012 1:51PM - 2:03PM |
Z30.00014: No Drama Quantum Theory? Andrey Akhmeteli Is it possible to offer a ``no drama'' quantum theory? Something as simple (in principle) as classical electrodynamics - a theory described by a system of partial differential equations (PDE) in 3+1 dimensions, but reproducing unitary evolution of a quantum field theory in the configuration space? The following results suggest an affirmative answer: 1. The scalar field can be algebraically eliminated from scalar electrodynamics; the resulting equations describe independent evolution of the electromagnetic field (EMF). 2. After introduction of a complex 4-potential (producing the same EMF as the standard real 4-potential), the spinor field can be algebraically eliminated from spinor electrodynamics; the resulting equations describe independent evolution of EMF. 3. The resulting theories for EMF can be embedded into quantum field theories. Another fundamental result: in a general case, the Dirac equation is equivalent to a 4th order PDE for just one component, which can be made real by a gauge transform. Issues related to the Bell theorem are discussed. A. Akhmeteli, Int'l Journal of Quantum Information, Vol. 9, Suppl., 17-26 (2011) A. Akhmeteli, Journal of Mathematical Physics, Vol. 52, 082303 (2011) A. Akhmeteli, quant-ph/1108.1588 [Preview Abstract] |
Friday, March 2, 2012 2:03PM - 2:15PM |
Z30.00015: Type 2 local realism is alive and well in the quantum world Jeffrey Boyd It is often, but incorrectly said, that there is no local realism in the quantum world. This is wrong because there is a second type of local realism. Type two local realism is drastically different than how most people think of their world, including Einstein. It starts with the idea that we are immersed in a sea of invisible elementary waves, traveling in all directions, at all wavelengths. Whenever a photon or other particle moves, it follows one of these waves in the reverse direction. Although this may sound preposterous, this theory can explain several quantum experiments: Jacques (2007) Wheeler thought experiment with delayed choice, Kim (1999), Quantum eraser experiment with delayed choice, all the Bell test experiments with delayed choice, and the double slit experiment. Using the Theory of Elementary Waves (TEW), all these experiments can, surprisingly, be explained using this unconventional type of local realism, with time going forward. [Preview Abstract] |
Session Z31: Topological Insulators: Junctions and Interfaces
Sponsoring Units: DCMPChair: Shoucheng Zhang, Stanford University
Room: 260
Friday, March 2, 2012 11:15AM - 11:27AM |
Z31.00001: Topological p-n Junction Jing Wang, Shou-Cheng Zhang (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ is an ideal topological insulator with truly insulating bulk and tunable surface state across the Dirac point. We consider a junction between surface p-type and surface n-type on these ideal topological insulators in which carrier type and density in two adjacent regions are locally controlled by electrostatic gating or planar grade doping. Such junction setting on topological insulators are fundamental to device development. We find a gapless chiral edge state localized at the p-n interface arises when applying a magnetic field, which can be detected by scanning tunneling microscopy. The two-terminal conductance of such p-n junction in the quantum Hall regime will be 1/4 times the quantum of conductance e$^2$/h, which signifies the half-quantum Hall effect of a topological insulator surface. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z31.00002: Andreev Reflection in two-dimensional Topological Insulators Awadhesh Narayan, Stefano Sanvito A metal-superconductor interface may reflect an incident electron from the metal as a positively charged hole with opposite spin, while a cooper pair is formed in the superconductor. This electron-hole conversion is Andreev reflection (AR) and has served as a useful probe for spin-polarized currents. In this work we study AR at topological insulator-superconductor interface, for both time-reversal symmetric ($Z_2$) and time-reversal broken ({\it Chern}) cases. We model $Z_2$ insulators using the proposal of Kane and Mele, while for {\it Chern} insulator we use a spinful version of the Haldane model. By employing Landauer-B\"{u}ttiker scheme we find for both cases perfect AR, which is highly robust to disorder and persists as long as the edge states are present. Further, we propose an experiment to distinguish between the two types of topological insulators. The proposal involves a local doping with magnetic impurities at one of the edges of the two-dimesional material. This suppresses one of the channels for reflection and the AR coefficient drops by a factor of two. No such suppression is seen for the {\it Chern} insulator. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z31.00003: Theory of Odd-Parity Superconductivity: from Gap Function to Topological Invariants and Surface Andreev Bound States Yang Qi, Liang Fu Three-dimensional superconductors with a nodeless pairing gap can be classified by an integer topological invariant, and gapless Andreev bound states exist on the surface when the topological invariant is nontrivial. In this letter we give a general criterion for determining the topological invariant for centrosymmetric superconductors with \emph{odd-parity} pairing. We show that in a general system with spin-orbit coupling, the superconducting gap function can be expressed by a pseudospin d-vector, and the topological invariant can be determined from the total winding number of the pseudospin d-vector on the Fermi surfaces. We also discuss Andreev surface states, and we found gapless surfaces states when the topological invariant in the bulk is topologically nontrivial. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z31.00004: Current-phase relation for Josephson effect through helical metal Erhai Zhao, Chris Olund We compute the current-phase relation of Josephson junctions fabricated on the surface of three-dimensional topological insulators. The Josephson coupling between two superconductors is mediated by the two-dimensional helical metal. It gives rise to the so-called fractional Josephson effect. A short junction is previously known to be a quantum wire of Majorana fermions. We discuss the dependence of the current-phase relation on the length of the junction, the chemical potential of the helical metal, and temperature. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z31.00005: Interface engineering of quantum Hall effects in digital transition-metal oxide heterostructures Satoshi Okamoto, Di Xiao, Wenguang Zhu, Ying Ran, Naoto Nagaosa Based on tight-binding modeling and first-principles calculations, we investigate possible quantum Hall effects in transition-metal oxide heterostructures. Bilayers of perovskite-type transition-metal oxides grown along the [111] crystallographic axis are found to be potential candidates for two-dimensional topological insulators. The topological band structure of these materials can be tune-tuned by changing dopant ions, substrates, and external gate voltages. We predict that LaAuO$_3$ bilayers have a topologically-nontrivial energy gap of about 0.15 eV, which is sufficiently large to realize the quantum spin-Hall effect at room temperature. We also discuss intriguing phenomena associated with the nearly flat topologically-nontrivial bands found in eg systems, such as fractional quantum Hall effect. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z31.00006: Proximity effects at semiconductor/topological insulator interfaces Gufeng Zhang, Jie Wang, Xiaoguang Li, Dimitrie Culcer, Zhenyu Zhang Using phenomenological model Hamitonians, we study the spatial distribution of topological surface states (TSS) in semiconductor/topological insulator (TI) heterostructures. Due to proximity effects induced by the TI substrate, the location of the TSS can be shifted perpendicularly to the interface. We show that both the direction and magnitude of the shift can be tuned by the cooperative effects of the spin-orbit coupling within the hybrid system, the bandgap of the overlayer, and the thickness of the overlayer. Potential technological applications of these salient properties of the TSS will also be discussed. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z31.00007: Abrikosov Vortex Lattice in 3D Topological Insulator -- Superconductor Heterostructures Hsiang-Hsuan Hung, Taylor Hughes, Matthew Gilbert Majorana fermions have been predicted to exist on the surface of the three-dimensional (3D) topological insulator/s-wave superconductor heterostructures by proximity effects [Phys. Rev. Lett. {\bf 100}, 096407 (2008)]. In the diffuse vortex limit, the physics of these non-abelian anyons is theoretically well-understood c.f. Phys. Rev. B {\bf 84}, 144507 (2011). However, the dilute vortex limit is unlikely to be available in experimental systems. In this work, we study the dense vortex limit in 3D topological insulator/s-wave superconductor heterostructures using the self-consistent Bogoliubov-de Gennes (BdG) equations under the application of a uniform magnetic flux. We find that as we approach the dense limit of vortices on the surface, that the hybridization between the vortices leads to the formation of a ``Majorana bandstructure'' which exists within the superconducting gap.We describe the physics of the system as we move from the dilute limit to the the dense limit as we vary the surface chemical potentials and the magnetic field magnitudes. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z31.00008: Quantum Hall Super uids in Topological Insulator Thin Films Dagim Tilahun, Byounghak Lee, Ewelina Hankiewicz, Allan MacDonald Three-dimensional topological insulators have protected Dirac-cone surface states. In this work we argue that gapped excitonic superfluids with spontaneous coherence between top and bottom surfaces can occur in the TI-thin-film quantum-Hall regime. We find that the large dielectric constants of TI materials increase the layer separation range over which coherence survives and decrease the superfluid sound velocity, but have little influence on the superfluid density or on the charge gap. The coherent state at total Landau-level filling factor $\nu_T = 0$ is predicted to be free of edge modes, qualitatively altering its transport phenomenology compared to the widely studied case of $\nu_T = 1$ in GaAs double quantum wells. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z31.00009: Junction between surfaces of two topological insulators Diptiman Sen, Oindrila Deb We study scattering from a line junction which separates the surfaces of two three-dimensional topological insulators; some aspects of this problem were recently studied in Takahashi and Murakami, Phys. Rev. Lett. 107, 166805 (2011). The velocities of the Dirac electrons on the two surfaces may be unequal and may even have opposite signs; in the latter case, we find that the electrons must, in general, go into the two-dimensional interface separating the two topological insulators. We also study what happens if the two surfaces are at an angle $\phi$ with respect to each other. We find in this case that there are bound states which propagate along the line junction with a velocity and direction of spin which depend on the bending angle $\phi$. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z31.00010: Dirac cones in the gapless interface states between two topological insulators Ryuji Takahashi, Shuichi Murakami When two topological insulators are attached together, the states on the interface become gapped due to the hybridization between the surface states. We have shown that if the two topological insulators have the opposite signs for the Dirac velocities, there exist gapless interface states [1]. In the last March meeting we showed a general proof for the existence of the gapless states using the mirror Chern number, which fixes the chirality of the surface states. In this presentation, we report the dispersions of these gapless interface states. They are in general a collection of Dirac cones. For example, if the system has threefold rotational symmetry, the interface states have six Dirac cones. By using the Fu-Kane-Mele model, which is the tight-binding model on the diamond lattice with the spin-orbit interaction, we calculate the dispersion of this gapless interface states, and discuss the relationship with the mirror Chern number.\\[4pt] [1] R. Takahashi, S. Murakami, Phys. Rev. Lett. 107,166805 (2011). [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z31.00011: Conductance through step junctions in 3D topological insulators Mireia Alos-Palop, Rakesh P. Tiwari, Miriam Blaauboer An effective continuous model for low-energy surface states of a 3D topological insulator was presented by Zhang {\it et al.}, {\it Nat. Phys. }{\bf 5}, 438 (2009). We present a general solution for this 3D model in a surface different from the standard (111)-surface. In our solution, surface states consist of a single Dirac cone with a Fermi velocity different from the one in (111)-surfaces, and the energy has an elliptical dispersion in $k$-space. We then study transport through a step junction composed of a (111)-surface -- side-surface -- (111)-surface and predict that the conductance saturates at 2/3 G$_0$, independent of eccentricity and velocity mismatch at the interfaces. We compare our model with a junction in a plane with only (111)-states where conductance saturation does depend on velocity mismatch. We also analyze the Fano factor and highlight experimentally relevant situations where our predictions could be tested. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z31.00012: Inter-band Tunneling between Doped Topological Insulator Surface States Gen Yin, Darshana Wickramaratne, Roger Lake Thin films of 3D topological insulators (TIs) have been experimentally synthesized recently. Impurity doping of the TI surface has been reported to modify the position of the Fermi level and generate Rashba-like splitting of the surface band structure. Our research uses a Non-Equilibrium Green's Function (NEGF) method to simulate inter-surface transport properties for TI thin films. A tight-binding model is established by discretizing a 4 x 4 k.p Hamiltonian for 3D TIs. Because of confinement, thin TI slabs of several nanometers allows inter-surface tunneling at quantum number matching states. The tunneling intensity can be tuned by surface Coulomb impurity doping or applying an external bias. Unlike regular topologically trivial surface states, inter-band tunneling between TI surfaces presents a conduction minimum when the dispersions of the two surfaces align perfectly over each other. The suppression of transport originates from the momentum coupling with time reversal symmetry, leading to significant non-linear I-V properties for the P-N tunneling at forward bias. This leads to a NDR current minimum when an external bias completely compensates the built-in potential. The study on inter-surface tunneling in TI thin films benefits the understanding of the transport behavior of TI surface states, which calls for further experimental investigations in the future. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z31.00013: Spin Texture on the Fermi Surface of Strained HgTe Saad Zaheer, Steve Young, Daniel Cellucci, Jeffrey Teo, Charles Kane, Eugene Mele, Andrew Rappe We present \emph{ab initio} and ${\bf k\cdot p}$ calculations of the Fermi surface of strained HgTe obtained by stretching the Zinc-Blende lattice along the (111) axis. Near the Fermi level, strained HgTe exhibits point-like accidental degeneracies between a two-fold degenerate and two non-degenerate bands along the (111) axis. The three bands disperse linearly in all directions about the degenerate points and their low energy physics is described by an effective four band ${\bf k\cdot p}$ Hamiltonian. The Fermi surface consists of two ellipsoids which contact only at the point where the Fermi level crosses the two-fold degenerate band along the (111) axis. The spin expectation value on both ellipsoids is constrained to vanish along the (111) axis due to mirror symmetry about a plane that contains that axis. Furthermore the winding number of spins around the two ellipsoids changes from one end to the other indicating the existence of singular points in the spin texture. Indeed, the \emph{ab initio} and ${\bf k\cdot p}$ calculations confirm the existence of such spin singularities on the Fermi ellipsoids. We show that doping HgTe with Zinc atoms chemically strains the HgTe Zinc-Blende lattice and present \emph{ab initio} calculations on HgZnTe that confirm the above results. [Preview Abstract] |
