Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session U1: Isakson Prize, Adler Award, Nicholson Medal Session
Sponsoring Units: DCMP DMPChair: David Pine, New York University
Room: Morial Convention Center LaLouisiane AB
Thursday, March 13, 2008 8:00AM - 8:36AM |
U1.00001: TBD Invited Speaker: |
Thursday, March 13, 2008 8:36AM - 9:12AM |
U1.00002: Frank Isakson Prize Talk: Using ultrafast to probe the slow Invited Speaker: The field of ultrafast optics exploded on the scene with the development of mode-locked lasers, and continues to grow as technology evolves. Although often associated with highly-nonequilibrium phenomena, ultrafast experiments can be performed in a low-power regime in which electronic systems are tickled, rather than blasted. The amplitude and phase stability of today's laser oscillators allows detection of very small changes in optical response that result from weak laser excitation. While these changes can be viewed as a form of linear response, they often reveal properties that are not detected by traditional probes such as electrical conductivity or magnetic susceptibility. In this talk I will describe two examples of this approach, in the fields of high-Tc superconductivity and spin propagation in semiconductors. Somewhat paradoxically, the use of ultrafast techniques allows the observation of some rather slow effects. In the high-Tc materials, the lifetime of the photoexcited state diverges as the optical energy per laser pulse is lowered. The slow dynamics in this regime provide a window to the intrinsic inelastic scattering rate of quasiparticles, a new collective mode, and an abrupt transition in dynamics that takes place as a function of doping. In GaAs quantum wells the stability of the laser oscillator enables phase-sensitive detection of a transient spin-polarization wave generated by the interference of two excitation pulses. Measuring dynamics as function of wave vector fully characterizes the spin propagation, revealing effects such as ballistic to diffusive crossover and spin Coulomb drag. In these systems we again have focused on ``slow'' phenomena. I will describe some of our recent attempts to create and detect a long-lived, ``persistent spin-helix'' state, predicted to occur at special points in the spin-orbit coupling parameter space. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:48AM |
U1.00003: Frank Isakson Prize Talk: Optical Probes of $\pi $-Conjugated Polymers Invited Speaker: We review several \textit{optical probes} that have been applied to $\pi $-conjugated polymers over a time period of $\sim $ 30 years. These include linear and nonlinear optical spectroscopies, resonant Raman scattering, transient and steady state photomodulation, photoluminescence and laser action, and optically detected magnetic resonance spectroscopy. The application of these techniques has revealed a myriad of important information on the interaction that govern the optical, electrical and magnetic properties of these materials; including electron-phonon interaction, electron-electron (e-e) and electron-hole (e-h) interactions, interchain coupling, spin-lattice and spin-orbit coupling. These properties are very important for various \textit{optoelectronic applications}, in which the polymers serve as active layers. The following picture of the excited state properties of these polymers has emerged. (i) The e-e and e-h interactions are substantial, and as important as the electron phonon interaction. This leads to relatively large intrachain exciton binding energy of $\sim $0.5 eV, and exchange energy between the singlet and triplet lowest states of $\sim $0.7 eV. (ii) There are few important excited states with odd and even parity symmetry that govern the nonlinear optical spectra of these materials. (iii) The primary photoexcitations are intrachain excitons in isolated chains, and both excitons and polaron pairs in chains coupled by interchain interaction. (iv) The most strongly coupled phonons are amplitude modes of which frequencies and oscillator strengths are very sensitive to the existence of excess charges on the chains. (v) Excess charges are accommodated on the chains in the form of polarons with relatively large relaxation energy ranging from 0.1 to 0.5 eV. (vi) The spin orbit coupling is very weak in these materials, but can be tuned by involving heavy atoms in the polymer building blocks. (vii) The spin relaxation time for spin $\raise.5ex\hbox{$\scriptstyle 1$}\kern-.1em/ \kern-.15em\lower.25ex\hbox{$\scriptstyle 2$} $ polarons is relatively long of $\sim $ 1 microsecond. The two latter properties may lead to new applications in the field of Organic Spintronics. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:24AM |
U1.00004: David Adler Lectureship Award Talk: Lattice instabilities and ferroelectricity in complex oxides Invited Speaker: In perovskite oxides, layered perovskites and other complex-structured oxide families, a wide variety of distorted equilibrium structures can be realized, including ferroelectric, antiferroelectric, antiferrodistortive, and mixed-character structures. For an individual material, the equilibrium structure can be understood as being produced by the freezing-in of one or more lattice instabilities of an appropriate high-symmetry reference structure; this concept is central to the long-established soft-mode theory of ferroelectricity. In fact, first-principles phonon-dispersion calculations show that the high-symmetry reference structures of many complex oxides have entire ranges of instabilities that do not contribute to the bulk ground state structure. In this talk, we discuss how the information from first-principles studies of these systems provides guidance for altering the balance of the competition of instabilities of different character through changes in electrical and mechanical boundary conditions characteristic of epitaxial thin films, superlattices, and nanoparticles, leading to the realization of non-bulk phases. For example, it has been shown both theoretically and experimentally that SrTiO$_3$, which has a nonpolar bulk ground state, can be driven ferroelectric by epitaxial strain. To illustrate the further development and application of these ideas, we present results for CaTiO$_3$ and discuss other materials which, while nonpolar in bulk, can be driven in this way through a phase boundary to become ferroelectric. New ferroelectrics thus obtained could have combinations of tunable properties, including switchable polarization, magnetic ordering, and dielectric and piezoelectric response, desirable for current and future technological applications. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 11:00AM |
U1.00005: Nicholson Medal Talk: Simulational Physics in a Shrinking World Invited Speaker: Over the past few decades simulational physics has been elevated to a major new methodology of quantitative science, {\it en par} with theory and experiment. Once, only researchers in the developed world with access to powerful supercomputers were able to carry out state-of-the-art simulational studies; but with the advent of fast, low cost microprocessors, simulational research can now be performed anywhere in the world. In 1986 the Center for Simulational Physics was founded at the University of Georgia, and the subsequent introduction of an annual Workshop has brought together countless individuals from different countries in a highly interactive atmosphere. (2008 marks the 21st such Workshop.) This ongoing Workshop series has brought together countless ``students'' who have gone on to teach other students and also to interact with each other. Similar Workshop series have now been created in Brazil and in China. This talk will review other aspects of the internationalization of simulations research and instruction and will emphasize the importance of human outreach activities at all different levels. [Preview Abstract] |
Session U2: Electron Nematics
Sponsoring Units: DCMPChair: Jan Zaanen, University of Leiden
Room: Morial Convention Center LaLouisiane C
Thursday, March 13, 2008 8:00AM - 8:36AM |
U2.00001: Transport anisotropy as a signature of electron nematicity Invited Speaker: Strong electron correlations often give rise to novel phenomena that are never found in ordinary materials. One of such phenomena is the emergence of electron nematicity (EN), which was theoretically proposed in 1998 to occur due to a combined effect of electron self- organization and quantum fluctuations. Experimentally, the EN was first discovered in 1999 in the two-dimensional electron gas (2DEG) at high Landau levels, where a clear transport anisotropy was found to grow upon lowering temperature in the mK region. In search for the signatures of the EN in cuprates, we have done extensive transport measurements of La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) and YBa$_2$Cu$_3$O$_y$ (YBCO) systems using high-quality single crystals. We discovered in 2001 that the in-plane resistivity anisotropy in untwinned single crystals of LSCO and YBCO in the lightly hole-doped region grows below $\sim$150 K with decreasing temperature, similar to the case in 2DEG, albeit the much higher temperature scale. In those samples, the easy transport axis was apparently dictated by the orthorhombic crystal structure; however, the orthorhombicity $\eta$ was only up to 1.5\%, while the resistivity anisotropy was up to a factor of 3, which was obviously too large for the small $\eta$. Furthermore, the anisotropy in YBCO was found to be {\it enhanced} with decreasing $y$ below $\sim$6.5 despite the {\it decreasing} $\eta$ until the crystal structure turns to tetragonal at $y \simeq$ 6.30. While this result gave strong evidence for the self-organized EN in high-$T_c$ cuprates, it was not completely conclusive because of the existence of the orthorhombicity that chooses the preferred direction; also, the lack of support from neutron scattering kept the skepticism remain. However, very recently, neutron scattering has finally found corroborating anisotropy in YBCO and convincing evidence for EN in a related oxide Sr$_3$Ru$_2$O$_7$ was obtained, which together strengthened the case for cuprates considerably. \\ \\ In collaboration with Kouji Segawa, Seiki Komiya, and A. N. Lavrov. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 9:12AM |
U2.00002: Theory of the nematic quantum critical point in a nodal superconductor Invited Speaker: In the last several years, experimental evidence has accumulated in a variety of highly correlated electronic systems of new quantum phases which (for purely electronic reasons) spontaneously break the rotational (point group) symmetry of the underlying crystal. Such electron ``nematic'' phases have been seen in quantum Hall systems[1], in the metamagnetic metal Sr$_3$Ru$_2$O$_7$[2], and more recently in magnetic neutron scattering studies of the high temperature superconductor, YBCO[3]. In the case of a high $T_c$ superconductor, the quantum dynamics of nematic order parameter naturally couples strongly to quasiparticle (qp) excitations. In this talk, I will discuss our recent results on the effects of the coupling between quantum critical nematic fluctuations and the nodal qp's of a d-wave superconductor in the vicinity of a putative quantum critical point inside the superconducting phase. We solve a model system with $N$ flavors of quasiparticles in the large $N$ limit[4]. To leading order in $1/N$, quantum fluctuations enhance the dispersion anisotropy of the nodal excitations, and cause strong scattering which critically broadens the quasiparticle peaks in the spectral function, except in the vicinity of ``the tips of the banana,'' where the qp's remain sharp. We will discuss the possible implications of our results to ARPES and STM experiments. \par [1] M.P. Lilly, K.B. Cooper, J.P. Eisenstein, L.N. Pfeiffer, and K.W. West, PRL {\bf 83}, 824 (1999). \par [2] R. A. Borzi and S. A. Grigera and J. Farrell and R. S. Perry and S. J. S. Lister and S. L. Lee and D. A. Tennant and Y. Maeno and A. P. Mackenzie, Science {\bf 315}, 214 (2007). \par [3] V. Hinkov, D. Haug, B. Fauqu\'e, P. Bourges, Y. Sidis, A. Ivanov, C. Bernhard, C. T. Lin, B. Keimer, unpublished. \par [4] E.-A. Kim, M. Lawler, P. Oreto, E. Fradkin, S. Kivelson, cond-mat/0705.4099. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:48AM |
U2.00003: Metamagnetic Nematic Phase of Sr$_{3}$Ru$_{2}$O$_{7}$ Invited Speaker: In this talk I will review our group's recent observations that a quantum phase with pronounced electrical transport anisotropies forms in the vicinity of a metamagnetic quantum critical point in Sr$_{3}$Ru$_{2}$O$_{7}$. The behaviour, which is strongly dependent on disorder and is only seen in the highest purity crystals, has phenomenological similarities with prior observations on two-dimensional electron gases in semiconductor devices [1,2]. Its appearance in bulk in Sr$_{3}$Ru$_{2}$O$_{7}$ has allowed us to perform a number of thermodynamic measurements, and also offers the promise of study using modern surface-based spectroscopies such as angle resolved photoemission and spectroscopic imaging scanning tunneling microscopy. References [1] For example M.P. Lilly \textit{et al.}, Phys. Rev. Lett. \textbf{82}, 394 (1999); ibid \textbf{83}, 824 (1999) [2] W. Pan \textit{et al}., Phys. Rev. Lett. \textbf{83}, 820 (1999). Collaborators: S.A. Grigera$^{1}$, R.A. Borzi$^{1,2}$, A. Rost$^{1}$, J.F. Mercure$^{1}$, J. Farrell$^{1}$, R.S. Perry$^{3}$, A.G. Green$^{1}$, M. Allan$^{1}$, M. Wang$^{4}$, J. Lee$^{1}$, F. Baumberger$^{1}$, S.J.S Lister$^{1}$, S.L. Lee$^{1}$, J.C.S. Davis$^{1,4}$, Z.X. Shen$^{5}$, Y. Maeno$^{6}$. $^{1}$ University of St Andrews, Scotland $^{2 }$INFTA, La Plata, Argentina $^{3}$ University of Edinburgh, Scotland $^{4 }$Cornell University, USA $^{5}$ Stanford University, USA $^{6}$ Kyoto University, Japan [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:24AM |
U2.00004: Electronic Nematic Liquid in Correlated Systems Invited Speaker: It was proposed that the electronic nematic phase with a broken rotational symmetry is a generic ground state of a doped Mott insulator, and it has attracted much attention with the discovery of anisotropic quantum Hall phases in GaAs-heterostructures in large magnetic fields. A series of recent experiments on the bilayer ruthenate Sr$_3$Ru$_2$O$_7$ also suggest the existence of an anisotropic metallic phase which is uncovered by tuning the magnetic field. In this talk, I will show that the two consecutive metamagnetic transitions, a large residual resistivity, and an anisotropic magnetoresistance observed in the bilayer ruthenate can be understood via the electronic nematic order and its domains. I will also discuss an SO(4) invariance at the critical point between the electronic nematic and d-wave superconducting states, and its relation to the similar SO(4) invariance between the d-density wave and d-wave superconducting states. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 11:00AM |
U2.00005: Spins in cuprates near the edge of the superconducting phase - incoherent pairing, fluctuations on all timescales and observation of pseudogap energies. Invited Speaker: When doping is just enough to create the superconducting phase spins behave very differently from heavily doped cuprates. Nonetheless low doping reveals the key ingredients for the coherence of superconducting pairs. The ordered spins of the insulating antiferromagnet are replaced by a hedgehog phase with isotropically polarized spins, and a commensurate central mode with slow short-range spin correlations extending over four planar unit cells. A subcritical 3D enhancement is destroyed by temperature and energy. The spins follow not via the coherence of charge pairs but the structures caused by hole doping, so that glassy short range spin order occurs within both the superconducting and normal phase. The tumbling regions of spins exhibit a power-law spectrum similar to 1/f noise down to microeV energies. Excitations are overdamped with a millieV relaxation rate a thousand times faster, unlike the well-defined resonance familiar at large doping. Below 50 K the scale length is geometric and not linked by velocity to dynamic widths. As excited states are depopulated spin weight transfers to the central mode. At energies 300,000 times larger, decay of paramagnons into pseudogap states has been detected [1], a strong indication that it is the large density of high-energy spin states that provides the superconducting glue, rather than the resonance which is absent as a sharp spectral feature. \newline [1] C.Stock et al, Phys. Rev. B 75,172510 (2007). [Preview Abstract] |
Session U3: Simple Views on Polymers at Surfaces and Interfaces: Symposium Honoring P G de Gennes
Sponsoring Units: DPOLYChair: E. Raphael
Room: Morial Convention Center RO2 - RO3
Thursday, March 13, 2008 8:00AM - 8:36AM |
U3.00001: Polymer adsorption Invited Speaker: The aim of this talk is to review Pierre-Gilles deGennes' work on polymer adsorption and the impact that it has now in our understanding of this problem. We will first present the self-consistent mean-field theory and its applications to adsorption and depletion. De Gennes most important contribution is probably the derivation of the self-similar power law density profile for adsorbed polymer layers that we will present next, emphasizing the differences between the tail sections and the loop sections of the adsorbed polymers. We will then discuss the kinetics of polymer adsorption and the penetration of a new polymer chain in an adsobed layer that DeGennes described very elegantly in analogy with a quantum tunneling problem. Finally, we will discuss the role of polymer adsorption for colloid stabilization. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 9:12AM |
U3.00002: Polymer brushes Invited Speaker: The polymer brush is an ensemble of macromolecules end-tethered to a substrate. The so-called Alexander-de Gennes polymer brush model (S. Alexander, J. Physique 1977, and P.-G. de Gennes, Macromolecules 1980) opened a new field in polymer science and provided a theoretical framework to look at brush-like polymer systems. In this presentation, we will first briefly review the ideas and concepts behind the Alexander-de Gennes model of a planar polymer brush and summarize its major findings. We will then focus on the general impact of this seminal work by demonstrating numerous theoretical developments initiated by the Alexander-de Gennes model with specific emphasis on polyelectrolyte brushes and biological brush-like systems. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:48AM |
U3.00003: Adhesion Invited Speaker: Adhesion is a highly practical subject in which the vast majority of published work is either chemical in nature, concerned with chemistry that is thought to occur at an adhesive junction or chemistry of adhesives, or essentially mechanical, concerned with the mechanics of testing and failure of adhesive systems. The role of polymer physics in general and de Gennes' work in particular is to discover what happens at the scale of the polymer chain and hence form a bridge between these two approaches. A distinguishing feature of Gennes' work in adhesion is the way he developed simple models that permitted us to see the essential physics of the situation. This is particularly true in his work in viscoelastic effects on toughness (the de Gennes trumpet) where more sophisticated mechanics had been done but the physical situation was obscure. Much of his work was concerned with the effects of connector molecules in toughening an interface in both elastomeric and glassy materials. This work has been extended by a number of authors and forms the basis of our current understanding of the area. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:24AM |
U3.00004: Slippage Invited Speaker: After a brief overview of the usual assumptions made to fix the boundary conditions for the flow velocity at a solid wall, we shall present the early conjecture made by Pierre Gilles de Gennes in 1979, predicting huge slip at the wall for polymer melts flowing against smooth non adsorbing surfaces. We shall then discuss how interplay between theory and experiments has allowed to produce a refined picture of the molecular mechanisms of friction at polymer interfaces, which quantitatively accounts for the three different friction regimes which have been experimentally identified when increasing the shear rate in simple shear experiments of polymer melts. The key idea (Brochard and de Gennes, 1992) focuses on the effect of a few surface anchored chains. If these chains are not rigid, they will deform under the effect of the friction forces resulting from entanglements between surface and bulk chains. When increasing the shear rate, the surface chains are thus progressively stretched, and can disentangle from bulk chains: a dynamic decoupling between surface and bulk polymer occurs. We shall discuss the available sets of experimental data (and the corresponding techniques which have been developed to either directly characterize wall slip or yield friction forces measurements). At present, the low surface density regime is fully understood, while series of data in the case of large grafting densities are available, but still lack of an adequate model. We shall finally draw lines of possible extensions of the ideas to other systems which start to be investigated as regards to friction: polyelectrolytes - grafted polyelectrolyte chains, rigid polymers, other complex fluids. We shall also discuss what is known at present on slip at the wall in the case of simple fluids, a situation which appear much more difficult to model than the polymer case. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 11:00AM |
U3.00005: Polymers in Confined Geometry Invited Speaker: Thanks to P. G. de Genne's famous~``$n$ = 0'' theorem (relating the configuration of a polymer chain to a magnetic phase transition with an order parameter of $n$ = 0 component), the swelling exponent \textit{$\nu $} ($R=N^{\nu }a)$ was calculated in terms of space dimension $d$ and fitted the qualitative Flory calculation. Using scaling laws, or the~``blob'' picture, it became possible to derive the conformation of a chain confined in a tube or a slit. The dynamics of confined polymers followed immediately. The new feature was the screening of the hydrodynamic interactions in confined geometry leading to Rouse-like behaviour. In a third step, de Gennes focused on the forced penetration of polymers (linear, branched, stars, neutral or charged) in narrow tubes, when they are submitted to a flow (or an electrical field for DNA). From all these calculations, the conclusion was that the threshold velocity (or field) corresponds to the penetration of the first blob. Afterwards, the friction force increases linearly with the penetrated length, whereas the confinement force remains constant. In the last few years, with his collaboration we studied the penetration of DNA in ~``soft'' tubes. We predicted a transition from extended to globular DNA observed experimentally. We also showed that semi flexible polymer chains, like DNA, are ideal in 3d, but swollen effects are more pronounced in confined geometries. We will also discuss experiments inspired by his work on confined polymers, in the last thirty years. [Preview Abstract] |
Session U4: Interferometry in the Quantum Hall Regime
Sponsoring Units: DCMPChair: Steven Simon, Lucent Tech/Bell Labs-Murray
Room: Morial Convention Center 206
Thursday, March 13, 2008 8:00AM - 8:36AM |
U4.00001: Measuring Fractional Statistics with Fabry-Perot Quantum Hall Interferometers Invited Speaker: Laughlin quasiparticles are the elementary excitations of a highly-correlated fractional quantum Hall electron fluid. They have fractional charge and obey fractional statistics. The quasiparticles can propagate quantum-coherently in chiral edge channels, and constructively or destructively interfere. Unlike electrons, the interference condition for Laughlin quasiparticles has a non-vanishing statistical contribution that can be observed experimentally. Two kinds of interferometer devices have been realized. In the primary-filling interferometer, the entire device has filling 1/3, and the e/3 edge channel quasiparticles encircle identical e/3 island quasiparticles. Here the flux period is h/e, same as for electrons, but the back-gate charge period is e/3. In the second kind of interferometer, a lower density edge channel at filling 1/3 forms around a higher density island at filling 2/5, so that e/3 edge quasiparticles encircle e/5 island quasiparticles. Here we observe superperiodic oscillations with 5h/e flux and 2e charge periods, both corresponding to excitation of ten island quasiparticles. These periods can be understood as imposed by the anyonic braiding statistics of Laughlin quasiparticles. This work was done in collaboration with Fernando E. Camino, Ping Lin and Wei Zhou. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 9:12AM |
U4.00002: Interactions and Disorder in Quantum Hall Interferometers Invited Speaker: Quantum Hall (QH) devices are supposed to be an ideal laboratory for the study of interference effects, because within a conductance plateau, the bulk of a sample is insulating and the current is confined to conducting edge states. Closed interference paths can be defined with the help of two narrow constrictions, which mediate tunneling from one edge to the other. Quantum interference should then manifest itself in flux- and gate-voltage-dependent conductance oscillations. When there is an integer quantized Hall state within the constrictions, a region between them, with higher electron density, may form a compressible island. Electron-tunneling through this island can lead to residual transport, modulated by Coulomb-blockade type effects. Then, the coupling between the fully occupied lower Landau levels and the higher partially occupied level gives rise to flux subperiods smaller than one flux quantum [1]. We generalize this scenario to other geometries and to fractional quantum Hall systems, and compare our predictions to experiments. For interferometers probing non-abelian statistics in the $\nu=5/2$ QH state, current-carrying quasiparticles flow along edges that encircle $N_{qp}$ bulk quasiparticles, which are localized at impurities. The interference pattern depends on whether $N_{qp}$ is even or odd, and is affected by a coupling that allows tunneling of neutral Majorana fermions between the bulk and edge. While at weak coupling this tunneling degrades the interference signal, at strong coupling the bulk quasiparticle becomes essentially absorbed by the edge and the interference signal is fully restored [2]. These works have been done in collaboration with B.I.~Halperin, S.H.~Simon, and A.~Stern. \\ $[1]$ B. Rosenow and B.I.~Halperin, Phys. Rev. Lett. 98, 106801 (2007).\\ $[2]$ B.~Rosenow, B.I.~Halperin, S.H.~Simon, and A.~Stern, arXiv:0707.4474. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:48AM |
U4.00003: Quantum Oscillations and the $\nu = 5/2$ Fractional Quantum Hall State in Mesoscopic Quantum Hall Interferometers Invited Speaker: Magnetotransport study of mesoscopic quantum Hall corrals fabricated from a high mobility GaAs/AlGaAs quantum well structure will be presented. Prominent Aharonov-Bohm-like quantum oscillations are observed at magnetic fields just below even integer quantum Hall plateaus at low temperatures. We establish the fundamental flux period of these oscillations as $\Phi_{0}/f$, where $\Phi_{0}$ is the universal flux quantum and $f$ is the integer number of fully filled Landau levels. The flux period of the observed quantum oscillations thus fundamentally differs from that of Aharonov-Bohm effect which has a period of one flux quantum, $\Phi_{0}$. The observed quantum oscillations in the quantum Hall corrals can be understood within the Coulomb blockade model of quantum Hall interferometers [1] as forward tunneling and backscattering, respectively, through the center island of the corral from the bulk and the edge states. In the second Landau level, we observe an extended series of oscillations with flux period of $\Phi_{0}/2$. The Aharonov-Bohm-Like oscillations are found to coexist with the $\nu = 5/2$ fractional quantum Hall effect. We detail the transport properties of the $\nu = 5/2$ fractional quantum Hall state and the mesoscopic quantum Hall corral in the second Landau level. \newline \newline [1] R. Rosenow and B.I. Halperin, Phys. Rev. Lett. {\bf 98}, 106801 (2007). [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:24AM |
U4.00004: Non-Abelian Interferometry Invited Speaker: Topologically-ordered phases supporting excitations with non-Abelian braiding statistics are expected to occur at several observed fractional quantum Hall plateaux. These states are of particular interest as they may provide a platform for topological quantum computation. Interferometric experiments are likely to play a crucial role in both determining the non-Abelian nature of these states and in their potential applications for quantum computing. I will discuss interferometric experiments designed to detect such non-Abelian quasiparticle statistics -- one of the hallmark characteristics of the Moore-Read and Read-Rezayi states, which are likely candidates for the observed fractional quantum Hall plateaux at $\nu=5/2$ and $12/5$ respectively. Aside from their potential utility for experimental verification of non-Abelian anyonic statistics, such interferometric experiments would provide the most promising route to qubit read-out in a topological quantum computation. With these potential applications in mind, I will also address interferometric measurements of states having superpositions of anyonic charges and discuss their measurement collapse behavior. [Preview Abstract] |
Session U5: Epitaxial Oxide/Semiconductor Systems
Sponsoring Units: FIAPChair: Alex Demkov, University of Texas at Austin
Room: Morial Convention Center RO1
Thursday, March 13, 2008 8:00AM - 8:36AM |
U5.00001: Structural and Electronic Properties of Epitaxial Complex Oxide-Silicon Interfaces Invited Speaker: Following the discovery of a method to deposit a crystalline complex oxide, SrTiO$_{3}$, directly on Si, a significant effort has been devoted to understand the nature of crystalline oxide-semiconductor interfaces. This research has in part been motivated by the desire to find a suitable high dielectric constant (high-k) insulator to replace SiO$_{2}$ in Si-based field effect transistors. In this talk, we present results on the detailed structural and electronic properties of epitaxial SrTiO$_{3}$-Si and LaAlO$_{3}$-SrTiO$_{3}$-Si heterostructures grown by oxide molecular beam epitaxy. Using synchrotron crystal truncation rod analysis and reflection high energy electron diffraction, we place constraints on the detailed positions of the atoms at the oxide-semiconductor interface. These results are compared with predictions based on first-principles calculations of the interface. We also discuss capacitance-voltage and inelastic tunneling spectroscopy measurements of the electronic and dielectric properties of these high-k oxide-semiconductor systems. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 9:12AM |
U5.00002: From GaAs MOSFETs to epitaxial oxides on silicon : old and new MBE stories. Invited Speaker: 50 years of intense development in chip technology did not fundamentally change the initial concept: the capability to modulate charges right at the interface between two dissimilar materials. This concept allowed the whole microelectronic industry to develop exponentially and to disseminate its products all over our environment. Two simple reasons can be given to such a success: i) device scaling was a simple and cost-effective method to make chip faster; ii) faster chips simply allowed our computing environment to perform new functions. None of the two reasons given will remain true in the next few years. Scaling has come to an end. The materials properties will be scaled instead of the device itself. The recent introduction of high-k materials perfectly illustrates such a transition. The future success for chip makers might then depends on new rules: i) many new materials will be developed, and interfaces, still a key element for a device to perform better, will multiply; ii) The future technology developments will be more expensive and generate smaller performance gains. The added value might be then in the integration of functions implemented in these new materials. A few years ago, molecular beam epitaxy allowed band-gap engineering in compound semiconductors to build new devices and, more recently, was successfully used to explore the physics and chemistry of complex perovskites. During the last years, new developments have been made to combine oxides and semiconductors. In particular, many groups have reported the growth of epitaxial oxides on silicon surfaces. The recent and renewed interest in compound semiconductor MOSFETs structures might indeed be seen as a logical conclusion for this evolution. This presentation will review the latest developments in the field, with a focus on the activities taking place at IBM Zurich. It will also put them in perspective with the new rules the microelectronic industry might follow. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:48AM |
U5.00003: First-principles modeling of functional oxides-semiconductor interfaces Invited Speaker: Search is ongoing for new classes of materials and structures based on complex oxides. One important example is the multiferroics which combine ferromagnetism or antiferromagnetism and ferroelectricity in a single phase. The coexistence, and occasionally the coupling of the two order parameters, has opened new opportunities for multifunctional applications. A promising route for practical applications is to employ thin films and multilayered structures, the properties of which can be readily manipulated at the nanoscale. Epitaxial multiferroic films are currently being developed through integration with semiconductors. Despite the progress in synthesis and experimental characterization, the roles of the interface phenomena, including strains, chemistry, etc., on the ferroelectric and magnetic properties of multiferroic thin films is not fully understood and difficult to differentiate experimentally. In this talk we illustrate the utility of theoretical methods based on density functional theory in understanding these technologically relevant structures. We describe examples of oxide-semiconductor interfaces based on YMnO$_3$ on GaN that have been synthesized recently and show how interfacial spins behave differently from those in the bulk. The interfacial effects lead to an intriguing behavior of the band offsets. We also discuss our ongoing investigation of electric field doping interfaces. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:24AM |
U5.00004: Thin Film Synthesis of New Complex Titanates. Invited Speaker: Thin film deposition methods allow for one to synthesize rationally specific compositions in targeted crystal structures. Because most of the thermodynamic and kinetic variables that control the range of materials that can be synthesized are unknown for specific compounds/processes, epitaxial stabilization and design of artificially layered crystals are driven through empirical investigations. Using examples taken primarily from the family of complex titanates, which exhibit a range of interesting physicochemical behaviors, the thermodynamic and kinetic factors that control materials design using thin film deposition are discussed. The phase competition between the pyrochlore and the (110) layered perovskite structure in the \textit{RE}$_{2}$Ti$_{2}$O$_{7}$ family (\textit{RE} = rare-earth, Bi) will be explored, using pulsed laser deposition as a synthesis method. For \textit{RE} = Gd, Sm, Nd, and La, the phase stability over a wide range of conditions is dictated entirely by substrate choice, indicating that the free energies of the phases are similar enough such that by controlling nucleation one controls the phase formation. In a related fashion, the growth of \textit{AE}Ti$_{2}$O$_{5}$ films (\textit{AE} = Ba or Sr) will be discussed with respect to the formation of single-phase films or films that phase separate into \textit{AE}TiO$_{3}$ and TiO$_{2}$. The entire Ba$_{1-x}$Sr$_{x}$Ti$_{2}$O$_{5}$ series was grown and will be discussed with respect to growth technique (using MBE and PLD) and/or substrate choice. In this case, rock-salt substrates, which are not expected to interact strongly with any phase in the system, allow for the formation of single-phase films. Finally, several examples will be discussed with respect to the (SrO)$_{m}$(TiO$_{2})_{n}$ system, which includes the perovskite SrTiO$_{3}$ and the Ruddlesden-Popper phase Sr$_{2}$TiO$_{4}$, grown using layer-by-layer molecular beam epitaxy. The solid phase epitaxial formation of the perovskite SrTiO$_{3}$ from superlattices of rock-salt SrO and anatase TiO$_{2}$ is discussed from both a kinetic and thermodynamic perspective by exploring the growth of a range of $m$ and $n$ values. Using similar arguments for stability, new layered intergrowths in the Sr$_{m}$TiO$_{2+m}$ family are presented and their structures are discussed. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 11:00AM |
U5.00005: Semiconductor-on-epitaxial insulator: towards ultrathin and nonclassical semiconductor devices Invited Speaker: The microelectronics industry is currently moving from bulk Si field-effect transistors (FETs) with silicon-dioxide gate insulators to high-k gate dielectric FETs and semiconductor-on-insulator (SOI) substrates, with alternative non-Si channel materials and nonplanar device layouts on the horizon. The possibility of integrating epitaxial insulator layers with well-controlled bandgaps and near-monolayer thickness control may open up new opportunities for nonclassical devices and possibly optical sources. Unlike their III-V counterparts, where epitaxial heterostructures have been available for decades, epitaxial oxide-based SOI devices have the crucial advantage of potential integrability with dominant silicon technology. This talk will discuss the examples of tunneling FETs and real-space transfer devices, as well as a proposed tunneling-based SOI intersubband laser. At this point, all of the proposed devices require epitaxial control and material quality that exceeds the state-of-the-art. Still, the unique characteristics deriving from quantum mechanical tunneling make such devices an interesting playground for innovative device research, essentially replicating the III-V heterostructure device platform in the silicon-dominated microelectronics industry just as standard Si FET technology heads towards the long-predicted end of the miniaturization paradigm. [Preview Abstract] |
Session U6: Computational and Theoretical Challenges in Predicting Climate Change
Sponsoring Units: DCOMPChair: David Bader, Lawrence Livermore National Laboratory
Room: Morial Convention Center RO4
Thursday, March 13, 2008 8:00AM - 8:36AM |
U6.00001: Radiative Transfer in Climate Models Invited Speaker: Radiation is a key physics element in both the maintenance of the climate as well as in driving climate change. The absorption of the Sun's radiant energy by the surface-atmosphere system in the ultra-violet, visible and near-infrared spectral regions, and the absorption and emission of infrared radiation by the surface and atmosphere, together govern the planetary energy balance. The importance of radiative processes enters in both the ``forcing'' of the climate system and in the ``feedbacks'' that amplify the response to perturbations of the energy balance. As examples, we will examine the natural and anthropogenic radiative forcings that have occurred over the 20$^{th}$ century, our understanding of the governing processes and the challenges in representing them in climate models. The quantitative description comprises the determination of the forcing at the surface and in the atmosphere due to: emissions of the long-lived greenhouse gases (e.g., carbon dioxide), ozone precursors, and pollution particulates (e.g., sulfate and black carbon); changes in land surface properties; changes in solar irradiance; and particulates arising due to episodic volcanic eruptions. The various types of forcings are governed by fundamentally different underlying mechanisms, have distinct space-time dependencies and uncertainties, and exert varying signatures in terms of the climate system responses. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 9:12AM |
U6.00002: Climate Feedbacks and Their Simulation in Coupled Ocean Atmosphere Models Invited Speaker: The response of Earth's climate to an increase in greenhouse gases depends on a complex superposition of feedback processes. These processes act to either amplify or dampen the climate's response to an initial perturbation in the Earth's radiative energy budget. Differences in the representation of these feedback processes in current models represent a primary source of uncertainty in model projections of future climate change. Progress in reducing uncertainties in model predictions of climate change therefore requires an accurate assessment of the differences in various feedback strengths between models. In this talk I will review the key climate feedback processes and assess their range of values in current models. Attention will be focused on the feedbacks from water vapor and clouds, which represent the most important climate feedbacks in current models. My presentation will describe the prevailing view behind these feedbacks and review observational evidence used in assessing the fidelity of their representation in current models. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:48AM |
U6.00003: Objective methods for detecting climate change and attribution of causes Invited Speaker: |
Thursday, March 13, 2008 9:48AM - 10:24AM |
U6.00004: Predicting climate change: Uncertainties and prospects for surmounting them Invited Speaker: General circulation models (GCMs) are among the most detailed and sophisticated models of natural phenomena in existence. Still, the lack of robust and efficient subgrid-scale parametrizations for GCMs, along with the inherent sensitivity to initial data and the complex nonlinearities involved, present a major and persistent obstacle to narrowing the range of estimates for end-of-century warming. Estimating future changes in the distribution of climatic extrema is even more difficult. Brute-force tuning the large number of GCM parameters does not appear to help reduce the uncertainties. Andronov and Pontryagin (1937) proposed \textit{structural stability} as a way to evaluate model robustness. Unfortunately, many real-world systems proved to be structurally unstable. We illustrate these concepts with a very simple model for the El Ni\~{n}o--Southern Oscillation (ENSO). Our model is governed by a differential delay equation with a single delay and periodic (seasonal) forcing. Like many of its more or less detailed and realistic precursors, this model exhibits a Devil's staircase. We study the model's structural stability, describe the mechanisms of the observed instabilities, and connect our findings to ENSO phenomenology. In the model's phase-parameter space, regions of smooth dependence on parameters alternate with rough, fractal ones. We then apply the tools of random dynamical systems and \textit{stochastic structural stability} to the circle map and a torus map. The effect of noise with compact support on these maps is fairly intuitive: it is the most robust structures in phase-parameter space that survive the smoothing introduced by the noise. The nature of the stochastic forcing matters, thus suggesting that certain types of stochastic parametrizations might be better than others in achieving GCM robustness. This talk represents joint work with M. Chekroun, E. Simonnet and I. Zaliapin. [Preview Abstract] |
Session U7: Physics Demonstrations and Strategies for Teaching and Public Outreach
Sponsoring Units: FEdChair: James McGuire, Tulane University
Room: Morial Convention Center RO5
Thursday, March 13, 2008 8:00AM - 8:36AM |
U7.00001: Bringing Nano to the Public through Informal Science Education Invited Speaker: Researchers in nanoscale science and engineering communicate all the time. We give talks, present lectures, and write papers regularly. But the general public---the consumers who will use the products of our work and the voters who indirectly set the national research agenda---do not often hear us. Informal science education---including museums, TV, public lectures, popular press, etc.---is a way to connect with broader audiences in a variety of fun and effective ways. Museums, which are visited by hundreds of millions of people each year in the U.S., are popular because they are skilled at making abstract and complex phenomena comprehensible to people from all walks of life and at making the whole experience fun. This talk will provide an introduction the ``informal science education'' field, discuss the art of honing your message into clear and realistic learning goals, describe methods for understanding your audience and their background, and help researchers to appreciate the limits of what can be learned in one experience. It will also review what the public currently understands about nanoscale science and engineering and the challenges that these (mis)understandings create for museums and researchers. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 9:12AM |
U7.00002: Preparing minority undergraduate students for successful science careers. Invited Speaker: Xavier University of Louisiana is well known for being number one in graduating the most minority students in physical and biological sciences. The reason for this success is built on the concept of Standards with Sympathy in the Sciences (Triple S). This is an outgrowth of over twenty years of planning and development by the Xavier science faculty to devise a program for preparing and retaining students in the sciences and engineering. Xavier has been successfully conducting for over ten years, Summer Science Academy (SSA) for middle and high school students; Science Technology, Engineering and Mathematics (STEM) Scholars and Howard Hughes Biomedical programs for in-coming freshmen. Recently, through a grant from NSF, we have developed the Experiential Problem-solving and Analytical Reasoning (EPsAR) summer bridge program for in-coming freshmen who were given conditional admission to the university (i.e., those students who scored below the acceptable range for placement into degree mathematics courses). In this program, EPsAR participants will be engaged in problem-solving and critical thinking activities for eight hours per day, five days per week, for six weeks. Additionally, an interdisciplinary approach is taken to convey the mathematical skills learned to relate to physics, chemistry, biology, and computer science. Sixty-six students have participated in the last two years in the EPsAR program. During the first year 23 of 28 students successfully bi-passed the algebra review course and were placed into a degree credit course in mathematics. In the second year, thirty-one (31) of the 38 were advanced to a higher-level mathematics course. Twenty-three (23) out of 38 went on to degree credit math course. To retain students in the sciences peer tutoring in all the science disciplines are made available to students throughout the day for 5 days per week. Faculty and students are available to give guidance to the needed students. The University has established a Graduate Placement Office and a Center for Undergraduate Research to facilitate students' pursuit of gradate studies. The results of these efforts indicate a 40 percent graduation rate in four years and increased to 90 percent in six years in the natural sciences and 50 percent of these graduates pursue graduate/professional careers. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:48AM |
U7.00003: ``Wow'' is good, but ``I see'' is better - techniques for more effective Physics demonstrations Invited Speaker: The use of demonstrations to assist in Physics education at all levels is commonplace, but frequently lacks optimal effectiveness. In many cases, the choice of demonstration is not at issue, but rather the manner in which it is presented to the audience. Modern educational research reveals a number of simple ways to improve instruction of this kind, including objective setting, audience evaluation, concept building, and promoting engagement. These techniques and considerations will be reviewed, explained, and modeled through a demonstration of ``Why Mr. Fork and Mr. Microwave Oven don't get along.'' [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:24AM |
U7.00004: Gravity - The Engine of the Universe Invited Speaker: The pervasive nature of gravity ought to give us pause. Since gravity is inescapable, one could reasonably claim that it is an overarching theme in our universe. In this session we will investigate some demonstrations and strategies for teaching gravity concepts. We will use education research results on student misconceptions related to gravity to focus the activities. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 11:00AM |
U7.00005: Sparks Fly With Physics Invited Speaker: |
Session U8: Colloidal Manipulation Techniques
Sponsoring Units: DFDChair: Vinothan Manoharan, Harvard University
Room: Morial Convention Center RO6
Thursday, March 13, 2008 8:00AM - 8:12AM |
U8.00001: Local perturbation caused by a particle driven through a 2-D colloidal suspension Cara Hageman, Vikram Prasad, Eric R. Weeks When polystyrene colloids are placed at a decane-water interface they form different phases based on their area fraction. These phases are: liquid, liquid-hexatic, hexatic and crystalline. In particular, the hexatic-crystal transition is characterized by a change in the functional form of the correlation functions and the density of defects. We study this system for area fractions near the hexatic-crystal transition. Using a laser tweezer we trap and drag a particle along the interface and observe its effect on the surrounding colloids. We observe a change in the local density of defects and a decay in the perturbed motion of colloids away from the trapped particle, revealing a length scale. We measure this length scale as a function of area fraction of the colloids and the applied velocity of the trapped particle. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U8.00002: Microrheology of a sticking transition Shobo Bhattacharya, Prerna Sharma, Shankar Ghosh The phenomenon of colloidal deposition in presence of shear is studied by using an optical tweezer to hold a colloidal particle in close proximity of a plate subject to a sinusoidal in-plane shaking. Through the measurement of the real and imaginary parts of the particle's response function, the coupling between the particle and the plate is found to evolve from a viscous regime to an elastic regime through an intermediate regime of time-dependent enhancement of viscoelasticity, reminiscent of aging in glasses. The sticking transition can be described in a scenario of hindered Stokes-Einstien diffusion and the Maxwell model of viscoelasticity. Upon changing the bead -plate interaction or the strength of the drive, three regimes of response: stick, stick-slip and slip are observed. The observed behavior has analogies to jamming in granular materials and the glass transition in viscous liquids. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U8.00003: A New Diffusion NMR Experimental Model System for Studies of Bidisperse Colloids Anand Yethiraj, Swomitra Palit A method to prepare monodisperse colloidal particles that are simultaneously NMR-visible and fluorescent is described. A systematic approach to obtain spectrally resolved diffusion coefficients for every component (colloid and solvent) in a monodisperse colloidal suspension is presented. We also prepared bidisperse colloidal suspensions where each colloid component has a distinct NMR spectral signature, and obtained the diffusion coefficient of both colloid species and solvent simultaneously, in concentrated colloidal suspensions with volume fractions between 20 and 50 $\%$. This colloidal model system enables the study of bidisperse colloids at different size ratios and number ratios. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U8.00004: Universal exponential tails in the displacement distribution observed in an attractive colloidal glass Yongxiang Gao, Maria Kilfoil Dynamical heterogeneities exist ubiquitously in glassy materials. They manifest themselves as a non-Gaussian distribution of the constituent particle displacements, that is, the self part of the van Hove correlation function. Though the shape of the tail of the distribution looks exponential or nearly exponential, not until recently has serious attention been paid to it. We observe pure exponential behavior---neither stretched nor compressed---over a wide range of volume fractions and time scales in an attractive colloidal system on the route to attractive glass transition. We observe universal behavior as all the distributions over a wide range of $\tau $ and $\phi $ can be scaled together. The tails arise from the mobile sub-component of the constituent particles. If time permits, I will also show our studies on the structure of colloidal gels and attractive glasses in terms of a translational order parameter and an orientational order parameter, under different interaction strength, volume fraction and buoyancy matching conditions. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U8.00005: Point response of a 2D packing of soft colloidal spheres near the jamming transition. Peter J. Yunker, Daniel T. N. Chen, Zexin Zhang, Arjun G. Yodh We have created a 2D jammed packing by confining a bidisperse mixture of thermoresponsive NIPA microgel spheres between two glass slides with a thickness of roughly the larger sphere diameter. The packing is subjected to a point compression created by local heating with optical tweezers. We use particle tracking microscopy to characterize the response as a function of particle volume fraction both above and below the jamming transition. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U8.00006: Local perturbations of dense colloidal suspensions Gianguido C. Cianci, Eric R. Weeks A rapid temperature quench can transform a liquid into a disordered solid: a glass. We model glassy materials using dense colloidal suspensions, where the transition is induced by increasing the number density rather than decreasing temperature. This transition has drawn significant attention because it poses numerous fundamental questions. For example, close to the glass transition temperature a small decrease in temperature can cause the viscosity of the liquid to increase by 14 orders of magnitude. Meanwhile the structure remains essentially unchanged --- there is no growing static length scale accompanying the transition. Fast laser scanning confocal microscopy allows us to directly observe and track thousands of colloidal particles in real time. We add a small number of superparamagnetic colloids in the sample and pull them with an external magnet. The motion of a magnetic probe locally perturbs the dense suspension and highlights its heterogeneous structure. We examine the dependence of the affected region's size on density and applied magnetic force. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U8.00007: Understanding the dynamics of closely packed microgel particles Melaku Muluneh, Hans Wyss, Giovanni Romeo, Johan Mattsson, Alberto Fernandez-Nieves, Jinwoong Kim, David Weitz Soft particles such as microgel suspensions have important applications in industry, which exploit their unusual structural and rhelogical properties. Despite their relevance, the fundamental physics that controls their behavior remains poorly understood. Intriguingly, microgels act as fluid even at high density. Techniques such as rheology, microscopy, and light scattering have been used to probe the macroscopic properties of these materials -- however, the underlying physical mechanisms demand further investigation. We use confocal microscopy to image the local dynamics of highly packed microgels. The gel particles are tracked over time to obtain information about the short and long range correlations of the local particle motion. We probe their response to changes in environmental factors such as temperature or pH using light scattering. The results obtained not only help us understand the origins of the observed macroscopic behavior, but also give us information on the dynamics of glassy arrest in general. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U8.00008: Colloidal Particle Geometry and Its Effect on Optical Trapping Rachael Harper, Alex Levine Recent experiments by Wilking and Mason (Europhysics Letters, in press) on the laser trapping of colloids of various shapes (the letters of the alphabet) show that for identical chemistries the trapping force varies wildly with particle shape. In fact, certain shapes do not trap at all. Motivated by these experimental results, we explore the trapping of particle of variable shape using a ray-optics simulation. This numerical tool allows us to perform Monte Carlo integrations of the total trapping forces and torques for a series of objects such as a cross (the letter ``x'') or a beam (the letter ``I''). We find that certain shapes feature bi-stable trapping positions/orientations, and some, indeed, do not allow for trapping at all. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U8.00009: Modeling pore formation in lipid membranes via Janus nanoparticles Alexander Alexeev, William E. Uspal, Anna C. Balazs Phospholipid membranes, which separate the cytoplasm from the extracellular environment in biological cells, embed a large diversity of proteins. Some proteins form pores for the free transport of small molecules and ions across the membrane. Here, we use coarse grained numerical simulations to design a synthetic membrane, where pores can be formed ``on demand.'' Specifically, we use dissipative particle dynamics to probe the interaction between bilayer membranes and nanoparticles. The particles are nanoscopic Janus beads that comprise both hydrophobic and hydrophilic portions. We demonstrate that when the membrane is ruptured due to an external stress, these nanoparticles diffuse to the free edge of the membrane and form stable pores, which persist after the stress is released. Pore size depends on the architecture and concentration of the nanoparticles. Once a pore is formed, a small increase in membrane tension readily reopens the pore allowing rapid transport through the membrane. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U8.00010: Three Dimensional Rotational Motion of Colloidal Clusters Justin Caram, Vinothan Manoharan We will present the results of a study into the three dimensional rotational and translational diffusion of colloidal clusters, especially dimers and trimers. We will have accomplished this study using both diffraction pattern and holograph analysis, as well as depolarized dynamic light scattering. We believe that trimers break into discrete rotational diffusion constants dependent on their geometries. These findings can be matched to the results for the rotational decay in the correlation function generated by DDLS. Understanding these diffusion constants is important to the understanding of protein and liquid crystal dynamics in solution. Furthermore, developing depolarized dynamic light scattering experimental strategy for non ellipsoidal systems may help to determine 3-dimensional hydrodynamic extent of such systems in solution. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U8.00011: Anomalous Rotation of a Pair of Spherical Particles in AC Electric Fields Pushkar Lele, Eric Furst Suspensions of colloidal particles are observed to form angled bands and vortices near surfaces in AC electric fields. We map the critical frequencies and field biases at which particles phase separate in to bands and the vortices gradually set in. The results from such mapping experiments are interpreted based on measurements of anomalous rotation on a pair of colloidal particles held in blinking optical tweezers. Our observations suggest that particle pair rotation is the characteristic motion in vortices and that the polarization of double layer around the particles influences the angular velocity of the vortex revolution. Increasing the electrolyte concentration of the medium or the frequency of the electric field results in reduction of the rotation. Based on these results, the suspension behavior can be ``tuned'' by changing the ionic strength, field strength, field frequency and particle size. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U8.00012: Interferometric and holographic imaging combined: correlating interface deformations with 3D tracking of interfacial particles David Kaz, Vinothan Manoharan We employ the techniques of digital holography and interference phase mapping to investigate particles on interfaces. Digital holography is used to track the positions of small (micron sized) particles to within a few nanometers in three dimensions, while optical interferometry maps interfacial deformations to within a few tens of nanometers. By utilizing both techniques simultaneously, we correlate the 3D position of particles trapped on an interface with deformations of that interface at up to 1000 frames per second. Such comprehensive data will serve to answer questions regarding the capillary interactions of particles on an interface. [Preview Abstract] |
Session U9: Fluctuations, Phase Transitions and the Polar Kerr Effect
Sponsoring Units: DCMPChair: Jeff Tallon, Industrial Research Ltd., New Zealand
Room: Morial Convention Center RO7
Thursday, March 13, 2008 8:00AM - 8:12AM |
