Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session X1: Quantum and Classical Phenomena in Josephson Junction Arrays
Sponsoring Units: DCMPChair: John Clarke, University of California, Berkeley
Room: Ballroom A1
Thursday, March 24, 2011 2:30PM - 3:06PM |
X1.00001: Quantum Coherence of the Fluxonium Superconducting Artificial Atom Invited Speaker: Artificial atoms built from superconducting tunnel junctions illustrate the problem of engineering a controllable electrodynamic quantum system, starting from basic elements. Can circuit architecture mitigate or even eliminate coherence limitations due to defects in the basic electrical constituents? This central question will be discussed from the perspective of recent experimental results of our group, obtained on the fluxonium[1], a novel superconducting quantum circuit. It consists of a Cooper-pair box junction which is shunted by a long array of larger junctions. Immunity to offset charge noise and only a weak sensitivity to flux noise is observed for the qubit transition. The combination of the very large inductance of the array, which has negligible parasitic resistance, and large phase fluctuations of the small junction, distinguishes fundamentally the fluxonium from the flux qubit. Significant improvement of the relaxation time has been obtained, when one compares with qubits of the same family. Finally, fluxonium displays the type of 3-level-atom physics which should prove useful for continuous, high-fidelity monitoring of a state. Work supported by the IARPA, ARO and NSF. \\[4pt] [1] V.E. Manucharyan, Jens Koch, L.I. Glazman and M.H. Devoret, Science 326, 113-116 (2009). [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:42PM |
X1.00002: Measurement of Quantum Phase-Slips in Josephson Junction Chains Invited Speaker: Quantum phase-slip dynamics in Josephson junction chains could provide the basis for the realization of a new type of topologically protected qubit [1] or for the implementation of a new current standard [2]. I will present measurements of the effect of quantum phase-slips on the ground state of a Josephson junction chain. We can tune in situ the strength of the phase-slips. These phase-slips are the result of fluctuations induced by the finite charging energy of each junction in the chain. Our measurements demonstrate that a Josephson junction chain under phase bias constraint behaves in a \textit{collective }way [3]. I will also show evidence of coherent phase-slip interference, the so called Aharonov-Casher effect. This phenomenon is the dual of the well known Aharonov-Bohm interference.\\[4pt] In collaboration with I.M. Pop, Institut Neel, C.N.R.S. and Universite Joseph Fourier, BP 166, 38042 Grenoble, France; I. Protopopov, L. D. Landau Institute for Theoretical Physics, Kosygin str. 2, Moscow 119334, Russia and Institut fuer Nanotechnologie, Karlsruher Institut fuer Technologie, 76021 Karlsruhe, Germany; and F. Lecocq, Z. Peng, B. Pannetier, O. Buisson, Institut Neel, C.N.R.S. and Universite Joseph Fourier. \\[4pt] [1] I. M. Pop, O. Buisson, K. Hasselbach, I. Protopopov, W. Guichard and B. Pannetier, Phys. Rev. B, 78, 104504(2008) \\[0pt] [2] W. Guichard and F. Hekking, Phys. Rev. B 81, 064508 (2010) \\[0pt] [3] I. M. Pop, I. Protopopov, F. Lecocq, Z. Peng, B. Pannetier, O. Buisson and W. Guichard, Nature Physics, 6, 589 (2010). [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 4:18PM |
X1.00003: Coherent Terahertz Emission of Intrinsic Josephson Junction Stacks in the Hot Spot Regime Invited Speaker: Having small sized active and tunable devices operating at frequencies up to the Terahertz (THz) range is one of the goals of modern electronics. However, there is still a lack of good active or passive devices, often referred to as the ``Terahertz gap.'' Intrinsic Josephson junctions formed by the layered crystal structure of high temperature superconductors such as Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8}$ have the potential to operate in this regime. While for a long time the research on THz generation with this type of junctions was carried out with perhaps only modest success, recently synchronous emission, with an estimated output power in the $\mu $W range, of stacks consisting of several hundred intrinsic Josephson junctions was achieved [1]. We report on the investigation of THz electromagnetic wave generation in intrinsic junction stacks (mesas) of different geometries, using a combination of transport measurement, direct electromagnetic wave detection and Low Temperature Scanning Laser Microscopy [2,3]. At high enough input power a hot spot (a region heated to above the superconducting transition temperature) coexists with regions being still in the superconducting state. In the ``cold'' regions cavity resonances can occur, synchronizing the ac Josephson currents and giving rise to strong and stable coherent THz emission. We discuss possible scenarios of the hot spot/wave interaction and its relation to the generation of coherent THz radiation. \\[4pt] [1] L. Ozyuzer, et al., Science \textbf{318}, 1291 (2007). \\[0pt] [2] H.~B. Wang, et al., Phys. Rev. Lett. \textbf{102}, 017006 (2009). \\[0pt] [3] H. B. Wang, et al., Phys. Rev. Lett. \textbf{105}, 057002 (2010). [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:54PM |
X1.00004: Vortex pinning in ferromagnet-superconductor bilayer with tunable domain patterns Invited Speaker: Ferromagnet superconductor hybrids provide a fascinating example of systems in which there is a rich interplay between two seemingly incompatible collective phenomena. Particularly interesting is the impact of the ferromagnet on the dynamics of vortices in the superconductor. The magnetic domains control the location of the vortices. Exquisite control of the dynamics can be achieved by careful tuning of the geometry of the magnetic domains. In this talk I will present the results of recent experiments on superconductor(S)-ferromagnet(F) bilayers with a focus on understanding the hitherto unexplained seemingly unpredictable dependence of the critical current density on the parameters of the experiment. In our experiments the S layer is made of niobium, the F layer is a Co/Pt multilayer with perpendicular magnetic anisotropy, and a thin insulating layer in-between eliminates proximity effect. We use various demagnetization procedures to define different domain patterns in the F layer. We show that some domain patterns produce highly inhomogeneous flux penetration and strong vortex confinement at the sample edge, while for others there is remerkable enhancement of the critical current density in excess of 15. This is the highest value reported to date. We have measured, for the first time in a single tunable structure, the dependence of the activation energy for vortex pinning on the domain width, temperature, and magnetic field. In collaboration with L.Y. Zhu, X. M. Cheng and C. L. Chien (Johns Hopkins), Z. Adamus (Polish Acad. Sci.) and M. Konczykowski (Ecole Polytechnique). [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:30PM |
X1.00005: Tuning superconductivity by carrier injection Invited Speaker: All high-T$_c$ cuprates are stacking sequences of CuO$_2$ layers and charge reservoir layers consisting of metal oxides. Upon doping the CuO$_2$ layers, antiferromagnetic order is destroyed and metallic conductivity is established. Usually doping is achieved by a non-stoichiometric composition of the charge reservoir layer. However, we already have shown that we can change the carrier concentration of Bi$_2$Sr$_2$CaCu$_2$O$_ {8+\delta}$ single crystals by current injection along the c- axis [1]. Critical temperature, c-axis resistivity and critical current of intrinsic Josephson junctions can be tuned in a large range from underdoping to extreme overdoping. This effect is persistent up to annealing temperatures of approximately 270 K. Using current injection at higher bias, we were able to reduce the carrier concentration again. We investigated in detail the superconducting properties by performing macroscopic quantum tunneling experiments of intrinsic Josephson junctions. The experiments have been carried out repeatedly on samples, whose properties were changed only by current injection. An exponential increase of the critical current density with hole concentration was observed. At the same time, the capacitance of intrinsic Josephson junctions increased significantly. Finally, only by current injection, we were able to convert into the superconducting state a nonsuperconducting, oxygen depleted sample. This work was done in collaboration with Y. Koval, X.Y. Jin, S. Probst, Y. Simsek, C. Steiner (Universit\"at Erlangen), H. B. Wang (NIMS, Tsukuba), and G. Behr, B. B\"uchner (IFW Dresden). \\[4pt] [1] Y. Koval, X.Y. Jin, C. Bergmann, Y. Simsek, L. \"Ozy\"uzer, P. M\"uller, H. B. Wang, G. Behr, B. B\"uchner, Appl. Phys. Lett. \textbf{96}, 082507 (2010). [Preview Abstract] |
Session X2: Coexistence Between Antiferromagnetism and Superconductivity in Fe-pnictides
Sponsoring Units: DCMPChair: Joerg Schmailian, Iowa State University
Room: Ballroom A2
Thursday, March 24, 2011 2:30PM - 3:06PM |
X2.00001: Competing phases in the iron pnictides Invited Speaker: In this work, we present a theoretical model that consistently describes the interplay between the magnetic, elastic, and superconducting degrees of freedom of the iron pnictides, comparing our results to several experimental observations. First, we show that the outcome of the competition between the antiferromagnetic (AFM) and the superconducting (SC) order depends on the symmetry of the pairing state. In particular, we demonstrate that a conventional phonon-mediated superconducting state cannot coexist microscopically with the itinerant magnetic phase [1,2]. We also show that the magneto-elastic coupling in these materials is mediated by Ising-nematic degrees of freedom, which emerge from the degeneracy of the magnetic ground state. As a result, in the tetragonal phase, nematic fluctuations lead to the \emph{softening} of the lattice in the normal state but to its \emph{hardening} in the SC state, due to the competition between SC and AFM [3]. Accordingly, in the orthorhombic phase, nematic order is suppressed below the SC transition temperature, causing the suppression of the orthorhombic order parameter [4].\\[4pt] [1] R. M. Fernandes \emph{et al.}, Phys. Rev. B \textbf{81}, 140501(R) (2010).\\[0pt] [2] R. M. Fernandes and J. Schmalian, Phys. Rev. B \textbf{82}, 014521 (2010).\\[0pt] [3] R. M. Fernandes \emph{et al.}, Phys. Rev. Lett. \textbf{105}, 157003 (2010).\\[0pt] [4] S. Nandi \emph{et al.}, Phys. Rev. Lett. \textbf{104}, 057006 (2010). [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:42PM |
X2.00002: Superconductivity and Magnetism in iron-pnictides: co-existence or not? Invited Speaker: In this talk I will review the weak-coupling approach to describe the interplay of two electronic orders: superconductivity (SC) in the form of Cooper pairs, and magnetism in the form of the spin-density waves (SDW). The two orders, traditionally thought as incompatible, are close neighbors in magnetically-active Fe-based superconductors with surprisingly high $T_c$. Complex multi-band structure, multiple interactions and many families of these materials create a range of possible states of mingling between superconductivity and magnetism. I will present a list of different parameters, including (a) the Fermi surface shape, (b) the order parameter structure, (c) the relative strength of SC and SDW interactions, (d) the external magnetic field, and describe which properties, or their combinations, lead to co-existence or avoidance of SC and SDW orders, and how transition between the two orders occurs upon doping. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 4:18PM |
X2.00003: Superconductivity and magnetism in 111 iron pnictides Invited Speaker: We study different 111 materials at ultra low temperatures by means of angle-resolved photoemission spectroscopy (ARPES). The measurements provide a direct access to the information on the low energy electronic structure, which includes the detailed knowledge of the Fermi surface, band renormalization, electronic self-energy and symmetry of the superconducting order parameter. The results suggest a direct correlation between the fermiology and fundamental physical properties throughout the phase diagram of 111 iron superconductors. In particular, the Van Hove singularity is identified as playing a primary role for the superconductivity. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:54PM |
X2.00004: Antiferromagnetic phase in iron-based superconductors: selection of magnetic order, spin excitations, competition with superconductivity Invited Speaker: Recent discovery of superconductivity in the iron-based layered pnictides with Tc ranging between 26 and 56K generated enormous interest in the physics of these materials. The superconductivity has been discovered in oxygen containing RFeAsO (R=La, Nd, Sm) as well as in oxygen free AFe2As2 (A=Ba, Sr, Ca). Like the cuprates, the pnictides are quasi-two-dimensional systems, their parent material shows antiferromagnetic long-range order below 150K and superconductivity occurs upon doping of either electrons or holes into the FeAs layers. In my talk I will analyze the properties of the magnetically ordered state. In particular, I will discuss the selection of the stripe magnetic order in the unfolded BZ within itinerant description. Selecting one hole and two electron pockets we find that SDW order is highly degenerate if electron pockets are circular and interactions involved are between holes and electrons only. Repulsive charge interactions between two electrons as well as ellipticity of the electron pockets break the degeneracy and select metallic (0, $\pi )$ [($\pi $,0)] SDW state in the unfolded BZ -- the same order as seen in the experiments. I will argue that the SDW state remains a metal even for the case of a perfect nesting because one combination of the two hole operators and one combination of two electron operators decouple from the SDW mixing. We also demonstrate that the quasi-one-dimensional nanostructure identified in the quasiparticle interference (QPI) is a consequence of the interplay of the magnetic (\textit{$\pi $}, 0) spin-density wave (SDW) order with the underlying electronic structure. Finally, we address the salient experimental features of the magnetic excitations in the spin-density-wave phase of iron-based superconductors. We use a multiband random-phase approximation treatment of the dynamical spin susceptibility. Weakly damped spin waves are found near the ordering momentum and it is shown how they dissolve into the particle-hole continuum. We show that ellipticity of the electron bands accounts for the anisotropy of the spin waves along different crystallographic directions and the spectral gap at the momentum conjugated to the ordering one. *Work done with A. V. Chubukov, J. Knolle, R. Moessner, and A. Akbari. \\[4pt] [1] J. Knolle, I. Eremin, A. Akbari, and R. Moessner, Phys. Rev. Lett. 104, 257001 (2010). \\[0pt] [2] J. Knolle, I. Eremin, A.V. Chubukov, and R. Moessner, Phys. Rev. B 81, 140506(R) (2010) \\[0pt] [3] I. Eremin and A.V. Chubukov, Phys. Rev. B 81, 024511 (2010) [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:30PM |
X2.00005: Optical Investigations of the Superconducting State in 122 Iron-Pnictides Invited Speaker: The new high-Tc iron pnictide superconductors have a pronounced multiband character, which complicates the electronic properties and allows for a variety of possible superconducting ground states. We have used the infrared spectroscopy--one of the most powerful tools to investigate the low-energy electrodynamic properties of superconductors--to study several pnictide families. We made a comparison between them with the aim to answer the following questions: (1) Is it possible to have more than one superconducting gap in iron pnictide? (2) Can their order parameters be distinct from each other? (3) How does the coupling between different bands influence the gap? (4) Do the gaps have a three-dimensional character? (5) Is the gap scenario universal for all the iron pnictides? We could show that the pairing condition depends sensitively on the similarity of geometry and dimension between hole and electron Fermi-surfaces. [Preview Abstract] |
Session X3: Topological Vortices in Magnets, Ferroelectrics, and Multiferroics
Sponsoring Units: DCMPChair: Sang-Wook Cheong, Rutgers University
Room: Ballroom A3
Thursday, March 24, 2011 2:30PM - 3:06PM |
X3.00001: Magnetic vortices: From a “hidden parameter” to novel switching modes Invited Speaker: This abstract not available. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:42PM |
X3.00002: Skyrmion Lattices in Chiral Magnets Invited Speaker: Skyrmions are topologically stable field configurations with particle-like properties. Using neutron scattering and measurements of the Hall effect we identified the formation of two-dimensional lattices of skyrmion lines, a new form of magnetic order, in metallic and semiconducting B20 compounds, namely MnSi [1,2], Mn$_{1-x}$Co$_x$Si [3], Mn$_{1-x}$Fe$_x$Si [3] and Fe$_{1-x}$Co$_x$Si [4]. The existence of individual skyrmions and skyrmion lattices has recently been confirmed by Lorentz force microscopy for Fe$_{1-x}$Co$_x$Si ($x=0.5$) [5]. The skyrmion lattices in chiral magnets share remarkable similarities with vortex lattices in type II superconductors -- they may be understood as vortex lattices of transverse spin supercurrents exhibiting domain formation and complex morphologies. Our studies establish magnetic materials lacking inversion symmetry as an arena for new forms of order composed of topologically stable spin configurations. \\[4pt] [1] S. M{\"u}hlbauer, et al. Science \textbf{323}, 915 (2009).\\[0pt] [2] A. Neubauer, et al. Phys. Rev. Lett. \textbf{102}, 186602 (2009).\\[0pt] [3] C. Pfleiderer, et al. J. Phys. Cond. Matter \textbf{22}, 164207 (2010).\\[0pt] [4] W. M{\"u}nzer, et al. Phys. Rev. B (R) \textbf{81}, (2010).\\[0pt] [5] X. Z. Yu, et al. Nature \textbf{465}, 901 (2010); C. Pfleiderer, A. Rosch, Nature (N\&V) \textbf{465}, 880 (2010). [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 4:18PM |
X3.00003: Skyrmion crystal and topological Hall effect in B20-type transition-metal compounds Invited Speaker: Topological objects in solids such as domain walls and vortices have been attracting much attention for long. Among them the spin texture called skyrmion is an unusual topological object, in which the spins point in all the directions wrapping a sphere. The skyrmion hosts finite spin chirality, and therefore is anticipated to induce novel electromagnetic phenomena such as topological Hall effect. In B20-type transition metal compounds MnSi and Fe$_{1-x}$Co$_x$Si, the crystallization of skyrmions was observed by the neutron diffraction studies.\footnote{S. M\"{u}hlbauer {\it et al.,} Science {\bf 323,} 915 (2009).}$^,$\footnote{W. M\"{u}nzer {\it et al.,} Phys. Rev. B {\bf 81,} 041203 (2010).} Recently, we have observed the real-space images of skyrmion crystal in thin films of related compounds (Fe$_{0.5} $Co$_{0.5}$Si and FeGe) using Lorentz transmission electron spectroscopy.\footnote{X. Z. Yu, {\it et al.}, Nature, {\bf 465,} 901 (2010).}$^,$\footnote{X. Z. Yu, {\it et al.,} Nature material, {\it inpress}.} We have observed the hexagonal arrangement of skyrmions including the topological defects (chiral domains and dislocations) under the magnetic field normal to the films, and found that the two dimensional skyrmion crystal phase is fairly stabilized by the thin film form of the samples. We have also studied the topological Hall effect caused by the spin chirality of the skyrmion crystal in a related material MnGe. In terms of the Hall measurement, they have shown the real space nature of the fictitious magnetic field caused by the magnetic configuration of the skyrmion crystal, in contrast with the momentum-space fictitious field in another spin chirality system, Nd$_2$Mo$_2$O$_7$.\footnote{Y. Taguchi {\it et al.,} Science {\bf 291,} 2573 (2001).} This work was done in collaboration with X. Z. Yu, N. Kanazawa, J. H. Park, J. H. Han, K. Kimoto, W. Z. Zhang, S. Ishiwata, Y. Matsui, N. Nagaosa, and Y. Tokura. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:54PM |
X3.00004: Multiferroic vortices in hexagonal manganites Invited Speaker: Hexagonal rare earth manganites (\textit{RE}MnO$_3$) show a unique improper ferroelectricity induced by structural trimerization. Extensive research on these systems has been carried out due to its potential application in memory and the intriguing multiferroicity (coexistence of ferroelectricity and antiferromagnetism). However, the true relationship between ferroelectric domains and structural domains has never been revealed. Using transmission electron microscopy (TEM) and conductive atomic force microscopy (cAFM), we observed an intriguing conductive ``cloverleaf'' pattern of six domains emerging from one point, all distinctly characterized by polarization orientation and structural antiphase relationships in hexagonal manganites.\footnote{T. Choi, et al, ``Insulating interlocked ferroelectric and structural antiphase domain walls in multiferroic YMnO$_3$'' Nature Materials, \textbf{9}, 253-258 (2010).} The nanoscale electric conduction between a sharp tip and the surface is intrinsically modulated by the ferroelectric polarization.\footnote{W. Wu, et al, ``Polarization-Modulated Rectification at Ferroelectric Surfaces'' Phys. Rev. Lett., \textbf{104}, 217601 (2010).} The cloverleaf defects are structural vortices where the phase angle goes successively through all six phases.\footnote{M. Mostovoy, ``a whirlwind of opportunities,'' Nature Materials, \textbf{9}, 188-190 (2010).} In addition, we discovered that the ferroelectric domain walls and structural antiphase boundaries are mutually locked. Correlated with previous observation of coupled ferroelectric and antiferromagnetic domain walls,\footnote{M. Fiebig, et al, ``Observation of coupled magnetic and electric domains,'' Nature, \textbf{419}, 818 (2002).} our results suggest that these cloverleaf defects are indeed multiferroic vortices. These fascinating results reveal the rich physics of the hexagonal system with a semiconducting bandgap where structural trimerization, ferroelectricity, magnetism and charge conduction are intricately coupled. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:30PM |
X3.00005: Ferroelectric vortices from atomistic simulations Invited Speaker: In 2004, the use of a first-principles-based effective Hamiltonian [1] led to the prediction of a novel structure in zero-dimensional ferroelectrics, in which the electric dipoles organize themselves to form a vortex [2]. Such structure exhibits the so-called spontaneous toroidal moment, rather than the spontaneous polarization, as its order parameter [2]. Subsequently, various original phenomena, all related to vortices, were predicted in ferroelectric nanostructures. Examples of such phenomena are: (i) the existence of a new order parameter, denoted as the hypertoroidal moment, that is associated with many complex dipolar structures (such as double-vortex states) [3]; (ii) the possible control of single and double vortex states by electric fields, via the formation of original intermediate states [4-8]; (iii) the discovery of a new class of quantum materials (denoted as incipient ferrotoroidics), for which zero-point vibrations wash out the vortex state and yield a complex local structure [9]; (iv) the existence of chiral patterns of oxygen octahedral tiltings that originate from the coupling of these tiltings with the ferroelectric vortices [10]. The purpose of this talk is to discuss some of these striking phenomena, as well as, to reveal others (if time allows). These studies are done in collaboration with A.R. Akbarzadeh, H. Fu, I. Kornev, I. Naumov, I. Ponomareva, S. Prosandeev, Wei Ren and D. Sichuga. \\[4pt] [1] L. Bellaiche, A. Garcia and D. Vanderbilt, Phys. Rev. Lett. 84, 5427 (2000). \\[0pt] [2] Ivan I. Naumov, L. Bellaiche and Huaxiang Fu, Nature (London) 432, 737 (2004). \\[0pt] [3] S. Prosandeev and L. Bellaiche, Phys. Rev. B 77, 060101(R) (2008). \\[0pt] [4] S. Prosandeev, I. Ponomareva, I. Kornev, I. Naumov and L. Bellaiche, Phys. Rev. Lett. 96, 237601 (2006). \\[0pt] [5] I. Naumov and H. Fu, Phys. Rev. Lett. 98, 077603 (2007). \\[0pt] [6] S. Prosandeev and L. Bellaiche, Phys. Rev. Lett. 101, 097203 (2008). \\[0pt] [7] S. Prosandeev, I. Ponomareva, I. Kornev, and L. Bellaiche, Phys. Rev. Lett. 100, 047201 (2008). \\[0pt] [8] I. Naumov and H. Fu, Phys. Rev. Lett. 101, 197601 (2008). \\[0pt] [9] S. Prosandeev, A. R. Akbarzadeh and L. Bellaiche, Phys. Rev. Lett. 102, 257601(2009). \\[0pt] [10] David Sichuga, Wei Ren, Sergey Prosandeev, and L. Bellaiche, Phys. Rev. Lett. 104, 207603 (2010). [Preview Abstract] |
Session X4: Functional Gels
Sponsoring Units: DPOLYChair: Igal Szleifer, Northwestern University
Room: Ballroom A4
Thursday, March 24, 2011 2:30PM - 3:06PM |
X4.00001: Micromechanical study of mitotic chromosome structure Invited Speaker: Our group has developed micromanipulation techniques for study of the highly compacted mitotic form of chromosome found in eukaryote cells during cell division. Each metaphase chromosome contains two duplicate centimeter-long DNA molecules, folded up by proteins into cylindrical structures several microns in length. Native chromosomes display linear and reversible stretching behavior over a wide range of extensions (up to 5x native length for amphibian chromosomes), described by a Young modulus of about 300 Pa. Studies using DNA-cutting and protein-cutting enzymes have revealed that metaphase chromosomes behave as a network of chromatin fibers held together by protein-based isolated crosslinks. Our results are not consistent with the more classical model of loops of chromatin attached to a protein-based structural organizer or ``scaffold". In short, our experiments indicate that metaphase chromosomes can be considered to be ``gels" of chromatin; the stretching modulus of a whole chromosome is consistent with stretching of the chromatin fibers contained within it. Experiments using topoisomerases suggest that topological constraints may play an appreciable role in confining chromatin in the metaphase chromosome. Finally, recent experiments on human chromosomes will be reviewed, including results of experiments where chromosome-folding proteins are specifically depleted using siRNA methods. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:42PM |
X4.00002: A Molecular View of Liquid Crystalline Elastomers and Gels Invited Speaker: A combination of Monte Carlo and molecular dynamics simulations is used to examine the order-disorder transitions that arise in model liquid crystalline elastomers and colloidal gels as a function of concentration and strain, respectively. Two models are considered. In the first, a lattice model is used to represent a colloidal gel of nematogens and nanoparticles. In the second, a cross-linked elastomer of Gay-Berne mesogens is adopted to examine the order-disroder transition that arises as a function of strain. The results of simulations are compared to those of recent experiments for these two classes of systems. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 4:18PM |
X4.00003: Gelation of mucin: Protecting the stomach from autodigestion Invited Speaker: In this talk I will describe the molecular mechanisms involved in the remarkable ability of the mucus lining of the stomach for protecting the stomach from being digested by the acidic gastric juices that it secretes. These physical properties can be attributed to the presence of a high molecular weight glycoprotein found in mucus, called mucin. Rheology and other measurements show that gastric mucin forms a gel under acidic pH. A model of gelation based on the interplay of hydrophobic and electrostatic interactions will be discussed. Molecular Dynamics simulation studies of folding and aggregation of mucin domains provide further support for this model. The relevance of gelation to the motion of the ulcer causing bacterium H. pylori will be discussed. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:54PM |
X4.00004: Responsive gels and membranes Invited Speaker: We use computer simulations and analytical method to explore and analyze effects of composition heterogeneities in elastic membranes and gels. In particular, we focus on shape pattern formation in thin networks and elastic closed membranes driven by the presence of composition heterogeneities. We study the crumpling of multicomponent elastic membranes in response to changes in external conditions, as well as the spontaneous buckling transition of heterogeneous elastic shells into regular and irregular polyhedra. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:30PM |
X4.00005: ABSTRACT WITHDRAWN |
Session X5: The Corporate Feel: Atomic Force Microscopy in Industry
Sponsoring Units: FIAPChair: Jason Cleveland, Asylum Research
Room: Ballroom C1
Thursday, March 24, 2011 2:30PM - 3:06PM |
X5.00001: Accelerated design and quality control of impact modifiers for plastics through atomic force microscopy (AFM) analysis Invited Speaker: Standard polymer resins are often too brittle or do not meet other mechanical property requirements for typical polymer applications. To achieve desired properties it is common to disperse so called ``impact modifiers'', which are spherical latex particles with diameters of much less than one micrometer, into the pure resin. Understanding and control of the entire process from latex particle formation to subsequent dispersion into polymer resins are necessary to accelerate the development of new materials that meet specific application requirements. In this work AFM imaging and nanoindentation techniques in combination with AFM-based spectroscopic techniques were applied to assess latex formation and dispersion. The size and size distribution of the latex particles can be measured based on AFM amplitude modulation images. AFM phase images provide information about the chemical homogeneity of individual particles. Nanoindentation may be used to estimate their elastic and viscoelastic properties. Proprietary creep and nanoscale Dynamic Mechanical Analysis (DMA) tests that we have developed were used to measure these mechanical properties. The small size of dispersed latex inclusions requires local mechanical and spectroscopic analysis techniques with high lateral and spatial resolution. We applied the CRAVE AFM method, developed at NIST, to perform mechanical analysis of individual latex inclusions and compared results with those obtained using nanoscale DMA. NanoIR, developed by Anasys Inc., and principal component confocal Raman were used for spectroscopic analysis and results from both techniques compared. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:42PM |
X5.00002: Scanning Probe Evaluation of Electronic, Mechanical and Structural Material Properties Invited Speaker: We present atomic force microscopy (AFM) studies of a range of properties from three different classes of materials: mixed ionic electronic conductors, low-k dielectrics, and polymer-coated magnetic nanoparticles. (1) Mixed ionic electronic conductors are being investigated as novel diodes to drive phase-change memory elements. Their current-voltage characteristics are measured with direct-current and pulsed-mode conductive AFM (C-AFM). The challenges to reliability of the C-AFM method include the electrical integrity of the probe, the sample and the contacts, and the minimization of path capacitance. The role of C-AFM in the optimization of these electro-active materials will be presented. (2) Low dielectric constant (low-k) materials are used in microprocessors as interlayer insulators, a role directly affected by their mechanical performance. The mechanical properties of nanoporous silicate low-k thin films are investigated in a comparative study of nanomechanics measured by AFM and by traditional nanoindentation. Both methods are still undergoing refinement as reliable analytical tools for determining nanomechanical properties. We will focus on AFM, the faster of the two methods, and its developmental challenges of probe shape, cantilever force constant, machine compliance and calibration standards. (3) Magnetic nanoparticles are being explored for their use in patterned media for magnetic storage. Current methods for visualizing the core-shell structure of polymer-coated magnetic nanoparticles include dye-staining the polymer shell to provide contrast in transmission electron microscopy. AFM-based fast force-volume measurements provide direct visualization of the hard metal oxide core within the soft polymer shell based on structural property differences. In particular, the monitoring of adhesion and deformation between the AFM tip and the nanoparticle, particle-by-particle, provides a reliable qualitative tool to visualize core-shell contrast without the use of additional contrast enhancing agents. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 4:18PM |
X5.00003: Nanomechanical characterization of polypropylene-based materials with multifrequency atomic force microscopy (AFM)-based methods Invited Speaker: Atomic force microscopy (AFM) is a powerful technique with broad applications to characterization of surfaces, primarily used for nanoscale quantitative topographic measurements and qualitatively distinguishing between material properties on the surface. We describe recent advances in our capabilities to quantify nanoscale mechanical measurements of surface properties using recently developed high frequency and multifrequency methods. Initial focus of this work is for polymeric materials (and specifically polypropylene based blends), where nanomechanical characterization is critical for effective understanding of structure-property relationships, especially for more complicated multi-component materials such as blends and composites. SPM techniques rely on complicated tip-sample interactions that must be effectively separated and understood if we are to ultimately identify and quantify specific materials and material properties at the nanoscale. We describe different approaches to this problem utilizing a number of AFM based techniques including force curves, bimodal imaging and contact resonance imaging. Ultimately, these techniques yield quantitative maps of conservative and dissipative tip-sample interactions that are then converted into elastic and viscous moduli maps. We describe initial applications of these methods to measure mechanical properties such as storage and loss moduli of model polypropylene containing blends including polypropylene/rubber and polypropylene/polystyrene blends. Finally, quantitative moduli values obtained by methods described above are compared to those obtained by bulk methods. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:54PM |
X5.00004: Probing Photovoltaic Performance Invited Speaker: A wide range of nanostructured materials including organic bulk heterojunction blends, solution processed colloidal semiconductors, and hybrid organic/inorganic thin films are being explored for solar energy applications. These sytems typically exhibit nanoscale heterogenity in their electronic and optical properties. Scanning probes are critical for building a microscopic picture of the performance of new nanostructured and thin film photovoltaic materials--and may ultimately prove to be a valuable metrology tool for process control during production--because scanning probe microscopy provides a unique opportunity to correlate local charge generation, recombination and transport with local structure in these systems. In this talk I will focus on techniques developed and lessons learned during our group's study of thin film solar cell materials with a particular emphasis on nanostructured organic bulk heterojunction blends. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:30PM |
X5.00005: Challenges and opportunities for probe-based information technology Invited Speaker: Scanning probe microscopes have become standard tools for characterization of materials and devices at the nanoscale. But what about ``OEM'' versions for information technology? The standard answer is that probe-based lithography or storage is not practical because it cannot scale-their cost and complexity will never allow useful devices to be made with probes. Such was not always the conventional wisdom in the industrial community. The Millipede Project,\footnote{http://www.zurich.ibm.com/st/storage/concept.html} pioneered by Gerd Binnig at IBM and pursued at other companies such as Hitachi and Seagate, sought to scale the number of probes to $\sim$1000. In fact, they were successful, but not enough to be competitive with FLASH memory. Since then, order of magnitude improvements have been made both in scaling up to the number of probes past ten million,\footnote{F. Huo et al, Science ${\bf 321}$, 1658 (2008).} and in scaling down the minimum bit size below two nanometers.\footnote{C. Cen et al, Science, ${\bf 323}$, 1026 (2009).} Combining these two approaches may well justify the statement: ``There's plenty of room for probes at the bottom.'' [Preview Abstract] |
Session X6: Pairing in Imbalanced Fermi Mixtures
Sponsoring Units: DCOMP DAMOPChair: Richard Scalettar, University of California, Davis
Room: Ballroom C2
Thursday, March 24, 2011 2:30PM - 3:06PM |
X6.00001: Correlated phases in the Fermi Hubbard model with spin and mass imbalance Invited Speaker: One dimensional attractive fermions with unequal spin populations provide a direct realization of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluid state, in which Cooper pairs condense at finite momentum. The presence of an additional mass asymmetry between the two components gives rise to multi-particle bound states, including trimers made of one light and two heavy fermions. I first discuss the stability of these bound states through the exact solution of the three-body problem. Based on Density Matrix Renormalization Group simulations and bosonization theory, I then show that at finite and commensurate densities the ground state of the system is a Luttinger liquid of trimers. In this new phase superconducting FFLO correlations are exponentially suppressed. Finally I explain how the mass asymmetry changes the topology of the grand-canonical phase diagram of the Fermi Hubbard model. [GO, E. Burovski and T. Jolicoeur, PRL 104, 065301 (2010); E. Burovski, GO and T. Jolicoeur, PRL 103, 215301 (2009)] [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:42PM |
X6.00002: The phase diagram of imbalanced Fermi gases Invited Speaker: Recent experimental and theoretical research has focused on the phases of strongly interacting Fermi gases under an imposed population imbalance between the fermion species. The large difference in chemical potential between the majority and minority species disrupts conventional singlet s-wave pairing, yielding a rich phase diagram including regions of phase separation, Fulde-Ferrell-Larkin-Ovchinnikov superfluidity and magnetic superfluidity. I will discsuss these predicted phases, as well as the behavior at large imbalance where the minority species can induce an effective attraction among the majority fermions and a resulting instability towards p-wave superfluidity. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 4:18PM |
X6.00003: Pair formation in Fermi systems with population imbalance in one- and two-dimensional optical lattices Invited Speaker: I will discuss pairing in fermionic systems in one- and two-dimensional optical lattices with population imbalance. This will be done in the context of the attractive fermionic Hubbard model using the Stochastic Green Function algorithm in d=1 while for d=2 we use Determinant Quantum Monte Carlo. This is the first exact QMC study examining the effects of finite temperature which is very important in experiments on ultra-cold atoms. Our results show that, in the ground state, the dominant pairing mechanism is at nonzero center of mass momentum, i.e. FFLO. I will then discuss the effect of finite temperature in the uniform and confined systems and present finite temperature phase diagrams. The numerical results will be compared with experiments. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:54PM |
X6.00004: Spin imbalanced Fermi gases in 1D and the crossover to 3D Invited Speaker: The search for the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) phase, a polarized superfluid with a spatially varying order parameter, has generated large interest in both condensed matter and cold atoms communities. To date, there has been only indirect experimental evidence of FFLO in the heavy fermion superconductor CeCoIn5. A strongly interacting 3D polarized Fermi gas exhibits three phases at low temperature: an unpolarized superfluid, a partially polarized and a fully polarized normal phase, which phase separate in an optical trap. There is no experimental evidence for an FFLO phase in the 3D system and theory predicts that it occupies only a small region in the phase diagram. In a 1D polarized Fermi gas, the FFLO phase is predicted to occupy a large region of the phase diagram. We have implemented a 2D optical lattice in order to explore the phase diagram of an imbalanced spin mixture of $^6$Li. In in-situ density distributions, we observe in the center of the cloud a partially polarized region surrounded by an either fully polarized or an unpolarized superfluid shell depending on the spin imbalance. The density profiles are quantitatively well described by a finite temperature Bethe ansatz and can be used to extract the 1d phase diagram of the imbalanced 1d Fermi gas. Moreover, the quantitative agreement of experiment and theory paves the way for directly observing the elusive FFLO phase in the system. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:30PM |
X6.00005: Universal Spin Transport in Strongly Interacting Fermi Gases Invited Speaker: Ultracold gases of fermionic atoms have emerged as a unique platform to study strongly interacting fermion systems. Here we study spin transport in a two-state mixture of fermionic atoms near a Feshbach resonance. Starting with two separate spin domains in an atom trap, we observe the subsequent evolution of the spin mixture towards the eventual ground state, a superfluid of fermion pairs. Initially, the gas clouds of unlike spins almost perfectly bounce off each other, despite densities a million times thinner than air. Only over several seconds, about 100 000 collision times, the spins slowly diffuse into each other and, below a critical temperature, form a superfluid of fermion pairs. We determine the transport properties in this gas as a function of interaction strength and temperature. In particular, we find the spin diffusion coefficient in the strongly interacting, degenerate regime to take on the universal value for a ``perfect fluid'', $D \simeq \hbar/m = \frac{(100 \mu \rm m)^2}{{\rm s}}$, where $m$ is the mass of the $^6$Li atoms. At high temperatures, we find the universal law $D = \alpha \frac{\hbar}{m} \left(T/T_F\right)^{3/2}$ with a constant $\alpha$. The ratio of spin conductivity and spin diffusion coefficient yields the spin susceptibility in these gases, showing the Curie law at high temperatures and a departure from the compressibility at low temperatures, that we interpret as a signature for entering the Fermi liquid regime. Our transport experiments near and far equilibrium have implications on other strongly interacting Fermi systems, suggesting a fundamental lower limit to the spin diffusion coefficient, in the absence of localization, on the order of $\hbar/m$ - a conjecture already made by Onsager. Our spin susceptibility measurements appear to exclude a ferromagnetic ground state on the repulsive side of the Feshbach resonance. [Preview Abstract] |
Session X7: Quantitative Approaches to DNA Replication
Sponsoring Units: DBPChair: John Bechhoefer, Simon Fraser University
Room: Ballroom C3
Thursday, March 24, 2011 2:30PM - 3:06PM |
X7.00001: Replication domains are self-interacting structural chromatin units of human chromosomes Invited Speaker: In higher eukaryotes, the absence of specific sequence motifs marking the origins of replication has been a serious hindrance to the understanding of the mechanisms that regulate the initiation and the maintenance of the replication program in different cell types. In silico analysis of nucleotide compositional skew has predicted the existence, in the germline, of replication N-domains bordered by putative replication origins and where the skew decreases rather linearly as the signature of a progressive inversion of the average fork polarity. Here, from the demonstration that the average fork polarity can be directly extracted from the derivative of replication timing profiles, we develop a wavelet-based pattern recognition methodology to delineate replication U-domains where the replication timing profile is shaped as a U and its derivative as a N. Replication U-domains are robustly found in seven cell lines as covering a significant portion (40-50{\%}) of the human genome where the replication timing data actually displays some plasticity between cell lines. The early replication initiation zones at U-domains borders are found to be hypersensitive to DNase I cleavage, to be associated with transcriptional activity and to present a significant enrichment in insular-binding proteins CTCF, the hallmark of an open chromatin structure. A comparative analysis of genome-wide chromatin interaction (HiC) data shows that replication-U domains correspond to self-interacting structural high order chromatin units of megabase characteristic size. Taken together, these findings provide evidence that the epigenetic compartmentalization of the human genome into autonomous replication U-domains comes along with an extensive remodelling of the threedimensional chromosome architecture during development or in specific diseases. The observed cell specific conservation of the replication timing between the human and mouse genomes strongly suggests that this chromosome organization into self-interacting structural and functional units is a general feature of mammalian organisms. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:42PM |
X7.00002: Thermal Replication Trap Invited Speaker: The hallmark of living matter is the replication of genetic molecules and their active storage against diffusion. We have argued in the past that thermal convection can host the million-fold accumulation even of single nucleotides and at the same time trigger exponential replication [1]. Accumulation is driven by thermophoresis and convection in elongated chambers, replication by the inherent temperature cycling in convection. Optothermal pumping [2,3] allows to implement the thermal trap efficiently in a toroidal [4] or linear [5] geometry. Based on this method, we were in a position to combine accumulation and replication of DNA in the same chamber [5]. As we are missing a solid chemistry of prebiotic replication, we used as a proxy reaction for to replication the polymerase chain reaction. Convective flow both drives the DNA replicating polymerase chain reaction (PCR) while concurrent thermophoresis accumulates the replicated 143 base pair DNA in bulk solution. The time constant for accumulation is 92 s while DNA is doubled every 50 s. The length of the amplified DNA is checked with thermophoresis. Finite element simulations confirm the findings. The experiments explore conditions in pores of hydrothermal rock which can serve as a model environment for the origin of life and has prospects towards the first autonomous evolution, hosting the Darwin process by molecular selection using the thermophoretic trap. On the other side, the implemented continuous evolution will be able to breed well specified DNA or RNA molecules in the future. \\[4pt] [1] Baaske, Weinert, Duhr, Lemke, Russell and Braun, PNAS 104, 9346 (2007) \\[0pt] [2] Weinert, Kraus, Franosch and Braun, PRL 100, 164501 (2008) \\[0pt] [3] Weinert and Braun, Journal of Applied Physics 104, 104701 (2008) \\[0pt] [4] Weinert and Braun, Nano Letters 9, 4264 (2009) \\[0pt] [5] Mast and Braun, PRL 104, 188102 (2010) [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 4:18PM |
X7.00003: Single-molecule measurements of replisome composition and function reveal the mechanism of polymerase exchange Invited Speaker: A complete understanding of the molecular mechanisms underlying the functioning of large, multiprotein complexes requires experimental tools capable of simultaneously visualizing molecular architecture and enzymatic activity in real time. I will describe a novel single-molecule assay that combines the flow-stretching of individual DNA molecules to measure the activity of the DNA-replication machinery with the visualization of fluorescently labeled DNA polymerases at the replication fork. By correlating polymerase stoichiometry with DNA synthesis of T7 bacteriophage replisomes, we are able to quantitatively describe the mechanism of polymerase exchange. We find that even at relatively modest polymerase concentration (2 nM), soluble polymerases are recruited to an actively synthesizing replisome, dramatically increasing local polymerase concentration. These excess polymerases remain passively associated with the replisome through electrostatic interactions with the T7 helicase for 50 seconds until a stochastic and transient dissociation of the synthesizing polymerase from the primer-template allows for a polymerase exchange event to occur. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:54PM |
X7.00004: Defects and DNA replication: a role for stochasticity Invited Speaker: When a cell replicates its DNA, each base must be copied once and only once per cell cycle. A failure to complete replication normally can lead to cell death, or worse. In this talk, I will discuss how ideas from statistical physics can help understand how replication is organized and controlled. In particular, we describe a formalism based on rate equations similar to those used to describe the kinetics of crystal growth and show how it can describe the normal course of DNA replication. In practice, replication must also deal with numerous kinds of problems. For example, the machinery of replication may stall or strands may be broken. We show how to extend our formalism to include the effects of such damage and conclude that there are two regimes: a normal regime, where the influence of defects is local, and an initiation-limited regime, where the influence of defects is long range. In the latter regime, defects have a global impact on replication. We show that normal, healthy cells have defect densities in the normal regime but cells with ``problems'' have defect densities that approach the crossover value. The overall conclusion is that passive stochastic control and physical effects such as diffusion are more relevant for DNA replication than had been believed until recently. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:30PM |
X7.00005: Universal Temporal Profile of Replication Origin Activation in Eukaryotes Invited Speaker: The complete and faithful transmission of eukaryotic genome to daughter cells involves the timely duplication of mother cell's DNA. DNA replication starts at multiple chromosomal positions called replication origin. From each activated replication origin two replication forks progress in opposite direction and duplicate the mother cell's DNA. While it is widely accepted that in eukaryotic organisms replication origins are activated in a stochastic manner, little is known on the sources of the observed stochasticity. It is often associated to the population variability to enter S phase. We extract from a growing \textit{Saccharomyces cerevisiae} population the average rate of origin activation in a single cell by combining single molecule measurements and a numerical deconvolution technique. We show that the temporal profile of the rate of origin activation in a single cell is similar to the one extracted from a replicating cell population. Taking into account this observation we exclude the population variability as the origin of observed stochasticity in origin activation. We confirm that the rate of origin activation increases in the early stage of S phase and decreases at the latter stage. The population average activation rate extracted from single molecule analysis is in prefect accordance with the activation rate extracted from published micro-array data, confirming therefore the homogeneity and genome scale invariance of dynamic of replication process. All these observations point toward a possible role of replication fork to control the rate of origin activation. [Preview Abstract] |
Session X8: Migrations of Physicists
Sponsoring Units: FHP FIPChair: Noemie Koller, Rutgers University
Room: Ballroom C4
Thursday, March 24, 2011 2:30PM - 3:06PM |
X8.00001: Migration of scientists and the International Centre for Theoretical Physics -- a Personal and Professional Perspective Invited Speaker: Scientists migrate for a variety of reasons: political problems with their governments, lack of professional opportunities in their countries, the lure of better lives, financial security for them and their families, better education for their off-springs, and so forth. Migration usually occurs from poor and oppressed countries--the two categories are not one and the same--to the rich and the open. It has created, over time, a dilemma for the poor countries: in the midst of all their other problems, how to justify spending on higher education and research when that investment often results in the most enterprising of its citizens to leave their countries behind? (When migration has reversed direction occasionally, it is because of certain necessities of the scientists to be back in their countries or for opportunities that some individuals see for wielding greater scientific power.) The ideal of keeping the best scientists in their own countries, still ensuring that they remain scientifically productive and inspiring to the youth, is what provided the motivation for the creation of the International Centre for Theoretical Physics in Trieste, Italy, where I served as Director for some seven years. In this talk, I will present the story behind the formation of the Centre in 1964, explain its rationale, and analyze its evolution over time to accommodate the changing scene in world; I will discuss how some things have remained the same even as more of them have changed over time. Finally, I will remark on the broad needs of developing countries even though such needs are often very specific to a country. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:42PM |
X8.00002: Physicists' Forced Migrations under Hitler Invited Speaker: When the Nazis came to power in early 1933 they initiated formal and informal measures that forced Jews and political opponents from public institutions such as universities. Some physicists retired and others went into industry, but most emigrated. International communication and contact made emigration a viable option despite the desperate economic times in the Great Depression. Another wave of emigrations followed the annexation of Austria in 1938. Individual cases as well as general patterns of migration and adaptation to new environments will be examined in this presentation. One important result of the forced migrations was that many of the physicists expelled under Hitler played important roles in strengthening physics elsewhere, often on the Allied side in World War II. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 4:18PM |
X8.00003: Migrations and the Rise of High Energy Physics in Brazil Invited Speaker: There have been five phases in the development and evolution of High Energy Physics (HEP) in Brazil, that I review from a personal perspective. The Founding Phase (1938 - 1955) was followed by the Brasilia Phase that included an attempt by R. Salmeron, the first Brazilian physicist to work at CERN in its early years, to found an HEP institute at the University of Brasilia in 1960 - 1965. This was followed by my return from France during the redemocratization of 1985, after 21 years of miltary rule. The Expansion Phase (1984 - 1999) followed an ambitious plan that led to the spread of high energy physics to many key institutions throughout the country. The Evolutionary Phase (1984 - 2010), initiated with the strong help of Leon Lederman (Nobel Laureate, 1988) began with the strengthening of HEP groups and their participation in the major experimental collaborations, at the Tevatron at Fermilab and more recently at the LHC at CERN, with the strong support of the Brazilian funding agencies. The fifth Partnership phase (2011 - ) has just begun, as the high energy physics community and its supporting agencies work to make Brazil an Associate Member of CERN, and in this way to make the Brazilian community a full partner in the global effort aimed at discoveries at the Energy Frontier. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:54PM |
X8.00004: Russian, Soviet, and post-Soviet scientific migration: history and patterns Invited Speaker: Immigrant scientists from other European countries (predominantly German) were crucial in establishing the tradition of modern science in the Russian Empire of the 18th and 19th centuries. Since the 1860s, however, outgoing waves of scientific migration started originating in Russia, bringing important innovations to international science. The scale and patterns of migration varied greatly with the turbulent time. The talk will describe several landmark stages of the proceess and their cultural consequences: from opening higher education possibilities for women during the late 19th century, to post-1917 academic refugees and Soviet defectors, to the 1960s brain drain provoked by the launch of Sputnik, and to what can be called the first truly global scientific diaspora of Russophone scientists after 1990. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:30PM |
X8.00005: Chinese/American Physicists: A Transnational History Invited Speaker: As part of a broader project on ``Chinese/American Scientists: Transnational Science during the Cold War and Beyond,'' this paper examines the movements of American-trained Chinese physicists following the founding of the People's Republic of China in 1949. While a majority of these physicists chose to stay in the US (the ``stayees''), a number went back to China in the 1950s (the ``returnees'') against many obstacles during the McCarthy era. After the reopening of US-China relations in the 1970s, the two groups joined hands in promoting China-US scientific and educational exchanges, leading eventually to the coming to the US of a new generation of Chinese physics students and the return to China of some of the original ``stayees.'' This transnational history of Chinese/American physicists aims to illustrate the nature and extent of the Americanization of international science and the internationalization of American science in the post-World War II era. [Preview Abstract] |
Session X9: Coordination, Coherence and Synchronization through Hydrodynamic Interactions
Sponsoring Units: DFDChair: Thomas Powers, Brown University
Room: D220
Thursday, March 24, 2011 2:30PM - 3:06PM |
X9.00001: Collective motion and density fluctuations in swimming bacteria Invited Speaker: The emergence of collective motion such as in fish schools, mammal herds, and insect swarms is a ubiquitous self-organization phenomenon. Such collective behavior plays an important role in a range of problems, such as spreading of deceases in animal or fish groups. Current models have provided a qualitative understanding of collective motion, but progress in quantitative modeling in hindered by the lack of experimental data. Here we examine a model microscopic system, where we are able to measure simultaneously the positions, velocities, and orientations of up to a thousand bacteria in a colony. The motile bacteria form closely-packed dynamic clusters within which they move cooperatively. The number of bacteria in a cluster exhibits a power-law distribution truncated by an exponential tail, and the probability of finding large clusters grows markedly as bacterial density increases. Mobile clusters cause anomalous fluctuations in bacterial density as found in mathematical theories and numerical models. Our results demonstrate that bacteria are an excellent system to study general phenomena of collective motion. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X9.00002: Collective behavior of spinning particles at fluid/fluid interface Yaouen Fily, Aparna Baskaran, M. Cristina Marchetti Rotating particles in a viscous fluid can exhibit interesting behavior due to hydrodynamic interactions. When the particles are driven by an external torque, these interactions result in an effective azimuthal force, leading to swirling motion. It has been shown that small numbers of such particles form precessing atom-like structures. The behavior of large collections of spinning particles is, however, still relatively unexplored. We study the phase diagram of a collection of spinning particles in two dimensions using molecular dynamics simulations. The rotors interact via hydrodynamic interactions and short-range repulsion, in the presence of thermal noise. The repulsive interaction yields transitions from a solid to a liquid to a gas as the density of rotors is decreased. The azimuthal hydrodynamic interaction modifies each of these phases in a distinct way. Both long-range and screened hydrodynamic interactions are considered. Some properties of the various phases are shown to depend critically on the range of the interaction. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X9.00003: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X9.00004: Synchronization phenomena in systems with magnetodipolar interactions Andrejs Cebers, Mihails Belovs Rich pattern formation phenomena under the action of AC field for two dimensional systems of magnetic dipoles floating on the surface of liquid are observed and reproduced numerically by the first-principles model [1]. Here by the study of dynamics of two dipoles interacting with weak dipolar forces it is found that due to series of bifurcations the motion of dipoles in AC field is synchronized. If the dipoles orientation is confined to the plane the synchronous oscillation regime by infinite period bifurcation transforms to the regime of synchronous rotation. This regime is unstable for intermediate values of the field strength and the motion of dipoles is periodic or quasi-periodic. Above the critical value of field strength these regimes transform to rotational regime and the dipoles synchronously rotate in plane. Estimate of the critical parameters of the synchronization according to the dimensionless parameters used in the first-principles model [1] show that the synchronization of the dipoles rotation should be inherent in this model.\\[4pt] [1] M.Belkin, A.Glatz, A.Snezhko, and I.S.Aranson, Phys.Rev.E, 82, 051301(R), (2010) [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X9.00005: Pattern Formation in a Rotating Suspension of Non-Brownian Buoyant Particles Penger Tong, Makrand Kalyankar, Bruce Ackerson, W.R. Matson This study examines concentration and velocity patterns observed in a horizontal rotating cylinder completely filled with a monodisperse suspension of non-Brownian buoyant particles. The unique patterns or phases are mapped by varying both the rotation rate and the solvent viscosity. Individual phases are identified using both frontal and axial views. Phase boundaries are compared to those obtained recently for suspensions of non-buoyant particles. Expressing the boundaries in terms of dimensionless parameters unifies the data for several samples at low rotation rates. When centrifugal force dominates, the behavior becomes quite different from previous studies. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X9.00006: Shear-induced hydrodynamic diffusion of a flowing suspension of elastic capsules Marcus Hwai Yik Tan, Duc Vinh Le, Keng-Hwee Chiam In flowing suspensions of soft and deformable elastic capsules, the shear flow causes hydrodynamic interaction among the capsules, resulting in an effective hydrodynamic diffusion that is not Brownian in origin. Recent experiments have suggested that hydrodynamic diffusion of red blood cells may play an important role in the pathophysiological processes of vasoocclusion and thrombosis. To study hydrodynamic diffusion further, we have developed accurate three-dimensional numerical simulations based on the immersed boundary method and thin shell theory to study the deformation of a large number of elastic capsules enclosed by thin shells moving in a shear flow. Using these simulations, we have calculated the effective hydrodynamic diffusion coefficient and showed how it varies with bulk flow velocity and capsule properties such as the volume fraction, size, and stiffness of spherical and biconcave capsules. We also compared them to scaling arguments and experimental measurements done for red blood cell suspensions. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X9.00007: Artificial Microfluidic Squirmers Shashi Thutupalli, Ralf Seemann, Stephan Herminghaus While there is a growing consensus on the propulsion mechanisms of swimmers at low Reynolds' numbers, many questions remain open regarding the hydrodynamic effects on such swimmers, in particular the coupling between swimmers. Here we present experiments on artificial swimmers, where hydrodynamics is seen to be responsible for a wide range of collective behavior and interactions. Using droplet microfluidics with a surfactant laden continuous oil phase, we create monodisperse aqueous droplets containing chemicals that produce a steady source of Bromine ions. The surfactant (mono-olein) reacts at the droplet interface with the Bromine produced within the droplets, and a dynamic instability leads to gradients of interfacial tension at the droplet interface. These gradients set up Marangoni flows propelling the droplets, in a manner similar to the classical squirmer model of swimming. The flow around the swimmers as well as its effect on the droplet motion are measured using particle image velocimetry (PIV). The PIV analysis reveals the far field flows generated by the swimmers in the surrounding liquid, leading to the emergence of bound states and oriented clusters. We discuss the interaction mechanisms and compare it to previous theoretical work and simulations. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X9.00008: Characterizing particle transport due to actuated cilia with adhesive tips Amitabh Bhattacharya, Gavin Buxton, Alexander Alexeev, O. Berk Usta, Anna C. Balazs Biological tissues and organisms commonly utilize arrays of cilia to manipulate microparticles of different sizes. Motivated by biology, we use numerical simulations to study the interaction of microparticles with an array of actuated cilia, immersed in fluidic microchannel. For each cilium in the array, one end is tethered to the wall, while the other end is actuated by an external periodic force. Also, an adhesive force is introduced between the cilia tip and the microparticle. The simulations are performed using the Lattice Boltzmann Method for the flow, with a chain of point-forces, connected by springs, used to represent each cilium. We observe that a combination of hydrodynamic and adhesive forces can lead to size-specific control of microparticle transport. For instance, for certain adhesion strength and particle sizes, it is possible to trap and release particles by varying the actuation frequency. Also, for a given actuation frequency, the average particle speed is maximized at a particular adhesion strength. We will present the parameter range where we can observe the above behavior. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X9.00009: Harnessing self-oscillating polymer gels to design active ciliated surfaces Pratyush Dayal, Amitabh Bhattacharya, Olga Kuksenok, Anna C. Balazs Via theory and simulations, we design active surfaces capable of replicating characteristics of biological cilia. Our approach harnesses the use of polymer gels that undergo photosensitive Belousov-Zhabotinsky (BZ) reaction. Powered by internalized BZ reaction these polymer gels swell and de-swell autonomously due to the chemo-mechanical transduction and therefore are ideal materials for designing our system. We have successfully developed an efficient hybrid approach by combining our three dimensional gel lattice spring model (3D-gLSM) and Lattice Boltzmann Method (LBM) which allows us to capture the interactions between the cilia and the surrounding fluid. Using our gLSM-LBM hybrid model we determine the factors that govern the bending and beating of individual cilium and also their collective dynamic behavior. Our findings provide guidelines for designing ciliated surfaces that can exhibit biomimetic functionality. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X9.00010: Designing active cilial sensors Yi Yang, Alexander Alexeev We employ a hybrid lattice Boltzmann / lattice spring computational model to simulate the three-dimensional hydrodynamic interactions among actuated and sensory elastic cilia tethered to a wall of a microfluidic channel. These actuated and sensory cilia are arranged a chessboard pattern on the channel wall. The actuated cilia are driven by a sinusoidal force applied to their free ends and induce periodic oscillations of a viscous fluid filling the microchannel. The passive, sensory cilia are used to measure the force arising due to fluid oscillations. We show that the combination of sensory and actuated cilia allows us to evaluate distances to solid objects located in a fluid-filled microchannel, thereby yielding a useful active sensor for microfluidic and biomedical applications. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X9.00011: Nonlinear dynamics of flagellar bundling Pieter Janssen, Michael Graham Flagella are long thin appendages of microscopic organisms used for propulsion in low-Reynolds environments. In many bacterial species, helical-shaped flagella driven by a molecular motor will bundle up. This bundling process is poorly understood, and the exact roles of hydrodynamic interactions, helix elasticity, and mechanical contact are unclear. To investigate the bundling, we consider two flexible helices next to each other, as well as several flagella attached to a spherical body. Each helix is modeled as several prolate spheroids connected by springs. For HI, we consider the flagella to made up of point forces, while the finite size of the body is incorporated via Fax\'{e}n's laws. Before flagella can bundle, they must synchronize. Synchronization occurs fast relative to the bundling process. For flagella next to each other, the initial stage of bundling is governed by rotlet interactions generated by the rotating helices. At longer times, once bundling has occurred, we find that a sharp distinction can be made between ``tight'' and ``loose'' bundles, indicated by the local distance between the flagella. As function of the anchor point distance, a sharp transition from tight to loose is found when starting from the completely unbundled state. Incremental steps from stationary situations give multiple stationary for a single anchor distance. We show that the balance between elasticity and strong non-linear hydrodynamic interactions is responsible for this bifurcation behavior. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X9.00012: Spontaneous transitions in the synchronisation states of a Chlamydomonas mutant Kirsty Wan, Kyriacos Leptos, Marco Polin, Idan Tuval, Raymond Goldstein The mechanisms by which eukaryotic flagella are found to synchronise is poorly understood; the origins being dependent upon the hydrodynamics, as well as the underlying molecular biochemistry. Exemplifying how available phenotypic variations in a species may be exploited to extend our mathematical models for flagellar coupling, we turn to ptx1 - a non-phototactic mutant strain of the biflagellated alga \textit{Chlamydomonas} with seemingly intact flagellar apparatus, which does not exhibit any gross motility defects. Intriguingly however, our high-speed imaging analysis of flagellar dynamics in ptx1 have revealed that rather unlike their wildtype predecessors, which beat mostly in synchrony interrupted by brief periods of drifts or slip [1], the two flagella of ptx1 are observed to consistently revert from synchrony to a state of stable, coupled, anti-phase beating dynamics. Incorporating the interpretation of the flagella pair as coupled noisy oscillators, we show how such behaviour corroborates readily with a secondary contribution to the coupling, which is further conjectured to be inherent in the wildtype. \newline [1] Polin M et al. \textit{Science}, 487-490, \textbf{2009}. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X9.00013: Emergence of synchronisation in flagella of variable length Marco Polin, Idan Tuval, Raymond Goldstein {\it Chlamydomonas reinhardtii} is a unicellular green alga that can swim by the concerted breaststroke-like beating of its two flagella. When the flagella are synchronised the organism moves along a straight helical path, while a large difference in the two beating frequencies induces sharp turns. Even in the synchronous state, however, the two flagella have slightly different intrinsic frequencies, and synchrony is guaranteed only by the presence of a sufficiently strong interflagellar coupling. Although the magnitude of this coupling is consistent with the value derived from a rough hydrodynamic estimate, no direct experimental test for the role of hydrodynamic in interflagellar coupling is available. In order to better understand the origin of interflagellar coupling, we employ high- speed imaging to study the dynamics of the two flagella of \textit{Chlamydomonas} as they regrow after mechanically induced deflagellation. Our results show that the duration of synchronised motion is strongly dependent on flagellar length. We discuss this dependence in light of hydrodynamic models of flagellar synchronisation. [Preview Abstract] |
Thursday, March 24, 2011 5:30PM - 5:42PM |