Friday, March 2, 2012 1:51PM - 2:03PM |
Z31.00014: Spin-transfer torque and spin-polarization in topological-insulator-based magnetic tunnel junctions Farzad Mahfouzi, Naoto Nagaosa, Branislav Nikoli\' c We derive a nonequilibrium Green function-based formula for spin-transfer torque (STT) exerted by the conduction electrons on the magnetization of a free ferromagnetic (F) layer where {\em strong} spin-orbit coupling (SOC) is present either in the bulk or at the interface of the F layer. This nonequilibrium Born-Oppenheimer approximation-type formula is employed to predict unconventional STT in N$|$TI$|$F semi- magnetic tunnel junction (MTJ) containing a three-dimensional topological insulator (TI). The STT is driven by the SOC on the surface of TI, as well as by the charge current becoming spin-polarized in the direction of transport as it flows from the normal metal (N) through the bulk of the TI layer. The in- plane and perpendicular STT components in N$|$TI$|$F semi-MTJ are an order of magnitude larger than in conventional F$^\prime|$I$|$F MTJ, or N$|$I$|$F semi-MTJ with the strong Rashba SOC at the I$|$F interface, assuming comparable resistance of all three junctions. [Preview Abstract] |
Friday, March 2, 2012 2:03PM - 2:15PM |
Z31.00015: Time reversal symmetric Kitaev model and topological superconductor Ryota Nakai, Shinsei Ryu, Akira Furusaki We study a time reversal symmetric quantum spin model in two dimensions that is introduced as a higher spin extension of the Kitaev model and is exactly solvable [1]. The ground state of the topological phase of this model can be viewed as a time reversal symmetric topological superconductor in two dimensions. The helical Majorana edge modes which appear in time reversal pair in the topological phase are explained by topological argument. The correlation functions along the edge are derived from the gapless edge theory.\\[4pt] [1] R. Nakai, S. Ryu, and A. Furusaki, arXiv:1111:1230. [Preview Abstract] |
Session Z32: Topological Insulators: General Theory II
Sponsoring Units: DCMPChair: Jens Bardarson, University of California, Berkeley
Room: 261
Friday, March 2, 2012 11:15AM - 11:27AM |
Z32.00001: Room temperature quantum spin Hall thin film for buckled honeycomb structures of silicon, germanium, and tin Tay-Rong Chang, Hsin Lin, Horng-Tay Jeng, M.Z. Hasan, Arun Bansil We have carried out first-principles calculations on buckled honeycomb structures of silicon, germanium and tin. When the spin-orbit coupling is included in the computations, these two dimensional (2D) systems are found to be in a quantum spin Hall phase with nontrivial topological invariant Z$_2$=-1, and a band gap that increases with increasng atomic number. In particular, we predict that buckled honeycomb Sn thin film is a room temperature quantum spin Hall insulator with 0.25 eV band gap. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z32.00002: Topological order and semions in a strongly correlated quantum spin Hall insulator Andreas Ruegg, Gregory A. Fiete We provide a self-consistent mean-field framework to study the effect of strong interactions in a quantum spin Hall insulator on the honeycomb lattice. We identify an exotic phase for large spin-orbit coupling and intermediate Hubbard interaction. This phase is gapped and does not break any symmetry. Instead, we find a four-fold topological degeneracy of the ground state on the torus and fractionalized excitations with semionic mutual braiding statistics. Moreover, we argue that it has gapless edge modes protected by time-reversal symmetry but a trivial $Z_2$ topological invariant. Finally, we discuss the experimental signatures of this exotic phase. Our work highlights the important theme that interesting phases arise in the regime of strong spin-orbit coupling and interactions. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z32.00003: Spontaneous breakdown of time reversal symmetry in the doped honeycomb lattice with enlarged unit cell Adolfo G. Grushin, Eduardo V. Castro, Alberto Cortijo, Mar\'{\i}a A. H. Vozmediano, Bel\'en Valenzuela, Fernando de Juan Enlarging the unit cell of simple lattice models is proposed as a simple means to get spontaneous breakdown of time reversal symmetry ($\mathcal{T}$) and to stabilize non trivial topological phases. As a case study we explore the nearest neighbor tight binding model for spinless fermions interacting through short range Coulomb interactions in the honeycomb lattice. Using a variational mean field approach and an enlarged six atom unit cell we obtain a very rich phase diagram as doping and interaction strength are varied. Two broken $\mathcal{T}$ phases, with orbital currents (fluxes) arranged in a Kekul\'e pattern, show up above the Van Hove filling. One of them realizes a topological Fermi liquid with a finite anomalous Hall conductivity. Instabilities towards charge modulated and superconducting phases are also discussed. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z32.00004: Synchronous and Asynchronous Mott Transitions in Topological Insulator Ribbons Amal Medhi, Vijay B. Shenoy, H.R. Krishnamurthy We address how the nature of linearly dispersing edge states of two dimensional (2D) topological insulators evolves with increasing electron-electron correlation engendered by a Hubbard like on-site repulsion $U$. We consider finite ribbons of two systems of topological band insulators with local electronic interactions incorporated. Using an inhomogeneous cluster slave rotor mean-field method developed here, we show that electronic correlations drive the topologically nontrivial phase into a Mott insulating phase via two different routes. In a synchronous transition, the entire ribbon attains a Mott insulating state at one critical $U$. In the second, asynchronous route, Mott localization first occurs on the edge layers at a smaller critical value of electronic interaction which then propagates into the bulk as $U$ is futher increased until all layers of the ribbon become Mott localized. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z32.00005: Interaction-driven topological and nematic phases on the Lieb lattice Wei-Feng Tsai, Chen Fang, Hong Yao, JiangPing Hu We investigate the interaction-driven instabilities of the band crossing point (BCP), which occurs in the band structure resulting from spinless/spinful fermions moving on the (extended) Lieb lattice. In the non-interacting limit, we show the topological stability of the BCP both from momentum and real space arguments, provided time reversal and $C_4$ point group symmetries are preserved. With short-range repulsive interactions, we find that at zero temperature this BCP is marginally unstable against infinitesimal repulsions and results in topological quantum anomalous/spin Hall, charge nematic, and nematic-spin-nematic phases, separately, depending on the interaction strengths. Possible physical realizations and the existence of a topological nearly flat band are also discussed. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z32.00006: Geometric effects on surface states in topological insulator Bi$_{2}$Te$_{3 }$ nanowire Parijat Sengupta, Tillman Kubis, Michael Povolotskyi, Gerhard Klimeck Bismuth Telluride (BT) is a 3D topological insulator (TI) with surface states that have energy dispersion linear in momentum and forms a Dirac cone at low energy. In this work we investigate the surface properties of a BT nanowire and demonstrate the existence of TI states. We also show how such states vanish under certain geometric conditions. An atomistic model (sp3d5s* TB) is used to compute the energy dispersion in a BT nanowire. Penetration depth of the surface states is estimated by ratio of Fermi velocity and band-gap. BT possesses a tiny band-gap, which creates small localization of surface states and greater penetration in to the bulk. To offset this large spatial penetration, which is undesirable to avoid a direct coupling between surfaces, we expect that bigger cross-sections of BT nanowires would be needed to obtain stable TI states. Our numerical work validates this prediction. Furthermore, geometry of the nanowire is shown to influence the TI states. Using a combined analytical and numerical approach our results reveal that surface roughness impact electronic structure leading to Rashba type splits along z-direction. Cylindrical and square cross-sections are given as illustrative examples. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z32.00007: Spinless massless and massive Dirac fermions in a checkerboard lattice magnet Jorn Venderbos, Maria Daghofer, Jeroen van den Brink, Sanjeev Kumar We investigated the theory of the interplay of itinerant electrons and localized magnetic moments on the frustrated checkerboard lattice as function of the super-exchange interaction between the localized moments and the band filling of fermions. We find that at half filling a very robust magnetic ``flux'' phase is lowest in energy. The ordering of the localized spins induces an effective gauge field flux of $\pi$ for the electrons. Consequently, this phase preserves time-reversal symmetry and the low-energy effective theory of the electrons is that of massless Dirac fermions, resembling the situation in graphene except that the spin degree of freedom is absent here. The robustness of this state originates from the geometrical frustration of the checkerboard lattice. In the crossover regime from this flux state and the satured FM state at vanishing super-exchange coupling, these Dirac fermions become massive with opposite sign of the mass at the two degeneracy points. This chiral spin state is then equivalent to a time-reversal breaking anomalous Quantum Hall phase, precisely in the way once envisioned by Haldane in graphene. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z32.00008: Kondo lattice on the edge of a 2D topological insulator Joseph Maciejko Much attention has been devoted recently to the experimental and theoretical study of the effect of magnetic impurities on the stability of the gapless boundary modes of topological insulators. When the quantum dynamics of the impurities is considered, those boundary modes constitute novel types of fermionic baths which may affect the nature of possible impurity phases and phase transitions. We study a regular one-dimensional array of quantum magnetic impurities interacting with the helical edge liquid of a two-dimensional time-reversal invariant topological insulator. Exact solutions at the special Toulouse and Luther-Emery points as well as a renormalization group analysis \`{a} la Anderson-Yuval allow us to construct a phase diagram in the space of Kondo coupling, electron-electron interaction strength, and electron density. We point out similarities and differences with the Kondo lattice in a ordinary one-dimensional electron gas. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z32.00009: Non-Abelian Berry transport, spin coherent states, and Majorana points Yun Liu, Abhishek Roy, Michael Stone We consider the adiabatic evolution of Kramers degenerate pairs of spin states in a half-integer spin molecular magnet as the molecule is slowly rotated, for which it is possible to encounter non-Abelian Berry's phase. To understand the full details of the adiabatic quantum evolution, we invoke Majorana's parametrization of a general spin-$j$ state in terms of the $2j$ Majorana points. As an illustration we consider molecular magnets of the $j=9/2$ Mn4 family and compute the frequency with which the magnetization varies. This frequency is generally different from the frequency of the rotation. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z32.00010: Exploring Diabolical points and Berry Phases in the Majorana Representation Abhishek Roy Diabolical points or level crossings are often observed in spin hamiltonians, where they give rise to a Berry phase. We study their geometric structure using the Majorana representation which associates a spin state to a set of points on the Bloch sphere. Each crossing carries a Chern number which is found to be directly related to the wrapping of Majorana points around it. We apply our method to study model hamiltonians for molecular magnets in an external magnetic field, in which a rich pattern of diabolical points are seen. Our result enables a simple visualization of the Berry phases without the use of perturbation theory. [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z32.00011: Quasiparticle effects in the bulk and surface-state bands of Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ topological insulators Oleg V. Yazyev, Emmanouil Kioupakis, Joel E. Moore, Steven G. Louie We investigate the bulk band structures and the surface states of Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ topological insulators using first-principles many-body perturbation theory based on the \textit{GW} approximation. The quasiparticle self-energy corrections introduce significant changes to the bulk band structures, surprisingly leading to a decrease in the direct band gaps in the band-inversion regime as opposed to the usual situation without band inversion. Parametrized ``scissors operators'' derived from the bulk studies are then used to investigate the electronic structure of slab models which exhibit topologically protected surface states. The introduction of self-energy corrections results in significant shifts of the surface-state Dirac point energies relative to the bulk bands and in enlarged gap openings from the interactions between the surface states across the thin slab, both in agreement with experimental data. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z32.00012: Search for new topological insulators: ternary Li$_2$AgSb-class semiconductors and related compounds Hsin Lin, Tanmoy Das, Y.J. Wang, L.A. Wray, S.-Y. Xu, M.Z. Hasan, Arun Bansil Topological insulators host a rare quantum phase of electrons which is characterized by a topological invariant number of bulk states of combined spin-orbit and time-reversal symmetry origin. Despite recent progress the available classes of topological insulators are still quite limited for use in device applications and experimental exploration of exotic topological phenomena. For this reason, the search for new materials with greater structural flexibility and tunability in various local order broken symmetry phases is continuing worldwide with great intensity. Here we discuss our effort based on first-principles calculations to show that the adiabatic continuation method can provide a very powerful tool for predicting non-trivial topological phases with the example of ternary intermetallic series, Li$_2M'X$ ($M'$=Cu, Ag, Au, and Cd, $X$=Sb, Bi, and Sn) as well as other compounds with zinc-blende type sublattice. [1-3] Work supported by the Office of Basic Energy Sciences, US DOE.\\ \mbox{[1]} H. Lin, {\it et al.} Nature Materials \textbf{9}, 546 (2010). \\ \mbox{[2]} Y. J. Wang, {\it et al.} New J. Phys. {\bf 13}, 085017 (2011). \\ \mbox{[3]} H. Lin, {\it et al.}, arXiv:1007.5111. [Preview Abstract] |
Session Z40: Biomechanics - Organismic Motion
Sponsoring Units: DBIOChair: James Valles, Brown University
Room: 156A
Friday, March 2, 2012 11:15AM - 11:27AM |