U9.00001: Fluctuations, mean-field $T_c$ and energy gaps in cuprate superconductors Jeffery Tallon, James Storey, John Loram We have carried out an analysis of Gaussian fluctuations about $T_c$ in the specific heat of (Y,Ca)Ba$_2$Cu$_3$O$_{7-x}$ and Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$. The analysis employs a full ARPES derived dispersion, including the pseudogap. This enables us to calculate the doping dependence of the mean-field transition temperatures, $T_c^{mf}$, in the absence of fluctuations. The values lie well above $T_c$ especially for lower doping where $T_c^{mf}$ is trending towards 180K. As a result, while the observed $T_c$ follows the well-known parabolic doping dependence, the values of $T_c^{mf}$ decrease monotonically with doping along with the superconducting gap parameter, $\Delta_0$, such that $2\Delta_0/k_BT_c = 5$. Using this result we offer explanations for a number of anomalous observations. The line $T_c^{mf}(p)$ should not however be confused with the $T^*(p)$ line which is associated with the pseudogap and falls abruptly to zero at critical doping ($p_ {crit}$=0.19 holes/Cu). [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U9.00002: Universal values for the static and dynamic critical exponents in thin-film and bulk crystalline YBCO C.J. Lobb, Su Li, Hua Xu, M.C. Sullivan, K. Segawa, Yoichi Ando, S.M. Anlage Many researchers have used scaling of current vs.\ voltage curves to study the normal-superconducting phase transition of the high-temperature superconductors, searching for the static and dynamic critical exponents; however, there is little consensus among experimentalists as to the values of the exponents. We have studied this phase transition in optimally-doped YBa$_2$Cu$_3$O$_{7-\delta}$ thin films and bulk crystals. We consistently find $z=1.5\pm0.2$ for the dynamic critical exponent in films (when finite-size effects are taken into consideration) and in crystals (where there are no finite size effects). We also find for the static critical exponent $\nu=0.68\pm0.1$ for crystals and $\nu=0.63 \pm 0.1$ for films. The failure to account for finite-thickness effects in thin films may account for the wide ranges of values for $\nu$ and $z$ previously reported in the literature. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U9.00003: Dynamic critical behavior of YBCO thin films/crystals Hua Xu, Su Li, Christopher Lobb, Steven Anlage The zero-field phase transition of high $T_c$ superconductors has been studied by a number of techniques. However transport properties (such as the conductivity) which probe the dynamics near $T_c$ are less explored, and a wide range of critical exponents have been reported experimentally. We studied fluctuation effects of YBa$_2$Cu$_3$O$_{7-\delta}$ thin films around $T_c$ by doing frequency-dependent microwave conductivity and DC nonlinearity measurements. The length scales involved in the measurements and their influence on the fluctuation conductivity have been examined systematically, and this helps to clarify the historical discrepancy between experimental results and scaling predictions. Our results give a dynamical scaling exponent $z=1.55\pm0.15$, which indicates the superconducting to normal phase transition of high-$T_c$ materials likely belongs to the model E-dynamics. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U9.00004: Two-Dimensional Superconducting Fluctuations in Stripe-Ordered La$_{2-x}$Ba$_{x}$CuO$_{4}$ Qiang Li, Markus Hucker, Genda Gu, Alexei Tsvelik, John Tranquada Recent spectroscopic observations of a d-wave-like gap in stripe-ordered La$_{2-x}$Ba$_{x}$CuO$_{4}$ with x=1/8 have led us to critically analyze the anisotropic transport and magnetization properties of this material. The data suggest that concomitant with the spin ordering is an electronic decoupling of the CuO$_{2}$ planes. We observe a transition (or crossover) to a state of two-dimensional (2D) fluctuating superconductivity. Thus, it appears that the stripe order in La$_{2-x}$Ba$_{x}$CuO$_{4}$ frustrates three-dimensional superconducting phase order, but is fully compatible with 2D superconductivity and an enhanced $T_{c}$ --[Ref. Q. Li, et al., PRL \textbf{99, }067001 (2007)] [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U9.00005: Disorder-induced quantum critical point in an anisotropic gap superconductor Victor Galitski We consider an inhomogeneous anisotropic gap superconductor in the vicinity of the quantum critical point, where the transition temperature is suppressed to zero by disorder. Starting with the BCS Hamiltonian, we derive the Ginzburg-Landau action for the superconducting order parameter. It is shown that the critical theory corresponds to the marginal case in two dimensions and is formally equivalent to the theory of an antiferromagnetic quantum critical point, which is a quantum critical theory with the dynamic critical exponent, z=2. This allows us to use a parquet method to calculate the non-perturbative effect of quantum superconducting fluctuations on thermodynamic properties. We also discuss mesoscopic disorder fluctuations, which lead to the spatial variations of the local pairing temperature and the formation of superconducting islands above the mean-field transition. This disorder-induced Griffiths phase is described by a network of superconducting islands and metallic regions with a strongly suppressed density of states due to superconducting fluctuations. We argue that the phenomena associated with mesoscopic disorder fluctuations may also be relevant to high-temperature superconductors, in particular, to recent STM experiments, where gap inhomogeneities have been explicitly observed. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U9.00006: Fermi arcs in phase fluctuating d-wave superconductors Ehud Altman, Erez Berg One of the most puzzling aspects of the high $T_c$ superconductors is the appearance of Fermi arcs in the normal state of the underdoped cuprate materials. These are loci of low energy excitations covering part of the fermi surface, that suddenly appear above $T_c$ instead of the nodal quasiparticles. Based on a semiclassical theory, we argue that partial Fermi surfaces arise naturally in a d-wave superconductor that is destroyed by thermal phase fluctuations. Specifically, we show that the electron spectral function develops a square root singularity at low frequencies for wave-vectors positioned on the bare Fermi surface. We predict a temperature dependence of the arc length that can partially account for results of recent angle resolved photo emission (ARPES) experiments. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U9.00007: Vortex dynamics and Nernst effect in fluctuating superconductors Daniel Podolsky, Srinivas Raghu, Ashvin Vishwanath, David Huse We present a new method to study the Nernst effect and diamagetism of an extreme type II superconductor dominated by phase fluctuations. We work directly with vortex variables and our method allows us to tune vortex parameters (eg. core energy and number of vortex species). We find that diamagnetic response and transverse thermoelectric conductivity ($\alpha_{xy}$) persist well above the Kosterlitz-Thouless transition temperature, and become more pronounced as the vortex core energy is increased. However, they \textit{weaken} as the number of internal vortex states are increased. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U9.00008: Nernst effect in the phase-fluctuating superconductor InO$_x$ Panayotis Spathis, Herve Aubin, Alexandre Pourret, Cigdem Capan, Kamran Behnia We present a study of the Nernst effect in the amorphous superconductor InO$_x$. The low carrier density in this system implies a weak superfluid stiffness and consequently, strong phase fluctuations of the superconducting order parameter are expected. Measurements as function of temperature show that the Nernst signal evolves continuously through the superconducting transition as previously observed in underdoped cuprates. This contrasts with the abrupt jump expected at a BCS transition, as observed previously in Nb$_{0.15}$Si$_{0.85}$. In the last system, the Nernst signal due to vortices below T$_{c}$ and by Gaussian fluctuations above are clearly distinct [1]. The behavior of the ghost critical field in InO$_x$ points to a correlation length which does not diverge at the Cooper-pair forming temperature $T_c$, a temperature below which the amplitude fluctuations freeze, but phase fluctuations survive. [1] Pourret et al, Nature Physics 2, 683 (2006) [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U9.00009: Bosonic Magnetic Field Driven Superconductor-Insulator Transitions in Amorphous Nano-honeycomb Films M.D. Stewart, Jr., Aijun Yin, J.M. Xu, James M. Valles, Jr. We have observed multiple magnetic field driven superconductor-insulator transitions (SIT) in amorphous Bi films perforated with a nano-honeycomb (NHC) array of holes. The magneto-resistance across the SITs is periodic, with a period $H=H_M=h/2eS$, where $S$ is the area of a unit cell of holes. These transitions are, therefore, boson dominated. In constant field the temperature dependence of the resistance can be parameterized by $R(T)=R_0(H)\exp(T_0(H)/T)$ on both sides of the transition so that the evolution between the superconducting and insulating states is controlled by the vanishing of $T_0\to0$. We compare these data to the thickness driven transition in NHC films and the field driven transitions in unpatterned Bi films, other materials, and Josephson junction arrays. Our results suggest a structural source for similar behavior found in some materials and that despite the clear bosonic nature of the SITs, quasiparticle degrees of freedom likely also play an important part in the evolution of the SIT. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U9.00010: Excess Voltage Noises in the Superconducting Transition in Tin Films Hengsong Zhang, Fulin Zuo We report voltage noise studies in the superconducting transition of thin Tin (Sn) films. Voltage noises are measured as a function of temperature and applied current. The noise spectral power S$^{1/2}$ is peaked during the superconducting transition, with the peak temperature shifted downward from that of dR/dT. Comparison with the dc noise measurement shows the S$^{1/2}$ is much larger with ac current than dc. I-V characteristics and voltage noises are measured simultaneously to correlate the voltage noise with the vortex motion. The noise depends on the voltage with a characteristic $\sqrt V $ dependence for small V, suggesting shot noise nature for the excess noises. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U9.00011: Time-reversal symmetry breaking by a $(d+id)$ density-wave state in underdoped cuprate superconductors Victor Yakovenko, Sumanta Tewari, Chuanwei Zhang, Sankar Das Sarma It was proposed that the $id_{x^2-y^2}$ density-wave state (DDW) may be responsible for the pseudogap behavior in the underdoped cuprates. Here we show that the admixture of a small $d_{xy}$ component to the order parameter of the DDW state breaks macroscopic time reversal symmetry, leading to a non-zero polar Kerr effect. The $d_{xy}$ component breaks the symmetry between the counter-propagating orbital currents of the DDW state, which is ultimately responsible for the non-zero Kerr signal. From the results of the recent experiments by Xia \textit{et al.}, arXiv:0711.2494, we deduce that the amplitude of the $d_{xy}$ admixture is quite small compared to the ordered DDW component. \\ Reference: S. Tewari, C. Zhang, V. M. Yakovenko, S. Das Sarma, arXiv:0711.2329. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U9.00012: Kerr Effect in Superconductor. Sang Boo Nam A magnetic field H is to make the time reversal symmetry of the system be broken. Using the formulation [1], neglecting H dependence of $\Delta $(T) and for the pair cyclotron frequency $\Omega $ = (2e/2m)H/c less than the photon frequency 2$\pi $ c/$\lambda $, the Kerr angle is obtained as $\theta _{{\rm K}}$(T) = $\theta _{K}$(0)[$\Delta $(T)/$\Delta $(0)]tanh [$\Delta $(T)/2k$_{B}$T], where $\theta _{{\rm K}}$(0) = A $\lambda ^{3} \quad \Omega $ / (8 $\pi ^{3}$ c N $\lambda _{L}^{2})$, A = (3$\lambda $/4, L)/$\xi _ {BCS}$ in the (non-local, local) limit, with mean free path length L and BCS coherence length $\xi _{BCS}=\hbar $v$_{F}$/$\pi \Delta $(0). N = (n-1) n (n+1) with index of refraction n. For Sr2RuO4 [2], $\lambda $ = 1550 nm, L = 1 $\mu $m, v$_{F}$ = 100Km/s, n=3 and the London penetration depth length $\lambda _{L}$ = 3 $\mu $m [3], T$_{C}$ = 1.5 K . In the strong coupling case [4], $\Delta $(0) = 2T$_{C}$. The effective H is sum of the external applied and internal (by pair current) magnetic fields, to maintain the fluxoid quantization. After cooling a sample in the external magnetic field, turning it off, before warming a sample, is not necessary to make H vanish, since the pair current was set in a sample during cooling it. Then, H$_{C2}$ = 750 Gauss [3], in the normal vortex core, is considered as H. With all values of parameters given above, we obtain $\theta _ {K}$(0) = (44, 38) nrad in the (non-local, local) limit in satisfactory agreement with data of 65nrad [2]. The fluxoid quantization makes the Kerr angle same within a range of the external applied magnetic fields. [1] Nam, PR. \textbf{156}, 470, 487 (67). [2] Xia et al., PRL. \textbf{97}, 167002 (06). [3] Mackenzie et al., RMP.\textbf{ 75}, 657 (2003). [4] Nam, PL. \textbf {A193}, 111 (94); (E) ibid\textbf{. A197}, 458 (95). [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U9.00013: Possible Weak Ferromagnetism in Time Reversal Violating State of Underdoped Cuprates Vivek Aji, Chandra Varma Recent polar Kerr effect measurements on underdoped YBCO have provided evidence for time reversal symmetry breaking near the pseudogap temperature. These results are consistent wih the existance of a ferromagnetic moment of order 10\^{}(-4) Bohr magneton along the c-axis. We discuss the conditions for the possible occurrence of ferromagnetism with moments perpendicular to the Copper-Oxideplanes, accompanying the loop current orbital magnetic order, in the underdoped phase of Cuprates. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U9.00014: Electromagnetic response of a chiral p-wave superconductor Rahul Roy, John Berlinsky, Catherine Kallin We study the response of a time-reversal symmetry breaking $p_x + i p_y$ superconductor to an external electromagnetic wave and calculate the ac Hall conductivity, paying particular attention to gauge invariance, the effects of the topological Chern Simons term in the effective action and contributions from collective modes. We also consider the effects of a long range interaction such as the Coulomb interaction on the collective modes and the Hall conductivity. These results will also be discussed in the context of the recent Kerr rotation experiments on Strontium Ruthenate by Xia et al. [1] \newline \newline [1] J. Xia, Y. Maeno, P. T. Beyersdorf, M. M. Fejer, and A. Kapitulnik, \textit{Phys. Rev. Lett.} \textbf{97}, 167002 (2006) [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U9.00015: Absence of magnetic field (B $\leq$ 33~T) induced effects in the mid-infrared properties of La$_{2-x}$Sr$_{x}$CuO$_4$ films with 0~$\leq$~x~$\leq$~0.16 S.V. Dordevic, L.W. Kohlman, A. Gozar, G. Logvenov, I. Bozovic, L.C. Tung, Y.-J. Wang We have performed magneto-transmission measurements on a series of La$_{2-x}$Sr$_{x}$CuO$_4$ films with magnetic fields up to 18 Tesla. Studied samples include doping levels x= 0, 0.1, 0.3, 0.45, 0.6, 0.8, 0.10. In addition, an optimally doped film (x= 0.16) was studied in magnetic fields as high as 33 Tesla, both below and above its superconducting critical temperature T$_c$=\,41\,K. In neither of the studied samples we could detect any field-induced changes of transmission in the mid- infrared energy range (between about 1000 and 3500 cm$^{-1}$). We discuss how these observations can enhance our current understanding of medium energy range excitations in the cuprates, and their relation to high temperature superconductivity. [Preview Abstract] |
Session U10: Superconductivity: Magnetic Field Effects
Sponsoring Units: DCMPChair: Neil Sullivan, University of Florida
Room: Morial Convention Center RO8
Thursday, March 13, 2008 8:00AM - 8:12AM |
U10.00001: Peak Effect in Co$_{x}$NbSe$_{2}$ Single Crystals Maria Iavarone, R. Di Capua, G. Karapetrov, A. Koshelev, D. Rosenmann, H. Claus, W.K. Kwok We report a pronounced peak effect in the magnetization of Co$_{x}$NbSe$_{2}$ single crystals having critical temperatures ranging between 7.1 K and 5.0 K. Magnetization studies reveal that the magnetic irreversibility below the peak effect regime is higher in samples with lower concentration of Co while exhibits a nearly reversible magnetization over a wide range of magnetic field for samples with higher concentration of Co. However, in the peak effect regime the situation is different. The irreversibility is a non-monotonic function of the Co content, and therefore of the critical temperature of the sample. This behavior cannot be explained as a crossover between collective to single pinning regimes as suggested for NbSe$_{2}$, since this should be a monotonic function of number of pinning centers. Furthermore, we investigated the peak effect regime with low temperature STM at 4.2 K and 1.8 K. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U10.00002: Effect of the sample geometry on the intermediate state in mesoscopic 3D Type-I superconductors Golibjon Berdiyorov, Alexander Hernandez, Francois Peeters The intermediate state (IS) of type-I superconductors (SC) has recently became a topic of increasing interest [1,2]. Direct imaging of type-I SC reveals two distinct topologies of the intermediate state: flux tubes are formed upon magnetic field penetration and laminar patterns appear upon flux exit [2]. However, spheres and cones show no hysteresis with flux tubes dominating the IS [1]. In this work we investigate the effect of the sample topology on the formation of the flux patterns in mesoscopic type-I SC using the phenomenological Ginzburg-Landau theory. We carry out simulations on three-dimensional samples of different geometries. We show that in the samples with sharp boundaries (cubes and disks) laminar structures are mostly located along the boundary, whereas radial distribution of the flux patterns is obtained for cones and spheres. The effect of the edge defects on the observed structures will also be studied. [1] R.Prozorov, Phys. Rev. Lett. \textbf{98}, 257001 (2007). [2] M. Menghini et al., Phys. Rev. B \textbf{75}, 014529 (2007). [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U10.00003: Pauli Paramagnetic Effects on the Vortices in Superconducting TmNi$_2$B$_2$C L. DeBeer-Schmitt, M.R. Eskildsen, M. Ichioka, K. Machida, N. Jenkins, C.D. Dewhurst, S.L. Bud'ko, P.C. Canfield The magnetic field distribution around the superconducting vortices in TmNi$_2$B$_2$C in the paramagnetic phase above $T_N$ was studied experimentally as well as theoretically. Using small-angle neutron scattering we imaged the vortex lattice (VL). Using the magnitude of the VL scattering vector, we obtain a direct measure of the magnetic induction, $B$, which is found to exceed $\mu_0 H$ at all fields up to $H_{c2}$. Measurements of the VL reflectivity yielded a form factor which remains essentially constant up to $\sim 0.6 H_{c2}$, above which it decreases rapidly but remains measurable up to the upper critical field. This field dependence of the form factor is in striking contrast to the usual exponential suppression. The measured form factor is well fitted by model based on the Eilenberger equations, extended to include paramagnetic effects due to the exchange interaction with the localized 4$f$ Tm moments. The model shows how the induced paramagnetic moments around the vortex cores act to maintain the field contrast probed by the form factor. The results will be compared to our recent measurements of the VL form factor in CeCoIn$_5$, which also indicate strong paramagnetic effects. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U10.00004: Pauli-Limited Superconductors with Magnetic Fluctuations Robert Beaird, Anton Vorontsov, Ilya Vekhter We examine the temperature ($T$) and magnetic field ($B$) phase diagram of a paramagnetically-limited superconductor in the presence of fluctuating magnetic moments. We derive a Ginzburg- Landau free energy functional for the superconducting order from microscopics and include classical fluctuations of the magnetization. We consider the full wave vector ($q$) dependence of the expansion coefficients and examine the onset of both the uniform and spatially modulated superconducting phases along critical magnetic field $B_c(T)$. In the absence of fluctuations, both superconducting transitions are second order in 2D and meet at a tricritical point $B_c (T^\star)$. We find that including magnetic fluctuations introduces first order transitions into the superconducting phase. We find the tricritical points where these transitions meet and determine $B_c(T)$ and $q(T)$ as functions of the coupling between superconducting order and the fluctuating magnetization. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U10.00005: Evidence for Field-Induced Quantum Criticality in an Overdoped Cuprate Takasada Shibauchi, Lia Krusin-Elbaum, Masashi Hasegawa, Yuichi Kasahara, Ryuji Okazaki, Yuji Matsuda In current views, the putative quantum phase transitions in high-$T_{\rm c}$ superconductors are deemed driven by charge doping. Here we uncover an unanticipated transition from a non-Fermi- to a Fermi-liquid state driven by magnetic field in a highly overdoped Tl$_2$Ba$_2$CuO$_{6+x}$ with $T_{\rm c}\approx 15$~K. From the $c$-axis resistivity measured up to 45~T, we show that the Fermi-liquid $\rho_c = \rho_c(0)+ AT^2$ features, accompanied by a (quantum) field-linear magnetoresistance, appear above a temperature-dependent field $H_{\rm FL}$, which decreases linearly with decreasing temperature and points to a quantum critical point near the upper critical field $H_{\rm c2}(0)$. The observed field-induced quantum criticality with a power-law diverging $A(H)$ bears a striking resemblance to that of heavy-fermion superconductor CeCoIn$_5$, suggesting a common underlying physics in these strongly correlated electron systems. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U10.00006: Superconducting Vortices in CeCoIn$_5$: Beyond the Abrikosov-Ginzburg-Landau Paradigm A. D. Bianchi, M. Kenzelmann, J. Mesot, M. Zolliker, J. Kohlbrecher, L. DeBeer-Schmitt, M. R. Eskildsen, J. S. White, E. M. Forgan, Z. Fisk, R. Movshovich, E. D. Bauer, J. L. Sarrao, C. Petrovic We report on the magnetic field ($H$) dependence of the form factor $|F|^2$ of the vortex lattice (VL) in CeCoIn$_5$ obtained by small angle neutron scattering for $H$ applied along the crystallographic $c$-axis. Superconductivity (SC) in CeCoIn$_5$ has several unconventional aspects to it: The $d$-wave SC is in competition with antiferromagnetic order, as suggested by the presence of an magnetic QCP located at the upper critical field $H_{c2}$ determined by the Pauli effect. At both 50 and 500~mK we observe an $H$-independent $|F|^2$ up to 2~T. With further increasing $H$, $|F|^2$ continues to increase all the way up to $H_{c2}$. This finding is in contrast to that normally observed in type-II SC's, where $|F|^2$ decreases with increasing $H$. It suggests a departure from the Abrikosov-Ginzburg-Landau paradigm, where the properties of the vortex state can be described by the coherence length $\xi$ and the penetration depth $\lambda$. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U10.00007: Reversal of specific heat oscillations with planar magnetic field in 2D d-wave superconductors: Doppler shift beyond the nodal approximation. G.R. Boyd, P.J. Hirschfeld, Ilya Vekhter Experiments on several novel superconducting compounds found oscillations of the specific heat when an applied magnetic field is rotated with respect to the crystal axes. The results are commonly interpreted as arising from the nodes of an unconventional order parameter, but the identifications of nodal directions are sometimes controversial. While the semiclassical method predicted the minima for the field aligned with the nodes in the low-energy range, inclusion of vortex scattering was shown to lead to an inversion of the oscillation pattern at finite energy. Here we show with a simple model calculation that even within semiclassical approach with no scattering on the vortices, either minima or maxima can occur in thermodynamic quantities depending on the the temperature and the magnetic field scale. Using a d-wave BCS model and approximating the quasiparticle excitations semiclassically, we find a sign reversal of the field angle oscillations as temperature is increased. Absence of this effect in earlier work within the same approach is attributed to the use of the nodal approximation, which breaks down at higher energies. The result indicates that the inversion of oscillations is an intrinsic feature of unconventional superconductors. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U10.00008: Effect of Charge Carrier Density on the Vortex Regimes in Y$_{1-x}$Pr$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ Single Crystals P. Gyawali, V. Sandu, C.C. Almasan, B.J. Taylor, M.B. Maple We report the evolution of the vortex matter state in the temperature and field range where the second magnetization peak SMP is present by studying the magnetization and magnetic relaxation of a series of Y$_{1-x}$Pr$_{x}$Ba$_{2}$Cu$_{3}$O$_{7-\delta}$ ($x=0.13$, $T_{c}=82$ K; $x=0.34$, $T_{c}=50$ K; $x=0.47$, $T_{c}=34$ K) single crystals. Our study has shown that the main ingredient that controls the evolution of the vortex matter through the different regimes is the charge carrier density. The SMP is first enhanced and then suppressed as $Pr$ concentration increases. The reason for this behavior is the softening of the elastic moduli, which makes the vortex lattice less stable to defect invasion. Within the collective creep theory, we determined the apparent activation energy. Its evolution with current density has shown that the vortex system is predominantly elastically pinned below the SMP, while above there is a smooth crossover to a vortex regime most likely dominated by the proliferation of dislocations. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U10.00009: Skyrmion Flux Lattices and their $\mu$SR signature Qi Li, John Toner, Dietrich Belitz Recently, topological excitations known as skyrmions were predicted to exist in p-wave superconductors [1]. The elastic theory of an induced skyrmion lattice was developed in [2], and its melting curve was found to be qualitatively different from that for vortex lattices. Here we show that the muon spin resonance ($\mu$SR) signatures of the two types of lattices are also very different. $\mu$SR has been applied extensively to study the magnetic properties of vortex flux lattices [3]. The observable in this technique is the $\mu$SR line shape $n(B)$, which is the probability density that a muon experiences a local magnetic induction $B$. In a vortex lattice, for small $B$, $n(B) \propto \ln(1/B)/B$. By contrast, for a skyrmion lattice we predict $n(B) \propto B^ (-3/2)$. This difference provides another way to easily distinguish between vortex and skyrmion flux lattices, and can thus help to identify p-wave superconductors. [1] A. Knigavko, B. Rosenstein, and Y.F. Chen, Phys. Rev. B 60, 550 (1999). [2] Qi Li, John Toner, and D. Belitz, Phys.Rev. Lett. 98, 187002 (2007). [3] J. E. Sonier, J.H. Brewer, and R. F. Kiefl, Rev. Mod. Phys. 72, 769 (2000). [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U10.00010: Non-linear magnetization effects within the Kosterlitz-Thouless theory Lara Benfatto, Claudio Castellani, Thierry Giamarchi Recent experiments in cuprate superconductors have attracted the attention on the role of vortex fluctuations. Measurements of the field-induced magnetization showed that the correlation length diverge exponentially, as predicted within the Kosterlitz-Thouless (KT) theory. However, it is somehow puzzling thepersistence of strong non-linear magnetization effects at low field. Here we address this issue by means of a new theoretical approach to the KT transition at finite magnetic field, based on the sine-Gordon model. This approach is particularly useful in two respects. First, it leads to a straightforward definition of the field-induced magnetization as a function of the external magnetic field $H$ instead of the magnetic induction $B$, which is crucial to get a consistent description of the Meissner phase. Second, it allows us to identify the cross-over field $H_{cr}$ from linear to non-linear magnetization both below and above the transition. Above $T_{KT}$ $H_{cr}$ turns out to scale as the inverse correlation length, so that it decreases as the transition is approached. As a consequence, the fact that only the non-linear regime is accessible experimentally should be interpreted as a typical signature of the fast divergence of the correlation length within the KT theory. L.Benfatto, C.Castellani and T.Giamarchi, Phys. Rev. Lett. 99, 207002 (2007) [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U10.00011: Theory of diamagnetic response in layered superconductor above ``Tc" PeiJen Lin, DingPing Li, Baruch Rosenstein Recent work by Wang et al. (Phys. Rev. Lett. 95, 247002) on field induced diamagnetic properties above Tc attracted a lot of attention from both theoretical and experimental physicist. In this talk, we will show that the phenomenon can be understood using conventional Ginsburg Landau theory for anisotropic materials. Above Tc, where the thermal fluctuations are strong, the effective description based on GL becomes non trivial due to the important contribution of higher landau levels. In previous works, some progress was achieved when certain additional assumptions (such as Lowest Landau Level approximation ) were made. However, the validity of these assumptions is under debate. In our study of this system, we include the contribution of higher landau levels with nonperturbative method.Comparison will be made with Varlamov-Larkin results. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U10.00012: Theory of quantum magneto-oscillations in underdoped cuprate superconductors Sasha Alexandrov Magneto-oscillations in kinetic and magnetic response functions of a few underdoped cuprates are perhaps one of the most striking observations since many probes of underdoped cuprates clearly point to a non Fermi-liquid normal state. Their observation in the vortex state well below the upper critical field raises a doubt concerning their normal state origin. Here I propose an explanation of the magneto- oscillations as emerging from the quantum interference of the vortex lattice and checkerboard modulations of the electron density of states revealed by STM with atomic resolution in some cuprate superconductors. The checkerboard effectively pins the vortex lattice, when the period of the latter is commensurate with the period of the checkerboard. This condition yields $1/\sqrt{B}$ periodicity of the response functions, rather than $1/B$ periodicity of conventional normal state oscillations periodic versus inverse magnetic field $B$. Our solution of the Gross-Pitaevskii-type equation for composed charged bosons accounts for the d-wave symmetry of the order-parameter and its checkerboard modulations, and describes well changes in resonant frequency of the tunnel-diode oscillator circuit with YBa$_2$Cu$_4$O$_8$ and the oscillatory part of the Hall resistance in the mixed state of YBa$_2$Cu$_3$O$_{6.5}$. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U10.00013: Enhanced tunneling in a magnetic field Boris Ivlev As known, a probability of quantum tunneling through a static potential barrier $U(x)$ can be substantially reduced by a static magnetic field $H_{z}$. This happens due to increase of the effective barrier height caused by Landau's gauge potential in a magnetic field (the same potential results in Landau levels). There is an exponentially small current in the direction of tunneling, $x$. An underbarrier current in the direction perpendicular to tunneling, $y$, is not small. If the potential barrier $U(x,y)$ depends also on the coordinate $y$, a new unexpected scenario can occur. Now the partial de Broglie waves, generated under the barrier, are not collected to the current in the $y$ direction only but can be reflected by the potential $U(x,y)$. An interference of those underbarrier waves after reflections can result in a peak of the particle density at a classically allowed region close to the conventional exit point from under the barrier. At the certain magnetic field, $H_{z}=H_{R}$, the peak amplitude is not exponentially small (Euclidean resonance). The same phenomenon can occur in tunneling through nonstationary barriers and is expected for photon tunneling when a refractive index is slightly inhomogeneous in the tunnel region. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U10.00014: Current induced properties in bulk YBCO above the transition temperature George Zimmerman Simultaneous measurement of the electrical resistivity at high current densities and magnetic susceptibility of YBCO superconducting material reveals interesting behavior of the samples above the transition temperature. In addition to anomalies which appear at temperatures between 85K and 140K, the susceptibility seems to be affected by the electrical current up 200K. The electrical current, of density between 8 and 400 A/cm$^2$ seems to induce the behavior, with a lowered resistivity, which suggests a first order phase transition, possibly meta-stable, and persists despite the repeated cycling between 77K and 300K. The samples of bulk polycrystalline cylindrical rods 1.22 mm diameter and between 6cm and 20 cm long have a density of 5.4 gm/cm$^3$ and were prepared sintering and annealing from a YBCO powder. Most were 10 to 15 years old. At 77K most samples exhibit relaxation times of several minutes in their magnetic and resistive behavior. The details of the measurements as a function of temperature, current density, and low magnetic field, will be described along with possible implications as to the nature of the pseudo-gap and other competing theories. [Preview Abstract] |
Session U11: Focus Session: MgB2-like: Enhancement of Superconducting Properties
Sponsoring Units: DMPChair: Alex Koshelev, Argonne National Laboratory
Room: Morial Convention Center RO9
Thursday, March 13, 2008 8:00AM - 8:12AM |
U11.00001: Introduction of Carrier Scattering in MgB2, and its Effect on both Normal and Superconducting Properties, especially Hc2 N. Newman, Y. Shen, R. Singh, J. Rowell, D. Larbalestier, F. Hunte The low Hc2 values seen in pure and well ordered MgB2 can be raised dramatically, to 35 T or more, by introducing carrier scattering by native and impurity defects. We describe three means to do this.~ First, He ion irradiation is used to tune Tc from 39K to less than 10K, while at Tc near 33K, Hc2 reached a maximum value. Similar behavior has been reported for neutron damage and carbon doping. Second, we introduced oxygen in the films, either in-situ or ex-situ, and again, high Hc2 values were seen and in these films, very high Jc values as well. Finally, a novel route has been investigated. We deposited MgB2 films on room temperature substrates, then annealed at temperatures just sufficient to produce crystallinity, giving Tcs in the range of 10 to 30K. Such films exhibit large dHc2/dT values near Tc, sometimes larger than 2 T/K. This work is of practical importance and gives an improved understanding of how intraband and interband carrier scattering in the ``2-gap'' superconductor MgB2 determine its Hc2, resistivity and Tc. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U11.00002: Upper critical field enhancements of TMB HPCVD magnesium diboride F. Hunte, J. Jaroszynski, A. Gurevich, D.C. Larbalestier, Y. Zhu, P.M. Voyles, R.H. Wilke, X.X. Xi The $H_{c2}$ of four well textured carbon-doped MgB$_2$ films grown by HPCVD from tri-methyl boron (TMB) at flow rates from 2.5 to 10 sccm was measured in fields up to 45T. $H_{c2}$ derived from low- current, four-point magnetoresistance clearly increases with increasing TMB flow rate. TMB appears to be a more uniform dopant than the (C$_6$H$_7$)$_2$Mg used earlier. These earlier films exhibited $H_{c2}^{\parallel}(0)$ up to 70 T but also were imperfectly connected ($\rho(50\mbox{ K})\sim200-800$~$\mu\Omega$cm) due to excess amorphous C-rich phases observed between the MgB$_2$ grains. In strong contrast, $\rho(50\mbox{ K})$ was only $10-20$~$\mu\Omega$cm for the TMB films. When first measured, the linearly extrapolated $H_{c2}^{\parallel}(0)$ reached $\sim$40 T for the film with the highest TMB flow rate, but after about 3 months of aging, this value rose to $\sim50$~T. The angular dependence of $H_{c2} $ for this sample was measured up to 45~T yielding $H_{c2}^{\parallel}(4.2\mbox{ K}) = 45.8$~T and showing the good Ginzburg-Landau scaling with an $H_{c2}$ anisotropy of 2.88 at 4.2 K. These results are discussed in terms of the theory of dirty two-gap superconductors as a part of an in-depth study of the effect of ternary doping of magnesium diboride. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U11.00003: Kinetic roughening of magnetic flux penetration in MgB$_{2}$ thin films Andrea Lucarelli, Gunter Luepke, Brian Moeckly, Yue Zhao, Shi Dou Mg$_{\mbox{B2}}$thin films exhibit pronounced instabilities such as finger-like structures, flux jumps or dendritic patterns, which endanger electronic devices and lead to energy dissipation. We investigated the magnetic flux behavior of Mg$_{\mbox{B2}}$ thin films samples grown by in situ pulsed laser deposition and in situ reactive deposition technique on different substrates. We performed time-resolved magneto-optical imaging (TRMOI) measurements as a function of applied static field and for a static filed plus ac current to visualize the kinetic roughening of the flux penetration front. The TRMOI images are analyzed by employing dynamic scaling concepts used in the studies of interface roughening of stochastic systems. For both static field and ac current the resulting critical state shows self-affine structure characterized by universal exponents. [1] Dynamic scaling-laws determined in both cases are consistent with the directed percolation depinning model, placing the vortex dynamics of Mg$_{\mbox{B2}}$ in the same universality class as YBCO and Nb. \newline [1] Lucarelli et al. Appl. Phys. Lett. 91, 22 (2007) [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U11.00004: Evidence of new pinning centers in irradiated MgB$_{2}$ C. Tarantini, A. Martinelli, P. Manfrinetti, A. Palenzona, I. Pallecchi, M. Putti, C. Ferdeghini, M.R. Cimberle It has been shown that C or SiC addictions can strongly enhance upper critical field of MgB$_{2}$, leading to an in-field increase of critical current, but without introducing pinning centers other than grain boundaries. On the contrary neutron irradiation introduces new pinning centers, as highlighted by a significant shift of the maximum of pinning force and by a strong improvement of J$_{c}$ at high field. This effect can be correlated to the defects that neutron irradiation produces. In fact TEM images show the presence of nanometric amorphous regions whose sizes are compatible with the coherence length and such as to act as pinning centers through two different mechanisms. The influence that neutron irradiation induces on MgB$_{2}$ is also confirmed by magnetization decays that, differently by doped samples, show an important enhancement of pinning energies at high field. These measurements highlight as the increase of pinning energy with irradiation fluence is strongly correlated with J$_{c}$ improvement. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U11.00005: Flux Pinning and Connectivity in MgB2 M.D. Sumption, M. Susner, M. Bhatia, E.W. Collings The transport and pinning properties of in-situ MgB2 bulks and strands are discussed. The influence of SiC, excess Mg, B4C, TiC, and their combination on Birr and Bc2 as distinct from connectivity and flux pinning is the focus of the work. SiC dopants increase Bc2 and Birr predominantly, with little influence on connectivity or flux pinning. Excess Mg improves the transport current, changes the grain microstructure, and also leads local maxima in Bc2 and Birr at excess Mg levels of 15\% mol fraction. Fp curves are consistent with grain boundary pinning for the binary materials over the whole temperature range. This is also true for SiC and TiC doped materials at lower fields and temperatures, while higher temperatures show a deviation from surface pinning. These higher temperature deviations are consistent with the size and distribution of these nanoparticulate additions. Normal state resistivity measurements and models are used to extract residual resistivity values, percent connectivity, and Debye temperatures. Debye temperatures are seen to be depressed by SiC doping, an effect which is confirmed by heat capacity measurements. Residual resistivity values are seen to correlate with Bc2 and Birr enhancements, consistent with B site substitution with C as evidenced by XRD extracted lattice parameter shifts. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U11.00006: Upper critical field study of MBE grown MgB$_2$ thin films J. Jaroszynski, F. Hunte, A. Gurevich, D. C. Larbalestier, Y. Zhu, P. M. Voyles, Y. Shen, R. Gandikota, R. Singh, J. Rowell, N. Newman "Normal" alloying of MgB$_2$ enhances the electron scattering, as does radiation damage. As a result, the upper critical field $H_{c2}^{\parallel}(0)$ parallel to the ab planes doubles from about 18 to 35 T. ASU has been growing films by non-equilibrium MBE methods and either intentionally doping with oxygen during growth, or by deposition at room temperature with subsequent annealing {\em ex situ} at rather low temperature: {\em e.g.} 350 $^\circ$C for 36 hrs followed by 600 $^\circ$C for 30 min. The resistive transitions of the films have been measured in fields up to 45 T at different temperatures. The measurements revealed strong enhancement of $H_{c2}^{\parallel}(0)$. In particular, the cold-deposited film remains superconducting at 45 T at 5 K, while extrapolation yields $H_{c2}^{\parallel}(0)$ higher than 65 T, almost as high as the best C-doped HPCVD films. At the same time, the film is strongly inhomogeneous, the resistivity is as high as ~30 m$\Omega$cm, while the transitions are very broad, and the critical temperature is lowered to 24 K. However, $dH_{c2}/dT$ at $T_c$ reaches a record high value of 2.7 T/K. These observations open up another way to get exceptional $H_{c2}$ values in MgB$_2$ films. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:48AM |
U11.00007: Novel properties in the normal state and the mixed state due to multiband effect in MgB$_{2}$ Invited Speaker: Based on high quality MgB$_{2}$ thin films, we measured the longitudinal and transverse resistivity of seven different samples with variable disorders characterized by the residual resistance ratio (\textit{RRR}) ranging from 4.0 to 33.3. Strong nonlinear Hall effect and strong magnetoresistance have been found in clean samples and they decrease gradually with the increase of scattering centers or temperature. By fitting to the theoretical model for a four-band system, for the first time, we derived the scattering rates for each band. Nontrivial difference between the transport properties of these four bands are unraveled. In the mixed state, a non-vanishing dissipation has been observed in the low temperature regime. The Hall Effect measurement confirms that it is induced by the vortex motion. Together with the data of I-V curves, point-contact tunneling and the magnetization relaxation, we conclude that this non-vanishing dissipation in the zero temperature limit is induced by the proliferation of the pi-band quasiparticles, in association with the multigap feature. In collaboration with H. Yang, Y. Jia, J. R. Shi, L. Shan, C. Ren and Y. Z. Zhang at IOP, CAS and C. G. Zhuang, Z. K. Liu, Qi Li, Yi Cui, and X. X. Xi at Penn State University. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U11.00008: Structural-Microstructural Characteristics and its Correlations with the Superconducting Properties of in-situ PIT Processed MgB$_{2}$ Tapes with Ethyltoulene and SiC Powder added Anjana Asthana, H. Yamada, N. Uchiyama, A. Matsumoto, H. Kitaguchi, Y. Matsui, H. Kumakura The structure and microstructures of pure MgB$_{2}$, ethyltoulene and ethyltoulene + SiC added MgB$_{2}$ tapes have been investigated by using selected area electron diffraction, bright field, dark field and high resolution electron microscopy. As reported, the Jc values of the ethyltoluene and ethyltoulene + SiC added MgB$_{2}$ tapes are much higher than the pure MgB$_{2}$ tape sample. Analysis of the microstructures shows that pure MgB$_{2}$ tape~ sample consist of grains of 100-200nm. With the addition of ethyltoulene and ethyltoulene + SiC to the starting powder of \textit{in situ} processed MgB$_{2}$ tapes, the grain size decreases drastically to an average size of about 20-50nm. The higher Jc value of~ the ethyltoulene and ethyltoulene + SiC added MgB$_{2}$ tapes as compared to the pure MgB$_{2}$ tapes has been attributed~to the decrease in grain size and better connectivity of the grains and also presence of pinning centers as some precipitates and Mg$_{2}$Si particles of size less than 100nm. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U11.00009: Properties of MgB$_{2}$ Thin Films Grown at Different Temperatures by Hybrid Physical-Chemical Vapor Deposition Menno Veldhorst, Ke Chen, Che-Hui Lee, Qi Li, Xiaoxing Xi MgB$_{2}$ films grown by Hybrid Physical-Chemical Vapor Deposition (HPCVD) at high temperature excel in $T_{c}$, cleanness, and crystallinity. MgB$_{2}$ films have been grown at temperatures from 350$^{^{\circ}}$C to 750$^{^{\circ}}$C by a HPCVD system with separate Mg and substrate heaters. The 100 nm MgB$_{2}$ film grown on a (001) SiC substrate at 350$^{^{\circ}}$C has a $T_{c0}$ of about 36K and a residual resistance ratio of about 1.4. X-ray diffraction and atomic force microscopy show that the film is polycrystalline. The low-temperature grown MgB$_{2}$ films are promising as the top electrode for sandwich-type all-MgB$_{2}$ junctions to preserve the integrity of the barrier layer. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U11.00010: Superconductivity in Sc, Y, Lu, and CaLi$_{2}$ under Exreme Pressures M. Debessai, J.J. Hamlin, A.K. Gangopadhyay, J.S. Schilling, T. Matsuoka, K. Shimizu Following the first experiments by Sizoo and Onnes in 1925 on Sn, studies of superconductivity under high pressures have made important contributions by furthering our understanding of this exotic state and creating many new and novel superconducting materials, including O, Si, Fe, I, and Cs. Indeed, the number of elemental superconductors across the periodic table has almost doubled through the application of extreme pressures. MgB$_{2}$ exhibits the highest value of T$_{c}$ of any known binary compound, but the T$_{c}$ values of elemental superconductors under extreme pressures are not far behind. We have recently used pressures as high as nearly 2 Mbar to induce superconductivity in Sc, Y, and Lu, as well as to search for superconductivity in CaLi$_{2}$. T$_{c}$ values as high as 20 K are obtained, comparable to the highest values observed for the A15 compounds. These studies thus allow an investigation into the question: what is the maximum possible value of T$_{c}$ in a phonon-mediated superconductor? [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U11.00011: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U11.00012: Superconductivity in MgCNi$_{3}$.Tunneling and heat capacity on single crystals Peter Samuely, Zuzana Pribulova, Jozef Kacmarcik, Pavol Szabo, Christophe Marcenat, Thierry Klein, D.-J. Jang, H.-G. Lee, H.-S. Lee, S.I. Lee MgCNi$_{3}$ reveals superconductivity despite a large molar volume of Ni atoms. The origin of superconductivity in this material has not yet been clarified. There is a lot of discrepancy in experimental results and physical interpretation where even unconventional pairing or a two-band model have been proposed for the system, but these suggestions are based on measurements on polycrystalline samples. Here we present the point-contact tunneling spectroscopy and ac-calorimetry measurements on single crystals of very good quality. Measurements have been performed in the temperature range from 0.7 K and in magnetic fields up to 8 T. The temperature dependence of the energy gap of the system is presented and compared to the BCS model. [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U11.00013: No Evidence for Spin Density Waves in Pb using Phonon Imaging Timothy Head, James Wolfe Phonon-imaging in superconducting Pb has proven to be a sensitive probe of quasiparticle density due to the highly anisotropic absorption of ballistic phonons by quasiparticles. Slower than expected temperature dependences of quasiparticle density observed previously by Wolfe and Short (\textit{Physica B} \textbf{316,} 107 (2002)) are explained here by taking into account the effects of nonequilibrium quasiparticles. Minimizing the effects of nonequilibrium quasiparticles enables us to extract a value of the zero-temperature superconducting gap parameter. We measure $\Delta $=1.32 $\pm $0.07 meV consistent with tunneling measurements in Pb and the conventional BCS picture, and inconsistent with earlier specific heat data that motivated the proposal of a spin-density-wave ground state in Pb (Overhauser and Daemen, \textit{PRL} \textbf{61}, 1885 (1988)). [Preview Abstract] |
Session U12: Strongly Correlated Electron Systems: Quantum Phase Transitions
Sponsoring Units: DCMPChair: Andrew Millis, Columbia University
Room: Morial Convention Center 203
Thursday, March 13, 2008 8:00AM - 8:12AM |
U12.00001: Quantum phase transition of a magnetic impurity in a dissipative environment Mengxing Cheng, Kevin Ingersent, Matthew Glossop We study the quantum phase transition (QPT) induced by dissipation in the Bose-Fermi Anderson model of a magnetic impurity that hybridizes with a metallic host and is also coupled (via its charge) to a bosonic bath having a spectral density proportional to $\omega^s$. For sub-Ohmic bath exponents $0 < s < 1$, numerical renormalization-group calculations show that upon increasing the coupling to the bosonic bath from zero, there is a crossover from a conventional (spin-sector) Kondo effect to a charge-Kondo effect. Further increase of the bosonic coupling results in a zero-temperature transition to a phase in which charge fluctuations on the impurity site are frozen out. Critical exponents describing the response of the impurity charge to a locally applied electric field are found to obey the hyperscaling relations characteristic of an interacting critical point. The numerical value of these exponents suggests that the QPT lies in the same universality class as that of the sub-Ohmic spin-boson model. Results for the Ohmic case $s=1$ will also be presented. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U12.00002: Quantum Criticality of the Pseudogapped Kondo Problem: Finite Temperature Scaling and Conformal Invariance Matthew T. Glossop, Stefan Kirchner, Qimiao Si The critical destruction of the Kondo effect is of interest as a potential mechanism for quantum-critical heavy-fermion metals. Here, we study the pseudogapped Kondo model [1], with a conduction-electron density of states proportional to $|\epsilon|^r$, which provides a paradigm for understanding critical local-moment fluctuations. In general, an interacting quantum critical point (QCP), at a finite critical Kondo coupling $J_c$, separates Kondo-screened and free local-moment phases [2]. We focus on finite-$T$ scaling properties in the vicinity of the QCP, obtained using a dynamical large-N method for an SU(N) generalization of the model. Though the bulk lacks conformal invariance for $r>0$, we show that correlation functions assume the form expected of a boundary conformal field theory, implying an enhanced symmetry at the QCP. We also address these and related issues in the N=2 model using a continuous-time quantum Monte Carlo impurity solver [3], which involves a stochastic evaluation of an expansion in the host-impurity hybridization. [1] D. Withoff and E. Fradkin, Phys. Rev. Lett. 64, 1835 (1990) [2] K. Ingersent and Q. Si., Phys. Rev. Lett. 89, 076403 (2002). [3] P. Werner et al., Phys. Rev. Lett. 97, 076405 (2006) [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U12.00003: Quantum-criticality in models of an impurity coupled to fermionic and bosonic baths Kevin Ingersent, Matthew Glossop Impurity models exhibiting quantum phase transitions (QPTs) have attracted interest in connection with impurities in cuprate superconductors, heavy-fermion quantum criticality, and quantum-dot devices. This talk focuses on three models describing an impurity level coupled both to a band of fermions (either spinful or spinless) with a density of states varying as $|\epsilon|^r$ around the Fermi energy $\epsilon=0$, and to a dissipative bosonic bath having a spectral function $\propto \omega^s$. Each of these models features a QPT between a phase in which the fermionic band dominates the impurity dynamics and a second phase in which the bosons freeze out the impurity degrees of freedom. We study these QPTs using a recently developed numerical renormalization-group technique. Over much of the parameter space spanned by the exponents $r$ and $s$, the QPT in all three models falls into the universality class of the pure-bosonic spin-boson model, with exponents that are independent of $r$. However, for sufficiently strong fermionic pseudogaps (large values of $r$), new universality classes of QPT emerge. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U12.00004: The Two-Impurity Anderson Model at Quantum Criticality David Mross, Henrik Johannesson We propose a realization of the two-impurity Anderson model in a double quantum-dot system. When charge transfer between the dots is suppressed the system exhibits a non-Fermi liquid critical line parameterized by the amount of charge localized on the dots. Employing conformal field theory techniques we identify the critical exponents that govern transport and thermodynamics in the vicinity of the critical line. We also determine the dynamical exponent that sets the time scale for buildup of the non-Fermi liquid state after the system is shifted into the critical region, e.g. by a sudden change of a nearby gate voltage. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U12.00005: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U12.00006: Absence of quantum phase transition in two-state quantum dot Xin Wang, Andrew J. Millis We use continuous-time quantum Monte Carlo methods to study a model of a spinless-fermion two state quantum dot which was argued in Ref. [1] to exhibit a quantum phase transition. We find instead a smooth behavior as parameters are varied. The generalization of the model to the spinful case is also presented. [1] D. I. Golosov and Y. Gefen, {\em Phys. Rev. B} {\bf 74}, 205316 (2006). [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U12.00007: Kondo destruction in the Bose-Fermi Kondo model with a singular dissipative spectrum: Exact solutions and their implications Jianhui Dai, C.J. Bolech, Qimiao Si Quantum dissipation induces a critical destruction of the Kondo screening, which is of interest in the contexts of quantum critical heavy fermions and magnetic mesoscopic structures. The sub-Ohmic Bose-Fermi Kondo (BFK) model provides a setting to study such an effect. Here, we show that this many-body problem is exactly solvable when the spectrum of the dissipative bosonic bath, $J(\omega)$, is singular, such that $J (\tau)={\rm const.}$. We determine the exact results for the local spin correlation functions, which imply that the singular longitudinal fluctuations of the bosonic bath play a dominant role. We also demonstrate how the large-N limit of an SU(N) generalization of the same model fails to capture the $N=2$ physics in the cases of a singular dissipative bosonic spectrum, due to an interesting under-treatment of the longitudinal fluctuations. Our results resolve an apparent inconsistency between the previous results respectively found using numerical renormalization group and large-N treatments, providing evidence that the local quantum critical solution of the extended dynamical mean field approach to the Kondo lattice model indeed has a zero residual entropy. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U12.00008: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U12.00009: Quantum Criticality out of Equilibrium: Kondo Destruction and $V/T$ Scaling in a Magnetic Single-Electron Transistor Stefan Kirchner, Qimiao Si Non-equilibrium quantum phase transitions have so far received only limited attention despite the long-standing strong interest in classical out-of-equilibrium phase transitions. This is in part due to the fact that dynamics and statics are already intermixed at an equilibrium quantum phase transition. Nanostructured devices constitute simplified systems, both theoretically and experimentally, to study well-defined out-of-equilibrium states that give rise to unique steady-state limits. We recently showed that such a system, a magnetic single-electron transistor, can be tuned through a continuous quantum phase transition as the applied gate voltage is tuned [1,2]; the Kondo effect is critically destroyed across the quantum critical point, an effect that is also of interest in some bulk strongly correlated systems such as heavy fermions[3]. To address the non-linear electronic transport near the transition, we generalize the system to a large-N limit, where an exact quantum Boltzmann treatment becomes possible. We determine the universal scaling functions for the I-V characteristics in the linear and non-linear regime, and demonstrate a $V/T$ scaling in the quantum critical state out of equilibrium. [1] S. Kirchner et al., Proc.Natl.Acad.Sci. 102 (2005) 18824 [2] S. Kirchner, Q. Si, Physica B (2007), doi:10.1016/j.physb.2007.10.297 [3] S. Kirchner and Q. Si, Phys. Rev. Lett. in press; arXiv:0706.1783v1. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U12.00010: Effects of dissipation on a quantum critical point with disorder Thomas Vojta, Jose Hoyos, Chetan Kotabage We study the effects of dissipation on a disordered quantum phase transition with O$(N)$ order parameter symmetry by applying a strong-disorder renormalization group to the Landau-Ginzburg-Wilson field theory of the problem. We find that Ohmic dissipation results in a non-perturbative infinite-randomness critical point with unconventional activated dynamical scaling while superohmic damping leads to conventional behavior. We discuss applications to the superconductor-metal transition in nanowires and to Hertz' theory of the itinerant antiferromagnetic transition. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U12.00011: Current-Flow-Driven Nonequilibrium Paramagentic-Ferromagnetic Phase Transitions Aditi Mitra, Igor Aleiner, Andrew Millis We study a 2d itinerant electron system near a ferromagnetic-paramagnetic quantum critical point, which has been driven out of equilibrium by current flow through its bulk. The lack of Galilean invariance in physically realistic models implies that there is no co-moving frame of reference where the physics is identical to that in the absence of current. In the vicinity of the equilibrium critical point the main effect of current flow is shown to be an effective temperature, with current induced drift giving subleading corrections. The current can also destabilize a classical order, and may give rise to new kinds of ordered or quasi-ordered phases. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U12.00012: Theory of a smeared quantum phase transition Jose Hoyos, Thomas Vojta We present a comprehensive strong-disorder renormalization group theory of the quantum phase transition in the dissipative random quantum Ising chain. For Ohmic dissipation, we solve the renormalization group flow equations analytically, yielding asymptotically exact results for the low-temperature properties of the system. We find that the interplay between quantum fluctuations and Ohmic dissipation destroys the quantum critical point by smearing. We also determine the phase diagram and the behavior of observables in the vicinity of the smeared quantum phase transition. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U12.00013: Rounding of a first order quantum phase transition to a quantum critical point Pallab Goswami, David Schwab, Sudip Chakravarty We give a heuristic argument for disorder rounding of a first order quantum phase transition into a continuous phase transition. From both weak and strong disorder analysis of the the $N$-color quantum Ashkin-Teller model in one spatial dimension, we find that for $N \geq 3$, the first order transition is rounded to a continuous transition and the physical picture is the same as the random transverse field Ising model for a limited parameter regime. The results are strikingly different from the corresponding classical problem in two dimensions where the fate of the renormalization group flows is a fixed point corresponding to $N$-decoupled pure Ising models. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U12.00014: Criticality in Inhomogeneous magnetic systems: Applications to Quantum Ferromagnets R. Saha, T.R. Kirkpatrick, D. Belitz In standard phase transitions such as the liquid-gas transition, a homogeneous order parameter (OP) vanishes as one crosses from the ordered phase to the disordered one. An external field may preclude a homogeneous OP. This happens for a fluid in a gravitational field, where the transition becomes smeared[1] in the sense that the OP is nonzero everywhere, albeit very small in some regions of the phase diagram. A ferromagnet (FM) subject to mechanical stress is another realization of a system in an external field that has an inhomogeneous OP. We first investigate a classical Heisenberg FM, which is modeled by a $\phi^{4}$ theory with a spatially dependent mass $r(x)$. In contrast to the fluids case, we find a sharp phase transition where the envelope of the local magnetization vanishes uniformly, and mean-field critical exponents. The first order transition in quantum itinerant FMs also remains sharp and the fluctuation effects leading to a tricritical point are suppressed, and one recovers a quantum critical point with mean field exponents[2]. [1] J.V. Sengers and J.M.J. van Leeuwen, Physica A, 116, 345 (1982). [2] D. Belitz, T.R. Kirkpatrick, and R. Saha, Phys. Rev. Lett., 99, 147203(2007). [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U12.00015: Non-Ginzburg-Landau Type Universality in Quantum Metamagnetism Induced by Topological Change of Fermi Surface: Applications to a Weak Itinerant-Electron Ferromagnet ZrZn$_{2}$ Youhei Yamaji, Takahiro Misawa, Masatoshi Imada We clarify that metamagnetic transitions show unconventional properties as quantum phase transitions if they are accompanied by changes in Fermi-surface topology. Topological change of the Fermi surface makes the phase diagram qualitatively different from that of the conventional metamagnetic transitions; the quantum critical endpoint becomes not only the terminal of the finite-temperature critical line, but also the terminal of a quantum critical line of continuous Lifshitz transitions. Around the \textit{quantum critical terminal}, power-law singularities of thermodynamic quantities are determined by the Fermi-surface topology and, therefore, are characterized \textit{neither} by the Ising symmetry breaking \textit{nor} by the Ginzburg-Landau-Wilson scheme proposed by Moriya, Hertz and Millis for the conventional quantum criticalities. We propose that such an unconventional universality indeed accounts for the metamagnetic transitions observed in ZrZn$_{2}$. [Preview Abstract] |
Session U13: Focus Session: Frontiers in Electronic Structure Theory II
Sponsoring Units: DCOMP DCPChair: Claudia Ambrosch-Draxl, University of Leoben-Austria
Room: Morial Convention Center 204
Thursday, March 13, 2008 8:00AM - 8:36AM |
U13.00001: Self-consistent van der Waals density functional: Development and Applications Invited Speaker: The inability of density functional theory (DFT), with standard exchange-correlation functionals, to correctly describe van der Waals/dispersion (vdW) interactions has severely limited its applicability to sparsely packed systems, such as organic and biological molecules. Numerous attempts have been made to correct these deficiencies; however, many of them either require extensive reparameterization for each new situation or scale poorly with system size. In this paper, I will discuss the development and implementation of an exchange-correlation functional which correctly incorporates non-local vdW interactions within DFT (vdW-DF)\footnote{M. Dion, H. Rydberg, E. Schr\"{o}der, B. I. Lundqvist and D. C. Langreth, Phys. Rev. Lett., {\bf 92}, 246401 (2004)}. In addition, I will present our recent development of the corresponding exchange- correlation potential ($V_{\rm xc}$)\footnote{T. Thonhauser, V. R. Cooper, S. Li, A. Puzder, P. Hyldgaard, and David C. Langreth, Phys. Rev. B, {\bf 76}, 125112 (2007)}. The $V_{\rm xc}$ gives us the ability to compute Hellmann-Feynman forces, allowing for structural relaxations and molecular dynamics simulation. Using the $V_{\rm xc}$ I will examine the nature of the van der Waals bond between molecules. Finally, to demonstrate the power of the vdW-DF, I will discuss our relatively large scale application of the functional to study the influence of stacking interactions on the structure and stability of DNA. Here, I will show how these interactions are crucial for defining the twist and base pair separation in DNA and how methyl-nucleobase and methyl-methyl interactions give additional stability to DNA. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 9:12AM |
U13.00002: Stochastic Time-Dependent Current-Density Functional Theory Invited Speaker: Static and dynamical density functional methods have been applied with a certain degree of success to a variety of closed quantum mechanical systems, i.e., systems that can be described via a Hamiltonian dynamics. However, the relevance of open quantum systems - those coupled to external environments, e.g., baths or reservoirs - cannot be overestimated. To investigate open quantum systems with DFT methods we have introduced a new theory, we have named Stochastic Time-Dependent Current Density Functional theory (S-TDCDFT) $[1]$: starting from a suitable description of the system dynamics via a {\it stochastic} Schr\"odinger equation $[2]$, we have proven that given an initial quantum state and the coupling between the system and the environment, there is a one-to-one correspondence between the ensemble-averaged current density and the external vector potential applied to the system.\\ In this talk, I will introduce the stochastic formalism needed for the description of open quantum systems, discuss in details the theorem of Stochastic TD-CDFT, and provide few examples of its applicability like the dissipative dynamics of excited systems, quantum-measurement theory and other applications relevant to charge and energy transport in nanoscale systems.\\ $[1]$ M. Di Ventra and R. D'Agosta, Physical Review Letters {\bf 98}, 226403 (2007)\\ $[2]$ N.G. van Kampen, {\it Stochastic processes in Physics and Chemistry}, (North Holland, 2001), 2nd ed. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:48AM |