X9.00014: Fluid dynamics and noise in bacterial scattering Jorn Dunkel, Knut Drescher, Luis Cisneros, Sujoy Ganguly, Raymond Goldstein Bacterial communication through chemical and physical channels is permanently challenged by internal and external noise. While the role of stochastic fluctuations in quorum sensing has been widely studied both theoretically and experimentally, our understanding of hydrodynamic interactions between bacteria is limited by the absence of empirical data. Here, we report the first direct measurement of the fluid flow generated by an individual bacterium far away from and near to a wall. The experiments show that the micro-hydrodynamics of E. coli are considerably different from that of more complex eucaryotes as, for example, Chlamydomonas algae. We discuss the implications of our results for bacterial cell-cell and cell-wall interactions. [Preview Abstract] |
Thursday, March 24, 2011 5:42PM - 5:54PM |
X9.00015: ABSTRACT WITHDRAWN |
Session X10: Focus Session: Growth, Structure, Dynamics, and Function of Nanostructured Surfaces and Interfaces -- Organic Molecules
Sponsoring Units: DMPChair: Jay Gupta, Ohio State University
Room: D221
Thursday, March 24, 2011 2:30PM - 2:42PM |
X10.00001: Identification of the Atomic Scale Structures of the Molecule-Metal Interfaces of Single-Molecule Nanowires Firuz Demir, George Kirczenow We show theoretically how inelastic tunneling spectroscopy can identify the atomic scale structures of the molecule-metal interfaces of single-molecule nanowires bridging pairs metal electrodes, and thus resolve a long standing problem that is central to the field of single-molecule nanoelectronics. As an example we consider the propanedithiol (PDT) molecules bridging gold nanocontacts in the recent experiment of Hihath {\em et al.} [Nano Lett. \textbf{8}, 1673 (2008)]. Based on {\em ab initio} density functional and semi-empirical calculations we identify the features observed in the experimental inelastic tunneling spectra of these molecules at phonon energies near 46, 40 and 42 meV as arising from sulfur atoms (that have lost their thiol hydrogen atoms) bonding to the gold contacts in top site, bridge site and mixed bridge-top site geometries respectively. PDT molecules in which the sulfur atoms retain their thiol hydrogen atoms and bond strongly to gold in the top site geometry give rise to an IETS feature in the phonon energy range 54-57 meV. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X10.00002: The adsorption geometries of C60 monolayer on Ag(111) and Au(111) Renee Diehl, Heekeun Shin, Katariina Pussi C$_{60}$ films on metal surfaces are of particular interest as model van der Waals systems, and for applications such as molecular electronics. The electronic properties of these films are known to depend strongly on their structures and the relative molecular orientations of the C$_{60}$ molecules, yet there are few detailed structure determinations for C$_{60}$ films. When grown at room temperature and annealed to a sufficiently high temperature, C$_{60}$ on Au(111) and Ag(111) form ($2\sqrt{3}\times 2\sqrt{3}$)R30$^{\circ}$ structures with one C$_{60}$ molecule per unit cell. We present a LEED study of their surface geometries, which are similar in some ways, but differ in others. They both form vacancy site structures that are thermally activated, they both form monolayers that are composed of a mixture of hex-down and 6:6 bond down molecules. The details of the 6:6 bond molecule geometries are different on both substrates, and the temperature dependence of the mixture is different. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X10.00003: Mechanical Properties of a vdW molecular monolayer at a metal surface: Structural Polymorphism leading to facile compression Dezheng Sun, Daeho Kim, Duy Le, {\O}yvind Borck, Kristian Berland, Kwangmoo Kim, Wenhao Lu, Yeming Zhu, Miaomiao Luo, Jon Wyrick, Zhihai Cheng, T.L. Einstein, Talat Rahman, Per Hyldgaard, Ludwig Bartels Intermolecular force plays an important role in self-assembly and surface pattern formation. Anthracene and similar unsubstituted arenes attach to a metallic substrate predominantly through van der Waals interaction leading. In this contribution we present images how anthracene on Cu(111) forms a large number of highly ordered patterns that feature a broad array of structural motifs. Density functional theory modeling including vdW interactions allows us to model the energetic of the pattern formation at high fidelity. Moreover, it allows us to deduce the strain energy associated with films of varying coverage. From this work, we obtain the Young's modulus and Poisson Ratio of a molecular monolayer, which resemble properties conventionally found for porous materials. These patterns are in marked contrast to those found after introduction of functional groups in the molecules, such as carbonyls or thiols. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X10.00004: Monte Carlo Study of the Fish-like Patterns of Anthracenes on Cu(111) Kwangmoo Kim, T.L. Einstein, Dezheng Sun, Dae-Ho Kim, Ludwig Bartels Using Monte Carlo calculations of the two-dimensional triangular lattice with a 2-component 3-state Potts model, we demonstrate a mechanism for the spontaneous formation of fish-like patterns of anthracene (AC) molecules on Cu(111) by sputtering and annealing, then cooling to $\sim$ 80 K. The two components are an AC on a hollow site and another on a bridge site of Cu(111).\footnote{Dezheng Sun {\it et al.}, Phys.\ Rev.\ B {\bf 82}, xxx(R) (2010).} The liquid crystal model with two separate parts, positional and orientational, only explains a part of the fish-like pattern, not the whole regular pattern. Our model fixes the positional order of AC's into the triangular lattice and the orientational order into three angles as observed in the experiments. The variation of the coverages of AC's is reflected in the change of the ratio of two components in our model. We also try to understand the compression of AC's with the introduction of Gaussian dispersion of AC's about their triangular lattice sites. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X10.00005: Self-assembly of metal phthalocyanines modulated by different substrates Wende Xiao, Yuhang Jiang, Jichun Lian, Liwei Liu, Zhihai Cheng, Li Gao, Shixuan Du, Hongjun Gao The self-assembly of organic molecules on solid surfaces has made tremendous progresses due to potential applications in organic nano-devices. Among the organic molecular building blocks, metal phthalocyanines (MPcs) have been attracting considerable interests because of their novel electronic and magnetic properties. The self-assembly and physical properties of MPcs on various surfaces have been investigated by scanning tunneling microscopy and spectroscopy (STM/STS). In this presentation, we will report on the self-assembly of iron phthalocyanine (FePc), manganese phthalocyanine (MnPc) and nickel phthalocyanine (NiPc) on Pb(111) and monolayer graphene (MG) epitaxy on Ru (0001) by means of low temperature (LT) STM. Highly ordered close-packed islands with square lattice are observed for all three kinds of MPcs growth on Pb(111), whereas regular dislocation lines are formed in the molecular islands of FePc on Pb(111). We find that the Kondo resonance of MnPc on Pb(111) is strongly location-dependant. For FePc, MnPc and NiPc growth on MG, dispersive single molecules, dispersive molecular lines and small patches of Kagome lattice are observed, respectively. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X10.00006: Structural and electronic properties of polymer-silicon semiconductor heterojunctions using hybrid functionals Joseph Turnbull, Wenchang Lu, Jerry Bernholc Combining organic and inorganic components to form semiconductor heterostructures provides the basis for an enormous number of potential optoelectronic device designs. We report here on the use of hybrid-DFT calculations to study the structural and electronic properties of semiconductor interfaces between silicon and pi-conjugated polymers. Using large supercells and exact-exchange-corrected hybrid functionals, we explore different attachment motifs for polymer monolayers, as well as the role of screening, in particular in the context of predicting semiconductor band-offsets. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X10.00007: Tuning Structural and Mechanical Properties of Two-Dimensional Molecular Crystals: The Roles of Carbon Side Chains H.-J. Gao, H.Y. Cun, Y.L. Wang, S.X. Du, L. Zhang, L.Z. Zhang, W.A. Hofer, S.J. Pennycook Organic-molecule based flexible electronics has been of significant interest due to its potential to challenge conventional silicon-based technologies. The crucial properties of these devices rely on the largely invariant physical properties of organic compounds or films when they are mechanically deformed. In this present work, by choosing quinacridone with flexible carbon chains as a model system, we developed a theoretical scheme to evaluate the contributions of various interactions to the molecular self-assembly process and find that such a process should be considered as a collective interaction between molecules and substrates rather than from the viewpoint of an isolated molecule. Importantly, the data provide insight into the origin and an estimate of the magnitude of the Young's modulus of the molecular film, which suggests that the elastic properties of molecular films can be tuned through control of side chain length. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X10.00008: Formation of Molecular Networks: Tailored Quantum Boxes and Behavior of Adsorbed CO in Them Jon Wyrick, Dezheng Sun, Dae-Ho Kim, Zhihai Cheng, Wenhao Lu, Yeming Zhu, Miaomiao Luo, Yong Su Kim, Eli Rotenberg, Kwangmoo Kim, T.L. Einstein, Ludwig Bartels We show that the behavior of CO adsorbed into the pores of large regular networks on Cu(111) is significantly affected by their nano-scale lateral confinement and that formation of the networks themselves is directed by the Shockley surface state. Saturation coverages of CO are found to exhibit persistent dislocation lines; at lower coverages their mobility increases. Individual CO within the pores titrate the surface state, providing crucial information for understanding formation of the network as a result of optimization of the number N of electrons bound within each pore. Determination of N is based on quinone-coverage-dependent UPS data and an analysis of states of particles in a pore-shaped box (verified by CO's titration); a wide range of possible pore shapes and sizes has been considered. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X10.00009: Origin of the Giant Honeycomb Network of Quinones on Cu(111) T.L. Einstein, Kwangmoo Kim, Jon Wyrick, Zhihai Cheng, Ludwig Bartels, Kristian Berland, Per Hyldgaard We discuss the factors that lead to the amazing regular giant honeycomb network formed by quinones on Cu(111). Using a related lattice gas model with many characteristic energies, we can reproduce many experimental features. These models require a long-range attraction, which can be attributed to indirect interactions mediated by the Shockley surface state of Cu(111). However, Wyrick's preceding talk gave evidence that the network self-selects for the size of the pore rather than for the periodicity of the superstructure, suggesting that confined states are the key ingredient. We discuss this phenomenon in terms of the magic numbers of 2D quantum dots. We also report calculations of the effects of anthraquinones (AQ) in modifying the surface states by considering a superlattice of AQ chains with various separations. We discuss implications of these results for tuning the electronic states and, thence, superstructures. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X10.00010: Tunneling and Time-Reversal Invariance in the Diffusion of Polyatomic Molecules at a Metal Surface Zhihai Cheng, Eric Chu, Dezheng Sun, Daeho Kim, Yeming Zhu, MiaoMiao Luo, Greg Pawin, Kin Wong, Ki-Young Kwon, Robert Carp, Michael Marsella, Ludwig Bartels Rectangular molecules with 1 or 2 oxygen substrate linkers attached to each of their long sides diffuse in a uniaxial fashion, despite the threefold symmetry of the Cu(111) substrate. They achieve this by sequential placement of their substrate linkers and are hence dubbed ``molecular walkers''. VT-STM monitoring of their motion reveals a striking difference between the diffusion prefactors of the quadrupedal and bipedal species, with the latter being very low. DFT modeling of the diffusion barrier and WBK-based estimation of the potential for tunneling suggest that this discrepancy lies in the prevalence of tunneling for species, whose motion is only blocked by a barrier affecting one of their substrate linkers. In contrast, if the diffusion barrier affects two substrate linkers simultaneously, tunneling will not occur and conventional prefactors are observed. This finding may actually have far-reaching implications for the modeling of molecular motion in general, as it highlights that blocking of a single atoms is insufficient for confinement of molecular motion. We also investigated an asymmetric ``molecular walkers'', showing a symmetric diffusion in agreement with time-reversal invariance despite a saw-tooth shape of the diffusion barrier experienced. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X10.00011: Hydrogen Bonding Controls the Dynamics of Catechol Adsorbed on a TiO$_{2}$(110) Surface Ulrike Diebold, Shao-Chun Li, Li-Na Chu, Xue-Qing Gong Direct studies of how organic molecules diffuse on metal oxide surfaces can provide insights into catalysis and molecular assembly processes. We studied individual catechol molecules, C$_{6}$H$_{4}$(OH)$_{2}$, on a rutile TiO$_{2}$(110) surface with scanning tunnelingmicroscopy. Surface hydroxyls enhanced the diffusivity of adsorbed catecholates. The capture and release of a proton caused individual molecules to switch between mobile and immobile states within a measurement period of minutes. Density functional theory calculations showed that the transfer of hydrogen from surface hydroxyls to the molecule and its interaction with surface hydroxyls substantially lowered the activation barrier for rotational motion across the surface. Hydrogen bonding can play an essential role in the initial stages of the dynamics of molecular assembly. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X10.00012: Reversible rectification in sub-monolayer molecular heterojunctions Joe Smerdon, Chris Giebink, Matthias Bode, Nathan Guisinger, Jeffrey Guest Pentacene and C$_{60}$ are archetypal molecules for optically active acceptor-donor heterojunctions and have been used as the active materials in bilayer solar cells. We will discuss UHV STM and STS measurements on these bi-molecular films deposited sequentially to form heterojunctions on Cu(111). It is observed that rectification can be detected at the single-junction limit, and that the direction of rectification flips in accordance with the polarity of the heterojunction. The morphology of heterolayers will also be discussed. It is found that the density of a Pn monolayer can be affected by choice of growth conditions, and in turn can affect the overall morphology of the heterolayer and the rectification behavior of the heterojunctions. We will also briefly discuss progress towards probing the photophysical behavior of these systems. The correlation of atomic-scale structure and electronic behavior at the single-junction limit has important implications for applications of such heterojunctions, such as solar cells or OLEDs. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X10.00013: Molecular self-assembly of dichloropentacene and C$_{60}$ for a model organic solar cell heterojunction Jun Wang, Amanda Brown, Irvinder Kaur, Jian-Ming Tang, Glen Miller, Karsten Pohl Organic molecular self-assembly is a bottom-up approach to create molecular architectures that are suitable for a variety of applications including functional materials and molecular electronics. Specifically, the co-assembly of functionalized acenes (electron-donor materials) and fullerenes (electron-acceptor materials) on metal substrates provides a model for studying the structural and electronic properties for novel organic photovoltaic heterojunctions. Previously, we have fabricated a persistent self-assembled monolayer composed of single-domain 6,13-dichloropentacene (DCP) over large areas on the stepped Au(788) surface. Here we present the subsequent growth of C$_{60}$ on a DCP monolayer covered gold surface by STM investigation and ab-initio calculations. After mild annealing C$_{60}$ molecules are forced to the Au(788) step edges forming very long C$_{60}$ chains, in contrast to the short C$_{60}$ chains reported on bare vicinal gold steps. These final structure formations are attributed to the delicate intermolecular interactions and molecule-substrate interactions. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X10.00014: BEEM Study of Interface Properties in PVDF-graphite Heterojunctions Le Zhang, Clay Upton, Andrei Sokolov Since the prediction of giant electroresistance, tunnel junctions with ferroelectric (FE) barrier attract significant experimental and theoretical attention due to potential application as non-volatile data storage devices. The ferroelectric polymer, polyvinylidene fluoride (PVDF), is of particular interest thanks to its outstanding electromechanical, dielectric, and mechanical properties. However, direct experimental study of organic FE interface properties as a function of its polarization is extremely challenging. We employ ballistic electron emission microscope (BEEM) technique for nanometer size characterization of P(VDF-TrFE)/HOPG heterostructure. This method offers STM scale of spatial resolution and is immune to the contact quality of top electrode. By comparing the voltage dependence of ballistic current for different polarization states, we observe the change in tunnel barrier properties as the sign of FE polarization reverses.~High-quality thin films of PVDF were fabricated using a Langmuir-Blodgett (LB) technique. Results will be also compared with thermally evaporated PVDF film and its isomorphic analog, polyethylene. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X10.00015: Reverse self assembly: (111)-oriented gold crystallization at thiol monolayer templates Ahmet Uysal, Benjamin Stripe, Pulak Dutta, Binhua Lin, Mati Meron Certain microorganisms can reduce gold ions from aqueous solutions to form gold nano/micro particles in a controlled way [1]. Understanding how biomolecules control the crystallization process may result in cheap and environment-friendly techniques in many different applications, including organic-inorganic hybrid molecular technologies and shape controlled gold nanoparticle production. To better understand the organic-inorganic interactions, we studied the crystallization of gold under octadecanethiol monolayers at the air-water interface. We used synchrotron x-rays in the grazing incidence geometry to determine the orientation of the gold crystals as well as the organic monolayer structure \textit{in situ}. These x-rays also act as the gold reducing agent in this experiment. We see that the (111) faces of gold crystals are parallel to the monolayer surface. The monolayer structure changes with time and becomes commensurate with the gold (111) face, similar to a $\sqrt 3 \times \sqrt 3 $ self-assembled monolayer of thiol on gold. \\[4pt] [1] K. B. Narayanan, and N. Sakthivel, Advances in Colloid and Interface Science 156, 1 (2010). [Preview Abstract] |
Session X11: Integer Quantum Hall Effect
Sponsoring Units: FIAPRoom: D222
Thursday, March 24, 2011 2:30PM - 2:42PM |
X11.00001: Imaging quantum Hall Coulomb islands inside a quantum ring Frederico Martins, Benoit Hackens, Sebastien Faniel, Vincent Bayot, Marco Pala, Hermann Sellier, Serge Huant, Ludovic Desplanque, Xavier Wallart In the quantum Hall regime near integer filling factors, electrons are transmitted through edge states confined at the borders of the device. In mesoscopic samples, however, edge states may be sufficiently close to allow electrons to tunnel, or to be transmitted through localized states (``Coulomb islands'') [1]. Here, we use the biased tip of a low temperature scanning gate microscope to alter tunneling through quantum Hall Coulomb islands localized inside a quantum ring patterned in an InGaAs/InAlAs heterostructure. Simultaneously, we map the quantum ring resistance and observe different sets of concentric resistance fringes, due to charging/discharging of each Coulomb island. Tuning the magnetic field and the tip voltage, we reveal the rich and complex behaviour of these fringes [2].\\[4pt] [1] B. Rosenow and B. I. Halperin, PRL 98, 106801 (2007).\\[0pt] [2] B. Hackens et al., Nature Communications 1, 39 (2010). [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X11.00002: Emergent Dissipation in the $\nu=1$ Quantum Hall Bilayer Ganpathy Murthy, Herbert Fertig Disorder is known to be central to the $\nu=1$ bilayer [1]. Building on our previous study of the bilayer $\nu=1$ system in a periodic potential [2] to capture the nonperturbative effects of disorder, we construct a $T=0$ effective theory, in which the $XY$ angle is coupled to an emergent Ising spin. We uncover a $z=2$ quantum phase transition with emergent dissipation. Calculations of the interlayer tunnelling conductance and counterflow conductivity will be presented. \\[4pt] [1] H. A. Fertig and G. Murthy, Phys. Rev. Lett. {\bf 95}, 156802 (2005). \\[0pt] [2] J. Sun, G. Murthy, H. A. Fertig, and N. Bray-Ali, Phys. Rev. B, {\bf 81}, 195314 (2010). [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X11.00003: Microwave induced electron heating in the regime of radiation-induced magnetoresistance oscillations in the GaAs/AlGaAs system A.N. Ramanayaka, R.G. Mani, W. Wegscheider We examine the influence of microwave photoexcitation on the amplitude of Shubnikov-de Haas (SdH) oscillations at large filling factors in a two dimensional GaAs/AlGaAs electron system. A SdH lineshape analysis indicates that increasing the incident microwave power has a weak effect on the amplitude of the SdH oscillations, in comparison to the influence of modest temperature changes at liquid Helium temperatures on the dark-specimen SdH effect. The results indicate negligible electron heating under modest microwave photoexcitation, in good agreement with theoretical predictions for this regime. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X11.00004: Transport study under microwave photoexcitation in epitaxial graphene Ramesh Mani, John Hankinson, Claire Berger, Walt de Heer Single layers of carbon known as graphene are a promising new electronic material with potential for high frequency applications. For electronics, top-gated graphene field-effect transistors fabricated on large area epitaxial graphene wafers have already indicated switching cutoff frequencies up to 100 GHz [1]. Microwave and terahertz radiation-sensing constitutes another area of interest. Hence, we examine the electrical photo-response of graphene devices in the microwave band, and report transport measurements under microwave photo-excitation (f $<$ 120 GHz) carried out on micron sized Hall bars at liquid Helium temperatures. \\[4pt] [1] Y-M Lin et al., Science 327, 662 (2010). [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X11.00005: Quantized Anomalous Hall Insulator in a Nanopatterned Two-Dimensional Electron Gas Yongping Zhang, Chuanwei Zhang We propose that a quantum anomalous Hall insulator (QAHI) can be realized in a nanopatterned two-dimensional electron gas (2DEG) with an in-plane magnetic field. The Berry curvatures originating from the in-plane magnetic field and Rashba and Dresselhaus spin-orbit coupling, in combination with a nanoscale honeycomb lattice potential modulation, lead to topologically nontrivial insulating states in the 2DEG. In the bulk insulating gaps, the anomalous Hall conductivity is quantized -e$^{2}$/h, corresponding to a finite Chern number -1. There exists one gapless chiral edge state on each edge of a finite size 2DEG. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X11.00006: Remote sensing of transport in microwave photo-excited GaAs/AlGaAs heterostructure devices Tianyu Ye, G. Chand, A.N. Ramanayaka, R.G. Mani, W. Wegscheider The GaAs/AlGaAs two dimensional electron system (2DES) exhibits magnetoresistance oscillations under microwave and terahertz photo-excitation at liquid Helium temperatures. Such oscillations are understood in terms of the displacement and inelastic models for photo-excited transport in this system. In order to identify the relative physical contributions, we report on transport measurements and concurrent ``remote'' sensing of the 2DES. Hence, measurements under microwave photo-excitation were carried out on Hall bars fabricated from high mobility GaAs/AlGaAs single heterostructures, as a sensor above the specimen served to look for concurrent changes in response. We report here on the observed noticeable changes in the remote sensor and correlate the observations with the observed transport response of the photo-excited 2DES. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X11.00007: Microscopic conductivity imaging of the quantum Hall edge states by a microwave impedance microscope Keji Lai, Worasom Kundhikanjana, Michael Kelly, Zhi-Xun Shen, Javad Shabani, Mansour Shayegan Spatially resolved studies of the quantum Hall edge channels are usually challenging because most high mobility two-dimensional electron gas (2DEG) systems are buried underneath the surface. Using a cryogenic microwave impedance microscope, we demonstrate the conductivity mapping of the bulk and edge states in a GaAs/AlGaAs 2DEG. Narrow strips with either metallic or insulating screening properties are observed along edges of the 2DEG. The sizes and positions of these strips as a function of the magnetic fields agree with the self-consistent electrostatic picture. The quantitative local conductivity information provides a complete microscopic description of the evolution through the bulk filling factor $\nu $ = 2. The imaging was performed without DC electrodes, vividly manifesting that the quantum Hall edges are equilibrium states and do not depend on externally supplied currents. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X11.00008: Study of integer quantum Hall transition in long-ranged potentials Ravindra Bhatt, A. Chandran We present results of a numerical study of a two-dimensional system of noninteracting electrons in a random correlated potential in the lowest Landau level in the presence of a perpendicular magnetic field. We use spatially uncorrelated and unbiased random gaussian potentials as well as potentials $V(r) $ with long-range, power-law correlations $\langle V(0)V(r) \rangle \propto r^{-\alpha}$ for different exponents $\alpha$ as models of disorder. We compute the Hall conductance $\sigma_{xy} $ as well as the Thouless conductance as a function of size $L$ of the sample, and use finite size scaling to determine the exponent $\nu$ characterizing the divergence of the localization length $\xi$ at the quantum Hall transition. We also study the scaling of the diagonal conductivity as a function of $L$ and compare our results to those obtained previously through different methods. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X11.00009: High-Resolution Tunneling Spectroscopy of 2D Holes in the Quantum Hall Regime B. Hunt, O.E. Dial, R.C. Ashoori, L.N. Pfeiffer, K.W. West We use Time-Domain Capacitance Spectroscopy (TDCS){[}1{]}, a method for extracting precise, high-resolution tunneling spectra, to determine the single-particle spectrum of the 2D hole system (2DHS) in the presence of high magnetic fields. The 2DHS has a variable density from zero to $3\times10^{11}\,\mathrm{cm^{-2}}$ and $T=100\,\mathrm{mK}$. Owing to the heavy mass of holes in GaAs quantum wells, much higher values of $r_{s}$ are attainable compared to 2D electron systems(2DES). Basic structure in the spectra appear very different from those observed in the 2DES{[}1{]}. For instance, a magnetic-field-induced Coulomb gap {[}1{]} appearing about the Fermi energy has a strong dependence on electron density (with a larger gap at low densities) that is not present for the 2DES. In addition, structure created by the exchange enhancement of spin splittings has an entirely different appearance from that seen in the 2DES. Ultimately, at lower temperatures, a high-resolution TDCS study of the 2DHS may show features related to the 2D metal-insulator transition. \\[4pt] [1] O.E. Dial et al, Nature 448, 176-179 (2007). [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X11.00010: High quality two-dimensional hole system on hydrogen terminated silicon (111) surfaces Binhui Hu, Tomasz M. Kott, Robert N. McFarland, Bruce E. Kane We have previously developed a novel field effect transistor structure, in which high mobility two-dimensional electrons are induced at a hydrogen-terminated Si(111) surface by a positive gate voltage through an encapsulated vacuum dielectric [1]. In this talk, we will demonstrate that a similar structure can also be used to define a high quality two-dimensional hole system (2DHS) at the H-Si(111) surface with a negative gate voltage. Hole concentrations up to $7.1\times 10^{11}$ cm$^{-2}$ are obtained. The longitudinal and Hall resistivities are measured as functions of magnetic fields up to 12 T. Preliminary data show Shubnikov-de Haas (SdH) oscillations at B $>$ 3 T at T= 5 K. Until now the studies on 2DHSs on Si(111) surfaces are limited, primarily due to the lack of high quality 2DHSs on them. The high quality 2DHS here can provide some new opportunities.\\[4pt] [1] K. Eng, R. N. McFarland, and B. E. Kane, Appl. Phys. Lett. 87, 052106 (2005). [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X11.00011: Microwave photo-voltaic oscillations in the GaAs/AlGaAs system Ganesh Chand, T. Ye, A. Ramanayaka, R.G. Mani, W. Wegscheider Microwave photo-excitation of the GaAs/AlGaAs system produces oscillations in the diagonal resistance that lead into novel zero-resistance states in the low temperature limit. Such photo-excitation also produces concomitant photo-voltage oscillations. Here, we examine this microwave photo-voltaic effect and correlate the results with observed magneto-transport over the frequency range $30 \le f \le 120 GHz$ in Hall bars fabricatedfrom material characterized by n = 2x $10^{11} cm^{-2}$. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X11.00012: Terahertz Coherent Control of Cyclotron Resonance in the Quantum Hall Regime T. Arikawa, X. Wang, J. Kono, D.J. Hilton, J.L. Reno, W. Pan We report on the creation and coherent control of a superposition of many-electron quantum states (or a qubit) in a Landau-quantized GaAs two-dimensional electron gas (2DEG) using a sequence of coherent terahertz (THz) pulses. The first pulse excites electrons from the highest-filled Landau level (LL) to the lowest-unfilled LL, creating a superposition of the two LLs which re-emits a coherent THz wave. We found that the second THz pulse incident within the decoherence time stops or enhances the THz re-emission depending on its arrival phase. These results show that an arbitrary coherent control of the LL qubit is possible using THz pulses. We also performed a simulation within the framework of single-particle optical Bloch equations, which reproduced the experimental results surprisingly well. This agreement shows that the 2DEG behaves in the same way as a single-electron two-level system despite the fact that it contains a large density of interacting electrons. This finding extends the Kohn's theorem to a more general level of coherent dynamics. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X11.00013: THz quantum Hall conductivity in a two dimensional electron gas A.V. Stier, H. Zhang, C.T. Ellis, D. Eason, G. Strasser, B.D. McCombe, J. Cerne We investigate the THz Hall conductivity through measurement of the Faraday effect at 84 cm$^{-1}$ near the cyclotron resonance (CR) in a two dimensional electron gas formed at a GaAs/(AlGa)As interface. Motivated by predictions of novel step-like features in the optical Hall conductivity ($\sigma _{xy})$ by Morimoto et.al. (Phys. Rev. Lett. 2009), we measure the THz $\sigma _{xy}$ as a function of filling factor and temperature using polarization modulation techniques (Grayson, Phys. Rev. Lett. 2002). We observe plateaus in the Faraday rotation near integer filling factors of 1, 2 and 3 which we attribute to the THz integer quantum Hall effect. In electron density dependent studies, we observe a slight non-monotonic shift of the plateaus as a function of filling factor at magnetic fields above CR. A comparison of this effect with the shift in temperature shows that this cannot be explained by a simple electronic heating effect. This research was funded through NSF-DMR1006078. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X11.00014: Superconducting Transport Mediated by Quantum Hall Edge Modes Stephanie Law, Michael Vissers, Allison Dove, Nadya Mason, James Eckstein We report transport measurements between superconducting leads separated by a small gap consisting of quantum Hall edge modes. The NbTi superconducting layer is grown in-situ on top of the semiconducting heterostructure to allow good contact. The samples are then fabricated into Hall bars with narrow gaps between the superconducting leads. Differential resistance and IV characteristics are measured in two and four terminal setups at 300mK both on and off quantum Hall plateaus. We will show that in the smallest gaps when we are on a plateau, we see a chiral supercurrent the direction of which is controlled by the field. With larger gaps we see either a zero bias resistance minimum or maximum. Data taken on and off plateaus show marked differences, indicating that quantum Hall modes are crucial. Results for samples with different heterostructures will also be shown. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X11.00015: In-plane Field Tuned Subband Quantum Hall Ferromagnetism Dagim Tilahun, Allan MacDonald Motivated by the recent experimental work of Guo \textit{et al.} (Phys. Rev. B 78, 233305 (2008)), we study the effects of an in-plane magnetic field on quantum Hall states in which subband, Landau level, and spin degrees of freedom compete. We find that the phase diagram identified by these authors can be explained qualitatively by using only single-electron properties, whereas the energy gap behavior can be explained only by considering electron-electron interactions. We predict a series of in-plane field tuned first order phase transitions in high-mobility samples. [Preview Abstract] |
Session X13: Focus Session: Continuum Description of Particulate Media
Sponsoring Units: GSNPChair: Ken Kamrin, Massachusetts Institute of Technology
Room: D225/226
Thursday, March 24, 2011 2:30PM - 2:42PM |
X13.00001: A nonlocal enhancement to granular elasto-plasticity Ken Kamrin, Georg Koval A general, three-dimensional law to predict granular flow in an arbitrary geometry has been an elusive goal for decades. Recently, an elasto-plastic continuum model has shown the ability to approximate steady flow and stress profiles in multiple inhomogeneous flow environments. However, the model does not capture some of the characteristic phenomena observed in the slow, creeping flow regime. As normalized flow-rate decreases, granular stresses are observed to become largely rate-independent and a dominating length-scale emerges in the mechanics. This talk attempts to account for these effects with a nonlocal correction term that modifies the continuum law when the inertial number drops below a critical value. The correction depends on stress and strain-rate gradients and brings in a natural dependence on the particle diameter. We implement the modified law in multiple geometries and validate its predictions against discrete particle simulations. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X13.00002: Effective temperature in elastoplasticity of amorphous solids Ido Regev, Laurent Boue, Jacques Zylberg, Itamar Procaccia, George Hentschel An effective temperature $T_{\rm eff}$ which differs from the bath temperature is believed to play an essential role in the theory of elasto-plasticity of amorphous solids. The definition of a measurable $T_{\rm eff}$ in the literature on sheared solids suffers however from being connected to a fluctuation-dissipation theorem which is correct only in equilibrium. Here we introduce a natural definition of $T_{\rm eff}$ based on measurable structural features without recourse to any questionable assumption. The value of $T_{\rm eff}$ is connected, using theory and scaling concepts, to the flow stress and the mean energy that characterize the elasto-plastic flow. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X13.00003: Shock driven jamming and periodic fracture of particulate rafts Mahesh Bandi, Tuomas Tallinen, L. Mahadevan A tenuous monolayer of hydrophobic particles at the air-water interface often forms a scum or raft. When such a monolayer is disturbed by localized surfactant introduction, a radially divergent shock emanates and packs the particles into a jammed, compact, annular band that grows with time. The resulting two-dimensional, disordered, elastic solid locally has a packing fraction that saturates at random close packed density ($\phi_{RCP}$) and fractures as it is driven radially outwards, to form periodic triangular cracks with robust geometrical features. We find that the number of cracks $N$ varies monotonically with the initial particulate packing fraction $\phi_{init}$, as does the compaction band radius $R^*$ at fracture onset. However, its width $W^*$ is constant across all $\phi_{init}$. A simple geometric theory that treats the compaction band as an elastic annulus, and accounts for mass conservation allows us to deduce that $N \simeq 2\pi R^*/W^* \simeq 4\pi \phi_{RCP}/\phi_{init}$, a result that we experimentally verify over the range ($0.1 \le \phi_{init} \le 0.64$). [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:42PM |
X13.00004: Application of Classical Nucleation Theory to Cavitation in Metallic Glass Invited Speaker: In order to predict the fracture toughness of amorphous solids such as metallic glasses it is necessary to understand the physics of the process zone. Theories of plastic deformation provide information about response to shear, but on their own these theories provide limited insight into the microscopic mechanisms that mediate the free surface generation critical to crack propagation. Previous molecular dynamics simulations indicate that cavitation likely plays this role. We have undertaken a series of molecular dynamics simulations of cavitation under hydrostatic tension in a binary metallic glass analog using pair-wise potentials. We compare the rate of cavity nucleation directly to homogeneous nucleation theory to examine the role of surface energy and irreversible deformation in the cavitation process. We find that both the reduction of the surface energy at small cavity size and the plastic deformation required for the cavity to grow play important roles in setting the strain-dependent free energy barrier to cavitation.\\[4pt] Work done in collaboration with Michael Spector, Materials Science and Engineering, Johns Hopkins University, Baltimore; Shuo Lu, Materials Science and Engineering, Beihang University; and Pavan K. Valavala, Materials Science and Engineering, Johns Hopkins University. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X13.00005: Macroscopic transport and topological transitions in ordered suspensions in parallel-wall channels Jerzy Blawzdziewicz, Nidhi Khurana, Eligiusz Wajnryb Our recent investigations of ordered suspensions in parallel-wall channels revealed complex nonlinear dynamics, including formation of defects in a particle lattice, dynamic order-disorder transitions, buckling of particle lattice, and fingering instabilities. We will describe hydrodynamic mechanisms that govern this collective particle behavior. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X13.00006: Continuum Mean-Field Theories for Molecular Fluids, and Their Validity at the Nanoscale C.B. Hanna, F. Peyronel, C. MacDougall, A. Marangoni, D.A. Pink We present a calculation of the physical properties of solid triglyceride particles dispersed in an oil phase, using atomic- scale molecular dynamics. Significant equilibrium density oscillations in the oil appear when the interparticle distance, $d$, becomes sufficiently small, with a global minimum in the free energy found at $d \approx$ 1.4 nm. We compare the simulation values of the Hamaker coefficient with those of models which assume that the oil is a homogeneous continuum: (i) Lifshitz theory, (ii) the Fractal Model, and (iii) a Lennard-Jones 6-12 potential model. The last-named yields a minimum in the free energy at $d \approx$ 0.26 nm. We conclude that, at the nanoscale, continuum Lifshitz theory and other continuum mean-field theories based on the assumption of homogeneous fluid density can lead to erroneous conclusions. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X13.00007: Coarse-Graining of a Physical Granular System Jie Zhang, Isaac Goldhirsch, Robert Behringer We present results, including particle displacements and rotations, as well as strain and stress fields, obtained by applying a resolution-controlled coarse-graining method to an experiment comprised of bidisperse disks subject to pure shear. We briefly review the experimental methods which involve determing inter-particle contact forces using the photoelastic properties of the disks. We then consider the philosophical and technical approaches of the coarse-graining methods used here. We particularly consider the emergence of shear bands, which are manifest in the displacements, rotations, and some strain fields, but not in the stress. Correlations of the displacement fluctuations decay on a very small scale, of the order of a few particle diameters, even close to jamming, which in this case, is induced by shear. We report an unexpected but simple correlation between particle rotation angles and the rotation field. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:54PM |
X13.00008: Nonequilibrium Thermodynamics of Driven Disordered Materials Invited Speaker: We present a nonequilibrium thermodynamic framework for describing the dynamics of driven disordered solids (noncrystalline solids near and below their glass temperature, soft glassy materials such as colloidal suspensions and heavily dislocated polycrystalline solids). A central idea in our approach is that the set of mechanically stable configurations, i.e. the part of the system that is described by inherent structures, evolves slowly as compared to thermal vibrations and is characterized by an effective disorder temperature. Our thermodynamics-motivated equations of motion for the flow of energy and entropy are supplemented by coarse-grained internal variables that carry information about the relevant microscopic physics. Applications of this framework to amorphous visco-plasticity (Shear-Transformation-Zone theory), glassy memory effects (the Kovacs effect) and dislocation-mediated polycrystalline plasticity will be briefly discussed. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X13.00009: Breakdown of Granular Constitutive Relations for Flow through a Narrow Vertical Channel Donald Candela, Kevin Facto We have used NMR/MRI techniques to study flow profiles and fluctuations in the dense, gravity-driven flow of a granular medium through a relatively narrow vertical channel (channel diameter approximately 20 grain diameters). Although the flow is macroscopically steady, the NMR experiments reveal large velocity fluctuations that can be characterized as random, short-lived jamming of the flow. Constitutive relations have been successfully developed for granular shear flows in constant pressure conditions, such as flow in open chutes or wide vertical channels. For the narrow-channel flow probed in our experiments, the constant-pressure constitutive relations are not appropriate. An alternative equation of state based on constant-volume conditions may be appropriate for the narrow-channel case, or it can be modeled using a piling-jamming model that abandons the constitutive-equation approach altogether. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X13.00010: Dynamic Structure Factor and Transport Coefficients of a Homogeneously Driven Granular Fluid in Steady State Katharina Vollmayr-Lee, Annette Zippelius, Timo Aspelmeier We study the dynamic structure factor of a granular fluid of hard spheres, driven into a stationary nonequilibrium state by balancing the energy loss due to inelastic collisions with the energy input due to driving. The driving is chosen to conserve momentum, so that fluctuating hydrodynamics predicts the existence of sound modes. We present results of computer simulations which are based on an event driven algorithm. The dynamic structure factor $F(q,\omega)$ is determined for volume fractions 0.05, 0.1 and 0.2 and coefficients of normal restitution 0.8 and 0.9. We observe sound waves, and compare our results for $F(q,\omega)$ with the predictions of generalized fluctuating hydrodynamics which takes into account that temperature fluctuations decay either diffusively or with a finite relaxation rate, depending on wave number and inelasticity. We determine the speed of sound and the transport coefficients and compare them to the results of kinetic theory. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X13.00011: Discrete-continuum mapping for fiber network mechanics Catalin Picu, Ali Shahsavari, Hamed Hatami-Marbini Semi-flexible random fiber networks are the structural element of many biological and non-biological systems such as the cytoskeleton, artificial tissue and cellulose-based products. We have shown that in these systems the density, as well as mechanical fields (elastic moduli, strain energy etc), are long-range power-law correlated. The correlation length evolves during deformation. A procedure to map the elasticity of the discrete system to continuum representations is developed. The method is used to solve boundary value problems defined over large fiber network domains. However, the mapping can be performed only in some situations, limitations which are discussed in this talk. [Preview Abstract] |
Session X14: Focus Session: Extreme Mechanics: Elasticity and Deformation IV
Sponsoring Units: GSNPChair: Narayanan Menon, University of Massachusetts, Amherst
Room: D227
Thursday, March 24, 2011 2:30PM - 3:06PM |
X14.00001: Harnessing Instabilities in Polymers under Electric Fields Invited Speaker: Subject to a voltage, a layer of a polymer reduces thickness and expands area, so the same voltage will induce an even higher electric field. The positive feedback may cause the polymer to thin down drastically, resulting in an electrical breakdown. This electromechanical instability has been long recognized in the electrical power industry as a major failure mode for polymer insulators. In this talk, we will present recent new observations and understandings of the electromechanical instability. For example, what will happen if the polymer is bonded on a rigid substrate to prevent the area expansion? We show that a new mode of instability will set in. Once the electric field reaches a critical value, the initially flat surface suddenly folds upon itself, deforming into a pattern of creases. As the electric field further rises, the creases increase in size and decrease in density, and strikingly evolve into holes in the polymer. The critical electric field for the creasing instability scales with square root of the polymer's modulus. We show that linear stability analysis overestimates the critical electric field for the instability. A theoretical model has been developed to predict the critical field by comparing the potential energies in the creased and flat states. The theoretical prediction matches consistently with experimental results. We further show that the instability can be harnessed with promising applications in many areas including high-breakdown-field organic capacitors, electrostatic lithography, dynamic pattern formations, fabrication of semi-permeable membranes, and energy harvesting. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X14.00002: Ultrasoft Electronics for Hyperelastic Strain, Pressure, and Direct Curvature Sensing Carmel Majidi, Rebecca Kramer, Robert Wood Progress in soft robotics, wearable computing, and programmable matter demands a new class of ultrasoft electronics for tactile control, contact detection, and deformation mapping. This next generation of sensors will remain electrically functional under extreme deformation without influencing the natural mechanics of the host system. Ultrasoft strain and pressure sensing has previously been demonstrated with elastomer sheets (eg. PDMS, silicone rubber) embedded with microchannels of conductive liquid (mercury, eGaIn). Building on these efforts, we introduce a novel method for direct curvature sensing that registers the location and intensity of surface curvature. An elastomer sheet is embedded with micropatterned cavities and microchannels of conductive liquid. Bending the elastomer or placing it on a curved surface leads to a change in channel cross-section and a corresponding change in its electrical resistance. In contrast to conventional methods of curvature sensing, this approach does not depend on semi-rigid components or differential strain measurement. Direct curvature sensing completes the portfolio of sensing elements required to completely map hyperelastic deformation for future soft robotics and computing. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X14.00003: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X14.00004: Tensile shock waves in rubber Krishnaswamy Ravi-Chandar, Johnathan Niemczura We examine the propagation of waves of finite deformation in rubbers through experiments and analysis; in particular attention is focused on the propagation of one-dimensional tensile shock waves in strips of latex and nitrile rubber. Tensile wave propagation experiments were conducted at high strain-rates by holding one end fixed and displacing the other end at a constant velocity. A high-speed video camera was used to monitor the motion and to determine the evolution of strain and particle velocity in rubber strips. Shock waves have been generated under tensile impact in pre-stretched rubber strips; analysis of the response yields the tensile shock adiabat for rubbers. The propagation of shocks is analyzed by developing an analogy with the theory of detonation; it is shown that the condition for shock propagation can be determined using the Chapman-Jouguet shock condition. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X14.00005: Kinetic features of pattern transformation and recovery in periodic hydrogel membranes Xuelian Zhu, Rong Dong, Ji Feng, Chi-Mon Chen, Shu Yang Pattern transformation triggered by mechanical instabilities is an attractive bottom-up method to create complex structures over a wide range of length scales. However, how to dynamically control the transformation and its recovery is yet to be studied. Here, we present a systematic study of the kinetic pattern transformation and its recovery using a model system from poly(2-hydroxyethyl methacrylate) hydrogel membrane with a square lattice of micron-sized cylindrical holes. The hydrogel membrane undergoes (1) a breathing mode (i.e. the hole reduces size but retains the shape) when exposed to DI-water; (2) a phase transition to a diamond plate pattern driven by capillarity during drying process; and (3) a recovery upon re-exposure to water. During drying, many antiphase boundaries (APBs) appear in the diamond plate pattern, which then act as embryos that determine the kinetic path for recovery. The boundary morphology (either random or aligned) can be manipulated by the moving speed of the waterfront. To reveal the underlying mechanism of pattern transformation and APB arrangement, as well as the role of APB in recovery, we utilized the dynamic Monte Carlo method to simulate the kinetic process of pattern transformation and recovery, which qualitatively matched well with the experiments. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X14.00006: Cavitation in elastomeric solids: A defect-growth theory Oscar Lopez-Pamies, Martin Idiart, Toshio Nakamura A new theory is introduced to study the phenomenon of cavitation in soft solids that, contrary to existing approaches, simultaneously: (i) applies to large (including compressible and anisotropic) classes of nonlinear elastic solids, (ii) allows to consider general 3D loading conditions with arbitrary triaxiality, and (iii) incorporates direct information on the initial shape, spatial distribution, and mechanical properties of the underlying defects at which cavitation can initiate. The basic idea is to cast cavitation in elastomeric solids as the homogenization problem of nonlinear elastic materials containing random distributions of zero-volume cavities, or defects. In spite of the generality of the proposed approach, the relevant calculations amount to solving tractable Hamilton-Jacobi equations, in which the initial size of the cavities plays the role of ``time'' and the applied load plays the role of ``space.'' [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X14.00007: The aerodynamics of jumping rope Jeffrey Aristoff, Howard Stone We present the results of a combined theoretical and experimental investigation of the motion of a rotating string that is held at both ends (i.e. a jump rope). In particular, we determine how the surrounding fluid affects the shape of the string at high Reynolds numbers: the string bends toward the axis of rotation, thereby reducing its total drag. We derive a pair of coupled non-linear differential equations that describe the shape, the numerical solution of which compares well with asymptotic approximations and experiments. Implications for successful skipping will be discussed. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X14.00008: Helical Root Buckling: A Transient Mechanism for Stiff Interface Penetration Jesse Silverberg, Roslyn Noar, Michael Packer, Maria Harrison, Itai Cohen, Chris Henley, Sharon Gerbode Tilling in agriculture is commonly used to loosen the topmost layer of soil and promote healthy plant growth. As roots navigate this mechanically heterogeneous environment, they encounter interfaces between the compliant soil and the underlying compacted soil. Inspired by this problem, we used 3D time-lapse imaging of Medicago Truncatula plants to study root growth in two-layered transparent hydrogels. The layers are mechanically distinct; the top layer is more compliant than the bottom. We observe that the roots form a transient helical structure as they attempt to penetrate the bi-layer interface. Interpreting this phenotype as a form of buckling due to root elongation, we measured the helix size as a function of the surrounding gel modulus. Our measurements show that by twisting the root tip during growth, the helical structure recruits the surrounding medium for an enhanced penetration force allowing the plants access to the lower layer of gel. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X14.00009: Increasing Digging Efficiency Using Two Biologically-Inspired Techniques Dawn Wendell, Peko Hosoi The mechanics of digging through granular materials often neglect the inhomogeneities present in granular packings. This work reports on two biologically-inspired mechanisms that aim to increase the efficiency of digging through granular materials by taking advantage of the variety of forces found in granular packings. First, flexible diggers demonstrate that a slight increase in flexibility can lead to more efficient digging using a completely passive mechanism. Secondly, a digger with an actuated tip is investigated to find optimum parameters for energy efficient digging with actuated mechanisms. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X14.00010: Pattern switches in granular crystals Katia Bertoldi, JongMin Shim, Fatih Goncu, Stephen Willshaw, Tom Mullin, Stefan Luding We report an experimental and numerical study of a pattern transformation in a regular array of macroscopic cylindrical particles with contrasting dimensions and stiffnesses. The initial structure is a square lattice with a pair of large (soft) and small (hard) particles at each lattice site. The application of a uniaxial compression produces a new periodic structure and the transformation principally depends on the size ratio of the particles. At small ratios it is homogeneous and approximately reversible i.e. the initial geometry is almost recovered after unloading. In contrast, when the size ratio is increased the final pattern is reached after a sudden rearrangement of the particles which involves the formation of a shear band. The structural reorganization of the granular crystal will have a significant effect on wave propagation properties and we suggest that this could have interesting applications in phononic and photonic crystals. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X14.00011: Evidence for a mechanical instability, via folding, of the vein network in leaves Pilnam Kim, Manouk Abkarian, Howard A. Stone The venation pattern of leaves is the archetype of a self-organized transport network whose efficiency and robustness stems from the connectivity of its hierarchical branching structure, but whose underlying principles of formation are not understood. Here we propose that the folding instability of the inner tissues of the leaf provides such a hierarchical venation pattern. Using a multi-layered polymeric system under an equibiaxial compressive stress, which mimics both growth and the layered structure of a leaf tissue, we show that a repetitive wrinkling-to-folding transition can achieve a hierarchical network of folds by continual, local reorganization of the stress field. We find that the resulting network topology, including closed loops, is the result of a spontaneous evolution of both terminal and segmental branching of the fold network and shares basic topological properties with venation patterns. This folding transition gives new insights into the role of mechanical stress as a possible feedback mechanism for cell differentiation in early veins. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X14.00012: Minimal resonances in annular non-Euclidean strips Bryan Chen, Christian Santangelo Differential growth processes play a prominent role in shaping leaves and biological tissues. Using both analytical and numerical calculations, we consider the shapes of closed, elastic strips which have been subjected to an inhomogeneous pattern of swelling. The stretching and bending energies of a closed strip are frustrated by compatibility constraints between the curvatures and metric of the strip. To analyze this frustration, we study the class of ``conical'' closed strips with a prescribed metric tensor on their center line. The resulting strip shapes can be classified according to their number of wrinkles and the prescribed pattern of swelling. We use this class of strips as a variational ansatz to obtain the minimal energy shapes of closed strips and find excellent agreement with the results of a numerical bead-spring model. We derive and test a surprising resonance condition for strips with minimal bending energy along the strip center line to exist. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X14.00013: Deflation of elastic surfaces Catherine Quilliet The deflation of elastic spherical surfaces has been numerically investigated, and show very different types of deformations according the range of elastic parameters, some of them being quantitatively understood through simple theoretical considerations. In particular, the role of the Poisson ratio is closely investigated. This work allowed to retrieve various shapes observed on hollow deformable shells (from colloidal to centimeter scale), on lipid vesicles, or on some simple biological objects. Conversely, it shows how high deformations can tell observers about mechanical properties of a body. Such investigations have been extended to other geometries, in order to provide clues to understand deformations of vegetal or animal tissues. [Preview Abstract] |
Session X15: Focus Session: Spins in Semiconductors - Manipulation of Dopant Spins
Sponsoring Units: DMP GMAG FIAPChair: Paul Koenraad, TU/Eindhoven
Room: D171
Thursday, March 24, 2011 2:30PM - 3:06PM |
X15.00001: Quantum information in silicon: Initialization, manipulation, storage and readout Invited Speaker: Spin qubits in silicon are exciting because of their long coherence times [1] and the electrical readout of the state of one electron spin [2]. In a single experiment we demonstrate initialization [3], manipulation, storage and electrical readout of quantum information with a small ensemble of phosphorus electronic and nuclear spins in silicon [4]. Our electrical readout does not destroy the electron spin coherence which is limited instead by naturally-occurring $^{29}$Si nuclear spins. These experiments require a pulsed electron spin resonance spectrometer operating at high magnetic fields [5]. Silicon quantum computers would benefit from having a second dopant species which can be addressed selectively [6-8], and we find that bismuth atoms are well suited for this role [9]. They offer long spin coherence times [9,10] as well as new opportunities [11] when compared with phosphorus. \\[4pt] [1] A M Tyryshkin {\&} S A Lyon, Phosphorus electron spin coherence time can be over 10 s, Private communication (2010) \newline [2] A Morello\textit{ et al}, Nature \textbf{467}, 687 (2010) \newline [3] D R McCamey, J van Tol, G W Morley {\&} C Boehme, Phys Rev Lett \textbf{102}, 027601 (2009) \newline [4] G W Morley\textit{ et al}, Phys Rev Lett \textbf{101}, 07602 (2008) \newline [5] G W Morley, L-C Brunel {\&} J van Tol, Rev Sci Instrum \textbf{79}, 064703 (2008) \newline [6] A M Stoneham, A H Harker {\&} G W Morley, J Phys Condens Matter \textbf{21}, 364222 (2009) \newline [7] A M Stoneham, A J Fisher {\&} P T Greenland, J Phys Condens Matter \textbf{15}, L447 (2003) \newline [8] P T Greenland\textit{ et al}, Nature \textbf{465}, 1057 (2010) \newline [9] G W Morley\textit{ et al}, Nature Mater \textbf{9}, 725 (2010) \newline [10] R E George\textit{ et al}, Phys Rev Lett \textbf{105}, 067601 (2010) \newline [11] M H Mohammady, G W Morley {\&} T S Monteiro, Phys Rev Lett \textbf{105}, 067602 (2010) [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X15.00002: Crystal orientation induced spin Rabi beat oscillations of point defects at the c-Si(111)/SiO$_{2}$ interface Seoyoung Paik, Sang-Yun Lee, Christoph Boehme Spin-dependent electronic transitions such as certain charge carrier recombination and transport processes in semiconductors are usually governed by the Pauli blockade within pairs of two paramagnetic centers. One implication of this is that the manipulation of spin states, e.g. by magnetic resonant excitation, can produce changes to electric currents of the given semiconductor material. If both spins are changed at the same time, quantum beat effects such as beat oscillation between resonantly induced spin Rabi nutation becomes detectable through current measurements [1]. Here, we report on electrically detected spin Rabi beat oscillation caused by pairs of $^{31}$P donor states and P$_{b}$ interface defects at the phosphorous doped Si(111)/SiO$_{2 }$interface. Due to the g-factor anisotropy of the P$_{b}$ center we can tune the intra pair Larmor frequency difference (so called Larmor separation) through orientation of the sample with regard to the external magnetic field. As the Larmor separation governs the spin Rabi beat oscillation, we show experimentally how the crystal orientation can influence the beat effect. \\[4pt] [1] D. R. McCamey, et al. \textit{Phys. Rev. Lett.} \textbf{104}, 017601 (2010). [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X15.00003: Dynamic Response of the Kondo Resonance in a Single-Electron Transistor in the Presence of Magnetic Field Bryan Hemingway, Tai-Min Liu, Andrei Kogan, Steven Herbert, Michael Melloch We report a sharp peak in the differential conductance of a Single-Electron Transistor (SET) in the Kondo regime irradiated with microwaves, plotted as function of an external, in-plane magnetic field, $B$. The peak emerges at frequencies, $hf,$ above $\sim T_K/2$ and shifts approximately linearly with the microwave signal frequency. At frequencies significantly below the Kondo scale, $T_K/h$ (M. Hettler and H. Schoeller Phys. Rev. Lett. 74, 4907-4910 (1995)), no such peak is present and the conductance data agree with the predictions based on static measurements. In the Coulomb Blockade regime, we find good agreement with the photon-assisted resonant tunneling model and experiments (T.H. Oosterkamp et al., Phys. Rev. Lett. 78 (1997)) . Our SETs are fabricated lithographically using GaAs/AlGaAs heterostructure with sheet density $4.8$ x $10^{11}$ cm$^{-2}$ and mobility $5$ x $10^{5}$ cm$^{2}$V$^{-1}$sec$^{-1}$ and have the lithographic dot size approximately $130$ nm in diameter. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X15.00004: Jahn-Teller induced multiple ferromagnetic exchange interactions in magnetic semiconductors Hannes Raebiger, Takeshi Fujita Ferromagnetic interactions among $3d$ impurities in compound semiconductors (II-VI, III-V, etc) are usually rationalized via ``double exchange'', ``$p$-$d$ exchange'', and ``superexchange'' type interactions, which ensue a description of the $3d$ impurity electronic configuration based on host symmetries. Obviously Jahn-Teller distortions break these symmetries and yield completely different, closed shell, electronic configurations for the impurities, which most theories~[1] simply describe as magnetically inactive. Nonetheless, such Jahn-Teller distorted $3d$ impurities e.g. in the AlN host exhibit strong short range ferromagnetic interactions. Superexchange models may offer some insight to ``closed shell'' magnetic interactions, not applicable to the present case, however. We investigate such peculiar magnetic interactions via density-functional calculations, and find that a Jahn-Teller distortion can trigger the formation of a 3d-3d chemical bond that stabilizes ferromagnetism via direct exchange~[2], and further facilitates a double exchange type interaction. This multiple exchange is what aligns parallel the spins of e.g. Cr impurities in AlN.\\[4pt] [1] A. Zunger, S. Lany, and H. Raebiger, Physics {\bf 3}, 53 (2010).\\[0pt] [2] H. Raebiger, S. Lany, and A. Zunger, PRL {\bf 99}, 167203 (2007). [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 4:18PM |
X15.00005: Detecting excitation and magnetization of individual dopants in a semiconductor two-dimensional electron gas Invited Speaker: Magnetic atoms doped into a semiconductor are the building blocks for bottom up spintronic and quantum logic devices. They also provide model systems for the investigation of fundamental effects. In order to correlate the dopant's atomic structure with its magnetism magnetically sensitive techniques with atomic resolution are a prerequisite. Here, I show electrical excitation and read-out $\left[1\right]$ of single magnetic dopant associated spins in a two-dimensional electron gas (2DEG) confined to a semiconductor surface $\left[2\right]$ using spin-resolved scanning tunneling spectroscopy $\left[3\right]$. I will review our real-space study of the quantum Hall transition in the 2DEG $\left[2\right]$ and of the magnetic properties of the dopants $\left[1\right]$. Finally, I will demonstrate that the dopant serves as an atomic scale probe for local magnetometry of the 2DEG. This work was done in collaboration with A. A. Khajetoorians, B. Chillian, S. Schuwalow, F. Lechermann, K. Hashimoto, C. Sohrmann, T. Inaoka, F. Meier, Y. Hirayama, R. A. R{\"o}mer, M. Morgenstern, and R. Wiesendanger.\\[4pt] $\left[1\right]$ A. A. Khajetoorians {\it et al.}, Nature {\bf 467}, 1084 (2010).\\[0pt] $\left[2\right]$ K. Hashimoto {\it et al.}, Phys. Rev. Lett. {\bf 101}, 256802 (2008).\\[0pt] $\left[3\right]$ J. Wiebe {\it et al.}, Rev. Sci. Instrum. {\bf 75}, 4871 (2004). [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X15.00006: Coherent spin precession of multiple spins in an inhomogeneous environment V. Kortan, M.E. Flatt\'e Mn dopants in GaAs, whose core spins are bound anti-aligned to a hole, forming a J=1 ground state of the neutral acceptor, are very sensitive to their environment, including strain [1] and electric fields [2]. This sensitivity affects spin precession by broadening resonance lines and shifting/adding resonant frequencies. Using a low energy Hamiltonian developed for a single Mn ion-hole complex in GaAs [1,2] we have studied spin dynamics of a small collection of spins in the presence of bias electric fields and strain fields. Each Mn ion-hole complex is locally subject to a random electric field in addition to bias fields to determine if coherent spin precession persists. Using these calculations we predict the possible observation of coherent spin precession of small numbers of Mn spins via optical polarization measurements [3], and estimate the strength of the random field necessary to destroy the signal of coherent spin precession. This work supported by NRI through WIN.\\[4pt] [1] A. M. Yakunin, et al, Nature Mat. 6, 512 (2007).\\[0pt] [2] J.-M Tang, Jeremy Levy, and M. E. Flatt\'e, Phys. Rev. Lett. 97, 106803 (2006).\\[0pt] [3] R. C. Myers, et al, Nature Mat. 7, 203 (2008). [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X15.00007: Study of spin interactions between InAs surface electrons and local magnetic moments by antilocalization measurements Yao Zhang, R.L. Kallaher, V. Soghomonian, J.J. Heremans Spin-orbit coupled electrons in the InAs surface accumulation layer can be used as a sensitive system to experimentally study the interactions and exchange between electrons and local magnetic moments in semiconductors. We use antilocalization measurements as a probe of quantum states, by comparing measurements on patterned InAs accumulation layers where Sm$^{3+}$, Gd$^{3+}$ and Ho$^{3+}$ have been deposited, with those where no magnetic species are deposited. The randomly distributed ions modify the spin-orbit scattering time and the magnetic spin-flip time, identified via the antilocalization signal and characterized over temperature. The magnetic spin-flip time carries information about magnetic interactions. Experiments indicate that the spin-orbit scattering times display a weak temperature dependence. The Sm$^{3+}$ and Gd$^{3+}$ cases yield temperature-independent magnetic spin-flip times, while Ho$^{3+}$ shows a spin-flip time obeying T$^{-1/2}$ at low temperatures. Similar results as observed in the Ho$^{3+}$ case have in the literature been attributed to Kondo-like behavior. We thus interpret the results as indicative of a Kondo interaction with a Kondo temperature considerably above 5 K, of which antilocalization measurements can identify the low-temperature tail (partial support from DOE DE-FG02-08ER46532). [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X15.00008: Spin relaxation dynamics for an electron gas with localized magnetic impurities near the ferromagnetic transition Matthew Mower, Giovanni Vignale We study the spin relaxation dynamics of electrons in Mn-doped GaAs. Modeling the Mn as magnetic impurities embedded in an electron gas, we construct effective electron-electron and impurity-impurity interactions. This model exhibits a ferromagnetic transition as the temperature is lowered. Near the ferromagnetic transition, strong spin fluctuations cause an enhancement of the electron scattering rate, which affects the spin relaxation time of spin polarized electrons. This is especially notable in the D'yakonov-Perel' spin relaxation time which is proportional to the electron scattering rate. We will elucidate the behavior of the spin relaxation time and other spin-dependent kinetic coefficients near the ferromagnetic transition. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X15.00009: Intrinsic Magnetism at Silicon Surfaces Steven Erwin, Franz Himpsel It has been a long-standing goal to create magnetism in a nonmagnetic material by manipulating its structure at the nanometer scale. Many structural defects have unpaired spins; an ordered arrangement of such defects can give rise to a magnetically ordered state. Here we predict theoretically [1] that stepped silicon surfaces stabilized by adsorbed gold realize this goal by self-assembly, creating linear chains of polarized electron spins with virtually perfect structural order. The spins are localized at the silicon step edges, which have the form of graphitic hexagonal ribbons. The predicted magnetic state is indirectly supported by recent experimental observations, such as the coexistence of double- and triple-period distortions and the absence of edge states in photoemission. Ordered arrays of spins at a surface offer access to local probes with single spin sensitivity, such as spin-polarized scanning tunneling microscopy. The integration of structural and magnetic order is crucial for technologies involving spin-based computation and storage at the atomic level. \\[4pt] [1] S.C. Erwin and F.J. Himpsel, Nature Communications 1:58 (2010). [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X15.00010: Magnetic Si Atoms at the Step Edges of Si(553)-Au Paul Snijders, Nathan Guisinger, Phillip Johnson, Steven Erwin, Franz Himpsel A recent calculation predicts the possibility of magnetism at step edges of vicinal Si(111) surfaces decorated with gold [1]. Graphene-like Si ribbons are formed, which contain spin-polarized Si atoms at their edges. Those atoms form a six-fold superlattice at low temperature. Scanning tunneling spectroscopy (STS) of the magnetic broken bond orbitals reveals two peaks below 50 K. They match the calculated majority and minority spin states. The peaks merge into a single, broad peak at 300 K due to rapid spin fluctuations. \\[4pt] [1] Steven C. Erwin and F. J. Himpsel, Nature Communications 1:58 (2010). [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X15.00011: Electronic structure of Gd impurities in GaN on Ga, N and Ga-N adjacent sites and the role of N interstitials Tawinan Cheiwchanchamnangij, Walter Lambrecht Gd-doped GaN is one of the most interesting dilute magnetic semiconductors. However, the origins of its magnetic properties are still unclear. Previous studies have focused on the role of intrinsic defects, such as Ga vacancies and N or O interstitials. Here, we study Gd doped in pairs on adjacent Ga and N sites, which were suggested to be required to explain the X-ray linear dichroism signals in Gd L-edge spectra by Ney et al. JMMM 322, 1162 (2010). By using the FP-LMTO method in the LSDA+U, we find that the Gd on N site is pushed to the interstitial site after the relaxation and there is no extra magnetic moment besides the seven Bohr magneton from the 4f half-filled shell on each Gd atom. In spite of the relaxation, we find the energy of formation of this cluster to be of order 10 eV, which shows that the Gd doped on Ga-N adjacent sites is unlikely to occur. We also study Gd doped on a single N site and find an excess magnetic moment of 3 Bohr magneton which is spread over Gd, the nearest neighbor Ga atoms, and second nearest neighbor N atoms. However, its energy of formation is also large that this kind of impurity is unlikely to occur. We critically examine previous work on the role of N interstitials in the magnetism of Gd-doped GaN by studying the magnetic properties of the split interstitials, their interaction with each other and with Gd. [Preview Abstract] |