Z40.00001: The mechanics of anisotropic cytoskeletal networks Tao Zhang, Moumita Das, D.A. Quint, J.M. Schwarz At the leading edge of a crawling cell, the actin cytoskeleton extends itself via a branched, crosslinked network of filaments, otherwise known as the lamellipodium. The filaments in this network have an average preferred orientation of around $\pm$ 30 degrees with respect to the normal of the leading edge. This preferred orientation of filaments leads to a material that is structurally anisotropic. To better understand the forces generated by the lamellipodium, we analytically and numerically study the mechanical properties of a model branched and crosslinked filamentous network where the filaments are preferentially oriented along one direction. We investigate the interplay between geometry, elasticity and anisotropy in the network. In particular, we show how anisotropy modulates the onset of rigidity and non-linear mechanical response of the network. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z40.00002: Self-Organized Cell Motility from Motor-Filament Interactions XinXin Du, Konstantin Doubrovinski, Miriam Osterfield Cell motility is driven primarily by the dynamics of the cell cytoskeleton, a system of filamentous proteins and molecular motors. It has been proposed that cell motility is a self-organized process; that is, local short-range interactions determine much of the necessary dynamics required for the whole-cell organization that leads to polarization and directional motion. Here we present a mesoscopic meanfield description of filaments, motors, and cell boundaries; this description gives rise to a dynamical system exhibiting multiple self-organized states. We discuss several qualitative aspects of the asymptotic states and compare them to those of living cells. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z40.00003: Modeling of a crawling C. elegans in a micro-structured environment Amar Patel, Venkat Padmanabhan, Deepak Solomon, Zeina Khan, Frank Van Bussel, Siva Vanapalli, Kendra Rumbaugh, Jerzy Blawzdziewicz A simple curvature-based model is used to study crawling C. elegans in a micro-structured environment of periodic pillars. In our model system, the shape of the worm is described by a simple sinusoidal expression in curvature representation. The gait of the worm is determined by a set of parameters including the amplitude, frequency, and phase of the curvature. The moving worm is subject to the friction with the underlying substrate, and the forces due to interaction of the worm body with pillars. If the friction is isotropic the worm has to interact with the pillars to move forward. We find that only a narrow range of worm gaits leads to efficient propulsion in this case. To investigate how the worm adjusts its gait to the environment microstructure, we implement a simple control system that chooses the right set of parameters based on the past interactions of the worm with its surroundings. Results of our simulations of the worm motion are compared with our experimental observations of C. elegans crawling in an agar environment containing an array of fabricated pillars. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z40.00004: Fluctuations, Dynamics, and the Stretch-Coil Transition of Single Actin Filaments in Extensional Flows Vasily Kantsler, Raymond E. Goldstein Semi-flexible polymers (actin filaments) subject to hydrodynamic forcing play an important role in cytoskeletal dynamics in the cell. The non-equilibrium problem of semi-flexible polymer dynamics is highly challenging due to the coupling between the objects deformations and the flow. This leads to a free-boundary hydrodynamic problem, where the object's shape is not given {\it a priori}, but determined by an interplay between the fluid stresses, bending energy and the length constrain of the actin filaments. We have investigated experimentally and analytically dynamics of actin filaments in elongational flow. Near hyperbolic stagnation points of the flow filaments experience a competition between bending elasticity and tension induced by the flow, and are predicted to display suppressed thermal fluctuations in the steady regime and a buckling instability under sudden change of the velocity gradient. Using a microfluidic cross-flow geometry we verify these predictions in detail, including a fluctuation-rounded stretch-coil transition of actin filaments. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z40.00005: Cell Shape Dynamics: From Waves to Migration Meghan Driscoll, Colin McCann, Xiaoyu Sun, John Fourkas, Carole Parent, Wolfgang Losert We observe and quantify wave-like characteristics of amoeboid migration. Using the amoeba Dictyostelium discoideum, a model system for the study of chemotaxis, we demonstrate that cell shape changes in a wave-like manner. Cells have regions of high boundary curvature that propagate from the leading edge toward the back, usually along alternating sides of the cell. Curvature waves are easily seen in cells that do not adhere to a surface, such as cells that are electrostatically repelled from surfaces or cells that extend over the edge of micro-fabricated cliffs. Without surface contact, curvature waves travel from the leading edge to the back of a cell at $\sim $35 $\mu $m/min. Non-adherent myosin II null cells do not exhibit these curvature waves. At the leading edge of adherent cells, curvature waves are associated with protrusive activity. Like regions of high curvature, protrusive activity travels along the boundary in a wave-like manner. Upon contact with a surface, the waves stop moving relative to the surface, and the boundary shape thus reflects the history of protrusive motion. The wave-like character of protrusions provides a plausible mechanism for the ability of cells to both swim in viscous fluids and to navigate complex 3-D topography. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z40.00006: A curvature-based description for the kinematics of \textit{C. Elegans} Venkat Padmanabhan, Zeina Khan, Deepak Solomon, Siva Vanapalli, Kendra Rumbaugh, Jerzy Blawzdziewicz Caenorhabditis Elegans is a free-living soil nematode that propels itself in various complex environments by producing undulatory body motion. Such nematodes display a rich variety of body shapes and trajectories during their locomotion. Here we show that the complex shapes and trajectories of \textit{C. Elegans }have a simple analytical description in curvature representation. Our model is based on the assumption that the curvature wave is generated in the head segment of the worm body and propagates backwards. We have found that a simple sinusoidal function for the curvature can capture multiple worm shapes during the undulatory movement. The worm body trajectories can be well represented by piece-wise sinusoidal curvature with abrupt changes in amplitude, frequency, and phase. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z40.00007: Paramecia Swim with a constant propulsion in Solutions of Varying Viscosity James M. Valles, Jr., Ilyong Jung, Harry Mickalide, Hojin Park, Thomas Powers Paramecia swim through the coordinated beating of the 1000's of cilia covering their body. We have measured the swimming speed of populations of Paramecium Caudatam in solutions of different viscosity, $\eta$, to see how their propulsion changes with increased drag. We have found the average instantaneous speed, V to decrease monotonically with increasing $\eta$. The product $\eta v$ is roughly constant over a factor of 7 change in viscosity suggesting that paramecia swim at constant propulsion force. The distribution of swimming speeds is Gaussian. The width appears proportional to the average speed implying that both fast and slow swimmers exert a constant propulsion. We discuss the possibility that this behavior implies that the body cilia beat at constant force with varying viscosity. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z40.00008: Response of Swimming Paramecia to {\it in situ} changes in their apparent weight Ilyong Jung, Harry Mickalide, James M. Valles, Jr. There is a class of marine micro-organisms that are small enough that low Reynold's number hydrodynamics dictates their swimming mechanics and large enough that the force of gravity exerts a noticeable influence on their motion. Experiments on populations of paramecia suggest that they exert a greater propulsion when swimming against gravity. This negative gravi-kinesis is surprising because it suggests that they sense their tiny apparent weight of about 80 pN. To understand this response in more detail, we are investigating how individual paramecia caudatum change their swimming speed and helical trajectories in response to changes in their apparent weight. We vary the apparent weight with the technique of Magnetic Force Buoyancy Variation employing a high field resistive magnet at the National High Magnetic Field Laboratory. We will present analysis of the swimming for apparent weight changes as large as a factor of 8. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z40.00009: Capillary Action may act as a cooling method in Plants and Animals Richard Kriske A capillary tube in a plant may lead from its roots to the leaves. It takes no work for the column of water to rise from the roots to the leaves, and if there is capillarity in the soil, it takes no work for the water to flow through the ground to the roots. It does take work for a molecule of water to evaporate from the tube into the atmosphere. When a molecule of water evaporates another molecule travels through the soil and up the plant to replace it. The lost molecule creates a ``hole'' in the water column which like a signal is sent to the root and the sea of water in the soil replaces it. Since the water molecules are not unique this is the same situation as if the water vapor where condensed back to a liquid in a refrigeration cycle. Another interesting aspect of this sort of refrigeration is that the ``hole'' itself may be used to do work along the wall of the capillary tube, which may have Fermi Levels in it. An Hydraulic Semi Conductor, and in it is a method of cooling the Semi Conductor. This may be applicable to other similar systems using other liquids, or substances such as nanotube systems, where the hole signals,cools and performs chemical reactions involving not only obitals but Fermi Levels, a transition between Quantum and Classical Mechanics, with surprises. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z40.00010: Data-Driven Classification of Animal Behavior Gordon Berman, William Bialek, Joshua Shaevitz The last decades have seen an explosion in our ability to characterize the molecular, cellular and genetic building blocks of life; the ingredients out of which we try to explain the rich and compelling behavior of living organisms. Our characterization of behavior itself, however, has advanced more slowly. Since modern ethology was founded over a century ago, behavioral experiments have focused largely on a restricted set of behaviors within the scope of a limited environment. Moreover, the set of behaviors to be examined is often user-defined, creating the potential for human bias and anthropomorphism. The research presented here describes a data-driven methodology for analyzing animal behavior, focusing on the fruit fly, Drosophila melanogaster, as a model system. Towards this end, we have built an imaging system that can track single flies as they move about a relatively unencumbered environment. Utilizing this capacity to generate large data sets of animal behavior, we have developed a method for automatically identifying behavioral states using techniques from image analysis, machine learning, and nonlinear dynamics. Identifying these states provides the starting point for many analyses and creates the possibility for automatic phenotyping of subtle behavioral traits. [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z40.00011: Multiscale Analysis of Head Impacts in Contact Sports Mark Guttag, Subham Sett, Jennifer Franck, Kyle McNamara, Eyal Bar-Kochba, Joseph Crisco, Janet Blume, Christian Franck Traumatic brain injury (TBI) is one of the world's major causes of death and disability. To aid companies in designing safer and improved protective gear and to aid the medical community in producing improved quantitative TBI diagnosis and assessment tools, a multiscale finite element model of the human brain, head and neck is being developed. Recorded impact data from football and hockey helmets instrumented with accelerometers are compared to simulated impact data in the laboratory. Using data from these carefully constructed laboratory experiments, we can quantify impact location, magnitude, and linear and angular accelerations of the head. The resultant forces and accelerations are applied to a fully meshed head-form created from MRI data by Simpleware. With appropriate material properties for each region of the head-form, the Abaqus finite element model can determine the stresses, strains, and deformations in the brain. Simultaneously, an in-vitro cellular TBI criterion is being developed to be incorporated into Abaqus models for the brain. The cell-based injury criterion functions the same way that damage criteria for metals and other materials are used to predict failure in structural materials. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z40.00012: The fern sporangium: an ultrafast natural catapult Xavier Noblin, Mederic Argentina, Jared Westbrook, Coraline Llorens, Nicolas Rojas, Jacques Dumais Plants have developed fascinating mechanisms to create ultra fast movements that often reach the upper limit allowed by physical laws. Inspiration for new technologies is one of the reasons for the strong interest for these mechanisms, along with the deep interest of understanding complex, natural systems. The fern sporangium is a capsule that contains the spores, it is surrounded by a row of cells called the annulus which acts as a beam. Due to the water evaporation from its cells, the annulus bends strongly and induces elastic energy storage during an opening phase. The tension in the cells breaks when cavitation bubbles appear in the cells, leading to a fast release of the elastic energy. The fern sporangium then acts as a catapult which ejects rapidly its spores by closing back to the initial closed shape. We have analyzed the slow opening motion and the fast catapulting mechanism. We found that the catapult motion involves two time scales, showing a very original behavior. In man-made catapults, the recoil motion needs to be arrested by a cross bar so that the projectile is released from the arm. We show here that the fern sporangium replaces the essential cross bar by an elegant poroelastic damping, leading to a completely autonomous, efficient device. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z40.00013: Hydrodynamics of Active Permeating Gels Andrew Callan-Jones, Frank J\"ulicher We present a hydrodynamic theory of active viscoelastic gels in which a polymer network is embedded in a background fluid. This work is motivated by active processes in the cell cytoskeleton in which motor molecules generate elastic stresses in the network which can drive permeation flows of the cytosol. Our approach differs from earlier ones by considering the elastic strain in the polymer network as a slowly relaxing dynamical variable. We discuss a specific case that illustrates the role of permeation in active gels: the self-propulsion of a thin slab of gel relative to a substrate driven by filament polymerization and depolymerization. [Preview Abstract] |
Session Z41: Focus Session: Non-Covalent Protein Interactions
Sponsoring Units: DBIO GSNPChair: Corey O'Hern, Yale University
Room: 156B
Friday, March 2, 2012 11:15AM - 11:51AM |