U13.00003: Investigating interaction-induced chaos using time-dependent density functional theory Invited Speaker: Systems whose underlying classical dynamics are chaotic exhibit signatures of the chaos in their quantum mechanics. In this talk I will discuss the possibility of using time-dependent density functional theory (TDDFT) to study the case when chaos is induced by electron interaction alone. Nearest-neighbor level- spacing statistics are in principle exactly and directly accessible from time-dependent density functional theory (TDDFT). Can the linear response formalism of TDDFT reveal the mechanism of chaos induced by electron-interaction alone? A simple model of a few-electron quantum dot highlights the necessity to go beyond the adiabatic approximation in TDDFT. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U13.00004: Time-dependent NEGF calculations of extended systems Alexander Prociuk, Barry Dunietz A non-equilibrium GF (NEGF) model based on time dependent perturbation theory is developed to propagate electronic structure and molecular conductance of extended electrode-molecule-electrode nanostructures. In this model, we take advantage of the two time variable nature of the KB equations in order to formulate a mixed time-frequency representation for the lesser GF. This allows us to include bulk affected electrodes with non-trivial energy representations in our propagation. It also allows us to express dynamical observables such as current with highly informative Wigner distributions that shed light on the physical causes for certain dynamic features. Preliminary calculations, performed on simple systems, reveal that the dynamic current has both a direct and an alternating contribution. The direct current is due to a bulk affected state and the alternating component is due to a superposition of states. The amplitude of the alternating current can be changed dramatically by adjusting the bias pulse. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U13.00005: Many-Body Density Matrix Perturbation Theory C.J. Tymczak One fundamental limitation of quantum chemical methods is the accuracy of the approximate many-body theoretical framework that is utilized. Accurate many- body formalisms for quantum chemical methods do exist, but these methods are computationally very expensive. Methods also exist that are much less computationally expensive such as Hatree-Fock, Density Functional and the Hybrid Functional theories, but at a reduced representation of the exact many-body ground state. This severely limits either the system size that can be addressed accurately, or the accuracy of the representation of the many-body ground state. What is essential is a method which represents the many-body ground state accurately, but with a low computational cost. Recently, a method for determining the response, to any order of the perturbation, within the density matrix formalism has been discovered. This method is very simple and computationally efficient, and it immediately opens up the possibility of computing the variational many- body ground state to unprecedented accuracy within a simplified computational approach. Within this article, we report on the theoretical development of this methodology, which we refer to as Many Body Density Matrix Perturbation Theory. This theory has many significant advantages over existing methods. One, its computational cost is equivalent to Hartree-Fock. Two it is a variational upper bound to the exact energy. And three, it has no self-interaction. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U13.00006: Conformational hierarchies of weakly bonded systems: Accuracy of dispersion corrected DFT Alexandre Tkatchenko, Volker Blum, Matthias Scheffler It is well known that long-range dispersion interactions, important for stabilization of, e.g., molecular crystals, biomolecules and physisorption, are badly described by state-of-the-art \textit{xc} functionals in DFT, but naturally included in post-HF methods or empirically in force field simulations. We have implemented a semi-empirical $C_6/R^6$ correction [1,2] in the numeric atom-centered orbital based code FHI-aims [3] and obtained correction parameters by fitting to a database of high quality \textit{ab initio} calculations [2], improving on previous results due to the more accurate basis set (0.5 kcal/mol average error for binding energies using PBE+$C_6$). We assess the accuracy and impact of the correction on conformational energy \emph{hierarchies} of (i) (H$_2$O)$_n$ clusters ($n$=2-6), (ii) Ala$_2$ and Ala$_4$, and (iii) infinite polyalanine conformers, comparing to published post-HF results for (i) and (ii). Even though the relative energies are not changed for small H$_2$O clusters and Ala$_2$ compared to \textsc{DFT-GGA}, the impact of dispersion on the conformation hierarchy of larger systems is surprisingly large, reaching $\sim$1-4 kcal/mol for different polyalanine conformers. [1] S. Grimme, J. Comput. Chem. 25, 1463 (2004) [2] P. Jurecka et al., J. Comput. Chem. 28, 555 (2007) [3] V. Blum et al., FHI ab initio molecular simulations (FHI-aims) project. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U13.00007: Iterative computation of dielectric eigenmodes Hugh Wilson, Francois Gygi, Giulia Galli We present an iterative method for the calculation of the eigenvectors of dielectric matrices of materials and nanostructures, based on Density Functional Theory, within a linear response framework. We show that by computing a relatively small number of eigenvectors via iterative dielectric response calculations, one may reconstruct the full dielectric matrix of a given system to high accuracy. The proposed method bypasses the need for the calculation of a large number of excited states required by earlier dielectric matrix computations based on the Random Phase Approximation. The scaling of the algorithm and the efficiency of the approach will be demonstrated by the calculation of the static dielectric properties of a variety of nanostructures, including silicon rods and slabs. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U13.00008: Comparison of vibrational and electronic contributions to van der Waals interactions Mark R. Pederson, Kyungwha Park, Amy Y. Liu The van der Waals interaction can be caused by either ionic vibrations or instantaneous electronic motion relative to the atomic center. In this study, the vibrational contribution to the van der Waals interaction is formulated by considering the interaction between induced dipoles caused by the infrared-active normal modes of a neutral molecule. Using the derived formula, the contribution is quantified, within the density-functional theory formalism, using a screened, i.e., self-consistent, vibrational polarizability. Applications for several neutral nonpolar dimers are presented. It is found that the vibrational contributions for the dimers are substantially smaller than their electronic contributions. The ratio of the vibrational to electronic contributions depends strongly on the ratio of the screened vibrational to electronic polarizabilities and on the ratio of the frequency of the strongest infrared-active mode to an ionization energy. [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U13.00009: Recent progress in ab initio density matrix renormalization group methodology Johannes Hachmann, Jonathan J. Dorando, Garnet Kin-Lic Chan We present some recent developments in the \textit{ab initio} density matrix renormalization group (DMRG) method for quantum chemical problems, in particular our local, quadratic scaling algorithm [1] for low dimensional systems. This method is particularly suited for the description of strong nondynamic correlation, and allows us to compute numerically exact (FCI) correlated energies for large active spaces, up to one order of magnitude larger then can be done by conventional CASCI techniques. Other features of this method are its inherent multireference nature, compactness, variational results, size-consistency and size-extensivity. In addition we will review the problems (predominantly organic electronic materials) on which we applied the \textit{ab initio} DMRG: \qquad 1) metal-insulator transition in hydrogen chains [1] \qquad 2) all-trans polyacetylene [1] \qquad 3) acenes [2] \qquad 4) polydiacetylenes [3]. \bigskip References [1] Hachmann, Cardoen, Chan, \textit{JCP} 125 (\textbf{2006}), 144101. [2] Hachmann, Dorando, Avil\'{e}s, Chan, \textit{JCP} 127 (\textbf{2007}), 134309. [3] \textit{unpublished}. [Preview Abstract] |
Session U14: Focus Session: Exotic Phases in Ultracold Fermi Gases
Sponsoring Units: DAMOPChair: Mehmet Oktel, Bilkent University
Room: Morial Convention Center 205
Thursday, March 13, 2008 8:00AM - 8:36AM |
U14.00001: FFLO states in resonant Fermi gases Invited Speaker: We discuss the possible phases of two-component Fermi gas with population imbalance. In particular, we consider the various states proposed by Fulde-Ferrell and Larkin-Ovchinnikov. We distinguish between the plane-wave state Delta $\sim $ e$^{iqr}$, where the magnitude of the order parameter is uniform in space but the phase varies continuously in space, from those where the order parameters are real but change sign from one spatial region to the other. The later states, considered first by Larkin and Ovchinnikov, occupy a much larger region in the uniform phase diagram than previously suggested by other authors. If time permits, we shall discuss also the situation in a harmonic trap. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U14.00002: Is There an FFLO Region in a Polarized Trapped Unitary Fermi Gas? William Schneider, Rajdeep Sensarma, Roberto Diener, Mohit Randeria We have studied strongly interacting polarized gases in a harmonic trap beyond the local density (LDA) approximation using the Bogoliubov-deGennes equations. In particular, we are interested in the region separating an unpolarized superfluid core in the center and a fully polarized majority gas in the outer edge. Several authors have found that in this region the order parameter oscillates in a way similar to an FFLO phase. We will present the results of a detailed analysis of the properties of this system as a function of polarization, system size, and high energy cutoff used in the calculations. We find that the order parameter oscillations are an artifact of a finite (not sufficiently large) cutoff. Moreover, we find that the intermediate region shows a scaling with number of particles which makes it consistent with an interface. Our BdG calculation thus gives us a microscopic theory of the interface in a trapped unitary fermi gas. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U14.00003: Polarized Fermi superfluids between one and three dimensions Meera Parish, Stefan Baur, Erich Mueller, David Huse We theoretically explore the phase diagram of a polarized two-component Fermi gas divided into an array of tubes by a two-dimensional optical lattice. By increasing the intensity of the optical lattice one suppresses the inter-tube hopping, and one can drive a crossover from three to one-dimensional behavior. We show that the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) inhomogeneous superfluid phase is stabilized by the lattice, and we argue that the most promising parameters for observing the FFLO phase are in the intermediate lattice limit, where the anisotropy in the atomic motion enhances Fermi surface `nesting', but there is still a small amount of tunneling between the tubes to provide long-range order. Finally we discuss the spatial arrangement of phases in a trap: in 3D the homogenous superfluid phase sits in the center of the trap, while in 1D it lies on the edge. We explain how this pattern evolves as one changes the lattice intensity. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U14.00004: Dimensional crossover from Quasi-1D to 3D in spin-polarized Fermi superfluids Stefan Baur, Meera Parish, Erich Mueller, David Huse We use a zero temperature Bogoliubov de Gennes mean field theory to study the evolution of the exotic FFLO superfluid in a spin-imbalanced Fermi gas as one progresses from quasi-1D to 3D by changing the coupling between an array of tubes. The boundary between the uniform BCS superfluid and the FFLO state is determined by examining the energetics of a single $\pi$-domain wall in the superfluid order parameter. In the quasi-1D limit, each tube contains a single excess particle at the center of each domain wall and the spectrum of single particle excitations is gapped. As one approaches 3D there is a phase transition where this commensurability condition is relaxed and gapless single particle excitations can be found. [1] M. M. Parish, S. K. Baur, E. J. Mueller, and D. A. Huse, arXiv:0709.1120 [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U14.00005: Effect of the surface tension on the shape of the superfluid region in population imbalanced unitary Fermi gases. Theja De Silva, Erich Mueller We use a variational approach to determine the shape of the central superfluid shell of apopulation imbalanced unitary Fermi gases. We find that the surface tension between superfluid and normal regions significantly distorts the superfluid shell from an ellipsoidal shape to a cylindrical shape as experimentally seen in highly anisotropic traps. Comparing with experimental data, we find that the surface tension has strong temperature dependence and this allows us to compare the temperatures of various available experiments. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U14.00006: Shape of the Normal-Superfluid Boundary in Polarized Fermi Gases Stefan Natu, Erich Mueller We model the normal-superfluid boundary in a trapped polarized Fermi gas as an elastic membrane and calculate the density profile. For weak trapping anisotropy, the normal-superfluid boundary remains elliptical, in agreement with the LDA. However, for strong anisotropy, the boundary becomes distorted into a capsule-like shape. As one moves axially from the edge of the trap to the center, the radius of the boundary almost discontinuously jumps. In addition to full numerical calculations, we present a simple model that predicts the density profile in the limit of large trapping anisotropy. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U14.00007: Imbalanced Fermi superfluids beyond mean-field. Jacques Tempere, Serghei Klimin, Jozef Devreese Cold atomic Fermi gases undergo pairing transitions leading to superfluidity, as has been demonstrated in recent experiments. In superconducting metals, the number of spin-up and spin-down fermions forming the Cooper pairs is always equal, but in the experiments with atomic superfluids, the number of each pairing partner can be tuned individually. This allows to probe the effect of population imbalance on the pairing properties, and has rekindled much theoretical interest in these systems. Here, we describe how to tackle fluctuations beyond mean field, and at nonzero temperature through an extension of the path-integral scheme developed by Randeria and co-workers. The results are discussed in the context of the recent (sometimes conflicting) experimental observations of imbalanced Fermi superfluids. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U14.00008: Evolution from weak to strong coupling pairing of Dirac Fermions Shi-Quan Su, Ka-Ming Tam, Hai-Qing Lin We study the pairing of Dirac Fermions with attractive interaction from weak to strong coupling regime, highlighting the differences and resemblances with that of the BCS-BEC crossover in the systems with extended Fermi surface. Dirac Fermions model at low doping limit is solved by mean field approximation. Exact Quantum Monte Carlo method, auxiliary field Quantum Monte Carlo, is used to simulate the single band attractive Hubbard model on a honeycomb lattice. Quantities for probing the crossover, double occupancy, spin susceptibility, on-site pair correlation, and kinetic energy are obtained impartially. We find that these quantities indicate the BCS-BEC crossover of the model. This can be interpreted as a competition between Fermionic modes and Bosonic modes which coexist in the single band Hubbard model on a honeycomb lattice with attractive interaction. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U14.00009: P-wave Pairing in Two-Component Fermi System with Unequal Population near Feshbach Resonance Renyuan Liao, Florentin Popescu, Khandker Quader We explore $p$-wave pairing in a single-channel two-component Fermi system with unequal population near Feshbach resonance. Our analytical and numerical study reveal a rich superfluid (SF) ground state structure as a function of imbalance. In addition to the state $\Delta_{\pm 1} \propto Y_{1\pm 1}$, a multitude of ``mixed'' SF states formed of linear combinations of $Y_{1m}$'s give global energy minimum under a phase stability condition; these states exhibit variation in energy with the relative phase between the constituent gap amplitudes. States with local energy minimum are also obtained. We provide a geometric representation of the states. A $T$=0 polarization vs. p-wave coupling phase diagram is constructed across the BEC-BCS regimes. With increased polarization, the global minimum SF state may undergo a quantum phase transition to the local minimum SF state. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U14.00010: P-wave Paired Ground state Structure of Two-component Population Imbalanced Fermi Systems with Unequal Mass Florentin Popescu, Renyuan Liao, Khandker Quader Effects of unequal mass on p-wave pairing in population imbalanced two-component Fermi systems is studied using a single-channel fermionic Hamiltonian. Provided certain phase criteria are satisfied among the orbital gap parameters, a rich structure of the ground state is obtained. The global and local minima ground state energies are determined analytically. By numerically solving the gap and number equations, we construct T=0 polarization versus p-wave coupling phase diagrams across the BEC-BCS regime for different mass ratios. This shows the existence of two superfluid phases, SF1 and SF2, corrsponding to the global and local energy minima respectively; phase separation occurs at higher polarizations. For small mass ratios, SF1 is enhanced; SF2 is not significantly affected. Competition between mass ratio and polarization is studied; the superfluid transition temperature shows interesting behavior versus the mass ratio at high polarizations. At large polarizations, a stable p-wave superfluid would survive only for small mass ratios, and for mass ratios close to unity. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U14.00011: Orbital ordering in an atomic Mott insulator of p-band fermions Erhai Zhao, W. Vincent Liu We derive the low energy effective model describing the orbital degrees of freedom of strongly interacting spinless p-orbital fermionic atoms in 2D optical lattices. Virtual hopping processes of $p_x$ and $p_y$ fermions give rise to direct and multi-particle orbital exchanges in the strong coupling regime. For the square lattice, we show that the effective orbital Hamiltonian is equivalent to a quantum spin-1/2 XXZ model. In the limit where the transverse hopping is much smaller than the longitudinal hopping, the XXZ model reduces to an antiferromagnetic Ising model. Thus the atomic Mott insulator is antiferro-orbitally ordered. We also present results for other simple 2D lattices and discuss the experimental signatures of various orbital ordering. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U14.00012: Density waves and supersolidity in rapidly rotating atomic Fermi gases Gunnar Moller, Nigel R. Cooper We study theoretically the low-temperature phases of a two-component atomic Fermi gas with attractive $s$-wave interactions under conditions of rapid rotation [1], a problem related to the discussion of high-field superconductivity in the solid state [2]. The regime of interest for atomic gases differs substantially from solid state conditions: the rotation does not lead to any Zeeman splitting which might suppress high-field SC order; the short-range interactions allow density wave order to develop (contrary to long-range Coulomb interactions). We show that the low-temperature phases of an atomic Fermi gas with attractive interactions involve an interesting interplay between CDW and superconducting phases. In the extreme quantum limit, when only the lowest Landau level is occupied, we employ a renormalization group approach [3] to show that the system is unstable to CDW order along the rotation axis. At lower rotation rates, we show how CDW and SC can coexist, leading to supersolid behaviour.\\[0pt] [1] G. M\"oller and N. R. Cooper, Phys. Rev. Lett {\bf 99}, 190409 (2007).\\[0pt] [2] Z. Tesanovic, M. Rasolt and L. Xing, Phys. Rev. Lett. {\bf 63}, 2425 (1989).\\[0pt] [3] V.~M. Yakovenko, Phys.~Rev.~B {\bf 47}, 8851 (1993). [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U14.00013: Quantum Hall Transition near a Feshbach Resonance in Fast Rotating Fermi Gases Kun Yang, Hui Zhai We consider two-species of fermions in a rotating trap that interact via an s-wave Feshbach resonance, at total Landau level filling factor two (or one for each species). We show that the system undergoes a quantum phase transition from a fermion integer quantum Hall state to a boson fractional quantum Hall state as the pairing interaction strength increases, with the transition occurring near the resonance. The effective field theory for the transition is shown to be that of a (emergent) massless relativistic bosonic field coupled to a Chern-Simons gauge field, with the coupling giving rise to semionic statistics to the emergent particles. [Preview Abstract] |
Session U15: Focus Session: Open Quantum Systems and Decoherence
Sponsoring Units: GQIChair: Sergio Boixo, University of New Mexico
Room: Morial Convention Center 207
Thursday, March 13, 2008 8:00AM - 8:36AM |
U15.00001: Coherence and control of single electron spins in quantum dots Invited Speaker: Following our earlier work on single-shot read-out and relaxation of a single spin~in a quantum dot, we now demonstrate coherent control of a single spin (detection is done using a second spin in a neighbouring dot). First, we manipulate the spin using conventional magnetic resonance. Next, we show that we can also rotate the spin using electric fields instead of magnetic fields. In both cases, 90 rotations can be realized in about 50 ns or less. We use these control techniques to probe decoherence of an isolated electron spin. The spin dephases in about 30 ns, due to the hyperfine interaction with the uncontrolled nuclear spin bath in the host material of the dot. However, since the nuclear spin dynamics is very slow, this dephasing can be largely reversed using a spin-echo pulse. Echo decay times of about 0.5 us are obtained at 70 mT. In parallel, we have started work on quantum dots in graphene, which is expected to offer superior coherence times. As a first step, we have succeeded in opening a bandgap in bilayer graphene, necessary for electrostatic confinement of carriers. \newline \newline F.H.L. Koppens et al., Nature 446, 56 (2006). \newline K.C. Nowack et al., Science Express, 1 Nov 2007. \newline F.H.L. Koppens et al., arXiv:0711.0479. \newline J.B. Oostinga, Nature Mat., in press. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U15.00002: Non-Markovian thermalization for a few qubit system Pedro Manrique, Ferney Rodriguez Non-Markovian dynamics in the thermalization process of a single and coupled-qubit systems are analyzed by means on an effective master equation. Memory effects are included in a time dependent relaxation constant which is obtained from a proper bosonic bath spectral function. For different initial states, the population and qubit coherences are studied as a function of the qubit-bath coupling strenghts and bath temperature. Clear signatures of non-exponential decays for the qubit density matrix elements are found in a short-time regime corresponding to the back action of the qubit system on the bath dynamics. In the case of realistic two-qubit systems, such as quantum dots, entanglement oscillations should be observable. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U15.00003: Decoherence of coupled electron spins via nuclear spin dynamics in quantum dots Wen Yang, Ren-bao Liu Decoherence of coupled electron spins due to electron-nuclear hyperfine interaction in double quantum dots is a major issue of solid-state quantum computation. Using an interacting nuclear spin bath model, we show theoretically that the exchange interaction between the two electron spins renormalizes the pair- flip excitation energy in the bath and modifies the non- Markovian bath dynamics, which in turn changes the electron singlet-triplet (S-T) decoherence arising from electron-nuclear entanglement. As the energy renormalization varies with the Overhauser field mismatch between the quantum dots, the S-T decoherence depends on the sampling of nuclear spin states from an ensemble, leading to the transition from super-exponential decoherence in single-sample dynamics to power-law decay under ensemble average,[1] in contrast with the sample-independent super-exponential decoherence of a single electron spin in one dot. \newline \newline [1] W. Yang and R. B. Liu, arXiv:0707.2529v1. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U15.00004: Rabi oscillations decay from interaction with dynamical spin environments V.V. Dobrovitski, A.E. Feiguin Studying decoherence of spins/qubits interacting with a spin bath is important for quantum computation, high-precision metrology, coherent spintronics. Measurements of the Rabi oscillations decay provide much information about the decoherence dynamics and properties of the spin bath in single-spin quantum dots, dopant spins in a solid-state matrix, etc. [1] Also, for a static bath, application of a large Rabi field suppresses decoherence, changing fast exponential decay into slow power-law one. [1] However, internal dynamics of spin environment is important in such systems as NV centers in diamonds, magnetic molecules, and rare-earth dopant spins in solid state, but decay of Rabi oscillations for dynamical spin bath has been little studied. We present a detailed theoretical investigation of Rabi oscillations decay for a dynamic spin bath, demonstrating new unusual decay regimes useful for characterization of the bath and decoherence suppression. \newline [1] F. H. L. Koppens et al, Phys. Rev. Lett. 99, 106803 (2007); S. Bertaina et al, Nature Nano. 2, 39 (2007); V. V. Dobrovitski et al, quant-ph/0112053. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U15.00005: Environmentally-Induced Rabi Oscillations and Decoherence in Phase Qubits Kaushik Mitra, Carlos Sa de Melo, Christopher Lobb We study decoherence effects in a dc SQUID phase qubit caused by an isolation circuit with a resonant frequency. The coupling between the SQUID phase qubit and its environment is modeled via the Caldeira-Leggett formulation of quantum dissipation/coherence, where the spectral density of the environment is related to the admittance of the isolation circuit. When the frequency of the qubit is at least two times larger than the resonance frequency of the isolation circuit, we find that the decoherence time of the qubit is two orders of magnitude larger than the typical ohmic regime, where the frequency of the qubit is much smaller than the resonance frequency of the isolation circuit. Lastly, we show that when the qubit frequency is on resonance with the isolation circuit, an oscillatory non-Markovian decay emerges, as the dc SQUID phase qubit and its environment self-generate Rabi oscillations of characteristic time scales shorter than the decoherence time. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U15.00006: Boundary Conditions for Open Rotating Quantum Systems Arthur Davidson The customary boundary conditions for a 1 D rotational system (e.g. a rigid rotor on a surface) are continuity of the complex wave function and its gradient. These four boundary conditions are sufficient if the potential energy satisfies rotational symmetry, but fail for non-rotational potentials. However, classical systems with a rotational coordinate and non- rotational potential are easily solved if the gradient of the potential, the force, is rotational. A solution is thus needed for Schroedinger's equation with a rotational coordinate and force, but non- rotational potential. Such solutions emerge if the boundary conditions are modified, allowing a discontinuous phase in the wave function related to the discontinuous potential energy. It will be shown that the modified boundary conditions are continuity of three real quantities: the probability density, the gradient of the probability density, and the probability current density. Moreover, with these boundary conditions and non-rotational potential, energy can flow both ways between the system and its environment. The discontinuous wave functions obey the new boundary conditions, but nonetheless are not generally superposable. A subset of the discontinuous wave functions can be superposed, however, yielding the usual result for angular momentum states. The non-superposable wave functions offer an alternate interpretation of the Schroedinger's cat paradox. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U15.00007: Non-Markovian Open Quantum Systems Cesar Rodriguez-Rosario, E.C.G. Sudarshan A generalized non-markovian master equation is derived from the dynamical map of systems initially correlated with their environment. We study the connection between the initial correlations and the non-markovian memory effects. The significance of not-complete positive maps in order to obtain a consistent theory of non-markovian quantum dynamics is discussed. Previous specific instances of non-markovian master equations are examined in this framework. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U15.00008: Finite representations of continuum environments Michael Zwolak Understanding dissipative and decohering processes is fundamental to the study of non-equilibrium systems and quantum computing, and such processes can even induce quantum phase transitions. A typical construction is to have a system connected to a continuum environment, which acts as the source of dissipation or decoherence, or as a reservoir of particles. If the connection is strong or the environment has long-range correlations in time, the system dynamics are not easily separated from the dynamics of the environment. To study this situation numerically, one option is to simulate both the system and environment. This is a viable option so long as an efficient finite representation of the environment can be constructed. We will discuss a procedure to construct finite representations based on the concept of two-site recurrence and increasing smoothness. For solvable cases of non-interacting bosons, the procedure gives an exponential reduction in the number of discrete modes necessary to achieve some given accuracy in a real-time simulation. We will also discuss application of this procedure to interacting systems. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U15.00009: Induced decoherence and entanglement by an interacting spin chain Pochung Chen, Cheng-Yan Lai, Jo-Tzu Hung, Chung-Yu Mou We study the reduced dynamics of a single or two qubits coupled to an interacting spin chain using time-dependent density matrix renormalization group (TD-DMRG) technology. By using TD-DMRG we can go beyond the uniform coupling central spin model and evaluate nonperturbatively the reduced dynamics even when the coupling between qubits and the chain is non-uniform. Furthermore, the qubit-bath interaction can be of Ising, XY, or Heisenberg type. This allows us to go beyond pure dephasing model. For single qubit we use Loschmidt echo to gauge the decoherence and investigate how the short time decay parameter and large time behavior are linked to the phase of spin chain. We use concurrence to quantify the (dis)-entanglement process of two qubits due to spin chain. We show that one can induced entanglement for an initially disentangled pair of qubit. The competition between induced decoherence and entanglement is discussed. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U15.00010: Temperature Exchange in a System of Two Harmonic Oscillators Antonia Chimonidou, George Sudarshan We study the process by which quantum correlations are created when an interaction Hamiltonian is repeatedly applied to a system of two harmonic oscillators for some characteristic time interval, under what we call the ``interact-refresh-repeat'' scheme. We show that, for the case where the oscillator frequencies are equal, the initial Maxwell-Boltzmann distributions of the uncoupled parts evolve to a new Maxwell-Boltzmann distribution through a series of transient Maxwell-Boltzmann distributions, or quasi-stationary, non-equilibrium states. Further, we discuss why the equilibrium reached when the two oscillator frequencies are unequal, is not a thermal one. More specifically, we show that the ratio of the harmonic oscillator temperatures at the new equilibrium state is completely determined by the ratio of the inverse harmonic oscillator frequencies. We conclude that the selection rules imposed by the interaction Hamiltonian override the expected statistical mechanical effects. All the calculations are exact and the results are obtained through an iterative process, without using perturbation theory. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U15.00011: Impact of classical forces and decoherence in three-terminal Aharonov-Bohm devices Elia Strambini, Vincenzo Piazza, Fabio Beltram, Giorgio Biasiol, Lucia Sorba Multi-terminal Aharonov-Bohm (AB) rings are ideal building blocks for quantum networks (QNs) thanks to their ability to transform input states into controlled coherent superpositions of output states. We report on experiments performed on three-terminal GaAs/AlGaAs AB devices and compare our results with a scattering-matrix model of our device including Lorentz forces and decoherence. Our devices were studied as a function of external magnetic field ($B)$ and gate voltage ($V_{g})$ down to a T=350 mK. The total output current from two terminals while applying a small bias to the third lead was found to be symmetric with respect to $B$ with clear AB oscillations showing abrupt phase jumps between 0 and $\pi $ at different values of $V_{g}$, reminiscent of the phase-rigidity constraint due to Onsager-Casimir relations. Surprisingly, the individual outputs show an almost linear dependence of the oscillation phase on the external electric field. We emphasize that a simple scattering-matrix approach does not explain the observed behavior and show how to extend this model in order to fully describe the observed phenomena. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U15.00012: The Behavior of Electronic Interferometers in the Non-Linear Regime. Izhar Neder, Eran Ginossar We investigate theoretically the behavior of the current oscillations in an electronic Mach-Zehnder interferometer (MZI) as a function of its source bias. Recently, The MZI interference visibility showed an unexplained lobe pattern behavior with a peculiar phase rigidity. Moreover, the effect did not depend on the MZI paths difference. We argue that these effects have a fundamental reason. A simple invariance argument leads to an additional interaction term that must be added to the non-interacting Hamiltonian. It causes correlations inside each of the two MZI arm, resulting in the electrons affecting each other's phase. An approximate solution shows that the interference visibility has a lobe pattern with applied bias with a period proportional to the average path length (and independent of the paths difference), together with a phase rigidity. [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U15.00013: What is quantum about quantum trajectory equations? Jay Gambetta, Howard Wiseman, Steve Jones, Eric Cavalcanti Quantum trajectory equations are stochastic equations for the state of an open quantum system conditioned on a monitoring i.e. a continuous-in-time measurement of a bath to which it is coupled. They are closely related to classical stochastic equations for classical probability distributions called filtering equations (e.g. the Kalman filter). Given this close relation, the question arises: what is quantum about quantum trajectory equations? In this talk I suggest that the answer lies in the ability of an experimenter to choose different monitoring schemes. Moreover, I propose that there is an experimental way to distinguish between cases where this choice does demonstrate the quantum nature of the noise, and those where it does not. [Preview Abstract] |
Session U16: Focus Session: Medical Physics and Radiation Biology
Sponsoring Units: DBPChair: Richard Britten, Eastern Virginia Medical School
Room: Morial Convention Center 208
Thursday, March 13, 2008 8:00AM - 8:36AM |
U16.00001: Image-Guided Radiation Therapy: the potential for imaging science research to improve cancer treatment outcomes Invited Speaker: The role of medical imaging in the planning and delivery of radiation therapy (RT) is rapidly expanding. This is being driven by two developments: Image-guided radiation therapy (IGRT) and biological image-based planning (BIBP). IGRT is the systematic use of serial treatment-position imaging to improve geometric targeting accuracy and/or to refine target definition. The enabling technology is the integration of high-performance three-dimensional (3D) imaging systems, e.g., onboard kilovoltage x-ray cone-beam CT, into RT delivery systems. IGRT seeks to adapt the patient's treatment to weekly, daily, or even real-time changes in organ position and shape. BIBP uses non-anatomic imaging (PET, MR spectroscopy, functional MR, etc.) to visualize abnormal tissue biology (angiogenesis, proliferation, metabolism, etc.) leading to more accurate clinical target volume (CTV) delineation and more accurate targeting of high doses to tissue with the highest tumor cell burden. In both cases, the goal is to reduce both systematic and random tissue localization errors (2-5 mm for conventional RT) conformality so that planning target volume (PTV) margins (varying from 8 to 20 mm in conventional RT) used to ensure target volume coverage in the presence of geometric error, can be substantially reduced. Reduced PTV expansion allows more conformal treatment of the target volume, increased avoidance of normal tissue and potential for safe delivery of more aggressive dose regimens. This presentation will focus on the imaging science challenges posed by the IGRT and BIBP. These issues include: \textit{Development of robust and accurate nonrigid image-registration (NIR) tools:} Extracting locally nonlinear mappings that relate, voxel-by-voxel, one 3D anatomic representation of the patient to differently deformed anatomies acquired at different time points, is essential if IGRT is to move beyond simple translational treatment plan adaptations. NIR is needed to map segmented and labeled anatomy from the pretreatment planning images to each daily treatment position image and to deformably map delivered dose distributions computed on each time instance of deformed anatomy, back to the reference 3D anatomy. Because biological imaging must be performed offline, NIR is needed to deformably map these images onto CT images acquired during treatment. \textit{Reducing target and organ contouring errors}: As IGRT significantly reduces impact of differences between planning and treatment anatomy, RT targeting accuracy becomes increasingly dominated by the remaining systematic treatment-preparation errors, chiefly error in delineating the clinical target volume (CTV) and organs-at-risk. These delineation errors range from 1 mm to 5 mm. No single solution to this problem exists. For BIBP, a better understanding of tumor cell density vs. signal intensity is required. For anatomic CT imaging, improved image reconstruction techniques that improve contrast-to-noise ratio, reduce artifacts due to limited projection data, and incorporate prior information are promising. More sophisticated alternatives to the current concept fixed boundary anatomic structures are needed, e.g., probabilistic CTV representations that incorporate delineation uncertainties. \textit{Quantifying four-dimensional (4D) anatomy}: For adaptive treatment planning to produce an optimal time sequence of delivery parameters, a 4D anatomic representation, the spatial trajectory through time of each tissue voxel, is needed. One approach is to use sequences of deformation vector fields derived by non-rigidly registering each treatment image to the reference planning CT. One problem to be solved is prediction of future deformed anatomies from past behavior so that time delays inherent in any adaptive replanning feedback loop can be overcome. Another unsolved problem is quantification 4D anatomy uncertainties and how to incorporate such uncertainties into the treatment planning process to avoid geometric ``miss'' of the target tissue. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U16.00002: Bridging the Gaps between IGRT Systems and R{\&}V Systems Yulong Yan, Xuejun Weng, Jose Penagaricano, Vaneerat Ratanatharathorn Image Guided Radiation Therapy (IGRT) is the next-generation of technology for high precision radiotherapy. BrainLAB ExacTrac and Tomotherapy are two of them. Unfortunately, neither of the two communicates with any Record and Verify (R{\&}V) system for seamless radiation therapy workflow. So two dedicated software systems, iPump and ScreenBee, have been developed respectively to bridge the gaps between IGRT systems and the R{\&}V systems to allow remote image reviewing as well as consolidation of patient's medical records. As an image pumping utility, iPump periodically searches for new registered images, fuses them and sends them to the R{\&}V system via DICOM connection. The built-in instant messaging mechanism automatically notifies the attending radiation oncologists right after images were sent. ScreenBee is a DICOM screen dumper. Instead of sending unsupported treatment parameters, it captures their graphical presentations on the computer screen and sends them to the R{\&}V system. Both iPump and ScreenBee have been extensively tested and evaluated in our clinic. They reduce the cost and improve the efficiency and the safety of clinical procedures. They also act as key integral components of our facility on its way toward the digital and paperless future. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U16.00003: Hyperthermal heavy ion damage to DNA bases Sarvenaz Sarabipour, Zongwu Deng, Michael Huels Ionization and fragmentation of DNA is a key step in biological radiation damage. When heavy ions cross the cell, secondary ballistic ions, electrons and radicals are generated along the ion tracks. Here we report measurements of ionic fragments induced by 1-100eV Ar$^{+}$ irradiation of Adenine, Guanine and Cytosine films on Pt. Experiments are conducted with a UHV ion-beam apparatus consisting of a low energy ion source, a beam line with high resolution magnetic mass spectrometer (MS), a biomolecular film preparation system, and a reaction chamber with high-resolution quadrupole MS to monitor desorbing ion yields. Among the major fragments, NH$_{4}^{+}$ was identified in the desorption mass spectra of all bases examined, indicating efficient de-amination; in cells this results in pre-mutagenic lesions. Several important factors, e.g. intra/inter-molecular proton/hydrogen tunneling, tautomeric equilibrium and the molecular geometry of the bases in the films likely contribute to ion induced de-amination, and will be discussed here. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U16.00004: Physical and Clinical Evaluation of Standardized Uptake Values Cristina Lois, Bjoern W. Jakoby, Karl Hubner, Mario Ca\~nadas, David W. Townsend The Standardized Uptake Value (SUV) is often used in positron emission tomography (PET) to differenciate malignant from benign tumors, and to monitor the progress of the patient response to therapy. Despite its name, SUV may depend on both PET scanner hardware and software details, and will depend on the imaging protocol. In this paper, we present a study of the SUV variability according to these external factors. To study the influence of the PET device, phantom studies were performed on two different combined PET/CT scanners. SUVs were obtained using several reconstruction algorithms and different reconstruction matrix sizes. To study the influence of the imaging protocol, patients were injected with 370 MBq of 18F-FDG and scanned at 60 and 90 min post-injection. SUVs were obtained applying several clinical image reconstruction algorithms. Significant differences in SUVs were obtained depending on the PET scanner, reconstruction method, and imaging protocol. It is essential, therefore, to follow a strict protocol in order to reliably compare FDG uptake with SUVs. Our results may have a significant clinical impact in order to provide an unbiased SUV thereshold to determine malignancy. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:48AM |
U16.00005: The Dose Response Relationship for Radiation Carcinogenesis Invited Speaker: Recent surveys show that the collective population radiation dose from medical procedures in the U.S. has increased by 750{\%} in the past two decades. It would be impossible to imagine the practice of medicine today without diagnostic and therapeutic radiology, but nevertheless the widespread and rapidly increasing use of a modality which is a known human carcinogen is a cause for concern. To assess the magnitude of the problem it is necessary to establish the shape of the dose response relationship for radiation carcinogenesis. Information on radiation carcinogenesis comes from the A-bomb survivors, from occupationally exposed individuals and from radiotherapy patients. The A-bomb survivor data indicates a linear relationship between dose and the risk of solid cancers up to a dose of about 2.5 Sv. The lowest dose at which there is a significant excess cancer risk is debatable, but it would appear to be between 40 and 100 mSv. Data from the occupation exposure of nuclear workers shows an excess cancer risk at an average dose of 19.4 mSv. At the other end of the dose scale, data on second cancers in radiotherapy patients indicates that cancer risk does not continue to rise as a linear function of dose, but tends towards a plateau of 40 to 60 Gy, delivered in a fractionated regime. These data can be used to estimate the impact of diagnostic radiology at the low dose end of the dose response relationship, and the impact of new radiotherapy modalities at the high end of the dose response relationship. In the case of diagnostic radiology about 90{\%} of the collective population dose comes from procedures (principally CT scans) which involve doses at which there is credible evidence of an excess cancer incidence. While the risk to the individual is small and justified in a symptomatic patient, the same is not true of some screening procedures is asymptomatic individuals, and in any case the huge number of procedures must add up to a potential public health problem. In the case of radiation oncology, modern innovations such as Intensity Modulated Radiation Oncology or Proton Therapy both result in a substantial total-body dose to the patient, which must result in an increased incidence of second cancers. The technology exists to reduce these total body doses and the problem needs to be addressed. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:24AM |
U16.00006: Panel Discussion on Medical Physics and Radiation Biology and pathways to these fields Invited Speaker: Medical Physics is one of the less know physics field but one that has the most impact on our daily life. It is intrinsically linked to radiation biology as the latter provides crucial inputs to the former including treatment planning software packages, understanding of cancer treatment optimization, etc. This panel discussion, composed of renown experts in these fields and who are part of the four medical physics sessions to be presented during this 2008 APS March meeting, will provide an environment for the audience to fully understand what medical physics and radiation biology are about and the various pathways to become a successful practitioner or researcher for contributing in these fields. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U16.00007: Anomalous Effect of Surface Diffusion on NMR Signal in Restricted Geometry Neranjan Edirisinghe, Vadym Apalkov, Gennady Cymbalyuk The diffusion of magnetic molecules along the surface of restricted media and the coupling of the surface and the bulk translational motions can strongly modify the echo attenuation NMR signal in the pulse field gradient measurements. The origin of this strong effect is the change of the symmetry of the lowest diffusion eigenmode of the system. We illustrate the effect of surface diffusion for cylindrically symmetric system. We find the parameters of the system under which the anomalous behavior of echo signal can be observed. [Preview Abstract] |
Session U17: Proteins in Membranes and the Cytoskeleton
Sponsoring Units: DBPChair: Robijn Bruinsma, University of California, Los Angeles
Room: Morial Convention Center 209
Thursday, March 13, 2008 8:00AM - 8:12AM |
U17.00001: Hierarchy of Specific Lipid-Peptide Interactions Produces the Activity of Cell-penetrating and Cell-permeating Peptides Matthew Davis, Daniel Parente, Vernita Gordon, Abhijit Mishra, Nathan Schmidt, Lihua Yang, Robert Coridan, Abhigyan Som, Gregory Tew, Gerard Wong Protein transduction domains can cross cell membranes with high efficiency, even when carrying a variety of cargos, and thus has strong biotechnological potential. The molecular mechanism of entry, however, is not well understood. We use small-angle x-ray scattering (SAXS) and confocal microscopy to systematically study the interaction of the TAT and ANTP PTD with model membranes of variable composition. Their membrane transduction activity requires the presence of both PE and PS lipids in the membrane. Antimicrobial peptides (AMP's) are cationic amphiphiles that comprise a key component of innate immunity. Synthetic analogs of AMP's, such as the family of phenylene ethynylene antimicrobial oligomers (AMO's), recently demonstrated broad-spectrum antimicrobial activity, but the underlying molecular mechanism is unknown. PE lipid greatly enhances permeating activity of AMO in these membranes, showing the importance of specific lipid composition for the activity of cell-permeating peptides. Since bacterial cell membranes are richer in PE lipids than are eukaryotic cell membranes, this may indicate a mechanism for antimicrobial specificity. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U17.00002: Simulations of the pore structures for a M2G1yR derived channel forming peptide in membrane Ahlam N. Al-Rawi, Asma Al-Rawi, Jianhan Chen, Alvaro Herrera, John Tomich, Talat S. Rahman In an effort to develop a peptide-based compound suitable for clinical use as a channel replacement therapeutic for treating channelopathies such as cystic fibrosis, we present a reductionist model that appears to capture many of the biophysical properties of an intact ion channel using short channel-forming peptides. We have developed two anion selective channel-forming peptides with near native and altered properties from the peptides derived from the glycine receptor: NK$_{4}$-M2GlyR-p22 WT (KKKKPAR-VGLGITTVLTMTTQS) and NK$_{4}$-M2GlyR-p22 S22W (KKKKPARVGLGITTVLTMTTQW), respectively. Starting with the two structures determined by solution multidimensional NMR (800 MHz) in SDS, we used CHARMM and NAMD to perform molecular dynamics simulations on the monomers. Using the existing experimental data, we then built an initial 5- helix assembly by altering the tilted angle, rotational angle and pore radius. We investigated the impact of the single mutation at position 22 on the structure and dynamics of the pore formed in a membrane build in a hydrated POPC lipid bilayer. Probable structures for both assemblies are presented. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U17.00003: Membrane-mediated mechanism of amyloid oligomer toxicity in Alzheimer's Disease. Frank Heinrich, Yuri Sokolov, James E. Hall, Rima Budvytyte, Gintaras Valincius, Mathias Loesche There is strong evidence, that soluble amyloid $\beta $ (A$\beta )$ oligomers, involved in Alzheimer's Disease, are the primary toxic species of A$\beta $, although the mechanism of cell toxicity is very much debated [1]. Neutron reflectivity and electrical impedance spectroscopy assess the structural impact of A$\beta $ (1-42) oligomers and their effect on the electrical properties of a tethered phosphocholine model membrane. Two distinct and reversible peptide -- membrane interactions were revealed: At low A$\beta $ concentrations an equal incorporation of A$\beta $ into both lipid leaflets and a compaction of the lipid membrane takes place. A$\beta $ locally lowers the dielectric barrier for ion transport and the activation energy for ion transport through the bilayer remains significantly above that of a water-filled transmembrane pore. At high A$\beta $ concentrations, an additional membrane thinning is observed. [1] D. Eliezer, J. Gen. Physiol. 128:631 (2006). [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U17.00004: Molecular target of synthetic antimicrobial oligomer in bacterial membranes Lihua Yang, Vernita Gordon, Abhigyan Som, John Cronan, Gregory Tew, Gerard Wong Antimicrobial peptides comprises a key component of innate immunity for a wide range of multicellular organisms. It has been shown that natural antimicrobial peptides and their synthetic analogs have demonstrated broad-spectrum antimicrobial activity via permeating bacterial membranes selectively. Synthetic antimicrobials with tunable structure and toxicological profiles are ideal for investigations of selectivity mechanisms. We investigate interactions and self-assembly using a prototypical family of antimicrobials based on phenylene ethynylene. Results from synchrotron small angle x-ray scattering (SAXS) results and in vitro microbicidal assays on genetically modified `knock-out' bacteria will be presented. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U17.00005: Interaction of Arginine-Rich Peptides with Model Cell Membranes Abhijit Mishra, Nathan Schmidt, Vernita Gordon, Jianjun Cheng, Timothy Deming, Gerard Wong Cell-penetrating peptides have the ability to traverse the plasma membrane of eukaryotic cells. Furthermore, these peptides can transport cargo across a range of cell membranes, implying they have many potential biotechnological applications. In this study we compare the interaction of three commonly used arginine-rich cell-penetrating peptides, TAT, Penetratin, and pVEC, with model cell membranes of variable charge density and intrinsic curvature, using synchrotron small angle x-ray scattering (SAXS). To better understand the respective roles of arginine and hydrophobic residues in membrane reorganization we also examine the interaction of arginine-leucine (R60L20) block copolypeptides with model membranes, as well as the relationship between membrane composition and peptide induced changes in membrane topology. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U17.00006: Lipid and Protein Sorting during Membrane Tube Formation Hongyuan Jiang, Thomas Powers Motivated by recent experiments that implicate the mechanical properties of membranes in lipid sorting, we examine the interplay of lipid composition and curvature in membrane tubes. We study how the dependence of bending stiffness and surface tension on membrane lipid and protein composition affects tube formation. Drawing a tube from a vesicle leads to a rearrangement of composition in which the phase of higher flexibility segregates in the tube, the region of high mean curvature. For point forcing, the force vs. extension curve can have a sharp drop just as the tube begins to form. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U17.00007: Equilibrium Stability of Transmembrane Proteins : A Hard-Core Gas Problem. Karim Wahba, David Schwab, Robijn Bruinsma Hydropathy plots, a moving average of amino acid hydrophobicity over a sequence, can be used to predict potential protein structure, in particular transmembrane proteins. Traditionally transmembrane regions are identified by peaks above an empirical cutoff. We treat the transmembrane segments as a one-dimensional gas of hard rods in a correlated random energy landscape. At zero temperature, where the entropic contribution due to the loops is negligible, we calculate the density profile as a function of the chemical potential in the case of the original as well as randomly generated landscapes. The density profile exhibits plateaus indicating regions where a transmembrane segment has been established. For designed versus random sequences we explore the distribution of the sizes of these plateaus and attempt to infer characteristic features that may be interpreted in terms of the stability of the protein in its inserted state. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U17.00008: Diffusion of Transmembrane Proteins: Beyond the Saffman-Delbr\"{u}ck Model Tatiana Kouriabova, Mark Henle, Alexander J. Levine The hydrodynamic model of Saffmann and Delbr\"{u}ck [\textit{PNAS} \textbf{72} 3111 (1975)] predicts that the diffusion constant $D$ of proteins embedded in a fluid membrane exhibits a weak logarithmic dependence on the radius $a$ of the protein. However, recent experiments by Gambin \textit{et al.} [\textit{PNAS} \textbf{103} 2098 (2006)] have observed a much stronger 1/$a$ dependence for proteins embedded in model membranes. Local interactions between a transmembrane protein and the lipids that surround it can cause the lipids to deform by, for example, stretching or compressing their tails, or by tilting their long axis with respect to the membrane's surface. In this talk, we show that these deformations lead to additional sources of energy dissipation which cause the protein diffusion constant \textit{D $\sim $ 1/a}, as observed by Gambin \textit{et al.} Our model incorporates the lipid stretch and tilt degrees of freedom into a traditional hydrodynamic model by introducing additional scalar and vector fields, respectively. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U17.00009: Lateral organization of a non-equilibrium membrane model with immobile randomly-distributed impurities Andrew P. Paradis, Susan R. McKay, Samuel T. Hess Cell membranes are dynamic, far-from-equilibrium systems; transport, signaling, and other membrane functions ensure that membrane lateral organization is heterogeneous across several length scales. However, many studies and simulations consider membranes as equilibrium systems. Here, we present a model of cell membranes that includes simplified endo- and exocytosis and immobile randomly-distributed impurities. The impurities take the form of fixed protein sites within the membrane, which act as potential localization zones for micro-domain rafts. We analyze the lateral organization in terms of spatial statistics through a modified Ripley K-test. This model illuminates the role of protein in a 1:1:1 mixture of saturated lipids, unsaturated lipids, and cholesterol. Additionally, this model exhibits a realistic heterogeneity of cluster sizes and shapes, and suggests conditions under which we may observe a partitioning of cholesterol in the membrane. Such simulated observations of the direct interactions between cholesterol, lipids, and protein on the molecular scale can enhance our understanding of all biophysical processes occurring within cell membranes. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U17.00010: Multidrug efflux transporter activity in sea urchin embryos:Does localization provide a diffusive advantage? Xianfeng Song, Sima Setayeshgar, Bryan Cole, Amro Hamdoun, David Epel Experiments have shown upregulation of multidrug efflux transporter activity approximately 30 min after fertilization in the sea urchin embryo [1]. These ATP-hydrolyzing transporter proteins pump moderately hydrophobic molecules out of the cell and represent the cell's first line of defense againstexogenous toxins. It has also been shown that transporters are moved in vesicles along microfilaments and localized to tips of microvilli prior to activation. We have constructed a geometrically realistic model of the embryo, including microvilli, to explore the functional role of this localization in the efficient elimination of toxins from the standpoint of diffusion. We compute diffusion of toxins in extracellular, membrane and intracellular spaces coupled with transporter activity, using experimentally derived values for physical parameters. For transporters uniformly distributed along microvilli and tip-localized transporters we compare regions in parameter space where each distribution provides diffusive advantage, and comment on the physically expected conditions. [1] A. M. Hamdoun, G. N. Cherr, T. A. Roepke and D. Epel, Developmental Biology {\bf 276} 452 (2004). [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U17.00011: Observation of a single particle diffusive motion on the membrane by 3-dimensional particle tracking microscope. Seungyong You, Jing Yuan, David Van Winkle, Thomas Fischer Three-dimensional motion of particles on the surface of giant vesicle membranes has been tracked with nanometer scale resolution using a quadrant photodiode, which is used as a position sensitive detector (PSD), placed in the diffraction pattern formed behind a 100x microscopic objective. Two He-Ne beams were focused to trap a single particle in solution between cover slips with 100 micro-meter gap. The single particle is brought very close to the surface of a membrane system by optical tweezer. By reducing the optical power, the particle sits on the membrane surface and moves diffusively. The diffusing motion is measured using a fast-feedback controller designed to respond to the axial and the lateral position of the particle simultaneously in less than 250 micro-seconds. This work has better positional and temporal accuracy of 3-dimensional particle tracking than conventional video-tracking methods. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U17.00012: Oxygen dynamics in photosynthetic membranes. Sergei Savikhin, Shigeharu Kihara Production of oxygen by oxygenic photosynthetic organisms is expected to raise oxygen concentration within their photosynthetic membranes above normal aerobic values. These raised levels of oxygen may affect function of many proteins within photosynthetic cells. However, experiments on proteins \textit{in vitro }are usually performed in aerobic (or anaerobic) conditions since the oxygen content of a membrane is not known. Using theory of diffusion and measured oxygen production rates we estimated the excess levels of oxygen in functioning photosynthetic cells. We show that for an individual photosynthetic cell suspended in water oxygen level is essentially the same as that for a non-photosynthetic sell. These data suggest that oxygen protection mechanisms may have evolved after the development of oxygenic photosynthesis in primitive bacteria and was driven by the overall rise of oxygen concentration in the atmosphere. Substantially higher levels of oxygen are estimated to occur in closely packed colonies of photosynthetic bacteria and in green leafs. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U17.00013: Identification of co-evolving sites in the ligand binding domain of G protein-coupled receptors using mutual information Sarosh N. Fatakia, Stefano Costanzi, Carson C. Chow G protein-coupled receptors (GPCRs) are the largest superfamily of membrane proteins in humans. They are involved in signal transduction in numerous cellular processes and are the most common target for pharmacological intervention via activation or inhibition. Identification of functionally important sites is relevant for better understanding the ligand-receptor interaction and therefore for drug delivery. In a superfamily of proteins, functionally important but co-evolving sites are not easily identified in a multiple sequence alignment (MSA). Using a MSA of trans-membrane (TM) domains of GPCR superfamily, we identify sites which co-evolve, and may therefore be functionally important. Assigning the TM site as a node and the MI of site pairs as an inverse inter-node distance, a MI graph is established. Co-evolving sites are then identified via this graph. Nodes characterized by high connectivity are located within the commonly accepted ligand binding site of GPCRs, suggesting that concerted co-evolution of a number of neighboring residues gave rise to a multitude of subfamilies each recognizing a specific set of ligands. MI and graph analysis may serve as a tool for the identification of topologically conserved binding pockets in the families of evolutionarily related proteins. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U17.00014: Dynamics of Actin Cable Polymerization in Fission Yeast Hui Wang, Dimitrios Vavylonis In fission yeast, formin for3p nucleates actin filament bundles (cables) at cell tips which contribute to polarized cell growth. Actin cables reach a steady state of dynamic turnover involving for3p-mediated actin polymerization at the barbed ends near the plasma membrane, retrograde flow of polymerized actin toward the cell center, and cable disassembly. Formin for3p associates with actin at the cable tip where it transiently polymerizes actin filaments and subsequently follows the retrograde actin cable flow (Martin and Chang, Curr. Biol. 16, 1161, 2006). Because of the small number of formin nucleators, the actin cable dynamics are subject to spatial and temporal fluctuations. We studied actin cable dynamics with simple analytical models and whole cell computational models which combine deterministic simulation of actin diffusion with stochastic simulation of formin reaction and diffusion. Our model successfully explains a large number of experimental observations, such as density of formin speckles and variance of actin cable density. The model predicts significant spatial gradient of actin and formin molecules in the cytoplasm, powered by the retrograde flow of actin cables. [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U17.00015: Synchrotron X-ray Diffraction Study of Neurofilament Networks Interaction under Osmotic Pressure R. Beck, J. Deek, J.B. Jones, H. Hesse, M.C. Choi, C.R. Safinya Neurofilaments (NFs) are cytoskeletal proteins, which are found abundantly in nerve cell axons and impart mechanical stability and act as structural scaffolds for microtubules. The filaments assemble from 3 different subunit proteins to form a 10 nm diameter flexible polymers with radiating unstructured sidearms. At high protein concentration, the NFs form a nematic hydrogel network with a well-defined interfilament spacing as measured by synchrotron small angle x-ray scattering (SAXS) [1]. Here, NFs purified from bovine spinal cord are reassembled in vitro. Using analogous SAXS-osmotic pressure techniques [2] we study forces between NFs and directly probe the polyampholyte brush interactions between NF sidearms. We measure the interfilament spacing at different osmotic pressure, salt and sidearm concentrations. The study reveals the non-trivial electrostatic nature of the interfilament interaction within the NF hydrogel. [1] J. Jones, C.R. Safinya (submitted) [2] D. J. Needleman et al., PRL 93, 198104 (2004) [Preview Abstract] |
Session U18: Focus Session: Polymer Collapse and Protein Folding
Sponsoring Units: GSNP DPOLYChair: Tom Truskett, University of Texas at Austin
Room: Morial Convention Center 210
Thursday, March 13, 2008 8:00AM - 8:36AM |