Session X16: Focus Session: Spins in Carbon-Based Materials -- Graphene, CNT, and C60
Sponsoring Units: GMAG DMPChair: Luis Hueso, CIC nanoGUNE Consolider
Room: D173
Thursday, March 24, 2011 2:30PM - 2:42PM |
X16.00001: Enhanced spin injection and spin lifetime in Graphene Wei Han, Keyu Pi, Kathleen McCreary, Yan Li, Roland Kawakami Graphene is an attractive material for spintronics due to the low intrinsic spin-orbit and hyperfine coupling, which should lead to excellent spin transport properties. Earlier studies on spin injection and transport in graphene present two major challenges: low spin injection efficiency and short spin lifetimes compared to the theoretical predictions. In our work, we utilized TiO2 Seeded MgO barriers and achieved tunneling spin injection into single layer. As a result, large nonlocal magnetoresistances were observed at room temperature, with high spin injection efficiency up to 30{\%}. Surprisingly, enhanced spin lifetimes of graphene are obtained, which is due to reducing the contract-induced spin relaxation by inserting tunnel barrier between graphene and Co electrodes. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X16.00002: Electrical Detection of Spin Transport in Epitaxial Graphene Grown on the Si-face of Hexagonal SiC(0001) J. Abel, A. Matsubayashi, J. Garramone, C. Dimitrakopoulos, A. Grill, C.Y Sung, V. LaBella Graphene has great potential for use as a spin transport channel due to its low spin orbit coupling and high mobility. Spin diffusion lengths in the microns have been demonstrated on exfoliated graphene at room temperature\footnote{Wei Han, et al. PRL 105, 167202 (2010). }. We will present our measurements of spin relaxation in both exfoliated graphene and epitaxially grown graphene on SiC from IBM using non-local Hanle measurements as a function of temperature. The diffusion lengths on epitaxial graphene were comparable to those found in exfoliated flakes. The initial results show the diffusion length is limited by contact induced relaxation that occurs at the metal/graphene interface in agreement with results from exfoliated flakes. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X16.00003: Room Temperature Spin Transport in C$_{60}$-based spin valves. Luis Hueso, Marco Gobbi, Roger Llopis, Federico Golmar, Felix Casanova Carbon-based materials offer a unique playground for spin transport studies by merging relatively small spin relaxation mechanisms with the potential chemical versatility of some organic molecules. However, how the spin travels inside such materials is far from understood. In this work, we present magneto-transport studies in vertical spin valves containing a C$_{60}$ non-magnetic spacer and simple ferromagnetic (Co and Py) electrodes. Large magnetoresistance values (up to 5{\%}) are recorded at room temperature for fullerene thickness up to 30 nm. Remarkably, magnetoresistance is also present at relatively high bias (1 Volt), highlighting the robustness of the spin transport in this material. By choosing such a simple carbon system we are also able to introduce a simple multi-step tunneling model, which explain the electronic transport data and which is compatible with coherent spin transport over long distances. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:42PM |
X16.00004: Spin-orbit coupling in graphene: from single layers to graphite Invited Speaker: The spin-orbit interaction in graphene is full of contrasts. First, this relativistic interaction destroys the ideal relativistic ``touching cones'' electronic dispersion at the K points. A finite, albeit small, gap appears, giving a finite mass to the electrons. Then, while the spin-orbit splitting in the carbon atom is about 10 meV, the electronic states at the K points have a gap of only about 24 micro eV. Finally, it turns out that this quintessential sp material has its spin-orbit coupling derived almost exclusively from d orbitals. In this talk I will give first-principles [1] and tight-binding [2] perspectives on the spin-orbit coupling in graphene in the presence of a transverse electric field. The field, which would normally come from the substrate or the gates, breaks the space inversion symmetry and gives the extrinsic (Bychkov-Rashba) splitting of the states. It also brings interesting band-structure topologies, from gapped at low electric fields (the topological insulator phase), through a mixture of genuine touching Dirac cones and parabolic bands (the intrinsic and extrinsic spin-orbit strengths equal), to gapples, dominated by the extrinsic effects [1]. The intrinsic coupling is dominated by d orbitals, while the extrinsic by the field induced hybridization of the s and p orbitals. It turns out that similar physics holds for bilayer and trilayer graphene, ultimately also in graphite. I will also discuss the problem of the spin relaxation in graphene. The main issue is that conventional theories predict microseconds for the spin relaxation time, while experiments seem to consistently yield 100 ps. One possibility [3] is that that spin relaxation in graphene is due to adatoms that pull out the carbon-like spin-orbit coupling of the p electrons and lead to the larger spin relaxation of the Dyakonov-Perel type.\\[4pt] [1] M. Gmitra et al., Phys. Rev. B 80, 235431 (2009);\\[0pt] [2] S. Konschuh et al., Phys. Rev. B (in press);\\[0pt] [3] C. Ertler et al., Phys. Rev. B(R) 80, 041405 (2009). [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X16.00005: Ferromagnetism in Mn-implanted HOPG Samaresh Guchhait, Hendrik Ohldag, Domingo Ferrer, Sanjay Banerjee 20 keV energy Mn ions were implanted on HOPG samples at 300$^{\circ}$C. SQUID magnetometer measurements show ferromagnetic ordering and magnetic hysteresis at very low temperatures. Mn K-edge XAS spectra show presence of Mn and O in our sample and XMCD data shows ferromagnetic ordering of Mn at 14 K, but not at 300 K. SIMS data show presence of Mn and O with carbon, besides other elements. Raman spectroscopy results indicate disorder graphite phase and high resolution TEM images confirm amorphous Mn-implanted region with presence of nanocrystallites. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X16.00006: Organic spin-valves based on fullerene C60 Ran Lin, Fujian Wang, Markus Wohlgenannt, Chunyong He, Xiaofang Zhai, Yuri Suzuki Recent work suggests that the spin-transport length in organic semiconductors is limited by hyperfine coupling. Therefore, to potentially overcome this limitation, we fabricated spin-valves based on C60 for which the hyperfine coupling is minute. However, our devices do not show a significantly larger spin-diffusion length. This suggests that either a mechanism other than hyperfine coupling causes the loss of spin-polarization, or that in thick devices an increasing conductivity mismatch limits spin-injection. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X16.00007: On the Role of Spin-Orbit Coupling in the Spin Response of C$_{60}$-based Spintronics Devices Tho Nguyen, Fijian Wang, Xiao-Guang Li, Eitan Ehrenfreund, Valy Vardeny We report comprehensive studies of the spin response in C$_{60}$-based spintronics devices such as spin valves and diodes. The buckeyball C$_{60}$ molecules are composed of $\sim $99{\%} $^{12}$C carbon atoms having spinless nuclei with zero hyperfine interaction. Therefore it was believed that the spin diffusion length in C$_{60}$-based spin-valves is large, and the magnetoresistance (MR) in C$_{60}$ diodes is negligible small. Surprisingly, we obtained a small spin diffusion length which we believe to be due to a relatively strong spin-orbit (SO) coupling in the material. We also found that the MR in C$_{60}$ diodes is relatively small, with characteristic magnetic field response dominated by the SO coupling with strength, $\xi \approx \quad \raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} \quad \mu $eV, more than ten times larger than the HFI constant\textbf{. }This was verified by measuring the response of $^{13}$C-rich C$_{60}$ diodes. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X16.00008: Kondo effect of magnetic impurities on nanotubes Pier Paolo Baruselli, Alexander Smogunov, Michele Fabrizio, Erio Tosatti The effect of magnetic impurities on the ballistic conductance of nanocontacts is, as suggested in recent work, amenable to ab initio study [1]. Our method proceeds via a conventional density functional calculation of spin and symmetry dependent electron scattering phase shifts, followed by the subsequent numerical renormalization group solution of Anderson models -- whose ingredients and parameters are chosen so as to reproduce these phase shifts. We apply this method to investigate the Kondo zero bias anomalies that would be caused in the ballistic conductance of perfect metallic (4,4) and (8,8) single wall carbon nanotubes, ideally connected to leads at the two ends, by externally adsorbed Co and Fe adatoms. The different spin and electronic structure of these impurities are predicted to lead to a variety of Kondo temperatures, generally well below 10 K, and to interference between channels leading to Fano-like conductance minima at zero bias.\\[4pt] [1] P. Lucignano, R. Mazzarello, A. Smogunov, and E. Tosatti, Nature Materials 8, 563 (2009). [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X16.00009: Electric-field control of magnetism in graphene quantum dots: A route to spin field effect transistors Luis Agapito, Nicholas Kioussis, Efthimios Kaxiras Graphene is a promising candidate for all-carbon electronics because of its outstanding electrical, mechanical, and thermal properties. Also, the relentless drive for miniaturization leads to the use of ever smaller graphene fragments; at nanoscopic dimensions ($<$ 10nm), edge states become more relevant. Edge states are important because they lie in the vicinity of the Fermi level and hence are relevant to transport properties. Furthermore, edge states exhibit magnetism. We have employed ab-initio electronic structure and Landauer-B\"{u}ttiker transport calculations to study the magnetoelectro effects of graphene patches. We will present results of (1) how specific geometries (such as ``diamond'' shape) favor specific magnetic states, (2) how those magnetic states can be controlled by an external electric field [1], and (3) we will demonstrate how a graphene fragment containing different edge geometries can be employed as a spin-polarized field effect transistor.\\[4pt] [1] Agapito et al., PRB Rap. Com. 82, (2010) [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X16.00010: Evidence for magnetic behavior in chemically modified graphene Joel Therrien, Kyle Twarowski, Vaibhav Mathur, Antonio H. Castro-Neto Although graphene has exceptional electronic and structural properties, there is very little experimental evidence that graphene by itself shows strong electron-electron correlations. In fact, in spite of a large amount of theoretical work, recent experiments have shown that pure and clean graphene shows no signs of correlated many-body states such as magnetism or superconductivity. We will report on the observation of room temperature magnetism in mechanically exfoliated, chemically modified, graphene. The effect has been found using both magnetic force microscopy and magnetization tests. It was shown that the graphene can be brought back to a non-magnetized state by removing the surface chemistry. The search for correlated electronic states in graphene generates an enormous interest because of its low dimensionality, which is prone to strong quantum and thermal effects, and also because it would open up doors for a plethora of technological applications, from permanent two-dimensional magnets to spintronics. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X16.00011: QMC study of molecules for spintronics and photoswitching Matus Dubecky, Rene Derian, Lucia Horvathova, Lubos Mitas, Ivan Stich A combination of QMC and quantum chemistry (CAS-SCF) techniques are used to study two large molecules: azobenzene (AB) imporatnt as a photoswitch and vanadbenzene (VB), frequently used in spintronics [1]. In AB higher singlet state, S$_{2}$ the fingerprint of AB in excitation spectra has been calculated in addition to the low singlet states S and S$_{1}$ [2]. We have also calculated the lowest triplet T$_{1}$ vertical excitation, identified by EELS [3] as well as adiabatic T$_{1}$ excited state. All calculated energies are in excellent agreement with available experiments [3, 4]. In VB we focus initially on PES for dissociation and excited state of the vanadium cation. \\[4pt] [1] V.V. Maslyuk et al., Phys.Rev.Lett. 97, 201, (2006). [2] M. Dubecky et al., J.Chem.Phys, accepted (2010). [3] M. Allan, private communication. [4] J.-{\AA}. Andersson, R. Petterson, L. Tegn\'{e}r, J. Photochem. \textbf{20}, 17 (1982). [Preview Abstract] |
Session X17: Focus Session: Magnetic Oxide Thin Films - Multiferroic Heterostructures and Europium Oxide
Sponsoring Units: GMAG DMPChair: Tiffany Santos, Argonne National Laboratory
Room: D174
Thursday, March 24, 2011 2:30PM - 3:06PM |
X17.00001: Electric field control of magnetism in multiferroic heterostructures Invited Speaker: Much interest is being devoted to designing systems where magnetic and ferroelectric orders coexist (multiferroics), and where the presence of magnetoelectric coupling could enable the electrostatic control of magnetism in the solid state. In particular, proximity effects can be tailored to design novel electronic structures with enhanced magnetoelectric couplings in composite heterostructures [1]. A striking example of this approach is our recent demonstration of a large, charge-mediated, magnetoelectric coupling in epitaxial PZT/LSMO heterostructures [2], which explores the sensitivity of the magnetic properties of the doped manganites to charge. Through magnetic, electric, structural and spectroscopic characterization, we demonstrate that the magnetoelectric coupling in PZT/LSMO heterostructures is electronic in origin, and results from the modulation in the valency of the Mn upon switching the PZT ferroelectric polarization [3]. In particular, we conclude that the interfacial spin ordering is modified upon charge doping, which explains the large magnetoelectric response found in this system [4]. This ability to control spin via electric fields opens a new pathway for the development of novel spin-based technologies. \\[4pt] [1] Vaz et al. Adv. Mater. 22:2900, 2010.\\[0pt] [2] Molegraaf et al. Adv. Mater. 21:3470, 2009.\\[0pt] [3] Vaz et al. Phys. Rev. Lett., 104:127202, 2010.\\[0pt] [4] Vaz et al. Appl. Phys. Lett., 97:042506, 2010. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X17.00002: Ferroelectric field effect modulation of magnetism in composite multiferroics Jason Hoffman, Carlos Vaz, Yaron Segal, Matthew Marshall, Fred Walker, Charles Ahn This work harnesses the strong charge-driven magnetoelectric coupling in Pb(Zr$_{0.2}$Ti$_{0.8}$)O$_{3}$/La$_{0.8}$Sr$_{0.2}$MnO$_{3}$ (PZT/LSMO) heterostructures to explore the sensitivity to charge of the electron transport and magnetic behavior of complex oxides. Epitaxial LSMO films that exhibit a highly ordered crystalline structure, as determined by \emph{in situ} reflection high energy electron diffraction (RHEED) measurements and \emph{ex situ} x-ray diffraction and transmission electron microscopy are grown by oxide molecular beam epitaxy. Off-axis RF magnetron sputtering is used to grow the PZT gate dielectric, which is characterized by square polarization-electric field hysteresis loops, with a large polarization and low leakage current. We use a combination of low-field magneto-transport and magneto-optic Kerr effect (MOKE) magnetometry to study the ferroelectric field effect induced changes in the magnetic coercive field in PZT/LSMO bilayers. A reversible shift in the coercive field is observed for the two polarization states of the ferroelectric, with a larger coercive field in polarization state that accumulates hole carriers at the PZT/LSMO interface. The reversible electrical control of magnetism in engineered heterostructures is opens new directions in the field of spintronics. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X17.00003: Pulsed Laser Deposition of Cr$_{2-x}$Fe$_{x}$TeO$_{6}$ Thin Film Junlei Wang, Kirill D. Belashchenko, Peter A. Dowben, Christian Binek Promising spintronic concepts such as Cr$_{2}$O$_{3}$ based voltage-controlled exchange bias system [1] employ electric controlled boundary magnetization. Symmetry arguments reveal that equilibrium boundary magnetization is a generic property of magnetoelectric antiferromagnets [2]. However, experimental evidence of the boundary magnetization is scarce. Here we explore non-traditional growth of magnetoelectric oxides with tri-rutile structure using pulsed laser deposition (PLD) methodology. We grow and characterize structurally and magnetically various magnetoelectric thin films of the Cr$_{2-x}$Fe$_{x}$TeO$_{6}$ family starting from x=2 in order to take advantage of the reduced chemical complexity of Fe$_{2}$TeO$_{6}$ and the beneficial high temperature onset of antiferromagnetic order at 230K in comparison to 90K of Cr$_{2}$TeO$_{6}$. Our investigation aims on an experimental test of the predicted generality of the equilibrium boundary magnetization in magnetoelectric antiferromagnets. \\[4pt] [1]. He, Xi et al., Nature Materials 9, 579 - 585 (2010) \\[0pt] [2]. Belashchenko, K.D., Phys. Rev. Lett. 105, 147204 (2010) [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X17.00004: Properties of the Predicted Multiferroic Ca$_{3}$Mn$_{2}$O$_{7}$ - Experiment R. Misra, C. Adamo, N.A. Benedek, S.A. Denev, A. SenGupta, J.A. Mundy, J.H. Lee, D.A. Muller, V. Gopalan, C.J. Fennie, D.G. Schlom, P. Schiffer We have studied the properties of epitaxial films of Ca$_{3}$Mn$_{2}$O$_{7}$, an $n=2$ Ruddlesden-Popper phase. This material has been predicted to have novel multiferroic properties, including electric field switching of the magnetization [1]. 50 nm thick unstrained Ca$_{3}$Mn$_{2}$O$_{7}$ films were grown by reactive MBE on (110) YAlO$_{3}$ single crystal substrates. XRD shows that the Ca$_{3}$Mn$_{2}$O$_{7}$ films are single phase and epitaxial with (001) Ca$_{3}$Mn$_{2}$O$_{7}$ // (110) YAlO$_{3}$. Our films show a transition to a weakly ferromagnetic or canted antiferromagnetic state below 120K. The magnetic properties have strong anisotropy with a clear transition visible with an in-plane applied field, but none along the out of plane direction. Second harmonic generation results show that a weak polar order exists at room temperature and it persists until $\sim $700$^{\circ}$C. We also report on the low temperature dielectric properties of the material. \\[4pt] [1] N. A. Benedek and C. J. Fennie, arXiv:1007.1003v1. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 4:18PM |
X17.00005: Robust isothermal electric control of exchange bias at room temperature Invited Speaker: Voltage-controlled spintronics is of particular importance to continue progress in information technology through reduced power consumption, enhanced processing speed, integration density, and functionality in comparison with present day CMOS electronics. Almost all existing and prototypical solid-state spintronic devices rely on tailored interface magnetism, enabling spin-selective transmission or scattering of electrons. Controlling magnetism at thin-film interfaces, preferably by purely electrical means, is a key challenge to better spintronics. Currently, most attempts to electrically control magnetism focus on potentially large magnetoelectric effects of multiferroics. We report on our interest in magnetoelectric Cr$_{2}$O$_{3}$ (chromia). Robust isothermal electric control of exchange bias is achieved at room temperature in perpendicular anisotropic Cr$_{2}$O$_{3}$(0001)/CoPd exchange bias heterostructures. This discovery promises significant implications for potential spintronics. From the perspective of basic science, our finding serves as macroscopic evidence for roughness-insensitive and electrically controllable equilibrium boundary magnetization in magnetoelectric antiferromagnets. The latter evolves at chromia (0001) surfaces and interfaces when chromia is in one of its two degenerate antiferromagnetic single domain states selected via magnetoelectric annealing. Theoretical insight into the boundary magnetization and its role in electrically controlled exchange bias is gained from first-principles calculations and general symmetry arguments. Measurements of spin-resolved ultraviolet photoemission, magnetometry at Cr$_{2}$O$_{3}$(0001) surfaces, and detailed investigations of the unique exchange bias properties of Cr$_{2}$O$_{3}$(0001)/CoPd including its electric controllability provide macroscopically averaged information about the boundary magnetization of chromia. Laterally resolved X-ray PEEM and temperature dependent MFM reveal detailed microscopic information of the chromia (0001) surface magnetization and provide a coherent interpretation of our results on robust isothermal electric control of exchange bias. The latter promise a new route towards purely voltage-controlled spintronics and an exciting way to electrically control magnetism. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:54PM |
X17.00006: Europium Chalcogenide Magnetic Semiconducting Nanocrystals Invited Speaker: This abstract not available. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X17.00007: First-principles analysis of magnetic interaction in electron-doped EuO Joonhee An, Kirill Belashchenko Using linear response calculations within the linear muffin-tin orbital method, we analyze the exchange interaction in electron-doped EuO. The 4f shell is treated within the LDA+U method. Calculations in the virtual crystal approximation show that the RKKY interaction mediated by the conduction band qualitatively explains the observed doping dependence of the Curie temperature in EuO. Further, to understand the role of a particular rare earth dopant, we consider EuO supercells with a substitutional Gd atom, as well as with an oxygen vacancy. Important differences with the virtual crystal approximation are found. The behavior of the exchange interaction in real space is analyzed, and its mechanisms are sorted out. The applicability of the magnetic polaron picture to Gd-doped EuO is evaluated. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X17.00008: Curie temperature of electron-doped EuO - is there an intrinsic limit? A. Schmehl, T. Mairoser, A. Melville, T. Heeg, L. Canella, P. B\"oni, W. Zander, J. Schubert, D.E. Shai, E.J. Monkman, K. M. Shen, D.G. Schlom, J. Mannhart Increasing the Curie temperature ($T_{C})$ of the ferromagnetic semiconductor europium monoxide is the key problem to make this versatile material attractive for wide use. Its half-metallic behavior and its structural and electronic compatibility with Si, GaN and GaAs make EuO a promising material for semiconductor-based spintronics. By doping EuO with donor impurities, $T_{C}$ can substantially be increased. This increase is attributed to an additional exchange interaction that is mediated via the conduction electrons. Here we report on Hall measurements on Gd doped EuO films grown over a wide range of doping concentrations and growth conditions. We demonstrate that only a small fraction of the introduced impurities actually act as donors even for optimized growth parameters. Too high growth temperatures even render the dopants completely inactive. These results open the exciting question, if further raising the charge carrier density will elevate the Curie temperature way above today's maximum value of 170 K. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X17.00009: The Effect of Lu Doping on Ferromagnetic EuO Alexander Melville, Thomas Mairoser, Andreas Schmehl, Jochen Mannhart, Darrell Schlom Europium Oxide (EuO) is a poorly understood ferromagnetic semiconductor whose spin-ordering temperature (T$_{C})$ can be greatly influenced by the inclusion of dopants such as oxygen vacancies or one of several trivalent ions. The ability to grow high-quality crystalline and stoichiometric EuO by adsorption-controlled growth using molecular-beam epitaxy is imperative in separating the effect of oxygen vacancies from that of trivalent dopants. In this study, we have prepared 5{\%} Lu-doped EuO and characterized the effects of this doping on the magnetic and electronic properties. We show for the first time that Lu is a viable dopant material for EuO, increasing the T$_{C}$ up to 120K as a result of an increase in the carrier concentration to 1.8x10$^{26 }$m$^{-3}$ from 1.0x10$^{23 }$m$^{-3}$. This is on par with other EuO films grown in an adsorption-controlled environment and doped with La or Gd. Furthermore, we find that EuO maintains a high spin-polarization ($>$80{\%}) at this doping level. As a result of the simultaneously high T$_{C}$ and high spin-polarization, EuO can be considered for spintronic applications at much higher temperatures than possible for undoped EuO. [Preview Abstract] |
Session X18: Focus Session: Low D/Frustrated Magnetism - Spin Chains & Ladders
Sponsoring Units: GMAG DMPChair: Marcelo Jamie, Los Alamos National Laboratory
Room: D172
Thursday, March 24, 2011 2:30PM - 3:06PM |
X18.00001: Quantum criticality, kink confinement, and emergent symmetries in coupled Ising chains and ladders Invited Speaker: In this talk I cover the physics in three of the central quantum phase transitions in 1D. First, the transverse Ising model which is realized in CoNb2O6. While this is perhaps the simplest textbook case of a quantum phase transition, a remarkable emergence of E8 symmetry arises close to the quantum critical point. This manifests itself in an octave of bound states. We observe these experimentally and in particular the interval of the first two resonances on this octave which are found to match the golden ratio 1.618{\ldots} - just as predicted for the emergence of this extraordinary symmetry. I then plan to show with the example of the Heisenberg chain how we can probe the quantum critical volume experimentally and show the characteristic scaling behaviour in space and time. The third example is of a spin ladder CaCu2O3 which is near the long sought after Wess-Zumino-Novikov-Witten quantum critical point. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X18.00002: Evidence for strong orbital fluctuations below the Jahn-Teller transition in Sr$_{3}$Cr$_2$O$_8$ J. Deisenhofer, Zhe Wang, M. Schmidt, A. G\"{u}nther, S. Schaile, N. Pascher, F. Mayr, Y. Goncharov, H.-A. Krug von Nidda, A. Loidl, D.L. Quintero-Castro, A.T.M.N. Islam, B. Lake We report on the magnetic and phononic excitation spectrum of SrCrO~determined by THz and infrared (IR) spectroscopy, specific heat, and electron spin resonance measurements. We identify the singlet-triplet excitations in the dimerized ground state and observe an extended temperature range $T^{*}(=$ 125 K)$<$ $T<$ $T_{JT}(=$ 285 K$)$ below the Jahn-Teller transition, where the IR active phonons change only gradually with decreasing temperature. A clear anomaly in the specific heat marks the onset of orbital ordering at $T_{JT}$, but a detailed analysis of the orbital contribution to the specific heat shows the persistence of strong fluctuations down to $T^{*}$ in agreement with the IR data. Due to these fluctuations we can observe electron spin resonance absorptions only below $T^{*}$ with a linewidth \begin{math}\propto \exp{(-\Delta/k_BT)}\end{math} indicating an Orbach-type spin relaxation via the excited orbital state of the Cr $e$ doublet split by $\Delta/k_B$ = 388~K. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X18.00003: NMR in the high-field magnetic phase of LiCuVO$_4$ Wolfgang Kraetschmer, Norbert Buettgen, Alois Loidl, Leonid E. Svistov, Lyudmila A. Prozorova, Andrey Prokofiev LiCuVO$_4$ is a quasi-one-dimensional antiferromagnetic spin-$1/2$ system with strong magnetic frustration due to competing interactions within the Cu spin chain. Consequently, a complex magnetic phase diagram with three critical magnetic field values evolves [arXiv:1005.5668]. Furthermore, spiral spin order below $T_{\mathrm{N}}$=2.7K induces ferroelectricity thus rendering LiCuVO$_4$ multiferroic. The ferroelectric polarization can be switched by reorienting the spin helix through application of external magnetic fields [PRB \textbf{77}, 144101 (2008)]. Above $H>H_{\mathrm{c}2}$ LiCuVO$_4$ becomes paraelectric and the magnetic order changes to a collinear spin-modulated phase. Employing Nuclear Magnetic Resonance we study the nature of this high-field phase by analysis of $^7$Li and $^{51}$V spectra recorded around $H>H_{\mathrm{c}2}$=7.5T. We find, together with simulations for different scenarios, that Cu-spins of neighboring planes along the c-axis have a random phase-relation thus implying a two-dimensional character of the spin order [PRB \textbf{77}, 144101 (2008)]. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X18.00004: Density Functional Study of the magnetic structure on spin frustrated MnSb$_{2}$S$_{4 }$ and Sr$_{2}$MOsO$_{6}$ (M = Cu, Ni) Chuan Tian, Changhoon Lee, Erjun Kan, Fang Wu, Mike Whangbo We explored the electronic structures of two spin-frustrated magnetic systems monoclinic MnSb$_{2}$S$_{4}$ and Sr$_{2}$MOsO$_{6}$ (M = Cu, Ni) on the basis of first principles DFT calculations. The spin exchanges of MnSb$_{2}$S$_{4}$ are frustrated within each MnS$_{4}$ chain and between adjacent MnS$_{4}$ chains, which explains the observed helical spin order of MnSb$_{2}$S$_{4}$. We predict that MnSb$_{2}$S$_{4}$ is multiferroic with ferroelectric polarization of $\sim $14 $\mu $C/m$^{2}$ along the chain direction, and a field-induced reversal of the ferroelectric polarization occurs by reversing the direction of the helical spin rotation. The ordered double perovskites Sr$_{2}$MOsO$_{6}$ (M = Cu, Ni), reported to be half-metallic, are found to be magnetic insulators. The magnetic structures of Sr$_{2}$MOsO$_{6}$ were probed by evaluating their spin exchanges. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X18.00005: Ultra-slow Magnetic Order-Order Transition in the Spin Chain Antiferromagnet Ca3Co2O6 O.A. Petrenko, S. Agrestini, C.L. Fleck, L.C. Chapon, C. Mazzoli, A. Bombardi, M.R. Lees We report the observation of a highly unusual time-dependent
magnetic phenomena in which a transition from one long-range
magnetically ordered state to another occurs over a timescale of
several hours.
We have used powder neutron diffraction to
investigate the magnetic structure of Ising spin chain compound
$\rm Ca_3Co_2O_6$.
Our investigation focuses on the low-temperature regime
($T<14$~K$< |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X18.00006: Magnetic structure and superexchange pathways in CsV$_2$O$_5$ Andres Saul, Guillaume Radtke The study of low dimensional spin-1/2 quantum systems has been a very prolific field of condensed matter physics during the last decades. The family of vanadates, in particular, has provided a rich variety of compounds with different behaviours and topologies. Their magnetic structure and properties are primarily determined by the magnitude and the sign of the different exchange couplings arising between magnetic ions and therefore on the very details of their atomic and electronic structures. The sole consideration of the topology of a compound, based on idealized crystal structures, is often incomplete and even misleading. In this work, the magnetic properties of the layered compound CsV$_2$O$_5$ have been investigated using density-functional calculations. The results show that this compound is built from strongly dimerized alternating chains oriented along the ${\bf c}$ axis. Moreover, we demonstrate that the largest interaction along the chains direction arises \textit{between} the structural dimers, involving a superexchange pathway through the covalently bonded V$^{(5+)}$O$_4$ bridging groups. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X18.00007: Thermal Conductivity due to Magnon Heat Transport in Ca$_{2+x}$Y$_{2-x}$Cu$_{5}$O$_{10}$ Raheem Bello, Isaac Manzanera Esteve, John Markert In the spin-chain structure Ca$_{2+x}$Y$_{2-x}$Cu$_{5}$O$_{10}$, the thermal conductivity in some samples is observed to exhibit two peaks attributed to the thermal transport contributions by phonons and magnons. We have built a thermal conductivity probe, with a 2D rotation stage, to study the magnon contribution to the thermal conductivity of ceramic samples over the range 4--300 K. We plan to measure changes in the magnon contribution by varying the orientation of the lattice with respect to the magnetic field. We have prepared a set of samples with small deviations in oxygen stoichiometry to examine the effects of the defects on magnon thermal transport. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X18.00008: The origin of the incommesurate phase in the spin Peierls compound TiOCl Ariel Dobry, Diego Mastrogiuseppe, Claudio Gazza TiOX (X=Cl,Br) are recently characterized Spin-Peierls compounds. They are unusual due to the appearance of an intermediate incommensurate phase between the dimerized and the uniform ones. We show that the incommensurate phase is stabilized by a linear dependency of the phononic dispersion near the dimerized mode. A model based on antiferromagnetic chains with position dependent exchanges accounts for the evolution of the atomic coordinates with temperature within the incommensurate phase. The magnetic gap closes in the intermediate phase. Finally, we find that the magnetic static structure factor has incommensurate peaks situated at twice the wave vector of the structural ones. These peaks could be found in future elastic neutron scattering measurements. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X18.00009: Singlet-Triplet Excitations in the Unconventional Spin-Peierls System TiOBr J.P. Clancy, B.D. Gaulin, C.P. Adams, G.E. Granroth, A.I. Kolesnikov, T.E. Sherline, F.C. Chou TiOBr belongs to a select group of quasi-one-dimensional materials which undergo a spin-Peierls (SP) phase transition and develop a dimerized singlet ground state at low temperatures. However, unlike conventional SP systems, TiOBr exhibits not one, but two successive phase transitions upon cooling: a continuous transition into an incommensurate SP state at T$_{C2}$ $\sim$ 48 K, followed by a discontinuous transition into a commensurate SP state at T$_{C1}$ $\sim$ 27 K. We have performed time-of-flight neutron scattering measurements on powder samples of TiOBr using the fine-resolution Fermi chopper spectrometer (SEQUOIA) at the Spallation Neutron Source. These measurements reveal two branches of magnetic excitations within the commensurate and incommensurate SP phases, which we associate with n = 1 and n = 2 triplet excitations out of the singlet ground state. This study represents the first direct measure of the singlet-triplet energy gap in TiOBr, which we have determined to be E$_{g}$ = 21.2 $\pm$ 1.0 meV. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X18.00010: Field-Induced Gap in a Quantum Spin-1/2 Chain in a Strong Magnetic Field S. Zvyagin, M. Ozerov, J. Wosnitza, E. \v{C}i\v{z}m\'{a}r, R. Feyerherm, S.R. Manmana, F. Mila Magnetic excitations in copper pyrimidine dinitrate, a spin-1/2 antiferromagnetic chain with alternating $g$-tensor and Dzyaloshinskii-Moriya interactions that exhibits a field-induced spin gap, are probed by means of pulsed-field electron spin resonance spectroscopy. In particular, we report on a minimum of the gap in the vicinity of the saturation field $H_{sat}=48.5$ T associated with a transition from the sine-Gordon region (with soliton-breather elementary excitations) to a spin-polarized state (with magnon excitations). This interpretation is fully confirmed by the quantitative agreement over the entire field range of the experimental data with the DMRG investigation of the spin-1/2 Heisenberg chain with a staggered transverse field. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X18.00011: Non-equlibrium energy dynamics in spin chains and ladders Fabian Heidrich-Meisner, Stephan Langer, Markus Heyl, Ian McCulloch We investigate the real-time dynamics of the energy density in spin-1/2 chains and ladders, starting from initial states with an inhomogeneous profile of bond energies, extending our previous work on the dynamics of spin-density wave packets [1]. These simulations are carried out using the adaptive time-dependent density matrix renormalization group algorithm. We analyze the time-dependence of the spatial variance of the bond energies which yields necessary criteria for ballistic or diffusive energy dynamics. In the case of the XXZ chain, our results are consistent with ballistic behavior, both in the massless and the massive phase. For the massless regime, we compare our numerical results to predictions from bosonization for, e.g., the velocity that the initial perturbation spreads with. In the case of ladders, we find an involved dynamics whose qualitative interpretation is still under scrutiny. \\[4pt] [1] Langer et al. Phys. Rev. B 79, 214409 (2009) [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X18.00012: Nonmagnetic impurities in a frustrated spin ladder Erik Wulf, Sebastian M\"uhlbauer, Tatiana Yankova, Vasily Glazkov, Andrey Zheludev Sul-Cu$_2$Cl$_4$ is a representative of the spin S=1/2 4-leg ladders. Due to weak interladder interactions it shows almost perfect 1D character. The singlet ground state is separated from the excited triplet state by a gap of $\Delta$=0.52meV which can be closed by a critical field of H$_c$=3.7T. At H$_c$ the disordered spin liquid undergoes a phase transition to chiral helimagnetic order. By replacing nonmagnetic chlorine atoms by nonmagnetic bromine atoms random bond disorder is introduced in Sul-Cu$_2$(Cl$_{1-x}$Br$_x$)$_4$. Measurements of the magnetization and the specific heat show a drastically changed behavior in an applied field even at low bromine concentrations. At T$>$0 the material exhibits an intermediate phase between the spin liquid phase and the helimagnetic ordered phase for x=0.01, while the phase transition to the helimagnetic order is suppressed already for x=0.025. Nevertheless, the critical field H$_c$ to overcome the excitation gap is independent from the impurity concentration. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X18.00013: Exotic gapless Bose metals and insulators on multi-leg ladders Ryan V. Mishmash, Matt S. Block, Ribhu K. Kaul, Donna N. Sheng, Olexei I. Motrunich, Matthew P.A. Fisher We present recent work establishing compelling evidence for the existence of quasi-1D descendants of the $d$-wave Bose liquid (DBL), a novel 2D quantum phase of itinerant bosons first discussed in [1]. In particular, we study a model of hard-core bosons moving on the $N$-leg ladder square lattice with frustrating four-site ring exchange. In this talk, we focus on two novel phases: an incompressible gapless Mott insulator on the 3-leg ladder and a compressible gapless Bose metal on the 4-leg ladder. The former is a fundamentally quasi-1D phase that is insulating along the ladder but has two 1D gapless modes and power law transverse density-density correlations at incommensurate wave vectors; extensions of this phase to full 2D will be discussed. The latter, on the other hand, is conducting along the ladder and has five 1D gapless modes, one more than the number of legs; this represents a significant step forward in establishing the existence of the DBL in two dimensions. In both cases, we can understand the nature of the phase using slave-particle-inspired variational wave functions consisting of a product of two distinct Slater determinants, the properties of which compare impressively well to a DMRG solution of the model Hamiltonian. [1] O. I. Motrunich and M. P. A. Fisher, PRB {\bf 75}, 235116 (2007). [Preview Abstract] |
Session X19: Classical and Quantum Molecular Dynamics
Sponsoring Units: DCOMPChair: Isaac Tamblyn, Lawrence Berkeley National Laboratory
Room: D170
Thursday, March 24, 2011 2:30PM - 2:42PM |
X19.00001: Structure and Dynamics of Shock-Induced Nanobubble Collapse in Water Mohammad Vedadi, Amit Choubey, Ken-ichi Nomura, Rajiv Kalia, Aiichiro Nakano, Priya Vashishta, Adri van Duin Structure of water under shock and shock-induced collapse of nanobubbles in water are investigated with molecular dynamics simulations based on a reactive force field. Shock induces dramatic structural changes, including an ice-VII-like structural motif at a particle velocity of 1 km/s. The incipient ice VII formation and the calculated Hugoniot curve are in good agreement with experimental results. In the presence of a nanobubble, we observe a focused nanojet at the onset of nanobubble shrinkage and a secondary shock wave upon nanobubble collapse. The secondary shock wave propagates spherically backwards and induces high pressure as it propagates. Both the propagation velocity and the induced pressure are larger than those of the primary shock. We explored effects of nanobubble radius and shock amplitude on nanojet formation. The nanojet size increases by increasing particle velocity but the effect of increasing radius is more significant. The jet length scales linearly with the nanobubble radius, as observed in experiments on micron-to-millimeter size bubbles. Shock-induced collapse of a nanobubble in the vicinity of a cell membrane creates a transient nanopore when the nanojet impacts the membrane. Transient cell poration has potential applications in drug delivery. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X19.00002: Thermal conductivity of ultra high temperature ceramics (UHTC) $ZrB_2$ and $HfB_2$ from atomistic simulations John Lawson, Murray Daw, Charles Bauschlicher Ultra high temperature ceramics (UHTC) including $ZrB_2$ and $HfB_2$ are characterized by high melting point, good strength, and reasonable oxidation resistance. These materials are of interest for use as sharp leading edges for hypersonic vehicles among other applications. Progress in computational modeling of UHTCs has been limited in part due to the absence of suitable interatomic potentials. Recently, we developed Tersoff style parametrizations of such potentials for both $ZrB_2$ and $HfB_2$ appropriate for atomistic simulations. As an application, Green-Kubo molecular dynamics simulations were performed to evaluate the lattice thermal conductivity for single crystals of $ZrB_2$ and $HfB_2$. The atomic mass difference in these binary compounds leads to oscilations in the time correlation function of the heat current, in contrast to the more typical monotonic decay seen in monoatomic materials. Results at room temperature and at elevated temperatures will be reported. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X19.00003: A robust and monotonically convergent iterative algorithm for solving the Kohn-Sham equations in metallic systems Jean-Luc Fattebert We propose a new iterative algorithm to efficiently calculate the electronic structure in Density Functional Theory calculations of metallic systems and warm dense matter with high electronic temperature. This parameter-free algorithm directly searches for a set of wave functions and a compatible single particle density that minimizes the Mermin finite temperature functional. It is particularly useful for simulating physical systems considered difficult to converge, such as large systems with variable occupancies and presenting charge sloshing. We demonstrate the effectiveness of the proposed algorithm and its implementation by applying it to challenging large scale problem in First-Principles Molecular Dynamics simulations. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X19.00004: Molecular Dynamics with Quantum Fluctuations Ionut Georgescu, Jason Deckman, Vladimir Mandelshtam A new Quantum Dynamics approach, called Gaussian Molecular Dynamics (GMD), is introduced. As in the Centroid Molecular Dynamics (CMD), the N-body quantum system is mapped to an N-body classical system with an effective Hamiltonian arising within the Variational Gaussian Wave-packet approximation. The approach is exact for the harmonic oscillator and for the high-temperature limit, accurate in the short time limit and is computationally very efficient. GMD is furthermore used to estimate the diffusion constant and the spectrum of the velocity auto-correlation function of low pressure para-hydrogen at 14K and respectively 25K. The results are consistent with known experimental and theoretical results, such as CMD and RPMD. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X19.00005: Biased Monte Carlo technique to accelerate Molecular Dynamics simulations of rare events Pratyush Tiwary, Axel van de Walle We propose a hybrid Monte Carlo (MC) -Molecular Dynamics (MD) technique to study temporally rare event dynamics. By using biased MC sampling (Metropolis-Hastings), we avoid actually visiting low energy states in the MD and instead carry out a quick estimate of the mean escape time to be added to the computer clock. The method does not assume anything about the nature of the transition surfaces separating basins in the energy surface. We then apply the method to the case of dislocation kink movement in BCC metals at low temperatures. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X19.00006: Ab initio molecular dynamics simulations using a Chebyshev-filtered subspace iteration technique for modeling amorphous silicon dioxide Minjung Kim, Khoonghong Khoo, James Chelikowsky Ab initio molecular dynamics simulations are a powerful tool for examining liquids and amorphous materials; however, such simulations are often computationally intensive. We present a molecular dynamics method that dramatically reduces the computational load using a new algorithm based on Chebyshev-filtered subspace iteration. We apply this method to amorphous silicon dioxide. Amorphous silicon dioxide has been intensively studied owing to its broad applications to electronic devices and photonics. We perform ab initio molecular dynamics simulations to obtain the amorphous structure of silicon dioxide. We employ implement several new procedures to investigate the effect of quenching rates and system sizes. The calculated structure factor for our amorphous structure is in good agreement with experimental data. We performed structural relaxations to calculate the hyperfine splitting constants. Our calculated hyperfine splitting constants of $E^\prime_\gamma$ oxygen defect centers show excellent agreement with electron paramagnetic resonance experiments. We will also discuss statistical results of oxygen-related defect centers. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X19.00007: Molecular Dynamics simulations of Carbon-Oxygen mixtures in the Core of White Dwarf Stars Andre da Silva Schneider, Joe Hughto, Charles Horowitz, Don Berry A White Dwarf will be the final evolutionary state of most of the stars in our galaxy. The core of these faint and compact stars is a mixture of ions immersed in a degenerate electron gas. The latent heat of fusion of this mixture is important for White Dwarf cooling from which the age of stellar systems can be inferred. Assuming Carbon and Oxygen to be the most abundant elements we studied the phase diagram of the mixture using large classical molecular dynamics simulations. The ion interactions were modeled by a screened Coulomb potential and the system was kept in a half-solid half-liquid state. Understanding the chemical separation that takes place helps estimate the central abundance of these elements and is important for observations of White Dwarfs in globular star clusters [1]. \\[4pt] [1] C.J. Horowitz, A.S. Schneider, and D.K. Berry, Physical Review Letters 104, 231101 (2010) [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X19.00008: Diffusion in Yukawa Crystals in White Dwarfs and Neutron Stars Joseph Hughto, Charles Horowitz, Andre Schneider Compact stars, white dwarfs and neutron stars, contain strongly interacting liquid and solid systems that we model using screened Yukawa interactions. Diffusion of impurities can release significant gravitational energy. We calculate diffusion constants using Molecular Dynamics (MD) simulations for both multicomponent liquid and single component solid systems. Diffusion in the solid depends strongly on the number and nature of crystal defects. We are not aware of previous direct calculations of diffusion in Yukawa crystals. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X19.00009: MD Simulations of the Breaking Strain of Coulomb Crystals in Neutron Stars: Star mountains and gravitational waves Charles Horowitz, Joe Hughto, Andre Schneider, Don Berry Neutron stars --- collapses stars half again as massive as the sun but with a 10-kilometer radius --- have solid crusts made of dense coulomb crystals. We perform large-scale molecular dynamic simulations of the breaking strain (strength) of this crust including the effects of impurities, defects, and grain boundaries. We find neutron star crust to be the strongest material known, with a breaking stress 10 billion times stronger than steel [1]. This is because of the high density, high pressure, and the long-range nature of the coulomb interactions where each ion interacts with thousands of its neighbors. The crust can support massive mountains that, on a rapidly rotating neutron star, can radiate detectable gravitational waves. These oscillations of space and time, predicted by Einstein almost 100 years ago, should be detected in the next few years. \\[4pt] [1] C. J. Horowitz and Kai Kadau, Phys. Rev. Letters 102, 191102 (2009). [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X19.00010: Tracer Diffusion for Rough Hard Spheres Olga Kravchenko, Mark Thachuk We present a study of tracer diffusion in a rough sphere fluid. In such fluid collisions between particles exchange rotational and translational energy and momentum. As tracer particles grow in size, their diffusion constant is described by the Stokes-Einstein hydrodynamic result. In this limit, smooth hard spheres are shown to adopt ``slip'' boundary conditions. The current results show that rough hard spheres adopt boundary conditions proportional to their degree of roughness, defined by the radius of gyration. Spheres with maximum roughness adopt ``stick'' boundary conditions while those with intermediate roughness adopt values between the ``slip'' and ``stick'' limits. This dependence is found to be almost linear. Changes in the diffusion constants as a function of roughness are also examined and it is found that the dependence is stronger than suggested by the low-density, Boltzmann result. Rough hard spheres model the effect of inelasticity of a real collision and show that even without the presence of attractive forces, the boundary conditions for large particles can deviate from ``slip'' and approach ``stick.'' [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X19.00011: Solvation and thermal effects on the optical properties of naturaldyes: a case study on the flavylium cyanin Arrigo Calzolari, Baris Malcioglu, Ralph Gebauer, Daniele Varsano, Stefano Baroni We present a first-principles study of the effects of both hydration and thermal dynamics on the optical properties of a natural anthocyanin dye, namely, \textit{cyanin} (Cya), in aqueous solution. We combine Car-Parrinello molecular dynamics and time-dependent density functional theory (TDDFT) [1] approaches to simulate the time evolution of UV-vis spectrum of the hydrated Cya molecule at room temperature [2,3]. The spectrum of the dye calculated in the gas phase [4] is characterized by two peaks in the red and in the blue, which would bring about a greenish hue incompatible with the dark purple coloration observed in nature. Describing the effect of the water solvent through a polarizable continuum model does not modify qualitatively the resulting picture. An explicit simulation of both solvent and thermal effects using ab-initio molecular dynamics results instead in a spectrum that is compatible with the observed coloration. This result is analyzed in terms of the spectroscopic effects of molecular distortions, induced by thermal fluctuations. [1] \textit{turbo}-TDDFT, B. Walker, A. Saitta, R. Gebauer, S. Baroni, \textit{Phys. Rev. Lett. }\textbf{2006}, $96$, 113001. [2] A. Calzolari, et. al , \textit{J. Chem. Phys.} \textbf{132}, 114304 (2010). [3] O.B. Malcioglu, A. Calzolari, R. Ghebauer, D. Varsano, and S. Baroni, preprint (2010). [4] A. Calzolari, et al, \textit{J. Phys. Chem. A} \textbf{113} 8801 (2009). [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X19.00012: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X19.00013: Model inter-atomic potential for Cu-Zr system generated using a multicanonical simulation combined with a first-principles calculation Yoshihide Yoshimoto We can obtain an accurate force field for a molecular dynamics simulation from a first principles calculation. However, the available physical time for a direct first-principles molecular dynamics simulation is often limited to $\sim 10$ ps because of its high computational cost. If we want to achieve much longer physical time, a possible approach is to build a model inter-atomic potential from a first-principles calculation. As a kind of this approach, Yoshimoto has proposed the ``thermodynamic downfolding'' method[1,2] which generates an inter-atomic potential based on a multicanonical simulation combined with a first-principles calculation. With this method, we can expect that the thermodynamics of the system is conserved to a maximum extent. In this presentation, application of the method to Cu-Zr system will be reported. This system is interesting because at an composition this system become a bulk metallic glass which has several technologically attracting properties. The melting properties of the system will be covered.\\[4pt] [1] Y. Yoshimoto, J. Chem. Phys., 125, 184103 (2006)\\[0pt] [2] Y. Yoshimoto, J. Phys. Soc. Jpn., 79, 034602 (2010). [Preview Abstract] |
Session X20: Focus Session: Thermoelectric Materials: Theory
Sponsoring Units: DMP FIAP GERAChair: Marco Fornari, Central Michigan University
Room: D168
Thursday, March 24, 2011 2:30PM - 3:06PM |
X20.00001: Atomistic simulations of heat transport in nanostructures Invited Speaker: Engineering materials at the nanoscale allows for tuning several of their properties over a broad range. These holds particularly for thermoelectric performances of group IV semiconductors, such as silicon and germanium. Experiments [1,2] suggest that improvements of the thermoelectric figure of merit in nanostructured silicon are mostly related to a drop in the thermal conductivity of about two orders of magnitude with respect to the bulk. In spite of success of macroscopic empirical approaches, we argue that atomistic simulations are necessary to provide the correct physical behavior and achieve significant understanding of a complex phenomenon such as thermal transport at the nanoscale ($\sim 10$nm). By means of atomistic simulation methods, we address the issue of lattice thermal transport in silicon and SiGe nanostructures and nanostructured materials, e.g. nanowires, nanoporous and amorphous silicon thin films. We have reviewed and compared several simulation approaches (equilibrium and non-equilibrium molecular dynamics and anharmonic lattice dynamics), and developed a new method for large scale simulations, based on the scattering approach. We have identified strength, weaknesses and possible artifacts for each method, and established reliable simulation procedures to compute thermal transport properties. Our results shed light on the cooperative effects of dimensionality reduction, nanostructuring and disorder, in reducing the thermal conductivity of silicon-based nanostructured materials, stemming from prominent changes of lattice vibrational properties and enhancement of phonon scattering [3]. \\[4pt] [1] A. I. Hochbaum, et al. Nature (London) {\bf 451}, 163 (2008).\\[0pt] [2] J.-K. Yu, et al. Nature Nanotech. {\bf 5}, 718 (2010).\\[0pt] [3] D. Donadio and G. Galli, Phys. Rev. Lett. {\bf 102}; 195901 (2009), Nano Lett. {\bf 10}, 847 (2010); M. K. Y. Chan, et al. Phys. Rev. B {\bf 81}, 174303 (2010). [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X20.00002: Thermoelectric performance of Si-Ge heterostructured nanowires from first-principles Arash Mostofi, Matthew Shelley We present calculations of the thermoelectric figure of merit $ZT$ of both pristine and axially heterostructured Si/Ge nanowires as a function of their compositional disorder, growth direction and diameter. Our method is based on density-functional theory (DFT). Both charge and transport properties are calculated within the Landauer-Buttiker formalism. We compute $ZT$ for realistic nanowires (ca.~10,000 atoms and 100~nm in length) by using maximally-localized Wannier functions to map large-scale DFT calculations onto short-ranged model Hamiltonians with negligible loss of accuracy. The approach is fully automated and robust, such that large numbers of configurations of the system can be explored with high throughput and efficiency. While we focus here on their application to thermoelectric nanowires, the algorithms we have developed are generally applicable to other classes of disordered quasi-one-dimensional nanostructures such as DNA, carbon nanotubes and graphene nanoribbons. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X20.00003: Thermoelectric Properties of Ultra Narrow Silicon Nanowires from Atomistic Calculations Neophytou Neophytos, Hans Kosina The progress in nanomaterials' synthesis allows the realization of thermoelectric devices based on 1D nanowires (NWs). In these confined systems the electrical and thermal conductivities, and the Seebeck coefficient can be designed to some degree independently, providing enhanced ZT values as compared to the bulk material's value. We calculate the electrical conductivity, the Seebeck coefficient, and the electronic part of the thermal conductivity of scaled Si NWs. We use the atomistic sp3d5s*-spin-orbit-coupled tight-binding model and linearized Boltzmann transport. Our calculations include up to 5500 atoms, a task still computationally affordable within this model. We examine n-type and p-type NWs of diameters between 3nm and 12nm for [100], [110] and [111] transport orientations, at different doping levels. Using experimentally measured values for the lattice thermal conductivity, the expected ZT values of the nanowires are estimated. We further provide directions for power factor optimization with structure confinement. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X20.00004: Interface scattering and thermal conductivity in Si/SiGe alloy superlattices Zlatan Aksamija, Irena Knezevic $Si/Si_{1-x}Ge_{x}$ alloy superlattices (SLs) show promise for application as efficient thermoelectrics because of their low thermal conductivity, below that of the bulk $Si_{1-x}Ge_{x}$ alloy. Lattice thermal conductivity in these superlattices is dominated by scattering from the rough interfaces between layers, even at room temperature. Therefore, interface properties, such as roughness, orientation, and composition, are expected to play a significant role in thermal transport and offer additional degrees of freedom to control the thermal conductivity in semiconductor nanostructures based on superlattices. In this paper, we demonstrate the sensitivity of the lattice thermal conductivity in SLs to the interface properties, using a momentum-dependent model for scattering of phonons from rough material interfaces. Our results show excellent agreement with experimental data and explain the measured thickness and temperature dependence, as well as anisotropy of thermal conductivity in superlattices. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X20.00005: Molecular dynamics simulation of the thermal transport across Si/Al interfaces Woon Ih Choi, Kwiseon Kim, Sreekant Narumanchi Efficient heat dissipation is critical for power electronics where the device package consists of several layers of different materials. Conventional thermal interface materials are bottlenecks in heat removal. Detailed understanding of interfacial heat resistance would benefit efforts to improve the device design. We have chosen Si/Al interfaces for this thermal transport study. We construct Si-Al MEAM interatomic potential parameters based on the density functional theory (DFT) calculations. We generate various interface structures using the first-principles molecular dynamics (MD) simulations. Using the direct method to compute the thermal conductance, we investigated various interface structures. We will discuss the effect of the inter-diffused layers and roughness of the interfaces on the thermal boundary conductance. We will also compare our result with limited data in the literature. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X20.00006: Thermal Boundary Resistance at Ideal Gas Solid-Fluid Interfaces Sanghamitra Neogi, Gerald Mahan We study the thermal boundary resistance at the interface between an ideal gas solid and another ideal gas fluid. In the solid side, heat is mostly carried by phonons, and thermal resistance occurs due to the partial reflection of phonons at the interface. In the fluid side, the sound waves can carry diffusive heat from the interface into the bulk of the liquid. We include both longitudinal and transverse sound modes of the fluid in the theory. The sound modes in the fluid and the reflected phonons in the solid have the same frequency as the phonon incident at the interface from the solid side. The wave vector for the sound modes is then calculated using the knowledge of the fluid pair distribution function in the bulk. The pair distribution function near the interface is modified due to the presence of the solid atoms. We solve coupled equations of motion for the atoms at the interface to obtain the phonon reflection coefficients. The Kapitza resistance is then obtained using the knowledge of these reflection coefficients. The calculation provides a method for extending the Young-Maris theory to the fluid-solid substances. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X20.00007: Micro to Nano Scale Heat Conduction in Thermoelectric Materials Martin Maldovan Understanding and controlling heat transfer in solids is very important for increasing the efficiency of thermoelectric materials such as skutterudites, clatharates, superlattices, nanowires, and quantum dots. Although the mechanisms governing the thermal conductivity have been understood for years, a comprehensive theoretical method to calculate heat transfer, particularly at small scales, has not been available. This is mainly due to the complexity of anharmonic processes and phonon boundary scattering. We present a comprehensive theoretical model to calculate the thermal conductivity of thermoelectric materials at small length scales. The approach involves an exact calculation of the reduction of the phonon mean free paths due to boundary scattering and removes the need to solve the Boltzmann equation or to use adjustable terms as in the Callaway or Holland models. The analysis is based on the kinetic theory of transport processes and considers general expressions for dispersion relations, phonon mean free paths, and surface specularity parameters. The results show an excellent agreement with experiments for thin films, nanowires, and superlattices over a wide range of temperature and across multiple length scales. The theoretical approach can further be applied to a wide variety of problems involving the conduction of heat in micro/nanostructured thermoelectrics. This research was funded by the MIT Energy Initiative. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X20.00008: Quantum Thermoelectric Effects on the Nanoscale Justin Bergfield, Charles Stafford An exact expression for the heat current in a nanostructure coupled to multiple metallic electrodes is derived, including both electron-electron and electron-phonon interactions. We use this formalism to investigate quantum effects on the flow of charge and entropy, and find an enormous quantum enhancement of thermoelectric effects in the vicinity of higher-order interferences in the transmission spectrum of a nanoscale junction. A nonequilibrium quantum analysis of a single-molecule junction based on 3,3'-biphenyldithiol demonstrates a maximum operating efficiency of 27\% of the Carnot limit. Nonlocal quantum corrections to thermoelectric transport coefficients in multiterminal geometries are predicted. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X20.00009: Thermal transport in Si-based disordered systems: amorphous silicon and silicon germanium alloys Yuping He, Ivana Savic, Giulia Galli, Davide Donadio Understanding and modeling heat transport in structurally and mass disordered semiconductors (e.g. amorphous silicon--a-Si and SiGe alloys) have long been a challenging problem in solid state physics. Using a combination of techniques (equilibrium and non- equilibrium molecular dynamics and lattice dynamics), we analyze the nature of vibrations and compute the thermal conductivities (k) of a-Si, bulk and nanoporous SiGe. We find that in amorphous and mass disordered systems, two types of modes are present, phonons and diffusive modes. In a-Si, phonons ( who are only 3 \% of the total vibrations) contribute to approximately half of k [1]. The value of k critically depends on the morphology of the system [2], for example it considerably dereases if thin films or samples with nano-holes are considered. A discussion of how mean free paths and lifetimes change as a function of morphology and disorder will be presented, together with results showing the effect,on k, of disorder at pores or film surfaces.Work supported by grant DOE DE-FC02-06ER25777.\\[4pt] [1] Y.He, D.Donadio and G.Galli (submitted, 2010).\\[0pt] [2] Y. He, D. Donadio, Joo-H. Lee, J. C. Grossman and G. Galli (submitted, 2010) [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X20.00010: Atomistic study of heat transport in SiGe alloys Ivana Savic, Yuping He, Davide Donadio, Giulia Galli Semiconductor alloys, e.g. SiGe, are considered as promising materials to build efficient thermoelectric devices [1], and atomistic modeling of heat transport in these systems may help complement and guide experiments in optimizing their efficiency. We analyze strengths and weaknesses of several atomistic approaches in modeling the thermal conductivity of SiGe alloys, and we analyze in detail their range of validity. In particular, we focus on equilibrium molecular dynamics [2], an approach based on the solution of the Boltzmann transport equation [3] and Green function techniques [4]. Applications to both bulk and nanostructured SiGe will be presented. \\[4pt] [1] A. J. Minnich, M. S. Dresselhaus, Z. F. Ren, and G. Chen, Energy Environ. Sci. 2, 466 (2009). [2] See e.g. D. Donadio and G. Galli, Phys. Rev. Lett. 102, 195901 (2009); Nano Lett. 10, 847 (2010). [3] See e.g. J. E. Turney, E. S. Landry, A. J. J. McGaughey, and C. H. Amon, Phys. Rev. B, 79, 064301 (2009). [4] See e.g. I. Savic, N. Mingo, and D. A. Stewart, Phys. Rev. Lett. 101, 165502 (2008). [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X20.00011: Scattering of charge carriers and phonons in thermoelectric devices Giuseppe Romano, Lee Joo-Hyoung, Jeffrey Grossman We investigate the effects of the scattering of charge carriers and phonons on the figure of merit of thermoelectric devices. Despite many efforts devoted to the optimization of the figure of merit ZT, the commercial diffusion of such systems is still limited due to their low efficiency. The main problem behind the engineering of ZT is the interdependency between the Seebeck coefficients, electrical conductivity and thermal conductivity. ZT could be maximized by either increasing the Seebeck coefficient or decreasing the thermal conductivity. While the first approach involves the distortion of the electronic density of states [1], the thermal conductivity can be lowered by inserting nonporous in the bulk materials [2]. Recent works have shown a detailed comparison between np-Ge and np-Si material and investigated the effect of the porosity on ZT [3]. Here we couple classical molecular dynamics and continuous simulation to study the phonon-phonon, phonon-pore, electron-phonon and phonon-boundary scattering and their effects on the electrical and thermal conductivities. The knowledge gained about material properties is then used to perform simulations of thermoelectric devices. \\[4pt] [1] PRL \textbf{104}, 016602 (2010)\\[0pt] [2] PRB \textbf{80}, 155327 (2009)\\[0pt] [3] APL \textbf{95}, 013106 (2009) [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X20.00012: Many-body effects in frequency-dependent charge and thermal transport Jesus Cruz, James Freericks Recently, Shastry has proposed that thermoelectric properties (thermopower, Lorenz number, and figure of merit) can be determined accurately in strongly correlated materials by examining their high frequency behavior. He also has derived a sum rule similar to the f-sum rule in optical conductivity, for the frequency dependent thermal conductivity. We examine these ideas within the context of an exactly solvable model (the Falicov-Kimball model) with dynamical mean-field theory. We see that the low-frequency and high-frequency limits are not so close in this system. We also discuss the thermal conductivity sum rule. These results are important in trying to understand strong electron correlation effects in thermoelectrics. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X20.00013: Effective medium theory for thermoelectrics Paul Haney We report on the application of effective medium theory to binary compound thermoelectric materials. We find a range of parameters for the conductivity and thermopower of the constituent elements such that the compound has an enhanced power factor. The results of effective medium theory are compared to full numerical simulations of an ensemble of disordered systems, and good qualitative agreement is found between the two calculations. The effect of various tailored geometries are explored in the direct numerical solution of the compound thermoelectrics. [Preview Abstract] |
Session X21: Focus Session: Novel X-Ray Instrumentation and Measurement Techniques
Sponsoring Units: GIMSChair: Albert Macrander, Argonne National Laboratory
Room: D161
Thursday, March 24, 2011 2:30PM - 3:06PM |