Z41.00001: Novel Aspects of Hydrogen Bonding in Protein Function: Active Site Ionic Hydrogen Bonds Invited Speaker: Wouter D. Hoff We use photoactive yellow protein (PYP), a bacterial photoreceptor, to explore novel aspects of the role of hydrogen bonding in protein function. PYP exhibits photochemical activity based on its ionized p-coumaric acid (pCA) chromophore, which is hydrogen bonded to Tyr42 and Glu46. We report that these active site ionic hydrogen bonding interactions cause unexpected molecular and functional properties of PYP. First, we describe a novel spectroscopic isotope effect (SIE) in which dissolving PYP in D2O causes a red-shift in its electronic absorbance spectrum. We assign this SIE to the ionic hydrogen bond between pCA and Glu46, which--in contrast to standard hydrogen bonds--is weakened upon H/D exchange. These findings extend the effects of H/D exchange from kinetic isotope effects to include shifts in absorbance spectrum, and illustrate the biological relevance of ionic hydrogen bonding to protein active sites. Secondly, we examine how the protein environment achieves the unusual strong preference of the pCA to remain ionized in the protein interior. We use the rescue of the Y42F mutant of PYP by incorporation of a trans-locked analog of pCA to dissect the contributions of active site hydrogen bonding to the large down-shift in the pKa of the pCA. Together, the Tyr42 and Glu46 hydrogen bonds to the pCA account for $\sim$80\% of this shift, which can be quantitatively explained by the loss of ionic hydrogen bonding upon pCA protonation from the solvent. Since ionic hydrogen bonds occur in many proteins, this mechanism of pKa tuning is likely to be of general relevance. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:27PM |
Z41.00002: Computational design of protein interactions: designing proteins that neutralize influenza by inhibiting its hemagglutinin surface protein Invited Speaker: Sarel Fleishman Molecular recognition underlies all life processes. Design of interactions not seen in nature is a test of our understanding of molecular recognition and could unlock the vast potential of subtle control over molecular interaction networks, allowing the design of novel diagnostics and therapeutics for basic and applied research. We developed the first general method for designing protein interactions. The method starts by computing a region of high affinity interactions between dismembered amino acid residues and the target surface and then identifying proteins that can harbor these residues. Designs are tested experimentally for binding the target surface and successful ones are affinity matured using yeast cell surface display. Applied to the conserved stem region of influenza hemagglutinin we designed two unrelated proteins that, following affinity maturation, bound hemagglutinin at subnanomolar dissociation constants. Co-crystal structures of hemagglutinin bound to the two designed binders were within 1Angstrom RMSd of their models, validating the accuracy of the design strategy. One of the designed proteins inhibits the conformational changes that underlie hemagglutinin's cell-invasion functions and blocks virus infectivity in cell culture, suggesting that such proteins may in future serve as diagnostics and antivirals against a wide range of pathogenic influenza strains. We have used this method to obtain experimentally validated binders of several other target proteins, demonstrating the generality of the approach. We discuss the combination of modeling and high-throughput characterization of design variants which has been key to the success of this approach, as well as how we have used the data obtained in this project to enhance our understanding of molecular recognition. References: Science 332:816 JMB, in press Protein Sci 20:753 [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 1:03PM |
Z41.00003: The power of simple hard-sphere models in protein structure prediction Invited Speaker: Lynne Regan There are several force-fields that are currently used to describe the potential energy of biological macromolecules such as proteins. These typically include many parameters, derived from a combination of statistical, experimental sources. These work on average to describe a protein, but the large number of parameters moves this description further away from a true physical understanding than is desirable. Our approach is to investigate to what extent simple hard sphere models can be used to model and predict the behavior of different aspects of protein structure. We present the results of specific calculations. The distributions of the side-chain dihedral angle chi1 of Val and Thr in proteins of known structure show distinctive features: Val side chains predominantly adopt dihedral angle, chi1, of 180, whereas Thr side chains typically adopt a dihedral angle, chi1, of 60 or 300. Several hypotheses have been proposed to explain these differences, including inter-residue steric clashes and hydrogen-bonding interactions. In contrast, we show that the observed side-chain dihedral angle distributions for both Val and Thr can be explained using only local steric interactions in a dipeptide mimetic. Our results emphasize the power of a simple physics-based approaches and their importance for future advances in protein engineering and design. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z41.00004: Acetylation of LYS-16 of H4 Histone Tail May Sequester the Tail and Inhibit its Interactions with Neighboring Nucleosomes Davit Potoyan, Garegin Papoian Histone tails are highly flexible N terminal protrusions of histone proteins, which help to fold DNA into dense superstructures known as chromatin. On a molecular scale histone tails are poly-electrolites with high degree of conformational disorder, allowing them to function as bio-molecular ``switches,'' regulating various genetic regulatory processes via diverse types of covalent modifications. Because of being intrinsically disordered, the structural and dynamical aspects of histone tails are still poorly understood. Using multiple explicit solvent and coarse-grained MD simulations we have investigated the impact of the acetylation of LYS-16 residue on the conformational and DNA-binding propensities of H4 histone tail. The potential of mean force computed as a function of distance between a model DNA and histone tail center of mass showed a dramatic enhancement of binding affinity upon mono-acetylation of the H4 tail. The estimated binding free energy gain for the wild type is 2kT, while for the acetylated it reaches 4-5 kT. Additionally our structural analysis shows that acetylation is driving the chain into collapsed states, which get enriched in secondary structural elements upon binding to the DNA. We suggest a non-electrostatic mechanism that explains the enhanced binding affinity of the acetylated H4 tail. At last our findings lead us to propose a hypothesis that can potentially account for the celebrated chromatin ``fiber loosening effects'' observed in many experiments. [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z41.00005: Exploring copper chelation in Alzheimer's disease protein Frisco Rose, Miroslav Hodak, Jerry Bernholc Alzheimer's disease (AD) is a neurodegenerative disorder affecting millions of aging people in the U.S. alone. Clinical studies have indicated that metal chelation is a promising new approach in alleviating the symptoms of AD. Our study explores the as yet undetermined mechanism of copper chelation in amyloid-$\beta$, a protein implicated in AD. The structure of amyloid-$\beta$ is derived from experimental results and incorporates a planar copper-ion-binding structure in a semi-solvated state. We investigate the chelation process using the nudged elastic band method implemented in our {\it ab initio} real-space multigrid code. We find that an optimal sequence of unbonding and rebonding events as well as proton transfers are required for a viable chelation process. These findings provide fundamental insight into the process of chelation that may lead to more effective AD therapies. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z41.00006: Transition-metal prion protein attachment: Competition with copper Miroslav Hodak, Jerry Bernholc Prion protein, PrP, is a protein capable of binding copper ions in multiple modes depending on their concentration. Misfolded PrP is implicated in a group of neurodegenerative diseases, which include ``mad cow disease'' and its human form, variant Creutzfeld-Jacob disease. An increasing amount of evidence suggests that attachment of non-copper metal ions to PrP triggers transformations to abnormal forms similar to those observed in prion diseases. In this work, we use hybrid Kohn-Sham/orbital-free density functional theory simulations to investigate copper replacement by other transition metals that bind to PrP, including zinc, iron and manganese. We consider all known copper binding modes in the N-terminal domain of PrP. Our calculations identify modes most susceptible to copper replacement and reveal metals that can successfully compete with copper for attachment to PrP. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z41.00007: Steric clashes determine differences in side chain dihedral angle distributions: A study of Thr versus Val Alice Zhou, Corey O'Hern, Lynne Regan With the long-term goal to improve the design of protein-protein interactions, we develop a simple hard sphere model for dipeptides that can predict the side-chain dihedral angle distributions of Val and Thr in both the $\alpha$-helix and $\beta$-sheet backbone conformations. We find that it is essential to include the non-polar hydrogens in the model; indeed interatomic clashes involving the non-polar hydrogens largely determine the form of side-chain dihedral angle distributions. Further, we are able to explain key differences in the side-chain dihedral angle distributions for Val and Thr from intra-residue steric clashes rather than inter-residue steric clashes or hydrogen bonding. These results are the crucial first step in developing computational models that can predict the side chain conformations of residues at protein-peptide interfaces. [Preview Abstract] |
Session Z42: Focus Session: Physics of Biomaterials: Mechanics, Dynamics, and Transport
Sponsoring Units: DBIOChair: Lester Hedges, Lawrence Berkeley National Laboratory
Room: 156C
Friday, March 2, 2012 11:15AM - 11:27AM |
Z42.00001: Highly parallel computational study of amphiphilic molecules using the Wang--Landau method Thomas Vogel, David Landau The self-assembly process in amphiphilic solutions is a phenomenon of broad interest. Molecular dynamics simulations generally used to study micelle formation or lipid layer assembly in an explicit solvent are limited in time scale. Vast studies of structure formation processes via standard Markov-chain based Monte Carlo simulations are challenging, but the Wang--Landau method~[1] provides a way to efficiently study such systems in a generalized thermodynamic ensemble. This makes it possible, for example, to get results over a broad temperature range from a single simulation. In an attempt to develop highly parallel applications using this method, we study the thermodynamic behavior of a generic coarse-grained model for amphiphilic molecules~[2] as well as of a new coarse-grained lipid model specifically designed for dimyristoyl phosphatidylcholine (DMPC)~[3]. Here, we focus on the design and the performance of our parallel Wang--Landau simulation on multi-CPU and GPU systems.\\[4pt] [1] F. Wang and D.P. Landau, Phys. Rev. Lett. \textbf{86}, 2050 (2001)\\[0pt] [2] S. Fujiwara et al., J. Chem. Phys. \textbf{130}, 144901 (2009)\\[0pt] [3] W. Shinoda et al., J. Phys. Chem. B \textbf{114}, 6836 (2010) [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z42.00002: Spatial response variations within biosensor flow cells Nicola Cant, Sarah Harrison Biosensors are currently being developed for the detection of a wide range of analytes in a variety of scenarios. One such area is that of environmental monitoring for the presence of biological threats, from toxins through to viruses and bacteria. The varying nature, and in particular disparate size, of such a variety of analytes poses a significant challenge in the development of effective high confidence instruments. Many existing biosensors employ functionalised flow cells in which spatially defined arrays of surface immobilised recognition elements are present to specifically capture their analyte of interest. Experimental data obtained using a grating coupled SPR instrument, the BIAcore{\texttrademark} Flexchip, has revealed spatial dependency differences in response behaviours between proteinaceous and particulate analytes. In particular, the magnitude of responses seen with \textit{Bacillus anthracis} spores across the instruments flow cell appear to be influenced by shear and gravitational effects whilst those from soluble proteins are more uniform. We have explored this dependence to understand its fundamental impact on the successful implementation of multi-analyte environmental biological detection systems. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z42.00003: Size-dependent mechanical properties of low-dimensional materials: coupling between deformation modes Yiting Ding, Zhiping Xu Coupling between deformation modes, such as tension, bending, shear, and twisting, are widely observed in low-dimensional materials, especially biological materials. In this talk we will present our study on microtubules (MT) mechanics, where significance of mode coupling leads remarkable size-dependence in structural and mechanical properties. Using molecular dynamics (MD) simulations, we find a distinct dependence of bending rigidity on the contour length of a MT that agrees well with experimental data. We develop a simple model by including basic parameters to explain and predict this interesting phenomenon. An extended discussion will be made to general low-dimensional materials, with focuses not only on the mechanism, but also applications in optimal materials design. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z42.00004: A QM/MM method for simulating the sequencing of DNA using graphene nanopores: solvent efects on nucleobase selectivity Alexandre Rocha, Gustavo Feliciano, Yuhui He, Ralph Scheicher, Mauricio Coutinho The possibility of using graphene nanopores for DNA sequencing is driving a significant research effort with the aim of obtaining cheaper, more efficient, sequencing techniques. In this work we will present electronic transport calculations of graphene nanopores used for sequencing DNA strands. We consider both single layer and double layer graphene with different types of functionalization of the pore edges. The simulations were performed using a QM/MM method which allows us to treat the graphene sheet containing the nanopore and a segment of DNA within the pore via ab initio DFT methods (QM) whereas the effects of the water molecules, the counter-ions and the remainder of the DNA strand is taken into consideration using a classical potential (MM). The electronic transport properties along graphene are subsequently calculated using non-equilibrium Green's functions including the classical potential. Different time steps of classical molecular dynamics simulations of the DNA strand passing through the nanopore are considered in order to simulate the translocation process. This way we are able to address the effects of the solvent on the selectivity of the device to different nucleobases using atomistic methods. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z42.00005: First-Principles Study of Muon Trapping in Singlet and Triplet States of Oxyhemoglobin Roger Pink, S.R. Badu, T.P. Das, N. Sahoo, Lee Chow, R.H. Scheicher, K. Nagamine There is great current interest in the possibility of magnetic character in oxyhemoglobin (OxyHb) due to the detection [1] of muon spin-lattice relaxation in OxyHb. First-Principles variational Hartree-Fock Many Body Perturbation Theory (VHFMBPT) technique investigations on the singlet and triplet states of pure OxyHb have shown$^{2}$ that the triplet state is considerably higher than the singlet state ruling out magnetic character. However the charge distribution obtained by the VHFMBPT procedure in both states show a number of sites that have negative charges where the trapping of muon is being investigated to examine if the energy gap in the ordering of singlet and triplet states can be reduced or reversed leading to magnetic effects. Other possible sources of magnetism in Oxyhemoglobin will also be discussed. 1. K. Nagamine et al. Proc. Japan. Acad. B-Physics 83, 120 (2007); 2. S.R. Badu et al. Reported at Third Joint HFI-NQI International Conference, CERN, Geneva, September 2010. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z42.00006: Branched Polymer Models and the Mechanism of Multilayer Film Buildup Pradeep Waduge, Dhan Khadka, Donald Haynie The ``in and out diffusion'' hypothesis does not provide a sufficient explanation of the exponential buildup displayed by some polyelectrolyte multilayer film systems. Here, we report initial tests of an alternative view, on which the completion of each adsorption cycle results in an increase in the number of polymer binding sites on the film surface. Polycationic dendrimeric peptides, which can potentially bind several oppositely-charged peptides each, have been designed, synthesized and utilized in comparative film buildup experiments. Material deposited, internal film structure and film surface morphology have been studied by ultraviolet spectroscopy (UVS), circular dichroism spectroscopy (CD), quartz crystal microbalance (QCM) and atomic force microscopy (AFM). Polycations tended to contribute more to film buildup than did polyanions on quartz but not on gold. Increasing the number of branches in the dendrimeric peptides from 4 to 8 reproducibly resulted in an increase in the film growth rate on quartz but not on gold. Peptide backbones tended to adopt a $\beta $-strand conformation on incorporation into a film. Thicker films had a greater surface roughness than thin films. The data are consistent with film buildup models in which the average number of polymer binding sites will increase with each successive adsorption cycle in the range where exponential growth is displayed. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z42.00007: Reduction of Scattered Light by M\"uller Cells in the Human Retina Oliver Bendix, Ragnar Fleischmann, Theo Geisel It is a long standing question why in the mammalian eye photoreceptors are positioned at the back of the retina, forcing photons to travel through various neuronal layers of the retina before the light-sensitive rods and cones can detect them. Recent studies suggest that certain retinal glial cells--called M\"uller cells (MCs)--play an important role in answering that question. It has been experimentally shown that MCs extracted from the retina can act as optical fibers [1]. To understand the light guiding properties of the MC in the natural fluctuating optical environment, we developed a model to analyze the light reflection and transmission of MCs embedded in a random medium neuronal tissue. With these quantities and a simplified geometrical eye model we study how light is scattered in the eye. We found that MCs can lead to a substantial increase of the signal-to-noise ratio (SNR), the ratio of the intensity of direct incident light at a photoreceptor to the intensity of back-scattered light from other areas of the retina. The SNR is most pronounced in the vicinity of the fovea and can be more than an order of magnitude.\\[4pt] [1] Franze et. al., Proc Natl Acad Sci USA, 2007, 104, 8287-8292. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z42.00008: Quantifying the deformation of the red blood cell skeleton in shear flow Zhangli Peng, Qiang Zhu To quantitatively predict the response of red blood cell (RBC) membrane in shear flow, we carried out multiphysics simulations by coupling a three-level multiscale approach of RBC membranes with a Boundary Element Method (BEM) for surrounding flows. Our multiscale approach includes a model of spectrins with the domain unfolding feature, a molecular-based model of the junctional complex with detailed protein connectivity and a whole cell Finite Element Method (FEM) model with the bilayer-skeleton friction derived from measured transmembrane protein diffusivity based on the Einstein-Stokes relation. Applying this approach, we investigated the bilayer-skeleton slip and skeleton deformation of healthy RBCs and RBCs with hereditary spherocytosis anemia during tank-treading motion. Compared with healthy cells, cells with hereditary spherocytosis anemia sustain much larger skeleton-bilayer slip and area deformation of the skeleton due to deficiency of transmembrane proteins. This leads to extremely low skeleton density and large bilayer-skeleton interaction force, both of which may cause bilayer loss. This finding suggests a possible mechanism of the development of hereditary spherocytosis anemia. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z42.00009: Theoretical study of electron transport in DNA Bikan Tan, Miroslav Hodak, Wenchang Lu, Jerry Bernholc Multiple experiments have indicated high conductivity of DNA, but its origin has not yet been satisfactorily explained. In this work, we explore the conductivity of double stranded B-DNA using a nonequilibrium Green's function method based on density-functional theory. The DNA is sandwiched between metallic nanotube leads and we investigate the effect of various linkers connecting the DNA to the leads. Our results show that the alkane linker, $(CH_2)_n$, which is often used in experiments, dramatically decreases the conductivity due to its large band gap around the Fermi level. We also find that conductivity can be greatly enhanced by aligning the highest occupied molecule orbital energy of DNA with the Fermi level of the leads by applying a gate bias to the DNA. Finally, we examine the effects of misalignment and mismatches on conductivity of DNA. [Preview Abstract] |
Session Z43: Invited Session: Applications of Jamming
Sponsoring Units: GSNP DFDChair: Lisa Manning, Syracuse University
Room: 157AB
Friday, March 2, 2012 11:15AM - 11:51AM |
Z43.00001: Robotics using sand Invited Speaker: Heinrich Jaeger |
Friday, March 2, 2012 11:51AM - 12:27PM |
Z43.00002: Shocks in fragile matter Invited Speaker: Vincenzo Vitelli Non-linear sound is an extreme phenomenon typically observed in solids after violent explosions. But granular media are different. Right when they unjam, these fragile and disordered solids exhibit vanishing elastic moduli and sound speed, so that even tiny mechanical perturbations form supersonic shocks. Here, we perform simulations in which two-dimensional jammed granular packings are continuously compressed, and demonstrate that the resulting excitations are strongly nonlinear shocks, rather than linear waves. We capture the full dependence of the shock speed on pressure and compression speed by a surprisingly simple analytical model. We also treat shear shocks within a simplified viscoelastic model of nearly-isostatic random networks comprised of harmonic springs. In this case, anharmonicity does not originate locally from nonlinear interactions between particles, as in granular media; instead, it emerges from the global architecture of the network. As a result, the diverging width of the shear shocks bears a nonlinear signature of the diverging isostatic length associated with the loss of rigidity in these floppy networks. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 1:03PM |
Z43.00003: Controlling the jamming transition of sheared hard spheres Invited Speaker: Thomas Haxton Many applications require understanding how disordered materials flow under an external load such as a shear stress. Since external loads drive systems out of equilibrium, their behavior cannot be described solely in terms of equilibrium parameters like temperature and pressure. However, simulations and experiments show that sheared spherical particles possess an \textit{effective} temperature that relates low-frequency fluctuations of various observable quantities to their associated response functions. Here, we show that the mobility of a mixture of sheared hard spheres is largely controlled by the dimensionless ratio of effective temperature to pressure, $T_{\rm eff}/p\sigma^3$, where $\sigma$ is the sphere diameter. We define the effective temperature as the consistent value that relates the amplitudes of low-frequency shear stress and density fluctuations to their associated response functions. We find that the relaxation time $\tau$ characterizing the mobility depends on $T_{\rm eff}/p\sigma^3$ according to two distinct mechanisms in two distinct regimes. In the \textit{solid response} regime, the behavior at fixed packing fraction $\phi$ satisfies $\tau\dot\gamma\propto \exp(-cp\sigma^3/T_{\rm eff})$, where $\dot\gamma$ is the strain rate and $c$ depends weakly on $\phi$, suggesting that the effective temperature controls the average local yield strain. In the \textit{fluid response} regime, $\tau$ depends on $T_{\rm eff}/p\sigma^3$ as it depends on $T/p\sigma^3$ in equilibrium. This regime comprises a large part of the hard-sphere jamming phase diagram including both near-equilibrium conditions where $T_{\rm eff}$ is similar to the kinetic temperature $T_{\rm kin}$ and far-from-equilibrium conditions where $T_{\rm eff} \ne T_{\rm kin}$. In particular, the dynamic jamming transition is largely controlled by the fluid-response mechanism; like equilibrium hard spheres, sheared hard spheres can flow only if low-frequency fluctuations are large enough compared to the pressure. By presenting our results in terms of the dimensionless jamming phase diagram, we show how these mechanisms likely apply to systems with soft repulsive interactions. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:39PM |
Z43.00004: Density-Temperature-Softness Scaling of the Dynamics of Glass-forming Soft-sphere Liquids Invited Speaker: Magdaleno Medina-Noyola We employ the principle of dynamic equivalence between soft-sphere and hard-sphere fluids [Phys. Rev. E {\bf 68}, 011405 (2003); Phys. Rev. Lett. {\bf 107}, 155701 (2011)] to describe the interplay of the effects of varying the density $n$, the temperature $T$, and the softness (characterized by a softness parameter $\nu^{-1}$) on the dynamics of glass-forming soft-sphere liquids in terms of simple scaling rules. The main prediction is the existence of a dynamic universality class associated with the hard sphere fluid, constituted by the soft-sphere systems whose dynamic parameters, such as the $\alpha$-relaxation time and the long-time self-diffusion coefficient, depend on $n,\ T$, and $\nu$ only through the reduced density $n^*\equiv n \sigma_{HS}^3(n,T,\nu)$, where the effective hard-sphere diameter $\sigma_{HS}(n,T,\nu)$ is determined by the Andersen-Weeks-Chandler condition for soft-sphere--hard-sphere structural equivalence. A number of scaling properties observed in recent experiments and simulations involving glass-forming fluids with repulsive short range interactions are found to be a direct manifestation of this general dynamic equivalence principle. The self-consistent generalized Langevin equation (SCGLE) theory of colloid dynamics [Phys. Rev. E {\bf 76}, 041504, 062502 (2007)] is shown to accurately capture these scaling rules. The non-equilibrium extension of this theory [Phys. Rev. E {\bf 82}, 061503, 061504 (2010)] is employed to describe the manifestation of this scaling on the aging of instantaneously-quenched soft-sphere liquids. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 2:15PM |
Z43.00005: Active Jamming: Self-propelled particles at high density Invited Speaker: Silke Henkes What determines the mechanical properties of dense collections of active particles? The answer to this question is highly relevant to a wide range of physical and biological phenomena from tissue formation to the dynamics of vibrated granular layers. We present a numerical study of the phases and dynamics of a dense collection of self-propelled particles with soft repulsive interactions and polar alignment in a two-dimensional confined geometry. The phase diagram consists of a polar liquid phase at low packing fraction and high self-propulsion speed, and an active jammed phase at high density and low self-propulsion speed. The liquid phase exhibits local alignment and giant number fluctuations typical of the Vicsek class of models. The dynamics of the jammed phase is dominated by oscillations along the low frequency modes of the underlying packing. We show analytically that at long times the energy is carried entirely by the lowest available excitations of the system. Recent experiments on epithelial cell monolayers using force traction microscopy have revealed stress distributions that resemble those observed in granular materials. We measure and compare the local stresses in our active system, with added attraction, to both granular materials and the tissue experiments. [Preview Abstract] |
Session Z46: Invited Session: Self-Assembly of Proteins: From Capsids to Crystals
Sponsoring Units: DPOLY DBIOChair: Steve Whitelam, Lawrence Berkeley National Laboratory and Mike Hagan, Brandeis University
Room: 160AB
Friday, March 2, 2012 11:15AM - 11:51AM |
Z46.00001: Experimental and theoretical studies of capsid self-assembly Invited Speaker: Adam Zlotnick |
Friday, March 2, 2012 11:51AM - 12:27PM |
Z46.00002: Theoretical approach to crystallization: foundations and application to proteins Invited Speaker: James Lutsko A fundamental issue in the modern study of phase transitions is the description of the process of nucleation, i.e. the choices of nucleation pathways. Proteins, in particular, are well-known to sometimes crystallize by passing through a meta-stable amorphous state and simulation and theory have shown that this is also true of many other systems. The issue also arises in the important case of polymorphic materials. In all cases, the goal is to understand which pathway is favored and how this is affected by the external control parameters. In this talk, I discuss a theoretical description of nucleation that allows for the direct determination of nucleation pathways and of their relative probability of realization that takes into account both thermodynamics and kinetic effects. It is based on a formulation of nucleation as a fundamentally non-equilibrium process and fully incorporates the effect of free-energy landscapes, determined e.g. via Density Functional Theory, in a consistent manner. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 1:03PM |
Z46.00003: Elasticity Theory of Viral Capsids Invited Speaker: Robijn Bruinsma The continuum elasticity theory of icosahedral thin shells has been applied with success to shape transitions of the protein shell surrounding the viral genome, the capsid. The talk presents an extension of thin-shell elasticity theory that is applicable to aggregates of \textit{functional biomolecules } at length scales comparable to that of the component molecules themselves. Unlike classical elasticity theory, the stress and strain fields have a network of \textit{mathematical discontinuities} along the interfaces of the proteins, due to the conformational incompatibility of packing proteins as well as to conformational transitions of the proteins. The method is applied to the P-II to EI transition of the protein shell of the virus HK97 driven by hexon skewing. The combination of the intrinsic stresses of icosahedral shells and the conformational pre-stress turns the P-II state into a ``critical'' state whose shape is independent of the bending and Young's moduli. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:39PM |
Z46.00004: The Role of Multivalent Counterions in Protein Crystallization Invited Speaker: Fajun Zhang In this talk, I will give an overview of our recent studies on the phase behavior of model globular proteins in solution in the presence of multivalent counterions. We have shown that negatively charged globular proteins at neutral pH in the presence of multivalent counterions undergo a ``reentrant condensation (RC)'' phase behavior [1,2], i.e. a phase-separated regime occurs in between two critical salt concentrations, c* $<$ c**, giving a metastable liquid-liquid phase separation (LLPS) [3]. This reentrant phase behavior corresponds to an effective charge inversion of proteins as confirmed by zeta-potential measurements and supported by Monte Carlo simulations [1,2]. Crystallization from the condensed regime follows different mechanisms. Near c*, crystals grow following a classic nucleation and growth mechanism; near c**, the crystallization follows a two-step mechanism, i.e, crystals growth follows a metastable LLPS [3,4]. Nucleation rate is faster from the protein-poor phase than that from the protein-rich phase, which cannot be explained by the recent theories. SAXS measurements demonstrate that protein clusters act as precursors for crystal growth, which reduce the energy barrier of nucleation [4]. X-ray diffraction analyses on the high quality single crystals provide direct evidence of the crystal structure and cation binding sites [3]. The bridging effect of the metal cations explains the cluster formation.\\[4pt] [1] Zhang, F.; et al. \textit{Phys. Rev. Lett.} \textbf{2008}, 101, 148101.\\[0pt] [2] Zhang, F.; et al. \textit{Proteins }\textbf{2010}, 78, 3450.\\[0pt] [3] Zhang, F.; et al.\textit{ J. Appl. Cryst.} \textbf{2011}, 44, 755.\\[0pt] [4] Zhang, F.; et al. In preparation. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 2:15PM |