U18.00001: Folding peptides and proteins with all-atom physics: methods and applications Invited Speaker: Computational methods offer powerful tools for investigating proteins and peptides at the molecular-level; however, it has proven challenging to reproduce the long time scale folding processes of these molecules at a level that is both faithful to the atomic driving forces and attainable with modern commodity cluster computing. Alternatively, the past decade has seen significant progress in using bioinformatics-based approaches to infer the three dimensional native structures of proteins, drawing upon extensive knowledge databases of known protein structures [1]. These methods work remarkably well when a homologous protein can be found to provide a structural template for a candidate sequence. However, in cases where homology to database proteins is low, where the folding pathway is of interest, or where conformational flexibility is substantial---as in many emerging protein and peptide technologies---bioinformatics methods perform poorly. There is therefore great interest in seeing purely physics-based approaches succeed. We discuss a purely physics-based, database-free folding method, relying on proper thermal sampling (replica exchange molecular dynamics) and molecular potential energy functions. In order to surmount the tremendous computational demands of all-atom folding simulations, our approach implements a conformational search strategy based on a putative protein folding mechanism called zipping and assembly [2-4]. That is, we explicitly seek out potential folding pathways inferred from short simulations, and iteratively pursue all such routes by coaxing a polypeptide chain along them. The method is called the Zipping and Assembly Method (ZAM) and it works in two parts: (1) the full polypeptide chain is broken into small fragments that are first simulated independently and then successively re-assembled into larger segments with further sampling, and (2) consistently stable structure in fragments is detected and locked into place, in order to avoid re-sampling those degrees of freedom in subsequent steps. ZAM pursues all potential folding routes it finds, which may be mutually exclusive, and it ranks these by calculating free energies along the way. Importantly, it gives full conformational ensembles and folding pathways, features not captured by bioinformatics approaches. We also discuss ways in which the structural ensembles and folding pathways of ZAM can facilitate the rational design of peptide technologies. In particular, we examine the coupling of ZAM-produced structures with coarse-grained theories of transport and association, in order to model the interactions of peptides with membranes (for insertion processes), proteins (for binding processes), and other peptides (for aggregation processes). Importantly, this approach is able to capture highly sequence-specific effects due to the atomistic nature of the ZAM folding simulations, providing a predictive tool for targeted sequence mutations. 1. J. Moult, \textit{A decade of CASP: progress, bottlenecks and prognosis in protein structure prediction,} Curr. Opin. Struct. Biol. \textbf{15, }(2005). 2. K.M. Fiebig and K.A. Dill, \textit{Protein core assembly processes,} J. Chem. Phys. \textbf{98, }(1993). 3. S.B. Ozkan, G.H.A. Wu, J.D. Chodera, and K.A. Dill, \textit{Protein folding by zipping and assembly,} Proc. Natl. Acad. Sci. U. S. A. \textbf{104, }(2007). 4. M.S. Shell, S.B. Ozkan, V.A. Voelz, G.H.A. Wu, and K. Dill, \textit{Can molecular physics predict the native structures of globular proteins?,} \textit{under review}\textbf{, }(2007). [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U18.00002: Studies of Protein Folding in Non-Funeled Free Energy Landscapes Corey O'Hern, Gregg Lois, Jerzy Blawzdziewicz A theoretical framework is developed to understand the dynamics of protein folding. The key insight is that the search for the optimal conformation of the protein is influenced by the rate at which external parameters are adjusted to induce folding. A theory based on this insight predicts that (1) proteins with non-funneled free energy landscapes can fold reliably, (2) reliable folding can occur in equilibrium or out-of-equilibrium, and (3) reliable folding only occurs when the quench rate is below a limiting value, which can be calculated from measurements of the free energy. We test these predictions against numerical simulations of heteropolymers with hydrophobic and hydrophilic interactions and a single energy scale. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U18.00003: Exploring HP protein models using Wang-Landau sampling Thomas Wuest, David P. Landau The hydrophobic-polar (HP) protein model has become a standard in assessing the efficiency of computational methods for protein structure prediction as well as for exploring the statistical physics of protein folding in general. Numerous methods have been proposed to address the challenges of finding minimal energy conformations within the rough energy landscape of this lattice heteropolymer model. However, only a few studies have been dedicated to the more revealing - but also more demanding - problem of estimating the density of states which allows access to thermodynamic properties of a system at any temperature. Here, we show that Wang-Landau sampling, in connection with a suitable move set (``pull moves''), provides a powerful route for the ground state search and the precise determination of the density of states for HP sequences (with up to 100 monomers) in both, two and three dimensions. Our procedure possesses an intrinsic simplicity and overcomes the inevitable limitations inherent in other more tailored approaches. The main advantage lies in its general applicability to a broad range of lattice protein models that go beyond the scope of the HP model. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U18.00004: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U18.00005: Resolution of the unfolded state. Gregory Beaucage The unfolded states in proteins and nucleic acids remain weakly understood despite their importance to protein folding; misfolding diseases (Parkinson's {\&} Alzheimer's); natively unfolded proteins ($\sim $ 30{\%} of eukaryotic proteins); and to understanding ribozymes. Research has been hindered by the inability to quantify the residual (native) structure present in an unfolded protein or nucleic acid. Here, a scaling model is proposed to quantify the \textit{degree of folding} and the unfolded state (Beaucage, 2004, 2007). The model takes a global view of protein structure and can be applied to a number of analytic methods and to simulations. Three examples are given of application to small-angle scattering from pressure induced unfolding of SNase (Panick, 1998), from acid unfolded Cyt c (Kataoka, 1993) and from folding of \textit{Azoarcus} ribozyme (Perez-Salas, 2004). These examples quantitatively show 3 characteristic unfolded states for proteins, the statistical nature of a folding pathway and the relationship between extent of folding and chain size during folding for charge driven folding in RNA. Beaucage, G., \textit{Biophys. J.}, in press (2007). Beaucage, G., \textit{Phys. Rev. E}. \textbf{70}, 031401 (2004). Kataoka, M., Y. Hagihara, K. Mihara, Y. Goto \textit{J. Mol. Biol.} \textbf{229}, 591 (1993). Panick, G., R. Malessa, R. Winter, G. Rapp, K. J. Frye, C. A. Royer \textit{J. Mol. Biol.} \textbf{275}, 389 (1998). Perez-Salas U. A., P. Rangan, S. Krueger, R. M. Briber, D. Thirumalai, S. A. Woodson, \textit{Biochemistry} \textbf{43} 1746 (2004). [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U18.00006: Crowding Effects on the Thermodynamics of Apoflavodoxin Folding. Dirar Al Homouz The thermodynamics of folding in Apoflavodoxin protein are studied using coarse-grained molecular dynamics simulations as a function of volume fraction of crowding agents. The stability of the folded state is enhanced in the presence of crowding agents as can be seen from the free energy diagrams. The changes in the transition state ensemble are analyzed under different crowding conditions. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U18.00007: Protein Folding Simulation of Mutant Go Models of the Wild-Type Trp-cage Protein Apichart Linhananta, Junmin Liu For the past three decades, Go models of protein folding have played important roles in the understanding of how proteins fold from random conformations to their unique native structures. Unfortunately Go models reliance on known NMR or x-ray structures to construct Go interaction potentials severely limit their predictive powers. In this work, we introduce a novel method for constructing Go interaction potentials of mutant proteins based on Go interaction potentials of wild type proteins. As a template we employ the all-atom Go model of the 20-residue Trp-cage protein (A. Linhananta, J. Boer and I. MacKay, J. Chem. Phys., 2005, 122, 114901) as the wild type Go model. Trp-cage mutants are constructed by replacing a Trp-cage residue with a different residue. In particular the Pro-12 residue of the Trp-cage is substituted by Trp-12 to produce the Trp2-cage mutant, whose native structure is not yet known. Monte Carlo simulations, using CHARMM force fields, are performed to determine the ground-state structure mutant. The resulting mutant structures are used to construct the Go interaction potential of the Trp2-cage mutant Go model. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U18.00008: Statistical features of the rough energy landscape of proteins emerging from single molecule force-clamp spectroscopy Jasna Brujic, Maxime Clusel, Eric Corwin Following the complete folding trajectories of single ubiquitin molecules opens an unique window into the detailled mechanisms of protein folding. The biological importance of this problem motivated extensive studies using macroscopic biochemistry experiments and molecular dynamics simulations at the atomic scale, while little is known about the mesoscopic mechanisms of folding. To this end, our recent experiments combined with the tools of modern statistical mechanics reveal a wealth of new information. Using this single molecule approach, we have observed physical features reminiscent of glassy systems, exemplified by a power-law distribution of the rates of protein unfolding under a stretching force [1]. To further probe the signs of complexity in protein dynamics, we investigate memory effects and the influence of force on the folding trajectories, and more specifically the mechanism of formation of native interactions. The general aim of this research is to build a self-consistent picture of the free energy landscape of proteins. [1] J. Brujic \textit{et al.}, Nature Physics, vol 2, 282 (2006). [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U18.00009: Asymmetrical collapse of charged heterogeneous macromolecules Natalia Denesyuk, John Weeks We propose a new method based on local molecular field (LMF) theory to treat Coulomb interactions in simulations of ionic fluids. This method has been tested in Langevin dynamics simulations of a model protein, which consists of a random sequence of charged hydrophilic and neutral hydrophobic monomers, in salt solution. The concentration of salt ions in the simulation box is maintained by grand canonical Monte Carlo. Our general strategy is to perform averages over an ensemble of sequences in order to identify those general properties that are sequence independent. We find that, independently of their random sequence, heterogeneous polyelectrolytes undergo the asymmetrical collapse in which one of their quadruple moments vanishes. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U18.00010: A Model for the Thermally Induced Polymer Coil-to-Globule Transition David Simmons, Isaac Sanchez A quantitative mean-field model for the thermally-induced (heating-induced) polymer coil-to-globule transition (HCGT) is developed with no adjustable parameters. The transition temperature $\Theta $ is given for a long chain by the equation $\Theta =2T_p^{\ast} \left[ {1-\tilde {\rho }\left( \Theta \right)} \right]$ where $T_{p}^{\ast}$ is the characteristic temperature of the polymer for the lattice fluid model and $\tilde {\rho }\left( \Theta \right)$ is the reduced solvent density at the transition temperature $\Theta $. Calculated HCGT temperatures show good agreement with experimental LCSTs. The physics of the HCGT transition is shown to be consistent with the physics of the LCST transition. The predicted globular state is characterized by the dominance of attractive polymer self interactions over excluded volume interactions. This model can be easily generalized to treat cross-linked gels and their contraction-expansion characteristics. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U18.00011: Force Induced Globule-to-Coil Transition of Single Polymer Chains. Nikhil Gunari, Gilbert Walker Force induced structural transitions of individual homopolymer chains have been studied in different solvent conditions using single molecule force spectroscopy. Single molecule mechanics in the ``fly-fishing'' mode showed a first-order like transition for polystyrene (PS) in water exhibiting a characteristic three regime force extension curve. In contrast, poly methylmethacrylate (PMMA) showed a characteristic saw-tooth pattern reminiscent of multidomain disassembly behavior similar to that seen in modular protein mechanics. The plateau force for PS and the saw-tooth pattern for PMMA disappear when measured in aqueous guanidine hydrochloride solution and in other non-solvents showing that the characteristic deformational behavior observed for the two polymers in water may be due to hydrophobic interactions . [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U18.00012: Wang-Landau sampling for homopolymer collapse Daniel T. Seaton, Steven J. Mitchell, David P. Landau We explore the behavior of a continuum-homopolymer model using the Wang-Landau algorithm, concentrating on phase transitions such as the coil-globule and solid-liquid transitions. Using the density of states generated by the Wang-Landau algorithm, we calculate various thermodynamic quantities, e.g., the internal energy and specific heat. We also study how algorithmic parameters, such as sampling boundaries (maximum and minimum energies for random walks) and the final value of the modification factor, affect these quantities. In particular, we observe how the sampling boundaries can significantly alter the transition behavior. Our results are compared with two recent studies that yielded contradictory results, one using the bond-fluctuation model and the other using a continuum model similar to our own. We find that the transitions seen in our model are much more similar to those in the bond-fluctuation study. The careful analysis of the effects of algorithmic parameters on thermodynamic quantities should be relevant to the study of other polymeric/protein models. [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U18.00013: Stimuli-Responsive, Concentrated Aqueous Solutions of DMAEMA-containing Amphiphilic Di- and Triblock Copolymers Kyle Guice, Yueh-Lin Loo Poly(dimethyoaminoethyl methacrylate), poly(DMAEMA), has generated considerable interest due to its responsiveness to changes in temperature and pH. The pendant tertiary amine groups of DMAEMA are easily protonated below its pKa, and the polymer undergoes a hydrophilic-to-hydrophobic transition when heated above its lower critical solution temperature (LCST) in water. We have investigated di- and triblock copolymers containing statistical copolymers of DMAEMA and hydroxyethyl methacrylate (HEMA), a biocompatible but nonresponsive monomer, as stimuli-responsive concentrated aqueous solutions. The swelling characteristics of these concentrated aqueous block copolymer solutions depend highly on the DMAEMA composition. Further, by selecting an appropriate hydrophobic block, we are able to design stimuli-responsive concentrated aqueous solutions that undergo reversible phase transformations over a narrow temperature window. [Preview Abstract] |
Session U19: Computational Methods: Dynamics, Transport, and Plasma
Sponsoring Units: DCOMPChair: Michael Zachariah, University of Maryland
Room: Morial Convention Center 211
Thursday, March 13, 2008 8:00AM - 8:12AM |
U19.00001: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U19.00002: Charged particle and neutron energy deposition in an inertial confinement fusion plasma leading to internal tritium breeding Karabi Ghosh, S.V.G. Menon Plasma heating by charged particles and neutrons, energy exchange between ions and electrons and radiative losses are the primary mechanisms determining the ignition conditions in a thermonuclear plasma. In this work the energy leakage probability has been obtained numerically by including the effect of nuclear scattering, small and large angle Coulomb scattering and collective plasma effects. A simple multigroup approach has been developed for energy deposition by neutrons due to nuclear interaction with the ions. Using this accurate model for energy deposition, the concept of internal tritium breeding in DT fusion pellet has been re-evaluated by numerically solving the rate equations for various participating species and energy balance equations for ions, electrons and radiation within the three temperature model. Internal tritium breeding is found to occur even when all the radiation loss mechanisms such as bremsstrahlung and inverse compton scattering are fully accounted for. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U19.00003: Breakeven Fusion in Staged Z Pinch Hafiz Rahman, Paul Ney, Norman Rostoker, Frank Wessel We are studying the prospect for breakeven thermonuclear fusion considering a Mega joule (MJ) class, 100 ns, impulse generator using a modified version of MACH2, a 2-1/2 D, radiation-code. The load is a cylindrical, xenon plasma shell that implodes radially onto a co-axial, deuterium-tritium plasma target. Optimized plasma density and pinch radius lead to a fusion-energy output that is many times the stored capacitor bank energy. In this ``Staged Z-pinch'' shock fronts form that preheat the DT plasma to several hundred eV, before adiabatic compression. During compression, the Xe liner becomes Rayleigh-Taylor (RT) unstable while the DT target remains stable. Proper selection of the initial pinch radius and plasma density is crucial for optimum implosion efficiency. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U19.00004: Intensity correlations in wave transport through complex media Gabriel Cwilich, Luis Froufe-Perez, Antonio Garcia-Martin, Juan Jose Saenz The intensity-intensity correlations that appear when a wave propagates coherently through a random medium will be discussed within the framework of the random matrix theory (RMT) of transport. We will consider the case of transmitted-transmitted, reflected-reflected and transmitted-reflected correlations. In the case of transmission the spatial correlations can be expressed as the sums of three terms with distinctive spatial dependences. This result coincides with the one obtained in the diffusive regime from perturbative calculations, but here its validity is extended from the quasi ballistic to the localized regime. In the RMT framework, approximate solutions of the DMPK equations allow us to study the dependence of the correlations with the length of the system. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U19.00005: Modified Transition Matrix Methods David Yevick, Michael Reimer, Bjarne Tromborg Recently we adapted the transition matrix Monte-Carlo method to general communication systems problems [D. Yevick and M. Reimer, Photon.Technol. Lett. 1529 (2007), IEEE Trans. Commun., submitted, (2007)]. In this presentation, we compare the accuracy and parameter dependence of different multicanonical and transition-matrix methods. We find that the standard multicanonical method can be reformulated more simply and accurately for a single system observable (output variable) within a transition matrix formulation by constructing the intermediate probability density function (density of states) after a small number of Markov transitions from the ratios of the elements of the transition matrix between adjacent histogram bins. Further, we consider an alternative procedure in which transitions only occur either from a given state to itself or to states that have previously been less frequently sampled. Here we show that the numerical error is small unless the self-transition probability is considerable. In this case, despite the violation of detailed balance, numerical precision can be effectively restored by ensuring that the random walker thermalizes within each histogram bin before effecting a transition to a different bin. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U19.00006: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U19.00007: Critical phenomena of Site-Percolation Models with Two Different Sizes of Particles on a Square Lattice Ryoji Sahara, Hiroshi Mizuseki, Kiyoshi Kanie, Atsushi Muramatsu, Yoshiyuki Kawazoe The concept of percolation plays an important role in explaining various important physical phenomena, including transport, mechanical, and electromagnetic properties of disordered systems. To date, many percolation models have been developed. Contrary to the ordinary site percolation models with homogeneous particles, systems have a certain particle-size distribution. Such a distribution may affect the properties of the system in certain ways. In the present study, site-percolation models with two different sizes of particles are systematically introduced on a square lattice to understand the effect of nonhomogeneity of the particles in the system. To estimate the critical phenomena with high accuracy, a finite-size scaling analysis is performed with a Monte Carlo simulation. The critical coverage at the percolation threshold is examined as a function of the size distribution of elements in the system. Fractal dimension and the critical exponentials are also estimated. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U19.00008: Development of a Simple Sintering Law for Fractal Aggregates Composed of Unequal Sized Primary Particles Takumi Hawa, Michael Zachariah Sintering of silicon nanoparticle chain aggregates composed of unequal sized primary particles are investigated using molecular dynamics (MD) simulations at 1500 K. We consider straight chain aggregates consisting of up to 40 2.5 and 5.4 nm primary particles. The sintering time increases with increase in the total volume of the chain aggregate or with increase in the exposed initial surface area of the chain. A mathematical model was developed to describe the dynamics of sintering of such chain aggregates. The model is a power law modification of the Frenkel sintering equation with the Koch-Friedlander model to include primary particle size dependence. We found that the particle size effect is a local process, and important only at the initial stage of the sintering. Thus, the effect is not significant when the aggregate becomes large. The model is amenable for use in aerosol models that might include sintering effects. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U19.00009: Toward a new criteria of soliton/ domain wall creation in condensed matter systems? Andrew Beckwith We do an extension of prior work where we applied a quasi 1-Dimensional wavefunctional formulation of tunneling Hamiltonians to a physical transport problem characterized by a perturbed washboard potential. To do so beforehand in the quasi one dimensional situation, we considered tunneling between states that were modeled as wavefunctionals of a scalar quantum field. I-E curves that matched Zener curves --- were used to fit data from an experimental stand point with quasi one dimensional wavefunctionals congruent with the false vacuum hypothesis. We generalize this to the case of higher dimensional formulations of the wave functionals, and also present a minimum criteria for the formation of soliton/ instanton structure in higher dimensions. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U19.00010: Use of Space-Time Basis Sets for Solving Initial-Value Problems Charles Weatherford, Xingjun Zhang A new algorithm for solving Quantum Mechanical initial-value problems such as the time-dependent Schroedinger equation and the Liouville equation is described. The method avoids the use of the time-translation operator which inevitably results in an essentially sequential algorithm and instead turns the problem into the solution of simultaneous equations, which produces a highly parallelizable algorithm. A basis set in time as well as spatial degrees of freedom is used. The basis may be spectral, finite element, or spectral element and may be continuous or discrete (discrete variable representation--DVR). The time-axis may have an arbitrary size of time element including only one element. The larger the time step, the larger the size of the time basis that is required. The Hamiltonian may be time-independent or time-dependent. In the case of a time-independent Hamiltonian, an extremely efficient algorithm results. For the time-dependent case, the problem of time-ordering does not arise. Several applications involving laser-atom interactions will be given. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U19.00011: Propagating the nonlinear Schroedinger equation Frederick Strauch We derive an exact propagation scheme for nonlinear Schroedinger equations. This scheme, analogous to the propagation of linear Schroedinger equations, results from a special operator whose properties ensure the correct propagation. Using this scheme we prove the correctness of higher-order integrators for the Gross-Pitaevskii equation and its multi-component generalizations. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U19.00012: Reconstructing the dynamics of water sheared between charged plates using inelastic x-ray scattering Ghee Hwee Lai, Robert H. Coridan, Nathan W. Schmidt, Peter M. Abbamonte, Gerard C. L. Wong Understanding the dynamical behavior of water under confinement or near surfaces is fundamental to tribology and many transport processes in cell biology. To achieve angstrom and femtosecond resolution in water dynamics, we reconstruct the space-time longitudinal (density) response function from high-resolution inelastic x-ray scattering (IXS) studies of water and, together with linear response theory, investigate how water behaves between two moving 2-D charge lattices at different charge densities and inter-plate separations. We find that the density profile varies with plate separation with a periodicity close to the diameter of a water molecule ($\sim$2.6{\AA}), in agreement with surface forces apparatus measurements, and that the hydration patterns of charges on the surfaces are strongly velocity dependent. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U19.00013: Ergodicity of Isothermal Molecular Dynamics Method Hiroshi Watanabe A condition for equations of motion for isothermal dynamics is derived, and the Nos\'e--Hoover method is generalized on the basis of this condition. The ergodicity of the one-variable thermostats are studied, and it is shown that the one-variable thermostat coupled with the one-dimensional harmonic oscillator loses its ergodicity with large enough relaxation time. A stochastic process of the Nos\'e--Hoover method is also discussed based on the Markovian approximation. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U19.00014: Statistical Mechanics of the Fluctuating Lattice Boltzmann Equation Burkhard Duenweg, Ulf Schiller, Anthony J.C. Ladd The statistics of the occupation variables of a stochastic lattice Boltzmann simulation is analyzed in terms of a generalized lattice gas. We show that the most probable state of this model corresponds to the equilibrium distribution of the lattice Boltzmann equation. Stochastic collision rules are described in terms of a Monte Carlo process satisfying detailed balance. This allows a straightforward derivation of the discrete Langevin equation for the fluctuating modes. Detailed balance requires to thermalize all non-conserved modes. A Chapman--Enskog analysis shows that the approach is fully consistent with macroscopic fluctuating hydrodynamics. [Preview Abstract] |
Session U20: Focus Session: Metal Surfaces, Interfaces, and Thin Films
Sponsoring Units: DMPChair: Mina Yoon, Oak Ridge National Laboratory
Room: Morial Convention Center 212
Thursday, March 13, 2008 8:00AM - 8:12AM |
U20.00001: Surface islands nucleated by a beam of energetic self-ions on Pt(111): A low-energy electron microscopy study Michal Ondrejcek, C. Peter Flynn, Wacek Swiech Using low energy electron microscopy (LEEM), we observe the adatom and advacancy islands nucleate and evolve when clean Pt(111), in the temperature range 750-1300K, is bombarded by a beam of Pt$^{-}$ ions of various energies. The source of negative ion beam is incident on the sample at normal incidence with impact energies selectable in the range of 0 to 5 keV, and with current densities up to 40 $\mu $A/cm$^{2}$. We describe briefly initial experiments done with LEEM-Ion accelerator tandem namely the investigations of relaxing steps extending the range over which surface mass diffusion coefficient D$_{s}$ is known on Pt(111) and observed neutral energy of 245 eV, at which sputtering balances the self-ion input. The results reveal a previously unobserved symmetry between the chemical potentials $\mu $* required to nucleate the adatom and advacancy islands. Linear response theory is employed to relate $\mu $* to ion beam flux; the observations confirm that its use is valid above 1000K. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U20.00002: The effect of Fe atoms on the adsorption of a W atom on W surfaces Jeffery Houze, Sungho Kim, Seong-Jin Park, Randall German, Mark Horstemeyer, Seong-Gon Kim We report ab-initio calculations on the effect of iron (Fe) atoms on the adsorption of a tungsten (W) atom on W(100), W(110), and W(111) surfaces. The adsorption of a W atom on the clean W surfaces is compared with the adsorption of a W atom on a monolayer of Fe atoms covering the W surfaces. The total energy of the system is computed as the function of the height of the W adatom. For the W(100) surface I will show that the W atom first adsorbs onto the Fe monolayer. Then the W atom can replace one of the Fe atoms through a path with a moderate energy barrier and reduce its energy further. This intermediate site makes the adsorption (and desorption) of W atoms a two-step process in the presence of Fe atoms and lowers the overall adsorption energy by nearly 2.4 eV. Similar processes for adsorption will be presented for the (110) and (111) surfaces. Our result provides a fundamental mechanism that can explain the activated sintering of tungsten by Fe atoms. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U20.00003: Stability of V and Ti on Al surfaces: Searching for suitable interlayer materials to stabilize the Fe-Al interface Weerasinghe Priyantha, Hui Chen, Michael Kopczyk, Kasey Lund, Dan Tonn, Richard Smith, Ponnusamy Nachimuthu, Vaithiyalingam Shutthanandan There is considerable interest in fabricating thin film multilayer structures with sharp interfaces for a wide variety of applications. Interface intermixing may degrade the desired physical properties of a structure, but this may be reduced in some cases using stabilizing interlayers at the interface. Model calculations predict that both V and Ti will be effective stabilizing interlayers for the Fe-Al interface, a system well known for considerable intermixing at room temperature. We have used X-ray reflectometry (XRR) and Rutherford backscattering spectrometry (RBS) to characterize bilayers and trilayers of the Fe-V-Al and Fe-Ti-Al systems prepared using dc magnetron sputtering. Our analysis revealed that Fe-Al bilayer systems showed considerable intermixing, especially when the Fe layer was deposited on top of the Al. It was also found that with V or Ti as an interlayer at the interface, the intermixing of Fe and Al was reduced. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U20.00004: Magic planar Ag clusters Y.P. Chiu, C.M. Wei, C.S. Chang, Tien. T. Tsong The spontaneous assembly of atoms and molecules in a system has attracted many research interests and created numerous potential applications. Utilizing the periodic pattern found on the Pb quantum islands, which are grown on the Si(111) surface, we have recently discovered that self-organized Ag planar clusters formed on these templates exhibit enhanced stability at some particular sizes. Detailed calculations based on ab initio density functional theory have also been performed. The use of a density-functional optimization in geometrical structures and the corresponding binding energy support an examination of the genesis of these magic Ag nanoclusters and their relative stability. When the Ag nanopuck grows to a certain size, the geometrical effect takes hold from the electronic effect as the major attribute, which drive the Ag nanopucks towards well defined hexagonal crystalline structures. The theoretically related electronic and geometrical structures are also correlated with the experimentally energetically favorable structures of these magic clusters. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U20.00005: Crucial electronic contributions to measures of surface diffusion by He atom scattering Guido Fratesi, Gil Alexandrowicz, Mario Italo Trioni, Gian Paolo Brivio, William Allison In a He atom scattering (HAS) experiment, the position and motion of atoms or molecules at a surface is inferred indirectly, through the electron distribution at the sample surface. Nevertheless, surface diffusion measurements are typically analyzed assuming that the electron distribution simply follows the position of the surface atoms. We have examined theoretically recent HAS measurements of Na/Cu(001), identifying a non trivial relation between the dynamics of the electron distribution and that of the Na ions. The magnitude of the calculated variations in the charge density, and their dependence on the local density of adsorbates, account for the correlated 3D motion experimentally observed. The results of this study further highlight the sensitivity of HAS to the electron distribution of the sample and point out the role of electronic contributions in high-resolution measures of surface dynamics. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U20.00006: Sapphire Surface Polymorphs and The Growth of Pb Overlayers Hawoong Hong, Aaron Gray, T.-C. Chiang The surface structure of sapphire ($\alpha $-alumina) is an issue of long-standing interest, both scientifically and technologically. Molecular dynamics simulations showed the $\gamma $-alumina structure to have a lower energy than that of $\alpha $-alumina [1], thus suggesting a possibly modified surface structure when sapphire is treated at high temperatures. We have performed x-ray reflectivity measurements at the Advanced Photon Source to address this issue. Standard sapphire substrates were prepared by furnace annealing at 1600$^{\circ}$ C in air. The resulting surfaces showed large terraces with straight step edges. The substrates were then annealed in a UHV chamber at increasingly higher temperatures. Many new features emerged in the reflectivity curves, which could be attributed to various transition alumina structures, including the $\theta $'-, $\delta $-, and $\theta $- polymorphs [2]. Pb films were grown on these surfaces. The resulting structure and morphology was characterized. This talk will summarize our findings. [1] S. Blonski and S. H. Garofalini, Surf. Sci. \textbf{295}, 263 (1993). [2] I. Levin and D. Brandon, J. Am. Ceram. Soc. \textbf{81}, 1995 (1998). [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U20.00007: Molecular dynamics simulations of low temperature Cu/Cu(100) growth Valery Borovikov, Yunsic Shim, Jacques G. Amar Recent X-ray scattering studies [1] of Cu/Cu(100) growth indicate the existence of a sharp transition from epitaxial growth at high temperatures to growth with a high vacancy concentration at lower temperatures ($T < 150$ K). Here we present the results of molecular dynamics (MD) simulations carried out in order to understand the detailed mechanisms of incorporation of vacancies and voids (vacancy clusters) into the growing film during low temperature deposition. The dependence of compressive strain, vacancy concentration and vacancy-cluster size distribution in deposited thin film, as well as the surface roughness and morphology on deposition conditions, such as the growth temperature, deposition angle, and incident kinetic energy will be discussed and compared with experiments. [1] C. E. Botez, K. Li, E. D. Lu, W. C. Elliott, P. F. Miceli, E. H. Conrad, and P.W. Stephens, Appl. Phys. Lett. 81, 4718 (2002). [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U20.00008: Parallel temperature accelerated dynamics simulations of vacancy formation in low temperature Cu/Cu(100) growth Yunsic Shim, Valery Borovikov, Jacques G. Amar, Blas P. Uberuaga, Arthur F. Voter While molecular dynamics simulations may be used to study thin-film growth at very low temperatures and at very high deposition rates, in order to study growth over time scales close to experiment, accelerated dynamics simulations are needed. Here we present the results of parallel temperature-accelerated dynamics simulations of low-temperature Cu/Cu(100) growth carried out using our recently developed parallel temperature-accelerated dynamics (parTAD) method, in order to understand recent X-ray diffraction experiments showing a surprisingly large vacancy concentration in Cu(100) growth at low temperature. In general, we find that, due to the existence of increased surface relaxation and activated events, the vacancy concentration obtained in our parTAD simulations is smaller than the corresponding concentration obtained in molecular dynamics (MD) simulations. The dependence of compressive strain and vacancy concentration, as well as the surface roughness and morphology, on growth temperature and deposition angle will also be discussed and compared with experiments. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U20.00009: Atomic Processes responsible for the diffusion of 2D Cu islands on Ag(111): results from self learning KMC O. Trushin, H. Yildirim, A. Kara, T.S. Rahman Diffusion on Ag(111) of small 2D-Cu clusters (4 to 30 atoms) is examined using embedded atom method potentials, as a first step towards understanding hetero epitaxial growth. A combination of an Off-Lattice Self-Learning Kinetic Monte Carlo and spherical repulsion scheme for saddle point searches, has revealed novel diffusion mechanisms. For this size range, the diffusion of islands involve 3 classes of processes: i) collective concerted motion (gliding), found to be dominant for small sized islands (4-9); ii) processes involving shear mechanism in which some of the Cu island atoms are commensurate with the substrate and others are not ($>$ 9 atoms), finally iii) processes involving a ``breathing'' mechanism in which the island ``shrinks'' as a whole before ``relaxing'' to a less compact shape with a net displacement of the whole cluster equivalent to an fcc-hcp hop ($>$ 12 atoms). These processes were revealed during the first 100 KMC steps for each island. Diffusion coefficients as a function of temperatures, effective diffusion barriers and frequencies of the responsible events will be presented. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U20.00010: Semi-coherent Fe(001)/MX(001) interfaces Dan Fors, G\"{o}ran Wahnstr\"{o}m Using ab initio calculations we investigate interface energies and structures for the semi-coherent Fe(001)/MX(001) interface systems. We apply a continuum approach using the Peierls-Nabarro model in order to account for the elastic displacements arising from the lattice misfit and the periodic misfit dislocations in the interface. The chemical part of the interface energy is obtained by using density functional theory calculations. We find that the Fe/MN systems show decreasing trends along the 3d, 4d and 5d element rows corresponding to stronger bonding to Fe. In contrast the Fe/MC systems show a maximum for the Ti group. The trends and differences have been explored using projected density of states and charge density analysis. The results show a stong covalent bonding between Fe and C(N) when the two atoms are aligned on top of each other, but the hybridization itself can't account for the differences along the rows. Instead the trend appears to be due to the metallic interaction between the Fe atom and M atom in the interface layer. We also find that the Fe/MX-interfaces have large misfits, which causes that many atoms will be unfavorable positioned and the elastic energy will constitute a significant part of the interface energy. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U20.00011: Unstable and metastable states of dynamics governed by surface diffusion B. Davidovitch, H. King , C.D. Santangelo Under certain kinetic conditions, the dynamics of solid surfaces is governed by surface diffusion processes. This type of dynamics is relevant, for example, in high-temperature sintering processes, and in the coarsening of nanoporous metals coated by catalytic elements. For compact surfaces, the fixed points of this dynamics are surfaces of constant mean curvature (CMC). It is thus natural to ask whether there exist nontrivial CMC's which are stable under dynamics governed by surface diffusion. This question will be addressed in this talk. We will discuss some subtleties concerning an analytic approach to the problem, and will present some numerical results for simple CMC surfaces. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U20.00012: Morphology evolution of solid thin films in the presence of long range de-wetting interactions Adi Constantinescu, Artem Levandovsky, Leonardo Golubovic The thin films of metals, such as cobalt or silver on substrates such as sapphire exhibit a striking formation of multilayered islands that reach heights many times larger than the initial film thickness. Here, we theoretically elucidate these phenomena within an interface dynamics model which incorporates both Mullins type surface diffusion relaxation and long range de-wetting forces acting across the film, such as Van der Waals forces and Fermionic Casimir forces (Quantum size effect). The model is used to explore the scaling laws of multilayered island height growth as well as the coarsening laws governing surface evolution. At early times, the surface evolution is dominated by strong up-hill surface currents caused by long range Casimir-like forces. At late times however, the surface coarsening laws are universal and dominated by surface tension effects. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U20.00013: Charge Transfer Model for Disscociative Barrier Formation; A First Principles Study Shigeyuki Takagi, Hidekazu Tomono, Kazuo Tsumuraya The origin of the formation of the barriers has been explained, for two decades, using Pauli repulsion, i.e., exchange term, by Hammer et al.[1] Excluding the exchange term in the electronic total energy calculation, we have however obtained the monotonic increase of the potential energy surface in the dissociation process of the H$_{2}$ molecule on Au(111) system. So we propose another origin for the formation focusing on the charger transfer induced by electronegativity differences between the hydrogen molecule and Au metal surfaces using density functional calculations. We evaluate the charges that belong to atoms in the system during the dissociation process using Bader analysis. The calculated dissociation energy curve along the reaction path coincides with that of the isolated, separated and positively charged hydrogen molecule using the linear combination of the atomic orbital method in real space. No interaction between the hydrogen molecule and the Au surface has been found in the initial stage of the dissociation. The transfer elongates the inter-atomic distance of the hydrogen molecule that raises the energy of the molecule, leading to the formation of the energy barrier in the present case. This charge transfer model is confirmed to be applicable to not only the present system but also H$_{2}$/Mg, H$_{2}$/Pt, O$_{2}$/Pt systems. [1] B.Hammer and J.K.N{\o }rskov, Nature 376, 238 (1995). [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U20.00014: A Density Functional Study of Atomic Hydrogen and Oxygen Chemisorptions on the (0001) Surface of Double Hexagonal Close Packed Americium Pratik Dholabhai, Raymond Atta-Fynn, Asok Ray \textit{Ab initio} total energy calculations within the framework of density functional theory have been performed for atomic hydrogen and oxygen chemisorptions on the (0001) surface of double hexagonal packed americium using a full-potential all-electron linearized augmented plane wave plus local orbitals (FLAPW+lo) method. The three-fold hollow hcp site was found to be the most stable site for H adsorption, while the two-fold bridge adsorption site was found to be the most stable site for O adsorption. Chemisorption energies and adsorption geometries for different adsorption sites will be discussed. The change in work functions, magnetic moments, partial charges inside muffin-tins, difference charge density distributions and density of states for the bare Am slab and the Am slab after adsorption of the adatom will be discussed. The implications of chemisorption on Am 5$f$ electron localization-delocalization will also be discussed. [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U20.00015: Simulations of adsorption of hydrocarbons on decagonal AlNiCo quasicrystal surfaces Wahyu Setyawan, Renee D. Diehl, Milton W. Cole, Stefano Curtarolo Classical many-body interatomic potentials for hydrocarbon adsorptions on Al-Ni-Co systems are developed by using the Embedded-Atom Method. The potentials are fit to ab-initio energies of Al-Ni-Co ternary phases and hydrocarbons adsorbed on decagonal surface of Al-Ni-Co (d-AlNiCo) approximants. First principle data show that no dissociation occurs for all systems in the training set, indicating no chemisorptions. We extend the study and use the potentials to simulate adsorption of simple hydrocarbons on d-Al$_{73}$Ni$_{10}$Co$_{13}$ quasicrystal surfaces using Grand Canonical Monte Carlo method. Research sponsored by ACS and NSF. [Preview Abstract] |
Session U21: Surfaces and Interfaces I
Sponsoring Units: DCPChair: Arthur Reber, Virginia Commonwealth University
Room: Morial Convention Center 213
Thursday, March 13, 2008 8:00AM - 8:12AM |
U21.00001: Automated Analysis of Nanocar Molecules as Observed by VT-STM. A. J. Osgood, T. Sasaki, J. M. Tour, K. F. Kelly The observation and measurement of individual nanocar molecules by variable temperature scanning tunneling microscopy (STM) has uncovered a great deal of information regarding their electronic properties and dynamic abilities. While STM is particularly powerful in measuring the properties of individual molecules, it is often desirable and enlightening to obtain information of the ensemble as well. Many groups have previously worked on the automatic detection and recognition of molecules in STM images, however, the complex 4-lobed nature of the nanocar introduces additional challenges.~Therefore, we have developed an automated image processing routine that is more robust and able to overcome these problems.~We then apply this to the analysis of nanocars imaged by STM at various temperatures and demonstrate the recognition of spinning vs. stationary fullerene wheels on the nanocar molecules by correlating the rotational state with observed changes in their electronic properties. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U21.00002: Study of p-diaminobenzene Adsorption on Au(111) by Scanning Tunneling Microscopy Hui Zhou, Zonghai Hu, Daejin Eom, Kwang Rim, Li Liu, George Flynn, Latha Venkataraman, Alberto Morgante, Tony Heinz From the well-defined conductivity obtained for various individual diamino-substituted molecules spanning two gold contacts, as well as from theoretical analysis [1], researchers have suggested that amines adsorb preferentially to coordinatively unsaturated surface Au atoms through the N lone pair. To understand the nature of the amine binding, we have applied ultrahigh vacuum scanning tunneling microscope (STM) to investigate the adsorption of p-diaminobenzene molecules on the reconstructed Au(111) surface. The STM topography images (taken at 4 K) show that the molecules adsorb preferentially to step edges, corresponding to sites of reduced Au atom coordination. The adsorbed molecules are found to display a distinctive orientation along the step edges. The two-lobe topographic structure of each molecule seen by STM is compatible with the previously calculated charge density of the HOMO level. [1] L. Venkataraman at el., Nano Lett. 7, 502 (2007). [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U21.00003: Adsorption of colloids with Gaussian-size distribution on clean and pre-patterned substrates Joao F. Marques, A.B. Lima, Nuno A.M. Araujo, Antonio Cadilhe We performed extensive Monte Carlo simulations to study the influence of Gaussian size-distributed colloidal particles on film morphology. Also, we take the limit of irreversible adsorption, which leads to the study of a generalized random sequential adsorption model. We considered size dispersions ranging from $1\%$ up to $20\%$. The study concerns not only the jammed state but also the full kinetic dependence. Moreover, we also considered the influence of a patterned substrate, consisting of equal sized squares regularly distributed on the surface. Colloids can adsorb solely inside these squares. Results make understandable why colloids with size dispersions up to $4\%$ are considered monodisperse. Finally, we consider in the patterned substrate case cells with a prescribed number of colloids and characterize these deposits. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U21.00004: Binary mixture study of CF$_{4}$ and CF$_{3}$Cl on graphite Petros Thomas, Daniel Velazquez, George Hess In a binary mixture adsorption study of CF$_{4}$ and CF$_{3}$Cl on graphite from 60 K to 105 K, both the CF$_{3}$Cl - \textit{$\nu $}$_{4}$ and the CF$_{4}$ - \textit{$\nu $}$_{3}$ frequency shifts are measured using IRAS as the spreading pressure (chemical potential) of CF$_{4}$ is increased. Even though CF$_{3}$Cl has a much lower saturation vapor pressure (SVP) compared to CF$_{4}$ (at 80 K, SVP of CF$_{4}$ is $\sim $ 70 mT and that of CF$_{3}$Cl is $\sim $ 0.1 mT), the CF$_{4}$ either continuously displaces or adsorbs on top of CF$_{3}$Cl depending on the initial coverage of CF$_{3}$Cl on the graphite surface. For temperatures between 70 K and 105 K and lower coverage of CF$_{3}$Cl, where the molecules lie with their C -- Cl axis nearly parallel with the surface, CF$_{4}$ continuously displaces CF$_{3}$Cl from the surface. For saturated monolayer coverage of CF$_{3}$Cl, where the C -- Cl axis of the molecules are tilted relative to the surface, the CF$_{4}$ molecules adsorb on top of the CF$_{3}$Cl -- HOPG template. At 60 K, the displacement of the low-coverage CF$_{3}$Cl is only partial and the orientation of the remaining CF$_{3}$Cl is tilted relative to the surface from a nearly flat position. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U21.00005: DFT Estimation of Lateral Interactions in Lattice-gas Models of Br and Cl on Ag(100) T. Juwono, P.A. Rikvold We studied Br and Cl chemisorbed on a Ag(100) surface, using a latice-gas model and the Density Functional Theory (DFT) method. In this model the Br and Cl ions adsorb at the fourfold hollow sites of the Ag(100) surface, which yields a square lattice of adsorption sites. Five different coverages for each kind of adsorbate were calculated. For each adsorbate and coverage, we obtained the minimum-energy configuration, its energy, and its charge distribution. From these data we calculated dipole moments, lateral interaction energies, and binding energies. Our results showed that for Br the lattice-gas model obtained by fitting to the adsorption energies from the DFT calculation is consistent with long-range dipole-dipole lateral interactions using the dipole moments calculated from DFT charge distribution. For Cl we found less consistency, which indicates that long-range dipole-dipole interactions are not sufficient to describe the Chlorine system. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U21.00006: Modelling Interfaces in Liquid Crystal/isotropic fluid mixtures Colin Denniston, Dan Vriesinga We use all-atom molecular dynamics simulations of mixtures of a real liquid crystal (5CB) and water to study the 5CB/water interface. Properties of the director anchoring at the interface are studied in detail. We map our results onto a continuum model implemented using lattice Boltzmann simulations. We examine anchoring as a function of interface shape and droplet size and discuss the impact on liquid crystal colloidal suspensions. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U21.00007: Extended Analysis of a Fluid Configuration Experiment on the Space Shuttle Eric Barnett, Marcus Dejmek Glass cylinders, partially filled with water, were exposed to the near free-fall environment. In at least two of the cylinders, the liquid-vapour interface adopted a two-interface configuration, as previously predicted. An initial analysis was conducted on 20 images for one cylinder, resulting in contact angles of 6.7$\pm $2.7\r{ } at the upper three-phase line and 26.5$\pm $6.2\r{ } at the lower. Herein, the analysis has been extended to include all 12538 images recorded for each of two cylinders, in addition to correcting for optical distortion. An automated procedure to calculate the contact angles was developed, resulting in values of 2.7$\pm $2.8\r{ } and 16.5$\pm $5.3\r{ } for the same cylinder previously analyzed. The effective gravity (g$_{e})$ based on this analysis was inferred to be 3.3$\pm $2.1 x 10$^{-4}$ g/g$_{0}$, which differs from that previously reported. However, the standard deviation of g$_{e}$ is of the same order of magnitude as the RMS accelerations recorded. The difference in pressures between the two liquid phases was calculated to be 0.21$\pm $0.14 Pa. A Fourier analysis was conducted and no significant frequencies could be distinguished. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U21.00008: Dissipation at Moving Contact Lines: Effect of Interface Width and Slip Mark Robbins, Shengfeng Cheng, Colin Denniston Continuum mechanics predicts a diverging stress and total dissipation when the contact line between a fluid interface and a solid substrate is advanced. Several models for removing this divergence have been advanced. One is that the divergence is cutoff by a finite slip length. Another is that diffusion can remove the singularity for fluid interfaces of finite width. Extensive molecular dynamics simulations of partially miscible binary fluids were used to test these two pictures. The interfacial tension was changed by a factor of 20 and the interfacial width by an order of magnitude. The interface width had no direct effect on the dissipation and diffusion was orthogonal to the predicted direction. The dissipation only depended on system size, the dimensionless capillary number, and the slip length S associated with the flow boundary condition in the fluid far from the contact line. The divergence in stress is cut off at the sum of S and a distance of order the molecular diameter. The dissipation rises rapidly as the amount of slip is decreased. In all cases there is a first order transition where the advancing contact line becomes unstable and a film is entrained. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U21.00009: Counterintuitive connection between layering and mobility in confined fluids Gaurav Goel, William Krekelberg, Jeffrey Errington, Thomas Truskett Fluids confined to narrow spaces adopt a spatially inhomogeneous distribution of density due to the interactions between the fluid particles and the boundaries. This ``density profile'' is the most common measure of inhomogeneous structure in confined fluids, but its connection to fluid transport coefficients is poorly understood. We explore via molecular simulations how tuning particle-wall interactions to flatten or enhance the particle layering of a model Weeks-Chandler-Andersen (WCA) confined fluid impacts its self-diffusivity, viscosity, and entropy. Counterintuitively, interactions that eliminate particle layering significantly reduce confined fluid mobility, while those that enhance layering have the opposite effect. Excess entropy helps to both understand and predict these trends. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U21.00010: Effective interfacial tension between miscible fluids John Pojman, Nick Bessonov, Gloria Viner, Vitaly Volpert Isobutyric acid (IBA) and water have an Upper Critical Solution Temperature of 27 C. Using spinning drop tensiometry, we were able to demonstrate the existence of an effective interfacial tension by preparing a drop of isobutyric acid-rich phase below the UCST and then raising the temperature above the UCST. The capillary instability was also observed by rapidly reducing the rotation rate. We also demonstrated that such an effective interfacial tension is not unique to the IBA-water systems but can also occur in the cyclohexane -- aniline, which has a Lower Critical Solution Temperature. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U21.00011: The Au(111) electrolyte interface: A DFT investigation Timo Jacob, Sudha Venkatachalam, Felice Simeone, Dieter Kolb Density functional theory calculations have been performed to derive a detailed model of the electric double layer for Au(111) in contact with an aqueous H$_2$SO$_4$ electrolyte. At potentials of $E\ge +0.8$\,V vs. SCE various surface sensitive techniques found evidence for a ($\sqrt{3}\times \sqrt{7}$)R19.1$^\circ$ (bi)sulfate structure, but the nature of coadsorbates remains still unclear. Focusing on a sulfate adlayer, the coadsorption of H$_3$O$^+$ and/or H$_2$O has been studied [1]. The calculated binding energies show that the coadsorption of a single H$_3$O$^+$ per sulfate (stabilizing the adlayer by hydrogen bonds) is the most stable configuration. In addition, the charge density distribution within the adlayer well agrees with effective barrier heights deduced from recent distance tunnelling spectroscopy measurements [2]. Afterwards we studied the interfacial structure that forms at negative electrode potentials and found that water arranges near the electrode in an ice-like hexagonal structure with hydronium ions being located in the second water layer and non-specifically adsorbed. Again the calculated charge density distribution shows a perfect correspondence to distance tunnelling spectroscopy measurements. [1] S. Venkatachalam and T. Jacob, Z. Phys. Chem., {\bf 221}, 1393 (2007). [2] S. Venkatachalam {\it et al.}, Angew. Chem. Int. Ed., {\bf 46}, 8903 (2007). [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U21.00012: NMR Study of Organic Counterion Binding to Perfluorinated Micellar Structures Dobrin Bossev, Mustuo Matsumoto, Masaru Nakahara In this study we have applied our previously developed NMR method to study the adsorption of tetramethylammonium (TMA$^{+})$ and tetraethylammonium (TEA$^{+})$ counterions to micelles formed by perfluorooctylsulfonate (FOS$^{-})$ surfactant in water at 30 \r{ }C. These two counterions induce formation of threadlike surfactant structures that result in well pronounced viscoelastic properties of the solution. To selectively probe the degree of counterion binding we have used $^{1}$H and $^{19}$F NMR chemical shifts and self-diffusion coefficients that are sensitive to the \textit{Stern} and \textit{diffuse double layers}, respectively. The competitive adsorption of TMA$^{+}$ and TEA$^{+}$ was examined as a function of the TMA$^{+}$/TEA$^{+}$ ratio at a constant FOS$^{-}$ concentration of 100 mM. The two counterions were found to form Stern layer around the FOS$^{-}$ micelles with comparable packing; about one counterion per two micellized FOS molecules. When mixed at intermediate proportions, however, the TEA$^{+}$ counterion shows preferential binding; the concentration of TEA$^{+}$ in the \textit{Stern layer} is found to be twice higher than that of TMA$^{+}$ at equal total respective concentrations in the solution. These results are discussed in terms of counterion size and hydrophobicity and presented in parallel with those that involved the smaller and more hydrophilic lithium counterion. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U21.00013: Design and Fabrication of Micro-textures for Inducing a Superhydrophobic Behavior on Hydrophilic Materials Di Gao, Liangliang Cao, Anmin Cao, Hsin-Hua Hu Artificial superhydrophobic surfaces are typically fabricated by tuning the surface roughness of intrinsically hydrophobic surfaces. We here report the design and fabrication of micro-textures for inducing a superhydrophobic behavior on intrinsically hydrophilic hydrogen-terminated Si surfaces with an intrinsic water contact angle of about 74 degree. The micro-textures consist of overhang structures with well-defined geometries fabricated by microfabrication technologies, which provide positions to support the liquid and prevent the liquid from entering the indents between the micro-textures. As a result, water is in contact with a composite surface of solid and air, which induces the observed macroscopic superhydrophobic behavior. The principle is applied to fabricate non-aging superhydrophobic surfaces by packing flower-like micrometer-sized hematite particles. The as-fabricated superhydrophobic surfaces do not age even in extremely oxidative environments---they retain the superhydrophobicity after being stored in ambient laboratory air for 4 months, heated to 800 degree C in air for 10 hours, and exposed to ultraviolet ozone for 10 hours. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U21.00014: Heterogeneity and Fluctuations in Electrochemical Sensors Jean-Luc Fraikin, Michael Requa, Michael Stanton, Andrew Cleland Metal electrodes submerged in aqueous electrolytes biased with very small voltages frequently display a capacitive low-frequency electrical impedance, which is primarily imaginary but typically displays a 1/$f ^{\alpha }$ frequency dependence, with 0.7 $\le \quad \alpha \quad \le $ 1. This electrode-electrolyte interface is phenomenologically modeled as a constant phase element (CPE). There are a number of explanations for the observed frequency dependence, including geometric arguments based on the assumption of fractal surface geometries, but it is difficult to quantitatively match such models to experiment. We propose a new model to explain this phenomenon, as well as other low frequency electrical characteristics of the electrode-electrolyte interface, using a model that relies on microscopic heterogeneity, allowing for local variations in capacitance and diffusion coefficients. We will present the basic aspects of our model, and describe measurements under way to validate this model, using a combination of impedance measurements and electrochemical noise spectroscopy. [Preview Abstract] |
Session U22: Focus Session: Nonequilibrium Fluctuations in Biomolecules
Sponsoring Units: DPOLYChair: Christy Landes, University of Houston
Room: Morial Convention Center 214
Thursday, March 13, 2008 8:00AM - 8:36AM |