X21.00001: Inelastic X-ray Scattering at Third Generation Synchrotron Sources: Present Activities and Future Plans Invited Speaker: This talk will review present activities and future plans for utilizing inelastic x-ray scattering to study excitations in hard condensed matter systems. In particular, at current third generation sources it is now possible to observe the key elementary excitations in solids, including phonons, magnons, orbital excitations and electronic excitations such as plasmons and charge transfer excitations. The technique offers a number of advantages over existing methods for the study of these excitations, including especially, the ability to study very small sample volumes, the range of momentum and energy transfers available and the ability to work in disparate sample environments. A few recent illustrative examples are discussed. The first of these is a study of phonons in SmFeAs(O,F) which show an anomalous renormalization of certain phonons and for which momentum-dependent measurements of the electron-phonon coupling have been made. The second example will focus on work being performed at the Swiss Light Source in which spin waves in (La,Sr)CuO4 have been observed. Finally, the current and future state of inelastic x-ray scattering instrumentation in this country is discussed, including the upgrade plans at the Advanced Photon Source, and plans for new inelastic beamlines at the NSLS-II source currently under construction at Brookhaven National Laboratory [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X21.00002: The new X-ray absorption spectroscopy beamline at Diamond: B18 Silvia Ramos, Giannantonio Cibin, Stephen Parry, Andy J. Dent The new core XAS spectroscopy beamline at Diamond (B18) has been designed to provide a reliable spectrometer for a broad scientific community. The instrument first became operational in April 2010 and is currently combining further commissining with a rapidly growing user programme. The main goal of the optics design of B18 was to achieve high stability in an instrument that can operate over a wide energy range (2.05 to 35.0 keV). XAS measurements can be carried out using several detection methods: transmission, electron yield and fluorescence (with a Ge detector, a Si drift detector or a gas microstrip). It is also possible to carry out combined absorption and diffraction measurements. The instrument offers several sample environments: a liquid nitrogen cryostat capable of loading over 40 samples, a pulse tube cryostat with base temperature of 1.6 K and an infrared furnace with a maximum temperature of 800$^{o}$C and can integrate a variety of specialised set-ups designed by the users. In this talk we will present the beamline and selected examples to show its capabilities. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X21.00003: The soft x-ray materials research (SXR) instrument at the Linac Coherent Light Source Joshua J. Turner, Oleg Krupin, William Schlotter The soft x-ray materials science research (SXR) instrument completed commissioning in June 2010 and began experimental user operations shortly afterwards. This instrument delivers intense, ultra-short soft x-ray pulses from the Linac Coherent Light Source, the free-electron laser at the SLAC National Accelerator Laboratory. These are fully coherent and can contain up to $10^{13}$ photons per pulse (or about 3 mJ per pulse) with bunch lengths from 300 femtoseconds down to sub-10 femtoseconds. The instrument includes a monochromator whose energy range spans energies from 480 eV - 2000 eV and a Kirkpatrick-Baez mirror system to create a focal spot of a few microns in diameter. The SXR instrument has a diverse set of end stations available to conduct a large variety of experimental techniques such as coherent imaging, resonant x-ray diffraction, photoelectron spectroscopy, and x-ray emission and/or absorption. First studies include fields spanning liquid femtosecond chemistry and time-resolved resonant inelastic x-ray scattering to ordering in solids and ultrafast magnetization. An overview of the instrument and its capabilities will be given. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X21.00004: Polarization Depend Soft X-ray Scattering of Anisotropic Organic Thin Films H. Ade, E. Gann, B. Collins, H. Yan, J. Cochran, M. Chabinyc Crystalline, semi-crystalline and liquid-crystalline organic materials have locally large anisotropic bond orientation statistics. This strongly impacts the mechanical, optical and electronic properties. For example, charge transport in organic thin films is often highly anisotropic and overall transport depends upon domain size, degree of order within domains, domain correlations, and the domain boundaries. Knowing the relative impact of all parameters is necessary for a detailed understanding of organic thin film transistors. - We demonstrate a novel scattering method to characterize such films: Polarization Dependent Soft X-ray Scattering (P-SoXS). In scattering, the linear dichroic absorption often exploited in x-ray microscopy is accompanied by strong linear dichroic dispersion, leading to large contrast based on bond orientation. This can not be achieved with hard x-rays or neutrons. We demonstrate P-SoXS on devices based on prototypical materials such as pentacene, poly(2,5-bis(3-tetradecyllthiophen-2-yl)thieno[3,2-b]thiophene) (pBTTT) and poly(3-hexylthiophene) (P3HT).~The use of linear or circularly polarized x-rays allows the bond orientation contrast to be switched on and off, respectively, which is very useful to characterize the correlated and individual domain size. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X21.00005: Probing buried interfaces by Index-Matched Soft X-ray Scattering E. Gann, J. Seok, J. Cochran, M. Chabinyc, B. Collins, H. Ade Interfaces are often critical to function and performance in systems ranging from electronics to biology. Organic-organic interfaces are the location for charge transport in organic thin film transistors and exciton splitting in organic photovoltaics. Complete morphological characterization of buried interfaces is unfortunately difficult to achieve with conventional tools. We present a new method termed Index-Matched Soft X-ray Scattering (IM-SoXS). By matching the real part of the complex index of refraction of the top layer to that of vacuum through judicious choice of photon energy, we can minimize scatter from the top surface and substantially increase scatter from a buried interface, revealing both the spatial distribution and amplitude of the interfacial roughness. We demonstrate the method on samples with engineered and controlled interfacial roughness and provide examples of systems of scientific interest for which IM-SoXS should provide improved understanding of interface morphology and its relation to performance and function in systems. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X21.00006: Lensless x-ray imaging in reflection geometry Daniel Parks, Sujoy Roy, Keoki Seu, Run Su, Joshua Turner, Weilun Chao, Erik Anderson, Stefano Cabrini, Stephen Kevan We report on the development of a technique for lensless x-ray imaging in reflection geometry. In an approach similar to Fourier transform holography, we use a set of apertures to define object and reference waves from light which has already scattered from the sample. Back propagation from the apertures gives the image at the sample plane. This technique can be used with extended objects without additional masking, and can be used in reflection and transmission geometries. The extension of lensless x-ray imaging into reflection geometry opens the possibility of imaging surfaces in thin films, buried interfaces in multilayers, or Bragg planes in single crystals. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X21.00007: Determination of Total X-ray Absorption Coefficient using Non-Resonant X-ray Emission Andrew Achkar, Tom Regier, Eric Monkman, Kyle Shen, David Hawthorn Inverse partial fluorescence yield (IPFY) is a newly developed x-ray absorption spectroscopy (XAS) that utilizes non-resonant emission processes to measure the x- ray absorption of a material. Unlike XAS by traditional transmission, total electron yield and total fluorescence yield, IPFY is free of pinhole, saturation, and self-absorption effects. Moreover, IPFY exhibits a simple angle dependence that can be exploited to deduce the total x-ray absorption coefficient from a series of measurements performed with different experimental geometries. We quantitatively determine the total x-ray absorption coefficient of insulating NiO and NdGaO$_3$ single crystals at soft x-ray energies using this approach. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:54PM |
X21.00008: Hard X-ray Microscopy with Multilayer Laue Lenses Invited Speaker: The possibility of imaging at near-atomic resolution using x-rays has been a dream ever since the short-wavelength nature of x-rays was demonstrated by von Laue and coworkers nearly a century ago. Even today the scientific impact of atomic-scale focusing of electromagnetic radiation would be deep and broad, because x-ray microscopy provides capabilities (ability to penetrate, sensitive and accurate elemental and structural information) that are complementary to other high-resolution microscopies. Although hard x-rays can in principle be focused to spot sizes on the order of their wavelength (0.1 nm), this limit has never been approached because of the difficulty in fabricating the optics. Multilayer Laue lens(MLL) is a novel diffractive optic for hard x-ray nano-focusing, which can be fabricated by sputter deposition of zone plate structure on flat substrate. According to the theoretical results, MLL is capable of focusing x-rays to well below 1 nm. We have demonstrated 2-dimensional focusing of hard x-rays with MLLs to a spot size of 25 nm x 27 nm with an efficiency of 2{\%} at a photon energy of 12 keV, while 1-dimensional focus of 16 nm has been achieved. In this talk, we will present an overview of MLL microscopy and recent accomplishments for the determination of chemical composition in nanoscale systems. Lastly, we will give the capabilities of MLL microscopy that have the potential to significantly advance materials science, nanoscience, bio-medical science and environmental science. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X21.00009: Dynamical diffraction effects on beam focusing for x-ray back reflection from curved multi-plate x-ray crystal cavity Ying-Yi Chang, Sung-Yu Chen, Mau-Tsu Tang, M. Yabashi, Yi-Wei Tsai, Yu-Hsin Wu, Shih-Chang Weng, Chia-Hung Chu, Po-Yu Liao, Shih-Lin Chang We have recently observed diffraction enhanced beam-focusing in curved multi-plate x-ray crystal cavities of silicon using (12 4 0) as the back reflection at 14.4388 keV. The measurement on the transmitted x-ray beam size through the crystal cavities shows a reduced focal length and an extremely long beam waist at the focal point. This effect could be understood according to the dynamical theory of x-ray diffraction. Based on the consideration of the excitation of the dispersion surface for each curved crystal surface involved in the crystal device, beam focusing and beam splitting occur, leading to the observed focusing feature. Detailed dynamical calculations on the transmitted intensities at different positions near the focal point will be discussed. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X21.00010: High-Resolution Thermal Expansion Measurements of Single-Crystal Sapphire for Application as X-Ray Backscattering Monochromator John J. Neumeier, I. Sergeev, D. Bessas, R.P. Hermann We report measurements of the thermal expansion of high-purity single crystal sapphire along the $a$ and $c$ directions. The data were acquired using a thermal expansion cell that is constructed of fused silica with a relative resolution of approximately 3x10$^{-9}$. Comparison will be made to existing literature values determined from dilatometry and high-resolution x-ray diffraction. This project's main goal is the use of sapphire as x-ray backscattering monochromator for phonon spectroscopy using nuclear inelastic scattering. Tuning of the monochromator is done by varying the sapphire temperature, and the new thermal expansion values will improve the energy calibration. [Preview Abstract] |
Session X22: Metals: Bulk Properties and Nanostructures
Sponsoring Units: DCMPChair: Lin-Lin Wang, Ames Laboratory/Iowa State University
Room: D163
Thursday, March 24, 2011 2:30PM - 2:42PM |
X22.00001: Phonon self energy in transition metals Laurent Chaput, Atsushi Togo, Isao Tanaka, Gilles Hug We present \emph{ab initio} calculations of the phonon self energy of transition metals obtained using second order many body perturbation theory.\footnote{S. Narasimhan and D. Vanderbilt, Phys. Rev. B, 43, 4541 (1991)} The code we have implemented\footnote{L. Chaput, A. Togo, I. Tanaka and G. Hug, submitted to Phys. Rev. B} use the symmetry properties of the phonon-phonon interactions to express the self energy as a sum over irreducible triplets. It is analogous to the reduction of integration to the irreducible part of the Brillouin zone for one particle properties. The self energy of transition metals is then calculated. We show that the Peierls approximation\footnote{R. E. Peierls, Quantum Theory of Solids, Oxford University Press,1964} is in fact reasonable for \emph{bcc} and \emph{fcc} metals, but fails for the \emph{hcp}. The decays paths of phonons producing the self energy is finally analyzed using surfaces of reciprocal space defined by conservation law. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X22.00002: Observation of phonon softening in Cr near its Neel transition Ruqing Xu, Tai-Chang Chiang Chromium is a classic antiferromagnetic spin-density-wave system, with many unique properties yet to be fully understood despite the extensive experimental and theoretical efforts in the past. For instance, near its two magnetic transitions at 311K and 123K, the elastic constants of Cr have been observed to soften abruptly by ultrasonic experiments, indicating a strong lattice-spin interaction. However, such softening has never been confirmed in previous measurements of Cr's phonon dispersion relations. To address this issue we have carried out studies with inelastic x-ray scattering (IXS) as well as x-ray thermal diffuse scattering (TDS) at temperatures around the Neel transition (311K). While the IXS measurements did not find obvious changes in the overall phonon dispersion relations of Cr, abrupt changes in TDS intensities were clearly observed across the transition at wavevectors close to the Brillouin zone centers, unveiling a softening in the long-wavelength lattice excitations in Cr at the Neel transition. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X22.00003: First-principles study of phonon-phonon interaction in FCC metals at high temperatures Xiaoli Tang, Chen W. Li, Brent Fultz Third-order lattice anharmonicity induced phonon broadening of FCC metals (including Al and noble metals Cu, Ag, Au) were calculated from first-principles density functional theory (DFT) using the second-order perturbation theory, where anharmonic force constants were obtained from supercell finite displacement method combined with DFT calculations. For aluminum, the good agreement between our calculations and prior measurement of phonon linewidth at 300K and our new measurement of phonon density of states to 750K indicates the third-order phonon-phonon interactions can account for the lifetime broadenings of phonons in aluminum to at least 80\% of its melting temperature. A systematic study of noble metals further suggests that, despite of the similarity among these systems, scattering kinematics play an important role in determining the relative anharmonicity between the modes, while potential anharmonicity modulates the absolute phonon decay rate. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X22.00004: Dynamical behavior of coherent phonons in semimetals: Measuring fast electron decoherence rates with slow pulses Jian Chen, Jingjing Li, Stephen Fahy, Roberto Merlin, David Reis Coherent light illumination of solids above the band-gap leads not only to a population of photoexcited carriers, but also to the generation of coherent electronic states of particular symmetries which can drive vibrations of the same symmetry. For A1g and Eg symmetry phonons in Bi and Sb, there has been some controversy regarding the generation mechanism of coherent vibrations. Here, we use a combination of ultrafast stimulated Raman scattering (RS) and cw spontaneous RS to determine the lifetime of electronic coherences of A1g and Eg symmetry. Their lifetime can be inferred from a comparison between pump-probe measurements of the amplitude of the corresponding coherent phonons, and a determination of the spontaneous RS cross sections. Our results represent a new approach to probe extremely fast electron decoherence rates using much slower (50-100fs FWHM) laser pulses. The Eg electronic coherence, resulting from a fragile unequal distribution of carriers in three equivalent regions of the band structure, is extremely short lived. Its temperature-dependent lifetime is in the range 2-12 fs in Bi and 5-12 fs in Sb. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X22.00005: First-principles calculations of lattice stabilities in Mo Weston Nielson, Vidvuds Ozolins The determination of accurate lattice stabilities is of great importance in producing phase diagrams of metallic alloys using the CALPHAD approach. Ab-initio molecular dynamics simulations in combination with thermodynamic integration are used to determine the lattice stabilities of fcc and bcc phases in Molybdenum at a range of temperatures. We employ the so-called fixed-cell-shape molecular dynamics approach, which involves the calculation of free energies over varying lattice strains. Our results also predict that at high temperatures fcc Mo is harmonically unstable. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X22.00006: Competing orders in the Dirac-like electronic structure and the non-linear sigma model with the topological terms Pouyan Ghaemi, Shinsei Ryu The Dirac-like electronic structure can host a large number of competing orders in the form of mass terms. In particular, two different order parameters, can be said to be dual to each other, when a static defect in one of them traps a quantum number (or ``charge'') of the other. The complementary nature of the pair of the order parameters shows up in their dynamical properties (correlation functions) in the following sense: When a quantum phase transition is driven by one type of fluctuations in order parameter, approaching the transition from the disordered (paramagnetic) side, the order parameter correlation function at the critical point is reduced. On the other hand, such fluctuations enhances the correlation of the dual order parameter. Such complementary behaviors in the correlation function can be used to diagnose the nature of quantum fluctuations that is the driving force of the quantum phase transition. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X22.00007: Concentration dependence of the electron-phonon coupling from metals to semiconductors Andrei Sergeev, Michael Reizer, Vladimir Mitin We study dependence of the deformation potential (DP) on concentration of carriers in the wide range from metals to semiconductors. DP in metals and semiconductors has a different nature. In metals, DP is due to electron gas compressibility, while in semiconductors this contribution is negligible due to small carrier concentrations. DP in semiconductors originates from a shift of the conduction band edge under the deformation, while in metals such contribution is small because of strong screening. We investigate DP in the transition region and found that the electron-phonon coupling has a significant minimum at intermediate concentrations. The effects of disorder on the coupling are also investigated. Theoretical conclusions are compared with available data on semi-metals and highly-doped semiconductors. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X22.00008: Investigation of the electronic structure of TiTe$_{2}$ using ARPES Jianqiao Meng, Gey-Hong Gweon, Andrew LaForge, Sriram Shastry, Arthur Penn Ramirez, Zack Schlesinger, Kai Rossnagel TiTe$_{2}$ is considered a model Fermi liquid material in the field of angle-resolved photoelectron spectroscopy (ARPES). Over the years, many groups have contributed to improving the quality of the ARPES data on TiTe$_{2}$, helping to understand the connection between the ARPES data and the transport properties. However, some key questions remain unanswered, the most outstanding one being the anomalous temperature dependence in the Hall coefficient $R_{H}$. Here, we present a detailed high resolution ARPES data set in a wide range of temperature and momentum. This reveals some new features: temperature dependence in the band width, temperature dependence of the Ti 3d and Te 5p occupancies, and subtle features in the line shapes as the peak crosses the Fermi level. We discuss these new features in comparison with previous ARPES studies and known transport properties. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X22.00009: Anomalous Ordering in Inhomogeneously Strained Materials: Surface Critical Behavior in the Bulk Kevin E. Bassler, Charo I. Del Genio, Bo Li We study a continuous quasi-two-dimensional order-disorder phase transition that occurs in a simple model of a material that is inhomogeneously strained due to the presence of dislocation lines. Performing Monte Carlo simulations of different system sizes and using finite size scaling, we measure critical exponents describing the transition of $\beta =0.18 \pm 0.02$, $\gamma =1.0\pm 0.1$, and $\alpha =0.10\pm 0.02$. Comparable exponents have been reported in a variety of physical systems. These systems undergo a range of different types of phase transitions, including structural transitions, exciton percolation, and magnetic ordering. In particular, similar exponents have been found to describe the development of magnetic order at the onset of the pseudogap transition in hightemperature superconductors. Their common universal critical exponents suggest that the essential physics of the transition in all of these physical systems is the same as in our simple model. We argue that the nature of the transition in our model is related to surface transitions although our model has no free surface. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X22.00010: Finding saddle points using Gentlest ascent dynamics Amit Samanta, Xiang Zhou, Weinan E We present dynamical equations for determining transition states and escape paths from basins of attraction of a stable system on a potential energy landscape. It is shown that the stable fixed points of such dynamical systems are the index-1 saddle points. The method relies on determining the smallest eigenvalue of the Hessian matrix. The formalism is easy to extend to systems of higher dimensions and can be used to explore the free energy landscapes of systems whose large time scale separation makes the standard molecular dynamics inefficient. The utility of the algorithm is demonstrated by evaluating the activation parameters for homogeneous and heterogeneous dislocation nucleation. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X22.00011: An efficient method to treat low barriers in kinetic ART simulations Peter Brommer, Normand Mousseau In kinetic Monte Carlo (KMC) the time scale of the simulation is dominated by the height of the lowest energy barrier separating two states. Rapid back-and-forth movements across very low barriers called flickers are a major limitation of the technique, as they can cost considerable CPU time without advancing the simulation. To accelerate KMC simulations, an energy basin finding algorithm has been presented [1]. In the kinetic Activation-Relaxation technique (kART) [2], KMC events are constructed during the simulation, taking full care of elastic deformations while avoiding the need for complete event search at every step. To account for low energy barriers located in this package, we implement a basin identification scheme that works on the fly as well. We apply this method to interstitial diffusion in bcc iron. There, rapid diffusion paths with low barriers for properly aligned interstitial clusters limit the simulated time. With our method, we can prevent unproductive oscillations in this diffusive basin while maintaining an appropriate distribution of exit states. This considerably extends the time scales accessible to simulation.\newline \noindent [1] Puchala \emph{et al.}, \emph{J.\ Chem.\ Phys.} {\bf 132}, 134104 (2010)\newline [2] El-Mellouhi \emph{et al.}, \emph{Phys.\ Rev.\ B}, {\bf 78}, 153202 (2008). [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X22.00012: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X22.00013: Is the Debye-temperature a useful concept at the nanometer scale? Insights from\textit{ ab initio} free energy calculations of Au$_{13 }$and Au$_{12}$Fe nanoclusters Ghazal S. Shafai, Marisol Alcantara Ortigoza, Talat S. Rahman We have calculated the phonon density of states, specific heat, and mean-square vibrational amplitudes of the five lowest-lying isomers of Au$_{13}$ and two of Au$_{12}$Fe nanoparticles, as dictated by their Helmholtz free energy. We find the vibrational entropic contributions to not affect the energy ordering of the isomers. We show that the highest phonon frequencies shift to slightly higher energies in the hybrid clusters: a signature of alloying. As expected the density of vibrational states differs significantly from the Debye model for bulk systems. The definition of the ``Debye temperature'' for the nanocrystal thus becomes ambiguous and depends very much on how it is calculated. In particular it neither correlates uniquely with atomic bond strengths nor does it relate to the maximum phonon frequency. The discrete phonon spectrum of nanoparticles is thus needed to describe the \textit{exact} mean square displacement or the temperature dependency of the heat capacity. Work supported by DOE Grant DE-FG02-07ER46354 [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X22.00014: Insights on the anomalously soft and stiff modes of metal nanoparticles Marisol Alcantara Ortigoza, Talat S. Rahman, Rolf Heid, Klaus-Peter Bohnen The low- and high-energy tails of the phonon density of states (PDOS) of transition-metal nanoparticles is enhanced with respect to that of their bulk counterparts. For particles in the sub-nanometer scale, we propose a rationale for this fact based on ab initio calculations of their charge density and of the frequency and displacement pattern of their vibrational modes. We find that the radial breathing and non-radial vibrations -- analogous to the pulsations observed in variable stars -- correspond to the highest and lowest frequencies, respectively. This result is traced to the radial atomic distribution and the charge density distribution particular to the low-coordinated atoms, both of which give rise to modes that have no counterpart in the bulk. We find that the enhanced PDOS at low frequencies is at least partly due to the relatively small number of modes that nanoparticles can sustain and that clusters with bulk-like ordering render fewer and less stiff modes above the bulk limit. [Preview Abstract] |
Session X23: Focus Session: Iron Based Superconductors -- Gap Symmetry
Sponsoring Units: DMP DCOMPChair: Laura Greene, University of Illinois
Room: D165
Thursday, March 24, 2011 2:30PM - 3:06PM |
X23.00001: Nodal and nodeless Superconductivity in Iron-Based Superconductors Invited Speaker: The superconducting phase in Iron-based superconductors (pnictides) exhibits a variety of different properties depending on the doping regime and specific parameters such as band structure and interaction which describe the different compounds. The question of the symmetry of the superconducting order parameter combined with the role of interaction-induced anisotropies plays a decisive role to distinguish material- dependent effects from universal mechanisms in this family of compounds. In our talk we attempt to provide an overview of the superconducting phases currently discussed for different classes of pnictides. Specifically, we report on our work on functional renormalization group (FRG) calculations for the pnictides and how it can contribute to our understanding of the different compounds. We discuss from first principles why LaOFeAs shows nodeless while LaOFeP shows nodal anisotropic extended $s$-wave superconductivity, which we find to be dictated by the existence / non-existence of an additional hole pocket at $\rm{M}=(\pi,\pi)$ in the unfolded Brillouin zone. We also elaborate on the nodal phase in KFe${}_2$As${}_2$ as being of $d$-wave symmetry type and explain its microscopic origin related to the absence of electron pockets which are gapped out at large hole doping. In particular, we will draw a direct line from band structure and interaction parameter calculations to FRG which accomplishes to study Fermi surface instabilities from a truly ab initio starting point, and illustrate this approach for the LiFeAs compound. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X23.00002: Do theoretical calculations really predict nodes in Fe-based superconductors? Igor Mazin It is well established that calculations based on the LDA band structure and the Hubbard model, with the parameters $U\sim 1.3-1.6$ eV, and $J\sim 0.2-0.3 J$ (a ``UJ'' model), yield strongly anisotropic, and sometimes nodal gaps. The physical origin of this effect is well understood: the two leading terms in the model are $\sum{Un_{i\uparrow}n_{i\downarrow}}$ and $\sum ' {U n_{i}n_{j}}$. The former ensures that the coupling to spin fluctuations proceeds only through the like orbitals, and the latter, not being renormalized by the standard Tolmachev-Morel-Anderson logarithm, tends to equalize the positive and the negative order parameters. Both these features are suspect on a general physics basis: the leading magnetic interaction in itinerant systems is the Hund-rule coupling, which couples every orbital with all the others, and the pnictides, with the order parameter less than 20 meV, should have nearly as strong renormalization of the Coulomb pseudopotential as the conventional superconductors. I will argue that, instead of the UJ model, in pnictides one should use the ``I'' model, derived from the density functional theory (which is supposed to describe the static susceptibility on the mean field level very accurately). The ``I'' here is simply the Stoner factor, the second variation of the LSDA magnetic energy. Unfortunately, this approach is very unlikely to produce gap nodes as easily as the UJ model, indicating that one has to look elsewhere for the nodes origin. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X23.00003: Competing Pairing Symmetries in a Generalized Two-Orbital Model for the Pnictides A. Nicholson, W. Ge, X. Zhang, J. Riera, M. Daghofer, A. Oles, G. Martins, A. Moreo, E. Dagotto An extended ``$t$-$U$-$J$'' two-orbital model [1] for the pnictides will be introduced that includes Heisenberg terms deduced from the strong coupling expansion of the Hubbard model. This extension allows us to enhance the strength of the $(\pi,0)$-$(0,\pi)$ spin order and favors the presence of tightly bound pairing states even in the small clusters that are exactly diagonalized. The $A_{\rm 1g}$ and $B_{\rm 2g}$ pairing symmetries are found to compete in the realistic spin-ordered and metallic regime. The dynamical pairing susceptibility additionally unveils low-lying $B_{\rm 1g}$ states, suggesting that small changes in parameters may render any of the three channels stable. These results contribute to understanding the puzzling results in pnictides where both nodeless and nodal states have been reported.\\[4pt] [1] A. Moreo {\em et. al.}, Phys. Rev. {\bf B} 79, 134502 (2009) \\[0pt] [2] A. Nicholson {\em et. al.}, preprint [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X23.00004: What the magnitude of $2\Delta/T_c$ on the hole pockets can tell us about the structure of the gaps on electron pockets in Fe-based superconductors? Saurabh Maiti, Andrey Chubukov There is evidence from transport and penetration depth measurements that some Fe-based superconductors (pnictides) are nodal and some nodeless. Most notable example of nodal behavior is $BaFe_2(As_{1-x}P_x)_2$. But as of this date, there has been no direct probes of the gap structure in this material. ARPES is a direct probe to measure the gap evolution along the Fermi surfaces (FS), but in $BaFe_2(As_{1-x}P_x)_2$ accurate laser ARPES data are only available for hole FSs at $\Gamma$ point, along which the gaps are nearly identical and are nearly angle-independent. We addressed the issue whether one can use ARPES data for $2\Delta_h/T_c$ on the hole FSs to predict the gap structure and magnitude along the two electron FSs. For this, we considered the non-linear gap equations in realistic 2D multi-pocket models. We found that, in the 4-pocket model, at least in certain limits, the electronic gaps have accidental nodes if $2\Delta_h/T_c$ is below a certain value close to the BCS result, and have no nodes if $2\Delta_h/T_c$ exceeds this value. This, combined with the experimental input on the $2\Delta_h/T_c$, allows us to predict the forms of the electronic gaps based on the ARPES data for the gaps on the hole pockets The verification of these results by the ARPES measurements along the electron FSs will be a crucial test for 2D itinerant multi-pocket models for Fe-pnictides. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X23.00005: Observation of a ubiquitous three-dimensional superconducting gap function in optimally-doped Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ Y.-M. Xu, Y.-B. Huang, X.-Y. Cui, E. Razzoli, M. Radovic, M. Shi, G.-F. Chen, P. Zheng, N.-L. Wang, C.-L. Zhang, P.-C. Dai, J.-P. Hu, Z. Wang, H. Ding The knowledge of the quasi-three-dimensional (3D) superconducting (SC) gap is essential for understanding the superconducting mechanism of the iron-pnictides highlighted by their multiband and quasi-3D electronic structure. By using the $k_z$-capability of angle-resolved photoemission, we completely determined the SC gap on all five FSs in 3D on Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ samples. We found a significant $k_z$ dispersion of the SC gap which can only derive from interlayer pairing. Remarkably, the SC energy gaps can be described by a single 3D gap function with two energy scales characterizing the strengths of intra-layer ($\Delta_1$) and interlayer ($\Delta_2$) pairing. The anisotropy ratio $\Delta_2/\Delta_1$, determined from the gap function, is close to the $c$-axis anisotropy ratio of the magnetic exchange coupling $J_c/J_{ab}$ in the parent compound. The ubiquitous gap function for all the 3D FSs reveals that pairing is short-ranged and strongly constrain the possible pairing force in the pnictides. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X23.00006: Field dependence of the zero energy density of states of an anisotropic $s_{\pm}$ superconductor Yan Wang, Peter Hirschfeld, Siegfried Graser The pairing symmetry in iron-based superconductors (SC) is generally believed to be an $s_{\pm}$-wave state. Although ARPES suggests a mainly isotropic gap on all Fermi surface sheets, different thermodynamic and transport measurements are still inconclusive about the existence and orientation of gap nodes. Specific heat measurements in a magnetic field showing a square root like dependence of the Sommerfeld coefficient $\gamma(B)$ have been reported, contradicting the linear behavior expected for a fully gapped system. For a $d$-wave SC, $\gamma(B)\propto\sqrt{B}$ is well-known as Volovik effect. For a fully gapped $s_{\pm}$-wave SC with $\Delta_+\ne\Delta_-$, a similar concave field dependence is expected. To distinguish these two effects we apply a two-band model using the Riccati parametrization of the Eilenberger equation to study the density of states around a single vortex and compare it with self-consistent calculations in the vortex lattice. Different models for the momentum dependence of the gap on each band relevant to the iron-based SC, ranging from isotropic to strongly anisotropic and nodal gaps are investigated. Partial support was provided by DOE DE-FG02-05ER46236 (PJH). [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X23.00007: Measurement of a sign-changing two-gap superconducting phase in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ single crystals using scanning tunneling spectroscopy (STS) M.L. Teague, G.K. Drayna, G.P. Lockhart, T.P. Wu, N.-C. Yeh, P. Chen, B. Shen, H.-H. Wen We present STS studies of the iron pnictide superconductors Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ (x=0.06, 0.08, 0.12). Studies on single crystals, cleaved at room temperature in a pure argon atmosphere, reveal direct spectroscopic evidence for predominantly two-gap superconductivity. These gaps decrease with increasing temperature and vanish immediately above the superconducting transition, T$_{C}$. Fourier transformation of the tunneling conductance demonstrates slight doping- and energy-dependent quasiparticle scattering interferences (QPI) near the nesting wave-vectors ($\pm \pi $,0)/(0,$\pm \pi )$ and also near ($\pm $2$\pi $,0)/(0,$\pm $2$\pi )$. The dominant QPI near ($\pm \pi $,0)/(0,$\pm \pi )$ and the two-gap nature are consistent with sign-changing s-wave superconductivity. The excess zero-bias conductance and the large gap-to-T$_{C}$ ratios suggest significant unitary impurity scattering. Further studies of the magnetic field dependence will be discussed. This work was supported by NSF Grant DMR-0907251. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X23.00008: Local measurement of the superfluid density in the pnictide superconductors $Ba(Fe_{1-x} Co_x )_2 As_2 $ across the superconducting dome Lan Luan, T.M. Lippman, C.W. Hicks, O.M. Auslaender, J.A. Bert, Jiun-Haw Chu, J.G. Analytis, I.R. Fisher, K.A. Moler We locally measure the superfluid density $\rho _s (T)$ in $Ba(Fe_{1-x} Co_x)_2 As_2 $ single crystals with magnetic force microscopy and scanning SQUID susceptometry. These high-precision, local-probe-based techniques enable us to measure both the zero temperature value of the superfluid density $\rho _s (0)$ and the temperature variation, to distinguish homogeneous from spatially varying responses, and to report systematic behavior as a function of Co doping across the superconducting dome. We find that $\rho _s (T)$ increases sharply with decreasing temperature below the superconducting transition temperature $T_c $ of both optimally doped and underdoped compounds, and that $\rho _s (0)$ falls more quickly with $T_c $ on the underdoped side of the dome than on the overdoped. These observations, as well as the increasing temperature induced change of $\rho _s (T)$ at low temperatures upon underdoping, are consistent with magnetic fluctuation mediated pairing and the coexistence of magnetism and superconductivity. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X23.00009: A sign of change: pinning down the pairing symmetry of the iron-based superconductors Erez Berg, Netanel Lindner, Tamar Pereg-Barnea Understanding the structure of the order parameter of the iron-based superconductors is the key to unveil their pairing mechanism. Although there has been much theoretical and experimental indications that the order parameter changes its sign in momentum space, direct evidence is still lacking. The difficulty stems from the fact that the order parameter is likely to be of s-wave symmetry, and therefore designing a phase sensitive experiment that would clearly reveal the sign change is non-trivial. Here, we examine a contact between a sign-changing superconductor and an ordinary, uniform-sign superconductor. If the the barrier between the two superconductors is not too high, the frustration of the Josephson coupling between different portions of the Fermi surface across the contact can lead to surprising consequences, such as time-reversal symmetry breaking at the interface and unusual energy-phase relations with multiple local minima. We propose this mechanism as a possible explanation for the half-integer flux quantum transitions in niobium-iron pnictide loops, which were discovered in a recent experiment (C-T. Chen et. al., Nature Physics 6, 260 (2010)). [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X23.00010: Probing the Superconducting Order Parameter of Ba(Fe$_{1-x}$Co$_{x})_{2}$As$_{2}$ by Josephson Interferometry J.M. Atkinson, D.J. Van Harlingen, P. Canfield, N. Ni, J.D. Strand Since the discovery of the first Fe-based superconductors in 2006, extensive effort has been directed toward characterizing and modeling the novel properties of these exotic materials, in particular, the symmetry of their superconducting order parameter. We probe the order parameter in Co-doped BaFe$_{2}$As$_{2}$ single crystals by fabricating Josephson junctions on polished faces orthogonal to the c-axis. It has been proposed that the Fe-pnictides form electron and hole pockets in the Fermi surface that have s-wave Cooper pair symmetry but opposite phases, the so-called s$\pm $ model. The modulation of the critical current I$_{C}$ as a function of magnetic flux applied along the c-axis is different for junctions fabricated on a corner (between [100] and [110] faces) or on an edge (either [100] or [110]). In the same way, the product I$_{C}$R should be different for each type of junction. The combination of these effects may help us map the phase anisotropy and test for this pairing symmetry. We will present preliminary results of these studies and attempts to match them with existing theoretical models. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X23.00011: Robust nodal gap structure in BaFe$_2$(As$_{1-x}$P$_x$)$_2$ with P doping revealed by magnetic penetration depth measurements Kenichiro Hashimoto, Ryo Katsumata, Sho Tonegawa, Shigeru Kasahara, Takahito Terashima, Takasada Shibauchi, Yuji Matsuda, Alessandro Serafin, Antony Carrington A number of experimental studies show that the non-universal superconducting gap structures with and without nodes is realized in iron pnictides, depending on the doping materials and its doping level. It has been suggested that in a framework of s++ wave symmetry, vertical nodal gap structure occurs during the crossover from s++ to s+- state due to the competition between the orbital and magnetic fluctuations as well as the impurity scattering effect. On the other hand, within the spin-fluctuation mediated pairing mechanism, three dimensional nodal structures is discussed. Therefore, it is important to uncover the doping dependence of the superconducting gap structure and its impurity effect. Here we report the magnetic penetration depth results measured down to 100 mK in the P-doped Ba122 system indicate robust nodal gap structure. We especially focus on doping evolution of the superfluid density with P doping. We also discuss the impurity effect introduced by Pb heavy ion beam respective to the in-plane. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X23.00012: Two-Gap Paring of the Optimal Doped (M,K)Fe$_{2}$As$_{2}$ with M = Ba, Sr Fengyan Wei, Bing Lv, Feng Chen, Yuyi Xue, Chingwu Chu The gap structure revealed by the specific heat of iron pnictides remains unsettled. Not only do the reported characters vary for similar Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$ and Sr$_{0.55}$K$_{0.45}$Fe$_{2}$As$_{2}$, single gap and two-gap pairings have also been suggested in the crystals with the same nominal composition of Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$. It seems that either the gap structure is unusually sensitive to the sample details or some analysis procedures need to be refined Here we explored both the (Sr,K)Fe$_{2}$As$_{2}$ and (Ba,K)Fe$_{2}$As$_{2}$ systems, and different procedures were used to extract the phonon background. In the case of (Sr,K)Fe$_{2}$As$_{2}$, the phonon background seems to be insensitive to both the procedures and the potassium doping. For (Ba,K)Fe$_{2}$As$_{2}$, however, the data suggest a significant doping dependency of the soft phonons. The observations cast doubts on the previous procedures of using either BaFe$_{2}$As$_{2}$ or Ba(Fe$_{0.9}$Co$_{0.1})_{2}$As$_{2}$ to estimate the phonon background of Ba$_{0.6}$K$_{0.4}$Fe$_{2}$As$_{2}$. A new procedure, therefore, is developed. The result will be presented and discussed. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X23.00013: Superconducting gap measurements on Co-doped SrFe2As2 single crystals by point contact spectroscopy Cassandra R. Hunt, H.Z. Arham, W.K. Park, L.H. Greene, J. Gillett, S. Sebastian We present point contact spectroscopy results on single crystal Co-doped SrFe$_2$As$_2$. Two sets of Andreev-like enhancements in conductance are seen with nominally $c$-axis contacts. For temperatures up to $T_c$ = 14.5 K, the conductance is fit to a Blonder-Tinkham-Klapwijk (BTK) model extended to two independent bands with lifetime broadening [1]. We also consider recently proposed $s_\pm$-wave extensions to BTK [2,3]. Many recent reports claim multiple gaps in the 122 compounds, however care must be taken to distinguish the presence of Andreev peaks from other excitation modes. We find robust evidence of an SC gap at 6 meV and evidence of another conductance enhancement at 12 mV that tracks the inner gap. The origin of this feature, and of multi-gap features as measured by PCS, are discussed. [1] G. E. Blonder, M. Tinkham, and T. M. Klapwijk, PRB \textbf{25}, 45154532 (1982). [2] A. A. Golubov, \emph{et al}. PRL \textbf{103}, 077003 (2009). [3] I. B. Sperstad, J. Linder, A. Sudbo, PRB \textbf{80}, 144507 (2009). [Preview Abstract] |
Session X24: Focus Session: Quantum Transport Simulations and Computational Electronics -- Molecular Junctions
Sponsoring Units: DCOMPChair: Hong Guo, McGill University
Room: D167
Thursday, March 24, 2011 2:30PM - 2:42PM |
X24.00001: Efficient k.p method for first-principles calculation of Seebeck coefficient in quantum transport David A. Strubbe, Su Ying Quek, Hyoung Joon Choi, J.B. Neaton, Steven G. Louie Thermoelectric properties of molecular junctions reveal fundamental aspects of nanoscale charge transport at interfaces and are relevant to potential organic/inorganic hybrid thermoelectric materials. Quantum transport calculations typically evaluate the Seebeck coefficient S by finite differences of the transmission as a function of energy. However, in ab initio calculations this quantity is difficult to converge for realistic systems and can require very large k-grids. We derive a new analytic-derivative method to evaluate S via k.p perturbation theory, implement it in a DFT-based scattering-state transport code, and apply it to calculations of molecular junctions. This technique improves k-point convergence by avoiding critical points in the lead bandstructure and allows more efficient calculations of Seebeck coefficients. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X24.00002: First-Principles Studies of Charge Dynamics in Single-Molecule Junctions at Finite Bias Pierre Darancet, Hyoung Joon Choi, Jonathan R. Widawsky, Scott Berkley, Latha Venkataraman, Jeffrey B. Neaton Extending well-established measurements of low-bias conductance of single molecule junctions, new experiments report IV characteristics of organic molecules for biases as high as 1V [1]. Such measurements provide a unique probe of electronic properties of well-defined metal-organic nanointerfaces when driven out-of-equilibrium, and an opportunity to examine a still-missing quantitative theory of out-of-equilibrium charge dynamics at the nanoscale. Here we will present first-principles transport calculations for several amine-Au and pyridine-Au linked junctions at different levels of approximation: first mean-field, and then including electron-electron correlations at equilibrium and out-of-equilibrium. We show that incorporating electronic correlations at equilibrium already leads to a very good agreement with experiments [1], and discuss how these corrections might change out of equilibrium.\\[4pt] [1] Widawsky et al., Nanotechnology (2009). [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X24.00003: Understanding the Role of Direct Au-C Links to Electrodes in Single Molecule Junctions Hector Vazquez, Jonathan Widawsky, Zhang-Ling Cheng, Severin Schneebeli, Rachid Skouta, Ronald Breslow, Mark S. Hybertsen, Latha Venkataraman Recent experiments have shown that use of tri-methyl tin (SnMe3) link groups results in the formation of alkane single molecule junctions with measured conductance $\sim $100 times higher than found for any other link group previously used. Further evidence points to the in-situ formation of direct Au-C bonds to the electrode. In this work we use Density-Functional Theory based calculations to study the formation and structure of junctions based on direct Au-C link bonds. Transport calculations based on Non-Equilibrium Green's Functions for benzene and alkane molecules anchored through Au-C bonds show that the alkane backbone couples more strongly to the leads, resulting in a higher transmission as compared with other link groups. In the case of benzene, however, transport is primarily through the $\sigma $ system, yielding a smaller conductance increase. Finally, we discuss corrections to the position of molecular resonances found in the DFT-based calculations and the implications for conductance. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X24.00004: Phonon-assisted tunneling in two-level quantum dots or diatomic molecules Kevin Ingersent, Edson Vernek, Gisele Iorio Electron-electron interactions in nanoscale systems can be significantly modified by coupling to bosonic modes (photons, phonons, and plasmons) that act as sources of dissipation and decoherence (dephasing). Photon-assisted tunneling can take place through ground and excited states of various types of quantum-dot system, while signatures of vibrational modes are seen in transport through single-molecule transistors in the Coulomb blockade and Kondo regimes. We report numerical renormalization-group results for a quantum dot or diatomic molecule that has two active levels, taking into account both intra-level and inter-level Coulomb interactions. We focus on how decoherence induced by phonon-assisted inter-level transitions affects the formation of the many-body Kondo singlet between the dot/molecule and the leads, and quantify the consequent modification of the zero-bias electrical conductance. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X24.00005: Quantum current of a molecular photo-switch between two graphene sheets G.P. Brivio, C. Motta, M.I. Trioni, K.L. Sebastian Light responsive materials that reversibly change shape under alternate UV and visible irradiation have attracted much interest because they can be used as optical switches, since the isomers show different features in the dimension, HOMO-LUMO gap and transmission spectrum. In view to integrate the photo-switch in the carbon based electronics devices, the conductance of a system constituted by a photochromic molecule between two graphene electrodes is investigated. In this work the conductance of the junction formed by diarylperfluorocyclopentene between two semi-infinite graphene sheets was computed using the non-equilibrium Green's function method combined with density functional theory via the TranSiesta code. The results emphasize the role of the graphene and the molecular electronic states in the switching behaviour of this hybrid system. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X24.00006: Correlation between Raman scattering and conductance in a molecular junction Tae-Ho Park, Michael Galperin Raman spectroscopy of molecular junctions is a promising diagnostic and control tool. We present a model for non-resonant Raman spectroscopy, generalizing previous considerations to strong laser pulses of arbitrary time dependence. The model paves a way to realistic simulations of Raman spectroscopy experiments in molecular conduction junctions. We demonstrate within the model that the optical properties of molecular conduction junctions are strongly correlated with the electron transport properties. Feynman diagrams responsible for such similarity are analyzed for both processes, and possible explanation for observed (anti-)correlated behavior of Stokes signal and conductance is proposed. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X24.00007: Towards {\it single-atom-controlled} device Subhasish Mandal, Ranjit Pati {\it Single-atom-controlled} device has been explored recently in the context of molecular junction. Here, by using a codoping model, where a cation and an anion are introduced simultaneously into the host to maintain charge neutrality, we have probed the electron transport characteristics in a strongly coupled single molecular junction. We have used 1, 12-dicarba-{\it closo}-dodecaborane inorganic molecule as a precursor and have replaced one of the vertex carbon atoms by a boron atom and simultaneously decorated it with an endohedrally doped alkali atom (Li/Na) to look into the role of dopant atoms on the conductivity. The commonly used thiolate anchoring groups are used to attach the molecule in between two gold electrodes, and a parameter free, first-principles, nonequilibrium Green's function approach is used to calculate the current-voltage characteristics. Charge transfer from the alkali atom to the host is found to have a profound effect on the electronic structure causing a dramatic change in the conductivity. Since the single alkali atom controls the behavior of electron flow in this circuit, we term this device as a {\it single-atom-controlled} device. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X24.00008: Conformational and Voltage Gating in a Molecular Three Terminal Device Saikat Mukhopadhyay, Ravindra Pandey, Shashi Karna The effect of the conformational changes in the gate arm of a three terminal molecular device is investigated. The donor-acceptor molecular moieties connected through a ring describe the two arms, whereas a $\pi $-conjugated molecular wire is used as a gate in the proposed architecture. In the absence of the gate field, the device exhibits current switching between the non-planar and planar orientations of the $\pi $ -electron moieties with respect to each other with maximum I$_{(ON)}$/I$_{(OFF)}$ =14. When the gate field is applied, I$_{(ON)}$/I$_{(OFF)}$ ratio decreases, thus suggesting that the effects of the conformational changes in the gate arm and the applied gate field oppose each other in the architecture considered. Furthermore, the tunneling current corresponding to conformational gating in two different directions appears to exhibit oscillatory nature with a phase factor of $\pi $/2 in presence of the gate field. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X24.00009: Charge transport in strongly coupled molecular junctions: `In-phase' and `out of phase' contribution to electron tunneling Partha Pratim Pal, Ranjit Pati We report a first principles study on the evolution of charge transport in a two-terminal molecular scale device with the increase in the length of the molecular wire build out of cubane oligomers. In particular, for wires of three different lengths, we look into the relative contribution of the `in-phase' and the `out-of-phase' components of the total electronic current under the influence of an external bias. In the low bias regime, the `out-of-phase' contribution to total current is minimal and `in-phase' or elastic tunneling of the electrons is responsible for the net electronic current. This is true irrespective of the length of the molecular spacer. In this regime, the I-V characteristics follow Ohm's law and the conductance of the wires is found to decrease exponentially with length which is in agreement with experimental results. However, after a certain `off-set' voltage, the I-V characteristics become non-linear and the `out-of-phase' tunneling starts to contribute substantially to the net current. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X24.00010: Negative Differential Resistance at Low Bias: C60-Based Molecular Devices Wenchang Lu, Xiaohong Zheng, T.A. Abtew, Vincent Meunier, Jerry Bernholc Unlike single-C60-based devices, molecular assemblies based on two or more C60 can exhibit negative differential resistance (NDR). We evaluate electron transport properties of molecular devices built from two C60 connected by an alkane chain, using a non-equilibrium Green function technique implemented within the framework of linear-scaling DFT. We find that electronic conduction in these systems is mediated by C60's lowest unoccupied molecular orbitals (LUMOs), as in the case of a single-C60-based device. However, as the LUMOs' positions are pinned to the chemical potentials of their respective electrodes, their relative alignment shifts with applied bias and leads to an NDR at a very low bias. Furthermore, the position and magnitude of the NDR can be tuned by chemical modification of the C60s and by changing the length of the alkane linker. The flexibility and richness of C60-based molecular electronics components point to a potentially promising route for the design of molecular devices and chemical sensors. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X24.00011: Charge injection and transport across metal-C60 and C60-petacene interfaces: A first-principles study Yong-Hoon Kim Recent experiments demonstrated that [60]fullerene (C$_{60})$ molecules adsorbed on metal surfaces provide favorable energy level alignment for both electron and hole injections in the context of light-emitting diode applications [1,2]. The efficient hole injection across C$_{60}$ layers is rather surprising, since C$_{60}$s are highly electron-accepting molecules and should behave as a hole blocking (rather than hole injection) layer. To provide a microscopic understanding of these seemingly contradictory finding, we consider Au--C$_{60}$--pentacene--C$_{60}$--Au molecular junctions using a first-principles computational approach. We find the Fermi level pinning at the Au--C$_{60}$ interfaces and the strongly configuration-dependent charge transport efficiency at the C$_{60}$--pentacene interfaces. The former finding is in agreement with a recent experimental report [2] and our earlier conclusion from the study of polymerized C$_{60}$ wires [3]. We will explain the latter observation based on the nature of charge tunneling across $\pi $--$\pi $ orbitals [1] Lee, J.Y., Appl. Phys. Lett. \textbf{88}, 073512 (2006) [2] Wang, Z.B \textit{et al.}, Appl. Phys. Lett. \textbf{95}, 043302 (2009). [3] Lee, G.I., Kang, J.K., {\&} Kim, Y.-H., J. Phys. Chem. C \textbf{112}, 7029 (2008). [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X24.00012: Electron correlation enhancement of the diode property of asymmetric molecule Yoshihiro Asai, Hisao Nakamura, Joshua Hihath, Nongjian Tao Stimulated by the giant diode property found in tetraphenlydithiol derivative including dipyrimidinil-diphenly diblock [1], a possible mechanism of the giant diode property was investigated theoretically based on electron correlation. We found that the mean field theory (MFT) fails in describing the giant diode property, as it was confirmed by first principle calculation of ballistic electronic current through the diode molecule using GGA. Electron correlation effect on electric current taken into account within the self-consistent GW approximation using Keldysh Green's function theory was found to give the fair account of the giant diode property. We conclude that elastic electron collision beyond MFT enhances the diode property quite a lot. \\[4pt] [1] I.D\'iez-P\'erez et al, Nature Chemistry, 635 (2009). [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X24.00013: First Principles Study and Theoretical Analysis of a Single Molecular Diode by $p-n$ di-block molecules Hisao Nakamura, Yoshihiro Asai, Josh Hihath, Nongjian Tao The concept of a single molecular diode was first proposed by Aviram and Ratner, and there have been many studies of synthesis D-$\sigma $-A or $p-n$ di-block molecules and measurements of the current-voltage ($I-V)$ characteristics for relating molecular junctions. Recently, the $I-V $measurement in a symmetric tetraphenyl junction and non-symmetric dipyrimidinyl -diphenyl diblock junction was performed, and clear rectification was found in the latter system, which resembles the $p-n$ junction by the covalent connection between electron-deficient bypyrimidinyl and electron-rich biphenyl moieties, though an applied bias is much lower than the resonant level. In this presentation, we performed the first principles calculations of electron transport for the above tetraphenyl and dipyrimidinyl -diphenyl diblock junctions by the self-consistent nonequilibrium Green's function theory with the use of our HiRUNE program module. We carried out the systematic analysis of the rectification behavior and identified the change of electron-pathway in the bridge molecule relating to $p-n$ junction based on the first principles data. The relation between the rectifying action and molecular conformation, particularly, the torsion of diblock, will be discussed. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X24.00014: Temperature influence on a molecular switching under electric field: quantum transport ab initio calculation Maia Vergniory, Jose Grandino-Roldan, Aran Garc\'Ia-Lekue, Lin-Wang Wang A molecular transistor based on torsion-angle conformation change driven by gate electric field is designed and studied using {\it ab initio} calculations. This transistor consists of a ${\rm SH-C_6H_2F(CH_3)C_6H_2(CH_3)F-SH}$ molecule sandwiched between two Au(111) electrodes, where the interaction between the molecular dipole and a gate voltage induced electric field will cause the molecule to twist along its c-axis. This twist changes the quantum conductivity of the molecule. The effect of thermal fluctuation on the molecular conformation is studied, so is the ability of the transistor to shut off its current. The advantages and challenges of using such molecular conformation change as a mechanism for transistor gating are discussed [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X24.00015: Vibronic- and mechanical-spin control in spin-1 molecular devices David Ruiz-Tijerina, Pablo Cornaglia, Carlos Balseiro, Sergio Ulloa Using numerical renormalization group calculations, we study the effect of a vibronic mode on the electronic transport through a deformable spin-1 molecular device. We analyze the experimental situation of Parks et. al. [Science 328 1370 (2010)], where it is observed that stretching the molecule introduces a static magnetic anisotropy. The device is modeled as an interacting two-level system with only one level coupled to metallic leads, in which the static anisotropy is modulated by a vibronic mode. We performed calculations of the local spectral density, which indicate that this dynamic magnetic anisotropy can counter the static effects and drive the ground state into a non Fermi-liquid phase with non-zero spectral density at the Fermi level. It also renormalizes the couplings between the molecule and the metallic leads in an anisotropic fashion, reducing the spin-1 Kondo temperature of the device. [Preview Abstract] |
Session X25: Superconductivity: Optical, Raman and Other Spectroscopies
Sponsoring Units: DCMPChair: David Tanner, University of Florida
Room: D166
Thursday, March 24, 2011 2:30PM - 2:42PM |
X25.00001: How can we relate the critical temperature and the superconducting gap amplitude in cuprate superconductors? Alain Sacuto, Seabstien Blanc, Yann Gallais, Maximilien Cazayous, Marie Aude Measson, J.S. Wen, Z.J. Xu, Genda Gu We explore the superconducting state of hole-doped cuprates by electronic Raman scattering as a function of both temperature and doping level. We observe a loss of coherent quasi-particles in the anti-nodal region and show that coherent Bogoliubov quasiparticles are confined around the nodes. This contrasts to conventional superconductors where superconductivity develops uniformly along the normal-state Fermi surface. We define the fraction of coherent Fermi surface, f$_{c}$ around the nodes for which quasi-particles are well defined and superconductivity sets in. We establish that T$_{c} \quad \propto $ f$_{c}\Delta _{max}$. $\Delta _{max}$ is the maximum amplitude of the d-wave superconducting gap. This new relation differs from the standard BCS theory and gives us some clues for increasing T$_{c}$ in the cuprates. S. Blanc et al. Phys. Rev. B \textbf{82}, 144516 (2010); S. Blanc et al. Phys. Rev. B \textbf{80}, 140502 (2009). [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X25.00002: Raman response in density wave materials Elizabeth Nowadnick, Alexander Kemper, Brian Moritz, Thomas Devereaux Raman spectroscopy, which uses different incoming and outgoing light polarizations to measure different areas of the Brillouin zone, allows researchers to probe the nature of charge and spin density wave gaps. We present calculations of the Raman response for two density wave materials: rare earth tri-tellurides in the charge density wave state and the iron pnictides in the spin density wave state. Both of these materials have phase diagrams which can be further understood by clarifying the nature of the density wave state. For example, in the tri-tellurides, either one or two charge density wave gaps are present depending on the type of rare earth element in the compound. In the pnictides, which we treat with a multiband model, superconductivity coexists with or is in close proximity to a spin density wave state. We discuss what can be learned from our calculations and compare to experimental results. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X25.00003: Far-infrared spectroscopy of magnetic-field-induced pairbreaking in superconducting thin films Xiaoxiang Xi, J. Hwang, C. Martin, D.B. Tanner, G.L. Carr A magnetic field will break the time-reversal symmetry of the superconducting condensate pairing, giving rise to a pair-breaking effect. This pairbreaking has been confirmed by our recent far-infrared transmission and reflection measurement of a superconducting NbTiN thin film in an in-plane magnetic field. The complex optical conductivity was extracted, and the optical gap was obtained from its real part. Comparison with the pair-breaking theory of Abrikosov and Gor'kov yields good quantitative agreement, confirming directly the theory's validity for the optical conductivity. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X25.00004: Optical properties of the pseudogap state in deeply underdoped cuprates Adam Pound, Jules Carbotte, Elisabeth Nicol Recent optical measurements of deeply underdoped cuprates have revealed that a coherent Drude response persists well below the end of the superconducting dome in the phase diagram[1]. We show that this observation is consistent with the resonating valence bond spin-liquid model proposed by Yang, Rice, and Zhang[2]. Within this model, we analyze the three elements that cause the overall reduction in optical conductivity in the approach to the Mott insulating state: a Gutzwiller factor associated with increased correlations, which causes a reduction in the coherent part of the carrier Green's function; a shrinking of the Fermi surface defining the hole Luttinger contours; and an increase in optical effective mass. We show that each of these elements yields qualitative agreement with various experimental observations. Finally, we show how the increased magnitude of the pseudogap at low doping modifies the microwave conductivity and the Wiedemann-Franz law. \\[4pt] [1] W.J. Padilla et al., Phys. Rev. B 72, 060511 (2005)\\[0pt] [2] K.-Y. Yang, T.M. Rice, F.-C. Zhang, Phys. Rev. B 73, 174501 (2006) [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X25.00005: Point group sensitive probes of the pseudogap electronic structure in Bi2212 J.P. Hinton, J.D. Koralek, J. Orenstein, I. Firmo, M. Hamidian, K. Fujita, J.C. Davis We combine optical transient grating spectroscopy (TGS) and spectroscopic imaging scanning tunneling microscopy (SI-STM) to study the pseudogap electronic structure in the underdoped cuprate superconductor Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta }$. In TGS a pair of 50 fs pump pulses at 800 nm coincident on the sample surface generate a sinusoidal variation in the index of refraction. This index grating is phase sensitively probed, allowing us to clearly resolve two components in the optical response below Tc. We attribute one of the components to a coherent nonlinear optical process, whose properties are sensitive to the point group symmetry of the pseudogap electronic structure. We compare the results of these optical experiments with recent analysis of SI-STM data (M. J. Lawler \textit{et al} \textbf{Nature 466}$, $347 (2010)) which measures the amplitude of peaks at various reciprocal lattice vectors in the Fourier transform of atomically resolved images of the pseudogap electronic structure. The symmetry properties of the SI-STM Bragg amplitudes provide additional evidence relevant to the point group of the pseudogap electronic structure. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X25.00006: Evidence for symmetry breaking in the pseudogap phase of the single-layer Cuprate Pb-Bi2201 J.D. Koralek, J. Hinton, J. Orenstein, R.-H. He, M. Hashimoto, Z.-X. Shen, H. Karapetyan, A. Kapitulnik, H. Eisaki We use time-resolved optical spectroscopy, combined with angle resolved photoemission, and polar Kerr effect measurements, to study the single-layer Cuprate superconductor Pb$_{0.55}$Bi$_{1.5}$Sr$_{1.6}$La$_{0.4}$CuO$_{6+\delta }$ (Pb-Bi2201). Near optimal doping this material has convenient temperature scales with a $T_{c}$ of 38K and $T^{\ast }$ of 130K, allowing signals associated with the superconducting and pseudogap phases to be clearly separated in the raw data. The unusual time dependence of the pseudogap signal is suggestive of a coherent nonlinear optical process which is sensitive to changes in the electronic point group symmetry. This nonlinear signal turns on at $T^{\ast }$ and persists to low temperature. Angle resolved photoemission and polar Kerr effect measurements performed on the same batch of samples reveal the opening of a particle-hole asymmetric gap and the onset of Kerr rotation, both with strikingly similar temperature dependence to the nonlinear optical signal. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X25.00007: BCS-BEC Crossover Approach to the Optical Conductivity in high $T_c$ Superconductors Dan Wulin, Hao Guo, Chih-Chun Chien, Kathryn Levin We address the finite frequency $\omega$ conductivity in the cuprates. We presume that the pseudogap arises from stronger-than-BCS attraction, which leads to non-condensed pairs above and below $T_c$. Our theoretical formalism, which is consistent with gauge invariance and the transverse f-sum rule, yields a mid infrared peak associated with the energy needed to break pairs. It also leads to a situation in which very high $\omega$ spectral weight participates in the formation of the condensate. These observations, along with others reported here are consistent with experiment. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X25.00008: Kerr effect measurements in the pseudo-gap regime of LBCO and Pb-BSCO using high resolution Sagnac Hovnatan Karapetyan, Vikram Nathan, Ruihua He, Makoto Hashimoto, Zhi-Xun Shen, Aharon Kapitulnik, Hiroshi Eisaki, Jake Koralek, Jamie Hinton, Joe Orenstein, John Tranquada, Genda Gu, Markus Huecker Polar Kerr effect in several high-Tc superconductors systems was measured at zero magnetic field with high precision using a cryogenic Sagnac fiber interferometer with zero-area. We observed non-zero Kerr rotations of order $\sim 1 \mu$rad appearing near the pseudogap temperature $T^*$, and marking what appears to be a true phase transition. In this talk we will review our work on $\mathrm{YBa_2Cu_3O}_{6+x}$, $\mathrm{La_{1.875}Ba_{0.125}CuO_4}$ and $\mathrm{Pb_{0.55}Bi_{1.5} Sr_{1.6}La_{0.4}CuO_{6+\delta}}$. In particular, in Pb-BSCO we observe an emergence of Kerr signal that coincides with ARPES data showing an abrupt change at $T^*$ from a relatively simple one- band metal into a state with profoundly-altered electronic structure. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X25.00009: DC Transport in Pseudogapped Superconductors: The Role of the Fermi Arcs Hao Guo, Benjamin M. Fregoso, Dan Wulin, Chih-Chun Chien, Kathryn Levin We examine the dc conductivity $\sigma$ in a d-wave pseudogapped high $T_c$ superconductor for a range of different hole doping concentrations and temperatures $T$. Our approach is based on treating the cuprates as mid-way between BCS and Bose Einstein condensation and our correlation functions are demonstrably consistent with gauge invariance and the transverse f-sum rule. Studies of the $\omega \rightarrow 0$ dc conductivity below $T_c$ lead to a peak structure (observed experimentally) while above $T_c$ we show that pseudogap effects manifest themselves in the resistivity primarily through a depression in the effective carrier number with decreasing $T$. We discuss related implications for resistivity vs $T > T_c$ experiments and demonstrate that the trends with hole doping are compatible with the data, while the role of the Fermi arcs appears overall to be secondary. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X25.00010: Ultrafast transient grating and pump probe measurements in optimally doped La$_{2-x}$Sr$_{x}$CuO$_{4}$ thin films Darius Torchinsky, Fahad Mahmood, David Hsieh, James McIver, A. Bollinger, I. Bozovic, Nuh Gedik We have performed pump probe and transient grating measurements on high-T$_c$ thin films of optimally doped La$_{2-x}$Sr$_{x}$CuO$_{4}$. In these experiments, a pair of femtosecond pulses are interfered on the sample generating a sinusoidal intensity modulation that in turn induces a density grating of photoexcitations. The resulting change in reflectivity allows time-resolved optical measurement of the separate effects of recombination and diffusion. We describe the temperature and excitation density dependence of these measurements and discuss their implications on the nature of superconductivity in the cuprates. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X25.00011: Ultrafast quasiparticle dynamics of La$_{2-x}$Sr$_x$CuO$_4$ probed by time-resolved THz spectroscopy Alex Frenzel, Daniel Pilon, Anthony Bollinger, Ivan Bozovic, Nuh Gedik We have studied picosecond quasiparticle recombination dynamics in the superconducting state of the cuprate superconductor La$_{2-x}$Sr$_x$CuO$_4$. After excitation by a 1.5 eV optical pulse, the optical conductivity in the range 0.5 - 2 THz is measured at varying time delays using coherent time-domain terahertz spectroscopy. We show that the conventional two-fluid model, which successfully describes the optical conductivity in YBCO, is unable to accurately reproduce our results. At optimal doping, we observe a weak dependence on excitation density in the recovery rate at low fluence. We comment on the recovery rate of the superconducting state in terms of the bimolecular recombination dynamics described by the phenomenological Rothwarf-Taylor model. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X25.00012: Optical conductivity in dynamic Hubbard model Giang Bach, Jorge Hirsch, Frank Marsiglio The Dynamic Hubbard model is a candidate to capture the physics of two-band Hubbard models, such as the enhancement of critical Hubbard $U$ for the Mott transition. A pseudo-spin $1/2$ auxiliary field, which modifies the Coulomb $U$ interaction based on the on-site occupancy of electrons, breaks the electron-hole symmetry normally associated with the Hubbard model. The dependence of optical conductivity on the number of particles also reveals the effect of the pseudo-spin on the spectral weight distribution as a function of frequency. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X25.00013: Optical and DC conductivity in the two-dimensional $t-t'-t''$ Hubbard model near the antiferromagnetic quantum critical point Dominic Bergeron, Andr\'e-Marie S. Tremblay We calculate the conductivity of the two-dimensional Hubbard model with second and third nearest neighbor hoppings $t'$ and $t''$ for dopings in the vicinity of the antiferromagnetic quantum critical point (QCP) using the two-particle self-consistent approach. This approach is non-perturbative and was benchmarked against quantum Monte Carlo calculations from weak to intermediate coupling. We include vertex corrections that are the analogs of the Maki-Thompson and the Aslamazov-Larkin terms in the theory of paraconductivity, but for antiferromagnetic fluctuations. With these corrections the f-sum rule is satisfied and important effects in DC and optical conductivity are obtained. In the pseudogap regime induced by antiferromagnetic correlations, the resistivity increases with vertex corrections. This effect is stronger on the hole-doped side where the system changes from metallic to insulating. This is opposite to what is observed when $t'=t''=0$. On the non-magnetic side of the QCP, the resistivity decreases with vertex corrections. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X25.00014: Exact nonequilibrium model for time-resolved photoemission spectroscopy of an electronic charge density wave insulator at zero temperature Wen Shen, James Freericks We exactly solve the nonequilibrium problem of electrons moving in a lattice potential that corresponds to a checkerboard ordered charge density wave at zero temperature. The exact solution can be found in arbitrary dimensions by calculating a series of two-by-two evolution operators with the Trotter formula. We examine how the charge density wave responds to being excited into nonequilibrium by a large electric field femtosecond pulse. We find that the order parameter is rapidly reduced (but not to zero) and then rings with an oscillation frequency given by the potential scattering energy U. The density of states shows evidence of gap closing for short times, which then reforms for long times. We discuss the implications of the solution of this model for the nonequilibrium melting of charge density waves observed in recent experiments. [Preview Abstract] |