Z46.00005: Self-assembly and Evolution from protein complexes to DNA nanostructures Invited Speaker: Ard A. Louis The remarkable ability of biological matter to robustly self-assemble into well defined composite objects excites the imagination, suggesting that these processes could perhaps be emulated through the judicious design of synthetic building blocks. We use statistical mechanics to uncover the design rules for self-assembly into well defined three dimensional composite objects. In Nature, the rules for self-assembly emerge from an evolutionary process. We show how some patterns in protein complexes can be explained by their evolutionary origin [1]. We also introduce a coarse-grained rigid nucleotide model of DNA that reproduces the basic thermodynamics of short strands: duplex hybridization, single-stranded stacking and hairpin formation, and also captures the essential structural properties of DNA: the helical pitch, persistence length and torsional stiffness of double-stranded molecules, as well as the comparative flexibility of unstacked single strands [2]. We apply the model to calculate the detailed free-energy landscape of one full cycle of DNA ``tweezers,'' a simple machine driven by hybridization and strand displacement. We also study other nanomachines as well as processes such as force-induced melting, cruciform formation and the self-assembly of DNA tetrahedra.\\[4pt] [1] The self-assembly and evolution of homomeric protein complexes Gabriel Villar, et al., Phys. Rev. Lett. 102, 118106 (2009\\[0pt] [2] Structural and thermodynamic properties of a coarse-grained model of DNA, Thomas E. Ouldridge, Ard A. Louis, Jonathan P.K. Doye, J. Chem. Phys. 134 , 085101 (2011) [Preview Abstract] |
Session Z47: Self-Assembly and Polymer Composites
Sponsoring Units: DPOLYChair: Lutz Wiegart, Brookhaven National Laboratory
Room: 160C
Friday, March 2, 2012 11:15AM - 11:27AM |
Z47.00001: Highly piezoelectric Biocompatible and Soft Composite Fibers Antal Jakli, Jason Morvan, Ebru Buyuktanir, John West We report the fabrication of highly piezoelectric biocompatible soft fibers containing Barium Titanate (BT) ferroelectric ceramic particles dispersed in electrospun poly lactic acid (PLA). These fibers form mats that have two orders of magnitude larger piezoelectric constant per weight than single crystal barium titanate films. We demonstrate that the observed super-piezoelectricity results from the electrospinning induced polar alignment of the ferroelectric particles and the increased surface area compared to single crystal films. Due to the biocompatibility of PLA that encases the ferroelectric particles, these mats can be applied even in biological applications such as bio-sensors, artificial muscles and energy harvesting devices. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z47.00002: Phase Behavior of Polymer Nanocomposites from Gravity-based Combinatorial Simulations Dong Meng, Sanat Kumar The phase behavior of polymers mixed with nanoparticles, termed nanocomposites, has been of great current interest. Holding the promise of improved engineering properties, the property improvement is critically dependent on the spatial dispersion of nanoparticles, which is controlled by polymer-nanoparticle phase behavior. Enumerating the appropriate phase diagrams, however, still remain outside the realm of most simulation-based methods. The difficulty arises since inserting a large particle into a dense sea of small particles, critically for establishing phase equilibrium, is prohibitively difficult. Here we extend a gravity-based combinatorial method, first devise by Biben et al., and used primarily in the context of pure fluids to circumvent this insertion issue. Our results are validated against available simulations for colloid mixtures, but provide new insight into the phase behavior of polymers and nanoparticles. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z47.00003: Diffusive and re-orientation dynamics in lyotropic gels of self-assembled organic nanotubes Lutz Wiegart, Pierre Wiegart, Chiara Caronna It is known that dispersing lithocholic bile acid (LCA) in aqueous solutions of sodium hydroxide or ammonia leads to the formation of organic nanotubes with well-defined diameters on the nanometer scale. The suspensions appear to be tunable from liquid- to solid-like via the LCA concentration. The length of the tubes is decreasing as a function of temperature, favoring the formation of a phase where the tubes form hexagonally ordered bundles for an appropriate LCA concentration and temperature. In the present study, we have used X-ray Photon Correlation Spectroscopy (XCPS) to probe the diffusive dynamics of the nanotubes in these lyotropic gels under various conditions (concentration, counter ion, temperature). The multispeckle analysis of the coherent scattering pattern reveals domains in the sample with differing diffusive dynamics. The re-orientation process of different domains of nanotube bundles into a preferred parallel alignment has been observed. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z47.00004: Carbon Nanotube Liquid Crystals: Nematic Droplets and Coarsening Dynamics Natnael Behabtu, Bohdan Senyuk, Ivan Smalyukh, Matteo Pasquali On a fundamental basis, carbon nanotubes (CNTs) offer a new model molecule to explore the dynamics and phases of rigid rods and test theories. Their large aspect ratio (100 to 100,000) and persistence length ($\sim $ 100 microns) allow exploring the physics of nematic phases with high Frank elastic constant. Moreover, understanding of CNT liquid crystals is key to their rational processing into ordered materials such as fibers. Here we report the formation of elongated nematic droplets of CNTs in chlorosulfonic acid. In nematic droplets, a continuous transition from a homogeneous to bipolar nematic director field is expected theoretically, as a function of droplet volume; yet, experimental determination of such transition has been elusive. We show that CNT nematic droplets display such transition. We study the coarsening dynamics of positive and negative nematic droplets and observe that two or more droplets merge by matching their nematic director. Merging scenarios that lead to defect formation are not observed. Negative tactoids (isotropic phase in liquid crystalline continuum) merge through attractive forces induced by the nematic director distortion with quadrupolar symmetry. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z47.00005: Free volume in a granular nanocomposite Reghan J. Hill, Ahmad Mohaddespour Free volume plays a determinative role in the unusual molecular sieving and rheological characteristics of nanocomoposites. To help elucidate the nanoparticle influence on polymer-chain packing, we mixed acrylic spheres with aluminum ball-chains and ascertained from measurements of the bulk density the partial molar sphere volume at small but finite sphere volume fractions. This macroscopic mechanical model permits a systematic exploration of how the chain length, sphere size, and mixture composition influence free volume. We show that the nanoparticle radius to the chain loop size is the primary dimensionless parameter that bridges the solvent and continuum scaling regimes. In the intermediate regime, where the inclusion and polymer nanoscales are comparable, the primary sphere-chain interaction increases free volume---up to twice the intrinsic inclusion volume per particle---and the primary sphere-sphere interaction decreases free volume. Comparisons will be made with data gleaned from PALS and the bulk density of molecular nanocomposites reported in the literature. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z47.00006: Molecular packing in bone collagen fibrils prior to mineralization Benjamin Hsiao, Hong-wen Zhou, Christian Burger, Benjamin Chu, Melvin J. Glimcher The three-dimensional packing of collagen molecules in bone collagen fibrils has been largely unknown because even in moderately mineralized bone tissues, the organic matrix structure is severely perturbed by the deposition of mineral crystals. During the past decades, the structure of tendon collagen (e.g. rat tail) --- a tissue that cannot mineralize in vivo, has been assumed to be representative for bone collagen fibrils. Small-angle X-ray diffraction analysis of the native, uncalcified intramuscular fish bone has revealed a new molecular packing scheme, significantly different from the quasi-hexagonal arrangement often found in tendons. The deduced structure in bone collagen fibrils indicates the presence of spatially discrete microfibrils, and an arrangement of intrafibrillar space to form ``channels'', which could accommodate crystals with dimensions typically found in bone apatite. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z47.00007: Understanding the self-assembly of meso-tetra(4-sulfonatophenyl)porphyrin (H$_{2}$TPPS$^{4-})$ in aqueous solutions Javoris Hollingsworth, Allison Richard, Graca Vicente, Paul Russo The aggregation of \textit{meso}-tetra(4-sulfonatophenyl)porphyrin (H2TPPS4-) in phosphate solutions was investigated as a function of pH, concentration, time, ionic strength, and solution preparation (either from dilution of a freshly prepared 2 mM stock or by direct preparation of $\mu $M solution concentrations) using a combination of complementary analytical techniques. UV-Vis and fluorescence spectroscopy indicated the formation of staggered, side-by-side (J-type) assemblies. Their size and self-associative behavior was determined using analytical ultracentrifugation and small angle X-ray scattering. Our results indicate that in neutral and basic solutions of H$_{2}$TPPS$^{4-}$, porphyrin dimers and trimers are formed at $\mu $M concentrations and in the absence of NaCl to screen any ionic interactions. At these low concentrations and pH 4, the protonated H$_{4}$TPPS$^{2-}$ species self-assembles leading to the formation of particularly stable aggregates bearing 25 $\pm $ 3 macrocycles. At higher concentrations, these structures further organize or re-organize into tubular, rod-like shapes of various lengths which were imaged by cryogenic and freeze-fracture transmission electron microscopy. Micron-scale fibrillar aggregates were obtained even at $\mu $M concentrations at pH 4 when prepared from dilution of a 2 mM stock solution and/or upon addition of NaCl. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z47.00008: Self-Assembly of Polystyrene Tethered Hydrophilic POSS/C60 Nanoparticles Xinfei Yu, Stephen Z.D. Cheng Self-assembly of nanobuilding block provides unique opportunities to arrange nanoparticles into larger, functional ensembles. Anisotropy or symmetry breaking will lead to the formation of specific hierarchical structures with novel properties. Polymer tethered particle is one of the approaches to break the symmetry of nanobuilding blocks, which are promise to become the elementary building blocks for self-assembled materials. Shape and interactions are two important factors to determine the self-assembly of these molecules. Although a few examples have been reported during the last decade, little is known about ordered self-assembly structures from these anisotropic building blocks in bulk or solution by experiments. To solve this problem, hydrophilic POSS and C60 have been designed and synthesized, to which hydrophobic polystyrene tail(s) was tethered with controlled topology. The designed molecules could form micelles and colloidal particles in solutions. The effects of PS tail length, solvent properties, and molecular topology on self-assembly morphologies have been studied. The designed molecules could form ordered nanophase separation structures in bulk state with the feature sizes of sub-22 nm, which has great potential applications in advanced lithography technologies. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z47.00009: Self-Assembly of Narrowly Dispersed Brush Diblock Copolymers with Domain Spacing more than 100 nm Weiyin Gu, Benjamin Sveinbjornsson, Sung Woo Hong, Robert Grubbs, Thomas Russell Self-assembled structures of high molecular weight (MW), narrow molecular weight distribution brush block copolymers containing polylactic acid (PLA) and polystyrene (PS) side chains with similar MWs were studied in both the melt and thin films. The polynorbornene-backbone-based brush diblock copolymers containing approximately equal volume fractions of each block self-assembled into highly ordered lamellae with domain spacing over 100 nm, as revealed by SAXS, GISAXS and AFM. The domain size increased approximately linearly with backbone length, which indicated an extended conformation of the backbone in the ordered state. The length of side chains also played a significant role in terms of controlling the domain size. As the degree of polymerization (DP) increased, the symmetric brush diblock copolymers with longer side chains tended to form larger lamellar microdomains in comparison to those that have the same DP but shorter side chains. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z47.00010: Non-Classical Ordering of Sphere Forming SISO Tetrablock Terpolymers Jingwen Zhang, Scott Sides, Frank Bates ABAC tetrablock terpolymers represent the simplest symmetry breaking multiblock extension of ABC triblocks. The model system poly(styrene-b-isoprene-b-styrene-b-ethylene oxide) (SISO) tetrablock terpolymers were synthesized in relatively monodisperse form and the resulting morphologies were characterized by small-angle X-ray scattering, transmission electron microscopy, differential scanning calorimetry and dynamic mechanical spectroscopy. Two non-classical sphere-based orderedphases have been established in these single component materials with P$_{6}$/mmm (simple hexagonal) and P$_{42}$/mnm (tertragonal sigma phase) symmtery. A third state, tentatively associated with quasicrystalline order, has been identified at temperatures between the hexagonal and sigma phases, which occur at low temperatures, and prior to disordering, respectively. This unusual set of morphologies will be discussed in the context of segregation under the constraints associated with the tetrablock molecular architecture. Self-consistent mean-field theoretical calculations, obtained using the PolySwift++ software package, provide valuable insights into the molecular configurations associated with these morphologies. [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z47.00011: Tear rotation in reinforced natural rubber Paul Sotta, Brice Gabrielle, Arnaud Vieyres, Loic Vanel, Didier Long, Olivier Sanseau, Pierre-Antoine Albouy We analyze the impact of tear rotation, that is, an abrupt instability in the direction of propagation of a notch, on the tensile strength of natural rubber elastomers reinforced with carbon black or precipitated silica, in single edge notched samples stretched at constant velocity. As a consequence of tear rotation, the energy at break increases by a factor of 6 to 8 in some cases. We show how the tensile strength of a test sample is related to the presence of tear rotations and analyze semi-quantitatively this increase in tensile strength, based on energetic arguments, without entering into a detailed description of the elastic strain field in the vicinity of the tear tip. The proposed interpretation is based on the idea that tear rotations creates a macroscopic tip radius, which relaxes the local strain (or stress) at the tear tip. Materials reinforced with carbon black or precipitated silica aggregates show similar behavior. The relation to strain-induced crystallization is discussed. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z47.00012: Quantification of Folding in Sheet-like Nanostructures Durgesh Rai, Gregory Beaucage, Ramanth Ramachandran, Siddharth Pradhan Two-dimensional nanostructures are of interest to a wide range of scientists from biologists interested in membranes to polymer scientists producing grapheme reinforced nanocomposites. Considering two--dimensional structures as a class of nanomaterials, we could use the sheet thickness and lateral size as structural description. In addition to these parameters, two-dimensional structures are, at times, capable of folding or crumpling, largely depending on the interfacial chemistry and to some extent on thermodynamics. We have studied this crumpling behavior using small-angle neutron and x-ray scattering in a range of nano-materials, specifically, membrane bilayers, graphene oxide, as well as exfoliated nano-sheets of molybdenum sulfide, boron nitride, and tungsten sulfide. A new parameterization of crumpling in these two-dimensional nanostructures will be described with indications of how this quantification can lead to general categories of crumpling behavior that differ in the systems mentioned above. (A helpful discussion with Fyl Pincus assisted with this work.) [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z47.00013: Rheology of cube shaped particles in a suspension Rajesh Mallavajula, Donald Koch, Lynden Archer ~The rheological properties of suspensions of Brownian cube-shaped particles are interesting because of the greater increase in the translational freedom caused by layering relative to suspensions of Brownian spheres.~Theoretical solutions for a simple shear flow around an isolated cube were obtained using the geometric properties of cubes. Since stress-strain relationship is anisotropic, the suspension viscosity$_{~}$in the limit of zero shear rate can be obtained from the orientational average of the stresslet acting on the cube. The value of intrinsic viscosity was found to be 3.1. Brownian dynamics simulations were carried out with the obtained anisotropic stress relationship to understand hydrodynamic interactions between cubes at low to moderate volume fractions and lubrication hydrodynamic interactions at higher volume fractions. Interesting transitions are observed in rheological properties as the volume fraction of particles in suspension rises above a critical value substantially lower than the minimum close-packing. We have also synthesized model cube-shaped particles with two different chemistries and sizes: Iron oxide, Fe$_{3}$O$_{4}$, nanocubes (20nm and 100nm) and hollow Manganese Carbonate (MnCO$_{3})$ microcubes (1-2microns). MnCO$_{3}$ microspheres were also synthesized to compare their properties with the cubic particle suspensions. At low particle volume fractions, the experimentally determined intrinsic viscosity for the suspension of cubic particles is in excellent accord with expectations from theory. The talk will also compare results of rheological properties of spheres and cubes for both high and low volume fractions over a range of Peclet numbers. [Preview Abstract] |
Session Z48: Focus Session: Statistical Physics of Active Systems Away from Detailed Balance; Motors, Swimmers and All That
Sponsoring Units: DPOLY DBIOChair: Alexander Grosberg, New York University
Room: 161
Friday, March 2, 2012 11:15AM - 11:27AM |
Z48.00001: Mechanical manipulation and bifurcation dynamics of stereociliary bundles Seung Ji, Lea Fredrickson-Hemsing, Robijn Bruinsma, Dolores Bozovic We propose a numerical model for the mechanical response of hair cells of the inner ear. These mechanically sensitive cells have been described previously using systems of nonlinear differential equations, that captured the main features of the experimental phenomena. Here we extend the study to include the effects of static and time-dependent forcing, and show it to induce transitions across different types of bifurcations. We compare the theory to the experimental measurements of the phase-locked response of spontaneously oscillating hair cells of the bullfrog sacculus, under varying mechanical deflections. For a static deflection, when adaptation phenomena play an important role, the offset generates a critical point where the frequency but not the amplitude vanishes, as opposed to the expected Hopf bifurcation where the amplitude vanishes. On the other hand, for a periodic deflection, mode-locking of the spontaneous oscilliations to the drive period can proceed via different types of bifurcations, depending on the degree of detuning. We will present a simple dynamic systems framework that captures the main features of the experimentally observed behavior in the form of an Arnold Tongue. [Preview Abstract] |
Friday, March 2, 2012 11:27AM - 11:39AM |
Z48.00002: Towards an active hydrodynamic theory of the metaphase spindle Jan Brugues, Daniel Needleman he spindle is a dynamic steady-state structure composed of microtubules and a wide range of factors which control microtubule nucleation, growth, and motion. While many of the individual components of the spindle have been studied in detail, it is still unclear how these molecular constituents self-organize into this structure. Here I describe how we are using spatio-temporal correlations of microtubule density and orientation to formulate a continuum coarse grained theory for the spindle. Our results suggest that microtubule turnover plays a crucial role in stabilizing the giant density fluctuations predicted by traditional active hydrodynamic theories. [Preview Abstract] |
Friday, March 2, 2012 11:39AM - 11:51AM |
Z48.00003: A phase transition between collective and individual dynamics in suspension of swimming bacteria Andrey Sokolov, Igor Aronson We present the experimental study of the onset of collective behavior in suspension of swimming bacteria \textit{Bacillus Subtilis} in a liquid film. The system behavior is analyzed as the average swimming speed is varied in a wide range while the concentration of bacteria is constant. The average swimming speed was controlled by adjusting the concentration of dissolved oxygen in a fluid with bacteria. We obtained phase diagrams for the transition of the system between the state of collective swimming and the disordered state at different conditions. [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z48.00004: Chemotaxis of Phoretic Swimmers Suropriya Saha, Sriram Ramaswamy, Ramin Gloestanian An artificial phoretic swimmer in a uniform bath of reactant propels itself in a direction dictated by the polarity of the enzymatic and mobility patterning on its surface. We have have shown that a polar active particle of this type can also orient itself along an imposed gradient of reactant concentration. This amounts to a theoretical demonstration of a phoretic analogue of chemotaxis, that is, the ability of a self-propelled particle to align with respect to, and hence to move up or down, a chemical gradient. The nature of the chemotaxis depends on the shape of the particle, on the distribution of enzymatic sites on its surface, and on the surface mobility. We have also considered the type of motion that arises when the orientation time of the particle becomes comparable to the diffusion time of the cloud of reaction products around it. Lastly, we consider motion arising from the interaction of two or more such chemotactic particles. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z48.00005: Active viscosity of E-Coli suspensions Eric Clement, Jeremie Gachelin, Gaston Mino, Helene Berthet, Anke Lindner, Annie Rousselet Active suspension is the name borne by fluids laden with self-swimming entities such as bacteria, algae or artificial swimmers. Such fluids display emergent constitutive properties differing strongly from those of passive suspensions. Here, we present a recent work, were we measure the viscosity of a wild type E-Coli suspension in the dilute and semi-dilute limits. To this purpose, we use micro-fluidic device build as a Y shape micro-fluidic channel. On one side of the arm, the active suspension is injected and on the other arm, the suspending fluid in injected at the same flow rate. From the position of the interface between the pure fluid and the suspension, we extract the suspension relative viscosity. Varying bacteria density and flow rate, we show a regime specific to active fluids, where the relative viscosity is lower than the viscosity of the suspending viscous fluid. We discuss our results in the perspective of recent theoretical and experimental works. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z48.00006: Molecular Dynamics Study of Self-propelled Droplets on Solid Substrates Murat M\"ulayim, Marcus M\"uller We study, by molecular simulation, the statics and dynamics of chemically driven polymer droplets on regularly and finely corrugated substrates. Droplets are driven by a self-induced wettability gradient, which is formed under the droplet by changing the strength of the polymer-substrate interaction when the polymer get into contact with a reactive substrate with a pre-defined probability. The effect of reaction rate on droplet profile, slip and motion are investigated in the quasi steady-state. Moreover wetting properties of heterogeneous, partially reacted, systems are studied and a connection to experiments in the literature and thin film theoretical results on driven droplets are drawn, via exploring velocity profiles and dissipation mechanisms in the system. [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z48.00007: Self-assembled structures in immiscible systems of active, switchable nanocolloids Antonio Osorio-Vivanco, Igal Szleifer, Sharon Glotzer We consider the synthesis and fabrication of active building blocks that can dynamically switch between two or more states and assemble into novel structures. We present novel steady-state structures predicted by computer simulation to assemble in systems of switchable, immiscible building blocks. We discuss the dynamics that stabilize these structures, explore approaches to analyze the dissipative nature of the system, and provide a mapping to experimental colloidal systems where these concepts could be implemented. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z48.00008: Collective motion in active solids and active crystals Cristian Huepe, Eliseo Ferrante, Ali Emre Turgut We introduce a minimal model for self-propelled particles with strong attraction-repulsion interactions, but no explicit alignment rules, that displays collective polar or rotational motion. We describe a novel elasticity-based mechanism responsible for such collective motion and compute analytically its required conditions in the continuous elastic sheet approximation. By studying the mechanism's dynamics numerically and analytically, we show that it supports collective motion even for finite noise levels if the coupling between individual self-propulsion and elastic modes transfers energy towards larger and larger wavelengths. We hypothesize that this elasticity-based mechanism could provide an alternative explanation for collective motion in some biological groups without explicit alignment interactions, given their natural cohesion. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z48.00009: Statistics and dynamics in an optical vortex Yael Roichman, Yulia Sokolov, Derek Frydel, David Grier, Haim Diamant When two identical particles are driven along a straight line by an identical force there are no attractions forces between them. Surprisingly, when the same two particles are driven along a ring of light, with identical force, they attract and form a pair. The pairing mechanism is a pure non-equilibrium phenomena and is a result of symmetry breaking due to the path's curvature. We use experiment, simulation and theory to demonstrate and explain this effect. We show that dynamic limit-cycles and structure emerge due to this pseudo-potential in many particle systems, and demonstrate how they depend on temperature and trapping stiffness. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z48.00010: Induced Motion by Asymmetric Enzymatic Degradation of Hydrogels Jennifer Hou, Adam Cohen Biological hydrogels are continuously turned over through secretion and degradation. This non-equilibrium flux is critical to understanding cellular and molecular transport through biogels such as mucus and the extracellular matrix. Gel-digesting enzymes can drastically change the physical and chemical properties of the hydrogel environment. We report that a spatial gradient in the degradation of two gel/enzyme systems--gelatin/trypsin and hyaluronan/hyaluronidase--leads to directional motion of particles embedded in the gel in the direction of higher enzyme concentration. We study the rate at which the degradation front propagates through the gel and the ensuing velocity of the embedded particles, as functions of enzyme and gel concentrations. We propose that asymmetric degradation leads to asymmetric swelling, which transports particles up the enzyme concentration gradient. [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z48.00011: Possible mechanisms for initiating macroscopic left-right asymmetry in developing organisms Christopher L. Henley, Ricky Chachra, Jimmy X. Shen Systematic left-right (L/R) asymmetry in development --i.e. body axes satisfying a ``right-hand rule'' -- emerges at the organism level out of the microscopic handedness of biological molecules, not by the usual pattern-forming mechanisms of reactions (including regulation) plus diffusion, but rather (at the cell level) from the cytoskeleton and molecular motors -- usually in collective two-dimensional states associated with the cell membrane~\footnote{C. L. Henley, Landau 2008 conference\ (arxiv:0811.0055)}. I outline possible scenarios we are simulating for (i) snails and C. elegans, from a chiral shearing tendency in the actomyosin layer and/or (ii) for plant cells, from a precesson of the nematic order direction in the microtubule array. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z48.00012: Activation of nanoscale allosteric protein domain motion revealed by neutron spin echo spectroscopy Zimei Bu, Bela Farago, David Callaway NHERF1 is a multi-domain scaffolding protein that assembles the signaling complexes, and regulates the cell surface expression and endocytic recycling of a variety of membrane proteins. The ability of the two PDZ domains in NHERF1 to assemble protein complexes is allosterically modulated by a membrane-cytoskeleton linker protein ezrin, whose binding site is located as far as 110 angstroms away from the PDZ domains. Here, using neutron spin echo (\textbf{NSE}) spectroscopy, selective deuterium labeling, and theoretical analyses, we reveal the activation of interdomain motion in NHERF1 on nanometer length scales and on sub-microsecond time scales upon forming a complex with ezrin. We show that a much simplified coarse-grained model is sufficient to describe inter-domain motion of a multi-domain protein or protein complex. We expect that future NSE experiments will benefit by exploiting our approach of selective deuteration to resolve the specific domain motions of interest from a plethora of global translational and rotational motions. The results demonstrate that propagation of allosteric signals to distal sites involves the activation of long-range coupled domain motions on submicrosecond time scales, and that these coupled motions can be distinguished and characterized by NSE. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z48.00013: Effector CD8$^+$ T cells migrate via chemokine-enhanced generalized L\'evy walks Edward Banigan, Tajie Harris, David Christian, Andrea Liu, Christopher Hunter Chemokines play a central role in regulating processes essential to the immune function of T cells, such as their migration within lymphoid tissues and targeting of pathogens in sites of inflammation. In order to understand the role of the chemokine CXCL10 during chronic infection by the parasite {\it T. gondii}, we analyze tracks of migrating CD8$^+$ T cells in brain tissue. Surprisingly, we find that T cell motility is not described by a Brownian walk, but instead is consistent with a generalized L\'evy walk consisting of L\'evy-distributed runs alternating with pauses of L\'evy-distributed durations. According to our model, this enables T cells to find rare targets more than an order of magnitude more efficiently than Brownian random walkers. The chemokine CXCL10 increases the migration speed without changing the character of the walk statistics. Thus, CD8$^+$ T cells use an efficient search strategy to facilitate an effective immune response, and CXCL10 aids them in shortening the average time to find rare targets. [Preview Abstract] |