U22.00001: Driving proteins and DNA with mechanical forces: Pushing, pulling, and squeezing molecules using computer simulations. Invited Speaker: In living organisms, proteins and other biopolymers are often subjected to mechanical forces. Some of those forces are strong enough to cause proteins to unfold. For example, proteins driven across transmembrane pores may only enter the pores after they are mechanically denatured. Mechanically driven protein unfolding is often a non-equilibrium, irreversible process; Nature often takes advantage of the energy dissipation associated with such irreversible phenomena. For example, the ability of certain protein domains to dissipate large amounts of energy in the process of their mechanical unfolding is exploited in natural fibers and adhesives, which, as a result, display a remarkable combination of toughness and strength that is rarely achieved in artificial materials. In this talk, I will report on theoretical studies and computer simulations of several types of mechanical processes involving biopolymers. Examples include mechanical unfolding of proteins pulled at their ends, translocation of polymers across transmembrane pores, and stochastic dynamics of knots in tensioned polymer chains. I will discuss some of the computational challenges associated with the disparity between the time scales of simulation and experiments, comment on the molecular origins of high mechanical resistance displayed by some proteins, and compare our results with single-molecule pulling experiments. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U22.00002: Fluorescent resonant energy transfer: Correlated fluctuations of donor and acceptor Zhi-Gang Yu Mounting evidence suggests that in single-molecule flurescent resonant energy transfer (FRET) measurements, correlation between fluctuations in donor and acceptor may be important. We present a general theory to describe this correlation and its effect on the FRET rate [1]. The correlation arises from low-energy excitations (e.g. acoustic phonons) of the molecule to which a donor-acceptor pair is attached, and results in an effective interaction between local environments or baths associated with the donor and the acceptor. The correlation is found to reduce the transfer rate, in particular at short donor-acceptor distances. The theory can quantitatively explain recent measurements of polyproline peptides. [1] Z. G. Yu, J. Chem. Phys. {\bf 27}, 20xxxx (Communications) (2007). [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U22.00003: The water effects on long-distance charge transfer in polypeptides Nikolai Sergueev, Alexander Demkov Long-range electron transfer (ET) is one of the most intriguing reactions occurring in biological systems. Recent experiments indicate that water play an important role in the mechanism of charge transfer in proteins. In this talk we present the first-principles study of the effect of intervening water molecules on the electron tunneling processes in simple polypeptide bridges. The ET rate is related to the probability current that is computed using density functional theory and nonequilibrium Green's function formalism which takes into account the inelastic electron-phonon scattering in the bridge. Our results suggest that the effect of water is two fold. First, the insertion of water molecules changes the conformational and dynamic properties of the polypeptide molecule. Second, the presence of water modifies the electrostatics of the bridge. Both effects are found to have a significant effect on the electron transfer rate. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U22.00004: In$_{2}$O$_{3}$ nanowire based field effect transistor for biological sensors. Zhongming Zeng, Kai wang, Weilie Zhou Semiconductor nanowires (NWs) are attracting considerable attention due to their nanoscale dimensions and enormous surface-to-volume ratios. Many applications have been demonstrated in toxic gas, protein, small molecule and viruses sensing because of their superior sensing performances. Indium oxide (In$_{2}$O$_{3})$ NWs have been successfully applied for toxic gas and small organic molecule sensing. In our experiment, In$_{2}$O$_{3}$ NWs based field effect transistors (FET) are fabricated for virus (Ricin) detections. Single-crystalline In$_{2}$O$_{3}$ NWs with diameters around 100 nm were synthesized by the thermal evaporation. The nanodevice based on In$_{2}$O$_{3}$ NWs bridges the source/drain electrodes with a channel length of $\sim $5 $\mu $m. Basic transport properties of devices were measured before biological detection. The I-V curves with the gate voltage V$_{g}$=0 shows good ohmic contact and the resistance is about 10 M$\Omega $. The back-gate effect on the conductivity showed that In$_{2}$O$_{3}$ NW is working as $n$-type channel with obvious back-gate effect, which is much stronger than the reported results. The nanodevices used as virus detection will be also discussed. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U22.00005: Messenger RNA sequence and the translation process --a particle transport perspective Jiajia Dong, Beate Schmittmann, Royce K.P. Zia The translation process in bacteria has been under intensive study. A key question concerns the quantitative effect of different elongation rates, associated with different codons, on the overall translation efficiency. Starting with a simple particle transport model, the totally asymmetric simple exclusion process (TASEP), we incorporate the essential components of the translation process: Ribosomes, cognate tRNA concentrations, and messenger RNA (mRNA) templates correspond to particles, hopping rates, and the underlying lattice, respectively. Using simulations and mean-field approximations to obtain the stationary currents (the protein production rates) associated with different mRNA sequences, we are especially interested in the effect of slow codons, i.e., codons which are associated with rare tRNAs and are therefore translated very slowly. As the first step, we look at a ``designed sequence'' with one and two slow codons and quantify the marked impact of their spatial distribution to the currents. Extending the results to several mRNA sequences taken from real genes, we argue that an \textit{effective translation rate} including the information from the vicinity of each codon needs to be taken into consideration when seeking an efficient strategy to optimize the protein production. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U22.00006: Sticky-sphere model for phase separation of mixtures of the eye lens proteins gamma-B and alpha crystallin: non-monotonic dependence on mutual attraction George Thurston, Maurino Bautista, David Ross, Vern Lindberg, Hossein Shahmohamad We apply a multi-component extension of the Baxter sticky-sphere model to aqueous solutions of the eye lens proteins gamma-B crystallin and alpha crystallin. These mixtures show liquid-liquid phase separation influenced by gamma-B/gamma-B attraction, gamma-B/alpha size disparity and gamma-B/alpha attraction. We examine the dependence of the upper-consolute spinodal temperature surface on gamma-B/alpha attraction, previously found to influence stability. Gamma-B crystallin is modeled with a temperature-dependent stickiness parameter that reproduces both static light scattering and small-angle neutron scattering near its critical point. Alpha crystallin is modeled as a hard sphere. We find that the Barboy-Tenne model shows a non-monotonic dependence of the spinodal temperature surface on gamma-B/alpha attraction that is qualitatively consistent with recent molecular dynamics simulation results. Hard-sphere or very attractive gamma-B/alpha interactions lead to instability, and the spinodal surface shows a minimum in an intermediate range of gamma-B/alpha attraction strength. We examine the nature of the two types of instability. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U22.00007: Structural Isotopic Effects in the smallest chiral amino acid: Observation of a structural phase transition in fully deuterated alanine. Heloisa Bordallo, Joelma de Souza, Paulo de Tarso, Dimitri Argyriou A first study of possible changes instigated by deuteration in amino acids was carried out using neutron diffraction, inelastic neutron scattering and Raman scattering in L-alanine, C$_{2}$H$_{4}$(NH$_{2})$COOH. Careful analysis of the structural parameters shows that deuteration of L-alanine engenders significant geometric changes as a function of temperature, which can be directly related to the observation of new lattice vibration modes in the Raman spectra. The combination of the experimental data suggests that C$_{2}$D$_{4}$(ND$_{2})$COOD undergoes a structural phase transition (or a structural rearrangement) at about 170 K. Considering that this particular amino acid is a hydrogen-bonded system with short hydrogen bonds (O{\ldots}H $\sim $ 1.8 {\AA}), we evoke the Ubbelohde effect to conclude that substitution of hydrogen for deuterium gives rise to changes in the hydrogen-bonding interactions. The structural differences suggest distinct relative stabilities for the hydrogenous and deuterated L-alanine. De Souza et al. - Journal of Physical Chemistry B (Letters) \textbf{111}, 5034-39 (2007) [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U22.00008: Exploring the Electrical Conductivity of Myoglobin Debin Li, David Lederman, Peter M Gannett The electrical conductance of single myoglobin proteins was measured to study its electron transfer properties. We examined the electronic properties of myoglobin, using apomyoglobin (myoglobin without a heme group) as a reference. The differential conductivity of the proteins deposited on Pt nanometer-scale electrodes was measured using a lock-in technique as a function of bias and gate voltages. Nano- electrodes were fabricated by creating small Pt channels 100 nm - 300 nm wide via e-beam lithography and then creating a break junction by electromigration at low temperatures (4 K - 77 K). The conductance of apomyoglobin was very different from that of myoglobin, with a predominant peak at ~50 meV. On the other hand, myoglobin had a rich structure that we surmise results from the presence of the heme group. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U22.00009: Selective binding affinity of cationic antimicrobial peptides for lipid membranes: roles of peptide charge and hydrophobicity Sattar Taheri-Araghi, Bae-Yeun Ha Antimicrobial peptides selectively disrupt microbial membranes through hydrophobic insertion into the outer layers, which are known to carry a large fraction of anionic lipids. When the peptides are cationic, as is often the case, the interplay between hydrophobic and electrostatic interactions determines the selective binding affinity (thus antimicrobial activity) of the peptides. Here we present a detailed theoretical picture of how the selective binding is influenced by such factors as the charge and hydrophobicity of the peptides and the elasticity of target membranes. This effort not only accounts for some of the general trends observed in experimental studies, but it also leads to a theoretical model for optimizing the selectivity and antimicrobial activity. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U22.00010: Excitation dynamics in purple bacteria photosynthetic membranes under different light adaptation conditions Felipe Caycedo, Ferney Rodriguez, Luis Quiroga Photosynthetic membranes in {\it R. Sphaeriodes} purple bacteria adapt to light growth conditions such as the intensity level of radiation, which determine their amount of Reaction Centers (RCs), and also their global architecture. In any case, for both high and low intensity conditions the trend for core (LH1) and antenna (LH2) complexes clustering is mantained. Using a F\"orster hopping model for excitation transfer, we analize different adapted membranes for which we report results for the yield and lifetime of excitations under continuous illumination levels. We show that complexes stoichiometry obey to efficiency optimization under RC biochemical cycle constraints. By constrast to common belief, complexes aggregation does not directly show any strong dependence on excitation efficiency observables. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U22.00011: Microscopic Electrohydrodynamics of DNA electrophoresis Aleksei Aksimentiev, Binquan Luan Gel electrophoresis is currently the most successful yet costly method to sequence DNA. Electrophoresis of DNA through solid-state nanopores holds promise for reducing the costs and making personal genomics a reality. The underlying physics of DNA electrophoresis, however, remains controversial. Theoretical models of this process often invoke the notion of the effective charge of a DNA molecule $q_{\mathrm{eff}}$ to account for the reduced electric force on DNA in an external field $E$, i.e. $F= q_{\mathrm{eff}}E$. However, experimental estimates of $q_{\mathrm{eff}}$ can differ from each other by as much as ten times. To clarify the physical origin of the reduction of an electric force on DNA in electrophoresis, we investigated this process through extensive all-atom molecular dynamics simulations. Our results demonstrate that the effective screening of the DNA charge arises from the hydrodynamic drag of the electroosmotic flow, not from the counterion condensation. We show that the effective driving force $F$ of an applied electric field $E$ in a nanopore obeys the same law as in a bulk electrolyte: $F=\xi\mu E$. Here, $\xi$ and $\mu$ are, respectively, the friction coefficient and electrophoretic mobility of DNA that depend on the surface properties of a nanopore, such as its roughness. Based on the above law, a method for determining the effective driving force is suggested that does not require a direct force measurement. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U22.00012: Multivalent counterions inhibit DNA ejection from viral capsid Toan Nguyen Viral DNA packaged inside a bacteriophage is tighly bent. This stored bending energy of DNA is believed to be the main driving force to eject viral DNA into host cell upon capsid binding. One can control the amount of ejected DNA by subjecting the virus to a solution of PEG8000 molecules. The molecules cannot penetrate the viral capsid, therefore, they exert an osmotic pressure on the virus preventing DNA ejection. Experiments showed that for a given osmotic pressure, the degree of ejection also depends on the concentration of small ions in solution. Interestingly, for multivalent ions (such as Mg2+, Spd3+ or HexCo3+), this dependence is non-monotonic. We propose a simple electrostatic theory to explain this non-monotonic behavior. This is based on the fact that DNA molecules can invert its net charge at high enough multivalent counterion concentration. In other words, as multivalent counterion concentration is increased from zero, charge of DNA molecules change from negative to positive. At the concentration where DNA net charge is zero, the DNA molecules experience an attraction between different segments and DNA ejected amount is reduced. At low or high counterion concentration, DNA segments are charged (negatively or positively), repel each other and DNA ejected amount is increased. Fitting the result of the theory to experimental data, we obtain a numerical value for Mg2+ mediated DNA - DNA attraction energy to be -0.008kT per base. [Preview Abstract] |
Session U23: Focus Session: Manganite Thin Films
Sponsoring Units: DMP GMAGChair: Amlan Biswas, University of Florida
Room: Morial Convention Center 215
Thursday, March 13, 2008 8:00AM - 8:12AM |
U23.00001: Thickness dependence of the exchange bias in epitaxial manganite bilayers Alexey Kobrinskii, Maria Varela, Allen Goldman A series of thin ferromagnetic/antiferromagnetic (F/AF) bilayers of doped lanthanum manganites La2/3Ca1/3MnO3 (F) and La1/3Ca2/3MnO3 (AF) have been grown by ozone-assisted molecular beam epitaxy (OAMBE). The lattice of the substrate material (001) SrTiO3 is a good match to that of the manganites. Growth by the OAMBE method results in samples with sharp interfaces, which are suitable systems to study the interfacial phenomenon of exchange bias (EB). We present STEM and high-resolution X-ray diffraction data that verify the high structural quality of the samples. We have studied EB as a function of the AF layer thickness and determined two critical values of the thickness for the onset and for the saturation of the hysteresis loop shift which is traditionally used to measure the effect. The observed dependence of EB on the AF layer thickness can be described within the original or generalized Meiklejohn-Bean model. Using this simple approach we have estimated the interfacial coupling energy and the antiferromagnetic anisotropy constant. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U23.00002: Infrared studies of Phase Separated (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ Thin Films Naveen Margankunte, Tara Dhakal, Amlan Biswas, D.B. Tanner We report optical spectroscopy studies of phase separated (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ thin films grown on the substrate NdGaO$_{3}$. Reflectance measurements in the far and mid infrared were performed for a range of temperatures from 10 to 300 K. Particular attention was given to the narrow temperature range where the insulator-metal transition occurs. The optical constants were extracted by fitting the measured reflectance to a Drude-Lorentz dielectric function in conjunction with thin film optics and the measured properties of the substrate. Spectral weight analyzes show that the growth of low energy oscillator strength occurs well above the Curie temperature, indicating phase coexistence in the hysteresis regime seen in resistivity measurements. The optical conductivity results are contrasted with existing models for free carrier electrodynamics in manganites. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U23.00003: Anisotropic Magnetoresistance in (La,Pr)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ Films. Megumi Yamamoto, Chuhee Kwon, Anthony Davidson, Sanjay Adhikari, Rajeswari Kolagani The out-of-plane anisotropic magnetoresistance (AMR) was measured in mixed phase manganite (La,Pr)$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LPCMO) films. Two samples with different film thicknesses ($\sim $30 nm and $\sim $150 nm) on LaAlO$_{3}$ substrate were compared for the effects of stress on AMR. The thicker sample exhibits an insulator-metal resistive transition with a hysteresis typical of LPCMO with the peak temperature of 175 K and 250 K at H = 0 T and 8 T, respectively. While the resistance of the thinner sample is too high for our system to measure (Quantum Design PPMS) below 140 K at H = 0 T, the peak temperature at 8 T is 105 K. AMR shows a sinusoidal angular dependence typical of a ferromagnet for both samples. In this talk, we will present systematic AMR measurements of the LPCMO samples. We found that the peak position of AMR depends both on film thickness and on temperature. In addition, we observed time-dependent changes in resistance at lower temperatures indicating a long relaxation time for spins. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 9:12AM |
U23.00004: Interface magnetism in complex oxide heterostructures Invited Speaker: Magnetic oxides are an important class of materials from the perspectives of fundamental physics and technological applications. Advances in growth of high quality thin films and epitaxial oxide heterostructures over the years, have led to the realization of ideal condensed matter systems in which the complex and rich physics associated with cooperative phenomena can be explored. Examples of coupled phenomena in heterostructures include exchange bias effects, magnetoelectric coupling and interplay between magnetism and superconductivity. In this talk, I will focus on three classes of oxide heterostructures --PLD-grown M-type barium hexaferrite(BaM)/barium strontium titanate(BST), CVD-grown CrO$_{2}$/Cr$_{2}$O$_{3}$ bilayers and high-pressure sputtered LCMO/YBCO films. The common theme is the magnetic coupling across the interfaces. I will demonstrate that dynamic susceptibility and kinetic inductance experiments using a sensitive tunnel-diode oscillator (TDO) are effective probes of such coupled effects. In the case of CrO$_{2}$/Cr$_{2}$O$_{3}$ and LCMO/YBCO, the interface coupling results in anomalous anisotropy, exchange bias in the former and complex interaction between the LCMO magnetism and YBCO vortex lattice in the latter. In BaM/BST heterostructures, I will discuss how interfacial coupling influences the microwave response that is both electrically and magnetically tunable. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U23.00005: Atomic resolution STM study of Perovskite Manganite Thin Films Kenji Fuchigami, Zheng Gai, T. Zac Ward, Lifeng Yin, E. Ward Plummer, Jian Shen The perovskite manganites have attracted huge interest due to their intriguing electronic inhomogeneous nature which is believed to be responsible for colossal magnetoresistance. Scanning tunneling microscope (STM) is one of the most promising techniques for studying such electronic inhomogeneity in real space. In order to investigate electronic property at the surface of non-layered perovskite manganite, we have synthesized single crystal La5/8Ca3/8MnO3 (LCMO) thin film by laser MBE technique. In-situ thin film growth enables us to obtain atomically resolved STM image which has c-2x2 superlattice unit cell. In this talk, we will discuss the electronic properties as well as lattice structures of the LCMO surfaces. This research is sponsored by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory (ORNL), managed by UT-Battelle, LLC for the US Department of Energy under Contract No. DEAC05-00OR22725. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U23.00006: Linear Crack Arrays and Resistive Anisotropy in Nd$_{0.2}$Sr$_{0.8}$Mn$O_3$ Thin Films Under Tensile Strain$^{\ast}$ Krishna Neupane, Joshua Cohn, John Neumeier The structure, morphology, and electrical properties of epitaxial $a$-axis oriented thin films of Nd$_{0.2}$Sr$_{0.8}$Mn$O_3$ are reported for thicknesses $10\leq t\leq 150$~nm. Films with $t\geq 20$~nm grown under tensile stress on NdGaO$_3$ (100) and LSAT (110) substrates develop uniform linear crack arrays (cracks running along film $c$ axis) with a crack spacing (0.3-10~$\mu$m) that decreases with increasing thickness. Films grown under compression on LaAlO$_3$(110) substrates exhibit no cracks. The room-temperature in-plane electrical resistance ratio, $\rho_b/\rho_c$, increases approximately exponentially with increasing film thickness to values of $\sim 1000$ in the thickest films studied. The temperature dependencies for $\rho_b$ and $\rho_c$ are essentially identical, suggesting that very long effective transport paths perpendicular to the cracks are responsible for enhanced values of $\rho_b$. \vskip .1in \noindent $^{\ast}$ This material is based upon work supported by the National Science Foundation under grants DMR-0072276 (Univ. Miami) and DMR-0504769 (Montana State Univ.), the Research Corporation (Univ. Miami), and the U.S. DOE Office of Basic Energy Sciences (Grant No. DE-FG-06ER46269). [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U23.00007: Effect of strain and disorder in manganite thin films Sung Hee Yun, Rajiv Misra, Ben deGlee, Jacob Tosado, Tara Dhakal, Arthur Hebard, Amlan Biswas We have studied the effect of strain and disorder on the phase separated state in thin films of the manganite (La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$(LPCMO, $y$ = 0.4, 0.5, 0.6) grown on (110) NdGaO$_{3}$ substrates using pulsed laser deposition. Due to the competition between the charge-ordered insulating and ferromagnetic metallic phases, thin films of LPCMO display a fluid-like phase separation (FPS) near the insulator-to-metal transition temperature. By applying direct mechanical stress on the LPCMO thin films using a three-point beam bending technique, we observed a colossal piezoresistance in the FPS state of these materials. Our observations show that a small amount of strain ($\sim $10$^{-4})$ can move the phase boundaries in the FPS state. We then modified the extrinsic disorder in the thin films by bombarding them with Ar-ions. Our measurements show a reduction of piezoresistance in the ion-bombarded samples which suggests that such extrinsic disorder can pin the phase boundaries and reduce the fluidity of the FPS state. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U23.00008: Low Temperature Magneto-transport Measurements on Multilayered Manganite Films M. Spencer, P. Bhupathi, S. H. Yun, A. Biswas, Y. Lee We have performed out-of-plane resistance measurements on a micro-fabricated multilayered manganite thin film structure down to 50 mK. The structure is composed of a 26 nm thick (La$_{0.4}$Pr$_{0.6})_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LPCMO) film on top of a 60 nm thick La$_{0.67}$Ca$_{0.33}$MnO$_{3 }$(LCMO) layer grown on an (110) NdGaO$_{3}$ (NGO) substrate. Two gold electrodes were deposited on the LPCMO layer and the exposed LPCMO layer was then etched by ion plasma etching technique. We observed an upturn in resistance below 30 K for various current excitations ranging from 0.2 to 100 $\mu $A. Based on the electric and magnetic field dependence of the resistance, we attribute the upturn to the disorder-induced static phase separation of the LPCMO thin film at low temperature. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U23.00009: The influence of growth temperature on the physical properties of La1-xSrxMnO3 thin film by rf magnetron sputtering Yonghang Pei, Jiwei Lu, Stuart Wolf Perovskite La$_{1}-$xSrxMnO$_{3}$ has been of interest for such diverse applications as a spin injector for spintronic devices and for fuel cell electrodes due to its magnetic and transport properties, namely high spin polarization and ionic conduction. In this work, we prepared La$_{1}-$xSrxMnO$_{3}$ (LSMO) thin films by rf-magnetron sputtering. LSMO films were deposited on single crystal SrTiO$_3$ (100) substrates and platinized Si wafers concurrently in the temperature range from 450 to 600 $^{\circ}$C. X-Ray diffraction (XRD) determined that the LSMO film was epitaxial on either substrate. Magnetic hystersis loops were measured at temperatures between 10 and 300K and the saturation moment was significantly improved by increasing the growth temperature. In addition, the Curie temperature of LSMO was between 150 and 250 K and was also strongly dependent on the growth temperature. We will also discuss the impact of growth temperature on the temperature dependent transport and magnetic properties of LSMO. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U23.00010: Current-induced and Photoinduced Effects in Annealed Bi$_{1-x}$Ca$_{x}$MnO$_{3}$ Thin Films Vera Smolyaninova, K. Karki, Rajeswari Kolagani, G. Yong, R. Kennedy Doped rare-earth manganese oxides exhibit a wide variety of physical phenomena. Application of magnetic field, electric field, or electromagnetic wave irradiation drastically modifies electrical conductivity and refraction index of these materials. A photoinduced and current-induced insulator to metal transition in charge-ordered (CO) manganese oxides is especially interesting from the point of view of photonic and opto-electronic device development. We have found that 50 nm Bi$_{0.4}$Ca$_{0.6}$MnO$_{3}$ thin film grown on NdCaAlO$_{4}$ substrate is very susceptible to increase of current and illumination with laser light ($\lambda \sim $ 500 nm) [1]. Application of these stimuli partially destroys CO and produces charge-disordered conductive phase. Combined application of illumination and current completely destroys the charge ordering in this material, while the application of one of these factors separately is not sufficient to produce such effect. Current-induced and photoinduced properties of the Bi$_{0.4}$Ca$_{0.6}$MnO$_{3 }$thin films grown on different substrates will also be presented and current-voltage characteristics will be discussed. [1] V. N. Smolyaninova at al., Phys. Rev. B 76, 104423 (2007). [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U23.00011: Enhancement of Photoinduced Effects in Annealed Bi$_{1-x}$Ca$_{x}$MnO$_{3 }$Thin Films. K. Karki, Rajeswari Kolagani, G. Yong, R. Kennedy, K. DeMarchi, Vera Smolyaninova Doped rare-earth manganese oxides (manganites) exhibit a rich variety of interesting physical phenomena including their sensitivity to application of magnetic, electric fields, and electromagnetic wave irradiation. A photoinduced insulator to conductor transition in charge-ordered (CO) manganites is especially interesting from the point of view of creating photonic devices. Thin films of Bi$_{0.4}$Ca$_{0.6}$MnO$_{3}$ exhibit large photoinduced effects associated with melting of the charge ordering by visible light and can support conducting and insulating phase coexistence on a submicron scale [1]. We have found significant increase of the photoinduced resistivity changes and the life time of the photoinduced conducting phase after annealing. The changes in current-induced effects in annealed films will be also reported, and the possible origin of these effects will be discussed. [1] V. N. Smolyaninova at al., Phys. Rev. B 76, 104423 (2007). [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U23.00012: Insulating Domain Walls in Mixed Phase Manganite Guneeta Singh Bhalla, S. Selcuk, T. Dhakal, A. Biswas, A.F. Hebard We present here evidence of \textit{novel }insulating domain walls (IDWs) which allow direct tunneling of spin polarized currents in thin films of the mixed phase ferromagnet (La,Pr,Ca)MnO$_{3}$. Elastic interactions in the distorted perovskite structure of (La,Pr,Ca)MnO$_{3}$ coupled with magnetostatic interactions give rise to coexisting ferromagnetic metallic and insulating regions near the Curie temperature, $T_{C}$. Well below $T_{C}$, magnetization measurements reveal that the mixed phase metal/insulator state evolves into a fully saturated ferromagnetic metallic state. However, when either the film thickness is reduced, or a thicker film is patterned into a nanometer wide bridge structure, the formation of domain structure is modified as theoretically predicted for mixed phase ferromagnets, resulting in thin IDWs separating adjacent half-metallic domains. Experimentally we observe that upon cooling below $T_{C}$, a predominantly ferromagnetic supercooled state persists where remnants of the insulating regions behave as IDWs within the bridge. Tunneling across IDWs results in metastable, temperature-independent, high-resistance plateaus over a large range of temperatures below $T_{C}$. Upon application of fields on the order of the coercive field, neighboring domains align and the IDWs are extinguished resulting in sharp, colossal resistance drops. The presence of IDWs offers rich physical insights into ferromagnetic domain formation in mixed phase systems. [Preview Abstract] |
Session U24: Focus Session: Transport in Nanostructures VI: Nonequilibrium Phenomena and Noise
Sponsoring Units: DMPChair: Jeff Neaton, Lawrence Berkeley National Laboratory
Room: Morial Convention Center 216
Thursday, March 13, 2008 8:00AM - 8:36AM |
U24.00001: Imaging hot-electron transport using chemical reactions on metal surfaces Invited Speaker: We have investigated a new regime of single-molecule excitation in the scanning tunneling microscope, where hot electrons locally injected from the STM tip spread out via surface resonances over length scales of up to 100 nm and electronically excite surrounding molecules causing chemical reactions. Such non-local reactions were observed for several different molecules on the (111), (110) and (100) terminated surfaces of gold and copper. The hot-electron origin of these reactions was differentiated from the possible electric field effect in the tip-surface junction on the basis of the statistical analysis of the dissociation yield as well as the non-local excitation in the presence of artificially fabricated nanoclusters. One of the new opportunities provided by the non-local excitation is a direct measurement of hot-electron transport on a metal surface. Using a phenomenological kinetic model for the statistical analysis of the non-local reactions, it is shown that the reaction rate increases linearly with tunneling current and decays exponentially with the distance from the excitation pulse. Since the attenuation length of the non-local reaction has little dependence on the STM-tip and the parameters of the excitation, we argue that it is proportional to the inelastic mean-free path of hot-electrons in the surface resonance. The angular distribution of the reaction events is isotropic on Au(111), which is consistent with the symmetry of its surface resonances in the energy range of the non-local reaction. It is also shown that the total yield of the non-local reaction provides a measure of hot-electron transport across single-atom steps. Although the reflectance of the hot-electrons by single atom steps on Au(111) is less than 20{\%} at energies above 1.5 V, the yield of the reaction becomes surprisingly asymmetric if hot-electrons are injected in the immediate vicinity of the step. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U24.00002: Electromigration force, surface resistivity and low-frequency noise Ellen D. Williams, O. Bondarchuk, C.G. Tao, W. Yan, W.G. Cullen, P.J. Rous, T. Bole Scattering of charge carriers from surface structures will become an increasing factor in the resistivity as the structure decreases in size to the nanoscale. The measured effects of scattering at the most basic surface defect, a kink in a step edge, are 5x larger than for a freely diffusing surface atom. For thermally active materials, this yields a corresponding contribution of the fluctuating steps to the surface resistivity, which will exceed 1{\%} of the bulk resistivity as wire diameters decrease below 10s of nanometers. The temporal fluctuations of kink density will cause resistivity noise. Relating the known fluctuation spectrum of the step displacements to fluctuations in their lengths, the corresponding resistivity noise is predicted to show spectral signatures of $\sim f^{-1/2}$ for step fluctuations governed by random attachment/ detachment, and $\sim f^{-3/4}$ for step fluctuations governed by step-edge diffusion. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U24.00003: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U24.00004: Frequency-dependent counting statistics in interacting nanoscale conductors Ramon Aguado, David Marcos, Clive Emary, Tobias Brandes Following the considerable success of shot-noise in the understanding of transport through mesoscopic systems, attention is now turning towards the higher-order statistics of electron current. The so-called Full Counting Statistics (FCS) of electron transport yields all moments (or cumulants) of the probability distribution $P(n,t)$ of the number of transferred electrons during time $t$. The theory of FCS is now well established in the zero-frequency limit. However, this is by no means the full picture, since the higher-order current correlators at finite frequencies contain much more information than their zero-frequency counterparts. In this work [1], we present a formalism to calculate finite-frequency current correlations in interacting nanoscopic conductors. We work within the n-resolved density matrix approach and obtain a multi-time cumulant generating function that provides the fluctuation statistics solely from the spectral decomposition of the Liouvillian. We apply the method to the frequency-dependent third cumulant of the current through a single resonant level and through a double quantum dot. Our results, which show that deviations from Poissonian behaviour strongly depend on frequency, demonstrate the importance of finite-frequency higher-order cumulants in fully characterizing transport. [1] C. Emary, D. Marcos, R. Aguado and T. Brandes, Physical Review B, 76, 161404R, 2007. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U24.00005: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U24.00006: Nonequilibrium elastic quantum transport using plane waves Aran Garcia-Lekue, Lin-Wang Wang In this work, we present an \emph{ab initio} nonequilibrium electronic structure method for modeling the elastic electron transport through a nanostructure coupled to semi-infinite external electrodes and with an applied bias voltage. Our method is based on the scheme presented in Ref.\,[1], where the coherent quantum transport is calculated by means of the \emph{exact} scattering states of the system obtained using plane waves and for zero applied bias voltage. In the case of a finite bias voltage, the electronic system is in a nonequilibrium situation, and the problem needs to be solved self-consistently. Here, we present an approach to obtain the self-consistent charge density and potential of the system, which are then employed in the calculation of the nonequilibrium transmission coefficient and conductance. As an illustration, results for a model system made up of a di-thiol-benzene (DBT) molecule connected by two Cu wires are provided. [1] A. Garcia-Lekue and L.W. Wang, Phys. Rev. B. {\bf 74}, 245404 (2006). [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U24.00007: Allowed charge transfers between coherent conductors driven by a time-dependent scatterer Alexander Abanov, Dmitri Ivanov We derive constraints on the statistics of the charge transfer between two conductors in the model of arbitrary time-dependent instant scattering of non-interacting fermions at zero temperature. The constraints are formulated in terms of analytic properties of the generating function: its zeroes must lie on the negative real axis. This result generalizes existing studies for scattering by a time-independent scatterer under time-dependent bias voltage. We discuss the meaning and possible extensions of our results. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U24.00008: Ab-initio formulation of the 4-point conductance of interacting electronic systems and its implementation in the GWST method Peter Bokes, Matthieu Verstraete, Rex Godby The commonly employed linear-response expression for the conductance of quantum junctions suffers from an ambiguity of the definition of the applied potential difference. We show how this is resolved in terms of the formally as well as physically well defined 4-point conductance [P. Bokes, J. Jung, and R. W. Godby, Phys. Rev. B 76, 125433 (2007)]. Furthermore, expressing the 4 point conductance solely in terms of the density response function or polarizability, we obtain a computationally viable approach to go beyond mean-field, Green's function based descriptions of realistic ab initio models of quantum junctions. We will discuss the numerical implementation of the formalism within the GWST code for the real-space imaginary-time GW method [N. Rojas, R.W. Godby and R.J. Needs, Phys. Rev. Lett. 74 1827 (1995)] and present results for several simple systems. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U24.00009: Comparison of transport calculations using complex absorbing potentials and the Non-equilibrium Green's function formalism Kalman Varga, Joseph Driscoll In the Non-equilibrium Green's formalism (NEGF) the system is divided into left and right leads and a central region. To avoid spurious reflections from the boundaries one has to treat the leads as semi-infinite systems. Various efficient recursion methods are developed [1] for this purpose. Alternatively, one can use a complex absorbing potential (CAP) that absorbs the outgoing waves and one only has to deal with short finite leads. In this work we have compared the NEGF recursion and CAP approaches (1) on a simple analytically solvable example and (2) by calculating the transmission coefficients for a carbon nanotube device using a density functional Hamiltonian. Both approaches give very accurate results but the CAP method is orders of magnitude faster in calculating the self-energies. This work was supported by NSF grant ECS 0622146. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U24.00010: Systematic Study on Quantum Confinement and Waveguide Effects for Elastic and Inelastic Currents in Atomic Gold Wire: Importance of the Phase Factor for Modeling Electrodes Hisao Nakamura, Koichi Yamashita Quantum confinement of the electrodes is an important issue for electron transport through molecular or atomic wire junctions. To assess the importance of waveguide effects by quantum confinement of the electrodes, we have calculated elastic and inelastic conductance and inelastic electron tunneling spectra of atomic gold wire with gold electrodes for several models. The results show the quite important role of the phase factors between the modeled electrodes and the contact region. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U24.00011: Nonlinear transport properties of model metal--Mott-insulator--metal heterostructures Satoshi Okamoto Transport properties of heterostructures in which a finite number of correlated-insulator or correlated-metal layers are sandwiched by semi-infinite metallic leads are investigated by using the layer dynamical-mean-field method combined with the Keldysh Green's function technique. We use as impurity solvers the equation-of-motion decoupling method, the noncrossing approximation and the iteration perturbation method. Electron spectral functions in the interacting region are shown to evolve by an applied bias voltage. These effects control the current-voltage characteristics of the heterostructures. It is also shown that the deformed spectral functions strongly affect the optical response. These features differentiate a correlation-induced Mott insulator and a conventional band insulator. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U24.00012: Phonon Effects on Charge Transport Through a Two State Molecule Sergio E. Ulloa, Efta Yudiarsah We study the effect of local and non-local phonon on the transport properties of a molecule model described by two- electronic states. The local phonon interaction is tackled by means of a Lang–Firsov transformation [1,2]. The interaction with non-local phonons (phonon-assisted hopping) is considered perturbatively up to the first nonzero order in the self energy. The presence of different kinds of electron-phonon interaction open new transmission channels. In addition to the polaron shift and replicas due to local phonons, non-local phonons cause the appearance of new satellite states around the initial states. In the weak coupling regime of non-local phonon and electrons, states are shifted an amount proportional to square of the interaction. However, in the strong coupling regime, the non-linear effects emerge and display more interesting features on transport properties. Additional features on transport properties due to new transmission channel are shown to appear at finite temperatures. [1] G. D. Mahan, Many-particle physics, 3rd ed. (Plenum Publishers, New York, 2000). [2] R. Gutierrez \emph{et al.}, Phys. Rev. B. \textbf{74}, 235105 (2006). [Preview Abstract] |
Session U25: Theory and Simulation II
Sponsoring Units: DPOLYChair: Marina Guenza, University of Oregon
Room: Morial Convention Center 217
Thursday, March 13, 2008 8:00AM - 8:12AM |
U25.00001: Monte Carlo simulations of a coarse-grain model for block-copolymer melts: method and application Francois Detcheverry, Darin Pike, Paul Nealey, Juan de Pablo, Marcus Mueller A new Monte Carlo based approach has been developed for simulation of polymeric systems, including block copolymers. The approach represents the system at the level of a coarse-grain Hamiltonian, akin to that employed in widely used self-consistent field theoretic (SCFT) treatments. In contrast to traditional implementations of SCFT, however, molecules are treated explicitly and fluctuations are taken into account. We present two distinct implementations of the method; the first relies on a grid, and the second does not. While the grid-based method is highly computationally efficient, the gridless implementation permits simulations in arbitrary ensembles, including the grand-canonical and Gibbs ensembles, thereby facilitating study of phase transitions. The gridless implementation also gives access to the local mechanical properties. The performance of the two implementations is discussed in the context of several applications, including the directed assembly of multi-block copolymer thin films on patterned substrates, either chemical or topographical. In both cases we examine the ordering of the material and the effect of pattern or surface roughness. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U25.00002: Interaction between Polymer Grafted Particles: Self-Consistent-Field Study Jaeup Kim, Mark Matsen Recently, there has been an ongoing debate regarding a possible attraction between two polymer grafted nanoparticles. Using numerical self-consistent field theory (SCFT) we investigate the inter-particle potential, showing that only a monotonically increasing repulsive force is expected between the two particles regardless of the particle size and brush thickness. We also compared the exact mean-field result to approximate solutions using the Derjaguin approximation. The previously reported attraction is thought to be an anomaly caused by the use of bispherical coordinates. We avoid this problem by developing a new SCFT scheme using two separate spherical coordinate systems centered on each particle. The idea of using multiple coordinate systems is applicable for many polymeric problems involving complicated geometries. In this scheme, two or more coordinate systems share an overlapping volume. Then the statistics of the polymers belonging to a certain coordinate system are solved in trial fields, and the resulting polymer concentration is shared by all coordinate systems to find a self-consistent solution. This method has been tried for other problems such as the behavior of a Janus nanoparticle (solved in spherical coordinates) in block copolymer lamellar phase (solved in cylindrical coordinates). [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U25.00003: Discovering Ordered Phases of Block Copolymers: A New Fourier-space Approach Feng Qiu, An-Chang Shi, Zuojun Guo, Hongdong Zhang, Yuliang Yang A new method to solve the self-consistent field theory of block copolymers is developed. This method is based on the fact that, for any computational boxes with periodic boundary conditions, all spatially varying functions are spanned by the Fourier series determined by the size and shape of the box. This method is well suited for the discovery of ordered structures of block copolymer systems. The symmetry of the ordered structures emerges from the minimization of the free energy density. Application of the technique to diblock copolymers recovers all the previously known ordered structures plus a few new metastable ones. As an example of application, the method is used to construct a phase diagram for a model of frustrated triblock copolymers. A variety of stable or metastable three-dimensional ordered structures are discovered. Furthermore, the capability of the method to reproduce experimentally observed structures is demonstrated by the knitting pattern in triblock copolymers. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U25.00004: Cylindrical phase of diblock copolymers in thin films Marianne Breuer, Barbara Drossel We investigate the microphases of diblock copolymers confined in a thin film with walls attracting one of the monomer types.We focus on the possible structures of copolymers that form cylindrical phases in the bulk. We employ both self-consistent field theory and strong segregation theory to obtain the concentration profile minimizing the free energy of the system and to compare the free energy of possible morphologies. We present a phase diagram showing the possible microphases for a diblock copolymer with fixed volume fraction and fixed segregation parameter in dependence of the film thickness and the affinity of the walls. We study the effects of numerical inaccuracies on the appearance of different morphologies and their free energies. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U25.00005: Cubic Micellar Crystals of $A_nB_mA_n$ Block Copolymers from MD Chris Lorenz, Joshua Anderson, Alex Travesset Amphiphilic block copolymers exhibit a wide variety of phases in solution. One common phase of $A_nB_mA_n$ polymers is made up of spherical micelles with hydrophobic (B) cores and hydrophilic (A) coronas. At high enough concentrations, these micelles order on a lattice forming a micellar crystal. The dynamics during the formation of this phase are fascinating, being controlled almost entirely by the polymer transfer between micelles, as shown in Molecular Dynamics~simulations. Application of a standard nucleation and growth analysis shows that the micellar crystals grow extremely rapidly and are probably aided by the periodic box in the simulation. A study of the dynamics at equilibrium shows that polymer transfer is still a continuing process and can be understood in the context of transition state theory. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U25.00006: Brownian Dynamics Simulation of Kinetics of HEX Cylinders to FCC Spheres Transition in ABA Triblock Copolymer in Selective Solvent Minghai Li, Rama Bansil We report Brownian Dynamics simulations on 400 bead-spring chains of triblock copolymer, A$_{10}$B$_{10}$A$_{10}$, in a selective solvent for the A block using a FENE potential together with Lennard-Jones (LJ) for B-B attraction and a Weeks-Chandler-Anderson potential for A-B and A-A repulsions. On varying volume fraction and temperature T (in units of $\varepsilon $/k$_{B}$, where $\varepsilon $ is the well depth of the LJ interaction and k$_{B}$ the Boltzmann constant) we observe spheres in cubic phases, HEX cylinders, worm-like and disordered micelles. The time evolution following a quench from T= 0.8 to 0.5 shows a nucleation and growth mechanism where one cylinder breaks into spheres and induces neighboring cylinders to break into spheres. This observation is confirmed by calculating the density profile of each cylinder and Fourier transform of the density distribution. We also performed jumps at constant T = 0.8 by changing the LJ well depth ($\varepsilon )$ from 1 to various higher values. We found that for $\varepsilon \quad >$ 4 the cylinders are kinetically trapped, and the transition is fastest for $\varepsilon $ = 1.5. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U25.00007: Thermal and Mechanical Properties of Polymer Nanofibers from Molecular Simulations Sezen Curgul, Krystyn J. Van Vliet, Gregory C. Rutledge Polymer nanofibers exhibit new, emergent behavior as the diameter of the fibers are decreased from macroscopic to nanometer length scales. Since individual nanofibers are challenging to characterize experimentally due to their small size, computer simulations can be helpful in predicting the properties. We present the results of molecular dynamics (MD) simulations of polymer nanofibers to study their size-dependent properties. The fibers mimic the prototypical polymer polyethylene and have diameters in the range 2.0 to 23.0 nm. The fibers have been analyzed size dependent behavior in their thermal and mechanical properties. The glass transition temperature (Tg) of these amorphous nanofibers decreases with decreasing fiber diameter, and is independent of molecular weight over the range considered. Application of a volume averaged layer model for Tg shows that the cooperativity length scale compares well with previous estimates for polyethylene. Young's moduli of these nanofibers also decrease with decreasing fiber diameter, in agreement with T$_{g}$ depression. There is a significant decrease in modulus when the temperature increases above the glass transition temperature of the surface layer. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U25.00008: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U25.00009: Predicting glass transition temperatures from simulation studies Solomon Duki, Philip Taylor We have been seeking techniques by means of which the glass transition temperature $T_g$ of a polymer can be predicted with minimal computational effort. With this goal in mind, the glass transition in syndiotactic poly(methyl methacrylate) was studied through atomistic molecular-dynamics simulations performed at temperatures in the range from 320 K to 700 K. The mean squared deviations of atoms, monomers, and molecules from their initial positions were analyzed by several different techniques. The most direct method looks at the long-time diffusive motion, and detects a characteristic change in the diffusion constant at $T_g$. This approach required lengthy computer runs to achieve meaningful results. Other techniques study the velocity correlation functions and the short-time vibrational motion. All three yield identical values for the glass transition temperature, but it is found that the method that is most economical of computing resources is the analysis of the short-time departure from ballistic behavior. The apparent softening of the ``cage'' in which a monomer or chain segment oscillates coincides with the onset of diffusive motion. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U25.00010: Band Structure Controlled by Chiral Imprinting Adrian Reyes Cervantes, P. Castro-Garay, Ruben Ramos-Garcia Using the configuration of an imprinted cholesteric elastomer immersed in a racemic solvent, we find the solution of the boundary--value problem for the reflection and transmission of incident optical waves due to the elastomer. We show a significant width reduction of the reflection band for certain values of nematic penetration depth, which depends on the volume fraction of molecules from the solvent, whose handedness is preferably absorbed. The appearance of nested bandgaps of both handednesses during the sorting mixed chiral process is also obtained. This suggests the design of chemically controlled optical filters and optically monitored chiral pumps. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U25.00011: Coarse-graining and Multiscale Modeling of Polymeric Materials Ivan Lyubimov, Marina Guenza Dynamics of macromolecules are characterized by the presence of several length scales in which relevant phenomena take place. Theoretical models play a pivotal role in building the infrastructure that allows one to model multiscale properties. Starting from the Ornstein- Zernike equation we derive analytical methods that coarse-grain the structure of polymeric liquids (homopolymer melts, diblock copolymers, and polymer mixtures) at different length scales of interest. These methods provide effective potentials input to mesoscale simulations. Information obtained from simulations, performed at the united-atom and at the coarse-grained scales, is combined in an original multiscale modeling procedure, resulting in the complete physical picture of the system across the many length scales of interest. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U25.00012: Static properties of equilibrium polymers confined in ultrathin films Anna Cavallo, Joachim P. Wittmer, Albert Johner, Joerg Baschnagel The static properties of equilibrium polymer melts confined in ultrathin films are studied by means of Monte Carlo simulations of a lattice model: the bond fluctuation model. In this work we focus on the effects of ultrathin film confinement between two parallel and neutral walls on chain size and molecular weight distribution. We compare our numerical results to analytical calculations by Semenov and Johner [Eur. Phy. J. E, 12, 469 (2003)] who predicted for ultrathin films, logarithmic corrections to the leading mean-field behavior. Our simulation data are compatible with the theoretical results. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U25.00013: Promotion of the Polyfluorene Beta-Phase: A First Principles Study Elizabeth M. Lupton, Feng Liu, David G. Prendergast, Jeffrey B. Neaton Two configurations of polyfluorenes - potentially important for their blue emission properties in organic devices - have been identified in single molecule spectroscopy experiments: a glassy phase with random torsional angles between fluorene units, and a planar beta-phase. The twisted conformtaion is known to be the lowest energy structure, and the factors which promote the stabilization of the beta-phase are unclear. We present a density functional theory study of ways in which polyfluorene molecules could be manipulated to favor the formation of the photophysically more stable beta-phase. Extension along the molecular axis, which increases the stability of the planar conformation relative to the glassy phase, and the role of side groups are examined in fluorene oligomers and a polyfluorene infinite in the molecular axis. Implications for excited state properties, including fluorescence, will be discussed in the context of these results. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U25.00014: Atomic structures and electronic properties of poly(3-hexyl thiophene) on ZnO(110-1) surface. Sefa Dag, Lin-Wang Wang The atomic structures of adsorbed poly(3-hexyl thiophene) (P3HT) polymers on the non-polar ZnO surface (110-1) are studied with molecular dynamics using ab initio adjusted atomic force fields, and the electronic structures of the resulting systems arc studied with direct ab initio calculations. We investigated different P3HT attachment orientations on the ZnO surface. We also studied the influence of the crystallization among the P3HT polymers to the polymer - ZnO attachment. We found that the strength of the attachment depends strongly on the P3HT crystal orientation, and to the partial charge of the surface Zn, O atoms and the end atoms of the P3HT. We studied the temperature dependence of the attachment, and the effects of the details of the attachment atomic structures to the electronic properties of the interface. \newline \newline This work is supported by U.S. Department of Energy, BES, under contract No. DE-AC02-05CH11231 and it used the resource of the National Energy Research Scientific Computing Center. \newline [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U25.00015: Ab initio study of a promizing class of copolymers for application to high-efficiency photovoltaics Jean Fr\'ed\'eric Laprade, Michel C\^ot\'e In order to achieve high power conversion efficiency in bulk-heterojunction solar cells using PCBM as electron acceptor, it is essential to identify an electron donor polymer which i) harvests the largest part of the solar spectrum and ii) shows an electronic structure appropriate to PCBM. In the last few years, different groups synthetized copolymers based on either fluorene$^{i}$, carbazole$^{ii}$ or dibenzosilole$^{iii}$ with interesting results. This presentation will report the results of density-functional theory (DFT) and time-dependant density-functional theory (TDDFT) calculations on those copolymers and their units in order to better assess the impact of changing the fluorene's 9-atom on the electronic properties. We will focus our discussion on the interplay of the counits on the energy levels and on the oscillator strength of the oligomers. $^{i}$ O. Ingan\"{a}s {\&} \textit{al}. \textit{Appl. Phys. A}, \textbf{2004}, 79, 31 $^{ii}$ N. Blouin {\&} \textit{al}., \textit{Adv. Mater.}, \textbf{2007}, 19, 2295 $^{iii}$ P.L.T. Boudreault {\&} \textit{al}, Macromol. Rapid Commun., \textbf{2007}, 28, 2176 [Preview Abstract] |
Session U26: Focus Session: Advances in Atmospheric Aerosol Science II
Sponsoring Units: DCPChair: Ruth Signorell, University of British Columbia
Room: Morial Convention Center 218
Thursday, March 13, 2008 8:00AM - 8:36AM |
U26.00001: Chemistry of Individual Atmospheric Particles Invited Speaker: Aerosols are widely recognized as key elements in atmospheric environment. Chemical and morphological data of individual particles are of crucial importance for understanding of their formation, reactions, atmospheric history and aging. Microprobe analytical techniques have been extensively used in the past to characterize the size, morphology, phase and composition of particles collected in field and laboratory studies. These technique coupled with an appropriate time-resolved aerosol sampling are capable of generating time-resolved single-particle data, which then can be used to follow in detail the time evolution of specific types of aerosols. In this presentation we give a summary of recent research projects carried out in our laboratory that demonstrates how the use of complementary microprobe methods provides new insights into the atmospheric reactions of aerosols, their physical and chemical transformations, and in particular how the obtained data can be utilized to define future directions in laboratory and field studies of aerosols. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U26.00002: Heterogeneous oxidation reactions relevant to tropospheric aerosol chemistry studied by sum frequency generation Grace Stokes, Avram Buchbinder, Julianne Gibbs-Davis, Karl Scheidt, Franz Geiger Unsaturated organic molecules (terpenes) that commonly form molecular films on tropospheric aerosols can be oxidized by ozone, influencing the microphysics of cloud formation and thus the earth's climate. Using a laboratory approach that combines organic synthesis with surface spectroscopy, we track the ozone oxidation reactions of tropospherically relevant terpenes bound to glass surfaces that serve as mimics for mineral dust. Specifically, vibrational broadband sum frequency generation (SFG) is used to study a number of tailor-made terpene-modified glass surfaces and to track their interactions with ozone in real time. Exposure of these surfaces to ppm levels of ozone at 1 atm and 300 K yield initial reaction probabilities that are significantly higher than corresponding gas phase reactions. SFG spectra help elucidate the molecular orientations of the surface-bound terpenes and the accessibility of reactive C=C bonds. Our work shows the successful use of SFG spectroscopy to determine heterogeneous atmospheric reaction probabilities and bridges the gap between atmospheric aerosol science and surface spectroscopy. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U26.00003: Oxidation of the PAH Coronene by Ozone and OH Radical Erin Mysak, Jared D. Smith, John T. Newberg, Kevin R. Wilson, Hendrik Bluhm Reactivity of the polycyclic aromatic hydrocarbon (PAH) coronene to oxidation sources ozone and OH radical is examined. To probe the extent of chemical reaction, product formation, and change in surface morphology as a function of reaction, we examine coronene, adsorbed onto various substrates, from both a surface and bulk perspective, with ambient pressure photoemission spectroscopy (APPES) and aerosol mass spectrometry (AMS), respectively. For bulk on-line analysis, a 20nm thick layer of coronene adsorbed onto NaCl seed particles and reacted with either oxidant in a flow tube showed very little reactant conversion to product in the AMS. However, surface analysis by the APPES of the same reaction where coronene was adsorbed onto model substrates showed up to 50 per cent conversion of the carbon species to oxidized carbon, depending on coronene layer thickness (about 1.5-14A). Data obtained with these complimentary techniques provide evidence for a surface selective reaction. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U26.00004: Morphological effects on coated aerosol kinetics Elias Rosen, Eva Garland, Tomas Baer The fate of organic material in the atmosphere can be strongly dependent on the chemical environment under which oxidation takes place. We have investigated the reaction of gas-phase ozone and oleic acid adsorbed to the surface of polystyrene latex spheres and silica aerosols to better understand how the substrate influences heterogeneous kinetics. Flow tube experiments were performed with an Aerosol Time of Flight Mass Spectrometer using a two-laser vaporization/ionization scheme to minimize analyte fragmentation. Pseudo-first-order rate coefficients suggest that ozonolysis of oleic acid proceeds differently on the hydrophobic latex and hydrophilic silica particles. Mechanistic interpretation of these results has been complicated by the discovery that the morphology of oleic acid vapor deposition onto both particle types is non-uniform, which results in the formation of discrete areas of organic material on the particle surface as evidenced by AFM and SEM/EDS measurements. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:48AM |
U26.00005: Optical and microphysical properties of organic multicomponent aerosol particles Invited Speaker: Atmospheric aerosols affect Earth's climate in direct and indirect manners. The direct effect of aerosols on climate is by scattering and/or absorbing incoming solar and outgoing terrestrial radiation, which strongly modify Earth's radiation budget. In addition, aerosols acting as cloud condensation nuclei (CCN) indirectly affect climate and precipitation by modifying the microphysical properties of clouds and cloud coverage. These climatic effects depend on the chemical composition, size and morphology. We will present laboratory studies aiming at understanding how the organic component of atmospheric affect the climate system Specifically, we present the use of cavity ring down (CRD) spectrometer to derive the extinction and complex refractive index of aerosols containing a significant organic component. By precisely measuring extinction as a function of particle size the real and imaginary refractive indices are obtained, and the single scattering albedo may be calculated.. We will present results on aerosol particles containing humic like substances (HULIS). HULIS are a common component of aerosols in the atmosphere. They contribute to the CCN activity, hygroscopic properties and the density of aerosols. In addition, HULIS absorb throughout the visible range, and hence contribute to the direct climatic effect of aerosols. The absorption by organic aerosols is largely unaccounted for in models. Specifically, we will present how the absorption of aerosols containing HULIS and inorganic salts varies with wavelength, test various optical mixing rules and will present results on the extinction of core-shell particles. In addition, we will discuss how the presence of HULIS affects the surface tension of CCN at activation and of cloud droplets and its implications. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U26.00006: Oxidation of oleic acid monolayers at air/liquid interfaces Laura Voss Field studies of marine and continental aerosols find that fatty acid films form on aqueous tropospheric aerosols. Oxidation of oleic acid monolayers by ozone was studied to understand the fate of fat-coated aerosols from both fresh and salt water sources. Using vibrational sum frequency generation spectroscopy and reflection absorption infrared spectroscopy, we present a molecular-level investigation of fatty acid monolayers at the air-water and air-sodium chloride solution interface and explore reactions with atmospheric oxidants by these model systems. Coupling sum frequency generation spectroscopy with a Langmuir trough, concurrent spectroscopic and thermodynamic data were collected to obtain a molecular picture of the monolayers. No substantial difference was observed between oxidation of monolayers spread on water and on 0.6 molar sodium chloride solutions. Results indicate that depending on the size of the aerosol and the extent of oxidation, the subsequent oxidation products may not remain at the surface of these films, but instead be dissolved in the aqueous sub-phase of the aerosol particle. Results also indicate that oxidation of oleic acid could produce monolayers containing species that have no oxidized acyl chains. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U26.00007: Chemical and Spatial Microscopy of Individual Organic Aerosols Alexei V. Tivanski, Rebecca J. Hopkins, Mary K. Gilles Carbonaceous particles originating from biomass burning can account for a large fraction of organic aerosols in a local environment. Presently, their composition, physical, and chemical properties as well as their environmental effects are largely unknown. A distinct type of biomass burn particles, called ``tar balls'', have been observed in a number of field campaigns, both in fresh and aged smoke. They are characterized by their spherical morphology, high carbon content and ability to efficiently scatter and absorb light. Here, a combination of scanning transmission x-ray microscopy and near edge x-ray absorption fine structure spectroscopy is used to determine the shape, structure and size-dependent chemical composition of 150 individual tar ball particles ranging in size from 0.15 to 1.2 $\mu $m. Oxygen is present primarily as carboxylic carbonyls and oxygen-substituted alkyl functional groups. The observed chemical composition is distinctly different from black carbon and more closely resembles high molecular weight humic-like substances. A detailed examination of the carbonyl intensity as a function of particle size reveals the presence of a thin oxygenated interface layer on the tar balls, indicative of atmospheric processing of biomass burn particles. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U26.00008: OH oxidation of organic aerosols. Jared Smith, Erin Mysak, Musa Ahmed, Christopher Cappa, Stephen Leone, Kevin Wilson Ambient aerosols play a significant role in a variety of atmospheric processes such as direct and indirect effects on radiative forcing. Chemical composition can be an important factor in determining the magnitude of these effects. However, a major fraction of organic aerosols (OA) can not be resolved on a molecular level. Recent identification of high mass oligomeric species as a major component in laboratory and ambient OA has received much attention due to the possibility that these species may account for much of the unknown organic mass in ambient aerosols. Although, a few mechanisms have been proposed, the origin and formation processes of these compounds remain largely unknown. Here we provide strong evidence for a previously unidentified mechanism of extremely rapid oligomer formation, via OH radical initiated oxidation of OA. This process appears capable of converting a sizable fraction of an organic particle to higher mass oligomers within a day of exposure to OH radicals at typical atmospheric concentrations. Furthermore, we have found that rapid volatilization is also important for specific reaction systems, and can lead to the loss of a large fraction of the particle mass. We propose that such a rapid processing is possible due to a radical chain reaction which quickly propagates throughout the entire particle and is only initiated by the surface OH reaction. [Preview Abstract] |