Session X26: Focus Session: Iron Based Superconductors -- Fe(Se-Te)
Sponsoring Units: DMP DCOMPChair: Pengcheng Dai, University of Tennessee and ORNL
Room: D162/164
Thursday, March 24, 2011 2:30PM - 2:42PM |
X26.00001: Evidence for local moment magnetism in superconducting FeTe$_{0.35}$Se$_{0.65}$ Guangyong Xu, Zhijun Xu, Jinsheng Wen, Songxue Chi, Wei Ku, Genda Gu, John Tranquada We investigate the temperature evolution (from 5~K to 300~K) of low energy spin fluctuations in Fe-based superconductor FeTe$_{0.35}$Se$_{0.65}$ (T$_c \sim 14$~K) via inelastic neutron scattering. The magnetic excitation spectrum in the superconducting phase appears qualitatively similar to those observed in other Fe-based superconductors, with a spin gap (at about 5~meV) and a resonance peak at $\hbar\omega\sim6.5$~meV. At higher temperatures, the spectral weight of the low-temperature resonance is found to redistribute to lower energies below the spin gap. A significant moment ($\agt 0.26 \mu_B/$Fe) is found for the integrated spectral weight below merely $\hbar\omega\sim12$~meV, with nearly no temperature dependence up to 300K, indicating existence of strong local moments. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X26.00002: Low-energy magnetic excitations in the parent superconducting phases Fe$_{1+x}$Te for $x= 0.07-0.18$ Chris Stock, Efrain Rodriguez, Mark Green We present inelastic neutron scattering measurements of the phases Fe$_{1+x}$Te for varying amounts of interstitial iron in the lattice. The $x$ in Fe$_{1+x}$Te corresponds to interstitial iron located between the two-dimensional FeTe sheets, and the amount of $x$ greatly affects the nature of the crystallographic transition and the magnetic ordering at lower temperatures in this system. The low energy spectrum of the magnetic excitations from 0.5 meV to 10 meV using the Multi-Axis Crystal Spectrometer (MACS) will be presented for both the incommensurate and commensurate phases. Neutron polarized diffraction experiments that detail the nature of the magnetic ordering will also be presented. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X26.00003: Neutron Scattering Study of the Dependence of Magnetic Correlations on Se and Fe Content in the Fe(Te,Se) System Zhijun Xu, Jinsheng Wen, Guangyong Xu, Genda Gu, John Tranquada We have performed a series of neutron scattering and magnetization measurements on Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ with different Fe and Se compositions to study the interplay between magnetism and superconductivity.[1] FeTeSe is rather unique for possessing two different types of spin configurations: one is a ``bicollinear'' or ``E-type'' structure that corresponds to the static order near (0.5,0), and the other is a ``collinear'' or ``C-type'' spin configuration that gives rise to spin excitations near (0.5,0.5). [2] Short-range static magnetic order near the (0$.$5$, $0) in-plane wave-vector (using the two-Fe unit cell) is found in all non-superconducting samples. The static order disappears and bulk superconductivity emerges, as the spectral weight of the magnetic excitations shift to the region of reciprocal space near the in-plane wave-vector (0$.$5$, $0$.$5) with Se doping. Besides Se doping, Fe also plays an essential role in superconductivity and the magnetic correlations. Our results suggest that spin fluctuations associated with the collinear magnetic structure appear to be universal in all Fe-based superconductors, and there is a strong correlation between superconductivity and the character of the magnetic order/fluctuations in this system. [1] Zhijun Xu \textit{et al}., Phys. Rev. \textbf{B} 82, 104525 (2010) [2] Wei-Guo Yin \textit{et al}., Phys. Rev. Lett. 105, 107004 (2010) [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:42PM |
X26.00004: Spin Excitations in Fe(Se,Te) Invited Speaker: The full spectrum of magnetic excitations in both superconducting FeTe$_{0.51}$Se$_{0.49}$ (x=0.49) and non- superconducting Fe$_{1.04}$Te$_{0.73}$Se$_{0.27}$ (x=0.27) was studied using inelastic neutron scattering on single crystal samples. The magnetic excitations are two-dimensional in nature and are observed for energy transfers as high as 300 meV. The zero energy extrapolation of the measured dispersion shows incommensurate excitations emanating from a wavevector near (0.5,0.5), the location of the resonance in the superconducting material. For low energy transfers, the spectrum consists of a set of incommensurate spots, four-fold symmetric about the (1,0) (square lattice ($\pi$,$\pi$)) wavevector. At higher energies, these spots evolve into rings centered on Q=(1,0). These excitations are notably different than the cones of scattering expected from a long-range magnetically ordered material and likely reflect the itinerant nature of the magnetism. The qualitative evolution of the incommensurate excitation spectrum is similar that seen previously in the cuprates. Despite the incommensurate nature of the spectrum, the observed resonance in the x=0.49 sample remains peaked at the (0.5,0.5) wavevector as in other Fe-based superconductors. At low energies, the x=0.27 sample exhibits an additional feature in the excitation spectrum centered near Q=(0.5,0), the wavevector of magnetic order in Fe$_{1+y}$Te. This scattering persists for all energies below about 10 meV and forms the short range order observed for this concentration. This scattering is completely absent in the x=0.49 sample which contains no excess Fe. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X26.00005: Chemical tuning of magnetism and superconductivity in Fe$_{1+x}$(Te,Se) Efrain E. Rodriguez, Chris Stock, Nicholas P. Butch, Johnpierre Paglione, Mark Green We present evidence demonstrating how the magnetism and superconductivity can be tuned for the phases Fe$_{1+x}$Te and Fe$_{1+x}$Te$_{1+y}$Se$_{1+y}$. Through the use of iodine vapor as an oxidant, we can de-intercalate these materials to remove the interstitial iron, \textit{i.e} the $x$ in Fe$_{1+x}$(Te,Se). Our analysis of the neutron inelastic scattering indicates that paramagnetism from this interstitial iron is detrimental to superconducting properties, and magnetization measurements show that superconducting volume fraction is indeed increased as the amount of interstitial iron is removed. Diffraction results detailing changes in key structural parameters and magnetic ordering will also be presented. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X26.00006: Local lattice dynamic correlation in FeSe$_x$Te$_{1-x}$ Keeseong Park, Despina Louca, Jon Taylor, Jiaqiang Yan With the use of inelastic neutron scattering, the local lattice dynamics were determined for the new Fe-based superconductors, FeSe$_x$Te$_{1-x}$ with $x$=0.1, 0.5 and 0.9. The nature of the dynamic pair correlations was characterized above and below the phase transitions. In the $x$=0.1 sample that is not superconducting(SC), the nearest Fe-Te and Fe-Fe pair correlations gradually disappear with increasing energy by 35 meV. The same energy dependence is observed above and below the magnetic transition. This energy corresponds to the cut-off frequency of the phonon vibrational modes. On the other hand, in the SC $x$=0.5 and 0.9, the Fe-Fe correlations gain weight just above the elastic, only to be quickly suppressed by 15 meV. This effect is stronger below the transition than above. The Fe-Te correlations that overlap with the Fe-Fe bonds persist in $x$=0.5 (possibly in $x$=0.9 as well but are too weak). On the other hand, the Fe-Se correlations persist beyond this energy, and eventually disappear by 30 meV. These differences in the local lattice dynamics between the non-SC and SC might provide a clue towards understanding the phonon contribution to the mechanism of superconductivity in this system. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X26.00007: Magnetic field dependence of spin fluctuations in superconducting FeSe0.4Te0.6 V. Thampy, Y. Zhao, W. Bao, Z. Mao, J. Rodriguez, D. Argyriou, A. Savici, G. Granroth, A. Hiess, C. Broholm Spin fluctuations may play a key role in metals where superconductivity appears as a magnetic phase is suppressed under pressure or with chemical substitution. The suppressed magnetism is manifested as a gap in the spin fluctuation spectrum and a spin resonance to which the spectral weight is shifted. We have studied the effect of high magnetic fields on this resonance. While fine structure is observed, these features do not shift with field and persist in zero field and in the normal state. Temperature difference spectra are however, significantly broadened in high fields. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X26.00008: Neutron study of spin fluctuations in iron chalcogenide Songxue Chi, Taner Yildirim, Jeffrey Lynn, Chenglin Zhang, Jose Rodrigues, Pengcheng Dai, Daniel Phelan, Deepak Singh, Rick Paul The incommensurate spin excitations in the nonsuperconducting FeTe$_{0.72}$Se$_{0.28}$ have been studied using both cold and thermal neutron spectroscopy. At low energies spectrum weight shifts from (1/2,0) commensurate excitations to the incommensurate quartets about the (1,0) point, which disperse outward before the inward dispersion at higher energies. The steep dispersion is disturbed in the energy range between 20 meV and 32 meV, resulting in abnormal excitations that are also observed in the superconducting FeTe$_{0.62}$Se$_{0.38}$. Polarized neutron measurements were carried out and the origin of these abnormal excitations is discussed. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X26.00009: ARPES study of FeTe single crystal and FeTeO$_{x}$ films Yuefeng Nie, Martin Mansson, Yasmine Sassa, Christof Niedermayer, Genda Gu, Joseph Budnick, Barrett Wells We have performed an ARPES investigation of FeTe single crystals, films, as well as the novel superconducting film FeTeOx. Our results from the single crystals reflect the previously reported Fermi surface pocket around the X-point [($\pi $,0)], possibly connected to a spin-density wave (SDW) order [Y. Xia, PRL 103, 037002 (2009)] . Unlike this previous report, our results also reveal the presence of an energy gap which would be expected from the SDW order. The temperature dependence shows that the gap closes in the rough vicinity of the magnetic transition temperature, supporting it's interpretation as reflecting the SDW state. We were able to produce an ARPES quality surface by cleaving films of FeTe and FeTeO$_{x}$, with the FeTe films showing similar features as the bulk. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X26.00010: ARPES Study of the Phase Diagram of Fe$_{1+y}$Te$_{1-x}$Se$_x$ Zhongkai Liu, Ming Yi, Yulin Chen, Ruihua He, Donghui Lu, Rob Moore, Jin Hu, Tijiang Liu, Zhiqiang Mao, Zhi-Xun Shen Iron chalcogenide Fe$_{1+y}$Te$_{1-x}$Se$_x$ is a unique member among the iron-based superconductor family due to its simplicity in structure and richness in physics. The end member Fe$_{1+y}$Te has an antiferromagnetic order with Neel temperature $\sim$72K. Substitution of Se for Te suppresses this long-range magnetic order and enters a ``spin-glass'' phase where ($\pi$,0) short-range magnetic order contributes to weak charge carrier localization. Superconductivity emerges by further substitution of Se and suppression of the short-range magnetic order. Here we present ARPES study on this system, providing evidence of the underlying physics in the phase diagram by analyzing electronic structure information. The comparison of iron chalcogenide and other iron-based systems help us identify the governing physics in this new family of superconductors. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X26.00011: ARPES studies on FeTe$_{1-x}$Se$_{x}$ iron chalcogenides epitaxial thin films Davide Innocenti, Luca Moreschini, Young Jun Chang, Andrew Walter, Aaron Bostwick, Daniele Di Castro, Antonello Tebano, Pier Gianni Medaglia, Emilio Bellingeri, Ilaria Pallecchi, Carlo Ferdeghini, Giuseppe Balestrino, Eli Rotenberg The physics of iron-based chalcogenides raises fundamental questions on the interplay of magnetic order and electron pairing at the origin of the superconducting state. We have performed angle-resolved photemission spectroscopy (ARPES) studies on high-quality epitaxial thin films of FeTe$_{1-x}$Se$_{x}$, grown by \textit{in situ} pulsed laser deposition (PLD) on beamline 7.0.1 at the ALS. Specifically, we are able to show the evolution of the band structure as a function of x. We discuss our experimental results in comparison to the available theoretical band calculations. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X26.00012: ARPES Studies on $FeTe_{1-x}Se_{x}$ Hongbo Yang, Zhihui Pan, Genda Gu, Peter Johnson, Michael Weinert Angle-resolved Photoelctron Spectoscopy (ARPES) is used to study the electronic structure of Fe based superconductor, $FeTe_{1-x}Se_{x}$. Detailed comparisons are made between the measured Fermi surfaces and first principles FLAPW calculations. In particular we explore the origin of a Dirac like cone at the center of the Brillouin zone. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X26.00013: ABSTRACT WITHDRAWN |
Session X27: Quantum Computing and Simulation II
Sponsoring Units: GQIChair: Alan Aspuru-Guzik, Harvard University
Room: C155
Thursday, March 24, 2011 2:30PM - 2:42PM |
X27.00001: Analysis of quantum Monte Carlo dynamics for quantum adiabatic evolution in infinite-range spin systems Jun-ichi Inoue We analytically derive deterministic equations of order parameters such as spontaneous magnetization in infinite-range quantum spin systems obeying quantum Monte Carlo dynamics. By means of the Trotter decomposition, we consider the transition probability of Glauber-type dynamics of microscopic states for the corresponding classical system. Under the static approximation, differential equations with respect to macroscopic order parameters are explicitly obtained from the master equation that describes the microscopic-law. We discuss several possible applications of our approach to disordered spin systems for statistical-mechanical informatics. Especially, we argue the ground state searching for infinite-range random spin systems via quantum adiabatic evolution. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X27.00002: Ramsey numbers and adiabatic quantum computing Frank Gaitan, Lane Clark The graph-theoretic Ramsey numbers are notoriously difficult to calculate. In fact, only nine Ramsey numbers are currently known, with knowledge of other Ramsey numbers limited to bounds on their possible value. We describe a quantum algorithm for the computation of Ramsey numbers. We show how the problem of computing a Ramsey number can be mapped to a combinatorial optimization problem whose solution can be found using adiabatic quantum evolution (AQE). We numerically simulate this adiabatic quantum algorithm and show that it correctly determines the Ramsey numbers R(3,3) and R(2,s) for s $\leq$ 6. We also discuss experimental implementation of an adiabatic quantum computation of R(3,3). [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X27.00003: Two-particle quantum walks applied to the graph isomorphism problem John King Gamble, Mark Friesen, Dong Zhou, Robert Joynt, S.N. Coppersmith We show that an algorithm based on the dynamics of interacting quantum particles is more powerful than the corresponding algorithm for non-interacting particles. Specifically, our algorithm attempts to determine whether two graphs are isomorphic. We focus on strongly regular graphs (SRGs), a class of graphs with particularly high symmetry. By studying the dynamical evolution of two-particle quantum walks on pairs of non-isomorphic SRG's, we show that interacting particles can distinguish non-isomorphic graphs that noninteracting particles cannot. First, we prove that quantum walks of two noninteracting particles cannot distinguish pairs of non-isomorphic SRG's. Next, we demonstrate numerically that two interacting bosons are more powerful, in that their quantum walks distinguish all non-isomorphic pairs of SRGs we tried, including those with up to 64 vertices. Finally, we find a set of operators that determine these evolutions. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X27.00004: Quantum Walks on Trees with Disorder Steven Jackson, Teng Jian Khoo, Frederick Strauch Quantum walks on trees have the potential for exponential speedup compared to classical algorithms. It has been argued that disorder may limit this potential, due to Anderson localization. We report on an extensive numerical analysis of quantum walks with disorder and find evidence of a localization transition for large disorder, but a quantum-to-classical transition for intermediate disorder. These results suggest that quantum walks may yet retain their speedup for high-dimensional graphs with weak disorder. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X27.00005: Numerical investigations of quantum walks with hard-core bosons and the graph isomorphism problem Mark Wellons, John Gamble, Eric Bach, Mark Friesen, Robert Joynt, Kenneth Rudinger, Dong Zhou, Susan Coppersmith Gamble et al. investigated quantum walks of two hard-core bosons on a class of highly symmetric graphs called strongly regular graphs (SRGs) and showed that these walks will distinguish nonisomorphic graphs from the same family. However, J. Smith (arXiv:1004.0206) has shown that pairs of nonisomorphic graphs exist that cannot be distinguished by such quantum walks. Here we construct explicit counterexample graph pairs for 2 and 3-particle interacting and non-interacting walks. We also describe an algorithm that, given $k$ particles, generates two graphs indistinguishable by a $k$-boson quantum walk. We find that these indistinguishable graph pairs generated by our algorithm scale in size quadratically with the number of particles. It follows that distinguishing graphs via simulating quantum walks with classical computers will likely require exponential time in the size of the graph, while leaving open the possibility that a quantum computer could distinguish the graphs in polynomial time. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X27.00006: Simulating Quantum Dynamics On A Quantum Computer Nathan Wiebe, Dominic Berry, Peter Hoyer, Barry Sanders We develop an efficient quantum algorithm for simulating time-dependent Hamiltonian evolution of general input states on a quantum computer. Given conditions on the smoothness of the Hamiltonian, the complexity of the algorithm is close to linear in the evolution time, and therefore is comparable to algorithms for time-independent Hamiltonians. In addition, we show how the complexity can be reduced by optimizing the time steps. The complexity of the algorithm is quantified by calls to an oracle, which yields information about the Hamiltonian, and accounts for all computational resources. In contrast to previous work, which allowed an oracle query to yield an arbitrary number of bits or qubits, we assign a cost for each bit or qubit accessed. This per-bit or per-qubit costing of oracle calls reveals hitherto unnoticed simulation costs. We also account for discretization errors in the time and the representation of the Hamiltonian. We generalize the requirement of sparse Hamiltonians to being a sum of sparse Hamiltonians in various bases for which the transformation to a sparse Hamiltonian may be performed efficiently. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X27.00007: Preparing Ground States of Many-Body Systems by Simulated Cooling Dvir Kafri, Jacob Taylor Computational problems, such as satisfiability, can be rephrased in terms of the preparation of the ground state of a many-body Hamiltonian. More generally, a quantum simulator could provide information on many-body systems if the ground state can be appropriately prepared. Adiabatic preparation is a common technique for obtaining the ground state of a quantum mechanical system, by slowly varying the system Hamiltonian. A principle disadvantage is that its timing scales with the gap energy of the intermediate Hamiltonian, for which a gap may not be promised, rather than the final Hamiltonian which may be known to be gapped. We present an alternative approach, in which an arbitrary system of qubits is cooled to an effective many-body ground state, through the algorithmic interaction with a small number of ``bath'' qubits. We specify bounds for the parameters of the algorithm, show that cooling time scales with the system's gap, and present simulated results on a frustrated few-spin system. We further discuss possible experimental applications. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X27.00008: An Open-System Quantum Simulator with Trapped Ions Julio T. Barreiro, Markus Mueller, Philipp Schindler, Daniel Nigg, Thomas Monz, Michael Chwalla, Markus Hennrich, Christian F. Roos, Peter Zoller, Rainer Blatt The control of quantum systems is of fundamental scientific interest and promises powerful applications and technologies. Impressive progress has been achieved in isolating the systems from the environment and coherently controlling their dynamics, as demonstrated by the creation and manipulation of entanglement in various physical systems. However, for open quantum systems, engineering the dynamics of many particles by a controlled coupling to an environment remains largely unexplored. Here we report the first realization of a toolbox for simulating an open quantum system with up to five qubits. Using a quantum computing architecture with trapped ions, we combine multi-qubit gates with optical pumping to implement coherent operations and dissipative processes. We illustrate this engineering by the dissipative preparation of entangled states, the simulation of coherent many-body spin interactions and the quantum non-demolition measurement of multi-qubit observables. By adding controlled dissipation to coherent operations, this work offers novel prospects for open-system quantum simulation and computation. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X27.00009: Discrete quantum walk on a binary tree Zlatko Dimcovic, Ian Milligan, Dan Rockwell, Robert M. Burton, Thinh Nguyen, Yevgeniy Kovchegov We have recently constructed a framework for quantum walks, based on classical walks with memory. This framework reproduces known walks, while it can be used to build walks in systems that are difficult for current approaches. As our first example of its utility, we study a symmetric discrete quantum walk on the infinite binary tree. For a walk starting from a pure state at a given level in the tree, we compute the amplitude at the root, as a function of time and starting level. The result is strikingly different from the classical case, as its amplitude spans an order of magnitude, with a power law tail, while the classical one decays exponentially. (For example, for a delayed walk this property yields a polynomial vs. exponential speed up over the classical walk, in delay time.) The breadth of the probability peak indicates that any restriction of the extent of the tree, such as a matching tree, sinks or boundaries, would likely yield algorithms superior to classical. The calculation utilizes a variety of analytical techniques (memoried stochastic processes, combinatorics and path counting, transforms, steepest descent, orthogonal polynomials). This study also brings up interesting general questions about quantum processes on such structures. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X27.00010: Quantum Random Walks of Non-Interacting Bosons on Strongly Regular Graphs Kenneth Rudinger, John King Gamble, Mark Wellons, Mark Friesen, Dong Zhou, Eric Bach, Robert Joynt, S.N. Coppersmith We investigate the quantum dynamics of particles on graphs (``quantum walks"), with the aim of developing quantum algorithms for determining if two graphs are isomorphic and show that there are fundamental differences between the distinguishing power of two-particle and three-particle non-interacting quantum walks. We investigate quantum walks on strongly regular graphs (SRGs), a class of graphs with high symmetry. We show analytically that the two-particle walk always fails to distinguish non-isomorphic members of the same SRG family. We show numerically that the three-boson walk is able to distinguish 99.6\% of 70,712 SRG comparisons made and that this distinguishing power comes from different multiplicities of certain graph substructures in non-isomorphic graphs. We identify certain distinguishing substructures and examine ones that appear in the four-boson walk, discovering they are able to distinguish almost all of the graphs that the three-boson walk failed on. This indicates a positive correlation between number of bosons in the walk and distinguishing power. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X27.00011: Real-time Simulations of Quantum Spin $\frac{1}{2}$ Particles Coupled to Multiple Spin Baths Marta L. Guerra, M.A. Novotny, Hans De Raedt We present simulations in real time for one and two spin $\frac {1}{2}$ particles coupled to one or more baths of $\frac{1}{2} $-integer quantum spins. The simulations were performed using the algorithm and code of Prof. De Raedt [1,2]. We first simulated one spin coupled to one or two spin-baths with no interactions between the bath spins, as has been calculated theoretically [3]. We find in agreement with [3], that the quantum purity ${\cal P}(t)$ decays in both cases, exponentially for a single bath and algebraically for two baths. We extend these simulations by introducing random interactions between the bath spins in an attempt to reach the asymptotic decay rate at earlier times and for fewer spins in the baths. We also have performed similar studies for two spin $\frac{1}{2}$ quantum particles coupled to one, two, or more spin baths. The time- dependent quantum density matrix and ${\cal P}(t)$, as well as other quantities, are calculated in these simulations.\hfil\break [1] V.V. Dobrovitski and H.A. De Raedt, Phys. Rev. E {\bf 67} 056702 (2003).\hfil\break [2] S. Yuan, M.I. Katsnelson, and H. De Raedt, Phys. Rev. A {\bf 75} 052109 (2007).\hfil\break [3] D.D. B. Rao, H. Kohler and F. Sols, New J. Physicis {\bf 10} 115017 (2008). [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X27.00012: Effects of Decoherence in Quantum Simulations Nayeli Zuniga-Hansen, Mark S. Byrd, Yu-Chieh Chi We investigate the effects of decoherence in quantum simulations by observing the evolution of the system when the Quantum Information Processor is coupled to the environment. We simulate the noise as the interactions between the particles of the processor itself and observe the effects of varying the strength of the couplings. We perform these calculations for different quantum systems and compare the results of those that interact with the environment to the same system when it's completely isolated from it to observe the effects of the noise on the simulation and investigate ways to prevent the adverse effects of the noise. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X27.00013: Computational codes for simulating the Schr\"{o}dinger equation and the Master equation Nagendra Dhakal, Michael Leuenberger We developed new codes for simulating the Schr\"{o}dinger equation. We compared the codes with the FDTD codes and codes based on Quantum Monte Carlo method in 1, 2 and 3 dimensions. In addition, we simulated the Master equation for the purpose of studying the spatial and time evolution of the decoherence. Our main focus is to investigate the scalability of the codes and we found the Quantum Monte Carlo method is the most suitable for the simulation of the Master equation because it reduces the dimension of the problem to the dimension of Hilbert space, with the benefits of speeding up the process of calculation and at the same time reducing the memory. Our results are important for the implementation of quantum computing, quantum communication, and spintronics. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X27.00014: Experimental photonic quantum simulation of frustrated Heisenberg spins Philip Walther, Xiao-song Ma, Borivoje Dakic, William Naylor, Anton Zeilinger Quantum simulators are controllable quantum systems that can reproduce the dynamics of the system of interest, which are typically unfeasible for classical computers. The recent developments of quantum technology enable the precise control of individual quantum particles as required for studying complex quantum systems. In particular, quantum simulators capable of simulating frustrated Heisenberg spin systems provide a platform for understanding exotic matter such as high-temperature superconductors. Here we report the analog quantum simulation of arbitrary Heisenberg-type interactions among four spin-1/2 particles. This spin-1/2 tetramer is the two-dimensional archetype system whose ground state belongs to the class of valence-bond states. Depending on the interaction strength, frustration within the system emerges such that the ground state evolves from a localized to a resonating valence-bond state. This spin-1/2 tetramer is created using the polarization states of four photons. The precise single-particle addressability and a tunable measurement-induced interaction allows us to obtain fundamental insights into entanglement dynamics among individual particles by observing the frustration of entanglement, governed by quantum monogamy. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X27.00015: A realistic topological quantum computation platform using hole-doped semiconductor nanowires and s-wave superconductors Ming Gong, Li Mao, Sumanta Tewari, Chuanwei Zhang We show that two majorana fermions exist at the two ends of a hole-doped semiconductor nanowire that is in proximity contact with an s-wave superconductor. The required experimental parameters (carrier density, g-factor, spin-orbit coupling effect, magnetic field, etc.) for the observation of the Majorana fermions are within the experimentally reachable regime of InSb and InAs nanowires and the mini gap that provides the topological protection for the Majorana zero energy states is of the order of the s-wave superconducting gap. The Majorana zero energy states can be observed through the zero bias peak in the STM signal. The Josephson effects between two nanowire are studied. The proposed model provides a realistic experimental platform for observing non-Abelian statistics and performing topological quantum computation. This work is supported by DARP-MTO (FA955-10-1-0497), and DARPA-YFA (N66001-10-1-4025). [Preview Abstract] |
Session X28: Carbon Nanotubes and Related Materials: Theoretical and Computational Studies
Sponsoring Units: DCMPChair: Tahir Cagin, Texas A&M University
Room: C156
Thursday, March 24, 2011 2:30PM - 2:42PM |
X28.00001: Electron-phonon renormalization of the electronic structure of diamond Feliciano Giustino, Steven G. Louie, Marvin L. Cohen The calculation of band structures from first-principles has reached a high level of accuracy. Calculations combining density-functional theory with many-body perturbation theory often are in good agreement with measurements by photoemission, tunneling, and other spectroscopic probes. While significant efforts have been devoted to improving the description of electron-electron interactions in these calculations, the effect of lattice vibrations has largely been overlooked so far. In this work we study from first principles the electron-phonon renormalization of the band gap of diamond. The calculated temperature dependence of the gap and the broadening of the absorption edge are in excellent agreement with spectroscopic ellipsometry data. Interestingly we find a gap renormalization due to zero-point vibrations as large as 0.6 eV. We discuss the implications of our findings for the electronic structure of other carbon-based bulk materials and nanostructures. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X28.00002: Intrinsic Twisting and Electronic Properties of Carbon Nanotubes : A First-Principles Study Koichiro Kato, Takashi Koretsune, Susumu Saito We report the energetics and electronic structures of twisted single-walled CNTs in the framework of the density functional theory (DFT) with the local density approximation. As for very thin CNTs, we use conventional plane-wave DFT computational code. In order to utilize the periodic boundary condition implemented in the plane-wave DFT code, we study CNTs under several discretized twisting conditions. On the other hand, in the case of thicker nanotubes including experimentally abundant nanotube sizes, we use a real-space DFT computational code which can deal with twisted CNTs with only two atoms per ``helical'' unit cell. As a result, it is found that chiral CNTs become more stable in slightly twisted geometry. Our results suggest that chiral nanotubes would possess the intrinsic twisting. We also report the twisting-level dependence of the electronic structures. It is found that the fundamental gaps of most kinds of CNTs sensitively depend on twisting level. Importantly, the directions of the intrinsic twisting are the same as the directions of enlarging the fundamental gap except for very thin CNTs. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X28.00003: Excitonic Hierarchies in Gapped Carbon Nanotubes Robert Konik We present evidence that the strong electron-electron interactions in gapped carbon nanotubes lead to a hierarchy of excitons within a given nanotube subband. We study these hierarchies by employing a field theoretic reduction of the gapped carbon nanotube permitting electron-electron interactions to be treated exactly. We analyze this reduction by employing a Wilsonian-like numerical renormalization group. We are so able to determine the gap ratios of the one-photon excitons as a function of the effective strength of interactions. We also determine within the same subband the gaps of the two-photon excitons, the single particle gaps, as well as a subset of the dark excitons. The strong electron-electron interactions in addition lead to strongly renormalized dispersion relations where the consequences of spin-charge separation can be readily observed. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X28.00004: Biexcitonic Non-Linearities in Semiconducting Carbon Nanotubes Toros Torosyan, Igor Bondarev We obtained an analytical expression for the biexciton binding energy as a function of the inter-exciton distance and binding energy of constituent quasi-one-dimensional excitons in single-wall semiconducting carbon nanotubes. This allows one to trace biexciton energy variation and relevant non-linear absorption under external conditions whereby the exciton binding energy varies. In particular, we show the biexciton-plasmon coupling tunability by means of the quantum confined Stark effect, both for the ground-ground state and for the ground-excited state biexcitonic configurations. The non-linear absorption lineshapes calculated exhibit characteristic asymmetric Rabi splitting as the exciton energy is tuned to the nearest interband plasmon resonance. These results are useful for tunable optoelectronic device applications of optically excited semiconducting carbon nanotubes, including the strong excitation regime with optical non-linearities. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X28.00005: Spin manipulation in carbon nanotubes: All electrical spin filtering through spin-orbit interactions G. S. Diniz, A. Latg\'e, S. E. Ulloa Carbon nanotubes (CNTs) are known to exhibit interesting physical properties, such as metallic or insulating behavior for different chiral vectors. Application of external electric fields and the presence of spin-orbit interaction (SOI) result in modification of the energy level structure of CNTs and their conductance profiles. SOI couples spin and orbital degrees of freedom in these nanostructures, and we explore this effect in this work. We present calculations of the electronic transport of different single-wall CNTs in the presence of SOI. Our calculation uses a single-orbital tight-binding Hamiltonian representation and the equilibrium surface Green's function formalism [1] to calculate electronic transport. We consider the effects of both Rashba and intrinsic SOIs. Our results show possible implementations of carbon nanotubes as spin filtering devices for spatially asymmetric electric fields. We further discuss the spin polarization for different CNT size, chirality, field strength, and the spatially varying fields induced by the adsorption of DNA on their surface.\\ $\left[1\right]$ M. B. Nardelli, Phys. Rev. B \textbf {60}, 7828 (1999). [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X28.00006: Substantial reduction of thermal conductivity of defected carbon nanotubes Cem Sevik, Haldun Sevincli, Justin B. Haskin, Alper Kinaci, Gianaurelio Cuniberti, Tahir Cagin The influence of the structural details and defects on the thermal transport properties of carbon nanotubes (CNTs) are explored by molecular dynamics and real-space Kubo methodologies. A variety of randomly oriented and distributed defects, (mono- and di-vacancies, Stone Wales defects) on lattice thermal conductivity and anharmonic phonon mean free paths are studied for model systems in sizes up to 1000 nm. Substantial reduction in thermal conductivity, up to $\sim $80{\%} reduction compared to the pristine CNTs, is observed for $\sim $0.5$\backslash ${\%} defect concentrations. Additionally, nearly the same saturation value of lattice thermal conductivity for CNTs with different type of defects is predicted. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X28.00007: All About Chlorinated Carbon Nanotubes Dogan Erbahar, Savas Berber The halogens are viable alternatives to harsher chemicals in the post-process of purification of carbon nanotube production. However the chlorine is known to bind less agresively to carbon nanotubes than fluorine and hydrogen. Therefore, in principle the residual Cl left after the halogen gas treatment of the nanotubes can be removed without damaging the nanotube walls easier. We report ab initio density functional calculation results about pure and defective carbon nanotubes of various diameters interacting with single and multiple chlorine atoms. We first focus on pure nanotubes and investigate the adsorption of additional Cl atoms near the first adsorbtion site, investigate the clustering tendency and most favourable configurations. We report the energetics results as well as the alteration of electronic properties. We then focus on monovacancy and divacancy defects on carbon nanotubes. It is a known fact that the defective site to be more active in this case. We apply the same procedure as in the pure nanotubes but also investigate the effect of chlorination on reconstruction process and also electronic transport properties. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X28.00008: The origins of nanotube chirality: Is the edge-catalyst in control? Evgeni Penev, Yuanyue Liu, Boris Yakobson The chance for a nanotube of a chiral angle c to emerge from the ``primordial soup'' of carbon atoms on the catalyst is determined by their relative energies. Massive computations allow one to evaluate the ``elastic'' energies of the caps, and the energies of their edges [1], G(c+C), which appears to be dominating. Importantly, the latter contains a ``chemical phase shift'' C, so that the probability of different chiralities is determined by the chemical conditions at the edge. Preference for specific chirality can be achieved by tuning chemical potential of edge terminating chemical. This offers a rational way to control the tube chiral symmetry, a tantalizing yet so far elusive goal. \\[4pt] [1] Y. Liu, A. Dobrinsky, and B.I. Yakobson, Phys. Rev. Lett. in press (Dec 10 2010 issue). [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X28.00009: Laser-Shot-Induced Chemical Reactions inside Nanotubes: a TDDFT investigation Hong Zhang, Yoshiyuki Miyamoto, Angel Rubio We present the application of the time-dependent density functional theory (TDDFT) on ultrafast laser pulse which induces dynamics in molecules encapsulated by a nanotube. A strong laser pulse polarized perpendicular to the tube axis induces a giant bond-stretch of an HCl molecule inside both C and BN nanotubes. Depending on the initial orientation of the HCl molecule, the subsequent laser-induced dynamics is different [1]. We also observed a radial motion of the nanotube and vacancies appear on the tube wall when the HCl is perpendicular to tube axis. Furthermore, the disintegration of HCl molecules took place when their molecular axis tilted to tube axis. These simulations are important to analyze light-induced nanochemistry and manipulation of nanostructures encapsulated in organic and inorganic nanotubes. The computational scheme used in present work was a combination of the molecular dynamics and real-time propagation of electron wave functions under presence of strong optical field [2,3]. The energy conservation rule was checked to monitor the numerical stability. [1] Y. Miyamoto, H. Zhang, and A, Rubio, submitted., [2] O. Sugino and Y. Miyamoto, Phys. Rev. B59, 2579 (1999). [3] A. Castro, E. Rasanen, A. Rubio and E. K. U. Gross, Eur. Phys. Lett. 87, 53001 (2009). [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X28.00010: Ab initio study of the dependence of the reactivity upon carbon nanotube diameter Jonathan Laflamme Janssen, Jason Beaudin, Michel C\^ot\'e, Nicholas D.M. Hine, Peter D. Haynes One of the main research efforts of the recent years has been the development of an efficient way to select desired carbon nanotubes according to their size and their electronic properties. This selectivity would allow easier fabrication of field effect transistor and light-emitting diode devices with appropriate nanotubes. An appealing approach to assess this problem is to use the dependence of chemical functionalization thermodynamics on the material's properties. In this talk, ab initio studies of carbon nanotubes functionalized with bromophenyl will be presented. The radius dependence of the binding and activation energies of this functionalization will be reported. The purpose of this presentation is also to demonstrate the performance of linear-scaling density-functional theory code ONETEP, which provides the possibility carrying out large system simulations (up to several tens of thousands of atoms). Furthermore, the diameter dependence of the oxidation of carbon nanotubes by carbon dioxide will be presented. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X28.00011: Electric field response on hybrid C/BN nanostructures Miguel Alonso-Pruneda Synthesis of hybrid C/BN nanotubes [1] and nanosheets [2] offer a unique route for material engineering, by combination of the exciting properties of graphene with those of insulating polar BN. First principles (DFT) calculations of the zigzag-terminated edges between C and BN nanodomains will be presented, proving that unconventional physical effects similar to those observed at insulating oxide interfaces [3], can also exist in lower dimensions, opening alternative routes for tuning electronic properties at nanointerfaces. In particular, it will be shown that the magnetic character of the edge states in zigzag shaped graphene nanoribbons, and the polar BN edge, team up to give a spin asymmetric screening that induces half-semimetallicity at the interface [4]. This property is also observed in tubular geometries, where potential magnetoelectric effects will be discussed. \\[4pt] [1] Suenaga et. al. {\it Science} {\bf 278} 5338 (1997); Enouz et. al. {\it Nano Lett.} {\bf 7}, 1856 (2007).\\[0pt] [2] Ci et. al. {\it Nat. Materials} {\bf 9}, 430 (2010).\\[0pt] [3] Ohtomo \& Hwang {\it Nature} {\bf 427}, 423 (2004); Brinkman et. al. {\it Nat. Mater.} {\bf 6}, 493 (2007); Reyren et. al. {\it Science} {\bf 317}, 5842 (2007).\\[0pt] [4] Pruneda {\it Phys. Rev. B} {\bf 81}, 161409(R) (2010). [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X28.00012: Study of Singlet-Triplet Gaps in $\pi$-Conjugated Polymers versus Graphene Nanoribbons and Single-Walled Carbon Nanotubes. The Effect of Dimensionality Karan Aryanpour, Sumit Mazumdar, Hongbo Zhao We compute and compare the gap between the optical singlet and lowest triplet excitons in poly (para-phenylenevinylene) (PPV) with semiconducting graphene nanoribbons (GNRs) and single-walled carbon nanotubes (SWCNTs) within Coulomb correlated model Hamiltonian. The singlet-triplet gaps in semiconducting GNRs and SWCNTs are more than one order of magnitude smaller that in PPV. We ascribe this to two-dimensionality. Spatial distribution of the electron-hole separation in excitons reveals significant localization of the triplet state wave function compared to singlet state in PPV. In GNRs and SWCNTs however, singlet and triplet wave functions exhibit comparably extended spatial distributions. Singlet-triplet gap size is an indicator of the effective Coulomb interaction strength which in turn controls the exciton binding energies of these systems. Exciton binding energy plays a deciding role in light emission and device performance in photovoltaics. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X28.00013: Carbon-Based Zero-Bandgap Tunnel Transistors Youngki Yoon, Sayeef Salahuddin Tunnel field-effect transistors (TFET) have been proposed as a means of breaking the classical limit of voltage requirement and energy dissipation in electronic devices. However, a tunnel barrier severely reduces the current and hence the speed at which the transistor can be operated. In this work, by performing an atomistic quantum simulation, we propose a novel transistor involving a unique interface between a graphene nanoribbon (GNR) and a metallic carbon nanotube (CNT), such that (i) at low voltages it acts exactly like a tunnel transistor reducing voltage requirement below the classical limit and (ii) at a larger voltage the tunnel barrier is dramatically diminished, resulting in a large flow of current [Appl. Phys. Lett. \textbf{97}, 033102 (2010)]. Indeed, experimental fabrication of such an interface could be possible using recently demonstrated methods where carbon nanotubes are unzipped to open up narrow graphene ribbons. Our results show that orders of magnitude improvement in ON current can be obtained in this structure. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X28.00014: Josephson current in carbon nanotube quantum dots Ramon Aguado, Jong Soo Lim, Rosa Lopez, Mahn-Soo Choi We study theoretically the Josephson current through a carbon nanotube quantum dot coupled to superconducting leads. Due to the interplay between the curvature-induced spin-orbit effect and external magnetic fields, we find a rich $0-\pi$ phase diagram in various transport regimes ranging from noninteracting to Coulomb Blockade, cotunneling and the Kondo limit. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X28.00015: Kinetics of Gas Adsorption in Nanopores: A Computer Simulation Study Christopher E. Pueblo, M. Mercedes Calbi Motivated by a variety of experimental results concerning gas adsorption in open-ended carbon nanotubes, we present a series of results for the kinetics of adsorption of a gas inside a nanopore. The study is based on a Kinetic Monte Carlo simulation in combination with a lattice model of adsorption. This allows us to monitor the change in coverage with time and extract corresponding adsorption rates or equilibration times. Adsorption in nanopores presents several distinctive features when compared to open surfaces. The adsorption process is mainly controlled by the energy states close to the ends of the pore; we analyze the consequences of this effect on the equilibration times of the system and also on temperature programmed desorption spectra. [Preview Abstract] |
Session X29: Focus Session: Quantum Information for Quantum Foundations - Information Measures, Entanglement, and Entropies
Sponsoring Units: GQIChair: Mark Byrd, Southern Illinois University Carbondale
Room: C148
Thursday, March 24, 2011 2:30PM - 2:42PM |
X29.00001: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X29.00002: Uncertainty Relation for Smooth Entropies Marco Tomamichel, Renato Renner Uncertainty relations give upper bounds on the accuracy by which the outcomes of two incompatible measurements can be predicted. While the established uncertainty relations apply to cases where the predictions are based on purely classical data (e.g., a description of the system's state before the measurement), an extended relation which remains valid in the presence of quantum information has been proposed recently [Berta et al., Nature Physics 6, 659 (2010)]. Here we generalize this uncertainty relation to one formulated in terms of smooth entropies. Since these entropy measures are related to operational quantities, our uncertainty relation has various applications. As an example, we show that it directly implies security of quantum key distribution protocols. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X29.00003: Inadequacy of von Neumann entropy for characterising extractable work Oscar Dahlsten, Renato Renner, Elisabeth Rieper, Vlatko Vedral The lack of knowledge an observer has about a system limits the amount of work it can extract. This lack of knowledge is normally quantified using the Shannon/von Neumann entropy. We show that this standard approach is, surprisingly, only correct in very specific circumstances. In general one should use the recently developed smooth entropy approach. For many common physical situations, including large but internally correlated systems, the resulting values for the extractable work can deviate arbitrarily from those suggested by the standard approach. (For details see arXiv:0908.0424) [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X29.00004: Interpreting quantum discord through quantum state merging Vaibhav Madhok, Animesh Datta We present an operational interpretation of quantum discord based on the quantum state merging protocol. Quantum discord is the markup in the cost of quantum communication in the process of quantum state merging, if one discards relevant prior information. Our interpretation has an intuitive explanation based on the strong subadditivity of von Neumann entropy. We use our result to provide operational interpretations of other quantities like the local purity and quantum deficit. Finally, we discuss in brief some instances where our interpretation is valid in the single copy scenario. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X29.00005: The thermodynamic meaning of negative entropy Lidia del Rio, Renato Renner, Johan Aaberg, Oscar Dahlsten, Vlatko Vedral Landauer's erasure principle states that all irreversible operations, like the erasure of data stored in a system, have an inherent work cost. This work cost depends on our knowledge of the system: the less we know about its state, the more it costs to erase it. Here, we analyse erasure in a general setting, where our information about a system can be quantum mechanical. We show that the work cost of erasure is bounded by the entropy of the system conditioned on that quantum information. Our result implies that conditional entropies, originally introduced in the context of information theory, have a direct thermodynamic significance. Since these entropies can become negative, a particular consequence is that an observer who is strongly correlated to a system may gain work while erasing it. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X29.00006: Operational interpretations of quantum discord Marco Piani, Daniel Cavalcanti, Leandro Aolita, Sergio Boixo, Kavan Modi, Andreas Winter Quantum discord quantifies non-classical correlations going beyond the standard classification of quantum states into entangled and unentangled ones. Although it has received considerable attention, it still lacks any precise interpretation in terms of some protocol in which quantum features are relevant. Here we give quantum discord its first information-theoretic operational meaning in terms of entanglement consumption in an \emph{extended quantum state merging} protocol. We further relate the asymmetry of quantum discord with the performance imbalance in quantum state merging and dense coding. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X29.00007: Measures of non classical correlations Matthias Lang, Anil Shaji, Carlton Caves To quantify non classical correlations in a quantum state, much effort has been put into the investigation of entanglement and its properties. It is known, however, that entanglement does not capture all quantum correlations. Several entropic measures of non-classical correlations beyond entanglement have been proposed, quantum discord being the most popular amongst them. We have developed an entropic framework for formulating such measures. We discuss new measures that emerge from this framework, and relations among the various measures, and we present numerical results for the measures for two-qubit states. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X29.00008: Redundant imprinting of information in non-ideal environments: Quantum Darwinism via a noisy channel Michael Zwolak, Haitao Quan, Wojciech Zurek Quantum Darwinism provides an information-theoretic framework for the emergence of the classical world from the quantum substrate. It recognizes that we - the observers - acquire our information about the ``systems of interest'' indirectly from their imprints on the environment. Objectivity, a key property of the classical world, arises via the proliferation of redundant information into the environment where many observers can then intercept it and independently determine the state of the system. While causing a system to decohere, environments that remain nearly invariant under the Hamiltonian dynamics, such as very mixed states, have a diminished ability to transmit information about the system, yet can still acquire redundant information about the system [1,2]. Our results show that Quantum Darwinism is robust with respect to non-ideal initial states of the environment.\\[4pt] [1] M. Z., H. T. Q., W. H. Z., Phys. Rev. Lett. 103, 110402 (2009)\\[0pt] [2] M. Z., H. T. Q., W. H. Z., Phys. Rev. A 81, 062110 (2010) [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X29.00009: Quantum Darwinism in an Everyday Environment: Huge Redundancy in Scattered Photons Charles Riedel, Wojciech Zurek We study quantum Darwinism---the redundant recording of information about the preferred states of a decohering system by its environment---for an object illuminated by a blackbody. In the cases of point-source, small disk, and isotropic illumination, we calculate the quantum mutual information between the object and its photon environment. We demonstrate that this realistic model exhibits fast and extensive proliferation of information about the object into the environment and results in redundancies orders of magnitude larger than the exactly soluble models considered to date. We also demonstrate a reduced ability to create records as initial environmental mixedness increases, in agreement with previous studies. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X29.00010: Quantum systems as embarrassed colleagues: what do tax evasion and state tomography have in common? Chris Ferrie, Robin Blume-Kohout Quantum state estimation (a.k.a. ``tomography'') plays a key role in designing quantum information processors. As a problem, it resembles probability estimation -- e.g. for classical coins or dice -- but with some subtle and important discrepancies. We demonstrate an improved classical analogue that captures many of these differences: the ``noisy coin.'' Observations on noisy coins are unreliable -- much like soliciting sensitive information such as ones tax preparation habits. So, like a quantum system, it cannot be sampled directly. Unlike standard coins or dice, whose worst-case estimation \emph{risk} scales as $1/N$ for all states, noisy coins (and quantum states) have a worst-case risk that scales as $1/\sqrt{N}$ and is overwhelmingly dominated by nearly-pure states. The resulting optimal estimation strategies for noisy coins are surprising and counterintuitive. We demonstrate some important consequences for quantum state estimation -- in particular, that adaptive tomography can recover the $1/N$ risk scaling of classical probability estimation. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X29.00011: Quantum networks reveal quantum nonlocality Daniel Cavalcanti, Mafalda Almeida, Valerio Scarani, Antonio Acin The results of local measurements on some composite quantum systems cannot be reproduced classically. This impossibility, known as quantum nonlocality, represents a milestone in the foundations of quantum theory. Quantum nonlocality is also a valuable resource for information processing tasks, e.g. quantum communication, quantum key distribution, quantum state estimation, or randomness extraction. Still, deciding if a quantum state is nonlocal remains a challenging problem. Here we introduce a novel approach to this question: we study the nonlocal properties of quantum states when distributed and measured in networks. Using our framework, we show how any one-way entanglement distillable state leads to nonlocal correlations. Then, we prove that nonlocality is a non-additive resource, which can be activated. There exist states, local at the single-copy level, that become nonlocal when taking several copies of it. Our results imply that the nonlocality of quantum states strongly depends on the measurement context. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X29.00012: A generalization of Noether's theorem and the information-theoretic approach to the study of symmetric dynamics Iman Marvian, Robert Spekkens Information theory provides a novel approach to study of the consequences of symmetry of dynamics which goes far beyond the traditional conservation laws and Noether's theorem. The conservation laws are not applicable to the dissipative and open systems. In fact, as we will show, even in the case of closed system dynamics if the state of system is not pure the conservation laws do not capture all the consequences of symmetry. Using information theoretic approach to this problem we introduce new quantities called asymmetry monotones, that if the system is closed they are constant of motion and otherwise, if the system is open, they are always non-increasing. We also explain how different results in quantum information theory can have non-trivial consequences about the symmetric dynamics of quantum systems. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X29.00013: Closed Systems that Measure Particles Michael Steiner, Ronald Rendell The Measurement Problem has been of fundamental concern since the discovery of Schr\"{o}dinger's equation. We have been developing a framework for which this problem can be considered under the assumption that the particle and detector are jointly considered a closed system. The framework is based on imposing conditions on quantum state evolution that such a closed system meet, including conservation of energy and momentum, no-cloning and no-signaling, gauge invariance, and relativity constraints. Another requirement will be presented, which is a quantum mechanical generalization of Newton's first law. Based on these conditions, we will derive and present several new results. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X29.00014: Quantum Theory for a Total System with One Internal Measuring Apparatus Wen-ge Wang We propose a quantum theory for a total system including one internal measuring apparatus. The theory is based on three basic assumptions and a principle termed the principle of compatible description (PCD). The assumptions are: (i) Physical states of the total system can be associated with vectors in the Hilbert space. (ii) Dynamical evolution of a state vector obeys Schr\"{o} dinger equation. (iii) For a physical state of the total system described by a pure vector, in which a subsystem may play the role of an internal measuring apparatus, when certain stable condition is satisfied, the pure-vector description may be given a Born-type ensemble interpretation. The PCD states that different descriptions for the same state of the total system must give consistent predictions for results of measurements performed by the internal measuring apparatus. The proposed theory lies at a meeting point of Copenhagen, Everett's relative-state, and consistent-histories interpretations of quantum mechanics. While, it provides something new: For example, the PCD imposes a restriction to vectors that can be associated with physical states, which may effectively break the time-reversal symmetry of Schr\"{o}dinger equation. As an application of the theory, we derive a condition under which a two-level quantum system may have definite properties, such that it may play the essential role of a measuring apparatus. [Preview Abstract] |
Session X30: Nanowires: Electronic Transport, Experimental
Sponsoring Units: DCMPChair: Francois Leonard, Sandia National Laboratories
Room: C147/154
Thursday, March 24, 2011 2:30PM - 2:42PM |
X30.00001: Resistivity of Endotaxial Silicide Nanowires Measured with UHV-STM Nanoprobe Sam Tobler, Peter Bennett We have measured the resistivity of endotaxial silicide nanowires on silicon using a UHV STM nanoprobe in a 2-point configuration, which allows separation of intrinsic resistivity from contact resistance using a variable probe spacing. A fixed contact is provided by a thin metal film deposited by shadow evaporation with an edge profile 100nm wide and sheet resistance 500 ohms, while the second contact is provided by the STM tip. A controlled approach with 15 Ang displacement beyond the tunneling position allows for reliable and repeatable electrical contact without damage to the tungsten STM tip. Using this method, we have obtained resistivity values of 30 micro-ohm-cm and 120 micro-ohm-cm for CoSi2 nanowires of width 40nm and 20nm, respectively, on Si(110). The increase of resistivity with decreasing width is attributed to boundary scattering along the sidewalls of the nanowires. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X30.00002: Field Effect Transistor based on Single Crystalline InSb Nanowire Jia Lu, Yennai Wang, Karan Banerjee, Huijun Yao, Thomas Schaepers Semiconductor nanowires have attracted substantial scientific and technological interests due to their unique properties arising from the size confinement effects. Among III-V group, indium antimonide (InSb) has the smallest bandgap energy (170 meV) at room temperature and possess an extremely high bulk electron mobility. It has been widely used in infrared optoelectronics and high-speed devices, and has inspired significant interest for fundamental studies in their nanostructure form. In this work, InSb nanowires with precise stoichiometry and zincblende crystal structure are synthesized via pulsed-laser chemical vapor deposition. Raman spectroscopy shows stoke and anti-stoke peaks of transverse-optical mode with asymmetric broadening. The nanowire demonstrates $n$-type semiconductor behavior. Enhanced surface scattering due to size confinement leads to reduced electron mobility. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X30.00003: Realizing lateral wrap-gated nanowire FETs: Controlling gate length with chemistry rather than lithography Adam Micolich, Kristian Storm, Gustav Nylund, Lars Samuelson An important consideration in miniaturizing transistors is maximizing the coupling between the gate and the semiconductor channel. A semiconductor nanowire with a coaxial metal gate represents the optimum in gate-channel coupling, but has only been realized for vertically-oriented nanowire transistors. We report a method for producing laterally oriented fully wrap-gated nanowire field-effect transistors that provides exquisite control over the gate length via a single wet etch step, eliminating the need for additional lithography beyond that required to define the source/drain contacts and gate lead. Our design allows the contacts and nanowire segments extending beyond the wrap-gate to be controlled independently by biasing the doped substrate, significantly improving the sub-threshold electrical characteristics. Our devices provide stronger, more symmetric gating of the nanowire, operate at temperatures between 300 to 4 Kelvin, and offer new opportunities in applications ranging from studies of one-dimensional quantum transport through to chemical and biological sensing. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X30.00004: Transport studies of ultrathin YSi$_{2}$ nanowires Saban M. Hus, Hao Hu, Violeta Iancu, Hanno H. Weitering, An-Ping Li Extremely long YSi$_{2}$ nanowires have been fabricated via self-assembly during epitaxial growth of Y on the Si (100) 2x1 surface. The thinnest YSi$_{2}$ nanowires have a cross section of $\sim $0.4 x1.1 nm$^{2}$ and can grow up to 2 $\mu $m long. They are among the closest realizations of a one-dimensional conductor. Their electrical transport properties have been investigated with a variable-temperature four-tip scanning tunneling microscope (STM) using ex-situ fabricated contact pads. These ex-situ investigations indicated that the electrical conductivity of single nanowires is thermally activated, following an inverse Arrhenius law. In-situ contact fabrication has been accomplished via a field-induced atomic emission process from a gold STM tip. Details of the in-situ fabrication method and preliminary transport results will be presented. The research at Oak Ridge National Laboratory's Center for Nanophase Materials Sciences was sponsored by the Division of Scientific User Facilities, US Department of Energy. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X30.00005: Experimental observation of very large magnetoconductance in microbial nanowires Nikhil Malvankar, Madeline Vargas, Derek Lovley, Mark Tuominen Microbial nanowires are 2-5 nm-wide conductive proteinous pili filaments secreted by some bacteria, which can grow tens of micrometers long and may serve as a conduit for long-distance electron transport. Our previous studies demonstrated that pili of Geobacter sulfurreducens exhibit properties akin to disordered metals, and indicated a temperature-driven crossover from the regime of weak localization (WL) to strong localization (SL). Here we report a very large positive magnetoconductance (MC), up to 10,000 \%, at 300K. MC increased exponentially with magnetic field. A crossover from positive MC (WL regime) to negative MC (SL) was observed at $\sim$ 280K when the localization and the phase-breaking lengths are expected to become comparable. We attribute positive MC to destruction of the quantum interference of delocalized electron wavefunctions and negative MC to shrinkage of the localized electron wavefunctions due to applied magnetic field, which is consistent with the temperature dependence of conductivity. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X30.00006: Electron Transport in Gold Nanowires: Stable 1-, 2- and 3-Dimensional Atomic Structures and Non-Integer Conduction States Douglas Smith, Francesca Tavazza, Lyle Levine, Jon Pratt, Anne Chaka We report experimental conductivity measurements made during highly stable tensile deformation of Au nanowires showing a rich variety of behaviors, including non-integer quantum conductance plateaus, transitions and slopes. Using tight binding conductance calculations on simulated nanowires previously deformed using density functional theory calculations, we demonstrate that all of these phenomena can arise from structural transitions between highly stable ordered atomic configurations that self-organize during tensile deformation. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X30.00007: Gate-induced Fermi level tuning and ambipolar conduction control in InP nanowires Kristian Storm, Gustav Nylund, Magnus Borgstr\"om, Jesper Wallentin, Carina Fasth, Claes Thelander, Lars Samuelson Semiconducting nanowires are an interesting platform for studies of fundamental material transport properties in one dimension as well as for building blocks for various types of devices. Most conventional semiconductor devices are based upon doping for its operation, but as device dimensions are decreased, the random position of a few incorporated impurity atoms may come to dominate device characteristics. We present measurements of InP nanowires in which the Fermi level is tuned at efficiency close to the theoretical limit using semi-wrapped gates. Furthermore, we present ambipolar devices in which the Fermi level can be tuned across the entire bandgap of the semiconductor. We believe this will be of considerable importance and serve as a foundation for producing nanowire devices where the device behavior is induced by sequential gates wrapped around the nanowire channel, replacing the need for doping in certain types of devices. This way, the properties can be dynamically tuned using wrapgates, as opposed to statically set using the doping level. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X30.00008: Transport Measurements on Sb doped Silicon Nanowires Prathyusha Nukala, Marzieh Zare, Gopal Sapkota, Pradeep Gali, Usha Philipose Semiconductor nanowires (NWs) present an alternative approach for device scaling. N-type Si NWs are generally grown with silane as source with phosphine and arsenic as dopants, all of which are toxic in nature. We present a safe, cost-effective approach for synthesis of n- doped Si NWs using Sb. Structural and compositional characterization using electron microscopy and X-ray spectroscopy will be presented for crystallographic information on the quality and morphology. Ohmic contacts established to a single and on an array of doped and undoped NWs in an FET type of device configuration will provide information on several parameters such as type of majority carriers, mobility and concentration. We will highlight the promise of Sb doped Si NWs for electronic applications such as nano-scale field effect transistors and sensors. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X30.00009: Role of defect states in charge transport in semiconductor nanowires Dongkyun Ko, Xianwei Zhao, Kongara Reddy, Wolfgang Windl, Nitin Padture, Nandini Trivedi, Fengyuan Yang, Ezekiel Johnston-Halperin Charge transport characteristics are investigated in Se-doped InP nanowires in order to determine the nature of the defect states. I-V curves indicate that transport is limited by trapped space charges rather than by Schottky at high bias. In addition, mobility calculations show that hopping between defect states plays an important role at low bias. A transition between hopping mechanisms as a function of temperature can be determined from the behavior of the temperature-dependent resistance R(T). Nearest neighbor hopping (NNH) is dominant in the high temperature regime ($>$158K) , \textit{R$\sim $exp(T}$_{0}/T)^{1.03}$, and Efros-Shklovskii variable range hopping (ES-VRH) is dominant in the low temperature regime ($<$158K), \textit{R$\sim $exp(T}$_{ES}/T)^{0.49}$. Gate-bias dependence of the transition temperature and hopping parameters are also investigated: these results suggest that applying positive gate-bias changes the strength of electron correlations in these quasi-1D systems. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X30.00010: InAs Nanowire Transistors as Gas Sensor: the Role of Surface States Dong Liang, Juan Du, Hao Tang, Xuan P.A. Gao Utilizing the large surface-to-volume ratio, sensors of quasi one-dimensional semiconductor nanowires based electronic devices have been shown high sensitivity to the adsorption gaseous molecules or the binding of biomolecules in liquid, enabling a label-free sensing modality with high sensitivity and direct electrical readout. We report a study of the response of InAs nanowire field-effect transistor sensor devices to various gases and alcoholic vapors. It is concluded that the change in conductance of the device in response to chemical vapors is a combined result of both the charge transfer and modified electron mobility effects. In particular, we found that surface adsorption of most chemical molecules can reduce electron density in nanowires from $\sim $10$^{4}$ to $\sim $10$^{3}$/$\mu $m and enhance the electron mobility greatly (from tens to a few hundred of cm$^{2}$/(V s)) at the same time. These effects are attributed to the interactions between adsorbed molecules and the electron accumulation layer and rich surface states on the InAs nanowire surface. Journal reference: Nano Letters 9, 4348 (2009). [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X30.00011: Surface passivated pure Indium Oxide nanowires for gas sensors Pradeep Gali, Kiran Shrestha, Fang Lingkuo, Nigel Shepherd, Usha Philipose Indium oxide (In$_{2}$O$_{3})$ nanowires have applications in semiconductor electronics and gas sensing. We report on growth of stoichiometric In$_{2}$O$_{3}$ nanowires with diameter ranging from 40 to 80nm and lengths over 10 $\mu $m. Structural characterization done with SEM, XRD and TEM shows that the nanowires exhibits BCC structure and grow along the (100) direction. Energy Dispersive X-ray spectroscopy shows stoichiometric composition. Transport measurements on a single nanowire shows ohmic behavior and a resistance of about 100 K$\Omega $. Photoluminescence spectrum at room temperature shows strong emission peaks at 370nm and 415nm, corresponding to near band edge and defect related emission respectively. We present a technique of post-growth annealing of these nanowires to eliminate the defect induced emission and enhance band edge emission. Passivating the surface of these nanowires enhances their gas sensing abilities. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X30.00012: Synthesis and Characterization of InAs / InSb Nanowire Heterojunctions Minkyung Jung, Michael Schroer, Jason Petta InSb is a very promising material for both electronic and optoelectronic devices due to its unique features, including a very small band gap, large bulk mobility, enormous electronic g- factor and strong spin-orbit interaction. In particular, the small effective mass of InSb makes it straightforward to fabricate devices that display effects due to quantum confinement [1,2]. Here InAs/InSb nanowire heterostructures were grown by metal-organic vapor-phase epitaxy on InAs $\left (111\right)$B substrates. We investigated morphology changes of InAs/InSb nanowires with varying growth temperature and V/III ratio. The samples were characterized using scanning electron microscopy and high resolution transmission electron microscopy. In order to study the transport properties of InAs/InSb nanowires, field effect transistors were fabricated on SiO$_2$/Si substrates and characterized at room temperature and 4.2 K. \\[4pt] [1] H. A. Nilsson {\it et al.}, Nano Lett. {\bf 9}, 3151 (2009) \\[0pt] [2] P. Caroff {\it et al.}, Small {\bf4}, 878 (2008) [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X30.00013: Measurement of minority carrier diffusion length in individual silicon nanowires with an in-situ grown p-n junction A.D. Mohite, D.E. Perea, S. Singh, S.A. Dayeh, S.T. Picraux, H. Htoon We report a scanning photocurrent microscopy study across a p-n junction of individual in-situ doped Si nanowires (NWs). The measured photocurrent decreases exponentially as the laser spot is scanned away from the p-n junction in both directions. The photocurrent peak widens with increasing reverse bias, indicating the increase of depletion width. For a 40nm diameter NW, the fit of photocurrent decay to an exponential function gives minority carrier diffusion lengths of $L_{n}$=1.842 $\mu $m and $L_{p}$=1.45 $\mu $m for electrons and holes, respectively. Such relatively long minority carrier diffusion lengths are consistent with the low dopant incorporation we expected for our growth condition. This result further suggests that the diffusion length scales with doping concentration despite the impact of surface states of a 1D system. We will further discuss the dependence of the minority carrier diffusion length on diameter, doping concentrations, and back-gating. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X30.00014: Andreev tunneling enhanced by Coulomb oscillations in superconductor-semiconductor hybrid Ge/Si nanowire devices Xiaojie Hao, Tao Tu, Hai-Ou Li, Cheng Zhou, Gang Cao, Guang-Can Guo, Guo-Ping Guo, Wayne Fung, Zhongqing Ji, Wei Lu We explore the magneto-conductance of Ge/Si core/shell nanowire quantum dot devices contacted by superconducting leads. Significant magneto-conductance peaks around zero field are observed and show a periodic modulation with gate voltage as discrete states of the quantum dot are turned on- and off-resonance with the Fermi energy in the superconducting electrodes. The ability to create and control coherent transport in superconductor-semiconductor hybrid nanostructures allows for new opportunities in the study of various fundamental competing effects such as superconductivity and electron-electron interactions. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X30.00015: Minority Carrier Lifetimes and Surface Effects in VLS-Grown pn Junction Silicon Nanowires Yeonwoong Jung, Aleksandar Vacic, Daniel Perea, Tom Picraux, Mark Reed We study the minority carrier lifetimes and surface effects of pn junction Si nanowires. Axial pn junction Si nanowires with alternating p-n doped segments are grown based on the Au-catalyzed VLS process by an in-situ exchange of gas-phase dopants. As-grown nanowires display strong current rectification only after surface etching processes. By utilizing the reverse recovery transient of minority carriers, we directly characterize the minority carrier lifetimes and observe the decrease of the lifetimes with a decrease of nanowire diameters. Investigation of the diameter-dependent device ideality factor and current density strongly suggests that the surface recombination with an enhanced surface-to-volume ratio significantly governs the carrier transport. We also characterize the carrier lifetimes of nanowires with and without surface passivation layers, and observe an enhancement of the lifetimes in the surface-passivated ones. These studies elucidate the carrier transport mechanism in VLS pn junction Si nanowires and emphasize the importance of the surface passivation for efficient photovoltaic applications. [Preview Abstract] |