Session Z50: Focus Session: Micro and Nano Fluidics IV: Emulsions and Complex Fluids
Sponsoring Units: DPOLY DFDChair: Laura Adams, Harvard University
Room: 162B
Friday, March 2, 2012 11:15AM - 11:51AM |
Z50.00001: Active Emulsions: Synchronization of Chemical Oscillators Invited Speaker: Seth Fraden We explore the dynamical behavior of emulsions consisting of nanoliter volume droplets of the oscillatory Belousov-Zhabotinsky (BZ) reaction separated by a continuous oil phase. Some of the aqueous BZ reactants partition into the oil leading to chemical coupling of the drops. We use microfluidics to vary the size, composition and topology of the drops in 1D and 2D. Addition of a light sensitive catalyst to the drops and illumination with a computer projector allows each drop to be individually perturbed. A variety of synchronous regimes are found that systematically vary with the coupling strength and whether coupling is dominated by activatory or inhibitory species. In 1D we observe in- and anti-phase oscillations, stationary Turing patterns in which drops stop oscillating, but form spatially periodic patterns of drops in the oxidized and reduced states, and more complex combinations of stationary and oscillatory drops. In 2D, the attractors are more complex and vary with network topology and coupling strength. For hexagonal lattices as a function of increasing coupling strength we observe right and left handed rotating oscillations, mixed oscillatory and Turing states and finally full Turing states. Reaction -- diffusion models based on a simplified description of the BZ chemistry and diffusion of messenger species reproduce a number of the experimental results. For a range of parameters, a simplified phase oscillator model provides an intuitive understanding of the complex synchronization patterns. \\[4pt] ``Coupled oscillations in a 1D emulsion of Belousov--Zhabotinsky droplets,'' Jorge Delgado, Ning Li, Marcin Leda, Hector O. Gonzalez-Ochoa, Seth Fraden and Irving R. Epstein, \textit{Soft Matter}, \textbf{7}, 3155 (2011). [Preview Abstract] |
Friday, March 2, 2012 11:51AM - 12:03PM |
Z50.00002: Double Emulsion Templated Celloidosomes Laura R. Arriaga, Samantha M. Marquez, Shin-Hyun Kim, Connie Chang, Jim Wilking, Francisco Monroy, Manuel Marquez, David A. Weitz We present a novel approach for fabricating celloidosomes{\textregistered}, which represent a hollow and spherical three-dimensional self-assembly of living cells encapsulating an aqueous core. Glass- capillary microfluidics is used to generate monodisperse water-in-oil-in-water double emulsion templates using lipids as stabilizers. Such templates allow for obtaining single but also double concentric celloidosomes. In addition, after a solvent removal step the double emulsion templates turn into monodisperse lipid vesicles, whose membrane spontaneously phase separates when choosing the adequate lipid composition, providing the adequate scaffold for fabricating Janus-celloidosomes. These structures may find applications in the development of bioreactors in which the synergistic effects of two different types of cells selectively adsorbed on one of the vesicle hemispheres may be exploited. [Preview Abstract] |
Friday, March 2, 2012 12:03PM - 12:15PM |
Z50.00003: Viscoelastic Flow Instabilities of Worm-like Micellar Solutions in Microfluidic Devices Thomas Ober, Gareth McKinley Worm-like micellar (WLM) fluids are a unique class of complex fluids whose large deformation rate rheology is not fully understood. By combining mechanical pressure measurements, $\mu $-PIV and spatially-resolved measurements of flow-induced birefringence, we study the behavior of WLM solutions undergoing large deformation rates in microfluidic rectilinear and converging geometries, whose small characteristic dimensions facilitate experiments at high elasticity number (i.e. low inertia). In our experiments, we observe the extensional flow of a shear-banding WLM fluid in a planar hyperbolic contraction. We classify the flow regimes and observe the onset of spatio-temporally unsteady flow often referred to as ``elastic turbulence.'' We use pressure drop measurements to calculate the apparent extensional viscosity of both Newtonian fluids and WLM fluids. We also investigate the onset of elastically driven instabilities in flows nominally without streamwise curvature in a high aspect ratio straight channel. These latter experiments are aimed at determining if elastically-driven turbulence in dilute polymer solutions can be initiated and sustained in pressure-driven rectilinear flows. [Preview Abstract] |
Friday, March 2, 2012 12:15PM - 12:27PM |
Z50.00004: Elastic instability in straight channels Annie Colin, Hugues Bodiguel, Julien Beaumont Polymer solutions exhibit purely elastic flow instabilities even in the absence of inertia. The almost ubiquitous ingredient of such an elastic instability is the curvature of streamlines: polymers that have been extended along curved streamlines are taken by fluctuations across shear rate gradient in the unperturbed state which, in turn, couples the hoop stresses acting along the curved streamlines to the radial and axial flows and amplifies the perturbation. It has been tacitly assumed for over 30 years that in line with this instability scenario, visco-elastic parallel shear flows are stable, since the streamlines are straight. Recently, Saarloos and coworkers [1] derived a general instability criterion, which shows that these flows invariably exhibit a nonlinear instability. At this stage only a few studies support and validate this analysis [2]. In this work, we take advantage of microfluidic devices and study the flow of highly elastic polymers and surfactant solutions in a straight microchannel located after a constriction. The velocity of the solution is measured using Particle Imaging Velocimetry. The amplitude of the perturbation is controlled by the shape of the constriction. Using such devices, we present a comprehensive flow diagram in the parameter plane amplitude of the perturbation, Weissenberg number. 1/ Bernard Meulenbroek at all J. Non-Newtonian Fluid Mech. 116 (2004) 235--268 2/~Daniel Bonn et all Phys Rev E 84, 045301(R) (2011) [Preview Abstract] |
Friday, March 2, 2012 12:27PM - 12:39PM |
Z50.00005: Microfluidic Fabrication of Functional Capsules with ultra-thin membranes Shin-Hyun Kim, Alireza Abbaspourrad, David Weitz We have developed a new emulsification technique to produce monodisperse double-emulsion drops with an ultra-thin middle layer through a one-step emulsification. A biphasic flow, consisting of sheath of one fluid flowing along the capillary wall and surrounding a second fluid flowing through center of the capillary, is created in a form of either a jet or drops, which is emulsified into double-emulsion drops with ultra-thin middle layer. The ultra-thin middle phases provide stability to the double-emulsion drops by putting the fluid in the middle phase in the lubrication regime. We have employed such stable double-emulsion drops to make functional microcapsules using evaporation-induced consolidation. Simplest form is microcapsules with homogenous membrane. Using biodegradable polymers such as PLA or PLGA as a membrane material, we can achieve a long-term release of various bioactives from the capsules as the membrane degrades by hydrolysis. Heterogeneous membrane can also be prepared by using polymer blends. For example, a polymer blend of PMMA and PLA with small interaction parameter makes heterogeneous structure at nanoscale, while a polymer blend of PS and PLA with large interaction parameter makes their phase separation at one micrometer scale. [Preview Abstract] |
Friday, March 2, 2012 12:39PM - 12:51PM |
Z50.00006: Coupled oscillations in a 1D emulsion of Belousov--Zhabotinsky droplets Ning Li, Jorge Delgado, Hector Gonzalez Ochoa, Marcin Leda, Seth Fraden, Irving Epstein We experimentally and computationally study the dynamics of interacting oscillating Belousov--Zhabotinsky (BZ) droplets of 100 micron diameter separated by perfluorinated oil and arranged in a one-dimensional array. A microfluidic chip is used for mixing the BZ reactants, forming monodisperse droplets by flow-focusing and directing them into a hydrophobized 100 micron diameter capillary. In order to make quantitative comparison with theory, we use photosensitive Ru(bipy)$_{3}$ catalyzed BZ droplets and set both boundary and initial conditions of arrays of small numbers of oscillating BZ droplets with a programmable illumination source. The coupling strength is a function of malonic acid concentration and varying coupling strength leads to the generation of different dynamical attractors. In many cases, simulations agree well with experiments. [Preview Abstract] |
Friday, March 2, 2012 12:51PM - 1:03PM |
Z50.00007: Double emulsion templated monodisperse phospholipid liposomes incorporating Doxorubicin hydrochloride Mingtan Hai, David Weitz We present a novel approach for fabricating monodisperse phospholipid liposomes incorporating water soluble anticancer drug Doxorubicin hydrochloride using controlled w/o/w double emulsions as templates. Glass-capillary microfluidics is used to generate monodisperse w/o/w double emulsion templates and double emulsion droplet size is from 20 to 100 um according to different flow rates. We show that the high uniformity in size and shape of the templates are maintained in the final phospholipid liposomes after a solvent removal step by Nikon eclipse microscopy. The lipid bilayers encapsulating anticancer drug inside is retained after the emulsion drops are converted to vesicles. The liposomes vesicles are promising water soluble anticancer drug delivery vehicles. [Preview Abstract] |
Friday, March 2, 2012 1:03PM - 1:15PM |
Z50.00008: Drops of Yield-Stress Liquid Impacting a Solid Surface Qin Xu, Heinrich Jaeger We use high-speed video to investigate the drop impact process for yield-stress fluids under different initial conditions. Unlike Newtonian fluids, the impact dynamics of yield-stress liquids are greatly affected by the their viscoelasticity, which can be attributed to either a surface stress or bulk material properties. To explore these two different mechanisms, we perform impact experiments for two model fluids: liquid metals and particle suspensions, which both exhibit significant yield-stress in rheology. By controlling surface oxidation (for liquid metals) and packing density (for suspensions), we quantitatively vary the yield-stress within several orders of magnitude. In this way, we draw a direct comparison between the two fluids at various impact velocities to clarify the role of different sources of yield stress. Also, we build up an approach to bridge impact dynamics with rheological measurements. [Preview Abstract] |
Friday, March 2, 2012 1:15PM - 1:27PM |
Z50.00009: Oscillatory shear of non-colloidal fiber suspensions Alexandre Franceschini, Emmanouela Filippidi, Elizabeth Guazzelli, David Pine Concentrated suspensions of non-colloidal fibers under slow periodic strain undergo a phase transition from an absorbing state to an active fluctating state. Fiber trajectories are reversible in the absorbing state and irreversible in the fluctuating state. The activity, measured by the translational diffusivity between successive periods, vanishes in the absorbing state but reaches a finite value in the fluctuating state. We show that the transition is controlled by a collision cross-section, which is a function of the strain amplitude, concentration and fiber orientation. Over the course of an experiment, the activity drives the orientation toward the vorticity, subsequently reducing the cross section. We evaluate the influence of the control parameter decay on the phase transition and then focus on the fluctuating state dynamics. [Preview Abstract] |
Friday, March 2, 2012 1:27PM - 1:39PM |
Z50.00010: Dynamics in Semidilute Rod Suspensions Pramukta Kumar, Dan Blair, Jeffrey Urbach While shear-thinning in semidilute suspensions of rod-like particles has been widely observed, the underlying mechanisms are often unclear. We have developed a model system of fluorescent SU-8 rods suspended in a Glycerol/Ethylene-Glycol solution. This model system exhibits an order of magnitude difference in apparent viscosity at low shear rates as compared to high shear rates while showing no discernible difference in structure. Using a coupled confocal microscope and rheometer instrument along with fiber identification and particle tracking routines, we directly image and quantify the 3D structure and dynamics of our model system under shear flow in order to determine how particle interactions could be generating the observed shear thinning. In particular we look at how interactions modify Jeffery's orbits in semidilute suspensions as compared to the motion of an isolated rod or ellipsoid. [Preview Abstract] |
Friday, March 2, 2012 1:39PM - 1:51PM |
Z50.00011: The filed effect on local temperature distribution in natural convection in a magnetic fluid Jun Huang, Weili Luo Previously, we have reported the magnetic field effect on the flow front of natural convection in magnetic fluids. In this work we present the local temperature distribution as a function of applied field, from which flow patterns are constructed. We will also report, and discuss the possible mechanism for, a crossover field, above which the field dependence diminishes. [Preview Abstract] |
Friday, March 2, 2012 1:51PM - 2:03PM |
Z50.00012: Mixing dynamics of slurry in rotating drum Kiwing To, Chun Chung Liao, Shu-San Hsiau We study the effects of interstitial fluid viscosity on the rates of dynamical processes in a thin rotating drum half-filled with monodisperse glass beads. The rotating speed is fixed at the rolling regime such that a continuously flowing layer of beads persists at the free surface. While the characteristic speed of a bead in the flowing layer decreases with the fluid viscosity, the mixing rate of the beads is found to increase with the fluid viscosity. These findings are consistent to a simple model related to the thickness of the flowing layer. [Preview Abstract] |
Friday, March 2, 2012 2:03PM - 2:15PM |
Z50.00013: Bubble production using a Non-Newtonian fluid in microfluidic flow focusing device Yi-Lin Wang, Thomas Ward, Christine Grant We experimentally study the production of micrometer-sized bubbles using microfluidic technology and a flow-focusing geometry. Bubbles are produced by using a mixture containing aqueous polyacrylamide of concentrations ranging from 0.01-0.10\% by weight and several solution also containing a sodium-lauryl-sulfate (SLS) surfactant at concentrations ranging 0.01-0.1\% by weight. The fluids are driven by controlling the static pressure above a hydrostatic head of the liquid while the disperse phase fluid static pressure is held constant (air). In the absence of surfactant the bubble production is discontinuous. The addition of surfactant stabilizes the bubble production. In each type of experiment, the bubble length $\ell$, velocity U and production frequency $\omega$ are measured and compared as a function of the inlet pressure ratio. The bubbles exhibit a contraction in their downstream length as a function of the polymer concentration which is investigated. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700