Session U27: Correlated Electrons: Theory, Actinides, and Ising Systems
Sponsoring Units: DCMPChair: Makariy Tanatar, University of Sherbrooke
Room: Morial Convention Center 219
Thursday, March 13, 2008 8:00AM - 8:12AM |
U27.00001: Quantitative Calculation of the Spatial Extension of the Kondo Cloud Bergmann Gerd A recently developed compact solution for the singlet state of the Friedel-Anderson and the Kondo impurity is applied to investigate the old question of a Kondo cloud in the Kondo ground state. Wilson's states with an exponentially decreasing frame of energy cells towards the Fermi level are used. The Wilson states are expressed as free electron waves with a linear dispersion and integrated over the width of their energy cells. For the magnetic state of the Friedel-Anderson impurity one finds essentially no spin polarization in the vicinity of the d-impurity. However, for the magnetic \emph{component} of the singlet state a spin polarization cloud is observed which screens the spin (magnetic moment) of the d-electron. The range $\xi_{K}$ of this polarization cloud is investigated in detail for the Kondo impurity. The range is inversely proportional to the Kondo energy $\Delta _{K}$. The extent of the electron density in real space is a detector for a resonance in energy. The spatial extension $\xi $ and the resonance width $\Delta $ are reciprocal and given by the relation $\xi \Delta \approx \hbar v_{F}$. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U27.00002: Perturbative Cumulant Monte Carlo Study of LiHoF$_4$ in a Weak Transverse Magnetic Field S.M. Ali Tabei, Michel Gingras, Ying-Jer Kao, Taras Yavorskii Results from a recent quantum Monte Carlo (QMC) (P.B. Chakraborty {\it et al.}, Phys. Rev. B {\bf 70}, 144411 (2004)) study of the LiHoF$_4$ Ising magnetic material in an applied transverse magnetic field $B_x$ show a discrepancy with the experimental results, even for small $B_x$ where quantum fluctuations are small. This discrepancy persists asymptotically close to the classical ferromagnet to paramagnet phase transition. We numerically reinvestigate the phase diagram of LiHoF$_4$ in the regime of weak $B_x$. In this regime, we derive an effective temperature-dependent classical Hamiltonian that incorporates perturbatively the small quantum fluctuations in the vicinity of the classical phase transition at $B_x=0$. Via this effective classical Hamiltonian, we study the $B_x-T$ phase diagram via classical Monte Carlo simulations. In particular, we investigate the influence of various effects that may be at the source of the discrepancy. We also show how our method can be generalized to numerically study the diluted LiHo$_x$Y$_{1-x}$F$_4$ in the small $B_x$ regime. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U27.00003: A New Heavy Fermion Compound Yb$_{3}$Pt$_{4}$ Marcus Bennett, Peter Khalifah, Dmitriy Sokolov, Yiu Yuen, Moosung Kim, Carl Henderson, William Gannon, Meigan Aronson We report the synthesis of single crystals of a new binary heavy fermion system, Yb$_{3}$Pt$_{4}$. Magnetic susceptibility measurements find Yb$^{3+}$ local moment behavior above 150 K. Heat capacity measurements find a large weakly first order anomaly at 2.4 K, and the associated entropy indicates that magnetic order emerges from a doublet ground state. Magnetic field suppresses both the magnitude of the anomaly and the temperature at which the anomaly occurs, mapping out a first order phase line that ends at a tri-critical point, 1.75 T, 1.3 K. A weak cusp in the AC magnetic susceptibility indicates antiferromagnetic ordering. Above 0.2 T, the cusp becomes a step, which increases in height with increasing field indicating ferromagnetic order. The electrical resistivity of Yb$_{3}$Pt$_{4}$ is that of a good metal, and the quadratic temperature dependence of a Fermi liquid is found throughout the antiferromagnetically ordered state and continues into the high field paramagnetic state. Both the magnitude of the quadratic temperature dependence of the resistivity and of $\gamma$ are comparable to that found in heavy fermion compounds, indicating substantial quasiparticle mass enhancement. The Sommerfeld-Wilson ratio approaches 30 in the ordered state, suggesting strong ferromagnetic correlations among the quasiparticles. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U27.00004: Pressure and magnetic field effects in heavy-fermion UCu$_{3.5}$Al$_{1.5 }$ A. Alsmadi, H. Nakotte, V. Zapf, F. Fabris, T.D. Didn, A. Lacerda, J. Kamarad UCu$_{3.5}$Al$_{1.5}$ crystallizes in the hexagonal CaCu$_{5}$ structure and is described as a heavy fermion, which shows non-Fermi liquid behavior[1]. Here, we report on electrical resistivity, magnetic susceptibility, and magnetization results on polycrystalline UCu$_{3.5}$Al$_{1.5}$. The resistivity was measured under hydrostatic pressure up to 10kbar and in fields up to 18T. At ambient pressure and in zero field, the resistivity shows an anomaly at T$_{1}$=19K and then it goes through a maximum at T$_{max}$=2K. These two anomalies were also observed in the susceptibility data. The anomaly in the resistivity at T$_{1}$ goes to lower values with increasing fields and disappear at fields about 12T. T$_{max}$ on the other hand goes to higher values with increasing fields. We find relatively weak pressure dependence, where both T$_{1}$ and T$_{max}$ go down with increasing pressure. In the field scan at 2K and at ambient pressure, we find a change in the slope of the magnetoresistance at about 6.9T. Application of pressure causes a reduction of the magnetoresistance effect. \newline [1] H. Nakotte, et al., Phys. Rev. B 54, 12176 (1996) [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U27.00005: Optical spectra of the heavy fermion uniaxial ferromagnet UGe2 Violeta Guritanu, Peter Armitage, Riccardo Tediosi, Siddharth Saxena, Andrew Huxley, Dirk van der Marel We report on a detailed optical study of UGe$_{2}$ single crystalline material using infrared reflectivity and spectroscopic ellipsometry. The optical conductivity suggests the presence of a low frequency interband transition (~ 300 $cm^{-1}$) and a narrow free-carrier response with strong frequency dependence of the scattering rate and effective mass. We observe sharp changes in the low frequency mass and scattering rate below the upper ferromagnetic transition T$_{C_{1}}$. They recover their unrenormalized value above T$_{C_{1}}$ and for $\omega >$ 250 cm$^{-1}$. In contrast no sign of an anomaly is seen at T$_{C_ {2}} \sim$ 30 K, which is the lower transition of unknown nature. These observations are consistent with the weak anomaly observed at T$_{C_{2}}$ in transport and thermodynamic experiments. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U27.00006: Ferromagnetism of Silicon Doped with Uranium Investigated to Extremes of Magnetic Field (Beyond 100 tesla) Charles Mielke, Jason Cooley, William Hults The ferromagnetic (FM) phase of Si:U x At. \% (where x = 0.25, 0.5, 0.75, 1.0, and 50.0 (i.e. USi)) were studied in high magnetic fields as a function of temperature and U concentration. The effect of doping U into Si is investigated vis a vis the FM transition temperature (127K for x = 50.0) and high magnetic field saturation is discussed. The effect of the FM transition temperature is approached from the point of view of correlation effects in f-electron systems. Attention to the high magnetic field saturation is investigated as it is unusually high in the x = 50.0 intermetallic compound. Ultra-high field data extending to 185 tesla is reported for the system. Issues with homogenization of the dilute samples are presented as well. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U27.00007: Electronic Structure of Actinide Materials J.J. Joyce, T. Durakiewicz, K.S. Graham, D.P. Moore, L.A. Morales, J.M. Wills, R.L. Martin, J.-X. Zhu, L.E. Roy, C.G. Olson, G.E. Scuseria, I.D. Prodan Photoelectron spectroscopy results for both metallic and insulating actinide materials are reviewed and compared against model calculations. The dual nature of 5f electron characteristics is discussed for photoemission results and three different electronic structure calculations. Magnetic configurations as a means of f-electron localization are discussed for metallic materials. The photoemission results for U and Pu intermetallics are compared against mixed-level-model and dynamical-mean-field-theory calculations. The experimental results for the actinide oxide Mott insulators are compared against screened hybrid functional calculations. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U27.00008: Effects of full Coulomb interactions on electronic structure of \textit{$\delta $}-Pu Eugeny Gorelov, Tim Wehling, Hartmut Hafferman, Alexander Lichtenstein, Alexey Rubtsov, Alexander Landa, Chris Marianetti, Michael Fluss, Alexey Shorikov, Alexey Lukoyanov, Michael Korotin, Vladimir Anisimov We used the CTQMC method for the realistic simulation of electronic properties of correlated actinides. In particular, we focus on the spectral function of \textit{$\delta $}-Pu, which is described in terms of a 7-orbital $f$-impurity model interacting with a metallic bath. Our CTQMC implementation solves this model by calculating a weak coupling expansion of the partition function in the fermionic multiorbital path-integral representation and provides numerically exact results for relatively high temperature. We discussed how different terms in the full on-site Coulomb vertex affect the local density of states. The comparison of CTQMC results with only diagonal density-density like Coulomb interactions and with additional non-diagonal terms in the interaction part of the Hamiltonian related with so-called spin flips terms shows the importance of the full rotationally invariant Coulomb vertex on the electronic structure of \textit{$\delta $}-Pu. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U27.00009: First-principles study of electronic structure and local moment interactions in PuAm alloy Myung Joon Han, Xiangang Wan, Sergej Y. Savrasov Expected to provide a clue about the origin of zero moment in the bulk phase of Plutonium, Pu$_{1-x}$Am$_{x}$ alloy has attracted a great attention, in which as Am ratio, $x$, increases, Pu approaches from bulk to atomic limit. To understand the electronic structure and the magnetic properties of Pu in different crystal environments, we perform fully self-consistent first-principles calculations of the PuAm system based on the density functional theory. Electronic structure strongly depends on the level of approximation for correlation effects. The exchange interactions between Pu 5f electrons and the Kondo screening strength were estimated and compared, which provide a new insight to Pu magnetism. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U27.00010: Knight shifts around nonmagnetic impurities in a trinagular lattice spin 1/2 antiferromagnet: Case of $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$ Karol Gregor, Olexei Motrunich We study effects of nonmagnetic impurities in a spin-1/2 frustrated triangular antiferromagnet with the aim of understanding the observed broadening of $^{13}$C NMR lines in the organic spin liquid material $\kappa$-(ET)$_2$Cu$_2$(CN)$_3$. For high temperatures down to $J/2$, we calculate local susceptibility near a nonmagnetic impurity and near a grain boundary for the nearest neighbor Heisenberg model in the high temperature series expansion. We find that the local susceptibility decays to the uniform one very quickly (few lattice spacings), and with the suggested density of impurities would not explain the observed line broadening present already at elevated temperatures; more extended defects and/or longer-ranged interactions are probably needed. At low temperatures, we assume a gapless spin liquid with a Fermi surface of spinons. We calculate the local susceptibility in the mean field and also go beyond the mean field by Gutzwiller projection. Here the Knight shift decays with a power law and oscillates at $2 k_F$. However, with single site impurities the results fall short of the observed inhomogeneous broadening, calling for a better understanding of the appropriate models for the spins and impurities and of the possible ground states that are probed by such experiments. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U27.00011: Excitons in the 1D Hubbard Model: a Real-Time Study Khaled Al-Hassanieh, Adrian Feiguin, Fernando Reboredo, Ivan Gonzalez, Elbio Dagotto We study the real-time dynamics of a pair hole/doubly-occupied-site, namely a holon and a doublon, in a 1D Hubbard insulator with on-site and nearest-neighbor Coulomb repulsion. Our analysis shows that the pair is long-lived and the expected decay mechanism to underlying spin excitations is actually inefficient. For a nonzero inter-site Coulomb repulsion, we observe that part of the wave-function remains in a bound state. Our study also provides insight on the holon-doublon propagation in real space. Due to the one-dimensional nature of the problem, these particles move in opposite directions even in the absence of an applied electric field. The potential relevance of our results to solar cell applications is discussed. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U27.00012: Investigation of Dipole-Forbidden $d-d$ Excitations in Strongly Correlated Transition-Metal Oxides Using Higher-Order Multipole, Non-resonant Inelastic X-Ray Scattering B.C. Larson, J.Z. Tischler, C.-L. Yeh, C.-C. Lee, Wei Ku We have shown that quadrupole and higher order multipole non-resonant inelastic x-ray scattering (NIXS) at large wavevectors, $q$, provides direct access to dipole-forbidden $d-d$ excitations (Larson et al. \textit{Phys. Rev. Lett.} \textbf{99}, 026401 (2007)). NIXS measurements using the XOR/UNI beamline at the APS have shown that the large-$q $intensity of on-site excitons in NiO and CoO is highly anisotropic in $q$ and dominates the energy loss spectrum. Energy-resolved Wannier function analyses have shown that the anisotropies, including a nodal direction for NiO, provide direct information on the point-group symmetry of the particle-hole wave functions for transition-metal oxides. The interpretation of these large-$q$ NIXS measurements will be discussed in connection with energy-resolved Wannier function analyses and LDA+$U$ dynamical response calculations. Implications for the extension of such investigations to manganite systems will be considered. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U27.00013: Exact many-electron ground states on the diamond Hubbard chain Zsolt Gulacsi, Arno Kampf, Dieter Vollhardt Exact ground states of interacting electrons on the diamond Hubbard chain in a magnetic field are constructed which exhibit a wide range of properties such as flat-band ferromagnetism, correlation induced metallic, half-metallic, or insulating behavior [1]. The properties of these ground states can be tuned by changing the magnetic flux, local potentials, or electron density.The results show that the studied simple one-dimensional structure displays remarkably complex physical properties. The virtue of tuning different ground states through external parameters points to new possibilities for the design of electronic devices which can switch between insulating or conducting and nonmagnetic or (fully or partially spin polarized) ferromagnetic states, open new routes for the design of spin-valve devices and gate induced ferromagnetism. \break [1] Z. Gulacsi, A. Kampf, D. Vollhardt, Phys. Rev. Lett. {\bf 99}, 026404(2007). [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U27.00014: Phonon effect on elementary excitations in one-dimensional Mott insulators Hiroaki Matsueda, Akihiro Ando, Takami Tohyama, Sadamichi Maekawa We examine the interplay among phonons and elementary excitations in one-dimensional Mott insulators. For this purpose, we perform dynamical density matrix renormalization group calculations for the single-particle excitation and optical absorption spectra in the extended Hubbard-Holstein model. We find that the elementary excitations in the Mott insulators are quite easily modified by the phonons. In particular, the spinon branch in the single-particle excitation spectrum can be broadened, even when the phonons only couple with charge degrees of freedom. In addition, the exciton in the photoexcited state becomes polaronic, and this polaronic feature is enhanced by increasing the on-site Coulomb repulsion. We show the origins of these novel properties, and discuss implications of the present results in light of spectroscopic measurements in 1D cuprates. [Preview Abstract] |
Session U28: Focus Session: Semiconductor Qubit Approaches II
Sponsoring Units: DCMPChair: Malcolm Carroll, Sandia National Laboratories
Room: Morial Convention Center 220
Thursday, March 13, 2008 8:00AM - 8:12AM |
U28.00001: Radio-frequency single-electron transistor coupled to few-electron double quantum dot Feng Pan, Joel Stettenheim, Mustafa Bal, Weiwei Xue, Zhongqing Ji, Alexander Rimberg, L.N. Pfeiffer, K.W. West The radio frequency single-electron transistor (rf-SET) has been shown to be an ultra fast and highly sensitive electrometer, and also has been used as a qubit readout device operated close to the quantum noise limit [1]. The interplay between the rf-SET electrometer and a two-level system offers an interesting system for study. Here we report our progress on investigating rf-SETs capacitively coupled to few-electron double quantum dots (DQDs). We fabricate lateral-defined DQDs from an AlGaAs/GaAs heterostructure and the rf-SET from superconducting aluminum embedded in a tank circuit. The sensitivity and bandwidth of on-chip rf-SET electrometer can be used to probe DQD operated in the few-electron regime. Alternatively, the DQD can be used as high-frequency quantum noise detector to probe SET operation in the subgap region [2,3]. Recent experimental results will be discussed. [1] M. H. Devoret and R. J. Schoelkopf, Nature, \textbf{406}, 1039 (2000). [2] R. Aguado and L. P. Kouwenhoven, Phys. Rev. Lett., \textbf{84}, 1986 (2000). [3] O. Naaman and J. Aumentado, Phys. Rev. Lett. \textbf{98}, 227001 (2007). [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U28.00002: Decoherence of coupled spin qubits due to charge fluctuations Guy Ramon, Xuedong Hu One of the significant advantages of using the spin of quantum dot electrons as a qubit rather than their charge is their relative insulation from the environment. A number of recent works have utilized two-spin singlet and unpolarized triplet states in biased configuration to encode a logical qubit, which offer better control and coherence properties as compared with single spin states. When spin states are exchange coupled, however, they are potentially vulnerable to environmental fluctuations affecting charge qubits, since exchange coupling is electrostatic in nature. Here we carry out a quantitative calculation of the coupling between a biased two-spin qubit and a nearby charge fluctuator represented by a two-level-system (TLS), utilizing a multipole expansion up to and including the Quadrupole-Quadrupole order. The resulting coupling terms are used in a master equation formalism to study the dynamics of the open system that is formed by the spontaneous emission of the TLS coupled to the vacuum. We are thus able to provide a reliable estimate of the decoherence effects during various gate operations on the spin qubit as a function of the geometry and other characteristics of the system. Possible ways to alleviate the sensitivity of coupled spin qubits to charge fluctuations are also discussed. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U28.00003: Hyperfine-mediated gate-driven electron spin resonance Edward Laird, Christian Barthel, Emmanuel Rashba, Charles Marcus, Micah Hanson, Art Gossard An all-electrical spin resonance effect in a GaAs few-electron double quantum dot is investigated experimentally and theoretically. The magnetic field dependence and absence of associated Rabi oscillations are consistent with a novel hyperfine mechanism. The resonant frequency is sensitive to the instantaneous hyperfine effective field, and the effect can be used to detect and create sizable nuclear polarizations. A device incorporating a micromagnet exhibits a magnetic field difference between dots, allowing electrons in either dot to be addressed selectively. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 9:12AM |
U28.00004: Cluster Techniques to Study Spin Decoherence in a Spin Bath Invited Speaker: Noisy nuclear spin environments in many solid state materials pose a serious threat to the feasibility of solid-state spin quantum computation where localized electron spins, as qubits, may interact with millions of lattice nuclei [1, 2, 3, 4]. Such nuclear induced decoherence may be partially reduced through the application of a strong magnetic field that suppresses electro-nuclear flip-flops (due to a large mismatch of their gyromagnetic ratios). However, even in the limit where electro-nuclear flip-flops are completely suppressed, dephasing decoherence, known as spectral diffusion, occurs as a result of fluctuations of the nuclear field that is caused by dipolar (or other) interactions among the nuclear bath spins. While a direct approach to this problem is impossible due to the intractable Hilbert space of many interacting spins, we have devised a cluster method to formally solve this problem. Direct application of perturbation theories are futile due to the large size of the bath. Perturbation methods become effective, however, in the cluster expansion framework. These techniques will be discussed and qubit decoherence calculation results will be shown, including effects of dynamical decoupling pulse sequences [5, 6] that prolong qubit coherence. [1] W. M. Witzel, Rogerio de Sousa, S. Das Sarma, Phys. Rev. B 72, 161306(R) (2005). [2] W. M. Witzel, S. Das Sarma, Phys. Rev. B 74, 035322 (2006). [3] W. M. Witzel, S. Das Sarma, Phys. Rev. Lett. 98, 077601 (2007). [4] W. M. Witzel, Xuedong Hu, S. Das Sarma, Phys. Rev. B 76, 035212 (2007). [5] W. M. Witzel, S. Das Sarma, arXiv:0707.1037. [6] B. Lee, W. M. Witzel, S. Das Sarma, arXiv:0710.1416. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U28.00005: Spin-Dependent Scattering off Neutral Donors in Silicon Field-Effect Transistors C.C. Lo, J. Bokor, T. Schenkel, J. He, A.M. Tyryshkin, S.A. Lyon One promising route towards single donor spin readout for donor qubits in silicon is by detecting spin-dependent scattering of conduction electrons by the neutral donors. We use accumulation-mode field-effect transistors formed in isotopically enriched silicon to study this effect. Only small ensembles of donor spins are present in our devices, ruling out bolometric effects caused by bulk donors. Spin-dependent scattering was detected using electrically detected magnetic resonance (EDMR) where spectra show resonant changes in the source-drain voltage for conduction electrons and electrons bound to donors. The utilization of spin-dependent scattering for the readout of donor spin-states in silicon based quantum computers will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U28.00006: Time-Reversal Symmetry and Electron Spin Relaxation of Lithium Donors in Silicon V.N. Smelyanskiy, A.G. Petukhov, A.M. Tyryshkin, S.A. Lyon We report theoretical and experimental studies of longitudinal electron spin relaxation time of interstitial shallow Li donors in Si. Ground state of a donor electron has a unique ten-fold (near) degeneracy which is only slightly lifted by stresses, magnetic field and spin-orbital interaction. Despite this degeneracy, we predict, on the basis of the time-reversal symmetry and weakness of the umklapp phonon processes, an extraordinary long relaxation times for lithium donor electron spin for the temperatures below 0.3 K. Strong temperature dependence of the spin relaxation time is due to activation-type processes with several activation exponents. Experimentally observed traces of magnetization reversal and longitudinal spin relaxation times at $T=$ 2.1~K and $T=4.5$~K are in remarkably close agreement with the theory. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U28.00007: Spin relaxation of exchange-coupled donors in silicon A.M. Tyryshkin, S. Shankar, S.A. Lyon The exchange interaction (J-coupling) between electron spins provides a natural way to accomplish two-qubit operations in a spin-based, solid-state quantum processor. The J-coupling, because of its electrostatic (Coulombic) nature, is susceptible to charge noise in the environment, and hence turning on the J- coupling may cause fast decoherence of the interacting spin qubits. To clarify the effect of J-coupling on spin coherence, we performed spin relaxation measurements for exchange-coupled donors in dimers and trimers randomly formed in bulk-doped natural silicon and isotopically-purified $^{28}$Si. The longitudinal relaxation time, T$_1$, for donors in exchange- coupled dimers is found to be identical to that of isolated donors at temperatures 8-15K, solely determined by a two-phonon Orbach mechanism. The transverse relaxation time, T$_2 $, for dimers is even longer than that of isolated donors because of the lower density of the dimers in our samples and thus substantially reduced dipole-dipole interactions. In natural silicon containing 5\% $^{29}$Si magnetic nuclei, an additional decoherence results from the nuclear-induced spectral diffusion. The spectral diffusion decoherence of J-coupled dimers and trimers is also identical to that of isolated donors. We conclude that J-coupling does not induce any additional decoherence in bulk donors in Si. However, the situation may change for donor dimers placed closer to the surface where more charge noise is expected. Supported by LPS/ARO. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U28.00008: Solid state quantum memory using the $^{31}$P nuclear spin J.J.L. Morton, A.M. Tyryshkin, S. Shankar, A. Ardavan, T. Schenkel, J.W. Ager, S.A. Lyon Nuclear spins benefit from long coherence times compared to electron spins, but are slow to manipulate and suffer from weak thermal polarisation. A powerful model for quantum computation is thus one in which electron spins are used for processing and readout while nuclear spins are used for storage. Here we demonstrate the coherent transfer of an electron spin superposition to the nuclear spin using a combination of microwave and radiofrequency pulses applied to $^{31}$P donors in an isotopically pure $^{28}$Si crystal. The state is left in the nuclear spin on a time scale long compared with the electron T$_2$ and then coherently transferred back to the electron spin, thus demonstrating the $^{31}$P nuclear spin as a solid-state quantum memory. The transfer fidelity is about 84$\%$ each way, attributed to imperfect rotations which could be corrected using composite pulses [JJL Morton et al. Phys Rev Lett 95, 200501 (2005)]. Varying the time for which the state is stored in the nuclear spin permits the direct measurement of the nuclear spin T$_2$, which we have studied in the range 6.5 to 10~K. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U28.00009: Detection of low energy single ion impacts in silicon transistors Christoph Weis, Arunabh Batra, Stefano Cabrini, Jeffrey Bokor, Cheuk Lo, Thomas Schenkel We report a technique for single ion doping of field effect transistors through monitoring of changes in the source-drain currents at room temperature [1]. Implant apertures are formed in the interlayer dielectrics and gate electrodes of planar, micro-scale transistors by electron beam assisted etching. Device currents increase due to the generation of positively charged defects in gate oxides when ions (121Sb12+, 14+, Xe6+; 50 to 70 keV) impinge into channel regions. Implant damage is repaired by rapid thermal annealing, enabling iterative cycles of device doping and electrical characterization. We discuss integration of single ion doping for the development of silicon based quantum computer structures with donor electron and nuclear spin qubits. [1] A. Batra, et al., Appl. Phys. Lett. 91, 193502 (2007) [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U28.00010: Real-time decoherence of hyperfine-coupled electrons in quantum dots Jordan Kyriakidis, Jean-Marc Samson We approach the study of the electron spin decoherence due to the Fermi contact hyperfine interaction with the density matrix formalism of quantum relaxation. We consider an s-type electron in the ground state of a quantum dot interacting with a thermal distribution of nuclear spins. We directly compute the time dependence of the reduced density matrix by solving the system of integro-differential equations resulting from the Liouville equation at the Born (but not Markov) approximation. We show how the spin precession can, under certain circumstances, slow down and even reverse its rotation sense. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U28.00011: Coupling of Mechanical Modes and Shot Noise in a Radio Frequency Quantum Point Contact J. Stettenheim, Feng Pan, Z. Ji, Mustafa Bal, W.W. Xue, Madhu Thalakulam, L.N. Pfeiffer, K.W. West, A.J. Rimberg Interesting interactions exist between the electrical and mechanical degrees of freedom in GaAs quantum nanostructures due to the piezoelectric nature of the substrate. Here, we report measurements in radio-frequency quantum point contacts (RF-QPCs) in which the photon assisted shot noise (PASN) in the vicinity of the carrier wave shows strong frequency dependence correlated with sample dimensions. In particular we find that the rf drive excites surface acoustic waves (SAWs) of frequency $f\approx v_{s}/2l, $where $v_{s}$ = 3010 m/s is the speed of sound in GaAs and $l$ is a length of the device or substrate chip. As a result by varying the sample dimensions, we can manipulate the shot noise spectrum so that the resulting noise is excluded from a chosen bandwidth. Implications for use of the RF-QPC as a fast charge detector will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U28.00012: Intrinsic noise measurement of an ultra-sensitive radio-frequency single electron transistor W.W. Xue, Z. Ji, Feng Pan, A.J. Rimberg The radio-frequency single electron transistor (rf-SET) has been the focus of intense interest since its invention in 1998[1]. Using cryogenic ultra-thin film evaporation techniques [2] and an improved on-chip superconducting matching network [3], we have consistently fabricated rf-SETs with charge sensitivity of 1.7--5$\mu e/\sqrt{\mathrm{Hz}}$ and uncoupled energy sensitivity 1.1--5$\hbar$. Using our 1GHz resonant circuit, intrinsic noise in the SET arising from a dc voltage bias was measured in the white noise limit. We measured the offset charge dependence of the intrinsic noise in the vicinity of the Josephson-quasiparticle and double Josephson-quasiparticle transport cycles. In regions for which the offset charge and resistance noise are strongly suppressed, we can determine the SET shot noise in the sup-gap regime. We discuss the effects of correlations between charge carriers on the measured Fano factor. [1] R.J.Schoelkopf et al., Science 280,1238 (1998); [2] N.A.Court et al., Cond-mat 0706.4150 (2007); [3] W.W.Xue et al., Appl.Phys.Lett. 91, 093511 (2007). [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U28.00013: Coherent electron-phonon states in suspended quantum dots: decoherence and dissipation effects Luis G.C. Rego The dynamics of coherent electron-phonon (el-ph) states is investigated for a suspended quantum dot structure. Exact quantum dynamics calculations reveal that electron and phonons (comprising a thermal bath) couple quantum mechanically to perform coherent oscillations with periods in the range of tens of nanoseconds, despite the finite temperature of the phonon bath. Mechanical energy dissipation due to clamping loss is taken into account in the calculations. Although the lifetime of the coupled el-ph states decreases with the temperature, well defined Rabi oscillations are obtained for temperatures up to 100 mK. The dynamics of the coupled electron-phonon state is susceptible to various forms of external control. For instance, a weak external magnetic field can be used to control the dynamics of the system, by decoupling the electron from the phonon bath. The results cast light upon the underlying physics of a yet unexplored system that could be suitable for novel quantum device applications. [Preview Abstract] |
Session U29: Focus Session: Carbon Nanotubes and Related Materials XII: Graphene Transport
Sponsoring Units: DMPChair: Allan MacDonald, University of Texas
Room: Morial Convention Center 221
Thursday, March 13, 2008 8:00AM - 8:12AM |
U29.00001: Scanning Tunneling Microscopy and Spectroscopy of Rotated Phases on C-Face Epitaxial Graphene Joanna Hass, Nikhil Sharma, Jorge-Enrique Millan-Otoya, Michael Sprinkle, Claire Berger, Walter deHeer, Edward Conrad, Phillip First Diffraction data have shown that multilayer graphene grown on the (000-1) polar face of SiC forms with a high density of rotational stacking faults [1]. We present STM evidence of these rotated phases along with rationale for the particular angles observed. Topographic images show flat, micron scale domains with surface modulation periods corresponding to moir\'{e} patterns generated by rotational stacking faults near the surface. The modulation periods are in agreement with surface x-ray diffraction and low energy electron diffraction data. STS data will be presented and the effects of the observed rotated domains on the electronic structure of C-face multilayer graphene films will be discussed. [1] J. Hass, F.Varchon, J. E. Mill\'{a}n-Otoya, M. Sprinkle, W.A. de Heer, C. Berger, P.N. First, L. Magaud, E.H. Conrad (\textit{to be published}), http://arxiv.org/abs/0706.2134 [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U29.00002: Gate-tunable Graphene Flakes Probed by Scanning Tunneling Spectroscopy at Atomic Scale Yuanbo Zhang, Victor Brar, Feng Wang, Caglar Girit, Yossi Yayon, Melissa Panlasigui, Alex Zettl, Michael Crommie The nanometer scale electronic properties of mechanically cleaved graphene flake devices having tunable back-gates are resolved using scanning tunneling microscopy and spectroscopy. We observe an energy gap feature in the graphene tunneling spectrum that is unexpectedly pinned to the Fermi level ($E_{F}$ ) for different gate-induced electron densities. The Dirac point, on the other hand, is shifted by the back-gate by an amount prescribed by the graphene linear band structure. This energy gap is found to arise from a suppression of elastic electronic tunneling to graphene states near $E_{F}$ and a significant enhancement of tunneling (seen as a more than factor of 10 increase in the conductance) at higher energies due to a phonon-mediated inelastic channel. This work reveals important new tunneling processes in gate-tunable graphitic layers. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U29.00003: Local Variations of Graphene Electronic Structure Probed by STM. Victor Brar, Yuanbo Zhang, Caglar Girit, Alex Zettl, Michael Crommie Transport measurements on devices made from exfoliated graphene sheets have shown that graphene has a high mobility and long mean free path. However, the role that disorder plays in these measurements remains unknown, as does the source of the disorder. In order to better understand the causes of disorder on the local scale, we have performed scanning tunneling spectroscopy measurements on gated graphene flakes at 4.2K in an UHV environment. Our spectroscopy measurements show local variations in graphene electronic properties at different length scales. These variations are analyzed in terms of graphene 2D electronic structure. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 9:12AM |
U29.00004: Electron Transport in Graphene and its Nanostructures Invited Speaker: |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U29.00005: Probing Chiral Quasiparticles using Local-Density-of-States Measurements in Graphene Tamar Pereg-Barnea, Allan H. MacDonald We show that STM local-density-of-states (LDOS) measurements in weakly disordered graphene sheets probe the pseudospin chirality of states near Dirac points. The Fourier transformed LDOS $N({q},\omega)$ has both intravalley contributions centered near reciprocal lattice vectors and intervalley contributions displaced by the wavevector $Q$ which connects graphene's two distinct Dirac points. We explain the qualitative differences between these two features in $N(q,\omega)$ on the basis of analytic calculations starting from graphene's continuum model Dirac equation, and comment on the sensitivity of both $N(q,\omega)$ features to the mix of atomic length scale and smooth disorder sources. For on-site disorder, the LDOS $N(q,\omega)$ measured on A sites due to an A site potential has the periodicity of the Brillouin zone, whereas the pattern produced by a B site potential is periodic with a primitive cell three times larger. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U29.00006: Mesoscopic valley-Hall effect in graphene Johan Nilsson An intriguing property of graphene is the existence of a degree of freedom associated with the two inequivalent valleys in the Brillouin zone. A controlled manipulation of this degree of freedom may potentially be used in novel electronic devices. We study the mesoscopic valley-Hall effect that may provide a route toward the desired control. The effect appears when the inversion symmetry of the crystal is broken, and it can generate a transverse valley current in response to an applied electric field. We look at a few sample setups and discuss the dependence on the geometry and the appearance of valley-Hall edge states. We also compare and contrast our results with those obtained from linear response theory in bulk samples. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U29.00007: Electronic screening in graphite James Reed, Young Il Joe, Peter Abbamonte Nonlocal screening in highly-oriented pyrolitic graphite was investigated with inelastic x-ray scattering. Measurements were performed over a sufficiently broad range of momentum and energy to permit complete inversion of the loss function, $-Im [1/\epsilon(k,\omega)]$, allowing real-time, microscopic imaging of the induced electron density around a charged impurity. In addition, we found evidence for a sign change in the zero-frequency dielectric function, $\epsilon(k,0)$, over a sizeable range of momentum. This ``antiscreening" should cause the Coulomb interaction to be attractive, perhaps assisting superconductivity in this system. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U29.00008: Band Structure of K(2x2) on graphene Jessica McChesney, Aaron Bostwick, Taisuke Ohta, Thomas Seyller, K.V. Emtsev, Karsten Horn, Eli Rotenberg The electronic structure of K(2x2) on graphene, the same stochiometry as bulk KC$_{8}$, was studied using angle-resolved photoemission spectroscopy (ARPES). In addition to bands derived from the graphene $\pi $ states an intercalant induced ``interlayer band'' is observed centered at $\Gamma $. Of these two bands, the dominant mass renormalization occurs in the $\pi $-derived bands, as determined by characterization of the ``kinks'' in the dispersion measured by ARPES. This suggests that the superconductivity in bulk KC$_{8}$ has a more important role than the interlayer band. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U29.00009: Coulomb scattering and transport in graphene Dmitry Novikov The exact transport cross-section off a Coulomb impurity in graphene [1] is proportional to the carrier wavelength. Unexpectedly, the relativistic Coulomb scattering also exhibits a pronounced attraction-repulsion asymmetry [1,2]: Massless carriers are scattered more strongly when they are attracted to a charged impurity than when they are repelled from it. This finding, confirmed recently [3], can be used to separately determine the surface density of donors and acceptors in a graphene monolayer [2]. I will outline quantitative and qualitative differences between the exact result [1] and the commonly used Born approximation for charged impurity scattering. [1] D. S. Novikov, arXiv:0706.1391, Phys. Rev. B (in press); [2] D. S. Novikov, Appl. Phys. Lett. 91, 102102 (2007); [3] J. H. Chen, C. Jang, M. S. Fuhrer, E. D. Williams, M. Ishigami, arXiv:0708.2408v2. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U29.00010: Resonance Raman investigation of monolayer and bilayer graphene Marcos Pimenta, Leandro Malard, Daniela Mafra, Juliana Brant, Daniel Elias, Georgii Samsonidze, Johan Nilsson, Antonio Castro Neto, Flavio Plentz, Elmo Alves The Raman spectra of graphene samples exhibit a band around 2700 cm$^{-1}$, the so called G$'$ band, that is ascribed to a double resonance Raman process involving electrons and phonons in the vicinity of the Dirac point. A dispersive behavior in the position and shape of this band is observed when we change the laser energy used in the Raman experiment, showing that it can be used to probe experimentally the dispersion of electrons and phonons near the Dirac point of graphene. We will present a resonance Raman investigation of monolayer and bilayer graphene using many different laser lines in the visible and near IR range. By the analysis of the dispersive behavior of the G$'$ band we can obtain information about the electronic structure of monolayer and bilayer graphene, such as the intralayer and interlayer tight-binding parameters. Our results reveals a significant asymmetry between the electronic dispersion in the valence and conduction bands of bilayer graphene. We are also able to obtain experimental values for the velocity of the TO and LA phonons near the Dirac point of graphene. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U29.00011: Effect of contact induced states on minimum conductivity in graphene Roksana Golizadeh-Mojarad, Supriyo Datta Recent experiments show that the conductivity of graphene tends to a minimum value in the range of $\sim $2-12${e^2} \mathord{\left/ {\vphantom {{e^2} h}} \right. \kern-\nulldelimiterspace} h$ as the Fermi energy $E_{f}$ approaches the charge neutral Dirac points ($E=0)$. We point out that contact induced states can help explain the structure dependence of the minimum conductivity observed experimentally even if the samples were purely ballistic. Contact induced states are similar to the well-known metal induced gap states (MIGS) in metal-semiconductor Schottky junctions, which typically penetrate only a few atomic lengths into the semiconductor, while the depth of penetration decreases with increasing band gap. However, in graphene we find that these states penetrate a much longer distance of the order of the width of the contacts. As a result, ballistic graphene samples with a length less than their width can exhibit a resistance proportional to length that is not `Ohmic' in origin, but arises from a reduced role of contact-induced states. While actual samples are probably not ballistic and involve scattering processes, our results show that these contact induced effects need to be taken into account in interpreting experiments and minimum conductivity depends strongly on the structure and configuration (two- vs. four-terminal). [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U29.00012: Theory of superconductivity by the edge states in graphene Ken-ichi Sasaki, Masahiro Suzuki, Riichiro Saito Superconductivity in graphite intercalation compound and carbon nanotubes has been attracting much attention due to its high superconducting transition temperature above 10 K. However, the density of states (DOS) near the Fermi energy of graphene is not sufficient to explain the observed high transition temperature. Thus, the mechanism of the superconductivity is an important issue. The STS measurements (Kobayashi et al., PRB73,125415, Niimi et al., PRB73,085421) show an anomalous DOS near the Fermi level of graphene which is relevant to localized edge states. The edge states significantly enhance the local DOS near the zigzag edge. Thus, it is valuable to examine the effect of the edge states on the superconductivity. Using the Eliashberg equation, we obtain an appreciable transition temperature for the edge states. We found that the effects of the Coulomb interaction and Fermi energy position are sensitive to the formation of superconducting gap. We will discuss the condition for observing the edge state superconductivity. (Sasaki et al., J. Phys. Soc. Jpn. 76, 033702 (2007)) [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U29.00013: Ground-state carrier density in graphene Enrico Rossi, Sankar Das Sarma We calculate the carrier density spatial distribution for the ground state of a single layer graphene sheet in presence of randomly distributed charged impurities. In our calculation we include the effects due to the exchange and correlation energy. We carefully study how the distance $d$ of the charge impurities from the graphene layer and an external bias affect the spatial distribution of the carrier density. At zero bias we find that the carrier density is characterized by the presence of electron and hole puddles with equal probability and that, for $d\approx 1$, the typical size of the puddles is of the order of $30\;{\rm nm}$ in agreement with recent experiments \footnote{J. Martin {\em et al.}, Nature Physics (2007)}. With the same approach we study the situation when a tunable barrier potential is applied locally and a bipolar junction within the graphene sheet is formed. This work is supported by NRI-NSF. [Preview Abstract] |
Session U30: Electronic Properties of Graphene and Related Structures IV
Sponsoring Units: DCMPChair: Mike Mehl, Naval Research Laboratory
Room: Morial Convention Center 222
Thursday, March 13, 2008 8:00AM - 8:12AM |
U30.00001: Ab-initio study of polarization in graphene films Eric Yu, Derek Stewart, Sandip Tiwari We present an ab-initio analysis of polarization of multilayer graphene systems under applied electric fields. The effects of applied electric fields are calculated using a Berry phase approach with a plane-wave density functional formalism. We have determine polarizability values for graphene films and carbon nanotubes and find that the polarizability of graphene films follows a linear relationship with the number of layers. We also examined changes in the induced charge distribution as a function of graphene layers. We focus in particular on bilayer graphene and find that the induced charge accumulates primarily on the B sublattice sites. This induced charge distribution was also confirmed by a separate tight-binding Green's function calculation. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U30.00002: Large-Scale Self-Consistent Simulation of Multilayered Graphene Devices Denis Areshkin, Branislav K. Nikoli\' c We use the Density Functional Theory-based Self-Consistent Environment-Dependent Tight-Binding (SC-EDTB) and self-consistent Non-equilibrium Green function formalism (NEGF) to test the {\it all-graphene} multilayer circuit concept. The key element of multi-layered circuits, which are expected to become available through press-print technology, is the heavily perforated graphene layer. The latter serves as an electrical insulator due to its relatively large band gap, and poor ballistic coupling to the conductive parts of the circuit. High bias $I-V$ characteristics for various normally-ON and normally-OFF transistor configurations were simulated, and transistor tolerance to manufacturing defects and imperfections was tested. The usage of SC-EDTB-NEGF makes it possible to model quantum transport through {\em realistic} devices composed of large number of carbon atoms ($\sim 10000$), which are within the reach of presently available processing techniques. Other circuit elements, such as electric interconnects between different layers, wire crossings, and electric interconnects within the same layer are also considered. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U30.00003: Energy bands of multilayer graphenes Wen-Ying Ruan, Jia-An Yan, Mei-Yin Chou The energy bands of L-layer graphenes have been obtained using first-principles calculations. We found that after the introduction of interlay coupling the linear valence and conduction bands of isolated layers can either remain or develop into parabolic bands or bands with a very flat top(bottom), depending on the stacking geometry. A theoretical explanation and some general rules have been developed based upon the tight-binding model with only the nearest-neighbor interactions. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U30.00004: Effect of strain on the electronic structure of graphene Edgar Martinez, Eduardo Cifuentes, Romeo de Coss Graphene has been attracting interest due to its remarkable physical properties resulting from an electron spectrum resembling relativistic dynamics (Dirac fermions). Thus, is desirable to know methods for controling the charge carriers in graphene. In this work, we propose that the electronic properties of graphene can be modulated via isotropic and uniaxial strain. We have studied the electronic structure of graphene under mechanical deformation by means of first principles calculations. We present results for the charge distribution, electronic density of states, and band structure. We focus the analysis on the behavior of the Dirac cones and the number of the charge carriers as a function of strain. We find that an isotropic tensile strain increases the effective mass of carriers and an isotropic compression strain decrease it. Uniaxial tensile strain induce a similar behavior, as strain increase effective mass increase. Thus, our results show that strain allows controllable tuning of the graphene electronic properties. This research was supported by Consejo Nacional de Ciencia y Tecnolog\'{\i}a (Conacyt) under Grant No. 43830-F. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U30.00005: Band-gap engineering in graphene systems for electronic applications Sujata Paul, Marco Buongiorno-Nardelli Ultrathin graphite films including monolayers, bilayers and graphene nanoribbons are intensely studied for the development of future electronic and optoelectronic devices. In this work we will present first principles electronic/phonon structure calculations to elucidate the role of geometry and interactions (electric field, edge functionalization, gating etc.) in the determination of the electronic properties of a wide variety of graphene systems (multi-layers, ribbons etc.). One preliminary objective of this task is to explore ways to accurately control the band gap through the careful design of the active graphitic systems. The coupling between electron and lattice dynamics will be analyzed via the evaluation of the electron-phonon coupling parameters and phonon dispersions calculations. We will discuss the growth of graphene layers on a preferred substrate e.g. SiC an Si. We will identify possible graphene-substrate geometries through first principles calculations and investigate the role of interfacial bonding in the modification and engineering of the band gap. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U30.00006: Effect of Disorders in Graphene Nanoribbon Field-Effect Transistors Youngki Yoon, Gianluca Fiori, Seokmin Hong, Giuseppe Iannaccone, Jing Guo Recent progress on the graphene and graphene nanoribbon (GNR) has provoked strong interests in GNR field-effect transistors (FETs) for future digital and analog nanoelectronics applications. In this work, device characteristics of GNRFETs are calculated by solving the non-equilibrium Green's function (NEGF) transport equation in an atomistic p$_{Z}$ orbital basis set self-consistently with three-dimensional (3D) Poisson equation. The effects of a lattice vacancy, ionized impurity, and edge roughness on transistor performance and characteristics are examined by the atomistic simulations. We show that even a single disorder can have a significant effect on the device characteristics of GNRFETs due to the atomically thin and nanometer-wide channel geometry. For example, a single lattice vacancy can affect the on-current of a GNRFET by 40{\%}. Localized states in the GNR band gap energy range can be induced by the disorders, which affect quantum transport and self-consistent electrostatics. Significant variations between devices are expected due to disorders, but the GNRFETs still switch in the presence of moderate amount of disorders. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U30.00007: Diluted Graphene Antiferromagnet Herbert Fertig, Luis Brey, Sankar Das Sarma We study RKKY interactions between local magnetic moments for both doped and undoped graphene. We find in both cases that the interactions are primarily ferromagnetic for moments on the same sublattice, and antiferromagnetic for moments on opposite sublattices. This suggests that at sufficiently low temperatures dilute magnetic moments embedded in graphene can order into a state analogous to that of a dilute antiferromagnet. We find that in the undoped case one expects no net magnetic moment, and demonstrate numerically that this effect generalizes to ribbons where the magnetic response is strongest at the edge, suggesting the possibility of an unusual spin-transfer device. For doped graphene we find that moments at definite lattice sites interact over longer distances than those placed in interstitial sites of the lattice ($1/R^2$ vs. $1/R^3$) because the former support a Kohn anomaly that is suppressed in the latter due to the absence of backscattering. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U30.00008: Magnetization measurement of highly oriented pyrolytic graphite surface with a spin-polarized metastable helium beam Shiro Entani, Mitsunori Kurahashi, Yasushi Yamauchi Magnetic ordering in nanometerscale graphitic materials has attracted much interest in recent years. Some theoretical studies have predicted that the origin of ferromagnetism in these materials was attributed to defects in graphitic structures, such as edges and topological defects. Employing surface analytical technique is favorable for elucidation of these predictions by experimental studies, because there exist a number of pores and steps at the graphite surface and the structure not in an equilibrium phase might be realized at a surface state. In this work, we have successfully detected the magnetization in highly oriented pyrolytic graphite (HOPG) surface using a spin-polarized metastable helium (He*) beam under high magnetic field up to 5 T. The He* beam is an extremely surface-sensitive probe and the surface magnetization can be analyzed by measuring the asymmetry of sample current induced by the He* spin direction [1]. The observed value of the asymmetry shows a clear temperature dependence and is much larger than that of magnetic impurities measured by Auger electron spectroscopy. Thus, we could conclude that this surface magnetism is an intrinsic property of the HOPG itself other than the diamagnetism. \newline [1] M. Kurahashi and Y. Yamauchi Rev. Sci. Instrum. 77, 023904 (2006). [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U30.00009: Structural and electronic properties of few-layer graphenes from frist-principles M.-H. Tsai, J.-R. Huang, J.-Y. Lin, B.-H. Chen Using first-principles calculation method, it is found that the calculated layer spacing for the two-layer AB stacked FLG is only 2.725{\AA}, which is substantially reduced from the calculated value of 3.257{\AA} for bulk graphite. The average interlayer spacing for 2-, 3-, 4-, 5-, 6-, 7- and 8-layer FLG's are found to oscillate and approach that of the bulk graphite. The two-layer AA stacked FLG is found to remain metallic for the external electric potential up to 4.5Volts considered in this study. In contrast, the two-layer AB stacked FLG is found to exhibit a semi-metal-semiconductor transition under an external electric potential qualitatively in agreement with previous theoretical studies. However, the energy gap is not limited at 0.3eV as obtained in previous first-principles calculation due to the substantially reduced interlayer spacing. The threshold of the semi-metal-semiconductor transition is 0.04 Volts. The external electric potential also induced energy gaps in 3- and 4-layer AB stacked FLG's. However, in these two thicker FLG's, the induced energy gaps are small within 0.1eV. Based on this study, only the two-layer FLG is useful as a nanoscale electronic switch. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U30.00010: Band structure engineering of graphene by strain Jianxin Zhong, Gui Gui, Jin Li We have investigated the electronic structure of graphene under different planar strain distributions using the first principles pseudopotential planewave method and the tight-binding approach. We found that graphene with a symmetrical strain distribution is always a zero band gap semiconductor and its pseudogap decreases linearly with the strain strength in the elastic regime. However, asymmetrical strain distributions in graphene result in opening of band gaps at the Fermi level. For the graphene with a strain distribution parallel to C-C bonds, its band gap continuously increases to its maximum width of 0.486 eV as the strain increases. For the graphene with a strain distribution perpendicular to C-C bonds, its band gap continuously increases only to 0.170 eV. The anisotropic nature of graphene is also reflected by different Poisson ratios in different directions. We found that the Poisson ratio is 0.079 and 0.255 for the strain distributions parallel to or perpendicular to C-C bonds, respectively. These findings are important for understanding and controlling the transport properties of graphene systems. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U30.00011: Electronic properties of nanotube-graphene composite carbon systems Yen-Hung Ho, C.P. Chang, M.F. Lin Band structures of nanotube-graphene hybrid carbon systems are calculated by the tight-binding model. The Lennard-Jones potential is used to determine the optimal geometry for a single-walled carbon nanotube and a monolayer graphene. There exist many one-dimensional energy bands. The low energy bands are drastically changed by the interlayer atomic hoppings, such as the destruction of state degeneracy, alteration of Fermi-momentum states, creation of extra band-edge states, and modulation of energy gap. The composite systems are metals or semiconductors, which depends on the alignment and the geometry of carbon nanotube. The main characteristics of electronic structures are directly reflected in density of states. DOS exhibits a lot of asymmetric prominent peaks. The predicted results could be verified by the experimental measurements from the scanning tunneling spectroscopy. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U30.00012: Edge states of zigzag bilayer graphite nanoribbons Jun-Won Rhim, Kyungsun Moon Electronic structures of the zigzag bilayer graphite nanoribbons(Z-BGNR) with various ribbon width $N$ are studied within the tight binding approximation. Neglecting the small inter-layer hopping parameter $\gamma_4$, there exist two fixed Fermi points $\pm k^*$ independent of the ribbon width with the peculiar energy dispersion near $k^*$ as $\varepsilon (k) \sim \pm (k-k^*)^N$. By investigating the edge states of the Z-BGNR, we notice that the trigonal warping of the bilayer graphene sheets are reflected on in the edge state structure of the Z-BGNR. With the inclusion of $\gamma_4$, the above two Fermi points are not fixed, but drift toward the vicinity of the Dirac point with the increase of the width $N$ as shown by the finite scaling method and the peculiar dispersions change to the parabolic ones. [Preview Abstract] |
Session U31: Focus Session: Multiferroics II: Theory and LuFe2O4
Sponsoring Units: DMP GMAGChair: Nicola Spaldin, University of California, Santa Barbara
Room: Morial Convention Center 223
Thursday, March 13, 2008 8:00AM - 8:12AM |