Session X31: Amorphous Solids, Glasses & Liquids I
Sponsoring Units: DCMPChair: Punit Boolchand, University of Cincinnati
Room: C145
Thursday, March 24, 2011 2:30PM - 2:42PM |
X31.00001: Evidence of Fatigue Damage in the Local Structure of Zr-based Bulk Metallic Glasses Despina Louca, Peng Tong, Peter Liaw, Gongyao Wang, Yoshihiko Yokoyama, Anna Llobet, Rick Spence Bulk metallic glasses (BMG) are particularly vulnerable to fatigue damage, where catastrophic failure may occur without observable macroscopic changes. The local atomic structure of two BMGs with compositions of Zr$_{50}$Cu$_{40}$Al$_{10}$ and Zr$_{60}$Cu$_{30}$Al$_{10 }$was investigated by synchrotron X-ray and neutron diffraction via the pair density function analysis. Under a load of 1600 MPa, the number of compression cycles ranged from 0 -- 10$^{7}$ at 10 Hz. At room temperature, a subtle but irreversible change is observed in the local structure due to fatigue. Upon cooling down to 10 K, however, a significant structural re-organization is observed especially in the short range that is proportional to the number of fatigue cycles. The effect becomes more pronounced with increasing the number of loading cycles. The changes are beyond the usual narrowing from reducing thermal vibrations. The results indicate that hardening occurs after fatigue. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X31.00002: Analysis of Amorphous Iron Surface Energies and Bulk Properties using DFT Charles Newnam, Michael Mehl, Daniel Finkenstadt From Ab Initio calculations, we compare the energy of amorphous Iron to bcc and fcc Iron structures, both at zero pressure and high pressure. From these calculations we draw conclusions on the properties of metallic glass structures over a range of pressure. Additionally, we address the adsorption energy of Oxygen on amorphous surfaces versus the structure's cell size and compared against bcc and fcc Iron structures. The adsorption energies allow us to evaluate the corrosion potential of an amorphous structure versus a typical crystalline surface. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X31.00003: Super-localization of atomic dynamics in liquid Iron Madhusudan Ojha, David J. Keffer, Don M. Nicholson, Takeshi Egami Lattice dynamics in crystals is well described in terms of phonons. However, phonons cannot give precise description of the atomic dynamics in liquids because they are highly damped. We carried out MD simulations of liquid iron at high temperatures. The results are presented in terms of the dynamic pair-density function (DPDF), which describes the atomic correlation, or the distribution of atomic distances over time, at the angular frequency $\omega $. Our analysis shows that the atomic dynamics in liquid iron above the boson mode (BM) is confined to only the nearest neighbors, resulting in super-localization of atomic dynamics. The dynamics of nearest neighbor shells is well described in terms of atomic level stresses. This super-localization of atomic dynamics in liquid iron implies that the dynamics of the local atomic level stresses represents the normal modes in liquids at high temperatures, and justifies the equipartition law observed for the atomic level stresses [1]. \\[4pt] [1] V. Levashov, et al. Phys. Rev. B 78,064205 (2008) [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X31.00004: Data-mining for hidden order in metallic liquids and glasses Xiaowei Fang, C.Z. Wang, Y.X. Yao, Z.J. Ding, K.M. Ho Although metallic liquids and glasses look quite homogenous macroscopically, most of them exhibit structural and chemical orders at the atomic scale. This short-range (SRO) or medium-range order (MRO) occurs on a length scale of 5-20 {\AA}. However, they are generally difficult to discern at the macroscopic scale due to random orientations of the ordered units. In this paper, we develop an efficient computational algorithm to align the neighborhood cluster around each atom to reveal the hidden symmetry and order contained in the system. In our alignment algorithm, we put the center atoms into a common origin and rigidly rotate the clusters to maximize their common registry to reveal any existing SRO or MRO. The results determine what are the major competing orders and the strengths of various orders in the system. Such atomic scale information are very difficult to acquire by experiments and are critical for understanding the mechanism of glass formation and phase selections during the rapid solidification from the metallic liquids. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X31.00005: Metallic States of Multicomponent Glasses Terrence Jach The K$\alpha _{3}$ and K$\alpha _{4}$ satellite lines in x-ray fluorescence result from two-electron shake-up transitions. The ratio of these lines in some solids is known to be highly sensitive to the valence band of the material and a well-defined indicator of metallic vs. oxide states. The good energy resolution of a microcalorimeter x-ray detector allows us to determine the ratio in the fluorescence x-ray spectrum of glasses. An investigation of the satellite ratios of Mg and Al K$\alpha $ lines in a multi-component glass used as a NIST Standard Reference Material shows that these elements appear to be in a metallic state, despite the original constituents of the glass. This result would be hard to determine by x-ray photoemission spectroscopy because of charging effects in the glass. It remains to be determined whether the effect is due merely to incomplete oxidation of the atoms or actual nanoparticle-sized metallic phases in the glass. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X31.00006: An NMR study of homogenous deformation-induced ordering in La$_{50}$Ni$_{15}$Al$_{35 }$ Magdalena Sandor, Yue Wu, Haibo Ke, Wei Hua Wang The mechanism of mechanical deformation is currently an unresolved issue of fundamental importance. $^{27}$Al NMR nutation experiments in La$_{50}$Ni$_{15}$ Al$_{35}$ bulk metallic glasses (BMG) were carried out to probe local structural changes induced by elastostatic compression at room temperature. It was observed that compression enhances local symmetry at Al sites with compression time. Modulated differential calorimetry studies were also performed to understand how free volume changes with compression time. Results provide insight into the nature of homogenous deformation and the interplay of free volume with local structural changes. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X31.00007: Understanding intrinsic ductility from Poisson's ratio for amorphous solids through force-field tuning Yunfeng Shi, Jian Luo This work is motivated by recently observed empirical relationship between the Poisson's ratio and the fracture energy for a range of metallic glasses and oxide glasses. Glassy solids with low Poisson's ratio are brittle and vice versa, with a critical Poisson's ratio of about 0.31. Here we used a force-field tuning scheme to investigate how a near-equilibrium elastic constant determines far-from-equilibrium fracture behavior. By modifying a well-studied binary Lennard-Jones system, we obtained a family of glassy systems with different Poisson's ratio ranging from 0.2 to 0.4. Interestingly, the model glasses with low Poisson's ratio exhibit brittle fracture in tension and vice versa, which agrees with experimental observations. Finally, we will discuss how ductility of amorphous solids can be comprehended in terms of the structure and bonding of the amorphous solids, both of which also dictate the Poisson's ratio. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X31.00008: Universal Sound Attenuation in Amorphous Solids Dervis Can Vural A large class of amorphous materials, including glasses, polymers, disordered crystals and in some cases quasi-crystals and proteins, show a striking degree of universality in their low temperature acoustic and thermal properties. Among the least understood is the dimensionless acoustic mean path $l/\lambda \sim 150$. Although many theories have been proposed to explain the universality of this constant, they rely on detailed phenomenological assumptions, such as the existence of tunneling two-state systems. In this talk, I present the many-body acoustic response of elastically coupled random matrices to demonstrate that the universality is a property of a general class of theories, and emerges regardless of the detailed assumptions regarding the constituents of the amorphous solid. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X31.00009: Finite-temperature critical point of a glass transition Yael Elmatad, Robert Jack, Juan Garrahan, David Chandler We generalize the simplest kinetically constrained models of a glass-forming liquid by softening kinetic constraints, allowing them to be violated with a small rate. We demonstrate that these models support a first-order dynamical (space-time) phase transition between active (fluid) and inactive (glass) phases. The first-order phase boundary in these softened model ends in a finite-temperature dynamical critical point, which may be present in natural systems. In this case, the glass phase has a very large but finite relaxation time. We discuss links between the dynamical critical point and quantum phase transitions, showing that dynamical phase transitions in $d$ dimensions map to quantum transitions in the same dimension, and hence to classical thermodynamic phase transitions in $d + 1$ dimensions. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X31.00010: Nonequilibrium relaxation and aging scaling properties of the Coulomb glass Matthew T. Shimer, Uwe C. T\"auber, Michel Pleimling Using Monte Carlo simulations, we analyze the two-time density autocorrelation function for the two- and three-dimensional Coulomb glass with various long-range interaction potentials. A full aging scaling ansatz is sufficient to describe the nonequilibrium relaxation properties of these highly correlated disordered systems. By investigating the trends of the scaling exponents, we find that they are non-universal, and depend on temperature, charge density, and interaction strength. \par \noindent Reference: EPL {\bf 91}, 67005 (2010). [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X31.00011: Vibrational excitations and elastic phases in Sodium Borate Glasses K. Vignarooban, P. Boolchand, M. Micoulaut Glass Transition temperatures (T$_g$s) and non-reversing enthalpy ($\Delta$H$_{nr}$) at T$_g$ of dry (Na$_2$O)$_x$(B$_2$O$_3$)$_{100-x}$ glasses across the 0\% $<$ x $<$ 44\% soda range are measured. Trends in $\Delta$H$_{nr}$(x) show a reversibility window in the 20\% $<$ x $<$ 40\% range, and fix the Intermediate Phase (IP). IR and Raman vibrational modes including Boson modes are also examined. At low x ($<$ 20\%), the Raman active 808 cm$^{-1}$ mode of boroxyl rings steadily lowers in scattering strength and red-shifts with increasing x, suggesting that the stressed-rigid quasi 2D network of B$_2$O$_3$ glass at x = 0, steadily softens with a characteristic optical elastic power-law (p$_1$ = 0.85(2)). In the 26\% $<$ x $<$ 40\% range, a mode near 770 cm$^{-1}$ rapidly grows in strength and red shifts with increasing x with a power- law of p$_2$= 1.05(5) characteristic of IPs observed earlier$^a$ in other 3D covalent and ionic networks. In addition, many other modes are observed, some blue-shift, some red-shift and some remain unchanged with x. These data will be discussed in relation to glass structure evolution with composition.\\[4pt] D.Novita et al. J. Phys. Condens. Matter 21, 205106 (2009) [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X31.00012: Accelerated kinetics of amorphous silicon using an on-the-fly off-lattice kinetic Monte-Carlo method Jean-Francois Joly, Fedwa El-Mellouhi, Laurent Karim Beland, Normand Mousseau The time evolution of a series of well relaxed amorphous silicon models was simulated using the kinetic Activation-RelaxationTechnique (kART), an on-the-fly off-lattice kinetic Monte Carlo method [1]. This novel algorithm uses the ART nouveau algorithm to generate activated events and links them with local topologies. It was shown to work well for crystals with few defects but this is the first time it is used to study an amorphous material. A parallel implementation allows us to increase the speed of the event generation phase. After each KMC step, new searches are initiated for each new topology encountered. Well relaxed amorphous silicon models of 1000 atoms described by a modified version of the empirical Stillinger-Weber potential [2] were used as a starting point for the simulations. Initial results show that the method is faster by orders of magnitude compared to conventional MD simulations up to temperatures of 500 K. Vacancy-type defects were also introduced in this system and their stability and lifetimes are calculated. \\[4pt] [1] El-Mellouhi et al., ,Phys Rev. B, 78, 153202 (2008)\\[0pt] [2] Vink et al., J. Non-Cryst. Sol. 282, 248 (2001) [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X31.00013: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X31.00014: Properties of fluids under strong confinement: a mode coupling approach Saroj Nandi, Sarika Bhattacharya, Sriram Ramaswamy We extend the mode coupling theory (MCT) of glass transition in bulk fluids to the case of confinement, which enhances the feedback mechanism to drive the system to a glassy state. Confinement enters the theory in terms of an external potential that produces an inhomogeneous density background, which in turn forces the fluid to relax diffusively. Below a certain density, the MCT transition becomes continuous and the critical density of continuous to discontinuous transition depends on the nature of the external potential. If the control parameters are in the proper region of phase space, the fluid shows a three-step relaxation scenario. We also incorporate shear in our theory and thereby show that the fluid, when confined, shows shear thinning at much lower shear rate compared to a bulk fluid. [Preview Abstract] |
Session X32: Focus Session: Frontiers in Computational Thermodynamics of Materials II
Sponsoring Units: FIAP DCOMPChair: Stefano Curtarolo, Duke University
Room: C144
Thursday, March 24, 2011 2:30PM - 3:06PM |
X32.00001: Low temperature phase transition predicted in the compound B13C2/B4C Invited Speaker: The experimental phase diagram of boron-carbon exhibits the compound boron-carbide over a broad composition range that extends to low temperatures, in seeming contradiction to the third law of thermodynamics. First principles total energy calculations suggest the presence of two energy-minimizing structures in the boron-carbon phase diagram, B13C2 and B4C. Both distribute boron and carbon atoms on the same 15-atom rhombohedral unit cell (hR15), consisting of 12-atom icosahedra at cell vertices plus three-atom chains at cell centers. However, only B13C2 respects the rhombohedral symmetry, while B4C breaks the symmetry by replacing one of the icosahedral boron atoms with carbon. Because B4C is incompatible with the experimentally observed rhombohedral symmetry, it must lose thermodynamic stability at elevated temperatures. We report a study of the configurational ensemble obtained by substitution of boron or carbon on different sites using a semi-grand canonical ensemble. Varying chemical potential at low temperature, we find sharp transitions from beta-rhombohedral boron to B13C2 then to B4C and finally to graphitic carbon. Only the rhombohedral-symmetry phase B13C2 survives at high temperature while the symmetry-broken phase B4C loses stability around room temperature. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X32.00002: Thermodynamic modeling of Pt-Al and Pd-Al Derek Carr Pure platinum and pure palladium are too soft for typical jewelry applications. Adding small amounts of other metals can significantly increase their performance. However, international hallmarking standards require the alloys to be 95{\%} pure by weight. How does one achieve significant improvements in performance adding only small amounts (5 wt-{\%}) of other metals? Significant improvements are possible even with small additions if precipitate hardening can be induced. Using a combination of first-principles, cluster expansion, and Monte Carlo modeling, we have identified new Pt-rich/Pd-rich phases in Pt-Al and Pd-Al that should be useful in precipitate hardening. Thermodynamical modeling indicates that the phases are experimentally feasible (not kinetically inhibited). [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X32.00003: Equation of State and Viscosity of Tantalum and Iron from First Principles Ljubomir Miljacic, Steven Demers, Axel van de Walle To understand and model at continuum level the high-energy-density dynamic response in transition metals like Tantalum and Iron, as it arises in hypervelocity impact experiments, an accurate prediction of the underlying thermodynamic and kinetic properties for a range of temperatures and pressures is of critical importance. The relevant time scale of atomic motion in a dense gas, liquid, and solid is accessible with \textit{ab-initio} Molecular Dynamics (MD) simulations. We calculate EoS for Ta and Fe via Thermodynamical Integration in 2D (V,T) phase space throughout different single and two-component phases. To reduce the \textit{ab-initio} demand in selected regions of the space, we fit available gas-liquid data to the Peng-Robinson model [1] and treat the solid phase within the Boxed-quasi-harmonic approximation [2]. In the fluid part of the 2D phase space, we calculate shear viscosity via Green-Kubo relations, as time integration of the stress autocorrelation function. \\[4pt] [1] Ind. Eng. Chem., Fundam \textbf{15}, 59 (1976) \\[0pt] [2] A. van de Walle and G. Ceder, \textit{Rev. Mod. Phys.} \textbf{74} 11 (2002) [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X32.00004: Binary Magnesium Alloys: Searching for Novel Compounds by Computational Thermodynamics Richard Taylor, Stefano Curtarolo, Gus Hart Magnesium alloys are among the lightest structural materials and are of considerable technical interest. We use the high-throughput framework AFLOW to make T = 0 K ground state predictions by scanning a large set of known candidate structures for thermodynamic minima. The study presented here encompasses 34 Mg-X systems of interest (X=Al, Au, Ca, Cd, Cu, Fe, Ge, Hg, Ir, K, La, Pb, Pd, Pt, Mo, Na, Nb, Os, Rb, Re, Rh, Ru, Sc, Si, Sn, Sr, Ta, Tc, Ti, V, W, Y, Zn, Zr). Avenues for further investigation revealed by this study include stable phases found in addition to experimental phases and compound forming systems thought to be either immiscible or non-compound forming. The existence of potentially novel ordered phases presents new opportunities for materials design. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X32.00005: High-throughput combinatorial search of novel topological insulators Kesong Yang, Wahyu Setyawan, Shidong Wang, Jeffrey Mulllen, Marco Buongiorno-Nardelli, Stefano Curtarolo In recent years, topological insulators (TIs) have attracted lots of attentions not only because of their interesting electronic characteristics induced by spin-orbit coupling but also their potential applications. So far, experimentally observed topological insulators mainly include HgTe/CdTe quantum well structure, semiconducting alloy Bi$_{1-x}$Sb$_x$, and so-called second-generation TI materials, i.e., the family of Bi$_2$Se$_3$, Bi$_2$Te$_3$, and Sb$_2$Te$_3$. Later theoretical simulations predict more TIs such as TlBiQ$_2$ and TlSbQ$_2$ (Q=Te, Se, and S), LaBiTe$_3$ as well as half-Heusler alloys, LuPtSb and ScPtBi. Numerous attempts are being made to look for more TIs. In this presentation, we will introduce our high-throughput combinatorial approach to find novel TI materials based on the AFLOW framework and distributed libraries. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X32.00006: Classification of the ICSD by crystal Prototype Junkai Xue, Wahyu Setyawan, Stefano Curtarolo An efficient way to determine the prototypes of the ICSD database has been implemented within our high-throughput formalism. This presentation illustrates how we use this tool to search for structure/properties correlations in an automatic fashion. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X32.00007: Novel Occurences of L1$_1$ and L1$_3$ found using the synnergy between High Throughput and Cluster Expansion Lance Nelson, Gus Hart, Stefano Curtarolo Despite their geometric simplicity, L1$_1$(CuPt) and L1$_3$ (CdPt$_3$) fail to appear as groundstates in experimental systems. ( L1$_1$ appears in CuPt only) Are these crystal structures actually energetically unfavorable, or have they simply been overlooked in experimental studies? Here we investigate, using computational methods, the energetic stability of these phases in all binary inter-metallic systems. We combine the results of two techniques,namely High Throughput (HT) and Cluster Expansion (CE), to maximize efficiency and ensure thoroughness. HT results show L1$_1$(L1$_3$) to be stable in the following systems: AgPd, AgPt, CuPt, PdPt(CdPt,CuPt,PdPt,LiPd,LiPt). Cluster expansions constructed for these systems verify the HT findings in all cases, with the exception of the HT groundstate PdPt-L1$_1$.(D$_4$ is found to be energetically more favorable) Monte Carlo simulations, which are used to identify order-disorder transition temperatures, were performed for all occurences of these two phases. While the transition temperatures for some systems are found to be extremely low, others appear to be physically realizable. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X32.00008: Ab Initio Insights on the Shapes of Nanocrystals Roman Chepulskyy, Stefano Curtarolo Catalytic, chemical, optical and electronic properties of nanocrystals are strongly influenced by their faceting. A variational approach based on quantum mechanical energies is introduced to evaluate stable and metastable shapes of nanocrystals. The method leads to a nanoscale equation of state, which is solved self-consistently. Using platinum as example, it is found that the surface energy dependence on the lattice parameter is the key factor controlling the equilibrium stability of the crystal shapes. The energies of different surfaces versus lattice parameter are calculated from first principles in high-throughput fashion. Considering several crystal shapes and using Wulff's construction, the transitions between stable and metastable shapes are predicted below 3 nm in diameter. Our variational approach explains experimental results and establishes a direction to search for better catalysts. [Preview Abstract] |
Session X33: Quantum Fluids and Solids I
Sponsoring Units: DCMPRoom: C143/149
Thursday, March 24, 2011 2:30PM - 2:42PM |
X33.00001: Characterization of MEMS Devices for the Study of Superfluid Helium Films Miguel Gonzalez, Byoung Hee Moon, Pradeep Bhupathi, Pan Zheng, George Ling, Erik Garcell, Ho Bun Chan, Yoonseok Lee Measurements on the mechanical attributes of MEMS resonators were performed at room and low temperatures. Specially devised resonators which can be actuated for shear motion were designed and fabricated using a state-of the-art multi-user MEMS process. The devices consist of a pair of parallel plates with a well-defined gap whose size can be controlled with high accuracy down to the sub-micron range. A full study of resonance properties at various pressures was performed at room temperature. Details of design, fabrication, and operation will be presented along with results from a preliminary study of a resonator immersed in liquid $^{4}$He. The devices show potential for use in low temperature experiments and to investigate novel phenomena in quantum fluids at the micro/nano scale such as superfluid $^{3}$He films. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X33.00002: Interplay of Aerogel Anisotropy and Textures in Superfluid $^3$He Jia Li, Johannes Pollanen, Charles Collett, William J. Gannon, William P. Halperin We have performed pulsed NMR on $^3$He-B in $98.1\%$ porosity aerogel with different anisotropy. The aerogel anisotropy was characterized with an optical, cross-polarization technique [1]. In the isotropic aerogel sample at P = 26 bar and $T<1.2mK$, we find a single peak with a positive frequency shift relative to the Lamor frequency indicating an n-texture that is predominately perpendicular to the field. Upon warming, we find a crossover at $T\approx1.2mK$ from $n\perp \mathbf{H}$ to a texture where n is predominately parallel to the field. Near the crossover the NMR intensity is distributed among two components indicating an inhomogeneous texture. We have also studied an anisotropic aerogel which was compressed along its cylinder axis by $22.5\%$. At the same pressure, we find a homogeneous texture for all $T$ and a similar textural crossover from $n\perp \mathbf{H}$ to $n \parallel \mathbf{H}$, but for this sample the textural crossover happens near $T_{caerogel}$. We have introduced a model to account for the interplay of aerogel anisotropy and n-textures. Currently we are studying the tip angle dependence of NMR frequency shifts in these aerogels. This work was supported by the National Science Foundation, DMR-0703656.\\[0pt] [1] J. Pollanen et al. \emph{J. of Non-Crystalline Solids} \textbf{354}, 4668 (2008). [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X33.00003: Equal-Spin Pairing Superfluid State of $^3$He in Radially Compressed Aerogel J. Pollanen, J. Li, C. Collett, W.J. Gannon, W. P. Halperin Anisotropic quasiparticle scattering has been predicted to stabilize anisotropic superfluid states of $^3$He [1,2]. We have performed pulsed nuclear magnetic resonance (NMR) experiments on $^3$He in a homogeneously anisotropic $97.5\%$ porosity aerogel. From the NMR frequency shifts on warming at P = 26 bar we find a single superfluid state exists between $0.7 mK$ and $T_{caero} = 1.67mK$. Susceptibility measurements indicate this phase is an equal-spin pairing (ESP) state. The anisotropy of our cylindrical aerogel sample was induced during the growth and drying stages in the form of $14.3\%$ radial compression. The sample was characterized with an optical, cross-polarization technique [3] to confirm the presence of a homogeneous optical axis aligned with the cylinder axis. Similar experiments and characterization have been performed on a homogeneously isotropic $98.1\%$ aerogel and, in this case, we find the non-ESP aerogel B-phase is the stable state. We are currently studying the tip angle dependence of the NMR frequency shift to identify which of the ESP states we have observed and to explore the full P-T phase diagram of superfluid $^3$He in these aerogels. This work was supported by the National Science Foundation, DMR-0703656. [1] C.L. Vicente, et al., PRB 72, 094519 (2005). [2] K. Aoyama and R. Ikeda, PRB 73, 060504(R) (2006). [3] J. Pollanen et al., JNCS 354, 4668 (2008). [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X33.00004: Moderate Magnetic Field Transverse Acoustics in Superfluid $^3$He-B C. Collett, S. Sasaki, J.P. Davis, J. Pollanen, W.J. Gannon, J. Li, W.P. Halperin We present the results of transverse acoustics studies in superfluid $^3$He-B at fields up to 0.1 T. Using acoustic cavity interferometry, we observe the acoustic Faraday effect\footnote{G.F. Moores and J.A. Sauls, J. Low Temp. Phys. {\bf 91}, 13 (1993).}$^,$\footnote{Y. Lee {\it et al.}, Nature {\bf 400}, 431 (1999).} for a transverse sound wave propagating along the magnetic field, and we measure Faraday rotations of the polarization of the sound up to 990$^\circ$, significantly more extensive than has been previously reported. We use these results to extend previous calculations of the Land\'e \textit{g} factor. We also find the field dependence of cavity interference oscillations resulting from coupling to the imaginary squashing mode (ISQ), a collective mode of the order parameter with total angular momentum $J=2$. Measurements in large magnetic fields were performed at frequencies up to the pair breaking threshold, where there has been a recent report\footnote{J.P. Davis {\it et al.}, Nature Physics {\bf 4}, 571 (2008).} of a new collective mode with $J=4$. The discovery of Faraday rotations coming from this new mode is reported, along with their intersection with rotations from the ISQ. Support for this work from the NSF, grant DMR-0703656, is gratefully acknowledged. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X33.00005: L\'evy Flights and Anomalous Diffusion in Liquid $^3$He-Aerogel James Sauls The transport of heat by liquid $^3$He impregated into silica aerogel is limited at low temperatures by elastic scattering of quasiparticles by the aerogel. The gossamer structure of silica aerogel is a realization of a random fractal - a solid with no long-range order, but power-law scaling of the density correlation function. Complementary to fractal scaling of the particle-particle correlation function is the appearance of a power law distribution of {\sl free flight paths}. The open structure shown in the DLCA simulations of low-density aerogel leads to a distribution of exceedingly long flight paths governed by a L\'evy distribution. I describe a theory for anomalous diffusion of quasiparticles in which the L\'evy distribution of long free paths is interrupted by inelastic collisions between quasiparticles. These rare events lead to finite temperature corrections to the thermal diffusion coefficient of the form, $\kappa/T = K_{0} - K_{1}\,(T/T^{\star})^{\beta}$, where $T^{\star}$ is the temperature at which the elastic and inelastic mean free paths are equal and $\beta$ is related to the fractal dimension of the L\'evy distribution. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X33.00006: Signatures of Crystalline Phases and Domain Walls in Superfluid $^3$He Thin Films Anton Vorontsov, James Sauls Thin films of superfluid $^3$He may spontaneously break translation symmetry in the plane of the film.\footnote{Phys. Rev. Lett. 98,045301 (2007).} Near a critical film thickness, $D_{c_1}\approx 13\,\xi_0$, a one-dimensional ``stripe phase'' develops as a periodic array of domain walls separating degenerate, but inequivalent B-phases, $(\Delta_{||},\Delta_{||},+\Delta_{\perp})$ and $(\Delta_{||},\Delta_{||},-\Delta_{\perp})$. These defects have a unique spectrum of topological excitations bound to the domain wall. We present results for the order parameter and Fermionic spectrum, and their observable signatures, for a single domain wall and for the stripe phase. The combination of particle-hole asymmetry and broken translational symmetry of the order parameter leads to a weak modulation of the density, $\delta n \sim \ln\left(E_f/k_B T_c\right)\left(k_B T_c/E_f\right)^2\,\bar{n}$, where $\bar{n}$ is the mean particle density. This leads to a modulation of the van der Walls attraction, and thus a small, static modulation of the film thickness. We report theoretical results for the density modulation, film thickness profile and optical reflectivity for the crystalline phases of superfluid $^3$He. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X33.00007: Visualization of counterflow dynamics using frozen nanoparticles Enrico Fonda, Matthew S. Paoletti, Katepalli R. Sreenivasan, Daniel P. Lathrop We study the dynamics of quantized vortices and quantum turbulence utilizing a particle tracking visualization technique. This is accomplished by using sub-micron and micron-sized hydrogen or atmospheric ice particles injected into He$^{4}$ flows that get trapped on the vortices. This technique has been used to observe and characterize reconnection of quantized vortices and thermal counterflow. We present the latest results using nano-sized ice particles. These sub-micron particles are superior to larger particles in a number of ways. In particular, being less affected by Stokes drag, they stay trapped on faster moving vortices and remain trapped closer to the lambda transition. Using these particles, we have made additional observations of counterflows at higher heat fluxes to shed light on the particle-vortex interaction mechanism. The technique has also been extended for visualization for fluid dynamics experiments using liquid nitrogen. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X33.00008: Numerical Real Space Renormalization of a 2D Random Boson Model Shankar Iyer, Gil Refael Interest in the random boson problem originated in experiments on Helium adsorbed in Vycor, but the problem arises in many contexts, including Josephson junction arrays and disordered cold atom systems. Recently, Altman, Kafri, Polkovnikov, and Refael have studied a rotor model description of interacting bosons subjected to quenched disorder in one dimension. Using a real space renormalization approach, they have identified a random fixed point that marks the transition between superfluid and Mott-glass phases. Here, we describe work that numerically extends their approach to the random boson problem in two dimensions. We first test the validity of the real space renormalization by comparison to exact diagonalization of small systems. Then, we move to larger systems and explore what the renormalization scheme can tell us about the nature of the insulating and superfluid phases. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X33.00009: Third Sound in Superfluid $^{4}$He Films Adsorbed on Packed Multiwall Carbon Nanotubes Emin Menachekanian, Gary A. Williams An investigation of third-sound propagation is carried out with thin $^{4}$He films adsorbed on multiwall carbon nanotubes. At an average diameter of 12 nm and a length of several microns, the powder of nanotubes is lightly packed into a cylindrical resonator, with a resistor bolometer at the cylinder end to detect the temperature oscillations accompanying the waves. The lowest standing-wave mode in the cavity is excited by mechanical vibrations, with FFT analysis allowing measurement of the sound speed as well as the dissipation. The Kosterlitz-Thouless onset transition is observed with increasing film thickness for temperatures between 1.3 and 1.7 K. At higher thicknesses capillary condensation becomes important, probably at connection points where the nanotubes touch. Layering effects in the third-sound velocity, associated with the relatively strong van der Waals coupling between helium and carbon, are not observed, and measurements below 1 K may be necessary to see this. There is also no indication of any effect of superfluidity attributable to the adsorption of helium on the inner surfaces of the nanotubes. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X33.00010: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X33.00011: Vortex-Loop Thermodynamics of Superfluid $^4$He Under Pressure Andrew Forrester, Gary A. Williams The thermodynamic quantities of pressurized superfluid $^4$He near the $\lambda$-transition are calculated using a vortex-loop renormalization method. The superfluid density, specific heat, vortex core size, and vortex core energy are determined as functions of pressure and temperature, and compared with experiments. The theory predicts exponents describing the critical behavior of the superfluid density and specific heat that are in agreement with recent high-precision theoretical simulations. The vortex core size is found to increase with pressure, while the core energy decreases, the behavior found experimentally for both parameters. The specific heat, though strongly dependent on both of these parameters, is found to scale with pressure in agreement with experimental measurements. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X33.00012: Topological and geometrical interactions between quantum vortices near zero temperature Ran Cheng, Xiao Li, Qian Niu With new velocity-dependent term discovered, various types of interactions between quantum vortices in 2-d superfluid Helium and BEC near zero temperature are unified via Berry Phase theory. Originated from the finite compressibility of the fluid, the topological statistical gauge field of an vortex breaks down to geometrical gauge field mediating local interactions. This new interaction modifies the cyclotron motion of a pair of identical vortices, and changes the pattern of orbits of a pair of vortex-antivortex. Damping effect due to finite temperature is treated phenomenologically, which does not invalidate our essential conclusions. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X33.00013: Semiclassical dynamics of vortices in superfluid helium thin films Xiao Li, Ran Cheng, Qian Niu Based on the Berry phase theory, we consider the case of two vortices in Bosonic superfluids and try to extract the interaction between them. Under the adiabatic approximation, we use semiclassical Lagrangian formalism to describe the system and found that in addition to the universal background ``magnetic field'' which results in the Magnus force, there exists a new interaction mediated by the density profile of the background fluid due to its finite compressibility. Finally, numerical solutions from the nonlinear Schrodinger equation were employed to gain better insight into this problem. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X33.00014: Theory of the Bose-glass states in Br-doped Nickel-Tetrakis Thiourea (DTN) Rong Yu, Stephan Haas, Tommaso Roscilde We present extensive Quantum Monte Carlo calculations on bond-disordered coupled spin chains with strong single-ion anisotropy, modeling the behavior of Br-doped Nickel-Tetrakis Thiourea (DTN). Our model quantitatively describes the phase diagram of the experimental compound - in particular the low-temperature magnetization curve and the critical temperature for magnetic Bose-Einstein condensation as a function of the field. Hence it provides fundamental insight into the nature of the Bose-glass phases appearing at low temperature close to the two critical fields for condensation. Br-doped bonds act as nucleation centers of magnetic quasiparticles in the low-field Bose glass, while at high fields the Br-doped bonds represent the localization centers of magnetic quasiholes. The quantitative understanding of Br-doped DTN opens the way to a detailed investigation of Bose-glass physics in quantum magnets. [Preview Abstract] |
Session X34: Nanostructures: Assembly, Growth, and Characterization
Sponsoring Units: DCMPChair: Eric Stinaff, Ohio University
Room: C141
Thursday, March 24, 2011 2:30PM - 2:42PM |
X34.00001: Vapor-Liquid-Solid Glancing Angle Deposition (VLS-GLAD): A New Approach to Fabricate Crystalline Semiconductor Nanowires Arif Sinan Alagoz, Tansel Karabacak Vapor-liquid-solid (VLS) method has become one of the few and most powerful bottom-up single crystal nanowire growth techniques. On the other hand, control of growth direction and crystal orientation of semiconductor nanowires stand as major issues in VLS technique. In order to overcome these challenges, we developed a new vapor-liquid-solid glancing angle deposition (VLS-GLAD) fabrication approach for the growth of semiconductor nanowire arrays with a controlled geometry and crystal orientations. VLS-GLAD is a physical vapor deposition nanowire fabrication approach based on selective deposition of nanowire source atoms onto metal catalyst nanoislands placed on a crystal wafer. In this technique, collimated obliquely incident flux of source atoms selectively deposit on catalyst islands by using ``shadowing effect''. Geometrical showing effect combined with conventional VLS growth mechanism leads to the growth of tilted crystalline semiconductor nanowire arrays. In this presentation, we show the morphological and structural properties of tilted single crystal Si and Ge nanowire arrays fabricated by utilizing a conventional thermal evaporation system for VLS-GLAD. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X34.00002: LEEM and STM observations of Growth of Nanowires of Ag on Ge(110) and Surface Structural Phases of Ir on Ge(111) Cory Mullet, Marshall van Zijll, Emilie Huffman, Shirley Chiang We have used both low energy electron microscopy (LEEM) and scanning tunneling microscopy (STM) to characterize the growth of silver on Ge(110) and iridium on Ge(111) as a function of coverage, deposition temperature, and annealing temperature. We observed 1D island growth along [-110] as Ag is deposited onto Ge(110) above 430 C. Island dimensions varied with deposition temperature. At 480 C, Ag islands are $\sim $100nm wide and 1-20 $\mu $m long for 9 ML coverage. Between 380 C and 430 C, we observed two novel low coverage phases, with the higher coverage phase completing at 0.12 ML. Ir deposited onto the Ge(111) c(2x8) above 400 C forms a ($\surd $3x$\surd $3)R30$^{o}$ phase, with island size dependent upon substrate temperature during deposition. Deposition at 400-425 C produces Ir islands, which are 1-20 nm in diameter at 0.5 ML coverage. Island heights range from one to several atomic layers, and exhibit a unique growth mode with islands connected by ``streamers'' of Ir. We observed Stranski-Krastanov growth in LEEM at 670 C. Ir desorbs from Ge(111) at 870 C, beginning from areas of high step density. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X34.00003: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X34.00004: Alignment of Gold Nanorods in Thermoresponsive Hydrogels Heung-Shik Park, Oleg Lavrentovich The unique optoelectronic properties of the anisotropic metallic nanorods (NRs) are of great interest because of their potential applications in biomedical science, transformative optics and materials science. In order to utilize metallic NRs for the practical devices, the control of orientation and immobilization of NRs in bulk materials are essential. We report an experimental study of gold NR embedded in thermoresponsive gels which can align NRs by volume-contraction transition. When temperature increases, an NR hydrogel stripe experiences an abrupt shrinkage in two lateral (x,y) directions; in the third z-direction, the size remains fixed as the stripe is clamped. The shrunk stripes show high birefringence and anisotropic absorption associated with alignment of the NRs. The alignment of NRs in anisotropically shrunk hydrogels can be achieved also when one uses aggregates of side-by-side preassembled NRs rather than individual NRs. These aggregates can be transferred into a polymer hydrogel preserving their structural and optical features. The hydrogel stripes with preassembled NRs show optical anisotropy opposite to that one of stripes with isolated NRs. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X34.00005: Programmable Nanofabrication of Nanoparticle Assemblies of arbitrarily Shapes on DNA Templates Mauricio Pilo-Pais, Sarah Goldberg, Enrique Samano, Henok Mebrahtu, Thomas LaBean, Gleb Finkelstein We present a method for producing metallic structures with nanoscale dimensions and programmable design. Rectangular ``DNA origami'' structures ($\sim $90x70nm) were modified to have uniquely coded binding sites and adsorbed onto silicon dioxide substrates. Gold nanoparticles functionalized with a complimentary DNA sequence were attached to these binding sites in a highly controllable fashion. The seed nanoparticles were then enlarged (and even fused, if desired) by a silver reduction chemistry. Using this method we constructed a variety of metallic structures, including parallel wires, H-shapes, and rings. Due to the flexibility of the design and the multiply parallel nature of the method, these structures may offer great promise for plasmonic applications. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X34.00006: Controlling Nanostructure Self-assembly for Design of Three-dimensional Semiconductor Heterostructures Santino D. Carnevale, J. Yang, P.J. Phillips, M.J. Mills, R.C. Myers We examine the control of vertical and coaxial growth in self-assembled GaN/AlN nanowires grown on Si (111) by plasma assisted molecular beam epitaxy. To grow nanowires vertically and not radially a two-step growth method is used. Nanowires are nucleated at low temperatures and grown vertically at high temperatures, allowing for independent control of density and height and constant radius. A second method is used to promote radial growth. GaN nanowire cores are formed, then growth temperature is reduced and growth continues. Vertically and coaxially oriented AlN/GaN heterostructures grown using these methods are presented. We discuss the structural and optical properties of these GaN/AlN quantum disk and core-shell heterostructures using scanning electron microscopy, scanning transmission electron microscopy, and temperature dependent photoluminescence measurements. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X34.00007: Preparation of monodisperse silicon nanocrystals through density-gradient unltracentrifugation in organic solvents Joseph B. Miller, Austin Van Sickle, Swathi Iyer, Rebecca A. Anthony, Uwe R. Kortshagen, Erik K. Hobbie Monodisperse colloidal suspensions of ligand-coated silicon nanocrystals, synthesized through a nonthermal low-pressure plasma reaction, have been prepared through density-gradient ultracentrifugation in mixed organic solvents. Density-gradient profiles of mixed chloroform and m-xylene are used to tune and control the settling speed of the nanoparticles and hence optimize their transient separation by size along the depth of polyoxymethylene ultracentrifuge tubes. The mean size and polydispersity of the extracted fractions are characterized through photoluminescence spectroscopy and transmission electron microscopy, and the self-assembly of fractions into close-packed crystal lattices is achieved using an immiscible two-fluid evaporation scheme. The photophysical properties of the nanocrystal lattices are compared with those of the starting materials and suspensions, and the influence of atmospheric oxygen on the stability of the nanocrystal photoluminescence is measured. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X34.00008: Self-assembled Au Nanoparticle Arrays with Engineered Hot Spots for SERS A. Chen, U. Welp, V. Vlasko-Vlasov, A.E. DePrince III, A. Demortiere, A. Joshi-Imre, E.V. Shevchenko, S.K. Gray We demonstrate a cost-effective bottom-up self-assembly of 80 nm Au nanoparticles (NPs) with controllable regular arrays of hot spots for high-fidelity and high-sensitivity sensor applications. The self-assembly of gold NPs is implemented using solvent evaporation techniques. By careful control of surface stabilizers on NPs and optimization of assembly conditions, we fabricated hcp arrays of NPs extended over more than 200 $\mu$m. Electromagnetic hot spots localized in the nanometer gaps between Au NPs are well defined and reproducible over large areas of the arrays. UV-Vis-NIR extinction spectra of our 2D plasmonic crystals exhibit unique resonances due to strong particle-particle interactions, in a good agreement with results of our finite-difference-time-domain (FDTD) simulations. We experimentally demonstrate large enhancements of both photoluminescence and surface enhanced Raman scatterings of 5nm CdSe quantum dots coated on 80nm Au NP arrays. High-resolution SEM imaging of quantum dots gave a precise estimate of their density and positions and allowed direct evaluation of the enhancement factors. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X34.00009: Fine tuning nanoparticle spacing in freestanding membranes through ion and electron beams Pongsakorn Kanjanaboos, Alexandra Joshi-Imre, Xiao-Min Lin, Heinrich Jaeger Freestanding membranes of ligated nanoparticles can be assembled in a one-step drying-mediated process. These 2D sheets have remarkable mechanical properties, in particular extreme bending flexibility coupled with effective Young's moduli in the range of 1-20GPa, depending on the nanoparticle and ligand types and sizes. We report on experiments in which used a focused Ga ion beam to strategically place cuts into freestanding monolayer membranes. Exposed to electron beams, the cuts expand and the membranes act as if additional strains were deposited. Given that the exposed membranes behave like strained elastic sheets, we can easily design various strain patterns. With calibration, the electron beam dose serves as a knob to fine-tune interparticle distances in these patterns. \\ K. E. Mueggenburg, X. Lin, R. H. Goldsmith, and H. M. Jaeger, Nature Materials 6, 656 (2007). J. He, P. Kanjanaboos, N. L. Frazer, X. Lin, and H. M. Jaeger, Small 6, 1449 (2010) W. Cheng et al. Nature Materials 8, 519 (2009) [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X34.00010: Mechanics of colloidal nanoparticle arrays Jie Yin, Markus Retsch, Edwin L. Thomas, Mary C. Boyce Hollow colloidal nanoparticles have become a focal point of studies for applications in drug delivery and nanostructured materials. The mechanical properties of individual nanoparticle and the collective behavior of colloidal nanoparticle arrays are of great importance. In this paper, the mechanics of colloidal arrays of hollow amorphous silica spherical nanoparticles during microindentation are explored. The study reveals that the consecutive contact process of nanoparticles during indentation results in highly nonlinear indentation load-displacement curves. The contacted nanoparticles successively become flattened and locally bend and buckle to form a localized dimple as the indentor encounters each particle. By using the contact mechanics model of single hollow particle, the indentation load-displacement formula is obtained for indentation on hollow spherical nanoparticle arrays and the Young's modulus of an individual particle is extracted from the measured load-displacement behavior of an array. The reduced Young's modulus is consistent with the measurement of single hollow amorphous silica nanoparticle by using AFM. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X34.00011: Au/Fe nanoparticles prepared by multilayers annealing Aida Serrano, Oscar Rodriguez de la Fuente, Miguel Angel Garcia Metallic nanoparticles supported onto a substrate can be obtained by thin film deposition and subsequent annealing. The stress relief after the thermal annealing due to the difference of thermal expansion coefficient between the metal and the substrate promotes hillock formation and subsequent hole nucleation, growth and percolation leading to the formation of nanoparticles layers. The nanoparticle size and inter-particle distance can be tuned by controlling the initial film thickness and the annealing time, temperature and atmosphere, providing a simple and low cost method to prepare NPs layers over large areas. The method has been successfully applied to obtain nanoparticles from a single metallic layer in the past. We report here the formation of complex nanoparticles ensembles by deposition and annealing of Au-Fe multilayers. The optical properties of gold nanoparticles (surface plasmon resonance absorption) and the magnetic properties of Fe/Fe oxide ones as well as cross-over effects are studied as a function of multilayers structure and annealing conditions. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X34.00012: Effects of ligand binding strength and facet coverage on the morphology of nanocrystal superlattices Clive Bealing, Richard Hennig Nanocrystals (NCs) of lead-salt have been proposed for a number of photovoltaic applications. These NCs consist of an inorganic core, in the rock salt structure, whose surface is usually passivated by oleate ligands. The self-assembly of NCs from colloidal solutions into mesoscale superlattices provides a path to materials with tunable electronic, physical and chemical properties that are promising for applications. The self-assembly is controlled by the NC shape and by ligand-mediated interactions between NCs; to understand this, it is necessary to know the effect of the ligands on the surface energies, as well as the relative coverage of the different facets. Our density functional calculations of the binding energies of carboxylic acid-based ligands on PbSe and PbS show that the ligands exhibit a strong energetic preference to particular facets. The results suggest that the transformation of the NC superlattice structure from \emph{fcc} to \emph{bcc} in aged NC assemblies is caused by the preferential detachment of ligands from particular facets, leading to anisotropic ligand coverage. Combined with the experimental results, our calculations present a potential route to greater control over the morphology of the NC superlattice assembly. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X34.00013: Scanning Tunneling Microscopy and Spectroscopy of Rare Earth-Monopnictide Nanostructures Embedded in a Semiconducting Matrix Jason Kawasaki, Rainer Timm, Trevor Buehl, Edvin Lundgren, Anders Mikkelsen, Arthur Gossard, Chris Palmstr{\O}m The atomic and electronic structure of molecular beam epitaxy (MBE)-grown rare earth-monopnictide nanostructures embedded within a III-V semiconductor matrix are examined via scanning tunneling microscopy (STM) and spectroscopy (STS). We examine several systems, including ErSb nanoparticles embedded in GaSb, ScAs nanoparticles embedded in GaAs, and ErAs nanoparticles and nanorods embedded in GaAs. Tunneling current I(V) and differential conductance dI/dV spectra show that for both ErAs nanoparticles and nanorods the local density of states (LDOS) exhibits a sharp but finite minimum at the Fermi level, demonstrating that both the particles and rods are semimetallic and not semiconducting. This observation lies in contrast to previous models of quantum confinement in ErAs. We also use STS to measure the LDOS across the ErAs/GaAs interface and discuss the formation of interface states and band bending at the interface. Finally, we discuss possible changes in the LDOS with varying nanoparticle size and varying levels of doping in the semiconductor matrix. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X34.00014: \textit{Ab Initio }Studies of Si$_{m}$Ge$_{n }$(m+n $\leq$ 5) Nanoclusters Sarah Duesman, Asok Ray Electronic and geometric structure properties of Si$_{m}$Ge$_{n}$(m+n $\leq$ 5) nanoclusters have been investigated using hybrid density functional B3LYP, 6-311G (3df, 3pd) basis set, and the GAUSSIAN 03 software. For the Si atom, the computed values of the ionization potential and electron affinity are 8.11 and 1.10eV, and for the Ge atom, the values are 7.90 and 1.14eV. The experimental values are 8.15, 1.39, 7.90, and 1.23eV, respectively. Various possible geometries have been spin-optimized to determine the global minimum for each nanocluster. We will present the electronic and geometric structures of the isomers of each nanocluster, including bond length, symmetry group, electronic state, binding energy, HOMO-LUMO gap, ionization potential, and electron affinity. In addition, the harmonic frequencies, fragmentation energies, average coordination number and Mulliken atomic charges will also be discussed for the ground states of the nanoclusters. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X34.00015: Competition of Nonlinear Optical Properties in ZnO Nanoparticles Jie Lin, Antonio Llopis, Benny Urban, Yasuhisa Fujita, Arup Neogi ZnO nanoparticles have attracted increased attention due to its large exciton binding energy. Moreover it has enhanced nonlinear optical properties due to its noncentrosymmetric crystal structure which results in a second order nonlinearity. The presence of oxygen vacancy and modified surface states also yields third order nonlinearity such as two photon absorption which yield significant two-photon emission. However, the presence of high second order nonlinearity in a system can result in the retardation of the third order nonlinearity. We thereby present the relative efficiencies of the second and the third-order nonlinear processes in ZnO nanoparticle system. Using tunable femtosecond laser irradiation the recombination lifetime due to single and two-photon induced electron-hole recombination process has been studied. Our results show that the second harmonic generation (SHG) process compete with the two photon emission(TPE) process in the region 700nm---900nm. The TPE process is more efficient in 700 nm-740nm whereas the SHG process is more efficient from 745-900nm) region. We also observed the increase of the two photon emission with excited energy is caused by the increased life of its virtual state. [Preview Abstract] |
Session X35: Topological Insulators: Theory III
Sponsoring Units: DCMPChair: Andreas Ruegg, University of Texas at Austin
Room: C140
Thursday, March 24, 2011 2:30PM - 2:42PM |
X35.00001: Search for New Topological Insulators Hsin Lin, L.A. Wray, S.-Y. Xu, M.Z. Hasan, T. Das, Y.J. Wang, R.S. Markiewicz, Arun Bansil Topological insulators (TIs) host a novel quantum phase of electrons which is characterized by topologically protected surface states originating from the effects of spin-orbit and time-reversal symmetries. While several families of TIs have already been found, the intense world-wide search for new classes of TIs continues unabated. This interest is driven by the need for materials with greater structural flexibility and tunability to enable viable applications in spintronics and quantum computing. We have used first-principles band theory computations in combination with angle-resolved photoemission experiments to successfully predict many new classes of topologically interesting materials, including Bi2Se3 series, the ternary half-Heusler compounds, thallium-based chalcogenides, and the Li2AgSb and Ge$_n$Bi$_{2m}$Te$_{3m+n}$ families. [1-5] Work supported by the Office of Basic Energy Sciences, US DOE.\\[4pt] [1] H. Lin, R. S. Markiewicz, L. A. Wray, L. Fu, M. Z. Hasan, and A. Bansil, Physical Review Letters \textbf{105}, 036404 (2010). \\[0pt] [2] H. Lin, L. A. Wray, Y. Xia. S. Y. Xu, S. Jia, R. J. Cava, A. Bansil, and M. Z. Hasan, Nature Materials \textbf{9}, 546 (2010). \\[0pt] [3] W. Al-Sawai {\it et al.}, Physical Review B \textbf{82}, 125208 (2010). \\[0pt] [4] L. A. Wray {\it et al.}, Nature Physics (2010, in press).\\[0pt] [5] S.-Y. Xu {\it et al.}, arXiv:1007.5111 (2010). [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X35.00002: Spin-texture of three-dimensional topological insulators: Bi$_2$Te$_3$, Bi$_2$Se$_3$ and Sb$_2$Te$_3$ Susmita Basak, Hsin Lin, L.A. Wray, S.-Y. Xu, M.Z. Hasan, A. Bansil We have investigated the nature of surface states in the Bi$_2$Te$_3$, Bi$_2$Se$_3$ and Sb$_2$Te$_3$ family of 3D topological insulators using first-principles calculation as well as $k\cdot p$ scheme [1]. Recent spin-resolved photoemission experiments suggest that electrons on the surface of a topological insulator behave as massless relativistic particles with an intrinsic angular momentum (spin) which is locked to their translational momentum [2,3]. We have computed the in-plane spin-textures of all three aforementioned compounds to demonstrate the `spin-helical' nature of the 2D fermions. In addition, the spin must acquire a finite out-of-the-plane component to preserve the bulk topological invariant [1]. We study this quantity in particular since there are possibilities of observing new quantum effects. Work supported by the US DOE.\\[4pt] [1] L. Fu, Phys. Rev. Lett. {\bf 103}, 266801, (2009). \\[0pt] [2] D. Hsieh {\it et al.}, Science {\bf 323}, 919 (2009). \\[0pt] [3] D. Hsieh {\it et al.}, Nature {\bf 460}, 1101 (2009). [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X35.00003: First principles analysis of quantum transport in Bi2Se3 3D topological insulators Yonghong Zhao, Yibin Hu, Lei Liu, Yu Zhu, Hong Guo By carrying out density functional theory (DFT) within the Keldysh nonequilibrium Green's function formalism (NEGF), we have investigated quantum transport properties of the Bi2Se3 topological insulator from atomistic first principles without any phenomenological parameters. Using the scattering states, our results vividly reveal the surface Dirac fermions and helical edge spin states in the momentum space. We have also determined the real-space distribution of the helical edge spin states which provide the penetration depth of the surface topological conducting channels into the bulk Bi2Se3 crystal. Our first principles calculations take into account the full non-collinear spin structure and spin-orbit interaction, the details of these technical advances within the NEGF-DFT quantum transport formalism will also be briefly discussed. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X35.00004: Electrically controllable surface magnetism on the surface of topological insulator Jia-Ji Zhu, Dao-Xin Yao, Shou-Cheng Zhang, Kai Chang We study theoretically the RKKY interaction between magnetic impurities on the surface of a three dimensional topological insulator, mediated by the massless and massive helical Dirac electrons. Exact analytical expression of RKKY interaction shows that the spin- spin interaction consists of the Heisenberg-like, Ising-like and Dzyaloshinskii-Moriya (DM)-like terms caused by the helicity of the topological surface states. It provides us a new way to realize various spin models, e.g., DM model, XXZ model and XZ model, and control surface magnetism by tuning the Fermi energy, and/or the distance between the two local spins. The gap opened by doped magnetic ions can lead to a short-range Bloembergen- Rowland interaction via the virtual interband interaction when the Fermi energy is located in the gap. The competition among the Heisenberg, Ising and DM terms leads to rich spin configurations and anomalous Hall effect on different lattices. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X35.00005: Ordering of magnetic impurities and tunable electronic properties of topological insulators Dmytro Pesin, Dmitry Abanin We study collective behavior of magnetic adatoms randomly distributed on the surface of a topological insulator. As a consequence of the spin-momentum locking on the surface, the RKKY- type interactions of two adatom spins depend on the direction of the vector connecting them, thus interactions of an ensemble of adatoms are frustrated. We show that at low temperatures the frustrated RKKY interactions give rise to two phases: an ordered ferromagnetic phase with spins pointing perpendicular to the surface, and a disordered spin-glass-like phase. The two phases are separated by a quantum phase transition driven by the magnetic exchange anisotropy. Ferromagnetic ordering occurs via a finite-temperature phase transition. The ordered phase breaks time-reversal symmetry spontaneously, driving the surface states into a gapped state, which exhibits an anomalous quantum Hall effect and provides a realization of the parity anomaly. We find that the magnetic ordering is suppressed by potential scattering. Our work indicates that controlled deposition of magnetic impurities provides a way to modify the electronic properties of topological insulators. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X35.00006: Possible Strong Topological Insulator Phase in Li$_{2}$IrO$_{3}$ Heung Sik Kim, Choong Hyun Kim, Hosub Jin, Jaejun Yu Recently Na$_{2}$IrO$_{3}$, a layered 5$d$ transition metal oxide compound, was suggested to be a possible topological insulator (TI) based on the $j_{eff}=1/2$ states induced by the strong spin-orbit coupling of Ir 5$d$ states, but its realization has not been clarified yet. In search of the TI phase in transition metal oxides, we propose Li$_{2}$IrO$_{3}$ to be a candidate for the three-dimensional strong TI. By carrying out Wannier function analysis based on first-principles calculations, we constructed a low energy effective Hamiltonian, which leads to a three-dimensional extension of the Kane-Mele model with third-nearest-neighbor hopping within the Ir honeycomb layer and a significant inter-layer coupling. The nature of spin-orbit coupled states near the Fermi level depends on the change of the trigonal crystal field driven by the lattice deformations. A competition between the third next-nearest-neighbor hopping parameter and the trigonal crystal field is found to play a key role in determining the topological character of Li$_{2}$IrO$_{3}$. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X35.00007: Half-Heusler Topological Insulators: A First-Principle Study with the Tran-Blaha Modified Becke-Johnson Density Functional Wanxiang Feng, Di Xiao, Ying Zhang, Yugui Yao We systematically investigate the topological band structures of half-Heusler compounds using first-principles calculations. The modified Becke-Johnson exchange potential together with local density approximation for the correlation potential (MBJLDA) has been used here to obtain accurate band inversion strength and band order. Our results show that a large number of half-Heusler compounds are candidates for three-dimensional topological insulators. The difference between band structures obtained using the local density approximation (LDA) and MBJLDA potential is also discussed. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X35.00008: Toplogical electronic structure in half-Heusler topological insulators Wael Al-Sawai, Hsin Lin, Robert Markiewicz, L. Wray, Y. Xia, S. Xu, M. Hasan, A. Bansil We investigate the details of electronic band structure of a series of 28 ternary half-Heusler compounds MM$^\prime$X of MgAgAs-type where M = (Lu, La, Sc, Y) and M$^{\prime}$X=(PtBi, AuPb, PdBi, PtSb, AuSn, NiBi, PdSb). Our results show that the $Z_2$ topological order is due to a single band inversion at the $\Gamma$-point. Half-Heusler compounds can be either topologically nontrivial semimetals, nontrivial metals, or trivial insulators. Our analysis reveals a straightforward relationship between the band inversion strength (extent of deviation from the critical point), the atomic charge of constituents, and the lattice parameter. Our findings suggest a general method for identifying $Z_2$ topological insulators in nonmagnetic ternary compounds. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X35.00009: Topological insulating behavior in conducting property of crystalline Ge-Sb-Te Jeongwoo Kim, Jinwoong Kim, Seung-Hoon Jhi Phase-change random access memory (PRAM) is one of the most promising materials for data storage application. Especially, Ge-Sb-Te$_{ }$(GST) is considered as the best candidates for next generation nonvolatile memories because of the rapid and reversible cycles between the crystalline and amorphous structures. GeTe and Sb$_{2}$Te$_{3 }$are the main components of GSTs, and have finite band gaps in the bulk phase. Sb$_{2}$Te$_{3}$ is topological insulator that has gapless edge states while maintaining bulk energy gap. These surface states are robust to external perturbations because they are protected by time-reversal symmetry. We report a discovery, through first-principles calculations, that crystalline GST phase-change materials exhibit the topological insulating property. Our calculations show that the materials become topological insulator or develop conducting surface-like interface states depending on the layer stacking sequence. It is shown that the conducting interface states originate from topological insulating Sb$_{2}$Te$_{3}$ layers in GSTs and can be crucial to the electronic property of the compounds. These interface states are found to be quite resilient to atomic disorders but sensitive to the uniaxial strains. We presented the mechanisms that destroy the topological insulating order in GSTs and investigated the role of Ge migration that is believed to be responsible for the amorphorization of GSTs. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X35.00010: Ab initio study of topological order induced by symmetry breaking in PbTe Jinwoong Kim, Seung-Hoon Jhi Topological insulator (TI) is a new class of materials that have an energy gap in bulk phase but contain linear and chiral band dispersions on their surface. The topological insulating order can be initiated by parity inversion in time-reversal symmetric momenta. We studied the topological insulating properties of PbTe under uniaxial strain using first-principles methods. PbTe is a narrow-gap semiconductor with trivial topological insulating order. While it is known to have band inversion under pressure at a time-reversal symmetric k-point, the degeneracy at the k-point prevents the overall parity inversion which is needed to induce the TI order. In this presentation, we show that uniaxial strain can break the symmetry and thus induce topologically nontrivial order in PbTe. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X35.00011: Electronic structure of the side surface of Bi$_{2}$Se$_{3}$ Chang-Youn Moon, Jinhee Han, Hyungjun Lee, Hyoung Joon Choi We investigate the electronic band structure of a side surface geometry, other than the conventional [111] surface, of the topological insulator Bi$_{2}$Se$_{3}$ using the first-principles pseudopotential calculations. As Bi$_{2}$Se$_{3}$ is known to be a strong topological insulator, it is expected that an arbitrary surface would have the topological surface state characterized by Dirac-cone-like band dispersion and spin-momentum coupling. Here we indeed obtain surface states with linear band dispersion around the Gamma point, but with a strong anisotropy with different group velocities along different k-directions. Low energy effective hamiltonian is proposed, and physical implications of the anisotropic Dirac fermions are also discussed. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X35.00012: Spatial characters of metallic surface states of topological insulators Jinhee Han, Hyungjun Lee, Hyoung Joon Choi We study the electronic structure of metallic surface states in Bi$_{2}$Se$_{3}$, Bi$_{2}$Te$_{3}$, and Sb$_{2}$Te$_{3}$ using an ab-initio pseudopotential density-functional method. We implemented the spin-orbit interaction into the SIESTA in a form of additional fully non-local projectors. For surface states on (001) surface, we used a supercell containing 10 quintuple layers. We obtained bulk and surface electronic structures of topological insulators Bi$_{2}$Se$_{3}$, Bi$_{2}$Te$_{3}$, and Sb$_{2}$Te$_{3}$, which are close to previous theoretical results and consistent with Dirac-cone band dispersions measured by angle-resolved photoemission spectroscopy. Then, we analyzed the wavefunctions of the metallic surface states near the Fermi level to find out spatial distributions of the surface-state wavefunctions, which turn out to be localized in the surface region with a typical spread of about 2 quintuple layers, and the shapes of the wavefunctions around Bi (or Sb) atoms close to the surface. This work was supported by the NRF of Korea (Grant No. 2009-0081204) and KISTI Supercomputing Center (Project No. KSC-2008-S02-0004). [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X35.00013: Chern-Simons orbital magnetoelectric coupling in generic insulators Sinisa Coh, David Vanderbilt, Andrei Malashevich, Ivo Souza The isotropic Chern-Simons coupling $\theta$ is a component of the orbital contribution to the magnetoelectric coupling.\footnote{A. Malashevich {\it et al.}, New J. Phys. {\bf 12}, 053032 (2010); A.~M. Essin {\it et al.}, Phys. Rev. B {\bf 81}, 205104 (2010). } In a generic insulator it can have any value, while it must be exactly $\pi$ in a strong Z$_2$ topological insulator. The results of our first-principles density-functional calculations for the ordinary magnetoelectrics Cr$_2$O$_3$, BiFeO$_3$ and GdAlO$_3$ confirm that the Chern-Simons contribution is quite small in these materials.\footnote{S. Coh {\it et al.}, arXiv:1010.6071.} We discuss various strategies for finding insulators for which $\theta$ is large but not equal to $\pi$. For example, we show that if the spatial inversion and time-reversal symmetries of the Z$_2$ topological insulator Bi$_2$Se$_3$ are broken by hand, large induced changes appear in the Chern-Simons magnetoelectric coupling. We also perform an analysis based on space-group representation theory to determine the simplest possible magnetic structures which allow for a non-zero and possibly large value of $\theta$. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X35.00014: Theoretical prediction of new topological insulators in filled skutterudites Binghai Yan, Lukas Muechler, Xiao-Liang Qi, Claudia Felser, Shou-Cheng Zhang We have reported a unique class of topological insulators, filled skutterudite (FS) compounds, using ab initio calculations. We find that several FSs are not only two-dimensional topological insulators as quantum wells like HgTe, but also three-dimensional topological Kondo insulators. Different from previously reported topological insulators, they have unique band inversion feature in band structures. Their advantages are discussed to realize superconductivity proximity and other topological phenomena. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X35.00015: Three-Dimensional Topological Insulators in I-III-VI$_2$ and II-IV-V$_2$ Chalcopyrite Semiconductors Di Xiao, Wanxiang Feng, Jun Ding, Yugui Yao Using first-principles calculations, we investigate the band topology of the ternary chalcopyrite family. Our method is based on the adiabatic continuity of the Hamiltonian combined with direct calculation of the Z2 topological invariants in inversion-symmetry breaking systems. We show that a large number of these compounds are candidates for three-dimensional topological insulators. Moreover, The topological order can be tuned and controlled by lattice strain. The excellent physical properties of these compounds make them an appealing platform for novel quantum phenomena. [Preview Abstract] |
Thursday, March 24, 2011 5:30PM - 5:42PM |
X35.00016: Edge states and the bulk-boundary correspondence in Dirac Hamiltonians Vasudha Shivamoggi, Roger Mong We present an analytic prescription for computing the edge dispersion $E(k)$ of a tight-binding Dirac Hamiltonian terminated at an abrupt crystalline edge. Specifically, we consider translationally invariant Dirac Hamiltonians with nearest-layer interaction. The result is a geometric formula that relates the existence of surface states as well as their energy dispersion to properties of the bulk Hamiltonian. We give examples of how the formula can be used to find the edge state dispersion in various topologically ordered systems. We further prove the bulk-boundary correspondence between the Chern number and the chiral edge modes for quantum Hall systems within the class of Hamiltonians studied here. Our results can be extended to the case of continuum theories which are quadratic in momentum, as well as other symmetry classes. [Preview Abstract] |
Session X36: Graphene: Quantum Hall Effect
Sponsoring Units: DCMPChair: Eva Andrei, Rutgers University
Room: C142
Thursday, March 24, 2011 2:30PM - 2:42PM |
X36.00001: Graphene in a periodically alternating magnetic field: an unusual quantization of the anomalous Hall effect Patrick Bruno, Mathieu Taillefumier, Vitalii K. Dugaev, Benjamin Canals, Claudine Lacroix We study the energy spectrum and electronic properties of graphene in a periodic magnetic field of zero average with a symmetry of triangular lattice. The periodic field leads to formation of a set of minibands separated by the gaps, which can be manipulated by external field. The Berry phase, related to the motion of electrons in $k$ space, and the corresponding Chern numbers characterizing topology of the energy bands are calculated analytically and numerically. In this connection, we discuss the anomalous Hall effect in the insulator state, when the Fermi level is located in the minigap. The results of calculations show that in the model of gapless Dirac spectrum of graphene the anomalous Hall effect can be treated as a sum of fractional quantum numbers, related to the nonequivalent Dirac points. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X36.00002: Quantum Hall Edge States in Bilayer Graphene Ribbons Herbert Fertig, Victoria Mazo, Efrat Shimshoni We study the low energy edge states of bilayer graphene ribbons subject to a strong perpendicular magnetic field $B$, and show that they can be described within a continuum model (the Dirac equation). We are mainly interested in investigating the energy- band structure of ribbons with a zigzag termination. At the zero Landau Level there are eight degenerate bands, whose degeneracy can be broken and controlled by an external inter- layer voltage bias $V$. This leads to the opening of a gap in the bulk. On the edges, due to a mixture of hole- and particle- like bands (from the same valley), an avoided crossing occurs which can be understood within a perturbative expansion in the inter-layer hopping. On the other hand, edge states from different valleys are protected from mixing by a long-range disorder potential. Hence, hole- and particle-like states can cross without mixing, and the system has properties of a topological insulator. In the presence of interactions, the rich behavior of crossing single-electron edge states may lead to a variety of collective edge-modes, whose properties dominate the transport behavior of this system. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X36.00003: Hofstadter's Fractal Energy Spectrum in Twisted Bilayer Graphene Zhengfei Wang, Feng Liu, M.Y. Chou Hofstadter butterfly, the fractal spectrum of 2D lattice electrons in a magnetic field, has been studied theoretically for a few prototypical systems. However, due to the small unit cell in traditional materials, it is difficult to directly observe such a structure in the experiment. In this work we demonstrate that the Hofstadter butterfly structure can be detected in twisted bilayer graphene with a reasonable strength of the magnetic field. Based on the recursive tight-binding method, we have systematically studied the landau level dependence on the magnetic field as a function of the twist angle, with the underlying electronic structure ranging from the parabolic dispersion of Bernal bilayer graphene to the linear dispersion of decoupled graphene layers. The signature of transition is characterized by some low-lying landau levels in slightly twisted bilayer graphene, which are related to the flat bands induced in the layer decoupling process. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X36.00004: Multi-Domain Model of Bilayer Graphene with Broken Time Reversal Symmetry Gilad Ben-Shach, Amir Yacoby, Bertrand I. Halperin Recent experiments suggest strange new states for bilayer graphene at zero electric and magnetic fields~[1]. We consider recent models for the ground state of bilayer graphene with interactions~[2,3]. These models predict domains with non-zero local Hall conductance but an expected overall average Hall conductance of $\sigma_{xy}=0$, and should exhibit broken time reversal symmetry. We examine theoretical models for random four-probe measurements for various domain geometries in bilayer graphene at zero electric and magnetic fields. We find non-zero Hall conductance of magnitude dependent on domain geometry and network structure. \\[4pt] [1] Weitz, R.T., Allen, M.T., Feldman, B.E., Martin, J. Yacoby, A. Science. {\bf330}, 6005 (2010). \\[0pt] [2] Nandkishore, R., Levitov, LPhys. Rev. Lett. {\bf104}, 156803 (2010) \\[0pt] [3] Zhang F., Jung J., Fiete, G.A., Niu, Q., MacDonald, A.H. arXiv:1010.4003v1 (2010) [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X36.00005: Fractional quantum Hall effect in graphene: multicomponent states and tunable interactions Zlatko Papic, Dmitry Abanin, Nicolas Regnault, Mark Goerbig We study the fractional quantum Hall (FQH) states in graphene using exact diagonalization and taking into account the multicomponent degrees of freedom and the possibility of tuning the interaction potential. The recently observed graphene FQH state at a filling factor $\nu_G=1/3$ is found to be adiabatically connected to the 1/3 Laughlin state in the upper spin branch, with SU(2) valley-isospin ferromagnetic ordering and a completely filled lower spin branch. At the experimentally relevant values of the Zeeman field, however, the state possesses characteristic low-energy spin-flip excitations (different from the magneto-roton expected at large Zeeman fields) that may be unveiled in inelastic light-scattering experiments. We also discuss the possibility of realizing other Abelian and non-Abelian FQH states in graphene by modifying the effective interaction potential using a combination of insulating substrates. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X36.00006: Decomposition into half-integer quantum Hall numbers from Dirac cones in a graphene-related lattice model Haruki Watanabe, Hatsugai Yasuhiro, Hideo Aoki In field theory it is well-known that the Hall conductivity of a massive Dirac particle is equal to $1/2\,\,{\rm sgn}\, (m) $ (in units of $-e^2/h$) when the Fermi energy lies in the mass gap. By contrast, any lattice model must have an integer quantum Hall number when we consider noninteracting electrons, as dictated by the TKNN formula arising from the periodicity in the Brillouin zone. Usually, this is understood to be consistent with the fact that in lattice models such as graphene honeycomb lattice Dirac dispersions tend to appear in pairs (as dictated by the Nielsen-Ninomiya theorem for chiral cases), but this does obscure a half-integer contribution from each Dirac cone. To resolve this, here we show that it is possible to identify half-integer contributions by constructing a lattice model with systematically shifted Dirac points [1]. Edge-state spectrum also confirms this. \\[4pt] [1] H. Watanabe, Y. Hatsugai and H. Aoki, arXiv:1008.0130, to appear in Phys. Rev. B (R). [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X36.00007: Lattice Theory of Pseudospin Ferromagnetism in Bilayer Graphene: Competing Orders and Interaction Induced Quantum Hall States Jeil Jung, Fan Zhang, Allan MacDonald In mean-field-theory bilayer graphene's massive Dirac fermion model has a family of broken inversion symmetry ground states with charge gaps and spin-valley flavor dependent spontaneous charge transfers between layers. We use a lattice Hartree-Fock model to explore some of the physics which controls whether or not this type of broken symmetry state, which can be viewed as a pseudospin ferromagnet, occurs in nature. We find that inversion symmetry is still broken in the lattice model and estimate that transferred areal densities are $\sim 10^{-5}$ per carbon atom, that the associated energy gaps are $\sim 10^{-2} {\rm eV}$, the ordering condensation energies are $\sim 10^{-7} eV$ per carbon atom and the energy differences between competing orders at the neutrality point to be of the order of $\sim 10^{-9} eV$ per carbon atom. We explore the quantum phase transitions between different states induced by external magnetic fields and by externally controlled electric potential differences between the layers. We find, in particular, that in an external magnetic field coupling to spontaneous orbital moments favors broken time-reversal-symmetry states that have spontaneous quantized anomalous Hall effects. Our theory predicts a non monotonic behavior of the band gap as a function of electric field in qualitative agreement with recent experiments. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X36.00008: $\nu=0$ quantum Hall ferromagnet in a monolayer graphene: bulk ground states and charged edge excitations Maxim Kharitonov The $\nu=0$ quantum Hall state in a defect-free graphene sample is studied within the framework of the quantum Hall ferromagnetism. Starting from the low-energy electron Hamiltonian, in which all allowed by symmetry sublattice- and valley-anisotropic terms due to the Coulomb and leading electron-phonon interactions are taken into account, the energy functional for the quantum Hall ferromagnet is derived. Paying special attention to the signs of anisotropies, we find that the anisotropy due to the repulsive Coulomb interactions always favors the spin-polarized pseudospin-singlet state. On the other hand, the anisotropy due to the phonon-mediated attractive interactions favors the $XY$ pseudospin-polarized spin-singlet state. It is then demonstrated that, in the case of the $XY$ pseudospin bulk order and armchair boundary, the Skyrmion-type charged excitations are gapped at the edge, which makes the whole sample insulating. These findings suggest that the experimentally observed insulating $\nu=0$ state is an $XY$ pseudospin ferromagnet favored by electron-phonon interactions. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X36.00009: Disorder-induced magnetooscillations in bilayer graphene at high bias Mikhail Raikh, Vagharsh Mkhitaryan Energy spectrum of biased bilayer graphene near the bottom has a ``Mexican-hat''-like shape. For the Fermi level within the Mexican hat we demonstrate that, apart from conventional magnetooscillations which vanish with temperature, there are additional magnetooscillations of capacitance and conductance which are weakly sensitive to temperature. These oscillations are also insensitive to a long-range disorder. Their period in magnetic field scales with bias, $V$, as $V^2$. The origin of these oscillations is the disorder-induced scattering between electron-like and hole-like Fermi-surfaces, specific for Mexican hat. At low temperatures, oscillations transform into quantum Hall plateaus in $\sigma_{xy}$. We predict that evolution of $\sigma_{xy}$ with magnetic field is highly non-trivial. This is because the contributions to $\sigma_{xy}$ from electron-like and hole-like Landau levels have opposite signs. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X36.00010: Dynamical scaling analysis of the optical Hall conductivity in the quantum Hall regime Takahiro Morimoto, Yshai Avishai, Hideo Aoki We study the optical Hall conductivity $\sigma_{xy}(\varepsilon_F, \omega)$ in two-dimensional electron gas (2DEG) and in graphene in the quantum Hall regime, which is measurable by the Faraday rotation. It was previously demonstrated that both conductivities retain their plateau structure at finite frequency, up to the optical frequency regime. Physically, the robustness of the plateau structure in the ac optical regime can be attributed to the localization of electrons in the QHE. To quantify this picture, a dynamical scaling analysis of $\sigma_{xy}(\varepsilon_F, \omega)$ is performed for the $n=0$ Landau level in graphene as well as for the conventional quantum Hall system. This analysis examines whether the system size dependence of $\sigma_{xy}(\varepsilon_F, \omega)$ can be captured with a universal scaling function that involves the localization exponent $\nu$ and the dynamic critical exponents $z$. Based on exact diagonalization of these systems with potential disorder, employing the Kubo formula, it is shown that $\sigma_{xy}(\varepsilon_F,\omega)$ obeys a well-defined dynamical scaling behavior. For both systems, the static exponents $\nu$ are similar and the dynamical exponents $z$ are found to be $\approx 2$. Our quantitative analysis indicates that the plateau structure in the ac Hall conductivity should be robust and experimentally testable in the THz regime. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X36.00011: Disorder Effect of Quantum Anomalous Hall effect in Graphene Zhenhua Qiao, Shengyuan A. Yang, Wang-Kong Tse, Yugui Yao, Jian Wang, Qian Niu We investigate the possibility of realizing quantum anomalous Hall effect in graphene. We show that a bulk energy gap can be opened in the presence of both Rashba spin-orbit coupling and an exchange field. We calculate the Berry curvature distribution and find a nonzero Chern number for the valence bands and demonstrate the existence of gapless edge states. Inspired by this finding, we also study, by first-principles method, a concrete example of graphene with Fe atoms adsorbed on top, obtaining the same result. We further study the disorder effect of this quantum anomalous Hall effect and show how this state is localized in the presence of strong disorders. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X36.00012: Multiferro\"ic-like behavior in the quantum Hall ferromagnetic states of a graphene bilayer Rene Cote, Jules Lambert In a quantizing magnetic field, a graphene bilayer has an octet of degenerate states in the Landau level $N=0$. An electron in this level must be described by three quantum numbers: its spin, its valley index $K$ or $K'$ and an orbital quantum number $n=0,1$. In the Hartree-Fock approximation, the ground states of the graphene bilayer at integer filling factors $\nu \in [-3,4]$ can be described as different kinds of quantum Hall ferromagnets (QHF's) with finite interlayer, inter- orbital, or inter-spin coherence. In this talk, we present the phase diagram of the two-dimensional electron gas (2DEG) in $N=0 $ when the filling factor or a finite interlayer voltage, $\Delta_{B}$ is varied. A finite density of $\it electric$ dipoles is either spontaneously present in the QHF phases with inter-orbital coherence or can be generated by applying an external electric field in the plane of the layers. We show that by changing the strength of this electric field, and so the coupling with the electric dipoles, it is possible to control the degree of $\it magnetic$ polarisation of the 2DEG. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X36.00013: Longitudinal Conductivity of Bilayer Graphene in the Integer Quantum Hall Regime Rohit Hegde, Allan MacDonald We investigate the frequency dependent conductivity of disordered bilayer graphene near neutral filling, in a strong magnetic field. Absent Zeeman coupling, and with two independent valleys, a graphene bilayer's lowest Landau level is eightfold-degenerate, comprising two Landau orbitals of equal energy. Its spectral properties are altered by an inter-layer bias potential, which can open a gap between the constituent orbitals. We establish the dependence of the one and two-particle disorder-averaged Greens' functions on inter-layer bias, and show that the longitudinal conductivity exhibits the signature of disorder-induced Landau orbital mixing. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X36.00014: Transverse Thermoelectric Conductivity of Bi-layer graphene in quantum Hall Regime Wei-Li Lee, Chang-Ran Wang, Wen-Sen Lu We performed electric and thermoelectric transport measurements of bilayer graphene in a magnetic field up to 15 Tesla. The transverse thermoelectric conductivity $\rm\alpha_{xy}$, determined from four transport coefficients, attains a peak value of $\rm\alpha_{xy, peak}$ whenever chemical potential lies in the center of a Landau level. The temperature dependence of $\rm\alpha_{xy, peak}$ is dictated by the disorder width $\rm W_L$. For $\rm k_BT/W_L\leq$0.2, $\rm\alpha_ {xy, peak}$ is nominally linear in temperature, which gives $\rm\alpha_{xy,peak}/T=0.19 \pm 0.03 n A/K^2$ independent of the magnetic field, temperature and Landau Level index. At $\rm k_BT/W_L\geq$0.5, $\rm\alpha_{xy, peak}$ saturates to a value close to the predicted universal value of $\rm 4\times(ln2) k_Be/h$ according to the theory of Girvin and Jonson. We remark that an anomaly is found in $\rm\alpha_{xy}$ near the charge neutral point, similar to that in single-layer graphene. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X36.00015: Interface Landau levels in graphene monolayer-bilayer junction Mikito Koshino, Takeshi Nakanishi, Tsuneya Ando Electronic structure of graphene monolayer-bilayer junction in a magnetic field is studied within an effective-mass approximation. The energy spectrum is characterized by interface Landau levels, i.e., the locally flat bands appearing near the boundary region, resulting in a series of characteristic peaks in the local density of states. Their energies are independent of boundary types such as zigzag or armchair. In the atomic scale, the local density of states shows a Kekul\'{e} pattern due to the valley mixing in the armchair boundary, while does not in the zigzag boundary. [Preview Abstract] |
Session X37: Focus Session: Graphene Structure, Dopants, and Defects: Magnetism and Nanoribbons
Sponsoring Units: DMPChair: Young-Woo Son, Korea Institute for Advanced Study
Room: C146
Thursday, March 24, 2011 2:30PM - 2:42PM |
X37.00001: Oxygen- and Sulfur- driven Ferromagnetism in Graphitic Fragments: Ab-Initio Study Ivan Naumov, Yakov Kopelevich, Alexander Bratkovsky We study the origins of high-temperature ferromagnetic behavior in graphite by means of unbiased ab-initio calculations and compare them with our data. The experimental results show that oxygen/sulfur-induced edges of graphitic fragments (via unzipping effect) play an essential role in this phenomenon, and that the finite magnetic moment appears if edges in a graphitic ribbon are occupied asymmetrically by either oxygen or sulphur. In particular, our ab-initio calculations performed within the LDA and GGA approximations showed that in the case of pure graphene ribbon, its zig-zag edge carbon atoms carry large magnetic moment ($\sim $1 $\mu _{B}$/C). In an oxidized or sulfurized graphene, however, the magnetic moment at the edge with absorbed atoms gets considerably reduced, leading to effective ferromagnetic (more precisely, ferri-magnetic) behavior of the sample. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X37.00002: Defect Induced Resonances and Magnetic Patterns in Graphene Yi chen Chang We investergate the effects of point and line defects in monolayer graphene was investigated within the framework of the Hubbard model, using a self-consistent mean field theory. These defects are found to induce characteristic patterns into the electronic density of states and cause non-uniform distributions of magnetic moments in the vicinity of the impurity sites. Specifically, defect induced resonance bound states in the local density of states are observed at energies close to the Dirac points. The magnitudes of the frequencies of these resonance states are shown to decrease with the strength of the scattering potential, whereas their amplitudes decay algebraically with increasing distance from the defect. Furthermore, non-trivial impurity induced magnetic patterns are observed in the presence of line defects: zigzag line defects are found to introduce stronger-amplitude magnetic patterns than single line defect and armchair line defects. When the scattering strength of these topological defects is increased, the induced patterns of magnetic moments become more strongly localized. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X37.00003: Theory of Magnetic Edge States in Chiral Graphene Nanoribbons Rodrigo Capaz, Oleg Yazyev, Steven Louie Using a model Hamiltonian approach including electron Coulomb interactions, we systematically investigate the electronic structure and magnetic properties of chiral graphene nanoribbons. We show that the presence of magnetic edge states is an intrinsic feature of any smooth graphene nanoribbons with chiral edges, and discover a number of structure-property relations. Specifically, we describe how the edge-state energy gap, zone-boundary edge-state energy splitting, and magnetic moment per edge length depend on the nanoribbon width and chiral angle. The role of environmental screening effects is also studied. Our results address a recent experimental observation of signatures of magnetic ordering at smooth edges of chiral graphene nanoribbons and provide an avenue towards tuning their properties via the structural and environmental degrees of freedom. This work was supported by National Science Foundation Grant No. DMR10-1006184, the U.S. Department of Energy under Contract No. DE-AC02-05CH11231 and the ONR MURI program. RBC acknowledges financial support from Brazilian agencies CNPq, FAPERJ and INCT-Nanomateriais de Carbono. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X37.00004: Spin-orbit interactions in graphene nanoribbons : Effects of the edge profile Jun-Won Rhim, Kyungsun Moon In graphene, it has been shown by Kane and Mele that the spin orbit coupling (SOC) connects the Dirac particles to the low- lying $p_x$ and $p_y$ orbitals so that the quantum spin Hall effect(QSHE) arises at the edges of the graphene. Their theory has drawn considerable attention as a realization of Haldane's idea of quantum Hall effect without magnetic field and as a trigger for the surging field of topological insulator. In the work, we study the band structure of the zigzag nanoribbons with the spin-orbit interaction and argue that the role of graphene edge should be considered more carefully since the realization of the QSHE is found to be largely dependent on the edge profile such as the kinds of molecules passivated. When the edge $p_x$, $p_y$ and $s$ orbitals are dangling without any passivation, the Dirac states at the edges seem to be no longer chiral for each spin species and the QSHE is not guaranteed to occur. We notice that upon the hydrogen passivation at the edges, the spin filtered chiral edge states become available. We will explain that these are due to the interaction between $\pi$-edges states and $\sigma$-edge states. The similar calculations are also performed for the armchair nanoribbons and compared with those of zigzag nanoribbons. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X37.00005: Quantum Monte Carlo Study of Edge Magnetism in Nanoribbons of Graphene Z.Y. Meng, H. Feldner, T.C. Lang, S. Wessel, A. Honecker, F. Assaad We study the electronic and magnetic properties of graphene nanoribbons, employing projective quantum Monte Carlo simulations within the Hubbard model description of electrons in graphene. We also compare our numerical results to a self-consistent mean field approximation in the weak coupling regime. Motivated by recent STM experiments about electronic resonance around atomic vacancies on multilayer graphene and graphene nanoisland, we in particular examine the local density of states throughout the sample. From this, we verify that interacting zig-zag ribbons develop an insulating ground state with a finite single particle gap from the localized edge modes observed in the non-interacting limit. In addition, we observe a drastic increase of the spin-spin correlation length along the zig-zag edge with the ribbon width. Effectively, on our finite samples ferromagnetic edges appear already for moderately wide zig-zag ribbons. This ferromagnetism is accompanied by an essentially gapless edge magnon mode, that we identify in the spin excitation spectrum. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X37.00006: First principles study of edge effects in electronic structures of graphene nanoflakes and nanoribbons Chengbo Han, Wenchang Lu, Jerry Bernholc Graphene is a promising material for future nanoelectronics. Understanding of the edge effects on the electronic structure of graphene nanoflakes and nanoribbons is important for its nanoscale applications. Using the real space multigrid method within density functional theory, we systematically simulate STM images of nanoflakes and nanoribbons with both zigzag and armchair edges. Our results explain several STM patterns seen in experiments [1], such as triangular and hexagonal lattices for different shapes of flakes. We also find that localization of edge states in zigzag flakes depends on the interior angle between two edges. Furthermore, we show that the influence of Si(001)-2x1-H substrate on the local density of states of graphene nanoflakes is not significant when the graphene layer is 0.3 nm above the substrate.\\[4pt] [1] K. A. Ritter and J. W. Lyding, Nature Materials 8, 235 (2009). [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 4:18PM |
X37.00007: Spin states in graphene quantum dots Invited Speaker: Graphene quantum dots [1,2], douple dots [3], rings [4] and nanoribbons [5] have been fabricated by electron beam lithography and dry etching. The orbital [1] properties of graphene quantum dots have been investigated in perpendicular magnetic fields and the details of the electron-hole crossover in graphene leads to a situation where electron (hole) states move down (up) in magnetic field opposite to what has been observed in standard semiconductor based quantum dots. Graphene quantum dots are thought to be good candidates for spin-based quantum information processing since spin-orbit interactions and hyperfine coupling are both expected to be weak. We investigated graphene quantum dots in the single-level transport regime in in-plane magnetic fields where orbital effects are expected to have a minor effect [6]. The g-factor is found to be $g\approx 2$ and the spin filling sequence of orbital levels can be understood in view of the strength of the exchange interaction which is independent of carrier density in graphene. \\[4pt] [1] J. Guttinger, C. Stampfer, F. Libisch, T. Frey, J. Burgdoerfer, T. Ihn, K. Ensslin, Phys. Rev. Lett. 103, 046810 (2009) \\[0pt] [2] T. Ihn, J. Guttinger, F. Molitor, S. Schnez, E. Schurtenberger, A. Jacobsen, S. Hellmuller, T. Frey, S. Droscher, C. Stampfer, and K. Ensslin, Materials Today 13, 44 (2010) \\[0pt] [3] F. Molitor, H. Knowles, S. Droscher, U. Gasser, T. Choi, P. Roulleau, J. Guttinger, A. Jacobsen, C. Stampfer, K. Ensslin and T. Ihn, Europhys. Lett. 89, 67005 (2010) \\[0pt] [4] M. Huefner, F. Molitor, A. Jacobsen, A. Pioda, C.Stampfer, K. Ensslin and T. Ihn, N. J. of Phys. 12, 043054 (2010) \\[0pt] [5] C. Stampfer, J. Guttinger, S. Hellmuller, F. Molitor, K. Ensslin, and T. Ihn, Phys. Rev. Lett. 102, 056403 (2009) \\[0pt] [6] J. Guttinger, T. Frey, C. Stampfer, T. Ihn, and K. Ensslin, Phys. Rev. Lett. 105, 116801 (2010) [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X37.00008: Complex edge effects in graphene nanoribbons due to hydrogenation Biplab Sanyal, Sumanta Bhandary, Mikhail Katsnelson, Olle Eriksson We have performed density-functional calculations as well as employed a tight-binding theory, to study the effect of hydrogenation of zigzag graphene nanoribbons (ZGNR). We show that each edge C atom bonded with 2 H atoms open up a gap and magnetism collapses for small widths of the nanoribbon. However, a re-entrant magnetism accompanied by a metallic electronic structure is observed from eight rows and thicker nanoribbons. The electronic structure and magnetic state are quite complex for this type of termination, with $sp^{3}$ bonded edge atoms being nonmagnetic whereas the nearest neighboring atoms are metallic and magnetic. We have also evaluated the phase stability of several thicknesses of ZGNR and demonstrate that $sp^{3}$ bonded edge atoms with 2 H atoms at the edge can be stabilized over 1 H atom terminated edge at high temperatures and pressures. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X37.00009: Magnetism in bulk and finite size graphene multilayers and its effect on the band gaps Bhagawan Sahu, Hongki Min, Sanjay Banerjee, Allan MacDonald In this talk, we will address the edge state magnetism and the resulting modulation of band gaps induced by quantum confinements in multilayer graphene ribbons and flakes. The magnetism arising from random point defects such as vacancies in bulk graphene layers will also be presented. The robustness of magnetism with respect to the edge disorder and the saturating agents in finite size graphene layers and with respect to the defect concentrations in bulk graphene layers will be discussed. A numerical approach based on density functional theory which uses plane-wave basis set and pseudopotentials for ion-electron interactions will be used for elucidating the complex interplay of magnetism, external electric field applied perpendicular to the layers and the resulting band gaps. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X37.00010: Exploring the Structure of Graphene Nanoribbons Using Scanning Tunneling Microscopy Yen-Chia Chen, Juanjuan Feng, Chenggang Tao, Liying Jiao, Xiaowei Zhang, Oleg Yazyev, Rodrigo Capaz, Alex Zettl, Steven Louie, Hongjie Dai, Michael Crommie The confined dimension and edges of graphene nanoribbons (GNRs) are predicted to result in novel magnetic edge states and tunable energy gaps. Such properties should be strongly dependent on GNR nanoscale structure. Here we report a scanning tunneling microscopy (STM) study of the structure of GNRs derived from unzipped carbon nanotubes that are deposited onto different substrates. These GNRs are found to have different chiralities and widths, and show some unexpected geometrical structure near the edges. We will also present new results obtained from GNRs with disordered edges. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X37.00011: STM and STS studies of CVD grown graphene nanoribbons Xiaoting Jia, Minghu Pan, Sreekar Bhaviripudi, Vincent Meunier, Jing Kong, Mildred Dresselhaus Graphene nanoribbons (GNRs) are quasi one dimensional structures which have unique transport properties, and have a potential to open a bandgap at small ribbon widths. They have been extensively studied in recent years due to their high potential for future electronics applications. We have experimentally found some GNRs in our CVD grown graphene layers. In this work, we investigated the morphology and electronic properties of the GNRs on top of a graphene layer transferred to a SiO$_{2}$ substrate by using scanning tunneling microscopy. Our results suggest that these GNRs have a surprisingly high crystallinity with one side folded. Atomic resolution images were obtained on the folded layer and the bottom layer of the GNR, which enables clear identification of the chirality for both layers. By combining with theoretical modeling we conclude that a (5,7) line defect exists at the zone of maximum curvatures to help reducing the strain energy of the folding. Low temperature spectroscopic measurements suggest that different electronic states may exist at GNR edges, when compared to the ribbon interior regions. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X37.00012: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X37.00013: Large intrinsic energy band gaps in annealed nanotube-derived graphene nanoribbons J. Haruyama, T. Shimizu, D.C. Marcano, D.V. Kosinkin, J.M. Tour, K. Hirose, K. Suenaga The usefulness of graphene for electronics is diminished by an absent energy bandgap. While graphene nanoribbons have non-zero bandgaps, lithographic fabrication methods introduce defects which decouple the bandgap from electronic properties and compromise performance [1]. Here, we present direct measurements of a large intrinsic energy bandgap of approximately 50 meV in 100 nm-width level nanoribbons fabricated by high-temperature annealing of unzipped carbon nanotubes [2]. The activation energy is seven times greater than those in [1], and is close to the width of the transport gap in the differential conductance. This similarity suggests that the activation energy is in fact the intrinsic bandgap. High-resolution TEM and Raman spectroscopy, along with an absence of hopping conductance and stochastic charging effects, suggest a low defect density. [1] M.Y. Han, P. Kim et al., PRL 104, 056801 (2010) [2] J.Haruyama, J.M.Tour, et al., Nature Nanotech. (December 2010) [Preview Abstract] |
Session X38: Focus Session: Non-Equilibrium Insights into Single Molecules and Cell Function I
Sponsoring Units: DCP DBPChair: Norbert Scherer, University of Chicago
Room: A130/131
Thursday, March 24, 2011 2:30PM - 3:06PM |
X38.00001: The Statistical Mechanics of Trajectories and Weights: Applications to Gene Expression Invited Speaker: Many fascinating questions concerning the behavior of systems ranging from chemical reaction patterns to the patterns of gene expression in living systems do not concern their terminal states, but rather the various microscopic trajectories connecting those states. Some of the most intriguing examples of these kinds of phenomena center on the time evolution of the many molecular machines that populate living cells. Motivated by studies of the time evolution of gene expression, this talk will review both classic approaches to time evolution using rate equations (but couched in the language of trajectories and weights) and more controversial ideas based upon the principle of maximum entropy. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:42PM |
X38.00002: Challenges in Characterizing and Controlling Complex Cellular Systems Invited Speaker: Multicellular dynamic biological processes such as developmental differentiation, wound repair, disease, aging, and even homeostasis can be represented by trajectories through a phase space whose extent reflects the genetic, post-translational, and metabolic complexity of the process - easily extending to tens of thousands of dimensions. Intra- and inter-cellular sensing and regulatory systems and their nested, redundant, and non-linear feed-forward and feed-back controls create high-dimensioned attractors in this phase space. Metabolism provides free energy to drive non-equilibrium processes and dynamically reconfigure attractors. Studies of single molecules and cells provide only minimalist projections onto a small number of axes. It may be difficult to infer larger-scale emergent behavior from linearized experiments that perform only small amplitude perturbations on a limited number of the dimensions. Complete characterization may succeed for bounded component problems, such as an individual cell cycle or signaling cascade, but larger systems problems will require a coarse-grained approach. Hence a new experimental and analytical framework is needed. Possibly one could utilize high-amplitude, multi-variable driving of the system to infer coarse-grained, effective models, which in turn can be tested by their ability to control systems behavior. Navigation at will between attractors in a high-dimensioned dynamical system will provide not only detailed knowledge of the shape of attractor basins, but also measures of underlying stochastic events such as noise in gene expression or receptor binding and how both affect system stability and robustness. Needed for this are wide-bandwidth methods to sense and actuate large numbers of intracellular and extracellular variables and automatically and rapidly infer dynamic control models. The success of this approach may be determined by how broadly the sensors and actuators can span the full dimensionality of the phase space. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 4:18PM |
X38.00003: Maximizing efficiency of molecular machines Invited Speaker: I will discuss how to locate protocols that minimize dissipation in non-equilibrium, molecular scale processes, adapting ideas from finite-time thermodynamics. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:54PM |
X38.00004: Autonomous Boolean models for logic, timing, and stability in regulatory networks Invited Speaker: The dynamics of gene expression in a cell is controlled by a dizzying array of biochemical processes. Natural selection, however, has created regulatory systems with a level of logical organization that can be modeled without detailed knowledge of the biochemistry. In cases where graded responses are not relevant, autonomous Boolean network (ABN) models can effectively represent the logic of gene regulation. These are models in which Boolean logic governs the output value of each node and the timing of updates is determined according to delay parameters associated with each link. An advantage of ABNs over synchronous or random asynchronous Boolean networks is that noise associated with molecular concentrations or transport times can be represented through fluctuations in the timing of updates. We have used ABN models to investigate the stability of oscillations in a model of transcriptional oscillations in yeast and the parameter constraints in a model of segment polarity maintenance in the fly embryo, and also to characterize chaotic dynamics observed in a free--running digital electronic circuit. The yeast study highlights architectural and dynamical features of oscillators that rely on pulse transmission rather than a frustrated feedback loop; the fly study reveals timing constraints that are hidden in ODE models; and the electronics study shows that Boolean chaos can occur if and only if time delays are history dependent. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X38.00005: Maximum Caliber Analysis of Ion-Channel Gating Roy Campbell The principle of maximum caliber, MaxCal, is a generalization to nonequilibrium statistical mechanics of the principle of maximum entropy, MaxEnt. E. T. Jaynes introduced the MaxEnt approach to equilibrium statistical mechanics in 1957 and its MaxCal generalization in 1980. MaxCal has recently been used to derive dynamical laws of transport, analyze single particle two-state dynamics, and study few state models of non-equilibrium processes. We use MaxCal to analyze ion-channel gating data and make logical inferences concerning the underlying dynamics. The inferred trajectory probabilities are used to calculate the fluctuations responsible for channel noise. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X38.00006: Driving denaturation: Nanoscale thermal transport as a probe of DNA melting Yonatan Dubi, Kirill Velizhanin, Chih-Chun Chien, Michael Zwolak The microscopic dynamics of DNA denaturation have long been a subject of intense study but many aspects of this phenomenon remain poorly understood. Experiments typically measure the degree of denaturation versus temperature which, unfortunately, introduces only a relatively weak constraint: Although many existing models reproduce this denaturation transition well, they give, e.g., incorrect time scales for fluctuations in base pair unbinding. Here, we propose a critical test of DNA models based on driving DNA out of thermal equilibrium via two heat reservoirs. Contrary to what might be expected, we find that the preeminent model of denaturation predicts the thermal conductance to increase substantially as DNA melts. Furthermore, we show that different models can possess qualitatively different thermal transport properties. Measuring the thermal conductance of DNA will thus shed new light on the nonlinear physics of this important molecule and may lead to novel thermal technologies, such as a DNA thermal switch. [Preview Abstract] |
Session X39: Biomechanics: From Subcellular to Multicellular Scales
Sponsoring Units: DBP DCOMPChair: Gabor Forgacs, University of Missouri
Room: A124/127
Thursday, March 24, 2011 2:30PM - 3:06PM |
X39.00001: Simple, Voltage Dependent Statistics Governing Cell-Substrate Contact Times Invited Speaker: The distribution of contact times between a nanofilament-based contact sensor and individual pseudopods of \textit{D. discoideum} have been measured as a function of voltage applied to the filament. The distributions are well described by exponential distributions. The average duration of the pseudopod-filament contact was found to increase across the +20 mV to -50 mV range of filament-voltages. These results are consistent with the predictions of a simple model based on rather general considerations of energy usage by the cell. This analysis indicates that the exponential functionality (of the contact time distributions) results from competition between a large number of cellular processes for the available energy. The evolution of these distributions across the +20 mV to -50 mV voltage range suggests that the negatively biased filament enhances adhesion to the filament by activating additional adhesion molecules to bind to its surface. These results will be discussed in the context of recent findings on the coupling of voltage gated ion channels and cellular adhesion. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:42PM |
X39.00002: Multiscale modeling of the dynamics of multicellular systems Invited Speaker: Describing the biomechanical properties of cellular systems, regarded as complex highly viscoelastic materials, is a difficult problem of great conceptual and practical value. Here we present a novel approach, referred to as the Cellular Particle Dynamics (CPD) method, for: (i) quantitatively relating biomechanical properties at the cell level to those at the multicellular and tissue level, and (ii) describing and predicting the time evolution of multicellular systems that undergo biomechanical relaxations. In CPD cells are modeled as an ensemble of cellular particles (CPs) that interact via short range contact interactions, characterized by an attractive (adhesive interaction) and a repulsive (excluded volume interaction) component. The time evolution of the spatial conformation of the multicellular system is determined by following the trajectories of all CPs through integration of their equations of motion. Cell and multicellular level biomechanical properties (e.g., viscosity, surface tension and shear modulus) are determined through the combined use of experiments and theory of continuum viscoelastic media. The same biomechanical properties are also ``measured'' computationally by employing the CPD method, the results being expressed in terms of CPD parameters. Once these parameters have been calibrated experimentally, the formalism provides a systematic framework to predict the time evolution of complex multicellular systems during shape-changing biomechanical transformations. By design, the CPD method is rather flexible and most suitable for multiscale modeling of multicellular system. The spatial level of detail of the system can be easily tuned by changing the number of CPs in a cell. Thus, CPD can be used equally well to describe both cell level processes (e.g., the adhesion of two cells) and tissue level processes (e.g., the formation of 3D constructs of millions of cells through bioprinting). [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X39.00003: Theoretical estimation of the breakage intensity of microtubules at resonance using ultrasound waves Abdorreza Samarbakhsh, Jack Tuszynski Microtubules (MTs) are protein filaments forming a major part of the cytoskeleton of all eukaryotic cells which directly contribute to the process of cell division by forming mitotic spindles and providing force for the segregation of chromosomes. In this work first we show the resonance condition for MTs subject to ultrasound wave by solving the beam equation for MT analytically. Then we estimate the required minimum intensity of the ultrasound at the location of the MT in order to break it. We have shown that this intensity is of the order of 100KW per unit of area which corresponds to 170 dB. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X39.00004: Flexural Rigidity of MCF-7 Microtubules Measured from Thermal Fluctuations in Shape Mitra Shojania Feizabadi, Kiryako Mutafopulos, Adam Behr Microtubules play a key role in the mechanical and elastic properties of eukaryotic cells. For this reason, measuring the flexural rigidity of bovine brain microtubules have been extensively investigated through different methods of measurement. Beta tubulin isotypes, a noticeable trait to consider as we transfer from mammalian neural microtubules to mammalian non-neural microtubules, are assembled differently in distributions among various types of microtubules. Different studies have shown that microtubules made from different beta-tubulin isotypes express unique polymerization and dynamic behavior. This study focuses on measuring mechanical properties of one of non-neural microtubules, MCF-7. We will discuss the structure differences between brain bovine microtubules and MCF-7, along with the rigidity of single microtubules polymerized from MCF-7 tubulin through monitoring the curvature of microtubule due to thermal fluctuations. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X39.00005: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X39.00006: Persistence Length of Stable Microtubules Taviare Hawkins, Matthew Mirigian, M. Selcuk Yasar, Jennifer Ross Microtubules are a vital component of the cytoskeleton. As the most rigid of the cytoskeleton filaments, they give shape and support to the cell. They are also essential for intracellular traffic by providing the roadways onto which organelles are transported, and they are required to reorganize during cellular division. To perform its function in the cell, the microtubule must be rigid yet dynamic. We are interested in how the mechanical properties of stable microtubules change over time. Some ``stable'' microtubules of the cell are recycled after days, such as in the axons of neurons or the cilia and flagella. We measured the persistence length of freely fluctuating taxol-stabilized microtubules over the span of a week and analyzed them via Fourier decomposition. As measured on a daily basis, the persistence length is independent of the contour length. Although measured over the span of the week, the accuracy of the measurement and the persistence length varies. We also studied how fluorescently-labeling the microtubule affects the persistence length and observed that a higher labeling ratio corresponded to greater flexibility. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X39.00007: Modeling actin waves in dictyostelium cells Vaibhav Wasnik, Ranjan Mukhopadhyay Actin networks in living cells demonstrate a high capacity for self-organization and are responsible for the formation of a variety of structures such as lamellopodia, phagocytic cups, and cleavage furrows. Recent experiments have studied actin waves formed on the surface of dictyostelium cells that have been treated with a depolymerizing agent. These waves are believed to be physiologically important, for example, for the formation of phagocytic cups. We propose and study a minimal model, based on the dendritic nucleation of actin polymers, to explain the formation of these waves. This model can be extended to study the dynamics of the coupled actin-membrane system. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X39.00008: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X39.00009: Force Generated by Actin Array Konstantinos Tsekouras, David Lacoste, Kirone Mallick, Jean-Francois Joanny We study a theoretical model for a group of parallel filaments growing against a barrier held by a constant force. An array of N filaments nucleate on a fixed surface and grow towards a rigid barrier which is held in place by a constant force. Filaments are coupled only by mechanical contact against the barrier. We obtain the filament density distribution in terms of the distance from the barrier, and force-velocity curves. We apply our model to the case of an array of actin filaments. All results are validated by extensive Monte-Carlo simulations. For a small value of N we find the stall force to be N times the stall force of a single filament ($f_{stall}\approx Nf_{stall}^1$). For large N we find that the velocity \textit{appears} to be considerably smaller, an effect due to its exponential decrease as the theoretical stall force is approached. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X39.00010: BSDB: the Biomolecule Stretching Database Marek Cieplak, Mateusz Sikora, Joanna I. Sulkowska, Bartlomiej Witkowski Despite more than a decade of experiments on single biomolecule manipulation, mechanical properties of only several scores of proteins have been measured. A characteristic scale of the force of resistance to stretching, $F_{max}$, has been found to range between $\sim$10 and 480 pN. The Biomolecule Stretching Data Base (BSDB) described here provides information about expected values of $F_{max}$ for, currently, 17 134 proteins. The values and other characteristics of the unfolding proces, including the nature of identified mechanical clamps, are available at www://info.ifpan.edu.pl/BSDB/. They have been obtained through simulations within a structure-based model which correlates satisfactorily with the available experimental data on stretching. BSDB also lists experimental data and results of the existing all-atom simulations. The database offers a Protein-Data-Bank-wide guide to mechano-stability of proteins. Its description is provided by a forthcoming Nucleic Acids Research paper. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X39.00011: The nonequilibrium thermodynamics and kinetics of focal adhesion dynamics Krishna Garikipati, Joseph Olberding, Michael Thouless, Ellen Arruda We consider a focal adhesion (FA) to be made up of molecular complexes consisting of ligands, integrins, and associated plaque proteins. Free energy changes drive the binding and unbinding of these complexes, thus controlling the FA's dynamic modes of growth, treadmilling and resorption via the following mechanisms: (\romannumeral 1) work done during the addition of molecular complexes, (\romannumeral 2) the chemical free energy of addition of a molecular complex, (\romannumeral 3) the elastic free energy of deformation of FAs and the cell membrane, and (\romannumeral 4) the work done on a molecular conformational change. We have developed a treatment of FA dynamics as a nonlinear rate process driven by out-of-equilibrium thermodynamic driving forces, and modulated by kinetics. The mechanisms governed by the above four effects allow FAs to exhibit a rich variety of behavior, predicting growth, treadmilling and resorption. Treadmilling requires symmetry breaking between the ends of the focal adhesion, and is achieved by driving force (\romannumeral 1) above. In contrast, the remaining mechanisms cause symmetric growth or resorption. These findings hold for a range of conditions: temporally-constant force or stress, and for spatially-uniform and non-uniform stress distribution over the FA. This treatment of FA dynamics can be coupled with models of cytoskeleton dynamics and contribute to the understanding of cell motility. [Preview Abstract] |
Session X40: Biological Networks and Systems Biology
Sponsoring Units: DBPChair: Luis Cruz Cruz, Drexel University
Room: A122/123
Thursday, March 24, 2011 2:30PM - 2:42PM |
X40.00001: Computer Simulations of Loss of Organization of Neurons as a Model for Age-related Cognitive Decline Luis Cruz, Elene Fengometidis, Frank Jones, Srinivas Jampani In normal aging, brains suffer from progressive cognitive decline not linked with loss of neurons common in neurodegenerative disorders such as Alzheimer's disease. However, in some brain areas neurons have lost positional organization specifically within microcolumns: arrays of interconnected neurons which may constitute fundamental computational units in the brain. This age-related loss of organization, likely a result of micron-sized random displacements in neuronal positions, is hypothesized to be a by-product of the loss of support from the surrounding medium, including dendrites. Using a dynamical model applied to virtual 3D representation of neuronal arrangements, that previously showed loss of organization in brains of cognitively tested rhesus monkeys, the relationship between these displacements and changes to the surrounding dendrite network are presented. The consequences of these displacements on the structure of the dendritic network, with possible disruptions in signal synchrony important to cognitive function, are discussed. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X40.00002: Coupled feedback loops govern bistability properties in gene networks Abhinav Tiwari, Oleg Igoshin Positive feedback is a necessary component for network bistability - the simplest design being a positive autoregulatory circuit. Then why some biological systems have multiple feedback loops? We hypothesize that the presence of multiple additively or multiplicatively coupled feedback loops affects the net cooperativity of the system, thereby influencing the possibility of bistability. We find that additively coupled feedback loops in the MprAB-SigE-RseA network in mycobacteria do not lead to bistability. Only the inclusion of post-translational regulation of SigE by RseA makes the system robustly bistable. In general we find that if two one-feedback networks are individually monostable, then only multiplicative coupling can generate bistability in the combined circuit. We analytically perform pair-wise controlled comparisons between the autoregulation circuit, additively and multiplicatively coupled two-gene circuits that reveal neither of the circuits has an advantage with regards to bistability range. We numerically validate our results by employing Monte Carlo parameter sampling for the comparisons. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X40.00003: Better Bet-Hedging with coupled positive and negative feedback loops Jatin Narula, Oleg Igoshin Bacteria use the phenotypic heterogeneity associated with bistable switches to distribute the risk of activating stress response strategies like sporulation and persistence. However bistable switches offer little control over the timing of phenotype switching and first passage times (FPT) for individual cells are found to be exponentially distributed. We show that a genetic circuit consisting of interlinked positive and negative feedback loops allows cells to control the timing of phenotypic switching. Using a mathematical model we find that in this system a stable high expression state and stable low expression limit cycle coexist and the FPT distribution for stochastic transitions between them shows multiple peaks at regular intervals. A multimodal FPT distribution allows cells to detect the persistence of stress and control the rate of phenotype transition of the population. We further show that extracellular signals from cell-cell communication that change the strength of the feedback loops can modulate the FPT distribution and allow cells even greater control in a bet-hedging strategy. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X40.00004: Attractor Distribution in Random Biological Networks Described by ODEs and Diminished Order-Chaos Transition Zhiyuan Li, Chao Tang Ordinary Differential Equations (ODEs) are widely used to model biological network in a continuous manner. The state of an ODE system after infinitely long time is called attractor, which indicates the ultimate fate of the corresponding biological system. Even though the attractor behaviors of many biological systems have been understood, yet the distribution of attractors for networks followings biological reaction rules is in general unknown. In our work, we study the final state for all 3 nodes networks that follow transcriptional regulation or enzymatic reaction rules, under random parameter sets. Surprisingly, mono-stable behavior appears most frequently, while bi-stable and tri-stable behavior is less frequently observed. Oscillations are rarely seen, and chaos is almost never observed. We extend the study to random networks with a large number of nodes, and the outcome does not change qualitatively. Furthermore, with increased connectivity, the transition from order to chaos predicted by discrete models is not observed. Our results provide a null-distribution for attractors in bio-networks, and have important implication for cell fate decision. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X40.00005: Beyond Critical Exponents in Neuronal Avalanches Nir Friedman, Tom Butler, Robert DeVille, John Beggs, Karin Dahmen Neurons form a complex network in the brain, where they interact with one another by firing electrical signals. Neurons firing can trigger other neurons to fire, potentially causing avalanches of activity in the network. In many cases these avalanches have been found to be scale independent, similar to critical phenomena in diverse systems such as magnets and earthquakes. We discuss models for neuronal activity that allow for the extraction of testable, statistical predictions. We compare these models to experimental results, and go beyond critical exponents. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X40.00006: Stochastic Modeling of Regulation of Gene Expression by Multiple Competing Small RNAs Charles Baker, Tao Jia, Rahul Kulkarni A wealth of new research has highlighted the critical roles of small RNAs (sRNAs) in diverse processes such as quorum sensing and cellular responses to stress. The pathways controlling these processes often have a central motif comprised of a key protein regulated by multiple sRNAs. However, the regulation of stochastic gene expression of a single target gene by multiple sRNAs is currently not well understood. To address this issue, we analyze a stochastic model of regulation of gene expression by multiple sRNAs. For this model, we derive exact analytic results for the regulated protein distribution including compact expressions for its mean and variance. The derived results provide novel insights into the roles of multiple sRNAs in fine-tuning the noise in gene expression. In particular, we show that, in contrast to regulation by a single sRNA, multiple sRNAs provide a mechanism for independently controlling the mean and variance of the regulated protein distribution. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X40.00007: The up and down states of cortical networks Maryam Ghorbani, Alex J. Levine, Mayank Mehta, Robijn Bruinsma The cortical networks show a collective activity of alternating active and silent states known as up and down states during slow wave sleep or anesthesia. The mechanism of this spontaneous activity as well as the anesthesia or sleep are still not clear. Here, using a mean field approach, we present a simple model to study the spontaneous activity of a homogenous cortical network of excitatory and inhibitory neurons that are recurrently connected. A key new ingredient in this model is that the activity-dependant synaptic depression is considered only for the excitatory neurons. We find depending on the strength of the synaptic depression and synaptic efficacies, the phase space contains strange attractors or stable fixed points at active or quiescent regimes. At the strange attractor phase, we can have oscillations similar to up and down states with flat and noisy up states. Moreover, we show that by increasing the synaptic efficacy corresponding to the connections between the excitatory neurons, the characteristics of the up and down states change in agreement with the changes that we observe in the intracellular recordings of the membrane potential from the entorhinal cortex by varying the depth of anesthesia. Thus, we propose that by measuring the value of this synaptic efficacy, one can quantify the depth of anesthesia which is clinically very important. These findings provide a simple, analytical understanding of the spontaneous cortical dynamics. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X40.00008: Adiabatic and Non-Adiabatic Non-Equilibrium Stochastic Dynamics of Single Regulating Genes Haidong Feng, Bo Han, Jin Wang We explore the stochastic dynamics of self regulative genes from fluctuations of molecular numbers and of on and off switching of gene states due to regulatory protein binding/unbinding to the genes. We found when the binding/unbinding is relatively fast (slow) compared with the synthesis/degradation of proteins in adiabatic (non-adiabatic) case, the self regulators can exhibit one or two peak (two peak) distributions in protein concentrations. This shows even with the same architecture (topology of wiring), networks can have quite different functions (phenotypes), consistent with recent single molecule single gene experiments. We derive the non-equilibrium phase diagrams of mono-stability and bi-stability in adiabatic and non-adiabatic regimes. We study the stability and robustness of the systems through mean first passage time (MFPT) from one peak (basin of attraction) to another. In addition, using the new method for quantifying the paths and the associated weights for complex systems in discrete state space (Markov chains), we identified the dominant paths among all possible paths from the ``off'' basin to the ``on'' basin for self-activators, and observe turnover kinetic behavior of transitions and MFPT from non-adiabatic to adiabatic regimes. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X40.00009: Steady state growth of \textit{E. Coli} in low ammonium environment Minsu Kim, Barret Deris, Zhongge Zhang, Terry Hwa Ammonium is the preferred nitrogen source for many microorganisms. In medium with low ammonium concentrations, enteric bacteria turn on the nitrogen responsive (ntr) genes to assimilate ammonium. Two proteins in \textit{E. coli}, Glutamine synthetase (GS) and the Ammonium/methylammonium transporter AmtB play crucial roles in this regard. GS is the major ammonium assimilation enzyme below 1mM of NH$_{4}^{+}$. AmtB is an inner membrane protein that transports NH$_{4}^{+}$ across the cell membrane against a concentration gradient. In order to study ammonium uptake at low NH$_{4}^{+}$ concentration at neutral pH, we developed a microfluidic flow chamber that maintains a homogenous nutrient environment during the course of exponential cell growth, even at very low concentration of nutrients. Cell growth can be accurately monitored using time-lapse microscopy. We followed steady state growth down to micro-molar range of NH$_{4}^{+}$ for the wild type and $\Delta $amtB strains. The wild type strain is able to maintain the growth rate from 10mM down to a few uM of NH$_{4}^{+}$, while the mutant exhibited reduced growth below $\sim $20~uM of NH$_{4}^{+}$. Simultaneous characterization of the expression levels of GS and AmtB using fluorescence reporters reveals that AmtB is turned on already at 1mM, but contributes to function only below $\sim $30~uM in the wild-type. Down to $\sim $20~uM of NH$_{4}^{+}$, \textit{E.~coli} can compensate the loss of AmtB by GS alone. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X40.00010: Threshold response and bimodality in non-cooperative auto-activation circuits Rutger Hermsen, David Erickson, Terence Hwa In prokaryotes as well as in eukaryotes, many transcription factors (TFs) activate their own gene. For that reason the benefits of auto-activation have been studied extensively. However, little attention is paid to the fact that many TFs are modified by a signal, usually through phosphorylation or binding of a ligand. Typically only one version of the TF---the modified or the unmodified one---can activate transcription. Consequently the TF's expression level responds to changes in the signal. Here, we use stochastic models to study the response properties of such circuits. In real examples the auto-activation is often mediated by a single binding site. Surprisingly, in that case we find that an arbitrarily sensitive threshold response can be obtained, while the bistability and hysteresis associated with multiple cooperative binding sites are avoided. Also, we find that the steady-state probability distributions of the TF expression level can be bimodal even though the system is not bistable. This is not caused by slow TF--DNA binding kinetics or bursty protein production, as in earlier studies, but by strongly reduced production and degradation rates at low expression levels. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X40.00011: Inferring Complex Network Topology from Spatio-Temporal Spike Patterns Frank Van Bussel, Birgit Kriener, Marc Timme The problem of reconstructing or reverse-engineering the connectivity of networks consisting of dynamically interacting units has become an active area of study in fields such as genetics, ecology, and neuroscience. The collective dynamics of such networks is often sensitive to the presence (or absence) of individual interactions, but there is commonly no direct way to probe for their existence. We present an explicit method for reconstructing neuronal networks from their spiking activity. The approach works well for networks in simple collective states, but is also applicable to networks exhibiting complex spatio-temporal spike patterns. In particular, stationarity of spiking time series is not required. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X40.00012: ABSTRACT WITHDRAWN |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X40.00013: Temporal competition between differentiation programs determines cell fate choice Anna Kuchina, Lorena Espinar, Tolga Cagatay, Alejandro Balbin, Alma Alvarado, Jordi Garcia-Ojalvo, Gurol Suel During pluripotent differentiation, cells adopt one of several distinct fates. The dynamics of this decision-making process are poorly understood, since cell fate choice may be governed by interactions between differentiation programs that are active at the same time. We studied the dynamics of decision-making in the model organism \textit{Bacillus subtilis} by simultaneously measuring the activities of competing differentiation programs (sporulation and competence) in single cells. We discovered a precise switch-like point of cell fate choice previously hidden by cell-cell variability. Engineered artificial crosslinks between competence and sporulation circuits revealed that the precision of this choice is generated by temporal competition between the key players of two differentiation programs. Modeling suggests that variable progression towards a switch-like decision might represent a general strategy to maximize adaptability and robustness of cellular decision-making. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X40.00014: Comparison of Control Approaches in Genetic Regulatory Networks by Using Stochastic Master Equation Models, Probabilistic Boolean Network Models and Differential Equation Models and Estimated Error Analyzes Mehmet Umut Caglar, Ranadip Pal Central dogma of molecular biology states that ``information cannot be transferred back from protein to either protein or nucleic acid''. However, this assumption is not exactly correct in most of the cases. There are a lot of feedback loops and interactions between different levels of systems. These types of interactions are hard to analyze due to the lack of cell level data and probabilistic - nonlinear nature of interactions. Several models widely used to analyze and simulate these types of nonlinear interactions. Stochastic Master Equation (SME) models give probabilistic nature of the interactions in a detailed manner, with a high calculation cost. On the other hand Probabilistic Boolean Network (PBN) models give a coarse scale picture of the stochastic processes, with a less calculation cost. Differential Equation (DE) models give the time evolution of mean values of processes in a highly cost effective way. The understanding of the relations between the predictions of these models is important to understand the reliability of the simulations of genetic regulatory networks. In this work the success of the mapping between SME, PBN and DE models is analyzed and the accuracy and affectivity of the control policies generated by using PBN and DE models is compared. [Preview Abstract] |
Session X41: Focus Session: Electronic Structure and Applications to Energy Conversion II
Sponsoring Units: DCPChair: Troy van Voorhis, Massachusetts Institute of Technology
Room: A115/117
Thursday, March 24, 2011 2:30PM - 3:06PM |