U31.00001: Spontaneous currents and charge redistribution in Mott insulators Daniel Khomskii, Lev Bulaevskii, Christian Batista, Maxim Mostovoy The standard view is that at low energies Mott insulators exhibit only magnetic properties while charge degrees of freedom are frozen out as the electrons become localized by a strong Coulomb repulsion. We demonstrate that this is in general not true: for certain spin textures \textit{spontaneous circular electric currents} or \textit{nonuniform charge distribution} exist in the ground state of Mott insulators. The latter can give a purely electronic mechanism of \textit{multiferroic behaviour}. In addition, low-energy ``magnetic'' states contribute comparably to the dielectric and magnetic functions, leading to interesting phenomena such as the electric field-induced ``ESR'' transitions, rotation the electric field polarization and resonances which may be common for both functions producing a negative refraction index in a window of frequencies. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U31.00002: First-Principles Study of Large Magnetoelectric Coupling in Triangular Lattices Kris T. Delaney, Maxim Mostovoy, Nicola A. Spaldin We investigate, using density functional theory, the magnetoelectric coupling in a frustrated antiferromagnet in which the combination of frustration with magnetic interactions mediated by Anderson superexchange leads to a unconventional and large coupling between polarization and magnetic order. The nature of the superexchange mechanism can be manipulated through induced polarization of the lattice upon application of an electric field, leading to a strong magnetoelectric effect. We demonstrate the effect in a Mn-based triangular lattice that is closely related to the Kagom\'e structure, with modifications to avoid self-compensation of the induced magnetic order. For our study, we employ the VASP software package with LSDA+U for describing electron exchange and correlation effects. A fully noncollinear treatment of the spinors is essential to describe the complex spin structures that the system adopts. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U31.00003: Trends in spin exchange interactions and ferroelectric polarization in the orthorhombic $R$MnO$_{3}$ series Silvia Picozzi, Kunihiko Yamauchi, Biplab Sanyal, Frank Freimuth, Stefan Blugel, Elbio Dagotto Recently, magnetic ferroelectricity induced by Heisenberg-type interactions has been theoretically predicted in E-type antiferromagnetic (AFM) HoMnO$_{3}$ \footnote{S. Picozzi et al., Phys. Rev. Lett. (in press)}. In order to fully clarify this unconventional microscopic mechanism, we have studied the structural, magnetic and ferroelectric properties for the entire family of orthorhombic $R$MnO$_{3}$ ($R$= rare earth ions), based on first-principles density functional calculations. The ferromagnetic exchange interaction between nearest-neighbor Mn sites decreases with the ionic radius of $R$ (concomitantly with the in-plane Mn-O-Mn bond angle), whereas the anti-ferromagnetic next-nearest neighbor interaction stays rather constant in the series. The competition of these exchange interactions results in a complicated magnetic phase diagram. The decrease in the Mn-O-Mn angle also affects the hopping integrals between Mn ions (as determined from Wannier functions), so that the calculated electric polarization in E-type AFM $R$MnO$_{3}$ is remarkably reduced throughout the rare earth series. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U31.00004: First-principles study of magnetoelecric coupling in TbMnO$_3$ Andrei Malashevich, David Vanderbilt At room temperature, the perovskite TbMnO$_3$ forms an orthorhombically distorted lattice with the {\it Pbnm} space group. Below $\sim$27~K the magnetic moments on the Mn atoms develop incommensurate cycloidal order, and simultaneously a polarization appears. We present a first-principles study of this low-temperature phase in which the ordering of the Mn$^{3+}$ moments is forced to be commensurate in a 60-atom supercell, approximating the experimental wavevector. The calculations are based on a non-collinear spin treatment of density-functional theory in the local-density approximation, with the polarization computed using the Berry-phase technique. We confirm that the electric polarization appears only when the spin-orbit coupling is turned on. Both electronic and lattice-mediated contributions to the polarization appear, the latter being dominant. We make a normal-mode analysis of the lattice contribution and discuss the ability of a model based on local Dzyaloshinskii-Moriya interactions to reproduce the computed pattern of forces. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U31.00005: Magnetoelectric Chains: Quasi-One-Dimensional Multiferroics Turab Lookman, Avadh Saxena Recently discovered pyroxenes represent an example of a multiferroic containing quasi-one dimensional (Q1D) building blocks of zig-zag chains of edge- sharing octahedra along the c-axis of the crystal. This leads us to the natural question: what types of Q1D symmetries would allow for the simultaneous presence of polarizartion and magnetization? Moreover, what kinds of phase transitions can exist between two different Q1D magnetic phases? From this perspective we study quasi- one-dimensional magnetic symmetry in 3D space (magnetic rod groups), enumerate Q1D magnetic point groups that allow for magnetoelectricity and illustrate their role in low-dimensional multiferroic phase transitions. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U31.00006: Influence of oxygen defects and lattice distortions on the energy gap formation and magnetic properties of multi-ferroic materials Jiji Pulikkotil, Vladimir Antropov Full potential density functional calculations are performed to study the effects of O-vacancies on the structural parameters and electronic properties of $RMnO_{3-\delta}$, (R=Y, rare-earths). We find that vacancies at the 2a-position of the hexagonal unit cell can introduce a small gap of magnitude 0.2eV. Although significantly lower than the experimentally observed value of 2.5eV, we claim that already such defects, which in general are inevitable in oxides, can introduce the energy gap in addition to the most commonly accepted Coulomb correlation mechanism. Besides, the off-plane O-vacancies modify the magnetic properties of the system and induce small magnetic polarization on the in-plane O-sites and at the interstitials. The electronic structure and magnetic properties modifications as a function of several lattice distortions have been analyzed. We also discuss the mechanisms of the exchange coupling and its most effective path. In addition we compare previously known results of the electronic structure calculations for these systems. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U31.00007: Domain structure and magnetization reversal in multiferroic LuFe$_2$O$_4$ Weida Wu, Soonyong Park, Chenglin Zhang, S.-W. Cheong We report real space magnetic imaging of single crystal multiferroic LuFe$_2$O$_4$ via variable temperature magnetic force microscopy (VT-MFM). The magnetization reversal of LuFe$_2$O$_4$ is investigated in detail with MFM in magnetic fields up to 8 tesla at several temperatures below T$_N$=230 K. Our results suggest that the domain structure and the magnetization reversal of LuFe$_2$O$_4$ are different from those of conventional FM magnets with a uniaxial anisotropy. These unconventional behaviors may originate from the low dimentionality and the unusual spin-charge frustration of LuFe$_2$O$_4$. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U31.00008: Interplay of charge order and magnetism in LuFe$_{2}$O$_{4}$ M. Angst, R.P. Hermann, A.D. Christianson, W. Tian, R. Jin, B.C. Sales, D. Mandrus Ferroelectricity in LuFe$_{2}$O$_{4}$ may originate from charge order and seems to be coupled the magnetism as well. For both charge order and magnetism, conflicting reports have been published. We have recently grown single crystals exhibiting features in magnetization sharper than previously reported and suggesting an additional transition around 175 K. Neutron scattering experiments have revealed 3D magnetic and charge order. We will present recent synchrotron, calorimetry, and magnetization measurements on these crystals, which indicate a subtle interplay of magnetism, charge order, and structural distortions. Superstructure reflections studied include (1/3,1/3,n and n/2) (exhibiting small systematic deviations away from 1/3), satellites to (0,0,3/2n) and (1/3,1/3,n and n/2), and two other types of reflections. Supported by the Division of Materials Sciences and Engineering, Office of Basic Energy Sciences, US Department of Energy. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U31.00009: Three dimensional magnetic correlations in LuFe$_{2}$O$_{4}$ M.D. Lumsden, A.D. Christianson, M. Angst, Z. Yamani, W. Tian, R. Jin, S.E. Nagler, B.C. Sales, D. Mandrus LuFe$_{2}$O$_{4}$ has recently attracted much attention due to a novel electronically driven ferroelectric transition and concomitant indications of coupling between magnetic degrees of freedom and a large spontaneous polarization. To examine the behavior of LuFe$_{2}$O$_{4}$ in further detail, we have performed extensive polarized and unpolarized neutron diffraction experiments on high quality single crystal specimens. These measurements reveal two phase transitions involving magnetic degrees of freedom below 300 K. At 240 K we find the onset of three dimensional ferrimagnetic order. The refined ferrimagnetic spin structure is a symmetry allowed magnetic structure of the parent R-3m space group with a propagation vector of (1/3 1/3 0). Below 175 K many of the magnetic Bragg peaks become significantly broadened and a broad diffuse component to the magnetic scattering becomes evident. In addition, a new set of satellites is observed indexed as (1/3$\pm \delta $ 1/3$\pm \delta $ 3L/2) where $\delta \sim $0.027. Polarized neutron diffraction measurements indicate that these satellites have a substantial magnetic component. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U31.00010: Optical properties of LuFe$_2$O$_4$ Xiaoshan Xu, Tatiana Brinzari, Janice Musfeldt, Manuel Angst, David Mandrus We measured the optical properties of single crystalline LuFe$_2$O$_4$ as a function of temperature and compared the results with recent electronic structure calculations. The 300 K optical gap is found to be ${\sim}$ 0.4 eV. The optical conductivity is very sensitive to temperature and shows a sharp transition at around 170 K structural transition. The Fe$^{2+}$ to Fe$^{3+}$ charge transfer transition (at approximately 1.5 eV) sharpens dramatically at low temperature, a trend that may be connected with the complex charge order of the material. Preliminary high energy magneto-dielectric data will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U31.00011: Effect of oxygen deficiencies on charge ordering in RFe$_{2}$O$_{4-\delta }$ (R=Lu and Y) Y. Horibe, S. Mori, S. Shinohara, Y. Matsuo, N. Ikeda, S-W. Cheong Charge ordering (CO) of Fe$^{2+}$ and Fe$^{3+}$ on the triangular lattice in RFe$_{2}$O$_{4-\delta}$ (R=Lu and Y) is suggested to play an important role in the physical properties such as ferroelectricity. Herein, we report changes in the CO structures due to the oxygen deficiencies in RFe$_{2}$O$_{4-\delta}$ by transmission electron microscopy. At room temperature, characteristic superlattice reflections at (1/3 1/3 1/2)-type positions can be observed in the nearly stoichiometric YFe$_{2}$O$_{4-\delta}$, while the diffuse streaks along c*-axis can be seen clearly in the non-stoichiometric YFe$_{2}$O$_{4-\delta}$. It is suggested that the correlations between the Fe-O bilayers are suppressed due to the oxygen vacancies and therefore the two-dimensional charge ordering appears in the non stoichiometric samples at room temperature. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U31.00012: Ferroelectricity in an Ising Chain Magnet V. Kiryukhin, Y.J. Choi, H.T. Yi, S. Lee, S-W. Cheong, Q. Huang The concept of magnetism-driven ferroelectricity has recently drawn significant attention. Among the simplest model systems showing this effect are magnetic spiral compounds, and frustrated collinear chain magnets with alternating charge order. While many experimental realizations of the former systems exist, no undisputed examples of the latter have been reported so far. Herein, we report discovery of an experimental realization of this model in an Ising chain compound with an up- up-down-down magnetic order. Unlike in the spiral magnetoelectrics where antisymmetric exchange coupling is active, the symmetry breaking in this system occurs through exchange striction associated with symmetric superexchange coupling. Since the latter can be large, this observation may help identify candidate systems with large magnetoelectric coupling and significant magnetoelectric effects. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U31.00013: Magnetoelectric effect in Cr$_{2}$O$_{3}$ thin films Xi He, Yi Wang, Sarbeswar Sahoo, Christian Binek Magnetoelectric materials experienced a recent revival as promising components of novel spintronic devices [1, 2, 3]. Since the magnetoelectric (ME) effect is relativistically small in traditional antiferromagnetic compounds like Cr$_{2}$O$_{3}$ (max. $\alpha _{zz}\approx $4ps/m ) and also cross- coupling between ferroic order parameters is typically small in the modern multiferroics, it is a challenge to electrically induce sufficient magnetization required for the envisioned device applications. A straightforward approach is to increase the electric field at constant voltage by reducing the thickness of the ME material to thin films of a few nm. Since magnetism is known to be affected by geometrical confinement thickness dependence of the ME effect in thin film Cr$_{2}$O$_{3 }$is expected. We grow (111) textured Cr$_{2}$O$_{3}$ films with various thicknesses below 500 nm and study the ME effect for various ME annealing conditions as a function of temperature with the help of Kerr-magnetometry. [1] P. Borisov et al. Phys. Rev. Lett. \textbf{94}, 117203 (2005). [2] Ch. Binek, B.Doudin, J. Phys. Condens. Matter \textbf{17}, L39 (2005). [3] R. Ramesh and Nicola A. Spaldin 2007 \textit{Nature Materials} \textbf{6} 21. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U31.00014: First-principles approach to lattice-mediated magnetoelectric effects Jorge I\~niguez I will present a first-principles scheme for the computation of the magnetoelectric (ME) response of magnetic insulators. The method focuses on the \textit{lattice-mediated} part of the magnetic response to an electric field, which can be expected to be the dominant contribution in materials displaying a strong coupling, thus avoiding the technical difficulties associated to the treatment of a purely electronic ME effect. I will describe results of calcultions for Cr$_{2}$O$_{3}$ and other model magnetoelectric compounds. [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U31.00015: Chirality, Handedness and Pseudovectors Thomas A. Kaplan, S. D. Mahanti, Kyle Wardlow Chirality has been, to our knowledge, universally defined as a symmetry property, namely, lack of mirror symmetry of a physical object (e.g. a molecule) or, more generally, a physical situation (e.g. light propagation). It is understood that the mirror can be followed by a proper rotation and/or translation. The word chirality (Greek: hand), was added by Lord Kelvin after Pasteur noted the physical importance of right- and left-handed molecules, and, in this context, is considered identical to handedness. However, there is another context, equally important, where handedness has a different meaning, namely the handedness associated with the definition of the cross-product of two vectors. We call the former def. 1, the latter, def. 2. We show that the two meanings are essentially different by giving examples which are simultaneously handed (def. 2) and not chiral. These are drawn from light waves, spin spirals and multiferroics. Thus we show that there must be a distinction between chirality and the general idea of handedness. [Preview Abstract] |
Session U32: Focus Session: Magnetic Multilayers and Nanostructures
Sponsoring Units: GMAG DMPChair: Ilya Krivorotov, University of California, Irvine
Room: Morial Convention Center 225
Thursday, March 13, 2008 8:00AM - 8:12AM |
U32.00001: Unusual resonant response in [Fe(001)/Cr(001)]$_{10 }$/ MgO(001) magnetic multilayers in magnetic field Vladimir Pryadun, Farkhad Aliev, Etienne Snoeck We report on experimental observation of unusual electromotive resonances in [Fe/Cr]$_{10}$ multilayers epitaxially grown on MgO(001) substrates and measured by using balanced excitation and detection schemes. Electric voltage resonances with quality factor exceeding 10$^{3}$ induce strong enhancement of Hall resistance for specific frequencies. Surprisingly, the continuum mechanics model for suspended Fe/Cr layers accounts well for the observed phenomena. Cross-sectional electron microscopy analysis of the multilayers confirms that we could be dealing with non-suspended nanoelectromechanical system. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U32.00002: Field Dependence of the Magnetic Roughness of CoFe(B)/MgO. Yves Idzerda, Joe Dvorak, Ezana Negusse, Alex Lussier, S.R. Shinde, Y. Nagamine, S. Furukawa, K. Tsunekawa, D.D. Djayaprawira We have measured the field dependent roughness of the magnetic interface of CoFe and CoFeB films covered by MgO by using diffuse X-ray resonant magnetic scattering (diffuse-XRMS) of circular polarized light. The samples studied were 3.0 nm of either a Co(70)Fe(30) or a Co(60)Fe(20)B(20) film covered by 1.8 nm MgO created by UHV sputtering system (ANELVA C-7100). By comparing the specular scattering map and the diffuse scattering map for a large range of incidence angles for photons resonantly tuned near the Co L3-edge as the applied magnetic field is swept through the coercive field, we have determined the chemical and magnetic roughness as a function of applied field. For the CoFe films, the magnetic scattering of the X-rays increases significantly as the film passes through the coercive field where the magnetic in-plane correlations are relatively short range, indicating the presence of small magnetic domains during moment reversal. For the CoFeB films, there is no significant increase in magnetic scattering at the coercive field, consistent with large domain switching. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U32.00003: Enhanced Electronic Density of States Observed in Fe-Cr Magnetic Multi-Layers David W. Cooke, Frances Hellman, Matthew Carey Magnetic multi-layer structures have garnered much interest over the past two decades particularly because of the giant magnetoresistance (GMR) effect and its application to information storage technology. Iron-chromium multi-layer structures have been studied extensively, but there remain many questions in the field due to the complex behavior of the anti-ferromagnetic layer (Cr). Using our silicon micro-machined calorimeters, we examine the low temperature specific heat for a range of sputtered MML films grown under similar conditions to those used in industry. We have observed an enhanced electronic density of states in the Fe-Cr MMLs far beyond that of the iron or chromium individually. We compare this enhancement to the observed GMR behavior through a systematic study varying the thickness of the spacer layer. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U32.00004: High bias voltage effect on spin-dependent conductivity and low frequency noise in epitaxial Fe/MgO/Fe magnetic tunnel junctions Farkhad Aliev, Ruben Guerrero, David Herranz, Raul Villar, Fanny Greullet, Coriolan Tiusan, Michel Hehn, Francois Montaigne Low temperature (10K) high voltage bias dynamic conductivity (up to 2.7V) and shot noise (up to 1V) were studied in epitaxial Fe(100)/Fe-C/MgO(100)/Fe(100) magnetic tunnel junctions, as a function of the magnetic state. The MTJs show large TMR (185{\%} at 300K and 330{\%} at 4K). Multiple sign inversion of the magnetoresistance is observed for bias polarity when the electrons scan the electronic structure of the bottom Fe-C interface. The shot-noise shows a Poissonian character validating the high structural quality of the MgO barrier [1]. We have found that the normalized 1/f noise (Hooge factor) asymmetry between parallel and antiparallel states may strongly depend on the applied bias and its polarity. These MTJs exhibit record low Hooge factors being at least one order of magnitude smaller than previously reported [2]. ( [1] R. Guerrero, et al., Appl. Phys. Lett. \textbf{91}, 132504 (2007); [2] F.G. Aliev, et. al., accepted to Appl. Phys. Lett.). [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U32.00005: Giant Magnetoresistance in Nanogranular Magnets Andreas Glatz, Igor Beloborodov, Valerii Vinokur I discuss the giant magnetoresistance of nanogranular magnets in the presence of an external magnetic field and finite temperature. It is shown that the magnetization of arrays of nanogranular magnets has hysteretic behavior at low temperatures leading to a double peak in the magnetoresistance which coalesces at high temperatures into a single peak. The magnetization of magnetic domains and the motion of domain walls in this system is calculated numerically using a combined mean-field approach and a model for an elastic membrane moving in a random medium, respectively. From the obtained results, the electric resistivity as a function of magnetic field and temperature is obtained. The findings show excellent agreement with various experimental data. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U32.00006: Influence of dipolar interactions on the formation of domains in layered Ni/Al$_{2}$O$_{3}$ nanocomposites R. Das, A. Hebard, A. Gupta, D. Kumar, S. Oh, S. Pennycook Pulsed laser deposition has been used to fabricate Ni/Al$_{2}$0$_{3}$ multilayer composites in which Ni nanoparticles of uniform size in the range of 3-60 nm are embedded as layers in an insulating Al$_{2}$0$_{3}$ host. At fixed temperatures, the coercive fields show well-defined peaks which define a critical size that delineates a crossover from single domain (SD) to multiple domain (MD) behavior. Most applications require that the particles be single domain with a uniform magnetization that remains stable with a sufficiently large anisotropy energy to overcome thermal fluctuations and beat the superparamagnetic limit, which establishes a temperature-dependent \textit{lower bound} to the particle size (superparamagnetic limit). These considerations must take into account the effect of interactions on magnetic properties as is evident for high-density recording media where particles are very close to each other. The effect of dipolar interactions on the establishment of an \textit{upper bound} to particle size ($d_{c})$, which defines the crossover from SD to MD behavior will be discussed. We show using coercivity measurements that, with increasing temperature, $d_{c}$ increases and then saturates due to attenuated dipolar interactions from thermally induced motions of neighboring randomly oriented particles. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U32.00007: An investigation of quantum well states and magnetic properties of Co/Au/Ru(0001) J. Choi, J. Wu, F. El Gabaly, A.K. Schmid, Z.Q. Qiu Quantum well state of Au/Ru(0001) and its effect on the magnetic properties of Co/Au/Ru(0001) was investigated using Spin Polarized Low Energy Electron Microscopy (SPLEEM). Epitaxially grown Au on Ru(0001) at room temperature was annealed to $\sim $300\r{ }C. Upon annealing, Au forms islands with atomically flat tops across stepped regions of Ru, forming local wedges of different Au thickness. Energy scans reveal clearly the existence of quantum well states in Au/Ru(0001). After depositing Co film on top of the Au, we found that the Curie temperature and the spin reorientation transition of Co film on the flat-top Au islands are different from on the Au wetting layer. However the quantum well states of the Au layer have no effect on the Curie temperature and the spin reorientation transition of Co film. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U32.00008: Angular Dependent Magnetic Susceptibility with Photoexcitation Studies on Prussian Blue Analog Thin Films D.M. Pajerowski, J.-H. Park, M.W. Meisel, F.A. Frye, D.R. Talham Prussian blue analog systems are the topic of ongoing research because of their novel physical effects. One such effect is persistent photoinduced magnetism, found in CoFe analogs. For such an optical system, in an attempt to maximize the material's interaction with incident photons, a thin film geometry is often utilized; this geometry can produce new effects [1]. Samples of different starting materials have been characterized with respect to photoinduced states, angular dependent susceptibility, film thickness, and chemical formula. Notably, magnetic anisotropies present in the systems show a dependence on the studied factors. One class of interesting starting materials are Rb$_{j}$Ni$_{k}$[Cr(CN)$_{6}$]$_{l}$.nH$_{2}$O and Rb$_{j}$Co$_{k}$[Fe(CN)$_{6}$]$_{l}$.nH$_{2}$O heterostructures, generated by sequential adsorption on a Melinex substrate, that display behavior different than a noninteracting admixture of the two materials by themselves. \newline [1] J.-H. Park, E. Cizmar, M. W. Meisel, Y. D. Huh, F. Frye, S. Lane, and D. R. Talham, Appl. Phys. Lett. 85, 3797 (2004). [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U32.00009: Ferroelectric control of magnetism in BaTiO$_{3}$/Fe heterostructures Sarbeswar Sahoo, Srinivas Polisetty, Chun Gang Duan, Sitaram Jaswal, Evgeny Tsymbal, Christian Binek Multiferroics can offer the possibility to manipulate the cross coupled order parameters by conjugate electric and magnetic fields. Switching off ferromagnetic order by an electric field for instance promises significant impact in the design of novel spintronic devices. Here we report on the reversible control of magnetism for a Fe thin film in proximity of a BaTiO$_{3}$ single-crystal. Large magnetization changes emerge in response to ferroelectric switching and structural transitions of BaTiO$_{3}$ controlled by applied electric fields and temperature, respectively.$^{\dag }$ Interface strain coupling is the primary mechanism altering the induced magnetic anisotropy. As a result, coercivity changes up to 120{\%} occur between the various structural states of BaTiO$_{3}$. Up to 20{\%} coercivity change is achieved via electrical control at room temperature. Our all solid state ferroelectric-ferromagnetic heterostructures open viable possibilities for new technological applications. $^{\dag }$S. Sahoo, S. Polisetty, C.-G. Duan, S. S. Jaswal, E. Y. Tsymbal, and Ch. Binek, Phys. Rev. B \textbf{76}, 092108 (2007). [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U32.00010: Griffiths phase and parimagnetism in ErCo$_{2}$ Fernando Bartolome, Luis M. Garcia, Julia Herrero-Albillos, Anthony T. Young, Tobias Funk A systematic study of the paramagnetic phase of ErCo$_{2}$ has recently allowed to identify the inversion of the net magnetization of the Co net moment with respect to the applied field well above the ferrimagnetic ordering temperature, $T_{C}$. This phenomenon, which we have denoted \textit{parimagnetism}, may be related with the onset of a Griffiths-like phase in paramagnetic ErCo$_{2}$. We have measured SANS and ac susceptibility on ErCo$_{2}$ as a function of temperature, applied field, and excitation frequency. Several characteristics shared by systems showing a Griffiths phase are present in ErCo$_{2}$, namely the formation of ferromagnetic clusters in the disordered phase, the loss of analyticity of the magnetic susceptibility and its extreme sensitivity to an applied field. Our XMCD study of the Co magnetic moment flipping process show the ocurrence of a pseudo-violation of the third Hund's rule at the para- to parimagnetic ``transition''. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U32.00011: Magnetostriction close to the phase transition in Gd$_{5}$(Si$_{x}$Ge$_{1-x}$)$_{4}$ R. L. Hadimani, Y. Melikhov, J. E. Snyder, D. C. Jiles Gd$_{5}$(Si$_{x}$Ge$_{1-x}$)$_{4}$ is a potential material for magnetic refrigeration. It has the highest magnetocaloric effect observed for the composition 0.41$\leq$x$\leq$0.5 near its first order coupled magnetic-structural phase transition. We have investigated the relation between the magnetic transition from ferromagnetic to paramagnetic phase and the structural transition from monoclinic to orthorhombic. A series of measurements have been carried out showing magnetostrictive strain as a function of temperature at various magnetic field strengths and magnetostrictive strain as a function of magnetic field at various temperatures with a magnetic field of up to 7 Tesla. There was fine structure observed in the magnetostriction curve $\lambda$ vs. H near the critical point. The magnetostriction measurements show that close to the critical temperature there is a sudden increase in the magnetostriction of about 100 ppm just before the field induced first order phase transition. This anomaly was observed for both strain vs. magnetic field at various temperatures and for strain vs. temperature at various magnetic field strengths measurements. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U32.00012: Magnetostatic interaction between two thin nanotubes Eugenio E. Vogel, Omar Su\'arez, Patricio Vargas We consider here the magnetic interaction between two identical tubes, characterized by: total magnetization M, length 2L, external radius Re, internal radius Ri. Following most of the experimental realizations we consider very thin tubes, namely, (Re-Ri)$<<$Ri. We begin by considering the two nanotubes in perfect parallel alignment and we vary the separation distance D. The continuous magnetization approach is invoked using different methods to compute the interacting energy: analytic expression valid for D$<$2L, analytic integration over the interaction of elements on each tube, and numeric integration for general cases. These results are compared with two independent results: a) the tubes are far apart so they can be considered solid nanowires: b)each tube is considered as a set of elementary nanowires and a series expansion is obtained and truncated. The advantages and disadvantages of each method are discussed. The ranges of applicability of the ``handy'' approximate expressions are obtained. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U32.00013: Kondo-like features in chemically pure magnetic atomic-size contacts M. Reyes Calvo, Joaquin Fernandez-Rossier, Carlos Untiedt The influence of magnetism in the electronic transport in atomic sized contacts is not yet clear. However, certain features systematically appear in the conductance measurements of magnetic atomic contacts. Specifically, the spectroscopy of atomic size contacts of Ni, Fe or Co reveal the existence of a characteristic significant peak or dip at zero bias that is not present in the case of non-magnetic materials. We have measured the differential conductance as a function of bias at 4K on two houndred monoatomic contacts of Ni, Fe and Co fabricated by STM. The zero bias anomaly has been analyzed as using the Kondo- Fano lineshape typical of magnetic adatoms in non-magnetic surfaces. The statistical analysis of the data results in Kondo temperatures around 250 K, 120 K and 80 K for Ni, Co and Fe respectively. A Kondo-like behaviour could arise in chemically pure magnetic contacts if tip atoms behave different due to their smaller coordination. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U32.00014: ABSTRACT HAS BEEN MOVED TO SESSION K1 |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U32.00015: Spin-density wave in polycrystalline Cr films from infrared reflectivity Zoe Boekelheide, Erik Helgren, Frances Hellman The spin-density wave properties of polycrystalline chromium thin films were determined by using infrared reflectivity to determine the gap energies. The incommensurate spin density wave of bulk chromium is highly sensitive to perturbations from stress, disorder, and finite size effects, such as those found in polycrystalline films. Films prepared under various conditions display three different types of spin density wave behavior: incommensurate, commensurate, and mixed. Unexpectedly, the mixed phase includes the incommensurate spin density wave and two different forms of commensurate spin density wave. A phenomenologically determined low temperature phase diagram is created to describe the spin density wave in chromium in the stress-disorder plane. The effects of stress and disorder on the spin density wave of chromium films are analogous to the effects of dilute alloying in bulk chromium. In this case, tensile stress has a similar effect to Mn impurities while disorder has a similar effect to Al. [Preview Abstract] |
Session U33: Focus Session: Theory and Simulations of Magnetism I
Sponsoring Units: DCOMP DMP GMAGChair: Xiaoguang Zhang, Oak Ridge National Laborattory
Room: Morial Convention Center 224
Thursday, March 13, 2008 8:00AM - 8:36AM |
U33.00001: Tailoring Magnetism in Bulk Semiconductors and Quantum Dots Invited Speaker: Carrier-mediated magnetism in semiconductors shows important and potentially useful differences from their metallic counterparts [1]. For example, in magnetically doped semiconductors the change in carrier density induced by light or bias could be sufficient to turn the ferromagnetism on and off. However, there remain many important challenges to fully understand these materials. Our density functional theory study of Mn- doped II-IV-V$_2$ chalcopyrites [2] reveals that variation of magnetic properties across 64 different materials cannot be explained by the dominant models of ferromagnetism in semiconductors. We observe no qualitative similarity with the suggested Curie temperature scaling with the inverse cube of the lattice constant [3]. In contrast to most of the theoretical studies, we explicitly include the temperature dependence of the carrier density and propose a model which permits analysis of the thermodynamic stability of the competing magnetic states [4]. As an example we analyze the stability of a possible reentrant ferromagnetic semiconductor and discuss the experimental support for this prediction. An increasing temperature leads to an increased carrier density such that the enhanced coupling between magnetic impurities results in the onset of ferromagnetism as temperature is raised. We also use the real space finite-temperature local spin density approximation to examine magnetically doped quantum dots in which the interplay of quantum confinement and strong Coulomb interactions can lead to novel possibilities to tailor magnetism. We reveal that, even at a fixed number of carriers, the gate induced changes in the screening [5] or deviations from isotropic quantum confinement [6] could allow for a reversible control of magnetism and switching between zero and finite magnetization. Such magnetic quantum dots could also provide versatile voltage-control of spin currents and spin filtering. The work done in collaboration with S. C. Erwin (Naval Research Lab), A. G. Petukhov (South Dakota School of Mines and Technology), R. M. Abolfath (SUNY Buffalo) and P. Hawrylak (NRC, Canada). [1] T. Jungwirth et al., Rev. Mod. Phys 78, 1311 (2006); I. Zutic, J. Fabian, and S. Das Sarma, Rev. Mod. Phys. 76, 323 (2004). [2] S. C. Erwin and I. Zutic, Nature Mater. 3, 410 (2004). [3] T. Dietl et al., Science 287, 1019 (2000). [4] A. G. Petukhov, I. Zutic, and S. Erwin, Phys. Rev. Lett. 99, 257202 (2007) [5] R. M. Abolfath, P. Hawrylak, and I. Zutic, Phys. Rev. Lett. 98, 207203 (2007); New J. Phys. 9, 353 (2007). [6] R. M. Abolfath, A. G. Petukhov, and I. Zutic, arXiv:0707.2805. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U33.00002: Effect of vacancies on ferromagnetism in GaN:Mn dilute magnetic semiconductors from first principles Paul Larson, Sashi Satpathy In spite of considerable interest in ferromagnetism of the dilute magnetic semiconductor GaN:Mn, the nature of ferromagnetism is still quite controversial. Experimental values for the Curie temperature T$_C$ vary widely depending upon the details of the the impurity concentrations. We have performed {\it ab initio} density functional studies of the magnetic interactions in GaN in the presence of nitrogen and gallium vacancies. Previous studies have found the nitrogen vacancy has the lowest formation energy. The nitrogen vacancy releases electrons in the system which changes the Mn $d^4$ state to a half-filled Mn $d^5$ state, so that the antiferromagnetic superexchange becomes dominant. The naive picture of Ga vacancies is the release of holes into the system which should increase ferromagnetism. However, we find an antiferromagnetic interaction for the Ga vacancy as well, in agreement with Mahadevan's work$^2$. This can be attributed to the localized nature of the hole states which do not participate in the transport. This hole localization from the Ga vacancy has been demonstrated using the virtual crystal approximation. Thus, both the nitrogen and gallium vacancy are found to impede ferromagnetism. This work is supported by AFOSR-FA 9550-05-1-0462. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U33.00003: Effect of magnetic short-range order on spin disorder resistivity. Aleksander Wysocki, Kirill Belashchenko, Mark van Schilfgaarde, Julian Velev Spin-disorder resistivity (SDR) of Fe and Ni is studied using the noncollinear density functional theory. The Landauer conductance is averaged over random disorder configurations and fitted to Ohm's law. In the fully disordered state, SDR for Fe is close to the experiment, while for fcc Ni it exceeds the experimental value by a factor of 2.3. This indicates either strong magnetic short-range order (MSRO) or reduced local moment above T$_{C}$ for Ni. The temperature dependence of SDR for Fe was studied using the mean-field approximation and the Monte Carlo method applied to the classical nearest-neighbor Heisenberg model. Both methods gives the same magnetization dependence of SDR that is in excellent agreement with the results for the isotropic s-d model. Further using the Reverse Monte Carlo method we generated disordered spin structures with strong MSRO. We found that resulting SDR is not significantly different than for Monte Carlo and mean-field methods. This result indicates that for Fe MSRO is not very important for SDR. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U33.00004: First-Principle theory of Longitudinal Spin-Fluctuations at high temperatures for itinerant ferromagnets. Sergii Khmelevskyi, Andrei V. Ruban, Peter Mohn, Borje Johansson We have developed a framework for calculating parameters of effective magnetic Hamiltonian, which includes transverse as well as longitudinal spin fluctuations (LSF) on equal footing. The method is based on the set of constrained calculations within a Local Spin-Density Approximation and Coherent Potential Approximations. The used effective Hamiltonian is similar to those derived in Moriya-Takahashi theory approximating between local and weak itinerant limits of magnetism. The Curie temperatures, paramagnetic susceptibilities and magnetic specific heat have been calculated for bcc Fe and fcc Ni in good agreement with experiment. The importance of LSF contribution even for qualitatively correct description of magnetism of Ni is demonstrated. The first principles criteria for magnetic moment `itineracy', based on fixed spin moment constrained calculations of a magnetic impurity in the Disordered Local Moment host, is established and applied to various magnetic systems. In particular, the famous Rhodes-Wohlfarth plot has been revisited. It is found that in some cases, like VAu$_{4}$, the magnetic moments have very local character in contrast to their long-standing interpretation as weak itinerant ferromagnets. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U33.00005: Calculation of diamagnetic susceptibility in Cu, graphite and Bi from band-structure G. Samolyuk, J. Schmalian, B. Harmon, S. Bud'ko, P. Canfield Since early pioneering work on the orbital diamagnetism of free electrons[1] the problem of calculation of diamagnetic susceptibility has attracted attention in systems such as graphite and bismuth. Recent interest in this problem has been motivated by the unconventional electronic properties of mono- and multilayered graphenes. These materials demonstrate large orbital diamagnetism caused by a specific type of band dispersion: a crossing of two bands, each with linear dispersion near the Fermi level. Significant progress in the theoretical description of orbital diamagnetism of electrons in periodic potentials was achieved by Fukuyama[2], with an exact expression for diamagnetic susceptibility, but with an approximation for band dispersion put in by hand. As an alternate approach, we use band structure obtained from a first-principles calculation (LMTO). The orbital susceptibility was calculated for Cu, as an example of a metal with small orbital diamagnetism, as well as for graphite and Bi, materials with known, large diamagnetic susceptibilities. [1] L. Landau, Z. Physik. {\bf 64}, (1930) 629. [2] H. Fukuyama, Prog. Theor. Phys. {\bf 45}, (1971) 704. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U33.00006: Magnetic Moment of MnSi Robert Collyer, Dana Browne MnSi is a metallic helimagnet below 29 K. Density functional theory predicts that it has a moment of 1.0 $\mu_B$/Mn, which is much larger than the measured value of ~0.4 $\mu_B$/Mn. By adding a Hubbard-U correction, we have found a ground state with a moment consistent with the experimental value. These solutions posess a novel quadrupolar spin ordering. We discuss their behavior under pressure and in a magnetic field. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U33.00007: ABSTRACT WITHDRAWN |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U33.00008: Piezomagnetic effect in Mn-based antiperovskites Renat Sabirianov, Pavel Lukashev, Kirill Belashchenko We predict large piezomagnetic effect in Mn-based antiperovskites. The magnetic ground state is determined to be the non-collinear $^{5g}$ structure, which can be viewed as a sequence of alternating layers of clockwise and counterclockwise triangular spin currents in (111) plane, resulting in the zero total magnetization of the system. We use PAW method (VASP) within PBE generalized gradient approximation. We find that the system develops non-zero magnetization under biaxial strain due to the lowering of the crystal symmetry from cubic to tetragonal. The system exhibits linear magnetization dependence on the applied strain over the moderate range (up to 1{\%}) of the latter. The magnetization under strain appears as a result of the rotation of local magnetic moments (LMM) on Mn atoms in Mn$_{2}$N plane. Besides, the system exhibits biaxial anisotropy under strain. We propose using the observed piezomagnetic effect to build the magnetoelectric perovskite ferroelectric -- antiperovskite piezomagnetic heterostructures. The estimated magnetoelectric coefficient is $\sim $ 2*10$^{-9 }$s/m. Because of the piezomagnetic effect, Mn-based antiperovskites can be used in memory cells with electric control of magnetization. It can be also applicable in spintronics, as the system developes a net polarization of up to 30{\%} under external strain. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U33.00009: Spin Flip in the Presence of a Complex Absorbing Potential Frank Marsiglio, Fatih Dogan, Cindy Blois, Wonkee Kim We examine the impact of a complex absorbing potential on electron transport, both in the continuum and on a lattice. This requires the use of non-Hermitian Hamiltonians; the required formalism is briefly outlined. The lattice formulation allows us to study the interesting problem of an electron interacting with a stationary spin, and the subsequent time evolution of the electron and spin properties as the electron is absorbed after the initial interaction. Remarkably, the properties of the localized spin are affected `at-a-distance' by the interaction of the (now entangled) electron with a complex potential. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U33.00010: Giant Zeeman electric-dipole resonance in antiferromagnetic conductors. Revaz Ramazashvili Essential dependence of the electron g-factor on the quasiparticle momentum is a fundamental and, so far, largely overlooked property of antiferromagnetic conductors. It leads to a number of remarkable phenomena, such as excitation of spin flip transitions by AC *electric* field. Absorption intensity of these transitions exceeds that of the Electron Spin Resonance by some four orders of magnitude. I develop a theory of this phenomenon in a weakly doped antiferromagnetic insulator. The predictions may be relevant for a number of antiferromagnetic conductors, ranging from chromium to electron-and hole-doped cuprates, to organic conductors with spin density wave, and to heavy fermion antiferromagnetic metals. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U33.00011: Electronic properties and magnetic moments of Mn$_{x}$Si$_{1-x}$ for x $<$ 5{\%}. Michael Shaughnessy, Ryan Snow, Ching Yao Fong Recently, there have been experimental reports about Mn$_{x}$Si$_{1-x}$ alloys. All show great promise for room temperature spintronic applications. We report on theoretical studies of the electronic properties of Mn$_{x}$Si$_{1-x}$ for x $<$ 5{\%}, using first principles density functional methods. For the Mn-doped Si, we consider three configurations of the Mn impurities: nearest neighbor (nn), second nearest neighbor (snn), and a three-atom chain configuration. For the nn and snn configurations, the ferromagnetic and antiferromagnetic phases have been compared. The magnetic moment/unit-cell for the nn and ferromagnetic configurations is smaller than for the ferromagnetic snn and chain configurations. The reason will be given. Supported in part by NSF grant: ESC-0725902. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U33.00012: Monte Carlo simulation of giant magnetoresistance Gaston Barberis Monte Carlo method was used to simulate giant magnetoresistance in solids. A square lattice, composed by Ising spins, was generated by the usual Monte Carlo method, using periodic conditions for the borders. Resistivity was measured considering the number of clusters connected between right and left sides of the lattice, with and without applied magnetic field. The calculation of the number and surface of the connecting clusters was calculated using the Hoshen-Kopelman algorithm [1]. As we developed previously pseudorandom non-periodic numbers [2], which allows that the sizes of the clusters as big as necessary, and the steps near the transition as small as desired. This allowed a detailed study near the percolation region, over and below the magnetic transition. Three dimensional lattices, and models other than Ising for the spin coupling are natural extension of the calculation. [1]J. Hoshen and R. Kopelman, Phys. Rev. B14, 3438 (1976) [2]G.E. Barberis,Physica B 398, 468 (2007) [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U33.00013: Propagators for Hamiltonians with Spin-Orbit Coupling Bailey Hsu, Jean-Francois Van Huele Quantum mechanical propagators can be used to understand the dynamics of electrons in confined electromagnetic environments. We extend the propagator formalism to include the spin degree of freedom for spin-orbit coupling potentials~in two-dimensional geometries.~The 2x2 spin propagators allow us to follow the evolution in time and space of the spin-components of localized wave packets. We apply the technique to Rashba and Dresselhaus interactions and present graphical displays of the corresponding spin motions.~ [Preview Abstract] |
Session U35: Focus Session: Hydrogen Storage IV: Theoretical Predictions
Sponsoring Units: DMP FIAPChair: Karl Johnson, University of Pittsburgh
Room: Morial Convention Center 227
Thursday, March 13, 2008 8:00AM - 8:12AM |
U35.00001: Optimization of metal dispersion and hydrogen adsorption strength in doped graphitic materials Seung-Hoon Jhi, Gyubong Kim, Noejung Park, Steven Louie, Marvin Cohen The non-covalent hydrogen binding on transition metal atoms dispersed on carbon clusters and graphene is studied with the use of the pseudopotential density functional method. It is found that the presence of acceptor-like states in the absorbents is essential for enhancing the metal-absorbent binding strength and for increasing the number of hydrogen molecules attached to the metal atoms. Particular configurations of boron or nitrogen substitutional doping are found to be very efficient for providing such states and thus enhancing storage capacity. Optimal doping conditions are suggested based on our calculations for the binding energy and ratio between metal and hydrogen molecules. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U35.00002: Determination of best models for adsorption of hydrogen in boron-doped carbon nanopores Raina Cepel, Matthew Beckner, Carlos Wexler, Peter Pfeifer Nanoporous carbon offers significant hydrogen storage capacities at low pressure and reversible conditions. Storage is achieved by physical adsorption of molecular hydrogen (H$_{2}$) on the surface of nanometer-size pores in the carbon matrix. Within the ALL-CRAFT collaboration (http://all-craft.missouri.edu), we conduct a proof-of-concept study of the prediction that boron-doped nanoporous carbon can store as much as 8 weight{\%} at 47 bar and room temperature. By comparing theoretical and experimental H$_{2}$ adsorption isotherms for intrinsic and doped carbon, we determine which adsorption models and scenarios (e.g.\ localized vs.\ mobile) are consistent with experimental evidence. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U35.00003: Metal clustering and catalytic spillover on the nanotubes and graphene for hydrogen storage Feng Ding, Pavel Krasnov, Yu Lin, Boris I. Yakobson Energies and kinetic barriers associated with transition metal (Sc) clustering on a single-walled carbon nanotube (SWNT) and graphene were studied by all-electron density functional method. The analysis shows that the binding energy of Sc atom on SWNT is highly sensitive to the tube diameter and chirality. The metal atoms do cluster on common SWNT, with diameters $\sim $1-2 nm. Hydrogen binds to the metal cluster chemically and thus opens a way for hydrogen storage via catalytic spillover. However the hydrogen chemisorption on graphene receptor-substrate is difficult to reconcile with a single H atom binding to carbon being weaker than it is within initial molecular H$_{2}$. This paradox is resolved by presenting the process as phase nucleation. Atomistic calculations bridge remarkably with the macroscopic-continuum description, and show a feasible path to 7.7 wt{\%} H-content at nearly ambient conditions. P. O. Krasnov, F. Ding, et.al., J. Phys. Chem. C, in press (2007). [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U35.00004: Hydrogen Storage in Ti Doped Nano Porous Graphene Sa Li, Puru Jena Clustering of Ti on carbon nanostructures has proved to be an obstacle in their use as hydrogen storage materials. Using density functional theory we show that Ti atoms will not cluster when doped into nanoporous graphene. With each Ti atom binding up to four hydrogen molecules with an average binding energy of 0.54 eV/H2, this material can be ideal for storing hydrogen. Equally important, nanoporous graphene is magnetic with or without Ti doping, but magnetism disappears when fully saturated with hydrogen. This novel feature suggests that nanoporous graphene can also be used as a hydrogen sensor. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U35.00005: Graphenic C$_3$N$_4$: A New Template for Metal Decoration and Hydrogen Adsorption Yi Zhang, Hong Sun, Changfeng Chen From density functional theory calculations we identify a graphenic C$_3$N$_4$ (g-C$_3$N$_4$) structure as an excellent template for stable and well dispersed decoration of alkali and transition metal atoms which, in turn, exhibits a high capacity for hydrogen adsorption with binding energies (a few tenths of eV) suitable for mobile applications. The unique porous micro-structural sites of g- C$_3$N$_4$ accommodate the excessive N lone-pair electrons and promote strong hybridization between the orbitals of N and metal atoms. It plays a key role in overcoming the tendency of metal-atom clustering that has plagued other proposed hydrogen storage media. These metal decorated g- C$_3$N$_4$ may also prove useful in a variety of catalytic and sensing applications. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U35.00006: Hydrogen Storage in Titanium-decorated Boron Buckyball Jia Li, Gang Zhou, Wenhui Duan, Hoonkyung Lee, Jisoon Ihm Using first-principles electronic structure calculations, we investigate the potential of Ti-decorated B$_{80}$ for hydrogen storage medium. The Ti-decorated B$_{80}$ has the merit of an unexpected large binding energy of a Ti atom to B$_{80}$ which can overcome the problem of metal clustering. Up to four hydrogen molecules are found to be adsorbed on a single Ti atom coated on B$_{80}$. At high Ti coverage, we show that the Ti-decorated B$_{80 }$can adsorb up to 5 wt{\%} hydrogen and the calculated binding energy falls in the desirable range of 0.2-0.6eV/H$_{2}$ which is suitable for reversible hydrogen storage at room-temperature, near-ambient-pressure conditions. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U35.00007: Sequential Dissociative Chemisorption of H$_{2}$ on Ti$_{13}$ Cluster T.J. Dhilip Kumar, P. Tarakeshwar, N. Balakrishnan Ti nanoparticles have received much attention due to their superior catalytic property in potential hydrogen storage materials for fuel cell applications. In this study, we show that the energetically stable distorted icosahedral Ti$_{13}$ cluster has excellent H$_{2}$ adsorption and desorption properties and lead to stable structures upon hydrogen cycling. H$_{2}$ adsorption initially leads to a highly stable Ti$_{13}$H$_{20}$ cluster and on further saturation yields the Ti$_{13}$H$_{30}$ cluster. The chemisorbed H atom in Ti$_{13}$H$_{20}$ occupies above the face of the triangular planes of Ti$_{13}$ whereas in Ti$_{13}$H$_{30}$, H atoms remain dangling above the apex Ti edges. The three coordinated H in Ti$_{13}$H$_{20}$ has higher chemisorption and desorption energies than the fully saturated Ti$_{13}$H$_{30}$ cluster. This type of multi-center H-bonds with varied chemisorption energies is structurally significant since adsorption and desorption rate processes could be controlled and deserve attention as potential candidates for hydrogen storage materials. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U35.00008: An Ab Initio Study of Molecular Hydrogen Interaction with SiC Nanotube -- A Precursor to Hydrogen Storage Souptik Mukherjee, Asok Ray First principles calculations have been performed to study the adsorption of molecular hydrogen (H2) on three types of armchair (9,9) silicon carbide nanotubes. The distances of H2 from the outer walls of the nanotubes have been optimized using the B3LYP and PW91 functionals. For the PW91 functional, the carbon top site for type 1, the second hollow site for type 2 and the C-C bridge site for type 3 nanotubes are the most preferred adsorption sites. For the B3LYP functional, the C-Si normal bridge site for type 1, and the C-C bridge site for type 2 and type 3 nanotubes are the most preferred sites. The adsorption energies using the PW91 functional are found to be always higher than those using the B3LYP functional; however, the adsorption distances using the B3LYP functional are greater than the corresponding distances using the PW91 functional. Current studies indicate that silicon carbide nanotubes can possibly be used as a proper media for hydrogen storage at ambient conditions. [Preview Abstract] |
Session U36: Nanoinstrumentation for Biological and Other Applications
Sponsoring Units: GIMSChair: Robert Guertin, Tufts University
Room: Morial Convention Center 228
Thursday, March 13, 2008 8:00AM - 8:12AM |
U36.00001: Cocaine detection using piezoresistive microcantilevers Bernadeta Srijanto, Christine P. Cheney, David L. Hedden, Anthony Gehl, Thomas L. Ferrell Sensitive and inexpensive sensors play a significant role in the analysis of drugs and drug metabolites. Specifically, reliable \textit{in vivo} detection of cocaine and cocaine metabolites serves as a useful tool in research of the body's reaction to the drug and in the treatment of the drug addiction. We present here a promising cocaine biosensor to be used in the human body. The sensor's active element consists of piezoresistive microcantilevers coated with an oligonucleotide-based aptamer as the cocaine binder. \textit{In vitro} cocaine detection was carried out by flowing a cocaine solution over the microcantilevers. Advantages of this device are its low power consumption, its high sensitivity, and its potential for miniaturization into an implantable capsule. The limit of detection for cocaine in distilled water was found to be 1 ng/ml. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U36.00002: Bioinspired optical sensing of picomolar concentrations of lead in solution. Anushree Saha, Vladislav Yakovlev Lead poisoning is a life threatening medical condition, which can cause irreversible neurological, cardiovascular and reproductive damage. Despite of an extensive research, the minimum amount of lead to be considered hazardous is not yet known. The biophysical interactions of minute quantities of lead with blood are also poorly understood. Albumin being the most important binder and transporter in blood, is known to interact with lead ions in solution. In this report, we present the first experimental evidence that picomolar concentrations of lead strongly affect albumin properties in solution. High precision difference Raman and excitation-emission fluorescence spectroscopies are employed to identify the effect of lead ions on albumin. Both spectroscopies proved to be very effective in detecting lead poisoning at a very early stage, setting a new course for bio-inspired inexpensive platform of lead sensing [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U36.00003: Nanofluidic redox-cycling in electrochemical biosensing applications Bernhard Wolfrum, Marcel Zevenbergen, Serge Lemay We have developed a chip-based nanofluidic device which amplifies the sensitivity in electrochemical biosensing applications by orders of magnitude. The amplification is based on rapid redox-cycling between plane parallel electrodes inside a nanochannel. We show that it is possible to monitor the signal of less than a hundred molecules residing in the active area of the nanofluidic sensor. The small number of molecules is reflected in the noise spectrum of the device. In particular, we obtain high sensitivities when detecting catecholamines, which comprise an important group of hormones and neurotransmitters such as serotonin and dopamine. Furthermore, due to the nanochannel design, the sensor is immune to interference by molecules undergoing irreversible redox reactions. We demonstrate the selectivity of the device by detecting target molecules in the presence of ascorbic acid whose oxidized form is only stable on the order of milliseconds. The interference of ascorbic acid is usually a challenge in the detection of catecholamines in biological samples. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U36.00004: Detection of Target Biomolecules by Magnetic Reporting Using Rod-Like Nanosensors R.P. Guertin, E. Goldberg, T.P. Harrah, S. Sonkusale, K. Park, S. Sun, J. I. Oh, M. Naughton We describe the ongoing development of a device to assay a variety of cellular, viral and molecular targets by measuring the increase of the Brownian relaxation time, $\tau $, in solution of magnetically-tagged nanoscale detectors. The shift shows as a frequency reduction of the peak of the complex magnetic susceptibility, $\chi (\omega )$''. Measurements of $\chi (\omega )$'' with 12 nm monodisperse nanoparticles of CoFe$_{2}$O$_{4}$ coated with polyethelyne glycol reveal spectra with the narrowest lines yet reported. Thin avidin coating of these particles reveals small shifts in $\chi (\omega )$''. Bacteriophage T4 tail fibers, engineered to specific lengths (30-150 nm), were employed as the platform for magnetic nanoparticle attachment and at the other end for an inserted target peptide epitope. Attachment of the nanoparticles to bacteriophage T4 tail fibers was successful, though no detectable shifts in $\chi (\omega )$'' were detected due to weak attachment. The advantages associated with non-spherical geometry detectors will be discussed, as will preliminary measurements with rare earth oxide magnetic nanoparticles. Progress on miniaturization and low power requirements of the electronic detection system will be reported. Supported by NERCE/BEID (NIAID). [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U36.00005: A Ringdown Breath Analyzer for Diabetes Monitoring: Breath Acetone in Diabetic Patients. Chuji Wang, Armstrong Mbi, Mark Shepherd It is highly desirable for millions of diabetic patients to have a non-blood, non-invasive, point-of-care device for monitoring daily blood glucose (BG) levels and the adequacy of diabetic treatment and control. Cavity ringdown spectroscopy, due to its unique capability of high sensitivity, fast-response, and relatively low cost for instrumentation, has the potential for medical application through non-invasive analysis of breath biomarkers. We report the first ringdown acetone breath analyzer for clinic testing with diabetic outpatients. The instrument was set in a clinic center and 34 outpatients (24 T1D and 10 T2D) were tested during a four-day period. 10 T1D subjects and 15 nondiabetic persons were tested in our laboratory. Three juvenile-onset T1D subjects were selected for a 24-hr monitoring on the variations of breath acetone and simultaneous BG level. In this talk, we present our research findings including the correlations of breath acetone with BG level and A1C. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U36.00006: Integrating Different Types of Nanowire Sensors in a Large Array Yaping Dan, Stephane Evoy, A.T. Charlie Johnson Biological olfactory systems have a key structural feature: different types of sensors in a large array. Humans, for example, possess several hundred distinct types of sensing cells, a level of sensor diversity not yet achieved in artificial olfactory systems. Here, we demonstrate a simple and low-cost electrochemical approach to integrate large numbers of different types of nanowire sensors in an array on the same silicon wafer. In our approach, nanowires are grown inside an on-chip nanochannel template by electrochemistry with each horizontal channel connected to a gold electrode. This design allows for addressable synthesis of a specific type of nanowire in specified channels by providing a voltage to the electrodes connecting to those channels. The process can be further repeated to produce different types of nanowires in other channels using different electroplating solutions. The scale and diversity of this array have a potential to compete with those of biological olfactory systems and the synthesis process is cost-effective enough for commercialization. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U36.00007: Compact Femtosecond-Millisecond Transient Absorption Spectrometer Elizabeth Carroll, Melissa Hill, Dorte Madsen, Konstantin Malley, Delmar Larsen The measurement of population dynamics in biological, chemical, and solid-state samples occurring over 10$^{-15}$-10$^{1}$ seconds requires a combination of transient absorption techniques, typically involving different laser systems and detection electronics (e.g. femtosecond transient absorption and nanosecond flash photolysis). The difficulty in exactly matching excitation conditions often prohibits connecting ultrafast and longer time measurements, particularly in samples exhibiting nonlinear kinetics. We present a simple solution to bridge the femtosecond and microsecond domains with an inexpensive modification of a kHz amplified Ti:Sapphire laser. By introducing a secondary pulse-picker between the laser oscillator (75 MHz) and amplifier, we can electronically delay unamplified 800-nm probe pulses in 13.3-ns steps. The 5-nJ pulses seed a photonic crystal fiber to produce a supercontinuum (450-1100 nm) for broadband probing. We demonstrate the system capability by resolving formation and decay dynamics, spanning 10 decades (10$^{-14}$-10$^{-4}$ s), of photoexcited solvated electrons in sinapic acid, and triplet states and quinonoid intermediates in Vitamin B$_{6}$. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U36.00008: Realization of Confocal and Hyperspectral Microscopy via Compressive Sensing Ting Sun, Dharmpal Takhar, Jason Laska, Marco Duarte, Vivek Bansal, Richard Baraniuk, Kevin Kelly Given its important role, factors such as sensitivity, resolution, dwell time, and bandwidth limit are critical parameters for detectors in modern microscopy. A new method known as compressive sensing has emerged, which greatly improves the imaging resolution of these detectors. In our configuration, a digital micromirror device randomly but controllably modulates the light before it is collected at the detector. This process simultaneously compresses the signal because the measurement projects the signal onto a white-noise basis. Subsequently, the data from this incoherent basis is reconstructed into a complete real-space image. Given its compressive nature, far fewer measurements are required than the total number of pixels which greatly decreases the acquisition time of the signal. In addition, the intensity of the compressed signal at the detector is much greater than its raster scan counterpart and therefore results in greater signal sensitivity and improved image quality. These advantages make compressive sensing particularly attractive for use in hyperspectral and confocal microscopy. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U36.00009: Fabrication of robust superconducting granular aluminum/palladium bilayer microbolometers with sub-nanosecond response Thomas Wilson We provide a convenient recipe for fabricating reliable superconducting microbolometers as acoustic phonon detectors with sub-nanosecond response, using image-reversal optical lithography and dc-magnetron sputtering, and our recipe requires no chemical or plasma etching. Our approach solves the traditional problem for granular aluminum bolometers of unreliable (i.e., non-Ohmic) electrical contacts by sequentially sputtering the granular aluminum film and then a palladium capping layer. We use dc calibration data, the method of Danilchenko et al., and direct nanosecond-pulsed photoexcitation to obtain the microbolometer's characteristic current, thermal conductance, characteristic relaxation time, and heat capacity. We also demonstrate the use of the deconvolution algorithm of Edwards et al., to obtain the phonon flux in a heat pulse experiment with nanosecond resolutio. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U36.00010: Highly Sensitive Photodetector Based on a Self-assembled Organic Single Submicrometer Ribbon Jian Wang, Yan Zhou, Lei Wang, Jian Pei We demonstrated a highly sensitive air-stable photodetector based on a single submicrometer organic crystalline ribbon self-assembled from a condensed benzothiophene via solution process. The low cost and simple solution process was employed in the device fabrication process from the submicrometer ribbons preparation to the deposition on the substrate. The photoconductivity gain is up to 1.3 $\times $ 10$^{3}$, while the responsivity is about 420 A/W at the field of 2 $\times $ 10$^{4}$ V/cm. The highest on/off ratio reaches around 1000. The performance is comparable to that of photodetectors based on inorganic nanowires, and even better than those based on carbon nanotube or other bilayer molecular self-assembled nanotubes. In addition, the photo-switching properties to those organic photodetectors were investigated with different metal electrodes. The results show that the surface states created by the thermal evaporation of the heavy gold atoms are responsible for the high photo gain and the slow photocurrent decay. To our best knowledge, this is the first report on photodetectors based on crystalline organic 1D submicrometer ribbons self-assembled via solution process, which combine both advantages of intrinsic properties of the 1D crystalline structure and the simplicity of the solution process. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U36.00011: Single carbon nanotube syringe: A model for the study of liquid transport through individual carbon nanotubes Guangyu Chai, Lee Chow The hollow structure of the carbon nanotubes (CNT) and their ability to translocate through plasma membrane of a living cell provide a significant chance to use them as a nano syringe for the delivery of therapeutically active molecules into a live cell. However, the size and the extremely high aspect ratio of the CNTs make the nano syringe device difficult to realize. We successfully prepared a monolithic multiwall CNT with a graphitic shield by chemical vapor deposition technique. The graphitic shield provides a handle which allows the manipulation of the supported CNTs. A single CNT syringe device is fabricated with focused ion beam technique. The well-controlled liquid transport through individual CNT is demonstrated. [Preview Abstract] |
Session U37: Transport Properties of Semiconductors: 2-D Systems
Sponsoring Units: FIAPChair: Giti Khodaparast, Virginia Polytechnic Institute and State University
Room: Morial Convention Center 229
Thursday, March 13, 2008 8:00AM - 8:12AM |