X41.00001: Fundamental understanding and computational design of thin-film photovoltaics materials Invited Speaker: The search for abundant and clean energy sources has placed photovoltaics at the focus of research over a variety of disciplines spanning physics, chemistry and materials science. However, the quest for more cost-efficient photovoltaics is challenged by limitations in efficiency of charge excitation and collection in the materials and their interfaces. We will present our recent \textit{ab initio} calculations aimed at understanding important microscopic mechanisms in solar photovoltaic materials. Our goal is to predict accurately key properties that govern the efficiency in these materials, including structural and electronic effects, interfacial charge separation, electron and hole traps, excited state phenomena, band level alignment, and binding energies. Examples of our work in the areas of organic and other thin-film photovoltaics will be presented. We use these examples to illustrate how accurate electronic structure approaches can improve our understanding and lead to more efficient materials. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X41.00002: Effect of environment and long range behaviour of exchange functional on polaron formation in $\pi$-conjugated polymers Iffat Nayyar, Enrique Batista, Sergei Tretiak, Avadh Saxena, Darryl Smith, Richard Martin Organic conjugated polymers find a variety of applications in devices such as solar cells, light emitting diodes and lasers. An accurate understanding of the role of nonlinear excitations as polarons in charge carrier transport is critical to improve the efficiency of these devices. PPV and MEH-PPV are the candidates of choice for the extensive experimental data and relative simplicity compared to other polymers. This motivated us to perform a density functional theory study to describe the charge defects in these systems. We emphasize on the role of surrounding dielectric medium and the amount of long range orbital exchange in the density functional to predict the polaron localization in agreement with experiment. The particle-hole symmetry observed in trans-geometries is broken by introducing certain cis defects. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X41.00003: Photo-induced modulation in the dipole moment of a donor-acceptor pair of organic molecules Yoshiyuki Miyamoto, Mina Yoon, Matthias Scheffler We have investigated the photo-induced electron dynamics in donor-acceptor pairs of organic molecules. Specifically we will discuss TTF and TCNQ molecules and study their electron dynamics under illumination by means of time-dependent density functional theory within the local-density approximation. In their stable molecular structure, we find that these molecules align in parallel and show maximum optical oscillator strength with an optical polarization parallel to their molecular axis. Without illumination, a dipole moment from TTF to TCNQ directs perpendicular to the molecular axis. This dipole-moment is further increased upon illumination with an optical polarization parallel to the molecular axis at resonant excitation energies of 2.00 eV and 3.55 eV. The light-induced increase of the dipole moment, which reflects the separation of electron and hole pair, is caused by the internal electric field between these molecules. Therefore, these molecules may have a high potential as building blocks of future organic photovoltaic devices. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 4:06PM |
X41.00004: Exciton transport and dissociation at organic interfaces Invited Speaker: This paper focuses on modeling studies of exciton transport and dissociation at organic interfaces and includes three parts: 1) Experiments have shown that the values of exciton diffusion length $L_{D}$ in conjugated polymers (CPs) are rather low, in the range of 5-10 nm, apparently regardless of their chemical structure and solid-state packing. In contrast, larger $L_{D}$ values have been reported in molecular materials that are chemically more well-defined than CPs. Here we demonstrate that energetic disorder alone reduces the exciton diffusion length more than one order of magnitude, from values typically encountered in molecules ($>$50nm) to values actually measured in CPs ($<$10nm). 2) A number of organic crystals show anisotropic excitonic couplings, with weak interlayer interactions between molecules that are more strongly coupled within the layers. The resulting energy carriers are intra-layer 2D excitons that diffuse along the interlayer direction. We model this analytically for infinite layers and using quantum-chemical calculations of the electronic couplings for anthracene clusters. We show that the exciton hopping rates and diffusion lengths depend in a subtle manner on the size and shape of the interacting aggregates, temperature and the presence of energetic disorder. 3) The electronic structure at organic/organic interfaces plays a key role, among others, in defining the quantum efficiency of organic-based photovoltaic cells. Here, we perform quantum-chemical and microelectrostatic calculations on molecular aggregates of various sizes and shapes to characterize the interfacial dipole moment at pentacene/C60 heterojunctions. The results show that the interfacial dipole mostly originates in polarization effects due to the asymmetry in the multipolar expansion of the electronic density distribution between the interacting molecules. We will discuss how the quadrupoles on the pentacene molecules produce direct electrostatic interactions with charge carriers and how these interactions in turn affect the energy landscape around the interface and therefore also the energy barrier for exciton dissociation into free carriers. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X41.00005: Correlating First-Principles Electronic Structure with Device Performance of Organic Photovoltaic Cells Eric B. Isaacs, Sahar Sharifzadeh, Biwu Ma, Jeffrey B. Neaton Organic photovoltaic cells (OPVs) are promising candidates for low-cost solar energy conversion. Here, we employ static and time-dependent density functional theory calculations to predict the excitation energy of the donor-acceptor charge transfer state (E$_{\textrm{CT}})$ at the interface between C$_{60}$ and several boron(subphthalocyanine)- and azadipyrromethene-based donor moieties, comparing to measured open-circuit voltage (V$_{\textrm{oc}})$ in bilayer heterojunction OPVs [1]. When E$_{\textrm{CT}}$ is approximated as the difference between the ionization potential and electron affinity of the isolated donor and acceptor molecules, respectively, we observe no apparent correlation between E$_{\textrm{CT}}$ and V$_{\textrm{oc}}$. Both bulk polarization and excitonic effects at the interface are found to influence the energetics significantly, the latter being strongly morphology dependent. We demonstrate that a linear relationship between V$_{\textrm{oc}}$ and E$_{\textrm{CT}}$ may be obtained once the interface morphology is considered. We acknowledge support from DOE, NSF-NCN, and NERSC. \\[4pt] [1] C. E. Mauldin \textit{et al.}, ACS Appl. Mater. Interfaces \textbf{2}, 2833 (2010). [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X41.00006: Photo-induced Charge Separation in Nanoscale Donor-Bridge-Acceptor Systems: Theory and Experiment Peter Doak, Pierre Darancet, Kasper Moth-Poulsen, Jesse Jenkins, Rachel Segalman, Don Tilley, Jeff Neaton Understanding and control of light-harvesting processes at the molecular-scale remains a fundamental challenge in solar energy conversion. Donor-bridge-acceptor molecules (DBAM), with atomically-defined interfaces made by a covalently bound bridge between donor and acceptor moieties, allow probing of excited states relevant to optical absorption and charge separation. In close collaboration with experiment, we use first-principles many-body perturbation theory, within the GW approximation and the Bethe-Salpeter equation approach, to compute excited states for six DBAMs. We compare with experiments, and quantitative agreement is obtained. Implications of our results for nanoscale light-harvesting are thoroughly discussed. Support: DOE via the Molecular Foundry and Helios SERC, and NSF via NCN. Computational support provided by NERSC. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 5:06PM |
X41.00007: Exciton Scattering in Branched Conjugated Molecules: Towards Photoinduced Dynamics and Energy Transfer Invited Speaker: The exciton scattering (ES) approach attributes excited electronic states in quasi-one-dimensional (branched) conjugated molecules with perfect geometry to standing waves on the linear segments of a molecule formed by scattering of quantum quasi-particles (excitons). We extract their dispersion and frequency-dependent scattering matrices at termini, including donor/acceptor substitutions, joints, and branching centers from time-dependent density functional theory (TD-DFT) calculations, with applications to for conjugated phenylacetylene-based molecules. This allows electronic spectra for any structure of arbitrary size within the considered molecular family to be obtained with insignificant numerical effort. To extend the capability of the ES approach to treating photoinduced dynamics, including absorption and fluorescence lineshapes and energy transfer, the methodology should be modified to account for non-ideal molecular geometry. Geometry distortions break down translational symmetry of the linear segments, and excitations are not represented by perfect standing waves anymore. To overcome this difficulty we associate electronic excitations with the eigenstate of a quantum particle on an irregular lattice (graph), referred to as a tight-binding model. The morphology of the underlying lattice, together with the tight-binding parameters, can be identified by studying the topological and analytical properties of excitons at molecular termini, joints, and branching centers. The dependence of the tight-binding parameters on geometry distortions that controls effects of disorder and coupling to vibrational modes can be extracted from quantum chemical calculations by studying exciton scattering on localized geometry distortions, the latter considered as scattering centers. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X41.00008: NA-ESMD modeling of photoinduced dynamics in conjugated molecules Tammie Nelson, Sebastian Fernandez-Alberti, Vladimir Chernyak, Adrian Roitberg, Sergei Tretiak The evolution of electronic excitations in optically active molecules can generally be defined by non-adiabatic (NA) dynamics. A number of fundamental and complex processes are associated with NA dynamics. To treat ultrafast excited state dynamics we have developed a non-adiabatic excited state molecular dynamics (NA-ESMD) framework incorporating quantum transitions. Our calculations combine the Collective Electronic Oscillator (CEO) package with the Tully's fewest switches algorithm for surface hopping, and the actual potential energy surfaces of the excited states are used. This method is applied to model the photoinduced dynamics of distyrylbenzene. Our analysis shows intricate details of vibronic relaxation and identifies specific slow and fast nuclear motions that are strongly coupled to the electronic degrees of freedom. Non-adiabatic relaxation of the highly excited mAg state is predicted to occur on a femtosecond timescale at room temperature and on a picosecond timescale at low temperature. [Preview Abstract] |
Session X42: Polymeric Glasses
Sponsoring Units: DPOLYChair: Mark Ediger, University of Wisconsin--Madison
Room: A302/303
Thursday, March 24, 2011 2:30PM - 2:42PM |
X42.00001: Molecular Mobility on the Surface of Glassy Tris-naphthylbenzene (TNB) Zahra Fakhraai, Chad Daley, Stephen F. Swallen, Daniel Scifo, James A. Forrest, Mark D. Ediger Mechanical relaxation measurements on the surface of polymeric glasses show that as the bulk material falls out of equilibrium at Tg a thin layer at the surface behaves like a liquid with relaxation times that are orders of magnitude faster and more weakly temperature dependent compared to those of the bulk glass. However the origin of this phenomenon remains elusive. Recently exceptionally stable glasses of small organic molecules have been produced by physical vapor deposition at temperatures below Tg, suggesting that these glasses also exhibit enhanced surface mobility. In this study gold nanoparticles were used to probe micron size meniscus formation on the surface of organic glass former TNB below Tg, a direct evidence of surface mobility in this material. Neutron scattering measurements of inter-diffusion between stacks of d-TNB and regular TNB layers during the deposition suggest that the temperature dependence of the diffusion on the surface is very similar to what is observed on polymeric films. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X42.00002: Tuning the Dynamics of Penetrant Transport in Glassy Polymers through Network Structure Modification Adam Ekenseair, Nicholas Peppas The relative rates of the diffusional and relaxational processes during the absorption of penetrant molecules in glassy polymers determine the nature of the transport process and lead to Fickian, Case II, and anomalous absorption behavior. While previous models account for anomalous behavior, there is still a disconnect between theory and experiment, as data must be fit to the model with previously determined independent parameters. With trends leading to smaller device scales and increasingly complex polymer structures, there is a need for a quantitative understanding of the manner in which a polymer's network structure alters both the rate and the mode of penetrant transport. To this end, the effects of the basic network parameters of PMMA, including the degree of crosslinking, polymer mesh size, and the crosslink interchain bridge length, on the integral sorption of methanol were studied utilizing gravimetric integral sorption studies. The effects of sub-T$_{g}$ annealing/aging, temperature, and the presence of un-reacted monomer were also investigated. Controlling the relative timescale of the relaxational process by altering the polymer network structure was shown to directly influence the Case II front propagation velocity and control the overall nature of the observed transport behavior. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X42.00003: A Surrogate for Debye-Waller Factors from Stokes Shifts Marcus Cicerone, Qin Zhong, Madhusudan Tyagi We show that short-time relaxation behavior characteristic of the intermediate scattering function at $q$ near the peak in the static structure factor can be obtained from time-resolved Stokes shifts (TRSS) in glassforming materials. We extract Debye- Waller factor ($\langle u^{2}\rangle$) analogs from the TRSS data from four glassforming liquids and apply these to a proposed relationship between $\alpha$ relaxation and the Debye-Waller factor; $\tau_{\alpha}=\tau _{\infty }\; \mbox{E}xp\left[ \frac{a^{2}}{2\langle u^{2}\rangle}+\frac{\sigma _{a^{2}}^{2}}{8\langle u^{2}\rangle^{2}} \right]$. This putative relationship has previously been evaluated using experimental Debye-Waller factors obtained in the time range (40 to 2000) ps. We show that the relation yields physically meaningful fit values only when relaxation on a 1 ps timescale is considered. We also observe an unexpected dependence of short-time Debye-Waller factors on fragility. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X42.00004: Comparison of the KWW and BSW Model Descriptions of the Dynamic Responses of Polymeric and Colloidal Glass Formers Ben Xu, Gregory B. McKenna In this work, we present the results of the KWW and BSW\footnote{M. Baumg\"{a}rtel, A. Schausberger, H.H. Winter, Rheol. Acta. 29:400--408 (1990). } descriptions of the dynamic data for a colloid and a polymer (PVAc) in their respective glass transition regions. It was found that the KWW function is not able to describe the dynamic data for the colloidal system, while BSW function, provides an acceptable description to the dynamic response of the polymer. The fitting parameters n$_{e}$ and n$_{g}$ in the BSW function, which indicate the slopes of the relaxation spectrum, remain constant at different temperatures consistent with the validity of the time-temperature superposition principle. We also used the G$_{g}$ obtained from the KWW and BSW functions, where appropriate, to evaluate the Dyre shoving model.\footnote{J. C. Dyre, N. B. Olsen, T. Christensen, Physical Review B.53, 5 (1996).} Here, as is the case for small molecule glass formers, we found the temperature dependences of the G$_{g}$ highly sensitive to the model chosen to describe the experimental data. This suggests that evaluation of the shoving model requires very broad frequency and temperature experiments beyond those normally performed in dynamic rheometry. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X42.00005: Aging and structural recovery behaviors in epoxy films subjected to carbon dioxide plasticization jumps: Evidence for a new glassy state Gregory McKenna, Shankar Subramanian, Jing Zhao, Mataz Alcoutlabi, Lameck Banda Structural recovery and physical aging of glassy polymers after temperature jumps have been very well studied in the literature. On the contrary, there is only limited work available on the aging and recovery behaviors of glassy polymers subjected to plasticizer jumps. We have shown in our previous works, using strong and weakly polar plasticizers that qualitatively they mimic the behaviors of temperature jumps but quantitatively they are different [1, 2]. In this work, we further investigate this anomalous behavior by studying the structural recovery and physical aging of an epoxy film subjected to carbon dioxide pressure jumps and compare the results with temperature jump experiments such that the final conditions are identical. The results are surprising and we observe evidence for existence of a new glassy state. \\[4pt] [1] Zheng. Y., and McKenna, G.B., \textit{Macromolecules,} \textbf{36}, 2387-2396, 2003\\[0pt] [2] Alcoutlabi, M., Briatico-Vengosa, F., and McKenna, G.B., JPSB., \textbf{40}, 2050-2064, 2002 [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X42.00006: Effect of Quench Conditions on the Subsequent Physical Aging Rate of Polymer Glasses Laura Golick, Paul Yoon, Andy Pahner, Connie Roth We investigate the stability of polymer glasses when thermally quenched under different conditions. Ellipsometry is used to measure the physical aging rate of polystyrene (PS) films supported or transferred onto silicon wafers. The aging rate quantifies the time-dependent decrease in film thickness that results from the increase in average film density during aging. Although all films are subsequently aged in a supported state, we observe significant differences between films quenched in a free-standing compared to supported state. Films quenched in a free-standing state exhibit a strong thickness dependence to their physical aging rate at micron length scales, an order of magnitude or two larger than thicknesses where nanoconfinement effects on the glass transition and modulus are typically observed. In contrast, supported films do not display any film thickness dependence to their aging rate at this large length scale. This indicates that the physical aging of the material is strongly dependent on conditions during the formation of the glassy state. In an effort to determine the key factors underlying the aging dynamics, we have measured the physical aging rate of supported PS films quenched at various controlled rates. In addition, we have explored the effects of quenching free-standing films held on different frames such that either biaxial or uniaxial stress is applied. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X42.00007: Low Temperature Flow of PVC Chains Wei Chen, Gi Xue PVC is usually processed at temperature above 180 $^{\circ}$C, however, it starts to degrade at 130 $^{\circ}$C. If PVC can flow at temperatures below glass transition temperature (Tg), the manufacturing procedure will be energy-conserving and environment-friendly. We find that PVC powders with controlled inter-segment van der Waals attraction can be compressed into a transparent pellet with high modulus at low temperatures. The molecular mechanism underlying this phenomenon involves shear-induced unjamming transition. PVC chains are unjammed by cold-pressing freeze-dried powder with decreased packing density. Because the Tg of freeze-dried PVC is dramatically reduced to the test temperatures under compression, PVC chains are able to flow by applying pressure solely. These results help us better understanding glass transition and can possible to develop a theory for cold processes. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X42.00008: Evolution of Entanglements During Crazing of Glassy Polymers Ting Ge, Mark O. Robbins, Robert Hoy, Stefanos Anogiannakis, Christos Tzoumanekas, Doros Theodorou Craze formation increases the fracture energy of glassy polymers by orders of magnitude. The polymer volume expands by an extension ratio which is assumed to be determined by the entanglement network. We test this assumption with molecular simulations that use the Contour Reduction Topological Analysis (CReTA) algorithm to follow topological constraints (TCs) associated with the entanglement network. The TCs are identified with contacts between chains after applying CReTA. Within systematic errors, crazing does not change the number of TCs or the distribution of chemical distances between them. Moreover, about 75{\%} of the contacts remain between the same chains at nearly the same location. The 25{\%} of contacts that change do not reflect a comparable loss of entanglements. Instead, small displacements within the tube change which chains contact after CReTA. This interpretation is tested by adding fixed crosslinks to a sparse entanglement network and crazing preoriented samples. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X42.00009: Surface Softening in Polymers and Their Nanocomposites Determined by Surface Mechanical Properties through Spontaneous Particle Embedment Taskin Karim, Gregory McKenna In the present work, we have used the particle embedment technique with sub-micron particles to estimate the surface modulus of epoxy/POSS composites at a temperature far below the glass transition temperature. The embedment of the particle is determined from atomic force microscope measurements and the modulus was determined using the elastic analysis of Johnson, Kendall and Roberts (JKR) with surface energy estimates of the work of adhesion as the driving force for embedment. The surface modulus values were found much smaller than the macroscopic modulus values. The maximum embedment depth obtained for all surfaces was low enough so that it did not cause plastic deformation on the surface. The maximum stress values on all surfaces induced by the particle embedment were estimated to verify the expected response in close to the linear regime. \\[4pt] [1] K. L. Johnson, K. Kendall and A. D. Roberts, \textit{P. Royal Society of Lonodon A, }\textbf{324}, 301-313 (1971). \\[0pt] [2] J. H. Teichroeb and J. A. Forrest, \textit{Physical Review Letter, }\textbf{91}, 016104 (2003). [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X42.00010: Effect of molecular weight on gold nanoparticle embedding into polystyrene films near and below the bulk glass transition temperature Chad Daley, Dongping Qi, James Forrest We use gold nanoparticle embedding to probe the surface properties of glassy polystyrene films at temperatures ranging from a few degrees above to 10's of degrees below the bulk glass transition temperature (T$_{g})$. These studies employed monodisperse polymer samples with molecular weights (M$_{w})$ ranging from 3000-80000 kg/mol. A qualitative change in the surface response is observed between the high M$_{w}$ and low M$_{w}$ regimes. At low M$_{w}$ a buildup of polymer material forms around the base of the nanoparticles similar to the observed behavior in molecular glasses. For the higher molecular weights this buildup is not observed and the system instead relaxes through nanoparticle embedding. We also observe changes in the complete embedding process observed only near the bulk T$_{g}$. These changes suggest that nanoparticle embedding can be used as a probe of polymer entanglements. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X42.00011: Glass Transition and Free Volume Behavior in Epoxy-amine Network Glasses: Effect of Diiamine Isomers Sergei Nazarenko, Mukul Kaushik, Matthew Jackson, Jeffrey Wiggins A systematic investigation of the effect of meta and para isomers of diamino diphenyl sulfone (DDS) crosslinker on glass transition temperature (T$_{g})$ and free volume properties of DGEBF based epoxy-amine network was carried out. The pressure volume temperature (PVT) properties were measured experimentally from 0 to 120 MPa and 30\r{ }C to 240\r{ }C in a high pressure dilatometer-type PVT apparatus. It was observed that the glass transition temperature of epoxy system with para isomer is higher than the one consisting of meta isomer by 30 \r{ }C. PVT data were also fitted using Simha-Somcynsky, equation of state to calculate occupied and free volume. Positron annihilation lifetime spectroscopy (PALS) was used to calculate average hole free volume below and above glass transition temperature. The average free volume size in para isomer cured systems is larger than in meta isomer cured systems below their glass transition temperature, while in the melt state they are the same. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X42.00012: Difference in the heat capacity and the coefficient of thermal expansion responses during thermal cycling Grigori Medvedev, Eun-Woong Lee, James Caruthers An observation that different experimental methods give different values of Tg is part of the lore of the field of the glassy polymers. We report on a careful study of a series of polymeric systems both thermoplastic and thermoset, including PMMA, PC, PS, and 3,3' DDS Epon 825, conducted using DSC and TMA techniques. We found that for the same thermal history the heat capacity and the coefficient of thermal expansion (both measured upon heating) as functions of temperature transition from the glassy asymptote to the equilibrium asymptote at significantly different temperatures; this difference was in the range from 8 to 17 degrees, depending on the system. We argue that such a large difference in the enthalpy and volume responses during the same thermal history is inconsistent with the commonly used material clock models, but is consistent with the view of the glassy materials as containing dynamically heterogeneous regions. [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X42.00013: Formation of Glassy Polymer Films by Matrix Assisted Pulsed Laser Evaporation Rodney Priestley, Yunlong Guo, Craig Arnold The properties of glasses strongly depend on the path to glass formation. The most common method of making polymer glasses is by cooling from the liquid state. Recently, it has been shown that alternative routes to the vitreous state can lead to dramatically improved glassy-state stability. In this talk, we present our initial work on the thermal and kinetic stability of glassy polymer films prepared by Matrix Assisted Pulsed Laser Evaporation (MAPLE). In comparison to glassy films prepared by spin coating, MAPLE-deposited glassy films can exhibit higher glass transition temperatures and greater kinetic stability. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X42.00014: Characterization of pre-mature nanocomposite crazes Gregory N. Toepperwein, Juan J. de Pablo Crazing is a unique mode of failure by which polymer strands are stretched into a periodic array of columns. It has been shown that these crazes follow cavitation under deformation. Inclusion of nanoparticles drastically alters the glass transition temperature and the globally measurable mechanical properties of these polymer glasses. However, limited literature exists to explain the behavior on the lengths scales of the heterogeneous domains within the glass in the context of nanocomposites. In this work, we investigate the nucleation and growth of voids that precede craze formation to elucidate the role these inclusions play in failure and further characterize the pre-mature craze itself. Extensive Molecular Dynamics and Monte Carlo simulations of highly entangled polymer nanocomposites allow for calculation of local densities, local elastic moduli, and local orientation of additives. We find that the site of void formation is inexorably linked to the local mechanical properties of polymer. This relationship is more evident upon the inclusion of reinforcing additives which induce a broader distribution of local moduli leading to the nucleation of more, smaller voids. Within the developing craze, larger additives resist incorporation, but those that do are subject to ordering. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X42.00015: Structural Characterization of a Polymer of Intrinsic Microporosity: X-ray Scattering With Insight From Molecular Dynamics Simulations Amanda G. McDermott, Gregory S. Larsen, Peter M. Budd, Coray M. Colina, James Runt Polymers of intrinsic microporosity (PIMs) are high-T$_{g}$, amorphous materials exhibiting high gas permeability and a large concentration of pores smaller than 2 nm, arising from a combination of rigid segments and sites of contortion. Structures generated by molecular dynamics simulations accurately reproduce characteristic scattering features from PIM-1 at high $q$, allowing us to investigate their origin by examining partial structure factors. Unlike scattering patterns typical of nonporous amorphous polymers, broad $q$ range PIM scattering patterns include a shoulder at the size scale corresponding to pore sizes measured by other techniques. We discuss the development of a model for extracting pore sizes from scattering patterns. [Preview Abstract] |
Session X43: Focus Session: Assembly, Structure, & Instabilities in Polymer Films, Network Films, & Interfaces II
Sponsoring Units: DPOLYChair: Ryan Toomey, University of Southern Florida
Room: A306/307
Thursday, March 24, 2011 2:30PM - 3:06PM |
X43.00001: Hydrogels with Spatially and Temporally Controlled Properties to Control Cellular Interactions Invited Speaker: Stem cells (e.g., mesenchymal stem cells, MSCs) respond to many cues from their microenvironment, which may include chemical signals, mechanics, and topography. Importantly, these cues may be incorporated into scaffolding to control stem cell differentiation and optimize their ability to produce tissues in regenerative medicine. Despite the significant amount of work in this area, the materials have been primarily static and uniform. To this end, we have developed a sequential crosslinking process that relies on our ability to crosslinked functional biopolymers (e.g., methacrylated hyaluronic acid, HA) in two steps, namely a Michael-type addition reaction to partially consume reactive groups and then a light-initiated free-radical polymerization to further crosslink the material. With light exposure during the second step comes control over the material in space (via masks and lasers) and time (via intermittent light exposure). We are applying this technique for numerous applications. For example, when the HA hydrogels are crosslinked with MMP degradable peptides with thiol termini during the first step, a material that can be degraded by cells is obtained. However, cell-mediated degradation is obstructed with the introduction of kinetic chains during the second step, leading to spatially controlled cell degradability. Due to the influence of cellular spreading on MSC differentiation, we have controlled cell fates by controlling their spread ability, for instance towards osteoblasts in spread areas and adipocytes when cell remained rounded. We are also using the process of stiffening with time to investigate mechanically induced differentiation, particularly in materials with evolving mechanics. Overall, these advanced HA hydrogels provide us the opportunity to investigate diverse and controlled material properties on MSC interactions. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X43.00002: Hydrogel Stamping of Polyelectrolyte Multilayers for Directed Cell Growth Nicole Zacharia, Chungyeon Cho The authors have recently introduced the use of hydrogel stamp materials to pattern polyelectrolyte multilayer (PEM) films. It has been demonstrated that using a stamp equilibrated in either low or high pH can cause local swelling in these films, leading to patterns. It has also been shown that stamps soaked in high ionic strength salt solutions are able to locally etch PEM films. This hydrogel stamping technique gives both lateral control of surface properties and depth control over the film's properties. This technique is a promising way to pattern chemical reactions within PEM, phase transformation, and physical properties such as film thickness, Young's modulus, and swelling. By using hydrogels for the stamp material, stamping becomes a process of continuously delivering aqueous reagent of interest to a film, instead of merely a single layer of material, as is the case when using hydrophobic stamp materials such as PDMS. While chemical modification of only the surface may be desirable in some cases, the hydrogel stamping technique is more versatile. By creating local variations in swelling, we are able to pattern mechanical stiffness, and in turn cell adhesion. We demonstrate the creation of gradients in mechanical stiffness which we are able to use to direct cell growth and adhesion on these films. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X43.00003: Patterned Poly-N-isopropylacrylamide Surfaces for Culture and Harvest of Muscle Fibers Samuel DuPont, Kranthi Kumar Elineni, Nathan Gallant, Ryan Toomey Swelling of surface confined poly-N-isopropylacrylamide (pNIPAAm) structures leads to non-uniform patterns that can be switched by a thermal cue. Based on the geometry of the surface confined patterns, various forms of structural instabilities arise such as bulk buckling, differential lateral swelling and edge buckling. Instabilities that arise from the swelling of patterned pNIPAAm surfaces present a unique platform for tissue engineering applications. Recent work has demonstrated the attachment, survivability, and alignment of fibroblasts grown atop rectangular pNIPAAm surface extrusions. Detachment of contiguous and aligned fibroblasts grown on these surfaces was observed when the geometry of the structure was such that a bulk buckling instability formed upon thermally induced gel swelling. Current work is aimed at utilizing this switchable platform to culture aligned myoblasts, which upon differentiation, form multicellular myotubes, an important structure in skeletal muscle. Myotubes for tissue engineering can then be harvested by non-enzymatic detachment facilitated by thermally induced non-uniform gel swelling. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X43.00004: Controlled Release from Model Blended Polyelectrolyte Multilayer Films Bulent Akgun, Yeongseon Jang, Sushil Satija, Kookheon Char We propose a new concept of controlled release platforms based on the model blended multilayer films composed of positively charged weak polyelectrolyte (linear poly(ethylenimine),LPEI) layer and blended layer with negatively charged strong (poly(sodium-4-styrene sulfonic acid),PSS) and weak (poly(methacrylic acid),PMAA) polyelectrolytes. The blended multilayer films ((LPEI/PSS:PMAA)$_{n})$ with well-defined internal structure are prepared by spin-assisted LbL deposition method, and their release behavior is systematically characterized with combined techniques of neutron reflectivity, ellipsometry, AFM, QCM and FT-IR. Since PSS provides the robust skeleton within the multilayer films independently on pH variation, the burst erosion of multilayer films is dramatically suppressed, and the release kinetics of PMAA can be precisely controlled by simply changing PSS contents within the multilayer films. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X43.00005: Surface wrinkling driven by swelling and its applications Hyun Suk Kim, Alfred Crosby The nature of reversible surface wrinkling offers great promise for designing responsive or adaptive materials. We experimentally explore the reversibility and geometry of surface wrinkles driven by swelling an elstically-supported stiff plate by solvent vapor and liquid. We use crosslinked polydimethylsiloxane (PDMS) substrates with surfaces modified by ultraviolet-ozone (UVO) processing, creating materials with rigid, surface films that swell differentially in comparison to the underlying substrates. We observe the dependence of the wrinkle wavelength on the UVO time and thickness of elastomers. Furthermore, we identify a critical length scale for the swelling region below which wrinkle formation is suppressed. Taking advantage of the reversibility and geometric relations, we demonstrate advanced applications such as smart windows with swichable optical transparency and responsive channels in microfluidics. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X43.00006: Nanoporous Conductive Films Derived from Polymeric Bicontinuous Microemulsions Brad Jones, Kai-Yuan Cheng, Russell Holmes, Timothy Lodge Ternary blends of two homopolymers and a diblock copolymer can self-assemble into interpenetrating, 3D-continuous networks with a characteristic length scale of 100 nm. These polymeric bicontinuous microemulsions (B$\mu $E) can be designed to serve as templates for the synthesis of nanoporous materials with 3D-continuous pore networks. We have investigated the behavior of B$\mu $E-forming blends of polyolefins as precursors to nanoporous polyethylene (PE) films. The effect of interfaces in these films can drastically disrupt the B$\mu $E structure, leading to a macro-phase separated morphology. Proper consideration of several factors, including substrate surface energy, film thickness, and annealing time, is necessary to retain a B$\mu $E structure in such films. Finally, we use the B$\mu $E-like, nanoporous PE films as templates in the synthesis of nanoporous films of the conducting polymer poly(3,4-ethylenedioxythiophene), having potential application in organic electronic devices. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X43.00007: Particle Behavior at Anisotropically Curved Liquid Interfaces Kathleen McEnnis, Chuan Zeng, Benny Davidovitch, Anthony Dinsmore, Thomas Russell A particle bound to an anisotropically curved liquid interface, such as a cylinder or catenoid, cannot maintain a constant contact angle without deforming the interface. Theory suggests that the particles will experience a force that depends on the interfacial shape and migrate to minimize the total interfacial energy. To test these predictions, particles were deposited on top of liquid semi-cylinders of ionic liquid or melted polystyrene confined on chemically patterned surfaces. Particles were also deposited on liquid catenoid structures created by placing a melted polymer film under an electric field. The location of the particles on these structures was observed by optical, confocal, and scanning electron microscopy. The implications for the directed assembly of particles and stability of Pickering emulsions are also discussed. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X43.00008: Hierarchically Ordered Block Copolymer Micelles Formed by Controlled Evaporative Self-Assembly Wei Han, Myunghwan Byun, Zhiqun Lin Highly ordered gradient stripes of PS-b-P4VP block copolymer were obtained by combining the microscopic controlled evaporative self-assembly (CESA) of confined microfluid of PS-b-P4VP toluene solution in a ``cylinder-on-Si'' geometry with spontaneous self-assembly of micellar hexagonal arrays of PS-b-P4VP at the nanometer scale. The order of packed micelles within microstripes could be significantly improved by subsequent THF vapor annealing. The surface reconstruction of micelles led to the formation of nanoporous arrays when immersed in a selective solvent of the pore component. Gold nanoparticles were then selectively deposited into the core of micelles, and eventually forming the hexagonal arrays of gold nanoparticles after removal of polymer templates by oxygen plasma. The formation of gold particle arrays was verified by XPS measurement. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X43.00009: Viscoelastic properties of ultrathin polymer films using the liquid dewetting technique Jinhua Wang, Gregory McKenna There is considerable interest in studying the behavior of polymers at the nanoscale. Here we describe experiments using the Bidiguel and Fretigny's liquid dewetting technique in which no great glass transition temperature (Tg) reduction or rubbery plateau compliance change for polystyrene (PS) films was observed [1]. These results are contrary to observations by others of Tg reductions on free standing polystyrene films and of large rubbery stiffening observed in our lab using a bubble inflation method [2,3]. Preliminary results of PS film dewetting are consistent with the Bidiguel and Fretigny's results. Also, annealing time and confinement effects on the creep behavior of polystyrene thin film were examined. Then, the range of investigated materials is being expanded to polycarbonate (PC) and poly(methyl methacrylate) PMMA with the ultimate goal to determine the reasons for the differences between the bubble inflation method and liquid dewetting technique of polymer film charicterization. References: [1] H. Bodiguel and C. Fretigny, ``Viscoelastic dewetting of a polymer film on a liquid substrate,'' \textit{Eur.Phys. J. E}., 19, 185-193 (2006). [2] K. Dalnoki-Veress, J. A. Forrest, P. G. de Gennes and J. R. Dutcher, \textit{J. Phys. IV.}, 10, 221-226 (2000). [3] O' Connell P. A. and McKenna G. B., ``Rheological Measurements of the Thermoviscoelastic Response of Ultrathin Polymer Films'',\textit{ Science}, 307, 1760-1763 (2005). [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 4:54PM |
X43.00010: Elastic Moduli of Nanoparticle-Polymer Composite Thin Films via Buckling on Elastomeric Substrates Hongyi Yuan, Alamgir Karim Polymeric thin films find applications in diverse areas such as coatings, barriers and packaging. The dispersion of nanoparticles into the films was proven to be an effective method to generate tunable properties, particularly mechanical strength. However, there are very few methods for mechanical characterization of the composite thin films with high accuracy. In this study, nanometric polystyrene and polyvinyl alcohol films with uniformly dispersed cobalt and Cloisite nanoparticles at varying concentrations were synthesized via flow-coating and then transferred to crosslinked polydimethylsiloxane (PDMS) flexible substrates. The technique of Strain-Induced Elastic Buckling Instability for Mechanical Measurements (SIEBIMM) was employed to determine the elastic moduli of the films, which were calculated from the buckling patterns generated by applying compressive stresses. Results on moduli of films as a function of the concentrations of nanoparticles and the thicknesses of the composite films will be presented. *Corresponding author: alamgir@uakron.edu [Preview Abstract] |
Thursday, March 24, 2011 4:54PM - 5:06PM |
X43.00011: Effect of Chain Architecture on the Physical Aging of Thin Polymer Filims Bradley Frieberg, Emmanouil Glynos, Peter Green Physical aging, glassy structural relaxations, is an important phenomenon that has an important influence on a range of physical properties, such as optical, mechanical and electrical, of polymeric materials properties. When a polymeric material is cooled below its glass transition temperature (Tg) it resides in a non-equilibrium state, and over time it attempts to return to equilibrium via a structural relaxation process. We have previously demonstrated that chain architecture can influence the Tg in supported thin films. Specifically, star-shaped molecules possessing sufficiently high functionality (f) and low molecular weight of the arm (Mw), exhibit significant differences in vitrification trends from their linear analogs. In this presentation we show that when f is sufficiently high, or Mw is sufficiently low, the physical aging rate is suppressed compared to linear chains. Moreover, the aging rates of thin, supported films of star shaped molecules are strongly thickness dependent. [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X43.00012: Perturbing the Tg of Polymers by 50-100 K in Nanoconfined Freely Standing Films and by the Presence of Neighboring Layers of Other Polymers John Torkelson, Soyoung Kim We demonstrate via the temperature dependence of fluorescence intensity intrinsic to the polymer of interest or from dye labels that the glass transition temperature (Tg) of a polymer can be altered by 50-100 K by nanoconfinement in freely standing films and in multilayer systems in which the neighboring layers are different polymers. In the former case, Tg always decreases from bulk Tg; in the latter case, Tg decreases or increases depending on the Tg of the neighboring polymer layer and factors that may be related to fragility. We employ fluorescence to characterize the gradient in Tg from the perturbing interfaces. These studies reveal that the theory by de Gennes for the Tg reduction in freely standing films cannot be correct and that the perturbation to Tg by a neighboring layer of another polymer can extend as much as 100 nm into the layer of interest. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X43.00013: ABSTRACT WITHDRAWN |
Session X44: Focus Session: Polymer Colloids-Structure, Function, and Dynamics II
Sponsoring Units: DPOLY DFDChair: Alberto Fernandez De Las Nieves, Georgia Institute of Technology
Room: A309
Thursday, March 24, 2011 2:30PM - 2:42PM |
X44.00001: Nanoparticle Organic Hybrid Suspensions: Structure and Rheology Samanvaya Srivastava, Lynden Archer Nanoparticle Organic Hybrid Materials (NOHMs) are a new class of tethered nanoparticle systems with high grafting densities and behave as model systems for studying spherical polymer brushes. Here we report rheology and scattering measurements of NOHMs with a silica core and PEG corona suspended in PEG oligomers at varying volume fractions. Our rheology results reveal a liquid-glassy transition at strikingly low core volume fractions in these suspensions and prominent stress overshoots in flow startups indicative of yielding in the high volume fraction suspensions. Further, we elucidate the form of particle interactions in the glassy suspensions and compare them with established models. Also, a negative first normal stress difference in the moderate volume fraction suspensions is reported, which is in agreement with recent theoretical and experimental findings. We also report small angle scattering measurements of these suspensions to reveal their equilibrium structure, which are in qualitative agreement with a recent theoretical study (Langmuir, 2010, 26, 16801). [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X44.00002: Measuring and Modeling the Interactions Between DNA-Functionalized Colloids William Rogers, John Crocker DNA hybridization is an ideal tool to direct ``bottom-up'' assembly of complex materials and has been used to form crystalline assemblies of quantum dots, polymer microspheres and other materials made exclusively of DNA. In order to fully realize the potential of DNA-directed self-assembly, one must be able to quantitatively predict the binding energies and interaction potentials between the relevant ``building blocks.'' In this work, we use a scanning-line optical tweezers instrument to measure DNA-induced interactions between colloidal microspheres. We then use well-known concepts in statistical mechanics to model the pair-potentials, whose functional form and energetics of binding are intimately related to the equilibrium configurations of grafted polymers and polymer bridges. By measuring and modeling the pair interaction energies as a function of the essential system parameters (solution hybridization free energies, DNA concentrations, temperature, interparticle separation, etc.), we are able to develop simple, numerical tools that can be used to guide both experiment and simulation. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X44.00003: Rheological and scattering properties of cross-linker-free microgels Zhiyong Meng, Chinedum Osuji Microgel suspensions are intriguing tunable systems in part due to their pH/temperature responsivity at the single particle level. Particle collapse during volume transitions is heavily mediated by the presence of cross-links in the system. Here we examine the rheology and light scattering of microgel suspensions based on poly($N$-isopropylacrylamide-\textit{co}-acrylic acid) (pNIPAm-AAc) in the limit of vanishing cross-linking density. One issue of concern is centered on the nature of these fluids -- are they simple polymer solutions or real particulate suspensions? A combination of concentration-dependent viscometry and static light scattering demonstrates conclusively that these are particulate suspensions. The absence of cross-linkers provides a sharper volume collapse at the LCST in comparison with heavily cross-linked particles. Furthermore, at fixed mass content, cross-linker-free microgel suspersions display a much higher shear modulus than cross-linked counterparts due to their larger particle size, which implicates the use of these particles in rheological modification. We survey the frequency dependence and yielding response of these suspensions as a function of temperature and composition. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:42PM |
X44.00004: Normal Modes and Density of States of Disordered Colloidal Solids Invited Speaker: The normal modes and the density of states (DOS) of any material provide a basis for understanding its thermal and mechanical transport properties. In perfect crystals, normal modes take the form of planewaves, but they can be complex in disordered systems. I will show our recent experimental measurements of the normal modes, the DOS and dynamical structure factor (DSF) in disordered colloidal solids: disordered colloidal crystals composed of thermally sensitive micron-sized hydrogel particles at several different particle volume fractions, $\phi $. Particle positions are tracked over long times using optical microscopy and particle tracking algorithms in a single two dimensional (2D) [111] plane of a 3D face-centered-cubic single crystal. The dynamical fluctuations are spatially heterogeneous while the lattice itself is highly ordered. At all $\phi $, the DOS exhibits an excess of low frequency modes, a so-called boson peak (BP), and the DSF exhibits a crossover from propagating to non-propagating behavior, a so-called Ioffe-Regel (IR) crossover, at a common frequency somewhat below the BP for both longitudinal and transverse modes. As we tune $\phi $ from 0.64 to 0.56, the Lindemann parameter grows from \textit{$\sim $}3{\%} to \textit{$\sim $}8{\%}, however, the shape of the DOS and DSF remain largely unchanged when rescaled by the Debye level. This invariance indicates that the effective degree of disorder and the structure of the underlying normal modes remain essentially unchanged even in the vicinity of melting. This work was supported by NSF through grants DMR-0645596 {\&} DMR-0619424, the Sloan Foundation and American Chemical Society Petroleum Research Fund. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X44.00005: Signatures of Aging: Comparison between Colloidal and Molecular Glasses Xiaojun Di, K.Z. Win, Gregory McKenna, T. Narita, F. Lequeux, S. Pullela, Z. Cheng Colloids near to the glass concentration are often taken as models for molecular glass formers. Yet, one of the most important aspects of the dynamics of molecular glasses, structural recovery, remains to be examined in colloids. We use DWS to investigate structural recovery in a thermosensitive PNIPAM colloidal suspension in the glass concentration range. The three classical aging signatures observed in molecular glasses: intrinsic isotherms, asymmetry of approach and memory effect, are investigated with this colloid and the results are compared with those typical of molecular glasses. We find: 1 for the intrinsic isotherms, the colloid shows dramatic changes in relaxation time at equilibrium while the times required to reach the equilibrium state are nearly independent of the concentration; 2 for the asymmetry of approach, the observed nonlinearity is similar to that in molecular glasses; 3 for the memory experiment, while the memory effect is seen in the colloid, the response is qualitatively different than in the molecular glass. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X44.00006: Packings of soft disks Primoz Ziherl, Marija Vidmar We explore the stability of 2D ordered structures formed by soft disks treated as isotropic solid bodies. Using a variational model, we compute the equilibrium shapes and the elastic energy of disks in regular columnar, honeycomb, square, and hexagonal lattice. The results reproduce the Hertzian interaction in the regime of small deformations. The phase diagram of elastic disks is characterized by broad regions of phase coexistence; its main feature is that the coordination number of the stable phases decreases with density. These results may provide an insight into structure of the non-close-packed lattices observed in certain nanocolloidal systems. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X44.00007: Theory of effective interactions and dispersion of soft nanoparticles in polymer melts Jian Yang, Kenneth Schweizer Integral equation theory is employed to investigate the consequences of nanoparticle softness (surface fluctuations) and corrugation (discrete roughness) on the equilibrium behavior of polymer-particle mixtures in the dilute filler limit. Monomer-particle pair correlations exhibit qualitatively different features relative to hard spheres which depend on both roughness and softness. Under athermal nonadsorbing polymer conditions, depletion effects on the interparticle potential-of-mean-force (PMF) are qualitatively modified by surface corrugation and/or fluctuations. As particle softness increases, monomer-scale PMF oscillations are destroyed, and the strongest attraction occurs at a particle separation and attraction depth that depends sensitively on surface fluctuation amplitude, as does the dependence on monomer-nanoparticle size asymmetry ratio (R). For corrugated particles, the most attractive nanoparticle separation does not occur at contact, and is far weaker and less sensitive to R than for hard spheres. Second virial coefficient calculations are performed to estimate how particle softness/roughness modifies miscibility in chemically matched blends. How surface corrugation and softness modifies bridging and sterically stabilized states has also been studied.. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X44.00008: Density functional theory for the structure and dynamics of solvent-free nanoparticle--organic hybrid materials Hsiu-Yu Yu, Donald Koch Nanoparticle--organic hybrid materials consist of inorganic nanocores functionalized with oligomeric organic molecules. They exhibit fluid behavior in the absence of solvent with the fluidity provided by the attached oligomers. We present a density-functional theory for the equilibrium structure and transport properties of these materials based on an assumption that the intercore forces are mediated by entropic effects associated with the conformations of the hairs subject to the constraint that the oligomer fluid is incompressible. Because each core particle carries its share of the fluid phase, the structure factor at zero wave number is equal to zero. When the radius of gyration of the oligomers is large compared with the core radius, each core experiences weak interactions with many other cores residing in its neighborhood. Exploiting this limit, the transport properties can be determined in a quasi-analytical manner based on a solution of the non-equilibrium probability density for pairs of particles experiencing a non-pairwise-additive intercore potential. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 5:06PM |
X44.00009: Yielding mechanisms and particle rearrangements in colloidal glasses and gels under shear Invited Speaker: Steady and oscillatory rheology was utilized to study the mechanical response of colloidal glasses and gels with particular emphasis in the way these are shear melted (yield) [1,2]. We used suspensions of hard sphere colloids with short-range depletion attractions induced by the addition of non-adsorbing linear polymer. The linear viscoelasticity and the yielding mechanisms at different regimes of colloid volume fraction and particle attractions are discussed. While hard sphere glasses exhibit a single step yielding due to cage breaking, attractive glasses show a two-step yielding reflecting bond and cage breaking respectively [1]. Here we present experimental data both along a line of equal attraction, varying the particle volume fraction, from an attractive glass to a low volume fraction gel as well as at intermediate and high volume fractions with increasing the attraction strength. In attractive gels yielding remains a two step process until very low $\phi $'s. The first yield strain is related with in-cage or inter-cluster bond braking while the second yield point is attributed to braking of cages or clusters into smaller constituents [3]. The latter increases as volume fraction is decreased due to enhancement of structural inhomogeneities. When the range of attraction was increased, both yield strains increase, scaling with the range of attraction and accompanied structural changes. Brownian Dynamics simulations and Dynamic Light scattering under shear (LS-echo) provide information on the microscopic particle rearrangements and structural changes during yielding and flow such as the size and structure of clusters that change under steady shear as a function of shear rate. Work in collaboration with: N. Koumakis, (FORTH), M. Laurati, S.U. Egelhaaf (U. Duesseldorf) and J. F. Brady (Caltech). \\[4pt] [1] K. Pham et al. J. Rheology 52, 649 (2008)\\[0pt] [2] M. Laurati, J. Chem. Phys. 130, 134907 (2009)\\[0pt] [3] Koumakis and Petekidis, submitted (2010); Laurati et al, submitted (2010) [Preview Abstract] |
Thursday, March 24, 2011 5:06PM - 5:18PM |
X44.00010: Ridge formation of charged end group ligands grafted on faceted nanoparticle Peijun Guo, Rastko Sknepnek, Monica Olvera de la Cruz We have investigated the conformations of charged end group ligands grafted on icosahedral nanoparticles, using a coarse-grained molecular dynamics approach. Due to a competition between the electrostatic repulsion and the hydrophobic ligand-ligand attraction, the ligand coatings form a variety of different conformations. These conformations have been compared with the case of non-charged grafted ligands. We have found that the electrostatic interaction between the charged ends drives the formation of a ridge-like structure of the ligands, which makes the nanoparticle surface highly anisotropic. We argue that the ridge-like ligand structure induces controllable directional interaction between the nanoparticles, and can drive the self-assembly of the nanoparticles into crystalline structures. [Preview Abstract] |
Thursday, March 24, 2011 5:18PM - 5:30PM |
X44.00011: Dynamics of Polymers in Colloidal Flows Hsieh Chen, Alfredo Alexander-Katz This research is motivated by recent studies on the von Willebrand factor (vWF), a large multimeric protein that plays an essential role in the initial stages of blood clotting in blood vessels. Recent experiments substantiated the hypothesis that the vWF is activated by shear stress in blood flow that causes its shape to transform from a compact globule to an extended state [1], and biological function is obtained only in the extended state. Simple simulations (which only consider a single polymer in bulk shear flow) have successfully reproduced the observed dynamics of the vWF [2]. However, a more refined model is still demanding for the better understanding of the behaviors of this biomolecule in the physiological environments. Here we refine the existing model by adding the drifting colloids into the flows to mimic the presence of the blood cells in the bloodstream. Preliminary result shows that colloids greatly influence the dynamics of the polymers. It is observed that the average extensions of polymers along and perpendicular to the shear flow direction are both increased with the presence of the colloids. \\[4pt] [1] S.W. Schneider, et. al. PNAS (2007) 104 19 7899-7903\\[0pt] [2] A. Alexander-Katz, et. al. Phys. Rev. Lett. (2006) 97 13 138101 [Preview Abstract] |
Thursday, March 24, 2011 5:30PM - 5:42PM |
X44.00012: Spontaneous asymmetry in coated spherical nanoparticles in solution and at liquid-vapor interfaces J. Matthew D. Lane, Gary S. Grest Nanoparticles in solution are often stabilized with functional coatings to prevent aggregation. We'll present recent simulations results showing that small spherical nanoparticles produce highly asymmetric coating arrangements, when coated with simple polymer chains. These coatings are not symmetric even when extremely uniform grafting arrangements and full coverages are employed. I will also discuss the geometric properties which dictate the coating shape. When particles are placed in an anisotropic environment, such as the liquid/vapor interface, the asymmetric coatings are amplified and oriented by the surface. Particle shape and its responsive behavior is seen to strongly influence interactions. Implications and examples of controlled self-assembly will be presented. [Preview Abstract] |
Session X45: Focus Session: Nanocomposite Physics I-Dispersions and Physical Properties
Sponsoring Units: DPOLYChair: Frederick Beyer, Army Research Laboratory
Room: A310
Thursday, March 24, 2011 2:30PM - 2:42PM |
X45.00001: Dynamical Aspects of Percolation Networks of Carbon Nanotubes in Polymer Composites Gyemin Kwon, Bong June Sung Carbon nanotubes (CNTs) form a percolating network easily in polymer nanocomposites due to their high aspect ratios, thus improving both electrical and mechanical properties of composites. However, poor dispersion of CNTs has been a stumbling block to their application in industry. Therefore, extensive studies on the structure and thermodynamics of CNTs have been carried out to enhance the dispersion of CNTs in composites and find optimal conditions for better electrical and mechanical properties. But little attention has been paid to the dynamic aspects of percolation networks of CNTs, which should be also a critical factor to determine physical properties of composites. In this study, we investigate the 1st order survival rate, the assortative coefficients, and the bond connectivity time correlation function of percolation networks by using molecular dynamics simulations. We find that the CNT network dynamics becomes significantly slow and the CNT networks become dynamically stable as the concentration of CNTs increases beyond the percolation threshold concentration. We also investigate the effect of intermolecular interaction between CNTs and polymers on the dynamic behaviors of CNT networks. [Preview Abstract] |
Thursday, March 24, 2011 2:42PM - 2:54PM |
X45.00002: Langevin Approach to Optimizing Thermal Conductivity in Composite Materials Abdellah Ait Moussa, K.G.S.H. Gunawardana, Kieran Mullen The quest for high thermal conductivity materials has lead to nano-composites incorporating materials with excellent thermal conductivity in a matrix of poorer thermal conductivity. To minimize the interface thermal resistance the stiff, incorporated materials can be chemically functionalized with various side chains. We report here an efficient theoretical method to evaluate different choices for functionalization. We use this method to examine how effective different alkane chains improve the heat flux through a graphene nano-sheet. [Preview Abstract] |
Thursday, March 24, 2011 2:54PM - 3:06PM |
X45.00003: Use of embedded metal nanoparticles as photothermal heaters in polymer nanocomposites Somsubhra Maity, Jason Bochinski, Laura Clarke ~Embedded metallic nanoparticles within polymer nanofibers can internally heat and thus thermally-modify (soften, melt, or bond) polymer composites when irradiated with visible light via excitation and non-radiative relaxation of the nanoparticle surface plasmon resonance. ~Because the heating originates at the nanoparticle surface and propagates outward, a strong spatial temperature gradient exists. ~We discuss a non-contact, temperature-sensitive fluorescence technique to determine local temperature within the composite, which utilizes changes in the emission spectrum of perylene......,\footnote{Bur, A. J.; Vangel, M. G.; Roth, S. \textit{Applied Spectroscopy }\textbf{2002,} 56, (2), 174-181.} in addition to determining temperature from changes in polymer morphology. The efficacy of plasmonic heating in different morphologies (nanofibers/films) as well as its effect on material mechanical properties when heated between T$_{g}$ and T$_{m}$ is discussed. The spatial specificity of the photothermal heating as determined by the nanoparticle location represents a unique nanoprocessing tool. [Preview Abstract] |
Thursday, March 24, 2011 3:06PM - 3:18PM |
X45.00004: Resistive switching in bulk polymer nanocomposites containing silver nanowires Karen Winey, Sadie White, Patrick Vora, Jay Kikkawa, Rose Mutiso Traditionally, bulk nanocomposites of electrically conducting particles and insulating polymers have been categorized as either insulating or conducting when the nanoparticle concentration is below or above the percolation threshold, respectively. We present the first examples of reversible resistive switching in bulk, glassy polymer nanocomposites. At compositions close to the electrical percolation threshold, silver nanowire-polystyrene nanocomposites demonstrate reversible resistive switching upon increase voltage at room temperature. Nanocomposites with compositions outside of this range exhibit either irreversible switching, or no switching at all. We propose that resistive switching in these materials is the result of the field-induced formation of silver filaments that bridge adjacent nanowire clusters, extending the percolation network and decreasing the sample's bulk resistivity. We also describe the temperature-dependent characterization of resistive switching in these nanocomposites between 10 and 300K. These findings break from the usual dichotomy of insulating or conducting properties in polymer nanocomposites and could inspire new devices that capitalize on this responsive behavior in these versatile materials. [Preview Abstract] |
Thursday, March 24, 2011 3:18PM - 3:30PM |
X45.00005: The Development of Structure in Nanoscale Colloidal Silica -- Polymer Nanocomposites Jeff Meth, J. David Londono, Changzai Chi, Barbara Wood, Patricia Cotts, Sangah Gam, Karen Winey, Russell Composto Controlling the state of dispersion or agglomeration in polymeric nanocomposites has a profound impact on their properties. Many nanocomposites are manufactured by a solution process. In such processes, colloidal silica dispersed in a formulation possesses a certain interparticle structure, and this structure changes as the coating formulation dries. In this work, we have measured the structure of colloidal silica -- PMMA formulations as a function of solvent content using small angle X-ray scattering (SAXS). We found that the formulations dried in two stages: concentration and neutralization. In the concentrating stage, the charged colloid structure prevails, and the formulation simply concentrated down. In the neutralization stage, the colloid gradually lost its charge. Controlling the matrix viscosity enables one to control the final state of dispersion. These findings explain how and why it is possible to create good nanodispersions in some material systems. These general findings are applicable to a wide range of material systems. [Preview Abstract] |
Thursday, March 24, 2011 3:30PM - 3:42PM |
X45.00006: Dispersion Behavior of Au Nanorods in Polymer Thin Films Mediated by Brush-Matrix Interactions Michael J.A. Hore, Russell J. Composto Moderate volume fractions ($\sim $5 v{\%}) of poly(ethylene glycol) or polystyrene-functionalized Au nanorods are incorporated into poly(ethylene oxide), poly(methyl methacrylate), or polystyrene thin films (thickness $\sim $30 nm). Their dispersion is characterized via TEM, AFM, and x-ray reflectivity. When the chemical species of the brush is identical to that of the matrix, nanorod dispersion is dominated primarily by entropy and controlled by the ratio of the chain lengths of the brush and matrix. When there is a favorable enthalpic interaction between the brush and matrix, the dispersion is independent of the molecular weights of the brush and matrix. These experimental data are compared to Monte Carlo simulations. [Preview Abstract] |
Thursday, March 24, 2011 3:42PM - 3:54PM |
X45.00007: Particle networks through aggregation in polymer nanocomposites Meisha Shofner, Jasmeet Kaur, Ji Hoon Lee Structure-property research in polymer nanocomposites has often focused on producing systems that are homogeneously dispersed in order to capitalize on the large amount of specific surface area available from nanoparticles. However, inhomogeneous dispersion is often obtained and in some cases has been deliberately sought to enhance functional properties through the formation of particle networks. In this research, we are seeking to understand how particle aggregation impacts network formation in polymer nanocomposites as a function of native particle shape. Specifically, we are characterizing nanocomposites comprised of calcium phosphate particles with different shapes and a polyhydroxybutyrate matrix. Experimental results concerning the effect of particle aggregation and shape on polymer crystalline structure, thermal transitions and mechanical properties are presented to correlate particle aggregation to network formation and understand structure-property relationships in these materials. [Preview Abstract] |
Thursday, March 24, 2011 3:54PM - 4:06PM |
X45.00008: A silica nanoparticle based ionic material Nikhil Fernandes, Zubair Azad, Emmanuel Giannelis We report an ionic fluid consisting of silica nanoparticles as the anion, and amine-terminated polyethylene glycol as the cation. Unlike previous work that has required chemical functionalization of the silica surface, the charge on the nanoparticle anion is carried by the intrinsic surface hydroxyls, simplifying the synthesis, and thus making this a simple test system to probe the physics of these nanoscale ionic materials. Charge and steric factors result in excellent dispersion of the nanoparticles in the polymer matrix. The resulting material is a soft glass that has thermal and rheological properties that depend on the silica:polymer ratio. In particular, at a critical silica:polymer ratio, the ionic material shows a significant depression of the normalized heat of melting and the melting temperature compared to samples with higher or lower silica content (showing eutectic-like behaviour), and to controls without the ionic interaction between the polymer and the particle. [Preview Abstract] |
Thursday, March 24, 2011 4:06PM - 4:18PM |
X45.00009: Structure-Properties Relationship in Segmented Polyurethane/Silica Nanoparticle Composites Matthew Hood, James Sands, John La Scala, Frederick Beyer, Christopher Li Segmented polyurethanes (SPUs) phase separate into hard and soft domains due to differences in segment composition, resulting in extraordinary mechanical properties. We have synthesized a set of SPU/nanoparticle composites possessing 25, 35 or 45wt.{\%} hard segment content and loaded with less than 5wt.{\%} silica nanoparticles (SiNPs). SiNPs were added either during SPU synthesis or blended after. Drastic effects on morphology and mechanical properties were observed. Blended composites, due to their destabilizing of the hard domain, showed decreased mechanical robustness. When particles are added, at very low SiNP concentrations, during SPU synthesis the SiNPs are covalently attached to the SPU matrix and hard domains are intact which enhanced elongation to break and tensile strength considerably. With increasing SiNP concentration this effect was reversed and hard domain crystallization was hindered. Thermal, mechanical and diffraction experiments were used to correlate the relationship between interfacial chemistry of the SiNP and SPU matrix and the mechanical properties of the composites. [Preview Abstract] |
Thursday, March 24, 2011 4:18PM - 4:30PM |
X45.00010: Polyurethane Nanocomposites Reinforced with Core-shell Magnetic Particles for Microwave Absorption Applications Zhanhu Guo, Jiahua Zhu, Rahul Patil, Neel Haldolaarachchige, David Young, Suying Wei Iron-silica core-shell particles with controlled shell thickness are fabricated using a sol-gel method. Polyurethane nanocomposites are fabricated with a surface initialized polymerization (SIP) method. The thermal stability of iron-silica NPs and its corresponding PNCs is significantly enhanced due to the barrier effect of silica shell. The anti-corrosive property of the core-shell particle is dramatically improved which is able to keep stable in 1M acid solutions. Salt fog exposure tests on PNCs reveal a better anti-corrosive performance with the incorporation of core-shell particles. By embedding different NPs, unique physical properties such as enlarged coercivity and dielectric constant (real permittivity) are observed. After coating a silica layer on iron NPs, the PNCs show lower real permittivity as compared to the PNCs filled with pure NPs. However, it is interesting to observe that only slight difference in real permeability is observed in both samples at the same loading. The permittivity and permeability of the PNCs are investigated with frequency ranging from 2-18 GHz. Results indicate that the PNCs reinforced with core-shell NPs exhibit a reflection loss in a wider frequency ranges. The maximum reflection loss is around -20 dB. [Preview Abstract] |
Thursday, March 24, 2011 4:30PM - 4:42PM |
X45.00011: Studies of Microwave Absorption Properties of Carbon Nanotubes-Epoxy Composites Z. Ye, Z. Li, J.A. Roberts, G.L. Zhao Less weight, excellent mechanical properties, and high efficiency in absorbing electromagnetic (EM) wave make carbon nanotubes (CNTs) composites attractive for microwave technology applications. Six groups of multi-walled carbon nanotube (MWCNT)-epoxy composite samples with various outside diameter (OD) distributions were fabricated. The weight percentages of MWCNTs in the polymer composites were controlled in the range from 1 to 10{\%}. A microwave resonant cavity technique was utilized to measure the microwave absorption properties of all the sixty samples near a central frequency of 9.968 GHz. The results show that the maxima of EM wave absorptions for the six groups of samples were all around 7{\%} MWCNTs weight percentage. In general, the MWCNTs with smaller diameters have higher microwave absorption at 9.968 GHz. However, the sample group M5 (OD$<$8nm) shows unusual results, a lower microwave absorption than other samples. SEM was used to study the morphologies of the MWCNT samples. Based on the SEM analysis and microwave absorption measurements, it was found that the efficiency of the microwave absorption of MWCNT-Epoxy composites is also affected by the morphologies/structures of MWCNTs in individual bundles. *The work is funded in part by AFOSR, NSF, and Louisiana Board of Regents. [Preview Abstract] |
Thursday, March 24, 2011 4:42PM - 5:18PM |
X45.00012: Ecobionanocomposites: a new class of green materials Invited Speaker: This abstract not available. [Preview Abstract] |
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