U37.00001: Resistance oscillations in two-dimensional electron systems due to resonant acoustic phonon scattering Michael Zudov, Anthony Hatke, Wenhao Zhang, Loren Pfeiffer, Ken West A few years ago a new class of resistance oscillations was discovered in two-dimensional electron systems subject to weak magnetic fields and elevated temperatures [1]. It was proposed that oscillations originate from resonant interaction with acoustic phonons made possible by virtue of a special selection rule which favors electron backscattering. In contrast to other types of magneto-resistance oscillations, such as those appearing under application of microwave or dc electric fields, phonon-induced resistance oscillations (PIRO) have not received much attention and remain poorly understood. Of particular interest are the period and the phase of PIRO, relative contribution and nature of different phonon modes, and the effect of temperature and sample parameters. This talk will briefly review prior and new experimental results and discuss open issues. \newline [1] M.A. Zudov, I.V. Ponomarev, A.L. Efros, R.R. Du, J.A. Simmons, and J.L. Reno, Phys. Rev. Lett. {\bf 86}, 3614 (2001) [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U37.00002: Effect of dc electric field on resonant acoustic phonon scattering in two-dimensional electron systems Wenhao Zhang, Michael Zudov, Loren Pfeiffer, Ken West We study [1] the effect of dc electric field on transport properties of two-dimensional electron systems in which resonant acoustic phonon scattering dominates linear response resistivity. We observe that dc electric field strongly modifies phonon resonances, transforming resistance maxima into minima and back into maxima. Further, phonon resonances are enhanced dramatically in the non-linear dc response and can be detected even at low temperatures. Most of our observations can be explained by dc-induced (de)tuning of the acoustic phonon resonances and intra-Landau level impurity scattering. We also observe a dc-induced zero-differential resistance state and a resistance maximum which occurs when the electron drift velocity approaches the speed of sound. \newline [1] W. Zhang, M.A. Zudov, L.N. Pfeiffer, and K.W. West, arXiv:0711.1547v1, Phys. Rev. Lett. (accepted for publication). [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U37.00003: Effect of electron-phonon scattering on magneto-transport in 2DES M. G. Vavilov We evaluate the contribution of electron-phonon scattering to the non-linear magneto-resistance of two-dimensional electron systems (2DES). Both linear [1] and differential [2] magneto-resistances show oscillatory dependence on the applied magnetic field in high mobility 2DES. These oscillations originate from the relation between the change of electron energy in a scattering event and the cyclotron frequency. In case of electron scattering with phonons in dc electric fields, the change in electron energy is equal to the sum of the energy of an emitted or absorbed phonon and the change of the electrostatic electron energy due to the shift of an electron cyclotron trajectory. We show that the electrostatic contribution in sufficiently strong fields moves the position of maxima and minima of the differential magnetoresistance. We also explain why the phonon-induced magneto-oscillations exist in the linear response regime only at moderately high temperatures, but appear at significantly lower temperatures in stronger electric fields. [1] M.A. Zudov, et al., Phys. Rev. Lett. {\bf 86}, 3614 (2001). [2] W. Zhang, et al., arXiv:0711.1547v1 (2007). [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U37.00004: Non-linear dc response in microwave-irradiated two-dimensional electron systems: interplay between ac and dc induced effects Anthony Hatke, Wenhao Zhang, Michael Zudov, Loren Pfeiffer, Ken West \def\eac{\epsilon^{\mbox{{\scriptsize ac}}}} We study nonlinear dc response of a high-mobility two- dimensional electron system subject to microwave (ac) excitation and weak magnetic fields. Recent experiments [1] studied resistance at different dc excitations as a function of the ratio of the microwave frequency to the cyclotron frequency, $\eac$. Here, we examine oscillations in microwave photoconductivity as a function of dc excitation, at different values of $\eac$. We find that, for the most part, the oscillation period is the same as in the dark resistivity, and the phase is determined by microwave-induced oscillations at zero dc bias, consistent with the earlier results. However, at some excitation values previously associated with resistance maxima, this approach revealed resistance minima indicating saddle points in the resistivity. We further observed that the oscillation amplitude itself oscillates as a function of $\eac$, with the oscillations strongly suppressed near half-integral values. These findings indicate the limitations of the simplified resonant condition proposed in Ref. 1 and might stimulate further theoretical studies. \newline [1] W. Zhang, M.A. Zudov, L.N. Pfeiffer, and K.W. West, Phys. Rev. Lett. {\bf 98}, 106804 (2007) [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U37.00005: Zero differential resistance state in 2D dimensional electron system in strong magnetic field. Sergey Vitkalov, Jing-Qiao Zhang, A. A. Bykov , A. K. Kalagin, A. K. Bakarov We report the observation of a zero differential resistance state (ZDRS) in response to direct current above a threshold value $I>I_{th}$ applied to a two-dimensional system of electrons at low temperatures in a strong magnetic field. Entry into the ZDRS, which is not observable above several Kelvins, is accompanied by a sharp dip in the differential resistance. Additional analysis reveals instability of the electrons for $I>I_{th}$ and an inhomogeneous, non-stationary pattern of the electric current. We suggest that the dominant mechanism leading to the new electron state is a redistribution of electrons in energy space induced by the direct current. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U37.00006: Influence of a Parallel Magnetic Field on the Microwave-Induced Resistance and Photovoltaic Oscillations Chi Zhang, Kristjan Stone, Rui-rui Du, Changli Yang, Loren Pfeiffer, Ken West Microwave induced photovoltaic (PV) and resistance oscillations (MIRO) were studied in high-mobility ($\mu >$ 8$\times $10$^{6}$ cm$^{2}$/V s) 2D electron gas in GaAs/Al$_{x}$Ga$_{1-x}$As Hall bar samples employing a two-axis magnet system (perpendicular field B$_{\bot }$ and parallel field B//). Consistent with the previous results, strong MIRO were observed and were found to diminish under a B// $\sim $ 1 T. We observed two types of PV oscillations: 1) PV oscillations that are periodic in 1/B$_{\bot }$, with a periodicity similar to MIRO, but are anti-symmetric with respect to B$_{\bot }$ = 0; and, 2) PV oscillations due to edge magnetoplasmon modes, which are periodic in B$_{\bot }$ and are symmetric with respect to B$_{\bot }$ = 0. Characteristically, the 1/B$_{\bot }$ oscillations in PV were completely suppressed by a B// $\sim $ 1 T, whereas the B$_{\bot }$-periodic oscillations retain their main features even in B// = 2 T. Experimental data and a brief discussion will be presented. The work at Rice was supported by NSF DMR-0706634. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U37.00007: Microwave-Induced Resistance Oscillations in Non-Faraday Configurations Kristjan Stone, Zhuoquan Yuan, Rui-Rui Du, Changli Yang, Loren Pfeiffer, Ken West The microwave-induced resistance oscillations (MIRO) are commonly observed in high-mobility GaAs 2D electron systems (2DES), typically using a Faraday configuration. In a Faraday configuration, the electromagnetic components (E$_{\omega }$ and H$_{\omega })$ coincide with the 2DES plane. We explore MIRO in a microstrip line geometry, in which the dominant excitation component in the 2DES plane is H$_{\omega }$. Our samples were 100 or 200 $\mu $m wide Hall bars of very-high mobility ($\mu \quad >$ 8 $\times $ 10$^{6}$ cm$^{2}$/Vs) GaAs/Al$_{x}$Ga$_{1-x}$As heterojunctions or quantum wells with electron density ranging from 2.1 - 7.0 $\times $ 10$^{11}$/cm$^{2}$. Microwaves from a tunable source (2 GHz - 40 GHz) were fed via a semi-rigid coax cable to the microstrip line over the length of the Hall bar. In a temperature range of 0.3 K -- 2.0 K, we observed strong MIRO in all the samples measured. We have studied the fractional MIRO using both the microstrip line and dipole antenna geometries. By increasing the MW power, MIRO features associated with $\varepsilon $ = 1/2, 1/3, and 1/4 emerged, where $\varepsilon =\omega $/$\omega _{c}$, and $\omega _{c}$ is the cyclotron frequency. Experimental data as well as a brief discussion will be presented. The work at Rice was funded by NSF DMR-0706634. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U37.00008: Low Temperature Scanning Hall Probe Microscopy of 2D Electron Nanostructures Zhuoquan Yuan, Yanhua Dai, Ruirui Du, L.N. Pfeiffer, K.W. West Current distribution can provide key information on microscopic properties of 2D electron systems (2DES) in the regime of quantum transport. However, imaging coherent electron flow is proven to be experimentally challenging. We developed a method to image current distribution by using low temperature (0.3K), high spatial resolution ($<1\mu m)$ scanning Hall probe. We imaged the local magnetic field component perpendicular to the 2DES in GaAs/AlGaAs samples, and then used Fast Fourier Transform (FFT) technique to recover the current distribution from the data of magnetic field. As an example, we will present the data and a brief discussion of imaging geometrical resonance in anti-dot lattices patterned on a very high mobility 2DES. The research at Rice was supported by NSF DMR-0706634. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U37.00009: Spin-Orbit Interaction in High-$\kappa $ Dielectric Gated Rashba-2D Electron Gas and Mesoscopic Rings Yanhua Dai, Zhuoquan Yuan, Kristjan Stone, Rui-Rui Du, Min Xu, Peide Ye There is increasing current interest in the quantum interference effect in mesoscopic devices fabricated on a Rashba-2D electron gas (2DEG), where the spin-orbit interaction parameters can be tuned by a potential gate. We explore ring structures that use a gate consisting of thin (5nm-50nm) high-$\kappa $ dielectric Al$_{2}$O$_{3}$ or HfO$_{2}$ layer and nano-patterned metals. The 2DEG is provided by lattice-matched $In_{0.52} Al_{0.48} As/In_{0.53} Ga_{0.47} As/In_{0.52} Al_{0.48} As$ quantum wells that have a typical electron density n of $1.5\times 10^{12}/cm^2$ and mobility $\mu \ge 2\times 10^4cm^2/Vs$. The dielectric material was grown by atomic layer deposition. We will present the gate characteristics of Hall bars as well as magnetic transport data from gated mesoscopic rings. The work at Rice is funded by NSF DMR-0706634. Reference: M. Konig et al, Phys. Rev. Lett. 96, 076804 (2006); T. Bergsten et al, Phys. Rev. Lett. 97, 196803 (2006); B. Grbic et al, Phys. Rev. Lett. 99, 176803 (2007). [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U37.00010: Novel interaction-induced magneto-oscillations in ac conductivity of 2D electron gas Tigran Sedrakyan, Mikhail Raikh We demonstrate that electron-electron interactions in a high-mobility 2D electron gas give rise to the {\em oscillatory} correction, $\delta\sigma^{int}(\omega)$, to the ac magnetoconductivity, $\sigma(\omega)$. Similarly to the conventional single-particle harmonics of the cyclotron resonance, the oscillating correction is periodic in $\omega_c^{-1}$, where $\omega_c$ is the cyclotron frequency. However, unlike the single-particle oscillations, which are periodic with $\omega$, the interaction correction is periodic with $\omega^{3/2}$. Oscillatory behavior of the interaction-induced magnetoconductivity develops at very low magnetic fields, $\omega_c\ll\omega$; at such fields the conventional harmonics are suppressed by the disorder. The underlying physical process of the new effect is {\em double} backscattering of an electron from the impurity-induced Friedel oscillations. Unlike the case of single-particle oscillations, the electron travels only a {\em small portion} of the Larmour circle during the time $\sim\omega^{-1}$ between the two backscattering events. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U37.00011: Many-body local field corrections to spin Coulomb drag in a quasi-two-dimensional electron system Samvel Badalyan, Chang Sub Kim, Giovanni Vignale We investigate the effect of exchange and correlation on spin Coulomb drag in a quasi-two-dimensional electron gas of finite transverse width. We find that the finite transverse width of the electron gas causes a significant reduction of the spin Coulomb drag. This reduction, however, is largely compensated by the enhancement coming from the inclusion of many-body local field corrections beyond the random phase approximation. Our calculations are in very good agreement with and confirm the experimental observations of the spin Coulomb drag by C. P. Weber \textit{et al.}, Nature, \textbf{437}, 1330 (2005). [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U37.00012: Evidence of Coulomb Drag between Anderson Insulators Kareem Elsayad, John Carini, David Baxter We report observations of Coulomb drag between 200 Angstrom thick co-sputtered insulating amorphous silicon-niobium alloy films, separated by a thin silicon-oxide barrier. An apparent linear-response regime for the transresistance is found to only exist over a narrow range of layer separations ($\sim $100 Angstroms) and material parameters (niobium concentrations $\sim $ 7{\%}) at low driving currents ($\sim $1nA) and temperatures below $\sim $20 Kelvin. The temperature dependence, as well as the magnitude, of the transresistance in this regime is consistent with predictions for that between Anderson insulators with long ranged intra-layer Coulomb interactions, provided that the density of states of the silicon-niobium layers are taken to be that of effectively 3-dimensional systems. This is in contrast with measurements of the temperature dependence of the dc layer-conductivity in such bilayer systems, which suggest that transport should be effectively 2-dimensional at these energies. We will discuss the fabrication and characterization of bilayer samples, as well as possible explanations for the observed magnitude and temperature dependence of the transresistance. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U37.00013: Scaling of thermoelectric voltage induced by microwave radiation at the boundary between two-dimensional electron systems N. Romero Kalmanovitz, I. Hoxha, Y. Jin, S.A. Vitkalov, M.P. Sarachik, I.A. Larkin, T.M. Klapwijk We report measurements of the rectification of microwave radiation ($0.7$-$20$ GHz) at the boundary between two- dimensional electron systems created by a narrow gap split gate on a silicon surface for different temperatures, electron densities and microwave power. For frequencies above $4$ GHz and different temperatures, the rectified voltage $V_{dc}$ as a function of microwave power $P$ can be collapsed onto a single universal curve $V_{dc}^{*}=f^*(P^{*})$ using two scaling parameters. The scaled voltage, $V_{dc}^{*}$, is a linear function of power, $P^{*}$, for small power and proportional to $(P^*)^{1/2}$ at higher power. A theory is developed which attributes the observed voltage to the thermoelectric response associated with local heating by the microwave radiation of adjacent two-dimensional electron systems with different densities $n_1$ and $n_2$. Excellent quantitative agreement is obtained between theory and experiment. *The work at the City College of New York was supported by DOE grant DOE-FG02-84-ER45153. The work at International Center of Condensed Matter Physics, Brasilia, was supported by IBEM fund from Brazilian Ministry of Science and Technology. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U37.00014: Charged Excitations of a Two Dimensional Electron System Oliver Dial, Ray Ashoori, Loren Pfeiffer, Ken West Despite the central role that the tunneling (or single-particle) particle density of states (TDOS) plays in our theories of many- body systems, it has proven a difficult quantity to access experimentally in two dimensional electron systems (2DES). We have developed a technique, time domain capacitance spectroscopy, which allows measurement of the TDOS over a range of 30 meV centered about the Fermi surface, revealing the detailed structure present in these systems far from the Fermi energy. Remarkably, we observe a long-lived excitation in the 2DES whose creation requires more energy than is needed to eject an electron from the most tightly bound state in the 2DES. Based on its energy as a function of the 2D electron density and behavior when a magnetic field is applied, we identify this excitation as a hole in the 2DES coupled to a plasmon. Such a ``plasmaron'' has been predicted in calculations of the density of states for 3D [1] and 2D [2] electron gases, but it has never been definitively observed. These spectra may represent the first observation of this long-predicted quasiparticle in the 2DES. [1] L. Hedin, B.I. Lundqvist, and S. Lunqvist. Solid State Comm. 5, 237–239 (1967). [2] P. von Allmen. Phys. Rev. B 46, 13345 (1992). [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U37.00015: Effects of finite layer thickness on the differential capacitance of electron bilayers J.J. Durrant, C.B. Hanna We have calculated the effects of the finite thickness of electron or hole layers in double-quantum-well systems on the complete set of differential capacitances that can be measured in double-layer electron systems, with or without separately contactable layers.~ We~present results for~the regime of negligible interlayer tunneling, zero applied magnetic field, and low layer densities, when the compressibility of~one or~both layers is negative. [Preview Abstract] |
Session U38: Focus Session: Ferroelectric Oxide Superlattices and Oxide Thermoelectrics
Sponsoring Units: DCMPChair: Jean-Mare Triscone, University of Geneva
Room: Morial Convention Center 230
Thursday, March 13, 2008 8:00AM - 8:12AM |
U38.00001: Thermoelectric response of Lanthanum-doped SrTiO$_{3}$ thin-films grown under various oxygen partial pressures Matthew L. Scullin, Jayakanth Ravichandran, Mark Huijben, Choongho Yu, Subroto Mukerjee, Joel Moore, Arun Majumdar, R. Ramesh Doped strontium titanate is a strong candidate for the next-generation high-$Z$ bulk thermoelectric material due to both its wide tunability in transport properties and very large carrier effective mass. Thermoelectric thin-films of Sr$_{1-x}$La$_{x}$TiO$_{3-\delta }$ with various La content were grown via pulsed-laser deposition (PLD) on (001)-oriented LSAT substrates under various oxygen partial pressures. We find that electron transport is dominated by carriers from oxygen vacancies in samples grown at low $p_{O2} \quad <$ 10$^{-6}$ Torr, and that thermopower as high as 1000 \textit{$\mu $}V/K can be achieved even in heavily La-doped samples. Doping combinations that yield resistivities as low as 5 m$\Omega $-cm yield power factors @ 300K as high as 0.7 W/m-K, implying \textit{ZT} $>$ 0.1. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U38.00002: ``Pudding mold"-type band dispersion as the origin of large thermopower in Na$_x$CoO$_2$ Kazuhiko Kuroki, Ryotaro Arita Na$_x$CoO$_2$ is an interesting material in that it has large thermopower, non-trivial magnetic property, and becomes superconducting when water molecules are intercalated. Recently, in Ref. 1 and 2, we proposed that the magnetism and the superconductivity can have the same root. Namely they can originate from the peculiar band dispersion of a$_{1g}$ band. In this study, by using Boltzmann's equation, we calculated the thermopwer of this system. We found that the origin of the coexistence of the large thermopower and the low resistivity in this material is that not just the density of states, the effective mass, nor the band width, but indeed again the peculiar ``pudding mold''-type band dispersion of the a$_{1g}$ band which consists of a dispersive portion below the Fermi level and a dispersionless portion above the Fermi level[3]. \newline [1] K. Kuroki et al., Phys. Rev. B 73, 184503 (2006). \newline [2] K. Kuroki et al., Phys. Rev. Lett. 98, 13601 (2007). \newline [3] K. Kuroki and R. Arita, J. Phys. Soc. Jpn. 76 (2007) 083707 [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U38.00003: Thermoelectric properties of Bi$_{2}$Sr$_{2}$Co$_{2}$O$_{y}$ thin films grown by pulsed laser deposition Shufang Wang, Venimadhav Adyam, Shengming Guo, Qi Li, Xiaoxing Xi Epitaxial and c-axis preferred oriented oxide thermoelectric Bi$_{2}$Sr$_{2}$Co$_{2}$O$_{y}$ thin films have been deposited on LaAlO$_{3}$ (100), Al$_{2}$O$_{3 }$(0001) and fused silica substrates using pulsed laser deposition. At room temperature, the Seebeck coefficient and resistivity are of the order of 125 $\mu $V/K, 120 $\mu $V/K, 110 $\mu $V/K and 3 m$\Omega $ cm, 2 m$\Omega $ cm, 14 m$\Omega $ cm for the films on LaAlO$_{3}$ (100), Al$_{2}$O$_{3 }$(0001) and silica substrates respectively. A large negative in-plane magnetoresistance (MR) is observed in the films at low temperatures, with a MR reaching 41{\%} at 9 T and T=1.8 K in films on LaAlO$_{3}$ (100). We also observed a large bias current-dependent resistivity in the films at low temperature, which has been attributed to the suppression of spin-density-wave by electric field. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U38.00004: Localization of holes at oxygen sites in a thermoelectric rhodate revealed by photoemission spectroscopy Ishida Yukiaki, Baba Teruhisa, Eguchi Ritsuko, Matsunami Masaharu, Taguchi Munetaka, Chainani Ashish, Senba Yasunori, Ohashi Haruhiko, Okamoto Yoshihiko, Takagi Hidenori, Shin Shik We have performed soft-x-ray absorption and photoemission studies on Sr1-xRh2O4 [1], a thermoelectric material structurally and electronically analogous to NaxCoO2. Metal-insulator transition occurs with hole doping into the Rh 4d t2g band via introducing Sr vacancies [1]. Valence-band spectra of Sr1-xRh2O4 showed satellite structures $\sim $11 eV below the Fermi level, which cannot be explained by LDA or LDA+U calculations. The O 1s2p2p Auger peak in the photoemission spectra appeared from the 11-eV satellite in the vicinity of the O 1s absorption edge, indicating that the satellite is a sign of holes localized at oxygen sites. Concomitantly, doping dependent changes appeared mainly in the O 1s absorption spectra rather than in the Rh 3p3/2 absorption spectra. We discuss inhomogeneous evolution of the system with hole doping along with Sr-vacancy potential and d-p hybridization. [1] Y. Okamoto, et al., J. Phys. Soc. Jpn . 75, 023704 (2006). [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U38.00005: Quantum Critical Paraelectrics and the Casimir Effect in Time Lucia Palova, Premala Chandra, Piers Coleman We present a study of the quantum paraelectric-ferroelectric transition (QPFT) with special emphasis on temperature near a quantum critical point where it acts as a boundary condition in imaginary time. The effect of temperature on quantum critical fluctuations can be likened to the Casimir effect where critical electromagnetic fluctuations are probed through their sensitivity to boundary conditions in space. Exploiting this analogy, we use finite-size scaling methods to study the finite-temperature properties of a quantum paraelectic in the vicinity of a QPFT. Simple arguments are used to derive the $1/T^{2}$ divergence of the paraelectric susceptibility ($\chi$) previously found with diagrammatic techniques. Self-consistent mean-field theory is used to probe the classical-quantum crossover in $\chi$ and the resulting temperature-pressure phase diagram is presented. Observable consequences of our scaling approach for high pressure measurements on strontium titanate and potassium tantalate at low temperatures will also be discussed. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U38.00006: Effective Hamiltonian Approach to Ferroelectric Ordering in Oxide Superlattices Jun Hee Lee, Umesh V. Waghmare, Jaejun Yu We developed an effective Hamiltonian approach to the ferroelectric ordering in oxide superlattices. Despite that recent experiments have made a huge progress in making layer-by- layer heterostructures of ferroelectric superlattices, direct first-principles calculations are so far limited to the finite- size calculations, and electrostatic models are too simple to reflect intricate interactions of superlattices in atomic scale. We took the effective Hamiltonian method as one of candidates for the calculations of ferroelectric superlattices not only because it has enough degrees-of-freedom of energy terms to reflect complex interactions such as dipole-dipole interaction and short-range repulsion, but because it needs small amount of computational load compared with first- principles. All the parameters in the effective Hamiltonian are predetermined from the first-principles of constituent bulk components. As an application of the model parameters, we calculated the polarization of (BaTiO$_3$)$_n$/(SrTiO$_3$)$_m$, the results of which are in good agreement with those of previous first-principles calculations for not only of average polarizations but also of local polarizations. This effective Hamiltonian procedure can provide for developing a model for other kinds of oxide superlattices. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:48AM |
U38.00007: Predicting polarization and nonlinear dielectric response of arbitrary perovskite superlattice sequences Invited Speaker: A complete theory of epitaxial perovskite superlattices requires an understanding both of epitaxial strain effects and of electrostatic boundary conditions. Here, focusing on the latter issue, we\footnote{In collaboration with Massimiliano Stengel, Karin M. Rabe and David Vanderbilt.} have carried out first-principles calculations of the nonlinear dielectric properties of short-period ``bicolor'' and ``tricolor'' CaTiO$_3$/SrTiO$_3$/BaTiO$_3$ superlattices having the in-plane lattice constant of SrTiO$_3$. In particular, we have calculated the layer polarizations $p_j$ as defined using the Wannier-based method of Wu, Di\'eguez, Rabe and Vanderbilt\footnote{X. Wu, O. Di\'eguez, K. Rabe and D. Vanderbilt, Phys.\ Rev.\ Lett.\ {\bf 97}, 107602 (2006).} for each neutral BaO, SrO, CaO, or TiO$_2$ layer. We use a cluster expansion (CE) technique to model the layer polarizations $p_j$ of a selected set of bicolor superlattices as a function of the displacement field $D$ (which is uniform throughout the insulating superlattice), the chemical identity of the layer itself, and the chemical identity of its neighboring layers. We find that $p_j$ is a strongly localized function of its chemical environments at fixed $D$ field, i.e., the dependence on the identity of the neighboring layers decays rapidly with distance. This localized property enables us to arrive at a truncated and simplified CE model which can accurately predict $p_j(D)$ in arbitrary layer sequences, both bicolor and tricolor. A similar approach is used to model the dependence of the $c$ lattice constant. With all this information in hand, we can predict the polarization, piezoelectric and nonlinear dielectric response of arbitrary superlattice sequences. The power of the approach is demonstrated by showing that a model fitted only to calculations on inversion-symmetric bi-color superlattices can successfully predict the inversion symmetry breaking in tricolor superlattices such as 2SrTiO$_3$/1BaTiO$_3$/1CaTiO$_3$. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U38.00008: First-principles study of piezoelectric response in PbTiO$_{3}$/SrTiO$_{3}$ 1 x 1 superlattice Yanpeng Yao, Huaxiang Fu Artificial superlattices composed of two materials of similar structure but distinct properties are a promising group of new materials to achieve modified properties. For ferroelectric materials, it is well known that strain effects due to lattice mismatch can considerably alter the properties of ferroelectric thin films. Here we perform first-principles density functional calculations to study the structure and polarization response of PbTiO$_{3}$/SrTiO$_{3}$ (PT/ST) 1x1 superlattice to varied inplane strain. The superlattice is chosen to be grown along the [001] crystallographic direction, and is assumed to be tetragonal. For a given inplane strain, the out-of-plane lattice constant and atomic positions are relaxed. Similar calculations on PT, ST single materials are also performed for comparison. We find that by increasing strain, the superlattice has combined features as those of PT and ST single materials. Being unpolar without strain, the superlattice becomes ferroelectric at a critical inplane lattice constant larger than that of ST. The PT/ST 1x1 superlattice is shown to have a much larger piezoelectric response than PT, a property similar to ST. But, compared to ST, the large piezoelectric response in PT/ST is easier to realize, since a smaller inplane strain is required. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U38.00009: Improper ferroelectricity in perovskite oxide artificial superlattices Matthew Dawber, Nicolas Stucki, Celine Lichtensteiger, Jean-Marc Triscone, Eric Bousquet, Patrick Hermet, Philippe Ghosez In paraelectric/ferroelectric heterostructures with thick constituent layers electrostatics is the dominant interaction between layers and we have previously demonstrated that the key ferroelectric parameters, polarization and critical temperature can be tuned over a very large range in PbTiO$_{3}$/SrTiO$_{3}$ superlattices by varying the ratio of the layer thicknesses [1]. However, as the layers become thinner, a departure from the electrostatic model is observed, which manifests itself as an unusually high ferroelectric polarization and transition temperature and a high, but temperature independent, dielectric constant. Detailed examination of the phase transitions with temperature reveal that along with these enhanced characteristics there is a fundamental change in the nature of the ferroelectricity. The microscopic origin of this change, a form of improper ferroelectricity, is revealed by first principles calculations to occur through a coupling of oxygen rotations and the polarization mode at the interfaces in the material. \\ $[1]$ M. Dawber et al., Adv. Mat. (2007) [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U38.00010: A new hybrid exchange-correlation functional for accurate prediction of the electronic and structural properties of ferroelectric oxide bulks and nanostructures D.I. Bilc, R. Shaltaf, J. \'I\~niguez, Ph. Ghosez We report a systematic comparison of various DFT and hybrid exchange-correlation functionals for the prediction of the electronic and structural properties of prototypical ferroelectric oxides. We find that some, although not all, usual DFT functionals predict the structure with acceptable accuracy, but always underestimate the electronic band gaps. Conversely, common hybrids yield an improved description of the band gaps, but overestimate the volume and atomic distortions associated to ferroelectricity, giving rise to an unacceptably large $c/a$ ratio for the tetragonal phases of BaTiO$_3$ and PbTiO$_3$. This {\it super-tetragonality} is found to be induced mainly by the GGA exchange energy. We thus propose an alternative hybrid functional, B1-WC, that mixes exact exchange following the B1 scheme with the recently improved GGA proposed by Wu and Cohen. This B1-WC renders accurate description of both the structural and electronic properties of typical ferroelectric oxide at the bulk level. The case of Pt/BaTiO$_3$ heterostructures is also considered and a comparison of the of B1-WC and LDA results for atomic relaxation and electronic band alignment at the interface is presented. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U38.00011: First principles calculations at fixed dielectric displacement Massimiliano Stengel, David Vanderbilt, Nicola Spaldin With the experimental advances in growth and characterization of superlattices and thin-film capacitors based on ferroelectric perovskites, it is becoming more and more crucial to achieve a fundamental understanding of these structures by accurate first-principles modeling. Unfortunately, difficulties in the treatment of finite electric fields and macroscopic polarization in periodic systems - in particular when metallic electrodes are present - make the application of \emph{ab-initio} techniques to such systems particularly challenging. To address these issues, we present here a method to perform first-principles calculations of periodic systems at a fixed value of the macroscopic dielectric displacement. This technique, which complements previously established strategies to fix $P$ or $\mathcal{E}$, provides a simple and natural way to explore the phase diagram of a ferroelectric system as a function of a single electrical order parameter. We demonstrate the power of our approach by computing the electrical equations of state for two symmetrical ferroelectric capacitors which are based on the same combination of materials, Au and BaZrO$_3$, but are characterized by inequivalent interface geometries and stoichiometries. In particular, we use the information extracted from the centrosymmetric systems to accurately predict the electrical equation of state for an asymmetric capacitor of analogous composition. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U38.00012: Broadband Permittivity Measurements of Ruddlesden-Popper Sr$_{n+1}$Ti$_{n}$O$_{3n+1 }$(n=1,2,3) Thin Films N. Orloff, W. Tian, D. Schlom, J. Booth, I. Takeuchi In order to explore the microwave dielectric response of Sr$_{2}$TiO$_{4}$, Sr$_{3}$Ti$_{2}$O$_{7}$, and Sr$_{4}$Ti$_{3}$O$_{10}$ thin films, we have performed broadband in-plane quantitative complex permittivity($\varepsilon )$ measurements on Sr$_{n+1}$Ti$_{n}$O$_{3n+1 }$(n=1,2,3) thin films in the frequency range 100Hz-40GHz. The films, of approximately 160 nm thickness, were fabricated by molecular beam epitaxy[1], and standard lithographic techniques were used to define coplanar waveguide transmission lines and interdigitated capacitors using gold. We extracted $\varepsilon $ from the measured complex S-parameters (.01-40GHz) and the complex impedance (100Hz-.001GHz), which were measured at 70K, 150K, 200K, and 250K using a cyrogenic probe station. We found that below $\sim $10GHz the $\varepsilon $'s of these thin films were approximately constant with frequency: $\varepsilon \approx $38, 48, and 100 for Sr$_{n+1}$Ti$_{n}$O$_{3n+1 }$(n=1,2,3) respectively. In addition, the measured value for $\varepsilon $ of Sr$_{2}$TiO$_{4}$ is consistent with recent theoretical calculations [2]. We will discuss in detail the temperature and electric field dependence of the measured complex $\varepsilon $ for these material systems. [1] J.H. Haeni, \textit{et al} APL, \textbf{78}, 21 (2001) [2] C.J. Fennie and M.K. Rabe, PRB, \textbf{68}, 184111 \textbf{(}2003) [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U38.00013: Competing structural instabilities in Ti-based layered-perovskite-oxide superlattices Serge Nakhmanson Utilizing first-principles computational techniques, we have mapped out structural instabilities in the Ruddlesden-Popper homologous oxide superlattice families with a general chemical formula A$_{n-1}$A$^{\prime}_{2}$Ti$_{n}$O$_{3n+1}$, A = Sr, Ba, Pb (perovskite-type block) and A$^{\prime}$ = Sr (rocksalt-type block), for $n$ = 1--5. Our calculations show that each superlattice family has a unique set of ``instability footprints'' --- including the ferroelectric, antiferroelectric and antiferrodistortive types --- which may or may not have a strong coupling to epitaxial strain. Furthermore, the existence and strength of structural instabilities within each particular family change dramatically with an increasing number of perovskite-type layers $n$, granting us wide flexibility to fine-tune the properties of these materials for various device applications or, e.g., for integration into composites with magnetic Ru- or Mn-based layered-perovskite-oxide compounds. [Preview Abstract] |
Session U39: Focus Session: Structure and Dynamics of Complex Networks
Sponsoring Units: GSNPChair: Sidney Redner, Boston University
Room: Morial Convention Center 231
Thursday, March 13, 2008 8:00AM - 8:36AM |
U39.00001: From network dynamics to human activity and mobility patters Invited Speaker: The next challenge of network research is to go beyond the structure and quantify the dynamics of interconnected systems. A particular difficult facet of this research requires us to understand the temporal and spatial driving forces that govern social, technological and biological networks. In this talk I will focus on the dynamical mechanism that drive the activity of social networks. While none of us thinks of our daily activity pattern as random, most modeling efforts approximate human activity with fundamentally random spacial and temporal patterns. My purpose is to offer evidence of a series of significant deviations from this random expectation. I will talk about the bursty temporal character of human activity patterns and the travel patterns of individuals. I will show that both human activity and travel patterns are far more regular than the standard Poisson and diffusion models would predict, with implications on agent based models, epidemic modeling as well as the nature of time and space experienced by humans. The work was done in collaboration with Marta Gonzales, Cesar Hidalgo, Kwang-il Goh, Joao Oliveria, and Alexei Vazquez. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U39.00002: Finite Size Effects and Symmetry Breaking in the Evolution of Networks of Competing Boolean Nodes Kevin Bassler, Min Liu The effects of finite network size on the evolutionary dynamics of a Boolean network are analyzed. In the model considered, Boolean networks evolve via a competition between nodes that punishes those in the majority. Finite size networks evolve in a fundamentally different way than infinitely large networks do. The symmetry of the evolutionary dynamics of infinitely large networks that selects for canalizing Boolean functions is broken in finite size networks. In finite size networks there is an additional selection for input inverting Boolean functions. Classes of functions are found empirically to evolve with the same frequency. The classes depend on the symmetry of the evolutionary dynamics and correspond to orbits of the relevant symmetry group. The empirical results match analytic results, determined by utilizing Polya's theorem, for the number of orbits expected in both finite size and infinitely large networks. The reason for the symmetry breaking is due to the need for nodes in finite size networks to behave differently in order to cooperate to collectively perform efficiently. The results suggest that both finite size effects and symmetry are important for understanding the evolution of real-world complex networks, including genetic regulatory networks. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U39.00003: Attractors in continuous and Boolean networks Johannes Norrell, Joshua Socolar, Bj\"orn Samuelsson Random Boolean models of complex regulatory networks are known to exhibit rich dynamical behaviors, including an order/disorder transition. We show that implementation of the nominal Boolean logic of a network using differential equations involving sigmoidal switching functions generically leads to deviations from the Boolean predictions. On simple rings, the ``reliable'' set of Boolean attractors corresponds to the stable attractors of the analogous continuous system. For networks with more complex logic, however, the set of the continuous attractors is determined by non-Boolean characteristics of the switching events. In large random networks, the nature of the order/disorder transition is altered by collective effects associated with compositions of the sigmoidal switching functions. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U39.00004: Scale-Free Overlay Topologies with Hard Cutoffs for Unstructured Peer-to-Peer Networks Hasan Guclu, Murat Yuksel The topology have profound impact on the efficiency of search on unstructured peer-to-peer (P2P) networks as well as other networks. It has been well-known that search on scale-free (power-law) topologies offer outstanding search efficiency as good as $O(\ln \ln N)$ for a range of degree distribution exponents. However, generation and maintenance of such scale-free topologies are hard to realize in a distributed and potentially uncooperative environments as in the P2P networks. A key limitation of scale-free topologies is the high load (i.e. high degree) on very few number of hub nodes. In a typical unstructured P2P network, peers are not willing to maintain high degrees/loads as they may not want to store large number of entries for construction of the overlay topology. So, to achieve fairness and practicality among all peers, hard cutoffs on the number of entries are imposed by the individual peers. Thus, efficiency of the flooding search reduces as the size of the hard cutoff does. Interestingly, we observe that the efficiency of normalized flooding and random walk search algorithms increases as the hard cutoff decreases. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U39.00005: A Self--organized model for network evolution Guido Caldarelli, Andrea Capocci, Diego Garlaschelli Here we present a self-organized model for the evolution of complex networks. Vertices of the network are characterized by a variable evolving through an extremal dynamics process. The network topology is in turn shaped by the variable itself. More specifically, to each vertex a fitness is assigned; then, in the evolution, the vertex with minimum fitness and its neighbors are updated by extracting new fitnesses. For any given realization of fitnesses we can determine the edges in the network through a fitness dependent rule. We show analytically and numerically that this system self--organizes to a nontrivial state. A power--law decay of dynamical and topological quantities above a threshold emerges spontaneously, as well as a feedback between different dynamical regimes and the underlying network's correlation and percolation properties. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U39.00006: Money circulation networks reveal emerging geographical communities D. Brockmann, F. Theis, V. David Geographical communities and their boundaries are key determinants of various spatially extended dynamical phenomena. Examples are migration dynamics of species, the spread of infectious diseases, bioinvasive processes, and the spatial evolution of language. We address the question to what extend multiscale human transportation networks encode geographical community structures, how they differ from geopolitical classifications, whether they are spatially coherent, and analyse their structure as a function of length scale. Our analysis is based on a proxy network for human transportation obtained from the geographic circulation of more than 10 million dollar bills in the United States recorded at the bill tracking website www.wheresgeorge.com. The data extends that of a previous study (Brockmann et al., Nature 2006) on the discovery of scaling laws of human travel by an order of magnitude and permits an approach to multiscale human transportation from a network perspective. [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U39.00007: Functional structure through dynamic clustering of neuronal networks Sarah Feldt, Michal Zochowski We propose a new method for detecting functional structure in neuronal networks based solely upon the information derived from the spike timings of the neurons. Unlike traditional algorithms that depend on knowledge of the topological structure of the network to parse the network into communities, we dynamically cluster the neurons to build communities with similar functional interactions. We define means to derive optimal clustering parameters and investigate what conditions have to be fulfilled to obtain reasonable predictions of functional structures. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U39.00008: Citation analysis: Beyond the Journal Impact Factor Manolis Antonoyiannakis The journal impact factor is a robust measure of the average citation performance of a journal, but the number of citations varies widely from paper to paper within any journal. Therefore, it makes sense to look for additional ways of characterizing journals in terms of their impact. We introduce the ``citation density curve'' (citations per paper in a given year for papers published in the previous two years, plotted vs. the citation rank of these papers). This curve, which displays a Zipf's law behavior, contains all the pertinent information about a journal: its size, its impact factor, the maximum number of citations per paper, the relative size of the top-cited portion of the journal, how the citation density varies within the journal, etc. Being the ``fingerprint'' of a journal, the citation density curve can be used: (a) by editors, for strategic decisions affecting the future of their journal; (b) by citation analysts, for comparing (ranking) journals; and (c) by authors, for assessing the relative impact of their published work. Further, we identify a complementary metric to the impact factor, a single number that characterizes the top-cited portion of a journal. This metric reproduces the ranking of the citation density curves for various journals, and can be readily calculated from the same data used in the impact factor calculations. We propose that this new metric be used as an essential complement to the impact factor in assessing the true impact of journals. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U39.00009: Epidemics on adaptive networks with geometric constraints Leah Shaw, Ira Schwartz When a population is faced with an epidemic outbreak, individuals may modify their social behavior to avoid exposure to the disease. Recent work has considered models in which the contact network is rewired dynamically so that susceptibles avoid contact with infectives. We consider extensions in which the rewiring is subject to constraints that preserve key properties of the social network structure. Constraining to a fixed degree distribution destroys previously observed bistable behavior. The most effective rewiring strategy is found to depend on the spreading rate. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U39.00010: Some aspects on human preference in communication and friendship Diego Rybski, Hern\'an D. Rozenfeld, Fredrik Liljeros, Shlomo Havlin, Hern\'an A. Makse The objects of our investigation are social networks consisting of individual actants as nodes and their relations as links. Recently, on-line communities have gained immense popularity as indicated by millions of members participating in these platforms. Fortunately, the information given by member activity provides an ideal environment to study structural preferences of social behavior. In particular, we address the questions of how network topology benefits the establishment of new relations between the actants. Among others, we find that actants tend to get connected at a distance of 2. Further analysis indicates that the more common neighbors two actants have, the more likely they will be in relation with each other. We attribute this behavior to some kind of social pressure imposed by the neighborhood biasing the actants preferences. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U39.00011: Studying Human Dynamics Through Web Analytics Jose Ramasco, Bruno Goncalves When Tim Berners Lee, a physicist at the European Center for Nuclear Research (CERN) first conceived the World Wide Web (WWW) in $1990$ as a way to facilitate the sharing of scientific information and results among the centers different researchers and groups, even the most ingenious of science fiction writers could not have imagined the role it would come to play in the following decades. The increasing ubiquitousness of Internet access and the frequency with which people interact with it raise the possibility of using it to better observe, understand, and even monitor several aspects of human social behavior. Websites with large numbers of frequently returning users, such as search engines, company or university websites, are ideal for this task. The properly anonymized logs detailing the access history to Emory University's website is studied. We find that a small number of users is responsible for a finite fraction of the total activity. A saturation phenomenon is observed where, certain connections age, becoming less attractive to new activity over time. Finally, by measuring the average activity as a function of the day of the week, we find that productivity seems to be higher on Tuesdays and Wednesdays, with Sundays being the least active day. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U39.00012: Which Route Will You Choose to Use For Driving Home Tonight in Rush-Hour Traffic? Bogdan Danila, Yudong Sun, Kevin Bassler The best answer to the question posed in the title for a city of drivers requires knowing the optimal routes for congested traffic flow on complex networks. This is known to be an NP-hard problem. Despite this fact, we will present answers calculated in only polynomial time using extensions of a recently introduced heuristic algorithm [Danila, et al., PRE 74, 046106 (2006)] that, at least, scale optimally with network size. Using the optimal routes allows a network to support the maximum traffic load and significantly reduces the average travel time in congested traffic. The results presented apply to vehicular traffic and to traffic on wireless communication networks. [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U39.00013: Epidemic of cell phone virus Pu Wang, Marta Gonz\'alez , Albert-L\'aszl\'o Barab\'asi Standard operating systems and Bluetooth technology will be a trend for future cell phone features. These will enable cell phone viruses to spread either through SMS or by sending Bluetooth requests when cell phones are physically close enough. The difference in spreading methods gives these two types of viruses' different epidemiological characteristics. SMS viruses' spread is mainly based on people's social connections, whereas the spreading of Bluetooth viruses is affected by people's mobility patterns and population distribution. Using cell phone data recording calls, SMS and locations of more than 6 million users, we study the spread of SMS and Bluetooth viruses and characterize how the social network and the mobility of mobile phone users affect such spreading processes. [Preview Abstract] |
Session U40: Biomolecular Computation
Sponsoring Units: DBPChair: Margaret Cheung, University of Houston
Room: Morial Convention Center 232
Thursday, March 13, 2008 8:00AM - 8:12AM |
U40.00001: Hydrophobic polymers in nano-sized water droplets Buddhi Tilakaratne, Samina Masood, Margaret Cheung As simulations of biopolymers take place in confined and tight spaces, such as protein folding in the interior of bacteria chaperones or the exit tunnels of ribosomes, quantitative analyses of the confinement effects on both biopolymers and solvent molecules become the center of attention as the solvent-mediated interactions are too profound to solve analytically. We are in the progress to investigate the solvation of hexane molecules in various nano-sized water droplets. Free energy profiles for a single hexane molecule in droplets show that the droplet surfaces are favored. Averaged configurations of hexane molecules at the interior and the surface are computed using the umbrella sampling methods. The implications of our results for protein stability in confined spaces will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 8:12AM - 8:24AM |
U40.00002: A classical density functional for water David Roundy, Dennis Jackson We present a classical density functional for water that represents the short-range repulsive interaction using the fundamental-measure-theory hard-sphere functional. The parameters of this functional are chosen to reproduce the experimental liquid density, bulk modulus and surface tension of water, and to ensure coexistance of liquid and vapor phases. This functional inherits from the FMT functional its accurate description of reduced-dimensionality configurations. We will present computations of the hydrophobic hydration energy of hard-sphere solutes, demonstrating an accurate description at both large and small length scales. [Preview Abstract] |
Thursday, March 13, 2008 8:24AM - 8:36AM |
U40.00003: Hydrophobic interactions at molecular scale. Dubravko Sabo, Sameer Varma, Susan Rempe, Marcus Martin Structural and thermodynamic properties are investigated for one of the simplest hydrophobic solutes, a hydrogen molecule solvated in liquid water. The structural properties are calculated using different representations of the intermolecular interactions within molecular dynamics, Monte Carlo and ab initio molecular dynamics simulation frameworks. Although structural details differ in the radial distribution functions obtained by different force fields all approaches agree that 16 water molecules coordinate hydrogen. The thermodynamic properties are investigated using Monte Carlo molecular simulation and the quasichemical theory of liquids. Results show that the net hydration free energy arises from a balance between chemical association and molecular packing. Additionally, the results suggest the molecular packing is almost equally driven by unfavorable enthalpic and entropic components. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 8:48AM |
U40.00004: Computing absolute binding affinities via non-equilibrium unbinding simulations F. Marty Ytreberg We demonstrate that non-equilibrium unbinding simulations can be used to accurately estimate equilibrium absolute binding affinities ($\Delta G$). Utilizing the FKBP protein bound to two different ligands we estimate $\Delta G$ within less than 1.0 kcal/mol of experimental values. The methodology is straight-forward, requiring no modification to many modern molecular simulation packages. The approach makes use of a physical pathway, eliminating the need for complicated alchemical decoupling schemes. These results suggest that non-equilibrium simulation could provide a viable means to accurately estimate protein-ligand binding affinities. [Preview Abstract] |
Thursday, March 13, 2008 8:48AM - 9:00AM |
U40.00005: Macromolecular crowding effects on Brownian motion of protein GB1 Antonios Samiotakis, Margaret Cheung The effect of macromolecular crowding in the interior of a cell plays an important role in protein folding dynamics and its stability. In the present work the dependence of diffusion coefficients of the macromolecule on various crowding conditions is studied using a coarse-grained representation of protein G B1 domain that includes Go-like interactions. Using Brownian dynamics simulations, diffusion coefficients are computed as a function of the volume fraction of crowders $\phi _c $ and the ratio $\lambda$ of the sizes of the crowder over that of protein. Deviation from linear Stokes-Einstein relation will be discussed. [Preview Abstract] |
Thursday, March 13, 2008 9:00AM - 9:12AM |
U40.00006: Core-Shell Model of Folding-Unfolding Transitions (UFT) in Proteins Svetlana Aroutiounian There are $\sim $10$^{N}$ conformations for a protein of length $N$ to sort out randomly in search of lowest free energy state. Can protein folding be simple and fast? Core-shell model introduces principles, proposes mechanisms and scores residues of fast, reversible UFT in protein. According to it, during UFT the realm of intra-residual interactions leads the residue motion. The scaffold of hydrophilic residues forms external shell of unstructured, tube-like protein in unfolded state, just as the hydrophobic residues form internal scaffold -- core, of the protein in folded state. As UFT proceeds, residue slides into lowest-score position permitted by its structure. Model accounts for experimentally observed features of UFT. It is based on three principles: 1) During UFT protein is \textit{virtual} - its features or structure are inferred only statistically and with limited precision; 2) Mechanism of UFT memory is not longitudinal, but \textit{transverse}; 3) Native design overrides specific features of residues - the alphabet of amino acids assumes an \textit{intrinsic} score-function. Per-residue mechanism of UFT is proposed and score-function is described. Difference graphs of transitional score-function and average genome-wide abundance index show that our score-function \textit{is} the order parameter of UFT in protein and by virtue of being it, reveals transitional key residues. It echoes the multiple-tier and funnel concepts of FEL perspective. Monte Carlo simulations of UFT in myoglobin illustrate the idea. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:24AM |
U40.00007: Potential of mean force of Glycophorin A alpha-helix dimerization Lorant Janosi, Manolis Doxastakis Thansmembrane proteins recognition and association plays a crucial role in their assembly and function. In spite of the extensive studies, these processes are only partially understood. We investigate the association of the model transmembrane alpha-helices by means of potential of mean force (PMF) calculations. The model system consists of a pair of alpha-helices of Glycophorin A, a system that experimentally exhibits dimerization in lipid membranes. Using established coarse-grained models, we developed a Monte Carlo methodology to overcome sampling limitations imposed by long characteristic times present in lipid membrane simulations. A combination of the Expanded Ensemble Density of States formalism and hybrid molecular dynamics allow for efficient and accurate calculations on the association of the helices. The methodology developed offers unique insight into the mechanism of dimerization and provides a means to evaluate the effect of lipid composition and temperature on the association of Glycophorin A. [Preview Abstract] |
Thursday, March 13, 2008 9:24AM - 9:36AM |
U40.00008: Monte Carlo Simulations with reference interaction site model theory for simulating peptide molecules in aqueous solution Ayori Mitsutake, Yutaka Maruyama, Takashi Imai, Masahiro Kinoshita, Yuko Okamoto, Fumio Hirata We have developed Monte Carlo simulations with reference interaction site model theory for simulating proteins in aqueous solution. The reference interaction site model theory based on the liquid theory of statistical mechanics can treat solvent effect with solvent molecular shape and estimate solvation free energy around proteins. We have developed simulation algorithms which combine with generalized-ensemble algorithms and reference interaction site model theory. We showed results of a simulated annealing Monte Carlo simulation, a multicanonical Monte Carlo simulation, and a replica-exchange Monte Carlo simulation with one dimensional reference interaction site model theory for Met-Enkephalin, a penta-peptide [1,2,3]. Recently we have performed a Monte Carlo simulation with three-dimensional reference interaction site model theory for simulating C-peptide in aqueous solution. We will describe these attempts and discuss results of these simulations. [1] M. Kinoshita, Y. Okamoto, and F. Hirata, \textit{J. Am. Chem. Soc.}\textbf{ 120}, 1855 (1998) [2] A. Mitsutake, M. Kinoshita, Y. Okamoto, and F. Hirata, \textit{Chem. Phys. Lett.} \textbf{329}, 295 (2000) [3] A. Mitsutake, M. Kinoshita, Y. Okamoto, and F. Hirata, \textit{J. Phys. Chem. B} \textbf{108}, 19002 (2004) [Preview Abstract] |
Thursday, March 13, 2008 9:36AM - 9:48AM |
U40.00009: Glassy protein dynamics and gigantic solvent reorganization energy of plastocyanin David LeBard, Dmitry Matyushov This work focuses on the results of extensive explicit solvent Molecular Dynamics simulations of plastocyanin, a blue copper electron transfer protein involved in natural photosynthesis. Simulation data indicate that low-frequency non-ergodic fluctuations of the protein matrix tethered to the hydrating water are responsible for a very broad distribution of the vertical energy gaps of one-electron protein reduction/oxidation. The width of the corresponding free energy surfaces yields a reorganization free energy far larger than previously reported for any organic, inorganic, or biological chromophores. However, the Stokes shift is not affected by these slow motions and can be calculated from the polarization response function of the dipolar solvent using microscopic solvation models. The glassy nature of the protein-water interface breaks the direct link between the Stokes shift and the reorganization energy from equilibrium (ergodic) electron transfer theories. This suggests a mechanism that accounts for electron transfer in natural proteins, which are characterized by a low reaction free energy combined with a low activation barrier. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:00AM |
U40.00010: Modeling low frequency vibrational modes of large biomolecules Otto Sankey, Eric Dykeman Mechanical oscillations of proteins in their native state are relevant to understanding the flexibility of the protein assembly, the binding of substrates, the mechanical action involved in enzymatic activity, and the vibrational response to light scattering. Often, only the low frequency modes are of interest and coarse grained methods or other approximations are used due to the large size of the dynamical matrix. We introduce a computational approach, which exploits the methodology from electronic structure Order N methods, to find the vibrational modes below some frequency threshold (analogous to a Fermi-level in electronic structure theory). The approach allows systems to be described in atomistic detail. We use a generalized Born force field to model the interactions. Examples of normal modes for icosahedral viruses (e.g. satellite tobacco necrosis virus), tubular viruses (e.g. M13), and enzymes (e.g. lysozyme, HIV-protease, alpha-lytic protease) will be discussed. This effort is motivated by recent experimental work to produce high amplitude vibrations of viruses from impulsive stimulated Raman scattering. [Preview Abstract] |
Thursday, March 13, 2008 10:00AM - 10:12AM |
U40.00011: Monte Carlo simulations of Protein Adsorption Sumit Sharma, Sanat K. Kumar, Georges Belfort Amyloidogenic diseases, such as, Alzheimer's are caused by adsorption and aggregation of partially unfolded proteins. Adsorption of proteins is a concern in design of biomedical devices, such as dialysis membranes. Protein adsorption is often accompanied by conformational rearrangements in protein molecules. Such conformational rearrangements are thought to affect many properties of adsorbed protein molecules such as their adhesion strength to the surface, biological activity, and aggregation tendency. It has been experimentally shown that many naturally occurring proteins, upon adsorption to hydrophobic surfaces, undergo a helix to sheet or random coil secondary structural rearrangement. However, to better understand the equilibrium structural complexities of this phenomenon, we have performed Monte Carlo (MC) simulations of adsorption of a four helix bundle, modeled as a lattice protein, and studied the adsorption behavior and equilibrium protein conformations at different temperatures and degrees of surface hydrophobicity. To study the free energy and entropic effects on adsorption, Canonical ensemble MC simulations have been combined with Weighted Histogram Analysis Method(WHAM). Conformational transitions of proteins on surfaces will be discussed as a function of surface hydrophobicity and compared to analogous bulk transitions. [Preview Abstract] |
Thursday, March 13, 2008 10:12AM - 10:24AM |
U40.00012: Complexation of Flavonoids with Iron: Structure and Optical Signatures Jun Ren, Sheng Meng, Ch. E. Lekka, Efthimios Kaxiras Flavonoids exhibit antioxidant behavior believed to be related to their metal ion chelation ability. We investigate the complexation mechanism of several flavonoids, quercetin, luteolin, galangin, kaempferol and chrysin with iron, the most abundant type of metal ions in the body, through first- principles electronic structure calculations based on Density Functional Theory (DFT). We find that the most likely chelation site for Fe is the 3-hydroxyl-4-carbonyl group, followed by 4- carbonyl-5-hydroxyl group and the 3'-4' hydroxyl (if present) for all the flavonoid molecules studied. Three quercetin molecules are required to saturate the bonds of a single Fe ion by forming six orthogonal Fe-O bonds, though the binding energy per molecule is highest for complexes consisting of two quercetin molecules and one Fe atom, in agreement with experiment. Optical absorption spectra calculated with time- dependent DFT serve as signatures to identify various complexes. For the iron-quercetin complexes, we find a redshift of the first absorbance peak upon complexation in good agreement with experiment; this behavior is explained by the narrowing of the optical gap of quercetin due to Fe(d)--O(p) orbital hybridization. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 10:36AM |
U40.00013: When Cells Collide: A Model for Cell-Assisted Cell Growth based on Direct Contacts Carl Franck, Wui Ip, Albert Bae, Nathan Franck, Elijah Bogart, Thanhbinh Thi Le Although intercellular communication is frequently viewed as involving the transport of small molecules through an intracellular fluid medium, biologists have proposed chemical signaling with chemical specificity due to chemical recognition through direct contacts. Considering the collective computation behind the decision of a cell to divide when it senses the presence of a sufficient number of like neighbors, we offer a model for the transition from slow to exponential growth in shaken suspension cell culture of the model eukaryote, \textit{Dictyostelium discoideum}. Besides exploring an elegantly simple example of multicellular life, this discussion might well prove useful in considering the limits of cell culture on small spatial scales as required for contemporary massively parallel biotechnology. [Preview Abstract] |
Thursday, March 13, 2008 10:36AM - 10:48AM |
U40.00014: Do Porins Pass CAPs? C.B. Hanna, D.A. Pink, T.A. Gill, T.J. Beveridge, B.E. Quinn, J.J. Durrant, M.H. Jericho The cationic antimicrobial peptide (CAP) protamine is known to inhibit bacterial survival (Pink et al., \textit{Langmuir} \textbf{19}, 8852 (2003), and references therein), but the mechanism of attack is as yet undetermined. For Gram-negative bacteria, two pathways have been proposed: (a) self-promoted uptake, and (b) passage through porins. Here, we study the latter possibility, and model part of the outer membrane of a Gram-negative bacterium in an aqueous solution containing multivalent ions and CAPs. The intent is to determine whether CAPs could pass through porins and, if so, what aspects of external (e.g., ionic concentration) and internal (e.g., porin and O-sidechain characteristics) parameters affect their passage. This study is accomplished via Monte Carlo computer simulations of a ``minimal model'' of the outer membrane of a Gram-negative bacterium with an embedded porin. [Preview Abstract] |
Thursday, March 13, 2008 10:48AM - 11:00AM |
U40.00015: Multimode Analysis of SHG Signal from Complex Biological Systems: Parameterization of Regional and Global Features. Clayton Bratton, Karen Reiser, Andre Knoesen, Diego Yankelevich, Mingshi Wang, Israel Rocha-Mendosa We have continued development of our novel computational approach for quantifying structural disorder in biomolecular lattices with nonlinear susceptibility based on analysis of polarization-modulated second harmonic signal. Local disorder at the level of molecular organization is identified using a novel signal-processing algorithm sufficiently compact for near real-time analysis. Global and regional disorder within the biostructure is characterized using two-dimensional wavelet transform of the magnitude and phase of the second harmonic signal. Results suggest our signal processing method represents a robust, scaleable tool that allows us to detect both regional and global alterations in signal characteristics of biostructures with a high degree of discrimination. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700